PVDF-based film, PVDF-based polymer film, preparation method and application

Abstract

The invention discloses a PVDF (Polyvinylidene Fluoride)-based film, a PVDF-based polymer film as well as a preparation method and application thereof. The preparation method of the PVDF-based thin film comprises the following steps: 1) preparing a polyvinylidene fluoride-based material into a polymer solution; wherein the polyvinylidene fluoride based material is formed by grafting polyvinylidene fluoride with an olefin monomer containing a double bond; the olefin monomer contains an electron withdrawing group; 2) spreading the polymer solution in the step 1) on a substrate, and drying to obtain an initial film; and 3) carrying out thermal annealing treatment on the initial film at 100-150 DEG C to obtain the PVDF-based film. The PVDF-based thin film obtained by the preparation method disclosed by the invention and the PVDF-based polymer thin film are relatively good in piezoelectric property.

Description

Technical Field

[0001] This invention relates to a PVDF-based thin film, a PVDF-based polymer thin film, a preparation method thereof, and its uses. Background Technology

[0002] PVDF has various crystal structures. The most common, non-electroactive α phase is kinetically favorable and is inevitably produced during normal, slow, non-isothermal crystallization. However, obtaining high amounts of electroactive polar phases, such as β and γ phases, is relatively difficult.

[0003] Currently, there are also studies on the preparation of polar phases of PVDF, but the piezoelectric activity of the obtained polyvinylidene fluoride films is still relatively low.

[0004] CN112708154A discloses a method for improving the nucleation of the γ phase of polyvinylidene fluoride (PVDF), comprising: 1) preparing a 5% PVDF / carboxylated polyamide amine solution using N,N-dimethylformamide (DMF) as a solvent; 2) dropping the solution from step 1) onto a glass slide and curing it into a film in a vacuum oven at 70°C; 3) melting the cured film obtained in step 2) on a hot plate at 200°C to eliminate thermal history, and then rapidly cooling it to 155°C for isothermal crystallization until the blend film is completely crystallized. The piezoelectric activity of this film still needs to be improved.

[0005] CN114685916A discloses a polymer piezoelectric material and its preparation method. This polymer piezoelectric material is an induced co-crystallization composition of fluororesin A and fluororesin B. Fluororesin A is poly(vinylidene fluoride-trifluoroethylene), and the β-crystal content of fluororesin B accounts for 50-100% of the total crystal content of fluororesin B. Fluororesin B is a homopolymer or copolymer of vinylidene fluoride. The piezoelectric coefficient of this polymer piezoelectric material still needs further improvement.

[0006] CN114953296A discloses a method for fabricating a polycrystalline polyvinylidene fluoride (PVDF) film, comprising: coating a PVDF solution onto a substrate to form a film, and heating the PVDF solution on the substrate above its melting point to produce a first PVDF film; cooling the first PVDF film to obtain a semi-molten second PVDF film; producing multiple PVDF fibers having a β-phase; arranging the multiple PVDF fibers in parallel on the second PVDF film to obtain a third PVDF film having both α- and β-phases; and annealing the third PVDF film at a fixed temperature to produce the polycrystalline PVDF film having both β- and γ-phases. The piezoelectric coefficient of the PVDF film obtained by this method is still relatively low. Summary of the Invention

[0007] In view of the above, one object of the present invention is to provide a method for preparing a PVDF-based thin film, wherein the obtained PVDF-based thin film has piezoelectric properties. Another object of the present invention is to provide a method for preparing a PVDF-based polymer thin film, wherein the obtained PVDF-based polymer thin film has good piezoelectric properties, a high piezoelectric coefficient, and a γ-phase content greater than the α-phase content. Yet another object of the present invention is to provide a PVDF-based polymer thin film prepared according to the preparation method described above, or a PVDF-based thin film prepared according to the preparation method described above. A further object of the present invention is to provide the use of the PVDF-based polymer thin film or PVDF-based thin film prepared according to the above method in the preparation of electroactive materials.

[0008] The present invention achieves the above objectives using the following technical solutions.

[0009] On one hand, the present invention provides a method for preparing a PVDF-based thin film, comprising the following steps:

[0010] 1) The polyvinylidene fluoride material is formulated into a polymer solution; wherein the polyvinylidene fluoride material is formed by grafting a polyvinylidene fluoride with an olefin monomer containing a double bond; the olefin monomer contains an electron-withdrawing group;

[0011] 2) Spread the polymer solution from step 1) onto the substrate and dry it to obtain the initial film;

[0012] 3) The initial film is subjected to thermal annealing at 100-150℃ to obtain a PVDF-based film.

