Organic solvent resistant nanofiltration membrane and preparation method thereof

Abstract

The invention relates to an organic solvent-resistant nanofiltration membrane and a preparation method thereof, the nanofiltration membrane comprises a bottom support layer, a middle buffer layer, a compact selection layer and a top protection layer which are stacked in sequence, and the surface of the bottom support layer contains active functional groups; the compact selection layer is a cross-linked polyamide network; the top protection layer is formed by curing a fluorine-containing amphiphilic block copolymer; the middle buffer layer is a zwitterionic copolymer network containing a reactive functional group, and the zwitterionic copolymer network containing the reactive functional group is formed by curing a zwitterionic monomer mixed solution containing the reactive functional group; wherein the reactive functional group comprises an amino group, an epoxy group and a zwitterionic structure provided by a sulphobetaine methacrylate unit. According to the invention, a four-layer gradient structure is constructed, and three reactive functional groups of amino, epoxy group and zwitter-ion structure are introduced into the middle buffer layer, so that the nanofiltration membrane has excellent interface stability and separation performance under the washing of a strong-polarity organic solvent.

Description

Technical Field

[0001] This application relates to the field of membrane separation material preparation technology, and in particular to an organic solvent-resistant nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration, a membrane separation technology situated between ultrafiltration and reverse osmosis, is widely used in water treatment due to its advantages such as a molecular weight cutoff of 200-1000 Da, low operating pressure, high rejection rate for divalent ions and small organic molecules, no phase change, and ease of integration. With the rapid development of the pharmaceutical, fine chemical, and petrochemical industries, the demand for organic solvent-resistant nanofiltration membranes in the separation and purification of organic systems is increasingly urgent. Polyimide, with its excellent heat resistance, mechanical properties, and chemical stability, has become an ideal candidate material for preparing solvent-resistant nanofiltration membranes, and in recent years, a large amount of research has focused on its application in the separation processes of substances in organic solvent systems.

[0003] However, while polyimide materials possess many excellent properties, they also have significant drawbacks, primarily their susceptibility to dissolution in polar organic solvents. This deficiency severely restricts their development in the field of solvent-resistant nanofiltration membranes. To overcome this problem, existing technologies have attempted several improvement routes: first, improving the solvent resistance of polyimide through chemical crosslinking modification, but this often leads to a significant decrease in membrane flux and difficulty in controlling the degree of crosslinking; second, constructing multilayer composite membranes using layer-by-layer self-assembly, but electrostatic interactions are easily weakened in strongly polar solvents, leading to structural disintegration; third, developing novel solvent-resistant materials such as polybenzimidazole, but the film-forming process is complex and costly; and fourth, using interfacial polymerization to form a selective layer, but the selective layer and the support layer are only physically adsorbed together, and interfacial delamination easily occurs after long-term immersion in organic solvents.

[0004] In summary, the core challenges currently facing organic solvent-resistant nanofiltration membranes in industrialization are: firstly, insufficient long-term stability in highly polar aprotic solvents such as DMF and NMP, leading to swelling and deformation of the membrane material and a decline in separation performance; secondly, weak interfacial bonding between the separation layer and the support layer, making it difficult to withstand the shearing action of high-velocity organic solvent flow. Therefore, developing a novel organic solvent-resistant nanofiltration membrane structure that combines high stability and high separation performance has significant industrial application value. Summary of the Invention

[0005] In order to enable nanofiltration membranes to have both high stability and high separation performance under the scouring of organic solvents, this application provides an organic solvent-resistant nanofiltration membrane and its preparation method.

[0006] In a first aspect, this application provides an organic solvent-resistant nanofiltration membrane, which adopts the following technical solution: An organic solvent-resistant nanofiltration membrane comprises a bottom support layer, an intermediate buffer layer, a dense selective layer, and a top protective layer stacked sequentially. The surface of the bottom support layer contains active functional groups. The dense selective layer is a cross-linked polyamide network. The top protective layer is formed by curing a fluorinated amphiphilic block copolymer. The intermediate buffer layer is a zwitterionic copolymer network containing reactive functional groups, which is formed by curing a zwitterionic monomer mixture containing reactive functional groups, wherein the reactive functional groups include amino, epoxy, and zwitterionic structures provided by sulfobetaine methacrylate units.

[0007] This application designs an organic solvent-resistant nanofiltration membrane consisting of a bottom support layer, an intermediate buffer layer, a dense selective layer, and a top protective layer stacked sequentially. The inventors discovered that by introducing an intermediate buffer layer containing three types of reactive functional groups—amino, epoxy, and zwitterionic structures provided by sulfobetaine methacrylate units—between the bottom support layer and the dense selective layer, it not only acts as a bridge connecting the upper and lower layers but also effectively buffers the internal stress generated by the swelling of organic solvents, significantly improving the stability and separation performance of the nanofiltration membrane under the scouring of organic solvents.

