In part because of its anticipated application for faster and lower power-consuming electro-optic devices, small energy generator in industry and low operating voltage in multicolor LC display, which could revolutionize display technology, the bent-core nematic (BCN) liquid crystals has long been sought after. Due to their bow like shape, BCN gives rise to some interesting features including flexoelectricity (a coupling between electric polarization and elastic flexure) and viscoelastic properties. The viscoelastic parameters are relevant to performance of LC devices and to the understanding of the connection between unusual properties (like “giant flexoelectricity”) and structure (molecular organization) that gives rise to them. We have investigated basic properties- particularly the absolute elastic constants (K11 for splay, K22 for twist, and K33 for bend distortion of the uniaxial director, or optic axis) and the corresponding viscosities, ηsplay, ηtwist, ηbend, at a characteristic temperature in the nematic phases of three bent-core nematic liquid crystals, CIPbis10BB, DT6PY6E6 and 2832, using two techniques, dynamic light scattering (i.e., through the detection and characterization of the symmetry breaking fluctuation mode associated with the director) and magnetic field induced orientational distortion (Fredericksz transition) on well aligned samples. At certain fixed temperatures below the isotropic-nematic transitions, we have explored different experimental geometries and optical selection rules to optimize and probe the quantitative measurements of these parameters and our result shows that the orientational elastic constants of the BCN studied are significantly lower (particularly K22) than those of typical calamitics (5CB) [1] and that K11>K33≫K22 (for CIPbis10BB and 2832). A dramatic enhancement of orientational viscosities (4 to 100 times larger than calamitics) was also confirmed in BCNs. The unusual anisotropies of these parameters are discussed in terms of a nematic containing molecular clusters exhibiting short-range smectic-CP-type correlations.
We have conducted additional light scattering studies on one of these three bent-core nematic liquid crystals, designated DT6Py6E6, whose pyridine end group favors homeotropic anchoring (i.e., molecular long axis perpendicular) to the glass surfaces of the sample cell. This configuration allows one to achieve a scattering geometry in which the ordinary uniaxial director fluctuation modes are “dark”, which allows us to search for fluctuation modes that are not purely associated with the uniaxial director. Indeed, we observe two modes (hydrodynamic and non-hydrodynamic) in addition to the expected twist-bend director mode. We present a model for the additional modes based on fluctuations of the biaxial order parameter, which leads to an estimate of 10-100 nm for the correlation length associated with these fluctuations.We have analyzed our results in light of the available models – including formation of short-range smectic layered clusters – used to describe the microscopic structure of bent-core nematics.