Mechanism of yielding and cold flow in glassy polymers

This is the authors' abstract. We don't add key points for this paper.

Tensile and compressive yield stress as well as tensile and compressive flow stress are determined as functions of strain rate, temperature, and thermal history for atactic polystyrene, isotactic polystyrene, polycarbonate, and poly-(methyl methacrylate). An Eyring-type activated flow analysis is employed to calculate both activation energy and shear activation volume over the range of experimental conditions studied, and the results are interpreted in terms of a two-phase amorphous microstructure consisting of both ordered and disordered regions. Similar values for yielding activation energies and activation energies obtained from dynamic mechanical analysis suggest that at temperatures well below Tg, yielding is controlled by cooperative local segmental motion. With increasing temperature, increasing degrees of local segmental motion are involved. Only when the test temperature approaches Tg does a transition to cooperative long-chain motion occur. Equivalent values for shear activation volume in both tension and compression suggest that tensile and compressive yielding mechanisms are very similar and that both may be controlled by localized volume dilations.