JP2012041473A - Polyimide film and method of manufacturing the film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyimide film with which an auxiliary material having a strong coloring effect can be used by suppressing the coloring of the film caused by the auxiliary material such as a solvent or a catalyst during manufacturing, and both transparency and heat resistance of an original polymer structure without the influence of the kind of the auxiliary material.SOLUTION: The polyimide film having YI of at most 25 and haze of at most 5% is obtained by imidizing a dope solution prepared by mixing a polyamide acid, which is prepared by reacting an aromatic diamine and an aromatic acid dianhydride in an organic solvent, with an imidization catalyst composed of a tertiary amine, a dehydrating agent composed of an aliphatic acid anhydride, and a phosphoric acid ester.

Description

The present invention relates to a low-colored polyimide film. In particular, by using appropriate additives, it is possible to suppress coloring during production due to auxiliary materials such as solvents and catalysts, and materials for forming products or members that have high requirements for heat resistance and low coloring. The present invention relates to a polyimide film that can be suitably used as (for example, display device glass replacement) and a method for producing the same.

In recent years, with the rapid progress of displays such as liquid crystal, organic EL, and electronic paper, and electronics such as solar cells and touch panels, devices have been required to be thinner and lighter, and more flexible. . In these devices, various electronic elements such as thin transistors and transparent electrodes are formed on a glass plate. By changing this glass material to a film material, the panel itself becomes flexible, thin and lightweight. Can be achieved. The film material used in this application is required to have heat resistance in the formation process of the electronic element.

  As a general film material excellent in heat resistance, a polyimide film can be mentioned. Polyimide films are widely used in electronic devices and semiconductor applications, such as flexible printed wiring boards, taking advantage of their heat resistance and insulation. However, polyimide films are generally colored yellow and brown, and have a higher coefficient of linear expansion than glass. Therefore, transparency can be improved and a low coefficient of linear expansion can be realized to replace glass. It was a challenge. Attempts to solve these problems have been made in the past, for example, obtained from 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride and 2,2′-bis (trifluoromethyl) benzidine. Polyimide is excellent in transparency in addition to heat resistance and linear expansion coefficient, and there have been several reports so far (see, for example, Patent Document 1).

  As a general method for producing a polyimide film, a polyamic acid produced by reacting diamine and acid dianhydride in an organic solvent is cast on a support, formed into a film, and imidized by heating at a high temperature. can get. In order to promote the imidization reaction and improve the productivity, means for adding an imidization catalyst composed of a tertiary amine and a dehydrating agent composed of an aliphatic acid anhydride to the polyamic acid is also used. Further, for example, as a halogen-free flame retardant means for polyimide films, further additives are also used to improve characteristics such as using a phosphoric acid compound (see, for example, Patent Document 2).

  Since the transparency and colorability of polyimide are almost determined by the primary structure of the molecule, general polyimide films are originally colored yellow and brown, and there are limits to the improvement of transparency and colorability with additives. It was.

US Pat. No. 5,071,1997 JP 2006-37106 A

  This invention is made | formed in view of the above, Comprising: The objective is to provide the polyimide film by which coloring derived from an auxiliary material was suppressed.

  As a result of intensive studies in view of the above-mentioned problems, the present inventors have found that the YI of the resulting film can be reduced by using a phosphate ester during the formation of a polyimide film, and the present invention has been completed. That is, the present invention has the following configuration.

  1) An imidization catalyst comprising a tertiary amine, an acid, and a polyamic acid having a repeating unit represented by the following formula (1) obtained by reacting an aromatic diamine and an aromatic dianhydride in an organic solvent A polyimide film having a YI of 23 or less and a haze of 5% or less, which is obtained by imidizing a dope solution in which a dehydrating agent composed of an anhydride and a phosphate ester are mixed.

(In the formula, R ¹ represents a tetravalent organic group, and R ² represents a divalent organic group.)
2) The polyimide film according to 1), wherein R ¹ is a tetravalent organic group selected from the following general formula (2).

3) The polyimide film according to 1) or 2), wherein R ² is a divalent biphenylene group selected from the following general formula (3).

(In the formula, R ³ represents an alkyl group having 1 to 16 carbon atoms, a halogenated alkyl group having 1 to 16 carbon atoms, or halogen.)
4) The polyimide film as described in any one of 1) to 3), wherein N, N-dimethylformamide is used as the organic solvent.

