A kind of intermediate layer regulated polyamide membrane prepared with non-planar rigid twist monomer and its preparation method

Abstract

This invention relates to the field of membrane separation technology, specifically to a method for preparing a polyamide membrane with an interlayer controlled by a non-planar rigid twisted monomer. The nanofiltration membrane comprises a porous base membrane and a polyester interlayer and a polyamide separation layer sequentially formed thereon. The polyester interlayer is formed by interfacial polymerization of a non-planar rigid twisted monomer and trimesoyl chloride; the polyamide separation layer is formed by interfacial polymerization of a diamine monomer and TMC on the interlayer. The preparation method involves a two-step sequential interfacial polymerization: first, a polyester interlayer is formed on the base membrane, and then, using it as a diffusion control layer, the polyamide separation layer is constructed in situ. This method finely controls the diffusion of the amine monomer through the interlayer, resulting in a thinner polyamide layer with a nano-wrinkled structure, thereby synergistically improving the membrane's permeation flux and salt rejection rate. This effectively breaks the performance trade-off effect of traditional nanofiltration membranes and exhibits excellent long-term operational stability.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a method for preparing a polyamide membrane with an intermediate layer using a non-planar rigid twisted monomer. Background Technology

[0002] Global climate change and urbanization have exacerbated the scarcity of freshwater resources. my country's abundant saline resources (including seawater and brackish water) and large-scale industrial wastewater discharge provide a strategic resource foundation for the development of desalination and reclaimed water technologies. Therefore, breakthroughs in the development of efficient desalination and wastewater reuse technologies, and the construction of a diversified water supply system, have become urgent needs to ensure national water security.

[0003] Among numerous water treatment technologies, nanofiltration membrane technology, with its highly efficient separation capability of monovalent / divalent ions based on the Donnan effect, dielectric effect, and steric hindrance effect, combined with the characteristics of low operating pressure and high water flux, demonstrates unique advantages in wastewater regeneration and resource recovery. Currently, polyamide composite nanofiltration membranes prepared based on the interfacial polycondensation of diamines and acyl chlorides have become mainstream. However, these membranes generally face the trade-off between permeability and selectivity, meaning that achieving high rejection rates often leads to a significant reduction in water flux. The academic and industrial consensus is that effectively improving this long-standing "permeability-selectivity" trade-off, simultaneously enhancing membrane permeate flux and separation selectivity, is crucial for reducing system operating energy consumption and expanding the application scenarios of the technology, thus opening new pathways for the efficient utilization of water resources.

[0004] In traditional interfacial polymerization reactions, amine monomers (such as piperazine, PIP) in the aqueous phase readily diffuse rapidly into the organic phase and react with acyl chloride monomers (such as trimesoyl chloride, TMC) in the organic phase to form a relatively thick and dense polyamide separation layer. Excessive diffusion of aqueous monomers directly affects key physicochemical properties of the polyamide separation layer, such as its thickness, surface roughness, and pore size distribution. This often results in an overly dense separation layer structure, increasing resistance to water molecule mass transfer and thus reducing membrane permeability. Therefore, effectively controlling the diffusion behavior of aqueous monomers during interfacial polymerization has become a crucial key to optimizing the polyamide separation layer structure and improving the overall performance of nanofiltration membranes.

[0005] Currently, strategies for controlling the diffusion of aqueous monomers (represented by piperazine) mainly focus on introducing additives, constructing intermediate layers, and employing other emerging technologies. Among these, polymers of intrinsic microporosity (PIMs), as polymers composed of rigid and twisted molecular chains, can form a large number of inherently interconnected micropores smaller than 2 nanometers due to their low molecular chain stacking efficiency, providing a material basis for constructing novel functional layers. PIMs typically introduce conformationally locked, rigidly twisted structural units (such as spirocyclic centers, tripterene groups, etc.) and utilize their steric hindrance effect to prevent the molecular chains from closely aligning, thereby constructing a stable microporous structure.

