Evolution of the switching current during the preparation of polymer network-ferroelectric liquid crystal microcomposites

The polymer network-ferroelectric liquid crystal (FLC) microcomposites are prepared by UV crosslinking of a chiral prepolymer diluted in a FLC. A fast cross-linking process involving 20 mW cm^-2 UV intensity produces material with ferroelectric properties which have strongly deteriorated compared with the pure FLC. By a slow process using a UV intensity of 2 mW cm^-2 the ferroelectric properties are less affected. It is supposed that the very dense polymer network, which arises during the fast process, is responsible for hindering the switching process and decreasing the spontaneous polarization. The photopolymerization kinetics are determined by measuring the switching current during the UV irradiation. The characteristic time of cross-linking is mainly reflected in the evolution of the spontaneous polarization and rotational viscosity with time.     

Ferroelectric polymer nanocomposite alignment layer in twisted nematic liquid crystal devices for reducing switching voltage

Twisted nematic liquid crystal device (TNLCD) was fabricated using a ferroelectric zinc oxide (ZnO)-doped polyimide alignment layer. The ferroelectric nanoparticle can produce a local electric field, which can trigger the orientation of liquid crystal molecule and reduces the switching voltage. The uniform dispersion of ferroelectric ZnO nanoparticles in the alignment layer was studied using field emission scanning electron microscopy and atomic force microscopy. The ferroelectric property of ZnO-doped polyimide was investigated using dynamic contact electrostatic force microscopy. An increased local electric field due to the presence of nano ZnO was confirmed with the help of scanning tunnelling microscopy. An augmentation of capacitance was observed with an increase in concentration, which substantiates the reduction of switching voltage of TNLCD with the modification of ferroelectric nanoparticle-doped alignment layer.     

Optical switching property from a laser beam propagating in a polymer dispersed liquid crystal film

Optical switching has been studied in epoxy-based polymer dispersed liquid crystal films. A 'self-transparency' effect is observed due to refractive index variations thermally induced either by heating the sample with a hot stage, or by absorption of the incident laser power. The switching phenomena were studied for different liquid crystal contents in the composites, and experimental evidence of light modulation by a probe beam is also reported. The thermo-optical behaviour of the material is correlated with the morphology of the samples. The results obtained verify the possibility of employing this type of material in optical devices based on the thermo-optical switching effect.     

Optical switching property from a laser beam propagating in a polymer dispersed liquid crystal film

Optical switching has been studied in epoxy-based polymer dispersed liquid crystal films. A 'self-transparency' effect is observed due to refractive index variations thermally induced either by heating the sample with a hot stage, or by absorption of the incident laser power. The switching phenomena were studied for different liquid crystal contents in the composites, and experimental evidence of light modulation by a probe beam is also reported. The thermo-optical behaviour of the material is correlated with the morphology of the samples. The results obtained verify the possibility of employing this type of material in optical devices based on the thermo-optical switching effect.     

In situ observation of smectic layer reorientation during the switching of a ferroelectric liquid crystal

Abstract

A thin plastic cell containing the ferroelectric liquid crystal ZLI3654 (4 μm) was placed edge-on to a pin-hole collimated horizontal X-ray beam. In this way, the smectic layers were brought into register. Subsequently, triangular voltage waves with various peak (V _p) values were applied across the cell and diffraction photos were obtained during the application of the alternating voltage. Up to V _p = ± 30 V, no significant movements in the initial tilted orientation of the smectic layers with respect to the surfaces (chevrons) could be observed. During the application of V _p = ± 32 V an increasing fraction of the smectic layers changed their initial tilt angle with respect to the cell surfaces to make larger tilt angles. At a slightly higher voltage, the layers became upright (bookshelf structure). Upon removing the voltage and short circuiting the cell, the quasi-bookshelf structure was sustained. The new method described here can be used in combination with a fast detector for time resolved experiments.     

In situ observation of smectic layer reorientation during the switching of a ferroelectric liquid crystal

A thin plastic cell containing the ferroelectric liquid crystal ZLI3654 (4 μm) was placed edge-on to a pin-hole collimated horizontal X-ray beam. In this way, the smectic layers were brought into register. Subsequently, triangular voltage waves with various peak (V_p) values were applied across the cell and diffraction photos were obtained during the application of the alternating voltage. Up to V_p = ± 30 V, no significant movements in the initial tilted orientation of the smectic layers with respect to the surfaces (chevrons) could be observed. During the application of V_p = ± 32 V an increasing fraction of the smectic layers changed their initial tilt angle with respect to the cell surfaces to make larger tilt angles. At a slightly higher voltage, the layers became upright (bookshelf structure). Upon removing the voltage and short circuiting the cell, the quasi-bookshelf structure was sustained. The new method described here can be used in combination with a fast detector for time resolved experiments.     

