JPH06118390A - Liquid crystal composite film - Google Patents

Abstract

PURPOSE:To provide a liquid crystal composite film having high turbidity in spite of formation of a thin film of, for example, about <=10mum so that a good contrast and low-voltage driving is obtd. by evaporating a solvent from a uniform soln. prepd. by dissolving a liquid crystal material and a specific high polymer in the common solvent for both, thereby forming a micro phase separation structure. CONSTITUTION:This liquid crystal composite film is formed by evaporating the solvent from the uniform soln. prepd. by dissolving the liquid crystal material and the high polymer which is the polymer of >=1 kinds of monomers to be uniformly mixed with the liquid crystal material and satisfies the conditions of equation I and equation II in the common solvent for both. In the equation, equation III denotes a weight average mol. wt. in terms of polystyrene. In the equation II, equation IV denotes a number average mol. wt. in terms of polystyrene. Then, the equation II indicates that the variance of the mol.wt. expressed by the ratio of the weight average mol. wt. and the number average mol. wt. is 4. As a result, the liquid crystal composite film has the high turbidity in spite of formation of the thin film of, for example, about <=10mum. The good contrast is thus obtd. and the higher contrast and lower voltage driving of the thin film are resulted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal composite film used for a liquid crystal display device or the like, which exhibits a change between white turbidity and transparency (electro-optical effect) when an electric field is applied, and particularly a liquid crystal having a high white turbidity during white turbidity. It relates to a composite membrane.

[0002]

2. Description of the Related Art There are various types of liquid crystal composite films composed of a polymer and a liquid crystal material and exhibiting an electro-optical effect of opaque and transparent. For example, the liquid crystal material is encapsulated (JLFergason, (Japanese Patent Publication No. 58-501631), a liquid crystal is precipitated by thermosetting an oligomer (JW Doane, Japanese Patent Publication No. 61-502128), and a liquid crystal is precipitated by photo-curing an oligomer ( Vaz, Japanese Patent Laid-Open No. 62-2231) and the like are known. In all of these composite films, a liquid crystal material is dispersed in the film in the form of particles.

On the other hand, a method of forming a liquid crystal composite film by evaporating a solvent from a uniform solution in which a polymer and a liquid crystal material are dissolved in a common solvent is a group of professor Chisato Kajiyama of Kyushu University in 1979. First proposed by [CHEMIS
TRY LETTERS, p.679-682,1979]. The composite film has a structure in which the liquid crystal material is filled in a three-dimensional network in the continuous pores of the sponge-like polymer matrix due to phase separation between the polymer and the liquid crystal material during solvent evaporation (micro phase separation structure). ) Have been reported [J.App
L. Polym. Sci., 29, 3955-3964 (1984)]. In addition, the electro-optical effect of the composite film formed by the solvent evaporation method is
It is also disclosed in Japanese Patent Laid-Open No. 501512/1990, Japanese Patent Laid-Open No. 1-230693, and the like.

In the above liquid crystal composite film, when no voltage is applied, the liquid crystal molecules in the pores are in a random state, so that the incident light is scattered and the composite film is in an opaque state.
Then, when a voltage is applied between the pair of conductive base materials sandwiching the composite film, Δε> 0 [where Δε is the dielectric anisotropy, and the formula:

[0005]

[Equation 3]

It is represented by

[0007]

[Outside 4]

Is the dielectric constant in the molecular axis direction,

[0009]

[Outside 5]

Indicates the dielectric constant in the direction orthogonal to the molecular axis)], the liquid crystal molecules are oriented in the direction of the electric field, and the refractive index n _{P of the} polymer and the ordinary refractive index n _{O of the} liquid crystal are substantially the same. If so, the incident light can pass through the composite film without being scattered, and the electro-optical effect of converting the composite film to a transparent state is exhibited. The liquid crystal composite film having the above structure can be manufactured simply by applying and drying a solution containing a polymer and a liquid crystal material, and thus it is possible to increase the area of, for example, a liquid crystal display element. Moreover, since flexibility can be imparted to the composite film by selecting a polymer compound, by combining a flexible film or the like to which conductivity has been imparted by forming a conductive film on the surface as a conductive base material, the liquid crystal There is an advantage that flexibility can be given to the display element.

