Showing papers on "Liquid crystal published in 1995"

Optical Switching and Image Storage by Means of Azobenzene Liquid-Crystal Films

Abstract: Liquid crystals are promising materials for optical switching and image storage because of their high resolution and sensitivity. Azobenzene liquid crystals (LCs) have been developed, in which azobenzene moieties play roles as both mesogens and photosensitive chromophores. Azobenzene LC films showed a nematic phase in trans isomers and no LC phase in cis isomers. Trans-cis photoisomerization of azobenzene with a laser pulse resulted in a nematic-to-isotropic phase transition with a rapid optical response of 200 microseconds.

Liquid crystal dispersions

Abstract: Methods for Film Formation: Emulsification Methods Phase-Separation Methods. Droplet Configurations: Methods for Probing Droplet Structure Catalogue of Droplet Structures Transformations between Droplet Structures Gels and Porous Glasses. Electro-Optical Effects: Light Scattering Dichroic Effects Droplet Reorientation Fields Response Times Hysteresis and Persistence Effects Film Dielectric Properties Nonlinear Optical Effects. Applications: Projection Displays Direct-View Displays Light-Valve Applications.

A Helical Polymer with a Cooperative Response to Chiral Information

Abstract: Polyisocyanates, long studied as theoretical models for wormlike chains in dilute solution and liquid crystals, differ from their biological helical analogs in the absence of a pre-determined helical sense. These polymers have an unusual sensitivity to chiral effects that arises from a structure in which alternating right- and left-handed long helical blocks are separated by infrequent and mobile helical reversals. Statistical thermodynamic methods yield an exact description of the polymer and the cooperative nature of its chiral properties. Minute energies that favor one of the helical senses drive easily measurable conformational changes, even though such energies may be extremely difficult to calculate from structural theory. In addition, the chiral nature of the polymer can be used to test theoretical ideas concerned with cholesteric liquid crystals, one of which solves the problem of assigning the helical sense.

Mesostructure Design with Gemini Surfactants: Supercage Formation in a Three-Dimensional Hexagonal Array

Abstract: At low temperatures, liquid crystal-like arrays made up of inorganic-cluster and organic molecular units readily undergo reversible lyotropic transformations. Gemini surfactants, with two quaternary ammonium head groups separated by a methylene chain of variable length and with each head group attached to a hydrophobic tail, can be used to control organic charge sitting relative to the bivariable hydrophobic tail configurations. This approach has led to the synthesis of a mesophase (SBA-2) that has three-dimensional hexagonal (P6(3)/mmc) symmetry, regular supercages that can be dimensionally tailored, and a large inner surface area. This mesostructure analog of a zeolite cage structure does not appear to have a lyotropic surfactant or lipid liquid crystal mesophase counterpart. Through the modification of gemini charge separation and each of the two organic tails, these syntheses can be used to optimize templating effects, including the synthesis of MCM-48 at room temperature.

Wide-band reflective polarizers from cholesteric polymer networks with a pitch gradient

Abstract: CHOLESTERIC liquid crystals, in which the orientation of the molecules varies in a helical fashion, are used for optical filtering of circularly polarized light, for example in liquidcrystal displays. They reflect circularly polarized incident light of the same handed ness as the cholesteric helix, and in a wavelength band that depends on the helical pitch (repeat distance). This pitch can be selected by careful design of the liquidcrystal molecules or by mixing cholesteric materials with nematic (linearly oriented) liquid crystals, which tend to increase the pitch. Stable optical filters are produced by crosslinking the cholesteric molecules by photopolymerization1; these filters typically have a reflection wavelength bandwidth of ∼50 nm. Here we show that, by introducing a gradient in the pitch of the cholesteric helix, we can obtain reflection of one of the two circularly polarized components over the entire visible spectrum. Polarizers with such broadband reflectivity would greatly improve the light yield and energy efficiency of liquidcrystal display devices.

