Polyimide two-way drafted porous membrane and preparation method thereof

Abstract

The preparation method comprises the following steps: synthesizing a polyamic acid-amic acid ester block copolymer, preparing the polyamic acid-amic acid ester block copolymer into a gel film, carrying out two-way drafting, and carrying out heat setting to obtain the polyimide two-way drafted porous film. By controlling the molecular weight and proportion of polyamic acid ester chain segments, the size and size of a pore structure are regulated and controlled, and meanwhile, due to the fact that molecular chains are fixed under the action of tension during high-temperature imidization, formed gas is difficult to agglomerate to form macropores, so that the mechanical performance of the porous membrane is guaranteed; the prepared polyimide bidirectionally-drawn porous membrane is excellent in mechanical property, uniform in pore distribution and controllable in size, and can be widely applied to the industrial fields of electronics, electric appliances, automobiles, machinery, aviation, spaceflight, chemical engineering and the like.

Description

Technical Field

[0001] The invention belongs to the field of polyimide films, and in particular relates to a polyimide biaxially stretched porous film and a preparation method thereof. Background Art

[0002] Commercial polyimide films, such as DuPont's Kapton, Ube Industries' Upilex, and Kaneka Chemical's Apical series, exhibit excellent mechanical properties, heat resistance, dimensional stability, and dielectric properties, and have been widely used in fields such as high-temperature insulation and flexible electronics. However, applications in water treatment, gas separation, fuel cell membranes, and lithium-ion battery separators require polyimide films with a porous structure to facilitate material transport or separation. Therefore, research on methods for preparing polyimide films by introducing a continuous pore structure into polyimide materials to create porous membranes is becoming an important direction for material development. Combining the excellent properties of the polymer backbone with the functionality of the porous phase can meet the specific requirements of lightweight, safety, and weather resistance in fields such as environmental energy, microelectronics, and aerospace, and is expected to become an important direction for material development. However, the introduction of pores in thin film materials inevitably leads to a significant reduction in mechanical properties, thus limiting the service life and lifespan of polyimide porous membranes. Therefore, research and development of high-performance polyimide porous membranes and their preparation processes are of great significance.

[0003] The imidization methods of polyamic acid are divided into thermal imidization and chemical imidization. During the chemical imidization process of polyamic acid, the molecular chain segments are gradually transformed into insoluble polyimide units, and the solubility of the molecular chain decreases, causing the originally homogeneous solution to undergo a sol-gel transition, and a gel film with high stretching ratio can be obtained. In addition, chemical imidization can be carried out at low temperatures. As the degree of imidization increases, a large amount of solvent is retained inside the gel film, which is conducive to the rigid molecular chains being oriented and arranged regularly under external forces. Therefore, the use of chemical imidization for bidirectional stretching is an important process link in the industry to improve the mechanical strength and dimensional stability of polyimide films. U.S. Patent US5460890A describes a process for preparing a gel film by chemically controlling the imidization reaction of polyamic acid and performing bidirectional stretching on it, which can greatly improve the mechanical properties and dimensional stability of the film. Patent CN116496528A prepares a boron nitride nanosheet / polyamic acid precursor solution through in-situ polymerization. A certain amount of acetic anhydride and pyridine are then added to control the sol-gel transition to form a gel film. The film is then stretched and imidized to produce a composite polyimide film with excellent thermal conductivity and mechanical properties. If chemical imidization and biaxial stretching can be combined into the polyimide porous membrane preparation process, it is expected that a polyimide porous membrane with high mechanical properties will be obtained.

