We report x-ray reflectivity measurements on polystyrene thin films supported on silicon wafer. In annealing experiments, we found fast and slow contraction processes in the thin films above the glass transition temperature. The former is the normal relaxation (annealing) process observed in bulk, and the latter is unexpected and enhanced in thin films below approximately 20 nm. In addition, we found unexpected extremely slow reexpansion processes in the glassy state. These unexpected very slow processes are discussed in terms of lateral contraction and expansion processes driven by entropic changes at the interfaces and the difference of the expansivities between polystyrene and silicon wafer.

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.