GAS-TRANSPORT PROPERTIES OF LIQUID-CRYSTALLINE POLY(ETHYLENE TEREPHTHALATE-CO-P-OXYBENZOATE)

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 becomes ordered upon annealing above the glass transition. Permeability measurements were made for He, H2, O2, N2, and CO2 at 35-degrees-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.