Direct Measurement of Molecular Mobility in Actively Deformed Polymer Glasses
Hau-Nan Lee,
Keewook Paeng,
Stephen F. Swallen,
M. D. Ediger
When sufficient force is applied to a glassy polymer, it begins
to deform through movement of the polymer chains. We used an
optical photobleaching technique to quantitatively measure changes
in molecular mobility during the active deformation of a polymer
glass [poly(methyl methacrylate)]. Segmental mobility increases
by up to a factor of 1000 during uniaxial tensile creep. Although
the Eyring model can describe the increase in mobility at low
stress, it fails to describe mobility after flow onset. In this
regime, mobility is strongly accelerated and the distribution
of relaxation times narrows substantially, indicating a more
homogeneous ensemble of local environments. At even larger stresses,
in the strain-hardening regime, mobility decreases with increasing
stress. Consistent with the view that stress-induced mobility
allows plastic flow in polymer glasses, we observed a strong
correlation between strain rate and segmental mobility during
creep.
