Keywords: thermodurcissables biosourcés, degradation, bioraffinerie, propriétés thermomécaniques, analyse structurale, polymères furaniques Biobased thermoset, degradation, biorefinery, thermo-mechanical properties, structural analysis, furanic polymers

Cette thèse de doctorat s'est déroulée dans le cadre du projet ANR FUTURES (Furanic TUnable REsins for Sustainable materials, i.e., Résines furaniques modulables et durables). L'alcool polyfurfurylique (PFA) est un polymère thermodurcissable biosourcé qui possède d'excellentes propriétés chimiques et thermiques malgré un comportement cassant. L'objectif de ce projet est d'ouvrir la voie à de nouvelles applications pour le PFA en exploitant les réactions secondaires qui ont lieu lors de la polymérisation. Cela permettra d'apporter de nouvelles fonctionnalités au matériau réticulé et de jouer sur la fragilité du matériau. Les groupements C=O résultants de l'ouverture du cycle furanique lors de la polymérisation du PFA ont été quantifiés par potentiométrie et RMN quantitative. Les facteurs influençant le taux de carbonyles dans le polymère ont été évalués et la présence additionnelle d'eau au cours de la polymérisation s'est révélé être un facteur capital. L'environnement chimique des carbonyles a été apprécié par l'utilisation de RMN bidimensionnelle et un nouveau phénomène de réticulation de surface a été identifié et expliqué. L'impact de cette réticulation de surface sur les propriétés finales du matériau a été également mis en avant. Enfin, la présence de C=O résultant de l'ouverture de cycle a été exploitée pour fonctionnaliser le polymère post-réticulation. En utilisant des amines flexibles, le matériau passe progressivement d'un comportement cassant à un comportement plus ductile, permettant ainsi d'entrevoir de nouvelles applications. Les facteurs gouvernant les propriétés des matériaux fonctionnalisés par les amines ont également été étudiés.

Résumé détaillé

Cette thèse de doctorat s'est déroulée dans le cadre du projet ANR FUTURES (Furanic TUnable REsins for Sustainable materials, i.e., Résines furaniques modulables et durables).

L'alcool polyfurfurylique (PFA) est un polymère thermodurcissable biosourcé qui possède d'excellentes propriétés chimiques et thermiques malgré un comportement cassant. L'objectif de ce projet est d'ouvrir la voie à de nouvelles applications pour l'alcool polyfurfurylique en exploitant les réactions secondaires qui ont lieu lors de la polymérisation. Cela permettra d'apporter de nouvelles fonctionnalités au matériau réticulé et de jouer sur la fragilité du matériau.

Pour appréhender l'importance de ces ouvertures de cycle furanique pendant la polymérisation du FA, deux méthodes de quantification, par RMN du 19 F et par ont été adaptées.

En fonction de la méthode utilisée, il est possible d'obtenir des informations sur l'accessibilité chimique des groupements C=O au sein du PFA. D'autre part, le taux de carbonyles a été mesuré tout au long de la polymérisation. Également, deux types de résine PFA ont été préparés. Celle intitulée « PFA° » a été préparée uniquement en présence d'alcool furfurylique (FA) et d'un catalyseur. La résine intitulée « PFA+ » a été préparée dans un mélange 50/50 FA/eau. L'addition d'eau dès le début de la polymérisation permet d'augmenter significativement le taux de carbonyle.

En effet, à des taux de conversion élevés, le PFA° possède environ 2,2 mmol.g -1 de carbonyle contre 3.7 mmol.g -1 pour le PFA+, soit environ 1,7 fois plus. Une étude sur les leviers disponibles pour agir sur le taux de carbonyle a également été conduite (variation du catalyseur, solvent, température). Celle-ci a montré que l'eau est le facteur principal qui affecte le taux de C=O. D'autre part, celui-ci est limité à environ 3.7 mmol.g -1 pour des résines solubles.

Dans un deuxième temps, l'environnement chimique des carbonyles au sein de PFA° et PFA+ a été étudié par RMN bidimentionelle. Cette étude a permis de mettre en valeur les multiples environnements chimiques des composés carbonylés résultants de l'ouverture du cycle furane.

Cette étude a mis également en lumière la propension toute particulière qu'on les cycles furaniques en bout de chaine à s'ouvrir. En effet, les furanes terminaux au sein du PFA peuvent être soit sous forme de furanes méthylés soit mono substitués.

Dans le premier cas, les furanes méthylés sont susceptibles de se transformer en lactones pour les systèmes PFA°. Pour les systèmes PFA+, les furanes méthylés s'hydrolysent en unités levuliniques. Enfin, les furanes monosubtitués sont sensibles à l'oxydation. Leur oxydation par l'air engendre des groupements aldéhydes. Ceux-ci ont été quantifiés et peuvent représenter jusqu'à 20 % du total des carbonyles.

Il a été souligné que l'ouverture des furanes terminaux en aldéhydes conjugués par voie oxydative participe à un phénomène de réticulation de surface. En effet, ces aldéhydes conjugués sont susceptibles de réagir par cycloaddition Diel-Alder avec des furanes non conjugués. Cela se traduit par la formation d'une croute dure et riche en C=O en surface des résines PFAs. L'impact de cette croute a été étudié d'un point de vue macroscopique sur des échantillons de PFA oxydés et non-oxydés. Les effets sur les propriétés en traction et sur les propriétés thermomécaniques restent limités. L'épaisseur de la croute a été estimé à environ 20 µm. Enfin, les cyclo-adduits sont susceptibles de s'aromatiser lorsque le matériau est exposé à des températures supérieures à 150 °C.

La dernière partie de ce manuscrit est consacrée à la fonctionnalisation des C=O du PFA par des amines primaires. Dans un premier temps, une preuve de concept a été réalisée en utilisant la Priamine 1071, une amine flexible et biosourcée. Concrètement, en faisant varier le taux de Priamine, il est possible faire varier la température de transition vitreuse (Tg) du matériau entre 20 °C et 60 °C. Par conséquent, il est possible de multiplier par sept l'élongation à la rupture du matériau, tout en divisant par cent son module d'Young avec en optimum ténacité pour un rapport molaire NH2/C=O autour de 0.25.

Index of abbreviations

Introduction

Biomass and fossilized biomass (i.e. petroleum oil) represent the two sources of organic carbon available on earth. Refining and transforming crude oil allowed the development of modern life. In the course of the 20 th century, oil was employed at a constantly increasing rate , mostly to produce commodity chemicals, materials and to propel vehicles. However, our consumption of petroleum exceeds by far its natural formation. In addition, the uses of petroleum are associated with the release of massive amounts of greenhouses gases (GHGs) in the atmosphere. Ultimately, the GHGs are disturbing earth balance. International groups such as the Intergovernmental Panel on Climate Change (IPCC) are urging humanity to take actions to drastically reduce GHGs emissions. In addition, the fossilization process required to produce petroleum occurs in a geological timescale. To tackle these issues, biorefinery processes emerged in the past decade to generate chemical building blocks, biofuels and polymers from terrestrial plants and in particular lignocellulosic biomass. On a more European level, biorefineries could allow a partial independence from the massive use of petroleum derivatives.

As regards to the climate and oil-depletion concerns, the issues relative to human health and environmental pollution lead to new regulations. In 2006 the REACH legislation was ratified (Registration, Evaluation, Authorization and restriction of Chemicals). Nowadays, all chemicals manufactured and/or imported in Europe for volumes superior to one ton per years need to be registered to the ECHA (European CHemical Agency). The health and environmental risks of the registered chemicals may lead to high restrictions. The monomers required for the production of thermosetting resins fall under the REACH legislation. Indeed, these resins are widely employed in structural applications (furniture, transportation industry, …). Phenols, melamine, urea and formaldehyde are among the most commonly employed raw materials for thermosetting resins. Yet some of these compounds may be harmful to humans. For instance, the emission of the carcinogenic formaldehyde from furniture will be severely limited by 2026.

[1] Similarly, melamine is on the list of the substances of very high concern (SVHC). Phenols are also concerning compounds.

Yet, phenol, melamine, and formaldehyde are all commonly employed in the manufacture of thermosetting resins for structural purposes. Their excellent properties take roots in the rigidity of their molecular structure, especially the aromatic ones. Few sustainable alternatives are available. Lignins, polyphenols and tannins from woody biomass are promising alternatives. In addition, furanic molecules, such as furfuryl alcohol are also in the spotlight.

Furfuryl alcohol (FA) is one of the only compounds only manufactured from biomass in high volumes. It is produced from agricultural wastes, mainly sugarcane bagasse. FA can polymerize into poly(furfuryl alcohol) (PFA), a thermoset with properties somewhat similar to phenol, formaldehyde and melamine resins. Furfuryl alcohol is labeled both toxic and CMR.

Nonetheless, FA can be oligomerized and the residual FA removed. This allows the production of low VOC resins. In addition, FA is exclusively produced from biomass, more specifically from agricultural wastes. Hence, PFA could be a sustainable alternative to the aforementioned resins.

In the line of furanic compounds, industrial humins are produced as a side product of sugar conversion into furanic building blocks. One promising route of valorization is the use of humins as thermosetting resins. Humins are structurally close to PFA, i.e. a tight network of furanic units.

As a consequence, they share the same features, i.e., thermostability, difficult processing and brittleness. This PhD manuscript focus on the possibilities available to tune the properties and therefore provide new potential applications for both humins and PFA.

Introduction -14 -

The ring-opening reaction is a feature stemming from the unique reactivity of furanic macromolecule. Yet, the ring-opening side reactions were often neglected and disregarded.

Therefore, a gap of knowledge is present in this area.

The aim of this PhD manuscript is to take advantage of the carbonyls generated within the furanic network using PFA as a model in the scope of the project ANR JCJC FUTURES (FUranic TUnable REsins for Sustainable materials).

Objectives of the thesis

Exploiting the carbonyls formed within PFA and humins requires key knowledge on their occurrence. How many carbonyls are present and when do they form? Can the carbonyls be accurately and reproductively quantified? What are the leverages available to promote the furan ring-opening? In addition, precise knowledge on the chemical environment of the carbonyls is required to fully exploit their reactivity. Once the answers to these questions are established, the carbonyls present in PFA could be prone to chemical derivatization. To do so, this manuscript aims to pave the way to the functionalization of carbonyls, mainly using their reactivity with amines.

Once the feasibility of the functionalization of PFA's carbonyls will be proved, the properties of the resulting materials will be assessed. In other words, is it possible to achieve new amine functionalized PFA materials? Could their brittleness be mitigated? What is the array of properties achievable through amine-functionalization? Are they recyclable? Can the thermal stability of PFA be maintained?

The scheme on the next page summarizes the major objectives of the FUTUREs project.

Introduction -16 -

Chapter 4 focuses on the structural evolution of PFAs upon heating under inert and oxidative atmospheres. It aims at bridging the gap of knowledge regarding the structure of PFA between its polymerization and its graphitization. The effect of oxidation-induced surface crosslinking on the materials' properties is assessed.

Chapter 5 explores the possibilities of functionalization offered by the carbonyls within PFA.

This chapter exploits the reactivity of carbonyls with primary amines. After a proof of concept, the influence of several factors is determined. Namely, the amines structures, the post-curing environment, the advancement of PFA's polymerization and the influence of bases on furfuryl alcohol's acid-catalyzed polymerization.

Chapter 1. State of the art -19 -Chapter 1. State of the art 1.1 2 nd generation biomass: from wastes to value added products 1.1.1 Petroleum-based products at the crossroad of ubiquity and concern Since the late 1800s there has been a significant improvement in the life expectancy and overall quality of life for people worldwide. The industrial revolution allowed this change through a facilitation food production to sustain a growing population, the development of drugs to improve health, and modern comfort. In all these examples, the petroleum products played a crucial role either in the form of feedstock for the syntheses of fertilizers and drugs, in the form of fuels to motorize agriculture and transport goods, in the form of polymers to produce vehicles, machines and everyday household appliances.

However, in the past decades, concerns on the negative impacts of the petroleum industry rose dramatically. One ecological concern is the widespread burning of petroleum products in motorized vehicles or in heat and electricity generation (Table 1.1-1).

[2] Consequently, carbon dioxide, methane and other greenhouse gases are massively liberated and are increasingly disturbing the climate balance. In addition to ecological concerns, petroleum oil is a non-renewable resource which will only deplete in the future. Currently, about 14 % of the crude oil is used by the petrochemical industry. These 14 % play a major role in our lives, in the forms of fertilizers, drugs, specialty chemicals and mostly plastics. Hence, new feedstocks and new value chains are required to reduce ou dependence on petroleum. Table 1.1-2 provides an overview of the uses of plastics by sectors, all directly impacting us daily. Synthetic polymers are ubiquitous, they are found in the packaging used to increase food's lifespan, in our furniture either as a load-bearing elements or as glues, in paints, electric and electronic equipment biomedical implants and goods, textiles, vehicles and even in the coating of the cover of this manuscript. Mankind could decide to ban all these items to annihilate petroleum-induced issues. Yet, very few people would be willing to do so. Hence, new feedstocks and new value chains are required to reduce our dependance on petroleum.

Table 1.1-2. Shares of plastics production by Sectors, built on data from 2015. [START_REF] Geyer | Use, and Fate of All Plastics Ever Made[END_REF] Sector Yet, Europe is far from covering its needs by local production only. As a matter of facts, in 2021, Europe imported about 7.6 million of petroleum barrel per day which translates to about 1 000 000 tons per day.

[5] Therefore, increasing the share of renewable and locally available resources is essential to ensure Europe's security and independence in the face of future geopolitical perturbations and ultimately the depletion of fossil resources. In the following sections, the possibilities offered by living biomass, i.e. "carbons above the ground" to produce renewable chemicals and materials are overviewed.

1.1.2 Current and future opportunities for renewable feedstocks Renewable resources can be classed in several manners. One approach is to consider land use and edibility of the biomass. The biofuel industry commonly agrees on four generation of biomass, although one could extent this vision to all chemicals. They are illustrated in Figure 1.1-1.

The first generation corresponds to the use of edible biomass, such as starch (found in corn, wheat, …), to produce chemicals primarily through fermentation processes. It is the most developed generation with well-established industries such as Roquette Frères, NatureWorks or Total Corbion. For example, ethanol can be made in that manner to substitute or complement fuels.

As well, polymers such as poly(lactic acid) and bio-based poly(ethylene) may also be manufactured Chapter 1. State of the art -22 -from edible biomass. Although the chemicals from edible biomass are renewable, their production enters in competition with the production of food and impact its price. Furthermore, the reduction of GHGs may not be guaranteed when chemicals are sourced from renewable feedstocks. This is especially true in the case of not yet optimized production processes. [6] [7] Figure 1.1-1. Available and developing routes for the production of chemicals

The second generation encompasses resources obtained from non-edible biomass. Typical examples include wastes from crop production such as sugarcane or corn bagasse. Some chemicals such as furfural are produced by this route since decades. Yet, the production of other building blocks is still largely at laboratory or pilot scale. The furanic molecules emerging from the secondgeneration biomass often require a reduction reaction to be used as fuels. [START_REF] Lange | Furfural-A Promising Platform for Lignocellulosic Biofuels[END_REF] The products and value chains from this generation will be further detailed in section 1.2.

The third generation involves the non-edible and soil-less biomass. This route tames microorganisms such as algae and bacteria to produce chemicals. In most cases, the production of chemicals by this route is at a laboratory or pilot scale. Nonetheless, some polymers like xanthans, alginates and poly(hydroxy alkanoates) are produced for the food, pharmaceutical and cosmetic industries. [START_REF] Rehm | Bacterial Polymers: Biosynthesis, Modifications and Applications[END_REF] Chapter 1. State of the art -23 -

The third generation is highly promising as the yield of chemicals per m 2 is higher than the other routes. [START_REF] Srivastava | Effective Hydrolysis for Waste Plant Biomass Impacts Sustainable Fuel and Reduced Air Pollution Generation : A Comprehensive Review[END_REF] A fourth generation is emerging and utilizes genetic modifications to tailor the outputs of the third-generation biomass.

Another classifying method for renewable feedstocks is their fractionation in the key intermediates obtained from biorefineries. The International Energy Agency reported a series of major platform molecules. It should be noted that these intermediates are rarely single molecules but rather complex mixtures. Among the platforms, one can find pyrolysis oils, CO2, synthetic gases, bio-oils, organic solutions, lignins and sugars. [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF] To stay in the scope of this manuscript only the sugar and lignin platforms are discussed in the followings.

Lignocellulosic biomass

Before delving into the outputs of the sugar and the lignin value chain, a brief contextualization of their origin is helpful. The sugars and lignins come from the lignocellulosic biomass which constitute the major portion of the terrestrial plant kingdom. The macroorganization of plants is both complex and varying between species, thus it is not discussed here.

However, some key physico-chemical features are given.

First, the lignocellulosic biomass is composed mainly of three main macromolecules:

cellulose, hemicellulose and lignin. Cellulose is organized in microfibrils and gives the mechanical strength of plants. Lignins is the matrix embedding the cellulose fibrils. It brings hydrophobicity and further enhances the plants' structural integrity. [START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF] Finally, the hemicelluloses act as a compatibilizer between cellulose and lignin. They are chemically linked to lignin chains and interact trough hydrogen bonds with the celluloses. [START_REF] Faisal | Cell Wall Hemicellulose for Sustainable Industrial Utilization[END_REF] A simplistic illustration of this organization is provided in Figure 1.1-2. The interplay between the components of lignocellulosic biomass are source challenges for biorefinery as these physical interactions and chemical bonds need to be broken to produce platform chemicals. Furthermore, side-products are often generated during the treatment of the lignocellulosic biomass. This decreases the yields and it introduces a need to valorize the side-products.

Chapter 1. State of the art -24 -Figure 1.1-2. Illustration of the organization of celluloses, hemicellulose and lignins in lignocellulosic biomass. Drawing inspired by a previous illustration from Koddenberg et al. [START_REF] Rowell | Handbook of Wood Chemistry and Wood Composites 2nd Edition[END_REF] In terms of composition, significant variations are found between the plants as detailed in Table 1.1-4. Hence, biorefineries need to adapt their processes to accommodate the specificities of each feedstock. The feedstock composition is dependent both on the plant species and its growth conditions. The structures and applications of the three main macromolecules of lignocellulosic biomass are briefly reviewed in the following sections.

Table 1.1-4. Examples of lignocellulosic biomass composition Cellulose (%) Hemicellulose (%) Lignin (%) Reference Hardwood i 40 -55 24 -40 15-25 [13, 14, 15, 16, 17] Softwood 40 -50 25 -35 25-35 [13, 14, 16, 17] Agricultural residues ii 25-45 12-36 6-25 [17] Grasses 25-40 35-50 10-30 [17] i Hardwoods typically have broad leaves and are denser and darker and grow slower than softwoods. Hardwoods include species such as oak, elm, birch. Softwoods are conifers such as pine, fir, spruce, … ii Values for: wheet straw, rice straw/husk, barley straw, oat straw, corn cob/stalks, sugarcane bagasse Chapter 1. State of the art -25 -

Cellulose

Cellulose is the most abundant natural polymer. The structure of cellulose consists in a linear arrangement of D-glucose units. The most common structure is the 𝛼-1,4 cellulose, depicted in Scheme 1.1-1. [START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF] Scheme 1.1-1. Chemical structure of cellulose (type 𝛼-1,4)

Applications of cellulose developed through the ages, exploiting the properties of cellulose at different scales as illustrated by Scheme 1.1-2. The development of cellulose-based products began with use of fibers for textiles, followed by the rise of the paper industry. Then, the scope of cellulose uses expanded through the exploitation of its chemical reactivity. [START_REF] Arantes | The Current Status of the Enzyme-Mediated Isolation and Functionalization of Nanocelluloses: Production, Properties, Techno-Economics, and Opportunities[END_REF] The chemical derivatization of cellulose began with its nitration to nitrocellulose in the 1850s. Nitrocellulose was used as an explosive at that time and later found coating applications in the automotive and wood industries. [START_REF] Saunders | A Review of the Synthesis, Chemistry and Analysis of Nitrocellulose[END_REF] Once discovered possible, the chemical modifications of cellulose led to many cellulose-derived polymers still used nowadays. While the list is long, relevant examples include cellulose acetates mainly employed as membranes and filters [START_REF] Vatanpour | Chemosphere Cellulose Acetate in Fabrication of Polymeric Membranes : A Review[END_REF] and carboxymethyl celluloses as rheology modifiers in food, paints, as well as uses in drug delivery systems. [START_REF] Rahman | Recent Developments of Carboxymethyl Cellulose[END_REF] Other examples of cellulose derivatives include cellulose sulfates, nitrates and ethyl/methyl cellulose and so forth [START_REF] Liu | Trends in Food Science & Technology A Review of Cellulose and Its Derivatives in Biopolymer-Based for Food Packaging Application[END_REF] The uses of nano/micro fibrillated celluloses also gained interest in the past years. However, their production still faces challenges to reduce to production costs, energy consumption and routes to mitigate the hydrophilicity of the fibers. [START_REF] Rol | Recent Advances in Surface-Modified Cellulose Nanofibrils[END_REF] Chapter 1. State of the art -26 -Scheme 1.1-2. Cellulose based products technologies development, adapted from Arantes et al. [START_REF] Arantes | The Current Status of the Enzyme-Mediated Isolation and Functionalization of Nanocelluloses: Production, Properties, Techno-Economics, and Opportunities[END_REF] In contemporary times, research shifted towards increasingly complex applications such as cellulose nanocomposites [START_REF] Tanpichai | Review of the Recent Developments in All-Cellulose Nanocomposites : Properties and Applications[END_REF][START_REF] Sharma | Commercial Application of Cellulose Nano-Composites -A Review[END_REF], humidity sensors [START_REF] Li | Green and Sustainable Cellulose-Derived Humidity Sensors : A Review[END_REF] or additive manufacturing. [START_REF] Gauss | The Use of Cellulose in Bio-Derived Formulations for 3D / 4D Printing : A Review[END_REF] Another important field is the decomposition of cellulose to ethanol for biofuels to reduce petroleum consumption and mitigate greenhouse gases emissions. [START_REF] Robertson | Cellulosic Biofuel Contributions to a Sustainable Energy Future: Choices and Outcomes[END_REF][START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF] At last, cellulose can be used as a feedstock for the renewable production of platform chemicals. [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF][START_REF] Corma Canos | Chemical Routes for the Transformation of Biomass into Chemicals[END_REF] 1.1. [START_REF] Geyer | Use, and Fate of All Plastics Ever Made[END_REF]

.2 Lignin

The term "lignin" is a generic term to describe the products from the condensation of phenolic compounds called "monolignols". The most common ones are depicted in Scheme 1.1-3. Lignin is the most naturally abundant aromatic polymer on earth. [START_REF] Asif | Sustainability of Timber, Wood and Bamboo in Construction[END_REF][START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF] The chemical structure of lignin is both complex and highly dependent on the botanical origin. [30] An example of structure is provided in Scheme 1.1-4 although it is not representative of all the lignins, the main functional groups and linkages are depicted. The technical lignins are a sub-group of the lignins.

Technical lignins are mainly generated as byproducts of the paper industry and the production of cellulosic ethanol. [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF]31,32] In terms of uses, more than 95 % of the technical lignins are burned for energy recovery purposes. [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF]32] However, technical lignin derivatives called "lignosulfonates" are an established product in the construction market, especially as concrete admixtures. [31,32] Scheme 1.1-4. Example of lignin structure with some linkages, based on Ralph et al. [30] Chapter 1. State of the art -28 -Considerable resources are dedicated to the valorization of lignins. Ongoing research examples include the use of technical lignins in composites, in lead acid batteries, as dust suppressant, metal adsorbents, flocculants, or additives in polymers. [31,[START_REF] Aro | Production and Application of Lignosulfonates and Sulfonated Lignin[END_REF] Yet, the main field of research is the depolymerization of lignins. [START_REF] Asif | Sustainability of Timber, Wood and Bamboo in Construction[END_REF][START_REF] Wang | From Lignin to Valuable Products-Strategies, Challenges, and Prospects[END_REF] Depolymerizing technical lignins yields a variety of aromatic compounds which allow an increase in reactivity. The resulting monomers may be repolymerized to biobased polymers with properties competitive with conventional ones. [START_REF] Mahajan | Aromatics from Lignocellulosic Biomass: A Platform for High-Performance Thermosets[END_REF][START_REF] Decostanzi | Biobased Phenol and Furan Derivative Coupling for the Synthesis of Functional Monomers[END_REF] Lastly, the removal of functional groups allows access to aromatics such as benzenes, naphthenes and other hydrocarbons. [START_REF] Wang | From Lignin to Valuable Products-Strategies, Challenges, and Prospects[END_REF] 1.1. 3.3 Hemicellulose "Hemicellulose" is a general term originally proposed to refer to the extractable substances of plant cell walls under basic conditions. Nowadays, it is used to designate the non-cellulose polysaccharides parts of the plant's cell walls. The hemicelluloses are a mixture of six and five carbon saccharides and are typically connected with cellulose and lignin. The hemicelluloses can be divided in four main groups: the xyloglucans, xylans, mannans and 𝛽-glucans. Scheme 1.1-5

gives a general structure with the various linkage found in xylans, including the ferulic acid which serves as a covalent bridge to connect with lignin. [START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF][START_REF] Asif | Sustainability of Timber, Wood and Bamboo in Construction[END_REF] Chapter 1. State of the art -29 -Scheme 1.1-5. General structures of the various linkages found in a variety of xylan isolated from plant cell walls. This structure was adapted from Wertz et al. [START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF] The direct applications of hemicelluloses are currently limited by the high-cost associated with their extraction and purification. [START_REF] Arzami | Valorization of Cereal By-Product Hemicelluloses : Fractionation and Purity Considerations[END_REF][START_REF] Faisal | Cell Wall Hemicellulose for Sustainable Industrial Utilization[END_REF] Ongoing research aims to use native or derivatized hemicelluloses in packaging, cosmetics, food additives, biochar and drug delivery. [START_REF] Zhao | Hemicellulose-Based Film : Potential Green Films for Food Packaging[END_REF][START_REF] Figueiredo | Suberin and Hemicellulose in Sugarcane Cell Wall Architecture and Crop Digestibility : A Biotechnological Perspective[END_REF][START_REF] Arzami | Valorization of Cereal By-Product Hemicelluloses : Fractionation and Purity Considerations[END_REF][START_REF] Faisal | Cell Wall Hemicellulose for Sustainable Industrial Utilization[END_REF] Finally, as in the case of cellulose and lignins, hemicelluloses are investigated as a renewable feedstock for the production of fuels and platform chemicals. [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF][START_REF] Corma Canos | Chemical Routes for the Transformation of Biomass into Chemicals[END_REF][START_REF] Arzami | Valorization of Cereal By-Product Hemicelluloses : Fractionation and Purity Considerations[END_REF] 1.1. 3.4 Concluding remarks on the lignocellulosic biomass Among the options available to substitute fossil feedstock for renewable ones, the lignocellulosic biomass offers an array of opportunities and challenges. The major challenges include the selectivity of chemical reactions, the need for depolymerization and reduction of both polysaccharides and technical lignins. In the followings, some of these aspects will be discussed through the specific examples of furanic building blocks obtained from polysaccharides.

Chapter 1. State of the art -30 -

Furan Value Chain

Furanic compounds can be obtained from the lignocellulosic biomass via two routes. First, the hexose route is briefly reviewed followed by the pentose one.

1.2.1 Production and uses of furanic compounds from hexoses One of the main furanic compound derived from hexoses is 5-hydroxymethylfurfural (HMF). HMF can be obtained from a triple dehydration of hexoses, as shown in Scheme 1.2-1.

HMF is composed of three functional groups, an aldehyde, an alcohol and a furan heterocycle.

Scheme 1.2-1. Dehydration of hexoses to HMF.

In 2023, the market of HMF is still small with between 10 to 100 tons manufactured and/or imported in Europe each year. [START_REF](hydroxymethyl)-2-furaldehyde[END_REF] Yet the high functionality of HMF allows a wide range of derivatives and promising applications. [START_REF] Van Putten | A Versatile Platform Chemical Made from Renewable Resources[END_REF] Laboratory scale examples include the mild oxidation of HMF to furan-2,5-dicarbaldehyde. This dialdehyde was polymerized with amines to produce polymers with physical and chemical reprocessability. [START_REF] Liguori | Designed from Biobased for Recycling: Imine-Based Covalent Adaptable Networks[END_REF] HMF may also partially substitute formaldehyde in urea/formaldehyde resins. In addition, HMF and derivatives may be incorporated in other polymeric networks such as, poly(urethanes) or poly(acrylates). [START_REF] Thoma | Hydroxymethylfurfural and Its Derivatives: Potential Key Reactants in Adhesives[END_REF] A more mature application of HMF derivatives is the production of poly(ethylene furanoate) (PEF). PEF is advertised by the Avantium company as a replacement for poly(ethylene terephthalate), one of the most produced polymers (Mtons per year). In comparison with PET, PEF has the advantages of higher barrier properties, i.e., longer storage capability of food and beverages, higher Tg and elastics modulus. Moreover, PEF is compatible with the current shaping processes of PET. [START_REF] Forestier | Understanding of Strain Induced Crystallization Developments Scenarios for Polyesters: Comparison of Poly(Ethylene Furanoate), PEF, and Poly(Ethylene Terephthalate)[END_REF]45,[START_REF] Forestier | Comparative Analysis of the Mechanical Behaviour of PEF and PET Uniaxial Stretching Based on the Time/Temperature Superposition Principle[END_REF] Chapter 1. State of the art -31 -Additionally, PEF is compatible with the current recycling streams of PET. [START_REF] De | The Road to Bring FDCA and PEF to the Market[END_REF] Nowadays, the production of PEF is at the pilot scale. The process uses the methoxylated version of HMF (5-(methoxymethyl)-2-furaldehyde) which is oxidized to 2,5-furandicarboxylic acid (2,5-FDCA). 5-(methoxymethyl)-2-furaldehyde is employed instead of HMF for stability purposes. More specifically, the ethers of HMF form much faster than HMF decomposes under the acid conditions required for its formation. In addition, ethers of HMF are much less susceptible to ring-opening reactions and condensation into humins. [START_REF] Gruter | Furanics : Novel Fuel Opt¡ons from Carbohydrates[END_REF] 2,5-FDCA is then copolymerized with ethylene glycol to reach PEF. The route to PEF is illustrated in Scheme 1.2-2.

Scheme 1.2-2. Synthesis route of PEF from 5-(methoxymethyl)-2-furaldehyde.

Furan compounds are sensitive to ring opening reactions. Hydrolysis of HMF, yields levulinic acid (LA) and formic acid. If HMF is open in alcoholic solvent or if an ether form of HMF is used, alkyl levulinates are obtained. On one hand, methyl levulinate is a co-product of the sugar conversion to 2,5-FDCA in the Avantium process. [START_REF] De | The Road to Bring FDCA and PEF to the Market[END_REF] On the other hand, levulinic acid is produced by the petrochemical industry in relatively small volumes and from furfural in even smaller volumes. The yearly volumes of LA manufactured and/or imported in Europe are between 100 to 1000 tons. [49] Nowadays, LA has a few applications in the pharmaceutic, cosmetic and food industries. [START_REF] Van Der Waal | Avantium Chemicals: The High Potential for the Levulinic Product Tree[END_REF] Yet, more than a decade ago, LA was identified as one of the most promising platform chemicals and its potential still holds true today. [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF][START_REF] Bozell | Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates-the US Department of Energy's "Top 10[END_REF][START_REF] Okolie | Next-Generation Biofuels and Platform Biochemicals from Lignocellulosic Biomass[END_REF] Indeed, the carboxylic acid and ketone functionality of LA enables a wide range a derivatization and some of them are depicted in Scheme 1.2-3. LA can be transformed into specialty chemicals such as halogenated or amino compounds, acrylates, ketals, alcohols, carboxylic acids and so forth.

Chapter 1. State of the art -32 -

The highest potential of the levulinate derivatives resides in their performance as fuel additives. Indeed, the C1 to C4 esters of LA were labeled "sustainable fuel additives". They generally increase fuel properties such as the cold flow, spark property, flash point, stability and so forth. [START_REF] Badgujar | A Review on Catalytic Synthesis of Energy Rich Fuel Additive Levulinate Compounds from Biomass Derived Levulinic Acid[END_REF] Scheme 1.2-3. Examples of levulinic acid and levulinate derivatives. [START_REF] Van Der Waal | Avantium Chemicals: The High Potential for the Levulinic Product Tree[END_REF][START_REF] Rackemann | The Conversion of Lignocellulosics to Levulinic Acid. Biofuels, Bioprod[END_REF] 1.2.2 Production and uses of furanic compounds from pentoses

The five carbon saccharides within hemicellulose can be exploited for the production of furanic compounds. The biggest markets for the furan from hemicellulose are the furfural and furfuryl alcohol markets.

Furfural

Furfural or 2-furaldehyde (CAS 98-01-1) is among the few organic compound which are only produced from renewable feedstocks. The manufacture of furfural through the biomass route is often reported as more economically competitive than the petroleum route although it is not specified how much. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF][START_REF] Kabbour | Furfural as a Platform Chemical: From Production to Applications[END_REF]57] Chapter 1. State of the art -33 -Chemically, furfural is composed of a furan heterocycle moiety and an aldehyde group, as shown in Scheme 1.2-4. It is a good example of a naturally occurring compound which can be harmful to humans.

Furfural is found at low concentrations in coffee, tea, cocoa and some fruits. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF] In addition, furfural participate in the smell of old books.

[58] However, furfural is labeled as a toxic and CMR (carcinogenic, mutagenic and/or reprotoxic) substance. It's sister molecule, furfuryl alcohol suffers of the same safety issues. Hence, these compounds need to be handled carefully.

Fortunately, such safety issues can be circumvented in the case of polymerized FA, as discussed later in section 1.3.1.

Scheme 1.2-4. Chemical structure of Furfural.

Starting from the beginning, furfural was discovered in the 19 th century. The large-scale production of furfural began in the early 1920s with the Quarker oats company. At the time, the goal was to valorize cereal side-products, i.e., oat hulls. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF][START_REF] Kabbour | Furfural as a Platform Chemical: From Production to Applications[END_REF] Nowadays, it is estimated that 200 000 to 360 000 tons/year of furfural are produced worldwide, mainly from corn cobs and sugarcane bagasse in China, Dominican republic and South Africa. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF][START_REF] Kabbour | Furfural as a Platform Chemical: From Production to Applications[END_REF] According to the ECHA, furfural is registered for manufacture and/or import volumes between to 10 000 and 100 000 tons each year in the European economic area.

[59] The production of furfural usually follows the steps depicted in Scheme 1.2-5.

Scheme 1.2-5. Illustration of the two-step production process of furfural.

Chapter 1. State of the art -34 -First, the hemicellulose in the biomass is hydrolyzed to pentoses. The second step consists in the dehydration of monomeric sugars to furfural. These steps are usually carried out with nonoxidant mineral acids and at temperatures between 150 -190 °C. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF][START_REF] Kabbour | Furfural as a Platform Chemical: From Production to Applications[END_REF][START_REF] Yong | Furfural Production from Biomass Residues: Current Technologies, Challenges and Future Prospects[END_REF] Usually, furfural is continuously evacuated by steam distillation to limit the formation of side-products. [START_REF] Kabbour | Furfural as a Platform Chemical: From Production to Applications[END_REF] In terms of yields, the literature reports that around 45-50 %w of the pentoses are converted to furfural for most processes [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF][START_REF] Yong | Furfural Production from Biomass Residues: Current Technologies, Challenges and Future Prospects[END_REF]. The main feedstocks, i.e., corncobs and sugarcane bagasse, comprise up to 30 %w of pentosans. Consequently, the yield of furfural is below 20 %w, with respect to the dry biomass. More recently, the Biofine process tackled the yield issue by developing units capable of coproducing furfural, formic acid and levulinic acid. This aims at increasing economic and ecological viability. [32,[START_REF] Kapanji | Techno-Economics of Lignocellulose Biorefineries at South African Sugar Mills Using the Biofine Process to Co-Produce Levulinic Acid, Furfural and Electricity along with Gamma Valeractone[END_REF] According to Hoydonck et al. and Zeitsch et al., the major factor limiting the yield of furfural is the product from the acetalization of furfural, depicted in Scheme 1.2-6. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF]57] The acetals, saccharides and other side-products may condensate to give furan bearing macromolecules called humins, an infamous biorefinery side-product. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF]32] More details about humins are provided in section 1.2.4.

Scheme 1.2-6. Acetalization of furfural, adapted from. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF]57] Current research on the production of furfural is focused on the improvement of the furfural yield, diminution of steam requirements and uses of less corrosive catalysts. [START_REF] Yong | Furfural Production from Biomass Residues: Current Technologies, Challenges and Future Prospects[END_REF] In addition, water/organic solvents biphasic systems are studied to separate furfural from the water and thus limit furfural's degradation and condensation reactions, i.e., humins. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] Pawar | Sulfonic Acid Anchored Heterogeneous Acid-Catalyst DICAT-3 for Conversion of Xylose into Furfural in Biphasic Solvent System[END_REF][START_REF] Zhao | Enhanced Furfural Production from Biomass and Its Derived Carbohydrates in the Renewable Butanone-Water Solvent System[END_REF] Other works focus on increasing the selectivity either during the dissolution of the hemicellulose or the dehydration of the pentoses or both. [START_REF] Luo | The Production of Furfural Directly from Hemicellulose in Lignocellulosic Biomass: A Review[END_REF] Chapter 1. State of the art -35 -

Here, the goal is to preserve the cellulose and lignin so they can be used for other purposes.

In this line, the coproduction of furfural with ethanol and lignin was techno-economically assessed.

[65] Zhang et al. extensively reviewed articles investigating the production of furfural with innovative solvent systems and catalysts. [START_REF] Zhang | Recent Progress in Direct Production of Furfural from Lignocellulosic Residues and Hemicellulose[END_REF] Another relevant field of research regarding the production of furfural is the mitigation of furfural pollution in waste water. [START_REF] Ghosh | A General Review on the Use of Advance Oxidation and Adsorption Processes for the Removal of Furfural from Industrial Effluents[END_REF] Regarding its applications, furfural can either be employed directly or converted in other chemicals. An overview of the applications of furfural is available in Figure 1.2-1.

