X Yan

A Cayla

E Devaux

Belkacem Otazaghine

F Salaun

Keywords: polymer composites, micro/nanofiber, polypropylene (PP), melt-spinning, Janus particles

Simultaneous surface modification and mechanical enhancement of micro/nanofiber fabrics achieved by

Janus particles

Introduction

In the textile field, micro/nanofibers are being more and more widely utilized due to their high specific area for reinforcement, biomedical, filtration, and purification uses [START_REF] Fakirov | Converting of bulk polymers into nanosized materials with controlled nanomorphology[END_REF]. There are several strategies for manufacturing micro/nanofibers in the non-woven field, e.g., melt blowing [START_REF] Drabek | Meltblown technology for production of polymeric microfibers/nanofibers: A review[END_REF][START_REF] Eyer | Bimodal nanofiber and microfiber nonwovens by melt-blowing immiscible ternary polymer blends[END_REF], forcespinning [START_REF] Raghavan | Fabrication of melt spun polypropylene nanofibers by forcespinning[END_REF], and electrospinning [START_REF] Acik | Polypropylene microfibers via solution electrospinning under ambient conditions[END_REF]. Besides these methods, melt spinning combined with biphasic blending technology is also feasible for producing micro/nanofibers. There are two kinds of fibers with different implementation methods: segmented-pie or islands-in-the-sea bicomponent fibers [START_REF] Fedorova | High strength nylon micro-and nanofiber based nonwovens via spunbonding[END_REF], and the other is physically meltblended biphasic fibers. The former bicomponent fibers are manufactured by an especial bicomponent melt-spinning device with a specific spinneret, and this process gives the possibility of fabricating micro/nanofibers by subjecting them to mechanical or solvent actions [START_REF] Ayad | Effect of viscosity ratio of two immiscible polymers on morphology in bicomponent melt spinning fibers[END_REF][START_REF] Karthik | Nonwovens: Process, structure, properties and applications[END_REF]. The production of latter melt-blended biphasic fibers is conducted via the normal melt-spinning device of a relatively lower cost than the bicomponent melt-spinning one. In this case, due to the shear and extension flows, the dispersed phase is transformed into a fibrillar structure during the melt-spinning process in the form of short micro/nanofibers. Fakirov [START_REF] Fakirov | Nano-/microfibrillar polymer-polymer and single polymer composites: The converting instead of adding concept[END_REF][START_REF] Fakirov | Nanofibrillar polymer-polymer and single polymer composites via the 'converting instead of adding' concept -Examples of true polymer nanocomposite[END_REF] have conducted systematic researches of physically melt-blended biphasic fibers, and Simultaneous surface modification and mechanical enhancement of micro/nanofiber fabrics achieved by Janus particles X. Yan 1* , A. Cayla 1 , E. Devaux 1 , B. Otazaghine 2 , F. Salaün 1 1 ENSAIT, GEMTEX -Laboratoire de Génie et Matériaux Textiles, F-59000 Lille, France 2 Polymers Composites and Hybrides (PCH), IMT Mines Alès, Alès, France one of the core ideas is to manufacture the micro/ nanofibers through the removal of the matrix phase with a solvent. The selective phase extraction is also feasible to be placed after the production of the knitted fabrics from the multifilament yarns. Wang and Sun [START_REF] Wang | Formation and morphology of cellulose acetate butyrate (CAB)/polyolefin and CAB/polyester in situ microfibrillar and lamellar hybrid blends[END_REF] adopted PP as a dispersed phase for the manufacture of polypropylene micro/nanofibers by melt extrusion, drawing, and selective phase extraction. Their study focused on the changes in the morphology of minor phases. It was found that the design could produce the micro/nanofibers with a number averaged diameter of 2.5 µm (minimum/ maximum diameter: 0.4-6.5 µm). The corresponding matrix phase was cellulose acetate butyrate (CAB), of which the related solvent was organic solvent acetone instead of water. However, due to the improvement of people's environmental awareness, the importance of water-soluble polymers has been highlighted. Thermoplastic PVA is an outstanding candidate with an excellent fiber-forming property adopted in the textile industry. Robeson et al. [START_REF] Robeson | Microfiber formation: Immiscible polymer blends involving thermoplastic poly(vinyl alcohol) as an extractable matrix[END_REF] initially used polymer-PVA blends to fabricate micro/nanofibers. The approach is to blend PVA and the other polymer candidate by melt compounding and post orientation. Micro/nanofibers with the diameter of 0.1-5 μm were produced, including PP, polystyrene (PS), polyethylene (PE), poly(vinylidene fluoride) (PVDF), cellulose propionate, as well as an elastomer from poly (butylene terephthalate)polytetrahydrofuran block copolymer (Hytrel 4056, DuPont). Recycled polymer scrap was also successfully utilized to fabricate micro/nanofibers, while PVA as sacrificial polymer was proved to be also partially recyclable. Among the polymers, PP is a nonpolar thermoplastic polymer, vastly used in the textile industry, with low cost, density, and thermal conductivity, excellent chemical resistance, thermal stability, elasticity, and toughness. However, PP has some limits, e.g., sensitivity to fire, ultraviolet (UV) radiation, oxygen, and poor dyeability. The incorporation of nanoparticles near the PP surface has a potential to improve the related resistance and functionality. The clay particles are a branch of industrialized particles from natural materials with low cost, excellent thermal stability, and good dispersion properties [START_REF] Wu | Polypropylene/polystyrene/clay blends prepared by an innovative eccentric rotor extruder based on continuous elongational flow: Analysis of morphology, rheology property, and crystallization behavior[END_REF]. Clay particles possess a layered structure of tetrahedral silica oxide and octahedral Al, Mg, or Fe oxide. Clay minerals are generally classified into two categories, 1:1 and 2:1, which represent the ratio of the tetrahedral sheet to the octahedral one. Kaolin clays are 1:1 phyllosilicates and montmorillonite (e.g., kaolinite, dickite, nacrite, and halloysite). Kaolinite is very abundant and used mainly in classical applications (paper, ceramics, absorbents for pollution control) [START_REF] Dedzo | Functional nanohybrid materials derived from kaolinite[END_REF][START_REF] Su | Preparation of functionalized kaolinite/epoxy resin nanocomposites with enhanced thermal properties[END_REF]. It is discovered as anisometric platelets with aspect ratios of around 20:1-40:1 [START_REF] Hirsemann | Large-scale, low-cost fabrication of Janus-type emulsifiers by selective decoration of natural kaolinite platelets[END_REF] with the formula of Al 2 O 3 •2SiO 2 •2H 2 O. It is composed of a silica-containing tetrahedral sheet (TS) combined with an alumina-containing octahedral sheet (OS) [START_REF] Batistella | Fire retardancy of polypropylene/ kaolinite composites[END_REF]. The mineral particles have an inherent Janus character with different basal planes, which can be selectively modified (TS or OS) with hairy polymers. It is potential to enhance the compatibility with different phases [START_REF] Daitx | Kaolinite-based Janus nanoparticles as a compatibilizing agent in polymer blends[END_REF]. Not all the clay derivatives have inherent Janus properties, and for example, laponite and montmorillonite are symmetrical as 2:1-layered silicates without Janus properties [START_REF] Weiss | Hybrid Janus particles based on polymermodified kaolinite[END_REF]. Weiss et al. [START_REF] Weiss | Hybrid Janus particles based on polymermodified kaolinite[END_REF] tailored kaolinite via polymer chains to fabricate hybrid particles with Janus properties. The regioselective modifications were achieved for one side by simple cation exchange at the silica surfaces (TS layers). Meanwhile, the modifications were also for the other side by covalent grafting with catechol groups at the alumina surfaces (OS layers). The Janus kaolinite particles grafted with PS and PMMA were initially adopted for compatibilization of PS-PMMA blend via a solvent casting process. Daitx et al. [START_REF] Daitx | Kaolinite-based Janus nanoparticles as a compatibilizing agent in polymer blends[END_REF] used a similar methodology to synthesize Janus particles to further investigate their effects on the properties of the blends, e.g., thermal properties. Based on our previous research, it has been found that the regioselective grafting with phosphonic acid groups at the alumina surfaces is also a feasible method to endow the kaolinite with a significantly enhanced Janus morphology. We have found that the modification at single sides, which are alumina surfaces (OS layers), provides a facile method to fabricate Janus particles. The prepared particles realized the interfacial localization of the biphasic polymers PP-PVA [START_REF] Yan | Polypropylene/poly(vinyl alcohol) blends compatibilized with kaolinite Janus hybrid particles and their transformation into fibers[END_REF]. As for melt spinning of biphasic polymeric fibers, we have originally made efforts to thermodynamically control the localization of fillers towards the interface [START_REF] Yan | Porous fibers surface decorated with nanofillers: From melt-spun PP/PVA blend fibers with silica nanoparticles[END_REF]. Therefore, porous PP filaments decorated with fillers were successfully produced after selective phase extraction, where PVA was the dispersed phase. The related schematic figure is illustrated in Figure 1. If PVA becomes the matrix phase, it may provide a novel strategy to produce surface-modified micro/nano fibers. By this method, the surface of PP microfibers is easier to be exposed to the outside, which is beneficial for playing surface function. So far, there are no reports on the preparation of micro/nanofiber using the idea of thermodynamic control of fillers. Therefore, we aim to prepare surface-modified micro/ nanofibers with fillers as the basis for the development of novel functional fibers. In this study, we select PP as the minor phase (PP-PVA ratio: 30/70, wt/wt) to manufacture PP micro/nanofibers filled with nanoparticles and to control the fillers' migration towards the PP-PVA interface [START_REF] Yan | A green method to fabricate porous polypropylene fibers: Development toward textile products and mechanical evaluation[END_REF]. Due to their anisotropic surface, the Janus particles more easily adsorb at the biphasic interface. Herein, the kaolinite particles before and after regioselective modification were used in the melt spinning of PP-PVA biphasic polymer in order to fabricate the PP micro/nanofibers with surface functionality. Observations, including transmission electron microscopy (TEM) and scanning electron microscope (SEM) were applied to determine the morphology and dimension of the fibers. Also, mechanical tests were focused on monofilament fibers and knitted fabrics, before and also after selective phase extraction. Thermal behaviors of the obtained fibers were evaluated via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), to analyze the influence of kaolinite particles on the thermal stability and thermal properties of melting. These thermal analyses were done to determine the retention of particles on PP micro/nanofibers and the effect on their aggregate structure. [START_REF] Yan | Polypropylene/poly(vinyl alcohol) blends compatibilized with kaolinite Janus hybrid particles and their transformation into fibers[END_REF]. Octadecylphosphonic acid (ODPA) was supplied by Specific Polymers (Montpellier, France). In addition, ethanol and acetone were obtained by Fisher Chemical Company. The kaolinite (PARALUX™) was supplied by the Imerys Company (Paris, France), denoted as KL. Its specific surface area (measured by BET) is 12.2±1.5 m 2 /g, with the aspect ratio (α) of 10 and the average primary particle size (d el ) of 0.8 μm. The particles were also surface-modified to generate the Janus particles according to the work of Yah et al. [START_REF] Yah | Selective modification of halloysite lumen with octadecylphosphonic acid: New inorganic tubular micelle[END_REF] Briefly, 100 g of KL, 2.5 g of ODPA, and 300 ml of an ethanol/water solution (96/4) were poured in a 500 ml flask adapted with a condenser. The mixture was stirred under solvent reflux for 15 h, before being centrifuged to get rid of the liquid phase and washed three times with acetone. The modified clay particles were dried under vacuum for subsequent use and denoted as KJ.

