CN114103336A - P (VDF-CTFE) composite film with sandwich structure and preparation method thereof - Google Patents

P (VDF-CTFE) composite film with sandwich structure and preparation method thereof Download PDF

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CN114103336A
CN114103336A CN202010885942.8A CN202010885942A CN114103336A CN 114103336 A CN114103336 A CN 114103336A CN 202010885942 A CN202010885942 A CN 202010885942A CN 114103336 A CN114103336 A CN 114103336A
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ctfe
vdf
bnnss
film
layer
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叶会见
刘文清
徐立新
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New Materials Research Institute Of Zhejiang University Of Technology Pinghu City
Zhejiang University of Technology ZJUT
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New Materials Research Institute Of Zhejiang University Of Technology Pinghu City
Zhejiang University of Technology ZJUT
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Abstract

The application discloses a sandwich structure P (VDF-CTFE) composite film and a preparation method thereof. The sandwich structure P (VDF-CTFE) composite film comprises two BNNSs/P (VDF-CTFE) film layers and a PMMA layer sandwiched between the two BNNSs/P film layers; in the BNNSs/P (VDF-CTFE) thin film layer, the copolymer HBPE-g-PMMA is adsorbed on the surface of the BNNSs through electrostatic force, and the BNNSs are uniformly dispersed in P (VDF-CTFE) and are in orientation arrangement; the mass ratio of BNNSs to P (VDF-CTFE) is 0.1 to 1wt percent; the thickness of the BNNSs/P (VDF-CTFE) film layer is between 5 and 20 mu m, and the thickness of the PMMA layer is between 0.5 and 5 mu m. The sandwich structure P (VDF-CTFE) composite film prepared by the invention has higher dielectric constant, low dielectric loss and high breakdown field strength, and simultaneously keeps good flexibility.

Description

P (VDF-CTFE) composite film with sandwich structure and preparation method thereof
Technical Field
The application relates to the field of dielectric energy storage of high polymer films, in particular to a P (VDF-CTFE) composite film with a sandwich structure and a preparation method thereof.
Background
The novel dielectric material with low loss and high energy storage density has important application prospect in the fields of high-end electronic devices, pulse capacitors, new energy automobiles and the like. The polyvinylidene fluoride-based polymer has various easily controlled crystal forms, the all-trans crystal configuration beta phase of the polyvinylidene fluoride-based polymer provides stronger ferroelectricity, provides advantages for storage and transfer of charges, has higher dielectric constant (more than 10), and is widely used for constructing polymer energy storage films with high energy density by researchers at home and abroad. For a linear dielectric, the energy density is given by the formula Ue＝1/2ε0εrE2Denotes epsilon0Represents a vacuum dielectric constant of 8.854X 10-12，εrDenotes the dielectric constant of the material, and E denotes the breakdown strength of the material. It can be seen that the main way to increase the energy density is to increase the dielectric constant and the breakdown strength of the dielectric material, wherein due to the energy density UeProportional to the square of the breakdown strength E, increasing the breakdown strength is a more effective way to increase the energy density. Currently, it is common and effective to increase the dielectric constant by adding high dielectric inorganic nanoparticles, such as inorganic nano ferroelectric ceramic particles BaTiO3、Ba0.6Sr0.4TiO3Inorganic nano-particle ZnO and the like, and can greatly improve the dielectric constant of the composite material. However, due to the great difference between the dielectric properties of the nanoparticles and the polymer matrix, the prepared composite material tends to cause the concentration of an electric field at an interface, so that the composite material is causedThe material is easily broken down under a high electric field. Meanwhile, the agglomeration of the high-content inorganic nano-particles can weaken the toughness, mechanical property, easy processability and the like of the polymer-based composite material, and influence the wide use property of the polymer-based composite material. Therefore, the development of a polymer film capacitor having both high dielectric constant and high breakdown field strength at low filler content is the main research direction of researchers.
Disclosure of Invention
The invention aims to provide a P (VDF-CTFE) composite film with a sandwich structure and a preparation method thereof.
