JP3308013B2 - Birefringent film and liquid crystal display device using the film - Google Patents

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 using a twisted nematic liquid crystal or a cholesteric liquid crystal. Further, the present invention relates to a birefringent film used for correcting coloration and increasing the viewing angle of the liquid crystal device. More specifically, the present invention improves the brittleness, strength, and haze (transparency of the film) of the film, and corrects the coloration and the visual field. The present invention relates to a birefringent film having excellent angle increasing characteristics and a liquid crystal display device using the same.

[0002]

2. Description of the Related Art A liquid crystal display device has many features such as being directly connected to an IC circuit with low voltage and low power consumption, having various display functions, being capable of high productivity and light weight. And its use has expanded. At present, a twisted nematic liquid crystal display (hereinafter referred to as STN-LCD) having a twist angle of liquid crystal molecules of 160 ° or more has been put into practical use as a dot matrix type liquid crystal display used for OA related devices such as a word processor and a personal computer. I have. This is because STN-LCD is a conventional twisted nematic liquid crystal display (TN-LCD) having a twist angle of 90 °.
LCD) can maintain high contrast even during high multiplex driving.

[0003] The STN cannot make the hue of the appearance of the liquid crystal display device white, and exhibits a color from green to yellow-red, which is unsuitable as a display device. In order to solve this problem, there has been proposed a method of providing one or a plurality of three layers of optically anisotropic material between a pair of polarizing plates. In this case, when the linearly polarized light that has passed through one of the pair of polarizing plates has passed through the liquid crystal layer of the liquid crystal element and the optically anisotropic body, it has a wavelength of about 400 nm to about 700 nm.
In this wavelength range, elliptically polarized light substantially uniform in the major axis direction can be obtained. As a result, when the light passes through the other polarizing plate, a specific wavelength range is not blocked, and white light is obtained.

[0004] In addition, a patent application of a retardation plate used alone for removing color in STN-LCDs is also found. For example, JP-A-63-189804 relates to a polycarbonate film stretched in a uniaxial direction so that a measured value of retardation (a product of a birefringence value and a film thickness) by a polarizing microscope becomes 200 to 350 nm or 475 to 625 nm. is there.

Japanese Unexamined Patent Publication (Kokai) No. 63-167304 relates to a film laminate in which two or more films or sheets having birefringence which have been uniaxially stretched are stacked such that their optical principal axes are orthogonal to each other. Things. In the above invention, when two birefringent films (each of which has a retardation value of R ₁ and R ₂ ) are superposed orthogonally, a retardation film having a retardation of | R ₁ −R ₂ | Taking advantage of the fact that even if R ₁ and R ₂ have a large retardation value, | R ₁ −R ₂ |
-180 nm, 200-350 nm, 475-625 n
It aims at the effect that it can be adjusted to a range such as m.

All of the above-mentioned inventions are aimed at removing the color of the STN-LCD, and in that respect, the invention has been greatly improved, and an image close to white / black display has been obtained. In addition, a method using a polymer birefringent film (hereinafter referred to as a retardation film) has a cost advantage, and the demand is rapidly expanding.

However, in this retardation film, when the liquid crystal display is viewed from the front, the removal of coloring can be almost achieved. However, when the display is viewed from an oblique direction, coloring due to a slight change in angle and display contents of the screen disappear. However, the problem of viewing angle characteristics observed in general STN-LCDs has not been solved.
-Has become a major challenge for LCDs.

More specifically, the refractive index of a polymer having a positive intrinsic birefringence in the longitudinal uniaxially stretched film in the stretching axis direction is n _MD , the refractive index in the direction perpendicular to the stretching axis is n _TD , and the film surface normal is Assuming that the refractive index in the linear direction is n _ND , the magnitude relationship between the respective refractive indices is expressed by the following equation.

[0009]

(Equation 1)

Therefore, when the incident light is perpendicular to the film surface, Re = (n _MD -n _TD ) d. Next, when the incident light passes through a plane orthogonal to the stretching direction, the birefringence value changes in a range from Δn = n _MD −n _TD to Δn = n _MD −n _ND with a change in the incident angle. Here, the relationship of the following formula holds, so △
n is unchanged or increased by oblique incidence.

[0011]

(Equation 2)

On the other hand, since the optical path length increases with oblique incidence, Re = △ n · d sharply increases with oblique incidence.

When the incident light is made incident from the normal direction of the film in the direction of the stretching axis, Δn is from n _MD −n _ND to n _ND.
−n _TD, which cannot be compensated for by an increase in the optical path length, and cannot be compensated for by Re = 入射 n with oblique incidence.
D decreases rapidly. In principle, a uniaxially stretched film having the smallest rate of change in retardation is n _MD > n _TD = n
_In the case of _ND , also in this case, Re greatly changes due to an increase in the optical path length accompanying oblique incidence.

