Nanofiltration membrane for water softening based on a multivariate copolymer, and preparation method and application thereof

Abstract

The application discloses a kind of nanofiltration membrane based on multivariate copolymer for water softening and its preparation method and application.The application belongs to nanofiltration membrane technical field.The purpose of the application is to solve the technical problems that existing nanofiltration membrane cannot simultaneously meet high retention performance,high flux and long-term stability.The nanofiltration membrane of the application includes an ultrafiltration membrane support layer and a multivariate polymer functional layer loaded thereon,which is formed by copolymerization of imidazolium salt positive monomer,amphiphilic monomer containing sulfonic acid group or carboxylic acid group and monomer containing epoxy functional group and is cured by thermal crosslinking.The application realizes fixation of polymer chain segment and improvement of membrane layer stability on the basis of not weakening positive electric characteristics of imidazolium cation by monomer screening and combination and regulation of epoxy group content,regulates pore size structure and mass transfer resistance of separation layer simultaneously,realizes synergistic optimization of permeability and ion retention performance and can be applied to drinking water purification and water quality improvement field.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration membrane technology, specifically relating to a nanofiltration membrane for water softening based on multi-component copolymers, its preparation method, and its application. Background Technology

[0002] With economic development and improved living standards, moderate softening and deep hardness removal of drinking water have become important research directions for improving water supply quality and ensuring the safe operation of systems. Excessive hardness ions in water not only cause scaling problems, increasing equipment energy consumption and maintenance costs, but also affect the sensory taste of drinking water. Nanofiltration membranes are considered a key technology for achieving high-quality drinking water supply due to their ability to efficiently retain multivalent ions. However, traditional negatively charged nanofiltration membranes rely mainly on steric hindrance to remove hardness cations, limiting their application in efficient drinking water softening. Therefore, the preparation of positively charged nanofiltration membranes is gradually becoming an important development direction in the field of drinking water softening.

[0003] Currently, the construction of positively charged nanofiltration separation layers includes post-processing modification, functional grafting, and polymer coating crosslinking. Preparation methods based on polymer coating and in-situ crosslinking immobilization can introduce polymers containing cationic groups onto the membrane surface and utilize crosslinking reactions to enhance the stability of the functional layer, thereby adjusting the membrane surface charge properties and separation performance. However, for positively charged nanofiltration membranes, due to the random stacking of polymer segments and high crosslinking density, the resulting separation layer is generally thick, and the permeation performance still needs improvement.

[0004] Recent studies have shown that by designing polymer molecular structures to regulate the mass transfer channels and charge characteristics of nanofiltration separation layers, it is possible to achieve a synergistic improvement in membrane permeability and ion selectivity. For example, introducing a sulfobetaine structure into the polymer can maintain the positive charge of the membrane surface through electrostatic interactions, ensuring strong repulsion against hardness cations, while also utilizing the strong hydration of zwitterionic groups to impart excellent hydrophilicity and anti-fouling capabilities to the membrane surface. However, this strategy still faces two limitations: first, the positive charge of traditional quaternary ammonium salts depends on a single nitrogen center, making them susceptible to charge shielding under high ionic strength; second, the functional layer mainly relies on inter-segment physical interactions to maintain structural integrity, resulting in insufficient long-term operational stability. Therefore, obtaining nanofiltration membranes with high retention capacity, high flux, and long-term stability remains a core problem that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to solve the technical problem that existing nanofiltration membranes cannot simultaneously meet the requirements of high retention performance, high flux, and long-term stability. The invention provides a nanofiltration membrane for water softening based on a multi-component copolymer, its preparation method, and its applications.

[0006] The technical solution of the present invention is as follows: One objective of this invention is to provide a nanofiltration membrane for water softening based on a multi-component copolymer. The nanofiltration membrane includes an ultrafiltration membrane support layer and a multi-component polymer functional layer loaded thereon. The functional layer is formed by copolymerizing imidazolium salt positively charged monomers, zwitterionic monomers containing sulfonic acid groups, and epoxy monomers and then thermally crosslinking and curing them.

