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Search for biaxiality in a shape-persistent bent-core nematic liquid crystal
Oleg D. Lavrentovich
,Young-Ki Kim
,Madhabi Majumdar
2,Bohdan I. Senyuk
,Luana Tortora
,Jens Seltmann
3,Matthias Lehmann
4,Antal Jakli
,Jim T. Gleeson
2,Samuel Sprunt
2
Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, OH, 44242, USA
Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA.
2Department of Physics, Kent State University, Kent, OH 44242, USA.
3Institute of Chemistry, Chemnitz University of Technology, 09107 Chemnitz, Germany
4Institute of Organic Chemistry, Julius-Maximillians-Universit˘ĉat W˘ĉurzburg, 97074 W˘ĉurzburg, Germany
Using a range of optical techniques, we have probed the
nature of orientational order in a thermotropic bent-core
liquid crystal, which features a shape-persistent molecular
architecture designed to promote a biaxial nematic phase. In
the upper range of the nematic phase (enantiotropic regime),
dynamic light scattering reveals strong fluctuations
attributable to the biaxial order parameter, in addition to
the usual uniaxial director modes. Assuming a Landau-type
expansion of the orientational free energy, we estimate the
correlation length associated with these fluctuations to be
~100 nm. At lower temperatures, and mainly in the monotropic
regime of the nematic, we observe by optical conoscopy an
apparently biaxial texture, which develops when the sample
temperature is changed but then relaxes back to a uniaxial
state over time scales much longer than observed in the
light scattering measurements. A combination of fluorescence
confocal polarizing microscopy and coherent anti-Stokes
Raman scattering confirms that the conoscopic texture arises
from a flow-induced reorientation of the molecules,
associated with a large thermal expansion coefficient of the
material, rather than from the spontaneous development of a
macroscopic secondary optical axis. We discuss a model to
account for the observed behavior at both high and low
temperatures based on the temperature-dependent formation of
nanoscale, biaxially ordered complexes among the bent-core
molecules within a macroscopically uniaxial phase.
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