CN111205496A - Preparation method of polyvinylidene fluoride gamma-type crystal - Google Patents
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- C08J2327/12—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen; Derivatives of such polymers not modified by chemical after-treatment containing fluorine atoms
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Abstract
The invention discloses a preparation method of a polyvinylidene fluoride gamma-type crystal, which comprises the following steps: shearing the polyvinylidene fluoride film by using fibers at a shearing temperature of 180-260 ℃ to obtain a sheared polyvinylidene fluoride film; and crystallizing the sheared polyvinylidene fluoride film at a crystallization temperature of 150-165 ℃ to obtain a polyvinylidene fluoride gamma-type crystal. The method can obtain the polyvinylidene fluoride gamma-type crystal.
Description
Technical Field
The invention relates to a preparation method of a polyvinylidene fluoride gamma-type crystal, in particular to a preparation method of a high-purity polyvinylidene fluoride gamma-type transverse crystal under the action of shear stress.
Background
Polyvinylidene fluoride (PVDF) is a semi-crystalline polymer made of-CH2-CF2-a repeating unit composition. Due to-CH2-CF2Having a plurality of spatial arrangements, so that PVDF has different crystallographic structures, includingPVDF with different crystal structures, including α, β, gamma and delta, exhibit different properties.
The PVDF has pyroelectric property, piezoelectric property and ferroelectricity, PVDF in different crystal forms shows great difference in electrical property, the piezoelectric property and the pyroelectric property are derived from the orientation arrangement of dipoles in a PVDF crystal region, the ferroelectricity shows that the material can be subjected to spontaneous polarization and polarization overturning, a α crystal form has a symmetrical molecular structure, a dipole moment of 0 and no spontaneous polarization capability, so that the piezoelectric property and the pyroelectric property are very poor, and the ferroelectricity is not available, a β crystal form is the most widely used crystal form for electrical property research in PVDF materials due to high residual polarization value and polarization overturning capability, and is common in research in the directions of sensors, memories and the like, a gamma crystal also has spontaneous polarization capability, and the strength is about half of an β crystal form, the electrical property of the PVDF is closely related to a film processing mode, crystallization temperature and the like, and the more the β crystal form and the gamma crystal form are more contained, so the electrical property of the material is better.
The α crystal form is the most common crystal form of PVDF, α crystals can be directly obtained by a general solution pouring method and a spin coating film forming method, gamma spherulites are mainly obtained by high-temperature melt crystallization, other substances can be added into PVDF to improve the content of the gamma crystals in the crystals, for example, clay can be added into PVDF, and the gamma crystals with polarity are induced and generated through the ion-dipole interaction generated between the negative charges carried by a clay layer and PVDF molecular chains.
CN109320743A discloses a method for preparing a polyvinylidene fluoride film, which comprises the steps of preparing a polyvinylidene fluoride pretreatment film by a film dropping method, heating to 160-170 ℃ for annealing, and cooling to room temperature at a speed of 5 ℃/min to obtain the polyvinylidene fluoride film with high gamma-type crystal content. The method cannot obtain the gamma-type crystal PVDF film with the content of 100 percent.
CN103113602A discloses a method for preparing a high-orientation gamma-phase polyvinylidene fluoride PVDF film, which comprises the steps of 1) casting a solution of polyvinylidene fluoride PVDF into a film, heating, preserving heat and eliminating heat history, rapidly cooling the polyvinylidene fluoride PVDF film to 160-170 ℃ at a speed of 50 ℃/min, 2) applying pressure to a melt by using a polymethylsiloxane plate, applying shear stress to the melt, standing at 160-170 ℃, crystallizing to α crystal form at 160-163 ℃, annealing at the temperature range to generate conversion from α crystal form to gamma crystal form, crystallizing to α crystal nucleus at 164-167 ℃, and crystallizing to gamma crystal form at 168-170 ℃.
CN110092934A discloses a method for promoting the transformation of polyvinylidene fluoride crystal form, which dissolves polyvinylidene fluoride PVDF and other polymers in a solvent to form a blend solution; applying the blend solution on a substrate, and drying at 50-100 ℃ to obtain a PVDF blend film; heating the PVDF blend film to a crystallization temperature of 150-170 ℃, crystallizing for 60-960 min at the crystallization temperature, and then cooling to room temperature to form gamma-type crystals. In the method, other polymers are required to be mixed with PVDF, and other polymers are required to be removed, otherwise, the purity of the PVDF crystal form is influenced, and the purity of PVDF gamma crystals is easily influenced due to the residue of other polymers; and the other polymers removed in this process are not well recycled, resulting in higher costs.
