Depth-dependent inhomogeneous characteristics in supported glassy polystyrene films revealed by ultra-low X-ray reflectivity measurements

Abstract

This study reports X-ray reflectivity measurements of the glass transition of polystyrene thin films supported on Si substrates and heated at low heating rates that ranged from 0.14 to 0.01 °C min-1. At a heating rate of 0.14 °C min-1, the glass transition temperature Tg was independent of the film thickness down to a thickness of 6 nm. However, at a heating rate of 0.04 °C min-1, the value of Tg decreased with decreased thickness. The reduction in Tg was most significant at the ultra-low heating rate of 0.01 °C min-1. Furthermore, with decreased film thickness, the linear thermal expansivity in the glassy state αglass slightly decreased at a heating rate of 0.14 °C min-1, whereas αglass exhibited a significant increase at the ultra-low heating rate of 0.01 °C min-1. Reconstructed depth profiles of thermal expansivity, which were obtained by fitting the αglass values using an integral model, indicated a decrease in the thickness of the interfacial dead layer with a decrease in the heating rate, whereas the volume fraction of the free surface region increased under this condition. The observed reduction in Tg can be attributed to surface and interface effects perturbing the glass transition dynamics of the thin films under slower probing conditions.