In a first aspect, the present application provides a sandwich structure P (VDF-CTFE) composite film comprising two BNNSs/P (VDF-CTFE) thin film layers and a PMMA layer sandwiched therebetween;
in the BNNSs/P (VDF-CTFE) thin film layer, the copolymer HBPE-g-PMMA is adsorbed on the surface of the BNNSs through electrostatic force, and the BNNSs are uniformly dispersed in P (VDF-CTFE) and are in orientation arrangement; the mass ratio of BNNSs to P (VDF-CTFE) is 0.1 to 1wt percent;
the thickness of the BNNSs/P (VDF-CTFE) film layer is between 5 and 20 mu m, and the thickness of the PMMA layer is between 0.5 and 5 mu m.
In the BNNSs/P (VDF-CTFE) thin film layer, the copolymer HBPE-g-PMMA is adsorbed on the surface of the BNNSs through electrostatic force, and the PMMA chain segment and the PVDF chain segment have excellent compatibility, so that the BNNSs are uniformly dispersed in a PVDF matrix and form a strong interaction interface; and the BNNSs is oriented in P (VDF-CTFE), and the meaning of the oriented arrangement is that the high molecular material P (VDF-CTFE) is stretched along one direction, and the molecular chain and the filler BNNSs tend to be parallel arranged along the stretching direction. The intrinsic high and wide band gap of the boron nitride nanosheet enables the film to have high breakdown strength, and meanwhile, the BNNSs/P (VDF-CTFE) film provides enhanced interface polarization, so that the dielectric property of the film is remarkably improved, and the energy density of the composite material is enhanced. And the PMMA layer sandwiched between the two BNNSs/P (VDF-CTFE) thin film layers can effectively delay the electric breakdown under high voltage due to good electric insulation, and the dielectric property of the sandwich structure is improved as a whole.
In the sandwich structure composite film, the relative content F (beta) of a beta-phase crystal form in a polymer P (VDF-CTFE) is more than 60 percent.
Preferably, the sandwich structure P (VDF-CTFE) composite film is composed of two BNNSs/P (VDF-CTFE) thin film layers and a PMMA layer sandwiched therebetween.
In a second aspect, the present application provides a method for preparing a sandwich structure P (VDF-CTFE) composite film, comprising the steps of:
(1) mixing and stirring a solution of P (VDF-CTFE) and a boron nitride nanosheet dispersion liquid to form a uniform casting solution, wherein the mass ratio of BNNSs to P (VDF-CTFE) is 0.1-1 wt%; uniformly casting the casting solution on a flat glass sheet, drying to form a film, and cooling to room temperature after the solvent is completely volatilized to obtain a BNNSs/P (VDF-CTFE) film layer;
(2) casting a layer of PMMA polymer solution on the BNNSs/P (VDF-CTFE) film layer obtained in the step (1), drying to form a film, and tearing off the double-layer film on the glass sheet for storage for later use;
(3) stacking the BNNSs/P (VDF-CTFE) thin film layer obtained in the step (1) and the double-layer thin film prepared in the step (2), and performing hot pressing to obtain a composite thin film with a sandwich structure, wherein the middle layer of the composite thin film is a PMMA layer;
(4) and (3) performing uniaxial stretching on the composite film with the sandwich structure obtained in the step (3) to ensure that BNNSs are aligned in an oriented manner in P (VDF-CTFE) to obtain the composite film with the sandwich structure P (VDF-CTFE).
In the invention, the boron nitride nanosheet BNNSs is obtained by utilizing a method of HBPE-g-PMMA (HBPE-g-PMMA) and simultaneously stripping Hexagonal boron nitride (Hexagonal boron nitride, abbreviated as h-BN) and non-covalent modification of the boron nitride nanosheet. The hexagonal boron nitride crystal is commercially available, and the transverse dimension is about 1 mu m. The HBPE-g-PMMA is prepared by synthesizing Hyperbranched polyethylene-grafted polymethyl methacrylate (HBPE-g-PMMA) in a laboratory, and comprises the following specific synthesis steps:
(a) under a certain ethylene pressure, Pd-diimine is used as a catalyst, and ethylene and 2- (2-bromoisobutyryloxy) ethyl acrylate (BIEA) are synthesized into the HBPE @ Br macromolecular initiator through a chain walking mechanism.
