JP2548376B2 - Liquid crystal display element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in image quality of a liquid crystal display device using a twisted orientation of liquid crystal between upper and lower substrates of a panel.

2. Description of the Related Art At present, the mainstream of liquid crystal display devices utilizes twisted alignment of nematic liquid crystals such as TN and STN. The display principle of these systems is to twist the liquid crystal between the upper and lower substrates provided with the electrodes, and to eliminate the twist by applying an electric field. Along with this, the light diffusivity or the birefringence of the light changes, and a display can be performed by controlling the light transmission state or the light reflection state in combination with a polarizing plate. In order to realize the twisted light distribution of the liquid crystal necessary for display,
A liquid crystal is added with a chiral substance in order to align the direction of twisting in a fixed direction while providing an alignment film which is usually subjected to a uniaxial alignment treatment by a rubbing treatment on the upper and lower substrate surfaces.

Problems to be Solved by the Invention However, when the display density is increased and the pixel size is reduced, a reverse twist region is likely to occur. When the reverse twist region occurs, a disclination line appears at the boundary with the forward twist region, which causes an alignment defect. As a result, the display contrast is lowered, and the image quality of the liquid crystal display element is lowered, which is a problem. Such defects can be improved to some extent by performing heat treatment or voltage application at a temperature higher than the isotropic phase transition temperature of the liquid crystal, but it is very difficult to completely eliminate the once-twisted reverse twist defect.

One of the causes of the occurrence of the reverse twist defect is that the rubbing process cannot be performed uniformly and sufficiently over the entire substrate surface. As the pixel size becomes smaller, the pitch of the unevenness on the substrate surface becomes finer, and the fibers of the rubbing cloth and the surface of the alignment film are inadequately contacted, resulting in insufficient rubbing. Further, it is considered that the local variation in the pretilt angle of the liquid crystal is one of the factors. Although some improvement is recognized by strongly performing the rubbing treatment, there is no significant effect. If the rubbing is strengthened, the alignment film may be damaged or the film may be peeled off. Furthermore, in an active matrix panel using a TFT, the TFT is damaged by static electricity, and electrostatic breakdown may occur in some cases.

Although a countermeasure is taken by increasing the amount of the chiral compound in the liquid crystal, an increase in the amount of the chiral compound added is not preferable because generally the characteristics of the liquid crystal are improved. Further, the effect of increasing the addition amount of the chiral substance is small with respect to the reduction of the reverse twist defect within the allowable range of the liquid crystal characteristic change.

To solve the above problems, the present invention is to solve the above problems, in a liquid crystal display element utilizing twisted alignment of liquid crystal between the upper and lower substrates of a liquid crystal panel, at least one of the alignment film provided on the upper and lower substrate surface. The surface is characterized by containing the above-mentioned chiral group formed by surface modification with a compound having a chiral group.

When the liquid crystal is aligned in a region sandwiched between the alignment films that have been subjected to the uniaxial alignment treatment of the liquid crystal by a rubbing process or the like, the liquid crystal near the interface between the alignment film and the alignment film is first uniaxially aligned due to the interaction between the alignment film surface and the liquid crystal. It is considered that this orientation propagates in the liquid crystal bulk. When the liquid crystal is twisted and aligned by changing the direction of the uniaxial alignment treatment between the upper and lower substrates, the twist direction cannot be defined unless a chiral substance is present.
If there is no chiral substance near the alignment film surface,
From there a reverse twist region can develop and grow. It is considered that when the reverse twist region grows to a certain size, the region is present stably, and the reverse twist region is fixed. A large amount of energy is required to return the once twisted reverse twist region to the forward twist.

The liquid crystal display element of the present invention is characterized by having a chiral group on the surface of the alignment film. Since the chiral group always exists on the surface, the liquid crystal is uniaxially aligned at the interface of the alignment film and the twist direction is also defined, and the alignment region grows from the interface to the bulk with a certain twist. Therefore, in the liquid crystal display device of the present invention, a reverse twist defect does not occur and a high image quality is obtained.

Example The liquid crystal display device of the present invention contains a chiral group on the surface of its alignment film, and can prevent the generation of reverse twist domains and obtain high image quality. As a method of introducing a chiral group to the surface of the alignment film, there is a method of surface-treating a compound having a chiral group by dipping by vapor deposition, solution treatment such as spin coating, or the like. Depending on the type of chiral compound, it may be effective to use the LB method or the chemisorption method at the solid-liquid interface. In addition, there are cases where a chiral group can be introduced on the surface by utilizing the phase separation phenomenon of a mixed system.

