Light scattering study of the "pseudo-layer" compression elastic constant in a twist-bend nematic liquid crystal.
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 Beffand average director splay constant K. The magnitude of K 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, Beffcould differ substantially from the typical value of ∼106Pa 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 Beffin the range 103-104Pa. We show additionally that the temperature dependence of Beffat 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.
Department of Physics, Kent State University, Kent, Ohio 44242, USA. ssprunt@kent.edu.
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Parsouzi
Department of Physics, Kent State University, Kent, Ohio 44242, USA. ssprunt@kent.edu.
Shokir A
SA
Pardaev
G. W. Gray Centre for Advanced Materials, Department of Chemistry, University of Hull, Hull HU6 7RX, UK.
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C
Welch
G. W. Gray Centre for Advanced Materials, Department of Chemistry, University of Hull, Hull HU6 7RX, UK.
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Z
Ahmed
G. W. Gray Centre for Advanced Materials, Department of Chemistry, University of Hull, Hull HU6 7RX, UK.
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GH
Mehl
Department of Physics, Kent State University, Kent, Ohio 44242, USA. ssprunt@kent.edu.
A R
AR
Baldwin
Department of Physics, Kent State University, Kent, Ohio 44242, USA. ssprunt@kent.edu.
J T
JT
Gleeson
Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA.
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OD
Lavrentovich
Department of Physics, Kent State University, Kent, Ohio 44242, USA. ssprunt@kent.edu.
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Allender
Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA.
J V
http://orcid.org/0000-0002-4982-2457
JV
Selinger
Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA.
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A
Jakli
Department of Physics, Kent State University, Kent, Ohio 44242, USA. ssprunt@kent.edu.
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Sprunt