Spatially heterogeneous dynamics during physical aging far below the glass transition temperature

Abstract

Physical aging experiments after a deep quench were performed via an optical photobleaching technique to examine the evolution of the segmental dynamics of a polystyrene melt far below the glass transition temperature (Tg). Both rotational and translational diffusion measurements of dilute tetracene probes dispersed in the polymer matrix were monitored as a function of aging time. Following a temperature quench from Tg+2°C deep into the glass (with isothermal aging temperatures ranging from Tg-15 to Tg-67°C), either rotational or translational diffusion measurements were performed for up to 105 s of elapsed time. During this period, relaxation times for both observables were retarded by at least a factor of 3. Rotational and translational observables respond differently to the initial temperature quench and to the subsequent isothermal aging. These differences likely indicate that the dynamics responsible for physical aging in a quenched glass are spatially heterogeneous. Immediately following the quench, regions of faster mobility age toward equilibrium faster than regions of slower mobility. These findings compliment recent aging experiments close to Tg and bolster the conclusion that heterogeneous dynamics are important for the accurate description of physical aging.