Ultrahigh molecular weight polyvinylidene fluoride-based polymer and preparation method thereof

Abstract

The invention discloses an ultra-high molecular weight polyvinylidene fluoride-based polymer and a preparation method thereof, and belongs to the technical field of fluorine chemical industry. An emulsifier (a series of anionic fluorinated surfactants such as perfluoroalkyl carboxylic acid and salt thereof, perfluoroalkyl sulfonic acid and salt thereof, perfluoroalkyl polyether fluorocarbonic acid and salt thereof) used for fluorine-containing olefin emulsion polymerization is used as a protecting group for generating metal nanoparticles through in-situ reduction; in the emulsion polymerization reaction process, a silver salt precursor is subjected to in-situ reduction to synthesize silver nanoparticles, and then the silver nanoparticles interact with an initiator to initiate fluorine-containing olefin monomer polymerization. Through in-situ reduction, the problem that the efficiency of catalytic initiation polymerization reaction is reduced due to the fact that monodisperse metal nanoparticles are prone to aggregation for a long time is solved.

Description

Technical Field

[0001] The present invention belongs to the fields of nanomaterial synthesis and polymer polymerization, and specifically relates to a method for in-situ reducing and synthesizing silver nanoparticles in a polymerization system to initiate the polymerization of vinylidene fluoride-containing monomers to synthesize ultra-high molecular weight polymers, and the application of such polymers in lithium-ion battery binders. Background Art

[0002] The strong electronegativity and high bond energy of fluorine atoms endow fluorinated polymers with a series of unique advantages, such as low refractive index, low dielectric constant, low dissipation factor, low surface tension, and excellent chemical stability. These materials exhibit tolerance to acids, bases, and organic solvents, as well as high temperature and weather resistance, and hydrophobic and oleophobic properties. Polyvinylidene fluoride (PVDF) is a semi-crystalline thermoplastic fluoropolymer formed by free radical polymerization of 1,1-difluoroethylene (VDF) monomers, and has highly inert, tough, and stable properties. Its excellent overall properties, such as chemical resistance, high crystallinity, and thermal stability, make it one of the most recognized high molecular weight fluoropolymers in the market.

[0003] Lithium batteries are one of the most commonly used battery types today, and the binder, as an important component thereof, plays an important role in battery performance and life. Lithium battery electrode materials usually need to be combined with a binder to form an electrode sheet coated on a current collector. As a binder, PVDF has excellent mechanical strength, chemical stability, and binding force with electrode active materials, and can effectively fix the electrode materials on the current collector. In addition, PVDF can also improve the high temperature resistance performance and cycle life of the battery. The factors affecting the adhesiveness of PVDF mainly include molecular weight, crystallinity, PVDF modification, types of cathode materials, and conductive agents. Generally speaking, the larger the molecular weight, the stronger the intermolecular interaction force, the longer the long chain, and the greater the binding strength to the electrode material active substances. Therefore, increasing the molecular weight of PVDF as much as possible can effectively improve its binding performance and reduce the amount of PVDF used. According to the currently published materials, it is a great challenge to prepare ultra-high molecular weight PVDF.

[0004] Patent CN103387633A discloses a method for preparing ultra-high molecular weight polyvinylidene fluoride materials. This method copolymerizes vinylidene fluoride and acrylate monomers through emulsion polymerization, but it is difficult to synthesize ultra-high molecular weight polymers at too high polymerization temperatures (above 90°C), and high temperatures have a great impact on the coloring property of the product.

[0005] Patent CN110183562A discloses a polyvinylidene fluoride polymer that can be used as a lithium battery adhesive. The preparation process is divided into three stages: First, a PVDF homopolymer with a molecular weight of 1.6 million to 2 million is obtained by homopolymerization. Second, vinylidene fluoride and a second monomer are copolymerized to obtain a copolymer with a high molecular weight of 1 million to 1.3 million. Finally, the PVDF homopolymer and the copolymer are mixed. Although the vinylidene fluoride mixture resin prepared by this method has good solution viscosity, bonding performance, and flexibility, its production process is cumbersome and the equipment cost is high.

[0006] Patent CN104710550A discloses a high molecular weight polyvinylidene fluoride resin and its preparation method, which adopts a suspension polymerization process. Although the crystallinity of the PVDF product is reduced by heating, a large amount of low molecular weight PVDF resin is easily generated, resulting in poor swelling performance in the electrolyte and affecting its application performance in lithium batteries.

[0007] Patent CN114773660A proposes the catalytic polymerization of metal nanoparticles to synthesize ultra-high molecular weight fluorinated olefin nanospheres. However, due to the high price of monodisperse noble metal nanoparticles and their tendency to aggregate in solution during storage, resulting in a decrease in catalytic ability, these undoubtedly increase the difficulty of industrializing this method. At the same time, the suspension process has many problems such as long production time, low efficiency, high crystallinity of the product leading to poor toughness, and the electrode sheet is prone to peeling and breaking. Moreover, the PVDF homopolymer has poor bonding and conductive properties due to the lack of polar groups, which greatly limits the performance of lithium-ion batteries.

