JPH0261614A - liquid crystal element - Google Patents

Abstract

PURPOSE:To obtain good orientational property and bistability by forming a mixture composed of >=2 kinds of polymers which are different in orientational power as an oriented film for a liquid crystal and dispersing and forming the domains which are different in the mixing ratio of the polymers to the surface layer part of the oriented film. CONSTITUTION:The mixture composed of the polymer having horizontal orientational power and the polymer having perpendicular orientational power is used as the oriented film 4 for the liquid crystal of the liquid crystal element constituted by crimping the ferroelectric liquid crystal 5 between two sheets of substrates 1 having transparent electrodes 3 on the surfaces; in addition, the surface layer part of the oriented film 4 is formed to have the phase sepn. structure dispersed and formed with the domains varying in the mixing ratios of the polymers. Such oriented film 4 has the sea and island structure in which the polymer components (regions 2) having the perpendicular orientational power float in the polymer components (region 1) having the horizontal orientational power. The size and shape of the domains are controllable by changing the kinds of the polymers, dispersants, temp. or the mixing ratios of the polymer components. The good orientational property and bistability are developed in this way.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a liquid crystal element having a ferroelectric liquid crystal as a liquid crystal layer, and particularly relates to a liquid crystal display having bistable memory properties or a liquid crystal optical modulator. It is something.

[Conventional technology]

In conventional liquid crystal devices, methods for uniaxially aligning liquid crystal molecules on a substrate include a rubbing method in which a polymer film such as polyimide or polyvinyl alcohol is formed on the substrate surface and then rubbed in one direction with a cotton cloth, or a rubbing method in which a polymer film such as SiO One method is to deposit an inorganic material obliquely to a substrate to form small column-shaped protrusions, thereby obtaining uniaxial orientation through microscopic physical shapes.

Other methods of orientation include a method of forming periodic minute irregularities using a photocurable resin, and a method of using a temperature gradient.
There is a method of growing and orienting a single crystal from the edge of a polymer spacer.

Among these methods, the simplest and industrially advantageous orientation method is the polymer rubbing method. This method has been widely used as a method for forming alignment films for conventional TN cells, and is also being studied for alignment of ferroelectric liquid crystals.

[Problem to be solved by the invention]

Among the conventional techniques mentioned above, the most productive method is the polymer rubbing method, but in the case of ferroelectric liquid crystals, good results are obtained when alignment is performed using a conventional alignment film such as a polyimide film. Although uniaxial alignment can be obtained, there is a problem in that because the alignment ability is too strong, it is difficult to obtain bistability, which is one of the major features of ferroelectric liquid crystals.

A report on controlling the strength of rubbing as a way to solve this problem (IEICE Technical Report, Vol. 87, P. 77)
~) have been done. However, there are still many uncertain factors in the rubbing process, and it is difficult to uniformly control the strength of rubbing over a large area.

There is also a report (Japanese Patent Laid-Open No. 63-14122) that the pretilt angle of liquid crystal molecules is controlled by an alignment film provided with a region having two different types of alignment films. To provide a region with orientation ability, first, a first layer is formed, and then a desired pattern is masked with photoresist, etc., and another type of second layer is formed to disperse and form regions with different orientation abilities. Therefore, compared to the conventional rubbing method, it requires more than twice as many steps and has the problem of poor productivity.

On the other hand, cells using obliquely deposited films have excellent properties in terms of orientation and bistability, but it is difficult to uniformly form a deposited film in the same direction over a large area of the substrate. Moreover, there is a problem that productivity also deteriorates.

Therefore, an object of the present invention is to overcome the problems of conventional alignment methods by using a polymer rubbing method with good productivity in an alignment method for ferroelectric liquid crystals, and to produce a liquid crystal element that exhibits good alignment and bistability. This is what we are trying to provide.

[Means to solve the problem]

The gist of the present invention is to solve the above-mentioned problems in a liquid crystal element in which a ferroelectric liquid crystal is sandwiched between two substrates having transparent electrodes on the surface. Good alignment is achieved by using a mixture of a polymer having a vertical alignment ability and a polymer having a vertical alignment ability, and by forming a phase separation structure in which domains with different mixing ratios of the polymer are dispersed in the surface layer of the alignment film. The present invention has discovered that it is possible to produce a liquid crystal element having good properties and bistability, and has been able to solve the problems of the conventional alignment methods at once.

The liquid crystal element of the present invention will be explained in detail below.

