Segmental Dynamics in the Strain-Hardening Regime for Poly(methyl methacrylate) Glasses with and without Melt Stretching

Strain hardening is a feature of polymer glasses during large deformation, which helps to stabilize the glasses against breakage. Experimentally, little is known about the segmental dynamics during strain hardening, and such data are important for building a molecular-level theory of polymer glasses deformed in this regime. Here, using a photobleaching technique, we measured the segmental dynamics of lightly crosslinked poly­(methyl methacrylate) (PMMA) glasses with and without melt stretching, which were deformed into the strain-hardening regime with local engineering strain rates from 10–4.6 to 10–4 s–1 at T g-23 K and T g-33 K. We find that melt-stretched PMMA glasses show a more prominent strain-hardening feature and faster segmental dynamics by a factor of about 15% compared to PMMA without melt stretching. At a given true strain rate, the segmental dynamics of PMMA without melt stretching are accelerated in the deep strain-hardening regime from the value just beyond yield, by up to 40% at 0.8 true strain. Our observations are in agreement with previously published simulation results.