A high-beta-crystal-phase polyvinylidene fluoride fiber piezoelectric film and a preparation method thereof

Abstract

The application discloses a high-beta-crystal-phase polyvinylidene fluoride fiber piezoelectric film and a preparation method thereof, wherein the preparation method comprises the following steps: step one: polyvinylidene fluoride resin with a weight average molecular weight of 500,000-800,000, a long branched chain content of 1.2‰-5‰, and a 7% mass fraction NMP solution rotary viscosity of 5000-8000 mpa.s at 25 DEG C is prepared by emulsion polymerization of polyvinylidene fluoride monomer and difluoro-monochloroethane; and step two: the polyvinylidene fluoride resin obtained in step one is made into an electrostatic spinning precursor solution, and the electrostatic spinning precursor solution is spun and film-formed by an electrostatic spinning method. The polyvinylidene fluoride nanofiber film with finer and higher beta-crystal-phase content can be prepared by adjusting the content of the long branched chain and the viscosity of the electrostatic spinning solution.

Description

Technical Field

[0001] This invention belongs to the field of piezoelectric material technology, specifically relating to PVDF fiber piezoelectric film technology. Background Technology

[0002] Piezoelectric materials enable the conversion between mechanical energy and electrical energy, which is beneficial for energy conservation. When a mechanical force is applied to a piezoelectric polymer, an electric charge is generated on the polymer surface. This charge is conducted through electrodes and can be used by external electrical devices, thus realizing the conversion of mechanical energy into electrical energy. Piezoelectric ceramics, which exhibit a significant piezoelectric effect, are widely used in sensors, transducers, and other fields.

[0003] Polyvinylidene fluoride (PVDF) possesses outstanding piezoelectric properties—good flexibility, impact resistance, and abrasion resistance. These properties of PVDF will greatly expand the application fields of piezoelectric materials.

[0004] Currently, polyvinylidene fluoride (PVDF) has been found to possess five crystalline forms: α, β, γ, δ, and ε. The α-crystalline form exhibits the highest stability, while the β-crystalline form is a crucial factor in the piezoelectric properties of PVDF. Existing piezoelectric products made from PVDF primarily utilize piezoelectric films prepared by casting, achieving high β-crystalline content and β-crystalline orientation through film stretching and polarization. However, these preparation methods are demanding, complex, and require sophisticated equipment.

[0005] Electrospinning is currently considered a relatively simple method for preparing ultrafine fibers or nanofibers. By adjusting the process parameters of electrospinning (such as solution concentration, solvent ratio, solution conductivity, voltage, flow rate, and receiving distance), the morphology or aggregate structure of the fibers can be easily changed or controlled. During electrospinning, the polymer jet is stretched and thinned by the electric field force in a high-voltage electrostatic field, while the solvent evaporates and solidifies, ultimately forming fibers deposited on the receiving plate. During fiber formation, the stretching force of the electric field helps the α-crystalline form of PVDF fibers transform into the β-crystalline form, providing better conditions for preparing fibers with a high β-crystalline content. Numerous studies have demonstrated that electrospinning is a simple method for preparing PVDF submicron or nanofibers with a high β-crystalline content. However, the β-crystalline content in PVDF nanofibers obtained by electrospinning is currently generally less than 85%, resulting in low efficiency of the piezoelectric material. While methods such as nanodoping and blending have been used to increase the β-crystalline content, this still prevents the full realization of the excellent properties of PVDF. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film and its preparation method, solving the problems of low piezoelectric efficiency and poor mechanical properties of nanofibers obtained during the electrospinning process of polyvinylidene fluoride.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] First, a method for preparing a high β-crystal phase polyvinylidene fluoride fiber piezoelectric film is provided, comprising the following steps:

[0009] Step 1: Polyvinylidene fluoride monomer and difluorochloroethane are used for emulsion polymerization to prepare polyvinylidene fluoride resin with a weight average molecular weight of 500,000 to 800,000, a long branched chain content of 1.2‰ to 5‰, and a rotational viscosity of 5000 to 8000 mpa·s in 7% NMP solution at 25°C.

[0010] Step 2: Prepare an electrospinning precursor solution from the polyvinylidene fluoride resin obtained in Step 1, and spin the electrospinning precursor solution into a film using an electrospinning method. The concentration of the electrospinning precursor solution is ≤10%.

