JPH06289395A - Liquid crystal display element and formation of its horizontally oriented film - Google Patents

Abstract

PURPOSE:To provide the liquid crystal display device which is free from a defect in initial orientation of a liquid crystal and a defect in reverse tilt discrimination orientation at the time of impression of a driving voltage and has good display quality by orienting liquid crystal molecules by providing these molecules with a desired pretilt angle as the liquid crystal display element formed with horizontally oriented films consisting of polyimide by an LB method. CONSTITUTION:The horizontally oriented films 8, 9 consisting of polyimide films formed by laminating monomolecular films of amphiphatic compds. having hydrophobic groups and hydrophilic groups by the LB method and subjecting the laminated films to an imidization treatment are so formed by controlling the number of lamination of the monomolecular films that the long chain alkyl groups per unit area remain at a prescribed ratio. The liquid crystal molecules A are oriented at the pretilt angle phi meeting the remaining ratio of the long chain alkyl groups per unit areas of the horizontally oriented films 8, 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device in which liquid crystal molecules are horizontally aligned and a method for forming a horizontal alignment film thereof.

[0002]

2. Description of the Related Art Liquid crystal display devices in which liquid crystal molecules are horizontally aligned include TN (twisted nematic) mode, STN (super twisted nematic) mode, and ECB (electric field control birefringence). Among the modes, there are horizontal alignment type, modes using ferroelectric liquid crystal or anti-ferroelectric liquid crystal, and the like.

In these liquid crystal display elements, a transparent electrode and a horizontal alignment film for horizontally aligning liquid crystal molecules are formed on the surfaces of a pair of transparent substrates which face each other with a liquid crystal layer in between and which face each other. It is composed.

The horizontal alignment film provided on the substrate of these liquid crystal elements is conventionally a method of applying polyimide or the like on the substrate and rubbing the film surface, or silicon oxide (SiO ₂ ) or the like on the substrate. Was formed by the method of oblique vapor deposition, but recently, Langmuir-Blodgett (La
A method of forming a horizontal alignment film made of a polyimide film by the ngumuir-Blodgett method has been adopted.

In the Langmuir-Blodgett method (hereinafter referred to as the LB method), a monomolecular film is formed on a still water surface, and a substrate previously immersed in water vertically is pulled up at a constant speed while being pulled up on the water surface. This is a method of depositing a monomolecular film on a substrate.

The formation of the polyimide film by the LB method is
An amphipathic compound having a hydrophobic group and a hydrophilic group, for example, a monomolecular film of a polyamic acid derivative compound obtained by subjecting a polyamic acid and an amine having a long-chain alkyl group to an ionic bond reaction is deposited on a substrate by the LB method. This step is repeated a plurality of times to stack the monomolecular film on the substrate, and the stacked film is imidized to form a polyimide film.

In the monomolecular film of the amphipathic compound deposited on the substrate by the above-mentioned LB method, since molecules having a long molecular length are arranged along the pulling direction of the substrate, a laminated film of this monomolecular film is formed. The imidized polyimide film has an orientation property of uniformly aligning liquid crystal molecules in one direction, and therefore,
This polyimide film can be directly used as a horizontal alignment film without rubbing the film surface.

In the liquid crystal display element, the alignment state of the liquid crystal molecules has a great influence on the electro-optical characteristics of the liquid crystal display element. Therefore, in order to obtain a good display, it is necessary to improve the stability of the alignment of the liquid crystal molecules. is there.

The important factor that influences the stability of the alignment of the liquid crystal molecules is the pretilt angle of the liquid crystal molecules given by the alignment film (the angle formed by the long axis of the liquid crystal molecules with respect to the substrate surface).
Therefore, the larger the pretilt angle, the better the stability of the alignment of the liquid crystal molecules.

