JPH11326918A - Liquid crystal display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the reflection type liquid crystal display element which has superior bistability and good characteristics of a reflection factor and color purity. SOLUTION: This liquid crystal display element is formed by sandwiching a chiral nematic liquid crystal composition 21 showing a cholesteric phase at room temperature and a columnar structure made of macromolecular bodies arranged at specific intervals between substrates 11 and 12 where ITO(indium tin oxide) electrodes 13 and 14 are formed. The liquid crystal composition 21 contains a nematic liquid crystal component of <=2.00, preferably, <=1.30 in elastic constant ratio K33 /K11 represented as the ratio of a bend elastic constant K33 and a spray elastic constant K11 and a chiral component.

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, and more particularly to a bistable / reflective liquid crystal display device using a liquid crystal exhibiting a cholesteric phase at room temperature.

[0002]

2. Description of the Related Art In recent years, various researches have been made on reflection type liquid crystal display devices using a chiral nematic liquid crystal which shows a cholesteric phase at room temperature by adding a chiral material to the nematic liquid crystal. In this element, display is performed by switching the liquid crystal between a planar state and a focal conic state by turning on and off the voltage.

However, to date, no reflective liquid crystal display device using a chiral nematic liquid crystal has sufficiently satisfied the characteristics of reflectance and color purity. In a device in which a chiral nematic liquid crystal is merely sandwiched between a pair of substrates, there is a concern about long-term bistability of liquid crystal molecular alignment. Therefore, a network type in which a network-like composite film made of a liquid crystal material and a resin material is formed has been proposed. However, it is difficult to form a uniform network, and the problem that characteristics such as reflectance vary locally cannot be solved.

Accordingly, an object of the present invention is to provide a reflection type liquid crystal display device which is excellent in bistability and excellent in characteristics of reflectance and color purity.

[0005]

SUMMARY OF THE INVENTION, operation and effect] To achieve the above object, a liquid crystal display device according to the present invention, between at least one of a pair of transparent substrates, the bend elastic constant K ₃₃ and a spray elastic constant K ₁₁ A liquid crystal composition containing a nematic liquid crystal component and a chiral material having an elastic constant ratio K ₃₃ / K ₁₁ represented by a ratio of 2.00 or less and having a cholesteric phase at room temperature, and being arranged at a predetermined interval in a display region. A columnar structure made of a polymer is sandwiched.

A nematic liquid crystal component having an elastic constant ratio K ₃₃ / K ₁₁ of less than 2.00 has a large elastic constant K _{33 for} bend strain and a small elastic constant K ₁₁ for splay strain, and therefore the molecule itself is STN (super- When the liquid crystal molecules are not twisted as in the case of twisted nematic, the response of the liquid crystal molecules to the applied voltage is improved. Therefore, it shows a good planar arrangement,
Reflectivity and color purity (stimulation purity) are improved.

Further, in the present invention, by providing a columnar structure made of a polymer between a pair of substrates, the arrangement of the liquid crystal composition is stabilized, and variations in characteristics such as the reflectance of the liquid crystal are minimized. Can be Bistability is improved as compared with a case where only a chiral nematic liquid crystal is sandwiched between substrates.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the liquid crystal display device according to the present invention will be described with reference to the accompanying drawings.

(Configuration and Display Operation of First Embodiment) FIG. 1 shows a cross-sectional structure of a liquid crystal display device according to a first embodiment of the present invention. (A) shows a planar state (RGB colored state) when a high voltage pulse is applied, and (B) shows a focal conic state (transparent / black display state) when a low voltage pulse is applied. This liquid crystal display element has a memory property, and the planar state and the focal conic state are maintained even after the pulse voltage is applied.

In FIG. 1, 11 and 12 are transparent substrates.
On each surface, transparent electrodes 13 and 14 are formed in a matrix. It is preferable that an insulating thin film 15 is coated on the electrode 13. The substrate 12
The visible light absorbing layer 16
Is provided. Reference numeral 20 denotes a columnar structure, and reference numeral 21 denotes a liquid crystal composition exhibiting a cholesteric phase at room temperature. These materials and combinations thereof will be specifically described with reference to the following experimental examples. Reference numeral 22 denotes a seal for sealing the liquid crystal composition 21 inside the liquid crystal display device. Reference numeral 25 denotes a pulse power supply for applying a predetermined pulse-like voltage to the electrodes 13 and 14.