[0008] Therefore, developing a simple, efficient, and ultra-high molecular weight polyvinylidene fluoride synthesis method containing polar groups has great market application potential. Summary of the Invention

[0009] Aiming at the defects of the prior art, the present invention provides an ultra-high molecular weight polyvinylidene fluoride-based polymer and its preparation method, which solves the problem that monodisperse metal nanoparticles are prone to aggregation over a long time, resulting in a decrease in the efficiency of catalytic initiation polymerization.

[0010] To solve the above technical problems, the present invention adopts the following technical solutions:

[0011] First, a method for in-situ reducing and synthesizing silver nanoparticles in a polymerization system and initiating the polymerization of fluorinated olefin monomers to synthesize an ultra-high molecular weight polymer is provided.

[0012] Emulsion polymerization reaction is carried out in a sealed reaction kettle by replacing oxygen with nitrogen.

[0013] The reaction raw materials include deionized water, emulsifier, silver salt precursor, initiator, and reaction monomer.

[0014] During the polymerization reaction, silver salt precursors are reduced by adding reducing agents to in-situ generate silver nanoparticles;

[0015] Under the conditions of set reaction temperature and reaction pressure, the polymerization reaction is initiated, and the reaction pressure is maintained by adding reaction monomers additionally, and finally a ultra-high molecular weight polyvinylidene fluoride-based polymer is obtained.

[0016] Preferably, the set reaction temperature of the polymerization reaction is 40-80 °C and the reaction pressure is 4.5-8 MPa.

[0017] Preferably, the dosage of the silver salt precursor is 0.003-0.02% of the mass of the reaction monomer, and the dosage of the initiator is 0.05-0.2% of the mass of the reaction monomer.

[0018] Preferably, the dosage of deionized water is 100-250% of the mass of the reaction monomer; the dosage of the emulsifier is 0.5-2.5% of the mass of the reaction monomer.

[0019] Preferably, the reducing agent is one of sodium borohydride, ascorbic acid, lithium aluminum hydride, sodium citrate, potassium tartrate, and sodium hypophosphite.

[0020] Preferably, the silver salt precursor is one of silver trifluoroacetate, silver nitrate, silver acetate, and silver oxalate.

[0021] Preferably, the initiator is an organic halide, including organic bromides and organic chlorides. The organic bromides include 2-bromo-2-methylpropanoic acid, tribromoacetic acid, 2-bromopropanoic acid, 2-bromobutanoic acid, 3-bromopropanoic acid, 2-bromobenzoic acid, α-bromobenzoic acid, 2-bromophenylacetic acid, α-bromophenylacetic acid, 2-bromo-4-methylbenzoic acid, 2-bromocyclohexanecarboxylic acid, 2,2-dibromopropanoic acid, 3-bromo-2,2-dimethylpropanoic acid, 2-bromo-1,4-dimethylbenzoic acid, 3-bromo-2-hydroxypropanoic acid, and difluorobromoacetic acid; the organic chlorides include chloroacetic acid, dichloroacetic acid, trichloroacetic acid, 2-chloropropanoic acid, chlorobutanoic acid, α-chlorobenzoic acid, α-chlorophenylacetic acid, and α-chlorophenylpyruvic acid.

[0022] Preferably, the reaction monomer is vinylidene fluoride or a mixture of one or more of vinylidene fluoride and tetrafluoroethylene, fluoroethylene, trifluoroethylene, hexafluoropropylene, trifluorochloroethylene, ethylene, propylene, 2,3,3,3-tetrafluoro-1-propene, 1,3,3,3-tetrafluoro-1-propene, cis-1-chloro-3,3,3-trifluoropropene, trans-1-chloro-3,3,3-trifluoropropene, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoropropyl vinyl ether, acrylic acid, methacrylic acid, monomethyl maleate, and monomethyl itaconate, wherein the mass fraction of vinylidene fluoride in the mixture is 60%-99.99%.

[0023] Furthermore, a ultra-high molecular weight polyvinylidene fluoride-based polymer is also provided, which is prepared by the above method. The weight-average molecular weight of the ultra-high molecular weight polyvinylidene fluoride-based polymer is 2 million - 5 million, the intrinsic viscosity is 4.0 - 6.5 dL·g-1, and the molecular weight distribution index Mw / Mn is 1.5 - 3.0;

[0024] Polymers with different molecular weights are obtained by adjusting the amounts of silver salt precursor and initiator.

[0025] In addition, a binder for lithium ion battery electrode is also provided, and the binder comprises the above-mentioned ultra-high molecular weight polyvinylidene fluoride-based polymer.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The present invention uses the emulsifiers used in the emulsion polymerization of fluorinated olefins (a series of anionic fluorinated surfactants such as perfluoroalkyl carboxylic acids and their salts, perfluoroalkyl sulfonic acids and their salts, perfluoroalkyl polyether fluorocarbonic acids and their salts, etc.) as the protecting groups for in-situ reduction to generate metal nanoparticles. During the emulsion polymerization reaction process, the silver salt precursor is in-situ reduced to synthesize silver nanoparticles, which then interact with the initiator to initiate the polymerization of fluorinated olefin monomers.

[0028] Different from the traditional free radical polymerization, anion (cation) polymerization or coordination polymerization methods, the present invention does not use free radical polymerization initiators or anion (cation) polymerization initiators, nor does it use the coordination polymerization Ziegler-Natta catalyst. At the same time, the problem that monodisperse metal nanoparticles are prone to aggregation over a long time, resulting in a decrease in the efficiency of catalytic initiation of the polymerization reaction, is solved by in-situ reduction.

