A flexible electronic material made of PVDF fiber membrane, its preparation method and application

Abstract

This invention relates to a PVDF fiber membrane flexible electronic material, its preparation method, and its applications. The method includes: mixing polyvinylidene fluoride (PVDF), an anhydride crosslinking agent, an initiator, POE-g-GMA, and an organic solvent; reacting the mixture to obtain a spinning solution; electrospinning the solution; and then hot-pressing it to obtain a hot-pressed PVDF fiber membrane substrate. Liquid metal ink is then used to form a patterned circuit on the surface of the hot-pressed PVDF fiber membrane substrate, followed by mechanical pressure to obtain an activated PVDF fiber membrane. Multiple layers of the activated PVDF fiber membrane are then laminated to obtain the final product. This invention's material possesses advantages such as high strength, high toughness, high density, and seamless integration without through-holes. It fundamentally solves the problems of weak bonding and easy detachment between liquid metal and polymer substrates, ensuring the conductivity stability and durability of the circuit under repeated bending and torsion, and has broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronics technology, and specifically relates to a PVDF fiber membrane flexible electronic material, its preparation method, and its application. Background Technology

[0002] Flexible electronic devices have enormous application potential in wearable devices, piezoelectric energy harvesters, and protective electronic devices. Their development places stringent demands on flexible electronic materials, requiring them to possess high strength, high toughness, high conductivity, and high-density integration properties to ensure stable electrical performance during dynamic deformation, thus meeting the application requirements of small size and high flexibility. PVDF, due to its excellent chemical stability, piezoelectric and dielectric properties, is an ideal material for flexible electronic substrates. However, PVDF films prepared by traditional processes suffer from drawbacks such as high brittleness, low elongation at break (<50%), and insufficient toughness due to disordered molecular chain arrangement and simple fiber structure. Under various mechanical actions, such as repeated bending, folding, stretching, or impact loads, they are prone to failure. Even with polymer blending or nanofiller modification, the effects are limited and can easily affect other material properties. Although electrospinning technology can produce micro-nano PVDF fiber membranes that impart flexibility to materials, the fibers are only bound by physical entanglement and van der Waals forces. Under external forces, the fibers are prone to slippage and detachment, resulting in poor mechanical strength of the membrane, which makes it difficult to meet the stringent requirements for mechanical reliability of materials in the aforementioned application fields.

[0003] Liquid metals, due to their excellent conductivity, fluidity, and flexibility, are ideal conductors for constructing flexible circuits. However, their application on PVDF fiber membranes faces several bottlenecks. For example, the interfacial adhesion between liquid metals and the PVDF fiber membrane substrate is poor, making them prone to leakage under mechanical stress, leading to the failure of conductive pathways and severely affecting the electromechanical stability and durability of devices. Furthermore, liquid metals have high surface tension, making them prone to diffusion on the substrate after printing, reducing the accuracy of conductive patterns. In addition, existing technologies require mechanical or laser drilling to achieve vertical pathway connections, which can easily cause stress concentration at the through-hole locations, increasing the risk of material breakage during bending or stretching.

[0004] In summary, the core bottleneck that urgently needs to be overcome in the field of PVDF fiber membrane flexible electronic materials is how to construct high-density, high-precision three-dimensional circuits within a limited thickness or volume, while ensuring excellent mechanical deformation capability, mechanical stability, and electrical reliability. Therefore, developing a class of PVDF fiber membrane flexible electronic materials and their preparation methods that combine mechanical strength, deformation capability, high conductivity, and high integration is of great significance for promoting the development of flexible electronics technology. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a PVDF fiber membrane flexible electronic material, its preparation method and application, which overcomes the defects of existing PVDF fiber membrane flexible electronic materials such as difficulty in balancing mechanical and electrical properties, low circuit integration density, easy shedding and diffusion of liquid metal, etc. It provides a high-strength, high-toughness / high-density PVDF fiber membrane flexible electronic material and its preparation method, so as to achieve high mechanical reliability, high circuit integration and stable electrical performance without significantly increasing the thickness / volume of the material.

[0006] This invention provides a method for preparing a flexible electronic material using PVDF fiber membranes, comprising the following steps:

[0007] (1) Preparation of spinning solution: PVDF, acid anhydride crosslinking agent, initiator, POE-g-GMA and organic solvent are mixed and reacted to obtain spinning solution;

[0008] (2) Electrospinning and hot pressing: The spinning solution is electrospinned to obtain a PVDF fiber membrane; the PVDF fiber membrane is then hot-pressed to obtain a hot-pressed PVDF fiber membrane substrate.

[0009] (3) Preparation of liquid metal ink: Liquid metal, solvent and dispersant are mixed and ultrasonically dispersed, then a catalyst is added and stirred and dispersed thoroughly, and then concentrated by centrifugation to obtain liquid metal ink; wherein the liquid metal has hydroxyl groups on its surface;

[0010] (4) Patterned circuit and mechanical pressure activation: Liquid metal ink is printed on the surface of the PVDF fiber membrane substrate after hot pressing to form a patterned circuit, and the solvent is removed by high temperature drying; then a release film is covered on one side of the patterned circuit, and different mechanical pressures are applied in different areas to control the penetration depth of liquid metal, and each layer contains at least one area activated by high pressure of 5-10 MPa, so that liquid metal penetrates the through-hole area of ​​the substrate to obtain the activated PVDF fiber membrane; the high temperature drying temperature is 100-140℃.

[0011] (5) Multilayer circuit integration: At least two activated PVDF fiber membranes are precisely aligned and stacked, porous adhesive liner is laid between the layers and then hot-pressed composite, and components are assembled after cooling to obtain PVDF fiber membrane flexible electronic material.

[0012] PVDF is polyvinylidene fluoride; POE-g-GMA is an ethylene-octene copolymer grafted with glycidyl methacrylate.

