A polyvinylidene fluoride high-crystallinity oriented polar crystal phase, a preparation method and application thereof

Abstract

The application discloses a kind of polyvinylidene fluoride high crystallinity orientation polar crystal phase, its preparation method and application.The preparation method is by polyvinylidene fluoride particle raw material is placed in mould, first temperature is raised to above melting point and is maintained to eliminate thermal history, temperature is reduced to forming temperature 150~260 DEG C, the pressure of 50MPa~600MPa is applied immediately after being applied above shear flow field, shear flow field is applied for more than 10s, then constant pressure is reduced to sample crystallization is completed, temperature is reduced to room temperature after waiting, unloading pressure, obtain the polyvinylidene fluoride sample containing high crystallinity orientation polar crystal phase.The polyvinylidene fluoride product prepared by the method, polar crystal phase crystallinity is as high as 63%, and presents good orientation, crystalline morphology is shish-kebab structure, and related polyvinylidene fluoride sample shows excellent piezoelectric performance.The preparation method of the PVDF high crystallinity orientation polar crystal phase provided by the application is helpful to promote the application of PVDF product in piezoelectric flexible sensor field.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of polymer material processing, and specifically relates to a polyvinylidene fluoride high-crystallinity oriented polar crystal phase, a preparation method thereof and application thereof in the field of flexible sensors. BACKGROUND

[0002] Polyvinylidene fluoride (PVDF) is a typical crystalline polymer, which has corrosion resistance, high temperature resistance, and oxidation resistance, etc., and has a wide range of applications in lithium battery binder separators, building coatings, packaging films for photovoltaic power generation, water treatment membranes, etc. At the same time, PVDF has good piezoelectric, dielectric and thermoelectric properties, and has very broad application prospects in sensors and energy storage devices [Costa, C. M.; Cardoso Martins, P.; Correia, D. M. et al. J. Chem. Rev. Sci. 2023 ,123,19, 11392-11487.]. High-crystallinity oriented polar crystal phase is a prerequisite for the application of PVDF in sensors and energy storage devices.

[0003] PVDF can crystallize to form five different crystal phases, among which β , γ and δ are polar crystal phases. However, under normal processing conditions, PVDF crystallizes to form non-polar α phase. At present, the common method for preparing PVDF polar crystal phase is to first cast a PVDF film, and then to prepare it by stretching and annealing, including mechanical stretching [Ye, Q. Y.; Sun, J. H.; Huang, C. X.; Huang, H. D. et al. J. Macromolecules. Sci. 2024 ,57,8, 3671-3686.], quenching and annealing heat treatment [Hess, C. M.; Rudolph, A. R.; Reid, P. J. J. Phys. Chem. B. Sci. 2015 ,119,10, 4127-4132.], high-pressure crystallization [Ren, J. Y.; Zhang, G. Q.; Li, Y. et al. J. Polymer. Sci. 2018, 158204-212.], introduction of polar interaction such as introduction of nucleating agent or solution [Wang, J. C.; Fu, Q.; Zhang, Q. J . Polymer . Scisuch as the methods disclosed in the following documents: "Preparation of PVDF polar crystal phase by melt crystallization", Journal of Applied Polymer Science, 2012, 5455-5458, "Preparation of PVDF polar crystal phase by melt crystallization", Journal of Applied Polymer Science, 2012, 5455-5458, and "Preparation of PVDF polar crystal phase by melt crystallization", Journal of Applied Polymer Science, 2012, 5455-5458. The mechanical stretching can make the PVDF molecular chain or segment arrange orderly in a certain direction, which can induce the PVDF copolymer to transform from the disordered, star-shaped nanocrystal into the ordered, self-polarized fiber chain-shaped nanocrystal. However, the crystallinity of the PVDF oriented polar crystal phase prepared by the mechanical stretching is low, and the mechanical stretching of the PVDF is prone to introduce defects such as cracks and voids, which can cause the deterioration of the mechanical properties of the PVDF polar crystal phase material. Although the quenching heat treatment, high-pressure crystallization, and introduction of the polar interaction can obtain the polar crystal phase with high crystallinity, the polar crystal phase prepared by the above-mentioned methods is not oriented, and the process of the above-mentioned methods is complex, which can also affect other properties such as the mechanical properties.

