JPH06123808A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide the liquid crystal display device having improved brittleness and excellent heat resistance and improved visual angle characteristics by using a uniaxially stretched film of a styrene copolymer contg. a specific polymer as a phase difference plate. CONSTITUTION:This liquid crystal display device is constituted by using a liquid crystal element, which is formed by sandwiching a twist-oriented nematic liquid crystal between two sheets of electrode substrates facing each other and negative films as a polarizing plate and phase difference plate without using films having a positive intrinsic double refractive value. A uniaxially stretched film of a styrene copolymer contg. a graft polymer formed by addition polymn. of >=1 kinds of monomers contg. at least a styrene monomer to a polymer having unsatd. double bonds in the main chain or side chain is used as a film having a negative intrinsic double refractive value. The polymer having at least the unsatd. double bonds in the main chain or side chain of the styrene copolymer is so formed as to have the repeating unit derived from the monomer having a conjugated diene structure and to have 30 deg.C lower glass transition temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device using a twisted nematic liquid crystal and a birefringent film as a retardation plate.

[0002]

2. Description of the Related Art Liquid crystal display devices have many features such as low voltage and low power consumption, which can be directly connected to an IC circuit, various display functions, and weight reduction. It is widely used as a display device for word processors and personal computers. Among them, a twisted nematic liquid crystal display device (hereinafter STN-LCD) having a twist angle of liquid crystal molecules of 160 ° or more has a larger capacity than a conventional twisted nematic liquid crystal display device (TN-LCD) having a twist angle of 90 °. Since it is capable of high speed response and is excellent in high-speed response, it is currently the mainstream of liquid crystal display devices.

However, the STN-LCD has a drawback in that the background of display pixels is colored in blue or yellow (blue mode or yellow mode). Therefore, in black and white display, the contrast and the visibility are lowered, and the colorization is reduced. There was also the problem of being extremely difficult.

As means for solving the above-mentioned problems, Japanese Patent Laid-Open No. 63-
No. 167303, No. 63-167304, No. 63-1
89804, 63-261302, 63-14.
9624, JP-A-1-201607 and 1-201.
608, 1-105217, JP-A-2-2853.
No. 03, No. 2-59702, No. 2-24406, No. 2-146002, No. 2-257103, JP-A-3.
-23404, 3-126012, 3-181.
As described in Japanese Patent Publication No. 905, No. 3-194503, etc., a method using a retardation plate has been proposed, and STN-
It was found that the coloring of the image displayed by the LCD was significantly improved, but the viewing angle characteristics were hardly improved.

Therefore, in order to improve this viewing angle characteristic,
JP-A-2-385303 discloses a method of producing a birefringent film having a refractive index in the thickness direction larger than that in the direction perpendicular to the optical axis of the birefringence by electric field orientation and using the birefringent film as a phase plate. was suggested. According to this method, the change in contrast depending on the viewing angle is reduced and the viewing angle characteristics are improved, but the effect is still small, and it is necessary to apply a high voltage to the molten polycarbonate for a long time, and the manufacturing process is complicated. Therefore, it was difficult to reduce the cost. In addition, JP-A-2-160204
In the publication, a method is proposed in which a rod-shaped polycarbonate obtained by extrusion molding is cut into a plate shape and polished and used as a phase plate.
It was extremely difficult to produce a large area phase plate at low cost. Further, EP482620A2 publication proposes a method in which a film obtained by laminating a heat-shrinkable film on a polycarbonate film, uniaxially stretched, and then peeling the heat-shrinkable film is used as a retardation plate. At least the same as the phase plate, or 2
There is a problem that a heat-shrinkable film having a double or more area is required, and it cannot be produced at low cost.

Further, JP-A-2-256023, JP-A-3-141303, JP-A-3-14122 and JP-A-3-24.
In Japanese Patent No. 502, there is proposed a method in which one film each having a positive and negative intrinsic birefringence or a laminated film is used as a retardation plate. According to this method, the viewing angle characteristics can be improved by adjusting the birefringence of the two films according to the characteristics of the liquid crystal cell. However, two or more birefringent films prepared separately are used. Things are necessary, and the cost is higher.

