Polymer nanocomposites are mixtures of nanoparticles and polymer chains, where the filler particles are usually added to enhance mechanical properties, e.g. of car tire materials. Performance depends to a great extent on the structure of the filler, i.e. its dispersion state in the polymer matrix, which itself depends on mixing protocols, but also on the thermodynamics of the system. The latter can be tuned by performing either chemical surface modifications of the nanoparticles, or by playing with the mass and/or grafting properties of the polymer chains. In this talk, some straightforward ways to obtain information from small-angle scattering on the structure of nanocomposites will be presented [1]. In particular, we will review recent results on nanocomposites close to industrial applications, including what is probably one of the first quantitative analyses of such disordered structures by a combination of SAXS, TEM and numerical simulations [2]. This analysis will then be applied to the impact of small molecules on such structures [3]. Concerning chain structure, a quick outlook on why contrast-matching with SANS in nanocomposites often fails will be proposed [4]. REFERENCES1.A.-C. Genix and J. Oberdisse, Structure and dynamics of polymer nanocomposites studied by X-ray and neutron scattering techniques. Current Opinion in Colloid & Interface Science 2015, 20, 293-303 (2015).2. G. P. Baeza, A. C. Genix, C. Degrandcourt, L. Petitjean, J. Gummel, M. Couty and J. Oberdisse, Multiscale Filler Structure in Simplified Industrial Nanocomposite Silica/SBR Systems Studied by SAXS and TEM, Macromolecules, 46, 317-329 (2013).3. D. Musino, A.-C. Genix, C. Fayolle, A. Papon, L. Guy, N. Meissner, P. Weda, T. Bizien, T. Chaussée, J. Oberdisse, Synergistic effect of small molecules on large-scale structure of simplified industrial nanocomposites, submitted.4. A. Banc, A.-C. Genix, C. Dupas, M. Sztucki, R. Schweins, M.-S. Appavou, J. Oberdisse, Origin of small-angle scattering from contrast-matched nanoparticles: A study of chain and filler structure in polymer nanocomposites, Macromolecules, 48, 6596−6605 (2015).

Les matériaux nanocomposites obtenus par la dispersion de charges nanométriques dans un polymère ont de nombreuses applications industrielles à cause de l’amélioration des propriétés des matrices. La dispersion des charges pendant le procès de préparation des nanocomposites est déterminée par les interactions entre charges et avec le polymère. L’état final de dispersion impacte les propriétés structurales, dynamiques et mécaniques du matériau.Dans ce travail de thèse, nous proposons l’étude de nanocomposites produits par dispersion de billes de silice hydrophiles dans une matrice de polymère hydrophobe, i.e., styrene-butadiene (SB), couramment utilisé dans les pneumatiques. Nous avons notamment étudié l’impact des agents de recouvrement - des silanes réagissant avec la surface des charges – qui sont utilisés afin de promouvoir la compatibilité entre la silice et le SB.Dans un premier temps, nous avons étudié des matériaux composites industriels simplifiés obtenus par l’incorporation de silice hautement dispersible dans le SB. En couplant des expériences de diffusion des rayons-X aux petits angles (DXPA) avec de la microscopie, nous avons montré que : i) la présence d’un catalyseur (DPG) amplifie l’effet de l’agent de recouvrement; ii) l'augmentation de la quantité de silane favorise la réduction de la taille des agrégats de silice. De plus, un système modèle équivalent au système industriel simplifié a été développé, mais avec une silice colloïdale. Nous avons mis au point une nouvelle méthode pour modifier la surface des billes de silice en suspensions dans en mélange eau/éthanol. Ces nanoparticules sont ensuite stabilisées dans le même solvant utilisé pour la dissolution du SB, le MEK. Enfin, le composite modèle est obtenu par évaporation du solvant. La dispersion des billes de silice greffées a été étudiée en suspension (i.e., dans le mélange eau/éthanol et MEK) et dans le nanocomposite, en couplant DXPA avec des simulations de Monte-Carlo inverse. Nous avons montré que la qualité de la dispersion dépend du type de silane utilisé et qu’elle est transférée du solvant (MEK) au composite. De plus, l’impact des agents de recouvrement sur les propriétés dynamiques (relaxation α) du système modèle a été étudié par spectroscopie diélectrique, BDS. Nous avons découvert que les agents de recouvrement peuvent plastifier le SB pur, en induisant une baisse significative de la température de transition vitreuse. Dans les composites, la modification de l’état de surface des billes par l’agent de recouvrement n’altère pas la relaxation α. Cependant, la présence de molécules de silane libres dans la matrice polymère du nanocomposite peut également induire un effet de plastification.