[0007] Further specifying, the ultrafiltration membrane material is polysulfone (PSF), polyethersulfone (PES), polyvinylidene fluoride (PVDF), or polyacrylonitrile (PAN).

[0008] Further specifying, the positively charged monomer of the imidazolium salt is at least one selected from 1-vinyl-3-ethylimidazolium bromide (VEImBr), 1-vinyl-3-methylimidazolium bromide (VMImBr), 1-vinyl-3-butylimidazolium bromide (VBImBr), 1-(4-vinylbenzyl)-3-methylimidazolium chloride (VbzMImCl), and 1-vinyl-3-(3-sulfopropyl)imidazolium inner salt (VSPI).

[0009] Further specifying, the zwitterionic monomer containing sulfonic acid groups includes at least one of the following: methacrylate oxyethyl dimethylammonium propanesulfonic acid inner salt (SBMA), 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]ammonium]propane-1-sulfonic acid inner salt (DMAPS), and 3-(1-methyl-1H-imidazol-3-onthium-3-yl)propane-1-sulfonic acid inner salt (MIMS).

[0010] Further specifying, the epoxy monomer is at least one selected from glycidyl methacrylate (GMA), glycidyl acrylate (GA), allyl glycidyl ether (AGE), and methyl 3,4-epoxycyclohexyl methacrylate (ECHMA).

[0011] Further specified, the mass ratio of the imidazolium salt positively charged monomer, the zwitterionic monomer containing sulfonic acid group, and the epoxy monomer is (3~6):1:(0.5~1.5).

[0012] A second objective of this invention is to provide a method for preparing a nanofiltration membrane for water softening based on a multi-component copolymer, the method comprising the following steps: Step 1: Immerse the ultrafiltration membrane in an ethanol solution, remove it, wash it with deionized water, and store it at 4°C; Step 2: Under an inert atmosphere, imidazolium salt positively charged monomer, zwitterionic monomer containing sulfonic acid group and epoxy monomer are added to ultrapure water and mixed evenly to obtain a blend solution. Then, an azo compound is added as an initiator to initiate the copolymerization reaction. After the reaction is completed, the obtained polymer solution is freeze-dried and ground to obtain a multi-component polymer powder. Step 3: Dissolve the multi-polymer powder in ultrapure water to form a mother liquor, then add a polyamine crosslinking agent and continue stirring until a homogeneous and transparent solution is formed; Step 4: Using the ultrafiltration membrane processed in Step 1 as the support layer, adjust the surface temperature of the support layer, use the uniform and transparent solution obtained in Step 3 as the ultrasonic spraying liquid, ultrasonically atomize and spray it evenly onto the surface of the support layer to form a continuous functional layer. Step 5: Transfer the membrane obtained in Step 4 to an oven for drying and thermal crosslinking and curing, and then wash it with deionized water to obtain a nanofiltration membrane for water softening based on a multi-component copolymer.

[0013] Further specify that the volume fraction of the ethanol solution in step 1 is 20-40%, and the soaking time is 30-60 min.

[0014] Further specifying, the mass fraction of the co-mixed solution in step 2 is 15~25 wt%.

[0015] Further specifying, the azo compound in step 2 is at least one of 2,2'-azobis(2-methylpropionamide) dihydrochloride (V-50), 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044), and 2,2'-azobis(2-methylpropanediamine) dihydrochloride.

[0016] Further specified, in step 2, the initiator concentration in the co-polymer solution is 0.2~1 wt%, the copolymerization time is 1.5~3 h, and the copolymerization temperature is 40~60℃.

[0017] Further specifying, the concentration of the mother liquor in step 3 is 7.5~10 wt%.

[0018] Further specifying, the polyamine crosslinking agent in step 3 is at least one of terminal amino polyether, ethylenediamine, diethylenetriamine, and polyethyleneimine.

[0019] Furthermore, the amino-terminated polyether is polyetheramine D230 or D400.

[0020] Further specifying, in step 3, the mass of the polyamine crosslinking agent is 0.5 to 1.2 times the mass of the epoxy monomer.

[0021] Further specifying, the surface temperature of the ultrafiltration membrane support layer in step 4 is controlled at 35~50℃.

