Preparation method of polyvinylidene fluoride separation membrane in-situ hydrophilic modified by coordination cross-linking function coagulation bath

Abstract

This invention discloses a method for preparing an in-situ hydrophilic modified polyvinylidene fluoride (PVDF) separation membrane using a coordination crosslinking functional coagulation bath, belonging to the field of membrane separation materials technology. Polyacrylic acid and MOF-808 are added together to an aqueous functional coagulation bath. The carboxyl or carboxylate groups of polyacrylic acid form coordination crosslinks with the metal nodes of MOF-808. The nascent PVDF membrane is immersed in the functional coagulation bath for a non-solvent-induced phase inversion, allowing the coordination crosslinked composite of polyacrylic acid and MOF-808 to enter from the coagulation bath side and be at least partially retained on the membrane surface and in the membrane pores. The functional components are not pre-added to the casting solution, and no further surface activation, bonding, or crosslinking deposition is required after membrane formation. This invention improves the stability of the hydrophilic functional components in the membrane and addresses the shortcomings of single hydrophilic polymer modification in terms of selectivity, separation performance, antifouling performance, and complexity of the preparation process in existing PVDF membrane modification processes.

Description

Technical Field

[0001] This invention relates to the field of membrane separation materials technology, specifically to a method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modified polyvinylidene fluoride separation membrane. Background Technology

[0002] Wastewater from industries such as dyeing, textiles, and papermaking often contains dyes and charged organic pollutants, characterized by high color intensity, strong water solubility, stable chemical structure, and poor biodegradability, making it difficult to treat. Congo red is a typical anionic azo dye containing aromatic rings, azo groups, and sulfonic acid groups in its molecule. It has high water solubility and is often used as a pollutant model. Membrane separation technology has advantages such as simple process and ease of continuous operation, and is often used in water treatment processes. Polyvinylidene fluoride (PVDF) has good chemical stability, thermal stability, and mechanical properties, making it a commonly used separation membrane material. However, PVDF has low surface energy and strong hydrophobicity, which easily leads to insufficient wettability, low water flux, and severe membrane fouling when used in water treatment, limiting its further application in this field. Therefore, hydrophilic modification and pore structure control of PVDF membranes are key approaches to improving their water treatment performance. Currently, the main methods for hydrophilic modification of PVDF separation membranes are as follows:

[0003] Blending modification of casting solution: Inorganic or organic-inorganic hybrid nanoparticles, hydrophilic polymers, or amphiphilic copolymers are directly added to the PVDF casting solution to prepare hydrophilic composite membranes via phase inversion. This method is simple to operate, and the hydrophilic components can be distributed in the membrane bulk and on the membrane surface. However, high-viscosity PVDF casting solutions have poor compatibility with inorganic or porous particles, and the particles are prone to agglomeration, sedimentation, or uneven distribution. Agglomerates can also block membrane pores and reduce water flux, and particles may precipitate and detach during water washing or long-term operation.

[0004] Post-film formation surface modification: Due to the chemical inertness and low surface energy of PVDF, the bonding force between the hydrophilic layer or functional particles and the base membrane is usually weak. After PVDF membrane formation, hydrophilic layers or functional coatings are introduced onto the membrane surface through methods such as surface coating, chemical grafting, plasma treatment, or layer-by-layer self-assembly to improve the membrane's wettability and separation performance. These additional steps significantly increase process complexity and cost, and may cause membrane pore blockage or membrane structure damage.

[0005] Coagulation bath modification: In the process of non-solvent-induced phase inversion, hydrophilic polymers or functional additives are added to the coagulation bath, allowing the modified components to migrate and accumulate from the coagulation bath side to the primary membrane interface during solvent-non-solvent exchange, thereby simultaneously achieving membrane formation and surface or near-surface layer modification. Although hydrophilic components can be introduced simultaneously during the phase inversion process, if the selected hydrophilic polymer has an excessively large molecular weight or poor solubility and dispersion performance at the concentration used in the coagulation bath, it can easily cause excessively high local viscosity in the coagulation bath or the formation of heterogeneous clumps, making it difficult to stably participate in the rapid phase inversion process, affecting the repeatability of film formation and the modification effect. In addition, single hydrophilic substances are easily migrated and lost during long-term water washing and filtration, and their separation of pollutants mainly relies on pore size sieving and charge repulsion, with limited selective retention capacity and anti-fouling durability.

