Polyimide having high strength and high elongation at break, and preparation method therefor and use thereof

Abstract

Disclosed in the present invention are a polyimide having a high strength and a high elongation at break, and a preparation method therefor and the use thereof. The polyimide comprises a rigid rodlike polymer main chain and a polyimide linear macromolecular branch chain, wherein the rigid rodlike polymer main chain comprises an aromatic polyamide or a polyamide-imide, and the rigid rodlike polymer main chain is linked with the polyimide linear macromolecular branch chain via an imide group. Compared with traditional polyimide linear macromolecules, the polyimide of the present invention has outstanding mechanical properties, also has a high tensile strength (200-340 MPa) and a high elongation at break (>140%), and has an important application value in the fields of polyimide films, coatings, molding powders, honeycomb materials, fibers, composite materials, etc.

Description

A polyimide with high strength and high elongation at break and its preparation method and application Technical Field

[0001] The present invention belongs to the field of polymer materials, and in particular relates to a polyimide with high strength and high elongation at break, and a preparation method and application thereof. Background Art

[0002] Polyimide is a high-performance engineering plastic with outstanding heat resistance and good mechanical properties. Its films, coatings, molded sheets / rods, and machined parts are widely used in industry. Polyimides include thermosetting polyimides with three-dimensional cross-linked molecular networks and non-thermosetting polyimides with linear macromolecular structures. However, regardless of whether they are thermosetting polymers, pure polyimides have limited mechanical properties, which to some extent restricts their applications.

[0003] The strength and modulus of polyimide can be significantly improved by adding inorganic reinforcements, such as carbon fiber, glass fiber, and other inorganic fibers, as well as nano- or submicron inorganic materials such as graphene, carbon nanotubes, nanocarbon fibers, and whiskers. However, these inorganic reinforcements can lead to increased density and a significant decrease in elongation at break.

[0004] Furthermore, stretching and orientation are also effective methods for improving the mechanical properties of linear polyimide macromolecules. Biaxial stretching has been reported to increase the tensile strength of polyimide films to over 180 MPa. However, biaxial stretching requires significant investment in production equipment, is complex, and is costly. Furthermore, the elongation at break of biaxially stretched polyimide films is generally less than 80%. Therefore, there remains a need for a simpler technology that can further improve the strength and elongation at break of linear polyimide macromolecules without increasing the polyimide density.

[0005] Patent document No. CN109337065A discloses that a composition obtained by addition reaction or physical mixing of an amino-substituted soluble rigid rod polymer with a thermosetting resin has significantly improved strength and toughness compared to pure thermosetting resin.

[0006] Thermosetting resins are small-molecule compounds with a far greater number of reactive functional groups than those in amino-substituted soluble rigid-rod polymers. Therefore, when the two are mixed and reacted in a solvent, crosslinking, gelation, and precipitation are avoided. After removing the solvent and subjecting the polymer to high-temperature treatment to complete the chemical crosslinking reaction (curing), the resulting polymer system comprises a three-dimensional, random, cross-linked macromolecular network embedded with rigid-rod macromolecules, resulting in significantly improved mechanical properties.

[0007] However, the cross-linked structure of the thermosetting polymer greatly suppresses the vibration, rotation, stretching, slippage and other movements of the macromolecular chain. Therefore, even after toughening modification of the amino-substituted soluble rigid rod polymer, the elongation at break of the thermosetting polymer is difficult to exceed 10%. In addition, the improvement of its mechanical properties is also limited. Patent document CN109337065A and journal (Fully aminated rigid-rod aramid reinforced high strength epoxy resin and its composite with carbon fibers, Composites Science and Technology, 2022, 221; New reactive rigid-rod aminated aromatic polyamide for the simultaneous strengthening and toughening of epoxy resin and carbon fiber / epoxy composites; Composites Part B-Engineering, 197, 108044) show that the strength of the amino-substituted soluble rigid rod polymer modified thermosetting resin is only about 140MPa at most, and the elongation at break is less than 10%.

[0008] As mentioned above, non-thermosetting linear macromolecules obviously also have the need for reinforcement and toughening, but to date, there have been no reports on the effective reinforcement and toughening of linear macromolecules using rigid rod-shaped polymers.

[0009] Compared to low molecular weight thermosetting resins (molecular weight is generally in the hundreds), it is very difficult to use rigid rod polymers to strengthen and toughen linear macromolecules (molecular weight is generally in the thousands to tens of thousands). This is because amino-substituted soluble rigid rod polymers are multifunctional polymers. When they are able to chemically react with polyimide linear macromolecules in solution, the two can easily cross-link, causing the solution viscosity to increase sharply until it loses fluidity and forms a chemical gel. This situation makes the processing and forming of the solution very difficult, and even if it is barely formed, it will lead to serious unevenness in the internal structure of the polymer and a significant decrease in elongation at break. When there is no chemical reaction between the two, it is obvious that the loss of processability due to gelation of the solution can be avoided. However, the unreacted amino groups will significantly reduce the polymer's heat resistance, stability and other properties, making it lose its application value. Summary of the Invention

[0010] To solve the problems of the prior art, the present invention provides a polyimide with high strength and high elongation at break. The polyimide has extremely excellent mechanical properties and has important application value in the fields of polyimide films, coatings, molding powders, honeycomb materials, fibers and composite materials.

