JP3343338B2 - Liquid crystal display device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission-scattering type liquid crystal display device using a polymer-dispersed liquid crystal.

[0002]

2. Description of the Related Art Conventionally, as a liquid crystal display device, a TN type or an STN type using a nematic liquid crystal has been used. However, these types require a polarizing plate, so that the display is dark and the cell thickness is low. However, there is a problem that it is difficult to enlarge the screen because of the need for high-precision control, and the viewing angle is narrow. In recent years, studies to apply a polymer-liquid crystal composite system to a liquid crystal display device have been actively conducted as a solution to these problems and to provide a large and inexpensive liquid crystal display device with good brightness and contrast.

Most of the polymer-liquid crystal composite display devices developed so far use a nematic liquid crystal as a liquid crystal component, and have a structure in which the liquid crystal is dispersed in a polymer matrix as minute droplets. (Polymer dispersed liquid crystal: JW Doane, N.M.
A. Vaz, B .; G. FIG. Wu, S .; Zumer, App
l. Phys. Lett. , 48 , 27 (1986))
And a structure in which a liquid crystal is used as a continuous phase and a polymer is dispersed therein as a three-dimensional network or fine droplets (polymer dispersed liquid crystal: JP-A-2-28284, JP-A-2-55318). No. 2) has been studied. However, the response speed of a polymer-liquid crystal composite display device using a nematic liquid crystal is considerably slower than other liquid crystal display devices, and a relatively high driving voltage is required. In general, the falling operation of a polymer-dispersed liquid crystal display device is accelerated by making the dispersion structure finer, but when the dispersion structure is smaller than the wavelength of light, the contrast ratio is reduced due to a decrease in the scattering effect. It is difficult to obtain a falling speed of several ms or less while maintaining a practical contrast ratio.

Recently, a chiral nematic liquid crystal containing a chiral dopant in an amount of 5 to 10% by weight has been used as a liquid crystal component, and a helical twisting force has been added to the liquid crystal to increase the response speed of a polymer-dispersed liquid crystal display device and increase the contrast ratio. Attempts have been made to raise
Giken Hideo Fujikake, Kuniharu Takizawa, Hiroshi Kikuchi, Masakatsu Okada, 16th
Proceedings of the 1st Liquid Crystal Symposium p. 120 (1990)). According to this method, the response speed is considerably improved, but the voltage required for driving is rather high.
Problems remain for practical application.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal display device which can solve the above-mentioned problems and can be driven at a low voltage, has a high contrast and a high response speed, and can have a large area.

[0006]

As a result of intensive studies to solve the above problems, the present inventors have found that a polymer-dispersed liquid crystal display device using a chiral nematic liquid crystal containing a chiral dopant for a light modulating layer is disclosed. The present inventors have found that by setting the concentration of the chiral dopant in the liquid crystal within a specific range, low-voltage driving and high-speed response can be achieved, and the present invention has been accomplished. That is, the present invention has two transparent conductive substrates and a dimming layer supported between the substrates,
The light modulating layer is composed of a mixture of a transparent polymer solid of 10 to 15% by weight and a chiral nematic liquid crystal containing a chiral dopant of 0.05 to 0.5% by weight of a liquid crystal component.
And a liquid crystal component having a grain structure wrapped in a thin film of the transparent polymer , and a focal conic
-A liquid crystal display device having a polydomain structure .

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS It is well known that when a small amount of chiral dopant is mixed into a nematic liquid crystal, the whole system becomes a chiral nematic liquid crystal. In a polymer dispersed liquid crystal display device using a chiral nematic liquid crystal, that is, a cholesteric liquid crystal, the liquid crystal phase has a focal conic polydomain structure peculiar to the cholesteric liquid crystal when no voltage is applied. It shows remarkable light scattering property as compared with the case where only a nematic liquid crystal is used. Due to this effect, in the present invention, the contrast ratio of the display can be significantly improved by using a polymer having high transparency, without intentionally considering the refractive indexes of the polymer and the liquid crystal component.

