A Comparative Study of Mesophase Formation in Rigid Chain Polyesters with Flexible Side Chains

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Two series of rigid chain polyesters having flexible linear side chains from 6 to 16 carbon atoms were synthesized and studied by optical microscopy, differential scanning calorimetry, and X-ray diffraction. They were prepared by melt condensation polymerization of 2,5-dialkoxyhydroquinones with terephthalic acid in one case and with bicyclo[2.2.2]octane-l,4-dicarboxylic acid in the other. Thus one series of polymers has a wholly aromatic backbone while the other contains alternating rigid aliphatic and aromatic units. Both series show a low-temperature solid—solid transition attributed to the disordering of the side chains and similar melting temperatures, which decrease accordingly as the length of the side chains increases. However, while the polymers from bicyclo[2.2.2]octane-1,4-dicarboxylic acid melt into an isotropic liquid, most of the terephthalic acid polyesters melt into a liquid crystalline phase, thus suggesting that interchain interactions provided by the wholly aromatic backbone are necessary for the stabilization of a mesophase in this class of materials. In the solid state both series of polymers crystallize into similar layered structures. X-ray diffraction shows that the layer spacing increases linearly with increasing length of the side chains, and comparison with molecular dimensions suggests that the side chains are interdigitated and tilted with respect to the backbone. Wholly aromatic polyesters containing four flexible side chains per repeat unit were also prepared. When the side chains are of equal length, the material is highly crystalline and melts directly into an isotropic liquid. X-ray analysis indicates a bilayer structure in the solid state in which the side chains are tilted with respect to the backbone but not interdigitated. When the side chains are of different length and alternating along the backbone, their crystallization is greatly suppressed, thus giving rise to a broad mesomorphic temperature range in which the side chains are disordered. Most of this disorder is preserved on cooling to the solid phase. © 1989, American Chemical Society. All rights reserved.

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