[0006] Therefore, existing PVDF membrane modification technologies still have problems such as uneven introduction of functional materials, unstable retention of functional components, difficulty in controlling membrane structure, and complexity of preparation process. Summary of the Invention

[0007] To address the technical problems mentioned in the background section, this invention provides a method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modified polyvinylidene fluoride separation membrane, the technical solution of which is as follows:

[0008] Includes the following steps:

[0009] S1. Dissolve polyvinylidene fluoride in an organic solvent to prepare PVDF casting solution;

[0010] S2. Disperse MOF-808 in non-solvent water, add polyacrylic acid with a weight average molecular weight of 2,000 to 5,000 to dissolve and mix, and obtain an aqueous functional coagulation bath. MOF-808 is a metal-organic framework material, which is a type of porous crystalline material formed by the self-assembly of metal clusters and organic ligands through coordination bonds.

[0011] Polyacrylic acid has coordinating groups such as carboxyl groups and carboxyl groups, and forms a homogeneous aqueous solution under coagulation bath conditions;

[0012] MOF-808 has water-molecule-accessible pores and accessible Zr-group coordination sites;

[0013] The carboxyl and carboxylate groups of polyacrylic acid are at least partially coordinated with the accessible Zr group coordination sites of MOF-808 to form a polyacrylic acid-MOF-808 coordination crosslinked composite dispersed in the coagulation bath;

[0014] S3. Prepare a nascent liquid membrane from the PVDF casting solution in S1 and immerse it in the functional coagulation bath obtained in S2 for non-solvent-induced phase transformation. During solvent-non-solvent exchange and PVDF curing, the pre-formed polyacrylic acid and MOF-808 coordination crosslinking composite migrates from the coagulation bath side to the nascent liquid membrane interface or is retained, and is at least partially retained on the membrane surface and membrane pores of the formed membrane, forming a coordination crosslinking composite interface.

[0015] S4. The membrane formed by phase inversion is immersed in water for washing to remove residual organic solvents and unfixed components, thereby obtaining a hydrophilic modified PVDF separation membrane.

[0016] Furthermore, the concentration of polyacrylic acid in the functional coagulation bath is 0.01–75 g / L, and the concentration of MOF-808 in the functional coagulation bath is 0.001–0.5 g / L. Based on the presence of 200 mL of deionized water in the coagulation bath, the functional coagulation bath contains 0.002–15 g of polyacrylic acid and 0.2–100 mg of MOF-808.

[0017] Furthermore, based on a coagulation bath containing 200 mL of deionized water, the functional coagulation bath contains 1.8 g of polyacrylic acid and 2–50 mg of MOF-808.

[0018] Furthermore, when preparing the functional coagulation bath, the aqueous system containing MOF-808 is first ultrasonically dispersed, and then polyacrylic acid is added and stirred for 0.5 to 4 hours. During the period from the completion of the preparation of the functional coagulation bath to the completion of the phase transformation of the initial liquid film, no macroscopic sedimentation or flocculation affecting the uniformity of film formation occurs in the functional coagulation bath.

[0019] Furthermore, the organic solvent of the PVDF casting solution is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide, the solid content of the PVDF casting solution is 10-25 wt%, the wet film thickness of the nascent liquid film is 100-500 μm, and the water washing time in step S4 is 24-72 h.

[0020] Furthermore, the organic solvent is N,N-dimethylformamide, the wet film thickness of the nascent liquid film is 300 μm, the nascent liquid film is maintained in the functional coagulation bath until a self-supporting membrane is formed, the water washing time in step S4 is 48 h, and the water is replaced every 8 h.

[0021] Furthermore, the coordination crosslinking functional coagulation bath in-situ hydrophilic modified polyvinylidene fluoride separation membrane includes a polyvinylidene fluoride porous matrix and a coordination crosslinking composite interface distributed on the membrane surface and in the membrane channels of the polyvinylidene fluoride porous matrix.

[0022] The coordination crosslinking composite interface includes polyacrylic acid and MOF-808 with a weight average molecular weight of 2,000 to 5,000. The carboxyl and carboxylate groups of polyacrylic acid form coordination links with the Zr group nodes of MOF-808.

[0023] The coordination cross-linking composite interface is formed by the polyacrylic acid and MOF-808 coordination cross-linking composite pre-formed in an aqueous functional coagulation bath, which is introduced and fixed from the coagulation bath side during the solvent-inducible phase transformation of polyvinylidene fluoride.

[0024] Neither polyacrylic acid nor MOF-808 requires the pre-addition of PVDF casting solution;

[0025] The polyacrylic acid and MOF-808 coordination crosslinking composite is formed before the nascent liquid film comes into contact with the functional coagulation bath, and is introduced from the coagulation bath side and fixed in the film surface and membrane channels during the non-solvent-induced phase transformation process, without undergoing separate surface activation, crosslinking agent treatment and deposition growth steps after film formation.