[0008] In this process, the amino groups in the intermediate buffer layer form hydrogen bonds with the active functional groups (such as hydroxyl groups) on the surface of the bottom support layer, achieving anchoring between the intermediate buffer layer and the bottom support layer and preventing the intermediate buffer layer from detaching from the surface of the bottom support layer under solvent immersion. Meanwhile, during subsequent interfacial polymerization, the epoxy groups undergo ring-opening reactions with the residual amino groups in the cross-linked polyamide network of the dense selective layer, forming covalent bonds and thus producing chemical cross-linking. This enhances the interfacial bonding between the intermediate buffer layer and the dense selective layer, fundamentally solving the interlayer delamination problem caused by weak interfacial bonding in traditional composite membranes in organic solvents. The zwitterionic structure (containing quaternary ammonium cations and sulfonate anions) in the sulfobetaine methacrylate unit endows the intermediate buffer layer with unique hydrophilicity and charge balance capabilities. This, on the one hand, adjusts the interfacial polarity through hydrophilic / hydrophobic balance, making the nanofiltration membrane adaptable to different solvent systems; on the other hand, the compliant molecular chain segments of the zwitterionic side chains can effectively absorb and disperse the internal stress generated by the swelling of the dense selective layer, reducing the risk of stress concentration leading to cracking of the dense selective layer.

[0009] Because highly polar organic solvents (such as DMF and NMP) easily cause polyamide networks to swell, leading to increased chain segment movement, increased free volume, and decreased retention rate, the intermediate buffer layer in this application, formed by zwitterionic networks of sulfobetaine methacrylate units, effectively buffers swelling stress and limits excessive movement of polyamide chains. Simultaneously, the chemical bonding of epoxy groups firmly anchors the selective layer to the surface of the buffer layer, making it difficult to peel off from the interface even in the event of slight swelling. Furthermore, the high permeability of highly polar solvents easily leads to interfacial penetration between the selective layer and the support layer. The amino groups in the intermediate buffer layer form dense anchoring points with the active functional groups on the surface of the bottom support layer, preventing solvent penetration along the interface and effectively protecting the structural integrity of the bottom support layer.

[0010] This application selects a cross-linked polyamide network formed on the surface of the intermediate buffer layer via interfacial polymerization as a dense selective layer. This is not only because the cross-linked polyamide network has precise molecular sieving capabilities, enabling efficient retention of small organic molecules with molecular weights of 200-1000 Da, but also because the cross-linked structure endows it with certain solvent resistance. The synergistic effect with the intermediate buffer layer significantly improves the stability of the nanofiltration membrane.

[0011] The top protective layer is formed by curing a fluorinated amphiphilic block copolymer, in which the fluorinated segments have low surface energy and can form a barrier layer against solvent erosion on the top surface of the nanofiltration membrane, reducing the direct impact of high-flow-rate solvents on the selective layer.

[0012] Through the synergistic effect of the above four layers—the bottom support layer provides mechanical strength, the middle buffer layer realizes the triple function of "anchoring-bonding-buffering", the dense selective layer ensures accurate sieving, and the top protective layer provides anti-erosion effect—the synergy enables the organic solvent resistant nanofiltration membrane of this application to have both excellent interfacial stability and separation performance under the erosion of strong polar organic solvents.

[0013] In one specific implementation, the method for preparing the zwitterionic monomer mixed solution containing reactive functional groups includes the following steps: Glycidyl methacrylate, sulfobetaine methacrylate, and aminoethyl methacrylate are dissolved in an organic solvent, and a photoinitiator is added and mixed in the dark to obtain the zwitterionic monomer mixed solution containing reactive functional groups; preferably, the mass ratio of glycidyl methacrylate, sulfobetaine methacrylate, and aminoethyl methacrylate is (2-3):(1-2):1.

[0014] Three functional monomers—glycidyl methacrylate, sulfobetaine methacrylate, and aminoethyl methacrylate—were dissolved together with a photoinitiator in a polar organic solvent. Because the polarity of the solvent system matches the functional groups of the three monomers, the epoxy groups of glycidyl methacrylate, the zwitterionic structure of sulfobetaine methacrylate, and the amino groups of aminoethyl methacrylate form a molecular-level pre-assembly through hydrogen bonding and electrostatic interactions, resulting in a microscopically uniform distribution of the solution before coating.

