JP2012092262A - Method for producing crystalline polyimide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a solvent-soluble crystalline polyimide having a high molecular weight.SOLUTION: The solvent-soluble crystalline polyimide having a high molecular weight is obtained by subjecting a nylon salt type monomer comprising tetracarboxylic acid or its alkyl ester object (where the alkyl has the carbon number of 1-5) and diamine to solid phase polymerization as a first step, and successively by subjecting it to suspension polymerization in a poor solvent at a higher temperature than the solid phase polymerization temperature as a second step.

Description

  The present invention relates to a method for producing a solvent-soluble crystalline polyimide having a high molecular weight.

  Polyimide resins are useful engineering plastics with high thermal stability, high strength, and high solvent resistance due to molecular chain rigidity, resonance stabilization, and strong chemical bonding, and are applied in a wide range of fields. Moreover, since the crystalline polyimide can further improve its heat resistance, strength, and chemical resistance, it is expected to be used as a metal substitute.

  However, high-temperature solution polymerization or chemical polymerization, or suspension polymerization in which imidization proceeds as it is even if the polymer is precipitated after charging the raw material in a solvent and then precipitating the polymer (patented) Reference 1). However, in high temperature solution polymerization and chemical polymerization, synthesis is difficult due to high instability in solution due to high stability of the molecular structure of polyimide. In addition, since any of the above polymerization methods uses an acid anhydride, there is also a problem that thorough removal of water at the time of reaction preparation is required.

  In order to improve this, a method for synthesizing polyimide using a solvent containing a large amount of water has also been developed (see Patent Document 2). However, since this method requires a large amount of water (80% or more of the solvent), there is a problem that the pressure is increased at a high temperature for performing imidization.

  Therefore, for the synthesis of polyimide, a method of polymerizing in a solid phase may be effective (see Non-Patent Document 1). Since solid phase polymerization does not require a solvent, it is not necessary to select a solvent, and as a raw material for the nylon salt type monomer used for solid phase polymerization, tetracarboxylic acid and its diester can be used. Very simple.

  However, the polyimide obtained by solid-phase polymerization often has a problem that even a concentrated sulfuric acid is insoluble even though it originally has a structure that dissolves in a solvent. This phenomenon is caused by the fact that polyimide with strong intermolecular interaction exists in a more solid state called a solid phase, so that planar imide groups are arranged linearly or planarly to form rigid molecules, This is considered to be because the firmness is so strong that it is impossible to solve the meeting state (see Non-Patent Document 2). Moreover, since the movement of molecules is controlled in solid phase polymerization, there is a problem that the molecular weight does not increase depending on the structure.

  On the other hand, imparting high crystallinity to polyimide makes it possible to further improve its heat resistance and mechanical properties. Polyimide composed of long linear aliphatic diamine and aromatic tetracarboxylic acid, the linear aliphatic part is a soft segment, the aromatic part is a mesogen and exhibits liquid crystallinity, and the regular arrangement is maintained even during cooling, It can be a highly crystalline polyimide (see Non-Patent Document 1).

  However, it is necessary to perform precise molecular design in order to allow a polyimide that does not have a clear soft segment to exhibit a high crystallization speed that immediately crystallizes even during molding (see Patent Document 3). In the case of a polyimide having crystallinity but a slow crystallization speed, annealing for a long time for crystallization (see Patent Document 4) or addition of a polyimide originally having a high crystallization speed ( It is also a problem that it is necessary to refer to Patent Document 5).

Japanese Patent No. 2950489 Japanese Patent Laid-Open No. 2001-181389 Japanese Patent Laid-Open No. 62-236858 Japanese Patent Laid-Open No. 10-152558 Japanese Patent No. 3708348

Macromolecules, 28, 6368, 1995 "Development of new polyimide and technology for high functionality for next-generation electronics and electronic materials", Technical Information Association, p53, 2003

  An object of the present invention is to solve the above-described problems in the prior art, and to provide a production method capable of easily obtaining a crystalline, solvent-soluble and high molecular weight polyimide.

