KR102062939B1 - Polyimide and molded body thereof - Google Patents

Abstract

(화학식 중 R은 각각 독립적으로 수소 원자 또는 탄소 원자 수 1~6의 알킬기를 나타낸다. 단, 동일한 벤젠 고리에 결합하는 2개의 R 중 하나 이상은 알킬기이다.)An object of the present invention is to provide a polyimide having excellent transparency, having high heat resistance and low coefficient of linear thermal expansion, and exhibiting solvent processability (excellent solubility and film forming property) by a low hygroscopic solvent, and a method for producing the same.
In order to solve the said subject, the polyimide of this invention is characterized by including the structural unit represented by following formula (1).
[Formula 1]

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

Description

Polyimide and molded body thereof

The present invention relates to a polyimide.

In the field of display devices such as liquid crystal displays, organic electroluminescent (EL) displays, and electronic paper, a transparent electrode (ITO; Indium Tin Oxide) or a thin film transistor (TFT) is formed on a glass substrate during the manufacturing process. Electrodes and electronic elements, such as these, are formed. In order to form these elements, high heat resistance and excellent dimensional stability (low coefficient of thermal expansion) are required, and it is difficult to apply transparent materials other than glass substrates in the present technology.

On the other hand, in recent years, the request for weight reduction or flexibleization is increasing about these displays, and it is becoming difficult to respond with the conventional glass substrate. Accordingly, more flexible and flexible resin substrates are attracting attention as substrates replacing glass substrates. However, the resin substrate which combines the high transparency, dimensional stability, heat resistance, and the outstanding workability with which a glass substrate is equipped is not known until now.

Engineering plastics can be mentioned as a material candidate for a resin substrate. However, polyethersulfone having the highest heat resistance among the existing transparent engineering plastics has a problem in that its glass transition temperature (Tg) is 225 ° C., but the coefficient of linear thermal expansion is high.

On the other hand, as a material having excellent properties such as high heat resistance, excellent dimensional stability, mechanical strength, insulation and flexibility, the wholly aromatic polyimide is widely used in aerospace materials, heat resistant materials, electronic materials and the like. .

For example, it is reported that the polyimide obtained from the ester group containing tetracarboxylic dianhydride and diamine represented by following formula (5) and (6) shows high heat resistance and excellent dimensional stability (patent document 1, patent document 2, patent document). 3, etc.).

However, many of these polyimides and many wholly aromatic polyimides are strongly colored by intramolecular conjugates and intramolecular charge transfer interactions, making it difficult to achieve transparency as much as glass.

Thereby, polyimide with high transparency which suppresses coloring of a polyimide film is proposed. For example, by introducing a fluorine atom into the polyimide (see Non-Patent Document 1, etc.) or by using an alicyclic compound for one or both of the diamine component and the tetracarboxylic dianhydride component constituting the polyimide. A method of improving transparency by suppressing conjugate and charge transfer interactions has been proposed (see Patent Documents 4 and 5).

However, the polyimide into which the fluorine atom was introduced may sometimes have a high coefficient of linear thermal expansion. For example, the use of 4,4 '-(hexafluoroisopropylidene) diphthalic anhydride (hereinafter referred to as 6FDA) as the monomer is known to improve the transparency of the polyimide film, but its coefficient of linear thermal expansion is very high. This results in a lack of dimensional stability required at the time of manufacturing the device. Accordingly, when the fluorine-containing polyimide is used as a glass replacement substrate for the industrial field, a large linear thermal expansion coefficient difference occurs between an electrode such as ITO or TFT formed on the polyimide film, an electronic device, and the polyimide film, resulting in peeling or cracking. This causes a problem that the reliability of the electronic device is significantly lowered.

Furthermore, when a polyimide using an alicyclic compound is used as the diamine component, for example, an alicyclic diamine having high bendability such as 4,4'-methylenebis (cyclohexylamine), a colorless transparent polyimide film is obtained. There is a problem that causes a decrease in the temperature and an increase in the coefficient of thermal expansion. On the other hand, trans-1,4-diaminocyclohexane having a rigid structure is selected as the alicyclic diamine, and this and 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride (hereinafter referred to as BPDA). The technique which manufactures the polyimide film which has high transparency, high glass transition temperature (345 degreeC), and low linear thermal expansion coefficient (23 ppm / K) by combining these is disclosed (refer patent document 6). However, since this polyimide is insoluble in an organic solvent, it has the drawback of lacking solution processability.

As described above, many of the conventional polyimides are insoluble in organic solvents, and it is not easy to mold the polyimide itself. For this reason, tetracarboxylic dianhydride and diamine are generally equimolar reacted in an aprotic polar solvent such as N-methyl-2-pyrrolidone (NMP) to polymerize a high degree of polyimide precursor (polyamic acid). Then, this polyamic acid solution is cast and dried on a substrate to form a polyamic acid film, and then a polyimide film is produced by a two-step method of heating and dehydrating ring closure (thermal imidization) at 300 ° C or higher.

However, thermal imidization at high temperatures is one of the factors that color the film, which is not suitable for applications requiring high transparency, and also causes residual deformation in the film due to large reaction shrinkage that occurs during thermal imidization. There is also the possibility of causing warpage. Moreover, the problem that the haze by a film defect and a bubble is easy to generate | occur | produce by water by-product at the time of thermal imidization is also concerned.

Therefore, in order to avoid thermal imidation at high temperature, solvent-soluble polyimide is proposed (refer patent document 7 etc.). For example, 6FDA, 3,3 ', 4,4'-diphenylsulfontetracarboxylic dianhydride, etc. are used as tetracarboxylic dianhydride, and 1,3-bis (3-aminophenoxy is used as a diamine. ) Polyimide synthesize | combined from the combination of benzene, bis [4- (3-amino phenoxy) phenyl] sulfone, etc. shows solubility in an amide solvent etc. Solvent soluble polyimide commonly has a bulky substituent represented by a bent structure such as isopropylidene, sulfone, ether, meta bond, or trifluoromethyl group, thereby inhibiting aggregation and crystallization of polyimide chains. As a result, the solvent molecules easily enter between the polyimide chains.

In order to produce these soluble polyimides, tetracarboxylic dianhydride and diamine are equimolarly reacted in a high boiling point solvent, the solution is heated to 150 ° C. or higher in the presence of an azeotropic agent such as xylene to remove byproduct water from the system. The method of obtaining the polyimide of polymerization degree can be used. Furthermore, the method (chemical imidation method) which obtains a polyimide, without adding and heating a dehydration reagent, such as an acetic anhydride / pyridine, to a polyamic-acid solution is also applicable.

However, most of the solvent-soluble polyimides do not exhibit the low thermal expansion characteristics (dimension stability) required as the substrate for an image display device because the coefficient of thermal expansion is very high. In order to lower the coefficient of linear thermal expansion, it is necessary to align the polyimide main chain in parallel with the film plane (in-plane orientation). For this purpose, it is essential that the linearity and rigidity of the polyimide main chain are sufficiently high. Since such molecular design is contrary to the molecular design for improving solvent solubility described above, the compatibility of solvent solubility and low thermal expansion properties is in principle a very difficult task.

As a limited example having the above characteristics, BPDA having high linearity, pyromellitic dianhydride (hereinafter referred to as PMDA), and 2,2'-bis (trifluoromethyl) benzidine (hereinafter referred to as TFMB) are synthesized as main raw materials. It is reported that the polyimide to be solubilized only in N-methyl-2-pyrrolidone (hereinafter referred to as NMP), which is an amide solvent, and the cast film formed from the NMP solution has a relatively low coefficient of linear thermal expansion (patent). See Document 8.

As described above, most of the soluble polyimides are soluble only in amide solvents such as NMP, which have high solubility, and are almost insoluble in non-amide solvents having low solubility, particularly low hygroscopic solvents. However, when forming a film with a polyimide varnish composed of an amide solvent, a serious problem often occurs.

That is, in the case of continuous coating for a long time, the polyimide varnish absorbs moisture in the air due to the high hygroscopicity of the amide solvent, causing clogging of the coating device by thickening of the polyimide solution or precipitation of polyimide, which is often continuous coating. It can cause serious obstacles. Therefore, from the viewpoint of solution processability, it is preferable that the polyimide has high solubility in a low hygroscopic solvent, but a low thermally expandable polyimide dissolved in a solvent having a lower polarity than an amide solvent is not known until now.

As described above, the molecular design of the wholly aromatic polyimide which combines solution processability (low hygroscopic solvent solubility), low coefficient of thermal expansion, high heat resistance, and high transparency and satisfies all is very difficult. Practical transparent plastic substrate materials that satisfy all of the physical properties required as transparent substrates and transparent protective film materials such as substrates for electroluminescence (EL), substrates for electronic paper, and solar cell substrates are not known. not.

