Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2001-11-19
2003-06-24
Beck, Shrive P. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S508000, C427S553000, C427S558000, C427S595000, C427S162000, C427S385500, C349S124000, C349S135000, C428S001200
Reexamination Certificate
active
06582776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a photo-alignment layer used in a liquid crystal display device and, more particularly, to a method of manufacturing a photo-alignment layer, capable of aligning liquid crystal molecules by light exposure, without being subjected to a rubbing treatment.
2. Description of Related Art
In liquid crystal display devices, the state of molecular alignment of liquid crystals is changed by the action of an electric field and a change in optical characteristics attending the change is utilized for display. In general, liquid crystals are used while interposed in the space between two substrates. To align liquid crystal molecules in a specific orientation, the interior surfaces of the substrates are subjected to an alignment treatment.
The alignment treatment is usually performed by a rubbing method in which a layer made of a polymer such as a polyimide is formed on a substrate made of glass, for example, and then rubbed with a cloth in one direction. Consequently, liquid crystal molecules near the substrate are aligned so that their directors become parallel to the rubbing direction. For example, in a twist nematic (TN) cell, two substrates coated with an alignment layer on their interior surfaces face each other between two polarizers whose polarization directions are perpendicular to each other, and the substrates are further arranged so that their rubbing directions are parallel to the polarization direction of their adjacent polarizers, thereby making it possible to form a display which operates based on changes in light transmittance.
Although the rubbing method has the advantage that the manufacturing apparatus has a simple structure, a cleaning process is required after the alignment treatment because dust may adhere during the manufacturing process. Also in TFT liquid crystal cells, which have recently been widely used, TFT previously deposited on the substrate may be destroyed by static electricity which is generated during the rubbing process, thus lowering the manufacturing yield. In the liquid crystal display device, since liquid crystal molecules align in some direction, problems arise, such as viewing-angle dependence in which the display color or contrast varies depending on the viewing angle.
As one method of solving the problem, for example, Japanese Unexamined Patent Application, First Publication No. Sho 62-159119 has proposed a multi-domain method of providing different pretilt angles of the liquid crystal molecules for different regions obtained by dividing each pixel, while Japanese Unexamined Patent Application, First Publication No. Sho 63-106624 has proposed a multi-domain method of providing different alignment directions of the liquid crystal molecules for different regions obtained by dividing each pixel. Such multi-domains are not suited for rubbing alignment method because of the complicated process required.
To solve these problems, there has recently been interest in liquid crystal alignment controlling techniques which do not use rubbing. As non-rubbing alignment techniques, for example, the oblique evaporation method, the LB (Langmuir Blodgett's) layer method, photolithography methods, and photo-alignment methods have been studied. Among these methods, the photo-alignment method, in which a coating layer formed on a substrate is exposed to polarized light to provide a liquid crystal aligning property, is simple. Therefore, the photo-alignment method has been intensively studied. As the photo-alignment method, a method utilizing photoisomerization of a photo-alignment moiety (e.g., an azo group) capable of carrying out a photo-alignment function in organic molecules, a method utilizing photo dimerization of a cinnamoyl group, a coumarin group, or a chalcone group, a method utilizing photocrosslinking of a benzophenone group, and a method utilizing photodegradation of a polyimide resin have been reported.
As the material for the photo-alignment layer, which utilizes photoisomerization, photo dimerization, or photocrosslinking, a polymeric material, in which the photo-alignment moiety is introduced into the side chain or the main chain, is often used to obtain a uniform layer when coated on a substrate made of glass, for example. It is also possible to use a material prepared by using molecules having a photo-alignment property as guest molecules and dispersing them in a host compound consisting of a polymeric compound.
For example, U.S. Pat. No. 4,974,941 discloses a photo-alignment material which can align liquid crystals in a fixed direction to the polarization direction by mixing a dichroic dye with a resin such as polyimide and exposing the mixture to polarized light. However, this photo-alignment material has the problem that the alignment state obtained by exposure to polarized light easily changes by exposure to light having different directions of polarization because the photo-alignment material utilizes cis-trans isomerization of the dichroic dye and is reversible. The photo-alignment layer disclosed in said U.S. patent is unstable and is likely to change its alignment state even by exposure to natural light, and therefore, it is not suited for practical use in a liquid crystal display device. The low-molecular weight dichroic dye included in the photo-alignment layer has the problem that it gradually diffuses into the liquid crystal, and thus the characteristics (e.g., voltage holding ratio) of the liquid crystal display device are likely to be degraded, and also has the problem that the photo-alignment layer loses the liquid crystal alignment capability with the passage of time.
Japanese Unexamined Patent Application, First Publication No. Hei 8-328005 and U.S. Pat. No. 6,001,277 disclose a liquid crystal alignment layer obtained by exposing a film made of a resin having a photoisomerizable structural unit, which exhibits dichroizm, to polarized light. That is, a film made of the resin obtained by coating a resin solution having a structural unit, which exhibits dichroizm, on a substrate is exposed to polarized light, thereby causing alignment, and, furthermore, the state of alignment is fixed by a crosslinking group of the resin.
However, a layer having a structure in which the structural unit, which exhibits dichroizm, is incorporated into the resin has the problem that this structure makes reorientation of the dichroic structural unit by polarized light difficult, thereby it is difficult to obtain sufficient alignment property. Although the problems described above can be solved to some extent with respect to the layer made of a resin having a structural unit in the side chain which exhibits dichroizm, the alignment direction of the structural unit which exhibits dichroizm is not completely fixed, thereby causing the problem that the alignment state of the structural unit is gradually lost by heat and light, and thus the liquid crystal alignment capability cannot be maintained for a long period.
BRIEF SUMMARY OF THE INVENTION
An object to be achieved by the present invention is to provide a photo-alignment layer, which has excellent long-term stability to light and heat.
To achieve the object described above, the present invention provides a method of manufacturing a photo-alignment layer which comprises coating a material for the photo-alignment layer, which contains a dichroic dye having two or more polymerizable groups per molecule, on a substrate, and exposing the coating layer to polarized light, thereby imparting a photo-alignment function, and polymerizing the polymerizable groups by heating or light exposure.
According to the method of manufacturing a photo-alignment layer of the present invention, it is made possible to provide a photo-alignment layer, which has excellent photochemical long-term stability. According to the method of manufacturing a photo-alignment layer of the present invention, a uniform and stable alignment layer can be formed because the material for the photo-alignment layer is coated on the s
Chigrinov Vladimir G.
Fukuda Masanobu
Kozenkov Vladimir M.
Kwok Hoi S.
Prudnikova Elena K.
Beck Shrive P.
Hong Kong University of Science And Technology
Markham Wesley
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