Ester linkages in the nematic phase: a thermodynamic study

Abstract

A study of the ester linkage in nematic liquid crystals has been made in which the criteria for intermolecular binding and ordering are the enthalpy (ΔH_N→I) and entropy (ΔS_N→I), respectively, of the nematic–isotropic transition. The data were obtained by calorimetric differential thermal analysis. The first objective was to examine analogous σ- bonded oxygen and σ-bonded sulphur esters. The thiolesters had consistently higher nematic–isotropic transition temperatures (T_N→I) and usually higher transition heats and entropies. These results are explained in terms of geometrical effects of sulphur on liquid crystalline lattice interactions. The second study involved the position of the ester carbonyl in two sets of mesomorphs. These compounds are of the type R–COO–S and R–OOC–S. Those compounds in which conjugation of the terminal mothoxy-substituents with the ester carbonyls is possible show increased ΔH_N→I and ΔS_N→I relative to the unconjugated analogues. Whether these thermodynamic properties are due to electronic or to geometrical effects is discussed. From the data and conclusions presented here for the nematic phase, some inferences can be drawn concerning earlier data obtained for cholesteric esters which were previously not well understood.