JP5004354B2 - Birefringent film and polarizing element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To materialize a birefringent film which contains an aromatic polymer (an ester based polymer) highly soluble in a solvent with low polarity, and has the large birefringence &Delta;n<SB>xz</SB>(=n<SB>x</SB>-n<SB>z</SB>) in the thickness direction. <P>SOLUTION: The birefringent film includes an ester based polymer expressed by formula (I). In formula (I), R<SB>1</SB>and R<SB>2</SB>independently represent a straight chain or branched 1-10C alkyl groups, or a substituted or unsubstituted aryl group, and R<SB>3</SB>to R<SB>6</SB>independently represent a hydrogen atom, a halogen atom, a straight chain or branched 1-6C alkyl group, or a substituted or unsubstituted aryl group. At least one of R<SB>3</SB>to R<SB>6</SB>is not a hydrogen atom, and n represents an integer of 2 or more. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to an optical film, and more particularly to a birefringent film and a polarizing element.

  In a liquid crystal panel, a birefringent film is used to control the phase difference of light rays passing through the liquid crystal. As a birefringent film, a coating liquid mainly composed of an aromatic polymer such as aromatic polyimide or aromatic polyester is cast on a substrate such as a glass plate or polymer film or a metal drum or metal belt. A film in which an aromatic polymer is oriented is known (Patent Document 1).

Such an aromatic polymer is characterized by excellent heat resistance and mechanical strength, but has a drawback of poor solubility in organic solvents. For this reason, a birefringent film containing an aromatic polymer as a main component is conventionally cast into a solution after dissolving the aromatic polymer in a highly polar solvent (for example, cyclopentanone, methyl ethyl ketone, dichloroethane). And dried to form a film. However, in the conventional film formation method, the choice of a solvent capable of dissolving the aromatic polymer is limited, so that drying conditions are limited or expensive equipment is required. Therefore, there has been a demand for a novel aromatic polymer that is soluble in a low polarity solvent such as toluene.
JP 2004-70329 A

An object of the present invention includes a high aromatic polymer soluble in less polar solvents, it is possible to realize yet birefringent film birefringence index _{Δn xz (= n x -n z} ) is large in the thickness direction.

As a result of intensive studies to improve the relationship between the solubility and the birefringence index Δn _{xz in} the thickness direction, which has been a contradictory relationship, (1) the introduction of a thiophene ring into the polymer skeleton, 2) It has been found that this can be solved by introducing a specific substituent into any of R _{3 to} R ₆ and R _{9 to} R _{12 in} the general formula (I) or (II).

一般式（I）においてＲ_１およびＲ_２は独立して炭素数１〜１０の直鎖もしくは分枝のアルキル基、置換もしくは無置換のアリール基を表わし、Ｒ_３〜Ｒ_６は独立して水素原子、ハロゲン原子、炭素数１〜６の直鎖もしくは分枝のアルキル基、置換もしくは無置換のアリール基を表わし、Ｒ_３〜Ｒ_６の少なくとも一つは水素原子ではなく、ｎは２以上の整数を表わす。
（２）本発明の複屈折フィルムは前記一般式（I）において、Ｒ_１がメチル基、Ｒ_２が炭素数２〜４の直鎖もしくは分枝のアルキル基であることを特徴とする。
（３）本発明の複屈折フィルムは前記一般式（I）において、Ｒ_３〜Ｒ_６が独立して炭素数１〜４の直鎖もしくは分枝のアルキル基であることを特徴とする。
（４）本発明の複屈折フィルムは下記一般式（II）で表わされる共重合体エステル系ポリマーを含むことを特徴とする。

