Thermal expansion behavior of ultrathin polymer films supported on silicon substrate

The thermal expansion behavior of polystyrene (PS) thin films was investigated using x-ray reflectivity, focusing on ultrathin films below 100.3em0exnm. It was found that the glass transition temperature Tg decreases with thickness as reported by many researchers while it is almost independent of thickness and constant at 3540.3em0exK for films below ~100.3em0exnm. The thickness dependence of Tg was well reproduced by a two-layer model consisting of a mobile surface layer with Tg of 354.50.3em0exK and a bulklike layer with Tg of 3730.3em0exK0.3em0ex(=bulk0.3em0exTg), suggesting that the so-called immobile dead layer near the substrate is negligible or very thin in this system. This surface Tg of 3540.3em0exK was confirmed by the relaxation of surface roughness of as-deposited films at about 3540.3em0exK. It was also found that the thermal expansivity decreases with thickness in the glassy state as well as in the molten state while the reduction is smaller in the molten state.