A polyvinylidene fluoride fiber membrane and a method for manufacturing the same

Abstract

The application provides a polyvinylidene fluoride fiber membrane and a preparation method thereof. The polyvinylidene fluoride fiber membrane comprises a polyvinylidene fluoride matrix and a crosslinked double interpenetrating network structure penetrating through the polyvinylidene fluoride matrix; the crosslinked double interpenetrating network structure comprises a first crosslinked network and a second crosslinked network; the first crosslinked network is formed by crosslinking of a first functional monomer; the second crosslinked network is formed by crosslinking of a second functional monomer; and the second functional monomer comprises a hydrophilic functional group. The polyvinylidene fluoride fiber membrane of the application realizes the common improvement of the hydrophilicity and mechanical strength of the membrane material by constructing two types of completely interpenetrating crosslinked networks (the first crosslinked network and the second crosslinked network) and utilizing the synergistic effect of the double network and the PVDF matrix.

Description

Technical Field

[0001] This invention relates to a polyvinylidene fluoride fiber membrane, and more particularly to a polyvinylidene fluoride fiber membrane and its preparation method, belonging to the field of filter membrane technology. Background Technology

[0002] Membrane separation technology is a highly efficient, energy-saving, and multifunctional separation method, widely used in processes such as concentration, purification, and refining. Among various membrane materials, hollow fiber membranes have become one of the most widely used membrane forms due to their advantages such as good self-support, high packing density per unit volume, excellent fouling resistance, high recovery rate, and low replacement cost. They are extensively used in wastewater treatment, water purification, and material separation, concentration, and recovery processes in fields such as environmental protection, food industry, medical industry, and electronics industry.

[0003] The overall performance of hollow fiber membranes depends not only on the uniformity of their pore structure but also on the hydrophilicity and mechanical strength of the membrane material. Insufficient mechanical strength can easily lead to membrane rupture, deformation, or even collapse and blockage during operation or recycling, severely affecting the membrane's service life and separation stability. When the membrane surface lacks sufficient hydrophilicity, it easily adsorbs organic pollutants such as proteins, exacerbating membrane fouling and causing membrane flux decline, reduced separation efficiency, and decreased product recovery rate, thus limiting its application in fine separation fields such as biopharmaceuticals and the food industry. Therefore, in the structural design and material selection of hollow fiber membranes, it is necessary to simultaneously consider high hydrophilicity and high mechanical strength. Summary of the Invention

[0004] This invention provides a polyvinylidene fluoride (PVDF) fiber membrane. This PVDF fiber membrane achieves simultaneous improvement in the hydrophilicity and mechanical strength of the membrane material by constructing two types of fully interpenetrating cross-linked networks (a first cross-linked network and a second cross-linked network) within a PVDF matrix and utilizing the synergistic effect of the cross-linked networks with the PVDF matrix in terms of structural support and hydrophilic fixation.

[0005] The present invention also provides a method for preparing a polyvinylidene fluoride fiber membrane, which can be used to prepare a polyvinylidene fluoride fiber membrane that combines hydrophilicity and mechanical strength.

[0006] This invention provides a polyvinylidene fluoride (PVDF) fiber membrane, comprising a PVDF body and a cross-linked interpenetrating network (IPN) structure penetrating the PVDF body; the IPN structure comprises a first cross-linked network and a second cross-linked network; the first cross-linked network is formed by cross-linking a first functional monomer; the second cross-linked network is formed by cross-linking a second functional monomer; the second functional monomer includes hydrophilic functional groups.

[0007] The hydrophilic functional group includes at least one of hydroxyl, carboxyl, amide, sulfonic acid, and etheroxy groups;

[0008] The first functional monomer is methyl methacrylate.

[0009] And / or, the second functional monomer includes at least one of acrylic acid, hydroxypropyl acrylate, methacrylic acid, acrylamide, N-vinylpyrrolidone, 2-methoxyethyl acrylate, and styrene sulfonic acid.

[0010] In an optional embodiment, the mass ratio of the polyvinylidene fluoride, the first functional monomer, and the second functional monomer is 10-30:0.1-10:0.1-15.

[0011] In another aspect, the present invention provides a method for preparing a polyvinylidene fluoride fiber membrane, comprising the following steps:

[0012] A first mixture comprising polyvinylidene fluoride, a pore-forming agent, and a solvent is mixed with a second mixture comprising a first functional monomer, a second functional monomer, a crosslinking agent, and an initiator, and an in-situ crosslinking reaction is carried out at 100℃-180℃ to obtain a casting solution.

[0013] The casting solution is molded to obtain the polyvinylidene fluoride fiber membrane.

[0014] In an optional embodiment, the molding process includes the following steps:

[0015] The casting solution is extruded through a spinneret and then immersed in a coagulation bath to form a film.

[0016] In an optional embodiment, based on a total mass of 100 wt% for the first mixture, the mass fraction of the polyvinylidene fluoride is 10 wt%-30 wt%, the mass fraction of the porogen is 0.1 wt%-20 wt%, and the balance is solvent;

[0017] And / or, based on a mass fraction of 100 parts for the first mixture, the mass fraction of the first functional monomer is 0.1-10 parts, the mass fraction of the second functional monomer is 0.1-15 parts, the mass fraction of the crosslinking agent is 0.1-5 parts, and the mass fraction of the initiator is 0.1-3 parts.

[0018] In an optional embodiment, the initiator includes at least one selected from azobisisobutyronitrile, azobisisopropionitrile, benzoyl peroxide, dibenzoyl peroxide, divinyl peroxide, and tert-butyl peroxide.

[0019] And / or, the crosslinking agent includes at least one of trimethylolpropane trimethacrylate, glyceryl acrylate, and methylenebisacrylamide;

[0020] And / or, the pore-forming agent includes at least one of polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and hydroxyethyl cellulose;

[0021] Preferably, the coagulation bath includes water.