[0011] A polyimide with high strength and high elongation at break comprises a rigid rod-shaped polymer main chain and polyimide linear macromolecular side chains, wherein the rigid rod-shaped polymer main chain comprises aromatic polyamide or polyamide-imide, and the rigid rod-shaped polymer main chain and the polyimide linear macromolecular side chains are connected via imide groups.

[0012] The rigid rod-shaped polymer main chain itself has excellent mechanical properties and a reinforcing effect, and can react with the anhydride end groups of the branched chains through its amino side groups to form branched macromolecules, thereby enhancing the chain entanglement between the macromolecular chains, thereby making the polyimide have high strength and high elongation at break.

[0013] A polyimide composite material comprises the polyimide and a nanoscale dispersant, wherein the nanoscale dispersant comprises one or more of graphene, carbon nanotubes, carbon nanofibers, and aramid nanofibers. The graphene, carbon nanotubes, and the like can be surface-modified by oxidation or other methods to improve their uniform dispersion in the polyimide composite material.

[0014] The polyimide composite material of the present invention exhibits unprecedented, exceptional tensile strength and elongation at break, even with the addition of only a small amount of nanoscale dispersant. The nanoscale dispersant has a large specific surface area and interacts strongly with the polymer matrix, dispersing and withstanding stress, inhibiting crack initiation and propagation, thereby further improving the material's mechanical properties.

[0015] Preferably, the nano-scale dispersant accounts for 0.1-20 wt% of the polyimide composite material. When the nano-scale dispersant content in the polyimide composite material is less than 0.1 wt%, the reinforcing effect is not obvious; when the nano-scale dispersant content in the polyimide composite material is higher than 20 wt%, agglomeration occurs.

[0016] The present invention also provides a method for preparing the polyimide with high strength and high elongation at break, comprising the following steps: subjecting an amino-substituted soluble rigid rod-shaped polymer and a polyamic acid linear macromolecule having anhydride end groups, or a polyamic acid linear macromolecule having anhydride end groups and a monofunctional end-capping agent, to an amino-anhydride reaction in an organic solvent and then to an imidization reaction;

[0017] Wherein, the amino-substituted soluble rigid rod polymer includes amino-substituted aromatic polyamide, amino-substituted polyamide-amic acid copolymer or amino-substituted polyamic acid;

[0018] The monofunctional end-capping agent includes a monofunctional compound with anhydride or amino group.

[0019] The invention uses amino-substituted soluble rigid rod-shaped polymer and polyamic acid linear macromolecule with anhydride end groups or polyamic acid linear macromolecule with anhydride end groups and a monofunctional end-capping agent as raw materials, mixes them in an organic solvent, and generates polyamic acid-grafted rigid rod-shaped polymer through nucleophilic substitution reaction between amino groups and anhydride groups. The polyamic acid-grafted rigid rod-shaped polymer is then subjected to imidization reaction to dehydrate and ring-close the polyamic acid chain segments therein, thereby preparing the polyimide.

[0020] The amino-substituted soluble rigid-rod polymer can be prepared from a nitro-substituted soluble rigid-rod polymer in an organic solvent by a conventional method of reducing nitro groups to amino groups, such as Pd / C catalytic hydrogenation. For example, the amino-substituted aromatic polyamide, amino-substituted polyamide-amic acid copolymer, or amino-substituted polyamic acid can be prepared from a nitro-substituted aromatic polyamide, a nitro-substituted polyamide-amic acid copolymer, or a nitro-substituted polyamic acid, respectively, by catalytic hydrogenation reduction.

[0021] Preferably, the reaction temperature of the catalytic hydrogenation reduction reaction is 50-180° C., the reaction pressure is 0.1-5 MPa, and the reaction time is 2-48 h; and the amount of the catalyst added is 0.5-10% of the mass of the nitro-substituted soluble rigid rod polymer.

[0022] Preferably, the organic solvent in the hydrogenation reduction reaction is selected from dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, sulfolane, N-methylpyrrolidone or hexamethylphosphoramide.

[0023] Preferably, the nitro-substituted soluble rigid-rod polymer has at least 25 mol% of the benzene rings in the macromolecular chain bearing nitro pendant groups. If the molar percentage of nitro-substituted benzene rings is less than 25%, the polymer is insoluble in organic solvents, making it difficult to obtain amino-substituted soluble rigid-rod polymers by methods such as catalytic hydrogenation.

[0024] Preferably, the nitro-substituted aromatic polyamide is obtained by polycondensation of one or more aromatic diacids of general formulas (1) to (4) and one or more aromatic diamines of general formulas (5) to (8).

[0025] In general formulae (1) to (8), i and j represent integers of 0 to 4, and i and j cannot be 0 at the same time.

[0026] The aromatic diacid of the present invention can also be converted into aromatic diacid chloride first and then reacted with aromatic diamine. The nitro-substituted aromatic polyamide prepared by the present invention includes amino-terminated nitro-substituted aromatic polyamide and carboxyl-terminated nitro-substituted aromatic polyamide.