[0015] The solution was coated onto a surface-activated bottom support layer to form a wet film, followed by ultraviolet irradiation in a nitrogen atmosphere. The photoinitiator was excited to generate free radicals, triggering a copolymerization reaction of the methacrylate double bonds in the three monomers, gradually constructing a copolymer network with carbon chains as the backbone. The photocuring process consumed only the double bonds; the epoxy, sulfonic acid, and amino groups on the side chains did not participate in the free radical reaction and were thus entirely retained in the cured network. This indicates that while forming a stable structure, the buffer layer still completely retains the active sites required for subsequent chemical reactions.

[0016] Through extensive experiments, the inventors have verified that in the preparation of zwitterionic monomer mixed solutions, glycidyl methacrylate providing the epoxy group, sulfobetaine methacrylate providing the zwitterionic structure of quaternary ammonium cation and sulfonate anion, and aminoethyl methacrylate providing the amino group, when the mass ratio of the three is controlled within the range of (2-3):(1-2):1, can effectively achieve the excellent stability and high separation performance of nanofiltration membranes under organic solvent rinsing.

[0017] Within the above mass ratio range, the contents of the three types of functional groups reach equilibrium: the epoxy group content is sufficient to form enough covalent bonds with the dense selective layer, the zwitterion content is sufficient to effectively buffer swelling stress, and the amino group content is sufficient to achieve firm anchoring with the support layer. If the weight ratio deviates from this range, such as when the content of glycidyl methacrylate is too low, the insufficient number of epoxy groups will result in too few chemical bonding points with the polyamide layer, leading to a decrease in interfacial adhesion. This makes the interfacial micro-defects prone to occur and expand during long-term immersion in organic solvents, ultimately causing the selective layer to peel off and the retention rate to drop sharply. If the content of sulfobetaine methacrylate is too low, the insufficient content of zwitterions will result in poor hydrophilicity of the intermediate buffer layer, which will not be able to effectively absorb and disperse the internal stress generated by the swelling of the selective layer. This will cause stress concentration and transfer to the interface, resulting in cracking of the dense selective layer. It will also affect the uniformity of the subsequent protective layer coating, which may lead to pinhole defects in the top protective layer. This will cause the stability and separation performance of the nanofiltration membrane to be greatly reduced under the washing of organic solvents. If the content of aminoethyl methacrylate is too low, the insufficient amino groups will result in insufficient anchoring with the bottom support layer. The intermediate buffer layer will be prone to peeling off from the surface of the support layer, and the retention rate will drop significantly.

[0018] In one specific feasible embodiment, the fluorinated amphiphilic block copolymer is a fluorosulfonic acid-polyethylene glycol block copolymer, and the preparation steps of the fluorosulfonic acid-polyethylene glycol block copolymer are as follows: S1-1, under nitrogen protection, 2-acrylamido-2-methylpropanesulfonic acid, polyethylene glycol monomethyl ether acrylate, RAFT chain transfer agent, and RAFT initiator are added to a solvent and reacted. After post-treatment, a polymer containing sulfonic acid groups and polyethylene glycol segments is obtained. S2-1 Under nitrogen protection, the polymer and fluorinated monomer obtained in step S1-1 are added to water, the pH of the reaction system is adjusted to 7-8, the RAFT initiator is dissolved in water and added to the reaction, and the post-treatment yields the fluorosulfonic acid-polyethylene glycol block copolymer.

[0019] The above steps yield a high-yield, high-purity fluorosulfonic acid-polyethylene glycol block copolymer. Coating this copolymer onto the surface of a dense selective layer and curing it forms a top protective layer that effectively improves the nanofiltration membrane's erosion resistance, antifouling ability, and long-term stability. Specifically, hydrogen bonds are formed between the sulfonic acid anionic groups in the top protective layer and the active groups (such as amino and carboxyl groups) in the polyamide layer, effectively enhancing the interfacial bonding between the top protective layer and the dense selective layer, thus significantly improving the overall structural stability of the nanofiltration membrane. At this point, the fluorinated segments in the top protective layer, forming a dense film, effectively block solvent molecules from directly impacting the dense selective layer. Simultaneously, the compliant polyethylene glycol segments absorb the kinetic energy of organic solvent erosion, reducing the shear force borne by the dense selective layer, thereby effectively enhancing the fatigue life of the nanofiltration membrane.

[0020] In one specific implementation, the bottom support layer is a polyimide nonwoven fabric with hydroxyl groups on its surface; preferably, the preparation method of the polyimide nonwoven fabric with hydroxyl groups on its surface includes the following steps: The polyimide nonwoven fabric was ultrasonically cleaned in ethanol and deionized water in sequence, dried, and then immersed in Tris-HCl buffer solution containing dopamine with a pH of 8-9 for reaction. The post-treatment yielded the polyimide nonwoven fabric with hydroxyl groups on its surface.