[0013] In another aspect, the present invention provides a method for preparing a PVDF-based polymer film, comprising the following steps:

[0014] (1) The polyvinylidene fluoride material is formulated into a polymer solution; wherein the polyvinylidene fluoride material is formed by grafting a polyvinylidene fluoride with an olefin monomer containing a double bond; the olefin monomer contains an electron-withdrawing group;

[0015] (2) Spread the polymer solution from step (1) onto the substrate and dry it to obtain the initial film;

[0016] (3) The initial film is heated to eliminate thermal history at 185-250℃ and then cooled to below 140℃; then heated to 157-180℃ for partial melting and recrystallization, and then cooled to below 80℃; then heated to 100-140℃ for annealing to obtain a phase transformation film.

[0017] (4) The phase transition film is subjected to contact polarization treatment to obtain a PVDF-based polymer film.

[0018] According to the preparation method of the present invention, preferably, the olefin monomer is selected from at least one of acrylate, acrylamide, acrylonitrile and styrene.

[0019] According to the preparation method of the present invention, preferably, the polyvinylidene fluoride material is prepared by the following steps:

[0020] (1') Mix polyvinylidene fluoride with a polar solvent to form a polyvinylidene fluoride solution;

[0021] (2') The catalyst, olefin monomer and auxiliary ligand are added to the polyvinylidene fluoride solution and reacted to obtain a reaction mixture;

[0022] (3') The reaction mixture is precipitated, washed and dried to obtain polyvinylidene fluoride material.

[0023] According to the preparation method of the present invention, preferably, in step (1'), the polar solvent is selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethyl sulfoxide and acetone; the concentration of the polyvinylidene fluoride solution is 0.05-0.5 g / mL.

[0024] According to the preparation method of the present invention, preferably, in step (2'), the catalyst is selected from cuprous chloride or cuprous bromide; the mass ratio of the catalyst to polyvinylidene fluoride is 0.005 to 0.1:1; the auxiliary ligand is selected from one of hexamethyltriethylenetetramine, 2,2-bipyridine and pentamethyldivinyltriamine; the mass ratio of the auxiliary ligand to polyvinylidene fluoride is 0.02 to 0.1:1; the mass ratio of the olefin monomer to polyvinylidene fluoride is 4 to 8:1; the reaction temperature is 100 to 140°C, the reaction time is 0.5 to 48 h; and the reaction is carried out in an oxygen-free environment.

[0025] According to the preparation method of the present invention, preferably, in step (3'), the reaction mixture is precipitated with an aqueous solution of alcohol and washed with an alkane or alcohol; wherein the alcohol in the aqueous solution of alcohol is selected from at least one of methanol, ethanol and isopropanol; and the alkane is selected from at least one of n-hexane, cyclohexane and petroleum ether.

[0026] According to the preparation method of the present invention, preferably, in step (3), the heating rate is 4 to 50 °C / min; and in step (4), the electric field strength used for contact polarization is 20 to 400 MV / m.

[0027] Furthermore, the present invention also provides a PVDF-based polymer film, which is prepared according to the preparation method described above, wherein the γ phase content of the PVDF-based polymer film is greater than the α phase content; its piezoelectric coefficient value d 33 It is above 40 pC / N.

[0028] In another aspect, the present invention also provides the use of the PVDF-based polymer film prepared according to the above-described method or the PVDF-based film prepared according to the above-described method in the preparation of electroactive materials.

[0029] The PVDF-based film prepared by the method of the present invention exhibits good piezoelectric properties. The PVDF-based polymer film prepared by the method of the present invention also exhibits good piezoelectric properties and a high piezoelectric coefficient. According to a preferred embodiment of the present invention, polyvinylidene fluoride is grafted with an olefin monomer containing a double bond, and this olefin monomer contains electron-withdrawing groups. A specific heat treatment step is then performed, resulting in a PVDF-based film or PVDF-based polymer film with a higher γ-phase content and better piezoelectric properties. Attached Figure Description

[0030] Figure 1 Infrared spectra of the polyvinylidene fluoride material (PtBA-g-PVDF) and PVDF prepared in Example 1.

[0031] Figure 2 The 1H NMR spectrum of the polyvinylidene fluoride material (PtBA-g-PVDF) of Preparation Example 1 and the unextracted crude product.

[0032] Figure 3 The figures show experimental data of the phase transition film (PtBA-g-PVDF) and PVDF obtained in Example 1 during the complete melt recrystallization process, i.e., POM diagrams of the two films after complete crystallization at the same crystallization temperature.

[0033] Figure 4 The polarity content, crystallinity, and crystallization temperature of the phase transition film (PtBA-g-PVDF) and PVDF obtained in Example 1 at different annealing temperatures are respectively.

