Hierarchical Scattering Function for Silica-Filled Rubbers under Deformation: Effect of the Initial Cluster Distribution

In this study, the evolution of the scattering function of silica-filled styrene-butadiene copolymer rubbers (SBRs) under a continuous uniaxial strain cycle is investigated in situ with a complementary mechanical analysis. The microscopic hierarchical arrangement of aggregating nanofiller particles in the rubber matrix is determined by ultrasmall-angle X-ray scattering (USAXS) at different strain stages up to 100%. The application of strain during the collection of the scattering images allows a unique correlation of the evolution of the microstructure with a macroscopic deformation. Industrially mixed filled rubber compounds are investigated in the absence of ZnO, typically used as a curing activator and strongly contributing to the scattering function, to relate the scattering function and its evolution to the unambiguous contribution of the silica fillers. The effect of the deformation is reflected in the two-dimensional (2D) scattering patterns as well as in the sectorially averaged intensities along the principal axes of the deformation tensor. A scattering model, based on fractal concepts, is applied to the orientation-dependent intensities, allowing a quantitative correlation between the external strain and the induced structural changes on a 10–100 nm length scale. The singular role of the initial preferential orientation of the silica clusters is investigated in this work due to a direct correlation between the initial states of the clusters and the stress–strain behavior of the rubbery system. Conclusions on the observed hysteresis could be drawn by the combination of microscopic and macroscopic observations.