A graphene composite film and its preparation method and application

Abstract

The present invention relates to the technical field of graphene composite film preparation, and more particularly to a graphene composite film, its preparation method, and its applications. The present invention utilizes multiple modification processes, including amino lignin modification, carbon nanotube modification, polyimide modification, and glutaraldehyde-crosslinked polystyrene sulfonic acid modification, to prepare the graphene composite film. By optimizing the raw material ratio and preparation process, the graphene composite film significantly improves its tensile strength, elongation at break, electrical conductivity, and thermal stability, showing promising application prospects 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 in particular to a graphene composite film and a preparation method and application thereof. Background Art

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

[0003] Chinese patent application number CN202111674620.X discloses a graphene transparent conductive film, its preparation method, and application. The preparation method of the graphene transparent conductive film includes: a. using methane as a carbon source to mix with a protective gas and perform a chemical vapor deposition reaction to form a graphene film on a substrate; b. melt-blending the graphene film with low-density polyethylene in a mass ratio of 100:1-1.5 in a vacuum environment at 280°C-300°C to obtain a blend; c. preparing the blend into a transparent conductive graphene film by physical vapor deposition under vacuum conditions. This method solves the problems of wrinkles and discontinuities on the surface of graphene films prepared by vapor deposition, and the resulting 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. In a high-temperature environment, it may soften, deform, or even decompose, thereby affecting the performance and structural integrity of the graphene transparent conductive film. This limits the application of the film in some fields with high thermal stability requirements, such as high-temperature electronic devices. Chinese patent application number CN202210636122.4 discloses an amine compound modified graphene film and a preparation method thereof, the method comprising: first forming a film of graphene oxide solution to obtain a graphene oxide film; then sequentially treating the film with an amine compound and reducing it to obtain a modified film, wherein 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 the graphene sheets, and the conjugated structure of the large-area graphene sheet forms an extended π electron cloud, achieving high electron mobility. However, amine modification mainly improves the conductivity of the film, cannot meet multiple functional requirements, and lacks multi-dimensional performance regulation.

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

[0005] To address these issues, the present invention provides a graphene composite film, its preparation method, and its application. Through a multi-step modification process, this invention successfully addresses the deficiencies of conventional graphene films in mechanical properties, electrical conductivity, and thermal stability. This enables the preparation of high-performance composite films and expands their potential for application in high-end electronic devices.

[0006] The technical solutions adopted by the present invention to achieve the above-mentioned purpose are:

[0007] A method for preparing a graphene composite film comprises the following steps:

[0008] Step S1, adding silane coupling agent KH-550 and sulfonated lignin to anhydrous ethanol, reacting at pH = 5-6 and 60-80° C. for 5-6 hours, and obtaining amino lignin after purification;

[0009] Step S2, adding the amino lignin to the graphene oxide dispersion, ultrasonicating for 20-30 minutes, then adding 1-ethyl-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS), reacting at room temperature for 20-24 hours, and obtaining lignin-modified graphene after purification;

[0010] Step S3, mixing the lignin-modified graphene with the polyamic acid solution, stirring for 22-24 hours, forming a film, and drying to obtain a graphene film;

[0011] Step S4, immersing the graphene film in a carbon nanotube solution, shaking at 50-60° C. for 1-2 hours, rinsing, and drying to obtain a carbon nanotube-modified graphene film;

[0012] Step S5: immersing the carbon nanotube-modified graphene film in a polystyrene sulfonic acid / glutaraldehyde mixed solution, performing a cross-linking reaction at 50-60° C. for 2-3 hours, and then rinsing and drying to obtain a graphene composite film.

[0013] Furthermore, the preparation method of the sulfonated lignin in step S1 is: dissolving alkali lignin in sulfuric acid solution, stirring for 20-30 minutes, adding sodium sulfite and copper sulfate, reacting at 80-90° C. for 2-3 hours, and purifying to obtain the sulfonated lignin.