It has been studied whether the problem of the viewing angle of the STN-LCD can be improved by changing the three-dimensional refractive index of the retardation film. For example, Japanese Unexamined Patent Application Publication No. Hei 2-256603 discloses that the viewing angle dependency of retardation (Re) defined as the product of the birefringence value (Δn) and the thickness (d) of a film is closely related to the viewing angle of an LCD. On the basis of, a film substantially having an optical axis in the normal direction of the film, specifically, a laminated film of a biaxially stretched film having a negative intrinsic birefringence value and a uniaxially stretched film having a positive intrinsic birefringence value It has been disclosed that the viewing angle can be greatly improved by inserting between the liquid crystal cell and the polarizing plate.

JP-A-3-206422 discloses a laminate of a uniaxially stretched film of a polymer having a positive intrinsic birefringence value and a uniaxially stretched film of a polymer having a negative intrinsic birefringence value between a liquid crystal cell and a polarizing plate. It has been disclosed that the viewing angle characteristics of a liquid crystal display device can be significantly improved by inserting the liquid crystal display device into a liquid crystal display device. Similarly, regarding the improvement of the viewing angle problem based on lamination of films having positive and negative intrinsic birefringence,
The disclosure is also made in JP-A-3-24502. All of the films disclosed as films having a negative intrinsic birefringence used in the above patents are homopolymers or copolymers using a styrene monomer.

The present inventors have improved the viewing angle characteristics, heat resistance, brittleness (breakage during uniaxial stretching, cracking during processing, cracking due to thermal history after assembling as a liquid crystal display device) of the styrenic copolymer. We focused on improvement and focused on the improvement, and filed a patent application for a retardation film using a film having a negative intrinsic birefringence with improved performance depending on the copolymer structure and copolymerization ratio or the stretching conditions. Kaihei 4-21
5602, Japanese Patent Application No. 3-293648, 3-474
No. 42, No. 4-321955).

However, when this type of styrene copolymer is produced by an emulsion polymerization method, the resulting film has an insufficient transparency level, so that the display quality when assembled as a liquid crystal display device may become a problem. there were. When the latex obtained by emulsion polymerization is taken out as a powder, the aggregates having a filterable size obtained by freezing the latex, agglomeration using an inorganic salt, or agglomeration with an acid, etc. become a strong agglomerate. However, there is a problem that fine particles are originally regenerated in the filtration process, resulting in a decrease in filtration / washing efficiency or a decrease in yield due to clogging of filter cloth or the like.

[0018]

SUMMARY OF THE INVENTION An object of the present invention is to firstly enable efficient powderization by using an emulsion polymerization method.
Styrenic copolymer excellent in transparency of obtained film and birefringent film using the same (retardation film)
Is to provide. A second object of the present invention is to provide a birefringent film having the above-mentioned excellent characteristics, having excellent film physical properties, having a small change in birefringence value due to heat, and having an excellent viewing angle improving effect. is there. A third object of the present invention is to provide a liquid crystal display device having improved performance using a birefringent film having the above-mentioned excellent characteristics.

[0019]

The object of the present invention has been attained by the following means. (1) A stretched styrene copolymer film having negative intrinsic birefringence, wherein the copolymer has at least styrene with respect to a polymer having at least an unsaturated double bond in a main chain or a side chain. A graft polymer obtained by addition polymerization of one or more monomers including a series monomer, and a nonionic surfactant or -COOM (M represents a cation)
A birefringent film having negative intrinsic birefringence, which is a copolymer obtained by emulsion polymerization in the presence of a surfactant having the following.

A preferred embodiment of the film having a negative intrinsic birefringence of the present invention is the following (2), and a liquid crystal display device using the above (1) is the following (3). (2) The styrene-based copolymer is obtained by coagulating a latex obtained by emulsion polymerization under a strong acid condition and heat-treating the latex to obtain a powder. Birefringent film with negative intrinsic birefringence.

(3) A liquid crystal display device comprising a liquid crystal element having a twisted nematic liquid crystal sandwiched between two opposing electrode substrates and at least one film having negative intrinsic birefringence. A liquid crystal display device, wherein the film having negative intrinsic birefringence is the film according to (1) or (2). The stretching may be uniaxial stretching or biaxial stretching, but uniaxial stretching is preferred.

As the styrene copolymer, conventionally,
A copolymer containing a styrene monomer and acrylonitrile or methacrylonitrile was typical.

The styrene monomer is the same as the monomer (B) described later.