The liquid crystal element of the present invention is formed by forming a mixture of two or more types of polymers with different alignment abilities on a substrate as a liquid crystal alignment film, and the surface layer of the liquid crystal alignment film has a mixture of polymers having different mixing ratios. It has a phase-separated structure in which domains are dispersed, and controls the interaction between the liquid crystal alignment film and liquid crystal molecules, especially the surface energy of the alignment film, making it an alignment film that easily exhibits memory properties.

In the present invention, examples of the polymer having horizontal alignment ability include aromatic polyimide, polyvinyl alcohol, polyacrylonitrile, etc. Among these, aromatic polyimide, which is usually used as a TN type alignment film, is particularly preferred. can be mentioned. In addition, examples of the polymer having vertical alignment ability of the present invention include polytetrafluoroethylene, polyvinylidene fluoride, polydimethylsiloxane, polyfluoroalkyl methacrylate,
Examples include fluorine-containing polymers such as polypropylene and copolymers thereof, and among these, a particularly preferred example is a copolymer of tetrafluoroethylene and vinylidene fluoride.

There is no particular restriction on the mixing ratio of the above-mentioned polymer having horizontal alignment ability and the above-mentioned polymer having vertical alignment ability, but in order to form a phase-separated structure in which domains with different mixing ratios are dispersed in the surface layer, , the polymer with vertical alignment ability is 1.0% compared to the polymer with horizontal alignment ability.
Preferably, the mixing ratio is in the range of 50% by weight.

In the present invention, a preferred method of mixing a polymer having horizontal alignment ability and a polymer having vertical alignment ability is to dissolve a predetermined amount of the polymer in a solvent such as dimethylacetamide, dimethylformamide, N-methylpyrrolidone, etc. There is a method of mixing and dispersing it uniformly. In this case, the concentration of the solution is preferably about 0.1 to 5.0% by weight. Further, in this case, a solution may be prepared by mixing one or more types of polymers having horizontal alignment ability and one or more types of polymers having vertical alignment ability.

Examples of methods for forming the alignment film include methods such as spin coating, dip coating, roll coating, and spraying. By these methods, an alignment film is formed on a substrate having a transparent electrode. The film thickness at that time is 100 to 1500
Preferably within the human range. Further, during coating, the mixed solution may be thoroughly stirred with a Starcy machine or vibrated with an ultrasonic vibrator to ensure uniform dispersion of the polymers.

Further, after the coating film is formed, drying and curing may be performed in an oven or a hot plate at a temperature of 100 to 300°C, depending on the case.

The formed alignment film is further subjected to an alignment treatment by rubbing in one direction with a cotton cloth, polyester cloth, etc. using a rubbing method. The rubbing treatment may be applied to both alignment films or only one of them, but when it is applied to both alignment films, they are overlapped so that the rubbing directions are aligned during cell formation.

[For production]

As shown in Fig. 1, the alignment film formed on the substrate in this manner has a polymer component (Tf4 region 2) having vertical alignment ability floating in a polymer component (region 1) having horizontal alignment ability. It has a sea-island structure, that is, a phase-separated structure in which domains are dispersed.

At this time, the size and shape of the domains corresponding to the islands in the sea-island structure can be changed from the micron order to several hundred by changing the type of polymer with horizontal alignment ability and vertical alignment ability, the dispersant, the temperature, or the mixing ratio of the polymer components. It can be controlled in the range down to microns, and the best orientation and bistability can be expressed depending on the characteristics of the ferroelectric liquid crystal used.

〔Example〕

FIG. 2 shows a structure of the present invention using a substrate having an alignment film formed in this manner. FIG. 2 is a partial cross-sectional view showing the structure of an example of a liquid crystal element according to the present invention.

1 is a transparent substrate such as a glass plate, PMMA or polycarbonate, and 2 is an undercoat of StO□, which may be provided as necessary. 3 is an ITO transparent electrode, and 4 is an alignment film according to the present invention. 5 is a ferroelectric liquid crystal, and 6 is a spacer for controlling the cell gap.

Example 1 Aromatic polyimide (Hitachi Chemical: HIM^L) was used as a polymer having horizontal alignment ability. Further, as the polymer having vertical alignment ability, a copolymer of solvent-soluble polytetrafluoroethylene and polyvinylidene fluoride in a weight ratio of 81:19 was used.

