Thermal probe measurements of the glass transition temperature for ultrathin polymer films as a function of thickness

The glass transition temperature of polystyrene and poly(methyl methacrylate) films on polar and nonpolar substrates was measured as a function of thickness using a thermal probe in contact with a polymer film. Using a technique called local thermal analysis, heat loss into the film was monitored as the temperature of the probe was ramped from ambient temperature to temperatures as high as 200 degrees C. The glass transition temperature was determined from a change in slope in the heat loss versus temperature plot. The T-g of polystyrene on silicon oxide decreased by as much as 25 degrees C below the bulk value for films 13 nm thick. The same trend in the glass transition temperature was observed for polystyrene films on silicon oxide treated with hexamethyldisilizane (HMDS). The T-g of poly(methyl methacrylate) on silicon oxide increased by up to 7 degrees C above the bulk value for films 18 nm thick. For poly(methyl methacrylate) on silicon oxide treated with HMDS, the T-g decreased by 10 degrees C below the bulk value for films 21 nm thick. The glass transition temperatures measured with the thermal probe compared favorably with values of T-g determined by ellipsometry. Local thermal analysis is an alternative to methods of determining T-g based on thermally induced changes in film thickness. The technique, relying on changes in the heat capacity and modulus of the polymer at T-g, may help resolve the current controversies over the measurement of the T-g in thin films.