[0013] Preferably, in step (1), the anhydride crosslinking agent is one or more of maleic anhydride (MAH), phthalic anhydride, methyl hexahydrophthalic anhydride, hexahydrophthalic anhydride, methyl tetrahydrophthalic anhydride, itaconic anhydride, pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropene)phthalic anhydride, succinic anhydride, and glutaric anhydride; the initiator is an organic peroxide initiator; and the organic solvent includes one or more of N,N-dimethylformamide, acetone, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0014] Furthermore, the organic peroxide initiator is benzoyl peroxide (BPO).

[0015] Preferably, in step (1), the anhydride crosslinking agent is 0.5-10% of the mass of PVDF; the initiator is 0.1-5% of the mass of PVDF; the POE-g-GMA is 1-20% of the mass of PVDF, and the GMA grafting rate of the POE-g-GMA is 0.8-1.5 wt%; the concentration of PVDF in the spinning solution is 5-20 wt%.

[0016] Preferably, the reaction in step (1) is carried out at 60-150°C and stirred for 1-24 hours.

[0017] Preferably, the electrospinning process parameters in step (2) include: a spinning voltage of 10-25 kV, a feed speed of 0.5-5.0 mL / h, a receiving distance of 10-25 cm, and a receiving device that is a roller with a rotation speed of 1500-3000 r / min.

[0018] Preferably, in step (2), the fiber diameter of the PVDF fiber membrane substrate after hot pressing is 0.5-10 μm, and the pore size of the fiber membrane is 0.1-5 μm.

[0019] Preferably, in step (2), the hot pressing temperature is 60-100℃, the hot pressing pressure is 1-10 MPa, and the hot pressing time is 1-10 min.

[0020] Preferably, in step (3), the liquid metal is one or more of gallium-indium alloy, gallium-indium-tin alloy, and bismuth-indium-tin alloy; the solvent is one or more of water, ethanol, isopropanol, N,N-dimethylformamide, and dimethyl sulfoxide; the dispersant is one or more of polyvinylpyrrolidone, polyvinyl alcohol, carboxymethyl cellulose, sodium alginate, hyaluronic acid, quaternized chitosan, and polyacrylate. The catalyst is 2,4,6-tris(dimethylaminomethyl)phenol.

[0021] Preferably, the dispersant has a mass fraction of 1-8% in the solvent; the catalyst 2,4,6-tris(dimethylaminomethyl)phenol is added in an amount of 0.1-2.0% of the mass of the liquid metal; and the volume percentage concentration of the liquid metal in the liquid metal ink is 50-95%.

[0022] In step (3), the ultrasonic power for ultrasonic dispersion is 100-500W and the ultrasonic time is 10-120 min; the centrifugation speed is 2000-5000 rpm and the centrifugation time is 5-15 min.

[0023] In step (3), the average diameter (dLM) of the liquid metal particles is less than or equal to the average pore size (df) of the PVDF fiber membrane substrate after hot pressing.

[0024] In step (3), the liquid metal surface has hydroxyl groups: the surface of the liquid metal will spontaneously oxidize in the air environment to form a very thin (1-100 nm) metal oxide layer. When the oxide layer is exposed to the air, the oxygen atoms on its surface will undergo hydrolysis reaction with the water vapor in the environment, that is, the metal-oxygen bond reacts with water molecules to generate a metal-hydroxyl (M-OH) structure, thereby forming abundant hydroxyl active sites on the surface of the liquid metal, so that the liquid metal oxide layer has a large number of -OH active sites that can participate in the ring-opening reaction.

[0025] The process of forming the patterned circuit in step (4) includes one or more of the following: screen printing, mask printing, dispensing printing, inkjet printing, aerosol jet printing, laser direct writing, and template-assisted printing.

[0026] Preferably, applying different mechanical pressures in step (4) includes: adjusting the activation pressure to 0.1-10 MPa to control the vertical penetration depth of liquid metal along the pores of the PVDF fiber membrane; wherein a low pressure of 0.1-2 MPa achieves single-sided conductivity, and a high pressure of 5-10 MPa achieves complete penetration of liquid metal into the substrate and double-sided conductivity.

[0027] Furthermore, in step (4), applying different mechanical pressures involves using a rigid pressure head (0.5 mm tip diameter) fixed to a three-axis motion platform, and using a force sensor to collect the pen tip load in real time. The activation pressure is calculated based on P=F / S. The activation pressure is adjusted from 0.1 to 10 MPa to control the vertical penetration depth of the liquid metal along the pores of the PVDF fiber membrane: low pressure (0.1-2 MPa) achieves single-sided conductivity, while high pressure (5-10 MPa) achieves complete penetration of the liquid metal into the substrate and double-sided conductivity.

[0028] The release film is a PET film with a thickness of 10-200μm and a surface roughness Ra of less than 0.1μm. The function of the release film is to remove excess liquid metal ink under pressure and prevent the diffusion of patterned circuits.

[0029] Furthermore, the entire surface of one side of the patterned circuit is covered with a release film, and then pressure is applied by the pen tip to activate it. Low pressure activates the circuit (non-via area), and high pressure activates the via.

[0030] Applying different mechanical pressures to different areas: applying an activation pressure of 0.1-2 MPa to the non-perforated areas of the release film to embed the liquid metal into the pores, and applying an activation pressure of 5-10 MPa to the perforated areas to allow the liquid metal to penetrate the substrate.

[0031] Preferably, the porous adhesive liner in step (5) is TPU; the hot pressing process temperature is the melting temperature of the porous adhesive liner, 70-110℃, the pressure is 0.1-5 MPa, and the time is 0.5-10 min. Hot pressing realizes high-precision three-dimensional integration of multilayer flexible circuits.

[0032] In step (5), there are at least two layers.

[0033] In step (5), the components are assembled after cooling. After cooling, conductive silver paste is applied to the preset pads, electronic components are mounted, and after curing, the components are electrically connected to the liquid metal circuit.

[0034] This invention provides a PVDF fiber membrane flexible electronic material prepared by the method described above.

