1. Field of the Invention
The present invention relates to an optical alignment polymer, an alignment layer using the same, and a liquid crystal display device having the alignment layer, and more particularly, to an optical alignment polymer having excellent optical alignment performance and stability against heat or impact, an alignment layer formed using the optical alignment polymer and a liquid crystal display device having the alignment layer.
2. The Related Art
In general, a liquid crystal display device has upper and lower substrate, transparent electrodes and alignment layers formed on the upper and lower substrates and a liquid crystal layer between the upper and lower alignment layers.
In the LCD having the aforementioned structure, according to an externally applied voltage, the arrangement of liquid crystals is changed by the electric field. According to the changed arrangement, external light introduced to the LCD is shielded or transmitted. The LCD is driven by such a property. In other words, if a voltage is applied to the transparent electrode layers, an electric field is formed in the liquid crystal layer. Thus, liquid crystals are driven in a predetermined direction. The light introduced into the liquid crystals of the LCD is shielded or transmitted according to the driving of the liquid crystals.
The functions of the LCD as a display device, i.e., light transmittance, response time, view angle or contrast, are determined by the arrangement characteristic of the liquid crystal molecules. Therefore, controlling the alignment of the liquid crystal molecules uniformly is a very important factor.
The uniform alignment state of the liquid crystals is difficult to accomplish by merely interposing the liquid crystals between the upper and lower substrates. Thus, it is generally known to form the alignment layers for aligning liquid crystals on the transparent electrode layers.
The alignment layer is conventionally formed by a rubbing method in which a thin film made of an organic polymer material such as polyimide or polyamide is formed, cured and then rubbed with a special cloth.
The rubbing method is easy to conduct and the process thereof is simple. However, minute particles or materials such as cellulose may separate from the cloth used in the rubbing treatment and contaminate the alignment layer. Further, depending on the material for forming the alignment layer, the alignment may not be accomplished smoothly. A thin film transistor may be damaged by static electricity generated during the rubbing treatment.
To solve the above-described problems, an optical alignment technology has been developed in which dust, static electricity or other pollutant particles are not generated and cleanliness is maintained during the overall process. According to such a non-destructive alignment method, polarized light is irradiated onto the optical alignment layer to cause anisotropic photopolymerization. As a result, the optical alignment layer has alignment characteristic, thereby uniformly aligning the liquid crystals. The polymer for the optical alignment layer includes polyvinylcinnamate (PVCN) and polyvinylmethoxyxinnamate (PVMC). These polymer compounds have an optically aligning functional group introduced into either a main chain or a side chain thereof.
If the alignment layer is formed using the optical alignment composition having such a polymer, the alignment performance by light and thermal stability becomes poor, and a pretilt angle of a liquid crystal of liquid crystal display device having the alignment layer decreases to almost 0.degree. C.