Experimental section

Sample preparation 2.2.1. Melt extrusion of the polymers

The raw materials of polymers were dried to remove moisture in the oven at 80 °C for at least 12 h. All of the materials were melt-compounded by using a corotating intermeshing twin-screw extruder (Thermo Haake, screw diameter of 16 mm, L/D = 25) [START_REF] Yan | Microstructure evolution of immiscible PP-PVA blends tuned by polymer ratio and silica nanoparticles[END_REF]. The temperature profiles of the five heating zones were adjusted to 160°C/170°C/180°C/190°C/200°C, respectively. The screw rotational speed was set as 100 rpm, and the extruded rod out of the die was conveyed with the aid of the conveyer belt, being cooled with the air stream, and then cut by the granulator. The blends of PP and PVA without fillers were directly compounded. The weight ratio of the two polymers (PP to PVA) was 30/70 (wt/wt), denoted as PP 30 -PVA 70 . As for the compounds with kaolinite particles, the weight fraction of KL and KJ particles was fixed at 2 wt.%. The blends with particles were mixed by two steps. In the first step, kaolinite particles were compounded with PP, and in the second step, the prepared polymer composites were extruded again with PVA. The samples containing KL and KJ were denoted as PP 30 -PVA 70 -KL 2 and PP 30 -PVA 70 -KJ 2 , respectively.

Melt spinning of polymer blends

The extruded blends were melt-spun to manufacture the multifilament yarns. Before melt spinning, the extrudates were dried at 80 °C overnight to remove moisture. The melt-spinning process was carried out on a Busschaert Engineering Spinboy I (Deerlijk, Belgium). Table 1 summarizes the component fractions, the melt-spinning temperatures, the rotating speed of the volumetric pump, and the theoretical draw ratio (DR). The melt-spinning temperatures include temperatures of five heating zones in the extruder zone and those of the spin-head and spinneret. The blends were melted through the single screw extruder and passed through the filter, and two parallel dies with 40 circular holes, of which the diameter is 1.2 mm. A volumetric pump (15 rpm, flow rate: 52.5 cm 3 /min) was equipped to accurately transport the material to manufacture multifilament yarns composed of 80 monofilaments. The as-spun multifilament yarns were cooled down by the airflow and afterward lubricated by a spin finish oil. It was wateroil-surfactant mixtures to increase the cohesion between the filaments and reduce static electricity. The multifilament yarns were stretched due to the speed difference between the feed roll and the draw one.

The theoretical stretching draw ratio (DR) was controlled as DR = V 2 /V 1 , where V 1 is the speed of feed roll, and V 2 is the speed of draw roll. The stretched multifilament yarns were collected on the bobbin. Two different draw ratios (2 or 4) were adopted, 100 rpm of the first roll and 200 or 400 rpm of the second roll. The suffix -DR 2 and -DR 4 are used for the indication of different draw ratios of the meltspun fibers originated from the extrudates. For example, PP 30 -PVA 70 -KL 2 -DR 4 refers to the melt-spun fibers with DR = 4 from PP 30 -PVA 70 -KL 2 extrudates.

Preparation and selective phase extraction of the knitted fabrics

The multifilament yarns were further knitted to achieve fabric implementation. In the first step, the multifilament yarns were twisted (25 twists/meter, Z direction) via the twisting machine (Twistec, Barcelona, Spain). In the second step, the twisted multifilament yarns were knitted on a flat knitting machine (Dubied, Couvet, Switzerland), of which the gauge value is 7 needles/inch. The selected texture was 1×1 Rib stitch, where the loop notation refers to related literature [START_REF] Soyaslan | Determination of electromagnetic shielding performance of plain knitting and 1×1 rib structures with coaxial test fixture relating to ASTM D4935[END_REF]. In order to obtain the knitted fabrics composed of PP micro/nanofibers, the selective phase extraction experiment was carried out to remove the PVA phase of the knitted fabrics under magnetic stirring in a hot water bath. The fabrics were immersed in the distilled water with a temperature of 80 °C, and the extraction time is fixed at 5 h. Finally, the obtained samples were rinsed with warm water to clean the multifilament yarn surface, before being dried overnight in the oven at 50 °C in order to get rid of water. It was found that almost all the PVA is removable during the selective phase extraction experiment based on the weighing method. The suffix -Ex is adopted for the indication of the materials after the selective phase extraction experiment. For example, PP [START_REF] Astm D | Standard test method for thickness of textile materials[END_REF] 2.

Characterization 2.3.1. Morphology observation

Transmission electron microscopy (TEM) observation (Tecnai G2 F20 S-TWIN, FEI Company, USA) was conducted on the cross-section of the extrudates in Analytical & Testing Center, Sichuan University. The cross-section was exposed via slicing the sample to the thickness of tens of nanometers with the help of liquid nitrogen. The images of the knitted fabrics before and after the selective phase extraction experiment were acquired via a digital camera (Huawei P9 EVA-AL00, Shenzhen, China). The knitted fabrics from the PP 30 -PVA 70 -DR 4 fibers before and after selective phase extraction were metalized and afterward observed by field emission scanning electron microscopy (SEM) on Apreo S (ThermoFisher Scientific, USA) in Sichuan University (Chengdu, China). The accelerating voltages are from 5.0 to 20.0 kV. Also, to better observe the morphology of the biphasic fibers (PP 30 -PVA 70 ) with kaolinite particles before and after selective phase extraction, further characterization was performed on a scanning electron microscope (FEI Quanta 200 ESEM) in PCH laboratory, IMT Mines Alès (France), accompanied with the secondary electron (SE) and back-scattered electron (BSE) mode. The accelerated voltage was 12.5 kV. Before the SEM experiment, the samples were coated with a thin carbon layer. The aforementioned experiments were repeated at least three times.