Electro‐optical characteristics and switching behavior of the in‐plane switching mode

Abstract: Electro‐optical characteristics related to the threshold behavior of liquid crystals when using the in‐plane switching (IPS) mode were investigated with interdigital electrodes. In order to analyze the switching behavior of liquid crystals, an equation, which expresses the threshold transition, was derived using the continuum elastic theory. It was made clear that it was the electric field and not the voltage that drives the liquid crystals in the IPS mode. Significantly, an inversely proportional relationship between the threshold voltage and the gap between the substrates was found to hold. Furthermore, the electro‐optical characteristics were recognized to change with the variation of the gap between the substrates. This behavior is due to the independence of electric field on liquid crystal layer normal.

Thermotropic liquid crystals formed by intermolecular hydrogen bonding interactions

Abstract: The role of hydrogen bonding in the formation or stabilization of liquid crystalline phases has only recently been appreciated. Following the first, wellestablished examples of liquid crystal formation from the dimerization of aromatic carboxylic acids, through hydrogen bonding, several classes of compounds have recently been synthesized, the liquid crystalline behavior of which is also dependent on intermolecular hydrogen bonds between similar or dissimilar molecules. In this review the main classes of compounds exhibiting liquid crystallinity due to hydrogen bonding are presented to show the diversity of organic compounds that can be used as building elements in liquid crystals. The molecules are either of the rigid-rod anisotropic or amphiphilic types such as molecules appropriately functionalized with pyridyl and carboxyl groups, whose interaction leads to the formation of liquid crystals; amphiphilic carbohydrates and amphiphilic and bolaamphiphilic compounds with multiple hydroxyl groups whose dimerization or association is indispensable for the formation of liquid crystals; and certain amphiphilic carboxylic acids with monomeric or polymeric mesogens and amphiphilic-type compounds bearing different moieties, whose interaction may lead to the formation of mesomorphic compounds. Associated with the macroscopic display of liquid crystalline phases is the supramolecular structure, and therefore rather extended discussion of these structures are included in this review.

High Photorefractive Gain in Nematic Liquid Crystals Doped with Electron Donor and Acceptor Molecules

Abstract: Liquid crystalline composite materials have been prepared that are strongly photorefractive. Nematic liquid crystals were doped with both electron donor and electron acceptor molecules that undergo facile, photoinduced, electron transfer reactions to yield mobile ions. A photorefractive gain ratio of 1.88 was observed. This gain ratio was achieved with low applied electric fields (0.4 kilovolts per centimeter) requiring only a 1.5-volt battery and low light intensities (100 milliwatts per square centimeter) in samples 37 to 88 micrometers thick that showed no loss in gain over a 6-month period.

Novel Side-Chain Liquid Crystalline Polyester Architecture for Reversible Optical Storage

Abstract: New side-chain liquid crystalline polyesters have been prepared by melt transesterification of diphenyl tetradecanedioate and a series of mesogenic 2-[ω-[4-[(4-cyanophenyl)azo]phenoxy]alkyl]-1,3-propanediols, where the alkyl spacer is hexa-, octa-, and decamethylene in turn. The polyesters have molecular masses in the range 5000-89 000. Solution 13 C NMR spectroscopy has been employed to identify carbons of polyester repeat units and of both types of end groups. Polyester phases and phase transitions have been investigated in detail by polarizing optical microscopy and differential scanning calorimetry for the hexamethylene spacer architecture with different molecular masses. Using FTIR polarization spectroscopy, the segmental orientation in unoriented polyester films induced by argon ion laser irradiation has been followed and an irradiation-dependent order parameter for the cyanoazobenzene mesogens calculated. FTIR is also utilized to follow the temperature-dependent erasure of the induced orientation. Optical storage properties of thin unoriented polyester films are examined through measurements of polarization anisotropy and holography. A resolution of over 5000 lines/mm and diffraction efficiencies of about 40% have been achieved. Lifetimes greater than 30 months for information stored have been obtained, even though the glass transition temperatures are about 20 °C. Complete erasure of the information can be obtained by heating the films to about 80 °C, and the films can be reused many times without fatigue.

