Graphene composite film and preparation method and application thereof

Abstract

The invention relates to the technical field of graphene composite film preparation, in particular to a graphene composite film and a preparation method and application thereof. According to the preparation method disclosed by the invention, the graphene composite film is prepared by utilizing various modification processes such as aminated lignin modification, carbon nanotube modification, polyimide modification and glutaraldehyde cross-linked polystyrene sulfonic acid modification; the tensile strength, the elongation at break, the electric conductivity and the heat conduction stability of the graphene composite film are remarkably improved, so that the graphene composite film has a good application prospect in molecular devices such as molecular sensors and molecular field effect transistors.

Description

Technical Field

[0001] The present invention relates to the technical field of graphene composite film preparation, and particularly to a graphene composite film, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid development of technology, molecular devices have become a hot research field due to their unique properties and potential application prospects. Among the numerous materials used to construct molecular devices, graphene is regarded as a highly potential basic material due to its excellent electrical, mechanical, and thermal properties, such as extremely high electron mobility, excellent flexibility, and good thermal conductivity. However, when pure graphene is actually applied to molecular devices, there are still some limitations. In some fields, the performance of graphene films is still insufficient. Researchers have tried to compound graphene with other materials to prepare composite films with better performance to meet the growing high-performance requirements of molecular devices.

[0003] Chinese Patent with application number CN202111674620.X discloses a graphene transparent conductive film, a preparation method thereof, and an application thereof. The preparation method of the graphene transparent conductive film includes: a. Using methane as a carbon source and mixing it with a protective gas, and performing a chemical vapor deposition reaction to form a graphene film on a substrate; b. In a vacuum environment at 280°C - 300°C, melting and blending the graphene film and low-density polyethylene in a mass ratio of 100:1 - 1.5 to obtain a blend; c. Preparing the blend into a transparent conductive graphene film by physical vapor deposition in a vacuum condition. This method solves the problems of wrinkles and discontinuity on the surface of the graphene film prepared by the vapor deposition method, and the obtained composite film has both high transparency and high conductivity. However, the addition of low-density polyethylene will also reduce the overall thermal stability. The melting point of low-density polyethylene is relatively low, and in a higher temperature environment, it may soften, deform, or even decompose, thus affecting the performance and structural integrity of the graphene transparent conductive film, which limits the application of this film in some fields with high requirements for thermal stability, such as high-temperature electronic devices. Chinese Patent with application number CN202210636122.4 discloses an amine compound modified graphene film and a preparation method thereof. The method includes: first forming a graphene oxide solution into a film to obtain a graphene oxide film; then sequentially performing amine compound treatment and reduction treatment on the film to obtain a modified film, and the amine compound is an aromatic amine containing two or more amino groups. This invention significantly improves the mechanical strength of the graphene film, enhances the overall performance by improving the axial stress transfer between graphene sheets, and forms an extended π electron cloud in the conjugated structure of large-area graphene sheets, achieving high electron mobility. However, amine group modification mainly improves the conductivity of the film and cannot meet various functional requirements, lacking multi-dimensional performance regulation.

[0004] Therefore, it is of great practical significance to develop a graphene composite film with a simple preparation process and excellent comprehensive performance, as well as its preparation method, and to expand its application in molecular devices. Summary of the Invention

[0005] To solve the above problems, the present invention provides a graphene composite film, its preparation method, and application. Through a multi-step modification process, the invention successfully solves the deficiencies of traditional graphene films in terms of mechanical properties, electrical conductivity, and thermal stability, realizes the preparation of high-performance composite films, and expands their application potential in high-end electronic devices.

