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

Abstract

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 α_e and α_o are obtained for all the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal. α_e and α_o calculated in this way are used to obtain Δα. The polarizability anisotropy in the perfect order (absolute K) is calculated theoretically using the δ-function model developed by Lippincott et al. 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 α and Δα values considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence measurement and this value of S is compared with those obtained from field models.

Keywords:

• density
• differential scanning caloriemetry
• dilatometry
• molecular polarizabilities
• n.Om and nO.Om compounds
• order parameter
• refractive indices

Acknowledgments

The authors Dr. P. Pardha Saradhi and Dr. V. G. K. M. Pisipati express their thanks to The Head, ECE Department and the Management of K.L. University, Vaddeswaram 522 502, India for providing facilities. P.V. Datta Prasad acknowledges the S.S.D. Polymers for providing some facilities during the work. D. Venkata Rao thanks the management, S.V. Engineering College, Nellore for giving the support.