JPH01222220A - Organic film for optical compensation and its manufacturing method - Google Patents

Abstract

PURPOSE:To enable colorless display and high-contrast display by constituting the above film of a polymer film which has no selective scattering wavelength within a visible region and contains an oriented chiral nematic (cholesteric) liquid crystal having a spiral structure. CONSTITUTION:The org. film for optical compensation consists of the liquid crystal polymer film which has no selective scattering wavelength lambda0 in the visible region and has the spiral structure. The chiral nematic liquid crystal polymer film exhibits circularly polarized dichromaticity by generating selective scattering to the light of the specific wavelength corresponding to lambda0=p.nfcospsi (p is a spiral pitch, nf is the refractive index of the film, psi is the deviation angle between the orientation of the incident light and the orientation of the spiral axis). Such org. film for optical compensation has the function to return the elliptically polarized light which is emitted from the twisted nematic liquid crystal layer of TN and STN and has an optically rotatory power to linearly polarized light. Also the film has the function to compensate the wavelength dispersion of the double refraction exhibited by the nematic liquid crystal layer. The degree of coloration of the display is thereby decreased and the display contrast is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an organic film for optical compensation used in liquid crystal displays and the like.

2. Description of the Related Art At present, the mainstream of liquid crystal displays is an optical modulation method using twisted nematic liquid crystals such as TN (twisted nematic) and 5TN (spartwisted nematic), which utilizes an effective change in birefringence due to voltage application.

Here, the twist angle θ of TN liquid crystal is usually π/4 to π/2.
(4s° to 9o0), and is used in television displays (active matrix drive) that require fast response speed, high contrast, and gradation.On the other hand, STN liquid crystal has a twist angle of π-3π/2 (180 '~270') and is used for character and graphic displays (simple matrix drive) that do not require gradation.

Now, when linearly polarized light passes through a twisted nematic liquid crystal layer of TN and 197N, wavelength dispersion generally occurs in both optical rotation and birefringence, and the output light becomes elliptically polarized light with optical rotation. Therefore, simply installing an analyzer (polarizing plate) caused the problem that the liquid crystal display panel was colored and its display contrast was reduced. As a means to solve these problems, between the twisted nematic liquid crystal layer for light modulation and the polarizing plate,
It has already been proposed to provide another twisted nematic liquid crystal layer for optical compensation (the twist angle θ is the same, but the sense of twist is opposite) (Japanese Patent Laid-Open No. 66-88112
, Japanese Patent Publication No. 57-46227. Japanese Patent Publication No. 67-12591
9). According to this conventional technology, the elliptically polarized light emitted from the light modulation liquid crystal layer in the absence of an electric field can be returned to linearly polarized light by passing through the optical compensation liquid crystal layer, thereby reducing the degree of coloring of the display. Display contrast can be increased.

Problems to be Solved by the Invention This conventional technique requires the formation of another twisted nematic liquid crystal layer for optical compensation in addition to the twisted nematic liquid crystal layer for optical modulation, resulting in high costs. . Furthermore, manufacturing yield is reduced. When using transparent substrates such as glass to assemble a liquid crystal panel, even if a single glass substrate is used in common, the number of manufacturing steps will increase, and the thickness of the panel will increase due to the addition of at least one glass substrate. It had drawbacks such as being over 60% thicker.

The present invention eliminates the above-mentioned drawbacks, that is, provides an organic film having an optical compensation function that is inexpensive and hardly increases the panel thickness, and a method for manufacturing the same.

Means for Solving the Problem The organic film for optical compensation of the present invention is a chiral nematic film with a helical structure that does not have a selective scattering wavelength in the visible range.
Cholesteric) consists of a polymer film containing oriented liquid crystals. In addition, the method for producing an organic film for optical compensation is to apply an alignment treatment to the surface of a liquid crystal display substrate such as a transparent substrate, polarizing plate, or light diffusing plate, then apply a mixture of chiral nematic liquid crystal, and maintain it at a predetermined temperature. After uniformly aligning the liquid crystal so that its helical axis is perpendicular to the surface of the substrate, the liquid crystal is photopolymerized or thermally polymerized. Here, it is desirable that the chiral nematic liquid crystal be oriented such that the direction of its helical axis is perpendicular to the surface of the polymer film.

Furthermore, it is desirable that the helical sense of the chiral nematic liquid crystal is opposite to the torsional sense of the twisted nematic liquid crystal layer for optical modulation to be compensated, and that the helical pitch of the chiral nematic liquid crystal is equal to 2πdf/θ. Further, it is desirable that the product of the birefringence and the thickness of the polymer film is equal to the product of the birefringence and the thickness of the twisted nematic liquid crystal layer for light modulation to be compensated and is equal to λFΣ 62Σ7.