Japanese Patent Laid-Open No. 10-070157 Japanese Patent Publication No. 2006-013419 International Publication No. 2008/09110 Japanese Patent Application Laid-Open No. 07-56030 Japanese Patent Application Laid-Open No. 09-73172 Japanese Patent Publication No. 2002-161136 Japanese Patent Publication No. 2002-206057 Japanese Patent Publication No. 2006-206756

 Macromolecules, 24, 5001 (1991)

The present invention is a polyimide having excellent transparency, having high heat resistance and low coefficient of linear expansion, and exhibiting solvent processability (excellent solubility and film forming property) by a low hygroscopic solvent, a polyimide varnish formed by dissolving it in a low hygroscopic solvent, It is an object of the present invention to provide a film having a low coefficient of thermal expansion, high heat resistance and high transparency equivalent to the inorganic thin film obtained therefrom, and a method for producing the same.

MEANS TO SOLVE THE PROBLEM As a result of earnestly researching in order to solve the said subject, the present inventors discovered the polyimide containing the structural unit represented by following formula (1), This polyimide has the outstanding transparency, combines high heat resistance and low coefficient of linear thermal expansion, The present invention was completed by discovering that a polyimide varnish with a low hygroscopic solvent can be easily obtained by exhibiting solvent processability (excellent solubility and film forming property) by a low hygroscopic solvent and further obtaining a polyimide film having excellent transparency. It was.

The present invention is as follows.

1. Polyimide containing the structural unit represented by following formula (1).

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

2. Polyimide containing the structural unit represented by following formula (2).

3. The polyimide of 1 containing 70 mol% or more of the structural unit represented by following formula (1).

[Formula 1]

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

4. Polyimide varnish containing the polyimide and organic solvent in any one of 1-3.

5. The organic solvent is a low hygroscopic organic solvent selected from one or more of an ester solvent, an ether solvent, a carbonate solvent, a glycol solvent, a phenol solvent, and a ketone solvent, and the solid content concentration of the polyimide is 5% by weight. The polyimide varnish of 4 which is more than%.

6. Polyimide molded body containing a structural unit represented by following formula (1).

[Formula 1]

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

7. Polyimide molded body of 6 whose molded object is a film.

8. Polyimide film obtained by apply | coating the polyimide varnish of 4 or 5 on a board | substrate, drying, and peeling from a board | substrate.

9. The polyimide film according to 7 or 8, wherein the polyimide film has a light transmittance of 45% or more at 400 nm when the film thickness is 10 μm, or a total light transmittance of 80% or more when the film thickness is 20 μm.

10. The polyimide film according to 7 or 8, wherein the polyimide film has a light transmittance at 400 nm of 45% or more when the film thickness is 10 μm and a total light transmittance of 70% or more when the film thickness is 10 μm. .

11. A method for synthesizing a polyimide comprising a structural unit represented by the following general formula (1) in which a heating temperature is less than 150 ° C when imidating a polyamide precursor.

[Formula 1]

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

Since the polyimide of the present invention has two biphenylene structures bonded in the para position in the structural unit, the main chain structure becomes very linear and rigid, and reflecting this, the polyimide film obtained from such polyimide has a low It has a coefficient of thermal expansion and high heat resistance (high glass transition temperature).

Moreover, in the said field, it is known that the molecular design which has two parabiphenylene structures with high linearity will drastically reduce solvent solubility. However, the polyimide of this invention is excellent in solvent solubility and also excellent in transparency. This is because the polyimide of the present invention has a methyl group and a trifluoromethyl group in the 2,2 'position of the biphenylene group constituting the polyimide, so that the phenylene ring constituting the biphenylene is twisted due to the steric hindrance effect. It is thought that transparency is improved because the solvent solubility is greatly improved by suppressing agglomeration of the polymer chains and electron conjugates, and the charge transfer interaction peculiar to polyimide, which causes coloration, is also suppressed. Moreover, it is thought that the alkyl group of the 3rd position, 3 'position, 5th position, and / or 5' position of a biphenylene group contributes to the same stereo effect.

As described above, the polyimide of the present invention is soluble in various solvents, and in particular, when a low hygroscopic solvent is used as the solvent, the resulting polyimide varnish exhibits high solution stability, so that it is stably polyimide regardless of the coating conditions during film formation. It can be processed into a film. Moreover, since the obtained polyimide film has the high heat resistance, the low coefficient of linear thermal expansion, and the high light transmittance (transparency) which the polyimide has, the board | substrate for liquid crystal displays, the board | substrate for organic electroluminescent (EL), and the board | substrate for electronic papers It is useful as a transparent substrate and transparent protective film material, such as a transparent substrate for display, a transparent protective film material, and a solar cell board | substrate.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the infrared absorption spectrum of the polyimide obtained in Example 1.
2 is a diagram showing an infrared absorption spectrum of the polyimide obtained in Example 5. FIG.
3 is a diagram showing an infrared absorption spectrum of the polyimide obtained in Example 6. FIG.

The polyimide of this invention is a polyimide containing the structural unit represented by following formula (1).

[Formula 1]

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

In the formula, when R is an alkyl group, a linear or branched alkyl group having 1 to 6 carbon atoms is represented, and for example, methyl group, ethyl group, isopropyl group, n-propyl group, t-butyl group, isobutyl Group, n-hexyl group, n-pentyl group, etc. are mentioned. Preferably it is an alkyl group of 1-4 carbon atoms, More preferably, it is a methyl group. Moreover, it is preferable that both R couple | bonded with the same benzene ring are alkyl groups.

Therefore, a more preferable polyimide is polyimide containing the structural unit represented by following formula (2).

[Formula 2]

As described above, in order to lower the thermal expansion of a conventional polyimide to polyimide film, it is generally necessary to make the main chain structure as straight as possible and to increase the rigidity, and to suppress the decrease in the linearity of the main chain accompanying structural changes. However, this molecular design is disadvantageous in terms of solvent solubility.

In contrast, the polyimide of the present invention has a low thermal expansion property and high solubility in solvent by introducing a parabiphenylene group having a large backside angle twisted through an ester bond to increase the solvent solubility while making the main chain skeleton as straight and rigid as possible. It solves the very difficult problem of achieving transparency at the same time.

The polyimide represented by the general formula (1) or the general formula (2) of the present invention is not particularly limited, but, for example, tetracarboxylic dianhydride represented by the following general formula (3) and 2 represented by the following general formula (7) as diamine Reacting with 2'-bis (trifluoromethyl) benzidine (hereinafter sometimes abbreviated as TFMB) to obtain a polyamic acid containing a structural unit represented by the following formula (4); It can manufacture through the process of imidating.

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group, and the bonding position of the ester group is relative to the amide bond. Meta position or para position.)

In addition, the tetracarboxylic dianhydride represented by the formula (3) is obtained by a known esterification reaction using biphenyl-4,4 'diol and trimellitic acid represented by the following formula (8).

(In the formula, each R independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, provided that at least one of two R's bonded to the same benzene ring is an alkyl group.)

Such a method for producing the polyimide of the present invention will be further described in detail by way of example a polyimide including the structural unit represented by the following formula (2) when all R in the formula (1) is a methyl group. Moreover, also when R is another alkyl group or a hydrogen atom, it can manufacture similarly to the polyimide containing the structural unit represented by General formula (2).

[Formula 2]

The polyimide represented by the formula (2) is prepared using tetracarboxylic dianhydride represented by the following formula (9).

Tetracarboxylic dianhydride represented by the formula (9) is a diol represented by the following formula 10, that is, 2,2 ', 3,3', 5,5'-hexamethyl-biphenyl-4,4'-diol (hereinafter HM44BP may be abbreviated) or diacetate and trimellitic acid thereof to prepare by a known esterification reaction.

As trimellitic acid, trimellitic anhydride, a trimellitic anhydride halide, etc. are mentioned.

The structural feature of the tetracarboxylic dianhydride represented by the above formula (9), which is a raw material of the polyimide of the present invention (hereinafter sometimes abbreviated as TAHMBP), is characterized by the fact that the methyl substituent is bonded to the 2,2 'position of the central biphenylene group. This is a point where the biphenylene group is twisted greatly, and both ester groups connecting the biphenylene group and the phthalimide moiety are bonded at the para position. It is thought that this makes it possible to simultaneously realize heat resistance, low hygroscopic solvent solubility, transparency, and a linear thermal expansion coefficient equivalent to that of the inorganic thin film.

When polymerizing the precursor (polyamic acid) of the polyimide of the present invention, aromatic or aliphatic other than tetracarboxylic dianhydride represented by the above formula (9) in a range that does not significantly impair polymerization reactivity and required properties of the polyimide. Tetracarboxylic dianhydride can be used together as a copolymerization component.

Although it does not specifically limit as aromatic tetracarboxylic dianhydride which can be used at that time, For example, pyromellitic dianhydride, 3,3 ', 4,4'-biphenyl tetracarboxylic dianhydride, hydroquinone bis (tri Mellitate Anhydride), Methylhydroquinone-bis (Trimellitate Anhydride), 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid Dianhydrides, 3,3 ', 4,4'-benzophenonetetracarboxylic dianhydride, 3,3', 4,4'-biphenylethertetracarboxylic dianhydride, 3,3 ', 4,4 '-Biphenylsulfontetracarboxylic dianhydride, 2,2'-bis (3,4-dicarboxyphenyl) hexafluoropropanoic dianhydride, 2,2'-bis (3,4-dicarboxyphenyl) Propanoic dianhydride, and the like.

