JP5983300B2 - Diisopropyl fumarate-cinnamic acid copolymer and retardation film using the same - Google Patents

Description

  The present invention relates to a novel diisopropyl fumarate-cinnamic acid copolymer and a retardation film using the same, and more particularly, a retardation film having a high out-of-plane retardation even in a thin film, particularly a liquid crystal. The present invention relates to a novel diisopropyl fumarate-cinnamic acid copolymer suitable for an optical compensation film for a display element.

  A liquid crystal display is widely used as a most important display device in a multimedia society, such as a mobile phone, a computer monitor, a notebook computer, and a television. Many optical films are used for improving the display characteristics of the liquid crystal display. In particular, the retardation film plays a major role in improving contrast and compensating for color tone when viewed from the front or obliquely. As the conventional retardation film, polycarbonate or cyclic polyolefin is used, and these polymers are polymers having positive birefringence. Here, the sign of birefringence is defined as shown below.

  The optical anisotropy of the polymer film molecularly oriented by stretching or the like can be represented by a refractive index ellipsoid shown in FIG. Here, the refractive index in the fast axis direction in the film plane when the film is stretched is represented by nx, the refractive index in the film in-plane direction orthogonal thereto is represented by ny, and the refractive index in the thickness direction of the film is represented by nz. The fast axis indicates an axial direction having a low refractive index in the film plane.

  Negative birefringence means that the stretching direction is the fast axis direction, and positive birefringence means that the direction perpendicular to the stretching direction is the fast axis direction.

  That is, in the uniaxial stretching of the polymer having negative birefringence, the refractive index in the stretching axis direction is small (fast axis: stretching direction), and in the uniaxial stretching of the polymer having positive birefringence, the axial direction perpendicular to the stretching axis. Has a small refractive index (fast axis: direction perpendicular to the stretching direction).

  Many polymers have positive birefringence. As a polymer having negative birefringence, there are acrylic resin and polystyrene. However, acrylic resin has a small retardation, and the properties as a retardation film are not sufficient. Polystyrene has a large photoelastic coefficient in the low-temperature region, so the phase difference changes due to slight stress, the problem of stability of the phase difference, the problem of optical properties such as the large wavelength dependence of the phase difference, and heat resistance However, it is not used at present.

  Here, the wavelength dependence of the phase difference means that the phase difference changes depending on the measurement wavelength. The phase difference measured at the wavelength of 450 nm (R450) and the phase difference measured at the wavelength of 550 nm (R550). The ratio can be expressed as R450 / R550. In general, a polymer having an aromatic structure has a strong tendency to increase R450 / R550, and the contrast and viewing angle characteristics in a low wavelength region are deteriorated.

  A stretched polymer film exhibiting negative birefringence has a high refractive index in the thickness direction of the film, resulting in an unprecedented retardation film. For example, a super twist nematic liquid crystal display (STN-LCD) or a vertically aligned liquid crystal Useful as a retardation film for compensating viewing angle characteristics of displays (VA-LCD), in-plane oriented liquid crystal displays (IPS-LCD), reflective liquid crystal displays (reflective LCDs), and viewing angle compensation films for deflecting plates Therefore, the market demand is strong for retardation films having negative birefringence.

  A method for producing a film has been proposed in which a polymer having positive birefringence is used to increase the refractive index in the thickness direction of the film. One is a treatment method (for example, Patent Documents 1 to 3) in which a heat-shrinkable film is bonded to one side or both sides of a polymer film, the laminate is subjected to a heat stretching treatment, and a shrinkage force is applied in the film thickness direction of the polymer film. Reference). In addition, a method of uniaxial stretching in a plane while applying an electric field to a polymer film has been proposed (see, for example, Patent Document 4).

  In addition, a retardation film composed of fine particles having negative optical anisotropy and a transparent polymer has been proposed (for example, see Patent Document 5).

  However, the methods proposed in Patent Documents 1 to 4 have a problem that productivity is inferior because the manufacturing process becomes very complicated. Also, the control of the uniformity of the phase difference and the like is extremely difficult as compared with the conventional control by stretching.

