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 10 nm. It was found that the glass transition temperature T(g) decreases with thickness as reported by many researchers while it is almost independent of thickness and constant at 354 K for films below approximately 10 nm. The thickness dependence of T(g) was well reproduced by a two-layer model consisting of a mobile surface layer with T(g) of 354.5 K and a bulklike layer with T(g) of 373 K ( =bulk T(g) ), suggesting that the so-called immobile dead layer near the substrate is negligible or very thin in this system. This surface T(g) of 354 K was confirmed by the relaxation of surface roughness of as-deposited films at about 354 K. 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.