A rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with gradient crosslinking structure and a preparation method thereof

Abstract

The application belongs to the field of nanofiltration membranes, and proposes a rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with gradient crosslinking structure and a preparation method thereof, which comprises a support layer and a separation layer, the separation layer has a gradient asymmetric structure with gradually reduced crosslinking density from the interface of the support layer to the surface of the membrane, and the network topology is an interpenetrating network of rigid carbon-carbon covalent bond skeleton and flexible zwitterionic side chain. The nanofiltration membrane can be used stably for a long time in an extreme PH environment, the method adopts a gradient crosslinking density regulation process, and the crosslinking density gradient is realized from the interface of the base film to the surface of the membrane in the carbon-carbon bond acid and alkali resistant nanofiltration membrane, the high crosslinking bottom layer ensures excellent ion rejection rate and chemical corrosion resistance, and the low crosslinking surface layer provides higher water permeation channels, breaks through the trade-off limitation of "flux-rejection rate" in the traditional uniform crosslinking network, and is low in energy consumption, friendly to the environment, and easy to scale up.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration membranes, and relates to strong acid and strong alkali resistant nanofiltration membranes based on carbon-carbon covalent bonds, and particularly to a rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient crosslinking structure and its preparation method. Background Technology

[0002] Nanofiltration membranes are widely used in industrial wastewater treatment, hydrometallurgy, chemical separation, and pharmaceutical purification. However, actual operating conditions often involve extreme acid and alkaline environments (pH 0-14), and traditional polyamide composite membranes fail rapidly due to the easy hydrolysis of amide bonds. Acid and alkali resistant nanofiltration membranes are mainly divided into three categories: one is membranes based on non-amide bond chemical structures, such as sulfonated polyether ether ketone and polyurea-thiourea, but their flux or rejection rate is often insufficient; another is composite membranes based on silane coupling agents to crosslink polyvinyl alcohol, such as the composite nanofiltration membrane and its preparation method disclosed in Chinese invention patent CN105435646A, whose separation layer contains easily hydrolyzed Si-OC bonds and a large number of hydroxyl groups, limiting its long-term stability in strong alkalis; and the third is membranes based on carbon-carbon covalent bond networks.

[0003] For membranes based on carbon-carbon covalent networks, such as the acid- and alkali-resistant antifouling nanofiltration membrane and its preparation method disclosed in Chinese invention patent CN119186272A, hydrophilic carbon-carbon bond networks are formed on a polyethersulfone base membrane by UV-initiated polymerization using sulfobetaine (DMAPS) or acrylic acid as monomers, thus achieving acid and alkali resistance. However, this technical solution still has some shortcomings, specifically including: 1. The network structure is simple, consisting only of linear or slightly cross-linked hydrophilic polymers, lacking rigid framework support, and is prone to swelling after extreme acid and alkali immersion, leading to a decrease in retention rate; 2. The cross-linking density is uniform, failing to achieve regulation of the cross-linking gradient within the separation layer, making it difficult to balance high retention rate and high flux; 3. No interface functionalization treatment is performed on the base membrane, resulting in weak bonding between the separation layer and the support layer, making it prone to delamination during long-term operation; 4. Only a one-step UV method is used, limiting the precise control over the cross-linking process. Summary of the Invention

[0004] This invention addresses the technical problems of nanofiltration membranes based on carbon-carbon covalent networks by proposing a rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient cross-linked structure and its preparation method. Combining gradient temperature-controlled / oxygen-limited polymerization process with polydopamine interface anchoring process, it is beneficial to improve the long-term stability and separation efficiency of the membrane under extreme chemical environments.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The present invention provides a rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient crosslinking structure, comprising a support layer and a separation layer formed on the surface of the support layer. The separation layer is formed by in-situ free radical polymerization of a polymer liquid containing rigid crosslinking monomer A, anionic hydrophilic monomer B, and cationic hydrophilic monomer C under the action of an initiator, through gradient temperature control and oxygen limitation synergistic regulation. The separation layer has a gradient asymmetric structure in which the crosslinking density gradually decreases from the support layer interface to the membrane surface, and its network topology is an interpenetrating network in which a rigid carbon-carbon covalent backbone and flexible zwitterionic side chains interpenetrate each other. The nanofiltration membrane can be used stably for a long time in a strong acid and strong alkali environment with pH 0 to 14.