[0022] The polyvinylidene fluoride (PVDF) fiber membrane provided by this invention achieves a combined improvement in hydrophilicity and mechanical strength by constructing two types of fully interpenetrating cross-linked networks (a first cross-linked network and a second cross-linked network) and utilizing the synergistic effect of the dual networks and the PVDF matrix. The main reasons for this are: the first cross-linked network and the PVDF matrix have good compatibility, reducing the risk of phase separation during membrane formation and allowing the first cross-linked network to be more uniformly distributed within the membrane, forming a continuous interpenetrating support structure, thereby improving the mechanical strength and structural stability of the membrane material; simultaneously, the second cross-linked network interpenetrates with the first cross-linked network (polymer segments intertwine), and the second cross-linked network is anchored within the membrane by the first cross-linked network, making it less prone to migration or loss during use, thus maintaining long-term stable hydrophilicity. A stable hydrophilic layer can inhibit the adsorption of pollutants such as proteins and colloids, reducing membrane fouling and extending the membrane's service life. Attached Figure Description

[0023] Figure 1 SEM images of hollow polyvinylidene fluoride fiber membranes of Examples 1 and Comparative Examples 1-3;

[0024] Figure 2 Line graphs showing the contact angle changes of hollow polyvinylidene fluoride fiber membranes in Examples 1, 1, and 4 at the same time interval. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0026] In existing technologies, the modification of polyvinylidene fluoride fiber membranes is mainly achieved through the following two methods:

[0027] Physical blending method: Hydrophilic polymers (such as polyethylene glycol, polyvinylpyrrolidone) or inorganic nanomaterials (such as silica, zinc oxide) are directly blended into the casting solution and the membrane is formed by phase separation. Although this method can improve the hydrophilicity of the membrane surface, the hydrophilic components are prone to migration and shedding during operation or cleaning, which leads to a gradual decrease in the membrane's antifouling properties and flux stability.

[0028] Surface grafting: This method involves introducing hydrophilic polymers (such as polyacrylic acid and polyacrylamide) onto the membrane surface through chemical grafting or coating. For example, hydrophilic segments containing carboxylic acid groups can be introduced onto the membrane surface via free radical grafting, or a hydrophilic layer can be constructed through layer-by-layer self-assembly technology. However, such methods typically require additional post-processing steps, are complex, and the grafted layer is prone to peeling off during long-term use, making it difficult to achieve durable hydrophilicity.

[0029] Although the above modification schemes introduce hydrophilic components, they lack effective fixation and stabilization mechanisms. During long-term operation, these hydrophilic components are easily lost, leading to increased membrane fouling and flux decline. This invention provides the following technical solution:

[0030] This invention provides a polyvinylidene fluoride (PVDF) fiber membrane, which includes a plurality of porous structures. The PVDF fiber membrane comprises a PVDF body and a cross-linked interpenetrating network (IPN) structure penetrating the PVDF body. The IPN structure includes a first cross-linked network and a second cross-linked network. The first cross-linked network is formed by cross-linking a first functional monomer. The second cross-linked network is formed by cross-linking a second functional monomer, which includes a hydrophilic functional group. The hydrophilic functional group includes at least one of hydroxyl, carboxyl, amide, sulfonic acid, and etheroxy groups.

[0031] The first monomer is methyl methacrylate;

[0032] And / or, the second functional monomer includes at least one of acrylic acid, hydroxypropyl acrylate, methacrylic acid, acrylamide, N-vinylpyrrolidone, 2-methoxyethyl acrylate, and styrene sulfonic acid.

[0033] The above-mentioned polyvinylidene fluoride fiber membranes utilize two types of fully interpenetrating and functionally distinct cross-linked networks (the first cross-linked network and the second cross-linked network) constructed within the same membrane forming system. By leveraging the synergistic effect of the two networks, the hydrophilicity and mechanical strength of the membrane material can be synergistically improved. Specifically, the first cross-linked network and the polyvinylidene fluoride body form a continuous interpenetrating network structure at the microscale. This structure helps reduce the risk of phase separation between different components during membrane forming, thereby constructing a stable structural support system inside the membrane and enhancing the overall mechanical properties and structural stability of the polyvinylidene fluoride fiber membrane.

[0034] Meanwhile, hydrophilic functional groups can form an orderly and stable distribution on the surface and internal pore walls of polyvinylidene fluoride (PVDF) through a cross-linked double interpenetrating network structure, forming a continuous and stable hydrophilic layer. This hydrophilic interface can adsorb and maintain a layer of water molecules on the membrane surface through hydration, effectively blocking direct contact between pollutants such as proteins and colloids and the hydrophobic PVDF, thereby inhibiting pollutant adsorption and deposition and reducing membrane fouling. Crucially, the interpenetrating network stably anchors the second cross-linked network containing hydrophilic functional groups into the three-dimensional network system composed of the first cross-linked network and the PVDF, avoiding the problem of easy migration and detachment of hydrophilic functional groups in traditional surface modification. This allows the hydrophilic properties to be maintained for a long time under long-term use and hydraulic scouring, achieving stable membrane flux and extended service life.

[0035] The aforementioned hydrophilic functional groups can construct a stable hydration layer, thereby enabling the polyvinylidene fluoride (PVDF) fiber membrane to form an antifouling barrier. Specifically, the negatively charged carboxyl and sulfonic acid groups introduce electrostatic repulsion on the PVDF fiber membrane surface, effectively blocking common pollutants such as proteins and colloids that are also negatively charged. The amide and etheroxy groups enhance interfacial hydration and molecular-level compatibility. In some preferred embodiments, the hydrophilic functional groups include at least two of hydroxyl, carboxyl, amide, sulfonic acid, and etheroxy groups. This combination of multiple functional groups allows the membrane surface to establish multiple protective mechanisms against different types of pollutants, such as hydrogen bond hydration, electrostatic repulsion, and steric hindrance, thus achieving a more comprehensive and efficient antifouling effect in complex aquatic environments.

