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 B and 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, B could differ substantially from the typical value of ∼10 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 B in the range 10-10 Pa. We show additionally that the temperature dependence of B 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.

Physics of liquid crystals of bent-shaped molecules

“…This is consistent with the heliconic axis becoming normal to the shear plane via shear-induced alignment. From measurements of the dynamic modulus the compression modulus of the pseudo layers was estimated to be C ≈ 2 ×10 3 Pa , close to the data in light scattering experiments (Parsouzi, Pardaev, et al 2016).…”

“…x is the shift of one arch with respect to the other. Equation 31 later Parsouzi et al (Parsouzi, Shamid, et al 2016) (Parsouzi, Pardaev, et al 2016) developed coarse-grained elastic models of N TB in which the material macroscopic properties K and C are expressed through the nanoscale parameters. Experimental measurements by light scattering (Parsouzi, Pardaev, et al 2016) yield B in the range of 10 3 -10 4 Pa, which is substantially lower than the compressibility modulus in SmA with density modulation, ( C~10 6Pa).…”

“…Equation 31 later Parsouzi et al (Parsouzi, Shamid, et al 2016) (Parsouzi, Pardaev, et al 2016) developed coarse-grained elastic models of N TB in which the material macroscopic properties K and C are expressed through the nanoscale parameters. Experimental measurements by light scattering (Parsouzi, Pardaev, et al 2016) yield B in the range of 10 3 -10 4 Pa, which is substantially lower than the compressibility modulus in SmA with density modulation, ( C~10 6Pa). The splay constant K is expected(Challa et al 2014) to be close to the value of 11 K in the N U phase near UTB T , i.e., on the order to 10 pN (Cukrov et al 2017) (Babakhanova et al 2017).…”

Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexiblen-mers

“…The pseudo-layers have the symmetry of a smectic-A (or, more precisely, a smectic-A * ) phase, although they differ from "true" layers in that no mass density wave is observed. The N TB phase should therefore exhibit a smectic-like spectrum of fluctuation modes as well as other interesting properties such as an electroclinic effect, and indeed these have been confirmed by electro-optical and light scattering 27,28 measurements.…”

The induction of the Ntbphase in mixtures

“…The Ntb phase, characterized by spontaneous formation of chiral domains with a helical pitch on the 10 nm scale and pseudo-layer structure, formed by chemically non-chiral compounds is one of the recently observed examples of chiral symmetry breaking in liquid crystals [1][2][3][4][5][6][7]. Previously this effect has been more associated with higher ordered liquid crystal (LC) phases, such as lamellar, columnar and cubic arrays [8][9][10][11][12].…”