一般式（II）においてＲ_１およびＲ_２は独立して炭素数１〜１０の直鎖もしくは分枝のアルキル基、置換もしくは無置換のアリール基を表わし、Ｒ_３〜Ｒ_６は独立して水素原子、ハロゲン原子、炭素数１〜６の直鎖もしくは分枝のアルキル基、置換もしくは無置換のアリール基を表わし、Ｒ_３〜Ｒ_６の少なくとも一つは水素原子ではなく、Ｒ_７、Ｒ_８はそれぞれＲ_１、Ｒ_２と同様であり、Ｒ_９〜Ｒ_１２はそれぞれＲ_３〜Ｒ_６と同様であり、ｌおよびｍは２以上の整数を表わす。
（５）本発明の複屈折フィルムは前記一般式（II）で表わされる共重合体エステル系ポリマーにおいて、ｌ／（ｌ＋ｍ）の値が０．３〜０．８であることを特徴とする。
（６）本発明の複屈折フィルムは前記一般式（I）または（II）で表わされるエステル系ポリマーのガラス転移温度が１００℃以上、３００℃以下であることを特徴とする。
（７）本発明の複屈折フィルムはフィルム厚み方向の屈折率ｎ_ｚがフィルム面内の屈折率の最大値ｎ_ｘより小さいことを特徴とする。
（８）本発明の偏光素子は上記の複屈折フィルムと偏光子とを含むことを特徴とする。
（９）本発明の偏光素子は、上記の複屈折フィルム／接着層／偏光子／接着層／透明保護フィルムがこの順に積層されたことを特徴とする。
（１０）本発明の偏光素子は、（上記の複屈折フィルムと基材との積層体）／接着層／偏光子／接着層／透明保護フィルムがこの順に積層されたことを特徴とする。 The gist of the present invention is as follows.
(1) The birefringent film of the present invention comprises an ester polymer represented by the following general formula (I).

In the general formula (I), R ₁ and R ₂ independently represent a linear or branched alkyl group having 1 to 10 carbon atoms or a substituted or unsubstituted aryl group, and R _{3 to} R ₆ independently represent hydrogen. An atom, a halogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group, at least one of R _{3 to} R ₆ is not a hydrogen atom, and n is 2 or more Represents an integer.
(2) The birefringent film of the present invention is characterized in that, in the general formula (I), R ₁ is a methyl group and R ₂ is a linear or branched alkyl group having 2 to 4 carbon atoms.
(3) The birefringent film of the present invention is characterized in that, in the general formula (I), R _{3 to} R ₆ are each independently a linear or branched alkyl group having 1 to 4 carbon atoms.
(4) The birefringent film of the present invention includes a copolymer ester polymer represented by the following general formula (II).

In the general formula (II), R ₁ and R ₂ independently represent a linear or branched alkyl group having 1 to 10 carbon atoms or a substituted or unsubstituted aryl group, and R _{3 to} R ₆ independently represent hydrogen. An atom, a halogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group, and at least one of R _{3 to} R ₆ is not a hydrogen atom, but R ₇ , R ₈ Are the same as R ₁ and R ₂ , R _{9 to} R ₁₂ are the same as R _{3 to} R ₆ , respectively, and l and m represent an integer of 2 or more.
(5) The birefringent film of the present invention is characterized in that, in the copolymer ester polymer represented by the general formula (II), the value of l / (l + m) is 0.3 to 0.8.
(6) The birefringent film of the present invention is characterized in that the ester polymer represented by the general formula (I) or (II) has a glass transition temperature of 100 ° C. or higher and 300 ° C. or lower.
(7) birefringent film of the present invention, wherein the refractive index n _z in the thickness direction of the film is smaller than the maximum value n _x of the refractive index in the film plane.
(8) The polarizing element of this invention is characterized by including said birefringent film and a polarizer.
(9) The polarizing element of the present invention is characterized in that the birefringent film / adhesive layer / polarizer / adhesive layer / transparent protective film is laminated in this order.
(10) The polarizing element of the present invention is characterized in that (laminate of the above-mentioned birefringent film and substrate) / adhesive layer / polarizer / adhesive layer / transparent protective film are laminated in this order.

Since the birefringent film of the present invention contains an ester polymer represented by the general formula (I) or (II) that is highly soluble in a low polarity solvent, the degree of freedom of the substrate on which the ester polymer is applied is high. further it is possible to realize a birefringent film birefringence index _{Δn xz (= n x -n z} ) is large in the thickness direction.

