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Influence of Polymerization Degree and Its Distribution on Thermal Behavior of Thermotropic Nematic Model Compounds

A study of oligomers with the repeating unit of an ester of 4,4′-dihydroxy-biphenyl and sebacic acid was performed to provide information on the relations of the polymerization degree and its distribution with the liquid crystalline properties of backbone-chain type thermotropic polymeric liquid crystals. To eliminate the effect of the end group, p-phenylphenol was connected to each end of the oligomers. The oligomers were separated into single components to provide pure materials of the monomer B8B and dimer (B8)2B and trimer (B8)3B. Each mer was studied by DSC and polarizing microscopy. B8B and (B8)2B did not show a liquid crystalline phase while (B8)3B formed a monotropic nematic phase, indicating that a liquid cyrstalline phase develops as polymerization proceeds. (B8)3B has a phase transition entropy ratio, ΔSIN/ΔSKI of 0.19, and the degree of order in its nematic phase is suggested to be as high as that of the liquid crystal of the polymer. Mixtures of (B8)2B and (B8)3B formed an enantiotropic nematic phase and a nematic-isotropic (N+I) biphase when observed by optical microscopy, indicating that its distribution is also a key factor, in addition to polymerization degree, in the development of a polymeric liquid crystalline phase.