EFFECT OF STRONGLY FAVORABLE SUBSTRATE INTERACTIONS ON THE THERMAL-PROPERTIES OF ULTRATHIN POLYMER-FILMS

The thermal behavior of ultrathin films of poly-(2)-vinylpyridine spin -cast on acid-cleaned silicon oxide substrates is considered. The inte raction between the polymer and the substrate is polar in nature and v ery favorable. As a means of examining the thermal properties of the f ilms, x-ray reflectivity is used to measure the temperature dependence of the film thickness. This experimentally measured thickness-tempera ture data is used to determine transition temperatures and thermal exp ansivities. Significantly increased transition temperatures (20-50 deg rees C above the measured bulk glass transition temperature) are obser ved in ultrathin polymer films. The transition temperature increases w ith decreasing film thickness, while the degree of thermal expansion b elow the transition temperature decreases with decreasing film thickne ss. If one assumes that a region of reduced chain mobility exists near the solid substrate-polymer interface, an analysis of the measured th ermal expansion behavior below the transition temperature indicates th at the length scale of substrate interactions is on the order of the m acromolecular size.