A gradient membrane based on continuous distribution of GO-PAN components and controllable construction method thereof

Abstract

The application discloses a gradient membrane based on continuous distribution of GO-PAN components and a controllable construction method thereof, and belongs to the field of gradient membranes, and aims to prepare a gradient membrane with a wider change range of wettability. A hydrophobic material PAN and a hydrophilic modified material graphene oxide GO are selected to construct the wettability gradient membrane. The PAN has certain hydrophobicity, and a small amount of graphene oxide is added in the polyacrylonitrile fiber, so that the hydrophilic modification of the PAN fiber membrane can be realized. The pulling speed change of the PAN base film is controlled on a blade electrode, the deposition amount of GO / PAN fibers on the surface of the PAN membrane is realized to be changed in a gradient manner through electrospinning, the wettability gradient is constructed by using the change of the chemical components, that is, the change of GO, and the contact angle is changed from 0 to 133 degrees. The gradient membrane has a good application prospect in the directional spreading of liquid drops on the surface of the gradient membrane and a micro-reaction device.

Description

Technical Field

[0001] This invention belongs to the field of gradient membrane technology, and specifically relates to a gradient membrane based on the continuous distribution of GO-PAN components and its controllable construction method. Background Technology

[0002] Surface wettability is a fundamental property of solid surfaces, significantly impacting the functional properties and lifespan of materials. Generally, surfaces with a water contact angle (WCA) greater than 150° are considered superhydrophobic, while those with a WCA between 0° and 10° are considered superhydrophilic. The discovery of the lotus effect led to significant breakthroughs in surface wettability research. Several single-wetting surfaces have been developed, including hydrophilic, hydrophobic, superhydrophilic, superhydrophobic, and superoleophobic surfaces, for various applications related to interface engineering, such as corrosion protection, oil-water separation, antifouling, de-icing, and catalytic systems. However, with technological advancements, it is clear that single-wetting surfaces may be insufficient to meet the needs of these fields.

[0003] The wettable surfaces of organisms such as desert beetles, cacti, and spider silk are more complex and functional than those of single wettable surfaces. For example, the dorsal structure of a desert beetle has irregularly distributed hydrophilic protrusions and hydrophobic grooves, forming a hydrophilic-hydrophobic composite surface. This surface not only possesses the advantages of a single wettable surface, such as the ability to collect water from fog, but also promotes droplet aggregation through its hydrophobic properties. In oil-water separation, single wettable surfaces such as superhydrophilic / underwater superoleophobic surfaces mainly rely on physical sieving effects for separation, while Janus membrane interfaces can generate surface energy differences, thereby increasing the driving force for emulsion breakup. Furthermore, for wettable surfaces with special patterns, the interaction between their surface energy gradient and Laplace pressure gradient can provide a directional driving force for the spontaneous transport of droplets, thus enabling the construction of droplet arrays and biochips.

[0004] Compared to surfaces with single or complex wettability, wettability gradient surfaces refer to surfaces whose wettability changes continuously with position. Due to this continuously changing wettability characteristic, they have attracted widespread attention and research. In nature, the surface structure of the pitcher plant's mouth exhibits a wettability gradient, which is achieved through changes in its surface microstructure and hydrophilic / hydrophobic properties. This wettability gradient allows the pitcher plant to effectively guide water droplets from the mouth towards the interior of the trap, thus helping it capture and digest insects. Beyond nature, wettability gradient surfaces have many applications in real life. Currently, wettability gradient surfaces are used in various fields, such as agriculture, clothing, energy, and biomedical research. Specifically, they can be used to study water mist collection, improve heat exchange efficiency, and achieve self-transportation of water droplets. Therefore, the preparation of surfaces with continuously changing wettability gradients is of great significance. Summary of the Invention

[0005] The purpose of this invention is to provide a gradient membrane based on the continuous distribution of GO-PAN components and its controllable construction method, belonging to the field of gradient membranes. To prepare a gradient membrane with a wider range of wettability variations, a wettability gradient membrane is constructed using hydrophobic material PAN and hydrophilic modified material graphene oxide (GO). PAN has a certain degree of hydrophobicity; adding a small amount of graphene oxide to the polyacrylonitrile fiber membrane can achieve hydrophilic modification of the PAN fiber membrane. The traction speed of the PAN substrate membrane is controlled on the blade electrode, and the deposition amount of GO / PAN fibers on the PAN membrane surface is gradient-varied through electrospinning. The wettability gradient is constructed by utilizing the change in chemical composition, i.e., the change in GO, achieving a contact angle variation from 0° to 133°. This has excellent application prospects in droplet-oriented spreading and microreactors on the gradient membrane surface.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0007] A method for controllable construction of gradient membranes based on continuous component distribution of GO-PAN includes the following steps:

[0008] Step S1: Preparation of PAN spinning solution

[0009] Weigh out polyacrylonitrile (PAN) powder, dissolve it in a solvent, and stir to form a uniform spinning solution, which is the preparation of PAN spinning solution;

[0010] Step S2: Preparation of PAN fiber membrane

[0011] PAN fiber membranes were electrospun at room temperature and used as a base membrane to receive GO-PAN fiber filaments.

