JPH0915565A - Liquid crystal display element - Google Patents

Abstract

PURPOSE: To provide a polymer dispersion type liquid crystal display element which shows little change in the operational voltage with temp. and can be operated at low voltage. CONSTITUTION: This liquid crystal display element consists of a pair of substrates having pixel electrodes and a liquid crystal light-controlling layer between the substrates. The liquid crystal light-controlling layer has such a structure that a liquid crystal is held in the microstructure formed by a polymer matrix. In this element, the liquid crystal is a cholesteric liquid crystal having positive dielectric anisotropy and 10-200μm natural pitch.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art As a display system of a liquid crystal display element, there is used a deflector represented by a twisted nematic (TN) mode or a super twisted nematic mode, which utilizes birefringence and optical rotatory power of a liquid crystal, and a dynamic type. There is a method that utilizes light scattering by liquid crystals without using a polarizing plate such as a scattering mode or a phase transfer mode. Among them, the light-scattering method does not require a polarizing plate, and therefore has a feature that brighter display is possible without loss (absorption) of light by the polarizing plate. One of the light scattering methods is a polymer-dispersed liquid crystal display element, which has been actively developed in recent years. This has a structure in which a liquid crystal is dispersed in a droplet form by a support such as a resin or a liquid crystal dispersion film in which a resin network structure is formed in the liquid crystal is sandwiched by a substrate with electrodes. In this method, generally, when no voltage is applied, the orientation of the liquid crystal is disturbed by the support, and light is scattered due to minute fluctuations in the refractive index. When a voltage is applied to this element, when the liquid crystal has a positive dielectric anisotropy, the liquid crystal molecules are aligned in the direction of the electric field and the fluctuation of the refractive index is reduced, so that the liquid crystal layer becomes transparent. This method has the advantage that the response speed is fast in addition to the brightness of the display. Furthermore, higher reliability is possible compared to the conventional scattering type liquid crystal display element,
It has a feature that it can be operated at a lower voltage than a conventional scattering type liquid crystal display element depending on a material and a manufacturing method. A problem with such a light-scattering type liquid crystal display element is that its operating voltage greatly depends on temperature. For example, in the case of the TN type, the change rate of the saturation voltage per 1 ° C. is about 10 mV / ° C., but the polymer dispersion type is 40 mV / ° C. or more, and the temperature change of 40 ° C. is 1.6 V or more. The saturation voltage changes. Such a large change causes a large increase in the driving voltage at low temperatures, which causes a problem that sufficient contrast cannot be obtained with a limited power supply voltage. Further, there is a problem in that the cost is increased because the device or the display element cannot be used unless the function of adjusting the drive voltage is added.

[0003]

[Objective] The present invention has little change in operating voltage with temperature, and
It is another object of the present invention to obtain a polymer dispersed liquid crystal display device that operates at a low voltage.

[0004]

According to the present invention, a pair of substrates having pixel electrodes are provided,
In a liquid crystal display element sandwiching a liquid crystal light control layer in which a liquid crystal is held in a microstructure formed by a polymer matrix, the liquid crystal has a positive dielectric anisotropy and a natural pitch of 10 μm to 200 μm.
The present invention relates to a liquid crystal display device characterized by being a cholesteric liquid crystal of m. That is, the present invention achieves the above object by using a liquid crystal having a positive dielectric anisotropy and a natural pitch of 10 to 200 μm as the liquid crystal for the polymer-dispersed light control layer of the liquid crystal display device. . Hereinafter, the present invention will be described in more detail with reference to the drawings. FIG. 1 shows the temperature change of the voltage-transmittance characteristic of a polymer-dispersed liquid crystal using a nematic liquid crystal (TL203 manufactured by Merck Limited, trade name). The voltage-transmittance characteristic curve is largely shifted in the range of 0 ° C to 40 ° C, which is the operating temperature range of a general liquid crystal display device. FIG. 2 shows nematic liquid crystal (TL203 manufactured by Merck Limited, trade name) and E. FIG. 3 shows temperature changes of voltage-transmittance characteristics of a liquid crystal display device of the present invention using a liquid crystal to which 0.51% by weight of S811 (trade name) manufactured by Merck was added. It can be seen that the temperature change in FIG. 2 is about 1/2 of that in FIG. The above-mentioned effect is that the natural pitch of the cholesteric liquid crystal is 200 μm.
If it is longer than 10 μm and it is too short, a sufficient effect cannot be obtained, and if it is shorter than 10 μm and it is too short, the saturation voltage becomes high, or light scattering becomes weak when no voltage is applied, and Occurs. A more preferable range of the natural pitch is 15 to 100 μm.
Range.