(b) HBPE @ Br is taken as a macroinitiator, MMA is taken as a monomer, and CuBr and PMDETA are subjected to Atom Transfer Radical Polymerization (ATRP) in a toluene solvent at 90 ℃ to synthesize HBPE-g-PMMA.
The preparation method of the boron nitride nanosheet specifically comprises the following steps: pouring an organic solvent dissolved with an HBPE-g-PMMA polymer into a glass bottle filled with a certain amount of hexagonal boron nitride powder, sealing the bottle cover with a film, placing the bottle into an ultrasonic pool, carrying out ultrasonic treatment for 8-72 hours at room temperature, wherein the ultrasonic power can be selected to be 100-320W, taking out a mixed solution after the ultrasonic treatment, placing the mixed solution into a centrifugal tube, and centrifuging at the rotating speed of 1000-5000 rpm, wherein the centrifuging process mainly comprises the steps of removing blocky un-stripped hexagonal boron nitride, taking supernatant liquid for further centrifuging, wherein the rotating speed can be controlled to be 6000-10000 rpm, the main purpose is to remove excessive HBPE-g-PMMA polymer, and the centrifuged supernatant liquid is the obtained boron nitride nanosheet dispersion liquid. Preferably, the feeding mass ratio of the HBPE-g-PMMA to the hexagonal boron nitride crystal is 1:2-3, and the organic solvent can be one or more of chloroform, toluene, N-methylpyrrolidone and N, N-dimethylformamide.
In the preparation scheme of the boron nitride nanosheet, the hexagonal boron nitride crystal is stripped into BNNSs in an organic solvent with the aid of HBPE-g-PMMA, the method ingeniously utilizes that the HBPE-g-PMMA can strip h-BN in the solvent to obtain BNNSs, and the HBPE-g-PMMA is adsorbed on the surface of the BNNSs through CH-pi electrostatic interaction to prevent the BNNSs from agglomerating.
In the present invention, the polymer of P (VDF-CTFE) or PMMA can be prepared by using commercially available products or by itself according to a method reported in the literature.
In step (1) of the present invention, the solvent of the solution of P (VDF-CTFE) may be DMF, NMP or DMAc. Preferably, the solvent of the solution of P (VDF-CTFE) is the same as the solvent of the boron nitride nanoplate dispersion, such as DMF. The thickness of the BNNSs/P (VDF-CTFE) thin film layer is realized by controlling the concentration and volume consumption of the casting solution, preferably, the concentration of the P (VDF-CTFE) in the casting solution is 15-20 mg/mL, and the casting solution loaded on each piece of glass is 3-5 mL/20cm2More preferably 4mL/20cm2。
In steps (1) and (2) of the present invention, the conditions for drying and film forming are preferably as follows: drying for 4-10 h at 60-100 ℃.
In step (2) of the present invention, the solvent of the PMMA polymer solution can be selected from chloroform, toluene, N-dimethylformamide, preferably CHCl3. The thickness of the PMMA layer is realized by controlling the concentration and volume dosage of a PMMA polymer solution, preferably, the concentration of the PMMA polymer solution is 5-20 mg/mL, and the volume dosage is 2-3 mL/20cm in terms of the area of a BNNSs/P (VDF-CTFE) thin film layer2More preferably 2mL/20cm2。
In the step (3), the temperature in the hot pressing process can be controlled to be 160-200 ℃, the pressure is 8-15 MPa, and the hot pressing time is 10-20 min.
In step (4) of the present invention, the uniaxial stretching is specifically performed as follows: and (3) carrying out constant temperature treatment for 5-15min at 80-120 ℃ in an oven of a tensile machine, and uniaxially stretching to 2-5 times of the original length at a stretching speed of 10-100 mm/min to obtain the P (VDF-CTFE) composite film with the sandwich structure. Preferably, the stretching rate is 20mm/min, and the stretching is performed to 4 times the original length.