[0034] Figure 5 The image shows the PVDF-based polymer film (PtBA-g-PVDF) obtained in Example 1, and the piezoelectric test results of PVDF.

[0035] Figure 6 The image shows the PVDF-based polymer film (PMMA-g-PVDF) obtained in Example 2, and the piezoelectric test results of PVDF.

[0036] Figure 7 Infrared spectra of PVDF, PVDF-based thin films obtained in Examples 3 and 4.

[0037] Figure 8 The piezoelectric output results are for the PVDF-based thin film in Example 3.

[0038] Figure 9Piezoelectric output results of the PVDF-based film in Example 4.

[0039] Figure 10 Piezoelectric output results of the pure PVDF film. Detailed implementation manners

[0040] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0041] <Preparation method of PVDF-based film>

[0042] The preparation method of the PVDF-based film of the present invention includes the following steps: 1) a step of preparing a polymer solution; 2) a spreading and drying step; 3) a thermal annealing treatment step. Optionally, it further includes a step of preparing a polyvinylidene fluoride-based material. The following is a detailed description.

[0043] Preparation steps of polyvinylidene fluoride materials

[0044] The polyvinylidene fluoride-based material of the present invention is prepared by the following steps:

[0045] (1’) Mix polyvinylidene fluoride with a polar solvent to form a polyvinylidene fluoride solution;

[0046] (2’) Add a catalyst, an olefin monomer, and an auxiliary ligand to the polyvinylidene fluoride solution for reaction to obtain a reaction mixture;

[0047] (3’) Precipitate, wash, and dry the reaction mixture to obtain a polyvinylidene fluoride-based material.

[0048] In step (1’), the polar solvent is selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylsulfoxide, and acetone. Preferably, the polar solvent is selected from one of N-methylpyrrolidone (NMP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylsulfoxide, and acetone. More preferably, the polar solvent is N-methylpyrrolidone (NMP). The concentration of the polyvinylidene fluoride solution is 0.05 - 0.5 g / mL, preferably 0.08 - 0.3 g / mL, and more preferably 0.1 - 0.2 g / mL.

[0049] In step (2’), the catalyst is cuprous chloride or cuprous bromide, preferably cuprous chloride. The mass ratio of the catalyst to polyvinylidene fluoride is 0.005 - 0.1:1, preferably 0.007 - 0.05:1, and more preferably 0.01 - 0.03:1.

[0050] The auxiliary ligand is selected from one of hexamethyltriethylenetetramine, 2,2-bipyridine, and pentamethyldivinyltriamine, preferably hexamethyltriethylenetetramine. Hexamethyltriethylenetetramine is 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA). Pentamethyldivinyltriamine is N,N,N',N',N”-pentamethyldivinyltriamine (PMDETA). The mass ratio of the auxiliary ligand to polyvinylidene fluoride is 0.02 to 0.1:1, preferably 0.03 to 0.08:1, and more preferably 0.04 to 0.06:1.

[0051] The olefin monomer is an olefin monomer containing a single double bond, and the ortho-position group of the olefin monomer is an electron-withdrawing group, preferably a strongly electron-withdrawing group. The olefin monomer is selected from at least one of acrylates, acrylamide, acrylonitrile, and styrene. Preferably, the olefin monomer is selected from one of acrylates, acrylamide, acrylonitrile, and styrene. More preferably, the olefin monomer is selected from one of acrylates, acrylamide, and acrylonitrile. In this invention, the acrylate is selected from at least one of tert-butyl acrylate (tBA) and methyl methacrylate (MMA). The mass ratio of the olefin monomer to polyvinylidene fluoride is 4–8:1, preferably 5–8:1, and more preferably 6–7:1. This is beneficial for forming PVDF-based films or PVDF-based polymer films with higher piezoelectric activity.

[0052] The reaction temperature can be 100–140°C, preferably 110–140°C, and more preferably 120–130°C. The reaction time can be 0.5–48 h, preferably 1–40 h, and more preferably 1.5–36 h. For example, it can be 3 h, 5 h, 10 h, 15 h, 20 h, 24 h, 30 h, 32 h, etc. The reaction is carried out in an anaerobic environment.

[0053] In step (3'), the reaction mixture is precipitated with an aqueous solution of alcohol and washed with an alkane or alcohol; wherein the alcohol in the aqueous solution is selected from at least one of methanol, ethanol, and isopropanol, preferably methanol or ethanol, and more preferably methanol. In the aqueous solution of alcohol, the volume ratio of water to alcohol is 1:2 to 5, preferably 1:2 to 4, and more preferably 1:3 to 4. The alkane is selected from at least one of n-hexane, cyclohexane, and petroleum ether, preferably n-hexane or cyclohexane, and more preferably n-hexane. The alcohol used for washing with alcohol can be methanol or ethanol.