  5) The polyimide film according to any one of 1) to 4), wherein any one of β-picoline, isoquinoline, and 3,5-diethylpyridine is used as a tertiary amine.

  6) The polyimide film according to any one of 1) to 5), wherein the phosphate ester is added in a proportion of 0.3 to 30% by weight based on the solid content weight of the polyamic acid.

  7) A polyamic acid obtained by reacting an aromatic diamine and an aromatic dianhydride in an organic solvent is mixed with an imidization catalyst comprising a tertiary amine, a dehydrating agent comprising an acid anhydride, and a phosphate ester. A method for producing a polyimide film having a YI of 23 or less and a haze of 5% or less according to any one of 1) to 6), wherein a dope solution is obtained and the dope solution is imidized.

The polyimide film obtained by the present invention is suppressed in coloration by using a phosphate ester, and is suitably used for applications where transparency and heat resistance are required at the same time. Phosphoric esters are originally used for imparting flame retardancy, and it has not been confirmed so far that they have the coloring suppression effect shown in the present invention.

Embodiments of the present invention will be described below.
<Polyamide acid>
The polyimide film according to the present invention is obtained by imidizing a polyamic acid having a repeating unit represented by the following general formula (1) obtained by reacting an aromatic diamine and an aromatic dianhydride in an organic solvent. .

(In the formula, R ¹ represents a tetravalent organic group, and R ² represents a divalent organic group.)
Any known method can be used for producing the polyamic acid. Usually, aromatic dianhydride and aromatic diamine are dissolved in an organic solvent in a substantially equimolar amount, and polymerization of the acid dianhydride and diamine is completed under controlled temperature conditions. Produced by stirring until.

R ¹ in formula (1) is preferably a tetravalent organic group selected from the following general formula (2). Of the tetravalent organic groups listed in formula (2), a benzene or biphenyl skeleton is particularly preferable. Specific compounds having a benzene or biphenyl skeleton may include pyromellitic dianhydride and 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, respectively. ', 4,4'-biphenyltetracarboxylic dianhydride is preferred. The acid dianhydrides having a tetravalent organic group listed in formula (2) may be used alone or in combination of two or more.

R ² in formula (1) is preferably a divalent biphenylene group selected from the following general formula (3).

(Wherein R ³ represents an alkyl group having 1 to 16 carbon atoms, a halogenated alkyl group having 1 to 16 carbon atoms, or halogen).
R ³ is preferably an electron-withdrawing group such as halogen or alkyl halide from the viewpoint of transparency, heat resistance, and dimensional stability of the resulting polyimide, and the halogen of halogen or alkyl halide is preferably fluorine. A fluoromethyl group, particularly a trifluoromethyl group, is preferred. Diamines having a divalent organic group listed in Formula (3) may be used alone or in combination of two or more.

  A conventionally known organic solvent can be used as the solvent for the polyamic acid polymerization, but amides such as N, N-dimethylformamide and N, N-dimethylacetamide are considered in consideration of the solubility of the raw material and the produced polyamic acid. System solvents can be suitably used. Further, N, N-dimethylformamide is preferable in consideration of cost.

  In the case of polyimide with low coloring and high transparency, when N, N-dimethylformamide is used for polymerization of polyamic acid, the YI of the film obtained is higher than when N, N-dimethylacetamide is used. However, as a result of intensive studies, the present inventors have found that the YI of the film obtained can be suppressed even when N, N-dimethylformamide is used.

  Regarding the concentration of the polyamic acid solution at the time of synthesis, a lower concentration is preferable because the amount of the solvent contained in the polyamic acid solution increases and the mixing property with the imidization accelerator is improved. However, if the concentration is too low, it is difficult to produce a thick film. The concentration of the polyamic acid solution is preferably 5 to 30 wt%, and more preferably 10 to 20 wt%.

