Hydrophilic vinylidene fluoride copolymer, preparation method and application

Abstract

The invention discloses a hydrophilic vinylidene fluoride copolymer, a preparation method and application, the hydrophilic vinylidene fluoride copolymer is a block copolymer composed of a first block and a second block, the first block contains a vinylidene fluoride structural unit, and the content of the vinylidene fluoride structural unit accounts for 85-100 wt% of the total amount of the first block; the second block contains a vinylidene fluoride structural unit and a compound structural unit shown in a structural formula (1), the content of the vinylidene fluoride structural unit accounts for 50-95 wt% of the total amount of the second block, and the content of the compound structural unit shown in the structural formula (1) accounts for 5-50 wt% of the total amount of the second block. The filter membrane prepared from the hydrophilic vinylidene fluoride copolymer has excellent mechanical strength, good hydrophilic performance and good stability.

Description

Technical Field

[0001] The present invention relates to the field of polymers, and particularly to a hydrophilic polyvinylidene fluoride copolymer, a preparation method and an application thereof. Background Art

[0002] Fluorine-containing polymer materials have attracted much attention due to their unique chemical stability and mechanical properties. Among them, polyvinylidene fluoride is a partially fluorinated and semi-crystalline polymer, which has excellent chemical resistance, heat resistance, weather resistance, dielectric properties and other characteristics, and also has good molding processability. Therefore, it is widely used in fields such as membrane separation. Due to the strong hydrophobicity of polyvinylidene fluoride, the polyvinylidene fluoride membrane has defects such as low water flux, easy contamination by organic substances and short service life. Therefore, it is difficult to be directly applied to scenarios such as water purification, treatment and reuse of domestic and industrial wastewater, industrial separation and purification of pharmaceutical / food / chemical products, and blood purification.

[0003] Hydrophilic modification of polyvinylidene fluoride is the main means to solve the above problems. In many studies, hydrophilic polyvinyl alcohol, polyvinylpyrrolidone, graphene, nanoparticles, amphiphilic polymers, etc. are mainly used to blend with hydrophobic polyvinylidene fluoride to improve the hydrophilic and anti-fouling properties of the membrane. However, water-soluble polymers and nano substances are easy to lose from the membrane, resulting in poor reliability of the membrane performance.

[0004] Amphiphilic polyvinylidene fluoride has both hydrophilic and hydrophobic groups, has excellent water treatment ability, has a good removal effect on heavy metal ions, and has good biological stability and is not easily decomposed by microorganisms, so it has great potential in membrane modification. Patent CN108467451B discloses a preparation method of a hydrophilic polyvinylidene fluoride resin, which is mainly prepared by suspension copolymerization of vinylidene fluoride and itaconic acid. Patent CN104558363B discloses a preparation method of a fluorine-containing copolymer for a water treatment membrane, which is mainly prepared by random copolymerization of three monomers of vinylidene fluoride, hexafluoropropylene and 2-trifluoromethylacrylic acid. For the hydrophilic polyvinylidene fluoride resin prepared by random copolymerization of vinylidene fluoride and a hydrophilic monomer, when the content of hydrophilic groups in the resin is high, the mechanical properties of the membrane material are poor and the service life is short; when the content of hydrophilic groups is low, the water flux of the membrane material is low and it is easy to be contaminated by organic substances.

[0005] Amphiphilic block copolymers have certain particularities and advantages compared to amphiphilic random copolymers, mainly manifested in that one segment of the amphiphilic block copolymer shows hydrophilicity, while the other segment shows hydrophobicity. Fluorinated amphiphilic block polymers mainly refer to introducing fluorine atoms into the hydrophobic segment of the amphiphilic block polymer to endow the hydrophobic segment with more excellent properties, such as water and oil repellency, aging resistance, anti-cracking, and good biocompatibility. Its hydrophobic fluorinated chain segment endows the polymer membrane material with good mechanical properties, giving the material strength and structural stability. At the same time, the hydrophilic chain segment endows the membrane material with a high water flux and biocompatibility, giving the material hydrophilicity and other special functions, and is an ideal raw material for realizing the "unity of structure and function" of separation membrane materials.