The styrenic copolymer may contain a monomer other than a styrenic monomer, acrylonitrile or methacrylonitrile.

Examples of the monomer other than the styrene monomer, acrylonitrile or methacrylonitrile are the same as the examples of the monomer (B) described later.

Specific examples of the compound containing a styrene monomer and a copolymer containing acrylonitrile or methacrylonitrile are shown below (in parentheses, the weight percentage ratio of each component is shown).

S-1-4 Styrene / acrylonitrile copolymer (x / y) S-1 x / y = 80/20 S-2 x / y = 70/30 S-3 x / y = 65/35 S -4 x / y = 60/40 S-5-6 Styrene / methacrylonitrile copolymer (x / y) S-5 x / y = 70/30 S-6 x / y = 65/35

S-7-8 Styrene / acrylonitrile / methyl methacrylate copolymer (x / y / z) S-7 x / y / z = 65/30/5 S-8 x / y / z = 60/30 / 10 S-9-10 Styrene / α-methylstyrene / acrylonitrile copolymer (x / y / z) S-9 x / y / z = 60/15/25 S-10 x / y / z = 70 / 5/25 S-11 Styrene / pt-butylstyrene / acrylonitrile copolymer (60/10/30) S-12 Styrene / acrylonitrile / methacrylonitrile copolymer (70/20/10)

The retardation film of the present invention is not a copolymer containing a styrene monomer and acrylonitrile or methacrylonitrile as described above, but a polymer having at least an unsaturated double bond in the main chain or side chain ((A )), A graft polymer (referred to as polymer (C)) obtained by addition polymerization of at least one monomer (referred to as (B)) containing at least a styrene-based monomer.

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 at least an unsaturated double bond repeating unit in a main chain or a side chain.
This repeating unit is preferably derived from the polymerization of a monomer having a conjugated diene structure.

Specific preferred examples of the monomer 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, etc., of which 1,3
-Butadiene, isoprene, 2-chloro-1,3-butadiene are particularly preferred.

The polymer (A) constituting the backbone of the polymer (C) of the present invention may be copolymerized with a hydrophobic monomer other than the 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 or dicarboxylic acids (e.g., 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)

Alternatively, amides (eg, t-butylacrylamide, t-butylmethacrylamide), monoethylenically unsaturated compounds (eg, acrylonitrile, methacrylonitrile) and the like can be mentioned. Among them, ethylene, propylene, styrene, α-methylstyrene, esters of acrylic acid or methacrylic acid, acrylonitrile and methacrylonitrile are particularly preferred.

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

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, a so-called random copolymer in which each monomer is copolymerized at an arbitrary ratio may be used, or a block copolymer may be used. Regarding the polymer and the synthesis method of such a conjugated diene monomer, see, for example, Shunsuke Murahashi et al., “Synthetic Polymer II,
1975, Asakura Shoten, pp. 171 to 308.

Preferred examples of the polymer (A) described above include a styrene-butadiene copolymer (generally called SBR, a solution-polymerized SBR and an emulsion-polymerized SBR).
There is. Examples of the solution-polymerized SBR include the above-mentioned block copolymers (eg, butadiene-styrene block copolymer, styrene-butadiene-styrene block copolymer) in addition to the random polymer),

Butadiene homopolymer (for example, cis-1,
4-butadiene, trans-1,2-butadiene, or a rubber in which these are mixed with a trans-1,4-butadiene structure), an isoprene homopolymer (an example of a steric structure is the same as the butadiene polymer), styrene- Isoprene copolymers (random copolymers, block copolymers), ethylene-propylene-diene copolymers (diene monomers such as 1,4-hexadiene, dicyclopentadiene, and ethylidene norbornene),

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

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

Next, the monomer (B) used for obtaining the graft polymer (C) by addition polymerization with respect to the polymer (A) will be described. In the polymer (C) of the present invention, if the polymer (A) is called a trunk,
The polymer of the monomer (B) can be referred to as a branch having a repeating unit extending in a comb shape starting from (A).

In the present invention, the monomer (B) used for the graft polymerization is at least one kind of monomer containing at least a styrene monomer, and specifically, styrene and α-methylstyrene , O-methylstyrene, p-methylstyrene, p-chlorostyrene, p-nitrostyrene, p-aminostyrene, p-carboxylstyrene, p-phenylstyrene, 2,5-dichlorostyrene, pt-butylstyrene Such as styrene derivatives, of which
Styrene or a combination of styrene and other styrene derivatives is particularly preferred.

