Diamine monomer containing piperazine unit, polyimide acid salt separation membrane, and preparation method and use thereof

Abstract

The application discloses a diamine monomer containing a piperazine unit, a polyimide acid salt separation membrane and a preparation method and application thereof. The diamine monomer containing the piperazine unit is synthesized from piperazine and a benzene derivative containing at least a nitro group and a bromine substituent and is obtained by reduction, or is synthesized from a double benzene ring substituted product of a double piperazine substituted alkane and a benzene derivative containing at least a nitro group and a bromine substituent and is obtained by reduction. The polyimide acid salt separation membrane is obtained by sequentially performing polycondensation on a diamine monomer containing a piperazine unit, an aromatic diamine monomer and a tetracarboxylic dianhydride polyamide acid, imidizing the polyamide acid into a polyimide, acidizing the polyimide by using an inorganic acid or an organic acid, and forming a polyimide acid salt film, and by using different post-processing reactions, a gas separation membrane and a liquid separation membrane having a gas separation function and a liquid internal large-size particle separation function are obtained, the free volume of the gas separation membrane is adjustable, the membrane pore size of the liquid separation membrane is adjustable, and the gas separation membrane and the liquid separation membrane are widely applied in the fields of gas separation and water treatment.

Description

Technical Field

[0001] This invention relates to the field of polyimide membrane technology, and particularly to a diamine monomer containing piperazine units, a polyimide salt separation membrane, its preparation method, and its uses. Background Technology

[0002] Polyimides are polymers with excellent overall properties. Due to the presence of imide rings in their main chain, they possess superior heat resistance, low-temperature resistance, self-lubrication, and flame retardancy, along with excellent flexibility, strength, and low dielectric properties. The most common method for synthesizing polyimides is to perform low-temperature polycondensation of dianhydrides and diamines in an aprotic polar solution to obtain the polyamic acid precursor, followed by chemical or thermal imidization to yield the corresponding polyimide. After obtaining polyimides, many high-performance materials can be prepared, such as films, powders, fibers, foams, hybrid materials, and adhesives. Among these, film materials are the earliest commercially available polyimide products and are also the most widely used, earning the reputation of "golden films." They are widely used in aerospace, military, microelectronics, solar cells, high-temperature filtration, communications, OLEDs, and other fields.

[0003] Polyimide can be used to prepare separation membranes for applications in battery separators, gas separation, wastewater treatment, drinking water purification, food filtration, pharmaceuticals, and petrochemicals. Polyimide separation membranes can be categorized into gas separation membranes, reverse osmosis membranes, ultrafiltration membranes, pervaporation membranes, and nanofiltration membranes based on different preparation processes. Controlling the molecular structure and pore size of the separation membrane is crucial for achieving different separation functions. Preparation methods include: polymer structure design, coating methods, phase inversion methods, interfacial polymerization methods, immersion precipitation phase inversion methods, composite methods with porous materials, and stretching methods. Despite the numerous methods for preparing separation membranes, limitations in structural design and difficulties in pore size adjustment remain. Summary of the Invention

[0004] The purpose of this invention is to provide a diamine monomer containing a piperazine unit.

[0005] Another object of the present invention is to provide a method for preparing the above-mentioned diamine monomer containing a piperazine unit.

[0006] Another object of the present invention is to provide a method for preparing a polyimide salt membrane using a diamine monomer containing a piperazine unit as described above, and its use as a gas separation membrane.

[0007] Another object of the present invention is to provide a method for preparing a polyimide salt membrane using a diamine monomer containing a piperazine unit as described above, and its use as a liquid separation membrane.

[0008] Therefore, the technical solution of the present invention is as follows:

[0009] A diamine monomer containing a piperazine unit has the following structure:

[0010]

[0011] In the formula, Q1 is n1 is 1, 2, or 3; R is H, CF3, OCF3, or SO3H; R and Q1 are located at the ortho, meta, or para positions on the benzene ring; -NH2 group and Q1 are located at the meta or para positions on the benzene ring.

[0012] Furthermore, for diamine monomers containing piperazine units, when Q1 is... The preparation steps of the diamine monomer are as follows: 1) Under alkaline conditions, a benzene derivative containing at least nitro and bromine substituents undergoes a substitution reaction with piperazine to obtain a piperazine dibenzene ring substituted product; 2) Using a reducing agent, all nitro groups in the piperazine dibenzene ring substituted product are reduced to amino groups.

[0013] Specifically, the preparation steps of the above-mentioned diamine monomer containing a piperazine unit are as follows:

[0014] 1) Piperazine, a benzene derivative containing at least nitro and bromine substituents, and an alkaline catalyst are dissolved in a solvent and reacted at 80℃~120℃ for 24h~120h. After standing and cooling to room temperature, a crude product is obtained. The crude product is purified by recrystallization to obtain a dibenzene ring-substituted product of piperazine. The molar ratio of piperazine, benzene derivative, and alkaline catalyst is 1:(2~2.5):(3~4.5). The alkaline catalyst may be, but is not limited to, N,N-diisopropylethylamine. The solvent may be, but is not limited to, acetonitrile, and its amount is 1 to 4 times the total weight of piperazine and the benzene derivative containing at least nitro and bromine substituents.

[0015] 2) The piperazine dibenzene ring-substituted product was dissolved in a solvent, and a Pd / C catalyst was added. Hydrogen gas at a pressure of 0.1 MPa to 0.15 MPa was introduced at room temperature, and the reaction was carried out for 6 to 12 hours. After the reaction was completed, Pd / C was removed by filtration, methanol was removed by vacuum distillation, and the crude product was purified by recrystallization to obtain the aminated piperazine dibenzene ring-substituted product. The solvent can be, but is not limited to, methanol, and its amount is 3 to 9 times that of the piperazine dibenzene ring-substituted product. The amount of Pd / C catalyst is 0.02 to 0.05 times the weight of the piperazine dibenzene ring-substituted product.