Disclosure of Invention
In view of the above, the present invention aims to provide a method for preparing polyvinylidene fluoride gamma-type crystals, which can replace the existing method for preparing gamma-type crystals. Further, the method can obtain polyvinylidene fluoride gamma-type crystals with the purity close to 100%. In addition, the method has simple process and high crystallization efficiency, and does not need to add other polymers, thereby having low cost.
The invention achieves the above object through the following technical scheme.
A preparation method of polyvinylidene fluoride gamma-type crystals comprises the following steps:
shearing the polyvinylidene fluoride film by using fibers at a shearing temperature of 180-260 ℃ to obtain a sheared polyvinylidene fluoride film; then crystallizing the sheared polyvinylidene fluoride film at a crystallization temperature of 150-165 ℃ to obtain polyvinylidene fluoride gamma-type crystals;
the melting point of the fiber is TmThe shear temperature isTsThen T ism-Ts≥5℃。
According to the preparation method of the invention, preferably, the fibers are sheared along one direction of the plane of the polyvinylidene fluoride film.
According to the preparation method of the present invention, preferably, the polyvinylidene fluoride has an average molecular weight of 5 × 104～7.5×105。
According to the preparation method of the present invention, preferably, the fiber is selected from one of polyimide fiber, polyetheretherketone fiber, polytetrafluoroethylene fiber and polyacrylonitrile fiber, and the elongation at break of the fiber is 8% or less.
According to the preparation method of the invention, the shear rate is preferably 0.02-0.1 mm/s.
According to the preparation method, the shearing time is preferably 1-10 s.
The preparation method according to the present invention preferably comprises the following specific steps:
heating the polyvinylidene fluoride film at a shearing temperature of 180-260 ℃ for 5-15 min, and then shearing the polyvinylidene fluoride film by using fibers to obtain a sheared polyvinylidene fluoride film; and then crystallizing the sheared polyvinylidene fluoride film at the crystallization temperature of 150-165 ℃ for 10-22 h to obtain the polyvinylidene fluoride gamma-type crystal.
According to the production method of the present invention, preferably, the γ -type crystal is a γ -type transverse crystal.
According to the preparation method of the invention, preferably, the polyvinylidene fluoride film does not contain other polymers.
According to the preparation method of the present invention, preferably, the polyvinylidene fluoride thin film is prepared by the following steps:
applying a polyvinylidene fluoride solution on a substrate, and drying to obtain a polyvinylidene fluoride film;
the polyvinylidene fluoride solution contains 30-70 mg/mL of polyvinylidene fluoride, and a solvent of the polyvinylidene fluoride solution is one or more selected from dimethyl sulfoxide, acetonitrile, N-dimethylformamide and N, N-dimethylacetamide.
The invention provides a novel preparation method for forming polyvinylidene fluoride gamma type crystals, which is obviously different from the traditional preparation method. In addition, the present invention obtains polyvinylidene fluoride gamma-type crystals having a purity of almost 100% by heating and melting a polyvinylidene fluoride film, shearing the film with a fiber, and then crystallizing the film at a crystallization temperature. Furthermore, the invention can avoid adding other polymers, reduce the cost and reduce impurities. According to the preferable technical scheme, the high-purity polyvinylidene fluoride gamma-type transverse crystals can be obtained and the crystallization efficiency can be improved by controlling the shearing temperature, the shearing rate and the shearing time.
Drawings
FIG. 1 is a diagram of the polarization of gamma-type transverse crystals of the product of example 1 measured by a polarization microscope.
FIG. 2 is a diagram showing the polarization pattern of α type transverse crystals of the product of comparative example 1 measured by a polarization microscope.
FIG. 3 is a crystal form polarization diagram of the product of comparative example 7 tested by a polarization microscope.
FIGS. 4(a) -4 (c) are the crystal form polarization diagrams of the product of comparative example 7 tested by a polarization microscope under in-situ hot-stage in-situ melting; wherein, FIG. 4(a) is a polarization diagram before melting;
FIG. 4(b) is a polarization diagram at 170 ℃; FIG. (c) is a polarization diagram at 177 ℃.