Further, Japanese Patent Laid-Open No. 4-51101 discloses a method for improving the viewing angle problem by using only a film having a negative intrinsic birefringence as a retardation plate. However, the film having a negative intrinsic birefringence used in the above patent is a styrene monomer alone or,
It is a copolymer, and the polymer mainly composed of styrene-based monomers is brittle, and it is easy to break in the uniaxial stretching process, it is easy to break during processing (cutting, punching) of the film after stretching, or as a liquid crystal display device. There was a problem that cracks were likely to occur due to the thermal history after assembly.

[0008]

An object of the present invention is to solve the above problems, that is, by a method capable of producing a large area at a low cost, with excellent viewing angle characteristics, stretching or processing steps, and further liquid crystal display. It is an object of the present invention to provide a liquid crystal display device having improved viewing angle characteristics by using a retardation plate having improved brittleness when incorporated as a material.

[0009]

The above object of the present invention has been solved by the following means. (1) A negative film was used as a liquid crystal element and a polarizing plate and a retardation plate sandwiching a twisted nematic liquid crystal between two opposing electrode substrates, without using a film having a positive intrinsic birefringence value. In a liquid crystal display device, a graft polymer in which a film having a negative intrinsic birefringence value is addition-polymerized with at least one styrene-based monomer-containing polymer with respect to a polymer having an unsaturated double bond in a main chain or a side chain. A liquid crystal display device comprising a uniaxially stretched film of a styrene-based copolymer containing: (2) The polymer having at least an unsaturated double bond in the main chain or side chain of the styrene-based copolymer has a repeating unit derived from a monomer having a conjugated diene structure, and its glass transition temperature is The liquid crystal display device according to (1) above, which has a temperature of 30 ° C. or lower. (3) The liquid crystal display device according to the above (1) or (2), wherein the addition-polymerizable monomer of the styrene-based copolymer is at least one styrene-based monomer and acrylonitrile or methacrylonitrile.

The present invention will be described in detail below. The polymer used in the retardation film of the present invention is a polymer having at least an unsaturated double bond in the main chain or side chain (referred to as (A)), and at least one styrene-based monomer (referred to as a monomer). It is a graft polymer (referred to as the polymer (C) of the present invention) obtained by addition polymerization of (referred to as (B)).

First, the polymer (A) constituting the so-called trunk of the graft polymer (C) of the present invention will be described. The polymer (A) is specifically a polymer having a repeating unit of at least an unsaturated double bond in the main chain or side chain.
This repeating unit is preferably one derived by polymerization of a monomer having a conjugated diene structure.

Preferred examples of specific monomers having a conjugated diene structure include 1,3-butadiene, isoprene, 1,3-pentadiene, 2-ethyl-1,3-butadiene and 2-n-propyl-. 1,3-butadiene, 2,
3-dimethyl-1,3-butadiene, 2-methyl-1,
3-pentadiene, 1-phenyl-1,3-butadiene, 1-α-naphthyl-1,3-butadiene, 1-β-
Naphthyl-1,3-butadiene, 2-chloro-1,3-
Butadiene, 1-bromo-1,3-butadiene, 1-chlorobutadiene, 2-fluoro-1,3-butadiene,
2,3-dichloro-1,3-butadiene, 1,1,2-
Trichloro-1,3-butadiene, 2-cyano-1,3
-Butadiene and the like can be mentioned, of which 1,3
-Butadiene, isoprene, 2-chloro-1,3-butadiene are particularly preferred.

Further, the polymer (A) constituting the trunk of the polymer (C) of the present invention may be copolymerized with a hydrophobic monomer other than the above-mentioned monomer having a diene structure. Examples of such hydrophobic monomers include ethylene, propylene, 1-butene, isobutene, styrene, α-methylstyrene, vinyl ketone, monoethylenically unsaturated esters of aliphatic acids (eg vinyl acetate, allyl acetate). , Esters of ethylenically unsaturated monocarboxylic acids or dicarboxylic acids (for example, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, n-butyl acrylate, n-hexyl acrylate, 2- Ethylhexyl acrylate, t
-Butyl methacrylate, dodecyl methacrylate, 2
-Hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate)

Further, amides (eg t-butyl acrylamide, t-butyl methacrylamide), monoethylenically unsaturated compounds (eg acrylonitrile, methacrylonitrile) and the like can be mentioned. Of these, ethylene, propylene, styrene, α-methylstyrene, esters of acrylic acid or methacrylic acid, acrylonitrile, and methacrylonitrile are particularly preferable.

