Thermal Probe Measurements of the Glass Transition Temperature for Ultrathin Polymer Films as a Function of Thickness

Abstract

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 °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 °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 °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 °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.