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

The proportion of the trunk component polymer (A) in the graft copolymer (C) used in the present invention can be uniquely determined because its structure can contribute to the physical properties of the membrane. Although difficult, it is preferably 1 to 30% by weight, particularly preferably 3 to 3% by weight, considering that the amount required for improving the physical properties of the film or too large may cause the film to be too soft and the polymer molecular orientation may be relaxed by heat. 20% by weight.

The molecular weight of the polymer is not particularly limited unless it is particularly small. However, in consideration of the viscosity of the polymer solution when a film is formed from a solution, the weight average molecular weight of the portion excluding the gel component is 100,000. To 1,000,000, particularly preferably 150,000 to 500,000.
The proportion of the styrene-based monomer component in all the constituent components in the graft polymer (C) is preferably at least 50% by weight, particularly preferably 60%, from the viewpoint of developing birefringence characteristics.
% Or more.

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. Also,
Depending on the polymerization method, the polymer obtained in the present invention may be a graft polymer and another polymer such as a polymer (A) which has not undergone a graft reaction or a styrene random copolymer which has not undergone a graft reaction. Can be mixed.

Preferred examples of the graft polymer (C) of the present invention are as follows: (A) a polymer having at least an unsaturated double bond in the main chain or side chain (backbone); and (B) at least a styrene monomer. This is exemplified in the form of a polymer portion (branch) to which one or more monomers are added, but the present invention is not limited to this. That is, it is obtained by graft polymerization of (B) to (A). (A), (B) The ratio of the numbers in parentheses represents the copolymerization ratio (weight) of the monomer in each component, and (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) = 12.5: 87.5 P-5 (A) Styrene / butadiene copolymer (5
(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)
(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 copolymer 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

Among the above copolymers, particularly preferred from the viewpoint of the brittleness of the obtained film and the strength of the film,
It is a graft polymer obtained by adding at least one kind of monomer including a styrene-based monomer to a polymer having at least an unsaturated double bond in a main chain or a side chain.

In the present invention, the styrenic copolymer is nonionic or -COOM (M is a cation atom.
One of the characteristics is that it is obtained by emulsion polymerization in the presence of a surfactant having an alkali metal salt such as sodium or potassium, or an ammonium salt. Hereinafter, surfactants useful for producing the copolymer of the present invention will be exemplified, but the present invention is not limited thereto.

[0053]

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[0054]

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[0055]

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[0056]

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[0057]

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[0058]

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In the emulsion polymerization of the copolymer of the present invention, water or an organic solvent miscible with water (eg, methanol,
The monomer is emulsified in a mixed solvent of ethanol, acetone, isopropyl alcohol and the like, and is generally used at 30 ° C. to about 100 ° C., preferably 40 ° C. using a radical polymerization initiator.
C. to a temperature of about 90.degree. The amount of the organic solvent miscible with water is 0 to 100%, preferably 0 to 30%, by volume relative to water.

The polymerization reaction is usually carried out using 0.05 to 5% by weight of the radical polymerization initiator based on the monomer to be polymerized and about 0.1 to 10% by weight of the above-mentioned surfactant.
Examples of the polymerization initiator include azobis compounds, peroxides, hydroperoxides, and redox catalysts, such as potassium persulfate, ammonium persulfate, tert-butyl peroctoate, benzoyl peroxide, isopropyl percarbonate, and 2,4-dichlorobenzoyl peroxide. Oxide, methyl ethyl ketone peroxide, cumene hydroperoxide, dicumyl peroxide, azobisisobutyronitrile, 2,2'-azobis (2-amidinopropane) hydrochloride and the like.

Examples of the emulsifier that can be used include compounds other than the above-mentioned nonionic or surfactant represented by -COOM, specifically, sulfone as long as the transparency of the obtained film is not substantially impaired. An acid salt or a salt of a sulfuric acid monoester (for example, sodium laurate, sodium dodecyl sulfate, sodium dodecylbenzenesulfonate) or a cationic surfactant (for example, N-dodecylpyridinium chloride) may be used in combination. However, since the amount of these emulsifiers greatly affects the transparency of the film even in a small amount, the amount of the emulsifier used is 0.1% with respect to the styrene copolymer.
% By weight or less is preferred.

In the present invention, a graft polymer obtained by addition-polymerizing at least one kind of monomer containing at least a styrene-based monomer to a polymer having at least an unsaturated double bond in a main chain or a side chain is used. After the latex of the polymer (A) constituting the trunk is synthesized by emulsion polymerization of one or more monomers including a monomer having a diene structure, at least one monomer including a styrene-based monomer is added. To carry out a polymerization reaction.

At this time, one or more monomers including a styrene-based monomer may be added all at once, but from the viewpoint of avoiding the heat of polymerization, it is preferable to use means such as division or dripping. The surfactant may be added in its entirety at the time of preparing the polymer constituting the trunk, or may be added separately in each polymerization process.