Although it does not specifically limit as aliphatic tetracarboxylic dianhydride, For example, it is bicyclo [2.2.2] oct-7-ene-2,3,5,6- tetracarboxylic dianhydride, 5- as an alicyclic thing. (Dioxotetrahydrofuryl-3-methyl-3-cyclohexene-1,2-dicarboxylic acid anhydride, 4- (2,5-dioxotetrahydrofuran-3-yl) tetraline-1,2 -Dicarboxylic acid anhydride, tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride, bicyclo-3,3 ', 4,4'-tetracarboxylic dianhydride, 1,2, And 3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, etc. Moreover, it is also possible to use these two or more types together.

Of these tetracarboxylic dianhydrides, tetracarboxylic dianhydrides having a rigid and linear structure, ie, pyromellitic dianhydrides, 3,3 ', 4,4'-ratios in terms of low thermal expansion of polyimide films. Phenyltetracarboxylic dianhydride is suitable as the copolymerization component, and in particular, 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride is more preferable as tetracarboxylic dianhydride because of polymerization reactivity and availability. Do.

Content of the said aromatic or aliphatic tetracarboxylic dianhydride used together with the tetracarboxylic dianhydride represented by the said Formula (9) is 0-30 mol% of all the tetracarboxylic dianhydride usage-amount.

The polyimide containing the structural unit represented by the said Formula (2) of this invention is obtained by making 2,2'-bis (trifluoromethyl) benzidine (TFMB) react with such tetracarboxylic dianhydride.

As described above, in the polyimide of the present invention, when the raw material diamine is TFMB, the intermolecular force decreases due to the presence of the 2,2 'position of the trifluoromethyl group in the TFMB, thereby increasing the low hygroscopic solvent solubility of the polyimide. Moreover, since trifluoromethyl group also acts as an electron withdrawing group, it contributes to suppressing the charge transfer interaction which causes coloring, and to improve transparency of a polyimide film. It is also believed that the rigid parabiphenylene group in TFMB contributes to the low thermal expansion expression of the polyimide.

When superposing | polymerizing the precursor (polyamic acid) of the polyimide of this invention, aromatic or aliphatic diamines other than TFMB can be used together as a copolymerization component in the range which does not impair polymerization reactivity and the required characteristic of a polyimide significantly.

Although it does not specifically limit as aromatic diamine which can be used then, For example, p-phenylenediamine, m-phenylenediamine, 2, 4- diamino toluene, 2, 5- diamino toluene, 2, 4- diamino xylene, 2,4-diaminodurene, 4,4'-diaminodiphenylmethane, 4,4'-methylenebis (2-methylaniline), 4,4'-methylenebis (2-ethylaniline), 4,4 '-Methylenebis (2,6-dimethylaniline), 4,4'-methylenebis (2,6-diethylaniline), 4,4'-diaminodiphenylether, 3,4'-diaminodiphenyl Ether, 3,3'-diaminodiphenylether, 2,4'-diaminodiphenylether, 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, 4,4 ' -Diaminobenzophenone, 3,3'-diaminobenzophenone, 4,4'-diaminobenzanilide, 4-aminophenyl-4'-aminobenzoate, benzidine, 3,3'-dihydroxybenzidine, 3,3'-dimethoxybenzidine, o-tolidine, m-tolidine, 1,4-bis (4-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 1 , 3-bis (3-aminophenoxy) benzene, 4,4'-bis (4-aminophenoxy) biphenyl, bis (4- (3-aminophenoxy) phenyl) sulfone, bis (4- (4 -Aminophenoxy) phenyl) sulfone, 2,2-bis (4- (4-aminophenoxy) phenyl) propane, 2,2-bis (4- (4-aminophenoxy) phenyl) hexafluoropropane, 2, 2-bis (4-aminophenyl) hexafluoro propane, p-terphenylenediamine, etc. are mentioned.

In addition, as aliphatic diamine, it is a chain aliphatic to alicyclic diamine, Although it does not specifically limit as alicyclic diamine, For example, 4,4'- methylenebis (cyclohexylamine), isophorone diamine, trans-1, 4- diamino cyclo Hexane, cis-1,4-diaminocyclohexane, 1,4-cyclohexanebis (methylamine), 2,5-bis (aminomethyl) bicyclo [2.2.1] heptane, 2,6-bis (amino Methyl) bicyclo [2.2.1] heptane, 3,8-bis (aminomethyl) tricyclo [5.2.1.0] decane, 1,3-diaminoadamantane, 2,2-bis (4-aminocyclohexyl) Although it does not specifically limit as propane, 2, 2-bis (4-amino cyclohexyl) hexafluoro propane, and a chain aliphatic diamine, For example, 1, 3- propanediamine, 1, 4- tetramethylenediamine, 1, 5- Pentamethylenediamine, 1,6-hexamethylenediamine, 1,7-heptamethylenediamine, 1,8-octamethylenediamine, 1,9-nonamethylenediamine, diaminosiloxane And the like. Moreover, it is also possible to use two or more types together. At this time, content of the said diamine is the range of 0-30 mol% of the total amount of diamines used.

As a solvent used when superposing | polymerizing the precursor (polyamic acid) of the polyimide of this invention, N, N- dimethylformamide, N, N- dimethylacetamide, N-methyl- 2-pyrrolidone, dimethyl sulfoxide, etc. Although an aprotic solvent of is preferable, any solvent can be used without any problem as long as the raw material monomer, the resulting polyimide precursor, and the imidized polyimide are dissolved, and the solvent structure is not particularly limited.

Specifically, for example, amide solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, γ-butyrolactone, γ-valerolactone, δ- Ester solvents such as valerolactone, γ-caprolactone, ε-caprolactone, α-methyl-γ-butyrolactone, butyl acetate, ethyl acetate, isobutyl acetate, carbonate solvents such as ethylene carbonate and propylene carbonate, and diethylene Glycol solvents such as glycol dimethyl ether, triethylene glycol, triethylene glycol dimethyl ether, phenol solvents such as phenol, m-cresol, p-cresol, o-cresol, 3-chlorophenol and 4-chlorophenol, and cyclopenta Ketone solvents such as paddy, cyclohexanone, acetone, methyl ethyl ketone, diisobutyl ketone, methyl isobutyl ketone, tetrahydrofuran, 1,4-dioxane, dimethoxyethane, diethoxyethane, dibutyl ether, etc. Ether solvents, acetophenone, other general solvents, 3-dimethyl-2-imidazolidinone, sulfolane, dimethyl sulfoxide, propylene glycol methyl acetate, ethyl cellosolve, butyl cellosolve, 2-methyl cellosolve acetate, ethyl cellosolve acetate, butyl cello Solvent acetate, butanol, ethanol, xylene, toluene, chlorobenzene, terebin oil, mineral spirit, petroleum naphtha solvent, etc. can also be used, You may use them in mixture of 2 or more types.

After polyaddition of tetracarboxylic dianhydride (TAHMBP) represented by the formula (9) and TFMB to obtain a polyimide precursor, the resulting polyimide of the present invention can be obtained by imidating the polyimide precursor.

The polyimide of the present invention has a solution processability (low hygroscopic solvent solubility), low linear thermal expansion coefficient equivalent to that of an inorganic thin film, based on the chemical structural characteristics of the linear, rigidity, suitable substituent, and three-dimensional twist of the polymer chain. It is possible to set it as the material which has the physical property which cannot be obtained with the conventional material which has heat resistance and high transparency.

Usually, the polymerization reactivity of tetracarboxylic dianhydride and diamine has a big influence on the toughness of the polyimide film finally obtained. If the polymerization reactivity is not high enough, a high polymer may not be obtained, resulting in low entanglement of the polymer chains, resulting in a weak polyimide film. In this regard, since TAHMBP and TFMB used in the present invention exhibit sufficiently high polymerization reactivity, there is no such concern.

The method of synthesize | combining the polyimide of this invention is not specifically limited, A well-known method can be applied suitably. Specifically, it can synthesize | combine by the following method, for example. In the process of obtaining a polyimide precursor (polyamic acid), firstly dissolve TFMB in a polymerization solvent in a reaction vessel, gradually add equimolar TAHMBP powder substantially equal to TFMB to this solution, and then use a mechanical stirrer or the like to obtain a temperature of 0 to 100 ° C. It is in the range of 0.5 to 150 hours, preferably 1 to 48 hours at 20 to 60 ℃.

At this time, the raw material monomer concentration is usually in the range of 5 to 50% by weight, preferably in the range of 10 to 40% by weight. By polymerizing in such a monomer concentration range, a uniform and high degree of polymerization polyimide precursor (polyamic acid) can be obtained. When the polymerization degree of a polyimide precursor increases too much and it becomes difficult to stir a polymerization solution, it can also be diluted with the same solvent suitably. In view of the toughness of the polyimide film, the degree of polymerization of the polyimide precursor is preferably as high as possible. By polymerizing in the above monomer concentration range, the degree of polymerization of the polymer is sufficiently high, and the solubility of the raw material monomer and the produced polymer can also be sufficiently secured.

