阅读说明：本技术 一种聚酰亚胺/聚醚酰亚胺复合膜及其制备方法 (Polyimide/polyetherimide composite film and preparation method thereof ) 是由 王杰 贾南方 于 2021-08-27 设计创作，主要内容包括：本发明提供了一种聚酰亚胺纳米纤维/聚醚酰亚胺复合膜及其制备方法。所述的制备方法如下:以二胺和二酐为单体合成聚酰胺酸(PAA)作为纺丝溶液；将PAA溶液进行静电纺丝然后高温环化得到PI纳米纤维膜；将配制好的聚醚酰亚胺(PEI)溶液均匀涂覆在PI纳米纤维膜表面,因相分离形成多孔结构,得到PI/PEI复合多孔膜。本发明方法实施简便,成本低,且PEI的引入,极大提高了纤维膜的机械强度。同时,复合膜阻燃性大幅提升,在高温下还会出现热闭孔现象,保证了电池高温下的使用安全。PI/PEI复合膜在新型锂离子电池隔膜领域具有广阔的应用前景。(The invention provides a polyimide nanofiber/polyetherimide composite membrane and a preparation method thereof. The preparation method comprises the following steps of synthesizing polyamide acid (PAA) as a spinning solution by using diamine and dianhydride as monomers; carrying out electrostatic spinning on the PAA solution and then carrying out high-temperature cyclization to obtain a PI nanofiber membrane; and uniformly coating the prepared Polyetherimide (PEI) solution on the surface of the PI nanofiber membrane, and forming a porous structure due to phase separation to obtain the PI/PEI composite porous membrane. The method is simple and convenient to implement and low in cost, and the introduction of the PEI greatly improves the mechanical strength of the fiber membrane. Meanwhile, the flame retardance of the composite film is greatly improved, the phenomenon of thermal hole closure can occur at high temperature, and the use safety of the battery at high temperature is guaranteed. The PI/PEI composite membrane has wide application prospect in the field of novel lithium ion battery diaphragms.)

1. The polyimide/polyetherimide composite membrane is characterized by comprising a polyimide nanofiber membrane and a polyetherimide layer coated on the polyimide nanofiber membrane.

2. A polyimide/polyetherimide composite film according to claim 1, wherein the polyimide nanofibers have a diameter of 40nm to 3 μm, preferably 50nm to 2 μm; the thickness of the fiber film is 1-40 μm, preferably 2-30 μm; the diameter of the polyetherimide micropore is 0.2-15 μm, preferably 0.3-10 μm; the total thickness of the composite membrane is 3-80 μm, preferably 4-60 μm.

3. A method for preparing a polyimide/polyetherimide composite film according to claim 1 or 2, characterized by comprising the steps of:

a: dissolving monomer diamine in organic solvent, stirring to dissolve completely, adding monomer dianhydride into the mixed solution step by step, placing the reactor containing the mixed solution in ice-water bath, and stirring to react the monomer diamine and the mixed solution sufficiently to synthesize PAA solution;

b: taking the PAA solution prepared in the step A as a spinning solution, carrying out electrostatic spinning to obtain a PAA nanofiber membrane, and carrying out heat treatment to obtain a polyimide nanofiber membrane;

c: and C, preparing a polyetherimide solution, coating the polyetherimide solution on the surface of the polyimide nano fiber membrane prepared in the step B, and standing and solidifying in a constant temperature and humidity box to obtain the polyimide/polyetherimide composite porous membrane.

4. The method for preparing a polyimide/polyetherimide composite film according to claim 3, wherein the molar ratio of the monomer diamine to the monomer dianhydride in the step A is 1: 0.98-1.02; the monomer diamine is aromatic diamine, and the dianhydride is aromatic dianhydride; the solid content of the polyamic acid solution is 3 to 30 wt%, preferably 5 to 25 wt%.