[0016] Furthermore, for diamine monomers containing piperazine units, when Q1 is... The preparation steps of the diamine monomer are as follows: 1) Under alkaline conditions, piperazine undergoes a nucleophilic substitution reaction with dibromoalkanes to obtain bispiperazine-substituted alkanes; 2) Under alkaline conditions, a benzene derivative containing at least nitro and bromine substituents undergoes a substitution reaction with bispiperazine-substituted alkanes to obtain bisbenzene ring-substituted products of bispiperazine-substituted alkanes; 3) Using a reducing agent, all nitro groups in the bisbenzene ring-substituted products of bispiperazine-substituted alkanes are reduced to amino groups.

[0017] Specifically, the preparation steps of the above-mentioned diamine monomer containing a piperazine unit are as follows:

[0018] 1) Dissolve piperazine, dibromoalkanes and basic catalyst in a solvent and react at 70℃~90℃ for 6h~12h. After the reaction is complete, cool to room temperature and purify the crude product by recrystallization or column chromatography to obtain bispiperazine substituted alkanes; wherein the molar ratio of piperazine, dibromoalkanes and basic catalyst is 4:1:2.

[0019] 2) Dissolve a piperazine-substituted alkane, a benzene derivative containing at least nitro and bromine substituents, and an alkaline catalyst in a solvent, and react at 80℃~120℃ for 24h~120h. Allow the mixture to stand and cool to room temperature to obtain a crude product. Purify the crude product by recrystallization to obtain a dibenzene ring-substituted product of the piperazine-substituted alkane. The molar ratio of piperazine, benzene derivative, and alkaline catalyst is 1:(2~2.5):(4~4.5). The alkaline catalyst may be, but is not limited to, N,N-diisopropylethylamine. The solvent may be, but is not limited to, acetonitrile, and its application amount is 1 to 4 times the total weight of piperazine and the benzene derivative containing at least nitro and bromine substituents.

[0020] 3) Dissolve the diphenyl ring-substituted alkane of piperazine in a solvent, add a Pd / C catalyst, and pass hydrogen gas at a pressure of 0.1 MPa to 0.15 MPa at room temperature for 6 to 12 hours. After the reaction is complete, remove Pd / C by filtration, remove methanol by vacuum distillation, and purify the crude product by recrystallization to obtain the aminated diphenyl ring-substituted alkane of piperazine. The solvent can be, but is not limited to, methanol, and its amount is 3 to 9 times that of the diphenyl ring-substituted alkane of piperazine. The amount of Pd / C catalyst is 0.02 to 0.05 times the weight of the diphenyl ring-substituted alkane of piperazine.

[0021] A polyimide salt membrane prepared using the above-mentioned diamine monomer containing piperazine units is obtained by sequentially passing through: polyamic acid obtained by polycondensation of a diamine monomer containing piperazine units, an aromatic diamine monomer, and tetracarboxylic acid dianhydride in a molar ratio of x:(1-x):y; imidizing the polyamic acid to polyimide; acidifying the polyimide with an inorganic or organic acid to obtain a polyimide salt; and further forming a film; wherein 0.01≤x≤1, 0.95≤y≤1.05; the chemical structural formula of the polyimide salt is:

[0022]

[0023] In the formula, A1 is a tetravalent aliphatic ring group or a tetravalent aromatic ring group; A2 is a divalent aromatic ring group; A3 is an organic acid or an inorganic acid; n3 is the molar ratio of A3 to Q1 groups; when Q1 contains one piperazine structure, 0.15≤n3≤1; when Q1 contains two piperazine structures, 0.3≤n3≤2.

[0024] Preferably, the effective content of the polyimide solution is 5 wt.% to 10 wt.%.

[0025] Preferably, A1 is any one of the following tetravalent alicyclic groups:

[0026]

[0027] Preferably, A1 is any one of the following tetravalent aromatic ring groups:

[0028]

[0029]

[0030] Where R1 is

[0031] Preferably, the divalent aromatic ring group A2 is derived from an aromatic diamine monomer, specifically selected from p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfide, 3,4-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl ether, 3,4-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl ketone, 4,4'-diaminodiphenyl ketone, 3,4'-diaminodiphenyl ketone, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 2,2-bis(3-aminophenyl)propane, 2 bis(4-aminophenyl)propane, 2-(3-aminophenyl)-2-(4-aminophenyl)propane, 1-bis(3-amino-1-ylethane), 1,1-bis(4-aminophenyl)-1-phenylethane, 1,3-bis(3-aminophenyl)-1-(4-aminophenyl)-1-ylethane, 1,3-bis(3-aminophenoxy), 3-bis(4-aminophenyl), 4-bis(3-aminophenyl) 1,4-bis(4-aminophenoxy) 1,3-bis(3-aminobenzoyl) 1,3-bis(4-aminobenzoyl)benzene, 1,4-bis(3-aminobenzoyl)benzene, 1,4-bis(4-aminomethyl), 3-bis(3-amino-a,a-dimethyl)benzene, 1,3-bis(4-amino-a,a-dimethyl) 1,4-bis(3-aminophenyl)benzene 1,4-Bis(4-amino-a,a-dimethylcarboxy)benzene, 2,6-bis(3-aminooxy)pyrrolidone, 2,6-bis(3-aminophenoxy)pyrrolidone, 4,4-bis(3-aminophenoxy)bi-4,4-bis(4-aminophenoxy)biphenyl, bis[4-(3-aminophenoxy)phenyl], bis[4-(4-aminophenoxy)yl]bis[4-(3-aminophenoxy)phenyl]sulfide, bis[4-(4-aminooxy)yl]sulfide, bis[4-(3-amino)phenyl]sulfide, bis[4-(4-aminophenoxy)phenyl]bis[(3-aminophenoxy)yl]ether, bis[4-(4-aminophenoxy)phenyl]ether, 2,-bis[4-(3-amino)yl]propane, 2-bis [4-(4-aminophenoxy)phenyl]propane, 1,3-bis[4-(3-aminophenoxy)methyl], 3-bis[4-(4-aminophenoxy)benzoyl]benzene, 1,4-bis[4-(3-aminophenoxy)methyl], 1,4-bis[4-(4-aminophenoxy)benzoyl]benzene, 1,3-bis[4-(3-aminophenoxy)-a,a-dimethyl], 3-bis[-(4-aminophenoxy)-a,a-dimethyl]benzene, 1,4-bis[4-(3-aminophenoxy)-a,a-dimethyl]benzene, 1,4-bis[4-(3-aminophenoxy)-a,a-dimethyl]benzene, 1,4-bis[4-(4-aminophenoxy)-a,a-dimethyl]benzene, 4,4-bis[4-(aminophenoxy)benzoyl]diphenyl ether, 4,4'-bis[4-(4-amino-a,[α-dimethyl)phenoxy]benzophenone, 4,4-bis[4-(4-amino-α,dimethyl)phenoxy]benzophenone, 3,3'-diamino-4,4'-diphenoxybenzophenone, 3,3-diamino-,