[0026] The present invention has the following advantages:

[0027] Compared to directly adding MOF-808 to the PVDF casting solution, this invention places MOF-808 in a low-viscosity aqueous functional coagulation bath, which helps reduce the risk of its agglomeration and uneven distribution in the high-viscosity casting solution.

[0028] Compared with the method of depositing or growing MOF-808 after film formation, the present invention completes the introduction and coordination crosslinking fixation of PAA (polyacrylic acid) and MOF-808 in the phase transformation step of the functional coagulation bath, without the need for additional surface activation, adhesive layer, crosslinking agent, seed crystal layer or multi-step deposition process.

[0029] Materials can be screened based on the functional criteria of "water-soluble hydrophilic polymers with coordination functional groups and water-stable hydrophilic MOFs with coordination sites," eliminating combinations that are unsuitable for water-based coagulation baths or water treatment environments.

[0030] Hydrophilic polymers provide hydrophilic modification and charge effects, MOFs provide pore sieving and adsorption sites, and coordination networks further regulate pore size and the stability of the two types of functional components in the membrane, which can improve hydrophilicity, antifouling and separation performance. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the preparation process of the in-situ modified PVDF separation membrane by the coordination crosslinking functional coagulation bath of the present invention;

[0032] Figure 2 This is a schematic diagram showing the coordination and crosslinking composite interface formed by PAA and MOF-808 on the surface and pores of a PVDF membrane according to the present invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Please refer to Figure 1-2 ,in Figure 2 It should be noted that the actual microstructure of the PVDF membrane will vary depending on the type of solvent used and the coagulation bath conditions. This schematic diagram is only used to illustrate the existence and formation mode of the coordination crosslinking composite interface in the membrane. This invention is based on the technical concept of "pre-coordination composite and in-situ fixation of the phase transformation interface in a functional coagulation bath." It constructs a water-soluble coordination-type hydrophilic polymer / water-stable hydrophilic MOF coordination system in the same aqueous coagulation bath, and simultaneously introduces and fixes the functional components during the solvent-inducible phase transformation of PVDF, thereby obtaining a hydrophilic modified separation membrane with a coordination crosslinking composite interface.

[0035] Specifically, the core mechanism of this technology is as follows:

[0036] (I) Pre-coordination recombination mechanism in coagulation bath

[0037] Polyacrylic acid (PAA) with a weight-average molecular weight of 2,000–5,000 was co-dispersed with MOF-808 in a water-based functional coagulation bath. MOF-808 is a water-stable Zr-based metal-organic framework material with a crystal structure containing pores that allow water molecules to enter and accessible Zr coordination sites. The PAA molecular chain contains multiple carboxyl and / or carboxylate groups. In the aqueous environment of the coagulation bath, these groups can coordinate with the accessible Zr sites at the nodes of MOF-808 to form a PAA / MOF-808 coordination crosslinked composite. This composite is in an aqueous dispersion state in the functional coagulation bath, avoiding the agglomeration and uneven distribution problems that easily occur when MOF particles are directly dispersed in high-viscosity casting liquid.

[0038] (ii) In-situ interface fixation mechanism during phase transformation

[0039] When the nascent PVDF liquid film is immersed in the aforementioned functional coagulation bath, a bidirectional exchange of solvent (organic solvent) and non-solvent (water) immediately occurs. As the non-solvent diffuses into the interior of the nascent liquid film and the solvent diffuses outward, the PVDF gradually precipitates and solidifies to form a porous matrix. During this dynamic phase transformation, the PAA / MOF-808 coordination complex pre-formed in the coagulation bath migrates from the coagulation bath side to the interface region of the nascent liquid film under the drive of the concentration gradient. Simultaneously, the forming PVDF membrane structure physically traps and anchors this complex, ensuring that it is at least partially retained on the surface and in the pores of the final formed membrane. This results in the in-situ formation of a coordination crosslinking composite interface within the PVDF matrix, eliminating the need for post-film formation surface activation, adhesive layer construction, or crosslinking agent treatment.

