Short-range smectic order in bent-core nematic liquid crystals

Abstract

Small angle X-ray diffraction from the uniaxial nematic phase of certain bent-core liquid crystals is shown to be consistent with the presence of molecular clusters possessing short-range tilted smectic (smectic-C) order. Persistence of these clusters throughout the nematic phase, and even into the isotropic state, likely accounts for the unusual macroscopic behavior previously reported in bent-core nematics, including an anomalously large flexoelectric effect ($ 1000 times that of conventional calamitic nematics), very large orientational and flow viscosities ($ 10-100 and $ 100-1000 times, respectively, typical values for calamitics), and an extraordinary flow birefringence observed in the isotropic state.

Elastic constants and orientational viscosities of a bent-core nematic liquid crystal

“…In the theory, the splay constant A satisfactory explanation of the significantly lower twist and bend constants relative to calamitics requires a model that not only takes into account the molecular shape, but also the potential effects of local correlations among molecules of reduced symmetry that may exist in the nematic state. Recent small angle X-ray scattering studies on ClPbis10BB and similar BCNs [18][19][20], together with earlier light scattering experiments [6], indicate the presence of short-range, smectic-C correlated molecular "clusters" throughout the nematic range, which exist even in the absence of a lower temperature smectic phase and are characterized by nearly temperature-independent correlation lengths of the order a few molecular lengths.…”

“…In additional to quantitative disparities (most dramatic in the magnitude of the viscosities), we found qualitative differences in the anisotropy of these parameters between the BCN and conventional calamitic nematics. Our proposed explanation of these differences is based on structural evidence [18][19][20] that, while the macroscopic symmetry of the nematics is similar, the BCN is characterized by a random distribution of nanoscale, tilted polar smectic correlated volumes or "clusters". The potential chirality of such clusters, or a minority of them acting as dopants, may selectively soften K 22 and K 33 relative to K 11 .…”

“…The viscosity ratios also contrast markedly with ordinary calamitics, where η twist > η splay , η bend due to the well-known backflow effect [17], and where all three viscosities are substantially smaller (even when measured at lower temperatures) compared to the BCN. Motivated in part by recent small angle X-ray scattering studies [18][19][20], we argue that the unusual results for the elastic/viscous parameters in the BCN stem from short-range smectic order similar to the polar layer structure that is characteristic of the smectic-CP phases of bent-core mesogens.…”

Helical nanofilaments of bent-core liquid crystals with a second twist

“…The beamline was configured for a collimated beam (0.2 Â 0.3 mm 2 ) at energy 16 keV (0.775 Å). Details of the experimental conditions are described elsewhere 29 .…”

Light scattering from liquid crystal director fluctuations in steady magnetic fields up to 25 tesla

“…ClPbis10BB and a number of other bent-core compounds exhibit an unusual "cybotaxis" (short-range smectic-like molecular order) that is apparently not associated with pretransitional behavior above a smectic phase [8,24,25], which is the usual and well-known basis for "cybotactic" behavior in nematics. The More interesting is the proposition that the observed dependence of the slopes on q ⊥ arises from implicit field-dependence of the elastic constants K, viscosities η, and the refractive indices affecting the value of q ⊥ in Eq.…”