  When polycarbonate is used as the base film, the photoelastic coefficient at room temperature is large and the phase difference is changed by a slight stress, so that there is a problem in the stability of the phase difference. Furthermore, there is a problem that the wavelength dependence of the phase difference is large.

  The retardation film obtained in Patent Document 5 is a retardation film that exhibits negative birefringence by adding fine particles having negative optical anisotropy. From the viewpoint of simplification of production method and economical efficiency, fine particles are obtained. There is a need for retardation films that do not require the addition of.

  Moreover, the fumaric-acid diester type resin and the film which consists of it are proposed (for example, refer patent documents 6-10).

Japanese Patent No. 2818983 JP-A-5-297223 JP-A-5-323120 JP-A-6-88909 JP 2005-156862 A JP 2008-112141 A JP 2012-032784 A WO2012 / 005120 JP 2008-129465 A JP 2006-193616 A

  Although the fumaric acid diester-based resin proposed in Patent Documents 6 to 10 and a film made thereof have a high out-of-plane retardation, at present, a film having a higher out-of-plane retardation is also present in a thin film. It has been demanded.

  An object of the present invention is to provide a novel diisopropyl fumarate-cinnamic acid copolymer suitable for a retardation film having a high out-of-plane retardation even in a thin film using a specific copolymer and excellent in optical properties. There is.

  As a result of intensive studies to solve the above problems, the present inventors have found that a specific diisopropyl fumarate-cinnamic acid copolymer can solve the above problems, and have completed the present invention.

  That is, the present invention relates to a diisopropyl fumarate-cinnamic acid copolymer containing a diisopropyl fumarate residue unit and a cinnamic acid residue unit, and a retardation film using the same.

  Hereinafter, the diisopropyl fumarate-cinnamic acid copolymer suitable for the retardation film of the present invention will be described in detail.

  The diisopropyl fumarate-cinnamic acid copolymer of the present invention is a diisopropyl fumarate-cinnamic acid copolymer containing diisopropyl fumarate residue units and cinnamic acid residue units.

  The diisopropyl fumarate-cinnamic acid copolymer may contain other monomer residue units as long as the scope of the present invention is not exceeded. Examples of other monomer residue units include: Styrene residue units such as styrene residue units and α-methylstyrene residue units; (meth) acrylic acid residue units; (meth) methyl acrylate residue units, (meth) ethyl acrylate residue units, (Meth) acrylic acid ester residue units such as butyl (meth) acrylate residue units; vinyl ester residue units such as vinyl acetate residue units and vinyl propionate residue units; acrylonitrile residue units; methacrylonitrile Residue units; vinyl ether residue units such as methyl vinyl ether residue units, ethyl vinyl ether residue units, butyl vinyl ether residue units; N-methylmaleimide residue units, N N-substituted maleimide residue units such as cyclohexylmaleimide residue units and N-phenylmaleimide residue units; olefin residue units such as ethylene residue units and propylene residue units; di-n-butyl fumarate residue units And one or more selected from fumaric acid diester residue units other than diisopropyl fumarate residue units such as bis (2-ethylhexyl) fumarate residue units.

  Since the composition of the diisopropyl fumarate-cinnamic acid copolymer of the present invention is excellent in retardation characteristics and strength when used as a retardation film, diisopropyl fumarate residue units of 50 to 99 mol% and Cinnamic acid residue units are preferably 1 to 50 mol%, more preferably diisopropyl fumarate residue units 70 to 97 mol% and cinnamic acid residue units 3 to 30 mol%.

  Since the diisopropyl fumarate-cinnamic acid copolymer of the present invention has excellent mechanical properties, the number in terms of standard polystyrene obtained from an elution curve measured by gel permeation chromatography (GPC). The average molecular weight is preferably 30,000 to 500,000, and more preferably 30,000 to 300,000.

  The method for producing the diisopropyl fumarate-cinnamic acid copolymer of the present invention may be produced by any method as long as the diisopropyl fumarate-cinnamic acid copolymer can be obtained. It can be produced by radical polymerization of cinnamic acid.