When the polymerization is carried out at a concentration lower than the above range, the degree of polymerization of the polyimide precursor may not be sufficiently high, and when the polymerization is carried out at a higher concentration than the above monomer concentration range, the dissolution of the monomer and the resulting polymer may be insufficient. In addition, when aliphatic diamine is used, salt formation often occurs at the initial stage of polymerization, and polymerization is prevented. In order to increase the degree of polymerization so as to suppress salt formation, it is preferable to manage the monomer concentration at the time of polymerization in the above suitable concentration range.

Next, the process of imidating the obtained polyimide precursor (polyamic acid) is demonstrated. The imidation method of the polyimide precursor for obtaining the polyimide of this invention can use well-known methods, such as the thermal imidation method of thermally dehydrating and ring closing, and the chemical imidation method using a dehydrating agent.

However, it is preferable to imidize under mild conditions such as chemical imidization which does not require high temperature heat treatment. In the case of methods other than chemical imidization, for example, thermal imidization, tetracarboxylic dianhydride and diamine are equimolar reacted in a high boiling point solvent and heated to 150 ° C. or higher in the presence of azeotropic agents such as xylene to produce byproducts. Although the polyimide of high degree of polymerization can be removed in a system and can be obtained in a solution state, at 150 degreeC or more heating, a solvent etc. are colored and since this coloring component may cause coloring of a film, it is not preferable.

That is, the imidation method of the polyimide precursor (polyamic acid) of this invention specifically, for example, the polyimide which made the said polyimide precursor solution the suitable solution viscosity which is easy to stir with the same solvent used at the time of superposition | polymerization, for example. As a precursor solution, while stirring with a mechanical stirrer or the like, a dehydration ring closing agent (chemical imidating agent) consisting of an anhydride of an organic acid and a tertiary amine as a basic catalyst is added dropwise, and the temperature is 0 to 100 ° C, preferably 10 to 50 ° C. Chemical imidation is completed by stirring for 1 to 72 hours at.

Although it does not specifically limit as organic acid anhydride which can be used then, acetic anhydride, a propionic anhydride, etc. are mentioned. Acetic anhydride is suitably used because of ease of handling and purification of reagents. Moreover, although a pyridine, triethylamine, quinoline, etc. can be used as a basic catalyst, pyridine is used suitably, although it is easy to handle or isolate a reagent, It is not limited to these. The amount of the organic acid anhydride in the chemical imidating agent is in the range of 1 to 10 times mole of the theoretical dehydration amount of the polyimide precursor, and more preferably 1 to 5 times mole. The amount of the basic catalyst is in the range of 0.1 to 2 times the molar amount, and more preferably in the range of 0.1 to 1 times the mole relative to the amount of the organic acid anhydride.

In the reaction solution after chemical imidization, by-products (hereinafter referred to as impurities) such as chemical imidating agents and carboxylic acids are mixed, and it is necessary to remove these to purify the polyimide. Purification can use a well-known method. For example, as the simplest method, the imidized reaction solution is added dropwise to a large amount of poor solvents while stirring to precipitate polyimide, and then the polyimide powder is recovered, washed repeatedly until impurities are removed, and dried under reduced pressure to obtain polyimide. The method of obtaining a powder can be applied.

The solvent that can be used at this time is not particularly limited as long as it is a solvent that can precipitate impurities and efficiently remove polyimide, and is easy to dry. For example, water or alcohols such as methanol, ethanol and isopropanol are suitable, and these are mixed. May be used. If the concentration of the polyimide solution when dropping and depositing in the poor solvent is too high, the precipitated polyimide may form a particle mass and impurities may remain in the coarse particles or the time for dissolving the obtained polyimide powder in a solvent. There is a risk of requiring a long time.

On the other hand, if the concentration of the polyimide solution is too dilute, a large amount of poor solvent is required, which is not preferable because the environmental load increase by the waste solvent treatment and the manufacturing cost increase. Therefore, the density | concentration of the polyimide solution at the time of dripping in a poor solvent is 20 weight% or less, More preferably, it is 10 weight% or less. At this time, the amount of the poor solvent to be used is preferably equal to or greater than the polyimide solution, and preferably 1.5 to 3 times the amount. The obtained polyimide powder is recovered and the residual solvent is removed by vacuum drying or hot air drying. The drying temperature and time are not limited as long as the polyimide does not deteriorate and the residual solvent does not decompose, and drying is preferably performed for 48 hours or less in a temperature range of 30 to 200 ° C.

Although the polyimide of this invention can be suitably selected according to a use as an intrinsic viscosity, it does not have a restriction | limiting in particular, For example, when using as a polyimide film, it is preferable as an intrinsic viscosity of a polyimide from a viewpoint of toughness and handling of a solution, Preferably it is 0.1. It is the range of -10.0 dL / g, More preferably, it is the range of 0.5-5.0 dL / g.

In addition, the composition of the present invention represented by the formula (1) or (2) using tetracarboxylic dianhydride other than tetracarboxylic dianhydride which is the raw material of the present invention and / or diamine components other than the diamine which is the raw material of the present invention during the reaction. When using as a polyimide copolymer containing a unit, the polyimide copolymer containing 70 mol% or more of the structural unit represented by General formula (1) or general formula (2) is preferable.

Moreover, since the polyimide of this invention is excellent in solubility with respect to a solvent, it can melt | dissolve in various organic solvents and can be set as a polyimide varnish. Moreover, the obtained varnish can be used as a molded object, such as a polyimide film and a laminated body, for example.

In addition, since the polyimide of the present invention is usually obtained as a powder, it can be molded as the resin itself, and can be used as various molded products such as electronic parts, connectors, films, laminates, etc. by adopting a well-known molding method appropriately according to the purpose. .

When dissolving the polyimide of this invention in an organic solvent and using it as a varnish, a solvent can be suitably selected according to the use use and processing conditions of a varnish as an organic solvent. For example, in the case of continuous coating over a long time, the solvent in the polyimide solution may absorb moisture in the air, causing the polyimide to precipitate, thereby reducing low concentrations such as triethylene glycol dimethyl ether, γ-butyrolactone, or cyclopentanone. Preference is given to using hygroscopic solvents. Therefore, the polyimide of this invention can select the various solvent which shows low hygroscopicity, or mixed solvent.

The low hygroscopic solvent used is not particularly limited, and for example, γ-butyrolactone, γ-valerolactone, δ-valerolactone, γ-caprolactone, ε-caprolactone, α-methyl-γ-butyro Ester solvents such as lactone, butyl acetate, ethyl acetate, isobutyl acetate, carbonate solvents such as ethylene carbonate and propylene carbonate, glycol solvents such as diethylene glycol dimethyl ether, triethylene glycol, triethylene glycol dimethyl ether, and phenol phenolic solvents such as m-cresol, p-cresol, o-cresol, 3-chlorophenol and 4-chlorophenol, cyclopentanone, cyclohexanone, acetone, methyl ethyl ketone, diisobutyl ketone and methyl isobutyl Ketone solvents such as ketones, ether solvents such as tetrahydrofuran, 1,4-dioxane, dimethoxyethane, diethoxyethane, dibutyl ether, acetophenone, 1,3-dimethyl-2- Imidazolidinone, sulfolane, dime Sulfoxide, propylene glycol methyl acetate, ethyl cellosolve, butyl cellosolve, 2-methyl cellosolve acetate, ethyl cellosolve acetate, butyl cellosolve acetate, chloroform, butanol, ethanol, xylene, toluene, chlorobenzene , Turpentine oil, mineral spirits, petroleum naphtha solvents, and the like may also be used, and two or more kinds thereof may be mixed and used. In addition, even in the case of amide solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, which are hygroscopic solvents, the precipitation of polyimide can be suppressed by combining with the low hygroscopic solvent. Do.

Next, the polyimide varnish of this invention and the manufacturing method of the polyimide film obtained by shape | molding it are described in detail.

When the polyimide of the present invention is dissolved in a solvent to form a varnish, the solid content concentration can be appropriately selected depending on the use of the varnish, and there is no particular limitation. For example, when making into a film, although it changes also with the molecular weight of a polyimide, a manufacturing method, or the thickness of the film to manufacture, it is preferable to make solid content concentration into 5 weight% or more. If the solid content concentration is too low, it is difficult to form a film having a sufficient film thickness. On the contrary, if the solid content concentration is high, the solution viscosity may be too high and coating may be difficult. As a method when melt | dissolving the polyimide of this invention in a solvent, for example, the polyimide powder of this invention is added, stirring a solvent, in air or an inert gas in the temperature range from room temperature to the boiling point of a solvent-for 1 hour- It can melt | dissolve over 48 hours and can be set as a polyimide solution.

As a most preferable form which manufactures a film using the polyimide of this invention, a polyimide varnish is apply | coated on a support body, such as a glass substrate, for example using a well-known method, for example, a doctor blade, etc., and dried, and then polyimide Make a film. It is preferable that it is 30 ppm / K or less, and, as for the linear thermal expansion coefficient of the polyimide film obtained by this, it is more preferable that it is 25 ppm / K or less. Moreover, it is preferable that the glass transition temperature as an index of heat resistance is 250 degreeC or more, and it is more preferable that it is 270 degreeC or more from film forming conditions of inorganic thin films, such as ITO.

Regarding the transparency of the polyimide film of the present invention, the total light transmittance when the film thickness is 20 μm is preferably 80% or more, and more preferably 85% or more.

