Composite nanofiltration membrane based on functionalized interlayer interface regulation and preparation method thereof

Abstract

The application relates to the technical field of water treatment separation membrane preparation, and discloses a composite nanofiltration membrane based on functionalized intermediate layer interface regulation and a preparation method thereof. The composite nanofiltration membrane comprises a reinforced substrate, a porous polymer support layer, a composite intermediate layer and a polyamide selection layer which are sequentially formed on the reinforced substrate. The porous polymer support layer is formed by phase inversion after casting solution is coated on the surface of the reinforced substrate, the porous polymer support layer contains protonated PEI amine groups, and the composite intermediate layer is formed by self-assembly of sulfonic acid groups and protonated PEI amine groups.

Description

Technical Field

[0001] This invention relates to the field of water treatment separation membrane preparation technology, and more specifically, to a composite nanofiltration membrane based on functionalized intermediate layer interface regulation and its preparation method. Background Technology

[0002] Polyamide composite nanofiltration membranes are typically prepared by interfacial polymerization of piperazine (PIP) with polyfunctional acyl chloride monomers (such as trimesoyl chloride, TMC) on the surface of a porous support layer. During interfacial polymerization, PIP migrates from the aqueous phase to the organic phase interface, and its local distribution, effective mass transfer rate, and reaction kinetics with TMC collectively influence the polyamide selective layer structure. Existing support layers exhibit variations in pore size and orifice structure, which may lead to uneven local PIP supply and increase the possibility of intrapore reactions or localized overgrowth, thereby increasing ineffective mass transfer resistance.

[0003] Existing technologies (such as CN103111196A, CN117065575A, and US12296300) disclose methods to regulate the interfacial polymerization process by adding components such as PVA, PVP, PEG, and SDS to the aqueous phase of the polyamide polymer (PIP) layer. These methods can effectively alter PIP diffusion and the structure of the polyamide selective layer (PA). However, since the regulating components are in the free aqueous phase, they approach the interfacial polymerization region along with the PIP. Interfacial polymerization occurs instantaneously, and some macromolecular additives are momentarily locked in the nascent PA layer. During subsequent alkaline washing, these macromolecules hydrolyze, their molecular chains break, or they are eluted, resulting in defects in the desalination layer and decreased alkali resistance. Patent US4859384A points out that these surfactants affect the continuity of the PA selective layer (desalination layer) and reduce its chemical stability. Layer-by-layer self-assembly technology can utilize the electrostatic interaction between polyelectrolytes to construct selective layers, but the traditional multilayer LbL (layer-by-layer self-assembly) structure has a complicated production process and increases membrane thickness and hydraulic resistance, resulting in a decrease in membrane flux. Summary of the Invention

[0004] The purpose of this invention is to provide a composite nanofiltration membrane based on functionalized intermediate layer interface regulation, which aims to avoid the problem of increased hydraulic resistance and decreased membrane flux caused by traditional multilayer LbL structures.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution: The present invention provides a composite nanofiltration membrane based on functionalized intermediate layer interface regulation. The composite nanofiltration membrane includes a reinforcing substrate and a porous polymer support layer, a composite intermediate layer and a polyamide selective layer sequentially formed on the reinforcing substrate. The porous polymer support layer is formed by phase transformation after the casting liquid is applied to the surface of the reinforcing substrate. The porous polymer support layer contains protonated PEI amine groups. The composite intermediate layer is formed by a single self-assembly of sulfonic acid groups and protonated PEI amine groups.

[0006] Furthermore, a composite nanofiltration membrane based on functionalized intermediate layer interface regulation: the reinforcing substrate is polyester nonwoven fabric or polypropylene nonwoven fabric.

[0007] Furthermore, a composite nanofiltration membrane based on functionalized intermediate layer interface regulation: the casting solution comprises the following components by mass fraction: 10.0-25.0 wt% membrane matrix polymer, 0.01-5.0 wt% PEI, 1.0-10.0 wt% pore-forming agent, and the balance casting solution solvent.

[0008] Furthermore, a composite nanofiltration membrane based on functionalized intermediate layer interface regulation: the casting solution comprises the following components by mass fraction: 16.0-19.0 wt% membrane matrix polymer, 0.1-0.8 wt% PEI, 2.0-6.0 wt% pore-forming agent, and the balance casting solution solvent.

[0009] Furthermore, a composite nanofiltration membrane based on functionalized intermediate layer interface regulation: the membrane matrix polymer is selected from one or more of polysulfone, polyethersulfone, polyphenylsulfone, and thermoplastic polyimide; the PEI is selected from one or more of branched PEI, linear PEI, or hyperbranched PEI, and the weight-average molecular weight of the PEI is 1.8–70 kDa; the pore-forming agent is selected from one of PVPK15, PVP K30, PVP K90, PEG, or inorganic salts; and the casting solution solvent is selected from one or more of DMAc, NMP, DMF, and DMSO.