In the liquid crystal display device having the above configuration, display is performed by applying a pulse voltage from the power supply 25 to the electrodes 13 and 14. That is, when the liquid crystal composition uses a cholesteric phase, by applying a relatively high pulse voltage, the liquid crystal is in a planar state,
Light of a wavelength determined based on the cholesteric pitch and the refractive index is selectively reflected. By applying a relatively low pulse voltage, the liquid crystal is in a focal conic state and is in a transparent state. When the visible light absorbing layer 16 is provided as shown in FIG. 1, black is displayed in the focal conic state.

In the present liquid crystal display device, a matrix electrode 1
The area where 3 and 14 intersect is the display pixel. In this specification, a region where light modulation is performed by liquid crystal is referred to as a display region, and its periphery is outside the display region where light modulation is not performed.

(Substrate) At least one of the substrates 11 and 12 needs to be transparent. As a transparent substrate,
In addition to glass, flexible substrates such as polycarbonate, polyethersulfone, and polyethylene terephthalate can be used.

(Electrode) The electrodes 13 and 14 are made of ITO.
A transparent conductive film represented by (Indium Tin Oxide), a metal electrode such as aluminum or silicon, or a photoconductive film such as amorphous silicon or BSO (Bismuth Silicon Oxide) can be used. In order to form the electrodes 13 and 14 in a matrix, for example, an ITO film may be formed on the substrates 11 and 12 by a sputtering method and then patterned by a photolithography method.

(Insulating Film, Alignment Film) The insulating thin film 15 is an inorganic film such as silicon oxide or an organic film such as a polyimide resin or an epoxy resin, and prevents a short circuit between the electrodes 13 and 14 and a liquid crystal as a gas barrier layer. It has the function of improving the reliability of Further, an alignment film typified by a polyimide resin may be provided on the electrodes 13 and 14 as needed. Further, the same material as the polymer used for the columnar structure 20 may be used as the insulating film or the alignment film.

(Spacer) Although not shown in FIG. 1, a spacer may be inserted between the substrates 11 and 12. This spacer is a sphere made of resin or inorganic oxide, and
The gap between 1 and 12 is kept uniform.

(Liquid Crystal Composition) As the liquid crystal composition, various nematic liquid crystals can be used. A liquid crystal composition to which a chiral material is added so as to exhibit a cholesteric phase at room temperature is used. Good. Preferred examples of the liquid crystal component include a nematic liquid crystal containing, as a main component, at least one compound selected from the group consisting of a liquid crystalline trans compound, a liquid crystalline pyrimidine compound, a liquid crystalline ester compound, and a liquid crystalline cyanobiphenyl compound.

Nematic liquid crystals have a constant elastic constant under an electric field, and generate elastic strain energy simultaneously with electrostatic energy. There are three types of elastic constants: K ₁₁ for splay strain, K ₂₂ for twist strain, and K ₃₃ for bend strain. When the value of the elastic constant ratio K ₃₃ / K ₁₁ increases, the liquid crystal becomes so-called hard, and the response to an applied voltage decreases. As is clear from the following experimental examples and comparative examples, when the elastic constant ratio K ₃₃ / K ₁₁ is 2.00 or less, preferably 1.30 or less, good response is exhibited and the color purity is improved. did.

As a method for determining the elastic constant of the liquid crystal, a method using a magnetic field and a method using an electric field are known. In the following experimental examples and comparative examples, K ₃₃ / K is determined by the elastic constant obtained by the method using an electric field. Indicates the value of ₁₁ .

On the other hand, the color purity means the distance between the reference chromaticity point of the illumination light, the chromaticity point of the main wavelength on the spectrum locus of the chromaticity coordinates, the chromaticity point of the illumination light and the liquid crystal. It shows the ratio of the distance to the chromaticity point of the display element sample. That is,
With the same brightness, it can be said that the color purity is higher (the display quality is higher) when the chromaticity point of the element is farther from the chromaticity point of the illumination light. In the following experimental examples and comparative examples, the measurement of the Y value (luminous reflectance) and the color purity (stimulus purity) were performed using a spectrophotometer CM-3700d (manufactured by Minolta) having a white light source.

