Interpretation of shift of relaxation time with deformation in glassy polymers in terms of excess enthalpy

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, there was observed a decrease in the elastic contribution to the modulus. These two aspects of nonlinear viscoelasticity are interpreted in terms of the excess 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.