A feature of the production of the above composite film is that when a uniform mixed solution of a polymer and a liquid crystal material dissolved in a solvent is cast on a conductive substrate, the polymer and the liquid crystal material evaporate as the solvent evaporates. Becomes phase incompatible because 3 becomes incompatible.
It is to form a three-dimensional mesh-like structure. In essence, the phase separation is caused by solvent evaporation. The method for forming such a composite film is known from the aforementioned documents.

"Phase separation by solvent evaporation" in the production of the composite membrane, which is the basis of the present invention, is a unique method which is completely different from the other methods described above, and is the first published document by Kajiyama et al. It was done. In the continuous pores of the sponge-like polymer matrix in the film,
The liquid crystal material exists as a continuous phase having a three-dimensional network shape rather than a particle shape. This is also a major feature of the composite film of Kajiyama et al.

[0013]

The liquid crystal composite film of Kajiyama et al. Has excellent properties as described above.
To drive it, 20-30V or more, and in some cases 100
Since a high voltage of V or more is required, there is a problem that it is difficult to apply and develop the liquid crystal television. That is,
TF commonly used in liquid crystal televisions, etc.
A liquid crystal display element using a non-linear active element such as T or MIM for driving has a withstand voltage of a peripheral drive circuit of about several volts,
The liquid crystal composite film that requires a high voltage as described above cannot be used.

As a countermeasure for the above, it is conceivable to reduce the film thickness of the liquid crystal composite film in order to lower the driving voltage. In that case, the white turbidity of the liquid crystal composite film at the time of white turbidity (voltage OFF).
However, there is a problem in that the degree of light scattering at the time) is reduced, and the contrast due to the electric field ON-OFF is significantly reduced. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a liquid crystal composite film having a high white turbidity even when it is a thin film having a thickness of, for example, about 10 μm or less, and thus obtaining a good contrast.

[0015]

In order to solve the above problems, the inventors of the present invention have studied the relationship between the physical properties of the polymer and liquid crystal material constituting the liquid crystal composite film and the structure of the liquid crystal composite film to be formed. As a result of various studies, the following facts were found. That is, in order to form a uniform microphase-separated structure over the whole in a composite film having a phase-separated structure by solvent evaporation, a monomer as a raw material of a polymer and a liquid crystal material have excellent affinity. There is a need.

That is, when the affinity between the monomer and the liquid crystal material is small and they are incompatible with each other, it is difficult to find a solvent capable of uniformly dissolving the polymer obtained by polymerizing the monomer and the liquid crystal material, and Even if such a solvent is present, a uniform microphase-separated structure cannot be obtained by solvent evaporation, and it is unavoidable that macroscopically complete separation occurs. For example, when using a cyanobiphenyl-based liquid crystal material, when a fluorine-based or siloxane-based monomer is used, the two do not mix uniformly even at the monomer stage. Therefore, the polymer of these monomers is a cyanobiphenyl-based liquid crystal material. Even if it is combined with the above liquid crystal material, a uniform phase separation structure cannot be obtained.

Therefore, in the production of a liquid crystal composite film by solvent evaporation, it is necessary to use a polymer obtained by polymerizing a monomer having a good affinity with the liquid crystal material used.
The main cause of phase separation due to solvent evaporation is the difference in molecular weight between the liquid crystal material and the polymer. That is, the smaller the molecular weight difference between the two, the more unclear the phase separation, and the larger the molecular weight difference, the more clear the phase separation. In extreme terms,
When the monomer, which is the raw material for the polymer, and the liquid crystal material have excellent affinity as described above and are mixed in a uniform manner, the monomer and the liquid crystal material do not undergo phase separation by solvent evaporation, but are formed from this monomer. It is considered that the polymer has a phase-separated state as the degree of polymerization increases.

When the degree of polymerization of the polymer is low and phase separation from the liquid crystal material is not clear, more liquid crystal material is dissolved in the polymer matrix in the formed composite film, The index of refraction of the matrix approaches the average index of refraction of the liquid crystal (refractive index when the electric field is OFF), the white turbidity decreases, and sufficient contrast cannot be obtained due to ON-OFF of the electric field. On the other hand, the higher the degree of polymerization of the polymer and the clearer the phase separation from the liquid crystal material, the higher the white turbidity and the higher the contrast due to the electric field ON-OFF.

