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 similar to 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-(omega-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 130ms (at 280V rms) to 5ms at an applied voltage of 102V 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.