Structural model for oxygen permeability of a liquid crystalline polymer

This study examined the solid-state structure and oxygen transport properties of smectic poly(diethylene glycol 4,4'-bibenzoate) (PDEGBB). The polymer was quenched from the isotropic melt to the smectic liquid crystalline glass and subsequently isothermally crystallized by heating above the glass transition temperature. Crystallized PDEGBB was characterized by thermal analysis and X-ray diffraction. Gas transport properties were characterized by oxygen permeability, diffusivity, and solubility at 1 atm. Oxygen solubility decreased linearly with crystallinity as determined by the heat of melting. This result supported a simple two-phase model of impermeable crystals dispersed in a permeable liquid crystalline (LC) glass. Extrapolation to zero solubility gave a crystal density of 1.371 g cm(-3); the corresponding heat of the crystal-to-smectic transition was 10.1 M mol(-1). The solid-state morphology was examined by atomic force microscopy. A hierarchical structure was proposed for the smectic LC polymer in which mesogens organized into smectic layers, stacks of layers formed wavy lamellae, and assemblies of lamellae defined domains. The lamellar morphology remained after crystallization. It was suggested that crystallized PDEGBB possessed small crystals of low aspect ratio dispersed in the permeable LC phase. This model provided the structural basis for describing oxygen permeability in terms of the generalized Maxwell equation for spherical particles.