Physical aging and the viscoelastic response of glassy polymers: Comparison of observations in mechanical and dilatometric tests

Constitutive equations are developed for the linear viscoelastic behavior and volume recovery in glassy polymers. The model is based on the concept of cooperative relaxation that treats a polymer as an ensemble of cooperatively rearranged regions (CRRs). A rearrangement event is thought of as a hop of a CRR trapped in its potential well on the energy landscape to a liquid-like energy level. The energy landscape of a glassy polymer is assumed to be time-independent, whereas the position of the liquid-like state changes with time approaching the equilibrium energy level. Its evolution is described by a fictive temperature that obeys Tool's equation with a characteristic time proportional to the average time for rearrangement. Kinetic equations for volume recovery and stress-strain relations for the viscoelastic response are verified by fitting observations in dilatometric and mechanical (static and dynamic) tests. Fair agreement is demonstrated between the results of numerical simulation and the experimental data for polycarbonate, poly(arylene etherimide), poly(ether ether ketone), and poly(vinyl acetate).