A strain-insensitive anti-leakage liquid metal-based flexible electromagnetic shielding composite material and a preparation method thereof

Abstract

The present disclosure provides a strain-insensitive anti-leakage liquid metal-based flexible electromagnetic shielding composite material and a preparation method thereof, and belongs to the technical field of electromagnetic shielding materials. The flexible electromagnetic shielding composite material has a gradient Janus structure, and the composite material comprises a polymer matrix, a conductive filler and a functional filler. The polymer matrix is water-based polyurethane. The conductive filler is carboxylated cellulose nanofiber modified gallium-indium liquid metal. The functional filler is polydopamine modified graphene oxide nanosheet. The flexible electromagnetic shielding composite material of the present disclosure has high electromagnetic shielding efficiency, excellent flexibility, strain insensitivity and anti-leakage characteristics.

Description

Technical Field

[0001] This disclosure belongs to the field of electromagnetic shielding materials technology, specifically relating to a strain-insensitive, leak-proof liquid metal-based flexible electromagnetic shielding composite material and its preparation method. Background Technology

[0002] With the rapid advancement of urbanization and the high-speed development of modern information technology, electromagnetic interference (EMI) problems are becoming increasingly serious. The large-scale construction of communication base stations, the widespread adoption of smart homes, and the high-frequency use of various electronic devices have led to a significant increase in the density of electromagnetic waves in the environment. This type of non-contact radiation not only poses a potential threat to human health, increasing the risk of cardiovascular, nervous system, and other systemic diseases, but also severely interferes with the normal operation of precision instruments and equipment, causing signal disturbances, data transmission errors, component malfunctions, and other types of failures. Therefore, the development of high-performance electromagnetic shielding materials is of significant practical importance for improving the living environment and ensuring the normal operation of electronic equipment. Traditional electromagnetic shielding materials mostly use copper, aluminum, iron, and their alloys as the main base material, and their shielding function relies on the excellent high conductivity of the metal base material. When electromagnetic waves irradiate the material surface, the free electrons in the base material are forced to oscillate and reflect most of the electromagnetic waves. The remaining electromagnetic waves that penetrate drive the internal electrons to form an induced current, which is then converted into heat energy through the Joule effect, achieving the electromagnetic shielding effect. While these materials offer good electromagnetic shielding, their inherent rigidity makes them difficult to conform to complex curved surfaces or achieve effective stretching, thus hindering their application in wearable devices, intelligent robots, and flexible electronic components. Therefore, developing highly efficient electromagnetic shielding materials with excellent stretchability has become a critical and urgent problem to solve.

[0003] Conductive polymer composites based on elastomers, possessing both stretchability and conductivity, have become a research hotspot in the field of electromagnetic shielding. However, traditional rigid conductive fillers (such as graphene, carbon nanotubes, and Mxene) are prone to separation between filler contacts and disruption of the conductive network integrity due to matrix deformation under tensile or other external forces, ultimately leading to a significant decrease in electromagnetic shielding performance. Liquid metal (LM) combines the high conductivity of metals with the strong fluidity of fluids. When embedded as a conductive filler in an elastomer matrix, LM can adaptively flow and reconstruct itself with matrix deformation, effectively alleviating the separation and network breakage problems of traditional rigid fillers under strain, thus endowing the composite material with reversible conductivity under deformation.

[0004] However, in practical applications, the high surface tension of liquid metal makes it poorly compatible with the elastomer matrix, preventing efficient transfer of interfacial stress. Although liquid metal itself has excellent flow deformation capabilities, it cannot achieve coordinated deformation with the polymer matrix during stretching, ultimately causing local breakage of the conductive network under strain, resulting in increased resistance. In addition, liquid metal itself has a high density, and it is prone to settling and accumulating at the bottom of the material under gravity in the elastomer matrix. On the one hand, this leads to a reduction in the effective thickness of the conductive layer, resulting in a significant decrease in electromagnetic shielding performance. On the other hand, the liquid metal particles accumulated at the bottom are prone to merging to form large droplets, making the material highly susceptible to leakage of liquid metal under external forces, causing irreversible performance degradation and environmental pollution.

[0005] In summary, it is urgent to develop a novel flexible liquid metal-based electromagnetic shielding material that combines high electromagnetic shielding effectiveness, strain insensitivity, and leak-proof capability. Summary of the Invention

[0006] This disclosure aims to at least solve one of the technical problems existing in the prior art, and to provide a strain-insensitive leak-proof liquid metal-based flexible electromagnetic shielding composite material and its preparation method.

[0007] In one aspect, this disclosure provides a strain-insensitive, leak-proof, liquid metal-based flexible electromagnetic shielding composite material, the flexible electromagnetic shielding composite material having a gradient Janus structure, the composite material comprising: The polymer matrix is ​​an aqueous polyurethane; The conductive filler is a carboxylated cellulose nanofiber modified gallium-indium liquid metal; The functional filler is polydopamine-modified graphene oxide nanosheets.

