The twist-bend nematic phase of bent mesogenic dimer CB7CB and its mixtures

TL;DR

This study investigates the properties of binary mixtures of the twist-bend nematic liquid crystal CB7CB with 5CB, revealing their phase behavior, nanoscale modulations, and a structural transition at higher 5CB concentrations.

Contribution

It provides the first quantitative measurement of the twist-bend phase pitch in mixtures and identifies a structural transition at 50 wt% 5CB.

Findings

Mixtures are homogeneous with no phase segregation.

Addition of 5CB decreases viscosity and relaxes textures.

Ground-state pitch increases with 5CB, with a discontinuous jump at 50 wt%.

Abstract

Binary mixtures of the twist-bend nematic-forming liquid crystal CB7CB with the prototypical rod-like liquid crystal 5CB exhibit a twist-bend nematic phase with properties similar to those reported for neat CB7CB. The mixtures appear homogeneous, with no micron- or nano-scale segregation evident at any concentration. The linear dependence of the phase transition temperature on concentration indicates that these binary mixtures are nearly ideal. However, a decrease in the viscosity with the addition of 5CB allows the characteristic twist-bend stripe textures to relax into a state of uniform birefringence. We confirm the presence of nanoscale modulations of the molecular orientation in the mixtures by freeze-fracture transmission electron microscopy (FFTEM), further evidence of their twist-bend nature. We devise and implement a statistical approach to quantitatively measure the ground state pitch of the twist-bend phase and its mixtures using FFTEM. The addition of 5CB generally shifts the measured ground-state pitch distributions towards larger pitch. Interestingly, the pitch appears to increase discontinuously by ~10 nm at the 50 wt% concentration of 5CB, indicating that the twist-bend phase undergoes a structural transition at higher 5CB concentrations.