5. The method for preparing a polyimide/polyetherimide composite film according to claim 3, wherein the specific electrospinning parameters in the step B are spinning voltage: 15-50V; spinning temperature: 15-35 ℃; spinning humidity: 30-50%; the spinning time is 4-14 h; the heat treatment conditions are as follows: heating the mixture to 250-450 ℃ in air, and keeping the temperature for 10 min-3 h.

6. The preparation method of the polyimide/polyetherimide composite film according to claim 3, wherein the solid content of the polyetherimide prepared in the step C is 3-30 wt%, preferably 5-25%; the coating dosage of the polyetherimide solution is 0.003 to 0.15ml/cm²Preferably 0.01 to 0.1ml/cm²(ii) a The molecular weight of the polyetherimide is 30000-100000, preferably 40000-500000.

7. The method of claim 3, wherein the polyetherimide formed in step C comprises one or both of a copolymer polyetherimide and a homopolymer polyetherimide.

8. The method for producing a polyimide/polyetherimide composite film according to claim 7, wherein the homopolymeric polyetherimide is obtained by condensation polymerization of a diamine selected from one or a mixture of two or more of 3, 5-diaminobenzoic acid, m-phenylenediamine, 3,4 '-diaminodiphenylmethane, p-phenylenediamine, diaminodiphenyl ether, p-phenylenediamine, 4, 4' -diaminodiphenylmethane and 4,4 '-diamino-2, 2' -bistrifluoromethylbiphenyl, and a dianhydride selected from one or a mixture of two or more of bisphenol a type diether dianhydride and 3, 3,4, 4-triphendiether tetracid dianhydride; the copolymer polyetherimide is prepared by condensation polymerization of diamine and dianhydride, wherein the diamine is one or more selected from 3, 5-diaminobenzoic acid, m-phenylenediamine, 3,4 '-diaminodiphenylmethane, p-phenylenediamine, diaminodiphenyl ether, p-phenylenediamine, 4' -diaminodiphenylmethane, and 4,4 '-diamino-2, 2' -bis (trifluoromethyl) biphenyl; the dianhydride is selected from the mixture of diether anhydride and binary anhydride, wherein the mole percentage of the diether anhydride accounts for 90-100% of the mixed anhydride, the mole percentage of the binary anhydride accounts for 0-10% of the mixed anhydride, the diether anhydride comprises one or more of bisphenol A diether dianhydride and 3, 3,4, 4-triphenyl diether tetracid dianhydride, and the binary anhydride comprises one or more of biphenyl tetracid dianhydride, pyromellitic dianhydride, benzophenone tetracid dianhydride, diphenyl ether tetracid dianhydride and hexafluoro dianhydride.

9. The method for preparing a polyimide/polyetherimide composite film according to claim 3, wherein the coating method of the polyetherimide solution in the step C is a dip coating method, a transfer coating method, a squeeze coating method, a gravure coating method or a blade coating method; the temperature of the environment where the composite membrane is solidified is 20-80 ℃, and the humidity is 20-100%.

10. The method of claim 3, wherein the organic solvent is one or more of DMF, DMAC, NMP, and DMSO.

Technical Field

The invention belongs to the technical field of polyimide nanofiber membranes, and relates to a polyimide/polyetherimide composite membrane and a preparation method thereof.

Background

In recent years, high-power electrical equipment such as new energy automobiles is rapidly popularized, and lithium ion batteries with high safety have become hot spots for research. The separator, referred to as the "third electrode," plays a critical role in the manufacture of the battery. The current major commercial separator is a polyolefin separator. However, this makes the battery very dangerous to operate at high temperatures and even explodes due to the low melting point and poor thermal stability of the polyolefin. In addition, such a separator is a non-polar material and has poor wettability with a polar electrolyte, resulting in low ionic conductivity of the battery, thereby reducing the overall performance of the battery. The new generation of battery diaphragm with high temperature resistance, high permeability and high safety is actively developed to meet the development requirement of the future high-power lithium ion battery, and the method has important significance.

