CREEP OF GLASSY-POLYMERS IN THE ALPHA-RETARDATION AND BETA-RETARDATION REGIONS - PHYSICAL AGING AND NONLINEAR BEHAVIOR

Methods are summarized for analyzing the tensile creep curves of glassy polymers over wide time ranges at 23-degrees-C. The methods account for contributions to the creep compliance from secondary (beta) and glass-rubber (alpha) retardation processes, and also allow for the effects of physical aging. With increasing physical age of a polymer, a decrease is found in the magnitude or relaxed compliance of the beta process and an increase in mean retardation time for the a process. Quantitative relationships have been established for each of these effects. Nonlinear creep behavior at elevated stresses can be described by functions which account for a rapid stress-induced decrease in alpha-retardation time and a subsequent increase in this quantity during the creep ascribed to a reactivation of the aging. The nonlinear behavior is illustrated by recent creep data for poly(vinyl chloride) and poly(butylene terephthalate). The effects of aging and nonlinearity may be correlated with changes in structural temperature within the polymer.