Polyamic acid having three-dimensional network molecular structure, polyimide obtained therefrom and process for the preparation thereof

Abstract: A polyamic acid having a three-dimensional network molecular structure produced by a ring-opening poly-­ addition reaction of (A) at least one tetracarboxylic acid dianhydride; (B) at least one aromatic diamine; and (C) at least one polyamino compound selected from the group consisting of compounds represented by formula (I) or (II): n₁ is 0 or an integer from 1 to 4; n₂ is 0 or an integer from 1 to 3; X represents an acid; q represents the base number of the acid; R represents -O-, -CH₂-, -CO- or -­ SO₂-; and n₃ is 0 or 1; a polyimide derived from the polyamic acid; and a process for preparing a film of the polyamic acid or polyimide. The polyimide film has superior heat resistance and mechanical characteristics.

Description: The present invention relates to a polyamic acid which is a precursor of a polyimide resin known as a heat-resistant resin. More particularly, it relates to a novel polyamic acid having a three-dimensional network molecular structure capable of providing a gel. Furthermore, the present invention relates to the polyimide resin obtained from the polyamic acid. It also relates to processes for the preparation thereof and preparation of a film therefrom. Methods are known for obtaining polyimide resins by polycondensing a tetracarboxylic acid dianhydride with an aromatic diamine in an organic solvent to prepare a polyamic acid as a precursor of a polyimide resin, followed by the dehydration and cyclization thereof by a chemical dehydration reaction under heating or using a dehydrating agent. Generally, the ring opening polycondensation reaction proceeds in an organic solvent in such an amount as to give a polyamic acid concentration of 5 to 20% by weight in the solution. The solvent is removed from the solution of the polyamic acid uniformly dissolved in the organic solvent to obtain a film or a molded article. Polyimide resins are widely used in the fields of electrical and electronical materials, adhesives, coatings, composite materials, fiber and film materials, because they are excellent in heat resistance, wear resistance, chemical resistance, electrical insulating properties and mechanical characteristics. Polyimide resin films are also used in various ways as coating materials for electric wires, cables and wires and as insulating materials for transformers and printed-wiring boards, because of their excellent characteristics. Generally, polyimide films are prepared by a method wherein an organic solution of a polyamic acid is cast on a support and the dehydration-cyclization reaction of the polyamic acid is accelerated by a heat treatment or a chemical treatment to obtain a polyimide film. Alternatively, after an organic solvent solution of a polyamic acid is cast on a support, the solvent is removed by drying or the cast polyamic acid together with the support is immersed in a coagulating bath to coagulate the polymer solution, whereby a film can be obtained. The thus-obtained film is sometimes called a gel film. However, this gel film is formed by entanglement of polymer chains and does not have a three-dimensional network molecular structure formed through chemical bonds, therefore it is different from a gel film which is obtained by the present invention, which has a three-dimensional network structure. JP-A-57-109614 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a method wherein an organic solvent solution of a dehydrating agent and a polyamic acid and an organic solvent solution of a catalyst and a polyamic acid and are simultaneously applied to a support. After a gel film (obtained by entanglement of polymer chains) of the polyamic acid is formed by drying, the gel film is heat treated to obtain a polyimide film. In the production of polyimide films, films having high mechanical characteristics can be obtained depending on the combinations of tetracarboxylic acid dianhydrides and aromatic diamines to be polycondensed. A polyimide film obtained from the combination of pyromellitic anhydride with 4,4'-diaminodiphenyl ether is a typical film which has high heat resistance and is excellent in tensile properties. However, when heat resistance becomes higher, the film generally is brittle, and hence heat resistance is balanced against mechanical characteristics. Particularly, polyimide obtained from a combination of pyromellitic dianhydride with p-phenylenediamine has poor film properties and is brittle and extremely inferior in mechanical characteristics, though it is among the best with regard to physical and chemical heat resistance. In the polyimide of this type, the mechanical characteristics of the polyimide film can be improved by copolymerization techniques. To improve mechanical characteristics without reducing heat resistance, JP-A-63-254131 discloses a method wherein 4,4'-diaminodiphenyl ether as another component of the aromatic diamine is copolymerized in