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

Abstract

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/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 Tg 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 (ΔTg = Tgfilm − Tgbulk) scaled linearly with γSL irrespective of the chemistry of the polymer.