X-ray photon correlation spectroscopy on polymer films with molecular weight dependence

We have applied X-ray photon correlation spectroscopy (XPCS) to study the dynamics of surface fluctuations in thin supported polystyrene films as a function of wave vector, temperature, film thickness, and molecular weight. Molecular weights, ranging from 30 to 650 kg/mol, were used in the study. Lateral length scales probed are at least ten times smaller than those accessible in conventional dynamic light scattering. Good agreement between the experimental results and conventional theory permits us to determine the film viscosity. The viscosity obtained in thin films with a higher molecular weight shows lower viscosity than the one obtained from bulk polystyrene of corresponding molecular weight.