[0010]

However, the above L
When the horizontal alignment film is formed by the B method, conventionally, the number of laminated monomolecular films of the amphipathic compound is determined so that an alignment film uniformly covering the adhered surface can be obtained. In this alignment film, liquid crystal molecules do not pretilt (pretilt angle 0 °)
Orient in the state.

Therefore, the conventional liquid crystal display element has a problem that the initial alignment failure of the liquid crystal occurs or the reverse tilt disclination alignment failure occurs when the driving voltage is applied.

The present invention is a liquid crystal display device in which a horizontal alignment film made of polyimide is formed by the LB method, in which liquid crystal molecules are aligned with a desired pretilt angle, and the initial alignment failure of the liquid crystal and the reverse when a driving voltage is applied. It is an object of the present invention to provide a liquid crystal display device having good display quality without tilt disclination alignment defects and a method of forming a horizontal alignment film thereof.

[0013]

A liquid crystal display element of the present invention comprises a transparent electrode, a hydrophobic group and a hydrophilic group by the LB method, on the surfaces of a pair of transparent substrates facing each other across a liquid crystal layer. And a horizontal alignment film made of a polyimide film obtained by imidizing the laminated film by laminating monomolecular films of an amphipathic compound having, and the horizontal alignment film is a stack of the monomolecular films. The hydrophobic group is formed so as to remain at a predetermined ratio per unit area by controlling the number,
It is characterized in that the liquid crystal molecules are aligned with a pretilt angle according to the residual ratio of the hydrophobic groups per unit area of the horizontal alignment film.

The method for forming a horizontal alignment film of the present invention is
A monomolecular film of an amphipathic compound obtained by mixing a raw material having a hydrophobic group and a raw material having a hydrophilic group in a predetermined ratio by a LB method is laminated on a transparent substrate on which a transparent electrode is formed. A method of forming a horizontal alignment film made of a polyimide film by imidizing a laminated film under predetermined conditions, wherein the number of laminated monomolecular films is controlled, and the hydrophobic group has a predetermined ratio per unit area. The method is characterized in that a horizontal alignment film made of the polyimide film left over in step 1 is obtained.

[0015]

That is, in the liquid crystal display device of the present invention, a horizontal alignment film made of a polyimide film formed by the LB method is used as a unit of an amphipathic compound so that hydrophobic groups remain at a predetermined ratio per unit area. It is formed by controlling the number of laminated molecular films, and the larger the residual ratio of the hydrophobic group per unit area of the horizontal alignment film, the smaller the surface tension of the alignment film, and the liquid crystal molecules are Horizontally oriented with a pre-tilt angle according to.

Therefore, by controlling the number of stacked monomolecular films, the residual ratio of the hydrophobic groups per unit area of the horizontal alignment film is controlled to align the liquid crystal molecules with a predetermined pretilt angle. It is possible to obtain an alignment film to be used, and by using this alignment film, it is possible to obtain good display quality without initial alignment defect of liquid crystal or reverse tilt disclination alignment defect when a drive voltage is applied.

The method of forming a horizontal alignment film of the present invention is
When a monomolecular film of an amphipathic compound is laminated by the LB method, by controlling the number of laminated monomolecular films, the hydrophobicity of a polyimide film formed by imidizing the monomolecular film It controls the residual ratio of the group, determines the mixing ratio of the raw material of the amphipathic compound and its imidization conditions, by controlling the number of lamination of the monomolecular film according to them, the polyimide film Since the residual ratio of the hydrophobic group is determined, it is possible to obtain a horizontal alignment film that aligns liquid crystal molecules with a desired pretilt angle.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view of a liquid crystal display element. This liquid crystal display element includes a pair of transparent substrates 1 made of glass or the like.
2 are joined via a frame-shaped sealing material 3, and both substrates 1,
Liquid crystal is sealed in a region surrounded by the seal material 3 between the two, and transparent electrodes 4 and 5 are formed on the surfaces of the substrates 1 and 2 facing each other. Also, both of these substrates 1,
The electrode forming surface of No. ₂ is covered with transparent insulating films 6 and 7 made of silicon oxide (SiO ₂ ), etc., and horizontal alignment films 8 and 9 are formed on the insulating films 6 and 7. .

