A method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on an interface in-situ self-rearrangement-ion complex mechanism

Abstract

This invention provides a method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on an in-situ interfacial rearrangement-ion complexation mechanism, mainly comprising the following steps: (1) subjecting polysulfone / polyethersulfone to mild swelling aqueous phase treatment; (2) using hydrophilic polymers for in-situ enrichment at the interface; and (3) using polyvalent metal ions and polyamine-polyacid ion pairs for complexation. The advantage of this invention is that the pure water flux (PWF) of the separation membrane prepared by this invention, at an operating pressure of 0.1 MPa, is increased from 300 L·m⁻¹ before modification. ‑2 ·h ‑ Increased to 280–320 L·m ‑2 ·h ‑ Meanwhile, the membrane flux recovery rate (CIP) has been significantly improved from 60–70% to no less than 85%; in terms of BSA retention performance, it has been improved from 90% to ≥92%; more importantly, in a continuous operation test of up to 500 hours, the membrane performance degradation has been significantly reduced from 25% to less than 12%, demonstrating excellent long-term stability and antifouling ability.

Description

Technical Field

[0001] This invention belongs to the field of polymer gas separation membrane technology, specifically a method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on an in-situ interfacial rearrangement-ion complexation mechanism. Background Technology

[0002] Polysulfone (PSF) and polyethersulfone (PES), as high-performance polymer materials, possess excellent mechanical strength, thermal stability, and chemical resistance. Hollow fiber membranes prepared from them are widely used in water treatment, gas separation, biological separation, blood purification, and many other fields due to their advantages such as large specific surface area, high separation efficiency, and convenient operation. However, the surface of unmodified polysulfone / polyethersulfone hollow fiber membranes is strongly hydrophobic, which easily leads to problems such as pollutant adsorption and severe membrane fouling. At the same time, their key performance characteristics, such as flux, permeability, selectivity, and long-term operational stability, are difficult to meet the requirements of practical applications, thus limiting their further promotion.

