Oxygen-transport properties of liquid-crystalline polyesters based on 4,4'-bibenzoic acid

This study examined the oxygen-transport properties of aromatic liquid-crystalline (LQ polyesters of 4,4'-bibenzoic acid (1313) and an aliphatic diol. The effect of the spacer length was probed by the number of methylene groups in the diol being increased from 3 to 6. Parallel studies were performed on polymers in which the LC character was reduced or removed by copolymerization with 30 mol % nonmesogenic isophthalic acid (BB-301) or by the replacement of BB with 4,4'-oxybis(benzoic acid) (OBB). The oxygen-transport properties were interpreted in terms of the solid-state structure as revealed by the thermal behavior and wide-angle X-ray diffraction. The strong odd-even effect of the spacer length that characterized the thermal transitions of the BB polyesters was weak or absent from the oxygen-transport properties when the effect of crystallinity was considered. Permeability and diffusivity tended to increase with the spacer length. The lower oxygen permeability of all the BB polyesters compared with that of the BB-301 copolyesters and the OBB polyesters was due primarily to lower oxygen solubility. The low solubility of the noncrystalline LC phase was ascribed to a smaller free-volume hole size and possibly to a lower free-volume hole density in comparison with an amorphous glass. The solubility of oxygen in the noncrystalline regions of the BB-301 copolyesters and in the amorphous OBB polyesters conformed to the relationship between the solubility and the glass-transition temperature previously established for amorphous and glassy aromatic polyesters. (C) 2007 Wiley Periodicals, Inc.