Observation, prediction and simulation of phase transitions in complex fluids

Abstract: Preface. A: Experiment. Phase Transition of Spherical Colloids W.C.K. Poon, P.N. Pusey. Liquid Crystal Phase Transitions in Dispersions of Rodlike Colloidal Particles H.N.W. Lekkerkerker, P. Buining, J. Buitenhuis, G.J. Vroege, A. Stroobants. Phase Transitions in Colloidal Suspensions of Virus Particles S. Fraden. B: Theory. Colloidal Suspensions: Density Functional Theory at Work J.-P. Hansen. Bilayer Phases M.E. Cates. Liquid Crystal Interfaces M.M. Telo Da Gama. Statistical Mechanics of Directed Polymers D.R. Nelson. C: Simulation. Introduction to Monte Carlo Simulation M.P. Allen. Numerical Techniques to Study Complex Liquids D. Frenkel. Molecular Dynamics Techniques for Complex Molecular Systems M. Sprik. Gibbs Ensemble Techniques A.Z. Panagiotopoulos. Phase Transitions in Polymeric Systems K. Binder. Simulations and Phase Behaviour of Liquid Crystals M.P. Allen. D: Seminars. Equilibrium Sedimentation Profiles of Screened Charged Colloids. A Test of the Hard-Sphere Equation of State V. Degiorgio, R. Piazza, T. Bellini. Dynamics of N-Nonadecane Chains in Urea Inclusion Compounds as Seen by Incoherent Quasielastic Neutron Scattering and Computer Simulations M. Souaille, J.C. Smith, A.-J. Dianoux, F. Guillaume. Theory of Phase Equilibria in Associating Systems: Chain and Ring Aggregates, Amphiphiles, and Liquid Crystals R.P. Sear, G. Jackson. Can a Solid be Turned into a Gas Without Passing through a First Order Phase Transition? R. Lovett. Index.

Cryo-TEM and SANS Microstructural Study of Pluronic Polymer Solutions

Abstract: Cryogenic temperature transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) were combined to give complete microstructural characterization of aqueous solutions of Pluronic F127, an (EO) 99 (PO) 65 (EO) 99 triblock copolymer. The images of vitrified specimens observed by cryo-TEM provided direct information about the building blocks of these systems, i.e., spheroidal micelles. This confirmed the structural model obtained from analysis of SANS data, which, in addition, provide quantitative information about the system. Occasionally spheroidal micelles were pressed closer to each other during specimen preparation, thus giving rise locally to higher concentration. The cubic phase that was thus formed was directly imaged. The existence of this lyotropic liquid crystalline phase was confirmed by SANS of an originally higher concentration solution. When exposed to shear, the polycrystalline phase transforms into a monodomain crystal with cubic symmetry. The system was also used to demonstrate the potential of selective electron beam radiolysis to enhance contrast in radiation-sensitive, inherently low-contrast systems.

Disclination loops, standing alone and around solid particles, in nematic liquid crystals

Abstract: A suspended particle with specific director anchoring on its surface introduces a complex distortion field in a nematic liquid crystal matrix. Topological defects---disclination loops, boojums, and hedgehogs, are needed to match the director near the particle surface with that at the far distance, which is determined by boundary conditions on the sample. This paper analyzes the elastic energy and stability of a singular loop of wedge disclination and the first-order transition of the radial hedgehog into a wide singular loop, driven by an external magnetic field. The far field of distortions, created by a ``Saturn ring'' of disclination around the spherical radial particle, allows one to calculate the potential of interaction between such particles and with the surface of the liquid crystal. Particles are repelled from each other and from the rigidly anchored surface with the potential U\ensuremath{\sim}1/${\mathit{r}}^{3}$. If the sample surface has soft anchoring, the particle is attracted to it at close distances and is repelled, if beyond the anchoring coherence length ${\ensuremath{\xi}}_{\mathit{w}}$. Several experiments to test these conclusions are suggested.