[0004] Polyamic acid esters can be obtained by esterifying the carboxyl groups in polyamic acid, which has good solubility and excellent storage stability compared to polyamic acid. Unlike the imidization mechanism of polyamic acid, polyamic acid esters are difficult to complete the imidization reaction under the action of chemical imidization reagents, and polyimide is obtained by cyclization dealcoholization at high temperatures. When the esterified part is large in volume, it is removed at high temperature, leaving a pore structure in the material, thereby preparing a polyimide porous material. Patent CN118373990A discloses a method for preparing a low thermal conductivity polyimide foam insulation material, in which an aerogel with an active end group is added to the stage of preparing an esterification liquid by reacting a dibasic acid anhydride with a fatty alcohol, followed by foaming, curing, and finally high-temperature imidization to obtain a polyimide foam insulation material. CN115895254A prepares two polyimide precursors by polymerization of a first aromatic dianhydride and / or its esterified product, a reactive end-capping agent and / or its esterified product, and a first aromatic diamine, and foams and cures at a temperature of 350-400°C to obtain a polyimide foam material. CN114395158A discloses a method for preparing a low-density, high-temperature-resistant, highly uniform closed-cell polyimide rigid foam. The dianhydride and a small molecular fatty alcohol are subjected to an esterification reaction in an organic solvent to obtain an esterification reaction product. A diamine and a foam stabilizer are then added, stirred for reaction, and then dried and pulverized in sequence to obtain a foam precursor powder. The foam precursor powder is then mixed with a solid foaming agent and then formed by radiation crosslinking and thermal curing to obtain a polyimide foam. Although the process of foaming and pore formation using high-temperature imidization of polyamic acid esters is simple, it is difficult to control the pore density and size, resulting in a high closed-cell ratio and severely compromised mechanical strength. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a polyimide biaxially stretched porous membrane and a preparation method thereof. The polyimide biaxially stretched porous membrane has excellent mechanical properties, uniform pore distribution and controllable size.

[0006] The invention provides a polyimide biaxially stretched porous membrane. The polyimide biaxially stretched porous membrane is obtained by synthesizing a polyamic acid-amic acid ester block copolymer, preparing the copolymer into a gel membrane, biaxially stretching the membrane, and then heat-setting the membrane.

[0007] Preferably, the polyimide biaxially stretched porous membrane has a thickness of ≤25 μm, a porosity of ≥40%, and a stretching strength of ≥120 MPa.

[0008] The present invention also provides a method for preparing a polyimide biaxially stretched porous membrane, comprising:

[0009] (1) heating an acid anhydride to reflux in an excess alcohol reagent, separating and purifying it, then reacting it with an acyl chloride reagent, and obtaining an ester-containing monomer after recrystallization and drying; then reacting the ester-containing monomer with a diamine in an aprotic polar organic solvent to synthesize a polyamic acid ester oligomer solution containing an ester-terminated unit;

[0010] (2) using the same type of anhydride and diamine in step (1) as monomers, synthesizing an amino-terminated polyamic acid solution in a non-protonic polar organic solvent, and then mixing it with a polyamic acid ester oligomer solution containing an ester unit end-capping to obtain a polyamic acid-amic acid ester block copolymer solution;

[0011] (3) adding acetic anhydride and a basic catalyst to the polyamic acid-amic acid ester block copolymer solution, scraping the film and then heating to obtain a gel film;

[0012] (4) The gel film is subjected to biaxial stretching, followed by heat setting, removal of ester groups and imidization to obtain a polyimide biaxially stretched porous film.

[0013] Preferably, the acid anhydride in step (1) is at least one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-dibenzophenone tetracarboxylic dianhydride, 2,3',3,4'-biphenyltetracarboxylic dianhydride, and 4,4'-biphenyl ether dianhydride; the diamine is at least one of p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, and 2-(4-aminophenyl)-5-aminobenzimidazole.

[0014] Preferably, the molar ratio of the ester-containing monomer to the diamine in step (1) is 0.98:1 to 1.02:1, preferably 1:1; and the reaction time is 1 to 10 hours.

[0015] Preferably, the preparation method according to claim 2 is characterized in that: in the step (1), the acid anhydride is heated to reflux in an excess alcohol reagent using pyridine as a catalyst, and the molar amount of pyridine is 0.1 to 4.5 times that of the acid anhydride.

[0016] Preferably, the alcohol reagent in step (1) is at least one of isopropyl alcohol, tert-butyl alcohol, n-hexanol, and n-decanol, and the molar amount thereof is 1 to 10 times that of the acid anhydride.

[0017] Preferably, the acyl chloride reagent in step (1) is at least one of thionyl chloride, oxalyl chloride, and phosphorus trichloride, and the molar amount thereof is 1 to 10 times that of the acid anhydride.