The mixing ratio was 1% to 20% by weight of the copolymer of polytetrafluoroethylene and polyvinylidene fluoride to the polyimide. Dimethylacetamide was used as a solvent to give a concentration of 0.1 to 5% by weight, and the mixture was stirred well or treated with ultrasonic waves to obtain a mixed solution so that the polymer was uniformly and finely dispersed in the solution.

The thus obtained mixed solution was placed on a glass substrate on which a transparent electrode had already been formed, using a spinner (rotation speed: 300
A film having a thickness of 100 to 1,800 PPM) was formed and further heated at 150° C. for 2 hours to obtain a liquid crystal alignment film.

Using a solution with a polymer mixing ratio of 10% by weight and a solution concentration of 3% by weight, the orientation obtained by the above procedure is as shown in Figure 3, where a circular domain with a diameter of 40 microns has an area. It was present at a ratio of about 15%.

FIG. 4 is a chart when an X-ray microanalyzer is used to scan along the cotton in the center of FIG. 3 with the wavelength of the detector set to the characteristic X-rays from fluorine. This result shows that the domains corresponding to the islands of the sea-island structure have a high fluorine content and are mainly formed from a copolymer of tetrafluoroethylene and vinylidene fluoride.

In this way, it can be seen that the liquid crystal alignment film formed using a mixture of two types of polymers with different alignment abilities has a phase-separated structure, and the domains with different alignment abilities of the seabird structure are uniformly dispersed. .

FIG. 5 is a plot of the average domain size of the copolymer of tetrafluoroethylene and polyvinylidene fluoride when the mixing ratio of the copolymer of tetrafluoroethylene and polyvinylidene fluoride to the aromatic polyimide was varied. As shown in the figure, the size of the domains corresponding to the islands in the sea-island structure can be controlled at will by changing the mixing ratio.

Example 2.

In the same manner as in Example 1, the copolymer of tetrafluoroethylene and vinylidene fluoride was mixed with 1 part of the aromatic polyimide.
Using a transparent conductive glass substrate coated with an alignment film formed from a solution with a mixing ratio of 0% by weight, a ferroelectric liquid crystal cell was fabricated by rubbing only one side of the substrate, and its memory performance was evaluated. . The results are shown in FIG. The liquid crystal used was C5-1014 (nitrogen), and its phase transition temperature was
Spontaneous polarization and helical pitch are 5 nc/crA (at 25°C), helical pitch = 5 tm (at 25°C), and cell thickness is '1.3) am'. Further, the evaluation was performed with the applied voltage being a pulse width of 1 mS and a pulse height of ±40 V.

The ferroelectric liquid crystal cell using the alignment film according to the present invention has a sixth
As shown in the optical response waveform shown in the figure, good bistability was exhibited.

〔Effect of the invention〕

As explained above, according to the present invention, the liquid crystal alignment film of the liquid crystal element is made of a mixture of a polymer having horizontal alignment ability and a polymer having vertical alignment ability, and in the surface layer portion of the alignment film, the mixing ratio of the polymers is By dispersing and forming different domains to form a phase-separated structure, ferroelectric liquid crystal devices exhibiting good bistability can be easily obtained with a high yield.Furthermore, nematic liquid crystal devices can be produced using the conventional polymer rubbing method. Since the manufacturing process can be used as is, it has become possible to provide liquid crystal elements with high reliability and high productivity.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a liquid crystal alignment film according to the present invention, and FIG. 2 is a partial cross-sectional view showing an example of a liquid crystal element according to the present invention. Third
The figure is a plan view of an example of a liquid crystal alignment film, and FIG. 4 is a chart obtained by X-ray microanalysis of the liquid crystal alignment film of FIG. 3. FIG. 5 is a diagram in which domain size is plotted against the mixing ratio of polymers. FIG. 6 shows the optical response waveform of the liquid crystal element of the present invention. ■... Glass substrate, 2... Undercoat (SiO□), 3... Transparent electrode (ITO), 4... Liquid crystal alignment film, 5... Ferroelectric liquid crystal, 6... Spacer Figure 1 Figure 2 Figure 6 Subaser diagram

Claims (1)

[Claims] 1. In a liquid crystal element in which a ferroelectric liquid crystal is sandwiched between two substrates having transparent electrodes on the surface, the liquid crystal alignment film is made of a mixture of a polymer having horizontal alignment ability and a polymer having vertical alignment ability, A liquid crystal device characterized in that, in a surface layer portion of the alignment film, domains having different mixing ratios of the polymers have a phase-separated structure in which they are dispersed.