[0040] (III) The principle of synergistic adaptation in material selection

[0041] In this invention, the material properties of PAA and MOF-808 must meet the conditions for synergistic compatibility, specifically in the following two aspects:

[0042] (1) The selection of the molecular weight of PAA is the key to balancing the "operability of coagulation bath" and "interfacial function construction". PAA with a weight average molecular weight of 2,000 to 5,000 can quickly dissolve to form a homogeneous aqueous solution under the concentration and stirring conditions of this application, ensuring that the functional coagulation bath has good operational uniformity; at the same time, the chain length of PAA in this molecular weight range is moderate, which can not only migrate and enrich rapidly to the phase transformation interface, but also has enough carboxyl sites to form multi-point bridging with multiple MOF-808 particles, thereby constructing a three-dimensional cross-linked network, and will not cause excessive coiling due to excessive chain length, resulting in excessive densification of membrane pores.

[0043] (2) PAA plays a dual role in this system: it acts as a hydrophilic modifier to improve the wettability of the PVDF membrane and regulate the membrane pore size, and as an aqueous dispersion stabilizer and in-situ coordination crosslinking agent for MOF-808. Through the coordination of carboxyl groups and Zr groups, it stably fixes MOF-808 particles to the membrane interface region. This "hydrophilic polymer-metal-organic framework" coordination crosslinking interface is expected to synergistically exert the hydrophilicity and charge-carrying properties of PAA and the pore sieving function of MOF-808, while improving the retention stability of functional components in the membrane through the coordination crosslinking structure.

[0044] The amounts of PAA and MOF-808 together affect the dispersion state of the composite components in the functional coagulation bath, as well as the solvent-non-solvent exchange and phase transformation processes, and further influence the interfacial composition and pore structure of the resulting membrane.

[0045] This invention provides a method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modified polyvinylidene fluoride separation membrane, comprising the following steps:

[0046] S1. Dissolve polyvinylidene fluoride (PVDF) in an organic solvent to prepare a casting solution with a PVDF content of 10–25 wt%. The organic solvent is selected from N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. It should be able to fully dissolve PVDF and undergo solvent-non-solvent exchange with water to meet the requirements of the non-solvent-induced phase transformation process.

[0047] S2. Prepare the functional coagulation bath by first ultrasonically dispersing the aqueous system containing MOF-808, then adding polyacrylic acid with a weight-average molecular weight of 2,000–5,000 and stirring for 0.5–4 h. During the period from the completion of the preparation of the functional coagulation bath to the completion of the phase transformation of the initial liquid film, no macroscopic sedimentation or flocculation affecting the uniformity of film formation occurs in the functional coagulation bath. The concentration of polyacrylic acid in the functional coagulation bath is 0.01–75 g / L, and the concentration of MOF-808 in the functional coagulation bath is 0.001–0.5 g / L. Based on 200 mL of deionized water in the coagulation bath, the functional coagulation bath contains 0.002–15 g of polyacrylic acid and 0.2–100 mg of MOF-808.

[0048] MOF-808 possesses water-permeable channels and accessible Zr-group coordination sites. The carboxyl and / or carboxyl groups of polyacrylic acid form coordination links with the Zr-group nodes of MOF-808. A polyacrylic acid / MOF-808 coordination crosslinking composite is pre-formed in a functional coagulation bath. Polyacrylic acid with a weight-average molecular weight of 2,000–5,000 is selected to ensure rapid dissolution in the coagulation bath to form a homogeneous system and rapid migration and enrichment to the membrane surface during phase inversion. Simultaneously, its suitable chain length allows for coordination with multiple MOF-808 particles to construct a three-dimensional crosslinked network while avoiding excessive shrinkage leading to over-densification of the channels. A polyacrylic acid aqueous solution dosage of 0.01–75 g / L is selected to achieve an optimal balance between hydrophilicity improvement and pore structure control by regulating the carboxyl concentration and phase inversion rate in the coagulation bath. A dosage of 0.001–0.5 g / L is chosen. The purpose of adding g / L of MOF-808 is to introduce coordination crosslinking nodes and adsorption functional sites in an appropriate ratio to regulate separation performance, while taking into account the dispersion stability of MOF-808 in the coagulation bath.

[0049] S3. The casting solution of S1 is used to prepare a nascent liquid film and is immersed in the functional coagulation bath obtained in S2 for non-solvent-induced phase transformation. During the solvent-non-solvent exchange and PVDF curing process, the pre-formed polyacrylic acid / MOF-808 coordination crosslinking composite migrates from the coagulation bath side to the nascent liquid film interface and is trapped, and is at least partially retained on the surface and pores of the formed film to form a coordination crosslinking composite interface. The wet film thickness of the nascent liquid film is 100-500 μm.

