JP3062978B2 - Ferroelectric liquid crystal 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 liquid crystal device used for a liquid crystal display device or a liquid crystal optical shutter, and more particularly to a ferroelectric liquid crystal device. More specifically, the present invention has improved electro-optical characteristics by improving the alignment state of liquid crystal molecules. The present invention relates to a ferroelectric liquid crystal element.

[0002]

2. Description of the Related Art Clark and Lagerwall have proposed a display device of a type in which the amount of transmitted light is controlled in combination with a polarizer utilizing the refractive index anisotropy of ferroelectric liquid crystal molecules. (JP-A-56-107216, U.S. Pat. No. 4,436,792).
No. 4 specification). This ferroelectric liquid crystal generally shows a non-helical structure chiral smectic C phase (SmC ^* phase) to H phase (SmH ^* phase) in a specific temperature range,
In these phase states, it takes one of the first optically stable state and the second optically stable state in response to an applied electric field, and when no electric field is applied, it takes that state. It has the property of retaining, that is, bistability. Further, since the ferroelectric liquid crystal has a feature of quick response to application of an electric field, it is expected to be applied to a large-screen and high-definition display as a high-speed driving storage type display medium.

In order for an optical modulator using a bistable ferroelectric liquid crystal to exhibit desired driving characteristics,
It is necessary that the liquid crystal arranged between the pair of parallel substrates is in a molecular alignment state that switches between the two stable states stably, efficiently, and with good reproducibility.

On the other hand, in the case of a liquid crystal element utilizing the birefringence of liquid crystal, the light transmittance under crossed Nicols is I / I _O = s
in ² 4θ sin ² (Δndπ / λ), where I _O is the incident light intensity, I is the transmitted light intensity, θ is the apparent tilt angle between the two stable states, Δn is the refractive index anisotropy, d is the thickness of the liquid crystal layer, and λ is the wavelength of the incident light. In the above equation, the maximum transmittance is obtained at θ = 22.5 °, and to achieve high contrast, the angle between the average positions in the molecular long axis direction in the two states in the non-helical structure (apparent tilt) It is necessary to make the angle θ) close to ± 22.5 °.

It is generally known that the above-mentioned non-helical structure of ferroelectric liquid crystal takes a chevron structure in which the layer structure in the SmC ^* phase is bent. In this structure, the apparent tilt angle θ of the liquid crystal is smaller than the true tilt angle Θ of the liquid crystal molecules. Therefore, by applying an AC electric field, the layer structure is changed to change the apparent tilt angle θ to the true tilt angle. There has been proposed a method of approaching Θ (JP-A-62-16).
1123).

According to this technique, the alignment of liquid crystal molecules can be made uniform between the upper and lower substrates with C-directors in substantially the same direction, so that the transmittance in the dark state under crossed Nicols can be reduced, and high contrast display performance can be achieved. Can be expected.

[0007]

As described above, the ferroelectric liquid crystal element is characterized by switching between two different stable states by an external electric field.
Therefore, it is necessary to maintain the switching (driving) characteristics stable and with good reproducibility. Factors that change the switching characteristics include a surface memory effect called "burn-in" and a change in the alignment state from uniform alignment to spray alignment. is there. These are based on the interfacial interaction between the ferroelectric liquid crystal molecules and the alignment-treated substrate surface, that is, the surface of the rubbed polymer film or the surface of the obliquely deposited oxide film. As a means for suppressing the influence of this interfacial interaction, in a ferroelectric liquid crystal device having a chevron structure, it is effective to float the ferroelectric liquid crystal molecules from the substrate surface by introducing a high pretilt. In a system in which it is difficult to introduce a high pretilt such as an element whose layer structure is changed by application of an AC electric field, it is difficult to suppress a change in switching characteristics.

Accordingly, an object of the present invention is to suppress the interaction at the interface between a liquid crystal molecule switching between two stable states in a ferroelectric liquid crystal device and an alignment-treated substrate, thereby making it possible to perform electro-optical processing. An object of the present invention is to provide a ferroelectric liquid crystal device having improved characteristics.

[0009]

Means for Solving the Problems and Actions The present invention made to achieve the above object is achieved by providing a substrate between a pair of substrates each having an electrode and an orientation control film subjected to a uniaxial orientation treatment.
In ferroelectric liquid crystal device formed by sandwiching a ferroelectric liquid crystal exhibiting at least two stable states in the orientation state, strong
Permanent in the minor axis direction between the dielectric liquid crystal and the alignment control film
Consists of a racemic rod-like compound with a dipole moment
Ferroelectric liquid crystal device having an interface separation layer
It is.

