GAS-TRANSPORT PROPERTIES OF THERMOTROPIC LIQUID-CRYSTALLINE COPOLYESTERS .1. THE EFFECTS OF ORIENTATION AND ANNEALING

Gas transport properties are reported for two series of films prepared from copolyesters of 73 mol % hydroxybenzoic acid (HBA) and 27 mol % 2,6-hydroxynaphthoic acid (HNA) which systematically vary the degree of orientation and annealing time. Scanning electron microscopic (SEM) photomicrographs of the liquid-crystalline polymer (LCP) films showed evidence of a skin-core structure and polydomain texture. The degree of orientation in the films was quantified by analyzing the azimuthal intensity of the x-ray reflection associated with the lateral packing of the nematic mesophase. Using heat of fusion data from differential scanning calorimetry (DSC), the films were found to contain low levels of crystallinity estimated to be in the range of 5 to 15 wt %, which increased with annealing time. Permeability measurements were made for He, H-2, O2, N2, Ar, and CO2 at 35-degrees-C and the diffusivities were computed from time-lag data. The films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. A moderate reduction in permeability was observed with increased orientation, which could be attributed directly to a decrease in the effective diffusivity. The effect of increased crystallinity from annealing on the permeability coefficients was smaller than would be expected for similar changes in the crystallinity of conventional polymers. Analysis using a simple two-phase model suggests that a mechanism dominated by transport in a small volume fraction of boundary regions possibly could account for the bulk transport properties of these materials.