Cristina Javi

Alberto Silvia

Natalia Mónica

Samuel

Belén, Ester José Daniel

Eduardo Pablo Luis

A Gabriel Cendejas

Diego Cortizo

Javier Fernández

Santiago Pighin

Rafael Caballero

Luis José

José Ignacio

Paz… Marcos Juan

Manuela Pablo

Almudena, Isadora Michael

Pablo

Y También

Julián Natalia Daniel

Javier Pinto Sanz

J Pinto

S Pardo-Alonso

E Solórzano

J A Reglero-Ruiz

M Dumon

M A Rodriguez-Perez

J A Reglero

J M Tallon

M Pedros

P Viot Block

F García-Moreno

B Notario

J Escudero

J Martín De León

D Velasco

J A De Saja

S Pardo

Fabrication and Characterization of Nanocellular Polymeric Materials from Nanostructured Polymers

Keywords: polymeric nanofoam, block copolymer, heterogeneous nucleation, gas dissolution foaming, nanostructured polymers. Unités Future Perspectives ……………………………………………………………

This dissertation focuses on the production and study of nanocellular foams from PMMAbased (poly(methyl methacrylate) materials by CO2 gas dissolution foaming.

Due to the novelty of this research field several experimental techniques have been improved or adapted in order to provide valuable information from the systems under study.

Nanostructuration of PMMA-based blends induced by the addition of a block copolymer (MAM, poly(methyl methacrylate)-b-poly(butyl acrylate)-b-poly(methyl methacrylate)) and the cellular structure of the foams produced from these blends have been characterized and related; obtaining that the nanostructuration acts as a pattern for the cellular structure, allowing obtaining a wide range of cellular structures and in particular nanocellular foams.

It is demonstrated that processing parameters, such as pressure and temperature, allow differentiating between two foaming routes; and present a significant influence on the foaming process and final characteristics of neat PMMA foams, but not on PMMA/MAM blends. PMMA/MAM blends present a heterogeneous nucleation mechanism controlled by the nanostructuration that avoid the influence of the processing parameters in the cell nucleation. In addition, some PMMA/MAM blends also present a high stability during the cell growth, avoiding the cellular collapse and coalescence.

Finally, it has been studied the influence on the foams properties of the transition between the microcellular and the nanocellular ranges; obtaining that there is a clear influence on the thermal conductivity, which decreases in nanocellular foams due to the Knudsen effect, and the glass transition temperature, which increases in nanocellular foams due to the confinement of the polymer chains in the cell walls, but not on the Young's modulus.

Résumé :

Cette thèse porte sur la production et l'étude de mousses de polymères micro ou nano poreux à partir de mélanges nanostructurés à base de PMMA (poly(méthyl méthacrylate)) par dissolution et moussage avec CO2. D'autre part, plusieurs techniques expérimentales ont été améliorées ou adaptées afin de fournir de précieuses informations sur les systèmes étudiés.

La nanostructuration de mélanges solides denses à base de PMMA est induite par l'addition d'un copolymère à blocs (MAM, poly(méthyl méthacrylate)-co-poly(butylacrylate)co-poly(méthyl méthacrylate)). Les structures cellulaires des mousses produites à partir de ces mélanges ont été caractérisées et expliquées ; on a démontré que la nanostructuration agit comme un modèle (un gabarit) pour la structure cellulaire, permettant l'obtention d'un large éventail de structures cellulaires et en particulier des mousses nanocellulaires.

De plus il est démontré que les paramètres du procédé, tels que la pression et la température, permettent la différenciation entre les deux voies de moussage utilisées ; ceux-ci ont une influence significative sur les caractéristiques finales des mousses de PMMA seul, mais pue sur celles des mélanges PMMA/MAM. Les mousses dans ces mélanges présentent un mécanisme de nucléation hétérogène contrôlée par la nanostructuration, ce qui permet de limiter l'influence des paramètres de traitement thermique dans la nucléation de la cellule. En outre, certains mélanges de PMMA/MAM présentent également une remarquable stabilité de leur morphologie au cours de la croissance cellulaire, ce qui évite l'effondrement cellulaire et la coalescence.

Enfin, on a étudié l'influence de la transition entre les structures micro-cellulaires et les structures nano-cellulaires sur les propriétés : une nette diminution de la conductivité thermique en raison de l'effet de Knudsen que nous avons mis en évidence, une augmentation notable de la température de transition vitreuse en raison de l'isolement des chaînes de polymères dans les parois (les murs) de la cellule ; mais n'avons pas noté d'influence importante de cette transition sur le module de Young.

Mots clés : mousse polymère, nano poreux, copolymère à blocs, nucléation hétérogène, moussage pour dissolution de gaz, polymères nanostructurés.

Título: Fabricación y Caracterización de Materiales Poliméricos Submicrocelulares a partir de Polímeros Nanoestruturados.

Resumen:

Esta tesis se centra en la producción y estudio de de espumas poliméricas nanocelulares producidas a partir de materiales basados en PMMA (poli(metil metacrilato)), mediante la técnica de espumado por disolución de gas usando CO2.

Debido a la novedad de este campo de investigación ha sido necesario mejorar o adaptar varias técnicas experimentales para obtener la información necesaria de los sistemas bajo estudio.

Se han caracterizado y relacionado la nanoestructuración de mezclas basadas en PMMA, inducida por la adición de un copolímero de bloque (MAM, poli(metil metacrilato)-copoli(butil acrilato)-co-poli(metil metacrilato)), y la estructura celular de las espumas producidas a partir de esas mezclas; obteniéndose que la nanoestructuración actúa como patrón para la estructura celular, permitiendo obtener una amplia variedad de estructuras celulares y en particular de estructuras nanocelulares.

Se ha demostrado que los parámetros de procesado, como la presión y temperatura, permiten diferenciar entre dos rutas de espumado y presentan una influencia significativa en las características finales de las espumas de PMMA puro, pero no en las mezclas de PMMA/MAM. Estas mezclas presentan un mecanismo de nucleación heterogénea controlado por la nanoestructuración, que evita que los parámetros de procesado influyan en el proceso de nucleación de las celdas. Además, algunas mezclas de PMMA/MAM también presentan una alta estabilidad durante el crecimiento de las celdas, evitando el colapso de la estructura celular y la coalescencia.

Finalmente, se ha estudiado la influencia en las propiedades de las espumas de la transición entre el rango microcelular y el rango nanocelular; obteniéndose que hay una clara influencia sobre la conductividad térmica, que decrece en las espumas nanocelulares debido al efecto Knudsen, y sobre la temperatura de transición vítrea, que se incrementa debido al confinamiento de las cadenas poliméricas en las paredes de las celdas, pero no sobre el módulo de Young.

Palabras clave: espumas poliméricas nanocelulares, copolímero de bloque, nucleación heterogénea, espumado por disolución de gas, polímeros nanoestructurados AGRADECIMIENTOS Tras varios años de investigación y escribir las más de doscientas páginas que componen la tesis me ha llegado el momento de enfrentarme otra vez a un folio en blanco. Es hora de escribir las primeras páginas que la mayor parte de la gente leerá de mi tesis (y para que engañarnos, en muchos casos serán las únicas, todos lo hemos hecho alguna vez).

Me pongo de nuevo el mono de faena con la disposición de tratar de no olvidarme de nadie a quien deba agradecer haberme ayudado a llegar hasta aquí o haberme acompañado en el camino, tarea nada sencilla y empeño en el que si fallo pido disculpas de antemano a quien se sienta ausente de estas líneas.

Comenzando por mis directores de tesis, primero por detalles individuales, y luego por virtudes que comparten. He de agradecer a Miguel Ángel Rodríguez sus consejos antes de iniciar mi etapa doctoral, y tras ellos la oportunidad que me brindó de ser parte del laboratorio CellMat con una sorpresiva llamada a Segovia una tarde de verano. A Michel Dumon le agradezco el magnífico trato que me ha dado durante mis estancias en el LCPO, el haberme abierto las puertas de una línea de investigación que a la larga se convertiría en mi tesis, y el haberme ofrecido la posibilidad de llevar a cabo la cotutela que ha hecho de mi tesis lo que ha llegado a ser. A ambos he de agradecerles su cercanía y su trato cordial, todas las puertas que me han abierto y los "empujones" que me han dado para cruzarlas, y muy especialmente la libertad que me han dado y la confianza que han depositado en mí para orientar mi trabajo en las direcciones que más me motivaban.

Continuaré por el laboratorio CellMat. A José Antonio de Saja he de agradecerle igualmente la oportunidad de unirme al grupo de investigación, el haberme dado "la alternativa" en tareas docentes, y los consejos que oportunamente me ha dado.

A Blanca Calvo, Emy Andrés, y Laura Izaguirre he de agradecerlas lo fácil que resulta todo trabajando con ellas, son el incombustible motor de CellMat sin el cual nos sería imposible llevar a cabo nuestras investigaciones al ritmo actual, ya que estaríamos perdidos en un maremágnum de ensayos y papeleos.

A Eusebio Solórzano, con el que he tenido la suerte de trabajar codo con codo en multitud de ocasiones, tratando de aprender todo lo posible, ya fuera preparando presentaciones hasta horas intempestivas, montando "cachivaches", programando contrarreloj en Berlín, o debatiendo sobre cualquier resultado extraño con el que nos hubiéramos topado.

A Sergio Estravis, mi "socio" desde que empezamos la tesis, compañero de aventuras en el Máster, de "marrones" en el departamento, y de "armas" cuando hemos tenido que lidiar con la administración española y su peculiar versión de "las 12 pruebas de Hércules" para presentar cualquier impreso.

Financiación

He de agradecer la financiación para llevar a cabo esta investigación que he recibido de diversas instituciones, comenzando por una beca de Introducción a la Investigación de 3 meses concedida por la UVa, unos meses de beca concedida por la Junta de Castilla y León, y finalmente una beca FPU concedida por el Ministerio de Educación (AP2008-03603). Mientras que la realización de estancias y asistencia a congresos han sido en general financiadas por la beca FPU y la UVa. También he de agradecer la financiación recibida por el grupo CellMat para el estudio de materiales poliméricos nanocelulares proveniente de los siguientes proyectos:

F.1 Introduction

L'Union Européenne, dans la définition d'objectifs 2014-2020 (Horizon 2020) [3], définit l'une de ses priorités de la recherche en science des matériaux : "Manufacturing and control of nanoporous materials". La phrase suivante précise l'objectif, "a constantly growing interest in nanostructured porous materials, thanks to the many applications that can benefit from controlled porosity at the nanoscale".

Ainsi pour accroître les applications des matériaux poreux [4], outre le gain de masse, les nano et micro mousses de polymères ont des potentielles applications nombreuses dans la thermique [12], l'absorption de chocs, la filtration [17], la catalyse [18], l'optique, l'acoustique [13].

Notre travail contribue à ces objectifs et est intitulé "Fabrication et Caractérisation de Polymères Micro et Nano Cellulaires à partir de Polymères Nanostructurés à base PMMA".

Ce travail fut initié au LCPO lors d'un projet ANR 2008-2012, Listrac (Light Structures and Composites), qui a produit et étudié du point de vue mécanique, une gamme de matériaux cellulaires de taille de porosités variable pour l'absorption de chocs et d'impacts (sécurité passive). Une des familles de polymères étudiés est à base de mélanges d'un homo polymère PMMA avec un copolymère à blocs acryliques. Le procédé d'élaboration est le moussage physique par CO 2 .

Le travail de cette thèse prolonge et approfondit l'étude de ce système. Il généralise le concept de nano et micro mousses en milieu CO 2 à partir de polymères nano structurés; notamment l'effet du taux de copolymère, les relations entre les nano structures initiales et les porosités, les mécanismes de moussage, les propriétés mécaniques et thermiques.

Le manuscrit est composé d'une introduction -chapitre 0-décrivant le cadre du travail, l'état de l'art, les objectifs, un synopsis, les articulations des chapitres, les principales avancées technologiques et scientifiques, puis de 5 chapitres -I à V-correspondant à des publications acceptées, et une conclusion.

Cette collaboration entre deux laboratoires, CellMat Laboratory (Condensed Matter Physics Department, Universidad de Valladolid, España) et le Laboratoire de Chimie des Polymères Organiques (LCPO, Université de Bordeaux, France) fait l'objet d'une convention de cotutelle de thèse avec double diplôme.

La thèse est écrite entièrement en anglais. Des liens entre les publications sont toujours faits et chaque chapitre a sa propre bibliographie.

Le choix d'un homo polymère se porte sur un polymère amorphe, non formulé, le poly(méthylméthacrylate) (PMMA) et un copolymère tri-blocs (MAM) contenant deux blocs PMMA et un bloc polybutylacrylate (le poly(méthylméthacrylate)-co-poly(butylacrylate)-co-poly(méthyl méthacrylate, MAM), celui-ci donne une nano structuration quel que soit le taux de copolymère dans le Résumé en français F-4 PMMA homopolymère, avec des morphologies diverses à l'échelle nanométrique : micelles, « core-shell », « warm-like », lamelles. Un copolymère tri blocs (MAM) est choisi en ayant un bloc CO 2 -phile élastomère, il a déjà été utilisé à un taux de 10% (massique) dans le PMMA pour générer des mousses de taille « sous micronique » et dans certains cas nanométrique [START_REF] Román-Lorza | Cellular Structure of EVA/ATH Halogen-free Flame-retardant Foams[END_REF].

La fabrication et l'étude de polymères nanocellulaires à l'état de films est connue, soit à partir d'empreinte de nanostructures (pattern-transfer [16,[START_REF] Madou | Fundamentals of Microfabrication: The Science of Miniaturization, Second Edition[END_REF][START_REF] Hillmyer | Nanoporous materials from block copolymer precursors[END_REF][START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF]) ou par dégradation ou dissolution sélective de blocs [16,[START_REF] Hillmyer | Nanoporous materials from block copolymer precursors[END_REF][START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF][START_REF] Hedrick | Templating Nanoporosity in Thin-Film Dielectric Insulators[END_REF][START_REF] Hentze | Template synthesis of porous organic polymers[END_REF][START_REF] Jackson | Nanoporous membranes derived from block copolymers: From drug delivery to water filtration[END_REF][START_REF] Hedrick | Nanoporous polyimides[END_REF][START_REF] Lazzari | Block copolymers as a tool for nanomaterial fabrication[END_REF][START_REF] Park | Enabling nanotechnology with self assembled block copolymer patterns[END_REF]. D'autre part, la bibliographie montre que le moussage par un agent chimique traditionnel ne peut pas générer des nano mousses.

Le moussage par dissolution de gaz sous haute pression (CO 2 , N 2 ) est encore assez peu connu pour générer des mousses nano cellulaires [START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF][START_REF] Kazarian | Polymer Processing with Supercritical Fluids[END_REF][START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF][START_REF] Reverchon | Production of controlled polymeric foams by supercritical CO 2[END_REF] mais apparait comme une technique très efficace, peu onéreuse et "verte", notamment le CO 2 (ayant une excellente capacité de diffusion à l'état supercritique (31º C and 7.3 MPa). étape I. Saturation du polymère.

étape II. Désorption du gas, détente de pression.

étape III. Moussage à une température supérieure ou proche de la T g du polymère plastifié (T g,effective ).

Si la détente en pression déclenche la nucléation de bulles, le procédé est appelé one-step ou batch foaming [START_REF] Ruiz | Microcellular foaming of polymethylmethacrylate in a batch supercritical CO2 process: Effect of microstructure on compression behavior[END_REF][START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF] (cas d'une température proche de T g ou au-dessus).

Si le milieu est vitreux, l'expansion se fait lors d'une 2 ème étape par un chauffage au-dessus ou au voisinage de T g,eff qui amorce le moussage; le procédé est appelé two-step ou solid state foaming [START_REF] Kumar | A process for making microcellular thermoplastic parts[END_REF]. D'autre part les nano mousses demandent une densité de nucléation importante (N o , nombre de noyaux par cm 3 de polymère dense), classiquement 10 14 -10 15 noyaux/cm 3 . Les paramètres du procédé (T sat , la 2) Avoir une compréhension des mécanismes de moussage de polymères dans CO 2 , en particulier en milieu hétérogène c'est-à-dire en présence d'un copolymère à blocs.

Gas dissolution foaming

3) Etudier la transition entre mousses microcellulaire et nanocellulaire, en particulier sur les propriétés mécaniques et thermiques.

Du point de vue technologique, nous recherchons des matériaux d'accès facile, capables de se nanostructurer dans toutes les conditions de procédés (extrusion / injection / gonflement dans une enceinte CO 2 ); des mousses de densité réduite (-50% de la densité du matériau précurseur dense) et des structures nano cellulaires variées, ceci avec des matériaux de grande diffusion et de production industrielle.

Résumé en français

F-7

Du point de vue scientifique, nous voulons répondre aux questions suivantes :

• la nucléation est-elle homogène ou hétérogène (ou une combinaison des deux) ?

• peut-on découpler les mécanismes ?

• la nanostructure du solide dense est-elle réplicable dans les mousses obtenues par CO 2 ?

• quel est le rôle du copolymère et d'un bloc souple CO 2 -phile dans la nucléation et la croissance ?

• quelle est l'influence des paramètres du procédé (T, P, dP/dt) en présence de copolymère ?

F.3 Structure de cette thèse

Les objectifs décrits dans la section précédente sont abordés à travers cinq chapitres, comme indiqué dans le schéma suivant (Figure F.4). En outre les publications incluses dans chaque chapitre sont répertoriées dans le Table F-1.

F.4 Avantages des mélanges structurés amorphes pour fabriquer des nano mousses polymères

Nous avons étudié et montré la nano structuration du MAM dans le PMMA, quel que soit le taux de MAM et quel que soit le procédé de mélanges (extrusion / injection), on peut ainsi envisager une échelle semi industrielle.

Des mousses ouvertes ou fermées sont obtenues selon le taux de MAM, à l'état massique (bulk) sur des échantillons épais, sont obtenues. Les densités sont de l'ordre de 0.4 à 0.6 g/cm 3 , sur une gamme large de pressions de saturation (10 à 30 MPa).

La nucléation est quasi complétement hétérogène, contrôlée par le bloc souple polybutylacrylate PBA.

L'efficacité de nucléation est proche de 1 (rapport du nombre de noyaux des cellules des mousses sur le nombre d'objets dispersés du matériau dense).

La croissance est contrôlée par la plastification de la phase PMMA, on peut ainsi éviter la coalescence par la sélection des paramètres de procédé de moussage.

Une transition entre mousses micro and nano cellulaire est mise en évidence, en particulier sur la conductivité thermique.

F.5 Résultats principaux

Les mélanges PMMA/MAM nano structurés se révèlent d'excellents matériaux modèles et robustes pour la production de polymères nano cellulaires.

Les morphologies (la structuration ou l'autoassemblage) à l'état dense dépend du taux massique de MAM et montrent des dispersions d'objets allant du micellaires / core shells / warm like, à lamellaires ou quasi co continus.

La dissolution de gaz dans ces mélanges conduit à des nano ou des micro mousses. Les quantités de CO 2 dans chaque phase MAM, PBA et le PMMA ont été évaluées.

Les polymères seuls ne montrent des nano mousses qu'avec un choix étroit de conditions de moussage (i.e. à haute pression et températures proche ou en dessous de la température ambiante); alors que les mélanges PMMA/MAM sont robustes et des nano mousses sont possibles sur la gamme complète de pressions (de 10 à 30 MPa) et des températures jusqu'à 50º C.

Les cellules de vides (les pores) peuvent être fermées ou ouvertes voire allongées ou co continues. Le recuit sous CO 2 permet l'homogénéisation de ces nano structures cellulaires.

La nucléation quasi uniquement hétérogène est due à la dispersion parfaite d'objets et s'explique par la présence du bloc PolyButylAcrylate (PBA), de plus grande CO 2 -philie que le PMMA ainsi que son état caoutchoutique tout au long du moussage. Aussi les paramètres de procédé (i.e. pression de saturation et température, vitesse de détente) sont peu sensibles, ce qui est un avantage technologique important.

Les domaines de PBA constituent donc les noyaux pour la nucléation des cellules.

Néanmoins si la quantité de MAM (donc de PBA) augmente l'absorption de CO 2 dans le mélange, elle entraine aussi la diffusion plus rapide du CO 2 hors de la matrice, surtout si les structures sont cocontinues.

La présence de copolymère permet de découpler nucléation et croissance des cellules. En effet, ici, la croissance est gouvernée par la plastification de la phase PMMA et s'arrête à la vitrification de cette phase (dans la matrice ou dans les blocs).

La mesure de certaines propriétés thermomécaniques montre des effets de la transition micro -nano cellulaires. Ainsi la T g de la phase PMMA est augmentée de 8 à 12 °C, ce qui est attribué à l'effet des très faibles épaisseurs des parois des cellules qui réduit la mobilité des chaines. Le module d'Young (mesuré en flexion 3 points) est augmenté ; cet effet n'est pas relié explicitement à un effet de taille mais plus à une meilleure homogénéité macroscopique des mousses et à un effet de « renforcement structural ».

Un résultat important est la réduction de la conductivité thermique par l'effet Knudsen qui est montrée expérimentalement pour une des premières fois dans une mousse polymère lorsque l'on transite d'une micro à une nano mousse.

F.6 Références

Les références citées dans ce résumé peuvent être trouvées dans la section 0.7.

Resumen en español S.1 Introducción

El conocimiento humano y el desarrollo tecnológico no han crecido de manera constante desde el comienzo de la civilización. Al contrario, muchas áreas científicas y técnicas evolucionan de manera exponencial [1,2]. De hecho, se estima que más del 90% del conocimiento científico ha sido desarrollado desde el final de la II Guerra Mundial en 1945. Esta afirmación es válida prácticamente para cada campo científico, y en lo que respecta a la Ciencia de Materiales presenta dos lecturas complementarias.

Por una parte, los avances en este campo permiten desarrollar nuevos materiales con propiedades únicas, o bien mejorar las propiedades o rutas de producción de materiales de uso común, ampliando de esa forma su rango de aplicación o inclusive dando lugar a aplicaciones completamente nuevas para dichos materiales. Por otra parte los avances en otras ramas de la ciencia y el desarrollo de nuevas tecnologías frecuentemente requieren de nuevos o mejorados materiales para poder llevarse a cabo, y especialmente para poder llevar a cabo la transición desde su producción en laboratorio hasta ser capaces de suponer un impacto significativo sobre la sociedad. Debido a esta dualidad la Ciencia de Materiales es una de las principales áreas de interés tanto de organismos públicos de investigación como de centros de investigación privados.

De hecho, la Unión Europea (UE) estableció en diciembre de 2013 los objetivos de investigación del programa "Horizonte 2020" (Horizonte 2020, el programa de Investigación e Innovación de la UE para el periodo 2014-2020) [3], entre los que se encuentran múltiples objetivos centrados en la Ciencia de Materiales. En particular, dentro de esos objetivos una de las prioridades de la UE es la "Producción y control de materiales nanoporosos". La UE refleja así este tema "creciente interés en materiales porosos nanoestructurados, debido a las múltiples aplicaciones que pueden beneficiarse de una porosidad controlada en la nanoescala".

La presente tesis titulada "Fabricación y Caracterización de Materiales Poliméricos Submicrocelulares

a partir de Polímeros Nanoestructurados" se encuadra por tanto dentro de este campo de investigación prioritario, en la actualidad y en el futuro, para la UE.

De manera general los materiales espumados, o espumas, son un tipo de materiales porosos (es decir, con una fase sólida y una fase gaseosa). La palabra espuma es habitualmente utilizada para clasificar materiales de baja densidad relativa (independientemente de su tamaño de poro) producidos mediante procesos o técnicas de espumado, que implican la introducción y expansión de la fase gaseosa en dicho sólido. La producción de espumas tiene como finalidad mejorar las propiedades de materiales conocidos, o bien incrementar el rango de sus potenciales aplicaciones [4]. Además de ofrecer una reducción de la densidad del material de partida (obteniendo así piezas más ligeras para unas mismas dimensiones/aplicación), los materiales porosos y espumas pueden presentar mejores propiedades que los materiales sólidos en diferentes áreas/aplicaciones, como por ejemplo: menor conductividad térmica, mejor absorción de impactos, absorción acústica, etc.

Sin embargo los materiales porosos no pueden considerarse una invención de la humanidad, la naturaleza ha utilizado ampliamente esta aproximación durante la evolución de la vida sobre la Tierra, siendo posible encontrar materiales en estructuras vivas (vegetales o animales) con estructuras porosas, como por ejemplo la madera, corcho, las esponjas marinas, o los huesos de diferentes especies de vertebrados [5].

Pero reproducir estas estructuras vivas por medios artificiales dista mucho de ser simple. Las estructuras de organismos vivos se desarrollan con la estrategia denominada "bottom-up" ("de abajo a arriba"), con las células del organismo organizándose para formar la estructura, y respondiendo a las demandas externas a las que esa estructura es sometida. Por ejemplo, la estructura de los huesos humanos (Figura 0.1, izquierda) se organiza para proveer el soporte necesario al cuerpo humano, y puede reforzarse si esas demandas se incrementan (como por ejemplo con el ejercicio físico).

Figura 0.1: Estructura porosa de un hueso humano (izquierda) y estructura porosa de una espuma polimérica nanocelular (derecha)

Actualmente es posible fabricar materiales celulares a partir de múltiples materiales de partida, como líquidos, polímeros, metales, cerámicas, etc. [6][7][8][9][10]. Estos materiales pueden presentar propiedades mecánicas y densidades relativas bajas que les permiten encontrar aplicación en empaquetamiento, industria aeronáutica, industria del automóvil, industria naval, turbinas aerogeneradoras, etc. [11]. Otras aplicaciones de importancia de estos materiales se encuentran en el aislamiento térmico [12], aislamiento acústico [13], así como en su uso como scaffolds (materiales que sirven como "andamio" para el crecimiento de tejidos vivos) [14][15][16], filtros [17] y en procesos de catálisis debido a su estructura y su elevada área específica superficial [18].

La producción de espumas ha experimentado una progresión significativa con el desarrollo de la Nanotecnología. Este concepto fue primeramente introducido por el premio Nobel Richard P. Feynman en 1959 en su conferencia "There's Plenty of Room at the Bottom". Tal y como Feynman predijo (y otros investigadores como Norio Taguchi, quien propuso el término "nano-tecnología") gracias a la manipulación de la materia a escala atómica, molecular, o supramolecular comenzó una nueva era en la Ciencia de Materiales.

Correspondientemente, varias de las actuales fronteras en la investigación en espumas poliméricas son la adición de nanopartículas (con el fin de modificar el proceso de espumado o para mejorar/modificar la estructura y propiedades de la espuma), y la producción de espumas poliméricas con estructuras celulares en el rango nanométrico (1 nm = 10 -9 m) con porosidades intermedias o elevadas (densidades medias o bajas). Debido al enorme interés de este campo, hemos centrado este trabajo de investigación en la producción y comprensión de espumas poliméricas nanocelulares, utilizando para ello una aproximación "bottom-up" basada en la auto-organización de copolímeros de bloque como patrón de las estructuras nanocelulares. La Figura 0.1 (derecha) muestra la estructura típica de una de las espumas nanocelulares producidas en esta investigación.