[0006] The rigid crosslinking monomer A is one or both of N,N-methylenebisacrylamide and ethylene glycol dimethacrylate, and its mass accounts for 25% to 40% of the total mass of the monomers;

[0007] The anionic hydrophilic monomer B is 2-acrylamido-2-methylpropanesulfonic acid;

[0008] The cationic hydrophilic monomer C is [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide; the mass ratio of the anionic hydrophilic monomer B to the cationic hydrophilic monomer C is (2~3):1, and the sum of their masses accounts for 50%~70% of the total mass of the monomers, which is used to graft zwitterionic brushes with strong anti-polyelectrolyte behavior onto the backbone.

[0009] Preferably, the support layer is a polyethersulfone ultrafiltration membrane pretreated with polydopamine, which is used to anchor polymeric free radicals and enhance interfacial bonding strength.

[0010] Preferably, the initiator is a redox initiation system, which comprises ammonium persulfate and sodium bisulfite, and the total amount of the two is 0.5% to 2.0% of the total mass of the monomers.

[0011] Preferably, the polymerization solution includes a chain transfer agent accounting for 0.1% to 0.5% of the total monomer mass, and the chain transfer agent is 3-mercaptopropionic acid.

[0012] A method for preparing an acid- and alkali-resistant nanofiltration membrane with a gradient cross-linked structure includes the following steps:

[0013] S1. Support layer pretreatment: The polyethersulfone ultrafiltration membrane is immersed in dopamine hydrochloride buffer solution for surface modification to form a polydopamine layer;

[0014] S2. Preparation of polymerization solution: Dissolve rigid crosslinking monomer A, anionic hydrophilic monomer B and cationic hydrophilic monomer C in deionized water, add initiator and chain transfer agent, and remove oxygen;

[0015] S3. Gradient Aqueous Phase In-situ Polymerization: Immerse the pretreated support layer in the polymerization solution and perform the following steps:

[0016] S3-1. Under nitrogen protection at 5~10℃, perform low-temperature pre-initiation by ultraviolet irradiation for 2~3 minutes;

[0017] S3-2. Heat to 35~45℃, introduce oxygen-containing gas, maintain for 30~60min, and use the oxygen inhibition effect to form a crosslinking density gradient.

[0018] S4. Post-processing: Remove the membrane, wash it with deionized water until neutral, and dry it at room temperature to obtain the finished nanofiltration membrane.

[0019] Preferably, the oxygen flow rate of the oxygen-containing gas in S3-2 is 0.1~0.5 L / min.

[0020] Preferably, the operating environment for step S3 is a spin coater or a dynamic flow reactor, and the separation layer thickness is controlled between 50 and 200 nm.

[0021] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0022] 1. The present invention provides a rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient crosslinking structure. It has a rigid-flexible interpenetrating network structure of "rigid skeleton + flexible zwitterionic side chain", which is different from the single-component carbon-carbon bond network. By introducing the rigid crosslinking agent MBAA, a three-dimensional mechanical skeleton with high crosslinking density is constructed. At the same time, a flexible zwitterionic brush is formed by copolymerizing AMPS / sulfonate betaine. The two work together to achieve a unity of high mechanical stability and high hydrophilicity, and still maintain a low swelling rate after long-term immersion in extreme acid and alkali environments.

[0023] 2. The present invention provides a method for preparing a rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient crosslinking structure. The method adopts a gradient crosslinking density control process and achieves a gradient decrease in crosslinking density from the base membrane interface to the membrane surface in the carbon-carbon bond acid and alkali resistant nanofiltration membrane through a three-step strategy of "low temperature pre-initiation - temperature-increase polymerization - trace oxygen suppression". The highly crosslinked bottom layer ensures excellent ion rejection rate and chemical corrosion resistance, while the low crosslinked surface layer provides higher water permeation channels, breaking through the trade-off limitation of "flux-rejection rate" in traditional uniform crosslinking networks.

[0024] 3. The preparation method of this invention uses polydopamine interface anchoring technology to enhance the stability of the separation layer. A PDA layer is introduced on the surface of the support layer. Its abundant catechol / amino groups not only improve the wettability of the polymerization liquid, but also serve as free radical capture / anchoring points to covalently lock the separation layer onto the base film, significantly improving the interfacial bonding strength during long-term operation.

