Reduced Viscosity of Free Surface in Entangled Polymer Melt Films

The dynamics of polymer chains near the surface of a melt and within thin films remains a subject of inquiry along with the nature of the glass transition in these systems. By embedding ``dilute'' gold nanoparticles in single polystyrene thin films as ``markers'', we could probe the local viscosity of the free surface at temperatures far above the glass transition temperature ($T_{g})$. The technique used was X-ray photon correlation spectroscopy with resonance-enhanced X-ray scattering. The results clearly showed the viscosity was about 30 {\%} lower than the rest of the film. We found that this reduction is strongly associated with chain entanglements at the free surface rather than the reduction in $T_{g}$.