In addition, the light transmittance of 400 nm in the case of a film thickness of 10 μm is preferably 45% or more, more preferably 50% or more, still more preferably 60% or more, particularly preferably 80% or more, and furthermore, transparency For excellent reasons, it is preferable that the total light transmittance when the film thickness is 10 µm is 70% or more, more preferably 80% or more, and even more preferably 85% or more.

In this case, the preferred range of 400 nm or total light transmittance when the film thickness is 10 μm or 20 μm is that the polyimide film having a film thickness of 10 μm or 20 μm is used as Lambert Berg's law or the like. When the transmittance converted into the case of the film thickness of 10 micrometers or 20 micrometers based on it, or the transmittance | permeability measured by processing the film thickness to 10 micrometers or 20 micrometers, when it falls in the said preferable range, the polyimide film in such a case is also included. I shall do it.

In the case of the same quality polyimide film, the thinner the film thickness, the higher the transmittance of the film. It is included in a preferable polyimide film without the need for a measurement.

As a conversion method, for example, it is assumed that the product having a light transmittance of 400 nm of the polyimide film having a thickness of 21 μm obtained in Example 2 described later is 69.5%, but the product of the molar extinction coefficient and the concentration is constant. The conversion value in the case of 10 micrometers is 84.0% based on the law.

Moreover, additives, such as a mold release agent, a filler, a silane coupling agent, a crosslinking agent, a terminal sealing agent, antioxidant, an antifoamer, a leveling agent, can be added to the polyimide varnish of this invention as needed.

A polyimide film can be formed by forming into a film by a well-known method and drying using the obtained polyimide varnish. For example, a polyimide varnish is cast on a support such as a glass substrate using a doctor blade or the like, and is usually in the range of 40 to 300 ° C using a hot air dryer, an infrared drying furnace, a vacuum dryer, an inert oven, or the like. Preferably, a polyimide film can be formed by drying in 50-250 degreeC.

Example

Although an Example demonstrates this invention concretely below, it is not limited to these Examples. In addition, the physical property value in the following example was measured by the following method.

(Assessment Methods)

The material characteristic values described in this specification are obtained by the following evaluation method.

<Infrared absorption spectrum>

The infrared absorption spectrum of tetracarboxylic dianhydride was measured by the KBr method using a Fourier transform infrared spectrophotometer FT / IR350 (manufactured by Jasco). In addition, about the infrared absorption spectrum of polyimide, the thin film sample (about 5 micrometers in thickness) was produced and measured.

< ¹ H-NMR spectrum>

The ¹ H-NMR spectrum of tetracarboxylic dianhydride and chemical imidized polyimide powder in deuterated dimethyl sulfoxide was measured using Fourier transform nuclear magnetic resonance JNM-ECP400 (manufactured by JEOL). Tetramethylsilane was used as a standard.

<Differential scanning calving analysis (melting point)>

Melting | fusing point of tetracarboxylic dianhydride was measured at the temperature increase rate of 2 degree-C / min in nitrogen atmosphere using the differential scanning calorimeter DSC3100 (Bruker AXS company). The higher the melting point and the sharper the melting peak, the higher the purity.

<Intrinsic viscosity>

The reduced viscosity was measured at 30 ° C. using 0.5 wt% polyimide precursor solution or polyimide solution using an Ostwald viscometer. This value was considered inherent viscosity.

<Solubility Test for Organic Solvents of Polyimide Powder>

About 0.1 g of polyimide powder, 9.9 g (solid content concentration of 1 weight%) of the organic solvent of Table 2 was put into the sample tube, and it stirred for 5 minutes using the test tube mixer, and the dissolution state was visually confirmed. Chloroform (CF), acetone, tetrahydrofuran (THF), 1,4-dioxane (DOX), ethyl acetate, cyclopentanone (CPN), cyclohexanone (CHN), N, N-dimethylformamide as solvent (DMF), N, N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), m-cresol, dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL), triethylene Glycol dimethyl ether (Tri-GL) was used. The evaluation results indicated that the case of dissolving at room temperature was ++, the case of dissolving by heating, and the uniformity was maintained even after cooling to room temperature, +, the case of swelling / partially dissolving, and the case of insoluble as-. .

<Hygroscopic stability evaluation of polyimide varnish>

The density | concentration of the polyimide solution was 9 to 13 weight%, 2 mL of this solution was dripped on the glass substrate, and it left still for 24 hours in the environment of 40% of the relative humidity. After standing for 24 hours, (circle) and the polyimide precipitated and the solution whitened the state where there was no change compared with immediately after dripping the polyimide solution to x. If there is no change in a polyimide solution even after 24 hours in 40% of relative humidity, it shows that the coating property at the time of manufacturing a polyimide film is excellent.

<Glass transition temperature: Tg>

The glass transition temperature of the polyimide film was calculated | required from the loss peak at the frequency of 0.1 Hz and a temperature increase rate of 5 degree-C / min by dynamic viscoelasticity measurement using the Bruker AXS company thermomechanical analyzer (TMA4000).

<Linear thermal expansion coefficient: CTE>

The coefficient of linear thermal expansion of the polyimide film was made to Bruker AXS Co., Ltd. TMA4000 (sample size width 5 mm, length 15 mm), and the load was set to a film thickness (μm) x 0.5 g, and then to 150 ° C at 5 ° C / min. It heated up to 20 degreeC after raising a temperature (1st temperature rise), and also heated up at 5 degree-C / min (second temperature increase), and computed from the TMA curve at the time of 2nd temperature increase. Linear thermal expansion coefficient was calculated | required as an average value between 100-200 degreeC.

<5% weight reduction temperature: Td ⁵ >

The temperature at which the initial weight of the polyimide film (thickness 20 μm) decreased by 5% in the temperature rising process at a temperature rising rate of 10 ° C./min in air using a Bruker AXS thermogravimetric analyzer (TG-DTA2000) Measured. Higher values indicate higher thermal stability.

<Transmittance of the polyimide membrane: T ₄₀₀ >

The light transmittance at the wavelength of 400 nm was measured by measuring the light transmittance at 200-700 nm of the polyimide film (thickness 20 μm) using an ultraviolet visible near infrared spectrophotometer (V-650) manufactured by Jasco. Used as an indicator. In addition, the wavelength (cutoff wavelength) at which the transmittance is 0.5% or less was also obtained.

<Birefringence: Δn>

Using an Abe Refractometer (Abe 1T) manufactured by Atago, the refractive index _in the direction parallel to the polyimide film plane (n _in ) and in the vertical direction (film thickness direction) (n _out ) was measured using an Abe refractometer (using a sodium lamp, wavelength 589 nm). ), Birefringence (Δn = n _in -n _out ) was obtained from these refractive index differences. The higher the birefringence value, the higher the in-plane orientation of the polymer chain.

<Water absorption rate>

The polyimide film (film thickness of 20 to 30 μm) vacuum dried at 50 ° C. for 24 hours was immersed in water at 24 ° C. for 24 hours, and then the excess moisture was thoroughly wiped off with Kimwipes and the water absorption rate from the weight increase ( %) Was obtained. For most applications the lower this value is, the better.

<Dielectric constant: ε _opt >

Based on the average refractive index [n _av = (2n _in + n _out ) / 3] of a polyimide film using the Abe refractometer (Abe 1T) by an Atago company, a polyimide film by following formula: epsilon _cal = 1.1 * n _av ² The permittivity (ε _opt ) of was calculated.

<Tensile modulus (Young's modulus), breaking strength, elongation at break>

Using TENSILON UTM-2 (manufactured by A & D Co., Ltd.), a tensile test (drawing speed: 8 mm / min) was performed on a test piece (3 mm x 30 mm) of a polyimide film to obtain an elastic modulus from an initial gradient of the stress-strain curve. The elongation at break (%) was determined from the elongation rate when the film was broken. The higher the elongation at break, the higher the toughness of the film. In addition, breaking strength was calculated | required from the stress at the time of test piece fracture.

<Synthesis example 1>

Synthesis of tetracarboxylic dianhydride represented by the formula (9)

(synthesis)

Tetracarboxylic dianhydride (TAHMBP) represented by Formula 9 was synthesized as follows. 8.8493 g (42.0 mmol) of anhydrous trimellitic acid chloride was added to a branched flask, dissolved in 30 mL of dehydrated tetrahydrofuran (THF) at room temperature, septum sealed to adjust solution A (solute concentration 24.9 wt%). ). Further in a separate flask, 5.4044 g (20.0 mmol) of 2,2 ', 3,3', 5,5'-hexamethyl-biphenyl-4,4'-diol (HM44BP) was added to 38 mL of dehydrated THF at room temperature. Was dissolved in (solvent concentration 14.5% by weight), and 3.9 mL (48 mmol) of pyridine was added thereto and septum sealed to adjust solution B.

Solution B was slowly added dropwise to the solution A with a syringe while cooling and stirring in an ice bath, and then stirred at room temperature for 24 hours. After the reaction was completed, the white precipitate was filtered off and washed with THF and ion-exchanged water. The removal of pyridine hydrochloride confirmed that the white precipitate was not seen by adding an aqueous solution of silver nitrate to the washing solution. The washed crude product was collected and vacuum dried at 150 占 폚 for 12 hours. The obtained crude product was pale yellow white powder, yield was 6.1984 g and yield was 50.1%.

