Large Strain Stress Relaxation and Recovery Behavior of Amorphous Ethylene−Styrene Interpolymers

Abstract

Ethylene−styrene interpolymers (ESIs) are viewed as random copolymers of ethylene and ethylene−styrene dyads. The amorphous ESIs offer an approach to characterizing polyethylene without the constraints imposed by the inevitable crystallinity. Large strain, nonlinear stress relaxation and recovery of ESIs were studied at temperatures above the glass-transition temperature (Tg). The nonlinear stress relaxation curves superposed on the double-logarithmic scale by vertical shifting after a characteristic time τk that increased as the molecular weight increased and the testing temperature approached Tg. The experimental shift factors were independent of molecular weight and styrene content and correlated well to the damping function obtained in the tube theory. The instantaneous recovery after stress relaxation increased as the molecular weight increased or as the testing temperature decreased approaching Tg. Instantaneous recovery was correlated with stress relaxation in terms of mechanisms of molecular dynamics. The success of this approach led to a generalization of the model of two coexisting networks used previously.