Showing papers on "Homeotropic alignment published in 2000"

Photoinduced surface alignment for liquid crystal displays

Abstract: This review examines surface alignment of liquid crystals by exposure to light. Two distinct effects are discussed: illumination with polarized light induces a surface anisotropy to an alignment layer and hence a preferred in-plane orientation of the overlying liquid crystal director. Alternatively, a photochemical reaction of the alignment surface changes the liquid crystal anchoring conditions from homogeneous to homeotropic. We discuss how cis/trans isomerization, crosslinking and photodegradation are used to produce photoalignment layers and we show how the performance of the materials can be optimized by molecular design. Pretilted alignment is discussed and theoretical models are introduced to explain the photoalignment process. A range of display and non-display applications for photoalignment is presented.

Observation of saturn-ring defects around solid microspheres in nematic liquid crystals

Abstract: We report an experimental system based on rigid, monodisperse, microparticles with tunable surface properties for the study of topological defects that form about dispersed solid phases in anisotropic media. We clearly observe equatorial Saturn-ring defects to form a distance $(0.09\ifmmode\pm\else\textpm\fi{}0.01)R$ from the surface of isolated microspheres (with strong homeotropic anchoring) dispersed in nematic phases. These defects, some of which are observed to be stable for up to a month, are similar in size to past theoretical predictions (with strong homeotropic anchoring) and can be reversibly expanded in an electric field.

Mesomorphic phthalocyanines, tetraazaporphyrins, porphyrins and triphenylenes as charge‐transporting materials

Abstract: The columnar mesomorphism of triphenylene-, phthalocyanine-, tetraazaporphyrin- and porphyrin-based compounds is briefly reviewed and the potential use of this particular supramolecular structure for charge-conducting devices is discussed. Suitably substituted triphenylenes exhibit ideal material properties for photoconducting devices, and well-aligned layers have been shown to give a very high charge carrier mobility of 0.1 × 10-4m2V-1s-1 (time-of-flight measurements). From a molecular point of view, porphyrin systems should be superior to triphenylenes, but the properties of their mesophases do not support the advantageous homeotropic alignment of the columnar stacks. However, pulse radiolysis time-resolved microwave conductivity measurements revealed the highest charge carrier mobility value of 0.28 × 10-4m2V-1s-1 for mesomorphic phthalocyanines.

Bistable nematic liquid crystal device

Abstract: A bistable nematic liquid crystal device cell is provided with a surface alignment grating on at least one cell wall and a surface treatment on the other wall. Such treatment may be a homeotropic alignment or a planar alignment with or without an alignment direction, and zero or a non zero pretilt. The surface profile on the monograting is asymmetric with its groove height to width selected to give approximately equal energy within the nematic material in its two allowed alignment arrangements. The monograting may be formed by a photolithographic process or by embossing of a plastics material. The cell is switched by dc pulses coupling to a flexoelectric coefficient in the material, or by use of a two frequency addressing scheme and a suitable two frequency material. Polarisers either side of the cell distinguish between the two switched states. The cell walls may be rigid or flexible, and are coated with electrode structures, e.g. in row and column format giving an x,y matrix of addressable pixels on the cell.

Planar and Homeotropic Alignment of LC Polymers by the Combination of Photoorientation and Self-Organization

Abstract: Upon irradiation with linearly polarized light a photoorientation process occurs in spin-coated films of polymethacrylates with 4-hexyloxyazobenzene side groups containing para methoxy and trifluoromethoxy tail groups. It results in the induction of an oblate orientational distribution perpendicular to the electric field vector causing an optical in-plane anisotropy up to an order parameter of about 0.52. The annealing of the photoreoriented films above Tg results in a prolate homeotropic alignment in the center of the irradiated spot up to a degree of order of 0.78, whereas an amplification of the photoinduced in-plane anisotropy is observed in the interim region to the nonirradiated film area. The development of the in-plane and the out-of-plane components is compared for the photoorientation and the subsequent photoreorientation process in this series of polymers. In both cases, the photogenerated order in the glassy state acts as an initializing force for the thermotropic self-organization resulting i...

Topological defects in nematic droplets of hard spherocylinders

Abstract: Using computer simulations we investigate the microscopic structure of the singular director field within a nematic droplet. As a theoretical model for nematic liquid crystals we take hard spherocylinders. To induce an overall topological charge, the particles are either confined to a two-dimensional circular cavity with homeotropic boundary or to the surface of a three-dimensional sphere. Both systems exhibit half-integer topological point defects. The isotropic defect core has a radius of the order of one particle length and is surrounded by free-standing density oscillations. The effective interaction between two defects is investigated. All results should be experimentally observable in thin sheets of colloidal liquid crystals.

