Possible Second-Order Nematic—Smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>Phase Transition

Abstract

NMR measurements of the molecular orientational order in the homologous series 4-$n$-alkoxybenzylidene-4\ensuremath{'}-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-$A$ \ensuremath{\leftrightarrow} 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.