Polyimide (PI) nanofiber membranes are considered to be an excellent candidate for lithium ion battery separator because of their excellent thermal stability and high porosity. However, due to the loose packing characteristics of the nanofibers in the membrane, there is a lack of strong interaction between them, resulting in insufficient mechanical properties of the membrane, and the large pore size of the fiber membrane, during long-term operation of the battery, potential risks such as self-discharge and even short circuit may occur. In order to improve the performance of the fiber membrane, patent 202010713486.9 discloses coating a polyamic acid solution on a polyimide nanofiber membrane to obtain a composite membrane, and then immersing the composite membrane in a chemical imidization reagent to obtain a chemically imidized full polyimide composite membrane; the full polyimide composite diaphragm is subjected to heat treatment to decompose the pore-forming agent, so that the dual-structure polyimide composite battery diaphragm is obtained. Therefore, designing and developing a PI diaphragm which has high performance and can be prepared in a large amount by applying a simple process has great significance for improving the safety of the lithium ion battery.

The phase inversion method is a common method for preparing the porous membrane at present, and the mass transfer exchange is carried out between a homogeneous solution and a solvent and a non-solvent in the environment, the thermodynamic state of the homogeneous solution is changed, the homogeneous solution is changed from a stable state to an unstable state, the homogeneous solution is subjected to phase separation, and finally the homogeneous solution is solidified to form the membrane. The invention provides a method for preparing a high-performance PI composite membrane by compounding a porous PEI layer and PI nano fibers. Preparing a PI nanofiber membrane through electrostatic spinning, and then coating PEI on the fiber membrane to construct a layer of porous PEI membrane. During membrane formation, PEI exhibits a spongy, porous membrane morphology due to phase separation, and the morphology and properties of the membrane can be controlled by varying the solid content of PEI. In addition, a portion of the PEI penetrates into the PI nanofiber membrane, and the two layers of the membrane are interwoven to form a tightly bound architecture. Due to the introduction of the PEI porous membrane, compared with the PI nanofiber membrane, the mechanical strength of the composite membrane is greatly improved. Moreover, the composite membrane maintains high temperature resistance and high permeability, and the porosity of the nanofiber membrane is only slightly reduced. The PEI layer, tested by heating at high temperatures, exhibits a hot-blocking function that is believed to be of great value in preventing short circuits and thermal runaway in lithium ion batteries under certain hazardous conditions.

Disclosure of Invention

The invention aims to prepare a polyimide nanofiber membrane by an electrostatic spinning method, and then prepare a polyimide/polyetherimide composite membrane by a method of coating polyetherimide on the surface of the nanofiber membrane. The polyimide/polyetherimide composite film has the characteristics of high mechanical strength, high temperature resistance, good chemical stability, thermal closed hole and the like, and the flame retardance of the composite film is improved.

The polyimide/polyetherimide composite membrane is characterized by comprising a Polyimide (PI) nanofiber membrane and a Polyetherimide (PEI) layer coated on the PI nanofiber membrane.

Furthermore, the diameter of the polyimide nanofiber in the Polyimide (PI) nanofiber membrane is 40nm-3 μm, preferably 50nm-2 μm, and the thickness of the fiber membrane is 1-40 μm, preferably 2-30 μm.

Further, the polyetherimide micropore diameter is 0.2-15 μm, preferably 0.3-10 μm.

The thickness of the polyimide/polyetherimide composite film is 3-80 mu m, and preferably 4-60 mu m.

A preparation method of a polyimide/polyetherimide composite film comprises the following specific steps:

a: reacting at least one diamine and at least one dianhydride to obtain a polyamic acid (PAA) solution;

b: b, carrying out electrostatic spinning on the PAA solution prepared in the step A to obtain a PAA nanofiber membrane, and carrying out heat treatment to obtain a PI nanofiber membrane;

c: and preparing a PEI solution, coating the PEI solution on the surface of the PI nanofiber membrane, and standing in a constant temperature and humidity box to obtain the PI/PEI composite porous membrane.

Further, the monomer diamine and dianhydride in the step A are aromatic diamine and dianhydride; the aromatic diamine is one or more selected from octadecyl amine (ODA) and p-Phenylenediamine (PDA); the dianhydride is selected from one or more of pyromellitic dianhydride (PMDA), biphenyl tetracarboxylic dianhydride (BPDA) and hexafluoro dianhydride (6 FDA).

