Observation of Anomalous Viscosity in Entangled Polymer Films near the Glass Transition

We have studied the viscous relaxation of surface waves on molten polystyrene films of various molecular weights ($M_{W})$ using x-ray photon correlation spectroscopy. The relaxation time has been measured as a function of wave vector from high temperatures down to near the bulk glass transition temperature ($T_{g})$. We find a transition from a single exponential regime through a stretched exponential to another single exponential regime as the temperature decreases to $T_{g}$ where the effective viscosity saturates at that of chains with critical molecular weight for entanglement. These results are interpreted in terms of the freezing-out of relaxation modes involving full chains and large segments until only fluctuations of chain segments of critical entanglement length survive. We also find no evidence for a low-viscosity surface layer near $T_{g}$.