Multilevel network synergistic reinforced composite film based on mxene and cellulose and its preparation and application

Abstract

The application discloses a kind of based on MXene and cellulose multistage network synergistic reinforced composite film and its preparation and application.The preparation includes: MXene nanosheet water dispersion, bacterial cellulose water dispersion are mixed stirring and are concentrated processing, and MXene / bacterial cellulose hydrogel is prepared;Cellulose nanocrystal water dispersion and MXene / bacterial cellulose hydrogel are uniformly mixed, and MXene / bacterial cellulose / cellulose nanocrystal hydrogel is prepared;Water gel is applied to the surface of flexible porous substrate using doctor blade coating method, and wet film is prepared;The wet film is pre-dried, and it is immersed in glutaraldehyde aqueous solution to crosslink, and based on MXene and cellulose multistage network synergistic reinforced composite film is prepared.The composite film in the application has higher mechanical property and high humidity environment stability.

Description

Technical Field

[0001] This invention belongs to the field of nanocomposite thin film material preparation technology, specifically relating to a multi-level network synergistic reinforcement composite thin film based on MXene and cellulose, its preparation method and application. Background Technology

[0002] Two-dimensional nanosheet materials (such as titanium carbide) possess high specific surface area, high in-plane modulus, and designable interlayer structures, making them important materials for constructing high-performance structural / functional integrated films. However, the macroscopic mechanical properties of two-dimensional nanomaterial films are not solely determined by the intrinsic strength of individual nanosheets, but are highly dependent on multi-scale structural factors such as the regularity of nanosheet orientation in the film, interlayer packing density, interfacial or interlayer interaction strength, and defect morphology (wrinkles, pores, microcracks, etc.). Generally, high orientation and high packing density are beneficial for improving load transfer efficiency and suppressing stress concentration; strong interfacial interactions are beneficial for increasing interlayer shear strength and reducing interlayer slip; defects and pores can lead to crack initiation and shortened propagation paths, thus significantly weakening the strength and toughness of the film (Matter 2020, 3, 696-707).

[0003] Currently, strategies for improving the mechanical properties of two-dimensional nanomaterial films mainly include: achieving ordered stacking and densification of nanosheets through assembly process control; enhancing interlayer bonding by introducing interfacial bridging / crosslinking; improving overall load-bearing and energy dissipation capacity by filling pores or constructing multiphase frameworks; and eliminating wrinkles and improving orientation by external force traction or stretching (Science 2013, 341, 534-537; Science 2021, 374, 96-99; Nat. Mater. 2021, 20, 624-631). These strategies have been widely explored in two-dimensional material systems such as graphene and MXene, and have achieved improvements in strength or toughness under certain conditions. However, in actual wet chemical film formation processes, the above strategies often struggle to simultaneously meet the synergistic requirements of "high orientation" and "close packing," and are even more difficult to meet the more stringent service constraint of "mechanical property retention rate under high humidity conditions." When assembling two-dimensional nanosheet films using wet chemical methods such as vacuum filtration, casting, and coating, capillary shrinkage inevitably occurs during the drying process after film formation, accompanied by solvent evaporation. On the one hand, capillary forces can promote the convergence of layers, increasing local packing density; on the other hand, capillary pressure and its uneven distribution can trigger severe shrinkage and stress redistribution in the film, either overall or locally, causing undulations, wrinkles, and misaligned stacking, and inducing defects such as uneven pore closure or microcracks. For high aspect ratio nanosheets such as MXene, wrinkles and misaligned stacking significantly reduce the in-plane continuous load transfer path and effective load-bearing area, leading to a decrease in the film's macroscopic strength, modulus, and fracture toughness. Simultaneously, defect concentration areas can become crack initiation sources, thereby reducing the film's reliability and batch stability.

[0004] To avoid structural collapse and defect accumulation caused by capillary shrinkage, existing technologies include freeze-drying and supercritical drying to reduce or avoid capillary pressure at the liquid-gas interface (Joule 2018, 2, 778-787). However, these methods often result in numerous pores within the material, leading to a loose structure and ultimately insufficient density, weak interlayer bonding, and low mechanical properties. Furthermore, solvent displacement and adjustments to the wettability of nanosheets and solvents can mitigate capillary forces to some extent, enabling the film to be self-supporting or reducing macroscopic shrinkage. However, achieving both orientation and density remains a challenge. Therefore, simultaneously suppressing wrinkles and defects induced by capillary shrinkage and maintaining high orientation and close packing throughout the wet chemical assembly and drying curing processes remains a significant challenge in the field of mechanical reinforcement of two-dimensional nanosheet films.

[0005] Besides structural defects during the drying process, MXene and its composite films often face high-humidity service environments in practical applications. The oxygen-containing groups and cellulose components on the MXene surface exhibit significant hydrophilicity. Under high humidity conditions, the films are prone to moisture absorption and interlayer water ingress, leading to increased interlayer spacing, reduced interfacial friction, disruption of the hydrogen bond network by moisture, and plasticization and relaxation of chain segments or fiber networks. This results in enhanced interlayer slip and decreased mechanical properties. Especially for composite systems dominated by hydrogen bonding and electrostatic interactions, increased humidity significantly reduces interfacial shear strength and energy dissipation capacity, causing problems such as modulus reduction, strength decay, and even microcrack propagation under cyclic stress or long-term environmental exposure. Therefore, achieving high mechanical property retention in high-humidity environments requires not only improving initial strength but also constructing stable interfacial bonding and load transfer structures that remain stable under wet conditions (Chem. Eng. J. 2024, 495, 153343).

[0006] To improve wet stability and interfacial bonding strength, previous studies have proposed introducing chemical crosslinks or multi-point strong interactions into two-dimensional nanosheet films to fix the interlayer structure. Examples include using multifunctional small molecules, metal ion coordination, aldehyde crosslinking agents, or polymer binders to achieve interfacial reinforcement (CN113817230A; CN115341390A). These methods can improve initial strength or suppress interlayer slip to some extent, but they typically have the following limitations: First, insufficient crosslinking sites or uneven crosslinking distribution lead to localized embrittlement and stress concentration; second, some crosslinking systems require harsh conditions (strong acids, strong bases, high temperatures, or strong oxidizing environments), which may be detrimental to the MXene surface terminators and structural stability; third, while some polymer binders can improve toughness, they are prone to softening and absorbing water in high-humidity environments, leading to a decrease in interfacial shear strength and making it difficult to achieve long-term mechanical stability under high humidity.

