Transformation of polar nematic phases in the presence of an electric field.

Only a few years have passed since the discovery of polar nematics, and now they are becoming the most actively studied liquid-crystal materials. Despite numerous breakthrough findings made recently, a theoretical systematization is still lacking. In the present paper, we take a step toward systematization. The powerful technique of molecular-statistical physics has been applied to an assembly of polar molecules influenced by electric field. Three polar nematic phases were found to be stable at various conditions: the double-splay ferroelectric nematic N_{F}^{2D} (observed in the lower-temperature range in the absence of or at low electric field), the double-splay antiferroelectric nematic N_{AF} (observed at intermediate temperature in the absence of or at low electric field), and the single-splay ferroelectric nematic N_{F}^{1D} (observed at moderate electric field at any temperature below transition into paraelectric nematic N and in the higher-temperature range (also below N) at low electric field or without it. A paradoxical transition from N_{F}^{1D} to N induced by application of higher electric field has been found and explained. A transformation of the structure of polar nematic phases at the application of electric field has also been investigated by Monte Carlo simulations and experimentally by observation of polarizing optical microscope images. In particular, it has been realized that, at planar anchoring, N_{AF} in the presence of a moderate out-of-plane electric field exhibits twofold splay modulation: antiferroelectric in the plane of the substrate and ferroelectric in the plane normal to the substrate. Several additional subtransitions related to fitting the confined geometry of the cell by the structure of polar phases were detected.

Emergence of paraelectric, improper antiferroelectric, and proper ferroelectric nematic phases in a liquid crystal composed of polar molecules.

We have elaborated a theoretical approach for the description of polar nematic phases observed by Nishikawa et al. [Adv. Mater. 29, 1702354 (2017)0935-964810.1002/adma.201702354], their structures, and transitions between them. Specific symmetry contributions to the pair molecular potentials provide the molecular mechanisms responsible for the formation of proper and improper polarity on the macroscopic level. An improper antiferroelectric nematic M2 phase can arise between paraelectric nematic M1 and proper ferroelectric nematic MP in the temperature scale. The local polarization in M2 arises mostly due to the local splay deformation. The director distribution in M2 represents the conjugation of cylindrical waves with opposite splay and polarization signs. The director and polarization are parallel to the cylindrical domain axes in the middle of each cylinder but exhibit considerable (mostly radial) deformation on the periphery of each cylinder. Polarization vectors are mostly stacked antiparallel on the borders between the domains without the director disruption. The domain size decreases with the decreasing temperature, the percentage of the antiferroelectric decouplings increases, and M2 exhibits the first-order phase transition into proper ferroelectric MP. With the increasing temperature the domain size in the M2 phase increases, the domination of particular polar orientation of molecules reduces, and finally, the domain size diverges at particular temperature corresponding to the second-order phase transition from M2 to paraelectric M1. Variations of the polar and nonpolar orientational order parameters are estimated within each phase and between the phases. Our experimental and computer simulation results (also presented in the paper) fully support our theoretical findings.

Distinctive thermal behavior and nanoscale phase separation in the heterogeneous liquid-crystal B4 matrix of bent-core molecules.

By means of high-resolution calorimetry, we studied thermodynamic properties of the liquid-crystal B(4) phase where bent-core molecules form a helical nanofilament structure. Distinctive thermal behavior characterizing the growth process of the B(4) phase was obtained in undergoing the phase transition with many sharp peaks, indicating a highly heterogeneous structure. It has been demonstrated that such unusual behavior is commonly seen for two types of rodlike molecules as well as for various mixture compositions. We speculate that mixture systems involve a nanoscale phase-separated structure due to the remarkable aggregation effect in the bent-core molecules and that the helical nanofilament structure independently grows in the isotropic state of rodlike molecules. We also propose that the asymmetry in viscoelastic property plays a role in yielding unusual behavior.

Ferroelectric phases in a chiral bent-core smectic liquid crystal: dielectric and optical second-harmonic generation measurements

We report unambiguous evidence for ferroelectricity in a chiral bent-core liquid crystal. This evidence includes (i) a single current peak in the polarization reversal current, (ii) a strong dielectric response that is suppressed by applying a bias electric field, and (iii) strong optical second-harmonic generation in the absence of electric field even at normal incidence. We propose a phase structure that is anticlinic in tilt and ferroelectric in polar order, i.e., smectic-C(A)P(*)(F).