Advancement trends in dye-doped polymer dispersed liquid crystals—a survey review

At present there is significant interest in the doping of various types of dyes into polymer dispersed liquid crystals (PDLCs) in display films. Till to date, various types of dyes have been employed to improve the electro-optical properties of PDLC films in various situations by different research groups. Some of the advantageous features of dye-doped PDLC predict that such dye-doping into the PDLC films resolve the scattering effect, improve the films absorbance, enhance the degree of alignment, stability, and the electro-optical properties of the film such as reflectance, contrast ratio and better transmittance. The current article contains a short review on the present and past studies conducted on the dye-doped polymers dispersed liquid crystals films. The fabrication techniques and their various applications in the different display areas are reviewed, along with the new key findings are discussed.     

Nematic and Cholesteric PDLC Elaborated under Shear Stress

Abstract

The properties of nematic and cholesteric Polymer Dispersed Liquid Crystal (PDLC) with ellipsoidal droplets are reported. The shape of the droplets are modified by a shear stress during the preparation. Different values of the shear stress were used which permit an ellipticity ratio modification by a factor 10. The electro-optical properties are measured for different types of samples and compared with theoretical previsions: transmission or reflection under voltage, threshold voltage, response times. We also describe the preparation of the sample, the experimental apparatus used for the shear stress, and the expected theoretical quantities like response times. When the liquid crystals are nematics, we obtain an increase of the threshold voltages, as predicted. The relaxation time decreases with increasing deformation but the diminution is not as great as predicted. When the liquid crystals are cholesterics, the high deformations induce polygonal fields, which strongly modify the electro-optical properties of the micro-composite. The threshold voltages and response time at applied voltage decrease whereas the relaxation time is increasing.     

Lasing Pixels: A New Application for PDLCs

Abstract

Pixels in an intracavity polymer dispersed liquid crystal can switch lasing action on and off, yielding addressable lasing pixels for projection applications. The marriage between an efficient PDLC display element and laser illumination consolidates the positive attributes of two different approaches to projection into a very capable technology with greater application potential. This radically different approach to projection certainly meets many of the market needs where current technologies fall short. With these needs met, a new application of PDLC technology is on the horizon.     

Liquid Crystal Light Valves with Organic Polymeric Photoconductors

Abstract

It has been suggested to use organic polymerle photoconductors in the liquid crystal light valves instead of widely used inorganic ones. Light valves with photoconductive sensibilized polyimides with various types of electro-optical effects in liquid crystals were constructed and tested. The influences of the read-on light regimes and control voltages were analysed. High resolution up to 1500 mm^?1 at diffraction efficiency 0.1%, and 40% at space frequencies up to 100 mm^?1 were obtained in pulse regimes. Thin holograms can be effectively read on such modulators in reversible mode in real time. The results obtained permit to come to a conclusion about prospects of the organic polymeric photoconductor usage in liquid crystal light valves and applications of the latter for input and output of the optical information.     

Fabrication of Pre-tilted Vertical Alignment Layers for High-Speed Liquid Crystal Display Using Bi-functional Photoreactive Monomers

Abstract

To enable recent 3D and moving picture applications, liquid crystal displays (LCD) must exhibit fast response and wide viewing angle characteristics. Pre-tilted vertical alignment layers for high-speed LCD were fabricated using bi-functional photoreactive monomers. The monomers can be confined to the polyimide layer and photo-polymerized by UV exposure under voltage. Response characteristics of photo-controlled alignment films according to the structure were investigated. Vertical alignment properties were evaluated using a polarizing optical microscope, and electro-optical characteristics were compared through the voltage-transmittance curve and response time measurement. Faster response speed could be obtained by using photoreactive monomers having a long alkyl chain.     

Liquid Crystal Compounds Incorporating the Trans-1,4-Substituted Cyclohexane Ring System

A number of esters and other compounds containing a trans-1,4-substituted cyclohexane ring system-have been prepared, and the liquid crystal thermal stabilities of these new mesogens are discussed in relation to those of their analogues containing the 1,4-phenylene ring. The observed effects are discussed in terms of a possible thermal energy absorption by the cyclohexane ring in causing minor conformational changes.

Other physical properties of these new mesogens are also discussed with particular reference to their potential in applications. In relation to electro-optical display devices, such properties as birefringence, dielectric anistropy, viscosity etc., are important parameters, and the new materials have been assessed in this context as possible additives to the 4-n-alkyl- and 4-n-alkyloxy-4′-cyanobiphenyls for producing a favorable combination of physical parameters.