[0036] The polymer chain formed by the first functional monomer is well matched with the PVDF molecular chain in terms of polarity and solubility parameters. This compatibility allows the first functional monomer to achieve close molecular-level mixing and interpenetration with the PVDF matrix during the polymerization process to form the first crosslinking network. This enhances the interfacial bonding force between the first crosslinking network and the PVDF matrix and effectively reduces the risk of phase separation between polyvinylidene fluoride and the functional monomer system. As a result, it restricts the movement and structural relaxation of the PVDF molecular chain at the microscopic level, thereby significantly improving the overall mechanical properties and structural stability of the polyvinylidene fluoride fiber membrane.

[0037] The second functional monomers mentioned above form a stable hydrophilic network structure through cross-linking, and are fixed in the membrane interior and pore surface for a long time under the constraint of the interpenetrating network. Thus, without relying on additional surface treatment processes, the polyvinylidene fluoride fiber membrane is continuously endowed with stable hydrophilicity and excellent antifouling performance, effectively inhibiting the adsorption of proteins, colloids and organic pollutants, and improving the membrane flux stability and water treatment efficiency.

[0038] In one specific embodiment, the polyvinylidene fluoride fiber membrane comprises a plurality of pore structures, the average pore size of which is 15nm-30nm.

[0039] The pore structure of the polyvinylidene fluoride (PVDF) fiber membrane is provided by the PVDF vinyl body and a cross-linked double interpenetrating network structure that runs through the PVDF vinyl body. This pore structure further ensures the permeability of the PVDF fiber membrane, making it more suitable for water purification and the separation, concentration, and recovery of materials.

[0040] In some implementations, the method for testing the average pore diameter of the channel structure includes the following procedures:

[0041] At room temperature of 25℃, a 5cm sample of hollow polyvinylidene fluoride fiber membrane was taken and analyzed using a pore size analyzer (full-function membrane pore size analyzer, BSD-PBL, Beijing Best Instruments).

[0042] For example, the average pore size of the channel structure is any value of 15 nm, 18 nm, 20 nm, 25 nm, 30 nm, or a range of any combination of both.

[0043] The dual-network structure constructed in this invention provides stable and durable hydrophilic properties, which helps reduce the interfacial resistance during water molecule transport within the pores. This facilitates the transition of water transport from a high-resistance state to a low-resistance state within the pores and promotes capillary-dominated wetting transport, thus effectively converting the stable pore structure into a higher actual permeation flux. Therefore, in a specific embodiment, the pure water flux of the polyvinylidene fluoride fiber membrane is 400 L•m. -2 •h -1 •MPa -1 -700L•m -2 •h -1 •MPa -1 .

[0044] The above pure water flux of the polyvinylidene fluoride fiber membrane demonstrates that the membrane material has successfully achieved a synergistic improvement in hydrophilicity, permeability, and mechanical properties.

[0045] For example, the pure water flux of the polyvinylidene fluoride fiber membrane is 400 L•m. -2 •h -1 •MPa -1 450 L•m -2 •h -1 •MPa -1 500 L•m -2 •h -1 •MPa -1 550 L•m -2 •h -1 •MPa -1 600 L•m -2 •h-1 •MPa -1 650 L•m -2 •h -1 •MPa -1 700 L•m -2 •h -1 •MPa -1 The range of any value in the range, or any combination of both.

[0046] As an example, and not a limitation, the method for testing the pure water flux of polyvinylidene fluoride fiber membranes includes the following procedures:

[0047] The polyvinylidene fluoride fiber membrane to be tested was encapsulated into a module and placed in a membrane performance evaluation device for testing. The operating pressure was 0.1 MPa and the water temperature was 25℃. Before the test, the membrane was pre-pressurized with pure water for more than half an hour. The permeate flow rate (V) was collected and measured over a certain period of time (t). The effective area of ​​the filter membrane through which pure water permeates is defined as A, m. 2 Calculate the pure water flux corresponding to the membrane using the following formula:

[0048] .

[0049] Pure water refers to water with an electrical conductivity of 1μS / cm-5μS / cm.

[0050] In one specific embodiment, the water contact angle of the polyvinylidene fluoride fiber membrane is 45°-65°.

[0051] The polyvinylidene fluoride fiber membrane with the above water contact angle avoids the risk of excessive swelling or decreased mechanical strength of the membrane material due to excessive hydrophilicity, while still inhibiting the adsorption of pollutants.

[0052] In some embodiments, the tensile strength of the polyvinylidene fluoride fiber membrane is 4.7 MPa-6 MPa.

[0053] The above-mentioned polyvinylidene fluoride fiber membranes have good tensile strength and can be used in complex separation environments, avoiding problems such as cracking, deformation or even collapse and blockage during operation or recycling.

[0054] For example, the tensile strength of the polyvinylidene fluoride fiber membrane is any value or a range of any two of the following: 4.7 MPa, 5 MPa, 5.2 MPa, 5.4 MPa, 5.5 MPa, 5.7 MPa, 5.8 MPa, 6.0 MPa.

[0055] In some embodiments, the elongation at break of the polyvinylidene fluoride fiber membrane is 140%-250%.

[0056] The above-mentioned polyvinylidene fluoride fiber membranes have good elongation at break, can be used in complex separation environments, and further avoid problems such as cracking, deformation or even collapse and blockage during operation or recycling.

[0057] For example, the elongation at break of the polyvinylidene fluoride fiber membrane is any value or a range of any two of the following: 140%, 150%, 170%, 190%, 200%, 220%, 240%, 250%.

[0058] As an example, and not a limitation, the test methods for the tensile strength and elongation at break of polyvinylidene fluoride fiber membranes include the following procedures:

[0059] The dried polyvinylidene fluoride (PVDF) fiber membrane was placed in the fixture of an electronic universal testing machine (model: AGS-J 20N) with a gauge length of 50 mm. It was stretched at a speed of 250 mm / min until the PVDF fiber membrane broke. The tensile force at which the membrane fibers broke is the breaking strength. The breaking strength of the membrane fibers can be calculated using the following formulas: Breaking strength = Breaking strength / Cross-sectional area of ​​the PVDF fiber membrane; Elongation at break = Tension distance at break / Gauge length × 100%. The cross-sectional area of ​​the PVDF fiber membrane was calculated by measuring the inner and outer diameters of the cross-section using SEM.

