STRAIN RECOVERY BEHAVIORS AT HEATING FOR GLASSY POLYMETHYL METHACRYLATE LARGELY STRETCHED WITH STRAIN RATE JUMPS

Abstract

To analyze the effect of strain rate upon the state of internal strain energy accumulated in largely stretched glassy polymethyl methacrylate (PMMA), we studied its strain recovery behavior at heating. Specimens were subjected to up- and down-jump schemes of strain rate and stretched to various amounts of strain at a temperature below the glass transition temperature Tg. As an ultimate case of the down-jump scheme, the stress relaxation experiment was also performed for various durations. Comparing transient stress responses due to the strain rate jump imposed on the specimens with their strain recovery behaviors at heating, we found that to increase time length of viscoelastic stress relaxation had an effect of shifting a temperature range of sub-Tg strain recovery to higher temperatures. Since the strain recovery at less than Tg has been found to be accompanied by a release of the internal strain energy, the present result allowed us to conclude that molecular kinetics of nonlinear viscoelastic relaxation occurring in the largely deformed polymer had an effect to enhance the thermal stability of the internal strain energy accumulated in the deformed polymer.