[0006] In particular, 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirolinkindole (TTSBI), as a monomer with a rigid twisted spirocyclic structure, offers new possibilities for regulating interfacial polymerization processes due to its unique structural characteristics. Theoretically, the polyester layer formed using TTSBI can physically regulate the diffusion process of piperazine to the reaction interface, thereby affecting the reaction kinetics between piperazine and TMC, and ultimately controlling the formation and structure of the polyamide separation layer. If a polyester layer derived from TTSBI and possessing typical microporous properties of PIMs can be combined with a polyamide layer formed by the piperazine / TMC reaction, a novel hybrid separation layer is expected to be constructed. This structure is expected to integrate microporous properties and hydrophilicity, achieving precise sieving while enhancing water molecule permeability, providing an innovative approach to overcoming the performance bottlenecks of existing nanofiltration membranes. Summary of the Invention

[0007] To address the performance bottleneck of nanofiltration membranes, this invention provides a method for preparing a mid-layer regulated polyamide membrane using a non-planar rigid twisted monomer.

[0008] The specific technical solution is as follows: First, the present invention provides a polyamide composite nanofiltration membrane with intermediate layer control prepared by non-planar rigid twisted monomer, comprising a porous ultrafiltration base membrane, and a polyester intermediate layer and a polyamide separation layer sequentially formed on the base membrane; The polyester interlayer is formed by interfacial polymerization of a non-planar rigid twisted monomer and pyromellitic trimethylol chloride. The polyamide separation layer is formed by interfacial polymerization of diamine monomers and trimesoyl chloride. The core innovation of this membrane structure lies in the introduction of a novel polyester interlayer between the traditional "base membrane-polyamide layer." This interlayer is constructed from non-planar rigid twisted monomers, and its unique spatial structure physically blocks and regulates the diffusion rate of aqueous diamine monomers (such as piperazine) to the reaction interface during subsequent interfacial polymerization. This regulation of diffusion kinetics results in a thinner polyamide separation layer with a nano-wrinkled surface, thereby significantly reducing water mass transfer resistance and increasing water flux while maintaining a high rejection rate, effectively overcoming the "trade-off effect" of traditional nanofiltration membrane performance.

[0009] Furthermore, the non-planar rigid twisted monomer is a non-planar rigid twisted spirocyclic polyphenol monomer, selected from one or more of TTSBI, spirobisindane, 2,2'-hydroxy-9,9'-spirodifluorene, and bisphenol fluorene. Specifically, the non-planar rigid twisted monomer is defined as a spirocyclic polyphenol compound. These monomers possess a rigid spirocyclic skeleton and a twisted molecular configuration, enabling them to form polyester networks with inherent microporous (PIM) properties during interfacial polymerization. This microporous structure not only possesses sieving potential but, more importantly, its significant steric hindrance and molecular-level free volume are the key structural basis for effectively controlling the diffusion of upper-layer amine monomers and realizing the function of the intermediate layer.

[0010] The preferred monomer is TTSBI (5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirolinkindole). As shown in the comparative examples, compared with monomers such as phenolphthalein with smaller molecular cavities, TTSBI possesses a rigid spirocyclic bis-indene skeleton, greater steric hindrance of the spirocycle, and the resulting abundant intermolecular voids, enabling it to create a superior microporous environment and diffusion barrier effect when forming the polyester interlayer. Experiments have shown that nanofiltration membranes prepared using TTSBI achieve a sodium sulfate rejection rate >97% while maintaining a pure water flux of over 20.57 LMH / bar. The overall performance is significantly better than that of comparative examples using other twisted monomers, demonstrating the unexpectedly excellent effect of TTSBI in solving this technical problem.

[0011] Furthermore, the thickness of the polyester interlayer is 10-100 nm, and the thickness of the polyamide separation layer is 20-80 nm. By controlling the reaction conditions, the thickness of the polyester interlayer is optimized to 10-100 nm, and the thickness of the polyamide separation layer is optimized to 20-80 nm. A polyester layer within this thickness range is sufficient to effectively control diffusion without becoming too thick and increasing additional resistance, while the significantly thinner polyamide layer (e.g., only 46 nm in Example 1) directly reduces the water permeation resistance. Simultaneously, due to changes in diffusion kinetics, nanoscale wrinkles form on the surface of the polyamide layer, significantly increasing the effective filtration area. This synergistic optimization of thickness and morphology is the direct structural reason why the membrane of this invention simultaneously achieves high flux and high rejection rate.