[0008] Optionally, the mass ratio of the polymer matrix, the conductive filler, and the functional filler is 1:(0.8~2.5):(0.01~0.03).

[0009] Optionally, the particle size of the carboxylated cellulose nanofiber modified gallium indium liquid metal is 100-1000 nm; The polydopamine-modified graphene oxide nanosheets have a size of 1~5 μm.

[0010] In another aspect of this disclosure, a method for preparing the aforementioned leak-proof liquid metal-based flexible electromagnetic shielding composite material is provided, the method comprising: Preparation of gallium-indium liquid metal emulsion modified with carboxylated cellulose nanofibers; Preparation of polydopamine-modified graphene oxide nanosheet dispersion; A waterborne polyurethane solution, the carboxylated cellulose nanofiber modified gallium indium liquid metal emulsion, and the polydopamine-modified graphene oxide nanosheet dispersion were blended and subjected to high-speed shearing to obtain a WPU / CLM / PGO dispersion. The WPU / CLM / PGO dispersion was subjected to evaporation and drying treatment to obtain the WPU / CLM / PGO composite material; The composite material is subjected to high-temperature mechanical pressing to obtain WPU / CLM / PGO electromagnetic shielding composite material.

[0011] Optionally, the preparation of the carboxylated cellulose nanofiber modified gallium-indium liquid metal emulsion includes: Gallium-indium liquid metal was added to a carboxylated cellulose nanofiber solution, and the temperature was maintained at 0–10 °C. The solution was then ultrasonically treated for 10–30 min with an ultrasonic power of 40%–80% to obtain a gallium-indium liquid metal emulsion modified with carboxylated cellulose nanofibers.

[0012] Optionally, the solute mass fraction of the carboxylated cellulose nanofiber solution is 0.1% to 0.3%. The concentration of the carboxylated cellulose nanofiber-modified gallium-indium liquid metal emulsion is 0.05–0.1 g·mL. -1 ; The particle size of the carboxylated cellulose nanofiber modified gallium-indium liquid metal microdroplets is 100–1000 nm.

[0013] Optionally, the preparation of the polydopamine-modified graphene oxide nanosheet dispersion includes: Graphene oxide and dopamine hydrochloride were uniformly dispersed in deionized water. Tris-HCl buffer was added to adjust the pH of the system to 8.0-8.5. The mixture was stirred continuously at room temperature for 22-26 hours to allow dopamine hydrochloride to polymerize and form polydopamine, which was then uniformly coated on the surface of graphene oxide nanosheets, resulting in a polydopamine-modified graphene oxide nanosheet dispersion.

[0014] Optionally, the mass ratio of the graphene oxide to the dopamine hydrochloride is (0.1-0.3):(0.05-0.15); The concentration of the polydopamine-modified graphene oxide nanosheet dispersion is 10–20 mg·mL. -1 .

[0015] Optionally, the solute mass fraction of the aqueous polyurethane solution is 40% to 60%. The mass ratio of the polydopamine-modified graphene oxide nanosheet dispersion to the carboxylated cellulose nanofiber-modified gallium indium liquid metal emulsion is 1:(4-12). The mass ratio of the aqueous polyurethane solution to the carboxylated cellulose nanofiber modified gallium indium liquid metal emulsion is (0.6~2):(4~12).

[0016] Optionally, the high-speed shearing rate is 12,000~14,000 rpm, and the shearing time is 4-6 min; The evaporation drying process is carried out at a temperature range of 60°C to 90°C for a time range of 3 to 6 hours, with an evaporation rate of 0.1 to 0.3 kg / m³. -2 h -1 ; The high-temperature mechanical pressing process has a temperature range of 110°C to 140°C, a pressure range of 5 to 10 MPa, and a time range of 25 to 30 min.

[0017] This disclosure presents a strain-insensitive, leak-proof, liquid metal-based flexible electromagnetic shielding composite material and its preparation method. The material has a gradient Janus structure and comprises: a polymer matrix, wherein the polymer matrix is ​​aqueous polyurethane; a conductive filler, wherein the conductive filler is gallium-indium liquid metal modified with carboxylated cellulose nanofibers; and a functional filler, wherein the functional filler is polydopamine-modified graphene oxide nanosheets. The material of this disclosure mainly consists of three parts: a polymer matrix, a conductive filler, and a functional filler. The aqueous polyurethane (WPU) imparts excellent flexibility and mechanical properties to the material, and has low biotoxicity. The introduction of carboxylated cellulose nanofibers significantly improves the dispersibility of liquid metal droplets, achieving delayed phase separation of the solution and greatly enhancing its leak-proof capability. The functional filler promotes the interaction between the liquid metal and the polymer network, improving the material's mechanical strength and conductivity while enhancing the co-deformation capability of the liquid metal and the polymer matrix, thus making the composite material strain-insensitive. This material possesses high electromagnetic shielding effectiveness while also exhibiting excellent flexibility, strain insensitivity, and leak-proof characteristics. Attached Figure Description