And B, adding the dianhydride in the step A into the mixed solution step by step, placing the whole reaction system in an ice water bath, and stirring to enable the dianhydride and the mixed solution to react fully.

The solid content of the polyamic acid solution in the step A is 3-30 wt%, preferably 5-25 wt%.

Further, the specific electrostatic spinning parameters in the step B are spinning voltage: 15-50V; spinning temperature: 15-35 ℃; spinning humidity: 30-50%; the spinning time is 4-14 h; the thermal imidization conditions are as follows: heating the mixture to 250-450 ℃ in air, and keeping the temperature for 10 min-3 h.

Further, the solid content of the polyetherimide solution prepared in the step C is 3-30%, preferably 5-25%.

The coating dosage of the polyetherimide solution is 0.003 to 0.15ml/cm²Preferably 0.01 to 0.1ml/cm²(ii) a The molecular weight of the polyetherimide is 30000-100000, preferably 40000-500000.

Further, the polyetherimide in the step C comprises one or a mixture of two of copolymerization type polyetherimide and homopolymerization type polyetherimide.

Further, the homopolymerization type polyetherimide is formed by condensation polymerization of diamine and dianhydride, wherein the diamine is selected from one or more of 3, 5-diaminobenzoic acid, m-phenylenediamine, 3,4 '-diaminodiphenylmethane, p-phenylenediamine, diaminodiphenyl ether, p-phenylenediamine, 4' -diaminodiphenylmethane and mixture of more than two of 4,4 '-diamino-2, 2' -bistrifluoromethyl biphenyl; the dianhydride is one or more selected from bisphenol A type diether dianhydride and 3, 3,4, 4-triphenyl diether tetracid dianhydride.

Further, the copolymerization type polyetherimide is obtained by condensation polymerization of diamine and dianhydride, wherein the diamine is selected from one or more of 3, 5-diaminobenzoic acid, m-phenylenediamine, 3,4 '-diaminodiphenylmethane, p-phenylenediamine, diaminodiphenyl ether, p-phenylenediamine, 4' -diaminodiphenylmethane and 4,4 '-diamino-2, 2' -bistrifluoromethylbiphenyl; the dianhydride is selected from a mixture of diether anhydride and dibasic anhydride, wherein the mole percentage of the diether anhydride accounts for 90-100% of the mixed anhydride, and the mole percentage of the dibasic anhydride accounts for 0-10% of the mixed anhydride. The diether dianhydride comprises one or more of bisphenol A diether dianhydride and 3, 3,4, 4-triphenyl diether tetracid dianhydride, and the dibasic anhydride comprises one or more of biphenyl tetracid dianhydride, pyromellitic dianhydride, benzophenone tetracid dianhydride, diphenyl ether tetracid dianhydride and hexafluoro dianhydride.

Further, the temperature of the environment where the composite film is solidified in the step C is 15-85 ℃, preferably 20-80 ℃, and the humidity is 15-100%, preferably 20-100%. The coating method includes dip coating, transfer coating, extrusion coating, gravure coating, and blade coating.

Further, the organic solvent is one or more of DMF, DMAC, NMP and DMSO.

Compared with the prior art, the method has the following technical effects:

1. the invention provides a simple, convenient and effective strategy, which combines an electrospinning technology with a coating process, coats PEI on the surface of a PI nano-fiber membrane, and dries in the air under a proper environment to obtain the composite porous membrane.

2. The polyimide/polyetherimide composite membrane prepared by the method has the characteristics of high temperature resistance, chemical stability, good flame retardance and the like, and due to the introduction of the PEI, a cross-linked structure is generated on the PI nanofiber membrane, the mechanical property is greatly enhanced, and the pore structure is more stable.

3. The polyimide/polyetherimide composite membrane prepared by the method shows a hole closing phenomenon at high temperature, which provides a safe guarantee for the lithium ion battery under an overheating condition or when an unexpected thermal runaway accident occurs.

