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Article
Search For Biaxiality in a Shape-Persistent Bent-Core Nematic Liquid Crystal
Soft Matter (2012)
  • Young-Ki Kim, Kent State University - Kent Campus
  • Madhabi Majumdar, Kent State University - Kent Campus
  • Bohdan I. Senyuk, Kent State University - Kent Campus
  • Luana Tortora, Kent State University - Kent Campus
  • Jens Seltmann
  • Matthias Lehmann
  • Antal Jakli, Kent State University - Kent Campus
  • Jim T. Gleeson, Kent State University - Kent Campus
  • Oleg Lavrentovich, Kent State University - Kent Campus
  • Samuel Sprunt, Kent State University - Kent Campus
Abstract

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 similar to 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.

Keywords
  • Raman Scattering Microscopy,
  • X-Ray-Diffraction,
  • Polarizing Microscopy,
  • Phase-Transitions,
  • Order,
  • Orientation
Disciplines
Publication Date
July 20, 2012
Publisher Statement
Available on publisher's site at http://dx.doi.org/10.1039/c2sm26278a
Citation Information
Young-Ki Kim, Madhabi Majumdar, Bohdan I. Senyuk, Luana Tortora, et al.. "Search For Biaxiality in a Shape-Persistent Bent-Core Nematic Liquid Crystal" Soft Matter Vol. 8 Iss. 34 (2012)
Available at: http://works.bepress.com/antal_jakli/6/