A modified polyimide-polyvinylidene fluoride double-layer fiber membrane and a preparation method thereof

Abstract

The application provides a modified polyimide@polyvinyl fluoride double-layer fiber membrane and a preparation method thereof, and belongs to the technical field of fiber membrane preparation; the preparation process comprises the following steps: preparation of NH2-MWCNT; preparation of casting solution and core solution; and preparation of the modified polyimide@polyvinyl fluoride double-layer fiber membrane. The modified polyimide@polyvinyl fluoride double-layer fiber membrane comprises an inner layer and an outer layer; the inner layer is polyvinyl fluoride; the outer layer is NH2-MWCNT modified polyimide; the polyimide is any one of Matrimid5218 and P84; the dry-wet method and the co-extrusion method are adopted; and then heat crosslinking is performed to obtain the NH2-MWCNT modified polyimide@polyvinyl fluoride double-layer fiber membrane.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fiber membrane preparation, in particular to a modified polyimide@polyvinylidene fluoride double-layer fiber membrane and a preparation method thereof. BACKGROUND

[0002] Helium is a valuable and non-renewable gas, due to its non-toxic, inert, light and non-flammable properties, it is widely used in various industries, scientific research and medical applications. At present, the enrichment of helium from natural gas is the only large-scale industrial method of helium production. Since the 1960s, cryogenic distillation and pressure swing adsorption have been the main technologies for separating helium from natural gas. However, due to the relatively low helium content in most natural gas reservoirs (0.05-1.0%), it takes 100-2000 cubic meters of liquefied natural gas to obtain 1 cubic meter of helium, which consumes a huge amount of energy. Therefore, new phase-change-free gas separation technologies have attracted widespread attention, and gas membrane separation technology provides a very promising alternative for helium separation due to its low energy demand, easy operation, compact structure and low maintenance cost.

[0003] Gas membrane separation technology has many advantages, but also has problems such as "Trade-off" effect, low selectivity, poor stability, etc. Literature [Journal of Membrane Science 591 (2019) 117292] reported a double-layer hollow fiber membrane with polysulfone as the support inner layer, polybenzimidazole and polyphenylsulfone as the outer layer selection layer, and chemical cross-linking and heat treatment with dibromo-p-xylene, the hydrogen flux is 19GPU, and the H2 / CO2 selectivity is 9.7. The flux and selectivity of this material are both low, and the preparation process is complex. Patent CN113166538B discloses a preparation method of a layered hollow fiber membrane based on poly(2,5-benzimidazole) (ABPBI), ABPBI copolymer and substituted polybenzimidazole (PBI) polymer, which can be applied to gas separation, the maximum helium flux can reach 1.21GPU, and the helium / nitrogen selectivity can reach 49.14. The gas flux of this method is low, and it is difficult to apply in industry. Patent CN111298664B discloses a preparation method of a hollow fiber membrane modified by carbon nanotubes, which is used for separating CO2, the CO2 flux reaches 3.138mol•m -2 •s -1 •MPa -1 , CO2 / N2 selectivity 17.8, CO2 / O2 selectivity 11.3, the preparation process of this method is complex, and the separation effect is not good.

[0004] Therefore, we propose a modified polyimide@polyvinylidene fluoride double-layer fiber membrane and a preparation method thereof. SUMMARY

[0005] In view of the deficiencies of the prior art, the purpose of the present application is to provide a modified polyimide-polyvinylidene fluoride double-layer fiber membrane and a preparation method thereof, which comprises an inner membrane layer and an outer membrane layer, the inner membrane layer is polyvinylidene fluoride, and the outer membrane layer is NH2-MWCNT modified polyimide, the polyimide is any one of Matrimid5218 and P84, and the NH2-MWCNT modified polyimide-polyvinylidene fluoride double-layer hollow fiber membrane is obtained by using a dry-wet method and a co-extrusion method and then re-crosslinking.