[0021] Polyimide itself possesses excellent mechanical strength and solvent resistance, providing a stable mechanical foundation for the entire membrane when used as a support layer. Furthermore, dopamine treatment introduces abundant hydroxyl functional groups into the surface of the polyimide nonwoven fabric. These hydroxyl groups can form a hydrogen bond network with the amino groups in the intermediate buffer layer, enhancing interlayer adhesion. This application chooses dopamine treatment because its mild conditions allow for surface functionalization modification without compromising the mechanical strength of the polyimide substrate.

[0022] In one specific implementation, the crosslinked polyamide network is formed by interfacial polymerization of a diamine monomer and an acyl chloride monomer; preferably, the diamine monomer comprises a diamine having a rigid twisted structure, the rigid twisted structure being selected from one or more of fluorenyl carbonyl pendant groups, biphenyl groups, and spirocyclic structures; more preferably, the acyl chloride monomer is selected from one or more of trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride.

[0023] Diamine monomers with rigid, twisted structures (such as diamines containing fluorenyl carbonyl pendant groups, biphenyl groups, or spirocyclic structures) form polyamide networks during interfacial polymerization. These networks exhibit higher segmental rigidity and tighter molecular packing, significantly improving the anti-swelling ability of the dense selective layer in polar organic solvents. The rigid, twisted structure effectively suppresses solvent-induced segmental rearrangement, ensuring structural stability of the polyamide network in strongly polar solvents such as DMF and NMP. Furthermore, compared to conventional m-phenylenediamine, the polyamide layer formed by rigid diamines has a higher free volume and better solvent permeability, contributing to high throughput while maintaining high rejection rates. The selection of acyl chloride monomers (pyromellitic phthaloyl chloride, isophthaloyl chloride, terephthaloyl chloride) can further regulate the degree of crosslinking and network structure, optimizing separation performance.

[0024] Secondly, this application provides a method for preparing an organic solvent-resistant nanofiltration membrane, employing the following technical solution: A method for preparing an organic solvent-resistant nanofiltration membrane includes the following steps: S1-2. A mixed solution of zwitterionic monomers containing reactive functional groups is coated onto the surface of the bottom support layer and crosslinked by ultraviolet light irradiation to form an intermediate buffer layer. S2-2. The bottom support layer with an intermediate buffer layer is subjected to interfacial polymerization: first, it is immersed in an aqueous solution containing amine monomers, then removed and immersed in an oil solution containing acyl chloride monomers, and then heat-treated to form a dense selective layer. S3-2. A fluorinated amphiphilic block copolymer is coated onto the surface of a dense selective layer and cured to form a top protective layer, thereby obtaining the organic solvent resistant nanofiltration membrane.

[0025] The nanofiltration membrane prepared using the above steps exhibits in-situ chemical bonding between its layers during the preparation process: ultraviolet irradiation crosslinks and solidifies the buffer layer copolymer while retaining some active functional groups; during interfacial polymerization, the epoxy groups in the buffer layer undergo ring-opening reactions with the amino groups in the polyamide layer, forming covalent bonds; heat treatment further increases the crosslinking density of the polyamide network and promotes the continued reaction of residual functional groups at the interface. This layer-by-layer construction and in-situ bonding process ensures the integrity and interfacial stability of the multilayer structure, enabling the final product to maintain excellent separation performance even after long-term operation in highly polar solvents.

[0026] In summary, this application includes at least one of the following beneficial technical effects: 1. This application constructs a four-layer gradient structure consisting of a bottom support layer, an intermediate buffer layer, a dense selective layer, and a top protective layer. In the intermediate buffer layer, three types of reactive functional groups, namely amino, epoxy, and zwitterionic structures, are introduced to achieve multiple chemical bonds and functional synergy between layers.

[0027] 2. This application introduces sulfonic acid groups into the top protective layer of the fluorinated amphiphilic block copolymer, which form hydrogen bonds with the amino groups in the dense selective layer, effectively enhancing the interfacial bonding between the protective layer and the selective layer.

[0028] 3. This application constructs a polyamide layer using a diamine with a rigid twisted structure, which makes the dense selective layer made of cross-linked polyamide in the nanofiltration membrane have a higher free volume and better solvent permeability, which helps to achieve high throughput while maintaining a high rejection rate. Detailed Implementation

[0029] The present application will be further described in detail below with reference to embodiments and comparative examples: Some of the raw materials used in the examples and comparative examples: Tris-HCl buffer at pH 10 (product number: T6455, from Sigma-Aldrich). Dopamine hydrochloride (CAS: 62-31-7, Product No.: H8502, from Sigma-Aldrich); S-1-Dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (CAS: 461642-78-4, Product No.: 723010, from Sigma-Aldrich). 2-Hydroxy-2-methyl-1-phenyl-1-propanone (CAS: 7473-98-5, Product No.: 405655, from Sigma-Aldrich); Hydroxyl-terminated fluorinated polyether silicone oil (model: BD-3365F-5, purchased from Hangzhou Baode New Material Technology Co., Ltd.)