Photo-Generation of Linearly Polymerized Liquid Crystal Aligning Layers Comprising Novel, Integrated Optically Patterned Retarders and Color Filters

Abstract: For the first time photo cross-linking of linearly polymerizable polymers (LPPs) is shown to induce uniaxial planar alignment in adjacent liquid crystal polymer (LCP)-layers on single substrates. Ways and novel materials allowing integration of LPP-aligning layers with optical retarders in patterned, hybrid LPP-LCP-configurations with freely adjustable optical axes are presented. The novel multifunctional, anisotropic photopolymer configurations are shown to render in-situ optical retarders and polarization interference filters for black-white and color liquid crystal displays (LCDs) feasible. The molecular mechanisms inducing the anisotropic film properties and their thermal and optical stabilities are outlined. The photo-patternable, high resolution hybrid configurations are shown to exhibit excellent thermal and light stability.

An electro‐optically controlled liquid crystal diffraction grating

Abstract: A structure for an electro‐optically controlled liquid crystal diffraction grating is proposed, which can dramatically simplify the fabrication process of liquid crystal optical gratings. The structure consists of two alternating stripes. Each stripe is a hybrid liquid crystal cell with adjacent stripes oriented perpendicularly. This kind of electro‐optically controlled diffraction grating in principle gives 100% diffraction efficiency and no polarization direction dependence. The detailed fabrication process is presented.

Zigzag Morphology of a Poly(styrene-b-hexyl isocyanate) Rod-Coil Block Copolymer

Abstract: The solid state morphology of an anionically synthesized P(S-b-HIC) rod-coil block copolymer was studied using a number of techniques and casting solvents. Liquid crystalline ordering was seen in concentrated solutions using optical microscopy (OM). Bulk and thin film samples cast from solutions in toluene and studied using transmission electron microscopy (TEM) revealed a new zigzag morphology. Electron diffraction (ED) experiments were able to show that the PHIC rods were tilted with respect to the interface separating the PS and PHIC domains. In addition, the PHIC rods were found to be highly crystalline, having an 8 3 or 8 5 helical conformation and packing in a two chain monoclinic or triclinic (pseudohexagonal) unit cell with a = b = 15.1 A, c = 15.6 A, γ = 120°, and a crystal density of 1.10 g/cm 3 . A model for the zigzag morphology which allows interdigitation of the rods is consistent with TEM and ED results as well as domain spacing predictions based on molecular weight information. The formation of such a morphology is also consistent with thermodynamic arguments based on a theory developed by Halperin for rod-coil block copolymers if, in addition, quantization of the allowed tilt angle by crystallization is taken into account. Solvent quality was found to profoundly affect the morphology formed from solution cast samples. In addition to the zigzag morphology, morphologies consisting of fragmented PS zigzags and micelle-like regions were also observed. The choice of solvent most likely determines what phases macrophase separate from the isotropic solution before microphase separation of the rod-coil and crystallization of the PHIC take place and also whether chain stretching or interfacial energy is more dominant in the thermodynamics of microphase separation.

Solidified liquid crystals of cellulose with optically variable properties

Abstract: Solid films with novel optical properties are produced from colloidal suspensions of cellulose crystallites; the colloidal suspensions are prepared by acid hydrolysis of crystalline cellulose under carefully controlled conditions; the solid materials possess a helicoidal arrangement of the constituent crystallites; by appropriate selection of conditions for preparation and treatment of the colloidal suspensions, solid films are produced that reflect circularly polarized visible light; the wavelength of the reflected light can be controlled to give colours across the visible spectrum from red to violet, and if necessary to infrared and ultraviolet wavelengths. The reflected iridescent colours arise from optical interference effects, and change with the viewing angle; this makes the materials ideally suited for optical authenticating devices, since no printing or photocopying technique can reproduce this effect; furthermore, they are easily distinguishable from other optical interference devices since thay have additional optical properties.