An azobenzene-containing side-on liquid crystal polymer

A new azobenzene-containing side-on LC polymer was synthesized. It shows a nematic phase. Its photochemical properties were characterized in solution and in thin film. The N-I phase transition can be induced upon irradiation in the π-π* absorption region of the polymer.     

Studies of polymer ball type polymer dispersed liquid crystal films

'Reverse' or 'polymer ball' polymer dispersed liquid crystal (PDLC) samples were prepared by a photopolymerization induced phase separation method. A detailed study of the effects of the sample preparation parameters, such as curing time, curing intensity, and liquid crystal concentration are reported. It was found that by adjusting these parameters, we were able to change the morphology of these 'polymer ball' PDLC samples and thus change their optical characteristics. Incomplete polymerization of the PDLC samples results in a higher threshold voltage and a lower ON state transmission. When the amount of monomer is too low, the shape of the resulting polymer ball becomes irregular, and the sample has a larger threshold voltage and a larger saturation voltage.     

Fast switching and luminance-controlled fringe-field switching liquid crystal device for vehicle display

ABSTRACT

Liquid crystal displays (LCDs) for vehicle displays should exhibit a fast response time in wide temperature range and wide-viewing angle in horizontal and downward directions without grey-scale inversion but limited brightness in the upward direction because the display images can be reproduced in the front window glass of a vehicle, affecting driver’s front visibility. Currently, fringe-field switching (FFS) liquid crystal device is widely commercialised for high resolution and wide-viewing-angle LCD; however, it needs to improve response times and limit the display brightness in the upward direction. As a solution, we propose a homogeneously aligned liquid crystal device in which liquid crystal director does tilt as well as twist deformation in a confined area by both vertical- and fringe-electric fields, exhibiting about two times faster decay response time than that of conventional FFS mode with suppressed luminance in the upward direction. The proposed liquid crystal device can be applied to LCDs for vehicle displays.     

Response speed of in-plane switching mode liquid crystal displays

The response speed of in-plane switching mode liquid crystal displays with three initial director configurations—homogeneous, twisted nematic and 180° super twisted nematic—was investigated via simulations. These simulations studied the relationship between the change in the optical axis and the optical transmission in the three configurations, allowing us to calculate the optical response times. The time-dependent change in the director was calculated using the Erickson-Leslie equations and these two results combined. These results predict that the response time during both the rise and decay periods of a super twist cell is about four times faster than the other two configurations.     

Effects of the fluorinated liquid crystal molecules on the electro-optical properties of polymer dispersed liquid crystal films

The different fluorinated liquid crystal (LC) molecules doped to E8 were used as LC component to prepare polymer dispersed liquid crystal (PDLC) films. The mass fraction of the LC mixture is fixed 50.0 wt%. Results indicate that doping 8.0 wt% fluorinated LC molecule ME3CP to E8 significantly reduced the driving voltage of the PDLC films, and the driving voltage reduced with the rise of mass fraction of ME3CP. Besides, the terminal flexible chain length of the fluorinated LC molecule influenced the LC mixture properties based on E8, such as the dielectric anisotropy, birefringence and viscosity of the LC mixture, and the morphology and the electro-optical properties of PDLC films were controlled not only by the physical properties of the LC mixture, but also by the terminal flexible chain length of the fluorinated LC molecule .     

Electro-optic characteristics of the fringe in-plane switching liquid crystal device for a liquid crystal with negative dielectric anisotropy

Fringe-field switching (FFS) liquid crystal (LC) mode is mainly used for high-end LC displays. At present, an LC with positive dielectric anisotropy is utilised, although light efficiency of the device in a white state is not maximised due to generation of tilt angle near the edge of electrodes along the field direction. In order to overcome the demerit, an LC with negative dielectric anisotropy has been challenged. In this article, FFS mode, which shows a high light efficiency and a low operating voltage, is investigated with the utilisation of fringe in-plane electric field. The optimised device shows improved electro-optic characteristics in comparison with not only conventional LC modes, but also previously proposed FFS device using a positive type of LC.     

