Segmental dynamics of normal and pre-oriented PMMA glasses in the strain-hardening regime*

Strain-hardening is a feature of polymer glasses during large deformation, which helps to stabilize the glasses against breakage. Little is known about the segmental dynamics during strain hardening, and such data is important for building a molecular-level theory of this process. Here, using a photobleaching technique, we measure the segmental dynamics of normal and pre-oriented PMMA glasses during deformation into strain-hardening regime at temperatures from T_g-30K to T_g-20K, with local strain rates from 3×10^-5 s^-1 to 9×10^-5 s^-1. The pre-oriented glasses were prepared by stretching above Tg followed by rapid cooling.  For both normal and pre-oriented glasses, we find that the segmental relaxation time follows a power law relationship with the strain rate in the strain-hardening regime with an exponent near 0.85. For the normal sample, the relationship between segmental relaxation time and strain rate is the same as in the early  flow regime. At the same engineering strain, the segmental dynamics of the normal and pre-oriented samples are almost the same, with the pre-oriented sample having dynamics about 15% faster.