[0011] Preferably, the concentration of the electrospinning precursor solution is ≤8%, the purity of the vinylidene fluoride monomer is 99.5% to 99.9%, and the content of difluorochloroethane is 500 ppm to 1000 ppm.

[0012] Preferably, when preparing polyvinylidene fluoride resin by emulsion polymerization, a redox initiator is used to initiate the polymerization, and the polymerization is carried out in the presence of a dispersant, a chain transfer agent, and a co-dispersant. The polymerization temperature is 25-70℃, and the reaction pressure is 2.5-5MPa.

[0013] Preferably, the oxidant is one of ammonium persulfate, potassium persulfate, sodium persulfate, hydrogen peroxide, succinic acid peroxide, methyl ethyl ketone peroxide, cyclohexane peroxide, tert-butyl hydroperoxide, and potassium permanganate; and / or, the reducing agent is one of sodium metabisulfite, sodium hypochlorite, sodium sulfite, ferrous sulfate, ascorbic acid, sodium citrate, sodium formaldehyde sulfoxylate, oxalic acid, and N,N-dimethylaniline.

[0014] Preferably, the molar ratio of the oxidant to the reducing agent is 1:0.8 to 1:3.5.

[0015] Preferably, the dispersant is one or more of polyethyleneimine, polyvinylpyrrolidone, polyethylene glycol-maleimide, and stearic acid-polyethylene glycol-maleic acid imide; and / or, the amount of the dispersant relative to the mass fraction of the vinylidene fluoride monomer participating in the reaction is 0.5‰ to 5‰.

[0016] Preferably, the electrospinning method for spinning and film formation involves the following steps:

[0017] S1: Dissolve polyvinylidene fluoride in a mixed solution of N-methyl-2-pyrrolidone and acetone, heat in a water bath at 60-80°C and stir magnetically for 4-8 hours until homogeneous to obtain a precursor solution for electrospinning.

[0018] S2: Place the electrospinning precursor solution in an electrospinning device for electrospinning;

[0019] S3: The vinylidene fluoride nanofiber membrane is subjected to heat treatment to further remove the solvent. The heat treatment temperature is 80-130℃.

[0020] Preferably, the polyvinylidene fluoride is dissolved in powder form; and / or, the mass concentration of polyvinylidene fluoride in the precursor solution is 4% to 20%.

[0021] Preferably, in step S1, the mass ratio of N-methyl-2-pyrrolidone to acetone in the mixed solution is 85:10 to 75:10; and / or, in step S1, the mass ratio of polyvinylidene fluoride, N-methyl-2-pyrrolidone, and acetone is 5 to 15:75 to 85:10; and / or, the electrospinning process parameters are: inner diameter of the metal needle 0.4 mm to 0.8 mm; spinning voltage 12 kV to 20 kV; receiving distance 15 cm to 35 cm; spinning solution propulsion speed 0.5 mL / h to 4 mL / h; and the polyvinylidene fluoride membrane is collected with aluminum foil.

[0022] In addition, a high β-crystal phase polyvinylidene fluoride piezoelectric film is also provided, which is prepared by the above preparation method and has a 50-100 nm nanoscale fiber structure and a β crystal form greater than 85%.

[0023] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0024] By employing emulsion polymerization, a polyvinylidene fluoride resin with a certain long-chain content was obtained. By adjusting the content of long-chain components and the viscosity of the electrospinning solution, finer polyvinylidene fluoride nanofiber membranes with higher β-phase content can be prepared.

[0025] The polyvinylidene fluoride (PVDF) contains 1.2‰ to 5‰ of long-branched chains. When the long-branched chain content of PVDF is low, there are fewer entanglement points in the low-concentration electrospinning solution, which is not conducive to the preparation of nanofiber membranes with complete morphology at low concentrations. When the long-branched chain content is excessive, the interaction between PVDF molecular chains is more pronounced. Therefore, controlling the long-branched chain content is more beneficial for creating more entanglement points at low concentrations, allowing for sufficient stretching in the electric field during electrospinning. This leads to the better preparation of finer nanofiber membranes with higher β-phase content.

