  We investigate the relaxation behavior of thin films of a polyamide random
copolymer, PA66/6I, with various film thicknesses using dielectric relaxation
spectroscopy. Two dielectric signals are observed at high temperatures, the
$\alpha$-process and the relaxation process due to electrode polarization (the
EP-process). The relaxation time of the EP-process has a Vogel-Fulcher-Tammann
type of temperature dependence, and the glass transition temperature, $T_{\rm
g}$, evaluated from the EP-process agrees very well with the $T_{\rm g}$
determined from the thermal measurements. The fragility index derived from the
EP-process increases with decreasing film thickness. The relaxation time and
the dielectric relaxation strength of the EP-process are described by a linear
function of the film thickness $d$ for large values of $d$, which can be
regarded as experimental evidence for the validity of attributing the observed
signal to the EP-process. Furthermore, there is distinct deviation from this
linear law for thicknesses smaller than a critical value. This deviation
observed in thinner films is associated with an increase in the mobility and/or
diffusion constant of the charge carriers responsible for the EP-process. The
$\alpha$-process is located in a high frequency region than the EP-process at
high temperatures, but merges with the EP-process at lower temperatures near
the glass transition region. The thickness dependence of the relaxation time of
the $\alpha$-process is different from that of the EP-process. This suggests
that there is decoupling between the segmental motion of the polymers and the
translational motion of the charge carriers in confinement.

Soft Condensed Matter;Materials Science