[0006] Amphiphilic poly(vinylidene fluoride) block copolymers have received a great deal of attention and reports. For example, Patent CN104610518B discloses a preparation method of a poly(vinylidene fluoride)-polyacrylic acid block copolymer, Patent CN104610519B discloses a preparation method of a poly(vinylidene fluoride)-polyethylene glycol block copolymer, Patent CN104558452B discloses a preparation method of a poly(vinylidene fluoride)-polyvinylpyrrolidone block copolymer, and Patent CN109096453B discloses a comb-shaped amphiphilic poly(vinylidene fluoride)-based block copolymer and its application. In these amphiphilic poly(vinylidene fluoride) block copolymers, the hydrophilic polymer segments mostly use strongly hydrophilic segments such as polyacrylic acid, polyethylene glycol, polyvinylpyrrolidone, and polyvinyl alcohol. These strongly hydrophilic segments have problems with compatibility with the fluorinated chain segments. At the same time, they will spontaneously form micelles in the solvent, and factors such as different assembly methods lead to the micro-nano structures of such block polymers presenting some other special morphologies such as microspherical, cylindrical, vesicular, etc., resulting in large pore sizes of the membrane material, poor filtration effects, complex film-making processes, and other uncertain factors.

[0007] Therefore, it is very necessary to provide a hydrophilic poly(vinylidene fluoride) copolymer that has good compatibility with the fluoropolymer chain segment and has excellent mechanical strength and hydrophilic properties. Summary of the Invention

[0008] Technical problems to be solved by the invention

[0009] In order to solve the problems of low mechanical strength, poor hydrophilic properties, large pore sizes, poor separation effects, and poor compatibility between the hydrophilic chain segment and the fluorinated chain segment of the membrane material prepared from the hydrophilic poly(vinylidene fluoride) copolymer, the inventors of the present invention found that introducing vinylidene fluoride structural units into the hydrophilic chain segment can enhance the compatibility with the hydrophobic chain segment and increase the overall mechanical strength and stability of the material; in addition, introducing the hydrophilic group of the compound shown in structural formula (1) into the hydrophilic chain segment can enhance the overall hydrophilic properties of the material.

[0010] Technical solutions for solving the problems

[0011] In a first aspect, the present invention provides a hydrophilic vinylidene fluoride copolymer, which is a block copolymer composed of a first block and a second block.

[0012] The first block contains vinylidene fluoride structural units, and the content of the vinylidene fluoride structural units accounts for 85-100 wt% of the total amount of the first block;

[0013] The second block contains vinylidene fluoride structural units and the structural units of the compound shown in formula (1). The content of the vinylidene fluoride structural units accounts for 50-95 wt% of the total amount of the second block, and the content of the structural units of the compound shown in formula (1) accounts for 5-50 wt% of the total amount of the second block.

[0014]

[0015] wherein, R 1 , R 2 , R 3 are independently selected from hydrogen, halogen, and perfluoroalkyl groups with 1-6 carbon atoms. At least one of R 1 , R 2 , R 3 is halogen or a perfluoroalkyl group with 1-6 carbon atoms.

[0016] n is an integer from 0 to 12.

[0017] R 4 is selected from hydrogen, lithium, sodium, potassium, ammonium, alkylammonium or alkylarylammonium ions with 1-6 carbon atoms, and the compound shown in formula (2).

[0018]

[0019] m is an integer from 1 to 100.

[0020] Furthermore, in order to improve the hydrophilic property of the material, the compound shown in formula (1) is selected from at least one of 2-(trifluoromethyl)acrylic acid and 2-fluoroacrylic acid.

[0021] The second block of the present invention further contains first olefin structural units, and the content of the first olefin structural units accounts for 0-5 wt% of the total amount of the second block.

[0022] The first olefin is selected from at least one of hexafluoropropylene, chlorotrifluoroethylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluoroethyl vinyl ether, vinyl fluoride, methyl acrylate, methyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, acrylic acid, methacrylic acid, itaconic acid, maleic ester, citraconic ester, vinyl acetate, vinylene carbonate or acrylonitrile.

[0023] The first block of the present invention further contains a first fluorinated olefin structural unit, and the content of the first fluorinated olefin structural unit accounts for 0-15 wt% of the total amount of the first block.

[0024] The first fluorinated olefin is selected from at least one of hexafluoropropylene, chlorotrifluoroethylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluoroethyl vinyl ether, vinyl fluoride.

[0025] For the hydrophilic vinylidene fluoride copolymer of the present invention, the content of the first block accounts for 50-95 wt% of the total amount of the hydrophilic vinylidene fluoride copolymer; the content of the second block accounts for 5-50 wt% of the total amount of the hydrophilic vinylidene fluoride copolymer.

[0026] In a second aspect, the present invention provides a method for preparing a hydrophilic vinylidene fluoride copolymer, and the preparation method adopts an iodine transfer living radical polymerization method.