[0012] Step S3: Preparation of GO-PAN spinning solution

[0013] Weigh out graphene oxide (GO) and add it to a solvent. Disperse it using ultrasound. Then weigh out PAN powder and add it to the graphene oxide dispersion. Stir until fully dissolved to form a uniform spinning solution; this is the GO-PAN spinning solution.

[0014] Step S4: Preparation of a gradient film with continuous distribution of GO-PAN components

[0015] The PAN fiber membrane prepared in step S2 is placed on the blade electrode, and the PAN fiber membrane traction speed is set to decrease linearly. Then, electrospinning is performed above the blade electrode using the GO-PAN spinning solution prepared in step S3. The GO / PAN spinning solution is received to form a gradient membrane with continuously varying GO-PAN component content on the surface of the PAN fiber membrane substrate.

[0016] Furthermore, in step S1, the concentration of PAN powder in the solvent is 10%-20% (m / v) by mass volume.

[0017] Furthermore, in step S1, the solvent is DMF, and the stirring time is more than 10 hours.

[0018] Furthermore, the spinning parameters in step S2 are as follows:

[0019] Under spinning parameters of 10-20 kV high voltage, roller collection speed of 10-100 r / min, collection distance of 10-20 cm, and push speed of 0.5-2 mL / h, the spinning time is 30-120 min.

[0020] Furthermore, in step S3, the concentration of graphene oxide in the solvent is 0.2%~2% (m / v) by mass volume, and the ultrasonic dispersion is performed for 30 minutes.

[0021] Furthermore, in step S3, the mass ratio of graphene oxide to PAN powder is 2:100 to 25:100.

[0022] Furthermore, in step S3, the solvent is DMF, and the stirring time is more than 10 hours.

[0023] Furthermore, the spinning parameters in step S4 are as follows:

[0024] Under positive voltage conditions of 10-20 kV and negative voltage conditions of -3 to -12 kV; the GO-PAN spinning solution push rate is 0.5-2 mL / h; the electrospray distance between the spinning needle and the blade electrode is 5-20 cm; and the spinning time is 0-60 min.

[0025] Furthermore, in step S4, the PAN fiber membrane traction speed decreases linearly from 2 mm / min to 0.01 mm / min.

[0026] A gradient membrane based on the continuous distribution of GO-PAN components is prepared using any of the above-mentioned preparation methods.

[0027] The beneficial effects of this invention are as follows:

[0028] This invention discloses a gradient membrane based on the continuous distribution of GO-PAN components and its controllable construction method, belonging to the field of gradient membranes. To prepare a gradient membrane with a wider range of wettability variations, a wettability gradient membrane is constructed using hydrophobic material PAN and hydrophilic modified material graphene oxide (GO). PAN has a certain degree of hydrophobicity; adding a small amount of graphene oxide to the polyacrylonitrile fiber membrane can achieve hydrophilic modification of the PAN fiber membrane. The traction speed of the PAN substrate membrane is controlled on the blade electrode, and the deposition amount of GO / PAN fibers on the PAN membrane surface is gradient-varied through electrospinning. The wettability gradient is constructed by utilizing the change in chemical composition, i.e., the change in GO, achieving a contact angle variation from 0° to 133°. This membrane has excellent application prospects in droplet-oriented spreading and microreactors on the gradient membrane surface. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the electrospinning apparatus used to prepare the gradient film in this invention.

[0030] Figure 2 This is a molecular structure diagram of the material in Example 1 of this invention;

[0031] Figure 3 The static water contact angle test results are for the gradient-free membranes prepared in the embodiments of the present invention with a GO to PAN powder mass ratio of 0:100 to 15:100.

[0032] Figure 4 This is a graph showing the dynamic changes in the water contact angle of gradient-free membranes with different proportions of GO-PAN in the embodiments of the present invention.

[0033] Figure 5 This is a scanning electron microscope (SEM) image of the PAN fiber membrane obtained by electrospinning without blade electrodes in an embodiment of the present invention.

[0034] Figure 6 This is a scanning electron microscope (SEM) image of the PAN fiber membrane obtained by electrospinning in the presence of a blade electrode in an embodiment of the present invention.

[0035] Figure 7 The images show the scanning electron microscope (SEM) results at points A, B, C, and D of the gradient fiber membrane prepared in Example 1 of this invention, along with the corresponding energy dispersive spectral (EDS) results for N and O elements.