[0014] Furthermore, 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 amount of the sulfonated lignin added to the anhydrous ethanol is 0.05-0.1 g / mL.

[0015] Furthermore, the concentration of the graphene oxide dispersion in step S2 is 0.2-0.5 mg / mL, and the mass ratio of amination lignin, graphene oxide, 1-ethyl-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide is 1:0.9-1.1:0.8-1:0.4-0.6.

[0016] Furthermore, 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.

[0017] Furthermore, the concentration of the carbon nanotube solution in step S4 is 1-2 mg / mL.

[0018] Furthermore, 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 the polystyrene sulfonic acid, glutaraldehyde and deionized water is 10:1-1.2:90-95.

[0019] The graphene composite film prepared by the above method is used in molecular sensors and molecular field effect transistors.

[0020] The present invention has the following beneficial effects:

[0021] The present invention first prepares amination lignin by reacting silane coupling agent KH-550 with sulfonated lignin, then mixes the amination lignin with graphene oxide dispersion, reacts under EDC and NHS catalysis to obtain lignin-modified graphene, then mixes the lignin-modified graphene with a polyamic acid solution to form a film and dries it, then immerses the obtained film in a carbon nanotube solution for oscillation treatment, finally immerses it in a mixed solution of polystyrene sulfonic acid and glutaraldehyde for cross-linking, and obtains a graphene composite film after rinsing and drying. The graphene composite film has excellent tensile strength, elongation at break, electrical conductivity and thermal stability, which is mainly due to the optimization of the raw material ratio and the specific modification process. The addition of amination 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-lignin prevent excessive stacking of graphene sheets, improve the efficiency of electron transfer, and thus enhance the conductivity of the composite film. Carbon nanotubes provide additional charge transfer pathways, while polar groups such as amino and carboxyl groups on the polyamic acid molecular chains 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 and reduces the breakage of the conductive path caused by sheet slippage, thereby stabilizing the conductive network of the graphene film and significantly improving the conductivity of the composite film. The polystyrene sulfonic acid / glutaraldehyde mixed solution undergoes a cross-linking reaction 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 cross-linking effect inhibits the sliding and thermal motion of the molecular chains, thereby improving the tensile strength of the material; on the other hand, it strengthens the stability of the thermal conduction path, making the composite film exhibit better thermal conductivity. DETAILED DESCRIPTION

[0022] The following will be combined with the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0023] All raw materials used in the following examples are commercially available products. Alkali lignin with an active ingredient content of 90% was purchased from Wuhan Lanabai Pharmaceutical Chemical Co., Ltd.; graphene oxide with a fixed carbon content of 99.9%, a particle size of 2-3 nm, and a purity of 99.9% was purchased from Changzhou Yaobang Friction Material Factory; polyamic acid with an active ingredient content of 99% was purchased from Hubei Shineng Chemical Technology Co., Ltd.; carbon nanotubes with a particle size of 2-3 nm and a purity of ≥99.9% were purchased from Ningbo Luofei Nanotechnology Co., Ltd.; and polystyrene sulfonic acid with a molecular weight of 70,000 was purchased from Shanghai Yuanye Biotechnology Co., Ltd.

[0024] Example 1: A method for preparing a graphene composite film, comprising the following steps:

[0025] Step S1: add silane coupling agent KH-550 and sulfonated lignin to anhydrous ethanol, stir and react at 60°C for 5 hours, and then dialyze with anhydrous ethanol for 72 hours, changing anhydrous ethanol every 8 hours. The molecular weight cut-off of the dialysis is 3.5. kDa dialysis membrane, and then freeze-drying the dialyzate at -50°C for 48 hours to obtain amino lignin, wherein the mass ratio of silane coupling agent KH-550 and sulfonated lignin is 0.05:1, and the amount of sulfonated lignin added to anhydrous ethanol is 0.05 g / mL. The preparation method of sulfonated lignin is as follows: dissolving alkali lignin in a 15% sulfuric acid solution by mass fraction, stirring for 20 minutes, and then adding sodium sulfite and copper sulfate, wherein the mass ratio of alkali lignin, sodium sulfite, and copper sulfate is 1:0.5:0.05, and the amount of alkali lignin added to the sulfuric acid solution is 0.1 g / mL, stirring at 80°C for 2 hours, standing for stratification, and then filtering, washing the filtrate with deionized water until the pH is neutral, and then concentrating and cooling for crystallization to obtain;