The response speed becomes more complicated, and the rise time (τ _r ) in a polymer dispersed liquid crystal display device using a chiral nematic liquid crystal corresponds to the time required for forcibly aligning molecules by an electric field. Therefore, the higher the voltage, the shorter the time tends to be. On the other hand, the fall time (τ _d ) corresponds to the time required for the spontaneous recovery of the polydomain structure, so that the chiral nematic twisting force and the wall effect, that is, the anchoring of liquid crystal molecules due to the interaction at the polymer-liquid crystal interface, are mainly observed. Determined only by strength. Increasing the torsion force of chiral nematics by increasing the concentration of chiral dopant will give τ
_{Although d} can be reduced, the voltage required for driving is increased because a strong force is required for the rise involving the collapse of the cholesteric structure. That, tau speed and acceleration of tau _d of _r becomes a reverse relationship to each other in magnitude of the chiral dopant concentration and drive voltage, the achieve practical low-voltage drive at high speed of response, the polymeric components It is necessary to adjust the cholesteric torsional force to an optimum state by adjusting the concentration and selecting the chiral dopant concentration and properties.

Assuming that the above conditions are satisfied, the inventors of the present invention set the concentration of the polymer component in the light control layer to 1% by weight or more and 50% by weight or less to provide a wall effect to some extent, and to increase the chiral dopant concentration in the liquid crystal component. Is set to 0.05 to 5% by weight, driving is performed at a relatively low voltage of 3 to 15 V, τ _r is 2 to 10 ms, τ _d is 6 to 14 m
s, maximum contrast ratio (T ₁₀₀ / T ₀ , where T ₁₀₀ : maximum transmittance (%) when voltage is applied; T ₀ : minimum transmittance, ie, transmittance (%) when no voltage is applied. )
It has been found that a polymer dispersed liquid crystal display device having an excellent performance of 580: 1 or more can be obtained.

In particular, in the case where two kinds of liquid crystal mixtures of a cholesteric liquid crystal and a ferroelectric liquid crystal exhibiting a chiral smectic C phase are used as the chiral dopant in the above-mentioned composition, the driving power is further reduced, and the polydomain property is reduced. It is possible to further improve the light scattering ability.

It is not clear why excellent characteristics can be obtained by using a mixture of a cholesteric liquid crystal and a ferroelectric liquid crystal exhibiting a chiral smectic C phase as a chiral dopant. The liquid crystal system has a short spontaneous recovery time to a polydomain structure even though the twisting force is weak, and the liquid crystal system has a relatively high twisting power and a spontaneous recovery time to a polydomain structure. Therefore, it is considered that the device responds even at a relatively low voltage as a whole and shows a relatively fast response.

The substrate used in the device of the present invention is not particularly limited as long as it is a transparent substrate having a conductive layer, and a glass, a transparent resin sheet or the like can be used.

The nematic liquid crystal material used for the light control layer has a sufficient electric field response at room temperature, and when mixed with a polymer component, the polymer phase and the liquid crystal phase are mixed with each other as different phases. It is necessary that the liquid crystal be formed in the polymer phase between the two substrates so as to form continuous domains. A general nematic liquid crystal can be used as long as it satisfies such conditions, for example, biphenyl, phenylcyclohexane, cyclohexylcyclohexane, cyanobiphenyl, cyanophenylcyclohexane, cyanocyclohexylcyclohexane, or these. And mixtures thereof. Among them, particularly preferred are cyanobiphenyl-based, cyanophenylcyclohexane-based, and cyanohexylcyclohexane-based ones having excellent electric field response.

Cholesteric liquid crystal used as chiral dopant in liquid crystal component and chiral smectic C
The ferroelectric liquid crystal exhibiting a phase may be any as long as it has good miscibility with a nematic liquid crystal material and can give a sufficient helical twisting force to the nematic liquid crystal material. Specifically, the material is not particularly limited as long as it independently exhibits a cholesteric phase and a chiral smectic C phase at normal temperature, but a material having a relatively non-bulky structure is preferable.

As described above, the concentration of the chiral dopant is preferably 0.05 to 5% by weight of the liquid crystal component, more preferably 0.3, in consideration of the driving voltage and the response speed.
~ 0.5% by weight.

As the components of the chiral dopant, those having a cholesteric liquid crystalline property and those having a ferroelectric property exhibiting a chiral smectic C phase are used alone or as a mixture. The component ratio when mixed and used is not particularly limited, but in order to sufficiently exhibit the effect of mixing two kinds of torsional forces, a ferroelectric liquid crystal exhibiting a chiral smectic C phase in the total amount of chiral dopants must be used. It is preferably 40 to 60% by weight.

