JP2002069447A - Liquid crystal display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display element capable of obtaining good alignment in an alignment film having reduced imidization degree. SOLUTION: A liquid crystal 13 including a fluorocyanoester compound of 30 wt.% or more which is a single compound or mixture of compounds represented by formula (1) [Wherein, R represents a straight-chain, alkyl, alkoxy, alkenyl or alkenyloxy groups; X represents a single bond, 1,4- cyclohexylene or 1,4-phenylene groups; Y represents H or F.] is utilized.

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 particular, the present invention relates to a plastic liquid crystal display device using a low-temperature firing type alignment film using polyamic acid as a precursor.

[0002]

2. Description of the Related Art FIG. 5 shows a structure of a plastic liquid crystal display element generally used conventionally as an example of a liquid crystal display element. The plastic liquid crystal display panels 1 face each other. It has two plastic substrates 2.

The two plastic substrates 2 are adhered to each other while maintaining a predetermined gap by a peripheral sealing material 3. A liquid crystal 5 is provided in a space surrounded by the pair of substrates 2 and 2 and the peripheral sealing material 3. Is enclosed.

[0004] Transparent electrodes 6 are formed on both inner surfaces of both plastic substrates 2 facing each other.
On each transparent electrode 6, an alignment film 7 for controlling the alignment of the liquid crystal 5 is formed.

In general, the alignment film 7 is made of a polyimide-based alignment film 7 from the viewpoints of mechanical strength that can withstand rubbing treatment and solvent resistance in cleaning after rubbing treatment.
Is often used.

The most common polyimide-based alignment film 7 is formed by reacting pyromellitic dianhydride as a tetracarboxylic dianhydride with diaminodiphenyl ether as a diamine compound. However, since the alignment film 7 is insoluble in a solvent, a solution in which polyamic acid as a precursor is dissolved in the solvent has been commercialized as an alignment film material.

On the other hand, for example, a polyimide obtained by reacting 2,3,5-tricarboxycyclopentyl acetic acid dianhydride with diaminodiphenylmethane has a chemical structure that shows solubility in a solvent itself, and therefore, as a soluble polyimide, polyimide is used. It is commercialized as a solution.

Then, the solution of the polyamic acid or the solution of the soluble polyimide is applied to the plastic substrate 2 and baked to form a polyimide film as the alignment film 7 on the plastic substrate 2. It is supposed to.

In the case where a polyimide film is formed using the above-mentioned polyamic acid, it is necessary to cause an imidization reaction during firing, so that firing has to be performed at a high temperature of 250 to 350 ° C.

On the other hand, when a polyimide solution is used, since the polymer which has completely become polyimide is already dissolved in the solvent, it is only necessary to remove the solvent for forming the polyimide film. 120 to 180
It can be fired at a low temperature of ° C.

Since the plastic liquid crystal display element 1 could not be fired at a temperature of 150 ° C. or higher due to the problem of heat resistance of the substrate, the alignment film 7 was formed by firing the solution of the soluble polyimide at a low temperature. The used polyimide film was used.

However, the solution of the soluble polyimide has poor transferability (paintability), and has drawbacks such as unevenness of Vth (threshold voltage) under an intermediate voltage such as gradation display.

For this reason, recently, a low-temperature sintering type polyamic acid type alignment film 7 has been developed,
The sintering temperature of ° C. enabled the formation of the alignment film 7.

[0014]

The polyamic acid type alignment film 7 of the low temperature firing type at 120 to 150 ° C. has an imidization ratio of 20 to 70%.
It's just

Here, the imidization ratio is generally calculated by infrared spectroscopy (IR) measurement. That is, when the alignment film 7 is used as a sample, measurement is performed by transmitting infrared light through the alignment film 7, and the peak of the imide bond portion and the peak that can be normalized without changing before and after imidization (for example, in the polymer chain). (The peak based on the aromatic ring).