[0032] 4,4-Diphenyloxybenzophenone, 3,3'-Diamino-4-phenoxydiphenyl, 3,3-Diamino-4-biphenoxydiphenyl, 6,6-bis(3-aminophenoxy)-3,3,33-tetramethyl-1,1'-bisspiro, 6,6-bis(4-aminophenoxy)-3,3,3,3-tetramethyl-1,1'-bisspiro, 1,4-diamino-2-fluorophenyl, 1,4-diamino-2,3-difluoro-1,4-diamino -2,5-Difluoro-1,4-diamino-2,6-difluorobenzene, 1,4-diamino-2,3,5,6-tetrafluorobenzene, 1,4-diamino-2-(trifluoromethyl)benzene, 1,4-diamino-2,3-di(trifluoromethyl)benzene, 1,4-diamino-2,5-di(trifluoromethyl)benzene, 1,4-diamino-2,6-di(trifluoromethyl)benzene, ,4-diamino-2,3,5-tri(trifluoromethyl)benzene, 1, 4-Diamino-2,3,5,6-Tetra(trifluoromethyl)benzene, 2-benzidine, 3-benzidine, 2,3-difluorobenzidine, 2,5-difluorobenzidine, 2,6-difluorobenzidine, 2,3,5-trifluorobenzidine, 2,3,6-trifluorobenzidine, 2,3,5,6-tetrafluorobenzidine, 2,2'-difluorobenzidine, 3,3'-difluorobenzidine, 2,3'-difluorobenzidine, 2,2'-difluorobenzidine 3-Trifluorobenzidine, 2,3,3'-Trifluorobenzidine, 2,2',5-Trifluorobenzidine, 2,2',6-Trifluorobenzidine, 2,3',5-Trifluorobenzidine, 2,3',6-Trifluorobenzidine, 2,2',3,3'-Tetrafluorobenzidine, 2,2',5,5'-Tetrafluorobenzidine, 2,2',6,6'-Tetrafluorobenzidine, 2,2',3,3',6,6'-Hexafluorobenzidine

[0033] 2,2',3,3',5,5”,6,6'-Octafluorobenzidine, 2-(trifluoromethyl)benzidine, 3-(trifluoromethyl)benzidine, 2,3-Di(trifluoromethyl)benzidine, 2,5-Di(trifluoromethyl)benzidine, 2,6-Di(trifluoromethyl)benzidine

[0034] 2,3,5-Tris(trifluoromethyl)benzidine, 2,3,6-Tris(trifluoromethyl)benzidine, 2,3,5,6-Tetra(trifluoromethyl)benzidine, 2,3'-Di(trifluoromethyl)benzidine, 2,2,3-Tris(trifluoromethyl)benzidine, 2,3,3'-Tris(trifluoromethyl)benzidine, 2,2',5-Tris(trifluoromethyl)benzidine, 2,2',6-Tris(trifluoromethyl)benzidine, 2,3',5-Tris(trifluoromethyl)benzidine, 2,3,6-Tris(trifluoromethyl)benzidine, 2,2',3,3'-Tetra(trifluoromethyl)benzidine, 2,2',5,5'-Tetra(trifluoromethyl)benzidine The mixture of any one or more of the following in any proportion: amine, 2,2',6,6-tetra(trifluoromethyl)benzidine, 2,2'-di(trifluoromethyl)diaminobiphenyl, 4,4-diaminobenzoylaniline, 9,9-di(4-amino-phenyl)fluorene, 9,9-di(4-amino-3-fluorophenyl)fluorene, 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline], 2,2'-bis(trifluoromethoxy)-benzidine, 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, and 2,2-bis(4-aminophenyl)hexafluoropropane.

[0035] Preferably, the inorganic acid is hydrochloric acid, sulfuric acid, phosphoric acid, perchloric acid, hypochlorous acid, or chlorous acid; the organic acid is trifluoroacetic acid, trichloroacetic acid, aromatic sulfonic acid, fatty sulfonic acid, or oxalic acid; wherein, the aromatic sulfonic acid is a... Compounds with structure R 1 It is an H or C1-C18 alkyl chain; fatty sulfonic acids are those with R 2 Compounds with the -SO3H structure, R 2 It is a CF3 or C1-C18 alkyl chain.

[0036] A method for preparing the above-mentioned polyimide salt film includes the following steps:

[0037] S1. Dissolve a diamine monomer containing a piperazine unit, or a diamine monomer containing a piperazine unit and an aromatic diamine monomer in an organic solvent, and add tetracarboxylic acid dianhydride in multiple portions under stirring to form a reaction system.

[0038] S2. Place the reaction system in an ice-water bath and stir it at 0℃~10℃ for 12h~48h. The reaction system undergoes a polycondensation reaction at low temperature to obtain a polyamic acid solution with a solid content of 5wt.%~10wt.%.

[0039] S3. Add acetic anhydride and triethylamine to the polyamic acid solution and stir at room temperature for at least 24 hours to obtain a polyimide solution; then, add an inorganic acid or organic acid to the polyimide solution and stir at room temperature for 30 to 60 minutes to obtain a polyimide salt solution; wherein the molar amount of acetic anhydride is twice the molar amount of tetracarboxylic acid dianhydride, and the molar amount of triethylamine is the same as the molar amount of acetic anhydride.

[0040] S4. Apply the polyimide solution to a glass plate using a blade coating method, with a coating thickness of 20μm to 300μm. Then, place the glass plate coated with the polyimide solution in a vacuum dryer at 80℃ to 120℃ for 1h to 4h to remove the solvent. Next, place it in a muffle furnace under a nitrogen atmosphere and heat it to 150℃ to 180℃ at a rate of 1℃ / min, and heat it at 150℃ to 180℃ for 1h to 5h. After cooling, peel off the film from the glass plate to obtain the polyimide film.

