Comparative study of orientational order of some liquid crystals from various homologous series

The long-range orientational order of liquid crystals from homologous series of 4-n-alkyl-4'-cyanobiphenyl, 4-n-alkoxy-4'-cyanobiphenyl, trans-4-n-alkyl(4'-cyano-phenyl)-hexane, and 4-(trans-4'-n-alkylcyclohexyl)-isothiocyanato-benzene was studied by means of classical methods of optical spectroscopy: absorption of the polarized light, fluorescence depolarization, and Raman scattering depolarization. The absorption, emission, and Raman scattering spectra of linearly polarized light were recorded as a function of temperature in the whole range of the mesophase. On the basis of these spectra the order parameters (P-2) and (P-4) as well as the orientational distribution function were determined. The results obtained for members of various series with the same number of carbon atom in the alkyl chain were compared. It was found that the orientational order of liquid crystal molecules depends on the structure of the rigid core, on the kind of the terminal group as well as on the alkyl chain length. The odd-even effect for the order parameters, explained as the alteration of interaction between alkyl chains, was observed.