La fabricación de polímeros nanoporosos a partir de films poliméricos es bien conocida, ya sea mediante el empleo de nanoestructuras (técnicas de transferencia de patrones [16,[START_REF] Madou | Fundamentals of Microfabrication: The Science of Miniaturization, Second Edition[END_REF][START_REF] Hillmyer | Nanoporous materials from block copolymer precursors[END_REF][START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF]) o mediante la degradación o disolución selectiva de bloques [16,[START_REF] Hillmyer | Nanoporous materials from block copolymer precursors[END_REF][START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF][START_REF] Hedrick | Templating Nanoporosity in Thin-Film Dielectric Insulators[END_REF][START_REF] Hentze | Template synthesis of porous organic polymers[END_REF][START_REF] Jackson | Nanoporous membranes derived from block copolymers: From drug delivery to water filtration[END_REF][START_REF] Hedrick | Nanoporous polyimides[END_REF][START_REF] Lazzari | Block copolymers as a tool for nanomaterial fabrication[END_REF][START_REF] Park | Enabling nanotechnology with self assembled block copolymer patterns[END_REF]. Por otra parte, la bibliografía muestra que no es posible obtener materiales nanocelulares mediante el empleo de técnicas de espumado empleando un agente de espumado químico.

Por el contrario, el proceso de espumado por disolución de gas a altas presiones (típicamente N 2 o CO 2 )

es quizás menos conocido [START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF][START_REF] Kazarian | Polymer Processing with Supercritical Fluids[END_REF][START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF][START_REF] Reverchon | Production of controlled polymeric foams by supercritical CO 2[END_REF], pero es una técnica versátil, eficaz, barata y medioambientalmente sostenible, especialmente cuando se emplea CO 2 (que presenta una excelente capacidad de difusión en el estado supercrítico (por encima de 31º C y 7.3 MPa)) y con el potencial de crear polímeros nanocelulares.

El espumado mediante disolución de gas está compuesto de tres etapas (Figura 0.2):

Figura 0.2: Esquema del proceso de disolución de gas -Etapa I: Saturación del polímero bajo presión.

-Etapa II: Desorción del gas tras liberar la presión.

-Etapa III: Espumado a una temperatura superior o cercana a la temperatura de transición vítrea efectiva del polímero plastificado (T g,ef ).

Si la nucleación de las celdas se produce durante la liberación de la presión el proceso es calificado como un proceso de espumado en una etapa [START_REF] Ruiz | Microcellular foaming of polymethylmethacrylate in a batch supercritical CO2 process: Effect of microstructure on compression behavior[END_REF][START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF] (también conocido como batch foaming), para lo cual la temperatura del polímero durante el proceso de liberación de la presión deberá ser próxima o superior a su T g,ef .

Por el contrario, si durante la liberación de la presión el polímero se encuentra en estado vítreo será necesario llevar a cabo el proceso de espumado en una segunda etapa, calentando la muestra polimérica saturada de gas a una temperatura cercana o superior a su T g,ef . En este caso el proceso de espumado será de dos etapas [START_REF] Kumar | A process for making microcellular thermoplastic parts[END_REF] (también conocido como solid state foaming).

La producción de polímeros nanocelulares mediante el proceso de espumado por disolución de gas requiere alcanzar una densidad de nucleación muy elevada (N o , número de núcleos por cm 3 de la muestra sin espumar), típicamente con valores del orden de 10 14 -10 15 núcleos/cm 3 . Existen diversas aproximaciones para incrementar la densidad de nucleación como se muestra en la Figura 0.3. En materiales que presentan un mecanismo de nucleación homogéneo la densidad de nucleación está controlada por los parámetros del proceso de espumado (principalmente la presión de saturación y la velocidad de liberación de la presión) [START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF][START_REF] Goel | Generation of microcellular polymeric foams using supercritical carbon dioxide. I: Effect of pressure and temperature on nucleation[END_REF]102].

Mientras que en los materiales que presentan mecanismos de nucleación heterogénea es posible incrementar la tasa de nucleación mediante la adición de partículas (talco, óxido de titanio, caolín, nanosílicas, etc.), siempre y cuando dichas partículas estén bien dispersadas en la matriz polimérica y que la matriz de poliestireno. Estos autores necesitaban llevar a cabo un enfriamiento de la muestra tras la liberación de la presión para preservar la estructura nanocelular, obteniendo de esa manera tamaños de celda en torno a los 15-30 nm, con una eficiencia de nucleación baja (es decir, con una densidad de nucleación significativamente inferior a la densidad de nanodominios), y densidades relativas generalmente altas (cercanas a 1).

S.2 Marco de la tesis

Este trabajo es el resultado de la colaboración entre dos laboratorios de investigación, el Laboratorio de Materiales Celulares (CellMat, Departamento de Física de la Materia Condensada, Universidad de Valladolid, España) y el Laboratorio de Química de Polímeros Orgánicos (LCPO, Universidad de Burdeos, Francia). Dicha colaboración se estableció mediante la firma de un acuerdo de cotutela de la tesis entre ambas instituciones, siendo dicha tesis financiada por el Ministerio Español de Educación (programa FPU, referencia AP2008-03603).

La actividad del grupo CellMat comenzó en 1999 con la tesis doctoral de Miguel Ángel Rodríguez Pérez [19]. A partir de entonces, bajo la dirección del profesor Miguel Ángel Rodríguez y del profesor José Antonio de Saja se han desarrollado trece tesis doctorales y se han publicado en torno a 150 artículos en revistas internacionales en el ámbito de los materiales celulares [20][21][22][23][24][25][26][27][28][29][30][31]. En la actualidad CellMat mantiene cinco líneas de investigación principales: espumas microcelulares y nanocelulares [START_REF] Rodriguez-Perez | Mechanical Response of Polyethylene Foams with High Densities and Cell Sizes in the Microcellular Range[END_REF][START_REF] Rodríguez Pérez | Mechanical Behaviour at Low Strains of LDPE Foams with Cell Sizes in the Microcellular Range: Advantages of Using these Materials in Structural Elements[END_REF][START_REF] Ruiz | Production, cellular structure and thermal conductivity of microcellular (methyl methacrylate)-(butyl acrylate)-(methyl methacrylate) triblock copolymers[END_REF],

nanocomposites celulares [START_REF] Verdejo | Physical properties of silicone foams filled with carbon nanotubes and functionalized graphene sheets[END_REF][START_REF] Velasco | Foams based on low density polyethylene/hectorite nanocomposites: Thermal stability and thermo-mechanical properties[END_REF][START_REF] Román-Lorza | Cellular Structure of EVA/ATH Halogen-free Flame-retardant Foams[END_REF][START_REF] Ma | X-ray Microtomographic Study of Nanoclay-Polypropylene Foams[END_REF], espumas a partir de bioplásticos [START_REF] Wang | Thermal and Thermomechanical Behaviour of Polycaprolactone and Starch/Polycaprolactone Blends for Biomedical Applications[END_REF][START_REF] Simoes | Tailoring the structural properties of PVDF and P(VDF-TrFE) by using natural polymers as additives[END_REF][START_REF] Rodriguez-Perez | Foaming of EVA/starch blends: Characterization of the structure, physical properties, and biodegradability[END_REF], espumas metálicas [START_REF] Solórzano | Mechanical Behaviour of Internal Reinforced Aluminium Foams[END_REF][START_REF] Solórzano | Thermal Conductivity of Cellular Metals Measured by the Transient Plane Sour Method[END_REF][START_REF] Lázaro | Early anisotropic expansion of aluminium foam precursors[END_REF][START_REF] Garcia-Moreno | Kinetics of coalescence in liquid aluminium foams[END_REF], y desarrollo de nuevas técnicas experimentales y dispositivos para el estudio de los mecanismos de espumado [START_REF] Solórzano | Application of a microfocus X-ray imaging apparatus to the study of cellular polymers[END_REF][START_REF] Solórzano | X-ray radioscopy in-situ studies in thermoplastic polymer foams[END_REF][START_REF] Solórzano | Optical expandometry: A technique to analyze the expansion kinetics of chemically blown thermoplastic foams[END_REF][START_REF] Pardo-Alonso | 3D Analysis of the progressive modification of the cellular architecture in polyurethane nanocomposite foams via X-ray microtomography[END_REF].

El laboratorio LCPO fue creado en 1985 por el profesor M. Fontanille, siendo actualmente dirigido por el profesor Henri Cramail. LCPO es uno de los principales centros de investigación sobre polímeros en Francia y Europa, con una calificación A + desde 2009. Su producción científica se encuentra en torno a 80 publicaciones/año en revistas internacionales, entre 7 y 10 patentes cada año, y desarrolla en la actualidad 7 proyectos de investigación europeos y 20 proyectos de investigación nacionales.

S.3 Objetivos

Los objetivos generales de este trabajo son estudiar la producción, los mecanismos de espumado, y las propiedades de poliméros nanocelulares obtenidos a partir de mezclas de PMMA (poli(metil metacrilato))

y MAM (copolímero de bloque compuesto por un bloque de PMMA, un bloque de PBA (poli(butil acrilato)) y otro bloque de PMMA). Este sistema polímero/copolímero no ha sido estudiado sistemáticamente con anterioridad como precursor de materiales espumados, de manera que se ha llevado a cabo un estudio completo sobre el espumado de estos materiales.

Tradicionalmente existen cuatro principales áreas de interés en el estudio de un nuevo material celular: su ruta de producción, su estructura celular, sus propiedades, y sus potenciales aplicaciones. Esos aspectos están fuertemente interrelacionados, tal y como muestra el tetraedro de la Figura 0.4. 1. Solventar algunas de las principales desventajas o limitaciones de las actuales estrategias de producción utilizadas para obtener espumas poliméricas nanocelulares ("Producción").

2. Proporcionar una mejor comprensión de los mecanismos de espumado involucrados en la producción de espumas poliméricas nanocelulares. Para ello se analizará la estructura celular de los materiales fabricados y su relación con la morfología de los precursores utilizados para el espumado y con los parámetros de procesado ("Producción  Estructura").

3. Estudiar si existe una transición en las propiedades de estos materiales cuando el tamaño celular pasa del rango microcelular al rango nanocelular ("Propiedades").

Aunque los principales objetivos se hayan centrado en estas tres áreas a lo largo del los distintos capítulos se describen algunas potenciales aplicaciones de estos materiales, así como del conocimiento generado.

Asimismo se han definido objetivos parciales dentro de los objetivos principales, de cara a definir precisamente el enfoque y la extensión de cada uno de ellos.

Desde el punto de vista de la técnica de producción queremos obtener espumas poliméricas nanocelulares con los siguientes requisitos:

-Los materiales de partida deben ser fácilmente accesibles, sin una síntesis excesivamente compleja y preferiblemente ser polímeros comerciales.

-Las mezclas sólidas polímero/copolímero deben presentar nanoestructuración mediante autoorganización.

-La ruta de producción debe ser escalable industrialmente, evitando requerimientos como presiones de saturación excepcionalmente elevadas, velocidades de despresurización extremadamente altas, enfriamiento de la muestra tras la liberación de la presión, etc.

-Debe obtenerse una reducción de la densidad significativa (por encima del 50%), manteniéndose intacta la estructura nanocelular.

Desde el punto de vista científico, en torno a los mecanismos de espumado el trabajo se ha centrado en las siguientes cuestiones:

-¿El proceso de nucleación es homogéneo o heterogéneo? (O una combinación de ambos mecanismos). ¿Esos mecanismos pueden ser identificados de manera independiente?

-¿Está la nanoestructura de las muestras solidas relacionada con la estructura celular de las muestras espumadas?

-¿Cuál es el papel del bloque blando del copolímero MAM en los procesos de nucleación y crecimiento de las celdas?

-¿Cuál es el papel de la matriz de PMMA en los procesos de nucleación y crecimiento de las celdas?

-¿Cuál es la influencia del porcentaje de MAM y la morfología de las nanoestructuras en la densidad y estructura celular de las espumas de PMMA/MAM? -¿Cuál es la influencia de los parámetros de procesado (presión de saturación de CO 2 y temperatura de saturación) en las espumas de PMMA/MAM? Y finalmente, hemos centrado parte de nuestra investigación en determinar la influencia de la reducción del tamaño celular, desde el rango micrométrico al nanométrico, sobre las siguientes propiedades:

-Conductividad térmica: se espera que la conductividad térmica de la fase gaseosa de las espumas nanocelulares sea menor que la de las espumas microcelulares debido al efecto Knudsen [124].

-Temperatura de transición vítrea de la matriz polimérica: la temperatura de transición vítrea en films delgados puede incrementarse debido al confinamiento de las cadenas poliméricas [126].

Por tanto, los polímeros nanocelulares podrían presentar un efecto similar debido al bajo espesor de las paredes y aristas de las celdas.

-Módulo de Young: en espumas microcelulares de PMMA se ha descrito una mejora de sus propiedades mecánicas (módulo de Young entre otras) con la reducción del tamaño de celda [127]. Se espera por tanto que las espumas nanocelulares puedan presentar mejores propiedades mecánicas que las espumas microcelulares [START_REF] Schmidt | Templated cross-linking reactions for designing nanoporous materials[END_REF].

S.4 Estructura de la tesis

El trabajo de investigación presentado en esta tesis se ha realizado en el marco de un acuerdo de cotutela entre dos universidades de diferentes países. Además esta tesis se presenta para optar al grado de Doctor con mención Internacional. Por ambos motivos el cuerpo principal de la tesis está redactado en inglés.

Adicionalmente, de acuerdo con las normativas específicas del acuerdo de cotutela se incluyen dos resúmenes de la tesis, uno en francés y otro en español.

Además, esta tesis está escrita como compendio de publicaciones. Incluyendo seis artículos enviados a revistas internacionales (uno de ellos todavía pendiente de ser aceptado) y un proceeding de una comunicación oral en un congreso internacional. Adicionalmente, se han incluido algunos resultados no publicados para proporcionar una mejor comprensión del trabajo y para alcanzar los objetivos previamente definidos. La Tabla 0-1 muestra los artículos incluidos en la tesis, indicando en que capítulo se encuentran.

Otras publicaciones en revistas con proceso de revisión por pares, contribuciones a congresos internacionales (relativas a la tesis u otros campos de investigación), estancias de investigación en otras instituciones, y la participación en proyectos de investigación durante los últimos cuatro años (en ámbitos diferentes a las espumas basadas en PMMA) se muestran en las tablas 0-2, 0-3, 0-4, 0-5 y 0-6, respectivamente.

La tesis está dividida en seis capítulos, incluyendo la siguiente información:

El capítulo 1 introduce las técnicas experimentales utilizadas en este trabajo. Las técnicas convencionales son enumeradas, mientras que las técnicas no convencionales (algunas de ellas desarrolladas dentro de esta investigación) son descritas en detalle.

El capítulo 2 describe los materiales de partida, PMMA y MAM, y las rutas de producción de las mezclas de PMMA/MAM y de las espumas. La morfología de la nanoestructuración de las mezclas sólidas de PMMA/MAM y la estructura celular de las espumas fabricadas son estudiadas y descritas en este capítulo.

El capítulo 3 define las relaciones entre la nanoestructuración de los precursores de PMMA/MAM y la estructura de las espumas obtenidas mediante espumado por disolución de gas (CO 2 ). Además se analizan el proceso de nucleación y la influencia de la presión de saturación y el contenido de MAM en la estructura celular. A partir de estas relaciones se describen los procesos de nucleación y crecimiento de las celdas para estos materiales.

El capítulo 4 proporciona información sobre el papel de la temperatura de saturación y la de postespumado en el proceso de espumado de las mezclas de PMMA/MAM. Determinándose el límite entre procesos de espumado de una y dos etapas, y proporcionándose una descripción de ambos procesos teniendo en cuenta los mecanismos de nucleación de las mezclas de PMMA/MAM previamente descritos.

El capítulo 5 estudia las propiedades de las espumas nanocelulares comparadas con las propiedades de espumas microcelulares de la misma composición química. Este capítulo se divide en tres secciones, correspondientes al estudio de la conductividad térmica, la temperatura de transición vítrea, y el módulo de Young.

El último capítulo recoge las principales conclusiones de este trabajo, e incluye sugerencias para la continuación de esta línea de investigación en el futuro.

Con el ánimo de proporcionar una mejor comprensión de la estructura de la tesis y las relaciones entre los diferentes capítulos se ha incluido el esquema de la Figura 0.5.

S.5 Principales resultados

A continuación se van a introducir las principales conclusiones obtenidas en este trabajo de investigación:

-Las mezclas de PMMA/MAM nanoestructuradas se han mostrado como excelentes materiales para la producción de materiales poliméricos nanocelulares. Siendo estos materiales de partida comerciales y de fácil acceso.

-Las morfologías de la nanoestructuración auto-organizada de las mezclas sólidas de PMMA/MAM dependen del contenido de MAM, mostrando desde objetos micelares, a estructuras tipo gusano o lamelares. Además esta nanoestructuración bien definida se presenta después de los procesos de extrusión e inyección (procesos industrialmente escalables).

-Se han obtenido de manera controlada espumas nanocelulares con dos morfologías diferentes claramente definidas (celda cerrada y celda abierta). El tipo de estructura celular puede elegirse ajustando el contenido del copolímero de bloque en las mezclas de PMMA/MAM.

-Se han obtenido espumas nanocelulares a partir de muestras sólidas de varios milímetros de espesor con tiempos de saturación de 24 horas.

-Se han obtenido espumas nanocelulares con significativas reducciones de densidad (densidades relativas desde 0.4 a 0.6).

-Se han obtenido espumas nanocelulares con presiones de CO 2 no excesivamente altas, ni por medio de velocidades de despresurización extremadamente altas, ni necesitando enfriar la muestra tras la liberación de la presión.

-El proceso de nucleación de las celdas en las mezclas de PMMA/MAM es completamente heterogéneo. Estando controlado por el bloque blando y con mayor afinidad por el CO 2 del MAM. La eficiencia de nucleación (la relación entre el número de núcleos de celdas y el número de nanoestructuras dispersas) es cercana a la unidad.

-El crecimiento de las celdas es controlado por la plastificación de la matriz de PMMA, siendo posible prevenir la coalescencia de las celdas mediante una apropiada selección de las condiciones de procesado.

-Como consecuencia de los dos resultados previos, es posible obtener espumas nanocelulares a partir de mezclas de PMMA/MAM en un amplio rango de presiones de saturación, desde 10 a 30

MPa.

-Se ha encontrado una clara transición entre las espumas microcelulares y nanocelulares de Generally speaking, foams belong to porous materials; the word "foam" is often used to name relatively low density materials (irrespective of the pore size) and is related to the production process or technique (use of an expansion method). But production of foams (i.e. porous/cellular materials) is one approach to improve several properties of known materials, or to increase the range of their potential applications [4].

Indeed if mass gain is both a technical and society concern, porous materials and foams do offer additional properties (thermal, shock absorption, filtration, catalysis, optical, acoustical, etc.).

Nature also used this approach during the evolution of life in Earth, and it is possible to find materials in living organisms (vegetables or animals) with a porous structure, such as wood, cork, sponges or bones of different species [5]. However, reproducing living structures by artificial means is never simple.

Structures of living materials are built in a "bottom-up" way, with the individual living cells organizing themselves, and responding to the external demands. For instance, structure of the human bones (Figure 0.1, left) is organized to provide support to the human body, and can be reinforced if the support demands are increased (e.g. by physical exercise). Nowadays foams can be manufactured from several different materials, such as liquids, polymers, metals, ceramics, etc. [6][7][8][9][10]. Applications of manufactured foams due to their mechanical properties and low relative density are found in packaging, aeronautic industry, automotive industry, shipbuilding industry, wind turbines, etc. [11]. Other important applications of this kind of materials are their use in thermal insulation [12], acoustic insulation [13], and their use as scaffolds [14][15][16] and filtration [17] and catalysis processes due to their structure and high specific surface area [18].

The production of foams has experienced a significant progression with the development of Nanotechnology. This concept was firstly introduced by the Nobel Prize Richard P. Feynman in 1959 in his talk "There's Plenty of Room at the Bottom". As Feynman predicted (and other scientists like Norio Taniguchi, who introduced the term "nano-technology") with the manipulation of matter at an atomic, molecular or supramolecular scale began a new era for Materials Science.

Accordingly, some of the current frontiers in polymer foams research are the addition of nanoparticles (to modify the foaming process or to enhance/modify the structure and properties of the foam) and the production of polymers with cellular structures in the nanometric range at medium to high void fractions (i.e. medium to low densities). Due to the enormous interest of this field, we focus this research work in the production and understanding of polymer nanofoams, using a "bottom-up" approach with the selfassembly of block copolymers as a pattern of the nanocellular structures. This introducing chapter (Chapter 0) is divided into five sections. First is a description of the scientific framework where this work was carried out. Secondly, an overall view of the state of the art on nanocellular polymeric foams is provided. Then, the scope and objectives are described. Also, an explanation about the body of the manuscript, its organization and a global schematic view of the investigation are included. Finally, the main novelties and contributions of this work to the field of nanocellular polymeric foams field are enumerated. Thus the Listrac work was focused in the production of foams from nanostructured polymer-copolymer blends by gas dissolution foaming, using CO 2 as physical blowing agent.

Framework of this thesis

In order to provide an academic, work, neat unformulated (or as little as possible), unfilled, uncharged commodity amorphous polymers as polystyrene (PS) and poly(methyl methacrylate) (PMMA) [START_REF] Ruiz | Production, cellular structure and thermal conductivity of microcellular (methyl methacrylate)-(butyl acrylate)-(methyl methacrylate) triblock copolymers[END_REF][START_REF] Ruiz | Foaming Behaviour and Compressive Properties of Microcellular Nanostructured Polystyrene[END_REF][START_REF] Ruiz | Microcellular foaming of polymethylmethacrylate in a batch supercritical CO2 process: Effect of microstructure on compression behavior[END_REF][START_REF] Ruiz | Two-step micro cellular foaming of amorphous polymers in supercritical CO2[END_REF][START_REF] Ruiz | Micro and nano cellular amorphous polymers (PMMA, PS) in supercritical CO2 assisted by nanostructured CO2-philic block copolymers -One step foaming process[END_REF][START_REF] Reglero | Foaming of amorphous polymers and blends in supercritical CO2: Solubility versus block copolymers addition[END_REF] were chosen because these polymers are homogeneous at the nanometer level Then nanostructuration of these amorphous polymers is coupled, which is well known in the field of polymer blends, compatibilization and reinforcement. The field of self-assembly, nano structuration, nano patterning is well known with di/tri block copolymers and abundant literature exits (we will not give a review on this field in Chapter 0, but a summary can be found in Chapter II). Polymer nanostructuration consists in dispersing a second separated phase at the nanometer level; the phase being ordered at either nano or micro ranges as micelles, core-shells, lamella, and worm-like. The choice is such that the second phase is compatible enough to leave the precursor blend material transparent before foaming.

In this manuscript nanostructuration of PMMA is induced by the addition of a tri block acrylic copolymers with a CO 2 -philic block, with the aim to increase the gas sorption and increase the nucleation ratio of these foams.

Summary of the state of the art

In the present section prior knowledge on the production and properties of nanocellular polymeric foams is briefly presented. Up to date literature on this topic and on other specific topics (image analysis, cell size calculation, thermal conductivity, etc.) can be found in the journal articles included in the corresponding chapters. It is important to mention that there has been a significant evolution of this topic and an increase of the papers published in the last three years.

Nanocellular polymer foams

Cellular materials are two-phase structures composed of a continuous solid/liquid phase and either continuous or discontinuous gaseous phase [4,6,8,9]. As explained before it is possible to find a large variety of materials with a cellular structure (polymers, wood, metals, ceramics, etc.). Cellular structures can be classified according to their topology; on one hand would be the open cell foams, which show continuity of the gas and solid phases, and on the other hand would be the closed cell foams in which the gas is enclosed in the pores (i.e. continuous solid phase and discontinuous gas phase).

Another classification of foams, and in particular of polymer foams, can be done in terms of their average cell size. For instance, conventional polymer foams present cell sizes larger than 100 µm, whereas microcellular polymer foams present cell sizes in the order of 10 µm. Microcellular polymers and corresponding technologies have been under development since the early 1980s [START_REF] Martini-Vvedensky | Microcellular closed cell foams and their method of manufacture[END_REF][START_REF] Kumar | Process Synthesis for Manufacturing Microcellular Thermoplastic Parts[END_REF][START_REF] Kumar | A process for making microcellular thermoplastic parts[END_REF]; these materials show significant improvements of compressive and bending strength, impact strength and fatigue life, etc.; for example, microcellular plastics exhibit higher Charpy impact strength, toughness, fatigue life, thermal stability, light reflectivity, and lower thermal conductivity than unfoamed plastics, and much better mechanical properties than conventional foams [START_REF] Shimbo | Mechanism of Strength Improvement of Foamed Plastics Having Fine Cell[END_REF][START_REF] Nadella | Tensile and Flexural Properties of Solid-State Microcellular Abs Panels[END_REF][START_REF] Kumar | Experimental Characterization of the Tensile Behavior of Microcellular Polycarbonate Foams[END_REF][START_REF] Kumar | The effect of additives on microcellular PVC foams: Part II. Tensile behaviour[END_REF]. Continuing with the classification of polymer foams in terms of their average cell size the next group of foams is the nanocellular polymers, with cell sizes under the micron.

Then, nanocellular polymer foams (i.e. nanocellular polymers produced by foaming processes) (Figure 0.2) are two-phase materials composed by a polymeric matrix and a cellular structure with nanometric pores filled by gas (either individual pores or connected ones). Just as microcellular foams present better properties than conventional foams, it is expected that nanocellular foams will present enhanced properties due to their lower cell size. For instance, they could have better mechanical properties, such as better toughness, higher impact energies, and strain to failure [START_REF] Miller | Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide II. Tensile and impact properties[END_REF]; also, they could present an improved thermal insulation behavior due to the Knudsen effect [START_REF] Schmidt | Templated cross-linking reactions for designing nanoporous materials[END_REF]. More details about the expected properties of nanocellular polymer foams will be provided in the last section of this literature survey.

The terms "nanoporous polymer" and "nanocellular polymer foams" often refer to different materials, and literature surveys give different papers if inquiring both keywords. However among nanocellular polymer foams produced by foaming processes, rather few (a few tens) papers are given in the last 10 years. More papers are found about nanoporous polymers in high T g matrices or biopolymers (e.g. see papers of Antonietti, Hentze, Hillmyer, Hedrick, etc.) and we will not give a complete survey but only the following remarks. Fabrication of thin film nanocellular polymeric materials was often carried out from nanostructured polymers by pattern-transfer techniques [16,[START_REF] Madou | Fundamentals of Microfabrication: The Science of Miniaturization, Second Edition[END_REF][START_REF] Hillmyer | Nanoporous materials from block copolymer precursors[END_REF][START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF] or by the selective removal of one of the blocks in nanostructured block neat copolymers. This route was been employed generally in high-T g polymers/copolymers by thermal degradation of one part of the polymer/co-polymer or in co-polymers by the use of organic solvents to remove one of the co-polymer phases [16,[START_REF] Hillmyer | Nanoporous materials from block copolymer precursors[END_REF][START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF][START_REF] Hedrick | Templating Nanoporosity in Thin-Film Dielectric Insulators[END_REF][START_REF] Hentze | Template synthesis of porous organic polymers[END_REF][START_REF] Jackson | Nanoporous membranes derived from block copolymers: From drug delivery to water filtration[END_REF][START_REF] Hedrick | Nanoporous polyimides[END_REF][START_REF] Lazzari | Block copolymers as a tool for nanomaterial fabrication[END_REF][START_REF] Park | Enabling nanotechnology with self assembled block copolymer patterns[END_REF]. The traditional foaming chemical process (using chemical blowing agents, CBA) is not suitable to our knowledge to provide nanofoams.