Compared with the prior art, the invention has the beneficial effects that:
(1) according to the invention, the hyperbranched polyethylene grafted polymethyl methacrylate is subjected to liquid phase stripping in a common organic solvent to obtain the boron nitride nanosheet, and the interaction between the hyperbranched structure and the surface of the nanosheet enables the target polymer to be adsorbed on the surface of the nanosheet, so that the surface modification of the filler is realized, and the filler has good dispersibility in a polymer matrix and strong interface interaction.
(2) The composite membrane with the sandwich structure is prepared by hot pressing and stretching, wherein the upper layer and the lower layer are BNNSs/P (VDF-CTFE) nano composite layers, and nano fillers are arranged in an oriented mode in the stretching process, so that the polarization performance of the sandwich structure is improved; the PMMA is used as the middle layer, the good electrical insulation property of the PMMA effectively blocks the development of the electrical tree under high voltage, and the high voltage resistance of the sandwich structure is improved. The experimental result shows that compared with a two-layer structure composite film without a PMMA (polymethyl methacrylate) middle layer, the sandwich structure composite film has lower dielectric loss under high frequency.
The nano composite material prepared by the invention has higher dielectric constant, low dielectric loss and high breakdown field strength, and simultaneously keeps good flexibility.
Drawings
FIG. 1 is a schematic view of a process for preparing a composite film;
FIG. 2 is an infrared spectrum of the prepared composite thin film material;
FIG. 3 shows the content of beta phase in the composite film under different PMMA contents;
FIG. 4 shows the dielectric constants of P (VDF-CTFE) composites with different PMMA content of the middle layer;
FIG. 5 shows the dielectric loss of P (VDF-CTFE) composites with different PMMA content of the middle layer;
FIG. 6 is a graph of dielectric constant and dielectric loss for a tri-layer structure with different BNNSs content in the upper and lower layers;
FIG. 7 is a graph of dielectric constant and dielectric loss for a three layer structure at different stretching rates;
fig. 8 is a graph of dielectric constant and dielectric loss for a three-layer structure at different stretch ratios.
Detailed Description
The following examples are given to further illustrate the technical solutions of the present invention, but it should be noted that the following examples should not be construed as limiting the scope of the present invention, and those skilled in the art should make non-essential improvements and modifications to the present invention according to the above disclosure.
Example 0
The HBPE-g-PMMA is prepared by synthesizing Hyperbranched polyethylene grafted polymethyl methacrylate (HBPE-g-PMMA) in a laboratory, and comprises the following specific synthesis steps:
(1) a clean and strictly dry 100mL Schlenk flask was taken and charged proportionally with 342mmol of MMA monomer, 12mL of anhydrous toluene, 0.38mmol of HBPE @ Br and 0.76mmol of PMDETA.
(2) The mixed solution is frozen, vacuumized and unfrozen for three times to ensure the anhydrous and oxygen-free environment in the polymerization process, and then 0.38mmol of CuBr is rapidly added in the nitrogen atmosphere, and the mixture reacts for 12 hours at 90 ℃ after being sealed.
(3) After the reaction is carried out for a preset time, the reaction bottle is immersed into an ice-water bath to terminate the reaction, then the solution is blown to be dry by cold air, THF is added to dissolve the product, then methanol is continuously dripped to precipitate the product, and finally the supernatant is poured out. This step was repeated 3 times to purify the product.
(4) And transferring the product into a centrifugal tube, and drying the product in a vacuum oven at 60 ℃ for 24h to obtain white solid powder, namely HBPE-g-PMMA.
Example 1
The method comprises the following steps: weighing 0.1g of HBPE-g-PMMA in a glass bottle, adding 10mL of DMMF, stirring to assist the copolymer HBPE-g-PMMA to be fully dissolved, adding 0.24g of hexagonal boron nitride (h-BN) powder into the glass bottle, finally adding 50mL of DMMF (the mass of h-BN in the solvent is recorded as 4mg/mL), and sealing the bottle cap.
