Dielectric Relaxations in Nematic Phase of Hydrogen Bonded Liquid Crystal Homologous Series

Abstract

A novel homologues series of linear inter hydrogen bonded liquid crystals have been isolated. Hydrogen bonds are formed between Mandelic acid (MDL) and various homologues of p-n-alkyloxy benzoic acids (nOBA). FTIR studies on these compounds substantiate the formation of hydrogen bond. Optical textural and DSC studies confirm the rich polymorphism of the series. Phase diagram of this series has been constructed from the above data. It has been observed that all the synthesized mesogens exhibit nematic phase expect the pentyloxy benzoic acid complex. Furthermore, odd-even effect with respect to the enthalpy values obtained at isotropic to nematic phase transitions in various members of the homologues series has been analyzed and discussed. Dielectric relaxations in nematic phase have been carried out in five hydrogen bonded complexes. From the Cole-Cole plots the activation energies and distribution parameter (α) have been investigated. A quantitative approach for the origin of the relaxation in nematic phase with reference to the chemical molecular structure is proposed. The variation of distribution parameter with alkyloxy number and temperature are discussed.