The Use of Azo Dyes in Guest-Host Displays

Azo dye molecules are cylindrically shaped, and therefore, due to their resemblance to liquid crystal molecules, would seem to be suitable for use in guest-host mixtures. The dichroic ratio of a dye in a given nematic host depends on the molecular length of the dye molecules, the order parameter of the liquid crystal host and its molecular dimensions. It is possible to obtain a guest-host mixture with an order parameter which is higher than that of the pure host. Lateral substitution of mono-azo dyes lowers the order parameter, whilst lateral substitution of the corresponding bis-and tris-azo molecules results in only a slight change in their order parameter.

The stability of these compounds to light is poor, and depends on the wavelength for maximum absorption, and on the host material. Whilst lateral substitution increases the fastness of azo compounds when used for textile dying, this is not the case when they are employed with liquid crystal mixtures. The kinetics of degradation are zero order in dye concentration.

It has been found that azo dyes are not in general suited to applications where prolonged exposure to sunlight may be encountered, e.g. watch applications.     

Order Parameter and the Performance of Nematic Guest-Host Displays

A simple model of a nematic guest-host display is presented in which the limiting “on” and “off” states are considered to be uniform arrays of molecules oriented perpendicular and parallel (or at a small tilt angle), respectively, to the plane of the display. Calculated contrast ratio and brightness curves are presented for several angles of view and of all planes of view as functions of dye order parameter, pretilt andploarizer efficiency. The effects of host refractive index and front surface reflectivity are discussed.     

Assessment of Nematic Guest-Host Systems for Application to Integrated Liquid Crystal Displays

An optimized configuration of the nematic guest-host effect is described in which a 90° twist cell is used. In order to assess the suitability of nematic guest-host systems for use in integrated displays in which a significant dc voltage may appear across the liquid crystal, several dye/host systems were tested with up to 1.5V dc superimposed on the normal 5V r.m.s. drive signal. Degradation was monitored by measurement of tilt, clearing point and absorbance. One system tested showed no significant degradation after 9 × 10³ hr. Excellent correlation was observed between stability of a dye/host system in a driven display and the reversibility of the reduction of the dye in acetonitrile solution at a platinum electrode.     

Color Gamut of Liquid Crystal Polysiloxanest

Polysiloxanes with suitable side-chains form a cholesteric liquid crystal phase that can be applied to a substrate to produce a durable coating which exhibits strong colours below the glass transition temperature. The peak reflectance is almost 0.5 and a nearly 1.0 peak reflectance can be obtained using a half-wave retardation plate coated on both sides with the material and viewed from one direction against a black background. Values below 0.5 and between 0.5 and 1.0 have been also obtained using suitable solvents and alignment techniques. Many desired spectral reflectance distributions can be then obtained because of the additive color properties of such cholesteric coatings. As a result it is possible to produce a larger colour gamut than with real surface colours such as pigments and dyes. Employing commercially available material experimental work was carried out demonstrating a greatly expanded colour gamut on the chro-maticity diagram. Also it is possible to modify the spectral reflectance distribution of coloured substrates by coating them with a material having a reflectance peak at the desired band of wavelength.     

White and Taylor Type Guest Host Displays Without Scattering Effects Using the Tilted Boundary Conditions

White and Taylor type guest host displays using the tilted boundary condition have been reported. The parallel or the perpendicular boundary condition is usually used for these displays and both conditions cause scattering phemomena after the electric field removal. The present paper reports the relaxation processes of samples with the tilted boundary after the field removal. Using the tilted boundary, two types of nonscattering conditions depending on the thickness of the liquid crystal layer (d), the natural pitch (Po) and the boundary condition (φ) exist. Both types of conditions are discussed in detail.     

