JPH05107525A - Liquid crystal display element - Google Patents

Abstract

(57) [Summary] [Object] A liquid crystal display device using a polymer-dispersed liquid crystal is intended to reduce a driving voltage. [Structure] A liquid crystal display device is constituted by adding an ion-adsorptive substance or an ion-exchangeable substance to a polymer constituting a polymer dispersed liquid crystal.

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 the resistivity of liquid crystal is improved to enable low voltage driving.

There are various methods for displaying information, but liquid crystal display (abbreviated as LCD) is generally widespread. Here, the liquid crystal display element is composed of a liquid crystal sandwiched between transparent substrates such as glass on which transparent electrodes are patterned. A simple matrix system is used as a drive system for the display electrodes and a transparent substrate. There is an active matrix type that is driven by a large number of thin film transistors (abbreviated as TFTs) formed in a matrix shape on the top, and large-area display elements are being put into practical use.

The present invention relates to an improvement in a liquid crystal display device which uses a polymer dispersed liquid crystal and adopts an active matrix system.

[0004]

2. Description of the Related Art FIG. 3 shows a conventional twisted nematic (Tw
FIG. 3 is a schematic cross-sectional view illustrating the configuration and operation of an isted Nematic (TN type) liquid crystal display element.

That is, a solid solution of indium oxide (In ₂ O ₃ ) and tin oxide (SnO ₂ ) (
Transparent electrodes 2 and 2'made of abbreviated ITO) are patterned, and the transparent electrodes 2 and 2'covered with alignment films 3 and 3'are made to face inward, and spacers are used. The liquid crystal molecules 4 are filled with the liquid crystal molecules 4 held at equal intervals.

Liquid crystal display elements are constructed by providing polarizing plates 5 and 5'on the outside of the glass substrates 1 and 1 '. When no voltage is applied between the transparent electrodes 2 and 2'of the unit elements formed in a matrix (V
_OFF ), the arrangement of the liquid crystal molecules is in a twisted state, and even if linearly polarized light is incident through the polarizing plate 5, the light twisted along the liquid crystal is shielded by the polarizing plate 5 ′ and is dark. It becomes a state.

On the other hand, when a voltage is applied between the transparent electrodes 2 and 2 ′ of the unit element (V _ON ), the liquid crystal molecule arrangement is oriented in the direction of the electric field, so that the linearly polarized light incident through the polarizing plate 5 is Since the light passes through the polarizing plate 5 ', it becomes a bright state, and a display is performed.

However, the problem with this TN type display element is that the transmitted light is attenuated and the viewing angle is narrow because a polarizing plate is used. On the other hand, FIG. 2 is a schematic cross-sectional view for explaining the structure and operation of a liquid crystal display device using polymer dispersed liquid crystal, in which the glass substrate 1, 1 ′ provided with the transparent electrodes 2, 2 ′ is shown in FIG. 3. Similarly, the polymer 6 is distributed in the form of a mesh, and the liquid crystal molecules 7 are filled in the gaps to form a liquid crystal display element.

When no voltage is applied between the transparent electrodes 2 and 2'of the unit elements formed in a matrix (V _OFF ), the liquid crystal molecules 7 are randomly oriented in the gaps between the polymers. The incident light passes through the polymer, hits the liquid crystal molecules 7 and is scattered, and does not pass through the glass substrate 1 ′, resulting in a dark state.

On the other hand, when a voltage is applied between the transparent electrodes 2 and 2'of the unit element (V _ON ), the liquid crystal molecules 7 in the gap between the polymers are oriented in the direction of the electric field, so that the incident light is The light is transmitted through the glass substrate 1 ', and the display is performed.

That is, the bright and dark display is realized by using the phenomenon of transmission and absorption of incident light in the display element using the TN type liquid crystal, while the phenomenon of transmission and scattering is realized in the display element using the polymer dispersed liquid crystal. Since the latter does not require a polarizing plate, the intensity of transmitted light is large, and the viewing angle is wide, which is excellent as a display element.

[0012]

It is necessary to lower the driving voltage of the liquid crystal in order to drive the active matrix system using the TFT using the polymer dispersed liquid crystal.

For that purpose, it is necessary to use liquid crystal molecules having a large difference in refractive index (Δn) and a large difference in dielectric constant (Δε). Here, Δn and Δε are the difference between the refractive index (n) or dielectric constant (ε) of the liquid crystal molecule in the major axis direction and n or ε in the minor axis direction.

However, since liquid crystal molecules having large Δn and Δε have polar groups, it is easy to take in impurity ions, which causes a phenomenon that the resistivity becomes low. for that reason,
Conventionally, liquid crystal molecules having a large Δn and Δε cannot be used, and in order to obtain a sufficient contrast using such a liquid crystal, it is necessary to increase the driving voltage of the TFT.

However, the increase of the driving voltage is not preferable from the viewpoint of the withstand voltage of the TFT, and the use of the TFT with the high withstand voltage is not preferable from the viewpoint of cost. From the above, it is an object to eliminate the decrease in resistivity due to the incorporation of impurity ions in the liquid crystal having large values of Δn and Δε.

[0016]

The above problems can be solved by forming a liquid crystal display device by adding an ion-adsorbing substance or an ion-exchangeable substance to the polymer constituting the polymer-dispersed liquid crystal.

[0017]

When liquid crystal molecules having large values of Δn and Δε are used, a threshold voltage is low in driving and display with high contrast becomes possible.

However, it is easy to take in ions, and in this case, the resistivity decreases, so that the drive voltage must be increased. Therefore, the present invention removes impurity ions contained in the liquid crystal by adding an ion adsorbent or an ion exchange resin into the polymer.