[0004] In summary, the existing preparation methods of the PVDF polar crystal phase have the problems of low crystallinity, easy introduction of defects, and no orientation of the obtained PVDF polar crystal phase, which are difficult to prepare the PVDF high-crystallinity oriented polar crystal phase. SUMMARY

[0005] In order to solve the problems in the prior art, the present application provides a method for preparing a PVDF high-crystallinity oriented polar crystal phase, that is, the PVDF high-crystallinity oriented polar crystal phase is prepared by applying the pressure, temperature and flow field to the melt PVDF, so as to overcome the problems of low crystallinity, easy introduction of defects such as cracks and voids after the mechanical stretching, and no orientation of the polar crystal phase of the PVDF polar crystal phase prepared by the existing preparation methods.

[0006] In order to achieve the above-mentioned purpose, the present application provides the following technical scheme:

[0007] The present application utilizes the coupling effect of the pressure, temperature and flow field, and the PVDF crystallization directly obtains the high-crystallinity oriented polar crystal under the combined action of the pressure, temperature and shear flow field, and the crystallinity of the polar crystal phase is as high as about 63%. The melt preparation method of the PVDF high-crystallinity oriented polar crystal phase comprises the following specific steps:

[0008] S1, the PVDF raw material is added to the mold;

[0009] S2, the mold filled with the PVDF raw material is heated to above the melting point and maintained, and then the temperature of the melt PVDF raw material is adjusted;

[0010] S3, the pressure is applied to the PVDF, and then the shear flow field is immediately applied to the PVDF for orientation, and then the PVDF is cooled under the pressure, and the pressure is removed after the temperature is reduced to room temperature, to obtain the PVDF sample with the high-crystallinity oriented polar crystal phase.

[0011] Further, in step S2, the PVDF raw material is heated to 30-50 DEG C above its melting point and maintained for 5-10 minutes.

[0012] Further, in step S2, the temperature of the melted PVDF raw material is adjusted to 150-260 DEG C.

[0013] Further, in step S3, the cooled PVDF raw material is subjected to a pressure in the range of 50-600 MPa. The application of a pressure field to the PVDF melt promotes the generation of polar crystal phases of PVDF.

[0014] Further, in step S3, the PVDF raw material subjected to the pressure is immediately subjected to a shear flow field, with a shear rate of 100-10000 s-1. The flow field is applied for more than 10 seconds. The application of a flow field to the PVDF melt promotes the orientation of molecular chains and improves the crystallinity of polar crystal phases of PVDF.

[0015] Further, in step S3, after the PVDF raw material is subjected to the shear flow field, it is subjected to a constant-pressure cooling process, and then the pressure is released when the temperature drops to room temperature, to obtain a PVDF sample containing high-crystallinity oriented polar crystal phases.

[0016] During the orientation process, the molecular chains or segments of the melted PVDF are arranged along the direction of the shear flow field under the action of external shear stress, which promotes the arrangement of dipoles in the same direction, generating oriented polar crystal phases and reducing the generation of defects such as cracks and voids. The ordered arrangement of molecular chains or segments of the oriented PVDF makes the arrangement of dipoles more ordered, further enhancing the overall polarity of the material.

[0017] The application also discloses the use of the PVDF high-crystallinity oriented polar crystal phases obtained by the above preparation method in the field of piezoelectric flexible sensors.

[0018] The PVDF piezoelectric flexible sensor is obtained by sequentially pasting electrode sheets and insulating films on both sides of the PVDF high-crystallinity oriented polar crystal phase sample, and can respond to mechanical stress to obtain electrical signals. The electrode sheets pasted on both sides of the PVDF sample are conductive copper foils or silver foils, and the insulating films are polyimide or polyethylene films.