[0006] A preparation method of a modified polyimide-polyvinylidene fluoride double-layer fiber membrane, comprising the following steps:

[0007] S1: preparation of NH2-MWCNT

[0008] The multi-walled carbon nanotubes, toluene and 3-aminopropyl triethoxysilane are mixed and heated for reaction, and the reaction product is centrifuged, washed and dried to obtain NH2-MWCNT;

[0009] S2: preparation of casting solution and core solution

[0010] The polyvinylpyrrolidone is dispersed in a solvent and then polyvinylidene fluoride is added after ultrasonic treatment, and the mixture is stirred and vacuum degassed to obtain the inner membrane layer casting solution, the NH2-MWCNT is dispersed in a solvent, and then polyimide is added after ultrasonic treatment, and the mixture is stirred and vacuum degassed to obtain the outer membrane layer casting solution, and the sodium chloride is dissolved in deionized water and then added to nitrogen-methyl pyrrolidone to obtain the core solution;

[0011] S3: preparation of modified polyimide-polyvinylidene fluoride double-layer fiber membrane

[0012] The flow rates of the inner membrane layer casting solution, the outer membrane layer casting solution and the core solution are controlled and simultaneously extruded from the spinneret, and then coagulation is performed to obtain a double-layer hollow fiber membrane, the double-layer hollow fiber membrane is soaked in a solvent, and then dried after soaking, and then heat crosslinking is performed to obtain the modified polyimide-polyvinylidene fluoride double-layer fiber membrane.

[0013] Further, the step S1 of preparing NH2-MWCNT specifically comprises the following steps:

[0014] S1.1: The multi-walled carbon nanotubes are dried at 100-150 DEG C for 8-12 h, 1 part by weight of the dried multi-walled carbon nanotubes is added to 30-60 parts by weight of toluene, and then 2-4 parts by weight of 3-aminopropyl triethoxysilane is added, and the mixture is stirred and refluxed at 60-100 DEG C for 12-24 h to obtain a reaction product;

[0015] S1.2: The reaction product is centrifuged and washed for 3-5 times, and then the washed reaction product is vacuum dried at 80-100 DEG C for 8-12 h to obtain NH2-MWCNT, which is ready for use.

[0016] Further, the preparation of the casting solution and the core solution in step S2 specifically includes the following steps:

[0017] S2.1: Disperse 1-2 parts by weight of polyvinylpyrrolidone in 15-17 parts by weight of solvent, after ultrasonic treatment for 30-60 min, add 2-3 parts by weight of polyvinylidene fluoride, stir at 50-80℃ for 12-24h, and then vacuum degassing for 8-12h after cooling to obtain the inner layer casting solution of the membrane, ready for use;

[0018] S2.2: Disperse 1-5 parts by weight of NH2-MWCNT in 69-77 parts by weight of solvent, ultrasonic treatment for 30-60 min, then add 22-26 parts by weight of polyimide, stir at 60-80℃ for 10-22h, and then vacuum degassing for 8-12h after cooling to obtain the outer layer casting solution of the membrane, ready for use;

[0019] S2.3: Add 3 parts by weight of sodium chloride to 5-8 parts by weight of deionized water to dissolve, then add 89-92 parts by weight of nitrogen-methyl pyrrolidone after dissolution, stir and mix for 2-4h to obtain the core solution, ready for use.

[0020] Further, the preparation of the modified polyimide@polyvinylidene fluoride double-layer fiber membrane in step S3 specifically includes the following steps:

[0021] S3.1: Control the flow rate of the inner layer casting solution of the membrane, the outer layer casting solution of the membrane and the core solution to be extruded from the spinneret at the same time, and the membrane filament is coagulated in the water coagulation bath to collect the double-layer hollow fiber membrane;

[0022] S3.2: Replace and soak the collected double-layer hollow fiber membrane with solvent for 1-2d, then dry at 40-60℃ for 8-12h, and finally heat crosslink at 200-300℃ for 1-2h to obtain the modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0023] Further, the molecular weight of polyvinylpyrrolidone in step S2.1 is 8000-58000.

[0024] Further, the solvent in step S2.1 is any one of nitrogen-methyl pyrrolidone, N,N-dimethylacetamide.

[0025] Further, the solvent in step S2.2 is any one of nitrogen-methyl pyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide, and the polyimide is any one of Matrimid5218 and P84.

[0026] Further, the volume flow rate ratio of the inner layer casting solution, the outer layer casting solution and the core solution in step S3.1 is 3:3-5:1-2, the height of the spinneret from the coagulation bath is 7-10 cm, the spinneret temperature is 50-65 DEG C, and the coagulation bath temperature is 25-35 DEG C.

[0027] Further, the solvent in step S3.2 is any one of water and ethanol.

[0028] A modified polyimide@polyvinyl fluoride double-layer fiber membrane is prepared by the preparation method of the modified polyimide@polyvinyl fluoride double-layer fiber membrane.