Figure 1.2-1. Applications of furfural. Scheme built on references. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF]57 ] Chapter 1. State of the art -36 -

The direct applications of furfural are mainly as a solvent. Here, the ability of furfural to interact strongly with C=C bonds and weakly with alkyl compounds is exploited. Examples include the extraction of aromatic compounds in hydrocarbon systems such as lubricants, dyes, diesel, and so forth. In these cases, furfural is used for its high affinity with aromatic compounds and thus to remove them from the product. Another use of furfural as an extraction agent is to recover unsaturated molecules from vegetable oils or tall oil, a residue of the paper industry. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF]57] Furfural can be found in pesticides (fungicides, herbicides, nematicides). In addition, furfural may be used as a wetting agent in bitumen systems. In this case, furfural increases the binding strength between the bitumen and the gravel agent which induces a better cohesion and thus the road's lifespan. For this process about 600 kg of furfural are required by kilometers of road.

[57]

Furfural can enter in the composition of furan resins. It may be alone or compounded with other molecules such as FA, urea, acetone, phenols and so forth. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF] On its own, furfural can polymerize overtime or when exposed to acids or bases. Finally, furfural was highlighted as one of the most promising platform chemicals. [START_REF] Bozell | Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates-the US Department of Energy's "Top 10[END_REF] Thanks to its aldehyde group and aromatic reactivity, furfural can give birth to more than eighty molecules, some of them are depicted in Figure 1.2-1.

[69] Nowadays, the majority of furfural is reduced to furfuryl alcohol. The synthesis and uses of FA are detailed in the next section of this chapter.

Furfuryl Alcohol

Furfuryl alcohol (FA) (CAS 98-00-0) is a colorless to pale yellow liquid. The chemical structure of FA is depicted in Scheme 1.2-8. . Scheme 1.2-8. Chemical structure of furfuryl alcohol.

Industrially, FA is produced through the hydrogenation furfural, mainly in vapor phase using copper-chromium catalysts with a selectivity superior to 99 % and yields above 95 %. The reaction is depicted in Scheme 1.2-9. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF] The production of FA represents 60 to 70 % of the worlds' consumption of furfural. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF] In terms of volumes, between 10 000 and 100 000 tons of FA are manufactured and/or imported in Europe each year. [70] The hydrogenation of furfural to FA is a well-established process. However, research is still conducted on this synthesis, especially to replace the chromium catalyst for less polluting ones.

[71] Additionally, researchers are also looking for ways to produce FA directly from sugars in onepot systems or using alternative solvents such as deep-eutectic solvents. [START_REF] Sixta | Towards the Green Synthesis of Furfuryl Alcohol in A One-Pot System from Xylose : A Review[END_REF][START_REF] Qin | Efficient Synthesis of Furfuryl Alcohol from Corncob in a Deep Eutectic Solvent System[END_REF] Scheme 1.2-9. Industrial synthesis of FA from furfural.

Chapter 1. State of the art -39 -

In terms of applications, the major use of FA is the manufacture of poly(furfuryl alcohol) and more generally furan resins. This subject is thoroughly detailed in section 1.3.1. FA also finds other uses as a solvent. It can be a reactive solvent in phenolic resins, a viscosity reducer for epoxy resins and a component of paint strippers. [START_REF] Hoydonckx | Furfural and Derivatives H. In Encyclopedia of industrial chemistry[END_REF] The use of FA as an anti-freeze agent was also reported in the 1970s. [START_REF] Schmitt | Alcohol Resins[END_REF] Furthermore, FA can enter in the composition of hypergolic mixtures.

They were used to propel early French and American rockets.

[57] Although FA-propelled rockets are more than sixty years old, research is still conducted nowadays to valorize its fuel wastes. [START_REF] Chalmpes | Nanocarbon from Rocket Fuel Waste: The Case of Furfuryl Alcohol-Fuming Nitric Acid Hypergolic Pair[END_REF] The fact that FA is able to form hypergolic mixture further supports the need for careful handling.

This is especially true when exposing FA to strong acids and/or oxidizers.

Furfuryl alcohol can serve as a building block. Indeed, some compound such as tetrahydrofuran and an anti-ulcer drug (ranitidine) can be manufactured from FA. However, a promising use of FA as building block is the manufacture of levulinic acid. As detailed in section 1.2.1 LA is a promising platform molecule. It can be manufactured from HMF and also from FA.

The mechanism of FA's opening to LA was investigated by Gonzales et al. and their suggested mechanism is available in Scheme 1.2-10. [START_REF] Maldonado | Experimental and Theoretical Studies of the Acid-Catalyzed Conversion of Furfuryl Alcohol to Levulinic Acid in Aqueous Solution[END_REF] The manufacture of levulinate esters can also be performed using alcohols to yield alkyl levulinates. [START_REF] Chappaz | Selective Conversion of Concentrated Feeds of Furfuryl Alcohol to Alkyl Levulinates Catalyzed by Metal Triflates[END_REF] In all cases, humins are produced and their structures might be close to PFA's.

Scheme 1.2-10. Proposed mechanism for the ring opening of FA to LA in aqueous media. [START_REF] Maldonado | Experimental and Theoretical Studies of the Acid-Catalyzed Conversion of Furfuryl Alcohol to Levulinic Acid in Aqueous Solution[END_REF] Chapter 1. State of the art -40 -

Routes from furan to molecules

To summarize, the lignocellulosic biomass is composed of lignin, cellulose and hemicellulose. The lignocellulosic biomass is a renewable source of aromatic compounds as well as sugars containing five or six carbons. These later can be into furanic compounds. Furanic compounds have a developing potential as chemical feedstock and already developed applications such as thermosetting resins. The term « humins » can have several meanings. It can refer to the part of soil organic matter that remains insoluble in alkaline solutions. [START_REF] Hayes | Its Composition and Importance in Soil Organic Matter[END_REF] In the context of this manuscript, the term "humins" is used to describe the variety of biorefinery side products from the treatment of lignocellulosic biomass. [START_REF] Van Putten | A Versatile Platform Chemical Made from Renewable Resources[END_REF] Humins can be formed through the condensation of sugars with dehydrated sugars, i.e., furans, in acid conditions. They are typically obtained in biorefinery processes aiming at the production of furanic molecules. The humins can also be the result of processes using furanic molecules as feedstock. Indeed, in acidic conditions furan-on-furan condensation reaction can occur. For example, humins are formed during the production of levulinic derivatives from furfuryl alcohol.

The structure of humins was extensively studied in the past decade, although a comprehensive understanding is beyond the scope of this section. [START_REF] Velasco | Mechanistic Investigation into the Formation of Humins in Acid-Catalyzed Biomass Reactions[END_REF][START_REF] Zandvoort | Molecular Structure, and Morphology of Humins in Biomass Conversion : Influence of Feedstock and Processing Conditions[END_REF][START_REF] Herzfeld | Molecular Structure of Humin and Melanoidin Via Solid State NMR[END_REF][START_REF] Shen | Evolution Process and Controlled Synthesis of Humins with 5-Hydroxymethylfurfural (HMF) as Model Molecule[END_REF] Yet, it is worth mentioning that the structure of humins is variable depending on the chemical processes and feedstocks. Generally, the chemical structure of humins consists in furanic moities, conjugated and aliphatic moieties functionalized with ketones, aldehydes, alcohol and carboxylic acid. As examples, the proposed structure for xylose derived and glucose derived humins are displayed in Scheme 1.2-11.

Nowadays, humins are a constraint on the growth of economically viable biorefineries due to several unwanted effects. Firstly, the formation of humins reduce the yield of targeted products (LA, furans, …). Secondly, humins deposits can deactivate heterogeneous catalysts. Finally, humins complicate reactor design as they can cause plugging and fouling on the reactor walls. The deposits interfere with heat transfer and increase the maintenance of the reactors. [START_REF] Van Putten | A Versatile Platform Chemical Made from Renewable Resources[END_REF] To address these issues, companies are looking for ways to decrease the yield of humins and/or altering their physical state. For example, Avantium uses alcoholic solvents in their process to obtain highly viscous humins instead of solid ones. [32,[START_REF] De | The Road to Bring FDCA and PEF to the Market[END_REF] Chapter 1. State of the art -42 -As humins are currently unavoidable, the humins are valorized trough heat generation or composting. [START_REF] Van Putten | A Versatile Platform Chemical Made from Renewable Resources[END_REF][START_REF] De | The Road to Bring FDCA and PEF to the Market[END_REF] Regarding the composting, an ecotoxicological study was performed on Avantium's humins and showed no immediate concern. [START_REF] Muralidhara | Humins in the Environment: Early Stage Insights on Ecotoxicological Aspects. Biofuels, Bioprod[END_REF] Scheme 1.2-11. Proposed structure for xylose and glucose derived humins. [START_REF] Zandvoort | Molecular Structure, and Morphology of Humins in Biomass Conversion : Influence of Feedstock and Processing Conditions[END_REF] Numerous studies focused on the valorization of humins. Examples include the use of humins as feedstock to produce formic and acetic acid by oxidation of humins.

[84] The liquefaction of humins to produce biofuel additives or chemical feedstock was also foreseen. [START_REF] Wang | Catalytic Liquefaction of Humin Substances from Sugar Biore Fi Neries with Pt/C in 2 -Propanol[END_REF] The field of carbonaceous compounds also investigated the humins as precursor to microporous materials such as sorbents for wastewater or catalyst support, [START_REF] Björnerbäck | Microporous Humins Prepared from Sugars and Bio-Based Polymers in Concentrated Sulfuric Acid[END_REF][START_REF] Kang | Valorization of Biomass Hydrolysis Waste: Activated Carbon from Humins as Exceptional Sorbent for Wastewater Treatment[END_REF] carbonaceous foams [START_REF] Tosi | Auto-Crosslinked Rigid Foams Derived from Biorefinery Byproducts[END_REF] or electrodes for supercapacitors. [START_REF] Chernysheva | Sustainable Utilization of Biomass Refinery Wastes for Accessing Activated Carbons and Supercapacitor Electrode Materials[END_REF] In the material field, it was demonstrated that once thermally activated, humins behave as a thermosetting resin. [START_REF] Sangregorio | Humins from Biorefineries as Thermoreactive Macromolecular Systems[END_REF] In addition, humins were studied in copolymerization with epoxies [START_REF] Dinu | Cross-Linked Polyfuran Networks with Elastomeric Behaviour Based on Humins Biorefinery by-Products[END_REF][START_REF] Dinu | Reprocessable Humins Thermosets and Composites[END_REF], with FA in jute composite [START_REF] Sangregorio | Natural Fibre Composites with Furanic Thermoset Resins . Comparison between Polyfurfuryl Alcohol and Humins from Sugar Conversion[END_REF] with FA in cellulose composites [START_REF] Pin | Valorization of Biorefinery Side-Stream Products: Combination of Humins with Polyfurfuryl Alcohol for Composite Elaboration[END_REF], as wood reinforcing agent [START_REF] Sangregorio | Humin Based Resin for Wood Modification and Property Improvement[END_REF] or bitumen additives. [START_REF] Sangregorio | Furanic Humins from Biorefinery as Biobased Binder for Bitumen[END_REF] Chapter 1. The main application of PFA and FA-based furan resins i in general is the manufacture of foundry molds. The fabrication of foundry molds typically involves 0.8 to 2 % of binder, the rest being aggregates.

[97] The formulation of the furan resins may comprise FA alone, i.e., leading to PFA, or in combination with molecules such as formaldehyde, furfural, urea, phenol or resorcinol.

The production of foundry molds and cores can be performed following several processes using numerous formulations. The conditions are dependent on the size of the molds, production volume, mechanical strength and type of metal or alloy desired. [98,[START_REF] Pizzi | D20220101-20221231&q=nftxt%20%3D%20%22foundry%22%20AND%20nftxt%20%3D%20%22furan%20[END_REF] The pictures in Figure 1. [113] and the molds' mechanical properties [114], effect of reclaimed sand (i.e. recycled sand)

[115], assessment of the release of aromatic compounds during the casting of metals [116] and the list goes on. In addition, 3D printing technologies using sand and furan resins are also investigated.

[117, 118] industries where harsh sanitizers are required. These systems are however not tolerant to oxidizers. [START_REF] Schmitt | Alcohol Resins[END_REF]97,[START_REF] Pizzi | D20220101-20221231&q=nftxt%20%3D%20%22foundry%22%20AND%20nftxt%20%3D%20%22furan%20[END_REF] (B) (A) In the past twenty years, researchers explored the properties and applications of furan-based polymer concretes. This includes investigations on of the mechanical properties and effect of the aggregate type and size on the resulting concrete. [121,122,123] In addition, investigation of furan resins to consolidate sand in oil wells [124,125], soil stabilizers for road and helipads [126] were conducted together with the formulation of "all weather" repair resins for concretes and asphalt structures. [127] The reinforcement of oil wells with thermoset such as PFA seems to be a relatively active field as patents affiliated to sand consolidation are still issued nowadays. As for most thermosets, PFA is also employed in the composite industry. PFA has applications in fiber-glass composites were resistance to acids or halogenated solvents is required.

Uses in pipes, tanks and scrubbers were reported. With the increasing interest for more sustainable materials, a significant number of PFA reinforced with natural fibers composites were studied. For instance, semi-synthetic fibers (Rayon)

were impregnated with PFA and characterized by Malaba et al. [159] The carbon storage capacity and mechanical properties of hemp reinforced PFA were investigated as well. [160,148] Kenaf fibers composites were also studied by Deka et al. [161] In addition, systems comprising PFA and flax fibers were thoroughly investigated. The studies include the effect of aligned and/or random fibers on the properties and processability of PFA and flax composites. [142,148,162,163] Overall, in adhesive systems, introducing FA or PFA in the formulations often aims at either reducing the amount of formaldehyde or replacing phenolic compounds.

Over the years, a wide array of PFA nanocomposites were prepared and characterized. First, the polymerization kinetics of FA catalyzed by montmorillonite clays was investigated [179,180] along with the mechanical and thermal properties of the resulting nanocomposites. [181,182,183] Other publications using inorganic non-carbonaceous fillers are mostly focused on PFA reinforced with silica. In some cases, the thermal degradation pathways and fire resistance where at the center of the articles. [184,185] [188,189,190] Scheme 1.3-1. Example of surface functionalization strategies proposed for silica particles (top)

[186] and carbon nanotube (bottom) [192,198].

A significant part of the nanocomposites reinforcing a PFA matrix involve carbonaceous nanoparticles. First, the works involving monodimentional fillers. For instance, phenylsulfonate functionalized single-walled carbon nanotubes [191] and COOH functionalized ones [192] were mixed with PFA. Ionic and esters bonds were proposed as the interface between the fillers and the matrix. The tribological properties of PFA filled with multiwalled carbon nanotubes functionalized with various chemicals were studied by Men et al.[193,194] As a matter of fact, PFA is recognized since at least the 1970s for its performance as a carbon precursor. Industrially, solutions of FA are impregnated on carbon scaffolds, polymerized and then carbonized under inter atmosphere. This densifying process densifies the material and the voids are removed. The impregnation may be repeated one or more time. As a consequence, products such as large carbon electrodes, heat exchanger tubing and anti-corrosion bearing for vacuum pumps can be manufactured. [74 97, 57] Both phenolic and furan resins can be used to impregnate and to densify carbon scaffolds. An example of impregnated graphite with a phenolic resin is available in Figure 1.3-8.

. Oddly, articles are also dealing with photopolymerization and FA. As a matter of fact, some compound called photo-acids can decompose upon exposure to light and release acids capable of initiating the polymerization of FA. More oddly, when using such initiation "wrinkled" materials can be achieved. An example wrinkled PFA is provided in Figure 1. 3-9. A few publications are dedicated to the understanding and control of the morphology of the wrinkles as they may give better results than lithography if large areas are targeted. [261,262] In the late 1990s a series of publications investigated the use of PFA with acrylic molecules to produce negative photoresists.

[263] Other procedures where PFA prepolymers were spin-coated on silicon wafer or solvent casted on glass plates prior to an exposure to UV-light were investigated. A photopolymerization pathway for the curing of PFA prepolymers prepared with trifluoroacetic acid was suggested. [264,

265,] Nonetheless, it is not clear if the film produced in these papers is the consequence of photopolymerization or the combination of the heat from the UV lights and left-over acids.

Chapter 1. State of the art -58 - [261]

Most of the applications and potential applications of PFA mentioned earlier aimed at the manufacture of materials. Nonetheless, PFA oligomers can be fuel precursors. Indeed, small oligomers (C5-C25) from FA's polycondensation can be hydrogenated to produce renewable fuels.

Examples of studies in this field include the investigation of the thermodynamics and reaction pathways of the oligomerization of FA [266] or the kinetic of polymerization [267]. In addition, To conclude, PFA is mostly used in composite applications were thermal and chemical stability are required. Nowadays, the use of PFA is often branded as an eco-friendly material thanks to its renewable sourcing and possibility to be used as a low VOCs prepolymer. Finally, the applications of furan resins, including PFA, reviewed in this chapter are summarized in Figure 1. The polymerization of FA in acidic medium is complex and debated since the 1940s.

Nowadays, there is a consensus on the major structural features and crosslinking mechanism. Yet, the proportions of each repeating units are still debated and were never quantified. The following sections are dedicated to the known features of PFA.

The polymerization of FA: structural features the polymerization

The understanding of the polymerization mechanisms of FA began in the 1940s around the same time as the Quaker Oats process was launched. In their article, Dunlop and Peters, employees of Quaker Oats, studied the "insoluble form of furfuryl alcohol". They observed a release of water upon heating FA. From this and in combination with elemental analysis and chemical function tests they deduced the formation of oligomers according to Scheme 1. 3-3. [270] This linear structure is still widely accepted nowadays with a number of repeating units reaching from four and ten. However, two major features of PFA were still not understood at the time. First, while FA is colorless, PFA oligomers range from yellow to dark red and the final material is black, as showcased in Figure 1. 3-11. i However, none of the molecules in Scheme 1.3-4 display signs of colored organic molecules, such as high conjugation. Moreover, oligomers of PFA with between four to ten units are soluble and were isolated several times. Yet, the polymers would undergo a rapid crosslinking after a certain threshold. This left researchers with only hypotheses on the crosslinking mechanisms of PFA.

i Somethimes, a dark green color can be observed at the very beginning of the polymerization Coupling NMR, UV/vis spectroscopy, a trityl cation and inspiration from an earlier paper of Furthermore, the authors concluded that the crosslinking reactions in PFA involve the conjugated 2,5-dihydrofurans. They suggested a crosslinking pathway through the Diels-Alder 2,4 cycloaddition depicted in Scheme 1. 3-9. Although no direct evidence of the reaction was given in this paper, one could look at papers providing 13 C NMR of cured PFA. In several of them, both old and recent, a broad 13 C NMR signal can be observed between 120 and 135 ppm and interpreted as non-conjugated the C=C in the Diels-Alder cycloadduct. [278,282,283,284] Scheme 1. An important feature of the polymerization of FA is its high exothermicity. As a matter of fact, the polymerization of FA with concentrated strong mineral acids was more than once reported as potentially explosive. [START_REF] Schmitt | Alcohol Resins[END_REF]127,182] The heat of polymerization can be easily visualized using a thermal camera. Figure 1. [START_REF] Geyer | Use, and Fate of All Plastics Ever Made[END_REF][4][5][START_REF] Srivastava | Effective Hydrolysis for Waste Plant Biomass Impacts Sustainable Fuel and Reduced Air Pollution Generation : A Comprehensive Review[END_REF][START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] Lange | Furfural-A Promising Platform for Lignocellulosic Biofuels[END_REF][START_REF] Rehm | Bacterial Polymers: Biosynthesis, Modifications and Applications[END_REF][START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF][START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF][START_REF] Faisal | Cell Wall Hemicellulose for Sustainable Industrial Utilization[END_REF] showcases the heat produced when about 2 mL or FA mixed with 0.5 mL of 50 %v H2SO4. Under these conditions, in about 10 seconds the outsides of the test tube reached nearly 100 °C. More quantitatively, the total heat release of FA's polymerization using mild catalysts is comprised between 400 and 700 J.g -1 depending on the conditions. [180] Such high values of enthalpy make DSC a technic of choice when studying the polymerization of FA.

DSC can be used to determinate the conversion degree of a given resin using Equation 1.3-1.

Chapter 1. State of the art -69 - The conventional methods to prepare PFA are either at ambient temperature with strong mineral acids or with mild acids and conventional heating in an oil bath or an oven. Some articles focused on the effect of other processes. The polymerization of FA under UV light was already reported in section 1. suggested that the formed PFA oligomers may contribute to the brown color of roasted coffee.

[314]

Chapter 1. State of the art -70 -Finally, it should be mentioned that shaping a PFA material is not an easy task. This challenge is the result of the polycondensation reactions, i.e., the release of a substantial amount of water. Indeed, as detailed in Table 1.3-4, 100 g of FA yield about 80 g of PFA and 20 g of water.

If no precautions are taken, the samples end-up in foamy structures which are usually unwanted.

This may be counter acted by the use of slow curing, i.e. over tens of hours, with gradually increased temperature. [192,231] 1935. [317] For instance, the carbonyls of bio-oils and lignins were quantified by oximation [318,319] and were confronted to further non-wet carbonyl quantification techniques using UV spectroscopy, gas chromatography [320, 321] and 13 C NMR [322]. Finally, large arrays of hydrazine-based derivatization methods followed by 19 F NMR quantification were dedicated to the carbonyl classification and quantification in bio-oils, lignins and humins. [323,324,325,326] Among all the studies mentioned in this chapter, the quantification of carbonyls in contained in PFAdue in particular to the ring opening -was never performed. Furthermore, means to control the ring-opening reaction were not investigated. Finally, the reaction between C=O and amines were suggested to occur in PFA/Epoxy/Amine systems.

[218] Yet, the derivatization of the C=O as a source of tunability in furanic resin has never been a focus of research. In the following chapters, the quantification and control of C=O over the course of FA's polymerization is performed. The nature and chemical environment of PFA's carbonyls is also discussed. Finally, the final chapter is dedicated to the chemical functionalization of the C=O in PFA to tune its properties.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -77 -Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA

In the previous chapter the state of the art on PFA was detailed. Although numerous studies were dedicated to PFA, very few of them had the ring-opening side reaction as their focus. This chapter aims at breaking through the knowledge of the ring-opening side reaction in PFA. The following questions will be addressed. How can one quantify carbonyls in PFA? How many carbonyls are present in PFA systems? When does the side-reaction occur? How can one optimize the carbonyl content of PFAs?

Section 2.1 regroups the experimental details of this chapter.

Section 2.2 deals with the development of C=O quantification methods adapted to furanic macromolecules. In addition, the rate of formation of carbonyls in two kinds of PFA is also assessed. Furthermore, lab-scale humins are prepared and their C=O content quantified.

Section 2.3 deals with the synthesis of laboratory-scale humins and their characterization, more specifically their carbonyl content.

Section 2.4 is dedicated to the probing of the leverages available to control the carbonyl content of PFAs such as the type of catalyst and amount of water.

Finally, section 2.5 gives insight on the chemical nature of carbonyls within PFAs. Water was distilled with a SI analytics distillation unit (conductivity ≈1 µS/cm).

All chemicals were used as received, excepted FA which was used as received (i.e. commercial grade FA). Or was fractionally distilled two times at atmospheric pressure (distilled FA). The purity of distilled FA was investigated by DSC and found to be better than 98%.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -79 -

Preparation of PFA resins:

For section 2.2, two kinds of PFA resins, namely PFA° and PFA+, were prepared. PFA° or PFA 100/0 refer to a PFA prepared in neat conditions and with 2 %w of MA. PFA+ or PFA 50/50 was prepared in 50 % aqueous solution from the beginning of polymerization and 2 %w of MA. It may be noted that FA and MA both have a molecular weight of 98.1 g.mol -1 . Thus, in a solution of FA, 2 %w of MA = 2 %mol of FA. In all cases, the percentage of catalyst is calculated on the amount of FA.

Typical syntheses of PFA were performed as follow: First, a room temperature mixing was conducted to ensure complete homogenization of components in 150 mL glass round bottom flask equipped with water condenser before initiating the polymerization. In this chapter, the batches of polymers were about 20-40 g, based on the initial amount of FA. Heating was controlled with regulated oil bath. The solutions were then gradually heated. In most cases, the temperature was set to 80-85 °C for 1 h, then the condenser was removed, and the temperature gradually increased up to 120 °C over the course of 1 to 5 h, depending on the conversion degree desired. In Section 2.4, many PFAs were prepared and analyzed. Their conditions of preparation are reported in Table 2.7-1 and Table 2.7-2. Samples were collected at different moments in the process, and stored at 4

°C. The sampling process is illustrated in Scheme 2.1-1.

Scheme 2.1-1. Sampling process.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -80 -

Carbonyl quantification by wet methods:

Based on previous work of Faix et al. and Black et al. [318,319] This protocol was replicated with phenylhydrazine hydrochloride (ΦNHNH2.HCl) as a replacement of NH2OH.HCL. Only difference was the need to preheat ΦNHNH2.HCl solution at 80 °C for solubility purposes.

Acids quantifications were conducted as follows. To the DMSO -PFA solution 2 mL of 80 %v EtOH were added followed by 2 mL of 0.52 N triethanolamine solution in 96 %v EtOH.

Samples were triplicated and controls prepared every time.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -81 -

In each case, vials were placed in an oven at 80 °C for 24 h before titration by 0.08 N HCL.

Samples were allowed to cool down to room temperature before titration. HCl concentration was determined with dry Na2CO3. Carbonyl contents were calculated using Equation 2.1-1. V and V0

are the endpoints of samples and blanks, respectively. Endpoint were determined using inflexion point (3.5 < pH < 4.0). mPFA is the mass of PFA. The DOS obtained from this method is called DOSTitri.

𝐶 = 𝑂 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 = (𝑉 0 -𝑉) 𝑚 𝑃𝐹𝐴 * [𝐻𝐶𝑙] Equation 2.1-1
To calculate degree of open structures within the resins, Equation 2.2-1was re-adapted in Equation 2.1-2 taking into account the C=O content.

𝐷𝑒𝑔𝑟𝑒𝑒 𝑜𝑓 𝑜𝑝𝑒𝑛 𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒𝑠 (𝐷𝑂𝑆) = 1 2 . 1000. 𝐶𝑂𝑐𝑜𝑛𝑡𝑒𝑛𝑡. 𝑀 (𝑓𝑢𝑟𝑓𝑢𝑟𝑦𝑙 𝑢𝑛𝑖𝑡) Equation 2.1-2

As the oximation kinetics of carbonyls within PFA was unknown, experiments were carried out to determine the optimal duration of oximation. Carbonyl quantification by 19 F NMR:

Based on previous work of Constant et al. [326], the following protocol was adopted, it is illustrated in Scheme 2.1-2. In 1.5 mL vial, 70 -130 mg of PFA were introduced, followed by about 140 mg of 4-(trifluoromethyl)-phenylhydrazine and (CF3ΦNHNH2), 20 mg of 1,4bis(trifluoromethyl)benzene internal standard. Then, 400 µL of d6-DMSO were added before being shaken for ≈ 10 mins followed by 5 s centrifugation. Contents of vials were transferred to NMR tubes before being heated at 40 °C for 24 h. Just before analysis, 100 µL of 28 mg.mL -1 relaxing agent -chromium acetyl acetonate-in d6-DMSO were added and homogenized.

As highlighted by Constant et al.,[326] commercial CF3ΦNHNH2 contains about 3 % of an impurity which overlaps with hydrazones. Using a reference containing CF3ΦNHNH2 and the internal standard, the impurity amount was determined for every batch and used as a correction factor. Carbonyl contents were calculated using Equation 2.1-3.

Fourier-Transform Insfrared Spectroscopy (FTIR):

A Thermoscientific Nicolet IS 20 equipped with an ATR device (Pike technologies gladiator -3 mm diamond crystal diameter-) was used. Spectra were recorded from 400 to 4 000 cm -1 with accumulation of 64 scans and a resolution of 4 cm -1 with spectrum of air as background.

OMNIC and Origin softwares was used for data analysis.

Throughout the polymerization of FA, a very intense band is observed around 1010 cm -1 .

It is mainly attributed to FA's furan ring C-O stretching at 1012 cm -1 which overlaps with 2,5disubstituted ring C-H in plane wagging at 1018 cm -1 (among others), leading to a progressive shift. [293,294] Overall, the band at ≈ 1000 cm -1 is intense and stable enough to be used for normalization. Normalization is mandatory here as it reduces the influence of samplediamond interface which affects peaks intensities. Especially as PFA samples present progressive modification of their physical states from liquid, viscous and rubbery resins.

Lab scale humins

Lab-scale humins were prepared using process similar to van Zandvoort et al. [80,329] Fructose (1 M in distilled water) was used as a sugar source and H2SO4 was employed for acidification. Humins were either prepared in a Teflon lined autoclave (AC, Figure 2.1-5) or in a round bottom flask (RBF). Table 2.1-1 reports the conditions of preparation of the lab scales humins and their yield. The crude humins were washed with distilled water, dried at 50 °C, ground, washed with distilled water and dried again. Figure 2.1-5 depicts the typical aspect of lab-scale humins. The yield of humins was calculated using the masses of washed and dried humins. The C=O content was determined by titrimetry only. Three kinds of humins were prepared, in a attempt to have more or less condensed humins. In Table 2.1-1 "Hu1" correspond to the most condensed humins, i.e the harshest conditions, Hu3 is the least consented humins and Hu2 is an intermediary. Two categories of PFA resins were synthetized. In the first type of resin, FA was polymerized in presence of an acidic initiatormaleic anhydride (MA)without addition of any protic polar solvent to limit as much as possible the furan ring opening (PFA°). On the contrary, the second type of resin was prepared in 50 % aqueous solution (PFA+) to favor the ring opening reaction without compromising the polymerization reaction. For the first time, the carbonyl content was quantified with accurate methodologies (i.e. NMR quantification or titration) and not only quantitatively assessed. Interestingly the evolution of carbonyl content and consequently the C=O content was investigated with the course of polymerization. Different resins were prepared at different time/temperature of polymerization and the conversion degree of polymerization of each resin was precisely assessed from DSC and FTIR data.

Carbonyls in PFA were determined and quantified via two strategies which take advantage of the high reactivity of carbonyls towards hydrazine and hydroxylamines compounds to respectively form hydrazones and oximes. The first method was based on the oximation reaction of carbonyls with hydroxylamine hydrochloride followed by potentiometric titration. A preliminary investigation was necessary to understand the role played by the oximation time, and presence of acids. The second methodology involved a 19 F NMR quantification employing a fluorinated phenyl hydrazine. Finally, 13 C quantitative solid-state NMR was carried out to corroborate with the other methods.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -89 -

Validation of the quantification methods

The main objective of this section is to breakthrough knowledge on the carbonyl formation in PFA resins through the comparison of the oximation with the 19 NMR quantification.

First, PFA+ samples were collected during the polymerization (from undistilled FA) to understand the process of carbonyls formation. A progressive darkening associated with crosslinking development is observed and it is correlated with a progressive increase of the viscosity (Figure 2.1-4). For each sample, the conversion degree of FA (𝛼) was determined and employed to monitor the evolution of polymerization independently of time or temperature conditions.

However, the complex polymerization process of FA involves several reactions. Therefore, highly exothermic reactions can mask nearly athermic ones. This effect is especially significant for solid state reactions. [301] To circumvent this, the polymerization was also monitored using FTIR. This spectroscopic technique is complementary to DSC as it will be more sensitive to structural changes when athermic reactions in the solid state are becoming predominant. End of polymerization stages will be barely distinguishable with DSC while FTIR bands can vary strongly. This is the case for C=O stretching which can be seen from 1650 to 1870 cm -1 in FTIR spectra. [330]. As demonstrated by Faix et. al, the normalized FTIR C=O area is linear with the C=O content of lignins, provided the absence of species such as ketals and esters.

[319] The same rationale was employed herein. Figure 2.2-1 shows the FTIR spectrum of undistilled FA (𝛼 = 0) and the spectral evolution with the conversion of FA. The formation of PFA network is clearly seen in the progressive increase of the peak at 1560 cm -1 that corresponds to the C=C stretching of 2,5-disubstituted furan rings. In the meantime, a broad peak between 1650 cm -1 and 1800 cm -1 is developing with conversion. This region corresponds to the C=O stretching of carbonyls. The appearance of this peak can be also noticed at early stage (𝛼 = 0.12). Such evolution is the direct consequence of the progressive carbonyl formation in the system. Therefore, the area of this peak can be taken to monitor the formation of carbonyls during FA polymerization. The oximation and the 19 F NMR methods are in the same range in the first stage of polymerization. On the other hand, there is a clearcut difference at the end of polymerization. When NH2OH.HCl is employed, the carbonyl content continue to increase while a plateau is observed at the end of polymerization when the two phenylhydrazines are employed. Therefore, the nature and/or the size of the derivatizing agent is the most influencing factor on the quantification of carbonyl rather than the method itself (i.e. titration or quantitative NMR). It is likely that bigger derivatizing agent -i.e. CF3ΦNHNH2 or ΦNHNH2.HCl -with much more steric hindrance than the mobile NH2OH.HCl -will have more difficulty to react with carbonyls. This will be especially the case for the late phase of polymerization where carbonyls will become less accessible with the progressive increase of the cross-link density. Besides, it could be argued that the oximation method leads to further ring opening as it liberates HCl in the medium. However, ΦNHNH2.HCl method exhibits a plateau in the same way as the CF3ΦNHNH2 derivatization method. Liberated HCl is therefore not inducing further furanic ring openings in significant amounts.

On a side note, Constant et al. obtained 2.35 mmol.g -1 of C=O in industrial furanic humins using the 19 F method. [326] This is consistent with the most advanced PFA samples studied here (2.46 mmol.g -1 for PFA+ with 𝛼 = 0.92).

During the polymerization of FA, (hemi)acetals and/or (hemi)ketals might form the carbonyls as alcohol and water are present in the present acidic system. Effect of (hemi)acetals or (hemi)ketals was investigated using bromoacetaldehyde dimethyl acetal as an acetal model and subjecting it to carbonyl quantification methods. Results from this investigations are reported in In Figure 2.2-3, four peak can be distinguished. The one on the extreme left corresponds to CF3ΦNHNH2. The three other peaks are products of the reaction between CF3ΦNHNH2 and 2,5hexanedione. When the three later peaks are integrated a C=O content of 17.0 mmol.g -1 is obtained and is equal to the theoretical one (M2,5-hexanedione = 114.14 g.mol -1 , with two C=O per molecule).

From this it can be deduced that the 19 F NMR method does not induce the formation of pyrroles, at least in the conditions used here. Hence, pyrroles are not responsible for the inconsistencies observed in Figure 2.2-2. The presence of three peaks can be attributed to the various isomers of the product (Z/Z, Z/E and E/E). Separated 19 F chemical shifts for diastereoisomers was demonstrated by Constant et al. [326] Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -94 - When opened the C2 and C5 carbons of the furan ring will be transformed in two aliphatic carbons, considering an opening within the macromolecular chain. Therefore, the carbonyl content can be approximated using 208 ppm peak corresponding to the ketonic species. The integration of ketonic carbons at 208 ppm (13.6) rationalized to the integration from C3 + C4 furanic carbonsset to 100 -gives a carbonyl content of 3.4 mmol.g -1 for PFA+ at 𝛼 = 0.92. These results are in perfect agreement with the oximation results (Figure 2.2-2) for which the carbonyl content is 3.5 ± 0.2 mmol.g -1 at 𝛼 = 0.92. Where NC=O is the number of carbonyls (ketones) and Nfuranic is the number of furanic units.

In the example of Scheme 2.2-1 there are C=O for 20 furanic units (N.B: the cycloadduct count for two furanic units) which thus give a DOS = 0.15. It means that 15 % of furanic units have been opened is this example Scheme 2.2-1. Simplified scheme of PFA and its potential opened structures.

As highlighted in Scheme 2.2-1 and Equation 2.1-2, the DOS is a more visual concept than the carbonyl content per gram of PFA. The following section aims at comparing the DOS evolution during polymerization both in PFA+ system, i.e 50 % aqueous, and in a neat PFA° system. For this purpose, both titration and 19 NMR quantification were employed to quantify the DOS in PFA° and PFA+.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -97 - Overall, at a given conversion, PFA+ system exhibits higher DOS than PFA° (Figure 2.2-5).

This difference is particularly marked for 𝛼 > 0.20 and suggest higher ring opening rate for PFA+.

It therefore confirms that a polymerization in 50 % aqueous solution of FA will favor the formation of carbonyl groups. This difference can be explained by the increased rate of formation of furfuryl carbocations in polar solvents as shown by Kim et al. [289] Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -98 -

The formation of the furfuryl carbocation is a rate limiting step for the condensation reactions. Correlations can also be made with previous studies in which silylated furfuryl alcohol furfuryloxytrimethylsilane -and furfuryl acetate were employed to avoid hydrolytic ring opening due to water produced during condensation reactions. [281, 276] Furthermore, Shi et al. also reported the importance of water in the formation of humins and hydrolytic furan ring opening.

[332]

The DOS' increase throughout the polymerization ( In the present study, a good correlation coefficient is obtained for PFA+ (employing the distilled and undistilled batches) for a part of the polymerization process. This underlines the existence of other phenomenons which affects either the normalized FTIR C=O area or the DOS or both. The presence of water in the samples could be an explanation as it might dilute the PFAs oligomers and increase normalized FTIR C=O area. Notwithstanding, contamination with water can be seen in FTIR with the broad peak at 1640 cm -1 appearing in the spectrum of PFA+ at 𝛼 = 0.12 (Figure 2.2-1). Accordingly, such points have been removed from Figure 2.2-6. Moreover, samples with higher conversion degree PFA is highly apolar, leading to very low residual water.

For PFA°, the deviation might lie in the presence of a shouldering around 1750 cm -1 (Figure 2.7-1).

Indeed, such shape suggests that other carbonyl species are present in this system, possibly endchain esters or lactones as previously postulated. Considering all these data, the mean DOSNMR/DOSTitri ratio for PFA° is 0.57 ± 0.07 and this ratio increase to 0.73 ± 0.03 for PFA+. The gap between titration and 19 F NMR quantification is significantly more marked for PFA° than for PFA+. As explained in the first section, a high content of (hemi)acetal or (hemi)ketals in PFA° would increase the DOSTitri and thus decrease the DOSNMR/DOSTitri ratio. On the other hand, the DOSNMR might be more sensitive to steric hindrance around the carbonyls than the DOSTitri. The higher gap between NMR and titration for PFA° would indicate that its carbonyls are generally less accessible than in PFA+. They may either be less accessible from a physical point of view (steric hindrance) or from a chemical point of view (protected ketals) or both.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -101 -On a side note, one could define a "FTIR conversion degree" using the heights of the disubstituted furans (1560 cm -1 ) and the monosubstituted ones (1500 cm -1 ). When these heights are extracted from a normalized spectrum and are used in a conversion degree formula such as Equation 2.1-4, Equation 2.2-2 is obtained.