This liquid crystal display device is of a TN mode or STN mode, and the molecules A of the liquid crystal enclosed between the substrates 1 and 2 are oriented films 8 and 9 on the substrates 1 and 2 side.
The respective alignment directions of the liquid crystal molecules A in the vicinity of are regulated by the alignment films 8 and 9, and the substrates 1 and 2 are twist-aligned at a predetermined twist angle. However, in FIG. 1, for convenience, the liquid crystal molecules A are not twisted.

Each of the horizontal alignment films 8 and 9 is an amphipathic compound having a hydrophobic group and a hydrophilic group, for example, an amine having a polyamic acid and a long-chain alkyl group (hereinafter referred to as a long-chain alkylamine). It is composed of a polyimide film obtained by imidizing a film obtained by stacking several monolayers of a compound obtained by reacting with and several layers to several tens of layers.

The horizontal alignment films 8 and 9 are formed by the following method. Although the formation of the horizontal alignment film 8 provided on one substrate 1 will be described here, the horizontal alignment film 9 provided on the other substrate 2 is also formed in the same manner.

The above polyamic acid is represented by the following structural formula [Chemical Formula 3], and this polyamic acid is composed of the tetracarboxylic dianhydride represented by the structural formula [Chemical Formula 1] and [Chemical Formula 2]. It is obtained by synthesizing the diamine represented by the structural formula.

[0024]

[Chemical 1]

[0025]

[Chemical 2]

[0026]

[Chemical 3]

The long-chain alkylamine is for imparting hydrophobicity to the hydrophilic polyamic acid, and the long-chain alkylamine is represented by the following structural formula [Chemical formula 4].

[0028]

[Chemical 4]

A solution prepared by dissolving the polyamic acid in a solvent and a solution prepared by dissolving the long-chain alkylamine in the same solvent are mixed at a predetermined ratio to cause an ionic bond reaction between the polyamic acid and the long-chain alkylamine, A solution of a polyamic acid derivative compound (polyamic acid salt) represented by the following structural formula [Chemical Formula 5] is prepared. The solvent for the polyamic acid and the long-chain alkylamine is NM.
P (N-methyl-2-pyrrolidinone) and benzene 1:
A mixed solvent mixed at a ratio of 1 is used.

[0030]

[Chemical 5]

The horizontal alignment film 8 is formed of the transparent electrode 4
LB is formed on the substrate 1 on which the insulating film 6 is formed.
A monomolecular film of the above polyamic acid derivative compound is laminated on a predetermined layer by the method, and the laminated film of the monomolecular film is imidized to form the film. FIG. 2 shows a method of depositing a monomolecular film of a polyamic acid derivative compound on the substrate 1 by the LB method. The deposition of this monomolecular film is performed as follows. First, the monomolecular film adhered surface of the substrate 1 (the insulating film 6
The substrate 1 is subjected to hydrophilic treatment, and the substrate 1 is vertically immersed in water in the water tank 10.

Next, the solution of the above polyamic acid derivative compound is dropped on the water surface between the bar-shaped moving barrier 11 provided at the water surface height and the substrate 1 to spread the monomolecular film a on the water surface. Let

Next, the moving barrier 11 is moved toward the substrate so that the single molecules on the water surface are densely packed and the surface pressure of the monomolecular film a is adjusted to a constant pressure (25 dyn / cm), and then the moving barrier 11 is placed on the substrate. The monomolecular film a is moved in a predetermined direction (2 mm / min) while pushing the monomolecular film a toward the substrate, and the substrate 1 is pulled up in synchronization with this to deposit the monomolecular film a on the water surface onto the substrate 1. .

