JPH09152631A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make driving by a simple matrix driving system possible by incorporating monomers into liquid crystals, selecting an unstable metastable state of the two metastable states in the relaxation state after voltage impression and potopolymerizing or thermally polymerizing the monomers. SOLUTION: A liquid crystal layer contg. the monomers and chiral nematic liquid crystals is formed between a pair of transparent electrode substrates 5 provided with liquid crystal oriented films 2. The chiral nematic liquid crystals have a twisted structure of a twist angle ϕ in an initial state. The monomers are incorporated at 1 to 5wt.% to the total weight of the liquid crystal layer. The voltage at which Frederick's transition occurs in the initial state is applied on the liquid crystal layer and the unstabler metastable state of the two different metastable states in the relaxation state after this voltage impression is selected. The monomers are polymerized in this state to form a high polymer. The liquid crystal layer contg. the high polymer and the chiral nematic liquid crystals is held between the transparent electrode substrates 5, by which this liquid crystal display device is constituted.

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 using a chiral nematic liquid crystal, and more particularly to a liquid crystal display device containing a polymer that is driven in a simple matrix by utilizing large metastability, and a manufacturing method thereof.

[0002]

2. Description of the Related Art As a liquid crystal display device, a so-called super twisted nematic (STN) mode or a twisted nematic (T) driven by a thin film transistor (TFT) is used.
N) mode liquid crystal display devices have been developed and are used as display parts for word processors and personal computers, for example.

Since these liquid crystal display elements do not have a memory effect, they are driven by a simple matrix driving method by a voltage averaging method or an active matrix driving method in which an active element is provided in each pixel.

In addition to this, liquid crystal display systems using bistable switching, such as surface-stabilized ferroelectric liquid crystals and anti-ferroelectric liquid crystals, have been proposed. Further, Japanese Unexamined Patent Publication No. 6-230751 (Prior Art 1) discloses a driving principle between metastable states of a bistable cholesteric and its application technology to a simple matrix driving system.

Particularly, in Conventional Example 1, a simple matrix for driving between bistable cholesteric metastable states in a device having a cell thickness of 2 μm or less is shown. The conventional example 1 is intended to obtain a high-speed response and a wide viewing angle. Also, E
P-0613115A2 (conventional example 2) discloses a technique for improving a drive waveform including a delay time.

[0006]

However, the above prior art has the following problems. When the TN type liquid crystal is driven by the voltage averaging method, as described in Conventional Example 1, if the number of scanning lines is N, the voltage V ON and OFF for selecting the ON state can be set. The ratio of the voltage Voff for selection is given by the following equation (1).

[0007]

(Equation 1)

From this equation, it can be understood that the contrast ratio decreases as VON / VOFF approaches 1 as N increases. Considering current electro-optical characteristics of liquid crystal, delay of driving pulse, etc., the upper limit of N is about 500. Therefore, it is difficult to use it in a display device that requires higher density display. In addition, there is a drawback that the display characteristics have visual dependency and the switching time to turn on and off is long.

Regarding an operation mode having bistability or a plurality of stable states, if it is possible to selectively switch between these states by an appropriate voltage pulse, it can be applied to a high-definition display having a large number of scanning lines. However, even in this case, there still remain problems peculiar to each.

For example, in Conventional Example 1, a simple matrix for driving between bistable cholesteric metastable states in a device having a cell thickness of 2 μm or less is shown. However, it is difficult to drive between metastable states in a wide temperature range because the physical properties of the liquid crystal change greatly depending on the temperature change. In order to deal with this, it is possible to use the technique of Conventional Example 2, but the allowable range for temperature change is not yet wide enough.

In an operation mode having bistability or a plurality of stability, the stable state used for display is often not sufficiently stable in terms of energy, so that driving between arbitrary metastable states is stably performed. Often impossible.

The present invention solves the above-mentioned problems.
The object is to provide a high-quality, high-definition liquid crystal display device that can be driven by a simple matrix driving method.

