The effect of molecular orientation on the physical ageing of amorphous polymers-dilatometric and mechanical creep behaviour

The objective of this study was to determine whether molecular orientation has an effect on the rate of physical ageing in amorphous glassy polymers. To achieve this, samples of atactic polystyrene and bisphenol-A polycarbonate were uniaxially hot-drawn to various stretch ratios and then quenched into the glassy state to freeze in orientation. Physical ageing rates were then measured as a function of orientation with dilatometry and tensile creep measurements. The volume relaxation rate, β, was approximately 50% higher for the stretched samples and did not vary with orientation over the range of stretch ratios tested. This was true despite the fact that the initial free volume was actually decreasing with increasing elongation. In contrast, creep measurements showed a slight decrease in the horizontal shift rate, μ_h, upon stretching (i.e. a decreased ageing rate). Possible explanations for these unusual trends in terms of a stretch-induced activated state are discussed.