Elastic and viscous properties of nematic dimer CB7CB

TL;DR

This study comprehensively measures the optical, dielectric, elastic, and viscous properties of the nematic liquid crystal dimer CB7CB across its temperature range, revealing complex behavior of elastic constants and viscosities related to its unique bent molecular structure.

Contribution

It provides detailed temperature-dependent measurements of elastic and viscous properties of CB7CB, including independent determination of splay, twist, and bend elastic constants, highlighting their unusual temperature behaviors.

Findings

K11 is larger than 2 times K22 across the nematic range.

K33 exhibits a non-monotonous temperature dependence with negative slope.

Viscosities increase sharply near the N-NTB transition.

Abstract

We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rod-like cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist bend nematic, NTB, and the oblique helicoidal cholesteric, ChOH, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pre-transitional regions near the N-NTB and N-to-isotropic (I) transitions. The splay constant K11 is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K33 and twist K22 constants are measured by DLS. K33 being the smallest of the three constants, shows a strong non-monotonous temperature dependence with a negative slope in both N-I and N-NTB pretransitional regions. The measured ratio K11/K22 is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist and bend fluctuations in the N phase are comparable to those of nematics formed by rod-like molecules. All three show strong temperature dependence, increasing sharply near the N-NTB transition.