[0006] The technical solutions adopted by the present invention to achieve the above objectives are as follows: A preparation method of a graphene composite film, comprising the following steps: Step S1: Add silane coupling agent KH-550 and sulfonated lignin into absolute ethanol, react at pH = 5 - 6 and 60 - 80 °C for 5 - 6 h, and obtain amino-functionalized lignin after purification; Step S2: Add the amino-functionalized lignin into the graphene oxide dispersion, ultrasonicate for 20 - 30 min, then add 1-ethyl-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS), and react at room temperature for 20 - 24 h. After purification, obtain lignin-modified graphene; Step S3: Mix the lignin-modified graphene with the polyamic acid solution, stir for 22 - 24 h, form a film and dry it to obtain a graphene film; Step S4: Immerse the graphene film in the carbon nanotube solution, oscillate at 50 - 60 °C for 1 - 2 h, rinse and dry it to obtain a carbon nanotube-modified graphene film; Step S5: Immerse the carbon nanotube-modified graphene film into the polystyrene sulfonic acid / glutaraldehyde mixed solution, carry out cross-linking reaction at 50 - 60 °C for 2 - 3 h, rinse and dry it to obtain the graphene composite film.

[0007] Further, the preparation method of the sulfonated lignin in Step S1 is as follows: Dissolve alkali lignin in sulfuric acid solution, stir for 20 - 30 min, add sodium sulfite and copper sulfate, react at 80 - 90 °C for 2 - 3 h, and purify to obtain it.

[0008] Further, the mass ratio of the silane coupling agent KH-550 to the sulfonated lignin in Step S1 is 0.05 - 0.1:1, and the addition amount of the sulfonated lignin in absolute ethanol is 0.05 - 0.1 g / mL.

[0009] Further, the concentration of the graphene oxide dispersion liquid in step S2 is 0.2 - 0.5 mg / mL, and the mass ratio of amino - modified lignin, graphene oxide, 1 - ethyl - (3 - dimethylaminopropyl) carbodiimide, and N - hydroxysuccinimide is 1:0.9 - 1.1:0.8 - 1:0.4 - 0.6.

[0010] Further, the mass ratio of the lignin - modified graphene to polyamic acid in step S3 is 1.3 - 1.5:1, and the concentration of the polyamic acid solution is 0.09 - 0.1 g / mL.

[0011] Further, the concentration of the carbon nanotube solution in step S4 is 1 - 2 mg / mL.

[0012] Further, the polystyrene sulfonic acid / glutaraldehyde mixed solution in step S5 is prepared by dissolving polystyrene sulfonic acid and glutaraldehyde in deionized water, and the volume ratio of polystyrene sulfonic acid, glutaraldehyde, and deionized water is 10:1 - 1.2:90 - 95.

[0013] For the graphene composite film prepared by the above - mentioned method, the application of the graphene composite film in molecular sensors and molecular field - effect transistors.

[0014] The present invention has the following beneficial effects: The present invention first prepares amino-functionalized lignin by reacting silane coupling agent KH-550 with sulfonated lignin. Subsequently, the amino-functionalized lignin is mixed with a graphene oxide dispersion and reacted under the catalysis of EDC and NHS to obtain lignin-modified graphene. Then, the lignin-modified graphene is mixed with a polyamic acid solution to form a film and dried. Next, the obtained film is immersed in a carbon nanotube solution and subjected to oscillating treatment. Finally, it is immersed in a mixed solution of polystyrene sulfonic acid and glutaraldehyde for crosslinking. After rinsing and drying, a graphene composite film is obtained. This graphene composite film has excellent tensile strength, elongation at break, conductivity, and thermal stability, which is mainly due to the optimization of the raw material ratio and the specific modification process. The addition of amino-functionalized lignin not only improves the mechanical properties of the composite film but also plays a thermal buffering role through chemical bonds such as amino and hydroxyl groups in its molecular structure, further enhancing the thermal stability of the composite film. In addition, the polar groups of amino-functionalized lignin prevent the excessive stacking of graphene sheets, improve the electron transport efficiency, and thus enhance the conductivity of the composite film; carbon nanotubes provide additional charge transport paths, and polar groups such as amino and carboxyl groups on the polyamic acid molecular chain can form hydrogen bonds or electrostatic interactions with oxygen-containing functional groups such as hydroxyl and carboxyl groups on the graphene surface. This interfacial bonding enhances the binding force between graphene sheets, reduces the breakage of the conductive path caused by sheet slippage, and thus stabilizes the conductive network of the graphene film, significantly improving the conductivity of the composite film; the crosslinking reaction of the polystyrene sulfonic acid / glutaraldehyde mixed solution occurs on the surface of the graphene composite film, constructing a denser three-dimensional network structure. This structural optimization significantly enhances the mechanical properties and thermal stability of the composite film: on the one hand, the crosslinking effect inhibits the sliding and thermal movement of molecular chains, thereby improving the tensile strength of the material; on the other hand, it strengthens the stability of the heat conduction path, making the composite film exhibit more excellent thermal conductivity. Detailed implementation manners

