Résumé
Mixing inorganic nanoparticles (ca. 10-20 nm) with polymer leads to the formation of nanocomposites. Their main reason of existence is that they combine the best of both worlds, i.e. nanoparticle strength and polymer flexibility, which has important applications, e.g. for the car tire industry. In this talk, a simplified industrial model system based on filler particles of industrial origin but with a highly reduced chemistry will be presented. It is generally recognized that the filler structure has a strong impact on the rheological properties, and it is thus important to be able to characterize it in detail. Here, microscopic observations of the dispersion of the nanoparticles in the polymer matrix by electron microscopy and scattering will be discussed and related to the microscopic structure to their macroscopic rheological properties. 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 is 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 is in the 40 nm range. These aggregates are themselves concentrated in large-scale fractal branches (thickness ca. 150 nm, extending over microns), cf. the TEM cliché below. A quantitative analysis of the TEM data is used to estimate the volume fraction of fractal branches, which is an input for structure factor calculations of aggregates inside branches using numerical simulations as indicated in the Figure on the right. Then, the influence of polymer grafting is explored, yielding a strong effect on the aggregate mass [2]. 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.