At this time, since the monomolecules on the water surface are pulled up by adhering to the substrate 1 whose hydrophilic portion has been subjected to the hydrophilic treatment, the monomolecular film a has the molecules arranged in almost one direction. In this state, it is deposited on the substrate 1. The following is the monolayer a
The above-mentioned deposition process is repeated to laminate the monomolecular film a on the substrate 1 into a predetermined layer.

After laminating the monomolecular film a of the polyamic acid derivative compound on the substrate 1 in this manner, a heat treatment of heating at 200 ° C. or higher for about 1 hour is performed. By this heat treatment, imidization of the laminated film on the substrate 1 progresses, and the horizontal alignment film 8 in which the alkylamine remains is formed.

In this case, the monomolecular laminated film of the polyamic acid derivative compound, which is a compound in which the polyamic acid and the long-chain alkylamine are ion-bonded, is dehydrated and ring-closed while the alkylamine is removed. Although it is imidized to be a polyimide having a structure as described above, the imidization is not completed, and alkylamine remains at a predetermined ratio. The residual amount of this alkylamine is
When the heat treatment conditions and the like are constant, it changes depending on the number of stacked monomolecular films a. Therefore, the residual ratio of long-chain alkyl groups can be controlled by appropriately setting the number of stacked monomolecular films a.

[0037]

[Chemical 6]

In the liquid crystal display device, the horizontal alignment film 8 made of a polyimide film on both substrates 1 and 2 is used.
9 is set by appropriately setting the number of laminated monomolecular films a,
The horizontal alignment films 8 and 9 are formed by controlling the residual ratio of the long-chain alkyl group R ₃ in the long-chain alkylamine shown in [Chemical Formula 4] per unit area.
Are oriented with a desired pretilt angle φ.

FIGS. 3 and 4 are views schematically showing the residual state of the long-chain alkyl group R ₃ of the horizontal alignment films 8 and 9 made of a polyimide film formed by controlling the number of laminated monomolecular films a. 3 shows the long-chain alkyl group residual state when the number of monomolecular films a laminated is small, and FIG. 4 shows the long-chain alkyl group residual state when the number of monomolecular films a laminated is large.

As shown in FIGS. 3 and 4, the horizontal alignment films 8 and 9 made of the polyimide film formed by the LB method.
The residual ratio of the long-chain alkyl group R ₃ changes depending on the number of stacked monomolecular films a, and as the number of stacked monomolecular films a increases, the residual ratio of the long-chain alkyl group R ₃ increases, The distribution is also uniform.

In the above liquid crystal display element, L
The horizontal alignment films 8 and 9 made of the polyimide film formed by the method B are formed so that the long-chain alkyl group R ₃ remains at a predetermined ratio per unit area by controlling the number of laminated monomolecular films. Therefore, the liquid crystal molecules are set to the desired pretilt angle φ.
Can be oriented.

This is because the long-chain alkyl group has hydrophobicity, and as the residual ratio of the long-chain alkyl group R ₃ per unit area of the horizontal alignment films 8 and 9 is increased, the alignment films 8 and 9 are increased. , The liquid crystal molecules A are aligned with a pretilt angle φ.

Therefore, by controlling the number of stacked monomolecular films a to control the residual ratio of the long-chain alkyl group R ₃ per unit area of the horizontal alignment films 8 and 9, the liquid crystal molecule A is desired. By orienting with a pretilt angle, it is possible to obtain good display quality without initial alignment failure of liquid crystal or reverse tilt disclination alignment failure when a drive voltage is applied.

In the horizontal alignment films 8 and 9 having the long-chain alkyl group residual ratio as described above, the number of laminated layers of the monomolecular film a is set to a preset mixing ratio of the raw material of the polyamic acid derivative compound, that is, The polyamic acid and the long-chain alkylamine are formed by controlling according to the mixing ratio and the imidization treatment conditions.

