Coupling of shear rheology and SAXS of rubber-carbon black nanocomposites

Abstract

A combined SAXS-shear analysis of polymer nanocomposites for the first time between parallel plates is presented. This set-up allows investigating microstructural changes accompanying macroscopic rheological phenomena, such as the Payne effect. The latter is known to induce filler network destruction and subsequent flocculation when the shear strain amplitude is increased or decreased, respectively. First, a detailed SAXS analysis of the microstructure of carbon black in styrene-butadiene nanocomposites is applied to a series of increasing filler loading. The average size and mass of primary particles and small aggregates are estimated using a model of inter-aggregate polydisperse hard-sphere interactions. In a second time, the impact of a new geometry of the shear experiments with a horizontal X-ray beam passing through a disc-shaped nanocomposite on the SAXS pattern is analyzed in terms of sample heterogeneity (different positions), anisotropy (orientations), and sample thickness (transmission and heterogeneity). Application of our quantitative analysis shows that hot molding induces a slight anisotropy of the aggregate shapes into prolate ellipsoids oriented parallel to the rubber sheet. The Payne effect is then followed by scattering, surprisingly showing no modification of the intensity under oscillatory shear. Thus, the aggregate structure observed on the scale of standard SAXS is not broken up during Payne experiments, hinting at either an averaging effect over oscillation periods, or to the reorganization of large-scale agglomerate or network structures.