[0030] Unless otherwise specified, all raw materials used in the examples and comparative examples are commercially available products.

[0031] Preparation Example 1 Tris-HCl buffer containing 2 mg / mL dopamine was prepared as follows: Add hydrochloric acid to 80 mL of pH 10 Tris-HCl buffer to adjust the pH to 8.5, add 0.2 g of dopamine hydrochloride and mix well. Transfer to a volumetric flask and add deionized water to make up to 100 mL to obtain a Tris-HCl buffer containing 2 mg / mL dopamine.

[0032] Preparation Example 2 The bottom support layer was prepared as follows: Polyimide nonwoven fabric was ultrasonically cleaned in ethanol for 15 minutes, then ultrasonically cleaned in deionized water for 15 minutes, followed by vacuum drying at 60°C for 2 hours. It was then immersed in Tris-HCl buffer solution containing 2 mg / mL dopamine, prepared in Example 1, and reacted at room temperature for 3 hours. After the reaction was complete, the fabric was removed, rinsed with deionized water, and dried with nitrogen to obtain a bottom support layer with hydroxyl active groups on its surface.

[0033] Preparation Example 3 The preparation of a mixed solution of zwitterionic monomers containing reactive functional groups is as follows: Dissolve 50g glycidyl methacrylate, 30g sulfobetaine methacrylate, and 20g aminoethyl methacrylate in 500mL DMF. Add 5g 2-hydroxy-2-methyl-1-phenyl-1-propanone and stir for 15 minutes in the dark. Place the resulting mixture in an ultrasonic cleaner and degas for 5 minutes to obtain a zwitterionic monomer mixed solution containing reactive functional groups.

[0034] Preparation Example 4 The only difference between Preparation Example 4 and Preparation Example 3 is that the 50g glycidyl methacrylate, 30g sulfobetaine methacrylate, and 20g aminoethyl methacrylate in Preparation Example 4 are replaced with 100g glycidyl methacrylate.

[0035] Preparation Example 5 The only difference between Preparation Example 5 and Preparation Example 3 is that the 50g glycidyl methacrylate, 30g sulfobetaine methacrylate, and 20g aminoethyl methacrylate in Preparation Example 5 are replaced with 100g sulfobetaine methacrylate.

[0036] Preparation Example 6 The only difference between Preparation Example 6 and Preparation Example 3 is that the 50g glycidyl methacrylate, 30g sulfobetaine methacrylate, and 20g aminoethyl methacrylate in Preparation Example 6 are replaced with 100g aminoethyl methacrylate.

[0037] Preparation Example 7 The only difference between Preparation Example 7 and Preparation Example 3 is that the 50g glycidyl methacrylate, 30g sulfobetaine methacrylate, and 20g aminoethyl methacrylate in Preparation Example 7 are replaced with 40g glycidyl methacrylate, 35g sulfobetaine methacrylate, and 25g aminoethyl methacrylate.

[0038] Preparation Example 8 The only difference between Preparation Example 8 and Preparation Example 3 is that the 50g glycidyl methacrylate, 30g sulfobetaine methacrylate, and 20g aminoethyl methacrylate in Preparation Example 8 are replaced with 55g glycidyl methacrylate, 20g sulfobetaine methacrylate, and 25g aminoethyl methacrylate.

[0039] Preparation Example 9 The only difference between Preparation Example 9 and Preparation Example 3 is that the 50g glycidyl methacrylate, 30g sulfobetaine methacrylate, and 20g aminoethyl methacrylate in Preparation Example 9 are replaced with 55g glycidyl methacrylate, 35g sulfobetaine methacrylate, and 10g aminoethyl methacrylate.

[0040] Preparation Example 10 The preparation of the fluorosulfonic acid-polyethylene glycol block copolymer is as follows: Under nitrogen protection, 20.7 g of 2-acrylamido-2-methylpropanesulfonic acid, 30.0 g of polyethylene glycol monomethyl ether acrylate, 0.36 g of S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate, and 0.16 g of azobisisobutyronitrile were added to 200 mL of DMF, and the mixture was heated in an oil bath to 70 °C for 8 hours. After the reaction was completed, the reaction solution was cooled to room temperature and added dropwise to excess anhydrous diethyl ether with stirring to precipitate. The precipitate was filtered, washed three times with diethyl ether, and dried in a vacuum drying oven at 40 °C to constant weight to obtain a polymer containing sulfonic acid groups and polyethylene glycol segments. S2-1 Under nitrogen protection, 10.0 g of the polymer obtained in step S1-1 and 15.0 g of hexafluorobutyl acrylate were added to water and stirred until evenly dispersed. The pH of the reaction system was adjusted to 7.5 with 0.5 mol / L sodium hydroxide solution. 0.05 g of 2,2-azobis(2-methylpropylimidazolium) hydrochloride was dissolved in 5 mL of water and added to the above mixture. The mixture was heated in an oil bath to 75°C and reacted for 6 hours. After the reaction was completed, the reaction solution was cooled to room temperature and transferred to a dialysis bag (molecular weight cutoff 3500 Da). Dialysis was performed in deionized water for 48 hours, with the water changed every 8 hours. The dialyzed solution was freeze-dried to obtain a fluorosulfonic acid-polyethylene glycol block copolymer. Example Example