  For the radical polymerization, any known polymerization method, for example, bulk polymerization method, solution polymerization method, suspension polymerization method, precipitation polymerization method or emulsion polymerization method can be employed.

  Examples of the polymerization initiator used for radical polymerization include benzoyl peroxide, lauryl peroxide, octanoyl peroxide, acetyl peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, and dicumyl peroxide. , Organic peroxides such as t-butylperoxyacetate and t-butylperoxybenzoate; 2,2′-azobis (2,4-dimethylvaleronitrile), 2,2′-azobis (2-butyronitrile), 2 Azo initiators such as 1,2′-azobisisobutyronitrile, dimethyl-2,2′-azobisisobutyrate, 1,1′-azobis (cyclohexane-1-carbonitrile), and the like.

  And there is no restriction | limiting in particular as a solvent which can be used in solution polymerization method, suspension polymerization method, precipitation polymerization method, emulsion polymerization method, For example, aromatic solvents, such as benzene, toluene, xylene; Methanol, ethanol, propanol, butanol, etc. Cyclohexane, dioxane, tetrahydrofuran, acetone, methyl ethyl ketone, dimethylformamide, isopropyl acetate, water and the like, and mixed solvents thereof.

  Moreover, since the polymerization temperature at the time of performing radical polymerization can be suitably set according to the decomposition temperature of the polymerization initiator and the control of the reaction is easy, it is generally performed in the range of 30 to 150 ° C. It is preferable.

  When an optical film using the diisopropyl fumarate-cinnamic acid copolymer of the present invention is obtained, since it has a high out-of-plane retardation even in a thin film and is excellent in optical properties, it can be a retardation film. preferable.

  In the retardation film of the present invention, the refractive index in the fast axis direction in the film plane is nx, the refractive index in the film in-plane direction perpendicular thereto is ny, and the refractive index in the thickness direction of the film is nz. A retardation film characterized in that the relationship is nx ≦ ny <nz. By satisfying nx ≦ ny <nz, the viewing angle of STN-LCD, IPS-LCD, reflective LCD, transflective LCD, etc. The retardation film is excellent in compensation performance. In general, since the control of the three-dimensional refractive index of the film is performed by stretching the film and the like, the manufacturing process and quality control are complicated. However, the retardation film of the present invention is unstretched and has a refractive index in the film thickness direction. Has been found to exhibit a unique behavior of high.

In addition, since the retardation film of the present invention becomes a retardation film having more excellent optical properties, an out-of-plane retardation (Rth) measured at a wavelength of 550 nm represented by the following formula (a) is −50 to −. It is preferably 2000 nm, and more preferably −100 to −500 nm.
Rth = ((nx + ny) / 2−nz) × d (a)
(Here, d indicates the thickness of the film.)
Since the retardation film of the present invention has a high out-of-plane retardation even in a thin film, the relationship between the film thickness and the out-of-plane retardation is preferably 4.5 nm / film thickness (μm) or more in absolute value. 15 nm / film thickness (μm) is more preferable.

  The wavelength dependence of the phase difference can be expressed as a ratio R450 / R550 of the phase difference (R450) measured at a wavelength of 450 nm and the phase difference (R550) measured at a wavelength of 550 nm. In the retardation film of the present invention, the R450 / R550 is preferably 1.1 or less, more preferably 1.08 or less, and particularly preferably 1.05 or less.

  When the retardation film of the present invention is used in a liquid crystal display device, the image quality characteristics are good, and therefore, the light transmittance is preferably 85% or more, and more preferably 90% or more. . The haze (cloudiness) of the retardation film is preferably 2% or less, more preferably 1% or less.

  There is no restriction | limiting in particular as a manufacturing method of the phase difference film of this invention, For example, methods, such as a solution cast method and a melt cast method, are mentioned.