The natural pitch of the liquid crystal used in the liquid crystal display device of the present invention can be easily adjusted by a conventionally known method. For example, when an optically active substance such as a cholesteric liquid crystal is added to a nematic liquid crystal, the natural pitch becomes shorter in inverse proportion to the concentration added.Therefore, by selecting an appropriate amount of the optically active substance such as a cholesteric liquid crystal, the natural pitch It can be adjusted. However, since the proportionality coefficient largely depends on the intrinsic pitch of the optically active substance used and also slightly depends on the type of nematic liquid crystal, it is impossible to unambiguously exemplify the preferable addition amount from the natural pitch. As the optically active substance to be used, cholesteric liquid crystals are preferable in view of stability of liquid crystal phase, and cholesterol derivatives and liquid crystals of the formulas (1) to (8) exemplified below are typical. As cholesteric liquid crystal, product name S manufactured by Merck
Taking 811 as an example, the preferable addition amount is about 0.0
The range is 5% to 1% by weight. Taking the company's CB15 as an example, the range is 0.1% to 2% by weight.
For example, it is 0.3% by weight to 8% by weight.

[0006]

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The liquid crystal display device according to the present invention can be manufactured by a conventionally known manufacturing method. Generally, in order to form a polymer-dispersed liquid crystal light control layer, a polymerizable monomer and / or an oligomer (also referred to as a prepolymer) and a liquid crystal are mixed, and the mixture is polymerized to induce phase separation. Induced phase separation method and emulsion method in which liquid crystal is dispersed in polymer aqueous solution, and the dispersion is applied and dried, solvent-induced phase in which polymer and liquid crystal are dissolved in an organic solvent, and this is applied and dried to induce phase separation The separation method (or cast method) is typical. Although the present invention is not particularly limited to the production method, a polymerization-induced phase separation method is preferably used from the viewpoint of easy control of the dispersion structure. Among them, a radical-polymerizable monomer and / or oligomer such as acrylate or methacrylate is used as a prepolymer. The photopolymerization method described above is particularly preferably used. The nematic liquid crystal used as a mixture with the optically active substance has a positive dielectric anisotropy. Toluene-based, cyclohexanecarboxylic acid cyanophenyl ester-based, cyanophenylethane-based, and cyanophenyldioxane-based liquid crystal mixed compositions having a cyano group at the molecular end and having a positive dielectric anisotropy such as A nematic liquid crystal having a positive dielectric anisotropy can be exemplified by adding a liquid crystal having a substantially neutral dielectric anisotropy or a negative liquid crystal to the liquid crystal having a positive anisotropy in order to adjust the viscosity and the liquid crystal temperature range. . Examples of the nematic liquid crystal as described above include those of the following formulas (9) to (24).

Embedded image

Embedded image (In the above formula, R means an alkyl group or an alkoxy group.)

As the nematic liquid crystal, in the nematic liquid crystal having a cyano group, a halogen-substituted liquid crystal having a fluorine atom, a chlorine atom, a trifluoromethyl group or the like as an electron-withdrawing group in place of the cyano group can also be used. . In the case of the halogen-substituted liquid crystal, liquid crystal having substantially neutral dielectric anisotropy or negative liquid crystal can be added as in the case of the cyano-substituted liquid crystal. Especially in the polymerization-induced phase separation method, the compatibility of the prepolymer and the liquid crystal greatly influences the dispersion structure to be obtained, and when a liquid crystal having a high compatibility with the prepolymer is used, it operates at a low voltage and has a high contrast liquid crystal. A display element is obtained. Halogen-substituted liquid crystals have higher compatibility with prepolymers than cyano-substituted liquid crystals,
Therefore, halogen-substituted liquid crystals are particularly preferable in the polymerization-induced phase separation method. Examples of such halogen-substituted liquid crystals include those of the following formulas (25) to (39).