Self-assembly of amphiphilic block copolymers: The (EO)13(PO)30(EO)13-water-p-xylene system

Abstract: The self-assembly of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymer (Pluronic L64, (EO) 13 (PO) 30 (EO) 13 ) in the presence of water and p-xylene was investigated. The phase boundaries were identified using 2 H NMR of heavy water ( 2 H 2 O) and inspection under polarized light. Small-angle X-ray scattering was employed to ascertain the structure of the various liquid crystalline phases formed and to determine the structural lengths involved. A rich phase behavior with normal hexagonal, lamellar, bicontinuous cubic, and reverse hexagonal liquid crystalline regions, in addition to three separate liquid phases, was observed at 25 °C. The cubic phase was identified as having a structure associated with the Gyroid minimal surface. A very small liquid region, found between the normal hexagonal and lamellar phases on the binary water-polymer axis, is identified as a melted analogue of a bicontinuous cubic phase often present in this part of the phase diagram. The pure polymer exists at 25 °C as a disordered melt. Structure and segregation are induced by the addition of water. p-Xylene is soluble in the polymer melt but does not induce structure when added alone. For polymer concentrations above ∼50%, a sequence of liquid crystalline phases is observed when the water-to-oil ratio is varied. Here we can identify the volume fraction of apolar components, given by the sum of the PPO and oil volume fractions, as the major parameter governing the phase behavior.

Bragg reflection from cholesteric liquid crystals.

Abstract: In this paper we discuss the optical reflective properties of single and multidomain cholesteric liquid crystals both experimentally and theoretically. The multidomain system has been prepared by dispersing a low concentration of polymer in a cholesteric liquid crystal. This results in a (passive) bistable colorful reflective display. Here we discuss the role of the polymer in altering the reflective properties in regard to their spectra and viewing characteristics. Theoretically we offer an application of the well known Berreman method [J. Opt. Soc. Am. 62, 502 (1972)] suitable for systems composed of an ensemble of uncorrelated domains, each of which is composed of the same dielectric anisotropy, however with its own local orientation. Using this technique the reflective properties of a cholesteric liquid crystal possessing a distribution in the orientation of the helix axes are simulated. We furthermore illustrate how a small fluctuation in the pitch from one domain to another significantly reduces any interference fringes. From these simulations we will show how experimentally by using a polymer network one may control the extent to which the orientation of the helix axes are distributed.

Liquid crystal display panel and method and device for manufacturing the same

Abstract: A mixed material of liquid crystals and resin is dropped on at least one substrate in an amount greater than the amount needed to cover a display area for the LCD panel. The substrate is adhered to another substrate, and excess material is removed to the outside of a display area. The phase-separation of the liquid crystals and resin is carried out by irradiating light while pressure is added to at least one substrate, so that a liquid crystal display panel applied to a liquid crystal display device or a light shutter can be manufactured without applying a complex vacuum device.

Alignment tensor versus director: Description of defects in nematic liquid crystals.

Abstract: The core structure of nematic defects cannot be represented by the usual director field, but requires the description by the full orial calculus.

Holographic grating formation in dye- and fullerene C(60)-doped nematic liquid-crystal film.

Abstract: Transient and persistent holographic gratings have been observed in dye- and fullerene C60-doped nematic liquid-crystal films. The nature and time evolution of the underlying mechanisms, such as space-charge field production, flows, and dielectric- and conductivity-induced torques, and the resultant director axis reorientation and refractive-index gratings are examined.