[0007] In summary, existing technologies generally suffer from the following shortcomings: First, capillary shrinkage caused by drying during wet assembly is difficult to avoid, often leading to defects such as nanosheet wrinkles, misaligned stacking, pores, or microcracks, making it difficult to simultaneously achieve high orientation and high density packing. Second, cellulose-reinforced systems are prone to water absorption and plasticization in high-humidity environments, resulting in decreased interfacial bonding and interlayer friction, leading to insufficient mechanical property degradation and retention. Third, existing crosslinking / bonding strategies still have limitations in terms of crosslinking uniformity, process mildness, and wet stability. Therefore, there is an urgent need to develop a composite film preparation method that can balance sheet orientation, packing density, interfacial stability, and mechanical property retention under wet assembly and high-humidity service conditions to meet the application requirements of flexible structural materials in high-humidity environments. Summary of the Invention

[0008] The main objective of this invention is to provide a multi-level network synergistic reinforced composite film based on MXene and cellulose, its preparation method and application, so as to overcome the shortcomings of the prior art.

[0009] To achieve the aforementioned objectives, the technical solution adopted by this invention includes:

[0010] This invention provides a method for preparing a multi-level network synergistic reinforced composite film based on MXene and cellulose, comprising:

[0011] We provide aqueous dispersions of MXene nanosheets, bacterial cellulose, and cellulose nanocrystals (CNC); among which, the MXene nanosheets are Ti3C2T. x Nanosheets; the mass ratio of the MXene nanosheets to the bacterial cellulose and cellulose nanocrystals is 30:70 to 60:40; the content of cellulose nanocrystals is 5 to 15 wt% based on the total mass of bacterial cellulose and cellulose nanocrystals.

[0012] The MXene nanosheet aqueous dispersion and the bacterial cellulose aqueous dispersion were mixed and stirred and then concentrated to obtain the MXene / bacterial cellulose hydrogel (denoted as: MXene / BC hydrogel).

[0013] The cellulose nanocrystal aqueous dispersion was uniformly mixed with MXene / bacterial cellulose hydrogel to obtain MXene / bacterial cellulose / cellulose nanocrystal hydrogel (denoted as: MXene / BC / CNC hydrogel).

[0014] A wet film was prepared by applying the MXene / bacterial cellulose / cellulose nanocrystal hydrogel to the surface of a flexible porous substrate using a doctor blade coating method.

[0015] Furthermore, when the wet film is pre-dried until the surface of the wet film loses its fluidity while the interior remains wet, it is immersed in a glutaraldehyde aqueous solution for crosslinking, and then washed, vacuum dried, and peeled off from the surface of the flexible porous substrate to obtain a multi-level network synergistic reinforced composite film based on MXene and cellulose (denoted as: MCMG composite film).

[0016] The present invention also provides a multi-level network synergistic reinforced composite film based on MXene and cellulose prepared by the aforementioned preparation method, wherein the multi-level network synergistic reinforced composite film has a layered oriented stacking structure.

[0017] This invention also provides the application of the aforementioned multi-level network synergistic reinforcement composite film based on MXene and cellulose in the preparation of flexible conductive structural materials for use in high humidity environments.

[0018] The principle of this invention lies in the following: A stable in-plane shear field is generated during the film formation process using a doctor blade coating, causing MXene nanosheets to align and stack in a regular layered manner along the coating direction. Simultaneously, the BC (carbon fiber) provides a continuous nanofiber network framework, which, during the wet film stage, applies flexible constraints to the stacked sheets through entanglement and support, making it easier to fix and maintain the continuity of the nanosheet structure after shear orientation. This results in a macroscopically consistent layered structure with lower defect density. CNC (carbon fiber optic compositing) is introduced to construct a cross-scale stress transfer pathway. In-plane tensile loads are initially borne by the high in-plane stiffness MXene sheets and transferred to the BC network through interactions between sheets and at the sheet / cellulose interface. The BC network further disperses stress and inhibits crack propagation along a continuous path. CNC acts as a bridge and restraint at the micro / nano scale, enhancing interlayer shear capacity and reducing relative slippage between sheets, enabling efficient load transfer. GA (carbon fiber optic crosslinking) mainly forms crosslinked structures with the active sites related to the cellulose component and helps improve the bonding stability of the MXene / cellulose interface, thereby contributing to improved interfacial adhesion strength and interlayer shear strength. Compared with single hydrogen bonds, the cross-linked structure and more stable interfacial interaction formed after the introduction of GA help to improve the peel resistance. Even if some hydrogen bonds are competed for by water in a high humidity environment, the effective interfacial binding is still maintained, realizing wet locking of the lamellar orientation structure and dense packing, and significantly reducing the mechanical degradation caused by interlayer slip.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] (1) Based on the MXene / BC composite system, the present invention further introduces CNC. CNC, as a rigid nano-bridging unit, works in synergy with the BC continuous fiber network to form a multi-level stress transmission network of “MXene-BC-CNC”, which enhances the interfacial contact and restricts the relative slip between layers, so that the load can be efficiently transferred and dispersed between the sheet and the cellulose phase, thereby improving the tensile strength and Young's modulus of the composite film.

[0021] (2) Based on the "MXene-BC-CNC" multi-scale network, the present invention introduces glutaraldehyde (GA) to crosslink and cure the MXene / BC / CNC composite film, forming a more stable cellulose crosslink network and improving the interfacial bonding strength and interlayer shear bearing capacity, reducing the tendency of interfacial debonding and slip failure, thereby further improving the mechanical properties of the composite film on the basis of the uncrosslinked system;

[0022] (3) Compared with the problem that traditional MXene / cellulose composite films are prone to moisture absorption and plasticization and interface relaxation leading to mechanical decay under high humidity conditions, the present invention constrains the movement of cellulose chain segments and network deformation through GA crosslinking and achieves wet locking in the multi-scale stress transmission network, suppressing interlayer slip and structural relaxation caused by high humidity, so that the film still maintains a high mechanical level after being exposed at 25 ℃ and 75%RH for 168 h. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the preparation process and internal cross-linking structure of the MXene / BC / CNC / GA composite film in a typical embodiment of the present invention;

[0025] Figures 2a-2d The image shows a cross-sectional morphology of an MXene film, an MC composite film, an MCM composite film, and an MCMG composite film in a typical embodiment of the present invention.

[0026] Figure 3 This is an X-ray diffraction pattern of an MXene thin film, an MCM composite thin film, and an MCMG composite thin film in a typical embodiment of the present invention;

[0027] Figure 4a This is a comparison diagram of the azimuth angle distribution of MXene film, MCM composite film and MCMG composite film in a typical embodiment of the present invention;

[0028] Figures 4b-4d Two-dimensional WAXS plots of MXene thin films, MCM composite thin films, and MCMG composite thin films, respectively, along with their corresponding azimuth distribution curves and Herman orientation factor results.