Also reported is a liquid crystal mixture which has a 100°C nematic range and which contains only materials which are aliphatic. This mixture is assessed for its potential application as an anistropic solvent in analytical techniques such as ultra-violet spectroscopy and nuclear magnetic resonance spectroscopy.     

Polymer Dispersed Liquid Crystal (PDLC) “Industrial Technology and Devlopment in Europe”

Abstract

Following three decades of industrial progress and setback cycles, the Polymer Dispersed Liquid Crystal (PDLC) has currently become a growing worldwide business in the Smart Window markets. In this article, we provide a brief review of industrial evolution of PDLC film technology, its development history in Europe, the emerging products and a brief account of current global market situation.     

Birefringence of binary liquid crystalline mixtures of MBBA and PPMAECOBA in TCM,interferometric assessment

Abstract

The mutual influence of two mesogenic components, namely a nematic liquid crystal N-(p-methoxy-bezylidene)-butylaniline (MBBA) and a lyotropic liquid crystal poly-(phenyl-methacrylic) ester of cetyloxybenzoic acid (PPMAECOBA) in tetra-chloromethane (TCM) is studied in this paper. The ordinary and extraordinary refractive indices of the mixtures, nematic liquid crystal and lyotropic liquid crystal were measured at 589.3?nm with a Rayleigh interferometer and some electro-optical parameters were computed. The main refractive indices, the birefringence, the main normalized polarizabilities and their difference are dependent on the mixture volumetric concentrations, proving the existence of the collective orientational interactions between the two types of liquid crystals.     

Single‐layered microscale linear‐gradient PDLC material for electro‐optics

We report on single‐layered optical material of linear‐gradient microscale polymer‐dispersed liquid crystal (PDLC). E7/NOA65 composite films formed by pulsed UV laser photopolymerization‐induced phase separation exhibit two morphology types, namely a bipolar and a hybrid alignment of liquid crystal droplets. The specific structural properties of the produced PDLC layers, such as the droplet shape uniformity and alignment, as well as the droplet size control through the film thickness, facilitate the efficient control on the electro‐optical (EO) response, thus being of practical interest for EO device applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ferroelectricity in Tilted Smectics Doped with Optically Active Additives

The spontaneous polarization (P _s) induced by chiral dipolar additives in non-chiral smectic C liquid crystals was investigated. High values of P_s , are reported. A special type of smectic polymorphism was observed and novel phases, supposedly, of the anti-ferroelectric type were discovered. Fast linear electro-optical switching with low-temperature mixtures was demonstrated.     

Fabrication of a fast-switching liquid crystal device using reactive self-assembled polyimide alignment layer

ABSTRACT

We proposed a new vertical alignment method for simultaneously improving the alignment force and electro-optical properties. The key point of the new method is the self-assembly of the reactive monomer via hydrogen bonding with the polyimide alignment layer and the formation of pre-tilt using the reactive monomer on an alignment layer. Through the self-assembly of the reactive monomer and the generation of the pre-tilt, it is possible to obtain a higher alignment force and a fast response time. As a result, through a simple additional step, we can fabricate a fast-switching liquid crystal device using a reactive self-assembled alignment layer.     

A fast-switching vertically-aligned liquid crystal device based on a multi-functional mesogenic photocrosslinker

Abstract

A vertically-aligned liquid crystal (LC) device with fast-switching LC molecules and excellent electro-optical properties is proposed. The proposed method employs a mixture of a polyimide to achieve the desired vertical alignment, and a mesogenic photocrosslinker to obtain the fast LC response. More specifically, the photocrosslinkers were photopolymerized by UV irradiation under an electric field to pre-tilt the LC molecules. In addition, upon increasing the number of functional groups in the photocrosslinker, faster switching properties were exhibited due to enhanced pre-tilting. Following UV irradiation, the proposed system also exhibited stable homeotropic alignment properties similar to those of conventional polyimides.     

The Role of Host Molecules in the Crystalline Inclusion Compounds: Hexamethylenebis(trimethylammonium) dications as guest species

ABSTRACT

This report demonstrates hybrid homeotropic alignment of liquid crystals (LCs) using a self-assembled molecular (SAM) layer. 4-(4-Hep-tylphenyl)benzoic acid forms a SAM layer through hydrogen bonding with indium-tin oxide (ITO) substrate, and the LCs were aligned vertically without a polyimide (PI) layer on one side of substrate. The proposed hybrid structure featured a more stable homeotropic alignment than the LC device (LCD) with only half of a PI layer, and showed electro-optical characteristics similar to conventional LCDs with full PI layers. The SAM layer showed stable alignment and fast response in the LC cell by a simple doping method.     

