Influence of spatially heterogeneous dynamics on physical aging of polystyrene

Abstract

A photobleaching technique has been used to study the segmental dynamics of a polystyrene melt during isothermal physical aging. Both rotational and translational diffusion measurements of dilute tetracene probes dispersed in the polymer matrix were performed. Following a temperature quench from Tg+2 °C into the glass, either rotational relaxation from an induced anisotropic state or translational diffusion across a holographic grating was measured as a function of aging time. Relaxation times for the two observables evolved by nearly a factor of 10 during isothermal aging, and equilibration times ranged from 103 to 106 s at 98.6 to 95.6 °C. Immediately following the temperature jump, probe rotational diffusion has evolved by over half a decade in response to the quench, while probe translational diffusion has been retarded by only 0.2 decades. At longer elapsed times, for aging at 95.6 °C, the translational observable reaches equilibrium before the rotational observable. These differences in aging behavior indicate that the dynamics responsible for physical aging are spatially heterogeneous, and that regions of different mobility age at different rates. Furthermore, the results suggest that these heterogeneous dynamics could be responsible for causing other observables, such as volume and enthalpy, to equilibrate on different time scales—a phenomenon often associated with physical aging.