[0054] In some embodiments, when the olefin monomer used is tert-butyl acrylate (tBA), step (3') specifically includes: precipitating and washing the reaction mixture; after washing, extracting the washed solid using an extraction device; and then drying the extracted solid to obtain the polyvinylidene fluoride material. The extraction device may include a Soxhlet extractor. This allows for further purification of the polyvinylidene fluoride material and improvement of its piezoelectric properties.

[0055] Steps for preparing polymer solutions

[0056] The polyvinylidene fluoride-based material obtained above is formulated into a polymer solution. This is conducive to forming a thin film in the next step. According to an embodiment of the present invention, the polyvinylidene fluoride-based material is formed by grafting an olefin monomer containing a double bond onto polyvinylidene fluoride; the olefin monomer is selected from at least one of acrylate, acrylamide, acrylonitrile, and styrene.

[0057] The solvent used to prepare the polymer solution can be selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. It is preferably selected from N,N-dimethylformamide or N,N-dimethylacetamide. The concentration of the polymer solution can be 15 - 35 mg / mL, preferably 20 - 30 mg / mL, and more preferably 20 - 25 mg / mL.

[0058] Spreading and drying steps

[0059] The above-obtained polymer solution is spread on a substrate and dried to obtain an initial thin film. The spreading method can adopt those known in the art, such as solution casting method, casting method, etc.

[0060] The substrate can be a glass slide substrate. The drying temperature can be 55 - 90 °C, preferably 60 - 85 °C, and more preferably 70 - 80 °C. The drying time can be 12 - 36 h, preferably 20 - 30 h, and more preferably 24 - 28 h.

[0061] Heat annealing process

[0062] The initial thin film is subjected to a thermal annealing treatment at 100 - 150 °C to obtain a PVDF-based thin film. In this way, a PVDF-based thin film with coexistence of γ-phase and β-phase can be obtained, improving its piezoelectric properties. The temperature of the thermal annealing treatment can be 100 - 150 °C, preferably 110 - 145 °C, and more preferably 120 - 140 °C. The time of the thermal annealing treatment can be 6 - 72 h, preferably 8 - 70 h, and more preferably 10 - 48 h. For example, it can be 16 h, 20 h, 24 h, 28 h, 32 h, 36 h, 40 h, etc.

[0063] The PVDF-based thin film obtained by the present invention is a thin film with coexistence of γ-phase and β-phase, and there is no α-phase.

[0064] <Preparation Method of PVDF-based Polymer Thin Film>

[0065] The method for preparing the PVDF-based polymer film of the present invention includes the following steps: (1) preparing a polymer solution; (2) spreading and drying; (3) phase transformation; and (4) polarization treatment. Optionally, it also includes a step of preparing polyvinylidene fluoride material.

[0066] Preparation steps of polyvinylidene fluoride materials

[0067] The preparation steps of the polyvinylidene fluoride (PVDF) material described in this preparation method are the same as the corresponding steps in the above-mentioned preparation method of PVDF-based thin films, as detailed above, and will not be repeated here.

[0068] Steps for preparing polymer solutions

[0069] The steps for preparing the polymer solution in this preparation method are the same as the corresponding steps in the above-mentioned preparation method of PVDF-based thin films, as detailed above, and will not be repeated here.

[0070] Spreading and drying steps

[0071] The spreading and drying steps described in this preparation method are the same as the corresponding steps in the above-mentioned preparation method of PVDF-based thin films, as detailed above, and will not be repeated here.

[0072] Phase transition steps

[0073] The initial film is subjected to thermal history elimination at 185–250 °C, then cooled to below 140 °C; subsequently, it is heated to 157–180 °C for partial melting and recrystallization, then cooled to below 80 °C; and finally annealed at 100–140 °C to obtain a phase transition film. This process is beneficial for ensuring that the γ phase content is greater than the α phase content, which is conducive to improving piezoelectric activity.

[0074] The temperature required to eliminate thermal history can be 185–250°C, preferably 190–230°C, and more preferably 200–220°C. The time required to eliminate thermal history can be 5–180 min, preferably 5–120 min, and more preferably 5–100 min.

[0075] After eliminating the thermal history, the temperature is lowered to below 140°C, preferably to room temperature.

[0076] Partial melt recrystallization is the heat treatment procedure corresponding to the phase transformation process. Specifically, the film, having eliminated its thermal history, is held at a temperature slightly above its melting point for 10–180 min, then cooled at a constant rate, preferably to room temperature. The partial melt recrystallization temperature can be 157–180°C, preferably 165–180°C, and more preferably 170–175°C. The partial melt recrystallization time can be 10–180 min, preferably 10–120 min, and more preferably 12–100 min. The polar γ phase is key to obtaining high-performance PVDF-based materials; however, obtaining a high content of polar phase is difficult. After graft modification, the product can achieve a high γ phase content during partial melt recrystallization due to the melt memory effect, with a spherulite nucleation density reaching up to 99% during the process. The graft modification of this invention can effectively promote the formation of the γ phase.