[0029] Figure 5a This is a comparison of the full FTIR spectra of MXene film, MC composite film and MCM composite film in a typical embodiment of the present invention;

[0030] Figure 5b This is a magnified view of the high wavenumber region in the FTIR spectra of the MXene film, MC composite film and MCM composite film in a typical embodiment of the present invention;

[0031] Figure 5cThis is a magnified view of the low wavenumber region in the FTIR spectra of the MXene film, MC composite film and MCM composite film in a typical embodiment of the present invention;

[0032] Figure 5d This is a comparison of the full FTIR spectra of MCM composite film, MCMG composite film and d-MCMG composite film in a typical embodiment of the present invention;

[0033] Figure 5e The FTIR spectra of the MCM composite film, MCMG composite film, and d-MCMG composite film in a typical embodiment of the present invention show approximately 823.5 cm⁻¹. -1 A magnified view of the surrounding area;

[0034] Figure 6 The images show X-ray photoelectron spectra of MXene films, MC composite films, MCM composite films, and MCMG composite films in a typical embodiment of the present invention. Specifically, a is the XPS full spectrum of different samples; b is the O 1s high-resolution spectrum of the MC composite film; c is the C 1s high-resolution spectrum of the MCM composite film; d is the O 1s high-resolution spectrum of the MXene film; e is the O 1s high-resolution spectrum of the MCM composite film; and f is the C 1s high-resolution spectrum of the MCMG composite film. Detailed Implementation

[0035] In view of the shortcomings of the prior art, the inventors of this invention, through long-term research and extensive practice, have proposed the technical solution of this invention. Addressing the problems of capillary shrinkage, lamellar wrinkling, and low mechanical property retention in high humidity environments that easily occur during the film formation and drying process of MXene composite films, this invention employs a method combining blade coating and wet crosslinking to achieve the desired effect. x Nanosheets, bacterial cellulose, cellulose nanocrystals, and glutaraldehyde synergistically construct a multi-level structural network, which is conducive to the close stacking of MXene sheets and improves the stability of the composite system. The prepared MXene / BC / CNC / GA composite film has high mechanical properties and high humidity environment stability.

[0036] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] This invention first obtains a high-quality monolayer large-scale titanium carbide MXene nanosheet (statistical average sheet diameter of approximately 7.5 μm) aqueous dispersion by chemical etching and oscillatory exfoliation, and prepares a BC aqueous dispersion by ultrasonic-assisted centrifugation; secondly, MXene / BC / CNC in different mass ratios are assembled into a hydrogel film by a doctor blade coating method; finally, the hydrogel film is cross-linked in an aqueous solution of GA by immersion to obtain an MXene-based composite film with a multi-scale stress transfer network.

[0038] Specifically, as one aspect of the technical solution of this invention, a method for preparing a multi-level network synergistic reinforced composite film based on MXene and cellulose includes:

[0039] We provide aqueous dispersions of MXene nanosheets, bacterial cellulose, and cellulose nanocrystals; wherein the MXene nanosheets are Ti3C2T. x Nanosheets; the mass ratio of the MXene nanosheets to the bacterial cellulose and cellulose nanocrystals is 30:70 to 60:40; the content of cellulose nanocrystals is 5 to 15 wt% based on the total mass of bacterial cellulose and cellulose nanocrystals.

[0040] The MXene nanosheet aqueous dispersion and the bacterial cellulose aqueous dispersion were mixed and stirred and then concentrated to obtain the MXene / bacterial cellulose hydrogel.

[0041] The cellulose nanocrystal aqueous dispersion was uniformly mixed with MXene / bacterial cellulose hydrogel to obtain MXene / bacterial cellulose / cellulose nanocrystal hydrogel.

[0042] A wet film was prepared by applying the MXene / bacterial cellulose / cellulose nanocrystal hydrogel to the surface of a flexible porous substrate using a doctor blade coating method.

[0043] Furthermore, when the wet film is pre-dried until the surface of the wet film loses its fluidity while the interior remains wet, it is immersed in a glutaraldehyde aqueous solution for crosslinking, and then washed, vacuum dried, and peeled off from the surface of a flexible porous substrate to obtain a multi-level network synergistic reinforced composite film based on MXene and cellulose.

[0044] In some preferred embodiments, the preparation method specifically includes: etching, washing, peeling, and centrifuging the MAX phase to obtain an aqueous dispersion of MXene nanosheets;

[0045] Wherein, the MAX phase is Ti3AlC2; the concentration of the MXene nanosheet aqueous dispersion is 1–10 mg / mL;

[0046] The etching is performed using a system containing lithium fluoride and hydrochloric acid, at an etching temperature of 40–55 °C and an etching time of 24–30 h.

[0047] The MXene nanosheets in the aqueous dispersion have a diameter of 3–15 μm.

[0048] In some preferred embodiments, the preparation method specifically includes: dispersing bacterial cellulose in water and then subjecting it to ultrasonic dispersion and centrifugal separation to obtain a bacterial cellulose aqueous dispersion; wherein the ultrasonic dispersion time is 0.5 to 2 hours; and the concentration of the bacterial cellulose aqueous dispersion is 1 to 2 mg / mL.

[0049] In some preferred embodiments, the preparation method specifically includes: dispersing cellulose nanocrystals in water and centrifuging them to obtain an aqueous dispersion of cellulose nanocrystals with a concentration of 5–30 mg / mL.

[0050] Furthermore, the concentration of the aqueous dispersion of the cellulose nanocrystals is 20 mg / mL.

[0051] In some preferred embodiments, the preparation method specifically includes: adjusting the pH of the MXene nanosheet aqueous dispersion to 7-9, adjusting the pH of the bacterial cellulose aqueous dispersion to 8-10, then mixing and stirring for 0.5-2 h, and then concentrating the obtained dispersion into an MXene / bacterial cellulose hydrogel by filtration or centrifugation.

[0052] In some preferred embodiments, the mass ratio of the MXene nanosheets to the sum of bacterial cellulose and cellulose nanocrystals is 50:50.

[0053] In some preferred embodiments, the content of cellulose nanocrystals is 10 wt% based on the total mass of bacterial cellulose and cellulose nanocrystals.

[0054] In some preferred embodiments, the blade coating uses a blade gap of ≤600 μm.

[0055] In some preferred embodiments, the flexible porous substrate includes, but is not limited to, a PVDF filter membrane.