[0060] The polyvinylidene fluoride fiber membrane of the present invention has good hydrophilicity, mechanical strength and stability. In some embodiments, the water contact angle of the polyvinylidene fluoride fiber membrane is maintained in the range of 45°-65° during the effective service life of the polyvinylidene fluoride fiber membrane.

[0061] The polyvinylidene fluoride fiber membrane of the present invention can be made into different forms according to the application scenario, such as sheet membrane, fiber filament, fiber tube, etc.

[0062] In one specific embodiment, the polyvinylidene fluoride fiber membrane is a hollow polyvinylidene fluoride fiber membrane, which has a hollow inner cavity along the axial direction.

[0063] It can be understood that a hollow polyvinylidene fluoride fiber membrane includes a hollow inner cavity and a membrane wall that forms the hollow inner cavity; the membrane wall is the polyvinylidene fluoride fiber membrane.

[0064] In one specific embodiment, the mass ratio of polyvinylidene fluoride, the first functional monomer, and the second functional monomer is 10-30:0.1-10:0.1-15.

[0065] The mass ratio of polyvinylidene fluoride, the first functional monomer, and the second functional monomer can be adjusted according to different application requirements. However, in order to ensure the synergistic improvement of the hydrophilicity, permeability, and mechanical properties of the polyvinylidene fluoride fiber membrane, the preferred implementation is a mass ratio of polyvinylidene fluoride, the first functional monomer, and the second functional monomer of 15-25:3-7:6-12.

[0066] In another aspect, the present invention provides a method for preparing a polyvinylidene fluoride fiber membrane, comprising the following steps:

[0067] A first mixture comprising polyvinylidene fluoride, a pore-forming agent, and a solvent is mixed with a second mixture comprising a first functional monomer, a second functional monomer, a crosslinking agent, and an initiator, and an in-situ crosslinking reaction is carried out at 100℃-180℃ to obtain a casting solution.

[0068] The casting solution was molded to obtain a polyvinylidene fluoride fiber membrane.

[0069] The mixture of the first and second materials is stirred at 100℃-180℃, and the above reaction occurs. The reaction includes thermal decomposition of the initiator and free radical generation, which triggers the free radical polymerization of the first and second functional monomers. At the same time, under the action of the crosslinking agent, a first crosslinking network that is tightly bonded to the PVDF matrix and a second hydrophilic crosslinking network anchored in the interior of the membrane and on the surface of the pores are formed. The first and second crosslinking networks form a crosslinked double interpenetrating network structure. The first functional monomer is a monomer that can form a well-compatible system with polyvinylidene fluoride matrix during the in-situ crosslinking reaction and preferentially form a continuous crosslinked support structure during the membrane forming process.

[0070] In one specific embodiment, in order to improve the stability of the molding process, the casting liquid is further subjected to vacuum degassing to obtain a uniform casting liquid.

[0071] In one specific embodiment, the molding process includes the following steps:

[0072] The casting solution is extruded through a spinneret and then immersed in a coagulation bath to form the film.

[0073] The above-described method for preparing hollow polyvinylidene fluoride fiber membranes is a continuous method suitable for industrial production.

[0074] To obtain a hollow polyvinylidene fluoride fiber membrane, the spinneret has an annular die. In one specific embodiment, under driving pressure, the casting liquid is extruded through the spinneret with the annular die, so that the casting liquid first passes through the air section behind the spinneret and then enters the coagulation bath by traction. In the coagulation bath, the PVDF matrix undergoes non-solvent-induced phase separation, thereby forming a hollow polyvinylidene fluoride fiber membrane.

[0075] In one specific embodiment, the temperature of the coagulation bath is 0℃-40℃, preferably 0℃-20℃.

[0076] The spinneret employs a core liquid and skin liquid structure during the spinning process. The skin liquid is a casting liquid. The core liquid includes a non-solvent for polyvinylidene fluoride (PVDF), or a mixture of a solvent for PVDF and a non-solvent for PVDF.

[0077] In one specific embodiment, the injection rate of the core liquid can be adjusted according to the requirements of the inner diameter and wall thickness of the hollow fiber, preferably 5 mL / min-50 mL / min, and the core liquid temperature is preferably controlled within the range of 5℃-30℃ to ensure the interface stability during the spinning process and to effectively control the pore structure of the hollow fiber.

[0078] For example, the injection rate of the core fluid is any value or a range of any two of the following: 5 mL / min, 10 mL / min, 15 mL / min, 20 mL / min, 25 mL / min, 30 mL / min, 35 mL / min, 40 mL / min, 45 mL / min, 50 mL / min.

[0079] Under driving pressure, the casting liquid is extruded through a spinneret with an annular die. The driving pressure is preferably provided by an extruder or compressed gas, including but not limited to nitrogen.

[0080] In one specific embodiment, the temperature of the air section after the spinneret is 10℃-30℃. This temperature can stabilize the shape of the casting liquid and regulate the initial phase separation behavior before entering the coagulation bath. The residence time of the air section can be adjusted as needed, including but not limited to 10ms-300ms.

[0081] For example, the temperature of the air section after the spinneret is any value or a range of any two of 10°C, 15°C, 20°C, 25°C, and 30°C. The residence time of the air section is any value or a range of any two of 10ms, 50ms, 70ms, 100ms, 150ms, 200ms, 250ms, and 300ms.

[0082] In one specific embodiment, the extrusion speed of the hollow polyvinylidene fluoride fiber membrane is 20 m / min - 80 m / min. Exemplarily, the extrusion speed of the hollow polyvinylidene fluoride fiber membrane is any value or a range of any combination of 20 m / min, 30 m / min, 40 m / min, 50 m / min, 60 m / min, 70 m / min, 80 m / min, etc.

[0083] In order to better induce the initial non-solvent-induced phase separation and obtain a hollow polyvinylidene fluoride fiber membrane with uniform pores, in one specific embodiment, the mass fraction of the non-solvent of polyvinylidene fluoride in the coagulation bath is 0wt%-15wt%, preferably 1wt%-10wt%, specifically including but not limited to any value or a range of any combination of 0.1wt%, 5wt%, 10wt%, 15wt%, etc.

