Modified polyimide and polyvinylidene fluoride double-layer fiber membrane and preparation method thereof

Abstract

The invention provides a modified polyimide and polyvinylidene 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: preparing NH2-MWCNT (multi-walled carbon nanotubes); preparing a membrane casting solution and a core solution; and preparing the modified polyimide and polyvinylidene fluoride double-layer fiber membrane. The membrane comprises a membrane inner layer and a membrane outer layer, the membrane inner layer is polyvinylidene fluoride, the membrane outer layer is NH2-MWCNT modified polyimide, the polyimide is any one of Matrimid5218 and P84, and the NH2-MWCNT modified polyimide and polyvinylidene fluoride double-layer fiber membrane is obtained by adopting a dry-wet method and a co-extrusion method and then performing thermal crosslinking.

Description

Technical Field

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

[0002] Helium is a valuable and non-renewable gas that is widely used in various industries, scientific research and medical applications due to its non-toxic, inert, lightweight and non-flammable properties. Currently, enriching helium from natural gas is the only large-scale industrial method for helium production. Since the 1960s, cryogenic distillation and pressure swing adsorption have been the main technologies for separating helium from natural gas. However, since the helium content in most natural gas reservoirs is quite low (0.05-1.0%), 100-2000 cubic meters of natural gas needs to be liquefied to obtain 1 cubic meter of helium, which consumes huge energy. Therefore, new gas separation technologies without phase change have attracted widespread attention. Gas membrane separation technology provides a 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 it also has problems, such as "trade-off" effect, low selectivity, poor stability and other problems. The literature [Journal of Membrane Science 591 (2019) 117292] reported a double-layer hollow fiber membrane with polysulfone as the supporting inner layer, polybenzimidazole and polyphenylsulfone as the outer selective layer, and chemically cross-linked and heat-treated with dibromoparaxylene. 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 complicated. Patent CN113166538B discloses a preparation method for a layered hollow fiber membrane based on poly (2, 5-benzimidazole) (ABPBI), ABPBI copolymer and substituted polybenzimidazole (PBI) polymer, which can be used for 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 the industrial application is difficult. Patent CN111298664B discloses a method for preparing a hollow fiber membrane modified by carbon nanotubes for separating CO2, with a CO2 flux of 3.138 mol·m -2 ·s -1 MPa -1 , CO2 / N2 selectivity is 17.8, CO2 / O2 selectivity is 11.3. The preparation process of this method is complicated and the separation effect is poor.

[0004] Therefore, we proposed a modified polyimide@polyvinylidene fluoride double-layer fiber membrane and its preparation method. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a modified polyimide@polyvinylidene fluoride double-layer fiber membrane and a preparation method thereof, comprising an inner membrane layer and an outer membrane layer, wherein the inner membrane layer is polyvinylidene fluoride, and the outer membrane layer is NH2-MWCNT-modified polyimide, wherein the polyimide is any one of Matrimid5218 and P84, and a NH2-MWCNT-modified polyimide@polyvinylidene fluoride double-layer hollow fiber membrane is obtained by a dry-wet method and a co-extrusion method, and then thermal cross-linking.

[0006] A method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane comprises the following steps:

[0007] S1: Preparation of NH2-MWCNT

[0008] The multi-walled carbon nanotubes, toluene and 3-aminopropyltriethoxysilane are mixed and heated to react, and the reactants are centrifuged, washed and dried to obtain NH2-MWCNTs;

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

[0010] Dispersing polyvinyl pyrrolidone in a solvent, adding polyvinylidene fluoride after ultrasonic treatment, stirring and vacuum degassing to obtain a membrane inner layer casting solution, dispersing NH2-MWCNT in a solvent, adding polyimide after ultrasonic treatment, stirring and vacuum degassing to obtain a membrane outer layer casting solution, dissolving sodium chloride in deionized water, adding nitrogen-methyl pyrrolidone, and mixing to obtain a core solution;

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

[0012] The inner layer casting liquid, the outer layer casting liquid and the core liquid flow rate are simultaneously extruded from the spinneret and solidified to obtain a double-layer hollow fiber membrane. The double-layer hollow fiber membrane is immersed in a solvent, dried after immersion, and thermally cross-linked to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0013] Furthermore, the preparation of step S1NH2-MWCNT specifically includes the following steps:

[0014] S1.1: drying the multi-walled carbon nanotubes at 100-150° C. for 8-12 h, adding 1 part by weight of the dried multi-walled carbon nanotubes to 30-60 parts by weight of toluene, and then adding 2-4 parts by weight of 3-aminopropyltriethoxysilane, stirring and refluxing at 60-100° 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 the washed reaction product is vacuum dried at 80-100° C. for 8-12 h to obtain NH2-MWCNT for later use.