[0041] Preferably, in step S1 of the method for preparing the polyimide film, the organic solvent is one of, but not limited to, dimethylacetamide (DMAc), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP).

[0042] Preferably, in step S1 of the method for preparing the polyimide film, the amount of organic solvent used is 14 to 20 times the total weight of the monomer.

[0043] One application of the polyimide salt membrane prepared by the above method is to use the polyimide salt membrane as a gas separation membrane; the gas separation membrane is used for the separation of oxygen and nitrogen, hydrogen and oxygen, carbon dioxide and nitrogen, hydrogen and methane, and carbon dioxide and methane.

[0044] Another method for preparing a polyimide film, comprising the following steps:

[0045] S1. Dissolve a diamine monomer containing a piperazine unit, or a diamine monomer containing a piperazine unit and an aromatic diamine monomer in an organic solvent, and add tetracarboxylic acid dianhydride in multiple portions under stirring to form a reaction system.

[0046] S2. Place the reaction system in an ice-water bath and stir it at 0℃~10℃ for 12h~48h. The reaction system undergoes a polycondensation reaction at low temperature to obtain a polyamic acid solution with a solid content of 5wt.%~10wt.%.

[0047] S3. Add acetic anhydride and triethylamine to the polyamic acid solution and stir at room temperature for at least 24 hours to obtain a polyimide solution; then, add an inorganic acid or organic acid to the polyimide solution and stir at room temperature for 30 to 60 minutes to obtain a polyimide salt solution.

[0048] S4. Apply the polyimide solution to a glass plate using a blade coating method, with a coating thickness of 20μm to 300μm. Then, place the glass plate coated with the polyimide solution under vacuum drying at 80℃ to 120℃ for 1h to 4h to remove the solvent. Next, place it in a muffle furnace under a nitrogen atmosphere and heat it to 180℃ to 220℃ at a rate of 1℃ / min to 3℃ / min, maintaining the temperature at 180℃ to 220℃ for 1h to 2h. Then, continue heating it in a muffle furnace under a nitrogen atmosphere at a rate of 1℃ to 3℃ / min to 280℃ to 320℃, maintaining the temperature at 280℃ to 320℃ for 1h to 2h. After cooling, peel off the film from the glass plate and soak it in clean water for 10h to 24h. After removing it, dry it at 100℃ to 120℃ to obtain the polyimide film.

[0049] Preferably, in step S1 of the method for preparing the polyimide film, the organic solvent is one of, but not limited to, dimethylacetamide (DMAc), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP).

[0050] Preferably, in step S1 of the method for preparing the polyimide film, the amount of organic solvent used is 14 to 20 times the total weight of the monomer.

[0051] One application of the polyimide salt membrane prepared by the above method is specifically the use of the polyimide salt membrane as a liquid separation membrane; the liquid separation membrane is used to retain particles with a diameter greater than 8 nm in a liquid.

[0052] Compared with existing technologies, this application designs a diamine monomer containing piperazine units for preparing polyimide. Polyimide salts formed from this diamine monomer containing piperazine units, or from a diamine monomer containing piperazine units and other diamine monomers through polymerization, imidization, and acidification, can be used to prepare gas separation membranes and liquid separation membranes with gas separation and large-size particle separation functions within liquids. Furthermore, this polyimide salt separation membrane can achieve adjustable free volume during acidification by utilizing the structural diversity and modifiability of acids. The pore size of this polyimide salt separation membrane can be adjusted through post-treatment after membrane formation, enabling its wide application in various mixed gas separation fields and various types of water treatment. Attached Figure Description

[0053] Figure 1This is a schematic diagram of the NMR results of the diamine monomer containing piperazine units of polyimide prepared in Example 1 of the present invention. Detailed Implementation

[0054] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0055] Example 1

[0056] 11.88 g of 2-bromo-5-nitrotrifluorotoluene (compound 1) and 1.72 g of anhydrous piperazine were dissolved in 60 mL of acetonitrile, and then 10.95 g of N,N-diisopropylethylamine was added. The mixture was heated under reflux at 85 °C for 92 h, allowed to cool to room temperature, and purified to obtain compound 2 in 82% yield. 1 -NMR (400MHz, CDCl3): δ8.56(s,2H),8.38(d,J=8.8Hz,2H),7.42(d,J=8.8Hz,2H),3.29(s,8H);

[0057] 2 g of compound 2 was dissolved in 10 mL of methanol, and 60 mg of Pd / C catalyst was added. Hydrogen gas at a pressure of 0.15 MPa was introduced at room temperature, and the reaction was carried out for 6 h. After the reaction was complete, Pd / C was removed by filtration, methanol was removed by vacuum distillation, and the product was further purified to give compound 3, a white solid with a yield of 87%. (See also...) Figure 1 H 1 -NMR (400MHz, CDCl3): 7.30 (d, J = 8.4 Hz, 2H), 6.92 (s, 2H), 6.81 (d, J = 8.4 Hz, 2H), 3.71 (s, 4H), 2.93 (s, 8H).

[0058] Compound 3 is a diamine monomer of a polyimide containing a piperazine unit, and its specific chemical reaction formula is shown below:

[0059]

[0060] Example 2

[0061] 6.88 g piperazine, 3.7 g 1,2-dibromoethane and 4.04 g triethylamine were dissolved in 80 mL ethanol and heated under reflux at 70 °C for 12 h. After cooling to room temperature, the mixture was purified by post-treatment to obtain 2.4 g compound 4.

[0062] 1.98 g of compound 4, 5.5 g of 2-bromo-5-nitrotrifluorotoluene (compound 1), 5.6 g of N,N-diisopropylethylamine and 10 mL of acetonitrile were mixed evenly and refluxed at 85 °C for 92 h. After cooling to room temperature, compound 5 was obtained by purification (yield 80%).

[0063] 2.88 g of compound 5 was dissolved in 30 mL of methanol, 66 mg of Pd / C catalyst was added, hydrogen gas at a pressure of 0.11 MPa was introduced at room temperature and the reaction was carried out for 8 h. After the reaction was completed, Pd / C was removed by filtration, methanol was removed by vacuum distillation, and the product was further purified to obtain compound 6 (yield 92%).