In conventional aromatic polymers, if the polymer structure is linear and the birefringence Δn _xz in the thickness direction is increased, the solubility decreases, and if the polymer structure is bent and the solubility is increased, the birefringence in the thickness direction is increased. There was a problem that Δn _xz became small. Examples of the former include para-substituted 6-membered rings such as terephthalic acid, and examples of the latter include meta-substituted 6-membered rings such as isophthalic acid. That is, increasing the solubility and increasing the birefringence Δn _xz in the thickness direction were in a trade-off relationship.

As a result of intensive studies to improve the relationship between the solubility and the birefringence index Δn _{xz in} the thickness direction, which has been a contradictory relationship, (1) the introduction of a thiophene ring into the polymer skeleton, 2) It has been found that this can be solved by introducing a specific substituent into any of R _{3 to} R ₆ and R _{9 to} R _{12 in} the general formula (I) or (II). The specific substituent introduced into R _{3 to} R ₆ and R _{9 to} R ₁₂ is any one of a halogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms, and a substituted or unsubstituted aryl group. is there.
(1) Since the thiophene ring is a five-membered ring, the polymer chain can be bent appropriately to increase solubility, and the π-electron abundance enhances the interaction between aromatic rings in the thickness direction. The birefringence index Δn _xz can be increased.
(2) The specific substituent introduced into any of R _{3 to} R ₆ and R _{9 to} R ₁₂ is deformed so that two adjacent benzene rings are twisted with each other according to the size of the substituent. , Can increase the solubility. In addition, the linearity of the polymer structure can be maintained and a decrease in the birefringence Δn _xz in the thickness direction can be suppressed.

[Birefringent film]
The birefringent film of the present invention is characterized by containing an ester polymer represented by the general formula (I) or (II). In the present specification, the “birefringent film” refers to a transparent film having refractive index anisotropy in one or both of the in-plane and thickness directions. Birefringent film of the present invention, preferably, the refractive index n _z in the thickness direction of the film is smaller than the refractive indices n _x in the film plane, i.e. satisfying a relation of n _x> n _z.

Birefringence index in the thickness direction [Delta] n _xz at a wavelength 550nm of the birefringent film of the present invention ₍₌ n x -n _z) is preferably from 0.015 to 0.030. Since the birefringent film of the present invention has such a high birefringence index Δn _xz in the thickness direction, a birefringent film having a desired thickness direction retardation value Rth (= Δn _xz × film thickness) is made thinner. It becomes possible to do.

  Although the thickness of the birefringent film of this invention is suitably set according to a use or the phase difference value of the desired thickness direction, Preferably they are 1 micrometer-20 micrometers, More preferably, they are 1 micrometer-10 micrometers.

The transmittance of the birefringent film of the present invention at a wavelength of 400 nm is preferably 90% or more, more preferably 92% or more. The birefringent film of the present invention was able to obtain such a high transmittance by using an ester polymer represented by the general formula (I) or (II). This is because a specific substituent introduced into any of R _{3 to} R ₆ and R _{9 to} R ₁₂ is deformed so that two adjacent benzene rings are twisted with each other depending on the size of the substituent. This is because the solubility was increased and the linearity of the polymer structure was maintained, and the decrease in the birefringence Δn _xz in the thickness direction could be suppressed.

一般式（I）においてＲ_１およびＲ_２は独立して炭素数１〜１０の直鎖もしくは分枝のアルキル基、置換もしくは無置換のアリール基を表わし、Ｒ_３〜Ｒ_６は独立して水素原子、ハロゲン原子、炭素数１〜６の直鎖もしくは分枝のアルキル基、置換もしくは無置換のアリール基を表わす。ただしＲ_３〜Ｒ_６の少なくとも一つは水素原子ではない。ｎは２以上の整数を表わす。 [Ester polymer]
The birefringent film of the present invention includes an ester polymer represented by the following general formula (I).

In the general formula (I), R ₁ and R ₂ independently represent a linear or branched alkyl group having 1 to 10 carbon atoms or a substituted or unsubstituted aryl group, and R _{3 to} R ₆ independently represent hydrogen. An atom, a halogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms, or a substituted or unsubstituted aryl group is represented. However, at least one of R _{3 to} R ₆ is not a hydrogen atom. n represents an integer of 2 or more.