[0022] Further specifying the ultrasonic atomization spraying parameters in step 4: ultrasonic frequency of 40~120 KHz, distance between nozzle and support layer of 5~10 cm, spraying rate of 0.2~1.0 mL / min, and number of spraying layers of 3~10 layers.

[0023] Further specified, the curing temperature in step 5 is 60~80℃, and the curing time is 1~3 h.

[0024] The third objective of this invention is to provide an application of a nanofiltration membrane for water softening based on multi-component copolymers in the fields of drinking water purification and water quality improvement.

[0025] The advantages of this invention compared to existing technologies are: This invention proposes a multi-component copolymer functional layer constructed through molecular structure design, comprising a positively charged imidazolium salt monomer, a zwitterionic monomer with sulfonic acid groups, and an epoxy group monomer. The epoxy group serves as the core for structural regulation, and through cross-linking reactions with polyamines, a stable three-dimensional network structure is constructed within the functional layer. This achieves polymer chain segment fixation and improved membrane stability without weakening the positively charged cationic properties of imidazolium. Furthermore, by adjusting the epoxy group content to alter the cross-linking density and chain segment stacking state, the pore structure and mass transfer resistance of the separation layer can be controlled, achieving synergistic optimization of permeability and ion retention performance. Specific advantages are detailed below: (1) The present invention is constructed based on multi-polymer regulation and ultrasonic spraying thermal crosslinking process. The preparation process is simple, the reaction conditions are mild, and the dependence on the pore structure of the support layer is low. The process is highly controllable, which facilitates the continuous production of large-area membrane materials and has a good industrial application foundation. (2) This invention constructs a multi-component polymer functional layer through molecular structure design, introducing imidazolium cationic structures and zwitterionic structures to synergistically regulate membrane properties. The imidazolium cationic structure possesses stable heterocyclic positively charged centers, which can impart a continuous positive charge to the membrane layer and are less susceptible to environmental ionic strength, enhancing the membrane's electrostatic repulsion of hardness cations. Simultaneously, the zwitterionic structure improves the hydration environment of the membrane surface through strong hydration, effectively inhibiting inorganic scaling and improving membrane permeability, thereby achieving a synergistic improvement in membrane permeability, selectivity, and antifouling performance. Furthermore, and more importantly, it creatively introduces epoxy active groups, which, through ring-opening reactions with polyamine crosslinking agents, construct a stable three-dimensional chemical crosslinking network within the functional layer, effectively fixing polymer segments and functional groups, reducing membrane swelling and migration in long-term aquatic environments, and improving the structural stability of the separation layer. Simultaneously, by adjusting the epoxy group content, the crosslinking network density and segment stacking state can be altered, thereby regulating the pore volume, pore structure characteristics, and water transport resistance within the separation layer, achieving synergistic optimization of membrane structural stability, permeability, and ion retention performance. (3) The present invention uses ultrasonic spraying to replace the traditional dip coating or simple surface coating method, which can realize the uniform deposition and precise thickness control of the multi-polymer functional layer on the surface of the support membrane. Combined with the subsequent thermal crosslinking process, it enhances the bonding stability between the functional layer and the substrate, which helps to avoid problems such as uneven coating distribution, local accumulation and insufficient bonding strength in the traditional coating process, and improves the repeatability and large-scale application potential of the membrane material preparation process. (4) The membrane material prepared by the present invention exhibits excellent hardness ion rejection performance, maintains a high rejection rate of more than 95% for divalent cation salts (MgCl2, CaCl2), and has a water permeability of 18.24 LMH / bar, which meets the application requirements of high-quality drinking water softening treatment. Attached Figure Description