[0026] Step S2, adding the amination lignin to a 0.2 mg / mL graphene oxide dispersion, ultrasonically treating the dispersion at a power of 400 W and a frequency of 40 kHz for 20 min, then adding EDC and NHS, stirring at room temperature for 20 h, centrifuging at 8000 rpm for 20 min, and then dialyzing with deionized water for 48 h, changing the deionized water every 4 h, using a dialysis membrane with a molecular weight cutoff of 3.5 kDa, and then freeze-drying the dialyzate at -50 ° C for 24 h to obtain lignin-modified graphene, wherein the graphene oxide dispersion is prepared by adding graphene oxide to deionized water and ultrasonically treating the dispersion at a power of 300 W and a frequency of 40 kHz for 60 min, and the mass ratio of amination lignin, graphene oxide, EDC and NHS is 1:0.9:0.8:0.4;

[0027] Step S3, mixing the lignin-modified graphene with the polyamic acid solution, stirring for 22 hours, filtering through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, placing it in a vacuum oven at 60°C for 12 hours, and then peeling it off to obtain a graphene film, wherein the mass ratio of lignin-modified graphene to 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;

[0028] Step S4, immersing the graphene film in a carbon nanotube solution with a film mass to solution volume ratio of 1 g / 20 mL, shaking at 50° C. for 1 h, taking out and rinsing with deionized water three times, and then vacuum drying at 60° C. to constant weight to obtain a carbon nanotube-modified graphene film, wherein the solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 1 mg / mL;

[0029] Step S5, immersing the carbon nanotube-modified graphene film in a polystyrene sulfonic acid / glutaraldehyde mixed solution with a film mass to solution volume ratio of 1 g / 50 mL, cross-linking reaction at 50°C for 2 h, taking out and rinsing with deionized water three times, and then vacuum drying at 60°C to constant weight to obtain a graphene composite film, wherein 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.

[0030] Example 2: A method for preparing a graphene composite film, comprising the following steps:

[0031] Step S1: add silane coupling agent KH-550 and sulfonated lignin to anhydrous ethanol, stir and react at 80°C for 6 hours, and then dialyze with anhydrous ethanol for 72 hours, changing anhydrous ethanol every 8 hours. The molecular weight cut-off of the dialysis is 3.5. kDa dialysis membrane, and then freeze-drying the dialyzate at -50°C for 48 hours to obtain amino lignin, wherein the mass ratio of silane coupling agent KH-550 and sulfonated lignin is 0.1:1, and the amount of sulfonated lignin added to anhydrous ethanol is 0.1 g / mL. The preparation method of sulfonated lignin is as follows: dissolving alkali lignin in a 15% sulfuric acid solution by mass fraction, stirring for 30 minutes, and then adding sodium sulfite and copper sulfate, wherein the mass ratio of alkali lignin, sodium sulfite, and copper sulfate is 1:0.5:0.05, and the amount of alkali lignin added to the sulfuric acid solution is 0.1 g / mL, stirring at 90°C for 3 hours, standing for stratification, and then filtering, washing the filtrate with deionized water until the pH is neutral, and then concentrating and cooling to crystallize, thereby obtaining;