[0029] In addition, the initiator is mainly an organic halide, which introduces polar groups such as carboxyl groups into the polymer backbone while initiating the polymerization reaction, and can form a strong interaction with the surface of the material to be bonded, thereby enhancing the adhesion between PVDF and the material to be bonded.

[0030] In the technical field, the initiator used in the present invention does not belong to the initiator for conventional free radical polymerization and cannot initiate the polymerization reaction under conventional conditions. In the present invention, a reducing agent is added during the reaction process to reduce the precursor, and the generated silver nanoparticles further catalyze the decomposition of the initiator, enabling it to obtain the ability to initiate the polymerization reaction. Therefore, polymers with different molecular weights can be obtained by adjusting the amounts of silver salt precursor and initiator, realizing the precise control of the molecular weight and molecular weight distribution of the product, and further obtaining polymers with specific structures and properties.

[0031] In the present invention, the set reaction temperature for the polymerization reaction is 40 - 80°C, and the reaction pressure is 4.5 - 8 MPa. The polymerization reaction temperature is higher than the critical temperature of vinylidene fluoride (30.1°C). If the polymerization temperature is too low, the reaction time will be prolonged, and the crystallinity of the produced PVDF homopolymer will be too high, which is not conducive to its application in lithium batteries. If the temperature is too high, the polymerization reaction will be violent, and it will be difficult to synthesize a PVDF polymer with ultra-high molecular weight. Therefore, in the present invention, the temperature range of the polymerization reaction is between 40 and 80°C, preferably 50 - 70°C.

[0032] In the present invention, the polymerization reaction pressure is above the critical pressure of vinylidene fluoride (4.29 Mpa). If the reaction pressure is too low, the polymerization reaction rate will be slow and the polymerization time will be long. If the reaction pressure is too high, equipment with higher pressure resistance is required, and high pressure increases the risk of the polymerization process. Therefore, considering various factors, the polymerization pressure is between 4.5 - 8 Mpa, preferably 5 - 7 Mpa.

[0033] The reaction time of the method of the present invention is significantly shorter than that of the existing suspension polymerization process, achieving the goal of synthesizing ultra-high molecular weight polyvinylidene fluoride through an emulsion polymerization process, greatly improving production efficiency, and having broad development prospects.

[0034] The present invention synthesizes an ultra-high molecular weight polyvinylidene fluoride-based polymer with a weight average molecular weight of 2 - 4 million, an intrinsic viscosity between 4.0 - 6.5, and a molecular weight distribution between 1.5 - 3.0 through a composite initiation system of silver nanoparticles and organic haloacids. In addition, by using an organic haloacid as an initiator, polar groups can be introduced into the polymer main chain, further increasing the binding strength of the polymer as a binder. The polyvinylidene fluoride and its copolymers prepared by the above method belong to ultra-high molecular weight polymers, having both high mechanical strength and toughness, requiring less polymer dosage when used for binder preparation, and having a large binding strength.

[0035] The specific technical solutions and their beneficial effects of the present invention will be described in detail in the following specific embodiments in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be further described below in conjunction with the drawings and specific embodiments:

[0037] Figure 1 TEM image of silver nanoparticles formed by in-situ reduction in Example 1 of the present invention;

[0038] Figure 2 TEM image of silver nanoparticles formed by in-situ reduction in Example 2 of the present invention;

[0039] Figure 3 TEM image of silver nanoparticles formed by in-situ reduction in Example 3 of the present invention;

[0040] Figure 4 FT-IR spectra of the polymer synthesized in Example 1 of the present invention and the polymer synthesized in Comparative Example 1. Detailed implementation manners

[0041] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and in no way restrictive of the present invention and its application or use. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0042] The embodiment of the present invention provides a method for in-situ reduction synthesis of silver nanoparticles in a polymerization system and initiating the polymerization of fluorinated olefin monomers to synthesize ultra-high molecular weight polymers.

[0043] This embodiment uses emulsifiers (a series of anionic fluorinated surfactants such as perfluoroalkyl carboxylic acids and their salts, perfluoroalkyl sulfonic acids and their salts, perfluoroalkyl polyether fluorocarbonic acids and their salts, etc.) used in the emulsion polymerization of fluorinated olefins as protective groups for in-situ reduction to generate metal nanoparticles, in-situ reducing silver salt precursors to synthesize silver nanoparticles during the emulsion polymerization reaction process, and then interacting with initiators to initiate the polymerization of fluorinated olefin monomers.

[0044] Specifically, the method includes the following steps:

[0045] 1. Add an emulsifier, a silver salt precursor, an initiator, and deionized water to the reaction kettle in a ratio of 0.5 - 2.5:0.003 - 0.02:0.05 - 0.2:100 - 250, for example, 1:0.01:0.1:150. And add paraffin and silicone oil. Paraffin can effectively prevent the polymer from sticking to the kettle wall, while silicone oil improves the stability of the emulsion. Perform multiple vacuum pumping and nitrogen replacement on the reaction kettle until the oxygen content in the reaction kettle gas is less than 10 ppm.