[0026] Figure 1 Electron micrograph of the surface morphology of PSF substrate for ultrafiltration membrane; Figure 2 Here is an electron microscope image of the surface morphology of the nanofiltration membrane NF-SV in Comparative Example 1; Figure 3 Electron micrograph of the surface morphology of nanofiltration membrane NF-SVG-2 in Example 2; Figure 4 Attenuated total reflectance-Fourier infrared spectra of the ultrafiltration membrane substrate PSF and the nanofiltration membrane NF-SVG-2 of Example 2; Figure 5 The membrane surface potential diagrams are for the nanofiltration membrane NF-SV in Comparative Example 1 and the nanofiltration membrane NF-SVG in Examples 1-3; Figure 6 The graph shows the PEG rejection rates of the nanofiltration membrane NF-SV in Comparative Example 1 and the nanofiltration membrane NF-SVG in Examples 1-3. Figure 7 Bar charts showing the pure water flux of nanofiltration membrane NF-SV in Comparative Example 1 and nanofiltration membrane NF-SVG in Examples 1-3; Figure 8 Bar charts showing the rejection rates of common salts for nanofiltration membranes NF-SV (Comparative Example 1) and NF-SVG (Examples 1-3); Figure 9 The graph shows the rejection rate and flux of hardness ions for the nanofiltration membrane NF-SVG-2 in Example 2. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0029] Example 1: The preparation method of the nanofiltration membrane for water softening based on multi-component copolymers in this example is carried out according to the following steps: Step 1: Immerse a 10 kDa polysulfone (PSF) ultrafiltration membrane substrate (6 cm × 12 cm) in a 40% ethanol solution for 60 min, then remove it, wash it with deionized water, and store it in a 4°C refrigerator. Remove it again when needed. Step 2: Dissolve 0.8 g of oxyethyl dimethylammonium propanesulfonic acid inner salt (SBMA), 4 g of 1-vinyl-3-ethylimidazolium bromide (VEImBr), and 0.4 g of glycidyl methacrylate (GMA) in 25 mL of ultrapure water. After mixing evenly, a blend solution is obtained. Nitrogen gas is then introduced into the blend solution for 60 min for continuous bubbling deoxygenation. 0.125 g of 2,2'-azobis(2-methylpropionamide) dihydrochloride (V-50) is added as an initiator. The copolymerization reaction is carried out at a constant temperature of 45℃ for 3 h. After the polymerization reaction is completed, the reaction solution is added to 150.00 mL of anhydrous ethanol. Unreacted monomers and low molecular weight by-products are removed by precipitation separation. The polymer precipitate is collected and further freeze-dried and ground to obtain a multi-component polymer powder, named SVG-1. Step 3: Weigh 2.5 g of the multi-component polymer powder prepared in Step 2, add it to 25 mL of deionized water to form a mother liquor, then add 0.2 g of polyetheramine D230 crosslinking agent, and continue stirring until a homogeneous and transparent solution is formed; Step 4: Fix the polysulfone (PSF) ultrafiltration membrane treated in Step 1 in the fixture to ensure that the membrane substrate is flat and stable during the spraying process. Before spraying, check the membrane surface and adjust the substrate surface temperature to 35°C. Use the homogeneous and transparent solution obtained in Step 3 as the ultrasonic spraying liquid. Take 10 mL and add it to the liquid storage device of the spraying equipment. Use the ultrasonic spraying system to atomize and deposit the ultrasonic spraying liquid. Set the ultrasonic frequency to 80 kHz, the distance between the nozzle and the membrane surface to 8 cm, the spraying rate to 0.5 mL / min, and control the number of spraying cycles to 5 layers to make the polymer microdroplets uniformly deposited on the surface of the polysulfone (PSF) ultrafiltration membrane, forming a continuous functional layer on the surface of the support layer. Step 5: The membrane obtained by spraying in step 4 is heated at 60°C for 2 hours for thermal crosslinking and curing. The epoxy groups in the multi-component polymer undergo ring-opening crosslinking reaction with the polyetheramine crosslinking agent, which enhances the internal structure of the functional layer and the bonding stability between it and the ultrafiltration membrane substrate. After the heat treatment is completed, the membrane surface is thoroughly rinsed with deionized water and immersed in deionized water at 4°C to obtain a nanofiltration membrane for water softening based on multi-component copolymer, denoted as NF-SVG-1.

[0030] Example 2 The difference between this embodiment and Example 1 is that the amount of glycidyl methacrylate (GMA) in step 2 is adjusted from 0.4 g to 0.8 g, and the resulting nanofiltration membrane is designated as NF-SVG-2. The remaining steps and parameters are the same as in Example 1.

