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| - Thermal Probe Measurements of the Glass Transition Temperature forUltrathin Polymer Films as a Function of Thickness
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| - The glass transition temperature of polystyrene and poly(methyl methacrylate) films onpolar and nonpolar substrates was measured as a function of thickness using a thermal probe in contactwith a polymer film. Using a technique called local thermal analysis, heat loss into the film was monitoredas 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 versustemperature plot. The Tg of polystyrene on silicon oxide decreased by as much as 25 °C below the bulkvalue for films 13 nm thick. The same trend in the glass transition temperature was observed forpolystyrene films on silicon oxide treated with hexamethyldisilizane (HMDS). The Tg of poly(methylmethacrylate) on silicon oxide increased by up to 7 °C above the bulk value for films 18 nm thick. Forpoly(methyl methacrylate) on silicon oxide treated with HMDS, the Tg decreased by 10 °C below the bulkvalue for films 21 nm thick. The glass transition temperatures measured with the thermal probe comparedfavorably with values of Tg determined by ellipsometry. Local thermal analysis is an alternative to methodsof determining Tg based on thermally induced changes in film thickness. The technique, relying on changesin the heat capacity and modulus of the polymer at Tg, may help resolve the current controversies overthe measurement of the Tg in thin films.
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