Reduced Viscosity of the Free Surface in Entangled Polymer Melt Films
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By embedding ‘‘dilute’’ gold nanoparticles in single polystyrene thin films as ‘‘markers’’, we probe the local viscosity of the free surface at temperatures far above the glass transition temperature (Tg). The technique used was x-ray photon correlation spectroscopy with resonance-enhanced x-ray scattering. The results clearly showed the surface viscosity is 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 Tg.