Therefore, the polymer used for the composite membrane is
It is necessary that the average molecular weight is large, the dispersion of the molecular weight is small, and the low molecular weight component is not included in large amount. Therefore, the inventors of the present invention can obtain a sufficient white turbidity and thus a sufficient contrast even with a thin film of about 10 μm or less,
The range of the average molecular weight of the polymer and the range of the dispersion of the molecular weight in which the phase separation from the liquid crystal material becomes clear were further investigated. In addition, a general molecular weight of a commercially available liquid crystal material is about 200 to 1000, and in order to achieve clear phase separation such that a sufficient white turbidity can be obtained even in a thin film of about 10 μm or less, a polymer is required. Has a molecular weight of more than 100 times the molecular weight of the liquid crystal material, that is, a weight average molecular weight of more than 100,000, and a ratio of the weight average molecular weight to the number average molecular weight.

[0020]

[Outside 6]

The present invention has been completed by finding that the dispersion of the molecular weight represented by is required to be less than 4. Therefore, the liquid crystal composite film of the present invention comprises a liquid crystal material and a polymer, which is a polymer of one or more kinds of monomers uniformly mixed with the liquid crystal material, and which satisfies the following conditions, in both common solvents. The solvent was evaporated from the dissolved homogeneous solution to form a microphase-separated structure.

[0022]

[Equation 4]

[0023]

[Equation 5]

(However,

[0025]

[Outside 7]

Is a polystyrene-equivalent weight average molecular weight,

[0027]

[Outside 8]

Is a polystyrene-equivalent number average molecular weight,

[0029]

[Outside 9]

Indicates the molecular weight distribution. ) The liquid crystal composite film of the present invention having the above structure has a clear phase separation between the polymer and the liquid crystal material, and has a uniform micro phase separation structure over the entire composite film. As a thin film, it also has a high opacity, so that good contrast can be obtained.

The present invention will be described below. Used in the present invention
In order to obtain good light scattering properties, the liquid crystal material
Has large anisotropy Δn (preferably Δn> 0.15,
(Preferably within the range of Δn> 0.2) and good when an electric field is applied.
In order to obtain good transparency, the ordinary refractive index n as described above
_OIs the refractive index n of the polymer_PShould be approximately equal to
It Also, for high-speed response, the dielectric anisotropy Δε is large.
Better to hear. The liquid crystal material has a high volatility, which will be described later.
It must be soluble in the medium.

Examples of such liquid crystal materials include those having various skeletons such as biphenyl, terphenyl, ester, phenylcyclohexane, phenylpyridine, and tolan. Also, the dielectric anisotropy Δε
In order to increase, it is preferable to replace the terminal group of the skeleton with a -CN group or a -Cl group. In particular, when driven by the above-mentioned nonlinear active element, the liquid crystal material needs to have a high resistivity of preferably 10 ¹² Ω or higher, more preferably 10 ¹³ Ω or higher. In order to achieve the above, it is preferable to substitute the terminal group with a -CN group or a -Cl group. The molecular weight of the liquid crystal material is 2 as described above.
It is about 00 to 1000.

Specific examples of the liquid crystal material suitable for the present invention which satisfies the above requirements include, for example, commercially available mixed liquid crystal of nematic liquid crystal and cholesteric liquid crystal. As the polymer that constitutes the liquid crystal composite film of the present invention together with the above liquid crystal material, as described above, one or more kinds (preferably 3 to 4) which have a good affinity with the liquid crystal material at the monomer level and are uniformly mixed. Polymers of monomers of species) are used. Further, the polymer needs to be soluble in a highly volatile solvent described later. Specific examples thereof include polymers of vinyl polymerizable monomers such as acrylic acid esters, methacrylic acid esters, styrene, acrylonitrile, and vinyl ethers. Among them, polymers of acrylic acid esters and methacrylic acid esters are particularly preferable. It is most convenient in terms of control of polymerization degree, selection of monomer materials, affinity with liquid crystal materials, matching of refractive index with liquid crystal materials, solubility in solvents, and the like.

As described above, the above polymer has a polystyrene-equivalent weight average molecular weight.