[0029] Compared with the prior art, the present application has at least the following beneficial effects:

[0030] 1. The present application designs polyvinyl fluoride as the inner layer of the membrane, NH2-MWCNT modified polyimide as the outer layer of the membrane, and prepares a double-layer hollow modified polyimide@polyvinyl fluoride double-layer fiber membrane. The polyvinyl fluoride provides high flux for helium separation, and the porous structure of the multi-walled carbon nanotube can further improve the gas transmission efficiency. The amine functionalization of the multi-walled carbon nanotube can strengthen the interfacial compatibility with the polyimide, stabilize the polyimide segment structure, improve the stability and plasticization resistance, and make the modified polyimide@polyvinyl fluoride double-layer fiber membrane have excellent plasticization resistance.

[0031] 2. The present application modifies the polyimide by NH2-MWCNT, and uses the NH2-MWCNT modified polyimide as the outer layer of the membrane, thereby introducing NH2-MWCNT. The addition of NH2-MWCNT improves the selectivity and permeability of the modified polyimide@polyvinyl fluoride double-layer fiber membrane, and the helium flux reaches 378 Barrer, and the He / CH4 and He / N2 selectivities reach 385 and 370, respectively.

[0032] 3. The preparation method of the modified polyimide@polyvinyl fluoride double-layer fiber membrane of the present application is simple, can be produced on a large scale, and provides a new idea for the practical application of membrane separation technology. BRIEF DESCRIPTION OF DRAWINGS

[0033] The drawings incorporated herein and forming a part of the specification, illustrate embodiments of the present disclosure and together with the description are further used to explain the principles of the present disclosure and to enable one skilled in the relevant art to implement and use the present disclosure.

[0034] Figure 1 The preparation method flow chart of the modified polyimide@polyvinyl fluoride double-layer fiber membrane used in the embodiments of the present application;

[0035] Figure 2 The schematic diagram of the membrane module and the gas separation process used in the present application;

[0036] Figure 3 Plasticization resistance of the modified polyimide@polyvinylidene fluoride double-layer fiber membrane prepared for the present embodiment 1;

[0037] Figure 4 Plasticization resistance of the modified polyimide@polyvinylidene fluoride double-layer fiber membrane prepared for the present comparative example 1. DETAILED DESCRIPTION

[0038] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the following will combine the drawings in the embodiments of the present application to clearly and completely describe the technical solutions of the preparation method of a modified polyimide@polyvinylidene fluoride double-layer fiber membrane in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments of the present application.

[0039] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present application.

[0040] Embodiment 1

[0041] A preparation method of a modified polyimide@polyvinylidene fluoride double-layer fiber membrane, as shown in the following scheme, comprises the following steps: Figure 1

[0042] S1: Preparation of NH2-MWCNT

[0043] S1.1: Dry the multi-walled carbon nanotubes at 100°C for 8h, add 1 part by weight of the dried multi-walled carbon nanotubes to 30 parts by weight of toluene, then add 2 parts by weight of 3-aminopropyltriethoxysilane, stir and reflux at 60°C for 12h to obtain a reaction product;

[0044] S1.2: Centrifuge and wash the reaction product for 3 times, vacuum dry the washed reaction product at 80°C for 8h to obtain NH2-MWCNT, which is ready for use;

[0045] S2: Preparation of casting solution and core solution

[0046] S2.1: Disperse 1 part by weight of polyvinylpyrrolidone with a molecular weight of 8000 in 15 parts by weight of nitrogen-methylpyrrolidone, after ultrasonic treatment for 30min, add 2 parts by weight of polyvinylidene fluoride, stir at 50°C for 12h, and vacuum degassing for 8h after cooling to obtain the inner layer casting solution of the membrane, which is ready for use;

[0047] ​S2.2: 1 part by weight of NH2-MWCNT was dispersed in 77 parts by weight of nitrogen-methylpyrrolidone under ultrasonic for 30 min, then 22 parts by weight of Matrimid 5218 was added, stirred at 60℃ for 10 h, and defoamed under vacuum for 8 h after cooling to obtain a membrane outer layer casting solution, which was ready for use;

[0048] S2.3: 3 parts by weight of sodium chloride was added to 5 parts by weight of deionized water to dissolve, and 92 parts by weight of nitrogen-methylpyrrolidone was added after dissolution, and stirred and mixed for 2 h to obtain a core liquid, which was ready for use;

