Delayed yielding in glassy polymers

A theoretical description of the ductile transition of glassy polymers in tensile creep is given. It predicts the delay time for plastic yield as a function of stress. The model assumes first‐order rate expressions for the rate of change of loadbearing elements. The rate constants are given by a modification of the absolute reaction rate concepts of Eyring, where the energy barrier for breakdown of the bonding elements is asymmetric. This leads to an equation for the yield delay time as a function of the height of the energy barrier in the unstressed state, an activation volume, and a critical stress required to overcome the initial barrier asymmetry. The model has been compared to creep delay time experiments on polysulfone, polycarbonate, and poly(methyl methacrylate). It is able to predict the stress dependence for the onset of ductility in these materials with greater accuracy than previously reported equations.