Thermal Analysis of Ultrathin PS-r-PMMA Copolymer Films on Silica
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Composite materials represent a major part of man-made materials that are used in many applications. The interaction of polymers with surfaces plays a crucial role in the final properties of these materials and, by understanding the surface processes and adsorption mechanisms, better systems can be designed. Therefore, the behavior of polymer molecules at interfaces has been the topic of many studies in recent years and a rough picture has been obtained. Keddie et al. Obtained experimental results showing the dependence of the glass transition temperature (Tg) on the thickness of supported polystyrene films using ellipsometry. Later, they also investigated supported poly(methyl methacrylate)(PMMA) films. Their results indicated that a strongly attractive interaction between the polymer film and substrate (e.g. H-bonding between PMMA and the silicon native oxide) was responsible for an increase in Tg with decreasing film thickness. On the other hand, a weak interaction, as in PS with silicon oxide, resulted in a decrease in Tg with decreasing film thickness. It was suggested that the reduction in the Tg value was caused by the presence of a liquid-like layer at the polymer-air interface. Estimates based on the observed thermal expansivities suggest that the characteristic length scale for this layer is ~80-130 Å. Similar results were obtained by other techniques, such as Brillouin light scattering, X-ray reflectivity, positron annihilation lifetime spectroscopy, fluorescence recovery after patterned photobleaching, atomic force microscopy and magnetic resonance (NMR and ESR)3-7. Although many new techniques have been used and much useful information has been obtained, the understanding of the properties of polymers adsorbed on solid surfaces is still far from complete. Porter and Blum used modulated differential scanning calorimetry (MDSC) to investigate the thermal behavior of PMMA thin films adsorbed on silica. They found that the Tg of the adsorbed PMMA layer, at maximum adsorbed amount from toluene, was raised to 136o C and 158o C for half of that amount. Song et al. used MDSC to quantify the interfacial fractions in polymer blends. These studies suggest that MDSC may be a useful tool for investigating very small amounts of species on surfaces. We report use of MDSC to investigate silica adsorbed PS-r-PMMA copolymers as a function of the adsorbed amount and copolymer composition. The derivative of the heat capacity signal, dCp/dT, from MDSC was used to elucidate the fractions of different mobility. In this way we were able to see the changes of fractions with different mobilities directly from the DSC curves.