[0018] Figure 1 A flowchart illustrating a method for preparing a strain-insensitive, leak-proof, liquid metal-based flexible electromagnetic shielding composite material according to a specific embodiment of this disclosure; Figure 2 This is a cross-sectional scanning electron microscope image of the CL / PG1W composite material in Embodiment 1 of this disclosure; Figure 3 The electromagnetic shielding effectiveness of the CL / PG1W composite material in the X-band in Embodiment 1 of this disclosure is shown. Figure 4 The electrical conductivity diagrams are shown for the composite materials in Examples 1-3 and Comparative Example 1 of this disclosure; Figure 5The electromagnetic shielding effectiveness diagrams are shown for the composite materials in Examples 1-3 and Comparative Example 1 of this disclosure; Figure 6 This is a comparison graph showing the resistance stability of Embodiment 1 and Comparative Example 1 after 1000 tensile cycles under 150% strain conditions. Figure 7 The images show the surface liquid metal leakage conditions of Embodiment 1 and Comparative Example 2 after being adhered and removed with strong adhesive tape. Detailed Implementation

[0019] To enable those skilled in the art to better understand the technical solutions of this disclosure, the disclosure will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain this disclosure and represent a part of the embodiments of this disclosure, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the protection scope of this disclosure.

[0020] One aspect of this disclosure proposes a strain-insensitive leak-proof liquid metal-based flexible electromagnetic shielding composite material with a gradient Janus structure, comprising the following components: a polymer matrix, a conductive filler, and a functional filler; wherein the polymer matrix is ​​waterborne polyurethane (WPU), the conductive filler is carboxylated cellulose nanofiber modified gallium indium liquid metal (CLM), and the functional filler is polydopamine-modified graphene oxide nanosheets (PGO).

[0021] In some preferred embodiments, the mass ratio of the polymer matrix, the conductive filler, and the functional filler, based on dry weight, is 1:(0.8~2.5):(0.01~0.03).

[0022] In some preferred embodiments, the particle size of the carboxylated cellulose nanofiber modified gallium indium liquid metal microdroplets is 100–1000 nm.

[0023] In some preferred embodiments, the polydopamine-modified graphene oxide nanosheets have a size of 1~5 μm.

[0024] It should be understood that the Janus gradient structure is a gradient distribution of composition and properties in the thickness direction of the material. Based on the delayed phase separation process in the preparation process, the high-density modified liquid metal (CLM) microdroplets slowly and gradually deposit to the bottom during the solvent evaporation process, resulting in a high CLM content at the bottom of the material and a low content at the top, thus forming a gradient Janus structure with one side being highly conductive and the other side being relatively insulating. This structure is beneficial for the efficient shielding of electromagnetic waves in the conductive layer.

[0025] In this embodiment, waterborne polyurethane (WPU) is used to construct a flexible three-dimensional network matrix, endowing the material with excellent flexibility and mechanical properties, and low biotoxicity. Secondly, carboxylated cellulose nanofiber-modified liquid metal (CLM) droplets interconnect within the WPU matrix, particularly in the enriched region at the bottom of the material, forming a three-dimensional permeable conductive network. This is the basis for high conductivity and electromagnetic shielding effectiveness. The introduction of these carboxylated cellulose nanofibers significantly improves the dispersibility of the liquid metal droplets, achieving delayed phase separation between the liquid metal droplets and the WPU matrix, and promoting the formation of a three-dimensional WPU conductive layer. The U-network confines the liquid metal, significantly enhancing its leak-proof capability. Furthermore, polydopamine-modified graphene oxide (PGO) nanosheets are uniformly dispersed in the WPU matrix. On the one hand, due to their high specific surface area and conductivity, they form a large number of micro-liquid bridges with CLM microdroplets in the later stage of evaporation, strengthening capillary sintering force and connecting CLM microdroplets, promoting the construction of a continuous conductive network and enhancing overall conductivity. On the other hand, the polydopamine layer on its surface has good interaction with both WPU and CLM, enhancing the co-deformation capability of the liquid metal and the polymer matrix, thus making the composite material strain insensitive.

[0026] like Figure 1 As shown, in another aspect of this disclosure, a method S100 for preparing the leak-proof liquid metal-based flexible electromagnetic shielding composite material as described above is proposed, specifically including the following steps S110~S150: S110, prepare gallium-indium liquid metal emulsion modified with carboxylated cellulose nanofibers.

[0027] In step S110, forming a carboxylated cellulose nanofiber modified gallium-indium liquid metal emulsion includes the following process: Gallium indium liquid metal was placed in a centrifuge tube containing a carboxylated cellulose nanofiber solution. The bottom of a 6 cm diameter amplitude transformer was placed 1 cm away from the bulk gallium indium liquid metal. The centrifuge tube was placed in an ice bath and the temperature was maintained at 0–10 °C. The ultrasonic frequency was set to start for 2 seconds and stop for 1 second. The total ultrasonic operation time was 10–30 min and the ultrasonic power was set to 40%–80%. A gallium indium liquid metal (CLM) emulsion modified with carboxylated cellulose nanofiber was obtained.