FIG. 5 is a plot of the (micro-region) infrared spectra of the products of example 1 and comparative example 1.
FIG. 6(a) is an atomic force phase diagram of the product of example 1.
FIG. 6(b) is an atomic force phase diagram of the product of comparative example 1.
Detailed Description
The present invention will be further described with reference to the following specific examples, but the scope of the present invention is not limited thereto.
In the present invention, polyvinylidene fluoride may be abbreviated as pvdf, and the "pure γ type transverse crystal" in the present invention means that the γ type transverse crystal has a purity of almost 100%, and the "pure α type transverse crystal" means that the α type transverse crystal has a purity of almost 100%.
The preparation method of the polyvinylidene fluoride gamma-type crystal comprises the following steps: (1) preparing a polyvinylidene fluoride solution; (2) preparing a polyvinylidene fluoride film; (3) cutting the polyvinylidene fluoride film; (4) and (5) crystallizing. As described in detail below.
< preparation of polyvinylidene fluoride solution >
Dissolving polyvinylidene fluoride in a solvent to form a polyvinylidene fluoride solution. The average molecular weight of the polyvinylidene fluoride of the invention is 5 x 104～7.5×105Preferably 1X 105～7.5×105More preferably 3X 105～6.5×105. The above molecular weight can be determined by a conventional method such as high performance liquid chromatography. Controlling the average molecular weight of the polyvinylidene fluoride within the range is beneficial to the generation of the polyvinylidene fluoride gamma type crystal and improves the content of the polyvinylidene fluoride gamma type crystal.
The solvent of the polyvinylidene fluoride solution may be selected from one or more of dimethyl sulfoxide, acetonitrile, N-dimethylformamide and N, N-dimethylacetamide. Preferably, the solvent is selected from one of dimethyl sulfoxide, acetonitrile, N-dimethylformamide and N, N-dimethylacetamide. More preferably, the solvent is N, N-dimethylformamide, dimethyl sulfoxide or N, N-dimethylacetamide.
The polyvinylidene fluoride solution contains 30-70 mg/mL of polyvinylidene fluoride. That is, the concentration of the solute PVDF in the polyvinylidene fluoride solution of the invention can be 30-70 mg/mL, preferably 35-65 mg/mL, and more preferably 40-60 mg/mL. Thus, the thickness of the polyvinylidene fluoride film can be controlled, and the high-purity polyvinylidene fluoride gamma-type crystal can be obtained.
When the polyvinylidene fluoride solution is prepared, a stirrer can be adopted for stirring and dissolving. The stirrer is not particularly limited, and may be, for example, a magnetic stirrer. The dissolution temperature is 45-80 ℃, preferably 45-75 ℃, and more preferably 50-60 ℃. The dissolving time is 2-10 h, preferably 4-8 h, and more preferably 5-8 h. The stirring speed is 100 to 600rpm, preferably 200 to 500rpm, and more preferably 300 to 400 rpm. Thus being beneficial to obtaining the polyvinylidene fluoride film with uniform thickness.
According to one embodiment of the present invention, polyvinylidene fluoride is dissolved in N, N-Dimethylformamide (DMF) to form a polyvinylidene fluoride solution; average molecular weight of polyvinylidene fluoride is 1 × 105～7.5×105(ii) a The polyvinylidene fluoride solution contains 40-60 mg/mL of polyvinylidene fluoride.
The polyvinylidene fluoride solution of the present invention preferably contains no other polymers. Therefore, the cost can be reduced, other polymers do not need to be removed, the residues of other polymers are not generated, and the impurities are reduced.
< preparation of polyvinylidene fluoride film >
And applying the polyvinylidene fluoride solution on the substrate, and drying to obtain the polyvinylidene fluoride film. Substrates include, but are not limited to, glass, crystalline silicon, alumina, quartz, and metals. Glass and crystalline silicon are preferred. More preferably, the glass may be ordinary glass. According to one embodiment of the invention, the substrate is glass. The present invention is not particularly limited as to the manner in which the polyvinylidene fluoride solution is applied to the substrate. According to one embodiment of the present invention, the polyvinylidene fluoride solution may be pipetted and dropped onto the substrate. The drying temperature can be 50-100 ℃, preferably 60-100 ℃, and more preferably 60-90 ℃. The drying time can be 10-36 h, preferably 15-30 h, and more preferably 20-30 h. The drying method of the present invention is not particularly limited, and vacuum drying is preferable. This is beneficial to removing the influence of the solvent and ensures the thoroughness in eliminating the thermal history.