Thus, by introducing a thiophene ring into the polymer skeleton and introducing a specific substituent into any of R _{3 to} R ₆ , the solubility is increased and the birefringence Δn _xz in the thickness direction is increased. It is possible to do both.

Preferably, in general formula (I), R ₁ is a methyl group, R ₂ is a linear or branched alkyl group having 2 to 4 carbon atoms, and R _{3 to} R ₆ are linear or branched groups having 1 to 4 carbon atoms. It is an alkyl group. When the alkyl group of R _{1 to} R ₆ has too many carbon atoms (for example, 11 or more for R ₁ and R ₂ , 7 or more for R _{3 to} R ₆ ), the birefringence in the thickness direction becomes small, and heat resistance ( Glass transition temperature) may decrease.

一般式（II）で表わされるポリマーのシーケンスには特に制限はなく、ブロック共重合体でもランダム共重合体でもよい。一般式（II）で表わされるエステル系ポリマーにおいてチオフェン環を含む繰り返し単位の含有率、すなわちｌ／（ｌ＋ｍ）の値は、好ましくは０．３〜０．８である。ｌ／（ｌ＋ｍ）の値がこの範囲であれば、特に溶解性に優れ厚み方向の複屈折率の大きな複屈折フィルムが得られる。 The birefringent film of the present invention includes, as one embodiment, an ester polymer (copolymer) represented by the following general formula (II). In the general formula (II), l and m represents an integer of 2 or _more, R 1 to R ₆ is the same as in the general formula (I). R ₇ and R ₈ are the same as R ₁ and R ₂ , respectively, and R _{9 to} R ₁₂ are the same as R _{3 to} R ₆ , respectively.

The sequence of the polymer represented by the general formula (II) is not particularly limited, and may be a block copolymer or a random copolymer. In the ester polymer represented by the general formula (II), the content of repeating units containing a thiophene ring, that is, the value of l / (l + m) is preferably 0.3 to 0.8. When the value of l / (l + m) is within this range, a birefringent film having excellent solubility and a large birefringence index in the thickness direction can be obtained.

  Although there is no restriction | limiting in particular in the weight average molecular weight (Mw) of said ester polymer, Preferably it is 10,000-500,000. The glass transition temperature of the ester polymer is preferably 100 ° C. or higher from the viewpoint of heat resistance, and preferably 300 ° C. or lower from the viewpoint of moldability and stretchability.

  The birefringent film of the present invention contains the above ester-based polymer, preferably 50 wt% to 100 wt%, more preferably 80 wt% to 100 wt%, based on the total weight of the birefringent film. The birefringent film of the present invention comprises a polymer other than the ester polymer represented by the general formula (I) or the general formula (II) (for example, an imide polymer, an ether ketone polymer, an amidoimide polymer, a styrene polymer). If it is less than 50% of the total weight of the refractive film, it may be contained. In addition, the birefringent film of the present invention can contain an optional additive such as an ultraviolet absorber, an antistatic agent, and an antioxidant.

[Method of polymerizing ester polymer]
The ester polymer can be usually obtained by polycondensation of a biphenol compound and a dicarboxylic acid compound. The polycondensation method is not particularly limited, but an interfacial polymerization method in which a biphenol compound and a dicarboxylic acid compound are reacted in a two-phase system of an alkaline aqueous solution and a water-immiscible organic solvent in the presence of a phase transfer catalyst is preferably used. According to such a polymerization method, an ester polymer having a high molecular weight and excellent transparency can be obtained.

  Examples of the biphenol compound include 2,2-bis (3,5-dimethyl-4-hydroxyphenyl) -4-methylpentane, 2,2-bis (3-methyl-4-hydroxyphenyl) propane, 2, 2-bis (3,5-dimethyl-4-hydroxyphenyl) propane, 2,2-bis (3-methyl-4-hydroxyphenyl) butane and the like are used.