Two or more kinds of the above-mentioned monomer having the conjugated diene structure and the other hydrophobic monomer may be used. The unsaturated structure introduced into the polymer (A) by polymerization of a monomer having a conjugated diene structure may be a cis-1,4 bond as well known in the art, or a trans structure. 1,4-bond or transformer 1,2-
It may be a bond.

The polymer (A) may be a homopolymer derived from a monomer having a diene structure,
It may be a copolymer with another hydrophobic monomer. In the case of a copolymer, it may be a so-called random copolymer in which each monomer is copolymerized at an arbitrary ratio, or a block copolymer. Regarding the polymer and the synthesis method of such a conjugated diene monomer, for example, “Synthetic Polymer II,” edited by Shunsuke Murahashi et al.
Published by Asakura Shoten, 1975, pp. 171-308.

A preferred specific example of the polymer (A) described above is a styrene-butadiene copolymer (generally called SBR, which is solution-polymerized SBR and emulsion-polymerized SBR).
There is. As the solution-polymerized SBR, there are the above-mentioned block copolymers (for example, butadiene-styrene block copolymers and styrene-butadiene-styrene block copolymers) in addition to random polymers,

Butadiene homopolymer (eg cis-1,
4-butadiene, trans-1,2-butadiene, or rubber in which these and trans-1,4-butadiene structure are mixed), isoprene homopolymer (example of stereostructure is the same as butadiene polymer), styrene- Isoprene copolymer (random copolymer, block copolymer), ethylene-propylene-diene copolymer (as diene monomer, 1,4-hexadiene, dicyclopentadiene, ethylidene norbornene, etc.),

Acrylonitrile-butadiene copolymer,
Chloroprene copolymer, isobutylene-isoprene copolymer, butadiene-acrylic acid ester copolymer (as acrylic acid ester, ethyl acrylate, butyl acrylate, etc.), butadiene-acrylic acid ester-acrylonitrile copolymer (acrylic acid Examples of the ester include the same as above.

The glass transition temperature (T
g) is important for improving the film physical properties of the retardation film, and Tg is 50 ° C. or lower, preferably 30 ° C. or lower, and particularly preferably 0 ° C. or lower. The ratio of the conjugated diene monomer component in the polymer (A) is not particularly limited as long as it is within the above glass transition temperature range, but a range of 10 to 100% by weight is preferable.

Next, the monomer (B) used to obtain the graft polymer (C) by addition-polymerizing the polymer (A) will be described. In the polymer (C) of the present invention, if the polymer (A) is referred to as a trunk,
The polymer of the monomer (B) can be referred to as a branch having a repeating unit extending in a comb shape from the origin (A).

In the present invention, the monomer (B) used for the graft polymerization is at least one monomer containing at least a styrene monomer. Specific examples of the styrene-based monomer include styrene and α-methylstyrene, o-methylstyrene, p-methylstyrene, p-chlorostyrene,
Styrene derivatives such as p-nitrostyrene, p-aminostyrene, p-carboxylstyrene, p-phenylstyrene, 2,5-dichlorostyrene and p-t-butylstyrene are included, of which styrene or styrene and others Particularly preferred is a combination of the styrene derivatives.

Further, the monomer (B) may contain a monomer other than the above-mentioned styrenic monomer,
Such monomers include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, methyl methacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, acrylic acid, methacrylic acid, butadiene, Examples thereof include isoprene, maleic anhydride, vinyl acetate, ethylene, propylene and the like, and of these, acrylonitrile and methacrylonitrile are particularly preferable. These may be copolymerized alone with the styrene derivative, or a plurality of them may be used.

The proportion of the trunk component polymer (A) in the graft copolymer (C) used in the present invention cannot be uniquely determined because its contribution to the physical properties of the membrane can be changed depending on its structure. Although difficult, it is preferably 1 to 30% by weight, particularly preferably 3 to 30% by weight in view of the fact that the amount necessary for improving the physical properties of the film or the amount too large causes the film to be too soft and the relaxation of polymer molecular orientation due to heat may occur. It is 20% by weight.

The molecular weight of the polymer is not particularly limited as long as it is not particularly small, but considering the viscosity of the polymer solution when forming a film from a solution, the weight average molecular weight is 10
It is preferably in the range of 1,000,000 to 1,000,000, particularly preferably 15
It ranges from 10,000 to 500,000. The proportion of the styrenic monomer component in all the constituent components of the graft polymer (C) is preferably 50% or more by weight, particularly preferably 60% or more, from the viewpoint of manifesting birefringence characteristics.