(refine)

The obtained crude product was purified by recrystallization. 88 mL of γ-butyrolactone (GBL) was added to 5.9905 g of the crude product, which was dissolved by heating at 150 ° C., followed by natural cooling and standing overnight. The precipitated pale yellow white powder was collected by filtration and vacuum dried at 180 ° C. for 12 hours. The yield of the obtained pale yellow white powder was 3.6974 g, and the recrystallization yield was 66.2%. The product purified by recrystallization was carried out from the Fourier transform infrared spectrophotometer FT / IR350 (manufactured by Jasco) at 1861 cm ^-1 and 1772 cm ^-1 to the acid dianhydride C = O stretching vibration absorbing band, at 1745 cm ^-1 . The ester group C = O expansion vibration absorption band was confirmed. Furthermore, proton NMR measurement was performed using a Fourier transform nuclear magnetic resonance JNM-ECP400 (manufactured by JEOL). (DMSO-d ₆ , δ, ppm): 1.98 (s, -CH ₃ , 6H), 2.08-2.15 (m , -CH ₃ , 12H), 6.98 (brs, ArH, 2H), 8.33 (d, J = 7.9 Hz, ArH, 2H), 8.71-8.76 (m, ArH, 4H), product is TAHMBP It was confirmed. Moreover, when melting | fusing point was measured by the differential scanning calorimeter DSC3100 (Bruker AXS company), the product showed high purity from the sharp melting peak at 309.4 degreeC.

<Synthesis example 2>

Synthesis of tetracarboxylic dianhydride represented by the formula (5)

(synthesis)

Tetracarboxylic dianhydride (hereinafter referred to as TA44BP) represented by the formula (5) was synthesized as follows. 16.8457 g (80.0 mmol) of trimellitic anhydride chloride was added to a branched flask, dissolved in 71 mL of dehydrated N, N-dimethylformamide (DMF) at room temperature, septum sealed to adjust solution A (solute concentration 20% by weight). ). Further in a separate flask, 7.4483 g (40.0 mmol) of 4,4'-biphenol was dissolved in 32 mL of dehydrated DMF at room temperature (20% by weight solute), to which 19.3 mL (240 mmol) of pyridine was added. The solution B was adjusted by septum seal. Solution B was slowly added dropwise to the solution A with a syringe while cooling and stirring in an ice bath, and then stirred at room temperature for 12 hours. After the reaction was completed, the yellow precipitate was filtered off and washed with DMF and ion-exchanged water. The removal of pyridine hydrochloride confirmed that the white precipitate was not seen by adding an aqueous solution of silver nitrate to the washing solution. The washed crude product was collected and vacuum dried at 180 deg. The obtained crude product was yellow powder, the yield was 9.6930 g and the yield was 38.5%.

(refine)

The obtained crude product was purified by recrystallization. 120 mL of gamma -butyrolactone (GBL) was added to 7.9956 g of the crude product, and the mixture was heated and dissolved, and left to stand for 12 hours. The precipitated yellow plate crystals were collected by filtration and vacuum dried at 200 ° C for 12 hours. The yield of the obtained pale yellow white powder was 6.3165 g, and the recrystallization yield was 79%. The product purified by recrystallization was obtained at 1861 cm ^-1 and 1782 cm ^-1 from Fourier transform infrared spectrophotometer FT / IR350 (manufactured by Jasco). Acid Anhydride C = O Stretch Vibration Absorber, 1730 cm ^-1 to The ester C = O stretching vibration absorbing band was confirmed. Furthermore, proton NMR measurement was performed using Fourier transform nuclear magnetic resonance JNM-ECP400 (manufactured by JEOL). (DMSO-d ₆ , δ, ppm): 7.52 (d, ArH, 4H), 7.58 (d, ArH, 4H ), 8.51 (d, ArH, 2H), 8.6 (m, ArH, 4H), 8.71-8.76 (m, ArH, 4H), and the product was confirmed to be TA44BP. Moreover, when melting | fusing point was measured by the differential scanning calorimeter DSC3100 (Bruker AXS company), the product showed high purity from the sharp melting peak at 326 degreeC.

<Synthesis example 3>

Synthesis of tetracarboxylic dianhydride represented by the following formula (11)

(synthesis)

Tetracarboxylic dianhydride (TA23X-BP) represented by Formula 11 was synthesized as follows. 6.3207 g (30.0170 mmol) of anhydrous trimellitic acid chloride was placed in a branched flask, dissolved in 13.0 mL of dehydrated GBL at room temperature, and septum sealed to adjust solution A. Further, 2.4239 g (10.0032 mmol) of 2,2 ', 3,3'-tetramethyl-biphenyl-4,4'-diol (23X-BP) in a separate flask was dissolved in 37.3 mL of dehydrated GBL at room temperature. To this, 4.85 mL (59.9656 mmol) of pyridine was added and septum sealed to adjust solution B.

Solution B was added dropwise to Solution A with a syringe over about 20 minutes while cooling and stirring in an ice bath, and then stirred at room temperature overnight.

After the reaction was completed, the white precipitate was filtered off and washed with ion-exchanged water. The removal of pyridine hydrochloride confirmed that the white precipitate was not seen by adding an aqueous solution of silver nitrate to the washing solution. The washed crude product was collected and vacuum dried at 80 ° C for 1 hour and at 100 ° C for 12 hours. The obtained crude product was white powder, the yield was 5.6107 g and the yield was 95.0%.

(refine)

The obtained crude product was purified by recrystallization. 210 mL of GBLs were added to 3.6411 g of the crude product, and the resulting mixture was heated and dissolved at 100 ° C. The precipitate was collected by filtration and vacuum dried at 160 ° C. for 12 hours. The yield of the obtained white powder was 2.8178 g, and the recrystallization yield was 77.3%.

The product was purified by re-crystallization was confirmed that the acid anhydride C = O stretching vibration absorption band, ester C = O stretching vibration absorption band in 1741 to result ㎝ ^-1, 1780 ㎝ ^-1 measured by infrared absorption spectrum. Further, proton NMR measurement was performed. (DMSO-d ₆ , δ, ppm): 2.02 (s, -CH 3, 6H), 2.17 (s, -CH 3, 6H), 7.07-7.09 (d, J = 8.3 Hz, ArH, 2H), 7.24-7.26 (d, J = 8.2 Hz, ArH, 2H), 8.31-8.33 (d, J = 7.96 Hz, ArH, 2H), 8.70-8.72 (dd, J = 7.86 Hz, 1.22 Hz , ArH, 2H), 8.67 (s, ArH, 2H), and the product was confirmed to be TA23X-BP. Furthermore, when melting | fusing point was measured by differential scanning calorimetry, the product showed high purity from the sharp melting peak at 309.1 degreeC.

<Example 1>

(Polymerization of Polyimide Precursor)

0.9607 g (3 mmol) of 2,2'-bis (trifluoromethyl) benzidine (TFMB) was dissolved in 11.3 g of dehydrated N, N-dimethylacetamide (DMAc). 1.8558 g (3 mmol) of TAHMBP powder described in Synthesis Example 1 was slowly added thereto, followed by stirring at room temperature for 96 hours to obtain polyamic acid as a polyimide precursor (solid content concentration of 20.0 wt%). The intrinsic viscosity of the obtained polyamic acid was 1.84 dL / g.

(Chemical imidization reaction)

After diluting the obtained polyamic acid solution to 10.0 wt% of solid content with dehydrated DMAc, a mixed solution of 2.8 mL (30 mmol) of acetic anhydride and 1.2 mL (15 mmol) pyridine was slowly added dropwise at room temperature with stirring. And after completion | finish of dripping, it stirred for 24 hours. The obtained polyimide solution was slowly added dropwise to a large amount of methanol to precipitate polyimide. The white precipitate obtained was sufficiently washed with methanol and vacuum dried at 160 ° C. for 12 hours. The obtained fibrous polyimide powder was 2.5130 g. A proton NMR measurement was performed on the powder, suggesting that the chemical imidization reaction was completed because no COOH protons (near δ13 ppm) and NHCO protons (near δ11 ppm) specific to polyamic acid were observed. The intrinsic viscosity of the obtained polyimide was 3.16 dL / g, and was a high molecular weight body.

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder was redissolved in cyclopentanone (CPN) at room temperature to adjust a 11.4 wt% homogeneous solution. This polyimide solution was cast on the glass substrate, and it dried with the hot air dryer at 60 degreeC for 2 hours. Then, after heat-processing at 250 degreeC in the board | substrate vacuum for 1 hour, and standing to room temperature, the polyimide film was peeled off from the glass substrate. The polyimide film was once more heat treated at 250 ° C. for 1 hour in vacuo to remove residual strain. The obtained polyimide film was colorless and transparent. When the mechanical properties of this polyimide film (film thickness of 32 µm) were measured, the average elongation of 20 sheets of test sheets was 10.7%, the maximum elongation was 20.0%, the tensile modulus was 4.5 GPa, and the breaking strength was 0.19 GPa. In addition, when the thermal properties were measured, the coefficient of linear thermal expansion was 21.7 ppm / K, the glass transition temperature was 272 ° C, and the 5% thermogravimetric reduction temperature was 437 ° C (in air) in the polyimide film having a thickness of 24 µm. In addition, the dielectric constant calculated from the refractive index of the polyimide film was 2.80, and the water absorption was 0.04%. A characteristic is shown in Table 1 with other evaluation results. In addition, when the moisture absorption stability of the polyimide solution adjusted to 11.4 wt% with CPN was evaluated, no change was observed in the solution even under an environment of 40% relative humidity for 24 hours. In addition, solvent solubility in various solvents was also good, showing excellent solubility in low hygroscopic solvents such as CPN, GBL, and Tri-GL. This is due to the compact molecular design of the polyimide of the present invention. The solubility test results are shown in Table 2. In addition, the infrared absorption spectrum of a polyimide thin film is shown in FIG.

