Thermal stress and glass transition of ultrathin polystyrene films

The thermal stress of thin and ultrathin polystyrene (PS) films on Si substrate has been studied and the glass transition temperature ${\mathrm{(T}}_g)$ is determined from the thermal stress data. $T_g$ of PS turned out to be thickness independent for thick films but decreases when the film thickness is comparable to the end-to-end distance of the polymer chains $\text{(<100\hspace{0.05em}}\mathrm{nm}\mathrm{).}$ The thermal stress level and the slope of the stress temperature curve of the film also decrease as the film thickness decreases. The slope reduction indicates that the product of the biaxial modulus $\text{E/(1−ν)}$ and the coefficient of thermal expansion (CTE) of the film decreases with film thickness. Assuming that the CTE increases for ultrathin films, the modulus is found to decrease significantly with respect to the bulk value.