[0056] In some preferred embodiments, the preparation method specifically includes: pre-drying the wet film at 30-40 °C until there is no visible flowing liquid on the surface, and the film layer does not show obvious flow or slippage after tilting the substrate, and the film layer as a whole still remains moist and flexible, and then immersing it in a glutaraldehyde aqueous solution for crosslinking; wherein the concentration of the glutaraldehyde aqueous solution is 1-25 wt%; and the immersion time in the glutaraldehyde aqueous solution is 0.25-2 h.

[0057] Furthermore, the pre-drying temperature was 35 ℃, the concentration of glutaraldehyde aqueous solution was 5 wt%, and the soaking time was 0.5 h.

[0058] In some preferred embodiments, the vacuum drying temperature is 60–70 °C and the time is 18–30 h.

[0059] Furthermore, the vacuum drying temperature is 65 °C and the time is 24 h.

[0060] The typical evaluation conditions for high humidity environment in this invention are 25 °C and 75% RH exposure for 168 h.

[0061] This invention provides a method for preparing MXene / BC / CNC / GA composite films using a blade coating method, through Ti3C2T x A multi-scale network structure involving interface and cross-linking interactions is constructed between nanosheets, a continuous bacterial cellulose (BC) nanofiber network, rigid cellulose nanocrystal (CNC) bridging units, and glutaraldehyde (GA) molecules. During film formation and drying, the shear-induced sheet orientation and deformation constraint of the BC network, combined with the stress redistribution regulation effect of the wet network structure during drying, effectively suppress capillary shrinkage and nanosheet wrinkling caused by solvent evaporation. This induces highly regular orientation and close packing of MXene sheets, ultimately preparing an MXene / BC / CNC / GA composite film (MCMG film) with multi-scale stress transmission pathways. A typical MCMG film sample exhibits a tensile strength of 582.3 MPa, and retains 87.0% of its tensile strength after 168 h of storage at 75% RH. The improved mechanical properties and high-humidity retention capacity of the thin film are related to the following factors: the multi-point hydrogen bonding and physical entanglement formed between the MXene sheets and BC / CNC improve interfacial bonding and interlayer load transfer efficiency; the cross-linking effect of GA on the cellulose component further enhances the network stability of the composite system, which helps to suppress interfacial weakening and interlayer slip under high humidity conditions; simultaneously, the in-plane shear field formed during the coating process promotes the orientation alignment of the MXene sheets, BC provides continuous network constraints, and CNC plays a bridging and filling role, which helps to reduce structural defects and form a more continuous stress transfer path. Furthermore, wide-angle X-ray scattering (WAXS) characterization results show that the method of this invention is beneficial to improving the orientation degree of the thin film; scanning electron microscopy characterization of the thin film cross-section shows that the prepared thin film has high packing density and reduced structural defects such as wrinkles and pores, thus improving the mechanical stability of the thin film under high humidity conditions. The orientation degree is represented by the Herman orientation factor.

[0062] In some more specific embodiments, the method for preparing the multi-level network synergistic reinforced composite film based on MXene and cellulose includes the following steps:

[0063] (1) The raw material MAX phase is chemically etched with lithium fluoride (LiF) and hydrochloric acid (HCl) at 40-55 °C for 24-30 h. After washing, shaking and peeling and gradient centrifugation, a single-layer MXene nanosheet (sheet diameter 3-15 μm) aqueous dispersion is prepared; preferably, the MAX phase is Ti3AlC2; preferably, the MXene is Ti3C2T x ;

[0064] (2) Disperse bacterial cellulose gel in deionized water and prepare bacterial cellulose aqueous dispersion (approximately 1-2 mg / mL) by sonication (0.5-2 h, 500-800 W) and centrifugation (7000 rpm, 15 min); prepare cellulose nanocrystal (CNC) gel into a 20 mg / mL CNC aqueous dispersion.

[0065] (3) The aqueous dispersion of titanium carbide nanosheets (with a diameter of 3 to 15 μm) described in step (1) is mixed and stirred with the aqueous dispersion of bacterial cellulose (about 1 to 2 mg / mL) described in step (2) to obtain a uniform dispersion of MXene / BC. Preferably, the stirring time is 0.5 to 2 h and the solid content in the dispersion is 0.5 to 2 mg / mL.

[0066] (4) The MXene nanosheets and bacterial cellulose uniform dispersion obtained in step (3) are concentrated to obtain MXene / BC hydrogel; preferably, the concentration method is vacuum filtration and gradient centrifugation (10000 rpm, 1 h), and the concentration after concentration is about 20 mg / mL;

[0067] (5) The CNC aqueous dispersion prepared in step (2) is added dropwise to the gel obtained in step (4) according to the corresponding mass ratio, and stirred thoroughly to prepare MXene / BC / CNC hydrogel; preferably, the stirring time is about 0.5 h;

[0068] (6) The MXene / BC / CNC hydrogel obtained in step (5) is coated onto a flexible porous substrate by a doctor blade to form a film. The doctor blade gap is preferably less than 600 μm. The flexible porous substrate is a PVDF filter membrane with a pore size of 0.1 to 0.45 μm. Preferably, the PVDF filter membrane has a pore size of 0.22 μm.

[0069] (7) The hydrogel membrane on the flexible porous substrate obtained in step (6) is pre-dried until there is no visible flowing liquid on the surface, the membrane does not flow or slip significantly after tilting the substrate, and the membrane remains moist and flexible. Then it is immersed in a GA aqueous solution of a certain concentration for a certain period of time to prepare a GA crosslinked MXene / BC / CNC composite film. Preferably, the temperature during pre-drying is 35 °C. Preferably, the mass fraction of GA in the GA aqueous solution is 5 wt%.

[0070] (8) The GA crosslinked MXene / BC / CNC composite film obtained in step (7) is washed with deionized water multiple times to remove GA that has not participated in crosslinking.

[0071] (9) The wet GA crosslinked MXene / BC / CNC composite film obtained in step (8) is placed in a vacuum oven for drying. After drying, it is taken out and peeled off to obtain the final MXene / BC / CNC / GA composite film (MCMG). Preferably, the oven drying temperature is 65 ℃ and the time is 24 h.

[0072] Preferably, in step (1), the aqueous dispersion of monolayer titanium carbide MXene nanosheets is obtained by oscillation peeling and gradient centrifugation. Specifically, MXene is dispersed in water, oscillated in a closed environment for 2 to 15 minutes, and then separated by gradient centrifugation to obtain the aqueous dispersion of MXene nanosheets.

