Comparative analysis of anisotropic material properties of uniaxial nematics formed by flexible dimers and rod-like monomers

TL;DR

This study compares the temperature-dependent anisotropic properties of uniaxial nematic phases formed by flexible dimers and rod-like monomers, revealing significant differences in behavior related to molecular structure and phase transitions.

Contribution

It provides the first detailed comparison of elastic and optical properties of nematic phases formed by flexible dimers versus monomers, highlighting the impact of molecular flexibility.

Findings

Birefringence in monomers increases with decreasing temperature, following classic behavior.

Dimeric molecules exhibit non-monotonous birefringence and strong temperature-dependent elastic ratios.

DTC5C9 dimers tend to adopt bent configurations, leading to the twist-bend nematic phase.

Abstract

We report temperature dependencies of material properties such as dielectric anisotropy, birefringence, splay (K11), twist (K22), and bend (K33) elastic constants of the uniaxial nematic (N) phase formed by flexible dimers of DTC5C9 and compare their behavior to that of a corresponding monomer MCT5. DTC5C9 forms a twist-bend nematic (Ntb) at temperatures below the N phase. Anisotropic properties of MCT5 are typical of the rod-like mesogens. In particular, birefringence increases as the temperature is reduced, following the classic behavior, described by Haller. The elastic constants also follow the standard behavior, with their ratios being practically temperature-independent. In contrast, DTC5C9 shows a dramatic departure from the standard case. Birefringence changes non-monotonously with temperature, decreasing on approaching the N-Ntb phase transition. decreases strongly to 0.4 pN near the N - Ntb transition, although remains finite. The ratios of the elastic constants in DTC5C9 show a strong temperature dependence that can be associated with the bend-induced changes in the orientational distribution function. The measured elastic properties are consistent with the tendency of the dimeric molecules to adopt bent configurations that give rise to the Ntb phase.