FTIR conversion degree = 𝐻 1560 𝐻 1560 +𝐻 1500 Equation 2.2-2
Using Equation 2.1-4 and Equation 2.2-2 a DSC conversion degree and an "FTIR conversion degree" can respectively be obtained. These conversion degrees were plotted against each either to give Figure 2.2-7. Interestingly, rather high linear correlation coefficient may be obtained (R² > 0.987), especially if low conversion degrees are not taken into account. Such method could be useful to quickly characterize batches of PFAs prepared in the same conditions. Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -102 -

Conclusions section 2.2

The aim of this section was to investigate the degree of opened structures (DOS) within a PFA° -and in a PFA+ -MA intiated (i.e. in 50 % aqueous solution during polymerization stage).

To do so, two carbonyl quantification methods were adapted to PFA. Results showed an linear increase of DOS both for PFA° and PFA+ all along the polymerisation. However the rate of DOS increase was much faster for PFA+ resulting in final resins containing about two times more open structures than polymerization without additional water (PFA°). This is consistent the starting assumption of Falco et al. that water promotes ring opening reactions.

[283] This somehow highlights that, depending on conditions, the formation of carbonyl species can be controlled. Both quantification methodologies by means of wet titration and 19 F NMR spectral integration have agreed that DOS continously increase with the course polymerization but the carbonyl content is strongly dependent of the derivatizing agent's nature. Hinderred hydrazine employed for the 19 F NMR quantification lead to lower CO content compared to oximation with the small hydroxylamine hydrochloride. This somehow reflect the potential reactivity of these γ-diketones that are rather sensitive to the steric hindrance from the employed reagents or their protection under ketal forms. DOSNMR were invariably lower than DOSTitri and a DOS plateau was reached for DOSNMR in late polymerization stages. Finally, results from derivatization-based methods were confirmed by 13 C quantitative NMR.

Overall, the present investigation showed that the DOS of PFAsconsidered so far as a side reaction -could be somewhat controlled throughout the polymerization and maximized when using 50 % aqueous solution. The different pathways to determine the DOS can inform on the chemical availability of carbonyls within PFA. The following section is an investigation of the leverages available to control the ring opening reaction.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -103 -

Carbonyl content in Lab-scale humins

This section is dedicated to the C=O content of lab-scale humins. Here, three kinds of humins, more or less condensed, were prepared. Finally, several attemps of producing humin-based materials using the procedures developped in Chapter 5 were attempted. It was not possible to achieve materials using solely the lab-scale humins, most-likely due to their insolubility and highly consensed states. However, it might possible to integrate them in PFA-Humins systems. Another likely successful possibility would be to use consensed yet soluble humins.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -106 -

Conditions affecting the carbonyl content in PFA resins

The main purpose of this work was to study the furan ring opening focusing on certain aspects such as the effects of the acidic initiator, the presence or not of additional water and its ratio on the ring opening. In addition, the last sub-section reports all the others means attempted to increase the C=O content in PFA.

First, the role of the acidic initiator on the furan ring opening was studied by comparing Brönsted initiators such as citric acid, oxalic acid, acetic acid, with Lewis initiators such as boron trifluoride, iodine, alumina or montmorillonite K10 (MMT-K10). Most have already been used in the past [180,273,286,333]. A Brönsted superacid such as trifluoromethanesulfonic acid as well as initiators combining the two acidities such as cation exchanged montmorillonite with alkylammonium and bismuth triflate were also employed. Indeed, cation exchanged montmorillonite (Org-MMT) displays a dual acidity which, as highlighted by Zavaglia et al. leads to an acceleration of the FA polymerization rate.

[180] Therefore, the dual acidity of Org-MMT might arguably affect the ring opening occurring during polymerization. This goes also for bismuth triflate which was shown to promote the ring opening of FA to alkyl levulinates. [START_REF] Chappaz | Selective Conversion of Concentrated Feeds of Furfuryl Alcohol to Alkyl Levulinates Catalyzed by Metal Triflates[END_REF] The effect of additional water on the ring opening was investigated as well.

It should be noted that the control of the conversion degrees of PFA prepared with different catalyst is no easy task. They can behave in completely different manners, especially the MMT and Org-MMT PFAs. In addition, the amount of catalyst can significantly vary depending on the catalyst. For example, 10 %w of Al2O3 was necessary to obtain a resin while Tf(OH) had to be diluted in water. Otherwise, the polymerization with Tf(OH) occurred instantly at room temperature. In addition, some catalysts were heterogeneous and could not be removed from the resins without altering it. For all these reasons, PFA° and PFA+, both MA based, serve as references.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -107 -

Effect of the catalyst

As stated previously, different initiators were investigated. For the Brönsted type, citric acid and oxalic acid were chosen as they have already been used for preparing PFA, as well as acetic acid for its industrial accessibility [333].

A Brönsted superacid, namely trifluoromethanesulfonic acid (TfOH) was also investigated.

As for the Lewis type, boron trifluoride in methanol solution was chosen for being in liquid state. The results plotted in Figure 2.4-1, suggest that the nature of acidic initiator (i.e. Lewis vs.

Brönsted) does not particularly affect the DOS. Indeed, at a similar reaction progress, such as I2 and citric acid or BF3 and MA, Lewis and Brönsted acids do not notably stand out from one another.

Neither do the combined acidity of Org-MMT. Moreover, in all cases the DOSTitri were always below 0.10 for the PFAs prepared in neat conditions.

The results from the 19 F NMR method are available in Figure 2.7-4. A point worth noting is the undeniable difference between the titration and NMR results. They follow the same trend, however the DOSTitri is always superior to the DOSNMR as discussed in section 2.2.

These investigations confirmed that the DOS progressively increases with the conversion degree. However, the type of acidity does not particularly influence the DOS. Another aspect worthy of examination was the role of added water on the DOS.

Effect of Water in combination with catalysts

In order to determine whether the presence of additional water and its ratio affect the DOS two approaches were implemented.

First, the effect of additional water was studied by conducting several syntheses in which an array of initiators was tested with and without additional water in a same amount in the system, i.e. the initial water amount which will come in addition to the water generated during the polycondensation. The FA/water ratio was fixed to 50/50 as for PFA+ in section 2.2.

The results plotted in Figure 2.4-2 confirms that the presence of water does influence the DOS by increasing it. The 19 F NMR method in Figure 2.7-5 presents the same trend. Indeed, at a similar conversion degree the DOS is higher when the reaction was performed in the presence of additional water.

As stated, the results of this investigation show that the added water plays a role in the furan ring opening in agreement with both Falco et al. [283] and section 2.2. In addition to this investigation, it is clearly highlighted here that the increase in DOS is not strongly related to the acid initiator. In other words, all the systems show a DOS increase when additional water is present. These investigations allowed to determine that adding water plays a role in the opening of the furan ring but the ratio of added water does not impact the DOS that is reached near the end of polymerization. Yet, lower amount of additional water such as 10 %w of water allows higher DOS at lower conversion degrees and thus lower viscosities. This may be exploited to maximize the C=O content of PFAs while remaining at a convenient viscosity.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -111 -

Other attempts to increase the C=O content of PFAs

In this section, various complementary attempts to increase the C=O content of PFAs were conducted. The following experiments were rapid tests to check if any conditions yielded higher than usual C=O content. Most of the attempts reported in this section did not lead to significant increase of C=O content. The DOS was determined only with the potentiometric method and only the FTIR C=O area was used to compare the samples. The DOSNMR and the DSC conversion degree

were not determined here as they are more time consuming. The conditions of preparations of PFAs are reported in Table 2.7-2. The results of some test are reported in Figure 2.4-4, the values of PFA° and PFA+ may be used as reference lines. Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -112 -First, authors reported that H2O2 promoted ring opening reactions in PFA.

[201] Thus, the C=O content of such PFA was measured to check this claim. In Figure 2.4-4 the two "H2O2" samples showed rather high DOS but not so much higher than PFA+. However, the "H2O2" samples showed significant ester signals in FTIR which is unusual for PFAs prepared with 50 % additional water. A Baeyer-Villiger rearrangement may be responsible for this and may be exploited for derivatization purposes. It should also be noted that concentrated H2O2 strongly accelerate the polymerization of FA.

A series of catalyst and catalyst systems were tested in PFA systems with 50 % additional water (yellow symbols in Figure 2.4-4). The tests include mixture of Lewis and Brönsted acids (Bismuth triflate and a stoichiometric ratio of MA/BF3). Formic acid, an Amberlyst and AlCl3 were also tested. They all fit the PFA+ curves and thus does not improve significantly the C=O content.

In addition, the orange symbols in Figure 2.4-4 corresponds to a PFA prepared under ultrasonic solicitation and strong mechanical stirring. In the followings, other attempts of increasing the C=O content in PFA systems are briefly overviewed. In previous experiments, the approach was to increase the C=O content during the polymerization. Here, the approach was to first prepare a PFA+ and then expose it to various conditions to increase the C=O content without further polymerization.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -117 -

In his book, P. Merion reported the Fenton reagent to open furans though an oxidative pathway.

[316] The Fenton reagent is based on iron salts and H2O2. Pre-polymerized PFA resins were exposed, under magnetic stirring, to an aqueous solution of Fenton's reagent, a solution of Fe 2+ salt alone and another in a solution of H2O2. In addition, similar resins were magnetically stirred in presence of water and another in presence of acidified water (TfOH). The DOS was measured before and after a 60 h exposition at room temperature. All tests were yielded equal of lower DOS values possibly due to the immiscibility of PFA and water.

Consequently, attempts in which PFA was dissolved in solvents were performed using THF, 1,4-dioxane or toluene. In addition, a small amount of water (about 10 %vol) and a strong acid (H2SO4 or Tf(OH)) were added. After refluxing for a few hours, the solvent was removed using vacuum and rotavapor, leaving PFA phase and yellowish water phase. The water was removed and the DOS was measured. The values were either equal to the initial PFA or lower.

Hence, open furans are removed either by the rotavapor or were solubilized in the yellowish water.

Attempts at oxidizing PFA into carbonyls were also performed. A first one involving a prepolymerized PFA and slightly acidified KMnO4 was conducted. At room temperature, a highly vigorous reaction occurred and left a khaki PFA instead of the initial brownish PFA. This may indicate an oxidation of the conjugated moieties of PFA into carbonyls. It may be a way to increase the C=O content of PFAs although this method is labor intensive and means to mitigate the reaction speed are required. The uses of peroxides may also work.

Finally, it should be mentioned that during syntheses such as the PFA+ one, a demixion occur. The aqueous phase progressively evaporates to leave a yellowish liquid with a water-like viscosity. This liquid was recovered and left to evaporate. Its C=O content was measured and found to be 8.2 ± 0.3 mmol.g -1 including 2. The possibilities of copolymerization of FA with HMF to artificially increase the C=O content were assessed. Although it allows a significant increase C=O content, this greatly diminished the properties of the material.

The use of UV-light was investigated as a mean to open furan ring through an oxidative pathway. Such method could work although the development of a procedure is required. Finally, it was unsuccessfully attempted to increase the C=O content of PFA prepolymers by hydrolysis an oxidation in solvents. If one desires to further increase the C=O content of PFAs, an oxidation either using UV light or oxidants might be the most promising routes.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -119 -

Insight on the chemical structures of PFAs' carbonyls

All along this chapter, the hypothesis that all carbonyls result from the opening of furanic entities into alkyl 1,4-diketones was followed. Yet, the complexity of PFAs' polymerization process might induce production of several kind of carbonyls. This section is dedicated to giving insights on the chemical nature of carbonyls in PFA systems.

Insights on these carbonyls' nature can be given using the 19 F NMR spectra of derivatized PFAs. Figure 2.5-1 is an example of 19 [323,324,326] No statement can be made on the nature of carbonyls in PFA using Figure 2.5-1 and Figure 2.7-9 alone. Nevertheless, a significant proportion of conjugated carbonyls (i.e. more upfield hydrazones) can be highlighted in both system.

From the spectra in Figure 2.5-1, it can be deduced that aliphatic carbonyls (i.e. nonconjugated) such as 1,4-diketones or levulinate-like carbonyls are most likely present in both PFA° and PFA+. Indeed, levulinic acid's hydrazone has a chemical shift of -59.5 ppm (see 19 F spectra of derivatized LA in Figure 2. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] Lange | Furfural-A Promising Platform for Lignocellulosic Biofuels[END_REF][START_REF] Rehm | Bacterial Polymers: Biosynthesis, Modifications and Applications[END_REF][START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF] which is the major one in PFA°. However, PFA+ systems seem to have a larger extent of conjugated carbonyls. A plausible explanation for the presence of both aliphatic and conjugated carbonyls would be the co-existence of two pathways for furan ring opening, as illustrated in Figure 2.5-1. Literature reported hydrolytic ring opening of furan ring leading to saturated 1,4-diketones that were postulated by Conley & metil.[272] Nonetheless, unsaturated 1,4-diketones can also be formed through oxidation of furan ring. [335,336] Such oxidation might be happening due of air dioxygen.

Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -120 - In the past, it was highlighted several times that ketones are not the only carbonyls in PFA, esters groups are also present. [273,283,284] The FTIR spectra of PFA° and PFA+ at conversion degrees 𝛼 ≈ 0.5 and 𝛼 ≈ 0.8 are displayed in Figure 2.5-2. Both samples exhibit a C=O band at 1710 cm -1 . This band has been attributed to ketonic moieties. However, the neat PFA exhibits a shoulder around 1780 cm -1 which has been attributed to ester moieties [283,284]. prepared with additional water, the ones prepared in neat conditions and the ones prepared with additional IPA is portrayed in Figure 2.5-4. It is worth mentioning that both ketones and aldehydes functions are quantified by the oximation method but it is not the case for the ester function.

However, it is rather difficult to discriminate the ketones/aldehydes from the ester functions when integrating the more or less broad carbonyl peak in FTIR. Accordingly, if only ketones or aldehydes are present in the systems, the C=O content should increase linearly with the FTIR C=O area.

However, if other functional groups, such as esters, are taken into account in the C=O area, the DOS vs. the C=O area will not be linear. Moreover, in their work Wewerka et al. used Regarding the PFAs prepared with 50/50 IPA, they also belong to the ester bearing groups in Figure 2.5-4 as observed by Falco et al. [283]. Yet, they have a higher DOS than the neat PFA.

This suggests an opening of furan rings through the formation of compounds such as levulinates or lactones, thus increasing both the DOS and FTIR C=O area.

Finally, the 13 C NMR spectra of PFA+ in Figure 2.5-3 exhibits a significant number of carbonyl peaks from 194 to 210 ppm. Most of them are centered around 208 ppm which matches with 1,4-diketones. Similar peaks can be found in the neat PFA. However, at 𝛼 ≈ 0. Furthermore, chapter 2 highlighted a diversity of carbonylated structures in PFA although their exact structure could not be elucidated.

Hence, the aim of this chapter is to deeply investigated the structure of PFA using modern NMR tools with a strong focus on the carbonyl products. Section 3.2 is dedicated to the structural investigation of PFA+ using 2D NMR. The major known structural units are highlighted. In addition, the presence of several kinds of carbonyls is confirmed and new structures identified. This section also includes the quantification of functional groups by 1 H NMR in PFA+. In addition, the chapter 2 highlighted the formation of a C=O rich crust on the surface of PFA resins. This phenomenon is investigated in section 3.2.

Materials:

Furfuryl alcohol (96 %), maleic anhydride (99 %), and dimethyl terephthalate (TraceCERT®) were purchased from Sigma-Aldrich. DMSO-d6 and D2O were acquired from Eurisotop. All chemicals were used as received.

Preparation and characterization of PFA resins:

PFA+ and PFA° resins were synthetized and characterized as described in section 2.1. Their C=O content was determined by potentiometry and their conversion degree by DSC.

Nuclear Magnetic Resonance (NMR):

All NMR experiments were performed at 298 K on a Bruker Advance DRX 500 spectrometer equipped with a 5 mm PA DUL 500S2 C-H-D-05 Z probe. All the pulse sequences used were supplied by the manufacturer. The spectrometer operated at a frequency of 500.13 MHz for 1 H experiments and at 125.77 MHz for the 13 C experiments. The spectra were referenced to the DMSO peak ( 1 H 2.50 ppm; 13 C 39.52 ppm). About 100 mg of PFA resins were weighted and dissolved in 700 µL of DMSO-d6. The exact masses were recorded for the quantification calculations.

H experiments:

1 H experiments were conducted the "zg" pulse program . For quantitative measurements, the spinning was turned off. As many different aldehydes are present in the system the T1 were not determined. However, a relaxation delay of 30 s was employed to ensure their complete relaxation.

A total of 160 scans were accumulated. The presence of labile hydrogens under the areas of interest was checked by performing a control tube with PFA and D2O. The areas of interest were free of labile hydrogens. To determine the concentration of aldehydes in PFA resins, two different approaches were used and are described thereafter.

Chapter 3. The carbonyls in PFA: a structural investigation -137 -

Quantification using external standard:

The first quantification used dimethyl terephthalate as an external standard. About 2 mg of qNMR grade dimethylterephtalate (reference) were weighted on a Mettler Toledo microbalance XS3DU (d = 1 µg). Then they were dissolved in 700 µL of DMSO-d6.

On one hand, the reference spectrum of dimethylterephtalate was recorded using the previously described parameters and processed with Bernstein polynomial (order 3) baseline from 7 to 9 ppm. On the other hand, the spectra were recorded using the same parameters and the baseline was applied from 8.4 to 12.8 ppm to ensure accurate integration of the aldehyde peaks. If the baseline is applied on the whole spectrum, the area of the integrated peak would be highly overestimated as illustrated in Figure 3.5-1.

To determine the concentration of aldehydes within PFA resins, Equation 3.1-1 was used after adaptation from Soininen.

[337] In Equation 3.1-1, A and Ref subscripts stand for the aldehydes and reference, respectively. A is the absolute area of the peaks which were integrated The second approach used the furan rings in PFA as reference. For the calculation purpose, the macromolecular network of PFA was approximated as a repeating furfuryl unit (M = 80 g.mol - 1 ). In Chapter 2, such approximation was used to define the degree of open structures in PFA systems. In an opened furan, we assumed the formation of two carbonyls as the result of ring opening, thus giving Equation 3.1-2. Equation 3.1-2

Taking in account that only one aldehyde can be produced from one furanic entity, Equation 3.1-2 can be transformed into Equation 3.1-3. The relative proportion of aldehydes over all the C=O was calculated using Equation 3.1-5 Equation The 13 C NMR spectra were acquired using the "zgpg30" pulse sequence, 20 000 scans and a relaxation delay of 2 s. The DEPT-90 experiments used the pulse sequence "deptsp90", 8 600 scans and a relaxation delay of 2 s. The DEPT-135 spectra were acquired using the "deptsp135" pulse sequence, 12 000 scans were accumulated, and a relaxation delay of 2 s was used.

The HSQC and HMBC are 2D NMR tools correlate 1 H and 13 C signals with each either on NMR maps. As illustrated in Scheme 3.1-1, HSQC correlates hydrogen and carbon one bond away while the HMBC correlation are for atoms two to four atoms away, without heteroatoms in between.

Scheme 3.1-1. Correlations observed in HSQC (left) and HMBC (right).

For the HMBC experiments, the "hmbcetgpl3nd" pulse sequence was used with a relaxation delay of 2 s and 100 scans were accumulated. Finally, for the HSQC, the pulse sequence "hsqcedetgpsisp2.3" was used. 54 scans were recorded and a relaxation delay of 1.5 s was used.

HSQC J 1 correlations HMBC J 2 and J 3 correlations Chapter 3. The carbonyls in PFA: a structural investigation -140 -

MALDI ToF mass spectrometry

The samples were treated with a NaCl solution (1.5 μl of a 0.1 M) in a methanol/water mixture (1:1) to increase ion formation, and a drop placed on the MALDI target (3 mm diameter)

steel plate and dried. The samples and the matrix were then mixed in equal amounts, and 1.5 μl of the resulting slurry was placed on the MALDI target and dried at 40 °C for 2 h before being analyzed. A matrix of 2,5-dihydroxy benzoic acid was used. Red phosphorous (500-3000 g.mol -1 ) was used as reference for spectrum calibration. Finally, after evaporation of the solvent, the MALDI target was introduced into the spectrometer. The spectra were recorded on a KRATOS AXIMA Performance mass spectrometer from Shimadzu Biotech (Kratos Analytical Shimadzu Europe, Ltd., Manchester, UK). The irradiation source was a pulsed nitrogen laser with a wavelength of 337 nm. The length of one laser pulse was 3 ns. Measurements were carried out Among the means dedicated to structural elucidation, bidimensional NMR has proven to be a powerful tool. Indeed, due to their growing industrial interest lignins and their derivatives have been extensively studied with these techniques. [338,339,340,341] Nonetheless, to the best of our knowledge, bidimensional NMR was never used to elucidate the structure of PFA.

Bidimensional NMR investigations were employed to determine interconnections between lignin and PFA [137,295] or between epoxidized linseed oil and PFA.

General features in bidimentional NMR spectra

The main objective of this section is to identify the nature of the carbonyls within PFA+ and clarify their pathway of formation as well as their link to the macromolecular network. A PFA+ resin with a rather advanced conversion degree of 0.80 (based on FA polymerization), i.e when 80 % of the polymerization is completed, was chosen. This choice was driven by the high carbonyl content (≈ 3.0 mmol.g -1 ) of the resin while it could be easily dissolved in DMSO-d6.

First, PFA was investigated with monodimentional NMR using a high number of scans to highlight the carbonyl moieties as their concentration is relatively low. The 1 H and 13 C NMR spectra are reported in Figure 3.2-1. The most intense signals have already been assigned in previous investigations. [274,281,286] The main structure of PFA (I) as well as the terminal methyl and furan are easily identified using 1D NMR. Nonetheless, many signals overlap with each other thus rendering unambiguous structural elucidation highly complex for structures such as the dihydrofurans of (III) or the carbonyl moieties of Scheme 3.2-1 and Scheme 3.2-2. Regarding the rest of the spectrum, the cluster [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF] highlights the presence of enols and/or ketal species.

[330] Such note is of importance as these are derived from carbonyls and makes them less available for chemical reactions. Chapter 2 showed a gap between two quantification techniques. The first one was based on the derivatization of carbonyls by hydroxylamine hydrochloride followed by potentiometric titration. The second method, involved a derivation by a fluorinated phenyl hydrazine followed by 19 F NMR. Overall, the latter method yielded lower carbonyl content than the potentiometric one. This gap was mainly attributed to steric hindrance with a participation of ketal species. The data in Figure 3.2-2 outlines the fact that some carbonyls may not be directly available for derivatization. Indeed, the enol and ketals do not behave as carbonyls in terms of reactivity. Thus, a displacement of equilibrium from the enol/ketal towards the carbonyl may be required to fully exploit the potential of the carbonyl moieties in PFAs.

Chapter 3. The carbonyls in PFA: a structural investigation -146 -Interestingly, the presence of cluster [START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF] unambiguously highlights that aldehydes functions are present in PFA. Theses aldehydes can be seen in Figure 3.2-1 on the 1 H and DEPT spectra. At least seven types of aldehydes are present in the system. Their possible structures are discussed later. 6)

and ( 8), respectively. In the followings, the existence of structures C-I to C-V from Scheme 3.2-2 are discussed using the HMBC spectra in Figure 3.2-3. First, the clusters (3) form a spin system coherent with either levulinic acid (LA) or furfuryl levulinate (C-V). However, using potentiometric titration, about 0.15 mmol.g -1 of COOH can be found in PFA+ at this stage of polymerization. Moreover, theses PFA+ do not show signs of COOR bands in FTIR (Figure 2.5-2). Hence, it is more likely that free LA molecules are responsible for cluster (3) rather than structure C-V. The cluster [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF] at 2.8 ppm as well as cluster ( 9) at 3.8 ppm might correspond to structure C-I, postulated by Conley and Metil. [272] This kind of structure would emerge from the hydrolytic ring opening of furan within the chains of PFA.

The full HMBC spectrum of this sample is available in Figure 3.5-2 along with a magnification from 4.5 to 8.0 ppm in Figure 3. The cluster [START_REF] Wertz | Hemicelluloses and Lignin in Biorefineries[END_REF] has been assigned as a methyl to ketone correlation. Indeed, only two aldehydes have chemical shifts between 200 and 205 ppm and it is unlikely to see methylaldehyde neighbors in PFA systems. The cluster ( 4) is linked to correlation between CH2 and CH3.

Hence, a possibility for the origin of clusters ( 4) and ( 11) is the hydrolytic ring opening of methylfuran, structure (II), into structure C-VI, as depicted in Scheme 3.2-4. This hypothesis is backed-up by the cluster correlations Fu-(CH2) with a CH2 of cluster (1) and with a C=O of cluster [START_REF] Rehm | Bacterial Polymers: Biosynthesis, Modifications and Applications[END_REF]. Furthermore, the CH2 of cluster [START_REF] De | Bio-Based Chemicals: A 2020 Update[END_REF] around 2.8 ppm also point to structure C-VI. The CH2 to CH3 correlation is however not seen. Interestingly, the heterogeneity of cluster ( 11) is a good sign that these carbonyls are linked to the macromolecular chain rather than being parts of free molecules. These results are also consistent with section 2.5 where the presence of levulinic-like species was suggested by 19

C-VI

Chapter 3. The carbonyls in PFA: a structural investigation -149 -

Unexpected carbonyls

In the following, the focus is made on the aldehydes, using Figure 3.2-4A and B, extracts of HSQC and HMBC, respectively. Then, this conjugated aldehyde would be a suitable candidate for a Diels-Alder reaction with furans to yield the adduct C-VIII. Indeed, structure C-VII is a good dienophile with its two electron withdrawing groups.

[343] The spectra's complexity and low concentration of aldehydes does not allow full assignment of the structure.

Despite this, the HMBC extract in The literature can provide a few indications on the origin of these aldehydes. Indeed, some articles refer to the use of oxidative ring opening of furans to yield conjugated 1,4-dicarbonyls.

[ 316,345,346,347] In the case of furfuryl alcohol, the oxidative cleavage of the furan ring is often used in the Achmatowitcz rearrangement. Typical oxidants are Br2, N-bromosuccinimide, mchloroperoxybenzoic acid or H2O2. [342,348,349,350] As well, few papers mention the use of dioxygen as an oxidant for the ring opening reaction of furans in photocatalyzed reactions. [351,352,353,354] In the past, Gonzalez et al. investigated the polymerization of FA under inter atmosphere or air. They did not find any significant influence of dioxygen during the polymerization. Yet, they noticed that their PFA would still be soluble after 14 months when conserved in vacuum sealed ampoules i.e., with a limited contact with air. However, their synthetized PFA would become insoluble after only few hours exposed to air environment. [280,286] Moreover, if a PFA resin with a conversion degree of 0.80 (or more) is left in open air, a dark and hard crust forms as depicted in the photography of Figure 3.2-6. More specifically, in five minutes exposed to air, the red color shifts to black and the crust becomes hard in less than two hours. In addition, the C=O content of the crust was found to be up to 30 % higher than the heart of the resin in section 2.2.2. for soluble resins.

Finally, a pathway was proposed to explain the formation of a stiff crust on PFAs' surface.

The dioxygen present in air induces an oxidative ring opening at the surface of PFA thus leading to conjugated aliphatic aldehydes. The dienophilic character of these opened end-groups is enhanced by the electron-attracting effect from the aldehyde. Therefore, they are prone to make additional crosslinks via Diels-Alder cycloadditions thus leading to the formation of a stiffer surface compared to the core material. The peculiar sensibility of PFA for surface oxidation can have useful and interesting consequences when designing its processability.

Chapter 3. The carbonyls in PFA: a structural investigation -156 -

Structural comparison of PFA° and PFA+:

The aim of this section is to further apprehend the nature of the carbonyl groups in PFAs by comparing two types of PFA. The first one was prepared in neat conditions (PFA°), i.e., without additional water and a second one synthetized in 50 % aqueous solution (PFA+). The PFAs structures were compared using 2D NMR techniques (HMBC and HSQC) and 1 H NMR quantification. In addition, MALDI-TOF measurements were performed to further highlight the structural differences between the samples.

The CH3 groups

First, HSQC experiments of PFA° and PFA+ resins were performed to highlight their main structural differences. For an easier comparison, resins with a conversion degree of about 0.80 were chosen. Figure 3.3-1 displays a magnification of the HSQC spectra of PFA+ and PFA° located on the methyl area. In Figure 3.3-1, PFA°(brown) is stacked above PFA+ (orange). The opposite viewpoint in which the PFA+ HSQC spectrum is shown in Figure 3.5-4. This magnification highlights the main types of methyl found in PFA. Three groups with significatively different chemical environments can be identified in soluble PFA resins.

The first group centered around 2.2/14 ppm can be attributed to the methylated furans as depicted in Scheme 3.3-1. The methylated furans are formed during the conjugations of furanic chains through the loss of a hydride ion.

[281] Such methyls are observed in both PFA° and PFA+.

Nonetheless, a larger dispersion of chemical shift occurs in the case of PFA+, thus suggesting a higher heterogeneity of the furanic structures in PFA+ resins. Another distinct group of methyls at 1.95/15 ppm is observed only in the case of PFA+. This cluster does not spread as much as the main one. Thus, it might belong to a free molecule (i.e. dimers, trimers) rather than to a group linked to the main macromolecular chains. 2.2 ppm was assigned to the methylated furans and is found as well in the PFA° spectrum. An even broader signal is observed from 1.90 to 2.15 ppm. It was assigned to the end-chain levulinic units.

While the two PFAs are at similar conversion degrees, the 1.90-2.15 ppm signal is much broader in the case of PFA+ compared to PFA°. This is consistent with the higher C=O content in PFA+.

Methylated furan End-chain levulinic unit

Chapter 3. The carbonyls in PFA: a structural investigation -159 -

The CH2 groups

In this section, the discrepancies between the CH2 groups of PFA° and PFA+ are discussed. The carbonyl content of the studied PFA° is about 2.0 mmol.g -1 while PFA+ contains about 3.0 mmol.g -1 of carbonyls. Consequently, the intensity of the CH2 neighboring the carbonyls in PFA+ should be higher than in PFA°. Also, a wider distribution of chemical shifts is expected as the chemical environment of CH2 in PFA+ should be more diverse. The intensity and area of the 1 H signals from the CH2 cannot be compared accurately due to a lack of proper baseline.

Nonetheless, the HSQC spectra in Two methods were employed. The first one used dimethylterephtalate as an external standard. The second method employed the left-over furans as reference. This last method approximates the macromolecular network as a repetition of furfuryl unit. However, it does not require the sample mass for the calculations. The results are compiled in Table 3.3-1 and illustrated with the 1 H quantitative NMR spectra in Figure 3.3-6 In section 3.2.2 it was found that the concentration of aldehydes in PFA+ increase with the conversion degree, as well as the aldehyde ratio (over all the carbonyls). Table 3.3-1 reports a similar behavior for PFA°. With the method using an external standard, the aldehyde concentration increases very-slightly over time with the course of polymerization. Nonetheless, the aldehyde ratio remains steady during the whole polymerization of FA in the neat system. A similar trend is observed with the left-over furan method. Overall, PFA° comprised up to 0.13 mmol.g -1 of aldehydes, i.e., about 7 % of its carbonyls. In comparison, PFA+ resins comprise about 0.50 mmol.g -1 of aldehydes in resin with a conversion degree of 0.83 and about 0.20 mmol.g -1 of aldehydes for a resin with a conversion degree of 0.47. Consequently, polymerizing FA in a 50 % aqueous solution stimulates the production of aldehydes either from a water-based or an oxidative pathway.

Chapter 3. The carbonyls in PFA: a structural investigation -166 -

The esters

To further emphasize the structural discrepancies between PFA° and PFA+, the ester functions are now discussed.

In the past, the existence of ester functions in a PFA network was mentioned numerous times. [183,273,283,284,296] and further confirmed in chapter 2. The lactone structure in Scheme 3.3-5 was proposed as the source of these esters. In the followings, the HMBC method was used to investigate their nature. predict" software from University of Lausanne "NMR DB" and are available in Table 3.5-1.

[355]

The nature of the other methyl peaks in PFA° cannot be elucidated as easily. However, the peaks at 1.91, 1.75, 1.48 and 1.43 ppm all correlate with carbon peaks in the ester region, i.e from 150 to 185 ppm. The PFA+ HMBC spectrum in Figure 3.5-5, displays the 1.48/156 ppm and 1.91/172 ppm clusters as well. Nonetheless, the lactone is not observed in the PFA+ spectrum. This is consistent with previous work. Indeed, the FTIR spectra of PFA° resins exhibit the typical ester bands, cf Figure 2.7-1 and Table 1. From the comparison of the MALDI ToF spectra in Figure 3.5-9 and Figure 3.5-10 and Table 3.5-2 there seem to be more open structures and ketone structures in PFA+ than in PFA°. This is suggested from the greatest abundance of the structures at 326-327, 334 and 574-576 g.mol - 1 (Scheme 3. [START_REF] Geyer | Use, and Fate of All Plastics Ever Made[END_REF][4][5][START_REF] Srivastava | Effective Hydrolysis for Waste Plant Biomass Impacts Sustainable Fuel and Reduced Air Pollution Generation : A Comprehensive Review[END_REF]. The structure of 326-327 g.mol -1 is in line with the open structure already found in the previous sections and shown in the simpler structure corresponding to the MALDI peak at 198 g.mol -1 . As depicted in Scheme 3.3-6, the 334 g.mol -1 and 574-576 g.mol -1 ones are also examples of oligomers which includes some open ketone structure. The two structures at 334 g.mol - 1 and 574-576 g.mol -1 are also examples of how Diels-Alder rearrangement of furanic chains can participate in higher molecular weight oligomers. Scheme 3.3-6. MALDI ToF assignment of PFA° and PFA+.

For the 325 g.mol -1 the peak is much more intense in relation to the rest of the spectrum, and for the 334 g.mol -1 while the size is similar for the two PFA resins. PFA+ presents two peaks of equal height that indicates that the abundance of the structure is higher. The ketone carrying structure at 574-576 g.mol -1 has also a very much more pronounced peak for PFA+ and all its multiples are far more intense than in PFA°. The ketone structures at 360 and 440 g.mol -1 also present peaks clearly much more intense in PFA+. The structure at 361 g.mol -1 (including the 23 g.mol -1 for Na + ) is also one example of the structure determined in the NMR section . . The peak at 152 g.mol - 1 in the PFA+ that it is smaller than the one of 154 g.mol -1 in PFA°. In PFA°, the peak at 154 g.mol - 1 is closer to the non-ketone form (157 g.mol -1 ).

Scheme 3.3-7. MALDI ToF assignment of PFA° and PFA+.

There are several structures in which a charged atom is still present. One example is the peak at 161 g.mol -1 that could be interpreted as anyone of the two structures of Scheme 3.3-8. Scheme 3.3-8. MALDI ToF assignment of PFA° and PFA+ for the 161 g.mol -1 peak.

Both might be effectively present as well as being fabricated in the MALDI ToF experience itself. However, the presence of several other similarly charged structures at 176, 240-241, 481-483, and 496 g.mol -1 appear to indicate that these structures are not just a fabrication of the MALDI spectrometer.

The peak at 176 g.mol -1 (much stronger in PFA°) can be ascribed to two different structures contributing to the same peak (Scheme 3.3-9.). The much smaller residual of 176 g.mol -1 in PFA+ might be due to the open ketone structure. It could also be a dimer of FA. The peaks at 190 g.mol - 1 and 192 g.mol -1 , assigned to the structures in Scheme 3.3-10, are also corresponding to ketonic structures from ring opening reactions. Among the possible structures observed is also one structure identified in the NMR analysis in which the terminal unit of a furanic oligomer is a conjugated aldehyde resulting from the endchain oxidative ring-opening of furans in PFA resins (Scheme 3.3-13. ). This is observed at 404 g.mol -1 (no Na + ) and 428 g.mol -1 (with Na + , presenting the structure in Scheme 3.3-13. ) Scheme 3.3-13. MALDI ToF assignment of PFA° and PFA+ for the 404 g.mol -1 (no Na + ) and 428 g.mol -1 (with Na + ) peaks.

PFA+ PFA°

PFA+, PFA° PFA+, PFA° PFA+, PFA° PFA+, PFA°

Chapter 3. The carbonyls in PFA: a structural investigation -171 -Finally, structure formed by Diels-Alder cycloadducts seem to be present in the examined PFA resins, with the peaks at 310, 318, 334 g.mol -1 (with Na + ), suggesting again more open forms in PFA+, 456 g.mol -1 (no Na + ) and 476-478 g.mol -1 (with Na + ), 470 g.mol -1 (no Na + ) and 493 g.mol -1 (with Na + ), 481-483 g.mol -1 and 496 g.mol -1 and 574-575 g.mol -1 (Table 3.5-2) such as the species at 456 g.mol -1 and 470 g.mol -1 as follows in Scheme 3.3-14.

Scheme 3.3-14. MALDI ToF assignment of PFA° and PFA+.

Conclusions of section 3.3

In this section, the chemical structures of two types of PFA resins were investigated. The first PFA was prepared in neat conditions (PFA°) while the second one was prepared in a 50 %w aqueous solution (PFA+). To investigate the structures, both NMR and MALDI ToF were employed. Overall, the study highlighted a higher heterogeneity in PFA+ as exemplified by broader HSQC clusters. Such heterogeneities were attributed to water-induced reactions. They involve the formation of ketonic and aldehyde species. Moreover, it was further highlighted that the resulting ketones are sensitive to enolization and hydration. On the other hand, PFA° displays less ketone derivatives. In addition, the presence of aldehyde groups are about two times more important in PFA+ compared to PFA° thus indicating that polymerization in aqueous conditions favor their formation. Furthermore, the presence of lactone end-groups in PFA° was confirmed and it explains somehow the lower concentration of aldehydes for PFA°. Finally, the results from the MALDI ToF investigations corroborated the higher abundance of open-structures in PFA+ systems and helped to propose some potential structures.

g.mol -1

Chapter 3: Conclusion

This chapter was dedicated to improving the understanding of the chemical structure of PFA, with a strong focus on carbonyl moieties. Quantitative 1 H NMR, bidimentional NMR ( 1 H-13 C) and MALDi ToF spectrometry were employed to reach this objective.