The thickness of the polyvinylidene fluoride film can be 10-40 μm, preferably 15-35 μm, and more preferably 15-30 μm. By adopting the thickness of the polyvinylidene fluoride thin film, the generation of gamma-type crystals can be better promoted during shearing.
The polyvinylidene fluoride film of the present invention does not contain other polymers. Therefore, the cost can be reduced, other polymers do not need to be removed, the residues of other polymers are not generated, and the impurities are reduced.
< cutting step of polyvinylidene fluoride film >
Shearing the polyvinylidene fluoride film by using fibers at a shearing temperature of 180-260 ℃ to obtain a sheared polyvinylidene fluoride film; and crystallizing the sheared polyvinylidene fluoride film at a crystallization temperature of 150-165 ℃ to obtain a polyvinylidene fluoride gamma-type crystal. According to one embodiment of the invention, the polyvinylidene fluoride film is heated at a shearing temperature of 180-260 ℃ for 5-15 min, and then the polyvinylidene fluoride film is sheared by fibers to obtain the sheared polyvinylidene fluoride film. Preferably, the polyvinylidene fluoride film is heated for 6-13 min at a shearing temperature of 220-250 ℃. More preferably, the polyvinylidene fluoride film is heated for 7-11 min at the shearing temperature of 230-250 ℃. In the invention, polyvinylidene fluoride is heated and melted in advance, and then is sheared by fibers, thus being beneficial to obtaining high-purity polyvinylidene fluoride gamma-type crystals.
Assuming that the melting point of the fiber is TmShear temperature of TsThen T ism-TsNot less than 5 ℃. Preferably, Tm-TsMore preferably, T is 60 ℃ or morem-Ts≥100℃。Tm-Ts500 ℃ or less, preferably Tm-Ts350 ℃ or less, more preferably Tm-TsLess than or equal to 250 ℃. This avoids the adverse effect of fiber melting or deformation on the crystal form transformation. The fiber of the present invention may be selected from one of polyimide fiber, polyetheretherketone fiber, polytetrafluoroethylene fiber, and polyacrylonitrile fiber. Preferably, the fiber is selected from one of polyimide fiber and polyacrylonitrile fiber. More preferably, the fibers are polyimide fibers. The elongation at break of the fibers may be 8% or less, preferably 7% or less, more preferably 5% or less. The fibers had no interaction with PVDF in the static state. The fibers are sheared along one direction of the plane of the polyvinylidene fluoride film. This is advantageous for the formation of gamma-type crystals, especially for the formation of gamma-type transverse crystals.
The shearing temperature may be 180-260 deg.C, preferably 220-250 deg.C, and more preferably 230-250 deg.C. The shear rate may be 0.02 to 0.1mm/s, preferably 0.04 to 0.08mm/s, and more preferably 0.05 to 0.07 mm/s. The shearing time can be 1-10 s, preferably 2-9 s, and more preferably 2-8 s. The present inventors have surprisingly found that by controlling the shear rate or shear time, polyvinylidene fluoride gamma crystals can be obtained.
In the invention, a shearing machine can be used for shearing during shearing. The shearing machine is not particularly limited as long as effective shearing of the present invention can be achieved. The fibers may be placed on the base surface of the shear and the fibers secured, with the polyvinylidene fluoride film placed over the fibers (with the fibers as centered as possible in the polyvinylidene fluoride film). Adjusting the shearing rate and the shearing time, adjusting the shearing temperature to 180-260 ℃, heating the polyvinylidene fluoride film at 180-260 ℃ for 5-15 min, and then starting shearing. And pulling the fiber during shearing to generate shearing stress on the interface of the fiber and the polyvinylidene fluoride melt. And after shearing, rapidly shearing the fibers to obtain a sheared polyvinylidene fluoride film, and transferring the sheared polyvinylidene fluoride film to a crystallization hot table for crystallization. The length and width of the polyvinylidene fluoride film used in the shearing are not particularly limited to facilitate effective shearing, and may be, for example, 2X 2 mm. The elongation at break, modulus and melting point of the used fiber are all reasonable values, and the effective shearing of the polyvinylidene fluoride film can be ensured.