[0016] Furthermore, step S2 of preparing the casting liquid and the core liquid specifically comprises the following steps:

[0017] S2.1: Disperse 1-2 parts by weight of polyvinyl pyrrolidone in 15-17 parts by weight of solvent, add 2-3 parts by weight of polyvinylidene fluoride after ultrasonic treatment for 30-60 minutes, stir at 50-80°C for 12-24 hours, cool and degas in vacuum for 8-12 hours to obtain the inner layer casting solution for standby use;

[0018] S2.2: Disperse 1-5 parts by weight of NH2-MWCNT in 69-77 parts by weight of solvent, perform ultrasonic treatment for 30-60 minutes, then add 22-26 parts by weight of polyimide, stir at 60-80°C for 10-22 hours, cool and degas in vacuum for 8-12 hours to obtain the outer layer casting solution for standby use;

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

[0020] Furthermore, step S3 of preparing the modified polyimide@polyvinylidene fluoride double-layer fiber membrane specifically comprises the following steps:

[0021] S3.1: Control the flow rates of the inner layer casting liquid, the outer layer casting liquid and the core liquid to extrude from the spinneret at the same time, and the membrane fibers enter the water coagulation bath for coagulation, and then collect to obtain a double-layer hollow fiber membrane;

[0022] S3.2: The collected double-layer hollow fiber membrane is replaced and soaked with a solvent for 1-2 days, then dried at 40-60°C for 8-12 hours, and finally thermally cross-linked at 200-300°C for 1-2 hours to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0023] Furthermore, the molecular weight of polyvinyl pyrrolidone in step S2.1 is 8000-58000.

[0024] Furthermore, the solvent in step S2.1 is any one of nitrogen-methylpyrrolidone and N,N-dimethylacetamide.

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

[0026] Furthermore, in step S3.1, the volume flow rate ratio of the inner layer casting liquid, the outer layer casting liquid and the core liquid 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°C, and the coagulation bath temperature is 25-35°C.

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

[0028] A modified polyimide@polyvinylidene fluoride double-layer fiber membrane is prepared by any of the above-mentioned methods for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

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

[0030] 1. The present invention designs polyvinylidene fluoride as the inner layer of the membrane and NH2-MWCNT-modified polyimide as the outer layer of the membrane to prepare a double-layer hollow modified polyimide@polyvinylidene fluoride double-layer fiber membrane. Polyvinylidene fluoride provides high flux for helium separation. At the same time, the porous structure of multi-walled carbon nanotubes can further improve the gas transmission efficiency. The amine functionalization can enhance the interfacial compatibility with polyimide, stabilize the polyimide segment structure, improve stability and plasticization resistance, and make the modified polyimide@polyvinylidene fluoride double-layer fiber membrane have excellent plasticization resistance.

[0031] 2. The present invention modifies polyimide by NH2-MWCNT, and uses the polyimide modified by NH2-MWCNT 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@polyvinylidene fluoride double-layer fiber membrane. 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@polyvinylidene fluoride double-layer fiber membrane of the present invention is simple and can be mass-produced, providing a new idea for the practical application of membrane separation technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate embodiments of the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable those skilled in the relevant art to make and use the present disclosure.

[0034] Figure 1 A flow chart of a method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane used in an embodiment of the present invention;

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

[0036] Figure 3 The anti-plasticization performance of the modified polyimide@polyvinylidene fluoride double-layer fiber membrane prepared in Example 1 of the present invention;

[0037] Figure 4 The anti-plasticization performance of the modified polyimide@polyvinylidene fluoride double-layer fiber membrane prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme of a method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane in the embodiments of the present invention will be clearly and completely described below in combination with the drawings in the embodiments of the present invention. Obviously, the described embodiment is only a part of the embodiments of the present invention, not all of the embodiments.

