JPH0792568B2 - Liquid crystal display element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a liquid crystal display device.

In particular, it relates to improvement of an alignment film that controls the alignment of liquid crystals.

2. Description of the Related Art Display devices using liquid crystals have the advantages of 1) low power consumption, 2) low driving voltage, and 3) miniaturization and weight reduction, and are used in various applications such as calculators and watches. ing. However, in the case of a display device using a nematic liquid crystal, the electro-optical response is slow, and its application to fields requiring high-speed response, such as a television display device and an optical shutter device, has been limited.

Recently, researches have been actively conducted to apply the ferroelectric liquid crystal as an electro-optical device. Compared with the nematic liquid crystal which has been conventionally used for a liquid crystal display element, the ferroelectric liquid crystal has: 1) a high-speed electric field It has excellent responsiveness. 2) It has excellent characteristics such as the possibility of showing the memory effect of maintaining the display state even when no electric field is applied. If a display element is created using this ferroelectric liquid crystal, a simple matrix drive method that does not require TFT can be performed, and a large screen and high definition display can be realized. In order to realize this, development of ferroelectric liquid crystal materials and development of driving methods have been vigorously carried out.

Development of new liquid crystal materials and driving methods can be cited as problems to be solved in applying this ferroelectric liquid crystal.
One of the most important techniques for producing a display element is a technique for uniformly aligning liquid crystals.

Since the ferroelectric liquid crystal has a layer structure unlike the nematic liquid crystal, it is difficult to obtain uniform alignment as compared with the nematic liquid crystal. Therefore, various alignment methods have been proposed. In particular, the ferroelectric chiral smectic C (hereinafter, abbreviated as SmC ^* ), which is currently being actively investigated for practical use.
Examples of the alignment method proposed for a liquid crystal display device having a cell thickness of several μm or less include a sharing method, a temperature gradient method, an oblique vapor deposition method of SiO or the like, and a rubbing method.

Ferroelectric liquid crystal has an isotropic liquid phase (I
There is also a phase transition directly from the (phase) to the SmC ^* phase, but such a material is generally very difficult to be uniformly aligned. And most of them are cholesteric phase (Ch
Phase) or via smectic A phase (SmA phase) SmC ^*
Transition to phase. Among those passing through the Ch phase, it is said that the orientation is good especially when the helical pitch of the Ch phase is sufficiently longer than the cell thickness of the panel.

Among the above-mentioned orientation methods, the sharing method and the temperature gradient method are effective for obtaining uniform orientation in an area of several mm square or less at a laboratory level, but these methods are generally required for display elements. However, it is considered very difficult to obtain a uniform orientation in a larger area.

It has been reported that the orthorhombic vapor deposition method can obtain uniform alignment by increasing the vapor deposition angle to 80 ° or more, but since it has a large pretilt angle, the deviation between the direction of the electric field and the spontaneous polarization is large, and the electric field response is large. It has the problem of slowing down.
Further, since a vapor deposition device is required, the manufacturing cost will be high.

In contrast to these alignment treatment methods, the rubbing method can perform the alignment treatment by rubbing the organic polymer alignment film formed on the surface of the substrate in a certain direction with a cloth, which is expensive as in the oblique vapor deposition method. It does not require a special vapor deposition device, and can easily perform a large area alignment treatment. Although the mechanism of liquid crystal alignment by rubbing has not been completely clarified, by applying shear stress to the surface of the alignment film by rubbing, alignment of polymer chains near the surface occurs, and liquid crystal molecules are aligned according to the alignment of polymer chains. It is considered. At this time, the pretilt angle is 0 ° to several °, and the spontaneous polarization is oriented in a direction substantially parallel to the electric field, and a fast electric field response is expected. Therefore, if the alignment treatment is performed by the rubbing method, the fast-response ferroelectric liquid crystal display element can be manufactured most easily and inexpensively. In fact, there are many reports that good alignment was obtained by changing the type of alignment film or performing rubbing treatment on both substrates in the same direction, opposite directions, or on only one side.

However, the rubbing method also has the following problems. An element that has been subjected to an alignment treatment by the rubbing method generally cannot obtain a perfect memory effect. That is, when the voltage is turned off, the light transmittance of the liquid crystal panel is increased or decreased compared to when the voltage is turned on. This is because the arrangement of the ferroelectric liquid crystal molecules changes to a state different from that when a voltage is applied when no voltage is applied. Therefore, it is extremely difficult to maintain a bistable memory state.