the production of polyimide by using a combination of pyromellitic dianhydride with p-phenylenediamine. However, a large amount of 4,4'-diaminodiphenyl ether must be added to improve film forming properties and as a result, a loss of heat resistance results. As used herein, the term "free-standing gel" means that the gel is capable of retaining its shape without causing deformation at room temperature (25°C). For example, when the gel is formed on a support as a film, the film is capable of retaining its shape without flowing even the support is inclined. A first object of the present invention is to provide polyamic acids having a three-dimensional network molecular structure. A second object of the present invention is to provide a polyamic acid capable of repeatedly swelling and shrinking by absorption and desorption of an organic solvent, and capable in a swollen state of forming a free-standing gel. A third object of the present invention is to provide a polyimide having three-dimensional network molecular structure from the polyamic acid. A further object of the present invention is to provide a method for preparation of film from the polyamic acid and the polyimide. The invention comprises a polyamic acid having a three-dimensional network molecular structure produced by a ring-opening polyaddition reaction of (A) at least one tetracarboxylic acid dianhydride; (B) at least one aromatic diamine; and (C) at least one polyamino compound selected from compounds represented by formula (I) wherein A 1 represents or R represents -O-, -CH 2 -, -CO- or -SO 2 -; said polyamic acid comprises in a molar ratio of (A)/(B)/(C) of 100/82-100/2-8.5 and satisfies the following relationship: 0.95 x [moles of (B) + 2 x moles of (C)] < moles of (A) < 1.05 x [moles of (B) + 2 x moles of (C)]. Examples of tetracarboxylic acid dianhydrides include pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,3,3',4'-biphenyltetracarboxylic acid dianhydride, 2,2',3,3'-biphenyltetracarboxylic acid dianhydride, 2,2',6,6'-biphenyltetracarboxylic acid dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, 1,2,5,6-naphthalenetetracarboxylic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, bis(3,4-di-carboxyphenyl)sulfone dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, 3,4,9,10-perylenetetracarboxylic acid dianhydride, naphthalene-1,2,4,5-tetracarboxylic acid dianhydride, naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, benzene-1,2,3,4-tetracarboxylic acid dianhydride and ethylene glycol bis-(anhydrotrimellitate). These compounds may be used either alone or as a mixture of two or more of them. Examples of the aromatic diamines include m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminobenzophenone, 3,3'-diaminobenzophenone, 2,2'-bis(4-aminophenyl)propane, benzidine, 3,3'-diaminobiphenyl, 2,6-diaminopyridine, 2,5-diaminopyridine, 3,4-diaminopyridine, bis[4-(4-aminophenoxy)phenyl] sulfone, bis[4-(3-aminophenoxy) phenyl] sulfone, bis[4-(4-aminophenoxy)phenyl] ether, bis[4-(3-aminophenoxy)phenyl] ether, 2,2'-bis[4-(4-aminophenoxy)phenyl] propane, 2,2'-bis[4-(3-aminophenoxy)phenyl] propane, 4,4'-bis(4-aminophenoxy) biphenyl, 1,4-bis(4-aminophenoxy) benzene, 1,3-bis(4-aminophenoxy) benzene, 2,2'-bis[4-(3-aminophenoxy)-phenyl]-hexafluoropropane, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene and derivatives thereof (i.e., substituted diamine having a substituent(s)). The substituent for components (A) and (B) should not adversely affect the condensation reaction and should not deteriorate characteristics of the products obtained therefrom. Such substituents include an aliphatic group, and a halogen atom. These compounds may be used either alone or as a mixture of two or more of them. Typical examples of the polyamino compounds used as component (C) include 3,3',4,4'-tetraaminodiphenyl ether, 3,3',4,4'-tetraaminodiphenylmethane, 3,3',4,4'-tetraaminobenzophenone, 3,3',4,4'-tetraaminodiphenylsulfone, 3,3',4,4'-tetraaminobiphenyl 1,2,4,5-tetraaminobenzene. The above compounds usually exist in the form of hydrated compounds. These compounds may be used either alone or as a mixture of two or more of them. Since each of these components (A), (B) and (C) may be used as a mixture of two or more, the resulting polymers include copolymers. Furthermore, a polyamic acid gel may be obtained by mixing a first solution of a polyamic acid (prior to gelation) containing the specific components (A), (B) and (C) with a second solution of a polyamic acid (prior to gelation) wherein at least one of the aforesaid components (A), (B) and (C) is different from the above first solution, and then proceeding gelation. The polyamic acid of the present invention can be obtained by reacting the components (A), (B) and (C) in an organic solvent at a temperature of -10 to 80°C, preferably -5 to 50°C, more preferably 0 to 30°C, in an inert atmosphere. The reaction time is not longer than 10 hours, preferably not longer than 5 hours, more preferably not longer than 2 hours. When the