JPH0711636B2 - Liquid crystal element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a liquid crystal display device, a liquid crystal device such as a liquid crystal-optical shutter array, and more particularly to a liquid crystal device having an improved initial alignment state of liquid crystal molecules.

BACKGROUND ART Generally, a ferroelectric liquid crystal has a bistable state composed of a first optical stable state and a second optical stable state with respect to an electric field, and has a first optical stable state with respect to one electric field vector. The liquid crystal is oriented in the state and the liquid crystal is oriented in the second optically stable state with respect to the other electric field vector. Furthermore, this type of liquid crystal is considered to have the property of rapidly taking one of the two stable states in response to an applied electric field and maintaining that state when no electric field is applied. There is. However, in order for the liquid crystal element using such a ferroelectric liquid crystal to exhibit a predetermined driving characteristic, the liquid crystal disposed between the pair of parallel substrates has the above-mentioned two stable states regardless of the applied state of the electric field. It is necessary to be in a state of molecular alignment such that conversion between states can occur effectively. For example, for a ferroelectric liquid crystal having a chiral smectic C phase or H phase (SmC ^* or SmH ^* phase), the liquid crystal molecular layer having the SmC ^* or SmH ^* phase is perpendicular to the substrate surface, and therefore the liquid crystal molecular axis is It is necessary to form a region (mono domain) arranged in one direction substantially parallel to the substrate surface.

In order to form such a monodomain, at least one of the substrates is usually subjected to an appropriate alignment treatment. As a conventional alignment treatment method, a so-called rubbing method of rubbing the surface with an appropriate cloth or abrasive, and a method of obliquely depositing an inorganic substance are known.

However, in the liquid crystal element formed through such an alignment treatment, the change in the alignment between the first and second optically stable states due to the action of the electric field vector is often not the same. That is, the magnitude of the electric field required to change from the first stable state to the second stable state is not equal to the magnitude of the electric field required to change from the second stable state to the first stable state. That is actually happening. Furthermore, from the first stable state, due to the action of the electric field, it is completely changed to the second stable state, but when the electric field is cut off, it is observed that a certain nucleus gradually returns to the first stable state. There are also cases.

It is believed that these two developments are due to the fact that the two bistable states are not perfectly symmetrical and that either one is more stable than the other. Such a state can be called, so to speak, “two asymmetric stable states”.

When such "asymmetric two stable states" exist, there is a problem that the driving system becomes complicated due to a difference in threshold voltage when switching by an electric field between the two stable states, or Since the first stable state is mixed with the second stable state, there is a drawback that the contrast is lowered.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks of the prior art, and an object of the present invention is to provide a liquid crystal device capable of realizing excellent bistability.

SUMMARY OF THE INVENTION The present invention has been made by focusing on the above points,
By using a monomolecular film of an aromatic compound as an alignment film of a liquid crystal device, a liquid crystal device having excellent bistability was realized.

The cause of the appearance of the “two asymmetric stable states” may be the liquid crystal material, the alignment treatment film, the treatment conditions, the temperature, etc. In the present invention, the alignment treatment method is, so to speak, a rubbing method or a vapor deposition method. Realization of excellent bistability of ferroelectric liquid crystals by a method of forming a monomolecular film that acts by aligning liquid crystal molecules by applying a linear intermolecular force instead of the one mainly focusing on the physical alignment effect. It has been discovered.

As is generally well known, the molecule forming the monolayer contains a hydrophilic site and a hydrophobic site in its structure. However, since the generally known monolayer of rod-shaped molecules is arranged in the direction perpendicular to the substrate surface,
In such a monomolecular film, it has been difficult to horizontally align the liquid crystal molecules, which is the basis for forming the liquid crystal monodomain as described above.

Therefore, as a result of further studies on a monomolecular film capable of horizontally aligning liquid crystals, a compound having an aromatic ring having a large π-electron system that can be aligned in parallel with the substrate in the center, that is, a monomolecular film of an aromatic compound. It was found that the liquid crystal has the ability to horizontally align the ferroelectric liquid crystal.

The liquid crystal element of the present invention is based on such findings, and in a liquid crystal element in which a ferroelectric liquid crystal is sealed between a pair of substrates, a monomolecular film of an aromatic compound is provided on at least one of the pair of substrates. It is characterized in that an alignment film made of is formed.

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

Example The schematic structure of the liquid crystal device of the present invention is not particularly different from that of the conventional ferroelectric liquid crystal device except the structure of the alignment film.

FIG. 1 is a schematic cross-sectional view in the thickness direction of an embodiment of the liquid crystal element of the present invention. In this liquid crystal element 100, a pair of parallel substrates 101 made of a glass plate or a plastic plate is used.
Striped ITO (indium-tin-oxide) electrodes 102a and 102b (for example, respectively forming a scan electrode group and a signal electrode group) are formed on a and 101b, respectively, and further oriented (control). The films 103a and 103b are formed. If necessary, the electrodes 102a and 102b and the alignment control film 103
An insulating film (not shown) such as polyimide may be additionally provided between a and 103b.