[0019] Compared with the prior art, the application has at least the following beneficial effects:

[0020] The melt preparation method of the polyvinylidene fluoride high-crystallinity oriented polar crystal provided by the application promotes the molecular chain orientation by using the flow field, and improves the crystallinity of the PVDF polar crystal phase; the thermodynamic conditions for forming the PVDF polar crystal phase can be provided by adjusting the temperature and pressure field, and the shish-kebab structure of the oriented polar crystal phase of the PVDF can be induced by applying the flow field, so that the PVDF product with enhanced strength, modulus, transparency and corrosion resistance is prepared. Compared with the existing preparation method of the PVDF oriented polar crystal phase, i.e., the first flow casting and then stretching and annealing, the PVDF sample with the high-crystallinity oriented polar crystal phase can be directly prepared by the coupling of the temperature field, the pressure field and the flow field according to the application, and the stretching and annealing of the PVDF oriented polar crystal after film forming is not needed, so that the defects in the material are avoided, the crystallinity of the PVDF polar crystal phase is improved, and the preparation process is simpler. The shish-kebab structure of the oriented crystal of the PVDF is generated by the synergistic effect of the pressure and the flow field without adding any nucleating agent or solvent, so that the PVDF generates the high-crystallinity oriented polar crystal phase, thereby ensuring the piezoelectric performance of the PVDF. On the one hand, the generation of the shish-kebab oriented structure can greatly improve the mechanical properties of the PVDF, and on the other hand, the polar crystal phase also improves the piezoelectric performance of the PVDF material, so that the PVDF can be applied to the field of flexible sensors. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The structure diagram of the self-made pressure shear flow field mold;

[0022] Figure 2 The wide-angle X-ray diffraction curve of the polyvinylidene fluoride sample under the combined action of the temperature (150℃-200℃), the pressure (50MPa-600MPa) and the flow field (26-30 ) of the application examples 1-9;

[0023] Figure 3 The wide-angle X-ray diffraction curve of the polyvinylidene fluoride sample under the temperature (150℃-200℃) and the pressure (50MPa-500MPa) without the flow field of the comparative examples 1-5;

[0024] Figure 4 The azimuthal angle distribution curve of the small-angle X-ray scattering of the polyvinylidene fluoride crystalline sample under the combined action of the temperature (150℃-200℃), the pressure (50MPa-600MPa) and the flow field (26-30 ) of the application examples 1-9;

[0025] Figure 5Azimuthal distribution curve of small angle X-ray scattering of polyvinylidene fluoride crystalline sample under the temperature (150℃-200℃), pressure (50MPa-500MPa) and no flow field of comparative example 1-5 of the present application;

[0026] Figure 6 Scanning electron microscope photograph of quenched section of polyvinylidene fluoride crystalline sample under the temperature (180℃), pressure (100MPa) and flow field (27 ) of example 4 of the present application;

[0027] Figure 7 Scanning electron microscope photograph of quenched section of polyvinylidene fluoride crystalline sample under the temperature (180℃), pressure (100MPa) and flow field (10 ) of comparative example 7 of the present application;

[0028] Figure 8 Piezoelectric signal diagram of polyvinylidene fluoride film under the temperature (180℃), pressure (100MPa) and flow field (27 ) of example 4 of the present application;

[0029] Figure 9 Piezoelectric signal diagram of polyvinylidene fluoride film under the temperature (180℃), pressure (100MPa) and no flow field of comparative example 2 of the present application; DETAILED DESCRIPTION

[0030] In order to make the personnel in the art better understand the present application scheme, the technical scheme in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the person of ordinary skill in the art without making creative labor should belong to the scope of protection of the present application.

[0031] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or a chronological sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0032] The PVDF sample prepared by the application improves the content of the polar crystal phase of the PVDF material and makes the polar crystal phase oriented, so that the PVDF has more excellent piezoelectric response performance in the field of flexible sensors, and the most prominent feature is that the high-crystallinity oriented polar crystal phase has more excellent mechanical properties, thermal stability and transparency, so that the PVDF flexible sensor can play a role under more severe working conditions.

