Rubber-based nanocomposites prepared by solid-phase mixing with precipitated silica or carbon black are typically strongly aggregated systems with different levels of spatial organization, as highlighted by our group in the past [1, 2]. Our strategy is to investigate such systems based on the study of simplified industrial samples with ingredients limited to a strict minimum. Small-angle X-ray scattering (SAXS) can then be used to study the filler microstructure in rubber nanocomposites with different filler loading. The size and mass of primary particles and small aggregates are determined using a model of inter-aggregate polydisperse hard sphere interactions based on a correlation hole analysis. Another key feature of rubber nanocomposites is the influence of filler surfaces on polymer dynamics, and an original application of dielectric spectroscopy to measure the adsorption isotherm of coating agents on silica embedded in the polymer matrix will be presented. [3] A major advance was then to couple SAXS measurements on synchrotron beamlines with in-situ shear rheology to provide microstructural evidence for macroscopic rheological effects, which leads to disruption of the filler network and subsequent flocculation. The Payne effect has been characterized in rubber nanocomposites filled with carbon black, but with only small signatures in the scattering under oscillatory shear. In parallel, we have also coupled shear rheology with fast acquisition dielectric measurements to follow the change in electrical conductivity due to the destruction/formation of conductive pathways.

In order to explore the structural mechanisms responsible for the plastic behavior andpolyamorphism in silica glasses v-SiO2, we have performed Molecular Dynamics (MD)simulations using different approaches. Recently, a Density Functional Tight-Binding(DFTB) (1) study has shown that the structural changes from low- to high-densityamorphous structures in v-SiO2 occur through a sequence of percolation transitions (2).These transitions also explain some of the mechanical properties of v-SiO2.To gain a deeper insight into the properties of the clusters and percolations networks, wehave performed classical MD simulations using a reliable pair potential (3). Due to its lowcomputational load, this method has allowed us to significantly increase the size of thesimulated system and have access to large length scales, from 2.5 to 12.0 nm. Similarpercolation transitions have been identified also for these new models. Generating severalorders of magnitude of cluster sizes allows the extraction of scaling laws associated witheach property of the clusters such as mass, correlation length, order parameter etc... Thescaling of such properties is correlated to the critical exponents or the fractal dimensionalityof the clusters. The collection of these quantities related to the transitions for the differentconnectivities SiO4-SiO4, SiO5-SiO5, SiO6-SiO6 and SiO6-stishovite, leads to thecharacterization of the percolation model, providing compelling evidence for amorphous-amorphous “phase” transitions in compressed silica glasses.

Hyper-Raman scattering is used to study the temperature dependence of the longitudinal optic (LO) modes of three prototypical ferroelectric relaxors in a broad temperature range from 20 to 800 K. The three LO bands observed in all spectra of the three materials are linked to the three transverse optic (TO) modes in cubic relaxors where LO i is linked to TO i , with i = 1 to 3. The Last, Slater, and Axe eigenvector pictures of the TO modes are consistent with our observations on LO1, LO2, and LO3, respectively. Within this framework, the splitting of LO2 would mostly be linked to a structural disorder on the site while the strength of an anomaly of LO1 near the freezing temperature T f relies on the ability of the material to develop a long-range ordering. Moreover, the more pronounced the splitting, the more important the structural disorder and the lower the value of the dielectric constant. Published by the American Physical Society 2024

Despite the many experimental, theoretical, and numerical studies, there are still many unknowns about the microscopic understanding of the surface properties of silicate glasses. Nowadays, many applications rely on the control of surface composition, local structure, or morphology. Moreover, the disordered nature of the glass structure makes exploring and rationalizing some of these properties difficult on a microscopic level. MD simulations have been carried out to systematically examine the influence of alkali content and their chemical nature on the properties of alkali glass surfaces. We have shown how the production history, whether it's a melt-quench process or dynamic fracture, has a direct impact on both the atomistic-level and morphological properties