JPS62235927A - Ferroelectric liquid crystal element - Google Patents

Abstract

PURPOSE:To increase a tilt angle in a bistable state and to increase a contrast ratio by combining oriented films having the nature to orient the polarization direction of liquid crystal molecules near the boundary face to an orientation control layer side or liquid crystal layer side to form the orientation control layers. CONSTITUTION:This liquid crystal element consists of transparent substrates 1, 1' disposed in parallel, transparent electrode layers 2, 2' wired to the inside surfaces thereof, the orientation control layers 6, 6' formed on the inside surfaces of the layers 2, 2', and a ferroelectric liquid crystal layer 5 disposed therebetween. One of the layers 6, 6' is formed of single or double layers of either of the following oriented films A, B and the other is formed of the double layers of the films A, B. The film A is the film (e.g.; silane coupling agent) having the nature to orient the polarization direction of the molecules near the boundary in the layer 5 to the layer 6 or 6' side and the film B is the film (e.g.: polyimide polymer) having the property to orient the same conversely to the layer 5 side. The tilt angle is thereby increased and the element having excellent display contrast is obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a ferroelectric liquid crystal element applied to liquid crystal display elements, liquid crystal optical shutter arrays, etc. The present invention relates to a liquid crystal element that improves display contrast by increasing the tilt angle of the dielectric liquid crystal element.

[Prior Art] As a conventional liquid crystal element, for example, M-Shut (M,
5chadt) and W. Helfrich (W,
“Applied Physics Letters” by He1frich
tters”) Volume 18, No. 4 (February 15, 1971
(Published on the day).

+27 pages ~ 128 True “Voltage Devendant Optical Activity of a Twisted Nematic Liquid Crystal (” Vo
ltage Dependent 0ptical A
activity of a Twisted Nemat
ic Liquid Crystal”)
c) It is known whether it uses liquid crystal. This TN liquid crystal has a problem in that crosstalk occurs during time-division driving using a matrix electrode structure with high pixel density, so the number of pixels is limited.

Also, is there any known display element that connects a switching element using a thin film transistor to each pixel and switches each pixel individually?The process of forming a thin film transistor on a substrate is extremely complicated and creates a display element with a large area. There are some difficult issues to be solved.

Clark ((:
Iark) and Lage rwa I
I) (Japanese Unexamined Patent Publication No. 56-10721)
No. 6, U.S. Patent No. 4,367.924, L
:':g). Liquid crystal elements with bistability generally include chiral metallic C phase (SIIC) or H
A ferroelectric liquid crystal having a phase (Smll'') is used.

This liquid crystal has a bistable state 7g consisting of a first optically stable state and a second optically stable state with respect to an electric field, and therefore, unlike the optical modulation element used in the above-mentioned TN type liquid crystal, For example, the liquid crystal is oriented in a first optically stable state with respect to one electric field vector, and the liquid crystal is oriented in a second optically stable state with respect to the other electric field vector. Furthermore, this type of liquid crystal has the property of very quickly taking one of the above two stable states in response to an applied electric field, and maintaining that state IE when no electric field is applied. By utilizing such properties, considerable substantial improvements can be made to many of the problems of the conventional TN type elements mentioned above.

[The problem that the invention is trying to solve is, however, the conventional strength having bistability, 'A' + [
In the case of liquid crystal devices, the actual situation is that sufficient characteristics cannot be obtained because the liquid crystal is not necessarily formed in a uniformly aligned state. For this reason, methods have been proposed for aligning ferroelectric liquid crystals exhibiting bistability into a uniform alignment state in the presence of surfaces subjected to rubbing treatment or oblique evaporation treatment. The inventor of the present invention has found that a bistable ferroelectric liquid crystal having a uniform alignment state can be obtained by using a substrate that has been subjected to the above-mentioned rubbing treatment or oblique evaporation treatment.

However, according to the inventor's experiments, it has been found that the above-mentioned bistable state does not necessarily have the ideal bistable state shown in the above-mentioned literature published by Clark and Lagerwal.

