The glass transition temperature (Tg) of ultra thin films (thickness 80-18mm) of polystyrene (PS) and poly (methyl methacrylate) (PMMA) was measured on surfaces with an interfacial energy, gsl, ranging from 0.50 to 6.48 mJ/m2. The surface consisted of self-assembled monolayers of octadecyltrichlorosilane (OTS) that were exposed to x-rays in the presence of air. Exposure to x-ray radiation systematically modified the surface by incorporation of oxygen containing groups on to the OTS. The interfacial energy for PS and PMMA on the OTS surface was quantified using the Fowkes-van Oss-Chaudhury-Good model of surface tension. For both polymers, the interfacial energy increased with exposure dose. The Tg of the films was characterized using local thermal analysis, ellipometry and x-ray reflectivity. The results from all three methods of measuring Tg agreed within the resolution of the techniques. The results from all three methods of measuring Tg agreeded within the resolution of the techniques. At low values of gammasl, the Tg of PS and PMMA was below the bulk value. At high values of gammasl, the Tg was higher that the bulk value and increased monotonically with increasing gammasl. The deviation of the Tg from the bulk value increased with decreasing film thickness. For a specific film thickness, deltaTg = Tg (film) - Tg (bulk) scaled linearly with gamma irrespectivie of the chemistry of the polymer.
Issue: No 16
Pub Type: Journals
modified surface energy, OTS, Tg, ultra thin polymer film, x-ray reflectivity