[0028] In some preferred embodiments, the solute mass fraction of the carboxylated cellulose nanofiber solution is 0.1% to 0.3%. Within this concentration range, CNF can effectively suppress the aggregation of liquid metal (LM) droplets by providing sufficient electrostatic repulsion, thereby achieving uniform dispersion and system stabilization in the aqueous phase. Too low a concentration (<0.1%) results in insufficient coating and easy aggregation of liquid metal particles; too high a concentration (>0.3%) may lead to excessively high system viscosity, affecting subsequent blending uniformity and droplet size control. Simultaneously, under these concentration conditions, the electrostatic repulsion provided by CNF significantly decreases during solvent evaporation, accompanied by a significant increase in system viscosity. This effectively slows down the sedimentation and aggregation rate of liquid metal droplets from a kinetic perspective, thereby achieving the controllable construction of the gradient Janus structure.

[0029] In some other preferred embodiments, the concentration of the carboxylated cellulose nanofiber modified gallium-indium liquid metal emulsion is 0.05–0.1 g·mL. -1 This concentration range ensures that the liquid metal has sufficient filling volume to form a highly conductive network, while preventing severe agglomeration during ultrasonic emulsification or blending due to excessive concentration. It is also an important factor in effectively controlling the proportion of the conductive layer in the gradient structure, thereby optimizing the balance between shielding effectiveness and mechanical properties.

[0030] In some other preferred embodiments, the carboxylated cellulose nanofiber modified gallium indium liquid metal droplets have a particle size of 100–1000 nm. Liquid metal droplets within this particle size range can meet the requirements of kinetically controllable phase separation: when the particle size is too small, the droplets are uniformly dispersed in the system due to the balance between electrostatic repulsion and gravity, making it difficult for them to settle; when the particle size is too large, the gravity acting on the droplets is much greater than the hindrance provided by the viscosity of the system, and the settling rate is too fast, making it impossible to achieve delayed phase separation.

[0031] This embodiment modifies the surface of liquid metal with carboxylated cellulose nanofibers (CNF). CNF acts as a surfactant and stabilizer, coating the surface of liquid metal droplets. This significantly reduces the interfacial energy of the liquid metal, improves its dispersion stability in WPU aqueous solution and its compatibility with the matrix, and solves the problems of poor compatibility between liquid metal and polymer and easy aggregation and sedimentation. This allows the liquid metal to be deposited slowly and uniformly during the drying process, forming a stable gradient distribution.

[0032] S120. Preparation of polydopamine-modified graphene oxide nanosheet dispersion.

[0033] In step S120, forming a polydopamine-modified graphene oxide nanosheet (PGO) dispersion includes the following processes: 0.1-0.3 g of graphene oxide and 0.05-0.15 g of dopamine hydrochloride were uniformly dispersed in deionized water. The pH of the system was adjusted to 8.0-8.5 by adding Tris-HCl buffer. The mixture was continuously magnetically stirred at 1400-1600 rpm for 22-26 h at room temperature. Taking advantage of the self-polymerization property of dopamine hydrochloride in a weakly alkaline environment, dopamine hydrochloride was polymerized to form polydopamine, which was uniformly coated on the surface of graphene oxide nanosheets, thus obtaining a polydopamine-modified graphene oxide nanosheet (PGO) dispersion.

[0034] In the above steps, the graphene oxide (GO) sheets need to be large-sized (10-40 µm) monolayer / few-layer GO sheets prepared by a modified Hummer's method; the concentration of the PGO dispersion is 5-10 mg·mL. -1 .

[0035] In some preferred embodiments, the mass ratio of graphene oxide to dopamine hydrochloride is (0.1-0.3):(0.05-0.15).

[0036] In some other preferred embodiments, the concentration of the polydopamine-modified graphene oxide nanosheet dispersion is 10–20 mg·mL. -1 .

[0037] In this embodiment, dopamine hydrochloride is rich in active groups such as hydroxyl and amino groups, which can form strong hydrogen bonds and coordination interactions with the WPU matrix and CNF-modified CLM, acting as a molecular bridge between the liquid metal and the polymer matrix. During material stretching, the PGO sheets can effectively transfer stress, promote the synergistic deformation of the liquid metal and the polymer matrix, prevent the breakage of the conductive network, and thus maintain a high degree of resistance stability.

[0038] S130. The aqueous polyurethane solution, the carboxylated cellulose nanofiber modified gallium indium liquid metal emulsion, and the polydopamine-modified graphene oxide nanosheet dispersion are blended and subjected to high-speed shearing to obtain a WPU / CLM / PGO dispersion.

[0039] In some preferred embodiments, the solute mass fraction of the aqueous polyurethane solution is 40% to 60%.

[0040] In some other preferred embodiments, the mass ratio of the polydopamine-modified graphene oxide nanosheet dispersion to the carboxylated cellulose nanofiber-modified gallium indium liquid metal emulsion is 1:(4-12).

