Effects of Molecular Weight Reduction on Brittle-Ductile Transition and Elastic Yielding Due to Noninvasive γ Irradiation on Polymer Glasses

By exposing specimens to they irradiation, we show that polymer glasses including bisphenol A polycarbonate (PC) and poly(methyl methacrylate) (PMMA) turn from ductile to brittle due to chain scission by the irradiation that has noninvasively reduced their molecular weight. More importantly, by causing chain scission with the gamma irradiation, we examine the origin of mechanical stress produced from cold drawing of four ductile polymer glasses (PC, PMMA, polystyrene, and poly(2,6-dimethyl-1,4-phenylene oxide)). Specifically, we elucidate the origin of the residual stress embedded in the glassy state from the cold-drawing of these polymer glasses into the strain hardening regime by investigating the elastic yielding behavior of these cold drawn specimens. In contrast to the emergent retractive stress observed during annealing of the cold-drawn specimens above their storage temperature but still well below their glass transition temperatures, we find significantly lower retractive stress level and longer induction time for the retractive stress to show up when these specimens undergo sufficient dosage of the gamma irradiation whose effect on the molecular weight and its distribution has been measured with gel permeation chromatography.