Translation-Rotation Paradox for Diffusion in Glass-Forming Polymers: The Role of the Temperature Dependence of the Relaxation Time Distribution

Abstract

Comparisons are made of the translational and rotational diffusion of small-molecule probes in a polymer near its glass transition temperature, $T_g$. In the rubbery state, ${1.1T}_g>T>\mathrm{}{T}_g$, translational diffusion is much less temperature dependent than rotational reorientation; in a “quenched” glass, translation and rotation have similar temperature dependencies. This is explained to be a consequence of the fact that in the rubbery state near $T_g$ the breadth of the polymer relaxation distribution is strongly temperature dependent, while in the quenched glass it is temperature invariant.