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Temperature-ramping anisotropy measurements were used to probe the molecular mobility of fluorescent probes in polystyrene, poly(methyl methacrylate), and poly(2-vinyl pyridine) films supported upon silicon wafers with native oxide coatings. All polymer films showed evidence of high mobility at the free surface. The fraction of a film with high mobility was characterized as a mobile surface layer thickness, which increased with temperature. The mobile surface layer thickness for supported films of polystyrene and poly(methyl methacrylate) reasonably matched that previously deduced from freestanding films of these polymers; for poly(methyl methacrylate), enhanced mobility extends about 4 nm into the film from the free surface at Tg. For supported polystyrene and poly(methyl methacrylate) films, the results are consistent with no decrease in mobility near the solid substrate but do not eliminate this possibility. On the other hand, the mobility of supported poly(2-vinyl pyridine) thin films provides some evidence for slower-than-bulk relaxation near the solid substrate.
Paeng et al. (Mon,) studied this question.