[0039] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] Example 1

[0041] A method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane, such as Figure 1 As shown, the following steps are included:

[0042] S1: Preparation of NH2-MWCNT

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

[0044] S1.2: The reaction product was centrifuged and washed three times, and the washed reaction product was vacuum dried at 80°C for 8 h to obtain NH2-MWCNT for later use;

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

[0046] S2.1: Disperse 1 part by weight of 8000 molecular weight polyvinyl pyrrolidone in 15 parts by weight of nitrogen-methyl pyrrolidone, add 2 parts by weight of polyvinylidene fluoride after ultrasonic treatment for 30 minutes, stir at 50°C for 12 hours, cool and degas in vacuum for 8 hours to obtain the inner layer casting solution for use;

[0047] S2.2: Disperse 1 part by weight of NH2-MWCNT in 77 parts by weight of nitrogen-methylpyrrolidone, ultrasonicate for 30 minutes, then add 22 parts by weight of Matrimid5218, stir at 60°C for 10 hours, cool and degas in vacuum for 8 hours to obtain the outer layer casting solution for standby use;

[0048] S2.3: Add 3 parts by weight of sodium chloride to 5 parts by weight of deionized water to dissolve it, and then add 92 parts by weight of nitrogen-methyl pyrrolidone, and stir and mix for 2 hours to obtain a core liquid for later use;

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

[0050] S3.1: The volume flow rate ratio of the inner layer casting liquid, the outer layer casting liquid and the core liquid is controlled to be 3:3:1, and the membrane fibers are extruded from a spinneret at 50°C at the same time. The membrane fibers enter a water coagulation bath at 25°C for coagulation. The height of the spinneret from the coagulation bath is 7 cm, and a double-layer hollow fiber membrane is obtained by collecting.

[0051] S3.2: The collected double-layer hollow fiber membrane was replaced and soaked with water for 1 day, then dried at 40°C for 8 hours, and finally thermally cross-linked at 200°C for 1 hour to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0052] Example 2

[0053] A method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane, such as Figure 1 As shown, the following steps are included:

[0054] S1: Preparation of NH2-MWCNT

[0055] S1.1: drying the multi-walled carbon nanotubes at 150° C. for 12 h, adding 1 part by weight of the dried multi-walled carbon nanotubes to 60 parts by weight of toluene, and then adding 4 parts by weight of 3-aminopropyltriethoxysilane, stirring and refluxing at 100° C. for 24 h to obtain a reaction product;

[0056] S1.2: The reaction product was centrifuged and washed 5 times, and the washed reaction product was vacuum dried at 100°C for 12 hours to obtain NH2-MWCNT for later use;

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

[0058] S2.1: 2 parts by weight of polyvinyl pyrrolidone with a molecular weight of 58,000 is dispersed in 17 parts by weight of solvent N,N-dimethylacetamide, and 23 parts by weight of polyvinylidene fluoride is added after ultrasonic treatment for 60 minutes, and the mixture is stirred at 80°C for 24 hours, and vacuum degassed for 12 hours after cooling to obtain a membrane inner layer casting solution for standby use;

[0059] S2.2: Disperse 5 parts by weight of NH2-MWCNT in 69 parts by weight of N,N-dimethylacetamide, ultrasonicate for 60 min, then add 26 parts by weight of P84, stir at 80°C for 22 h, cool and vacuum degas for 12 h to obtain the outer layer casting solution for standby use;

[0060] S2.3: Add 3 parts by weight of sodium chloride to 8 parts by weight of deionized water to dissolve it, add 89 parts by weight of nitrogen-methyl pyrrolidone after dissolution, stir and mix for 4 hours to obtain core liquid for standby 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 liquid, the outer layer casting liquid and the core liquid is controlled to be 3:5:2, and the membrane fibers are extruded from a spinneret at 65°C at the same time. The membrane fibers enter a 35°C water coagulation bath for coagulation. The height of the spinneret from the coagulation bath is 10 cm, and a double-layer hollow fiber membrane is obtained by collecting.

[0063] S3.2: The collected double-layer hollow fiber membrane was replaced and soaked in ethanol for 2 days, then dried at 60°C for 12 hours, and finally thermally cross-linked at 300°C for 2 hours to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0064] Example 3

[0065] A method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane, such as Figure 1 As shown, the following steps are included:

[0066] S1: Preparation of NH2-MWCNT

[0067] S1.1: drying the multi-walled carbon nanotubes at 150° C. for 12 h, adding 1 part by weight of the dried multi-walled carbon nanotubes to 40 parts by weight of toluene, and then adding 3 parts by weight of 3-aminopropyltriethoxysilane, stirring and refluxing at 100° C. for 24 h to obtain a reaction product;

[0068] S1.2: The reaction product was centrifuged and washed three times, and the washed reaction product was vacuum dried at 100°C for 12 hours to obtain NH2-MWCNT for later use;

