A polyvinyl alcohol-based composite film and its preparation method and application

Abstract

The present invention discloses a polyvinyl alcohol-based composite film, which is composed of polyvinyl alcohol, fluorinated graphene nanosheets and carboxymethyl chitosan in a mass ratio of 100:1.1~8.2:8.7. The polyvinyl alcohol is connected to the fluorinated graphene nanosheets through hydrogen bonds and is oriented and crystallized at its edges. The carboxymethyl chitosan couples the adjacent polyvinyl alcohol crystal regions through hydrogen bonds, thereby forming an efficient heat-conducting network consisting of carboxymethyl chitosan-crystal region-fluorinated graphene. The present invention also discloses a preparation method and application of the polyvinyl alcohol-based composite film. Compared with the prior art, the polyvinyl alcohol-based composite film of the present invention has excellent electrical insulation performance, mechanical properties and thermal properties, and can be widely used in applications with high requirements for thermal conductivity and insulation.

Description

Technical Field

[0001] The invention relates to a polyvinyl alcohol-based composite film with high insulation and high thermal conductivity, belonging to the technical field of functional nanomaterials. Background Art

[0002] Power electronic components, the core of modern power electronics, have been adopted in an unprecedented range of applications across various fields. Due to the demands of their operating environments, they often require coating or forming one or more layers of protective films on their surfaces to protect them from adverse effects such as static electricity and oxidative corrosion, while also effectively and quickly dissipating the large amounts of heat generated during operation. This requires that the protective film material possess excellent mechanical, chemical, and thermal stability, as well as good insulation and thermal conductivity, while also requiring a simple and cost-effective manufacturing process.

[0003] Polyvinyl alcohol (PVA) is a non-toxic, odorless, biodegradable, insulating linear polymer. In recent years, it has been widely used in adhesives, hydrogels, photovoltaic substrates, pharmaceuticals, and biotechnology. While PVA exhibits excellent mechanical, chemical, and thermal stability, it also serves as a suitable matrix for thermal interface materials. However, its low thermal conductivity limits its further application in thermal management.

[0004] Currently, there are two approaches to constructing a continuous thermally conductive network in polyvinyl alcohol to improve its thermal conductivity: (1) increasing the coupling strength between chains through hydrogen bonding or chemical crosslinking. Although heterogeneous molecules can connect adjacent main chains through coupling to form a thermally conductive network, due to the low intrinsic thermal conductivity of polyvinyl alcohol, this connection method has limited or negligible contribution to thermal conductivity; (2) adding high thermal conductivity fillers such as graphene oxide, carbon nanotubes, and boron nitride. However, these fillers will reduce the crystallinity of polyvinyl alcohol, thus limiting the improvement in thermal conductivity.

[0005] Fluorinated graphene, a material with excellent thermal stability, chemical stability, and electrical properties, can be considered as a high-thermal-conductivity filler for addition to polyvinyl alcohol. However, existing research shows that excessive addition of fluorinated graphene can damage the mechanical properties and crystal structure of polyvinyl alcohol, reducing its flexibility and tensile strength, making it unfavorable for further application. When a small amount of fluorinated graphene is added to polyvinyl alcohol, polyvinyl alcohol connects the fluorinated graphene through hydrogen bonds and aligns and crystallizes at its edges. However, when it reaches the dispersion threshold in polyvinyl alcohol, it cannot form a thermal conductive network, limiting the significant improvement in thermal conductivity. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide a polyvinyl alcohol-based composite film and a preparation method thereof, wherein a high-efficiency thermal conductive network composed of carboxymethyl chitosan-crystalline region-fluorinated graphene is formed inside the composite film, and the composite film has excellent electrical insulation properties, mechanical properties and thermal properties.