[0046] 2. Raise the temperature of the reaction kettle to 40 - 80 °C and start stirring simultaneously.

[0047] 3. After the reaction kettle reaches the target temperature, inject the reducing agent into the reaction kettle, where the ratio of silver salt precursor to reducing agent is 0.003 - 0.02:0.0002 - 0.0015, for example, 0.01:0.0015; then press the reaction monomer into the reaction kettle to make the pressure in the kettle reach the target pressure (4.5 - 8 Mpa), and start the polymerization reaction. During the reaction process, maintain the reaction pressure constant by continuously adding the reaction monomer. The reaction time is generally 4 - 8 h.

[0048] 4. After the reaction reaches the target solid content, stop stirring, cool down the reaction kettle and relieve the pressure, recover the unreacted monomers, and discharge the polymer product emulsion through the discharge port.

[0049] 5. Coagulate the obtained emulsion with 10% CaCl2 solution, and wash the coagulated emulsion with deionized water until the conductivity is lower than 5 μS·cm -1 After that, dry it in a vacuum drying oven until constant weight to obtain the product.

[0050] In this embodiment, the amount of deionized water used is 100-250% of the mass of the reaction monomers, preferably 100-150%, such as 100%, 125%, 150%; the amount of emulsifier used is 0.5-2.5% of the mass of the monomers, preferably 0.5-1.2%, such as 0.5%, 0.9%, 1.2%; the amount of silver salt precursor used is 0.003-0.02% of the mass of the monomers, preferably 0.003-0.008%, such as 0.003%, 0.005%, 0.008%; the amount of initiator used is 0.05-0.2% of the mass of the monomers, preferably 0.08-0.15%, such as 0.08%, 0.12%, 0.15%.

[0051] In this embodiment, the reaction monomers are vinylidene fluoride or a mixture of one or more of vinylidene fluoride and tetrafluoroethylene, vinyl fluoride, trifluoroethylene, hexafluoropropylene, trifluorochloroethylene, ethylene, propylene, 2,3,3,3-tetrafluoro-1-propene, 1,3,3,3-tetrafluoro-1-propene, cis-1-chloro-3,3,3-trifluoropropene, trans-1-chloro-3,3,3-trifluoropropene, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoropropyl vinyl ether, acrylic acid, methacrylic acid, monomethyl maleate, monomethyl itaconate. In the application of battery binders, the copolymerization of vinylidene fluoride with some vinyl-containing monomers can endow the binder material with certain unique properties and improve the battery performance. However, a relatively high content of other monomers will lead to a decrease in the electrochemical corrosion resistance and electrolyte resistance of the binder. The mass fraction of vinylidene fluoride in the mixture needs to be 60%-99.99%, preferably 70%-99%.

[0052] In this embodiment, the emulsifier includes a series of anionic fluorinated surfactants such as perfluoroalkyl carboxylic acids and their salts, perfluoroalkyl sulfonic acids and their salts, perfluoroalkyl polyether fluorocarbonic acids and their salts, and one or more emulsifiers selected from the above can be used in any proportion. Perfluoropolyether fluorocarbonate is preferred.

[0053] In this embodiment, the silver salt precursor is one of silver trifluoroacetate, silver nitrate, silver acetate, and silver oxalate.

[0054] The reducing agent described in this embodiment is one of sodium borohydride, ascorbic acid, lithium aluminum hydride, sodium citrate, potassium tartrate, and sodium hypophosphite.

[0055] The initiator used in this embodiment is one or a mixture of any proportions of the following organic halides:

[0056] (1) Organic bromides, including 2-bromoisobutyric acid, tribromoacetic acid, 2-bromopropionic acid, 2-bromobutyric acid, 3-bromopropionic acid, 2-bromobenzoic acid, α-bromobenzoic acid, 2-bromophenylacetic acid, α-bromophenylacetic acid, 2-bromo-4-methylbenzoic acid, 2-bromocyclohexanoic acid, 2,2-dibromopropionic acid, 3-bromo-2,2-dimethylpropionic acid, 2-bromo-1,4-dimethylbenzoic acid, 3-bromo-2-hydroxypropionic acid, difluorobromoacetic acid;

[0057] (2) Organic chlorides, including chloroacetic acid, dichloroacetic acid, trichloroacetic acid, 2-chloropropionic acid, chlorobutyric acid, α-chlorobenzoic acid, α-chlorophenylacetic acid, α-chlorophenylpyruvic acid;

[0058] Preferably, it is one or a mixture of any proportions of more than two of 2-bromopropionic acid, tribromoacetic acid, difluorobromoacetic acid, trichloroacetic acid, and dichloroacetic acid.

[0059] Different from traditional free radical polymerization, anionic (cationic) polymerization or coordination polymerization methods, this embodiment does not use a free radical polymerization initiator or an anionic (cationic) polymerization initiator, nor does it use a coordination polymerization Ziegler-Natta catalyst. At the same time, the problem that monodisperse metal nanoparticles are prone to aggregation over a long time, resulting in a decrease in the efficiency of catalytic initiation of the polymerization reaction, is solved by in-situ reduction.