[0035] The PVDF fiber membrane flexible electronic material includes at least two layers with in-plane continuous conductive pathways made of liquid metal embedded inside, and the conductive pathways form interlayer vertical conductive pathways between adjacent two layers or continuous multilayer fiber membranes, thereby realizing the precise construction of a three-dimensional conductive network.

[0036] The interlayer vertical conductive pathways are achieved by liquid metal infiltrating into the fiber network.

[0037] This invention provides an application of the PVDF fiber membrane flexible electronic material in robotic electronic skin, wearable electronic devices, and protective electronic devices.

[0038] The mechanism by which the PVDF fiber membrane substrate of this invention achieves high strength and high toughness lies in the following: During high-speed electrospinning, the PVDF molecular chains and crystalline regions are highly oriented along the fiber axis, giving each fiber high axial load-bearing capacity and tensile strength. Subsequent hot-pressing treatment promotes melting, bonding, and interfacial fusion in the fiber contact area, further densifying the fiber network structure, reducing porosity, defects, and interfiber slippage, thereby improving the overall load transfer efficiency and mechanical strength of the membrane. Simultaneously, the POE-g-GMA epoxy groups and MAH anhydride groups undergo a ring-opening reaction under BPO and heat treatment, constructing a strong chemically bonded interface between the rigid PVDF matrix and the elastic POE-g-GMA dispersed phase. This improves compatibility and interfacial bonding strength, promotes efficient stress transfer, and induces multiple energy dissipation mechanisms, significantly enhancing elongation at break and toughness. Therefore, this invention addresses the shortcomings of existing PVDF substrates in achieving high strength and toughness through molecular chain orientation, hot-pressing densification, and interfacial crosslinking.

[0039] The mechanism by which the PVDF fiber membrane substrate and liquid metal ink achieve high bonding strength in this invention lies in the following: At the chemical level, the POE-g-GMA graft copolymer side chains introduced into the PVDF fiber membrane contain abundant highly active epoxy functional groups. When liquid metal is exposed to air for a long time, it spontaneously forms a surface oxide layer, the surface of which is rich in hydroxyl active sites. Under the action of a catalyst, the hydroxyl active sites of the oxide layer on the liquid metal surface can undergo a ring-opening reaction with the epoxy groups in POE-g-GMA, forming a stable chemical covalent bond interface, achieving chemical anchoring, and effectively improving the interfacial bonding strength between the liquid metal ink and the PVDF fiber membrane. At the physical level, the liquid metal is embedded and anchored in the three-dimensional nano-network framework structure constructed by highly oriented fibers, forming a mechanically interlocked structure. Combined with the "ridge-valley" microstructure generated by the uneven groove structure on the fiber surface, it can effectively shield external frictional stress, thus solving the problem of poor interfacial adhesion between the liquid metal and the PVDF fiber membrane substrate.

[0040] Beneficial effects

[0041] (1) Synergistic improvement of strength and toughness of PVDF fiber membrane substrate material: Through a multi-level synergistic strategy of "molecular chain orientation bearing - hot pressing densification - interfacial cross-linking", the limitations of traditional PVDF materials in achieving both strength and toughness are overcome. The resulting PVDF fiber membrane has a tensile strength of 72 MPa, an elongation at break of 886%, and a toughness of up to 417 MJ / m. 3 It is far superior to commercial films and conventional electrospun films, and can withstand extreme deformation without failure, significantly improving the mechanical reliability of flexible electronics in dynamic and harsh environments.

[0042] (2) The PVDF fiber membrane substrate and the liquid metal interface are firmly bonded: The liquid metal is embedded and anchored in the fiber network to form a stable fiber membrane flexible circuit. At the same time, the epoxy groups in POE-g-GMA react with the hydroxyl groups on the surface of the liquid metal to form chemical covalent bonds at the interface, which fundamentally solves the problem of weak bonding and easy detachment between the liquid metal and the PVDF fiber membrane substrate, and ensures the conductivity stability and durability of the circuit under repeated bending and twisting.

[0043] (3) High printing precision and customizable structure: Combining patterned printing and mechanical pressure activation control technology, the liquid metal is guided to flow along the grooves by highly oriented fibers, and the excess ink is selectively removed by the release film after being pressed, which inhibits lateral diffusion and breaks through the limitations of traditional printing precision. At the same time, by precisely controlling the penetration depth of liquid metal, the single-sided conductive or double-sided conductive structure can be flexibly customized to avoid stress concentration and breakage risks caused by mechanical or laser drilling.

[0044] (4) Balancing three-dimensional integration density and thinness: High-density three-dimensional circuit integration is achieved by using multilayer hot-pressing composite technology and introducing porous adhesive liner between layers. This process significantly improves the functional integration while avoiding the significant increase in device thickness caused by traditional stacking, thus achieving a balance between high density and thinness of flexible circuits.

[0045] (5) Broad prospects for industrial application: The method has stable process and good repeatability. The prepared material can simultaneously achieve high mechanical reliability (high strength and high toughness) and high functional integration (high density three-dimensional circuit) within a limited thickness / volume. It is suitable for flexible drive, wearable device, piezoelectric energy harvester and protective electronic device, providing an ideal solution for the next generation of high performance flexible electronics. Attached Figure Description

[0046] Figure 1 This is a schematic diagram illustrating the fabrication of the PVDF fiber membrane flexible electronic material in Example 1;

[0047] Figure 2 This is a SEM image of the PVDF fiber membrane substrate after hot pressing in Example 1;

[0048] Figure 3 This is the stress-strain curve of the PVDF fiber membrane substrate after hot pressing in Example 1;

[0049] Figure 4 This is a comparison chart of the mechanical properties of the PVDF fiber membrane after hot pressing in Example 1 and a commercial thin film;

[0050] Figure 5 This demonstrates the toughness of the PVDF fiber membrane substrate after hot pressing in Example 1;

[0051] Figure 6 This demonstrates the flexibility of the PVDF fiber membrane after hot pressing in Example 1;

[0052] Figure 7 These are SEM images of the liquid metal particles in the examples and comparative examples;

[0053] Figure 8 These are SEM images of the PVDF fiber membrane substrate printed with liquid metal after hot pressing in Example 1 before activation (left image) and after activation (right image);

[0054] Figure 9 This is the scratch / peel resistance test of the activated PVDF fiber membrane substrate in Example 1;

[0055] Figure 10 This is a bending durability test of the activated PVDF fiber membrane substrate in Example 1;

[0056] Figure 11 This is a torsion durability test of the activated PVDF fiber membrane substrate in Example 1;

[0057] Figure 12 It is a single-sided or double-sided conductive circuit prepared by adjusting the activation pressure of a PVDF fiber membrane substrate printed with liquid metal and hot-pressed in Example 1.