  As the dicarboxylic acid compound, thiophene-2,5-dicarbonyl chloride is preferably used. When a copolymer is used as in the general formula (II), terephthalic acid chloride, isophthalic acid chloride, phthalic acid chloride, biphenyldicarboxylic acid chloride, etc. are used in combination.

  The phase transfer catalyst is not particularly limited, and quaternary ammonium salts such as benzyltriethylammonium chloride, quaternary phosnium salts such as tetraphenylphosphonium chloride, and the like are used. Although there is no restriction | limiting in particular in said water immiscible organic solvent, Chloroform, a dichloromethane, etc. are used.

[Production method of birefringent film]
The birefringent film of the present invention can be formed by any method such as a solution casting method or a melt extrusion method. Ester-based polymer used in the present invention is a volatile process solvents, so that the refractive index n _z in the thickness direction of the film is smaller than the maximum value n _x of the refractive index in the film plane, the polymer ring is spontaneously oriented Has properties. For this reason, the solution casting method is preferably used for the birefringent film of the present invention from the viewpoint of expression of birefringence.

  The solution casting method is a method in which the above ester polymer is dissolved in a solvent to prepare a solution, and this solution is cast on the surface of a substrate, applied, and dried. The solvent is not particularly limited as long as it dissolves the above ester polymer, and for example, toluene, methyl isobutyl ketone, cyclopentanone, ethyl acetate, or the like is used. The concentration of the above solution is usually 1% to 40% by weight. As the casting and coating means, an arbitrary coater such as a spin coater, a die coater or a bar coater is used. As a drying means, an arbitrary drying device such as an air circulation drying oven or a thermal drum is used. The drying temperature is usually 40 ° C to 200 ° C.

  There is no restriction | limiting in particular in the base material for apply | coating said solution, The thing of a single layer may be sufficient and the laminated body (for example, anchor coat layer is included) of multiple layers may be sufficient. Specific examples of the substrate include glass plates and polymer films. When the substrate includes an anchor coat layer, the anchor coat layer is not particularly limited, but a vinyl alcohol polymer or a urethane polymer is preferably used in terms of good adhesion with an ester polymer. The thickness of the anchor coat layer is preferably 0.01 μm to 5 μm.

  The glass plate as the substrate is preferably used for a liquid crystal cell such as non-alkali glass. When a polymer film is used as the substrate, the substrate can be made flexible. The material of the polymer film used for the substrate is not particularly limited as long as it is a film-forming polymer, but a styrene polymer, a (meth) acrylic acid polymer, an ester polymer, an olefin polymer, a norbornene polymer, an imide polymer Examples include polymers, cellulose polymers, vinyl alcohol polymers, and carbonate polymers. The thickness of the substrate is not particularly limited except for the application, but is generally in the range of 1 μm to 1000 μm.

  Since the ester-based polymer represented by the general formula (I) or (II) is excellent in solubility, a solvent with little erosion of the polymer film, such as toluene, can be used. For this reason, it is also possible to use a base material that could not be used due to severe erosion with conventional ester polymers, for example, a film mainly composed of (meth) acrylic acid polymers or olefin polymers. This makes it possible to reduce the cost of the birefringent film.

[Polarizing element]
The polarizing element of the present invention includes the birefringent film of the present invention and a polarizer. Any polarizer may be used as long as it separates light into two orthogonal polarization components and transmits one polarization component and absorbs, scatters or reflects the other polarization component. The polarizer is obtained by, for example, staining polyvinyl alcohol with a dichroic dye and stretching it. Although there is no restriction | limiting in particular in the thickness of a polarizer, For example, they are 10 micrometers-200 micrometers.

  As one embodiment, the polarizing element of the present invention comprises (birefringent film of the present invention) / adhesive layer / polarizer / adhesive layer / transparent protective film laminated in this order. According to this configuration, since the birefringent film of the present invention also serves as a protective film for the polarizer, the polarizing element can be thinned.

  As another embodiment of the polarizing element of the present invention, (a laminate of the birefringent film of the present invention and a substrate) / adhesive layer / polarizer / adhesive layer / transparent protective film is laminated in this order. According to this configuration, since the laminate of the birefringent film and the base material obtained in the birefringent film manufacturing process can be used as a protective film for the polarizer, the step of peeling the birefringent film from the base material is omitted. The productivity of the polarizing element is improved.