[0018] Preferably, the heating reflux temperature in step (1) is 80-100° C., and the heating time is 2-10 h.

[0019] Preferably, the reaction time of the acyl chloride reagent in step (1) is 1 to 5 hours.

[0020] Preferably, in step (2), the molar ratio of the acid anhydride to the diamine is 0.98:1 to 1.02:1, preferably 1:1; and the reaction time is 1 to 10 hours.

[0021] Preferably, the aprotic polar organic solvent in steps (1) and (2) is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; and the solid content of the polyamic acid ester oligomer solution, the amino-terminated polyamic acid solution, and the polyamic acid-amic acid ester block copolymer solution is 10 wt % to 15 wt %, preferably 12 wt %.

[0022] Preferably, the time for synthesizing the amino-terminated polyamic acid solution and the reaction time for the polyamic acid-amic acid ester block copolymer solution in step (2) are both 1 to 10 hours.

[0023] Preferably, in the polyamic acid-amic acid ester block copolymer molecular chain in step (2), the molar proportion of the amic acid segment is 70% to 90%, preferably 80%; the molar proportion of the amic acid ester segment is 10% to 30%, preferably 20%. The degree of polymerization of the polyamic acid-amic acid ester block copolymer is 100 to 1000, preferably 600.

[0024] Preferably, the solid content of the polyamic acid-amic acid ester block copolymer solution in step (2) is 10-20 wt%, preferably 12 wt%.

[0025] Preferably, the alkaline catalyst in step (3) is at least one of pyridine, 2-methylpyridine, 3-methylpyridine, triethylamine, isoquinoline, and quinoline, and the molar amount is 0.1 to 10 times, preferably 6.0 times, of the acid anhydride in step (2).

[0026] Preferably, the molar amount of acetic anhydride in step (3) is 1.5 to 10 times that of the anhydride in step (2), preferably 6.0 times.

[0027] Preferably, the heating temperature in step (3) is 40-100° C. and the heating time is 10-30 min.

[0028] Preferably, the process parameters of the bidirectional stretching in step (4) are: stretching temperature is 20-80°C, preferably 50°C; longitudinal stretching ratio is 1.1-1.9, preferably 1.6; transverse stretching ratio is 1.1-1.9, preferably 1.6; stretching speed is 1-10 mm / s, preferably 5 mm / s.

[0029] Preferably, the heat setting process parameters in step (4) are: temperature of 300-450° C., preferably 400° C.; time of 30-90 min, preferably 60 min.

[0030] The present invention designs a polyamic acid-amic acid ester block copolymer, selectively catalyzing the imidization reaction of the amic acid units to adjust the polymer solubility and form a gel film. The gel film is then biaxially stretched to orient the molecular chains. The gel film's high-temperature imidization temperature is controlled to dealcoholize the amic acid ester segments at high temperatures, forming pores. By controlling the molecular weight and ratio of the polyamic acid segments, the size and shape of the pore structure are regulated. Furthermore, during high-temperature imidization, the molecular chains are fixed by tension, making it difficult for the gases formed to agglomerate and form large pores, thereby ensuring the mechanical properties of the porous membrane. The result is a polyimide biaxially stretched porous membrane with excellent mechanical properties, uniform pore distribution, and controllable pore size.

[0031] Beneficial effects

[0032] The present invention introduces amic acid ester units into the polyamic acid molecular chain to synthesize a polyamic acid-amic acid ester copolymer, and accurately controls the gel membrane preparation process according to the chemical imidization reaction activity of different chain segments; utilizes the bidirectional stretching and non-solvent-induced phase separation process of the gel membrane to achieve the improvement of the mechanical strength of the porous membrane and the regulation of the pore structure, and develops a new approach to prepare polyimide porous membranes based on the gelation-phase transition strategy, and prepares a polyimide bidirectional stretch porous membrane with excellent mechanical properties, uniform pore distribution and controllable size, which can be widely used in electronics, electrical appliances, automobiles, machinery, aviation, aerospace, chemical industry and other industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is an optical photograph of the gel film prepared in Example 1 during biaxial stretching.

