Effects of terminal mesogens on thermal properties of dimeric, trimeric, and tetrameric model compounds for main chain liquid crystalline polymers

Three series of oligomers as models of main chain liquid crystalline polyesters were synthesized from 4,4'-biphenol as internal mesogens, suberic acid as spacers, and para-substituted phenols as different terminal mesogens at both ends of the molecule. The oligomers were separated by a silica gel column and pure dimeric, trimeric, and tetrameric model compounds were obtained. The effects of the degree of polymerization and terminal mesogens on thermal properties of these model compounds were studied. No dimers showed a liquid crystalline phase. Trimers which had the terminal group CH3O or Ph were monotropic nematic, while the trimer which had the terminal group CN was enantiotropic nematic. Tetramers were enantiotropic nematic. Increasing the degree of polymerization generated the nematic phase and increased the regularity of nematic phase. The regularity of the nematic phase of trimers and tetramers is remarkably larger than that of low molecular liquid crystals. Terminal mesogens have effect on transition temperatures, but have almost no effect on entropies of nematic-isotropic transition. There existed Gray's order, Ph>CN>CH(3)Q between nematic-isotropic transition temperatures and terminal mesogens of tetramers.