Dielectric Relaxation Studies of Poly(4-hydroxybenzoic acid) and Copolyesters Based on 4-Hydroxybenzoic Acid and 6-Hydroxy-2-naphthoic Acid

Abstract

Dielectric relaxation measurements have been performed for two samples of 4-hydroxybenzoic acid homopolymer (PHBA) of different molecular weight and copolymers composed of 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA) with molar ratios of 73/27, 58/42, and 30/70 HBA/HNA, respectively; the materials were investigated at frequencies ranging from 0.4 kHz to 4 MHz at temperatures ranging from −100 to +480 °C (homopolymer) and −100 to +250 °C (copolymers). The homopolymer displays two dipolar relaxations with increasing temperature: a shallow, frequency-dependent relaxation process at low temperatures, which corresponds to motion of the HBA ester dipoles in noncrystalline regions of the material, and a stepwise increase in dielectric constant for all frequencies at ca. 340 °C, which corresponds to the first high-temperature transition in DSC. The characteristics of the latter transition are in excellent agreement with existing X-ray diffractometry and NMR results. The copolymers display two (frequency-dependent) relaxation processes with increasing temperature over the investigated frequency range: the lower temperature y relaxation follows an Arrhenius-type behavior that is virtually identical with that observed for the low-temperature relaxation in the PHBA homopolymer and that corresponds to rotation of the HBA units, while the higher temperature β relaxation displays isochronal loss maxima that can be described by a WLF-type relationship and that appears to correspond to a combination of both rotational and translational motions. Applications of the Onsager equation based on a model of full, unhindered ester dipole rotation for the various relaxations described above reveals measured dielectric strengths that are well below those expected for free rotation, indicating local conformational ordering that restricts the orientation of successive ester groups. Comparison of the (normalized) relaxation strengths for the γ transition in the copolymers with that observed at the high-temperature PHBA transition indicates that the local conformations and rotational motions of the HBA moieties in the copolymers are nearly identical with those present in the high-temperature phase of PHBA above ca. 340 °C. The increased dipolar relaxation strength observed for the β relaxation relative to the γ relaxation reflects a decrease in such conformational constraints at higher temperatures. © 1991, American Chemical Society. All rights reserved.