  In the solution casting method, a solution obtained by dissolving a diisopropyl fumarate-cinnamic acid copolymer in a solvent (hereinafter referred to as a dope) is cast on a support substrate, and then the solvent is removed by heating or the like to obtain a film. Is the method. In this case, as a method for casting the dope on the support substrate, for example, a T-die method, a doctor blade method, a bar coater method, a roll coater method, a lip coater method, or the like is used. Particularly, industrially, the most common method is to continuously extrude a dope from a die onto a belt-like or drum-like support substrate. Examples of the support substrate used include a glass substrate, a metal substrate such as stainless steel and ferrotype, and a film such as polyethylene terephthalate. In the solution casting method, when forming a film having high transparency and excellent thickness accuracy and surface smoothness, the solution viscosity of the dope is an extremely important factor, preferably 10 to 20000 cPs, More preferably, it is -10000 cPs.

  In this case, the coating thickness of the diisopropyl fumarate-cinnamic acid copolymer is preferably 1 to 200 μm after drying, more preferably 5 to 100 μm, and particularly preferably 10 to 50 μm because the film can be easily handled. is there.

  The melt casting method is a molding method in which a diisopropyl fumarate-cinnamic acid copolymer is melted in an extruder, extruded into a film form from a slit of a T die, and then cooled and cooled with a roll or air.

  The retardation film of the present invention can be used by being peeled off from a glass substrate as a base material or another optical film, and can also be used as a laminate with a glass substrate as a base material or another optical film.

  The retardation film of the present invention can be laminated with a polarizing plate to be used as a circular or elliptical polarizing plate, and can be laminated with a polarizer containing polyvinyl alcohol / iodine or the like to form a polarizing plate. is there. Furthermore, the retardation films of the present invention can be laminated with each other or with other retardation films.

  In the retardation film of the present invention, an antioxidant is preferably blended in order to increase the thermal stability of the retardation film itself or during film formation. Examples of the antioxidant include hindered phenol antioxidants, phosphorus antioxidants, and other antioxidants. These antioxidants may be used alone or in combination. And since an antioxidant effect | action improves synergistically, it is preferable to use together a hindered antioxidant and phosphorus antioxidant, and in that case, for example with respect to 100 weight part of hindered antioxidant More preferably, the phosphorus-based antioxidant is used in a mixture of 100 to 500 parts by weight. Further, the addition amount of the antioxidant is 0 with respect to 100 parts by weight of diisopropyl fumarate-cinnamic acid copolymer constituting the retardation film of the present invention in order to more effectively exhibit the antioxidant action. 0.01 to 10 parts by weight is preferable, and 0.5 to 1 part by weight is more preferable.

  Furthermore, as an ultraviolet absorber, for example, an ultraviolet absorber such as benzotriazole, benzophenone, triazine, or benzoate may be blended as necessary.

  The retardation film of the present invention is blended with other polymers, surfactants, polymer electrolytes, conductive complexes, inorganic fillers, pigments, antistatic agents, antiblocking agents, lubricants, etc. within the scope of the invention. May be.

  According to the present invention, optical properties such as a contrast film and a viewing angle characteristic compensation film and an antireflection film of a liquid crystal display having a large refractive index in the thickness direction, a large in-plane retardation, and a small wavelength dependency. It is possible to provide a diisopropyl fumarate-cinnamic acid copolymer suitable for an excellent retardation film.

  The diisopropyl fumarate-cinnamic acid copolymer of the present invention is a fumaric acid suitable for a retardation film having a high out-of-plane retardation even in a thin film and excellent optical properties such as a large refractive index in the thickness direction of the film. It is a diisopropyl-cinnamic acid copolymer and is particularly suitable for an optical compensation film for a liquid crystal display element.

Change in refractive index ellipsoid by stretching

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to these Examples.

  In addition, the various physical properties shown by an Example were measured with the following method.

<Composition of diisopropyl fumarate-cinnamic acid copolymer>
Using a nuclear magnetic resonance measuring apparatus (manufactured by JEOL, trade name JNM-GX270), it was determined by proton nuclear magnetic resonance spectroscopy ( ¹ H-NMR) spectrum analysis.

<Measurement of number average molecular weight>
Using a gel permeation chromatography (GPC) apparatus (manufactured by Tosoh Corporation, trade name C0-8011 (equipped with column GMH _HR- H)), tetrahydrofuran was measured as a solvent at 40 ° C., and the standard polystyrene equivalent value was obtained. Asked.