[0073] Preferably, in step (2), the ultrasonic power is 600 W and the time is 1 h. Then, the centrifugation is performed multiple times until there is no obvious sediment at the bottom of the centrifuge tube. The upper dispersion is the bacterial cellulose dispersion, which can be prepared to the required concentration.

[0074] Preferably, in step (3), the MXene aqueous dispersion and the bacterial cellulose aqueous dispersion are first adjusted to the aforementioned pH range and then mixed. The pH of the resulting MXene / BC aqueous dispersion is preferably controlled at around 10. The MXene dispersion is added dropwise to the bacterial cellulose aqueous dispersion and fed according to the target ratio of 30 wt% to 60 wt% of MXene in the final composite film.

[0075] Preferably, in step (5), the gel after the CNC dispersion is added has low adhesion to the container wall, making it easy to transfer and coat; preferably, the mass percentage of CNC is about 10 wt% of the total mass of the cellulose component.

[0076] Preferably, in steps (6), (7) and (8), the composite film does not undergo a complete drying process during assembly, crosslinking and washing.

[0077] Another aspect of the present invention provides a multi-level network synergistic reinforced composite film based on MXene and cellulose prepared by the aforementioned preparation method, wherein the multi-level network synergistic reinforced composite film has a layered oriented stacked structure.

[0078] In some preferred embodiments, the multi-level network synergistic enhancement composite film contains 30-60 wt% MXene, 35-65 wt% bacterial cellulose, and 2-10 wt% cellulose nanocrystals.

[0079] In some preferred embodiments, the thickness of the multi-level network synergistic enhancement composite film is 1–5 μm.

[0080] The high-quality monolayer titanium carbide MXene nanosheets described in this invention are two-dimensional layered materials composed of alternating C and Ti atomic layers. Their surfaces contain a large number of oxygen-containing functional groups, exhibiting good electrical conductivity and electrochemical activity. The bacterial cellulose (BC) is composed of high aspect ratio nanofibers, which can form a continuous network structure. Its fiber length can reach the micrometer scale, and its surface is rich in hydroxyl groups. The cellulose nanocrystals CNC are rigid rod-shaped nanocrystals, typically possessing nanoscale length and high axial modulus, and their surfaces are rich in hydroxyl groups and a small amount of sulfate half-ester groups. After MXene is stirred evenly with BC and CNC, a hydrogen bond network can be formed.

[0081] Another aspect of the present invention provides the application of the aforementioned multi-level network synergistic reinforced composite film based on MXene and cellulose in the preparation of flexible conductive structural materials for use in high humidity environments.

[0082] Unless otherwise specified, the mechanical tests involved in the experimental examples were conducted using the following methods.

[0083] The tensile strength test method includes the following steps: The prepared film sample is cut into test strips with a length of 1 cm and a width of 3 mm. The strips are fixed on a paper test template with a span of 0.5 cm. A Shimadzu AGS-X tensile tester equipped with a 100 N sensor is used to perform tensile performance testing on the test strips at a tensile speed of 0.5 mm / min to obtain the stress-strain curve of the film. Wherein, tensile strength = tensile force at break of the test strip / cross-sectional area of ​​the test strip.

[0084] Young's modulus is obtained by calculating the slope of the elastic region of the stress-strain curve.

[0085] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments and accompanying drawings. This embodiment is implemented on the premise of the technical solution of the invention, and provides detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0086] Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0087] Example 1

[0088] Add 5 mL of deionized water to a 100 mL PTFE reagent bottle, add 15 mL of concentrated hydrochloric acid (36 wt%), and stir at 800 rpm for 5 min. Slowly add 1.6 g of LiF while stirring, and stir for 5 min. Slowly add 1.0 g of Ti3AlC2 (Jilin Yiyi Technology Co., Ltd.) while stirring, and stir for 5 min. Tighten the PTFE reagent bottle and seal the opening with sealing film. Turn on the water bath, adjust the stirring speed to 800 rpm, and heat to 50 ℃, reacting for 30 h. Turn off the heating, remove the PTFE reagent bottle from the water bath, and cool to room temperature. Open the PTFE reagent bottle in a fume hood, add deionized water to the reaction product, and repeatedly centrifuge and wash (3500 rpm, 5 min) until the pH of the supernatant is approximately 6. Shake for 5 min, centrifuge at 1500 rpm for 30 min, and collect the supernatant. The collected supernatant was further centrifuged at 4500 rpm for 20 min, and the resulting precipitate was redispersed in water to prepare a dispersion with a concentration of 1 mg / mL, thus obtaining a large monolayer MXene dispersion with a sheet diameter of 3–15 μm. The dispersion was stored in a refrigerator at 2–4 °C for later use.

[0089] Example 2

[0090] A certain amount of bacterial cellulose gel (Guilin Qihong Technology Co., Ltd.) was dissolved in deionized water to prepare a bacterial cellulose aqueous dispersion. 150 mL of the bacterial cellulose aqueous dispersion was placed in a 250 mL blue-capped glass bottle and sonicated for 1 h using a cell disruptor (Ningbo Xinzhi Biotechnology Co., Ltd.) at 600 W. The resulting sonicated dispersion was then centrifuged multiple times at 7000 rpm for 15 min until no obvious precipitate appeared at the bottom. The supernatant was collected to prepare a 1 mg / mL dispersion, which was then stored at 2–4 °C for later use. Over time, the bacterial cellulose dispersion will spontaneously aggregate due to hydrogen bonding. Dispersions within one week of sonication can be used for sample preparation; dispersions older than this period need to be re-sonicated and prepared.

[0091] Example 3

[0092] A certain amount of cellulose nanocrystal gel (Guilin Qihong Technology Co., Ltd.) was placed in deionized water to prepare an aqueous dispersion. The dispersion was centrifuged at 5000 rpm for 30 min several times to remove cellulose particle aggregates. The treated CNC was prepared into a 20 mg / mL dispersion and stored in a refrigerator at 2-4 ℃ for later use.

[0093] Example 4

[0094] Take a certain amount of GA aqueous solution (Aladdin, 50 wt%) and dilute it with deionized water to prepare an aqueous solution with a mass fraction of 5 wt%. Store the solution in a refrigerator at 2-4 ℃ for later use.