[0025] 4. The preparation method of this invention is a green preparation process that is all-aqueous, low-temperature, and free of organic solvents. It eliminates the organic solvents required for traditional interfacial polymerization and uses a redox initiation system to achieve room temperature polymerization. It has low energy consumption, is environmentally friendly, and is easy to scale up for production.

[0026] 5. The present invention provides a rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient cross-linking structure, which has excellent comprehensive performance. After soaking for 60 days under pH 0 and pH 14 conditions, the water flux retention rate is ≥95%, the Na2SO4 rejection rate is ≥96.5%, and there is no delamination phenomenon at the interface after PDA anchoring. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A comparative table showing the performance tests of nanofiltration membranes prepared in Examples 1-3 and Comparative Examples 1-3. Detailed Implementation

[0029] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0030] Numerous specific details are set forth in the following description to provide a full understanding of the invention. However, the invention may be practiced in other ways than those described herein, and therefore, the invention is not limited to the specific embodiments disclosed in the following specification. The reagents and their abbreviations involved in the embodiments are: N,N-methylenebisacrylamide, MBAA; ethylene glycol dimethacrylate, EGDMA; [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide, betaine sulfonate; polydopamine, PDA; polyethersulfone, PES.

[0031] Example 1

[0032] S1. Support layer pretreatment: A polyethersulfone (PES) ultrafiltration membrane with a molecular weight cutoff of 50,000 Da (approximately 120 μm thick) was cut into 15 cm × 20 cm sheets and immersed in an aqueous solution containing 2 mg / mL dopamine hydrochloride and 10 mM Tris-HCl (pH=8.5) for surface modification. The membrane was then shaken on a shaker at room temperature for 3 hours. After removal, the membrane was rinsed three times with deionized water and dried in a 40°C oven for later use.

[0033] S2. Preparation of polymerization solution: Weigh the monomers according to the following mass ratio: 3.0g of MBAA, 5.0g of AMPS, and 2.0g of sulfonate betaine. Dissolve them in 90.0g of deionized water and stir until completely dissolved to obtain a polymerization solution with a total monomer concentration of 10wt%, wherein MBAA accounts for 30%, AMPS accounts for 50%, sulfonate betaine accounts for 20%, and B:C = 5:2. Add 0.08g of ammonium persulfate, 0.08g of sodium bisulfite, and 0.02g of 3-mercaptopropionic acid to the polymerization solution. Transfer to a three-necked flask, cool in an ice-water bath, and bubble with nitrogen gas to remove oxygen for 15min.

[0034] S3. Gradient aqueous phase in-situ polymerization: The PES membrane pretreated with PDA was fixed on a self-made polytetrafluoroethylene frame and completely immersed in the above-mentioned deoxygenated polymerization solution; low-temperature pre-initiation was carried out by irradiation with a 365nm ultraviolet lamp for 2.5 min under nitrogen protection at 8°C; then, the reaction system was rapidly heated to 40°C, the nitrogen was stopped, and air was introduced instead (oxygen flow rate of about 0.3 L / min), and the reaction was continued for 45 min, with the temperature increased to accelerate polymerization;

[0035] S4. Post-treatment: Remove the membrane and wash it three times each with deionized water, 0.5wt% dilute hydrochloric acid solution, and deionized water for 5 minutes each time to remove unreacted monomers and homopolymers and to fully protonate the zwitterionic side chains; place the membrane in the shade at room temperature for 12 hours to obtain the finished nanofiltration membrane.

[0036] Performance testing: Membrane performance was tested using a cross-flow filtration device with an effective membrane area of ​​28 cm². 2 The test temperature was 25℃, the operating pressure was 0.6MPa, the feed liquid was a 2000mg / L Na2SO4 aqueous solution, and the water flux and rejection rate were measured after pre-pressurization for 0.5h.

[0037] Initial performance: Pure water flux 58 L·m -2 ·h -1 The Na2SO4 rejection rate was 97.2%.

[0038] Acid resistance test: The membrane was immersed in a 2 mol / L H2SO4 solution at pH 0 for 60 days, then removed, cleaned, and tested. The water flux was 56 L·m. -2 ·h -1(Retention rate 96.6%), Na2SO4 rejection rate 96.8% (Retention rate 99.6%).