<Transparency evaluation method>
The total light transmittance and haze of the film were measured using a haze meter (trade name NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.).

<Measurement of refractive index>
An Abbe refractometer (manufactured by Atago) was used and measured according to JIS K 7142 (1981 version).

<Measurement of retardation and three-dimensional refractive index of film>
Measurement was performed using a fully automatic birefringence meter (trade name KOBRA-WR, manufactured by Oji Scientific Instruments).

Example 1 (Synthesis 1 of diisopropyl fumarate / cinnamic acid copolymer 1)
In a glass ampule with a capacity of 75 mL, 50 g (0.25 mol) of diisopropyl fumarate, 1.9 g (0.013 mol) of cinnamic acid, and 0.29 g (0.0016 mol) of tert-butyl peroxypivalate as a polymerization initiator. ), And after repeating nitrogen substitution and depressurization, sealing was performed under reduced pressure. The ampoule was placed in a thermostat at 50 ° C. and kept for 48 hours to carry out radical polymerization. After completion of the polymerization reaction, the polymer was taken out from the ampoule and dissolved with 400 g of tetrahydrofuran. This polymer solution was dropped into 3 L of methanol and precipitated, and then vacuum-dried at 80 ° C. for 10 hours to obtain 21 g of diisopropyl fumarate / cinnamic acid diester copolymer (yield: 41%). .

  The number average molecular weight of the obtained diisopropyl fumarate / cinnamic acid copolymer was 138,000.

The copolymer composition was confirmed by ¹ H-NMR measurement to be diisopropyl fumarate residue unit / cinnamic acid residue unit = 97/3 (mol%).

Example 2 (Synthesis 2 of diisopropyl fumarate / cinnamic acid copolymer)
In a glass ampule with a capacity of 75 mL, 50 g (0.25 mol) of diisopropyl fumarate, 6.5 g (0.044 mol) of cinnamic acid and 0.32 g (0.0018 mol) of tert-butyl peroxypivalate as a polymerization initiator. ), And after repeating nitrogen substitution and depressurization, sealing was performed under reduced pressure. The ampoule was placed in a thermostat at 50 ° C. and kept for 48 hours to carry out radical polymerization. After completion of the polymerization reaction, the polymer was taken out from the ampoule and dissolved with 400 g of tetrahydrofuran. This polymer solution was dropped into 3 L of methanol and precipitated, and then vacuum-dried at 80 ° C. for 10 hours to obtain 19 g of diisopropyl fumarate / cinnamic acid copolymer (yield: 34%).

  The number average molecular weight of the obtained diisopropyl fumarate / cinnamic acid copolymer was 122,000.

The copolymer composition was confirmed by ¹ H-NMR measurement to be diisopropyl fumarate residue unit / ethyl cinnamate residue unit = 90/10 (mol%).

Example 3 (Synthesis 3 of diisopropyl fumarate / cinnamic acid copolymer 3)
In a glass ampule with a capacity of 75 mL, 50 g (0.25 mol) of diisopropyl fumarate, 15.9 g (0.107 mol) of cinnamic acid and 0.21 g (0.0012 mol) of tert-butyl peroxypivalate as a polymerization initiator ), And after repeating nitrogen substitution and depressurization, sealing was performed under reduced pressure. The ampoule was placed in a thermostat at 50 ° C. and kept for 48 hours to carry out radical polymerization. After completion of the polymerization reaction, the polymer was taken out from the ampoule and dissolved with 400 g of tetrahydrofuran. This polymer solution was dropped into 3 L of methanol and precipitated, followed by vacuum drying at 80 ° C. for 10 hours to obtain 10 g of diisopropyl fumarate / cinnamic acid diester copolymer (yield: 15%). .

  The number average molecular weight of the obtained diisopropyl fumarate / cinnamic acid copolymer was 65,000.

The copolymer composition was confirmed by ¹ H-NMR measurement to be diisopropyl fumarate residue unit / cinnamic acid residue unit = 79/21 (mol%).