[0077] The annealing temperature can be 100–140°C, preferably 110–140°C, and more preferably 120–130°C. The annealing time can be 6–72 h, preferably 8–70 h, and more preferably 10–48 h, for example, 16 h, 20 h, 24 h, 28 h, 32 h, 36 h, 40 h, etc.

[0078] In the phase transformation step of the present invention, the heating rate during heating can be 4 to 50 °C / min, preferably 6 to 40 °C / min, and more preferably 10 to 30 °C / min. The cooling rate during cooling can be 4 to 50 °C, preferably 6 to 40 °C / min, and more preferably 10 to 30 °C / min.

[0079] Polarization treatment steps

[0080] The phase transition film was subjected to contact polarization treatment to obtain a PVDF-based polymer film.

[0081] The contact polarization process includes: keeping the phase transition thin film sample at a constant temperature of 50-75°C for 6-15 minutes to ensure uniform heating; after contact polarization for 20-40 minutes, transferring it to room temperature for 10-25 minutes of contact polarization; using an electric field strength of 20-400 MV / m; and cleaning the silicone oil off the surface of the thin film sample with detergent after contact polarization.

[0082] The constant temperature can be 50–75°C, preferably 55–70°C, and more preferably 60–65°C. The contact polarization time at 50–75°C is preferably 25–35 min, and more preferably 30–35 min.

[0083] The selected electric field strength can be 20 to 400 MV / m, preferably 25 to 350 MV / m, and more preferably 30 to 300 MV / m.

[0084] <PVDF-based thin film, PVDF-based polymer thin film>

[0085] The PVDF-based thin film and PVDF-based polymer thin film of the present invention are respectively prepared according to the preparation methods described above.

[0086] The thickness of the PVDF-based thin film obtained in the present invention can be 5 - 150 μm, preferably 7 - 120 μm, and more preferably 9 - 100 μm. This PVDF-based thin film has the coexistence of γ and β phases and no α phase.

[0087] The thickness of the PVDF-based polymer thin film obtained in the present invention can be 10 - 150 μm, preferably 15 - 120 μm, and more preferably 20 - 100 μm, for example, it can be 25 μm. In the PVDF-based polymer thin film obtained in the present invention, γ and α phases coexist, and the content of γ phase is greater than that of α phase; its piezoelectric coefficient value d 33 is above 40 pC / N, preferably above 45 pC / N, and more preferably above 50 pC / N.

[0088] <Use>

[0089] The present invention also provides the use of the PVDF-based thin film or PVDF-based polymer thin film as described above in the preparation of electroactive materials.

[0090] <Analysis method>

[0091] FTIR infrared test: The FTIR with the model of Spectrum 100 is produced by Perkin-Elmer Company in the United States. The transmitted IR mode is selected to conduct infrared tests on the thin film with a thickness < 10 μm, and the set resolution is 4 cm -1 , the number of scans is 8 times, and the scanning wavenumber range is 450 - 4000 cm -1 .

[0092] DSC test: A taq2000 scanning calorimeter is used.

[0093] Piezoelectric test: Mainly use the instrument composed of a linear motor device applying stress and a 6517A electrometer produced by Keithley Company in the United States, and the output values of voltage and charge quantity of the material under different stresses can be obtained.

[0094] The piezoelectric coefficients in the following examples are explained as follows:

[0095] S 33 and d 33 expression formula:

[0096] S 33 = dV / dF,

[0097] d33 =dQ / dF,

[0098] In the formula S 33 d 33 V, F, and Q represent piezoelectric sensitivity, piezoelectric coefficient, piezoelectric output, charge quantity, and applied stress, respectively.

[0099] The raw materials used in the following examples are described below:

[0100] Polyvinylidene fluoride (PVDF) (Mw = 180,000 g / mol) was purchased from Sigma-Aldrich. Tert-butyl acrylate (tBA) was purchased from TCI (Shanghai) Chemical Industry Development Co., Ltd.; methyl methacrylate (MMA) was purchased from Sigma-Aldrich. Both acrylates required neutral alumina column chromatography to remove stabilizers before use. N,N-dimethylformamide (DMF) and n-hexane were purchased from J&K Scientific or Beijing Tongguang Reagent Co., Ltd., and all were reagent grade. Unless otherwise stated, all other reagents were ready for use without special treatment.