[0027] As an implementation manner, the method for preparing the hydrophilic vinylidene fluoride copolymer of the present invention includes the following steps:

[0028] S1: Add deionized water, an emulsifier and paraffin into a polymerization reaction device, evacuate and replace with nitrogen to remove oxygen in the reactor;

[0029] S2: Add vinylidene fluoride into the polymerization reaction device;

[0030] S3: Heat, add part or all of a chain transfer agent and part of an initiator, and start the polymerization reaction. The polymerization reaction temperature is 40-120 °C, and the polymerization reaction pressure is 2.0-10.0 MPa;

[0031] S4: During the reaction process, supplement the remaining initiator and chain transfer agent, and supplement vinylidene fluoride to maintain the polymerization reaction pressure;

[0032] S5: After the feeding amount is completed, the polymerization reaction ends, and after degassing, demulsification, filtration, washing and drying, a polyvinylidene fluoride macromolecular chain transfer agent first block with iodine at the end is obtained;

[0033] S6: Add a certain amount of organic solvent to the polymerization reaction device to dissolve the polyvinylidene fluoride macromolecular chain transfer agent first block with iodine at its end, evacuate and replace with nitrogen to remove oxygen in the reactor;

[0034] S7: Add vinylidene fluoride and the compound shown in formula (1) to the polymerization reaction device, add part or all of the initiator, and start the polymerization reaction. The polymerization reaction temperature is 40 - 120 °C, and the polymerization reaction pressure is 2.0 - 10.0 MPa;

[0035] S8: During the reaction process, add the remaining initiator, and stop the polymerization reaction when the polymerization pressure drops to a certain value.

[0036] Further, in steps S2 and S4, in addition to adding vinylidene fluoride, a first fluorinated olefin is also added. The first fluorinated olefin is selected from at least one of hexafluoropropylene, chlorotrifluoroethylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluoroethyl vinyl ether, and vinyl fluoride. The content of the first fluorinated olefin accounts for 0 - 15 wt% of the total amount of vinylidene fluoride and the first fluorinated olefin.

[0037] Further, in step S7, in addition to adding vinylidene fluoride and the compound shown in formula (1), a first olefin is also added. The first olefin is selected from at least one of hexafluoropropylene, chlorotrifluoroethylene, pentafluoropropylene, tetrafluoropropylene, trifluoropropylene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluoroethyl vinyl ether, vinyl fluoride, methyl acrylate, methyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, acrylic acid, methacrylic acid, itaconic acid, maleic ester, citraconic ester, vinyl acetate, vinylene carbonate, or acrylonitrile. The content of the first olefin accounts for 0 - 5 wt% of the total amount of vinylidene fluoride, the compound shown in formula (1), and the first olefin.

[0038] Further, the initiator is a commonly used polymerization initiator in the art. Preferably, the initiator is selected from one or more of di-n-propyl peroxydicarbonate, di-isopropyl peroxydicarbonate, di-isobutyl peroxydicarbonate, di-isohexyl peroxydicarbonate, di-tert-butyl peroxide, tert-butyl peroxybenzoate, lauroyl peroxide, azobisisobutyronitrile, azobisisoheptonitrile, and dimethyl 2,2'-azobis(2-methylpropionate). The addition amount of the initiator is 0.01 - 5 wt% of the total amount of polymerization monomers, preferably 0.1 - 2 wt%.

[0039] Further, the emulsifier is a commonly used polymerization emulsifier in the art, including but not limited to perfluorocarboxylic acid, perfluorocarboxylate, perfluoropolyether, and perfluoropolyether carboxylate. Preferably, the emulsifier is a perfluoropolyether carboxylate having the following formula (3).

[0040] CF3CF2CF2O(CF(CF3)CF2O)o -2 The structural formula of CF(CF3)COOM is formula (3),

[0041] where: o is an integer from 2 to 6, and M is an alkali metal ion or an ammonium ion.

[0042] The dosage of the emulsifier only needs to satisfy the smooth progress of the polymerization reaction. The dosage of the emulsifier is 0.01 - 2 wt% of the total amount of the polymerization monomers. Preferably, it is 0.05 - 0.5 wt%.

[0043] Furthermore, the chain transfer agent can be any perfluoroiodoalkane that can adjust the molecular weight of the fluoropolymer. Preferably, the chain transfer agent is at least one of perfluoroiodobutane and perfluoroiodohexane. The addition amount of the chain transfer agent is 0.01 - 5 wt% of the polymerization monomers. Preferably, it is 0.1 - 2 wt%.

[0044] Furthermore, the organic solvent is any solvent that can dissolve the polyvinylidene fluoride macromolecular chain transfer agent. Preferably, the solvent is selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. More preferably, it is N,N-dimethylacetamide.

[0045] In the third aspect, the present invention provides an application of a hydrophilic vinylidene fluoride copolymer. The hydrophilic vinylidene fluoride copolymer is used as a component in the preparation of a porous film.