As a result of intensive research on the above three characteristics (high crystallinity, solvent solubility, and high molecular weight), the present inventors conducted a solid-state polymerization as a first step, followed by a solid-phase polymerization of a nylon salt monomer. It was found that the objective characteristics can be expressed by suspension polymerization in the second stage, and the present invention has been achieved.
That is, in the present invention, a nylon salt monomer comprising tetracarboxylic acid or an alkyl ester thereof (an alkyl group having 1 to 5 carbon atoms) and a diamine is subjected to solid phase polymerization as a first step, This is a polyimide production method in which suspension polymerization is carried out at a temperature higher than the solid phase polymerization temperature in a poor solvent in two stages.
Furthermore, the present invention is a polyimide having the following properties (1) to (3) obtained from pyromellitic acid and a diamine having an aliphatic ring.
(1) The calorific value of the crystallization exothermic peak during cooling at 20 ° C./min as measured by a differential scanning calorimeter is 5 J / g or more.
(2) Dissolve 0.1 g or more in 20 ml of concentrated sulfuric acid (96%) at 25 ° C.
(3) Using concentrated sulfuric acid (96%) as a solvent, the logarithmic viscosity value measured at a concentration of 0.5 g / dl and 30 ° C. is 1.0 dl / g or more.

  According to the production method of the present invention, crystallinity can be imparted, solvent solubility can be improved, and molecular weight can be increased.

  In the present invention, a nylon salt monomer composed of tetracarboxylic acid or an alkyl ester thereof (alkyl group has 1 to 5 carbon atoms) and a diamine is used as a polyimide raw material. The nylon salt monomer is preferably purified by recrystallization.

As the tetracarboxylic acid, either an aliphatic tetracarboxylic acid or an aromatic tetracarboxylic acid can be used, but it preferably contains an aliphatic ring or an aromatic ring. In the case of using an alkyl ester of tetracarboxylic acid (the alkyl group has 1 to 5 carbon atoms), the alkyl group preferably has 1 to 3 carbon atoms. Moreover, as the alkyl ester body of tetracarboxylic acid (the alkyl group has 1 to 5 carbon atoms), a dialkyl ester body is preferable.
For example, cyclobutane-1,2,3,4-tetracarboxylic acid, cyclopentane-1,2,3,4-tetracarboxylic acid, cyclohexane-1,2,4,5-tetracarboxylic acid, pyromellitic acid, 3 , 3 ', 4,4'-diphenylsulfonetetracarboxylic acid, 3,3', 4,4'-benzophenonetetracarboxylic acid, biphenyltetracarboxylic acid, naphthalene-1,4,5,8-tetracarboxylic acid, etc. It is recommended to use alkyl ester derivatives thereof. Of these, pyromellitic acid, cyclohexane-1,2,4,5-tetracarboxylic acid, 3,3 ′, 4,4′-benzophenone tetracarboxylic acid and the like can be suitably used.

As the diamine, any of linear aliphatic diamine, other aliphatic diamines, and aromatic diamines can be used, but it is preferable that the diamine contains an aliphatic ring or an aromatic ring.
For example, 1,4-phenylenediamine, 1,3-phenylenediamine, 2,4-toluenediamine, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 1,4 -Bis (4-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 1,3-bis (3-aminophenoxy) benzene, α, α'-bis (4-aminophenyl) 1, 4-diisopropylbenzene, α, α'-bis (3-aminophenyl) -1,4-diisopropylbenzene, 2,2-bis [4- (4-aminophenoxy) phenyl] propane, 4,4'-diaminodiphenyl Sulfone, bis [4- (4-aminophenoxy) phenyl] sulfone, bis [4- (3-aminophenoxy) phenyl] sulfone, 2,6-diaminonaphthalene, 1,5-diaminonaphthalene, ethylenediamine, hexamethylenediamine, Heptamethylene Amine, octamethylenediamine, decanediamine, dodecanediamine, polyethylene glycol bis (3-aminopropyl) ether, polypropylene glycol bis (3-aminopropyl) ether, 1,3-bis (aminomethyl) cyclohexane, 1,4-bis (Aminomethyl) cyclohexane, p-xylylenediamine, m-xylylenediamine, siloxane diamines, 4,4'-diaminodicyclohexylmethane, 4,4'-methylenebis (2-methylcyclohexylamine), isophoronediamine, norbornane Diamine, bis (aminomethyl) tricyclo [5.2.1.02,6] decane, etc. are recommended. Of these, 1,3-bis (aminomethyl) cyclohexane, hexamethylenediamine, bis (aminomethyl) tricyclo [5.2.1.02,6] decane, and the like can be suitably used.