[0058] Figure 13 These are photographs of the fabrication process of the PVDF fiber membrane flexible electronic material in Example 1;

[0059] Figure 14 It is a high-precision circuit obtained by mechanical activation of a PVDF fiber membrane substrate printed with liquid metal and hot-pressed in Example 1. Detailed Implementation

[0060] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0061] Related tests:

[0062] Tensile testing: Mechanical property tests were conducted on samples obtained from different examples according to GB / T 1040.1-2018. The specimens were dumbbell-shaped with an effective tensile length of 20 mm and a width of 10 mm. The tensile rate was set to 50 mm / min, and three parallel specimens were set for each group. Stress-strain curves were tested and collected to characterize the tensile strength, elongation at break, toughness, and other mechanical property parameters of the samples.

[0063] Interfacial bond strength test: Tensile shear tests were conducted on liquid metal-substrate lap specimens obtained from different examples, according to GB / T 7122. The lap width of the specimen was 25 mm, the effective lap length was 12.5 mm, the tensile rate was set at 50 mm / min, and the test was carried out at room temperature. At least 3 parallel specimens were used in each group. The maximum breaking load at the specimen interface failure was collected and the average value was calculated. This average value was then divided by the lap contact area to calculate the shear bond strength of the sample interface.

[0064] Conductivity test:

[0065] 1. In-plane conductivity test: The in-plane resistance performance of samples obtained from different examples was tested using a four-probe resistance meter. Multiple test points were selected laterally on the sample surface at room temperature, and the average in-plane conductivity was measured and calculated to characterize the planar conductivity of the sample.

[0066] 2. Interlayer resistance test: The interlayer resistance performance of samples obtained from different examples was tested. The electrodes were placed at the through holes (through hole diameter of 0.5 mm) on the upper and lower surfaces of the PVDF fiber film flexible electronic material. The interlayer conduction resistance was measured with a multimeter. The average value was taken after multiple parallel tests to characterize the interlayer conductivity and interface contact resistance characteristics of the samples.

[0067] Erasure / peel durability test: The durability of hot-pressed PVDF fiber film printed with liquid metal was evaluated in accordance with ASTM D3359 and in combination with a custom friction method. (1) Erasure test: Using a standard eraser (Shore hardness of about A60), a normal load of about 5 N was applied and the conductive pattern surface was wiped back and forth 50 times at a frequency of about 1 cycle / s. The resistance value was recorded once every 10 times and the resistance change rate (ΔR / R0) was calculated. (2) Tape peel test: Using 3M Scotch 610 transparent tape, it was manually pasted and peeled off at a peel angle of about 180°. This was repeated 50 times and the resistance value was recorded once every 10 times. The resistance change rate (ΔR / R0) was calculated.

[0068] Bending durability test: Cyclic bending durability of hot-pressed PVDF fiber membranes printed with liquid metal was evaluated. The sample was clamped at both ends onto the tensile fixtures of a flexible electronic multimodal testing system, with an effective clamping length of 40 mm and a bending angle of 180° (folded). 10,000 reciprocating bending cycles were performed at a speed of 250 mm / min. The resistance change was measured in real time using a Keithley 6510 electrometer, and the rate of change of resistance (ΔR / R0) was calculated.

[0069] Torsional durability test: Cyclic torsional durability was evaluated on a hot-pressed PVDF fiber membrane printed with liquid metal. The sample was clamped at both ends onto the torsional fixture of a flexible electronic multimodal testing system, with an effective clamping length of 40 mm and a torsional angle set at ±90°. 10,000 reciprocating torsional cycles were performed at approximately 10 revolutions / min. The resistance change was measured in real time using a Keithley 6510 electrometer, and the rate of change of resistance (ΔR / R0) was calculated.

[0070] Morphology and structural characterization: The surface / section morphology of the samples and the embedding state of the LM were tested by SEM; the flexibility, water bucket, and manufacturing process were demonstrated by taking photos with a mobile phone.

[0071] Raw material source:

[0072] Polyvinylidene fluoride (PVDF) is Arkema KYNAR® 720 / 721; maleic anhydride (MAH) is Shanghai Titan Technology Co., Ltd. 13585A; benzoyl peroxide (BPO) is Aladdin B104632; glycidyl methacrylate-grafted ethylene-octene copolymer (POE-g-GMA) is Hongcheng Plastics 2613, with a grafting rate of 1%; DMF is Sinopharm Group 81007718; acetone is Sinopharm Group 40064460; gallium indium alloy is Dongguan Dingguan Metal Technology Co., Ltd. 16-degree melting point; polyvinylpyrrolidone (PVP) is Maclean P816205; anhydrous ethanol is Shanghai Test 100092680; 2,4,6-tris(dimethylaminomethyl)phenol is Shanghai Bide Pharmaceutical Technology Co., Ltd. BD66389. The release film is from Jinjia Film Technology Co., Ltd.; the commercial PET film is from Binbo Co., Ltd.; the commercial PVC film is from CITIC Plastics Co., Ltd.; the commercial PVDF film is from Shanghai Tuohuang Industrial Co., Ltd.; and the commercial PE film is from Yixinrong Co., Ltd. All of the above commercial films have a thickness of 50 μm.

[0073] Note: Liquid metal surfaces have hydroxyl groups.