In section 3.2, the chemical structure of a C=O rich PFA+ was investigated. The NMR showed a predisposition of the terminal furan for ring-opening reactions. On one hand, the methyl furans open into levulinic-like moieties through a hydrolytic pathway. On the other hand, the monosubstituted terminal furan are sensitive to undergo oxidative ring opening to produce aldehydes. Quantitative 1 H NMR highlighted that up to 20 % of the total C=O in PFA+ are aldehydes.

Overall, whether they are aldehydes or levulinic end-chain units, the C=O in PFA seem to preferably form at the end of the macromolecular chains. This may be the reason why the carbonyl content of soluble PFAs could not be increased above 3.7 mmol.g -1 , i.e. 15 % of open furans. Section 3.3 further supported the findings of section 3.2 by comparing a C=O enriched PFA+ with a C=O poor PFA°. The PFA+ exhibited a more heterogeneous structure than PFA°. The presence of esters in PFA° was once again observed and the presence of terminal lactone was confirmed. This further highlights the higher reactivity of terminal furans in PFA systems.

Finally, a new oxidation-induced surface crosslinking pathway was proposed in this chapter.

This phenomenon gives birth to a hard crust on the surface of PFA resins. It is believed to emerge from the oxidation of terminal furans into conjugated 1,4-ketoaldehydes followed by a Diels-Alder cycloaddition. In the next chapter, the impact of this surface crosslinking on the properties of PFA materials is assessed. In chapter 3, an oxidation-induced surface crosslinking phenomenon was highlighted. This chapter aims at further understanding this phenomenon and its consequences on the properties of furan resins. To do so, two polymers were prepared under inert or oxidative atmosphere. The first one, called "PFA" was post-cured under N2 to avoid any oxidation while the second one "Ox-PFA" was post-cured under air. To avoid any confusion, in this chapter the term furan resin is used to refer to PFAs in general, regardless of the post-curing conditions.

After the experimental section in 4.1, section 4.2 briefly describes the known chemical transformation occurring when furan resins are thermally treated.

Section 4.3 deals with the structure of PFA and Ox-PFA using spectroscopic technics coupled with elemental analysis, thermogravimetric analysis, aldehyde detection tests and C=O quantifications. The goal is to determine the functional groups present on the surface of furan resins.

Section 4.4 complements the pathway found in chapter 3 and highlights some of its consequences on the materials' properties. In addition, the thickness of this oxidized layer is determined.

Section 4.5 is dedicated to the kinetics of oxidation, hardening and degradation of furan resins.

Finally, Section 4.6 deals with the macroscopic properties of oxidized and non-oxidized furan resins.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -188 -

Experimental section

Materials:

Maleic anhydride (99 %), silver nitrate (99 %), fructose (97 %), aqueous ammonia (25 %), sodium hydroxide (97 %) and were bought from Merck. Furfuryl alcohol (96 %) and ethyl acetate (98 %) were acquired from Carlo Erba. Ethanol (96 %v) was purchased from VWR. The volumic purity of the N2 used for the curing and TGA was superior to 99.995 %. All the chemicals were used without further purification.

Sample preparation:

The sample preparation procedure is summarized in Scheme 4.1-1.

To prepare the materials, a solution of furfuryl alcohol and maleic anhydride (100:2 mol) was first pre-polymerized. For this purpose, the mixture was heated at 100 °C for about 1 h 30 until a rubbery resin was obtained. The rubbery resin was then ground using a GT-300 ball-mill from Powtec. Two cycles at 1500 RPM for 2 mins were used. Prior to each cycle, the bowls were cooled down using liquid N2. The resulting powder was vacuumed overnight at 40 °C to fully dry it. Then, the dried powder was compression-molded at 150 °C under 20-25 bars for 2 hours, resulting in a solid piece of polymer.

A fraction of the materials was then cut for the tensile tests using a dumbbell shaped cutter (ISO 527-2-1BB, cf picture in Figure 4.8-1) and parallelepipeds for dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) experiments. The other fraction was ground using the ball-mill following the same protocol as described previously. Thereafter, both powders and pieces underwent either an oxidative atmosphere curing at 200 °C for 2 h 30 under air (named Ox-PFA) or an inert atmosphere curing at 200 °C for 2 h 30 (named PFA). The FTIR and ssNMR (CPMAS) spectra were acquired to the methods described in section 2.1. For the ssNMR, the spectra were referenced to the 151 ppm peak, i.e., the C=C of disubstituted furan rings. The height of this peak was set to 1 to compare PFA and Ox-PFA at a constant furan proportion. Such normalization could not be done on the FTIR spectra as the peaks overlapped with each either.

Elemental analysis:

The elemental analyses were conducted on a NCHS Thermo Electron Flash EA1112 from Thermo Fisher. About 1.5 mg of samples (powder form) were weighted in a flexible tin pan using a microbalance. After combustion, the gases were separated by gas chromatography and identified by a thermal conductivity detector. The resulting chromatogram was then integrated and the percentage of carbon and hydrogen were calculated. The oxygen content was deduced by subtraction.

Aldehyde detection (Tollens' reagent):

An aldehyde detection test was performed on PFA and Ox-PFA powders. This procedure was adapted from Malherbe et al. [356]. To do so, a 150 mL solution of 0.1 M silver nitrate in distilled water was prepared. Then, 2,4 g of sodium hydroxide were added, followed by 9 mL of 25 % ammonia solution. As a result, the reactions (1) to (3) occur.

AgNO3 + NaOH → AgOH + NaNO3 (1) 2 AgOH → Ag2O + H2O (2) Ag2O + 4 NH3 + H2O → 2 Ag(NH3)2 + + 2 -OH (3)
Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -191 -Then, the reagent's quality was tested using about 15 mL of solution and 100 mg of fructose. A few minutes after the addition of fructose, a silver mirror appeared as the reaction (4) occurred. Upon shaking the silver mirror dissociates from the glass, revealing an orange solution.

The silver mirror is a sign of the presence of aldehydes or alpha hydroxyketones. [356,357] R-CHO + 2 Ag + + 2 -OH → COOH + 2 Ag + H2O (4)

To test the presence of aldehydes in PFA and Ox-PFA, about 15 mL of the reagent and 400 mg of sample (powder form) were shaken and left to react for 24 h. A blank test was also conducted using only the tollens' reagent as it decomposes into Ag3N over time as well as a fructose reference.

Thermogravimetric Analysis (TGA):

The TGA experiments were conducted on a TGA 851e apparatus from Mettler-Toledo.

About 10 mg of samples were placed in 70 µL alumina pans. The temperature program started from 25 °C to reach 300 °C at 2 °C.min -1 under either 50 mL.min -1 of air or 50 mL.min -1 of N2.

Both PFA and Ox-PFA were subjected to these experiments (powder form). The experiments were duplicated. For kinetic analysis of the oxidation, heating rates of 1, 2, 4 and 8 °C.min -1 were employed. For kinetic analysis of the degradation, heating rates of 1, 2, 4, 8 and 16 °C.min -1 were employed.

Differential scanning calorimetry (DSC):

Oxidation The DMA experiments were conducted on DMA 1 from Mettler-Toledo. The tensile mode was used. The temperature program started from -80 °C to 300 °C at 2 °C.min -1 . The oscillation frequency was set at 1 Hz with a 0.1 % displacement amplitude. Both PFA and Ox-PFA were subjected to these experiments (piece form). These experiments were duplicated.

The curing time of 2 h 30 was determined using an isothermal experiment at 200 °C for 5 h. A strain amplitude of 0.1 % and a frequency of 1 Hz were used. The isotherm was preceded by a non-isothermal scan at 3 °C.min -1 , starting at 25 °C. The curve of the resulting E' as a function of time is available in Figure 4.1-1. For kinetic analysis of the hardening, heating rates of 0.5, 1 and 2 °C.min -1 were employed.

Uniaxial tensile and Shore hardness tests:

To measure the tensile properties, a Shimadzu EZ-LX instrument was used. A load cell of 1 kN and a gauge length of 21 mm were used. The samples were cut in the dumbbell shape (ISO 527-2-1BB) using a dumbbell cutter prior to their post-curing. The crosshead's speed was set to 1 mm/min. The strain as well as the stress were recorded, and the curves were exploited using Origin 2021. PFA and Ox-PFA samples were tested seven times each. The hardness tests were conducted using a shore D durometer from Kern & Sohn GmbH. Overall, 10 attempts were performed at different spots on the samples. The results are presented as the average and standard deviations (i.e., error bars) from the repetitions.

Goniometry:

The contact angles were measured using a Goniometer dataphysics OCA25. Prior to analysis, the samples were preconditioned for 30 mins under the atmospheric conditions of measurement. A 5 µL water droplet was placed on the samples then a picture was taken by a camera a few seconds after the deposition of the droplet. Six measurements were taken for each sample.

The results are presented as their average and standard deviation.

Carbonyl and carboxylic acid determination:

The C=O and COOH content were determined using the potentiometric method developed in section 2.1.

Scanning Electron Microscopy:

For scanning electron microscopy (SEM) observations, pieces of PFA samples were prepared as follow. From the fractured surface produced by the tensile test, cross section were cut with a razor blade and flat surfaces were prepared with a Reichert Ultracut E ultramicrotome. They were mounted on a SEM stub with carbon tape and subsequently coated with platinum (3 nm). SEM observations were carried out with a Tescan Vega 3 XMU scanning electron microscope (Tescan France, Fuveau, France) at an accelerating voltage of 5 kV.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -195 -

Current knowledge about the chemical transformation of furan resins at high temperatures

As mentioned in section 1.3.1, furan resins are widely used as carbon precursors. To explain the chemical structure of the carbonaceous materials, the mechanisms of carbonization were thoroughly investigated. [359,360,361,362,363,364] Scheme 4.2-1 reports example of structures found in carbonized furan resins. They mostly consist in a succession of aromatic rings that are more or less functionalized, depending on the carbonization conditions. Scheme 4.2-1. Aromatic structures found in carbonized furan resins. [361,362,363,364] Furthermore, other authors attributed the aromatic structures of the carbonized resins as the result of the aromatization of the Diels-Alder adducts, as depicted in Scheme 4.2-2. [185,365] The goal of this chapter is to shed new lights on the thermal phenomena occurring from 150 to 300 °C during the curing and/or post-curing treatment of furan resins. To do so, curing steps were performed under inert (N2) and oxidative (air) atmosphere. The resulting materials were subjected to thermomechanical, spectroscopic analysis and kinetic computations to establish the structure-property relationships of the fully cured furan resins. The objective is to breakthrough knowledge on the aromatization of furanic resins i.e. the transformation of furanic moieties into benzenic moieties. The consequence of such amortization on thermo-mechanical properties are discussed.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -197 -

Aromatization of furans: structural investigation

The goal of this section was to investigate the aromatization of furanic moieties within furan resins during their curing. For this purpose, two resins were prepared from the same batch and either a N2 or an air atmosphere was applied during the post-curing step. They resulted respectively in PFA and Ox-PFA samples.

First, the chemical structure of these samples will be discussed using FTIR and ssNMR spectroscopies. In Figure 4.3-1, the PFA spectrum exhibits the classical furanic bands respectively at 600, 1015, 1500 and 1550 cm -1 . In addition, the 1712 cm -1 band is also observed and it corresponds to the carbonyls from the furan opened-structures within the polymer.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -198 - However, in the Ox-PFA spectrum the furanic bands are either not observed (600 and 1500 cm -1 ) or highly dimmed (1015 and 1550 cm -1 ). Moreover, a broad signal is observed from 1060 to 1340 cm -1 . Furthermore, the 1600 cm -1 band is much more intense in the case of Ox-PFA relative to the furan peak at 1000 cm -1 . The 1060 -1340 cm -1 and 1600 cm -1 bands match with aromatic vibrations, respectively =C-H and C=C. [288, 361, 360, 364] Likewise, the 1712 cm -1 band in PFA is shifted to 1705 cm -1 . Such shifting to lower wavelength could be linked to the apparition of more conjugated carbonyls [328,330] In comparison with the furanic band at 1012 cm -1 the intensity of the C=O is much higher in the Ox-PFA spectrum than in PFA's. Figure 4.8-2 presents the magnified FTIR spectra of Ox-PFA and PFA from 2500 to 3400 cm -1 . For both samples, several bands superimpose in this area. Nonetheless, a COOH band is visible in the spectrum of Ox-PFA.

In addition, both samples seem to exhibit traces of OH groups. Finally, a shouldered peak a 2650 cm -1 is distinguishable for both samples, especially in the case of Ox-PFA. sample while the PFA one is mostly furanic. Thus, the oxidative treatment involves the aromatization of the system on top of the formation of C=O in furan resins.

As the FTIR analysis only takes into account the surface of the sample, an Ox-PFA piece was crushed to investigate the internal structure of the material. Ox-PFA. Again, the Ox-PFA peaks are broader than the ones of PFA. This is especially the case of the 39 ppm peak. Likewise, the 51 ppm peak visible on the PFA spectrum appears more as a shoulder in the case of Ox-PFA. This shoulder was also observed in the case of carbonized furan resins.

[361] These observations suggest once again a more heterogeneous structure of the furan resin if the post curing is performed under air rather than N2. As the time of experiment was rather long for a Tollen's test (usually minutes), a reference using only Tollen's reagent as well as a reference with fructose were performed. The solution with only the Tollen's reagent did not change in color, only a small solid precipitate formed. This precipitate results from the decomposition of the silver complex into AgN3. The test using fructose formed a silver mirror which fell from the glass upon shaking to expose a colored solution. Such phenomenon occurs when aldehydes are present in the system.

After the exposure of the PFA powder to the Tollen's reagent, a slight taint appeared without signs of silver mirror. In the case of Ox-PFA, the silver mirror could not be observed as well. However, the solution had a much deeper color, closer to the fructose test. Therefore, it would suggest the presence of aldehydes in the Ox-PFA sample. Hence, it is likely that the post curing in oxidative atmosphere induces the formation of aldehyde on the surface of the material. A positive Tollen's test could also be caused by the presence of 𝛼-hydroxyketones.

[357]

The previous experiments only focused on qualitative analyses. To compensate for this, carbonyl and carboxylic acid quantifications were performed as well as elemental analysis. The results from the quantifications are compiled in In order to deeper understand the previously highlighted phenomena, thermogravimetric analyses were performed on PFA and Ox-PFA in their powder form. The samples underwent TGA under air and N2 atmosphere. The curves are shown in Figure 4.3-5.

In Figure 4.3-5 the black lines correspond to the Ox-PFA samples. If the TGA is performed under N2, the material will decompose later than if the TGA is performed under air. This behavior is usual for carbon-based material as the dioxygen in the air favorizes their decomposition.

However, in the case of PFA's, the TGA scan under inert atmosphere shows a continuous mass loss earlier than the one observed under oxidative atmosphere. Moreover, when the TGA of PFA is performed under air a mass gain of about 0.5% is observed from 180 °C to 225 °C in the experimental conditions. Thus, it supports an introduction of oxygen in PFA when it is exposed to air above 170 °C. As well, this phenomenon delays the degradation of the polymer. Therefore, this oxidized layer might protect the polymer from decomposition. The plain lines correspond to the TGAs performed under N2.

To sum up, the atmospheric conditions (N2 or Air) during the post-curing of a furan resin implies surface modification. The oxidative curing induces the formation of surface groups, including ketones, aldehydes, carboxylic acids and aromatics. Moreover, the FTIR spectra showed that very few furans are present on the surface of the material while the NMR proved that they are still in majority when considering the whole bulky material. Finally, the TGA supported that oxygen is incorporated in PFA in significant amounts when subjected to oxidative thermal treatment above 170 °C.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -205 -

Origin of surface hardening

To further understand the effect of surface oxidation on the materials, three DMA experiments were performed. Both PFA and Ox-PFA samples were analyzed. In addition, an Ox-PFA piece was sanded prior to the DMA for removing the top surface layers of the material. Figure 4.4-1 presents the tan (𝛿) curves of these three samples. it increases to 0.23 for PFA (for similar peak-width). Thus, the surface oxidation induces a slightly lower damping capacity. In addition, the Ox-PFA has a Tg of 117 °C which is slightly higher than the one of PFA peaking at 113 °C. Secondly, the "Ox-PFA, sanded" curve superimposes nicely with PFA's. Thus, it confirms that the oxidation phenomenon is limited to a top surface layer. This implies that once the dioxygen reacted with PFA it becomes very difficult for the dioxygen molecules to enter into the core of the material and further react with other entities.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -206 -Hence, the top layer might, to some extent, act as a protection against further oxidation of the material. This could be a partial explanation for the well-known high thermal stability of the furan resins. In addition, it can be highlighted that the width of the tan (𝛿) peak remains constant, which suggests that the distribution of chain lengths is not or little impacted by the oxidative treatment. The elastic moduli curves of theses samples are highlighted in Figure 4.4-2. highlights an increase in moduli from 180 °C for both PFA and "Ox-PFA, sanded". An increase of moduli after reaching the rubbery plateau can be the sign of a partially cured material or a graphitization phenomenon. However, Ox-PFA and "Ox-PFA, sanded" underwent the same preparation procedure and came from the same batch. The only difference was the sanding process of Ox-PFA (blue curve in Figure 4.4-2) prior to DMA.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -207 -On top of that, the temperature is too low for the graphitization to occur. As a consequence, it can be deduced that this final increase in E' might be linked to a crosslinking occurring at the surface of the material.

To further emphasize the correlation between the oxidation and increase in moduli, the TGA curve of Hence, it explains the mass gain observed in TGA as well as the higher carbonyl contents. It is also possible that conjugated 1,4-diketone form following a similar process.

It is however challenging to determine the proportion of aldehydes and ketones. In either case, the resulting conjugated carbonyls are good dienophiles with their two electron withdrawing groups.

[343] Consequently, they are likely to form Diels-Alder cycloadducts with the linear furans.

Then, the aromatization would follow accompanied with a water release. As demonstrated earlier, this whole process seems to be limited to the surface.

The exact nature of the aromatized product is however unclear. The stability of the aldehydes is low, at least some of them might oxidize to carboxylic acid which could be followed by a decarboxylation. To assess the thickness of the oxidized layer, SEM was performed. Figure 4.4-4 showcases the surface of relevant Ox-PFA and PFA samples. In both PFA and Ox-PFA, the heart of the material is constituted of a pattern resulting from the cutting process. Yet, near its surface, Ox-PFA displays "lines of circles" almost perpendicular to the surface as highlighted by white dashed lines.

In PFA, this outer layer pattern is not so clearly visible nor organized although it appears different from the heart of the material. It should be noted that the sample preparation process with the microtome may induce edge effects. Thus, it may be a part of what is observed in Figure 4.4-4.

Nonetheless, if an edge effect alone was responsible both PFA and Ox-PFA should present it, which is not the case here.

Based on about thirty measures, the thickness of the outer layers was averaged at 24.0 ± 4.5 µm for Ox-PFA. For PFA, the thickness of the outer layers was estimated to 3.6 ± 0.3 µm although it might be an edge effect caused by the cutting process.

This hypothesis is supported by the observations made on Figure 4.3-1. According to the FTIR spectra, the surface of PFA and Ox-PFA are chemically different. However, the spectrometer used in this study detects depths comprised between 0.5 and 5 µm depending on the wavenumber.

More specifically, the penetration depth of the laser for ATR diamonds is typically around 2 µm at 1000 cm -1 . Hence, as furanic compounds only are detected in Figure 4.3-1 at 1000 cm -1 for PFA the thickness calculated earlier is not coming from an oxidized surface.

Kinetics of oxidation, hardening and degradation

To better understand the aromatization phenomenon, experiments and kinetic computations for kinetic analysis were performed in accordance with the ICTAC Kinetics Committee

Recommendations [368]. An advanced isoconversional method was used for this purpose and was described in previous papers [369,370]. 1 [372, 373]). As PFA's oxidation is progressing, the 𝐸 𝛼 value increase which indicates that the process is increasingly difficult.

The end of PFA's oxidation superimposes nicely with the beginning of the surface hardening observed in DMA. The 𝐸 𝛼 obtained from the E' rise in DMA rises as oxidation proceeds.

Then, as the temperature is increasing, the oxidation process is followed by a thermal hardening, which also presents increasing 𝐸 𝛼 values. This is coherent with the hypothesis emitted previously, i.e., the apparent difficulty encountered by dioxygen to penetrate the PFA's network. The end of the increase of E' is also coherent with the beginning of the degradation. The 𝐸 𝛼 of degradation of PFA and Ox-PFA are close and follow similar pathways, which indicates that the materials encounter the same degradation mechanisms once PFA has been fully oxidized.

Finally, Figure 4.5-1 shows that the effective activation energy at the beginning of the thermal degradation of PFA is slightly lower than the 𝐸 𝛼 of Ox-PFA's. As seen in Figure 4.3-5, oxidation of PFA is not complete once its thermal degradation starts. Thus, its oxidation degree is lower than for Ox-PFA and this perfectly corroborates the hypothesis of a lower stability of nonoxidated PFA.

Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -213 -

4.6

Properties of PFA and Ox-PFA

The effect of the surface crosslinking on the mechanical properties of uncured furan resins was qualitatively reported in chapter 2 and 3. Uncured furan resins tend to form a hard crust richer in carbonyls than its core. In the followings, the properties of Ox-PFA and PFA will be compared to better apprehend the impact of the surface crosslinking on the material. The elongations at break are of 1.2 ± 0.3 % for Ox-PFA and 1.9 ± 0.4 % for PFA while they have similar yield strength. In addition, the Shore D Hardness of Ox-PFA and PFA are slightly different. The higher E', lower elongation at break of Ox-PFA together with similar yield strength of the samples are consistent with the lower damping capacity observed for Ox-PFA in DMA.

Overall, the effect of the surface crosslinking on the tensile properties of furan resins is rather limited. This surface crosslinking occurring in Ox-PFA make the material slightly stiffer and more brittle.

To complete this study, the gel content, contact angle and Oxidation Onset Temperature (OOT) of Ox-PFA and PFA have been determined. The results from these experiments are gathered in Table 4.6-1. Finally, the degradation temperature of Ox-PFA and PFA were evaluated using the OOT method.

Conclusion

The formation of aromatic species, during the post-curing of furan resins was investigated.

A furan resin was post-cured under N2 to give PFA and was compared to a furan resin post-cured under air, i.e., Ox-PFA.

In terms of chemical structures, PFA presented the predominance of furanic structures. On the other hand, the Ox-PFA displayed signs of carbonyls, including aldehydes, as well as the presence of aromatic moieties. It was found that these groups are only located on the top surface.

A mechanism was proposed to explain their formation. It involved the oxidative ring opening reaction of the furan ring followed by a Diels-Alder crosslinking and the aromatization reaction of the adduct. This process is accompanied by a mass gain and a rise of the E' in the rubbery plateau.

From a thermomechanical point of view, this oxidative crosslinking is associated with a decrease of the damping capacity.

Overall, the observed oxidation process is limited to the surface, i.e., about 20 µm, and is protecting the material from further oxidation to some extent. Thus, such surface oxidation phenomenon might also explain the relatively high thermal stability obtained for furanic resins as highlighted by the kinetic study and OOT experiments. Finally, presence of an oxidized layer on the surface of PFA has little impact on the tensile properties of the material. In previous chapters, the C=O content of PFAs was quantified and somewhat optimized (chapter 2). In addition, their chemical environment was assessed (chapter 3) and an oxidationinduced surface crosslinking phenomena was observed and explained. Its impact on the properties of PFA materials was assessed (chapter 4).

In this chapter, the carbonyls presence in PFA are exploited to reach new materials with tunable properties. To do so, functionalization of carbonyls with amines was performed to give rise to PFA/Amine materials.

Section 5.1 regroups the experimental details common to all the sections. As the procedures vary between the sections, the experimental details specific to each section is reported directly inside the sections.

Section 5.2 consists in a preliminary study where Priamine 1071, a large and flexible amine, is employed as a softening agent. The chemical links between the amine and PFA are investigated.

In addition, the dependency of the materials' properties as function of the amount of Priamine introduced is assessed together with a short stability and recyclability study.

Section 5.3 has two main objectives. The first one is to evaluate the influence of different amines' structures on the properties of PFA/Amine materials. The second one is to assess the impact of the curing atmosphere (oxidative, non-oxidative) on the properties of PFA/Amine materials.

Section 5.4 deals with the influence of several parameters on the properties of PFA/Amine materials. First the influence of the starting PFA's conversion degree, i.e., prior to the reaction with amines, is assessed. Then, the impact of the basicity of amines on the acid-catalyzed polymerization of FA is also discussed.

Preparation of PFA resins

PFA resins were prepared in batches constituted of 150 g of furfuryl alcohol (FA) in a 500 mL round-bottom flask, 17 g of water and 3 g of maleic anhydride. The components were mixed at room temperature until completely homogeneous. Then the flask was heated in an oil bath. The heating time and temperature varied depending on the conversion degree desired. More details are available in Table 5.1-1.

Already reported characterization procedures

The determination of carbonyl contents (potentiometry), conversion degrees and acquisition of FTIR spectra were performed as in section 2.1. The elemental analyses were conducted as described in section 4.1. Conversion degrees and C=O content of PFA resins are reported in Table 5.1-1. 13 C NMR spectra were acquired following the procedure described in section 4.1.

Functionalization of PFA Resins and curing

The functionalization of PFA resins with Amines was performed as follows. The masses of amines were calculated using Equation 5.1-1 combined with Equation 5.1-2 or Equation 5.1-3.

Batches were between 20 and 50 g. PFAs were dissolved in 150 mL ethyl acetate in a 250 mL round-bottom flask. The amines were diluted with 20 mL of ethanol and added slowly to the flask.

The flask was placed in a heated oil bath under reflux and kept there for 4 hours to ensure conversion of ketones. The content of the round-bottom flask was poured into a crystal dish and left to evaporate overnight. The next steps are specific to each section and more details are given thereafter.

Thermogravimetric Analysis

A TGA 2 from Mettler-Toledo was used to investigate the degradation of the PFA/Amine materials under oxidative conditions. The scans started at 35 °C to reach 800 °C at a speed of 10 °C.min -1 under an air flow (50 mL.min -1 ).

Dynamic Mechanical Analysis

DMA1 from Mettler-Toledo was employed to assess the thermomechanical properties of the samples. The tensile mode was used. The DMA scans started at -80 °C to reach 300 °C at a speed of 2 °C.min -1 . The oscillation frequency was set to 1 Hz along with 0.1 % displacement amplitude and a force amplitude of 0.1 N. The maxima of tan (𝛿) was assimilated as the glass transition temperature (Tg) of the materials.

Crosslink density was calculated using the Fox-Flory Equation 5.1-4 where E' represents the elastic modulus at Tg + 50 °C, T is the temperature and R is the ideal gas constant. [374,375] 𝑣 = 𝐸′ 3𝑅𝑇 Equation 5.1-4

Uniaxial tensile tests

The uniaxial tensile tests were conducted on an EZ-LX from Shimadzu. Excepted for section 5.2, ISO 527-2-1BB, specimens were tested with a 1 kN load cell, a gauge length of 21 mm and at a crosshead speed of 1 mm.min -1 . The stress and strain were recorded for seven measurements. The results are presented as the average and standard deviation of the attempts. The tensile toughness was calculated as the area beneath the stress (MPa)strain (mm/mm) curve.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -227 -

Gel fraction

Gel fraction experiments were performed by submerging about 80 mg of sample in 15 mL of ethyl acetate for 48 h at 25 °C. Then, the solvent was removed and the samples washed with ethyl acetate before undergoing a vacuum drying step for 16 h. The gel fraction was calculated using Equation 4.1-1 were 𝑚 𝑖 stands for the initial mass and 𝑚 𝑑 the dry mass. The aim of this section is to expand the possible applications of PFA. Therefore, advantage can be taken from the so-called "self-functionalizing" ability of PFA. Indeed, chapter 2 demonstrated a significant occurrence of carbonyl moieties in PFA. They are produced within the PFA's structure during the polymerization through ring-opening side-reactions. Consequently, these 'self-generated carbonyls' can be used as anchor points for further functionalization without prior oxidation, as needed for polysaccharides. [376,377] Here, the focus is on the reaction between ketone and amine as a mean to tune the thermomechanical properties of PFA, with a focus on increasing the ductility of the material. To prove that PFA can be tuned via its open structures, a flexible C36 bio-based fatty diamine (i.e., Priamine 1071) was chosen to react with carbonyls.

The formation of connections between PFA's carbonyls and primary amine-bearing flexible segments should impact the mechanical properties of PFA, which is known to be a very hard and brittle material. The procedure for the preparation of samples for section 5.2 is illustrated in Scheme 5.2-1.

Here, the PFAs had conversion degree 0.70. Table 5.2-1 reports the sample labels together with their proportion of PFA and Priamine 1071. Priamine 1071 will be labeled "PA". The inflexion point was assimilated to the Tg of the polymers. The NH2/C=O stoichiometric ratio spanned from 0.1 to 1 and the thermo-mechanical properties of the modified PFA were assessed, with a special emphasis on the toughness. Scheme 5.2-2 schematically depicts the obtained thermoset and Table 5. In Figure 5.2-2A, the intensity of the ketone peak at 208 ppm decreases when the stoichiometry of introduced Priamine increases. The peak at 171 ppm in PFA is attributed to ester groups (cf Table 1.3-2). In PFA/PA samples, this peak increases and is slightly shifted to 165 ppm. This is consistent with the formation of imines bonds in the 160-170 ppm region. [385,386,387,388] Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -232 -In Figure 5.2-2B, the carbonyl stretching band at 1710 cm -1 in PFA (cf Table 1. progressively decreases with the increasing addition of Priamine. This decrease of the C=O band is accompanied by the increase of the C=N band at 1656 cm -1 , thus showing the condensation of carbonyl and amine moieties into imines. [388,389,390,391,392,393] The height ratio H1710/H1656 was calculated and found to have a good linear correlation with the amount of Priamine introduced in the system, as depicted in Figure 5.6-1. However, a small amount of carbonyl is detected in PFA/PA 1 sample in both NMR and FTIR spectra. The fact that a small proportion of carbonyl remains unmodified can be associated with the steric hindrance of Priamine, as it has been shown in chapter 2(SECTION) that large derivatizing agents employed for the C=O quantification can have difficulties reaching carbonyl moieties within PFA. Carbonyls can also form after the functionalization of PFA through the oxidation route (chapter 4). Earlier, it was proposed that the links between PFA and amines are only Schiff bases (C=N)

on the basis of the appearance of a peak at 170 ppm in 13 C NMR and a FTIR band at 1656 cm -1 .

Nonetheless, if the carbonyls in PFA were only based on the structure (I) in Scheme 5.2-3, their reaction with primary amines should yield pyrroles. Indeed, when a model compound such as 2,5hexanedione is reacted with a primary amine the major product is a pyrrole, as depicted in Scheme 5.2-4. On the 13 C NMR spectra presented in Figure 5.2-2A, a slight shift is visible from about 125 ppm to about 130 ppm. The magnified 13 C ssNMR spectra in Figure 5.6-2 showcases this shift.

Such shift could be consistent with the presence of pyrroles. However, determining the proportions of pyrroles compared to Schiff bases is very challenging as many functional groups are present in PFA/Amine systems which leads to overlapping signals.

Scheme 5.2-4. Formation of pyrroles from 1,4-diketones through the Paal-Knorr reaction.

Taking into account the structures in Scheme 5.2-3, the reaction of the carbonyls with primary amines should yield the products highlighted in Scheme 5.2-5. The structure (I) is probably mostly yielding the pyrrole structure (IV) and structure (V) in minor proportions. Finally, the enol structure (VI) and ketal structure (VII) might be responsible for Schiff base bonds. In the following parts, the chemical links between PFA and amines will be referred as C=N as it englobes the possible reactions of amines with ketones, i.e both imines and pyrroles. The Tg of PFAs may also be extracted from DSC scans performed at high speeds. Figure The above mentioned increase in flexibility is also well correlated with the Shore D measurements. Indeed, a large portion, i.e., from 80 to 30, of the Shore D range is covered by these systems, corresponding to extra hard for unmodified PFA to medium hard for PFA/PA-1.

Elongation at break and ultimate strength data were extracted from the stress-elongation curves to give Figure 5.2-5A and B, respectively. While classical PFA exhibits brittle behavior, i.e., ≈ 5 % elongation at break, the functionalization of PFA with Priamine allows it to reach up to ≈ 30 % elongation at break. This increase in ductility varies with the amount of Priamine introduced. However, such ductility comes with a decrease of the ultimate tensile strength, as can be seen in Figure 5.2-5B. Indeed, the ultimate tensile strength is reduced from 16 MPa to 1 MPa.

Thus, the properties can be optimized to meet the requirements needed for a specific application. This behavior is closely related to the crosslink density, which also doubles (440 to 950 mol.m -3 ).

Therefore, the Priamine has a double effect on PFA system. First, at low concentration (i.e., 25 %mol) it mostly toughens the system by introduction of flexible crosslink nods. On the other hand, at higher concentration the plasticizing effect of Priamine 1071 takes over. In this section, the stability of PFA and PFA/PA samples was investigated. To do so, the thermo-oxidative degradation was assessed by TGA. The chemical resistance was first evaluated by the measurements of the gel fraction in ethyl acetate. In addition, the stability of PFA/PA 1 towards hydrolysis was checked.

The thermal resistance of the copolymers was assessed by TGA (curves in Figure 5. and the T5% and T10% were extracted and are reported in Table 5.2-3. The thermal resistance of PFA is maintained for all samples. Moreover, for the highest Priamine concentrations (PFA/PA 1) both T5% and T10% increase of about 40 °C compared to unmodified PFA.

Table 5.2-3. Degradation temperatures and gel fraction of PFA and functionalized PFAs.

The gel fraction of the different samples was measured after 48 h immersion in EtOAc and the results are gathered in Table 5.2-3. The gel fraction increases from 87 wt% for PFA to 96 wt% for PFA/PA 0.5. It confirms that PFA is more cross-linked after functionalization with PA thus leading to higher stability in solvent. However, the slightly lower gel content for PFA/PA 1 (92 wt%) compared to PFA/PA 0.5 can be explained by its lower cross-link density (Figure 5. Imine groups can be hydrolyzed back to carbonyls. To highlight the stability of the newly formed imines, the PFA/PA 1 sample was exposed to different hydrolytic conditions (water, 0.1 M acidic water, acidic water/THF or acidic water/ethanol). Table 5.2-2 shows that the gel content after the different ageing remains very high (≈ 90 %w and above) and the released part is not correlated with the PA content (i.e 47.9 %w of PA in PFA/PA 1). Moreover, FTIR measurements conducted on the PFA/PA 1 samples and exposed to different hydrolytic conditions (Figure 5.2-7)

did not show significant differences with the reference material. The imine band at 1656 cm -1 remains unaffected indicating that the hydrolysis did not occur in these relatively mild conditions.

Only the sample exposed to 0.1 M acidic water showed a slight broadening of the imine band correlated with a slight increase of the carbonyl band (compared to the non-aged sample)

suggesting a somehow limited hydrolysis or a modification of the imine environment. Finally, a PFA/PA 1 sample was left to age for three months at room temperature in sealed bags. The DMA curves of a fresh and three months old PFA-PA 1 are available in Figure 5.2-9.

The E' curve shifts towards higher moduli and the tan (𝛿) peak is both larger and less gaussian for the aged sample. In addition, the Tg of the aged sample is higher by about 20 °C. These results highlight either an uncompletion of the curing or other phenomena occurred during the ageing period. The latter hypothesis being supported by the non-gaussian shape of the tan (𝛿) peak which suggests an inhomogeneous macromolecular network.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -241 -

Mechanical recycling of Priamine functionalized PFA

In this section, specimens of PFA and PFA/PA were ground with a ball-mill and the polymers were compression molded once again. The properties of the reprocessed specimens are briefly overviewed in the following. Unexpectedly, neat PFA showed a mechanical reprocessability. First, the successful realization of tensile test shows that PFA and PFA/PA cured at 150 °C and not aged can be reprocessed into cohesive samples. The Young's moduli of the samples behave similarly, as the E' at 20°C from Figure 5.2-10, i.e it increases after the reprocessing. This is less pronounced for higher amounts of Priamine. On the elongation at break, tensile toughness and ultimate strength it is hard to draw a conclusion as the standard deviation tend to overlap between the cycles. The exception being lower tensile toughness and ultimate strength for neat PFA.

To conclude on this part, it is possible to mechanically reprocess unaged PFA and PFA/PA if they were cured at 150 °C for less than 7 h. Yet, it is difficult to attribute this phenomenon to the presence of reversible reaction (imination, Diels-Alder). It could also very well be the presences of other, irreversible reactions, that did not occur during the fabrication of the materials (polycondensation, aromatization). The later hypothesis being more likely, especially as the DMA of PFA/PA 1 in Figure 5.2-9 shows a hardening of the polymer overtime and the fact that PFAs such as the one presented in Chapter 4 cannot be mechanically reprocessed as easily (cf annexes).

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -243 -

Conclusion Preliminary investigations

In summary, the aim of this section was to pave the way to use carbonyl moieties within poly(furfuryl alcohol) as a means to extend the possible applications of this thermoset. Indeed, PFA "self-functionalizes" during its polymerization though furan-ring opening. Consequently, chemically available ketones are ready to be the key to unlocking a latent potential for tunability.

In this work, PFA was treated with Priamine 1071. The resulting PFA/PA displayed a range of behavior from brittle to ductile. Elongation at break ranged from 5 to 35 % while tensile toughness could be doubled while maintaining the thermal resistance of PFA. Consequently, a fully bio-based and flexible thermoset with tunable properties can be prepared by exploiting the functionalities self-produced during polymerization.

However, further research is still needed to better understand the recyclability and variation of properties of Priamine-functionalized PFAs.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -244 -

Effect of the amines' structure and post-curing environment

The aim of this section is to highlight the factors influencing the thermomechanical properties of the PFA/Amine materials. First, the influence of the amines' structure is studied. Additionally, the effect of the post-curing environment (oxidative or non-oxidative) is assessed.

In a first part, different amines bearing various structures were selected to understand to which extent the chemical structures of the employed amines (or urea) can impact the tunability of PFA resins (amine structures in Scheme 5.3-1). For this purpose, flexible and rigid amines were selected and their impact on the macromolecular network was assessed.

Scheme 5.3-1. Structures of the amines and urea selected for derivatization.

In a second part, the impact of the curing atmosphere (i.e inert or oxidative atmosphere) on the properties of PFA/Amine materials was evaluated. As demonstrated in the chapters 3 and 4, the furans located on the surface of PFA materials are sensitive to oxidative ring-opening. When exposed to air, these furans at the top-surface can open into conjugated 1,4-dicarbonyls. These conjugated carbonyls are prone to Diels-Alder cycloaddition with other furans in PFA.