Optical compensatory sheet and liquid crystal display

Abstract: Disclosed is an optical compensatory sheet comprising a transparent support and an optically anisotropic layer provided thereon and having an optic axis inclined at 20 to 34 degrees from the normal of the sheet, wherein the optically anisotropic layer is formed from a discotic compound capable of forming a discotic nematic phase and an organic compound which has compatibility with the discotic compound and is capable of lowering a transition temperature of the discotic nematic phase by at least 10°C. Further, a liquid crystal display provided with the optical compensatory sheet is disclosed.

Chiroptical switching between liquid crystalline phases

Abstract: The modulation of the mesophases and physical properties of liquid crystalline (LC) materials by reversible photochemical reactions, i.e., photoswitching of LC phases, is a major challenge for information technology. The photochromic behavior of organic molecules has also attracted renewed interest with a view to the development of materials for optical data storage and molecular optical devices. It is well-known that small amounts of optically active guest molecules added to a nematic host induce a cholesteric phase and that the helical pitch in the twisted nematic phase is very sensitive to chemical modifications in the chiral guest molecule. Reversible switching of a liquid crystal between a twisted and a nontwisted phase by photochemical modification of the chirality of the doping molecule, although predicted, has thus far not been realized. In their pioneering efforts to develop a chiral liquid crystalline phototrigger, Schuster and co-workers 6 showed that photoracemization of optically active binaphthyl derivatives induces a cholesteric to nematic transition in a doped phase. We describe herein the first successful demonstration of the reversible photochemical modulation of the helical pitch of a twisted nematic LC phase and the reversible transition between cholesteric and nematic phases using as dopant an optically active photoresponsive molecule. The chiroptical molecular switch employed is based on the donor-acceptor substituted inherently dissymmetric alkenes cisand trans-2-nitro-7-(dimethylamino)-9-(2 ,3 -dihydro-1 H-naphtho[2,1-b]thiopyran1 -ylidene)-9H-thioxanthene (1 and 2) (Scheme 1). Addition of 1 wt % of enantiomerically pure P-trans-2 to the nematic phase, obtained from 4 -(pentyloxy)-4-biphenylcarbonitrile 3,

Nematic homogeneous photo alignment by polyimide exposure to linearly polarized uv

Abstract: Exposure of positive-type polyimide (PI) to linearly polarized deep UV (DUV) light (257nm) induced homogeneous alignment of nematic liquid crystals. The alignment state changed markedly at a particular energy of the UV light. Measurement of the UV absorption spectrum and retardation change generated in PI films by UV irradiation showed that anisotropic alignment was caused by a new mechanism, photodepolymerization of polyimide main chains parallel to the electric field of the DUV light.We induced nematic monodomain homogeneous alignment by exposing positive-type PI to linearly polarized DUV light. The alignment direction was perpendicular to the electric field of the DUV, and its condition suddenly changed at 7J/cm2. We explained this phenomenon as resulting from a new mechanism, anisotropic photodepolymerization of PI.

LCD columnar spacers made of a hydrophilic resin and LCD orientation film having a certain surface tension or alignment capability

Abstract: A liquid crystal display using a hydrophobic material as the material of resinous columns to orient the molecules of the liquid crystal parallel to the substrates. A hydrophilic material can also be used as the material of resinous columns to orient the molecules of the liquid crystal vertical to the substrates. The liquid crystal molecules are oriented horizontally by making the surface tension of the orientation film greater than the surface tension of the resinous columns. The liquid crystal molecules are oriented vertically by making the surface tension of the orientation film smaller than the surface tension of the resinous columns. A mixture of a liquid crystal material and an uncured resin is placed in a liquid crystal cell. The uncured resin is precipitated (deposited) out of the mixture. The molecules of the liquid crystal material are oriented. Then, the uncured resin is cured. Thereafter, an aging step for reorienting the molecules of the liquid crystal material is carried out. This prevents disturbance of the orientation around the resinous columns and leads to an improvement in the voltage holding ratio.