< crystallization step >
And crystallizing the sheared polyvinylidene fluoride film at the crystallization temperature of 150-165 ℃ to obtain the polyvinylidene fluoride gamma-type crystal. The γ -type crystal of the present invention is preferably a γ -type transverse crystal. The crystallization temperature may be 150 to 165 ℃, preferably 155 to 165 ℃, and more preferably 155 to 160 ℃. The crystallization time may be 10 to 22 hours, preferably 12 to 20 hours, and more preferably 15 to 20 hours. By adopting the crystallization temperature and the crystallization time, the high-purity polyvinylidene fluoride gamma-type transverse crystals with the purity of almost 100 percent can be obtained.
< test method >
(1) Polarizing microscope testing
The sample is heated and melted at a certain speed and photographed by using a polarizing microscope of Axioskop 40A in a transmission mode and by means of accurate temperature control of a heat distribution table, and melting of different crystal forms at different temperatures can be seen.
(2) Infrared Spectrum testing
The infrared scan was performed using an infrared spectrometer from a Spectrum 100FT-IR spectrometer to find the trabecular part of the sample under a microscope system. The scan area was 2 μm. Characteristic infrared peaks of samples of different crystal forms can be seen.
(3) Shape scanning of atomic force microscope
The crystalline sample was topographically scanned using a Bruker atomic force microscope. The area of the transverse crystals of the crystal sample was found under an optical system, and a size scan of 5X 5 μm was performed. It can be seen that samples of different crystal forms have different morphological features.
Preparation example 1
Dissolving polyvinylidene fluoride in N, N-dimethylformamide to form a polyvinylidene fluoride solution. Average molecular weight of polyvinylidene fluoride is 5.3X 105. The polyvinylidene fluoride solution contained 50mg/mL of polyvinylidene fluoride.
And uniformly dripping 200 mu l of polyvinylidene fluoride solution on the surface of the glass by a liquid transfer gun, and drying for 24h in a vacuum oven at 70 ℃ to obtain a polyvinylidene fluoride film with the thickness of 20 mu m.
Example 1
Heating the polyvinylidene fluoride film of preparation example 1 at 240 ℃ for 10min, then shearing the polyvinylidene fluoride film at 240 ℃ by using polyimide fibers with the elongation at break of 3%, wherein the polyimide fibers are positioned in the center of the polyvinylidene fluoride film during shearing, the shearing rate is 0.06mm/s, the shearing time is 8s, and after the shearing is finished, rapidly shearing the fibers to obtain the sheared polyvinylidene fluoride film;
and then transferring the sheared polyvinylidene fluoride film to a crystallization hot table, and crystallizing for 16 hours at 157 ℃ to obtain polyvinylidene fluoride gamma-type transverse crystals.
Comparative example 1
The difference from example 1 is that the shear rate is 0.6 mm/s.
Comparative example 2
The difference from example 1 is that the shear temperature is 270 ℃.
Comparative examples 3 to 6
The difference from comparative example 2 is that comparative examples 3 to 6 have shear rates of 0.15mm/s, 0.3mm/s, 0.6mm/s and 0.9mm/s, respectively.
Comparative example 7
The difference from comparative example 2 is that the shear rate is 0.3mm/s and the shear time is 2 s.
Comparative examples 8 to 10
The difference from comparative example 7 is that the shear times of comparative examples 8 to 10 were 4s, 6s, and 15s, respectively.
Examples of the experiments
The products of examples 1 to 3 and comparative examples 1 to 10 were respectively tested with a polarization microscope to obtain crystal form polarization diagrams, and the results were summarized to obtain table 1. To illustrate the results more clearly, the crystal form polarization diagrams of some products are given, see fig. 1, fig. 2 and fig. 3, respectively. Wherein, fig. 1 and 2 are crystal form polarization diagrams of the products of example 1 and comparative example 1, respectively. Fig. 3 is a crystal form polarization diagram of the product of comparative example 7. As is clear from table 1 and fig. 1 to 3, by searching and controlling the shearing time, shearing temperature and shearing rate, polyvinylidene fluoride γ -type transverse crystals having a purity of almost 100% were obtained.