Consequently, additional crosslinking nods are created which induces a surface hardening. This process is accompanied by a darkening of PFA. Hence, the importance of this phenomenon on PFA/Amine properties was assessed.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -245 -Section 5.3 aims at understanding, the influence of the amines' structure on the properties of PFA/Amine materials. In addition, the influence of the post-curing environment (oxidative or inert) was assessed. From here, two protocols were used. In most cases, the pieces were removed from the molds and cut in parallelepipeds for DMA, hardness and TGA measurements or in dumbbell shapes (ISO 527-2-1BB) using a specific cutter for the tensile tests (cf Figure 4.8-1). The cutting was performed at 150 °C to avoid breakage of the samples. Then, the specimens were post-cured in an oven at 150 °C for 3 hours.

In the second protocol, the samples were not removed from the molds and the post-curing was performed in the molds (150 °C, 3 h). Then the samples were cut using the same procedure.

In the study, when comparing the first and second protocol, the terms "OX" and "IN" serve as distinction. If nothing is mentioned, the post-curing occurred in the oven.

Effect of the amine's chemical structure

Section 5.3.2 is dedicated to the influence of the chemical structure of the amines on the properties of PFA/Amine materials. Scheme 5.3-1 presents the selected amines and their abbreviation. Their molar masses ranged from 60 g.mol -1 to 2 000 g.mol -1 . The selection included difunctional linear and cyclic amines as well as urea. The purpose of the larger aliphatic amines was to introduce flexibility in the macromolecular network. The aim of the shorter and cyclic ones was to potentially increase the rigidity of the network. It should be noted that the mass ratio of amines toward PFA is variable due to the large gap in molecular weight gap between the amines.

Table 5.3-1 displays the mass percent of PFA for each sample.

The reaction between the C=O in PFA and the NH2 was verified by means of FTIR spectroscopy in the same manner as in section 5.2. The magnified spectra of the PFA/Amine materials are available in Figure 5.3-1 and the spectra of pure amine are shown in Figure 5. 6-8. Here, all the materials showed less intense C=O peak than the standard PFA. Most materials exhibited the formation of a C=N bond around 1650 cm -1 . This confirms the linkages between the amines and PFA.

[ 328] Pyrroles are also likely formed as it was suggested by Mohajeri et al. in systems comprising an epoxy, an amine and a PFA. [219] Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -247 -

The case of urea is slightly more complex as other bonds than C=N and pyrroles are formed.

[394] Nonetheless, the diminution of the C=O compared with the furanic C=C (1550 cm -1 ) attest for interactions between PFA and urea. Even though the amines were introduced in stoichiometric In Figure 5.3-2, PFA/J2000 stands out from the other materials. The tan (𝛿) maxima of the PFA/J2000 is located at the lowest temperature (-44 °C). Moreover, the peak is at least three times higher than the other samples. Three orders of magnitude separate the E' value at 20 °C of PFA and the PFA/J2000 samples. According to Table 5.3-1, the shore D of the PFA/J2000 sample is about 8.5 times lower than the PFA's. Such behavior is consistent with the important amount of the flexible J2000 in the material (75 %w). Indeed, with such a high concentration of J2000, the macromolecular network of PFA is highly flexibilized. Finally, the T5% and T10% of the PFA/J2000 sample are about 10 °C lower than PFAs'. Thus, the good thermal stability of PFA is maintained although PFA represents only 25 %w of the total. Moreover, pure J2000 has a T5% near 240 °C and a T10% near 260 °C. So, PFA can also be seen as a thermal stabilizer when considering J2000 based materials. Regarding the other amines, both PFA/J230 presents lower Tg, E' at 20 °C and Shore D than PFA. However, as indicated by the T5% and T10% values, the thermal resistance of PFA is maintained despite the introduction of a less thermally resistant amine. Additionally, as shown in Figure 5.3-2, the rubbery plateau of PFA/J230 is lower than PFA's. Thus, J230 introduce some flexibility in the material. PFA/Urea shows a 13 °C increase of Tg together with a small increase of E' at 20 °C and no impact on the hardness, in comparison with PFA. Nonetheless, the T5% of PFA/Urea is more than 40 °C lower than PFA's.

PFA/IsoP has a Tg that is 20 °C higher than PFA and a slightly higher hardness. Its E' value at 20 °C and at the rubbery plateau are lower than PFA's. Yet, the rubbery plateau of PFA/IsoP is reached around 230 °C while PFA's is reached around 175 °C. The thermal resistance remains unchanged. Finally, the PFA/Furfu sample has a lower Tg and E' at 20 °C than PFA. The properties PFA/Furfu remain close to PFA's.

In complement with the previous characterizations, the materials were subjected to uniaxial tensile tests. PFA/J230 showed significantly lower E' and ultimate strength than PFA while its elongation at break was doubled. This is expected as its Tg are lower than the Tg of PFA. PFA/Urea, PFA/IsoP, samples showed lower ultimate strength and tensile toughness than PFA. Their elastic moduli and elongation at break are close to the ones of PFA. The PFA/Furfu sample showed similar tensile behavior but even closer to PFA's.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -251 -Overall, when reacted with a PFA at a conversion degree of 0.8 and at NH2/C=O stoichiometric ratio = 1, Urea and IsoP have negative effects on the tensile properties of PFA although they lead to an increase the Tg of the material. The introduction of these short amines has broadened the relaxation behavior of the materials and can be interesting for applications demanding low deformation. However, such short distance stiffening is not interesting when higher levels of deformation are needed. Interestingly, the more flexible amines such as J230 and J2000

increase the ductility of the material. This must be put in correlation with the weight percent of PFA in the samples (Table 5.3-1) which can be as low as 25 %w for PFA/J2000. The chemical structure of the amine also influences the properties of the material. The PFA/J230, and PFA/IsoP contain between 72 and 84 %w of PFA. Both samples retain the thermal stability of PFA. However, as their Tg are 70 °C apart, their mechanical properties measured at 24 °C are significantly different.

Hence, at somewhat similar weight percent of amine, the chemical structure of the amine (rigid or flexible) has a strong effect on the mechanical properties of the material.

Consequences of PFA's surface hardening

During the post-curing steps, a surface hardening phenomenon was observed on some samples. This surface hardening was especially frequent and obvious in the case of ductile materials. Figure 5.3-4 depicts a representative example of a ductile sample which was post-cured in a forced convection oven and freshly cut along its length. As seen in Figure 5.3-4, the sample has a soft and brownish core while its surface is hard and dark. Such color and texture are highly similar to the one of the PFA resin in Figure 3.2-6. Hence, it suggests the intervention of the oxidation-induced surface crosslinking.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -252 - In chapter 3 and 4, it was mentioned that PFA is sensitive to surface oxidation and surface crosslinking when exposed to air. In the case of some PFA/Amine samples, a similar surface hardening phenomenon was observed. It is especially visible for ductile samples such as the one in Figure 5.3-4. To further investigate this aspect, PFA/J2000 samples were prepared at three NH2/C=O stoichiometric ratios (0.1, 0.3 and 1.0). Two post-curing procedure were used in accordance with Scheme 5.3-2. For the first one, the post-curing occurred under air, i.e, in oxidative environment. In this case, the samples are labeled "OX". For the second procedure, the post-curing was performed in the molds, i.e., protected from air oxidation. In this case, the samples are labeled "IN". PFA references were also prepared following the two post-curing procedures. Figure 5.3-5 depicts the DMA curves of PFA and PFA/J2000 post-cured either in the oven or in their mold.

In Figure 5.3-5A, the E' curves of the reference PFA/OX and PFA/IN superimpose in the glassy state before the beginning of the glass transition. However, in the rubbery state, their evolution can be discriminated. PFA/OX exhibits a higher rubbery plateau and lower damping capacity than PFA/IN. These phenomena are specific to PFA and are linked to its ability to crosslink on its surface upon exposure to air. This oxidation-induced crosslinking was investigated in chapter 4 and is associated with a slight increase of E' when the material is in the rubbery plateau around 175 °C.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -253 - observations made with a PFA/Priamine system in section 5.2.

Nonetheless, when comparing the post-curing conditions, the thermomechanical behavior strikingly differs. PFA/J2000/0.1Eq/IN has its tan (δ) maxima at 10 °C while the one of PFA/J2000/0.1Eq/OX is around 42 °C. The shape of the tan (δ) peak of PFA/J2000/0.1Eq/IN is also much sharper than its "OX" counterpart. In addition, the rubbery plateau of PFA/J2000/0.1Eq/IN is lower than PFA/J2000/0.1Eq/OX. Similar comments can be made on the PFA/J2000/0.3Eq/IN and "OX" although the gap between the DMA curves is tighter. Finally, in the case of PFA/J2000/1Eq/IN and OX, the curves are almost superimposed.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -254 -Thus, it can be deduced that PFA is responsible for both the crust on the samples and variation seen in the thermo-mechanical properties. Hence, high concentrations of PFA (between 50 and 75 %w) in the materials have a pronounced effect on the thermomechanical behavior variation between the "IN" and "OX" procedures. Vice-versa, for lower concentrations of PFA (≤ 25 %w), the effect is minimal and the samples show little to no crust. In complement, TGA and shore D hardness tests were performed. close. The same conclusion also stands for PFA/J2000/1Eq samples. Nevertheless, for PFA/J2000/0.1 and 0.3 Eq, their E' at 20 °C are one to two orders of magnitude lower for the samples post-cured in inert conditions in comparison with the ones post-cured in a more oxidative environment. In this line, the hardness of PFA/J2000 samples is always higher in the case of the oven post-curing. This effect is especially strong for the PFA/J2000/0.1Eq samples which contain 75 %w of PFA. The PFA/OX and PFA/IN have similar hardness. However, their T5% are about 20 °C apart. For PFA/J2000/0.3Eq/IN the T5% is nearly 70 °C lower than its "OX" counterpart. For the PFA/J2000/0.1Eq/IN the effect is a little bit less significant with a T5% about 55 °C lower than PFA/J2000/0.1Eq/OX. In addition, the two PFA/J2000/1Eq samples have similar T5%. For all the samples the T10% values behave in a similar manner although the gaps between "OX" and "IN" are tighter.

To further study the impact of the crust formed in oxidative conditions, the PFA/J2000 samples were subjected to uniaxial tensile tests. As mentioned previously, the PFA/J2000/1Eq samples could not be tested. First, the PFA/IN and PFA/OX demonstrate very similar tensile properties, only a slightly lower ultimate strength is noticeable for PFA/IN. Thus, the impact of this formed crust on neat PFA on its tensile properties is limited. This is in agreement with chapter 4. In the case of PFA/J2000/0.1Eq and PFA/J2000/0.3Eq the oxidized samples have much higher Young's moduli than the non-oxidized counterparts, by order(s) of magnitude. The same comment can be made about the yield strength. In complement, the elongation at break of these samples is three to five times higher in the case of the non-oxidative curing procedure. Finally, the tensile toughness is only impacted in the case of the PFA/J2000/0.3Eq samples.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -256 - To sum up, surface hardening ability of PFA's has a tremendous impact on the properties of the more ductile PFA/J2000 materials. The crust results from an oxidative ring-opening pathways followed by a cycloaddition and potentially an aromatization of the adduct. Although this phenomenon has a limited impact on PFA alone, the PFA/Amine material may undergo significant variation in their properties. Upon surface hardening, the Tg of the materials can increase to more than 25 °C due to this surface hardening. Therefore, the polymer may shift from a rubbery behavior at room temperature to a glassy one and the mechanical properties may shift from ductile to brittle.

Furthermore, the thermal degradation is significantly affected by the presence of this stiffer top-layer that leads to an increase of onset temperatures of degradation once the crust is formed.

These effects are direct consequences of the unique reactivity of PFA. Thus, they are highly influenced by the weight percent of PFA in the materials. The dioxygen in the air plays a crucial role in the formation of the crust. Hence, if one wants to avoid its formation, protection from the air is required. Antioxidants and/or coating might be effective for that purpose.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -259 -

Conclusions

In this section the C=O present in PFA were derivatized with difunctionnal primary amines.

Three factors influencing the properties of PFA/Amine materials were investigated.

First, the effect of the amines' chemical structure was studied. Linear amines with long chains introduced flexibility in the material. The impact was especially significant for the larger amine, i.e, J2000. Materials based on rigid amines such as isophorone diamine gave materials with higher Tg than PFA. The impact of the amines on the thermo-oxidative degradation is limited in most cases.

Some of the PFA/Amine material presented the formation of a crust during their postcuring. This phenomena was investigated using PFA/J2000 materials at varying functionalization degree and post-curing environment (oxidative or non-oxidative). The materials post-cured in nonoxidative conditions were much more flexible than the ones post-cured in oxidative conditions. On similar samples, the post-curing environment can increase the Tg above room temperature. This ultimately leads to tensile propertiesultimate strength, Young's modulusdistant by orders of magnitudes. In addition, the oxidized PFA/J2000 samples presented superior thermo-oxidative degradation resistance as illustrated by an increase of T5% of about 70 °C compared to material post-cured in inert conditions. In addition, if the crust is removed, the material recovers its flexibility. The formation of the crust results from an oxidation-induced surface crosslinking pathway specific to PFA described in chapter 4. It could not be avoided for pure PFA/Amine materials if the weight percent of PFA was superior or equal to 50 %w.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -260 -

5.4

Effect of PFA's conversion degree and effect of the amines' basicity

The aim of this section is to further highlight the factors influencing the thermomechanical properties of the PFA/Amine materials. This section is divided in three parts. First, the precipitation of PFA upon reaction with DiHex in ethyl acetate is discussed. Then, the influence of the degree of polymerization of the starting PFA is evaluated. Finally, the last part deals with the impact of the amines' basicity on the acid-catalyzed polymerization of FA.

As discussed in this manuscript, the polymerization of FA is far from being straightforward.

To better understand this section, major facts are recalled in the followings. Briefly, the polymerization of FA goes through two main chain lengthening pathways. They are reported in Table 5. In a first phase, the impact of the conversion degree of PFA on the functionalization of its C=O, and consequently on the sample properties, was investigated. Indeed, the C=O content of PFA increase during the course of polymerization. Although highly polymerized PFA have the highest C=O content, their partial insolubility may hinder the C=Os' reactivity. Thus, PFAs at various conversion degrees (0.3 ≤ 𝛼 ≤ 0.95) were prepared, characterized and functionalized with 1,6-diaminohexane (DiHex).

In a second phase, investigations using a monoamine (hexylamine) and potassium hydroxide were conducted to assess the impact of the basicity on the polymerization and final properties of the materials. Indeed, the basicity of amines is in opposition with the acid-catalyzed polymerization of FA and it somehow impacts the final stages of the polymerization. 5.4.1 Experimental details specific to section 5.4

In section 5.4, the impact of the conversion degree of PFA before functionalization was studied using 1,6-diaminohexane (DiHex). In addition, the impact of bases on the polymerization was assessed. Accordingly, n-hexylamine (Hex) and potassium hydroxide (KOH) were employed.

The sample preparation procedures are illustrated in Scheme 5.4-1.

First, mixtures of FA comprising 10 %w of water and 2 %w of maleic anhydride were homogenized. The mixtures were then placed in a thermoregulated bath. To reach the desired conversion degrees the time and temperature conditions reported in Table 5.1-1 were used. The C=O content of these PFA prepolymers are also reported in Table 5.1-1. All the pre-polymers were soluble excepted the 𝛼 = 0.95 specimen who reached vitrification. Thus, it was ground using a ball mill prior to functionalization. To derivatize the C=O, about 30 g of PFA prepolymer were placed in a round bottom flask and dissolved in refluxed ethyl acetate (≈ 150 mL). In the case of PFA at 𝛼 = 0.95, only swelling was achievable. Then, a solution of amine or KOH in ethanol (20 mL) was quantitatively transferred to the round bottom flask. Depending on the conversion degree of the resin and the amine type, precipitation may occur at this stage. The PFA/Amine solutions were refluxed for 4 h before being left to evaporate in a crystalizing dish for 24 h with occasional stirring to break any chunks or floating layer. In some assays, the precipitate was filtered and washed thoroughly with ethyl acetate before drying. In this case, the sample name was suffixed with the letter "p". The yield of precipitate was calculated according to Equation 5.4-1. The washings were done with ethyl acetate. Then, the procedure mentioned previously (section 5.1) was used to grind the rubbers. The precuring and grinding steps were crucial to obtain bubble free and homogeneous materials. The resulting powders were then molded under 25 -35 bars at 150 °C and for 2 hours in a heated press.

𝑌𝑖𝑒𝑙𝑑 𝑜𝑓 𝑝𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑒 =

From here, the pieces were recovered from the molds and cut in various shapes. For DMA, hardness and TGA experiments parallelepipeds were used. For the tensile tests, dumbbell shapes (ISO 527-2-1BB) were used.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -265 -The cutting was done at 150 °C using with specific cutters. The heat is required to avoid fracture of the samples. Then, the materials were post-cured for 3 h at 150 °C in a forced-convection oven. To ensure proper interpretations, reference samples were prepared according to the same procedure using PFAs at 𝛼 of 0.30, 0.80 and 0.95 without any amine or base. In these cases, the precipitation phenomenon was not observed. Figure 5.6-14 (DMA curves), Figure 5.6-15 (TGA curves), Table 5.6-1 (thermomechanical properties) and Figure 5.6-16 (tensile data) display the results from characterization of the neat PFA samples i.e prepared without amines but following the same protocols. As variations can be observed between the PFAs, the "PFA" data are presented as the average and standard variation of the three controls. Finally, the yield of precipitate was evaluated as a function of the conversion degree of the starting PFA. Figure 5.4-3 depicts the mass ratio of insoluble part over the total mass of PFA and DiHex as a function of the PFA's conversion degree. Higher is the conversion degree and higher is the amount of insoluble part following a good linear correlation (R² = 0.989). This suggests that a minimal chain lengthor crosslink density -is required for the macromolecules to make this phase separation. The origin of this separation is most-likely due to the formation the C-N bonds. Indeed, formation of additional bonds by introduction of the amine would imply longer macromolecules and/or a tighter network. Hence, the solubility of the polymer would decrease. To sum up, the reaction of PFA's C=O with difunctional primary amines in ethyl acetate may lead to the precipitation of the resulting polymers. The yield of precipitate is highly correlated with the conversion degree of the starting PFA. In addition, the yield of precipitate may be up to 65 %w in respect with the initial amount of reagents. Finally, the PFA/DiHex materials are sensitive to surface ring opening when exposed to air and heat. This may be a factor explaining the presence of residual carbonyl groups on the surface of PFA/Amine materials.

Effect of the conversion degree on the properties of PFA/DiHex

In this section, the thermomechanical properties of PFA/DiHex materials are discussed. In addition, materials prepared from the precipitated macromolecules alone were also assessed. They were labeled with a "p" suffix.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -269 -First, the successful derivatization of the C=O was checked by FTIR. The amount of C=O in a PFA resin can be assessed by comparing the height of the C=O peak at 1715 cm -1 versus the 1550 cm -1 peak, corresponding to the C=C of disubstituted furan. [292,296,328] In Figure 5.4-5, the E' curves of the samples prepared with PFAs at 𝛼 ≤ 0.8 are all below the control PFA. Their surface of tan (𝛿) peak are higher than the one of reference PFA, thus it suggests a higher damping capacity for these samples. The sample prepared at a conversion degree of 0.3 (PFA/DiHex/𝛼0.3) presents a Tg around 20 °C and consequently the E' value at 20 °C reach 160 MPa, i.e., about 100 times lower than the E' value PFA at 20 °C. The shore D hardness of this sample is also much lower than PFA's. Its T5% and T10% are about 30 °C lower than PFA.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -271 -Finally, a second tan (𝛿) peak is observed at 210 °C. Insights on the nature of this peak are given in the last section of this paper. The samples prepared at conversion degrees of 0.7 and 0.8 place themselves between PFA and the PFA/DiHex/𝛼0.3. They are very close together apart from a higher T5% and T10% for PFA/DiHex/𝛼0.8.

The PFA/DiHex/𝛼0.9 sample is highly similar to PFA in most regards. Two small differences can however be highlighted. The T5% and T10% are increased by 5-10 °C and a secondary tan (𝛿) peak is observed around 175 °C. This peak is also visible on the PFA/DiHex/𝛼0.8 curve.

These differences are emphasized in the case of the PFA/DiHex/𝛼0.95 sample. For PFA/DiHex/𝛼0.95 the secondary tan (𝛿) peak at 175 °C appears more as a shouldering and its T5% and T10% are about 20 °C superior to PFAs'. The secondary tan (𝛿) peaks suggest a more rigid and heterogeneous macromolecular network than PFAs'. Such result is consistent with the higher diversity of chemical groups in PFA/DiHex materials. To complete this part of the study, two samples were prepared using only the precipitated macromolecules (PFA/DiHex/𝛼0.9p and PFA/DiHex/𝛼0.95p). PFA/DiHex/𝛼0.9p has a Tg 20 °C higher than its counterpart (PFA/DiHex/𝛼0.9). This comes along with a higher E' at 20 °C but without significant variation of the hardness. The T5% of PFA/DiHex/𝛼0.9p is 16 °C lower than PFA/DiHex/𝛼0.9 while their T10% are close. Similarly, the PFA/DiHex/𝛼0.95p samples shows a 10 -20 °C diminution of T5% and T10% in comparison with PFA/DiHex/𝛼0.95. However, the PFA/DiHex/𝛼0.95p displays a much higher Tg (+60 °C), and higher hardness than PFAs. In addition, the PFA/DiHex/ 𝛼0.95p does not show a rubbery plateau in Figure 5.4-5 B. The E' of PFA/DiHex/𝛼0.95p drops by about one order of magnitude upon passing it's Tg while the drop is of about two orders of magnitude of E' in the case of the non-precipitated sample.

These results suggested a higher crosslink density in the precipitated samples, possibly C=N crosslinks. To check this, elemental analyses were conducted. The results are displayed in Table 5.4-3. It clearly shows a higher proportion of nitrogen in the precipitated samples, about 1.4 times more than PFA/DiHex/𝛼0.95. than PFA. This is especially the case for the sample prepared with a starting PFA at 𝛼 = 0.95. The most-likely explanation is a higher concentration of C-N bonds in the insoluble fraction. This results in a higher Tg (i.e., 145 °C) and E' in the rubbery state. Such high Tg might be achievable trough the removal of small, plasticizing oligomers and/or FA. The removal low molecular weight species is a good point to avoid future release of potentially harmful compounds during the lifetime of a material or its recycling. Yet, it comes at the cost of a slightly reduced resistance to thermooxidative degradation. The cleavage of C-N bonds might be responsible for this. Regarding the PFA/DiHex/𝛼0.9 and PFA/DiHex/𝛼0.95, little differences are observed. The tensile properties of PFA/DiHex/𝛼0.95p could not be studied. The E' at temperature 150 °C and above of this sample is too high for it to be cut by the dumbbell cutter.

To sum up, 1,6-diaminohexane (DiHex) can be easily reacted with PFAs at conversion degree between 0.3 and 0.95 and materials can be obtained. PFA/DiHex samples prepared with a PFA at 𝛼 between 0.3 and 0.8 showed significantly diminished Tg, in comparison with PFA. Yet, they remained brittle and their thermal resistance was also decreased in some cases. The origin of their brittleness is discussed in the next section.

The PFA/DiHex samples prepared with PFA at 𝛼 > 0.8 showed thermal and mechanical properties close to standard PFA, with slightly higher Tg, T5% and T10%. Finally, during the synthesis of the PFA/amine materials, phase separation may occur. Materials were prepared using only the precipitate. The resulting specimens, especially PFA/DiHex/𝛼0.95p, displayed much higher mechanical properties at temperatures between 100 and 250 °C than all the other samples. This improvement however comes with a compromise on the starting degradation temperatures that are attributed to the cleavage of C-N bonds. The curves of PFA/Hex/0.25Eq are also close to PFA's until the tan (𝛿) maximum where the curves diverge. Regarding PFA/Hex/0.5Eq, its tan (𝛿) peak is both higher and wider than PFA's.

Furthermore, after 150 °C its values of tan (𝛿) matches the ones of PFA/KOH (tan (𝛿) ≈ 0.15).

Regarding the E' curve of PFA/Hex/0.5 Eq in Figure 5. In PFA/DiHex/ 𝛼0.3/1Eq, its C=O content was much lower than samples prepared with a starting PFA at a conversion degree of 0.8. Hence, a minimal concentration of C=O is required to produce PFA/DiHex materials with minimal intervention of pathway III.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -279 -5.4.5 Thermal and mechanical properties of PFA/KOH and PFA/Hex

As discussed earlier, it is possible to achieve PFA materials even if the polycondensation reactions were quenched with KOH and monoamines at a conversion degree of 0.8. Achieving

PFA materials in such conditions is possible thanks to the oxidative surface crosslinking pathway (Pathway III of Table 5.1-1). In the followings, the properties of these materials are investigated. The brittleness of PFA/KOH is highlighted through much lower yield strength elongation at break and Young's modulus than PFA. For the PFA/Hex samples, the properties progressively decrease with the increasing amount of hexylamine in the material, in coherence with the previous observations. Overall, although materials are achievable, introducing monoamines or KOH in a PFA at α = 0.8 results in a loss of mechanical properties, especially at the highest NH2/C=O stoichiometric ratios. The main reason behind this is likely the quenching of the final polycondensation reactions. Therefore, the loss of properties might be circumventable by the use of PFA at higher conversion degrees (α ≥ 0.9) to minimize oxidation-induced crosslinking (Pathway III).

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -281 -To sum-up, the fabrication of PFA materials using monoamines is feasible. However, the basicity of the amine stops the condensation reactions. The obtention of a thermoset is nevertheless achieved through the oxidation-induced crosslinking pathway (Pathway III of Table 5.4-1). As a consequence, the materials display lower mechanical properties compared to PFA. In the case of PFA/KOH, the loss of properties might also be linked to the amount of salt remaining in the material (≈ 15 %w)

In the PFA/Diamine materials the lack of polycondensation might be counteracted by the formation of C-N crosslinks. Hence, if one desires prepared PFA/Monoamine materials, it is highly suggested to use PFA with conversion degrees higher than 0.9 and preferably higher than 0.95 although solubility issues will be encountered.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -282 -

Conclusions:

To conclude, in this section the C=O in poly(furfuryl alcohol) (PFA) resins were reacted with primary amines to form PFA/Amine materials. The influence of the conversion degree of the starting PFA on the materials' properties was investigated. In addition, the effect of bases on the acid-catalyzed polymerization of FA was assessed.

The influence of the conversion degree (𝛼) of the PFA used in the preparation of the materials was evaluated using 1,6-diaminohexane (DiHex). PFAs with at 𝛼 comprised between 0.3 and 0.95 were studied. For 𝛼 ≤ 0.8, the condensation reactions of FA and oligomers were compromised by the basicity of the amines. Hence, the resulting material showed poor mechanical properties and little increase of flexibility. For 𝛼 ≥ 0.9 the PFA/DiHex materials had properties close to PFA. In addition, during the functionnalization of the C=O by NH2 a phase separation occured. The yield of precipitate may be up to 65 %w of the reagents weight. This insoluble phase was isolated and compression molded. The resulting material displayed a Tg of 145 °C although its thermal degradation resistance was slightly decreased in comparison with PFA. Hence, the use of the precipitated product is reccomended to produce high Tg bio-based materials. In addition, the use of PFA with 𝛼 ≥ 0.9 should be prefered to produce rigid PFA/Amine material.

The second part of the study involved the reaction of PFA with hexylamine to assess the effect of monoamines on PFA. Briefly, the resulting material showed decreased properties as the amount of hexylamine was increased. A reference PFA was also prepared using potassium hydroxide (KOH) as a polycondensation quencher. Although obtaining a hard and cohesive PFA in such conditions is achievable, it results in a brittle material. Hence, the basicity of the amines might prevent good mechanical properties by stopping the polymerization of PFA's oligomer if the conversion degree of the starting PFA is too low (𝛼 ≤ 0.8). The obtention of a PFA material with little polycondensation reactions is feasible through the oxidation-induced crosslinking. Such crosslinking pathway would be favored if the polycondensation are stopped and the pre-polymer exposed to enough air and heat.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -283 -

General conclusion of chapter 5

The aim of this chapter was to extend the possible applications of PFAs trough functionalization of the C=O within PFA with primary amine to form imines and pyrroles.

First, a proof a concept was conducted in section 5.2. Priamine 1071, a flexible bio-sourced fatty amine was reacted with the carbonyls PFA. By varying the stoichiometric ratio of Priamine, the properties of the materials could be drastically modified. A stoichiometric ratio of one C=O for one NH2 decreased the Tg by 40 °C which multiplied the elongation at break by seven while maintaining the thermal resistance of PFA and decreasing its ultimate strength, Young's modulus and hardness. In addition, the stability of Priamine functionalized PFAs towards hydrolysis in mild condition was confirmed.

Section 5.3 and 5.4 were dedicated to understanding the factors influencing the properties of the PFA/Amine materials.

Section 5.3 investigated the influences of the amines' structure. As expected, rigid amines decrease the mobility of the polymeric network while flexible ones increase it. In this section, the impact of the post-curing environment was also assessed. Briefly, if flexible PFA/Amine materials are exposed to air during their post-curing the samples form a crust which can tremendously impact the properties of the material by shifting their Tg from below to above room temperature. This crust is believed to take its origin from the oxidation-induced surface crosslinking detailed in chapter 3 and 4. The impact of the crust was the most pronounced for PFA weight fraction between 75 and 50 %w in a PFA/Jeffamine D2000 system.

Section 5.4 focused on the influence of the conversion degree of PFA prior to its functionalization with an amine together with the impact of the amines' basicity on the acidcatalyzed polymerization of FA.

Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -284 -

The conversion degree of PFA was found to be of major importance, especially when rigid materials are targeted. In addition, precipitation may occur during the functionalization of PFA with amines. The precipitate may be isolated and processed into a high Tg material (Tg > 140 °C).

To produce rigid materials, a conversion degree of 0.9 or above is recommended.

Regarding the effect of the amine's basicity, a C6 monoamine and potassium hydroxide were employed as models to avoid C=N crosslinking. This section highlighted the possibility of reaching a material from a PFA prepolymer even though the polycondensation are stopped at a conversion degree of 0.8. This it likely possible thanks to the oxidation-induced surface crosslinking of PFA (pathway III). In the introduction, our dependency to petroleum-based products was highlighted. One possibility to tackle this issue is to use renewable resources to manufacture materials. In this line, furfuryl alcohol (FA), a widely synthetized from crop wastes, can be employed to manufacture high-performance materials. Indeed, when FA is exposed to an acidic medium, it polymerizes into poly(furfuryl alcohol) (PFA).

During its polymerization, (PFA) undergoes a furan ring opening side-reaction leading to carbonyls. Although PFA has been studied since the mid-20 th century, the furan ring opening was often disregarded. This thesis aims at probing the possibilities offered by this "side-reaction".

Hence, the major goals of this thesis were to quantify, control and take advantage of this sidereaction. Ultimately, the aim was to give an extent of tunability to this biobased thermoset by functionalizing its carbonyls.

First, chapter 1 reported the current knowledge on the application, structure and general features of PFA. Chapter 2 answered the following questions. How can the carbonyls in PFA be quantified? Can the quantification be performed in an accurate and reproductible manner ? When do the carbonyl form during the polymerization? What are the leverages available to control the ring-opening reaction ?

The carbonyls in PFA can be quantified by at least three methods. Potentiometry, quantitative 19 F and 13 C NMR can be used for this purpose. The method used can inform on the chemical availability of the carbonyls within PFA (steric hindrance, ketals, stabilized enols). The carbonyls tend to form at a steady rate during most of the polymerization. Finally, the most efficient way to act on the carbonyl content of PFA was to introduce additional water from the beginning of the polymerization (PFA+). To deal with these questions, bidimensional NMR and MALdI-ToF were employed. This chapter highlights that the furans located in the end of the polymer chains are the most reactive ones. Indeed, in the case of PFA prepared in aqueous condition, the methyl furans tend to open into levulinic-like units through a hydrolytic pathway.

In the case of neat PFAs, the methylfurans tend to transform into terminal lactones. In both Other mechanical properties such as the ultimate strength or hardness followed the same trend. An optimum of tensile toughness was found for a NH2/C=O stoichiometric ratio of 0.25. In addition, the thermal stability of PFA, one of its key features, was maintained for all the PFA/Priamine materials. Moreover, the stability of PFA/Priamine towards mild hydrolysis was ensured. Finally, the possibility to mechanically recycle non-aged PFA and PFA/Priamine materials was highlighted. This proof of concept was followed by deeper investigations focusing on four factors susceptible of influencing the properties of PFA/Amine materials.

First, the influence of the amine's structure on the properties of the materials was studied.

Briefly, flexible amines (Priamine 1071, Jeffamine D2000) allow an increase of the molecular mobility while rigid amines such as isophorone diamine have the opposite effect.

Conclusions and perspectives

-299 -All studies combined, the Tg of PFA/Amine materials could be tuned between -44 and 145 °C, i.e. a range of almost 200 °C, while maintaining the thermal stability of PFA.

Chapter 3 and 4 highlighted air oxidation as a source of a structural evolution of PFA.

Hence, a similar study was performed on PFA/amine materials using Jeffamine D2000. Two postcuring environment, oxidative and inert, were compared agains't the proportion of amine in the material. For PFA alone, little effect on the properties were observed, in concordance with chapter 4. Similarly, samples with less than 25 %w showed little sensitivity to the post-curing environment.

However, between 75 and 50 %w, tremendous effects were observed. In these cases, the samples formed a crust, most-likely linked to the oxidation-induced surface crosslinking. The formation of the crust shifted the Tg of these samples by up to 40 °C, i.e. from below to above the room temperature. As a consequence, the mechanical properties (Young's modulus, ultimate strength, …) shifted by orders of magnitude towards a rigid behavior. Hence, if one desires to flexibilize PFA, it is recommended to either use large amounts of amines and/or protect the materials from oxidations. A coating or the inclusion of anti-oxidants might be effective for this purpose.

The conversion degree of PFA prior to its functionalization proved to be a factor of influence. In this study, 1,6-diaminohexane was used. PFAs at conversion degrees between 0.3 and 0.95 can all be employed to produce PFA/Amine materials. However, the basicity of amines may interfere with the acid-catalyzed polymerization of FA. As a consequence, if one wants to manufacture rigid PFA/amine materials, it is advised to employe PFAs at conversion degrees of 0.9 and above although solubility issues might be encountered.

Finally, in the state of the art, it was shown that knowledge on the structure of PFA was not required to employe it in a variety of applications for more than fifty years. Yet, the deeper knowledge of PFA acquired in recent studies and this manuscript might pave the way to a rebirth of PFA towards a set of highly tunable polymers, sourced from agricultural wastes (PFA) or biorefinery wastes (humins).

Conclusions and perspectives

-300 -

Perspectives

The perspective revolving around PFA and its functionalization are numerous. Three studies are still on-going at the time this section is being written. The first one deals with the reduction of The carbonyls could also be exploited to modify the surface properties of PFA coatings.

For instance, (super)hydrophobic, anti-oxidant, fire retardant or anti-bacterial moieties could be attached to PFA, providing enhanced properties.

The carbonyls could also be a source of recyclability. Indeed, their reactivity could allow aged PFAcontaining carbonyls -to be included in another material or in fresh PFA as a reactive charge. Up to 50 %w of oxidized PFA can be incorporated into fresh PFA and cohesive material can be achieved, as shown in annexes.

Finally, carbonyls are not the only reactive species in PFA. The furan by itself is susceptible to Diels-Alder cycloaddition. Such approach could be used to introduce new properties in PFA materials. This is the subject of a current collaboration with the university of Poitiers.

Annexes

-301 -

Annexes

Other work

Going back to PFA systems. The Leuckart reaction was attempted on insoluble humins, soluble PFAs and cured PFAs to transform the carbonyls into primary amines. This reaction is a two step reaction. First the formation of a formamide derivatives (T > 120°C) and then hydrolysis of the formamide to free a primary amine).

The first step was successfully achieved on cured PFA, at 170 °C and under nitrogen. This is shown by the orange FTIR spectrum next page though a shift of the C=O band towards a lower wavenumber typical of amides. Nitrogen is mandatory to avoid aromatization and further formation of carbonyls. The hydrolysis complexified the FTIR spectra. Altough bands can be attributed to amines (3000 -3550 cm -1 and 1580-1650 cm -1 ) no definitive attribution can be done. In addition, quantification of amines is complex for insoluble materials and amine presence tests can only be performed on soluble amines. Overall, the Leuckart reaction is most-likely feasible on PFA systems. However, such investigation would requires frequent access to equipment capable of handling insoluble materials (MALDI-ToF, ssNMR, elemental analyses, XPS,…). List of Figures, Tables andSchemes 5.9

  Scheme 1.1-3. Most common lignin precursors

  Figure 1.2-2 gives an overview of the value chain of furans, starting from lignocellulosic biomass.

Figure 1

 1 Figure 1.2-2. Simplified overview of the furan value chain.

Figure 1

 1 Figure 1.3-1. Pictures of a continuous foundry mixer taken before 1979 (A) [97] and furan resins casting molds nowadays (B).[119] In the line of sand binders, furan resins have commercial applications as polymer concretes since the 1940s. Polymers concretes based on furan resins are used to manufacture setting beds capable of withstanding highly corrosive and hot environment. The formulations of mortars typically comprise about 30 % of resin while in the grouts it may go by up to 46 %. The rest of the formulation is usually silica aggregates or carbon flour. The furan-based polymer concretes are typically used as a setting bed for bricks including fire clay brick and carbon brick. While the fire clay brick are more resilient to thermal shock and physical impact, the carbon bricks are capable of resisting to hydrofluoric acid. More generally, the furan-based flooring systems can be employed in conditions where the floors are exposed to highly acidic or alkaline conditions, for example

Figure 1

 1 Figure 1.3-2 showcases the fabrication and final results of furan-based flooring before 1979 and nowadays.

Figure 1

 1 Figure 1.3-2. Pictures of the fabrication (A) and a final brick lining (B) taken before 1979. [97] (C) shows a floor furan-based grout from the ATLAS minerals & chemicals company, 2022[120]

  [128] Furan based resins were also reported as bitumen rheology modifiers[129], to increase the heat resistance of bitumen[130] and the resistance of asphalt to rutting. [131] of furan resins, more specifically PFA is the reinforcement of softwood through a process called furfurylation. Pictures of furfurylated wood from the Kebony company are available in Figure1.3-3. A patent from 2004 depicts an example of furfurylation process. The process involves the penetration of a solution of FA and maleic anhydride (MA), as acidic initiator, into wood pieces followed by an oven curing.[132] The furfurylation allows softwoods to increase their properties towards the ones of hardwoods. The furfurylation enhances softwoods in terms of dimensional stability, hardness, resistance to fungal attacks and resistance to marine environments.[133, 134, 135] As a consequence, research focusing on the interactions of FA/PFA and wood components were conducted[136, 137] Additionally, studies are still performed to improve the processability and properties of furfurylated woods.[138, 139] The full extent of the current knowledge on furfurylated wood falls out of the scope of this section and the bibliography is far from being exhaustive here.