In other words, according to Clark and Lagerwal, the tilt angle of a chiral metalic phase with a non-helical structure that achieves bistability (angle 0 as shown in Figure 3 below) is compared with that of a chiral metalic phase with a helical structure. There are six angles that are the same as the tilt angle (apex angle ■ of the pentagonal pyramid shown in Figure 2 below), but in reality, the tilt angle of the non-helical structure is 0, or
The tilt angle is smaller than that of a spiral structure. Furthermore, it has been found that the reason why the tilt angle of the non-helical structure becomes smaller than 0 of the helical structure is due to the twisted arrangement of the liquid crystal molecules in the non-helical structure. In other words, in a chiral metatic phase with a non-helical structure, liquid crystal molecules twist continuously from the axis of liquid crystal molecules adjacent to the upper substrate to the axis of liquid crystal molecules adjacent to the lower substrate with respect to the normal to the substrate. This may be the reason why the tilt angle 0 of the non-helical structure is smaller than the tilt angle 0 of the helical structure.

By the way, in the case of a liquid crystal element that utilizes the birefringence of liquid crystal, the transmittance under crossed Nicols is expressed as 1/lo = 5in24θs in 2-πin. The tilt θ in the above-mentioned non-helical structure appears as an angle between the average molecular axes of the liquid crystal molecules twisted in the first and second alignment states. According to the above formula, the maximum transmittance occurs when the tilt θ is 22.5°, but the tilt angle O in a non-helical structure that achieves bistability is at most about 10°. Therefore, when considering its application as a display device, its transmittance is 3.
There are some problems where around 5% is not enough.

Therefore, it is an object of the present invention to solve the aforementioned problems, namely to increase the tilt angle in the chiral smectic phase of a non-helical structure that realizes at least two stable states, thereby increasing the transmittance when the pixel shutter opens. An object of the present invention is to provide a liquid crystal element with improved properties.

[Means for solving the problem] and [Operation] That is, the present invention has a layer structure of substrate/transparent electrode layer/alignment control layer/ferroelectric liquid crystal layer/alignment control layer/transparent electrode layer/substrate. In a liquid crystal element using ferroelectric liquid crystal, one alignment control layer is formed of a single layer consisting of either one of the following alignment films A and B, or a double layer of both, and the other alignment control layer or alignment film A A ferroelectric liquid crystal element is characterized in that it is formed from a double layer consisting of B and B.

(A) An alignment film A having a property of aligning the polarization direction of molecules near the interface of a ferroelectric liquid crystal toward the alignment control layer side.

(B) An alignment film B having a property of aligning the polarization direction of molecules near the interface of the ferroelectric liquid crystal toward the ferroelectric liquid crystal layer.

- Hereinafter, the present invention will be explained in detail.

FIG. 1 is a sectional view showing one embodiment of the ferroelectric liquid crystal element of the present invention. In FIG. 1, the ferroelectric liquid crystal element of the present invention includes a pair of glass or resin-based upper and lower transparent IN1 substrates 1. 1' and a transparent electrode layer 2.2' made of indium tin oxide (ITO) wired to each substrate, and on the transparent electrode layer 2.2', alignment films 3.4 and 3', An alignment control layer 6.6' formed of a double layer consisting of 4' is provided, and a ferroelectric liquid crystal layer 5 is disposed between the alignment control layer 6.6'. Orientation II! Either one of 23.3' and 4 and 4' is the alignment film A, and the other is the alignment film B, which can be arbitrarily selected.

In the present invention, each of the alignment control layers provided on the upper and lower substrates is formed of a double layer of alignment film A and alignment film B, or one of the alignment control layers is formed of either alignment film A or B. Alternatively, the orientation control layer may be formed of one single layer and the other orientation control layer may be formed of a double layer.

The alignment film A is a film formed from a material that has the property of aligning the molecules near the interface of the ferroelectric liquid crystal toward the orientation control layer in the polarization direction. Specifically, the film forming material is silane coupling. Agents etc. are preferred.

For orientation 1I2B, a film formed from a material having a property of aligning the polarization direction of molecules near the interface of the ferroelectric liquid crystal toward the ferroelectric liquid crystal side is used, and the film forming material is specifically a polyimide polymer. (PI), polyvinyl alcohol polymer (PVA), etc. are preferred.

