JPH0727139B2 - Chiral smectic liquid crystal element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a chiral smectic liquid crystal element applied to a liquid crystal display element, a liquid crystal-optical shutter array, etc., and more specifically, a chiral smectic liquid crystal element having at least two stable states. The present invention relates to a liquid crystal element that improves the display contrast by increasing the tilt angle of.

[Prior Art] Examples of conventional liquid crystal elements include M.Sc.
hadt) and W. Helfrich's "Applied Physics Letters"("Applied
Physics Letters ") Volume 18, Issue 4 (Published February 15, 1971), pages 127-128," Portage Dependant Optical Activity of a Twisted Nematic Liquid Crystal " (“Voltag
e Dependent Optical Activity of a Twisted Nematic
It is known that the twisted nematic liquid crystal shown in "Liquid Crystal") is used. This TN liquid crystal produces crosstalk during time-division driving using a matrix electrode structure with high pixel density. Due to the problems that occur, the number of pixels is limited.

In addition, a display element of a type in which a switching element using a thin film transistor is connected to each pixel and switching is performed for each pixel is known, but a step of forming a thin film transistor on a substrate is extremely complicated, and a large area display element is used. There are problems that are difficult to create.

As a solution to the above-mentioned drawbacks of conventional liquid crystal devices, the use of bistability liquid crystal devices has been proposed.
k) and Lagerwall (Japanese Patent Laid-Open No. 56-107216, U.S. Pat. No. 4,367,924, etc.). A liquid crystal element having bistability is generally a chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ).
A ferroelectric liquid crystal having is used. This liquid crystal has a bistable state consisting of a first optical stable state and a second optical stable state with respect to an electric field, and therefore, unlike the above-mentioned optical modulation element used in the TN type liquid crystal, for example, The liquid crystal is aligned in the first optically stable state with respect to one electric field vector,
The liquid crystal is aligned in the second optically stable state with respect to the other electric field vector. In addition, this type of liquid crystal has the property of taking one of the two stable states extremely quickly in response to an applied electric field and maintaining that state when no electric field is applied. By utilizing such a property, many of the problems of the conventional TN type element described above are solved.
A fairly substantial improvement is obtained.

[Problems to be Solved by the Invention] However, in a conventional ferroelectric liquid crystal device having bistability, a uniform alignment state of liquid crystal is not always formed satisfactorily, so that sufficient characteristics are obtained. The reality was that there was no such thing. Therefore, there has been proposed a method for aligning a ferroelectric liquid crystal exhibiting bistability in a uniform alignment state in the presence of a surface subjected to rubbing treatment or oblique vapor deposition treatment. The inventor has found that a bistable ferroelectric liquid crystal having a uniform alignment state can be obtained by using a substrate which has already been subjected to the rubbing treatment or the oblique vapor deposition treatment.

However, experiments by the present inventor have revealed that the above-mentioned bistable state does not necessarily have the ideal bistable state shown in the above-mentioned documents published by Clark and Lagerwall.

That is, according to Clark and Lager-Wol, the tilt angle in the chiral smectic phase of the non-helical structure (angle θ shown in FIG. 3 described later) that realizes the bistability is the tilt angle in the chiral smectic phase having the helical structure ( Although it should have the same angle as the apex angle Θ of the triangular pyramid shown in FIG. 2 described later, the tilt angle θ in the non-helical structure is actually smaller than the tilt angle Θ in the helical structure. ing.
Moreover, it has been found that the reason why the tilt angle θ in the non-helical structure is smaller than the tilt angle θ in the helical structure is attributed to the twist alignment of the liquid crystal molecules in the non-helical structure.
That is, in the chiral smectic phase having a non-helical structure, the liquid crystal molecules are continuously twisted and aligned with respect to the normal line of the substrate from the axis of the liquid crystal molecule adjacent to the upper substrate to the axis of the liquid crystal molecule adjacent to the lower substrate. This causes the tilt angle θ in the non-helical structure to be smaller than the tilt angle θ in the spiral structure.

By the way, in the case of a liquid crystal element using the birefringence of liquid crystal, the transmittance under orthogonal Nicols is It is represented by. The tilt θ in the above-mentioned non-helical structure appears as an angle in the average molecular axis direction of the twisted liquid crystal molecules in the first and second alignment states. According to the above formula, the maximum transmittance is obtained when the tilt θ is 22.5 °, but the tilt angle θ in the non-helical structure that realizes the bistability is about 10 ° at the maximum, and Considering its application as a display device, its transmittance is 3-5%.
There is a problem that the degree is not sufficient.