[0095] Example 5

[0096] Take 20 mL of the bacterial cellulose aqueous dispersion prepared in Example 2 and place it in a 50 mL glass bottle. Stir at 800 rpm for 0.5 h to ensure uniform dispersion of BC. Adjust the pH to 10 using 0.1 mol / L sodium hydroxide solution and stir at 800 rpm for 0.5 h. Then take 20 mL of the MXene aqueous dispersion prepared in Example 1 and place it in a 50 mL glass bottle. Adjust the pH to 8 and stir at 800 rpm for 0.5 h. Then add the MXene dispersion dropwise to the bacterial cellulose dispersion while stirring at 800 rpm during the dropwise addition. After the dropwise addition is completed, stir at 800 rpm for 1 h. After the reaction was completed, the dispersion was centrifuged at 10,000 rpm for 1 h, and a portion of the supernatant was discarded. The concentrated phase was retained and remixed to obtain a 20 mg / mL hydrogel. The 20 mg / mL CNC aqueous dispersion obtained in Example 3 was added as needed. A typical sample was MCMG with CNC accounting for approximately 10 wt% of the total cellulose component. 0.1 mL of the CNC aqueous dispersion obtained in Example 3 was added dropwise while stirring at 150 rpm. After the addition was complete, the mixture was stirred at 150 rpm for 0.5 h. The hydrogel containing MXene, BC, and CNC was placed in a 400 μm doctor blade and spread on a PVDF substrate at 100 mm / s. After spreading, the substrate was heated to 35 °C until the gel surface lost its fluidity and became wet. The substrate and the wet composite film were then immersed in the GA aqueous solution prepared in Example 4 for 0.5 h as needed. A typical GA aqueous solution had a concentration of 5 wt%. After soaking, the sample was washed five times with deionized water to remove residual GA aqueous solution. After washing, it was dried in a vacuum oven at 65 °C for 24 h and then peeled off from the flexible porous substrate to obtain the MXene / BC / CNC / GA composite film, which was denoted as MCMG. The thickness of the obtained MCMG composite film was approximately 2.0 μm.

[0097] Comparative Example 1

[0098] Take 20 mL of the bacterial cellulose aqueous dispersion prepared in Example 2 and place it in a 50 mL glass bottle. Stir at 800 rpm for 0.5 h to ensure uniform dispersion of BC. Adjust the pH to 10 using 0.1 mol / L sodium hydroxide solution and stir at 800 rpm for 0.5 h. Then take 20 mL of the MXene aqueous dispersion prepared in Example 1 and place it in a 50 mL glass bottle. Adjust the pH to 8 and stir at 800 rpm for 0.5 h. Then add the MXene dispersion dropwise to the bacterial cellulose dispersion while stirring at 800 rpm during the dropwise addition. After the dropwise addition is completed, stir at 800 rpm for 1 h. After the reaction was completed, the dispersion was centrifuged at 10,000 rpm for 1 h, and a portion of the supernatant was discarded. The concentrated phase was retained and remixed to obtain a 20 mg / mL hydrogel. The 20 mg / mL CNC aqueous dispersion obtained in Example 3 was added as needed. A typical sample was MCM with CNC accounting for 10 wt% of the total cellulose component. 0.1 mL of the CNC aqueous dispersion obtained in Example 3 was added dropwise while stirring at 150 rpm. After the addition was complete, the mixture was stirred at 150 rpm for 0.5 h. The hydrogel containing MXene, BC, and CNC was placed in a 400 μm doctor blade and spread on a PVDF substrate at 100 mm / s. After spreading, it was heated and dried at 35°C until the film was fully formed, resulting in an MXene / BC / CNC composite film (MCM). The thickness of the obtained MCM composite film was approximately 2.3 μm.

[0099] Comparative Example 2

[0100] Take 15 mL of the bacterial cellulose aqueous dispersion prepared in Example 2 and place it in a 20 mL glass bottle. Stir at 800 rpm for 0.5 h. After the reaction is complete, centrifuge the dispersion at 10000 rpm for 1 h, discard part of the supernatant, retain the concentrated phase, and remix to obtain a 20 mg / mL hydrogel. Add the CNC aqueous dispersion prepared in Example 3 dropwise to it and stir at 150 rpm for 0.5 h. Place the obtained hydrogel containing BC and CNC in a 400 μm doctor blade and spread it on a PVDF substrate at 100 mm / s. After spreading, heat it in an oven at 35 °C until the membrane naturally detaches, thus preparing a bacterial cellulose / cellulose nanocrystal composite film. The obtained composite film is denoted as MC. The mass of CNC in the MC composite film accounts for 10 wt% of the total mass of the cellulose component, and the thickness of the obtained MC composite film is approximately 3.5 μm.

[0101] Comparative Example 3

[0102] The 1 mg / mL monolayer MXene dispersion prepared in Example 1 was placed in a glass bottle and stirred at 800 rpm for 0.5 h. After the stirring reaction was completed, the dispersion was centrifuged at 8000 rpm for 0.5 h, and part of the supernatant was discarded. The concentrated phase was retained and remixed to obtain a 20 mg / mL hydrogel. After stirring and degassing, the hydrogel was placed in a 400 μm doctor blade and spread on a PVDF substrate at 100 mm / s. After spreading, the film was heated at 35 °C until it detached naturally, thus obtaining a pure MXene film with a thickness of approximately 1.8 μm.

[0103] Comparative Example 4

[0104] Take 15 mL of the bacterial cellulose aqueous dispersion prepared in Example 2 and place it in a 20 mL glass bottle. Stir at 800 rpm for 0.5 h, centrifuge at 10000 rpm for 1 h, discard part of the supernatant, retain the concentrated phase and remix to obtain a 20 mg / mL gel. Stir at 150 rpm for 0.5 h, place the obtained gel in a 400 μm doctor blade, and spread it on a PVDF substrate at 100 mm / s. After spreading, heat in an oven at 35 °C until the membrane detaches naturally to obtain a pure bacterial cellulose membrane. The thickness of the obtained pure bacterial cellulose membrane is approximately 3.2 μm.

[0105] Comparative Example 5

[0106] The method is the same as in Example 5, except that: the prepared MXene / BC / CNC hydrogel was spread using a doctor blade, heated at 35 °C until the film was completely dry and formed, and then the completely dried composite film was immersed in a 5 wt% glutaraldehyde aqueous solution for 0.5 h, followed by washing with deionized water and vacuum drying at 65 °C for 24 h to obtain a completely dried and re-crosslinked MXene / BC / CNC / GA composite film. The resulting composite film is denoted as d-MCMG.

[0107] Comparative Example 6

[0108] The method is the same as in Example 5, except that after preparing the MXene / BC hydrogel, no cellulose nanocrystal aqueous dispersion is added. The obtained MXene / BC hydrogel is directly placed in a 400 μm doctor blade and spread on a PVDF substrate at 100 mm / s. Subsequently, it is treated according to the conditions described in Example 5: pre-drying, cross-linking by soaking in glutaraldehyde aqueous solution, washing with deionized water, and vacuum drying to obtain the MXene / BC / GA composite film. The obtained composite film is denoted as MBG.