[0010] Specifically, PEI has high hydrophilicity, and some PEI can migrate or leach out during phase inversion and subsequent washing. Therefore, in this invention, a portion of PEI is retained after phase inversion and washing, forming PSS-accessible amine sites on the support layer surface. To increase the number of PEI retention sites and near-surface accessible sites, the molecular weight of PEI, PEI content, membrane matrix polymer concentration, pore-forming agent ratio, air residence time, and coagulation bath conditions can be adjusted. If the PEI content is too low, there will be insufficient PSS anchoring sites; if the PEI content is too high, it will alter the phase inversion behavior and the hydraulic resistance of the support layer. Furthermore, PEI contains primary, secondary, and tertiary amine groups, which can undergo different degrees of protonation in an aqueous environment depending on the pH. PSS contains strongly acidic sulfonate groups, maintaining a negative charge over a wide pH range. When the support layer comes into contact with the PSS aqueous solution, the protonated PEI amine groups and PSS sulfonate groups undergo multi-point electrostatic pairing, fixing the PSS to the support layer surface.

[0011] Furthermore, a composite nanofiltration membrane based on functionalized intermediate layer interface regulation: the composite intermediate layer is formed by the self-assembly of an assembly solution and protonated PEI amine groups in the porous polymer support layer; wherein, the assembly solution is obtained by dissolving a water-soluble polymer containing sulfonic acid groups in water, and the assembly solution may or may not contain an electrolyte; the concentration of the water-soluble polymer containing sulfonic acid groups in the assembly solution is 0.001–2.0 wt%, and the concentration of the electrolyte in the assembly solution is 1–200 mmol / L.

[0012] Furthermore, a composite nanofiltration membrane based on functionalized intermediate layer interface regulation: the polymer is selected from polyvinyl sulfonate or sodium polystyrene sulfonate, and the electrolyte is selected from sodium chloride or potassium chloride.

[0013] This invention also provides a method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface regulation, the method comprising the following steps: S1. Preparation of casting solution: Add PEI to the casting solution solvent and stir, then add the pore-forming agent and continue stirring, and finally add the membrane matrix polymer and stir to form a homogeneous casting solution. Let it stand or degas under reduced pressure for later use. S2. Preparation of porous polymer support layer: The casting solution is continuously coated onto the reinforcing substrate to form a wet film. After air retention, it enters a water coagulation bath for phase transformation and cleaning to form a porous polymer support layer on the reinforcing substrate. S3. Preparation of composite intermediate layer: The porous polymer support layer is brought into contact with the assembly liquid. The protonated PEI amine groups in the support layer are electrostatically paired with the sulfonic acid groups in the assembly liquid at multiple points to carry out self-assembly. Then, the layer is rinsed to form a composite intermediate layer on the support layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is brought into contact with an aqueous liquid, and then excess aqueous liquid on the surface is removed before it is brought into contact with an oil liquid to carry out an interfacial polymerization reaction to form a polyamide selective layer. Excess liquid on the surface is removed, heat treatment is performed, and cleaning is carried out to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer.

[0014] Furthermore, a method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface regulation includes the following steps: S1. Preparation of casting solution: Add PEI to the casting solution solvent and stir until homogeneous. Then add the pore-forming agent and continue stirring. Finally, add the membrane matrix polymer in at least two portions and stir at 45-75°C to form a homogeneous casting solution. Let it stand or degas under reduced pressure for 8-20 hours for later use. S2. Preparation of porous polymer support layer: The casting solution is continuously coated onto the reinforcing substrate to form a wet film with a thickness of 80-300 μm. After being left in the air for 1-30 seconds, it is placed in a water coagulation bath at 20-30°C for phase transformation. Then, it is washed with flowing deionized water to form a porous polymer support layer on the reinforcing substrate. S3. Preparation of composite intermediate layer: The porous polymer support layer is contacted with the assembly solution at 15-35℃ and pH 3.5-6.5 for 30-300 seconds. The protonated PEI amine groups in the support layer are electrostatically paired with the sulfonic acid groups in the assembly solution to carry out self-assembly. After self-assembly, the layer is rinsed with deionized water 2-5 times to form a composite intermediate layer on the support layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous phase liquid for 30-120 seconds, and then the excess aqueous phase liquid on the surface is removed. Then it is contacted with an oil phase liquid for interfacial polymerization reaction for 10-60 seconds to form a polyamide selective layer. The excess liquid on the surface is removed, and then it is heat-treated at 60-90°C for 1-10 minutes. Finally, it is thoroughly washed with deionized water to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer. The aqueous phase is obtained by dissolving a polyamine monomer in water, and may or may not contain an acid scavenger or a viscosity modifier; the oil phase is obtained by dissolving a polyacrylamide monomer in an organic solvent.

[0015] Furthermore, a method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface regulation is provided, wherein the concentration of polyamine monomer in the aqueous phase is 1.0–2.5 wt%, and the concentration of acid scavenger is 0.01–0.5 wt%; the concentration of polyacrylamide chloride monomer in the oil phase is 0.05–0.5 wt%; wherein the polyamine monomer is selected from one or more of piperazine, m-phenylenediamine, or polyethyleneimine; the acid scavenger is selected from one or more of Na3PO4, Na2CO3, NaHCO3, NaOH, and KOH; the viscosity modifier is selected from one or more of PVA, PVP, PEG, and SDS; the polyacrylamide chloride monomer is selected from one or more of trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride; and the organic solvent is selected from one or more of Isopar G, Isopar H, Isopar L, n-hexane, n-heptane, and cyclohexane.