  Moreover, about the viscosity of a polyamic-acid solution, since the lower one improves a miscibility with an imidation promoter, it is preferable. However, since lowering the viscosity leads to lowering the molecular weight of the polyamic acid, the resulting film may not exhibit the desired mechanical strength. When considering both compatibility and ensuring film strength, the viscosity of the polyamic acid solution is preferably 1000 to 3500 poise, and more preferably 1500 to 3000 poise. On the other hand, the viscosity of polyamic acid depends on the concentration. If the molecular weight is the same, the lower the concentration, the lower the viscosity. Therefore, the concentration of the polyamic acid may be adjusted so as to obtain a desired viscosity. However, in order to obtain a film having sufficient strength, the weight average molecular weight of the polyamic acid is preferably at least 100,000 or more.

<Imidation accelerator>
The polyimide film according to the present invention can be obtained by adding an imidization accelerator prepared by mixing an imidization catalyst composed of a tertiary amine and a dehydrating agent composed of an acid anhydride into the polyamic acid. Although there is a method of imidizing only by heating without adding the imidization accelerator, it is not suitable because the coefficient of linear expansion and hysteresis of the resulting film tend to deteriorate and the productivity is also inferior. As the imidization catalyst, tertiary amines such as aliphatic tertiary amines, aromatic tertiary amines, and heterocyclic tertiary amines can be suitably used. Among these, it is preferable to use any one of β-picoline, isoquinoline, and 3,5-diethylpyridine from the balance with the catalytic ability. These catalysts are preferably used alone or in combination.

  Examples of the dehydrating agent include aliphatic acid anhydrides such as acetic anhydride, and aromatic acid anhydrides such as phthalic anhydride. These dehydrating agents are preferably used alone or in combination. Of these, aliphatic acid anhydrides are preferred from the standpoints of cost, availability, and solubility.

  About the addition amount of an imidation catalyst and a dehydrating agent, it exists in the tendency for the linear expansion coefficient of the film obtained by increasing an amount to become a little low. In addition, productivity is improved because the time required for imidization is shortened. However, if the amount is excessive, the progress of imidization becomes too fast, and continuous casting for a long time may be difficult. On the other hand, if the amount is too small, desired film characteristics cannot be obtained, or imidization is difficult to proceed and productivity may be lowered. What is necessary is just to set suitably about the addition amount of an imidation promoter in consideration of the characteristic and production conditions of the film obtained.

  If the amount of the dehydrating agent and the imidization catalyst is intentionally illustrated, it is preferable that the number of moles of dehydrating agent / number of moles of amide groups in polyamic acid = 10 to 0.01, and the number of moles of amide groups in polyamic acid = 10 to 0. .01 is preferred. More preferably, the number of moles of dehydrating agent / the number of moles of amide groups in the polyamic acid is preferably 5 to 0.5, and the number of moles of amide groups in the imidization catalyst / polyamic acid is preferably 5 to 0.5.

<Phosphate ester>
As a result of intensive studies, the present inventors have found that the YI of the resulting polyimide film can be reduced by adding a phosphate ester. Commercially available phosphate esters can be selected and used as appropriate, but those with a low molecular weight may be decomposed and volatilized early by heating during imidization, resulting in insufficient YI suppression effects. is there. Therefore, the molecular weight of the phosphate ester used is preferably 400 or more. On the other hand, if the molecular weight is too large, phase separation may occur during mixing of the polyamic acid and the imidization accelerator, and film characteristics may not be uniform. Accordingly, the molecular weight of the phosphate ester used is preferably 800 or less. As commercially available phosphoric acid esters, for example, FP-600 and FP-700 (manufactured by Adeka Corporation) can be suitably used.

  On the other hand, if the amount of phosphate ester added is too small, the YI suppression effect may be insufficient. Conversely, if the amount is too large, a large amount will remain in the film after firing, and the linear expansion coefficient of the film due to the plasticizer effect. May be significantly higher. The addition amount is preferably 0.2 to 7.0% by weight, more preferably 0.4 to 5.0% by weight, based on the solid content weight of the polyamic acid.

  As a method of mixing the polyamic acid, the imidization accelerator, and the phosphoric acid ester, a conventionally known apparatus / method may be used. However, if the temperature in the reaction system is high, the imidization reaction proceeds too rapidly and it is difficult to form a film. Therefore, it is preferable to mix while cooling the entire reaction system. Further, the three components may be prepared individually and mixed at the end, or the phosphate ester may be mixed with either the polyamic acid or the imidization accelerator in advance.