[0074] Example 1

[0075] (1) Preparation of spinning solution: Dissolve 3 g of PVDF powder in a mixed solvent of 25 g DMF and acetone (mass ratio 7:3), and stir in a water bath at 110 ℃ for 6 hours until completely dissolved. Add 1% MAH, 0.3% BPO and 5% POE-g-GMA by mass of PVDF, and continue stirring for 2 hours to obtain a uniform and transparent spinning solution with a concentration of 10wt%.

[0076] (2) Electrospinning and hot pressing: The spinning solution was injected into a syringe, and spinning was carried out for 6 hours under the conditions of 15 kV voltage, 15 cm receiving distance, 2.5 mL / h feed rate and 3000 rpm receiving roller speed to obtain a thickness of about 100 μm. Then the fiber membrane was hot-pressed at 80℃ and 5 MPa for 1 minute to obtain a hot-pressed PVDF fiber membrane substrate with a thickness of about 50 μm and an average fiber diameter of about 2.5 μm.

[0077] (3) Preparation of liquid metal ink: Gallium-indium alloy and 5% PVP ethanol solution were mixed at a volume ratio of 1:2 and ultrasonically treated at 300W for 30 minutes to obtain a dispersion of liquid metal particles with an average particle size of about 5 μm. 0.1% of 2,4,6-tris(dimethylaminomethyl)phenol catalyst was added, and the mixture was stirred at low speed for 10 minutes. Then, it was centrifuged at 3000 rpm for 10 minutes, and the lower layer precipitate was removed to obtain concentrated ink with a liquid metal volume percentage concentration of 90%.

[0078] (4) Patterning and activation: Liquid metal ink is printed onto the surface of the hot-pressed PVDF fiber membrane using screen printing and dried at 140°C for 30 minutes. After covering with a release film, an activation pressure of 1 MPa is applied to the non-perforated areas to embed the liquid metal into the pores, and an activation pressure of 5 MPa is applied to the perforated areas to allow it to penetrate the substrate. The release film is then removed to obtain the activated PVDF fiber membrane.

[0079] (5) Multilayer circuit integration: Two activated fiber membranes are precisely stacked, with a porous adhesive backing placed between the layers, and hot-pressed at 70°C and 0.5 MPa for 5 minutes. After cooling, conductive silver paste is applied to the preset pads, electronic components are mounted, and after curing, electrical connection between the components and the liquid metal circuit is achieved, resulting in an integrated PVDF fiber membrane flexible electronic material.

[0080] like Figure 2 As shown, the surface and cross-sectional SEM images of the PVDF fiber membrane substrate obtained in step (2) after hot pressing show that the fiber structure on the surface of the fiber membrane melts after hot pressing, forming bonding points between the fibers, thereby achieving high mechanical properties. Its mechanical properties are excellent, such as... Figure 3 As shown, its tensile strength reaches 72 MPa, elongation at break is 886%, and toughness is 417 MJ / m. 3 Its toughness far exceeds that of commercially available films, such as... Figure 4 As shown. The PVDF fiber membrane substrate, after hot pressing, can lift a bucket of water weighing approximately 12 kg without significant deformation, such as... Figure 5 As shown. At the same time, it can be bent and twisted, exhibiting good flexibility, such as... Figure 6 As shown.

[0081] like Figure 8The image shows SEM images of a PVDF fiber membrane substrate before and after activation, after hot pressing with printed liquid metal. After activation by mechanical pressure, the liquid metal particles are embedded and anchored to the fiber network. Simultaneously, the epoxy groups in POE-g-GMA undergo a ring-opening reaction with the hydroxyl groups on the liquid metal surface, forming chemical covalent bonds at the interface. This enhances the bonding force between the circuit and the substrate, making the circuit less prone to detachment or breakage under frequent external forces such as bending, folding, and torsion, thus improving durability.

[0082] like Figure 9 The results of the scratch / peel resistance test are shown after the PVDF fiber membrane substrate has been activated. After 50 cycles of erasing with an eraser and peeling with adhesive tape, the resistance change rate was less than 5% and 3%, respectively, indicating that the material has good durability and stability.

[0083] like Figure 10 The image shows the flexural durability test results of the activated PVDF fiber membrane substrate. After 10,000 180° bends, the resistivity change rate is less than 9%, indicating that the material has good durability and stability. Figure 11 The results of the torsion durability test of the activated PVDF fiber membrane substrate are shown. After 10,000 torsion cycles, the resistance change rate is less than 3%, indicating that the material has good durability and stability.

[0084] In this embodiment, the bonding strength between the liquid metal and the hot-pressed PVDF fiber membrane substrate is 9.2 MPa, and the interfacial bonding is tight; the in-plane conductivity of the PVDF fiber membrane flexible electronic material is 3.06 × 10⁻⁶. 5 With a resistivity of S / m and an interlayer resistance of 1.5 Ω, the two-layer integration forms a complete and effective three-dimensional conductive network and a stable vertical conductive path. In summary, hot pressing promotes interfacial fusion between PVDF fibers, improves mechanical properties, and increases the densification of the fiber membrane to prevent excessive penetration of liquid metal. POE-g-GMA can act as a toughening phase to improve the flexibility of the membrane, and its epoxy groups can also chemically react with the liquid metal oxide layer to enhance interfacial bonding. The synergistic effect of these two processes achieves simultaneous improvement in material mechanics, conductivity, and multilayer integration stability.

[0085] like Figure 12 The diagram shows a single-sided or double-sided conductive circuit prepared by adjusting the activation pressure on a PVDF fiber membrane substrate printed with liquid metal after hot pressing in this embodiment.

[0086] like Figure 13 The image shown illustrates the fabrication process of the PVDF fiber membrane flexible electronic material in this embodiment. Two monolayers are precisely aligned and stacked, with a porous adhesive backing placed between the layers. They are then hot-pressed together, cooled, and components are installed to obtain an integrated PVDF fiber membrane flexible electronic material.

