Polyvinylidene fluoride based ferroelectric polymer and preparation method thereof

Abstract

The invention belongs to the field of flexible piezoelectric polymer materials, and discloses a polyvinylidene fluoride based ferroelectric polymer and a preparation method thereof. The preparation method comprises the following steps: (1) adding an inorganic alkaline reagent into a polyvinylidene fluoride-trifluoroethylene ferroelectric polymer solution, carrying out dehydrofluorination and nucleophilic addition-elimination reaction to generate a polymer with an enol structure, and then washing, precipitating and purifying a reaction product; and (2) re-dissolving by using an organic solvent, then forming a film by using a tape casting method, and carrying out ketone-enol tautomerization reaction in the film forming process of the tape casting method to finally prepare the polyvinylidene fluoride based ferroelectric polymer film. According to the effective modification and preparation method disclosed by the invention, an enol structure is promoted to be converted into a more stable ketone structure through a ketone-enol tautomerization reaction by utilizing a high-temperature environment in a film forming process, so that C = O double bonds are introduced into a polymer chain, and the piezoelectric property of the polymer is remarkably enhanced.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of flexible piezoelectric polymer materials, and more particularly, relates to a polyvinylidene fluoride-based ferroelectric polymer and a preparation method thereof. BACKGROUND

[0002] Polyvinylidene fluoride-based ferroelectric polymers have been widely used in various electronic and electromechanical devices in civil, medical, industrial and military fields due to their unique electrical and mechanical properties and lightweight advantages. However, the core problem restricting their further application in piezoelectric-related electronic devices is that their piezoelectric coefficients (d33) are generally low. For example, polyvinylidene fluoride (PVDF), which is the most widely used in current commercial applications, has a d33 value (~26 pC / N) that is still much lower than that of ceramic piezoelectric materials, despite extensive research over the past five decades. d 33 d 33

[0003] To improve the piezoelectric properties of polyvinylidene fluoride-based ferroelectric polymers, existing technologies have developed various piezoelectric property improvement methods, including the morphotropic phase boundary (MPB), high-field polarization, etc. In particular, since the MPB phenomenon was discovered in polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE) copolymer, its d33 value has been improved by more than twice, reaching 63.5 pC / N. In addition, existing research has reported that polyvinylidene fluoride-trifluoroethylene-fluorochloroethylene (P(VDF-TrFE-CFE) ternary copolymer modified by carbon-carbon double bond (C=C double bond) can obtain an ultra-high d33 value of more than 1000 pm / V under a specific constant electric field (40 MV / m), and its piezoelectricity is independent of the traditional non-centrosymmetric structure, similar to the phenomenon of inorganic piezoelectric oxide under electric field. However, the above methods, while improving piezoelectricity, also face certain limitations, such as the preparation of specific polymer components (50% / 50% molar ratio in P(VDF-TrFE)) materials due to the difference in reactivity between different monomers, which poses technical obstacles in precisely controlling the molar ratio between copolymer components, limiting their large-scale production; and the cyclic polarization method close to the material breakdown field strength (650 MV / m), which is strictly limited in sample size, usually only for laboratory research. Therefore, there are few reported methods that can achieve the preparation of polymer film materials with both large-scale production and high piezoelectric properties. d 33 d 33 SUMMARY

[0004] ​​​​In order to solve the above-mentioned defects or improvement needs of the prior art, the present application provides a polyvinylidene fluoride-based ferroelectric polymer and a preparation method thereof, which aims to introduce C=O double bond on the polymer chain to significantly enhance the effective modification and preparation method of the piezoelectric performance, thereby solving the technical problems that the piezoelectric performance of the polyvinylidene fluoride-based ferroelectric polymer is difficult to improve and the preparation is difficult to scale in the prior art.

[0005] In order to achieve the above-mentioned purpose, according to the first aspect of the present application, a preparation method of a polyvinylidene fluoride-based ferroelectric polymer is provided, which comprises the following steps: (1) polyvinylidene fluoride-trifluoroethylene ferroelectric polymer is dissolved in an organic solvent to obtain a polymer solution; an inorganic alkaline reagent is added to the polymer solution to perform dehydrofluorination and nucleophilic addition-elimination reaction to generate a polymer with an enol structure, and then the reaction product is purified; (2) the product prepared in step (1) is redissolved by an organic solvent, and then a film is formed by a casting method, and a ketone-enol tautomerization reaction occurs in the process of the casting method, and finally a polyvinylidene fluoride-based ferroelectric polymer film material is prepared.

[0006] As a preferred embodiment of the present application, in step (2), the temperature of the casting method is 160℃-260℃, and the time is 1h-12h.

[0007] As a preferred embodiment of the present application, in step (1), the molar content of vinylidene fluoride in the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer is 50%-85%.

[0008] As a preferred embodiment of the present application, in step (1), the organic solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, cyclohexanone or acetone.

[0009] As a preferred embodiment of the present application, in step (1), the inorganic alkaline reagent includes one of sodium hydroxide solution or potassium hydroxide solution.