[0060] In this embodiment, the polymerization reaction temperature is higher than the critical temperature of vinylidene fluoride (30.1 °C). If the polymerization temperature is too low, the reaction time will be prolonged, and the crystallinity of the generated PVDF homopolymer will be too high, which is not conducive to its application in lithium batteries; if the temperature is too high, the polymerization reaction will be violent, and it will be difficult to synthesize a PVDF polymer with ultra-high molecular weight. Therefore, the temperature range of the polymerization reaction in the present invention is between 40 and 80 °C, preferably 50 to 70 °C, such as 50 °C, 60 °C, 70 °C, etc.

[0061] In this embodiment, the polymerization reaction pressure is above the critical pressure of vinylidene fluoride (4.29 Mpa). If the reaction pressure is too low, the polymerization reaction rate will be slow and the polymerization time will be long; if the reaction pressure is too high, equipment with higher pressure resistance is required, and high pressure increases the risk of the polymerization process. Considering various aspects, the polymerization pressure is between 4.5 and 8 Mpa, preferably 5 to 7 Mpa, such as 5 Mpa, 6 Mpa, 7 Mpa, etc.

[0062] In the technical field, the initiator used in this embodiment does not belong to the initiators for conventional free radical polymerization and cannot initiate the polymerization reaction under conventional conditions. In the present invention, a reducing agent is added during the reaction process to reduce the precursor, and the generated silver nanoparticles further catalyze the decomposition of the initiator, enabling it to acquire the ability to initiate the polymerization reaction. Therefore, by adjusting the amounts of the silver salt precursor and the initiator, polymers with different molecular weights can be obtained, achieving precise control over the molecular weight and molecular weight distribution of the product, and thus obtaining polymers with specific structures and properties.

[0063] In this embodiment, the linear velocity of the stirring paddle tip is controlled at 0.4 - 1.2 m·s-1. If the stirring speed is too low, the reaction rate will be slow and the efficiency will be low; if the speed is too high, the lateral shear force of the paddle will increase, increasing the possibility of demulsification in the polymerization system.

[0064] The reaction time of this embodiment is significantly shorter than that of the existing suspension polymerization process, achieving the goal of synthesizing ultra-high molecular weight polyvinylidene fluoride through the emulsion polymerization process, greatly improving the production efficiency, and having broad development prospects.

[0065] This embodiment also provides an ultra-high molecular weight polyvinylidene fluoride-based polymer, prepared by the above method, with controllable molecular weight and molecular weight distribution. The weight average molecular weight is generally between 2 million and 5 million. In theory, polymer products with larger molecular weights can be obtained through the above method, but too large a molecular weight will greatly increase the solution viscosity of the polymer, which is not conducive to subsequent processing; at the same time, the intrinsic viscosity of the polymer is 4.0 - 6.5 dL·g-1; the molecular weight distribution index Mw / Mn is 1.5 - 3.0.

[0066] This embodiment also provides a binder for lithium ion battery electrodes, at least a part of the binder being an ultra-high molecular weight polyvinylidene fluoride-based polymer prepared by the above method.

[0067] Testing method

[0068] Using N,N-dimethylformamide (DMF) as the solvent, its number average molecular weight (M n ), weight average molecular weight (M w ) and molecular weight distribution index (M w / M n ) are determined by a GPC analysis system. Testing conditions: DMF containing 0.1% lithium bromide as the eluent, using a differential refractive index RI as the detector, polystyrene as the standard sample, temperature 50 °C, flow rate 0.5 mL / min.

[0069] The intrinsic viscosity is determined by an automatic intrinsic viscosity measuring instrument, using N-methylpyrrolidone (NMP) as the solvent, and the intrinsic viscosity value is obtained by the dilution extrapolation method.

[0070] The bonding performance of the binder is characterized by the peel strength. The positive electrode sheet is prepared using the product obtained by the present invention and measured using a universal testing machine according to GB / T 2792-2014.

[0071] The polar groups of the polymer are obtained by infrared spectroscopy (FT-IR) analysis.

[0072] The metal nanoparticles are characterized using a transmission electron microscope. By stopping the polymerization reaction at the initial stage of the reaction, the emulsion is taken out and centrifuged to precipitate the polymer. DMF is added to dissolve it and then centrifuged again. The bottom precipitate is redispersed in tetrahydrofuran for TEM characterization.

[0073] Examples

[0074] Example 1

[0075] In a 1L high-pressure reactor, 0.8g of paraffin wax and 0.6g of dimethyl silicone oil are added. Then, 600g of deionized water, 0.016g of silver nitrate, 0.3g of trichloroacetic acid, and 4g of perfluoropolyether fluorocarbon surfactant (FCD-108) are added in sequence. Immediately, the reactor is evacuated and replaced with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed is maintained at 400 r / min. 5g of 0.02% NaBH4 aqueous solution is added to the reactor through a feeding pump, 300g of vinylidene fluoride monomer is charged, and the reaction temperature is raised to 50°C. After the polymerization reaction starts, when the pressure in the reactor drops, vinylidene fluoride monomer is added to maintain the reaction pressure at 7 MPa. When 300g of vinylidene fluoride is added, the stirring is stopped, and the polymerization emulsion is centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0076] Example 2