<Film>
The polyimide film of the present invention is obtained by forming a dope solution obtained by mixing a polyamic acid, an imidization accelerator, and a phosphate ester into a film shape. A conventionally known apparatus / system can be used as the molding method. As a preferred example, the dope solution is extruded from a T-die or the like and cast onto a support such as a rotating drum or endless belt. The cast polyamic acid solution can be heated on the support to volatilize the solvent and advance imidization to some extent to obtain a gel film having self-supporting properties.

  The gel film is peeled off from the support, passed through a heating furnace with both ends in the width direction fixed, and removal of the remaining solvent and imidization are completed to obtain a polyimide film. The solvent residual ratio and the degree of imidization in the gel film state, the temperature setting of the heating furnace, and the heating time may be appropriately adjusted in view of the type of polyamic acid, the thickness of the resulting polyimide film, variations in physical properties, and the like. .

  In order to prevent long-time transportability and blocking during winding, inorganic particles may be added to the polyimide film according to the present invention as an anti-blocking material. The material of the inorganic particles is not particularly limited. However, when inorganic particles having a large particle diameter are added, the transmitted light is scattered and the haze of the film is increased. Therefore, the size of the inorganic particles is preferably 100 nm or less. The addition amount of the inorganic particles can be appropriately selected in consideration of the haze of the obtained film. Regarding the addition method, it may be added to the polyamic acid solution at the time of polymerization of the polyamic acid, may be added to the imidization accelerator at the time of preparation of the imidization accelerator, or the polyamic acid, the imidization accelerator, and the phosphate ester. You may add at the time of mixing.

  The polyimide film of the present invention can achieve both transparency and heat resistance by the above means without being affected by the auxiliary material. Specifically, YI is 23 or less and haze is 6% or less. In order to be suitably used as a film member, it is preferable that YI is 19 or less and haze is 4% or less.

  If YI and haze are within the above ranges, it can be suitably used as a film member application, but if there is a need for further reduction, it can be achieved by appropriately changing the polymerization solvent, imidization accelerator and firing conditions. Specifically, it is possible to further improve YI and haze by using β-picoline and 3,5-diethylpyridine among the tertiary amines used as catalysts for imidization. In particular, by using β-picoline, YI can be 17.5 or less and haze can be 2.8 or less.

  Since the polyimide film according to the present invention has high transparency and dimensional stability in addition to the original properties of polyimide such as heat resistance and insulation, fields and products in which these properties are effective, for example, inorganic Film member having thin film or inorganic microstructure on its surface, for example, film for thin film transistor, color filter film, film with transparent electrode, gas barrier film, inorganic glass or organic glass formed from silicon, metal oxide or organic substance It can be applied to films with laminated films. These members can be used for films for liquid crystal displays, films for organic EL, films for electronic paper, films for solar cells, films for touch panels, and the like.

EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to these. In addition, the evaluation method of YI of a polyimide film and a haze in an Example and a comparative example is as follows.

<YI>
The YI of the film was measured using HANDY COLORIMTER NR-3000 manufactured by Nippon Denshoku Industries Co., Ltd. The measurement was performed at five locations by changing the position of an 18 cm square sample, and the average value was taken as the measured value of the film.

<Haze>
The haze of the film was performed using a Nippon Denshoku Industries Co., Ltd. haze meter NDH5000. The measurement was performed at five locations by changing the position of an 18 cm square sample, and the average value was taken as the measured value of the film.

<Polyamic acid molecular weight>
The weight average molecular weight (Mw) of the polyamic acid was measured using a Waters GPC under the following conditions: (column: KD-806M 2 manufactured by Shodex, temperature 60 ° C., detector: RI, flow rate: 1 ml / min, Developing solution: DMF (lithium bromide 0.03 M, phosphoric acid 0.03 M), sample concentration: 0.2 wt%, injection amount: 20 μl, reference substance: polyethylene oxide).

