Gas transport properties of liquid crystalline poly (ethylene terephthalate‐co‐p‐oxybenzoate)

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Abstract
Gas transport properties are reported for a series of films prepared from thermotropic poly (ethylene terephthalate‐co‐p‐oxybenzoate), or PET/PHB, having compositions of 60 and 80 mol % PHB. The mesomorphic and crystalline morphology of the copolyester films was examined by cross‐polarized light microscopy, differential scanning calorimetry (DSC), and x‐ray diffraction. Melt‐processed films of both compositions appeared to exhibit an entirely anisotropic morphology with low levels of conventional crystallinity. Solution‐cast films prepared from the 60 mol % material were found to contain a large fraction of isotropic regions, which become ordered upon annealing above the glass transition. Permeability measurements were made for He, H2, O2, N2, and CO2 at 35°C and the diffusivities were computed from time‐lag data. The largely anisotropic films exhibit good barrier properties resulting from very low solubility coefficients. The partially isotropic 60 mol % films show much higher permeability coefficients driven primarily by increased solubility coefficients, while diffusivity is affected to a lesser extent. These results appear to contrast with what is observed in semicrystalline systems where increased crystalline order results in more dramatic reductions in penetrant mobility.