Distribution of glass transition temperature<i>T</i>_gin a polymer thin film by neutron reflectivity

Abstract

We report neutron reflectivity results on a three-layer polystyrene thin film on Si wafer, consisting of alternative stacking of deuterated layer ~20 nm thick and hydrogenated layer ~30 nm thick. In the experiments we have evaluated the film thickness of each layer and the surface roughness and interfacial widths as a function of temperature below and above the glass transition temperature Tg. It was found that the top layer has Tg lower than the bulk Tg by ~18 K, and the middle layer has almost the same Tg as the bulk. On the other hand, the bottom layer did show very high Tg above 130 °C. The results show the distribution of Tg in the thin film. It was also found that the interfacial width between the top and middle layers increased more rapidly than that between the middle and bottom layers above ~110 °C, showing higher mobility of polystyrene in the top layer because of the lower Tg.