Embedded image

Embedded image (In the above formula, R means an alkyl group or an alkoxy group, and X means F, Cl, CF ₃ or OCF _3. )

The dielectric anisotropy Δε of the liquid crystal composition used in the liquid crystal display device of the present invention is positive, preferably in the range of +5 to +20, and more preferably in the range of +7 to +15. △ ε
When is smaller than the above range, the operating voltage becomes high, and when it is too high, the voltage applied to the liquid crystal portion becomes low and the operating voltage rises due to the relationship with the dielectric constant of the polymer matrix. The size of the microstructure formed of the polymer matrix and holding the liquid crystal is preferably in the range of 1 μm to 4 μm, and more preferably in the range of 1.5 μm to 3.5 μm. If it is less than this range, the operating voltage is remarkably increased, and if it is too large, the light scattering property when no voltage is applied is lowered and the contrast is impaired. To control the size of the microstructure in this range, in the case of the photopolymerization method, it can be controlled mainly by controlling the phase separation rate, and specifically, the selection and addition amount of the polymerization initiator, the exposure intensity, Control of reaction initiation rate by control of wavelength, control of liquid crystal concentration to prepolymer, control of compatibility with liquid crystal by selection of prepolymer, control of reactive double bond amount in prepolymer, polyfunctional in prepolymer It can be controlled by controlling the kind and addition amount of the monomer or oligomer.

[0010]

The present invention will be described in more detail with reference to the following examples.

Example 1 A bifunctional acrylate [MANDA (trade name) manufactured by Nippon Kayaku], 2-ethylhexyl acrylate which is a monofunctional acrylate, and benzoin ethyl ether which is a photopolymerization initiator are used in an amount of 48.5: 48.5: 3. A prepolymer composition was prepared at a weight ratio of To the cyano-substituted liquid crystal BL046 (Δε = + 13.2) manufactured by Merck, a chiral nematic liquid crystal S811 manufactured by Merck was added at a rate of 0.5% by weight to prepare a liquid crystal composition having a natural pitch of 20 μm. The liquid crystal concentration of the prepolymer composition and the liquid crystal composition is 75% by weight.
And the mixture was injected into a liquid crystal cell having a gap of 7 μm. The cell was kept at 27 ° C. where the mixture was in a uniform phase and irradiated with light from a high pressure mercury lamp of 80 W / cm ² to prepare a polymer dispersion type liquid crystal display device in which liquid crystal droplets were dispersed in a size of about 2.5 μm. . When the voltage-transmittance characteristic of this element was measured, the transmittance was saturated at 5 V at room temperature, and the temperature coefficient of this saturated voltage was 25 mV / ° C.
The temperature coefficient was calculated by dividing the difference between the saturation voltage at 0 ° C and the saturation voltage at 40 ° C by the temperature range (= 40 ° C).

Comparative Example 1 When a liquid crystal display device was manufactured without adding S811 to the liquid crystal in Example 1, it was saturated at the same voltage at room temperature, but the temperature coefficient of the saturation voltage was very high at 70 mV / ° C. .

Example 2 A liquid crystal composition TL mainly composed of a halogen-substituted liquid crystal instead of BL046 as a nematic liquid crystal in Example 1.
A liquid crystal display device was produced in the same manner except that 203 (Δε = + 11) was used. The natural pitch of the liquid crystal was 22 μm. This device saturates at 3.2 V at room temperature and operated at a lower voltage than in Example 1 using a cyano liquid crystal. The temperature coefficient of the saturation voltage was -20 mV / ° C, which was excellent.

Comparative Example 2 A liquid crystal display device was produced in the same manner as in Example 2 without adding S811. Although this device was saturated at the same voltage at room temperature, the temperature coefficient of the saturation voltage was very high at 45 mV / ° C.

Example 3 A liquid crystal display device was produced in the same manner as in Example 2 except that the amount of S811 added was 0.2% by weight and the natural pitch was 50 μm. The temperature coefficient of the saturation voltage of this element is 2
It was 8 mV / ° C., which was improved compared to the comparative example.