Here, df is the thickness of the polymer film, θ is the twist angle (in radians) of the twisted nematic liquid crystal layer for light modulation, λ is the wavelength of light, and m is an integer larger than θ/π.

Function The organic film for optical compensation of the present invention has a selective scattering wavelength λ within the visible range. It is necessary that the liquid crystal polymer film has a helical structure. In general, chiral nematic liquid crystal polymer films have a wavelength of λ. =p @n fCO3ψ (p is the helical pitch, nf is the refractive index of the film, ψ is the deviation angle between the incident light direction and the helical glaze direction), causing selective scattering of light of a specific wavelength, and circularly polarized light Shows chromaticity. Therefore,
If λ0 is in the visible range 400 n7rL ~ 750 n7H
If this happens, the polymer film will be colored and its transmittance will be halved, making it unusable for liquid crystal displays. Here, the refractive index nf of the polymer film is usually 1.6 to 1.7, the direction of the helical axis is perpendicular to the surface of the polymer film, and ψ is usually zero. Therefore, it is necessary to select a chiral nematic liquid crystal material whose helical pitch p is adjusted so that the selective scattering wavelength λ0 is outside the visible range.

The organic film for optical compensation of the present invention includes TN and STN.
It has the function of returning optically polarized elliptically polarized light emitted from the twisted nematic liquid crystal layer to linearly polarized light.

This is because the polymer film of the present invention has a chiral nematic liquid crystal structure (helical structure) having a helical sense opposite to the torsional sense of the twisted nematic liquid crystal layer, and preferably the degree of twist of the helical structure is df/ This is because p (df: film thickness, p: helical pitch) is equal to the twist degree θ/2π (θ: twist angle of the liquid crystal layer) of the twisted nematic liquid crystal layer. In this way, the wavelength dispersion of optical rotation inherently exhibited by the light modulating twisted nematic liquid crystal layer is compensated by the polymer film of the present invention.

Furthermore, the polymer film of the present invention has the function of compensating for the wavelength dispersion of birefringence generally exhibited by a nematic liquid crystal layer. This is because the birefringence Δnf of the polymer film
By making the product of ΔnQ and the thickness df equal to the product of the birefringence Δng and the thickness dg of the twisted nematic liquid crystal layer, the retardation ΔnQ−dg generated in the twisted nematic liquid crystal layer is equal to the retardation Δn t ” of the same magnitude. This is because it is restored by the polymer film having a reverse twist structure by d t .

All of the above functions are effects when no electric field is applied to the twisted nematic liquid crystal layer for optical modulation.

On the other hand, when an electric field is applied to the twisted nematic liquid crystal layer, the effective phase difference due to the liquid crystal layer becomes small, and the above phase compensation relationship is no longer maintained.

That is, the light emitted from the organic film for optical compensation generally becomes elliptically polarized light. In order to eliminate this effect or to reduce it as much as possible, the retardation Δnf@df of the polymer film is determined by the Gooah and Tarry condition Δn t' d t=λ2Σr" (1, Phys, D
:AppQ, Phyg,,Vog, 8.197! ,
The material of the polymer film is designed to meet the requirements (ppl 575-1584).

As a result, linearly polarized light can be obtained even when an electric field is applied,
This enables high-contrast liquid crystal display without coloring.

Here, the wavelength λ of the light is usually set to green, which has high visibility.

As a result of the above-mentioned effects, the organic film for light compensation of the present invention has no coloring, high display contrast, and TN.
Liquid crystal displays and STN liquid crystal displays can be provided.

This effect can also be obtained in the same way even if the optical compensation organic film is placed on the light incident side of the TN liquid crystal layer and the STN liquid crystal layer.

Furthermore, the organic film for optical compensation of the present invention can be formed directly on a transparent substrate of a liquid crystal display panel, on a polarizing plate, or on a light diffusing plate, so it is easy to manufacture. Yes, it is low cost. Further, since the thickness of the organic film is usually several μm to several 100 μm, it has the effect that the overall thickness of the liquid crystal display panel hardly increases.

EXAMPLE The organic film for optical compensation of the present invention is a solid film of a liquid crystal polymer having a chiral nematic, ie, cholesteric liquid crystal structure. Thermotropic liquid crystal polymers and lyotropic liquid crystal polymers can be used as raw materials for obtaining such liquid crystal polymers.