[0064] Compound 6 is a diamine monomer of polyimide containing piperazine units, and its specific chemical reaction formula is shown below:

[0065]

[0066] Example 3

[0067] Polyimide films were prepared by reacting compounds 3, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride (PMDA), and p-toluenesulfonic acid prepared in Example 1. The preparation steps were as follows:

[0068] 398 mg of compound 3 and 200 mg of 4,4'-diaminodiphenyl ether were dissolved in 10 g of anhydrous DMAc. Then, pyromellitic dianhydride was added to the mixture under stirring, 145 mg each time, and the process was repeated three times to form a reaction system. The reaction system was placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 9.4 wt.%.

[0069] 406 mg of acetic anhydride and 402 mg of triethylamine were added to a polyamic acid solution and stirred at room temperature for 24 h to obtain a polyimide solution. Then, 172 mg of p-toluenesulfonic acid was added to the polyimide solution and stirred thoroughly for 30 min to obtain a p-toluenesulfonate solution of polyimide. The obtained p-toluenesulfonate solution of polyimide was coated onto a glass plate by a blade coating method and dried under vacuum at 120 °C for 2 h to remove the solvent. Then, it was placed in a muffle furnace under a nitrogen atmosphere and heated to 180 °C at a rate of 1 °C / min, and heated at 180 °C for 1 h. After cooling, the membrane on the glass plate was peeled off to obtain a p-toluenesulfonate gas separation membrane of polyimide with a thickness of 80 μm.

[0070] Example 4

[0071] A polyimide acid salt gas separation membrane was prepared by reacting compound 6 (prepared in Example 2), p-phenylenediamine, pyromellitic dianhydride (PMDA), and p-toluenesulfonic acid. The preparation steps are as follows:

[0072] 765 mg of compound 6 and 54 mg of p-phenylenediamine were dissolved in 15 g of anhydrous DMF. Then, pyromellitic dianhydride was added to the mixture under stirring, 147 mg each time, and this process was repeated three times to form a reaction system. The reaction system was then placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 7.7 wt.%.

[0073] 406 mg of acetic anhydride and 402 mg of triethylamine were added to a polyamic acid solution and stirred until homogeneous. The mixture was stirred at room temperature for 24 h to obtain a polyimide solution. Then, 300 mg of p-toluenesulfonic acid was added to the polyimide solution and stirred thoroughly for 30 min to obtain a p-toluenesulfonate solution of polyimide. The obtained p-toluenesulfonate solution of polyimide was coated onto a glass plate using a blade coating method and vacuum dried at 120 °C for 2 h to remove the solvent. The plate was then placed in a muffle furnace under a nitrogen atmosphere and heated to 150 °C at a rate of 1 °C / min. The temperature was maintained at 150 °C for 5 h. After cooling, the membrane on the glass plate was peeled off to obtain a p-toluenesulfonate gas separation membrane of polyimide with a thickness of 80 μm.

[0074] Example 5

[0075] A polyimide acid salt gas separation membrane was prepared by reacting compound 6 (prepared in Example 2), p-phenylenediamine, pyromellitic dianhydride, and dodecyl sulfonic acid. The preparation steps are as follows:

[0076] 765 mg of compound 6 and 54 mg of p-phenylenediamine were dissolved in 15 g of anhydrous DMF. Then, pyromellitic dianhydride was added to the mixture under stirring, 147 mg each time, and this process was repeated three times to form a reaction system. The reaction system was then placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 7.7 wt.%.

[0077] 406 mg of acetic anhydride and 402 mg of triethylamine were added to a polyamic acid solution and stirred at room temperature for 24 h to obtain a polyimide solution. Then, 200 mg of dodecyl sulfonic acid was added to the polyimide solution and stirred thoroughly for 30 min. The solution was then coated onto a glass plate using a blade coating method and dried under vacuum at 120 °C for 2 h to remove the solvent. The plate was then placed in a muffle furnace under a nitrogen atmosphere and heated to 150 °C at a rate of 1 °C / min, and heated at 150 °C for 1 h. After cooling, the membrane on the glass plate was peeled off to obtain a polyimide dodecyl sulfonate gas separation membrane with a thickness of 30 μm.

[0078] Example 6

[0079] Polyimide salt liquid separation membranes were prepared by reacting compounds 3, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride, and concentrated sulfuric acid prepared in Example 1. The preparation steps are as follows:

[0080] 404 mg of compound 3 and 200 mg of 4,4'-diaminodiphenyl ether were dissolved in 15 g of anhydrous DMF. Then, pyromellitic dianhydride was added to the mixture under stirring, 145 mg each time, and this process was repeated three times to form a reaction system. The reaction system was then placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 6.5 wt.%.

[0081] 406 mg of acetic anhydride and 402 mg of triethylamine were added to a polyamic acid solution and stirred at room temperature for 24 h to obtain a polyimide solution. Then, 60 mg of concentrated sulfuric acid (98 wt.%) was added to the polyimide solution and stirred thoroughly for 30 min. The solution was then coated onto a glass plate using a blade coating method and vacuum dried at 80 °C for 2 h to remove the solvent. The plate was then placed in a muffle furnace under a nitrogen atmosphere and heated to 150 °C at a rate of 1 °C / min, and maintained at 150 °C for 1 h. The temperature was then increased to 200 °C at a rate of 1 °C / min and maintained at 200 °C for 1 h. The temperature was then increased to 300 °C at a rate of 1 °C / min and maintained at 300 °C for 1 h. After cooling, the membrane was peeled off the glass plate and soaked in water for 12 h to remove excess acidic small molecule components. After drying at 120 °C, a polyimide filter membrane with a thickness of 80 μm was obtained.

[0082] Example 7

[0083] Polyimide salt liquid separation membranes were prepared by reacting compounds 3, 4,4'-diaminodiphenyl ether, pyromellitic dianhydride, and oxalic acid prepared in Example 1. The preparation steps are as follows:

[0084] 404 mg of compound 3 and 200 mg of 4,4'-diaminodiphenyl ether were dissolved in 10 g of anhydrous DMAc. Then, pyromellitic dianhydride was added to the mixture under stirring, 145 mg each time, and the process was repeated three times to form a reaction system. The reaction system was placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 9.4 wt.%.