[0013]

In order to achieve the above object, the present invention comprises a pair of transparent electrode substrates provided with a liquid crystal alignment film, and a liquid crystal containing a monomer and a chiral nematic liquid crystal between the transparent electrode substrates. A layer is formed, the chiral nematic liquid crystal has a twisted structure with a twist angle φ in the initial state, and the monomer is 1 to 5 wt% with respect to the total weight of the liquid crystal layer, and the voltage that causes the Frederickss transition in the initial state is A more unstable metastable state is selected from two different metastable states in the relaxed state after application of this voltage, and in this state, the monomer is polymerized to form a polymer, and the transparent electrode substrate Provided is a liquid crystal display device in which a liquid crystal layer containing a polymer and a chiral nematic liquid crystal is sandwiched therebetween.

Here, the monomer is a compound having one or more polymerizable functional groups in the molecule or a mixture thereof. It may be a mixture of a monomer having one polymerizable functional group in the molecule and a monomer having two or more polymerizable functional groups in the molecule. However, in order that the polymer is not substantially dissolved in the liquid crystal, it is preferable to use a monomer having two or more polymerizable functional groups in the molecule, the content of which is 15% by weight or more based on the total weight of the monomers. Is preferred.

First, a chiral nematic liquid crystal prepared by adding an optically active substance for adjusting the helical pitch and a polymerization initiator to a mixture of liquid crystal and a monomer is prepared. This is injected into a rubbing-treated liquid crystal cell which is formed on a pair of transparent electrode substrates.

The amount of the polymer is preferably about 1 to 5% by weight with respect to the total weight of the liquid crystal layer. If the amount is less than 1% by weight, the effect of stabilizing the metastable state by the polymer is not sufficient, and if it exceeds 5% by weight, light scattering due to the presence of the polymer increases, which is not preferable. Although the amount of light scattering depends on the refractive index of the polymer, it is preferable that the haze value when no voltage is applied is suppressed to 2% or less.

The polymerization reaction for forming the polymer is preferably chain polymerization, polyaddition or the like. On the other hand, a reaction such as polycondensation in which a volatile substance such as water is generated during the polymerization or a reaction in which a by-product which adversely affects liquid crystal properties is generated is not preferable. This is because there is a possibility that voids may be generated in the element or the liquid crystal characteristics may be deteriorated.

It is particularly preferable to use a photocurable monomer because it can be solidified in a short time without being affected by heat. In this case, usually 0.1 to 10 mJ / cm
^The polymerization reaction can be carried out by irradiating about ² ultraviolet rays.

Representative examples of monomers having one or more polymerizable functional groups in the molecule are described below. The first group is
Is a diester of acrylic acid. here,
X is a hydrogen atom, a fluorine atom or a methyl group. A represents a residue of diol. The first group also includes compounds generally referred to as acrylic oligomers having a molecular weight of 500 to tens of thousands.

[0020]

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The second group is the monoesters of acrylic acids represented by Chemical Formula 2. Here, X is a hydrogen atom, a fluorine atom or a methyl group. B indicates the residue of monool.

[0022]

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The polymerizable monomer preferably contains a liquid crystal monomer. By doing so, the change in the clearing point between the mixed solution and the original liquid crystal becomes small, and as a result, the change in the spiral pitch also becomes small. For example, 4-cyano-acryloyloxybiphenyl can be mentioned as a monomer exhibiting liquid crystallinity.

In particular, the above-mentioned liquid crystalline monomer is used when 3% by weight of this liquid crystalline monomer is added to the original liquid crystal,
The change in the clearing point of the mixed solution with respect to the clearing point of the original liquid crystal is preferably 3 degrees or less. This is because the change in the spiral pitch can be extremely reduced.

Bistable LC described in Conventional Example 1
The basic configuration of D will be described below. If the rubbing directions that define the alignment direction of the liquid crystal intersect each other at an angle of φ _R , the helical pitch of the chiral nematic liquid crystal is such that the twist angle in the initial state before applying a voltage is approximately φ _R. , Adjusted by the addition of chiral substances.

