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 motio n 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 tha t dynamics are spatially heterogeneous in glass-forming materials. A model of translational motion on heterogeneous lattices indicates that the size o f the heterogeneous regions is roughly 20 Angstrom. Thus the data presented here place a new constraint on models of molecular motion near T-g.