Studies on binary mixtures of nematic liquid crystals made of strongly polar molecules with identical cores and antagonistic orientation of permanent dipoles

TL;DR

This study investigates the optical, dielectric, and elastic properties of binary nematic liquid crystal mixtures with strongly polar molecules having identical cores but opposite dipole orientations, revealing the impact of dipole antagonism.

Contribution

It provides experimental insights into how antagonistic dipole orientations influence the anisotropic properties of nematic liquid crystal mixtures.

Findings

Antagonistic dipole orientation significantly affects elastic anisotropy.

Polar groups contribute notably to optical and dielectric anisotropies.

Elastic properties are explained by a model considering intermolecular association.

Abstract

We report experimental studies on optical (birefringence, $\Delta n$), dielectric $(\Delta \varepsilon)$ and bend-splay elastic anisotropies ($\Delta K=K_{33}-K_{11})$ of a few mixtures of two nematic liquid crystals, namely CCH-7 and CCN-47, made of highly polar molecules with identical cores and antagonistic orientation of permanent dipoles. In particular, the polar group (-CN) attached to the bicyclohexane core of CCH-7 is oriented along the longitudinal direction whereas, in CCN-47, it is oriented along the transverse direction. We show that apart from the significant contribution to the optical and dielectric anisotropies, the antagonistic orientation of strongly polar groups plays a crucial role in determining the bend-splay elastic anisotropy. The elastic properties are explained based on a model proposed by Priest, considering the effect of intermolecular association and the resulting length-to-width ratio of the molecules.