[0012] A method for preparing any of the above-described polyamide composite nanofiltration membranes includes the following steps: S1: Provides a first aqueous phase solution containing a nonplanar rigid twisted monomer A and NaOH; provides a second aqueous phase solution containing a diamine monomer; provides an oil phase solution containing an acyl chloride monomer; S2: The porous support layer is brought into contact with the first aqueous solution, and then into contact with the oil solution to perform the first interfacial polymerization, forming a polyester interlayer on the porous support layer; S3: The base membrane with the polyester interlayer is contacted with the second aqueous solution, followed by contact with the oil solution for a second interfacial polymerization, forming a polyamide separation layer on the polyester interlayer, thus obtaining the nanofiltration membrane. The core of this method is a two-step sequential interfacial polymerization. First, a polyester interlayer is constructed on the base membrane; then, using this interlayer as a "diffusion regulator," a second interfacial polymerization is performed on its surface to form the polyamide separation layer. This process cleverly achieves "in-situ control" of the formation process of the upper polyamide layer by the interlayer. Through simple two-step solution treatment, a three-layer hybrid composite membrane with a fine structure and excellent performance can be constructed. The process has good repeatability and is easy to scale up.

[0013] Further, in step S1, the mass concentration of the non-planar rigid twisted monomer A in the first aqueous solution is 0.1%~3%; the mass concentration of the diamine monomer in the second aqueous solution is 0.1%~3%; the mass concentration of the acyl chloride monomer in the oil solution is 0.01%~0.25%; and the molar ratio of the non-planar rigid twisted monomer A to the alkali is (0.25~2):(1~8). The concentration and ratio of each reactant monomer were optimized. The concentration of the non-planar rigid twisted monomer (0.1%-3%) is key to ensuring the formation of a continuous, uniform polyester interlayer with appropriate thickness and microporous structure. If the concentration is too low, the interlayer will be incomplete and the effect will be poor; if it is too high, the interlayer may be too thick or even have defects. The addition of alkali is used to neutralize the HCl generated in the reaction, promoting the forward polymerization reaction. Its molar ratio to the monomer (0.25~2):(1~8) ensures a suitable pH value for the reaction system, guaranteeing the high-quality formation of the interlayer. This concentration and ratio range is the optimized range that can obtain the best performance through a series of examples.

[0014] Further, in step S1, the diamine monomer is selected from one or more of piperazine, m-phenylenediamine, p-phenylenediamine, and o-phenylenediamine; the acyl chloride monomer is trimesoyl chloride. Piperazine (PIP) is preferred as the diamine monomer, and its reaction with the preferred trifunctional acyl chloride monomer trimesoyl chloride (TMC) can form a polyamide network with moderate crosslinking degree and both good separation performance and mechanical stability. This combination is a classic and efficient choice for preparing high-performance nanofiltration membranes, ensuring the basic performance of the separation layer.

[0015] Furthermore, in step S1, the oil phase solvent is selected from one or more of n-hexane, cyclohexane, and benzene. Using non-polar solvents such as n-hexane and cyclohexane as the oil phase effectively dissolves the acyl chloride monomer (TMC) and forms a clear liquid-liquid interface with the aqueous phase, which is a prerequisite for interfacial polymerization. These solvents also have moderate volatility, which is beneficial for forming a uniform and dense polymer separation layer after the reaction.