[0041] In some other preferred embodiments, the mass ratio of the aqueous polyurethane solution to the carboxylated cellulose nanofiber modified gallium indium liquid metal emulsion is (0.6~2):(4~12).

[0042] In some other preferred embodiments, the high-speed shearing step is performed by a spin coater at a shearing rate of 12,000 to 14,000 rpm for a shearing time of 4 to 6 minutes.

[0043] In this embodiment, the high-speed shearing process described above ensures that the components (WPU, CLM, PGO) are uniform and highly dispersed, forming an initial stable mixed system. The high energy provided by the high-speed shearing facilitates the physical contact and initial bonding between the components (such as CLM and PGO, WPU and PGO), which is conducive to the establishment of interactions such as electrostatic forces and hydrogen bonds. At the same time, by controlling the shearing rate, the size and distribution of CLM droplets can be adjusted, affecting the fineness of the final conductive network.

[0044] S140. The WPU / CLM / PGO dispersion is subjected to evaporation and drying treatment to obtain the WPU / CLM / PGO composite material.

[0045] In some preferred embodiments, the above WPU / CLM / PGO dispersion is subjected to evaporative drying in a forced-air drying oven at a temperature range of 60°C to 90°C for a time range of 3 to 6 hours; the evaporation rate is 0.1-0.3 kg m³ / h. -2 h -1 In the solvent evaporation process described above, unlike the traditional rapid phase separation process, carboxylated cellulose nanofibers (CNF) act as a modifier, significantly improving the dispersion stability of CLM in WPU aqueous solution. During the slow evaporation process, the presence of carboxylated cellulose nanofibers causes delayed phase separation of CLM droplets, which slowly and gradually deposit to the bottom of the system, forming a preliminary but not completely continuous conductive network. At the same time, during the evaporation process, WPU gradually solidifies as the moisture decreases, forming a flexible three-dimensional polymer network that encapsulates CLM and PGO, ultimately yielding a WPU / CLM / PGO composite material with a gradient Janus structure.

[0046] S150. The composite material is subjected to high-temperature mechanical pressing to obtain WPU / CLM / PGO electromagnetic shielding composite material.

[0047] In some preferred embodiments, the temperature range of the high-temperature mechanical pressing treatment is 110°C to 140°C, the pressure range is 5 to 10 MPa, and the time range is 25 to 30 min.

[0048] In this embodiment, the composite material undergoes high-temperature hot-pressing to obtain a WPU / CLM / PGO electromagnetic shielding composite material with a gradient Janus structure. The hot-pressing pressure directly destroys the native oxide layer on the LM surface, eliminating electron transport barriers and promoting in-situ fusion and complete connection of LM microdroplets to form a continuous three-dimensional conductive network. This significantly reduces contact resistance and interfacial impedance, resulting in a substantial increase in the material's conductivity. Simultaneously, the high temperature softens the WPU matrix, enhancing the chain segment mobility. Combined with a certain period of pressure holding, this fills the internal pores of the material and densifies the structure, significantly improving the material's mechanical properties and achieving synergistic optimization of conductivity and mechanical properties.

[0049] In the above method, on the one hand, by using waterborne polyurethane (WPU) as a matrix, the material is endowed with excellent flexibility and mechanical properties; on the other hand, carboxylated cellulose nanofiber modified gallium indium liquid metal (CLM) as a conductive filler not only significantly improves the material's electrical conductivity and electromagnetic shielding effectiveness, but also significantly improves the dispersibility of the liquid metal emulsion through the introduction of carboxylated cellulose nanofiber, achieving delayed phase separation of the solution and effectively preventing the liquid metal from seeping out during stress or use, thus greatly enhancing its leak-proof capability; furthermore, polydopamine-modified graphene oxide nanosheets (PGO) as a functional filler promote the interaction between the liquid metal and the polymer network, improving the material's mechanical strength and conductivity while enhancing the co-deformation capability of the liquid metal and the polymer matrix, thereby making the composite material strain insensitive.

[0050] In this implementation method, the preparation method mainly includes three steps: dispersion preparation, evaporation and drying, and high-temperature mechanical pressing. Specifically, the CLM emulsion and PGO dispersion are prepared and then mixed with WPU solution to form a dispersion through electrostatic force, hydrogen bonding and other interactions. After that, the dispersion is evaporated and dried in a forced-air drying oven. Finally, the conductive pathway is further activated by high-temperature mechanical pressing. This preparation method is simple and convenient.

[0051] In this embodiment, the WPU / CLM / PGO electromagnetic shielding composite material obtained by the above method has extremely high conductivity (~106 S·m). -1 When the thickness is greater than or equal to 200 µm, it achieves an electromagnetic shielding effectiveness of greater than or equal to 80 dB; secondly, the material has excellent flexibility and strain insensitivity, and its conductivity hardly decreases after being repeatedly stretched 1000 times under a deformation of 150%; thirdly, the material also exhibits excellent leak-proof properties, and even after mechanical pressing or strong adhesive tape, there is almost no leakage of liquid metal on its surface.