(Synthesis Example 1: Synthesis of polyamic acid solution)
In a reactor maintained at 20 ° C. under a nitrogen atmosphere, 758.5 g of N, N-dimethylformamide (hereinafter referred to as DMF) was added and stirred with 3,3 ′, 4,4′-biphenyltetracarboxylic acid dicarboxylic acid. 95.8 g of anhydride (hereinafter referred to as BPDA) was added. While stirring, 83.4 g of 2,2′-bis (trifluoromethyl) benzidine (hereinafter referred to as TFMB) was added. Stirring was continued for 2 hours. After visually confirming that there was no undissolved residue, 17.7 g of TFMB was added, and the mixture was further stirred for 1 hour. A solution in which TFMB was dissolved in DMF at a solid content of 7 wt% was separately prepared, and 41.6 g of this solution was gradually added to the reaction system. After completion of the addition, stirring was continued for 40 hours to obtain a polyamic acid solution having a viscosity of 2000 poise. The weight average molecular weight of the polyamic acid was 120,000.

(Synthesis Example 2: Synthesis of polyamic acid solution)
A polyamic acid solution having a viscosity of 2000 poise was obtained in the same manner as in Synthesis Example 1 except that N, N-dimethylacetamide (hereinafter referred to as DMAc) was used as the solvent instead of DMF. The weight average molecular weight of the polyamic acid was 120,000.

Example 1
In the polyamic acid solution obtained in Synthesis Example 1, an imidization accelerator composed of acetic anhydride / β-picoline / DMF (weight ratio 35/7/8) is 35% by weight with respect to the polyamic acid solution, and a phosphate ester. (FP-600 manufactured by Adeka Co., Ltd., molecular weight 600) is added at a weight ratio of 0.5% to polyamic acid (solid content of polyamic acid solution, the same applies hereinafter), continuously stirred with a mixer and extruded from a T-die. Cast on a stainless steel endless belt. After heating the resin film at 80 ° C. for 360 seconds, the self-supporting gel film is peeled off from the endless belt and fixed to the tenter clip, and dried at 150 ° C., 200 ° C., 250 ° C., and 300 ° C. for 5 minutes each. A polyimide film having a thickness of 50 μm was obtained by imidization.

(Example 2)
A polyimide film having a thickness of 50 μm was obtained by performing the same operation as in Example 1 except that phosphate ester (FP-700 manufactured by Adeka Corporation, molecular weight 700) was used.

(Example 3)
A polyimide film having a thickness of 50 μm was obtained by performing the same operation as in Example 1 except that phosphate ester (PFR-2 manufactured by Adeka Corporation, molecular weight 574) was used.

Example 4
A polyimide film having a thickness of 50 μm was obtained by performing the same operation as in Example 1 except that the addition amount of the phosphoric ester was 4% by weight with respect to the polyamic acid.

(Example 5)
To the polyamic acid solution obtained in Synthesis Example 1, an imidization accelerator composed of acetic anhydride / isoquinoline / DMF (weight ratio 24/6/20) was 35% by weight with respect to the polyamic acid solution, and phosphate ester (stock) FP-600 manufactured by Adeka Co., Ltd. was added at a weight ratio of 3% with respect to the polyamic acid, stirred continuously with a mixer, extruded from a T-die, and cast onto a stainless endless belt. After heating the resin film at 80 ° C. for 360 seconds, the self-supporting gel film is peeled off from the endless belt and fixed to the tenter clip, and dried at 150 ° C., 200 ° C., 250 ° C., and 300 ° C. for 5 minutes each. A polyimide film having a thickness of 50 μm was obtained by imidization.

(Example 6)
In the polyamic acid solution obtained in Synthesis Example 1, an imidization accelerator composed of acetic anhydride / 3,5-diethylpyridine / DMF (weight ratio 20/4/26) is 35% by weight with respect to the polyamic acid solution, Phosphoric acid ester (FP-600 manufactured by Adeka Co., Ltd.) was added at 3% by weight to the polyamic acid, continuously stirred by a mixer, extruded from a T-die, and cast on a stainless steel endless belt. After heating the resin film at 80 ° C. for 360 seconds, the self-supporting gel film is peeled off from the endless belt and fixed to the tenter clip, and dried at 150 ° C., 200 ° C., 250 ° C., and 300 ° C. for 5 minutes each. A polyimide film having a thickness of 50 μm was obtained by imidization.

(Comparative Example 1)
A polyimide film having a thickness of 50 μm was obtained in the same manner as in Example 1 except that the phosphate ester was not added.

(Comparative Example 2)
A polyimide film having a thickness of 50 μm was obtained by performing the same operation as in Example 5 except that the phosphate ester was not added.

