JPH02253233A - Electro-optical liquid crystal device - Google Patents

Abstract

PURPOSE:To increase the refractive index of an optically anisotropic film in a perpendicular direction and to make it possible to widen the visual angle of an optical liq. crystal device by using a uniaxially stretched polystyrene film as the optically anisotropic film having an optical axis in one direction of the surface and optically negative uniaxial property. CONSTITUTION:The principal chain of the molecular structure of a polystyrene film is a polymer of vinyl groups and the side chain is phenyl groups. When the polystyrene film is stretched, the principal chain elongates in the stretching direction and the phenyl groups are aligned in a direction perpendicular to the stretching direction. Since the refractive index in the direction of the aligned phenyl groups is generally higher than that of alkyl groups, the refractive index in a direction perpendicular to the stretching direction becomes higher than that in the stretching direction. Accordingly, the stretched polystyrene film is an optically anisotropic film having an optical axis in the stretching direction and causing negative double refraction. Since polystyrene can reduce the required film thickness by two figures, a low-cost film having high homogeneity is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a viewing angle compensator that widens the viewing angle of a liquid crystal electro-optical element that utilizes the birefringence effect.

[Conventional technology]

In order to widen the viewing angle range of a liquid crystal electro-optic device in which a homogeneously aligned liquid crystal is sandwiched between two electrode substrates, there is a method of applying a viewing angle compensation plate, which is an optically anisotropic film, to the liquid crystal electro-optic device. . Regarding this, the patent application 1986-198
506 (disclosed in detail), i.e., the optically anisotropic film has its three main refractive indices Nl01Neat N3
Of @, N5. are the other two refractive indices N10, N,. , Nlo and N2° are the same value, and the axis corresponding to N se is in one direction within the plane of the optically anisotropic film. By arranging this optically anisotropic film so that its Ns axis is aligned with the molecular axis of the liquid crystal, it has the effect of widening the viewing angle by canceling the change in △n of the liquid crystal that occurs in response to changes in the viewing angle. It is something that emanates.

By the way, there are two types of optically anisotropic films: a polymer film type and a discotic liquid crystal type, but the polymer film type is considered to be better in terms of cost and ease of production. Polymer film types, such as polymethyl methacrylate (PMMA) and polymethyl fluoroacrylate (PMFA), increase the refractive index in the direction perpendicular to the stretching direction by stretching, resulting in negative optical anisotropy. It is necessary that the polymer exhibits the following.

[Problems to be Solved by the Invention] However, the polymer film type optically anisotropic film in the above-mentioned prior art is mainly composed of PMMA ('PMFA),
These have a problem that the value of Δn generated by stretching is small and that it is necessary to form a thick film with a thickness of 1 to 2 mm, resulting in high cost and difficulty in manufacturing.

The present invention is intended to solve these problems, and its purpose is to increase the Δn generated by stretching and reduce the required thickness of the film, thereby achieving optical anisotropy that is inexpensive and easy to manufacture. It provides body membranes.

[Means to solve the problem]

The electro-optical device of the present invention includes a liquid crystal cell in which a homogeneously aligned liquid crystal is sandwiched between two opposing electrode substrates, an optically anisotropic film that widens the viewing angle characteristics of the device, and an optically anisotropic film on both sides sandwiching them. In a liquid crystal electro-optical element comprising a pair of arranged polarizing plates, an optically anisotropic film having an optical axis in one direction within the plane of the optically anisotropic film and having optically negative uniaxiality. but,
It is characterized by being a uniaxially stretched polystyrene film.

[Function J] By stretching the polymer film of the present invention, the refractive index in the direction perpendicular to the stretching direction becomes larger than the refractive index in the stretching direction, forming an optically negative refractive index anisotropic body. become. This principle will be explained here.

First, the case of a polymer film that forms a refractive index anisotropic body exhibiting positive birefringence through normal stretching will be described. This type of polymer film includes polycarbonate film and polyethylene film. These polymers are
The molecules extend in the direction of the main chain, and there are also groups in the main chain that increase the refractive index, such as benzene rings, so if the main chain is aligned in the stretching direction by stretching, the refractive index in the main chain direction will increase. ,
It becomes larger than the refractive index in the perpendicular direction. Therefore, in this case, the optically anisotropic film exhibits positive birefringence.

On the other hand, since the polymer film of the present invention is a polystyrene film, its monomolecular structure is a polymer in which the main chain is a vinyl group and the side chain is a phenyl group. Therefore, when a polystyrene film is stretched, the main chain extends in the direction of stretching, and the phenyl groups in the side chains extend in a direction perpendicular to the direction of stretching.

Generally, the refractive index in the direction in which the phenyl group faces is greater than the refractive index of the alkyl group, so the refractive index in the direction perpendicular to the stretching direction is greater than that in the stretching direction. Therefore,
The stretched polystyrene film becomes an optically anisotropic film exhibiting negative birefringence with the stretching direction as the optical axis.

In addition, since polystyrene film has phenyl groups in its side chains, it has a Δn
can be increased by about two orders of magnitude. Therefore, the required film thickness can be reduced by two orders of magnitude. Therefore, a film with high homogeneity can be obtained at low cost.

[Implementation example 1] This will be explained in more detail in Examples below.

Example 1 A liquid crystal (Δn=0.15) was placed between transparent electrode substrates and homogeneously aligned. The cell thickness was set to 6.0 μm, and the retardation ΔnXd was set to 0.90 μm.

On the other hand, an optically anisotropic film was created as follows. A commercially available polystyrene film (thickness: 50 μm) was stretched at 120°C at a speed of 0.5 mm/see to a film thickness of 40 μm.
A film with a stretching ratio of 1.4 was obtained. The ΔnXd of this film was 0.9 μm, and a film that perfectly matched with the liquid crystal cell was obtained. Layering film and liquid crystal cells,
It was found that when polarizing plates were placed on both sides to form a crossed nicol, the viewing angle was clearly widened.

By the way, for comparison, the PMMA film was stretched under the 0 condition, in which a sheet with a film thickness of 2 mm was stretched at 150°C and stretched at 015 mm/
It was stretched at a speed of sec. A sheet with a film thickness of 1.2 mm was thus obtained, and ΔnXd was also 0.9 μm. If you try to make a film of the same △nXd like this, PMM
In A, the thickness must be made 40 times thicker than the polystyrene film.

When a polystyrene film is used in this way, the difference in refractive index caused by stretching is larger than that of conventional polymer films, and the film thickness can be made thinner. Therefore, it can be seen that the method of this example is very superior in terms of cost and ease of manufacture.

〔Effect of the invention〕

As described above, according to the present invention, the polystyrene film has a higher refractive index in the direction perpendicular to the stretching direction than in the stretching direction, and can be made into an optically anisotropic film exhibiting negative birefringence. The anisotropic film obtained in this manner has the effect of widening the viewing angle of the liquid crystal optical element.

Claims (1)

[Claims] A liquid crystal cell consisting of a homogeneously aligned liquid crystal sandwiched between two opposing electrode substrates, an optically anisotropic film that widens the viewing angle characteristics of the element, and a pair of polarizing plates placed on both sides of the cell. In a liquid crystal electro-optical element comprising: an optically anisotropic film having an optical axis in one direction within the plane of the optically anisotropic film and having negative optical uniaxiality, the optically anisotropic film has a uniaxially stretched polystyrene film A liquid crystal electro-optical element characterized by a body.