Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
  • G02F1/133711—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by organic films, e.g. polymeric films
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/1333—Constructional arrangements; Manufacturing methods
  • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
  • G02F1/13378—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by treatment of the surface, e.g. embossing, rubbing or light irradiation
  • G02F1/133788—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by treatment of the surface, e.g. embossing, rubbing or light irradiation by light irradiation, e.g. linearly polarised light photo-polymerisation

Definitions

• This invention relates to materials and methods for achieving alignment of liquid crystal materials on a solid surface, and to devices made using these methods.
• Liquid crystal devices commonly comprise a layer of a liquid crystal material contained between two cell walls. These walls carry electrode structures for applying an electric field across the layer so that the layer may be switched between different molecular arrangements, e.g. between light transmitting and light blocking states. Many devices carry surface mounted structures such as thin film transistors; these are used in active matrix displays. Most devices also have a surface alignment treatment on the cell walls which impart a preferred molecular alignment direction to contacting liquid crystal molecules.
• An object of this invention is to provide means of achieving a defined surface alignment of a liquid crystal material on a surface, which does not require mechanical rubbing or other physical contact which may damage the surface or structures on it.
• a further objective of the invention is to provide means by which the pretilt angle and surface anchoring energy may be altered without the need for mechanical rubbing or contact.
• Homeotropic alignment refers to an alignment in which the unique optical axis of a liquid crystal phase, especially a nematic liquid crystal phase, is held perpendicular to the adjacent surface.
• Planar alignment sometimes referred to as homogeneous alignment, refers to alignment in which the unique optic axis of the liquid crystal phase lies parallel to the adjacent surface. Planar alignment may also impose a direction in which the optic axis of the liquid crystal lies, in the plane of the adjacent surface.
• Tilted planar alignment or tilted homogenous alignment refer to alignment in which the liquid crystal unique optic axis lies at an angle, termed the pretilt angle from the plane of the adjacent surface.
• the pretilt angle may be as small as a fraction of one degree, or as large as several tens of degrees.
• Tilted homeotropic alignment refers to an alignment in which the optic axis of the liquid crystal lies tilted away from the normal to the adjacent surface. This deviation is again termed a pretilt angle.
• said alignment geometries are chosen and used in combination to achieve specific optical and electro-optic properties from the device and may be combined in new ways or with new liquid crystalline fluids to provide new types of devices.
• a disadvantage of this method is that the mechanical rubbing process may cause damage to structures on the surface, for example to the elements of an active switching matrix fabricated on the surface.
• a further disadvantage is that a static electric charge may be generated during the rubbing process, and may damage the surface or structures on the surface or connected to it.
• a planar alignment or tilted planar alignment may also be achieved by evaporating a variety of inorganic substances onto the surface from an oblique angle if incidence.
• a disadvantage of this method is that it requires slow and costly vacuum processing.
• the resulting evaporated layer may show a high capacity to absorb contaminants onto itself from the environment or from other materials used in construction of the device. These contaminants may adversely affect the operation of the device.
• a homeotropic alignment can be obtained by depositing a surfactant, for example, a quaternary ammonium salt onto the surface from solution in a suitable solvent.
• a surfactant for example, a quaternary ammonium salt
• a disadvantage of this treatment is that the resistivity of the liquid crystal device may be lowered by the surfactant, and the resulting alignment may also show poor stability.
• a photoactive polymer or polymer mixture comprising or containing at least one polymer of structure
• BP in which C represents a carbon atom
• A is selected from H, Cl, F, CN, CO 2 R, OCOR, OR, where R is a straight chain or branched alkyl group having 1 to 15 carbon atoms, X and Y are independently selected from the same groups as A, subject to at least one of X and Y on each C atom being selected from F and H B is a linking group comprising a single bond, or a chain of from 1 to 15 methylene groups in which one or more non-adjacent methylenes may be replaced by O, C0 2 , OCO,
• P is a photoactive group selected from cinnamate or anthracene groups substituted with at least one group selected from F, Cl, CN, CF 3 , OCF 3l Br.
• the total interfacial energy of the polymer surface is within 3.0 erg per square centimetre of the liquid crystal surface energy, and in the range 35 to 50 ergs per square centimetre.
• n is conventionally known as the number of repeat units and may typically be in the range 4-1000. Preferably n is in the range 20-300. a is 1.
