Terminal groups in the nematic and cholesteric phases: a thermodynamic study

Abstract

The effects of terminal substituents in the nematic and cholesteric phases have been investigated by examination of the enthalpy and entropy changes of the mesophase–isotropic transition by calorimetric differential thermal analysis. A series of nematic, unsymmetrical p-phenylene bis-(para-substituted benzoates) were chosen in which one substituent was varied among small, compact groups, and a series of cholesteryl para-substituted benzoates were also studied. The nematic series showed that the transition temperature and the transition heat are not directly proportional as is quite commonly assumed; ΔH_N→I was found to be strongly related to the electron donating character of the substituent. The cholesteric series showed ΔH_ch→I which were not correlatable with substituent polarity or electronic donating character; T_ch→I and ΔH_Ch→I were also not directly related. The heats and entropies were intimately related along both series suggesting that as intermolecular order increases so does intermolecular binding for these compounds. The nematic and cholesteric phases have very similar terminal group efficiencies when the transition temperature is taken as the criterion for mesophase stability, but they show no such similarities with regard to heats and entropies of the mesophase–isotropic transition.