Next, the process of pulverizing the latex obtained by the emulsion polymerization of the present invention will be described. A simple method for pulverizing the obtained latex is a method in which the latex is agglomerated to a filterable level, filtered and dried to obtain a powder. As is well known in the emulsion polymerization field, the latex coagulation method involves freezing the latex and then thawing it, adding a salt (eg, sodium chloride, etc.), or rapidly changing the pH of the solution. A method is conceivable.

Among them, the freeze-thaw method is a time-consuming process and is not practical, and the addition of salt (water) or the like is effective in coagulation, but the obtained coagulated material has insufficient fusion-bonding properties. It has a drawback that the filtration efficiency tends to decrease due to re-dispersion at the time of filtration and the like.

In the present invention, a sudden change in the pH of the solution,
More specifically, it is preferable to aggregate under strong acid conditions.
In addition, when this method is applied to a surfactant having -COOM as a surfactant to be used, the surfactant loses the ability to stabilize the latex due to neutralization, so that efficient aggregation can be performed. This is particularly preferred in that respect.

Further, from the viewpoint of improving the filterability of the obtained aggregate, it is effective to heat the aggregated liquid. This is considered to be due to the fact that the fusion between the respective latexes in the aggregate formed by the strong acid is strengthened by heating, so that an aggregate having a sufficient strength that does not break during filtration is generated.

The preferable range of the pH conditions and the heat treatment conditions at the time of the above aggregation may vary depending on the type and amount of the surfactant used, the structure of the styrene-based polymer, and the like. The liquid pH is preferably 3 or less, particularly preferably 2 or less. As the heating temperature, when the final temperature is 50 ° C. or more, particularly preferably 75 ° C. or more, sufficient filterability of aggregates can be obtained. Hereinafter, preferred methods for synthesizing the copolymer of the present invention will be exemplified, but the present invention is not limited thereto.

Synthesis Example 1 Synthesis of Exemplified Compound S-2 A 2-liter three-necked flask equipped with a stirrer and a reflux condenser was charged with 4.40 g of sodium oleate, 0.56 g of sodium hydrogen sulfite, and 1 mol of sodium hydrogen carbonate. Per liter of a solution and 1100 g of distilled water, and heated to 65 ° C. under a nitrogen stream. After adding 0.64 g of potassium persulfate dissolved in 40 g of distilled water, styrene 308 was added.
g, a mixed solution of acrylonitrile 132 g, and a solution prepared by dissolving potassium persulfate 0.32 g in distilled water 100 g were each started dropwise at a constant speed so that the dropwise addition was completed in 5 hours.

After completion of the dropwise addition, the mixture was heated and stirred for 1 hour, and then a solution prepared by dissolving 0.32 g of potassium persulfate in 40 g of distilled water was added. The mixture was further heated and stirred for 3 hours. The mixture was cooled to room temperature and filtered to obtain 1,720 g of a latex (average particle size: 80
nm: Coulter submicron analyzer (Nikkaki)
Measurement).

0.32% sulfuric acid aqueous solution (pH = 1.48)
1000 g of the above latex was dropped into a three-liter three-necked flask containing 1000 g with stirring over 30 minutes to coagulate the latex. After dropping, 3
The flask was heated over 0 minutes until the temperature inside the flask reached 85 ° C., and the heating and stirring were continued at 85 ° C. for 20 minutes. After cooling to room temperature, the agglomerated polymer was filtered under reduced pressure using filter paper, washed with water and dried to give a white powder 245 of Exemplified Compound S-2.
g was obtained. The weight average molecular weight of the obtained powder was 420,000.

Synthesis Example 2 Synthesis of Exemplified Compound P-3 A 2-liter three-necked flask equipped with a stirrer and a reflux condenser was charged with 4.40 g of sodium oleate, 0.56 g of sodium hydrogen sulfite, and 1 mol of sodium hydrogen carbonate. / Liter solution 7.0 g, isopropyl alcohol 110 ml, distilled water 990 g, styrene / butadiene (23/77: weight ratio) latex (using 3% by weight of sodium oleate as an emulsifier based on the monomer, particle diameter 85 nm, solid content 40) 108.6 g) at 65 ° C. under a nitrogen stream.
Heated.

0.64 g of potassium persulfate was added to 40 g of distilled water.
Then, a mixture of 352 g of styrene, 88 g of acrylonitrile, and 0.1 g of potassium persulfate was added.
A solution prepared by dissolving 32 g in 100 g of distilled water was dropped at a constant speed so that dropping was completed in 5 hours.