Further, when the alignment control layer is composed of a single layer, it is preferable to use an alignment film made of a relatively weak polar silane coupling agent or the like.

The thickness of the alignment films A and B is usually 20 to 3000 Å, preferably 50 to 2000 Å.

Next, the ferroelectric liquid crystal used in the present invention will be explained.

FIG. 2 schematically depicts an example of a ferroelectric liquid crystal cell using a helical structure. 21a and 21b are 1n2
02, 5n02 and ITO (Indium Tin 0
It is a substrate (glass plate) coated with a transparent electrode such as (Xide), between which a plurality of liquid crystal molecular layers 22 are oriented perpendicularly to the glass surface, and S C8 (chiral smectic C phase) liquid crystal is sealed. has been done. Thick line 23
represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P) 24 in a direction perpendicular to the molecule. At this time, the angle forming 1/2 of the apex angle of the triangular silver represents the tilt angle of 0 in the chiral smectic phase of the helical structure. When a voltage equal to or higher than a certain threshold is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and the alignment direction of the liquid crystal molecules 23 is changed so that all dipole moments (P↓) 24 are oriented in the direction of the electric field. You can change it. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if polarizers are placed above and below the glass surface in a crossed nicol positional relationship, It is easily understood that the liquid crystal optical modulation element is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, Ig), the helical structure of the liquid crystal molecules unwinds even when no electric field is applied, as shown in Figure 3.
It becomes a non-helical structure, and its dipole moment Pa or p
b takes either an upward (34a) or downward (:14b) state, and a bistable state is formed. When an upper field Ea or Eb of different polarity above a certain dark value is applied to such a cell as shown in FIG.
a or downward 34b, and accordingly the liquid crystal molecules are oriented in either the first stable state 79333 or the second stable state 33b. At this time, the tilt angle corresponds to 1/2 of the angle formed by the first and second stable states or 0.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the orientation of the liquid crystal molecules has bistability. To explain the second point, for example, with reference to FIG.
This state is stable even when the electric field is turned off. Furthermore, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented in a second stable state 75:l:lb, and either change the orientation of the molecules or remain in this state even after the electric field is cut off. .

Further, as long as the electric field Ea does not exceed a certain threshold value, each orientation state yE is maintained. In order to effectively realize such fast response speed and bistability,
It is preferable that the cell be as thin as possible, and generally 0.5p to 20g, particularly Ig to 5g, is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using ferroelectric liquid crystals of this type has been proposed, for example, by Clark and Lagarhar in US Pat. No. 4:157,924.

Furthermore, the present invention can be applied not only to the above-mentioned bistable ferroelectric liquid crystal element but also to a ferroelectric liquid crystal element r having a more stable state.

As the ferroelectric liquid crystal that can be used in the liquid crystal element of the present invention, for example, p-decyloxybenzylidene-p'-amino-2-methylbutylcinnamate (DOBAMBC
), P-hexyloxybenzylidene-p'-amino-2-chloropropyl cinnamate (I+ [] B
A CP C), p-decyloxybenzylidene-p'
-Amino-2-methylbutyl-α-cyanocinnamate (DOBAMBCG), p-tetradecyloxybenziritine-p'-amino-2-methylphthyl-α-cyanocinnamate (TDOBAMBC:C), p-octyloxybenziritine- p'-amino-2-methylbutyl-α-chlorocinnamate (OOBAMBG(:)
, p-cutyloxybenzylidene-p'-amino-2
-Methylphthyl-α-methylcinnamate, 4,4'-
Azoxycinnamic acitobis(2-methylbutyl) ester, 4-o-(2-methyl)notylresocylidene-4'-octylaniline, 4-(2'-methylbutyl)phenyl-4'-octyloxy biphenyl-4
-carboxylate, 4-hexyloxyphenyl-4
-(2ζmethyltutyl)biphenyl-4'-carboxylate, 4-octyloxyphenyl-4-(2''-methylbutyl)biphenyl-4'-carboxylate, 4
-hebutylphenyl-4-(4''-methylhexyl)biphenyl-4'-carboxylate, 4-(2''-methylbutyl)phenyl-4-(4'-methylhexyl)biphenyl-4'-carboxylate, etc. These can be used alone or in combination of two or more, and other cholesteric liquid crystals or smectic liquid crystals can be included as long as they exhibit ferroelectricity.