As the chiral material to be added, various conventionally known chiral materials such as an ester compound, a pyrimidine compound, an azoxy compound and a tolan compound can be used. The addition amount is 5 to 45% by weight, preferably 5 to 40% by weight based on the liquid crystal component. If it is less than 5 wt%, a desired selective reflection wavelength may not be realized, and 45 wt%
Above, the cholesteric phase may not be exhibited at room temperature in some cases.

As the dye to be added, various conventionally known dyes such as an azo compound, a quinone compound, an anthraquine compound or a dichroic dye can be used. The amount of addition is 3 wt.
% Or less is preferable.

(Column Structure) First, the structure of the column structure 20 will be described. Columnar structure 20
Are, for example, a columnar body, a square columnar body, and an elliptical columnar body arranged at a predetermined interval in a predetermined pattern such as a lattice arrangement. Stripes arranged at predetermined intervals may also be used. The columnar structures 20 are not arranged at random, but are arranged at equal intervals, an array in which the intervals gradually change, and an array in which a predetermined arrangement pattern is repeated at a constant period.
It is preferable that the arrangement is such that the twelve gaps can be appropriately maintained and the image display is not disturbed. By providing columnar structures, the bistability of liquid crystal molecules is improved,
In addition, the strength as a liquid crystal display element, particularly the strength against bending and pressing is improved. The size of the columnar structure is not particularly limited, but is preferably 5 to 500 μm, more preferably 10 to 100 μm, and still more preferably 10 to 50 μm.
And has a width of 5 to 1000 μm, more preferably 10 to 500 μm, and still more preferably 25 to 20 μm.
Desirably, they are arranged at intervals of 0 μm. The preferred height of the columnar structure is 1 to 15 μm, more preferably 5 to 10 μm.
μm.

Next, the materials will be described. The columnar structure 20 is configured as a polymer having an acrylate compound as a main component. The acrylate ester is an acrylate compound or a methacrylate compound having two or more acrylate groups or methacrylate groups, and may have a structure such as an aromatic ring on the main chain. , CO ₂ , CH ₂ , O, and the like. The acrylate compound also includes an epoxy acrylate compound, a urethane acrylate compound, and the like.

Specific structural formulas (E ₁ ) to (E ₇ ) of the acrylate ester are shown below. The following equation (E ₁ )
In (E ₇ ), the substituents represented by A and R,
The integers represented by n and m can take any values within the range described for each formula, and may include a plurality of types of materials represented by the same general formula.

[0026]

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[0027]

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Next, a method for manufacturing the columnar structure 20 will be described. The method for producing the columnar structure 20 is not particularly limited, but, for example, a mask exposure method is a preferable production method. In the mask exposure method, first, an ultraviolet curable compound (column structure composition) is sandwiched between a substrate on which an ITO electrode is formed and a mask on which a predetermined pattern is formed, or an ultraviolet ray is placed on the electrode surface of the substrate. The curable compound is applied and covered with a mask, and this is irradiated with ultraviolet rays. Next, the mask is peeled off, the unexposed portion of the compound is washed with a predetermined solvent, dried and cured.

According to such a mask exposure method, a columnar structure having a desired shape and size can be accurately formed and arranged. In addition, by forming a columnar structure and then injecting a liquid crystal composition between the substrates sandwiching the columnar structure by a vacuum injection method or the like, by sandwiching the liquid crystal composition between the substrates to produce a liquid crystal display element, A liquid crystal display element with little variation in element characteristics and excellent in driving voltage and contrast can be obtained. Of course, a mixture of the liquid crystal composition and the photocurable resin material is sandwiched between the substrates, and then exposed through a mask in which a regular pattern is formed, thereby separating the resin into a polymerized phase. The columnar structure may be formed by precipitation.

(Configuration of Second Embodiment) FIG. 2 shows a cross-sectional structure (in a planar state when a high voltage pulse is applied) of a liquid crystal display element according to a second embodiment of the present invention. This liquid crystal display element is different from that of the first embodiment shown in FIG.
A small columnar structure 20 'extending to an intermediate portion between the gaps 1 and 12.
Is formed. 2, the same members as those in FIG. 1 are denoted by the same reference numerals. As in the second embodiment, not all the columnar structures need to be in contact with both substrates, and a liquid crystal display element excellent in driving voltage and contrast is obtained as in the first embodiment. be able to.