Figure 1 . 3 - 3 .

 133 Figure 1.3-3. Pictures of a terrace made of furfurylated wood (Kebony) in Porto-Vecchio (A)[140] and comparison between untreated softwood (light) and a furfurylated one (dark) (B).[141]

Figure

  Figure 1.3-4. Scrubber made from PFA and glass fibers, picture taken before 1979 (A) [97] and prototype of exterior vehicle panel made from PFA and glass fibers (B).[142]

  Figure 1.3-5. Cellulose and cellulose reinforced PFA foams ofLems et al. [204]

Figure 1 . 3 - 7 .

 137 Figure 1.3-7. Example of PFA-based superhydrophobic surfaces, (A) [240] PFA -PTFE system and (B) PFA-potassium stearate system. [242]

Figure 1 . 3 - 8 .

 138 Figure 1.3-8. Heat exchanger made of impregnated graphite from the CGThermal company.[248]

Figure 1 . 3 - 9 .

 139 Figure 1.3-9. Wrinkles obtained after photoinitiation of PFA, the white bar has a length of 1 mm.

10 :

 10 Figure 1.3-10: Overview of the applications of furan resins. The dashed lines correspond to industrially developed fields. The numbers correspond to the references.

3 .

 3 Scheme 1.3-3. Linear oligomerization of FA under acidic conditions.

11 .

 11 Figure 1.3-11. Illustration of the aspect of PFA over the course of its polymerization.

Figure 1 . 3 - 12 .

 1312 Figure 1.3-12. Visible and thermal vision of the polymerization of FA over the course of 10 s. Polymerization of 2 mL of FA with 0.5 mL of 50%v H2SO4. The pictures were taken one to two seconds apart and the emissivity was set to 0.95. 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑑𝑒𝑔𝑟𝑒𝑒 (𝛼) = 𝑇𝑜𝑡𝑎𝑙 ℎ𝑒𝑎𝑡 𝑟𝑒𝑙𝑒𝑎𝑠𝑒-𝑅𝑒𝑠𝑖𝑑𝑢𝑎𝑙 ℎ𝑒𝑎𝑡 𝑟𝑒𝑙𝑒𝑎𝑠𝑒 𝑇𝑜𝑡𝑎𝑙 ℎ𝑒𝑎𝑡 𝑟𝑒𝑙𝑒𝑎𝑠𝑒

  Scheme 2.1-2. Reaction schemes used for carbonyl quantifications methods.In 13 mL glass vials, 90-150 mg of PFA were accurately weighted and 2 mL of DMSO were added. Mixtures were heated at 80 °C for 5 mins then shaken until complete dissolution of PFA. To the cooled-down DMSO -PFA solution, 2 mL of 0.44 N hydroxylamine hydrochloride (NH2OH.HCL) solution in 80 %v EtOH were introduced followed by 2 mL of 0.52 N triethanolamine solution in 96 %v EtOH.

  Figure 2.1-1. DOSTitri of a PFA° as function of oximation time. Line is meant to guide the eyes.

Figure 2

 2 Figure 2.1-2. FTIR spectra of a PFA+ before and after titration with different derivatizing agents.

  AC and AIS respectively stand for hydrazones and internal standard areas. A 0.5 factor is used as the internal standard bears two CF3 groups. MIS and mIS are the molar mass and mass of internal standard. AH° and mH° are the area of the impurity and the mass of CF3ΦNHNH2 in the reference. The mass of hydrazine in the sample is mH and mPFA the mass of PFA. Then, Equation 2.1-2 and Equation 2.1-3 are used to calculated DOSNMR. 𝐶 = 𝑂 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑁𝑀𝑅 =

Figure 2

 2 Figure 2.1-3. Example of an exploitation of a thermogram of PFA and α calculus. DSC scan at 2 °C.min -1 from 20 to 300 °C.

  Figure 2.1-4. Visual appearance of PFA° and PFA+ with the conversion.

Figure 2

 2 Figure 2.1-5. Pictures of the autoclave (left) and humins (right).

Figure 2

 2 Figure 2.2-1. Normalized FTIR spectra of PFA+ (starting from undistilled FA) as function of conversion. 𝛼 = 0 corresponds to the initial spectrum of undistilled FA.

Chapter 2 .

 2 Quantification and control of the furan ring-opening side reaction in PFA -92 -Presence of carboxylic acids was investigated trough the use of COOH blanks, as suggested byFaix et al. for acidic lignins. [319] As highlighted in Figure2.2-2, a very low quantity of carboxylic groups (< 0.25 mmol.g -1 ) is measured in PFA+. The COOH quantity slightly increases in the final phase of polymerization. Anyhow, their amount is too low to explain differences seen between CF3ΦNHNH2 derivatization ( 19 F NMR method) and the oximation method with NH2OH.HCl. The pinpoint of such large difference -especially at the end of polymerization could be somehow exploited to monitor the carbonyl accessibility especially when the network of PFA becomes denser.

  Figure 2.2-3.

Figure 2

 2 Figure 2.2-3. 19 F NMR spectra of derivatized 2,5-hexanedione

Figure 2

 2 Figure 2.2-4. Quantitative 13 C solid state NMR (HPDEC) spectrum of a PFA+ (𝛼 = 0.92).

Figure 2

 2 Figure 2.2-5 presents the DOSTitri evolution for both PFA+ and PFA° as a function of the conversion degree 𝛼. PFA resins were either prepared with undistilled FA and distilled FA. First, as the oligomerization begins (0 < 𝛼 < 0.40) the DOSTitri quickly rises to 0.05 -i.e. 5 % of furan ring opened -for both PFA° and PFA+. Then, the polymerization keeps going and carbonyls form with a steady rate for both systems. Interestingly, the DOSTitri of both PFA° and PFA+ increase continuously for 𝛼 > 0.20 and this increase can be approximately linearly fitted.

Figure 2

 2 Figure 2.2-5. Evolution of DOSTitri against the conversion degree for PFA° (black squares) and PFA+ (red circles). Open and solid symbols are for samples prepared with undistilled FA and distilled FA respectively. Dot lines represent the linear fits. Error bars correspond to the standard deviation of triplicates.

  Figure 2.2-6 as function of the overall C=O area (1650 -1870 cm -1 ). These areas have been determined from the FTIR spectra of PFA° (Figure 2.7-1) and PFA+ (Figure 2.2-1).

Figure 2

 2 Figure 2.2-6. Evolution of DOSTitri against FTIR normalized C=O area for PFA° (black squares) and PFA+ (red circles). Open and solid symbols are for samples prepared with undistilled FA and distilled FA respectively. Dot lines represent the linear fits. Error bars correspond to the standard deviation of triplicates.

  [273, 283, 284] These species are not taken into account in the oximation method but are part of the normalized FTIR C=O area. Interestingly, this shouldering around 1750 cm -1 is absent in PFA+ systems. It might be caused by lactone opening or end-chain furan ring opening before lactone formation. The evolution of DOSNMR -either as function of 𝛼 or C=O area -is presented in Figure 2.7-2 and Figure 2.7-3 and follow the same increasing trend as observed for the DOSTitri (Figure 2.2-5, Figure 2.2-6). In agreement with the data of Figure 2.2-2 the DOSNMR is lower than DOSTitri.

Figure 2

 2 Figure 2.2-7. FTIR conversion degree as a function of DSC conversion degree for both PFA° (squares) and PFA+ (circles) both undistilled (open symbols) and distilled (filled symbols).

  Figure 2.3-1 reports the FTIR (A) and 13 C ssNMR (B) of the lab-scale humins. Overall, the spectra of the humins are highly similar and display signs of C=O and furanic bonds. In Figure 2.3-1A signals at ≈ 1000 cm -1 and ≈ 1500 cm -1 are more easily distinguished for the Hu3 sample. FTIR bands around 1000 and 1500 cm -1 may be attributed to furans. In the same manner Figure 2.3-1A shows signals around 150 and 110 ppm which are also typical peaks of furans. The Hu3 sample presents a 110 ppm peak clearer than the other humins. In addition, peaks around 70 -80 ppm could correspond to C-O bonds, possible sugar ones. Finally, the most condensed humins, especially Hu3 show non-furanic C=C bonds between 120 and 130 ppm. A possible explanation could be the reaction of furans towards aromatics when exposed at high temperatures in the autoclave. However, further investigation is required to shed light on these humins' structure.

Figure 2

 2 Figure 2.3-1. (A) Magnified FTIR spectra of lab-scale humins. (B) 13 C NMR (CPMAS) of labscale humins. Hu1 is more condensed than Hu2 which is itself more condensed than Hu3.

Figure 2

 2 Figure 2.3-2. Dependency of the C=O content of two batches of lab-scale humins (Hu1) on the oximation time.

Figure 2 . 3 - 3 .

 233 Figure 2.3-3. C=O content of three kinds of humins after 24 h of oximation. Hu1 is more condensed than Hu2 which is itself more condensed than Hu3.

  Alumina, iodine and MMT-K10 have been used in the past[180, 182, 273, 286]. Organically modified Montmorillonites (Org-MMT) were also used to investigated the effect of combined Lewis and Brönsted acidity. The results of the investigations on the role of the initiator are shown in Figure 2.4-1 for the titration results and Figure 2.7-4 for the NMR results.

Figure 2

 2 Figure 2.4-1. Comparison of the degree of open structures obtained from titration method synthesized with different initiators in function of the conversion degree.

Chapter 2 .

 2 Figure 2.4-2. Comparison of the degree of open structures obtained by titration method and synthesized with different initiators with and without additional water (50/50 w/w) as function of the conversion degree.

Figure 2

 2 Figure 2.4-3 exhibits the DOSTitri as a function of the conversion degree for PFAs synthetized with more or less additional water. The 19 F NMR results are depicted in Figure 2.7-6.

Figure 2

 2 Figure 2.4-3 and Figure 2.7-6 highlight that the DOS is continuously increasing with the conversion degree, independently of the FA/water ratio. The neat PFA shows the lowest DOS overall from the beginning to the near-end of the polymerization, i.e. 𝛼 ≈ 0.90.

Figure 2

 2 Figure 2.4-3. Comparison of the degree of open structures obtained with titration method and synthesized with different FA/Additional water ratios with in function of the conversion degree.

Figure 2

 2 Figure 2.4-4. DOS and C=O content as a function of the FTIR C=O area of a multitude of PFAs.

  Figure 2.4-5. Picture of a PFA prepared using only ultrasounds. A series of PFA were prepared using other solvent than water and mixtures of solvents and water. First, IPA was suggested by Falco et al. to promote the ring opening in PFA systems. Results of PFA/MA/IPA are displayed in Figure 2.4-6. The IPA PFAs are located in between PFA° and PFA+. Hence, if the IPA promotes ring-opening reaction it is either in small amounts or ketal species are formed or both.

Figure 2

 2 Figure 2.4-7A reports the C=O content of resinous FA/HMF samples. It clearly shows that the higher the proportion of HMF the higher the C=O content. The FA/HMF resins were further heated and compression molding was performed. i Yet a C=O rich, HMF like viscous liquid, leaked during the compression even at low pressures. This was observed by smell and FTIR. Samples containing 75 % of HMF could not be molded at all. Samples were post-cured anyways and grounded before undergoing C=O quantifications. The resulting C=O content of post-cured FA/HMF are displayed in Figure 2.4-7B.

Figure 2

 2 Figure 2.4-7. (A) C=O content of FA/HMF systems at resinous stages. (B) C=O content of FA/HMF samples cured, post cured and ground. All percentage are molar percentage considering only the furans.

Figure 2

 2 Figure 2.4-8 displays pictures taken during the polymerization. Interestingly, PFA first become green before further darkening which is unusual. Furthermore, after 2 hours of exposure a dark PFA is obtained with wrinkles as observed by other researchers.[261, 262]

Figure 2

 2 Figure 2.5-1. 19 F NMR spectrum of CF3ΦNHNH2 derivatized PFA+ and PFA°. To confirm this statement, a PFA+ resin (𝛼 ≈ 0.80) was left in open air environment for 2 months. A hard and dark crust formed at the surface. The C=O content of this surface crust was 3.87 ± 0.20 mmol.g -1 while the core resin (i.e. not exposed to air) was 3.04 ± 0.19 mmol.g -1 . It indicates that PFA exposed to long-term potential air oxidation has about 30 % higher C=O content than the heart. Moreover, Tondi et al. suggested a Diels-Alder reaction between unsaturated 1,4diketones and conjugated sequence PFA to give a saturated 1,4-diketones adduct (schematized in Scheme 2.2-1) which would induce both aliphatic and conjugated carbonyls.[284] Finally, opened structures adjacent to conjugated moieties of PFA could as well explain the wide range of hydrazones chemical shifts evidenced in Figure 2.5-1.

  Figure 2.5-3presents the13 C NMR of the same samples. The peaks corresponding to ketones appear between 195-210 ppm. As shown in Figure2.5-3 the sample PFA+ exhibits more peaks in this region compared to PFA°. In addition, in PFA+ there is at least five significantly different kinds of C=O in the ketone region. On the other hand, the chemical shifts attributed to esters are located between 160 and 178 ppm. PFA° shows more peaks attributed to esters compared to PFA+. In PFA+ at 𝛼 ≈ 0.8 , the 174 ppm peak is attributed to the COOH band from levulinic acid. Indeed, low levels of acid can be detected in such PFAs.

Figure 2

 2 Figure 2.5-2. Normalized FTIR spectra of PFA+ and PFA° neat at 𝛼 ≈ 0.5 and 𝛼 ≈ 0.8.

Figure 2

 2 Figure 2.5-4 shows the DOSTitri as function of the FTIR C=O area for all the investigated PFAs of this chapter and other prepared over the course of this thesis. A separation between PFAs

  Figure 2.5-4: Degree of open structures obtained with titration method against FTIR C=O area for PFA synthetized in neat, aqueous and 50/50 IPA systems. PFA have been synthetized with aforementioned catalysts. *"Other attempts" include experiments of Section 2.4.4. The numbers, for example 90/10 correspond to the ratio of FA-Water or IPA if specified.

Chapter 2 .

 2 Figure 2.7-1. Normalized FTIR spectra of PFA° (starting from undistilled FA) as function of conversion. α = 0 corresponds to the initial spectrum of undistilled FA.

Figure 2

 2 Figure 2.7-2. Evolution of DOSNMR against the conversion degree; for PFA° (black squares) and PFA+ (red circles). Open and solid symbols are for samples prepared with undistilled FA and distilled FA respectively. Dot lines represent the linear fits. Error bars correspond to the standard deviation of triplicates.

Figure 2 . 7 - 3 .

 273 Figure 2.7-3. Evolution of DOSNMR against the C=O area in FTIR, for PFA° (black squares) and PFA+ (red circles). Open and solid symbols are for samples prepared with undistilled FA and distilled FA respectively. Dot lines represent the linear fits.

Figure 2

 2 Figure 2.7-4. Comparison of the degree of open structures obtained from 19 F NMR method synthesized with different initiators in function of the conversion degree.

Figure 2

 2 Figure 2.7-5. Comparison of the degree of open structures obtained 19 F NMR method and synthesized with different initiators with and without additional water (50 % w/w) in function of the conversion degree.

Figure 2 . 7 - 6 .

 276 Figure 2.7-6. Comparison of the degree of open structures obtained with 19 F NMR method and synthesized with different FA/Additional water ratios with in function of the conversion degree.

Figure 2

 2 Figure 2.7-8. (A) DMAs scan of FA/HMF samples (2 °C.min -1 ). (B) TGA scans of FA/HMF samples (10 °C.min -1 ) under 50 mL.min -1 of air.

Figure 2

 2 Figure 2.7-10. 19 F NMR spectrum of Levulinic acid derivatized with CF3ΦNHNH2 .

Finally, section 3

 3 .3 deepens the findings of Section 3.2, this time by comparing a PFA° with a PFA+ with 2D NMR and MALDI ToF. Doing so should allow to identify reactions occurring only when additional water is added during the polymerization of FA. Chapter 3. The carbonyls in PFA: a structural investigation -

from 8 .

 8 8 to 10.3 ppm for the aldehydes and from 8.0 to 8.20 for the reference. 𝐶 𝑟𝑒𝑓 is the concentration of the reference, n the number of hydrogens responsible for the peak, V the volume of solution, and 𝑚 𝑃𝐹𝐴 the mass of PFA resin introduced in the tube. 𝐴𝑙𝑑𝑒ℎ𝑦𝑑𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (𝑚𝑚𝑜𝑙. 𝑔 -1 ) = 𝐶 𝑟𝑒𝑓 𝐴 𝐴 * 𝑛 𝐻𝑟𝑒𝑓 𝐴 𝑟𝑒𝑓 * 𝑛 𝐻𝐴 * 𝑉 𝑚 𝑃𝐹𝐴 Equation 3.1-1 Chapter 3. The carbonyls in PFA: a structural investigation -138 -Quantification using the left-over furans:

   Then, NAldehyde can be determined as the integrated area of all aldehydes in the NMR spectra, i.e from 8.8 to10.3 ppm (𝐴 8.8-10.3 ). The same can be done for NFuran using the area from 5.6 to 7.0 ppm. This area corresponds to the two hydrogens, respectively in positions C3 and C4 of the furan ring. Thus, 𝑁 𝐹𝑢𝑟𝑎𝑛 = 0.5 * 𝐴 5.6-7.0 . Therefore, Equation3.1-4 is obtained. To measure 𝐴 8.8-10.3 and 𝐴 5.6-7.0 accurately, baselines were applied in the 8.4 -12.8 ppm and 5.2 -7.3 ppm, respectively. Equation 3.1-4

Chapter 3 .

 3 Scheme 3.2-1. Structures found in PFA.Recently, a special emphasis was placed on side-structures within PFA afterFalco et al. showed that the furan ring opening side-reaction can be used to tune the thermomechanical properties of PFA.[283] This study was followed by structural studies[284, 296] focused on the carbonyl moieties formed by hydrolytic furan-ring opening of PFA. These different carbonyl structures of PFA reported in the literature are displayed in Scheme 3.2-2. Furthermore, chapter 2 showed that up to 15 % of the furan rings in PFA can be opened. As developed later in chapter 5 the side reactions during FA polymerization offer new possibilities.

  [224] Consequently, the focus of this section is to breakthrough knowledge on the structural elucidation of carbonyl moieties contained in PFA by means of bidimensional NMR and quantitative 1 H NMR.

Figure 3 .

 3 Figure 3.2-1. 1 H NMR spectrum of PFA (A), 13 C, DEPT-90 and DEPT-135 NMR spectra of PFA+ at a conversion degree of about 0.80.

Figure 3 .

 3 Figure 3.2-2. HSQC spectrum of a PFA+ resin with a conversion degree of about 0.80. From Figure 3.2-2, the structure (I) of PFA and FA can be easily identified. The cluster (2) and (8) are respectively the methylene bridge and the C3-C4 of the furan ring. The cluster (5) is the CH2-OH from FA or end-chain alcohols. [281] On one hand, the cluster (10) corresponds to the unsubstituted =CH of terminal furan. [281] On the other hand, the methyl groups at 10 ppm from cluster (1) have been identified as the methyl group of structure (II) formed concomitantly with the conjugated structure (III). [281] However, three kinds of methyl groups are observed in cluster (1).Structure (III) and (IV) are most-likely responsible for the cluster[START_REF] Rehm | Bacterial Polymers: Biosynthesis, Modifications and Applications[END_REF]. Indeed, C=C bonds of alkenes are often comprised between 110 and 150 ppm.[330]

Figure 3 .

 3 Figure 3.2-3 displays a part of the HMBC spectrum of the same PFA+ as earlier. From thisfigure, the linear PFA and methylfuran from structure (I) and (II) can be found through cluster[START_REF] Srivastava | Effective Hydrolysis for Waste Plant Biomass Impacts Sustainable Fuel and Reduced Air Pollution Generation : A Comprehensive Review[END_REF]

Figure 3 .

 3 Figure 3.2-3. Magnified HMBC spectrum of PFA+ at a converion degree of about 0.80.

  5-3. Only two clusters of HC= correlating to HC=O are observed. Moreover, the related carbon peaks -194.2 and 197.8 ppmare positive in the DEPT-90 spectra. Hence, these peaks are caused by aldehydes. In addition, Tondi et al. attributed a 13 C peak at 219 ppm to the structure C-II, which is not observed in our PFA. [284] Hence, structure C-II could not be observed here. Its existence cannot be completely ruled out as it could be in low concentration or masked as an enol or ketal. Other possibilities are that the formation of this kind of carbonyls could appear later in the polymerization or be dependent on the polymerization conditions. Finally, Tondi et al. reported that structure C-II would react with furans to give the Diels-Alder adduct C-III. The clusters (10) around 3.3 ppm might correspond to this structure. Chapter 3. The carbonyls in PFA: a structural investigation -148 -

  Scheme 3.2-4. Representation of the hydrolytic ring opening of terminal methylfuran.

Figure 3 .

 3 Figure 3.2-4. Magnification of the HSQC (A) and HMBC (B) of PFA+ at a converion degree of about 0.80.

Figure 3 .

 3 Scheme 3.2-5. Postulated aldehyde-based structures within PFA.

Figure 3 .

 3 Figure 3.2-5. 1 H NMR spectra of the quantified PFA+ resins.

Figure 3 .Chapter 3 .

 33 Figure 3.2-6. Photography of a PFA sample immediately taken after cutting the sample.

Chapter 3 .

 3 Figure 3.3-1. Magnified HSQC spectra of PFA+ (orange) and PFA° (brown) of the CH3 area. The brown spectrum is stacked on blue the blue one.

Chapter 3 .

 3 Scheme 3.3-1. Methyl groups found in PFA resins.

Figure 3 . 3 -

 33 Figure 3.3-3 displays a magnification of the HSQC spectra of PFA+ and PFA° centered in the CH2 region. In Figure 3.3-3, the spectrum of PFA° is stacked above the one of PFA+, while the inverse situation (i.e., PFA+ above PFA°) ° is shown in Figure 3.5-6.

Figure 3 . 3 - 3 .Chapter 3 .

 3333 Figure 3.3-3. Magnified HSQC spectra of PFA+ (orange) and PFA° (brown) of the CH2 area. The brown spectrum is stacked on top of the orange one. The clusters circled in black dashed lines are CH groups.

Figure 3 . 3 -

 33 3 displays a much higher heterogeneity of the CH2 comprised between 2.2 and 3.3 ppm for PFA+, in comparison with PFA°. On top of that, the PFA+ spectrum displays clusters, mainly around 3.8/41 ppm, 3.3/40 ppm, 2,8/42 ppm and 3.2/43 ppm, which are not found in PFA°. Thus, introducing water in the initial reaction medium induces ringopening reactions which are not occurring in a neat system. Chapter 3. The carbonyls in PFA: a structural investigation -161 -Finally, the PFA° spectrum in Figure 3.3-3 displays CH clusters around 3.2/47, 3.4/50 ppm not visible on the PFA+ spectrum. Hence, the side-reactions occurring in a neat polymerization are of another nature than the ones occurring in aqueous systems. Theses sets of side-reactions might explain the influence of the reaction media on the polymerization of PFA. [180, 283, 301, 307] 3.3.3 The C=C and C-O bonds In the following section, the differences in PFA+ and PFA° structures' of the C=C and the C-O areas are detailed.

Figure 3 .

 3 Figure 3.3-4 depicts partial HSQC spectra of PFA° and PFA+ with PFA° on top of PFA+.The opposite situation with PFA+ on top of PFA° is available in Figure3.5-7. In Figure3.3-4, three groups of clusters can be isolated.

Figure 3 . 3 -

 33 Figure 3.3-5 displays the HSQC spectra of PFA+ and PFA° focusing on the aldehyde region, with the spectrum of PFA° on top of the one of PFA+. The opposite situation is available in Figure 3.5-8.

Figure 3 .

 3 Figure 3.3-5. Magnified HSQC spectra of PFA+ (orange) and PFA° (brown) focusing on the aldehyde region. The PFA° spectrum is presented on top of the PFA+ spectrum.

Figure 3 . 3 - 6 .

 336 Figure 3.3-6. Quantitative 1 H NMR spectra of FA and PFA° resin focus on the aldehyde area.

Scheme 3 .Figure 3 . 3 -

 333  283, 284, 296]

Figure 3 . 3 - 7 .

 337 Figure 3.3-7. Magnified HMBC spectrum of PFA° at a conversion degree of 0.80.

Chapter 3 .

 3 The carbonyls in PFA: a structural investigation -169 -There are other indications of this, the structures in

Scheme 3 . 3 - 9 .Chapter 3 .

 3393 . MALDI ToF assignment of PFA° and PFA+ for the 176 g.mol -1 peak.

Figure 3 .

 3 Figure 3.5-4. Magnified HSQC spectra of PFA+ (orange) and PFA° (brown) of the CH3 area. The orange spectrum is stacked on top of the brown one.

Figure 3 .

 3 Figure 3.5-5. HMBC spectra of PFA° (brown) and PFA+ (orange) focusing on the methyls. The brown spectrum is on top of the orange one.

Figure 3 .

 3 Figure 3.5-6. Magnified HSQC spectra of PFA+ (orange) and PFA° (brown) of the CH2 area. The orange spectrum is stacked on top of the brown one.

Figure 3 .Figure 3

 33 Figure 3.5-7. Magnified HSQC spectra of PFA+ (orange) and PFA° (brown) of the dienes and ethers areas. The orange spectrum is stacked on top of the orange one.

  Figure 3.5-10. (A-D). MALDI ToF mass spectra of PFA+.

  surface hardening and aromatization of PFA: origin and consequences Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -187 -Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences

Chapter 4 .

 4 Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -189 -In the case of inert atmosphere curing, the oven was vacuumed then filled with N2 then vacuumed and filled again with N2. The 2 h 30 time refers to the time the samples spent at 200 °C and does not include the heating nor cooling times.An isothermal DMA test was performed at 200 °C to ensure that 2 h 30 are enough to fully cure the samples. As well, a few pieces of Ox-PFA were sanded with 80 grit sandpaper to remove their top layers (≈ 200 µm).On one hand, PFA and Ox-PFA pieces were subjected to DMA, tensile testing, gel content determination, Fourier-Transform Infra-Red (FTIR) spectroscopy and goniometry. On the other hand, PFA and Ox-PFA powders underwent C=O and COOH quantifications, TGA, oxidation onset temperature (OOT), elemental analysis, solid state nuclear magnetic resonance (ssNMR) spectroscopy, aldehyde detection, and FTIR spectroscopy.

  Scheme 4.1-1. Sample preparation procedure of PFA and Ox-PFA.

  Onset Temperature (OOT) experiments were performed on a DSC 1 from Mettler Toledo. The temperature program was adapted from Volpini et al. [358] The experiments were performed from 25 to 300 °C at 20 °C.min -1 under 50 mL.min -1 of air with 2-3 mg of sample in 30 µL open aluminum pan. The results are presented as the average and standard deviation of three experiments. Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -192 -Dynamic Mechanical Analysis (DMA):

Figure 4

 4 Figure 4.1-1. DMA curve used to determine the curing time of the PFAs.

Chapter 4 .

 4 Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -194 -Gel Content:To determine the gel content of PFA samples, rectangular pieces of about 150 mg were immersed in about 15 mL of ethyl acetate for 7 days at 25 °C. The ethyl acetate was then removed, the sample were washed two times with ethyl acetate. Then, they were vacuum dried at 40 °C overnight. Equation4.1-1 was used to determine the gel content. In Equation4.1-1, 𝑚 𝑑 stands for the dry mass while 𝑚 𝑖 corresponds to the initial mass of PFA or Ox-PFA. The gel content experiments were triplicated.

  Scheme 4.2-2. Potential aromatization pathway of furan resins during carbonization.[185, 365]

  Figure 4.3-1 displays the FTIR spectra of PFA and Ox-PFA. As a complement, the attributions of the main bands are reported inTable 4.3-1.

Figure 4

 4 Figure 4.3-1. FTIR spectra of PFA (red line) and Ox-PFA (black line) in their powder form.

Chapter 4 .

 4 Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -199 -Vibrations in this area are typically attributed to C-H stretching of aldehydes (usually doublets). [328] Finally, Figure 4.3-2 displays pictures of PFA and Ox-PFA (powder form). Here, Ox-PFA is much darker than PFA. This observation is also consistent with a more conjugated chemical structure.

Figure 4

 4 Figure 4.3-2. Picture of PFA and Ox-PFA in their powder form.

  Figure 4.8-4A, B and C present magnifications of Figure 4.3-3.

Figure 4 . 3 - 3 .

 433 Figure 4.3-3. Normalized ssNMR spectra of PFA (red line) and Ox-PFA (black line) in their powder form. The spectra are normalized to the 151 ppm peak.

Figure 4 . 8 -

 48 Figure 4.8-4B depicts a magnification of the CH/CH2 area of the NMR spectra of PFA and

Figure 4 . 8 -

 48 Figure 4.8-4C focuses on the carbonyl region of the NMR spectra of Ox-PFA and PFA.Both spectra have two main signals around 208 and 170 ppm. The latter one corresponds to ester and/or carboxylic acid and is both broader and more intense in the case of Ox-PFA. Moreover, a slight shouldering towards lower chemical shifts is also observed which would be consistent with the conjugation of the COOH groups. The other peak at 208 ppm has often been attributed to 1,4diketones emerging from the hydrolysis of the furan ring. Yet, chapter 3 showed that aldehydic species with a lower chemical shift can also form, especially when the furan resins are exposed to air. Here, the 208 ppm peak of PFA is both sharper and less intense than Ox-PFA. Besides, on the Ox-PFA spectrum a clear shouldering is visible from 185 to 200 ppm. This would be consistent with the presence of conjugated carbonyls (ketones or aldehydes). To confirm the presence of aldehydes, Tollens' tests were performed on the different samples and the pictures are shown in Figure4.3-4.

Chapter 4 .

 4 Figure 4.3-5. TGA curves of PFA (red lines) and Ox-PFA (black lines) in their powder form at 2 °C.min -1 from 25 to 300 °C. The dashed-dot lines correspond to the TGAs performed under air.

Figure 4

 4 Figure 4.4-1. Tan (𝛿) curves from DMA experiments of PFA (red line), Ox-PFA (black line) and the sanded Ox-PFA (gray dashed-dot lines).

Figure 4

 4 Figure 4.4-1 highlights few facts. First, the Ox-PFA has a maximum of tan (𝛿) of 0.16 while

Figure 4

 4 Figure 4.4-2. Elastic moduli vs. temperature obtained from DMA experiments of PFA (red), Ox-PFA (black) and the sanded Ox-PFA (gray dashed-dot lines).

  Figure 4.3-5 has been put in perspective with the E' curve from Figure 4.4-2. This compilation is presented in Figure 4.4-3 for the PFA samples. The compiled curves from Ox-PFA are available in Figure 4.8-5.

Figure 4

 4 Figure 4.4-3 displays the evolution of E' from the DMA along with the mass percent from the TGA as a function of the temperature for a PFA sample. These results have been presented and discussed separately earlier. Nonetheless, Figure 4.4-3 allows to clearly highlight the correlation between the mass gain in TGA, i.e., due to incorporation of oxygen, and the increase of E' in DMA, i.e., due to new crosslinks.

Figure 4 . 4 - 3 .

 443 Figure 4.4-3. Magnified superposition of DMA (plain lines) and TGA (dashed-dot lines) of PFA.The same heating rate of 2 °C.min -1 was applied.

  Scheme 4.4-1. Proposed mechanism for the crosslinking occurring sample's surface during post curing under air, based on the opening of furans.

Chapter 4 .

 4 Figure 4.4-4. SEM pictures of Ox-PFA (left) and PFA (right). The white dashed lines highliths what was attributed to the oxidized layers. The white arrows is a representative example of how the thickness was measured.

  Figure 4.8-6, presents tha data obtained from DMA an TGA and Figure 4.5-1 depicts the dependence of the effective activation energy (𝐸 𝛼 ) with temperature (T) computed from these data for the reaction occurring when exposing PFA and Ox-PFA from 130 to 600 °C.

Figure 4

 4 Figure 4.5-1. Dependence of the effective activation energy (𝐸 𝛼 ) with temperature (T).

Figure 4 . 6 -

 46 Figure 4.6-1 shows the tensile properties obtained on PFA and Ox-PFA samples. The elastic modulus of PFA is slightly lower than Ox-PFA, respectively 2.2 ± 0.3 GPa and 3.1 ± 0.3 GPa.

Figure 4

 4 Figure 4.6-1. Tensile properties and Shore D hardness of Ox-PFA (black) and PFA (red).The gel content was determined for both materials. Ox-PFA had a gel content of 92.4 ± 1.8 % while the one of PFA was of 94.0 ± 1.5 %. Hence, the resistance to solvents is not substantially affected by the oxidized surface layer. Furthermore, goniometry experiments were conducted on PFA and Ox-PFA. The analysis gave a contact angle of 86.1 ± 2.7 ° for Ox-PFA and 85.3 ± 3.1 ° for PFA. Thus, the hydrophobicity brought by the aromatization is most likely counteracted by the presence of oxygenated groups on the surface of the material.

  Figure 4.8-7 displays example of curves from the OOT determination, the results from three repetitions are compiled in Table 4.6-1. Ox-PFA yielded 253.6 ± 2.3 °C of OOT while PFA had an OOT of 203.1 ± 3.1 °C meaning that Ox-PFA is less sensitive to oxidation than PFA. Such results further emphasize the role of the aromatized surface layer of furan resins as it protects the material from further oxidation. The exotherm might also comprise the heat generated during the surface oxidation. Chapter 4. Oxygen-induced surface hardening and aromatization of PFA: origin and consequences -215 -

Chapter 4 .

 4 Figure 4.8-1. ISO 527-2-1BB specimen (left) and dumbbell cutter (right).

Figure 4

 4 Figure 4.8-2. FTIR spectra of an Ox-PFA sample before (black) and after (red) magnified from 3700 to 2400 cm -1 .

Figure 4

 4 Figure 4.8-3. Magnified FTIR spectra of an Ox-PFA sample before (black) and after (red) being grinded by ball milling.

Figure 4

 4 Figure 4.8-4. Magnified NMR spectra of PFA (red) and Ox-PFA (black) on the C=C area (A), alkyl area (B) and C=O area (B).

Figure 4 . 8 - 6 .

 486 Figure 4.8-6. Extent of conversion (𝛼) with temperature for PFA (red) and Ox-PFA (black). The dot lines correspond to the data calculated from the first mass variation in TGA, the dash-dot lines to the increase of E' in DMA and the dash lines to the TGA degradation at high temperature. All the experiments were performed under air.

  The hardness (Shore D) of samples thicker than 5 mm was measured with a Kern & Sohn GmbH durometer. Ten attempts were done on several spots of each specimen. The results are presented as the average and standard deviation of these attempts.Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -226 -

  Figure 5.2-1. Section 5.2 graphical highlights

  [183] Moreover, with between 100 and 1 000 tons manufactured or imported into Europe per year, the Priamine family belongs to the small group of commercially available biobased amines.[378, 379] These Priamines are usually employed as epoxy cross-linkers [375], for polyamides[380], and as soft blocks in polyurethanes[381, 382], polymethacrylates[383] and polyimines.[384] Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -229 -5.2.1 Experimental details specific to section 5.2 Section 5.2 was a preliminary study on the functionalization of PFA with amines using Priamine 1071. The preparation procedures were improved for the next studies.

  Scheme 5.2-1. Process of sample preparation relative to section 5.2. Once evaporated, the PFA/PA were precured 50 mins at 130 °C. The samples were then compression molded at 150 °C and ≈ 10 bars for 2 h and post-cured in their molds for five additional hours. For the mechanical recycling, PFA/PA materials were ground using a ball mill and the powders were compression molded at 150 °C under 25-35 bars for two hours. For the uniaxial tensile tests, PFA/PA samples were cut in parallelepiped rectangles and a gauge length of 10 mm was used.

  Scheme 5.2-2. Schematic representation of the imination of PFA -prepolymer by Priamine 1071. First, the presence of C=N bonds was validated by means of Fourier-Transform Infra-Red spectroscopy (FTIR) and solid state 13 C NMR investigations. The spectra of cured resins are shown in Figure 5.2-2A (NMR) and Figure 5.2-2B (FTIR).

Figure 5

 5 Figure 5.2-2. CPMAS 13 C Solid state NMR (A) and magnified FTIR (B) spectra of PFA and Priamine-functionalized PFA.

  Scheme 5.2-5. Example of amine-derivatized carbonyl moieties in PFAs.Following this spectroscopic confirmation that PFA was functionalized with Priamine, the rest of this section is dedicated to the evaluation of the thermomechanical properties. Typical stressstrain curves are depicted in Figure5.2-3, which illustrates the progressive variations of properties using functionalization of high carbonyl content PFA with soft flexible segments. Thus, the mechanical properties of PFA resins can be tuned from a very brittle behavior for non-modified PFA towards a ductile behavior through Priamine modification (Figure5.2-3). It thus opens the prospect to expand the portfolio of applications of PFA resins.

Figure 5

 5 Figure 5.2-4 exhibits the elastic modulus, the glass transition temperature and the Shore D obtained for the different samples. Imination of PFA with Priamine creates an array of elastic moduli starting from 1 600 MPa for unmodified PFA to 20 MPa for PFA/PA 1 (E' at 20 °C DMA, extracted from DMA curves in Figure 5.6-3A). This decrease of moduli is directly linked to the Tg of the samples maximum of tan (𝛿), extracted from DMA curves in Figure 5.6-3B. Priamine's long aliphatic chains with intrinsic flexibility increase the free volume between chains and the possibility of molecular movements in the polymer.

5. 6 -Figure 5

 65 Figure 5.2-3. Examples of stress-elongation curves obtained from PFA and PFA/PA.

Figure 5

 5 Figure 5.2-4. Variation of modulus (Young's and E' at 20 °C) and glass transition of Priaminefunctionalized PFAs with the amount of Priamine introduced. The Tg was obtained from the maximum of the tan (𝛿) peak from DMA curves.

Figure 5

 5 Figure 5.2-5. Variation of the elongation at break (A) and ultimate tensile strength (B) of Priaminefunctionalized PFAs with the amount of Priamine introduced.