In addition, in the present invention, chiral smecti is used as a ferroelectric liquid crystal.
Specifically, chiral smectic C phase (SmC"), H phase (S11"),
It is possible to use the I phase (Sml').

Next, the alignment state of the ferroelectric liquid crystal using the alignment control layer in the ferroelectric liquid crystal element of the present invention will be explained. Fourth
Figures (a) to (d) are schematic diagrams showing the alignment state of ferroelectric liquid crystal molecules using an alignment film.

This schematic diagram was obtained from optical experiments of liquid crystal elements using various alignment films.

4(a) to 4(d) are projections of the liquid crystal molecule long axis 42 and polarization direction 43 of the ferroelectric liquid crystal molecules 41 onto the smectic phase in FIG. This figure shows the stable state in which Figure 4 (
Figure 4(b) shows the one using alignment film A on both sides, the one using alignment film A on both sides, and the one in Figure 4(c) with alignment film A on one side and alignment 1!2 on the other side. B is used. It can be seen that each alignment film defines the polarization direction of the ferroelectric liquid crystal molecules near the interface.

On the other hand, the alignment control layer in the present invention is shown in FIG. 4(d), and is composed of a double layer of the above-mentioned alignment films A and B. This shows the ideal state of a ferroelectric liquid crystal element that is oriented toward the ferroelectric liquid crystal layer and stabilized.

As explained above, in the present invention, the alignment film is made of two layers, one layer having the above-mentioned alignment ll2A, and the other layer having the alignment film B, so that an alternating current electric field (5 to 5
By applying K11z, 1~:1O1lV), it becomes possible to realize a more stable and ideal bistable state, thereby preventing the tilt angle from decreasing due to the above-mentioned twisting of the liquid crystal molecules in the normal direction of the substrate. I can do it.

The maximum transmittance/if! under crossed nicols for liquid crystals with twisted alignment eliminated! Liquid crystal devices that provide luminous index contrast and have a bistable state with twisted alignment can obtain maximum contrast under non-orthogonal Nicols, but at this time, the contrast has viewing angle dependence that differs depending on the viewing direction. However, in addition to eliminating the torsion arrangement, it is also possible to eliminate the viewing angle dependence described above.

[Example] Hereinafter, the present invention will be explained in more detail with reference to Examples.

In the examples, the following two liquid crystal compositions were used as ferroelectric liquid crystal materials.

Liquid crystal composition A CS 1011 (manufactured by Chisso ■) Liquid crystal composition B mB Example 1 A 0.7 m thick film formed of ITO with a film thickness of 1000 A
A polyimide film of 1000 A was formed on a +* glass substrate by spinner coating. As the polyimide, rPIQ manufactured by Hitachi Chemical Co., Ltd. was used. After baking and heating this polyimide, Milan Coupling '5I160204 manufactured by Toshi Silicone Company was coated with a spinner and then baking and heating to form a film thickness of several tens of amperes on the polyimide film.

'PI(b g yoU 'SI+5020J ha+
Leso J'L l”E Mukai JFJ Om layer and LCD group J
&The orientation state was observed for objects A and B, but each r
PTQ" is the alignment film B, rs116020.
was confirmed to be the alignment film A mentioned above.

'PIQi (7) V100OA upper '5I160
20J (7) A two-layer alignment film having a coating number of 1OA was subjected to a rubbing treatment using a terene cloth. 2 produced in this way
The two substrates were attached so that the rubbing axes were parallel to each other.

At this time, in order to keep the liquid crystal layer thickness constant, a particle size of 1.5
1 L of spherical spacers were sprinkled.

