JPS62293228A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To attain stable display of a phase transfer type liquid crystal over a large area by using a copolymer or block polymer between PVA and polyethylene as an orientation film. CONSTITUTION:The copolymer or block polymer between polyethylene and polyvinyl alchole (PVA) is formed like a film on a glass substrate provided with opposite electrodes. In order to reduced the surface tensile force of a coat solution, the content of polyethylene forming the copolymer or block polymer is set up to >=10%, and in order to improve humidity resistance and remove the discoloration of the electrode patterns and the deterioration of liquid crystal, it is necessary to set up the content of polyethylene to >=20%. Consequently, hysteresis width formed by a transmissivity curve to an impressed voltage can be widely secured, the generation of uneven coat can be suppressed and liquid crystal orientation film highly resisting water can be formed, so that stable display can be attained over a wide are.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention [Summary] As a method for improving the ratio of hysteresis width to drive voltage, polyethylene and polyvinyl alcohol were deposited on a glass substrate provided with a counter electrode constituting a liquid crystal display element. A liquid crystal display element formed by coating a random copolymer to form a film.

[Industrial application field]

The present invention relates to a liquid crystal display element capable of stably displaying a large area over a long period of time.

As shown in the schematic diagram of FIG. 3, a liquid crystal display element has glass substrates 2 each having an electrode pattern 1, at least one of which is made of a transparent metal, facing each other in a matrix shape with the electrode pattern 1 inside. It is possible to apply an electric field by enclosing a

For example, in the case of a dot matrix type, the line width is 200 to 300 μm on the inside of the glass substrate 2 made of parallel flat plates.
A transparent electrode and a counter electrode are made by forming a large number of parallel patterns with a line width of about 100 μm, and these electrode patterns 1 are separated by a separator made of Mylar film or glass fiber Hartisop with a thickness of about 10 μm. Electrode patterns 1 are arranged so as to face each other with a minute interval and to be orthogonal to each other, and a liquid crystal 4 is sealed in this gap. .

Here, the transparent electrode is formed using a metal electrode with good reflectance, such as aluminum (A#) instead of indium oxide (InzO3).

To drive the display element, scanning is performed by applying a scanning voltage and a signal voltage to orthogonal X and Y electrodes, and both voltages are superimposed and applied to the selected point, which is necessary for the phase transition of the liquid crystal to occur. configured to reach an electric field.

The present invention is based on the electric field.
When a nematic phase change liquid crystal, which is a mixture of a liquid crystal with chirality and a liquid crystal with chirality, is sealed in a cell made of two opposing glass substrates with transparent electrodes, and an external voltage is applied, the result is as shown in Figure 2. As shown, the light transmittance curves with respect to the applied voltage and draws a hysteresis curve.

゛And ζWhen the applied voltage is Vd, H with large light transmittance
' state (homeotropic state) and F with small light transmittance
1 state (hocalconic state) exists.

A method of displaying using these two states is a phase change display element, which is currently the mainstream of liquid crystal display (TN).
Twist Nematic:')? Compared to TN displays using crystals, ■ The viewing angle is wide because it utilizes the light scattering phenomenon.

■ Large capacity forms are possible.

■ There are few images on Flicker.

It has such characteristics.

However, if the voltage Vd is maintained in the H' state in Figure 2, the light transmittance will gradually decrease along the light transmittance line 5, and will eventually reach the same transmittance as the F state. .

The display becomes impossible.

On the other hand, if the applied voltage is maintained at Vd', which is greater than Vd, in order to keep the H' state stable over time, the initial H' state is maintained, but the light transmittance of the F state is is increased to Fl, resulting in a display with very poor contrast.

The reason for this is that the dregs transition in liquid crystal display elements occurs due to differences in the magical field strength, which changes the light transmittance. This shows that there are areas that can be displayed and areas that cannot be displayed.

In order to avoid this, it is necessary to use a flat laminated substrate and to keep the spacing between the substrates accurately constant, but there are limits to this.

Therefore, in order to realize a phase change type display element, it is necessary to ensure that the light transmittance curve changes sharply at the dark value voltage, and that the voltage width (hysteresis width Δ) of the rising and falling curves is kept as large as possible. .

Here, the hysteresis width Δ is defined as the difference between the applied voltage value at which the light transmittance is 90% on the falling curve and the applied voltage value at which the light transmittance is 20% on the rising curve. In order to perform display, it is expected that the ratio of the hysteresis width Δ to the drive voltage will be 7% or more.

However, maintaining this ratio is not easy.

Now, it is known that the value of the hysteresis width Δ changes not only depending on the composition of the liquid crystal but also depending on the liquid crystal alignment film applied to the surface of the electrode pattern.
It is not necessarily effective in the direction of increasing Δ.

For example, in display devices that use the TN display method, polyimide, silane force, and knobling agents are used as liquid crystal aligning agents, and the driving voltage of the liquid crystal cell prototyped using these is about 30 V and Δ is about 1. .2V, and the ratio of the hysteresis width to the drive voltage is as narrow as 4%, making it impossible to perform display with a constant voltage.

