In this paper, we demonstrate strong flexoelectric coupling in bimesogenic liquid crystals. This strong coupling is determined via the flexoelectro-optic effect in chiral nematic liquid crystals based on bimesogenic mixtures that are doped with low concentrations of high twisting power chiral additive. Two mixtures were examined: one had a pitch length of p∼300nm, the other had a pitch length of p∼600nm. These mixtures exhibit enantiotropic chiral nematic phases close to room temperature. We found that full-intensity modulation, that is, a rotation of the optic axis of 45° between crossed polarizers, could be achieved at significantly lower applied electric fields (E<5Vμm−1) than previously reported. In fact, for the condition of full-intensity modulation, the lowest electric-field strength recorded was E=2Vμm−1. As a result of a combination of the strong flexoelectric coupling and a divergence in the pitch, tilt angles of the optic axis up to 87°, i.e., a rotation of the optic axis through 174°, were observed. Furthermore, the flexoelastic ratios, which may be considered as a figure-of-merit parameter, were calculated from the results and found to be large, ranging from 1.3to2C∕Nm for a temperature range of up to 40°C.
In this paper, we report on the flexoelastic and viscoelastic ratios for a number of bimesogens compounds with the same generic structure. Values are obtained indirectly by measuring the flexoelectro-optic response in the chiral nematic phase. By varying the molecular structure we alter the bend angle, transverse dipole moment, and length of the molecule. First, to examine the influence of the bend angle we use a homologous series whereby the only alteration in the molecular structure is the number of methylene units in the aliphatic spacer, n . Results show that the flexoelastic ratio, e/K , and the effective flexoelectric coefficient, e , both exhibit an odd-even effect with values for n=odd being greater than that for n=even . This is understood in terms of an increase in the bend angle of the molecule and an increase in the transverse dipole moment. Second, in order to investigate the impact of the dipole moment, we have altered the mesogenic units so as to vary the longitudinal dipole moment and used different linkages in the aliphatic spacer in an attempt to alter the transverse dipole moment. Qualitatively, the results demonstrate that the odd-spaced bimesogen with larger transverse dipole moments exhibit larger flexoelastic ratios.
Two new series of racemic dimeric liquid crystals containing terminal alkyl chains with methyl branches are presented; in one the dimers are symmetric and in the other they are non-symmetric. Comparisons are made with the structurally isomeric series with no methyl branches in their terminal alkyl chains. The stability and clearing temperatures of the liquid crystal phases are markedly reduced for the dimers with methyl branches, particularly for the symmetric dimers. Chiral analogues of both series were also examined. The liquid crystal behaviour of the non-symmetric dimers is of particular interest. Four homologues were studied with the number of methylene units in the spacer ranging from six to nine. An odd-even effect was observed for the chiral properties of these materials. Blue phase I behaviour was exhibited by the heptane and nonane homologues but not by the even membered hexane and octane members of the series.
In this paper, a device type is presented in which the conventional geometry for the flexoelectro-optic effect is rotated, utilizing planar-aligned short-pitch chiral nematic and in-plane electric fields. The cell is optically neutral at zero applied field due to having its helix axis lie in the direction of light propagation, and at optical communication wavelengths ͑1550 nm͒ polarization rotation is insignificant due to the helical pitch of the material being shorter than the illuminating wavelength. An electric field, applied in the plane of the cell, has been found to induce a birefringence via a combination of dielectric helix unwinding and flexoelectric deformation of the director helix. The magnitude of the birefringence and direction of the induced optic axis in the plane of the cell are dependent on the amplitude and direction of the applied electric field, providing potential for use in a fast endlessly rotatable polarization controller. Herein, the chiral nematic materials utilized in the cell are bimesogenic liquid crystals designed to optimize the contribution from the flexoelectro-optic effect, and eliminate dielectric helix unwinding. The materials are also polymer network stabilized to preserve the texture against degradation in the applied fields. The results presented show a progression from a combined dielectric and flexoelectrically induced birefringence of 0.016 at field strengths up to 6.8 V / m, to a purely flexoelectric-induced birefringence of 0.0135, sufficient for a quarter wave plate in a 29-m-thick cell. Response times are of the order of hundreds of microseconds for both reaction to an applied field and relaxation upon removal.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.