  Dynamics of thin films of poly(vinyl acetate) (PVAc) and poly(methyl
methacrylate) (PMMA) have been investigated by dielectric relaxation
spectroscopy in the frequency range from 0.1Hz to 1MHz at temperatures from
263K to 423K. The $\alpha$-process, the key process of glass transition, is
observed for thin films of PVAc and PMMA as a dielectric loss peak at a
temperature $T_{\alpha}$ in temperature domain with a fixed frequency. For PMMA
the $\beta$-process is also observed at a temperature $T_{\beta}$. For PVAc,
$T_{\alpha}$ decreases gradually with decreasing thickness, and the thickness
dependence of $T_{\alpha}$ is almost independent of the molecular weight
($M_{\rm w}\le2.4\times10^5$). For PMMA, $T_{\alpha}$ remains almost constant
as thickness decreases down to a critical thickness $d_c$, at which point it
begins to decrease with decreasing thickness. Contrastingly, $T_{\beta}$
decreases gradually as thickness decreases to $d_c$, and below $d_c$ it
decreases drastically. For both PVAc and PMMA, the broadening of the
distribution of the relaxation times in thinner films is observed and this
broadening is more pronounced for the $\alpha$-process than for the
$\beta$-process. It is also observed that the relaxation strength is depressed
as the thickness decreases for both the polymers.