That is, the horizontal alignment films 8 and 9 are formed by controlling the number of laminated monomolecular films a when laminating the monomolecular films a of the polyamic acid derivative compound by the LB method. It is intended to control the residual ratio of the long-chain alkyl group R ₃ in the polyimide film formed by imidizing the laminated film of the monomolecular film a. When controlled according to the mixing ratio with the chain alkylamine and the imidization conditions thereof, the residual ratio of the long-chain alkyl groups in the polyimide film is determined accordingly, so that the liquid crystal molecules A are aligned with a desired pretilt angle φ. The horizontal alignment films 8 and 9 can be obtained.

FIG. 5 shows the relationship between the number of stacked monomolecular films a, the mixing ratio of polyamic acid and long-chain alkylamine, its imidization temperature, and the surface tension of the formed alignment film. Here, the mixing ratio (molar ratio) of the polyamic acid and the long-chain alkylamine is two, that is, polyamic acid: long-chain alkylamine = 1: 3 and polyamic acid: long-chain alkylamine = 1: 5. 2 shows an example in which each is imidized at an imidization temperature of 200 ° C.

As shown in FIG. 5, the alignment film made of the polyimide film formed with the mixing ratio of the polyamic acid and the long-chain alkylamine being 1: 3 has 18 monolayer films a.
When the number of layers is equal to or less than the number of layers, the surface tension is too large to pretilt the liquid crystal molecules A (pretilt angle φ = 0). Pretilt angle φ (for example, the number of stacked layers is 23
In the case of a layer, the surface tension for orientation is φ = 3.2 °. The surface tension becomes smaller as the number of stacked monomolecular films a increases, and accordingly, the liquid crystal molecules A
The pretilt angle φ of is increased.

Further, the orientation film made of the polyimide film formed with the mixing ratio of the polyamic acid and the long-chain alkylamine being 1: 5 has a surface tension when the number of monomolecular films a is about 10 or less. It is too large to pretilt the liquid crystal molecules A, but if the number of stacked monomolecular films a is larger than that, the surface tension causes the liquid crystal molecules A to be aligned with the pretilt angle φ. In this case, the pretilt angle φ is, for example, φ = 0.4 ° when the number of stacked layers is about 11, and φ = 13 ° or more when the number of stacked layers is about 17 or more. Also in this case, the surface tension of the alignment film decreases as the number of stacked monomolecular films a increases, and the pretilt angle φ of the liquid crystal molecules A increases accordingly, but when the number of stacked layers exceeds about 22 layers. The pretilt angle φ approaches 90 °, and the liquid crystal molecules A are vertically aligned.

Even if the amount of the long-chain alkylamine with respect to the polyamic acid is 3 times or less (mixing ratio 1: 3),
The pretilt alignment of the liquid crystal molecules A can be performed by increasing the number of laminated monomolecular films a. However, when the number of laminated monomolecular films a is increased, the alignment films 8 and 9 become thicker and the applied voltage is aligned. It drops significantly at the membrane.

Therefore, the mixing ratio of the polyamic acid and the long-chain alkylamine is preferably in the range of polyamic acid: long-chain alkylamine = 1: 3 to 1: 5. It is possible to obtain the horizontal alignment films 8 and 9 which align the liquid crystal molecules A with a desired pretilt angle φ without increasing the number of laminated films a.

In the above embodiment, the imidization treatment of the laminated film of the monomolecular film a is performed by heat treatment. However, this imidization treatment may be performed by chemical treatment with a solution of acid anhydride or the like. Both the treatment and the heat treatment may be performed in combination, and in that case also, the number of stacked monomolecular films a is controlled in accordance with the mixing ratio of the polyamic acid and the long-chain alkylamine and its imidization conditions. This makes it possible to obtain a horizontal alignment film that aligns liquid crystal molecules with a desired pretilt angle.