Therefore, an object of the present invention is to solve the above-mentioned problems, namely to increase the tilt angle in the chiral smectic phase of the non-helical structure which realizes at least two stable states, and thereby to increase the transmittance at the opening of the pixel shutter. Another object is to provide a chiral smectic liquid crystal device having improved liquid crystal display.

[Means for Solving Problems] and [Operation] That is, the present invention provides a first substrate provided with a first transparent electrode layer and a first alignment control layer, a second transparent electrode layer, and a second transparent electrode layer. A second substrate provided with a second orientation control layer, a first substrate and a second substrate.
A chiral smectic liquid crystal device having a chiral smectic liquid crystal arranged between the first substrate and the first substrate,
Of the alignment control layer and the second alignment control layer of the following alignment layers A and B are formed, and the alignment film B is provided on the alignment film A, and the alignment film B is rubbed. It is a chiral smectic liquid crystal device characterized by being provided with.

(A) An alignment film A having a horizontal alignment force for aligning the long axis of liquid crystal molecules of chiral smectic liquid crystal in parallel or substantially parallel to a substrate.

(B) An alignment film B having a vertical alignment force for aligning the long axis of liquid crystal molecules of chiral smectic liquid crystal vertically or substantially vertically to the substrate.

Hereinafter, the present invention will be described in detail.

FIG. 1 is a cross-sectional view showing an embodiment of a chiral smectic liquid crystal element (hereinafter referred to as a ferroelectric liquid crystal element) of the present invention. Referring to FIG. 1, a ferroelectric liquid crystal device according to the present invention comprises a pair of glass or resin-based upper and lower transparent substrates 1, 1 ′ arranged in parallel, and indium-tin oxide (ITO) wiring on each substrate. Transparent electrode layer consisting of 2,
An alignment control layer 6, which is provided with 2'and is formed on the transparent electrode layers 2, 2'by a double layer consisting of alignment films 3, 4 and 3 ', 4'.
6'is provided, and the ferroelectric liquid crystal layer 5 made of chiral smectic liquid crystal is arranged between the orientation control layers 6, 6 '. One of the alignment films 3, 3'and 4, 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.

The alignment film A is formed of a material having a horizontal alignment force for aligning the long axis of chiral smectic liquid crystal (hereinafter, referred to as ferroelectric liquid crystal) liquid crystal molecules in parallel or substantially parallel to the substrate. , Polyesterimide, polyparaxylylene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polyvinyl acetate, polyamide, polystyrene, cellulose resin, melamine resin, urea resin, organic resin such as acrylic resin or photoresist resin or SiO , SiO ₂ and TiO ₂
It is possible to use a film formed of an inorganic substance such as.

The alignment film B has a weak horizontal regulation force, and is preferably formed of a material having a vertical alignment force for aligning the long axis of liquid crystal molecules of the ferroelectric liquid crystal vertically or substantially vertically to the substrate, and specifically, a silane cup. A coating formed of a ring agent or the like can be used.

The thickness of the alignment films A and B is usually 20 to 3000Å, preferably 50Å to 2
000Å is desirable.

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

FIG. 2 is a schematic drawing of an example of a ferroelectric liquid crystal cell using a spiral structure. 21a and 21b are In ₂ O ₃ , Sn0 ₂
A substrate (glass plate) coated with a transparent electrode such as ITO or Indium Tin Oxide (ITO), and multiple liquid crystal molecular layers between them.
Liquid crystal of SmC ^* (chiral smectic C phase) oriented so that 22 is perpendicular to the glass surface is enclosed. The thick line 23 represents a liquid crystal molecule.
_{Has a} dipole moment (P _⊥ ) 24 in a direction orthogonal to its molecule. 1/2 of the apex angle of the triangular pyramid at this time
It represents the tilt angle Θ in the chiral smectic phase of the helical structure which takes an angle of When a voltage above a certain threshold is applied between the electrodes on the substrates 21a and 21b, the liquid crystal molecules 23
The orientation of the liquid crystal molecules 23 can be changed so that the helical structure is unwound and all the dipole moments (P _⊥ ) 24 are oriented in the electric field direction. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the major axis direction and the minor axis direction thereof. Therefore, for example, if polarizers arranged in a crossed Nicols positional relationship are placed above and below a glass surface, respectively. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. Further, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μ), the helical structure of the liquid crystal element is unwound and becomes a non-helical structure even when no electric field is applied, as shown in FIG. The moment Pa or Pb is either upward (34a) or downward (34b),
A bistable state is formed. When an electric field Ea or Eb having a polarity different from a certain threshold value is applied to such a cell as shown in FIG. In response, the liquid crystal molecules are aligned in either the first stable state 33a or the second stable state 33b accordingly. Half of the angle formed by the first and second stable states at this time corresponds to the tilt angle θ.