Another promising technique used to produce polymeric foams is gas dissolution foaming process and in particular the high-pressure or supercritical CO 2 gas dissolution foaming, where CO 2 is used as a physical blowing agent [START_REF] Olson | Templating Nanoporous Polymers with Ordered Block Copolymers †[END_REF][START_REF] Kazarian | Polymer Processing with Supercritical Fluids[END_REF][START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF][START_REF] Reverchon | Production of controlled polymeric foams by supercritical CO 2[END_REF]. Also, this gas is one of the best options for this kind of process because of its excellent characteristics of diffusion in the supercritical state, and the relatively mild conditions to reach this state (31º C and 7.3 MPa). Last but not least, carbon dioxide is a green solvent that can be removed without residue or production of any pollutant compound [START_REF] Jacobs | Sustainable polymer foaming using high pressure carbon dioxide: a review on fundamentals, processes and applications[END_REF][START_REF] Wells | CO2 technology platform: an important tool for environmental problem solving[END_REF].

Gas dissolution foaming process

Gas dissolution foaming consists of the following steps (Figure 0. molecules the free space between the polymer chains [START_REF] Vieth | A model for diffusion in a glassy polymer[END_REF][START_REF] Crank | The Mathematics of Diffusion1956[END_REF]. This stage usually ends when the polymer sample is completely saturated (i.e. it reaches the maximum amount of gas that can absorb at the pressure and temperature under study).

-Stage II. Desorption Stage: gas pressure is released, then the sample enters into a supersaturated state. The polymer starts to release the excess of gas, either by diffusion to the outside or by the formation of discontinuities/voids inside the matrix. These voids will act as nuclei for the cells formation [START_REF] Goel | Generation of microcellular polymeric foams using supercritical carbon dioxide. I: Effect of pressure and temperature on nucleation[END_REF][START_REF] Colton | Nucleation of Microcellular Foam: Theory and Practice[END_REF].

-Stage III. Foaming Stage: when the saturated sample reaches a temperature over or close to its glass transition temperature (T g ) the nuclei previously formed can grow into cells. Cell growing is promoted by the pressure difference between the gas inside the sample and the external pressure.

As a consequence of the increment of the cell size the sample presents a macroscopic expansion and, consequently, a density reduction. Final cell size is controlled by the magnitude of the pressure inside the sample, and the viscoelastic properties of the polymeric matrix at the foaming temperature (depending on these properties the polymer will show more or less opposition to the cell grown). Other factor that can influence the final cell size is the phenomena of coalescence, whereby several cells can collapse into one single cell [START_REF] Goel | Generation of microcellular polymeric foams using supercritical carbon dioxide. II: Cell growth and skin formation[END_REF].

Furthermore, the plasticization effect of the CO 2 on amorphous polymers decreases the effective value of the glass transition temperature (T g,ef ) [START_REF] Zhang | An in-situ study of plasticization of polymers by high-pressure gases[END_REF][100][101]. The relationships between this effective glass transition temperature and the temperature of the saturation stage allows differentiating two main gas types of processes.

On one hand, when the saturation temperature is higher than the effective glass transition temperature of the polymer (i.e. the polymer is in the rubbery state at the end of the saturation stage) the foaming occurs during the pressure release, being the foaming triggered by the pressure difference. This process is called one-step or batch foaming [START_REF] Ruiz | Microcellular foaming of polymethylmethacrylate in a batch supercritical CO2 process: Effect of microstructure on compression behavior[END_REF][START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF]. In this foaming process the desorption stage disappears, then, after the pressure release the cell nucleation and growth processes take place immediately one after another.

On the other hand, at saturation temperatures lower than the effective glass transition temperature of the polymer (i.e. the polymer is in the glassy state at the end of the saturation stage) the foaming process will be triggered in a second stage by heating the sample over its T g,ef . This process is usually called two-step or solid state foaming [START_REF] Kumar | A process for making microcellular thermoplastic parts[END_REF].

Development of polymer nanocellular foams by gas dissolution foaming requires reaching very high cell nucleation densities (N o , number of nuclei/cells per cubic centimeter of the unfoamed material) and reduced coalescence.

Coalescence can be avoided, or maintained without significant influence, by an appropriate selection of the polymer matrix and selection of the foaming temperature in comparison with the effective glass transition temperature.

Main difficulty of foaming processes to produce nanocellular structures resides in the thermodynamics of the polymer-gas systems. Formation of nanometric cells leads to a huge increment of the interfacial (surface) area and the interfacial (surface) free energy. Moreover, the nucleation mechanisms of the cells are also governed by thermodynamics; in gas dissolution foaming processes the abrupt modification of the process parameters (pressure or temperature) reduces the gas solubility into the polymer, inducing gas phase segregation and a new disposal of the gas into cells that will tend to reduce the free energy of the foaming systems. After the nucleation process, each nucleus should overcome an energy barrier to become into a cell (Gibbs free energy barrier). This energy barrier on homogeneous polymers (i.e. one phase polymers without particles or impurities) depends on the surface tension of the polymer matrix in the presence of CO 2 and the pressure difference between the bubble and the bulk (related to the CO 2 concentration); whereas in heterogeneous polymers (i.e. several phases or with particles or impurities) the dispersed particles or phases can provide potential nucleation sites with lower energy barrier than the surrounding polymeric matrix (more details about cell nucleation mechanisms can be found in the articles included in Chapter III).

Nanocellular polymer foams produced by CO 2 gas dissolution foaming

Several approaches were used to promote the cells nucleation up to the desired levels (typically higher than 10 14 -10 15 nuclei/cm 3 ) using CO 2 gas dissolution foaming processes, depending on the nucleation mechanisms involved.

On homogeneous materials (i.e. systems with a homogeneous nucleation) there are two approaches to promote the nucleation (Figure 0.4) by varying the processing parameters [START_REF] Tsivintzelis | Foaming of polymers with supercritical CO2: An experimental and theoretical study[END_REF][START_REF] Goel | Generation of microcellular polymeric foams using supercritical carbon dioxide. I: Effect of pressure and temperature on nucleation[END_REF]102]. Saturation pressure controls the amount of gas molecules dissolved into the polymer sample, being directly related to the CO 2 uptake. Increasing the amount of gas molecules into the polymer will enhance the probability of aggregation between molecules to produce a nucleus/bubble, and consequently, the nucleation density.

Therefore, a higher nucleation ratio can be promoted by increasing the saturation pressure.

The other processing parameter that can enhance the nucleation is the pressure drop rate during the pressure release. Higher pressure drop rates produce higher thermodynamic instability, and therefore a higher probability of nuclei formation.

Nawaby et al.

[106] study the retrograde behavior of amorphous acrylonitrile-butadiene-styrene copolymers (ABS) and syndiotatic poly(methyl methacrylate) (sPMMA); due to this particular behavior they produced nanocellular foams from these materials at saturation temperatures of 0º C and saturation pressure of 3.4 MPa. ABS foamed at 60º C (after the pressure release) presented a cell size about 470 nm and a relative density of 0.74; whereas PMMA foamed at 90º C (after the pressure release) presented a cell size about 800 nm and a relative density of 0.2.

Gas dissolution foaming

Process parameters

Particles Block copolymers

CO 2
Increase Pressure Increase Pressure Drop rate

Homogeneous nucleation Heterogeneous nucleation

Nucleation agent Nucleation agent

Janani et al.

[107] designed a specific high-pressure system able to produce an instantaneous pressure release (about 300 MPa/sec) and quench the sample just after the pressure release in order to preserve the cellular structured induced by the pressure release (avoiding coalescence). Using this system they produced foams from polystyrene (PS) in a temperature range of 90-180º C and pressure of 25 MPa, obtaining cell sizes near to the nanometric range (between 500 nm and 1 µm) and relative densities about 0.5.

Heterogeneous materials (i.e. systems in which and heterogeneous nucleation takes place) offer other two main approaches to increase the nucleation independently of the processing parameters.

On the one hand, addition of particles to the polymer (such as talc, titanium oxide, kaolin, nanosilica, and other nanoparticles) can increase the nucleation ratio. Particles should be well dispersed to increase the potential nucleation sites (individual particles volumetric density should be of the same order, or higher, than the desired nucleation density). Also, size of the individual particles is a main issue, they should present a size of the same order of magnitude, or higher, than the critical nucleation radius of the polymer-CO Moreover, they concluded that influence of the nanoclays on the nucleation process is small in comparison with the influence of the saturation parameters.

On the other hand, additional nucleation sites or interfaces to promote nucleation can be generated by heterogeneous polymer/polymer or polymer/block copolymer blends (either A-B diblock or A-B-A triblock copolymers) [113]. Well chosen block or random copolymers are readily miscible with their corresponding homopolymer, and can lead to the formation of nanostructures. Low contents of copolymer (< 15 wt%) can self-assemble into a large number of spherical micelles (up to 10 13 -10 14 micelles/cm 3 ) that can act as nucleation sites. In addition, the use of CO 2 -philic blocks can increase the effectiveness of these nanostructures as nucleating agents, becoming CO 2 reservoirs. The main difficulty of this approach lies in the selection and synthesis of the block copolymers.

Spitael et al.

[108] found the first evidences about the influence of block copolymer micelles with a CO 2philic block as nucleation agent with blends of polystyrene (PS) and polystyrene-blockpoly(dimethylsiloxane) diblock copolymer (PS-b-PDMS); however, nucleation increment was not enough to produce nanocellular structures. They suggested that the influence of the block copolymer micelles on the nucleation can be enhanced by using block copolymers that produce micelle cores with higher CO 2philicty and lower surface tensions than the surrounding polymer matrix. Finally, Nemoto et al.

[123] produced nanocellular foams from immiscible blends of poly(ether ether ketone) and poly(ether imide) (PEEK/PEI). They found that cell nucleation and growth were highly controlled in the PEI domains, obtaining cell sizes about 100 nm but without a significant foam macroscopic expansion ratio.

Properties of nanocellular polymer foams

Although it is commonly expected that nanocellular polymer foams will present improved properties than conventional or microcellular foams, there is just little evidence that this may be true. Main limitation to characterize several properties of nanocellular foams is the production of sufficiently large samples that can be subjected to standard test protocols.

Miller and Kumar [START_REF] Miller | Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide II. Tensile and impact properties[END_REF] compared the mechanical properties of microcellular (cells in 2-5 µm range) and nanocellular (cells in the 50-100 nm range) PEI foams produced by CO 2 gas dissolution foaming [105], in a range of relative densities comprised between 0.75 and 0.9. They found that nanofoams showed a significantly higher strain to failure, with an improvement of the modulus of toughness up to 350% compared to microcellular foams. Also, they found a brittle-to-ductile transition in nanofoams, resulting in better response under impact tests, with impact energies up to 600% higher than the microcellular foams. Moreover, they found an increment of the T g of the polymer matrix, related to the confinement of the polymer chains inside the thick cell walls.

No evidences of improved thermal insulation behavior of nanocellular polymer foams can be found in the literature. However, this behavior has been proved on analogous structures. Schmidt et al. [START_REF] Schmidt | Templated cross-linking reactions for designing nanoporous materials[END_REF] measured the thermal conductivity of porous materials obtained by cross-linking reactions, templated either by microemulsions and colloidal particles. These porous materials present open cell structure with cell sizes about 100-200 nm and relative densities lower than 0.15. Contribution of the gaseous phase of was found lower in nanocellular materials than in microcellular ones, due to the Knudsen effect [124], resulting in a lower total thermal conductivity for nanocellular materials.

Finally, it has been speculated that nanofoams produced from amorphous polymers with a well-defined cell structure with cell sizes under the wavelength of the visible radiation could be transparent. In fact, there are strong evidences about this behavior on aerogels; however, production of aerogels with high transparency requires pore/particle sizes of the order of the tenth of the light wavelength (i.e. below 40 nm) and extremely low relative densities (typically below 0.05 g/cm 3 ). These requirements are far away from the actual status on the production of nanofoams, and therefore the production of transparent nanofoams will be extremely difficult [125].

Objectives

This work aims at studying the production and foaming mechanisms of nanocellular polymer foams from blends of PMMA and MAM. This polymer/co-polymer system was not systematically studied before as a foam precursor, and therefore a complete and novel study about the foaming of the PMMA/MAM system has been carried out. Traditionally, there are four main topics on the study of a new cellular material: its production route, its cellular structure, its properties and its potential applications. These topics are strongly interrelated, as the materials tetrahedron of Figure 0.5 shows.

Figure 0.5: Tetrahedron of research areas and their relationships for foamed materials

Our research has been focused on the base of the tetrahedron, trying to produce valuable scientific knowledge about these materials that could allow finding applications. Then, the three main scientific and technical objectives are as follows:

1. To overcome some of the main disadvantages or limitations of the current production strategies used to produce nanocellular polymer foams ("Production").

2. To provide a better understanding of the foaming mechanisms involved in the production of nanocellular foams and to analyze the cellular structure of the produced materials and its relation with the morphology of the precursors used for foaming and the production parameters ("Production  Structure").

3. To study the property transition between microcellular and nanocellular foams ("Properties").

Although the main objectives of this research have been focused on these three areas, some potential applications of these nanocellular foams and the generated knowledge are discussed in the journal articles included in the following chapters. Some partial objectives have been defined inside each of the main objectives to define precisely the scope and extent of each of them.

From the production and technical point of view, we aim at obtaining nanocellular foams with the following requisites:

-Raw polymers should be easily accessible, without a very complex synthesis and preferably being commercial polymers.

-Solid blends should show a self-assembly nanostructuration.

-Production route should be industrially scalable, avoiding requirements as exceptionally high saturation pressures, extremely high pressure release rates, quenching of the sample before/after pressure release, etc.

-Significant density reduction (over 50%) should be obtained keeping the nanocellular structure.

From the scientific point of view, about the foaming mechanisms the work has been focused on answering the following questions:

-Is the nucleation process homogeneous or heterogeneous? (Or a combination of both mechanisms). Can these mechanisms be identified and decoupled?

-Is the nanostructure of solid blends related to the cellular structure of foams?

-What is the role of the soft-block of the MAM copolymer in the nucleation and cell growth processes?

-What is the role of the PMMA matrix in the nucleation and cell growth processes?

-What is the influence of the MAM amount and morphology on PMMA/MAM foams?

-What is the influence of the processing parameters (saturation CO 2 pressure and temperature) on PMMA/MAM foams?

And finally, we have focused part of the research on determining the influence of the reduction of the cellular size from the micrometric to the nanometric range on the following properties:

-Thermal conductivity: It is expected that a significant decrease of the thermal conductivity of the gaseous phase when the pore size falls in the nanometric range, due to an effect known as Knudsen Effect [124].

-Glass transition temperature of the polymer matrix: Glass transition temperature of thin films can increase due to the confinement of the polymer chains [126]. Nanocellular foams can present a similar effect due to the low thickness of their cell walls and edges.

-Young's modulus: In microcellular PMMA foams, enhancement of the mechanical properties with the reduction of the cell size has been described [127]. Also, it is expected that nanocellular foams can present better mechanical properties than microcellular foams [START_REF] Schmidt | Templated cross-linking reactions for designing nanoporous materials[END_REF].

Structure of this thesis

The research presented on this thesis has been done in the frame of a co-tutelle agreement between two universities from different countries. Also this thesis is presented to obtain the degree of Doctor in Philosophy (Ph. D.) with an International Mention. Then, for both reasons the main body of the thesis is written in English. In addition, according to the specific regulations of the co-tutelle agreement a summary of the thesis is included, both in French and Spanish.

This thesis is written as a compendium of publications. Six regular papers sent to international journals (one of them still pending for acceptance) and one proceeding from an oral communication in an international conference have been included. In addition, some unpublished results have been included in several chapters in order to provide a better understanding of the work and reach the objectives previously defined. Table 0-1 summarizes the papers included in this thesis, with their enclosing chapter in the right column.

Other peer-reviewed publications, contributions to international conferences (related to the thesis and on other topics), research stages in other institutions, and participation in research projects during the last four years (out of the direct scope of PMMA blends) are summarized in tables 0-2, 0-3, 0-4, 0-5 and 0-6 respectively.

The manuscript is divided into six chapters including the following information:

Chapter 1 introduces the experimental techniques used in this work. Standard techniques are enumerated, while non-usual techniques (some of them developed or adapted for this research) are further described.

Chapter 2 describes the raw polymers, PMMA and MAM, and the production route of PMMA/MAM solid blends and foams. Nanostructuration of PMMA/MAM solid blends (morphologies) and cellular structure of the obtained foams are studied and described in this chapter.

Chapter 3 defines the relationships between the nanostructuration of PMMA/MAM solid precursors and the cellular structure of foams obtained by CO 2 gas dissolution foaming. In addition, the influence of the saturation pressure and MAM amount on the final cellular structure and nucleation process are discussed.

From these previous relationships nucleation and cell growth mechanisms are described for these materials.

Chapter 4 provides information about the role of the saturation temperature and the post-foaming temperature in the foaming process of PMMA/MAM blends. The boundary between a one-step and a modified two-step foaming processes are determined, and a description of both foaming processes taking into account the nucleation mechanism of PMMA/MAM foams is proposed.

Chapter 5 studies the properties of the nanocellular foams compared to the properties of microcellular foams. This chapter is divided in three sections, corresponding to the study of the thermal conductivity, the study of the glass transition temperature and the study of the Young's modulus.

Last chapter collects the main conclusions of this work, and includes several research topics for future research.

To provide a better understanding of the thesis structure a schematic view can be found in Figure 0.6.

Advantages of amorphous structured blends for nanofoams

To conclude this introduction the major novelties or advantages of amorphous structured blends for the production of nanofoams introduced by this work over the literature are pointing out:

-The initial precursor materials for the production of nanocellular foams are commercial, easily accessible and well defined polymers.

-PMMA/MAM blends present a well-defined nanostructuration after extrusion/injection processes (industrially scalable processes).

-Two different morphologies of nanocellular foams (open and closed structures), are obtained in a controlled way by adjusting the amount of block copolymer from the same PMMA/MAM system.

-Nanocellular foams have been obtained from bulk samples of several millimeters of thickness with saturation times of 24 hours.

-Nanocellular polymer foams with significant density reduction have been produced (relative densities from 0.4 to 0.6).

-Nanocellular foams have been produced with medium-high, but not extremely high saturation pressures and pressure release rates, and without quenching of the polymer before/after the depressurization.

-Cells nucleation process is completely heterogeneous, being controlled by the soft and CO 2 -philic block of the PMMA/MAM blends. Nucleation efficiency (ratio between the number of cell nuclei and the number of dispersed nanostructured) is close to one.

-Cells growth process is completely controlled by the plasticization of the PMMA matrix, being able to avoid the coalescence phenomena with an appropriate selection of the processing conditions.

-As a consequence of the two previous statements, nanocellular foams from PMMA/MAM can be produced in a wide range of saturation pressures, from 10 to 30 MPa.

-A clear transition between the micro and nanocellular foams have been found on the thermal conductivity and glass transition temperature of PMMA/MAM foams.

Chapter I: Experimental techniques

Chapter I. Experimental techniques

Several experimental techniques were used to characterize the solid samples, the foaming process, the CO 2 -polymer interactions, the cellular structure and properties of foams described in this dissertation.

I.1 Non-standard characterization techniques

One of the main challenges found during this work was the selection of appropriate techniques to characterize the solid and foamed samples, and some of their properties. In the present section the techniques used for the characterization of nanostructuration in solid blends (SEM, AFM and TEM), the cellular structure of foamed blends (using a specific software developed in this work), the density distribution of foamed samples (x-ray radiography) and the thermal conductivity of nanocellular foams (TPS) are detailed.

I.1.1 Nanostructuration characterization

As explained in the introduction the main scope of this work is the production of nanocellular foams from nanostructured precursors. Therefore, a detailed characterization of the nanostructuration is essential. This important topic was covered using three techniques: SEM, AFM and TEM. General details about these techniques are well known. Therefore, the following explanations are focused on the specific protocols or operation modes used to study the nanostructuration of our samples.

I.1.1.1 SEM

First attempts to study the nanostructuration of PMMA/MAM blends were carried out by SEM. Injected samples of PMMA/MAM were cut by microtome and then stained with ruthenium tetraoxide (RuO 4 ).

Microtoming was done at room temperature with low PBA amount samples, and at cryogenic conditions with medium-to-high PBA amount samples. Staining protocol is described below.

[1]

-A 2 % solution of RuO 4 is prepared by adding 0.2 g of RuO 4 to 10 ml of a 50/50 mixture between distilled water and concentrated commercial bleach at 9.6 % of active chlorine.

-Thin samples (cut by microtome) were collected on cooper grids.

-4 droplets of RuO 4 solution are placed in a Petri dish, and the cooper grid with the samples is placed nearby.

-Samples are exposed to the RuO 4 solution vapors during 1-2 minutes.

-Samples are rinsed and dried (RuO 4 is neutralized by sodium bisulfate and then rinsed with water), being ready for SEM observations.

Although this staining protocol was developed for TEM observations [1] our first attempts with SEM observations on 90/10 PMMA/MAM blends seemed to show the nanostructuration [2].

However, results obtained by this method presented poor reproducibility and questionable accuracy.

Therefore this technique was discarded in the beginning of this work and other options were studied.

I.1.1.2 AFM

AFM experiences were carried out using a Multimode AFM (Veeco Instruments) in the Instituto of Microelectrónica de Madrid (INM, CSIC) with the help of Dr. Christian Dietz and Dr. Ricardo García.

Identification of polymer phases can be done with the AFM operating in the amplitude modulation mode (tapping mode AFM) [3]. Using AFM in this mode the topography, tip oscillation amplitude, and the phase shift signals are recorded. From this data polymer phases can be well identified in the AFM phase images. These images show a better contrast on polymer blends than the topography images because they detect changes in the energy dissipated by the AFM tip [4].

AFM images were obtained from PMMA/MAM blends on flat surfaces obtained by cutting with a microtome using a glass knife. Microtoming was done at room temperature with low PBA amount samples, and at cryogenic conditions with medium-high PBA amount samples (-50º C). Samples obtained by this procedure present surfaces with differences in height lower than 100 nm, being suitable to the AFM technique.

Main issue using this technique is the correct identification of the polymer phases. Although the better contrast is presented on AFM phase images, the information of the three channels is needed (topography, amplitude and phase) to reach a correct interpretation of the AFM images. AFM phase images from both materials show areas with high contrast (i.e., areas presenting significant different phase shift). Also, in both cases these areas correspond to signals of, apparently, lower height in the topography image. So, to identify if these areas are just voids or a different polymeric phase (a softer one that allows a little indentation of the surface) the information of the amplitude signal is required.

When the AFM tip reaches a void the amplitude is increased (tip loss contact with the sample's surface and then its oscillation amplitude is increased), whereas the phase shift is decreased. On the contrary, when the AFM tip reaches a different polymeric phase, in our case the soft-block PBA, the change on the amplitude signal has the same behavior as the phase signal; both are decreased due to the changes in the tip-sample interactions between the PMMA and the PBA phases. Therefore, high contrast areas of the figure 1 correspond to voids in the samples surfaces, and high contrast areas of the figure 2 correspond to a different polymeric phase.

Accurate identification of the PBA phase on AFM phase images allows measuring some parameters of the nanostructuration. On one side, the area fraction filled by the nanostructures can be determined by image analysis procedures. Contrast between the PBA and PMMA phases allows quantifying independently the area of each phase, and therefore obtaining the nanostructuration area fraction.

On the other side, AFM phase images of blends with dispersed nanodomains allow quantifying the volumetric density of domains (N n , domains/cm 3 ) and the average diameter of the domains. Volumetric density of domains can be obtained using Eq. I-1.

2 / 3       = A n N n [I-1]
Where n is the number of dispersed domains observed in the AFM phase image and A is the area of the image in cm 2 . Average diameter of the domains can be obtained by measuring the diameter of several domains (about 100), or by obtaining the average diameter from the measured area and density of domains (assuming spherical domains).

I.1.3 TEM

Last technique used to study the nanostructuration of PMMA/MAM blends is the transmission electron microscopy. TEM (model Hitachi H7650 with Orius camera) experiences were carried out in the Bordeaux Imaging Center BIC (Université Bordeaux Segalen) with the help of Dr. Etienne Gontier, Dr. Soizic Lacampagne and Dr. Melina Petrel.

In the case of PMMA-MAM blends, two acrylate polymers need to be differentiated, i.e. poly(butyl acrylate) (PBA) from poly(methyl methacrylate) (PMMA), even two different PMMA's might be differentiated (blocks from matrix). TEM imaging of similar polymers is often difficult, phosphotungstic acid (PTA) staining is one solution suggested for organic polymers and copolymers by Harris et al. [5]. Structural features were revealed (core-shell particles, blends, amphiphilic block copolymers).

Phosphotungstic acid (PTA) is a good contrasting staining agent at nanometric levels in polyethylacrylate-polyhydroxyethylacrylate (PEA-PHEA) interpenetrating networks [6] or poly(methyl methacrylate)-co-polyacrylonitrile (PMMA-PAN) composite particles or epoxy-acrylate particles [7].

Generally PTA is known to react with or to graft to chemical functions such as OH, COOH, NH [8]. For example, polyamides (PA) are very well contrasted by PTA in PA-PMMA blends; PA appears dark due to the NH groups while PMMA appears light grey. In PEA-PHEA blends, PHEA is stained due to the OH groups while PEA is not stained by PTA and appears white.

In PMMA-PAN systems, PTA interacts selectively with PAN and PMMA is not stained in the composite particles.

In the epoxy-acrylate latex particles where the acrylate cores are composed of butylacrylate (BA)methylmethacrylate (MMA) -styrene random copolymers, only the epoxy shell is stained. The styreneacrylate core is not affected by PTA.

In sequentially synthesized BA/MMA/epoxy latex particles, the three layered structure is evidenced by PTA showing a black shell, a black epoxy core and a light grey middle MMA layer. In another work, dealing with BA-MMA particles [9] where BA and MMA are homopolymers, only the BA cores are stained by PTA. Authors claim that the solubility (or maybe the differential swelling capacity) difference in water and ester could be the reason for PTA selective staining but no mechanism is explained, and all the explanations remain unclear.

Therefore, PMMA alone does not seem to be stained by PTA whatever the pH conditions (acidic or neutral) [7,9].

For Maiez et al. [10], in PMMA-DMA-PBA BCP's, PBA is white in the presence of RuO 4 while PMMA-DMA copolymer is dark (stained). As a strong oxidant RuO 4 is supposed to bind to carbon double bonds (unsaturated, aromatic compounds) but also to ester, alcohol, amine functions. Thus PBA is said to be stained in certain conditions by RuO 4 [11]. Nevertheless PMMA is not stained neither by RuO 4 (nor OsO 4 ).

In our case, it was checked that classical ruthenium oxide is not suitable to contrast the MAM copolymer neither the PMMA-MAM blends as dense materials (there is a very poor contrast, no distinction of an inner structure).

