Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening.

A simple theory for glassy-polymeric mechanical response that accounts for large-scale chain relaxation is presented. It captures the crossover from perfect-plastic response to Gaussian strain hardening as the degree of polymerization N increases, without invoking entanglements. By relating hardening to interactions on the scale of monomers and chain segments, we correctly predict its magnitude. Strain-activated relaxation arising from the need to maintain constant chain contour length reduces the characteristic relaxation time by a factor ~εN during active deformation at strain rate ε. This prediction is consistent with results from recent experiments and simulations, and we suggest how it may be further tested experimentally.