Measurement of the Interior Structure of Thin Polymer Films Using Grazing Incidence Diffuse X-Ray Scattering
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A method is developed for calculating the small-angle x-ray scattering originating from within the interior of a thin film under grazing incidence illumination. This offers the possibility of using x-ray scattering to probe how the structure of polymers is modified by confinement. When the diffuse scattering from a thin film is measured over a range of incident angles, it is possible to separate the contributions to scattering from the interfaces and the contribution from the film interior. Using the distorted-wave Born approximation the structure factor, S q , of the film interior can then be obtained. We apply this method to analyze density fluctuations from within the interior of a silicon supported molten polystyrene PS film. Measurements were made as a function of film thickness ranging from one to ten times the polymer radius of gyration Rg . The compressibility, calculated by extrapolating the measured S q to q=0, agrees well with that of bulk PS for thick films, but thinner films exhibit a peak in S q near q=0. This peak, which grows with decreasing thickness, is attributed to a decreased interpenetration of chains and a consequent enhanced compressibility.