The above-mentioned liquid crystal composition A was sealed between the substrates, and the temperature was raised to the tomoyoshi phase, and then slowly cooled at 2° C./+1 to perform an alignment treatment. After applying an alternating current electric field of 5011 z and ±30 V for 2 seconds between the substrates of this liquid crystal cell, IBI observation of the alignment state using a polarized light m microscope in the orthogonal Nicol state of x30 times revealed that defect-free monodomains were obtained. , I got the following results.

Tilt angle in bistable state 0:21° Contrast ratio (bright state: dark state); 1:18 Maximum transmittance in bright state; 18% Example 2 In Example 1, liquid crystal composition A was changed to liquid crystal composition %lB. A liquid crystal element was obtained in exactly the same manner except for the following changes. In this case as well, uniform monodomains with few defects can be obtained, and 50 I
After applying an alternating current of t z and ±20 V (for 2 seconds), the following results were obtained by polarizing microscopic observation of crossed Nicol lenses and measurement of transmitted light intensity.

Tilt angle in bistable state: 0; 20° Contrast ratio (bright state: dark state); l: 12 Maximum transmittance in bright state, 18% Example 3 In Example 1, the rPIQ of alignment film B was manufactured by Nippon Gosei Chemical Co., Ltd. company rEG25. A liquid crystal element was obtained in exactly the same manner except that (polyvinyl alcohol-based polymer) was used.

Polarized light microscopy observation under crossed Nicols revealed a uniform monodomain, and the following results were obtained.

Tilt angle of bistable state: 0:15° Contrast ratio (bright state: candy-like m); 1:In Maximum transmittance of bright state: 15% According to the invention, it is possible to obtain monodomains of ferroelectric liquid crystals, in particular of ferroelectric liquid crystals, which have at least two stable states obtained by a non-helical structure. Also, under at least two stable states, particularly under a bistable state, developed by the non-helical structure of the ferroelectric liquid crystal.

This has an excellent effect of increasing the tilt angle θ (that is, under a memory state) and obtaining a high value of contrast ratio.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing one embodiment of the ferroelectric liquid crystal element of the present invention, FIG. 2 is a perspective view schematically showing a liquid crystal element using a ferroelectric liquid crystal with a spiral structure, and FIG. A perspective view schematically showing a liquid crystal element using a ferroelectric liquid crystal with a helical structure, and FIGS. 4(a) to 4(d) are schematic diagrams showing the alignment state of ferroelectric liquid crystal molecules using an alignment film. . 1.1'...Transparent substrate 2, 2'--Transparent electrode layer 3.3', 4. C...Alignment film 5...Ferroelectric liquid crystal layer 6, 6'--Alignment control layer 21
a, 21b one substrate 22...liquid crystal molecule layer 23
...Liquid crystal molecule 24...Dipole moment 33a.
...First stable state 33b...Second stable state 34a...Upward dipole moment 34b...Downward dipole moment 41...Ferroelectric liquid crystal molecule 42...Liquid crystal molecule long axis 43 ...Polarization direction■...Tilt angle O in helical structure...Tilt angle Ea, Eb...Electric field in non-helical structure

Claims (3)

[Claims] (1) In a liquid crystal element using a ferroelectric liquid crystal having the layer structure of substrate/transparent electrode layer/alignment control layer/ferroelectric liquid crystal layer/alignment control layer/transparent electrode layer/substrate, one of the alignment control layers is A strong film characterized in that it is formed of a single layer or a double layer of either one of the following alignment films A and B, and the other alignment control layer is formed of a double layer of alignment films A and B. Dielectric liquid crystal element. (A) An alignment film A having a property of aligning the polarization direction of molecules near the interface of a ferroelectric liquid crystal toward the alignment control layer side. (B) An alignment film B having a property of aligning the polarization direction of molecules near the interface of the ferroelectric liquid crystal toward the ferroelectric liquid crystal layer. (2) The ferroelectric liquid crystal element according to claim 1, wherein the ferroelectric liquid crystal has at least a first and a second stable state, and has a memory effect that maintains each stable state even when no electric field is applied. . (3) The ferroelectric liquid crystal device according to claim 1, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal.