A method for preparing a chimeric structure positively charged nanofiltration membrane by interfacial polymerization step by step

Abstract

This invention belongs to the field of polymer material synthesis technology, specifically relating to a step-by-step controlled interfacial polymerization method for preparing a chimeric, positively charged nanofiltration membrane. This invention introduces a dual-surfactant step-by-step control strategy, using SDS pretreatment to modify the hydrophilicity of the base membrane and PEG400 bislaurate to regulate the TMC diffusion rate, synergistically optimizing the interfacial reaction environment. This results in a tighter integration between the PA layer and the base membrane in the prepared nanofiltration membrane, a significantly reduced pore size distribution deviation rate, a substantial improvement in structural stability, and a marked improvement in permeate flux and retention accuracy. The nanofiltration membrane prepared by this method can be used in membrane separation fields, such as drinking water purification, industrial water treatment, and wastewater treatment in the water treatment industry. It can also be applied in the biopharmaceutical and chemical dye industries, meeting the separation needs of different scenarios and possessing good application prospects. Furthermore, the process parameters are controllable, the operation is simple, and it is conducive to large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of polymer material synthesis technology, specifically relating to a stepwise controlled interfacial polymerization method for preparing a chimeric structure positively charged nanofiltration membrane. Background Technology

[0002] The nanofiltration layer is the core of the composite nanofiltration membrane filtration process. The essence of preparing a nanofiltration membrane is to construct nanoscale pores on the surface of a supporting material through a certain method, thereby endowing it with nanofiltration function. Interfacial polymerization technology is widely used due to its convenient, efficient, and stable technical advantages. The nanofiltration membrane prepared by this technology consists of two parts: a selective separation membrane and a base membrane. It has the characteristics of high stability, high water flux, and ease of industrial production.

[0003] Interfacial polymerization is an advanced film-forming technology that utilizes the rapid condensation of two immiscible monomers at the interface to form a polymeric thin film. As a core process for preparing high-performance composite nanofiltration membranes, this technology is widely used in the preparation of composite nanofiltration membranes, reverse osmosis membranes, microcapsules, and functional coatings due to its rapid reaction and excellent film-forming properties. It plays a crucial role in water treatment, substance separation, and sustained-release drug delivery. Composite nanofiltration membranes prepared using this technology exhibit excellent separation selectivity and stability, and have become core materials for seawater desalination and industrial wastewater treatment.

[0004] Traditional interfacial polymerization technology utilizes the condensation reaction of polyamine monomers and acyl chloride monomers dissolved in oil-water phases on the surface of microfiltration or ultrafiltration porous membranes to form a dense polyamide film with a thickness of 50-100 nm. This film is bonded to the surface of the base membrane and plays a role in selective separation. However, the polyamide layer (PA layer) prepared by traditional interfacial polymerization technology still suffers from insufficient bonding strength between the active layer and the base membrane. This defect not only affects the long-term stability of the membrane structure but also directly leads to a decrease in separation efficiency and a shortened service life, thereby increasing application costs and severely restricting the further application prospects of nanofiltration membranes in high-end separation fields. Summary of the Invention

[0005] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments.

[0006] As one aspect of the present invention, the present invention provides a method for the step-by-step controlled interfacial polymerization preparation of a chimeric structure positively charged nanofiltration membrane, which includes the following steps: Step (1): Immerse the dried nanofiltration membrane base in a 0.1~1 g / L sodium dodecyl sulfate aqueous solution for 1~5 minutes for pretreatment; Step (2): Dissolve polyethyleneimine with a molecular weight of 6000~10000 Da in water to prepare an aqueous solution with a concentration of 0.8~1.5 wt%; mix pyromellitic acid chloride, polyethylene glycol dilaurate and n-heptane to prepare an oil solution containing 0.05~0.2 wt% pyromellitic acid chloride and 0.02~0.2 g / L polyethylene glycol dilaurate. Step (3): Immerse the base film pretreated in step (1) into the aqueous solution obtained in step (2) for 3-5 minutes, take it out, and then immerse it in the oil solution obtained in step (2) for 3-5 minutes to carry out interfacial polymerization reaction; Step (4): Rinse the membrane obtained in step (3) with ethanol, and then heat-cur it at 65°C for 8-12 minutes to obtain a nanofiltration membrane with polyamide layer and base membrane intercalated.

[0007] As a preferred embodiment of the method described in this invention, the nanofiltration membrane base membrane in step (1) is selected from poly(p-phenylene terephthalamide) membrane, polysulfone membrane or polyethersulfone membrane, and the base membrane is supported by a non-woven fabric.

[0008] As a preferred embodiment of the method described in this invention, the nanofiltration membrane base membrane in step (1) is dried for 5 to 15 minutes at 20 to 25°C and 40% to 60% air humidity.

[0009] As a preferred embodiment of the method described in this invention, the polyethyleneimine in step (2) has a molecular weight of 8000 Da and a concentration of 1.0~1.2 wt%.

[0010] As a preferred embodiment of the method described in this invention, the concentration of pyromellitic chloroformyl chloride in the oil phase solution in step (2) is 0.1~0.2 wt%.

