Solid state structure and oxygen transport properties of copolyesters based on smectic poly(hexamethylene 4,4′-bibenzoate)

This study examined the solid state structure and oxygen barrier properties of copolyesters based on smectic poly(hexamethylene 4,4′-bibenzoate) (PHBB) and non-liquid crystalline poly(hexamethylene isophthalate) (PHI). The isophthalate content was varied from 10 to 75 mol%. Differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), dynamic mechanical thermal analysis (DMTA), and atomic force microscopy (AFM) were employed to characterize the polymers. The strong ordering tendency of 4,4′-bibenzoate was demonstrated by the persistence of ordered PHBB structures in copolymers with large amounts of the kinked isophthalate comonomer. Copolymers with up to 50 mol% isophthalate gave evidence of liquid crystalline (LC) character in the precursor melt. Copolymers with up to 75 mol% isophthalate crystallized in the PHBB α-crystal form with only small perturbations of the unit cell. The copolymers provided insight into the low gas permeability of LC polymers. Changes in both solubility and diffusivity contributed to the lower oxygen permeability of the smectic glass compared to the amorphous glass. Smaller free volume hole size of the smectic glass gave rise to lower oxygen solubility and contributed to lower diffusivity. The extended chain conformation in the smectic glass, which reduced the fraction of glycol units in gauche conformations, was a second factor that contributed to lower diffusivity.