Liquid crystal display device comprising an orientation layer

Abstract: An orientation layer for a liquid crystal display device is formed from a mixture of at least one reactive liquid-crystalline material and at least one non-reactive liquid-crystalline material. The mixture is brought into contact with a surface of a mould having an orienting effect, as a result of which the molecules in the mixture are oriented. Subsequently, the reactive liquid-crystalline material is made to cure, thereby forming an oriented polymer network in which the non-reactive liquid-crystalline material is dispersed. The orientation layer thus manufactured is very suitable for use in an active display device.

Description: The invention relates to a method of manufacturing a liquid crystal display device comprising an orientation layer, said method comprising forming said orientation layer in contact with the surface of a mould, and subsequently removing said mould. The invention also relates to a liquid crystal display device comprising an orientation layer and switchable liquid-crystalline material. A liquid crystal display device comprises a cell with switchable liquid-crystalline material between two flat substrates on which electric conductor tracks may be present which can be used to switch the liquid-crystalline material between an "on" and an "off" state in accordance with a desired pattern. To this end, one or both substrates of a so-called active display device may additionally be provided with switching elements such as transistors. Orientation layers are frequently applied between a substrate having conductor tracks and the switchable liquid-crystalline material to provide the molecules of the liquid-crystalline material with a parallel (homogeneous), perpendicular (homeotropic) or oblique (tilted) preferred orientation. It is possible to provide an orientation layer on one or on both sides of the cell, i.e. on one or on both substrates. Polymer layers of, for example, polyethylene or polyimide are often used for this purpose, which layers are rubbed in one direction, for example, by means of a cloth. This has the disadvantage, however, that underlying electric elements such as wiring or transistors may be damaged, for example, mechanically or by a static charge. Another type of orientation layer consists of a layer of silicon oxide which is provided at an angle by vacuum evaporation, but such layers cannot readily be manufactured on a large scale. An alternative orientation layer for a liquid crystal display device is described in an abstract relating to the published Japanese Patent Application JP 1-94318, in Patent Abstracts of Japan, Volume 13, Number 329 (P-904) on page 122 (1989). In said abstract the orientation layers are formed in contact with the surface of a mould which is subsequently removed. The surface of the mould contacting the orientation layers to be formed is provided with a saw-tooth structure which is transferred to the surface of the orientation layers. Said orientation layers, which consist of polyimide, are rubbed in the desired directions after which the cell is filled with ferroelectric liquid-crystalline material and spacers in a manner which is known per se, and then sealed. A disadvantage of said known method resides in the necessity of using a mould having an accurately mechanically machined surface. Said method can only be used to obtain an orientation having a small angle of inclination. Further, an unsmooth intermediate face in the display device is undesirable because of the effects it has on the optical properties. An other alternative orientation layer for a liquid crystal display is described in JP-A-62/245.218. This orientation layer is prepared by mixing and dispersing liquid crystalline molecules in a high polymer resin, arranging them in one direction with a magnetic or electric field and fixing them. No mould is used in this prior art method. It is an object of the invention to provide, inter alia, a simple method of manufacturing an orientation layer in a liquid crystal display device. The invention particularly aims at providing a method which can suitably be used for the manufacture of an active display device. It is an object of the invention to provide a liquid crystal display device comprising an orientation layer having a very effective orienting effect. The invention further aims at providing a display device having a flat and smooth orientation layer. According to the invention these objects are achieved by a method as described in the opening paragraph, characterised by the surface of the mould contacting the orientation layer to be formed being a flat surface that has been treated so as to provide to the orientation layer an orienting effect, by forming the orientation layer from a mixture of at least one polymerisable liquid-crystalline material and at least one non-polymerisable liquid-crystalline material, and by curing the polymerisable liquid-crystalline material in the oriented state. In a preferred embodiment of the method according to the invention the surface of the mould having an orienting effect consists of a rubbed polymer layer. Polymer layers which are customarily used for this purpose, such as layers of polyethylene or polyimide, can suitably be used, which layers are rubbed in one direction, for example, with a non-fluff cotton cloth before they are used. To fix the orientation of the molecules in the orientation layer in an effective and rapid manner it is advantageous, according to the invention, to cure the polymerisable liquid-crystalline material by means of actinic radiation. Actinic radiation is to be understood to mean herein, for example, radiation with UV light, electrons, X-rays, gamma rays or high-energy particles. In the method according to the invention, the orientation layer is preferably cured by UV light in order to prevent the underlying electric elements from being damaged. when UV light is used, in general, a small quantity of a light-sensitive initiator will be added to the reactive material according to a method which is known per se. In a very efficacious embodiment of the method according to the invention, the polymerisable liquid-crystalline material is cured, thereby forming a polymer network in which the non-polymerisable liquid-crystalline material is dispersed. A network can be manufactured by applying a monomer polymerisable material having at least two polymerisable groups per molecule. By virtue of the use of a network a stable material is obtained the properties of which are only slightly governed by, for example, temperature. By employing a network, it suffices to use a proportionately small quantity of polymerisable material relative to the quantity of non-polymerisable material. The choice of the non-polymerisable material enables the properties of the material of the orientation layer to be adapted to the requirements within wide limits. According to the invention, the object of providing a liquid crystal display device comprising an orientation layer and switchable liquid-crystalline material is achieved by a display device in which the orientation layer consists of a polymer which molecules are oriented in a network in which a non-polymerisable liquid-crystalline material is dispersed. An anisotropic gel consisting of a network of uniaxially oriented polymer and a liquid-crystalline material dispersed therein is described per se in the non-prepublished Netherlands Patent Application NL 9000808 which is filed by Applicants. A very strong interaction between the orientation layer and the switchable liquid-crystalline material and, consequently, a very effective orienting effect is obtained when the non-polymerisable liquid-crystalline material in the orientation layer has the same composition as the switchable liquid-crystalline material. The invention can be used for the manufacture of a passive display device in which electric conductor tracks, for example of transparent indium oxide/tin oxide, may be situated on the substrate. The invention can very suitably be used for the manufacture of an active display device in which the orientation layer is provided on a substrate comprising electric switching elements. Such switching elements are constructed, in known manner, as semiconductor switches comprising, inter alia, transistors. The invention will be explained in greater detail by means of an exemplary embodiment and a drawing, in which Fig. 1 diagrammatically shows a liquid crystal display device according to the invention, Fig. 2 shows the structural formula of a polymerisable liquid-crystalline material, and Fig. 3 shows the structural formulae of the components of a mixture of non-polymerisable liquid-crystalline materials. Exemplary embodiment. A liquid crystal display device was manufactured from two flat substrates 1,2 of glass which were provided with electric conductor tracks 3,4 of indium oxide/tin oxide according to a desired pattern, see Fig. 1. If desired, the substrates may be provided with switching elements such as transistors which are constructed as, for example, thin layers of amorphous silicon. A mixture of liquid-crystalline materials was manufactured from 10% by weight of a polymerisable liquid-crystalline material, 88% by weight of a non-polymerisable liquid-crystalline material and 2% by weight of the light-sensitive initiator 2,2-dimethoxy-2-phenyl-aceto phenone. The diacrylate compound C6M, the structural formula of which is shown in Fig. 2, was used as the polymerisable liquid-crystalline material. The mixture E7 from BDH Chemicals Limited, the composition of which in % by weight is shown in Fig. 3, was used as the non-polymerisable liquid-crystalline material. The mixture was provided on the substrates 1, 2 in the form of thin layers having a thickness of 10 µm and brought into contact with the surface of rubbed polyethylene foils. As a result thereof, the molecules of the mixture were oriented in the direction in which the foils were rubbed. The diacrylate compound C6M was made to cure by exposing it to UV light, thereby forming the orientation layers 5, 6. The intensity of the UV light was 0.2 mW/cm 2 at the location of the layers 5, 6. By means of the above-mentioned curing operation a polymer network was formed in which the non-polymerisable material was dispersed. After curing the polyethylene foils were removed. According to a method which is known per se, a cell was manufactured by locating the substrates 1 and 2, including the layers provided thereon, opposite each other and providing a switchable liquid-crystalline material, according to the example the mixture E7, between said substrates. Glass fibres 8 having a diameter of 10 µm and serving as spacers were provided between the substrates. The cell was sealed in a customary manner by applying an epoxy resin adhesive, not shown in the Figure, along the edges of the cell. It was found that the orientation layers 5 and 6 very effectively induce the desired orientation in the switchable liquid-crystalline material 7. The desired angle of inclination can be obtained within wide limits by adapting the composition of the orientation layers, in particular by the choice of the non-polymerisable liquid-crystalline material. In addition, the orientation layers have favourable optical properties such as transparency and flatness.