Drawings

FIG. 1 is a polyetherimide layer topography at 2000 times magnification of a polyimide/polyetherimide composite film prepared according to example 1;

FIG. 2 is a polyetherimide layer topography at 2000 times magnification of a polyimide/polyetherimide composite film prepared according to example 2;

FIG. 3 is a polyetherimide layer topography at 2000 times magnification of a polyimide/polyetherimide composite film prepared in accordance with example 3;

FIG. 4 is a topographical view of a polyimide/polyetherimide composite film prepared in accordance with example 4, at 2000 times magnification.

The specific implementation mode is as follows:

the invention will be further illustrated with reference to the following specific examples. It should be noted that: the following examples are only for illustrating the present invention and are not intended to limit the technical solutions described in the present invention. Thus, while the present invention has been described in detail with reference to the following examples, it will be understood by those skilled in the art that the present invention may be modified and equivalents may be substituted; all such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.

Example 1

Firstly, preparing a PMDA/ODA system polyamic acid solution with the solid content of 12%, preparing a polyamic acid nanofiber membrane through electrostatic spinning, and performing thermal imidization to obtain the polyimide nanofiber membrane.

(1) According to the mol ratio of 1: 1 weighing 2.03g of pyromellitic dianhydride (PMDA) and 1.84g of 4, 4' -diaminodiphenyl ether (ODA), completely dissolving ODA in 30ml of N, N-Dimethylformamide (DMF), and after complete dissolution, adding PMDA in batches to prepare a polyamic acid solution with the solid content of 12%. Preparing a polyamic acid nano-fiber membrane from a polyamic acid solution by an electrostatic spinning technology, wherein the parameters of an electrostatic spinning machine are as follows: 20 kV; spinning temperature: 25 ℃; spinning humidity: 40 percent; diameter of syringe needle: number 12; receiving roller rotating speed: 500 r/min; receiving distance: 20 cm. And (3) carrying out thermal imidization treatment on the prepared PAA nanofiber membrane, heating to 300 ℃ in air at a heating rate of 5 ℃/min, and keeping the temperature for 2 hours to obtain a 15.4 mu mPI nanofiber membrane, wherein the diameter of the PI nanofiber is 200-300 nm.

(2) A PEI solution having a solid content of 25% was prepared, 10.28g of PEI solid particles were weighed and dissolved in 30ml of N-methylpyrrolidone (NMP), and stirred in an oil bath at 60 ℃ until completely dissolved.

(3) Coating the surface of a PI nanofiber membrane with the copolymerization type PEI solution, wherein the relative coating amount is 0.015ml/cm²(ii) a Placing in a constant temperature and humidity box with 30 ℃ and 60% humidity for 12h for airing. Thus preparing the polyimide/polyetherimide composite membrane with the total thickness of 20.4 mu m. The PEI layer morphology of the composite film is shown in FIG. 1, wherein the polyetherimide micropore diameter is 0.6 μm, and the tensile strength is 78.6 MPa.

Comparative example 1: a PI nanofiber membrane was prepared according to step 1 of example 1, having a tensile strength of 14.1 MPa.

Example 2

Firstly, preparing a PMDA/ODA system polyamic acid solution with the solid content of 12%, preparing a polyamic acid nanofiber membrane through electrostatic spinning, and performing thermal imidization to obtain the polyimide nanofiber membrane.

(1) According to the mol ratio of 1: 1 weighing 2.03g of pyromellitic dianhydride (PMDA) and 1.84g of 4, 4' -diaminodiphenyl ether (ODA), completely dissolving ODA in 30ml of N, N-Dimethylformamide (DMF), and after complete dissolution, adding PMDA in batches to prepare a polyamic acid solution with the solid content of 12%. Preparing a polyamic acid nano-fiber membrane from a polyamic acid solution by an electrostatic spinning technology, wherein the parameters of an electrostatic spinning machine are as follows: 20 kV; spinning temperature: 25 ℃; spinning humidity: 40 percent; diameter of syringe needle: number 12; receiving roller rotating speed: 500 r/min; receiving distance: 20 cm. And (3) carrying out thermal imidization treatment on the prepared PAA nanofiber membrane, heating to 300 ℃ in air at a heating rate of 5 ℃/min, and keeping the temperature for 2 hours to obtain a 15.4-micrometer PI nanofiber membrane, wherein the diameter of the PI nanofiber is 200-300 nm.