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 schematically illustrates an example of the ferroelectric liquid crystal device of the present invention. In the figure, reference numerals 11a and 11b denote In ₂ O ₃ and ITO (Indium Tin O _2), respectively.
xide) is a substrate (glass substrate) covered with transparent electrodes 12a and 12b, and is made of polyimide, polyamide, polyvinyl alcohol (PVA) or the like, and has an alignment control film 13a with a thickness of several nm to several tens nm. And 13b are respectively laminated. On the surfaces of the orientation control films 13a and 13b, interface separation layers 14a and 14b each having a thickness of only a single molecular layer or several molecular layers are formed by a method such as a Langmuir-Blodgett method, an adsorption method, or a spin coating method. ing. Here, the interface separation layers 14a and 14b
Molecule is a layer of a mixture of even One LCD enantiomer each other a permanent dipole moment Ps in the short axis direction (a layer of substantially racemic without a Ps).

A ferroelectric smectic C liquid crystal 15 is disposed between the substrates 11a and 11b.
1b is the distance between the ferroelectric smectic C liquid crystal 15 and
The distance is set to a distance small enough to suppress the formation of the helical array structure (for example, 0.1 to 3 μm), and the ferroelectric smectic C liquid crystal 15 has a bistable alignment state.

The above-described sufficiently small distance is held by bead spacers 16 (silica beads, alumina beads, etc.) disposed between the substrates 11a and 11b. 17
Reference numerals a and 17b denote polarizing plates placed in orthogonal Nicols.

The interface separation layers 14a and 14b are not only composed of components different from the ferroelectric liquid crystal 15, but also composed of, for example, a racemic body composed of the ferroelectric liquid crystal 15 and an enantiomer of the ferroelectric liquid crystal 15. You can also.

That is, it suffices that a region composed of liquid crystal molecules having substantially no Ps exists at the interface between the alignment control films 13a and 13b and the ferroelectric liquid crystal 15.

Incidentally, the uniaxial orientation of the ferroelectric liquid crystal can be obtained by an alignment treatment on the substrate surface, specifically, by rubbing a polymer film of polyimide, polyamide, PVA or the like. This is considered to be caused by interaction such that the major axis directions of the liquid crystal molecules and the liquid crystal molecules become parallel. However, since the ferroelectric liquid crystal has a large Ps in the molecular minor axis direction, there is an interaction between the Ps and the alignment layer surface. FIG. 2 schematically shows the relationship between two stable states of liquid crystal molecules and the upper and lower substrate surfaces. The director of the liquid crystal molecules at each position is projected on the bottom surface of the cone, and this is viewed from the bottom direction. FIG. As shown in FIG.
In the stable state of (a), the substrate direction (outward) on the upper substrate side, the liquid crystal layer direction (inward) on the lower substrate side, and in the stable state of (b), inward on the upper substrate side and outward on the lower substrate side. It is. The difference in the interface state between the two stable states changes the interaction between the liquid crystal molecules and the substrate surface at the interface, and causes the above-mentioned "burn-in". On the other hand, when the substrate surface has a large force for regulating the direction of the Ps of the liquid crystal molecules, the Ps of the liquid crystal molecules is directed inward (or outward), so that the splay alignment is more easily achieved than the uniform alignment. FIG. 3 shows an example of the arrangement of C-directors of liquid crystal molecules in the uniform alignment and the splay alignment. In the figure, 31 is a uniform orientation, 32 is a splay orientation restricted to inward, and 33 is a splay orientation restricted to outward.

As described above, in a ferroelectric liquid crystal device having a large Ps of liquid crystal molecules, an alignment control film (polyimide or the like) having a strong alignment regulating force is used, and the alignment control film is in direct contact with the ferroelectric liquid crystal. In this case, a surface memory effect such as “burn-in” occurs, which causes a change in switching characteristics of the element and a decrease in electro-optical characteristics.

[0018] Now, the present invention, the orientation control film and the ferroelectric liquid crystal, because never contact more substantially directly to the interface separation layer previously described, the surface memory effect does not occur. That is, switching between two stable states due to an electric field or the like does not occur at the interface or very close to the interface, and the liquid crystal molecules in contact with the substrate surface are always kept in one stable state as shown in FIG.

Further, the ferroelectric liquid crystal device of the present invention can suppress the change in switching characteristics without introducing a high pretilt angle. Therefore, the AC electric field (for example, ± 20 V / μm , 10 Hz rectangular wave) to change the layer structure of the liquid crystal molecular layer and increase the apparent tilt angle, the switching characteristics do not change.

