Modeling the effect of physical aging on the stress response of amorphous polymers based on a two-temperature continuum theory

Abstract

Physical aging widely exists in amorphous polymers, which refers to the nonequilibrium structure of amorphous polymers evolving towards the equilibrium state. Aging can significantly influence the thermomechanical properties and subsequently the macroscopic response of polymers. In our recent work, we performed a series of uniaxial compression tests on amorphous thermoplastic poly(ethyleneterephthalate)-glycol (PETG) subject to different annealing temperature and time. The results showed that both annealing time and temperature have a significant influence on the yield strength. Here we apply a two-temperature continuum model to simulate the stress response of PETG with different thermal treatments. The model employs the effective temperature as a variable to characterize the nonequilibrium state. The physical aging is characterized by the evolution of the effective temperature. The effective temperature is also coupled with the viscoplastic deformation to describe strain softening induced by mechanical rejuvenation. The simulation results can reasonably reproduce the main experimental observations with some discrepancies. The possible reasons for the discrepancies between experiments and simulations are also critically analyzed. The theory is also used to simulate the response of amorphous glassy polymers in the creep and strain-rate-switching tests. The model successfully captures the important features of experimental observations in the literature.