In this work small angle x-ray scattering (SAXS) and a new reverse Monte Carlo (RMC) technique were used to characterize the evolution of filler structure in silica-filled polydimethylsiloxane and polydimethyldiphenylsiloxane elastomers. Elastomers are soft, brittle, and have poor fatigue properties. The addition of filler particles, usually carbon black or fumed metal oxides, greatly increases the toughness, maximum elongation, tensile strength, stiffness, and fatigue resistance.

Small angle x-ray scattering with in situ mechanical testing provides a way to directly measure structural changes during deformation that have proved difficult to measure using traditional microscopy techniques. Traditional analytical techniques as well as a new reverse Monte Carlo (RMC) method were used to provide insight into the nature of the structural changes during deformation.

During deformation the filler particles form raft-like structures that are aligned with their long axes perpendicular to the tensile axis. These structures separate in an affine matter with the sample extension ratio until a critical extension ratio is reached that is independent of silica surface treatment. At extensions above the critical extension ratio the rafts maintain the same relative spacing despite an increasing sample extension ratio. Above the critical extension Poisson compression combined with a high bulk modulus appears to breakup and deform the rafts. During unloading from extension ratios above the critical extension ratio the rafts show a less ordered structure and eventually return to their initial structure when completely unloaded. The patterns for the samples with high particle-filler adhesion show that the filler rich regions are disoriented by rotation. On subsequent cycles there is less correlated orientation between filler particles up to the point of previous maximum extension ratio. Above that extension ratio the structure is identical to that of a monotonically deformed sample. This provides a partial explanation for the strain softening phenomena known as the Mullins effect.