reaction time exceeds 10 hours, a hydrolysis reaction often occurs. Usually, the reaction time is at least 5 minutes. From the viewpoint of handling that the amount of the organic solvent used during the reaction be controlled so that the polyamic acid gel contains 3 to 50% by weight, preferably 5 to 30% by weight of the resulting polyamic acid. When the concentration of the polyamic acid is lower than 3% by weight, the resulting gel is poorly free-standing, while when the concentration exceeds 50% by weight, the solids content is too high, the viscosity of the solution is increased during the course of the polymerization and the polymerization reaction does not proceed as expected. It is necessary that the organic solvent used for the reaction be inert to the reaction and capable of dissolving the components (A), (B) and (C) to be reacted. Typical examples of the organic solvent include N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide, N,N-diethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, N,N-dimethylmethoxyacetamide, hexamethylphosphoamide, pyridine, dimethyl sulfone, tetramethylene sulfone, phenols such as cresol, phenol and xylenol, benzene, toluene, xylene, benzonitrile, dioxane and cyclohexane. These solvents may be used either alone or as a mixture of two or more of them. The reaction of the components (A), (B) and (C) is a polymerization reaction of the carboxylic anhydride and amino compounds takes place. The component (A) may be added to a solution of the components (B) and (C) dissolved in an organic solvent, in an inert atmosphere such as nitrogen gas. The Component (A) may be added in the form of a solid or a solution thereof in a solvent. Alternatively, the components (B) and (C) may be added to a solution of the component (A) dissolved in an organic solvent. If desired, the component (C) may be added during the course of the reaction of the component (A) with the component (B). However, it should be noted that when the component (B) is added to a reaction system where the predetermined amounts of the components (A) and (C) are reacted, there is a possibility that the reaction will not proceed sufficiently and that gelation may not occur depending on preparation conditions. The molecular weight of the polyamic acid is preferably 10,000 to 300,000, more preferably 30,000 to 300,000. The degree of the network molecular structure can be increased by heating (usually 50 to 80°C), subjecting to light (such as ultraviolet ray) or by applying pressure (usually up to 3.0 kg/cm 2 ). In addition to the components (A), (B) and (C), the reaction mixture may contain small amounts (usually up to 10 wt% based on the component (A)) of a di- or tricarboxylic acid such as trimellitic acid, a carboxylic acid anhydride or a substituted compound thereof. The three-dimensional structure of the polyamic acid containing about 50 to 97% (based on the total weight of the polyamic acid and the solvent) by weight of the above-described organic solvent is a free-standing gel. The content of the solvent in the three-dimensional structural product of the novel polyamic acid obtained by the present invention can be controlled by swelling or shrinkage by the absorption or desorption of the solvent. Deswelling can be conducted by heating preferably at up to 80°C under reduced pressure or by solvent replacement. The number of linkages in the chemical structure of the gel can be increased by external excitement such as temperature, light and pressure. The polyamic acid of the present invention which has a three-dimensional network structure is useful in functional high-molecular materials such as a material separating membrane, or a chemical switch using the swelling-shrinking (deswelling) function. The polyimide resin can be obtained by subjecting the polyamic acid to a conventional dehydration and cyclization reaction. For example, imidation can be completed by a high-temperature treatment or a chemical dehydrating treatment using a dehydrating agent. When the polyamic acid is in a gel state, the gel is preferably heated at 30 to 80°C for 0.5 to 10 hours, more preferably at 30 to 50°C for 1 to 5 hours. Usually, the temperature is gradually elevated in order to maintain the form of the polyamic acid (usually it is film). When the polyamic acid is in a deswollen state heating is usually conducted at 150 to 500°C for 10 seconds to 10 hours, preferably at 200 to 400°C for 1 to 5 hours. The imidation can be confirmed by observing infrared spectral absorption at 1780 cm -1 and 1720 cm -1 . The imidation of the polyamic acid can be conducted while retaining the polyamic acid in gel state, for example, by a method wherein dehydration and cyclization are chemically carried out by impregnating the polyamic acid with a solution of a dehydrating agent. Alternatively, the gel may be subjected to a chemical treatment after the solvent is removed from the gel or the solvent is replaced with a poor solvent. It is effective to use acid anhydrides as dehydrating agents