  As the solid phase polymerization conditions for the first stage, in the TG / DTA (10 ° C./min) measurement, it is preferable to carry out within the range from the temperature at which imidization proceeds and weight loss starts to the temperature at which imidization is completed. If the heating temperature in the solid phase polymerization is too high, the solvent solubility tends to decrease, which is not preferable. The solid phase polymerization time is preferably 10 to 120 minutes, more preferably 40 to 80 minutes. The heating atmosphere can be performed in the air, but is preferably performed in a nitrogen atmosphere, in a vacuum-free state such as under vacuum.

  In the second stage, suspension polymerization is carried out in a poor solvent at a temperature higher than the solid phase polymerization temperature, preferably at a temperature higher than the temperature at which the imidization is completed. The poor solvent is a solvent in which the solid obtained after solid-phase polymerization does not dissolve at room temperature, more preferably a solvent in which the solid does not dissolve even at suspension polymerization temperature, and more preferably an affinity with the solid obtained after solid-phase polymerization. It is a high solvent. As the poor solvent, two or more kinds of mixed solvent systems can be used. When the poor solvent is hydrophilic, it is possible to add water which has an adverse effect on the polymerization of polyimide. In addition, since a high boiling point solvent can be used, the pressure in the reaction vessel can be kept low even at a high temperature exceeding 200 ° C. Examples of these poor solvents are water, benzene, toluene, xylene, acetone, hexane, heptane, chlorobenzene, methanol, ethanol, n-propanol, isopropanol, methyl glycol, methyl triglycol, hexyl glycol, phenyl glycol, ethylene glycol, ethylene Glycol monomethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monoisobutyl ether, methyl propylene glycol, methyl propylene diglycol, propyl propylene glycol, phenyl propylene glycol, N-methyl-2-pyrrolidone, N, N-dimethylacetamide, N, N-diethylacetamide, N, N-dimethylformamide, N, N-diethylformamide, N-methylcaprolactam, hexamethylphos Luamide, tetramethylene sulfone, dimethyl sulfoxide, m-cresol, phenol, p-chlorophenol, 2-chloro-4-hydroxytoluene, diglyme, triglyme, tetraglyme, dioxane, γ-butyrolactone, dioxolane, cyclohexanone, cyclopentanone, Examples include, but are not limited to, dichloromethane, chloroform, 1,2-dichloroethane, 1,1,2-trichloroethane, dibromomethane, tribromomethane, 1,2-dibromoethane, 1,1,2-tribromoethane, etc. Not.

  In the above suspension polymerization, it is possible to carry out the polymerization in a pressurized system, which is useful when using a poor solvent having a boiling point equal to or lower than the imidization completion temperature, but the solvent and the polyimide are in contact with each other during the suspension polymerization. Otherwise, the characteristics of the present invention tend to be difficult to express. That is, a combination of a single hydrophobic solvent and a strong hydrophobic polyimide can be used but is not preferred.

When the production method of the present invention is applied to the production of a polyimide obtained from pyromellitic acid and a diamine having an aliphatic ring, a polyimide having the following properties (1) to (3) can be obtained.
(1) The calorific value of the crystallization exothermic peak during cooling at 20 ° C./min as measured by a differential scanning calorimeter is 5 J / g or more.
(2) Dissolve 0.1 g or more in 20 ml of concentrated sulfuric acid (96%) at 25 ° C.
(3) Using concentrated sulfuric acid (96%) as a solvent, the logarithmic viscosity value measured at a concentration of 0.5 g / dl and 30 ° C. is 1.0 dl / g or more.

  EXAMPLES Next, although an Example is given and this invention is demonstrated in more detail, this invention is not limited to this. The physical properties in each example were evaluated by the following methods.