Example 4 A liquid crystal display device was produced in the same manner as in Example 2, except that the amount of S811 added was 0.1% by weight and the natural pitch was 100 μm. Also in this element, the temperature coefficient was improved as compared with the comparative example.

Specific embodiments of the present invention will be described below. 1. In a liquid crystal display element in which a liquid crystal dimming layer in which liquid crystal is held in a microstructure formed by a polymer matrix is sandwiched between a pair of substrates having pixel electrodes, the liquid crystal has a positive dielectric anisotropy and a natural pitch. A liquid crystal display device, which is a liquid crystal of 10 μm to 200 μm. 2. In the liquid crystal display element of 1, the natural pitch is 15
A liquid crystal display device having a range of 100 μm. 3. In the liquid crystal display device of 1 or 2, the liquid crystal light control layer is composed of a phase-separated structure obtained by a polymerization reaction of liquid crystal and a polymerizable monomer and / or oligomer, and the liquid crystal is a nematic liquid crystal to which an optically active substance is added. A liquid crystal display device which is a composition. 4. The liquid crystal display device according to the above 3, wherein the optically active substance is a cholesteric liquid crystal. 5. The liquid crystal display element according to the above-mentioned 4, wherein the addition amount of the cholesteric liquid crystal is about 0.05 to 8% by weight.

6. The liquid crystal display element according to any one of 4 to 5 above, wherein the nematic liquid crystal has a cyano group at a molecular end. 7. The liquid crystal display element according to any one of 4 to 6 above, wherein the nematic liquid crystal has a fluorine atom and / or a chlorine atom, or a substituent containing a halogen atom thereof. 8. 7. The liquid crystal display element according to the above 7, wherein the nematic liquid crystal has at least one electron-withdrawing group selected from the group consisting of a fluorine atom, a chlorine atom and a trifluoromethyl group. 9. The liquid crystal display device according to any one of 1 to 8 above, wherein the liquid crystal composition has a dielectric anisotropy Δε of +5 to +20, more preferably +7 to +15. 10. In the liquid crystal display device of 1 to 9, the size of the microstructure in which the liquid crystal of the liquid crystal light control layer is held is 1 to 4 μm.
m, preferably a liquid crystal display device having a range of 1.5 to 3.5 μm.

[0019]

【effect】

1. Claims 1 and 2 A polymer-dispersed liquid crystal display device having a small change in operating voltage with temperature can be obtained. 2. According to the third aspect, in addition to the effect of the first aspect, a liquid crystal display device that operates at a lower voltage can be obtained. Therefore, the liquid crystal display element of the present invention,
In particular, it is suitable as a display element for a device which does not have a liquid crystal drive voltage adjusting function in a small portable device.

[Brief description of the drawings]

FIG. 1 is a diagram showing a temperature change of a voltage-transmittance characteristic of a polymer-dispersed liquid crystal using a conventional nematic liquid crystal (TL203 manufactured by Merck Limited, trade name).

FIG. 2 shows a liquid crystal display device of the present invention [TL203 + E. Merck S811, product name 0.5
It is a figure which shows the temperature change of the voltage-transmittance characteristic of [weight%, pitch = 21 micrometers].

Claims (3)

[Claims] 1. A liquid crystal display device having a liquid crystal dimming layer in which a liquid crystal is held in a microstructure formed by a polymer matrix, sandwiched between a pair of substrates having pixel electrodes, and the liquid crystal has a positive dielectric anisotropy. And a liquid crystal display element, which is a cholesteric liquid crystal having a natural pitch of 10 to 200 μm. 2. The liquid crystal display device according to claim 1,
A liquid crystal display device, wherein the liquid crystal light control layer has a phase-separated structure obtained by a polymerization reaction of liquid crystal with a polymerizable monomer and / or oligomer, and the liquid crystal is a composition in which an optically active substance is added to nematic liquid crystal. 3. The liquid crystal display element according to claim 1 or 2, wherein the liquid crystal is mainly composed of a liquid crystal having a substituent containing a fluorine atom and / or a chlorine atom or a halogen atom thereof.