Possible Second-Order Nematic—Smectic-APhase Transition

Abstract

NMR measurements of the molecular orientational order in the homologous series 4-$n$-alkoxybenzylidene-4′-phenylazoaniline are compared with the McMillan smectic-$A$ theory. The agreement is remarkable. The measurements show a discontinuous drop in the order parameter at the first-order smectic-$A$ ↔ nematic phase transition. The magnitude of the drop decreases with decreasing length of the alkyl end chain, finally approaching that characteristic of a second-order phase transition predicted by McMillan.