in the presence of amines as catalysts in the chemical dehydration reaction. Examples of the acid anhydrides include aliphatic acid anhydrides such as acetic anhydride, propionic anhydride and butyric anhydride and benzoic anhydride. These compounds may be used either alone or as a mixture of two or more of them. Examples of the amines which can be used as catalysts include tert-amines such as trimethylamine, triethylamine, triethylenediamine, hexamethylenetetraamine, tributylamine, dimethyl-aniline, pyridine, α-picoline, β-picoline, γ-picoline, isoquinoline and lutidine. At least one amine selected from the group consisting of tert-amines is used as a catalyst. The amount of the acid anhydride to be added in the chemical dehydration reaction is preferably 1 to 5 equivalents, more preferably 1 to 3 equivalents per equivalent of carboxyl group present in the polyamic acid. The amount of the catalyst is preferably 0.01 to 1.5 equivalents, more preferably 0.2 to 1 equivalent per equivalent of carboxyl group present in the polyamic acid. The acid anhydride and the amine are dissolved to an organic solvent for these compounds to form a solution used for the dehydration reaction. Examples of solvents include the above-described solvents for components (A), (B) and (C). The concentration of the acid anhydride is usually 1 to 10% by weight. The chemical treatment for imidation is preferably conducted at 0 to 80°C for 0.5 to 48 hours. A polyimide resin having a specific molecular stereostructure can be prepared by subjecting the polyamic acid to the above-described dehydrating-cylization reaction. A polyimide gel having a three-dimensional network molecular structure can be obtained when the imidation is conducted using the polyamic acid gel. A polyimide gel containing the above-described organic solvent in an amount depending on preparation conditions can be obtained. The state of the three-dimensional network structure of the novel polyimide resin obtained by the present invention can be changed by swelling or shrinking by absorption or desorption of the solvent. The resulting polyimide resin has excellent heat resistance and hence the resulting polyimide resin is useful as a functional high-molecular material having a material separating ability, an excitation response effect or a catalytic effect. The process for the preparation of polyimide film includes the steps of casting or coating an organic solvent solution of the polyamic acid of the present invention on a support to prepare a gel film of the polyamic acid which gives a free-standing three-dimensional network structure, and subjecting it to a dehydration-cyclization reaction after peeling or without peeling the film from the support. Examples of support material include glass, metals, ceramics, and polymer resins such as a polyester resin. Typical examples of the tetracarboxylic acid dianhydride preferably used as the component (A) for preparation of the polyimide film in the present invention include pyromellitic dianhydride, benzophenonetetracarboxylic acid dianhydride or biphenyltetracarboxylic acid dianhydride alone or a mixture of two or more thereof as the tetracarboxylic acid dianhydride. They are preferred because a polyimide film having high heat resistance and excellent mechanical characteristics (e.g., tensile strength) can be obtained. Typical examples of the aromatic diamine preferably used as the component (B) for preparation the polyimide film of the present invention include p-phenylenediamine, m-phenylenediamine and 4,4-diaminodiphenyl ether alone or a mixture of two or more thereof. They are preferred because a polyimide film having high heat resistance and excellent mechanical characteristics can be obtained. When the reaction temperature of components (A), (B) and (C) is lower than -10°C, there is difficulty in handling the reaction system and further, there is a possibility that the reaction temperature itself is too low for the reaction to proceed sufficiently. When the reaction temperature is higher than 80°C, the reaction proceeds too rapidly until gelation is caused, and there is a possibility that gelation will occur before film formation, and a uniform film cannot be obtained. Accordingly, when a film is prepared from a solution of a polyamic acid obtained by polyaddition of the component (A) with the components (B) and (C), it is preferred that the solution of the polyamic acid and a substrate for use in the preparation of the film are controlled to a temperature of from 0 to +30°C prior to the preparation of the film and then the solution of the polyamic acid is cast or coated on a support to prepare the film. When the components are blended in a composition outside the range defined above, for example, in proportions such that (moles of A) < 0.95x[(moles of B) + 2x(moles of C of formula (I))], and reacted, the molecular weight of polymer is low and the properties such as mechanical properties of the resulting polymer film are remarkably poor. On the other hand, when