Dronet et al. [8] and Ruzette et al. [1] show MAM micrographs where PMMA is said to be colored by PTA (grey or black) and PBA appears white. They claim that PTA binds to amide (Dronet), but also to methacrylate ester functions (thus PMMA is grey) while it does not bind to acrylate esters (thus PBA is white).

As a conclusion the selective staining power of PTA seems to depend on reactivity (grafting), solubility, hydrophobicity of the nano structure and maybe on the surrounding phases. Another hypothesis for preferred staining is the preferential swelling of soft materials or soft phases with alkyl groups (such as the poly(butyl acrylate) areas) by benzyl alcohol from the PBA solution, therefore enabling a better staining.

If the contrasting effect of PTA is proved, the precise assessment of the PBA and PMMA phases is sometimes contradictory. Nevertheless, nano structures are always proved (or revealed) by PTA staining.

In our particular situation of two non reactive polyacrylate blends, no chemical reactive functions are available; if our case can be related to some previous cases found in literature, we could expect that PTA staining may contrast PBA selectively and not or little PMMA.

Therefore, the TEM preparation protocol applied on PMMA/MAM samples can be summarized as follows:

-Thin cuts (60-70 nm) of the solid PMMA/MAM samples are obtained by microtome carried out at -50º C with a diamond knife.

-Cuts are deposited on formvar cooper grids and treated by a water solution with 2 % in weight of phospohotungstic acid (PTA) + 2 % in weight of benzyl alcohol during 2 minutes at 60º C.

Benzyl alcohol apparently acts as a dyeing assistant [1].

-Samples are rinsed and dried.

Following this procedure, PTA staining of TEM samples seems to be the best method to determine the nanostructuration of solid PMMA/MAM samples.

TEM micrographs obtained by this procedure allow obtaining the same parameters of the nanostructure than AFM phase images. Fraction area can be obtained in the same way by image analysis procedures, and also the volumetric density and average diameter of dispersed domains. From TEM micrographs the volumetric density of dispersed domains can be calculated using Eq. II-2.

V n N n = [I-2]
Where V the volume of the sample in cm 3 (calculated from the area of the image and the thickness of the TEM sample) and n is the number of core-shell nodules observed in a TEM image. As TEM image is a two-dimensional projection of the three-dimensional structure of the sample some assumptions are needed to estimate the number of nanosized domains. Completely black structures in TEM images are assumed to be core-shell structures completely contained in the sample and counted as units, and structures with a dark shell and gray core are assumed to be core-shell nodules partially contained in the sample, because the gray core is due to the absence of the lower or upper part of the dark shell (not contained in the sample), and counted as halves.

In addition some attempts to determine the nanostructuration of foamed samples were carried out.

Foamed samples were stiffened to allow the cutting by microtome and avoid damage to the nanocellular structure. With this aim, voids were filled by a TEM-contrasting liquid hardening formulation, composed by a crosslinking solution (formulation remains confidential and not publishable). This solution is allowed to fill the voids without solubilization of PMMA, and the overall system gelifies and hardens rapidly (< 30 minutes). After this procedure, the TEM foamed samples were prepared following the same protocol than the solid ones.

I.1.2 Cellular structure characterization

Characterization of the cellular structure is one of the main issues on the study of cellular materials.

Consequently, there are several methods to measure the main parameters such as average cell size, cells anisotropy, cells density, cells nucleation density or cell size distribution.

For nanocellular foams the characterization of the cellular structure is based on micrographs obtained by high resolution scanning electron microscopy. Other techniques such as tomography have not yet been developed enough to characterize these materials (spatial resolution is not as high as needed for these materials).

However, nor the standard neither the theoretical existent methods based on micrographs allow obtaining a complete and quick characterization. Due to this time-consuming manual measurements are usually needed to complete the characterization. It should be noticed that all the results showed in this paper are from microcellular materials. These materials were selected in order to provide broader test conditions to the software (different cell size distributions, number of cells per image, different anisotropies, etc.) and not for their average cell size.

Nonetheless, characterization of the nanocellular structures showed in this dissertation was carried out using this software.

I.1.3 Density homogeneity of foamed samples

Study of the polymer/fraction of voids spatial distribution on polymeric cellular materials can provide valuable information about the foaming mechanisms of these materials. In addition, the presence of structural defects or inhomogeneities can produce a reduction of the expected properties of these materials (e.g. structural defects can reduce the mechanical properties of a cellular material providing starting points for a mechanical failure).

A main characteristic of the cellular materials produced by solid state foaming is the presence of a foamed core and an outer solid skin. This structure appears due to the changes in the gas concentration profile inside the polymer sample during the desorption stage, and the existence of a minimum gas concentration needed to trigger the homogeneous cell nucleation [12]. Characterization of the solid skin thickness is usually carried out by SEM [12], being necessary to extract samples from the studied foam, and to take a large number of SEM images to obtain an average value. Therefore, study of the core-solid skin structure requires a destructive characterization method.

Also, the presence of internal defects or density inhomogeneities is usually studied by destructive Basic fundamentals of this technique and a description of the experimental device used on this research are summarized in the following subsections.

I.1.3.1 X-ray imaging fundamentals

The typical energy of X-rays photons ranges from 100 eV up to 20 MeV (see Figure I.4). Therefore this penetrating radiation seems to be appropriate for obtaining structural information of matter since the distance in between atoms is 10 -10 m approximately. The penetration power of X-rays is rather high and thus it allows for transmission imaging. However, for imaging purposes the useful energies are 5-150 keV, except in some special cases (industrial processes) where 500 keV can be applied. Too soft X-rays do not penetrate deeply enough and too hard X-rays simply are not absorbed leading to poor contrast The attenuation coefficient, µ, also varies with incident radiation wavelengths and energy. It is also important to note that a relationship of the captured intensity (non-absorbed by the material) and the density (ρ) can be established. In the specific case of monochromatic beam and pure element, the attenuation coefficient can be further extended and related to atomic number, Z. High atomic weight (Z) materials such as metals will stop X-ray radiation immediately (either by absorbing, reflecting or dispersing it), whereas materials with light molecular weight made for instance from carbon and hydrogen (polymers) will let almost all light to pass. This is shown in Figure I.6. It is also important to comment Imaging mainly works on absorption phenomena since we have seen that matter attenuates X-ray radiation. Therefore, in transmission imaging, the transmitted radiation is the information captured by the detection system as 2D image (radiography).

I.1.3.3 Microfocus X-ray system at CellMat

The X-ray imaging device used in this work has been recently built in CellMat. It is a valuable tool able to obtain information about polymer foaming processes. Both the X-ray tube and the detector needed careful selection since polymers present low X-rays absorption coefficient and foaming is a process evolving rapidly. In this sense low energies of the emitted X-rays are suitable for imaging polymers although detectors are not conceived for low energies. The high sensitivity of the detector response is thus of extreme importance. Moreover, dimensions of cellular structure are in the order of microns and therefore high resolution is also relevant, thus needed small spot focus size and small pixel size in the detector.

Microfocus X-ray source L10101

A closed air-cooled microfocus X-ray source L10101 (Hamamatsu, Japan) is used to produce the X-rays.

In this tube an ultrafine electron beam produces X-rays, striking on a tungsten target in high vacuum atmosphere. Then the tungsten surface that is irradiated by the electrons produces X-rays. X-rays come out the source through a Beryllium 150 µm-thick window forming an X-ray conic beam of 39º. The microfocus spot size of 5 µm tends to slightly broaden as electron acceleration voltage or current This flat panel detection technology consists in deposited scintillator material CsI, layer directly in front of a CMOS detector. One of its main advantages yields in the small dimensions of the complete high resolution imaging apparatus. The scintillating material is grown in special needle-like shape in order to improve light transmission in the normal direction and thus reduce photons scattering. It is important that the diameter of the grown needled material is smaller than or similar to the CMOS pixel size, i.e. 50 microns for our particular case.

System configuration X-ray tube and FP detector are settled on in front of each other inside a X-ray shielded lead cabinet, as seen in Figure I.9. In principle, it is possible to position them at any distance, SDD, but in general optimum distances are in the range of 0.3 to 1.2 m As the beam is cone-spread the more distant the less radiation the FP-detector receives, which, in principle, is positive for its lifetime but reduces image quality since different artifacts effects increase with distance. In this particular device, the box dimensions limit this distance and it was chosen at 580 mm, becoming a compromise distance for all the involved factors. The objects to be imaged can be placed at any position in between the detector and the source.

The fact of having diverging (cone-beam) X-ray beam allows for object magnification. Magnification factor (M) is a function of the object to source distance (OSD) compared to the total distance from the source to the detector (SDD). Thus magnification is defined as indicated in Eq. I-5.

: Appropriate samples from PMMA and PMMA/MAM foams were obtained using a precision saw (mod. 1000, Isomet) with a thickness about 2 mm. X-ray radiographies from these samples were taken using the following parameters: V = 35 kV, I = 120 µA, and t = 1200 ms (V and I are the voltage and current intensity of the X-ray source, and t is the exposition time of the detector used to take the image). Moreover, a background and brigthfield images were taken with the same parameters (the background is taken with the X-ray source off, while the brigthfield is taken without sample and with the X-ray source on) to be used in the correction of the images previous to their analysis. Density profiles were obtained from the X-ray radiographies using a specific software based on FIJI/ImageJ developed in CellMat Laboratory. Inputs of the software are the average density of the sample (measured independently by Archimedes' method) and the selection of one area of the image filled by the sample, and other area of the image without sample. The software measures the average radiation intensity value from these areas, and relates then with their densities (average density of the sample in the first case, and zero (no sample, just air) in the second case) using expressions derived from Eq. I-3. By this way a calibration between the radiation intensity and the samples density is obtained.

Then, using this calibration the software can calculate the density profile of the area of the sample selected to be measured, as Figure I-3 shows. More details can be found in the previously mentioned paper "Application of a microfocus X-ray imaging apparatus to the study of cellular polymers" [13].

I.1.4 Thermal conductivity of foams

Thermal conductivity of microcellular and nanocellular foams was measured by the Transient Plane Source (TPS) technique using a thermal conductivimeter mod. HDMD (Hotdisk). TPS is a standard technique for thermal properties characterization of different materials (metals, ceramics, polymers, liquids, etc.) [14].

I.1.4.1 An introduction to the TPS method

Measurement of thermal conductivity by means of the TPS method has been demonstrated elsewhere [15][16][17][18][19][20][21]. The basic principle of this method relies on a plane element which acts both as temperature sensor and heat source. This element consists of an electrically conducting pattern of thin nickel foil ( 10 It is important to remark that this is a contact method, so special care has to be taken to minimize thermal contact resistance. The good heat transition through two different materials is mainly associated to contact pressure and surface roughness [22,23].

Moreover, the TPS equipment is able to compensate the heat capacity of the sensor and other thermal delays of the heat flow by introducing a time correction. Nevertheless, it is necessary to suppress the first 20-30 points of each measurement (of a total of 200 points of the heating curve) to eliminate the heat capacity of the sensor that cannot be fully compensated by the time correction [17,18,24].

Finally, to avoid residual temperature drifts on the samples, it is necessary to fix a time spam between individual experiments on each specimen.

I.1.4.2 Experimental procedure

The CellMat TPS device is composed by the TPS sensor, TPS controller unit associated to a computer, a vacuum chamber, rotary vacuum pump (Edwards, mod. Samples preparation and measurement were carried out following the next steps: 1º Samples were studied by X-ray radiography to exclude those foams presenting defects or inhomogeneities that could be an important source of inaccuracy in the thermal conductivity measurements.

2º Samples were polished using a polishing machine (mod. LaboPol2-LaboForce3, Struers) equipped with a silicon carbide grinding paper (P 600) to assure an appropriate contact with the sensor faces and to remove the outer solid or densified skin (if any). After polishing, samples had an average thickness about 5 mm. 3º Samples were dried during 4 h at 50º C. 4º Samples were stored at controlled temperature and humidity (23º C and 50% humidity) at least 3 days before the measurements. 5º Samples were placed in the experimental set-up, inside the vacuum chamber, with a constant pressure between the two pieces. After samples placement the vacuum chamber was closed. 6º Samples and experimental set-up were allowed to reach an equilibrium temperature (experiments were carried out in an environment with controlled temperature) before the beginning of the measurements (typically 30 minutes). 7º Measurement parameters (power and time) were adjusted to ensure accurate results. Typical parameters to measure PMMA-based foams are about 0.006 watt and 50 seconds. 8º Samples were measured five times at atmospheric pressure. Time span to avoid temperature drift was fixed on 5 minutes. 9º After the measurement under atmospheric pressure the rotary vacuum pump was activated.

10º After reaching the desired vacuum conditions, samples and experimental set-up were allowed again to reach an equilibrium temperature before the beginning of the measurements (typically 60 minutes). 11º Measurement parameters (power and time) were adjusted to ensure accurate results. It should be noticed that measurement parameters usually change between atmospheric pressure and vacuum conditions. Parameters used at atmospheric pressure can damage the sample and the sensor due to the lower heat dissipation under vacuum conditions. In general, samples under vacuum conditions require lower power and higher measuring time (e.g., atmospheric pressure: w = 0.006 W and t = 50 s; vacuum: w = 0.005 W and t = 60 s).

12º Samples were measured twelve times at vacuum conditions. Time spam to avoid temperature drift was fixed on 60 minutes. Also these measurements during 12 hours allowed detecting if an evolution of the thermal conductivity occurs (due to a slow and progressive extraction of the gas inside the foam).

13º Thermal conductivities of the samples, both measured at atmospheric pressure and under vacuum conditions, were calculated using the software provided with the TPS device.

Calculations were made selecting the "Standard Analysis" and neglecting the first 25 measured points to eliminate the heat capacity of the sensor. 14º Taking into account the average cell size and density of the measured foams, their total thermal conductivity (λ t ) is contributed only by the thermal conductivity through the solid (λ s ) and gaseous (λ g ) phases (more details can be found in Chapter V). Measurements carried out at atmospheric pressure allow obtaining λ t ; whereas measurements under vacuum conditions provide λ s (assuming that the gas inside the voids of the foam is completely extracted); therefore, λ g can be calculated from the previous values, obtaining by this way a complete characterization of the thermal conductivity of these foams.

Chapter II: PMMA/MAM blends and foams production and consequent nanostructuration and cellular structure

Chapter II. PMMA/MAM blends and foams production and consequent nanostructuration and cellular structure

In this chapter the characteristics of the polymers employed in this work are presented. First, the main properties of PMMA and MAM, and the production route of PMMA/MAM blends are described.

As explained before PMMA/MAM blends present a phase-separated morphology due to self-assembly of incompatible but covalently linked PBA-PMMA blocks (these nano separated regular patterns produce a nanostructuration). For all MAM concentrations, PMMA/MAM blends are transparent and different nanostructures are evidenced. A wide selection of micrographs of the PMMA/MAM morphologies by AFM and TEM are included in this chapter. Morphological considerations about the influence of the MAM amount on the nanostructuration, homogeneity and anisotropy of injected bulk samples, and the influence of the CO 2 saturation process over the nanostructure are discussed.

Second, the production route of PMMA/MAM foams is explained. For all MAM concentrations, PMMA/MAM foams can be produced by gas dissolution foaming at different pressure and temperature conditions. This chapter includes a wide selection of SEM micrographs of the PMMA/MAM cellular structures, analyzing general aspects of the cellular structure such as the homogeneity, anisotropy, and influence of the CO 2 saturation pressure.

Quantification of the nanostructuration characteristics (% of area of nanodomains, density of dispersed nanodomains), a detailed description of the cellular structure (average cell size, porosity, density of cells), and the relationships between the nanostructure and the cellular structure of the foams can be found in Chapter III.

II.1 Materials

This research was carried out using PMMA and MAM as raw materials, both polymers are optically transparent, and were kindly supplied in the form of pellets by Arkema Company (France).

II.1.1 Poly(methyl methacrylate) (PMMA)

PMMA is an amorphous polymer, whose chemical formula is shown on Particular PMMA in our study is a pure homopolymer (without impact modifier), with a density (ρ) of 1.18 g/cm 3 , glass transition temperature (T g ) about 112º C, and the following characteristics M w ≈ 83000 g/mol, M n ≈ 43000 g/mol, I p ≈1.9 (molar mass values are related to polystyrene standards).

II.1.2 Poly(methyl methacrylate)-co-poly(butyl acrylate)-co-poly(methyl methacrylate) (MAM)

MAM copolymers are a family of "ABA" block copolymers synthesized by controlled radical polymerization with a DEPN (N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide) [1,2].

They have a rather high dispersity index (I p around 1.9 to 2.

II.1.3 Literature summary on neat block copolymers (BCP)

All MAM neat (pure) copolymers after a temperature annealing exhibit lamellar structures at PBA contents between 30 and 40 wt% approximately (equal block length) [3,4]; and regular curved lamellars with quite a long range development are characterized, their thickness being around 20 to 30 nm.

At PBA contents around 50 %, the PBA-rich phases are cylindrical, and then become micellar above 60 % of PBA. The equilibrium conditions are considered achieved after 48 h annealing at 180° C on films obtained from evaporation of a good solvent solution. The order-disorder temperature (T ODT , the temperature at which the micro/nano domain structure disappears completely, giving rise to a disordered homogeneous phase) of different MAMs lie well above 120°C [4], i.e. above T g 's of both phases. This temperature is probably overcome during a melt blending process. But MAM was observed to be structured even at certain high processing temperatures (180-200°C); and MAM nanostructures are still observed in bulk injected samples without any particular thermal treatment [2,[4][5][6]. Indeed an advantage of these copolymers is their robustness for self-assembly in several process conditions.

However, if MAM (equal block length, 30-40 wt% PBA) is not annealed, e.g. if the structure is observed directly after injection molding, the structures are poorly defined and worm-like or co-continuous structures are observed on neat MAMs.

In addition, some examples of nanostructuration of polymer/MAM blends can be found in the literature.

II.1.4 Literature summary on blends of block copolymers (BCP + epoxy systems)

Blends based on MAM-like BCP (with dimethylacrylamide in the PMMA block) were studied in epoxyamine monomers [7], they exhibit several nano structures: micelles or core-shell nodules which diameter ranges between 25 and 40 nm at low BCP content (< 15-20 wt%), the shell of these nano structures being the one block soluble with the matrix.

The nano structures are of different types: at high BCP contents, they are lamellar (> 75 % MAM), cylindrical micelles or worm-like structures ("elongated micelles") (≈ 40 % MAM), then spherical micelles (≈ 30 % MAM). The average length of worm-like structures is 100 to 200 nm and their thickness lies between 15 and 25 nm.

After thermal polymerization of the reactive epoxy-amine crosslinking systems, the structuration remains but depends on the polymerization temperature: the lower the temperature the more organized is the nano AFM micrographs allow differentiating dispersed nanodomains on blends with low MAM content (from 5 to 20 wt%). Size of the nanodomains seems to increase proportionally to the MAM amount, being in a range between 20 and 50 nm. Also, the nanodomains of 80/20 PMMA/MAM blend present a more pronounced alignment effect than 90/10. It seems that nanodomains of 80/20 are ellipsoids.

Blends with a high MAM content (50 and 75 wt%) present a worm-like nanostructuration on AFM micrographs, with no clear differences between different MAM amounts. It is also not possible to detect alignment effects. Measured values about the PBA/dark fraction obtained from a MAM surface micrograph should be about bidimensional projection of three-dimensional information. That implies that in samples with a lamellar nanostructuration the micrograph can present higher dark fractions if the nanostructuration is not perpendicular to the sample surface (Figure II.7). Also, in samples with dispersed nanostructuration the apparent PBA/dark fraction will be higher than expected due to the detection on the TEM micrograph of dispersed nanostructures from different planes of the sample, and not only from the surface. As a conclusion the MAM structures in PMMA blends are always present, being well or ill defined, even at very low content (5 wt%), even with a classical processing (e.g. single screw extruder with thermal treatment) the type of structures are rather classical and expected after the processing we used (extrusion + injection molding).

II.4 Morphology analysis of PMMA/MAM dense solid blends by TEM

II.5 Comparison between AFM and TEM results

Characterization of PMMA/MAM's nanostructuration by AFM and TEM will be compared for PMMA/MAM blends with low MAM (5 and 10 wt%), medium MAM (50 wt%) and high MAM contents (75 wt%). By performing this analysis we try to confirm the findings of the previous sections.

II.5.1 Low MAM content:

In

II.5.2 Medium MAM content:

At medium MAM contents both techniques still reveals the nanostructuration (Figure II.17). However, TEM shows the worm-like domains and orientation with a better definition. Also, only on TEM micrographs the inner structure of the nodules can be identified. However, lamellar structures of similar copolymers were studied using AFM in the literature [8]; therefore, this limitation must be due to the samples preparation and not to the AFM technique. Taking into account the samples preparation routes, the main difference that can be found resides in the knife used in the microtoming process. TEM samples were prepared with a diamond knife, while AFM samples were prepared using a glass knife.

As a conclusion, AFM and TEM techniques are appropriated to study the nanostructuration of PMMA/MAM blends with low MAM amounts. Blends with medium-high MAM amounts present better resolution using TEM; however, AFM results could be improved by following an improved sample preparation route probably using a microtome diamond knife.

The main advantage of the TEM technique is the possibility to study the inner structure of the nanostructures. On the other hand, information about the PBA/nanostructuration fraction obtained from AFM micrographs can be directly related to the PBA content of the sample, therefore this technique allows quantification (see Chapter III); while TEM micrographs do not allow studying the PBA/nanostructuration fraction due to the projection of three-dimensional information of the sample on a bidimensional micrograph.

Therefore, both techniques should be used in a complementary way to carry out a complete characterization of the PMMA/MAM nanostructuration.

II.6 Foams production route

Foams from neat PMMA, neat MAM, and PMMA/MAM blends with different MAM amounts (from 5 to 75 wt%) were produced by gas dissolution foaming, using CO 2 as physical blowing agent. Two high pressure vessels were employed in the production of these foams.

The first one is a commercial high pressure vessel provided by TOP Industry (Vaux Le Pénil, France), with a capacity of 300 cm 3 Another high pressure vessel with glass windows self-designed at CellMat Laboratory has been used.

This equipment has additional instrumentation (inner heaters and thermocouples, pressure control by means of a gas booster, control of the pressure release rate, etc.) with a capacity of 500 cm Both pressure vessels allow recording the pressure and temperature evolution during the entire process.

Saturation experiences in the TOP Industry high pressure vessel were carried out at pressures between 10 and 30 MPa, temperatures between room temperature and 70º C, and pressure release rates from 10 to 30 MPa/min. On the other hand, saturation experiences in our self-designed pressure vessel were carried out at room temperature and 7 MPa, while the average pressure release rate ranged from 1.4 to 45 MPa/min.

In both pressure vessels the samples remained at controlled pressure and temperature for 24 h, to ensure that the dissolution of CO 2 into the polymer reaches its equilibrium/maximum value.

Depending on the saturation temperature (see Chapter IV) the cellular structure was triggered during or after the pressure drop. At this saturation conditions samples of neat PMMA or blends with low amounts of MAM (e.g. 95/5) present a similar expansion ratio in every direction, and creep in the gravity direction during the process.

However, blends with medium amounts of MAM (from 10 to 50) present a significant decrease of their length, which is also aligned with the gravity direction, and creeping is not detected. Finally, blends with high amounts of MAM (e.g. 25/75) show neither creep nor significant expansion anisotropy.

These different macroscopic expansion behaviors can be a result of the foaming process or a result of an annealing of the solid samples during the saturation process (saturation annealing). Annealing is a thermally induced process that allows the polymer chains to reorganize them-selves reaching a more energetically favorable organization. Commonly, in amorphous polymers this process is induced increasing the temperature of the polymer up to values near or over the glass transition temperature (T g ).

At these temperatures the polymer chains have an increased mobility that allows the reorganization.

Obviously, the temperature employed during these saturation processes (60º C) is far from the T g of the PMMA (112º C for the particular PMMA grade selected). However, T g depletion induced by the CO 2 sorption can decrease the T g,ef of the PMMA down to values near or below room temperature [10,11]. In addition, swelling induced by the CO 2 sorption increases the mobility of the polymer chains. Therefore, the condition of the saturation process in the foaming tests may induce an annealing process of the PMMA/MAM samples due to the saturation with CO 2 .

PMMA/MAM samples extracted from the pressure vessel after the saturation process at these conditions are already foamed, thus it is no easy to obtain information from these samples about the effect of annealing because foaming also modifies the sample volume.

Then, in order to detect if this behavior is produced by an annealing we carry out thermal annealing processes in PMMA/MAM blends; trying to reproduce this behavior. First attempts were carried out Therefore, we make the hypothesis that the dimensional changes are due to an annealing induced by the CO 2 sorption during the saturation process. Secondly, looking for an explanation of the different evolutions of the samples it is possible to relate each kind of macroscopic evolution with different nanostructurations.

Samples without nanostructuration (neat PMMA) or micelles not aligned or elongated in the injection direction (95/5) are just creeping when the molecules have enough mobility.

Samples with micelles or nodules aligned and elongated in the injection direction (90/10, 80/20, 75/25, 50/50) present a significant decrease of their length and an increase of their width and thickness.

On the contrary samples with a co-continuous nanostructure and no orientation (25/75) do not present changes in their dimensions nor creep.

From these comparative qualitative observations between nanostructuration and macroscopic evolution during annealing we propose the following hypothesis:

During annealing (both thermal and CO 2 annealing) elongated micelles or nodules evolve in the direction of adopting their equilibrium shape (spherical). In this way the total length of the sample is decreased while their width and thickness are increased. Samples with no nanostructuration or spherical micelles

II.7.1 Effect of CO 2 annealing on final morphology

In order to provide a demonstration of this hypothesis the nanostructuration of CO As a conclusion, the nanostructuration presence and its particular morphology modify the macroscopic evolution of PMMA/MAM samples during the saturation and foaming processes at high temperatures has been demonstrated. We believe the process conditions and microstructure affect the dimensional changes upon foaming; we have shown a first benefit of nanostructuration: no macroscopic creep in some structures even above T g,ef .

II.8 Cellular structures of PMMA/MAM foams: general description and evolutions

Neat PMMA, neat MAM, and PMMA/MAM foams were studied by SEM in order to characterize their cellular structure. This section summarizes the typical cellular structures that can be obtained with the materials under study by modifying the amount of MAM in the PMMA and the processing conditions.

We will show the tremendous variety of structures that can be produced (micro and nanocellular, closed and open cell, isotropic and anisotropic, etc.) without trying to explain the origin of each cellular structure produced. These explanations are included in the following chapters.

II.8.1 Presence of solid skin-foamed core structure

In Chapter I it was explained that polymeric foams produced by gas dissolution foaming usually present an outer solid skin and a foamed inner core. SEM observations on some PMMA/MAM samples confirm that these foams can present this structure; however, it is not possible to study the thickness of the skin Taking into account this structure, the study of the cellular structure of PMMA-based foams the SEM micrographs was carried out in the foamed core (i.e. at least 500 µm below the samples surface).

