  The $\alpha$-process (segmental motion) of thin polystyrene films supported
on glass substrate has been investigated in a wider frequency range from
10$^{-3}$ Hz to 10$^4$ Hz using dielectric relaxation spectroscopy and thermal
expansion spectroscopy. The relaxation rate of the $\alpha$-process increases
with decreasing film thickness at a given temperature above the glass
transition. This increase in the relaxation rate with decreasing film thickness
is much more enhanced near the glass transition temperature. The glass
transition temperature determined as the temperature at which the relaxation
time of the $\alpha$-process becomes a macroscopic time scale shows a distinct
molecular weight dependence. It is also found that the Vogel temperature has
the thickness dependence, i.e., the Vogel temperature decreases with decreasing
film thickness. The expansion coefficient of the free volume $\alpha_f$ is
extracted from the temperature dependence of the relaxation time within the
free volume theory. The fragility index $m$ is also evaluated as a function of
thickness. Both $\alpha_f$ and $m$ are found to decrease with decreasing film
thickness.
