Physical aging in amorphous polymers near the glass transition point

A model is derived for the effect of physical aging on the linear viscoelastic response of amorphous polymers in the vicinity of the glass transition temperature T-g. A polymer is treated as an ensemble of flow units rearranged at random times. Any rearrangement event is thought of as a hop of a relaxing region trapped in its cage to some liquid-like state, where the region forgets its previous configuration. Stress-strain relations are developed and are verified by fitting observations in shear oscillation tests for poly(vinyl acetate). It is revealed that the inhomogeneity of the equilibrium energy landscape decreases with temperature and disappears at some crossover temperature T-c about 10 K above T-g. Kinetic equations are derived for out-of-equilibrium transformations of the energy landscape after cooling from above T-c to some temperature Tin the sub-T-g region. It is evidenced that the Kohlrausch-Williams-Watt (KWW) formula ensures fair prediction of experimental data for aged specimens. (C) 2000 Elsevier Science B.V. All rights reserved.