After completion of the dropwise addition, the mixture was heated and stirred for 1 hour, and then a solution prepared by dissolving 0.32 g of potassium persulfate in 40 g of distilled water was added. The mixture was further heated and stirred for 3 hours. After cooling to room temperature and filtering, 1810 g of latex (average particle size 13
5 nm: measured with a Coulter submicron analyzer (Nikkaki).

0.32% sulfuric acid aqueous solution (pH = 1.48)
The above-mentioned latex (1000 g) was dropped into a 3-liter three-necked flask containing 1000 g over 20 minutes while stirring to coagulate the latex. 30 after completion of dropping
The mixture was heated until the internal temperature of the flask reached 85 ° C. over a period of minutes, and heating and stirring were continued at 85 ° C. for 20 minutes. After cooling to room temperature, the aggregated polymer component was filtered under reduced pressure using filter paper,
After washing with water and drying, 257 g of white powder of Exemplified Compound P-3 was obtained. The weight average molecular weight of the obtained powder was 396,000. The birefringent film of the present invention can be produced by dissolving the copolymer obtained as described above in an organic solvent, casting, and stretching by a usual stretching method. The retardation value can be adjusted by adjusting the film thickness and the stretching ratio.

When the film having the negative intrinsic birefringence of the present invention is introduced into a liquid crystal display device in combination with a liquid crystal element having a twisted aligned nematic liquid crystal interposed therebetween, the film having the negative intrinsic birefringence of the present invention is obtained. Only one film may be used, or two or more retardation films having the same or different negative intrinsic birefringence may be used. It may be used in combination with at least one film having positive intrinsic birefringence. Although the film may be uniaxially stretched or biaxially stretched, preferred uniaxially stretched films will be mainly described.

A preferred embodiment of a laminate of a uniaxially stretched film formed from a polymer having a positive intrinsic birefringence value and a uniaxially stretched film formed from a polymer having a negative intrinsic birefringence value will be described below. The retardation in the linear direction is added to each other and can be freely controlled according to the purpose, such as a film with a very small change in retardation for omnidirectional oblique incidence and a film with an appropriate retardation change without being extinguished. Has an effect.

In particular, in the case where these effects are remarkable, a uniaxially stretched film of a polymer having a positive intrinsic birefringence value and a uniaxially stretched film of a polymer having a negative intrinsic birefringence value are arranged so that their stretching axes are orthogonal to each other. This is when the layers are stacked. A film laminate having a similar effect, i.e., a small change in retardation in all directions, is formed from a uniaxially stretched film formed from a polymer having a positive intrinsic birefringence value and a polymer having a positive intrinsic birefringence value. Orthogonal laminate of uniaxially stretched film and uniaxially stretched film formed from a polymer having a negative intrinsic birefringence value and uniaxially stretched film formed from a polymer having a negative intrinsic birefringence value. Are not realized together, and are realized only by the configuration of JP-A-3-206422.

Now, in a laminate of a uniaxially stretched film formed of a polymer having a positive intrinsic birefringence and a uniaxially stretched film formed of a polymer having a negative intrinsic birefringence, By controlling the orientation level of the molecule by stretching or the like, it is possible to freely control the retardation of the laminated body from viewing angle dependence or to give a moderate change, so that it can be controlled according to the optical characteristics of the STN-LCD. It has been found that the viewing angle characteristics of the retardation can be adapted, so that the viewing angle of the STN-LCD can be greatly increased when the retardation film is provided between the polarizing plate and the liquid crystal cell in the STN-LCD.

More specifically, 90 ° or more, especially 1 °
Eliminates the coloring phenomenon caused by the birefringence of the liquid crystal cell in a liquid crystal display device using a twisted nematic liquid crystal or a cholesteric liquid crystal having a twist angle of 80 ° to 330 °, and enables the expansion of the viewing angle and high contrast range. The retardation in the normal direction of the film, the polymer formed from a polymer having a positive intrinsic birefringence value in uniaxial stretching of the film and a polymer having a negative intrinsic birefringence value And the added value of the retardation in the uniaxial stretching of the film formed from.

However, when the stretching axes of the uniaxially stretched films of the polymer having the positive and negative intrinsic birefringence values coincide with each other, the retardation is canceled, which is not preferable. Therefore, it is preferable that the stretching axes of the film laminate are arranged substantially orthogonal to each other. Specifically, the angle formed by the stretching axis of the film is most preferably 70 ° to 110 °.