[0046] Furthermore, the porous film is used as a membrane module for sterilization filtration, seawater desalination, sewage separation, and blood purification.

[0047] In the fourth aspect, the present invention provides an ultrafiltration membrane, and the ultrafiltration membrane contains the hydrophilic vinylidene fluoride copolymer.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] In the hydrophilic vinylidene fluoride copolymer of the present invention, a vinylidene fluoride structural unit is introduced into the hydrophilic chain segment, which can enhance the compatibility with the hydrophobic chain segment, increase the overall mechanical strength and stability of the material, and avoid problems such as too large pore size and poor filtration efficiency caused by phase separation when preparing a porous film; in addition, a hydrophilic group of the compound shown in structural formula (1) is introduced into the hydrophilic chain segment, enhancing the overall hydrophilic property of the material, and having a high water flux and biocompatibility when preparing a porous film. Specific Embodiments

[0050] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternative solutions, improvement solutions, and equivalent solutions that may be included within the scope of the claims.

[0051] Example 1

[0052] In a 5L high-pressure polymerization reactor equipped with a stirrer, 3 kg of deionized water, 3 g of CF3CF2CF2O(CF(CF3)CF2O)CF(CF3)COONa, and 10 g of refined paraffin with a melting point of about 60 °C were added. The reactor was closed, the stirrer was started, and vacuum nitrogen replacement was carried out until the oxygen content in the reactor was less than 10 ppm. A certain amount of VDF was added to the reactor until the pressure in the reactor reached 2.5 MPa, and the temperature was raised to 80 °C. VDF was continuously added until the pressure in the reactor reached 4.5 MPa. 3 g of initiator diisopropyl peroxydicarbonate (IPP) and 3 g of chain transfer agent perfluoroiodohexane were added to start the polymerization reaction. When the pressure in the polymerization reactor dropped to 4.0 MPa, the pressure in the reactor was maintained between 4.0 - 4.5 Mpa by adding VDF. A total of 1000 g of VDF was added. A total of 10 g of initiator and 10 g of chain transfer agent were added in stages intermittently. When the specified weight of VDF was added and the reaction pressure in the reactor dropped to 1.0 MPa, the polymerization ended. The emulsion was demulsified, coagulated, filtered, washed, and vacuum dried to obtain the first block of polyvinylidene fluoride macromolecular chain transfer agent with iodine at the end.

[0053] 500 g of the first block of polyvinylidene fluoride macromolecular chain transfer agent, 2500 g of N,N-dimethylacetamide, and 30 g of comonomer 2-(trifluoromethyl)acrylic acid (MAF) were added to a 5L high-pressure polymerization reactor. Vacuum nitrogen replacement was carried out to remove oxygen. 600 g of VDF was added to the reactor, the temperature was raised to 65 °C, and it was stirred and dissolved for 30 minutes. 5 g of initiator azobisisobutyronitrile (AIBN) was added to start the polymerization reaction. When the reaction pressure in the reactor dropped to 2.5 MPa, the polymerization ended. It was precipitated with absolute ethanol, washed, filtered, and vacuum dried to obtain a hydrophilic polyvinylidene fluoride copolymer, where the first block is a VDF chain segment and the second block is a VDF / MAF chain segment.

[0054] Example 2

[0055] In a 5L high-pressure polymerization reactor equipped with a stirrer, 3 kg of deionized water, 3 g of CF3CF2CF2O(CF(CF3)CF2O)CF(CF3)COONa, and 10 g of refined paraffin with a melting point of about 60 °C were added. The reactor was closed, stirring was started, and vacuum nitrogen replacement was carried out until the oxygen content in the reactor was less than 10 ppm. A certain amount of VDF was added to the reactor until the pressure in the reactor reached 2.5 MPa, and the temperature was raised to 80 °C. VDF was continuously added until the pressure in the reactor reached 4.5 MPa. 3 g of initiator diisopropyl peroxydicarbonate (IPP) and 3 g of chain transfer agent perfluoroiodohexane were added to start the polymerization reaction. When the pressure in the polymerization reactor dropped to 4.0 MPa, the pressure in the reactor was maintained between 4.0 - 4.5 Mpa by adding VDF. A total of 1000 g of VDF was added. A total of 10 g of initiator and 10 g of chain transfer agent were added in stages intermittently. When the specified weight of VDF was added and the reaction pressure in the reactor dropped to 1.0 MPa, the polymerization ended. The emulsion was demulsified, coagulated, filtered, washed, and vacuum dried to obtain the first block of polyvinylidene fluoride macromolecular chain transfer agent with iodine at the end.