[0032] Step S2, adding the amination lignin to a 0.5 mg / mL graphene oxide dispersion, ultrasonically treating the dispersion at a power of 400 W and a frequency of 40 kHz for 30 min, then adding EDC and NHS, stirring at room temperature for 24 h, centrifuging at 8000 rpm for 20 min, and then dialyzing with deionized water for 48 h, changing the deionized water every 4 h, using a dialysis membrane with a molecular weight cutoff of 3.5 kDa, and then freeze-drying the dialyzate at -50 ° C for 24 h to obtain lignin-modified graphene, wherein the graphene oxide dispersion is prepared by adding graphene oxide to deionized water and ultrasonically treating the dispersion at a power of 300 W and a frequency of 40 kHz for 60 min, and the mass ratio of amination lignin, graphene oxide, EDC and NHS is 1:1.1:1:0.6;

[0033] Step S3, mixing the lignin-modified graphene with the polyamic acid solution, stirring for 24 hours, filtering through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, placing it in a vacuum oven at 60°C for 12 hours, and then peeling it off to obtain a graphene film, wherein 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;

[0034] Step S4, immersing the graphene film in a carbon nanotube solution with a film mass to solution volume ratio of 5 g / 100 mL, shaking at 60° C. for 2 h, taking out and rinsing with deionized water three times, and then vacuum drying at 60° C. to constant weight to obtain a carbon nanotube-modified graphene film, wherein the solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 2 mg / mL;

[0035] Step S5, immersing the carbon nanotube-modified graphene film in a polystyrene sulfonic acid / glutaraldehyde mixed solution with a film mass to solution volume ratio of 1 g / 50 mL, cross-linking reaction at 60°C for 3 h, taking out and rinsing with deionized water 3 times, and then vacuum drying at 60°C to constant weight to obtain a graphene composite film, wherein 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 10:1.2:95.

[0036] Example 3: A method for preparing a graphene composite film, comprising the following steps:

[0037] Step S1, adding silane coupling agent KH-550 and sulfonated lignin to anhydrous ethanol, stirring and reacting at 70 ° C for 5.5 hours, then dialyzing with anhydrous ethanol for 72 hours, changing anhydrous ethanol every 8 hours, and using a dialysis membrane with a molecular weight cutoff of 3.5kDa. The dialyzate is then freeze-dried at -50 ° C for 48 hours to obtain amino lignin, wherein the mass ratio of silane coupling agent KH-550 to sulfonated lignin is 0.08:1, and the addition of sulfonated lignin to anhydrous ethanol is 0.08:1. The amount of sulfonated lignin added is 0.07 g / mL. The preparation method of sulfonated lignin is as follows: alkali lignin is dissolved in a 15% sulfuric acid solution, stirred for 25 minutes, and then sodium sulfite and copper sulfate are added. The mass ratio of alkali lignin, sodium sulfite and copper sulfate is 1:0.5:0.05. The amount of alkali lignin added to the sulfuric acid solution is 0.1 g / mL. Stir at 85°C for 2.5 hours, let it stand for stratification and then filter. The filtrate is washed with deionized water until the pH is neutral, and then concentrated and cooled for crystallization to obtain the product.

[0038] Step S2, adding the amino lignin to a 0.3 mg / mL graphene oxide dispersion, ultrasonically treating the dispersion at a power of 400 W and a frequency of 40 kHz for 25 min, then adding EDC and NHS, stirring at room temperature for 24 h, centrifuging at 8000 rpm for 20 min, and then dialyzing with deionized water for 48 h, changing the deionized water every 4 h, using a dialysis membrane with a molecular weight cutoff of 3.5 kDa, and then freeze-drying the dialyzate at -50 ° C for 24 h to obtain lignin-modified graphene, wherein the graphene oxide dispersion is prepared by adding graphene oxide to deionized water and ultrasonically treating the dispersion at a power of 300 W and a frequency of 40 kHz for 60 min, and the mass ratio of the amino lignin, graphene oxide, EDC and NHS is 1:1:0.9:0.5;

[0039] Step S3, mixing the lignin-modified graphene with the polyamic acid solution, stirring for 23 hours, filtering through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, placing it in a vacuum oven at 60°C for 12 hours, and then peeling it off to obtain a graphene film, wherein 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;

