Effects of stress and physical ageing on nonlinear creep behavior of poly(methyl methacrylate)

Abstract

The effects of stress, ageing time and ageing temperature on creep behavior of poly(methyl methacrylate) were studied. After annealing above its glass transition temperature for a period of time to eliminate the stress and thermal history, the specimens were quenched and aged at various ageing temperatures for different ageing time, and then the short-term creep tests under different stress levels were carried out at room temperature. The creep strains were modeled by means of time-ageing time equivalence and time-stress equivalence, and the master creep curves were constructed via ageing time shift factors and stress shift factors. The results indicate that the creep rate increases with stress, while decreases with ageing time, and the ageing temperature history obviously affects the creep rate. For linear viscoelastic material, the ageing shift rate is independent on imposed stress, while for nonlinear viscoelastic material, the ageing shift rate decreases with increasing stress. The unified master creep curve up to 540 d at reference state was constructed by shifting the creep curves horizontally along the logarithmic time axis to overlap each other. It is demonstrated that the time-stress equivalence, united with the time-ageing time equivalence, provides an effective accelerated characterization technique in the laboratory to evaluate the long-term creep behavior of physical ageing polymers.