[0052] The electromagnetic shielding material disclosed herein can be applied to application scenarios that require both high material flexibility and efficient electromagnetic shielding performance, such as intelligent robots, flexible electronic devices, and wearable device surfaces.

[0053] The preparation method of electromagnetic shielding materials will be further explained below with reference to specific embodiments: Example 1 The method for preparing the strain-insensitive, leak-proof, liquid metal-based flexible electromagnetic shielding composite material in this embodiment includes the following steps: S1. Add 2 mL of 1% carboxylated cellulose nanofiber solution to 8 mL of deionized water to dilute and obtain a 0.2% carboxylated cellulose nanofiber solution. Place 1 g of gallium indium liquid metal in a centrifuge tube containing the above carboxylated cellulose nanofiber solution. Place the bottom of a 6 cm diameter amplitude transformer 1 cm away from the bulk gallium indium liquid metal. Place the centrifuge tube in an ice bath and maintain the temperature between 0 and 10 °C. Set the ultrasonic frequency to start for 2 seconds and stop for 1 second. The total ultrasonic running time is 10 min. Set the ultrasonic power to 40%. Obtain the carboxylated cellulose nanofiber modified gallium indium liquid metal (CLM) emulsion.

[0054] S2. Weigh 0.3 g of graphene oxide powder and 0.15 g of dopamine hydrochloride powder and disperse them evenly in 30 mL of deionized water. Add Tris-HCl buffer to adjust the pH of the system to 8.5. Stir continuously with magnetic force at 1500 rpm for 24 h at room temperature. Utilize the self-polymerization property of dopamine hydrochloride in a weakly alkaline environment to polymerize dopamine hydrochloride to form polydopamine and uniformly coat the surface of graphene oxide nanosheets to obtain a polydopamine-modified graphene oxide nanosheet (PGO) dispersion.

[0055] S3. 10 mL of the CLM emulsion formed in step S1, 1 mL of the PGO dispersion formed in step S2, and 1.5 g of a 60% waterborne polyurethane (WPU) solution are mixed and subjected to high-speed shearing using a spin coater to obtain a uniform WPU / CLM / PGO dispersion. The shearing rate is 12000 rpm and the shearing time is 5 min.

[0056] S4. Place the above WPU / CLM / PGO dispersion in a petri dish and use a forced-air drying oven to evaporate and dry the dispersion at a temperature of 80°C to finally obtain a WPU / CLM / PGO composite material with a gradient Janus structure.

[0057] S5. The above WPU / CLM / PGO composite material is subjected to high-temperature mechanical pressing treatment. The temperature is set to 130°C, the pressure is set to 5 MPa, and the time is set to 30 min. Finally, a WPU / CLM / PGO electromagnetic shielding composite material with a gradient Janus structure is obtained, which is abbreviated as CL / PG1W.

[0058] The cross-sectional scanning electron microscope image of the CL / PG1W composite material in Example 1 is shown below. Figure 2 As shown, the cross-section of the material exhibits a gradient distribution of liquid metal in the WPU matrix and a uniform distribution of PGO sheets within the material. The overall thickness of the composite material is approximately 200 µm.

[0059] like Figure 3 As shown, referring to the test method in GJB 8820-2015, the electromagnetic shielding efficiency of the CL / PG1W composite material in the X-band was measured to be approximately 80 dB using the waveguide method.

[0060] Example 2 The preparation method of the strain-insensitive leak-proof liquid metal-based flexible electromagnetic shielding composite material in this embodiment differs from the preparation method in Example 1 as follows: In step S3, 10 mL of the CLM emulsion formed in step S1, 0.5 mL of the PGO dispersion formed in step S2, and 1.5 g of a 60% waterborne polyurethane (WPU) solution are blended together and subjected to high-speed shearing using a spin coater to obtain a uniform WPU / CLM / PGO dispersion. The shearing rate is 12000 rpm and the shearing time is 5 min.

[0061] The other preparation steps are the same as in Example 1, resulting in a WPU / CLM / PGO electromagnetic shielding composite material with a gradient Janus structure, abbreviated as CL / PG0.5W.

[0062] Example 3 The preparation method of the strain-insensitive leak-proof liquid metal-based flexible electromagnetic shielding composite material in this embodiment differs from the preparation method in Example 1 as follows: In step S3, 10 mL of the CLM emulsion formed in step S1, 2 mL of the PGO dispersion formed in step S2, and 1.5 g of a 60% waterborne polyurethane (WPU) solution are blended together and subjected to high-speed shearing using a spin coater to obtain a uniform WPU / CLM / PGO dispersion. The shearing rate is 12000 rpm and the shearing time is 5 min.

[0063] The other preparation steps are the same as in Example 1, resulting in a WPU / CLM / PGO electromagnetic shielding composite material with a gradient Janus structure, abbreviated as CL / PG2W.