[0035]

[Equation 6]

And the molecular weight dispersion

[0037]

[Equation 7]

Must be When the weight average molecular weight of the polymer is below the above range, the phase separation from the liquid crystal material becomes unclear, the white turbidity of the composite film decreases, and the contrast becomes insufficient. Further, when the dispersion of the molecular weight exceeds the above range, since a large amount of low-molecular weight components are included, the phase separation from the liquid crystal material becomes unclear, and the white turbidity of the composite film decreases, resulting in a poor contrast. Becomes insufficient.

The weight average molecular weight is preferably in the range of more than 500,000, more preferably in the range of more than 1,000,000. The molecular weight dispersion is preferably less than 2, and more preferably less than 1.5. The upper limit of the weight average molecular weight is not particularly limited, but considering solubility in a solvent and the like, it is preferably about 5,000,000 or less.

Further, in the above-mentioned polymer, a side chain-bonding group capable of binding to a cross-linking agent is introduced into the side chain, and this is reacted with an appropriate cross-linking agent to cross-link during or after phase separation. Good. In this case, there is an advantage that the phase separation is further clarified by the increase in the molecular weight due to the crosslinking, the white turbidity of the composite film is improved, and the contrast is further improved. Further, the polymer may be blended with the second polymer component, or may be blended with various organic or inorganic additives.

As a method for synthesizing such a polymer, various conventionally known polymerization methods such as radical polymerization method, ionic polymerization method (anionic polymerization method, cationic polymerization method), photopolymerization method and coordination polymerization method can be adopted. However, the radical polymerization method and the anion polymerization method are practical for producing a polymer satisfying the above requirements. In the radical polymerization method, it is generally easy to produce a high-molecular weight polymer, although it depends on the composition (combination) of the monomers to be copolymerized. By adjusting conditions such as the type of solvent used for polymerization, the concentration of monomers, the reaction temperature, and the type of polymerization initiator used, it is possible to produce a polymer having a weight average molecular weight of several million at the maximum. However, in the radical polymerization method, the higher the molecular weight, the more the dispersion of the molecular weight tends to spread, and in order to make the dispersion less than 4, it is necessary to classify the produced polymer by the molecular weight by a reprecipitation method or the like. .

On the other hand, by the anionic polymerization method, a polymer having a smaller molecular weight dispersion than usual (about 1.1 to 1.3) can be obtained, but the degree of polymerization of the polymer tends not to be so high. However, since a polymer having a weight average molecular weight of about 200,000 can be obtained by selecting a polymerization initiator,
The anionic polymerization method is more ideal as the method for producing the polymer used in the present invention.

In the present invention, another important factor for obtaining a liquid crystal composite film having an ideal structure is selection of a solvent used for phase separation. In order to produce a liquid crystal composite film by phase separation, it is necessary to use a solvent having high volatility and a high evaporation rate. In addition, the solvent, the liquid crystal material and the polymer,
Both must be soluble. Specific examples of the solvent satisfying the above requirements include, for example, dichloromethane, 1,
2-dichloroethane, chloroform, methyl acetate, ethyl acetate and the like can be mentioned.

A structure in which the liquid crystal material and the polymer are microscopically phase-separated can be obtained by rapidly drying after applying the solution using a highly volatile solvent. When a solvent having a slow evaporation rate is used, the domain diameter of the liquid crystal becomes large, the white turbidity of the composite film decreases, and the contrast becomes insufficient. Even if the solvent has a slow evaporation rate, the evaporation rate can be improved by heating. However, as the kinetic energy of the liquid crystal material and the polymer increases due to heating, many liquid crystal materials dissolve in the polymer matrix, and the refractive index of the polymer matrix approaches the average refractive index of the liquid crystal. . Further, since the domain diameter of the liquid crystal is still large, the white turbidity of the composite film is lowered and the contrast becomes insufficient.

The thickness of the composite film is not particularly limited in the present invention, but according to the present invention, for example, by combining a polymer having an ideal molecular weight characteristic with a liquid crystal material having a large refractive index anisotropy Δn. Good contrast can be obtained even with a thin film of, for example, about 7 to 8 μm. Further, when a polymer having a large dielectric constant is used, the liquid crystal composite film having a film thickness of about 7 to 8 μm can be driven at a low voltage of about 5 V, and a liquid crystal of a liquid crystal composite film having excellent characteristics can be obtained. It can be applied to televisions and other applications.