[0085] 406 mg of acetic anhydride and 402 mg of triethylamine were added to a polyamic acid solution and stirred at room temperature for 24 h to obtain a polyimide solution. Then, 90 mg of anhydrous oxalic acid was added to the polyimide solution and stirred thoroughly for 30 min. The solution was then coated onto a glass plate using a blade coating method and dried under vacuum at 100 °C for 2 h to remove the solvent. The plate was then placed in a muffle furnace under a nitrogen atmosphere and heated to 150 °C at a rate of 1 °C / min for 1 h. The temperature was then increased to 200 °C at a rate of 1 °C / min and heated for 1 h. The temperature was then increased to 300 °C at a rate of 1 °C / min in a muffle furnace under a nitrogen atmosphere and maintained at 300 °C for 1 h. After cooling, the membrane was peeled off the glass plate and soaked in water for 12 h to remove excess acidic small molecule components. After drying, the membrane was obtained at 120 °C with a thickness of 80 μm.

[0086] Example 8

[0087] A polyimide acid salt gas separation membrane was prepared by reacting compound 6 (prepared in Example 2), p-phenylenediamine, pyromellitic dianhydride, and p-toluenesulfonic acid. The preparation steps are as follows:

[0088] 765 mg of compound 6 and 54 mg of p-phenylenediamine were dissolved in 15 g of anhydrous DMAc. Then, pyromellitic dianhydride was added to the mixture under stirring, 147 mg each time, and this process was repeated three times to form a reaction system. The reaction system was then placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 7.7 wt.%.

[0089] 406 mg of acetic anhydride and 402 mg of triethylamine were added to a polyamic acid solution and stirred at room temperature for 24 h to obtain a polyimide solution. Then, 150 mg of p-toluenesulfonic acid was added to the polyimide solution and stirred thoroughly for 30 min. The solution was then coated onto a glass plate using a blade coating method and dried under vacuum at 120 °C for 2 h to remove the solvent. The plate was then placed in a muffle furnace under a nitrogen atmosphere and heated to 150 °C at a rate of 1 °C / min for 5 h. The temperature was then increased to 220 °C at a rate of 1 °C / min and heated for 1 h. The temperature was then increased to 300 °C at a rate of 1 °C / min in a muffle furnace under a nitrogen atmosphere and maintained at 300 °C for 1 h. After cooling, the membrane was peeled off the glass plate and soaked in water for 12 h to remove excess acidic small molecule components. After drying at 120 °C, a polyimide filter membrane with a thickness of 80 μm was obtained.

[0090] Example 9

[0091] A polyimide acid gas separation membrane was prepared by reacting compound 6 (prepared in Example 2), p-phenylenediamine, pyromellitic dianhydride, and p-oxalic acid. The preparation steps are as follows:

[0092] 765 mg of compound 6 and 54 mg of p-phenylenediamine were dissolved in 15 g of anhydrous DMAc. Then, pyromellitic dianhydride was added to the mixture under stirring, 147 mg each time, and this process was repeated three times to form a reaction system. The reaction system was then placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 7.7 wt.%.

[0093] Add 406 mg of acetic anhydride and 402 mg of triethylamine to a polyamic acid solution and stir at room temperature for 24 h to obtain a polyimide solution. Then add 120 mg of anhydrous oxalic acid to the polyimide solution and stir thoroughly for 30 min. Then coat the solution onto a glass plate using a blade coating method and vacuum dry at 100 °C for 2 h to remove the solvent. Then place it in a muffle furnace under a nitrogen atmosphere and heat it at 150 °C at a rate of 1 °C / min for 1 h. Then heat it at 220 °C at a rate of 1 °C / min for 1 h. Then continue to heat it in a muffle furnace under a nitrogen atmosphere at a rate of 1 °C / min to 300 °C and maintain it at 300 °C for 1 h. After cooling, peel the membrane off the glass plate and soak it in water for 12 h to remove excess acidic small molecule components. After taking it out, dry it at 120 °C to obtain a polyimide filter membrane.

[0094] In Examples 6 to 9 above, the polyimide salt separation membrane obtained after acidification was further heated at high temperature in a muffle furnace to control the membrane pore size by thermally decomposing part of the polyimide.

[0095] Example 10

[0096] Polyimide salt liquid separation membranes were prepared by reacting compound 3 (prepared in Example 1), pyromellitic dianhydride, and oxalic acid. The preparation steps are as follows:

[0097] 808 mg of compound 3 was dissolved in 12 g of anhydrous DMAc, and then pyromellitic dianhydride was added to it under stirring, 145 mg each time, repeated three times to form a reaction system; the reaction system was placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, and a polyamic acid solution with a solid content of about 9.3 wt.% was obtained.

[0098] 406 mg of acetic anhydride and 402 mg of triethylamine were added to a polyamic acid solution and stirred at room temperature for 24 h to obtain a polyimide solution. Then, 340 mg of p-toluenesulfonic acid was added to the polyimide solution and stirred thoroughly for 30 min to obtain a p-toluenesulfonate solution of polyimide. The obtained p-toluenesulfonate solution of polyimide was coated onto a glass plate by a blade coating method and dried under vacuum at 120 °C for 2 h to remove the solvent. Then, it was placed in a muffle furnace under a nitrogen atmosphere and heated to 180 °C at a rate of 1 °C / min, and heated at 180 °C for 1 h. After cooling, the membrane on the glass plate was peeled off to obtain a p-toluenesulfonate gas separation membrane of polyimide with a thickness of 80 μm.

[0099] Example 11

[0100] The polyimide acid gas separation membrane was prepared using compound 6 prepared in Example 2, pyromellitic dianhydride, and reaction with oxalic acid. The preparation steps are as follows:

[0101] 1.02 g of compound 6 was dissolved in 15 g of anhydrous DMAc, and then pyromellitic dianhydride was added to it under stirring, 147 mg each time, repeated three times to form a reaction system; the reaction system was placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, and a polyamic acid solution with a solid content of about 8.9 wt.% was obtained.