[0056] 500 g of the first block of polyvinylidene fluoride macromolecular chain transfer agent, 2500 g of N,N-dimethylacetamide, and 30 g of comonomer 2-fluoroacrylic acid (FAA) were added to a 5L high-pressure polymerization reactor. Vacuum nitrogen replacement was carried out to remove oxygen. 600 g of VDF was added to the reactor, and the temperature was raised to 65 °C. Stirring was carried out for 30 minutes to dissolve. 5 g of initiator azobisisobutyronitrile (AIBN) was added to start the polymerization reaction. When the reaction pressure in the reactor dropped to 2.5 MPa, the polymerization ended. It was precipitated with absolute ethanol, washed, filtered, and vacuum dried to obtain a hydrophilic vinylidene fluoride copolymer, where the first block was a VDF chain segment and the second block was a VDF / FAA chain segment.

[0057] Example 3

[0058] In a 5L high-pressure polymerization reactor equipped with a stirrer, 3 kg of deionized water, 3 g of CF3CF2CF2O(CF(CF3)CF2O)CF(CF3)COONa, and 10 g of refined paraffin with a melting point of about 60 °C were added. The reactor was closed, the stirrer was started, and vacuum nitrogen replacement was carried out until the oxygen content in the reactor was less than 10 ppm. A certain amount of the initial mixed gas of VDF and HFP was added to the reactor until the pressure in the reactor reached 2.5 MPa, where the VDF content was 95 wt%. The temperature was raised to 80 °C, and VDF was continuously added until the pressure in the reactor reached 4.5 MPa. 3 g of the initiator diisopropyl peroxydicarbonate (IPP) and 3 g of the chain transfer agent perfluoroiodohexane were added, and the polymerization reaction was started. When the pressure in the polymerization reactor dropped to 4.0 MPa, the pressure in the reactor was maintained between 4.0 - 4.5 Mpa by adding VDF. A total of 1000 g of the initial mixed gas of VDF and HFP was added. A total of 10 g of the initiator and 10 g of the chain transfer agent were added in stages intermittently. When the specified weight of the initial mixed gas of VDF and HFP was added and the reaction pressure in the reactor dropped to 1.0 MPa, the polymerization ended. The emulsion was demulsified, coagulated, filtered, washed, and vacuum dried to obtain the first block of the polyvinylidene fluoride macromolecular chain transfer agent with iodine at the end.

[0059] 500 g of the first block of the polyvinylidene fluoride macromolecular chain transfer agent, 2500 g of N,N-dimethylacetamide, and 30 g of the comonomer 2-(trifluoromethyl)acrylic acid (MAF) were added to a 5L high-pressure polymerization reactor. Vacuum nitrogen replacement was carried out to remove oxygen. 600 g of the initial mixed gas of VDF and HFP was added to the reactor, where the VDF content was 95 wt%. The temperature was raised to 65 °C, and stirring and dissolution were carried out for 30 minutes. 5 g of the initiator azobisisobutyronitrile (AIBN) was added, and the polymerization reaction was started. When the reaction pressure in the reactor dropped to 2.5 MPa, the polymerization ended. It was precipitated, washed, and filtered with absolute ethanol and vacuum dried to obtain the hydrophilic vinylidene fluoride copolymer, where the first block was the VDF / HFP chain segment and the second block was the VDF / HFP / MAF chain segment.

[0060] Example 4

[0061] In a 5L high-pressure polymerization reactor equipped with a stirrer, 3 kg of deionized water, 3 g of CF3CF2CF2O(CF(CF3)CF2O)CF(CF3)COONa, and 10 g of refined paraffin with a melting point of about 60 °C were added. The reactor was closed, the stirrer was started, and vacuum nitrogen replacement was carried out until the oxygen content in the reactor was less than 10 ppm. A certain amount of the initial mixed gas of VDF and HFP was added to the reactor until the pressure in the reactor reached 2.5 MPa, where the VDF content was 95 wt%. The temperature was raised to 80 °C, and VDF was continuously added until the pressure in the reactor reached 4.5 MPa. 3 g of the initiator diisopropyl peroxydicarbonate (IPP) and 3 g of the chain transfer agent perfluoroiodohexane were added, and the polymerization reaction was started. When the pressure in the polymerization reactor dropped to 4.0 MPa, the pressure in the reactor was maintained between 4.0 - 4.5 Mpa by adding VDF. A total of 1000 g of the initial mixed gas of VDF and HFP was added. The initiator was added in stages intermittently for a total of 10 g, and the chain transfer agent was added in stages intermittently for a total of 10 g. When the specified weight of the initial mixed gas of VDF and HFP was added and the reaction pressure in the reactor dropped to 1.0 MPa, the polymerization ended. The emulsion was demulsified, coagulated, filtered, washed, and vacuum dried to obtain the first block of the polyvinylidene fluoride macromolecular chain transfer agent with iodine at the end.