Step two: and (3) putting the mixed solution treated in the step one into a water bath ultrasonic pool at room temperature for ultrasonic treatment for 48 hours, wherein the ultrasonic power is 240W.
Step three: and D, taking out the mixed solution subjected to ultrasonic treatment in the step II, putting the mixed solution into a centrifugal tube, and centrifuging the mixed solution at 3000rpm for 30min to mainly remove the non-peeled blocky h-BN. And then taking the supernatant for storage, namely the dispersion liquid of the Boron Nitride Nanosheets (BNNSs).
Step four: and (3) taking 40mL of the dispersion liquid in the third step, centrifuging the dispersion liquid in a centrifuge at 7000rpm for 30min, mainly aiming at removing the excessive HBPE-g-PMMA polymer, taking out the supernatant, collecting the bottom product, and drying the bottom product in a vacuum oven to constant weight to obtain BNNSs, wherein the HBPE-g-PMMA accounts for 19.6% of the weight of the BNNSs. DMF (1 mg/mL based on the mass of BNNSs in the solvent) was then added and re-sonicated for 2h until ready for use.
Step five: dissolving 200mg of P (VDF-CTFE) powder in 10mL of DMF, fully and magnetically stirring until the solution is transparent and uniform, adding 1mL of 1mg/mL BNNSs dispersion liquid, fully and magnetically stirring the mixed solution, taking 4mL of the mixed solution, casting on a 4X 5cm glass sheet in a forced air drying oven, evaporating the solvent at 60 ℃ to form a film, and removing the film after 8 hours, wherein the film is marked as a BNNSs/P (VDF-CTFE) composite film with the thickness of 20 μm.
Step six: 0.05g PMMA was dissolved in 10mL CHCl3And standing for later use after the PMMA is fully dissolved. And repeating the fifth step, casting the mixed solution on a glass sheet to evaporate the solvent for 2h, then casting a layer of PMMA polymer solution on the BNNSs/P (VDF-CTFE) composite film uniformly by using a dropper from 2mL of the PMMA solution, continuing to evaporate the solvent at 60 ℃ to form a film, and removing the film after 8h to prepare the two-layer composite film with the BNNSs/P (VDF-CTFE) composite film layer and the PMMA layer, wherein the thickness is 25 mu m.
PMMA obtained from Shanghai Jingqi Polymer materials Co., Ltd, and having a weight average molecular weight Mw＝63650。
Step seven: and (3) placing the two-layer composite membrane structure prepared in the step six on the lower layer, placing the single-layer membrane prepared in the step five on the upper layer, and hot-pressing the single-layer membrane into a three-layer membrane under a tablet press by taking a high-temperature-resistant PET membrane with the thickness of 0.2mm as a mould pressing isolation layer, wherein the hot-pressing temperature is set to be 170 ℃, the pressure is 11MPa, and the time is 15 min. The composite film with a BNNSs/P (VDF-CTFE) composite film as the upper layer and the lower layer and a PMMA three-layer structure as the middle layer is uncovered from the isolation layer.
Step eight: cutting the composite film with the sandwich structure prepared by hot pressing in the seventh step into a sample strip with the length of 3cm and the width of 1cm, loading the sample strip on a film clamp, keeping the temperature of 100 ℃ in an oven of a tensile machine for 10min, and uniaxially stretching to 4 times of the original length at the stretching rate of 20mm/min to obtain the stretched sample strip. The stretched sample film is the prepared composite material with a three-layer structure, and the composite film has high dielectric constant, low dielectric loss and good processability.
Examples 2 to 4
The mass of PMMA in step six of example 1 was changed to 0.1g (example 2), 0.15g (example 3), 0.2g (example 4) PMMA dissolved in 10mL CHCl3And (4) preparing the composite membrane material with the three-layer structure under the same other conditions.