[0049] S3: Preparation of modified polyimide@polyvinylidene fluoride double-layer fiber membrane

[0050] S3.1: The volume flow rate ratio of the membrane inner layer casting solution, the membrane outer layer casting solution and the core liquid was controlled to be 3:3:1, and was extruded from the spinneret at 50℃ at the same time, the membrane filaments were coagulated in the water coagulation bath at 25℃, the distance between the spinneret and the coagulation bath was 7 cm, and the double-layer hollow fiber membrane was collected;

[0051] S3.2: The collected double-layer hollow fiber membrane was replaced and soaked with water for 1 d, then dried at 40℃ for 8 h, and finally heat crosslinked at 200℃ for 1 h to obtain the modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0052] Example 2

[0053] A preparation method of a modified polyimide@polyvinylidene fluoride double-layer fiber membrane, as shown in Figure 1 , comprises the following steps:

[0054] S1: Preparation of NH2-MWCNT

[0055] S1.1: Multi-walled carbon nanotubes were dried at 150℃ for 12 h, 1 part by weight of the dried multi-walled carbon nanotubes was added to 60 parts by weight of toluene, and 4 parts by weight of 3-aminopropyltriethoxysilane was added, and stirred and refluxed at 100℃ for 24 h to obtain a reaction product;

[0056] S1.2: The reaction product was centrifuged and washed for 5 times, and the washed reaction product was vacuum dried at 100℃ for 12 h to obtain NH2-MWCNT, which was ready for use;

[0057] S2: Preparation of casting solution and core liquid

[0058] S2.1: 2 parts by weight of polyvinylpyrrolidone with a molecular weight of 58000 was dispersed in 17 parts by weight of solvent N,N-dimethylacetamide, and 23 parts by weight of polyvinylidene fluoride was added after ultrasonic for 60 min, and stirred at 80℃ for 24 h, and defoamed under vacuum for 12 h after cooling to obtain a membrane inner layer casting solution, which was ready for use;

[0059] S2.2: 5 parts by weight of NH2-MWCNT were dispersed in 69 parts by weight of N,N-dimethylacetamide, ultrasonic for 60 min, then 26 parts by weight of P84 was added, stirred at 80℃ for 22h, vacuum degassing for 12h after cooling, to obtain the outer layer casting solution of the membrane, ready for use;

[0060] S2.3: 3 parts by weight of sodium chloride was added to 8 parts by weight of deionized water, and dissolved, after dissolution, 89 parts by weight of nitrogen-methyl pyrrolidone was added, and stirred for 4h, to obtain the core solution, ready for use;

[0061] S3: Preparation of modified polyimide@polyvinylidene fluoride double-layer fiber membrane

[0062] S3.1: The volume flow rate ratio of the inner layer casting solution of the membrane, the outer layer casting solution of the membrane and the core solution was controlled to be 3:5:2, and was extruded from the spinneret at 65℃ at the same time, the membrane filament was solidified in the water coagulation bath at 35℃, the distance between the spinneret and the coagulation bath was 10cm, and the double-layer hollow fiber membrane was collected;

[0063] S3.2: The double-layer hollow fiber membrane collected was replaced and soaked with ethanol for 2d, then dried at 60℃ for 12h, and finally heat crosslinked at 300℃ for 2h, to obtain the modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0064] Example 3

[0065] A preparation method of a modified polyimide@polyvinylidene fluoride double-layer fiber membrane, as shown in Figure 1 , comprises the following steps:

[0066] S1: Preparation of NH2-MWCNT

[0067] S1.1: Multi-walled carbon nanotubes were dried at 150℃ for 12h, 1 part by weight of dried multi-walled carbon nanotubes was added to 40 parts by weight of toluene, and then 3 parts by weight of 3-aminopropyl triethoxysilane was added, stirred and refluxed at 100℃ for 24h, to obtain a reaction product;

[0068] S1.2: The reaction product was centrifuged and washed for 3 times, and the washed reaction product was vacuum dried at 100℃ for 12h, to obtain NH2-MWCNT, ready for use;

[0069] S2: Preparation of casting solution and core solution

[0070] S2.1: 1 part by weight of polyvinylpyrrolidone with a molecular weight of 24000 was dispersed in 10.67 parts by weight of N,N-dimethylacetamide, ultrasonic for 30 min, then 2 parts by weight of polyvinylidene fluoride was added, stirred at 80℃ for 24h, vacuum degassing for 12h after cooling, to obtain the inner layer casting solution of the membrane, ready for use;