[0040] Step S4, immersing the graphene film in a carbon nanotube solution with a film mass to solution volume ratio of 5 g / 100 mL, shaking at 55° C. for 1.5 h, taking out and rinsing with deionized water three times, and then vacuum drying at 60° C. to constant weight to obtain a carbon nanotube-modified graphene film, wherein the solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 1.5 mg / mL;

[0041] Step S5, immersing the carbon nanotube-modified graphene film in a polystyrene sulfonic acid / glutaraldehyde mixed solution with a film mass to solution volume ratio of 1 g / 50 mL, cross-linking reaction at 55°C for 2.5 h, taking out and rinsing with deionized water three times, and then vacuum drying at 60°C to constant weight to obtain a graphene composite film, wherein 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.

[0042] Comparative Example 1:

[0043] A method for preparing a graphene composite film comprises the following steps:

[0044] Step S1, same as step S1 in embodiment 3;

[0045] Step S2, adding the amination lignin to a 0.1 mg / mL graphene oxide dispersion, ultrasonically treating the dispersion at a power of 400 W and a frequency of 40 kHz for 10 min, then adding EDC and NHS, stirring at room temperature for 5 h, centrifuging at 8000 rpm for 20 min, and then dialyzing with deionized water for 48 h, changing the deionized water every 4 h, using a dialysis membrane with a molecular weight cutoff of 3.5 kDa, and then freeze-drying the dialyzate at -50 ° C for 24 h to obtain lignin-modified graphene, wherein the graphene oxide dispersion is prepared by adding graphene oxide to deionized water and ultrasonically treating the dispersion at a power of 300 W and a frequency of 40 kHz for 60 min, and the mass ratio of amination lignin, graphene oxide, EDC and NHS is 1:2:0.9:0.1;

[0046] Step S3, mixing the lignin-modified graphene with the polyamic acid solution, stirring for 10 hours, filtering through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, placing it in a vacuum oven at 60° C. and drying it for 12 hours, and then peeling it off to obtain a graphene film, wherein 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;

[0047] Step S4, immersing the graphene film in a carbon nanotube solution with a film mass to solution volume ratio of 5g / 100mL, shaking at room temperature for 1.5h, taking out and rinsing with deionized water three times, and then vacuum drying at 60°C to constant weight to obtain a carbon nanotube-modified graphene film, wherein the solvent in the carbon nanotube solution is N-methylpyrrolidone, and the concentration of the carbon nanotube solution is 1.5mg / mL; Step S5, immersing the carbon nanotube-modified graphene film in a polystyrene sulfonic acid / glutaraldehyde mixed solution, cross-linking reaction at room temperature for 1h, taking out and rinsing with deionized water three times, and then vacuum drying at 60°C to constant weight to obtain a graphene composite film, wherein 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.

[0048] Comparative Example 2:

[0049] A method for preparing a graphene composite film is prepared according to the method described in Example 3, except that step S5 is omitted.

[0050] Comparative Example 3:

[0051] A method for preparing a graphene composite film comprises the following steps:

[0052] Step S1, adding alkali lignin to a graphene oxide dispersion having a concentration of 0.3 mg / mL, ultrasonically treating at a power of 400 W and a frequency of 40 kHz for 25 min, then adding EDC and NHS, stirring at room temperature for 24 h, centrifuging at 8000 rpm for 20 min, and then dialyzing with deionized water for 48 h, changing the deionized water every 4 h, using a dialysis membrane with a molecular weight cutoff of 3.5 kDa, and then freeze-drying the dialyzate at -50 ° C for 24 h to obtain lignin-modified graphene, wherein the graphene oxide dispersion is prepared by adding graphene oxide to deionized water and ultrasonically treating at a power of 300 W and a frequency of 40 kHz for 60 min, and the mass ratio of alkali lignin, graphene oxide, EDC and NHS is 1:1:0.9:0.5;

[0053] Step S2, mixing the lignin-modified graphene with the polyamic acid solution, stirring for 23 hours, filtering through a mixed cellulose ester membrane with a pore size of 0.22 μm to form a film, placing it in a vacuum oven at 60°C for 12 hours, and then peeling it off to obtain a graphene film, wherein 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;

[0054] Step S3 is the same as step S4 in Example 3, to obtain a graphene composite film.

