Gas transport properties of thermotropic liquid‐crystalline copolyesters. II. The effects of copolymer composition

Gas transport properties are reported for a series of compression‐molded films prepared from copolyesters of hydroxybenzoic acid (HBA) and 2,6 hydroxynaphthoic acid (HNA) having 30/70, 58/42, 73/27, 75/25, and 80/20 mol % HBA/HNA. The mesomorphic and crystalline morphology of the materials was characterized using dynamic mechanical thermal analysis (DMTA), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X‐ray diffraction As evidenced by DMTA, the phenyl and naphthyl moieties of the HBA/HNA materials exhibit a significant degree of segmental mobility below the glass transition temperature. The nonlinear nature of the naphthyl unit leads to a more hindered rotation about the chain axis. Permeability measurements were made for He, H2, O2, N2, Ar, and CO2 at 35°C and the diffusivities were computed from time‐lag data. As previously observed in these materials, the films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. The liquid‐crystalline copolyester: (LCP) materials with the highest HNA content exhibit the best barrier properties. It appears that the more hindered motions of the naphthyl unit restrict penetrant mobility. The reduction in permeability with increased naphthyl unit content is accompanied by a very dramatic increase in selectivity between gas pairs. Fractional free volume analysis was used to correlate the transport properties of the LCP materials and other conventional polymers. A “two‐phase” modification of the free volume correlation suggests that transport may likely occur in a small volume fraction of a less dense boundary phase.