the proportions are such that (moles of A) > 1.05x[(moles of B) + 2x(moles of C)], unreacted monomers are left in the polymer solution and the characteristics of the film obtained by the evaporation and drying of the polymer solution are adversely affected. For example, the thermal properties and hygroscopic properties of the finally obtained polyimide film may be lowered. When the molar ratio of (C)/(A) is higher than 8.5/100, the crosslinking points of the three-dimensional network structure are increased, the film properties of the gel structure are adversely affected, and the film is apt to be brittle. Accordingly, the molar ratio of (C) /(A) is in the range of (2 to 8.5)/100. In the film of the polyamic acid prepared from the components (A), (B) and (C) of the present invention, gelation is caused after the solution of the polyamic acid is cast on a support, whereby a gel film of the polyamic acid is obtained. The gel film is free-standing when it contains about 50 to 97% by weight of the organic solvent based on the total amount of the gel film. It is preferred that when a polyimide film is prepared from the gel film of the polyamic acid as a precursor, the solvent is removed at a temperature of not higher than 100°C under atmospheric pressure or vacuum to adjust the solvent content of the gel film 0 to 50% by weight, preferably not higher than 40% by weight, and the resulting film is subjected to a dehydration and cyclization reaction to give a polyimide film. According to this method, there can be obtained a film with remarkably few defects that is excellent in physical properties (such as tensile strength). Further, a polyimide film can be obtained in the following manner. The gel film of the polyamic acid which contains the solvent is immersed in a coagulating bath comprising a poor solvent for the polyamic acid, whereby the organic solvent left in the film is removed by the substitution of the solvent or the washing of the film is carried out. Thereafter, the film is peeled off from the support and treated at a high temperature or chemically treated to obtain a polyimide film. In this operation, water is generally used as a coagulating liquid. Alcohols such as methanol and ethanol may be used with water in an amount of not more than 50% by weight based on the total amount of the coagulating liquid. Further, ketones such as acetone, amide solvents such as N,N-dimethylacetamide and chlorine-containing solvents such as 1,2-di-chloroethane may be used in an amount of not more than 20% by weight based on the total amount of the coagulating liquid. When the above-described washing of the film is intensified, impurities such as chlorine contained in the film can be removed. As a method for allowing imidation while retaining the three-dimensional network molecule structuree, imidation can be conducted in a manner similar to those used for conventional polyimide resins. For example, when imidation is to be completed by a high-temperature treatment, the free-standing gel film of the polyamic acid is dried until the amount of the solvent is reduced to 0 to 10% by weight at a temperature of 50 to 200°C for at least 10 seconds and further heat-treated at an elevated temperature of 150 to 500°C, preferably 200 to 450°C. When the above treatment is carried out while fixing both ends in the longer length direction of the film by means of a fixing frame, a fastener or a pin guide, a film excellent in dimensional stability and mechanical characteristics can be obtained. This method is very effective, because the gel film of the polyamic acid is greatly shrunk by drying or heat treatment in particular. Further, a method can be used wherein the three-dimensional network structural gel film of the polyamic acid is immersed in a solution of a dehydrating agent or a solution of a dehydrating agent is applied by penetrating in the gel film by coating or spraying onto the gel film as described hereinbefore for imidation processing, to perform chemical dehydration and cyclization. The polyimide film obtained from the gel film of the novel polyamic acid of the present invention has a three-dimensional network molecular structure. Hence, the mechanical characteristics of the film itself are improved and excellent heat resistance and mechanical strength with toughness are imparted to the film. According to the present invention a uniform polyimide film can be formed from the polyamic acid gel. A polyimide film having a specific performance can be prepared by the mechanical treatment such as orientation of the film or the preparation of a coagulated porous film by the substitution of the solvent in a coagulating bath. In particular polyimides having high heat resistance (a) a combination of one or more of pyromellitic dianhydride, benzophenonetetracarboxylic acid dianhydride and biphenyltetracarboxylic acid dianhydride as the tetracarboxylic acid dianhydride component, (b) one or more members of p-phenylenediamine, m-phenylenediamine and 4,4'-diaminodiphenyl ether