Memory-free conic anchoring of liquid crystals on a solid substrate

Abstract: We show that the anchoring memory of liquid crystals on solid substrates can be completely removed by grafting highly mobile polymer chains. Using grafted polystyrene, we obtain uniform, stable, and reproducible memory-free and conically degenerated anchoring of the nematic 5CB. The symmetry of the conic anchoring enables two different zenithal anchoring breakings, towards homeotropic and planar states. Under external torques we observe both of them, with weak thresholds.

Light-induced anchoring transition in 4,4´-disubstituted azobenzene nematic liquid crystal

Abstract: The behaviour of a liquid crystalline azobenzene material possessing a nematic phase was studied under UV illumination. It was found that a uniformly aligned planar sample of the azobenzene undergoes a reversible orientation transition to homeotropic; this can be interpreted as an anchoring transition caused by the photo-isomerization of the azobenzene moiety in the molecular structure of the nematogen. A simple model taking into account the changes in the molecular shape, as well as the magnitude of the molecular net dipole moment, and the changes in the surface density of the cis-isomers with UV exposure time is proposed to explain the light-induced anchoring transition in the azobenzene nematic.

Photoinduced anchoring transitions in a nematic doped with azo dyes

Abstract: Recently, we reported on a light-induced anchoring transition of an azobenzene nematic from planar to homeotropic alignment. In the proposed model of the transition, the changes in shape of the liq...

Ultrahigh-resolution liquid crystal display with gray scale

Abstract: An atomic force microscope was used to write planar alignment patterns on a polyimide-coated glass substrate. Paired with a substrate treated for homeotropic alignment, the resulting hybrid liquid crystal cell produced fixed gray scale images with pixel sizes of order 1 μm. The physics and efficacy of this architecture are compared to a cell having planar alignment at both substrates.

Phase diagrams and morphology of polymer dispersed liquid crystals based on nematic-liquid-crystal-monofunctional-acrylate mixtures

Abstract: Phase diagrams of unpolymerized and UV-polymerized 2-ethyl hexyl acrylate (EHA) mixtures with the liquid crystal E7 are established using optical microscopy and differential scanning calorimetry. Both diagrams show upper critical solution temperature behavior. From 50 to 90 wt % liquid crystal (LC), the $(\mathrm{I}+\mathrm{I})$ phase located between the $(\mathrm{N}+\mathrm{I})$ and (I) phases was clearly shown. The nematic phase inside the droplets exhibits a twisted radial structure indicating that homeotropic anchoring occurs at the polymer interface. The experimental phase diagrams were successfully analyzed using a model based on the Flory-Huggins theory of isotropic mixing supplemented with the Maier-Saupe theory of nematic order. The LC solubility limit in the polymer matrix and the fractional amount of LC contained in the droplets were deduced from the calorimetric measurements. For the specific composition EHA/E7 (50:50), the scattering and morphological properties of the films were studied as a function of time elapsed after UV exposure. Drastic changes in the size, shape, spatial distribution, and number density of nematic droplets were observed and analyzed in terms of coalescence/diffusion phenomena.

Method for fabricating a liquid crystal cell

Abstract: A method for fabricating a liquid crystal cell and related device includes providing an alignment layer of a light sensitive material on a substrate; and exposing the alignment layer to unpolarized or partially polarized light, to provide pretilt for the molecules of the alignment layer.

Photorefractive Mesogenic Composites

Abstract: Mesogenic composites, consisting of low-molar-mass liquid crystals, polymers, and photoconductive sensitizers, constitute novel organic materials possessing high-performance photorefractivity Polymeric materials in the photorefractive mesogenic composites play a very important role in terms of improving the resolution, stabilizing the homeotropic alignment, and functionalizing the materials

Effect of multidirection rubbing on the alignment of nematic liquid crystal

Abstract: We have investigated the alignment properties of liquid crystals induced by multiple rubbing of the surfaces in different directions. Experiments were carried out using homeotropic and hybrid-aligned samples. It is experimentally found that the alignment of the liquid crystals is along neither of the rubbing directions, but instead lies along an axis intermediate between these two directions, and that the direction depended on the relative strength of rubbing along the two axes. A model that assumed the grooves along two rubbing directions is proposed, and the relation between the orientation of the liquid crystal and the relative rubbing strength is analyzed. We found that this model can explain the observed experimental results.