The graft polymer (C) of the present invention can be synthesized by a commonly used radical polymerization method (eg emulsion polymerization method, solution polymerization method). The graft polymer (C) of the present invention may be used as a mixture of two kinds having different compositions and structures. Further, a polymer other than the graft polymer (C) may be mixed. Further, the polymer obtained in the present invention may be a graft polymer and another polymer such as a polymer (A) which does not undergo a graft reaction or a styrene-based random copolymer which does not undergo a graft reaction, although it depends on the polymerization method. Polymers may be mixed.

Preferred specific examples of the graft polymer (C) of the present invention are as follows: (A) a polymer (stem) having at least an unsaturated double bond in its main chain or side chain (B) containing at least a styrene monomer It is exemplified in the form of a polymer portion (branch) to which one or more kinds of monomers are added, but the present invention is not limited thereto. That is, it represents a product obtained by graft-polymerizing (B) with respect to (A). The ratio of the numbers in parentheses in (A) and (B) represents the copolymerization ratio (weight) of the monomers in each component,
(A): (B) represents the weight percentage ratio of the polymer of the corresponding trunk component / branch component.

P-1,2 (A) Styrene / butadiene copolymer (20/80) (B) Styrene / acrylonitrile / α-methylstyrene (60/20/20) P-1 (A) :( B) = 10: 90 P-2 (A) :( B) = 5: 95 P-3,4 (A) Styrene / butadiene copolymer (2
0/80) (B) Styrene / acrylonitrile (80/20) P-3 (A) :( B) = 10: 90 P-4 (A) :( B) = 7: 93 P-5 (A) Styrene / Butadiene copolymer (5
0/50) (B) Styrene / acrylonitrile (75/25) (A) :( B) = 12.5: 87.5

P-6,7 (A) Styrene / butadiene copolymer (50/50) (B) Styrene / acrylonitrile / α-methylstyrene (60/30/10) P-6 (A) :( B) = 15: 85 P-7 (A) :( B) = 10: 90 P-8 (A) Styrene / butadiene copolymer (5
0/50) (B) Styrene / acrylonitrile (75/25) (A): (B) = 10: 90 P-9 (A) polybutadiene (B) Styrene / acrylonitrile (70/30) (A): (B ) = 5: 95 P-10 (A) Polybutadiene (B) Styrene / acrylonitrile / methacrylonitrile (75/15/10) (A) :( B) = 10: 90

P-11 (A) Styrene / butadiene copolymer (50/50) (B) Styrene (A) :( B) = 12: 88 P-12 (A) Styrene / butadiene copolymer (2)
3/77) (B) Styrene / methyl methacrylate / acrylonitrile (70/10/20) (A) :( B) = 10: 90 P-13 (A) Polyisoprene (B) Styrene / t-butylstyrene (70) / 30) (A) :( B) = 10: 90

P-14 (A) acrylonitrile / butadiene copolymer (50/50) (B) styrene / acrylonitrile (80/20) (A) :( B) = 10: 90 P-15 (A) acrylonitrile / Butadiene copolymer (25/75) (B) Styrene / acrylonitrile / α-methylstyrene (60/20/20) (A) :( B) = 12: 88 P-16 (A) Ethyl acrylate / butadiene co-polymer Polymer (50/50) (B) Styrene / methyl methacrylate (80/20) (A) :( B) = 10: 90 P-17 (A) Ethyl acrylate / styrene / butadiene copolymer (40/30) / 30) (B) Styrene / methacrylonitrile (75/25) (A) :( B) = 15: 85

In contrast to the example graft polymer of the present invention, which is mainly in the form of an aqueous dispersion prepared by preparing the polymer represented by (A) by an emulsion polymerization method or purchasing a commercially available product, ( An aqueous dispersion of the graft polymer obtained after polymerization was carried out at 65 ° C. using a redox initiator of potassium persulfate and sodium sulfite while adding or dropping the monomer represented by B) all at once. Was synthesized under a strong acid condition or by addition of a salt (for example, common salt), or by freeze-thawing of the liquid, followed by filtration and drying.