(2) A PEI solution with a 20% solids content was prepared, and 7.71g of PEI solid particles were weighed and dissolved in 30ml of N-methylpyrrolidone (NMP) and stirred in an oil bath at 60 ℃ until completely dissolved.

(3) Coating the surface of a PI nanofiber membrane with the copolymerization type PEI solution, wherein the relative coating amount is 0.015ml/cm²And then the polyimide/polyetherimide composite membrane is placed in a constant temperature and humidity box with the temperature of 30 ℃ and the humidity of 60% for drying for 12h, so that the polyimide/polyetherimide composite membrane with the diameter of 20.3 mu m is prepared, the appearance of the PEI layer of the composite membrane is shown in figure 2, wherein the diameter of the polyetherimide micropore is 1.1 mu m, and the tensile strength is 59.7 MPa.

Comparative example 2: a PI nanofiber membrane was prepared according to step 1 thereof according to the method of example 2, and the tensile strength thereof was 14.1 MPa.

Example 3

Firstly, preparing a PMDA/ODA system polyamic acid solution with the solid content of 12%, preparing a polyamic acid nanofiber membrane through electrostatic spinning, and performing thermal imidization to obtain the polyimide nanofiber membrane.

(1) According to the mol ratio of 1: 1 weighing 2.03g of pyromellitic dianhydride (PMDA) and 1.84g of 4, 4' -diaminodiphenyl ether (ODA), completely dissolving ODA in 30ml of N, N-Dimethylformamide (DMF), and after complete dissolution, adding PMDA in batches to prepare a polyamic acid solution with the solid content of 12%. Preparing a polyamic acid nano-fiber membrane from a polyamic acid solution by an electrostatic spinning technology, wherein the parameters of an electrostatic spinning machine are as follows: 20 kV; spinning temperature: 25 ℃; spinning humidity: 40 percent; diameter of syringe needle: number 12; receiving roller rotating speed: 500 r/min; receiving distance: 20 cm. And (3) carrying out thermal imidization treatment on the prepared PAA nanofiber membrane, heating to 300 ℃ in air at a heating rate of 5 ℃/min, and keeping the temperature for 2 hours to obtain a 15.4-micrometer PI nanofiber membrane, wherein the diameter of the PI nanofiber is 200-300 nm.

(2) A PEI solution having a solid content of 15% was prepared, and 5.44g of PEI solid particles were weighed and dissolved in 30ml of N-methylpyrrolidone (NMP), and stirred in an oil bath at 60 ℃ until completely dissolved.

(3) Coating the surface of a PI nanofiber membrane with the copolymerization type PEI solution, wherein the relative coating amount is 0.015ml/cm²And placing the mixture in a constant temperature and humidity box with the temperature of 30 ℃ and the humidity of 60% for drying for 12 h. Thus preparing the polyimide/polyetherimide composite membrane with the thickness of 20.1 microns, wherein the appearance of the PEI layer of the composite membrane is shown in figure 3, the diameter of the polyetherimide micropore is 1.4 microns, and the tensile strength is 40.3 MPa.

Comparative example 3: a PI nanofiber membrane was prepared according to step 1 thereof according to the method of example 3, and the tensile strength thereof was 14.1 MPa.

Example 4

Firstly, preparing a PMDA/ODA system polyamic acid solution with the solid content of 12%, preparing a polyamic acid nanofiber membrane through electrostatic spinning, and performing thermal imidization to obtain the polyimide nanofiber membrane.