[0016] Furthermore, in steps S2 and S3, the contact time is independently 10-600 seconds; the interfacial polymerization reaction time is independently 10-300 seconds, the reaction temperature is 10-40℃, and the relative humidity of the reaction environment is 20%-80%. After the interfacial polymerization reaction in steps S2 and / or S3, a heat treatment step is included, with a heat treatment temperature of 20-90℃ and a time of 5-30 minutes; and after step S3, a step is included, immersing the obtained film in deionized water at 3-6℃ for 2-24 hours. Key process parameters for interfacial polymerization and post-treatment have been optimized. Contact time and reaction time together determine the monomer adsorption amount and reaction degree, thus affecting the thickness and crosslinking degree of each functional layer. Suitable temperature (10-40℃) and humidity (20%-80%) are important conditions for controlling the reaction rate and avoiding defect formation. Heat treatment (20-90℃, 5-30 minutes) promotes the hydrolysis of unreacted acyl chloride groups and improves the ordered arrangement of polymer chains, thereby enhancing the density and chemical stability of the separation layer. Low-temperature water immersion helps remove residual trace molecules from the membrane and stabilizes membrane performance.

[0017] Furthermore, the porous support layer is made of polysulfone, polyethersulfone, polyacrylonitrile, zirconium oxide, alumina, or titanium oxide, with a pore size of 5–100 nm. Selecting a porous ultrafiltration membrane with the aforementioned materials and pore size range as the support layer provides excellent mechanical support and low mass transfer resistance for the upper functional layers. Its abundant microporous structure on the surface facilitates the wetting of the first aqueous phase solution and the uniformity of subsequent interfacial polymerization, forming the basis for the formation of a defect-free composite nanofiltration membrane.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Synergistic improvement of membrane performance: By introducing a polyester interlayer to regulate the interfacial polymerization process, the resulting polyamide separation layer is thinner and has a nano-wrinkled structure. This significantly improves water flux while maintaining a high salt rejection rate (Na2SO4 rejection rate >97%), thus improving the trade-off between permeability and selectivity in traditional nanofiltration membranes.

[0019] 2. Controllable structural innovation: The design of the intermediate layer enables effective control over the thickness and surface morphology of the polyamide separation layer, which is the key structural basis for its performance improvement.

[0020] 3. Key Monomer Selection: The use of the specific non-planar rigid torsion monomer TTSBI is crucial for achieving excellent intermediate layer functionality, and its effect is superior to other structurally similar monomers (such as phenolphthalein), demonstrating the outstanding advantages of this invention.

[0021] 4. Stable and reliable performance: The obtained nanofiltration membrane exhibits stable separation performance in long-term testing and has good practical potential.

[0022] 5. Simple preparation method: The "two-step sequential interfacial polymerization" process adopted has mild conditions, a wide parameter range, and is easy to operate and implement. Attached Figure Description

[0023] Figure 1 This is a surface electron microscope (SEM) image of the nanofiltration membrane prepared in Example 1 of the present invention; Figure 2 This is a surface electron microscope (SEM) image of the nanofiltration membrane prepared in Example 2 of the present invention; Figure 3 This is a surface electron microscope (SEM) image of the nanofiltration membrane prepared in Example 3 of the present invention; Figure 4 Long-term stability test diagram of the nanofiltration membrane prepared in Example 1 of this invention. Detailed Implementation

[0024] The technical solution of the present invention will be described in detail below through embodiments, but the scope of protection of the present invention is not limited to the embodiments described.

[0025] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0026] The materials obtained in the embodiments of the present invention were subjected to performance testing according to the following method: The nanofiltration membrane prepared by the method of this invention was tested using Na₂SO₄ with a salt concentration of 1000 ppm as a model saline solution, and its SO₄²⁻ content was measured. 2- Selectivity, test temperature 25°C, operating pressure 6 bar.

[0027] The nanofiltration membrane was subjected to a long-term stability test using a 1000 ppm Na2SO4 aqueous solution at a temperature of 25°C, a pressure of 6 bar, and a duration of 50 h.