As the former thermotropic liquid crystal polymer, a side chain type liquid crystal polymer is useful. For example, it is a copolymer containing a nematic liquid crystal group and a chiral group in its side chain. Here, by adjusting the content ratio of nematic liquid crystal groups and chiral groups, a chiral nematic liquid crystal having a desired helical pitch can be obtained. A concentrated solution of such a copolymer dissolved in an appropriate organic solvent is applied onto a transparent substrate that has been subjected to an orientation treatment, and the organic solvent is evaporated through a drying process to form a liquid crystal polymer film of a predetermined thickness. get.

Alternatively, monomers and oligomers can be used. For example, a mixture of a polymerizable (unsaturated group-containing) nematic liquid crystal monomer and a polymerizable chiral monomer is coated to a predetermined thickness on a transparent substrate that has been subjected to an orientation treatment, and then a spiral glaze is applied perpendicular to the surface of the substrate while being maintained at a constant temperature. Oriented to
After fixing to a predetermined helical pitch and obtaining a uniform chiral nematic liquid crystal state, the monomer mixture in the liquid crystal state is polymerized by light irradiation and heating to obtain a chiral nematic liquid crystal polymer film.

Next, an example of a lyotropic liquid crystal polymer obtained by polymerizing lyotropic liquid crystal will be described.

Mixtures of polymerizable (unsaturated group-containing) monomers and oligomers and chiral nematic liquid crystal substances (low molecules or polymers); mixtures of polymerizable monomers and oligomers, nematic liquid crystal substances (low molecules or polymers), and chiral substances; Useful materials include mixtures of polymerizable chiral nematic liquid crystal materials and organic solvents, mixtures of polymerizable nematic liquid crystal materials, chiral materials, and organic solvents, and mixtures of polymerizable nematic liquid crystal materials, polymerizable chiral materials, and organic solvents. It is. These lyotropic liquid crystal materials are coated on a transparent substrate (or polarizing plate) that has been aligned in advance, and the helical axis is oriented perpendicular to the substrate surface (or polarizing plate surface) while being maintained at a constant temperature. After fixing the helical pitch and obtaining a uniform chiral nematic liquid crystal state, photopolymerization is performed to obtain a chiral nematic liquid crystal polymer. Note that when a liquid crystal polymer film is directly formed on a polarizing plate made of a stretched polymer (for example, polyvinyl alcohol), there is an advantage that no surface alignment treatment is required.

Next, we will discuss the material design of such a chiral nematic liquid crystal polymer film for optical compensation, that is, the film thickness at,
How to design the average refractive index nf, birefringence Δnf, and helical pitch p will be described. When light is incident parallel to the helical axis of chiral nematic liquid crystal, p-nf
When selective scattering (circular dichroism) occurs around a light wavelength equal to the value of It is necessary to design so that such a phenomenon does not occur in the visible range. From the point of view of material selection, p
It is easier to bring the value of @nt to the longer wavelength side than the visible range. Therefore, first p ” n f > 0, 76
Design to satisfy the μm condition. Here, the value of the refractive index "f" of chiral nematic liquid crystal polymer is usually 1.6 to 1.
．． 7, so the size of the helical pitch p is 0.46 μm
It is necessary to select the material of the liquid crystal polymer as described above. Next, the retardation of the polymer film is determined by Gooch and T
It is desirable to design so that the condition of arry is satisfied. That is, the birefringence Δnf and the thickness df of the polymer film are adjusted so that Δnf"dt = λW. For example, if the twist angle θ of the twisted nematic liquid crystal layer for light modulation is rr/2
(90' T N type), if m is 1 and the wavelength λ is green 0.546717 m, Δnt' d
f = 0.472μ . Δnf of liquid crystal polymer
varies depending on the material, but is usually a value of 0.03 to 0.16. Here, Δn f: 0.1 (20'C, λ
:0,545/J771), the optimum thickness df of the polymer film is 4.72 μm.

Furthermore, since it is desirable that the relationship between the helical pitch p and the thickness df of the liquid crystal polymer satisfies p=df・2π/θ, p
The optimum value of is 18.9 μm. This value is p ” n
The conditions of f>0 and 75 μm are fully satisfied.

To summarize the above, when a 90° right-handed TN liquid crystal layer is used as a light modulation liquid crystal layer, for example, a chiral nematic liquid crystal polymer with a birefringence of 0.1 and a left-handed helical pitch of 19 μm is controlled to have a thickness of 4.7 μm. It can be said that it is desirable to use an organic film for optical compensation.

Example 1 First, a 90° right-handed twisted TN liquid crystal panel was prepared as a twisted nematic liquid crystal layer for light modulation. This panel was sandwiched between two parallel polarizing plates whose polarization directions were parallel to each other, and the average contrast was determined to be 6o:1 over wavelengths in the visible range of 400 nm to 750 nm. In addition, the transmitted light was reddish.