(Comparative Example 3)
A polyimide film having a thickness of 50 μm was obtained by performing the same operation as in Example 6 except that the phosphate ester was not added.

(Reference Example 1)
To the polyamic acid solution obtained in Synthesis Example 2, an imidization accelerator composed of acetic anhydride / β-picoline / DMAc (weight ratio 35/7/8) was added at a weight ratio of 35% with respect to the polyamic acid solution. The mixture was stirred with a mixer, extruded from a T-die, and cast onto a stainless steel endless belt. After heating the resin film at 80 ° C. for 360 seconds, the self-supporting gel film is peeled off from the endless belt and fixed to the tenter clip, and dried at 150 ° C., 200 ° C., 250 ° C., and 300 ° C. for 5 minutes each. A polyimide film having a thickness of 50 μm was obtained by imidization.

(Reference Example 2)
To the polyamic acid solution obtained in Synthesis Example 2, an imidization accelerator composed of acetic anhydride / isoquinoline / DMAc (weight ratio 24/6/20) was added at a weight ratio of 35% with respect to the polyamic acid solution, and continuously. The mixture was stirred with a mixer, extruded from a T-die, and cast on a stainless steel endless belt. After heating the resin film at 80 ° C. for 360 seconds, the self-supporting gel film is peeled off from the endless belt and fixed to the tenter clip, and dried at 150 ° C., 200 ° C., 250 ° C., and 300 ° C. for 5 minutes each. A polyimide film having a thickness of 50 μm was obtained by imidization.

(Reference Example 3)
To the polyamic acid solution obtained in Synthesis Example 2, an imidization accelerator consisting of acetic anhydride / 3,5-diethylpyridine / DMF (weight ratio 20/4/26) was added at a weight ratio of 35% with respect to the polyamic acid solution. The mixture was continuously stirred by a mixer, extruded from a T die, and cast onto a stainless steel endless belt. After heating the resin film at 80 ° C. for 360 seconds, the self-supporting gel film is peeled off from the endless belt and fixed to the tenter clip, and dried at 150 ° C., 200 ° C., 250 ° C., and 300 ° C. for 5 minutes each. A polyimide film having a thickness of 50 μm was obtained by imidization.

The evaluation results of the obtained film are shown in Table 1. The film of the Example which added the phosphate ester at the time of film preparation resulted in YI and a haze improving compared with the film of the comparative example which is not added. In particular, YI is remarkably improved.
Moreover, imidation catalysts such as isoquinoline and 3,5-diethylpyridine are colored by themselves, and deteriorate YI of the resulting film, but an improvement effect has been confirmed by adding a phosphate ester.

Claims (7)

A polyamic acid having a repeating unit represented by the following formula (1) obtained by reacting an aromatic diamine and an aromatic acid dianhydride in an organic solvent, an imidization catalyst comprising a tertiary amine, an acid anhydride A polyimide film having a YI of 23 or less and a haze of 5% or less, obtained by imidizing a dope mixed with a dehydrating agent and a phosphate ester.

(In the formula, R ¹ represents a tetravalent organic group, and R ² represents a divalent organic group.) The polyimide film according to claim 1, wherein R ¹ is a tetravalent organic group selected from the following general formula (2).

The polyimide film according to claim 1 or 2, wherein R ² is a divalent biphenylene group selected from the following general formula (3).

(In the formula, R ³ represents an alkyl group having 1 to 16 carbon atoms, a halogenated alkyl group having 1 to 16 carbon atoms, or halogen.)   The polyimide film according to claim 1, wherein N, N-dimethylformamide is used as the organic solvent.   The polyimide film according to claim 1, wherein any one of β-picoline, isoquinoline, and 3,5-diethylpyridine is used as a tertiary amine.   The polyimide film according to any one of claims 1 to 5, wherein the phosphoric acid ester is added in a proportion of 0.3 to 30% by weight based on the solid content weight of the polyamic acid.   Dope solution by mixing polyamic acid obtained by reacting aromatic diamine and aromatic dianhydride in organic solvent with imidization catalyst consisting of tertiary amine, dehydrating agent consisting of acid anhydride, and phosphate ester The polyimide film having a YI of 23 or less and a haze of 5% or less according to any one of claims 1 to 6, wherein the dope solution is imidized.