[0007] The present invention specifically adopts the following technical solutions to solve the above technical problems:

[0008] A polyvinyl alcohol-based composite membrane is composed of polyvinyl alcohol, fluorinated graphene nanosheets (FGN) and carboxymethyl chitosan (CMC) in a mass ratio of 100:1.1 to 8.2:8.7. The polyvinyl alcohol is connected to the fluorinated graphene nanosheets through hydrogen bonds, and is directionally arranged and crystallized at their edges. The carboxymethyl chitosan couples adjacent polyvinyl alcohol crystal regions through hydrogen bonds, thereby forming a highly efficient thermal conductive network consisting of carboxymethyl chitosan-crystal region-fluorinated graphene.

[0009] Preferably, the mass ratio of polyvinyl alcohol, fluorinated graphene nanosheets and carboxymethyl chitosan is 100:1.1 to 3.4:8.7.

[0010] More preferably, the mass proportion of the fluorinated graphene nanosheets is 3%.

[0011] The preparation method of the polyvinyl alcohol-based composite film as described in any of the above technical solutions is as follows: a polyvinyl alcohol aqueous solution with a concentration of 20 to 50 mg / mL and a fluorinated graphene nanosheet aqueous dispersion of 0.5 to 1 mg / mL are mixed evenly according to the mass ratio, and then a carboxymethyl chitosan aqueous dispersion of 20 to 30 mg / mL is added and mixed evenly. After vacuum drying at a temperature of 60°C to 80°C to form a film, it is peeled off from the carrier.

[0012] Preferably, the preparation method of the fluorinated graphene nanosheets is as follows: dispersing the fluorinated graphene in a solvent, and subjecting the solvent to ultrasonic and centrifugal treatment to obtain exfoliated fluorinated graphene nanosheets.

[0013] Further preferably, the ultrasonic treatment is performed using a water bath sonicator with a power of 30 to 100 W and an ultrasonic time of 18 to 32 h; the rotation speed of the centrifugal treatment is 3000 to 6000 rpm and the treatment time is 15 to 30 min.

[0014] More preferably, the solvent is one or more of isopropyl alcohol, N-methylpyrrolidone, N,N-dimethylformamide and dichloromethane.

[0015] Preferably, the fluorinated graphene nanosheet aqueous dispersion, the polyvinyl alcohol aqueous solution and the carboxymethyl chitosan aqueous dispersion are uniformly mixed by ultrasound and stirring.

[0016] The polyvinyl alcohol-based composite film as described in any of the above technical solutions is used as a protective film on the surface of power electronic components.

[0017] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0018] (1) The composite membrane proposed in the present invention uses polyvinyl alcohol as the polymer matrix and is modified with low-filler fluorinated graphene nanosheets and carboxymethyl chitosan. The polyvinyl alcohol connects to the fluorinated graphene through hydrogen bonds, aligning and crystallizing it at its edges. The carboxymethyl chitosan couples adjacent crystalline regions through hydrogen bonds, forming a unique, highly efficient thermal conductive network consisting of carboxymethyl chitosan, crystalline regions, and fluorinated graphene.

[0019] (2) The composite film of the present invention has excellent electrical insulation properties, mechanical properties and thermal properties. When the contents of fluorinated graphene nanosheets and carboxymethyl chitosan are 3 wt.% and 7.76 wt.% respectively, the in-plane thermal conductivity of the polyvinyl alcohol-based composite film is 12.22 W / (m·K), which is 5.2 times that of polyvinyl alcohol; the out-of-plane thermal conductivity is 0.41 W / (m·K), which is 5.1 times that of polyvinyl alcohol, and the thermal decomposition temperature is increased by 57°C. It can be widely used in applications with high requirements for thermal conductivity and insulation, such as when applied to the surface of power electronic components, while ensuring electrical insulation, it can greatly increase the upper temperature limit of the components through good heat dissipation.

[0020] (3) The present invention adopts a solution casting method to connect fluorinated graphene nanosheets and carboxymethyl chitosan in polyvinyl alcohol and form a continuous thermal conductive network. The preparation process is simple, the preparation cost is low, and it is suitable for large-scale production. After preparation, the film has a tensile strength close to that of polyvinyl alcohol and excellent insulation properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The figures are scanning electron microscope images of the film of comparative example 1 and the films of examples 2 and 3; wherein (a), (b), and (c) correspond to the surfaces of the three films, and (d), (e), and (f) correspond to the brittle fracture surfaces of the three films.

