:We describe an apparatus for performing constant strain rate deformations of polymer glasses while simultaneously measuring the segmental mobility with an optical probe reorientation method. Poly(methyl methacrylate) glasses were deformed at T g -19 K, for local strain rates between 3.7x10 -5 and 1.2x10 -4 s -1 . In these experiments, the mobility initially increases in the pre-yield regime, by a factor of 40 to 160, as compared to the undeformed PMMA glass. The mobility then remains constant after yield, even as the stress is decreasing due to strain softening. This is consistent with the view that the sample is being pulled higher on the potential energy landscape in this regime. Higher strain rates lead to higher mobility in the post-yield regime and, for the range of strain rates investigated, mobility and strain rate are linearly correlated. We observe that thermal history has no influence on mobility after yield and that deformation leads to a narrowing of the distribution of segmental relaxation times. These last three observations are consistent with previously reported constant stress experiments on PMMA glasses. The experimental features reported here are compared to computer simulations and theoretical models.

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 T g – 27 K to T g – 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 kT g. 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 dynamics relative to the undeformed polymer, independent of temperature.

We performed constant strain rate deformation and stress relaxation on a poly(methyl methacrylate) glass at Tg – 19 K, utilizing three strain rates and initiating the stress relaxation over a large range of strain values. Following previous workers, we interpret the initial rate of decay of the stress during the relaxation experiment as a purely mechanical measure of mobility for the system. In our experiments, the mechanical mobility obtained in this manner changes by less than a factor of 3 prior to yield. During these mechanical experiments, we also performed an optical measurement of segmental mobility based on the reorientation of a molecular probe; we observe that the probe mobility increases up to a factor of 100 prior to yield. In the post‐yield regime, in contrast, the mobilities determined mechanically and by probe reorientation are quite similar and show a similar dependence on the strain rate. Dynamic heterogeneity is found to initially decrease during constant strain rate deformation and then remain constant in the post‐yield regime. These combined observations of mechanical mobility, probe mobility, and dynamic heterogeneity present a challenge for theoretical modeling of polymer glass deformation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1957–1967