Arrhenius-type temperature dependence of the segmental relaxation below T_g

In a recent paper DiMarzio and Yang [J. Res. Natl. Inst. Stand. Technol. 102, 135 (1997)] predicted that transport properties such as viscosity and diffusion coefficient do not follow the typical Williams, Landel, and Ferry (WLF) [J. Am. Chem. Soc. 77, 3701 (1955)] or Vogel-Fulcher-type of temperature dependence as the glass transition is approached. Rather, a transition to an Arrhenius-type of temperature dependence is predicted. Here we describe long term aging experiments that explore the temperature dependence of the viscoelastic response of polycarbonate in the vicinity of the glass transition. Aging the material for long times below the nominal glass transition temperature, assures that equilibrium is attained and we can directly test the DiMarzio-Yang prediction. In tests in which glassy samples of polycarbonate were aged into equilibrium at temperatures up to 17°C below the conventionally measured glass transition temperature, we find that the results are consistent with a transition from Vogel-Fulcher or WLF-type behavior to Arrhenius-type behavior. Our results are discussed within the context of other measurements on nonpolymeric glasses and other recent results on polymeric glass formers.