[0011] As a preferred embodiment of the method described in this invention, the concentration of polyethylene glycol dilaurate in step (2) is 0.05~0.1 g / L.

[0012] As a preferred embodiment of the method described in this invention, the rolling speed of the rubber roller in step (3) is 5 cm / s, and the number of rolling cycles is 2.

[0013] As a preferred embodiment of the method described in this invention, the thermosetting temperature in step (4) is 65°C and the time is 15 minutes.

[0014] As a preferred embodiment of the method described in this invention, when the nanofiltration membrane base is a polysulfone-based membrane or a polyethersulfone-based membrane, the base membrane is first subjected to hydrophilic modification treatment before step (1) to reduce its surface water contact angle to below 60°; the hydrophilic modification treatment method includes any one of plasma treatment, ultraviolet irradiation grafting, surface coating with a hydrophilic layer, or chemical grafting of hydrophilic monomers.

[0015] The present invention also provides a chimeric structure positively charged nanofiltration membrane prepared by the method.

[0016] The beneficial effects of this invention are as follows: Compared with traditional interfacial polymerization methods, this invention introduces a dual surfactant stepwise regulation strategy. By modifying the hydrophilicity of the base membrane through SDS pretreatment and regulating the TMC diffusion rate with PEG400 bislaurate, the interfacial reaction environment is synergistically optimized. This results in a tighter integration between the PA layer and the base membrane in the prepared nanofiltration membrane, a significantly reduced pore size distribution deviation rate, a substantial improvement in structural stability, and a marked improvement in both permeate flux and retention accuracy. The nanofiltration membrane prepared by this method can be used in membrane separation fields, such as drinking water purification, industrial water treatment, and wastewater treatment in the water treatment industry. It can also be applied in the biopharmaceutical and chemical dye industries, meeting the separation needs of different scenarios and possessing good application prospects. Furthermore, the process parameters are controllable, the operation is simple, and it is conducive to large-scale production. Detailed Implementation

[0017] To make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to specific examples.

[0018] Preparation of poly(p-phenylene terephthalamide) based membrane: A PPTA (poly(p-phenylene terephthalamide)) based membrane supported by PET nonwoven fabric was prepared according to a standardized procedure (refer to "Preparation and Characterization of Poly(p-phenylene terephthalamide) Porous Membranes", Wang Chun, 2014). The thickness of the base membrane was 300 μm.

[0019] Example 1: A step-by-step controlled interfacial polymerization method for preparing a polyamide (PA) composite nanofiltration membrane with a chimeric structure, comprising the following steps: (1) Place the poly(p-phenylene terephthalamide) (PPTA, Teijin Aramid, Netherlands, Twaron-5011) base film in a constant temperature and humidity drying oven and dry it for 15 minutes at 25 ℃ and 50% relative humidity (RH) to ensure that there is no visible moisture on the film surface; (2) Prepare 100 mL of 0.3 g / L SDS aqueous solution, and completely immerse the dried base film in the solution at room temperature (23±2℃) for 3 minutes; (3) Measure 0.5 g of polyethyleneimine (PEI, molecular weight 8000 Da, Shanghai Maclean Biochemical Technology Co., Ltd.) and dissolve it in 49.5 g of deionized water. Stir for 15 minutes until completely dissolved to obtain an aqueous solution containing 1.0 wt% PEI. Sonicate for 5 minutes. (4) Measure 100 mL of n-heptane into a beaker, add 0.1 g TMC and 0.01 g PEG400 dilaurate in sequence, stir for 10 minutes until completely dissolved, and prepare an oil phase solution containing TMC and PEG400 dilaurate. Seal and store in the dark. (5) Immerse the base film treated in step (2) in the PEI aqueous solution for 3 minutes. After taking it out, use a rubber roller to wipe the residual solution on the surface twice along the length of the base film at a uniform speed (roller speed 5 cm / s) to ensure that the liquid layer is evenly distributed. Then immediately immerse it in the oil phase solution prepared in step (4) and let it stand for 3 minutes to initiate the interfacial polymerization reaction. (6) Tilt to remove excess oil phase solution, lay the film flat on the shaping plate, and fix the edges with buckles to ensure that the film surface is flat and wrinkle-free. Transfer the composite film to the drying oven and heat cure at 65°C for 15 minutes to complete crosslinking. (7) Rinse the composite membrane treated in step (6) three times with anhydrous ethanol, 50 mL each time, for 1 minute each time; then immerse it in deionized water for storage. Repeat the above experiment multiple times and perform corresponding performance tests.

[0020] The polyamide (PA) composite nanofiltration membrane prepared in this embodiment was tested and found to have an average pore size of 0.58 nm, a pore size distribution range of 0.19–0.65 nm, a water permeation flux of 25.6 L / (m²·h·bar), a water flux of 23.04 L / (m²·h·bar) after 60 hours with a decay rate of 10%, and a water flux of 22.4 L / (m²·h·bar) after 80 hours with a decay rate of 12.5%. Under conditions of 60°C and 0.5 MPa, the rejection rate of 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ solutions was 91.2%, the rejection rate was 84.7% after 60 hours with a decay rate of 6.5%, and the rejection rate was 83.9% after 80 hours with a decay rate of 8%, demonstrating good long-term operational stability. This is attributed to the fact that after treatment with the dual-surfactant stepwise controlled interfacial polymerization process described in this invention, a stable intercalation structure is formed between the PA separation layer and the base membrane in the resulting polyamide (PA) nanofiltration membrane, and the degree of cross-linking within the PA layer is significantly improved, resulting in a denser and more uniform structure. While maintaining a high water permeation flux, this membrane exhibits a significantly improved rejection rate for the aforementioned salt solution, and its performance degradation is much slower. This fully verifies the outstanding effectiveness of the dual-surfactant strategy in constructing a structurally stable and high-performance PA separation layer.

[0021] Example 2: A stepwise controlled interfacial polymerization method for preparing a polyamide (PA) composite nanofiltration membrane with a chimeric structure is disclosed, comprising the following specific steps: (1) Place the polyethersulfone (PES) base film in a constant temperature and humidity drying oven and dry it for 10 minutes at 25°C and 50% relative humidity; (2) Prepare 100 mL of 0.6 g / L SDS aqueous solution, immerse the dried base film completely in it, and soak at room temperature (23±2℃) for 3 minutes; (3) Measure 0.5 g of polyethyleneimine (PEI, molecular weight 8000 Da) and dissolve it in 49.5 g of deionized water. Stir for 15 minutes until completely dissolved to obtain an aqueous solution containing 1.0 wt% PEI. Sonicate for 5 minutes. (4) Measure 100 mL of n-heptane into a beaker, add 0.1 g TMC and 0.01 g PEG400 dilaurate in sequence, stir for 10 minutes until completely dissolved, and prepare an oil phase solution containing TMC and PEG400 dilaurate. Seal and store in the dark. (5) Immerse the pretreated base film in the aqueous solution for 3 minutes, take it out and use a rubber roller to wipe the residual solution on the surface twice along the length of the base film at a uniform speed (roller speed 5 cm / s) to make the liquid layer evenly distributed; then immediately immerse it in the oil solution and let it stand for 3 minutes to initiate the interfacial polymerization reaction. (6) Tilt to remove excess oil phase solution, lay the film flat on the shaping plate, and fix the edges with clips to ensure that the film surface is flat and wrinkle-free. Transfer the composite film to the drying oven and heat cure at 65°C for 15 minutes to complete the crosslinking. (7) Rinse the composite membrane treated in step (6) three times with anhydrous ethanol, 50 mL each time, for 1 minute each time; then immerse it in deionized water for storage. Repeat the above experiment multiple times and perform corresponding performance tests.

[0022] After repeated experiments and performance tests, the composite nanofiltration membrane prepared in this embodiment has an average pore size of 0.52 nm, a pore size distribution range of 0.20~0.70 nm, a water permeation flux of 20.6 L / (m²·h·bar), a water flux of 18.85 L / (m²·h·bar) after 60 hours of measurement with an attenuation rate of 8.5%, and a water flux of 18.54 L / (m²·h·bar) after 80 hours of measurement with an attenuation rate of 10%. Under conditions of 60℃ and 0.5 MPa, the rejection rate of 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ solutions was 92.2%, the rejection rate was 86.2% after 60 hours of measurement with an attenuation rate of 6%, and the rejection rate was 85% after 80 hours of measurement with an attenuation rate of 7.2%.

[0023] Compared to Example 1, Example 2 significantly enhanced the hydrophilic modification of the base membrane surface (especially the microporous structure) by increasing the SDS concentration, effectively improving the uniformity of PEI dispersion in the aqueous phase, thus providing a more uniform and reactive substrate for interfacial polymerization. The resulting polyamide (PA) separation layer structure is more compact, with higher internal cross-linking and stronger interfacial intercalation with the base membrane. Thanks to this structural optimization, the modification process promotes more uniform and dense cross-linking within the separation layer, thereby enhancing the membrane's separation performance and long-term stability.

[0024] Example 3: A stepwise controlled interfacial polymerization method for preparing a polyamide (PA) composite nanofiltration membrane with a chimeric structure is disclosed, comprising the following specific steps: (1) Place the polysulfone (PSF) base film in a constant temperature and humidity drying oven and dry it for 10 minutes at 25°C and 50% relative humidity; (2) Prepare 100 mL of 0.6 g / L SDS aqueous solution, immerse the dried base film completely in it, and soak for 3 minutes at room temperature (23±2℃); (3) Measure 0.6 g of polyethyleneimine (PEI, molecular weight 8000 Da) and dissolve it in 49.4 g of deionized water. Stir for 15 minutes until completely dissolved to obtain an aqueous solution containing 1.2 wt% PEI. Sonicate for 5 minutes. (4) Measure 100 mL of n-heptane into a beaker, add 0.1 g TMC and 0.01 g PEG400 dilaurate in sequence, stir for 10 minutes until completely dissolved, and prepare an oil phase solution containing TMC and PEG400 dilaurate. Seal and store in the dark. (5) Immerse the pretreated base film in the aqueous solution for 3 minutes, take it out and use a rubber roller to wipe the residual solution on the surface twice along the length of the base film at a uniform speed (roller speed 5 cm / s) to make the liquid layer evenly distributed; then immediately immerse it in the oil solution and let it stand for 3 minutes to initiate the interfacial polymerization reaction. (6) Tilt to remove excess oil phase solution, lay the film flat on the shaping plate, and fix the edges with clips to ensure that the film surface is flat and wrinkle-free. Transfer the composite film to the drying oven and heat cure at 65°C for 15 minutes to complete the crosslinking. (7) Rinse the composite membrane treated in step (6) three times with anhydrous ethanol, 50 mL each time, for 1 minute each time; then immerse it in deionized water for storage. Repeat the above experiment multiple times and perform corresponding performance tests.

