Since 1992 and the apparition of highly dispersible silica as a nanofiller in the green-tire formulation, the structure and dynamics of precipitated silica-SBR nanocomposites produced by solid phase mixing is of a great interest for manufacturers. In this work, we make use of SAXS and TEM to investigate the organization of silica nanoparticles (Rsi 10nm) varying the filler fraction while dynamics is studied by broadband dielectric spectroscopy (BDS) and rheology. The structural analysis of such complex materials revealed a multi-scale filler organization based on a 3D fractal network built up from aggregates made of nanoparticles [1]. The characteristics (size, average aggregation number, compacity...) of such aggregates are extracted from a combined analysis of SAXS and TEM data taking into account the aggregate polydispersity (Nagg50, 35%). Jointly with mechanical experiments, this analysis enables us to estimate the critical aggregation volume fraction at which the network fully percolates (Figure 1a). In the same context, the ionic conductivity () measured by BDS at high temperature displays a jump from 12.7% of silica that we associate with the percolation threshold observed in rheology (Figure 1b). Moreover, at lower temperatures, a Maxwell-Wagner-Sillars process related to the charges blocked at the interface between silica and polymer is observed at temperatures higher than the glass transition in the nanocomposites.

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.

Etude de nanocomposites modèles par diffusion du rayonnement aux petits angles

Distribution de particules et conformation de chaînes dans des matériaux nanocomposites

Simplified silica (Zeosil 1165 MP) and SBR (140k carrying silanol end-groups) nanocomposites have been formulated by mixing of a reduced number of ingredients with respect to industrial applications. The thermo-mechanical history of the samples during the mixing process was monitored and adjusted to identical final temperatures. The filler structure on large scales up to micrometers was studied by transmission electron microscopy (TEM) and very small-angle X-ray scattering (SAXS). A complete quantitative model extending from the primary silica nanoparticle (of radius ≈10 nm), to nanoparticles aggregates, up to micrometer-sized branches with typical lateral dimension of 150 nm is proposed. Image analysis of the TEM-pictures yields the fraction of zones of pure polymer, which extend between the branches of a large-scale filler network. This network is compatible with a fractal of average dimension 2.4 as measured by scattering. On smaller length scales, inside the branches, small silica aggregates are present. Their average radius has been deduced from a Kratky analysis, and it ranges between 35 and 40 nm for all silica fractions investigated here (Φsi = 8−21% vol.). A central piece of our analysis is the description of the interaggregate interaction by a simulated structure factor for polydisperse spheres representing aggregates. A polydispersity of 30% in aggregate size is assumed, and interactions between these aggregates are described with a hard core repulsive potential. The same distribution in size is used to evaluate the polydisperse form factor. Comparison with the experimental intensity leads to the determination of the average aggregate compacity (assumed identical for all aggregates in the distribution, between 31% and 38% depending on Φsi), and thus aggregation number (ca. 45, with a large spread). Because of the effect of aggregate compacity and of pure polymer zones, the volume fraction of aggregates is higher in the branches than Φsi. The repulsion between aggregates has a strong effect on the apparent isothermal compressibility: it leads to a characteristic low-q depression, which cannot be interpreted as aggregate mass decrease in our data. In addition, the reinforcement effect of these silica structures in the SBR-matrix is characterized with oscillatory shear and described with a model based on the same aggregate compacity. Finally, our results show that it is possible to analyze the complex structure of interacting aggregates in nanocomposites of industrial origin in a self-consistent and quantitative manner.