as the aromatic diamine component and (c) component (C). The improvement of film due to the formation of the three-dimensional molecular structure of the polyamic acid by the addition of the component (C) is remarkable. Since the film thus obtained has a specific molecular structure, the state of the film structure can be changed by swelling or shrinkage (by penetration of the solvent or deswelling), or by changing the number of linkage in the molecular structure by external excitement such as temperature, light or pressure. Further, the film has excellent heat resistance and the film is useful as a functional high-molecular material having a material separating ability, an excitation response effect or a catalytic effect. Various additives, for example, metallic compounds, low-molecular organic compounds (other than the solvent), high-molecular compounds (for example, a polyamic acid derived from components (A) and (B) and plyimide derived from components (A) and (B), inorganic fillers, coloring materials, and/or reinforcing fiber may be present in the three-dimensional molecular network structure of the polyamic acid or polyimide of the present invention. Polyamic acids or polyimides of the present invention having different components (A), (B) and (C) or different amounts of each of components may be used as a mixture. The polyamic acids may also be mixed with the polyamide. The invention is now illustrated in greater detail with reference to the following specific examples, but the present invention is not to be construed as being limited thereto. Unless otherwise indicated, all parts, percents and ratios are by weight. In order to determine heat resistance of the polymer obtained in Examples, the temperature of 5% by weight reduction was measured by thermogravimetric analysis (in a nitrogen atmosphere at a heating rate of 10°C/min.) EXAMPLE 1 In a 500 ml four-necked separable flask, there were placed 0.038 mol (7.608 g) of purified 4,4'-diaminodiphenyl ether (component B, abbreviated to 4,4'-DPE) and 0.001 mol (0.214 g) of 3,3',4,4'-tetraaminobiphenyl (component C, abbreviated to TAB). To the flask was then added 115 g of distilled N,N-dimethylacetamide (solvent, abbreviated to DMAc). The mixture was stirred to dissolve them. The temperature of an external water bath was controlled to 15°C in a nitrogen atmosphere. While stirring the solution obtained above, 0.04 mol (8.732 g) of purified anhydrous pyromellitic dianhydride (component A, abbreviated to PMDA) in the form of a solid was gradually added thereto with care so that the temperature of the solution was not raised. With the completion of the addition of all of PMDA, stirring was stopped and the solution was left to stand for 2 hours at 30°C. The resulting product was a gel-form polymer, exhibiting N-H absorption of amic acid at 3280 cm -1 by infrared absorption analysis. The gel-form polymer was treated at 150°C for one hour, 200°C for one hour, at 300°C for one hour and at 370°C for one hour. Absorption bands specific to an imide group were observed at 1780 cm -1 and 1720 cm -1 . Hence, it was confirmed that the polymer was a polyamic acid which was a precursor of a polyimide. In order to determine the heat resistance of the resin the temperature of the resin was raised until the polyimide decomposed. The temperature at which 5% weight reduction of the polyimide obtained by the heat treatment occurred was 550°C. In Table 1 below, the blending ratios of monomers used in the reaction are indicated by molar ratios. COMPARATIVE EXAMPLE 1 In this Comparative Example Component (C) was omitted from the composition of the present invention to obtain a conventional polyamic acid. In a 500 ml four-necked separable flask, there was placed 0.04 mol (8.008 g) of purified 4,4'-DPE. To the flask was then added 115 g of distilled DMAc and the mixture was stirred to dissolve it. Thereafter, a polyamic acid was prepared in the same manner as in Example 1. The state of the resulting polymer was observed. The resulting polymer was not gelled, but was a viscous liquid. The intrinsic viscosity thereof was 1.2 dl/g, at 25°C. EXAMPLES 2 TO 8 AND COMPARATIVE EXAMPLES 2 TO 3 In these Comparative Examples the molar ratios of the components were outside the scope of the invention. Into a 500 ml four-necked separable flask, there were charged monomers and solvents indicated in Table 1 in amounts given in Table 1. In the same manner as in Example 1, polyamic acids were prepared. The states of the polyamic acids were observed. In Examples 2 to 8, gel-form polymers were obtained, and it was confirmed that the polymers were gel-form polyamic acids by infrared absorption spectrum. EXAMPLE 9 The preparation of the polyamic acid was conducted by using a 500 ml four-necked separable flask and the same monomer composition and reaction conditions as those in Example 1. Before the polymer was gelled, the solution was cast on a glass plate in an amount of 460 g/m 2 to prepare a gel film of the polyamic acid. The resulting gel