[0050] S4. The membrane formed by phase inversion is immersed and washed in water to remove residual organic solvents and unfixed components, resulting in a hydrophilic modified polyvinylidene fluoride separation membrane. Neither polyacrylic acid nor MOF-808 is added to the PVDF casting solution beforehand. The polyacrylic acid / MOF-808 coordination crosslinking composite is formed before the nascent liquid membrane comes into contact with the functional coagulation bath, and is introduced from the coagulation bath side and fixed to the membrane surface and membrane channels during the non-solvent-induced phase inversion process. No separate surface activation, adhesive layer construction, crosslinking agent treatment, seed crystal or deposition growth steps are performed after film formation.

[0051] The hydrophilic modified polyvinylidene fluoride separation membrane obtained by the above method can be used to separate aqueous solutions containing dyes and organic pollutants, so that pollutants are retained by at least one of the following: membrane pore size sieving, coordination cross-linking interfacial charge interaction, and MOF material affinity adsorption.

[0052] In this application, the water-soluble coordination-type hydrophilic polymer refers to a hydrophilic polymer that can form a homogeneous aqueous solution under the actual usage concentration, temperature, and stirring conditions of the functional coagulation bath, and contains at least one functional group capable of coordinating with MOF metal nodes, defect sites, or terminal metal ligands. The functional group can be selected from carboxyl groups, carboxyl groups, hydroxyl groups, amino groups, imidazole groups, pyridinyl groups, phosphonic acid groups, and catechol groups. In addition to polyacrylic acid, polymethacrylic acid, polyaspartic acid, polyglutamic acid, alginate, carboxymethyl cellulose, acrylic acid maleic acid copolymer, hydrolyzed polymaleic anhydride, polyethyleneimine, water-soluble polymers containing phosphonic acid groups, and water-soluble polymers containing catechol groups can also be selected, but water-soluble polymers containing carboxyl groups or carboxyl groups are preferred, and polyacrylic acid with a weight average molecular weight of 2,000 to 5,000 is more preferred.

[0053] Water-stable hydrophilic MOFs refer to porous framework materials composed of metal ions or metal cluster nodes and organic ligands through coordination bonds, simultaneously satisfying the following conditions: maintaining the main crystalline framework in the functional coagulation bath and membrane operating water environment; pore walls containing hydrophilic functional groups such as hydroxyl, carboxyl, amino, aldehyde, sulfonic acid, terminal coordinated water, and terminal hydroxyl groups; possessing water-permeable channels; possessing accessible metal sites that can coordinate with the coordination functional groups of hydrophilic polymers; and being able to be retained under water washing and filtration conditions after being introduced into the membrane along with hydrophilic polymers, contributing to separation performance or operational stability. Optional MOFs include Zr. The MOFs include Ti-based, Ti-based, and water-stable Fe-based, Al-based, or Cr-based MOFs, as well as ZIF materials belonging to the MOF subclass that meet the above conditions, such as MOF-808, UiO-66, UiO-66-NH2, UiO-66-(COOH)2, NU-1000, MIL-53, MIL-101, MIL-125-NH2, ZIF-90, ZIF-93, and ZIF-8 or ZIF-67 that have undergone hydrophilic functionalization or defect regulation. The above materials still need to meet the conditions of water stability, water inlet channels, and coordination fixation before they can be used, with MOF-808 being the preferred choice.

[0054] In this application, "functional coagulation bath" refers to an aqueous coagulation bath containing PAA and MOF-808 during the non-solvent-induced phase transformation of the nascent liquid film, and the formation of the PAA / MOF-808 composite interface is completed during this phase transformation process, rather than forming a film in the first coagulation bath and then transferring it to the second crosslinking bath.

[0055] In this application, "homogeneous aqueous solution" refers to a solution that, under the conditions of actual concentration in the functional coagulation bath and stirring at room temperature for 0.5 to 4 hours, does not contain any undissolved PAA clumps visible to the naked eye and can be used for uniform film coating and phase inversion.

[0056] In this application, "no visible sedimentation or flocculation affecting the uniformity of film formation" means that during the film-forming period from the completion of the preparation of the functional coagulation bath to the completion of the phase transformation of the nascent liquid film, no visible deposits or flocs appear in the system that would cause obvious particle accumulation, streaks or through-hole defects on the film surface. This limitation does not exclude the normal Brownian motion, slight concentration fluctuations or reversible aggregation of nano- or micro-scale particles in the system that does not affect film formation.

[0057] In this application, the "coordination cross-linking composite interface" refers to the coexistence of PAA and MOF-808 on the surface and in the pores of the PVDF membrane, and the formation of a multi-point connected composite region by the coordination between the carboxyl or carboxylate groups of PAA and the Zr group nodes of MOF-808. It can be characterized by one or more methods such as Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, elemental analysis or surface scanning, Zr element retention before and after water washing, particle size or rheological changes.