[0109] Comparative Example 7

[0110] The method is the same as in Example 5, except that the MXene / BC / CNC hydrogel was applied to the PVDF substrate surface using a conventional casting method, without using a doctor blade coating method for directional spreading. Subsequently, it was treated according to the conditions described in Example 5: pre-drying, cross-linking by immersion in glutaraldehyde aqueous solution, washing with deionized water, and vacuum drying to obtain a conventionally cast MXene / BC / CNC / GA composite film. The resulting composite film is denoted as c-MCMG.

[0111] Experimental Example 1

[0112] Initial mechanical property testing of thin film samples

[0113] Thin film samples prepared in Example 5 and Comparative Examples 1-7 were cut into tensile test strips with a length of 1.0 cm and a width of 3.0 mm, and fixed onto a paper test template with a span of 0.5 cm. Tensile properties were tested using a Shimadzu AGS-X tensile testing machine equipped with a 100 N sensor at a tensile speed of 0.5 mm / min, and the stress-strain curves of the samples were obtained. Tensile strength = maximum tensile force at break / cross-sectional area of ​​the test strip; Young's modulus was obtained by calculating the slope of the elastic region of the stress-strain curve. The initial tensile strength test results and the tensile strength retention rate after high humidity treatment (25 ℃, 75% RH exposure for 168 h) are shown in Table 1.

[0114] Experimental Example 2

[0115] Tensile strength retention test of thin film samples under high humidity conditions

[0116] Thin film samples prepared in Example 5 and Comparative Examples 1-7 were cut into tensile test strips with a length of 1.0 cm and a width of 3.0 mm. The strips were fixed onto paper test templates with a span of 0.5 cm, and then the paper test templates with the strips were placed in petri dishes. A suitable amount of saturated sodium chloride solution was added to the bottom of a vacuum desiccator with an inner diameter of 180 mm, and the desiccator was sealed and pre-equilibrated for 48 h to stabilize the relative humidity inside the desiccator at 75% RH. The desiccator was then quickly opened, the petri dishes containing the strips were placed on the upper layer of the desiccator and resealed, and exposed to 25°C and 75% RH for 168 h before being taken out for testing. Tensile properties were tested using a Shimadzu AGS-X tensile testing machine equipped with a 100 N sensor, with the tensile speed set to 0.5 mm / min, and the stress-strain curves of the strips were obtained. Tensile strength retention rate = tensile strength after high humidity exposure / initial tensile strength × 100%. Independent samples were used for testing at each time point. To minimize the impact of environmental humidity fluctuations on the test results, the tests should be completed as soon as possible after the samples are taken out.

[0117] Table 1 Comparison of initial tensile strength and tensile strength retention rate after high humidity treatment for different thin film samples

[0118] As shown in Table 1, the initial tensile strength and tensile strength retention rate of the MCMG composite film were 582.3 MPa and 87.0%, respectively, both higher than those of other comparative samples. The tensile strength retention rate of the MCM composite film without GA treatment decreased to 66.3%, indicating that GA treatment is beneficial for improving the network stability of the composite film under high humidity conditions. The initial tensile strength of the MBG composite film without CNC was reduced to 410.1 MPa, indicating that CNC bridging units are beneficial for improving interlayer stress transfer. The initial tensile strength and tensile strength retention rate of the d-MCMG composite film after complete drying followed by GA treatment were both lower than those of MCMG, indicating that wet GA treatment is more beneficial for fixing the composite network. The initial tensile strength and tensile strength retention rate of the c-MCMG composite film without doctor blade coating were significantly lower than those of MCMG, indicating that doctor blade coating is beneficial for forming a more effective oriented stacking structure. The above results indicate that the synergistic effect of blade coating orientation, CNC bridging, wet GA treatment, and MXene / BC network enables the MCMG composite film to possess both high initial mechanical properties and high humidity stability.

[0119] Experimental Example 3

[0120] Characterization of cross-sectional morphology, interlayer structure and orientation structure of thin film samples

[0121] The cross-sectional morphology, interlayer structure, and orientation characteristics of the thin films prepared in this invention were characterized by scanning electron microscopy, X-ray diffraction, and wide-angle X-ray scattering. The results are as follows: Figures 2a-2d , Figure 3 , Figures 4a-4d As shown. Figures 2a-2d Cross-sectional morphology images of MXene films, MC composite films, MCM composite films, and MCMG composite films are shown. It can be seen that compared to pure MXene films, the MC composite films, MCM composite films, and MCMG composite films all exhibit varying degrees of layered stacking structures. Among them, the MCM composite films and MCMG composite films have more continuous cross-sectional structures and more regular interlayer stacking, indicating that the introduction of BC continuous networks and CNC bridging units is beneficial to improving the structural integrity of the films. Figure 3 It is evident that MXene films, MCM composite films, and MCMG composite films all exhibit corresponding layered structure characteristics, indicating that the composite components and GA treatment affect the stacking state of MXene sheets. Figure 4a A comparison of the azimuth angle distributions of MXene thin films, MCM composite films, and MCMG composite films. Figures 4b-4dTwo-dimensional WAXS plots and corresponding azimuth distribution curves and Herman orientation factors are shown for MXene films, MCM composite films, and MCMG composite films, respectively. It can be seen that MXene films, MCM composite films, and MCMG composite films all exhibit obvious orientation distribution characteristics, with the MCMG composite film maintaining a higher Herman orientation factor. These results indicate that blade coating-induced sheet orientation, combined with BC continuous network constraints, CNC bridging filling, and GA treatment, is beneficial for improving the orientation degree, packing regularity, and structural stability of composite films, thus providing a structural basis for improving their mechanical properties.

[0122] Test Example 4

[0123] FTIR and XPS characterization of thin film samples

[0124] The Fourier transform infrared spectroscopy characterization results of the samples prepared according to this invention show that different thin film samples exhibit significant differences in the positions of characteristic absorption peaks and local chemical environments, as shown in the following figures. Figures 5a-5e As shown. By Figure 5a It is evident that, compared to pure MXene films and MC composite films, the characteristic absorption peak positions and peak shapes of the MCM composite films have undergone certain changes; combined with Figure 5b and Figure 5c The magnified results of the high and low wavenumber regions show that the infrared response of the MCM composite film in the corresponding characteristic absorption range differs from that of the comparison sample, indicating that the interfacial chemical environment changes after MXene is composited with the BC / CNC component. Furthermore, from... Figure 5d As can be seen, compared with the MCM composite film, the MCMG composite film has a thickness of approximately 3326.7 cm⁻¹. -1 The absorption characteristics in the vicinity showed differences; meanwhile, the d-MCMG composite film obtained after complete drying and subsequent GA treatment differed from the MCMG composite film obtained after wet treatment at approximately 1695.2 cm⁻¹. -1 and 823.5 cm -1 The surrounding area also exhibited different infrared responses. Combined with... Figure 5e Approximately 823.5 cm -1 The magnified results in the nearby area further show that different GA introduction methods affect the local chemical environment and network structure state inside the composite system. Among them, wet GA treatment is more conducive to the role of GA in the composite network.