[0077] In a 1L high-pressure reactor, 0.8g of paraffin wax and 0.6g of dimethyl silicone oil are added. Then, 600g of deionized water, 0.020g of silver trifluoroacetate, 0.36g of trichloroacetic acid, and 4g of perfluoropolyether fluorocarbon surfactant (FCD-108) are added in sequence. Immediately, the reactor is evacuated and replaced with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed is maintained at 400 r / min. 7g of 0.02% NaBH4 aqueous solution is added to the reactor through a feeding pump, 300g of vinylidene fluoride monomer is charged, and the reaction temperature is raised to 50°C. After the polymerization reaction starts, when the pressure in the reactor drops, vinylidene fluoride monomer is added to maintain the reaction pressure at 7 MPa. When 300g of vinylidene fluoride is added, the stirring is stopped, and the polymerization emulsion is centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0078] Example 3

[0079] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Then, 600 g of deionized water, 0.015 g of silver acetate, 0.2 g of trichloroacetic acid, and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Immediately, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 5 g of 0.02% NaBH4 aqueous solution was added to the reactor through a feeding pump, 220 g of vinylidene fluoride monomer was charged, and the reaction temperature was raised to 70 °C. After the polymerization reaction started, when the pressure in the reactor decreased, vinylidene fluoride monomer was added to maintain the reaction pressure at 7 MPa. When 300 g of vinylidene fluoride was added additionally, the stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0080] Example 4

[0081] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Then, 600 g of deionized water, 0.016 g of silver nitrate, 0.3 g of trichloroacetic acid, and 4 g of perfluoropolyether sulfonic acid surfactant (Capstone FS-10) were added in sequence. Immediately, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 5 g of 0.02% NaBH4 aqueous solution was added to the reactor through a feeding pump, 300 g of vinylidene fluoride monomer was charged, and the reaction temperature was raised to 50 °C. After the polymerization reaction started, when the pressure in the reactor decreased, vinylidene fluoride monomer was added to maintain the reaction pressure at 7 MPa. When 300 g of vinylidene fluoride was added additionally, the stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0082] Example 5

[0083] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Then, 600 g of deionized water, 0.016 g of silver nitrate, 0.3 g of tribromoacetic acid, and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Immediately, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 5 g of 0.02% NaBH4 aqueous solution was added to the reactor through a feeding pump, 300 g of vinylidene fluoride monomer was charged, and the reaction temperature was raised to 50 °C. After the polymerization reaction started, when the pressure in the reactor decreased, vinylidene fluoride monomer was added to maintain the reaction pressure at 7 MPa. When 300 g of vinylidene fluoride was added additionally, the stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0084] Example 6

[0085] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Subsequently, 600 g of deionized water, 0.016 g of silver nitrate, 0.3 g of difluorobromoacetic acid, and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Then, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 5 g of 0.02% aqueous NaBH4 solution was added to the reactor through a feed pump, 300 g of vinylidene fluoride monomer was charged, and the reaction temperature was raised to 50 °C. After the polymerization reaction started, when the pressure in the reactor dropped, vinylidene fluoride monomer was added to maintain the reaction pressure at 7 MPa. When 300 g of vinylidene fluoride was added additionally, the stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0086] Example 7

[0087] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Subsequently, 600 g of deionized water, 0.016 g of silver nitrate, 0.3 g of difluorobromoacetic acid, and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Then, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 5 g of 0.02% aqueous NaBH4 solution was added to the reactor through a feed pump, 350 g of vinylidene fluoride monomer was charged, and the reaction temperature was raised to 50 °C. After the polymerization reaction started, when the pressure in the reactor dropped, vinylidene fluoride monomer was added to maintain the reaction pressure at 8 MPa. When 300 g of vinylidene fluoride was added additionally, the stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0088] Example 8

[0089] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Subsequently, 600 g of deionized water, 0.016 g of silver nitrate, 0.3 g of difluorobromoacetic acid, and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Then, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 5 g of 0.02% aqueous NaBH4 solution was added to the reactor through a feed pump, 210 g of vinylidene fluoride monomer was charged, and the reaction temperature was raised to 50 °C. After the polymerization reaction started, when the pressure in the reactor dropped, vinylidene fluoride monomer was added to maintain the reaction pressure at 5 MPa. When 300 g of vinylidene fluoride was added additionally, the stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight polyvinylidene fluoride.

[0090] Example 9

[0091] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Then, 600 g of deionized water, 0.016 g of silver nitrate, 0.3 g of trichloroacetic acid, and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Immediately, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 6 g of 0.02% aqueous NaBH4 solution was added to the reactor through a feeding pump. 300 g of vinylidene fluoride monomer and 15 g of tetrafluoroethylene were charged, and the reaction temperature was raised to 50 °C. After the polymerization reaction started, when the pressure in the reactor decreased, vinylidene fluoride monomer was added to maintain the reaction pressure at 7 MPa. After the reaction ended, stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight poly(vinylidene fluoride-tetrafluoroethylene) copolymer.

[0092] Example 10

[0093] In a 1 L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Then, 600 g of deionized water, 0.016 g of silver nitrate, 0.3 g of trichloroacetic acid, and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Immediately, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor less than 10 ppm. The stirring speed was maintained at 400 r / min. 6 g of 0.02% aqueous NaBH4 solution was added to the reactor through a feeding pump. 300 g of vinylidene fluoride monomer and 22 g of chlorotrifluoroethylene were charged, and the reaction temperature was raised to 50 °C. After the polymerization reaction started, when the pressure in the reactor decreased, vinylidene fluoride monomer was added to maintain the reaction pressure at 7 MPa. After the reaction ended, stirring was stopped, and the polymerization emulsion was centrifuged, washed, and dried to obtain ultra-high molecular weight poly(vinylidene fluoride-chlorotrifluoroethylene) copolymer.

