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SynopsisTwo closely related series of polyesters that contain mesogenic units interconnected by flexible spacers along the main chain were prepared and characterized for their liquid crystal properties. All of these polymers showed theotropic behavior, which was examined by DSC, hot-stage microscopy on a polarizing microscope, small-angle light and wide-angle x-ray scattering methods, and visual observation of stir-opalescence of the polymer melts. The effect of the length of the flexible spacer and the nature of the substituent, which is on the central aromatic ring of the mesogenic unit, on the stability and molecular order of the mesophase for each of the polymers was investigated and the results are discussed on the basis of the thermodynamic data obtained.

Several different series of rigid and flexible polyesters with main chain liquid crystalline units were prepared and their properties were examined in relation to their structures. l'hc first group o f polymers were rigid aromatic copolyesters with mesogenic groups based on eithcr chloro or methyl hydroquinone terephthalate units combined with varying amounts of different types of bisphenol terephthalate units The bisphenol conionomers used contained the structure:in which X was none, -C(CH,),-, -CH2-, -0-, 4-, and -S02--. It was observed that the bisphenols with the bulkier X group were more efficient in destroying thermotropic liquid cryFtallinity of the resulting copolymers. The second group of polymers studied were flexible polyesters consisting of various types of mesogenic units which were connected together by different lengths of polymethylene flexible spacers. The liquid crystalline behaviours of these polymers, particularly their transition temperatures, were correlated with their structures. A brief review of previous studies on the synthesis of thermotropic liquid crystalline polyesters is included.

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