There are two advantages of using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has bistability. The second point will be explained with reference to FIG. 3, for example. When an electric field Ea is applied, the liquid crystal molecules are aligned in the first stable state 33a, but this state is stable even when the electric field is cut off. When a reverse electric field Eb is applied, the liquid crystal molecules are in the second stable state.
It is oriented to 33b and changes its molecular orientation, but it remains in this state even when the electric field is turned off.

Further, unless the applied electric field Ea exceeds a certain threshold value, the respective alignment states are also maintained. In order to effectively realize such a high response speed and bistability, it is preferable that the cell is as thin as possible, and generally, it is 0.
5μ to 20μ, especially 1μ to 5μ are suitable. Liquid crystal having a matrix electrode structure using this type of ferroelectric liquid crystal
An electro-optical device has been proposed, for example, by Clark and Lagabal in US Pat. No. 4,367,924.

In addition to the above-described bistable ferroelectric liquid crystal element, the present invention can be applied to a ferroelectric liquid crystal element having a stable state higher than that.

Ferroelectric liquid crystals that can be used in the liquid crystal device of the present invention include, for example, p-decyloxybenzylidene-p′-amino-2-methylbutyl cinnamate (DOBAMBC), p
-Hexyloxybenzylidene-p'-amino-2-chloropropyl cinnamate (HOBACPC), p-decyloxybenzylidene-p'-amino-2-methylbutyl-α-
Cyanocinnamate (DOBAMBCC), p-tetradecyloxybenzylidene-p'-amino-2-methylbutyl-α
-Cyanocinnamate (TDOBAMBCC), p-octyloxybenzylidene-p'-amino-2-methylbutyl-
α-chlorocinnamate (OOBAMBCC), p-octyloxybenzylidene-p′-amino-2-methylbutyl-
α-methylcinnamate, 4,4′-azoxycinnamic acid-bis (2-methylbutyl) ester, 4-
o- (2-Methyl) butyl resorcylidene-4'-octylaniline, 4- (2'-methylbutyl) phenyl-
4'-octyloxybiphenyl-4-carboxylate, 4-hexyloxyphenyl-4- (2 "-methylbutyl) biphenyl-4'-carboxylate, 4-octyloxyphenyl-4- (2" -methylbutyl) biphenyl -4'-carboxylate, 4-heptylphenyl-4- (4 "-methylhexyl) biphenyl-4 '
-Carxylate, 4- (2 "-methylbutyl) phenyl-4- (4" -methylhexyl) biphenyl-4'-
Examples thereof include carboxylates, which may be used alone or in combination of two or more, and may contain other cholesteric liquid crystals or smectic liquid crystals in a range showing ferroelectricity.

In the present invention, a chiral smectic phase can be used as the ferroelectric liquid crystal, and specifically, a chiral smectic C phase (SmC _* ), H phase (SmH _* ), I phase (SmI _* ) can be used. it can.

As described above, in the present invention, by forming the alignment film into two layers, one layer having the above-mentioned alignment film A and the other layer having the two-layer structure of the alignment film B, an alternating electric field (5 to 5 KHz) is applied between the substrates. ,
By applying 1 to 300 V), it is possible to realize a more stable and ideal bistable state, and it is possible to prevent the tilt angle from decreasing due to the twist of the liquid crystal molecules in the substrate normal direction.

The liquid crystal element that eliminates the twisted arrangement can obtain the maximum transmittance / light blocking ratio contrast under the orthogonal Nicols, and the liquid crystal element having the twisted arrangement bistable state can obtain the maximum contrast under the non-orthogonal Nicols. Although it has a viewing angle dependency in which the contrast varies depending on the direction of observation at times, it is possible to eliminate the above-mentioned viewing angle dependency as well as eliminating the twisted arrangement.

[Examples] Hereinafter, the present invention will be described more specifically with reference to Examples.

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

Liquid crystal composition A Liquid crystal composition B Example 1 A polyimide film 1000Å was formed by spinner coating on a 0.7 mm-thick glass substrate on which ITO with a film thickness 1000Å was formed. The polyimide is "PIQ" manufactured by Hitachi Chemical Co., Ltd.
Was used. After baking and heating this polyimide, an alignment agent "FS116" manufactured by Daikin Industries, Ltd. was applied by a spinner, and then baked to form a film having a thickness of 50 Å on the polyimide film.