The pair of substrates 101a, 101b processed as described above,
The striped electrodes 102a and 102b are combined so as to be orthogonal to each other, and are fixed via a spacer 104 so that the distance between them is, for example, 1 μ. A cell is formed. Ferroelectric liquid crystal 106, which is generally heated to an isotropic phase, is injected into this blank cell, and an injection hole (not shown) is sealed with epoxy resin. Then, the entire cell is filled with liquid crystal 106 in a chiral smectic phase. A liquid crystal cell is obtained by gradually cooling to a temperature at which the liquid crystal cell is sandwiched by a pair of polarizing plates 107 and 108 arranged in a crossed Nicols relationship to form a liquid crystal element 100.

According to the present invention, in the liquid crystal element structure as described above, at least one of the alignment films 103a and 103b is composed of a monomolecular film of an aromatic compound. An aromatic compound having an aromatic ring that constitutes a π-electron system is generally made of glass, a plastic plate, or the like and is parallel to the substrate with the aromatic ring as the center when subjected to a monomolecular film forming method as described below. Those having a plurality of hydrophobic groups or moieties, for example, 2 to 40 hydrophobic groups or moieties, are preferably used in order to improve the arrayability. Preferred examples of the hydrophobic group include long-chain alkyl groups having 5 to 30 carbon atoms.

Examples of preferable aromatic compounds include compounds (I) to (IV) whose structures are shown in FIG.

Especially, when a monomolecular film of an aromatic compound having a plurality of hydrophobic groups is formed, the arrangement of the molecules on the substrate is schematically shown in FIG. 3, for example. That is, the hydrophobic groups 3 such as long-chain alkyl groups are arranged vertically to the surface of the substrate 1 having the ITO electrode 2, and the plane 4 of the π-electron system including the aromatic ring is horizontally aligned with respect to the substrate 1. Become. The interaction between the π-electron system of this monomolecular film and the central skeleton of the ferroelectric liquid crystal enables horizontal alignment of the ferroelectric liquid crystal.

Next, a method for forming such a monomolecular film and an apparatus therefor will be described. 4 and 5 are a perspective view and a longitudinal sectional view, respectively, of an example of such a device.

In these figures, 21 is a liquid tank in which the water phase is contained. Inside the liquid tank 21, a polypropylene frame 22 functioning as a two-dimensional cylinder is hung horizontally to partition a water surface 23.

Inside the frame 22, a float 24 made of polypropylene, which functions as a two-dimensional piston, is floated. The width of the float 24 is made slightly narrower than the inner width of the frame 22 so that the float 24 can move smoothly in the left-right direction in the figure as a two-dimensional piston.

The lateral movement of the float 24 is performed by a weight 26 that pulls the float 24 rightward via a pulley 25. As shown in Fig. 5, the float 24 moves to the left and stops.
This is performed by the repulsive force of the magnet 27 provided on the magnet 24 and the counter magnet 28 that can move left and right by a holding mechanism (not shown).

Suction nozzles 29 for cleaning the water surface 23 by rinsing the liquid surface 23 in the frame 22 are provided on the left and right of the liquid tank 21. The suction nozzle 29 is connected to a suction pump (not shown) via a suction pipe 30.

On the water surface 23 on the right side of the figure, a carrier upper and lower arm 32 that holds the carrier 31 and is movable in the vertical direction is provided. By the movement of the carrier upper and lower arms, the carrier 31 is vertically moved across the water surface 33.

In order to form a monomolecular film of an aromatic compound using the apparatus as described above, the aromatic compound is dissolved in a suitable volatile solvent (for example, chloroform), and the solution is appropriately spread on the water surface 23. To do. Then after the solvent is evaporated off, the float
By moving 24, the development area of the monomolecular film left on the water surface 23 is contracted, and the surface pressure is increased to a certain value (for example, 20 dyn / cm). Then, while keeping the surface pressure constant, the substrate 31 as a carrier is gently moved up and down at a constant speed (for example, 10 mm / min) to transfer the monomolecular film onto the substrate 31.

The mode at this time is shown in FIG. In the part indicated by (a) on the left side, the molecules forming the monolayer are arranged with the hydrophobic site facing upward on the water surface, which is commonly referred to as "X type" (b) Only when the substrate 31 is lowered as described above, one layer of the film is transferred, and eventually a monomolecular film as shown in FIG. 3 is obtained.

Although it is possible to form a plurality of monolayers by the above-mentioned method, when the monolayers are multilayered, the monolayers have absorption in the visible region and hinder the transparency. It is preferable to form only a monolayer.

Hereinafter, the present invention will be described more specifically according to production examples.

