Affinity of Polystyrene Films to Hydrogen-Passivated Silicon and Its Relevance to the Tg of the Films

Qualitatively different thickness dependences have been observed in the glass transition temperature, Tg, of polystyrene (PS) films supported by hydrogen-passivated silicon (H-Si). It has been suggested that upon annealing at high temperatures in air, the polymer/substrate interface of these films (i.e., PS/Si), though buried underneath the PS layer, might be oxidized, rendering the films a different polymer/ substrate interface (i.e., PS/SiOx-Si), which may account for the different thickness dependences of the Tg observed. In this experiment, we examine if the buried substrate interface of PS/H-Si films can indeed be oxidized by annealing the films at 150 C in air. Our result shows that a residual film does form on top of the H-Si surface, but it is a bound layer of PS. X-ray photoelectron spectroscopic (XPS) analyses and independence of the residual film on the initial PS thickness evidence that the H-Si substrate buried underneath a PS film is not oxidized by annealing. We discuss a possible explanation to how the different thickness dependences may be observed in the Tg of these films.