Bimesogen-enhanced flexoelectro-optic behavior of polymer stabilized cholesteric liquid crystal

Abstract

We explore flexoelectro-optic effect in polymer stabilized cholesteric liquid crystals (CLC) with uniform lying helix texture. A method to enlarge the flexoelectric response of the CLC mixture is demonstrated by mixing two nematic liquid crystal host materials with large and small effective flexoelastic coefficients. By a choice of material composition of CLC mixture, a large angle of flexoelectrically driven in-plane rotation of helical axis with a submillisecond response time is obtained. Our findings will be useful for material design of future CLC mixtures for applications in flexoelectro-optic displays.

Giant Flexoelectro-optic Effect with Liquid Crystal Dimer CB7CB

“…Small positive dielectric anisotropy of CB7CB3435 and of LC mixture MLC-2048 is an important benefit of these materials for operation in flexoelectro-optic switching mode. The CLC mixtures were obtained by mixing LC host materials with laboratory synthesized chiral dopant iso-(6OBA) 2 (D-Glucitol,1,4:3,6-dianhydro-,2,5-bis[4-(hexyloxy)benzoate])19. Additionally, reactive monomer RM257 (Merck), and a small amount of photoinitiator Ir651 (Ciba) were added into CLC mixtures for polymer stabilization of a nonpermanent ULH CLC texture.…”

“…Samples with nonpermanent ULH CLC texture were exposed to ultraviolet (UV) light to form surface localized polymer network and achieve stabilization of the ULH CLC texture, similarly to previously reported processing56719. Photo-polymerization was performed using a UV light emitting diode (LED) light source (from UV ATA) with emission peak at ~365 nm, full width at half maximum (FWHM) of 9.2 nm, at operating temperature for each of CLC mixtures.…”

“…A convenient value characterizing flexoelectric properties of a LC material is flexo-elastic coefficient e f / K 8910111213141516171819. Flexoelectric rotation of the HA always competes with undesired unwinding of the ULH CLC texture into a non-helical state due to dielectric coupling to applied electric field.…”

“…A few of experimental works have reported generation of a well-aligned ULH texture using a combination of controlled rate cooling of a CLC and shearing of cells substrates with applied electric field12131617. A well-aligned ULH CLC texture can be obtained by application of pulsed electric field with empirically optimized duration and magnitude to the sample cell91819. Alternatively, a spontaneous formation of the ULH CLC texture have been demonstrated by treatment of the surface of substrates creating periodic micro-grooves1525, periodic anchoring conditions1426, controlling pretilt angle of LC molecules27, periodic polymer channels28 or polymer walls29.…”

Alternative generation of well-aligned uniform lying helix texture in a cholesteric liquid crystal cell

“…When fixing the temperature as a constant, ULH alignment can also be generated by a sustaining AC voltage, 14 by gradual increase in applied AC voltage with promoted uniformity by repeatedly adjusting the voltage between higher and lower magnitudes. 15,16 Another electric-field method, enabling the ULH state to be obtained stably via the electrohydrodynamic effect and to electrically switch to the wellknown stable Grandjean planar (P) or the focal conic (FC) state, has been proposed for CLCs with large positive dielectric anisotropy and a tristable CLC device is thus introduced. 17 Unfortunately, high-resistivity LC materials adopted in the current display industry are hardly applicable for this electrohydrodynamic-generated ULH texture since a considerable amount of ions is essential to induce the electrohydrodynamic flow.…”