(Experimental Example 1) A chiral material CB15 (Merck) was added to a nematic liquid crystal material BL046 (Merck) containing a liquid crystalline cyanobiphenyl compound as a main component.
2.6 wt% were mixed to prepare a liquid crystal composition having a selective reflection wavelength of around 680 nm. The elastic constant ratio K ₃₃ / K ₁₁ of the liquid crystal material is 1.30. Further, one side of a pair of transparent substrates on which a transparent electrode is formed by patterning is formed by a mask exposure method into a square pillar having a side length of 50 μm and a height of 7 μm.
Were formed at regular intervals of 50 μm. At this time, a photocurable resin material containing a diacrylate compound as a main component was used as a material of the columnar structure. Then, the other substrate was superimposed on the substrate on which the columnar structure was formed, and the liquid crystal composition was injected to obtain a liquid crystal display device having a configuration shown in FIG. In such a liquid crystal display element,
When a pulse voltage of 90 V is applied between the electrodes for 5 msec,
A red state was exhibited, the Y value was 40.5, and the color purity was 22%.

(Experimental Example 2) A chiral material CB15 (manufactured by Merck) was added to a nematic liquid crystal material BL001 (manufactured by Merck) containing a liquid crystalline cyanobiphenyl compound as a main component.
A liquid crystal composition having a selective reflection wavelength of around 680 nm was prepared by mixing 2.7 wt%. The elastic constant ratio K ₃₃ / K ₁₁ of the liquid crystal material is 1.54. Further, one side of a pair of transparent substrates on which a transparent electrode is formed by patterning is formed by a mask exposure method into a square pillar having a side length of 50 μm and a height of 7 μm.
Were formed at regular intervals of 50 μm. At this time, a photocurable resin material containing a diacrylate compound as a main component was used as a material of the columnar structure. Then, the other substrate was superimposed on the substrate on which the columnar structure was formed, and the liquid crystal composition was injected to obtain a liquid crystal display device having a configuration shown in FIG. In such a liquid crystal display element,
When a pulse voltage of 80 V is applied between the electrodes for 5 msec,
It showed a red state, with a Y value of 40.2 and a color purity of 19%.

(Experimental Example 3) A chiral material CB15 (manufactured by Merck) was added to a nematic liquid crystal material BL005 (manufactured by Merck) containing a liquid crystalline cyanobiphenyl compound as a main component.
2.0 wt% were mixed to prepare a liquid crystal composition having a selective reflection wavelength near 680 nm. The elastic constant ratio K ₃₃ / K ₁₁ of the liquid crystal material is 1.99. Further, one side of a pair of transparent substrates on which a transparent electrode is formed by patterning is formed by a mask exposure method into a square pillar having a side length of 50 μm and a height of 7 μm.
Were formed at regular intervals of 50 μm. At this time, a photocurable resin material containing a diacrylate compound as a main component was used as a material of the columnar structure. Then, the other substrate was superimposed on the substrate on which the columnar structure was formed, and the liquid crystal composition was injected to obtain a liquid crystal display device having a configuration shown in FIG. In such a liquid crystal display element,
When a pulse voltage of 80 V is applied between the electrodes for 5 msec,
It shows a red state, with a Y value of 35.8 and a color purity of 19.2%.
showed that.

Comparative Example 1 A chiral material CB15 (manufactured by Merck) was added to a nematic liquid crystal material BL034 (manufactured by Merck) containing a liquid crystalline cyanobiphenyl compound as a main component.
2.6 wt% were mixed to prepare a liquid crystal composition having a selective reflection wavelength of around 680 nm. The elastic constant ratio K ₃₃ / K ₁₁ of the liquid crystal material is 2.08. Further, one side of a pair of transparent substrates on which a transparent electrode is formed by patterning is formed by a mask exposure method into a square pillar having a side length of 50 μm and a height of 7 μm.
Were formed at regular intervals of 50 μm. At this time, a photocurable resin material containing a diacrylate compound as a main component was used as a material of the columnar structure. Then, the other substrate was superimposed on the substrate on which the columnar structure was formed, and the liquid crystal composition was injected to obtain a liquid crystal display device having a configuration shown in FIG. In such a liquid crystal display element,
When a pulse voltage of 90 V is applied between the electrodes for 5 msec,
A red state was exhibited, the Y value was 34.0, and the color purity was 18%.