[0064] Comparative Example 1 The preparation method of the strain-insensitive leak-proof liquid metal-based flexible electromagnetic shielding composite material in this comparative example differs from the preparation method in Example 1 as follows: This comparative example does not involve the preparation and addition of polydopamine-modified graphene oxide nanosheets (PGO) dispersions in step S2.

[0065] In step S3, 10 mL of the CLM emulsion formed in step S1 and 1.5 g of a 60% waterborne polyurethane (WPU) solution are blended together and subjected to high-speed shearing using a spin coater to obtain a uniform WPU / CLM dispersion. The shearing rate is 12000 rpm and the shearing time is 5 min.

[0066] The other preparation steps are the same as in Example 1, resulting in a WPU / CLM electromagnetic shielding composite material with a gradient Janus structure without the addition of PGO sheets, abbreviated as CL / PG0W.

[0067] Comparative Example 2 The preparation method of the strain-insensitive leak-proof liquid metal-based flexible electromagnetic shielding composite material in this comparative example differs from the preparation method in Example 1 as follows: This comparative example does not include the preparation and addition of the carboxylated cellulose nanofiber modified gallium indium liquid metal (CLM) emulsion in step S1.

[0068] In step S1, 1 g of gallium indium liquid metal is placed in a centrifuge tube containing 10 mL of deionized water. The bottom of a 6 cm diameter amplitude transformer is placed 1 cm away from the block of gallium indium liquid metal. The centrifuge tube is placed in an ice bath and the temperature is maintained between 0 and 10 °C. The ultrasonic frequency is set to start for 2 seconds and stop for 1 second. The total ultrasonic running time is 10 min and the ultrasonic power is set to 40%, thus obtaining a gallium indium liquid metal (LM) emulsion.

[0069] In step S3, 10 mL of the LM emulsion formed in step S1, 1 mL of the PGO dispersion formed in step S2, and 1.5 g of a 60% waterborne polyurethane (WPU) solution are mixed and subjected to high-speed shearing using a spin coater to obtain a uniform WPU / LM / PGO dispersion. The shearing rate is 12000 rpm and the shearing time is 5 min.

[0070] The other preparation steps are the same as in Example 1, resulting in a leaky WPU / LM / PGO electromagnetic shielding composite material without carboxylated cellulose nanofiber modification, abbreviated as C0L / PG1W.

[0071] Figure 4The conductivity diagrams are for the composite materials in Examples 1-3 and Comparative Example 1, based on... Figure 4 It can be seen that as the PGO lamellar content increases, the electrical conductivity of the composite material first increases and then decreases. Excessive PGO content can actually reduce the material's electrical conductivity. In Example 1, the electrical conductivity of the composite material reached 3.3 × 10⁶ S·m. -1 .

[0072] Figure 5 The electromagnetic shielding effectiveness diagrams of the composite materials in Examples 1-3 and Comparative Example 1 in the X-band are shown below. Figure 5 It can be seen that the electromagnetic shielding effectiveness of composite materials and Figure 4 The electrical conductivity of the materials is positively correlated. As the content of PGO sheets increases, the electromagnetic shielding effectiveness of the composite material also shows a trend of first increasing and then decreasing.

[0073] Figure 6 The resistance changes of the composite materials of Example 1 and Comparative Example 1 after 1000 tensile cycles at 150% strain were compared. The results showed that the composite material with PGO sheets (Example 1) exhibited excellent resistance stability, with almost no change in resistance value. In contrast, the resistance of the composite material without PGO (Comparative Example 1) increased significantly, confirming the key role of PGO sheets in maintaining the stability of the conductive network structure.

[0074] Figure 7 The amount of residual liquid metal on the surface of the adhesive tape after the composite materials of Example 1 and Comparative Example 2 were peeled off was compared. The results showed that the delayed phase separation effect induced by the modification of carboxylated cellulose nanofibers effectively suppressed the leakage of liquid metal, while the unmodified composite material exhibited serious liquid metal leakage behavior, highlighting the necessity of this modification strategy in leak-proof design.

[0075] This disclosure proposes a strain-insensitive, leak-proof liquid metal-based flexible electromagnetic shielding composite material and its preparation method, which has the following beneficial effects: First, this invention is prepared by a delayed phase separation-solvent evaporation-mechanical activation method. The material consists of three parts: aqueous polyurethane, carboxylated cellulose nanofiber modified gallium indium liquid metal, and polydopamine-modified graphene oxide nanosheets. This method is simple and easy to operate, and the introduction of highly conductive liquid metal endows the material with high overall conductivity, enabling it to achieve an electromagnetic shielding effectiveness of over 80 dB.

[0076] Secondly, the present invention uses waterborne polyurethane as the matrix, which endows the material with excellent flexibility; at the same time, the introduction of polydopamine-modified graphene oxide nanosheets enables the composite material to exhibit excellent electrical resistance stability. After 1000 tensile cycles at 150% strain, its resistance value hardly changes, achieving strain insensitivity.

