Anomalous Translational Diffusion: A New Constraint for Models of Molecular Motion Near the Glass Transition Temperature

A photobleaching technique has been used to measure the translational motion of tetracene in polystyrene near T_g. The intermediate scattering function S(q,t) was observed to become increasingly non-exponential with decreasing temperature. Thus a clear transition from Fickian diffusion to non-Fickian translational motion has been observed. These results support the view that dynamics are spatially heterogeneous in glass-forming materials. A model of translational motion on heterogeneous lattices indicates that the size of the heterogeneous regions is roughly 20 Å. Thus the data presented here place a new constraint on models of molecular motion near T_g.