[0003] To address the aforementioned shortcomings, various modification technologies have been developed in this field, becoming a current research hotspot in membrane materials. Existing modification technologies for polysulfone / polyethersulfone hollow fiber membranes have formed several mainstream schemes, specifically including: First, bulk blending modification, which involves incorporating hydrophilic polymers such as polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), sulfonated polyetheretherketone (SPEEK), and 2-methacryloyloxyethylphosphonic choline (MPC) into the polysulfone / polyethersulfone casting solution to enhance membrane hydrophilicity and regulate pore structure. This technology has reached a mature industrial level. Second, surface modification technology, which utilizes plasma grafting, ultraviolet (UV) grafting, and chemical coatings (such as heparinized coatings and PEG coatings) to improve membrane surface energy and reduce pollutant adsorption tendency. This technology is currently in the transitional stage from laboratory to pilot-scale production. Thirdly, nanocomposite modification involves doping membrane materials with nanofillers such as titanium dioxide (TiO2), graphene oxide (GO), iron tetroxide (Fe3O4), and molecular sieves to improve membrane porosity and enhance permeability and selectivity; this technology has progressed to the pilot-scale stage. Fourthly, crosslinking modification uses chemical crosslinking agents such as diisocyanates or thermal crosslinking methods to improve the mechanical strength and solvent resistance of the membrane; some technologies have already been industrialized. Fifthly, process optimization modification involves adjusting phase transformation process parameters, using double / multilayer spinning technology, and optimizing the types and amounts of non-solvent additives to control the pore morphology of the membrane (such as finger pores and sponge pores); this type of technology also has a mature industrial foundation. Although the aforementioned modification technologies have improved some properties of polysulfone / polyethersulfone hollow fiber membranes to a certain extent, many objective defects remain insurmountable in practical applications. The core issues are concentrated in key performance dimensions such as flux, permeability, selectivity, and attenuation rate, specifically manifested as follows: 1. Insufficient flux and permeability with poor stability: The water contact angle of unmodified polysulfone / polyethersulfone hollow fiber membranes is typically greater than 70°, exhibiting strong hydrophobicity and readily adsorbing contaminants such as proteins, oils, and colloids, leading to a rapid decline in membrane flux. Its pure water flux is generally only 30–80 L / (m²). 2In complex feed systems, flux decline can reach 30%–50% after 1–3 days of operation (·h·bar). Existing modification technologies not only fail to fundamentally solve this problem, but some modification methods also cause side effects: for example, while hydrophilic additives such as PVP and PEG in bulk blends can increase flux in the short term, they are prone to dissolution during long-term operation, causing the flux to drop to less than 60% of the initial value after 30–90 days of operation. Moreover, excessive addition of such additives can significantly reduce the mechanical strength of the membrane. Agglomeration is prone to occur during the doping process of nanofillers, causing membrane pore blockage and reducing membrane permeability. When the amount of filler added exceeds 2 wt%, the flux decline can reach 15%–25%. If the coating thickness is not properly controlled, surface coating modification can significantly increase mass transfer resistance, reducing permeability by 20%–40%. In addition, insufficient adhesion between the coating and the membrane substrate can easily lead to peeling during long-term operation, further deteriorating flux stability. Furthermore, traditional phase inversion methods have inherent limitations. The resulting membrane skin thickness is typically 2–5 μm, significantly increasing mass transfer resistance and leading to low permeability. Moreover, when the transmembrane pressure difference (TMP) increases to 0.3–0.5 MPa, the membrane is prone to pressure-induced phenomena, further reducing flux. 2. There is an inherent contradiction between flux and selectivity, making it difficult to simultaneously achieve separation accuracy: The flux and selectivity of polysulfone / polyethersulfone hollow fiber membranes are significantly negatively correlated, which is a core bottleneck that existing modification technologies struggle to overcome. To increase membrane flux, it is usually necessary to increase the pore size or improve porosity, but this directly leads to a decrease in membrane rejection rate. For example, in the field of ultrafiltration, the rejection rate for bovine serum albumin (BSA) may drop from 99.5% to below 95%. If the pore size is reduced or the membrane skin is densified to enhance selectivity, it will result in a flux loss of 30%–60%. Meanwhile, existing modified membranes have poor adaptability to complex separation systems: in gas separation scenarios (such as CO2 / N2 separation), the separation selectivity of modified membranes is typically only 20–50%, far lower than the more than 100 of inorganic membranes; in oil-water emulsion separation, for small-diameter oil droplets (particle size <5 μm) stable by surfactants, the rejection rate is less than 90%, which is difficult to meet the requirements of high-precision separation. In addition, during long-term operation, the dissolution of additives in blended modified membranes or the migration of nanofillers in nanocomposite membranes can lead to irreversible changes in the pore structure of the membrane, causing the molecular weight cutoff (MWCO) of the membrane to drift by ±20%–30%, further compromising the stability of separation precision. 3. High long-term operation degradation rate and insufficient stability: Existing modified membranes have poor long-term service stability and a high degradation rate, mainly reflected in three aspects: pollution degradation, chemical stability degradation, and mechanical performance degradation.Regarding fouling attenuation, even after modification, the membrane surface still exhibits a certain tendency to adsorb pollutants. After 72 hours of continuous operation, the flux attenuation rate of the unmodified membrane can reach 35%–45%, and the flux recovery rate (FRR) is only 50%–60%. Even with hydrophilic modification or antifouling coating modification, the flux attenuation rate still exceeds 25% after 180 days of operation in highly polluted systems (such as sludge water from membrane bioreactors (MBR)). Regarding chemical stability degradation, membranes in practical applications often come into contact with chlorine-containing cleaning agents (such as 500–1000 ppm NaClO), strong acids, or strong alkalis. Long-term contact leads to significant degradation of membrane performance: chlorine-containing cleaning agents can cause the main chain of polysulfone / polyethersulfone to break down; after 1000 hours of contact, the membrane permeability decreases by 15%–20%, and the mechanical strength decreases by 30%. In strong acid / alkali environments with pH < 2 or pH > 12, the dissolution rate of hydrophilic additives in blended membranes is significantly accelerated, and nanofillers in nanocomposite membranes are prone to aggregation or dissolution, resulting in a sharp increase in membrane performance degradation. Regarding mechanical performance degradation, nanocomposite modification or blend modification can, to some extent, damage the integrity of the membrane's microstructure. Under high-pressure operation (pressure > 0.3 MPa), the membrane is prone to creep and compaction; after 1000 pressure cycles, the flux reduction can reach 20%–30%, and hollow fiber membranes are prone to fiber breakage, severely affecting operational reliability. 4. Limited Large-Scale Application and Derivative Defects: While some existing high-performance modification technologies have shown good performance improvement effects at the laboratory level, they are difficult to scale up and suffer from derivative defects in terms of cost and safety. For example, in nanocomposite modification, the uniform dispersion of nanofillers requires complex pretreatment processes, leading to a 30%–80% increase in membrane production costs; surface grafting and coating modification technologies are difficult to adapt to continuous industrial production processes, with product yields of only 70%–85%. In addition, during chemical crosslinking modification and surface grafting modification, unreacted active groups (such as isocyanate groups) may remain on the membrane surface. These residual groups can cause blood compatibility problems such as increased hemolysis rates, limiting the application of modified membranes in medical fields such as blood purification. In summary, while existing polysulfone / polyethersulfone hollow fiber membrane modification technologies can specifically improve some membrane properties, they still suffer from numerous drawbacks, such as mutual incompatibility between flux and selectivity, insufficient permeability and long-term stability, high attenuation rates, and difficulties in large-scale application. These limitations make it difficult to meet the high-performance requirements of various application fields. Therefore, developing a polysulfone / polyethersulfone hollow fiber membrane modification technology that can balance flux, permeability, selectivity, and long-term stability, and is easily industrially producible, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on the interface in-situ self-rearrangement-ion complexation mechanism, including the following steps: (1) subjecting polysulfone / polyethersulfone to mild swelling aqueous phase treatment: immersing polysulfone / polyethersulfone in a mixture A of ethanol and water with a volume ratio of 20-40% by volume, the amount of mixture A being sufficient to cover the polysulfone / polyethersulfone, the temperature being 25-40 ℃, and the immersion time being 30-120 s.