Fluorescence behaviour of cyanobiphenyl liquid crystal molecules in liquid crystal/polymer composite films

The fluorescence behaviour of the liquid crystal, 4-cyano-4-pentylbiphenyl (5CB), in composite thin films prepared by the photopolymerization of 5CB/diacrylate mixtures, was investigated by means of three different excitation methods, in which the total-internal-reflection or surface-limited excitation method was used for analysis of the fluorescence from an ultra-thin interface layer ( 100nm) in contact with the substrate surface, whereas the fluorescence from the interior bulk was analysed by the through-film excitation method. It was found that intensity ratios of the monomer and excimer emissions of 5CB are significantly lower in the interface layer than in the interior bulk, depending upon photopolymerization conditions as well as upon the structures of the diacrylates used. Scanning electron microscopic observations and light-scattering measurements of some typical composite films showed possible relationships between morphological features and fluorescence characteristics depending upon the diacrylate structures and polymerization conditions. The different fluorescence behaviour has been discussed in terms of differences in mobility and/or aggregation degrees of 5CB molecules arising from dominant molecular interactions with the substrate and polymer surfaces.     

Control of liquid crystal droplet configuration in polymer dispersed liquid crystal with macro-iniferter polystyrene

A polystyrene macro-iniferter was applied to control the alignment of liquid crystal molecules at the droplet wall of polymer dispersed liquid crystal (PDLC) films. The aspects of the alignment were monitored by observing the droplet in the PDLC film. With increasing the macro-iniferter polystyrene in the composition, the configuration of LC droplets changes from bipolar to radial. This is because the high concentration of the macro-iniferter polystyrene results in a small surface interaction between the LC and the polymer matrix, which favours the formation of radial configuration. The radial configuration was stable under our conditions. However, increasing the LC and the initiator concentrations resulted in the change from radial to bipolar.     

Sandwich structure containing liquid crystal polymer grafted on polymer support

Graft post-polymerization of mesogenic monomers onto fluorine-containing polymer support was initiated by the simultaneous action of vacuum ultraviolet radiation and atomic oxygen. The resultant two-layer structure possesses the combined physical–mechanical properties of the polymer-supporting film and the optical characteristics of the anisotropic liquid crystal layer.     

The influence of nematic liquid crystal content on the electro-optical properties of a polymer dispersed liquid crystal prepared with monodisperse liquid crystal microcapsules

Highly monodisperse poly(methylmethacrylate) (PMMA)/liquid crystal (LC) microcapsules were prepared by the solute codiffusion method (SCM). In SCM, the solvency of the dispersion medium and the swelling temperature were controlled to ensure complete swelling of the LC into the PMMA substrate particles. After the solvent evaporation procedure, mononuclear LC domains were formed in every particle, and overall the LC microcapsules maintained a spherical shape, monodispersity and smooth surface. PDLC cells prepared using the LC microcapsules exhibited an excellent contrast ratio and transmittance. By increasing the LC content in these LC microcapsules, the electro-optical properties of PDLC cells have been greatly improved.     

Rubbing angle effect on in-plane switching liquid crystal displays

The rubbing angle effect on transmissive in-plane switching liquid crystal displays is analysed by the Jones matrix method. Simulation results show that the optimum rubbing angle is around 30°-40°; the cell gap/birefringence product (dΔn) is about 0.33 µm. Increasing the rubbing angle can shorten the rise time and enlarge the grey scale voltage intervals. The optical characteristics are similar in two cells with different rubbing angles. These effects are particularly attractive for liquid crystal TV applications.     

Interface modification of polymer stabilized cholesteric liquid crystal

Films were made by incorporating various quantities of fluoroalkyl acrylate (FA) into the chain termini of polyurethane acrylate oligomer of polymer stabilized cholesteric liquid crystals (PSCLCs). The effects of switching voltage, scanning electron and polarized optical microscope morphologies, texture transition and reversibility, and contact angle of the films were then studied. It was found that the switching voltage of the film decreased with the addition of FA up to 0.8 wt%, beyond which it increased. The decrease and increase were respectively explained in terms of decreased interface energy as noted from the contact angle of the LC on polymer surface, and too small droplet size as noted from the SEM. Copyright © 2008 John Wiley & Sons, Ltd.