Alignment control of liquid crystals on surface relief gratings

Abstract: Liquid crystal alignment layers of a high T g polymer containing an azobenzene moiety are prepared by photofabrication of a surface relief grating (SRG) The interference pattern of a circular and linearly polarized Ar+ laser beam generated the surface relief grating and the morphology was detected by atomic force microscope The optical anisotropy of the films was investigated by polarizing optical microscopy The orientation of the optical axis of the film mainly depends on the direction of the initial polarization plane Nematic liquid crystals were aligned parallel to the direction of the grating, but the pretilt angles of the liquid crystals were nearly zero Irradiation with homogeneous linearly polarized light could also align liquid crystals, but this alignment capability was weaker than that of the SRG film

A smectic T phase of 1,4-dialkyl-1,4-diazoniabicyclo[2.2.2]octane dibromides

Abstract: An unidentified phase X of 1,4-dialkyl-1,4-diazoniabicyclo[2.2.2]octane dibromides reported by Nogami et al. in 1985 was reinvestigated by differential scanning calorimetry, polarizing microscopy and temperature-dependent X-ray diffraction. The X phase was thus established as a smectic tetragonal (ST) mesophase which shows spontaneous homeotropic alignment.

Photosensitive Triethoxysilane Derivatives for Alignment of Liquid Crystals

Abstract: Synthesis of a new class of triethoxysilane materials for photoinduced orientation of liquid crystals is reported. A feature of these silanes is the incorporation of a photosensitive chromophore in the chemical structure. This leads to the formation of a complete monolayer with smooth surface morphology. The effect of varying the chemical structure on the kinetics of monolayer formation as well as on the alignment of liquid crystals is discussed. Short hydrocarbon chains at the far end of the silane molecule lead to a uniform planar alignment while longer chains induce a perpendicular alignment of liquid crystal molecules.

Liquid crystal alignment structures and optical devices containing same

Abstract: An alignment structure useful in a liquid crystal display device comprising a substrate having disposed thereon an oriented film of a lyotropic nematic liquid crystalline material. Particularly useful lyotropic materials include a class of nematic liquid crystalline compounds known as chromonics. The substrates can be made by coating the lyotropic liquid crystal material onto the substrate to provide an oriented film of the lyotropic material. The alignment structure can also include one or more polarizing dyes or other additives and can thus be made to perform polarizing, retarding and/or color filtering functions in addition to alignment and orientation functions. Optical devices containing such alignment structures and methods of making such devices are also described.

Multi-domain homeotropic aligned liquid crystal display having cruciform bumps formed around pixel electrodes

Abstract: A multi-domain homeotropic aligned liquid crystal display has cruciform bump structures formed around pixel electrodes. The cruciform bump structures are fabricated on a substrate of a color filter or a thin film transistor. The bump structures orient liquid crystal molecules with a pre-tilted angle which results in the alignment of liquid crystal molecules to form four-domain textures after a voltage is applied. A standard photo-lithographic process or a back side exposure method is used to manufacture the bump structures. The back side exposure method has the advantages of being simple and clean. It avoids electric static damage and particle pollution. The liquid crystal panel provides an extremely wide viewing angle of more than 140°, a high contrast ratio of more than 700, and fast response of less than 30 ms. The light intensity of the liquid crystal mode can reach up to 55% of that of a conventional 90°-twisted nematic LCD, and is 20% higher than that of a conventional vertically aligned wide-viewing angle LCD.

29.4: Homeotropic Alignment by Single Oblique Evaporation of SiO2 and Its Application to High Resolution Microdisplays

Abstract: We have discovered that single evaporation of SiO2 can align liquid crytstal either homogeneously or homeotropically depending on the dielectric anisotropy of liquid crystal being positive or negative, respectively. Van der Waals potential taking into account the anisotropy of LC polarizabilities have been used to explain the alignment mechanism The order electricity model proposed by Monkade and Barbero et al. have been extended to qualitatively explain the generation of the pretilt angles. The observed surface morphology of the evaporated SiO2 layer as revealed by the AFM and SEM images is consistent in support of the alignment model. We have applied our alignment method to fabricate a tilted homeotropic microdisplay with a resolution of 1024 by 1280. Its electro-optical properties are also reported.