The logarithmic viscosity μ is obtained by drying the obtained polyimide at 140 to 180 ° C. for 2 hours, then dissolving 0.1 g of polyimide in 20 ml of concentrated sulfuric acid (96%, Kanto Chemical), and using a Canon Fenceke viscometer at 30 ° C. Measurements were made at Using the value of logarithmic viscosity as an index of molecular weight, it was determined that the molecular weight was increased when the logarithmic viscosity was increased. The logarithmic viscosity μ was determined by the following formula.
μ = ln (ts / t ₀ ) / C
t ₀ : Solvent flow time
ts: Flow time of dilute polymer solution
C: 0.5g / dl

  The crystallinity is evaluated with a differential scanning calorimeter (DSC-6220) manufactured by SII Nanotechnology. The measurement conditions are 10 ° C / min for heating in a nitrogen atmosphere, 20 ° C / min for cooling, 10 ° C for reheating. One cycle of ° C / min. As a basic idea, a polymer that is crystallized and has a high degree of crystallinity has less amorphous parts, so that Tg is difficult to observe. In addition, even if a polymer is crystallized immediately after synthesis according to the present invention, a polymer having a low crystallization rate is not observed again after melting once in this DSC measurement.

  The imidization start temperature and imidation completion temperature were measured at a temperature increase of 10 ° C / min in a nitrogen atmosphere using a differential thermal and thermogravimetric simultaneous measurement device (TG / DTA-6200) manufactured by SII Nanotechnology. , The temperature at which weight loss is first observed and the temperature at which weight loss is no longer observed.

[Example 1]
In the reactor, 70.0 g (0.28 mol) of pyromellitic acid (Mitsubishi Gas Chemical) was dissolved in 500 ml of a mixed solvent of methanol (Mitsubishi Gas Chemical) / water = 1/1 to 65 ° C. A solution of 39.2 g (0.28 mol) of 1,3-bis (aminomethyl) cyclohexane (Mitsubishi Gas Chemical Co., Ltd.) dissolved in 300 ml of a mixed solvent of methanol / water = 1/1 was dropped into the reactor, and the heat generation stopped. After cooling to room temperature. The precipitated crystals were filtered and dried by heating at 80 ° C. to obtain 102 g (yield 93%) of a nylon salt monomer (nylon salt monomer 1). The imidization start temperature was 210 ° C., and the imidization completion temperature was 249 ° C.
150 g of the obtained nylon salt monomer 1, 150 g, was placed in a hot air dryer and heated to 220 ° C. in an air atmosphere. One hour after raising the temperature to 220 ° C., the reaction product was taken out and put into a 3 l autoclave.
Further, 1200 g of 2- (2-methoxyethoxy) ethanol (manufactured by Kishida Chemical Co., Ltd.) and 300 g of ion-exchanged water were added as a poor solvent, and after sealing, nitrogen substitution was performed. After raising the temperature to 250 ° C and reacting for 2 hours from the point where the gauge pressure became 2.2MPa, recovery, filtration and washing were performed, and further drying at 140 ° C for 2 hours with a dryer to obtain 116g of polyimide 1 Obtained. As a result of DSC measurement of this polyimide 1, Tm was only observed at 400 ° C. at the first temperature increase, and Tg and Tc were not observed (having high crystallinity). During cooling, Tc was observed at 340 ° C. (calorific value 18.6 J / g), confirming that it had high crystallinity. At the second temperature increase, Tg was observed at 260 ° C and Tm at 400 ° C. The logarithmic viscosity was measured and found to be 1.6 dl / g.

[Comparative Example 1]
A comparative example in which only the solid phase polymerization of the nylon salt monomer 1 is performed is shown below.
Nylon salt type monomer 1, 150 g was placed in a heat dryer, and the temperature was raised to 250 ° C. under a nitrogen atmosphere. After 2 hours, the reaction product was taken out (Polyimide 1-2) and evaluated for physical properties. The logarithmic viscosity could not be measured because polyimide 1-2 was insoluble in concentrated sulfuric acid. In DSC measurement, Tg was observed at 260 ° C, Tc at 340 ° C, and Tm at 400 ° C at the first temperature increase, but Tc and Tm were weak. In addition, Tc was not observed during cooling. At the second temperature increase, the same peak as the first temperature increase was observed. As compared with polyimide 1 obtained in Example 1 which is the production method of the present invention, significant differences in crystallinity and solubility were observed.