Production Example A liquid crystal element having a structure substantially as shown in FIG. 1 was produced as follows.

First, on a pair of glass substrates 101a and 101b, ITO electrodes 102a, 1
02b was patterned into a stripe shape with a thickness of 1000Å.

Then, as the alignment film, the monomolecular film 103a by the above method,
One layer of 103b was formed. The above-mentioned aromatic compounds (I) to (IV) were used as the monomolecular film forming materials, and these aromatic compounds were dissolved in chloroform at a concentration of 3 × 10 ⁻⁴ mol /. As the aqueous phase 23, distilled water containing cadmium chloride at a concentration of 5 × 10 ⁻³ mol /, adjusted to pH 6.3 with sodium hydrogen carbonate, was used at 20 ° C.

Spacers 104 were formed of polyimide to a height of 1 μm, and were patterned by photoetching and cured. Subsequently, as the sealing member 105, the upper and lower substrates were attached using the same material as the spacer 104.

After that, DOBAMBC was sealed as the ferroelectric liquid crystal 106 having a chiral smectic phase, and four sides were sealed to form a cell structure.

Next, this cell was heated until the liquid crystal layer 106 was in an isotropic phase state, and then gradually cooled at 0.5 ° C./hour for alignment.

The substrates 101a and 101b of the liquid crystal cell created as described above
Polarizers 107 and 108 as crossed Nicols state on both sides of
Four types of liquid crystal elements 100 were formed by arranging them respectively. In this liquid crystal element 100, the liquid crystal 106 is provided between the electrodes 102a and 102b.
When the DC voltage exceeding the threshold of
Or, it will be oriented in the second stable state, and optical modulation will occur, and switching under this condition was observed.

Separately, as the alignment film, the polyimide coatings 103a and 103b were formed by spinner coating on the ITO electrode substrate instead of the monomolecular film.
A comparative liquid crystal element was produced by the same method as that of the above-described example except that a film having a thickness of 00Å was formed, cured, and then rubbed by a predetermined method.

The switching characteristics of the comparative example and the four types of liquid crystal elements obtained above were measured under the conditions of 70 ° C. and a pulse width of 1 msec. The results are summarized in Table 1 below.

First, in a comparative example using a rubbing film as an alignment film, when switching between stable states with a voltage pulse width of 1 msec, an “asymmetric stable state” is shown, and a more stable state (“state A” in the table above). The threshold voltage from (shown) to the more unstable state (shown as "state B") was 21.8V, while the threshold voltage from state B to state A was 30.2V.

Further, when the state A was switched to the state B, a phenomenon (indicated as “return phenomenon”) of returning to the more stable state A 1 to 2 seconds after the switching was observed.

On the other hand, in Examples 1 to 4 using the monomolecular film as the alignment film, the threshold voltage value was smaller than that in the comparative example, and further, the asymmetry at the time of switching A → B and B → A was also
It was small and good bistability was obtained.

In addition, the "return phenomenon" that appeared in the comparative example was not observed. In Examples 1 to 4, the threshold voltage was smaller in the monomolecular films of the compounds (III) and (IV) having a relatively large π electron system. This result shows that the threshold voltage can be reduced by the correlation between the length of the ferroelectric liquid crystal in the long axis direction and the size of the π-electron system of the monomolecular film.

EFFECTS OF THE INVENTION As described above, according to the present invention, a ferroelectric liquid crystal device having improved bistability is provided by using a monomolecular film of an aromatic compound as an alignment film of a ferroelectric liquid crystal,
Moreover, the switching characteristics including the driving voltage characteristics are also improved.

[Brief description of drawings]

FIG. 1 is a schematic sectional view in the thickness direction of an embodiment of a liquid crystal device of the present invention, FIG. 2 is a structural formula of an example of an aromatic compound used in the present invention, and FIG. 3 is a single molecule of an aromatic compound on a substrate. FIG. 4 is a conceptual cross-sectional view showing the arrangement state of the membranes, FIGS. 4 and 5 are a perspective view and a vertical cross-sectional view of the monomolecular film forming apparatus, respectively, and FIG. 6 is a schematic diagram showing a transfer aspect of the monomolecular film. 1, 101a, 101b ... substrate, 2, 102a, 102b ... electrode film, 3 ... hydrophobic group, 4 ... π electron system, 100 ... liquid crystal element, 103a, 103b ... alignment film, 106 ... Ferroelectric liquid crystal, 107, 108 ... Polarizing plate. Representative figure: Figure 1

Claims (2)

[Claims] 1. A liquid crystal device in which a ferroelectric liquid crystal is sealed between a pair of substrates, wherein an alignment film made of a monomolecular film of an aromatic compound is formed on at least one of the pair of substrates. Liquid crystal element. 2. The liquid crystal device according to claim 1, wherein the aromatic compound is an aromatic compound having a plurality of hydrophobic groups.