The schematic diagram of the preparation process of the composite membrane material is shown in fig. 1, and as can be seen from fig. 1, the three-layer composite membrane has good toughness. Fig. 2 is an infrared spectrum of the composite film, fig. 3 is a relative content of a β phase calculated by Lambert-Beer law, and it can be seen that F (β) in a two-layer structure without a PMMA layer in the middle is 31.8%, and as the content of the middle PMMA increases, the content of the β phase increases to a maximum value of 97% when the mass of the middle PMMA is 10 mg; the beta phase content then slowly decreases, but overall is higher than the beta phase content of the two-layer composite membrane. Meanwhile, the dielectric constant and the dielectric loss thereof measured at room temperature depending on the frequency are shown in fig. 4 and 5.
Characterization and testing
The surface of the obtained composite membrane material is coated with a layer of conductive silver layer with the thickness of 1-3 mu m as an electrode, and the area of the conductive silver layer is about 1cm2The frequency dependent capacitance and loss angle were measured with a precision impedance analyzer (4294ALCR, Agilent, USA) over a frequency range of 102～107Hz, the dielectric constant and dielectric loss of each composite were calculated.
Test result comparison and analysis
As can be seen from FIG. 2, the absorption peak of the polar beta phase of P (VDF-CTFE) in the three-layer composite film is obvious, which indicates that the composite film is subjected to stretching treatment to induce the transformation of P (VDF-CTFE) from alpha crystal form to beta crystal form, which is beneficial to the improvement of dielectric constant. Meanwhile, absorption peaks of C ═ O and C — O from the middle layer PMMA occurred in the three-layer structure composite film.
As can be seen from fig. 4, at the same frequency, the dielectric constant of the composite film decreases as the PMMA content of the intermediate layer increases. But generally the decrease is not much and the composite film with the least PMMA interlayer content is the highest dielectric constant. The insulating layer of the middle PMMA can effectively prevent the migration of carriers and prevent the dielectric from generating a breakdown phenomenon prematurely under a high field due to charge concentration.
As can be seen from the dielectric loss of fig. 5, the dielectric loss angles of the respective three-layer composite film and two-layer composite film are below 0.05 at low frequencies, indicating that the dielectric loss is maintained low while the high dielectric constant is maintained by the structural design. At high frequencies 106～107Within Hz, the dielectric relaxation of the composite film is gradually weakened along with the addition of the PMMA layer, and the loss under high frequency is obviously inhibited.
Examples 5 to 9
Composite membrane materials with a three-layer structure were prepared by changing the amounts of BNNSs added in step five of example 1 to 0 wt% (example 5), 0.1 wt% (example 6), 0.3 wt% (example 7), 0.8 wt% (example 8), and 1.0 wt% (example 9), respectively, without changing other conditions.
It can be seen from FIG. 6 that the dielectric constant shows a tendency of increasing first and then decreasing with the increase of the BNNSs content in the BNNSs/P (VDF-CTFE) of the upper and lower layers; at the same time, the dielectric loss is 103The Hz is below 0.05, which effectively keeps the property of high dielectric and low loss.
Examples 10 to 13
The stretching rate in the eighth step in example 1 was changed to 10mm/min (example 10), 30mm/min (example 11), 40mm/min (example 12), and 50mm/min (example 13), and the other conditions were not changed to prepare a composite film material having a three-layer structure.
As can be seen from fig. 7, the dielectric constant tends to decrease with increasing stretching rate, but the overall decrease is not significant; the dielectric loss thereof is also maintained at a low level.
Examples 14 to 17
A composite film material having a three-layer structure was prepared by changing the draw ratio in step eight of example 1 to 0 times (example 14), 2 times (example 15), 3 times (example 16), and 5 times (example 17) under the same conditions.
The dielectric properties of the three-layer structure composite film at different stretch ratios are depicted in fig. 8. From the graph analysis, compared with the unstretched composite film, the dielectric constant of the three-layer composite film is improved from 17 to about 29, and the dielectric loss is at a lower level, which shows that the dielectric property of the composite film can be obviously improved by stretching.