However, this does not apply when the films having the positive and negative intrinsic birefringence values are disposed on both sides via the liquid crystal cell. That is, the films are not always laminated and used, and may be disposed on both sides of the liquid crystal cell, or may be used also as a protective film on the liquid crystal cell side of the polarizing plate. In particular, when used as a protective film for a polarizing plate, there is an advantage that the cost can be reduced together with the function of expanding the viewing angle. Further, the film in the present invention includes not only a film generally considered, but also a film-like material applied to a certain base material. The uniaxially stretched film is an object of the present invention as long as it functions not only as a pure uniaxial film but also as a uniaxial film even if it is slightly biaxial.

Therefore, the uniaxial stretching by the tenter method, the uniaxial stretching by utilizing the difference in the peripheral speed between the rolls, and the case where the natural shrinkage in the stretching in the width direction is performed or limited. In the present invention, the film having a positive intrinsic birefringence value preferably has a light transmittance of 70% or more and is achromatic, and more preferably has a light transmittance of 90% or more and is achromatic. Here, the intrinsic birefringence value (△ n °) means a birefringence value when molecules are ideally oriented in one direction,
It is approximately expressed by the following equation.

[0084]

(Equation 3)

The polymer used for the film having a positive intrinsic birefringence value is not limited, but specific examples thereof include polycarbonate, polyarylate, polyethylene terephthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone, and polyamide. Imide, polyimide, polyolefin,
Polyvinyl chloride, cellulose, polyester-based polymers, etc. are preferred, especially polycarbonate-based polymers, polyarylate-based polymers, polyester-based polymers, etc., which have large intrinsic birefringence values and form planar homogeneous films by solution casting. Easy polymers are preferred.

The above polymer is not limited to a homopolymer but may be a copolymer, a derivative thereof, a blend or the like. Also in the film having a negative intrinsic birefringence value in the present invention, it is preferable that the film has an achromatic color with a light transmittance of 70% or more, and more preferably an achromatic color with a light transmittance of 85% or more.

Although the absolute value of the intrinsic birefringence value is small, it can be used sufficiently by increasing the thickness or increasing the draw ratio, but in order to avoid these restrictions, the intrinsic birefringence value is preferably 0.02 or more in absolute value,
More preferably, it is 0.04 or more. Further, in order to prevent the molecules once oriented by stretching from relaxing the orientation due to the temperature rise during the LCD manufacturing process or display, the Tg (glass transition point) of the material is 90 ° or more, more preferably 100 ° or more, and further preferably 110 ° or more. Degrees or more. The thickness of the film having a birefringence value after being uniaxially stretched is not particularly limited.
The range of μ to 1 mm is preferred. Hereinafter, the present invention will be described in detail with reference to examples.

[0088]

Example 1 Copolymer latexes were synthesized in exactly the same manner as in Synthesis Examples 1 and 2, except that the surfactants were changed as shown in Table 1 for polymers S-2 and P-3. . Latex 7 obtained
Of sulfuric acid solution (basically, Synthesis Examples 1 and 2
Same as above, but the pH of the sulfuric acid solution and the temperature of the heat treatment were partially changed. ) Or agglutination by freezing and then thawing, and treating the solution with an ADVANT having an inner diameter of 18.5 cm.
20 torr using EC TOYO 131 (manufactured by Toyo Roshi)
Filtration was performed at a reduced pressure, and the time required for filtration was examined. Table 1 shows the results.

[0089]

[Table 1]

As shown in Table 1, the polymer using the emulsifier according to the present invention has excellent filterability by being treated with an acidic aqueous solution, and this effect is remarkable under low pH and high heating temperature treatment conditions. . On the other hand, when no emulsifier is used or a sulfonate-based or sulfate ester-based emulsifier is used, even if it is subjected to an acidic aqueous solution or a freezing treatment, its filterability is poor.

Example 2 180 g of each of the polymers shown in Table 1 including the polymer of the present invention was dissolved in 820 g of methylene chloride.
This solution was dried under a condition of 10% relative humidity to have a film thickness of 10%.
It was cast on a glass plate so as to have a thickness of 0 μm, left at room temperature for 5 minutes, dried with hot air at 45 ° C. for 20 minutes, and peeled off from the glass plate. Paste the peeled film on the frame 70
After drying at 110 ° C. for 1 hour and further at 110 ° C. for 15 hours, the sample was subjected to 200% uniaxial stretching using a commercially available tensile tester under the condition of glass transition temperature (Tg) of polymer + 10 ° C. . Next, the obtained sample was subjected to the following evaluation.

(1) Haze The haze of the prepared film was measured at five locations using a haze meter (Model 1001DP, Nippon Denshoku Co., Ltd.), and the haze of the film was determined from the average value. (2) Brittleness 10 mm in the stretching direction (MD) and a direction perpendicular to the stretching direction (T
A sample piece of 35 mm was cut out in D), and as shown in FIG.
Fold it over 3 seconds to make a fold parallel to D,
The angle of failure θ was read. 0 ° if not damaged
And θ is preferably small, and particularly preferably 10 ° or less.

