Glass transitions and dynamics in thin polymer films: dielectric relaxation of thin films of polystyrene

The glass transition temperature $T_{\rm g}$ and the temperature $T_{\alpha}$ corresponding to the peak in the dielectric loss due to the $\alpha$-process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in $T_{\rm g}$ was observed with decreasing film thickness, while $T_{\alpha}$ was found to remain almost constant for $d>d_{\rm c}$ and to decrease drastically with decreasing d for $d<d_{\rm c}$. Here, $d_{\rm c}$ is a critical thickness dependent on molecular weight. The relaxation time of the $\alpha$-process was found to have a d dependence similar to that of $T_{\alpha}$. The relaxation function for the $\alpha$-process was obtained by using the observed frequency dependence of the peak profile of the dielectric loss. The exponent $\beta_{{\tiny KWW}}$, which was obtained from the relaxation functions, decreases as thickness decreases. This suggests that the distribution of relaxation times for the $\alpha$-process broadens with decreasing thickness. The thickness dependence of $T_{\rm g}$ is directly related to the distribution of relaxation times for the $\alpha$-process, not to the relaxation time itself.