Time and Temperature Dependence of Recovery Behavior of Residual Strains in Largely Deformed Glassy Poly (Methyl Methacrylate)

Time and temperature dependence of recovery behavior of residual strains in largely compressed glassy poly (methyl methacrylate) (PMMA) has been studied. At several temperatures lower than the glass transition temperature Tg, uniaxially compressed specimens were isothermally kept in the stress-free state for a variation of time tf, and then subjected to thermally stimulated strain recovery measurement. Thermally stimulated recovery of residual strains in specimens right after the compression began at heating temperatures lower than Tg. With increasing time tf, residual strains recoverable at heating temperatures below Tg decreased simply from those recoverable at the lowest heating temperature. The decrease of residual strains recovering at a heating temperature less than Tg was examined as a function of time tf and was found to be expressed by a single exponential retardation function. Thus, the sub-Tg strain recovery was revealed to be a simple viscoelastic process with distribution of retardation times. Temperature dependence of the retardation time allowed us to calculate activation energy of sub-Tg strain recovery as a function of heating temperature. The activation energy of sub-Tg strain recovery was found to be approximately the same function of temperature as that of the linear viscoelastic relaxation of the polymer. This result led us to a conclusion that, as a first approximation, the molecular mechanism governing the sub-Tg recovery of strains given by large deformation is almost the same as that of linear viscoelastic relaxation.