[0015] Next, the technical solutions in the embodiments of the present application will be described clearly and completely in conjunction with 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 those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0016] All raw materials used in the following examples are ordinary commercially available products. The effective ingredient content of alkali lignin is 90%, purchased from Wuhan Lanaibai Medical and Chemical Co., Ltd.; the fixed carbon content of graphene oxide is 99.9%, the particle size is 2 - 3 nm, and the purity is 99.9%, purchased from Changzhou Yaobang Friction Materials Factory; the effective ingredient content of polyamic acid is 99%, purchased from Hubei Shineng Chemical Technology Co., Ltd.; the particle size of carbon nanotubes is 2 - 3 nm, and the content is ≥99.9%, purchased from Ningbo Luofei Nano Technology Co., Ltd.; the molecular weight of polystyrene sulfonic acid is 70000, purchased from Shanghai Yuanye Bio-Technology Co., Ltd.

[0017] Example 1: A preparation method of a graphene composite film, comprising the following steps: Step S1: Add silane coupling agent KH-550 and sulfonated lignin into absolute ethanol, stir and react at 60 °C for 5 h, then dialyze with absolute ethanol for 72 h, changing absolute ethanol every 8 h. The dialysis membrane with a cut-off molecular weight of 3.5 kDa is selected. Then freeze-dry the dialysate at -50 °C for 48 h to obtain amino-lignin. The mass ratio of silane coupling agent KH-550 to sulfonated lignin is 0.05:1, and the addition amount of sulfonated lignin in absolute ethanol is 0.05 g / mL. The preparation method of sulfonated lignin is as follows: dissolve alkali lignin in a sulfuric acid solution with a mass fraction of 15%, stir for 20 min, then add sodium sulfite and copper sulfate. The mass ratio of alkali lignin, sodium sulfite, and copper sulfate is 1:0.5:0.05. The addition amount of alkali lignin in the sulfuric acid solution is 0.1 g / mL. Stir at 80 °C for 2 h, let it stand for layering and then filter. Wash the filtrate with deionized water until the pH is neutral, and then concentrate and cool to crystallize to obtain it. Step S2: Add amino-lignin into a graphene oxide dispersion with a concentration of 0.2 mg / mL, ultrasonicate at a power of 400 W and a frequency of 40 kHz for 20 min, then continue to add EDC and NHS, stir at room temperature for 20 h, centrifuge at 8000 rpm for 20 min, and then dialyze with deionized water for 48 h, changing deionized water every 4 h. The dialysis membrane with a cut-off molecular weight of 3.5 kDa is selected. Then freeze-dry the dialysate at -50 °C for 24 h to obtain lignin-modified graphene. The graphene oxide dispersion is prepared by adding graphene oxide into deionized water and ultrasonically treating it at a power of 300 W and a frequency of 40 kHz for 60 min. The mass ratio of amino-lignin, graphene oxide, EDC, and NHS is 1:0.9:0.8:0.4. Step S3: Mix the lignin-modified graphene with the polyamic acid solution, stir for 22 h, filter and form a film through a mixed cellulose ester membrane with a pore size of 0.22 μm, place it in a vacuum oven at 60 °C and dry for 12 h, then peel it off to obtain a graphene film. The mass ratio of the lignin-modified graphene to the polyamic acid is 1.3:1, the concentration of the polyamic acid solution is 0.09 g / mL, and the solvent is N,N-dimethylacetamide; Step S4: Immerse the graphene film in the carbon nanotube solution, with the mass ratio of the film to the solution volume being 1 g / 20 mL, oscillate at 50 °C for 1 h, take it out and rinse it 3 times with deionized water, and then vacuum dry it at 60 °C until constant weight to obtain a carbon nanotube-modified graphene film. The solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 1 mg / mL; Step S5: Immerse the carbon nanotube-modified graphene film into the polystyrene sulfonic acid / glutaraldehyde mixed solution, with the mass ratio of the film to the solution volume being 1 g / 50 mL, carry out a cross-linking reaction at 50 °C for 2 h, take it out and rinse it 3 times with deionized water, and then vacuum dry it at 60 °C until constant weight to obtain the graphene composite film. The polystyrene sulfonic acid / glutaraldehyde mixed solution is prepared by dissolving polystyrene sulfonic acid and glutaraldehyde in deionized water, and the volume ratio of polystyrene sulfonic acid, glutaraldehyde, and deionized water is 10:1:90.