[0039] Alkali resistance test: The membrane was immersed in a 2 mol / L NaOH solution at pH 14 for 60 days, then removed, cleaned, and tested. The water flux was 55.5 L·m. -2 ·h -1 (Retention rate 95.7%), Na2SO4 rejection rate 96.5% (Retention rate 99.3%).

[0040] Interface observation: After acid and alkali immersion, no bubbling or delamination was observed on the membrane surface, indicating that the PDA anchoring layer effectively enhanced the interfacial bonding force.

[0041] Example 2

[0042] S1. Support layer pretreatment: Except for the PDA processing time being extended to 4 hours, the other operations are the same as in Example 1. Extending the PDA processing time can increase the density of surface anchor points.

[0043] S2. Preparation of polymerization solution: Weigh the monomers according to the following mass ratio: EGDMA 3.2g, AMPS 5g, and sulfonate betaine 1.8g, dissolve them in 90g of deionized water, and stir until completely dissolved to obtain a polymerization solution with a total concentration of 10wt%; wherein, EGDMA accounts for 32%, AMPS accounts for 50%, sulfonate betaine accounts for 18%, and B:C = 5:1.8; add 0.07g of ammonium persulfate, 0.07g of sodium bisulfite, and 0.015g of 3-mercaptopropionic acid; the deoxygenation method is the same as in Example 1;

[0044] S3, gradient aqueous phase in-situ polymerization: low temperature pre-initiation conditions: 6°C, nitrogen protection, UV irradiation for 3 min; heating polymerization conditions: heat to 38°C, introduce air (oxygen flow rate 0.2 L / min), react for 50 min, other operations are the same as in Example 1;

[0045] S4. Post-processing is the same as in Example 1.

[0046] Initial performance: Pure water flux 62 L·m -2 ·h -1 The Na2SO4 rejection rate was 96.8%.

[0047] Acid resistance test: Test conditions were the same as in Example 1 (pH 0, 60 days), with a water flux of 59.5 L·m. -2 ·h -1 (Retention rate 96.0%), Na2SO4 rejection rate 96.3% (Retention rate 99.5%).

[0048] Alkali resistance test: Test conditions were the same as in Example 1 (pH 14, 60 days), with a water flux of 58.8 L·m. -2 ·h-1 (Retention rate 94.8%), Na2SO4 rejection rate 96.1% (Retention rate 99.3%).

[0049] Interface observation: No layering.

[0050] Performance testing and interface observation show that, by adjusting the monomer ratio and process parameters, this embodiment 2 obtained a membrane with slightly higher flux and slightly lower rejection rate, demonstrating the wide adaptability of the present invention.

[0051] Example 3

[0052] S1. The pretreatment of the support layer is the same as in Example 1.

[0053] S2. Preparation of polymerization solution: Weigh the monomers according to the following mass ratio: 4g of MBAA, 4.5g of AMPS, and 1.5g of sulfonate betaine, and dissolve them in 90.0g of deionized water, with a total monomer concentration of 10wt%; wherein, MBAA accounts for 10%, AMPS accounts for 45%, and sulfonate betaine accounts for 15%, with a B:C ratio of 4:1.5; add 0.05g of ammonium persulfate, 0.05g of sodium bisulfite, and 0.01g (0.1%) of 3-mercaptopropionic acid; the deoxygenation method is the same as in Example 1.

[0054] S3, Gradient Aqueous Phase In-situ Polymerization: Low-temperature pre-initiation: 10°C, nitrogen protection, UV irradiation for 2 min. Heating polymerization: Heat to 45°C, introduce air (oxygen flow rate 0.5 L / min), react for 35 min, other operations are the same as in Example 1;

[0055] S4. Post-processing is the same as in Example 1.

[0056] Initial performance: Pure water flux 55 L·m -2 ·h -1 The Na2SO4 rejection rate was 97.5%.

[0057] Acid resistance test: Test conditions were the same as in Example 1 (pH 0, 60 days), with a water flux of 53.2 L·m. -2 ·h -1 (Retention rate 96.7%), Na2SO4 rejection rate 97.1% (Retention rate 99.6%).