[0055] Comparative Example 4:

[0056] A method for preparing a graphene composite film is prepared according to the method described in Example 3, except that: in step S3, the polyamic acid solution is replaced with N,N-dimethylacetamide; and step S4 is omitted.

[0057] Mechanical properties: According to the requirements of GB / T1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for film and sheeting", 150mm×10mm samples were prepared at a tensile rate of 10mm / min, and the tensile strength and elongation at break of the film were tested. Electrical conductivity: According to ASTM F84, the conductivity coefficient of the film was tested using a four-probe resistance meter. Thermal conductivity stability: A hot and cold cycling experiment was conducted at -10℃-80℃, with the number of cycles being 0, 200, and 400, and the thermal conductivity test was performed in sequence.

[0058] Table 1 Performance test results

[0059]

[0060] As shown in Table 1, the graphene composite films prepared in Examples 1-3 have excellent tensile strength, elongation at break, electrical conductivity and thermal stability. Compared with Examples 1-3, the various properties of the graphene composite films prepared in Comparative Example 1 have all declined, indicating that the raw material ratio ranges proposed in the present invention are the optimal ranges for preparing graphene composite films. Compared with Examples 1-3, the tensile strength, elongation at break and thermal stability of the graphene composite films prepared in Comparative Example 2 have declined. Comparative Example 2 lacks the step of cross-linking glutaraldehyde and polystyrene sulfonic acid on the film surface during the preparation process. 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 groups in polystyrene sulfonic acid. The cross-linking reaction forms a chemical bond between the polystyrene sulfonic acid molecular chains, thereby forming a relatively dense cross-linked network. This cross-linked network can significantly improve the mechanical strength of the graphene film. The presence of the cross-linked network makes sliding between the molecular chains more difficult, thereby improving the tensile strength of the material. The flexible segments of polystyrene sulfonic acid can relieve stress concentration, avoid brittle fracture, thereby improving the flexibility of the film. The improved thermal stability of Examples 1-3 is due to the chemical bonds formed by the cross-linked network enhancing intermolecular interactions and improving the thermal stability of the graphene film. The cross-linked network also restricts the thermal motion of the polystyrene sulfonic acid chains at high temperatures, making them less susceptible to thermally induced changes such as decomposition, degradation, and phase transitions. Furthermore, the cross-linked network and graphene synergize to rapidly conduct heat and maintain structural stability at high temperatures, helping to evenly distribute heat throughout the film and prevent localized overheating that could lead to material performance degradation.

[0061] Compared with Examples 1-3, the tensile strength, elongation at break, electrical conductivity and thermal stability of the graphene composite film prepared in Comparative Example 3 are reduced. During the preparation process, Comparative Example 3 lacks the cross-linking of glutaraldehyde and polystyrene sulfonic acid on the film surface and the introduction of amino lignin. Therefore, one reason for the better tensile strength of Examples 1-3 is that amino lignin has higher mechanical strength and good flexibility. When amino lignin is mixed with graphene, the fibrous structure of lignin can interact with the two-dimensional structure of graphene to form an enhanced composite network. The mechanical reinforcement of amino lignin and the cross-linked network on the film surface synergistically enhance the mechanical properties of the graphene film. In addition, the thermal conductivity stability of Examples 1-3 is better, because the molecular structure of the amino lignin contains a large number of chemical bonds, which can absorb a certain amount of energy when heated and play a role in thermal buffering. At the same time, the interaction between the amino lignin and the graphene forms a stable composite structure between the two. This composite structure limits the thermal motion of the molecular chain, increases the thermal decomposition temperature of the material, maintains the thermal stability of the film, and has a synergistic effect with the cross-linked network on the surface of the film. Although the electrical properties of the amino lignin itself are general, its introduction changes the stacking mode of the graphene sheets. The polar groups of the amino lignin will produce a certain amount of steric hindrance between the graphene sheets, preventing the excessive stacking of the graphene sheets, which is conducive to the transmission of electrons between the sheets. Therefore, the conductivity of the graphene films prepared in Examples 1-3 is higher.

