Anomalous Diffusion of Probe Molecules in Polystyrene: Evidence for Spatially Heterogeneous Segmental Dynamics

Abstract

A holographic fluorescence recovery after photobleaching technique has been used to measure the translational motion of tetracene and rubrene (tetraphenyltetracene) in polystyrene (PS) from T g -10 K to T g + 100 K (T g = 373 K). At 363.5 K, the translational motion oftetracene is not diffusive ; i.e., the mean-square displacement is not linear in time on the length scale (60 nm) and time scale (10 h) of the experiment. Above 370 K, diffusive transport is observed for both probes with diffusion coefficients in the range of 10 -6 -10 -15.5 cm 2 /s. The observed diffusion coefficients have a significantly weaker temperature dependence than rotation times for these two probes in PS. At low temperatures, the root-mean-square displacement for tetracene approaches 60 nm in one rotational correlation time. It is not necessary to conclude, however, that individual probe molecules translate large distances without rotating. All these observations can be rationalized without invoking a change in the local relationship between rotation and translation if the local dynamics of PS are assumed to be spatially heterogeneous.