Dielectric Spectroscopy of Side Chain Siloxane Liquid Crystal Copolymers

Side chain siloxane liquid crystal polymers (LCPs) are being investigated as potential optical storage media. Dielectric relaxation spectroscopy has been used to examine the real and imaginary components of permittivity as a function of frequency and temperature. The spectra obtained are discussed from the standpoint of both their molecular significance and their device implications. Spectra of liquid crystal homopolymers and a particular class of copolymers have been found to exhibit different features due to the larger free volume available to the mesogenic moieties in the copolymers. The relaxation behaviour of five LC copolymers is discussed and then two equivalent pairs of homopolymer and copolymer LCPs are considered in more detail. One pair shows anomalous relaxation behaviour and extra relaxation processes are attributed to internal reorientations.     

Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

Optical birefringence measurements for a calamitic lyotropic liquid crystal containing sodium lauryl sulphate/decanol/water are presented. A value of the order of 10^‐3 was obtained, much smaller than that of thermotropic liquid crystals. The order parameter temperature dependence is evaluated using an extrapolating method and taking into accounts the influence of the micelle shape. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monte Carlo Study of Slot-waveguide Liquid Crystal Phase Shifters

Abstract

We present detailed Monte Carlo simulations of a simple model of a nematic liquid crystal slot waveguide shifter, investigating the effect of an applied electric external field. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the planar alignment as in Silicon Organic Hybrid waveguides and the homeotropic anchoring appropriate for Polydimethylsiloxane polymer walls. The external field is modeled by adding a term to the Hamiltonian which describes its coupling to the mesogenic molecules. We have investigated the effect of the external field on the optical transmission and the ordering across the cell.     

Preparation of a Carbon Nanotube-Reinforced UV/Heat Dual-Curable Adhesive for Liquid Crystal Displays

Abstract

There is strong demand for narrow-bezel liquid crystal displays (LCDs) in the display industry. Adhesive materials for narrow-bezel LCDs require excellent adhesive properties in order to provide the same level of adhesive strength as conventional sealants, even when applied in small amounts. In this study, we prepared a previously unknown, highly adhesive, carbon nanotube-reinforced (CNT-reinforced) UV/heat dual-curable adhesive for narrow-bezel LCDs. Single-walled and multi-walled CNTs (SWCNTs and MWCNTs) were employed as fillers, resulting in superior adhesive properties; in particular, the inclusion of SWCNTs improved both adhesion and resistance to water permeability compared to a conventional adhesive.     

Liquid Crystal Channel Waveguides: A Monte Carlo Investigation of the Ordering

We present detailed Monte Carlo (MC) simulations of a nematic cell with homeotropic boundary conditions at the four confining surfaces. The simulations are based on the Lebwohl-Lasher lattice spin model with boundary conditions chosen to mimic the cell anchoring. We have investigated the model using a standard Metropolis Monte Carlo method to study the optical transmission and the ordering through the cell.     

Synthesis and Crystal Structure of Mono-Substituted Ferrocene-Containing Liquid Crystals

Abstract

Ten mono-substituted ferrocene-containing liquid crystals, which containing Schiff base or ester group, were prepared and their mesogenic behaviors were investigated. Two molecular structures were analyzed by X-ray. 4-ferrocenylphenyl 3′-chloro-4′-(2-methylbutyloxy)-benzoate, triclinic crystal system, P1 space group, a=12.035(2)Å, b=14.402(3)Å, c=15.137(3)Å, α =103.07(3)Å, ß = 102.01(3)Å, γ = 96.70(3)Å, Z=4, 8141 independent reflections, R=0.0528, Rw=0.1724. 4-ferrocenylphenyl 3′-chloro-4′-decanyloxybenzoate, triclinic crystal system, P1 space group, a= 11.663(2)Å, b= 11.729(2)Å, c=22.005(4)Å, α=76.10(3)Å, ß =87.56(3)Å, γ = 77.89(3)Å, Z=4, 7935 independent reflections, R=0.0784, Rw=0.2265.     