[0020]

Embodiments of the present invention will be described below.

Example 1 (Reference Example) First, an ITO film was formed on a glass substrate by sputtering for about 1 hour.
It is formed to a thickness of 50 nm, and 0.4% NMP / polyamic acid (Semico Fine SP710) manufactured by Toray Co., Ltd.
NBC (1: 1) mixed solution is applied with a spinner, and 300
The resultant was fired at 1 ° C. for 1 hour to form a polyimide film having an average thickness of 3 nm. Rubbing treatment conditions for this polyimide film (roller pushing amount: 0.4 mm, roller rotation speed: 16.7 per second)
Times, roller feed speed: 12 mm / sec, roller radius:
(100 mm). Next, the liquid crystal that takes a nematic phase at room temperature was spin-coated on the surface of the substrate subjected to the alignment treatment, left for 24 hours, and washed with hexane to remove only the liquid crystal molecules adsorbed on the substrate surface. .

A pair of substrates having been subjected to the above-described processing are rubbed in the same direction and in the same direction for about 1.5 μm.
The substrates were bonded to each other at an interval of m to produce a liquid crystal cell.
The cell is a multi-component mixed liquid crystal containing a phenylbenzoate-based liquid crystal as a main component, and has a tilt angle of 22.2 ° at 30 ° C.
Of ferroelectric liquid crystal was injected. The phase transition series of the liquid crystal was as follows, and Ps was −24 nC / cm ² .

[0023]

(Equation 1) The cell into which the ferroelectric liquid crystal was injected was converted from an isotropic phase to SmC
^{* During} the slow cooling process to the phase, an orientation state having a chevron structure was obtained, and the apparent tilt angle was 8.5 °. Next, when an alternating electric field of ± 20 V / μm, 10 Hz was applied to this cell at 30 ° C. for 1 minute to change the layer structure, the apparent tilt angle spread to 19.5 ° and was uniform over the entire pixel area. Uniform uniform orientation was obtained. The cell was switched between two stable states, but no change in alignment state appeared. In addition, when the switching characteristics were measured after being left for 36 hours in one of the two stable states, the change in the switching threshold value was found in a conventional cell in which a nematic liquid crystal layer was not formed at the interface of the polyimide film. It was much smaller than that.

Embodiment 2 In the same cell structure as in Embodiment 1, an example is shown in which a multi-component mixed liquid crystal containing the above-mentioned racemic phenylbenzoate liquid crystal at a ratio of about 1: 1 is used instead of using a nematic phase liquid crystal. Is shown. In this embodiment, when the liquid crystal was spin-coated, the substrate temperature was heated to 70 ° C. or higher.

In this cell, the switching characteristics were evaluated in the same procedure as in Example 1. As a result, no change in the orientation state at the time of switching was observed, and the change in the switching threshold was almost the same as that in Example 1. Obtained.

[0026]

As described above, the device of the present invention has the following features.
By eliminating the interaction between the liquid crystal layer that switches between the two stable states and the substrate surface on which the alignment control film is formed, uniform uniform alignment without changing the alignment state is obtained, and high-contrast display performance is achieved. . Changes in switching threshold ("burn-in") have been reduced and electro-optical properties have been improved.

Further, as a secondary effect, the time required for injecting the liquid crystal into the cell is reduced, and uneven injection is less likely to occur, so that display quality is improved without uneven display.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing one example of a ferroelectric liquid crystal device of the present invention.

FIG. 2 is a diagram for explaining a relationship between liquid crystal molecules and a substrate surface.

FIG. 3 is a diagram for explaining an alignment state of liquid crystal molecules.

FIG. 4 is a diagram for explaining a relationship between liquid crystal molecules and a substrate surface according to the present invention.

[Explanation of symbols]

 11a, 11b Glass substrate 12a, 12b Transparent electrode 13a, 13b Alignment control film 14a, 14b Interface separation layer 15 Ferroelectric liquid crystal 16 Spacer 17a, 17b Polarizer

Claims (1)

(57) [Claims] 1. A ferroelectric material comprising a pair of substrates each having an electrode and an orientation control film subjected to a uniaxial orientation treatment, wherein a ferroelectric liquid crystal exhibiting at least two stable states in an oriented state is sandwiched. In a liquid crystal device, between the ferroelectric liquid crystal and the alignment control film,
From a racemic rod-like compound with a dipole moment
A ferroelectric liquid crystal device having an interfacial separation layer .