Structural interpretation of nonlinear viscoelasticity in polymeric solids

Abstract

The mechanical relaxation time in a glassy polymer depends on the magnitude of strain. The stress relaxation modulus of a styrene acrylonitrile and polybutadiene composite system (ABS) was measured at strains ranging from 0.005 to 0.10. The relaxation time was observed to shorten by up to four orders of magnitude. In addition, a decrease in the elastic contribution to the modulus was observed. These two aspects of nonlinear viscoelasticity are interpreted in terms of the excess entropy and enthalpy associated with dilatation under strain, a crucial factor for ductile behavior and the formation of crazes. Up to 0.9 cal/g of the excess enthalpy associated with the stress‐induced dilatation is obtained from the differential scanning calorimetry study. As a corollary, brittle behavior following annealing is explained by the fact that it would take a greater magnitude of strain to regain a critical amount of excess enthalpy by the stress induced dilatation.