Furthermore, in the above-mentioned Examples, the polyamic acid derivative compound obtained by reacting polyamic acid with a long-chain alkylamine was used as the amphipathic compound having a hydrophobic group and a hydrophilic group. The amphiphilic compound is
The compound is not limited to the polyamic acid derivative compound as long as it can be converted into polyimide by imidization treatment.

In addition, the liquid crystal display device of the above embodiment is TN
Mode or STN mode, the present invention
The present invention can also be applied to a horizontal alignment type liquid crystal display element of an ECB (electric field control type birefringence) mode, a liquid crystal display element using a ferroelectric liquid crystal or an antiferroelectric liquid crystal, and the like.

[0054]

In the liquid crystal display device of the present invention, a horizontal alignment film made of a polyimide film formed by the LB method is used as a unit of an amphipathic compound so that hydrophobic groups remain in a predetermined ratio per unit area. Since it is formed by controlling the number of laminated molecular films, liquid crystal molecules are aligned with a desired pretilt angle, and the initial alignment failure of the liquid crystal and the reverse tilt disclination alignment failure at the time of applying a drive voltage may occur. It is possible to obtain good display quality.

The method for forming a horizontal alignment film of the present invention is
When laminating the monomolecular film of the amphipathic compound by the LB method, by controlling the laminated number of the monomolecular film according to a preset mixing ratio of the raw materials and its imidization condition, Since it controls the residual ratio of long-chain alkyl groups in a polyimide film formed by imidizing a laminated film of molecular films, it is easy to obtain a horizontal alignment film that aligns liquid crystal molecules with a desired pretilt angle. be able to.

[Brief description of drawings]

FIG. 1 is a sectional view of a liquid crystal display element showing an embodiment of the present invention.

FIG. 2 is a diagram showing a method for laminating a monomolecular film on a substrate by the LB method.

FIG. 3 is a diagram schematically showing a residual state of a long-chain alkyl group R _{3 in} a horizontal alignment film having a small number of laminated monomolecular films.

FIG. 4 is a diagram schematically showing a residual state of a long-chain alkyl group R _{3 in} a horizontal alignment film having a large number of laminated monomolecular films.

FIG. 5: Number of laminated monolayers and composition ratio of amphipathic compound (mixing ratio of polyamic acid and long-chain alkylamine)
FIG. 6 is a diagram showing the relationship between the imidization temperature and the surface tension of the formed alignment film.

[Explanation of symbols]

1, 2 ... Substrate 4, 5 ... Electrode 6, 7 ... Insulating film 8, 9 ... Horizontal alignment film made of polyimide film formed by LB method A ... Liquid crystal molecule

Claims (2)

[Claims] 1. A single molecule of an amphipathic compound having a transparent electrode and a hydrophobic group and a hydrophilic group by the Langmuir-Blodgett method on the surfaces of a pair of transparent substrates that face each other with a liquid crystal layer between them, facing each other. A liquid crystal display device comprising a horizontal alignment film made of a polyimide film obtained by stacking films and imidizing the stacked film, wherein the horizontal alignment film controls the number of stacked monomolecular films to form a unit area. The hydrophobic group is formed so as to remain in a predetermined ratio, and the liquid crystal molecules are aligned with a pretilt angle according to the residual ratio of the hydrophobic group per unit area of the horizontal alignment film. Characteristic liquid crystal display element. 2. An amphipathic compound obtained by mixing a starting material having a hydrophobic group and a starting material having a hydrophilic group in a predetermined ratio by a Langmuir-Blodgett method on a transparent substrate having a transparent electrode formed thereon. A method of forming a horizontal alignment film comprising a polyimide film by laminating a monomolecular film and subjecting the laminated film to an imidization treatment, wherein the number of laminated monomolecular films is controlled, and the hydrophobic group per unit area is controlled. A method for forming an alignment film, which comprises obtaining a horizontal alignment film made of a polyimide film in which a predetermined amount is left.