[0058] Example 1: MOF-808 raw material

[0059] This embodiment uses MOF-808 powder. MOF-808 can be a commercially available product or prepared using existing solvothermal methods. Before use, it can be confirmed by X-ray diffraction that it has the main structural features of MOF-808. In order to avoid the influence of differences in MOF-808 preparation process on the membrane preparation results, the same batch of MOF-808 was used in the same set of comparative experiments.

[0060] Example 2: Preparation of PVDF casting solution

[0061] Weigh 15 g PVDF and add it to 85 mL N,N-dimethylformamide. Stir for 6 h until a uniform and transparent casting solution is formed. Let the casting solution stand for 4–12 h to remove bubbles before use.

[0062] Comparative Example 1: Unmodified PVDF membrane

[0063] The PVDF casting solution obtained in Example 2 was uniformly poured onto a clean glass plate. A nascent liquid film was uniformly scraped using a film scraper with a wet film thickness of 300 μm. The film was then immediately immersed in a coagulation bath consisting of 200 mL of deionized water. The nascent liquid film was kept until the phase transformation was completed and a self-supporting membrane was formed. After the membrane solidified and peeled off from the glass plate, it was transferred to fresh deionized water and soaked for 48 h. The water was changed every 8 h to obtain an unmodified PVDF membrane, denoted as PVDF-0.

[0064] Comparative Examples 2-4: PAA coagulation bath modified PVDF membrane

[0065] 0.9 g, 1.8 g, and 2.7 g of PAA with a weight average molecular weight of approximately 3,000 were measured and added to coagulation baths containing 200 mL of deionized water, respectively. The mixtures were stirred for 2 h to form homogeneous PAA coagulation baths. Following the same membrane preparation, phase inversion, and washing steps as in Comparative Example 1, PAA-0.9, PAA-1.8, and PAA-2.7 membranes were obtained, respectively.

[0066] Comparative Example 5: Coagulation Bath Suitability of High Molecular Weight PAA

[0067] When PAA with a weight-average molecular weight of approximately 450,000 was substituted for PAA with a weight-average molecular weight of approximately 3,000, and an aqueous coagulation bath was prepared according to the same target addition amount and room temperature stirring conditions as PAA-1.8, the dissolution and homogenization rate of this high molecular weight PAA system was slow at the concentration and stirring time of 2 h, and local high viscosity or inhomogeneity was prone to occur. It was difficult to form an operable coagulation bath suitable for rapid and homogeneous phase transformation. This result only indicates that it is not applicable under the concentration, temperature and processing time of this embodiment.

[0068] Examples 3-6: PAA / MOF-808 functional coagulation bath modified PVDF membrane

[0069] 5 mg, 10 mg, 15 mg, and 20 mg of MOF-808 were added to four systems containing 200 mL of deionized water, respectively. The mixture was first ultrasonically dispersed, and then 1.8 g of PAA with a weight-average molecular weight of approximately 3,000 was added to each system. The mixture was stirred for 2 h to obtain four functional coagulation baths. During the preparation of the functional coagulation baths and membrane fabrication, no visible macroscopic sedimentation or flocculation that would affect the uniformity of membrane scraping and phase inversion was observed in the system. The membranes were fabricated according to the same scraping, phase inversion, and washing steps as in Comparative Example 1 to obtain PM-5, PM-10, PM-15, and PM-20 membranes, respectively.

[0070] Example 7: Membrane Performance Testing Method

[0071] (1) Water contact angle: The dry membrane is flat and fixed, deionized water is dropped on the membrane surface, and the contact angle after the droplet stabilizes is recorded. Three sets of samples are taken for each type of membrane, and at least three different positions are tested in each set and the average value is calculated.

[0072] (2) Pure water permeability: Using a pressure-driven filtration device, at room temperature, transmembrane pressure of 3 bar, and effective membrane area of ​​7.065 cm², the results were obtained. 2 Test under the following conditions. Pre-pressurize at 3 bar for at least 30 min before testing. Pure water permeability P is calculated according to formula (1):

[0073] P = V / (A·Δt·ΔP) (1)

[0074] In the formula, V is the permeate volume, A is the effective membrane area, Δt is the filtration time, and ΔP is the transmembrane pressure.

