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The temperature dependent thicknesses of ultrathin polystyrene films under vacuum on Si (111) substrates were investigated via x-ray reflectivity in situ. The contraction of ultrathin polymer films was directly observed for the first time to the author's knowledge. The degree of contraction depends on the initial thickness of the ultrathin polystyrene film, with the magnitude of contraction increasing with decreasing initial film thickness. This contraction ranged from 0--17 % and occurred at temperatures well below the reported bulk polystyrene glass transition temperature.
Orts et al. (Mon,) studied this question.