[0033] The application will be further described below in combination with the drawings and specific embodiments:

[0034] The high-crystallinity oriented polar crystal of polyvinylidene fluoride, its preparation method and application of the application have the following specific steps:

[0035] S1, the PVDF raw material (the manufacturer is Shanghai Huayi Sanai New Material Co., Ltd.) is added to the self-made pressure shear flow field mold (Yang, S.G.; Zhang, Z.C.; Zhang, L.Q.; Zhou, D. et al. J. Polym. Chem., 2015 ,6, 4588-4596), the mold structure is shown in the accompanying drawings Figure 1 ;

[0036] S2, the PVDF raw material is heated to 30-50 DEG C higher than its melting point and maintained for 5-10 min, so as to eliminate the thermal history of the raw material;

[0037] S3, the temperature of the PVDF is adjusted to the experimental temperature (150-260 DEG C), then the pressure (50-600 MPa) is applied and the shear force (the shear rate is greater than or equal to 260 , the shear flow field is applied for greater than or equal to 10 s), then the temperature is lowered at constant pressure, and the PVDF sample is taken out after the temperature is lowered to room temperature and the pressure is released.

[0038] The prepared PVDF sample is made into a piezoelectric flexible sensor: the electrode sheet and the insulating film are pasted on both sides of the PVDF sample in sequence to make the PVDF piezoelectric flexible sensor, which can respond to mechanical stress to obtain an electrical signal, the electrode sheet can be a conductive copper foil or a silver foil, and the insulating film can be a polyimide or a polyethylene film.

[0039] Example 1

[0040] S1, the PVDF particles are placed in the high-pressure shear flow field mold;

[0041] S2, the mold filled with the PVDF is heated to 220 DEG C and kept at this temperature for 5 min to eliminate the thermal history;

[0042] S3, continue to cool the PVDF to 150℃, apply a pressure field of 50 MPa, immediately after the pressure rises to 50 MPa, apply a shear flow field (30 ), the shear flow field lasts for 10 s, then cool at constant pressure, and after the temperature drops to room temperature, release the pressure and take out the sample.

[0043] Example 2

[0044] S1, put the PVDF particles into a high-pressure shear flow field mold;

[0045] S2, heat the mold filled with PVDF to 220℃ and keep it at this temperature for 5 min to eliminate thermal history;

[0046] S3, continue to cool the PVDF to 160℃, apply a pressure field of 100 MPa, immediately after the pressure rises to 100 MPa, apply a shear flow field (30 ), the shear flow field lasts for 10 s, then cool at constant pressure, and after the temperature drops to room temperature, release the pressure and take out the sample.

[0047] Example 3

[0048] S1, put the PVDF particles into a high-pressure shear flow field mold;

[0049] S2, heat the mold filled with PVDF to 220℃ and keep it at this temperature for 5 min to eliminate thermal history;

[0050] S3, continue to cool the PVDF to 170℃, apply a pressure field of 100 MPa, immediately after the pressure rises to 100 MPa, apply a shear flow field (28 ), the shear flow field lasts for 15 s, then cool at constant pressure, and after the temperature drops to room temperature, release the pressure and take out the sample.

[0051] Example 4

[0052] S1, put the PVDF particles into a high-pressure shear flow field mold;

[0053] S2, heat the mold filled with PVDF to 210℃ and keep it at this temperature for 6 min to eliminate thermal history;

[0054] S3, cool the PVDF to 180℃, apply a pressure field of 100 MPa, immediately after the pressure rises to 100 MPa, apply a shear flow field (27 ), the shear flow field lasts for 15 s, then cool at constant pressure, and after the temperature drops to room temperature, release the pressure and take out the sample.

[0055] S4, paste conductive copper foil electrode pieces and insulating polyethylene film on both sides of the PVDF sample in turn to make a PVDF piezoelectric flexible sensor, which can then respond to mechanical stress to obtain electrical signals.

[0056] Example 5

[0057] S1, put PVDF particles into the high pressure shear flow field mold;

[0058] S2, heat the mold to 200°C and keep constant temperature for 10 min to eliminate thermal history;

[0059] S3, cool the PVDF to 190°C, apply pressure field 200 MPa, immediately after the pressure rises to 200 MPa, apply shear flow field (27 ), shear flow field duration 20 s, then constant pressure cooling, after the temperature drops to room temperature, pressure relief and take out the sample.

[0060] Example 6

[0061] S1, put PVDF particles into the high pressure shear flow field mold;

[0062] S2, heat the mold filled with PVDF to 200°C and keep constant temperature for 10 min to eliminate thermal history;

[0063] S3, cool the PVDF to 190°C, apply pressure field 300 MPa, immediately after the pressure rises to 300 MPa, apply shear flow field (27 ), shear flow field duration 20 s, then constant pressure cooling, after the temperature drops to room temperature, pressure relief and take out the sample.