[0010] As a preferred embodiment of the present application, in step (1), the dehydrofluorination and nucleophilic addition-elimination reaction is specifically: The polyvinylidene fluoride-trifluoroethylene ferroelectric polymer is mixed with the inorganic alkaline reagent, and the reaction is carried out at a temperature of 10℃-60℃ under air or inert atmosphere for 0.1h-24h.

[0011] As a preferred embodiment of the present application, in step (1), the molar ratio of the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer to the inorganic alkaline reagent is 100: (0.2-5), preferably 100: (0.6-1.5).

[0012] As a preferred embodiment of the present application, in step (1), the reaction product is purified by multiple precipitations, specifically: The product of the dehydrofluorination and nucleophilic addition-elimination reaction is washed by multiple precipitations in a mixed solution of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 1: (1-5).

[0013] As a preferred embodiment of the present application, in step (2), the organic solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, cyclohexanone or acetone.

[0014] According to a second aspect of the present application, there is provided a polyvinylidene fluoride-based ferroelectric polymer prepared by the method according to any one of the first aspect of the present application, and the structural formula of the polyvinylidene fluoride-based ferroelectric polymer is as follows: .

[0015] Overall, compared with the prior art, the above technical solutions conceived by the present application mainly have the following technical advantages.

[0016] 1. The present application provides a new and effective modification path for improving the piezoelectric performance of polyvinylidene fluoride-based ferroelectric polymers. Specifically, the present application uses inorganic strong alkaline reagents as modifiers, adds inorganic alkaline reagents to the polymer solution, and introduces carbon-oxygen (C=O) double bonds on the polyvinylidene-trifluoroethylene polymer chain through dehydrofluorination, nucleophilic addition-elimination reaction and ketone-enol tautomerism reaction in sequence, changes the relative stability between different conformations of polyvinylidene-trifluoroethylene, regulates the energy barrier in the morphotropic phase boundary (MPB) region, and realizes significant enhancement of the piezoelectric performance of the material. The prepared polyvinylidene-trifluoroethylene modified film material d 33 up to 74.8 pC / N, and d 33 The uniformity change is less than 5%. At the same time, the preparation of the present application is easy to realize large-scale production by the flow casting process, and the thickness uniformity change of the modified film material is less than 6% in the range of 20-80 μm. In addition, the piezoelectric response sensitivity of the piezoelectric sensor manufactured by the modified polyvinylidene-trifluoroethylene copolymer film material prepared by the method of the present application is improved by 58% and 70% compared with commercial polyvinylidene fluoride and polyvinylidene-trifluoroethylene, respectively, which is beneficial to its application in the fields of flexible electronics, wearable devices, etc.

[0017] 2. The present application preferably controls the temperature of the flow casting film forming process to ensure that the enol structure generated on the polymer chain is converted into the thermodynamically more stable ketone structure, i.e. the generation of C=O double bond.

[0018] 3、The inorganic alkaline reagent is added into the polymer solution to carry out dehydrofluorination and nucleophilic addition-elimination reaction, preferably the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer and the inorganic alkaline reagent are matched according to the molar ratio of 100:0.2 to 100:5, more preferably 100:0.6 to 100:1.5, the content of the introduced carbon-oxygen (C=O) double bond can be controlled by controlling the molar ratio of the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer and the inorganic alkaline reagent, so that the performance of the piezoelectric film can be effectively controlled. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The reaction mechanism diagram involved in the preparation method for improving the piezoelectric performance of the polyvinylidene fluoride-based ferroelectric polymer of the application.

[0020] Figure 2 The Fourier transform infrared spectrum (FTIR) of the polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymer raw material, the modified polyvinylidene fluoride-trifluoroethylene copolymer in Example 1 of the application and the modified polyvinylidene fluoride-trifluoroethylene copolymer in Comparative Example 1, wherein a is the wave number in the range of 1200 cm -1 ~1800 cm -1 , b is the wave number in the range of 1500 cm -1 ~1800 cm -1 .

[0021] Figure 3 The X-ray diffraction (XRD) spectrum of the polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymer raw material and the modified polyvinylidene fluoride-trifluoroethylene copolymer in Example 1 of the application.

[0022] Figure 4 The comparative data of the piezoelectric coefficient of the polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymer before modification in Examples 1-4 of the application.

[0023] Figure 5 The thickness uniformity data (a) and the piezoelectric coefficient data (b) of Examples 1, 5-8 of the application.

[0024] Figure 6 The atomic force microscope morphology test of the commercial polyvinylidene fluoride (a) in Comparative Example 6 and the product (b) of Example 1 of the application.

[0025] Figure 7 The dielectric constant and dielectric loss data with frequency change of the commercial polyvinylidene fluoride (a) in Comparative Example 6, the polyvinylidene fluoride-trifluoroethylene (b) in Comparative Example 5 and the film material (c) of Example 1 of the application.

[0026] Figure 8 Stress-strain curves of the commercial polyvinylidene fluoride in Comparative Example 6, polyvinylidene fluoride-trifluoroethylene in Comparative Example 5, and the film material of Example 1.