Melt flow index (MFI)

The melt flow index (MFI) measurement was realized on the Melt Flow Tester (Thermo Haake, USA), according to the standard ASTM D1238 [START_REF] Astm D | Standard test method for melt flow rates of thermoplastics by extrusion plastometer[END_REF]. The piston and the tested polymer were pre-heated for 4 and 3 minutes, respectively. The temperature was kept at 190 °C, and the load during the experiment was selected as 2.16 kg. The MFI value was recorded with the unit [g/10 min].

Thermal behaviors

To evaluate the thermal stability of the samples, the thermogravimetric analysis (TGA) was carried out via a TGA/DSC 3+ (Mettler Toledo, Columbus, Ohio, USA) under nitrogen and air atmosphere at a purge rate of 50 ml/min. For each experiment, a sample of approximately 10 mg was used. To remove the physisorbed water, an isothermal step (10 min, 100 °C) was conducted before starting the analysis and then the samples were heated to 700 °C at a heating rate of 10 °C/min. The thermal properties of all the fiber samples after the selective phase extraction were observed with differential scanning calorimetry (DSC) with TGA/ DSC 3+, under constant nitrogen flow (50 ml/min). Each milligram sample was placed in a hermetically sealed aluminum pan. The scanning procedure involved heating from 20 to 220 °C at a rate of 10 °C/min, during which the melting endotherm was recorded. The parameters including melting temperature (T peak , maximum of the peak), onset, endset, and range of the melting peaks (T onset , T endset , ΔT = T endset - T onset ), and corresponding enthalpies were measured. In addition, Equation (1) was used to calculate the crystallinity degree (X C ): [START_REF] Fakirov | Converting of bulk polymers into nanosized materials with controlled nanomorphology[END_REF] where ΔH f is the heat of fusion of the tested samples [J/g], and ΔH f 0 is the reference value of the heat of fusion for a perfect 100% crystalline polymer. As for PP material, ΔH f 0 can be fixed as 209 J/g [START_REF] Brandrup | Polymer handbook[END_REF]. Herein, due to the almost complete extraction of PVA and low addition amount of kaolinite particles, we ideally only consider the impact of PP to make a semiquantitative analysis.

The aforementioned experiments were repeated at least three times.

Mechanical properties

To test the mechanical properties of the filaments, tensile tests were carried out on single monofilaments extracted from the 80-monofilament multifilament yarns, on a machine with the model name of Zwick 1456 (Germany), according to the ISO 5079 standard [START_REF]Textile fibres -Determination of breaking force and elongation at break of individual fibres[END_REF]. The load cell of the machine was 10 N, the test length was 20 mm and the deformation speed The mechanical properties of the knitted fabrics from PP 70 -PVA 30 -DR 3 before and after selective phase extraction were also tested on an electromechanical universal test system (MTS Criterion Model 43, Eden Prairie, USA) with a load cell of 10 kN. The width of the selected fabrics was near 40 mm, and the gauge length of the sample was selected as 60 mm, and the deformation speed was set as 30 mm/min. In addition, the thicknesses of the knitted fabrics (millimeter level) were tested by a fabric thickness tester (Sodemat, France), based on the standard ASTM D 1777-96 (2007) [START_REF] Astm D | Standard test method for thickness of textile materials[END_REF].

X H H 100 c f f 0 $ D D =

Air permeability

The air permeability of textile fabrics is evaluated based on the rate of flow of air passing perpendicularly through a given area of fabric by measuring its difference across the fabric test area at a given pressure over a given period. Transverse air permeability was measured with a machine named 'Permeabilimetre à l'air' (Emi Développement, Bréviandes, France) with a pressure applied of 100 Pa and a test area of 20 cm 2 , based on ISO 9237:1995 [START_REF]Textiles-determination of the permeability of fabrics to air[END_REF]. Air permeability tests were also carried out in a standard atmosphere (relative humidity: 65±5%; temperature: 20±2 °C).

Results and discussion 3.1. Melt-extruded polymer blends (rod)

PP and PVA with pristine and modified kaolinites were melt-compounded with the twin-screw extruder. Due to the difference of the particles in the surface tension, the localization is expected to differ within the polymer blends. Thus, TEM observation was conducted to observe the localization of kaolinite particles before and after regioselective modification. The related localization is illustrated in Figure 2, where the dark-colored phase represents the PP phase, and the light-colored phase is the PVA phase. As for unmodified KL particles, there is a more pronounced agglomeration with a stacked structure due to stronger hydrogen bonds. KL particles are distributed at the interface and also in the bulk of the PVA phase. The hydrophilicity of KL particles leads to their migration from the PP phase towards the PVA phase during the melt extrusion process. As for the Janus kaolinite particles, the octahedral layer (alumina face) becomes hydrophobic via regioselective grafting of octadecyl groups (C 18 H 37 -). As a result, the polarity and dispersibility of the kaolinite particles are tuned. From the TEM images, it is observed that KJ particles are mainly localized at the interface of PP and PVA due to the regioselective modification of the kaolinite particles. Furthermore, the octahedral structure of KJ particles with a thinner layer is observed in Figure 2c. Generally, the modification of kaolinite particles into Janus morphology is beneficial for interfacial localization. The obtained extrudates were prepared to be meltspun, and their fluidity during the high-temperature processing strongly influences the melt spinnability.

For the melt-spinning machine, the temperature at which the MFI index is between 15 and 30 g/10 min

Morphology of the knitted fabrics

The production of fabrics was realized by twisting, and then the knitting process. Afterward, the knitted fabrics were selectively extracted in the water bath to remove the PVA phase. The PP micro/nanofibers in the PP phase were released after the PVA removal, as the target product of the research. The schematic process of the selective phase extraction of PP micro/nanofiber fabrics is illustrated in Figure 4. The shape of knitted fabrics is maintained, and the color transforms from yellow to white, exhibiting the natural color of PP material. In order to discover the microscopic structures of the obtained samples, SEM observations were realized. It was started from the observation of the samples without particles, shown in Figure 5. Figure 5a and 5d illustrate the morphology of the fabric before selective phase extraction, where the knitting structure Figure 5a and the surface of the biphasic fiber Figure 5d are observable.

After the selective phase extraction of PVA in hot water, the physical aspect of the knitting structure is maintained (Figure 5b), with the presence of micro/ nanofibers (Figure 5e, 5c and 5f). The mean diameters are about hundreds of nanometers, corresponding to an increase of the specific surface area. The quantitative calculation of PP micro/nanofiber diameter is difficult to be carried out in the case of the diameter deviations (tens of micrometer in diameter) of the PP 30 -PVA 70 -DR 4 fibers before selective phase extraction. This is common for the melt-spun multifilament yarns from the immiscible polymer blends due to uncontrollable swelling coefficient and inhomogeneous morphology. Furthermore, it is distinguished in Figure 5f that the micro/nanofibers are not entirely isolated. Instead, the micro/nanofibers are interconnected, which is also the reason for the well maintaining of the monofilament and knitting structure after selective phase extraction. Furthermore, to investigate the morphology of the fibers filled with kaolinite particles, the related SEM images are displayed in Figure 6. A previous study has explained and successfully demonstrated the method of detecting the particle localization by BSE mode [START_REF] Yan | Polypropylene/poly(vinyl alcohol) blends compatibilized with kaolinite Janus hybrid particles and their transformation into fibers[END_REF]. The white dots are the kaolinite particles on the BSE images. The surface of the fibers is rough, and the fibrillar structure is visible, even before the selective phase extraction, and some particles have migrated towards the surface. In order to further reveal the localization of fillers at the biphasic interface, fibers after selective phase extraction were also observed via SEM characterization coupled with BSE mode. The related results are illustrated in Figure 7. Firstly, the morphology of a large number of micro/nanofibers is observable after the selective phase extraction (Figure 7a and7d), with the diameter of hundreds of nanometers (Figure 7c and 7f). Secondly, some kaolinite particles are retaining on the PP surface according to the BSE mode observation (Figure 7b and7e).

For the pellets and the obtained fibers, we have found that the PVA is completely accessible during the selective phase extraction experiment with hot water by weighing method. In order to investigate if some kaolinite particles retain on the PP micro/nanofibers, an indirect method is also feasible by using the thermogravimetric analysis (TGA). The detailed results are displayed in Figure 8. The TGA analyses of kaolinite particles have been done in the previous study to reveal their thermal stability [START_REF] Yan | Polypropylene/poly(vinyl alcohol) blends compatibilized with kaolinite Janus hybrid particles and their transformation into fibers[END_REF]. Based on the TGA data, the residues at 700 °C are summarized in Table 3. In the previous study, it has been observed that the initial decomposition temperatures (5% decomposition) of KL and KJ particles were around 500 °C under nitrogen and air atmospheres. In addition, their residues at 700 °C were approximately about 85-86%, of which the modified kaolinite exhibits a higher weight loss for the degradation of the grafted ODPA. There is no residue for unfilled PP micro/nanofibers at 700°C, but those for the PP micro/nanofibers filled with kaolinite are over 2.7% in any circumstances. Thus, it proves that both KL and KJ particles are embedded with the PP micro/ nanofibers and partially exposed to the external environment, from SEM observation and TGA analysis.