[0084] In one specific embodiment, the non-solvent is preferably water, or a mixture of water and one or more of ethanol, n-propanol, isopropanol, and glycerol.

[0085] In one specific embodiment, the polyvinylidene fluoride (PVDF) fiber membrane obtained through molding further includes extraction and drying processes. The extractant is a non-solvent for PVDF and capable of dissolving residual solvents in the membrane, preferably at least one of ethanol, n-propanol, isopropanol, and deionized water. The drying process includes baking or air drying, the purpose of which is to remove residual extractant and moisture from the PVDF fiber membrane to ensure stable membrane structure and excellent performance.

[0086] In one specific embodiment, the mixture further includes a pore-forming agent, which can form more porous structures in the polyvinylidene fluoride fiber membrane, making the polyvinylidene fluoride fiber membrane suitable for different scenarios.

[0087] In one specific embodiment, based on a total mass of 100 wt% for the first mixture, the mass fraction of polyvinylidene fluoride is 10 wt%-30 wt%, the mass fraction of the porogen is 0.1 wt%-20 wt%, and the balance is solvent;

[0088] And / or, based on a mass fraction of 100 parts for the first mixture, the mass fraction of the first functional monomer is 0.1 to 10 parts, the mass fraction of the second functional monomer is 0.1 to 15 parts, the mass fraction of the crosslinking agent is 0.1 to 5 parts, and the mass fraction of the initiator is 0.1 to 3 parts.

[0089] For example, the mass fraction of polyvinylidene fluoride is any value or a combination of any two of the following: 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%. The mass fraction of the porogen is any value or a combination of any two of the following: 0.1 wt%, 15 wt%, 20 wt%. The mass fraction of the first functional monomer is any value or a combination of any two of the following: 0.1 parts, 5 parts, 10 parts. The mass fraction of the second functional monomer is any value or a combination of any two of the following: 0.1 parts, 5 parts, 10 parts, 15 parts. The mass fraction of the crosslinking agent is any value or a combination of any two of the following: 0.1 parts, 1 part, 2 parts, 3 parts, 4 parts, 5 parts. The mass fraction of the initiator is any value or a combination of any two of the following: 0.1 parts, 1 part, 2 parts, 3 parts.

[0090] The crosslinking agent and initiator in the above ratio can effectively initiate the free radical polymerization of the first functional monomer and the second functional monomer, thereby forming an interpenetrating crosslinked network of the two monomers in the PVDF matrix.

[0091] In one specific embodiment, the initiator includes any one of azobisisobutyronitrile, azobisisopropionitrile, benzoyl peroxide, dibenzoyl peroxide, divinyl peroxide, and tert-butyl peroxide. These initiators exhibit good thermal decomposition rates and free radical generation efficiency within the temperature range of 100℃-180℃, enabling them to efficiently initiate the free radical polymerization of the first functional monomer and the second functional monomer, thereby forming an interpenetrating crosslinked network in the PVDF matrix.

[0092] In one specific embodiment, the crosslinking agent includes at least one selected from trimethylolpropane trimethacrylate, glyceryl acrylate, and methylenebisacrylamide. These crosslinking agents can efficiently promote the formation of an interpenetrating crosslinked network between the first functional monomer and the second functional monomer in the PVDF matrix.

[0093] The first crosslinking network is formed by the free radical polymerization of methyl methacrylate under the action of a free radical initiator and the crosslinking agent. The above-mentioned free radical polymerization and crosslinking reaction can be represented by the following general formula:

[0094] .

[0095] The second crosslinked network is formed by the free radical polymerization of vinyl-containing hydrophilic functional monomers under the action of a free radical initiator, and the formation of a crosslinked network structure under the action of a crosslinking agent. The above free radical polymerization and crosslinking reaction can be represented by the following general formula:

[0096] .

[0097] In one specific embodiment, the pore-forming agent includes at least one selected from polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and hydroxyethyl cellulose. These pore-forming agents can create more porous structures in the polyvinylidene fluoride (PVDF) fiber membrane, making the PVDF fiber membrane suitable for different applications. The pore-forming agent can be stably present in the casting solution and dissolved and removed in the subsequent coagulation bath, thereby creating several porous structures.

[0098] The following specific embodiments illustrate the polyvinylidene fluoride fiber membrane and its preparation method provided by the present invention.

[0099] Example 1

[0100] The polyvinylidene fluoride fiber membrane in this embodiment is a hollow polyvinylidene fluoride fiber membrane, and its preparation method includes the following steps:

[0101] Step (1): Preparation of casting solution

[0102] 20 wt.% of polyvinylidene fluoride, 10 wt.% of polyethylene glycol, and 70 wt.% of methyl 5-(dimethylamino)-2-methyl-5-oxovalerate (Polarclean) were mixed and stirred thoroughly to obtain a first mixture. The total mass of the first mixture was counted as 100 parts. The mass percentage of each component in the following examples was calculated based on the first mixture. Subsequently, 5 parts of methyl methacrylate (first functional monomer), 10 parts of 2-methoxyethyl acrylate (second functional monomer), 2 parts of N,N'-methylenebisacrylamide, and 2 parts of azobisisobutyronitrile were added to the system in sequence and mechanically stirred at 150°C for 6 hours until a uniform casting solution was formed. The casting solution was then placed in a vacuum environment for degassing treatment to remove air bubbles from the casting solution system.

[0103] Step (2): Preparation of the original membrane.

[0104] The casting solution from step (1) is extruded through a spinneret with an annular die under nitrogen-driven conditions. Simultaneously, water is injected into the cavity of the hollow polyvinylidene fluoride fiber membrane as the core liquid. The core liquid injection rate is 20 mL / min. The casting solution is drawn in the air section at a speed of 30 m / min. The temperature of the air section is controlled at 25°C, and the residence time in the air section is about 20 ms. Subsequently, the extruded membrane fibers enter deionized water at a temperature of 0°C and undergo phase separation to form a pre-cured hollow polyvinylidene fluoride fiber membrane.