film of the polyamic acid was immersed in a solution containing 240 ml of DMAc, 24.2 g of acetic anhydride and 6.3 g of pyridine and treated at room temperature for 24 hours while fixing the gel film so as to avoid warping or deformation. The imidation of the polyamic acid was thus performed, and the agar-agar-form gel was changed to a rigid gel containing the solvent. It was confirmed that the three-dimensional molecular structure of the rigid polyimide resin was could be repeatedly swollen and shrunk by the penetration of the solvent (which can be used for preparation of the polyamic acid according on the present invention) and deswelling. After the solvent was removed (to about 6 wt%) from the three-dimensional structure of the polyimide resin, heat treatment was carried out under the same conditions as those of Example 1. The temperature of 5 wt% weight reduction of the resulting film analyzed by thermogravimetric analysis was 550°C. An infrared absorption spectrum analysis of the three-dimensional structure of the polyimide resin was made. There were found absorption bands specific to imide group at 1780 cm -1 and 1720 cm -1 . Thus, it was confirmed that the imidation was completed. COMPARATIVE EXAMPLE 4 In this comparative example, the molar ratio of components being outside the scope of the invention. In a 500 ml four-necked separable flask, there were placed 0.039 mol (7.80 g) of purified 4,4'-DPE and 0.0005 mol (0.107 g) of TAB. To the flask was then added 115 g of distilled DMAc. The mixture was stirred to dissolve them. Subsequently, the preparation of the polyamic acid was carried out in the same manner as in Example 1. The resulting polymer was not gelled, but was a viscous liquid. The intrinsic viscosity thereof was 1.58 dl/g at 30°C. The polymer was heat-treated in the same manner as in Example 1. The temperature of 5 wt% weight reduction of the resulting film analyzed by thermogravimetric analysis was 560°C. EXAMPLES 10 TO 15, COMPARATIVE EXAMPLES 5 TO 6 In Comparative Example 5, Component (C) was omitted, and in Comparative Example 6 the molar ratios of components were outside the scope of the invention. Into a 500 ml four-necked separable flask, there were charged monomers and solvents indicated in Table 3 in amounts given in Table 3. In the same manner as in Example 1, polyamic acids were prepared. Polymers which were gelled were subjected to a chemical dehydration reaction in the same manner as in Example 2 to prepare the three-dimensional molecular structure of polyimide resins. After the preparation of the resins, it was conformed that the polymers were converted into the polyimide resin by infrared absorption spectrum. In Examples 10 to 15, the three-dimensional molecular structures of polyimide resins were prepared from gel-form polyamic acids. The temperature of 5 wt% weight reduction of each resin was measured. The results are shown in Table 5.

Claims: A polyamic acid having a three-dimensional network molecular structure produced by a ring-opening polyaddition reaction of (A) at least one tetracarboxylic acid dianhydride; (B) at least one aromatic diamine; and (C) at least one polyamino compound selected from compounds represented by formula (I) wherein A 1 represents or R represents -O-, -CH 2 -, -CO- or -SO 2 -; said polyamic acid comprises in a molar ratio of (A)/(B)/(C) of 100/82-100/2-8,5 and satisfies the following relationship: 0.95 x [moles of (B) + 2 x moles of (C)] < moles of (A) < 1.05 x [moles of (B) + 2 x moles of (C)]. The polyamic acid as claimed in claim 1, wherein said tetracarboxylic acid dianhydride is selected from pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2,3,3',4'-biphenyltetracarboxylic acid dianhydride, 2,2',3,3'-biphenyltetracarboxylic acid dianhydride, 2,2',6,6'-biphenyltetracarboxylic acid dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, 1,2,5,6-naphthalenetetracarboxylic acid dianhydride, 2,2-bis(3,4-dicarboxyphenyl)pzopane dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, 3,4,9,10-perylenetetracarboxylic acid dianhydride, naphthalene-1,2,4,5-tetracarboxylic acid dianhydride, naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, benzene-1,2,3,4-tetracarboxylic acid dianhydride and ethylene glycol bis-(anhydrotrimellitate). The polyamic acid as claimed in claim 1, wherein said aromatic diamine is selected from m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminobenzophenone, 3,3'-diaminobenzophenone, 2,2'-bis(4-aminophenyl)propane, benzidine, 3,3'-diaminobiphenyl, 2,6-diaminopyridine, 2,5-diaminopyridine, 3,4-diaminopyridine, bis[4-(4-aminophenoxy)phenyl] sulfone, bis[4-(3-aminophenoxy) phenyl] sulfone, bis[4-(4-aminophenoxy)phenyl] ether, bis[4-(3-aminophenoxy)phenyl] ether, 2,2'-bis[4-(4-aminophenoxy)phenyl] propane, 2,2'-bis[4-(3-aminophenoxy)phenyl] propane, 4,4'-bis(4-aminophenoxy) biphenyl, 1,4-bis(4-aminophenoxy) benzene, 1,3-bis(4-aminophenoxy) benzene, 2,2'-bis[4-(3-aminophenoxy)phenyl]-hexafluoropropane, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene