Dependence of the Glass Transition Temperature of Polymer Films on Interfacial Energy and Thickness

The glass transition temperatures (T g'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/m2. 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 T g values of PS and PMMA films were below the respective bulk values of the polymers. At high values of γSL, the T g values of PS and PMMA films were higher than the bulk values and increased monotonically with increasing γSL. The deviation of the T g 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 T g of the film and T g of the bulk polymer (ΔT g = T g film − T g bulk) scaled linearly with γSL irrespective of the chemistry of the polymer.