Dielectric and Dilatometric Studies of Glass Transitions in Thin Polymer Films

Dielectric relaxation and thermal expansion spectroscopy were made for thin polystyrene films in order to measure the temperature $T_{\alpha}$ corresponding to the peak in the loss component of susceptibility due to the $\alpha$-process and the $\alpha$-relaxation time $\tau$ as functions of film thickness $d$. While the glass transition temperature $T_{\rm g}$ decreases with decreasing film thickness, $T_{\alpha}$ and $\tau$ were found to remain almost constant for $d>d_{\rm c}$ and decrease drastically for $d<d_{\rm c}$ for high temperatures. Here, $d_{\rm c}$ is a critical thickness. Near the glass transition temperature, the thickness dependence of $T_{\alpha}$ and $\tau$ is more prominent. The relation between the fragility index and non-exponentiallity is discussed for thin films of polystyrene.