[0028] Example 1

[0029] This embodiment provides a method for preparing a nanofiltration membrane, specifically as follows: (1) Aqueous solution is obtained by dissolving 0.3% TTSBI monomer and NaOH in water; (2) Dissolve piperazine monomers with a mass concentration of 0.6% in water to obtain an aqueous solution; (3) Dissolve pyromellitic chloride in n-hexane and stir until homogeneous to obtain a 0.1% pyromellitic chloride oil phase solution; (4) The Psf-based membrane was immersed in TTSBI aqueous solution for 300 s. After immersion, excess solution on the membrane surface was removed using a rubber roller. Then, the oil phase solution was poured out to carry out interfacial polymerization to form a cross-linked polyester interlayer. After 120 s, the membrane was poured out and the residual oil phase monomer on the surface was cleaned with n-hexane. The membrane was then heat-treated at 85°C for 10 min. After immersion, the membrane was immersed in piperazine aqueous solution for 120 s. After immersion, excess solution on the membrane surface was removed using a rubber roller. Then, the oil phase solution was poured out to carry out interfacial polymerization to form a cross-linked polyamide separation layer. After 30 s, the membrane was poured out and the residual oil phase monomer on the surface was cleaned with n-hexane. The membrane was then heat-treated at 60°C for 5 min. After immersion, it was soaked in deionized water for 12 h to obtain a membrane for SO4 interception. 2- Nanofiltration membrane.

[0030] The nanofiltration membrane prepared in this embodiment is as follows: Figure 1 As shown.

[0031] Example 2

[0032] (1) Aqueous solution is obtained by dissolving 0.3% TTSBI monomer and NaOH in water; (2) Dissolve pyromellitic chloride in n-hexane and stir until homogeneous to obtain a 0.1% pyromellitic chloride oil phase solution; (3) The Psf base membrane was immersed in TTSBI aqueous solution for 300 s. After immersion, excess solution on the membrane surface was removed using a rubber roller. Then, the oil phase solution was poured out to carry out interfacial polymerization reaction to form a cross-linked polyester interlayer. After 120 s, the membrane was poured out and the residual oil phase monomer on the surface was cleaned with n-hexane. The above membrane was heat-treated at 85°C for 10 min and then immersed in deionized water for 12 h to obtain a membrane for SO4 interception. 2- Nanofiltration membrane.

[0033] The nanofiltration membrane prepared in this embodiment is as follows: Figure 2 As shown.

[0034] Example 3

[0035] (1) Dissolve piperazine monomers with a mass concentration of 0.6% in water to obtain an aqueous solution; (2) Dissolve pyromellitic chloride in n-hexane and stir until homogeneous to obtain a 0.1% pyromellitic chloride oil phase solution; (3) The Psf-based membrane was immersed in a piperazine aqueous solution for 120 s. After immersion, excess solution on the membrane surface was removed using a rubber roller. Then, the oil phase solution was poured out to carry out interfacial polymerization reaction to form a cross-linked polyamide separation layer. After 30 s, the membrane was poured out and the residual oil phase monomer on the surface was washed with n-hexane. The membrane was then heat-treated at 60°C for 5 min and then immersed in deionized water for 12 h to obtain a membrane for SO4 interception.2- Nanofiltration membrane.

[0036] The nanofiltration membrane prepared in this embodiment is as follows: Figure 3 As shown.

[0037] Example 4

[0038] The difference between this embodiment and Example 1 is that the mass concentration of TTSBI monomer in the aqueous solution is adjusted to 0.15%, while the rest of the preparation process is the same as in Example 1, and a nanofiltration membrane is obtained.

[0039] Example 5

[0040] The difference between this embodiment and Example 1 is that the mass concentration of TTSBI monomer in the aqueous solution is adjusted to 0.5%, while the rest of the preparation process is the same as in Example 1, and a nanofiltration membrane is obtained.

[0041] Example 6

[0042] The difference between this embodiment and Example 1 is that the mass concentration of TTSBI monomer in the aqueous solution is adjusted to 1%, while the rest of the preparation process is the same as in Example 1, and a nanofiltration membrane is obtained.