Multirings Aromatic Aldehyde Liquid Crystal with Azo Linkage and Their Photosensitivity in Mesophase

Ten new rod-like aromatic aldehyde liquid crystalline molecules with azo linkage were synthesized, in which bi(trans-cyclohexyl), cyclohexyl phenyl, and biphenyl carboxylic acid mesogenic cores with terminal ethyl, n-propyl, n-butyl, and n-pentyl substituents were esterified with azo benzoic aldehyde. These molecules were designed in an attempt to construct a series of new azo liquid crystalline molecules to investigate the influence of ultraviolet (UV) light on their mesophase. All compounds have been characterized on the basis of their spectral data, differential scanning calorimeter (DSC), and hot stage polarizing optical microscope (HS-POM). All these compounds exhibited liquid crystalline phase that belonged to nematic and photosensitive properties. Their temperature ranges of mesophase are from 101 °C to 150 °C. Under irradiated 365 nm UV light, they showed photosensitivity in the solvent of methanol. Observed under HS-POM, the UV light also did change the textures of these compounds. The result showed that terminal ethyl is enough for these molecules to exhibit wider temperature range of mesophase, and these new molecules have photosensitivities observed under illumination of UV light not only in solution but also in mesophase due to the change of their structures from trans isomer to cis one.     

Smectic A-Nematic and Nematic-Isotropic Liquid Transitions in 8CB Liquid Crystal: Comparative Analysis

A differential heat flux calorimeter has been used to study the Smectic A-Nematic and Nematic-Isotropic Liquid transitions in 8CB liquid crystal. The Faktor and Hanks model under our working conditions enables us to distinguish first-order from second-order transitions by analysis of differential signal. The critical exponent values obtained justify the proportionality between Cp evolution and the normalized differential signal (differential signal divided by the scanning rate).     

Determination of the Birefringence,the Twist Angle and the Thickness of the Nematic Liquid Crystal Sample by Renormalized Transmission Ellipsometry

Abstract

The birefringence, the twist angle and the thickness of the nematic liquid crystal sample were determined by means of the renormalized transmission ellipsometry. It was proposed that conventional sandwich-type cell is applicable to this ellipsometry measurement and the simplified numerical fitting procedure based on the 4 × 4 matrix method can provide the dispersion of the ordinary and the extraordinary refractive indices. The resolution of the twist angle and cell thickness measurement reaches to 0.05 deg. and 0.05 μm, respectively.     

A Positive Contrast Guest-Host Display Using a Liquid Crystal of Negative Dielectric Anisotropy

Recently we described a novel guest-host cell which exhibits a positive contrast (dark digits on a clear background) employing a mixture of a nematic liquid crystal with negative dielectric anisotropy, a chiral component, and a dichroic dye. In this cell, the cell thickness and the alignment layers are chosen such that in the absence of a field, the mixture shows a uniform homeotropic structure. When a voltage greater than the threshold value is applied, a helicoïdal structure is adopted, which absorbs the incident illumination.

In this work the expression for the threshold voltage is derived and compared with experimental results. Experimental results are also presented which indicate, for a given cell thickness, the influence of the pitch of the mixture on the contrast and the response time.     

Design of Optical Directional Couplers Made of Polydimethysiloxane Liquid Crystal Channel Waveguides

We present numerical simulations of a directional coupler based on three-dimensional waveguides made of a nematic liquid crystal, acting as the waveguide core, infiltrated in polydimethysiloxane channels. Modeling is based on the combination of minimization of Oseen-Frank energy of the liquid crystal molecules with a beam propagation algorithm. Design of the coupler waveguides is optimized to minimize coupling lengths and maximise efficiencies. Such components can be made at low cost on flexible plastic substrates and can be also integrated with optofluidic devices for biomedical applications.     

Liquid Crystal Television

The application of liquid crystal to display devices was first proposed by Heilmeier et al. in 1968, and a liquid crystal television (LC-TV) addressed by electron beam was made as a trial in the same year. Since then, it has taken about 15 years for practical LC-TVs to be developed. Full-color, higher resolution and greater pixel number have been attained, and presently, at least for small size LC-TVs, image quality has become very close to that of CRTs.