Example 4
The diisopropyl fumarate / cinnamic acid copolymer obtained in Example 1 was dissolved in methyl isobutyl ketone to give a 15% by weight resin solution, cast on a polyethylene terephthalate film with a coater, and dried at 130 ° C. for 10 minutes. By doing this, a film using a 30 μm thick diisopropyl fumarate / cinnamic acid diester copolymer was obtained.

  The obtained film had a total light transmittance of 92%, a haze of 0.5%, and a refractive index of 1.474.

  The three-dimensional refractive index was nx = 1.4716, ny = 1.4716, nz = 1.4773, and the obtained film showed a large value of refractive index in the thickness direction of nx = ny <nz. . Further, the out-of-plane retardation Rth was as negative as −170 nm, and the phase difference ratio (R450 / R550) (wavelength dependence) was 1.05. The absolute value of the film thickness and the out-of-plane retardation was 5.7.

  From these results, the obtained film has a negative birefringence, a large refractive index in the thickness direction, a large negative out-of-plane retardation, and a high out-of-plane retardation even in a thin film. It was suitable for a phase difference film.

Example 5
The diisopropyl fumarate / cinnamic acid copolymer obtained in Example 2 was dissolved in a toluene / methyl isobutyl ketone mixed solvent to give a 15% by weight resin solution, which was cast on a polyethylene terephthalate film by a coater. For 10 minutes to obtain a film using a 30 μm thick diisopropyl fumarate / cinnamic acid diester copolymer.

  The obtained film had a total light transmittance of 92%, a haze of 0.6%, and a refractive index of 1.482.

  The three-dimensional refractive index was nx = 1.4796, ny = 1.4796, nz = 1.4861 and the obtained film showed a large value of refractive index in the thickness direction of the film, nx = ny <nz. . The out-of-plane retardation Rth was as negative as −194 nm, and the phase difference ratio (R450 / R550) (wavelength dependence) was 1.07. The absolute value of the film thickness and the out-of-plane phase difference was 6.5.

  From these results, the obtained film has a negative birefringence, a large refractive index in the thickness direction, a large negative out-of-plane retardation, and a high out-of-plane retardation even in a thin film. It was suitable for a phase difference film.

Example 6
The diisopropyl fumarate / cinnamic acid copolymer obtained in Example 3 was dissolved in a mixed solvent of ethyl acetate / methyl isobutyl ketone to give a 15% by weight resin solution, which was cast on a polyethylene terephthalate film by a coater. A film using a 15 μm thick diisopropyl fumarate / cinnamic acid diester copolymer was obtained by drying at 10 ° C. for 10 minutes.

  The obtained film had a total light transmittance of 92%, a haze of 0.7%, and a refractive index of 1.495.

  The three-dimensional refractive index was nx = 1.4925, ny = 1.4925, nz = 1.4991, and the obtained film showed a large value of refractive index in the thickness direction of nx = ny <nz. . The out-of-plane retardation Rth was as negative as −99 nm, and the phase difference ratio (R450 / R550) (wavelength dependence) was 1.10. The absolute value of the film thickness and the out-of-plane retardation was 7.6.

  From these results, the obtained film has a negative birefringence, a large refractive index in the thickness direction, a large negative out-of-plane retardation, and a high out-of-plane retardation even in a thin film. It was suitable for a phase difference film.

Comparative Example 1 (Synthesis 1 of Fumarate Diester Copolymer (Diisopropyl Fumarate / Bis-2-ethylhexyl Fumarate Copolymer) 1)
In a 1 L autoclave equipped with a stirrer, a condenser tube, a nitrogen inlet tube and a thermometer, 1.6 g of hydroxypropylmethylcellulose (manufactured by Shin-Etsu Chemical Co., Ltd., trade name Metrose 60SH-50), 520 g of distilled water, 196 g of diisopropyl fumarate, bis of fumarate 84 g of 2-ethylhexyl and 1.9 g of t-butyl peroxypivalate as a polymerization initiator were added, nitrogen bubbling was performed for 1 hour, and then the suspension was kept at 50 ° C. for 24 hours while stirring at 400 rpm. Polymerization was performed. After cooling to room temperature, the resulting suspension containing polymer particles was filtered off and washed with distilled water and methanol to obtain a fumaric acid diester copolymer (yield: 66%).