[0027] Preferably, the preparation steps of the nitro-substituted aromatic polyamide are: adding aromatic diacid (or its dichloride), aromatic diamine, organic solvent, acid absorbent, activator and salt into a reactor, stirring, and performing polycondensation reaction to obtain the nitro-substituted aromatic polyamide.

[0028] Preferably, the aromatic diacid comprises one or more of the aromatic diacids of general formulas (1) to (4); the aromatic diamine comprises one or more of the aromatic diamines of general formulas (5) to (8); the molar ratio of the aromatic diacid to the aromatic diamine is (2:1) to (1:2), and the mass concentration of the aromatic diacid in the solution is 0.1 to 30 wt%.

[0029] Preferably, in the preparation process of the nitro-substituted aromatic polyamide, the organic solvent includes one or more of N,N-dimethylacetamide, N-methylpyrrolidone, N,N-dimethylformamide or hexamethylphosphoramide.

[0030] Preferably, the acid absorbent includes one or more of pyridine, NaOH solution, Ca(OH)2 solution or triethylamine, and the mass concentration of the acid absorbent in the solution is 1 to 40 wt%.

[0031] Preferably, the activator includes one or more of Ph3P / C2Cl6 (molar ratio 1:1 to 1:1.5), triphenyl phosphite or trinitrochlorobenzene; when an aromatic diacid monomer is selected, the mass concentration of the activator in the solution is 1 to 50 wt%; when an aromatic diacid chloride monomer is selected, the activator may not be used.

[0032] Preferably, the salt comprises one or more of lithium chloride and calcium chloride; the mass concentration of the salt in the solution is 0.1 to 20 wt%.

[0033] Preferably, the polycondensation reaction temperature is -20°C to 130°C, and the time is 1 to 24 hours.

[0034] Preferably, the nitro-substituted polyamide-acid copolymer is obtained by copolymerizing an amino-terminated and nitro-substituted aromatic polyamide segment (segment A) and an anhydride-terminated and nitro-substituted polyamide acid segment (segment B).

[0035] Preferably, the mass ratio of segment A to segment B is 100:1-100.

[0036] Preferably, the copolymerization temperature is -20°C to 100°C, and the time is 1 to 24 hours.

[0037] Preferably, the anhydride-terminated and nitro-substituted polyamic acid is obtained by reacting aromatic tetracarboxylic dianhydrides (without flexible linking groups such as ether, ketone, sulfone, methylene, isopropylidene, etc.) in general formulas (9) to (12) with nitro-substituted aromatic diamines in general formulas (5) to (8) under stirring at room temperature for more than 10 minutes.

[0038] In general formulae (9) to (12), i and j represent integers of 0 to 2.

[0039] Preferably, the molar ratio of the aromatic tetracarboxylic dianhydride in the general formulas (9) to (12) to the nitro-substituted aromatic diamine in the general formulas (5) to (8) is 1:0.1 to 0.9.

[0040] Preferably, the nitro-substituted polyamic acid is obtained by reacting aromatic tetracarboxylic dianhydrides of general formulas (9) to (12) and nitro-substituted aromatic diamines of general formulas (5) to (8) at room temperature with stirring for more than 10 minutes.

[0041] Preferably, the molar ratio of the aromatic tetracarboxylic dianhydride in the general formulas (9) to (12) to the nitro-substituted aromatic diamine in the general formulas (5) to (8) is 1:0.9 to 1.1.

[0042] Preferably, the mass ratio of the amino-substituted soluble rigid rod polymer to the polyamic acid having anhydride end groups is 1:20-1000.

[0043] Preferably, the polyamic acid having anhydride end groups is obtained by reacting a dianhydride and a diamine monomer at -20°C to 100°C with stirring for at least 10 minutes, wherein the molar ratio of the dianhydride to diamine monomer is 1:0 to 0.999. The dianhydride and diamine are not particularly limited and can be aromatic or aliphatic, with or without a flexible linking group.

[0044] Preferably, the mass ratio of the amino-substituted soluble rigid rod polymer to the polyamic acid having anhydride end groups and a monofunctional end-capping agent is 1:5-1000.

[0045] Preferably, the polyamic acid having anhydride end groups and a monofunctional end-capping agent is obtained by reacting a dianhydride, a diamine monomer and a monofunctional end-capping agent under stirring for more than 10 minutes at room temperature, wherein the molar ratio of the dianhydride, the diamine monomer and the monofunctional end-capping agent is 1:0.5~0.999:0.001~0.5. Wherein, the monofunctional end-capping agent is a monofunctional compound with anhydride or amino group, without special limitation, and can be phthalic anhydride, naphthalene dicarboxylic anhydride, aniline, naphthylamine, benzylamine or maleic anhydride, phenyl benzoic anhydride, alkynyl aniline, naphthalene dicarboxylic anhydride, etc. The monofunctional end-capping agent cannot guarantee that all polyamic acid macromolecular chains have only one anhydride end group, but it can greatly reduce the content of polyamic acid macromolecular chains with anhydride end groups at both ends, thereby greatly reducing the probability of crosslinking and gelation.

[0046] The present invention can reduce the risk of cross-linking structure formation and gelation by controlling the addition amount of the amino-substituted soluble rigid rod polymer and introducing a monofunctional end-capping agent, thereby ensuring the fluidity, uniformity and processability of the solution.