In the initial state, if _{one of the} angles formed by the director vector of the liquid crystal molecules in contact with the liquid crystal alignment film and the substrate surface is θ ₁ and the other is θ ₂ , then θ ₁ and θ ₂ have opposite signs. Become. At this time, if the twist angle of the initial state of the chiral nematic liquid crystal is 180 °, the above two director vectors are substantially parallel to each other.

The fact that the chiral nematic liquid crystal has two metastable states different from the initial state as relaxation states means that, for example, if the initial state has a twist structure with a twist angle of 180 °, one of them is twisted as a metastable state. The angle is 0 °, and the other is a twist angle of 360 °. Which one of the metastable states is relaxed after the Freedericksz transition depends on the waveform of the driving pulse applied, and each of the metastable states has a characteristic of spontaneously relaxing to the initial state.

Next, a more unstable metastable state is selected from these two metastable states, and a polymer obtained by polymerizing a photopolymerizable or thermopolymerizable monomer in this state is added to the liquid crystal layer. To form a liquid crystal display device. The selected metastable twist angle may be φ + 180 ° or φ-180 °.

Scanning electrode groups and signal electrode groups are respectively arranged on the pair of transparent electrode substrates, and the pixels formed by these groups are driven in a time division manner.

The obtained liquid crystal display device can be driven in a time-division manner because the relationship between the applied voltage and the transmittance takes two metastable states in which the energies are almost equal to each other. FIG. 2 shows that the bright state is maintained when the voltage is set to 0 V after the voltage of V ₂ or more is applied starting from the dark state. It also shows that the dark state is maintained when the voltage is set to 0 V after applying a voltage of V ₂ or more starting from the bright state. That is, when it is desired to change from dark to bright, a voltage of V ₂ may be applied and maintained at this voltage as it is, or a sudden change from V ₂ to 0 V may be performed and then maintained at 0 V.

[0031] If you want to change from light to dark by applying a voltage of V ₂ as it is 0V from V ₂ may be kept at this voltage
It may be kept at 0V after being suddenly changed to.

The time division driving method is not limited to the above method, and the methods disclosed in Conventional Example 1 and Conventional Example 2 may be applied.

The voltage applied during the polymerization depends on the dielectric anisotropy and elastic constant of the liquid crystal used and the thickness of the device.

The power source applied during polymerization may be direct current or alternating current. In the case of alternating current, the frequency is 1 because of the ease of application.
00 kHz or less, usually 10 Hz to 1 kHz is preferable. The cell thickness is 2 μm or more, more preferably 4 μm or more.

The basic structure of the liquid crystal cell sandwiching the liquid crystal layer is as follows. ITO patterned in a desired pattern on the surface of a substrate such as plastic or glass
A transparent electrode such as (In ₂ O ₃ —SnO ₂ ) or SnO ₂ is provided to form a substrate with an electrode. The electrode layers are arranged in a matrix in which the electrode groups face each other in correspondence with the display, and this makes it possible to control ON / OFF for each dot. The method for forming the electrode layer is not particularly limited to this, but from the viewpoint of making the layer thickness uniform, a vapor deposition method, a sputtering method, or the like is preferably used.

[0036] In the present invention, an insulating film of SiO _2, TiO _2, etc. above or below the electrode as needed, a phase difference film, polarizing film, reflective film, the photoconductive layer or the like may be formed.

On the surface of this substrate with electrodes, an alignment control film made of a film such as polyimide or polyamide whose surface is rubbed, or a film such as obliquely deposited SiO or the like is formed. 2
A sheet of the substrate is prepared so as to sandwich the liquid crystal layer.

An insulating film made of TiO ₂ , SiO ₂ , Al ₂ O ₃ or the like is provided between the electrodes and the orientation control film to prevent a short circuit between the substrates, or a transparent electrode made of a material such as Al, Cr or Ti is used. A resistance lead electrode may be provided together.

