Abstract
Gas transport properties are reported for several main-chain liquid crystalline polymers (LCP) including thermotropic copolyester and lyotropic polyamide systems. Films were processed by melt extrusion, compression molding, and solution casting. Permeability measurements were made for helium, H sub 2 , O sub 2 , N sub 2 , Ar, and CO sub 2 at 35 deg C and the diffusivities were computed from time-lag data. In general, the liquid crystalline polymers exhibited extraordinary gas barrier properties which, in some cases, exceeded those of polyacrylonitrile. The low permeabilities were attributed to the efficient chain packing of the mesomorphic microstructure and the low free volume available for transport. A dichotomy emerged between the transport behavior of the polyester and polyamide LCP materials. While the polyester LCPs exhibited low gas solubility with relatively high penetrant mobility, the opposite behavior was shown in the polyamide films. Several series of films were prepared from thermotropic copolyesters based on p-hydroxy benzoic acid. The degree of drawdown during extrusion, annealing time, and copolymer composition were systematically varied and examined for their effects on gas transport properties. The films showed evidence of a skin--core structure, polydomain texture, and crystallinity within the mesophase. The texture of the polydomain morphology was examined and quantified using an X-ray diffraction technique. The most dramatic reductions in permeability correlated very well with increased orientation of the polydomain morphology. By substituting more hindered aromatic moieties along the polymer main-chain, the permeability and diffusion coefficients decreased dramatically suggesting that issues of segmental mobility control the transport process in LCP materials. Two general transport mechanisms were considered and analyzed through the use of a two phase model and fractional free volume theory. A series of films was prepared from anisotropic solutions of poly(phenylene terephthalamide), PPT. The crystallinity level and crystal structure were characterized using X-ray diffraction methods. Transport properties were compared with those of amorphous films prepared from a structural isomer of PPT which contains a large fraction of meta linkages along the polymer main-chain. The oxygen permeability coefficients of the amorphous PPT-isomer are over an order of magnitude larger than those for the anisotropic PPT materials. The estimated solubility coefficients for the noncrystalline regions of the PPT and PPT-isomer, however, were very close. (DA9200747).