II.8.2 Homogeneity along directions

First topic under study was the homogeneity of the cellular structure along the foamed specimens. With previously explained (i.e. CO 2 saturation annealing). Finally, at 70º C the cellular structure tends to be "semi co-continuous" without homogeneous distribution (i.e. areas with co-continuous cellular structure and areas without cells can be found in these foams; these solid areas could be related to the saturation annealing at these saturation conditions). As a conclusion, we found that the addition to PMMA of various MAM amounts generates a broad range of cellular structures, which are also dependent on the processing conditions; but this latter dependence is very sensitive (or related) to the MAM amount, whereas the processing conditions (i.e. saturation temperature, saturation pressure and foaming temperature) play a much more negligible role in the presence of MAM than in neat PMMA. The tremendous variety of structures produced in this system would be very helpful in the rest of the research, both for the analysis of foaming mechanisms and for the analysis of structure-property relationships.

A further study of these cellular structures, and moreover the relationship between the nanostructuration, cellular structure, and processing parameters is provided in the following chapters.

Chapter III. Relationship between PMMA/MAM blends nanostructures and cellular structures. Foaming mechanisms

In this chapter the cellular structures of foams produced from neat PMMA and PMMA/MAM blends produced by CO 2 gas dissolution foaming are presented. A selection of samples to be foamed was undertaken taking into account the nanostructuration of PMMA/MAM blends (see Chapter II). Thus, PMMA/MAM blends with well-defined dispersed or co-continuous morphologies (micelles -core/shell nodules or lamellas) were selected to produce foams.

From the comparison between the characteristics of the cellular structure of foamed samples and the nanostructure of the solid samples used for foaming we focus the study on understanding the cells nucleation and growth mechanisms. Also the influence of foaming parameters (saturation temperature, saturation pressure, and pressure release drop rate) provides valuable information about the mechanisms involved in the formation of the cellular structure.

Experimental procedures employed to determine these mechanisms, as well as the results obtained and discussion are included in the journal articles included in this chapter that are briefly summarized in the following paragraphs. From the study of the CO 2 sorption during the saturation process it was found that the CO 2 uptake of the samples is increased with the MAM amount. This increment of the CO 2 sorption is related to the higher CO 2 -philicity of PBA, and therefore, the nanostructures formed by PBA enable a CO 2 concentration higher than the PMMA matrix (CO 2 is preferentially located in the PBA phase).

Neat PMMA and PMMA/MAM bulk samples were foamed at different saturation pressures (from 10 to 30 MPa) to study the influence of the saturation pressure on the cellular structure of the foams. Neat PMMA foams present a strong dependence on the saturation pressure: their average cell size and cell nucleation density (N o , nuclei/cm 3 of the unfoamed sample) varying in the studied pressure range from 3.3 µm to 90 nm and from 2.76 x 10 10 to 2.94 x 10 15 cells/cm 3 , respectively. However, PMMA/MAM foams present a constant cell nucleation density (about 4 x 10 14 nuclei/cm 3 , measured in the plane perpendicular to the injection direction), and their average cell size is slightly increased from 150 to 190 nm with the saturation pressure. Those results show that neat PMMA foaming is controlled by a homogeneous cell nucleation, while PMMA/MAM foaming is controlled by a heterogeneous cell nucleation mechanism, the micelles being the agents that control cells nucleation. Moreover, the role of PMMA/MAM micelles as nucleating sites is supported by qualitative theoretical considerations about the nucleation energy barrier (expected to be lower in the PBA micelles than in the PMMA matrix).

The study about the relationship between the PMMA/MAM nanostructuration and cellular structures are Both types of cellular structures present a nucleation process located on the PBA nanostructures, without influence of the CO 2 saturation parameters. Therefore, the saturation parameters are only used to control the global porosity and the average pore size.

It should be noticed that the previous articles mention that the increase in the MAM amount in samples with micelles (5 to 20 wt% of MAM) produces an increment of the micelles average diameter, and not of the micelles volumetric density of the sample. These results suggest that the increment of the MAM amount increase the number of MAM molecules on each micelle (Figure III.1). However, in a micellar system is expected that the size of micelles remains nearly constant above the critical micellar concentration (CMC) in a fluid medium at equilibrium, thus the increment of concentration of the block copolymer should result in an increment of the micelles volumetric density. Therefore, we can assume that the nanostructuration of PMMA/MAM blends is formed out of equilibrium, and the nanostructured domains observed are micellar objects/core-shell created in a viscous liquid. The results of the two previous papers permitted to have a clear understanding of the cellular structure produced in each system and the key foaming mechanism taking place. Therefore, the inner cellular structure could be understood in terms of the concepts and ideas proposed in the two previous papers.

However the foams produced by this process also can present a solid skin.

The last article, published in Defect and Diffusion Forum Vols. 326-328 (2012) and entitled "Solid Skin

Characterization of PMMA/MAM Foams Fabricated by Gas Dissolution Foaming over a Range of

Pressures", is focused in the cells growth mechanism and in the understanding of the mechanisms creating the skin-core morphology of the foams under study.

The density profiles of neat PMMA and PMMA/MAM blends (with MAM amounts from 5 to 10 wt%)

were obtained by X-ray radiography. From these density profiles it was possible to study the thickness and densification of the outer solid skin, a common characteristic of the foams produced by the solid state foaming process.

Neat PMMA foams present thicker solid skins than PMMA/MAM foams; also the skins of neat PMMA are completely densified and present a sharp transition between the foamed core and the solid skin, while PMMA/MAM skins are not completely densified, presenting a progressive densification from the core to the outside of the sample.

This behavior can be expected due to the different nucleation mechanisms of these materials. Neat PMMA, with a homogeneous nucleation mechanism, present typical solid skins, which are produced by the existence of a CO 2 concentration threshold to trigger the nucleation process (Figure III.2). Areas of the neat PMMA samples with lower CO 2 concentration than the threshold at the beginning of the foaming will not present nuclei, and thus no cells will appear on these areas. However, after a time t (time needed for the sample to reach RT after the cooling produced by the pressure release) the concentration profile evolves, and the concentration of outer parts of the sample can fall under this concentration threshold. Nuclei are present on the PBA domains of the entire sample, but in the areas where the PMMA matrix is below their T g,ef the cells growth will be hindered (Figure III.4).

Main difference between both models is that the T g is not a sharp threshold, but a continuous one, below this temperature the polymer chain mobility decreases significantly but some foaming is possible depending on the elastics properties of the polymer. That is why these foams present a progressive decrease of the porosity, but not a complete solid skin.

Chapter IV. Influence of temperature on the foaming processes, mechanisms, and foams structures

In the previous chapter, it was demonstrated that some processing parameters, such as saturation pressure and pressure release rate, have or little influence on the cell nucleation process of PMMA/MAM. Thus foams with a cell nucleation density high enough to produce nanocellular structures can be produced in a wide range of processing conditions. So, the next challenge is to produce nanocellular foams from PMMA/MAM blends with lower relative densities (ρ r ). Due to the constant cell nucleation density of these materials, the key to reduce the density is to understand and to have a better control of the cells growth.

We know that PMMA/MAM blends with low amounts of MAM present a rather low decrease (-10 %) of their density by increasing the MAM amount between 5 and 20 wt%. Also the saturation pressure can produce a decrease (down to -25 %) for 90/10 PMMA/MAM foams produced at 10 and 30 MPa. It was explained that the main factor controlling the cells growth is the plasticization of the PMMA matrix. A possible strategy to increase the cells growth can be to perform the expansion at higher temperatures, so the sample lies in the rubbery state more time during the foaming stage.

Therefore, in this chapter the influence of the temperature on the foaming process and on the foam structure of PMMA/MAM blends will be studied with the goal of reducing the overall density.

First, the influence of the saturation temperature (i.e. the temperature during the saturation process) on the cellular structure and density of neat PMMA, neat MAM and 90/10 PMMA/MAM blends is studied.

Moreover, the foaming behavior of these materials at different temperatures will allow differentiating between foaming process with desorption stage between the pressure release and the foaming (i.e. solid state foaming) and without desorption stage (i.e. batch foaming).

Secondly, the influence of the foaming temperature (i.e. the temperature at which the solid sample is subjected after the pressure release in solid state foaming processes) and the post-foaming temperature (i.e. the temperature at which the foamed sample is subjected after the pressure release in batch foaming processes) on the cellular structure and density of 90/10 PMMA/MAM blends and neat PMMA is determined.

IV.1 Influence of the CO 2 saturation temperature on foaming process, morphology and density of PMMA/MAM foams

The analysis of the influence of the saturation temperature on the foaming process, and the cellular In order to estimate the plastification evolution of these materials, the CO 2 desorption of PMMA, MAM and 90/10 PMMA/MAM blend was measured after the pressure release at 30 MPa/min.

The PMMA and 90/10 systems presented the same behavior: a slow CO 2 diffusion out of the sample, leaving nearly all the CO 2 inside the polymer matrix. On the contrary, MAM alone loses the gas very quickly. PMMA/MAM blends with low amounts of MAM present dispersed nanodomains, thus the CO 2 absorbed both by the PMMA matrix and the PBA domains should diffuse through the PMMA matrix to go out of the sample. Therefore these low content blends will present a similar desorption behavior as neat PMMA. On the contrary, neat MAM and PMMA/MAM blends with medium and high amounts of MAM present "co-continuous or semi co-continuous" PBA domains that allow the CO 2 to diffuse out of the sample mainly through the nanostructures.

Foams were produced at 30 MPa and saturation temperatures between room temperature (RT) and 70º C.

At RT all samples present nanocellular structure (cell sizes below 200 nm) and similar relative density (about 0.45). But when temperature is increased to 40º C, that quasi stable density completely disappears.

Density of PMMA foams remains constant, while 90/10 and MAM foams densities decrease by -10 % and -50 %, respectively. Above this temperature, density of the three materials does not show any more significant variations.

However, the evolution of the average cell size is quite different. Neat PMMA increase its average cell size steadily with the temperature from nanometric to micrometric ranges; cells of 90/10 PMMA/MAM blends remains in the nanometric range (below 300 nm) up to 50º C, increasing to 1 µm due to coalescence at 60º and 70º C; and neat MAM present a sharp change between RT and 40º C, from nanocellular (below 200 nm) to micrometric (about 8 µm), remaining constant over 40º C.

Therefore, only 90/10 PMMA/MAM blends present a significant density reduction (about 10%) keeping the nanocellular structure. Neat PMMA present no significant density reductions, whereas neat MAM foams present a high density reduction but lose their nanocellular structure.

These behaviors are difficult to explain. In the following journal article an hypothesis about the foaming processes of the PMMA/MAM systems is provided taking into account the previous observations (i.e.

important differences between RT and 40º C), the evolution of pressure and temperature during the pressure release, and some observations about the samples state when are extracted from the pressure vessel. Gas dissolution foaming processes, with and without desorption stage, are suggested.

Introduction

Development of nanocellular polymeric materials arouses a significant interest during the last years. Open pores in the nanometric range (20 to 100nm) allows using these materials in micro and nanofiltration processes [1], where different particles can be selectively removed by an accurate control of the pore size distribution and the overall porosity of the material. Furthermore, when the pore size reaches values around the free path of the gas molecules contained in the pores (e.g. ~75 nm in case of air), the conduction contribution of the gas to the thermal conductivity of the material is drastically decreased due to the Knudsen effect [2]. The combination of this effect with a high porosity content allow obtaining materials with very low thermal conductivity [3], with significant applications in thermal insulation industry. Other applications due to the high surface area of these materials is their use as catalyst or as scaffolds.

Nanocellular polymer foams can be produced by different processes. Such materials were obtained by the selective removal of one component in nanostructured polymers [4][5][6][7] or by pattern-transfer techniques [4,8]. However, from a "production" point of view, one of the most promising techniques to obtain nanocellular polymer foams is the high-pressure or supercritical CO 2 gas dissolution foaming, where CO 2 is used as physical blowing agent.

Commonly, the plasticization effect of CO 2 on amorphous polymers decreases the effective value of the polymer glass transition (T g,ef ) [9][10][11]. Considering this, two main types of process can be described, according to effective temperature at the moment of the foam trigger. On one hand, at saturation temperatures higher than the T g,ef (i.e. the polymer is in the rubbery state) the foaming process occurs during the pressure release. This process is usually called one-step or batch foaming [12,13]. On the other hand, when the saturation temperature is lower than the T g,ef (i.e. the polymer is in the glassy state) the foaming process is triggered in a second stage by heating the sample above its T g,ef . This process is called two-step or solid state foaming [14].

First attempts to produce nanocellular foams using CO 2 gas dissolution foaming were carried out in thin films or sheets [15][16][17][18], obtaining high relative densities (>0.4 g/cm 3 ). However, different approaches have appeared in the last years to increase the cell nucleation density, with the aim of fabricating nanocellular foams with medium to low relative densities in bulk polymers (high thickness).

Main issue to produce nanocellular foams is to increase the nucleation ratio to values high enough to produce cells in the nanometric range. Addition of particles that could act as nucleating agents (both common inorganic additives and nanoparticles) has been used to increase the potential nucleation sites in several works [19,20]. Also the use of copolymers, both random copolymers [21] and block copolymers [22][23][24] can promote the nucleation up to values appropriate to produce nanocellular foams. Recently, different approaches have been used with significant results. Nanocellular foams with cell sizes below 100 nm and relative densities from 0.3 to 0.4 were obtained using blends of styrenic and acrylic polymers [25]. Also foams with cell sizes between 40 and 100 nm and relative densities between 0.36 and 0.75 were produced from homogeneous polyetherimide (PEI) [26].

Using the block copolymer approach micro and nanocellular foams can be obtained from blends of PMMA and a tri block copolymer (i.e. Methylmethacrylate-ButylAcrylate-Methylmethacrylate (MAM)) [24,[27][28][29]. Previous results showed that these materials present a decoupling between the cell nucleation and some process parameters such as the saturation pressure and the pressure release rate [24]. This particular behavior allows the production of nanocellular foams even at relative low CO 2 pressures (about 10 MPa, when nanocellular foams production usually requires saturation pressures from 20 to 40 MPa).

Furthermore, the constant nucleation ratio allows to study separately the influence of the processing parameters on other foaming phenomena (cells growth or coalescence).

Up to date no information about the influence of the saturation temperature on the foaming process of these materials has been provided. As these blends show a self-assembled nanostructuration a significant influence is expected on the gas saturation and foaming.

The present study is an investigation about the influence of the saturation temperature and of the final cellular structure. The role of the block copolymer on the high-pressure CO 2 sorption and desorption behavior is studied.

Experimental section

Materials

Poly(methyl methacrylate) (PMMA) and block copolymer (BCP) Methylmethacrylate-ButylAcrylate-Methymethacrylate (MAM) were kindly supplied by Arkema Company (France). Both materials are obtained in the form of pellets, and are optically transparent.

PMMA is a homopolymer (M n = 43000 g/mol, M w = 83000 g/mol) without impact modifier, with a density (ρ) of 1.18 g/cm 3 and glass transition temperature (T g ) of 112º C. The MAM BCP used in our study (M n = 85000 g/mol, M w = 170000 g/mol) presents a density of 1.08 g/cm 3 , and has 36 % wt. of the soft block, poly(butyl acrylate) (PBA) which T g is about -50º C. Production details of this kind of MAM copolymers can be found in the literature [30,31].

Fabrication of bulk solid blends

PMMA/MAM blend with 10% wt. of MAM was produced by extrusion. First, both polymers were dried in vacuum (680 mm Hg) at 80º C during 4 h. A Scamex CE02 single screw extruder (L/D = 28, d = 45 mm), with a temperature profile from 165 to 225º C and screw speed of 60 rpm, was sufficient to generate macroscopically homogeneous transparent blends. Pellets from each blend were obtained using a continuous cutting machine operating at the end of the extrusion line.

Solid samples of neat polymers and PMMA/MAM blends were injection molded. Pellets, both raw materials and blends, were first dried in the same conditions as the extrusion process. Then, dried pellets were injected into bulk pieces (50 x 15 x 3 mm 3 ) using a small scale injection molding machine (DSM Xplore). Process parameters were set at 240º C for the melt temperature, 60º C for the mold temperature and 1 MPa during 8 seconds for the injection pressure. Obtained pieces were optically transparent, both for raw materials and blends, and without injection defects. These bulk specimens were subsequently foamed under different temperature and pressure conditions.

Foaming route

Foamed samples were produced following a gas dissolution foaming technique, i.e a CO 2 saturation step (at fixed pressure and fixed temperature) followed by a foaming step (depressurization step, with or without external heating). Solid samples of both neat materials and PMMA/MAM blends were saturated with high-pressure or supercritical CO 2 using a pressure vessel with temperature control provided by TOP Industrie (France), which has a capacity of 300 cm 3 and can operate up to 40 MPa and 250º C. Pressure was set at 30 MPa (300 bar) during 24 h for every foaming experiment, while different temperatures were fixed between room temperature (RT) and 70º C. The period of 24 h was enough to complete saturation of the gas in the polymer (maximum and homogeneous gas uptake). Then, pressure was quickly released at the (uncontrolled) maximum drop rate allowed for the device considered (i.e. at an average of 30

MPa/min). Pressure and temperature were monitored during the entire process.

Characterization techniques

Sorption of CO 2 on PMMA was studied measuring the CO 2 uptake during saturation. This value was calculated as the percentage of weight increment of the sample due to the CO 2 sorption. Samples were weighted before (m 0 ) and just after the saturation (m 1 ), and the CO 2 uptake was calculated using equation 1.

(

)

100 0 0 1 2 - = m m m uptake CO [1]
Time needed to extract the samples from the pressure vessel after the pressure release, around three minutes, can be a significant source of inaccuracy of these measurements. To determine the deviation of these values from the real CO 2 uptake a study of the CO 2 desorption was carried out by weighting several samples every minute during ten minutes after the samples extraction.

Nanostructuration of solid samples was determined by Transmission Electron Microscopy (TEM, model

Hitachi H7650 with Orius camera) at BIC (Bordeaux Imaging Center, Université Bordeaux Segalen).

TEM observations were performed between 60 to 80 kV, at magnifications from x30000 to x330000. TEM samples were obtained from solid materials using a microtome with diamond knife operating at -50º C. Then, the thin cuts (60-70 nm) were treated by a water solution of 2 % wt. phosphotungstic acid (PTA) and 2 % wt. benzyl alcohol. After 10 minutes at 60º C samples were rinsed 3 times on a drop of distilled water. After this treatment the selective staining of poly(butyl acrylate) by PTA reveals the nanostructuration.

Density of both solids (ρ s ) and foams (ρ f ) were determined by the water-displacement method, based on Archimedes' principle, obtaining an average value from at least three measurements of each sample.

Relative density (ρ r ) of foamed samples was obtained dividing their density (ρ f ) by the solid density (ρ s ).

Due to the presence of an outer solid skin in some samples, density measurements were carried out after removing this skin mechanically.

Micrographs of the cellular structure were taken by Scanning Electron Microscopy (SEM, model Quanta 200FEG, FEI) at magnifications between 10000x and 100000x. Samples were immersed in liquid nitrogen to freeze the cellular structure, and then were fractured to assure that the microstructure remained intact. Fractured surfaces within the cellular structure were coated with gold, using a sputter coater model SC 004 provided by Balzers Union. Characterization of the micrographs was carried out by an own-developed software [32] based on FIJI/ImageJ [32,33]. Main parameters of the cellular structure were obtained, such as the average cell size (Φ) and the volumetric cell nucleation density (N o ), defined as the number of cells nucleated (or nuclei) per cubic centimeter of the unfoamed sample) [14]. The volumetric cell nucleation density is calculated from the volumetric cell density (N v , defined as the number of cells per cubic centimeter of the foamed sample) taking into account the expansion of the foam. This calculation only provides accurate values when there is no coalescence between cells. In this work this magnitude will be calculated both in foams with and without coalescence effects with the aim to compare the values and then to estimate the magnitude of the coalescence. Therefore, it will be called as apparent cell nucleation density.

Glass transition temperature (T g ) of neat materials and blends were obtained by Differential Scanning Calorimetry (DSC, model DSC 862, Mettler). The T g was measured at the second heating, from 20º C to 160º C with the heating rate of 10º C/min in a N 2 environment (flow rate 60 ml/min).

Results and discussion

CO 2 sorption and desorption behavior

Several saturation tests with neat PMMA and MAM, and 90/10 PMMA/MAM blend were performed at 30 MPa and at RT. Desorption behavior after the depressurization stage was measured as a function of time for at least two samples of each material with identical results. Typical desorption curves are shown in Figure 1, where time is measured from the pressure release (30 MPa/min). Samples were dense unfoamed materials. A simple estimation of the CO 2 uptake at the saturation conditions can be obtained by a linear extrapolation to t = 0 s of the first part of the desorption curves which is almost linear. Following this procedure, a small deviation of the CO 2 uptake measurements from the estimated values by extrapolation was found (around a 5% over the total amount of CO 2 uptake) for PMMA and 90/10 samples. Due to the good accuracy of the simple measurement, this value was used in the following discussions for samples with low MAM contents. However, neat MAM presents a different desorption behavior (see Figure 1), and the deviation of the measurements can be higher than 70-90%. MAM has much faster desorption kinetics. Depending upon the type of phase separated structure, the PBA desorption is more of less constrained. The phase separation of the PBA phase is either a core/shell or micellar object or a lamellar structure (i.e. blend nanostructuration), which is presented in the TEM images of Figure 2. The lamellar structure of this block copolymer allows a higher CO 2 diffusivity through the soft block phase. Also it was checked that blends with high MAM contents (50 to 75 % wt., results not included in this paper) present a similar behavior as neat MAM. As a consequence CO 2 uptake in pure MAM cannot be measured within 3 minutes because during this time there is a strong desorption. Thus MAM bulk neat copolymer is discarded to further analysis of the CO 2 sorption and desorption behavior but kept for the cellular structure analysis.

In previous works [27,29] lower values of the CO 2 uptake of PMMA and 90/10 samples have been obtained due to the different pressure release rates employed ( ≤ 15 MPa/min), being the measurements taken at longer times from the initial pressure release and might be consequently less quantitative. Also estimations of neat MAM CO 2 uptake were provided, however the present study about the CO 2 desorption behavior of MAM shows that these estimations could be not accurate.

In contrast, the dispersed objects of 90/10 PMMA/MAM (Figure 2) allows the PMMA matrix to control the CO 2 desorption, and consequently PMMA/MAM blends with dispersed nanostructuration present a similar desorption behavior than neat PMMA. Influence of the saturation temperature on the sorption behavior of PMMA and 90/10 PMMA/MAM was studied at a saturation pressure of 30 MPa. Figure 3 shows the CO 2 uptake at temperatures between RT and 70º C. As expected, the CO 2 sorption is decreased with the increase of temperature [34]. It should be noticed that samples produced over 40º C are already foamed at the moment of the measurement.

However, these samples present a similar desorption behavior as un-foamed samples (produced at RT and measured before the foaming process starts) due to their closed cell structure (gas should diffuse trough the PMMA matrix to get out of the sample). Therefore, accurate measurements of the CO 2 uptake in these samples can be carried out. In spite of a faster desorption (Figure 3) shows that the addition of MAM increases the CO 2 uptake up to 60º C. At higher temperatures the assumption of an equal diffusion behavior between neat PMMA and 90/10 PMMA/MAM blends starts to fail. CO 2 diffusion starts to be too fast in blends to provide accurate results with the measurement method employed.

These results, showing an increase of the CO 2 uptake due to the addition of MAM in a temperature range up to 70º C are in good agreement with a concomitant study of the CO 2 sorption of neat PMMA and PMMA/MAM blends where saturation pressure is varied in a range of pressures (from 10 to 30 MPa) [24]. Values for CO 2 /polymer cloud points found in the work of Rindfleisch et al. [35] on PBA and PMMA although of different molar masses are quite consistent with our measurements.

So independently of the foaming conditions (pressure or temperature) the CO 2 uptake increases with the amount of MAM added to the blends, due to the higher CO 2 solubility in PBA and its higher swelling compared to PMMA.

Influence of the saturation temperature on the foam properties

Foamed samples of neat PMMA and MAM, and the 90/10 PMMA/MAM blend, have been produced to study the influence of the saturation temperature on the relative density and the cellular structure of the foams. Saturation pressure was set at 30 MPa, with different saturation temperatures from room temperature (RT) to 70º C.

First, it is noticed a significant difference between foams produced at room temperature and foams produced at temperatures over 40º C. After saturation stages carried out at room temperature the samples were extracted in a solid state from the pressure vessel (samples remain transparent), and start to foam a few minutes after the extraction (increasing their volume and becoming opaque). On the contrary, after saturation stages carried out at temperatures over 40º C the samples were extracted already foamed.

Therefore, it can be expected important differences between the properties of the foams produced at room temperature and the foams produced above 40º C.

Figure 4 presents the evolution of the relative density with saturation temperature. As it can be seen, at room temperature the three materials present practically the same relative density. When temperature is increasing up to 40º C a sharp change is found, neat PMMA presents no changes in its relative density, however relative density of 90/10 PMMA/MAM blend and neat MAM decrease by about 10% and 50% respectively. Over 40º C relative density does not present significant changes as a function of the saturation temperature, with only a small reduction in PMMA and 90/10 PMMA/MAM foams as temperature increases. Expansion ratio of neat MAM foams is higher than that of neat PMMA ones, as expected due the lower T g and the higher CO 2 sorption of the PBA. Addition of MAM to PMMA also decreases the foams density. At low amounts of MAM (like 90/10) this reduction can be explained by the higher CO 2 sorption of the PBA micelles which act as reservoirs [29].

Influence of the saturation temperature on the cellular structure has been studied over SEM micrographs of the foams. Some of the most representative micrographs are shown in Figure 5. Focusing on PMMA and 90/10 PMMA/MAM foams, there is no obvious correlation between their variation of the cell size (figure 5) and density (figure 4). Increment of the cell size without a reduction of the density when the processing temperature is increased can be explained by two phenomena: the cell nucleation ratio can decrease due to the reduction of the CO 2 uptake and the coalescence of the cells due to the lower stiffness of the cell walls. In figure 6, 90/10 PMMA/MAM curve is not in between PMMA and MAM curves, this non linearity is a sign of the role of nano dispersed reservoirs acted by PBA objects.

As explained before, 90/10 PMMA/MAM blend presents a constant nucleation ratio independently of the processing conditions [24]. This constant nucleation ratio is controlled by the micelles volumetric density (about 4 x 10 14 micelles/cm 3 ). Therefore, by studying the evolution of the apparent cell nucleation density of these foams it will be possible to identify the presence, or not, of coalescence. Figure 7 shows the evolution of the apparent cell nucleation density of neat PMMA and 90/10 PMMA/MAM foams. Cell nucleation density of 90/10 PMMA/MAM foams produced at RT (N o = 4.17 x 10 14 ) corresponds to the micelles density of this materials (heterogeneous cell nucleation), then, no coalescence happens at this temperature and this value can be taken as a reference value. Based on this reference it is possible to estimate the magnitude of the coalescence as the temperature is increased. At temperatures of 40º and 50º

C coalescence has a small influence, with an average value of 1.7 and 3.5 cells becoming one, respectively. However, at higher temperatures coalescence starts to have a strong effect, with more than 100 cells becoming one at temperatures of 60º and 70º C. Growing of the cells and therefore coalescence are controlled mainly by the properties of the PMMA matrix of the PMMA/MAM blends. A similar behavior can be expected in neat PMMA foams, with the supplementary effect of a homogeneous nucleation ratio variation depending upon the saturation temperature.