[0026] The applicant's research revealed that if the side chains are sufficiently long, i.e., the molecular weight of the side chains reaches or exceeds three times the critical molecular weight (Mb > 3Mc), the side chains themselves become entangled. In this case, the PVDF molecular chains form a "self-crosslinking" system due to mutual entanglement, characterized by high or ultra-high solution viscosity. This system can achieve high viscosity at low concentrations, enabling the production of finer nanofibers and a higher proportion of β-crystals in electrospinning. Typically, the concentration of the electrospinning solution is above 8%, but to reduce the diameter of the nanofibers, it is preferable to lower the concentration to below 8%. Therefore, for the viscosity of polyvinylidene fluoride resin, the design is a rotational viscosity of 5000–8000 mPa·s for a 7% NMP solution at 25°C. This allows for the production of high viscosity even at such low concentrations, facilitating spinning and obtaining finer nanofibers.

[0027] Moreover, this nanofiber membrane has good application prospects in the field of piezoelectric materials.

[0028] The specific technical solution of the present invention and its beneficial effects will be described in detail in the following specific embodiments. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Those skilled in the art will understand that, without conflict, the features in the following embodiments and implementations can be combined with each other.

[0031] The applicant focuses on the polymer system. The main factors affecting the viscosity of polymer solutions are concentration, solvent, and polymer branching. Many existing patents have studied the first two aspects. This application addresses the influence of long polymer branching on polymer solution viscosity. If the branching is quite long, i.e., the molecular weight of the branching reaches or exceeds three times the critical molecular weight (Mb > 3Mc), the branching itself becomes entangled. In this case, the PVDF molecular chains form a "self-crosslinking" system due to mutual entanglement. A key characteristic of this system is that the polymer has high or ultra-high solution viscosity. This system allows the spinning solution to maintain a high viscosity even at lower PVDF resin concentrations than in existing technologies, enabling the acquisition of finer nanofibers and a higher proportion of β-crystals in electrospinning.

[0032] To overcome the problems of low piezoelectric efficiency and poor mechanical properties in nanofibers obtained during the electrospinning process of polyvinylidene fluoride (PVDF), based on the above research on the influence of long polymer branches on the viscosity of polymer solutions, this invention provides a method for preparing a high β-crystalline phase PVDF fiber piezoelectric film, comprising the following steps:

[0033] Step 1: Polyvinylidene fluoride monomer and difluorochloroethane are used for emulsion polymerization to prepare polyvinylidene fluoride resin with a weight average molecular weight of 500,000 to 800,000, a long branched chain content of 1.2‰ to 5‰, and a rotational viscosity of 5000 to 8000 mpa·s in 7% NMP solution at 25°C.

[0034] Step 2: Prepare a polyvinylidene fluoride solution from the polyvinylidene fluoride resin obtained in Step 1, and then spin the polyvinylidene fluoride solution into a film using electrospinning.

[0035] Regarding the above technical solution, the present invention obtains polyvinylidene fluoride resin with long branches through emulsion polymerization. Using this resin as raw material, a low-concentration electrospinning solution is prepared, and polyvinylidene fluoride nanofibers with a high β-phase ratio and small fiber size are obtained through electrospinning.

[0036] The content of long branched chains is 1.2‰ to 5‰. The long branched chain content mentioned in this invention refers to the percentage of carbon atoms containing long branched chains on the polymer backbone. When N out of 1000 carbon atoms have long branched chains, the long branched chain content is N‰. In vinylidene fluoride polymers, long branched chains refer to branches containing side chains of -CH2CF2-CH(CH2CF2-)CF2-; the specific degree of branching can be calculated using NMR fluorine spectra.

[0037] When the content of long-branched polyvinylidene fluoride (PVDF) is low, there are fewer entanglement points in the low-concentration electrospinning solution, which is not conducive to the preparation of nanofiber membranes with complete morphology at low concentrations. When the content of long-branched chains is high, the interaction between PVDF molecular chains is more obvious. Therefore, controlling the content of long-branched chains is more beneficial for creating more entanglement points at low concentrations, allowing for sufficient stretching in the electric field during electrospinning. This leads to the better preparation of finer nanofiber membranes with higher β-phase content. However, the content of long-branched chains should not be too high, as it can easily cause the electrospinning solution to gel, making it unprocessable. Preferably, the content of long-branched polyvinylidene fluoride is 2‰ to 5‰, for example, 3‰ to 4‰.