A pair of polarizing plates are arranged on both outer sides of this substrate. The polarizing plate itself is generally placed outside the substrate that constitutes the cell, but if performance permits, the substrate itself may be configured with a polarizing plate or provided as a polarizing layer between the substrate and the electrode. Good.

It is also possible to use a color filter together. By forming this color filter on the inner surface of the cell, it is possible to perform more precise color display without causing a shift due to the viewing angle. Specifically, it may be formed below the electrode, or may be formed above the electrode. Further, a color filter for color correction or a color polarizing plate may be used together, a dye may be added to the liquid crystal layer, or illumination having a specific wavelength distribution may be used.

The element generally produces an interference color. As a compensation layer for the interference color, a liquid crystal cell having a reverse twist with the display cell and having substantially the same twist size is laminated,
It is also possible to stack so-called phase difference plates and perform black-and-white display.

Driving is performed by connecting driving means for applying a voltage to the electrodes of the liquid crystal cell having such a structure. That is, an anisotropic conductive rubber is used for the connection terminal portion led to the end of the board, an external circuit board made of a flexible board or the like is connected using a heat seal, an anisotropic conductive adhesive, or a TAB board. Or connect.

The present invention can be applied to both transmission type and reflection type, and its application range is wide. When the transmission type is used, a light source is arranged on the back side. Of course, a light guide and a color filter may be used together with this. Furthermore, in the case of using the transmissive type, the background portion other than the pixels can be covered with a light shielding film by printing or the like.

In addition to the above, the present invention can be applied to various techniques used in ordinary liquid crystal display devices within a range that does not impair the effects of the present invention.

The twist angle φ in the initial state is not limited to 180 ° and can be set to any angle. For example, when the twist angle φ is 90 ° in the initial state, the metastable state has a twist structure with twist angles of −90 ° and 270 °, and switching is possible between those metastable states. Confirmed experimentally.

The details of the mechanism for stabilizing the structure of the liquid crystal molecules by the polymer are unknown, but the polymer structure formed during the polymerization affects the orientation of the liquid crystal, and is metastable even when the cell thickness is 4 μm or more and in a wide temperature range. It is considered that it functions to stabilize the state. In particular, since the more unstable metastable state is selected from the two different metastable states and this state is stabilized by the polymer, the energy levels of the two metastable states become almost equal. Shows a voltage-transmittance curve such as

[0047]

EXAMPLES The present invention will be described in detail below with reference to specific examples. FIG. 1 is a cross section of a liquid crystal display device. Reference numeral 1 is a liquid crystal molecule, 2 is an alignment film, 3 is an insulating layer, 4 is a transparent electrode, 5
Is a glass substrate, 6 is a flattening layer, 7 is a polarizing plate, 8 is a light-shielding layer between pixels, θ ₁ and θ ₂ are pretilt angles of liquid crystal molecules at the interface, and 9 is a major axis direction of the liquid crystal molecule 1 at the interface. It is a director vector. Then, the ITO transparent electrode pattern 4 is formed on the glass substrate 5, the polyimide alignment film 2 is applied, and the substrate having the surface subjected to the rubbing treatment is arranged facing each other with a desired gap interposed therebetween through a suitable spacer to form a cell. did.

Example 1 For a liquid crystal composition exhibiting a nematic phase at room temperature (Δn = 0.10), A in Chemical formula 1 is a 4,4′-biphenylene compound represented by Chemical formula 4 below 4 2% by weight of 4'-diacryloxybiphenyl
Add 0.03% by weight of benzoin isobutyl ether,
Further, an optically active additive (S811 manufactured by Merck & Co., Inc.) was added to adjust the helical pitch to p = 7 μm.