[0047] Preferably, the conditions of the amino-anhydride reaction are: mixing the amino-substituted soluble rigid rod polymer with a polyamic acid linear macromolecule having anhydride end groups, or a polyamic acid linear macromolecule having anhydride end groups and a monofunctional end-capping agent in an organic solvent at -20°C to 100°C to obtain a mixed solution, and stirring the mixed solution at room temperature for more than 10 minutes.

[0048] Preferably, the organic solvent includes one or more of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, sulfolane, N-methylpyrrolidone or hexamethylphosphoramide, and the viscosity of the mixed solution system is 1-30%.

[0049] Preferably, the imidization reaction includes a thermal imidization reaction or a chemical imidization reaction.

[0050] Preferably, the conditions of the thermal imidization reaction are: preparing the product of the amino-anhydride reaction into a film, and reacting it at 100-400° C. for 3-6 hours.

[0051] Preferably, the nanomaterial, an amino-substituted soluble rigid rod polymer, and a polyamic acid linear macromolecule with anhydride end groups, or a polyamic acid linear macromolecule with anhydride end groups and a monofunctional end-capping agent are added together into an organic solvent to carry out an amino-anhydride reaction and then an imidization reaction to prepare a polyimide composite material containing the nanomaterial.

[0052] The present invention also provides the use of the polyimide or polyimide composite material in the preparation of polyimide films, coatings, molding powders, honeycomb materials, or fibers. The polyimide or polyimide composite material obtained by the present invention has extremely excellent mechanical properties. In the absence of biaxial stretching, its tensile strength reaches 200 to 340 MPa and its elongation at break can reach over 140%. It has important application value in the fields of polyimide films, coatings, molding powders, honeycomb materials, fibers, and composite materials.

[0053] Compared with the prior art, the present invention has at least the following beneficial effects:

[0054] (1) The present invention prepares polyimide by subjecting an amino-substituted soluble rigid rod-shaped polymer to an amino-anhydride reaction with a polyamic acid linear macromolecule having anhydride end groups, or a polyamic acid linear macromolecule having anhydride end groups and a monofunctional end-capping agent, and then subjecting the polyimide to an imidization reaction in an organic solvent. The rigid rod-shaped polymer main chain itself has excellent mechanical properties and a reinforcing effect, and can react with the anhydride end groups of the branched chains through its amino side groups to form branched macromolecules, thereby enhancing the chain entanglement between the macromolecular chains. The polyimide thus prepared has extremely excellent mechanical properties.

[0055] (2) The polyimide of the present invention has a tensile strength of 200-340 MPa and an elongation at break of over 140% without biaxial stretching. These properties are not only far superior to thermosetting resins modified with amino-substituted soluble rigid rod-shaped polymers, but also double the tensile strength and more than triple the elongation at break of linear polyimide macromolecules. They can even surpass DuPont's commercial biaxially stretched polyimide film, Kapton (tensile strength ~252 MPa, elongation ~72%). These improvements in mechanical properties are unattainable with currently known technologies and overcome the problem of inorganic fiber or nanomaterial reinforcement technologies increasing polyimide strength while simultaneously reducing elongation at break. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] FIG1 is a curve showing the relationship between shear viscosity and shear rate of the polyamic acid solution of Comparative Example 1 and the polyamic acid mixed solutions of Examples 11-15.

[0057] FIG2 is a tensile stress-strain curve of the polyimide of Comparative Example 1 and Examples 11-16.

[0058] FIG3 is a tensile stress-strain curve of the polyimide of Comparative Example 2 and Examples 17-20.

[0059] FIG4 is a tensile stress-strain curve of the polyimide of Comparative Example 2 and Examples 21-25.

[0060] FIG5 is a tensile stress-strain curve of the polyimide composite materials containing graphene oxide (GO) of Comparative Example 1 and Examples 43-47. DETAILED DESCRIPTION

[0061] The present invention will be further described below in conjunction with embodiment and comparative example, but the present invention is not limited to these embodiments. In addition, those skilled in the art can make various changes or modifications to the present invention after reading the content of the present invention, and these equivalent forms also fall within the scope limited by the appended claims of the application.

[0062] The tensile stress-strain curves in FIG. 2 to FIG. 5 were obtained by testing the polyimide specimens of the examples and comparative examples using a universal electronic testing machine according to standard ASTM D-638.

[0063] Examples 1-10

[0064] The synthesis steps of amino-substituted soluble rigid rod aromatic polyamide are as follows:

[0065] (1) adding the diacid or diacid chloride monomer, diamine monomer, solvent, acid absorbent, activator and salt specified in Table 1 into a reaction kettle and stirring them uniformly, and polycondensing them for a period of time, adding water for precipitation, washing, filtering and drying to obtain nitro-substituted aromatic polyamide;

[0066] (2) The nitro-substituted aromatic polyamide obtained in step (1) is redissolved in 500 mL of a solvent such as DMAc, and a palladium-carbon catalyst is added, wherein the amount of the palladium-carbon catalyst accounts for 1%-20% of the mass of the nitro-substituted polyamide, and catalytic hydrogenation is performed at 0.1-5.0 MPa for 2-48 hours, followed by cooling, precipitation, filtering, and drying to obtain an amino-substituted aromatic polyamide.

