PLASTIC-DEFORMATION MECHANISM IN PMMA UNDER CREEP STRESS

A rate analysis applied to the plastic flow under creep stress in glassy polymethyl methacrylate (PMMA) is presented. This analysis leads to the conclusion that the structure of the PMMA glass is changed into liquidlike structures during the large deformation under creep stress. Comparisons of various mechanical data obtained from the creep experiment for the polymer with those from the constant-rate deformation experiment indicate that the process of the glass-to-liquid structural change is controlled by the strain and strain rate being imposed on the polymer almost independently of the loading modes.