Single Molecule Experiments Reveal the Dynamic Heterogeneity and Exchange Time Scales of Polystyrene near the Glass Transition

In a polymeric material near its glass transition temperature, segmental dynamics of a given spatial region may differ considerably from that of neighboring regions without apparent structural origin, analogous to the supercooled liquid state of low molecular weight glass formers. Given that the supercooled liquid state is a (metastable) equilibrium state, spatial variations in dynamics are expected to average out in time, consistent with ergodicity of the system. By probing the rotations of fluorescent guest molecules, local segmental dynamics of polystyrene was scrutinized molecule by molecule. Two perylene dicarboximide dyes were investigated as potential reporters, and one of these was found to report a substantial proportion of the dynamic heterogeneity of the host polystyrene. Using this probe, we demonstrate that the polystyrene system is ergodic and characterize time scales over which molecules experience changes to their dynamics. We identify a characteristic time scale of exchange much longer than the structural relaxation of the host segmental dynamics, consistent with both previous studies on polystyrene and studies on small molecule glass formers. Moreover, we show that dynamic exchange spans a wide range of time scales from <150 to ∼35 000 times the segmental relaxation time of the polystyrene.