[0022] Figure 2 DSC curves of the films of Comparative Examples 1 to 3 and Examples 1 to 4;

[0023] Figure 3 The stress-strain curves and volume resistivity diagrams of the films of Comparative Examples 1 to 3 and Examples 1 to 4, wherein (a) is the stress-strain curve and (b) is the volume resistivity;

[0024] Figure 4 The thermal conductivity histograms of the films of Comparative Examples 1 to 3 and Examples 1 to 4 are shown, where (a) is the in-plane thermal conductivity and (b) is the out-of-plane thermal conductivity.

[0025] Figure 5 The figures are red thermal imaging comparisons of the films of comparative examples 1 to 3 and example 2; (a) is an infrared thermal imaging image of an LED bulb with the films of comparative examples 1 to 3 and example 2 as substrates, respectively; and (b) is an infrared thermal imaging image of the films of comparative examples 1 to 3 and the example film in the vertical direction. DETAILED DESCRIPTION

[0026] The inventors discovered that by using polyvinyl alcohol as the polymer matrix and modifying it with low-filler fluorinated graphene nanosheets and carboxymethyl chitosan, the polyvinyl alcohol connects to the fluorinated graphene through hydrogen bonds and is directionally arranged and crystallized at the edges of the fluorinated graphene nanosheets to form a large number of crystalline regions. The carboxymethyl chitosan couples the adjacent crystalline regions through hydrogen bonds, thereby forming a unique and efficient thermal conductive network consisting of carboxymethyl chitosan-crystalline regions-fluorinated graphene. This thermal conductive network enables the composite film to have extremely excellent electrical insulation, mechanical and thermal properties.

[0027] Based on the above findings, the inventors proposed the following technical solutions:

[0028] A polyvinyl alcohol-based composite membrane is composed of polyvinyl alcohol, fluorinated graphene nanosheets and carboxymethyl chitosan in a mass ratio of 100:1.1 to 8.2:8.7. The polyvinyl alcohol is connected to the fluorinated graphene nanosheets through hydrogen bonds and is directionally arranged and crystallized at their edges. The carboxymethyl chitosan couples adjacent polyvinyl alcohol crystal regions through hydrogen bonds, thereby forming a highly efficient thermal conductive network consisting of carboxymethyl chitosan-crystal region-fluorinated graphene.

[0029] Preferably, the mass ratio of polyvinyl alcohol, fluorinated graphene nanosheets and carboxymethyl chitosan is 100:1.1 to 3.4:8.7.

[0030] More preferably, the mass proportion of the fluorinated graphene nanosheets is 3%.

[0031] The preparation method of the polyvinyl alcohol-based composite film as described in any of the above technical solutions is as follows: a polyvinyl alcohol aqueous solution with a concentration of 20 to 50 mg / mL and a fluorinated graphene nanosheet aqueous dispersion of 0.5 to 1 mg / mL are mixed evenly according to the mass ratio, and then a carboxymethyl chitosan aqueous dispersion of 20 to 30 mg / mL is added and mixed evenly. After vacuum drying at a temperature of 60°C to 80°C to form a film, it is peeled off from the carrier.

[0032] Preferably, the preparation method of the fluorinated graphene nanosheets is as follows: dispersing the fluorinated graphene in a solvent, and subjecting the solvent to ultrasonic and centrifugal treatment to obtain exfoliated fluorinated graphene nanosheets.

[0033] Further preferably, the ultrasonic treatment is performed using a water bath sonicator with a power of 30 to 100 W and an ultrasonic time of 18 to 32 h; the rotation speed of the centrifugal treatment is 3000 to 6000 rpm and the treatment time is 15 to 30 min.

[0034] Preferably, the solvent is one or more of isopropyl alcohol, N-methylpyrrolidone, N,N-dimethylformamide and dichloromethane.

[0035] Preferably, the fluorinated graphene nanosheet aqueous dispersion, the polyvinyl alcohol aqueous solution and the carboxymethyl chitosan aqueous dispersion are uniformly mixed by ultrasound and stirring.

