Extensive studies on polymer thin films to date have revealed their interesting but unusual properties such as film thickness dependence of glass transition temperature Tg and thermal expansivity. Recent studies have shown that the lower Tg 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 Tg and the molecular mobility in thin films.

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High-resolution (ΔE/E ∼ 2−3%) incoherent inelastic neutron scattering spectra of polyisobutylene and cis-1,4-polyisoprene have been obtained on the TFXA spectrometer at the ISIS pulsed neutron scattering facility. The observed bands are assigned with reference to available infrared and Raman data, and the differences observed are discussed. Particular emphasis is placed on the methyl group torsions which are not observable with the optical techniques. These bands are significantly broadened compared to the instrumental resolution, and this is attributed to a Gaussian distribution of local environment potentials for the methyl group, in agreement with present theoretical models. For polyisoprene, this assignment is confirmed by independent molecular dynamics simulations.