[0077] Third, this invention utilizes the delayed phase separation effect induced by carboxylated cellulose nanofibers modifying liquid metal to effectively curb the leakage of liquid metal. After repeated application and removal of the composite material with strong adhesive tape, almost no liquid metal residue remains on the tape surface, demonstrating excellent leak-proof properties.

[0078] Fourth, the composite material described in this invention is inexpensive and has a simple preparation process, making it widely applicable in fields such as intelligent robots, flexible electronic devices, and wearable device surfaces. It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of this disclosure; however, this disclosure is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and substance of this disclosure, and these modifications and improvements are also considered to be within the scope of protection of this disclosure.

Claims

1. A strain-insensitive, leak-proof, liquid metal-based flexible electromagnetic shielding composite material, characterized in that, The flexible electromagnetic shielding composite material has a gradient Janus structure, and the composite material includes: The polymer matrix is ​​an aqueous polyurethane; The conductive filler is a carboxylated cellulose nanofiber modified gallium-indium liquid metal; The functional filler is polydopamine-modified graphene oxide nanosheets.

2. The leak-proof liquid metal-based flexible electromagnetic shielding composite material according to claim 1, characterized in that, The mass ratio of the polymer matrix, the conductive filler, and the functional filler is 1:(0.8~2.5):(0.01~0.03).

3. The leak-proof liquid metal-based flexible electromagnetic shielding composite material according to claim 1, characterized in that, The particle size of the carboxylated cellulose nanofiber modified gallium-indium liquid metal is 100–1000 nm. The polydopamine-modified graphene oxide nanosheets have a size of 1~5 μm.

4. A method for preparing a leak-proof liquid metal-based flexible electromagnetic shielding composite material as described in any one of claims 1 to 3, characterized in that, The method includes: Preparation of gallium-indium liquid metal emulsion modified with carboxylated cellulose nanofibers; Preparation of polydopamine-modified graphene oxide nanosheet dispersion; A waterborne polyurethane solution, the carboxylated cellulose nanofiber modified gallium indium liquid metal emulsion, and the polydopamine-modified graphene oxide nanosheet dispersion were blended and subjected to high-speed shearing to obtain a WPU / CLM / PGO dispersion. The WPU / CLM / PGO dispersion was subjected to evaporation and drying treatment to obtain the WPU / CLM / PGO composite material; The composite material is subjected to high-temperature mechanical pressing to obtain WPU / CLM / PGO electromagnetic shielding composite material.

5. The method according to claim 4, characterized in that, The preparation of the carboxylated cellulose nanofiber modified gallium-indium liquid metal emulsion includes: Gallium-indium liquid metal was added to a carboxylated cellulose nanofiber solution, and the temperature was maintained at 0–10 °C. The solution was then ultrasonically treated for 10–30 min with an ultrasonic power of 40%–80% to obtain a gallium-indium liquid metal emulsion modified with carboxylated cellulose nanofibers.

6. The method according to claim 5, characterized in that, The solute mass fraction of the carboxylated cellulose nanofiber solution is 0.1% to 0.3%. The concentration of the carboxylated cellulose nanofiber-modified gallium-indium liquid metal emulsion is 0.05–0.1 g·mL. -1 ; The particle size of the carboxylated cellulose nanofiber modified gallium-indium liquid metal microdroplets is 100–1000 nm.

7. The method according to claim 4, characterized in that, The preparation of the polydopamine-modified graphene oxide nanosheet dispersion includes: Graphene oxide and dopamine hydrochloride were uniformly dispersed in deionized water. Tris-HCl buffer was added to adjust the pH of the system to 8.0-8.

5. The mixture was stirred continuously at room temperature for 22-26 hours to allow dopamine hydrochloride to polymerize and form polydopamine, which was then uniformly coated on the surface of graphene oxide nanosheets, resulting in a polydopamine-modified graphene oxide nanosheet dispersion.

8. The method according to claim 7, characterized in that, The mass ratio of the graphene oxide to the dopamine hydrochloride is (0.1-0.3):(0.05-0.15); The concentration of the polydopamine-modified graphene oxide nanosheet dispersion is 10–20 mg·mL. -1 .

9. The method according to claim 4, characterized in that, The solute mass fraction of the aqueous polyurethane solution is 40%–60%. The mass ratio of the polydopamine-modified graphene oxide nanosheet dispersion to the carboxylated cellulose nanofiber-modified gallium indium liquid metal emulsion is 1:(4-12). The mass ratio of the aqueous polyurethane solution to the carboxylated cellulose nanofiber modified gallium indium liquid metal emulsion is (0.6~2):(4~12).

10. The method according to claim 4, characterized in that, The high-speed shearing rate is 12000~14000 rpm, and the shearing time is 4-6 min; The evaporation drying process is carried out at a temperature range of 60°C to 90°C for 3 to 6 hours, with an evaporation rate of 0.1 to 0.3 kg / m³. -2 h -1 ; The high-temperature mechanical pressing process has a temperature range of 110°C to 140°C, a pressure range of 5 to 10 MPa, and a time range of 25 to 30 min.

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