Liquid Crystal Alignment and Switching in Porous Chiral Thin Films

Abstract: Optimization of optical properties in liquid crystal (LC) devices requires control over the long-range orientation of the LC molecules. A variety of techniques are used to produce alignment in LC devices such as the familiar liquid crystal display (LCD). One well-known technique involves surface treatment of cell substrates by coating with thin polymer films such as polyimides that are subsequently rubbed. These treatments are used to produce a known orientation of the LC molecules at the surface. For example, rubbed layers may be used to align rod-like nematic LC molecules near the substrate with their long axes parallel to the direction of rubbing. The twisted nematic LCD, as an example, has the rubbing direction of the two substrates of the display cell perpendicular to each other to generate a 90 twist in the nematic orientation aided by a small amount of chiral dopant. A second technique for controlling LC orientation is the use of obliquely deposited thin films as alignment layers which generate a certain atilto of the LC molecules at the substrates. These types of orientation layer techniques influence LC orientation near the substrate surfaces only and are reliant on the LC molecules asettlingo into some minimum potential energy arrangement which gives the desired properties. The major drawback of these surface alignment layer techniques is that they are unable to provide significant influence on LC orientation further from the substrate surfaces. Maintenance of the desired LC orientation in thicker switching cells, particularly with chiral or cholesteric LCs (CLC), becomes especially difficult. This leads for instance to irreversible switching when the LCs do not settle back into their original arrangement under influence of the alignment layer after having been addressed. For better control over LC alignment, a technique which induces LC alignment throughout the cell is required, rather than influencing the LC orientation near the substrate surfaces only. This can be accomplished, for instance, by phase separation in polymer-dispersed liquid crystals (PDLC) or photopolymerization to afreezeo in the desired LC alignment. These techniques, however, may come at the sacrifice of the ability to switch the LC. A recent innovation in LC alignment was made by Robbie, Broer, and Brett who embedded LCs into porous thin films with engineered microstructures. These porous thin films are fabricated using glancing angle deposition (GLAD), a physical vapor deposition technique which allows the fabrication of highly porous thin films (Fig. 1) with columnar microstructure controllable on the sub-micrometer scale. GLAD uses highly oblique or glancing angle deposition (typically at vapor incidence angles of a > 75 , measured relative to the substrate normal) to accentuate the atomic shadowing effects, leading to thin films with porosities tunable from 10 % to 90 %. Computer control of substrate motion during

Liquid crystal display apparatus and a temperature compensation method therefor

Abstract: Disclosed is a liquid crystal display apparatus including a plurality of chiral nematic liquid crystal display layers stacked on each other. To compensate temperature dependency, a control unit, in accordance with a temperature detected by a temperature detection unit, adjusts at least one of a voltage level and a pulse width of a pulse signal be applied to at least one of the liquid crystal display. In one embodiment, the control unit retrieves the detected temperature before driving the liquid crystal display successively, and commonly uses the detected temperature for the successive drives. In another embodiment, the control unit does not adjust a voltage level nor a pulse width of a first reset pulse signal that is for setting the liquid crystal material to a homeotropic phase regardless of the detected temperature.

Orientational photorefractive effect in nematic liquid crystal with externally applied fields

Abstract: Light-induced space-charge fields and the related orientational photorefractive effects in nematic liquid crystal cells are studied both theoretically and experimentally. Analytical solutions for the refractive index change distribution are obtained under the single-elastic-constant approximation with various configurations of externally applied electric and magnetic fields and of the nematic liquid crystal director. The effects of the rigid boundary condition are investigated in detail. The grating spacing for which the refractive index change is maximized depends on the strength of external electric and magnetic fields. The responses of parallel cells differ from those of homeotropic cells, both upon the dependencies on the grating spacing and on the external fields. Under the same conditions, the orientational photorefractive effects are stronger in parallel cells than in homeotropic ones. It is also shown that the investigated effects will be greatly enhanced near the Freedericksz transition. Experimental verifications of the photorefractive response in parallel aligned cells confirm the necessity to consider the boundary conditions explicitly.

Modeling the Interface Region of Command Surface 2. Spectroscopic Evaluations of Azobenzene/Liquid Crystal Hybrid Langmuir-Blodgett Films under Illumination

Abstract: Langmuir−Blodgett (LB) films composed of the mixture of an amphiphilic polymer containing azobenzene (Az) side chain (6Az10-PVA) and 4‘-pentyl-4-cyanobiphenyl (5CB) were prepared to mimic the two-dimensional contacting region of the LC/Az interface of the command surface which photochemically switches the LC alignment. UV−visible absorption and Fourier transform infrared spectroscopic measurements were carried out under illumination. These procedures allowed separate and simultaneous evaluations of the static state and dynamic molecular motions of both Az and LC molecules, which probably reflect the initial triggering step of the “domino-mode” response of LC. The spectroscopic data indicated the induction of reversible perpendicular/tilt orientational changes of both the Az side chain and 5CB molecule upon alternative irradiation of 365 and 436 nm light. Thus, 6Az10-PVA/5CB hybrid LB film can be regarded as a satisfactory interface model of a command surface that promotes the homeotropic/planer alignment ...