(3) Heat resistance Using a Thomson punching machine of 18 cm × 16 cm shown in FIG.
The film was punched out so that D was 45 ° C. on the long side. The punched sample was attached to a glass plate using an adhesive, and the MD of the sample was placed between two crossed Nicols polarizing plates.
Is placed at 45 ° with respect to the polarization axis, and the retardation is measured from the wavelength dependence of the transmitted light intensity. Was determined (%). In practice, it is required to be less than 3%. Also, the heat treatment should not cause cracks. Table 2 shows the evaluation results.

[0094]

[Table 2]

It is clear that the present invention can significantly improve only the haze without changing the level of brittleness and heat resistance. Further, the acrylonitrile copolymer was improved in terms of brittleness and heat resistance as compared with polystyrene, and the graft polymer was further improved in brittleness remarkably.

Example 3 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
The film was uniaxially stretched between rolls having different peripheral speeds at a temperature of 70 ° C. to obtain a polycarbonate uniaxially stretched film having a retardation value of 430 nm. An adhesive sheet was attached to one side of the obtained polycarbonate film, and a protective film made of vinyl chloride was attached to the other side to obtain a sample.

Next, the retardation films before and after the liquid crystal cell of the word processor WD-A550 manufactured by Sharp Corporation having the configuration shown in FIG. 3A were removed, and the polycarbonate film was disposed in place of the later retardation film. In place of the previous retardation film, a film made of the polymer of the present invention having a good haze and having a retardation value of 430 nm was provided among the styrene-based films of Example 2. However, the optical axis of each film is shown in FIG.
As shown in (2), it was exactly the same as when purchased. As display characteristics of the obtained liquid crystal panel, a contrast ratio and a degree of coloring from the front in a driving state and a non-driving state, and an angle (viewing angle) at which the contrast ratio becomes 2 or more were evaluated.

Comparative Example 1 Word Processor WD-A550 Used in Example 3
Were evaluated for display characteristics at the time of purchase.

Comparative Example 2 In Example 3, display characteristics were evaluated by using a polycarbonate film instead of the front and rear retardation films. Reference Example Display characteristics were evaluated in the same manner as in Example 3 except that a retardation film containing a copolymer containing a styrene monomer and acrylonitrile or methacrylonitrile was used as the styrene copolymer. Table 3 shows the results of Example 3, Comparative Examples 1 and 2, and Reference Example.

[0100]

[Table 3]

As can be seen from Table 3, almost complete black-and-white display was obtained with respect to the degree of coloring when viewed from the front, but the viewing angle was higher for the polymer of the present invention than for Comparative Examples 1 and 2. Example 3 in which the film was used was much better. Note that Comparative Examples 1 and 2 had almost the same display characteristics.

[Brief description of the drawings]

FIG. 1 shows a sample attached to a brittleness measuring instrument.

FIG. 2 shows a cross section of a Thomson punching machine.

FIG. 3A shows a configuration of a liquid crystal cell of WD-A550. (2) shows the optical axis configuration viewed from the front of 正面.

[Explanation of symbols]

θ: bending angle a: sample b: holding plate (made of vinyl chloride) c: hard iron blade d: rubber mat e: wooden base BL: backlight P ₁ : polarizing plate D ₁ : rear retardation plate LC : Liquid crystal cell D ₂ : previous retardation plate P ₂ : polarizing plate

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-141322 (JP, A) JP-A-4-91296 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 5/30 C08F 2/00-2/60

Claims (3)

(57) [Claims] 1. A stretched styrene-based copolymer film having a negative intrinsic birefringence, wherein the copolymer has at least an unsaturated double bond in a main chain or a side chain thereof. And a graft polymer obtained by addition polymerization of one or more monomers including a styrene-based monomer, and in the presence of a nonionic surfactant or a surfactant having —COOM (M represents a cation). A birefringent film having negative intrinsic birefringence, which is a copolymer obtained by emulsion polymerization. 2. The method according to claim 1, wherein the styrene-based copolymer is obtained by coagulating a latex obtained by emulsion polymerization under a strong acid condition and heat-treating the latex. A birefringent film having the negative intrinsic birefringence described. 3. A liquid crystal display device comprising a liquid crystal element having a twisted nematic liquid crystal interposed between two opposing electrode substrates and at least one film having negative intrinsic birefringence. A liquid crystal display device, characterized in that the film having the intrinsic birefringence of (1) is the film according to claim 1 or 2.