and derivatives thereof. The polyamic acid as claimed in claim 1, wherein said polyamic acid has a molecular weight of from 10,000 to 300,000. The polyamic acid as claimed in claim 1, wherein said polyamic acid is in the form of a free-standing gel capable of retaining its shape without causing deformation at 25°C. The polyamic acid as claimed in claim 5, wherein said gel contains an organic solvent in an amount of from 50 to 97% by weight based on the total weight of the gel. The polyamic acid as claimed in claim 5, wherein said free-standing gel is in the form of a film. A process for making a polyamic acid having a three-dimensional network molecular structure comprising conducting a ring-opening polyaddition reaction of (A) at least one tetracarboxylic acid dianhydride, (B) at least one aromatic diamine, and (C) at least one polyamino compound selected from compounds represented by formula I in an organic solvent at a temperature in a range of 0 to 80°C wherein A 1 represents or R is -O-, -CH 2 -, -CO- or -SO 2 -; said polyamic acid comprises in a molar ratio of (A)/(B)/(C) of 100/82 -100/2-8,5 and satisfies the following relationship: 0.95 x [moles of (B) + 2 x moles of (C)] < moles of (A) < 1.05 x [moles of (B) + 2 x moles or (C)]. The process as claimed in claim 8, wherein said polyamic acid reaction product is in the form of a free-standing gel capable of retaining its shape without causing deformation at 25°C. The process as claimed in claim 8, wherein said solvent is used in an amount of 50 to 97% by weight of the total weight of the polyamic acid to be produced and the solvent. The process as claimed in claim 8, comprising the further step of casting the solution formed in said step as a film on a support, wherein said reaction is completed on said support to provide a gel film of said polyamic acid three dimensional network molecular structure. A process for producing a polyimide resin comprising the steps of: (a) conducting a ring-opening polyaddition reaction of (A) at least one tetracarboxylic acid dianhydride; (B) at least one aromatic diamine; and (C) at least one polyamino compound selected from compounds represented by formula I wherein A 1 represents or R represents -O-, -CH 2 -, -CO- or -SO 2 -; said polyamic acid comprises in a molar ratio of (A)/(B)/(C) of 100/82-100/2-8,5 and satisfies the following relationship : 0.95 x [moles of (B) + 2 x moles of (C)] < moles of (A) < 1.05 x [moles of (B) + 2 x moles o (C)]. (b) dehydrating and cyclizing said polyamic acid to produce a polyimide resin. The process as claimed in claim 12, wherein said dehydrating and cyclizing step is conducted after drying the reaction product to remove the solvent and then heating the polyamic acid. The process as claimed in claim 12, wherein said dehydrating and cyclizing step is conducted by heating said polyamic acid in a form of a gel. The process as claimed in claim 14, wherein said polyamic acid gel contains 50 to 97% by weight of said organic solvent. The process as claimed in claim 12, wherein said dehydrating and cyclizing step is conducted by contacting said polyamic acid with a solution of dehydrating agent. The process as claimed in claim 12, wherein said dehydrating and cyclizing step comprises the steps of removing at least a portion of said organic solvent from said polyamic acid and subsequently contacting said polyamic acid with a solution of dehydrating agent. The process as claimed in claim 12, wherein said dehydrating agent is an acid anhydride and said dehydrating and cyclizing reaction is conducted in the presence of an amine as a catalyst. The process as claimed in claim 12, comprising the further step of casting the solution formed in step (a) as a film on a support, wherein said reaction in step (a) is completed on said support to provide a gel film of said polyamic acid three dimensional network molecular structure. The process as claimed in claim 12, comprising the additional steps of: dehydrating and cyclizing said polyamic acid gel film to provide a polyimide film. A polyimide resin produced by dehydrating and cyclizing a polyamic acid having three-dimensional network molecular structure, said polyamic acid being produced by ring-opening polyaddition reaction of (A) at least one tetracarboxylic acid anhydride; (B) at least one aromatic diamine; and (C) at least one polyamino compound selected from compounds represented by formula I wherein A 1 represents or R represents -O-, -CH 2 -, -CO- or -SO 2 -; said polyamic and comprises in a molar ratio of (A)/(B)/(C) of 100/82-100/2-8,5 and satisfies the following relationship: 0.95 x [moles of (B) + 2 x moles of (C)] < moles of (A) < 1.05 x [moles of (B) + 2 x moles of (C)]. The polyimide resin as claimed in claim 21, wherein said polyimide resin is in the form of a film.

Inventors: NAGATA YASUHISA, NAGATA, YASUHISA, NAGATA, YASUHISA, C/O TOHO RAYON CO., LTD.
Applicants: TOHO RAYON KK, TOHO RAYON CO., LTD.
Application year: 1990
Main classification: C08G73
IPC codes: C08G73