Layered structure and Xe sorption and diffusion properties of low-density liquid-crystalline polyesters with n-alkyl side chains

Four low-density liquid-crystalline polyesters with n-alkyl side chains, B-Cn (n is a carbon number of n-alkyl group), were synthesized from the 1,4-di-(n-alkyl ester) of 1,2,4,5-benzenetetracarboxylic acid and 4,4′-biphenol, to clarify the effect of side chain length on higher ordered structure in the solid state and gas sorption and diffusion properties of these polyesters. All the B-Cn (n=6, 10, 14, 18) samples behaved as a thermotropic liquid crystal and formed a layered structure composed of alternating rigid aromatic main chain layers and flexible n-alkyl side chain layers. In the crystal phase, the distance between the rigid aromatic main chain layers, namely the layer spacing, linearly increased with n, indicating that the conformational feature of n-alkyl side chains was trans-rich. This finding was supported from the 13C NMR chemical shift values of methylene carbons in B-C14 and B-C18. Xe sorption of B-Cn was restricted to the side chain layer, which is almost a liquid-like environment, and these sorption isotherms obeyed Henry's law. The density of side chain layers was estimated from experimental values of layer spacing and density, under some speculate assumptions, but that determined individually by 129Xe NMR spectroscopy was coincidence. The estimated density of side chain layers became higher with increasing n, as well as n-alkane liquid, and closed to that of the polyethylene in rubbery state. Xe solubility coefficient corrected with the estimated side chain layer's density decreased with increasing n. On the other hand, the diffusion coefficient of Xe in B-Cn increased with n, and this was supported from the NMR spectral width of the 129Xe in B-Cn. These results also indicate that the main chain layers of B-Cn were very dense and could not sorb Xe.