[0034] Figure 2 This is an optical photograph of the polyimide biaxially stretched porous membrane prepared in Example 1.

[0035] Figure 3 This is a cryo-electron microscopy image of the cross-section of the polyimide biaxially stretched porous membrane prepared in Example 1. DETAILED DESCRIPTION

[0036] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0037] Example 1

[0038] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of isopropyl alcohol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90 ° C for 5 h, cooled to room temperature, added with 0.3 mol of thionyl chloride, reacted for 4 h, recrystallized and dried under vacuum at 50 ° C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 168.12 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0039] (2) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask to make the molar proportion of the amic acid ester chain segment 20%. The reaction was continued for 6 h to obtain a polyamic acid-amic acid ester block copolymer solution with a solid content of 12 wt%.

[0040] (3) 0.24 mol of pyridine and 0.24 mol of acetic anhydride were added dropwise into a three-necked flask at -10°C while stirring. After 30 minutes, the flask was centrifuged for degassing. An 800 μm liquid film was scraped on a clean glass cup and then placed in an oven at 50°C for 20 minutes to obtain a partially imidized polyimide-amic acid ester gel film.

[0041] (4) The gel film was fixed in a biaxial stretching machine and biaxially stretched at a speed of 5 mm / s, a longitudinal stretching ratio of 1.6, a transverse stretching ratio of 1.6, and a stretching temperature of 50°C. Subsequently, heat treatment was performed according to the temperature control program of 100°C for 1 h, 200°C for 1 h, 300°C for 1 h, and 400°C for 1 h to obtain a polyimide biaxially stretched porous membrane.

[0042] The microscopic morphology of the polyimide biaxially stretched porous membrane prepared in this embodiment is as follows: Figure 3 shown.

[0043] Example 2

[0044] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of tert-butyl alcohol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90°C for 5 h, cooled to room temperature, added with 0.3 mol of thionyl chloride, reacted for 4 h, recrystallized and dried under vacuum at 50°C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 176.92 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0045] (2) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask to make the molar proportion of the amic acid ester chain segment 20%. The reaction was continued for 6 h to obtain a polyamic acid-amic acid ester block copolymer solution with a solid content of 12 wt%.

[0046] (3) 0.24 mol of pyridine and 0.24 mol of acetic anhydride were added dropwise into a three-necked flask at -10°C while stirring. After 30 minutes, the flask was centrifuged for degassing. An 800 μm liquid film was scraped on a clean glass cup and then placed in an oven at 50°C for 20 minutes to obtain a partially imidized polyimide-amic acid ester gel film.

[0047] (4) The gel film was fixed in a biaxial stretching machine and biaxially stretched at a speed of 5 mm / s, a longitudinal stretching ratio of 1.6, a transverse stretching ratio of 1.6, and a stretching temperature of 50°C. Subsequently, heat treatment was performed according to the temperature control program of 100°C for 1 h, 200°C for 1 h, 300°C for 1 h, and 400°C for 1 h to obtain a polyimide biaxially stretched porous membrane.

[0048] Example 3

[0049] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of n-hexanol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90°C for 5 h, cooled to room temperature, added with 0.3 mol of thionyl chloride, reacted for 4 h, recrystallized and dried under vacuum at 50°C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 185.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0050] (2) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask to make the molar proportion of the amic acid ester chain segment 20%. The reaction was continued for 6 h to obtain a polyamic acid-amic acid ester block copolymer solution with a solid content of 12 wt%.

[0051] (3) 0.24 mol of pyridine and 0.24 mol of acetic anhydride were added dropwise into a three-necked flask at -10°C while stirring. After 30 minutes, the flask was centrifuged for degassing. An 800 μm liquid film was scraped on a clean glass cup and then placed in an oven at 50°C for 20 minutes to obtain a partially imidized polyimide-amic acid ester gel film.

[0052] (4) The gel film was fixed in a biaxial stretching machine and biaxially stretched at a speed of 5 mm / s, a longitudinal stretching ratio of 1.6, a transverse stretching ratio of 1.6, and a stretching temperature of 50°C. Subsequently, heat treatment was performed according to the temperature control program of 100°C for 1 h, 200°C for 1 h, 300°C for 1 h, and 400°C for 1 h to obtain a polyimide biaxially stretched porous membrane.