[0075] Congo red rejection: Using a 200 mg / L Congo red aqueous solution as the feed solution, the mixture was filtered at room temperature and a transmembrane pressure of 3 bar. The concentration of Congo red in the feed solution and the permeate was determined by UV-Vis spectrophotometry. The rejection rate R was calculated according to formula (2):

[0076] R = (1 - C p / Cf) × 100% (2)

[0077] In the formula, C p Cf is the concentration of Congo red in the permeate and the concentration of Congo red in the feed solution. The permeability of the Congo red solution is calculated using the same pressure normalization method as Equation (1).

[0078] Example 8: Effect of PAA Addition Amount

[0079] When the amount of PAA with a weight-average molecular weight of approximately 3,000 increased from 0 g to 1.8 g, the water contact angle on the membrane surface showed a decreasing trend, while the water permeability gradually increased from 3 L·m -2 ·h -1 ·bar -1 (PVDF-0) increased to 14.5 L·m -2 ·h -1 ·bar -1 (PAA-1.8) Further increasing PAA to 2.7g resulted in decreased water permeability. In the Congo red rejection experiment, the rejection rates of Congo red for PAA-0.9, PAA-1.8, and PAA-2.7 membranes were approximately 60%, 70%, and 80%, respectively. These results indicate that low molecular weight PAA can form a homogeneous system in the coagulation bath and participate in phase transformation, improving the surface hydrophilicity of the membrane and regulating its pore structure. However, excessive addition may lead to a denser pore structure or increase water transport resistance. Considering both water permeability and Congo red rejection performance, 1.8g of PAA was selected as the base dosage for the PAA / MOF-808 functional coagulation bath.

[0080] Example 9: Effect of MOF-808 Addition Amount

[0081] After adding 5 mg, 10 mg, 15 mg, and 20 mg of MOF-808 to a 1.8 g low molecular weight PAA aqueous coagulation bath, respectively, the water contact angles between the PM series membranes remained relatively stable at approximately 70°. The rejection rates of Congo red for PM-5, PM-10, PM-15, and PM-20 were approximately 75%, 90%, 95%, and 97%, respectively. Compared to the Congo red rejection rate of PAA-1.8 (approximately 70%), the Congo red rejection rate continuously increased with increasing MOF-808 dosage, with PM-20 showing a pure water permeability of approximately 13 L·m⁻¹. -2 ·h -1 ·bar -1 Meanwhile, the Congo Red retention rate reached approximately 97%, demonstrating a good balance between water transport and dye retention.

[0082] Example 10: Verification method for functional coagulation bath and coordination interface

[0083] To further confirm that the functional coagulation bath and the resulting membrane conform to the technical features of this invention, the membrane preparation process and membrane samples can be verified as follows: observe whether there is any visible sedimentation or flocculation affecting film formation during the period from the completion of preparation to the end of phase transformation of the functional coagulation bath; observe the Zr element distribution by scanning electron microscopy and elemental surface scanning of the membrane surface and cross-section; examine the changes in PAA carboxyl environment and Zr-related signals by Fourier transform infrared spectroscopy or X-ray photoelectron spectroscopy; evaluate the retention of MOF-808 by the Zr element content in the membrane or the Zr content in the leachate before and after thorough water washing. The above methods are used for verification and do not constitute additional limitations on the scope of protection.

[0084] The performance of the examples and comparative examples is summarized in the table below:

[0085]

[0086] The performance test results from the examples and comparative examples show that the water contact angle of the unmodified PVDF-0 membrane is approximately 120°, and the pure water permeability is approximately 3 L·m. -2 ·h -1 ·bar -1 The Congo red rejection rate was approximately 55%. After using a low molecular weight PAA coagulation bath, the overall water contact angle of the membrane decreased, and the pure water permeability significantly increased. Specifically, the pure water permeability of the PAA-1.8 membrane reached approximately 14.5 L·m⁻¹. -2 ·h -1 ·bar -1 After further introducing MOF-808 into the 1.8 g PAA base, the water contact angle of each PM series membrane remained relatively stable at approximately 70°. With increasing MOF-808 addition, the Congo red rejection rate increased from approximately 75% to approximately 97%. These results indicate that the PAA coagulation bath can improve the hydrophilicity and water transport performance of PVDF membranes. Introducing MOF-808 further enhances the dye rejection performance of the membranes, thus achieving a better balance between water transport and separation performance.