"PIQ" and "FS116" each had an alignment film as a single layer, and the alignment state was observed with respect to liquid crystal compositions A and B. "PIQ" has a horizontal alignment force and "FS116" has a vertical alignment force. Was confirmed.

A two-layer alignment film of 50 Å of "FS116" was rubbed on 1000 Å of "PIQ" with a teren cloth. The two substrates thus manufactured were attached to each other so that the rubbing axes were parallel to each other. At this time, spherical spacers having a particle diameter of 1.5 μ were dispersed on the substrate in order to keep the liquid crystal layer thickness constant.

The liquid crystal composition A was sealed between the substrates, heated to an isotropic phase, and then slowly cooled at 2 ° C./h for alignment treatment. After applying an AC electric field of 50Hz, ± 30V between the substrates of this liquid crystal cell for 2 seconds, and observing the alignment state with a polarizing microscope in a crossed Nicols state with a magnification of 30 times, a defect-free monodomain was obtained. Results were obtained.

Bistable tilt angle θ; 19 ° Contrast ratio (bright state: dark state); 1:18 Maximum transmittance in bright state; 18% Example 2 In Example 1, the liquid crystal composition A was changed to the liquid crystal composition B. A liquid crystal element was obtained in the same manner except that the liquid crystal element was replaced. Even in this case, a uniform monodomain with few defects is obtained, and it is 50Hz ± 20.
The following results were obtained by applying an alternating current of V (2 seconds), observing with a polarizing microscope under a crossed Nicols, and measuring the transmitted light intensity.

Bistable tilt angle θ; 21 ° Contrast ratio (bright state: dark state); 1:12 maximum transmittance in bright state; 18% Example 3 Vertical alignment in Example 1 manufactured by Daikin Industries, Ltd. A liquid crystal element was obtained in exactly the same manner except that the aligning agent "FS116" in Example 1 was replaced with "OSD-E" manufactured by Chisso Corporation. Observation with a polarizing microscope under crossed nicols produced a uniform monodomain, and the following results were obtained.

Bistable tilt angle θ; 15 ° Contrast ratio (bright state: dark state); 1:10 Maximum transmissivity in bright state; 15% [Effect of the Invention] Alignment control by the ferroelectric liquid crystal device of the present invention For example, it is possible to obtain a ferroelectric liquid crystal, particularly a monodomain of a ferroelectric liquid crystal having at least two stable states obtained by a non-helical structure. Further, it is possible to increase the tilt angle θ under at least two stable states which are exhibited by the non-helical structure of the ferroelectric liquid crystal, particularly under the bistable state (that is, under the memory state), and obtain a high contrast ratio value. It has an excellent effect.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a ferroelectric liquid crystal device of the present invention, FIG. 2 is a perspective view schematically showing a liquid crystal device using a ferroelectric liquid crystal having a helical structure, and FIG. FIG. 3 is a perspective view schematically showing a liquid crystal element using a ferroelectric liquid crystal having a spiral structure. 1,1 '... Transparent substrate, 2,2' ... Transparent electrode layer 3,3 ', 4,4' ... Alignment film, 5 ... Ferroelectric liquid crystal layer 6,6 '... Alignment control layer, 21a, 21b …… Substrate 22 …… Liquid crystal molecular layer, 23 …… Liquid crystal molecule 24 …… Dipole moment, 33a …… First stable state 33b …… Second stable state 34a …… Upward dipole moment 34b… … Downward dipole moment Θ …… Tilt angle in helical structure θ …… Tilt angle in non-helical structure Ea, Eb …… Electric field

Claims (2)

[Claims] 1. A first substrate provided with a first transparent electrode layer and a first alignment control layer, a second substrate provided with a second transparent electrode layer and a second alignment control layer, and a second substrate provided with the first alignment control layer. In a chiral smectic liquid crystal device having a chiral smectic liquid crystal arranged between a first substrate and a second substrate, the first alignment control layer and the second alignment control layer have the following two alignment films A and B. A chiral smectic liquid crystal device comprising a multi-layer structure, the alignment film B provided on the alignment film A, and the alignment film B subjected to a rubbing treatment. (A) An alignment film A having a horizontal alignment force for aligning the long axis of liquid crystal molecules of chiral smectic liquid crystal in parallel or substantially parallel to a substrate. (B) An alignment film B having a vertical alignment force for aligning the long axis of liquid crystal molecules of chiral smectic liquid crystal vertically or substantially vertically to the substrate. 2. The chiral smectic liquid crystal device according to claim 1, wherein the chiral smectic liquid crystal has at least first and second stable states and has a memory effect of keeping each stable state even when no electric field is applied. .