[0064] Example 7

[0065] S1, put PVDF particles into the high pressure shear flow field mold;

[0066] S2, heat the mold filled with PVDF to 200°C and keep constant temperature for 10 min to eliminate thermal history;

[0067] S3, cool the PVDF to 200°C, apply pressure field 400 MPa, immediately after the pressure rises to 400 MPa, apply shear flow field (26 ), shear flow field duration 30 s, then constant pressure cooling, after the temperature drops to room temperature, pressure relief and take out the sample.

[0068] Example 8

[0069] S1, put PVDF particles into the high pressure shear flow field mold;

[0070] S2, heat the mold filled with PVDF to 210°C and keep constant temperature for 5 min to eliminate thermal history;

[0071] S3, cool down the PVDF to 200℃, apply pressure field 500 MPa, immediately after the pressure rises to 500 MPa, apply shear flow field (27 ), shear flow field duration 30 s, then constant pressure cooling, after the temperature drops to room temperature, discharge the sample.

[0072] Example 9

[0073] S1, put the PVDF particles into the high-pressure shear flow field mold;

[0074] S2, warm up the mold filled with PVDF to 210℃ and keep constant temperature for 5 min to eliminate thermal history;

[0075] S3, cool down the PVDF to 200℃, apply pressure field 600 MPa, immediately after the pressure rises to 600 MPa, apply shear flow field (26 ), shear flow field duration 30 s, then constant pressure cooling, after the temperature drops to room temperature, discharge the sample.

[0076] Comparative Example 1

[0077] S1, put the PVDF particles into the high-pressure shear flow field mold;

[0078] S2, warm up the mold filled with PVDF to 210℃ and keep constant temperature for 5 min to eliminate thermal history;

[0079] S3, cool down the PVDF to 150℃, apply pressure field 50 MPa, after the pressure rises to the experimental pressure, constant pressure cooling, after the temperature drops to room temperature, discharge the sample.

[0080] Comparative Example 2

[0081] S1, put the PVDF particles into the high-pressure shear flow field mold;

[0082] S2, warm up the mold filled with PVDF to 210℃ and keep constant temperature for 5 min to eliminate thermal history;

[0083] S3, cool down the PVDF to 180℃, apply pressure field 100 MPa, after the pressure rises to the experimental pressure, constant pressure cooling, after the temperature drops to room temperature, discharge the sample.

[0084] Comparative Example 3

[0085] S1, put the PVDF particles into the high-pressure shear flow field mold;

[0086] S2, warm up the mold filled with PVDF to 210℃ and keep constant temperature for 5 min to eliminate thermal history;

[0087] S3, the PVDF was cooled to 190℃, a pressure field of 200 MPa was applied, and after the pressure rose to the experimental pressure, the temperature was lowered at a constant pressure, and after the temperature dropped to room temperature, the pressure was released to take out the sample.

[0088] Comparative Example 4

[0089] S1, the PVDF particles were placed in a high-pressure shear flow field mold;

[0090] S2, the mold filled with PVDF was heated to 210℃ and kept at a constant temperature for 5 min to eliminate thermal history;

[0091] S3, the PVDF was cooled to 190℃, a pressure field of 300 MPa was applied, and after the pressure rose to the experimental pressure, the temperature was lowered at a constant pressure, and after the temperature dropped to room temperature, the pressure was released to take out the sample.

[0092] Comparative Example 5

[0093] S1, the PVDF particles were placed in a high-pressure shear flow field mold;

[0094] S2, the mold filled with PVDF was heated to 210℃ and kept at a constant temperature for 5 min to eliminate thermal history;

[0095] S3, the PVDF was cooled to 200℃, a pressure field of 500 MPa was applied, and after the pressure rose to the experimental pressure, the temperature was lowered at a constant pressure, and after the temperature dropped to room temperature, the pressure was released to take out the sample.

[0096] Comparative Example 6

[0097] S1, the PVDF particles were placed in a high-pressure shear flow field mold;

[0098] S2, the mold filled with PVDF was heated to 210℃ and kept at a constant temperature for 5 min to eliminate thermal history;

[0099] S3, the PVDF was cooled to 200℃, a pressure field of 400 MPa was applied, and after the pressure rose to the experimental pressure, the temperature was lowered at a constant pressure, and after the temperature dropped to room temperature, the pressure was released to take out the sample.