On the other hand, the inventors have found that the hysteresis width Δ increases when polyvinyl alcohol (hereinafter abbreviated as PVA) with a high degree of saponification is used as the alignment film.

However, since PVA is a water-soluble resin, the following problems arise when using it.

■ Due to the high surface tension of the coating liquid, it has poor wettability to the substrate, which tends to cause coating unevenness in the coating film.

■ If used in a high humidity atmosphere, moisture will enter the panel, but this moisture will cause the alignment film to swell, causing a decrease in Δ and deterioration of the electrode quality.

Therefore, it has been desired to realize an alignment film that does not have such drawbacks and has a large Δ like PVA.

[Problem that the invention seeks to solve]

As described above, in order to stably perform a phase change type liquid crystal display over a large area, it is necessary that the width of Δ is large.

Here, the inventors have found that PVA has the effect of increasing Δ, but its drawbacks are that it is easily soluble in water and has a high surface tension, which tends to cause uneven coating.

[Means for solving problems]

The above problem can be solved by using a liquid crystal display element in which a film of a copolymer or block polymer of polyethylene and polyvinyl alcohol is formed on a glass substrate provided with a counter electrode.

[Effect]

The present invention provides a PV that has the effect of increasing the hysteresis width Δ.
While retaining the characteristics of A, P
A copolymer or block polymer of VA and polyethylene is used as an alignment film instead of PVA.

Alignment films made of this material are almost insoluble in water, which improves moisture resistance, and because it is soluble in organic solvents, it is possible to create a coating liquid with low surface tension, which eliminates uneven coating. .

As a result of experiments, the content of polyethylene forming the copolymer or block polymer needs to be 10 mol % or more in order to lower the surface tension of the coating liquid.

Further, in order to improve moisture resistance and eliminate discoloration of the electrode pattern and deterioration of the liquid crystal, the polyethylene content must be 20 mol % or more.

In this way, it is possible to eliminate discoloration due to reduction of the ITO electrode and decrease in resistivity of the liquid crystal, which are observed when using a liquid crystal cell using PVA as an alignment film.

〔Example〕

A random polymer of polyethylene and PVA was synthesized by varying the content of polyethylene from 0 to 40 mol%, and a 3% by weight solution was prepared using dimethyl sulfoxide as a solvent.
The coating solution was filtered through a 0.45 μm filter.

On the other hand, a glass plate having a size of 10011 square and a thickness of 1.11 was used as a substrate, and ITO with a sheet resistance of 50 Ω was evaporated thereon to form a pattern of transparent electrodes.

Next, a coating solution was applied onto the glass substrate on which the electrodes were formed by a spin cord method, and dried at 150° C. for 1 hour to form a coating film.

In addition, as a nematic liquid crystal, 87% by weight of an ethane-based, bicyclohexane-based, and ester-based mixed liquid crystal and 13% by weight of a chiral nematic liquid crystal having two asymmetric carbons are mixed at a temperature above the isotonic phase transition temperature to produce a phase change type liquid crystal. made.

Then, a liquid crystal cell was formed using a 9 μm glass fiber as a spacer using the same method as before.

FIG. 1 shows the actual measurement results of applied voltage and light transmittance for a liquid crystal cell with a polyethylene content of 38 mol %, and it was possible to obtain a large hysteresis width Δ of about 10 % when the driving voltage was around 42 V.

Note that the values in Figure 1 are 10% after writing with a writing time of 4ms.
This is the light transmittance value after seconds have elapsed.

FIG. 4 shows the hysteresis width Δ with respect to the drive voltage obtained in this way versus the content of polyethylene, and shows that it maintains a substantially constant value even if the ratio of polyethylene changes.

As shown in the above results, the copolymer or block polymer of Pv^ and polyethylene has the same effect of widening the hysteresis width Δ as PVA; It can be seen that the problem has been resolved.

〔Effect of the invention〕

According to the present invention, it is possible to secure a wide hysteresis width formed by a light transmittance curve with respect to applied voltage, and it is possible to form a liquid crystal alignment film that is less likely to cause uneven coating and is resistant to water. Therefore, stable display can be performed over a wide area over a long period of time.

[Brief explanation of drawings]

Figure 1 is a diagram of the relationship between applied voltage and transmittance for the example; Figure 2 is a diagram of the relationship between applied voltage and light transmittance for phase change type liquid crystal; Figure 3 is a cross-sectional diagram illustrating the structure of the liquid crystal display element. Figure 4 is a diagram showing the relationship between the hysteresis width ratio and the polyethylene content. In the figure, 1 is an electrode pattern, 2 is a glass substrate, and 4 is a liquid crystal. Hardstone Ware I

Claims (1)

[Scope of Claim] A liquid crystal display element formed by sealing a nematic-cholesteric phase change type liquid crystal having a hysteresis effect between glass substrates having a transparent counter electrode on at least one side, comprising the counter electrode. A liquid crystal display element comprising a film of a copolymer or block polymer of polyethylene and polyvinyl alcohol formed on a glass substrate.