[0025] The composite nanofiltration membrane prepared in this embodiment was tested and found to have an average pore size of 0.45 nm, a pore size distribution range of 0.21–0.63 nm, and a water permeation flux of 19.8 L / (m²·h·bar). After 60 hours of measurement, the water flux was 18.51 L / (m²·h·bar) with a decay rate of 6.5%, and after 80 hours of measurement, the water flux was 18.22 L / (m²·h·bar) with a decay rate of 8%. Under conditions of 60°C and 0.5 MPa, the rejection rate of 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ solutions was 94.8%. After 60 hours of measurement, the rejection rate was 89.6% with a decay rate of 5.2%, and after 80 hours of measurement, the rejection rate was 89.1% with a decay rate of 5.7%.

[0026] In this embodiment, the diffusion and reaction kinetics of PEI and the oil-phase monomer TMC during interfacial polymerization were optimized by adjusting the concentration of the PEI aqueous phase. XPS test results showed that this adjustment significantly increased the internal crosslinking density of the polyamide (PA) separation layer and enhanced the strength of the interfacial interlocking structure between the PA layer and the base membrane. These results further confirm that the dual-surfactant stepwise control strategy can achieve precise control of the PA membrane microstructure, effectively balancing water flux while improving separation performance and structural stability. This also demonstrates the good flexibility and operability of this method in membrane structure design and performance control.

[0027] Example 4: A stepwise controlled interfacial polymerization method for preparing a polyamide (PA) composite nanofiltration membrane with a chimeric structure is disclosed, comprising the following specific steps: The poly(p-phenylene terephthalamide) (PPTA) base film was placed in a constant temperature and humidity drying oven and dried for 15 minutes at 25°C and 50% relative humidity. Prepare 100 mL of 0.5 g / L SDS aqueous solution, and completely immerse the dried base film in it. Wet at room temperature (23±2℃) for 3 minutes. Measure 0.5 g of polyethyleneimine (PEI, molecular weight 8000 Da) and dissolve it in 49.5 g of deionized water. Stir for 15 minutes until completely dissolved to obtain an aqueous solution containing 1 wt% PEI. Sonicate for 5 minutes. Measure 100 mL of n-heptane into a beaker, add 0.2 g of TMC and 0.1 g of PEG400 dilaurate in sequence, stir for 10 minutes until completely dissolved, and prepare an oil phase solution containing TMC and PEG400 dilaurate. Seal and store in the dark. The pretreated base film was immersed in the aqueous solution for 3 minutes. After being removed, the residual solution on the surface was wiped twice by rolling a rubber roller along the length of the base film at a uniform speed (roller speed 5 cm / s). Then it was immediately immersed in the oil solution and left to stand for 3 minutes to initiate the interfacial polymerization reaction. Tilt to remove excess oil phase solution, lay the membrane flat on the shaping plate, and fix the edges with clips to ensure that the membrane surface is flat and wrinkle-free. Transfer the composite membrane to a forced-air drying oven and heat-cur it at 65°C for 15 minutes to complete cross-linking. The composite membrane treated in step (6) was rinsed three times with 50 mL of anhydrous ethanol for 1 minute each time; then it was immersed in deionized water for storage. The above experiment was repeated multiple times and the corresponding performance tests were performed.

[0028] The composite nanofiltration membrane prepared in this embodiment was tested and found to have an average pore size of 0.32 nm, a pore size distribution range of 0.22–0.58 nm, a water permeation flux of 18.5 L / (m²·h·bar), a water flux of 17.67 L / (m²·h·bar) after 60 hours of measurement with a decay rate of 4.5%, and a water flux of 17.39% after 80 hours of measurement with a decay rate of 6%. Under conditions of 60°C and 0.5 MPa, the rejection rate of 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ solutions reached 98.6%. The MgCl₂ rejection rate was 95.4% after 60 hours of measurement with a decay rate of 3.2%, and the rejection rate was 94.5% after 80 hours of measurement with a decay rate of 4.1%.

[0029] The nanofiltration membrane prepared using a conventional interfacial polymerization method was tested at 60℃ and 0.5 MPa. Its water permeation flux was 15.11 L / (m²·h·bar), decreasing to 14.2 L / (m²·h·bar) after 60 hours with a decay rate of 5%, and to 14.05 L / (m²·h·bar) after 80 hours with a decay rate of 7%. The rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ were only 90% and 82%, respectively. After 60 hours, the rejection rate was 77.85% with a decay rate of 13.5%, and after 80 hours, the rejection rate was 76.5% with a decay rate of 15%. These experimental results demonstrate that, compared to membranes prepared using conventional methods, the membrane obtained in this invention maintains a higher water flux while exhibiting not only a higher initial rejection rate but also a smaller decay rate over time, resulting in significantly better long-term stability.