Figure 5 . 2 - 6 .

 526 Figure 5.2-6. Variation of the tensile toughness and crosslink density of Priamine-functionalized PFAs with the amount of Priamine introduced.

  Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -239 -

Figure 5

 5 Figure 5.2-7. FTIR spectra of the PFA/PA-1 powder exposed to different hydrolytic conditions.

Figure 5

 5 Figure 5.2-8. DMA scans showing E' curves (plain lines) and tan (𝛿) curves (dash dot lines). The samples are a non-immersed PFA/PA 1 sample, and a PFA/PA 1 sample immersed in water (25 °C) respectively for two days and six days.

Figure 5

 5 Figure 5.2-9. DMA scan of two PFA/PA 1 samples. The orange one is less than a week old and the black one is three-month-old. Plain lines correspond to the storage modulus and the dash-dot lines to the tan (𝛿).

Figure 5

 5 Figure5.2-10 displays the thermomechanical properties of the recycled PFA and PFA/PA samples. Over the reprocess cycles, the Tg of the polymers tended to increase by 5-10 °C at each cycle, excepted the PFA/PA 1. The E' at 20 °C from the DMA curves also tend to increase although the higher the Priamine content the lower the rise of E'. In addition, both the crosslink density and the hardness increased with the reprocessing. The samples were also subjected to tensile tests. The results are gathered in Figure5.6-6.

5. 3 . 1

 31 Scheme 5.3-2. Process of sample preparation relative to section 5.3. Once the PFA/Amine solutions were evaporated and dried, the functionalized PFAs were heated at 130 °C in a forced convection oven for up to 90 mins with occasional stirring to reach the rubbery state. Then, the rubbers were ground using a ball mill. The powders were then compression molded at 150 °C under 25 -35 bars for 2 hours in a heated press. The pictures in Figure 5.6-7 illustrate the molds, backing mold and heated press used for the experiments.

  amount with respect to the carbonyls, C=O signals are still visible in all the materials as in PFA/PA systems. The properties of the PFA/Amine materials were assessed by DMA, TGA and shore D hardness. The corresponding DMA curves are available in Figure 5.3-2. Table 5.3-1 gathers data obtained from the evaluation of the DMAs and TGA scans and the Shore D values. The TGA scans of the materials and the pure amines are available in Figure 5.6-9 and Figure 5.6-10.

Figure 5

 5 Figure 5.3-1. Magnified FTIR spectra of PFA reacted with several amines. The samples were prepared at NH2/C=O stoichiometry.

Figure 5

 5 Figure 5.3-2. DMA Curves of the PFA/Amine materials, (A) E' and (B) tan (𝛿) curves. The samples were prepared at NH2/C=O stoichiometric ratio = 1.

  Figure 5.3-3. Tensile properties of the PFA/Amine materials. The samples were prepared at NH2/C=O stoichiometry.

Figure 5

 5 Figure 5.3-4. Picture of a PFA/J2000/0.3Eq/OX sample freshly cut along the length.

Figure 5

 5 Figure 5.3-5. DMA curves, (A) E' and (B) tan (𝛿), of the PFA/J2000 materials post-cured either in the oven or in their mold. The plain lines correspond to samples post-cured in their mold (IN) and the dash-dot ones to samples post-cured in the oven (OX). The numbers in the legend correspond to the NH2/C=O stoichiometric ratio.

  Figure 5.3-6 displays the results from these experiments along with the E' values at 20 °C obtained from the DMA curves of Figure 5.3-5. The TGA curves of the samples are available in Figure 5.6-12.

Figure 5 . 3 - 6 .

 536 Figure 5.3-6. Dependency of the thermal and mechanical properties of the PFA/J2000 materials on the NH2/C=O stoichiometric ratio, weight percent of PFA and post-curing environment. The postcuring was either performed in the oven (OX dash-dot lines) or in the molds (IN, plain lines).

  Figure 5.3-7 gathers the tensile properties of the samples and Figure 5.6-13 gives representative examples of the tensile curves of the samples.

Figure 5 . 3 - 7 .

 537 Figure 5.3-7. Dependency of the tensile properties of PFA/J2000 materials on the NH2/C=O stoichiometric ratio, weight percent of PFA and post-curing environment. The post-curing was either performed in the oven (OX, dash-dot lines) or in the molds (IN, plain lines).

Figure 5 . 3 - 8 .

 538 Figure 5.3-8. DMA curves of PFA/J2000/0.3Eq post-cured at 200 °C for 2 h 30 under air. The black lines correspond to a sample with crust (black). The red lines correspond to a sample without crust (removed with a scalpel). The plain lines correspond to the E' curves and the dash-dot ones to the tan (𝛿) curves.

*

  4-1. The condensation reactions (Pathway I) are the first step of polymerization. Once a degree of polymerization of four to ten is achieved, a loss of hydride ion occurs and leads to conjugated 2,5-dihydrofurans. Then, the conjugated and non-conjugated moieties react together by Diels-Alder (Pathway II), thus providing crosslinking nods. Pathway II is believed to be the main source of crosslinking in PFA. [281] Furthermore, if a PFA resin is exposed to air, the furan ring can open into conjugated 1,4dicarbonyls, especially 1,4-ketoaldehydes. The conjugated carbonyls can then undergo a Diels-Alder cycloaddition with non-conjugated furans to yield oxidation-induced crosslinking nods (Pathway III). Pathway III is observable through a rise of elastic moduli starting around 175 °C. It should be noted that Pathway III was only observed at the PFA-Air interface and is limited to a thin layer (≈ 20 µm). Finally, the oxidation-induced cycloadducts may be aromatized if exposed to further air and heat, as discussed in chapter 4. Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -261 -Table 5.4-1. Chain lengthening pathways during the polymerization of FA. The monosubstituted furans are especially sensitive to this reaction Section 5.2 and 5.3 highlighted the potential of expanding the portfolio of applications of PFA materials trough functionalization of the C=O. Yet, there are still phenomena which need to be understood to fully exploit the potential of C=O in PFAs. For this purpose, two features of the materials were investigated in this study. Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -262 -

Chapter 5 .

 5 Scheme 5.4-1. Process of sample preparation relative to section 5.4.

  Scheme 5.4-2. Illustration of the formation of PFA/Hex and PFA/KOH crust during the precuring step.

  5.4.2 Phase separation phenomenon during the functionalization of PFA's C=OWhen the carbonyls of PFA are reacted with difunctional primary amines, a phase separation may occur. This section is dedicated to this phase separation phenomenon in the case of PFA/DiHex.The phase separation phenomenon is illustrated in Figure5.4-1 with the example of PFA/DiHex/ 𝛼0.9. Pictures were taken at several step of the sample preparation and are presented in chronological order. During the synthesis, particles were observed in the flask together with a fluffy coating that readily formed on the flask surface as shown in Figure5.4-1A. Figure 5.4-1B, C and D present photos of these particles before and after evaporation. The evaporation process induces a darkening of the insoluble part with a formation of an external crust after long evaporation time (Figure 5.4-1D). These phenomena could be attributed to the drying process. Nevertheless, after the pre-curing step at 130 °C for 30 mins a hard and black solid (Figure 5.4-1E) is obtained which becomes orange-brown once ground (Figure 5.4-1F). Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -266 -Then, after compression molding the samples become black again (Figure 5.4-1G). To explain such color variations, FTIR spectra of the samples in Figure 5.4-1(E) and (F) were acquired and are presented in Figure 5.4-2.

Figure 5

 5 Figure 5.4-1. Pictures taken along the process of fabrication of PFA/Amine materials, case of PFA/DiHex, 𝛼 =0.9, NH2/C=O stoichiometry. The alphabetical order is also chronologic.

Chapter 5 .

 5 Figure 5.4-3. Dependency of the mass ratio of precipitate on the conversion degree of the starting PFA. The samples were prepared at NH2/C=O stoichiometry.

  Figure 5.4-4 displays the FTIR spectra of the PFA/DiHex materials. In this Figure, the C=O band at 1710 cm -1 in the PFA/DiHex samples is greatly diminishing in comparison with the furan band at 1550 cm -1 . Moreover, at C=N band around 1650 cm -1 is visible in the PFA/DiHex. [328] Finally, a small signal can be observed between 1510 and 1540 cm -1 for the PFA/DiHex. This may be attributed to pyrroles moieties. [328] Overall, Figure 5.4-4 showcases the successful reaction of PFA's C=O with NH2 to yield either imines or pyrroles. The proportion of each is however hard to pin-point.

Figure 5

 5 Figure 5.4-4. FTIR spectra of PFA/DiHex at NH2/C=O stoichiometry using PFAs at different conversion degrees (0.3 to 0.95). The numbers in the legend indicate the conversion degree of each starting PFA. The letter "p" indicates that only the precipitated macromolecules were used (dashdot lines). The reference PFA was prepared using a PFA at 𝛼 = 0.8.

Figure 5

 5 Figure 5.4-5. DMA curves, (A) E' and (B) tan (𝛿), of the PFA/DiHex prepared at NH2/C=O stoichiometry and using PFAs at different conversion degrees (0.3 to 0.95). The numbers in the legend indicate the conversion degree (𝛼) of the starting PFA. The letter "p" indicates that only the insoluble macromolecules were used in the preparation of the materials (dash-dot lines). The reference PFA was prepared using a PFA at 𝛼 = 0.8.

Chapter 5 .

 5 Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -273 -PFA/DiHex samples were subjected to tensile tests. Figure 5.4-6 displays a summary of the tensile data. Figure 5.6-18 presents representative examples of the tensile curves. In Figure 5.4-6, the PFA columns comprise the average and standard deviation of three reference materials prepared with PFAs at 𝛼 of 0.3, 0.8 and 0.95. The data of each reference PFA is available in Figure 5.6-16.

Figure 5 . 4 - 6 .

 546 Figure 5.4-6. Tensile data of the PFA/DiHex at NH2/C=O stoichiometry using PFAs at various conversion degrees (0.3 to 0.95). The "p" suffix indicates that only the precipitated macromolecules were used. The "PFA" columns comprise three references prepared with starting 𝛼 of 0.3, 0.8 and 0.95.

  5.4.4 Effect of bases on the polymerization of FAThe aim of this section is to better understand the impact of the basicity of amines on the polymerization of PFA. In this section, the polymerization of PFA was quenched with potassium hydroxide (KOH) and n-hexylamine (Hex). Even though the polycondensation reactions were stopped, crosslinked materials could still be obtained. Mechanistic insights on the crosslinking of quenched PFAs are given. Finally, the properties of a PFA quenched with KOH (PFA/KOH) and PFAs modified with hexylamine (PFA/Hex) at varying functionalizing degrees are investigated.Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions -275 -After being processed and shaped, the reference PFA, PFA/KOH and PFA/Hex samples were analyzed by DMA. Figure5.4-7 reports the DMA scans of PFA and PFA/KOH. In Figure5.4-7, the PFA sample exhibits a rise of E' beginning at 175 °C. This rise of E' is an indication that the Pathway III of Table5.4-1 is occurring during the DMA scan, in agreement with chapter 4.

Figure 5

 5 Figure 5.4-7. Magnified E' dependency on the temperature of PFA and PFA/KOH. The PFA/KOH was prepared at -OH/C=O stoichiometry. The samples were prepared using a starting PFA at α = 0.8.

Figure 5

 5 Figure 5.4-8. E' (A) and tan (δ) (B) dependency on the temperature of the PFA/Hex samples, PFA/KOH and PFA. The numbers in the legend correspond to the functionalization degree of the C=O. The samples were prepared with a starting PFA at α = 0.8.

  Figure 5.4-9. DMA curves, E' (A) and tan (δ) (B) of several PFA/Amine samples along with PFA. All the samples were prepared at NH2/C=O stoichiometry.

Finally, Figure 5

 5 Figure 5.4-10. Tensile data of the PFA/Hex at varying NH2/C=O stoichiometry PFA-KOH was prepared at -OH/C=O stoichiometry. All the samples were prepared using a PFA at α = 0.8.

Chapter 5 .

 5 Figure 5.6-1. Height ratio of the 1656 cm -1 over the 1710 cm -1 band from the normalized FTIR spectra against the NH2 equivalents introduced.

Figure 5

 5 Figure 5.6-2. Magnified 13 C ssNMR spectra of PFA/PA samples

Figure 5 . 6 - 6 . 3 Figure 5 . 6 - 7 .

 5663567 Figure 5.6-6. Mechanical Recycling PFA/PFA: tensile properties

Figure 5 . 6 -

 56 Figure 5.6-10. TGA curves of the pure amines under air flow.

Figure 5 . 6 -

 56 Figure 5.6-12. TGA curves of pure J2000 (dot line) and PFA/J2000 at varying NH2/C=O stoichiometry post cured in the oven (OX, dash-dot lines) or within the mold (IN, plain lines). The numbers in the legend are the NH2/C=O stoichiometric ratios. The TGAs were performed under air flow.

Figure 5 . 6 -

 56 Figure 5.6-15. TGA curves of the reference PFA materials prepared with PFA at various conversion degrees (numbers in the legend). The plain lines are the E' curves and the dash-dot lines the tan (𝛿) curve.

Figure 5 . 6 -

 56 Figure 5.6-16. Tensile data of the reference PFA materials prepared with PFA at various conversion degrees.

Figure 5

 5 Figure 5.6-17. TGA curves of the PFA/DiHex at NH2/C=O stoichiometry using PFAs at different conversion degrees (0.3 to 0.95). The numbers in the legend indicate the conversion degree of each starting PFA. The letter "p" indicates that only the precipitated macromolecules were used (dashdot lines).

Figure 5 . 6 - 18 .

 5618 Figure 5.6-18. Examples of tensile curves of the PFA/DiHex at NH2/C=O stoichiometry using PFAs at different conversion degrees (0.3 to 0.95). The numbers in the legend indicate the conversion degree of each starting PFA. The letter "p" indicates that only the precipitated macromolecules were used (dash-dot lines).

Figure 5 . 6 -

 56 Figure 5.6-19. TGA curves of PFA/Hex samples at varying NH2/C=O stoichiometry. PFA-KOH was prepared at -OH/C=O stoichiometry. All the samples were prepared using a PFA at α = 0.8.

Figure 5 . 6 - 20 .

 5620 Figure 5.6-20. Representative tensile curves of PFA/Hex samples at varying NH2/C=O stoichiometry. PFA-KOH was prepared at -OH/C=O stoichiometry. All the samples were prepared using a PFA at α = 0.8

  systems, the monosubstituted terminal furans are sensitive to oxidation by air. As a consequence, they open into conjugated 1,4-ketoaldehydes. Overall, the aldehydes account for 10 to 20 % of the carbonyls present in PFA. Finally, conjugated aldehydes are prone to react with furans through a Diels-Alder cycloaddition. This additional crosslinking leads to a progressive darkening and hardening of the surface of PFA resins.Two questions were at the center of chapter 4. What is the structural evolution of PFA after its polymerization and before its carbonization ? What is the impact of the oxidation-induced surface crosslinking on the properties of the materials ?Chapter 4 investigated the influence of oxidative and inert atmospheres at 200 °C on the properties of PFAs. It oxidized samples displayed a significant amount of aromatic spectrometric signals on the surface of the polymer while non oxidized samples displayed only furanic signals.Conclusions and perspectives-298 -It was suggested that the cycloadduct resulting from the oxidation-induce surface crosslinking is prone to aromatization. The oxidation and crosslinking can be observed in TGA and DMA. The thickness of the oxidized layer was average to about 20 µm. The oxidized layer had little effect on the mechanical properties of the materials, aside from a small decrease of damping capacity. In addition, this oxidized layer allowed a 50 °C increase of oxidation onset temperature in comparison with non-oxidized PFA.Finally, chapter 5 dealt with the questions related to the functionalization of PFA's selfgenerated carbonyls. Are the carbonyls within PFA suitable for amine functionalization ? Can amines mitigate the brittleness of PFA while maintaining the thermal stability of PFA ? What is the extent of tunability offered by the amines ?First, a proof of concept was performed using Priamine 1071, a diamine sourced from fatty acids. The chemical reactions between PFA and Priamine yielded both imines and pyrroles. The stoichiometric ratio of NH2/C=O was varied between 0.1 and 1.0. As a result, the Tg of the PFA/Priamine could range from 60 to 20 °C for the highest proportions of Priamine. As a result, the elongation at break could be multiplied by seven and the Young's modulus divided by a hundred.

  PFA's carbonyls to hydroxyl and the properties of the resulting materials. The second one is dedicated to the manufacture and characterization of PFA/Chitosan composites. The third one focuses on toughening PFA by incorporating poly(butadiene) rubber in the material.The carbonyls of PFA could be exploited in a number of ways. The transformation of carbonyls into primary amines was attempted. Comments on these attempts are available in annexes together with other observations made during this thesis. An exhaustive list of the carbonyls' chemical reaction would not be appropriate here. Yet, relevant examples include the conversion of carbonyls to epoxies or aminophosphates for the manufacture of high-performance thermosetting resins. In addition, the transformation of non-conjugated C=O to C=C by the Wittig reaction could open room for post functionalization or radical polymerization.

FTIRFully

  spectra of PFA, and a PFA subjected to the first (post-amidation) and second (posthydrolysis) steps of the Leuckart reaction. Annexes -302 -Other attempts to transform the C=O of PFA into primary amines were conducted by bubbling NH3 into a solution of PFA followed by NaBH3 reduction. Theses attempts did not lead to primary amines, possibly due to the instability of imines. It might be possible to perform similar reaction using high pressure systems involving NH3 and H2 or using NH3 dissolved in dry ethanol or methanol Work indirectly link to the thesis During my PhD thesis I also participated un FUR4Sutain COST action (Meeting, summer school, HH, STSM program). During my STSM (Short Term Scientific Mission), I spent 3 weeks in the group of Dr. Andreia Sousa at the university of Aveiro. We investigated the recyclability humins and PFA using innovative solvents in addition to acquiring the ssNMR spectra of humins. In addition, several other parallel studies were conducted. The first one was a collaboration with Politecnico Di Torino. The goal was to use furan allyl derivatives for the production of coatings by photopolymerization. In this work, I performed the UV assisted synthesis of ciscyclobutane-1,2-dicarboxylic acid from trans-3-(2-furyl) acrylic acid. [334] The second collaboration was with the university of Thessaloniki in the scope of a STSM grant (FUR4Sustain COST action). Here, I participated to the fabrication of composite made from cellulose nanofibers and PFA. The work conducted during this period lead to a publication currently under submission. Annexes -303 -Mechanical reprocessability of PFA In chapter 5, the reprocessability of non-aged PFA was observed. It was not clear if it was caused by reversible chemical reactions or lack of curing. This led to the following investigation. PFA and Ox-PFA powders from chapter 4 were compression molded at 200 °C for 2 h at 30 bars. Pictures of the samples are displayed bellow. The sample made from Ox-PFA powder was not cohesive. For PFA, a cohesive sample yet extremally brittle sample was achieved. In addition, it was tested to mix Ox PFA powder with uncured PFA powder (𝛼 = 0.95) and mold the powders for 2 h at 150 °C under 30 bars. The picture bellow showcase samples comprising 25 %w and 50 %w of Ox PFA. Both weigth fractions lead to cohesive materials.This work could be continued to prove the concept that Ox PFA could be recycled in fresh resins without impact on the properties During my PhD, I tried to soften PFA using Jeffamine m600 (a large monoamine). When the Jeffamine m600 was mixed with PFA, no materials could be achieved at 150 °C, even after 15 h of curing. So, the temperature was increased to 200 °C for 3 hours and then to 250 °C for 3 h.The picture bellow displays the sample that came out of the oven. The sample had a hard crust and a soft heart. Interestingly, the topology of the surface is somewhat similar to the wrinkled PFAs obtained under UV light. In addition, the FTIR spectra bellow correspond to the sides of the sample facing the air and the aluminum mold. The spectra clearly show structural differences between the air-exposed and air-protected sides. In addition, the air-side spectrum show resemblance with a PFA prepared using UV (cf section 2.4.4), i.e. little furan signals, potential aromatic C=C, and qualitatively high amount of potential C=O and COOH. Maybe, the oxidative ring-opening and surface crosslinking take somehow part in the formation of wrinkles in these PFAs.PFA/Jeffamine m600 sample FTIR spectra of the PFA/Jeffamine m600 sample of both sides (air-facing and mold-facing)

  FIGURE 2.2-5. EVOLUTION OF DOSTITRI AGAINST THE CONVERSION DEGREE FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. ERROR BARS CORRESPOND TO THE STANDARD DEVIATION OF TRIPLICATES. ................................................... FIGURE 2.2-6. EVOLUTION OF DOSTITRI AGAINST FTIR NORMALIZED C=O AREA FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. ERROR BARS CORRESPOND TO THE STANDARD DEVIATION OF TRIPLICATES. .................................... FIGURE 2.2-7. FTIR CONVERSION DEGREE AS A FUNCTION OF DSC CONVERSION DEGREE FOR BOTH PFA° (SQUARES) AND PFA+ (CIRCLES) BOTH UNDISTILLED (OPEN SYMBOLS) AND DISTILLED (FILLED SYMBOLS). ............................................................................ FIGURE 2.3-1. (A) MAGNIFIED FTIR SPECTRA OF LAB-SCALE HUMINS. (B) 13 C NMR (CPMAS) OF LAB-SCALE HUMINS. HU1 IS MORE CONDENSED THAN HU2 WHICH IS ITSELF MORE CONDENSED THAN HU3. .......................................................................... FIGURE 2.3-2. DEPENDENCY OF THE C=O CONTENT OF TWO BATCHES OF LAB-SCALE HUMINS (HU1) ON THE OXIMATION TIME. .......... FIGURE 2.3-3. C=O CONTENT OF THREE KINDS OF HUMINS AFTER 24 H OF OXIMATION. HU1 IS MORE CONDENSED THAN HU2 WHICH IS ITSELF MORE CONDENSED THAN HU3. ....................................................................................................................... FIGURE 2.4-1. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED FROM TITRATION METHOD SYNTHESIZED WITH DIFFERENT INITIATORS IN FUNCTION OF THE CONVERSION DEGREE. ................................................................................................. FIGURE 2.4-2. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED BY TITRATION METHOD AND SYNTHESIZED WITH DIFFERENT INITIATORS WITH AND WITHOUT ADDITIONAL WATER (50/50 W/W) AS FUNCTION OF THE CONVERSION DEGREE. ..................... FIGURE 2.4-3. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED WITH TITRATION METHOD AND SYNTHESIZED WITH DIFFERENT FA/ADDITIONAL WATER RATIOS WITH IN FUNCTION OF THE CONVERSION DEGREE. .............................................. FIGURE 2.4-4. DOS AND C=O CONTENT AS A FUNCTION OF THE FTIR C=O AREA OF A MULTITUDE OF PFAS. .................................. FIGURE 2.4-5. PICTURE OF A PFA PREPARED USING ONLY ULTRASOUNDS. ................................................................................. A SERIES OF PFA WERE PREPARED USING OTHER SOLVENT THAN WATER AND MIXTURES OF SOLVENTS AND WATER. FIRST, IPA WAS SUGGESTED BY FALCO ET AL. TO PROMOTE THE RING OPENING IN PFA SYSTEMS. RESULTS OF PFA/MA/IPA ARE DISPLAYED IN FIGURE 2.4-6. THE IPA PFAS ARE LOCATED IN BETWEEN PFA° AND PFA+. HENCE, IF THE IPA PROMOTES RING-OPENING REACTION IT IS EITHER IN SMALL AMOUNTS OR KETAL SPECIES ARE FORMED OR BOTH.........................................................................FIGURE 2.4-7. (A) C=O CONTENT OF FA/HMF SYSTEMS AT RESINOUS STAGES. (B) C=O CONTENT OF FA/HMF SAMPLES CURED, POST CURED AND GROUND. ALL PERCENTAGE ARE MOLAR PERCENTAGE CONSIDERING ONLY THE FURANS. ...................................... FIGURE 2.4-8. PICTURES TAKEN OVER THE POLYMERIZATION OF FA INITIATED BY UV LIGHT. ......................................................... FIGURE 2.4-9. FTIR SPECTRA OF THE UV INITIATED PFA, EXPOSED TWO HOURS. ........................................................................ FIGURE 2.5-1. 19 F NMR SPECTRUM OF CF3ΦNHNH2 DERIVATIZED PFA+ AND PFA°. ................................................................ FIGURE 2.5-2. NORMALIZED FTIR SPECTRA OF PFA+ AND PFA° NEAT AT 𝛼 ≈ 0.5 AND 𝛼 ≈ 0.8. ..................................................FIGURE 2.5-3. 13 C NMR SPECTRA OF PFA+ AND NEAT PFA° AT 𝛼 ≈ 0.5 AND 𝛼 ≈ 0.8 WITH AN ACCUMULATION OF 6000 SCANS. ..... FIGURE 2.5-4: DEGREE OF OPEN STRUCTURES OBTAINED WITH TITRATION METHOD AGAINST FTIR C=O AREA FOR PFA SYNTHETIZED IN NEAT, AQUEOUS AND 50/50 IPA SYSTEMS. PFA HAVE BEEN SYNTHETIZED WITH AFOREMENTIONED CATALYSTS. *"OTHER ATTEMPTS" INCLUDE EXPERIMENTS OF SECTION 2.4.4. THE NUMBERS, FOR EXAMPLE 90/10 CORRESPOND TO THE RATIO OF FA-WATER OR IPA IF SPECIFIED. ................................................................................................................................... FIGURE 2.7-1. NORMALIZED FTIR SPECTRA OF PFA° (STARTING FROM UNDISTILLED FA) AS FUNCTION OF CONVERSION. Α = 0 CORRESPONDS TO THE INITIAL SPECTRUM OF UNDISTILLED FA. ........................................................................................ FIGURE 2.7-2. EVOLUTION OF DOSNMR AGAINST THE CONVERSION DEGREE; FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. ERROR BARS CORRESPOND TO THE STANDARD DEVIATION OF TRIPLICATES. .................................................

  FIGURE 2.7-3. EVOLUTION OF DOSNMR AGAINST THE C=O AREA IN FTIR, FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. .................................................................................................................................................... FIGURE 2.7-4. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED FROM 19 F NMR METHOD SYNTHESIZED WITH DIFFERENT INITIATORS IN FUNCTION OF THE CONVERSION DEGREE. ................................................................................................. FIGURE 2.7-5. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED 19 F NMR METHOD AND SYNTHESIZED WITH DIFFERENT INITIATORS WITH AND WITHOUT ADDITIONAL WATER (50 % W/W) IN FUNCTION OF THE CONVERSION DEGREE. ....................... FIGURE 2.7-6. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED WITH 19 F NMR METHOD AND SYNTHESIZED WITH DIFFERENT FA/ADDITIONAL WATER RATIOS WITH IN FUNCTION OF THE CONVERSION DEGREE. ............................................................. FIGURE 2.7-7. FTIR SPECTRA OF THE YELLOWISH LIQUID FOUND IN PFAS. ................................................................................. FIGURE 2.7-8. (A) DMAS SCAN OF FA/HMF SAMPLES (2 °C.MIN -1 ). (B) TGA SCANS OF FA/HMF SAMPLES (10 °C.MIN -1 ) UNDER 50 ML.MIN -1 OF AIR. .................................................................................................................................................. FIGURE 2.7-9. 19 F NMR SPECTRA OF PFA+ (LEFT) AND PFA° (RIGHT) AT VARIOUS CONVERSION DEGREES. ..................................... FIGURE 2.7-10. 19 F NMR SPECTRUM OF LEVULINIC ACID DERIVATIZED WITH CF3ΦNHNH2 . ....................................................... FIGURE 3.2-1. 1 H NMR SPECTRUM OF PFA (A), 13 C, DEPT-90 AND DEPT-135 NMR SPECTRA OF PFA+ AT A CONVERSION DEGREE OF ABOUT 0.80. ........................................................................................................................................................ FIGURE 3.2-2. HSQC SPECTRUM OF A PFA+ RESIN WITH A CONVERSION DEGREE OF ABOUT 0.80. ................................................FIGURE 3.2-3. MAGNIFIED HMBC SPECTRUM OF PFA+ AT A CONVERION DEGREE OF ABOUT 0.80................................................ FIGURE 3.2-4. MAGNIFICATION OF THE HSQC (A) AND HMBC (B) OF PFA+ AT A CONVERION DEGREE OF ABOUT 0.80. .................. FIGURE 3.2-5. 1 H NMR SPECTRA OF THE QUANTIFIED PFA+ RESINS. ....................................................................................... FIGURE 3.2-6. PHOTOGRAPHY OF A PFA SAMPLE IMMEDIATELY TAKEN AFTER CUTTING THE SAMPLE. .............................................. FIGURE 3.3-1. MAGNIFIED HSQC SPECTRA OF PFA+ (ORANGE) AND PFA° (BROWN) OF THE CH3 AREA. THE BROWN SPECTRUM IS STACKED ON BLUE THE BLUE ONE. ............................................................................................................................. FIGURE 3.3-2. MAGNIFIED 1 H NMR SPECTRA OF PFA° (BROWN) AND PFA+ (ORANGE) AT A CONVERSION DEGREE OF ABOUT 0.80. ... FIGURE 3.3-3. MAGNIFIED HSQC SPECTRA OF PFA+ (ORANGE) AND PFA° (BROWN) OF THE CH2 AREA. THE BROWN SPECTRUM IS STACKED ON TOP OF THE ORANGE ONE. THE CLUSTERS CIRCLED IN BLACK DASHED LINES ARE CH GROUPS. ............................... FIGURE 3.3-4. MAGNIFIED HSQC SPECTRA OF PFA+ (ORANGE) AND PFA° (BROWN) OF THE DIENES AND ETHERS AREAS. THE BROWN SPECTRUM IS STACKED ON TOP OF THE ORANGE ONE. .................................................................................................... FIGURE 3.3-5. MAGNIFIED HSQC SPECTRA OF PFA+ (ORANGE) AND PFA° (BROWN) FOCUSING ON THE ALDEHYDE REGION. THE PFA° SPECTRUM IS PRESENTED ON TOP OF THE PFA+ SPECTRUM. ........................................................................................... FIGURE 3.3-6. QUANTITATIVE 1 H NMR SPECTRA OF FA AND PFA° RESIN FOCUS ON THE ALDEHYDE AREA. ...................................... FIGURE 3.3-7. MAGNIFIED HMBC SPECTRUM OF PFA° AT A CONVERSION DEGREE OF 0.80. ........................................................ FIGURE 3.5-1. 1 H NMR SPECTRA OF A PFA RESIN, CONVERSION DEGREE OF ABOUT 0.80, IN BLUE WITH A BASELINE APPLIED FROM 0 TO 14 PPM AND IN RED FROM 5.2 TO 7.3 (RIGHT) AND 8.4 TO 12.8 PPM (LEFT). ................................................................. FIGURE 3.5-2. HMBC SPECTRA OF A PFA+ RESIN AT CONVERSION DEGREE OF ABOUT 0.80. ........................................................ FIGURE 3.5-3. MAGNIFIED HMBC SPECTRA OF A PFA+ RESIN AT CONVERSION DEGREE OF ABOUT 0.80. ........................................ FIGURE 3.5-4. MAGNIFIED HSQC SPECTRA OF PFA+ (ORANGE) AND PFA° (BROWN) OF THE CH3 AREA. THE ORANGE SPECTRUM IS STACKED ON TOP OF THE BROWN ONE. ....................................................................................................................... FIGURE 3.5-5. HMBC SPECTRA OF PFA° (BROWN) AND PFA+ (ORANGE) FOCUSING ON THE METHYLS. THE BROWN SPECTRUM IS ON TOP OF THE ORANGE ONE. ............................................................................................................................................. FIGURE 3.5-6. MAGNIFIED HSQC SPECTRA OF PFA+ (ORANGE) AND PFA° (BROWN) OF THE CH2 AREA. THE ORANGE SPECTRUM IS STACKED ON TOP OF THE BROWN ONE. .......................................................................................................................

FIGURE

  FIGURE 5.6-18. EXAMPLES OF TENSILE CURVES OF THE PFA/DIHEX AT NH2/C=O STOICHIOMETRY USING PFAS AT DIFFERENT CONVERSION DEGREES (0.3 TO 0.95). THE NUMBERS IN THE LEGEND INDICATE THE CONVERSION DEGREE OF EACH STARTING PFA. THE LETTER "P" INDICATES THAT ONLY THE PRECIPITATED MACROMOLECULES WERE USED (DASH-DOT LINES). ......................................... 294 FIGURE 5.6-19. TGA CURVES OF PFA/HEX SAMPLES AT VARYING NH2/C=O STOICHIOMETRY. PFA-KOH WAS PREPARED AT -OH/C=O STOICHIOMETRY. ALL THE SAMPLES WERE PREPARED USING A PFA AT Α = 0.8. .................................................................. 295 FIGURE 5.6-20. REPRESENTATIVE TENSILE CURVES OF PFA/HEX SAMPLES AT VARYING NH2/C=O STOICHIOMETRY. PFA-KOH WAS PREPARED AT -OH/C=O STOICHIOMETRY. ALL THE SAMPLES WERE PREPARED USING A PFA AT Α = 0.8 ................................. 295

  [] ......................................................................................................................................................................... TABLE 1.1-4. EXAMPLES OF LIGNOCELLULOSIC BIOMASS COMPOSITION ....................................................................................... TABLE 1.3-1. MAIN 1 H NMR PEAKS OF PFA. ........................................................................................................................ TABLE 1.3-2. MAIN 13 C NMR PEAKS OF PFA. ....................................................................................................................... TABLE 1.3-3. MAIN FTIR BANDS OF PFA. ............................................................................................................................. TABLE 1.3-4. ESTIMATION OF THE PRODUCTION OF WATER DURING FA'S POLYMERIZATION. ........................................................... TABLE 2.1-1. CONDITIONS OF THE PREPARATION OF HUMINS AND YIELDS ....................................................................................

  SCHEME 1.1-1. CHEMICAL STRUCTURE OF CELLULOSE (TYPE 𝛼-1,4) ........................................................................................... SCHEME 1.1-2. CELLULOSE BASED PRODUCTS TECHNOLOGIES DEVELOPMENT, ADAPTED FROM ARANTES ET AL. [18] ........................... SCHEME 1.1-3. MOST COMMON LIGNIN PRECURSORS ............................................................................................................. SCHEME 1.1-4. EXAMPLE OF LIGNIN STRUCTURE WITH SOME LINKAGES, BASED ON RALPH ET AL. [30] .............................................. SCHEME 1.1-5. GENERAL STRUCTURES OF THE VARIOUS LINKAGES FOUND IN A VARIETY OF XYLAN ISOLATED FROM PLANT CELL WALLS.THIS STRUCTURE WAS ADAPTED FROM WERTZ ET AL. [11]...................................................................................................... SCHEME 1.2-1. DEHYDRATION OF HEXOSES TO HMF. ............................................................................................................. SCHEME 1.2-2. SYNTHESIS ROUTE OF PEF FROM 5-(METHOXYMETHYL)-2-FURALDEHYDE. ............................................................. SCHEME 1.2-3. EXAMPLES OF LEVULINIC ACID AND LEVULINATE DERIVATIVES. [50, ] ..................................................................... SCHEME 1.2-4. CHEMICAL STRUCTURE OF FURFURAL. ............................................................................................................. SCHEME 1.2-5. ILLUSTRATION OF THE TWO-STEP PRODUCTION PROCESS OF FURFURAL. .................................................................. SCHEME 1.2-6. ACETALIZATION OF FURFURAL, ADAPTED FROM. [55, 57] ................................................................................... SCHEME 1.2-7. RESINIFICATION OF FURFURAL, ADAPTED FROM GANDINI ET AL. [68] .................................................................... SCHEME 1.2-8. CHEMICAL STRUCTURE OF FURFURYL ALCOHOL. ................................................................................................. SCHEME 1.2-9. INDUSTRIAL SYNTHESIS OF FA FROM FURFURAL. ................................................................................................ SCHEME 1.2-10. PROPOSED MECHANISM FOR THE RING OPENING OF FA TO LA IN AQUEOUS MEDIA. [76] ........................................ SCHEME 1.2-11. PROPOSED STRUCTURE FOR XYLOSE AND GLUCOSE DERIVED HUMINS.[80] ........................................................... SCHEME 1.3-1. EXAMPLE OF SURFACE FUNCTIONALIZATION STRATEGIES PROPOSED FOR SILICA PARTICLES (TOP) [186] AND CARBON NANOTUBE (BOTTOM) [192, 198]. ............................................................................................................................ SCHEME 1.3-2. FORMATION OF A PYRROLE CYCLE FROM 2,5-DIKETONES AND A PRIMARY AMINE BY THE PAAL-KNORR REACTION. .......... SCHEME 1.3-3. LINEAR OLIGOMERIZATION OF FA UNDER ACIDIC CONDITIONS. ............................................................................. SCHEME 1.3-4. IDENTIFIED MOIETIES FOUND IN PFA OLIGOMERS BEFORE 1980. .......................................................................... SCHEME 1.3-5. LIBERATION OF FORMALDEHYDE FROM ETHER LINKAGES. ..................................................................................... SCHEME 1.3-6. STRUCTURE OF CONJUGATED 2,5-DIHYDROFURANS WITH FURAN. ......................................................................... SCHEME 1.3-7. METHYLENE LINKAGES PROPOSED BY CHUANG ET AL. STRUCTURE (A): CONDENSATION OF CH2-OH. STRUCTURE (B): FORMALDEHYDE-INDUCED CROSSLINKING. [278, 279] ................................................................................................... SCHEME 1.3-8. FORMATION OF CONJUGATED MOIETIES IN PFA OLIGOMERS. [281]...................................................................... SCHEME 1.3-9. PROPOSED CROSSLINKING PATHWAY IN PFA: DIELS-ALDER CYCLOADDITION. .......................................................... SCHEME 1.3-10. PROPOSED CROSSLINKING PATHWAY IN PFA: ELECTROPHILIC SUBSTITUTION (LEFT, ORANGE) OR CONDENSATION REACTION (RIGHT, DARK MAUVE). ............................................................................................................................... SCHEME 1.3-11. PROPOSED CROSSLINKING PATHWAY IN PFA: ELECTROPHILIC ATTACK. [287] ........................................................ SCHEME 1.3-12. EXAMPLE OF PFA STRUCTURE, BASED ON RESEARCH EARLIER THAN 2010. ........................................................... SCHEME 1.3-13. OPEN FURANIC STRUCTURES PROPOSED BY FALCO ET AL. [283] ......................................................................... SCHEME 1.3-14. DIELS-ALDER REACTION BASED ON OPEN FURAN PROPOSED BY TONDI ET AL. [284] ............................................... SCHEME 1.3-15. ROUTES TO OPEN FURAN RINGS.................................................................................................................... SCHEME 2.1-1. SAMPLING PROCESS. .................................................................................................................................... SCHEME 2.1-2. REACTION SCHEMES USED FOR CARBONYL QUANTIFICATIONS METHODS. ................................................................ SCHEME 2.2-1. SIMPLIFIED SCHEME OF PFA AND ITS POTENTIAL OPENED STRUCTURES. ................................................................. SCHEME 3.1-1. CORRELATIONS OBSERVED IN HSQC (LEFT) AND HMBC (RIGHT). ...................................................................... SCHEME 3.2-1. STRUCTURES FOUND IN PFA. ......................................................................................................................