Optical and Electrooptical Properties of “Guest-Host” Effect in Twisted Liquid Crystal Layers

Abstract

Optical and electro-optical characteristics of a positive dichroic dye-doped Twisted Nematic Liquid Crystal Layer (TNLCL) for two different orientations of the input light polarization to the NLC director α (α = 0°, α = 90°) and various NLC optical path lengths A = Δnd/λ (Δn = real part of birefringence, d = cell thickness, λ = wavelength) are investigated. In the case of one polarizer, the maximum contrast ratio is obtained for A ? 1.2 (α = 0°) and A ～ 0 (α = 90°), regardless of whether Δn or d is changed. The viewing characteristics of a dye doped TNLCL with one polarizer are shown to be more uniform than those for a “pure” TNLCL between parallel polarizers.     

Effects of the Anthraquinone Dye on the Electro-Optical Properties of Dye Guest-Host Polymer Dispersed Ferroelectric Liquid Crystal

Abstract

The effects of the anthraquinone dye on the electro-optical characteristics of DGHPDFLC have been investigated. The results show that PDFLC doped with dichroic dye was found to have no influence on phase separation. When dye concentration is over M483-1.0%, it causes the response time of DGHPDFLC to become slower, the frequency becomes dependent, the rotational viscosity increases, and the tilt angle becomes lower, but, on the whole, electro-optical properties are still very practical.     

Polypyrrole - Semiconductor photovoltaic devices

Abstract

A new approach is given for obtaining fast electro-optical response and good angular-viewing characteristics in a liquid-crystal variable-retardation device. The improved characteristics make the new device well suited for use in field-sequential color systems.     

Modal liquid crystal lenses

Abstract

When considering the possibility of commercially viable applications for discotic liquid crystals it is worth noting that in the case of calamitic liquid crystals some 80 to 90 years elapsed between their discovery in 1888 and their first application in displays. Since discotic mesophases were only discovered in 1977 [ll, it is not surprising that the first applications are yet to be realized.     

Effect of Cd1−xZnxS/ZnS core/shell quantum dot on the optical response and relaxation behaviour of ferroelectric liquid crystal

Dielectrics, polarizing optical microscopic and electro-optical measurements have been carried out on a core/shell quantum dot Cd_1?xZn_xS/ZnS dispersed ferroelectric liquid crystal (FLC). In the present study, quantum dots were dispersed into two different concentrations of 0.1 and 0.25 wt./wt.% in pure FLC. The electro-optical parameters of pure and QDs dispersed FLC were carried out as a function of applied voltage. A significant improvement in optical response time of QDs dispersed FLC system is one of the major finding of the present study which may be useful for fabrication of faster liquid crystal system.     

Polarisation‐independent liquid crystal devices

Liquid crystal (LC) devices can be used as amplitude modulators and phase modulators. Most LC devices are polarisation‐dependent and require at least one polariser. As a result, the optical efficiency is greatly reduced. In this paper, we review some of our recently developed polarisation‐independent LC devices. For amplitude modulation, we report two polariser‐free devices which combine light scattering with dye absorption: dye‐doped polymer‐dispersed liquid crystal (PDLC) and dye‐doped LC gels. For phase modulation, we also present two examples: residual phase type, e.g., voltage‐biased PDLC, Polymer‐Stabilised Cholesteric Texture (PSCT), and homeotropic LC gels; and double‐layered structure, such as thin polymer film‐separated orthogonal LC layers, and double‐layered LC gels. Potential applications of these polarisation‐independent LC devices for displays, laser beam steering and adaptive optics are emphasised.     

Electrical elements of the optical systems based on hydrogel - electrochromic polymer composites

Abstract

The results of studies of the electrically controlled elements of optical systems based on hydrogel - electrochromic polymer with conjugated π-electron system are shown. One important feature of conjugated polymers is the ability to change their optical characteristics under an external electric field. The electrochromic effect of these polymers may find applications in optical gating and filtering systems, optical sensors, ‘‘smart windows’’, optical memories, IR-switching, and electrochromic displays. Such electrochromic polymers were embedded in the matrix of a macroporous polymer hydrogel by the oxidative chemical polymerization method. The obtained samples based on hydrogel - polyaniline and hydrogel - polyorthotoluidine composites demonstrate the electrochromic behavior. In these composites, the electrolyte, as well as the electrochromic polymer, are located directly in the volume of hydrogel.