[0036] The polyvinyl alcohol-based composite film as described in any of the above technical solutions is used as a protective film on the surface of power electronic components.

[0037] To facilitate public understanding, the technical solution of the present invention is described in detail below through several specific embodiments and in conjunction with the accompanying drawings. The polyvinyl alcohol, fluorinated graphene and carboxymethyl chitosan used in each embodiment are all commercial materials purchased from the market, wherein the molecular weight of polyvinyl alcohol is 1.45×10 5 The size of the fluorinated graphene is 5 to 10 μm, and the fluorine-carbon ratio is 1.1:1; the degree of substitution of carboxymethyl chitosan is ≥80%.

[0038] Example 1

[0039] (1) A suspension of fluorinated graphene and N-methylpyrrolidone was refluxed at 80°C for 2 h, ultrasonicated (40 W) for 24 h, and then centrifuged at 3000 and 6000 rpm for 30 min, respectively; the precipitate at 6000 rpm was washed with deionized water, filtered, and freeze-dried to obtain fluorinated graphene nanosheets; finally, the precipitate was redispersed in water under ultrasonication to obtain a 0.50 mg / mL aqueous dispersion of fluorinated graphene nanosheets.

[0040] (2) Add 2 g of polyvinyl alcohol particles to a beaker containing 100 mL of deionized water, stir at 80°C until completely dissolved, and add deionized water to the 100 mL mark to obtain a 20 mg / mL polyvinyl alcohol aqueous solution.

[0041] (3) Add 2 g of carboxymethyl chitosan powder to a beaker containing 100 mL of deionized water and stir until dissolved to obtain a 20 mg / mL carboxymethyl chitosan aqueous solution.

[0042] (4) First, 6.83 mL of polyvinyl alcohol aqueous solution was slowly dripped into 3.00 mL of fluorinated graphene nanosheet dispersion under stirring; the mixed solution was ultrasonicated for 10 min and stirred for 30 min; then, 0.59 mL of carboxymethyl chitosan aqueous dispersion was slowly dripped into it under stirring, and stirring was continued for 30 min; finally, the prepared mixed solution was poured into a quartz watch glass and dried in a conventional oven at a temperature of 60°C to obtain a uniform light yellow transparent polyvinyl alcohol-based composite film, wherein the mass ratio of polyvinyl alcohol, fluorinated graphene nanosheets, and carboxymethyl chitosan was 100:1.1:8.7. The polyvinyl alcohol-based composite film prepared in this example is hereinafter referred to as PCF-1.

[0043] Example 2

[0044] (1) A suspension of fluorinated graphene and N-methylpyrrolidone was refluxed at 80°C for 2 h, ultrasonicated (40 W) for 24 h, and then centrifuged at 3000 and 6000 rpm for 30 min, respectively; the precipitate at 6000 rpm was washed with deionized water, filtered, and freeze-dried to obtain fluorinated graphene nanosheets; finally, the precipitate was redispersed in water under ultrasonication to obtain a 0.5 mg / mL aqueous dispersion of fluorinated graphene nanosheets.

[0045] (2) Add 3 g of polyvinyl alcohol particles to a beaker containing 100 mL of deionized water, stir at 80°C until completely dissolved, and add deionized water to the 100 mL mark to obtain a 30 mg / mL polyvinyl alcohol aqueous solution.

[0046] (3) Add 2 g of carboxymethyl chitosan powder to a beaker containing 100 mL of deionized water and stir until dissolved to obtain a 20 mg / mL carboxymethyl chitosan aqueous solution.

[0047] (4) First, 4.46 mL of polyvinyl alcohol aqueous solution was slowly dripped into 9.00 mL of fluorinated graphene nanosheet dispersion under stirring, and the mixed solution was ultrasonicated for 10 min and stirred for 30 min; then, 0.58 mL of carboxymethyl chitosan aqueous dispersion was slowly dripped into it under stirring, and stirring was continued for 30 min; finally, the prepared mixed solution was poured into a quartz watch glass and dried in a conventional oven at a temperature of 60°C to obtain a uniform light yellow transparent polyvinyl alcohol-based composite film, wherein the mass ratio of polyvinyl alcohol, fluorinated graphene nanosheets, and carboxymethyl chitosan was 100:3.4:8.7. The polyvinyl alcohol-based composite film prepared in this example is hereinafter referred to as PCF-3.