Next, a chiral nematic liquid crystal polymer film for optical compensation of the present invention was produced. A polyimide alignment film was applied on the outer surface of the glass substrate of the same TN liquid crystal panel as above, and the polyimide alignment film was rubbed in a direction perpendicular to the rubbing direction of the light-emitting side alignment film inside the panel. In addition, 97 mol% of the nematic liquid crystal monomer shown in the following chemical formula (1) and 3 mol% of the levorotatory chiral monomer shown in the chemical formula (2)
and CH=C-C0O-(CH2)2-0-...
...(1) CH2=C-Coo-(CH2) ■"-Coo Ch
og+ (Ususa) - - (2) The mixed chiral nematic liquid crystal was coated and photopolymerized while being maintained at a constant temperature of 200'C, and then cooled to room temperature to give a film thickness (df) of 6.
A chiral nematic liquid crystal polymer (copolymer) film was obtained having a birefringence (Δnf) of 7 μm, 0.07, and a cholesteric left-handed helical pitch (p) of 27 μm (the helical axis was oriented perpendicular to the substrate surface). A TN liquid crystal panel equipped with such an organic film for optical compensation of the present invention is sandwiched between two parallel polarizing plates whose polarization directions are orthogonal to each other, and the display is performed over a visible wavelength range of 400 nm to 750 nm.
The average contrast was found to be 100:1 or more. Moreover, the transmitted light was colorless.

Example 2 First, as a twisted nematic liquid crystal layer for light modulation, 2400
A left-handed STN liquid crystal panel was prepared. This panel was sandwiched between two parallel polarizing plates so that the polarization direction made an angle of 30' with respect to each other, and the visible range was 400 nm to 750 nm.
The result of finding the average contrast over the wavelengths is 1o:
It was 1. Moreover, the transmitted light showed a magenta color.

Next, a chiral nematic liquid crystal polymer film for optical compensation of the present invention was produced. A polyimide alignment film was applied on the outer surface of the glass substrate of the same STN liquid crystal panel as above, and the polyimide alignment film was rubbed in a direction perpendicular to the rubbing direction of the light-emitting side alignment film inside the panel. in addition,
Nematic liquid crystal monomer 94 shown in the following chemical formula (3).
6 mol% and 6.6 mol% of the dextrorotatory chiral monomer shown in chemical formula (4) and C)i2=C-Coo-(CH2)2-
0-・・・・・・・・・(3) CI(2=c-Coo-(C)12),--COO
A chiral nematic liquid crystal mixed with ChoQ (L-form)...P4) was applied, photopolymerized while maintaining the temperature at 200'C, and then cooled to room temperature to obtain a film thickness (d,).
11.6 μm, birefringence ゛(Δn1) 0.07,
and cholesteric stone winding helical pitch (P) 17.4
A chiral nematic liquid crystal polymer (copolymer) film consisting of .mu.m (the spiral glaze is oriented perpendicular to the substrate surface) was obtained. An ST equipped with such an organic film for optical compensation of the present invention
N liquid crystal panel is sandwiched between two parallel polarizing plates whose polarization directions are orthogonal to each other, and the visible range is 400 nm to 750 nm.
The result of determining the average contrast over the wavelengths of 40:
It was 1 or more. Moreover, the transmitted light was colorless.

Effects of the Invention As can be seen from the examples above, the organic film for optical compensation comprising the chiral nematic liquid crystal polymer of the present invention has the following effects:
By mounting it on TN and STN liquid crystal display panels, colorless display and high contrast display can be achieved. Furthermore, even when the organic film of the present invention is mounted, the thickness of the entire panel increases by several hundred micrometers or less, and light scattering is therefore small. In addition, the organic film of the present invention can be formed directly on a panel substrate or a polarizing plate, so that the manufacturing process is simple and easy, and a low-cost liquid crystal display panel can be provided.

Claims (3)

[Claims] (1) An organic film for optical compensation consisting of a polymer film containing aligned chiral nematic (cholesteric) liquid crystal with a helical structure that does not have a selective scattering wavelength in the visible range. (2) The organic film for optical compensation according to claim 1, wherein the chiral nematic liquid crystal has its helical axis oriented perpendicularly to the surface of the polymer film. (3) After alignment treatment is applied to the surface of the liquid crystal display substrate, a mixture of chiral nematic liquid crystal is applied and maintained at a predetermined temperature so that the helical axis of the liquid crystal is perpendicular to the substrate surface. A method for producing an organic film for optical compensation, which comprises aligning and then photopolymerizing or thermally polymerizing the liquid crystal.