In order to use the styrene copolymer film of the present invention as a retardation plate, it is necessary to uniaxially stretch the film to exhibit birefringence. The uniaxial stretching in this case includes not only pure uniaxial stretching but also what is considered to be essentially uniaxial stretching even if a little biaxiality is imparted. Therefore, horizontal uniaxial stretching by the tenter method and longitudinal uniaxial stretching utilizing the difference in peripheral speed between rolls are also included in the uniaxial stretching of the present invention. Thus, when the styrene-based copolymer film of the present invention is uniaxially stretched and used as a retardation plate, it is preferable that the retardation plate has a light transmittance of 70% or more and is achromatic. Is preferably 90% or more and is achromatic.

The retardation value indicating the birefringence of the retardation plate is determined by the thickness of the retardation plate and the stretching conditions.
The thickness is preferably 125 nm to 1200 nm, more preferably 380 nm to 700 nm, and the thickness of the retardation plate is preferably 5 μm to 1000 μm, more preferably 40 μm to 10 μm.
It is in the range of 0 μ.

A method of incorporating the retardation plate of the present invention into a liquid crystal display device will be described. The retardation plate of the present invention may be composed of a uniaxially stretched styrene-based copolymer film alone, but usually a polarizing plate, a protective film, or another film having birefringence is used as an adhesive or the like. It is configured to be laminated by using.

FIG. 1 illustrates a retardation plate obtained by laminating a birefringent film containing the uniaxially stretched styrene copolymer of the present invention on a polarizing plate. Reference numeral 1 is a birefringent film, 2 is an adhesive layer, and 3 is a polarizing film. The polarizing plate used is not particularly limited. Generally, a polarizing film or a polarizing film reinforced or protected by a resin layer or a glass layer is used. The polarizing film can be formed, for example, by treating a hydrophilic film such as a polyvinyl alcohol-based stretched film with a dichroic dye such as iodine, or by appropriately treating a resin film to orient the polyene. Although the adhesive is not limited, an acrylic adhesive or an adhesive having good transparency such as an adhesive is preferably used. The wavelength characteristic of birefringence in the retardation plate can be changed by the combination of laminated birefringent films, the number of laminated layers, the crossing angle of the optical axes, and the like. Therefore, the number of stacked layers and the like may be appropriately determined according to the wavelength property of the required birefringence, the phase difference to be compensated, and the like. However, from the viewpoint of suppressing the deterioration of the transmittance and the visibility due to the absorption loss and the reflection loss at the laminated interface, the smaller number of laminated layers is advantageous. The liquid crystal display device of the present invention has a retardation plate disposed on one side or both sides of a birefringent liquid crystal cell. The constitution [I] of the liquid crystal display device uses one retardation plate of the present invention, and the constitution [II] of the liquid crystal display device uses two retardation plates of the present invention. The retardation value of the retardation plate arranged in this liquid crystal display device depends on the type of the liquid crystal display device to be mounted, but it is preferable to match it with the retardation value of the liquid crystal itself. When two sheets are used, the same one may be used, but the range can be changed within ± 20 nm. Hereinafter, the invention will be described in detail with reference to Examples.

[0037]

【Example】

Example 1 The above-mentioned graft polymer of the present invention was used as polymers P3 and P9.
170 g of each was dissolved in 830 g of methylene chloride. This solution was cast on a glass plate so that the film thickness after drying was 100 μm, allowed to stand at room temperature for 5 minutes, and then at 45 ° C.
Was dried with warm air for 20 minutes and peeled from the glass plate. The peeled film was attached to a frame, dried at 70 ° C. for 1 hour, further dried at 110 ° C. for 15 hours, and then uniaxially stretched 430 at a temperature of 115 ° C. using a commercially available tensile tester 430, A roll film having a retardation value of 440 nm and 570 nm was obtained and used as a sample. Next, the obtained sample was used for the following evaluations.

(1) Brittleness 10 mm in the stretching direction (MD), the direction perpendicular to the stretching direction (T
A 35 mm sample piece was cut out in D) and MD as shown in FIG.
The angle θ at which the sheet was bent slowly and was broken so that a crease parallel to was formed was read. When it was not damaged, it was set at 0 °. It is preferable that θ is small, and it is necessary that it is actually 10 ° or less.