[0027] Figure 9 (a) Schematic diagram of the flexible sensor prepared based on the commercial polyvinylidene fluoride in Comparative Example 6, polyvinylidene fluoride-trifluoroethylene in Comparative Example 5, and the film material of Example 1, (b) piezoelectric sensitivity comparison, and (c) pressure-voltage signal of the flexible sensor prepared in Example 1. DETAILED DESCRIPTION

[0028] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and not used to limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0029] In the following examples, the instruments and the like not marked with the manufacturer are all conventional products that can be purchased through regular channels. The methods are all conventional methods unless otherwise specified, and the raw materials can be obtained from public commercial channels unless otherwise specified.

[0030] The present application provides a preparation method for improving the piezoelectric performance of polyvinylidene fluoride-based ferroelectric polymers. Inorganic strong alkaline reagents are used as modifiers to successfully introduce carbon-oxygen (C=O) double bonds on the polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) polymer chain, change the relative stability between different conformations of polyvinylidene fluoride-trifluoroethylene, regulate the energy barrier in the morphotropic phase boundary (MPB) region, and realize significant enhancement of the piezoelectric performance of the material.

[0031] As shown in the reaction mechanism shown in Figure 1 , the reaction mechanism mainly includes: The hydrogen atoms and fluorine atoms of the polyvinylidene fluoride unit on the polymer chain are removed by the inorganic strong alkaline reagent; the hydroxyl ion (OH - ) further replaces the fluorine atom on the olefin structure to generate an enol structure (-CH=C(OH)-). At this time, due to the keto-enol tautomerism reaction, a ketone structure (-C(=O)-CH2-) begins to appear in the solution; after purification of the reaction sample, film casting and high-temperature heating are performed to promote the transformation of the enol structure (-CH=C(OH)-) in the polymer into a more stable ketone structure (-C(=O)-CH2-), thereby introducing carbon-oxygen (C=O) double bonds in the polyvinylidene fluoride-based ferroelectric polymer.

[0032] The reaction process is mainly realized by the following steps: (1) Dissolving polyvinylidene fluoride-based ferroelectric polymer in an organic solvent to obtain a polymer solution; adding an inorganic strong alkaline reagent to the polymer solution to perform dehydrofluorination and nucleophilic addition-elimination reaction to generate an enol structure, and then washing and purifying the reaction product.

[0033] In the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer, the molar content of polyvinylidene fluoride is 50% to 85%.

[0034] (2) Redissolving the product of step (1) by using an organic solvent, and preparing a film material by using a casting method. In the process of film formation by the casting method, the carbon-oxygen (C=O) double bond defect is introduced by the ketone-enol tautomerism reaction under high temperature environment, so as to effectively enhance the response ability of the polyvinylidene fluoride-based ferroelectric polymer to external electric field or mechanical stress, thereby realizing excellent electromechanical coupling performance.

[0035] In the process of film formation by the casting method, the temperature is strictly controlled to ensure that the enol structure generated in advance on the polymer chain is converted into a more thermodynamically stable ketone structure, i.e. the generation of C=O double bond.

[0036] Preferably, in step (1), the inorganic alkaline reagent is added to the polymer solution to perform dehydrofluorination and nucleophilic addition-elimination reaction. The polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) is matched with the inorganic alkaline reagent at a molar ratio of 100:0.2 to 100:5, more preferably 100:0.6 to 100:1.5. The reaction is carried out under the atmosphere of air or inert gas (such as nitrogen, argon, helium and carbon dioxide gas) at a temperature of 10°C to 60°C for 0.1h to 24h.

[0037] The inorganic alkaline reagent can be dissolved in water in the form of an aqueous solution and blended with the polymer solution, including one of sodium hydroxide solution or potassium hydroxide solution.

[0038] Preferably, in step (1), the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer is uniformly dissolved. The organic solvent includes one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), cyclohexanone (CYC) or acetone.

[0039] Preferably, in step (1), the reaction product is further purified by multiple precipitation, specifically: The product of dehydrofluorination and nucleophilic addition-elimination reaction is washed multiple times in a mixed solution of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 1: (1-5).

[0040] Preferably, in step (2), the organic solvent includes one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), cyclohexanone (CYC), or acetone.

[0041] Preferably, in step (2), the film is formed by a casting method, and the ketone-enol tautomerization reaction occurs during the casting method, and finally a polyvinylidene fluoride-based ferroelectric polymer film with a C=O double bond is obtained.

[0042] The present application also provides a polyvinylidene fluoride-based ferroelectric polymer prepared by the above method for preparing a high-pressure ferroelectric polymer, and the structural formula is as follows: .

[0043] Preferably, the thickness of the polyvinylidene fluoride-based ferroelectric polymer film prepared by the above method is in the range of 20-80 μm, and the piezoelectric performance can be improved.

[0044] The present application will be further described in detail below with specific examples.