Homeotropic polar anchoring energy of a nematic liquid crystal using the fully leaky waveguide technique

Abstract: Optical excitation of a series of fully leaky guided modes has been used to explore the director distortion of a hybrid-aligned nematic liquid crystal with a negative dielectric anisotropy (Merck-BDH, MLC 6608) under application of an ac electric field. Hybrid alignment is realized by the use of a lecithin film on the top glass plate and rubbed polyimide on the bottom plate, both plates having indium–tin–oxide coatings. Continuum theory is used to model the director profile through the cell. Fitting the fully leaky guided mode data both in transmission and reflection using optical multilayer theory together with continuum modeling gives the director profile both with and without applied fields. From this fitting the changes in the surface tilt angle and its gradient are obtained. Using this information the polar anchoring coefficient of the homeotropic surface Wθ is deduced. At 23.1 °C we find Wθ=(4.6±0.2)×10−4 J m−2.

Alignment Control in Holographic Polymer Dispersed Liquid Crystal.

Abstract: Alignment control is promising approach for improving the characteristics of holographic polymer dispersed liquid crystal (HPDLC). This paper describes the methods of controlling the alignment of LC molecules in HPDLC (by using an electric field, shear force, and LC monomer) and the resulting characteristics of the devices. When the alignment is controlled, the diffraction characteristics can have polarization selectivity. The diffraction efficiency is also improved, because the optical anisotropy of LC molecules can be fully used.

Brightness enhancement for bistable cholesteric displays

Abstract: The present invention is directed to a cholesteric LCD {C} that includes a homogeneous alignment surface effective to provide increased brightness, low focal conic reflectance and/or reflective light that is to a significant degree circularly polarized {C}. The homogeneous alignment surface substantially homogeneously aligns the liquid crystal director adjacent thereto. The homogenous alignment surface may be disposed on one or both sides of a cell of the display. In the case of a cell in which the homogeneous alignment surface is disposed on only one side, the substrate with the inhomogeneous alignment surface may be upstream or downstream of the substrate with the inhomogeneous alignment surface relative to a direction of incident light {LS}.

Helical superstructure from an achiral banana-shaped liquid crystal

Abstract: In the banana-shaped achiral molecular system reported here, 1,3-phenylene bis[4-(3-chloro-4-n-octyloxy-phenyliminomethyl) benzoate] (PBCOB) and 1,3-phenylene bis[4-(3-fluoro-4-n-octyloxy-phenyliminomethyl) benzoate] (PBFOB), their smectic mesophases, including a switchable banana phase B7, were identified by differential scanning calorimetry (DSC), X-ray scattering and polarizing optical microscopy (POM) studies. Both the left- and right-handed helical domains are spontaneously formed upon cooling from isotropic liquid to the switchable banana phase (By). The homeotropic alignment and ferroelectric properties of banana-shaped liquid crystal are investigated. The spontaneous polarization for PBCOB is about 50 nC/cm2. Doping the PBCOB with a cyanohydrin-contain chiral smectic liquid crystal resulted in more right-handed helical structures.

Effect of Conformations of Trans- and Cis-Azobenzenes on Photoinduced Anchoring Transitions in a Nematic Liquid Crystal

Abstract: Two azobenzenes with different positional substituents, 3,3'-dihexyloxy-2,2'-dimethylazobenzene (3,3'-azo) and 4,4'-dihexyloxy azobenzene (4,4'-azo), were doped onto a nematic liquid crystal (LC) to form photoresponsive guest-host LC mixtures The photoinduced anchoring transition of the LC, triggered by trans-cis photoisomerization of the two dyes subjected to linearly polarized UV light was found to be different Whereas the photoisomerization of 3,3'-azo only induced a transition from random planar (RP) to uniform planar (UP) (in-plane) alignment, that of 4,4'-azo additionally induced a transition from uniform planar to homeotropic (H) (out-of-plane) alignment of the LC at a longer exposure time The anchoring behavior of the two azobenzenes is discussed in terms of the conformations of their trans- and cis-isomers and the interactions of the isomers with the solid surface of the substrate