[0067] Among them, Example 4 uses 2-nitroterephthalic acid (2-nitroterephthaloyl chloride can also be used) and p-phenylenediamine as raw materials to prepare amino-substituted polyamide, referred to as NH2'-PPTA; Example 5 uses 2-nitroterephthalic acid (2-nitroterephthaloyl chloride can also be used) and 2-nitro-p-phenylenediamine as raw materials to prepare amino-substituted aromatic polyamide, referred to as (NH2)2-PPTA.

[0068] The raw materials and reaction conditions for preparing nitro-substituted aromatic polyamide in step (1) of Examples 1-10 are shown in Table 1, and the specific reaction conditions for preparing amino-substituted aromatic polyamide in step (2) are shown in Table 2.

[0069] Table 1 Raw materials and reaction conditions for preparing nitro-substituted aromatic polyamides in step (1) of Examples 1-10

[0070] Note: DMF: N,N-dimethylformamide; DMAc: N,N-dimethylacetamide; DMSO: dimethyl sulfoxide; NMP: N-methylpyrrolidone; HMPA: hexamethylphosphoramide; Py: pyridine; TEAE: triethylamine. TPC: terephthaloyl chloride; TPA: terephthalic acid; NDCA: 2,6-naphthalenedicarboxylic acid; DNNDCA: dinitro-2,6-naphthalenedicarboxylic acid; NTPA: 2-nitroterephthalic acid; 2,5-DNTPA: 2,5-dinitroterephthalic acid; BPDA: 4,4'-biphenyldicarboxylic acid; DNBPDA: 2,2'-dinitrobiphenyldicarboxylic acid; NDPA: 2-nitro-p-phenylenediamine; p-PDA: p-phenylenediamine; DNBZD: dinitrodiaminobenzidine; TPP: triphenyl phosphite; PC: trinitrochlorobenzene.

[0071] Table 2 Reaction conditions for preparing amino-substituted aromatic polyamides in step (2) of Examples 1-10

[0072] Examples 11-16

[0073] The preparation steps of polyimide containing (NH2)2-PPTA are as follows:

[0074] (1) Dissolve 0.1 mol of 4,4'-diaminodiphenyl ether in 250 mL of DMAc and stir evenly. Add 0.102 mol of pyromellitic dianhydride in a salt bath (-20°C) under a nitrogen atmosphere in portions, mechanically stir, and react for 4 hours. Dilute with DMAc to 10 wt% to obtain a polyamic acid solution A.

[0075] (2) Dissolve 10 g of (NH2)2-PPTA in 990 g of DMAc and stir evenly to obtain a solution B with a concentration of 1 wt%.

[0076] (3) Solution B was added to solution A in a certain proportion, mixed and stirred for 2 hours to obtain a uniform and viscous mixed solution. The mass ratios of solution A to solution B in Examples 11-16 were 9.98:0.2, 9.97:0.3, 9.96:0.4, 9.94:0.6, 9.92:0.8 and 9.9:1.0, respectively. FIG1 is a curve showing the relationship between the shear viscosity and shear rate of polyamic acid solution A (the same as the polyamic acid solution in Comparative Example 1) and the mixed solution after adding different amounts of (NH2)2-PPTA. It can be seen that the viscosity of the solution increased significantly after adding (NH2)2-PPTA, which is because the reaction between the two greatly increased the molecular weight.

[0077] (4) After vacuum defoaming the mixed solution obtained in step (3), the film was scraped onto a clean glass plate using a coater. The film was baked at 80°C for 2 hours to remove residual solvent, and then transferred to a heating furnace and heated at 120°C, 200°C, 250°C, and 300°C for 1 hour to complete imidization, and finally heated at 380°C for 0.5 hour.

[0078] The (NH2)2-PPTA contents in Examples 11-16 were 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, and 1.0 wt%, respectively. The tensile stress-strain curves are shown in FIG2 . Example 13 exhibited the best mechanical properties, with a tensile strength of 201 MPa and an elongation at break of 166%.

[0079] Examples 17-20

[0080] The preparation steps of polyimide containing (NH2)2-PPTA and phthalic anhydride end-capping agent are as follows:

[0081] (1) Dissolve 0.3 mol of 4,4'-diaminodiphenyl ether in 500 mL of DMAc and stir evenly. Add 0.015 mol of phthalic anhydride and 0.3 mol of pyromellitic dianhydride as end-capping agents in an ice-water bath (0-4°C) under a nitrogen atmosphere and mechanically stir. React for 6 hours. Dilute with DMAc to 10 wt% to obtain polyamic acid solution A.

[0082] (2) Dissolve 10 g of (NH2)2-PPTA in 990 g of DMAc and stir evenly to obtain a solution B with a concentration of 1 wt%.

[0083] (3) Solution B was added to Solution A in a certain proportion, mixed and stirred at 30°C for 2 hours to obtain a uniform and viscous solution. The mass ratios of Solution A to Solution B in Examples 17-20 were 9.946:0.54, 9.892:1.08, 9.838:1.62, and 9.784:2.16, respectively.