[0087] like Figure 14 The image shows a high-precision circuit obtained after mechanical activation of a PVDF fiber membrane substrate printed with liquid metal and hot-pressed. The highly oriented structure of the fibers guides the liquid metal to penetrate along the fiber grooves, reducing lateral diffusion. Simultaneously, by utilizing the difference in adsorption properties of the liquid metal on the surfaces of smooth and rough fiber membranes, the smooth release membrane can carry away excess liquid metal ink, preventing lateral diffusion and thus achieving high-precision printing.

[0088] Comparative Example 1

[0089] (1) Preparation of spinning solution: Dissolve 3 g of PVDF powder in a mixed solvent of 25 g DMF and acetone (mass ratio 7:3), and stir in a water bath at 110°C for 6 hours until completely dissolved. Add 1% MAH, 0.3% BPO and 5% POE-g-GMA by mass of PVDF, and continue stirring for 2 hours to obtain a uniform and transparent spinning solution with a concentration of 10 wt%.

[0090] (2) Electrospinning and hot pressing: The spinning solution was injected into a syringe, and spinning was carried out for 6 hours under the conditions of 15 kV voltage, 15 cm receiving distance, 2.5 mL / h feed rate and 500 rpm receiving roller speed to obtain a PVDF fiber membrane with a thickness of about 100 μm. Then the fiber membrane was hot-pressed at 80℃ and 5 MPa for 1 minute to obtain a hot-pressed PVDF fiber membrane substrate with a thickness of about 50 μm and an average fiber diameter of about 3.2 μm.

[0091] (3) Preparation of liquid metal ink: Gallium-indium alloy and 5% PVP ethanol solution were mixed at a volume ratio of 1:2 and ultrasonically treated at 300W for 30 minutes to obtain a dispersion of liquid metal particles with an average particle size of about 5 μm. 0.1% of 2,4,6-tris(dimethylaminomethyl)phenol catalyst was added, and the mixture was stirred at low speed for 10 minutes. Then, it was centrifuged at 3000 rpm for 10 minutes, and the lower layer precipitate was removed to obtain concentrated ink with a liquid metal volume percentage concentration of 90%.

[0092] (4) Patterning and activation: Liquid metal ink is printed onto the surface of the hot-pressed PVDF fiber membrane using screen printing and dried at 140°C for 30 minutes. After covering with a release film, an activation pressure of 1 MPa is applied to the non-perforated areas to embed the liquid metal into the pores, and an activation pressure of 5 MPa is applied to the perforated areas to allow it to penetrate the substrate. The release film is then removed to obtain the activated PVDF fiber membrane.

[0093] (5) Multilayer circuit integration: Two activated fiber membranes are precisely stacked, with a porous adhesive backing placed between the layers, and hot-pressed at 70°C and 0.5 MPa for 5 minutes. After cooling, conductive silver paste is applied to the preset pads, electronic components are mounted, and after curing, electrical connection between the components and the liquid metal circuit is achieved, resulting in an integrated PVDF fiber membrane flexible electronic material.

[0094] Performance: Without high-speed electrospinning, the mechanical properties of PVDF fiber membranes decrease significantly, with a tensile strength of only 31.9 MPa, an elongation at break of 586%, and a toughness of 287.25 MJ / m. 3 This phenomenon mainly stems from the fact that the PVDF molecular chains and crystalline regions were not sufficiently stretched and oriented during the spinning process, resulting in insufficient axial load-bearing capacity of individual fibers and thus weakening the overall mechanical reinforcement effect of the fiber network. Simultaneously, the insufficient orientation structure also adversely affects its electrical properties as a flexible electronic material. The in-plane conductivity of this fiber membrane is only 1.83 × 10⁻⁶. 5 The interlayer resistance increased to 4.1 Ω (S / m). The highly oriented fiber structure formed during high-speed spinning typically provides continuous, directional surface trench channels for liquid metal, guiding its spread and penetration along the fiber axis, thereby reducing disordered diffusion and improving the continuity of the conductive network. However, in the absence of a highly oriented fiber structure, the directional transport of liquid metal is limited, leading to insufficient construction of conductive pathways and ultimately a decrease in conductivity. Therefore, high-speed electrospinning not only helps improve the molecular chain orientation and mechanical load-bearing capacity of PVDF fiber membranes but also optimizes the distribution and penetration behavior of liquid metal in the fiber membrane, making it a key process step for synergistically improving the mechanical and electrical properties of materials.

[0095] Comparative Example 2

[0096] (1) Preparation of spinning solution: Dissolve 3 g of PVDF powder in a mixed solvent of 25 g DMF and acetone (mass ratio 7:3), and stir in a water bath at 110°C for 6 hours until completely dissolved. Add 1% MAH, 0.3% BPO and 5% POE-g-GMA by mass of PVDF, and continue stirring for 2 hours to obtain a uniform and transparent spinning solution with a concentration of 10 wt%.

[0097] (2) Electrospinning: The spinning solution was injected into the syringe and spun for 6 hours under the conditions of voltage 15 kV, receiving distance 15 cm, feed rate 2.5 mL / h and receiving roller speed 3000 rpm to obtain a PVDF fiber membrane with a thickness of about 100 μm and an average fiber diameter of about 5 μm.

[0098] (3) Preparation of liquid metal ink: Gallium-indium alloy and 5% PVP ethanol solution were mixed at a volume ratio of 1:2 and ultrasonically treated at 300W for 30 minutes to obtain a dispersion of liquid metal particles with an average particle size of about 5 μm. 0.1% of 2,4,6-tris(dimethylaminomethyl)phenol catalyst was added, and the mixture was stirred at low speed for 10 minutes. Then, it was centrifuged at 3000 rpm for 10 minutes, and the lower layer precipitate was removed to obtain concentrated ink with a liquid metal volume percentage concentration of 90%.

[0099] (4) Patterning and activation: Liquid metal ink is printed onto the surface of the PVDF fiber membrane using screen printing and dried at 140°C for 30 minutes. After covering with a release film, an activation pressure of 1 MPa is applied to the non-perforated areas to embed the liquid metal into the pores, and an activation pressure of 5 MPa is applied to the perforated areas to allow it to penetrate the substrate. The release film is then removed to obtain the activated PVDF fiber membrane.

