Organosiloxane liquid crystals for fast-switching bistable scattering devices

Abstract

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.

Evaluation of 4-alkoxy-4′-nitrobiphenyl liquid crystals for use in next generation scattering LCDs

“…11 The smectic A host Table 3 The average smectic layer spacing (D mean ,Å), molecular length obtained on geometry optimised at the DFT(B3LYP/6-311G(d)) level (L,Å), and the corresponding D/L ratio for compounds 6-9 obtained from small angle X-ray scattering experiments as described in the text employed in prototype scattering devices are -to the best of our knowledge -almost exclusively nitrile based materials. 1,4,5,11,33,34 While the nitro group offers reduced clearing points relative to those of the analogous nOCB compounds (Tables 1 and 2) these materials remain attractive due to their reduced switching voltage. This provides a fresh impetus to prepare not only new nitro-terminated materials (such as those incorporating bulky terminal substituents) but also to examine other highly polar functional groups such as -SF 5 and -SF 4 CF 3 as well as ortho uorinated cyano-and nitro-biphenyls.…”

Lowering the excitation threshold of a random laser using the dynamic scattering states of an organosiloxane smectic A liquid crystal

“…The combination of the organosiloxane monomesogen and the nematogen mixture enhances the temperature range of the smectic A phase (I (78 C) N (74 C) S A ( < À50 C) Kr), increases the birefringence, and reduces the operating voltage. 18 This mixture was then doped with 0.1 wt. % cetyltrimethylammonium bromide in order to ensure that there was sufficient conductivity present to induce the scattering states.…”

“…[16][17][18] Organosiloxane LCs have been shown to be particularly effective materials in which scattering can be readily induced by electrical means at relatively low voltages. 17 is possible to compare directly the emission characteristics for both static and dynamic (in which the local director is in a constant state of flux) regimes.…”

Topographically induced hierarchical assembly and geometrical transformation of focal conic domain arrays in smectic liquid crystals

“…In LCs, average local molecular orientations assume geometries that can be controlled by boundary conditions (1,2) and external fields (3,4), and the resulting mechanical and electric anisotropies of LCs provide powerful tools in controlling the propagation of light and the assembly of soft materials (5)(6)(7)(8)(9)(10). A quintessential example is the blue-phase LC organized around a 3D disclination network (11,12); as a display component, it offers rapid response time without surface alignment (13). The ability to tailor LCs with complex, topologically structured geometries will be necessary for the next generation of display technologies and beyond.…”