Claims: A method of manufacturing a liquid crystal display device comprising an orientation layer, said method comprising forming said orientation layer in contact with the surface of a mould and subsequently removing said mould, characterized by-the surface of the mould contacting the orientation layer to be formed being a flat surface that has been treated so as to provide to the orientation layer an orienting effect, by forming the orientation layer from a mixture of at least one polymerisable liquid-crystalline material and at least one non-polymerisable liquid-crystalline material, and by curing the polymerisable liquid-crystalline material in the oriented state. A method as claimed in Claim 1, characterized in that the surface of the mould having an orienting effect consists of a rubbed polymer layer. A method as claimed in Claim 1, characterized in that the polymerisable liquid-crystalline material is made to cure by means of actinic radiation. A method as claimed in Claim 3, characterized in that when the polymerisable liquid-crystalline material is cured it forms a polymer network in which the non-polymerisable liquid-crystalline material is dispersed. A liquid crystal display device comprising an orientation layer and switchable liquid-crystalline material, characterized in that the orientation layer consists of a polymer whose molecules are oriented in a network in which a non-polymerisable liquid-crystalline material is dispersed. A liquid crystal display device as claimed in Claim 5, characterized in that the non-polymerisable liquid-crystalline material in the orientation layer has the same composition as the switchable liquid-crystalline material. A liquid crystal display device as claimed in Claim 5 or 6, characterized in that the orientation layer is provided on a substrate comprising electric switching elements.

Inventors: HIKMET RIFAT ATA MUSTAFA, HIKMET, RIFAT ATA MUSTAFA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: G02F1
IPC codes: G02F1