[0102] Add 406 mg of acetic anhydride and 402 mg of triethylamine to a polyamic acid solution and stir at room temperature for 24 h to obtain a polyimide solution. Then add 320 mg of anhydrous oxalic acid to the polyimide solution and stir thoroughly for 30 min. Then coat the solution onto a glass plate using a blade coating method and vacuum dry at 100 °C for 2 h to remove the solvent. Then place it in a muffle furnace under a nitrogen atmosphere and heat it at 150 °C at a rate of 1 °C / min for 1 h. Then heat it at 220 °C at a rate of 1 °C / min for 1 h. Then continue to heat it in a muffle furnace under a nitrogen atmosphere at a rate of 1 °C / min to 300 °C and maintain it at 300 °C for 1 h. After cooling, peel the membrane off the glass plate and soak it in water for 12 h to remove excess acidic small molecule components. After drying, the membrane is obtained at 120 °C.

[0103] Comparative Example 1

[0104] Polyimide gas separation membranes were prepared by reacting compounds 3, 4,4'-diaminodiphenyl ether (prepared in Example 1) with pyromellitic dianhydride. The preparation steps are as follows:

[0105] 398 mg of compound 3 and 200 mg of 4,4'-diaminodiphenyl ether were dissolved in 7 g of anhydrous DMAc. Then, pyromellitic dianhydride was added to the mixture under stirring, 145 mg each time, and the process was repeated three times to form a reaction system. The reaction system was placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 13 wt.%.

[0106] The polyamic acid solution was coated onto a glass plate using a blade coating method and dried under vacuum at 120°C for 2 hours to remove the solvent. Then, it was placed in a muffle furnace under a nitrogen atmosphere and heated to 200°C at a rate of 1°C / min, and heated at 200°C for 1 hour. Then, it was heated to 300°C in the muffle furnace under a nitrogen atmosphere at a rate of 1°C / min and maintained at 300°C for 1 hour. After cooling, the film on the glass plate was peeled off to obtain polyimide film 1 with a thickness of 20 μm.

[0107] Comparative Example 2

[0108] Polyimide films were prepared by reacting compound 6 (prepared in Example 2), p-phenylenediamine, and pyromellitic dianhydride (PMDA). The preparation steps are as follows:

[0109] 765 mg of compound 6 and 54 mg of p-phenylenediamine were dissolved in 8 g of anhydrous DMAc. Then, pyromellitic dianhydride was added to the mixture under stirring, 147 mg each time, and this process was repeated three times to form a reaction system. The reaction system was then placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 13.6 wt.%.

[0110] The polyamic acid solution was coated onto a glass plate using a blade coating method and dried under vacuum at 120°C for 2 hours to remove the solvent. Then, it was placed in a muffle furnace under a nitrogen atmosphere and heated to 200°C at a rate of 1°C / min, and heated at 200°C for 1 hour. Then, it was heated to 300°C in the muffle furnace under a nitrogen atmosphere at a rate of 1°C / min and maintained at 300°C for 2 hours. After cooling, the film on the glass plate was peeled off to obtain polyimide film 2 with a thickness of 30 μm.

[0111] Comparative Example 3

[0112] 216 mg of p-phenylenediamine was dissolved in 5 g of anhydrous DMF, and then pyromellitic dianhydride was added to it under stirring, 147 mg each time, repeated three times to form a reaction system; the reaction system was placed in an ice-water bath and stirred for 12 h to allow the reaction system to undergo a polycondensation reaction at low temperature, resulting in a polyamic acid solution with a solid content of approximately 11.6 wt.%.

[0113] 203 mg of acetic anhydride and 201 mg of triethylamine were added to the reaction system and stirred until homogeneous. The mixture was stirred at room temperature for 24 h. The mixture was then coated onto a glass plate using a blade coating method and dried under vacuum at 120 °C for 2 h to remove the solvent. The plate was then placed in a muffle furnace under a nitrogen atmosphere and heated to 150 °C at a rate of 1 °C / min, and maintained at 150 °C for 1 h. The temperature was then increased to 200 °C at a rate of 1 °C / min and maintained at 200 °C for 1 h. The temperature was then increased to 300 °C at a rate of 1 °C / min in the muffle furnace under a nitrogen atmosphere and maintained at 300 °C for 1 h. After cooling, the membrane was peeled off the glass plate and soaked in water for 12 h to remove excess acidic small molecule components. After drying at 120 °C, the polyimide filter membrane 3 with a thickness of 80 μm was obtained.

[0114] Performance testing:

[0115] (I) Gas separation performance test of polyimide salt separation membrane:

[0116] Test I: The gas separation performance of the films prepared in Examples 3, 4, 5, 10, and Comparative Examples 1, 2, and 3 was tested. Specifically, the permeability and selectivity of each film under pure gas at 0.1 MPa were tested. The test results are shown in Table 1-1 below.

[0117] Table 1-1:

[0118]

[0119] As can be seen from the test results in Table 1-1, the polyimide sulfonate films prepared in Examples 3, 4, 5 and 10 have good permeability to oxygen, hydrogen and carbon dioxide, but poor permeability to nitrogen, and can achieve good separation of oxygen and nitrogen, oxygen and carbon dioxide, nitrogen and carbon dioxide, or hydrogen and oxygen.

[0120] Compared with the polyimide films prepared in Comparative Examples 1 and 2, the permeability of polyimide p-toluenesulfonate film and polyimide dodecyl sulfonate film to gases is significantly enhanced. Specifically, by introducing p-toluenesulfonic acid or dodecyl sulfonic acid, the free volume of polyimide is adjusted, that is, the porosity of the membrane is adjusted, which greatly improves the permeability and selectivity of the membrane, especially the selectivity for the two gas combinations of O2 / N2 and CO2 / N2 is significantly enhanced.

[0121] (II) Separation performance test of polyimide salt separation membrane for colloids in water:

[0122] The separation performance of the films prepared in Examples 6, 7, 8, 9, 11, and Comparative Examples 1, 2, and 3 on colloids in water was tested. Specifically, the water flux of each film under a pressure of 0.4 MPa and its retention capacity for spherical carbon nanoparticle suspensions with a particle size range of 10 nm to 40 nm in ink and bovine serum albumin with a particle size of about 8 nm were tested. The test results are shown in Table 3 below.