Comparative example 1
The method comprises the following steps: weighing 0.1g of HBPE-g-PMMA in a glass bottle, adding 10mL of LDMF, stirring to assist the copolymer HBPE-g-PMMA to be fully dissolved, adding 0.24g of hexagonal boron nitride powder into the glass bottle, finally adding 50mL of DMF (the mass of h-BN in the solvent is recorded as 4mg/mL), and sealing the bottle cap.
Step two: and (3) putting the mixed solution treated in the step one into a water bath ultrasonic pool at room temperature for ultrasonic treatment for 48 hours, wherein the ultrasonic power is 240W.
Step three: and D, taking out the mixed solution subjected to ultrasonic treatment in the step II, putting the mixed solution into a centrifugal tube, and centrifuging the mixed solution at 3000rpm for 30min to mainly remove the non-peeled blocky h-BN. And then taking the supernatant for storage, namely obtaining the dispersion liquid of the boron nitride nanosheet.
Step four: and (3) centrifuging 40mL of the dispersion liquid in the third step for 30min at 7000rpm in a centrifuge, mainly aiming at removing the excessive HBPE-g-PMMA polymer, pouring out the supernatant, collecting the bottom product, and drying in a vacuum oven to constant weight to obtain the Boron Nitride Nanosheet (BNNSs). DMF (1 mg/mL based on the mass of BNNSs in the solvent) was then added and re-sonicated for 2h until ready for use.
Step five: dissolving 200mg of P (VDF-CTFE) powder in 10mL of DMF, fully and magnetically stirring until the solution is uniform and transparent, adding 1mg/mL of BNNSs dispersion liquid (containing 1mg of BNNSs), fully and magnetically stirring the mixed solution, taking 4mL of the mixed solution, casting on a 4X 5cm glass sheet in an air-blowing drying oven, evaporating the solvent at 60 ℃ to form a film, and removing the film after 8 hours to obtain the BNNSS/P (VDF-CTFE) composite film.
Step six: and (4) overlapping two composite films prepared in the fifth step, using a high-temperature resistant PET film with the thickness of 0.2mm as a die pressing isolation layer, and hot-pressing the two composite films into two films under a tablet press, wherein the hot-pressing temperature is set to be 170 ℃, the pressure is 11MPa, and the time is 15 min. And removing the upper and lower layers of the composite film which is BNNSs/P (VDF-CTFE) from the isolation layer.
Step seven: and cutting the BNNSs/P (VDF-CTFE) composite film with the two-layer structure prepared by hot pressing in the sixth step into a sample strip with the length of 3cm and the width of 1cm, loading the sample strip on a film clamp, keeping the temperature of 80 ℃ for 10min in an oven of a tensile machine, and uniaxially stretching to 4 times of the original length at the stretching rate of 20mm/min to obtain the stretched sample strip. The stretched sample film is a prepared composite material with a two-layer structure.

Claims (10)

1. A sandwich structure P (VDF-CTFE) composite film comprises two BNNSs/P (VDF-CTFE) film layers and a PMMA layer sandwiched between the two BNNSs/P (VDF-CTFE) film layers;
in the BNNSs/P (VDF-CTFE) thin film layer, the copolymer HBPE-g-PMMA is adsorbed on the surface of the BNNSs through electrostatic force, and the BNNSs are uniformly dispersed in P (VDF-CTFE) and are in orientation arrangement; the mass ratio of BNNSs to P (VDF-CTFE) is 0.1 to 1wt percent;
the thickness of the BNNSs/P (VDF-CTFE) film layer is between 5 and 20 mu m, and the thickness of the PMMA layer is between 0.5 and 5 mu m.
2. The sandwich structure P (VDF-CTFE) composite film according to claim 1, characterized in that: in the sandwich structure P (VDF-CTFE) composite film, the relative content F (beta) of a beta-phase crystal form in the polymer P (VDF-CTFE) is more than 60 percent.