The retention of particles is mainly ascribed to two reasons. The first is their interfacial localization within the biphasic polymers, especially KJ particles. The second is their large dimension compared with the phase size of polymers. The average primary particle size of kaolinite particles is 800 nm, of the equivalent scale of micro/nanofiber diameters and the spacing between the micro/nanofibers, which is distinguishable from Figure 5 and Figure 7. It indicates that the particles within the PVA phase of the extrudates (referable to Figure 2) have the possibility to be bridged with the PP phase during the melt-spinning process, for the scale of the PVA phase drops sharply and cannot fully accommodate kaolinite particles.

Thermal properties

The crystallization degree is an important and easily measurable parameter to describe the aggregate structure and is inferable from the DSC results for the thermal properties. Therefore, the DSC measurement was carried out towards the obtained PP micro/ nanofibers. The DSC curves are displayed in Figure 9, and related details are summarized in Table 4.

There is a single peak for the PP micro/nanofibers when the draw ratio is 2, with a melting point of 165 °C. In addition, the introduction of the Janus kaolinite allows shifting the melting point to the higher temperatures, but there is no significant modification of the crystallinity degree. When the draw ratio is 4, the crystallinity degree is increased from 30.4 to 32.1%, and the melting point is also increased from 165 to 168 °C. The incorporation of Janus kaolinite particles gives rise to the further increment of crystallinity degree towards 33.4%. Furthermore, the endothermic curve becomes multi-peak when the draw ratio is enhanced, especially when Janus KJ particles are introduced. It is ascribed to the difference of crystal forms or the degree of their perfection due to different recrystallization situation at high temperatures during drawing [START_REF] Dabrowska | Compounding and spinning of polypropylene nanocomposites with kaolinite[END_REF][START_REF] Cho | Structural properties and superhydrophobicity of electrospun polypropylene fibers from solution and melt[END_REF].

Mechanical behaviors

The incorporation of kaolinite particles inevitably influences the mechanical properties of the multifilament yarns. The tensile tests were realized for the monofilaments randomly extracted from them. The mechanical properties of the PP 30 -PVA 70 filaments with and without kaolinite particles and the related representative stress-strain curves are illustrated in Table 5 and Figure 10, respectively. A low draw ratio (chosen as 2) during the melt-spinning process is usually not conducive to crystallization and orientation development with good mechanical properties of fibers. The related fibers exhibit low tenacity and high breaking elongation, which is ascribed to the low molecular chain orientation and low crystallization degree. The addition of kaolinite particles into the polymer blend significantly enhances Young's modulus and tenacity, while the breaking elongation is slightly decreased. With the increment of the draw ratio from 2 to 4, the mechanical properties are enhanced significantly. Young's modulus is increased sharply from 842 to 2916 MPa, while the tenacity increases from 5.8 to 19.4 cN/Tex. Meanwhile, from the shape of the curves, it is illustrated that the ductility of the fibers is significantly decreased, and breaking elongation is decreased from 47 to 21%. The incorporation of kaolinite particles becomes detrimental to Young's modulus as well as the tenacity. Based on the stressstrain curves, it is observed that within a certain degree of strain, the stress is enhanced due to the kaolinite particles. Meanwhile, the particles as stress concentration points lead to the advance of breakage, especially KJ particles. The diameter of the PP micro/nanofibers is in the same order of magnitude as the diameter of the kaolinite particles. Therefore, the increase of draw ratio brings about more stress concentration points.

As for the knitted fabrics, the related mechanical behaviors were also investigated via the tensile tests. Through these tests, the effect of the selective phase extraction was also taken into consideration. A higher draw ratio during the melt-spinning process leads to the enhancement of the mechanical properties of the monofilament fibers with a reduced thickness, which is beneficial for the knitting process. Therefore, the knitted fabrics from multifilament yarns with DR of 4 were tested for mechanical properties. The mechanical properties, as well as the air permeability of the knitted fabrics from PP 30 -PVA 70 fibers before and after selective phase extraction, are illustrated in Table 6. The stress-strain curves of knitted fabrics from PP 30 -PVA 70 fibers with DR = 4 are also displayed in Figure 11. As for the fabrics before selective phase extraction, the elongation at maximum force of the fabrics from PP 30 -PVA 70 -DR 4 -Knit is 86.3%. The addition of KL and KJ particles significantly enhances the maximum stress from 8.6 to 10.8 and 11.3 MPa with an increment of 26 and 31%, respectively. Meanwhile, the corresponding elongation is decreased with the incorporation of KJ particles. More detailed information lies in the stress-strain curves, and the enhancement effect of kaolinite particles is obviously distinguished from the sharp increment of the slope, especially with Janus particles. The stress concentration points control the breakage of the monofilament fibers, and the effect is relatively weakened in the form of the fabrics from multifilament yarns. It is ascribed that some monofilament fibers with less stress concentration points and better mechanical properties play an important role in the mechanical behaviors of the knitted fabrics. The PP micro/nanofiber fabrics are manufactured after the selective phase extraction. For their mechanical behaviors, the new thickness is taken into account. It is found that the maximum stress is decreased, and the corresponding elongation is significantly increased, mainly due to the 'sliding effect' of obtained PP micro/nanofibers between loops. With the incorporation of unmodified kaolinite particles KL, the maximum stress and corresponding elongation are decreased. Furthermore, KJ particles have no obvious impact on its corresponding elongation value and enhance the maximum stress from 5.8 to 6.8 MPa with an increment of 17%. The mechanical enhancement of the Janus kaolinite particles upon the fabrics still exists after PVA removal, attributed to the optimized aggregate structure of the PP phase. In conclusion, while imparting surface modification to the micro/nanofiber fabrics, the Janus kaolinite particles also endow them with better mechanical properties.

In other aspects, the thicknesses of the fabrics, as well as their areal density, are decreased after the removal of PVA via selective phase extraction. Furthermore, the air permeability is slightly increased due to the loss of material. The most visible increment appears in the samples with Janus kaolinite particles after selective phase extraction.

Conclusions

In this paper, the ratio of PP and PVA blends is 30 wt.%/70 wt.% for manufacturing the PP-PVA composite fibers, in which PP is transformed into micro/nanofibers in the PVA matrix phase. With the aid of green solvent water, the PP micro/nanofibers are obtained after the removal of the PVA phase. Kaolinite particles are incorporated into the PP 30 -PVA 70 blends, of which raw kaolinite and Janus one particles are selected. TEM measurement confirms that Janus particles have a higher tendency to be confined at the interface. The multifilament yarns composed of the binary and ternary blends were successfully spun, and the knitting technology was adopted to create textile products followed by selective phase extraction. Numerous PP micro/nanofibers with diameters of hundreds of nanometers are released, and the knitting structure still maintains. The results demonstrate that some kaolinite particles are embedded with the PP phase, no matter which kaolinite particles are selected. The introduction of kaolinite particles may bring about more stress concentration points in the biphasic fibers; however, the enhancement effects still exist for the knitted fabrics with the Janus particles even after the selective phase extraction, with the optimized aggregate structure reflected in the increased crystallization degree. It sets an innovative example of manufacturing the thermoplastic polymer micro/nanofiber surface-modified and simultaneously mechanically enhanced with Janus particles.

Figure 1 .

 1 Figure 1. Schematic figure of fabrication of surface-modified porous fibers and micro/nanofibers.

  -PVA 70 -KL 2 -DR 4 -Ex refers to the micro/nanofibers after extraction from initial PP 30 -PVA 70 -KL 2 -DR 4 fibers. The suffix -Knit is placed after the indication of the PP-PVA blend fibers to denote the corresponding knitted fabrics. For example, PP 30 -PVA 70 -KL 2 -DR 4 -Knit refers to the knitted fabrics from PP 30 -PVA 70 -KL 2 -DR 4 fibers. Similarly, the corresponding knitted fabrics after selective phase extraction is PP 30 -PVA 70 -KL 2 -DR 4 -Knit-Ex. All the denotements of samples are summarized in Table

Figure 2 .