[0030] Building upon Example 3, this embodiment further optimizes the reactivity of the oil-phase monomers by adjusting the concentration ratio of TMC to PEG400 dilaurate in the oil phase. This adjustment strategy not only significantly increases the internal crosslinking density of the polyamide (PA) separation layer and maintains the strength of the interfacial intercalation structure between the PA layer and the base membrane, but also enhances the loading rate of surface groups, thereby further improving the retention performance for positively valence metal salts. This result further confirms that the dual-surfactant stepwise adjustment strategy can effectively control the structural morphology of the PA membrane through precise adjustment of reaction parameters, achieving simultaneous optimization of separation performance and structural stability. This strategy has high universality and flexibility in membrane material design and controllable preparation, demonstrating promising prospects for industrial application.

[0031] Compare with Example 1: Compared to Example 4, the base film described in step (1) was placed in a constant temperature and humidity drying oven and dried for 15 minutes at 25°C and 50% relative humidity, which was adjusted to 5 minutes; the rest of the preparation methods (steps 2, 3, 4, 5, 6, 7) were the same as in Example 4.

[0032] After relevant performance tests, the composite nanofiltration membrane prepared in this embodiment has an average pore size of 0.67 nm and a pore size distribution range of 0.35~0.82 nm. Under conditions of 60℃ and 0.5 MPa, its water permeation flux is 27.2 L / (m²·h·bar). After 60 hours of measurement, the water flux is 23.17 L / (m²·h·bar) with a decay rate of 14.8%. After 80 hours of measurement, the water flux is 22.17 L / (m²·h·bar) with a decay rate of 18.5%. Regarding separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ are only 58% and 60%, respectively. After 60 hours of measurement, the rejection rate for MgCl₂ is 48.37% with a decay rate of 16.6%, and after 80 hours, the rejection rate is 46.69% with a decay rate of 19.5%.

[0033] This comparative example demonstrates that drying time is a key parameter for controlling the microenvironment on the base membrane surface before interfacial polymerization. When the drying time was shortened to 5 minutes, residual moisture in the pores of the base membrane was not completely removed. This suboptimal wetted surface state, during subsequent pretreatment with SDS solution, failed to provide a uniform and highly active hydrophilic substrate for the subsequent PEI aqueous phase dispersion, thus restricting the uniform dispersion of PEI on the base membrane surface. This resulted in the subsequent interfacial polymerization reaction failing to induce the formation of a dense, highly cross-linked polyamide separation layer strongly integrated with the base membrane, ultimately leading to poor overall separation performance and long-term operational stability of the nanofiltration membrane.

[0034] Compare with Example 2: Compared to Example 4, the dried base film is not completely immersed in the SDS aqueous solution, and interfacial polymerization is carried out directly on the base film, that is, the process of step (2) is not carried out. The rest of the preparation methods (steps 1, 3, 4, 5, 6, 7) are the same as those in Example 4.

[0035] After relevant performance tests, the composite nanofiltration membrane prepared in this embodiment has an average pore size of 0.65 nm and a pore size distribution range of 0.38~0.82 nm. Under conditions of 60℃ and 0.5 MPa, its water permeation flux is 26.6 L / (m²·h·bar). After 60 hours of measurement, the water flux is 22.58 L / (m²·h·bar), with a decay rate of 15.1%. After 80 hours of measurement, the water flux is 21.60 L / (m²·h·bar). 2 The retention rate was 18.8% (·h·bar). In terms of separation performance, the retention rates for 1 g / L MgCl2 and 1 g / L Zn(Ac)2 were only 74% and 75% respectively. After 60 hours of measurement, the retention rate for MgCl2 was 58.7% with a retention rate of 15.3%, and after 80 hours of measurement, the retention rate was 55.8% with a retention rate of 18.2%.

[0036] This comparative example demonstrates that, due to the lack of SDS aqueous solution pretreatment of the base membrane, the hydrophilicity of the base membrane surface was not improved, resulting in uneven dispersion of the aqueous monomer PEI on the base membrane surface and failure to form a complete initial reaction layer. Therefore, subsequent interfacial polymerization, due to the uneven substrate conditions, significantly weakened the interfacial intercalation structure between the polyamide separation layer and the base membrane. The resulting polyamide separation layer exhibited defects such as low structural density, poor continuity, and weak intercalation with the base membrane, affecting the nanofiltration membrane's rejection rate and long-term operational stability.

[0037] Compare with Example 3: Compared to Example 4, the concentration of the aqueous monomer in step (3) was adjusted from 1 wt% PEI to 0.5 wt%, and it was used after sonication for 5 minutes. The remaining preparation steps (steps 1, 2, 4, 5, 6, and 7) were the same as in Example 4.