[0071] S2.2: 3 parts by weight of NH2-MWCNT were dispersed in 69 parts by weight of N,N-dimethylacetamide for 60 min under ultrasonic, then 26 parts by weight of P84 was added, stirred at 80℃ for 22 h, and vacuum degassing for 12 h after cooling to obtain the outer layer casting solution of the membrane, ready for use;

[0072] S2.3: 3 parts by weight of sodium chloride were added to 8 parts by weight of deionized water to dissolve, and after dissolving, 89 parts by weight of nitrogen-methyl pyrrolidone was added and stirred for 4 h to obtain the core liquid, ready for use;

[0073] S3: Preparation of modified polyimide@polyvinylidene fluoride double-layer fiber membrane

[0074] S3.1: The volume flow rate ratio of the inner layer casting solution of the membrane, the outer layer casting solution of the membrane and the core liquid was controlled to be 3:3:1, and extruded from the spinneret at 65℃ at the same time. The membrane filaments were coagulated in a water coagulation bath at 35℃, the distance between the spinneret and the coagulation bath was 7 cm, and a double-layer hollow fiber membrane was collected;

[0075] S3.2: The double-layer hollow fiber membrane collected was replaced and soaked with ethanol for 2 d, then dried at 60℃ for 12 h, and finally heat crosslinked at 250℃ for 1 h to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0076] Comparative Example 1

[0077] Comparative Example 1 is different from Example 1 in that Comparative Example 1 removes NH2-MWCNT in steps S1 and S2.2, and the rest of the steps remain unchanged to modify the polyimide@polyvinylidene fluoride double-layer fiber membrane, which is denoted as Comparative Example 1.

[0078] Comparative Example 2

[0079] Comparative Example 2 is different from Example 1 in that Comparative Example 2 removes 3-aminopropyl triethoxysilane in step S1.1, and the rest of the steps remain unchanged to modify the polyimide@polyvinylidene fluoride double-layer fiber membrane, which is denoted as Comparative Example 2.

[0080] Gas separation performance tests were performed on the modified polyimide@polyvinylidene fluoride double-layer fiber membranes prepared in Examples 1-3 and Comparative Examples 1-2:

[0081] The modified polyimide@polyvinylidene fluoride double-layer fiber membranes prepared in Examples 1-3 and Comparative Examples 1-2 were cut and packaged in an assembly using epoxy resin, and the assembly schematic and separation process are shown in Figure 2 The test pressure was 0.4 Mpa, the temperature was room temperature, and argon was used for sweeping on the retentate side and the permeate side. The test gases were CH4, He, CO2 and N2, and the test results are shown in Table 1.

[0082] Table 1. Gas separation performance test results of examples 1-3 and comparative examples 1-2

[0083]

[0084] As can be seen from the data in Table 1, the modified polyimide-polyvinylidene fluoride double-layer fiber membrane after adding NH2-MWCNT has higher flux and selectivity, and the helium flux can reach 378 GPU, and the He / CH4, He / N2, He / CO2selectivity performance reaches 385, 370, 356. The modified polyimide-polyvinylidene fluoride double-layer fiber membrane without adding NH2-MWCNT has a lower helium flux, only 15 GPU, and the He / CH4selectivity is only 71, which shows that NH2-MWCNT can play a role in efficient gas transmission and has excellent gas selectivity function. The modified polyimide-polyvinylidene fluoride double-layer fiber membrane without adding 3-aminopropyl triethoxysilane has a helium flux of 966 GPU, but the selectivity is low. This is because the multi-walled carbon nanotube is not compatible with the polymer interface, resulting in the formation of non-selective gaps between the multi-walled carbon nanotube and the polymer.

[0085] With the ternary mixed gas He / CO2 / CH4and He / CO2 / N2as the feed gas, the plasticization resistance of examples 1 and comparative examples 1 was tested, and the test results are shown in Table 2. Figure 3 and Figure 4 .

[0086] From Figure 3 it is found that as the feed pressure increases, the helium selectivity does not decrease significantly, while from Figure 4 it is found that the helium selectivity decreases significantly, which shows that the introduction of NH2-MWCNT makes the modified polyimide-polyvinylidene fluoride double-layer fiber membrane have excellent plasticization resistance.

[0087] The above examples only illustrate the principles and effects of the present application, and are not used to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed by the present application should be covered by the claims of the present application.