 2 Figure 2. The TEM images of the extrudates in rod form (cross-section) from (a) PP 30 -PVA 70 -KL 2 ; (b) PP 30 -PVA 70 -KJ 2 ;(c) zoomed picture of (b).

  is a recommended reference, especially for the spinhead and spinneret to succeed in the spinnability. Therefore, the extrudates were characterized by the melt flow indexer. The melt flow indexes of neat polymers PP and PVA and their blends at 190 °C are presented in Figure3. The melt flow index of PP is 11.8 g/10 min, while that of PVA is 19.3 g/10 min. After the blending of the polymers, the melt flow index increases up to 41.6 g/10 min. The increment in the fluidity of the blends results from the weak interactions of the two immiscible components PP and PVA[START_REF] Ku | Rheological properties of thermoplastic polyvinyl alcohol and polypropylene blend melts in capillary extrusions[END_REF]. The kaolinite particles slightly decrease the melt flow index. The PP 30 -PVA 70 -KL 2 exhibits the lowest value as 37.2 g/10 min among the five groups of samples. KL particles are also localized in the PVA matrix phase, which has a more noticeable impact upon the fluidity compared with the KJ particles mostly localized at the biphasic interface. It is originated from the different impacts of particles on the fluidity of the PVA phase and the interaction of the two components.

Figure 3 .

 3 Figure 3. Melt flow index (190 °C, 2.16 kg) of PP, PVA, and the blends with/without kaolinite particles.

Figure 4 .

 4 Figure 4. The selective phase extraction process of PP 30 -PVA 70 -DR 4 fabrics. (a) The fabric before extraction; (b) the fabric after extraction.

Figure 5 .

 5 Figure 5. SEM images of knitted PP 30 -PVA 70 -DR 4 fabrics with increasing magnification. (a), (d) before selective phase extraction; (b ), (c), (e), (f) after selective phase extraction.

Figure 6 .

 6 Figure 6. SEM images of (a) PP 30 -PVA 70 -KL 2 -DR 4 fibers; (b) BSE of (a); (c) PP 30 -PVA 70 -KJ 2 -DR 4 fibers; (d) BSE of (c).

Figure 7 .

 7 Figure 7. SEM images of (a), (b), (c) PP 30 -PVA 70 -KL 2 -DR 4 fibers after extraction; (d), (e), (f) PP 30 -PVA 70 -KJ 2 -DR 4 fibers after extraction ((b), (e) BSE mode of (a), (d)).

Figure 8 .

 8 Figure 8. TGA results of the PP micro/nanofibers extracted from PP 30 -PVA 70 blend fibers with different draw ratios under different atmospheres. (a) DR = 2, N 2 atmosphere; (b) DR = 4, N 2 atmosphere; (c) DR = 2, air atmosphere; (d) DR = 4, air atmosphere.

Figure 9 .

 9 Figure 9. DSC spectra of PP micro/nanofibers after extraction (a) from PP 30 -PVA 70 with/without kaolinite particles with DR = 2; (b) from PP 30 -PVA 7 0 with/without kaolinite particles with DR = 4.

Figure 10 .

 10 Figure 10. Stress-strain curves of (a) PP 30 -PVA 7 -DR 2 fibers with/without kaolinites; (b) PP 30 -PVA 7 -DR 4 fibers with/without kaolinites.

Figure 11 .

 11 Figure 11. Stress-strain curves of knitted fabrics from PP 30 -PVA 70 fibers (a) before selective phase extraction; (b) after selective phase extraction.

2.1. Materials

	melt flow index is 15 g/10 min (200 °C, 2.16 kg).
	Poly(vinyl alcohol) (PVA) (OKS-8042P), with a
	melting temperature of about 178 °C, and a melt flow
	index of about 23 g/10 min (200 °C, 2.16 kg), was
	supplied by Nichigo Gohsei (Osaka, Japan, spinning
	grade)
	Polypropylene (PP) (PPH 9069) was purchased from
	Total S.A. Company (Courbevoie, France, spinning
	grade). Its melting temperature is 168 °C, and the

Table 1 .

 1 The melt-spinning parameters of the blends with kaolinite particles.

			Mass fraction	Temperature
	Denotement	PP	[%] PVA	Filler KJ or KL	Extruder	[°C]	Spinhead Spinneret	Pump [rpm]	DR
	PP 30 -PVA 70	30.0	70.0	0	175/180/185/190/192	187	15	2 or 4
	PP 30 -PVA 70 -KL 2 PP 30 -PVA 70 -KJ 2	29.4	68.6	2	175/180/185/190/194	189	15	2 or 4

Table 2 .

 2 Denotements of the samples. PVA 70 -DR 2 -Knit PP 30 -PVA 70 -DR 2 -Ex PP 30 -PVA 70 -DR 2 -Knit-Ex PP 30 -PVA 70 -DR 4 4 PP 30 -PVA 70 -DR 4 -Knit PP 30 -PVA 70 -DR 4 -Ex PP 30 -PVA 70 -DR 4 -Knit-Ex PP 30 -PVA 70 -KL 2 PP 30 -PVA 70 -KL 2 -DR 2 2 PP 30 -PVA 70 -KL 2 -DR 2 -Knit PP 30 -PVA 70 -KL 2 -DR 2 -Ex PP 30 -PVA 70 -KL 2 -DR2-Knit-Ex PP 30 -PVA 70 -KL 2 -DR 4 4 PP 30 -PVA 70 -KL 2 -DR 4 -Knit PP 30 -PVA 70 -KL 2 -DR 4 -Ex PP 30 -PVA 70 -KL 2 -DR 4 -Knit-Ex PP 30 -PVA 70 -KJ 2 PP 30 -PVA 70 -KJ 2 -DR 2 2 PP 30 -PVA 70 -KJ 2 -DR 2 -Knit PP 30 -PVA 70 -KJ 2 -DR 2 -Ex PP 30 -PVA 70 -KJ 2 -DR 2 -Knit-Ex PP 30 -PVA 70 -KJ 2 -DR 4 4 PP 30 -PVA 70 -KJ 2 -DR 4 -Knit PP 30 -PVA 70 -KJ 2 -DR 4 -Ex PP 30 -PVA 70 -KJ 2 -DR 4 -Knit-Ex was 20 mm/min. Tensile tests were carried out in a standard atmosphere (relative humidity of 65±5% with a temperature of 20±2 °C) [29]. Measurements were conducted for 10 different single monofilaments from the same yarn. In order to examine the fineness [dTex] of each single monofilament before extraction prior to tensile tests, a vibroscope machine (Vibroskop, Lenzing Instruments, Germany) was operated based on the NF G 07-306 standard. The fineness is used to calculate Young's modulus in [MPa] and also tenacity in [cN/Tex].

	Extrudates	Denotement	Melt-spun fibers DR Knitted fabrics	Micro/nanofibers with the removal of PVA	Fabrics with the removal of PVA
	PP 30 -PVA 70	PP 30 -PVA 70 -DR 2	2	PP 30 -

Table 3 .

 3 Residues (at 700 °C) of kaolinite particles and PP micro/nanofibers extracted from PP 30 -PVA 70 blend fibers with different draw ratios under different atmospheres.

		Residue in N 2	Residue in air
	Samples	atmosphere	atmosphere
		[%]	[%]
	KL particles	86.1	86.1
	KJ particles	85.1	84.7
	PP 30 -PVA 70 -DR 2 -Ex	0.0	0.0
	PP 30 -PVA 70 -KL 2 -DR 2 -Ex	3.0	3.3
	PP 30 -PVA 70 -KJ 2 -DR 2 -Ex	2.7	2.9
	PP 30 -PVA 70 -DR 4 -Ex	0.1	0.0
	PP 30 -PVA 70 -KL 2 -DR 4 -Ex	4.0	3.8
	PP 30 -PVA 70 -KJ 2 -DR 4 -Ex	3.7	3.0

Table 4 .

 4 Thermal properties of PP micro/nanofibers extracted from PP 30 -PVA 70 with/without kaolinite particles with different draw ratios.

		Draw ratio	Enthalpy [J/g]	X c [%]	T onset [°C]	T endset [°C]	ΔT [°C]	T peak [°C]
	PP 30 -PVA 70 -DR 2 -Ex	2	63.48	30.4	150.6	174.0	23.4	164.8
	PP 30 -PVA 70 -KL 2 -DR 2 -Ex	2	67.94	32.5	151.6	172.4	20.8	164.6
	PP 30 -PVA 70 -KJ 2 -DR 2 -Ex	2	62.94	30.1	151.9	173.4	21.5	167.1
	PP 30 -PVA 70 -DR 4 -Ex	4	67.10	32.1	150.2	173.9	23.7	167.9
	PP 30 -PVA 70 -KL 2 -DR 4 -Ex	4	67.81	32.4	155.5	173.4	17.9	166.5
	PP 30 -PVA 70 -KJ 2 -DR 4 -Ex	4	69.91	33.4	155.7	175.8	20.1	166.6

Table 5 .