[0045] Example 1: 12 g of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio 55%) was added to 200 mL of dimethylformamide (DMF) to prepare a polymer solution with a concentration of 40 mg / mL. 6.65 mL of 0.01 g / mL NaOH solution (corresponding to a polymer to NaOH molar ratio of 100:1.0) was added to the solution. The reaction was stirred at 40°C at a rate of 300 rpm / min for 4 hours. After the reaction was completed, the supernatant was centrifuged and directly poured into a 1:3 ethanol / water mixed solution for washing and precipitation. The precipitated sample was redissolved in DMF, and the above washing and precipitation process was repeated twice. The last obtained precipitated sample was vacuum dried, redissolved in DMF, and stirred at 40°C at a rate of 300 rpm / min for 12 hours. Subsequently, different amounts of polymer solution were poured onto a glass plate of a fixed area to control the film thickness, and placed in an oven at 200°C for 4 hours to obtain a 20 μm carbon-oxygen (C=O) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material, and the piezoelectric coefficient d 33 =74.8 pC / N.

[0046] Example 2: Example 1 : 12 g of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio of 55%) was added to 200 mL of dimethylformamide (DMF) to configure a polymer solution with a concentration of 60 mg / mL. To this solution, 13.3 mL of NaOH solution with a concentration of 0.01 g / mL (corresponding to a polymer to NaOH molar ratio of 100:2.0) was added. The reaction was stirred at 40 °C at a rate of 300 rpm / min for 4 hours. After the reaction was completed, the supernatant was directly poured into a 1:3 ethanol / water mixed solution for precipitation. The precipitated sample was redissolved in DMF, and the above washing and precipitation process was repeated 2 times. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at 40 °C at a rate of 300 rpm / min for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate of a fixed area, and placed in an oven at 200 °C for 4 hours to obtain a carbon-oxygen (C=0) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a thickness of 20 μm, and a piezoelectric coefficient of =70.5 pC / N. d 33 =70.5 pC / N.

[0047] Example 3: 12 g of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio of 55%) was added to 200 mL of dimethylformamide (DMF) to configure a polymer solution with a concentration of 60 mg / mL. To this solution, 6.65 mL of NaOH solution with a concentration of 0.01 g / mL (corresponding to a polymer to NaOH molar ratio of 100:1.0) was added. The reaction was stirred at 30 °C at a rate of 300 rpm / min for 2 hours. After the reaction was completed, the supernatant was directly poured into a 1:4 ethanol / water mixed solution for washing and precipitation. The precipitated sample was redissolved in DMF, and the above washing and precipitation process was repeated 2 times. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at 40 °C at a rate of 300 rpm / min for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate of a fixed area, and placed in an oven at 240 °C for 6 hours to obtain a carbon-oxygen (C=0) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a thickness of 20 μm, and a piezoelectric coefficient of =74.4 pC / N. d 33 =74.4 pC / N.

[0048] Example 4: Example 1 : 12 g of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio 55%) was added to 200 mL of N,N-dimethylacetamide (DMAc) to configure a polymer solution with a concentration of 60 mg / mL. To this solution, 6.65 mL of NaOH solution with a concentration of 0.01 g / mL was added (corresponding to a polymer to NaOH molar ratio of 100: 1.0). The reaction was stirred at 40 °C at a rate of 300 rpm / min for 4 hours. After the reaction was completed, the supernatant was directly poured into a 1:3 mixed solution of ethanol / water for precipitation. The precipitated sample was redissolved in DMAc, and the washing and precipitation process described above was repeated 2 times. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMAc and stirred at 40 °C at a rate of 300 rpm / min for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate of a fixed area, and placed in an oven at 170 °C for 4 hours, finally obtaining a 20 μm carbon-oxygen (C=0) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient d 33 = 68.5 pC / N.

[0049] Example 5: 12 g of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio 55%) was added to 200 mL of dimethylformamide (DMF) to configure a polymer solution with a concentration of 60 mg / mL. To this solution, 6.65 mL of NaOH solution with a concentration of 0.01 g / mL was added (corresponding to a polymer to NaOH molar ratio of 100: 1.0). The reaction was stirred at 60 °C at a rate of 300 rpm / min for 4 hours. After the reaction was completed, the supernatant was directly poured into a 1:3 mixed solution of ethanol / water for precipitation. The precipitated sample was redissolved in DMF, and the washing and precipitation process described above was repeated 2 times. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at 40 °C at a rate of 300 rpm / min for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate of a fixed area, and placed in an oven at 220 °C for 6 hours, finally obtaining a 30 μm carbon-oxygen (C=0) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient d 33 = 74.1 pC / N.

[0050] Example 6: 12 g of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), 55% VDF molar ratio) was added to 200 mL of dimethylformamide (DMF) to prepare a polymer solution with a concentration of 80 mg / mL. A 0.01 g / mL NaOH solution was added to this solution, maintaining the same molar ratio of NaOH to polymer as in Example 1. The reaction was stirred at 40°C at 300 rpm / min for 4 hours. After the reaction, the solution was centrifuged and the supernatant was directly poured into a 1:3 ethanol / water mixture for precipitation. The precipitate was redissolved in DMF, and the washing and precipitation process was repeated twice. The final precipitate was vacuum dried, redissolved in DMF, and stirred at 40°C at 300 rpm / min for 12 hours. Subsequently, the film thickness was controlled by pouring different amounts of polymer solution onto a glass plate of a fixed area, and the film was placed in an oven at 200°C for 4 hours to finally obtain a 40 μm carbon-oxygen (C=O) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient of d 33 =74.2 pC / N.

