Light scattering study of the 'pseudo-layer' compression elastic constant in a twist-bend nematic liquid crystal
| Autor: | 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., Jakli, A., Sprunt, S. |
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| Rok vydání: | 2016 |
| Předmět: | |
| Druh dokumentu: | Working Paper |
| DOI: | 10.1039/C6CP06292J |
| Popis: | 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 $B_{eff}$ and average director splay constant $K_1^{eff}$. The magnitude of $K_1^{eff}$ 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, $B_{eff}$ could differ substantially from the typical value of $\sim 10^6$ 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 $B_{eff}$ in the range $\sim 10^3-10^4$ Pa. We show additionally that the temperature dependence of $B_{eff}$ 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. Comment: 9 pages, 7 figures |
| Databáze: | arXiv |
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