Strain Softening of Styrene-Isoprene-Styrene Copolymers under Large Amplitude Oscillatory Shear for Clarifying Payne Effect in Rubbers and Their Nanocomposites

To illustrate mechanisms of Payne effect in rubbers and their nanocomposites experiencing large amplitude oscillatory shear (LAOS), comparison studies were performed in styrene-isoprene-styrene (SIS) copolymers and their selectively crosslinked materials at temperatures below and above glass transition temperature of the polystyrene (PS) phase. It was found that under periodic dynamic shear, the strain softening is reversible when the polyisoprene (PI) phase, either crosslinked or not, is restricted by hard PS domains but it shows hysteresis once the PS domains disassociate. The strain softening can happen at the time scale of intrinsic Rouse relaxation of elastically active network strands. Critical stress of strain softening scales with number density of elastically active network strands, a simple relation being verified not only in the selectively crosslinked SIS copolymers but also in PI gum vulcanizates and carbon black filled PI compounds. Payne effect is traditionally used to term strain softening of highly filled rubber vulcanizates under LAOS deformation while evidenced herein is that the Payne effect of highly filled rubber vulcanizates shares the mechanism being common to the strain softening of SIS copolymers.