[0058] Alkali resistance test: Test conditions were the same as in Example 1 (pH 14, 60 days), with a water flux of 52.5 L·m. -2 ·h -1 (Retention rate 95.5%), Na2SO4 rejection rate 97.0% (Retention rate 99.5%).

[0059] Interface observation: No layering.

[0060] Performance tests and interface observations show that Example 3, using a lower proportion of MBAA (25%) and a shorter polymerization time, still achieved excellent acid and alkali resistance and interface stability, and the retention rate was slightly higher than that of Examples 1 and 2, further verifying the technical effect of the present invention.

[0061] Comparative Example 1

[0062] Except for step S1, in which the original PES membrane is used directly and no PDA pretreatment is performed, the operation is the same as in Example 1.

[0063] Test results: Initial performance is similar (water flux 56 L·m) -2 ·h -1 The Na2SO4 rejection rate was 96.8%, but after 30 days of acid and alkali immersion, local blistering and peeling appeared on the membrane surface, the interfacial adhesion decreased significantly, and the Na2SO4 rejection rate dropped to below 85%.

[0064] Comparative Example 2

[0065] Except for step S3, which involves low-temperature pre-initiation conditions with no oxygen suppression and nitrogen protection throughout, the operation is the same as in Example 1.

[0066] Test results: The membrane separation layer has excessive cross-linking, resulting in an initial water flux of only 32 L·m. -2 ·h -1 The Na2SO4 rejection rate was 98.1%, and the flux was too low to be practical.

[0067] Comparative Example 3

[0068] Using DMAPS as the sole monomer (concentration 10wt%), benzophenone was added as a photoinitiator, and polymerization was carried out on an unmodified PES film under UV irradiation for 10 min in a gradient-free process.

[0069] Test results: Initial water flux 50 L·m -2 ·h -1 The Na2SO4 rejection rate was 94.5%; after 60 days of acid and alkali soaking, the Na2SO4 rejection rate dropped to 86.2% (retention rate 91.2%), and there was slight stratification at the interface.

[0070] like Figure 1As shown in Examples 1-3 and Comparative Example 1, the present invention employs polydopamine interface anchoring technology to enhance the stability of the separation layer. Introducing a PDA layer on the surface of the support layer helps ensure the Na2SO4 rejection rate of the nanofiltration membrane in extreme acidity and alkalinity environments, exhibits good long-term stability, and demonstrates high interfacial bonding strength. The catechol / amino groups in the PDA layer not only improve the wettability of the polymerization solution but also serve as free radical capture / anchoring points, covalently locking the separation layer to the base membrane, anchoring polymerization free radicals, and enhancing the interfacial bonding strength between the separation layer and the support layer, significantly improving the interfacial bonding strength during long-term operation.

[0071] Examples 1-3 and Comparative Examples 2 and 3 show that introducing nitrogen gas throughout the gradient aqueous phase in-situ polymerization stage leads to excessively high crosslinking of the membrane separation layer, resulting in a low initial flux of the nanofiltration membrane, which is detrimental to practical applications. The nanofiltration membranes prepared in Examples 1-3 employ a gradient crosslinking density control process. Through a three-step strategy of "low-temperature pre-initiation - heated polymerization - trace oxygen suppression," a gradient decrease in crosslinking density from the base membrane interface to the membrane surface is achieved in carbon-carbon bonded acid and alkali resistant nanofiltration membranes. The highly crosslinked bottom layer ensures excellent ion rejection and chemical corrosion resistance, while the low-crosslinked surface layer provides higher water permeation channels, overcoming the trade-off limitation of "flux-rejection rate" in traditional uniform crosslinked networks.

[0072] Unlike the single-component carbon-carbon bond network of the nanofiltration membrane in Comparative Example 3, the nanofiltration membrane separation layer network topology prepared in Examples 1-3 is an interpenetrating network in which a rigid carbon-carbon covalent backbone and flexible zwitterionic side chains interpenetrate each other, i.e., a rigid-flexible interpenetrating network structure of "rigid backbone + flexible zwitterionic side chains". This, combined with the three-dimensional mechanical backbone with high crosslinking density constructed by introducing a rigid crosslinking agent of MBAA, and the flexible zwitterionic brush formed by copolymerizing AMPS / sulfonate betaine, achieves a balance between high mechanical stability and high hydrophilicity, and maintains a low swelling rate even after long-term immersion in extreme acid and alkaline environments.