[0084] (4) After vacuum defoaming the mixed solution obtained in step (3), the film was scraped using a coating machine. The film was baked at 80°C for 2 hours to remove residual solvent, and then transferred to a heating furnace and heated at 120°C, 200°C, 250°C, and 300°C for 1 hour to complete imidization, and finally heated at 380°C for 1 hour.

[0085] The (NH2)2-PPTA contents in Examples 17-20 were 0.54 wt%, 1.08 wt%, 1.64 wt%, and 2.16 wt%, respectively, and the tensile stress-strain curves were shown in Figure 3. Among them, Example 18 had the best mechanical properties, with a tensile strength of 240 MPa and a maximum elongation at break of 132%.

[0086] Examples 21-25

[0087] The preparation steps of polyimide containing NH2'-PPTA and phthalic anhydride end-capping agent are as follows:

[0088] (1) Solution A with a concentration of 10 wt% was prepared according to the method of Examples 17-20.

[0089] (2) NH2'-PPTA was dissolved in DMAc to obtain a solution B with a concentration of 1 wt% according to the conditions of Examples 17-20.

[0090] (3) Solution B was added to Solution A in a certain proportion, mixed, stirred, and reacted for 2 hours to obtain a uniform and viscous solution. The mass ratios of Solution A to Solution B in Examples 21-25 were 9.973:0.27, 9.946:0.54, 9.892:1.08, 9.838:1.62, and 9.784:2.16, respectively.

[0091] (4) After vacuum defoaming the mixed solution obtained in step (3), the film was scraped off using a coating machine and imidization was carried out according to the conditions of step (4) of Examples 17-20.

[0092] The NH2'-PPTA contents in Examples 21-25 were 0.27 wt%, 0.54 wt%, 1.08 wt%, 1.62 wt%, and 2.16 wt%, respectively, and the tensile stress-strain curves are shown in Figure 4. Among them, Example 22 had the best mechanical properties, with a tensile strength of 283 MPa and a maximum elongation at break of 144%.

[0093] Examples 26-30

[0094] The synthesis steps of amino-substituted soluble rigid aromatic polyamide amic acid copolymer are as follows:

[0095] (1) To the solution of the amino-terminated nitro-substituted polyamide obtained in step (1) of Example 5, an anhydride-terminated nitro-substituted polyamic acid solution synthesized from the dianhydride and nitro-substituted diamine shown in Table 3 was added, and the mixture was stirred at -20°C to 100°C to obtain a nitro-substituted polyamide-acid copolymer solution;

[0096] (2) diluting the solution of the nitro-substituted polyamide-amic acid copolymer obtained in step (1) to a concentration of 1 wt %, adding a palladium-carbon catalyst, wherein the amount of the palladium-carbon catalyst accounts for 10% of the mass of the nitro-substituted polyamide, catalytically hydrogenating the mixture at 150° C. and 1 MPa for 24 h, cooling, filtering and removing the catalyst to obtain an amino-substituted aromatic polyamide-amic acid copolymer solution.

[0097] Table 3 Preparation raw materials and reaction conditions of nitro-substituted polyamide amic acid copolymers Note: DNNDA: dinitro-1,5-naphthalene diamine; NDTFMB: 2,2'-bis(trifluoromethyl)-4,4'-diamino-dinitrobiphenyl

[0098] Examples 31-35

[0099] The synthesis steps of amino-substituted soluble rigid aromatic polyamic acid are as follows:

[0100] (1) Selecting the dianhydride and nitro-substituted diamine monomers of Examples 26-30 shown in Table 3, 0.2 mol of each dianhydride and diamine were dissolved in 200 mL of DMAc, and mechanically stirred and reacted at 30° C. under a nitrogen atmosphere for 6 hours to synthesize a nitro-substituted polyamic acid solution;

[0101] (2) The solution of the nitro-substituted polyamic acid obtained in step (1) was diluted to 1 wt %, 0.1 wt % of palladium-carbon catalyst was added, and catalytic hydrogenation was performed under heating and pressurizing conditions. The specific conditions were the same as those in Example 1. After the reaction was completed, the solution was cooled and the catalyst was removed by filtration to obtain an amino-substituted aromatic polyamic acid solution.

[0102] Examples 36-38

[0103] The preparation steps of the polyimide containing amino-substituted aromatic polyamide amic acid copolymer are as follows:

[0104] (1) Select solvent, dianhydride and diamine monomer according to Table 4, react with mechanical stirring in an ice-water bath at 0° C. and N 2 atmosphere for 6 hours, and dilute to 10 wt % to obtain polyamic acid solution A.

[0105] (2) Solution A was mixed with the amino-substituted aromatic polyamide-acid copolymer solutions described in Examples 26-28 in a certain proportion and stirred for 2 hours to obtain a mixed solution.

[0106] (3) The mixed solution obtained in step (2) is imidized according to the conditions of step (4) of Example 11 to obtain polyimide.

[0107] The ratio of the two solutions in step (2) is such that the contents of amino-substituted aromatic polyamide-acid copolymer in the polyimides obtained in Examples 36-38 are 0.1 wt%, 0.8 wt% and 1.5 wt%, respectively.