[0018] Example 2: A preparation method of a graphene composite film, comprising the following steps: Step S1: Add the silane coupling agent KH-550 and sulfonated lignin to absolute ethanol, stir and react at 80 °C for 6 h, then dialyze with absolute ethanol for 72 h, changing the absolute ethanol every 8 h. The dialysis membrane used has a cut-off molecular weight of 3.5 kDa. Then freeze-dry the dialysate at -50 °C for 48 h to obtain amino-functionalized lignin. The mass ratio of the silane coupling agent KH-550 to the sulfonated lignin is 0.1:1, the addition amount of sulfonated lignin in absolute ethanol is 0.1 g / mL. The preparation method of the sulfonated lignin is as follows: Dissolve alkali lignin in a sulfuric acid solution with a mass fraction of 15%, stir for 30 min, then add sodium sulfite and copper sulfate. The mass ratio of alkali lignin, sodium sulfite, and copper sulfate is 1:0.5:0.05. The addition amount of alkali lignin in the sulfuric acid solution is 0.1 g / mL. Stir at 90 °C for 3 h, let it stand for layering and then filter, wash the filtrate with deionized water until the pH is neutral, and then concentrate and cool to crystallize to obtain it; Step S2: Add the aminated lignin into the graphene oxide dispersion with a concentration of 0.5 mg / mL, ultrasonicate for 30 min under the conditions of a power of 400 W and a frequency of 40 kHz, then continue to add EDC and NHS, stir at room temperature for 24 h, centrifuge at 8000 rpm for 20 min, and then dialyze with deionized water for 48 h, changing the deionized water every 4 h. The dialysis membrane with a molecular weight cut-off of 3.5 kDa is selected. Then freeze-dry the dialysate at -50 °C for 24 h to obtain lignin-modified graphene. The graphene oxide dispersion is prepared by adding graphene oxide into deionized water and ultrasonicating for 60 min under the conditions of a power of 300 W and a frequency of 40 kHz. The mass ratio of aminated lignin, graphene oxide, EDC, and NHS is 1:1.1:1:0.6; Step S3: Mix the lignin-modified graphene with the polyamic acid solution, stir for 24 h, filter through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, place it in a vacuum oven at 60 °C and dry for 12 h, and then peel it off to obtain a graphene film. The mass ratio of lignin-modified graphene to polyamic acid is 1.5:1. The concentration of the polyamic acid solution is 0.1 g / mL, and the solvent is N,N-dimethylacetamide; Step S4: Immerse the graphene film in the carbon nanotube solution, with the mass ratio of the film to the solution volume being 5 g / 100 mL, oscillate at 60 °C for 2 h, take it out and rinse it 3 times with deionized water, and then vacuum dry it at 60 °C to constant weight to obtain a carbon nanotube-modified graphene film. The solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 2 mg / mL; Step S5: Immerse the carbon nanotube-modified graphene film into the polystyrene sulfonic acid / glutaraldehyde mixed solution, with the mass ratio of the film to the solution volume being 1 g / 50 mL, carry out a cross-linking reaction at 60 °C for 3 h, take it out and rinse it 3 times with deionized water, and then vacuum dry it at 60 °C to constant weight to obtain the graphene composite film. The polystyrene sulfonic acid / glutaraldehyde mixed solution is prepared by dissolving polystyrene sulfonic acid and glutaraldehyde in deionized water. The volume ratio of polystyrene sulfonic acid, glutaraldehyde, and deionized water is 10:1.2:95.