(Experimental Example 4) A chiral material CB15 (manufactured by Merck) was added to a nematic liquid crystal material BL046 (manufactured by Merck) having a liquid crystalline cyanobiphenyl compound as a main component.
0.0 wt% was mixed to prepare a liquid crystal composition having a selective reflection wavelength of around 550 nm. The elastic constant ratio K ₃₃ / K ₁₁ of the liquid crystal material is 1.30. Further, one side of a pair of transparent substrates on which a transparent electrode is formed in a pattern is formed by a mask exposure method into a quadrangular prism having a side length of 50 μm and a height of 9 μm.
Were formed at regular intervals of 50 μm. At this time, a photocurable resin material containing a diacrylate compound as a main component was used as a material of the columnar structure. Then, the other substrate was superimposed on the substrate on which the columnar structure was formed, and the liquid crystal composition was injected to obtain a liquid crystal display device having a configuration shown in FIG. In such a liquid crystal display element,
When a pulse voltage of 90 V is applied between the electrodes for 5 msec,
Shows green state, Y value is 43.5, color purity is 44.6%
showed that.

(Experimental Example 5) A chiral material CB15 (manufactured by Merck) was added to a nematic liquid crystal material BL005 (manufactured by Merck) containing a liquid crystalline cyanobiphenyl compound as a main component.
9.0 wt% was mixed to prepare a liquid crystal composition having a selective reflection wavelength of around 550 nm. The elastic constant ratio K ₃₃ / K ₁₁ of the liquid crystal material is 1.30. Further, one side of a pair of transparent substrates on which a transparent electrode is formed in a pattern is formed by a mask exposure method into a quadrangular prism having a side length of 50 μm and a height of 9 μm.
Were formed at regular intervals of 50 μm. At this time, a photocurable resin material containing a diacrylate compound as a main component was used as a material of the columnar structure. Then, the other substrate was superimposed on the substrate on which the columnar structure was formed, and the liquid crystal composition was injected to obtain a liquid crystal display device having a configuration shown in FIG. In such a liquid crystal display element,
When a pulse voltage of 80 V is applied between the electrodes for 5 msec,
It showed a green state, a Y value of 35.7 and a color purity of 35%.

(Comparative Example 2) A chiral material CB15 (manufactured by Merck) was added to a nematic liquid crystal material BL034 (manufactured by Merck) having a liquid crystalline cyanobiphenyl compound as a main component.
0.0 wt% was mixed to prepare a liquid crystal composition having a selective reflection wavelength of around 550 nm. The elastic constant ratio K ₃₃ / K ₁₁ of the liquid crystal material is 2.08. Further, one side of a pair of transparent substrates on which a transparent electrode is formed in a pattern is formed by a mask exposure method into a quadrangular prism having a side length of 50 μm and a height of 9 μm.
Were formed at regular intervals of 50 μm. At this time, a photocurable resin material containing a diacrylate compound as a main component was used as a material of the columnar structure. Then, the other substrate was superimposed on the substrate on which the columnar structure was formed, and the liquid crystal composition was injected to obtain a liquid crystal display device having a configuration shown in FIG. In such a liquid crystal display element,
When a pulse voltage of 90 V is applied between the electrodes for 5 msec,
It showed a green state, a Y value of 34.4 and a color purity of 37%.

The liquid crystal display device according to the present invention is not limited to the above-described embodiments and experimental examples, but can be variously changed within the scope of the invention. In particular, in the above-mentioned experimental example, red and green display are shown, but the same effect can be obtained for blue display.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a first embodiment of a liquid crystal display device according to the present invention, wherein (A) shows a planar state and (B) shows a focal conic state.

FIG. 2 is a sectional view showing a second embodiment of the liquid crystal display device according to the present invention.

[Explanation of symbols]

 11, 12 ... substrate 13, 14 ... electrode 15 ... insulating thin film 16 ... visible light absorbing layer 20, 20 '... columnar structure 21 ... liquid crystal composition 25 ... power supply

Claims (2)

[Claims] 1. A nematic liquid crystal component having an elastic constant ratio K ₃₃ / K ₁₁ represented by a ratio of a bend elastic constant K ₃₃ and a splay elastic constant K _{11 of} at least 2.00 between a pair of transparent substrates. And a chiral material, a liquid crystal composition comprising a liquid crystal composition exhibiting a cholesteric phase at room temperature, and a columnar structure made of a polymer arranged at predetermined intervals in a display region. Display element. 2. The liquid crystal display device according to claim 1, wherein said elastic constant ratio K ₃₃ / K ₁₁ is 1.30 or less.