Phase Transitions and Order Parameter Studies from Polarizabilities in 3.Om and 3O.Om Liquid Crystalline Compounds

As a part of our systematic studies on liquid crystals of nO.m, n.Om, nO.Om and n.m homologous series, n-(p-n-alkoxy/alkylbenzylidene)-p-n-alkyl/alkoxy anilines, we present in this article the nature of phase transitions across isotropic-nematic exhibited by all the compounds mentioned below. Further, the orientational order parameter in the nematic phase of N-(p-n-propyl/propyloxy benzylidene)-p-n-alkoxy anilines, 3.Om and 3O.Om with m = 6-8 are estimated from the molecular polarizabilities calculated using the experimental refractive indices and density results. The molecular polarizabilities alpha(e) and alpha(o) are obtained for all the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal alpha(e) and alpha(o) calculated in this way are used to obtain . The polarizability anisotropy in the perfect order (absolute K) is calculated theoretically using the delta-function model developed by Lippincott etal. and molecular vibration method. These enabled the calculation of the orientational order parameter S. The values of polarizability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain alpha and Delta alpha values considering different local field models, a simple procedure developed by Kuczynski etal. was used for evaluation of S, based solely on birefringence measurement and this value of S is compared with those obtained from field models.