[0019] Example 3: A method for preparing a graphene composite film, comprising the following steps: Step S1: Add silane coupling agent KH-550 and sulfonated lignin into absolute ethanol, stir and react at 70 °C for 5.5 h, then dialyze with absolute ethanol for 72 h, changing absolute ethanol every 8 h. The dialysis membrane with a cut-off molecular weight of 3.5 kDa is selected. Then freeze-dry the dialysate at -50 °C for 48 h to obtain amino-functionalized lignin. The mass ratio of silane coupling agent KH-550 to sulfonated lignin is 0.08:1, the addition amount of sulfonated lignin in absolute ethanol is 0.07 g / mL. The preparation method of sulfonated lignin is as follows: Dissolve alkali lignin in a sulfuric acid solution with a mass fraction of 15%, stir for 25 min, then add sodium sulfite and copper sulfate. The mass ratio of alkali lignin, sodium sulfite, and copper sulfate is 1:0.5:0.05. The addition amount of alkali lignin in the sulfuric acid solution is 0.1 g / mL. Stir at 85 °C for 2.5 h, let it stand for layering and then filter. Wash the filtrate with deionized water until the pH is neutral, and then concentrate and cool to crystallize to obtain it; Step S2: Add amino-functionalized lignin into a graphene oxide dispersion with a concentration of 0.3 mg / mL, ultrasonicate at a power of 400 W and a frequency of 40 kHz for 25 min, then continue to add EDC and NHS, stir at room temperature for 24 h, centrifuge at 8000 rpm for 20 min, and then dialyze with deionized water for 48 h, changing deionized water every 4 h. The dialysis membrane with a cut-off molecular weight of 3.5 kDa is selected. Then freeze-dry the dialysate at -50 °C for 24 h to obtain lignin-modified graphene. The graphene oxide dispersion is prepared by adding graphene oxide into deionized water and ultrasonicate at a power of 300 W and a frequency of 40 kHz for 60 min. The mass ratio of amino-functionalized lignin, graphene oxide, EDC, and NHS is 1:1:0.9:0.5; Step S3: Mix lignin-modified graphene with polyamic acid solution, stir for 23 h, filter through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, place it in a vacuum oven at 60 °C and dry for 12 h, then peel to obtain a graphene film. The mass ratio of lignin-modified graphene to polyamic acid is 1.4:1, the concentration of the polyamic acid solution is 0.095 g / mL, and the solvent is N,N-dimethylacetamide; Step S4: Immerse the graphene film in a carbon nanotube solution, with the mass ratio of the film to the solution volume being 5 g / 100 mL, oscillate at 55 °C for 1.5 h, take it out and rinse 3 times with deionized water, and then vacuum dry at 60 °C to constant weight to obtain a carbon nanotube-modified graphene film. The solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 1.5 mg / mL; Step S5: Immerse the carbon nanotube-modified graphene film into the polystyrene sulfonic acid / glutaraldehyde mixed solution with a film mass to solution volume ratio of 1 g / 50 mL, conduct a crosslinking reaction at 55 °C for 2.5 h, take it out and rinse it 3 times with deionized water, and then vacuum dry it at 60 °C until constant weight to obtain the graphene composite film. The polystyrene sulfonic acid / glutaraldehyde mixed solution is prepared by dissolving polystyrene sulfonic acid and glutaraldehyde in deionized water, and the volume ratio of polystyrene sulfonic acid, glutaraldehyde, and deionized water is 10:1.1:93.