[0048] Example 3

[0049] (1) A suspension of fluorinated graphene and N-methylpyrrolidone was refluxed at 80°C for 2 h, ultrasonicated (40 W) for 24 h, and then centrifuged at 3000 and 6000 rpm for 30 min, respectively; the precipitate at 6000 rpm was washed with deionized water, filtered, and freeze-dried to obtain fluorinated graphene nanosheets; finally, the precipitate was redispersed in water under ultrasonication to obtain a 1.0 mg / mL aqueous dispersion of fluorinated graphene nanosheets.

[0050] (2) Add 4 g of polyvinyl alcohol particles to a beaker containing 100 mL of deionized water, stir at 80°C until completely dissolved, and add deionized water to the 100 mL mark to obtain a 40 mg / mL polyvinyl alcohol aqueous solution.

[0051] (3) Add 3 g of carboxymethyl chitosan powder to a beaker containing 100 mL of deionized water and stir until dissolved to obtain a 30 mg / mL carboxymethyl chitosan aqueous solution.

[0052] (4) First, 3.28 mL of polyvinyl alcohol aqueous solution was slowly dripped into 7.50 mL of fluorinated graphene nanosheet dispersion under stirring, and the mixed solution was ultrasonicated for 10 min and stirred for 30 min; then, 0.38 mL of carboxymethyl chitosan aqueous dispersion was slowly dripped into it under stirring, and stirring was continued for 30 min; finally, the prepared mixed solution was poured into a quartz watch glass and dried in a conventional oven at a temperature of 60°C to obtain a uniform light yellow transparent polyvinyl alcohol-based composite film, wherein the mass ratio of polyvinyl alcohol, fluorinated graphene nanosheets, and carboxymethyl chitosan was 100:5.7:8.7. The polyvinyl alcohol-based composite film prepared in this example is hereinafter referred to as PCF-5.

[0053] Example 4

[0054] (1) A suspension of fluorinated graphene and N-methylpyrrolidone was refluxed at 80°C for 2 h, ultrasonicated (40 W) for 24 h, and then centrifuged at 3000 and 6000 rpm for 30 min, respectively; the precipitate at 6000 rpm was washed with deionized water, filtered, and freeze-dried to obtain fluorinated graphene nanosheets, which were finally redispersed in water under ultrasonication to obtain a 1.0 mg / mL aqueous dispersion of fluorinated graphene nanosheets.

[0055] (2) Add 5 g of polyvinyl alcohol particles to a beaker containing 100 mL of deionized water, stir at 80°C until completely dissolved, and add deionized water to the 100 mL mark to obtain a 50 mg / mL polyvinyl alcohol aqueous solution.

[0056] (3) Add 3 g of carboxymethyl chitosan powder to a beaker containing 100 mL of deionized water and stir until dissolved to obtain a 30 mg / mL carboxymethyl chitosan aqueous solution.

[0057] (4) First, 2.57 mL of polyvinyl alcohol aqueous solution was slowly dripped into 10.50 mL of fluorinated graphene nanosheet dispersion under stirring, and the mixed solution was ultrasonicated for 10 minutes and stirred for 30 minutes; then, 0.37 mL of carboxymethyl chitosan aqueous dispersion was slowly dripped into it under stirring, and stirring was continued for 30 minutes; finally, the prepared mixed solution was poured into a quartz watch glass and dried in a conventional oven at a temperature of 60°C to obtain a uniform light yellow transparent polyvinyl alcohol-based composite film, wherein the mass ratio of polyvinyl alcohol, fluorinated graphene nanosheets, and carboxymethyl chitosan was 100:8.2:8.7. The polyvinyl alcohol-based composite film prepared in this example is hereinafter referred to as PCF-7.