[0016] The beneficial effects of this invention are: The composite nanofiltration membrane based on functionalized intermediate layer interface regulation provided here changes the way polyelectrolyte intermediate layers are fixed. Traditional post-coated polycationic layers rely entirely on their adsorption on the surface of the finished base membrane. However, this invention pre-adds the polycationic component (PEI) to the casting solution so that it becomes part of the support layer after phase transformation. Then, the PSS is fixed by the surface-accessible cationic segments. The interface formed in this way is no longer a simple superposition of two materials. The PSS and the support layer form an ion anchoring relationship through the built-in polycations.

[0017] This invention does not follow the traditional LbL approach of increasing retention by continuously adding bilayers. In this invention, self-assembly is only used to construct a thinner composite intermediate layer (interface control layer), and the final selectivity is still undertaken by the polyamide selective layer. Therefore, it retains the characteristics of polyelectrolyte positive and negative charge pairing, self-limiting adsorption and adjustable chain conformation, while avoiding the multilayer PEM (polyelectrolyte multilayer membrane) itself becoming the main hydraulic resistance. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0019] Example 1

[0020] A composite nanofiltration membrane based on functionalized intermediate layer interface regulation is provided. The composite nanofiltration membrane includes a reinforcing substrate (polyester nonwoven fabric) and a porous polymer support layer, a composite intermediate layer and a polyamide selective layer sequentially formed on the reinforcing substrate. The porous polymer support layer is formed by phase transformation after the casting liquid is applied to the surface of the reinforcing substrate. The porous polymer support layer contains protonated PEI amine groups. The composite intermediate layer is formed by a single self-assembly of sulfonic acid groups and protonated PEI amine groups.

[0021] The preparation method of the composite nanofiltration membrane in Example 1 above includes the following specific steps: S1. Preparation of casting solution: Add 3.0g of branched PEI (weight average molecular weight 25kDa) to 777.0g of casting solution solvent (DMAc) and stir until homogeneous. Then add 40.0g of pore-forming agent (PVP K30) and continue stirring. Finally, add 180.0g of membrane matrix polymer (polysulfone) in two portions and stir at 62℃ for 8 hours to form a macroscopically homogeneous casting solution (1000.0g) with no visible precipitation and obvious gel particles. Let stand for degassing for 8 hours and set aside. S2. Preparation of porous polymer support layer: The above casting solution is continuously coated on polyester nonwoven fabric to form a wet film with a thickness of about 200 μm. After being left in the air for 10 seconds, it is placed in a water coagulation bath at 25°C to complete the phase transformation. Then, it is washed with flowing deionized water for 24 hours to remove residual solvent and PEI that is not retained by the support layer, so as to form a porous polymer support layer on polyester nonwoven fabric. S3. Preparation of the composite intermediate layer: The porous polymer support layer is contacted with an assembly solution at pH 5.0 for 120 seconds (the assembly solution is obtained by dissolving sodium polystyrene sulfonate (PSS) in water at a concentration of 0.08 wt%, and the assembly solution also contains 50 mmol / L sodium chloride). The protonated PEI amine groups in the support layer and the sulfonic acid groups in the assembly solution undergo multi-point electrostatic pairing to perform self-assembly. After self-assembly, the layer is rinsed twice with deionized water for about 30 seconds each time to remove the unstable PSS and form a composite intermediate layer on the support layer. This step forms a PEI / PSS ionic composite intermediate layer with PEI as the built-in cation anchoring point and PSS as the outer negative charge layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous phase liquid for 60 seconds, then excess aqueous phase liquid on the surface is removed, and then it is contacted with an oil phase liquid for interfacial polymerization reaction for 30 seconds to form a polyamide selective layer. Excess liquid on the surface is removed, and then heat-treated at 75°C for 5 minutes, and then thoroughly washed with deionized water to obtain a composite nanofiltration membrane based on functionalized intermediate layer interface regulation (PEI / PSS intermediate layer regulated PIP / TMC composite nanofiltration membrane). The aqueous phase is obtained by dissolving piperazine (PIP) in water with a PIP concentration of 2.0 wt%, and the aqueous phase also contains 0.1 wt% Na3PO4; the oil phase is obtained by dissolving trimesoyl chloride (TMC) in Isopar G with a TMC concentration of 0.15 wt%.

[0022] Specifically, the composite nanofiltration membrane of Example 1 sequentially comprises a reinforcing substrate, a porous polymer support layer, a PEI / PSS ion-composite intermediate layer, and a polyamide selective layer. PEI is added to the casting solution during the support layer preparation stage. After phase inversion and water washing, at least a portion of the PEI remains within, on the surface of, and / or near the surface of the porous polymer support layer. The porous polymer support layer is then contacted with the assembly solution (PSS aqueous solution), allowing PSS sulfonate groups to form ion pairings with accessible PEI amine groups. The outer layer of PSS forms a negatively charged termination interface and simultaneously undertakes PIP adsorption, transient retention, in-plane redistribution, and controlled release during subsequent PIP aqueous phase contact. Finally, PIP and TMC form a PA selective layer above this intermediate layer.