[0020] Comparative Example 1: A preparation method of a graphene composite film, comprising the following steps:

[0021] Step S1: The same as Step S1 in Example 3; Step S2: Add the amino-functionalized lignin to the graphene oxide dispersion with a concentration of 0.1 mg / mL, ultrasonicate it for 10 min under the conditions of a power of 400 W and a frequency of 40 kHz, then continue to add EDC and NHS, stir at room temperature for 5 h, centrifuge at a rotation speed of 8000 rpm for 20 min, and then dialyze it with deionized water for 48 h, changing the deionized water every 4 h. The dialysis membrane used has a cut-off molecular weight of 3.5 kDa. Then freeze-dry the dialysate at -50 °C for 24 h to obtain the lignin-modified graphene. The graphene oxide dispersion is prepared by adding graphene oxide to deionized water and ultrasonicating it for 60 min under the conditions of a power of 300 W and a frequency of 40 kHz. The mass ratio of amino-functionalized lignin, graphene oxide, EDC, and NHS is 1:2:0.9:0.1; Step S3: Mix the lignin-modified graphene with the polyamic acid solution, stir for 10 h, filter it through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, place it in a vacuum oven at 60 °C and dry it for 12 h, and then peel it off to obtain the graphene film. The mass ratio of lignin-modified graphene to polyamic acid is 1:1, the concentration of the polyamic acid solution is 0.095 g / mL, and the solvent is N,N-dimethylacetamide; Step S4: Immerse the graphene film in the carbon nanotube solution with a film mass to solution volume ratio of 5 g / 100 mL, oscillate at room temperature for 1.5 h, take it out and rinse it 3 times with deionized water, and then vacuum dry it at 60 °C until constant weight to obtain the carbon nanotube-modified graphene film. The solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 1.5 mg / mL; Step S5: Immerse the carbon nanotube-modified graphene film into the polystyrene sulfonic acid / glutaraldehyde mixed solution, carry out cross-linking reaction at room temperature for 1 h, take it out and rinse it 3 times with deionized water, and then vacuum dry it at 60 °C until constant weight to obtain the graphene composite film. The polystyrene sulfonic acid / glutaraldehyde mixed solution is prepared by dissolving polystyrene sulfonic acid and glutaraldehyde in deionized water, and the volume ratio of polystyrene sulfonic acid, glutaraldehyde, and deionized water is 10:0.1:100.

[0022] Comparative Example 2: A preparation method of a graphene composite film is prepared according to the method described in Example 3, except that: Step S5 is omitted.

[0023] Comparative Example 3: A preparation method of a graphene composite film includes the following steps: Step S1: Add alkali lignin to the graphene oxide dispersion with a concentration of 0.3 mg / mL, ultrasonicate it for 25 min under the conditions of a power of 400 W and a frequency of 40 kHz, then continue to add EDC and NHS, stir at room temperature for 24 h, centrifuge at 8000 rpm for 20 min, and then dialyze it with deionized water for 48 h, changing the deionized water every 4 h. The dialysis membrane used for dialysis has a cut-off molecular weight of 3.5 kDa. Then freeze-dry the dialysate at -50 °C for 24 h to obtain lignin-modified graphene. The graphene oxide dispersion is prepared by adding graphene oxide to deionized water and ultrasonicate it for 60 min under the conditions of a power of 300 W and a frequency of 40 kHz. The mass ratio of alkali lignin, graphene oxide, EDC, and NHS is 1:1:0.9:0.5; Step S2: Mix the lignin-modified graphene with the polyamic acid solution, stir for 23 h, filter it through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, place it in a vacuum oven at 60 °C and dry it for 12 h and then peel it off to obtain the graphene film. The mass ratio of lignin-modified graphene to polyamic acid is 1.4:1, the concentration of the polyamic acid solution is 0.095 g / mL, and the solvent is N,N-dimethylacetamide; Step S3: The same as Step S4 in Example 3 to obtain the graphene composite film.

[0024] Comparative Example 4: A preparation method of a graphene composite film, prepared according to the method described in Example 3, with the difference that: in step S3, the polyamic acid solution is replaced with N,N-dimethylacetamide; and step S4 is omitted.