[0041] The preparation of organic solvent-resistant nanofiltration membranes is as follows: S1-2. The zwitterionic monomer mixture solution containing reactive functional groups obtained in Preparation Example 3 was coated onto the surface of the bottom support layer obtained in Preparation Example 2, and the wet film thickness was controlled to be 50 μm. The coated support layer was immediately placed in an ultraviolet curing chamber for irradiation crosslinking under nitrogen protection. The ultraviolet light source wavelength was 365 nm, the light intensity was 100 mW / cm², and the irradiation time was 5 minutes. After irradiation, the film was immersed in DMF for 3 minutes to remove unreacted monomers, and then dried with nitrogen to form an intermediate buffer layer. S2-2. The bottom support layer with an intermediate buffer layer is subjected to interfacial polymerization: First, it is immersed in an aqueous solution consisting of 2% (R)-2,2',3,3'-tetrahydro-1,1'-spirobis[1H-indene]-7,7'-diamine hydrochloride and 1% sodium hydroxide aqueous solution. After immersion for 2 minutes, it is removed, and excess droplets on the surface are removed. Then, it is immersed in a 0.15% (w / w) solution of trimesoyl chloride and hexane for 1 minute to carry out interfacial polymerization. After removal, it is heat-treated in hot air at 80°C for 10 minutes to obtain a dense selective layer.

[0042] S3-2. The fluorosulfonic acid-polyethylene glycol block copolymer obtained in Preparation Example 10 was dispersed in ethanol to form a 0.5% (w / w) solution. The solution was then spin-coated onto the surface of the dense selective layer at a spin speed of 2000 rpm for 30 seconds. The coated membrane was first dried at 60°C for 30 minutes, and then transferred to a vacuum oven at 120°C for heat treatment for 2 hours to obtain an organic solvent-resistant nanofiltration membrane. Example

[0043] The difference between Example 2 and Example 1 is that in step S1-2 of Example 2, the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 3 is replaced by the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 7. Example

[0044] The difference between Example 3 and Example 1 is that in step S1-2 of Example 3, the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 3 is replaced by the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 8. Example

[0045] The difference between Example 4 and Example 1 is that in step S1-2 of Example 4, the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 3 is replaced by the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 9. Example

[0046] The only difference between Example 5 and Example 1 is that in step S3-2 of Example 5, the fluorosulfonic acid-polyethylene glycol block copolymer prepared in Example 10 is replaced with hydroxyl-terminated fluorinated polyether silicone oil. Example

[0047] The difference between Example 6 and Example 1 is that in step S2-2 of Example 6, the 2% (R)-2,2',3,3'-tetrahydro-1,1'-spirobis[1H-indene]-7,7'-diamine hydrochloride and the 1% sodium hydroxide aqueous solution are replaced with the 2% m-phenylenediamine aqueous solution.

[0048] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that in step S1-2 of Comparative Example 1, the zwitterionic monomer mixed solution containing reactive functional groups prepared in Preparation Example 3 is replaced by the zwitterionic monomer mixed solution containing reactive functional groups prepared in Preparation Example 4.

[0049] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that in step S1-2 of Comparative Example 2, the zwitterionic monomer mixed solution containing reactive functional groups prepared in Preparation Example 3 is replaced by the zwitterionic monomer mixed solution containing reactive functional groups prepared in Preparation Example 5.

[0050] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that in step S1-2 of Comparative Example 3, the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 3 is replaced by the zwitterionic monomer mixed solution containing reactive functional groups prepared in Example 6.

[0051] The nanofiltration membranes prepared in each embodiment and comparative example were subjected to separation performance and solvent resistance stability tests in accordance with GB / T 34242-2017 "Nanofiltration Membranes Test Methods". The test procedures are as follows: 1. Water flux and retention rate test The nanofiltration membranes prepared in each embodiment and comparative example were installed in a nanofiltration membrane evaluation cell (where the effective area of ​​the membrane was 2.5 × 10⁻⁶). -3 m 2 The test was conducted at a test temperature of 25±1℃ and an operating pressure of 0.69 MPa. After 30 minutes of stable operation, the test was started, and the time t (h) required to collect 20 mL of filtrate was recorded.

