Effect of polymer concentration on stabilized large-tilt-angle flexoelectro-optic switching

Abstract

In this letter, the uniform lying helix ͑ULH͒ liquid crystal texture, required for the flexoelectro-optic effect, is polymer stabilized by the addition of a small percentage of reactive mesogen to a high-tilt-angle ͑ Ͼ 60°͒ bimesogenic chiral nematic host. The electro-optic response is measured for a range of reactive mesogen concentration mixtures, and compared to the large-tilt-angle switch of the pure chiral nematic mixture. The optimum concentration of reactive mesogen, which is found to provide ample stabilization of the texture with minimal impact on the electro-optic response, is found to be approximately 3%. Our results indicate that polymer stabilization of the ULH texture using a very low concentration of reactive mesogen is a reliable way of ruggedizing flexoelectro-optic devices without interfering significantly with the electro-optics of the effect, negating the need for complicated surface alignment patterns or surface-only polymerization. The polymer stabilization is shown to reduce the temperature dependence of the flexoelectro-optic response due to "pinning" of the chiral nematic helical pitch. This is a restriction of the characteristic thermochromic behavior of the chiral nematic. Furthermore, selection of the temperature at which the sample is ultraviolet cured allows the tilt angle to be optimized for the entire chiral nematic temperature range. The response time, however, remains more sensitive to operating temperature than curing temperature. This allows the sample to be cured at low temperature and operated at high temperature, providing simultaneous optimization of these two previously antagonistic performance aspects.

Uniform Lying Helix Alignment on Periodic Surface Relief Structure Generated via Laser Scanning Lithography

“…So far, only a few methods have been proposed to overcome this problem but a viable solution has still to be found. In order to stabilize the ULH texture, the use of reactive mesogens has been proposed to create a polymeric structure that locks the cholesteric into the ULH texture [13][14][15]. The use of an appropriate concentration of reactive mesogens leads to stability of the ULH without having deterioration in the tilt magnitude or the switching speed.…”

Giant Flexoelectro-optic Effect with Liquid Crystal Dimer CB7CB

“…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.…”

“…Most commonly the ULH is generated in cells with a uniform planar alignment layer as a result of a controlled rate cooling of CLC mixture from isotropic phase down to chiral nematic phase with small electric field applied across the cell gap167202122. Another method to generate a well-aligned ULH CLC texture is mechanical deformation or shearing of cell substrates with respect to each other with58101123 or without224 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.…”

Uniform Lying Helix of Cholesteric Liquid Crystals Aligned by means of Slit Coater Method with Electric Treatment

“…In order to maintain the ULH, similar to the polymer-stabilized blue phase LCD, the polymer stabilization technique has been introduced [4]. As for the substrate, a glass plate (t = 1.1 mm) or polycarbonate (PC) film (t = 0.1 mm) with transparent electrodes (Indium-Tin-Oxide film, ITO) whose rectangular electrode area is 15 × 22 mm 2 was used.…”