[0062] 500 g of the first block of the polyvinylidene fluoride macromolecular chain transfer agent, 2500 g of N,N-dimethylacetamide, and 30 g of the comonomer 2-fluoroacrylic acid (FAA) were added to a 5L high-pressure polymerization reactor. Vacuum nitrogen replacement was carried out to remove oxygen. 600 g of the initial mixed gas of VDF and HFP was added to the reactor, where the VDF content was 95 wt%. The temperature was raised to 65 °C, and stirring and dissolution were carried out for 30 minutes. 5 g of the initiator azobisisobutyronitrile (AIBN) was added, and the polymerization reaction was started. The polymerization ended when the reaction pressure in the reactor dropped to 2.5 MPa. It was precipitated with anhydrous ethanol, washed, filtered, and vacuum dried to obtain the hydrophilic vinylidene fluoride copolymer, where the first block was the VDF / HFP chain segment and the second block was the VDF / HFP / FAA chain segment.

[0063] Comparative Example 1

[0064] In a 5L high-pressure polymerization reactor equipped with a stirrer, 3 kg of deionized water, 3 g of CF3CF2CF2O(CF(CF3)CF2O)CF(CF3)COONa, and 10 g of refined paraffin with a melting point of about 60 °C were added. The reactor was closed, the stirrer was started, and vacuum nitrogen replacement was carried out until the oxygen content in the reactor was less than 10 ppm. A certain amount of VDF was added to the reactor until the pressure in the reactor reached 2.5 MPa, and the temperature was raised to 80 °C. Then, VDF was continuously added until the pressure in the reactor reached 4.5 MPa. 3 g of initiator diisopropyl peroxydicarbonate (IPP) and 3 g of chain transfer agent perfluoroiodohexane were added to start the polymerization reaction. When the pressure in the polymerization reactor dropped to 4.0 MPa, the pressure in the reactor was maintained between 4.0 - 4.5 Mpa by adding VDF. A total of 1000 g of VDF was added. The initiator was added in stages intermittently for a total of 10 g, and the chain transfer agent was added in stages intermittently for a total of 10 g. When the specified weight of VDF was added and the reaction pressure in the reactor dropped to 1.0 MPa, the polymerization ended. The emulsion was demulsified, coagulated, filtered, washed, and vacuum dried to obtain the first block of the polyvinylidene fluoride macromolecular chain transfer agent with iodine at the end.

[0065] 500 g of the first block of the polyvinylidene fluoride macromolecular chain transfer agent and 2500 g of N,N-dimethylacetamide were added to a 5L high-pressure polymerization reactor. Vacuum nitrogen replacement was carried out to remove oxygen. 630 g of the initial mixture of VDF and HFP was added to the reactor, where the VDF content was 95 wt%. The temperature was raised to 65 °C, and it was stirred and dissolved for 30 minutes. 5 g of initiator azobisisobutyronitrile (AIBN) was added to start the polymerization reaction. When the reaction pressure in the reactor dropped to 2.5 MPa, the polymerization ended. It was precipitated, washed, and filtered with absolute ethanol and then vacuum dried to obtain the hydrophilic vinylidene fluoride copolymer, where the first block was the VDF chain segment and the second block was the VDF / HFP chain segment.

[0066] Comparative Example 2

[0067] 3 kg of deionized water, 3 g of CF3CF2CF2O(CF(CF3)CF2O)CF(CF3)COONa, and 10 g of refined paraffin with a melting point of about 60 °C were added to a 5 L high-pressure polymerization reactor equipped with a stirrer. The reactor was closed, the stirrer was started, and the reactor was evacuated and purged with nitrogen until the oxygen content in the reactor was less than 10 ppm. A certain amount of VDF was added to the reactor until the pressure in the reactor reached 2.5 MPa, and the temperature was raised to 80 °C. VDF was continuously added until the pressure in the reactor reached 4.5 MPa. 3 g of initiator diisopropyl peroxydicarbonate (IPP) and 3 g of chain transfer agent perfluoroiodohexane were added, and the polymerization reaction was started. When the pressure in the polymerization reactor dropped to 4.0 MPa, the pressure in the reactor was maintained between 4.0 - 4.5 Mpa by adding VDF. A total of 1000 g of VDF was added. A total of 10 g of initiator and 10 g of chain transfer agent were added in stages intermittently. The polymerization was completed when the specified weight of VDF was added and the reaction pressure in the reactor dropped to 1.0 MPa. The emulsion was demulsified, coagulated, filtered, washed, and vacuum dried to obtain the first block of polyvinylidene fluoride macromolecular chain transfer agent with iodine at the end.

