Change in the temperature dependence of segmental dynamics in deeply supercooled polycarbonate

Abstract

The equilibrium and nonequilibrium segmental dynamics of bisphenol-A-polycarbonate (PC) have been investigated near the glass transition by studying the rotational and translational diffusion of probe molecules in the polymer matrix. The temperature dependence of equilibrium dynamics was studied from 149 to 124 °C. We report a change in the temperature dependence of equilibrium molecular motion near 134 °C. Below this temperature, the activation energy of both the translational diffusion coefficient DT and the rotational correlation time τc decreases by nearly a factor of 3. This change is not caused by the transition to the nonequilibrium glassy state, but occurs in equilibrium. Additionally, we present evidence that the equilibrium dynamics of PC are spatially heterogeneous. Translational diffusion of rubrene shows an enhancement of 2.5 decades over what would be expected in a homogeneous system at Tg and the mean-square displacement of tetracene translation is not linear with time at low temperatures. In the nonequilibrium glassy state, differences in the physical aging behavior of rotational and translational observables suggest that physical aging dynamics are spatially heterogeneous. During isothermal aging, regions of faster mobility initially age towards equilibrium faster than regions of slower mobility.