  The transparent protective film in the above configuration is not particularly limited as long as it is transparent and has a low birefringence, and includes, for example, a cellulose polymer and a norbornene polymer in addition to the above (meth) acrylic acid polymer and olefin polymer. A film is used.

[Uses of birefringent films and polarizing elements]
The birefringent film and polarizing element of the present invention are used for various optical elements by taking advantage of optical anisotropy. In particular, various liquid crystal panels, for example, OA equipment such as personal computer monitors, notebook personal computers, copiers, mobile phones, watches, Digital cameras, personal digital assistants (PDAs), portable devices such as portable game devices, household devices such as video cameras, televisions, and microwave ovens, back monitors, car navigation, car audio and other in-vehicle devices, store monitors, etc. It is suitably used for liquid crystal panels such as exhibition equipment, security equipment such as monitoring monitors, nursing monitors, and medical monitors.

[Example]
As shown in Table 1, in a reaction vessel equipped with a stirrer, 2.00 g of 2,2-bis (3,5-dimethyl-4-hydroxyphenyl) -4-methylpentane and 0.07 g of benzyltriethylammonium chloride. Was dissolved in 30 ml of 1M aqueous sodium hydroxide solution. To this solution, a solution prepared by dissolving 0.62 g of terephthalic acid chloride and 0.64 g of thiophene-2,5-dicarbonyl chloride in 25 ml of chloroform was added all at once with stirring, followed by stirring at room temperature (23 ° C.) for 90 minutes. . Thereafter, the polymerization solution was allowed to stand and separate to separate a chloroform solution containing the polymer, washed with acetic acid water, further washed with ion-exchanged water, and then poured into methanol to precipitate the polymer. The precipitated polymer was filtered and dried under reduced pressure to obtain 2.57 g of a white polymer of the following structural formula (III).

The glass transition temperature Tg of this polymer is 200 ° C., the weight average molecular weight Mw is 52,000, and the solubility in each solvent (cyclopentanone, toluene, methyl isobutyl ketone, ethyl acetate) is as shown in Table 2. 20% by weight or more was dissolved in the above solvent.

The obtained polymer was dissolved in toluene, cast and applied onto a glass substrate by a spin coating method, dried at 70 ° C. for 5 minutes, and further dried at 110 ° C. for 30 minutes to produce a birefringent film. The thickness of the birefringent film after drying was 5.5 μm, the transmittance was 92%, and the birefringence Δn _xz (550) in the thickness direction at a wavelength of 550 nm was 0.020 as shown in Table 2.

[Comparative Example 1]
As shown in Table 1 in a reaction vessel equipped with a stirrer, 1.50 g of 2,2-bis (3,5-dimethyl-4-hydroxyphenyl) -4-methylpentane and 0.06 g of benzyltriethylammonium chloride. Was dissolved in 30 ml of 1M aqueous sodium hydroxide solution. A solution prepared by dissolving 0.47 g of terephthalic acid chloride and 0.47 g of isophthalic acid chloride in 15 ml of chloroform was added to this solution all at once with stirring, and the mixture was stirred at room temperature (23 ° C.) for 90 minutes. Thereafter, the polymerization solution was allowed to stand and separate to separate a chloroform solution containing the polymer, washed with acetic acid water, further washed with ion-exchanged water, and then poured into methanol to precipitate the polymer. The precipitated polymer was filtered and dried under reduced pressure to obtain 1.83 g of a white polymer of the following structural formula (IV).

  The glass transition temperature Tg of this polymer is 205 ° C., the weight average molecular weight Mw is 61,000, and the solubility in each solvent (cyclopentanone, toluene, methyl isobutyl ketone, ethyl acetate) is as shown in Table 2. 20% by weight or more was dissolved in the above solvent.

The obtained polymer was dissolved in toluene, cast and applied onto a glass substrate by a spin coating method, dried at 70 ° C. for 5 minutes, and further dried at 110 ° C. for 30 minutes to produce a birefringent film. The thickness of the birefringent film after drying was 3.2 μm, the transmittance was 92%, and the birefringence Δn _xz (550) in the thickness direction at a wavelength of 550 nm was 0.014 as shown in Table 2.