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

[0070] S2.1: Disperse 1 part by weight of 24,000 molecular weight polyvinyl pyrrolidone in 10.67 parts by weight of N,N-dimethylacetamide, add 2 parts by weight of polyvinylidene fluoride after ultrasonic treatment for 30 minutes, stir at 80°C for 24 hours, cool and degas in vacuum for 12 hours to obtain the inner layer casting solution for standby use;

[0071] S2.2: 3 parts by weight of NH2-MWCNT were dispersed in 69 parts by weight of N, N-dimethylacetamide, and ultrasonicated for 60 minutes, and then 26 parts by weight of P84 were added, stirred at 80°C for 22 hours, and vacuum degassed for 12 hours after cooling to obtain the outer layer casting solution for standby use;

[0072] S2.3: Add 3 parts by weight of sodium chloride to 8 parts by weight of deionized water to dissolve it, add 89 parts by weight of nitrogen-methyl pyrrolidone after dissolution, stir and mix for 4 hours to obtain core liquid for standby 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 liquid, the outer layer casting liquid and the core liquid is controlled to be 3:3:1, and the membrane fibers are extruded from a spinneret at 65°C at the same time. The membrane fibers enter a 35°C water coagulation bath for coagulation. The height of the spinneret from the coagulation bath is 7 cm. A double-layer hollow fiber membrane is obtained by collecting the membrane fibers.

[0075] S3.2: The collected double-layer hollow fiber membrane was replaced and soaked in ethanol for 2 days, then dried at 60°C for 12 hours, and finally thermally cross-linked at 250°C for 1 hour to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

[0076] Comparative Example 1

[0077] Compared with Example 1, the difference of Comparative Example 1 is that Comparative Example 1 removes NH2-MWCNT in step S1 and step S2.2, and the remaining steps remain unchanged to modify the polyimide@polyvinylidene fluoride double-layer fiber membrane, which is recorded as Comparative Example 1.

[0078] Comparative Example 2

[0079] Compared with Example 1, the difference of Comparative Example 2 is that Comparative Example 2 removes 3-aminopropyltriethoxysilane in step S1.1, and the remaining steps remain unchanged to modify the polyimide@polyvinylidene fluoride double-layer fiber membrane, which is recorded as Comparative Example 2.

[0080] The gas separation performance test was conducted 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 membrane prepared in Example 1-3 and Comparative Example 1-2 was cut and encapsulated in a component using epoxy resin. The component schematic diagram and separation process are shown in FIG. Figure 2 As shown. The test pressure is 0.4Mpa, the temperature is room temperature, and the retentate side and the permeate side are purged with argon. The test gases are CH4, He, CO2 and N2. The test results refer to Table 1.

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

[0083]

[0084] It can be seen from the data in Table 1 that the modified polyimide@polyvinylidene fluoride double-layer fiber membrane after adding NH2-MWCNT has higher flux and selectivity, the helium flux can reach 378GPU, and the He / CH4, He / N2, and He / CO2 selectivity performances reach 385, 370, and 356, while the modified polyimide@polyvinylidene fluoride double-layer fiber membrane without adding NH2-MWCNT has a lower helium flux of only 15GPU and a He / CH4 selectivity of only 71, which indicates that NH2-MWCNT can play a role in efficient gas transmission and has excellent gas selectivity function; while the modified polyimide@polyvinylidene fluoride double-layer fiber membrane without adding 3-aminopropyltriethoxysilane has a helium flux of 966GPU, but a lower selectivity, because the multi-walled carbon nanotubes are incompatible with the polymer interface, resulting in the formation of non-selective gaps between the multi-walled carbon nanotubes and the polymer.

[0085] The anti-plasticization performance of Example 1 and Comparative Example 1 was tested by using ternary mixed gas He / CO2 / CH4 and He / CO2 / N2 as feed gas and controlling the feed pressure. The test results are shown in Table 1. Figure 3 and Figure 4 .

[0086] from Figure 3 It was found that with the increase of feed pressure, the helium selectivity did not decrease significantly, but Figure 4 It was found that the helium selectivity was significantly reduced, indicating that the introduction of NH2-MWCNT gave the modified polyimide@polyvinylidene fluoride double-layer fiber membrane excellent anti-plasticization properties.