[0023] After PEI / PSS assembly, some PSS sulfonate groups form ion pairs with protonated PEI amine groups, while other sulfonate groups located in chain segments, rings, or regions with incomplete charge compensation face outwards towards the aqueous phase. PIP exists in a certain proportion of its protonated form in the aqueous phase, and its positively charged piperazine-onium sites can interact with these exposed SO3 groups. - Reversible electrostatic attraction occurs. As PIP migrates from the aqueous phase of the support layer to the organic phase reaction interface, it continuously transitions between free and transient bound states, undergoing diffusion-adsorption-dissociation-re-diffusion. This reverse effect, generated by a fixed negative potential point, can be considered as interfacial electrostatic dragging of the PIP. The result is an increased average migration time for the PIP across the intermediate layer, reducing the apparent diffusion rate and instantaneous flux of the PIP reaching the TMC reaction zone. Insufficient electrostatic dragging leads to inadequate control, while excessive dragging may result in insufficient PIP supply and underreaction of PA. The PEI content, PSS loading, and assembly conditions can control the electrostatic dragging balance.

[0024] After the fixed PSS layer is applied over different pores, the local peak of PIP, which was originally dominated by the size of the support pores, will first pass through the same charged mass transfer interface. PIP undergoes transient adsorption and dissociation at this interface, buffering the instantaneous high flux at the large pores. At the same time, PIP can undergo a certain degree of in-plane redistribution along the hydrated polyelectrolyte network. The composite intermediate layer can also reduce PIP deep pore intrusion and intrapore polymerization, allowing the PA layer to form more concentratedly on the surface of the support layer.

[0025] Specifically, in Example 1, SO3 was fixed on the surface of the composite intermediate layer. -The reversible electrostatic dragging of protonated PIPs, unlike the method of slowing down the movement of all components by increasing the viscosity of the aqueous phase, exhibits significant interfacial localization and charge selectivity: PIPs undergo transient adsorption-dissociation at the interlayer, reducing the effective diffusion rate to the TMC reaction zone, thereby limiting excessive PA growth and localized thickening. The combined effect of electrostatic dragging and the pore coverage of the composite interlayer simultaneously reduces the instantaneous peak flux of PIPs, decreases deep-pore intrusion and ineffective polymerization within pores, reduces excessively thick PAs that do not participate in effective sieving, and reduces PAs within pores, shortening the effective water mass transfer path and increasing flux. In this embodiment, the polycationic layer and the base membrane phase conversion process are combined, requiring only one additional PSS contact treatment after film formation. Compared to multiple LbL bilayers, it is easier to integrate into continuous roll-to-roll film fabrication processes, demonstrating good process compatibility.

[0026] Specifically, Example 1 does not use a multilayer polyelectrolyte membrane as a selective layer, but instead utilizes a thin polyelectrolyte interlayer to regulate interfacial polymerization mass transfer. This application does not address the known problem of "whether PIP diffusion can be slowed down," but rather how to achieve similar or even more direct mass transfer regulation using a fixed, stable, and thickness-controlled composite interlayer without adding diffusion-regulating polymers or surfactants to the aqueous PIP phase. To this end, Example 1 moves the LbL polycationic layer forward to the support layer phase transition stage, and then forms a fixed negatively charged interface through a single PSS anti-charge self-assembly. The PSS does not exist as a free additive in the aqueous PIP phase, but is fixed as an interlayer on the necessary path for PIP migration from the support layer to the organic phase. The sulfonate groups on the fixed PSS surface generate a reversible electrostatic attraction to the partially protonated PIP, which can be understood as electrostatic dragging of the PIP towards the oil phase interface: during the outward diffusion process, the PIP continuously undergoes adsorption, dissociation, and re-diffusion, making its effective migration rate lower than the free diffusion rate on a conventional support membrane. Therefore, the instantaneous flux of PIP can be reduced without significantly increasing the thickness of the intermediate layer, limiting the rapid and excessive growth of PA and reducing PA in the pores. The ultimate goal is to reduce ineffective mass transfer resistance and obtain nanofiltration membranes with higher flux while maintaining PA continuity and the necessary degree of crosslinking.

[0027] Example 2

[0028] A method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface modulation is provided, comprising the following specific steps: S1. Preparation of casting solution: Add 5.0g of branched PEI (weight average molecular weight 25kDa) to 775.0g of casting solution solvent (DMAc) and stir until homogeneous. Then add 35.0g of pore-forming agent (PVP K90) and continue stirring. Finally, add 160.0g of membrane matrix polymer (polysulfone) in two portions and stir at 65℃ for 6 hours to form a macroscopically homogeneous casting solution (1000.0g) with no visible precipitation and obvious gel particles. Let stand for 10 hours to remove bubbles and set aside. S2. Preparation of porous polymer support layer: The above casting solution is continuously coated on polyester nonwoven fabric to form a wet film with a thickness of about 200 μm. After being left in the air for 10 seconds, it is placed in a water coagulation bath at 25°C to complete the phase transformation. Then, it is washed with flowing deionized water for 24 hours to achieve the formation of a porous polymer support layer on polyester nonwoven fabric. S3. Preparation of the composite intermediate layer: The porous polymer support layer is contacted with an assembly solution at pH 4.5 for 120 seconds (the PSS concentration in the assembly solution is 0.12 wt%, and the assembly solution also contains 50.0 mmol / L sodium chloride). The protonated PEI amine groups in the support layer and the sulfonic acid groups in the assembly solution undergo multi-point electrostatic pairing to perform self-assembly. After self-assembly, the layer is rinsed twice with deionized water for about 30 seconds each time to remove the unstable PSS and form a composite intermediate layer on the support layer. This step forms a PEI / PSS ionic composite intermediate layer with PEI as the built-in cation anchoring point and PSS as the outer negative charge layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous phase liquid for 60 seconds, and then the excess aqueous phase liquid on the surface is removed. Then it is contacted with an oil phase liquid for interfacial polymerization reaction for 30 seconds to form a polyamide selective layer. The excess liquid on the surface is removed, and then it is heat-treated at 75°C for 5 minutes. Then it is thoroughly washed with deionized water to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer. The aqueous phase contains 2.2 wt% PIP and 0.1 wt% Na3PO4. The oil phase is obtained by dissolving Isopar G in trimesoyl chloride (TMC) with a TMC concentration of 0.18 wt%.