<Example 2>

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder obtained by the method described in Example 1 was redissolved in γ-butyrolactone (GBL) at room temperature to adjust a 9.9% by weight solution. This polyimide solution was cast on the glass substrate, and it dried with the hot air dryer at 80 degreeC for 2 hours. Then, after heat-processing at 250 degreeC in the board | substrate vacuum for 1 hour, and standing to room temperature, the polyimide film was peeled off from the glass substrate. The polyimide film was once more heat treated at 250 ° C. for 1 hour in vacuo to remove residual strain. The obtained polyimide film was colorless and transparent. When the mechanical properties of this polyimide film (film thickness: 22 µm) were measured, the average elongation of 20 test sheets was 7.1%, the maximum elongation was 9.0%, the tensile modulus was 4.4 GPa, and the breaking strength was 0.18 GPa. In addition, when the thermal properties were measured, the coefficient of linear thermal expansion was 25.1 ppm / K in the polyimide film having a film thickness of 23 µm, the glass transition temperature was 274 ° C, and the 5% thermogravimetric reduction temperature was 437 ° C (in air). In addition, the dielectric constant calculated from the refractive index of the polyimide film was 2.80 and the water absorption was 0.05%. A characteristic is shown in Table 1 with other evaluation results. In addition, when the stability of the polyimide solution adjusted to 9.9% by weight with GBL was evaluated, no change was observed in the solution even under an environment of 40% relative humidity for 24 hours.

<Example 3>

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder obtained by the method described in Example 1 was redissolved in triethylene glycol dimethyl ether (TriGL) at room temperature to adjust a 10% by weight solution. This polyimide solution was cast on the glass substrate, and it dried with the hot air dryer at 100 degreeC for 2 hours. Then, after heat-processing at 250 degreeC in the board | substrate vacuum for 1 hour, and standing to room temperature, the polyimide film was peeled off from the glass substrate. The polyimide film was once more heat treated at 250 ° C. for 1 hour in vacuo to remove residual strain. The obtained polyimide film was colorless and transparent. When the mechanical properties of this polyimide film (film thickness of 30 µm) were measured, the average elongation of 20 sheets of test sheets was 10.2%, the maximum elongation was 25.5%, the tensile modulus was 4.2 GPa, and the breaking strength was 0.18 GPa. Moreover, when thermal characteristics were measured, the coefficient of linear thermal expansion was 25.7 ppm / K and the glass transition temperature was 280 degreeC in the polyimide film with a film thickness of 23 micrometers. Moreover, the dielectric constant calculated from the refractive index of the polyimide film was 2.79, and the water absorption was below the detection limit (<0.01%). A characteristic is shown in Table 1 with other evaluation results. In addition, when the stability of the polyimide solution adjusted to 10% by weight with TriGL was evaluated, no change was observed in the solution even under an environment of 40% relative humidity for 24 hours.

<Example 4>

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder obtained by the method described in Example 1 was redissolved in tetrahydrofuran (THF) at room temperature to adjust a 11.5% by weight solution. The polyimide film was produced like the method of Example 1 except having changed the solvent. The obtained polyimide film was colorless and transparent, and the coefficient of linear thermal expansion was 22.3 ppm / K in the polyimide film with a film thickness of 12 micrometers. In addition, when the stability of the polyimide solution adjusted to 11.5 wt% with THF was evaluated, no change was observed in the solution even under an environment of 40% relative humidity for 24 hours.

<Example 5>

(Polymerization of Polyimide Precursor)

0.9607 g (3 mmol) of TFMB was dissolved in 14.3 g of dehydrated DMAc. 1.2990 g (2.1 mmol) of TAHMBP powder and 0.2648 g (0.9 mmol) of 3,3 ', 4,4'-diphenyltetracarboxylic dianhydride (BPDA) powder described in Synthesis Example 1 were slowly added to this solution. And it stirred at room temperature for 72 hours, and obtained polyamic acid (solid content concentration 15 weight%). The intrinsic viscosity of the obtained polyamic acid was 1.46 dL / g.

(Chemical imidization reaction)

The polyamic acid solution was diluted with dehydrated DMAc to a solid content of 9.6 wt%, and then a mixed solution of 2.8 mL (30 mmol) of acetic anhydride and 1.2 mL (15 mmol) pyridine was slowly added dropwise at room temperature with stirring. It stirred after that for 24 hours. The obtained polyimide solution was added to a large amount of methanol to precipitate. The fibrous white precipitate thus obtained was sufficiently washed with methanol and vacuum dried at 160 ° C for 12 hours.

Proton NMR measurement was performed on the powder, suggesting that the chemical imidation reaction was completed because no COOH protons (near δ13 ppm) and NHCO protons (near δ11 ppm) specific to polyamic acid were observed. The intrinsic viscosity of the obtained polyimide was 2.96 dL / g.

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder was redissolved in cyclopentanone (CPN) at room temperature to adjust 12.5% by weight of the solution. This polyimide solution was cast on the glass substrate, and it dried with the hot air dryer at 60 degreeC for 2 hours. Then, after heat-processing at 250 degreeC in the board | substrate vacuum for 1 hour, and standing to room temperature, the polyimide film was peeled off from the glass substrate.

The polyimide film was once more heat treated at 250 ° C. for 1 hour in vacuo to remove residual strain. The obtained polyimide film was colorless and transparent. When the mechanical properties of this polyimide film (film thickness of 25 µm) were measured, the average elongation of 20 test sheets was 11.2%, the maximum elongation was 17.7%, the tensile modulus was 4.5 GPa, and the breaking strength was 0.19 GPa. In addition, when the thermal properties were measured, the coefficient of linear thermal expansion was 22.5 ppm / K, the glass transition temperature was 273 ° C., and the 5% thermogravimetry reduction temperature was 449 ° C. (in air) in the polyimide film having a film thickness of 22 μm.

In addition, the dielectric constant calculated from the refractive index of the polyimide film was 2.84 and the water absorption was 0.05%. A characteristic is shown in Table 1 with other evaluation results. In addition, when the stability of the polyimide solution adjusted to 12.5% by weight with CPN was evaluated, no change was observed in the solution even under an environment of 40% relative humidity for 24 hours. In addition, solvent solubility in various solvents was also good, showing excellent solubility in low hygroscopic solvents such as CPN, GBL, and Tri-GL. In general, when a monomer having a rigid structure such as BPDA is used as a copolymerization component, a small amount of the monomer usually causes a dramatic decrease in the solvent solubility of the polyimide, but the polyimide copolymer of the present invention still maintains excellent solubility. . This is due to the compact molecular design of the polyimide of the present invention. The solubility test results are shown in Table 2. In addition, the infrared absorption spectrum of a polyimide copolymer thin film is shown in FIG.

<Example 6>

(Polymerization of Polyimide Precursor)

0.32 g (1 mmol) of TFMB was dissolved in 2.12 g of dehydrated DMAc. 0.59 g (1 mmol) of TA23X-BP powder described in Synthesis Example 3 was slowly added thereto, followed by stirring at room temperature for 72 hours to obtain polyamic acid as a polyimide precursor (solid content concentration of 30% by weight).

The intrinsic viscosity of the obtained polyamic acid was 1.18 dL / g.

(Chemical imidization reaction)

The resulting polyamic acid solution was diluted with dehydrated DMAc to a solid content concentration of 12.0% by weight, and then a mixed solution of 1.0 g (10 mmol) of acetic anhydride and 0.4 mL (5 mmol) of pyridine was slowly added dropwise at room temperature with stirring. And after completion | finish of dripping, it stirred for 24 hours. The obtained polyimide solution was slowly added dropwise to a large amount of methanol to precipitate polyimide. The white precipitate obtained was sufficiently washed with methanol and vacuum dried at 100 ° C for 12 hours. The obtained polyimide powder was 0.819 g. Proton NMR measurement was performed on the powder, suggesting that the chemical imidation reaction was completed because no COOH protons (near δ13 ppm) and NHCO protons (near δ11 ppm) specific to polyamic acid were observed. The intrinsic viscosity of obtained polyimide was 1.81 dL / g.