[0051] Example 7: 12 g of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), 55% VDF molar ratio) was added to 200 mL of dimethylformamide (DMF) to prepare a polymer solution with a concentration of 100 mg / mL. A 0.01 g / mL NaOH solution was added to this solution, maintaining the same molar ratio of NaOH to polymer as in Example 1. The reaction was stirred at 40°C at 300 rpm / min for 4 hours. After the reaction, the solution was centrifuged and the supernatant was directly poured into a 1:3 ethanol / water mixture for precipitation. The precipitate was redissolved in DMF, and the washing and precipitation process was repeated twice. The final precipitate was vacuum dried, redissolved in DMF, and stirred at 40°C at 300 rpm / min for 12 hours. Subsequently, the film thickness was controlled by pouring different amounts of polymer solution onto a glass plate of a fixed area, and the film was placed in an oven at 200°C for 4 hours to finally obtain a 50 μm carbon-oxygen (C=O) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient of d 33=74.3 pC / N.

[0052] Example 8: 12 g of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), 55% VDF molar ratio) was added to 200 mL of cyclohexanone to prepare a polymer solution with a concentration of 120 mg / mL. To this solution, a 0.01 g / mL NaOH solution was added, maintaining the molar ratio of NaOH to polymer consistent with that in Example 1. The reaction was stirred at 40°C at 300 rpm / min for 4 hours. After the reaction, the mixture was centrifuged and the supernatant was directly poured into a 1:3 ethanol / water mixture for precipitation. The precipitate was redissolved in DMF, and the washing and precipitation process was repeated twice. The final precipitate was vacuum dried, redissolved in DMF, and stirred at 40°C at 300 rpm / min for 12 hours. Subsequently, the film thickness was controlled by pouring different amounts of polymer solution onto a glass plate of a fixed area, and the film was placed in an oven at 200°C for 4 hours to finally obtain a 60 μm carbon-oxygen (C=O) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film with a piezoelectric coefficient of d 33 =74.3 pC / N.

[0053] Example 9: 12 g of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), VDF molar ratio 85%) was added to 200 mL of dimethylformamide (DMF) to prepare a polymer solution with a concentration of 40 mg / mL. To this solution, 6.65 mL of a 0.01 g / mL NaOH solution (corresponding to a polymer:NaOH molar ratio of 100:0.9) was added. The reaction was stirred at 40°C at 300 rpm / min for 4 hours. After the reaction, the mixture was centrifuged and the supernatant was directly poured into a 1:3 ethanol / water mixture to wash the precipitate. The precipitate was redissolved in DMF, and the washing process was repeated twice. The final precipitate was vacuum dried, redissolved in DMF, and stirred at 40°C at 300 rpm / min for 12 hours. Subsequently, the film thickness was controlled by pouring different amounts of polymer solution onto a glass plate of a fixed area, and the film was placed in an oven at 200°C for 4 hours to finally obtain a 20 μm carbon-oxygen (C=O) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film with a piezoelectric coefficient of d 33=28.4 pC / N.

[0054] Example 10: 12 g of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), 80% VDF molar ratio) was added to 200 mL of dimethylformamide (DMF) to prepare a polymer solution with a concentration of 40 mg / mL. To this solution, 6.65 mL of a 0.01 g / mL NaOH solution (corresponding to a polymer:NaOH molar ratio of 100:0.9) was added. The reaction was stirred at 40°C at 300 rpm / min for 4 hours. After the reaction, the mixture was centrifuged and the supernatant was directly poured into a 1:3 ethanol / water mixture to wash the precipitate. The precipitate was redissolved in DMF, and the washing process was repeated twice. The final precipitate was vacuum dried, redissolved in DMF, and stirred at 40°C at 300 rpm / min for 12 hours. Subsequently, the film thickness was controlled by pouring different amounts of polymer solution onto a glass plate of a fixed area, and the film was placed in an oven at 200°C for 4 hours to finally obtain a 20 μm carbon-oxygen (C=O) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film with a piezoelectric coefficient of d 33 =34.3 pC / N.

[0055] Example 11: 12 g of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE), VDF at a molar ratio of 50%) was added to 200 mL of dimethylformamide (DMF) to prepare a polymer solution with a concentration of 40 mg / mL. To this solution, 6.65 mL of a 0.005 g / mL NaOH solution (corresponding to a polymer:NaOH molar ratio of 100:0.5) was added. The reaction was stirred at 40°C at 300 rpm / min for 4 hours. After the reaction, the mixture was centrifuged and the supernatant was directly poured into a 1:3 ethanol / water mixture to wash the precipitate. The precipitate was redissolved in DMF, and the washing process was repeated twice. The final precipitate was vacuum dried, redissolved in DMF, and stirred at 40°C at 300 rpm / min for 12 hours. Subsequently, the film thickness was controlled by pouring different amounts of polymer solution onto a glass plate of a fixed area, and the film was placed in an oven at 200°C for 4 hours to finally obtain a 20 μm carbon-oxygen (C=O) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film with a piezoelectric coefficient of d33 = 57.4 pC / N.