[0029] Compared with Example 1, Example 2 is a composite nanofiltration membrane with higher PSS anchoring. By increasing the PEI accessibility sites and PSS coverage, Example 2 enhances the interfacial drag effect of the intermediate layer on PIP, but still maintains that the intermediate layer itself is not used as the final nanofiltration selection layer.

[0030] Example 3

[0031] A method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface modulation is provided, comprising the following specific steps: S1. Preparation of casting solution: Add 2.0g of branched PEI to 788.0g of casting solution solvent (DMAc) and stir until homogeneous. Then add 40.0g of pore-forming agent (PVP K30) and continue stirring. Finally, add 170.0g of membrane matrix polymer (polysulfone) in two portions and stir at 60℃ for 8 hours to form a macroscopically homogeneous casting solution (1000.0g) with no visible precipitation and obvious gel particles. Let stand for degassing for 9 hours and set aside. S2. Preparation of porous polymer support layer: The above casting solution is continuously coated on polyester nonwoven fabric to form a wet film with a thickness of about 150 μm. After being left in the air for 15 seconds, it is placed in a water coagulation bath at 20°C to complete the phase transformation. Then, it is thoroughly cleaned with flowing deionized water to achieve the formation of a porous polymer support layer on polyester nonwoven fabric. S3. Preparation of the composite intermediate layer: The porous polymer support layer is contacted with an assembly solution at pH 5.5 for 60 seconds (the PSS concentration in the assembly solution is 0.05wt%, and the assembly solution also contains 25.0mmol / L sodium chloride). The protonated PEI amine groups in the support layer and the sulfonic acid groups in the assembly solution undergo multi-point electrostatic pairing to perform self-assembly. After self-assembly, the layer is rinsed twice with deionized water for about 30 seconds each time to remove the unstable PSS and form a composite intermediate layer on the support layer. This step forms a PEI / PSS ionic composite intermediate layer with PEI as the built-in cation anchoring point and PSS as the outer negative charge layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous phase liquid for 60 seconds, and then the excess aqueous phase liquid on the surface is removed. Then it is contacted with an oil phase liquid for interfacial polymerization reaction for 30 seconds to form a polyamide selective layer. The excess liquid on the surface is removed, and then it is heat-treated at 70°C for 5 minutes. Then it is thoroughly cleaned with deionized water to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer. The aqueous phase contains 1.5 wt% PIP and 0.05 wt% Na3PO4. The oil phase is obtained by dissolving Isopar G in trimesoyl chloride (TMC) at a concentration of 0.12 wt%.

[0032] Compared with Example 1, Example 3 is a composite nanofiltration membrane with a lower composite intermediate layer loading. Example 3 provides a complete implementation with a lower PEI / PSS loading and a lower interfacial monomer concentration.

[0033] Example 4

[0034] A method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface modulation is provided, comprising the following specific steps: S1. Preparation of casting solution: Add 3.0g of hyperbranched PEI to 787.0g of casting solution solvent (NMP) and stir until homogeneous. Then add 40.0g of pore-forming agent (PVP K30) and continue stirring. Finally, add 170.0g of membrane matrix polymer (polyethersulfone) in two portions and stir at 70℃ for 5 hours to form a macroscopically homogeneous casting solution (1000.0g) with no visible precipitation and obvious gel particles. Let stand for degassing for 9 hours and set aside. S2. Preparation of porous polymer support layer: The above casting solution is continuously coated on polyester nonwoven fabric to form a wet film with a thickness of about 250 μm. After being left in the air for 25 seconds, it is placed in a 30°C water coagulation bath to complete the phase transformation. Then, it is thoroughly cleaned with flowing deionized water to achieve the formation of a porous polymer support layer on polyester nonwoven fabric. S3. Preparation of the composite intermediate layer: The porous polymer support layer is contacted with an assembly solution at pH 5.0 for 120 seconds (the PSS concentration in the assembly solution is 0.1 wt%, and the assembly solution also contains 50.0 mmol / L sodium chloride). The protonated PEI amine groups in the support layer and the sulfonic acid groups in the assembly solution undergo multi-point electrostatic pairing to perform self-assembly. After self-assembly, the layer is rinsed twice with deionized water for about 30 seconds each time to remove the unstable PSS and form a composite intermediate layer on the support layer. This step forms a PEI / PSS ionic composite intermediate layer with PEI as the built-in cation anchoring point and PSS as the outer negative charge layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous phase liquid for 60 seconds, and then the excess aqueous phase liquid on the surface is removed. Then it is contacted with an oil phase liquid for interfacial polymerization reaction for 30 seconds to form a polyamide selective layer. The excess liquid on the surface is removed, and then it is heat-treated at 75°C for 5 minutes. Then it is thoroughly washed with deionized water to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer. The aqueous phase contains 2.0 wt% PIP and 0.10 wt% Na3PO4. The oil phase is obtained by dissolving Isopar G in trimesoyl chloride (TMC) at a concentration of 0.15 wt%.

