Orientational ordering in uniaxial liquid crystals

Abstract

Deuterium N.M.R. has revealed pronounced variations in the orientational order along the alkyl chains which form an integral part of most mesogenic molecules. The model developed by Emsley, Luckhurst and Stockley to account for such variations is found to work reasonably well for the 4-n-alkyl-4′-cyanobiphenyls. However the agreement between theory and experiment can be improved considerably if the potential of mean torque for each conformer is modified to include contributions from the C-D bonds as well as the C-C bonds in the alkyl chains and from the aromatic core. Analysis of the observed order parameters for the C-D bonds in 4-n-pentyl and 4-n-octyl-4′-cyanobiphenyl allow the temperature dependences of the parameters in the potential of mean torque to be determined. The temperature variation of these parameters is found to be consistent with the predictions of molecular field theories of orientational order in mesogens composed of non-rigid molecules.