[0100] (5) Multilayer circuit integration: Two activated fiber membranes are precisely stacked, with a porous adhesive backing placed between the layers, and hot-pressed at 70°C and 0.5 MPa for 5 minutes. After cooling, conductive silver paste is applied to the preset pads, electronic components are mounted, and after curing, electrical connection between the components and the liquid metal circuit is achieved, resulting in an integrated PVDF fiber membrane flexible electronic material.

[0101] Performance: The lack of hot-pressing interface fusion resulted in significant degradation of material properties; the PVDF fiber membrane had a tensile strength of only 40.5 MPa, an elongation at break of 465%, and a toughness of 155 MJ / m. 3 The fiber membrane structure is loose and easily broken; the bonding strength between the liquid metal and the PVDF fiber membrane substrate is only 4.3 MPa, indicating insufficient interfacial adhesion; the in-plane conductivity of the PVDF fiber membrane flexible electronic material is 2.51 × 10⁻⁶. 4 The interlayer resistance (S / m) is 52.3 Ω, and it deteriorates significantly. After 100 bends, the resistance change rate is approximately 85%, resulting in reduced conductivity stability. Poor interlayer contact after multilayer integration prevents the formation of a stable three-dimensional conductive network. The core reason is that the lack of hot pressing weakens the bonding between PVDF fibers and leads to excessive porosity, causing excessive penetration of liquid metal. The loose fiber network cannot support the conductive layer. Even with some interfacial bonding provided by POE-g-GMA, it cannot compensate for the structural defects, and the conductive layer is prone to breakage during mechanical bending. Therefore, hot pressing is a necessary step to improve the mechanical properties of the material and stabilize multilayer circuit interconnects.

[0102] Comparative Example 3

[0103] (1) Preparation of spinning solution: Dissolve 3 g of PVDF powder in a mixed solvent of 25 g DMF and acetone (mass ratio 7:3), and stir in a water bath at 110°C for 6 hours until completely dissolved. Add 1% MAH and 0.3% BPO by mass of PVDF, and continue stirring for 2 hours to obtain a uniform and transparent spinning solution with a concentration of 10wt%.

[0104] (2) Electrospinning and hot pressing: The spinning solution was injected into a syringe, and spinning was carried out for 6 hours under the conditions of 15 kV voltage, 15 cm receiving distance, 2.5 mL / h feed rate and 3000 rpm receiving roller speed to obtain a PVDF fiber membrane with a thickness of about 100 μm. Then the fiber membrane was hot-pressed at 80℃ and 5 MPa for 1 minute to obtain a hot-pressed PVDF fiber membrane substrate with a thickness of about 50 μm and an average fiber diameter of about 2.5 μm.

[0105] (3) Preparation of liquid metal ink: Gallium-indium alloy and 5% PVP ethanol solution were mixed at a volume ratio of 1:2 and ultrasonically treated at 300W for 30 minutes to obtain a dispersion of liquid metal particles with an average particle size of about 5 μm. 0.1% of 2,4,6-tris(dimethylaminomethyl)phenol catalyst was added, and the mixture was stirred at low speed for 10 minutes. Then, it was centrifuged at 3000 rpm for 10 minutes, and the lower layer precipitate was removed to obtain concentrated ink with a liquid metal volume percentage concentration of 90%.

[0106] (4) Patterning and activation: Liquid metal ink is printed onto the surface of the hot-pressed PVDF fiber membrane using screen printing and dried at 140°C for 30 minutes. After covering with a release film, an activation pressure of 1 MPa is applied to the non-perforated areas to embed the liquid metal into the pores, and an activation pressure of 5 MPa is applied to the perforated areas to allow it to penetrate the substrate. The release film is then removed to obtain the activated PVDF fiber membrane.

[0107] (5) Multilayer circuit integration: Two activated fiber membranes are precisely stacked, with a porous adhesive backing placed between the layers, and hot-pressed at 70°C and 0.5 MPa for 5 minutes. After cooling, conductive silver paste is applied to the preset pads, electronic components are mounted, and after curing, electrical connection between the components and the liquid metal circuit is achieved, resulting in an integrated PVDF fiber membrane flexible electronic material.

[0108] Performance: The PVDF fiber membrane after hot pressing has a tensile strength of 54 MPa, an elongation at break of 778%, and a toughness of 300.5 MJ / m. 3Without the addition of POE-g-GMA, hot pressing effectively improved the strength of the PVDF fiber membrane substrate and increased the fiber membrane density to prevent excessive penetration of liquid metal. However, due to the lack of toughening effect from flexible segments, the material's toughness was poor. Furthermore, without POE-g-GMA, the bonding strength between the liquid metal and the hot-pressed PVDF fiber membrane substrate was only 3.4 MPa, indicating poor interfacial adhesion. The in-plane conductivity of the PVDF fiber membrane flexible electronic material was 2.97 × 10⁻⁶. 5 With a resistivity of S / m and an interlayer resistance of 3.6 Ω, conductivity significantly decreased; after 100 bends, the resistance change rate was approximately 150%, indicating poor conductivity durability. The core reason is that without POE-g-GMA, the GMA groups in its molecular structure cannot form chemical bonds with the oxide layer on the liquid metal surface. The liquid metal is only fixed by mechanical interlocking through penetration into the fiber pores, resulting in weak bonding. Under mechanical deformation, it is easily peeled off from the substrate, thereby destroying the conductive pathway and reducing conductivity. Therefore, although a preliminary vertical conductive pathway can be formed after multilayer integration, the liquid metal is prone to detachment, leading to an unstable conductive network and preventing long-term effective conductivity. This fully demonstrates that POE-g-GMA is crucial for improving the interfacial bonding and conductivity durability of materials.

