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

Parsouzi, Z.; Pardaev, Shokir A.; Welch, C.; Ahmed, Z.; Mehl, G. H.; Baldwin, A. R.; Gleeson, J. T.; Lavrentovich, O. D.; Allender, D. W.; Selinger, J. V.; Jákli, A; Sprunt, S.

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.

Citation

Parsouzi, Z., Pardaev, S. A., Welch, C., Ahmed, Z., Mehl, G. H., Baldwin, A. R., …Sprunt, S. (2016). Light scattering study of the “pseudo-layer” compression elastic constant in a twist-bend nematic liquid crystal. Physical chemistry chemical physics : PCCP, 18(46), 31645-31652. https://doi.org/10.1039/C6CP06292J

• Journal Article Type: Article
• Acceptance Date: Nov 3, 2016
• Online Publication Date: Nov 3, 2016
• Publication Date: Nov 3, 2016
• Deposit Date: Nov 11, 2016
• Publicly Available Date: Nov 11, 2016
• Journal: Physical chemistry chemical physics
• Print ISSN: 1463-9076
• Electronic ISSN: 1463-9084
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 18
• Issue: 46
• Pages: 31645-31652
• DOI: https://doi.org/10.1039/C6CP06292J
• Keywords: Physical and Theoretical Chemistry; General Physics and Astronomy
• Public URL: https://hull-repository.worktribe.com/output/445074
• Publisher URL: http://pubs.rsc.org/en/content/articlelanding/2016/cp/c6cp06292j#!divAbstract
• Additional Information: : This document is Similarity Check deposited; : O. D. Lavrentovich (ResearcherID); : J. V. Selinger (ORCID); : J. V. Selinger (ResearcherID); : The PCCP Owner Societies have an exclusive publication licence for this journal; : Single-blind; : Received 12 September 2016; Accepted 3 November 2016; Accepted Manuscript published 3 November 2016; Advance Article published 11 November 2016; Version of Record published 23 November 2016