Phase transitions from density and order parameter from polarisabilities in symmetric dimeric liquid crystals

Abstract

As a part of our systematic studies on liquid crystal dimers, we present in this article the nature of phase transitions across isotropic-nematic and nematic-smectic-A exhibited by DLCs, α,ω-bis-(4n-alkylaniline benzylidene-4ʹ-oxy) alkanes. Further, the orientational order parameter in the nematic phase of these DLCs are estimated from the molecular polarisabilities calculated using the experimental refractive indices and density results. The molecular polarisabilities α e and α o are obtained for 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 polarisability anisotropy in the perfect order (absolute K) is calculated semiempirically using the δ-function model developed by Lippincott et al. and molecular vibration method. The values of polarisability 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 δn = (n e-n o) and this value of S is compared with those obtained from field models.