[0068] 500 g of the first block of polyvinylidene fluoride macromolecular chain transfer agent, 2500 g of N,N-dimethylacetamide, and 30 g of comonomer acrylic acid (AA) were added to a 5 L high-pressure polymerization reactor. The reactor was evacuated and purged with nitrogen to remove oxygen, and the temperature was raised to 65 °C. The mixture was stirred and dissolved for 30 minutes. 5 g of initiator azobisisobutyronitrile (AIBN) was added, and the polymerization reaction was started. The reaction was completed after 24 hours. It was precipitated with anhydrous ethanol, washed, filtered, and vacuum dried to obtain a hydrophilic vinylidene fluoride copolymer, where the first block was a VDF chain segment and the second block was an AA chain segment.

[0069] Comparative Example 3

[0070] In a 5L high-pressure polymerization reactor equipped with a stirrer, 3 kg of deionized water, 30 g of comonomer 2-(trifluoromethyl) acrylic acid (MAF), 3 g of CF3CF2CF2O(CF(CF3)CF2O)CF(CF3)COONa, and 10 g of refined paraffin with a melting point of about 60 °C were added. The reactor was closed, stirring was started, and vacuum nitrogen replacement was carried out until the oxygen content in the reactor was less than 10 ppm. A certain amount of the initial mixed gas of VDF and HFP was added to the reactor until the pressure in the reactor reached 2.5 MPa, where the VDF content was 95 wt%. The temperature was raised to 80 °C, and VDF was continuously added until the pressure in the reactor reached 4.5 MPa. 3 g of initiator diisopropyl peroxydicarbonate (IPP) and 3 g of chain transfer agent perfluoroiodohexane were added to start the polymerization reaction. When the pressure in the polymerization reactor dropped to 4.0 MPa, the pressure in the reactor was maintained between 4.0 - 4.5 Mpa by adding VDF. A total of 1600 g of the initial mixed gas of VDF and HFP was added. A total of 10 g of initiator and 10 g of chain transfer agent were added in stages intermittently. When the specified weight of the initial mixed gas of VDF and HFP was added and the reaction pressure in the reactor dropped to 1.0 MPa, the polymerization was completed. The emulsion was demulsified, coagulated, filtered, washed, and vacuum dried to obtain a ternary random copolymer of VDF / HFP / MAF.

[0071] Using the resins prepared in the examples and comparative examples, porous films were prepared. The film preparation method was as follows:

[0072] (1) 90 g of the prepared hydrophilic vinylidene fluoride copolymer resin, 5 g of pore-forming agent polyvinylpyrrolidone, and 405 g of N,N-dimethylacetamide were mixed evenly and dissolved in an environment at 60 °C for 24 h. Vacuum degassing was carried out, and it was left standing for 6 h to prepare a casting solution.

[0073] (2) Using the film scraping method, the casting solution was coated on a glass plate using a frame-type film coater. The thickness of the film coater was adjusted to 150 μm, and on a clean glass plate, the casting solution was evenly scraped into a film.

[0074] (3) The glass plate carrying the casting solution was immersed in deionized water at 30 °C, and the film slowly peeled off from the glass plate.

[0075] (4) The prepared film was soaked in water for 24 h, taken out after air drying, and the finished product was obtained.

[0076] The films prepared using the resins in the examples and comparative examples were subjected to performance tests. The test results are shown in Table 1 in detail. The test methods are as follows:

[0077] Contact angle: 3.5 μL of distilled water was dropped onto the surface of the prepared hydrophilic film using a micro syringe, and the contact angle was measured using a contact angle measuring instrument (model OCA-20). The better the hydrophilicity, the smaller the contact angle.

[0078] Rejection rate: Measured by the bovine serum albumin method (BSA) according to the standard of GB / T 32360-2015.

[0079] Tensile strength: Cut the prepared film into several strip films with a size of 60mm×10mm, and then soak them in pure water for standby. Clamp the soaked strip film in a microcomputer-controlled universal electronic testing machine, accurately measure its thickness with a micrometer, and then stretch the film at a speed of 10mm / min until it breaks, and read and record the relevant data from the computer. Each sample is measured at least three times and the average value is taken.

[0080] Pure water flux: Under room temperature conditions, using the closed-end method, first pre-press the film with deionized water at 0.2MPa for 30min, and then supply deionized water to one side surface of the film at 0.1MPa, measure the amount of permeated water, and then express it by the permeation amount per unit time, the permeation amount per unit membrane area, and the permeation amount per unit pressure. The higher the pure water permeability, the more likely the separation membrane is to exhibit excellent hydrophilicity.

