Novel features of the Mullins effect in filled elastomers revealed by stretching measurements in various geometries.

Abstract

Stretching experiments with various geometries are performed using a custom-built tensile tester to reveal the intriguing features of the mechanical softening phenomena of filled elastomers in loading-unloading cycles, commonly known as the Mullins effect. The dissipated energy (D), residual strain (ε^r), and dissipation factor (Δ; the ratio of D to input strain energy) in the loading-unloading cycles are evaluated as a function of the maximum stretch in cyclic loading (λ_m) using three types of extension, i.e., uniaxial, planar, and equibiaxial extension, for silica-filled elastomers with various filler contents, and with or without a silane coupling agent. The dissipated energy D and ε^r increase with an increase in λ_m, and they depend on the type of extension when compared at the same λ_m: D and ε^r increase in the order of equibiaxial, planar, and uniaxial extension. In contrast, the values of Δ obtained for various degrees and types of extension are collapsed into a single curve when the first invariant of the deformation tensor (I_1,m) corresponding to λ_m is employed as a variable: Δ steeply increases with an increase in I_1,m in the small deformation regime of I_1,m < 3.2, while Δ levels off in the large deformation regime of I_1,m > 3.5. The plateau values of Δ increase with an increase in filler content. The characteristic dependence of Δ on I_1,m in each of the small and large deformation regimes is expected to reflect the destruction process of the inherent structures, including filler networks and the filler-polymer interface, and the friction between the fillers and the rubber matrix, respectively.