[0058] In order to verify the technical effect of the present invention, polyvinyl alcohol (PVA) film, polyvinyl alcohol / fluorinated graphene nanosheet (PVA / FGN) composite film, polyvinyl alcohol / carboxymethyl chitosan (PVA / CMC) composite film were prepared respectively, and the performance was compared with the polyvinyl alcohol-based composite film of the present invention; the preparation methods of polyvinyl alcohol film, polyvinyl alcohol / fluorinated graphene nanosheet composite film, and polyvinyl alcohol / carboxymethyl chitosan composite film are shown in Comparative Examples 1 to 3 respectively:

[0059] Comparative Example 1

[0060] 3g of polyvinyl alcohol particles were added to a beaker containing 100mL of deionized water and stirred at 80°C until completely dissolved. Deionized water was then added to the 100mL mark to obtain a 30mg / mL polyvinyl alcohol aqueous solution. The polyvinyl alcohol solution was placed in a quartz watch glass, dried, and peeled off. The in-plane and out-of-plane thermal conductivities were 0.28W / (m·K) and 0.10W / (m·K), respectively.

[0061] Comparative Example 2

[0062] (1) A suspension of fluorinated graphene and N-methylpyrrolidone was refluxed at 80°C for 2 h, ultrasonicated (40 W) for 24 h, and then centrifuged at 3000 rpm and 6000 rpm for 30 min, respectively; the precipitate at 6000 rpm was washed with deionized water, filtered, and freeze-dried to obtain fluorinated graphene nanosheets; the precipitate was then redispersed in water under ultrasonication to obtain a 0.5 mg / mL aqueous dispersion of fluorinated graphene nanosheets.

[0063] (2) Add 5 g of polyvinyl alcohol particles to a beaker containing 100 mL of deionized water, stir at 80°C until completely dissolved, and add deionized water to the 100 mL mark to obtain a 50 mg / mL polyvinyl alcohol aqueous solution.

[0064] (3) First, 2.91 mL of the polyvinyl alcohol solution was slowly added dropwise to 9.00 mL of the fluorinated graphene nanosheet dispersion under stirring. The mixed solution was ultrasonicated for 10 min and stirred for 30 min. The prepared mixed solution was poured into a quartz watch glass and dried in a conventional oven at 60°C to obtain a composite film containing 3 wt.% of fluorinated graphene nanosheets.

[0065] The in-plane and out-of-plane thermal conductivities of the composite film (containing 3 wt.% of fluorinated graphene nanosheets) are 2.34 W / (m·K) and 0.08 W / (m·K), respectively.

[0066] Comparative Example 3

[0067] (1) Add 5 g of polyvinyl alcohol particles into a beaker containing 100 mL of deionized water, stir at 80°C until completely dissolved, and add deionized water to the 100 mL mark to obtain a 50 mg / mL polyvinyl alcohol aqueous solution.

[0068] (2) Add 3 g of carboxymethyl chitosan powder to a beaker containing 100 mL of deionized water and stir until dissolved to obtain a 30 mg / mL carboxymethyl chitosan aqueous solution.

[0069] (3) First, 2.76 mL of polyvinyl alcohol solution was slowly dripped into 0.40 mL of carboxymethyl chitosan aqueous dispersion under stirring, and stirring was continued for 30 min; the prepared mixed solution was poured into a quartz watch glass and dried in a conventional oven at a temperature of 60°C to obtain a composite film containing 8 wt.% of carboxymethyl chitosan.

[0070] The in-plane and out-of-plane thermal conductivities of the composite film (containing 8 wt.% of carboxymethyl cellulose) are 0.81 W / (m·K) and 0.21 W / (m·K), respectively.