Here, the polymer-dispersed liquid crystal is such that when a mixed liquid of an ultraviolet curable resin, a photopolymerization initiator and a liquid crystal is irradiated with ultraviolet rays, a polymer in the form of a network is formed, and liquid crystal molecules are formed in the network. A sealed product is formed.

Therefore, in the present invention, an ion-adsorptive substance or an ion-exchangeable substance is added at the same time when the ultraviolet curable resin, the photopolymerization initiator and the liquid crystal are mixed, and impurity ions are adsorbed or exchanged during the mixing. As a result, the polymerization proceeds in a state where harmful impurity ions are eliminated from the liquid crystal, and thus the ion-adsorptive substance or ion-exchangeable substance is incorporated into the polymer.

FIG. 1 is a cross-sectional view showing the structure of a liquid crystal display device provided with a polymer dispersed liquid crystal to which the present invention is applied, in which glass substrates 1 and 1'provided with transparent electrodes 2 and 2'inside are provided. An ion-adsorptive substance or an ion-exchangeable substance 9 is dispersed in a polymer 6 formed in a mesh shape, and liquid crystal molecules 7 are formed between the mesh-shaped polymers 6.
Is filled.

Here, the necessary conditions for the ion-adsorptive substance or ion-exchangeable substance 9 are that it is a transparent material, that the refractive index n is close to that of a polymer, and that the transmission of incident light is good. In addition, it is necessary to eliminate scattering.

As a result of investigating materials satisfying this condition, the inventors have found that silica gel is used as the ion-adsorbing substance and styrene is used as the ion-exchangeable substance.
It has been found that an ion exchange resin composed of a divinylbenzene copolymer or an acrylonitrile polymer is suitable.

In this way, by removing alkali ions such as Na ⁺ and ions and K ⁺ , and halogen ions such as Cl ⁻ and F ⁻ from the liquid crystal, the following liquid crystals having large Δn and Δε values are obtained. The use of molecules has become possible. 4-cyanophenyl 4 ´-hexyloxy benzoate, 4-cyano 4 ´
-pentyloxy biphenyl, 4-cyano 4'-heptoxy biphenyl
Such,

[0025]

【Example】

Example 1: In ₂ O ₃ and SnO ₂ were co-evaporated on a glass substrate having a thickness of 1.1 mm to form a transparent conductive film having a sheet resistance of 50 Ω / cm ² , and a stripe-shaped transparent electrode was formed by using a photo-etching technique. Was patterned, and two such glass substrates were arranged so as to face each other so that the transparent electrodes were orthogonal to each other with a sealing material interposed therebetween, to obtain a test element. Next, UV curable resin ・ ・ ・ Product name HX-620 (Nippon Kayaku) ・ ・ ・ ・ ・ ・ 75% by weight photoinitiator ・ ・ ・ Product name Darocure-1173 (Merck) ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ 2〃 Liquid crystal ・・ ・ Product name E-44 (Merck) ・ ・ ・ ・ ・ ・ 20〃 Silica gel ・ ・ ・ Product name 7729 (Merck) Particle size 63μm or less ・ ・ 5〃 is well mixed and injected into the test device, then glass substrate Ultraviolet rays were radiated through to form a polymer-dispersed liquid crystal.

When the liquid crystal display device thus formed was compared with a liquid crystal display device provided with a polymer-dispersed liquid crystal containing no silica gel, the liquid crystal resistance could be increased by about 50%.

Example 2: Using the same test element as in Example 1, UV curable resin: Product name HX-620 (Nippon Kayaku Co., Ltd.) 75% by weight photopolymerization initiator Product name Darocure-1173 (Merck) ・ ・ ・ 2 〃 Liquid crystal ・ ・ ・ Product name E-44 (Merck) ・ ・ ・ ・ ・ ・ 20 〃 Ion exchange resin ・ ・ ・ Product name Amberlite 200, for cations・ 2.5〃〃 ・ ・ ・ Product name Amberlite IRA-904, for anions ・ ・ 2.5〃 was mixed and injected into a test element, and then irradiated with ultraviolet rays through a glass substrate to form a polymer-dispersed liquid crystal.

When the liquid crystal display device thus formed was compared with a liquid crystal display device provided with a polymer-dispersed liquid crystal containing no silica gel, the liquid crystal resistance was similarly increased by about 50%.

[0029]

By implementing the present invention, it is possible to suppress a decrease in resistivity even when a liquid crystal having large values of Δn and Δε is used, and by implementing the present invention, the reliability of an active matrix driven liquid crystal display device is improved. be able to.

[Brief description of drawings]

FIG. 1 is a principle diagram of the present invention.

FIG. 2 is a schematic cross-sectional view illustrating the configuration and operation of a polymer dispersion type liquid crystal display device.

FIG. 3 is a schematic cross-sectional view illustrating the configuration and operation of a TN type liquid crystal display element.

[Explanation of symbols]

1,1 'Glass substrate 2,2' Transparent electrode 6 Polymer 7 Liquid crystal molecule 9 Ion-adsorbing substance or ion-exchangeable substance

Claims (3)

[Claims] 1. A display device using a polymer-dispersed liquid crystal, wherein an ion-adsorptive substance or an ion-exchangeable substance is added to a polymer constituting the polymer-dispersed liquid crystal. 2. The liquid crystal display device according to claim 1, wherein the ion-adsorptive substance is silica gel. 3. The liquid crystal display device according to claim 1, wherein the ion exchange material is an ion exchange resin.