[0123] Table 2-1:

[0124]

[0125] As can be seen from the test results in Table 2-1, the polyimide liquid separation membranes prepared in Examples 6-9 and Example 11 exhibit good water flux characterization results, and their filtration performance for particles larger than 8 nm in the liquid is superior to that in Comparative Examples 1-3. This indicates that the introduction of inorganic or organic acids can adjust the porosity and pore size of polyimide membranes containing piperazine structures in the main chain, enabling the membrane structure to reach ultrafiltration levels. The polyimide membranes prepared in Comparative Examples 1-3 do not possess these performance characteristics. Furthermore, the test results in Table 2-1 show that the pore size of the polyimide liquid separation membranes prepared using the methods in Examples 6-9 and Example 11 is controlled between 8 nm and 10 nm.

[0126] Furthermore, comparing the test results of Example 9 with those of Example 7, it can be seen that adjusting the amount of acid used for acidification can regulate the water flux through the filter membrane (e.g., increasing the amount of oxalic acid can correspondingly adjust the water flux of the membrane), but reduces the retention capacity for bovine serum albumin. This indicates that adjusting the amount of acid added can adjust the porosity of the filter membrane, thereby achieving adjustment of the filtration performance of the polyimide filter membrane.

Claims

1. A diamine monomer containing a piperazine unit, characterized in that, It has the following structure: ， In the formula, Q1 is n1 is 1, 2 or 3, R is CF3 or OCF3; R and Q1 are located at the ortho, meta or para position on the benzene ring; -NH2 group and Q1 are located at the meta or para position on the benzene ring.

2. A polyimide film prepared using a diamine monomer containing a piperazine unit, characterized in that, It is obtained by: polyamic acid obtained by polycondensation of diamine monomers containing piperazine units, aromatic diamine monomers and tetracarboxylic acid dianhydride in a molar ratio of x:(1-x):y; imidizing the polyamic acid into polyimide; acidifying the polyimide with inorganic or organic acid to obtain polyimide salt; and further forming a film; wherein, 0.01≤x≤1, 0.95≤y≤1.05; The structure of the diamine monomer containing the piperazine unit is as follows: ， In the formula, Q1 is or n1 is 1, 2 or 3, R is CF3 or OCF3; R and Q1 are located at the ortho, meta or para position on the benzene ring; -NH2 group and Q1 are located at the meta or para position on the benzene ring; The aromatic diamine monomer is selected from p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfide, 3,4-diaminodiphenyl sulfide, and 4,4'-diaminodiphenyl sulfide. Tetracarboxylic dianhydride is selected from pyromellitic dianhydride; The inorganic acids are hydrochloric acid, sulfuric acid, phosphoric acid, perchloric acid, hypochlorous acid, or chlorous acid; The organic acid is oxalic acid or aromatic sulfonic acid, and aromatic sulfonic acid has the following properties: Compounds with structure R 1 It is an H or C1~C18 alkyl chain.

3. A method for preparing a polyimide film as described in claim 2, characterized in that, The steps are as follows: S1. Dissolve a diamine monomer containing a piperazine unit, or a diamine monomer containing a piperazine unit and an aromatic diamine monomer in an organic solvent, and add tetracarboxylic acid dianhydride in multiple portions under stirring to form a reaction system. S2. Place the reaction system in an ice-water bath and stir it at 0℃~10℃ for 12h~48h. The reaction system undergoes a polycondensation reaction at low temperature to obtain a polyamic acid solution with a solid content of 5wt.%~10wt.%. S3. Add acetic anhydride and triethylamine to the polyamic acid solution and stir at room temperature for at least 24 hours to obtain a polyimide solution; then, add an inorganic acid or organic acid to the polyimide solution and stir at room temperature for 30 to 60 minutes to obtain a polyimide salt solution. S4. Apply the polyimide solution to the glass plate using a blade coating method, with a coating thickness of 20μm~300μm; then, place the glass plate coated with the polyimide solution in a vacuum dryer at 80℃~120℃ for 1h~4h to remove the solvent; then place it in a muffle furnace under a nitrogen atmosphere and heat it to 150℃~180℃ at a rate of 1℃ / min, and heat it at 150℃~180℃ for 1h~5h before removing it. After cooling, the film on the glass plate is peeled off to obtain a polyimide film.

4. The use of the polyimide membrane according to claim 3 as a gas separation membrane, characterized in that, Gas separation membranes are used for the separation of oxygen and nitrogen, and carbon dioxide and nitrogen.

5. A method for preparing a polyimide film as described in claim 2, characterized in that, The steps are as follows: S1. Dissolve a diamine monomer containing a piperazine unit, or a diamine monomer containing a piperazine unit and an aromatic diamine monomer in an organic solvent, and add tetracarboxylic acid dianhydride in multiple portions under stirring to form a reaction system. S2. Place the reaction system in an ice-water bath and stir it at 0℃~10℃ for 12h~48h. The reaction system undergoes a polycondensation reaction at low temperature to obtain a polyamic acid solution with a solid content of 5wt.%~10wt.%. S3. Add acetic anhydride and triethylamine to the polyamic acid solution and stir at room temperature for at least 24 hours to obtain a polyimide solution; then, add an inorganic acid or organic acid to the polyimide solution and stir at room temperature for 30 to 60 minutes to obtain a polyimide salt solution. S4. Apply the polyimide solution to a glass plate using a blade coating method, with a coating thickness of 20μm~300μm. Then, place the glass plate coated with the polyimide solution under vacuum drying at 80℃~120℃ for 1h~4h to remove the solvent. Next, place it in a muffle furnace under a nitrogen atmosphere and heat it to 180℃~220℃ at a rate of 1℃ / min~3℃ / min, maintaining the temperature at 180℃~220℃ for 1h~2h. Then, continue heating it in a muffle furnace under a nitrogen atmosphere at a rate of 1~3℃ / min to 280~320℃, maintaining the temperature at 280~320℃ for 1h~2h. After cooling, peel off the film from the glass plate and soak it in clean water for 10h~24h. After removing it, dry it at 100℃~120℃ to obtain the polyimide film.

6. The use of the polyimide membrane according to claim 5 as a liquid separation membrane, characterized in that, Liquid separation membranes are used to retain particles larger than 8 nm in liquids.

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