[0071] Figure 1 Scanning electron microscope images of the films of Examples 2 and 3 of Comparative Example 1 are shown; wherein (a), (b), and (c) correspond to the surfaces of the three films in sequence, and (d), (e), and (f) correspond to the brittle fracture surfaces of the three films in sequence. Through scanning electron microscopy, it can be observed that the fluorinated graphene nanosheets in the Example 2 film prepared in Example 2 are uniformly distributed in polyvinyl alcohol, without filler accumulation, and the intersheet spacing is about 0.5 μm, while the fluorinated graphene in the Example 3 PCF-5 film is mostly uniformly dispersed, but there is a small amount of filler accumulation, and the intersheet spacing is significantly smaller than that of the Example 2 film, indicating that the amount of fluorinated graphene in the Example 2 film has reached its dispersion threshold in polyvinyl alcohol.

[0072] Figure 2The DSC graphs of the films of Examples 1 to 4 and Comparative Examples 1 to 3 are shown, and Table 1 lists the data related to the DSC graphs and the crystallinity of the composite films. The above data show that the crystallinity of the composite films shows a trend of first increasing and then decreasing with the increase in the amount of fluorinated graphene added, and the crystallinity of the films of Comparative Example 2 and Example 2 is the highest. It can be inferred that polyvinyl alcohol will be oriented and crystallized at the edges of the fluorinated graphene, and the permeation threshold of polyvinyl alcohol is when the fluorinated graphene is 3wt.%. The difference between the two is that the melting point and glass transition temperature of the film of Example 2 are significantly higher than those of the film of Comparative Example 2, indicating that the film of Example 2 has a complete thermal conductivity network and can transfer heat well when heated.

[0073] Table 1

[0074]

[0075] Figure 3 (a) shows the stress-strain curves of the films of Examples 1 to 4 and Comparative Examples 1 to 3, wherein the tensile strength of Example 2 is 61.2 MP, which is lower than the films of Comparative Examples 1 and 3, but significantly higher than the film of Comparative Example 2. This indicates that although the crystallinity of the composite film increases after the addition of fluorinated graphene nanosheets, the random arrangement of the fluorinated graphene nanosheets leads to an increase in the disorder of the crystal regions, and the tensile strength decreases instead. The addition of carboxymethyl chitosan can effectively connect adjacent crystal regions, thereby improving the tensile strength. Figure 3 (b) shows the volume resistivity graph of the films of Examples 1 to 4 and Comparative Examples 1 to 3. The volume resistivity of all films is greater than >10 11 Ω·m, indicating that all prepared films have good insulating properties.

[0076] Figure 4 The histograms of the in-plane and out-of-plane thermal conductivity coefficients of the films of Examples 1 to 4 and Comparative Examples 1 to 3 show that when 3 wt.% of fluorinated graphene and 8 wt.% of carboxymethyl chitosan are added to polyvinyl alcohol, respectively, the in-plane and out-of-plane thermal conductivities of the films of Comparative Examples 2 and 3 are only slightly improved compared to that of the film of Comparative Example 1, while the thermal conductivity of the film of Example 2 is significantly improved, with in-plane and out-of-plane thermal conductivities of 12.22 W / (m·K) and 0.81 W / (m·K), respectively, which are 43.6 and 8.1 times that of the film of Comparative Example 1, indicating that a good thermal conductive network is formed inside the membrane after the addition of carboxymethyl chitosan.

[0077] Figure 5 (a) shows infrared thermal images of LED bulbs with films from Comparative Examples 1 to 3 and Example 2 as substrates. The film from Example 2 can reduce the temperature of the bulb to 52°C in 180s, which is significantly better than the films from Comparative Examples 1 to 3. Moreover, the temperature has basically stabilized after 120s, indicating that the composite film has good comprehensive thermal conductivity. Figure 5(b) shows the infrared thermal imaging images of the films of Comparative Examples 1 to 3 and Example 2 in the vertical plane direction. It can be seen that the temperature of the film of Comparative Example 2 is the lowest at 15s, indicating that its out-of-plane thermal conductivity is the lowest. Although it has a large degree of crystallinity, the disordered arrangement of the crystal regions is not conducive to heat transfer. The temperature of Comparative Example 2 is the highest, indicating that the carboxymethyl chitosan connects the adjacent crystal regions and forms a good thermal conductive network, and its out-of-plane thermal conductivity is significantly improved.

