The effect of physical ageing on the properties of poly(ethylene terephthalate)

Physical ageing rates of poly(ethylene terephthalate) have been measured, and ageing is interpreted to be associated with the conventional glass formation process, which occurs at a more rapid rate at higher temperatures. Ageing is accompanied by a marked change in mechanical properties, increased tensile yield stress and drawing stress, more localized yielding of the polymer and a marked decrease in impact strength. The fracture results have been attributed to the increased yield stress and a change in contribution of plane stress and plane strain conditions in the samples. Fracture surfaces show evidence of mixed modes of fracture.