• the polymer used may be a single polymer or a mixture of polymers.
• one or more polymers of structure 1 may be mixed together or may be mixed with other photoactive polymers in order to adjust the interfacial energy of the polymer surface.
• the types of polymer included in the current invention may be any of the known types of polymer including homo and co polymers.
• the interfacial energy of the surface may be measured by examining the contact angles made between the surface and reference liquids, according to the method described by Shohei Naemura, PhD Thesis, University of Kyoto, 1982.
• Suitable polymers include derivatives of poly(vinyl cinnamate), poly(2- cinnamoyloxyethylacrylate) and poly(2-(anthracene-5-carboxy)ethyl) acrylate.
• a method of providing an alignment layer on a surface of a liquid crystal cell wall includes the step of depositing a layer of a polymer containing at least one polymer of structure I on the surface, followed by exposure to actinic light, and controlling the exposure time and/or intensity of light used to provide a selected value of pretilt in a liquid crystal placed in contact with the exposed layer.
• a liquid crystal device comprises a layer of a liquid crystal material contained between two cell walls both carrying electrodes structures and surface treated to provide an alignment to liquid crystal molecules;
• the surface treatment is a layer of the polymer of structure 1 and exposed to actinic light.
• Figure 1 is a plan view of a liquid crystal device
• Figure 2 is a cross sectional view of Figure 1.
• the device of Figures 1 , 2 comprises a liquid crystal cell 1 formed by two cell walls 2, 3 spaced typically 1 to 15 ⁇ m apart by a spacer ring 4 to contain a layer 5 of a liquid crystal material.
• the inside faces of both walls 2, 3 are coated with a indium tin oxide layer 6, 7 forming transparent electrodes.
• the electrodes may be of sheet like form covering the complete wall, or formed into e.g. strip electrodes to provide an array of addressable electrode intersections.
• the walls are also coated with an aligning layer 8, 9 of polymer. These layers provide both an alignment direction, indicated by A, and A b as orthogonally arranged. If the material 5 is nematic then the device may be the known twisted nematic device. In this case polarisers 10, 11 are used to distinguish between the device voltage ON and OFF states.
• the liquid crystal material may be nematic, cholesteric, or smectic material.
• the device may be used as a display device, e.g. displaying alpha numeric information, or an x,y matrix displaying information.
• the device may operate as a shutter to modulate light transmission, e.g. as a spatial light modulator, or as a privacy window.
• the alignment layer may be produced as described, by way of example only, in the following examples:
• Poly(vinyl 4-chlorocinnamate) was prepared by reaction of 4-chloroacryloyl chloride with poly(vinyl alcohol) in pyridine at room temperature overnight. The product was recovered by removal of the pyridine on a rotary evaporator, washed with water, and purified by successive dissolution in dichloromethane followed by precipitation from ethyl alcohol until the colour was pale yellow.
• a solution of poly(vinyl 4-chlorocinnamate) was dissolved in propylene glycol methyl ether acetate at a concentration of 6%.
• the solution was deposited by spinning onto an indium tin oxide coated glass substrate at 3000rpm for 6 seconds.
• Samples of the substrate were exposed to linearly polarised light from a helium cadmium laser source providing radiation at a wavelength of 325nm. After exposure, the samples were assembled into cells and these were filled with liquid crystal mixture E7 (available from Merck).
• the tilt angle in each cell was measured by determining the symmetry point of the optical interference pattern which resulted from rotating the cell in a beam of light of wavelength 633nm, between crossed polarising filters. The results obtained are tabulated below:
• a liquid crystal cell was constructed having one wall surface coated with the alignment polymer of example 3, exposed to 0 5J/m 2 of 325nm linear polarised light, liquid crystal molecules align perpendicular to the polarisation direction
• the other wall surface was coated with Probimide 32, a polyimide commercially available from Ciba-Geigy chemicals, and mechanically rubbed, liquid crystal molecules align along the rubbing direction
• the cell was assembled with the alignment direction on the two walls mutually perpendicular The walls were spaced apart using plastic bead spacers by a distance of 6 ⁇ m
• the cell was filled with the commercially available liquid crystal mixture ZLI-2293 doped with 0 8% S-811 , both materials obtained from E Merck, Darmstadt The resulting electro-optic response showed a steep switching characteristic with no intrusion of an undesirable stripe characteristic

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Mathematical Physics (AREA)
• Chemical & Material Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Liquid Crystal (AREA)

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• 1996
  • 1996-04-03 GB GBGB9607059.4A patent/GB9607059D0/en active Pending
• 1997
  • 1997-03-19 JP JP9535937A patent/JPH11508063A/ja active Pending
  • 1997-03-19 EP EP97907225A patent/EP0830634A1/de not_active Withdrawn
  • 1997-03-19 WO PCT/GB1997/000745 patent/WO1997038349A1/en not_active Application Discontinuation

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