[0043] To demonstrate the superior performance of the specific monomer TTSBI selected in this invention, the applicant also experimented with other structural monomers for comparison. Experiments showed that, using phenolphthalein monomer, under the same preparation conditions, the overall performance of the resulting nanofiltration membrane (such as the synergistic effect of water flux and salt rejection rate) was significantly lower than that of Example 1 of this invention. This proves that the rigid spirocyclic biindene framework, large steric hindrance of the spirocyclic structure, and abundant intermolecular voids of TTSBI play an unexpectedly crucial role in achieving the technical effects of this invention. In contrast, the phenolphthalein monomer, with its twisted molecular structure, small cavity, and lack of large steric hindrance of the spirocyclic structure, results in a smaller free volume and lower permeation flux after membrane formation.

[0044] Comparative Example 1

[0045] This comparative example provides a method for preparing a nanofiltration membrane, specifically as follows: (1) Aqueous solution is obtained by dissolving 0.5% phenolphthalein monomer and NaOH in water; (2) Dissolve piperazine monomers with a mass concentration of 0.5% in water to obtain an aqueous solution; (3) Dissolve pyromellitic chloride in n-hexane and stir until homogeneous to obtain a 0.1% pyromellitic chloride oil phase solution; (4) The Psf-based membrane was immersed in TTSBI aqueous solution for 120 s. After immersion, excess solution on the membrane surface was removed using a rubber roller. Then, the oil phase solution was poured out to carry out interfacial polymerization to form a cross-linked polyester interlayer. After 60 s, the membrane was poured out and the residual oil phase monomer on the surface was cleaned with n-hexane. The membrane was then heat-treated at 60°C for 5 min. After immersion, the membrane was immersed in piperazine aqueous solution for 120 s. After immersion, excess solution on the membrane surface was removed using a rubber roller. Then, the oil phase solution was poured out to carry out interfacial polymerization to form a cross-linked polyamide separation layer. After 30 s, the membrane was poured out and the residual oil phase monomer on the surface was cleaned with n-hexane. The membrane was then heat-treated at 60°C for 5 min. After immersion, it was soaked in deionized water for 12 h to obtain a membrane for SO4 interception. 2- Nanofiltration membrane.

[0046] Comparative Example 2

[0047] The difference between this comparative example and Comparative Example 1 is that the mass concentration of phenolphthalein monomer in the aqueous solution was adjusted to 0.25%, and the concentration of piperazine monomer in the aqueous solution was adjusted to 0.8%. The rest of the preparation process was the same as that of Comparative Example 1, and a nanofiltration membrane was obtained.

[0048] Comparative Example 3

[0049] The difference between this comparative example and Comparative Example 1 is that the mass concentration of phenolphthalein monomer in the aqueous solution was adjusted to 0.75%, and the concentration of piperazine monomer in the aqueous solution was adjusted to 0.3%. The rest of the preparation process was the same as that of Comparative Example 1, and a nanofiltration membrane was obtained.

[0050] The nanofiltration membranes prepared in the above embodiments and comparative examples were subjected to performance tests, and the results are shown in Table 1.

[0051] Table 1 Example 1 97.74% 15.33 22.76 Example 3 93.23% 14.94 20.57 Example 4 97.21% 11.97 13.64 Example 5 97.83% 14.3 18.51 Example 6 98.65% 13.32 14.99 Comparative Example 1 96.42 8.16 - Comparative Example 2 97.83 3.30 - Comparative Example 3 96.16 8.67 - As shown in the table above, adjusting the amount of TTSBI monomer added significantly affects the performance of the nanofiltration membrane. This is because the TTSBI-TMC polyester layer slows down the diffusion rate of PIP monomer from the aqueous phase to the organic phase. Simultaneously, the TTSBI-TMC polyester interlayer can control the membrane thickness of the PIP-TMC polyamide layer and also regulate the surface morphology of the PIP-TMC polyamide layer, resulting in a wrinkled structure that increases the contact area between the solution and the membrane. According to the results in the table, the optimal technical effect is achieved when the mass concentration of TTSBI in the aqueous solution is 0.3%.

