Flexoelectric Polarization Studies in Bent-Core Nematic Liquid Crystals

The flexoelectric polarization (Pf) of four bent-core nematic liquid crystals (LCs) has been measured using the pyroelectric effect. Hybrid aligned nematic cells are fabricated for measuring the pyroelectric response over the entire range of the nematic phase. It is found that the magnitude of flexoelectric polarization Pf and the sum of the flexoelectric coefficients |e1 +e3| for the bent-core LCs studied here are three to six times higher than for the calamitics. Pf is found to depend on the transverse dipole moment of LC molecules. However,|e1 +e3| values are by no means giant as |e3| alone had been reported for a bent-core nematic system previously. The dependence of the sum of “splay and bend flexoelectric coefficients” is discussed in terms of the shape of the molecule and of the dipole moment directed normal to the molecular axis

Ferroelectric C* phase induced in a nematic liquid crystal matrix by a chiral non-mesogenic dopant

We report on a ferroelectric chiral smectic C (C*) phase obtained in a mixture of a nematic liquid crystal (NLC) and a chiral nonmesogenic dopant. The existence of C* phase was proven by calorimetric, dielectric and optical measurements, and also by X-rays analysis. The smectic C* which is obtained in such a way can flow, allowing to restore the ferroelectric liquid crystal layer structure in the electro-optical cells after action of the mechanical stress, as it happens with the cells filled with NLC. The proposed method of obtaining smectic C* material allows us to create innovative electro-optical cell combining the advantages of NLC (mechanical resilience) and smectic C* (high switching speed

SYNTHESIS AND DIELECTRIC PROPERTIES OF LIQUID CRYSTALS CONTAINING TWO CYANO GROUPS

Des séries de cristaux liquides contenant deux fonctions nitrile dans les positions latérale et terminale d'une molécule et appartenant aux classes de stilbènes, azobenzènes et benzoates de phényl ont été synthétisées. Les températures de transition et constantes diélectriques ont été mesurées et comparées avec celles des dérivés monocyano. Les résultats expérimentaux ont été analysés en employant les magnitudes et directions des moments dipolaires constants calculés.A series of liquid crystalline stilbenes, azocompounds and esters containing terminal and lateral cyano groups were prepared. Their transition temperatures and dielectric permittivities were compared with those for their monocyano analogues. The experimental data are discussed in terms of the values and directions of their molecular dipole moments

Anomalous optical properties of photoactive cholesteric liquid crystal doped with single-walled carbon nanotubes

<p>Optical transmission and selective reflection data are reported for suspensions of single-walled carbon nanotubes (CNTs) in photoactive nematic material ZhK-440 with a mesogenic chiral dopant M5. At small concentrations of CNTs (<i>C</i> ≈ 0.01–0.05%), the preferential localisation of CNTs at oily sticks (cholesteric topological defects) and suppression of the network of oily streaks by CNTs were observed. At the same time, the optical density <i>D</i> was shown to be essentially non-linear and a minimum at certain concentration of CNTs, <i>C</i> ≈ 0.05–0.08%, was observed. This anomalous behaviour was explained by the presence of the structural transition from the loose (ramified) aggregates with highly anisotropic shape (oriented along the anchoring direction on rubbed polyvinyl alcohol) to the compact aggregates with denser packing. The location of this minimum, as well as the selective reflection maximum (helical pitch), was sensitive to partially reversible UV-induced <i>trans–cis–trans</i> isomerisation effects. The UV-controlled helical pitch variation was shown to be only slightly affected by introduction of CNTs.</p

New chiral dopant possessing high twisting power

We have synthesized and investigated a new chiral dopant. It belongs to the derivatives of p-terphenyldicarboxylic acid and has twisting power in the smectic C* mixture much higher than any chiral compound of this series known before. The ferroelectric liquid crystal mixtures on the base of achiral SmC matrix and the new chiral dopant have been elaborated and there optical and electro-optical properties were investigated