[0038] After relevant performance tests, the composite nanofiltration membrane prepared in this embodiment has an average pore size of 0.69 nm and a pore size distribution range of 0.41~0.83 nm. Under conditions of 60℃ and 0.5 MPa, its water permeation flux is 27.9 L / (m²·h·bar). After 60 hours of measurement, the water flux is 23.21 L / (m²·h·bar) with a decay rate of 16.8%. After 80 hours of measurement, the water flux is 22.40 L / (m²·h·bar) with a decay rate of 19.7%. Regarding separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ are only 54% and 52%, respectively. After 60 hours of measurement, the rejection rate for MgCl₂ is 37.5% with a decay rate of 16.5%, and after 80 hours, the rejection rate is 33.8% with a decay rate of 20.2%.

[0039] This comparative example demonstrates that the concentration of the aqueous monomer PEI is a key parameter determining the structural performance of the polyamide separator. An excessively low PEI concentration directly leads to poor crosslinking, structural compactness, and intercalation with the base membrane in the resulting polyamide separator. While this structural defect may result in an initial increase in flux, it inevitably leads to a decrease in retention rate and long-term operational stability.

[0040] Compare with Example 4: Compared to Example 4, the immersion time of the pretreated base film in the aqueous solution in step (5) was adjusted from 3 minutes to 1.5 minutes, while the remaining preparation steps (steps 1, 2, 3, 4, 6, and 7) remained the same as in Example 4.

[0041] After relevant performance tests, the composite nanofiltration membrane prepared in this embodiment has an average pore size of 0.71 nm and a pore size distribution range of 0.40~0.85 nm. Under conditions of 60℃ and 0.5 MPa, its water permeation flux is 28.4 L / (m²·h·bar). After 60 hours of measurement, the water flux is 24.08 L / (m²·h·bar) with a decay rate of 15.2%. After 80 hours of measurement, the water flux is 23.63 L / (m²·h·bar) with a decay rate of 16.8%. In terms of separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ are only 46% and 53%, respectively. After 60 hours of measurement, the rejection rate for MgCl₂ is 33.5% with a decay rate of 12.5%, and after 80 hours, the rejection rate is 31.8% with a decay rate of 14.2%.

[0042] This comparative example demonstrates that shortening the soaking time of the base membrane in the aqueous solution in step (5) to 1.5 minutes directly leads to a significant reduction in the adsorption amount and distribution uniformity of the aqueous monomers on the membrane surface and in the underlying pore structure. This results in incomplete interfacial polymerization, causing significant defects in the crosslinking density, intercalation morphology, and overall continuity of the polyamide separation layer. Consequently, the initial retention performance of the nanofiltration membrane is severely reduced, significantly deteriorating its retention performance and long-term operational stability. This result confirms from a process perspective that sufficient aqueous soaking time is a key parameter to ensure that the monomers fully penetrate the membrane surface and pores, thereby guaranteeing the integrity of the interfacial polymerization reaction and forming a high-performance nanofiltration membrane. This clarifies the necessity and practical value of the soaking time in this step in Example 4.

[0043] Compare with Example 5: Compared to Example 4, in step (4) when preparing the oil phase solution, only TMC and n-heptane were added, without adding PEG400 bislaurate, to obtain an oil phase solution containing only TMC, which was then sealed and stored away from light. The remaining preparation steps (steps 1, 2, 3, 5, 6, and 7) were consistent with those in Example 4.

[0044] After relevant performance tests, the composite nanofiltration membrane prepared in this embodiment has an average pore size of 0.73 nm and a pore size distribution range of 0.42~0.84 nm. Under conditions of 60℃ and 0.5 MPa, its water permeation flux is 28.8 L / (m²·h·bar). After 60 hours of measurement, the water flux is 24.05 L / (m²·h·bar) with a decay rate of 16.5%. After 80 hours of measurement, the water flux is 22.52 L / (m²·h·bar) with a decay rate of 21.8%. In terms of separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ are only 54% and 61%, respectively. After 60 hours of measurement, the rejection rate for MgCl₂ is 38.4% with a decay rate of 15.6%, and after 80 hours, the rejection rate is 34.2% with a decay rate of 19.8%.

[0045] Compared to Example 4, this comparative example did not add PEG400 bislaurate to the oil phase, resulting in significant changes in the chemical composition, crosslinking density, and interfacial interlocking structure of the polyamide separation layer. Due to the lack of PEG400 bislaurate, the monomer reactivity in the oil phase was insufficient, leading to a reduced degree of crosslinking in the interfacial polymerization. This resulted in a more porous and less uniform microstructure of the separation layer, and a weakened interfacial bond strength with the base membrane. These structural defects significantly reduced the overall separation performance of the nanofiltration membrane and exhibited poor stability under long-term hydraulic action, specifically manifested in an accelerated decline in both water flux and rejection rate.

[0046] Compare with Example 6: Compared to Example 4, the curing temperature of the heat treatment process after interfacial polymerization was adjusted. The heat curing temperature of the base film in step (6) was reduced from 65°C in Example 4 to 45°C, while the heat curing time remained at 15 minutes. The remaining preparation steps (steps 1, 2, 3, 4, 5, and 7) were consistent with those in Example 4.