[0056] Example 12: Example 12: 12 g of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio of 65%) was added into 200 mL of dimethylformamide (DMF) to configure a polymer solution with a concentration of 40 mg / mL. To this solution, 6.65 mL of NaOH solution with a concentration of 0.05 g / mL (corresponding to a polymer to NaOH molar ratio of 100:4.9) was added. The reaction was stirred at a rate of 300 rpm / min at 40 °C for 4 hours. After the reaction was completed, the supernatant was directly poured into a 1:3 mixed solution of ethanol / water for washing and precipitation. The precipitated sample was redissolved in DMF, and the washing and precipitation process was repeated twice. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at a rate of 300 rpm / min at 40 °C for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate with a fixed area, and placed in an oven at 200 °C for 4 hours to obtain a carbon-oxygen (C=0) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a thickness of 20 μm, and a piezoelectric coefficient of d 33 = 51.4 pC / N.

[0057] Comparative Example 1: Example 12: 12 g of polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio of 65%) was added into 200 mL of dimethylformamide (DMF) to configure a polymer solution with a concentration of 40 mg / mL. To this solution, 6.65 mL of NaOH solution with a concentration of 0.05 g / mL (corresponding to a polymer to NaOH molar ratio of 100:4.9) was added. The reaction was stirred at a rate of 300 rpm / min at 40 °C for 4 hours. After the reaction was completed, the supernatant was directly poured into a 1:3 mixed solution of ethanol / water for washing and precipitation. The precipitated sample was redissolved in DMF, and the washing and precipitation process was repeated twice. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at a rate of 300 rpm / min at 40 °C for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate with a fixed area, and placed in an oven at 200 °C for 4 hours to obtain a carbon-oxygen (C=0) double bond modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a thickness of 20 μm, and a piezoelectric coefficient of d 33= 45.5 pC / N.

[0058] Comparative Example 2: P(VDF-TrFE) (VDF molar ratio 55%) was added to 200 mL of cyclohexanone to make a polymer solution with a concentration of 120 mg / mL. To this solution, a NaOH solution with a concentration of 0.005 g / mL (corresponding to a polymer to NaOH molar ratio of 100:0.5) was added. The reaction was stirred at 40 °C at a rate of 300 rpm / min for 1.0 hour. After the reaction was completed, the supernatant was directly poured into a 1:3 ethanol / water mixed solution for precipitation. The precipitated sample was redissolved in DMF, and the washing and precipitation process was repeated 2 times. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at 30 °C at a rate of 300 rpm / min for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate with a fixed area, and placed in an oven at 50 °C for 4 hours, and finally obtained 20 μm modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient d 33 = 25.5 pC / N.

[0059] Comparative Example 3: P(VDF-TrFE) (VDF molar ratio 55%) was added to 200 mL of cyclohexanone to make a polymer solution with a concentration of 120 mg / mL. To this solution, a NaOH solution with a concentration of 0.001 g / mL (corresponding to a polymer to NaOH molar ratio of 100:0.1) was added. The reaction was stirred at 20 °C at a rate of 300 rpm / min for 0.5 hour. After the reaction was completed, the supernatant was directly poured into a 1:3 ethanol / water mixed solution for precipitation. The precipitated sample was redissolved in DMF, and the washing and precipitation process was repeated 2 times. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at 40 °C at a rate of 300 rpm / min for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution onto a glass plate with a fixed area, and placed in an oven at 170 °C for 4 hours, and finally obtained 20 μm modified polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient d 33 =40.5 pC / N.

[0060] Comparative Example 4: To 200 mL of cyclohexanone, 12 g of polyvinylidene-fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio of 55%) was added to configure a polymer solution with a concentration of 120 mg / mL. To this solution, a NaOH solution with a concentration of 0.005 g / mL (corresponding to a polymer to NaOH molar ratio of 100:0.5) was added. The reaction was stirred at 40 °C at a rate of 300 rpm / min for 0.5 hours. After the reaction was completed, the supernatant was directly poured into a 1:3 ethanol / water mixed solution for precipitation. The precipitated sample was redissolved in DMF, and the washing and precipitation process was repeated 2 times. After the last obtained precipitated sample was vacuum dried, it was redissolved in DMF and stirred at 40 °C at a rate of 300 rpm / min for 12 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution on a glass plate with a fixed area, and placed in an oven at 90 °C for 4 hours, and finally obtained a 20 μm modified polyvinylidene-fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient d 33 = 34.5 pC / N.