[0105] Step (3): Extraction and drying.

[0106] The pre-cured membrane fibers obtained in step (2) were immersed in deionized water for extraction for 72 hours, and the deionized water was replaced intermittently to completely remove the residual solvent in the membrane. After extraction, the hollow polyvinylidene fluoride fiber membrane was dried at 60°C for 30 minutes to remove the residual moisture in the membrane, and finally the hollow polyvinylidene fluoride fiber membrane was obtained.

[0107] Example 2

[0108] The only difference from Example 1 is that the concentration of methyl methacrylate in step (1) is increased to 15 parts, while other conditions remain unchanged.

[0109] Example 3

[0110] The only difference from Example 1 is that the concentration of methyl methacrylate in step (1) is reduced to 2 parts, while other conditions remain unchanged.

[0111] Example 4

[0112] The only difference from Example 1 is that the concentration of 2-methoxyethyl acrylate in step (1) is reduced to 2 parts, while other conditions remain unchanged.

[0113] Example 5

[0114] The only difference from Example 1 is that the concentration of 2-methoxyethyl acrylate in step (1) is increased to 18 parts, while other conditions remain unchanged.

[0115] Example 6

[0116] The only difference from Example 1 is that in step (1), the concentration of N,N'-methylenebisacrylamide is reduced to 1 part and the concentration of azobisisobutyronitrile is reduced to 0.2 parts, while other conditions remain unchanged.

[0117] Example 7

[0118] The only difference from Example 1 is that in step (1), 2-methoxyethyl acrylate is replaced with hydroxypropyl acrylate, while other conditions remain unchanged.

[0119] Comparative Example 1

[0120] The only difference from Example 1 is that methyl methacrylate was not added in step (1), while other conditions remained the same.

[0121] Comparative Example 2

[0122] The only difference from Example 1 is that 2-methoxyethyl acrylate was not added in step (1), while other conditions remained the same.

[0123] Comparative Example 3

[0124] The only difference from Example 1 is that the first functional monomer, the second functional monomer, the crosslinking agent and the initiator were not added in step (1), and the casting solution was prepared only by PVDF, polyethylene glycol and solvent, while the other conditions remained unchanged.

[0125] Comparative Example 4

[0126] Step (1): Preparation of cross-linked PMMA.

[0127] 20 g of methyl methacrylate and 100 g of N,N-dimethylformamide were added to a 250 mL three-necked reaction flask. Nitrogen gas was purged for 30 min to replace oxygen in the solution and prevent inhibition of free radical polymerization. After nitrogen purging, 1 g of N,N'-methylenebisacrylamide as a crosslinking agent and 0.08 g of azobisisobutyronitrile as a free radical initiator were added to the system, and the mixture was stirred thoroughly to ensure uniform dispersion of the components. Subsequently, the reaction flask was placed in a constant temperature oil bath and reacted at 75 °C for 24 h. During the reaction, the mixture was continuously stirred and nitrogen protection was maintained. After polymerization, the reaction solution was slowly poured into 500 mL of n-hexane to precipitate the polymer. The precipitate was collected, and an appropriate amount of tetrahydrofuran (THF) was added to dissolve the polymer. If 10 mL of THF was not completely dissolved, 10 mL of THF could be added each time. 15 times the amount of n-hexane required to dissolve the polymer was added to the solution, and the polymer was precipitated again. This operation was repeated 3 times to thoroughly remove residual monomers and solvents. Finally, the precipitate was collected and dried in a vacuum oven at 60°C for 12 hours to obtain cross-linked PMMA.

[0128] Step (2) Crosslinking PMEA Synthesis

[0129] 20 g of 2-methoxyethyl acrylate and 100 g of N,N-dimethylformamide were added to a 250 mL three-necked reaction flask. Nitrogen gas was purged for 30 min to replace oxygen in the solution and prevent inhibition of free radical polymerization. After nitrogen purging, 1 g of N,N'-methylenebisacrylamide as a crosslinking agent and 0.08 g of azobisisobutyronitrile as a free radical initiator were added to the system, and the mixture was stirred thoroughly to ensure uniform dispersion of the components. Subsequently, the reaction flask was placed in a constant temperature oil bath and reacted at 75 °C for 24 h. During the reaction, the mixture was continuously stirred and nitrogen protection was maintained. After polymerization, the reaction solution was slowly poured into 500 mL of n-hexane to precipitate the polymer. The precipitate was collected, and an appropriate amount of tetrahydrofuran (THF) was added to dissolve the polymer. If 10 mL of THF was not completely dissolved, 10 mL of THF could be added each time. 15 times the amount of n-hexane required to dissolve the polymer was added to the solution, and the polymer was precipitated again. This operation was repeated 3 times to thoroughly remove residual monomers and solvents. Finally, the precipitate was collected and dried in a vacuum oven at 60°C for 12 hours to obtain crosslinked PMEA.

[0130] Step (3): Blending of raw materials.

[0131] 20 wt.% PVDF, 10 wt.% polyethylene glycol, and 70 wt.% Polarclean were mixed and thoroughly stirred to obtain a first mixture. The total mass of the first mixture was calculated as 100 parts. Subsequently, 5 parts PMMA and 10 parts PMEA were added sequentially to this system, and the mixture was mechanically stirred at 150°C for 6 hours until a homogeneous casting solution was formed. After mixing, the casting solution was placed in a vacuum environment for degassing treatment to remove air bubbles from the system.

[0132] The remaining steps are the same as in Example 1.

[0133] In this comparative example, the hydrophilic polymer was physically blended with PVDF in the form of a pre-synthesized crosslinked polymer, and no interpenetrating crosslinked network structure was formed during the film forming process.

[0134] Comparative Example 5

[0135] The only difference from Example 1 is that methyl methacrylate in step (1) is replaced with butyl vinyl ether, and 2-methoxyethyl acrylate is replaced with tert-butyl methacrylate.

