Effect of Temperature on Postyield Segmental Dynamics of Poly(methyl methacrylate) Glasses: Thermally Activated Transitions Are Important

Abstract

Optical probe reorientation measurements were utilized to investigate the effect of temperature on the segmental dynamics of a glassy polymer during deformation. Constant strain rate deformations were performed on poly(methyl methacrylate) glasses at temperatures from Tg – 27 K to Tg – 11 K and engineering strain rates from 3 × 10–6 to 3 × 10–5 s–1. Deformation enhances segmental mobility by up to a factor of 100 in these experiments. In the postyield flow state at a given strain rate, the effect of temperature on dynamics is reduced, relative to the undeformed polymer glass. However, we still observe a significant effect of temperature on segmental dynamics during flow, with calculated free energy barriers of ∼39 kTg. The Kohlrausch–Williams–Watts parameter βKWW, a measure of spatially heterogeneous dynamics, is observed to increase in the postyield regime with increasing strain rate and decreasing temperature, indicating more homogeneous dynamics. βKWW is correlated with the enhancement of segmental dyn...