[0094] Comparative Example

[0095] Comparative Example 1

[0096] In a 1-L high-pressure reactor, 0.8 g of paraffin wax and 0.6 g of dimethyl silicone oil were added. Subsequently, 600 g of deionized water and 4 g of perfluoropolyether fluorocarbon surfactant (FCD-108) were added in sequence. Then, the reactor was evacuated and purged with nitrogen repeatedly to make the oxygen content in the reactor lower than 10 ppm. The stirring of the reactor was started, and the reactor was heated to 80 °C. VDF monomer was introduced into the reactor until the pressure reached 4.0 MPa. 10 g of 2% aqueous potassium persulfate solution and 0.3 g of ethyl acetate as a chain transfer agent were pumped into the polymerization kettle using a metering pump, and the polymerization reaction was started. When the pressure in the kettle decreased, VDF monomer was replenished into the polymerization kettle to keep the pressure between 4.0 and 4.5 MPa. After the reaction proceeded for 45 min, 10.0 g of 1% aqueous potassium persulfate solution and 0.6 g of the chain transfer agent were replenished at intervals. When 300 g of vinylidene fluoride was replenished, the addition of the initiator and the chain transfer agent was stopped, and the reaction was terminated. The polymerization product was centrifuged, washed, and dried to obtain PVDF solid powder.

[0097] The properties of the polyvinylidene fluoride resins obtained in the above examples and comparative examples are shown in Table 1 below:

[0098] Table 1. Performance parameters of polymers in examples and comparative examples

[0099]

[0100] As Figures 1 to 4 shown, it can be observed from the high-resolution transmission electron microscope that using anionic perfluorosurfactant as a protective agent can in-situ reduce different silver salt precursors into silver nanoparticles with different morphologies and sizes in the polymerization system. The size of the silver nanoparticles synthesized with silver nitrate as the precursor in Example 1 was 13.2 ± 3.8 nm, and the morphology was mainly spherical nanoparticles. The size of the silver nanoparticles synthesized with silver trifluoroacetate as the precursor in Example 2 was 18.5 ± 4.1 nm, and the morphology of the nanoparticles showed triangular or polyhedral shapes and there were larger-sized nanoparticles. In Example 3, when silver acetate was used as the precursor, there were many irregularly shaped nanoparticles, such as triangular and rhombic morphologies, and obvious aggregation existed.

[0101] Moreover, it can be observed from the provided tabular data that by using the in-situ reduction silver salt method, various silver precursors and different types of anionic perfluorosurfactants as protective agents can effectively initiate the polymerization reaction and ultimately synthesize ultra-high molecular weight polymers. Among them, the molecular weight decreases in the order of Examples 1, 2, and 3. By comparing the morphological sizes of the nanoparticles, it is found that the nanoparticle size in Example 1 is smaller and the dispersibility is better. Due to the size effect of nanomaterials, as the particle size decreases, the surface unsaturated coordination environment of the nanoparticles changes, the more the surface active sites of the metal particles are exposed, the higher the initiation efficiency, and the more favorable for the synthesis of high molecular weight polymers. And we found that the nanoparticles have better activity compared to other morphological nanomaterials. This is because spherical nanoparticles usually have a larger specific surface area, increasing the contact area with the reactants. At the same time, the higher dispersibility makes the nanoparticles show a more uniform active site distribution during the catalytic process, manifested as a narrower molecular weight distribution of the polymer.

[0102] The increase in the polymer molecular weight enhances the interaction force between polymer molecules. At the same time, due to the presence of polar groups in the initiator, the interaction between the binder and the surface of the material to be bonded is greatly enhanced, thereby greatly improving the bonding strength of the PVDF binder.

[0103] Combined with the above description, the present invention proposes a method for synthesizing ultra-high molecular weight polyvinylidene fluoride by the in-situ reduction silver salt method. Different from traditional free radical polymerization, anion (cation) polymerization or coordination polymerization methods, the present invention does not use free radical polymerization initiators or anion (cation) polymerization initiators, nor does it use coordination polymerization Ziegler-Natta catalysts. The core of this method is to use an emulsifier as a protective group for nanoparticles, directly synthesize silver nanoparticles in an emulsion polymerization system, and these silver nanoparticles then interact with the initiator to jointly initiate the polymerization reaction, effectively solving the problem that noble metal nanoparticles are difficult to store for a long time in a monodispersed state, and solving the problem that monodispersed metal nanoparticles are prone to aggregation over a long time, resulting in a decrease in the catalytic initiation polymerization reaction efficiency. This synthesis strategy has universality, is applicable to a variety of silver salt precursors and perfluorosurfactants, is easy to operate and the reaction conditions are mild, showing good industrialization potential. The molecular weight of the obtained polymers all exceeds one million, showing excellent molecular weight characteristics.

[0104] The initiator is mainly organic haloacid, which introduces polar groups such as carboxyl groups into the polymer backbone while initiating the polymerization reaction, and can form a strong interaction with the surface of the material to be bonded, thereby enhancing the adhesion between PVDF and the material to be bonded.