[0136] Test case

[0137] The properties of the hollow polyvinylidene fluoride fiber membranes in the examples and comparative examples were characterized using the following test methods:

[0138] 1. Characterization of the inner and outer surface morphology of hollow polyvinylidene fluoride fiber membranes

[0139] The hollow polyvinylidene fluoride fiber membrane to be tested was dehydrated to constant weight in a 60℃ drying oven, cut to an appropriate size, and then fixed to the sample stage with the outer surface facing upwards using conductive adhesive for observation of the outer surface morphology. Subsequently, the sample stage was placed in a vacuum evaporator, and a platinum layer was uniformly sprayed at 2 kV and 8-10 mA to improve the conductivity of the sample. The surface and cross-section morphology of the membrane were characterized and images were taken using a scanning electron microscope (SEM, model JSM7401, JEOL) at 3 kV.

[0140] in, Figure 1 The images show SEM images of hollow polyvinylidene fluoride fiber membranes from Examples 1 and Comparative Examples 1-3. As can be seen, in Example 1, by constructing a cross-linked double interpenetrating network structure, the polyvinylidene fluoride fiber membrane exhibits a more uniform pore size distribution on both the surface and cross-section, with the average pore size concentrated in the range of 19nm-28nm. The average pore size of Comparative Example 2 is approximately 36nm and that of Comparative Example 3 is approximately 43nm.

[0141] 2. Determination of average pore size and pore size distribution

[0142] At room temperature of 25℃, a 5 cm thick sample of hollow polyvinylidene fluoride fiber membrane was taken and analyzed using a pore size analyzer (full-function membrane pore size analyzer, BSD-PBL, Beijing Best Instruments).

[0143] 3. Measurement of water contact angle

[0144] Hollow polyvinylidene fluoride (PVDF) fiber membranes with uniform inner and outer diameters and a length of approximately 2 cm were selected and dried in an oven at 60°C for 1 h. The dried PVDF fiber membranes were then smoothly attached to a glass slide and placed on the platform of a contact angle meter (KRUSS-DSA100). The static water contact angle of the membrane was measured with approximately 0.1 μL of pure water. Measurements were repeated at three different locations for each sample, and the average value was taken as the average water contact angle of the hollow PVDF fiber membrane material.

[0145] 4. Pure water flux test

[0146] The hollow polyvinylidene fluoride fiber membrane to be tested was encapsulated into a module and placed in a membrane performance evaluation device (low-pressure membrane flux tester, model SF-SA, Hangzhou Saifei Membrane Separation Technology Co., Ltd.) for testing. The operating pressure was 0.1 MPa and the water temperature was 25℃. Before testing, the membrane was pre-pressurized with pure water for 30 minutes, and the permeate flow rate (V) was collected and measured over a certain time (t). The effective area of ​​the filter membrane through which pure water permeates is defined as A, in m². 2 Finally, the pure water flux corresponding to the membrane is calculated using the following formula. The unit is L•m -2 •h -1 •MPa -1

[0147] .

[0148] 5. Determination of protein adsorption properties

[0149] The hollow polyvinylidene fluoride (PVDF) fiber membrane to be tested was dried to constant weight at 60℃, and its mass was recorded as the initial mass M0. Subsequently, the hollow PVDF fiber membrane was thoroughly wetted and washed successively with ethanol and deionized water, and then placed in a simulated protein-containing solution prepared with bovine serum albumin (BSA, isoelectric point 4.7, negatively charged) for adsorption experiments. The BSA solution used ultrapure water as the solvent, and the BSA concentration was 1 mg / mL. The hollow PVDF fiber membrane was immersed at 25℃ for 24 h to allow BSA to be fully adsorbed onto the membrane surface and inside the pores.

[0150] After adsorption, the unadsorbed proteins on the membrane surface were rinsed with deionized water, and the membrane was dried again at 60°C to constant weight. The mass M of the membrane after adsorption was recorded. p The protein adsorption capacity of the membrane can be calculated using the following formula: Protein adsorption capacity = (Mp - M0) / A

[0151] In the formula, the protein adsorption capacity is expressed in μg / cm³. 2 A represents the effective area of ​​the membrane surface in contact with the protein-containing solution, in cm². 2 .

[0152] 6. Mechanical performance testing

[0153] The dried hollow polyvinylidene fluoride fiber membrane filaments were placed in the fixture of an electronic universal testing machine (AGS-J 20N, Shimadzu Corporation, Japan), with a gauge length of 50 mm. The membrane filaments were stretched at a speed of 250 mm / min until they broke. The tensile force at which the membrane filaments broke is the breaking strength. The breaking strength and elongation at break of the membrane filaments were calculated using the following formulas: breaking strength = breaking strength / cross-sectional area of ​​the membrane filaments, and elongation at break = stretching distance at break / gauge length × 100%. The cross-sectional area of ​​the membrane filaments was calculated by measuring the inner and outer diameters of the membrane filament cross-section using SEM.

[0154] 7. Long-lasting hydrophilicity test

[0155] Hollow polyvinylidene fluoride (PVDF) fiber membranes were cut to a length of 30 cm and thoroughly wetted with deionized water until equilibrium was reached. They were then installed in a constant-pressure cross-flow filtration device and continuously operated at 0.1 MPa. Operation was stopped at preset time intervals (e.g., 24 hours), and membrane samples were removed. Free water on the membrane surface was gently blotted dry, and the static water contact angle was measured within a defined time period. The stability of the hydrophilic layer and the long-term hydrophilic properties of the membrane material in a long-term aquatic environment were evaluated by observing changes in the contact angle at different operating time points. Figure 2 Line graphs showing the contact angle changes of hollow polyvinylidene fluoride fiber membranes at different time intervals for Example 1, Comparative Example 1, and Comparative Example 4.

[0156] The test results are summarized in Table 1.