[0038] Based on the principle of electrospinning, the lower the solution concentration, the finer the nanofibers and the higher the proportion of β-crystals. However, the solution concentration cannot be infinitely low; there is a minimum concentration. Too low a concentration leads to non-uniform nanofibers and reduced utilization value. Typically, the concentration of the electrospinning solution is between 8% and 15%. This invention controls the spinning solution concentration to ≤10%, preferably between 3% and 8%, such as 3%, 5%, and 8%. For preparing nanofibers with finer diameters, too low a concentration results in an unstable spinning process prone to breakage, while too high a concentration leads to nanofibers with larger diameters that fail to meet the expected performance targets.

[0039] The rotational viscosity of the electrospinning solution is mainly affected by the degree of branching and the average molecular weight. When the degree of branching is high or the molecular weight is large, the molecular chains are easily entangled, resulting in a high viscosity of the electrospinning solution and gelation after prolonged standing. When the degree of branching is low or the molecular weight is small, there is less entanglement between the polyvinylidene fluoride (PVDF) molecular chains, resulting in a lower rotational viscosity of the spinning solution. This makes it difficult to stabilize the nanofiber size during the spinning process. Therefore, given the aforementioned long branch content, controlling the molecular weight within the range of 500,000 to 800,000 can yield a more stable nanofiber membrane. Preferably, the weight-average molecular weight of the PVDF is 600,000 to 750,000, for example, 650,000, 700,000, or 750,000.

[0040] The present invention prepares a polyvinylidene fluoride resin with a rotational viscosity of 5000-8000 mpa·s at 25°C using a 7% mass fraction NMP solution. This resin is then used for electrospinning, which allows it to maintain a high solution viscosity even under the aforementioned low concentration conditions. This enables the production of finer nanofibers with a higher proportion of β-crystals during electrospinning.

[0041] The present invention uses an emulsion polymerization method to prepare the above-mentioned polyvinylidene fluoride resin. The polymerization is initiated by a redox initiator and carried out in the presence of a dispersant, a chain transfer agent, and a co-dispersant. The polymerization temperature is 25-70℃ and the reaction pressure is 2.5-5MPa.

[0042] Generally speaking, controlling the molecular weight of polyvinylidene fluoride resin can be achieved relatively easily through conventional methods. However, increasing the content of long-chain branches is more complex. Existing technologies typically achieve this by increasing the reaction temperature, using graft polymerization, or using cross-linking methods, but precise control is difficult to achieve, and the process is cumbersome.

[0043] Polyvinylidene fluoride resin with a higher β-phase content can be obtained by using redox-initiated emulsion polymerization. Therefore, this application has carried out further research based on this method.

[0044] The applicant discovered through experiments that polymerization using low-purity vinylidene fluoride monomer produces more long-branched chains than polymerization using high-purity monomer, particularly with increased difluorochloroethane content. The applicant hypothesized that multiple chain transfers and re-initiation processes involving difluorochloroethane during the reaction might lead to the formation of multi-armed structures. However, increasing the difluorochloroethane content to improve the long-branched PVDF content resulted in excessively low reaction efficiency and a low molecular weight product. Subsequently, the applicant was surprised to find that certain polyamine surfactants promote the formation of long-branched chains, enabling the addition of these substances to increase the long-branched PVDF content in the presence of a certain amount of difluorochloroethane.

[0045] Based on the above findings, preferably, the low-purity vinylidene fluoride monomer has a purity of 99.5% to 99.9%, for example, ~99.8% or 99.9%. The dichlorofluoroethane content is 500 ppm to 1000 ppm, for example, 600 ppm or 800 ppm. The polyamine surfactant is one or more of polyethyleneimine, polyvinylpyrrolidone, polyethylene glycol-maleimide, and stearic acid-polyethylene glycol-maleic acid imide. The applicant uses one or more of these surfactants in combination as a co-dispersant in the preparation process of polyvinylidene fluoride. The amount of co-dispersant relative to the mass fraction of the vinylidene fluoride monomer is 0.5‰ to 5‰. The amount is adjusted according to the required long-chain branch content.

