Dependence of the glass transition temperature of polymer films on interfacial energy and thickness

The glass transition temperatures (Tg's) of ultrathin films (thickness 80-1 8 nm) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) were measure d on surfaces with interfacial energies (gamma (SL)) ranging from 0.50 to 6 .48 mJ/m(2). The surfaces consisted of self-assembled films of octadecyltri chlorosilane (OTS) that were exposed to X-rays in the presence of air. Expo sure to X-ray radiation systematically modified the OTS by incorporating ox ygen-containing groups on the surface. The interfacial energy for PS and PM MA 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 t he films were characterized by three complementary techniques: local therma l analysis, ellipsometry, and X-ray reflectivity. Within the resolution of the techniques, the results were in agreement. At low values of gamma (SL), the T-g values of PS and PMMA films were below the respective bulk values of the polymers. At high values Of gamma (SL), the T-g values of PS and PMM A films were higher than the bulk values and increased monotonically with i ncreasing gamma (SL). 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 a nd T-g of the bulk polymer (DeltaT(g) = T-g(film) - T-g(bulk)) scaled linea rly with gamma (SL) irrespective of the chemistry of the polymer.