[0052] The test solution used was a magnesium sulfate aqueous solution with a concentration of 2000 ppm.

[0053] The water flux F is calculated according to formula (1):

[0054] In the formula, F represents the water flux, with units of L·m -2 ·h -1 V represents the filtrate volume in liters (L); A represents the effective membrane area in square meters (m²). 2 t represents the filtration time, in hours (h).

[0055] The retention rate R is calculated according to formula (2):

[0056] In the formula, C p This indicates the magnesium ion content in the permeate, expressed in mg / L; C f This indicates the magnesium ion content in the test, expressed in mg / L.

[0057] 2. Solvent resistance stability test The nanofiltration membranes prepared in each embodiment and comparative example were immersed in pure DMF and pure NMP solvents, respectively, and stored in a sealed container at room temperature (25±2℃) for 7 days. After immersion, the membranes were removed, rinsed with pure water, and then the water flux and rejection rate of the membranes were re-measured according to the above method. The performance data after immersion were recorded.

[0058] The specific test results are recorded in Table 1: Table 1. Performance test results of nanofiltration membranes prepared in each embodiment and comparative example.

[0059] When nanofiltration membranes are soaked in highly polar organic solvents, their structures swell to varying degrees, generally resulting in a decrease in rejection rate and an increase in water flux.

[0060] Combining Examples 1-6 with Comparative Examples 1-3 and Table 1, it can be seen that Example 1 exhibits the best water flux and rejection rate before immersion in organic solvents, indicating that its initial separation performance is the best. Furthermore, its performance changes after immersion in organic solvents are minimal, demonstrating optimal solvent resistance stability. This is likely because Example 1, due to the synergistic effect of the ternary functional groups in the intermediate buffer layer, the combined effect of the rigid twisted structure selective layer, and the fluorinated block copolymer protective layer, effectively suppresses membrane swelling and interfacial delamination. Therefore, even after immersion in DMF and NMP for 7 days, it maintains a high rejection rate and stable flux, significantly superior to the other examples and comparative examples.

[0061] Specifically, combining Examples 1 and 2-4, Comparative Examples 1-3, and Table 1, we can conclude that: Examples 2-4 used a buffer layer monomer ratio outside the optimal range. Although the initial performance was good, after immersion in a highly polar solvent, the decrease in rejection rate and the increase in flux were significantly greater than in Example 1, indicating that an imbalance in the functional group ratio of the buffer layer weakens the interfacial bonding strength and the swelling stress buffering capacity. Comparative Examples 1-3 used only a single functional group component to prepare the intermediate buffer layer, lacking the synergistic effect of amino, epoxy, and zwitterionic structures. The membrane layer was prone to interfacial bonding failure and structural swelling and cracking in the solvent, resulting in a significant decrease in rejection rate, a sharp increase in flux, and stability far inferior to Example 1. This proves that the ternary monomer ratio and multi-functional group synergistic design of the buffer layer specified in this application are key to ensuring the solvent resistance stability of the membrane structure.

[0062] Combining Examples 1 and 5, and referring to Table 1, it can be concluded that: Example 5, using ordinary fluorinated polyether silicone oil instead of the fluorinated sulfonic acid-polyethylene glycol block copolymer of this application as the top protective layer, exhibits slightly lower initial performance than Example 1, and its performance degradation is more pronounced after immersion in DMF and NMP. This is because ordinary fluorinated materials are only physically bonded to the dense selective layer, lacking the chemical anchoring effect brought by the zwitterionic structure. Under long-term immersion and rinsing in organic solvents, local defects are prone to occur, failing to effectively protect the structural stability of the selective layer. This fully demonstrates that the fluorinated amphiphilic block copolymer protective layer used in this application has irreplaceable advantages in improving the erosion resistance and interfacial bonding stability of the membrane surface.

[0063] Combining Examples 1 and 6, and referring to Table 1, we can conclude that: Example 6, using conventional m-phenylenediamine as the interfacial polymerization diamine monomer and not using a diamine with a rigid twisted structure, achieved a higher initial throughput. However, after immersion in a highly polar organic solvent, the rejection rate decreased significantly, while the throughput increased markedly. This is because the conventional polyamide network lacks rigidity, making it more prone to swelling and chain segment relaxation in highly polar solvents, leading to larger pore sizes and reduced sieving capacity. In contrast, the rigid twisted structure in Example 1 effectively inhibits chain segment movement and improves the anti-swelling ability of the selective layer, thereby achieving more stable separation performance while ensuring high throughput.