The biggest cause of the occurrence of alignment defects in the device subjected to the alignment treatment by the rubbing method and the inability to obtain a sufficient memory effect is that the alignment film in the SmC ^* liquid crystal phase exerts the liquid crystal alignment direction regulating force in the direction parallel to the rubbing It is thought to be because they still have it.

When the phase transition from the SmA phase or Ch phase to the SmC ^* phase occurs, the liquid crystal molecules or layers tend to tilt by an angle corresponding to the tilt angle θ. That is, the rubbing causes the tilt to deviate by θ from the azimuth defined in the direction parallel to the rubbing. However, at this point, if the liquid crystal alignment azimuth regulating force is present in the direction parallel to the rubbing, the molecules near the interface of the alignment film are restricted and cannot be tilted by the original tilt angle θ, and the tilt angle becomes small. Therefore, distortion is likely to occur near the interface, and alignment defects are likely to occur. In addition, when no voltage is applied, molecules near the interface return to the direction bound by rubbing, which appears as a change in light transmittance of the panel, and a bistable memory effect cannot be obtained.

Means for Solving the Problems In a liquid crystal display element consisting of a pair of substrates whose surfaces are uniaxially oriented and a liquid crystal, a pair of substrates whose surfaces are uniaxially oriented are aligned in a direction parallel to the uniaxial orientation direction. A polymer mixture composed of at least one polymer having a regulation force and a polymer having a liquid crystal orientation azimuth regulation force in a direction orthogonal to the uniaxial orientation treatment direction, or a polymer mixture thereof. An alignment film formed of a high molecular weight copolymer composed of a polymer is used.

Action According to the above configuration, the alignment film has a smaller liquid crystal alignment orientation regulating force in the direction parallel to the rubbing with respect to the liquid crystal. Therefore, for example, when the ferroelectric liquid crystal is used as the liquid crystal, the ferroelectric liquid crystal can be tilted at almost the same angle as the original tilt angle even near the interface of the alignment film. Therefore, no interface strain is generated and uniform alignment is obtained in the SmC ^* phase. Further, even when no voltage is applied, the direction inclined from the rubbing direction by a certain angle is a stable state, so that it becomes bistable and a complete memory effect can be obtained.

Example The alignment film in the liquid crystal display device according to the present invention is formed of at least two kinds of polymers.

One is a polymer having a liquid crystal alignment direction regulating force in a direction parallel to the uniaxial alignment treatment direction, for example, a polymer that aligns liquid crystal in a direction parallel to the rubbing direction when rubbing treatment is performed (first Figure (a)).

The other is a polymer having a liquid crystal alignment direction regulating force in a direction orthogonal to the uniaxial alignment treatment direction, for example, a polymer that aligns liquid crystal in a direction orthogonal to the rubbing direction when rubbing treatment is performed ( Figure 1 (b)).

In the case of the liquid crystal display device of the present invention, one or more kinds of polymers for orienting the liquid crystal in such a direction are combined to obtain a polymer mixture, or a monomer of those polymers is reacted. Create a high molecular weight copolymer.
These polymer mixtures and polymer copolymers have a function as a polymer without losing the properties of each polymer and can be used as an alignment film. Therefore, the alignment direction of the liquid crystal can be set to a direction inclined by an arbitrary angle from the uniaxial alignment treatment direction depending on the mixing or copolymerization ratio (FIG. 2).

For the above reasons, when the alignment film of the liquid crystal display device of the present invention is used, the liquid crystal alignment direction regulating force in the direction parallel to the rubbing is reduced with respect to the liquid crystal. Therefore, for example, when the ferroelectric liquid crystal is used as the liquid crystal, the ferroelectric liquid crystal can be tilted at almost the same angle as the original tilt angle even near the interface of the alignment film. Therefore, no interface strain is generated and uniform alignment is obtained in the SmC ^* phase. Further, even when no voltage is applied, the direction inclined from the rubbing direction by a certain angle is a stable state, so that it becomes bistable and a complete memory effect can be obtained.