It can be concluded that 90/10 PMMA/MAM foams presents a significant thermal stability between saturation temperatures from room temperature to 50º C, allowing to obtain nanocellular foams with cell sizes below 300 nm in this range of temperatures. On the other hand, nanocellular foams can be produced from neat PMMA and MAM foams just at room temperature.

Foaming mechanisms of PMMA/MAM blends

According to the previous results it is possible to differentiate two foaming mechanisms for the neat PMMA and MAM, and PMMA/MAM blends in the range of temperatures studied. In the first one, with the saturation process carried out at room temperature, samples started to foam after being extracted of the pressure vessel. In the second one, with saturation temperatures over 40º C, samples foamed before being extracted of the pressure vessel. In addition, this variation of the foaming process involves important changes on the foams structure. Then, an explanation about this threshold in the foaming processes and schemes of the complete foaming process of PMMA/MAM blends, taking into account their particular nucleation behavior, is as follows.

Figure 8 shows the evolution of pressure and temperature during the depressurization process when the initial saturation temperature is either RT (23°C) or 40° C with an initial saturation pressure of 30 MPa.

At these high pressure drop ratios the temperature of the vessel decreases during the process, and can reach values down to -30º C (from RT) or -10º C (from 40º C), eventually inducing vitrification of the material or even of some part of the micro phase separated system. Under CO 2 pressure, PMMA can be in the rubbery state at room temperature, even at saturation pressures lower than 30 MPa [15,[36][37][38], so it is expected that foaming takes place during the pressure release at saturation temperatures equal or higher to room temperature (batch foaming process). However, depending on the pressure release rates and the saturation temperatures used, the cooling of the system may limit foaming due to vitrification and quenching of cell growth (see Fig. 8 for temperature evolution).

An equivalent high pressure drop rate is used for all blends saturated at the same pressure (pressure is completely released in about one minute), but depending on the saturation temperature, samples may remain transparent (T sat ≤ RT) when they are taken out of the pressure vessel, keeping nearly "all the CO 2 inside" and they will be foamed in a second stage by increasing the temperature. On the other hand, when the saturation temperature is higher (T sat ≥ 40°C) the foaming occurs during the pressure release. The temperature at which samples start to foam is not known, but this foaming is probably triggered near room temperature.

In both cases, it is expected that the cell nucleation, located in the micelles [24], occurs during the pressure release. Thus, the main difference between the two foaming processes lies in the cell growing controlled by the plasticization of the PMMA matrix [39]. Nuclei of samples saturated at room temperature cannot grow into cells until the samples reach an appropriate temperature (after the cooling due to the pressure release, some time is necessary after releasing pressure). On the other hand, nuclei in samples saturated at higher temperatures can grow just after, or during, the pressure release. Finally, both foaming processes are stopped when the CO 2 desorption from the polymer matrix increases the T g,ef of the sample above the room temperature. Both mechanisms are shown in Figure 9. Presence of a desorption stage in the foaming process at room temperature explains the presence of solid skins in neat PMMA foams and the semi-solid skins in 90/10 PMMA/MAM foams found in previous work [39].

Foaming carried out at temperatures between RT and 40º C presents an intermediate behavior.

Conclusions

Nanocellular and microcellular polymers from neat PMMA, neat MAM and a 90/10 PMMA/MAM blend have been produced by means of high-pressure CO 2 saturation process. The influence of the saturation temperature has been studied, allowing the description of two types of gas dissolution foaming processes in the range of temperatures studied. Significant differences in the foam structures were found between both processes. On one hand, foaming experiments carried out at room temperature allows obtaining nanocellular foams with different pore connectivity, open or semi-open cell structures are obtained from neat polymers, while well defined closed cell structures are obtained from the 90/10 PMMA/MAM blend.

On the other hand, higher saturation temperatures produce microcellular foams from neat polymers and allow studying coalescence in 90/10 PMMA/MAM foams; and shows that cells of these foams hardly collapse, remaining nanocellular and with a small density variation from RT to 50º C. Moreover, the morphology of MAM based-foams showed remarkable thermal cell size and morphology stability from 40º to 70º C, with no cell size or density variations. On the other hand, PMMA present a high sensibility of their cell size upon saturation temperature on their cell size, but not on the density.

The role of PBA chains in the CO 2 sorption and desorption behavior of PMMA/MAM blends has been determined, leading to a better understanding of the foaming process. It was found that nanostructuration of the PBA block affects the CO 2 diffusion in PMMA/MAM blends. Co-continuous structures of PBA allow a higher diffusivity of the CO 2 out of the sample. On the other hand, isolated nanodomains of PBA surrounded by the PMMA matrix increase the CO 2 uptake without influence on the diffusivity.

IV.2 Influence of the foaming temperature and post-foaming on the morphology and density of PMMA/MAM foams

The study of the influence of the foaming temperature on the properties of PMMA/MAM foams was carried out with samples that hold nearly all the CO 2 inside after the pressure release. Thus, samples of 90/10 and 80/20 PMMA/MAM foams were selected.

In order to obtain saturated samples with high levels of CO 2 uptake the samples were saturated at the maximum pressure (30 MPa) and low temperatures (between RT and 50º C). The foaming or postfoamimg temperature (in a second step) was controlled with a thermostatic water bath, in which the samples were introduced after being extracted of the pressure vessel (about 3 minutes from the beginning of the pressure release).

First, the foaming stage was carried out on samples saturated at RT (presenting desorption stage) at temperatures between 60º and 90º C, during times in general between 60 and 180 seconds. Samples foamed at 70º C were studied also at foaming times of 15 and 30 seconds. The maximum foaming time, 180 seconds, was fixed due to the absence of gas diffusion out of the sample at this time. At lower times gas bubbles appears on the water bath from the gas diffusion out of the sample; therefore, the absence of bubbles/gas diffusion implies that no more gas is contained inside the sample, and the cells growth process has finished.

Density evolution of the 90/10 PMMA/MAM samples with the foaming temperature and time is shown in found that no density reductions are obtained at any conditions. Best results, just a small density increase, are found for the lower temperatures (60º and 70º C) at times between 60 and 120 seconds. In general, the density values (0.44 to 0.55) present small differences between adjacent temperatures and times. These small differences may be within the experimental error, and then it is difficult to obtain clear conclusions from these results. However, it seems that the lower densities can be obtained using the lower foaming temperature (and low-intermediate foaming times). More experiments at lower temperatures (below 60º C) should be done trying to find better foaming conditions, which allow decreasing the density.

IV.2.1 Post-foaming vs. two-step foaming

Previous results were obtained by a foaming process -saturation at RT-with a desorption stage (as the solid state foaming process, see Chapter III), therefore the samples were extracted from the pressure vessel in solid state and foamed at the selected temperatures and times.

On the contrary, samples saturated at higher temperatures (40º and 50º C) do not present a desorption stage (as the batch foaming process, see Chapter III); and the foaming stage happens just after the pressure release, being the samples already foamed when they are extracted from the pressure vessel. In this case, some samples were nevertheless subjected to a post-foaming step into the thermostatic water bath to establish some initial trends about the possibility of reducing the density by this method.

Samples of 90/10 and 80/20 PMMA/MAM blends were produced at a saturation pressure of 30 MPa and saturation temperatures of 40º C and 50º C. Then, the post-foaming stage was carried out at 70º C during 120 seconds. Foams produced from 80/20 present a rather significant density reduction due to the post-foaming (about 10-20 %); even so, density of the post-foamed foams is higher than the density of the 80/20 foams produced with the saturation stage carried out at RT. Therefore, this strategy is not able to produce 80/20 foams with lower relative densities than those produced by saturating at RT.

On the contrary, small density reductions (about a 5%) are found in 90/10 samples saturated at 40º C and post-foamed. While no differences between post-foamed and non post-foamed samples can be found in 90/10 samples saturated at 50º C.

Then, the best result obtained by post-foaming was a 5% density reduction on 90/10 PMMA/MAM nanocellular foams. Taking into account the results of the previous study, maybe lower post-foaming temperatures and/or times can produce lower densities.

IV.2.2 Comparison of PMMA and PMMA/MAM post-foaming

In order to understand the difficulties found in the reduction of the density of nanocellular PMMA/MAM foams, neat PMMA was subjected to the same processing conditions as the lowest density previously Along with the increase in the average cell size it is also observed that the cell size distribution becomes wider, resulting in a less homogeneous cellular structure. Moreover, their apparent cell nucleation density decreases 23 times from 5.21 x 10 12 to 2.31 x 10 11 nuclei/cm 3 (N o , cells nuclei per cubic centimeter of the unfoamed material; see the journal article included in this chapter for more details). Therefore, due to the coalescence 23 cells converge in one on average during the post-foaming. In addition to the coalescence the cells should increase their volume about 3 times more to reach their final volume. This cell volume increment corresponds to the macroscopic relative density reduction of the foam from 0.47 to 0.21.

On the other hand, 90/10 PMMA/MAM foams presents small changes. Their average cell size is increased by the post-foaming from 210 to 220 nm, while their cell nucleation density remains practically constant from 3.03 x 10 14 to 2.71 x 10 14 nuclei/cm 3 . Thus, no significant influence of the post-foaming process on the cell size distribution can be found on these foams (Figure IV.5). 90/10 foams present the advantage that no coalescence is induced by the post-foaming. However, also no significant expansion is obtained; the average cell volume is only increased 1.15 times, corresponding to the small relative density reduction found in these foams.

IV.3 Conclusions

As a conclusion of this chapter, we found that the cellular structure of neat PMMA and MAM foams present a clear threshold between RT and 40º C as saturation temperatures. Both materials presented a nanocellular structure when the saturation stage was carried out at RT, whereas at higher temperatures microcellular structures were obtained. In addition, the density of neat PMMA foams remained rather constant for the entire range of saturation temperatures studied, while the density of neat MAM foams presents once again a threshold between RT and 40º C as saturation temperatures. Neat MAM foams presented relative densities about 0.45 at RT and about 0.2 at temperatures over 40º C.

On the contrary, we have observed that there is a small effect of the saturation and foaming temperatures of PMMA/MAM blends with low MAM amounts on the final foam density. Also there is a small incidence of coalescence for these foams, which is a consequence of the nanocellular structure. Variation of the saturation temperature only provides a density decrease of about 10 % keeping the nanocellular structure, whereas no density decrease is obtained by modifying the foaming temperature in the range studied, and post-foaming process can decrease the density just a 5 % keeping the nanocells. Foaming and post-foaming parameters could be improved, but the results obtained comparing the post-foaming effects on 90/10 PMMA/MAM nanocellular foams with the effects on neat PMMA microcellular foams show that, even optimizing these parameters, no significant density reductions can be expected, due to the quick CO 2 diffusion out of the thick cell walls of nanocellular foams.

A possible strategy to avoid early CO 2 diffusion could be carrying out a foaming or post-foaming stage in a CO 2 -rich environment. For instance, after the saturation stage at 30 MPa the pressure release can be stopped at 5-10 MPa. Pressure difference between the sample and the environment may be enough to induce the cell nucleation on the PBA domains and promote the cells growth. Then, the temperature could be increased to a post-foaming temperature and the remaining pressure released to promote cell growth.

By selecting an appropriate combination of the remaining CO 2 pressure and the post-foaming temperature (inside the pressure vessel, just after the pressure release) the PMMA matrix could be maintained in the rubbery state, and perhaps nanocellular foams with lower densities could be obtained.

Chapter V. Transition between microcellular and nanocellular foams; consequences on mechanical and thermal properties

As explained in the introduction chapter it is expected that nanocellular foams will present improved properties compared to conventional ones [1][2][3][4][5]. Although the main focus of this research work is to understand the phenomena leading to nanocellular PMMA-based structures induced by block copolymer and gas dissolution foaming, a study was also performed on the evolution of some properties when the cells are modified from the micrometric to the nanometric scale. In particular, the system under study, PMMA/MAM blends, is very convenient for this purpose because as we have seen in the previous chapters by modifying the processing conditions it is possible to produce nanocellular and microcellular materials with similar composition and density.

In particular, the influence of the cell size is presented on the following characteristics i) the Young's modulus (measured by a three-point bending DMA test), ii) glass transition temperature (T g ) of the polymer matrix, and iii) thermal conductivity of the foams. Focusing on the properties, the article suggests that the low thickness of the pore walls, near or below 100 nm, leads to a reduction of the PMMA chains mobility; resulting in an increment of T g of the PMMA/MAM nanocellular foams in comparison with the solid material.

Earlier indications of this influence on the

Next, a mechanical characterization is presented. 90/10 PMMA/MAM foams are studied by DMA threepoint bending, to measure their Young's modulus (E). It is shown that 90/10 PMMA/MAM foams present higher values of E than experimental results for conventional PMMA foams from the literature [3].

Both studies, about T g and E, are continued and extended in sections V.1 and V.2 of chapter V, after the included article. In addition, results about the thermal conductivity are included in section V.3.

V.1 Young's Modulus

In order to study some mechanical properties neat PMMA foams and 90/10 PMMA/MAM blends foams were produced at different processing conditions (Table V-1). Four measurements were carried out for each foam, using a three-point bending set-up in a Dynamic Mechanical Analyzer (Perkin-Elmer, Mod. DMA-7), to determine the Young's modulus (E) at room temperature. Samples for DMA tests were prepared using a precision saw (mod. 1000, Isomet), with dimensions 15 x 2 x 2 mm 3 . DMA experiments were carried out at room temperature, using a static force of 1293 mN and a dynamic force of 1077 mN with a frequency of 1 Hz. Mechanical properties of foams are strongly related to the apparent macroscopic density. In particular, the Young's modulus of foams (E f ) can be estimated from the value of the solid precursor (E s ) using the following expression (Equation V-1) [6].

𝐸 𝑓 = 𝐸 𝑠 • 𝜌 𝑟 𝑛 [V-1]
Where ρ r is the relative density of foam and the exponent n takes values between 1 and 2. Solid neat In the previous chapter it was showed that the high structural stability of 90/10 PMMA/MAM avoids coalescence, and their controlled nucleation process results in cellular structures with higher homogeneity than that of neat PMMA foams. Therefore, the better mechanical behavior of 90/10 PMMA/MAM foams could be related to the homogeneity of their cellular structure.

In addition, cells of 90/10 PMMA/MAM foams present anisotropy, with the cells elongated along the injection direction. It is know that the anisotropy of the cells has a great influence on the Young's modulus. Foams with anisotropic cells present enhanced values of E in the direction of the long axis of the cells; on the contrary, values of E measured in a direction perpendicular to the long axis of the cells are significantly lower than the expected [6].

In this study, determination of E on 90/10 PMMA/MAM foams was carried out in a direction perpendicular to the long axis of their cells (dimensions of PMMA/MAM foamed samples did not allow preparing samples to measure E in the direction of the long axis of the cells with appropriate dimensions to our DMA measurement system); therefore it can be expected that these foams present higher values of E in the direction of the long axis of their cells, whereas neat PMMA foams present an isotropic cellular structures and their E should be the same independently of the measured direction.

As the previous results show a quadratic dependence of the Young's modulus with the relative density, No relationship between the cell size and the normalized Young's modulus is found. This result is in good agreement with previous works, which shown that cell size (in the microcellular range) has no influence on the Young's modulus [7,8].

Therefore the Young's modulus of 90/10 PMMA/MAM foams seems to depend on the relative density and also on other structural characteristics such as the homogeneity of the cellular structure, anisotropy, etc., but not on the cell size. Because of the narrow range of cell sizes under study, we cannot make a definitive conclusion on this topic. Optimization of the post-foaming processes should allow producing PMMA/MAM foams with higher cell sizes, which would be appropriate to extend this study in the future.

Also, characterization of other mechanical properties and the study of the influence of the anisotropy of the cells on these properties could provide valuable information about the advantages of nanocellular foams.

V.2 Glass Transition Temperature

Increment of the T g of 90/10 PMMA/MAM nanocellular foams is first related to the confinement of the PMMA chains inside the walls of the cells when they fall into the nanometric range. This confinement effect has been widely studied in the literature on films of amorphous polymers such as PMMA or polystyrene (PS) [9][10][11][12][13][14], and some evidences has been found on PEI nanofoams [5]. Results from the literature demonstrate the existence of this confinement effect, but rather few explanations about its phenomenon are provided. We give arguments coherent with our system and some literature data and interpretation, but there also are contradictory arguments in literature so we stay at the hypothesis state.

Some studies related the increment of T g to the reduction of the free volume/polymer chains mobility [9,10], and the existence of hydrogen bonds [10]. With the aim to estimate the range of dimensions (for the confinement) which can influence the polymer chain mobility of PMMA, we calculated the theoretical length of a PMMA chain (L, assuming the chain completely stretched). This length can be estimated from an average degree of polymerization of the PMMA chain (X n = M n /M o , where M n = 43000 g/mol is the number average molecular mass of our PMMA and M o = 100.12 g/mol is the molecular mass of the

𝐿 = 𝑋 𝑛 • 𝑁 • 𝑑 • 𝑠𝑖𝑛 � 𝜃 2 � [V-2]
Obtained PMMA chain length is about 107 nm. Although the polymer chains are never completely stretched inside the polymer matrix, it can be expected that confinement of the polymer chains below these dimensions can have a significant influence on the chain mobility, and therefore on the T g .

In order to provide more experimental evidences of this effect two sets of measurements were carried out.

First, the T g was measured in samples produced at constant pressure (30 MPa) and different saturation No real influence of the saturation temperature was found, and all the foams present values of their T g higher than the corresponding value of their solid precursor (Figure V.4). It should be noticed that all the cell sizes of these foams are below the micron, and that the T g is increased when the cell size is decreased. It is found that saturation pressure does not show any direct influence on the T g 's evolution. Foams with cell sizes below the micron present significant increments of their T g in comparison with the solid precursors, while foams with higher cell sizes present values close to that of the solid precursors (Figure V.5). Once again the T g is increased when the cell size is decreased. As expected a relationship between the increment of the T g and the reduction of this product, related to Therefore we can make the hypothesis that PMMA and PMMA/MAM foams present a confinement effect when cell sizes lie below the micron, due to the associated reduction of their cell walls (below 100 nm for these cell sizes). Reduction of the PMMA chains mobility under these circumstances allows obtaining and increment of the glass transition temperature up to 11º C.

V.3 Thermal Conductivity

One of the most extended applications of cellular polymers is thermal insulation (buildings, pipes, fridges, transport, cryogenic applications…), with a high social, environmental, and economic impact. It is estimated that about 13 % of the total energy consumption in the European Union is related to the climatization of residential buildings. Some studies about thermal insulation obtain that an optimal thermal insulation (with current technology and materials) results in a 75 % power consumption and CO 2 emissions savings (Dynamic simulation from BASF (Passiv Haus Institut)).

Therefore, development of improved thermal insulation materials can have a significant impact on the economy and on the environmental problems associated to the CO 2 emissions.

The main advantage of cellular materials as thermal insulators is that the thermal conductivity of the polymer matrix, usually not so high, is reduced by the addition of the gaseous phase (with a much lower thermal conductivity). However, the contribution of the gaseous phase becomes the main component of the foam thermal conductivity at the relative densities usually employed in thermal insulation (below 0.1), as Figure V.9 shows. For instance for a relative density of 0.1 the gas phase contribution is around 60%, the solid phase contributes with 30%, and the radiation term accounts for around 10% of the total conductivity. Then, the main challenge in thermal insulation with foams is to reduce the thermal conductivity of the gaseous phase. One of the approaches employed is to substitute the gas inside the cells, usually air whose Also, the fitting of the contribution of the solid phase to usual models requires the introduction of a structural factor (g) different for microcellular foams and for nanocellular foams that accounts for the different tortuosity of the materials.

Conclusions and Perspectives

This final section summarizes all the main conclusions and remarkable achievements of this research work. The section is written according to the scope and objectives of this thesis. The conclusions cover the optimization of experimental techniques, the study of nanostructured polymers and nano cellular PMMA-based polymer foams, the understanding of the phenomena involved in the elaboration of these materials, and the main properties of these systems.

As a general conclusion, PMMA/MAM blends have proven to be exceptional materials for the production and study of nanocellular foams. These materials allow obtaining foams with different cellular structures in the nanometric range, presenting significant density reductions (down to around 0.4 g/cm 3 ). In addition, they exhibit well defined and controlled cell nucleation and growth mechanisms, which enable the production of nanocellular foams in a range of processing conditions wider than other polymers studied previously in the literature. Both nanocellular and microcellular foams are produced from these blends with a tunable cell size, cell interconnectivity (open and closed cells are possible), and density.

First, the advances on some experimental techniques are listed. And then the principal conclusions according the three main objectives of this research are presented (i.e. production of nanocellular foams, understanding of the foaming mechanisms, and properties of nanocellular foams).

Improved experimental techniques or technological progress for characterization:

The study of the PMMA/MAM system has required the improvement or adaptation of some experimental techniques. Main novelties on this topic are as follows:

-Procedures to study the PMMA/MAM nanostructuration by AFM and TEM have been -Specific software for the cellular structure characterization has been developed. It has been demonstrated that it enables the analysis of SEM micrographs from materials with different cellular structures and densities. This software provides a detailed characterization of the cellular structure of porous materials. This methodology combines a better accuracy than the standard methods with a short processing time, presenting significant advantages to become a useful tool in the cellular materials science.

-A TPS thermal conductivimeter was adapted to develop a procedure that allows determining the contribution of the gaseous phase to the thermal conductivity of cellular materials. The methodology employed also involves the use of X-ray radiography as a non-destructive technique to analyze the homogeneity of the samples.

These optimized techniques have demonstrated their suitability to improve the characterization of our materials. They have provided essential information to be able to achieve the objectives of this research.

Production of nanocellular foams:

A new route to produce nanocellular foams has been established using PMMA/MAM blends precursors.

Our materials present several advantages.

-Blends of PMMA and MAM, both commercial grades, can be easily produced by extrusion and injection obtaining bulk pieces with a well defined nanostructuration. Self-assembly of PMMA/MAM blends appears for each MAM amount of the blend. The morphology depends on the amount of the soft-block (PBA), evolving from nodular independent objects, such as micelles/core shells, to lamellas/worm like or quasi co-continuous structures.

-Gas dissolution foaming provided nanocellular foams from neat PMMA and MAM, together with PMMA/MAM blends. Nanocellular foams from the neat materials can only be obtained with a selection of specific processing parameters (i.e. near the maximum saturation pressure available (30 MPa) and temperatures near or below room temperature); while PMMA/MAM foams with cell sizes in the nanometric range can be produced in the large range of pressures (from 10 to 30 MPa) and several temperatures (up to 50º C).

-No specific or uncommon procedure had to be used to obtain these nanocellular foams.

Requirements such as extremely high pressures (over 40 MPa), high pressure release rates (GPa/second), or quenching of the sample after or prior the pressure release, are not needed to obtain PMMA/MAM nanofoams.

-Nanocellular PMMA/MAM foams with significant density reduction (over 50 %) were obtained in a wide range of processing conditions (pressure and temperature). Also, different MAM amounts allow obtaining nanocellular foams with different pore morphologies, from closed cell spherical or elongated pores to open cell co-continuous pores.

-Some evidence of saturation annealing of the PMMA/MAM blends during the foaming process was found. This process could avoid macroscopic creep of the samples and produce a homogenization of the nanostructuration, avoiding defects or inhomogeneities resulting from the production route of the solid precursors. The resulting nanofoams are reproducible and homogeneous.

Therefore we have been able to obtain a wide range of nanocellular foams with different characteristics overcoming the main difficulties of the production of this kind of materials. We just use commercial polymers and conventional industrially scalable techniques to produce these nanocellular foams.

Therefore, we think that our work is a significant advance in the development of these materials, decreasing the gap between the laboratory scale and the industrial production.

Understanding of the foaming mechanisms:

From the study of the nanostructures of solid samples, the cellular structures of foamed samples, and the foaming behavior of these materials we have characterized the foaming process and understood the phenomena involved in the production of these materials.

-It was demonstrated that PMMA/MAM blends present a heterogeneous nucleation process, controlled by the soft-block (PBA) domains inside the PMMA matrix. This behavior is due to several factors: o PBA is more CO 2 -philic than PMMA.

o PBA is in the rubbery state during the entire process, due to its low T g about -50º C.

o PBA probably presents a lower energy barrier for the nucleation process than PMMA due to their intrinsic characteristics.

-No influence of the processing parameters (i.e. saturation pressure and temperature, pressure release ratio) was found over the nucleation process.

-PBA domains of the solid samples act as an exact pattern of the cellular structures of the foamed samples:

o PMMA/MAM blends presenting dispersed nanostructures (i.e. micelles or nodules)

shown a nucleation efficiency ratio close to unity, which implies that each nucleating agent (dispersed nanostructure) becomes a cell. In addition, cell morphology reproduced the shape of the nanostructure: micelles become spherical cells, while elongated nodules result in anisotropic cells. Attempts to study the nanostructuration on the foams suggest that the nucleation process happens inside the PBA phase of the core-shell structure of the micelles/nodules. We reach this conclusion because the PBA shell appears swollen in the foamed samples with an inner diameter of similar size as the average cell size of these foams.

o PMMA/MAM blends with co-continuous morphologies result in co-continuous cellular structures. No information about the PBA domains after foaming can be derived from these materials. However, it is expected that the nucleation happens also inside the PBA domains.

-Increment of the MAM amount (between 5 to 20 wt%) seems to increase the CO 2 uptake and therefore increases the cell size and decrease the relative density, remaining constant a high cell nucleation density close to 4 x 10 14 nuclei/cm 3 . At higher MAM contents a similar behavior can be expected about the CO 2 sorption, however, the co-continuous or semi co-continuous nanostructure allows a quicker diffusion of the gas out of the sample after the pressure release, avoiding the study of the CO 2 sorption by the gravimetric method employed in this work.

-Cell growth in PMMA/MAM blends is controlled by the plasticization of the PMMA matrix.

Nucleation happens in the PBA domains along the entire sample, but these nuclei only can grow into a cell if the surrounding PMMA is plasticized enough to allow the growth of the nuclei.

Therefore, nucleation and cell growth in PMMA/MAM blends are independent processes, and can be controlled separately.

-As a consequence of the heterogeneous nucleation mechanism of PMMA/MAM blends, the formation of the solid skins, typical of solid state foaming processes, does not fit the model used on conventional foams with homogeneous nucleation. Therefore, a new qualitative model that explains the formation of the solid or semi-solid skins on PMMA/MAM blends has been proposed, taking into account the nucleation and cell growth mechanisms previously determined.

This model is based on the evolution of the plasticization of the PMMA matrix due to the desorption of CO 2 , and how this evolution controls the growth of the nuclei present on the PBA domains.

-It was found that the saturation temperature, and the evolution of the sample's temperature during the pressure release, control the presence of a desorption stage on the foaming process. Thus, quenching of the samples due to the pressure release from saturation temperatures near room temperature causes a desorption stage (like in the two-steps route or the solid state foaming process); while higher saturation temperatures (over 40º C) avoid this desorption stage (like in the one-step or batch foaming).

Differences between both processes are huge on the neat PMMA and MAM polymers. In fact, nanocellular foams from the neat polymers can be only obtained by the two-step foaming process.

On the contrary, PMMA/MAM blends present "a higher thermal stability", with small differences between both processes and a negligible incidence of the coalescence phenomena up to a saturation temperature over 60º C. This thermal stability of PMMA/MAM foams is a great advantage to obtain nanocellular foams in a wide range of processing conditions.

