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/SiO x-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.
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