[0035] Example 5

[0036] A method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface modulation is provided, comprising the following specific steps: S1. Preparation of casting solution: Add 3.0g of branched PEI (weight average molecular weight 10kDa) to 767.0g of casting solution solvent (DMAc) and stir until homogeneous. Then add 50.0g of pore-forming agent (PEG) and continue stirring. Finally, add 180.0g of membrane matrix polymer (polysulfone) in two portions and stir at 50℃ for 10 hours to form a macroscopically homogeneous casting solution (1000.0g) with no visible precipitation and obvious gel particles. Let stand for degassing for 8 hours and set aside. S2. Preparation of porous polymer support layer: The above casting solution is continuously coated on polyester nonwoven fabric to form a wet film with a thickness of about 120 μm. After being left in the air for 10 seconds, it is placed in a water coagulation bath at 25°C to complete the phase transformation. Then, it is thoroughly cleaned with flowing deionized water to achieve the formation of a porous polymer support layer on polyester nonwoven fabric. S3. Preparation of the composite intermediate layer: The porous polymer support layer is contacted with an assembly solution at pH 5.0 for 120 seconds (the PSS concentration in the assembly solution is 0.08 wt%, and the assembly solution also contains 50.0 mmol / L sodium chloride). The protonated PEI amine groups in the support layer and the sulfonic acid groups in the assembly solution undergo multi-point electrostatic pairing to perform self-assembly. After self-assembly, the layer is rinsed twice with deionized water for about 30 seconds each time to remove the unstable PSS and form a composite intermediate layer on the support layer. This step forms a PEI / PSS ionic composite intermediate layer with PEI as the built-in cation anchoring point and PSS as the outer negative charge layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous phase liquid for 90 seconds, and then the excess aqueous phase liquid on the surface is removed. Then it is contacted with an oil phase liquid for interfacial polymerization reaction for 35 seconds to form a polyamide selective layer. The excess liquid on the surface is removed, and then it is heat-treated at 75°C for 5 minutes. Finally, it is thoroughly washed with deionized water to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer. The aqueous phase contains 2.0 wt% PIP and 0.10 wt% Na3PO4. The oil phase is obtained by dissolving Isopar G in trimesoyl chloride (TMC) at a concentration of 0.15 wt%.

[0037] Comparative Example 1

[0038] The difference between Comparative Example 1 and Example 1 is that the support layer in Comparative Example 1 was prepared in the same manner as in Example 1, but branched PEI was not added to the casting solution, and the casting solution solvent (DMAc) was increased to 780.0 g. After cleaning, the porous polymer support layer was not treated with PSS, and interfacial polymerization was carried out directly using the same aqueous and oil phases as in Example 1. That is, Comparative Example 1 had no branched PEI and no PSS, and the other conditions were the same as in Example 1.

[0039] Comparative Example 2

[0040] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 does not perform PSS self-assembly on the porous polymer support layer; instead, it directly uses the same interfacial polymerization conditions as Example 1 to form the PA layer. Comparative Example 2 is used to eliminate the influence of PEI on the pore structure and hydrophilicity of the porous polymer support layer itself.

[0041] Comparative Example 3

[0042] Comparative Example 3 first prepared a conventional support layer (without PEI) as in Comparative Example 1. Then, the PEI-free support layer was contacted with a 0.10 wt% PEI aqueous solution for 120 seconds, rinsed, and then contacted with the assembly solution for 120 seconds (the assembly solution was the same as in Example 1). Next, the PA layer was formed using the same interfacial polymerization conditions as in Example 1. Comparative Example 3 involved the post-coating formation of a PEI / PSS intermediate layer. Comparative Example 3 was used to compare the structural and stability differences between the complete PEI first layer after film formation and the PEI first layer embedded during the casting stage.

[0043] Comparative Example 4

[0044] Comparative Example 4 used the same method as Comparative Example 1 to prepare a conventional support layer (without PEI), without pre-forming a PSS intermediate layer. Instead, 0.08 wt% PSS was added to the same PIP aqueous phase as in Example 1, followed by interfacial polymerization in the same oil phase as in Example 1. In Comparative Example 4, PSS was used as a PIP aqueous phase additive to distinguish between two PIP diffusion regulation methods: a fixed PSS intermediate layer and a free PSS aqueous phase additive.