[0081] Table 1 Performance test results

[0082]

[0083] It can be seen from the data in Table 1 that the membrane materials prepared from the hydrophilic polyvinylidene fluoride copolymer prepared in the examples have significantly better comprehensive performance than the comparative examples. Introducing a small amount of hexafluoropropylene structural units into the copolymer will not have an obvious impact on the performance of the material. At the same time, compared with the random copolymer, the block copolymer exhibits more excellent performance and has a large pure water flux. In addition, introducing hydrophilic groups into the second block can significantly reduce the contact angle between deionized water and the membrane material. However, if the second block does not contain fluorine-containing structural units and is only composed of hydrophilic segments, resulting in poor compatibility with the fluorine-containing segments, the prepared membrane material, although having strong hydrophilicity, has a large reduction in both tensile strength and pure water flux.

[0084] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A hydrophilic vinylidene fluoride copolymer, characterized in that: The hydrophilic vinylidene fluoride copolymer is a block copolymer composed of a first block and a second block. The first block contains vinylidene fluoride structural units, and the content of the vinylidene fluoride structural units accounts for 85-100 wt% of the total amount of the first block. The second block contains vinylidene fluoride structural units and the structural units of the compound shown in formula (1). The content of the vinylidene fluoride structural units accounts for 50-95 wt% of the total amount of the second block, and the content of the structural units of the compound shown in formula (1) accounts for 5-50 wt% of the total amount of the second block. Among them, R 1 , R 2 , R 3 are independently selected from hydrogen, halogen, and perfluoroalkyl groups having 1 to 6 carbon atoms, and R 1 , R 2 , R 3 at least one of which is halogen or a perfluoroalkyl group having 1 to 6 carbon atoms. n is an integer from 0 to 12. R 4 selected from hydrogen, lithium, sodium, potassium, ammonium, alkylammonium or alkylarylammonium ions having 1 to 6 carbon atoms in the alkyl group, The compound shown in formula (2). m is an integer from 1 to 100.

2. The hydrophilic vinylidene fluoride copolymer according to claim 1, wherein: The compound shown in formula (1) is selected from at least one of 2-(trifluoromethyl)acrylic acid and 2-fluoroacrylic acid.

3. The hydrophilic vinylidene fluoride copolymer according to claim 1, characterized in that: The second block further contains first olefin structural units, and the content of the first olefin structural units accounts for 0-5 wt% of the total amount of the second block.

4. The hydrophilic vinylidene fluoride copolymer according to claim 3, wherein: The first olefin is selected from at least one of hexafluoropropene, chlorotrifluoroethylene, pentafluoropropene, tetrafluoropropene, trifluoropropene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluoroethyl vinyl ether, vinyl fluoride, methyl acrylate, methyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, acrylic acid, methacrylic acid, itaconic acid, maleic ester, citraconic ester, vinyl acetate, vinylene carbonate or acrylonitrile.

5. The hydrophilic vinylidene fluoride copolymer according to claim 1, characterized in that: The first block further contains first fluorinated olefin structural units, and the content of the first fluorinated olefin structural units accounts for 0-15 wt% of the total amount of the first block.

6. The hydrophilic vinylidene fluoride copolymer according to claim 5, characterized in that: The first fluorinated olefin is selected from at least one of hexafluoropropene, chlorotrifluoroethylene, pentafluoropropene, tetrafluoropropene, trifluoropropene, perfluorobutene, tetrafluoroethylene, trifluoroethylene, perfluoroethyl vinyl ether, vinyl fluoride.

7. The hydrophilic vinylidene fluoride copolymer according to claim 1, characterized in that: The content of the first block accounts for 50-95 wt% of the total amount of the hydrophilic vinylidene fluoride copolymer; the content of the second block accounts for 5-50 wt% of the total amount of the hydrophilic vinylidene fluoride copolymer.

8. A method for preparing the hydrophilic vinylidene fluoride copolymer according to any one of claims 1-7, characterized in that: The preparation method uses an iodine transfer living radical polymerization method.

9. Use of the hydrophilic vinylidene fluoride copolymer according to any one of claims 1-7, characterized in that: The hydrophilic vinylidene fluoride copolymer is used as a component in the preparation of a porous film.

10. The application of the hydrophilic vinylidene fluoride copolymer according to claim 9, characterized in that: The porous film is used as a membrane module for sterilization filtration, seawater desalination, sewage separation, and blood purification.

11. An ultrafiltration membrane, characterized in that: The ultrafiltration membrane contains the hydrophilic vinylidene fluoride copolymer according to any one of claims 1-7.

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