[0105] In addition, by using an organic haloacid as an initiator, polar groups can be introduced into the polymer backbone, avoiding the need for additional addition of polar monomers and eliminating the problems caused by the excessive differences in reactivity and polarity between polar comonomers and fluorinated comonomers. The prepared copolymer not only maintains high molecular weight and high adhesion strength but also has a low crystallinity, endowing the material with higher toughness.

[0106] These properties significantly improve the service life of PVDF as a lithium battery binder, making it of important application value in the field of energy storage.

[0107] As described above, the above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes but is not limited to the content described in the above specific embodiments. Any modification that does not deviate from the functional and structural principles of the present invention will be included in the scope of the claims.

Claims

1. A method for in-situ reducing and synthesizing silver nanoparticles in a polymerization system and initiating the polymerization of fluorinated olefin monomers to synthesize ultra-high molecular weight polymers, characterized in that, Emulsion polymerization reaction is carried out in a sealed reaction kettle by replacing oxygen with nitrogen; Wherein the reaction raw materials include deionized water, emulsifier, silver salt precursor, initiator and reaction monomer; During the polymerization reaction, the silver salt precursor is reduced by adding a reducing agent to in-situ generate silver nanoparticles; Under the set reaction temperature and reaction pressure conditions, the polymerization reaction is initiated, and the reaction monomer is added additionally to maintain the reaction pressure, and finally an ultra-high molecular weight polyvinylidene fluoride-based polymer is obtained.

2. The method for polymerizing and synthesizing ultra-high molecular weight polymers according to claim 1, wherein The set reaction temperature of the polymerization reaction is 40 - 80 °C, and the reaction pressure is 4.5 - 8 MPa.

3. The method for polymerizing and synthesizing ultra-high molecular weight polymers according to claim 1, wherein The dosage of the silver salt precursor is 0.003 - 0.02% of the mass of the reaction monomer, and the dosage of the initiator is 0.05 - 0.2% of the mass of the reaction monomer.

4. The method for polymerizing and synthesizing a ultra-high molecular weight polymer according to claim 1, wherein The dosage of the deionized water is 100 - 250% of the mass of the reaction monomer; the dosage of the emulsifier is 0.5 - 2.5% of the mass of the reaction monomer.

5. The method for polymerizing and synthesizing ultra-high molecular weight polymers according to claim 1, wherein The reducing agent is one of sodium borohydride, ascorbic acid, lithium aluminum hydride, sodium citrate, potassium tartrate, sodium hypophosphite.

6. The method for polymerizing and synthesizing a super high molecular weight polymer according to claim 1, characterized in that, The silver salt precursor is one of silver trifluoroacetate, silver nitrate, silver acetate, silver oxalate.

7. The method for polymerizing and synthesizing ultra-high molecular weight polymer according to claim 1, characterized in that, The initiator is an organic halide, including organic bromides and organic chlorides. The organic bromides include 2-bromo-iso-butyric acid, tribromoacetic acid, 2-bromopropionic acid, 2-bromobutyric acid, 3-bromopropionic acid, 2-bromobenzoic acid, α-bromobenzoic acid, 2-bromophenylacetic acid, α-bromophenylacetic acid, 2-bromo-4-methylbenzoic acid, 2-bromocyclohexanoic acid, 2,2-dibromopropionic acid, 3-bromo-2,2-dimethylpropionic acid, 2-bromo-1,4-dimethylbenzoic acid, 3-bromo-2-hydroxypropionic acid, difluorobromoacetic acid; The organic chlorides include chloroacetic acid, dichloroacetic acid, trichloroacetic acid, 2-chloropropionic acid, chlorobutyric acid, α-chlorobenzoic acid, α-chlorophenylacetic acid, α-chlorophenylpyruvic acid.

8. The method for polymerizing and synthesizing ultra-high molecular weight polymer according to claim 1, wherein, The reaction monomer is vinylidene fluoride or a mixture of one or more of vinylidene fluoride and tetrafluoroethylene, fluoroethylene, trifluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, ethylene, propylene, 2,3,3,3-tetrafluoro-1-propene, 1,3,3,3-tetrafluoro-1-propene, cis-1-chloro-3,3,3-trifluoropropene, trans-1-chloro-3,3,3-trifluoropropene, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoropropyl vinyl ether, acrylic acid, methacrylic acid, monomethyl maleate, monomethyl itaconate, wherein the mass fraction of vinylidene fluoride in the mixture is 60% - 99.99%.

9. A ultra-high molecular weight polyvinylidene fluoride-based polymer, characterized in that, Obtained by using the method according to any one of claims 1 to 8, the weight average molecular weight of the ultra-high molecular weight polyvinylidene fluoride-based polymer is 2 million - 5 million, the intrinsic viscosity is 4.0 - 6.5 dL·g-1, and the molecular weight distribution index Mw / Mn is 1.5 - 3.0; Polymers with different molecular weights are obtained by adjusting the dosages of the silver salt precursor and the initiator.

10. A binder for a lithium-ion battery electrode, characterized in that, The binder includes the ultra-high molecular weight polyvinylidene fluoride-based polymer according to claim 9.

Images