(1) According to the mol ratio of 1: 1 weighing 2.03g of pyromellitic dianhydride (PMDA) and 1.84g of 4, 4' -diaminodiphenyl ether (ODA), completely dissolving ODA in 30ml of N, N-Dimethylformamide (DMF), and after complete dissolution, adding PMDA in batches to prepare a polyamic acid solution with the solid content of 12%. Preparing a polyamic acid nano-fiber membrane from a polyamic acid solution by an electrostatic spinning technology, wherein the parameters of an electrostatic spinning machine are as follows: 22 kV; spinning temperature: 25 ℃; spinning humidity: 40 percent; diameter of syringe needle: number 12; receiving roller rotating speed: 500 r/min; receiving distance: 20 cm. And (3) carrying out thermal imidization treatment on the prepared PAA nanofiber membrane, heating to 350 ℃ in air at a heating rate of 3 ℃/min, and keeping the temperature for 2 hours to obtain the PI nanofiber membrane with the thickness of 10 microns.

(2) A PEI solution with a solid content of 15% is prepared, 15g of PEI solid particles are weighed and dissolved in 30ml of N-methylpyrrolidone (NMP), and the solution is stirred in an oil bath at 60 ℃ until the PEI solid particles are completely dissolved.

(3) Coating the surface of a PI nanofiber membrane with the copolymer PEI solution, wherein the relative coating amount is 0.025ml/cm²Air drying at 30 deg.C and humidity of 60% for 12 hr to obtain 25.4 μm thick polyimide/polyetherimide composite film. The morphology of the PEI layer of the composite film is shown in FIG. 4, wherein the diameter of the polyetherimide micropore is 1.6 μm, and the tensile strength is 52.6 MPa.

Comparative example 4: a PI nanofiber membrane was prepared according to step 1 thereof according to the method of example 4, and the tensile strength thereof was 14.1 MPa.

Example 5

Firstly, preparing a PMDA/ODA system polyamic acid solution with the solid content of 12%, preparing a polyamic acid nanofiber membrane through electrostatic spinning, and performing thermal imidization to obtain the polyimide nanofiber membrane.

(1) According to the mol ratio of 1: 1 weighing 2.03g of pyromellitic dianhydride (PMDA) and 1.84g of 4, 4' -diaminodiphenyl ether (ODA), completely dissolving ODA in 30ml of N, N-Dimethylformamide (DMF), and after complete dissolution, adding PMDA in batches to prepare a polyamic acid solution with the solid content of 12%. Preparing a polyamic acid nano-fiber membrane from a polyamic acid solution by an electrostatic spinning technology, wherein the parameters of an electrostatic spinning machine are as follows: 22 kV; spinning temperature: 25 ℃; spinning humidity: 40 percent; diameter of syringe needle: number 12; receiving roller rotating speed: 500 r/min; receiving distance: 20 cm. And (3) carrying out thermal imidization treatment on the prepared PAA nanofiber membrane, heating to 350 ℃ in air at a heating rate of 3 ℃/min, and keeping the temperature for 2 hours to obtain the PI nanofiber membrane with the thickness of 10 microns.

(2) A PEI solution with a solid content of 15% is prepared, 15g of PEI solid particles are weighed and dissolved in 30ml of N-methylpyrrolidone (NMP), and the solution is stirred in an oil bath at 60 ℃ until the PEI solid particles are completely dissolved.

(3) Coating the surface of a PI nanofiber membrane with the copolymer PEI solution, wherein the relative coating amount is 0.025ml/cm²And then the polyimide/polyetherimide composite membrane is placed in a constant temperature and humidity box with the temperature of 30 ℃ and the humidity of 60% for drying for 12 hours, so that the polyimide/polyetherimide composite membrane with the thickness of 25.4 microns is prepared. The morphology of the PEI layer of the composite film is shown in FIG. 4, wherein the diameter of the polyetherimide micropore is 1.3 μm, and the tensile strength is 57.6 MPa.

Comparative example 5: a PI nanofiber membrane was prepared according to the method of example 5, according to step 1 thereof, and had a tensile strength of 14.1 MPa.