Résumé:

The mechanical reinforcement of polymer matrices by nanoparticles is a fundamental problem with far reaching applications, e.g., for car tires. From a conceptual point of view, it is generally recognized that the filler structure has a strong impact on the mechanical properties, accompanied by the effect of chain structure evolving in the hard filler environment, and the filler-chain interactions. All these contributions are related to the filler structure, and it is thus important to be able to characterize it in detail. In this work, the structure of SBR-silica nanocomposites designed to reproduce key features of industrial samples, but of simplified composition, has been studied on length scales extending from the nanometric primary particles to microns. We propose an original method for scattering data analysis of such multi-scale systems by including self-consistent polydisperse form and structure factors of aggregates.[1] The complex structure of the silica within the nanocomposites will be quantitatively modeled in a step-by-step manner, starting with the primary silica beads as basic building units (10 – 20 nm range). These beads are found to be aggregated in small clusters, the typical radius of which (40 nm range) will be determined by Kratky plots. These aggregates are themselves concentrated in largescale fractal branches (thickness ca. 150 nm, extending over microns). Inside these branches, the small aggregates repel each other. Within our model, this is described with a hard-sphere excluded volume interaction potential, which induces a characteristic depression of the scattered intensity at intermediate angles. This depression is directly related to the local concentration of aggregates, which is higher than the nominal silica volume fraction due to the confinement in the fractal branches, and the presence of polymer inside the aggregates. Therefore, a quantitative TEM analysis was used to estimate the volume fraction of fractal branches. Secondly, we have set up an independent Monte Carlo simulation in order to calculate the low-q limit of the polydisperse inter-aggregate structure factor, which quantifies the depression. Using a polydisperse aggregate form factor obeying the same polydispersity, the mass of the small aggregates (or, equivalently, their internal silica volume fraction, here called compacity) and their concentration inside the fractal branches can be extracted from the scattered intensity. In parallel, the rheological properties of these silica structures in the SBR-matrix are characterized with oscillatory shear. The resulting reinforcement curve of the high-frequency storage modulus can be described using a combination of hydrodynamic reinforcement for silica fraction below a critical percolation fraction, and a percolation law above. It is interesting to note that the aggregate compacity obtained from the structural analysis (SAXS and TEM) is fully compatible with the reinforcement data. On the other hand, for future work, it may be important to be able to compare the results obtained here to model systems where the filler is a well-defined nanoparticle. In this case, we were able to follow the chain conformation in hard filler environments using contrast-variation small angle neutron scattering (SANS). In polyacrylate latex based nanocomposites where the silica structure is well characterized, it has been found that the radius of gyration is not disturbed by the silica loading in a given state of aggregation (typically ten primary silica particles per aggregate, percolating at high concentration).[2] This important result opens the way for a systematic study of the chain structure in complex environments. [1] G.P. Baeza, A.C. Genix, C. Degrandcourt, L. Petitjean, J. Gummel, M. Couty, J. Oberdisse, accepted in Macromolecules. [2] A.C. Genix, M. Tatou A. Imaz, J. Forcada, R. Schweins, I. Grillo, J. Oberdisse, Macromolecules 2012, 45 (3), 1663.