Claims

1. A method for preparing a PVDF fiber membrane flexible electronic material, characterized in that, Includes the following steps: (1) Preparation of spinning solution: PVDF, acid anhydride crosslinking agent, initiator, POE-g-GMA and organic solvent are mixed and reacted to obtain spinning solution; (2) Electrospinning and hot pressing: The spinning solution is electrospinned to obtain a PVDF fiber membrane; the PVDF fiber membrane is then hot-pressed to obtain a hot-pressed PVDF fiber membrane substrate. (3) Preparation of liquid metal ink: Liquid metal, solvent and dispersant are mixed and ultrasonically dispersed, then a catalyst is added and stirred and dispersed, and then concentrated by centrifugation to obtain liquid metal ink; The liquid metal surface has hydroxyl groups; (4) Patterned circuit and mechanical pressure activation: Liquid metal ink is printed on the surface of the PVDF fiber membrane substrate after hot pressing to form a patterned circuit, and the solvent is removed by high temperature drying; then a release film is covered on one side of the patterned circuit, and different mechanical pressures are applied in different areas to control the penetration depth of liquid metal, and each layer contains at least one area activated by high pressure of 5-10 MPa, so that liquid metal penetrates the through-hole area of ​​the substrate to obtain the activated PVDF fiber membrane; the high temperature drying temperature is 100-140℃. (5) Multilayer circuit integration: At least two activated PVDF fiber membranes are precisely aligned and stacked, porous adhesive liner is laid between the layers and then hot-pressed composite, and components are assembled after cooling to obtain PVDF fiber membrane flexible electronic material.

2. The preparation method according to claim 1, characterized in that, In step (1), the anhydride crosslinking agent is one or more of maleic anhydride (MAH), phthalic anhydride, methyl hexahydrophthalic anhydride, hexahydrophthalic anhydride, methyl tetrahydrophthalic anhydride, itaconic anhydride, pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropene)phthalic anhydride, succinic anhydride, and glutaric anhydride; the initiator is an organic peroxide initiator; and the organic solvent includes one or more of N,N-dimethylformamide, acetone, N,N-dimethylacetamide, and N-methylpyrrolidone.

3. The preparation method according to claim 1, characterized in that, In step (1), the anhydride crosslinking agent is 0.5-10% of the mass of PVDF; the initiator is 0.1-5% of the mass of PVDF; the POE-g-GMA is 1-20% of the mass of PVDF; the GMA grafting rate of the POE-g-GMA is 0.8-1.5 wt%; the concentration of PVDF in the spinning solution is 5-20 wt%; the reaction temperature is 60-150℃; and the time is 1-24h.

4. The preparation method according to claim 1, characterized in that, The electrospinning process parameters in step (2) include: spinning voltage of 10-25 kV, feed speed of 0.5-5.0 mL / h, receiving distance of 10-25 cm, receiving device is a roller with a rotation speed of 1500-3000 r / min.

5. The preparation method according to claim 1, characterized in that, In step (2), the hot pressing temperature is 60-100℃, the hot pressing pressure is 1-10 MPa, and the hot pressing time is 1-10 min. In step (2), the fiber diameter of the PVDF fiber membrane substrate after hot pressing is 0.5-10 μm, and the pore size of the fiber membrane is 0.1-5 μm.

6. The preparation method according to claim 1, characterized in that, In step (3), the liquid metal is one or more of gallium indium alloy, gallium indium tin alloy, and bismuth indium tin alloy; the solvent is one or more of water, ethanol, isopropanol, N,N-dimethylformamide, and dimethyl sulfoxide; the dispersant is one or more of polyvinylpyrrolidone, polyvinyl alcohol, carboxymethyl cellulose, sodium alginate, hyaluronic acid, quaternized chitosan, and polyacrylate; and the catalyst is 2,4,6-tris(dimethylaminomethyl)phenol. In step (3), the dispersant has a mass fraction of 1-8% in the solvent; the catalyst 2,4,6-tris(dimethylaminomethyl)phenol is 0.1-2.0% of the liquid metal mass; and the liquid metal ink has a volume percentage concentration of 50-95%; In step (3), the ultrasonic power for ultrasonic dispersion is 100-500W and the ultrasonic time is 10-120 min; the centrifugation speed is 2000-5000 rpm and the centrifugation time is 5-15 min. In step (3), the average diameter (dLM) of the liquid metal particles is less than or equal to the average pore size (df) of the PVDF fiber membrane substrate after hot pressing.

7. The preparation method according to claim 1, characterized in that, The process of forming the patterned circuit in step (4) includes one or more of the following: screen printing, mask printing, dispensing printing, inkjet printing, aerosol jet printing, laser direct writing, and template-assisted printing. Applying different mechanical pressures in step (4) includes: adjusting the activation pressure to 0.1-10 MPa to control the vertical penetration depth of liquid metal along the pores of the PVDF fiber membrane; Low voltage (0.1-2 MPa) enables single-sided conductivity, while high voltage (5-10 MPa) enables liquid metal to completely penetrate the substrate and conduct electricity on both sides. The release film in step (4) is a PET film with a thickness of 10-200μm and a surface roughness Ra of less than 0.1μm. The function of the release film is to remove excess liquid metal ink after being pressed, and to prevent the diffusion of patterned circuits.

8. The preparation method according to claim 1, characterized in that, In step (5), the multilayer consists of at least two layers; the porous adhesive liner is TPU; the hot pressing process temperature is the melting temperature of the porous adhesive liner, 70-110℃, the pressure is 0.1-5 MPa, and the time is 0.5-10 min.

9. A PVDF fiber membrane flexible electronic material prepared by the method according to any one of claims 1-8, characterized in that, The PVDF fiber membrane flexible electronic material consists of at least two layers with in-plane continuous conductive pathways made of liquid metal embedded inside, and the conductive pathways form interlayer vertical conductive pathways between adjacent two layers or continuous multilayer fiber membranes, thereby realizing the precise construction of a three-dimensional conductive network.

10. The application of the PVDF fiber membrane flexible electronic material of claim 9 in robotic electronic skin, wearable electronic devices, and protective electronic devices.

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