[0087] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane, characterized in that: The steps include: S1: Preparation of NH2-MWCNT The multi-walled carbon nanotubes, toluene and 3-aminopropyltriethoxysilane are mixed and heated to react, and the reactants are centrifuged, washed and dried to obtain NH2-MWCNTs; S2: Preparation of casting solution and core solution Dispersing polyvinyl pyrrolidone in a solvent, adding polyvinylidene fluoride after ultrasonic treatment, stirring and vacuum degassing to obtain a membrane inner layer casting solution, dispersing NH2-MWCNT in a solvent, adding polyimide after ultrasonic treatment, stirring and vacuum degassing to obtain a membrane outer layer casting solution, dissolving sodium chloride in deionized water, adding nitrogen-methyl pyrrolidone, and mixing to obtain a core solution; S3: Preparation of modified polyimide@polyvinylidene fluoride double-layer fiber membrane The inner layer casting liquid, the outer layer casting liquid and the core liquid flow rate are simultaneously extruded from the spinneret and solidified to obtain a double-layer hollow fiber membrane. The double-layer hollow fiber membrane is immersed in a solvent, dried after immersion, and thermally cross-linked to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

2. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 1, characterized in that: Step S1: Preparation of NH2-MWCNT, specifically comprising the following steps: S1.1: drying the multi-walled carbon nanotubes at 100-150° C. for 8-12 h, adding 1 part by weight of the dried multi-walled carbon nanotubes to 30-60 parts by weight of toluene, and then adding 2-4 parts by weight of 3-aminopropyltriethoxysilane, stirring and refluxing at 60-100° C. for 12-24 h to obtain a reaction product; S1.2: The reaction product is centrifuged and washed for 3-5 times, and the washed reaction product is vacuum dried at 80-100° C. for 8-12 h to obtain NH2-MWCNT for later use.

3. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 2, characterized in that: Step S2: Preparation of casting liquid and core liquid, specifically comprising the following steps: S2.1: Disperse 1-2 parts by weight of polyvinyl pyrrolidone in 15-17 parts by weight of solvent, add 2-3 parts by weight of polyvinylidene fluoride after ultrasonic treatment for 30-60 minutes, stir at 50-80°C for 12-24 hours, cool and degas in vacuum for 8-12 hours to obtain the inner layer casting solution for standby use; S2.2: Disperse 1-5 parts by weight of NH2-MWCNT in 69-77 parts by weight of solvent, perform ultrasonic treatment for 30-60 minutes, then add 22-26 parts by weight of polyimide, stir at 60-80°C for 10-22 hours, cool and degas in vacuum for 8-12 hours to obtain the outer layer casting solution for standby use; S2.3: Add 3 parts by weight of sodium chloride to 5-8 parts by weight of deionized water to dissolve it, and then add 89-92 parts by weight of nitrogen-methyl pyrrolidone, and stir and mix for 2-4 hours to obtain core liquid for use.

4. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 3, characterized in that: Step S3: Preparation of modified polyimide@polyvinylidene fluoride double-layer fiber membrane, specifically comprising the following steps: S3.1: Control the flow rates of the inner layer casting liquid, the outer layer casting liquid and the core liquid to extrude from the spinneret at the same time, and the membrane fibers enter the water coagulation bath for coagulation, and then collect to obtain a double-layer hollow fiber membrane; S3.2: The collected double-layer hollow fiber membrane is replaced and soaked with a solvent for 1-2 days, then dried at 40-60°C for 8-12 hours, and finally thermally cross-linked at 200-300°C for 1-2 hours to obtain a modified polyimide@polyvinylidene fluoride double-layer fiber membrane.

5. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 3, characterized in that: The molecular weight of polyvinyl pyrrolidone in step S2.1 is 8000-58000.

6. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 3, characterized in that: The solvent in step S2.1 is any one of nitrogen-methylpyrrolidone and N,N-dimethylacetamide.

7. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 3, characterized in that: The solvent in step S2.2 is any one of nitrogen-methylpyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide, and the polyimide is any one of Matrimid5218 and P84.

8. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 4, characterized in that: In step S3.1, the volume flow rate ratio of the inner layer casting liquid, the outer layer casting liquid and the core liquid 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°C, and the coagulation bath temperature is 25-35°C.

9. The method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to claim 4, characterized in that: The solvent in step S3.2 is any one of water and ethanol.

10. A modified polyimide@polyvinylidene fluoride double-layer fiber membrane, characterized in that: The membrane is prepared by the method for preparing a modified polyimide@polyvinylidene fluoride double-layer fiber membrane according to any one of claims 1 to 9.

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