Nanophase segregation of nonpolar solvents in smectic liquid crystals of bent-shape molecules

Abstract

Electro-optical, calorimetric, x-ray, and dielectric measurements are reported on a mixture containing a bent-core liquid crystal 4-chloro-1,3-phenylinebis [4-(4-n-tetradecyloxyphenyliminomethyl) benzoate] mixed with the nonpolar solvent n-hexadecane (HEX). It is observed that the addition of HEX depresses the isotropic-to-smectic phase transition temperature, but the crystallization temperature does not change considerably. Instead, the texture of the crystalline phase changes and, at sufficiently high concentrations of HEX (>20 wt %), an optically isotropic phase appears. Above 40 wt % HEX concentrations, the mesophase completely disappears and a direct isotropic solid-to-isotropic liquid transition takes place. At increasing HEX concentration, the transition enthalpies, the layer ordering, and the magnitude of the electric polarization decrease. X-ray studies reveal that HEX molecules pack in between the smectic layers, resulting in an increase of the layer spacing by about 3 A. The increase of the layer spacing saturates at 5 wt % of HEX. The phase segregation observed seems to be due to steric interactions between the flexible HEX molecules and the rigid bent cores of the liquid crystal molecules.