[Comparative Example 2]
As shown in Table 1, in a reaction vessel equipped with a stirrer, 2.28 g of 2,2-bis (4-hydroxyphenyl) propane (commonly called bisphenol A) and 0.07 g of benzyltriethylammonium chloride were added to 1M sodium hydroxide. Dissolved in 30 ml of aqueous solution. A solution prepared by dissolving 1.02 g of terephthalic acid chloride and 1.02 g of isophthalic acid chloride in 30 ml of chloroform was added to this solution all at once with stirring, and the mixture was stirred at room temperature (23 ° C.) for 90 minutes. Thereafter, the polymerization solution was allowed to stand and separate to separate a chloroform solution containing the polymer, washed with acetic acid water, further washed with ion-exchanged water, and then poured into methanol to precipitate the polymer. The precipitated polymer was filtered and dried under reduced pressure to obtain 3.37 g of a white polymer of the following structural formula (V).

  The glass transition temperature Tg of this polymer is 200 ° C., the weight average molecular weight Mw is 77,000, and the solubility in each solvent (cyclopentanone, toluene, methyl isobutyl ketone, ethyl acetate) is as shown in Table 2. Although dissolved in cyclopentanone by 20% by weight or more, it was insoluble in toluene, methyl isobutyl ketone, and ethyl acetate.

The obtained polymer is dissolved in cyclopentanone, cast and applied onto a glass substrate by spin coating, dried at 70 ° C. for 5 minutes, and further dried at 110 ° C. for 30 minutes to produce a birefringent film. did. The thickness of the birefringent film after drying was 3.2 μm, the transmittance was 92%, and the birefringence Δn _xz (550) in the thickness direction at a wavelength of 550 nm was 0.020 as shown in Table 2.

[Evaluation]
The ester polymer (structural formula (III)) in the examples shows good solubility in each solvent (cyclopentanone, toluene, methyl isobutyl ketone, ethyl acetate), and a compound obtained from this ester polymer. The refractive film exhibited a high birefringence (Δn _xz (550) = 0.020).

The ester polymer (Structural Formula (IV)) of Comparative Example 1 showed good solubility in each solvent (cyclopentanone, toluene, methyl isobutyl ketone, ethyl acetate), and was obtained from this ester polymer. The birefringent film has a problem that the birefringence is low (Δn _xz (550) = 0.014). Compared to the ester polymer of the example (structural formula (III)), when the thiophene ring is replaced with a benzene ring, the solubility does not change but the birefringence decreases.

For example, when a birefringent film having a thickness direction retardation value (= _birefractive index Δn _xz (550) × film thickness in the thickness direction) of 200 nm is prepared, the film thickness is obtained using the ester polymer of Comparative Example 1. Although the thickness is about 14 μm, if the ester-based polymer of the example is used, the film thickness may be about 10 μm, and a thinning of about 4 μm (about 30%) can be realized.

The ester-based polymer of Comparative Example 2 has the same birefringence Δn _xz (550) in the thickness direction as compared with the ester-based polymer of the example, but is inferior in solubility and dissolves in cyclopentanone, but toluene, methyl Not soluble in isobutyl ketone and ethyl acetate. Since the ester-based polymer of Comparative Example 2 is soluble only in a highly polar solvent such as cyclopentanone, a polymer base material (for example, a (meth) acrylic acid-based polymer or an olefin-based polymer that is easily eroded by the solvent) is used as a main component. (Film) cannot be applied. Therefore, the selection range of the base film that can be applied becomes narrow, which is disadvantageous in terms of cost.

