  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 Tg - 19 K, for local strain rates between 3.7x10-5/s and
1.2x10-4/s. 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.