[0052] Figure 1 This is a SEM image of the nanofiltration membrane prepared in Example 1 of the present invention. Figure 2 This is a SEM image of the nanofiltration membrane prepared in Example 2. Figure 3 This is a SEM image of the nanofiltration membrane prepared in Example 3 of the present invention. (Referring to the table above and...) Figure 1 ,2 From 3, it can be seen that... Figure 1 As can be seen from Figures 2 and 3, the surface morphology of the PIP-TMC membrane, the TTSBI-TMC membrane, and the TTSBI-TMC / PIP-TMC membrane are significantly different. The PIP-TMC membrane has a smooth and flat surface with obvious nodular structure, which is a typical characteristic of PIP-TMC membranes. The membrane pores are dense and there are no obvious defects. The membrane thickness is 80.2 nm. Figure 2 The TTSBI-TMC film has a thickness of 82 nm and a smooth, dense surface. It can be seen that, compared to... Figure 1 PIP-TMC membrane and Figure 2 The TTSBI-TMC / PIP-TMC membrane in this study has a thickness of 122 nm, with a TTSBI-TMC polyester interlayer thickness of 76 nm and a PIP-TMC polyamide layer thickness of only 46 nm. The polyamide layer is thinner, and the surface morphology of the TTSBI-TMC / PIP-TMC membrane exhibits a nano-wrinkled structure, which increases the contact area between the membrane surface and the solution. This is mainly because the TTSBI-TMC polyester layer slows down the diffusion rate of PIP monomers from the aqueous phase to the organic phase. The study demonstrates that the TTSBI-TMC polyester interlayer can control both the membrane thickness and the surface morphology of the PIP-TMC polyamide layer. Figure 4 The diagram shows the long-term stability of the nanofiltration membrane prepared in Example 1 of this invention. Under 5 bar conditions, the permeation flux remained at approximately 95 ± 1 L·m for 52 hours. ‒2 ·h ‒1 The Na2SO4 rejection rate remained above 98.5% without any performance degradation, demonstrating its excellent long-term stability. These results indicate that the combination of high separation performance and long-term stability of nanofiltration membranes reflects their great potential in seawater desalination and advanced water treatment.

[0053] The nanofiltration membrane prepared by this invention has the following excellent properties: Significantly Improved Polyamide Membrane Separation Performance: This invention achieves control over the thickness and surface morphology of the polyamide membrane by introducing a polyester interlayer into the nanofiltration membrane and utilizing the non-planar rigid twisted structure of TTSBI. In tests using Na₂SO₄ with a total salt concentration of 1000 ppm as a model saline solution, the PWP and salt rejection rate were 20.57 LMH / bar and 97.2%, respectively. This result demonstrates that the nanofiltration membrane of this invention can effectively retain SO₄²⁻. 2- The addition of ions significantly improves the separation performance of polyamide nanofiltration membranes, solving the problem that the flux and salt rejection of traditional polyamide nanofiltration membranes cannot be unified.

[0054] Excellent long-term stability: In the long-term stability test of 1000ppm Na2SO4 aqueous solution, at a test temperature of 25℃ and a test pressure of 6 bar, the Na2SO4 rejection rate remained at 98.2% after 50 hours of continuous testing. This indicates that the nanofiltration membrane of the present invention has good stability and reliability during long-term operation, and can maintain efficient brine separation performance, providing a strong guarantee for continuous and stable operation in industrial production.

[0055] Unique Non-planar Rigid Twisted Polyester Interlayer Structure: This invention introduces the TTSBI-TMC polyester interlayer into the polyamide nanofiltration membrane, forming a unique TFC nanofiltration membrane structure. This structure not only endows the membrane with excellent brine separation performance but also solves the problem of existing polyamide membranes failing to achieve a balance between flux and retention capacity, leading to a decline in separation performance during long-term operation. The strong bond between the polyester interlayer and the polyamide separation layer enhances the membrane's structural stability and extends its service life.