 5 Mechanical properties of the composite fibers from PP 30 -PVA 70 with/without kaolinite particles.

	Fibers	Young's modulus [MPa]	Tenacity [cN/Tex]	Breaking elongation [%]
	PP 30 -PVA 7 -DR 2	0842±124	5.8±1.0	47±14
	PP 30 -PVA 7 -KL 2 -DR 2	1638±446	7.0±1.0	35±13
	PP 30 -PVA 7 -KJ 2 -DR 2	1810±674	7.3±1.4	34±9
	PP 30 -PVA 7 -DR 4	2916±431	19.4±1.9	21±3
	PP 30 -PVA 7 -KL 2 -DR 4	1987±209	18.6±1.8	17±2
	PP 30 -PVA 7 -KJ 2 -DR 4	2165±225	15.8±3.7	13±3

Table 6 .

 6 Mechanical properties of the knitted fabrics from PP 30 -PVA 70 fibers.

	Fabrics	Thickness [mm]	Width [mm]	Areal density [g/m 2 ]	Maximum stress [MPa]	Elongation at maximum force [%]	Air permeability [mm/s]
	PP 30 -PVA 70 -DR 4 -Knit	2.451	42	838.7	8.6±0.2	86.3±3.5	2532±45
	PP 30 -PVA 70 -KL 2 -DR 4 -Knit	2.359	42	825.4	10.8±0.8	87.8±3.9	2780±109
	PP 30 -PVA 70 -KJ 2 -DR 4 -Knit	2.353	41	824.9	11.3±0.1	67.8±1.5	2780±109
	PP 30 -PVA 70 -DR 4 -Knit-Ex	1.840	41	319.3	5.8±0.5	153.9±14.3	2571±187
	PP 30 -PVA 70 -KL 2 -DR 4 -Knit-Ex	1.715	42	307.3	5.3±0.1	111.5±14.0	2634±98
	PP 30 -PVA 70 -KJ 2 -DR 4 -Knit-Ex	1.825	42	302.0	6.8±0.6	148.2±21.7	2991±190

Acknowledgements

The author Xiang Yan thanks for the financial support for PhD from China Scholarship Council.


Vincent Besse

Rémi Auvergne

Stéphane Carlotti
email: carlotti@enscbp.fr

Gilles Boutevin

Belkacem Otazaghine
email: belkacem.otazaghine@mines-ales.fr

Sylvain Caillol

Jean-Pierre Pascault

Bernard Boutevin
email: bernard.boutevin@enscm.fr

Synthesis of isosorbide based polyurethanes: An isocyanate free method

Keywords:

  L'archive ouverte pluridisciplinaire

Introduction

Generally, linear polyurethanes (PUs) are obtained by the reaction between an oligomeric diol (low molecular weight polymer with terminal hydroxyl groups), a short diol as chain extender and a diisocyanate. To prepare cross-linked PUs, polyols or isocyanates with functionality higher than 2 can be used. However the use of diisocyanate should be avoided for several reasons. Isocyanate reactants are generally very harmful for human health, particularly for people exposed during polyurethanes synthesis and could entail adverse health effects such as asthma, dermatitis, conjunctivitis and acute poisoning [1]. Therefore the synthesis of PUs, from step growth polyaddition of dicyclocarbonates and diamines should be favored. This method is particularly interesting since no hazardous isocyanates are used. Thus, this old reaction is currently gaining a lot of attention as a substitution route for the synthesis of PUs [2][3][4][5][6][7][8][9]. Following this route, polyhydroxyurethanes (PHUs) are obtained with inter-and intramolecular hydrogen bonds, which are expected to present higher chemical and hydrolysis resistances.

We previously reported several works on the synthesis of new dicyclocarbonates by esterification of diacids such as terephthalic acid with glycerol carbonate. These works led to the synthesis of PHUs with ester bonds which exhibited poor stability towards hydrolysis [10,[START_REF] Benyahya | [END_REF]. We overcame this issue with the synthesis of dicyclocarbonates from allyl-cyclic carbonate and dithiols by thiol-ene coupling [12]. But these dicyclocarbonate were based on aliphatic structures. Yet, PUs contain generally rigid segments made of aromatic groups such as toluene diisocyanate (TDI), which is a hazardous compound. Therefore we intended to synthesize new biobased PHU with rigid segments and with ether bonds. Isosorbide is obtained from the dehydration of sorbitol, which is a product of the sugar industry and contains two cycloaliphatic rings likely to provide a good rigidity to the polymer. Its structure is also composed of two secondary hydroxyl groups.

Isosorbide is a biobased platform chemical extensively studied in literature with various industrial applications (isosorbide nitrate, diesters, lubricant and plasticizers, green solvents, etc.) [13][14][15][16]. It has to be noticed that the synthesis of the corresponding isosorbide amine and isocyanate was already reported [17][18][19]. Applications of isosorbide in polymers and materials are even more important and are summarized in Scheme 1. As depicted, a lot of derivatives could be synthesized such as polyesters [13,20] polytriazoles [21][22][23] and polycarbonates [24][25][26]33]. A recent patent reports the synthesis of polycarbonates from isosorbide. However, few works deal with PUs synthesis from isosorbide [27]. A first study was reported by Dirlikov and Schneider [28]. Then other teams reported the use of isosorbide for the synthesis of PUs with high glass transition temperature (T g ) values, from 80 to 240 °C (Table S1, Supporting information) [29][30][31]. These works show that isosorbide leads to rigid PUs with T g values higher than those obtained from other diols such as butanediol, neopentyl glycol, dihydroxymethylcyclohexane) and reveals the interest of isosorbide for industrial PUs.

These results led Cognet-Georjon et al. to use isosorbide for rigid segments of PUs with longer diols such as hydroxylated polybutadiene for the soft segments [30]. Most of PUs from isosorbide are synthesized by reaction between isosorbide and methylenediphenyl-4,4 0 -diisocyanate (MDI) which is a carcinogenic mutagenic reprotoxic (CMR) compound [32]. This drawback reduces the interest of using isosorbide as a biobased diol and led to the synthesis of PHUs thereof. Moreover, PU materials for coatings should have a T g around or below 0 °C which is not described in the literature.

The synthesis of PHUs from step growth polyaddition of dicyclocarbonates and diamines was extensively reported in literature, particularly by Endo [3,4,6,34,35]. Indeed, several cyclocarbonates were synthesized and some PHUs were thereof characterized [35][36][37][38]. Several methods are used to synthesize five-membered cyclic carbonates [39][40][41][42][43][START_REF] Weissermel | Industrial Organic Chemistry[END_REF]. Most of these methods are based on epoxide or diol reactants. This is also the case of dicyclocarbonate syntheses [6,20,[START_REF] Proempers | [END_REF][46][47]. Two bis and polycarbonates families were reported in the literature. The first one is constituted of carbonate esters from glycerin carbonate, the second one is constituted of carbonate ethers from glycidyl ether carbonatation [10,[START_REF] Benyahya | [END_REF]. In a recent paper, Brocas et al. [48] have reported the synthesis of cyclocarbonates using mild conditions to synthesize crosslinked polyethers (Scheme S1, Supporting information). Cyclocarbonate groups were synthesized by carbonatation of the corresponding epoxide groups using a low carbon dioxide pressure (1 bar) and a catalytic amount of lithium bromide (LiBr) at 80 °C.

Our work aims to synthesize new cyclocarbonate from isosorbide by carbonatation. The originality of this work consists not only in the synthesis of isosorbide dicyclocarbonate but also in the elaboration of original building block to synthesize low T g PHU, designed for coating applications, whereas isosorbide was exclusively used for high T g in literature. Moreover, we studied the thermal degradation of PHUs and the compounds resulting of degradation, which was never studied to the knowledge of the authors. To the best of our knowledge, no paper or even patent reports the synthesis of these new PHUs, synthesized according the concept of ''green chemistry''.

Experimental section

Materials

Isosorbide, epichlorohydrin, lithium bromide and 1,10-diaminodecane were purchased from Sigma Aldrich and used as received. N,N-dimethylformamide (DMF) was purchased from SDS Carlo Erba (Val de Reuil, France). Before use, DMF was dried according to current methods, distilled and stored under argon atmosphere. Deuterated solvents (CDCl 3 and DMSO-d 6 ) were purchased from Eurisotop (Saint-Aubin, France).

