This study examined the solid state structure and oxygen transport properties of smectic poly(pentamethylene 4,4'-bibenzoate) (PP5BB). The polymer was quenched from the isotropic melt to the smectic liquid crystalline (LC) glass and subsequently isothermally crystallized above the glass transition temperature. Crystallized PP5BB was characterized by thermal analysis and X-ray diffraction. Gas transport properties were characterized at 23 degrees C and 1 atm pressure. Examination of the smectic LC glass by atomic force microscopy (AFM) revealed a hierarchical structure in which mesogens organized into smectic layers, stacks of layers formed wavy lamellae, and assembles of lamellae defined domains. The effect of crystallization on the oxygen transport properties supported a simple two-phase model of impermeable crystallites dispersed in a permeable smectic glass. Due to the low-crystallite aspect ratio, crystallization only slightly increased the tortuosity of the diffusion pathway. This result provided the structural basis for describing oxygen permeability in terms of the Nielsen model for low-aspect ratio particles. The hierarchical LC structure of PP5BB closely resembled that reported for poly(diethylene glycol 4,4'-bibenzoate) (PDEGBB), which has the same chemical structure as PP5BB except that the center methylene of the spacer is replaced with an ether oxygen. However, oxygen permeability of PP5BB was about three times higher than that of PDEGBB. Non-LC aromatic polyesters with pentamethylene diol spacers also had oxygen permeability about three times higher than the corresponding polyester with diethylene glycol spacers due primarily to higher diffusivity. The effect was traced to differences in the sub-ambient mechanical relaxation behavior. (c) 2006 Elsevier Ltd. All rights reserved.