[0031] Example 3 The difference between this embodiment and Example 1 is that the amount of glycidyl methacrylate (GMA) in step 2 is adjusted from 0.4 g to 1.2 g, and the resulting nanofiltration membrane is designated as NF-SVG-3. The remaining steps and parameters are the same as in Example 1.

[0032] Comparative Example 1 The difference between this comparative example and the previous one is that glycidyl methacrylate (GMA) was not added in step 2, and the resulting nanofiltration membrane is designated as NF-SV. The remaining steps and parameters are the same as in Example 1.

[0033] The characterization and performance test results of the nanofiltration membrane materials in Examples 1-3 and Comparative Example 1 are as follows: To investigate the effect of epoxy group content regulation on the microstructure of nanofiltration membranes, the surface morphology of the substrate membrane (PSF), the nanofiltration membrane of Comparative Example 1 (NF-SV), and the nanofiltration membrane of Example 2 (NF-SVG-2) were characterized using scanning electron microscopy (SEM). The results are as follows: Figure 1-3 As shown, the untreated PSF substrate exhibits a typical loose and porous structure with numerous micropores randomly distributed on its surface. After functional layer composite, the micropores on the surface of the NF-SV membrane in Comparative Example 1 are completely covered, forming a dense polymer layer, but many cracks are visible. This is due to the lack of epoxy group crosslinking fixation and uneven membrane shrinkage during thermal crosslinking. In contrast, the NF-SVG-2 membrane in Example 2 has a smooth and flat surface without obvious defects, which directly proves that the introduction of epoxy groups effectively improves the film formation uniformity and structural integrity of the separation layer.

[0034] To confirm the occurrence of polymerization and crosslinking and the introduction of functional groups, the surface chemical composition of the substrate membrane (PSF) and the nanofiltration membrane (NF-SVG-2) of Example 2 were analyzed using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The results are as follows: Figure 4As shown. Compared to the PSF film, NF-SVG-2 is shown in the gray shaded area (1745 cm²) in the figure. 1 1057 cm 1 and 1057 cm 1 Both exhibited new and significant characteristic absorption peaks, which were attributed to the CN group after crosslinking of epoxy and amino groups, and the -N group in the imidazolyl monomer, respectively. + =Groups and -SO in SBMA monomers 3 The stretching vibrations of the groups demonstrate that the VEImBr, SBMA, and GMA monomers have been loaded onto the membrane surface through a copolymerization crosslinking reaction.

[0035] The surface zeta potential of the nanofiltration membrane in Comparative Example 1 and the nanofiltration membranes in Examples 1-3 was characterized using a solid surface charge measurement instrument. Figure 5 As shown, due to the introduction of cationic groups, the NF-SV membrane of Comparative Example 1 exhibits significant positive charge across the tested pH range, which is beneficial for enhancing the retention capacity of hardness cations through the Donnan repulsion effect. With the increase of the proportion of epoxy functional monomers, the membrane surface potential did not show a significant decrease, indicating that the epoxy groups mainly regulate the membrane structure by participating in the crosslinking reaction, rather than consuming or weakening the positive charge provided by the cationic units. Therefore, the epoxy crosslinking process can effectively improve the stability of the functional layer structure while maintaining the positive charge characteristics of the membrane surface.

[0036] To characterize the pore size distribution of nanofiltration membranes, a series of polyethylene glycol (PEG) solutions with molecular weights ranging from 200 to 2000 Da and a concentration of 200 mg / L were subjected to a cross-flow membrane cell with a cross-flow velocity of 0.25 m / s. Figure 6 The PEG retention test results showed that with the introduction of the epoxy monomer GMA and its gradual increase in concentration, the molecular weight cutoff (MWCO, i.e., the molecular weight corresponding to a 90% retention rate) of the membrane continuously decreased, from 1476 Da for NF-SV in Comparative Example 1 to 1257 Da for NF-SVG-1, 1169 Da for NF-SVG-2, and 1058 Da for NF-SVG-3. This continuous decrease in MWCO, from a physical structural perspective, demonstrates that the participation of epoxy groups in the cross-linking reaction increases the cross-linking density of the polymer network, restricts the free movement of chain segments, promotes a more compact internal structure of the functional layer, and reduces the pore size and distribution of the functional layer (separation layer). Therefore, adjusting the epoxy content can achieve precise control over the polymer network structure and membrane pore size, enhance the steric hindrance effect and pore size sieving effect, and is beneficial to improving the retention performance of nanofiltration membranes for hardness ions.

