Using dynamic light scattering, we study orientational fluctuation modes in the nematic phase of a self-assembled lyotropic chromonic liquid crystal (LCLC) disodium cromoglycate and measure the Frank elastic moduli and viscosity coefficients. The elastic moduli of splay (K₁) and bend (K₃) are in the order of 10 pN while the twist modulus (K₂) is an order of magnitude smaller. The splay constant K₁ and the ratio K₁/K₃ both increase substantially as the temperature T decreases, which we attribute to the elongation of the chromonic aggregates at lower temperatures. The bend viscosity is comparable to that of thermotropic liquid crystals, while the splay and twist viscosities are several orders of magnitude larger. The temperature dependence of bend viscosity is weak. The splay and twist viscosities change exponentially with the temperature. In addition to the director modes, the fluctuation spectrum reveals an additional mode that is attributed to diffusion of structural defects in the column-like aggregates.

We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4′-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, ${\mathrm{N}}_{\mathrm{TB}}$, and the oblique helicoidal cholesteric, $\mathrm{C}{\mathrm{h}}_{\mathrm{OH}}$, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the $\mathrm{N}\text{−}{\mathrm{N}}_{\mathrm{TB}}$ and N-to-isotropic (I) transitions. The splay constant $K_{11}$ is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend $K_{33}$ and twist $K_{22}$ constants are measured by DLS. $K_{33}$, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and $\mathrm{N}\text{−}{\mathrm{N}}_{\mathrm{TB}}$ pretransitional regions. The measured ratio $K_{11}/K_{22}$ is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the $\mathrm{N}\text{−}{\mathrm{N}}_{\mathrm{TB}}$ transition.

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.

Odd–even effects, oscillations in properties of materials comprised of an odd or even number of connected repeating units, are well-known phenomena in materials science. In organic materials, they are usually associated with the number of methylene groups in aliphatic chains. In this work, we unveil multiple signatures of a new odd–even effect in liquid crystals that occurs at the larger scale of molecular moieties that by themselves express liquid crystalline behavior. We demonstrate that oligomeric liquid crystals, with n = 1–4 number of rigid mesogenic segments connected by flexible aliphatic chains with an odd number of methylene groups, produce an odd–even effect in optical anisotropy and the bend elastic constant of the liquid crystal oligomer. This effect is different from the usual odd–even effects with respect to the parity of carbon atoms in an aliphatic chain and can be understood in term of the average molecular shape and the associations between n-mers based on the packing of these shapes. We also show that, in spite of the fact that there is no long-range electron density modulation, careful analysis of synchrotron SAXS results can provide important information about the molecular associations in the N and N_TB phases that other techniques cannot access. This novel odd–even effect opens up a new mode to optimize phase and optical behavior.

We report a dynamic light-scattering study of the fluctuation modes in a thermotropic liquid crystalline mixture of monomer and dimer compounds that exhibits the twist-bend nematic (${\mathrm{N}}_{\mathrm{TB}}$) phase. The results reveal a spectrum of overdamped fluctuations that includes two nonhydrodynamic modes and one hydrodynamic mode in the ${\mathrm{N}}_{\mathrm{TB}}$ phase, and a single nonhydrodynamic mode plus two hydrodynamic modes (the usual nematic optic axis or director fluctuations) in the higher temperature, uniaxial nematic phase. The properties of these fluctuations and the conditions for their observation are comprehensively explained by a Landau-de Gennes expansion of the free-energy density in terms of heliconical director and helical polarization fields that characterize the ${\mathrm{N}}_{\mathrm{TB}}$ structure, with the latter serving as the primary order parameter. A “coarse-graining” approximation simplifies the theoretical analysis and enables us to demonstrate quantitative agreement between the calculated and experimentally determined temperature dependence of the mode relaxation rates.

Thermotropic liquid crystals exhibit strongly temperature-dependent positional correlations among molecules in the nematic phase above the transition to a smectic-A or C phase. However, even in the absence of a lower temperature smectic phase, nematics composed of reduced symmetry molecules (bent-core mesogens, for example) reveal small angle X-ray scattering (SAXS) patterns with features similar to those associated with pretransitional smectic order. We report on a quantitative analysis and comparison of two-dimensional SAXS data on a bent-core nematic compound (lacking a smectic phase) and a rod-like nematic (exhibiting a nematic to smectic-C transition). This analysis demonstrates that a nanostructure based on distinct, smectic-C ordered molecular clusters provides a more accurate account of the diffraction data from the bent-core nematic than the standard models used to describe fluctuating smectic order in rod-like nematics. The average dimensions of the clusters are weakly temperature dependent, while the interlayer spacing and layer tilt angle with respect to the average molecular long axis are temperature independent. Our results may help to explain interesting and unusual macroscopic properties recently discovered in the fluid phases of bent-core molecules.