[0053] Example 4

[0054] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of n-decanol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90°C for 5 h, cooled to room temperature, and 0.3 mol of thionyl chloride was added. After reacting for 4 h, the mixture was recrystallized and dried under vacuum at 50°C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 203.32 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0055] (3) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask to make the molar proportion of the amic acid ester chain segment 20%. The reaction was continued for 6 h to obtain a polyamic acid-amic acid ester block copolymer solution with a solid content of 12 wt%.

[0056] (4) 0.24 mol of pyridine and 0.24 mol of acetic anhydride were added dropwise into a three-necked flask at -10°C while stirring. After 30 minutes, the flask was centrifuged for degassing. An 800 μm liquid film was scraped on a clean glass cup and then placed in an oven at 50°C for 20 minutes to obtain a partially imidized polyimide-amic acid ester gel film.

[0057] (5) The gel film was fixed in a biaxial stretching machine and biaxially stretched at a speed of 5 mm / s, a longitudinal stretching ratio of 1.6, a transverse stretching ratio of 1.6, and a stretching temperature of 50°C. Subsequently, heat treatment was performed according to the temperature control program of 100°C for 1 h, 200°C for 1 h, 300°C for 1 h, and 400°C for 1 h to obtain a polyimide biaxially stretched porous membrane.

[0058] Example 5

[0059] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of isopropyl alcohol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90 ° C for 5 h, cooled to room temperature, added with 0.3 mol of thionyl chloride, reacted for 4 h, recrystallized and dried under vacuum at 50 ° C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 168.12 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0060] (2) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask to make the molar proportion of the amic acid ester chain segment 30%. The reaction was continued for 6 h to obtain a polyamic acid-amic acid ester block copolymer solution with a solid content of 12 wt%.

[0061] (3) 0.24 mol of pyridine and 0.24 mol of acetic anhydride were added dropwise into a three-necked flask at -10°C while stirring. After 30 minutes, the flask was centrifuged for degassing. An 800 μm liquid film was scraped on a clean glass cup and then placed in an oven at 50°C for 20 minutes to obtain a partially imidized polyimide-amic acid ester gel film.

[0062] (4) The gel film was fixed in a biaxial stretching machine and biaxially stretched at a speed of 5 mm / s, a longitudinal stretching ratio of 1.6, a transverse stretching ratio of 1.6, and a stretching temperature of 50°C. Subsequently, heat treatment was performed according to the temperature control program of 100°C for 1 h, 200°C for 1 h, 300°C for 1 h, and 400°C for 1 h to obtain a polyimide biaxially stretched porous membrane.

[0063] Example 6

[0064] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of isopropyl alcohol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90 ° C for 5 h, cooled to room temperature, added with 0.3 mol of thionyl chloride, reacted for 4 h, recrystallized and dried under vacuum at 50 ° C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 168.12 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0065] (2) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask to make the molar proportion of the amic acid ester chain segment 10%. The reaction was continued for 6 h to obtain a polyamic acid-amic acid ester block copolymer solution with a solid content of 12 wt%.

[0066] (3) 0.24 mol of pyridine and 0.24 mol of acetic anhydride were added dropwise into a three-necked flask at -10°C while stirring. After 30 minutes, the flask was centrifuged for degassing. An 800 μm liquid film was scraped on a clean glass cup and then placed in an oven at 50°C for 20 minutes to obtain a partially imidized polyimide-amic acid ester gel film.

[0067] (4) The gel film was fixed in a biaxial stretching machine and biaxially stretched at a speed of 5 mm / s, a longitudinal stretching ratio of 1.6, a transverse stretching ratio of 1.6, and a stretching temperature of 50°C. Subsequently, heat treatment was performed according to the temperature control program of 100°C for 1 h, 200°C for 1 h, 300°C for 1 h, and 400°C for 1 h to obtain a polyimide biaxially stretched porous membrane.