[0073] The nanofiltration membrane and its preparation method provided by this invention are simple, operate under mild conditions, and require no organic solvents, making them suitable for large-scale production. The membrane products prepared by this invention can operate stably for extended periods in harsh conditions such as extreme acidic / alkaline environments (pH 0-14), wastewater containing organic solvents, and high-temperature (≤80℃) hydrometallurgical solutions. They are widely used in water treatment and material separation processes in chemical, metallurgical, pharmaceutical, and environmental protection fields.

[0074] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A rigid-flexible interpenetrating network acid-alkali resistant nanofiltration membrane with a gradient cross-linked structure, comprising a support layer and a separation layer formed on the surface of the support layer, characterized in that, The separation layer is formed by in-situ free radical polymerization of a polymer liquid containing rigid crosslinking monomer A, anionic hydrophilic monomer B, and cationic hydrophilic monomer C under the action of an initiator, through gradient temperature control and oxygen restriction synergistic regulation. The separation layer has a gradient asymmetric structure with crosslinking density gradually decreasing from the support layer interface to the membrane surface, and its network topology is an interpenetrating network in which rigid carbon-carbon covalent backbone and flexible zwitterionic side chains interpenetrate each other. The nanofiltration membrane can be used stably for a long time in strong acid and strong alkali environments with pH 0 to 14. The rigid crosslinking monomer A is one or both of N,N-methylenebisacrylamide and ethylene glycol dimethacrylate, and its mass accounts for 25% to 40% of the total mass of the monomers; The anionic hydrophilic monomer B is 2-acrylamido-2-methylpropanesulfonic acid; The cationic hydrophilic monomer C is [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide; the mass ratio of the anionic hydrophilic monomer B to the cationic hydrophilic monomer C is (2~3):1, and the sum of their masses accounts for 50%~70% of the total mass of the monomers.

2. The rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient cross-linked structure according to claim 1, characterized in that, The support layer is a polyethersulfone ultrafiltration membrane pretreated with polydopamine, which is used to anchor polymeric free radicals and enhance interfacial bonding strength.

3. The rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient cross-linked structure according to claim 2, characterized in that, The initiator is a redox initiation system, which consists of ammonium persulfate and sodium bisulfite, and the total amount of the two is 0.5% to 2.0% of the total mass of the monomers.

4. The rigid-flexible interpenetrating network acid and alkali resistant nanofiltration membrane with a gradient cross-linked structure according to claim 3, characterized in that, The polymerization solution includes a chain transfer agent accounting for 0.1% to 0.5% of the total monomer mass, and the chain transfer agent is 3-mercaptopropionic acid.

5. A method for preparing an acid- and alkali-resistant nanofiltration membrane with a gradient cross-linked network structure as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Support layer pretreatment: The polyethersulfone ultrafiltration membrane is immersed in dopamine hydrochloride buffer solution for surface modification to form a polydopamine layer; S2. Preparation of polymerization solution: Dissolve rigid crosslinking monomer A, anionic hydrophilic monomer B and cationic hydrophilic monomer C in deionized water, add initiator and chain transfer agent, and remove oxygen; S3. Gradient Aqueous Phase In-situ Polymerization: Immerse the pretreated support layer in the polymerization solution and perform the following steps: S3-1. Under nitrogen protection at 5~10℃, perform low-temperature pre-initiation by ultraviolet irradiation for 2~3 minutes; S3-2. Heat to 35~45℃, introduce oxygen-containing gas, maintain for 30~60min, and use the oxygen inhibition effect to form a crosslinking density gradient. S4. Post-processing: Remove the membrane, wash it with deionized water until neutral, and dry it at room temperature to obtain the finished nanofiltration membrane.

6. The method for preparing a rigid-flexible interpenetrating network acid-alkali resistant nanofiltration membrane with a gradient cross-linked structure according to claim 5, characterized in that, The oxygen flow rate of the oxygen-containing gas in S3-2 is 0.1~0.5 L / min.

7. The method for preparing a rigid-flexible interpenetrating network acid-alkali resistant nanofiltration membrane with a gradient cross-linked structure according to claim 6, characterized in that, The operating environment for step S3 is a spin coater or a dynamic flow reactor, and the separation layer thickness is controlled between 50 and 200 nm.

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