[0046] Preferably, the redox initiator is one of the following: ammonium persulfate, potassium persulfate, sodium persulfate, hydrogen peroxide, succinic acid peroxide, methyl ethyl ketone peroxide, cyclohexane peroxide, tert-butyl hydroperoxide, and potassium permanganate. The reducing agent is one of the following: sodium metabisulfite, sodium hypochlorite, sodium sulfite, ferrous sulfate, ascorbic acid, sodium citrate, sodium formaldehyde sulfoxylate, oxalic acid, and N,N-dimethylaniline. The molar ratio of oxidant to reducing agent is 1:0.8 to 1:3.5, for example, 1:0.8, 1:2, or 1:3. Different combinations of oxidant and reducing agent are selected according to the reaction temperature.

[0047] Preferably, the redox initiator is composed of potassium persulfate and ascorbic acid in a molar ratio of 1:1 to 1:3, for example, 1:1, 1:2, or 1:3. Preferably, the dispersant is a conventional dispersant used in polyvinylidene fluoride emulsion polymerization; common fluorinated surfactants or non-fluorinated surfactants can be used.

[0048] Preferably, the chain transfer agent is a chain transfer agent commonly used in polyvinylidene fluoride emulsion polymerization, such as alkanes, alcohols, ketones, and other substances with chain transfer capabilities.

[0049] The preparation of polyvinylidene fluoride membranes with high β-phase content via electrospinning involves the following steps:

[0050] S1: Dissolve polyvinylidene fluoride with long branches in a mixed solution of N-methyl-2-pyrrolidone and acetone, heat in a water bath at 60-80°C and stir magnetically for 4-8 hours. After stirring evenly, a precursor solution for electrospinning is obtained.

[0051] S2: Place the electrospinning precursor solution in an electrospinning apparatus. The electrospinning process parameters are: inner diameter of the metal needle 0.4mm-0.8mm; spinning voltage 12kV-20kV; receiving distance 15cm-35cm; and spinning solution propulsion speed 0.5mL / h-4mL / h. Collect the polyvinylidene fluoride membrane with aluminum foil.

[0052] S3: The vinylidene fluoride nanofiber membrane is subjected to heat treatment to further remove the solvent. The heat treatment temperature is 80-130℃.

[0053] Preferably, in step S1, PVDF is in powder form, which makes it easier to dissolve in the organic solvent used. PVDF is first dissolved in a mixed solution of N-methyl-2-pyrrolidone and acetone. The heating temperature in step S1 is preferably 60–70°C.

[0054] Preferably, the heating time in step S1 is 4 to 6 hours.

[0055] The mass concentration of PVDF in the solution is 4% to 20%, preferably 5% to 15%. Because polyvinylidene fluoride has long branches, it has high viscosity even at low concentrations. Too high a concentration can easily cause needle clogging and result in discontinuous fibers; the concentration also cannot be too low, as this can easily lead to excessively fine fibers that break.

[0056] In step S1, the preferred ratio of the mixed solution of N-methyl-2-pyrrolidone and acetone is (85:10 to 75:10). The addition of acetone makes the solvent more volatile during electrospinning and also induces the formation of the β-phase. However, the acetone content should not be too high, as acetone is a poor solvent and adding too much will have a certain impact on the solubility of PVDF.

[0057] Preferably, in step S2, the electrospinning voltage is 14-18 kV. If the spinning voltage is too high, unstable flow may occur during solution jetting; if the spinning voltage is too low, the fibers received by the receiving device may contain nodes or particles. The feed rate affects the fiber speed and the spinning process. If the feed rate is too low, the continuous supply of spinning solution will be insufficient, affecting the continuity of the spun fibers. If the feed rate is too high, the instantaneous volume of spinning solution will be too large, easily clogging the needle and preventing spinning from proceeding. Preferably, the spinning feed rate is 0.5 ml / h to 1.0 ml / h.

[0058] Preferably, in step S3, the heat treatment temperature is 80-130°C. More preferably, the heat treatment temperature is 100-125°C.

[0059] The relative content of the β phase, F(β), refers to the proportion of the β phase content to the total crystalline composition of PVDF, which can be calculated from the peak area of ​​the absorption peak corresponding to the crystal form in the FTIR data.