[0068] Example 7

[0069] A polyimide biaxially stretched porous membrane was prepared according to the method of Example 1, except that in step (5), the longitudinal stretching ratio was 1.8 and the transverse stretching ratio was 1.8.

[0070] Example 8

[0071] A polyimide biaxially stretched porous membrane was prepared according to the method of Example 1, except that in step (5), the longitudinal stretching ratio was 1.4 and the transverse stretching ratio was 1.4.

[0072] Example 9

[0073] A polyimide biaxially stretched porous membrane was prepared according to the method of Example 1, except that in step (5), the longitudinal stretching ratio was 1.2 and the transverse stretching ratio was 1.2.

[0074] Comparative Example 1

[0075] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of isopropyl alcohol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90 ° C for 5 h, cooled to room temperature, added with 0.3 mol of thionyl chloride, reacted for 4 h, recrystallized and dried under vacuum at 50 ° C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 168.12 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0076] (2) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask to make the amide ester segment account for 20%, and the reaction was continued for 6 h to obtain a polyamic acid-amide ester block copolymer solution with a solid content of 12 wt%.

[0077] (3) 0.24 mol of pyridine and 0.24 mol of acetic anhydride were added dropwise into a three-necked flask at -10°C while stirring. After 30 minutes, the flask was centrifuged for degassing. An 800 μm liquid film was scraped on a clean glass cup and then placed in an oven at 50°C for 20 minutes to obtain a partially imidized polyimide-amic acid ester gel film.

[0078] (4) The gel film is heat-treated according to a temperature control program of 100°C for 1 hour, 200°C for 1 hour, 300°C for 1 hour, and 400°C for 1 hour to obtain a chemically treated polyimide porous membrane.

[0079] Comparative Example 2

[0080] (1) Under nitrogen protection, 0.15 mol of pyromellitic dianhydride, 0.6 mol of isopropyl alcohol and 0.05 mol of pyridine were added to a 250 mL three-necked flask, heated under reflux at 90 ° C for 5 h, cooled to room temperature, added with 0.3 mol of thionyl chloride, reacted for 4 h, recrystallized and dried under vacuum at 50 ° C to obtain an ester-containing monomer. Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 168.12 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of the ester-containing monomer was added under ice bath and the reaction was continued for 6 hours to obtain a polyamic acid ester oligomer solution containing ester-terminated units with a solid content of 12 wt%.

[0081] (2) Under nitrogen protection, 0.04 mol of 4,4'-diaminodiphenyl ether and 122.72 g of N,N-dimethylacetamide were added to a 250 mL three-necked round-bottom flask. After the solid was completely dissolved, 0.04 mol of pyromellitic dianhydride was added under ice bath and the reaction was continued for 6 h to obtain an amino-terminated polyamic acid solution with a solid content of 12 wt%. Subsequently, a polyamic acid ester oligomer solution containing ester-terminated units was added dropwise to the flask and the reaction was continued for 6 h to obtain a polyamic acid-amic acid ester block copolymer solution with a solid content of 12 wt%.

[0082] (3) The polyamic acid-amic acid ester block copolymer solution was centrifuged for degassing, and an 800 μm liquid film was scraped on a clean glass cup, and then placed in an oven at 50°C for 1 hour. The solution was then heat-treated according to the temperature control program of 100°C for 1 hour, 200°C for 1 hour, 300°C for 1 hour, and 400°C for 1 hour to obtain a thermal imidization polyimide porous membrane.

[0083] from Figure 1 It can be seen that the gel film has good stretching processability. Figure 1 Gel film, Figure 2 The porous color becomes darker and opaque, which is caused by the internal pore structure. Figure 3 It can be seen that there are a large number of evenly distributed and uniformly sized pores inside the porous membrane, indicating that pores can be generated after high-temperature imidization of the amide ester, and the size and distribution of the pores are controlled after chemical imidization-biaxial stretching; combined with the performance data in Table 1, the tensile strength, elongation at break and elastic modulus of Example 1 are also significantly improved after chemical imidization and biaxial stretching, indicating that this process also improves the mechanical strength of the porous membrane.