[0056] Optimized pore size distribution and membrane structure: The polyester interlayer and polyamide separation layer prepared by interfacial polymerization have a dense structure and uniform pore size distribution. After introducing the polyester interlayer, the thickness of the polyamide separation layer becomes significantly thinner and a wrinkled structure appears on its surface. This invention controls the thickness and surface morphology of the polyamide separation layer, enabling more precise separation of brine solutions. Furthermore, the introduction of the polyester interlayer improves the membrane's permeability, achieving dual optimization of retention performance and membrane flux.

[0057] Unless otherwise specified, all technologies mentioned above refer to existing technologies.

[0058] As described above, although the invention has been shown and described with reference to specific preferred embodiments, it should not be construed as limiting the invention itself. Various changes in form and detail may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A polyamide composite nanofiltration membrane with intermediate layer controlled by a non-planar rigidly twisted monomer, characterized in that, It includes a porous ultrafiltration base membrane, and a polyester interlayer and a polyamide separation layer sequentially formed on the base membrane; The polyester interlayer is formed by interfacial polymerization of a non-planar rigid twisted monomer and pyromellitic trimethylol chloride. The polyamide separation layer is formed by interfacial polymerization of a diamine monomer and pyromellitic trimethylol chloride.

2. The nanofiltration membrane according to claim 1, characterized in that, The nonplanar rigid twisted monomer is a nonplanar rigid twisted spirocyclic polyphenol monomer selected from one or more of TTSBI, spirobisindane, 2,2'-hydroxy-9,9'-spirobisfluorene, and bisphenol fluorene.

3. The nanofiltration membrane according to claim 1, characterized in that, The non-planar rigid torsion monomer is TTSBI.

4. The nanofiltration membrane according to claim 1, characterized in that, The thickness of the polyester interlayer is 10-100 nm, and the thickness of the polyamide separator layer is 20-80 nm.

5. A method for preparing a polyamide composite nanofiltration membrane as described in any one of claims 1-4, characterized in that, Includes the following steps: S1: Provides a first aqueous phase solution containing a nonplanar rigid twisted monomer A and NaOH; provides a second aqueous phase solution containing a diamine monomer; provides an oil phase solution containing an acyl chloride monomer; S2: The porous support layer is brought into contact with the first aqueous solution, and then into contact with the oil solution to perform the first interfacial polymerization, forming a polyester interlayer on the porous support layer; S3: The base membrane with the polyester interlayer is contacted with the second aqueous solution, and then contacted with the oil solution for a second interfacial polymerization to form a polyamide separation layer on the polyester interlayer, thus obtaining the nanofiltration membrane.

6. The method according to claim 5, characterized in that, In step S1, the mass concentration of the non-planar rigid twisted monomer A in the first aqueous solution is 0.1%~3%; the mass concentration of the diamine monomer in the second aqueous solution is 0.1%~3%; the mass concentration of the acyl chloride monomer in the oil solution is 0.01%~0.25%; and the molar ratio of the non-planar rigid twisted monomer A to the alkali is (0.25~2):(1~8).

7. The method according to claim 5, characterized in that, In step S1, the diamine monomer is selected from one or more of piperazine, m-phenylenediamine, p-phenylenediamine, and o-phenylenediamine; the acyl chloride monomer is pyromellitic trimethylolpropionate chloride.

8. The method according to claim 5, characterized in that, In step S1, the oil phase solvent is selected from one or more of n-hexane, cyclohexane, and benzene.

9. The method according to claim 5, characterized in that, In steps S2 and S3, the contact time is independently 10 to 600 seconds; the interfacial polymerization reaction time is independently 10 to 300 seconds, the reaction temperature is 10 to 40°C, and the relative humidity of the reaction environment is 20% to 80%. After the interfacial polymerization reaction in steps S2 and / or S3, the method further includes a heat treatment step on the obtained film, with a heat treatment temperature of 20 to 90°C and a time of 5 to 30 minutes; and after step S3, the method further includes immersing the obtained film in deionized water at 3 to 6°C for 2 to 24 hours.

10. The method according to claim 5, characterized in that, The porous support layer is made of polysulfone, polyethersulfone, polyacrylonitrile, zirconium oxide, alumina, or titanium oxide, and its pore size is 5~100 nm.

Images