[0100] Comparative Example 7

[0101] S1, the PVDF particles were placed in a high-pressure shear flow field mold;

[0102] S2, the mold filled with PVDF was heated to 210℃ and kept at a constant temperature for 5 min to eliminate thermal history;

[0103] S3, the PVDF was cooled to 180℃, a pressure field of 100 MPa was applied, and after the pressure rose to 100 MPa, a shear flow field (10 ) was immediately applied, the shear flow field lasted for 15 s, and then the temperature was lowered at a constant pressure, and after the temperature dropped to room temperature, the pressure was released to take out the sample.

[0104] Example 1

[0105] To investigate the content and orientation of polar crystalline phases in the polyvinylidene fluoride crystalline samples prepared according to this invention, the following tests were first conducted on samples from Examples 1-9 of this invention: temperature (150℃~200℃), pressure (50MPa~600MPa), and flow field (26~30℃). Wide-angle X-ray diffraction tests were performed on the samples of Comparative Examples 1-5 at temperatures (150℃~200℃) and pressures (50MPa~500MPa) without an applied flow field. See Appendix. Figure 2 , 3 Appendix Figure 2 As can be seen, a strongly polar crystalline phase appeared under the stated temperature, pressure, and flow field conditions. β and γ The presence of characteristic peaks (indicating the presence of polar crystalline phases) proves that the method described in this invention can prepare polar crystalline phases of polyvinylidene fluoride, and that the content of these polar crystalline phases is relatively high. Figure 3 Comparative Examples 1-5, using polyvinylidene fluoride (PVDF) samples without an applied flow field at temperatures (150℃~200℃) and pressures (50MPa~500MPa), show very weak characteristic peaks of the polar crystalline phase, indicating only a small amount of polar crystalline phase formation. Further analysis of wide-angle X-ray diffraction results, by fitting the diffraction peaks of the polar and non-polar crystalline phases, calculated the crystallinity of the polar crystalline phase using the following formula:

[0106]

[0107] In the formula, A α(111) 、A C They represent (111) respectively. α The area of ​​the peak and the area of ​​all crystallization peaks. The total crystallinity is obtained by dividing the crystallization peak area by the total diffraction peak area. The calculation results are shown in Table 1:

[0108] Table 1 Crystallinity of the polar crystalline phases in Examples 1-9 and Comparative Examples 1-5

[0109]

[0110] It is evident that polyvinylidene fluoride samples with high crystallinity (27%~63%) polar crystalline phase can be prepared using the method of the present invention. In Example 4, the temperature (180℃), pressure (100MPa), and flow field (27℃) were controlled. Under the influence of PVDF polar crystalline phase, the crystallinity is the highest, reaching 63%.

[0111] Example 2

[0112] In order to investigate the orientation degree of the PVDF crystalline sample, the crystal orientation of the PVDF crystalline sample under the temperature (150-200℃), pressure (50-600MPa) and flow field (26 ~30 Figure 4 、 5 The comparison can be seen from the attached Figure 4 、 5 Without the flow field, the SAXS scattering signal is almost uniformly distributed, indicating that the polyvinyl fluoride crystal is isotropic; and after the shear flow field is applied, the SAXS scattering signal presents two peaks at 90° and 265°, indicating that the polyvinyl fluoride crystal presents good orientation (the attached Figure 4 The Hermans orientation function is used for quantitative analysis of the polyvinyl fluoride crystal orientation, and the calculation formula is as follows:

[0113]

[0114] Wherein, the azimuth angle is represented by When is 1, it indicates that the crystal is perfectly oriented, and when is 0, the crystal is randomly distributed. The calculation results are shown in the attached Figure 4 、 Figure 5 Under the combined action of the temperature (150-200℃), pressure (50-600MPa) and flow field (26 ~30 ), the polyvinyl fluoride crystal presents good orientation. Further observation of the oriented polar crystal phase by SEM is shown in the attached Figure 6 The oriented polar crystal phase presents a typical shish-kebab structure, and the lamellar crystal is highly oriented perpendicular to the flow field direction. At the same time, the polyvinyl fluoride crystalline morphology under the combined action of the temperature (180℃), pressure (100MPa) and flow field (10 ) of comparative example 7 is shown in the attached Figure 7 The crystalline morphology is isotropic ring banded crystal. Therefore, under the combined action of the temperature (150-200℃), pressure (50-600MPa) and flow field (shear rate greater than or equal to 26 , shear application time greater than or equal to 10s), the polyvinyl fluoride high-crystallinity oriented polar crystal phase can be prepared.