[0084] Table 1 compares the mechanical properties of various examples and comparative examples, wherein tensile strength, elongation at break, and elastic modulus were tested according to the national standard GB / T 1040.3-2006. The specific testing steps are as follows: a long spline with a length of 150 mm, a width of 10 mm, and a thickness of less than 100 μm was cut, parallel markings were marked in the middle of the spline with a spacing of 50 mm, and the edges of the spline were smooth and without notches; the distance between the clamps of a tensile testing machine was fixed at 50 mm, the spline was vertically clamped in the clamps, and the width, thickness, and length were input and the test was performed at a tensile speed of 5 mm / min.

[0085] Table 1

[0086]

Claims

1. A polyimide biaxially stretched porous membrane, characterized in that: A polyimide biaxially stretched porous membrane is obtained by synthesizing a polyamic acid-amic acid ester block copolymer, preparing the copolymer into a gel membrane, biaxially stretching the membrane, and then heat-setting the membrane.

2. A method for preparing a polyimide biaxially stretched porous membrane, comprising: (1) heating an acid anhydride to reflux in an excess alcohol reagent, separating and purifying it, then reacting it with an acyl chloride reagent, and obtaining an ester-containing monomer after recrystallization and drying; then reacting the ester-containing monomer with a diamine in an aprotic polar organic solvent to synthesize a polyamic acid ester oligomer solution containing an ester-terminated unit; (2) using the same type of anhydride and diamine in step (1) as monomers, synthesizing an amino-terminated polyamic acid solution in a non-protonic polar organic solvent, and then mixing it with a polyamic acid ester oligomer solution containing an ester unit end-capping to obtain a polyamic acid-amic acid ester block copolymer solution; (3) adding acetic anhydride and a basic catalyst to the polyamic acid-amic acid ester block copolymer solution, scraping the film and then heating to obtain a gel film; (4) The gel film is subjected to biaxial stretching, followed by heat setting, removal of ester groups and imidization to obtain a polyimide biaxially stretched porous film.

3. The preparation method according to claim 2, wherein: The acid anhydride in step (1) is at least one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ketone tetracarboxylic dianhydride, 2,3',3,4'-biphenyltetracarboxylic dianhydride, and 4,4'-biphenyl ether dianhydride; and the diamine is at least one of p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, and 2-(4-aminophenyl)-5-aminobenzimidazole.

4. The preparation method according to claim 2, wherein: The molar ratio of the ester-containing monomer to the diamine in step (1) is 0.98:1 to 1.02:1; and the reaction time is 1 to 10 hours.

5. The preparation method according to claim 2, wherein: The alcohol reagent in step (1) is at least one of isopropyl alcohol, tert-butyl alcohol, n-hexanol, and n-decanol, and the molar amount is 1 to 10 times that of the acid anhydride; the acyl chloride reagent is at least one of thionyl chloride, oxalyl chloride, and phosphorus trichloride, and the molar amount is 1 to 10 times that of the acid anhydride.

6. The preparation method according to claim 2, wherein: The molar ratio of the acid anhydride to the diamine in the step (2) is 0.98:1 to 1.02:1; and the reaction time is 1 to 10 hours.

7. The preparation method according to claim 2, wherein: The aprotic polar organic solvent in steps (1) and (2) is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; and the solid content of the polyamic acid ester oligomer solution, the amino-terminated polyamic acid solution, and the polyamic acid-amic acid ester block copolymer solution is 10 to 15 wt %.

8. The preparation method according to claim 2, wherein: In the polyamic acid-amic acid ester block copolymer molecular chain in the step (2), the molar proportion of the amic acid segment is 70% to 90%, and the molar proportion of the amic acid ester segment is 10% to 30%; the polymerization degree of the polyamic acid-amic acid ester block copolymer is 100 to 1000.

9. The preparation method according to claim 2, wherein: The alkaline catalyst in step (3) is at least one of pyridine, 2-methylpyridine, 3-methylpyridine, triethylamine, isoquinoline, and quinoline, and the molar amount is 0.1 to 10 times that of the acid anhydride in step (2).

10. The preparation method according to claim 2, characterized in that: The molar amount of acetic anhydride in step (3) is 1.5 to 10 times that of the anhydride in step (2).

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