[0047] After relevant performance tests, the nanofiltration membrane substrate prepared in this embodiment has an average pore size of 0.76 nm and a pore size distribution range of 0.43~0.82 nm. Under conditions of 60℃ and 0.5 MPa, its water permeation flux is 29.2 L / (m²·h·bar). After 60 hours of measurement, the water flux is 24.47 L / (m²·h·bar) with a decay rate of 16.2%. After 80 hours of measurement, the water flux is 23.48 L / (m²·h·bar) with a decay rate of 19.6%. Regarding separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ are only 58% and 54%, respectively. After 60 hours of measurement, the rejection rate for MgCl₂ is 42.5% with a decay rate of 15.5%, and after 80 hours, the rejection rate is 39.3% with a decay rate of 18.7%.

[0048] This comparative example demonstrates that reducing the thermosetting temperature from 65°C to 45°C significantly affects the post-curing crosslinking process of the polyamide release layer. At lower temperatures, the molecular chain mobility weakens, leading to a decrease in the crosslinking density of the polyamide release layer, insufficient structural curing, and a decline in overall structural stability. This structural defect directly affects the performance of the nanofiltration membrane, not only significantly reducing its initial retention capacity but also causing unstable fluctuations and a continuous decline in water flux and retention rate during operation.

[0049] Compare with Example 7: Compared to Example 4, in step (4) when preparing the oil phase solution, PEG400 bislaurate was replaced with PEG400 monolaurate of equal concentration (0.1 g / L), and the remaining preparation steps (steps 1, 2, 3, 5, 6, 7) were consistent with those in Example 4.

[0050] After relevant performance tests, the composite nanofiltration membrane prepared in this comparative example had an average pore size of 0.67 nm and a pore size distribution range of 0.39–0.82 nm. Under conditions of 60 °C and 0.5 MPa, its water permeation flux was 24.8 L / (m²·h·bar), 21.08 L / (m²·h·bar) after 60 hours of measurement (with a decay rate of 15.0%), and 20.09 L / (m²·h·bar) after 80 hours of measurement (with a decay rate of 19.0%). Regarding separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ were 74% and 72%, respectively. After 60 hours of measurement, the MgCl₂ rejection rate was 60.7% with a decay rate of 13.3%, and after 80 hours of measurement, the rejection rate was 57.6% with a decay rate of 16.4%.

[0051] This comparative example demonstrates that when PEG400 monolaurate is used instead of PEG400 bislaurate, although PEG400 monolaurate also exhibits amphiphilicity, the core difference lies in the different number of lauric acid molecules bound to the polyethylene glycol (PEG400) chain in their molecular structures. This directly leads to differences in their hydrophilic-lipophilic balance (HLB) value and physicochemical properties, making PEG400 monolaurate more inclined towards emulsification and solubilization, and unable to achieve the same diffusion control effect as in this patent during interfacial polymerization. The resulting polyamide separation layer has lower density and weaker intercalation structure, resulting in significantly inferior retention performance and long-term operational stability of the nanofiltration membrane compared to Example 4, proving the necessity of the preferred PEG400 bislaurate in this invention.

[0052] Compare with Example 8: Compared to Example 4, in step (4) when preparing the oil phase solution, PEG400 dilaurate was replaced with PEG200 dilaurate (molecular weight 200) of equal concentration (0.1 g / L), and the remaining preparation steps (steps 1, 2, 3, 5, 6, 7) were consistent with those in Example 4.

[0053] After relevant performance tests, the composite nanofiltration membrane prepared in this comparative example had an average pore size of 0.71 nm and a pore size distribution range of 0.43–0.86 nm. Under conditions of 60 °C and 0.5 MPa, its water permeation flux was 27.3 L / (m²·h·bar). After 60 hours of measurement, the water flux was 22.93 L / (m²·h·bar) with a decay rate of 16.0%. After 80 hours of measurement, the water flux was 21.57 L / (m²·h·bar) with a decay rate of 21.0%. Regarding separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ were 68% and 62%, respectively. After 60 hours of measurement, the rejection rate for MgCl₂ was 43.2% with a decay rate of 36.47%, and after 80 hours of measurement, the rejection rate was 39.8% with a decay rate of 18.2%.

[0054] This comparative example demonstrates that PEG200 dilaurate has a shorter molecular chain and a higher hydrophilic-lipophilic balance value, making it difficult to form a stable, ordered arrangement at the oil-water interface. This results in insufficient control over TMC diffusion, leading to uneven interfacial polymerization and a loose, defect-prone polyamide layer with poor intercalation with the base film. Its overall performance is far lower than that of Example 4, indicating that the preferred PEG400 dilaurate of this invention has the optimal molecular weight range.

[0055] Compare with Example 9: Compared to Example 4, in step (4) when preparing the oil phase solution, PEG400 dilaurate was replaced with PEG600 dilaurate (molecular weight 600) of equal concentration (0.1 g / L), and the remaining preparation steps (steps 1, 2, 3, 5, 6, 7) were consistent with those in Example 4.

