Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – Alignment layer of specified composition
Reexamination Certificate
2001-02-21
2004-10-26
Huff, Mark F. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
Alignment layer of specified composition
C428S001200, C428S001260
Reexamination Certificate
active
06808766
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a liquid crystal alignment agent, a liquid crystal device produced by using the liquid crystal alignment agent thereof, and a method for alignment of liquid crystal molecules by using the liquid crystal alignment agent. In more detail, the present invention relates to a novel liquid crystal alignment agent used in a method of aligning liquid crystal molecules comprising irradiating the surface of a thin polymer film with light and aligning the liquid crystal molecules without any rubbing treatment, a liquid crystal device and a method for alignment of liquid crystal molecules by using the liquid crystal alignment agent. The liquid crystal alignment agent comprising a polymer having a specific unit structure has characteristics such that the resulting liquid crystal alignment films realize high sensitivity, high heat stability and high light resistance compared to the prior art.
BACKGROUND ART
Liquid crystal display devices are display devices utilizing electrooptical changes of liquid crystal. Attention has been drawn to their features such that the devices are small in size and light in weight, and power consumption is small. Accordingly, in recent years, they have undergone remarkable developments as display devices for various displays. Among them, an electric field effect type (TN type) is representative, wherein nematic liquid crystal having a positive dielectric anisotropy is used, liquid crystal molecules are aligned in parallel with substrates at the respective interfaces of a pair of mutually opposing electrode substrates, and the two substrates are combined so that the alignment directions of liquid crystal molecules will cross each other.
With such a TN type liquid crystal display device, it is important to align the long axial directions of liquid crystal molecules uniformly in parallel with the substrate surface and to align the liquid crystal molecules with a constant tilt alignment angle (hereinafter referred to as a pre-tilt angle) to the substrate.
As typical methods for aligning liquid crystal molecules in such a manner, two methods have heretofore been known. The first method is a method wherein an inorganic substance such as silicon oxide is vapor-deposited from an oblique direction to the substrate to form an inorganic film on the substrate, so that the liquid crystal molecules will be aligned in the direction of vapor-deposition. This method is not industrially efficient, although constant alignment with a predetermined tilt angle can be obtained.
The second method is a method wherein an organic coating film is formed on the substrate surface, and the film surface is rubbed in a predetermined direction with a cloth, for example of nylon or polyester, so that the liquid crystal molecules are aligned in the rubbing direction. An organic coating film (called liquid crystal alignment film or alignment film) is usually formed by coating the liquid crystal alignment agent over the surface of a substrate. By this method, constant alignment can be obtained relatively easily, and this method is industrially most commonly employed. As the organic film, polyvinyl alcohol, polyoxyethylene, polyamide or polyimide may, for example, be mentioned. However, polyimide is most commonly employed from the viewpoint of the mechanical strength, chemical stability, thermal stability, etc. As typical examples of polyimide used for such liquid crystal alignment films, those disclosed in JP-A 61-47932 may be mentioned.
The treating method for liquid crystal alignment by rubbing polyimide film is an industrially useful method that is simple and excellent in productivity. The demands for high precision and high performance of liquid crystal display devices have increased and new display systems corresponding to such demands have been developed. For example, a STN (Super Twisted Nematic) system wherein the twist angle of a TN type liquid crystal display is increased, an AM (Active Matrix) system wherein switching elements are formed for individual electrodes, and a FLC (ferroelectric) or AFLC (antiferroelectric) system wherein ferroelectric liquid crystal or antiferroelectric liquid crystal is employed, may be mentioned as such examples. However, various problems of the rubbing method have been pointed out. In the STN system, contrast is high and scratches on the alignment film surface formed by rubbing become display defects. In the FLC or AFLC system, it is difficult to satisfy both high speed response and uniform alignment of smectic liquid crystal only by simple rubbing treatment. In the AM system, the mechanical force or static electricity due to rubbing is likely to destroy the switching elements, and dusting by rubbing tends to lead to display defects. Since the AM system in particular drives liquid crystals with semiconductor devices such as TFT (thin film transistor) and basically requires absolute cleanliness in its semiconductor technology, the a process such as rubbing is not strictly speaking the best method in practical industrial production.
For the purpose of solving such problems, a so-called “rubbing-less” alignment method, wherein liquid crystals are aligned without rubbing, has been studied and various methods have been proposed. For example, a method wherein photochromic molecules are introduced to the alignment film surface so that molecules on the alignment film surface are aligned by light (JP-A-4-2844), a method wherein molecular chains constituting an alignment film are aligned by means of a LB film (Langmuir-Blodgett film) (S. Kobayashi et al, Jpn. J. Appl. Phys., 27,475 (1988)), and a method wherein an alignment film is press-bonded on a preliminary alignment-treated substrate to transfer the alignment (JP-A-6-43458) have been studied. However, when industrial productivity is taken into account, these methods can not be said to be useful as substitutes for the rubbing method.
Various methods have been proposed wherein periodic irregularities are artificially formed on the alignment film surface and liquid crystal molecules are aligned along such irregularities. The most simple method of this type is a method wherein a replica having periodic irregularities is preliminary prepared and a thermoplastic film is heated and pressed thereon to transfer the irregularities onto the film (JP-A-4-172320, JP-A-4-296820, JP-A-4-311926 etc.). By this method, it is certainly possible to prepare a film having periodic irregularities on its surface efficiently, but it has been impossible to obtain practical reliability as high as a polyimide film used in the rubbing method. A method having high reliability has been proposed in which a light with high energy, such as electron rays (JP-A4-97139), &agr;-rays (JP-A-2-19836), X-rays (JP-A-2-2515) or eximer laser (JP-A-5-53513), is applied to a polyimide film to form periodic, irregularities on the film surface. However, to use a light source of such high energy can not hardly be said to be an efficient treating method for alignment when industrial production, where the alignment treatment is continuously carried out uniformly over the entire surface of a large size substrate, is taken into consideration.
On the other hand, as an efficient method for forming periodic irregularities on the surface of a polyimide film having high reliability, a photolithographic method may be mentioned. The polyimide is used as an insulating film for semiconductors by virtue of its high insulating property and excellent electrical characteristics. In recent years, a so-called photosensitive polyimide has been developed which is a polyimide having a photocurable nature by itself. There has been an attempt to form periodic irregularities by a photolithographic method using this photocurable polyimide. By this method, irregularities have certainly been formed on the surface of the polyimide film, but since the photocurable polyimide was initially developed as an insulating film, the properties to align liquid crystals have been inadequate. Further, it has been necessary to apply a buffer coating layer
Akiyama Eiich
Endo Hideyuki
Fukuro Hiroyoshi
Miyama Yukihiro
Nagase Yu
Huff Mark F.
Nissan Chemical Industries Ltd.
Oliff & Berridg,e PLC
Sadula Jennifer R.
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