As the polymer component interposed in the continuous phase of the liquid crystal component, it is necessary to take a structure in which the wall surface of the liquid crystal continuous phase is dispersed in a thin film form in order to sufficiently exhibit the wall effect. In order to maintain the dispersed state, it is desirable to mix with the liquid crystal component in the form of a prepolymer or a monomer, to form a dispersed state, and then to solidify at room temperature by irradiation with ultraviolet light or the like. By performing such a treatment, the cholesteric liquid crystal is polymerized while maintaining the focal conic grain structure, so that the domains of the grains are wrapped in a polymer thin film. As such an ultraviolet-curable prepolymer, acrylic or methacrylic ones can be used.

Further, if the ratio of the polymer component is high, it is difficult to form a continuous liquid crystal domain. Therefore, the ratio of the polymer component in the light control layer is 10 to 15% by weight.

Other optional components include a polymerization initiator such as benzophenone and 1-hydroxycyclohexyl phenyl ketone, a chain transfer agent, a dye, etc., depending on the properties of the liquid crystal component and the polymer component or the type and purpose of the intended display device. And the like can be used by appropriately mixing them.

The liquid crystal display device according to the present invention can be manufactured as follows. That is, in the isotropic state of the nematic liquid crystal component, a chiral dopant is added and mixed well, and a prepolymer and an optional component are further added and further stirred. This mixture is inserted between two transparent conductive substrates set at a predetermined interval using a spacer, and irradiated with ultraviolet light through the substrates, whereby the polymer component is solidified to form an opaque dimming layer. You.

In the present invention, the thickness of the light control layer is not particularly limited, but is preferably 5 from the viewpoint of responsiveness.
To 60 μm, and more preferably 7.5 to 15 μm.

[0022]

According to the present invention, the driving voltage is 3 to 15 V.
Rise time (τ_r) Falls for 2 to 10 ms.
Time (τ_d) Is 6 to 14 ms, and the maximum contrast ratio (T
₁₀₀/ T ₀) 580: 1 or higher with excellent performance
A molecular dispersion type liquid crystal display device is obtained. This is the traditional
Polymer-liquid crystal composite type liquid crystal display device drives several tens of volts
Voltage is much lower than it needed.
Response speed even at low drive voltage
No less than that. Furthermore, chiral as liquid crystal component
Maximum contrast by using nematic liquid crystal
Ratio is obtained using only conventional nematic liquid crystals
Practically improved compared to about 2 to 14: 1
And a high-performance liquid crystal display device can be provided.

[0023]

EXAMPLES The present invention will be described in detail with reference to the following Examples, but it should not be construed that the present invention is limited thereto.

In order to measure the electro-optical characteristics of each device manufactured in Examples and Comparative Examples, a function generator 1920A (manufactured by NF Corporation) was used as a power supply, and a 150 W (100 V) halogen lamp was used as a light source. A parallel light beam obtained by using a lamp house for a halogen lamp and a white light filter for a microscope was applied to a sample placed at a distance of 40 cm from the filter to have a diameter of 5 cm.
10 mm from the sample
At a distance of 5 mm, the amount of light passing through a slit having a width of 5 mm was evaluated by a photodetector. The maximum contrast ratio (T ₁₀₀ / T ₀ ) was calculated by obtaining the ratio of the maximum transmittance (T ₁₀₀ ) when a voltage was applied and the transmittance (T ₀ ) when no voltage was applied under the above conditions. Also, a digital storage oscilloscope (40 MHz, Iwatsu) is used to measure rise time (τ _r ) and fall time (τ _d )
And a rectangular wave having a frequency of 500 Hz was applied.

