Extensive studies on polymer thin films to date have revealed their interesting but unusual properties such as film thickness dependence of glass transition temperature $T_{\text{g}}$ and thermal expansivity. Recent studies have shown that the lower $T_{\text{g}}$ is not always related to the higher mobility in polymer thin films, which contradicts our current understanding of the glass transition process. In this work, we report the results of inelastic neutron-scattering measurements on polystyrene thin films using two spectrometers with different energy resolutions as well as ellipsometry measurements. The results are interpreted in terms of cooperatively rearranging region and motional slowing down due to the surface effect that explain plausibly the anomalous relationship between the glass transition temperature $T_{\text{g}}$ and the molecular mobility in thin films.

• Received 22 May 2009

DOI:https://doi.org/10.1103/PhysRevE.80.031802