As a polymer having a liquid crystal orientation control force in a direction orthogonal to the uniaxial alignment treatment direction, a polymer having a ring structure in its side chain is considered, and its side chain has a cyclopentane ring, a cyclohexane ring, a benzene ring, or naphthalene. Ring, furan ring, oxolane ring, dioxolane ring, thiophene ring, pyrrole ring,
Examples of the polymer include a pyran ring, an oxane ring, a dioxane ring, a pyridine ring, a piperidine ring, a pyrimidine ring, and a pyrazine ring. As a typical one, polystyrene,
Examples thereof include polyphenylacetylene, polyvinyl pyridine, polyvinyl pyrimidine and their derivatives.

As a polymer having a liquid crystal alignment azimuth control force in a direction parallel to the uniaxial alignment treatment direction, a polymer having no ring structure in its side chain is considered, such as polyimide, polyvinyl alcohol, polyvinyl acetate, polymethyl methacrylate. and so on.

When measuring the dielectric or viscoelastic properties of a polymer over a wide temperature and frequency range, two or more types of absorption have been found for each polymer substance,
For the sake of convenience, these absorptions are referred to as α, β, etc. because of their high temperature absorption.

It has been clarified from various measurement results that the α absorption (high temperature side) is due to the micro Brownian motion of the main chain segment. This corresponds to a property generally called glass transition. β absorption is due to micro Brownian motion of the entire side chain, but is considered to be accompanied by local movement of the main chain. It is also possible to obtain a better liquid crystal display device by limiting the absorption temperature of the polymer and the phase transition temperature of the liquid crystal.

Example 1 FIG. 3 shows the structure of a ferroelectric liquid crystal display device according to the present invention. A device having this structure was prepared as follows. First, on glass substrates 1 and 9 having ITO electrodes 2 and 8, polyphenylacetylene as a polymer having a liquid crystal orientation azimuth regulating force in a direction orthogonal to the uniaxial orientation treatment direction, and As a polymer with a liquid crystal orientation control force, polymethylmethacrylate was prepared so that their mixing weight ratio was 40:60, and this was dried to obtain an alignment film 3,4.
Was coated by spin coating so that the film thickness was 1000 A, and dried at 150 ° C. for 1 hour.

Then, the alignment film on the glass substrate is rubbed with a cloth, and then the rubbing directions are antiparallel to each other, and the substrates are bonded via a 2.0 μm bee spacer 6, and a portion to be an injection port is removed. The periphery of the head was sealed with the sealing resin 5. Next, as the liquid crystal 7, in this embodiment, a ferroelectric liquid crystal that undergoes the following phase transition change in the temperature decreasing process is put under reduced pressure in the device,
Injection was performed in the temperature region of phase I.

I-phase → Ch-phase → SA-phase → SmC ^* phase After gradually cooling this element to room temperature (approximately −0.5 ° C / min),
The inlet was sealed.

Microscopic observation of this device revealed that uniform alignment was obtained. The tilt angle of this device was also measured. The tilt angle is an angle formed between the rubbing direction and the liquid crystal molecule long axis direction. Polarizer-side and analyzer-side polarizing plates were installed so as to be orthogonal to each other, and the tilt angle was the tilt angle between the liquid crystal molecule major axis direction and the polarizing axis of the polarizing plate. The tilt angle at this time was 20 °.

Further, a polarity reversal voltage waveform (pulse voltage) as shown in FIG. 4 was applied to this device, and its response time, contrast, and memory effect were also measured by measuring the change in light transmittance. For measurement, turn on the pulse voltage and pulse time from ON (2ms, + 10V) to OFF (18ms)
→ Set ON (2ms, -10V) → OFF (18ms) and examined. At this time, the angle of the polarizing plate is set so that one of the two polarizing plates has its polarization axis aligned with the major axis direction of the liquid crystal molecule, and the other polarizing plate has its polarization axis orthogonal to this. did. The response time is the time required for the transmittance to change 90% during voltage reversal. The contrast is represented by the ratio of the transmitted light intensities 14 ms after the application of the pulse voltages of +10 V and -10 V is completed.

This device has a response speed of 200 μs and a contrast of 30:
1 was obtained, indicating a sufficient memory property.