[0056] After relevant performance tests, the composite nanofiltration membrane prepared in this comparative example had an average pore size of 0.66 nm and a pore size distribution range of 0.38–0.80 nm. Under conditions of 60 °C and 0.5 MPa, its water permeation flux was 25.2 L / (m²·h·bar). After 60 hours of measurement, the water flux was 21.67 L / (m²·h·bar) with a decay rate of 14.0%. After 80 hours of measurement, the water flux was 20.66 L / (m²·h·bar) with a decay rate of 18.0%. Regarding separation performance, the rejection rates for 1 g / L MgCl₂ and 1 g / L Zn(Ac)₂ were 76% and 74%, respectively. After 60 hours of measurement, the rejection rate for MgCl₂ was 63.1% with a decay rate of 12.9%, and after 80 hours of measurement, the rejection rate was 59.9% with a decay rate of 16.1%.

[0057] This comparative example shows that when the molecular weight of PEG dilaurate increases to 600, its viscosity increases, its diffusion rate decreases, its migration and alignment efficiency at the interface decreases, and its micro-control effect on the interfacial polymerization reaction weakens. The resulting nanofiltration membrane exhibits lower density and interlocking strength compared to Example 4. This further verifies that a molecular weight of 400 is the optimal choice for this invention.

[0058] In summary, this invention employs a dual-surfactant stepwise regulation strategy. SDS, as an anionic surfactant, is used to achieve uniform hydrophilic modification of the base membrane, providing an ideal reaction substrate for interfacial polymerization. Amphiphilic PEG400 dilaurate (acid value ≤10 (mgKOH / g), saponification value 130~155 (mgKOH / g), HLB value 10.5) is used for micro-regulation at the reaction interface, optimizing monomer diffusion and reaction kinetics, and promoting the formation of an ultrathin, dense, and defect-free polyamide separation layer. By synergistically integrating the advantages of these two surfactants into the process, SDS ensures uniformity in the initial reaction stage, while PEG400 dilaurate governs the precision of the reaction process. Together, they effectively improve the distribution and reaction behavior of monomers in the interfacial region, enhancing the interfacial bonding between the polyamide layer (PA layer) and the base membrane. This significantly improves the structural uniformity, separation accuracy, and long-term operational stability of the nanofiltration membrane, achieving a synergistic improvement in overall performance and durability.

[0059] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for stepwise controlled interfacial polymerization preparation of a chimeric structure positively charged nanofiltration membrane, characterized in that: Includes the following steps: Step (1): Immerse the dried nanofiltration membrane base in a 0.1~1 g / L sodium dodecyl sulfate aqueous solution for 1~5 minutes for pretreatment; Step (2): Dissolve polyethyleneimine with a molecular weight of 6000~10000 Da in water to prepare an aqueous solution with a concentration of 0.8~1.5wt%; mix pyromellitic acid chloride, polyethylene glycol dilaurate and n-heptane to prepare an oil solution containing 0.05~0.2wt% pyromellitic acid chloride and 0.02~0.2 g / L polyethylene glycol dilaurate. Step (3): Immerse the base film pretreated in step (1) into the aqueous solution obtained in step (2) for 3-5 minutes, take it out, and then immerse it in the oil solution obtained in step (2) for 3-5 minutes to carry out interfacial polymerization reaction; Step (4): Rinse the membrane obtained in step (3) with ethanol, and then heat-cur it at 65°C for 8-12 minutes to obtain a nanofiltration membrane with polyamide layer and base membrane intercalated.

2. The method according to claim 1, characterized in that, The nanofiltration membrane base membrane mentioned in step (1) is selected from poly(p-phenylene terephthalamide) membrane, polysulfone membrane or polyethersulfone membrane, and the base membrane is supported by non-woven fabric.

3. The method according to claim 1 or 2, characterized in that, The nanofiltration membrane base membrane described in step (1) is dried for 5 to 15 minutes at 20 to 25°C and 40% to 60% air humidity.

4. The method according to claim 1 or 2, characterized in that, The polyethyleneimine mentioned in step (2) has a molecular weight of 8000 Da and a concentration of 1.0~1.2 wt%.

5. The method according to claim 1 or 2, characterized in that, The concentration of pyromellitic chloride in the oil phase solution in step (2) is 0.1~0.2 wt%.

6. The method according to claim 1 or 2, characterized in that, The concentration of polyethylene glycol dilaurate in step (2) is 0.05~0.1 g / L.

7. The method according to claim 1 or 2, characterized in that, The rolling speed of the rubber roller in step (3) is 5 cm / s, and the number of rolling cycles is 2.

8. The method according to claim 1 or 2, characterized in that, The thermosetting temperature in step (4) is 65°C and the time is 15 minutes.

9. The method according to claim 2, characterized in that, When the nanofiltration membrane base is a polysulfone-based membrane or a polyethersulfone-based membrane, the base membrane is first subjected to hydrophilic modification treatment before step (1) to reduce the surface water contact angle to below 60°; the hydrophilic modification treatment method includes any one of plasma treatment, ultraviolet irradiation grafting, surface coating with a hydrophilic layer or chemical grafting of hydrophilic monomers.

10. The interlocking positively charged nanofiltration membrane prepared by the method according to claim 1.