[0060] Compared with the prior art, the advantages of the present invention are:

[0061] A polyvinylidene fluoride (PVDF) resin with a certain long-chain branch content was obtained through specific emulsion polymerization. By adjusting the long-chain branch content and the viscosity of the electrospinning solution, finer PVDF nanofiber membranes with higher β-phase content were prepared. These nanofiber membranes show promising applications in the field of piezoelectric materials.

[0062] The technical solution of the present invention will be described below with reference to specific embodiments.

[0063] In this invention, the weight-average molecular weight was obtained by gel permeation chromatography (GPC), and the mobile phase was N,N-dimethylformamide (DMF).

[0064] The degree of branching was measured using nuclear magnetic resonance (NMR) and the 19F spectrum was analyzed.

[0065] The test method for long-branched chains is as follows:

[0066] Long branched chains refer to branches containing the side chain -CH2CF2-CH(CH2CF2-)CF2-, located at -99.5 ppm in the 19F spectrum; the long branched chain content C = I(-99.5 ppm) / I(-92 ppm to 118 ppm), where I(-99.5 ppm) refers to the peak integral intensity at -99.5 ppm in the 19F spectrum, and I(-92 ppm to 118 ppm) refers to the sum of the integral intensities of all peaks from -92 ppm to 118 ppm.

[0067] The β crystal form was measured using infrared spectroscopy.

[0068] Methods for measuring infrared spectroscopy:

[0069] PVDF contains three common crystal forms: α-crystal, β-crystal, and γ-crystal. In the infrared spectrum, the α-crystal form exhibits a peak at a wavenumber of 408 cm⁻¹. -1 532cm -1 612cm -1 766cm -1 795cm -1 855cm -1 976cm -1 1182cm -1 1400cm -1 The peak position of the β-crystal form is 445 cm⁻¹. -1 470cm -1 511cm -1 600cm -1 840cm -1 1279cm -1 The γ-crystal form has a peak position of 431 cm⁻¹. -1 512cm -1 776cm -1 812cm -1 833cm -1 1234cm -1 The relative content of β crystal form is calculated based on the intensity of each absorption peak and the area integral.

[0070] The diameter of the polyvinylidene fluoride nanofiber membrane was measured by scanning electron microscopy.

[0071] Scanning electron microscopy measurement methods:

[0072] The polyvinylidene fluoride nanofiber membrane obtained by electrospinning was sliced, glued to the test platform of a scanning electron microscope with conductive adhesive, and then sputtered with gold. After sputtering, electron microscopy was performed to measure the diameter and diameter distribution of the nanofibers.

[0073] Example 1

[0074] In a 15L autoclave, 10kg of deionized water, 30g of perfluoropolyether ammonium carboxylate dispersant, 3g of polyethyleneimine co-dispersant, and 3g of ethyl acetate chain transfer agent were added. The autoclave was then purged with nitrogen until the oxygen content was less than 30ppm. The autoclave was heated to 40℃, and vinylidene fluoride was added to the autoclave until the pressure reached 3.5MPa. 12g of potassium persulfate oxidant and 8g of ascorbic acid reducing agent were added to initiate the reaction. Vinylidene fluoride was continuously added during the reaction to maintain a constant pressure in the autoclave. The difluorochloroethane content in the vinylidene fluoride monomer was 500ppm. When the reaction volume reached 3kg, the reaction was stopped, and the autoclave was depressurized to obtain a polyvinylidene fluoride emulsion. The emulsion was filtered, washed, and dried to obtain polyvinylidene fluoride.

[0075] The test results are as follows: solution viscosity 6000 mPa·s (25℃, 7wt% PVDF / NMP solution), weight average molecular weight 700,000, and long branched content 1.3%.

[0076] 5g of the prepared PVDF powder was dissolved in a mixed solvent of 85g NMP and 10g acetone, and magnetically stirred at 70℃ for 4 hours. After cooling to room temperature, a 5wt% PVDF solution was prepared. Electrospinning was performed using this solution. The syringe had a capacity of 10ml, the needle was 27G (inner diameter 0.21mm, outer diameter 0.41mm), the spinning temperature was 25℃, the humidity was 25%, the spinning voltage was 16kV, the spinning capacity was 4ml, the feed rate was 0.8ml / h, the receiving distance was 15cm, the roller rotation speed was 2000rpm, and the spinning time was 5h. The prepared nanofibers were then heat-treated at 110℃ for 6h.