[0045] Comparative Example 5

[0046] The difference between Comparative Example 5 and Example 1 is that Comparative Example 5 uses the method of Example 1 to prepare a PEI-containing support layer, and then first treats it with a 0.08 wt% PSS assembly solution, followed by alternating assembly with a 0.10 wt% PEI aqueous solution and a 0.08 wt% PSS assembly solution (multiple bilayers LbL), so that the intermediate layer has more than one complete polyelectrolyte bilayer, and finally performs interfacial polymerization as in Example 1. Comparative Example 5 is used to illustrate that the present invention does not rely on the accumulation of multiple PEI / PSS layers to form the final selective layer.

[0047] test: (1) The performance of the membranes in Examples 1-4 and Comparative Examples 1-5 was tested. The performance evaluation was conducted under the conditions of 2000 mg / L salt solution, 25℃, and 0.48 MPa (approximately 70 psi). After the membranes were pre-compressed and stabilized, the water flux and salt rejection rate were recorded. The water permeability coefficient was expressed as LMH / bar; the PA layer thickness was characterized by cross-section and the average value of multiple locations was taken; the test results are shown in Table 1 below: Table 1 shows the performance test results of the membranes in Examples 1-4 and Comparative Examples 1-5.

[0048] As can be seen from the test results in Table 1, the PA layer formed in Comparative Example 2 is thicker, with a water permeability parameter of 6.9 LMH / bar. This indicates that adding PEI alone can improve the hydrophilicity of the base membrane, but has limited effect on the mass transfer regulation of PIP, thus resulting in a small increase in flux.

[0049] As can be seen from the results of Example 1 and Comparative Example 1, after the formation of the PEI / PSS composite intermediate layer in Example 1, the effective diffusion of PIP was restricted, the PA layer thickness decreased from about 92.0 nm to about 60.0 nm, the water permeability coefficient increased to about 11.0 LMH / bar, and the MgSO4 and Na2SO4 retention capacity were slightly improved. This indicates that the flux improvement mainly comes from reducing PA overgrowth rather than significantly sacrificing the integrity of the selective layer.

[0050] The test results from Examples 2 and 3 show that the drag effect is insufficient at lower PSS loads, resulting in limited improvement in flux and selectivity; MgSO4 rejection is further improved at higher PSS loads, but water flux decreases slightly, indicating the existence of a suitable intermediate layer load range; the medium load scheme of Example 1 has a better balance between flux and divalent salt rejection.

[0051] The test results of Comparative Example 5 show that although the multi-layer PEI / PSS can improve the retention of divalent salts, the added resistance of the intermediate layer reduces the water permeability coefficient and increases the NaCl retention, which is not conducive to nanofiltration applications that require the separation of divalent / monovalent salts.

[0052] In Comparative Example 4, PSS was directly added to the aqueous phase of PIP, resulting in a decrease in both flux and divalent salt rejection. In contrast, the fixed PSS intermediate layer in Example 1 was able to increase flux while maintaining more stable divalent salt rejection.

[0053] (2) Alkali washing stability evaluation tests were conducted on membranes prepared by two methods: direct addition of PSS to the aqueous phase and fixation of PSS using an intermediate layer. The membranes of Examples 1, 2, and Comparative Example 4 were washed with a 4.0 wt% sodium hydroxide alkaline solution at 40°C for 2 hours. The retention rate of the membranes before and after washing was then evaluated, and the chemical stability was assessed. The specific results are shown in Table 2 below: Table 2 shows the retention performance retention rate test results of Examples 1-2 and Comparative Example 4.

[0054] The test results in Table 2 show that the functional groups in the PSS intermediate layer of the present invention are anchored by PEI ions. Compared with the method of directly adding PSS to the aqueous phase in Comparative Example 4, they are less likely to migrate during post-treatment and cleaning, and therefore have a higher retention rate of alkali-resistant cleaning performance.

[0055] The above-described preferred embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of the invention. Any obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A composite nanofiltration membrane based on functionalized intermediate layer interface modulation, characterized in that, The composite nanofiltration membrane includes a reinforcing substrate and a porous polymer support layer, a composite intermediate layer and a polyamide selective layer sequentially formed on the reinforcing substrate; The porous polymer support layer is formed by phase transformation after the casting liquid is applied to the surface of the reinforcing substrate. The porous polymer support layer contains protonated PEI amine groups. The composite intermediate layer is formed by a single self-assembly of sulfonic acid groups and protonated PEI amine groups.

2. The composite nanofiltration membrane based on functionalized intermediate layer interface regulation according to claim 1, characterized in that, The reinforcing substrate is a polyester nonwoven fabric or a polypropylene nonwoven fabric.

3. The composite nanofiltration membrane based on functionalized intermediate layer interface regulation according to claim 1, characterized in that, The casting solution comprises the following components by mass fraction: 10.0–25.0 wt% of film matrix polymer, 0.01–5.0 wt% of PEI, 1.0–10.0 wt% of pore-forming agent, and the balance being the casting solution solvent.

4. The composite nanofiltration membrane based on functionalized intermediate layer interface regulation according to claim 3, characterized in that, The casting solution comprises the following components by mass fraction: The film matrix polymer is 16.0–19.0 ​​wt%, PEI is 0.1–0.8 wt%, pore-forming agent is 2.0–6.0 wt%, and the balance is casting solution solvent.