The number average molecular weight of the obtained fumaric acid diester copolymer was 86,000. The copolymer composition was confirmed by ¹ H-NMR measurement to be diisopropyl fumarate residue unit / bis2-ethylhexyl fumarate residue unit = 84/16 (mol%).

Comparative Example 2 (Synthesis 2 of fumarate diester copolymer (diisopropyl fumarate / di-n-butyl fumarate copolymer) 2)
In a 1 L autoclave equipped with a stirrer, a condenser tube, a nitrogen inlet tube and a thermometer, 1.6 g of hydroxypropylmethylcellulose (manufactured by Shin-Etsu Chemical Co., Ltd., trade name Metrose 60SH-50), 520 g of distilled water, 230 g of diisopropyl fumarate, difumarate di After adding 50 g of n-butyl and 2.1 g of t-butyl peroxypivalate which is a polymerization initiator and carrying out nitrogen bubbling for 1 hour, the suspension is kept at 50 ° C. for 24 hours while stirring at 400 rpm. Polymerization was performed. The mixture was cooled to room temperature, and the resulting suspension containing polymer particles was filtered off and washed with distilled water and methanol to obtain a fumaric acid diester copolymer (yield: 80%).

The number average molecular weight of the obtained fumaric acid diester copolymer was 150,000. The copolymer composition was confirmed to be diisopropyl fumarate residue unit / di-n-butyl fumarate residue unit = 87/13 (mol%) by ¹ H-NMR measurement.

Comparative Example 3
The fumaric acid diester copolymer obtained in Comparative Example 1 was dissolved in a toluene / methyl ethyl ketone = 50/50 mixed solvent to form a 20% by weight resin solution, which was cast on a polyethylene terephthalate film with a coater, at 70 ° C. A film having a thickness of 30 μm was obtained by drying for 10 minutes.

  The obtained film had a total light transmittance of 92%, a haze of 0.6%, and a refractive index of 1.473.

  The three-dimensional refractive index was nx = 1.4723, ny = 1.4723, nz = 1.4738, and the obtained film had a large refractive index in the thickness direction of nx = ny <nz. However, the out-of-plane retardation was as small as −45 nm, and the absolute values of the film thickness and out-of-plane retardation were as small as 1.5.

Comparative Example 4
The fumaric acid diester copolymer obtained in Comparative Example 2 was dissolved in a toluene / methyl ethyl ketone = 50/50 mixed solvent to give a 20% by weight resin solution, which was cast on a polyethylene terephthalate film by a coater, at 70 ° C. A film having a thickness of 15 μm was obtained by drying for 10 minutes.

  The obtained film had a total light transmittance of 92%, a haze of 0.6%, and a refractive index of 1.472.

  The three-dimensional refractive index was nx = 1.4712, ny = 1.4712, nz = 1.4743, and the obtained film had a large refractive index in the thickness direction of nx = ny <nz. However, the out-of-plane retardation was as small as -47 nm, and the absolute values of the film thickness and the out-of-plane retardation were as small as 3.1.

nx: Refractive index in the fast axis direction in the film plane.
ny: Refractive index in the film in-plane direction orthogonal to nx.
nz: The refractive index in the thickness direction of the film.

Claims (5)

A diisopropyl fumarate-cinnamic acid copolymer comprising diisopropyl fumarate residue units and cinnamic acid residue units. 2. The diisopropyl fumarate-cinnamic acid copolymer according to claim 1, comprising 50 to 99 mol% of diisopropyl fumarate residue units and 1 to 50 mol% of cinnamic acid residue units. The number average molecular weight in terms of standard polystyrene is 30,000 to 500,000, The diisopropyl fumarate-cinnamic acid copolymer according to claim 1 or 2. A retardation film using the diisopropyl fumarate-cinnamic acid copolymer according to any one of claims 1 to 3. The relationship when the refractive index in the fast axis direction in the film plane is nx, the refractive index in the film in-plane direction orthogonal to it is ny, and the refractive index in the thickness direction of the film is nz is nx ≦ ny <nz. The retardation film according to claim 4, wherein the retardation film is provided.

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