[0101] Preparation Example 1 - Preparation of Polyvinylidene Fluoride Materials

[0102] Mix 3.0 g PVDF with 30 mL NMP solvent, heat to 60 °C with stirring, and maintain at this temperature for 6 h until the polymer is completely dissolved to obtain a polyvinylidene fluoride solution.

[0103] The polyvinylidene fluoride (PVDF) solution system was evacuated twice to remove oxygen and water. 0.03 g of CuCl was added to the PVDF solution system, along with 18.02 g of tert-butyl acrylate (tBA) and 0.127 g of 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA). The system was then evacuated twice more to ensure an oxygen-free environment. The mixture was then heated to 120°C with stirring to obtain the reaction mixture (heating time is the reaction time).

[0104] The reaction mixture was precipitated using a mixed solvent of water and methanol (volume ratio 1:3), then the solid was washed with n-hexane and redissolved with NMP (60°C, 6 h); the precipitation and washing steps were repeated; the crude product was placed in a vacuum oven and dried at 70°C for 6 h. The dried product was then extracted with n-hexane for 2 days using an extraction device, and after extraction, it was dried in an oven for 24 h to obtain polyvinylidene fluoride material, denoted as PtBA-g-PVDF.

[0105] The analysis and testing are as follows:

[0106] Infrared spectroscopy was performed on the raw material PVDF and the extracted polyvinylidene fluoride material, respectively. The results are shown below. Figure 1The unextracted crude product was compared with the extracted polyvinylidene fluoride (PVDF) material using a 1H NMR spectrum. (See attached image.) Figure 2 .

[0107] exist Figure 2 In the NMR spectrum, impurity peaks (within the box) that do not belong to PtBA-g-PVDF appeared in the unextracted product, while the extracted product showed a clean NMR spectrum without any interference peaks.

[0108] exist Figure 1 In the infrared data, characteristic peaks specific to PtBA-g-PVDF were observed, confirming the successful synthesis of the PtBA-g-PVDF polymer. Furthermore, no double bond characteristic peaks belonging to the monomers were observed, indicating that the residual tert-butyl acrylate in the grafted product has been completely removed under the precision of infrared testing.

[0109] Example 1 - Preparation of PVDF-based polymer films

[0110] The polyvinylidene fluoride material (PtBA-g-PVDF) prepared in Preparation Example 1 was dissolved in N,N-dimethylformamide to prepare a polymer solution of 20 mg / ml.

[0111] The polymer solution was spread onto a glass substrate by solution casting and dried in a vacuum oven at 70°C for 24 hours to obtain the initial film.

[0112] The initial film was first isothermally treated at 200℃ for 5 min to eliminate thermal history, and then cooled to below 140℃; then heated to 175℃ for partial melting and recrystallization for 10 min, and then cooled to below 80℃; subsequently heated to 120℃ for annealing for 24 h, and then cooled to room temperature to obtain a phase transition film with a thickness of 25 μm; wherein the heating rate during heating was 10℃ / min, and the cooling rate during cooling was 10℃ / min;

[0113] The phase transition film was subjected to contact polarization treatment to obtain a PVDF-based polymer film; denoted as PtBA-g-PVDF polymer film;

[0114] The contact polarization process includes: keeping the phase transition thin film sample at a constant temperature of 60°C in an oil bath for 10 minutes to ensure uniform heating; after contact polarization for 30 minutes, transferring it to room temperature for 20 minutes of contact polarization; using an electric field strength of 30 MV / m; and cleaning the silicone oil on the surface of the thin film sample with detergent after contact polarization.

[0115] The analysis and testing are as follows:

[0116] (I) The phase transition thin films obtained in Example 1 were characterized using FTIR and DSC. FTIR was used to confirm the crystal form and polar phase content of the samples, while DSC was used to confirm the crystal type and crystallinity of the samples. The results are shown below. Figure 3 and Figure 4 . Figure 3 In the middle, the left figure shows the crystal type results of PVDF, and the right figure shows the crystal type results of PtBA-g-PVDF phase transition films.

[0117] Figure 3 In the PtBA-g-PVDF phase transition film sample, compared with PVDF, more γ spherulites with slightly smaller volume, no rings, and weaker birefringence appeared. The difference in the number of γ spherulites between the two is extremely significant, and the PtBA-g-PVDF phase transition film sample is more conducive to the growth of γ spherulites.