5. A composite nanofiltration membrane based on functionalized intermediate layer interface regulation according to claim 3, characterized in that, The membrane matrix polymer is selected from one or more of polysulfone, polyethersulfone, polyphenylsulfone, and thermoplastic polyimide; The PEI is selected from one or more of branched PEI, linear PEI, or hyperbranched PEI, and the weight-average molecular weight of the PEI is 1.8 to 70 kDa; the pore-forming agent is selected from one of PVP K15, PVP K30, PVP K90, PEG, or inorganic salts; the casting solution solvent is selected from one or more of DMAc, NMP, DMF, and DMSO.

6. A composite nanofiltration membrane based on functionalized intermediate layer interface modulation according to claim 1, characterized in that, The composite intermediate layer is formed by the self-assembly of the assembly liquid and the protonated PEI amine groups in the porous polymer support layer; The assembly solution is obtained by dissolving a water-soluble polymer containing sulfonic acid groups in water, and the assembly solution may or may not contain electrolytes; the concentration of the water-soluble polymer in the assembly solution is 0.001 to 2.0 wt%, and the concentration of the electrolyte in the assembly solution is 1 to 200 mmol / L.

7. A composite nanofiltration membrane based on functionalized intermediate layer interface modulation according to claim 6, characterized in that, The polymer is selected from polyvinyl sulfonate or sodium polystyrene sulfonate, and the electrolyte is selected from sodium chloride or potassium chloride.

8. A method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface regulation according to any one of claims 1 to 7, characterized in that, The method includes the following steps: S1. Preparation of casting solution: Add PEI to the casting solution solvent and stir, then add the pore-forming agent and continue stirring, and finally add the membrane matrix polymer and stir to form a homogeneous casting solution. Let it stand or degas under reduced pressure for later use. S2. Preparation of porous polymer support layer: The casting solution is continuously coated onto the reinforcing substrate to form a wet film. After air retention, it enters a water coagulation bath for phase transformation and cleaning to form a porous polymer support layer on the reinforcing substrate. S3. Preparation of composite intermediate layer: The porous polymer support layer is brought into contact with the assembly liquid. The protonated PEI amine groups in the support layer are electrostatically paired with the sulfonic acid groups in the assembly liquid at multiple points to carry out self-assembly. Then, the layer is rinsed to form a composite intermediate layer on the support layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous liquid, and then excess aqueous liquid on the surface is removed before contacting with an oil liquid to carry out an interfacial polymerization reaction to form a polyamide selective layer. Excess liquid on the surface is removed, and the membrane is heat-treated and cleaned to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer.

9. The method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface regulation according to claim 8, characterized in that, The method includes the following steps: S1. Preparation of casting solution: Add PEI to the casting solution solvent and stir until homogeneous. Then add the pore-forming agent and continue stirring. Finally, add the membrane matrix polymer in at least two portions and stir at 45-75°C to form a homogeneous casting solution. Let it stand or degas under reduced pressure for 8-20 hours for later use. S2. Preparation of porous polymer support layer: The casting solution is continuously coated onto the reinforcing substrate to form a wet film with a thickness of 80-300 μm. After being left in the air for 1-30 seconds, it is placed in a water coagulation bath at 20-30°C for phase transformation. Then, it is washed with flowing deionized water to form a porous polymer support layer on the reinforcing substrate. S3. Preparation of composite intermediate layer: The porous polymer support layer is contacted with the assembly solution at 15-35℃ and pH 3.5-6.5 for 30-300 seconds. The protonated PEI amine groups in the support layer are electrostatically paired with the sulfonic acid groups in the assembly solution to carry out self-assembly. After self-assembly, the layer is rinsed with deionized water 2-5 times to form a composite intermediate layer on the support layer. S4. Preparation of polyamide selective layer: The composite intermediate layer is contacted with an aqueous phase liquid for 30-120 seconds, and then the excess aqueous phase liquid on the surface is removed. Then it is contacted with an oil phase liquid for interfacial polymerization reaction for 10-60 seconds to form a polyamide selective layer. The excess liquid on the surface is removed, and then it is heat-treated at 60-90°C for 1-10 minutes. Finally, it is thoroughly washed with deionized water to obtain a composite nanofiltration membrane based on the interface regulation of the functionalized intermediate layer. The aqueous phase is obtained by dissolving a polyamine monomer in water, and may or may not contain an acid scavenger or a viscosity modifier; the oil phase is obtained by dissolving a polyacrylamide monomer in an organic solvent.

10. The method for preparing a composite nanofiltration membrane based on functionalized intermediate layer interface regulation according to claim 9, characterized in that, The concentration of polyamine monomer in the aqueous phase is 1.0–2.5 wt%, and the concentration of acid scavenger is 0.01–0.5 wt%; the concentration of polyacrylamide chloride monomer in the oil phase is 0.05–0.5 wt%. The polyamine monomer is selected from one or more of piperazine, m-phenylenediamine, or polyethyleneimine; the acid scavenger is selected from one or more of Na3PO4, Na2CO3, NaHCO3, NaOH, and KOH; the viscosity modifier is selected from one or more of PVA, PVP, PEG, and SDS; the polyacrylamide chloride monomer is selected from one or more of trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride; and the organic solvent is selected from one or more of Isopar G, Isopar H, Isopar L, n-hexane, n-heptane, and cyclohexane.

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