2H NMR measurements of the molecular orientational ordering in a reentrant liquid-crystalline mixture

Deuterium NMR measurements of the molecular orientational ordering (nematic order) are reported for a reentrant binary mixture of some alkoxy-eyanobiphenyls (nOCB) to which small amounts of perdeuterated p-xylene have been added. The results indicate that the degree of orientational order is enhanced at the smectic A to reentrant nematic phase transition. This effect is shown to be in agreement with the predictions of a Landau-type theory of the reentrant phase transition where-by this phase transition is explained as the result of a coupling between the smectic and the nematic order parameters.