[0062] Compared with embodiment 1-3, the conductivity coefficient and the thermal stability of the graphene composite film prepared by comparative example 4 decrease to some extent, comparative example 4 lacks the process compounded with carbon nanotube solution, polyamic acid solution in preparation process, carbon nanotube has excellent electrical conductivity, can significantly improve the electrical property of composite film, and the nanoscale structure of carbon nanotube can also provide extra charge transfer path, improves charge transfer efficiency. Polar groups such as amino group, carboxyl group on polyamic acid molecular chain can form hydrogen bond or electrostatic interaction with oxygen-containing functional groups such as hydroxyl group, carboxyl group on graphene surface, this interfacial bonding strengthens the bonding force between graphene sheets, reduces the conductive path fracture caused by sheet slippage, thus stabilizes the conductive network of graphene film, improves the electrical conductivity of film. And embodiment 1-3 thermal stability is better because carbon nanotube has good thermal conductivity, can conduct heat quickly, and polyamic acid has certain thermal stability, can not decompose rapidly at high temperature, this stability makes polyamic acid keep its structure and performance under high temperature environment, thus for graphene film provides extra thermal protection.

[0063] Although the embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations may be made to these embodiments without departing from the principles and spirit of the present application, and 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: The steps include: Step S1, adding silane coupling agent KH-550 and sulfonated lignin to anhydrous ethanol, reacting at pH = 5-6 and 60-80° C. for 5-6 hours, and obtaining amino lignin after purification; Step S2, adding the amino lignin to the graphene oxide dispersion, ultrasonicating for 20-30 minutes, then adding 1-ethyl-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, reacting at room temperature for 20-24 hours, and obtaining lignin-modified graphene after purification; Step S3, mixing the lignin-modified graphene with the polyamic acid solution, stirring for 22-24 hours, forming a film, and drying to obtain a graphene film; Step S4, immersing the graphene film in a carbon nanotube solution, shaking at 50-60° C. for 1-2 hours, rinsing, and drying to obtain a carbon nanotube-modified graphene film; Step S5: immersing the carbon nanotube-modified graphene film in a polystyrene sulfonic acid / glutaraldehyde mixed solution, performing a cross-linking reaction at 50-60° C. for 2-3 hours, and then rinsing and drying to obtain a graphene composite film.

2. The method for preparing a graphene composite film according to claim 1, wherein The preparation method of the sulfonated lignin in step S1 is as follows: dissolving alkali lignin in sulfuric acid solution, stirring for 20-30 minutes, adding sodium sulfite and copper sulfate, reacting at 80-90° C. for 2-3 hours, and purifying to obtain the sulfonated lignin.

3. The method for preparing a 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 amount of the sulfonated lignin added to the anhydrous ethanol is 0.05-0.1 g / mL.

4. The method for preparing a graphene composite film according to claim 1, wherein The concentration of the graphene oxide dispersion in step S2 is 0.2-0.5 mg / mL, and the mass ratio of amination 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 method for preparing a graphene composite film according to claim 1, wherein The mass ratio of the lignin-modified graphene to the 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.

6. The method for preparing a graphene composite film according to claim 1, wherein The concentration of the carbon nanotube solution in step S4 is 1-2 mg / mL.

7. The method for preparing a graphene composite film according to claim 1, wherein The polystyrene sulfonic acid / glutaraldehyde mixed solution in step S5 is prepared by dissolving polystyrene sulfonic acid and glutaraldehyde in deionized water. 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 according to any one of claims 1 to 7.

9. Application of the graphene composite film according to claim 8 in molecular sensors and molecular field effect transistors.

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