The chiral group introduced on the surface of the alignment film is effective even in a physically adsorbed state, but from the viewpoint of stability, it is formed by a chemical reaction such as covalent bond or hydrogen bond with a functional group existing on the surface of the alignment film substrate. Is more preferable. In this sense, the alignment film substrate preferably contains functional groups such as —OH, —NH ₂ , COOH, and unsaturated bonds. However, even a polyolefin or the like which originally does not have such a functional group can be introduced by surface treatment using ultraviolet irradiation or radiation treatment.

The uniaxial orientation treatment by rubbing or the like can be performed before or after the above-mentioned surface treatment with the chiral group.

In the present invention, the chiral group to be introduced on the surface may be plural kinds instead of one kind. In that case, it is preferable to use a chiral group in which the twist direction of the helical structure of the liquid crystal induced by the chiral group is the same. If a chiral group in the opposite direction is used, the effect of preventing the reverse domain is weakened, and if the twisting forces are balanced, the effect is completely lost as in the case of the racemic field.

In the present invention, the twist direction of the liquid crystal is defined by the chiral group present on the surface, but it is more effective to add the chiral substance to the liquid crystal. In that case, it is induced by the chiral group present on the surface of the alignment film. The twisting direction of the helical structure of the liquid crystal and the twisting direction of the helical structure of the liquid crystal induced by the chiral substance contained in the liquid crystal must be the same direction.

The introduction of the chiral group is effective only on the surface of the alignment film on one side, but it is more effective to introduce it on the surfaces of the alignment films on both sides.

Hereinafter, the present invention will be described with reference to specific examples. In order to show the effect of the present invention, in one of the following examples, one substrate for forming a liquid crystal cell was used.
The substrate having the surface shape described in 1. was used.

Example 1 The following experiment was conducted to confirm the effect of introducing a chiral group in the present invention. As a substrate for forming a liquid crystal display element, a glass nitride film having a thickness of 2000 A provided on an ITO electrode, and aluminum lines having a width of 10 μm and a height of 1 μm formed on the ITO electrode at intervals of 60 μm A substrate was used. As a substrate for the alignment film, a polyamic acid solution consisting of pyromellitic dianhydride and 4,4′-diaminodiphenyl ether was spin-coated on the substrate.
Heat treatment was performed at 0 ° C. to obtain a film having a thickness of 1000 A. This board 4-
(S-2-methylbutyl) benzoyl chloride 0.1M
Surface treatment was carried out by heating under reflux in ether dissolved at a concentration of. This causes a reaction with an amino group existing on the surface of the alignment film substrate, and a chiral group is introduced to the surface by an amide bond. Washed with carbon tetrachloride,
After drying, a nylon non-woven fabric was used for rubbing in a direction perpendicular to the aluminum line. This substrate was treated in exactly the same manner as a glass substrate with ITO having a flat surface and a rubbing direction shifted by 90 ° to produce a cell having a gap of 6.3 μm.

As a comparative example, a substrate having the same shape as in Example 1 was used.
A cell was prepared in exactly the same manner as in Example 1 except that the surface treatment with (S-2-methylbutyl) benzoyl chloride was not carried out.

Liquid crystal ZLI-2244-000 manufactured by Merck and added with 0.1% of 4- (S-2-methylbutyl) -4'-cyanobiphenyl as a chiral substance was injected into these cells, and the alignment state was examined. No reverse twist domain was observed in the cell of Example 1, but the reverse twist domain occurred frequently in the cell of the comparative example. The polarizing plate was applied in a normally black (NB) mode in which light was blocked when no voltage was applied, and the ratio of light transmittance between when a voltage was applied and when no voltage was applied (contrast ratio) was measured. The contrast ratio of 95 was obtained in the cell of Example 1, while it was only 24 in the cell of Comparative Example. This is due to the effect of light leakage when no voltage is applied at the boundary between the reverse twist region and the forward twist region.

(Example 2) Similar to Example 1, an experiment was conducted using a substrate on which irregularities were formed in stripes. An aqueous solution of polyvinyl alcohol (Kuraray Poval 117) was spin-coated as a substrate for the alignment film and heat-treated at 150 ° C to form a film having a thickness of 1000A. The substrate was surface-treated by heating under reflux in ether in which 4- (S-2-methylbutyl) benzoyl chloride was dissolved at a concentration of 0.1M. As a result, a reaction occurs with the —OH group existing on the surface of the alignment film substrate, and a chiral group is introduced to the surface by an ester bond. A rubbing treatment was performed using a nylon non-woven fabric in a direction perpendicular to the aluminum line. This substrate was treated in exactly the same manner as a glass substrate with ITO having a flat surface and a rubbing direction shifted by 90 ° to produce a cell having a gap of 6.3 μm.