[0108] The tensile strength of the prepared polyimides was higher than 200 MPa, and the elongation at break was higher than 120%. In comparison, the polyimides obtained by defoaming, scraping, imidizing and heat-treating the polyamic acid solution A synthesized in step (1) under the same conditions as in step (3) did not contain amino-substituted aromatic polyamide-acid copolymers, and thus had a tensile strength lower than 140 MPa and an elongation at break lower than 90%.

[0109] Table 4 Raw materials and preparation conditions for solution A in Examples 36-38

[0110] Examples 39-43

[0111] The preparation steps of polyimide containing amino-substituted aromatic polyamic acid are as follows:

[0112] (1) Prepare a polyamic acid solution A with a concentration of 10 wt% according to the conditions of step (1) of Example 36.

[0113] (2) Solution A and the amino-substituted aromatic polyamic acid solution (Solution B) described in Examples 31-35 were mixed in a certain proportion at room temperature and stirred for 5 hours.

[0114] (3) The mixed solution obtained in step (2) is imidized according to the conditions of step (4) of Example 11 to obtain polyimide.

[0115] When the ratio of solution A to solution B in step (2) is such that the contents of amino-substituted aromatic amide acid in Examples 39-43 are 0.3 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt% and 3.0 wt%, respectively, the tensile strength of the obtained polyimides is higher than 200 MPa, and the elongation at break is higher than 100%. In comparison, the tensile strength of the polyimides prepared directly from solution A obtained in step (1) under the same imidization conditions without adding amino-substituted aromatic polyamide acid is lower than 140 MPa, and the elongation at break is lower than 90%.

[0116] Examples 44-48

[0117] A polyamic acid solution (solution A) having a concentration of 10 wt% was prepared according to the method and conditions of step (1) of Example 13; solution A and a DMAc solution of (NH2)2-PPTA having a concentration of 1.0 wt% (solution B) were mixed and stirred at 30°C for 2 hours to obtain a (NH2)2-PPTA content of 0.4 wt% of the polyamic acid; a DMAc solution of graphene oxide having a concentration of 1 wt% (solution C) was added to obtain graphene oxide contents of 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt% and 1.0 wt% of the polyamic acid in Examples 44-48, respectively, and the mixture was stirred uniformly. The above solutions were vacuum defoamed and scraped, and then dried and thermally imidized according to the conditions of step (4) of Example 11. The mechanical properties of the obtained polyimide composite film are shown in FIG5 , wherein the tensile strength of Example 45 reaches 280 MPa, and the elongation at break reaches 132%, which are much higher than those of the polyimide film without adding (NH2)2-PPTA and graphene oxide.

[0118] Example 49

[0119] A polyamic acid solution (solution A) with a concentration of 10 wt% was prepared according to the method and conditions of step (1) of Example 13; solution A and a DMAc solution (solution B) with a concentration of 1.0 wt% (NH2)2-PPTA were mixed and stirred for 2 hours to make the content of (NH2)2-PPTA 0.4 wt% of the polyamic acid; a DMAc solution (solution C) containing 0.5 wt% of oxidized multi-walled carbon nanotubes and 0.5 wt% of graphene oxide was added to make the contents of oxidized multi-walled carbon nanotubes and graphene oxide 0.4 wt% of the polyamic acid, and stirred evenly. The above solution was vacuum defoamed and then scraped, and heated and dried according to the conditions of step (4) of Example 11 and thermal imidization was performed. The tensile strength of the obtained polyimide composite film reached ~340 MPa, and the elongation at break reached 114%, which was higher than that of the polyimide film (Example 13) without the addition of oxidized multi-walled carbon nanotubes and graphene oxide and with the same (NH2)2-PPTA content.

[0120] Example 50

[0121] A mixed solution of solution A and solution B was prepared according to the conditions of Example 49, so that the content of (NH2)2-PPTA was 0.4wt% of polyamic acid. A DMAc solution containing 2.0wt% of oxidized vapor-grown carbon fibers was added so that the content of oxidized vapor-grown carbon fibers was 2.0wt% of polyamic acid and stirred evenly. The above solution was defoamed in vacuo and the film was scraped. The polyimide composite film prepared according to the conditions of step (4) of Example 11 had a tensile strength of ~230MPa and an elongation at break of 102%, which were higher than the polyimide film (Example 13) to which oxidized vapor-grown carbon fibers were not added and the same (NH2)2-PPTA content was obtained.

[0122] Example 51

[0123] A mixed solution of solution A and solution B was prepared according to the conditions of Example 49, so that the content of (NH2)2-PPTA was 0.4wt% of the polyamic acid. A DMAc solution containing 3.0wt% of aramid nanofibers was added so that the content of aramid nanofibers was 1.0wt% of the polyimide, and the mixture was stirred evenly. The above solution was vacuum defoamed and then scraped into a film. After thermal imidization according to the method and conditions of step (4) of Example 11, the tensile strength reached ~240MPa and the elongation at break reached 140%, which were higher than those of the polyimide film (Example 13) without the addition of aramid nanofibers and with the same (NH2)2-PPTA content.

[0124] Comparative Example 1

[0125] Polyamic acid solution A was prepared according to the method of step (1) of Examples 11-16, and vacuum defoamed and then scraped. Polyimide was prepared according to the method of step (4) of Example 11. The tensile strength of the resulting polyimide was only 107 MPa, and the elongation at break was only 88% (see Figure 2), which is approximately 50% of the performance of Example 13.