Synthesis

Epoxy index determination

The epoxy index (EI) is defined by the mass of monomer containing one mole of epoxide group. This index was determined by an acid-base titration. Epoxide groups were ring-opened by an excess of hydrochloric solution and this excess was determined by titration with a sodium hydroxide solution with phenolphthalein as a color indicator.

Isosorbide diglycidyl ether (E1)

In a two necks round bottom flask equipped with an addition funnel, isosorbide (8.5 g, 37.6 mmol) and chloroform (50 mL) were introduced. Then, m-chloroperbenzoic acid (47.2 g, 273.4 mmol) solubilized in chloroform (200 mL) beforehand, was introduced in the funnel and added dropwise. The mixture was stopped after 16 h when no more double bond signals were found in 1 H NMR. The organic layer was washed three times: first with Na 2 SO 3 , then Na 2 CO 3 and water. Organic layers were put together and the solvent removed under reduced pressure. The crude was purified by column chromatography under silica (eluent:ethyl acetate). Evaporation of the solvent with a rotary evaporator gave the corresponding product (5 g, 14.2 mmol) as yellowish viscous oil.

Yield: 38% 3 J HH = 4.4 Hz, 3 J HH = 11.6 Hz, 4 J HH = 1.2 Hz, H 6 ); 4.60 (ddt, 1H, 3 J HH = 4.4 Hz, 3 J HH = 8.4 Hz, 4 J HH = 1.6 Hz, H 7 ). 13

Isosorbide diglycidylether oligomers (E2 and E3)

In a round bottom flask equipped with an addition funnel and a reverse Dean Stark surmounted by a condenser, isosorbide (1 eq.) and epichlorohydrin (10 eq.) were introduced. A sodium hydroxide solution (2.3 eq.) in water (50/50:wt/wt) was prepared and placed in the addition funnel. The mixture was heated at 115 °C under argon atmosphere. The sodium hydroxide solution was added dropwise. The reaction was stopped when no longer water was collected in the Dean Stark apparatus, after approximately 6 h. The mixture was then allowed to stay at room temperature and epichlorohydrin was added to precipitate NaCl. After filtration, the epichlorohydrin was removed by evaporation under reduced pressure to give the desired product as yellowish viscous oil.

Yield: 98% (epoxide/isosorbide = 1.24). Dosage: 216 g/eq. epoxide rating: 4.63 meq/g (monomer).

Carbonatation reaction: general procedure for DC1-DC3

An epoxidized isosorbide (10.0 mmol of epoxide groups) and LiBr (0.5 mmol, 5% mol) were dissolved in dimethylformamide (DMF, 45 mL). The solution was introduced into a reactor and the atmosphere was replaced with CO 2 (P = 6 bars). The solution was then allowed to stand at 80 °C with continuous stirring during 12 h. DMF was removed by distillation (60 °C, P = 0.01 bar). The pure product was obtained quantitatively as yellowish highly viscous oil.

FTIR (m, cm À1 ) = 1780 (CO), 2860 and 2890 (CAC).

Polyhydroxyurethane (PHU1-PHU6) syntheses: general procedure

Step growth polyaddition of dicyclocarbonates (DC1-2) (1 eq.) was realized with diamines (1 eq.) and a catalyst (in a course of patent registration) under stirring in distilled DMF (2 mL) and argon atmosphere during 12 h at room temperature [49]. At the end of reaction, the formed polymers are still soluble. PHU1-6 polymers, soluble in DMF, were precipitated in water (these PHUs are not soluble in it). After drying by filtration then centrifugation, PHU1-6 were quantitatively obtained. Both synthesized PHUs were characterized by 1 H NMR spectrometry, FTIR analysis and SEC in DMF. Only 1 H NMR spectra of the corresponding polyhydroxyurethane PHU2 is reported.

FTIR (m, cm À1 ) = 1534 and 3082 (NH); 1694 (CO); 3328 (NH).

Analytical techniques

All Nuclear magnetic resonance ( 1 H NMR) measurements were recorded on a Brucker AC-400 MHz spectrometer at room temperature in deuterated chloroform (CDCl 3 ) or dimethylsulfoxyde (DMSO). The chemical shifts were reported in parts per million relative to tetramethylsilane.

IR spectra were recorded with a Nicolet 210 FT-IR spectrometer. Differential scanning calorimetry (DSC) analyses were performed under inert atmosphere with a calorimeter DSC 200F3 from Netzsch calibrated with indium standard. The polymer was weighted in an aluminum pan and consecutively placed in the measurement heating cell. An empty pan was used as reference. All the samples were heated under inert atmosphere from À120 to 100 °C at a heating rate of 20 °C/min. Three runs were recorded and the glass transition temperature (T g ) values were measured during the second run and confirmed by a third run. T g s were calculated at the inflexion point of the heat capacity jump.

Thermogravimetric analyses (TGAs) were performed using a TGA Q50 W/MFC apparatus of TA Instruments under air and nitrogen flow (25 ml/min) from room temperature to 500 °C at a heating rate of 5 °C/min À1 . The analysis consisted in registering the weight loss of the sample as a function of temperature.

Size exclusion chromatography (SEC) was performed on a Varian ProStar Model 210 equipped with an RI refractive index detector. Two PLgel 5 lm MIXED-C 600 mm were used at 70 °C with a 0.8 mL min À1 flow rate of DMF, calibrated using PMMA standards.

Results and discussion

Firstly, isosorbide was converted into epoxidized oligoisosorbide and then into cyclocarbonate oligoisosorbide by a carbonatation reaction. Then the corresponding PHUs were synthesized by reaction with synthesized dicyclocarbonates and diamines. Their spectral and physical characterizations as well as the formation of the polymer network were investigated. The last part of the article is dedicated to the degradation study of PHUs by performing ATG-IR analyses in order to analyze the released products.

Synthesis of isosorbide diglycidylether oligomers (E1-3)

This part focuses on the synthesis of diglycidylether from three different isosorbide reactants with various lengths. Epoxidized isosorbide E1 was synthesized by allylation of isosorbide followed by its epoxydation using m-chloroperbenzoic acid. Isosorbide oligomers E2 and E3 were obtained by reaction of epichlorohydrin with isosorbide at room temperature. Monomers E2 and E3 are different by the length of the spacer between the two epoxide functions; this aspect will be discussed below (Scheme 2).

Two epoxidized oligoisosorbides E2 and E3 were obtained with an epoxy index respectively of 4.13 meq/g and 2.9 meq/g (Fig. 1). The ratio R of epoxy cycle to isosorbide cycle is given by the ratio between the integration of the epoxide protons (H 1a-b and H 2 ) and the isosorbide protons (H Ã ) following the method described by Chrysanthos et al. [50]. Both ratio R obtained by NMR spectroscopy and titration are in agreement either for epoxidized oligoisosorbide E2 and oligoisosorbide E3 (Table 1). Chrysanthos et al. [50] mentioned other linear and branched oligomers. Branched oligomers can be explained by a competition during the reaction with epichlorohydrin, between the secondary hydroxyl groups of isosorbide and the secondary hydroxyl groups formed after a first reaction of isosorbide diglycidyl ether with isosorbide. It means that some oligomers can have more than two glycidyl groups.

Synthesis of dicyclocarbonates DC1-3

The dicyclocarbonates DC1-3 were synthesized by carbonatation of isosorbide diglycidyl ether oligomers E1-3. This reaction was carried out owing to Brocas et al. [48] conditions during 18 h. Total conversion was confirmed by the total disappearance of the protons of the epoxide moiety between 2.56 and 2.82 ppm and also by the appearance of signals corresponding to the carbonate protons between 4.20 and 4.88 ppm (Fig. 1).

Polyurethanes synthesis (PHU1-6)

Then, step growth polyadditions of DC1-2 with commercial diamines were carried out in presence of a catalyst (in a course of patent registration) and yielded PHU1-6 PHUs at room temperature (Table 2). The various diamine backbones are expected to provide a large range of mechanical properties to the PHUs. This part is focused on the development of polymers for coatings so that one of the absolutely desirable milestones is a T g below 0 °C. According to the theory, a more flexible carbon chain should provide a lower T g . The reaction between a cyclocarbonate and a diamine leads to two regioisomers depending on the carbonate position attacked by the amine. Isomers such as primary/primary alcohol, primary/secondary alcohol and secondary/secondary alcohol were obtained [51]. We firstly report the results obtained with oligomers DC2 and DC3 when polymerized with DA10. The analyses show the high reactivity of the reactants, the mechanical properties of the PHUs but also the effect of the oligomer length on these properties. These two PHUs (PHU2-3) were characterized by 1 H NMR, FTIR and SEC analyses.