The liquid crystal composite film of the present invention is used for a liquid crystal display element or the like by being sandwiched between a pair of conductive base materials, like the conventional liquid crystal composite film. The method for producing a liquid crystal display device provided with the liquid crystal composite film of the present invention is the same as the conventional method, a uniform mixed solution of a polymer and a liquid crystal material dissolved in a solvent is applied onto a conductive substrate, and the solvent is removed. After evaporation, the liquid crystal composite film of the present invention is formed on the conductive substrate by phase separation, and then another conductive substrate may be laminated thereon.

As the conductive substrate, glass or plastic film [eg polyethylene terephthalate (PE
T), polyether sulfone (PES), etc., on the surface of a substrate such as ITO (Indium-Tin-Oxide) or SnO
_In addition to those obtained by forming a conductive film such as ₂ by a vapor deposition method, a sputtering method, a coating method, or the like, a transparent conductive glass or film used in a normal liquid crystal panel can also be used.
When the element is of a reflective type, a reflective film made of a metal thin film or the like may be formed on the back surface of one of the conductive base materials, or the counter electrode may be formed of a metal thin film to serve also as the reflective film. .

The composite film may contain various conventionally known dichroic dyes in order to make the liquid crystal display device a color display type.

[0049]

EXAMPLES The present invention will be described below based on Examples and Comparative Examples. Examples 1 to 3 and Comparative Examples 1 to 30 30 parts by weight of a polymer (acrylic / methacrylic copolymer, containing a hydroxyl group) having the molecular weight characteristics shown in Table 1 below, and a liquid crystal material (product number E31LV manufactured by Merck Japan Ltd., refraction) Anisotropy Δn = 0.23) 70 parts by weight is dissolved in dichloromethane, and a polyisocyanate cross-linking agent (manufactured by Takeda Pharmaceutical Co., Ltd.) is used, and the isocyanate group equivalent is the hydroxyl group equivalent in the polymer. A coating solution was prepared by adding the same amount as.

Next, this coating solution was applied to the conductive surface of transparent conductive glass by a bar coating method and dried at 100 ° C. for 1 hour to obtain a liquid crystal composite film having a film thickness shown in Table 1. Then, on this composite film, another transparent conductive glass same as the above is laminated so that the conductive surface is in contact with the composite film,
A liquid crystal display cell for testing was prepared. Then, set each cell in a spectrophotometer (model number UV160 manufactured by Shimadzu Corporation),
The opacity was evaluated by measuring the transmittance T ₀ (%) of light having a wavelength of 600 nm without applying an electric field. The results are shown in Table 1.
Shown in.

[0051]

[Table 1]

From the results shown in Table 1 above, the weight average molecular weight was 1
In each of Examples 1 to 3 in which the polymer having a molecular weight dispersion of more than 0,000 and less than 4 was used, the transmittance T ₀ was remarkably higher than that of Comparative Examples 1 to 3 in which the molecular weight dispersion was more than 4. It was small, which confirmed that it had a high white turbidity.

[0053]

As described above in detail, the liquid crystal composite film of the present invention has a high white turbidity even if it is a thin film of about 10 μm or less, and a good contrast can be obtained. Therefore, according to the present invention, a liquid crystal composite film having excellent characteristics can be realized as a thin film with high contrast and low voltage drive, and can be applied to liquid crystal televisions and the like.

Claims (2)

[Claims] 1. A uniform solution in which a liquid crystal material and a polymer which is a polymer of one or more kinds of monomers which are uniformly mixed with the liquid crystal material and which satisfies the following conditions are dissolved in a common solvent for both. The liquid crystal composite film is characterized in that a solvent is evaporated to form a microphase-separated structure. [Equation 1] [Equation 2] (However, [External 1] Is the polystyrene-equivalent weight average molecular weight, [External 2] Is the polystyrene equivalent number average molecular weight, and Indicates the molecular weight distribution. ) 2. The liquid crystal composite film according to claim 1, wherein the liquid crystal material has a positive dielectric anisotropy and a refractive index anisotropy Δn> 0.15.