[0118] Figure 4 In the PtBA-g-PVDF phase transition film, the Fγ value first increases and then decreases with increasing annealing temperature Ts. Its crystallinity Xc is independent of annealing temperature, remaining relatively stable at around 60%, while the crystallization temperature slowly decreases with increasing annealing temperature. The bottom curve is the melting DSC curve of the phase transition film. The data shows that the maximum Fγ value of the PtBA-g-PVDF film is significantly higher than that of PVDF, indicating that grafting modification can effectively induce the transformation from the α phase to the γ phase. The PtBA-g-PVDF film of this invention can obtain a higher content of polar γ phase, F... γ It can reach 0.76. However, the content of polar γ-phase F in pure PVDF after annealing is... γ It is 0.49.

[0119] (II) PtBA-g-PVDF (PVDF-based polymer film), piezoelectric test results of PVDF are shown below. Figure 5 . Figure 5 In this context, the two are denoted as PT. 18 08-A, PVDF 18 -A. Figure 5 This is a graph showing the change in charge as a function of stress. The slope obtained from linear fitting is the piezoelectric coefficient value d. 33 It is not difficult to find that the piezoelectric coefficient of PtBA-g-PVDF polymer film (53.33 pC / N) is much higher than that of PVDF film (12.67 pC / N).

[0120] Preparation Example 2 - Preparation of PMMA-g-PVDF

[0121] Mix 5.0 g PVDF with 40 mL DMAC, heat to 60 °C with stirring, and maintain at this temperature for 6 h until the polymer is completely dissolved to obtain a polyvinylidene fluoride solution.

[0122] The polyvinylidene fluoride (PVDF) solution system was purged twice to remove oxygen and water. 0.06 g CuCl, 0.1 g PMDETA (pentamethyldivinyltriamine), and 30 g MMA (stabilizer removed by column chromatography with neutral alumina before use) were added to the PVDF solution. The system was purged twice more to ensure an oxygen-free environment. The temperature was raised to 100 °C with stirring, and the reaction was carried out at 100 °C for 3 h to obtain the reaction mixture.

[0123] The reaction mixture was diluted with 30 mL of THF, and PMMA-g-PVDF was precipitated in methanol solution. After removing the solution by filtration, the mixture was soaked in a large amount of methanol for 4 h to ensure the removal of small molecules and self-polymerized PMMA. The mixture was stirred during soaking, and after filtration, the product was redissolved in DMAC, precipitated again in methanol, and washed. The filtered sample was then vacuum dried in an oven at 80 °C for 24 h to obtain polyvinylidene fluoride material, denoted as PMMA-g-PVDF.

[0124] Example 2

[0125] The PMMA-g-PVDF polyvinylidene fluoride material prepared in Preparation Example 2 was dissolved in DMF to prepare a polymer solution of 20 mg / mL;

[0126] The polymer solution was spread onto the substrate by solution casting and dried to obtain an initial film with a thickness of less than 10 μm.

[0127] The prepared initial film was first isothermally treated at 200℃ for 5 min to eliminate thermal history, then cooled to below 140℃; next, it was heated to 175℃ for partial melting and recrystallization for 10 min, then cooled to below 80℃, and then heated to 120℃ for annealing for 24 h, and finally cooled to room temperature to obtain a phase transition film; the heating rate was 10℃ / min, and the cooling rate was 10℃ / min. A PVDF-based polymer film was obtained, denoted as PMMA-g-PVDF polymer film.

[0128] The analysis and testing are as follows:

[0129] The analysis and test results of the piezoelectric properties are shown in [the table]. Figure 6 .Depend on Figure 6 It is known that the piezoelectric coefficient of PMMA-g-PVDF polymer film can be as high as 80 pC / N.

[0130] Example 3

[0131] The polyvinylidene fluoride material (PtBA-g-PVDF) prepared in Preparation Example 1 was dissolved in N,N-dimethylformamide to prepare a polymer solution of 20 mg / ml.

[0132] The polymer solution was spread onto a glass substrate by solution casting and dried in a vacuum oven at 70°C for 24 hours to obtain the initial film.

[0133] The initial film was thermally annealed at 120℃ for 24 hours to obtain a PVDF-based film, denoted as PVDF(PtBA) film.

[0134] Example 4

[0135] The PMMA-g-PVDF polyvinylidene fluoride material prepared in Preparation Example 2 was dissolved in DMF to prepare a polymer solution of 20 mg / mL;

[0136] The polymer solution was spread onto the substrate by solution casting and dried to obtain the initial film.

[0137] The initial film was heat-annealed at 120℃ for 24 hours to obtain a PVDF-based film, denoted as PVDF (PMMA) film.

[0138] The PVDF-based thin films obtained in Examples 3 and 4 were analyzed and tested as follows:

[0139] Infrared spectrum Figure 7 .Depend on Figure 7 It can be known that 1276cm -1 The location corresponds to the polar β phase, while 1234 / 840cm -1 The peak at 796 cm⁻¹ corresponds to the polar γ phase, and no characteristic peak belonging to the α phase (796 cm⁻¹) was observed. -1 From this, we can know that the film exists in a polar phase (γ / β) under these conditions.

