S. P. Perkins

THE FLEXOELECTRIC BEHAVIOUR OF A HYPERTWISTED CHIRAL NEMATIC LIQUID CRYSTAL

Abstract The flexoelectric behaviour of a hypertwisted chiral nematic bimesogenic liquid crystal is presented. Through detailed electro-optic measurements, with particular emphasis on the switching properties, we demonstrate remarkably high optical axis tilt angles. The material studied possessed a room temperature nematic phase and aligned easily on cooling under the application of a moderate electric field. Switching times of the order of 500 μs and contrast ratios of 90:1 are readily achieved. The tilt angles, measured using the rotating analyser technique, were found to be practically temperature independent and linear with the applied field. Tilt angles of 22.5° were obtained with moderate applied fields of 9.4 V/μm whilst fields of 25 V/μ yielded tilt angles of 45°. We believe these are the highest tilt angles ever recorded for such fields.

ENHANCED FLEXOELECTRIC SWITCHING BEHAVIOUR IN THE CHIRAL NEMATIC PHASE

Abstract In order to develop materials that exhibit enhanced flexoelectric switching in the chiral nematic phase we have identified mesogenic units that display inherently strong flexoelectric coupling capabilities. Here we examine the oxycyanobiphenyl (OCB) moiety: homologues from the nOCB series exhibit significant electro-optic switching effects when doped with a highly chiral additive. Here we have examined lower dielectric anisotropy materials, since they allow the flexoelectric response to be extended to high field amplitudes. We show that dielectric coupling strength can be low in symmetric bimesogenic molecules. The flexoelectric response of such a molecular structure is tested by doping a homologue from the series CBOnOCB with a chiral additive: very significantly we find that the optic axis is rotated through 2φ = 45° in < 50 μs on reversing the polarity of the field (amplitude E = ±6 V μm−1). Subsequently we have synthesized room temperature chiral nematic materials that exhibit 2φ > 90° at E = 10 V μ−1.

Low Molar Mass Organosiloxane Liquid Crystalline Dyes for Dye Guest Host Ferroelectric Display Devices

Abstract In low molar mass organosiloxane liquid-crystal materials the siloxane moieties micro-separate and aggregate in planes that could be regarded as an effective or virtual two-dimensional polymer backbone. We show that if a siloxane moiety is attached to a dichroic dye molecule, the micro-segregation of the siloxane moieties makes it possible to include a high concentration of the guest dye (more than 50%) in a host organosiloxane solution. This effect, combined with the temperature independent tilt angles achievable with ferroelectric organosiloxane liquid crystals, provide an ideal material for high-contrast surface-stabilised ferroelectric display devices. We present dyed ferroelectric materials with a temperature independent tilt angle greater than 42 degrees, a wide (room temperature to over 100[ddot]C) mesomorphic temperature range and a response time shorter than 500μs in the dye guest host mode.

Physical properties of some novel nematic bimesogens for the flexoelectric effect

Abstract In a chiral nematic liquid crystal, the flexoelectric effect consists of a fast and linear coupling with an applied electric field. One difficulty to overcome is the unwinding of the helix that occurs at higher fields due to dielectric coupling. The use of bimesogens, which possess very low molecular dielectric anisotropy can improve flexoelectric characteristics. New bimesogen compounds have recently been synthesised that exhibit switching angles of 45° for applied fields of about 9 V.μm−1. In this paper, results from dielectric, electro-optic and dynamic light scattering measurements are reported for the new bimesogenic mixture. The dielectric anisotropy Δε changes sign with temperature and its values range between -0.2 and 0.3 for the temperature range studied. For Δε weakly positive, no electric field Freedericksz transition could be induced but Williams domains are observed instead. The large decrease in the bend elastic constant to viscosity coefficient ratio is attributed to a large increase in the bend viscosity coefficient.

Novel Room Temperature Fluorinated Ferroelectric Organosiloxane Liquid Crystal Mixtures

Abstract In this paper we present the results of mixing fluorinated and non-fluorinated organosiloxane materials based on ferroelectric and antiferroelectric low molar mass organosiloxane (LMMO) liquid crystals. None of the fluorinated materials exhibit a mesophase, however when mixed with the non-fluorinated analogues a ferroelectric phase is observed. We present the resultant phase diagrams together with the spontaneous polarisation curves of the mixtures.

Novel fluorinated organosiloxane materials for use in ferro- and anti-ferroelectric liquid crystal mixtures

Abstract In this paper we present the synthesis and physical characteristics of novel fluorinated organosiloxane materials based on ferroelectric and antiferroelectric low molar mass organosiloxane (LMMO) liquid crystals. AH the fluorinated materials are isotropic at room temperature, however we have prepared mixtures with the analogous non-fluorinated organosiloxanes. We present the resultant phase diagram together with the spontaneous polarisations of the mixtures.

Perylene-based fluorescent liquid crystal dye guest-host mixtures

Abstract We present a study on a series of dye guest-host mixtures using fluorescent perylene-based molecules as the guest dye in an organosiloxane host. These hosts have temperature-independent switching, at room temperature, through 90° for fields of the order of 10 Vrms/μm. Perylene molecules have been grafted onto the organosiloxane moiety via an alkyl spacer producing novel and rugged fluorescent dyes that are readily miscible in the host. Micro-separation of the low molar mass siloxane groups in the mesophases tend to form smectic phases. These planes produce an effective two-dimensional polymer backbonethat engenders the rugged mechanical properties of polymeric liquid crystals onto these low molar mass ferroelectric liquid crystals. In this study we show how the introduction of the dye molecules affects the electro-optic properties of the organosiloxane host.

Photoisomerising effects in nitroazo flexoelectric dimeric nematic systems

Abstract The photoisomerisation of a flexoelectric chiral nematic bimesogen system dyed with an azo dye has been investigated. The host material has a pitch and field dependent tilt angle that are temperature independent. Upon illumination by ultra violet, the azo dye molecules undergo a shape change from their trans to cis isomer. The effect of the shape change of the dye on the mixture is to decrease the I-N* transition temperatures, to increase the response times and to decrease the transmitted optical intensity. For the same reduced temperatures, the tilt angles, pitch and threshold voltages for the transition from focal conic to homeotropic textures are unchanged. The macroscopic parameters observed suggest that the orientational order parameter of the system is reduced by UV illumination. The cis isomers do not appear to separate from the host material or significantly change the flexoelectric coefficient.

PHOTOISOMERISING EFFECTS IN NITROAZO FERROELECTRIC DYE GUEST HOST SYSTEMS

Abstract The effect of a photoisomerising dye in a ferroelectric liquid crystal host has been investigated. A nitroazo dye incorporating the same three-siloxane unit as an organosiloxane ferroelectric host has been used to insure good miscibility and high dye concentrations. The effect of photoisomerisation by 300–400 nm UV of up to 430 mW. cm−2 has been investigated in two mixtures with 25% and 50% molar weight of dye. The 25% mixture has an I-SmC*-crystal phase sequence, while the 50% mixture has I-SmA*-SmC*-crystal. Transition temperatures of both the mixtures are reduced by increased UV illumination. For the first mixture at a constant temperature, the spontaneous polarisation in the ferroelectric phase is reduced by UV illumination. For the second mixture both the spontaneous polarisation and tilt angle of the SmC* phase are reduced by UV illumination and the phase is changed from SmC* to SmA* at temperatures close to the transition. The change of tilt angle and observations of micrograph textures indicate that this is a macroscopic change of the material rather than any phase separation of the dye and host materials.