Molecular theories and structure. Some effects of molecular structural change on liquid crystalline properties

Abstract

The liquid crystalline properties of several series of phenyl, ω-phenylalkyl and branched chain alkyl esters of 4-p-substituted-benzylideneaminocinnamic acids have been examined by optical microscopy and differential thermal analysis. The incidence of smectic polymorphism in these esters is high. Three distinct smectic forms have been identified; two are almost certainly smectic A and smectic B, and the third remains unassigned. Enthalpies and entropies of the transitions have been measured. Those esters which form homologous series give regular changes in the mesomorphic transition temperatures as the series are ascended.

ω-Phenylalkyl esters with a p-cyano-group give a very high alternation of the nematic-isotropic liquid temperatures, whereas ω-isopropylalkyl and ω-phenylalkyl esters with p-acetoxy- or p-phenyl-groups give an alternation between smectic and smectic + nematic behaviour as the methylene chain lengthens. The effects of branching of the ester alkyl chain and of the alkyl chain of a p-alkoxy-group on the mesomorphic properties have also been examined and found to be different. These various effects are discussed in terms of molecular structure and geometry.