Organosiloxane liquid crystals for fast-switching bistable scattering devices

In this paper, we investigate the dependence of the bistable electro-optic response of organosiloxane based smectic A liquid crystals on key material properties; the modes are optically scattering (<100 Hz) and clear (>1 kHz), respectively. The scattering mode is generated by motion of doped ionic species and is dependent upon the conductivity anisotropy of the liquid crystal; the clear mode occurs through dielectric coupling at frequencies sufficient to inhibit ionic conduction. The pure siloxane liquid crystals exhibit unusually high anisotropic ionic conductivities; the ratio of the components parallel and perpendicular to the director is up to 0.005; for analogous non-siloxane based liquid crystals the value is ∼0.5. However, the pure materials possess low dielectric anisotropy compared with similar polar liquid crystals, e.g. 0.8 compared with 8.4. Based upon these observations, we prepare exemplar mixtures of organosiloxane with 4-(ω-pentyloxy)-4'-cyanobiphenyl (5OCB) as a dielectric additive. These systems show beneficial properties: highly anisotropic conductivity coupled with an increase in the dielectric anisotropy. As a result, the electro-optic responses of these mixtures show a significant improvement upon the pure compounds; for example, for the 49% w/w mixture it is found that the response time of the optically clear mode is significantly reduced, e.g. from 130 ms (at 280 V rms) to 5 ms at an applied voltage of 102 V rms. We conclude that simple mixtures, using an organosiloxane as the bulk component, have much promise for use in bistable, electro-optic storage and display devices.