[0061] Comparative Example 5: To 200 mL of cyclohexanone, 12 g of polyvinylidene-fluoride-trifluoroethylene (P(VDF-TrFE), VDF molar ratio of 55%) was added to configure a polymer solution with a concentration of 120 mg / mL. The reaction was stirred at 40 °C at a rate of 300 rpm / min for 0.5 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution on a glass plate with a fixed area, and placed in an oven at 120 °C for 8 hours, and finally obtained a 20 μm polyvinylidene-fluoride-trifluoroethylene (P(VDF-TrFE)) piezoelectric film material with a piezoelectric coefficient d 33 = 41.0 pC / N.

[0062] Comparative Example 6: To 200 mL of cyclohexanone, 12 g of commercial PVDF was added to configure a polymer solution with a concentration of 120 mg / mL. The reaction was stirred at 40 °C at a rate of 300 rpm / min for 0.5 hours. Subsequently, the film thickness was regulated by pouring different amounts of polymer solution on a glass plate with a fixed area, and placed in an oven at 120 °C for 8 hours, and finally obtained a 20 μm commercial PVDF piezoelectric film material.

[0063] The commercial polyvinylidene fluoride (PVDF) used in the embodiments of the present application was purchased from PolyK; other polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) with different VDF content were all powders purchased from Arkema. The above-mentioned raw materials were modified by the following examples, and the raw materials and the modified polymer materials were characterized for performance and tested for piezoelectric properties.

[0064] The structural characterization tests of the present application were Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XR), wherein the FTIR test was measured by a spectrometer in ATR mode. The piezoelectric property test of the present application was a quasi-static d 33 Tester, and specifically, each raw and modified polymer was measured by a dynamic force of 0.25 N at a frequency of 110 Hz under the condition of applying a static force of 1 N.

[0065] Figure 2 The FTIR of P(VDF-TrFE) raw material, modified P(VDF-TrFE) in Example 1 of the present application, and modified P(VDF-TrFE) in Comparative Example 1 is shown. Figure 2 It is shown that the modified P(VDF-TrFE) has a new characteristic peak at 1733 cm -1 nearby, which corresponds to the stretching vibration of carbon-oxygen double bond (C=O), and this peak is not observed in the raw P(VDF-TrFE). This fully proves that the method of the present application successfully introduces C=O double bond in P(VDF-TrFE). In addition, compared with Comparative Example 1, the C=O peak intensity of Example 1 of the present application is higher, indicating that the reaction temperature can more effectively promote the generation of C=O double bond.

[0066] Figure 3 The XRD patterns of P(VDF-TrFE) raw material and modified P(VDF-TrFE) in Example 1 of the present application are shown, wherein the raw P(VDF-TrFE) has a strong diffraction peak at 19.3°, which corresponds to its all-trans conformation. After modification by the present application, the diffraction peak intensity corresponding to 3 / 1 helical conformation at 18.9° is significantly increased. This change shows that by introducing carbon-oxygen (C=O) double bond defects, the relative stability between different conformations of P(VDF-TrFE) is effectively regulated, the energy barrier of the morphotropic phase boundary (MPB) region is changed, and thus the piezoelectric properties of the material are significantly improved.

[0067] Figure 4 The piezoelectric property test data of the above-mentioned Examples 1-4 and raw material corresponding polymers are shown. d 33The yield was significantly improved compared to the raw material. In Example 1 (0.9 mol% NaOH, reaction at 40°C), d 33 The value was improved from about 41.0 pC / N of the raw material to about 74.8 pC / N. The results determined that the concentration of the strong alkali reagent (1.0 mol% NaOH) and the reaction temperature and film forming temperature were the preferred conditions. In addition, compared with other examples, the method of the application effectively avoids harsh reaction conditions while achieving high piezoelectric performance, and the preparation process is simpler.

[0068] Figure 5 For the thickness (a) and piezoelectric coefficient (b) uniformity comparison of Examples 1, 5-8 of the application, the thickness uniformity in the range of 20-80 μm changed by less than 6%, d 33 changed by less than 5%, and d 33 maintained good consistency under different thickness samples; Figure 6 The morphology of the commercial PVDF (a) in Comparative Example 6 of the application and the film material of Example 1 (b) was observed under an atomic force microscope. Compared with the commercial PVDF film material (a) with rough surface after stretching, the surface morphology of the modified film material (b) of Example 1 was more flat. It shows that the modification method of the application optimizes the piezoelectric performance without introducing significant surface defects, which helps to improve the surface quality of the film material.

[0069] Figure 7 The dielectric constant and dielectric loss of the commercial PVDF (a) in Comparative Example 6 of the application, the original P(VDF-TrFE) (b) in Comparative Example 5 and the film material (c) of Example 1 as a function of frequency, wherein Example 1 exhibits higher dielectric constant in a wide frequency range. The increase in dielectric constant directly corresponds to the increase in capacitance, which demonstrates its great potential in device applications. Figure 8 The stress-strain curve of the commercial PVDF (a) in Comparative Example 6 of the application, the original P(VDF-TrFE) (b) in Comparative Example 5 and the film material (c) of Example 1, the elongation at break is improved by 152% and 55% respectively compared with the commercial PVDF and the original P(VDF-TrFE). Thus, the material prepared by the application is superior to the existing commercial materials in the comprehensive performance of machine-electric coupling performance, flexibility and processability, etc.