[0025] Mechanical properties: Samples of 150 mm × 10 mm were prepared according to the requirements in GB / T 1040.3-2006 "Plastics - Determination of tensile properties - Part 3: Test conditions for films and sheets", with a tensile rate of 10 mm / min, and the tensile strength and elongation at break of the film were tested; Electrical conductivity: According to ASTM F84, a four-probe resistivity meter was used to test the conductivity coefficient of the film; Thermal stability: A thermal cycling experiment was carried out at -10°C - 80°C, and the number of cycles was 0, 200, and 400 times, and the thermal conductivity was tested in turn.

[0026] Table 1 Performance test results

[0027] As can be seen from Table 1, the graphene composite films prepared in Examples 1-3 have excellent tensile strength, elongation at break, conductivity coefficient, and thermal stability. Compared with Examples 1-3, the performance of the graphene composite film prepared in Comparative Example 1 has decreased, indicating that the range of raw material ratios proposed in the present invention is the best range for preparing graphene composite films. Compared with Examples 1-3, the tensile strength, elongation at break, and thermal stability of the graphene composite film prepared in Comparative Example 2 have decreased. In the preparation process of Comparative Example 2, the step of cross-linking glutaraldehyde and polystyrene sulfonic acid on the film surface is missing. It can be seen from this that the better tensile strength of Examples 1-3 is due to the cross-linking reaction between glutaraldehyde and the sulfonic acid group in polystyrene sulfonic acid. The cross-linking reaction makes chemical bonds form between the molecular chains of polystyrene sulfonic acid, thus forming a relatively dense cross-linked network. This cross-linked network can significantly improve the mechanical strength of the graphene film. The existence of the cross-linked network makes the sliding between molecular chains more difficult, thus improving the tensile strength of the material; while the flexible chain segments of polystyrene sulfonic acid can relieve stress concentration and avoid brittle fracture, thus improving the flexibility of the film. The better thermal stability of Examples 1-3 is because the chemical bonds formed by the cross-linked network can enhance the intermolecular interaction and improve the thermal stability of the graphene film. The cross-linked network restricts the thermal movement of the molecular chains of polystyrene sulfonic acid at high temperatures, making it difficult to undergo thermal changes such as decomposition, degradation, or phase transition. At the same time, the cross-linked network and graphene have a synergistic effect, which can quickly conduct heat and maintain structural stability at high temperatures, helping to evenly disperse heat in the film and avoid the decline of material performance caused by local overheating.

[0028] Compared with Examples 1-3, the tensile strength, elongation at break, conductivity, and thermal stability of the graphene composite film prepared in Comparative Example 3 decreased. In the preparation process of Comparative Example 3, the crosslinking of glutaraldehyde and polystyrene sulfonic acid on the film surface and the introduction of amino-functionalized lignin were missing. Therefore, one reason for the better tensile strength of Examples 1-3 is that amino-functionalized lignin has high mechanical strength and good flexibility. When amino-functionalized lignin is mixed with graphene, the fibrous structure of lignin can interact with the two-dimensional structure of graphene to form a reinforced composite network. The mechanical enhancement of amino-functionalized lignin and the crosslinking network on the film surface work synergistically to further improve the mechanical properties of the graphene film. In addition, Examples 1-3 have better thermal stability because the molecular structure of amino-functionalized lignin contains a large number of chemical bonds, which can absorb a certain amount of energy when heated, playing a role in thermal buffering. At the same time, the interaction between amino-functionalized lignin and graphene forms a stable composite structure, which restricts the thermal motion of molecular chains, increases the thermal decomposition temperature of the material, maintains the thermal stability of the film, and has a synergistic effect with the crosslinking network on the film surface. Although amino-functionalized lignin generally has average electrical properties, its introduction changes the stacking mode of graphene sheets. The polar groups of amino-functionalized lignin will generate a certain steric hindrance between graphene sheets, preventing the excessive stacking of graphene sheets and facilitating the transmission of electrons between sheets. Therefore, the conductivity of the graphene film prepared in Examples 1-3 is higher.