[0064] This application effectively suppresses membrane swelling and interfacial peeling through the synergistic effect of the ternary functional groups in the intermediate buffer layer, the combined effect of the rigid tortuous structure selective layer and the fluorinated block copolymer protective layer, thereby enabling the organic solvent resistant nanofiltration membrane to have both excellent surface stability and separation performance under the scouring of strong polar organic solvents.

[0065] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A nanofiltration membrane resistant to organic solvents, characterized in that, The device comprises a bottom support layer, an intermediate buffer layer, a dense selective layer, and a top protective layer stacked sequentially. The surface of the bottom support layer contains active functional groups. The dense selective layer is a cross-linked polyamide network. The top protective layer is formed by curing a fluorinated amphiphilic block copolymer. The intermediate buffer layer is a zwitterionic copolymer network containing reactive functional groups, which is formed by curing a zwitterionic monomer mixture containing reactive functional groups. The reactive functional groups include amino, epoxy, and zwitterionic structures provided by sulfobetaine methacrylate units.

2. The organic solvent-resistant nanofiltration membrane according to claim 1, characterized in that, The preparation method of the zwitterionic monomer mixed solution containing reactive functional groups includes the following steps: Glycidyl methacrylate, sulfobetaine methacrylate, and aminoethyl methacrylate are dissolved in an organic solvent, and a photoinitiator is added and mixed in the dark to obtain the zwitterionic monomer mixed solution containing reactive functional groups.

3. The organic solvent-resistant nanofiltration membrane according to claim 2, characterized in that, The mass ratio of glycidyl methacrylate, sulfobetaine methacrylate, and aminoethyl methacrylate is (2-3):(1-2):

1.

4. The organic solvent-resistant nanofiltration membrane according to claim 1, characterized in that, The fluorinated amphiphilic block copolymer is a fluorosulfonic acid-polyethylene glycol block copolymer, and the preparation steps of the fluorosulfonic acid-polyethylene glycol block copolymer are as follows: S1-1, under nitrogen protection, 2-acrylamido-2-methylpropanesulfonic acid, polyethylene glycol monomethyl ether acrylate, RAFT chain transfer agent, and RAFT initiator are added to a solvent and reacted. After post-treatment, a polymer containing sulfonic acid groups and polyethylene glycol segments is obtained. S2-1 Under nitrogen protection, the polymer and fluorinated monomer obtained in step S1-1 are added to water, the pH of the reaction system is adjusted to 7-8, the RAFT initiator is dissolved in water and added to the reaction, and the post-treatment yields the fluorosulfonic acid-polyethylene glycol block copolymer.

5. The organic solvent-resistant nanofiltration membrane according to claim 1, characterized in that, The bottom support layer is a polyimide nonwoven fabric with hydroxyl groups on its surface.

6. The organic solvent-resistant nanofiltration membrane according to claim 5, characterized in that, The preparation method of the polyimide nonwoven fabric with hydroxyl groups on its surface includes the following steps: The polyimide nonwoven fabric was ultrasonically cleaned in ethanol and deionized water in sequence, dried, and then immersed in Tris-HCl buffer solution containing dopamine with a pH of 8-9 for reaction. The post-treatment yielded the polyimide nonwoven fabric with hydroxyl groups on its surface.

7. The organic solvent-resistant nanofiltration membrane according to claim 1, characterized in that, The cross-linked polyamide network is formed by interfacial polymerization of diamine monomers and acyl chloride monomers.

8. The organic solvent-resistant nanofiltration membrane according to claim 7, characterized in that, The diamine monomer comprises a diamine having a rigid twisted structure, wherein the rigid twisted structure is selected from one or more of fluorenyl carbonyl pendant groups, biphenyl groups, and spirocyclic structures.

9. The organic solvent-resistant nanofiltration membrane according to claim 7, characterized in that, The acyl chloride monomer is selected from one or more of pyromellitic methyl methacrylate (PMMA), isophthaloyl chloride (IMMA), and terephthaloyl chloride (THC).

10. A method for preparing an organic solvent-resistant nanofiltration membrane according to any one of claims 1-9, characterized in that, Includes the following steps: S1-2. A mixed solution of zwitterionic monomers containing reactive functional groups is coated onto the surface of the bottom support layer and crosslinked by ultraviolet light irradiation to form an intermediate buffer layer. S2-2. The bottom support layer with an intermediate buffer layer is subjected to interfacial polymerization: first, it is immersed in an aqueous solution containing amine monomers, then removed and immersed in an oil solution containing acyl chloride monomers, and then heat-treated to form a dense selective layer. S3-2. A fluorinated amphiphilic block copolymer is coated onto the surface of a dense selective layer and cured to form a top protective layer, thereby obtaining the organic solvent resistant nanofiltration membrane.

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