[0005] Furthermore, it also includes step (2): the polysulfone / polyethersulfone raw material treated in step (1) is enriched in situ at the interface using a hydrophilic polymer. The polymer material includes any one or more of polyvinyl alcohol, sulfonated polyvinyl alcohol, sodium carboxymethyl cellulose, and sodium polyacrylate. The molecular weight of the polyvinyl alcohol and sodium polyacrylate is 10,000-50,000 Daltons.

[0006] Furthermore, the polymer material has a mass concentration of 0.1–1.0 wt% in the polysulfone / polyethersulfone raw material and is impregnated for 1–5 min.

[0007] Furthermore, it also includes step (3): the polysulfone / polyethersulfone raw material treated in step (2) is complexed with any one or more of polyvalent metal ions and polyamine-polyacid ion pairs, wherein the molar concentration of the polyvalent metal ions in the polysulfone / polyethersulfone raw material is 0.01-0.1M; and the mass concentration of the polyamine-polyacid ion pairs in the polysulfone / polyethersulfone raw material is 0.1-0.5%.

[0008] Furthermore, the multivalent metal ions include Ca... 2+ Mg 2+ Al 3+ Fe 3+ One or more of the following; polyamine-polyacid ion pairs include chitosan.

[0009] Furthermore, the complexation time of the polysulfone / polyethersulfone raw materials in step (3) is 30-120s.

[0010] Furthermore, it also includes step (4), after the polysulfone / polyethersulfone raw material treated in step (3) is processed by a wet process to obtain the initial product of the separation membrane, it is rinsed with deionized water and then dried at 40-60℃.

[0011] By employing the above-mentioned systematic technical solution, the following synergistic effects are achieved: Leveraging the micro-swelling properties of the polysulfone membrane surface in a humid environment, weakly compatible hydrophilic polymers are introduced into the aqueous system, spontaneously and directionally enriching in the membrane pore area to form a stable and uniform hydrophilic layer. Subsequently, by introducing polyvalent metal ions or polyamine-polyacid ion pairs, a "physical-chemical dual stabilization" strategy is implemented, successfully constructing an ultrathin, continuous, and water-resistant hydrophilic interface structure. Simultaneously, this structure does not significantly penetrate into the membrane pores, effectively maintaining the membrane's separation performance. Its core innovations include: the hydrophilic layer is constructed in situ through a "self-limiting generation" mechanism, rather than relying on traditional external coating processes; a dual stabilization mechanism combining ion complexation and polymer chain entanglement is used to improve interface durability; the entire process is completed in an aqueous environment, featuring low temperature, silane-free processing, and no grafting reaction—a green process characteristic. The specific implementation process includes four key steps: The first step is the "controlled swelling-activation treatment" of the membrane surface (using a non-plasma method), which aims to moderately enhance the migration of polymer segments on the surface without damaging the membrane pore structure, creating favorable conditions for the subsequent enrichment of hydrophilic materials; the second step is that the hydrophilic polymer cannot enter the membrane pores with a pore size of 5–50 nm due to the size exclusion effect, thus selectively forming an enrichment layer with a naturally limited thickness (about 10–80 nm) at the pore opening and membrane surface; the third step utilizes the electrostatic complexation and coordination between polyvalent metal ions or polyamine-polyacid ion pairs and hydrophilic polymer segments to construct a strong three-dimensional cross-linked network in the pore opening region; the selected ion pairs have unique advantages: no additional chemical reaction is required, they are not easily detached by water washing, and they can withstand the CIP (clean in situ) process, maintaining long-term stability; the fourth step is to effectively remove free water in the system through low-temperature shaping and stabilization treatment, further enhancing the stability of the complex structure, while avoiding the initiation of polysulfone material segment rearrangement.