[0037] The pure water flux of the membrane was measured using a cross-flow membrane cell with a cross-flow velocity of 0.25 m / s. The results are as follows: Figure 7 As shown, the pure water flux of the membrane first increases and then decreases with increasing epoxy group content. This phenomenon stems from the dual influence of epoxy crosslinking on the membrane microstructure: the introduction of an appropriate amount of epoxy groups helps to form a uniform and stable crosslinked network, reducing functional layer defects and optimizing the transport path of water molecules; however, when the crosslinking density is too high, the polymer network shrinks excessively, increasing mass transfer resistance and leading to a decrease in flux. Therefore, by adjusting the epoxy group content, a balance can be achieved between structural stability and permeability performance, thereby obtaining a nanofiltration separation layer with both high stability and excellent flux.

[0038] The separation performance of the nanofiltration membrane for four typical salt solutions with a concentration of 1000 mg / L was tested in a cross-flow membrane cell at a cross-flow velocity of 0.25 m / s. Figure 8 The retention rate test results showed that the overall retention rates of different salts for each membrane sample followed the order MgCl2 > CaCl2 > Na2SO4 > NaCl, and the retention rates for divalent cations were much higher than those for anionic and monovalent salts, consistent with the separation characteristics of typical positively charged nanofiltration membranes based on electrostatic repulsion. Further analysis of the influence of membrane type revealed that as the epoxy group content increased, the cross-linked network gradually improved, the effective pore size of the membrane decreased, and the steric hindrance effect on divalent cations was enhanced, leading to a continuous increase in the retention rates of MgCl2 and CaCl2. However, while excessively high cross-linking density further improved the retention rate, it also resulted in an overly dense network, increased water transport resistance, and a decrease in permeate flux. Considering both retention performance and permeability, the NF-SVG-2 membrane was selected as the preferred choice.

[0039] The ion concentrations in the feed solution and permeate during the inorganic salt rejection test were determined using inductively coupled plasma (ICP) spectrometry, and the ion concentrations of the NF-SVG-2 membrane against Mg were calculated. 2+ and Ca 2+ The retention rates were 98.09% and 96.71%, respectively (see...). Figure 9 This directly demonstrates the membrane's excellent retention capacity for hardness ions. Simultaneously, the permeate flux measured in salt solution filtration shows that Mg... 2+ and Ca 2+ The corresponding permeability fluxes are 17.82 LMH / bar and 18.14 LMH / bar, respectively, which are basically the same as the permeability of pure water (18.24 LMH / bar) (see...). Figure 9 This indicates that while maintaining a high rejection rate, the membrane still maintains good water permeability.

[0040] The above description is merely a preferred embodiment of the present invention. These specific embodiments are different implementations based on the overall concept of the present invention, and the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A nanofiltration membrane for water softening based on a multi-component copolymer, characterized in that, The nanofiltration membrane includes an ultrafiltration membrane support layer and a multi-component polymer functional layer loaded thereon. The functional layer is formed by copolymerizing imidazolium salt positively charged monomers, zwitterionic monomers containing sulfonic acid groups, and epoxy monomers and then thermally crosslinking and curing them.

2. The nanofiltration membrane for water softening based on a multi-component copolymer according to claim 1, characterized in that, The ultrafiltration membrane material is polysulfone, polyethersulfone, polyvinylidene fluoride, or polyacrylonitrile.