H NMR spectrum of synthesized PHU2 is confirmed by the formation of carbamate group with signal of proton HANCOO at 7.10 ppm [52] and alcohol at 4.40 ppm (Fig. 2).

FTIR spectrum confirms the formation of PHU2 with the three characteristic bands of carbamate function: NAH bond stretching vibration, hydrogen bonded C@O stretching and NAH bond deformation are respectively observed at 3082 cm À1 , 1694 cm À1 and 1534 cm À1 . The large absorption of the hydroxyl group OH at 3328 cm À1 also appeared. Moreover, the absence of the absorption band of the carbonyl of the carbonate group at 1785 cm À1 reveals a total conversion of initial carbonated isosorbide (Fig. S1, Supporting information).

Size exclusion chromatography (SEC) analyses have been performed on PHU2 and PHU3. Molecular weights were found to be quite low, M n (7800 and 8600 g/mol for PHU2 and PHU3 respectively) and PDI were quite high (2.6 and 6.3 for PHU2 and PHU3 respectively) (Fig. S2 and Table S2, Supporting information).

The properties of these synthesized polyurethanes are similar to those found for isosorbide based polyurethanes described by Cognet-Georjon et al. [15] especially concerning the low value obtained for M n . The high value of PDI and low molecular weight is certainly due the presence of initial branched oligomers which are insoluble Scheme 2. Idealized structure of glycidylisosorbide oligomers and synthesis of isosorbidedicyclocarbonates by carbonatation. in DMF and can interact intensively with the GPC column. Both PHU2 and PHU3 are amorphous as determined by DSC analysis (Fig. S3, Supporting information). They exhibit similar T g values of 15 and 19 °C respectively. Compared to the previous results found in the literature (Table S1, Supporting information), and especially those with the aliphatic diisocyanate, hexamethylene diisocyanate (HDI), the first addition of glycidyl functions on the isosorbide before the synthesis of bis-carbonate can explain the lower T g observed and the increase softness of PHUs.

TGA analysis shows a 5% mass loss (Td 5%) at 243 and 237 °C for PHU2 and PHU3 respectively. The thermal degradation with formation of volatiles of the two PHUs is similar and proceeds in three steps under air as well as under nitrogen atmosphere (Fig. 3).

For example, for PHU2, the first step between 180 and 310 °C corresponds to a 28% weight loss. Then the second step, between 342 and 379 °C corresponds to a 48% weight loss. The third step between 420 and 451 °C corresponds to a 48% weight loss and the residue represents 11 wt.% of the initial weight at 500 °C. In both cases, no difference between oxidative or inert atmosphere was found so that no oxidation phenomenon occurs. The weakest bond is supposed to be the urethane one (E C-NH = 98 kJ/mol) and its degradation is supposed to be the same under oxygen and nitrogen atmosphere [START_REF]Taylor & Francis group[END_REF].

The next part of this article is devoted to the study of the diamine structure effect. Since the oligomer length shows no evidence in modifying the final polymer properties, the next part of the study will focus on PHUs obtained from DC2 and three other diamines e.g. Jeffamine D-400, diethylenetriamine and isophoronediamine (Scheme 3 and Table 2).

We first studied the thermal properties (Td 5% and T g ) of PHU4-6. Due to the rigid structure of IPDA, the T g obtained for PHU4 (59 °C) is higher than the one obtained with DA10 and other diamines. Then DETA, a soft aliphatic triamine, allows PHU5 to reach a T g of 0 °C. The last amine, Jeffamine D-400, is a long polyether chain diamine which confers high flexibility and toughness to PHU6 so that its T g is the lowest (À8 °C) (Table 3).

This result is compatible with a use in coatings. DMF and DMSO appear to be the best solvent as all materials are soluble in it whereas none of them are soluble in water (Table S3, Supporting information). The last part of our study is dedicated to the characterization of the released compounds during the degradation process of PHUs by the ATR-IR technique (Fig. 4). The decomposition of urethane bonds is depending on the structures of the reactants which were used for their synthesis. If the first decomposition is due to hard segments, the second and third decompositions were attributed to soft segments. Javni et al. have demonstrated that the first degradation step for polyurethanes from functionalized vegetable oils was the production of carbon dioxide. This carbon dioxide was suspected to come from the formation of carbodiimides but more credibly from the disruption of the urethane bond following mechanisms 2 and 3 [START_REF] Javni | [END_REF].

PHU2 and PHU4 were typically introduced into an ATG instrument and the released gases were analyzed by an infra-red apparatus after a given time (Fig. 4). After only 495s, the first degraded bond is the carbonyl of the urethane function (m COurethane = 1720cm À1 ) as expected [START_REF]Taylor & Francis group[END_REF]. Carbon dioxide and secondary amine bands (m CO 2 = 670, 2320, 3700 cm À1 and m NH = 1095, 1490, 3800 cm À1 ).

Alkane groups, probably PHUs spacers, were detected by the presence of two large bands between 2800 and 3000 cm À1 . Except the carbonyl urethane group, the same bands can be observed along the thermal analysis (i.e. CO 2 and NH bands). PHU4 gave same results as PHU2 concerning the degradation products. Moreover, we can clearly see that no isocyanate, alcohol or alkene were formed during the degradation process so that we can clearly demonstrate that PHUs degradation products are CO 2 and secondary amines. This result is of importance since it shows that not only PHUs are synthesized without isocyanates, but also they do not lead to isocyanate during thermal degradation.

Conclusions

Isosorbide was functionalized with glycidyl ether groups and the quantity of epoxide groups was determined by a titration. Results obtained by titration were in agreement with those obtained by 1 H NMR spectroscopy. Then functionalized oligoisosorbides were carbonated for the first time using mild conditions with a total conversion. This work led to the synthesis of new biobased PHUs using the cyclocarbonate-aliphatic amine chemistry in the presence of a catalyst. This reaction was found to be very effective as the reaction of cyclocarbonate groups is completed within 12 h. Linear and branched PHUs were obtained with T g values in the range from À8 to 59 °C, and low T g PHUs are suitable for coatings application. An acceptable thermal stability (Td between 234 and 255 °C) was determined owing to TGA for all PHUs. This work demonstrates that biobased PHUs can be obtained easily from isosorbide as hard segment and with ether bonds. Works are in progress for a better control of molecular weight and polydispersity. Last but not least, the compounds released by the degradation of PHUs were analyzed and found to be carbon dioxide and secondary amines. Interestingly, these new PHUs do not release isocyanates during thermal degradation. Choices of other aliphatic biobased diamines, shorter one like C5 or longer one like C36 will permit to prepare hard and soft PHUs.

  C NMR (100.6 MHz, CDCl 3 , d): 44.4 (C 1 & C 12 ); 50.9 (C 2 & C 11 ); 70.4 (C 5 & C 8 ); 73.5 (C 3 & C 10 ); 80.6 (C 4 & C 9 ); 86.0 (C 6 & C 7 ).

Scheme 1 .

 1 Scheme 1. Isosorbide based polymers.

Fig. 1 .

 1 Fig. 1. 1 H NMR spectrum of carbonated oligoisosorbide DC2.

Fig. 2 .

 2 Fig. 2. 1 H NMR spectrum (400 MHz, DMSO-d 6 ) of PHU2.

Fig. 3 .

 3 Fig. 3. TGA spectrum of PHU2 under air and nitrogen atmosphere.

Scheme 3 .

 3 Scheme 3. Structures of amines used in this study.

Table 1

 1 Determination of R using 1 H NMR.

		E2	E3
	Protons	I (Integration value)	I (Integration value)
	H1a + H1b	1.00 + 1.02	1.00 + 0.88
	H2	1.03	0.96
	H Ã (isosorbide cycle)	0.84 + 0.85	1.12 + 1.16
	Ratio R	1.20	0.83
	Ratio R (titration)	1.24	0.88
	n	0.66	1.32
	Molar mass (g mol À1 )	392	524

Table 2

 2 Table of composition of PHU1-6.

	Isosorbide	Amines	PHUs
	DC1	DA10	PHU1
	DC2	DA10	PHU2
		IPDA	PHU4
		DETA	PHU5
		JEFF400	PHU6
	DC3	DA10	PHU3

Table 3

 3 Thermal properties of PHU2 and PHU4-6.

	Oligomer	Amines	PHUs	Td 5% (°C)	T g (°C)
	DC2	IPDA	PHU4	255	59
		DA10	PHU2	243	15
		DETA	PHU5	234	0
		JEFF400	PHU6	242	À8

Acknowledgements

This work was supported by ANR project GreenCoat. The authors also thank Specific Polymers for the synthesis of isosorbide diglycidyl ether compounds under the reference SP 9S-5-001.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.reactfunctpolym. 2013.01.002.