[0077] The test results are as follows: the diameter of the polyvinylidene fluoride nanofibers is 110 nm, and the relative content of the β crystal form is 90%.

[0078] Example 2

[0079] Compared to Example 1, the experimental pressure was changed to 5 MPa, and the content of difluorochloroethane in the monomer vinylidene fluoride was changed to 800 ppm. Other conditions remained unchanged.

[0080] The test results are as follows: solution viscosity 6800 mPa·s (25℃, 7wt% PVDF / NMP solution), weight average molecular weight 750,000, and long branched chain content 2.28%.

[0081] The test results are as follows: the diameter of the polyvinylidene fluoride nanofibers is 100 nm, and the relative content of the β crystal form is 90%.

[0082] Example 3

[0083] Compared to Example 1, the experimental temperature was changed to 60°C, and the content of difluorochloroethane in the monomer vinylidene fluoride was 800 ppm. Other conditions remained unchanged.

[0084] The test results are as follows: solution viscosity 6500 mPa·s (25℃, 7wt% PVDF / NMP solution), weight average molecular weight 680,000, and long branched chain content 3.35%.

[0085] The test results are as follows: the diameter of the polyvinylidene fluoride nanofibers is 85 nm, and the relative content of the β crystal form is 93%.

[0086] Example 4

[0087] Compared to Example 1, the amount of polyethyleneimine was increased to 9g, while other conditions remained unchanged.

[0088] The test results are as follows: solution viscosity 6900 mPa·s (25℃, 7wt% PVDF / NMP solution), weight average molecular weight 640,000, and long branched chain content 2.75%.

[0089] The test results are as follows: the diameter of the polyvinylidene fluoride nanofibers is 90 nm, and the relative content of the β crystal form is 90%.

[0090] Comparative Example 1

[0091] Compared to Example 1, no reducing agent was added; the oxidizing agent was initially added at 40%, with the remaining 60% continuously added during the reaction. The reaction temperature was 90°C.

[0092] The test results are as follows: solution viscosity 4000 mPa·s (25℃, 7wt% PVDF / NMP solution), weight average molecular weight 470,000, and long branched chain content 2.12%.

[0093] In Example 1, the spinning solution concentration was insufficient for normal spinning, so the PVDF solution concentration was increased to 8%.

[0094] The test results are as follows: the diameter of the polyvinylidene fluoride nanofibers is 190 nm, and the relative content of the β crystal form is 74%.

[0095] Comparative Example 2

[0096] Compared to Example 1, the monomer vinylidene fluoride concentration was 99.95%, and the difluorochloroethane content was 10 ppm.

[0097] The test results are as follows: solution viscosity 3000 mPa·s (25℃, 7wt% PVDF / NMP solution), weight average molecular weight 810,000, and long branched chain content 0.3%.

[0098] In Example 1, the spinning solution concentration failed to achieve normal spinning, so the PVDF solution concentration was increased to 12%.

[0099] The test results are as follows: the diameter of the polyvinylidene fluoride nanofibers is 410 nm, and the relative content of the β crystal form is 88%.

[0100] Comparative Example 3

[0101] Compared to Example 1, polyethyleneimine is not added.

[0102] The test results are as follows: solution viscosity 3300 mPa·s (25℃, 7wt% PVDF / NMP solution), weight average molecular weight 710,000, and long branched content 0.9%.

[0103] In Example 1, the spinning solution concentration failed to achieve normal spinning, so the PVDF solution concentration was increased to 12%.

[0104] The test results are as follows: the diameter of the polyvinylidene fluoride nanofibers is 370 nm, and the relative content of the β crystal form is 87%.

[0105] As can be seen from Experimental Example 1 and Comparative Examples 2 and 3, difluorochloroethane promotes the formation of long-chain polyvinylidene fluoride (PVDF) resin, while polyethyleneimine has a significant promoting effect. PVDF with long-chain branches can be used to prepare finer PVDF nanofiber membranes via electrospinning at lower concentrations. This is mainly because the presence of long branches allows for better entanglement of PVDF even at low concentrations. In electrospinning, the low-concentration spinning solution, under the influence of the electric field, results in more thorough stretching, leading to finer nanofibers.