[0012] In summary, the beneficial effects of this invention are as follows: the separation membrane prepared by this invention achieves significant optimization in terms of water contact angle, which is significantly reduced from 72–80° before modification to 30–40°; the pure water flux (PWF) at an operating pressure of 0.1 MPa is reduced from 300 L·m⁻¹ before modification. -2 ·h - Increased to 280–320 L·m -2 ·h - Meanwhile, the membrane flux recovery rate (CIP) has been significantly improved from 60–70% to no less than 85%; in terms of BSA retention performance, it has been improved from 90% to ≥92%; more importantly, in a continuous operation test of up to 500 hours, the membrane performance degradation has been significantly reduced from 25% to less than 12%, demonstrating excellent long-term stability and antifouling ability. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0014] Example 1: The present invention provides a method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on an in-situ self-rearrangement-ion complexation mechanism at the interface, characterized by the following steps: subjecting polysulfone / polyethersulfone to a mild swelling aqueous phase treatment: immersing polysulfone / polyethersulfone in a mixture A of ethanol and water with a volume ratio of 20% (ethanol / water), wherein the amount of mixture A is sufficient to submerge the polysulfone / polyethersulfone, the temperature is 25°C, and the immersion time is 30s.

[0015] Step (2): The polysulfone / polyethersulfone raw material treated in step (1) is enriched in situ at the interface using a hydrophilic polymer. The polymer is polyvinyl alcohol with a molecular weight of 10,000 Daltons. The mass concentration of the polymer in the polysulfone / polyethersulfone raw material is 0.1 wt%, and the impregnation time is 1 min.

[0016] Step (3): The polysulfone / polyethersulfone raw material treated in step (2) is complexed with a multivalent metal ion, wherein the molar concentration of the multivalent metal ion in the polysulfone / polyethersulfone raw material is 0.01M; the multivalent metal ion is Ca. 2+ Any compound that provides metal ions can be used. The complexation time of the polysulfone / polyethersulfone raw material in step (3) is 30s.

[0017] It also includes step (4), after the polysulfone / polyethersulfone raw material treated in step (3) is processed by a wet process to obtain the initial product of the separation membrane, it is rinsed with deionized water and then dried at 40°C.

[0018] Example 2: The present invention provides a method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on an interfacial in-situ self-rearrangement-ion complexation mechanism, characterized by the following steps: subjecting polysulfone / polyethersulfone to a mild swelling aqueous phase treatment: immersing polysulfone / polyethersulfone in a mixture A of ethanol and water with a volume ratio of 40%, wherein the amount of mixture A is sufficient to submerge the polysulfone / polyethersulfone, the temperature is 40°C, and the immersion time is 120s.

[0019] Step (2): The polysulfone / polyethersulfone raw material treated in step (1) is enriched in situ at the interface using a hydrophilic polymer. The polymer is a mixture of sulfonated polyvinyl alcohol and sodium carboxymethyl cellulose, which can be mixed in any mass ratio, such as 1:1 or 1:2. The mass concentration of the polymer in the polysulfone / polyethersulfone raw material is 1.0 wt%, and the impregnation time is 5 min.

[0020] Step (3): The polysulfone / polyethersulfone raw material treated in step (2) is complexed with a polyamine-polyacid ion pair, wherein the molar concentration of the polyamine-polyacid ion pair in the polysulfone / polyethersulfone raw material is 0.1M; the amine-polyacid ion pair is chitosan; and the complexation time of the polysulfone / polyethersulfone raw material in step (3) is 120s.