The product of comparative example 7 was subjected to in-situ melting on an in-situ hot stage and tested by a polarizing microscope to obtain a crystal form polarization diagram, FIG. 4(a) is a polarization diagram before melting, FIG. 4(b) is a polarization diagram at 170 ℃ and FIG. 4(c) is a polarization diagram at 177 ℃, from the results of FIGS. 4(a) to 4(c) and the melting points of α and gamma-type crystals of PVDF (the melting point of α type crystal is 170 ℃ and the melting point of gamma-type crystal is 177 ℃), it was further confirmed that the long transverse crystal having strong birefringence was α type transverse crystal and the short transverse crystal having weak birefringence was gamma-type transverse crystal.
The products of example 1 and comparative example 1 were each tested with a (micro-area) infrared spectrometer to obtain an infrared spectrum. In fig. 5, a curve a is a test result of the product of example 1, and a curve b is a test result of the product of comparative example 1. Wherein, 763cm-1Is a characteristic peak of α type crystal, 811cm-1And 833cm-1It is a characteristic peak of γ type crystal fig. 5 further confirms that the long transverse crystal is α type transverse crystal and the short transverse crystal is γ type transverse crystal.
The products of example 1 and comparative example 1 were subjected to shape scanning with an atomic force microscope to obtain fig. 6(a) and 6(b), respectively, from fig. 6(a) and 6(b), it can be seen that the α type transverse crystal was a platelet having an edge-on structure in an atomic diagram, and the γ type transverse crystal had a curly cauliflower-like appearance.
TABLE 1
As is clear from table 1 and the accompanying drawings, the present invention can obtain polyvinylidene fluoride γ -type transverse crystals having a purity of almost 100% by cutting a polyvinylidene fluoride film having a predetermined thickness with a fiber.
The present invention is not limited to the above-described embodiments, and any variations, modifications, and substitutions which may occur to those skilled in the art may be made without departing from the spirit of the invention.
Claims (10)
1. A preparation method of polyvinylidene fluoride gamma-type crystals is characterized by comprising the following steps:
shearing the polyvinylidene fluoride film by using fibers at a shearing temperature of 180-260 ℃ to obtain a sheared polyvinylidene fluoride film; then crystallizing the sheared polyvinylidene fluoride film at a crystallization temperature of 150-165 ℃ to obtain polyvinylidene fluoride gamma-type crystals;
the melting point of the fiber is TmThe shear temperature is TsThen T ism-Ts≥5℃。
2. The method of claim 1, wherein said fibers are sheared along a direction in the plane of said polyvinylidene fluoride film.
3. The method according to claim 1, wherein the polyvinylidene fluoride has an average molecular weight of 5 x 104～7.5×105。
4. The production method according to claim 1, wherein the fiber is one selected from the group consisting of polyimide fiber, polyetheretherketone fiber, polytetrafluoroethylene fiber, and polyacrylonitrile fiber, and the elongation at break of the fiber is 8% or less.
5. The method of claim 1, wherein the shear rate is 0.02 to 0.1 mm/s.
6. The method according to claim 1, wherein the shearing time is 1 to 10 seconds.
7. The preparation method according to claim 1, comprising the following specific steps:
heating the polyvinylidene fluoride film at a shearing temperature of 180-260 ℃ for 5-15 min, and then shearing the polyvinylidene fluoride film by using fibers to obtain a sheared polyvinylidene fluoride film; and then crystallizing the sheared polyvinylidene fluoride film at the crystallization temperature of 150-165 ℃ for 10-22 h to obtain the polyvinylidene fluoride gamma-type crystal.
8. The production method according to claim 1, wherein the γ -type crystal is a γ -type transverse crystal.
9. The method of claim 1, wherein the polyvinylidene fluoride film is free of other polymers.
10. The preparation method according to any one of claims 1 to 9, wherein the polyvinylidene fluoride film is prepared by the following steps:
applying a polyvinylidene fluoride solution on a substrate, and drying to obtain a polyvinylidene fluoride film;
the polyvinylidene fluoride solution contains 30-70 mg/mL of polyvinylidene fluoride, and a solvent of the polyvinylidene fluoride solution is one or more selected from dimethyl sulfoxide, acetonitrile, N-dimethylformamide and N, N-dimethylacetamide.
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| CN111205496B (en) | 2021-02-26 |
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