Ref HAL: hal-00822935_v1
Résumé: 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.

Ref Arxiv: 1212.3945
DOI: 10.1103/PhysRevE.87.032306
Résumé: We study the freezing kinetics of colloidal polycrystals made of micelles of Pluronic F108, a thermosensitive copolymer, to which a small amount of silica nanoparticles of size comparable to that of the micelles are added. We use rheology and calorimetry to measure Tc, the crystallization temperature, and find that Tc increases with the heating rate \dot{T} used to crystallize the sample. To rationalize our results, we first use viscosity measurements to establish a linear mapping between temperature T and the effective volume fraction, {\phi}, of the micelles, treated as hard spheres. Next, we reproduce the experimental \dot{T} dependence of the crystallization temperature with numerical calculations based on standard models for the nucleation and growth of hard spheres crystals, classical nucleation theory and the Johnson-Mehl-Avrami-Kolmogorov theory. The models have been adapted to account for the peculiarities of our experiments: the presence of nanoparticles that are expelled in the grain boundaries, and the steady increase of T and hence {\phi} during the experiment. We moreover show that the polycrystal grain size obtained from the calculations is in good agreement with light microscopy data. Finally, we find that the {\phi} dependence of the nucleation rate for the micellar polycrystal is in remarkable quantitative agreement with that found in previous experiments on colloidal hard spheres. These results suggests that deep analogies exist between hard-sphere colloidal crystals and Pluronics micellar crystals, in spite of the difference in particle softness. More generally, our results demonstrate that crystallization processes can be quantitatively probed using standard rheometry.

Commentaires: Journal: Phys. Rev. E 87, 032306 (2013)

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.

Résumé: Model systems of nanocomposites are important for our understanding of the structural and dynamical contributions of the constituents to macroscopic properties, and in particular mechanical reinforcement. We study silica-latex nanocomposites obtained from drying an aqueous colloidal mixture of polyacrylate nanolatex and silica nanoparticles. In these systems, the filler microstructure is controlled by the precursor solution pH and it is possible to work at constant aggregation number for different filler fractions [1]. We have followed the evolution of the polymer structure during nanocomposite formation and annealing using small angle neutron scattering under zero-average contrast conditions for the silica nanoparticles. The progressive disappearance of the latex beads by chain interdiffusion and release in the nanocomposites is analyzed with a model for the scattered intensity of hairy latex beads and an RPA description of the free chains. In silica-free matrices and nanocomposites of low (7%v) and high (20%v) silica content, the annealing procedure results in a molecular dispersion of chains. However, the interdiffusion of individual chain is delayed in presence of 20% of silica. The radius of gyration was not found to be affected by the silica loading in these samples with similar aggregation state (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] M. Tatou et al., Macromolecules 44, 9029 (2011) [2] A.C. Genix et al., Macromolecules 45, 1663 (2012)

Résumé: nanocomposites: structure and rheology