In the present specification, the imidation ratio of a polyamic acid as a precursor that has not been calcined at all is treated as 0%. In addition, the ratio of the peak of the imide bond portion by infrared spectroscopy to the peak that can be standardized contained in the alignment film is saturated by the relationship with the change in the firing temperature. Treat as an alignment film.

For example, FIG. 6 shows a graph of a certain alignment film.
It shows the relationship between the firing temperature and the degree of imidization representing the imidization per unit volume of the alignment film. In the alignment film shown in the figure, the alignment film fired at a firing temperature of 200 ° C. or higher is used. An orientation film having an imidization ratio of 100% is obtained.

As can be inferred from FIG.
At a firing temperature of 0 to 150 ° C., the imidization ratio is limited to 20 to 70%. When an orientation film having a small imidization ratio is used, the orientation of the liquid crystal is poor. When an ordinary liquid crystal is used, poor orientation such as a so-called inversion domain, wall domain, or linear domain occurs.

Of particular interest are linear domains and inverted domains. Here, the linear domains are alignment defects that appear linearly between the spacers, and the inversion domains are:
This is caused when the liquid crystal molecules are not completely twisted in a minute portion surrounded by three or more spacers and are maintained in an incompletely twisted state. Each domain is often seen when the orientation of the orientation film is weak.

The present invention has been made in view of the above problems, and has as its object to provide a plastic liquid crystal display device capable of obtaining good alignment in an alignment film having a low imidization ratio and reduced. Things.

[0021]

In order to achieve the above object, a liquid crystal display device according to a first aspect of the present invention is directed to a liquid crystal display device in which two plastic substrates having an alignment film formed on a transparent electrode are aligned with each other. In a liquid crystal display device in which liquid crystals are sealed between the alignment films, the alignment films are coated with a polyamic acid solution as a precursor, dried, and then baked to have an imidization ratio of 20 to 70%. And the liquid crystal contains 30% by weight or more of a fluorocyanoester compound.

By employing such a structure, the polarity of the liquid crystal itself can be increased by using a fluorocyanoester compound having a long conjugated system, whereby the polyamic acid whose imidization ratio is reduced is reduced. Even when using an alignment film using an acid as a precursor,
Good orientation can be obtained.

Further, in the liquid crystal display device according to the present invention, the fluorocyanoester-based compound has a general formula

[0024]

Embedded image

(Wherein R represents a linear alkyl group, an alkoxy group, an alkenyl group or an alkenyloxy group;
Is a single bond, 1,4-cyclohexylene group or 1,4-
Phenylene group and Y represents hydrogen or fluorine, respectively) or a mixture thereof.

By adopting such a structure, the intermolecular force between the alignment film and the liquid crystal can be increased,
The orientation force can be improved.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a plastic liquid crystal display device as an example of a liquid crystal display device according to the present invention will be described below with reference to FIGS.

The same or similar parts as those of the conventional plastic liquid crystal display element 1 will be described using the same reference numerals.

As shown in FIG. 1, the plastic liquid crystal display element 10 according to the present embodiment has a thickness of 0.1 to 0.4 m.
It has a plastic substrate 11 having a thickness of m. Examples of the plastic material of the substrate 11 include polycarbonate, polyarylate, polyether sulfone, and the like.

Further, the alignment film 12 in this embodiment is
After applying and drying a solution of polyamic acid as a precursor, 1
It is formed by firing at a temperature of 20 to 150 ° C.

The alignment film 12 is changed into a chemical structure having an imide ring by the imidization reaction during the firing, and the imide ring strengthens the intermolecular force as compared with the precursor. In addition, the orientation of the liquid crystal 13 is exhibited through the intermolecular force.

This is because the orientation of the liquid crystal 13 by the orientation film 12 is determined by the coordination of a planar conjugate system between the molecules of the orientation film 12 and the molecules of the liquid crystal 13, that is, the intermolecular interaction as an interaction between dipoles. It can be explained by considering it as a weak attraction.

