Phenomenology of plastic recovery in high polymer glasses

Deformations in isotropic, strain-free polymer glasses are usually complete ly recoverable (at the test temperature or after warming to T-g), in sharp contrast with the behavior of low molecular weight glasses and crystals. Th e apparent 'plastic strain' which remains at the end of a creep or stress r elaxation experiment does not recover at the test temperature, but only aft er the sample is heated. It is proposed that the long time scales needed fo r entanglement reorganization in the glass are responsible for this delayed recovery. A phenomenological network model for thermally activated strain recovery in polymer glasses is analyzed. A superposition relation between t he stress and the strain history using a KWW (stretched exponential) memory kernel is employed. The recovery of plastic (i.e. residual) strain in non- crosslinked amorphous thermoplastics is a two-step process that may be inte rpreted in terms of the network model. In particular, recovery at sub-T-g t emperatures is associated with entanglement slippage, while recovery near-T -g is believed to involve reorganization at or near chain ends. (C) 2001 El sevier Science Ltd. All rights reserved.