[Comparative Example 2]
196 g (0.899 mol) of pyromellitic dianhydride (manufactured by Mitsubishi Gas Chemical) and 505 g of isopropyl alcohol (manufactured by Kanto Chemical) were charged into a 3 l autoclave. The mixture was heated and stirred at 80 ° C. for 3 hours, and then cooled to 50 ° C. After cooling, 128 g (0.899 mol) of 1,3-bisaminomethylcyclohexane and 252 g of ion-exchanged water were introduced with a syringe, the temperature was raised to 220 ° C., and the mixture was stirred for 2 hours. After cooling to room temperature, it was collected, filtered and washed, and further dried with a vacuum dryer at 100 ° C. for 18 hours to obtain 243 g of polyimide 1-3. As a result of DSC measurement, Tg and Tc were not observed at the first temperature rise, and Tm was 400 ° C. During cooling, Tc was not observed, and Tc and Tm were not observed even at the second temperature increase, but only Tg was observed. Thus, the crystallinity was remarkable as compared with polyimide 1 obtained by the production method of the present invention. The difference was confirmed. The logarithmic viscosity was 1.7 dl / g.

[Example 2]
In the reactor, 5.16 g (0.0443 mol) of pyromellitic acid was dissolved in 30 ml of methanol. Bis (aminomethyl) tricyclo [5.2.1.02,6] decane (Celanese Chemicals product) 7.50 g (0.0443 mol) dissolved in 30 ml of methanol was dropped into the reactor and cooled to room temperature after the heat generation stopped. did. The obtained solution was dropped into 300 ml of ethanol (manufactured by Kanto Chemical) to precipitate a solid, which was collected by filtration. The recovered solid is recrystallized with 95 ml of ethanol / water = 2/1 mixed solvent, and the precipitated crystals are filtered and dried by heating at 80 ° C. to obtain the desired nylon salt type monomer (nylon salt type monomer 2) 4.43 g (35% yield) was obtained. The imidization start temperature was 204 ° C, and the imidization completion temperature was 241 ° C.
The obtained nylon salt-type monomer 2, 1.0 g, was placed in a heat dryer, and the temperature was raised to 220 ° C. under a nitrogen atmosphere. One hour after raising the temperature to 220 ° C., the reaction product was taken out and put into a 20 ml autoclave.
Further, 8 g of 2- (2-methoxyethoxy) ethanol and 1 g of ion-exchanged water were added as a poor solvent, and after sealing, nitrogen substitution was performed. After raising the temperature to 250 ° C and reacting for 2 hours from the point where the gauge pressure became 1.5 MPa, recovery, filtration and washing were carried out, and further drying was carried out at 180 ° C for 2 hours with a dryer to obtain polyimide 2. . As a result of DSC measurement of this polyimide 2, Tg and Tc were not observed at the first temperature rise, and Tm was observed at 360 ° C., which confirmed that a crystallized polyimide was obtained. During cooling, Tc was not observed, and only Tg was observed at 272 ° C for the second temperature increase. The logarithmic viscosity was measured and found to be 0.74 dl / g.

[Comparative Example 3]
A comparative example in which only the solid phase polymerization of the nylon salt monomer 2 is performed is shown below. Polyimide 2-2 was synthesized in the same manner as in Comparative Example 1 except that the nylon salt monomer 2 was used, and the physical properties were evaluated. The logarithmic viscosity could not be measured because the polyimide became insoluble in concentrated sulfuric acid. In the DSC measurement, the Tg is only observed at 270 ° C. in each stage of the first temperature rise, at the time of cooling, and at the second temperature rise, Tm and Tc are not present, and the polyimide obtained by the production method of the present invention Compared to 2, remarkable differences in crystallinity and solubility were observed.

[Example 3]
In the reactor, 5.0 g (0.0197 mol) of pyromellitic acid was dissolved in 40 ml of methanol. A solution of 2.3 g (0.0197 mol) of 1,6-hexamethylenediamine (manufactured by Wako Pure Chemical Industries, Ltd.) dissolved in 40 ml of methanol is dropped into the reactor to precipitate a nylon salt-type monomer, and after the exotherm has stopped, the temperature reaches Cooled down. The resulting solid was collected by filtration and dried at 80 ° C. for 2 hours. The recovered solid was heated to 60 ml and dissolved at 90 ° C. for dissolution and recrystallization. The crystals were collected by filtration and dried by heating at 80 ° C. to obtain 6.9 g (yield 95%) of the desired nylon salt type monomer (nylon salt type monomer 3). The imidization start temperature was 237 ° C., and the imidization completion temperature was 267 ° C.
Synthesis was performed in the same manner as in Example 2 except that the solid-phase polymerization temperature was set to 230 ° C. using the nylon salt type monomer 3 to obtain polyimide 3. As a result of DSC measurement of the obtained polyimide 3, Tg and Tc were not observed at the first temperature rise, and Tm was observed at 455 ° C. During cooling, a Tc peak with a peak top at 389 ° C (21.5 J / g calorific value) was observed. At the second temperature increase, Tg was not observed, and Tm was observed at 413 ° C. The logarithmic viscosity was measured and found to be 2.1 dl / g.