3. The nanofiltration membrane for water softening based on a multi-component copolymer according to claim 1, characterized in that, The positively charged monomer of the imidazolium salt is at least one of 1-vinyl-3-ethylimidazolium bromide, 1-vinyl-3-methylimidazolium bromide, 1-vinyl-3-butylimidazolium bromide, 1-(4-vinylbenzyl)-3-methylimidazolium chloride, and 1-vinyl-3-(3-sulfopropyl)imidazolium inner salt. The zwitterionic monomer containing a sulfonic acid group includes methacrylate oxyethyl dimethylammonium propanesulfonic acid inner salt and 3-[N,N-dimethyl-[2-(2-methylprop-2-enoyloxy] The product is at least one of [ethyl]ammonium]propane-1-sulfonic acid inner salt and 3-(1-methyl-1H-imidazolium-3-onthium-3-yl)propane-1-sulfonic acid inner salt, and the epoxy monomer is at least one of glycidyl methacrylate, glycidyl acrylate, allyl glycidyl ether, and methyl methacrylate-3,4-epoxycyclohexyl methyl methacrylate. The mass ratio of the imidazolium salt positively charged monomer, the zwitterionic monomer containing sulfonic acid group and the epoxy monomer is (3~6):1:(0.5~1.5).

4. The method for preparing a nanofiltration membrane for water softening based on a multi-component copolymer according to any one of claims 1-3, characterized in that, The method described: Step 1: Immerse the ultrafiltration membrane in an ethanol solution, remove it, wash it with deionized water, and store it at 4°C; Step 2: Under an inert atmosphere, imidazolium salt positively charged monomer, zwitterionic monomer containing sulfonic acid group and epoxy monomer are added to ultrapure water and mixed evenly to obtain a blend solution. Then, an azo compound is added as an initiator to initiate the copolymerization reaction. After the reaction is completed, the obtained polymer solution is freeze-dried and ground to obtain a multi-component polymer powder. Step 3: Dissolve the multi-polymer powder in ultrapure water to form a mother liquor, then add a polyamine crosslinking agent and continue stirring until a homogeneous and transparent solution is formed; Step 4: Using the ultrafiltration membrane processed in Step 1 as the support layer, adjust the surface temperature of the support layer, use the uniform and transparent solution obtained in Step 3 as the ultrasonic spraying liquid, ultrasonically atomize and spray it evenly onto the surface of the support layer to form a continuous functional layer. Step 5: Transfer the membrane obtained in Step 4 to an oven for drying and thermal crosslinking and curing, and then wash it with deionized water to obtain a nanofiltration membrane for water softening based on a multi-component copolymer.

5. The preparation method according to claim 4, characterized in that, In step 1, the volume fraction of the ethanol solution is 20-40%, and the soaking time is 30-60 min.

6. The preparation method according to claim 4, characterized in that, In step 2, the mass fraction of the blended solution is 15-25 wt%, the azo compound is at least one of 2,2'-azobis(2-methylpropionamide) dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, and 2,2'-azobis(2-methylpropanediamine) dihydrochloride, the initiator concentration in the blended solution is 0.2-1 wt%, the copolymerization time is 1.5-3 h, and the copolymerization temperature is 40-60℃.

7. The preparation method according to claim 4, characterized in that, In step 3, the concentration of the mother liquor is 7.5~10 wt%, and the polyamine crosslinking agent is at least one of terminal amino polyether, ethylenediamine, diethylenetriamine, and polyethyleneimine. The mass of the polyamine crosslinking agent is 0.5~1.2 times the mass of the epoxy monomer.

8. The preparation method according to claim 4, characterized in that, In step 4, the surface temperature of the ultrafiltration membrane support layer is controlled at 35~50℃. The ultrasonic atomization spraying parameters are: ultrasonic frequency of 40~120 KHz, distance between nozzle and support layer of 5~10 cm, spraying rate of 0.2~1.0 mL / min, and number of spraying layers of 3~10 layers.

9. The preparation method according to claim 4, characterized in that, In step 5, the curing temperature is 60~80℃ and the curing time is 1~3 h.

10. The application of the nanofiltration membrane for water softening based on a multi-component copolymer as described in any one of claims 1-3, characterized in that, The nanofiltration membrane is used in the fields of drinking water purification and water quality improvement.

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