Molecular Ordering in Liquid Crystals and the Effect of End-Chains on the Even-Odd Effect

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Abstract
The second and fourth orientational order parameters [Pbar]2, [Pbar]4 of the homologous series of 4-n-alkyl-4′-cyanobiphenyls (nCB, n = 5 ∼ 8) in the nematic liquid crystalline state have been determined from the Raman depolarization ratios by the resonance Raman probe method. It was found out that not only [Pbar]2 but also [Pbar]4 exhibits evident even-odd effect when they are compared at the same reduced temperature. For understanding the mechanism of the even-odd effect, a theoretical calculation has been carried out on the basis of the mean field theory of Marcelja, in which the effect of the end alkyl-chain on the liquid crystalline state is taken explicitly into consideration. The observed trend in [Pbar]2, [Pbar]4, nematic-isotropic transition temperature were well reproduced. Detailed inspection of the results shows that the presence of anisotropic molecular field plays an important role in the appearance of the even-odd effect. The anisotropic molecular field produces redistribution of the statistical weight of various conformers in such a way that the end chains of odd-members align better along the direction of the rigid core part than those of even-members.