In recent years, materials with enhanced mechanical properties have attracted both fundamental and industrial interest. The present study concerns nanocomposites (NCs) made by dispersion of silica nanoparticles in a soft polymer matrix of styrene/butadiene (SB), which is commonly used in car tires. We work on both a simplified industrial system (solid mixing with millimetric silica pellets) and we developed a model system (solvent casting with surface-modified colloidal silica). The NC characteristics may be tuned by the grafting of silane coating agents (SCAs) which differ in the length of their alkyl groups (different hydrophobicity). These agents may influence filler dispersion via different polymer/filler and filler/filler interactions. The investigation of NC materials is essential for many industrial applications, in particular for the improvement of tire performances from an energetical and climatical point of view.

We use computer simulations to investigate how a catalytic reaction in a polymer sol can induce the formation of a polymer gel. To this aim we consider a solution of homopolymers in which freely-diffusing catalysts convert the originally repulsive A monomers into attractive B ones. We find that at low temperatures this reaction transforms the polymer solution into a physical gel that has a remarkably regular mesostructure in the form of a cluster phase, absent in the usual homopolymer gels obtained by a quench in temperature. We investigate how this microstructuring depends on catalyst concentration, temperature, and polymer density and show that the dynamics for its formation can be understood in a semi-quantitative manner using the interaction potentials between the particles as input. The structuring of the copolymers and the AB sequences resulting from the reactions can be discussed in the context of the phase behaviour of correlated random copolymers. The location of the spinodal line as found in our simulations is consistent with analytical predictions. Finally, we show that the observed structuring depends not only on the chemical distribution of the A and B monomers but also on the mode of formation of this distribution.

Gliadins are edible wheat storage proteins well known for their surface active properties1, 2. Although their foaming3 and emulsifying4 capability is useful in food industry, the physical mechanisms of their adsorption at an interface are not well understood and the resulting structure of the protein interfacial films not yet well characterized. In this talk, we will present experimental results on the interfacial properties of acidic gliadin solutions studied over 4 decades of concentrations from 0.01 up to 110 mg/ml.Dynamic pendant drop tensiometry reveals that the surface pressure of gliadin solutions builds up in a multistep process. We show that the series of curves of the time evolution of alpha.time collected at different protein concentrations can be merged onto a single master curve when surface pressure is plotted as a function of alpha.time where t is the time elapsed since the formation of the air/water interface and alpha is a shift parameter that varies with the bulk protein concentration as a power law with an exponent 1.6. The existence of a master curve indicates that the surface pressure is the relevant parameter to characterize the interfaces covered with gliadin proteins.Optical and elastic properties of the interfaces are studied by dilatational rheology and ellipsometry and provide a consistent physical picture of the adsorption processes. Correlated evolutions of ellipsometry and rheological properties show a simple and progressive coverage of the interface by individual proteins at low surface pressure. This stage is followed, at higher surface pressure, by conformational rearrangements of the protein film, which are identified by a strong increase of the dissipative viscoelastic properties of the film concomitantly with an anomalous evolution of its optical profile that we have quantitatively rationalized. In the last stage, at even higher surface pressure, the optical profile is not modified while the elasticity of the film dramatically increases resulting from the gelation of the film due to the formation of intermolecular bonds. [1] J.Ornebro, T. Nylander, A-C Eliasson (2000). Journal of Cereal Science 31, 195-221[2] R. C. A Keller, R. Orsel, R. J. Hamer (1997). Journal of Cereal Science 25, 175-183[3] Martin A. Bos, Bertus Dunnewind, Ton van Vliet (2003). Colloids and Surfaces B 31, 95-105[4] Y.Popineau, F.Pineau, P. Evon, S. Bérot (1999). Nahrung 6, S. 361-367