(2) Heat resistance The sample prepared in Example 1 was attached to a glass plate with a pressure sensitive adhesive so that the uniaxial stretching direction (MD direction) was parallel to the glass plate, and the light was polarized on the glass plate. The film was attached so that the polarization axis was at 45 ° with respect to the MD direction, and the film was stored under forced conditions of 90 ° C./80% for about 300 hours and observed whether cracking occurred. Comparative Example 1 Polystyrene was dissolved in methylene chloride to obtain 20% by weight.
It was a solution. This solution was cast on a stainless steel band, continuously peeled off and dried, and the film was uniaxially stretched between rolls having different peripheral speeds under a temperature condition of 111 ° C., 430,
A roll film having a retardation value of 440 nm and 570 nm was obtained and used as a sample. The obtained sample was used in Example 1.
The same evaluation as was done. The results are shown in Table 1.

[0040]

[Table 1]

Comparative Example 2 A polycarbonate having a weight average molecular weight of 100,000 was dissolved in methylene chloride to prepare a 20% by weight solution. This solution was cast on a stainless steel band, continuously peeled off and dried to obtain a polycarbonate film. The film 1
Uniaxially stretched between rolls having different peripheral speeds under a temperature condition of 70 ° C. to obtain a polycarbonate uniaxially stretched film having retardation values of 420 nm, 430 nm and 440 nm.

Next, in the samples obtained in Example 1 and Comparative Examples 1 and 2, the retardation value was adjusted so as to maximize the contrast ratio of the liquid crystal panel to which it was applied. Word processor WD-
A551 or Epson personal computer PC3
Configuration of liquid crystal display device in 86NW STN liquid crystal cell [I]
The contrast was obtained by measuring the Y value on the front face in the driving state and the non-driving state by mounting as shown in [II] and [III] by a luminance meter. Further, from the measurement of the Y value, the upper and lower viewing angle ranges in which the contrast ratio was 2: 1 or more were examined.
The results are shown in Table 2.

[0043]

[Table 2]

[0044]

The styrene copolymer film of the present invention is excellent in brittleness and heat resistance, and in a liquid crystal display device using the same, coloration of display is highly compensated, and high contrast in a wide viewing angle range. Has been achieved. That is, it is possible to obtain a liquid crystal panel having excellent visibility, and thus a display device.

[Structure of Liquid Crystal Display Device] The order from the viewing side was combined as shown in [I] to [IV]. The birefringent film means a film having a different retardation value shown in Table 2. [I] Protective film / adhesive layer / polarizing film / adhesive layer /
Birefringent film / adhesive layer / liquid crystal cell / adhesive layer / polarizing film [II] protective film / adhesive layer / polarizing film / adhesive layer /
Birefringent film- / Adhesive layer / Birefringent film-
/ Adhesive layer / Liquid crystal cell / Adhesive layer / Polarizing film [III] Protective film / Adhesive layer / Polarizing film / Adhesive layer / Birefringent film- / Adhesive layer / Liquid crystal cell / Adhesive layer /
Birefringent film- / Adhesive layer / Polarizing film

[Brief description of drawings]

FIG. 1 shows a cross section of a retardation plate in which birefringent films are laminated.

FIG. 2 shows a sample mounted on a brittleness measuring machine.

[Explanation of symbols]

 1: Birefringent film 2: Adhesive layer 3: Polarizing plate θ: Folding angle a: Sample

Claims (3)

[Claims] 1. A liquid crystal element comprising a nematic liquid crystal which is twisted and oriented between two opposing electrode substrates, a polarizing plate and a retardation plate without using a film having a positive intrinsic birefringence value and a negative film. In the liquid crystal display used, a graft film in which a negative film having an intrinsic birefringence value is addition-polymerized with one or more monomers including at least a styrene-based monomer to a polymer having an unsaturated double bond in a main chain or a side chain. A liquid crystal display device, which is a uniaxially stretched film of a styrene copolymer containing a polymer. 2. A polymer having at least an unsaturated double bond in the main chain or side chain of a styrene-based copolymer has a repeating unit derived from a monomer having a conjugated diene structure, and its glass transition. The liquid crystal display device according to claim 1, wherein the temperature is 30 ° C. or lower. 3. The liquid crystal display device according to claim 1 or 2, wherein the addition-polymerizable monomer of the styrene-based copolymer comprises at least one styrene-based monomer and acrylonitrile or methacrylonitrile.