Les nanocomposites de polymère sont des matériaux dont la matrice polymère est renforcée par des nanoparticules (NPs), comme par exemple des NPs de silice. Pouvoir contrôler la structuration de ces dernières au sein de la matrice est un vieux rêve, parce que l’on espère pouvoir ajuster les propriétés macroscopiques via la microstructure. Dans un premier temps, nous montrerons quelques exemples d’analyse structurale de nanocomposites par diffusion de rayonnement aux petits angles, simulation numérique, et microscopie électronique [1]. Ces études mettent en évidence le caractère en partie hors équilibre de ces structures. Ces dernières années, un paramètre de contrôle des structuration des NPs à l’équilibre est apparu [2-4] : le greffage de chaînes de polymère à la surface des NPs. Il permet de contrôler l’agrégation des NPs, et d’obtenir des structures anisotropes essentiellement visualisées par TEM. Même si cela n’a pas encore trouvé d’écho dans la littérature de mode, la coiffure semble donc être liée au rangement, au moins pour les toutes petites billes. Pour finir, nous discuterons d’un modèle structural développé pour des systèmes plus proches des applications industrielles, et nettement moins enclins à une structuration ordonnée, malgré les efforts de greffage [5]. Références: 1 A. Banc, A.-C. Genix, M. Chirat, C. Dupas, S. Caillol, M. Sztucki, J. Oberdisse, Macromolecules 2014, 47 (9), 3219–32302 A. El Harrak, G. Carrot, J. Oberdisse, Ch. Eychenne-Baron, F. Boué, Macromolecules 2004, 37, 6376-63843 P. Akcora et al, Nature Mat 2009, 8, 354-3594 S. Kumar, N. Jouault, B. Benicewicz, T. Neely Macromolecules 2013, 46, 3199-32145 G. P. Baeza, A.-C. Genix, C. Degrandcourt, L. Petitjean, J. Gummel, M. Couty, J. Oberdisse, Macromolecules 2013, 46, 317−329

TBA.

The structure of polymer nanocomposites has important consequences on final properties, like for instance mechanical reinforcement. While the structure of the hard filler nanoparticles (NPs) is usually characterized by electron microscopy and small-angle X-ray scattering (SAXS), the chain conformation can only be measured by small-angle neutron scattering (SANS). Two main difficulties may arise with such a structural analysis. First of all, extracting reliable structural information on filler dispersion may be hindered by lack of a priori knowledge of the system. In this talk, we will discuss two examples. In the first one, additional knowledge is due to the model character of well-characterized NPs, and it can be used to analyze scattering data using Monte Carlo simulations [1]. In the second one, industrial filler NPs of high structural complexity have been used, and structural characterization of this system was only possible by quantitatively combining MC simulations, transmission electron microscopy, and SAXS [2]. Concerning the determination of polymer conformation by SANS, a second and mostly unforeseen difficulty shows up: in spite of great care in contrast-matching (undesired) NP contributions to the SANS signal, such additional intensities are systematically found in the literature. Two model cases are discussed in this talk: the first one emphasizes large-scale polymer heterogeneities reminiscent of the solution structure before nanocomposite formation. This structure is described quantitatively by a ‘hairy bead’ model [3]. The second one, finally, is the result of a general study of incomplete filler NP matching, evidencing for the first time that smallest NPs may induce statistical fluctuations in H-D-density in a surrounding shell, making in principle matched filler visible. Incidentally, the typical size of such mismatched polymer shells is deduced from the data [4]. [1] A. Banc et al, Macromolecules 47 (9), 3219–3230 (2014); J. Oberdisse et al Soft Matter, 3, 476-485 (2007)[2] G. Baeza et al, Soft Matter 10, 6686-6695 (2014)[3] A.C. Genix et al, Macromolecules 45(3), 1663-1675 (2012)[4] A. Banc et al, submitted

The structure of polymer nanocomposites has important consequences on final properties, like for instance mechanical reinforcement. While the structure of the hard filler phase is usually characterized by electron microscopy and small-angle X-ray scattering, the chain conformation can only be measured by small-angle neutron scattering (SANS). Continuous efforts over the past 15 years have produced a body of sometimes contradictory, because system-dependent, results. In virtually all studies, however, a mismatch ruining the polymer form factor at low-angles has been observed, in spite of careful contrast-matching. In this study, the conformation of polymer chains in silica-latex-nanocomposites has been studied under zero-average contrast conditions using SANS.

The structure of polymer nanocomposites has important consequences on final properties, like for instance mechanical reinforcement. The structure of the hard filler phase is usually characterized by electron microscopy and small-angle X-ray scattering, and we will review recent examples of filler structure in model [1] or industrial nanocomposites [2]. The chain conformation can only be measured by small-angle neutron scattering (SANS). Continuous efforts over the past 15 years have produced a body of sometimes contradictory, because system-dependent, results, see e.g. [3]. In virtually all studies, however, a mismatch ruining the polymer form factor at low-angles has been observed, in spite of careful contrast-matching. In this study, the conformation of polymer chains in silica-latex-nanocomposites has been studied under zero-average contrast conditions using SANS. Samples have been prepared by drying colloidal suspensions of silica and polymer nanoparticles (NPs) for two different silica NPs (radius of 5 and 15 nm) and two chain molecular weights (17 and 100 kg/mol). By appropriate mixing of hydrogenated and deuterated polymer, chain scattering contrast is introduced, and in principle silica scattering suppressed. The silica structure consisting mostly of small fractal aggregates is characterized by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) on the same samples. The measurement of the chain structure by SANS, however, is perturbed by unwanted silica contributions, as often reported in the literature. Here, the contribution of contrast-matched silica is evidenced as a function of system parameters, namely chain mass, silica size, and volume fraction, and a model rationalizing these contributions for the first time is proposed. Via a statistical model, a nanometer-thick polymer shell surrounding silica NPs is shown to create contrast, which is presumably maintained by the reduced mobility of polymer close to interfaces or attractive polymer-silica interactions. This shell is proven to be quantitatively important only for the smallest silica NPs. As a consequence, the pure polymer scattering can be isolated, and the polymer radius of gyration is found to be independent of filler content and NP size [4].[1] A. Banc et al, Macromolecules 2014, 47 (9), 3219–3230 [2] G. Baeza et al, Soft Matter 2014, 10, 6686-6695.[3] A.C. Genix and J. Oberdisse, Current Opinion in Colloid and Interface Science, 2015, in press. [4] A. Banc et al, Macromolecules 2015, 48, 6596−6605.

The structure and dynamics of filler nanoparticles and macromolecules in polymer nanocomposites determine many aspects of macroscopic behavior, which is why these properties have attracted considerable interest in the scientific literature. In this article, recent progress using scattering techniques for the elucidation of structure and dynamics, of both filler nanoparticles and polymer chains, is reviewed. Small-angle X-ray and neutron scattering, more and more combined with direct imaging methods and simulations, are appropriate methods to investigate nanocomposite structure. While they can usually be applied to standard samples for filler structure, chain structure can only be accessed with neutrons and isotopic substitution, under the zero-average contrast condition to match the filler contribution. Quasi-elastic neutron scattering and X-ray photon correlation spectroscopy cover different time scales with spatial resolution in the nanoscale. The former allows studies of polymer motions from atomic or segmental relaxation to reptation of the entire chain in presence of filler, while the latter is increasingly used for investigations of nanoparticle dynamics.