Claims

1. A method for preparing a modified polyimide@polyvinylidene fluoride bilayer fiber membrane, characterized in that, It comprises the following steps: S1: preparation of NH2-MWCNT S1.1: dry the multi-walled carbon nanotubes at 100-150℃ for 8-12h, add 1 part by weight of the dried multi-walled carbon nanotubes to 30-60 parts by weight of toluene, then add 2-4 parts by weight of 3-aminopropyl triethoxysilane, stir and reflux at 60-100℃ for 12-24h to obtain a reaction product; S1.2: centrifuge and wash the reaction product 3-5 times, and vacuum dry the washed reaction product at 80-100℃ for 8-12h to obtain NH2-MWCNT, which is ready for use; S2: preparation of casting solution and core solution S2.1: disperse 1-2 parts by weight of polyvinylpyrrolidone in 15-17 parts by weight of solvent, add 2-3 parts by weight of polyvinylidene fluoride after ultrasonic treatment for 30-60min, stir at 50-80℃ for 12-24h, vacuum degassing for 8-12h after cooling to obtain the inner layer casting solution of the membrane, which is ready for use; S2.2: disperse 1-5 parts by weight of NH2-MWCNT in 69-77 parts by weight of solvent, ultrasonic treatment for 30-60min, then add 22-26 parts by weight of polyimide, stir at 60-80℃ for 10-22h, vacuum degassing for 8-12h after cooling to obtain the outer layer casting solution of the membrane, which is ready for use; S2.3: add 3 parts by weight of sodium chloride to 5-8 parts by weight of deionized water to dissolve, then add 89-92 parts by weight of nitrogen-methyl pyrrolidone after dissolution, stir and mix for 2-4h to obtain the core solution, which is ready for use; S3: preparation of modified polyimide@polyvinylidene fluoride double-layer fiber membrane Control the flow rate of the inner layer casting solution of the membrane, the outer layer casting solution of the membrane and the core solution to be extruded from the spinneret at the same time, and solidify to obtain a double-layer hollow fiber membrane, immerse the double-layer hollow fiber membrane in a solvent, dry after immersion, and heat crosslink to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

2. The method for preparing a modified polyimide-polyvinylidene fluoride double-layer fiber film according to claim 1, characterized in that, Step S3 for preparing the modified polyimide@polyvinylidene fluoride double-layer fiber membrane, specifically comprises the following steps: S3.1: control the flow rate of the inner layer casting solution of the membrane, the outer layer casting solution of the membrane and the core solution to be extruded from the spinneret at the same time, and solidify the membrane filaments in a water coagulation bath to obtain a double-layer hollow fiber membrane; S3.2: immerse the collected double-layer hollow fiber membrane in a solvent for 1-2d, then dry at 40-60℃ for 8-12h, and finally heat crosslink at 200-300℃ for 1-2h to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

3. The method for preparing a modified polyimide-polyvinylidene fluoride double-layer fiber film according to claim 1, characterized in that, The molecular weight of the polyvinylpyrrolidone in step S2.1 is 8000-58000.

4. The method for preparing a modified polyimide-polyvinylidene fluoride double-layer fiber film according to claim 1, characterized in that, The solvent in step S2.1 is any one of nitrogen-methyl pyrrolidone, N,N-dimethylacetamide.

5. The method for preparing a modified polyimide-polyvinylidene fluoride double-layer fiber film according to claim 1, characterized in that, The solvent in step S2.2 is any one of nitrogen-methyl pyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide, and the polyimide is any one of Matrimid5218 and P84.

6. The method for preparing a modified polyimide@polyvinylidene fluoride bilayer fiber membrane according to claim 2, characterized in that, The volume flow rate ratio of the inner layer casting solution, the outer layer casting solution and the core solution in step S3.1 is 3:3-5:1-2, the height of the spinneret from the coagulation bath is 7-10 cm, the spinneret temperature is 50-65 ℃, and the coagulation bath temperature is 25-35 ℃.

7. The method for preparing a modified polyimide@polyvinylidene fluoride bilayer fiber membrane according to claim 2, characterized in that, The solvent in step S3.2 is any one of water and ethanol.

8. A modified polyimide@polyvinylidene fluoride double-layer fiber membrane, characterized in that, The modified polyimide-polyvinylidene fluoride double-layer fiber membrane is prepared by the method in any one of claims 1-7.

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