-On the other hand, the thermal stability of PMMA/MAM foams makes difficult obtaining nanocellular foams with lower relative densities than 0.4 g/cm 3 . Saturation temperature just allowed a density decrease by about a 10 %; while some attempts to reduce the density by postfoaming only decreased the density by another 5 %. This limitation on the cell growth/density reduction is related to a quick diffusion of CO 2 out of the very thin cell walls, reducing the PMMA plasticization and limiting the cell growth.

Properties of PMMA/MAM foams:

The unique characteristics of PMMA/MAM foams, their easy production, and the variety of possible cellular structures make them ideal materials for the study of the foam's properties evolution from the microcellular to the nanocellular range. Efforts performed on this topic provided several results of significant interest.

-90/10 PMMA/MAM foams present an improved mechanical behavior (higher Young's modulus) than neat PMMA foams. Although PMMA foams present in general microcellular structures and PMMA/MAM foams present in general nanocellular structures, no evidences of a relation/link between the enhancement of the mechanical properties and the reduction of the cell size was found in the range of cell sizes studied (from 100 nm to 2 µm). It is suggested that the better mechanical performance is a result of the higher homogeneity of the cellular structure of the PMMA/MAM foams, due to their controlled nucleation mechanism.

-It has been demonstrated that the glass transition temperature of the PMMA matrix of nanocellular foams increases when the average cell size lies under the micron. Different studies about the influence of the saturation pressure and temperature have been carried out, resulting in the absence of influence of the processing parameters on this property. On the contrary, a clear relationship between the glass transition temperature and the average cell wall thickness of the foams has been found. This behavior is related to the confinement of the PMMA chains inside the cell wall. Finally, it is concluded that the glass transition temperature is significantly increased when the cell wall thickness lies below 100 nm.

-Study of the thermal conductivity of PMMA/MAM microcellular and nanocellular foams shows a clear threshold between the two cell size ranges. Nanocellular foams present a lower thermal conductivity than microcellular foams. This reduction of the thermal conductivity was related to the phenomenon called Knudsen effect. Experimental measurements allow validating the theoretical predictions (obtained by Knudsen equation) of the dependence of the thermal conductivity of the gaseous phase with the average cell size.

Therefore, the existence of a property threshold between microcellular and nanocellular foams has been demonstrated for the thermal conductivity, and for the glass transition temperature; however the values of the Young's modulus seems to show an enhancement, which is more related to another property of the cellular structure, such as their homogeneity, than to a decrease of the cell size.

Future Perspectives

This is the first Ph.D. investigation in CellMat and LCPO about nanocellular foams. That is the reason why this research has been mainly focused in the understanding of the phenomena leading to the production of nanocellular foams by CO 2 dissolution with medium densities.

As nanocellular foams are one of the current frontiers on cellular materials science, and raise great expectations due to their potential properties and applications, we expect that the knowledge acquired during this research could be a starting point for new research works, some of which could be focused on the following topics.

- o To look for other potential applications of PMMA/MAM foams.

-Transition between microcellular and nanocellular foams:

o To extend the study of the mechanical properties to other properties such as impact strength, toughness, fatigue, etc. (not only the Young's modulus), and to a wider range of cell sizes and relative densities. To be able to determine if nanocellular foams present improved mechanical properties as it is predicted in the literature.

o To extend the study of the thermal conductivity to a wider range of cell sizes and relative densities. To be able to provide an accurate validation and understanding of the Knudsen effect on polymer nanofoams.

o To study the evolution of other properties, such as electrical conductivity, dielectric permittivity, acoustic insulation, matrix tortuosity, etc., between microcellular and nanocellular foams.

o To analyze and understand the implications of the confinement of the polymer chains detected in this thesis.

-Looking for other polymers with similar characteristics: other block copolymers with CO 2 -philic soft blocks can lead to the production of nanocellular foams with higher nucleation densities if the block copolymer allows higher nanodomains density than MAM. From the accumulated knowledge on this research new block copolymers with optimal properties to enhance the nucleation can be looked for.

-Continuous production route: current production route requires long processing times (i.e.

saturation times about 16-24 h) in a non-continuous production route. Then, the development of a continuous and fast production route of nanocellular foams is the main issue required for the industrial production of these materials.

Titre:

  Fabrication et Caractérisation de Polymères Micro et Nano Cellulaires à partir de Polymères Nanostructurés à base PMMA.

Figure F. 1 :

 1 Figure F.1 : schéma des étapes de moussage par dissolution de gaz

  Pression de saturation et la vitesse de détente [89, 96, 102]) contrôlent essentiellement la nucléation pour des matériaux homogènes comme le montre la figure ci-dessous (Figure F.2).

Figure F. 2 :

 2 Figure F.2 : Possibilités de nucléation homogène et hétérogène

F. 2 ObjectifsFigure F. 3 : 1 )

 231 Figure F.3 : Tétrahèdre de notre recherche et inter relations

Figure F. 4 :

 4 Figure F.4: Schéma de la structure de la thèse et des chapitres I à V

Figure 0 . 3 :

 03 Figure 0.3: Métodos para incrementar la densidad de nucleación en los procesos de espumados por disolución de gas

  tengan un tamaño apropiado (del mismo orden o superior al radio crítico de nucleación del sistema polímero/gas)[108]. Por ejemplo,Siripurapu et al. [109] obtuvieron films nanocelulares de PMMA mediante la adición de nanopartículas de sílice con diámetros en torno a 10-12 nm, con presiones de saturación de CO 2 de 34.5 MPa.También es posible incrementar la nucleación heterogénea mediante el uso de copolímeros de bloque (de tipo A-B o A-B-A, siendo A y B diferentes cadenas poliméricas)[113]. Estos copolímeros permiten la formación de nanoestructuras en la matriz polimérica, que pueden actuar como puntos de nucleación para las celdas. Adicionalmente, si alguno de los bloques del copolímero presenta afinidad por el CO 2 las nanoestructuras podrán almacenar más gas para inducir el crecimiento de las celdas [108]. Siguiendo este procedimiento pueden encontrarse en la bibliografía espumas nanocelulares obtenidas a partir de films de copolímeros de poliestireno-b-poli(dimetilsiloxano) o de PMMA-b-PFOMA (PFOMA, poli(1,1-dihidroperfluorooctil metacrilato)) [109]. Sin embargo estas espumas presentan densidades relativas muy elevadas (en torno a 0.98), y es necesario evitar la difusión del gas a través de las superficies del film durante la liberación de la presión. Al inicio de esta investigación los mejores resultados obtenidos mediante el empleo de copolímeros habían sido obtenidos por Yokoyama et. Al [115] y Yokoyama y Sugiyama [118], que obtuvieron densidades de nucleación cercanas a los 10 16 nucleos/cm 3 , a partir de films delgados de PS-b-PFMA (poliestireno-b-poli(perfluorooctyl etilmetacrilato)) y PS-b-PFS (poliestireno-b-poly(perfluorooctyl propiloxi estireno)), presentando en ambos casos los bloques fluorados una mayor afinidad por el CO 2

Figura 0. 4 :

 4 Figura 0.4: Tetraedro de la ciencia de los materiales y sus inter-relaciones para los materiales espumados

Tabla 0- 5 : 2

 52 Estancias en otros centros de investigaciónEstancias en otros centros de investigaciónDossemanas en el Instituto de Microelectrónica (CSIC) de Madrid (Spain) en 2009 Objetivo: Aprendizaje del uso de Atomic Force Microscopy (AFM) Una semana en la Technical University of Berlin (Germany) en 2010 Objetivo: Aprendizaje de técnicas de análisis de imagen Tres meses en el Microcellular Plastics Laboratory (Mechanical Engineering Department) de la University of Washington (USA) en 2011 Objetivo: Producción de materiales celulares basados en HDPE y x-HDPE mediante espumado por disolución de gas (CO 2 ) Tabla 0-6: Participación en otros proyectos de investigación financiados en los últimos 4 años Proyectos de investigación financiados FP7 Comisión Europea. NANCORE 214148 Agencia Espacial Europea. MAP AO-99-075 Ministerio de Ciencia e Innovación (Programa Nacional de Materiales). MAT2009-14001-C02-01 Ministerio de Ciencia e Innovación (Programa Nacional de Materiales). MAT2012-34901 Junta de Castilla y León. VA174A12-Junta de Castilla y León. VA035U13

Figure 0 . 1 :

 01 Figure 0.1: Porous structure of a human bone (left) and porous structure of a nanocellular polymeric foam (right)

Figure 0. 1 (

 1 right) shows the typical structure of one of the nanocellular foams produced in this research.

Figure 0 . 2 :

 02 Figure 0.2: Typical cellular structure of a PMMA/MAM nanocellular foam

  3) [93]:

Figure 0 . 3 :

 03 Figure 0.3: Gas dissolution foaming process scheme

Figure 0 . 4 :

 04 Figure 0.4: Methods to enhance the cell nucleation in gas dissolution foaming processes

  2 system [108]. Siripurapu et al. [109] obtained nanocellular PMMA foamed films with cell sizes about 400 nm and high relative densities (about 0.98) by the addition of nanosilica particles (mean diameter 10-12 nm) using CO 2 saturation pressures up to 34.5 MPa. Fujimoto et al. [110] and Ema et al. [111] reduced the average cell size of polyactide-based foams up to 200 nm by the addition of nanoclays (montmorillonite (MMT)) with saturation pressures between 10 to 24 MPa. Relative density of the obtained nanocellular foams took values about 0.44.Urbanczyk et al.[112] also added MMT to poly(styrene-co-acrylonitrile) (SAN) obtaining foams with cell sizes between the nanometric and micrometric scales at saturation pressures up to 30 MPa. They found that the addition of MMT allow decreasing the average cell size from values near 1 µm to values below 500 nm, but at the same time the relative density of the foam is increased from 0.23 to 0.65.

  Siripurapu et al.[109] obtained microcellular foams with enhanced nucleation from films of blends of PMMA and PMMA-b-PDMS, and nanocellular foams with cell sizes between 100-300 nm and high relative densities about 0.98 from blends of PMMA and PMMA-b-PFOMA (poly(1,1dihydroperfluorooctyl methacrylate)); both materials were foamed using a constrained gas dissolution foaming process, avoiding the gas diffusion through the films surface[114].Yokoyama et al. [115] produced nanocellular foams with very high cell densities (i.e. 10 16 cells/cm 3 ), cell sizes between 10-30 nm, and relative density of 0.5 from monoliths of polystyrene-blockpoly(perfluorooctylethyl methacrylate) (PS-b-PFMA); they found that a quench of the films to 0º C before the pressure release and a slow pressure release rate are needed to obtain nanofoams from this copolymer.Li et al. [116, 117] followed the same procedure with thin and thick films of PS-b-PFMA to produce nanocellular foams at 10 MPa, obtaining cell sizes about 15-30 nm and relative densities about 0.8-0.9. Yokoyama and Sugiyama[118] extended this route to polystyrene-blockpoly(perfluorooctylpropyloxy styrene) (PS-b-PFS), obtaining similar results as PS-b-PFMA. Research of Yokoyama's group with fluorinated block copolymers found that the nanostructured fluorinated CO 2philic domains acts as nucleation sites, with a nucleation efficiency significantly lower than one (i.e. cell density is significantly lower than the nanodomains density). The frozen PS matrix (due to the temperature quench before the pressure release) controls the cell size and avoids coalescence (without the temperature quench the nanocellular structure disappears), obtaining average cell sizes about 20 nm and high relative densities near 1.Nemoto et al. [119, 120] produced foams with micrometric and nanometric cellular structures from blends of poly(propylene) (PP) and a thermoplastics polystyrene elastomers (TPS, specifically hydrogenated polystyrene-block-polybutadiene-block-polystyrene rubber, and hydrogenated polystyreneblock-polyisoprene-block-polystyrene) or a propylene-block-ethylene copolymer (PER); best results were obtained in PP/TPS thin films saturated at 20 MPa, leading to cellular structures with an average cell size about 200-400 nm and relative densities about 0.75.Otsuka et al.[121] produced nanocellular foams from blends of PS/PMMA; the morphology of PMMA is in a form of nanoscale nodules, acting as nucleants agent. At high pressure drop rates and low temperatures they obtain cellular structures with 40-50 nm of average diameter, cell density of 8.5 x 10 14 cells/cm 3 , and therefore a relative density of 0.94. Wang et al. [122] controlled the cell size of thin films of aromatic poly(aryl ether)s (PAE) by modifying the parameters of a two-step foaming process, obtaining nanoporous co-continuous structures (i.e. open cell) with short desorption stages (i.e. time between the saturation stage (at 20 MPa) and the foaming stage) in order to preserve high CO 2 concentration. They obtained pore sizes between 50 and 500 nm, and relative densities down to 0.4.

Figure 0 . 6 :

 06 Figure 0.6: Scheme of the thesis structure

Figure I. 1

 1 Figure I.1 shows the AFM images (from left to right, topography, amplitude and phase) and the recorded signal values corresponding to the selected profile (in gray over the images) of a neat PMMA sample. And figure I.2 shows the same information from a PMMA/MAM blend.

Figure I. 1 :

 1 Figure I.1: AFM topographic (left), amplitude (middle) and phase (right) images of neat PMMA (up) and signals values (down) corresponding to the selected profile (in gray over the images).

Figure I. 1 :

 1 Figure I.1: AFM topographic (left), amplitude (middle) and phase (right) images of PMMA/MAM blend (up) and signals values (down) corresponding to the selected profile (in gray over the images).

First journal article including

  in this dissertation is focused on this topic. It was published in Journal of Cellular Plastics Vol. 49 (2013) with the title "Characterization of the cellular structure based on userinteractive image analysis procedures". This publication is focused on the development of a specific software to characterize the cellular structure from foam's micrographs. This software is based on image analysis procedures and the introduction of some steps that require interaction of the software user with the software, avoiding the main problems associated to automatic image analysis procedures. Results obtained using this software are presented and compared to the results obtained from three other methods commonly used in the literature. From this comparison the developed software presents the best relationship between the number of parameters provided, the accuracy of the measurement and the time consumed (less than 3 minutes per micrograph). Due to the clear advantages of this method and its accuracy, this software became the standard cellular structure characterization method in CellMat Laboratory.

  methods. Internal defects can be detected by cutting the sample to reveal the internal structure, and inhomogeneities are studied by measuring the density of several samples extracted from different areas of the studied foam.An alternative to these destructive methods is the X-ray radiography technique (or X-ray radioscopy technique if the study also has time resolution). Application of these non-destructive techniques to the study of cellular materials is widely studied in the journal article "Application of a microfocus X-ray imaging apparatus to the study of cellular polymers" (E. Solórzano, J. Pinto, S. Pardo, F. García-Moreno, M. A. Rodriguez-Perez) published in Polymer Testing vol. 32 (2013)[13]. These techniques allow studying the internal structure and density distribution of the samples preserving their integrity. Figure I.3 shows examples of the density distribution determination and internal defects detection over X-ray radiographies of the nanocellular foams of this thesis.

Figure I. 3 :

 3 Figure I.3: Determination of the density distribution profile (up, average density profile is obtained from the region identified with a red rectangle) and detection of internal defects (down, defect are identified with red circles) on polymeric foams by X-ray radiography.

Figure I. 4 :

 4 Figure I.4: Electromagnetic spectrum of energies and wavelengths

Figure I. 5 :

 5 Figure I.5: X-ray and matter interaction producing elastic and inelastic phenomena as a result The physical fundamentals of radiation absorption are based on the Beer-Lambert equation shown in Equation I-3. This expression predicts the attenuation of incident monochromatic (it is usually not true in this simple form for polychromatic beams) X-ray radiation by an exponential function of the linear absorption coefficient (µ), the element concentration (c) and the thickness (t). Additionally, it is possible to estimate an effective linear attenuation coefficient, µ eff , if several elements are contained in the irradiated material and for polychromatic beam, as shown in Eq. I-4. Finally, it is sometimes used more conveniently mass attenuation coefficient by defining µ m =µ eff /c. ct e I I µ -= 0

  Photoemission PEEM Electron

Figure I. 6 :

 6 Figure I.6: Left: attenuation coefficient of elements typically contained in polymers: hydrogen and carbon. Right: attenuation coefficient variation with Z in the periodic table.

: 1 Z

 1 Photon energy /MeV

Figure I. 7 :Figure I. 8 :

 78 Figure I.7: X-ray tube L10101 from Hamamatsu. Maximum power output curve Table I-2: Technical characteristics of X-ray tube L10101 Spot size 5-20 µm Voltage 20-100 kV Current 0-200 µA Maximum power output 20 W

Figure I. 9 :[I- 5 ]

 95 Figure I.9: Magnification scheme obtained thanks to cone-beam geometry

Figure I. 10

 10 Figure I.10: X-ray device with all complementary connections

:

Figure I. 11 :

 11 Figure I.11: Hamamatsu software for the X-ray source beam control and the detector acquisition Complementarily, self-developed software has been programmed under LabView environment (Figure I.12). The main objective is to have an integrated software which controls temperature, X-ray tube, and image acquisition in a single window. The software is programmed based on a states machine workflow. The detector control by Labview is done thanks to SDK library already implemented by Hamamatsu Germany. This library includes the SubVIs necessary to run in a batch mode all the HiPic v8.3 functionalities.

Figure I. 12 :

 12 Figure I.12: Integrated control software developed on CellMat

  µm) spiral-shaped, embedded in an insulation layer usually made of Kapton (70 µm thick). The TPS element is located between two samples with both sensor faces in contact with the two surfaces as Figure I.13 depicts. Two samples of similar characteristics are required for this purpose.

Figure I. 13 :

 13 Figure I.13: Experimental set-up to perform TPS measurements and sensor shape

  RV3 A65201903), and a vacuum sensor (BOC Edwards, mod. WRG-S) (Figure I.14).

Figure I. 14 :

 14 Figure I.14: General view of the CellMat TPS device (up). Detail of the TPS sensor inside the vacuum chamber (down).

Figure II. 1 .

 1

Figure II. 1 :

 1 Figure II.1: Chemical formula of PMMA

  2), and average molar masses 65000 ≤ M n ≤ 90000 g/mol. Depending upon the "grade", the number average molar mass M n of the middle Poly(Butyl Acrylate) (PBA) block is ranging from 20000 to 27000 g/mol with a low dispersity I p ≤ 1.2. Chemical formula of PBA homopolymer is shown in Figure II.2. T g of PBA homopolymer is around -45º C.

Figure II. 2 :

 2 Figure II.2: Chemical formula of PBA

  First topic under study was the homogeneity and possible anisotropy of the nanostructuration along the injected specimens (50 x 15 x 3 mm 3 ). With this aim two injected samples were selected, one with a low MAM content (90/10) and other with high MAM content (25/75). Then, AFM samples were prepared and studied from each injected specimen. Areas studied on each PMMA/MAM sample, their location on the injected specimen, and nomenclature used to describe the AFM samples are shown in Figure II.3. (V: along injection flow; H: perpendicular to injection flow).

Figure II. 3 :

 3 Figure II.3: Location and nomenclature of AFM samples taken from injected samples

Figure II. 5 :

 5 Figure II.5: AFM micrographs from PMMA blends with MAM amounts between 5 and 75 wt% taken in the plane perpendicular to the injection direction (H, up), and in the plane containing the injection direction (V, down).

  MAM and PMMA/MAM solid blends with 5, 10, 25, 50 and 75 wt% of MAM were also studied by TEM. Neat MAM (Figure II.6) shows the typical long ranged lamellas; some of them are curved (due to an effect of processing).

Figure II. 6 :

 6 Figure II.6: TEM micrograph of neat MAM stained by PTA

Figure II. 7 :

 7 Figure II.7: Difference between the nanostructuration information obtained from a surface (left) and from the entire volume (right) of the sample

Figure II. 8 :

 8 Figure II.8: TEM micrographs of 95/5 PMMA/MAM blend stained by PTA. Micrographs shown the H (left) and V (right) planes.

Figure II. 9 :

 9 Figure II.9: TEM micrographs of 90/10 PMMA/MAM blend stained by PTA. Micrographs shown the H (left) and V (right) planes.

Figure II. 10 :

 10 Figure II.10: Core-shell structures from a TEM micrograph of 90/10 PMMA/MAM blend (left), and scheme (right) of these PMMA/MAM core-shell structures.

Figure II. 11 :

 11 Figure II.11: TEM micrographs of 75/25 PMMA/MAM blend stained by PTA. Micrographs shown the H (left) and V (right) planes.

Figure II. 12 :

 12 Figure II.12: Core-shell structures from a TEM micrograph of 75/25 PMMA/MAM blend (left), and scheme (right) of these PMMA/MAM core-shell structures.

Figure II. 13 :

 13 Figure II.13: TEM micrographs of 50/50 PMMA/MAM blend stained by PTA. Micrographs shown the H (left) and V (right) planes.

Figure II. 14 :

 14 Figure II.14: TEM micrographs of 25/75 PMMA/MAM blend stained by PTA. Micrographs shown the H (left) and V (right) planes.

  PMMA/MAM blends with low MAM contents both techniques allow identifying the nanostructuration with practically identical results. Shape, size and alignment can be equally detected by both techniques (Figure II.15). TEM micrographs show an apparent higher object density or higher dark area fraction, due to the characteristics of this technique explained in the previous section (projection in two dimensions of the full sample volume).

Figure II. 15 :

 15 Figure II.15: Comparison between AFM (in color) and TEM (in gray scale) micrographs of 95/5 H (up) and 90/10 V (down) PMMA/MAM blends. Compared images have the same scale.

Figure II. 16 :

 16 Figure II.16: Comparison between AFM (right, in color) and TEM (left, in gray scale) micrographs from a 90/10 PMMA/MAM blend. Inner structure is shown only in TEM micrographs.

Figure II. 17 :

 17 Figure II.17: Comparison between AFM (right, in color) and TEM (left, in gray scale) micrographs of 50/50 V PMMA/MAM blend.

Figure II. 18 :

 18 Figure II.18: Comparison between AFM (right, in color) and TEM (left, in gray scale) micrographs of 25/75 H PMMA/MAM blend.

  and capable of operating at a maximum temperature of 250º C and maximum pressure of 40 MPa (Figure II.19). This pressure vessel is equipped with an accurate pressure pump controller provided by Teledyne ISCO (Lincoln NE, U.S.), and controlled automatically to keep the temperature, by means of external heaters, and pressure at the desired values (Figure II.19).

Figure II. 19 :

 19 Figure II.19: Teledyne ISCO pressure pump (left), and TOP Industrie high pressure vessel (rigth).

  3 and maximum operation temperature and pressure of 200º C and 20 MPa, respectively (Figure II.20) [9].

Figure II. 20 :

 20 Figure II.20: Scheme (up) and picture (down) of the self-designed at CellMat Laboratory high pressure vessel

II. 7

 7 Nanostructuration evolution due to CO 2 saturationDuring the foaming experiments of PMMA/MAM systems we found some unexpected results about the macroscopic expansion of the foams. Foams produced at saturation pressures and temperatures of 30 MPa and 60º C present a strong dependence of the MAM amount on their macroscopic expansion(Figure II.21). It should be noticed that these foams are produced by a free foaming process, i.e. not a constraint foaming neither mold foaming, and therefore the final shape and expansion of the foams are not fixed.

Figure II. 21 :

 21 Figure II.21: Macroscopic shape of foams produced at 30 MPa and 60º C from different PMMA/MAM blends compared to the solid precursors shape and dimensions (left). The saturation time was 24 hours and the depressurization rate was around 30 MPa/min.

during 4 h

 4 at 135º C (about 20º C above the T g of PMMA) without differences between different PMMA/MAM blends (Figure II.22). The modifications on dimensions were similar for all the analyzed samples.

Figure II. 22 :

 22 Figure II.22: PMMA (left) and PMMA/MAM samples (80/20, 75/25 and 25/75) after an annealing process at 135º C during 4 h.

Figure II. 23 :

 23 Figure II.23: PMMA (left) and PMMA/MAM samples (75/25 and 25/75) after an annealing process at 165º C during 4 h. Before annealing the three samples had the same geometry and dimensions.

( 95 / 5 )

 955 do not suffer this process and evolve in a normal way under annealing (creep). Finally, samples with a co-continuous local nanostructuration (such as 25/75) evolve into a clear three-dimensional structuration in the mesoscale, which could avoid the appearance of creep (these mechanisms are proved in the following section). These mechanisms are schematically represented on Figure II.24.

Figure II. 24 :

 24 Figure II.24: Evolution of macroscopic shape and nanostructuration during annealing of neat PMMA (left), PMMA/MAM blends with elongated micelles or nodules (middle), and PMMA/MAM blends with co-continuous nanostructuration (right).

  2 annealed samples was studied by AFM and TEM. Annealed samples under CO 2 saturation (24 hours at 30 MPa and 60º C) were obtained from thin samples of 90/10 and 25/75 PMMA/MAM blends, carrying out a extremely slow pressure release (several hours) to avoid the foaming of the samples. AFM phase images of 90/10 in the plane containing the injection direction (V) (Figure II.25) reveals that after annealing (24 hours at 30 MPa and 60º C) the elongated micelles decrease their length and increase their width. Anisotropy of the micelles does not completely disappear.

Figure II. 25 :

 25 Figure II.25: AFM phase images showing the evolution of the nanostructuration of 90/10 PMMA/MAM blends due to a CO2 annealing (24 hours at 30 MPa and 60º C). Both AFM phase images have the same scale.

Figure II. 26 :

 26 Figure II.26: TEM micrographs showing the evolution of the nanostructuration of 25/75 PMMA/MAM blends due to an annealing process (24 hours at 30 MPa and 60º C).

Figure II. 27 :

 27 Figure II.27: SEM micrograph from a 25/75 PMMA/MAM foam produced at 30 MPa and 60º C showing a mesoscopic structuration.

  due to the nanometric size of the cellular structure (i.e. SEM micrographs with enough magnification to appreciate the cellular structure are too close to measure the skin thickness) (Figure II.28).

Figure II. 28 :

 28 Figure II.28: SEM micrographs of 90/10 PMMA/MAM foam produced at 30 MPa and RT. Upper micrograph was taken with the appropriate magnification to measure the skin thickness (overlaid in the micrograph). Lower micrographs show the structure of the foamed core (left) and the densified skin (right) with higher magnification.

this aim a 90

 90 /10 PMMA/MAM foam produced at 30 MPa and RT was selected, and 90 SEM micrographs were taken in different areas and orientations of the sample (see Figure II.29, V: along injection flow; H: perpendicular to injection flow). For each area three zones were selected to take the SEM micrographs: near the front face of the sample (B), in the middle of the sample (C), and near the back face of the sample (D). Also, each zone was studied by SEM at three magnifications (x 10 000, x 50 000, x 100 000).

Figure II. 29 :

 29 Figure II.29: Location and nomenclature of SEM samples taken from injected samples

Figure II. 30 :

 30 Figure II.30: SEM micrographs of 90/10 PMMA/MAM blend foamed at 30 MPa and RT: areas H1 (upper left), H2 (upper right), and H3 (down).

Figure II. 31 :

 31 Figure II.31: SEM micrographs of 90/10 PMMA/MAM blend foamed at 30 MPa and RT: areas H4 (upper left), H2 (upper right), and H5 (down).

Figure II. 32 :

 32 Figure II.32: SEM micrographs of 90/10 PMMA/MAM blend foamed at 30 MPa and RT: area H2; zones B (upper left), C (upper right), and D (down).