[0070] Figure 9is a schematic diagram of the flexible sensor prepared by the film material of Example 1 based on the commercial PVDF in Comparative Example 6, the original P(VDF-TrFE) in Comparative Example 5, (a), piezoelectric sensitivity comparison (b), and the pressure-voltage signal of the flexible sensor prepared in Example 1 (c), wherein the piezoelectric response sensitivity of Example 1 is improved by 58% and 70% compared with the commercial PVDF and the original P(VDF-TrFE), respectively.

[0071] In summary, the present application promotes the removal of hydrogen atoms and fluorine atoms on the polymeric chain of vinylidene fluoride units by inorganic strong alkaline reagent; the hydroxyl ion (-OH-) forms enolic structure (-CH=C(OH)-) through nucleophilic addition-elimination reaction, at this time, due to the keto-enol tautomerism reaction, the ketone structure (-C(=O)-CH2-) begins to appear in the solution; after the purification of the reaction sample, the film is formed by casting and heated at high temperature, which promotes the transformation of enolic structure (-CH=C(OH)-) in the polymer into more stable ketone structure (-C(=O)-CH2-) in the molten state, thereby introducing carbon-oxygen (C=O) double bond in the polyvinylidene fluoride-based ferroelectric polymer. The modification method can significantly improve the piezoelectric properties of P(VDF-TrFE) polymer, and the piezoelectric coefficient is greatly improved compared with the unmodified sample. The P(VDF-TrFE) modified film prepared by the present application d 33 can reach 74.8 pC / N, and d 33 The uniformity change is less than 5%. The thickness uniformity change of the modified film material in the range of 20-80 μm is less than 6%, which is easy to realize large-scale production by casting process. In addition, the piezoelectric response sensitivity of the piezoelectric sensor manufactured by the modified P(VDF-TrFE) film prepared by the method of the present application is improved by 58% and 70% compared with the commercial PVDF and the original P(VDF-TrFE), respectively, which can be widely used in the fields of flexible electronics, wearable devices, etc.

[0072] Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application belong to the scope of the present application and its equivalent technology, the present application also intends to include these modifications and variations. The above-described examples are only preferred examples for fully illustrating the present application, and the protection scope is not limited thereto. The equivalent substitutions or transformations made by those skilled in the art on the basis of the present application are within the protection scope of the present application.

Claims

1. A method for preparing a polyvinylidene fluoride-based ferroelectric polymer, characterized in that: The following steps are involved: (1) dissolving a polyvinylidene fluoride-trifluoroethylene ferroelectric polymer in an organic solvent to obtain a polymer solution; adding an inorganic alkaline reagent to the polymer solution to carry out dehydrofluorination and nucleophilic addition-elimination reaction to generate a polymer with an enol structure, and then purifying the reaction product; (2) The product obtained in step (1) is redissolved in an organic solvent and then formed into a film by a casting method. During the film forming process of the casting method, a keto-enol tautomerism reaction occurs, and finally a polyvinylidene fluoride-based ferroelectric polymer film material is prepared.

2. The method for preparing the polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (2), the temperature of the film-forming method by the casting method is 160° C. to 260° C., and the time is 1 h to 12 h.

3. The method for preparing the polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (1), the molar content of vinylidene fluoride in the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer is 50% to 85%.

4. The method for preparing the polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (1), the organic solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, cyclohexanone or acetone.

5. The method for preparing a polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (1), the inorganic alkaline reagent includes one of a sodium hydroxide solution or a potassium hydroxide solution.

6. The method for preparing the polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (1), the dehydrofluorination and nucleophilic addition-elimination reaction is specifically: The polyvinylidene fluoride-trifluoroethylene ferroelectric polymer is mixed with the inorganic alkaline reagent and reacted in air or an inert atmosphere at a temperature of 10° C. to 60° C. for 0.1 h to 24 h.

7. The method for preparing a polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (1), the molar ratio of the polyvinylidene fluoride-trifluoroethylene ferroelectric polymer to the inorganic alkaline reagent is 100: (0.2~5), preferably 100: (0.6~1.5).

8. The method for preparing a polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (1), the reaction product is subjected to multiple precipitation purification steps as follows: The product of the dehydrofluorination and nucleophilic addition-elimination reaction is washed and precipitated multiple times in a mixed solution of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 1: (1-5).

9. The method for preparing a polyvinylidene fluoride-based ferroelectric polymer according to claim 1, wherein: In step (2), the organic solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, cyclohexanone or acetone.

10. The polyvinylidene fluoride-based ferroelectric polymer obtained by the preparation method according to any one of claims 1 to 9, characterized in that: The structural formula of the polyvinylidene fluoride ferroelectric polymer is as follows: 。

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