[0125] Figure 6 X-ray photoelectron spectra of MXene thin films, MC composite thin films, MCM composite thin films, and MCMG composite thin films. Figure 6In Figure 'a', the characteristic element signals of the MC composite film, MXene film, MCM composite film, and MCMG composite film are all present, indicating that each component has been successfully introduced into the composite system. Figure 6 As shown in the O 1s high-resolution spectra (b, d, and e), the chemical environments of oxygen-containing functional groups differ among the MC composite film, MXene film, and MCM composite film, indicating that the oxygen-related chemical environment at the interface changes after MXene is composited with the cellulose component. Figure 6 As shown in C and F, the C 1s high-resolution spectra reveal significant differences in peak shape and position distribution between the MCM and MCMG composite films in terms of carbon-related chemical environments, indicating that the local electronic environment of the composite system was altered after the introduction of GA. The FTIR and XPS results corroborate each other, demonstrating interfacial interactions between MXene and the cellulose component, and that wet GA treatment is beneficial for regulating the local chemical environment within the composite system, enhancing network structural stability, and improving interfacial bonding levels.

[0126] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0127] It should be understood that the technical solutions of the present invention are not limited to the specific embodiments described above. Any technical modifications made to the technical solutions of the present invention without departing from the spirit and scope of the claims are within the scope of protection of the present invention.

Claims

1. A method for preparing a multi-level network synergistic reinforced composite thin film based on MXene and cellulose, characterized in that, include: An aqueous dispersion of MXene nanosheets was prepared by etching, washing, peeling, and centrifugation of the MAX phase. Bacterial cellulose was dispersed in water and then subjected to ultrasonic dispersion and centrifugation to obtain a bacterial cellulose aqueous dispersion; cellulose nanocrystals were dispersed in water and then subjected to centrifugation to obtain a cellulose nanocrystal aqueous dispersion; wherein the MXene nanosheets in the MXene nanosheet aqueous dispersion are Ti3C2T. x Nanosheets; the mass ratio of the MXene nanosheets to the bacterial cellulose and cellulose nanocrystals is 30:70 to 60:40; the content of cellulose nanocrystals is 5 to 15 wt% based on the total mass of bacterial cellulose and cellulose nanocrystals. The MXene nanosheet aqueous dispersion and the bacterial cellulose aqueous dispersion were mixed and stirred and then concentrated to obtain the MXene / bacterial cellulose hydrogel. The cellulose nanocrystal aqueous dispersion was uniformly mixed with MXene / bacterial cellulose hydrogel to obtain MXene / bacterial cellulose / cellulose nanocrystal hydrogel. A wet film was prepared by applying the MXene / bacterial cellulose / cellulose nanocrystal hydrogel to the surface of a flexible porous substrate using a doctor blade coating method. Furthermore, when the wet film is pre-dried until the surface of the wet film loses its fluidity while the interior remains wet, it is immersed in a glutaraldehyde aqueous solution for cross-linking, and then washed and vacuum dried to obtain a multi-level network synergistic reinforced composite film based on MXene and cellulose.

2. The preparation method according to claim 1, characterized in that, Specifically, it includes: An aqueous dispersion of MXene nanosheets was prepared by etching, washing, peeling, and centrifugation of the MAX phase. Wherein, the MAX phase is Ti3AlC2; the concentration of the MXene nanosheet aqueous dispersion is 1–10 mg / mL; The etching is performed using a system containing lithium fluoride and hydrochloric acid, at an etching temperature of 40–55 °C, for a time of 24–30 h. The MXene nanosheets in the aqueous dispersion have a diameter of 3–15 μm.

3. The preparation method according to claim 1, characterized in that, Specifically, it includes: Bacterial cellulose was dispersed in water and then subjected to ultrasonic dispersion and centrifugation to obtain a bacterial cellulose aqueous dispersion; wherein the ultrasonic dispersion time was 0.5 to 2 h; and the concentration of the bacterial cellulose aqueous dispersion was 1 to 2 mg / mL. And / or, the preparation method specifically includes: dispersing cellulose nanocrystals in water and centrifuging them to obtain an aqueous dispersion of cellulose nanocrystals with a concentration of 5-30 mg / mL.

4. The preparation method according to claim 1, characterized in that, Specifically, it includes: The pH of the MXene nanosheet aqueous dispersion was adjusted to 7-9, and the pH of the bacterial cellulose aqueous dispersion was adjusted to 8-10. The mixture was then stirred for 0.5-2 hours. The resulting dispersion was then concentrated into an MXene / bacterial cellulose hydrogel by filtration or centrifugation.

5. The preparation method according to claim 1, characterized in that: The blade coating uses a blade gap of ≤600μm; and / or, the flexible porous substrate includes a PVDF filter membrane.

6. The preparation method according to claim 1, characterized in that, Specifically, it includes: The wet film is pre-dried at 30–40 °C until there is no visible flowing liquid on the surface, the film layer does not flow or slip significantly after tilting the substrate, and the film layer remains moist and flexible as a whole. Then, it is immersed in a glutaraldehyde aqueous solution for crosslinking. The concentration of the glutaraldehyde aqueous solution is 1–25 wt%, and the immersion time in the glutaraldehyde aqueous solution is 0.25–2 h.

7. The preparation method according to claim 1, characterized in that: The vacuum drying temperature is 60–70 °C, and the time is 18–30 h.

8. The multi-level network synergistic reinforced composite film based on MXene and cellulose prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The multi-level network-coordinated reinforced composite film has a layered oriented stacking structure.

9. The multi-level network collaborative enhancement composite film according to claim 8, characterized in that: The multi-level network synergistic enhancement composite film contains 30–60 wt% MXene, 35–65 wt% bacterial cellulose, and 2–10 wt% cellulose nanocrystals. And / or, the thickness of the multi-level network synergistic enhancement composite film is 1–5 μm.

10. The application of the multi-level network synergistic reinforcement composite film based on MXene and cellulose as described in claim 8 or 9 in the preparation of flexible conductive structural materials for use in high humidity environments.

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