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder was redissolved in cyclopentanone (CPN) at room temperature to adjust a 8.0 wt% homogeneous solution. This polyimide solution was cast on the glass substrate, and it dried with the hot air dryer for 2 hours at 60 degreeC. Then, after heat-processing at 200 degreeC in the board | substrate vacuum for 1 hour, and standing to room temperature, the polyimide film was peeled off from the glass substrate. The polyimide film was once more heat treated at 200 ° C. for 1 hour in vacuo to remove residual strain. Although the obtained polyimide film had some turbidity, it was colorless and transparent. The thermal properties of this polyimide film were measured, and the coefficient of linear thermal expansion was 15.5 ppm / K in the polyimide film having a film thickness of 28.6 μm, and the glass transition temperature was 211 ° C. and 5% thermal weight in the polyimide film having a film thickness of 13.0 μm. The reduction temperature was 437 ° C. (in air) in a 20.0 μm polyimide film.

In addition, the infrared absorption spectrum of a polyimide thin film is shown in FIG.

<Reference example 1>

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder obtained by the method described in Example 1 was redissolved in N, N-dimethylacetamide (DMAc) at room temperature to adjust a 11.1% by weight solution. A polyimide film was produced in the same manner as in Example 1 except that the solvent was changed. The obtained polyimide film was colorless and transparent, and the coefficient of linear thermal expansion was 27.1 ppm / K in the polyimide film with a film thickness of 15 micrometers. In addition, when the stability of the polyimide solution adjusted to 11.1 wt% with DMAc was evaluated, the solution became cloudy and polyimide precipitated under an environment of 40% relative humidity for 24 hours. This indicates that the solution absorbed moisture in an environment of 40% relative humidity for 24 hours because of high hygroscopicity of DMAc.

<Reference example 2>

(Adjustment of Polyimide Solution and Film Production of Polyimide Film)

The polyimide powder obtained by the method described in Example 1 was redissolved in N-methyl-2-pyrrolidone (NMP) at room temperature to adjust a 9.8 wt% solution. A polyimide film was produced in the same manner as in Example 1 except that the solvent was changed. The obtained polyimide film was colorless and transparent, and the coefficient of linear thermal expansion was 26.7 ppm / K in the polyimide film with a film thickness of 17 micrometers. In addition, when the stability of the polyimide solution adjusted to 9.8 wt% with NMP was evaluated, the solution became cloudy and polyimide precipitated under an environment of 40% relative humidity for 24 hours. This is because the hygroscopicity of NMP is high similarly to the result by the reference example 1.

<Reference example 3>

(Polymerization of Polyimide Precursor)

0.9607 g (3 mmol) of TFMB was dissolved in 6.6 g of dehydrated DMAc. 1.8558 g (3 mmol) of TAHMBP powder was slowly added to this solution (solid concentration 30.0 wt%), further, dehydrated DMAc was added and stirred at room temperature for 11 days (solid concentration 20.0 wt%). The intrinsic viscosity of the obtained polyimide precursor (polyamic acid) was 1.26 dL / g.

(Production of a polyimide precursor film and preparation of a polyimide film)

The obtained polyamic acid solution was cast on a glass substrate and dried in a hot air drier at 60 ° C. for 2 hours. Thereafter, the substrate was thermally imidized at 200 ° C. for 0.5 hour in the vacuum at the substrate and then at 350 ° C. for 1 hour. After cooling to room temperature, the polyimide film was released from the glass substrate. This polyimide film was once more heat treated at 320 ° C. for 1 hour in vacuo to remove residual strain. The obtained polyimide film was cloudy.

The linear thermal expansion coefficient of this polyimide film was 66.5 ppm / K in the polyimide film with a film thickness of 26 micrometers. Thus, the value of the coefficient of linear thermal expansion of the polyimide film produced by the thermal imidation reaction is the linear thermal expansion of the polyimide film of the same composition produced via the chemical imidation of Examples 1-4, and then through the cast film forming of a polyimide varnish. It is found that the conventional two-step process of film formation followed by thermal imidation in the step of polyamic acid is not preferable because the film is much larger than the coefficient and the film becomes cloudy. A characteristic is shown in Table 1 with other evaluation results.

<Comparative Example 1>

(Polymerization of Polyimide Precursor)

0.9607 g (3 mmol) of TFMB was dissolved in 10.3 g of dehydrated NMP. 1.6033 g (3 mmol) of TA44BP powder described in Synthesis Example 2 was slowly added thereto, followed by stirring at room temperature for 5 days to obtain polyamic acid (solid content concentration of 20.0 wt%). The inherent viscosity of the polyamic acid was 1.99 dL / g.

(Chemical imidization reaction)

After diluting the obtained polyamic acid solution to 10.0 weight% of solid content by dehydration NMP, the mixed solution of 2.8 mL (30 mmol) of acetic anhydride and 1.2 mL (15 mmol) pyridine was slowly dripped at room temperature, stirring this. The reaction solution gelled, making it impossible to complete the chemical imidization reaction. This is because solvent solubility of this polyimide is insufficient. This result is due to no substituent on the central biphenylene group of TA44BP used, and shows how important the substituent in TAHMBP plays a role in solvent solubility.

<Comparative Example 2>

(Thermal imidation of polyimide precursor)

The polyamic acid solution polymerized in Comparative Example 1 was cast on a glass substrate and dried at 80 ° C. for 2 hours in a hot air dryer. Thereafter, the substrate was thermally imidized at 250 ° C for 1 hour, and then at 350 ° C for 1 hour in the vacuum. After cooling to room temperature, the polyimide film was released from the glass substrate.

This polyimide film was once more heat treated at 350 ° C. for 1 hour in vacuum. The obtained polyimide film was strongly colored yellow.

This result is attributable to the structure having no substituent on the central biphenylene group of TA44BP used, and shows how important the substituent in TAHMBP plays a role in suppressing the coloration of the film.

+ +: Soluble at room temperature, +: heated to near boiling point, soluble,-: insoluble,

±: swelling, some melting,

b) gelling after several days, c) homogeneous solution for several weeks

(Total Light Transmittance and Haze Value of Polyimide Membrane)

About the polyimide film of Examples 1-3, Example 5, Example 6, and the comparative example 2, using Haze Meter NDH 4000 by Nippon Denshoku Industries Co., Ltd., the total light transmittance according to JIS K 7361 and the haze according to JIS K 7136 The value was measured 5 times, and the average value was calculated | required and used as an index of transparency. According to this result, the polyimide of Examples 1-3 and 5 showed the value with a high total light transmittance sufficiently high, and the haze value was low. However, the polyimide film of the comparative example 2 is falling in these values, and it turns out that coloring could not be suppressed due to the structure which has no substituent on the central biphenylene group of TA44BP used at the time of the synthesis | combination of a polyimide.

In addition, for the polyimide film of Example 6, since the total light transmittance, the light transmittance of 400 nm, and the cut-off wavelength are much superior to those of the polyimide film of Comparative Example 2, the transparency of the polyimide film is clearly shown in Example 6 Better than 2 In addition, since the haze value of the polyimide film of Example 6 was measured at twice the film thickness of the polyimide film of Comparative Example 2, the haze value was slightly larger than that of Comparative Example 2, but considering the film thickness of each film, As for the polyimide film of Example 6, it is excellent.

Claims (11)

Polyimide containing the structural unit represented by following formula (1).
[Formula 1]

(In formula, each R independently represents a hydrogen atom or a linear or branched alkyl group having 1 to 6 carbon atoms. However, at least one of two R's bonded to the same benzene ring is an alkyl group.) Polyimide containing the structural unit represented by following formula (2).
[Formula 2]

The method of claim 1,
Polyimide containing 70 mol% or more of the structural unit represented by following formula (1).
[Formula 1]

(In formula, each R independently represents a hydrogen atom or a linear or branched alkyl group having 1 to 6 carbon atoms. However, at least one of two R's bonded to the same benzene ring is an alkyl group.) The polyimide varnish containing the polyimide and organic solvent in any one of Claims 1-3. The method of claim 4, wherein
The organic solvent is a low hygroscopic organic solvent selected from one or more of an ester solvent, an ether solvent, a carbonate solvent, a glycol solvent, a phenol solvent, and a ketone solvent, and the solid content concentration of the polyimide is 5% by weight or more. Polyimide varnish. Polyimide molded body containing a structural unit represented by the following formula (1).
[Formula 1]

(In formula, each R independently represents a hydrogen atom or a linear or branched alkyl group having 1 to 6 carbon atoms. However, at least one of two R's bonded to the same benzene ring is an alkyl group.) The method of claim 6,
The polyimide molded body whose molded object is a film. The polyimide film obtained by apply | coating the polyimide varnish of Claim 4 on a board | substrate, drying, and peeling from a board | substrate. The method of claim 8,
The polyimide film has a light transmittance of 45% or more at 400 nm when the film thickness is 10 μm, or a total light transmittance of 80% or more when the film thickness is 20 μm. The method of claim 8,
The polyimide film has a light transmittance of 45% or more at 400 nm when the film thickness is 10 μm, and a total light transmittance of 70% or more when the film thickness is 10 μm. A method for synthesizing a polyimide comprising a structural unit represented by the following formula (1), wherein the heating temperature is less than 150 ° C. when the polyamide precursor is imidized.
[Formula 1]

(In formula, each R independently represents a hydrogen atom or a linear or branched alkyl group having 1 to 6 carbon atoms. However, at least one of two R's bonded to the same benzene ring is an alkyl group.)

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