[0049]

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In the cell, a polyimide alignment film was subjected to rubbing treatment in the antiparallel direction (180 °) on the upper and lower substrates. Also,
The gap d was set to 4.0 μm. When the liquid crystal composition is enclosed, the interface pretilt angle becomes about 4 ° with the opposite sign in the vicinity of the upper and lower substrates, and p / 4 <d <3p / 4.
The orientation of liquid crystal molecules is 180 °, which has a helical axis in the direction normal to the substrate.
A twisted state was formed.

After applying an alternating voltage of 30 V to this liquid crystal element, the voltage of 2 V was applied and ultraviolet rays of 1 mJ / cm ² were irradiated for 1 minute to prepare an element of the present invention in which a 0 ° twist state was stabilized by a polymer. did.

When the stability of the device thus obtained in the metastable state was examined, it was stable from −10 ° C. to + 50 ° C. In the drive shown in FIG. 2, V ₂ = 4
As V, 1/240 duty drive was possible.

Comparative Example In Example 1, the helical pitch p = 7 was obtained by adding only the optically active additive to the liquid crystal composition.
It was adjusted to μm. After applying an alternating voltage of 30 V to this liquid crystal element, an alternating voltage of 2 V was applied to select a metastable state.
This metastable state was stable in the temperature range of 20 ° C to 40 ° C.

[0054]

As described above, in the present invention, a liquid crystal is mixed with a monomer, a voltage that causes a Freedericksz transition in the initial state is applied to the liquid crystal, and two quasi-states in a relaxed state after the voltage is applied are applied. By selecting the more unstable metastable state among the stable states and photopolymerizing or thermally polymerizing the monomer, the unstable metastable state is stabilized by the polymer and can be driven by the simple matrix driving method. A fine liquid crystal display device was obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a liquid crystal display device according to the present invention.

FIG. 2 is a characteristic diagram showing a relationship between applied voltage and transmittance in the liquid crystal display device according to the present invention.

[Explanation of symbols]

 1: Liquid crystal molecule 2: Alignment film 3: Insulating film 4: Transparent electrode 5: Glass substrate 6: Flattening layer 7: Polarizing plate 8: Inter-pixel light shielding layer

Claims (5)

[Claims] 1. A pair of transparent electrode substrates provided with a liquid crystal alignment film, a liquid crystal layer containing a monomer and a chiral nematic liquid crystal is formed between the transparent electrode substrates, and the chiral nematic liquid crystal has a twist angle φ in an initial state. The twisted structure of the monomer is 1 and the monomer is 1 with respect to the total weight of the liquid crystal layer.
A voltage that causes a Freedericksz transition to be applied in the initial state is set to ˜5% by weight, and a more unstable metastable state is selected from two different metastable states in the relaxation state after the application of this voltage. 2. A liquid crystal display device, wherein the monomer is polymerized to form a polymer, and a liquid crystal layer containing the polymer and a chiral nematic liquid crystal is sandwiched between the transparent electrode substrates. 2. The liquid crystal display device according to claim 1, wherein an angle formed by the director vectors of the liquid crystal molecules in contact with the liquid crystal alignment film in the initial state with each surface of the pair of transparent electrode substrates has mutually opposite signs. . 3. The liquid crystal display device according to claim 1, wherein the twist angles of the liquid crystal molecules in the two metastable states are φ-180 ° for one and φ + 180 ° for the other. 4. A liquid crystal according to claim 1, 2 or 3, wherein a scanning electrode group and a signal electrode group are arranged on each of the pair of transparent electrode substrates, and pixels constituted by these groups are driven in a time division manner. Display device. 5. The voltage applied to generate the Freedericksz transition is a voltage pulse equal to or higher than a threshold value in the initial state and two metastable states, and a more unstable metastable state of the two metastable states is set. To select, the applied voltage is a voltage pulse selected with reference to a critical value that produces one of two metastable states, a more unstable metastable state is selected, and the monomer is polymerized in this state. The liquid crystal display device according to claim 1, wherein the voltage applied at this time is a voltage pulse equal to or less than a threshold value in two metastable states.