[0087] This invention is simple to operate, convenient to use, and suitable for widespread promotion and application. Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modification of polyvinylidene fluoride separation membrane, characterized in that, Includes the following steps: S1. Dissolve polyvinylidene fluoride in an organic solvent to prepare PVDF casting solution; S2. Disperse MOF-808 in non-solvent water, add polyacrylic acid with a weight average molecular weight of 2,000 to 5,000 to dissolve and mix, and obtain an aqueous functional coagulation bath. Polyacrylic acid has coordinating groups such as carboxyl groups and carboxyl groups, and forms a homogeneous aqueous solution under coagulation bath conditions; MOF-808 has water-molecule-accessible pores and accessible Zr-group coordination sites; The carboxyl and carboxylate groups of polyacrylic acid are at least partially coordinated with the accessible Zr group coordination sites of MOF-808 to form a polyacrylic acid-MOF-808 coordination crosslinked composite dispersed in the coagulation bath; S3. Prepare a nascent liquid membrane from the PVDF casting solution in S1 and immerse it in the functional coagulation bath obtained in S2 for non-solvent-induced phase transformation. During solvent-non-solvent exchange and PVDF curing, the pre-formed polyacrylic acid and MOF-808 coordination crosslinking composite migrates from the coagulation bath side to the nascent liquid membrane interface or is retained, and is at least partially retained on the membrane surface and membrane pores of the formed membrane, forming a coordination crosslinking composite interface. S4. The membrane formed by phase inversion is immersed in water for washing to remove residual organic solvents and unfixed components, thereby obtaining a hydrophilic modified PVDF separation membrane.

2. The method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modification of polyvinylidene fluoride separation membrane according to claim 1, characterized in that, The concentration of polyacrylic acid in the functional coagulation bath is 0.01–75 g / L, and the concentration of MOF-808 in the functional coagulation bath is 0.001–0.5 g / L. Based on the presence of 200 mL of deionized water in the coagulation bath, the functional coagulation bath contains 0.002–15 g of polyacrylic acid and 0.2–100 mg of MOF-808.

3. The method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modification of polyvinylidene fluoride separation membrane according to claim 2, characterized in that, Based on a coagulation bath containing 200 mL of deionized water, the functional coagulation bath contains 1.8 g of polyacrylic acid and 2–50 mg of MOF-808.

4. A method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modified polyvinylidene fluoride separation membrane according to any one of claims 1 to 3, characterized in that, When preparing the functional coagulation bath, the aqueous system containing MOF-808 is first ultrasonically dispersed, and then polyacrylic acid is added and stirred for 0.5 to 4 hours. During the period from the completion of the preparation of the functional coagulation bath to the completion of the phase transformation of the initial liquid film, no macroscopic sedimentation or flocculation affecting the uniformity of film formation occurs in the functional coagulation bath.

5. A method for preparing a coordination crosslinking functional coagulation bath in-situ hydrophilic modified polyvinylidene fluoride separation membrane according to any one of claims 1 to 4, characterized in that, The organic solvent of the PVDF casting solution is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide. The solid content of the PVDF casting solution is 10-25 wt%, the wet film thickness of the initial liquid film is 100-500 μm, and the water washing time in step S4 is 24-72 h.

6. The preparation method according to claim 5, characterized in that, The organic solvent is N,N-dimethylformamide, the wet film thickness of the nascent liquid film is 300 μm, the nascent liquid film is maintained in the functional coagulation bath until a self-supporting membrane is formed, the water washing time in step S4 is 48 h, and the water is replaced every 8 h.

7. A coordination crosslinking functional coagulation bath in-situ hydrophilic modified polyvinylidene fluoride separation membrane prepared by the method according to any one of claims 1 to 6, characterized in that, It includes a polyvinylidene fluoride porous matrix and a coordination crosslinking composite interface distributed on the membrane surface and in the membrane channels of the polyvinylidene fluoride porous matrix; The coordination crosslinking composite interface includes polyacrylic acid and MOF-808 with a weight average molecular weight of 2,000 to 5,000. The carboxyl and carboxylate groups of polyacrylic acid form coordination links with the Zr group nodes of MOF-808. The coordination cross-linking composite interface is formed by the polyacrylic acid and MOF-808 coordination cross-linking composite pre-formed in an aqueous functional coagulation bath, which is introduced and fixed from the coagulation bath side during the solvent-inducible phase transformation of polyvinylidene fluoride. Neither polyacrylic acid nor MOF-808 requires the pre-addition of PVDF casting solution; The polyacrylic acid and MOF-808 coordination crosslinking composite is formed before the nascent liquid film comes into contact with the functional coagulation bath, and is introduced from the coagulation bath side and fixed in the film surface and membrane channels during the non-solvent-induced phase transformation process, without undergoing separate surface activation, crosslinking agent treatment and deposition growth steps after film formation.

Images