Chain orientational order and the re-entrant nematic phase

The orientational order in a binary mixture of 4-n-hexyloxy- and 4-n-octyloxy-4′-cyanobiphenyl which exhibits a re-entrant nematic phase has been studied using deuterium N.M.R. spectroscopy. Both components have been synthesized with fully deuterated alkyl chains and the order parameter profiles along the chains have been determined as a function of temperature in the liquid crystal phases formed by the binary mixture and the pure components. The order parameters are found to undergo subtle changes at the transition from the smectic A to the re-entrant nematic phase. Our results are compared with those obtained from the few other studies of the orientational order in systems which exhibit re-entrant polymorphism as well as the predictions of the theory proposed by Luckhurst and Timimi.