[Measuring method]
[Glass-transition temperature]
It measured according to JISK7121 (1987 plastics transition temperature measuring method) using the differential scanning calorimeter (the product name "DSC-6200" by Seiko). Specifically, a 3 mg powder sample was heated from room temperature to 220 ° C. at a temperature rising rate of 10 ° C./min in a nitrogen atmosphere (nitrogen gas flow rate 50 ml / min), and then 30 ° C. at a temperature falling rate of 10 ° C./min. The temperature was lowered until the first measurement. Next, the temperature was increased to 350 ° C. at a temperature increase rate of 10 ° C./min, and the second measurement was performed. The second measurement data was adopted as the glass transition temperature. The calorimeter was temperature corrected using a standard material (indium).

[Weight average molecular weight]
The weight average molecular weight was adjusted to 0.1% tetrahydrofuran solution for each sample, filtered through a 0.45 μm membrane filter, and then a gel permeation chromatograph (product name “HLC-8820GPC” manufactured by Tosoh Corporation) was used as the GPC body. It was measured using RI (built in the GPC main body) as a detector. Specifically, the column temperature was 40 ° C., the pump flow rate was 0.35 ml / min, and the data processing was performed in advance by using a standard polystyrene calibration curve with a known molecular weight to obtain the molecular weight from the polystyrene equivalent molecular weight. The column used was SuperHZM-M (diameter 6.0 mm × 15 cm), SuperHZM-M (diameter 6.0 mm × 15 cm) and SuperHZ2000 (diameter 6.0 mm × 15 cm) connected in series, and the mobile phase was Tetrahydrofuran was used.

[Birefractive index in the thickness direction]
Measurement was performed at a wavelength of 550 nm using a product name “KOBRA-WPR” manufactured by Oji Scientific Instruments. The birefringence index Δn _xz (550) in the thickness direction was obtained by calculating from the front phase difference value and the phase difference value (R ₄₀ ) when the sample was tilted by 40 degrees using a program attached to the apparatus. As the film thickness, a value obtained by a product name “Dektak” manufactured by Sloan was used.

[Solubility]
The polymer was added little by little to the sample bottle containing each solvent, and the degree of dissolution was visually observed.

Claims (10)

A birefringent film comprising an ester polymer represented by the following general formula (I):

(In the general formula (I), R ₁ and R ₂ independently represent a linear or branched alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted aryl group, and R _{3 to} R ₆ independently represent Represents a hydrogen atom, a halogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms, or a substituted or unsubstituted aryl group, wherein at least one of R _{3 to} R ₆ is not a hydrogen atom, and n is 2 or more Represents an integer.) In the general formula (I), the birefringent film according to claim 1 in which R _1, wherein the methyl group, R ₂ is a straight-chain or branched alkyl group having 2 to 4 carbon atoms. _3. The birefringent film according to claim 1, wherein in the general formula (I), R _{3 to} R ₆ are each independently a linear or branched alkyl group having 1 to 4 carbon atoms. A birefringent film comprising a copolymer ester polymer represented by the following general formula (II):

(In the general formula (II), R ₁ and R ₂ independently represent a linear or branched alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted aryl group, and R _{3 to} R ₆ are independently Represents a hydrogen atom, a halogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms, or a substituted or unsubstituted aryl group, and at least one of R _{3 to} R ₆ is not a hydrogen atom, but R ₇ , R ₈ is the same as R ₁ and R ₂ , R _{9 to} R ₁₂ are the same as R _{3 to} R ₆ , and l and m each represent an integer of 2 or more.)   The birefringent film according to claim 4, wherein the copolymer ester polymer represented by the general formula (II) has a value of l / (l + m) of 0.3 to 0.8.   6. The birefringent film according to claim 1, wherein the ester polymer represented by the general formula (I) or (II) has a glass transition temperature of 100 ° C. or higher and 300 ° C. or lower. Birefringent film according to any one of the refractive index n _z in the thickness direction of the film is smaller than the maximum value n _x of the refractive index in a film plane from claim 1, wherein 6.   A polarizing element comprising the birefringent film according to claim 1 and a polarizer.   A polarizing element comprising a birefringent film according to claim 1, an adhesive layer, a polarizer, an adhesive layer, and a transparent protective film laminated in this order.   (A laminated body of a birefringent film and a substrate according to any one of claims 1 to 7) / Adhesive layer / Polarizer / Adhesive layer / Transparent protective film is laminated in this order.