[0140] The piezoelectric data of the thin film products of Examples 3 and 4, as well as the thin film formed from PVDF, are shown in the figures below. Figure 8 , Figure 9 and Figure 10 .Depend on Figure 8 , Figure 9 and Figure 10 It can be seen that the piezoelectric output values ​​of the PVDF (PtBA) film product of Example 3 and the PVDF (PMMA) film product of Example 4 are 9.4V and 6V, respectively, while the piezoelectric output value of the film formed from pure PVDF is only 3V. Therefore, the PVDF-based film of the present invention has better piezoelectric properties.

[0141] This invention is not limited to the above-described embodiments. Any modifications, improvements, or substitutions that can be conceived by those skilled in the art without departing from the essential content of this invention fall within the scope of this invention.

Claims

1. A method for preparing a PVDF-based thin film, characterized in that, Includes the following steps: 1) The polyvinylidene fluoride material is formulated into a polymer solution; wherein the polyvinylidene fluoride material is formed by grafting a polyvinylidene fluoride with an olefin monomer containing a double bond; the olefin monomer contains an electron-withdrawing group; 2) Spread the polymer solution from step 1) onto the substrate and dry it to obtain the initial film; 3) The initial film is subjected to thermal annealing at 100-150℃ to obtain a PVDF-based film.

2. A method for preparing a PVDF-based polymer film, characterized in that, Includes the following steps: (1) The polyvinylidene fluoride material is formulated into a polymer solution; wherein the polyvinylidene fluoride material is formed by grafting a polyvinylidene fluoride with an olefin monomer containing a double bond; the olefin monomer contains an electron-withdrawing group; (2) Spread the polymer solution from step (1) onto the substrate and dry it to obtain the initial film; (3) The initial film is heated to eliminate thermal history at 185-250℃ and then cooled to below 140℃; then heated to 157-180℃ for partial melting and recrystallization, and then cooled to below 80℃; then heated to 100-140℃ for annealing to obtain a phase transformation film. (4) The phase transition film is subjected to contact polarization treatment to obtain a PVDF-based polymer film.

3. The preparation method according to claim 1 or 2, characterized in that, The olefin monomer is selected from at least one of acrylate, acrylamide, acrylonitrile, and styrene.

4. The preparation method according to claim 1 or 2, characterized in that, The polyvinylidene fluoride material is prepared by the following steps: (1') Mix polyvinylidene fluoride with a polar solvent to form a polyvinylidene fluoride solution; (2') The catalyst, olefin monomer and auxiliary ligand are added to the polyvinylidene fluoride solution and reacted to obtain a reaction mixture; (3') The reaction mixture is precipitated, washed and dried to obtain polyvinylidene fluoride material.

5. The preparation method according to claim 4, characterized in that, In step (1'), the polar solvent is selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethyl sulfoxide and acetone; the concentration of the polyvinylidene fluoride solution is 0.05 to 0.5 g / mL.

6. The preparation method according to claim 4, characterized in that, In step (2'), the catalyst is selected from cuprous chloride or cuprous bromide; the mass ratio of the catalyst to polyvinylidene fluoride is 0.005 to 0.1:1; the auxiliary ligand is selected from one of hexamethyltriethylenetetramine, 2,2-bipyridine and pentamethyldivinyltriamine; the mass ratio of the auxiliary ligand to polyvinylidene fluoride is 0.02 to 0.1:1; the mass ratio of the olefin monomer to polyvinylidene fluoride is 4 to 8:1; the reaction temperature is 100 to 140°C, the reaction time is 0.5 to 48 h; and the reaction is carried out in an oxygen-free environment.

7. The preparation method according to claim 4, characterized in that, In step (3'), the reaction mixture is precipitated with an aqueous solution of alcohol and washed with an alkane or alcohol; wherein the alcohol in the aqueous solution of alcohol is selected from at least one of methanol, ethanol and isopropanol; and the alkane is selected from at least one of n-hexane, cyclohexane and petroleum ether.

8. The preparation method according to claim 2, characterized in that, In step (3), the heating rate is 4 to 50 °C / min; in step (4), the electric field strength used for contact polarization is 20 to 400 MV / m.

9. A PVDF-based polymer film, characterized in that, The PVDF-based polymer film prepared according to the preparation method described in claim 2 has a higher γ phase content than α phase content. Its piezoelectric coefficient value d 33 It is above 40 pC / N.

10. Use of the PVDF-based polymer film according to claim 9 or the PVDF-based film prepared by the preparation method according to claim 1 in the preparation of electroactive materials.

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