Table 1 .

 1

1-1. Oil demand by sector, built on 2021 data from the organization of the petroleum exporting countries

[2]

Sector Share of crude oil demand (%) Transportation 56.2 Petrochemicals 14.2 Other industries 13.2 Residential, commercial, agricultural 11.4

	Chapter 1. State of the art
	Electricity generation	5.1
	-20 -

  State of the art -43 -The ANR JCJC FUTURES projects aims to pave to way for new means of valorization of humins. The ultimate goal of the project is to use the chemistry of the carbonyls in furanic macromolecules to provide new properties and therefore new applications. However, as the chemical structure of humins is highly dependent on the feedstock, poly(furfuryl alcohol) is used as a model. Indeed, PFA can be made with reproductible structure and in soluble and insoluble forms depending on the conversion degree of FA. These features will allow the development of Properties and applications of PFA The polymerization of FA is detailed later in section 1.3.2. Briefly, PFA is composed of repeating furfuryl units densely crosslinked together by Diels-Alder cycloadduct. As the furan ring is sensitive to ring opening, some of the furans open into diketones. In terms of properties, PFA has attractive thermal and chemical resistance, which makes it suitable for various applications.

		Chapter 1. State of the art
	1.3	Poly (furfuryl alcohol)
	Poly(furfuryl alcohol) being at the center of this manuscript, this section is dedicated to this
	polymer. First, both mature and developing applications of PFA are reviewed in section 1.3.1. As
	for section 1.3.2, it deals with the understanding of FA's polymerization from 1940s to nowadays.
		1.3.1
	robust C=O quantification protocols, furan ring-opening procedures and finally methods for
	preparing derivatized materials.
		-44 -

  Among the other articles, composites made of PFA and flax fibers were combined with cellulose acetate butyrate and characterized by Toriz et al.[164] Finally, Motaung et al. proposed an alkali treatment of flax fibers which improved the flexural properties and the Tg of their PFA composite. [165]

	Chapter 1. State of the art
	Continuing on PFA combined with natural fillers, PFA and cork composites were
	characterized and suggested as a mean of valorization of cork powder residues from the cork
	stoppers industry. [166]. Furthermore, PFA and isolated soda lignin systems were proposed as a
	mean to valorize this side product from the paper industry. For instance, plasticized lignin was
	blended with PFA and their potential compatibility was suggested using Fox's equation. [167]
	Later, the flexural and impact behavior of PFA reinforced lignin with up to 20 % of lignin were
	studied. [168] Additionally, a mixture of lignin, FA, glyoxal and DGEBA was proposed for
	adhesive applications [169] together with oxidized lignin in combination with PFA for plywood
	glues. [170]
	-50 -

In the field of adhesives, FA was suggested several times as a co-reactant for adhesive purposes. Examples include the bonding of particleboards with FA/tannins/lignosulfonate mixtures

[171, 172]

and FA/urea/formaldehyde mixtures. [173] A furan resin/urea/formaldehyde mixture was also proposed as a plywood adhesive.

[174]

In the line of plywood adhesives, FA/tannins systems were proposed with tannins alone

[175]

and tannins in combination with silk.

[176]

Furthermore, FA/aldehydes systems (formaldehyde, glutaraldehyde, glyoxal) [177] and combinations of FA/Furfural/DGEBA/starch were also explored as plywood adhesives.

[178]

  Additionally, Bosq et al. reported the functionalization of silica Much earlier, in the 1990s, Spange et al. produced a series of publications involving the polymerization of FA with silica microparticles and the potential interaction between PFA and silica.

	Chapter 1. State of the art
	nanoparticles with furanic moieties to increase the matrix-filler compatibility. Their
	functionalization strategy is illustrated in Scheme 1.3-1. [186] Furthermore, hollow microspheres
	of silica within a PFA matrix were suggested for applications requiring thermal insulation and fire
	resistance. [187]
	-51 -

Wavenumber (cm -1 ) Attribution Reference

	Chapter 1. State of the art Chapter 1. State of the art Chapter 1. State of the art Chapter 1. State of the art
	Finally, the authors suggested the possibility of crosslinking through the condensation of Indeed, understanding the interactions between PFA and wood is of great help to understand Table 1.3-1. Main 1 H NMR peaks of PFA. 1.3.2.2 The polymerization of FA: other features
	primary OH on the conjugated sequences as depicted on the right of Scheme 1.3-10. [281] In the following years, structural studies were conducted to highlight such pathways. [285, 286] Principe and improve the properties of furfurylated wood. Similar works were also performed using Chemical shift (ppm) Attribution Reference bidimensional NMR technics. [137, 295] 1.8 -2.2 Fu-CH3 [275, 281] 3.5 -4 Fu-CH2-Fu Apart from the mechanisms producing PFA's macromolecules, further features of the [275, 281, 285, 286] et al. highlighted the presence of alkyl CH moieties by DEPT 13 C NMR in PFA. [285] 4.2 -4.5 Fu-CH2-OH [274, 281] polymerization of FA were investigated over the years. The kinetic of FA's polymerization was
	5.7 -6.4 7 -7.2 studied at least since the 1970s. [297, 298] More than forty years later, the kinetics of FA's H3 and H4 of furans [ 274, 275, 281, 285, 286] H5 monosubstituted furan [274, 275, 281, 286, 285]
	polymerization are still investigated either to understand the effect of different catalysts, [180, 299,
	Table 1.3-2. Main 13 C NMR peaks of PFA. 300] to understand the solvent effects [301], to assess the pot-life of oligomers [302, 303], to
	control the polymerization for the production of diesel precursors [267], to better apprehend the Chemical shift (ppm) Attribution Reference furfurylation process [304] or to characterize the reactivity of PFA with other compounds such as
	13 25-30 2-furylethanol [305] or DGEBA [217, 218]. Most of the kinetic studies involve DSC or rheology, Methyl [275, 283, 284] Furan-CH2 [275, 280 , 284, 285, 286] Scheme 1.3-10. Proposed crosslinking pathway in PFA: electrophilic substitution (left, orange) or condensation reaction (right, dark mauve). 30-50 CH2-C=O, CH-CH2, Quaternary C in Diels-Alder adduct, CH however one could note the work of Vargas et al. who investigated the kinetics of FA's [275, 283, 284, 285, 286] 55 -65 Residual CH2-OH, residual ether bridge [275, 280, 282] polymerization using 1 H NMR, more specifically the relaxation time of the molecules. [306] In
	In the 2000s, investigations on the mechanisms ruling FA's polymerization were still conducted thanks to the evolution of theoretical analysis. For instance, Montero et al., used 110 [275, 280, 282, 283, 284, addition, a chemorheological study on PFA was performed to assess and it was suggested that the C3 and C4 Furan 285, 286] 120-130 C=C in Diels Alder Adducts [283] final crosslinking step of PFA is a diffusion-controlled process. [307] Using similar tools, other
	theoretical modeling to simulate the crosslinking in PFA. [287] The calculations approved the electrophilic attacks were also feasible according to the authors. They also detailed one of the 150 285, 286] C1-C5 disubstituted furan [275, 280, 282, 283, 284, feasibility of the Diels-Alder reaction although it is mitigated by steric hindrance. In addition, the 140 286] C5 monosubstituted furan [275, 280, 282, 283, 284, authors reported the dependence of the viscosity of PFA resins on the temperature. [308]
	170 mechanisms as reported in Scheme 1.3-11. Interestingly, the resulting structures may present Esters, Levulinic Acid [283, 284, 285] 200 -210 C=O [283, 284]
	features proposed by Chuang et al. in Scheme 1.3-7, although the means to reach it are different. [279] In the line with the theoretical simulations, Bertarione et al. combined FTIR, Raman and UV-Scheme 1.3-11. Proposed crosslinking pathway in PFA: electrophilic attack. [287] Table 1.3-3. Main FTIR bands of PFA.
	vis spectroscopy with simulations to study the formation of PFA oligomers in zeolite cavities.[288] Finally, a series of tables is proposed as a conclusion of this section. They sum up the main
	spectrometric assignments of PFA across the years. Table 1.3-1 reports the main 1 H NMR peaks, 600 -920 Several bands Furan [291 292] In addition, Kim et al. studied the polymerization of FA with a thermodynamical approach coupled Table 1.3-2 the main 13 C NMR peaks and Table 1.3-3 the main FTIR bands found in PFA systems. 1000-1020 Mixed Furan (C-O / C-H) [280, 293] with Raman spectroscopy. [289] This first paper was followed by two other investigations on the 1500-1510 C=C furan monosubstituted [286, 289, 296]
	1565 effect of the solvent during FA's polymerization and the influence of the catalyst type on the C=C 2,5-disubstituted furan [291, 293, 296] 1710 C=O, C=C in Diels-Alder adduct [271, 280, 283, 286, 291, 293, 296] structure of PFA's oligomers. [290, 291] Simulations were also employed by Barsberg et al. to 1740 Esters [271, 283]
	2930 predict and assign Raman and/or FTIR bands of furfuryl alcohol [292], PFA [293] and a lignin/PFA Aliphatics [280 286, 292] 3000-3150 Furan C-H [291 292]
	3-9. Proposed crosslinking pathway in PFA: Diels-Alder cycloaddition. hybrid. [294] The latter one being a consequence of the blossoming of the furfurylation. 3200-3700 OH stretch [281, 291, 292]
	-65 --66 --67 --68 -

Table 2

 2

.2-1. About 40 % of this acetal model reacted in the oximation method (NH2OH.HCl) while less than 5 % reacted with the CF3ΦNHNH2 derivatization method. When subjected to ΦNHNH2.HCl, 24 % of the acetal model reacted. On top of that, Constant et al. subjected 2-(diethoxymethyl)furan to CF3ΦNHNH2 derivatization. No significant amount of hydrazone was found. [326] Theses results leave out the possibility that (hemi)acetal or (hemi)ketal formation as a possible explanation for the lower carbonyl content obtained by NMR. Chapter 2. Quantification and control of the furan ring-opening side reaction in PFA -93 -

Table 2 .

 2 2-1. Data resulting from subjecting ketal model to carbonyl quantification methods.

	Derivatizing agent	% of Bromoacetaldehyde dimethyl acetal reacted
		≈ 40 %
		≈ 0 %
			Bromoacetaldehyde dimethyl
		≈ 24 %	acetal

One could also argue the formation of pyrroles during the derivatization by CF3ΦNHNH2 and ΦNHNH2.HCl. Indeed, Alvi et al. demonstrated that hydrazines can react with 1,4-diketones to form pyrrole-like structures.

[331]

In such case, two carbonyls would account as one when the by CF3ΦNHNH2 and ΦNHNH2.HCl derivatizing agents are used. Hence, a model compound (2,5hexanedione) was subjected to the 19 F NMR method.

Table 2 .

 2 To conclude, insights on the nature of the carbonyls contained in open structure of PFAs were given. The 19 F NMR spectra suggested that both alkyl and conjugated ketones coexist in PFA systems. Overall, at least five different types of C=O in the ketone region were observed by13 C NMR in PFA+. The existence of esters in PFA resins was highlighted especially when FA polymerization was conducted without additional water. On the other hand, FA polymerization conducted with additional water rather highlighted the formation of ketonic groups in their opened structures and much less esters were identified. The C=O in PFA were quantified by potentiometric methods,19 F and13 C NMR.The potentiometric and19 F NMR methods involved derivatization. Depending on the chemical nature of the derivatizing agent, the obtained C=O content might be higher (NH2OH.HCl) or lower 7-2. Conditions for the preparation of the PFAs in Section 2.4.4.

	80 new peaks

(CF3ΦNHNH2). This was attributed to steric hindrance issues and/or the presence of protected carbonyl functions such as ketals. The quantifications methods were applied at different conversion degrees and highlighted a continuous increase of the degree of open structures during the polymerization of FA.

Secondly, this chapter aimed at exploring the leverages available to control the degree of open structures of PFA systems. The major factor allowing to increase the C=O content of PFA is the presence of additional water during the polymerization of FA. As little as 10 %w of water allows the highest C=O content. Overall, PFAs prepared with additional water exhibited a DOS maximum of 0.15, i.e about 3.7 mmol.g -1 . PFAs prepared in neat conditions were capped at a DOS of 0.09, i.e about 2.2 mmol.g -1 .

Finally, the spectrometric tools used in this chapter namely FTIR,

19

F and

13

C NMR highlighted the complexity of PFA. More specifically, an array of carbonyls is present in PFA, alkyl C=O, conjugated C=O and esters.

i Conditions: 15 s ON, 10 s off, 60% amplitude, 2 h using a Fisher scientific FB505 sonicator

(500 W 20kHz)

Table 3 .

 3 2-1. Results from the aldehyde quantification experiments in a PFA+ system at different conversion degrees.

		External standard	Left-over furans
	Conversion degree	Aldehyde content	Aldehyde	Aldehyde content	Aldehyde
	(α)	(mmol.g -1 )	Ratio	(mmol.g -1 )	ratio
	0.12	0.05	0.04	0.08	0.06
	0.47	0.17	0.09	0.25	0.13
	0.69	0.24	0.09	0.36	0.14
	0.83	0.47	0.16	0.62	0.21

Table 3 .

 3 3-1. Results of the aldehydes quantification in the PFA° resin.

		External standard	Left-over furans
	Conversion degree	Aldehyde content	Aldehyde	Aldehyde content	Aldehyde
	(α)	(mmol.g -1 )	Ratio	(mmol.g -1 )	ratio
	0.00	0.04	0.21	0.04	0.20
	0.23	0.10	0.07	0.11	0.08
	0.55	0.09	0.05	0.13	0.07
	0.76	0.13	0.07	0.22	0.11

  Feb 2021 14:16 Cal: 20210205 5 Feb 2021 14:14 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 69, P.Ext. @ 2300 (bin 78)

	%Int.	16 mV[sum= 3134 mV] Profiles 1-200 Smooth Av 50
							190.5
	100
	90
	80						176.6
	70				160.5
	60
							192.4
	50
	40
												240.5
	30
	20	111.6	136.6	154.5			198.6
	%Int. 0 Data: PFA0001.A5[c] 5 307.6 18 mV[sum= 3559 mV] Profiles 1-200 Smooth Av 50 100 120 140 160 180 200 m/z 10 125.6 101.6 175.6 194.5 157.6 127.6 108.6 208.6	220 225.6	240	260 252.6	280 281.6	300 1[c].A5
			325.7
			334.7
		308.7
					360.5
			335.7
			320.5	336.6								440.4	477.4
		303.7	318.6	350.6	376.3 374.6	396.6 394.4	412.3	430.6	456.3	474.3	492.4
		300	320	340	360		380	400		420		440	460	480	500 1[c].A5
								m/z
												476.9
		401.0
							440.8
			416.8						458.8		466.9
												480.9	496.9
								452.9
			408.8 410.8	418.8 420.8	430.9 431.8	438.9	448.8	456.4	464.9	470.8 472.9	482.9	492.9 490.9

3.20110624: Mode Linear, Power: 84, Blanked, P.Ext. @ 2300 (bin 78) Data: PFA0001.A5[c] 5 Feb 2021 14:16 Cal: 20210205 5 Feb 2021 14:14 Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Linear, Power: 69, P.Ext. @ 2300 (bin 78)

Table 3 .

 3 5-1. 1 H and13 C NMR predicted chemical shift of a furfuryl lactone unit.

Table 3

 3

	________________________________________________________________________
	152 g.mol -1 =	; 157 g.mol -1 =
	161 g.mol -1 =	/
	176 g.mol -1 =	AND/OR
	190 g.mol -1 =	AND/OR
	194 g.mol -1 =

.5-2. Peak/structures possible assignments of the MALDI ToF spectra for PFA+ and PFA°. + , and 428 g.mol -1 with Na + , and example of an end-chain conjugated aldehyde resulting from the oxidative ring-opening of furans in PFA resins. It is present on PFA + but not really distinguishable in PFA°. 456 g.mol -1 = no Na + , protonated, calc. 455 g.mol -1 ; 476-478 g.mol -1 with Na+, calculated 477 g.mol -1 It shows that there are several more forms of this structure in PFA+.

It is unnecessary to calculate and assigned higher oligomers structures as several isomer structures become possible.

Table 4 .

 4 3-1. FTIR bands attributions of PFA and Ox-PFA.

		Wavenumber (cm -1 )	Attribution	References
		600	Furan ring: out of plane deformation [292, 293]
		650-840	Multiple furanic bands (collective)	[292, 293]
		1014	2-sub: ring C-H in plane wag	[292, 293]
	PFA	1018	2,5-sub: ring C-H in plane wag	[292, 293]
		1505	2-sub: Furan C=C stretch	[292, 328]
		1560	2,5-sub: C=C stretch	[293, 328]
		1712	C=O stretch	[283, 288]
		1060 -1340	=C-H aromatic (in plane deformation) [328, 360]
	Ox-PFA	1605	Aromatic CC stretch, C=C stretch	[328, 367]
		1705	C=O stretch	[328, 367]

Table 4 .

 4 Table4.3-2. 3-2. Results from the elemental analysis, C=O and COOH potentiometric titration. Table4.3-2 reports a C=O content about 1.7 times higher and a COOH content about 3 times higher for Ox-PFA compared with PFA. Additionally, the overall oxygen content of Ox-PFA is 1.3 times higher than PFA. Hence, the oxidative curing underwent by Ox-PFA incorporated oxygen in the material in the form of ketones/aldehydes and carboxylic acids. The presence of other oxygenated moieties is also a possibility.

	Reference	Fructose	PFA	Ox-PFA

Table 4 .

 4

	Sample	Gel content (%) Contact angle (°)	OOT (°C)
	Ox-PFA	92.4 ± 1.8	86.1 ± 2.7	253.6 ± 2.3
	PFA	94.0 ± 1.5	85.3 ± 3.1	203.1 ± 3.1

Table 5

 5

	Conversion	Pre-polymerization	Pre-polymerization	C=O content	Pre-curing
	degree (𝛼)	time (80 °C)	time (120 °C)	(mmol.g -1 )	time (130°C)
	0.3	30 mins	/	2.4 ± 0.1	1 h 30
	0.7	1 h 30	/	2.8 ± 0.1	50 mins
	0.8	2 h 40	/	3.2 ± 0.1	40 mins
	0.9	2 h 40	30 mins	3.62 ± 0.1	20 mins
	0.95	2 h 40	2 h 30	4.6 ± 0.2	20 mins

.1-1. Details regarding the unfunctionalized PFA pre-polymers (starting PFA).

Table 5 .

 5 2-1. Masses of compounds used for the functionalization of PFA with Priamine 1071.

	Sample name	NH2/C=O Stoichiometric ratio	Mass of PFA (g) Mass of Priamine (g) %w PFA
	PFA	0		20	0	100.0
	PFA/PA 0.1	0.1	20	1.8	91.7
	PFA/PA 0.25	0.25	20	4.6	81.3
	PFA/PA 0.5	0.5	20	9.2	68.5
	PFA/PA 1.0	1		20	18.4	52.1
	Table 5.2-2. Conditions used for the exposure of PFA/PA 1 to water and aqueous mixtures and
	gel fraction.
	Sample Name	Volume of water (mL)	Volume of co-solvent (mL)	Gel fraction (wt. %)
	EtOH/ Acidic water 0.1 M	12.5	12.5	93.7
	THF/acidic water 0.1 M	12.5	12.5	92.3
	Acidic water 0.1 M	25		0	91.1
	Water		25		0	89.5

The stability of PFA/PA 1 towards hydrolysis was pursued by exposing 750 mg of powdered PFA/PA 1 to various conditions before drying and FTIR analysis. The test conditions included respectively exposure to water, acidic water (0.1 M), 50/50 %v ethanol/acidic water (0.1 M) and 50/50%v THF/acidic water (0.1 M). Sulfuric acid was used to acidify the water. All experiments were conducted for 2 hours under reflux. After filtering, washing and vacuum drying, the gel fractions were calculated according to Equation 4.1-1 and the powders were analyzed by FTIR spectroscopy. The conditions and gel fractions are reported in Table

5

.2-2.

Table 5 .

 5 3-1. Thermo-mechanical properties of the PFA/Amine materials. The samples were prepared at NH2/C=O stoichiometry.

	Sample	%w PFA	E' 20°C (MPa)	Tan (𝛿) maxima (°C)	T5% (°C) T10% (°C) Shore D
	PFA/J2000	25	1.1	-44	319	350	9.6 ± 1.2
	PFA/J230	72	280	20	322	356	49.1 ± 3.6
	PFA/Urea	91	1800	76	284	332	79.3 ± 1.4
	PFA/IsoP	79	1210	93	336	367	87.7 ± 1.8
	PFA/Furfu	83	1130	52	341	364	80.4 ± 1.7
	PFA	100	1680	63	328	362	82.4 ± 2.1

Table 5 .

 5 4-2. Thermal and mechanical properties of PFA/DiHex materials prepared with PFAs at various conversion degrees. The samples were all prepared at NH2/C=O stoichiometry.

	Sample's conversion degree (𝛼)	%w PFA	E' DMA 20 °C (MPa)	Tan (𝛿) maxima (°C)	T5% (°C) T10% (°C)	Shore D
	0.3	88	160	20	296	334	38.4 ± 2.5
	0.7	86	560	43	330	358	69.3 ± 2.5
	0.8	84	600	42	341	369	69.8 ± 2.0
	PFA (0

.3, 0.8, 0.95)* 100 1857 ± 575 66 ± 11 324 ± 7 359 ± 6 82.8 ± 2.2 0.9

	Chapter 5. Exploring new horizons for poly(furfuryl alcohol) by exploiting side reactions
		83	1560	62	345	370	85.0 ± 1.6
	0.9p**	83	1940	84	329	367	83.9 ± 1.7
	0.95	79	2050	86	358	386	86.2 ± 1.4
	0.95p**	79	1780	145	335	374	88.6 ± 1.3
	*The PFA row comprises the average and standard deviation of three references prepared with
	PFA at 𝛼 of 0.3, 0.8 and 0.95.**The "p" suffix indicates that only the precipitated macromolecules
	were used in the making of the materials.
				-272 -

Table 5

 5

	.4-3. Elemental analyses of PFA and PFA/DiHex samples.
	Sample	%C %H %N %O
	PFA	71.6 5.1	0 23.3
	PFA/DiHex/𝛼0.95p 71.4 6.6 5.6 16.4
	PFA/DiHex/𝛼0.95 71.0 6.1 3.9 19.0
	Overall, PFA/DiHex from insoluble fractions have drastically different polymeric network

Table 5 .

 5 4-4 regroups data obtained from the DMAs, TGA and Shore D measurements of the PFA, PFA/Hex and PFA/KOH samples. The TGA curves are available in Figure 5.6-19. Table 5.4-4. Thermomechanical properties of the PFA/Hex and PFA/KOH materials. All the samples were prepared using a starting PFA at α = 0.8. KOH sample displays poor properties and very brittle behavior. The brittleness of the samples forbade the Shore D measurements, even on thick samples. The T5% and T10% values indicate a decrease of thermal stability for PFA/KOH in comparison with PFA.Regarding the PFA/Hex samples, the PFA/Hex/0.1 and 0.3 Eq have properties close to PFA although their E' at 20 °C are lower. The PFA/Hex/0.5 and 1 Eq samples have significantly lower E' at 20 °C than PFA. The T5% and T10% of the PFA/Hex materials are almost identical to PFA's.

	Sample	%w PFA	E' DMA 20 °C (MPa)	Tan (𝛿) maxima (°C)	T5% (°C) T10% (°C)	Shore D
	PFA	100	1680	63	328	362	82.4 ± 2.1
	1 Eq KOH	85	690	57	235	300	Not measurable
	0.1 Eq Hex	97	1250	68	327	361	85.4 ± 1.7
	0.25 Eq Hex	93	1160	60	333	362	82.7 ± 3.0
	0.5 Eq Hex	86	652	57	327	357	79.0 ± 2.9
	1 Eq Hex	76	420	53	338	352	Not measurable
	First, the PFA/

Table 5 . 6

 56

	Starting 𝛼 of PFA	E' DMA 20°C (MPa)	Tan (𝛿) maxima (°C)	T5% (°C) T10% (°C) Shore D
	0.30	1390	56	316	352	83.8 ± 2.9
	0.80	1680	63	328	362	82.4 ± 2.1
	0.95	2500	78	329	364	83.2 ± 2.6

-1. Thermomechanical properties materials prepared with starting PFAs at various conversion degrees.

  Indeed, at a given conversion degree, a neat PFA (PFA°) comprise about 2.2 mmol.g -1 of C=O, a PFA+ prepared in aqueous condition has nearly two times more carbonyls, i.e. 3.7 mmol.g - 1 . Another way of considering the carbonyl content of PFA is the notion of "Degree of Open

	Conclusions and perspectives
	Structure". In this case, 3.7 mmol.g -1 translates into 15 % of open furan ring. In other words, 1/6
	of PFA's furan rings may be opened
	Chapter 3 was dedicated to the structural investigation of carbonyls within PFA and dealt
	with the following questions. Where are the carbonyls located in the macromolecular chains of
	PFA ? What is their chemical environment ? Why may a crust forms of the surface of PFA resins
	?
	-297 -

Table of Figures

 of FIGURE 1.1-1. AVAILABLE AND DEVELOPING ROUTES FOR THE PRODUCTION OF CHEMICALS ............................................................. FIGURE 1.1-2. ILLUSTRATION OF THE ORGANIZATION OF CELLULOSES, HEMICELLULOSE AND LIGNINS IN LIGNOCELLULOSIC BIOMASS. DRAWING INSPIRED BY A PREVIOUS ILLUSTRATION FROM KODDENBERG ET AL. [] .................................................................. FIGURE 1.2-1. APPLICATIONS OF FURFURAL. SCHEME BUILT ON REFERENCES. [7, 55, 57 ] ............................................................. FIGURE 1.2-2. SIMPLIFIED OVERVIEW OF THE FURAN VALUE CHAIN. ............................................................................................ FIGURE 1.3-1. PICTURES OF A CONTINUOUS FOUNDRY MIXER TAKEN BEFORE 1979 (A) [97] AND FURAN RESINS CASTING MOLDS NOWADAYS (B).[].................................................................................................................................................... FIGURE 1.3-2. PICTURES OF THE FABRICATION (A) AND A FINAL BRICK LINING (B) TAKEN BEFORE 1979. [97] (C) SHOWS A FLOOR FURAN-BASED GROUT FROM THE ATLAS MINERALS & CHEMICALS COMPANY, 2022[] .................................................................... FIGURE 1.3-3. PICTURES OF A TERRACE MADE OF FURFURYLATED WOOD (KEBONY) IN PORTO-VECCHIO (A) [] AND COMPARISON BETWEEN UNTREATED SOFTWOOD (LIGHT) AND A FURFURYLATED ONE (DARK) (B). [] ......................................................................... FIGURE 1.3-4. SCRUBBER MADE FROM PFA AND GLASS FIBERS, PICTURE TAKEN BEFORE 1979 (A) [97] AND PROTOTYPE OF EXTERIOR VEHICLE PANEL MADE FROM PFA AND GLASS FIBERS (B).[142] ........................................................................................ FIGURE 1.3-5. CELLULOSE AND CELLULOSE REINFORCED PFA FOAMS OF LEMS ET AL. [204] ........................................................... FIGURE 1.3-6. EXAMPLE OF A PFA-TANNIN FOAM (A) [208] AND A CROSS SECTION OF A PFA FOAM FOR OIL RECOVERY (B) [209] ....... FIGURE 1.3-7. EXAMPLE OF PFA-BASED SUPERHYDROPHOBIC SURFACES, (A) [240] PFA -PTFE SYSTEM AND (B) PFA-POTASSIUM FIGURE 1.3-8. HEAT EXCHANGER MADE OF IMPREGNATED GRAPHITE FROM THE CGTHERMAL COMPANY.[] ....................................... FIGURE 1.3-9. WRINKLES OBTAINED AFTER PHOTOINITIATION OF PFA, THE WHITE BAR HAS A LENGTH OF 1 MM. [261] ....................... FIGURE 1.3-10: OVERVIEW OF THE APPLICATIONS OF FURAN RESINS. THE DASHED LINES CORRESPOND TO INDUSTRIALLY DEVELOPED FIELDS. FIGURE 1.3-11. ILLUSTRATION OF THE ASPECT OF PFA OVER THE COURSE OF ITS POLYMERIZATION. .................................................. FIGURE 1.3-12. VISIBLE AND THERMAL VISION OF THE POLYMERIZATION OF FA OVER THE COURSE OF 10 S. POLYMERIZATION OF 2 ML OF FA WITH 0.5 ML OF 50%V H2SO4. THE PICTURES WERE TAKEN ONE TO TWO SECONDS APART AND THE EMISSIVITY WAS SET TO FIGURE 2.1-1. DOSTITRI OF A PFA° AS FUNCTION OF OXIMATION TIME. LINE IS MEANT TO GUIDE THE EYES. ........................................ FIGURE 2.1-2. FTIR SPECTRA OF A PFA+ BEFORE AND AFTER TITRATION WITH DIFFERENT DERIVATIZING AGENTS. ............................... FIGURE 2.1-3. EXAMPLE OF AN EXPLOITATION OF A THERMOGRAM OF PFA AND Α CALCULUS. DSC SCAN AT 2 °C.MIN -1 FROM 20 TO 300 FIGURE 2.1-4. VISUAL APPEARANCE OF PFA° AND PFA+ WITH THE CONVERSION. ........................................................................ FIGURE 2.1-5. PICTURES OF THE AUTOCLAVE (LEFT) AND HUMINS (RIGHT). .................................................................................. FIGURE 2.2-1. NORMALIZED FTIR SPECTRA OF PFA+ (STARTING FROM UNDISTILLED FA) AS FUNCTION OF CONVERSION. 𝛼 = 0 CORRESPONDS TO THE INITIAL SPECTRUM OF UNDISTILLED FA. .......................................................................................... FIGURE 2.2-2. CARBONYLS CONTENT (IN MMOL.G -1 ) OBTAINED FOR PFA+ (PREPARED WITH UNDISTILLED FA) FROM OXIMATION WITH NH2OH.HCL (SQUARES) OR WITH ΦNHNH2.HCL METHOD (TRIANGLES) AND FROM 19 F NMR QUANTIFICATION CF3ΦNHNH2 (CIRCLE). DIAMONDS ARE FOR THE ACID CONTENT. THE DATA ARE PLOTTED AGAINST FTIR NORMALIZED C=O AREA (BOTTOM ABSCISSA AXIS) AND DSC CONVERSION DEGREE (UP ABSCISSA AXIS). ERROR BARS CORRESPOND TO THE STANDARD DEVIATION OF TRIPLICATES. ........................................................................................................................................................... FIGURE 2.2-3. 19 F NMR SPECTRA OF DERIVATIZED 2,5-HEXANEDIONE ....................................................................................... FIGURE 2.2-4. QUANTITATIVE 13 C SOLID STATE NMR (HPDEC) SPECTRUM OF A PFA+ (𝛼 = 0.92). ............................................... FIGURE 2.2-5. EVOLUTION OF DOSTITRI AGAINST THE CONVERSION DEGREE FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. ERROR BARS CORRESPOND TO THE STANDARD DEVIATION OF TRIPLICATES. ................................................... FIGURE 2.2-6. EVOLUTION OF DOSTITRI AGAINST FTIR NORMALIZED C=O AREA FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. ERROR BARS CORRESPOND TO THE STANDARD DEVIATION OF TRIPLICATES. .................................... FIGURE 2.2-7. FTIR CONVERSION DEGREE AS A FUNCTION OF DSC CONVERSION DEGREE FOR BOTH PFA° (SQUARES) AND PFA+ (CIRCLES) BOTH UNDISTILLED (OPEN SYMBOLS) AND DISTILLED (FILLED SYMBOLS). ............................................................................ FIGURE 2.3-1. (A) MAGNIFIED FTIR SPECTRA OF LAB-SCALE HUMINS. (B) 13 C NMR (CPMAS) OF LAB-SCALE HUMINS. HU1 IS MORE CONDENSED THAN HU2 WHICH IS ITSELF MORE CONDENSED THAN HU3. .......................................................................... FIGURE 2.3-2. DEPENDENCY OF THE C=O CONTENT OF TWO BATCHES OF LAB-SCALE HUMINS (HU1) ON THE OXIMATION TIME. .......... FIGURE 2.3-3. C=O CONTENT OF THREE KINDS OF HUMINS AFTER 24 H OF OXIMATION. HU1 IS MORE CONDENSED THAN HU2 WHICH IS ITSELF MORE CONDENSED THAN HU3. ....................................................................................................................... FIGURE 2.4-1. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED FROM TITRATION METHOD SYNTHESIZED WITH DIFFERENT INITIATORS IN FUNCTION OF THE CONVERSION DEGREE. ................................................................................................. FIGURE 2.4-2. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED BY TITRATION METHOD AND SYNTHESIZED WITH DIFFERENT INITIATORS WITH AND WITHOUT ADDITIONAL WATER (50/50 W/W) AS FUNCTION OF THE CONVERSION DEGREE. ..................... FIGURE 2.4-3. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED WITH TITRATION METHOD AND SYNTHESIZED WITH DIFFERENT FA/ADDITIONAL WATER RATIOS WITH IN FUNCTION OF THE CONVERSION DEGREE. ..............................................

STEARATE SYSTEM. [242] .......................................................................................................................................... THE NUMBERS CORRESPOND TO THE REFERENCES. ......................................................................................................... 0.95. .................................................................................................................................................................... °C. .......................................................................................................................................................................

  ..... FIGURE 2.7-2. EVOLUTION OF DOSNMR AGAINST THE CONVERSION DEGREE; FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. ERROR BARS CORRESPOND TO THE STANDARD DEVIATION OF TRIPLICATES. ................................................. FIGURE 2.7-3. EVOLUTION OF DOSNMR AGAINST THE C=O AREA IN FTIR, FOR PFA° (BLACK SQUARES) AND PFA+ (RED CIRCLES). OPEN AND SOLID SYMBOLS ARE FOR SAMPLES PREPARED WITH UNDISTILLED FA AND DISTILLED FA RESPECTIVELY. DOT LINES REPRESENT THE LINEAR FITS. .................................................................................................................................................... FIGURE 2.7-4. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED FROM 19 F NMR METHOD SYNTHESIZED WITH DIFFERENT INITIATORS IN FUNCTION OF THE CONVERSION DEGREE. ................................................................................................. FIGURE 2.7-5. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED 19 F NMR METHOD AND SYNTHESIZED WITH DIFFERENT INITIATORS WITH AND WITHOUT ADDITIONAL WATER (50 % W/W) IN FUNCTION OF THE CONVERSION DEGREE. ....................... FIGURE 2.7-6. COMPARISON OF THE DEGREE OF OPEN STRUCTURES OBTAINED WITH 19 F NMR METHOD AND SYNTHESIZED WITH DIFFERENT FA/ADDITIONAL WATER RATIOS WITH IN FUNCTION OF THE CONVERSION DEGREE. ............................................................. FIGURE 2.7-7.

  5.6-18. EXAMPLES OF TENSILE CURVES OF THE PFA/DIHEX AT NH2/C=O STOICHIOMETRY USING PFAS AT DIFFERENT CONVERSION DEGREES (0.3 TO 0.95). THE NUMBERS IN THE LEGEND INDICATE THE CONVERSION DEGREE OF EACH STARTING PFA. THE LETTER "P" INDICATES THAT ONLY THE PRECIPITATED MACROMOLECULES WERE USED (DASH-DOT LINES). ......................................... 294 FIGURE 5.6-19. TGA CURVES OF PFA/HEX SAMPLES AT VARYING NH2/C=O STOICHIOMETRY. PFA-KOH WAS PREPARED AT -OH/C=O STOICHIOMETRY. ALL THE SAMPLES WERE PREPARED USING A PFA AT Α = 0.8. .................................................................. 295 FIGURE 5.6-20. REPRESENTATIVE TENSILE CURVES OF PFA/HEX SAMPLES AT VARYING NH2/C=O STOICHIOMETRY. PFA-KOH WAS PREPARED AT -OH/C=O STOICHIOMETRY. ALL THE SAMPLES WERE PREPARED USING A PFA AT Α = 0.8 ................................. 295

		List of Figures, Tables and Schemes
	5.10	Table of Tables
		-319 -

TABLE 1 .

 1 1-1. OIL DEMAND BY SECTOR, BUILT ON 2021 DATA FROM THE ORGANIZATION OF THE PETROLEUM EXPORTING COUNTRIES [2] TABLE 1.1-2. SHARES OF PLASTICS PRODUCTION BY SECTORS, BUILT ON DATA FROM 2015. [] ......................................................... TABLE 1.1-3. WORLD CRUDE OIL PRODUCTION BY REGION, BUILD ON 2010 DATA FROM THE US ENERGY INFORMATION ADMINISTRATION.

TABLE 2 .

 2 2-1. DATA RESULTING FROM SUBJECTING KETAL MODEL TO CARBONYL QUANTIFICATION METHODS. ...................................... TABLE 2.7-1. FA'S POLYMERIZATION CONDITIONS FOR SECTION 2.4.1 AND 2.4.2 . .................................................................... TABLE 2.7-2. CONDITIONS FOR THE PREPARATION OF THE PFAS IN SECTION 2.4.4. .................................................................... TABLE 3.2-1. RESULTS FROM THE ALDEHYDE QUANTIFICATION EXPERIMENTS IN A PFA+ SYSTEM AT DIFFERENT CONVERSION DEGREES. . TABLE 3.3-1. RESULTS OF THE ALDEHYDES QUANTIFICATION IN THE PFA° RESIN. ........................................................................ TABLE 3.5-1.

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3.5

Supplementary information Figure 3.5-1. 1 H NMR spectra of a PFA resin, conversion degree of about 0.80, in blue with a baseline applied from 0 to 14 ppm and in red from 5.2 to 7.3 (Right) and 8.4 to 12.8 ppm (Left).