The glass transition temperatures (Tg's) of ultrathin films (thickness 80-18 nm) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were measured on surfaces with interfacial energies (γSL) ranging from 0.50 to 6.48 mJ/m 2 . The surfaces consisted of self-assembled films of octadecyltrichlorosilane (OTS) that were exposed to X-rays in the presence of air. Exposure to X-ray radiation systematically modified the OTS by incorporating oxygen-containing groups on the surface. The interfacial energy for PS and PMMA on the OTS surface was quantified as a function of X-ray dose using the Fowkes-van Oss-Chaudhury-Good model of surface tension. The T g values of the films were characterized by three complementary techniques: local thermal analysis, ellipsometry, and X-ray reflectivity. Within the resolution of the techniques, the results were in agreement. At low values of γSL, the Tg values of PS and PMMA films were below the respective bulk values of the polymers. At high values of γSL, the Tg values of PS and PMMA films were higher than the bulk values and increased monotonically with increasing γSL. The deviation of the Tg values of the films compared to the bulk values increased with decreasing film thickness. For a specific film thickness of PS and PMMA, the difference between the Tg of the film and Tg of the bulk polymer (∆Tg ) Tg film -Tg bulk ) scaled linearly with γSL irrespective of the chemistry of the polymer.

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The temperature dependent thicknesses of ultrathin polystyrene films under vacuum on Si(l 11) substrates were investigated via x-ray reflectivity in situ. The contraction of ultrathin polymer films was directly observed for the first time to the authors' 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.

Neutron reflectometry is used to measure polymer mobility over distances less than 200 Å as a function of distance from an attractive solid surface and molecular size. Effective diffusion coefficients, D eff , are determined from the rates of interdiffusion between supported bilayers of deuterated and hydrogenated poly(methyl methacrylate) (PMMA). The thickness of the d-PMMA layer is varied from less than the radius of gyration, R g , of the bulk polymer to 11R g for three molecular weight pairs of polymers. D eff dramatically decreases in samples with d-PMMA layers less than 3R g thick and approaches bulk interdiffusion rates from layers greater than 5R g thick.