[0029] Compared with Examples 1-3, the conductivity and thermal stability of the graphene composite film prepared in Comparative Example 4 decreased. In the preparation process of Comparative Example 4, the process of composite with carbon nanotube solution and polyamic acid solution was missing. Carbon nanotubes have excellent conductivity and can significantly improve the electrical properties of the composite film. Moreover, the nano-scale structure of carbon nanotubes can also provide additional charge transport paths to improve the charge transport efficiency. Polar groups such as amino and carboxyl groups on the polyamic acid molecular chain can form hydrogen bonds or electrostatic interactions with oxygen-containing functional groups such as hydroxyl and carboxyl groups on the graphene surface. This interfacial bonding enhances the binding force between graphene sheets, reduces the breakage of the conductive path caused by sheet slippage, and thus stabilizes the conductive network of the graphene film and improves the conductivity of the film. The better thermal stability of Examples 1-3 is because carbon nanotubes all have good thermal conductivity and can quickly conduct heat, while polyamic acid has a certain thermal stability and will not decompose rapidly at high temperatures. This stability enables polyamic acid to maintain its structure and properties in a high-temperature environment, thus providing additional thermal protection for the graphene film.

[0030] Although embodiments of the present application have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present application. The scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a graphene composite film, characterized in that, It includes the following steps: Step S1: Add silane coupling agent KH-550 and sulfonated lignin into absolute ethanol, react at pH = 5-6 and 60-80 °C for 5-6 h, and obtain amino-functionalized lignin after purification; Step S2: Add the amino-functionalized lignin into the graphene oxide dispersion, ultrasonicate for 20-30 min, then add 1-ethyl-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, and react at room temperature for 20-24 h. After purification, lignin-modified graphene is obtained; Step S3: Mix the lignin-modified graphene with the polyamic acid solution, stir for 22-24 h, form a film and dry it to obtain a graphene film; Step S4: Immerse the graphene film in the carbon nanotube solution, oscillate at 50-60 °C for 1-2 h, rinse and dry it to obtain a carbon nanotube-modified graphene film; Step S5: Immerse the carbon nanotube-modified graphene film in the polystyrene sulfonic acid / glutaraldehyde mixed solution, carry out a cross-linking reaction at 50-60 °C for 2-3 h, rinse and dry it to obtain the graphene composite film.

2. The preparation method of the graphene composite film according to claim 1, characterized in that The preparation method of the sulfonated lignin described in Step S1 is: dissolve alkali lignin in sulfuric acid solution, stir for 20-30 min, add sodium sulfite and copper sulfate, react at 80-90 °C for 2-3 h, and purify to obtain it.

3. The preparation method of the graphene composite film according to claim 1, wherein, In Step S1, the mass ratio of the silane coupling agent KH-550 to the sulfonated lignin is 0.05-0.1:1, and the addition amount of the sulfonated lignin in absolute ethanol is 0.05-0.1 g / mL.

4. The preparation method of the graphene composite film according to claim 1, characterized in that, In Step S2, the concentration of the graphene oxide dispersion is 0.2-0.5 mg / mL, and the mass ratio of the amino-functionalized lignin, graphene oxide, 1-ethyl-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide is 1:0.9-1.1:0.8-1:0.4-0.

6.

5. The preparation method of the graphene composite film according to claim 1, characterized in that, In Step S3, the mass ratio of the lignin-modified graphene to the polyamic acid is 1.3-1.5:1, and the concentration of the polyamic acid solution is 0.09-0.1 g / mL.

6. The preparation method of the graphene composite film according to claim 1, characterized in that, In Step S4, the concentration of the carbon nanotube solution is 1-2 mg / mL.

7. The preparation method of the graphene composite film according to claim 1, wherein, In Step S5, the polystyrene sulfonic acid / glutaraldehyde mixed solution is prepared by dissolving polystyrene sulfonic acid and glutaraldehyde in deionized water, and the volume ratio of the polystyrene sulfonic acid, glutaraldehyde and deionized water is 10:1-1.2:90-95.

8. The graphene composite film prepared by the method described in any one of claims 1-7.

9. The application of the graphene composite film described in claim 8 in molecular sensors and molecular field effect transistors.

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