[Comparative Example 4]
A comparative example in which only the solid phase polymerization of the nylon salt type monomer 3 is performed is shown below. Polyimide 3-2 was synthesized in the same manner as in Comparative Example 1 except that the nylon salt type monomer 3 was used, and the physical properties were evaluated. The logarithmic viscosity of the product dissolved in concentrated sulfuric acid was measured and found to be 0.94 dl / g, which was significantly different from the polyimide 3 obtained by the production method of the present invention. As a result of DSC measurement, Tg and Tc were not observed at the first temperature rise, Tm was observed at 455 ° C, and Tc (calorific value 15.3J / g) having a peak top at 389 ° C was observed during cooling. At the second temperature increase, Tg was not observed, and Tm was observed at 413 ° C. It was confirmed that the thermal properties were equivalent to those of polyimide 3 produced by the production method of the present invention.

[Comparative Example 5]
Pyromellitic dianhydride 1.50 g (0.00687 mol) and isopropyl alcohol 10.0 g were charged into a 20 ml autoclave. The mixture was heated and stirred at 80 ° C. for 3 hours, and then cooled to room temperature. After cooling, 0.799 g (0.00687 mol) of 1,6-hexamethylenediamine and 5.0 g of ion-exchanged water were introduced, the temperature was raised to 220 ° C., and the mixture was stirred for 2 hours. After cooling to room temperature, it was collected, filtered and washed, and further dried with a vacuum dryer at 100 ° C. for 18 hours to obtain 1.60 g (yield 89%) of polyimide 3-3. When polyimide 3-3 was dissolved in concentrated sulfuric acid, it was insolubilized, and a significant difference was observed compared to polyimide 3 obtained by the production method of the present invention. As a result of DSC measurement, since the decomposition temperature and melting point were extremely close, accurate thermophysical properties could not be obtained.

Claims (10)

  Nylon salt type monomer consisting of tetracarboxylic acid or its alkyl ester (alkyl group has 1 to 5 carbon atoms) and diamine is subjected to solid phase polymerization as the first stage, and then in a poor solvent in the second stage. And a method for producing polyimide by suspension polymerization at a temperature higher than the solid phase polymerization temperature.   The method for producing a polyimide according to claim 1, wherein the second stage is under pressure.   The manufacturing method of the polyimide of Claim 1 or 2 which has the process of obtaining the salt which consists of tetracarboxylic acid or its alkylester body (alkyl is C1-C5) and diamine by recrystallization.   The method for producing a polyimide according to any one of claims 1 to 3, wherein the tetracarboxylic acid is a tetracarboxylic acid having an aliphatic ring or an aromatic ring.   The method for producing a polyimide according to claim 1, wherein the tetracarboxylic acid is pyromellitic acid.   The method for producing a polyimide according to any one of claims 1 to 5, wherein the diamine is an diamine having an aliphatic ring or an aromatic ring.   The method for producing a polyimide according to any one of claims 1 to 5, wherein the diamine is a linear aliphatic diamine.   The method for producing a polyimide according to any one of claims 1 to 5, wherein the diamine is 1,3-bis (aminomethyl) cyclohexane. A polyimide having the following properties (1) to (3) obtained from pyromellitic acid and a diamine having an aliphatic ring.
(1) The calorific value of the crystallization exothermic peak during cooling at 20 ° C./min as measured by a differential scanning calorimeter is 5 J / g or more.
(2) Dissolve 0.1 g or more in 20 ml of concentrated sulfuric acid (96%) at 25 ° C.
(3) Using concentrated sulfuric acid (96%) as a solvent, the logarithmic viscosity value measured at a concentration of 0.5 g / dl and 30 ° C. is 1.0 dl / g or more.   The polyimide according to claim 9, wherein the diamine having an aliphatic ring is 1,3-bis (aminomethyl) cyclohexane.