Physical aging and equilibrium dynamics of polystyrene and polycarbonate: The role of spatially heterogeneous dynamics

An optical photobleaching technique has been used to investigate the segmental dynamics of polystyrene and polycarbonate during isothermal physical aging, as well as in equilibrium. Following a temperature quench from T _g + 2°C into the glass, either rotational or translational diffusion measurements of tetracene probes dispersed in the polystyrene matrix were performed as a function of elapsed time. For aging temperatures ranging from T_g-1.5 to T_g-4.5°C, relaxation times for the two observables evolved by nearly a factor of 10 during isothermal aging, and equilibration times ranged from 103 to 106 seconds. Deep in the glassy state, for temperatures ranging from T_g-15 to T_g-67°C, relaxation times for both observables were retarded by at least a factor of 3 over 105 s of elapsed time. Rotational and translational observables respond differently to the initial temperature quench and also to the subsequent isothermal physical aging. Immediately following the temperature jump, probe rotational diffusion evolves further towards equilibrium than probe translational diffusion. These differences indicate that the dynamics responsible for physical aging are spatially heterogeneous, and that regions of different mobility age towards equilibrium at different rates.; In polycarbonate, both equilibrium and nonequilibrium segmental dynamics were investigated near T_g by monitoring the rotational and translational diffusion of rubrene and tetracene probes in the polymer matrix. The temperature dependence of equilibrium dynamics was studied for temperatures ranging from 149 to 124°C (T_g = 140°C). At 134°C, an abrupt change in the temperature dependence of equilibrium molecular motion occurs. Here, the activation energy of both the translational diffusion coefficient D _T and the rotational correlation time τ_c decreases by nearly a factor of three. Additionally, we present evidence suggesting that equilibrium dynamics are spatially heterogeneous. Translational diffusion of rubrene shows an enhancement of 2.5 decades over what would be expected in a homogeneous system at T_g, and the mean-square displacement of tetracene translation is not linear in time. Out of equilibrium, we find that physical aging dynamics in polycarbonate are spatially heterogeneous. During isothermal aging, regions of faster mobility initially age faster than regions of slower mobility.