[0126] Comparative Example 2

[0127] Polyamic acid solution A was prepared according to step (1) of Examples 17-20, and polyimide was prepared from it according to step (4) of Example 11. The tensile stress-strain curves are shown in Figures 3 and 4. The tensile strength was only 134 MPa, and the elongation at break was only 43%, which were far lower than the performance of the other examples.

[0128] It can be seen from the above embodiments and comparative examples that the polyimide containing nanomaterials of the present invention can significantly improve tensile strength and elongation at break, overcoming the defect of traditional polyimide reinforcement methods that significantly reduces elongation at break while improving tensile strength.

Claims

1. A polyimide having high strength and high elongation at break, characterized in that: The polyimide has a rigid rod-shaped polymer main chain and polyimide linear macromolecular side chains, wherein the rigid rod-shaped polymer main chain comprises aromatic polyamide or polyamide-imide, and the rigid rod-shaped polymer main chain and the polyimide linear macromolecular side chains are connected through imide groups.

2. A polyimide composite material, characterized in that: The invention comprises the polyimide according to claim 1 and a nano-scale dispersant, wherein the nano-scale dispersant comprises one or more of graphene, carbon nanotubes, nano-carbon fibers, and aramid nanofibers, and the nano-scale dispersant accounts for 0.1 to 20 wt % of the polyimide composite material.

3. The method for preparing a polyimide according to claim 1, wherein The following steps are involved: An amino-substituted soluble rigid rod polymer and a polyamic acid linear macromolecule having an anhydride end group, or a polyamic acid linear macromolecule having an anhydride end group and a monofunctional end-capping agent, are subjected to an amino-anhydride reaction in an organic solvent and then to an imidization reaction; Wherein, the amino-substituted soluble rigid rod polymer includes amino-substituted aromatic polyamide, amino-substituted polyamide-amic acid copolymer or amino-substituted polyamic acid; The monofunctional end-capping agent includes a monofunctional compound with anhydride or amino group.

4. The preparation method according to claim 3, characterized in that The amino-substituted aromatic polyamide, amino-substituted polyamide-amic acid copolymer or amino-substituted polyamic acid is prepared from nitro-substituted aromatic polyamide, nitro-substituted polyamide-amic acid copolymer or nitro-substituted polyamic acid by catalytic hydrogenation reduction reaction in an organic solvent, wherein the molar percentage of nitro-substituted benzene rings in the nitro-substituted aromatic polyamide, nitro-substituted polyamide-amic acid copolymer or nitro-substituted polyamic acid is not less than 25%.

5. The method for preparing polyimide according to claim 4, wherein The nitro-substituted aromatic polyamide is obtained by polycondensation of one or more aromatic diacids of general formulas (1) to (4) and one or more aromatic diamines of general formulas (5) to (8), wherein the molar ratio of the aromatic diacid to the aromatic diamine is (2:1) to (1:2); In general formulae (1) to (8), i and j represent integers of 0 to 4, and i and j cannot be 0 at the same time.

6. The method for preparing polyimide according to claim 4, wherein: The nitro-substituted polyamide-amic acid copolymer is obtained by copolymerizing an amino-terminated and nitro-substituted aromatic polyamide and an anhydride-terminated and nitro-substituted polyamide acid, wherein the mass ratio of the amino-terminated and nitro-substituted aromatic polyamide to the anhydride-terminated and nitro-substituted polyamide acid is 100:1-100.

7. The method for preparing polyimide according to claim 6, wherein: The anhydride group-terminated and nitro-substituted polyamic acid is obtained by reacting aromatic tetracarboxylic dianhydride in general formulas (9) to (12) with nitro-substituted aromatic diamine in general formulas (5) to (8), wherein the molar ratio of aromatic tetracarboxylic dianhydride in general formulas (9) to (12) to nitro-substituted aromatic diamine in general formulas (5) to (8) is 1:0.1 to 0.9; In general formulae (9) to (12), i and j represent integers of 0 to 2.

8. The method for preparing polyimide according to claim 4, wherein: The nitro-substituted polyamic acid is obtained by reacting aromatic tetracarboxylic dianhydrides of general formulas (9) to (12) with nitro-substituted aromatic diamines of general formulas (5) to (8), wherein the molar ratio of aromatic tetracarboxylic dianhydrides of general formulas (9) to (12) to nitro-substituted aromatic diamines of general formulas (5) to (8) is 1:0.9 to 1.1; In general formulae (5) to (8), i and j represent integers from 0 to 4, and i and j cannot be 0 at the same time; in general formulae (9) to (12), i and j represent integers from 0 to 2.

9. The method for preparing a polyimide according to any one of claims 3 to 8, wherein: The mass ratio of the amino-substituted soluble rigid rod polymer to the polyamic acid with anhydride end groups is 1:20-1000; the mass ratio of the amino-substituted soluble rigid rod polymer to the polyamic acid with anhydride end groups and a monofunctional end-capping agent is 1:5-1000; and the viscosity of the reaction system is 1-30%.

10. Use of the polyimide according to claim 1 or the polyimide composite material according to claim 2 in the preparation of polyimide films, coatings, molding powders, honeycomb materials or fibers.

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