| Abstract
| - Incoherent neutron scattering measurements were performed on polycarbonate (PC), poly(methyl methacrylate) (PMMA), and poly(vinyl chloride) (PVC) films of variable thickness, ranging frombulklike down to 75 Å, or length scales comparable to the polymer's radius of gyration. The temperaturedependencies of the incoherent elastic scattering are analyzed in terms of a Debye−Waller factor toestimate the hydrogen-weighted mean-square atomic displacement 〈u2〉. We find a general reduction of〈u2〉 as the polymer films become increasingly thin, especially above the calorimetric glass transitiontemperature, Tg. However, below Tg this reduction depends strongly upon the relative amplitude of thedisplacement. Specifically, if 〈u2〉 in the bulk glass is especially large, as seen in PC, the extensive sub-Tgmotions are strongly suppressed by thin film confinement. On the contrary, glassy PVC displayscomparatively small-amplitude displacements in the glass and virtually no reduction of 〈u2〉 uponconfinement. These results are discussed in terms of a caging of the atomic motions as the degree of thinfilm confinement increases.
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