Study of Molecular Polarizabilities and Orientational Order Parameter in the Nematic Phase of 6.O12O.6 and 7.O12O.7

Parameswara Rao Alapati,P V Datta Prasad,P Pardhasaradhi,D Bhuyan,D Madhavi Latha,K N Singh,V G K M Pisipati

doi:10.4236/wjcmp.2011.14025

Abstract

As a part of our systematic study of the physical properties of the compounds of symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4’-oxy) alkanes (m.OnO.m), we report here an optical study of two nematogenic compounds of this series viz., 6.O12O.6 and 7.O12O.7 by carrying out the measurement of extraordinary and ordinary refractive indices using modified spectrometer. The molecular polarizability anisotropies are evaluated using Lippincott δ-function model, the molecular vibration method, Haller’s extrapolation method and scaling factor method. The molecular polrizabilities, αe and αo are calculated using Vuks’ isotropic field model and Neugebauer’s anisotropic field model. The orientational order parameter, S, is estimated by using the molecular polarizability values calculated from refractive index and denstity data as well as polarizability anisotropy values calculated as a function of temperature in the nematic phase of both of the dimers. These values are discussed with reference to the order parameter values obtained directly from the optical birefringence, Δn, data without assuming any internal field. A comparison of the order parameter values from all of these methods was carried out in the light of available literature data.

Highlights

In liquid crystals, the molecular polarizabilities and their anisotropy are considered to be important characteristic inherent molecular properties because the intermolecular interaction energies, according to different theoretical models are dependent on them
As a part of our systematic study of the physical properties of the compounds of symmetric liquid crystal dimer homologous series, α,ω-bis-(4-n-alkylaniline benzylidene-4’-oxy) alkanes (m.OnO.m), we report here an optical study of two nematogenic compounds of this series viz., 6.O12O.6 and 7.O12O.7 by carrying out the measurement of extraordinary and ordinary refractive indices using modified spectrometer
The orientational order parameter, S, is estimated by using the molecular polarizability values calculated from refractive index and denstity data as well as polarizability anisotropy values calculated as a function of temperature in the nematic phase of both of the dimers

Summary

Introduction

The molecular polarizabilities and their anisotropy are considered to be important characteristic inherent molecular properties because the intermolecular interaction energies, according to different theoretical models are dependent on them. Liquid crystal dimers, which are prepared by joining two mesogenic units through a flexible alkyl spacer, are attracting much interest because of their unusual properties compared with monomeric liquid crystals as well as their capability to act as model compounds for semi-flexible main chain liquid crystal polymers [1,2,3,4] They are classified into two categories: symmetric and non-symmetric dimers. In case of non-symmetric liquid crystal dimers, the thermal stability of the smectic phase shows different behavior with long and short terminal alkyl chains. This surprising observation was rationalized by proposing a novel intercalated structure for the smectic. Orientational order parameter was evaluated using different methods have been discussed in the nematic phase of these two compounds

Experimental

Results and Discussion

Estimation of Order Parameter