That is, since the coordination of the conjugated system between the molecules of the alignment film 12 and the molecules of the liquid crystal 13 is considered to be generated by the amount of conjugation in the molecule, a cyclic structure is formed in the molecule rather than the shape of the polyamic acid. It is thought that coordination is more likely to occur in a polyimide containing a large amount because electrons including a π electron and the like move easily and conjugation in a molecule is easily formed.

That is, the polyimide shown in FIG. 3 has a wider range in which electrons can move than the polyamic acid bonding system shown in FIG. 2 and a conjugated system is easily formed. You can think.

In particular, in the case of polyimide, the better orientation of the liquid crystal 13 molecules compared to other polymers can be considered to be due to the inclusion of a large number of cyclic structures in which a conjugated system is easily formed.

Accordingly, the polyimide as the alignment film 12 in the present embodiment preferably has, for example, a long conjugated diamine or acid anhydride structure from the viewpoint of easily generating dipoles.

However, as in the present embodiment, the low temperature (120
(150 ° C. to 150 ° C.), a cyclopentane ring may be used as the acid anhydride without a benzene ring.

In this case, although the conjugation amount (dipole strength) is weaker than that of the structure containing a benzene ring, the imidization causes the molecules to be rod-shaped, thereby generating dipoles. The amount of the dipole can be considered as a parameter that facilitates the coordination between the alignment film 12 and the liquid crystal 13, that is, a parameter of the alignment. The amount of the dipole can be increased as the imidization proceeds. Therefore, even if the acid anhydride does not contain a benzene ring, the alignment force of the liquid crystal 13 can be improved by imidization.

The liquid crystal display device 10 according to the present embodiment contains, as the liquid crystal 13, at least 30% by weight of a fluorocyanoester-based compound.

[0040]

Embedded image

(Wherein R represents a linear alkyl group, an alkoxy group, an alkenyl group or an alkenyloxy group;
Is a single bond, 1,4-cyclohexylene group or 1,4-
A phenylene group, and Y represents hydrogen or fluorine, respectively). The liquid crystal 13 preferably has a dielectric anisotropy (Δε) of 10 or more.

Further, the fluorocyano compound as shown in FIG. 4 has many conjugated systems in the molecule, and the liquid crystal in which the electron bias is large and the dipole is easily generated has a stronger intermolecular force and an orientation. The orientation with the film is considered to be good. In addition, R in FIG. 4 represents a linear alkyl group, an alkoxy group, an alkenyl group, or an alkenyloxy group.

Accordingly, in particular, in the case of the alignment film 12 in which the imidization ratio is reduced as low as 20 to 70% as in the present embodiment and the coordination with the molecules of the liquid crystal 13 is difficult, the conjugate amount on the liquid crystal 13 side is reduced It is effective to increase the orientation to improve the orientation.

For this reason, in the present embodiment, the liquid crystal 1
By increasing the polarity of 3 itself, good alignment can be obtained even if the alignment of alignment film 12 is weak.

It should be noted that a more effective effect can be obtained if the design is performed so that the ratio d / p between the cell gap d and the liquid crystal pitch p is 0.55 to 0.6.

Next, the results of panel lighting tests performed on the plastic liquid crystal display element 10 of the present embodiment and the conventional plastic liquid crystal display element 1 will be described.

In this test, lighting tests of three types of plastic liquid crystal display elements 10 having different imidization rates and different firing temperatures of the alignment film 12 were performed as the plastic liquid crystal display element 10 of the present embodiment.

As a conventional plastic liquid crystal display device 1, two kinds of plastic liquid crystal display devices 1 having different imidization ratios were subjected to lighting tests.

Note that both plastic liquid crystal display elements 1
Both 0 and 1 have a 0.2 mm-thick polycarbonate plastic substrate 11 or 2 on which a barrier coat layer made of SiOx (1 <x <2) is formed.