[0115] Example 3

[0116] ​In order to investigate the piezoelectric sensing performance of the PVDF high crystallinity oriented polar crystal phase prepared by the method of the application, piezoelectric test was performed on the PVDF sample under the joint action of the temperature (180℃), pressure (100MPa) and flow field (27 ) of Example 4, and the PVDF sample under the joint action of the temperature (180℃), pressure (100MPa) and no flow field of Comparative Example 2. The piezoelectric sensing performance test was performed by sequentially pasting conductive copper foil and insulating film polyimide on both sides of the PVDF sample. The test results are shown in Figure 8 、 9 , Example 4 generates a voltage signal of about 2V under the action of pulsed mechanical external force, and Comparative Example 2 generates a voltage signal of about 0.4V under the action of pulsed mechanical external force. Therefore, the PVDF high crystallinity oriented polar crystal phase sample prepared by the application can continuously generate voltage signals with good repeatability under the action of pulsed mechanical external force, and can be used in application fields such as energy harvesting, intelligent sensors and flexible actuators.

[0117] The above is only to illustrate the technical idea of the application, and cannot limit the protection scope of the application. Any modification made according to the technical idea of the application on the basis of the technical scheme falls within the protection scope of the claims of the application.

Claims

1. A method for preparing a high crystallinity oriented polar phase of polyvinylidene fluoride, characterized in that, The specific steps are as follows: S1, polyvinylidene fluoride raw material is added to a mold; S2, the mold filled with polyvinylidene fluoride raw material is heated to above the melting point of polyvinylidene fluoride and maintained, and then the temperature of the molten polyvinylidene fluoride raw material is adjusted to 150-260℃; S3, applying pressure to the polyvinylidene fluoride in the mold to 50 MPa to 600 MPa, immediately after the orientation of the polyvinylidene fluoride by applying a shear flow field, cooling the polyvinylidene fluoride at the pressure, and removing the pressure after the temperature is reduced to room temperature, to obtain the polyvinylidene fluoride high-crystallinity oriented polar crystal phase sample, wherein the shear rate of the shear flow field is The shear flow field is applied for 10 s or more. The crystal phase of the polyvinylidene fluoride high-crystallinity oriented polar crystal phase sample contains β and γ phases and has a shish-kebab structure.

2. The method for preparing the highly crystalline oriented polar phase of polyvinylidene fluoride according to claim 1, characterized in that, In S2, the polyvinylidene fluoride raw material is heated to 30-50℃ above its melting point.

3. The method of claim 1, wherein the polyvinylidene fluoride high crystallinity oriented polar crystalline phase is characterized by, In S2, the polyvinylidene fluoride raw material is maintained for 5-10 minutes after being heated to above its melting point to eliminate thermal history.

4. A high crystallinity oriented polar crystalline phase of polyvinylidene fluoride characterized by, Prepared by the method of any one of claims 1-3.

5. Use of the polyvinylidene fluoride high-crystallinity oriented polar crystal phase according to claim 4 in the field of flexible sensors.

6. Use of the high crystallinity oriented polar crystalline phase of polyvinylidene fluoride according to claim 5 in the field of flexible sensors, characterized in that, A polyvinylidene fluoride piezoelectric flexible sensor is prepared by sequentially pasting electrode sheets and insulating films on both sides of the polyvinylidene fluoride high-crystallinity oriented polar crystal phase sample.

7. Use of the polyvinylidene fluoride high crystallinity oriented polar crystalline phase according to claim 6 in the field of flexible sensors, characterized in that, The electrode sheets are copper foils or silver foils, and the insulating films are polyimide films or polyethylene films.

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