Example 2 As a polymer having a liquid crystal alignment azimuth regulating force in a direction orthogonal to the uniaxial alignment treatment direction, poly (α-methyl polystyrene), a polymer having a liquid crystal alignment azimuth regulating force in a direction parallel to the uniaxial alignment treatment direction , Polyvinyl acetate was prepared so that the mixing weight ratio thereof was 40:60, and was applied by spin coating so that the thickness of the alignment films 3 and 4 after drying would be 1000 A, and the temperature was 150 ° C. And dried for 1 hour. A ferroelectric liquid crystal display device was prepared in the same manner as in Example 1.

The tilt angle, response speed, and contrast of this device were measured in the same manner. The tilt angle was 22 °, the response speed was 230 μs, and the contrast was 25: 1, and uniform alignment and sufficient memory performance were obtained.

Example 3 As a polymer having a liquid crystal alignment azimuth regulating force in a direction orthogonal to the uniaxial alignment treatment direction, poly (α-methylstyrene), a polymer having a liquid crystal alignment azimuth regulating force in a direction parallel to the uniaxial alignment treatment direction , Polymethylmethacrylate was prepared such that their mixing weight ratio was 30:70. This was applied by spin coating so that the film thickness of the alignment films 3 and 4 after drying was 1000 A, and dried at 150 ° C. for 1 hour (the α absorption temperature of the polymer mixture used at this time was 145 ° C. , Β absorption temperature is 70 ° C).

A liquid crystal display device was prepared in the same manner as in Example 1. In this example, as a liquid crystal, a ferroelectric liquid crystal that undergoes a phase transition change at the following temperature during the temperature lowering process was used in the device under reduced pressure to generate a phase of 90 ° C. Injection was performed in the temperature range.

The tilt angle, response speed, and contrast of this device were measured in the same manner. The tilt angle was 22 °, the response speed was 170 μs, and the contrast was 50: 1, and uniform alignment and sufficient memory performance were obtained. In Example 3, the contrast showed a very large value as compared with Examples 1 and 2. It is considered that this is because the following relationship is established between the phase transition temperature of the liquid crystal and the α and β absorption temperatures of the polymer used as the alignment film.

(Alpha absorption temperature of the alignment layer)> (injection temperature of the liquid crystal)> (beta absorption temperature of the alignment layer)> (SmC ^* SmC from the hot side of the phase phase ^*
Temperature at which phase transitions) When the temperature of the alignment film is equal to or higher than β absorption temperature and equal to or lower than α absorption temperature, the main chain segment of the polymer hardly moves, and the entire side chain performs micro Brownian motion. Therefore, if the liquid crystal is injected at this temperature, the liquid crystal is oriented by recognizing the alignment direction of the main chain. At this time, the alignment direction of the main chains is parallel to the rubbing direction, and the liquid crystal is aligned parallel to the rubbing direction (FIG. 5 (a)).

After that, when the liquid crystal is cooled to the SmC ^* phase and the temperature of the alignment film becomes equal to or lower than the β absorption temperature, the micro Brownian motion of the side chain becomes small or becomes zero. At this time, when the polymer forming the alignment film contains a polymer having a ring structure in the side chain, the liquid crystal recognizes the alignment state of the side chain and aligns in that direction. That is, when the alignment film is in this temperature range, the liquid crystal is aligned at an angle as shown in FIG. 5 (b) with respect to the rubbing direction. It is more preferable that this angle be the same as the tilt angle of the ferroelectric liquid crystal.

Comparative Example A ferroelectric liquid crystal display device was prepared in the same manner as in Example 1 using a polymer of polymethacrylate alone.

The tilt angle, response speed, and contrast of this device were measured in the same manner. The tilt angle was 3 °, the response speed was 260 μs, and the contrast was 5: 1. A considerable zigzag defect occurred, and the memory property was not obtained.

In Examples 1, 2 and 3, a polymer having a liquid crystal orientation azimuth regulating force in a direction parallel to the uniaxial orientation treatment direction and a polymer having a liquid crystal orientation azimuth regulating force in a direction orthogonal to the uniaxial orientation treatment direction were used. Although the alignment film solution was prepared from the polymer mixture, it goes without saying that the alignment film may be formed by using a copolymer prepared by mixing the monomers of the respective polymers in an appropriate ratio.

