Thermally stimulated depolarization current studies on strain-induced relaxations in polymers

Thermally stimulated depolarization current (TSD) method has been used to study the molecular relaxation from room temperature up to the glass-transition temperature for three plastically deformed polymers. A well-defined distinct α′ peak has been observed in the vicinity of 70°C for polycarbonate, polysulfone, as well as poly(ethylene terephthalate), that had been annealed above T_g and subsequently cold-rolled. Since this relaxation is not observed after the initial measurements, it is a nonequilibrium relaxation. In the case of polycarbonate, the nonreversible α′ process had an activation energy ranging from 1.22 to 1.75 eV and an intensity proportional to the degree of deformation resulting from cold-rolling. The spontaneous polarization in the rolled material was believed to originate from the initial anisotropy which was subsequently enhanced by the rolling process. Density measurements, as well as the birefringence observation, have been performed on the rolled specimens. All the studies led to the conclusion that the molecular motion responsible for this α′ peak was the disorientation of local chain segments from their aligned conformation frozen in during the cold-work.