Example 1 10% by weight of a 60:40 (weight ratio) mixture of hydroxyethyl acrylate and phenoxyethyl acrylate as a polymer component, and a cholesteric liquid crystal CM-33 (manufactured by Chisso Corporation) as a chiral dopant as a liquid crystal material, the total amount of liquid crystal. 90% by weight of a nematic liquid crystal 5CB (manufactured by Merck) containing 0.4% by weight of the above, and inserted between two indium oxide-coated Nesa glass substrates with a spacing of 7.5 μm set by a polyimide film spacer. Then, a pre-polymer was cured by irradiating a parallel light beam of ultraviolet light from a high-pressure mercury lamp of 100 W at a room temperature of 22 ° C. for 5 minutes at a position 30 cm from the light-emitting side lens. The size of the light control layer formed between the two substrates is about 1 cm × 1 cm. When this structure was observed with a scanning electron microscope, the structure formed a polydomain grain focal conic structure, and each small grain was thin. It was observed that the polymer film was covered. When the electro-optical characteristics of this device were measured at a room temperature of 20 ° C., a maximum contrast ratio (T ₁₀₀ / T ₀ ) of 580: 1 or more, τ _r = 1.4 ms, and τ _d = 8.0 ms were obtained at a driving voltage of 15 V. Was.

Example 2 Hydroxyethyl acrylate and a polymer
60:40 (weight ratio) mixture of nonoxyethyl acrylate
10% by weight, and chiral dopant as liquid crystal material
Cholesteric liquid crystal CM-33 (manufactured by Chisso)
Ferroelectric liquid crystal CS-2 showing chiral smectic C phase
003 (manufactured by Chisso) in a 1: 1 (weight ratio) mixture
Nematic liquid crystal 5CB containing 0.4% by weight of the total amount
90% by weight well mixed with polyimide fill
2 pieces with the spacing set to 7.5 μm by the spacer
Inserted between Nesa glass substrates coated with indium oxide
UV light from a 100 W high-pressure mercury lamp at room temperature 22 ° C
5 minutes at a position 30 cm from the lens on the light exit side
Irradiation cured the prepolymer. Form between two substrates
The size of the formed dimming layer is about 1 cm x 1 cm.
When the structure of was observed using a polarizing microscope, the liquid crystal phase was
Create a polydomain grain focal conic structure
However, the size of the grains is about several tens
Voltage to switch from polydomain to uniform structure
It was observed that phases with different exchange rates were mixed. Room
Measure the electro-optical characteristics of this device at a temperature of 20 ° C.
It was found that the maximum contrast ratio was
(T₁₀₀/ T₀) 700: 1 or more, τ _r= 4.2 ms, τ_d
= 10.4 ms.

Comparative Example 1 10% by weight of a 60:40 (weight ratio) mixture of hydroxyethyl acrylate and phenoxyethyl acrylate as a polymer component, cholesteric liquid crystal CM-33 (manufactured by Chisso) as a chiral dopant as a liquid crystal material, and chiral smectic Ferroelectric liquid crystal CS-200 showing C phase
90% of nematic liquid crystal 5CB (manufactured by Merck) containing a 1: 1 (weight ratio) mixture of 3 (manufactured by Chisso) in an amount of 10% by weight of the total amount of the liquid crystal was mixed well, and the interval was set to 7.5 μm with a polyimide film spacer. Insert between the set two indium oxide coated Nesa glass substrates,
The prepolymer was cured by irradiating a parallel beam of ultraviolet light from a high-pressure mercury lamp of 100 W at 2 ° C. for 5 minutes at a position 30 cm from the light-emitting side lens. The size of the dimming layer formed between the two substrates is 1 cm × 1 cm, and the room temperature is 20 ° C.
The electro-optical characteristics of this device were measured at a driving voltage of 50 V, and the maximum contrast ratio (T
₁₀₀ / T ₀ ) 520: 1 or more, τ _r = 1.3 ms, τ _d = 1
It was 5 ms.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/1334

Claims (2)

(57) [Claims] 1. A light-adjusting layer supported between two transparent conductive substrates and the substrate, wherein the light-adjusting layer comprises 10 to 15% by weight.
With a mixture of a chiral nematic liquid crystal containing a transparent polymer solids and 0.05 to 0.5 wt% of chiral dopant in the liquid crystal component, said liquid crystal component is wrapped in a thin film of the transparent polymer It has a grain structure and
A liquid crystal display device having a focal conic polydomain structure . 2. The liquid crystal display device according to claim 1, wherein the content of the chiral dopant in the liquid crystal component is 0.3 to 0.5% by weight.