Figure II. 33 :II. 8 . 4

 3384 Figure II.33: SEM micrographs of 90/10 PMMA/MAM blends foamed at 30 MPa and RT: areas H1 (upper left), V1 (upper right), H2 (middle left), V2 (middle right), H3 (lower left), and V3 (lower right).

Figure II. 34 :

 34 Figure II.34: SEM micrographs (V) of neat PMMA foamed at 30 MPa and different temperatures: RT (upper left), 40º C (upper right), 50º C (middle left), 60º C (middle right), 70º C (down).

95/ 5

 5 PMMA/MAM foams produced at 30 MPa and RT present cells aligned in the injection direction, but with no elongation (Figure II.35). At higher temperatures up to 60º C, and the same pressure, alignment of cells disappears and the cell size is increased, while samples foamed at 70º C lose their well-defined cellular structure.

Figure II. 35 :

 35 Figure II.35: SEM micrographs (V) of 95/5 PMMA/MAM blends foamed at 30 MPa and different temperatures: RT (upper left), 40º C (upper right), 60º C (lower left), 70º C (lower right).

Figure II. 36 :

 36 Figure II.36: SEM micrographs (V) of 90/10 PMMA/MAM blends foamed at 30 MPa and different temperatures: RT (upper left), 40º C (upper right), 50º C (middle left), 60º C (middle right), 70º C (down).

Figure II. 37 :

 37 Figure II.37: SEM micrographs (V) of 75/25 PMMA/MAM blends foamed at 30 MPa and different temperatures: RT (upper left), 40º C (upper right), 50º C (middle left), 60º C (middle right), 70º C (down).

Figure II. 38 :

 38 Figure II.38: SEM micrographs (V) of 25/75 PMMA/MAM blends foamed at 30 MPa and different temperatures: RT (upper left), 40º C (upper right), 50º C (middle left), 60º C (middle right), 70º C (down).

Figure II. 39 :II. 8 . 5

 3985 Figure II.39: SEM micrographs (V) of neat MAM foamed at 30 MPa and different temperatures: RT (upper left), 40º C (upper right), 50º C (middle left), 60º C (middle right), 70º C (down).

Figure II. 31 :

 31 Figure II.31: SEM micrographs (H) of 90/10 PMMA/MAM blends foamed at room temperature and different pressures: 10 MPa (upper left), 15 MPa (upper right), 20 MPa (middle left), 25 MPa (middle right), 30 MPa (down).

  The first article was published in Chemical Engineering Journal 243C (2014) and entitled "Nanocellular CO 2 foaming of PMMA assisted by block copolymer nanostructuration", explains the methodology used to determine the cell nucleation mechanism in PMMA/MAM blends. The nanostructuration of PMMA/MAM blends with low amounts of MAM (5, 10 and 20 wt%) was studied by AFM and TEM, relating directly the dark area fraction on AFM micrographs to the PBA amount in each blend. In addition, a nearly constant micelles volumetric density (N n ≈ 4 x 10 14 micelles/cm 3 of the solid sample) was found on these blends both in AFM and TEM micrographs.

  continued in the second article, entitled "Block Copolymers Self-Assembly Allows Obtaining Tunable Micro or Nanoporous Membranes and Depth Filters Based on PMMA; Fabrication Method and Nanostructures" (published in The Journal of Physical Chemistry C 118 (2014)). This study is focused on the capacity of the PMMA/MAM nanostructure to be an exact pattern of the cellular structure, and not only the nucleation regulator, and it includes blends in a wider range than those showed in the first paper (i.e. includes materials with high MAM contents). Moreover, this article discusses the potential use of the obtained cellular structures for filtration processes with the possibility of producing membranes and depth filters. The anisotropy of nanostructure of PMMA/MAM blends presenting micellar objects is related to the anisotropy of the cellular structure of the foams. It is found that PMMA/MAM blends with spherical micelles (95/5) result into foams with cells without anisotropy, while PMMA/MAM blends with elongated micelles (90/10 and 80/20) result into foams with anisotropic pores and closed cell structures. PMMA/MAM blends with high MAM amounts (25/75) exhibiting a co-continuous nanostructuration result into a co-continuous cellular structure (open cell foams).

Figure III. 1 :

 1 Figure III.1: MAM structure and possible scheme about the micelles size increment related to the MAM amount increment

Figure III. 2 :

 2 Figure III.2: Schematic model of the solid skin formation on foams presenting homogeneous nucleation. Sample is extracted of the pressure vessel at time = 0 and foaming starts at time = t. Gas concentration is shown on axis Y, while the sample's width is shown on axis X (center of the sample corresponds to x = 0).

Figure III. 3 :

 3 Figure III.3: Schematic representation of the gas concentration profile at time = 0 and threshold concentration needed for the PMMA to be in a rubber state at RT. Gas concentration is shown on axis Y, while the sample's width is shown on axis X (center of the sample corresponds to x = 0).

Figure III. 4 :

 4 Figure III.4: Schematic model of the solid skin formation on foams presenting heterogeneous nucleation. Sample is extracted of the pressure vessel at time = 0 and foaming starts at time = t. Gas concentration is shown on axis Y, while the sample's width is shown on axis X (center of the sample corresponds to x = 0).

  structure of PMMA, MAM, and 90/10 PMMA/MAM blends has been summarized in the journal article entitled "Temperature Influence and CO 2 Transport in Foaming Processes of Poly(methyl methacrylate)-Block Copolymer Nanocellular and Microcellular Foams" (submitted to Journal of Supercritical Fluids, acceptance pending).

Figure 1 .

 1 Figure 1. Desorption curves of neat PMMA and MAM, and 90/10 PMMA/MAM. (Initial saturation conditions: 30 MPa and RT, all are dense unfoamed materials).

Figure 2 .

 2 Figure 2. Morphology (TEM image) of the 90/10 PMMA/MAM blend (left) and neat MAM (right) showing the nanostructuration

Figure 3 .

 3 Figure 3. CO2 uptake of PMMA and 90/10 PMMA/MAM blend CO2-saturated under 30 MPa and at different saturation temperatures ranging between room temperature and 70º C

Figure 4 .

 4 Figure 4. Relative density of neat PMMA and MAM, and 90/10 PMMA/MAM foams. Saturation pressure is fixed at 30 MPa with saturation temperatures between RT and 70º C.

Figure 5 .

 5 Figure 5. Cellular structure of several foams produced at 30 MPa and RT (left), 40º C (middle) or 70º C (right) from neat PMMA (up), 90/10 PMMA/MAM (middle) and neat MAM (down).

Figure 6

 6 Figure 6 shows the average cell size as a function of saturation temperature. A clear threshold between foams produced at room temperature and foams produced over 40º C is found again in neat PMMA and MAM foams; on the contrary, 90/10 PMMA/MAM foams present a rather constant average cell size up to 50º C, being the threshold in this case over 60º C.

Figure 6 .

 6 Figure 6. Cell size of neat PMMA and MAM, and 90/10 PMMA/MAM foams. Produced at 30 MPa and temperatures between RT and 70º C. All foams are triggered by depressurization at ≈ 30 MPa/min.

Figure 7 .

 7 Figure 7. Cell nucleation density of neat PMMA and 90/10 PMMA/MAM foams. Produced at 30 MPa and saturation temperatures between RT and 70º C.

Figure 8 :

 8 Figure 8: Pressure and temperature evolution during the depressurization process. (Saturation pressure 30 MPa and saturation temperatures: RT and 40º C).

Figure 9 :

 9 Figure 9: Foaming schemes / mechanisms for PMMA/MAM blend samples. a) With desorption stage due to the cooling of the system. b) Without desorption stage.

Figure IV. 1 .

 1 Figure IV.1. Taking into account that the relative density of 90/10 foams produced at RT is about 0.44 we

Figure IV. 1 :

 1 Figure IV.1: Relative density of 90/10 PMMA/MAM foams at different foaming temperatures and times. Saturation stage: 30 MPa and RT.

  obtained. PMMA samples were saturated at 30 MPa and 40º C, and post-foamed at 70º C during 120 seconds. Surprisingly, neat PMMA samples present a density decrease of about 55 %, from a relative density of 0.47 (before post-foaming) to a relative density of 0.21 (after post-foaming). Then, a study of the cellular structure of neat PMMA and 90/10 PMMA/MAM foams, produced at 30 MPa/40º C and with or without post-foaming at 70º C during 120 seconds, was carried out. SEM images obtained from these foams are shown in Figure IV.3.

Figure IV. 3 :Figure IV. 4 :

 34 Figure IV.3: SEM micrographs of neat PMMA (up) and 90/10 PMMA/MAM (down) foams. Saturation stage: 30 MPa and 40º C. Post-foaming: none (left) and 70º C during 120 seconds (right)

Figure IV. 5 :

 5 Figure IV.5: Cell size distribution of 90/10 PMMA/MAM foams produced at 30 MPa and 40º C. Before (left), and after (right) the post-foaming.

  Young's modulus and on the glass transition temperature were shown on the article published in Macromolecular Materials and Engineering 296 (2011), entitled "Low-Density Nanocellular Foams Produced by High-Pressure Carbon Dioxide". In this article the first PMMA/MAM nanocellular foams obtained were presented. They were produced from a 90/10 PMMA/MAM blend at room temperature (RT) and different pressures between 10 and 30 MPa.

PMMA and 90 / 1 .Figure V. 1 :

 9011 Figure V.1: Young's modulus of PMMA and PMMA/MAM foams as a function of relative density

Figure V. 2 :

 2 Figure V.2: Influence of the cell size of 90/10 PMMA/MAM foams on the normalized Young's modulus

  MMA unit), the number of C-C bonds of the MMA backbone (N C-C bonds = 2), and the length and angle of the C-C bonds (d C-C bond = 0.154 nm, θ C-C bond = 109º), using the following expression (Equation V-2).

temperatures (from 10 Figure V. 3 :

 103 Figure V.3: Comparison between the DSC curves of a 95/5 foamed and solid blends. Foamed sample shows a higher Tg.

Figure V. 4 :

 4 Figure V.4: Experimental Tg values from 95/5 (left) and 75/25 (right) foams compared to their solid precursors

Figure V. 6 :

 6 Figure V.6: Experimental increment of the glass transition temperature of neat PMMA and PMMA/MAM blends as a function of the product between their relative density and cell size.

  the cell wall thickness, is found. With the aim to obtain a further proof of this behavior the average cell wall thickness of these foams was estimated by measuring at least one hundred walls from each foam over SEM micrographs. Results of the T g increase as a function of the measured cell wall thickness are shown on Figure V.7. The results show that below cell wall thickness of 100 nm there is a clear increase of the glass transition temperature in the foams.

Figure V. 7 :

 7 Figure V.7: Experimental Tg increment from neat PMMA and PMMA/MAM blends foams vs. to their cell wall thickness.

  Figure V.8 shows a detail of the cell walls from foams presenting this 11º C increment of their T g . The thickness of the walls is clearly below 100 nm.

Figure V. 8 :

 8 Figure V.8: SEM micrograph showing cell walls of a 95/5 PMMA/MAM foam with a Tg of 126º C

Figure V. 9 :

 9 Figure V.9: Contribution (in percentage) of the solid phase, gaseous phase and radiation term to the thermal conductivity of a polymer foam (adapted from E. Solórzano et al., Adv. Eng. Mater, 11, 818 (2009)).

Figure V. 12 :

 12 Figure V.12: Schematic representation of the solid phase tortuosity increase from microcellular to nanocellular foams

  established. It has been determined the range of application (i.e. MAM amount on the PMMA/MAM blend) of each technique, and how both techniques can complement each other when both techniques are applicable. Also, quantification procedures to measure the nanodomains volumetric density have been developed for both techniques, obtaining comparable results.

oooo

  Research on PMMA/MAM blends: To analyze in detail the characteristics of PMMA/MAM foams with medium amounts of MAM. To extend the study of the post-foaming process on PMMA/MAM foams, trying to reach significant density reductions. Two approaches have been mentioned, post-foaming at low temperatures than those used, and post-foaming under pressure. o To study the influence of the MAM amount on the CO 2 diffusion and PMMA swelling during the saturation process by optical means. To try to produce PMMA/MAM nanocellular foams without integral skin by modifying the production route. To try to develop filtration devices based on PMMA/MAM nanofoams and characterize them.

Introduction 0.1 Introduction

	Human knowledge and technological development have not kept a steady growth since the advent of
	civilization; on the contrary, several scientific and technological fields have increased in an exponential
	way [1, 2]. In fact, it is estimated that more than 90% of the scientific knowledge has been developed
	since the end of the II World War in 1945. This affirmation virtually applies for every single scientific
	field, and in the Materials Science field has two main implications.
	On one side, advances on this field allow developing new materials with unique properties, or improving
	the properties or production routes of known materials. On the other side, advances on other scientific
	fields and development of new technological devices usually require new or tailored materials with
	specific properties. Due to this duality, Materials Science is one of the main fields of interest of both
	public and private research organizations.
	The European Union (EU) established in December 2013 the research objectives of the program "Horizon
	2020" (Horizon 2020, the EU Research and Innovation program for the period 2014-2020)[3]. Inside the
	materials science field one of the priorities will be the "Manufacturing and control of nanoporous
	materials". EU refers "a constantly growing interest in nanostructured porous materials, thanks to the
	many applications that can benefit from controlled porosity at the nanoscale".
	The present thesis entitled "

Fabrication and Characterization of Nanocellular Polymeric Materials from Nanostructured Polymers" is

  part of this priority research field for the EU present and next period.

Table 0 -1: Publications in international journals included in this dissertation; the Ph.D is main author or main participant.Table 0 -2: Other publications in international journals; the Ph.D is coauthor. Other publications submitted to international journals

 00

	M. Dumon, J. A. Reglero-Ruiz, J. Pinto, M. A. Rodriguez-Perez, J. M. Tallon, M. Pedros, P. Viot
	Block Copolymer-

Assisted Microcellular Supercritical CO 2 Foaming of Polymers and Blends

	Cellular Polymers 31, 207-222 (2012)
	E. Solórzano, J. Pinto, S. Pardo-Alonso, F. García-Moreno, M. A. Rodriguez-Perez
	Application of a Microfocus X-

Ray Imaging Apparatus to the Study of Cellular Polymers

	Polymer Testing 32, 321-329 (2012)
	B. Notario, J. Pinto, E. Solórzano, J. Escudero, J. Martín de León, D. Velasco, M. A. Rodriguez-Perez
	In-

Situ Optical Analysis of Structural Changes in Polyactic Acid (PLA) during Gas Dissolution Process

Defect and Diffusion Forum (accepted, pending publication)

Table 0 -4: Other communications in international conferences Other communications presented in international conferences

 0

	J. Escudero, J. Pinto, E. Solórzano, M. A. Rodriguez-Perez, J. A. de Saja
	Structural Foams vs. Conventional Foams: Structural Characterization and Mechanical
	Behavior
	Oral. XI Reunión del Grupo Especializado de Polímeros (GEP), Valladolid (Spain) (2009)
	M. A. Martínez, J. Abenojar, M. A. Rodriguez-Perez, J. Pinto, S. Estravis, J. Ecenero, E. Aldanondo, P.
	Álvarez
	Unión de Espumas

de Poliolefina y Poliuretano al Aluminio: Ensayos de Adhesión y su Modelización

Poster. X Congreso de Adhesión y Adhesivos, Alicante (Spain) (

2009

) P. Álvarez, A. Echeverría, M. M. Petite, M. A. Rodriguez-Perez, J. Pinto, S. Estravís, M. A. Martínez, J. Abenojar Fabricación, Caracterización y Modelización de Paneles Sándwich Compuestos por Cores de Espumas de Polietileno y Poliuretano y Pieles de Aluminio Poster. Comatcomp 09, San Sebastian (Spain) (2009) E. Solórzano, M. A. Rodriguez-Perez, J. Pinto, F. García-Moreno, J. A. de Saja A Novel

Procedure to in-situ Analyze the Foamability of Thermoplastic Foams using Optical Expandometry and X-Ray Radioscopy Oral

. Blowing Agents 2010, Köln (Germany) (2010) E. Solórzano, J. Escudero, J. Pinto, J. A. de Saja, M. A. Rodriguez-Perez

Gas Diffusion and Other Physical Mechanisms Involved in the Production of Structural Foams Oral

. DSL 2010, Paris (France) (2010) M. A. Rodriguez-Perez, E. Solórzano, J. Pinto, J. A. de Saja A

Novel Method to Analyze the Foamability of Polymer Systems using in-situ Optical Expandometry and X-Ray Radioscopy Oral

	ImageJ: Multifunctional Analysis in Materials Science
	Oral. ImageJ User and Developer Conference, Mondor les Bains (Luxembourg) (2010)
	M. Dumon, J. A. Reglero-Ruiz, J. M. Tallon, M. Pedros, E. Cloutet, J. Pinto, P. Viot
	Behavior

. FOAMS 2011, Seattle (USA) (2010) E. Solórzano, J. Pinto, F. García-Moreno, M. A. Rodriguez-Perez

of block copolymers in supercritical CO 2 and microcellular foaming of polymer blends Oral

DSL 2011, Algarve (Portugal)

(2011)

Table 0 -5: Stages in other research institutions Stages in other research institutions

 0

	Two weeks in the Instituto of Microelectrónica (CSIC) of Madrid (Spain) in 2009
	Topic: Training on Atomic Force Microscopy (AFM)
	One week in the Technical University of Berlin (Germany) in 2010
	Topic: Training on Image Analysis Techniques
	Three months in the Microcellular Plastics Laboratory (Mechanical Engineering Department) of the
	University of Washington (USA) in 2011
	Topic: Production

of cellular materials based on HDPE and x-HDPE by CO 2 solid state foaming

Table 0 -6: Participation to other research funded projects in the course of the 4 years Research funded projects

 0

FP7 European Commission. NANCORE 214148 European Spatial Agency. MAP AO-99-075 Ministerio de Ciencia e Innovación (National Materials Program). MAT2009-14001-C02-01 Ministerio de Ciencia e Innovación (National Materials Program). MAT2012-34901 Junta of Castile and Leon. VA174A12-2 Junta of Castile and Leon. VA035U13

Table I -

 I 1 lists them in order to provide a general view of the characterizing methods used. Each publication briefly describes the techniques employed, however, in this chapter further explanations, about some non-conventional techniques developed, improved or adapted in this work, are provided.

Table I -1: Experimental techniques employed in this work and chapter in which they are used

 I

	Experimental technique	Chapter
	Size-exclusion chromatography (SEC)	II
	Transmission electron microcopy (TEM)	II-III
	Atomic force microscopy (AFM)	II-III
	Scanning electron microscopy (SEM)	II-V
	Differential scanning calorimetry (DSC)	II-V
	Archimedes densimetry	II-V
	CO 2 uptake gravimetric measurement	III-IV
	Cell structure characterization assisted by software	III-V
	X-ray radiography	III & V
	Dynamic mechanical analysis (DMA)	V
	Thermal conductimetry: Transient plane source (TPS)	V

Table II -1: Density and glass transition temperature (Tg) of PMMA/MAM blends

 II

	PMMA/MAM wt%	Density / g/cm 3	T g / º C
	95/5	1.18	115
	92.5/7.5	1.17	115
	90/10	1.17	115
	85/15	1.17	115
	80/20	1.16	114
	75/25	1.16	113
	50/50	1.16	108
	25/75	1.14	104
	II.3

Morphology analysis of PMMA/MAM dense solid blends by AFM

PMMA/MAM solid blends with 5, 10, 20, 50 and 75 wt% of MAM were studied by AFM.

2: Relative density of 80/20 PMMA/MAM foams at different foaming temperatures and times. Saturation stage: 30 MPa and RT.

  80/20 PMMA/MAM foams produced at RT present a relative density near 0.41. Results from the 80/20 foaming at higher temperatures can be found on Figure IV.2; in this case, significant density increments are found in the entire range of temperatures and times studied. Rather in contradiction to the density of 90/10 system the general tendency is an increase of density with time and temperature: there is no sign of asymptotic behavior.

		0.58
			60º C
		0.56	70º C
			80º C
		0.54	90º C
	Relative density	0.46 0.48 0.50 0.52
		0.44
		0.42
		0	30	60	90	120	150	180
					Time / s
	Figure IV.

Table IV -

 IV 1 shows the relative densities of 90/10 and 80/20 foams saturated at 30 MPa and 40º/50º C, with and without post-foaming stage.

Table IV -1. Relative densities of PMMA/MAM foams with and without post-foaming. Saturation stage: 30 MPa and 40º/50º C. Post-foaming stage: 70º C and 120 seconds

 IV

	PMMA/MAM	40º C	40º C + PF	50º C	50º C+ PF
	90/10	0.41	0.39	0.40	0.40
	80/20	0.52	0.48	0.60	0.48

Table V -1. Processing conditions and properties of the produced foams

 V

	Sample	Saturation Pressure (MPa)	Saturation Temperature (º C)	Cell size (nm)	ρ r	E (Pa)
	90/10 PMMA/MAM	10	23	150	0.58	7.82 x 10 8
	90/10 PMMA/MAM	15	23	160	0.54	9.15 x 10 8
	90/10 PMMA/MAM	20	23	170	0.49	6.38 x 10 8
	90/10 PMMA/MAM	25	23	190	0.43	5.15 x 10 8
	90/10 PMMA/MAM	30	23	180	0.44	7.25 x 10 8
	90/10 PMMA/MAM	30	40	220	0.41	5.01 x 10 8
	90/10 PMMA/MAM	30	50	280	0.40	4.69 x 10 8
	90/10 PMMA/MAM	30	60	1000	0.37	4.80 x 10 8
	90/10 PMMA/MAM	30	70	950	0.36	4.70 x 10 8
	Neat PMMA	30	40	760	0.47	4.09 x 10 8
	Neat PMMA	30	50	720	0.48	2.90 x 10 8
	Neat PMMA	30	60	1870	0.44	3.41 x 10 8

Figura 0.5: Esquema de la estructura de la tesis

Acknowledgements

Financial assistance from MCINN (MAT2009-14001-C02-01 and MAT 2012-34901), the Junta of Castile and Leon (VA035U13), and FPU grant AP2008-03603 (J. Pinto) from the Spanish Ministry of Education is gratefully acknowledged. The help and expertise of Dr. Etienne Gontier, Dr. Soizic Lacampagne and Dr. Melina Petrel (BIC, Université Bordeaux Segalen) is indeed acknowledged for TEM observations and samples preparation.

structure. But the solubility also changes along the reaction (due to molar mass increase) and the more organized structures (i.e. the cylinders) tend to disappear for micelles.

Another example is a blend of two BCP's, di + tri butyl-methylmethacrylate; this blend still shows lamella but the layers are randomly oriented and are thicker than the neat "monodisperse" MAM.

II.2 Blends production route

Several blends of PMMA/MAM with different MAM amounts (from 5 to 75 wt%) were produced by extrusion using a Scamex CE02 single screw extruder (IUT Bordeaux 1, France). The extruder had a diameter of 45 mm and a length/diameter ratio (L/D) of 28.

Both materials, PMMA and MAM, were dried in vacuum (680 mm Hg) at 80º C during 4 h prior to the extrusion. Then, they were introduced in the extruder at the appropriate proportions, and processed with a temperature profile from 165º to 225º C and a screw speed of 60 rpm. Pellets from each blend were obtained using a continuous cutting machine operating at the end of the extrusion line.

PMMA/MAM blends generated following this procedure were transparent and macroscopically homogeneous.

Solid samples of neat polymers and PMMA/MAM blends were injection molded. Pellets, both raw materials and blends, were first dried in the same conditions as the extrusion process. Then, dried pellets were injected into bulk pieces (50 x 15 x 3 mm 3 ) using a small scale injection molding machine (DSM Xplore). Process parameters were set at 240º C for the melt temperature, 60º C for the mold temperature and 1 MPa during 8 seconds for the injection pressure. Obtained pieces were optically transparent, both for raw materials and blends, and without injection defects.

Main characteristics (density and glass transition temperature) of PMMA/MAM blends are shown on C/min and N 2 flow of 60 ml/min, temperature range was from 20º to 160º C; thus the T g of the PBA (about -50º C) is not detected. Therefore, only the T g (midpoint) of the PMMA (mixture of homopolymer and block) is measured.

Neat PMMA foams produced at RT and pressures between 10 and 30 MPa show a clear dependence of the cell size with the saturation pressure (Figure II.32). Foams produced at pressures up to 20 MPa present microcellular structures, while at higher pressures the cell sizes lie under the micron; reaching an average size about 90 nm in foams produced at 30 MPa.

Chapter III:

Relationship between PMMA/MAM blends nanostructuration and cellular structures. Foaming mechanisms

Chapter IV:

Influence of temperature on the foaming processes, mechanisms, and foams structures TEMPERATURE INFLUENCE AND CO 2 TRANSPORT IN FOAMING PROCESSES OF POLY(METHYL METHACRYLATE)-BLOCK COPOLYMER NANOCELLULAR AND MICROCELLULAR FOAMS

Javier Pinto Although the cell size of the foams and the evolution of their T g seems to be related, it was explained before that this effect should be related to the cell walls thickness, and not necessarily to the cell size. Cell walls thickness (δ) can be estimated from the cell size (Φ) and the relative density (ρ r ) using the following expression (Equation V-3) [6].

Chapter V: Transition between microcellular and nanocellular foams; consequences on mechanical and thermal properties

Where f s is the fraction of polymer mass in the edges, and C is a constant that depends in the cell shape.

Assuming f s as constant the cell wall thickness should be proportional to the product of the relative density and the cell size. Then, in order to estimate the influence of the cell wall thickness on the T g evolution, the measured temperature increment (∆T g = T g, foam -T g, solid ) can be represented as a function of

thermal conductivity is about 0.026 W/m•K, by gases with lower thermal conductivity. But this approach presents the disadvantages that this gas should be retained inside the foam (which is in fact very difficult in the long-term), avoiding diffusion or interchange with the environment, otherwise the foam will lose their improved insulation performance. Another disadvantage of this approach is that the typical gases with good performance are not very convenient from the environmental point of view, because they contribute to the global warming and/or to the ozone depletion.

A second approach to reduce the thermal conductivity is the use of nanocellular foams, because it is expected than nanocellular foams will present the so called Knudsen effect [1,4]. This effect implies that when the cell size size is comparable or smaller than the mean free path of the liquid or gas, the molecules of the latter collide more often with the molecules forming the solid part that among them. A schematic representation of this effect can be found in Figure V.10, where the gas molecules are represented by a sphere with radius equal to their mean free path (l g ). The measurement of the solid precursors shows that the thermal conductivity is not affected by the MAM amount; thus the thermal conductivity of all the foams under study can be analyzed just according to their relative density and cell size. When the normalized thermal conductivity (calculated as the relative thermal conductivity divided by the relative density) is represented as a function of the cell size a clear threshold is found between the nanocellular and the microcellular foams, being the conductivity of the nanocellular foams smaller. Taking into account the relative density of the analyzed materials (around 0.5) the thermal conductivity by radiation can be neglected (Figure V.9) and then our study can be focused on the two remaining mechanisms (conductivity through the gas and solid phase).

That difference is explained in terms of an increment of the polymer matrix tortuosity (Figure V.12), which reduces the thermal conductivity of the solid phase, and the contribution of the Knudsen effect.