Dielectric relaxation of thin films of polyamide random copolymers

TL;DR

This study examines dielectric relaxation in thin polyamide copolymer films, revealing how film thickness influences relaxation processes, charge mobility, and glass transition behavior, with implications for polymer physics and materials engineering.

Contribution

It provides new insights into the thickness-dependent dielectric relaxation and charge dynamics in polyamide copolymer thin films, highlighting deviations from linear behavior at critical thicknesses.

Findings

Dielectric signals include $oldsymbol{ ext{α}}$-process and electrode polarization (EP) process.

EP-process relaxation time follows Vogel-Fulcher-Tammann temperature dependence.

Thickness reduction increases fragility index and affects charge mobility.

Abstract

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.