[0157] Table 1

[0158]

[0159] Table 2

[0160]

[0161] Comparing Tables 1 and 2, it can be seen that the cross-linked double interpenetrating network structure composed of a first cross-linking network and a second cross-linking network constructed in the PVDF matrix of the embodiments reduced the water contact angle of the hollow polyvinylidene fluoride fiber membrane from approximately 96° of the original PVDF membrane (Comparative Example 3) to approximately 45°, indicating a significant improvement in the wettability and hydrophilicity of the membrane surface. Simultaneously, due to the double cross-linked network structure, hydrophilic segments are distributed and immobilized on the membrane matrix and surface in a cross-linked interpenetrating manner, resulting in a corresponding reduction in the adsorption capacity of pollutants such as proteins by the membrane material of the embodiments. The BSA adsorption capacity decreased from approximately 36.8 μg / cm³ in Comparative Example 3. 2 Reduced to 4.9 μg / cm³ in Example 1. 2 This indicates that the cross-linked bi-interpenetrating network structure helps to inhibit the adsorption and deposition of pollutants on the membrane surface. In contrast, although Comparative Examples 1 and 4 introduced hydrophilic components, the hydrophilic segments were prone to migration or loss during use due to the lack of a stable interpenetrating cross-linked structure, resulting in relatively limited improvement in their hydrophilicity and antifouling performance.

[0162] In terms of mechanical properties, the tensile strength and elongation at break of the embodiments were higher than those of the original PVDF membrane (Comparative Example 3) and Comparative Examples 1 and 4, which did not form a cross-linked double interpenetrating network structure. These results indicate that constructing a continuous interpenetrating double cross-linked network structure in the PVDF matrix helps to provide structural support for polymer segments at the microscale, thereby improving the overall structural integrity of the hollow polyvinylidene fluoride fiber membrane and enabling the resulting membrane material to exhibit more stable mechanical properties under higher stress or complex operating conditions.

[0163] Further analysis of the flux data in Table 1 shows that although the average pore size of the membrane material in the examples is smaller than that in Comparative Example 3, its pure water flux remains at a high level, compared to 197.3 L·m⁻¹ in Comparative Example 3. -2 ·h -1 MPa -1 Increased to 400 L·m in the example -2 ·h -1 MPa -1 The results indicate that, under the stabilizing effect of the cross-linked double interpenetrating network structure, the hydrophilic functional groups introduced into the second cross-linked network can improve the wetting state of the inner surface of the pores and help reduce the transport resistance of water molecules in the pores, thus achieving high permeability even with a reduced pore size.

[0164] according to Figure 2The results of the long-term water contact angle test shown indicate that, in Comparative Example 4, the hollow polyvinylidene fluoride fiber membrane with hydrophilic particles introduced through blending showed a significant increase in water contact angle after immersion for a period of time. This suggests that blending is insufficient to form a stable cross-linked interpenetrating network structure, and the hydrophilic components are prone to migration or elution in the aqueous environment, making it difficult to maintain the hydrophilicity of the membrane surface for a long period. In Comparative Example 1, the membrane with only a single interpenetrating network maintained a certain level of hydrophilicity in the initial immersion stage, but its water contact angle also showed an upward trend as the immersion time increased, indicating that the single network structure still has limited ability to fix hydrophilic segments. In contrast, the membrane with the double cross-linked interpenetrating network structure constructed in Example 1 maintained a relatively stable water contact angle during long-term immersion, with no significant changes observed. This further demonstrates that the cross-linked interpenetrating network structure constructed in this invention helps improve the stability and durability of the membrane material's hydrophilic properties.

[0165] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A polyvinylidene fluoride fiber membrane, characterized in that, The polyvinylidene fluoride fiber membrane comprises a polyvinylidene fluoride body and a cross-linked interpenetrating network structure penetrating the polyvinylidene fluoride body; the cross-linked interpenetrating network structure comprises a first cross-linked network and a second cross-linked network; the first cross-linked network is formed by cross-linking a first functional monomer; the second cross-linked network is formed by cross-linking a second functional monomer; the second functional monomer includes hydrophilic functional groups. The hydrophilic functional group includes at least one of hydroxyl, carboxyl, amide, sulfonic acid, and etheroxy groups; The first functional monomer is methyl methacrylate. And / or, the second functional monomer includes at least one of acrylic acid, hydroxypropyl acrylate, methacrylic acid, acrylamide, N-vinylpyrrolidone, 2-methoxyethyl acrylate, and styrene sulfonic acid.

2. The polyvinylidene fluoride fiber membrane according to claim 1, characterized in that, The mass ratio of the polyvinylidene fluoride, the first functional monomer, and the second functional monomer is 10-30:0.1-10:0.1-15.

3. A method for preparing a polyvinylidene fluoride fiber membrane as described in any one of claims 1-2, characterized in that, Includes the following steps: A first mixture comprising polyvinylidene fluoride, a pore-forming agent, and a solvent is mixed with a second mixture comprising a first functional monomer, a second functional monomer, a crosslinking agent, and an initiator, and an in-situ crosslinking reaction is carried out at 100℃-180℃ to obtain a casting solution. The casting solution is molded to obtain the polyvinylidene fluoride fiber membrane.

4. The preparation method according to claim 3, characterized in that, The molding process includes the following steps: The casting solution is extruded through a spinneret and then immersed in a coagulation bath to form a film.

5. The preparation method according to claim 3 or 4, characterized in that, Based on a total mass of 100 wt% for the first mixture, the mass fraction of the polyvinylidene fluoride is 10 wt%-30 wt%, the mass fraction of the porogen is 0.1 wt%-20 wt%, and the remainder is solvent; And / or, based on a mass fraction of 100 parts for the first mixture, the mass fraction of the first functional monomer is 0.1-10 parts, the mass fraction of the second functional monomer is 0.1-15 parts, the mass fraction of the crosslinking agent is 0.1-5 parts, and the mass fraction of the initiator is 0.1-3 parts.

6. The preparation method according to claim 5, characterized in that, The initiator includes at least one of azobisisobutyronitrile, azobisisopropionitrile, benzoyl peroxide, dibenzoyl peroxide, and tert-butyl peroxide; And / or, the crosslinking agent includes at least one of trimethylolpropane trimethacrylate, glyceryl acrylate, and methylenebisacrylamide; And / or, the pore-forming agent includes at least one of polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and hydroxyethyl cellulose; And / or, the coagulation bath includes water.

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