[0078] In the present invention, after adding fluorinated graphene to polyvinyl alcohol, the crystallinity of the prepared composite film was found to increase significantly by DSC calculation results, and since there are a large number of F bonds that can form hydrogen bonds with polyvinyl alcohol at the edge of fluorinated graphene, it is speculated that the increase in crystallinity is the result of polyvinyl alcohol directional arrangement and crystallization at the edge of fluorinated graphene. However, at this time, the random arrangement of fluorinated graphene causes the disorder degree of the crystalline region to increase, and its thermal conductivity increases to a limited extent, and its out-of-plane thermal conductivity is even lower than that of polyvinyl alcohol. After adding carboxymethyl chitosan, its tensile strength and thermal conductivity are greatly improved compared to the polyvinyl alcohol-based composite film to which only fluorinated graphene is added, indicating that carboxymethyl chitosan can connect adjacent crystalline regions through hydrogen bond coupling, thereby forming an efficient heat-conducting network consisting of carboxymethyl chitosan-crystalline region-fluorinated graphene.

[0079] Since the polyvinyl alcohol-based composite film of the present invention has excellent electrical insulation properties, mechanical properties and thermal properties, it can be widely used in applications with high requirements for thermal conductivity and insulation, and is suitable for closed or continuous working scenarios. For example, it can be applied as a protective film on the surface of power electronic components, thereby ensuring electrical insulation while significantly increasing the upper temperature limit of the components through good heat dissipation.

Claims

1. A polyvinyl alcohol-based composite film, characterized in that It is composed of polyvinyl alcohol, fluorinated graphene nanosheets and carboxymethyl chitosan in a mass ratio of 100:1.1~8.2:8.

7. Polyvinyl alcohol connects to the fluorinated graphene nanosheets through hydrogen bonds, and directionally arranges and crystallizes at their edges. Carboxymethyl chitosan couples adjacent polyvinyl alcohol crystal regions through hydrogen bonds, thus forming an efficient thermal conductive network consisting of carboxymethyl chitosan-crystal region-fluorinated graphene.

2. The polyvinyl alcohol-based composite film according to claim 1, wherein The mass ratio of polyvinyl alcohol, fluorinated graphene nanosheets and carboxymethyl chitosan is 100:1.1 to 3.4:8.

7.

3. The polyvinyl alcohol-based composite film according to claim 2, wherein Fluorinated graphene nanosheets account for 3% of the mass.

4. The method for preparing a polyvinyl alcohol-based composite film according to any one of claims 1 to 3, wherein: A polyvinyl alcohol aqueous solution with a concentration of 20-50 mg / mL and a fluorinated graphene nanosheet aqueous dispersion with a concentration of 0.5-1 mg / mL are mixed evenly according to the mass ratio, and then a carboxymethyl chitosan aqueous dispersion with a concentration of 20-30 mg / mL is added and mixed evenly. After vacuum drying at a temperature of 60°C to 80°C to form a film, it is peeled off from the carrier.

5. The preparation method according to claim 4, wherein: The preparation method of the fluorinated graphene nanosheets is as follows: dispersing the fluorinated graphene in a solvent, and subjecting the solvent to ultrasonic and centrifugal treatment to obtain exfoliated fluorinated graphene nanosheets.

6. The preparation method according to claim 5, characterized in that: The ultrasonic treatment was performed using a water bath sonicator with a power of 30-100 W and an ultrasonic time of 18-32 h; the rotation speed of the centrifugal treatment was 3000-6000 rpm and the treatment time was 15-30 min.

7. The preparation method according to claim 5, characterized in that: The solvent is one or more of isopropyl alcohol, N-methylpyrrolidone, N,N-dimethylformamide, and dichloromethane.

8. The preparation method according to claim 4, characterized in that: The fluorinated graphene nanosheet aqueous dispersion, the polyvinyl alcohol aqueous solution and the carboxymethyl chitosan aqueous dispersion are uniformly mixed by ultrasound and stirring.

9. Use of the polyvinyl alcohol-based composite film according to any one of claims 1 to 3 as a protective film on the surface of power electronic components.

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