[0106] As can be seen from Examples 1, 2, 3 and Comparative Example 1, using a redox system to initiate the reaction at a lower reaction temperature can effectively increase the content of the β-phase.

[0107] The above description is merely a specific embodiment of the present invention, but the scope of protection 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 modifications that do not depart from the functional and structural principles of the present invention will be included within the scope of the claims.

Claims

1. A method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film, characterized in that, Includes the following steps: Step 1: Polyvinylidene fluoride (PVDF) resin with a weight-average molecular weight of 500,000 to 800,000 and a long-branched chain content of 1.2‰ to 5‰ is prepared by emulsion polymerization of PVDF monomer and difluorochloroethane. The rotational viscosity of a 7% (w / w) PVDF / NMP solution is 5000 to 8000 mPa·s under test conditions at 25°C. The purity of the PVDF monomer is 99.5% to 99.9%, and the content of difluorochloroethane is 500 ppm to 1000 ppm. When preparing PVDF resin by emulsion polymerization, a redox initiator is used, and the polymerization is carried out in the presence of a dispersant, chain transfer agent, and co-dispersant. The polymerization temperature is 25-70°C, and the reaction pressure is 2.5-5 MPa. Step 2: Prepare an electrospinning precursor solution from the polyvinylidene fluoride resin obtained in Step 1, and spin the electrospinning precursor solution into a film using electrospinning. The concentration of the electrospinning precursor solution is ≤10%.

2. The method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film according to claim 1, characterized in that, The concentration of the electrospinning precursor solution is ≤8%.

3. The method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film according to claim 1, characterized in that, The oxidizing agent is one of ammonium persulfate, potassium persulfate, sodium persulfate, hydrogen peroxide, succinic acid peroxide, methyl ethyl ketone peroxide, cyclohexane peroxide, tert-butyl hydroperoxide, and potassium permanganate; and / or, the reducing agent is one of sodium metabisulfite, sodium hypochlorite, sodium sulfite, ferrous sulfate, ascorbic acid, sodium citrate, sodium formaldehyde sulfoxylate, oxalic acid, and N,N-dimethylaniline.

4. The method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film according to claim 1 or 3, characterized in that, The molar ratio of oxidant to reducing agent is 1:0.8 to 1:3.

5.

5. The method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film according to claim 1, characterized in that, The dispersant is one or more of polyethyleneimine, polyvinylpyrrolidone, polyethylene glycol-maleimide, and stearic acid-polyethylene glycol-maleic acid imide; and / or, the amount of the dispersant relative to the mass fraction of the vinylidene fluoride monomer participating in the reaction is 0.5‰ to 5‰.

6. The method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film according to claim 1, characterized in that, The steps for electrospinning to produce a film are as follows: S1: Dissolve polyvinylidene fluoride in a mixed solution of N-methyl-2-pyrrolidone and acetone, heat in a water bath at 60-80°C and stir magnetically for 4-8 hours until homogeneous to obtain a precursor solution for electrospinning. S2: Place the electrospinning precursor solution in an electrospinning device for electrospinning; S3: The vinylidene fluoride nanofiber membrane is subjected to heat treatment to further remove the solvent. The heat treatment temperature is 80-130℃.

7. The method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film according to claim 6, characterized in that, Polyvinylidene fluoride is dissolved in powder form.

8. The method for preparing a high β-crystalline phase polyvinylidene fluoride fiber piezoelectric film according to claim 6, characterized in that, In step S1, the mass ratio of N-methyl-2-pyrrolidone to acetone in the mixed solution is 85:10 to 75:10; and / or, in step S1, the mass ratio of polyvinylidene fluoride, N-methyl-2-pyrrolidone, and acetone is 5 to 15:75 to 85:10; and / or, the electrospinning process parameters are: inner diameter of metal needle 0.4 mm to 0.8 mm, spinning voltage 12 kV to 20 kV, receiving distance 15 cm to 35 cm, spinning solution propulsion speed 0.5 mL / h to 4 mL / h, and the polyvinylidene fluoride piezoelectric film is collected with aluminum foil.

9. A high β-crystalline phase polyvinylidene fluoride piezoelectric film, characterized in that, The fiber is prepared by any one of the preparation methods described in claims 1 to 8, and has a 50-100 nm nanoscale fiber structure and a β crystal form content greater than 85%.