Regarding the phase transition of liquid crystal, I-Ch-SmA-SmC ^* may be used, but Ch phase or SmA phase may or may not exist between the I phase and the SmC ^* phase (I-SmC ^{*). *} , I-Ch-SmC ^* , I-
SmA-SmC ^* etc).

EFFECTS OF THE INVENTION With the liquid crystal display device of the present invention, the liquid crystal near the interface of the alignment film can be tilted at substantially the same angle as the original tilt angle.
Therefore, no interface strain is generated and uniform alignment is obtained in the SmC ^* phase. Further, even when no voltage is applied, the direction inclined from the rubbing direction by a certain angle is a stable state, so that it becomes bistable and a complete memory effect can be obtained.

[Brief description of drawings]

FIGS. 1 (a) and 1 (b) show liquid crystal molecules when a liquid crystal is aligned by using an alignment film made of a polymer having a liquid crystal alignment azimuth regulating force in a direction parallel and a direction perpendicular to a uniaxial alignment treatment direction, respectively. FIG. 2 shows the alignment state of liquid crystal molecules when the liquid crystal is aligned using the alignment film in the liquid crystal display element of the present invention, and FIG. 3 shows one embodiment of the present invention. 4 is a cross-sectional view showing the configuration of the liquid crystal display element in FIG. 4, FIG. 4 is a waveform diagram showing a polarity reversal voltage applied to the liquid crystal display elements in Examples and Comparative Examples, and FIGS. 5 (a) and 5 (b) are
FIG. 8 is a diagram showing an alignment state of liquid crystal molecules when a ferroelectric liquid crystal in Example 3 is aligned. 1,9 …… Glass substrate, 2,8 …… ITO electrode, 3,4 …… Alignment film,
5 ... Seal resin, 6 ... Bead spacer, 7 ... Liquid crystal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Narihiro Sato 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Yoshihiro Matsuo, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP-A-58-68721 (JP, A)

Claims (7)

[Claims] 1. A liquid crystal display device comprising a pair of substrates whose surfaces are uniaxially oriented and a liquid crystal, wherein the pair of substrates whose surfaces are uniaxially oriented are aligned in a direction parallel to the direction of the uniaxial orientation. At least one polymer selected from the group consisting of polyimide, polyvinyl alcohol, polyvinyl acetate, polyacrylate, and polymethacrylate, and a polymer having a liquid crystal alignment azimuth regulating force in a direction orthogonal to the uniaxial alignment treatment direction. A liquid crystal display device comprising an alignment film formed of a polymer mixture composed of one or more kinds of polymers or a polymer copolymer composed of monomers thereof. 2. The liquid crystal display device according to claim 1, wherein the polymer having a liquid crystal alignment azimuth regulating force in a direction orthogonal to the uniaxial alignment treatment direction has a ring structure in a side chain. 3. An aromatic ring, an aliphatic ring, as a side chain ring structure,
The liquid crystal display device according to claim 2, having a heterocycle or a condensed ring. 4. The liquid crystal display device according to claim 1, wherein the uniaxial alignment treatment is performed on the alignment film provided on the substrate, and the alignment film is formed by a rubbing method. 5. A polymer having a liquid crystal alignment azimuth regulating force in a direction parallel to the uniaxial alignment treatment direction and a polymer having a liquid crystal alignment azimuth regulating force in a direction orthogonal to the uniaxial alignment treatment direction. Claims characterized in that the α absorption temperature (glass transition temperature) in the polymer mixture consisting of the above polymers or the polymer copolymer consisting of monomers of these polymers is higher than the injection temperature of the liquid crystal. 2. A liquid crystal display device according to item 1. 6. The liquid crystal display device according to claim 1, wherein the liquid crystal is a ferroelectric liquid crystal. 7. A polymer having a liquid crystal alignment azimuth regulating force in a direction parallel to the uniaxial alignment treatment direction and a polymer having a liquid crystal alignment azimuth regulating force in a direction orthogonal to the uniaxial alignment treatment direction. The β absorption temperature (the temperature at which the side chain undergoes Micro Brownian motion) in the polymer mixture consisting of the above polymers or in the polymer copolymer consisting of the monomers of these polymers is lower than the injection temperature of the liquid crystal, and 7. The liquid crystal display device according to claim 6, wherein the liquid crystal display device has a temperature higher than a temperature at which a phase higher than the chiral smectic C phase transitions to the chiral smectic C phase.