The plastic liquid crystal display element 10 according to the present embodiment has dielectric anisotropy (Δε) for all three types.
And the conventional plastic liquid crystal display element 1 has a Δε of 9 for both types.

The results of this test are shown in Table 1 below.

[0052]

[Table 1]

The alignment film materials of Examples 1, 2 and Comparative Example 1 shown in Table 1 were PIA-X3 manufactured by Chisso Corporation.
70-001, and the alignment film material of Example 3 and Comparative Example 2 is JALS-4004-R1 manufactured by JSR Corporation.

As shown in Table 1, 3 in the present embodiment
Kinds of Plastic Liquid Crystal Display Elements 10 (Embodiment 1)
In any of 3) to 3), no display failure occurred, and good liquid crystal display was obtained.

On the other hand, in the two types of conventional plastic liquid crystal display elements 1 (conventional examples 1 and 2), display defects occurred.

This is because the plastic liquid crystal display element 10 according to the present embodiment contains a fluorocyanoester compound having a large conjugate amount of 30% by weight or more, so that the polarity of the liquid crystal 13 is strengthened and the alignment property is improved. It is presumed that it has been done.

Further, among the plastic liquid crystal display elements 10 according to the present embodiment, the plastic liquid crystal display element 10 baked at the allowable maximum temperature (150 ° C.) that the plastic substrate 11 can withstand has the highest display quality. You can see that. This is presumed to be due to the fact that the higher the firing temperature, the higher the imidization ratio and the stronger the intermolecular force, resulting in better orientation.

Therefore, according to the present embodiment, since the polarity of the liquid crystal 13 itself can be strengthened by including the fluorocyanoester-based compound in the liquid crystal 13 in an amount of 30% by weight or more, the imidization ratio is reduced to a low level. Good alignment can be obtained even when the alignment film 12 using the polyamic acid as a precursor is used. Further, since the interaction between the liquid crystals 13 can be strengthened, the disorder of the alignment can be eliminated.

It should be noted that the present invention is not limited to the above-described embodiment, but can be variously modified as needed.

[0060]

As described above, according to the liquid crystal display device of the present invention, good alignment can be obtained even when an alignment film having a low imidization ratio and a reduced polyamic acid precursor is used. it can.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a plastic liquid crystal display device in an embodiment of a liquid crystal display device according to the present invention.

FIG. 2 is a view showing a chemical structure of a polyamic acid as a precursor of a polyamide in the embodiment of the liquid crystal display device according to the present invention.

FIG. 3 is a view showing a chemical structure of polyimide as an alignment film in the embodiment of the liquid crystal display device according to the present invention.

FIG. 4 is a diagram showing the electron bias of a fluorocyanoester compound contained in a liquid crystal in an embodiment of the liquid crystal display device according to the present invention.

FIG. 5 is a cross-sectional view showing a conventionally employed liquid crystal display device.

FIG. 6 is a graph showing the relationship between the firing temperature of the alignment film and the degree of imidization.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Plastic liquid crystal display element 11 Plastic substrate 12 Alignment film 13 Liquid crystal

Claims (2)

[Claims] 1. A liquid crystal display device comprising two plastic substrates having an alignment film formed on a transparent electrode, wherein the alignment film surfaces are opposed to each other, and a liquid crystal is sealed between the alignment films. Is a film obtained by applying a polyamic acid solution as a precursor, drying and calcining the film to give an imidization ratio of 20 to 70%, and the liquid crystal contains a fluorocyanoester compound in an amount of 30% by weight or more. A liquid crystal display element characterized by containing. 2. The method of claim 1, wherein the fluorocyanoester compound is represented by the general formula: (Wherein, R represents a linear alkyl group, an alkoxy group, an alkenyl group or an alkenyloxy group, X represents a single bond,
2. The liquid crystal display device according to claim 1, wherein the compound is a compound represented by the formula: 1,4-cyclohexylene group or 1,4-phenylene group, and Y represents hydrogen or fluorine. .