Issue 46, 2016

Light scattering study of the “pseudo-layer” compression elastic constant in a twist-bend nematic liquid crystal

Abstract

The nematic twist-bend (TB) phase, exhibited by certain achiral thermotropic liquid crystalline (LC) dimers, features a nanometer-scale, heliconical rotation of the average molecular long axis (director) with equally probable left- and right-handed domains. On meso to macroscopic scales, the TB phase may be considered as a stack of equivalent slabs or “pseudo-layers”, each one helical pitch in thickness. The long wavelength fluctuation modes should then be analogous to those of a smectic-A phase, and in particular the hydrodynamic mode combining “layer” compression and bending ought to be characterized by an effective layer compression elastic constant Beff and average director splay constant Keff1. The magnitude of Keff1 is expected to be similar to the splay constant of an ordinary nematic LC, but due to the absence of a true mass density wave, Beff could differ substantially from the typical value of ∼106 Pa in a conventional smectic-A. Here we report the results of a dynamic light scattering study, which confirms the “pseudo-layer” structure of the TB phase with Beff in the range 103–104 Pa. We show additionally that the temperature dependence of Beff at the TB to nematic transition is accurately described by a coarse-grained free energy density, which is based on a Landau-deGennes expansion in terms of a heli-polar order parameter that characterizes the TB state and is linearly coupled to bend distortion of the director.

Graphical abstract: Light scattering study of the “pseudo-layer” compression elastic constant in a twist-bend nematic liquid crystal

Article information

Article type
Paper
Submitted
12 Sep 2016
Accepted
03 Nov 2016
First published
03 Nov 2016

Phys. Chem. Chem. Phys., 2016,18, 31645-31652

Light scattering study of the “pseudo-layer” compression elastic constant in a twist-bend nematic liquid crystal

Z. Parsouzi, S. A. Pardaev, C. Welch, Z. Ahmed, G. H. Mehl, A. R. Baldwin, J. T. Gleeson, O. D. Lavrentovich, D. W. Allender, J. V. Selinger, A. Jakli and S. Sprunt, Phys. Chem. Chem. Phys., 2016, 18, 31645 DOI: 10.1039/C6CP06292J

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements