Effect of Annealing on the Physical Properties of Biaxially Oriented Liquid Crystalline Copolyester Films

Abstract

Biaxially oriented liquid crystalline aromatic copolyester (LCP) films based on p-hydroxybenzoic acid (HBA), 4,4'-hydroxybiphenol (BP), terephthalic acid (TA), and isophthalic acid (IA) were studied in order to evaluate the effect of annealing on the thermal transitions, thermomechanical properties, microstructure, and dielectric relaxation behavior. By the analysis of DSC data, annealing near the liquid crystalline transition of the as-received films led to an increase in the degree of crystallinity as well as an increase in the transition temperature by 60-70 °C. This phenomenon appears to arise from the improved packing of the rodlike chains with a corresponding increase in dipole interactions. Thermomechanical analysis revealed that the coefficients of thermal expansion (CTE) of the as-received film A increased with increasing temperature : however, that of the as-received film B decreased with increasing temperature below 200 °C. Annealing appears to decrease the CTE of the two as-received films. A wide-angle X-ray diffraction (WAXD) study in the reflection mode was performed. The as-received films displayed a single X-ray peak characteristic of one-dimensional order. On the other hand, the annealed films showed two peaks with a new previously unreported peak. Dielectric measurements made from 10 kHz to 1 MHz and from -100 to +220 °C demonstrated that annealing near the liquid crystalline transition has little influence on the dielectric constant. The results obtained in this study are interpreted in terms of structural changes occurring during annealing.