As a comparative example, a substrate having the same shape as in Example 2 was used.
A cell was prepared in exactly the same manner as in Example 2 except that the surface treatment was not performed with (S-2-methylbutyl) benzoyl chloride.

4- (S-2-methylbutyl) -4 'in these cells
Liquid crystal ZLI-2244-000 manufactured by Merck & Co. in which 0.1% of -cyanobisphenyl was added was injected to examine the alignment state. In the cells of Example 2, no reverse twist domain was observed, but in the cells of Comparative Example, many reverse twist domains occurred. A polarizing plate was attached to a normally black (NB) mode in which light is blocked when no voltage is applied, and the ratio of light transmittance when voltage is applied and when no voltage is applied (contrast company) was measured. The contrast ratio of 114 was obtained in the cell of Example 2, but only 27 in the cell of Comparative Example. This is due to the effect of light leakage when no voltage is applied at the boundary between the reverse twist region and the forward twist region.

(Example 3) Similar to Example 1, an experiment was performed using a substrate on which unevenness was formed in a stripe shape. A solution of inorganic coating agent NHC-A-20-14 manufactured by Nissan Chemical Co., Ltd. was used as a substrate for the alignment film and was spin-coated at 300 ° C. to form a 1000 A thick SiO ₂ film. This substrate was surface-treated by heating under reflux in tetrahydrofuran decomposed with 4- (S-1-methylheptyloxy) biphenyl-4'-carbonyl chloride to a concentration of 0.1M. As a result, a reaction occurs with the —OH group existing on the surface of the alignment film substrate, and a chiral group is introduced to the surface by an ester bond. A rubbing process was performed on this in the direction perpendicular to the aluminum line. This substrate was treated in exactly the same manner and was combined with a glass substrate with ITO having a flat surface with the rubbing direction being shifted by 90 ° to prepare a cell with a gap of 3.7 μm.

As a comparative example, a substrate having the same shape as in Example 3 was used.
A cell was prepared in exactly the same manner as in Example 3 except that the surface treatment with (S-1-methylheptyloxy) biphenyl-4'-carbonyl chloride was not carried out.

Liquid crystal ZLI-1565 manufactured by Merck & Co., Inc. was injected into these cells to examine the alignment state. In the cell of Example 3, no reverse twist domain was observed, but in the cell of Comparative Example, the reverse twist domain occurred frequently. A polarizing plate was attached in a normal black (NB) mode in which light was blocked when no voltage was applied, and the ratio of light transmittance (contrast ratio) when voltage was applied and when no voltage was applied was measured. In the cell of Example 3, the contrast ratio
62 g was obtained, but only 12 for the comparative cell.

This is due to the effect of light leakage when no voltage is applied at the boundary between the reverse twist region and the forward twist region.

(Example 4) Similar to Example 1, an experiment was performed using a substrate on which unevenness was formed in a stripe shape. An aqueous solution of polyvinyl alcohol (Kuraray Poval 117) was spin-coated as a substrate for the alignment film and heat-treated at 150 ° C to form a film having a thickness of 1000A. This substrate was immersed in a carbon tetrachloride solution in which R-1-methylheptyloxytoluchlorosilane was dissolved at a concentration of 5 × 10 −2 M for 10 minutes, and then thoroughly washed with chloroform. These operations were carried out in a stream of dry nitrogen to avoid the influence of water. By this operation, a chiral group is introduced to the surface by a Si-O bond. After treatment at 120 ° C. for 30 minutes, rubbing treatment was performed in the direction perpendicular to the aluminum line. This substrate was treated in exactly the same way as the ITO-equipped glass substrate with a flat surface, and the rubbing direction was shifted by 90 °, and then combined.
A cell having a gap of 7.1 μm was produced. 4 in these cells
-(S-2-methylbutyl) -4'-cyanobiphenyl 0.1% added Merck liquid crystal ZLI-1840 was injected,
When the alignment state was examined under a polarizing microscope, the occurrence of reverse twist domain was not observed. When a polarizing plate was attached in a normally black (NB) mode and the ratio of light transmittance (contrast ratio) when voltage was applied and when no voltage was applied was measured, a contrast ratio of 68 was obtained.

EFFECTS OF THE INVENTION In the liquid crystal display element of the present invention, a high contrast ratio and a high image quality can be obtained without causing a reverse twist defect.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Fumiko Yokotani 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor Yoshihiro Matsuo 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP-A-51-110351 (JP, A) JP-A-63-75730 (JP, A)

Claims (1)

(57) [Claims] 1. A liquid crystal display device utilizing twisted alignment between upper and lower substrates of a liquid crystal panel, wherein at least one surface of an alignment film provided on the upper and lower substrates is formed by surface modification with a compound having a chiral group. A liquid crystal display device comprising a chiral group.