[0021] It also includes step (4), after the polysulfone / polyethersulfone raw material treated in step (3) is processed by a wet process to obtain the initial product of the separation membrane, it is rinsed with deionized water and then dried at 60°C.

[0022] Example 3: The present invention provides a method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on an in-situ self-rearrangement-ion complexation mechanism at the interface, characterized by the following steps: subjecting polysulfone / polyethersulfone to a mild swelling aqueous phase treatment: immersing polysulfone / polyethersulfone in a mixture A of ethanol and water with a volume ratio of 40%, wherein the amount of mixture A is sufficient to submerge the polysulfone / polyethersulfone, the temperature is 30°C, and the immersion time is 60s.

[0023] Step (2): The polysulfone / polyethersulfone raw material treated in step (1) is enriched in situ at the interface using a hydrophilic polymer. The polymer is sodium polyacrylate with a molecular weight of 50,000 Daltons. The mass concentration of the polymer in the polysulfone / polyethersulfone raw material is 0.5 wt%, and the impregnation time is 3 min.

[0024] Step (3): The polysulfone / polyethersulfone raw material treated in step (2) is complexed with a polyvalent metal ion, wherein the polyvalent metal ion is Mg. 2+ Al 3+ Fe 3+ The mixture can be mixed in any mass ratio, such as 1:1 or 1:2. The molar concentration of the polyvalent metal ions in the polysulfone / polyethersulfone raw material is 0.05M, and the complexation time of the polysulfone / polyethersulfone raw material in step (3) is 60s.

[0025] It also includes step (4), after the polysulfone / polyethersulfone raw material treated in step (3) is processed by a wet process to obtain the initial product of the separation membrane, it is rinsed with deionized water and then dried at 50°C.

[0026] The separation membrane obtained by this invention has a continuous hydrophilic ion complex layer on its surface with a thickness of 10–80 nm and a pore size variation of <10%. The pore opening is hydrophilic, and the original structure is maintained inside the pore. There is no independent coating interface.

[0027] The following items were tested using existing testing methods, and the results were compared with those before modification to illustrate the beneficial effects of the present invention. It should be noted that the test data were obtained through hundreds of parallel repeated experiments.

[0028] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for modifying hydrophilic polysulfone / polyethersulfone hollow fiber membranes based on an in-situ interfacial rearrangement-ion complexation mechanism, characterized in that, The steps include: (1) Mildly swelling the polysulfone / polyethersulfone in an aqueous phase: Immerse the polysulfone / polyethersulfone in a mixture of ethanol and water with a volume ratio of 20-40% by volume. The amount of mixture A should be enough to cover the polysulfone / polyethersulfone. The temperature is 25-40 °C and the immersion time is 30-120 s.

2. The method according to claim 1, characterized in that, It also includes step (2): the polysulfone / polyethersulfone raw material after step (1) is enriched in situ at the interface using a hydrophilic polymer. The polymer material includes any one or more of polyvinyl alcohol, sulfonated polyvinyl alcohol, sodium carboxymethyl cellulose, and sodium polyacrylate. The molecular weight of the polyvinyl alcohol and sodium polyacrylate is 10,000-50,000 Daltons.

3. The method according to claim 2, characterized in that, The polymer material is present in a concentration of 0.1–1.0 wt% in the polysulfone / polyethersulfone raw material and is impregnated for 1–5 min.

4. The method according to claim 2, characterized in that, The process also includes step (3): the polysulfone / polyethersulfone raw material treated in step (2) is complexed with any one or more of polyvalent metal ions and polyamine-polyacid ion pairs, wherein the molar concentration of the polyvalent metal ions in the polysulfone / polyethersulfone raw material is 0.01-0.1M; and the mass concentration of the polyamine-polyacid ion pairs in the polysulfone / polyethersulfone raw material is 0.1-0.5%.

5. The method according to claim 4, characterized in that, The multivalent metal ions include Ca. 2+ Mg 2+ Al 3+ Fe 3+ One or more of the following; polyamine-polyacid ion pairs include chitosan.

6. The method according to claim 4, characterized in that, The complexation time of the polysulfone / polyethersulfone raw materials in step (3) is 30-120s.

7. The method according to claim 4, characterized in that, It also includes step (4), after the polysulfone / polyethersulfone raw material treated in step (3) is processed by a wet process to obtain the initial product of the separation membrane, it is rinsed with deionized water and then dried at 40-60℃.

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