3. The sandwich structure P (VDF-CTFE) composite film according to claim 1 or 2, characterized in that: the sandwich structure P (VDF-CTFE) composite film consists of two BNNSs/P (VDF-CTFE) film layers and a PMMA layer sandwiched between the two BNNSs/P film layers.
4. A method for preparing a sandwich structure P (VDF-CTFE) composite film according to claim 1, comprising the steps of:
(1) mixing and stirring a solution of P (VDF-CTFE) and a boron nitride nanosheet dispersion liquid to form a uniform casting solution, wherein the mass ratio of BNNSs to P (VDF-CTFE) is 0.1-1 wt%: 1; uniformly casting the casting solution on a flat glass sheet, drying to form a film, and cooling to room temperature after the solvent is completely volatilized to obtain a BNNSs/P (VDF-CTFE) film layer;
(2) casting a layer of PMMA polymer solution on the BNNSs/P (VDF-CTFE) film layer obtained in the step (1), drying to form a film, and tearing off the double-layer film on the glass sheet for storage for later use;
(3) stacking the BNNSs/P (VDF-CTFE) thin film layer obtained in the step (1) and the double-layer thin film prepared in the step (2), and performing hot pressing to obtain a composite thin film with a sandwich structure, wherein the middle layer of the composite thin film is a PMMA layer;
(4) and (4) performing uniaxial stretching on the composite film with the sandwich structure obtained in the step (3) to ensure that BNNSs are aligned in the orientation mode in P (VDF-CTFE) to obtain the composite film with the sandwich structure P (VDF-CTFE).
5. The method of claim 4, wherein: the preparation method of the boron nitride nanosheet specifically comprises the following steps: pouring an organic solvent dissolved with an HBPE-g-PMMA polymer into a glass bottle filled with a certain amount of hexagonal boron nitride powder, sealing the bottle cover with a film, placing the bottle into an ultrasonic pool, carrying out ultrasonic treatment for 8-72 hours at room temperature, selecting the ultrasonic power of 100-320W, taking out a mixed solution after the ultrasonic treatment, placing the mixed solution into a centrifugal tube, centrifuging at the rotating speed of 1000-5000 rpm, taking the supernatant, further centrifuging at the rotating speed of 6000-10000 rpm, and obtaining the centrifuged supernatant, namely the obtained boron nitride nanosheet dispersion; wherein the feeding mass ratio of the HBPE-g-PMMA to the hexagonal boron nitride crystal is 1:2-3, and the organic solvent is selected from one or more of chloroform, toluene, N-methylpyrrolidone and N, N-dimethylformamide.
6. The method of claim 4, wherein: in the step (1), the solvent of the solution of P (VDF-CTFE) is DMF, NMP or DMAc; the concentration of P (VDF-CTFE) in the casting solution is 15-20 mg/mL, so that the casting solution loaded on each piece of glass is 3-5 mL/20cm2。
7. The method of claim 4, wherein: in the steps (1) and (2), the drying film forming conditions are as follows: drying for 4-10 h at 60-100 ℃.
8. The method of claim 4, wherein: in the step (2), chloroform, toluene or N, N-dimethylformamide is selected as a solvent of the PMMA polymer solution; the concentration of the PMMA polymer solution is 5-20 mg/mL, and the volume dosage of the PMMA polymer solution is 2-3 mL/20cm based on the area of a BNNSs/P (VDF-CTFE) film layer2。
9. The method of claim 4, wherein: in the step (3), the temperature in the hot pressing process is controlled to be 160-200 ℃, the pressure is controlled to be 8-15 MPa, and the hot pressing time is 10-20 min.
10. The method of claim 4, wherein: in the step (4), the uniaxial stretching is specifically performed as follows: performing constant temperature treatment for 5-15min at 80-120 ℃ in an oven of a tensile machine, and uniaxially stretching to 2-5 times of the original length at a stretching rate of 10-100 mm/min to obtain a composite film with a three-layer structure; preferably, the stretching rate is 20mm/min, and the stretching is performed to 4 times the original length.
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