[0036] Figure 8 In this invention Figure 7 Statistical results of the energy spectrum scan corresponding to region A in the middle;

[0037] Figure 9 In this invention Figure 7 Statistical results of the energy spectrum scan corresponding to region B in the middle;

[0038] Figure 10 In this invention Figure 7 Statistical results of the energy spectrum scan corresponding to region C in the middle;

[0039] Figure 11 In this invention Figure 7 Statistical results of energy spectrum scans corresponding to the four regions in the middle D region;

[0040] Figure 12 This is a photograph of the gradient film prepared in Example 1 of this invention.

[0041] Figure 13 This is a graph showing the water contact angle test results from left to right on the surface of the gradient membrane prepared in Example 1 of this invention;

[0042] Figure 14 This is a graph showing the test results of water contact angle at different positions on the gradient membrane surface under different negative voltages in this invention;

[0043] Figure 15 The diagram shows the spreading behavior of droplets on the gradient membrane and the hydrophilic non-gradient membrane prepared in Example 1 of this invention.

[0044] Figure 16 This is a diagram showing the spreading length of droplets of different volumes on the gradient membrane (GO / PAN) and the hydrophilic non-gradient membrane (GO@PAN) prepared in Example 1 of this invention;

[0045] Figure 17 This is a top view of the continuous liquid transport behavior of the liquid on the gradient membrane surface in this invention, showing the directions of increasing hydrophilicity and increasing hydrophobicity, respectively.

[0046] Figure 18 This is a side view of the continuous liquid transport behavior of the liquid on the gradient membrane surface in this invention, showing the flow along the directions of increasing hydrophilicity and increasing hydrophobicity. Detailed Implementation

[0047] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0048] PAN fiber membranes exhibit high hydrophobicity, and continuous hydrophilic modification of the PAN fiber membrane surface can further enhance the wettability difference per unit distance in gradient membranes. Polyacrylonitrile (PAN) possesses good thermal stability (317°C), maintaining its performance at high temperatures, which is crucial for high-temperature applications.

[0049] Graphene oxide (GO) contains many active groups at its edge structure, most of which are hydrophilic groups such as carboxyl and hydroxyl groups, making it suitable as a hydrophilic modifier. This invention adds a small amount of GO to a PAN fiber membrane to achieve hydrophilic modification. After determining the appropriate GO content, the traction speed of the PAN substrate membrane is controlled on the blade electrode. A gradient of GO / PAN fiber deposition is achieved on the PAN membrane surface through electrospinning, utilizing the change in chemical composition, specifically the change in GO, to construct a wettability gradient.

[0050] Example 1: Gradient film preparation

[0051] like Figure 1 As shown, the specific experimental steps for preparing the GO / PAN gradient film are as follows:

[0052] 1. Weigh 0.01, 0.03, 0.05, 0.07 and 0.09 g of graphene oxide respectively and add them to 5 mL of DMF solvent. Disperse by ultrasonication for 30 min. Then weigh 0.4 g of PAN powder into the graphene oxide dispersion and stir magnetically for more than 10 h to fully dissolve and form a uniform spinning solution.

[0053] 2. Weigh 0.7g of PAN powder, dissolve it in 5 mL of DMF solvent, and stir magnetically for more than 10 hours to form a uniform spinning solution.

[0054] 3. Under spinning parameters of 14 kV high voltage, 60 r / min collection speed, 15 cm collection distance and 1 mL / h push speed, and a spinning time of 60 min, PAN fiber membranes were electrospun at room temperature and used as the base membrane for receiving GO@PAN fiber filaments.

[0055] 4. Under the conditions of positive voltage 14 kV and negative voltage -9 kV, the GO-PAN spinning solution push rate is 1 mL / h; the PAN fiber membrane prepared in step S2 is placed on the blade electrode, the PAN fiber membrane traction speed is set to decrease linearly, and the electrospray distance between the spinning needle and the blade electrode is 15 cm; the traction speed of the receiving membrane PAN decreases linearly from 2 mm / min to 0.01 mm / min, and then the GO-PAN spinning solution prepared in step S3 is electrostatically sprayed above the blade electrode to receive the GO / PAN spinning solution, forming a gradient membrane with varying GO / PAN content on the substrate membrane surface, and the spinning time is 30 min.

[0056] The gradient films obtained above are evaluated as follows:

[0057] 1. Selective regulation of wettability

[0058] Characterization of wetting properties: A 10 μL water droplet was added to the fiber membrane, and the contact angle was measured using a JC2000D1 contact angle meter. Similarly, using the JC2000D1 contact angle meter, the fiber membrane was immersed in water, and a 5 μL air bubble was extruded onto the membrane surface. The underwater air bubble contact angle of the fiber membrane was measured.

[0059] like Figure 2 As shown, GO / PAN fiber membranes with GO contents of 0 wt%, 2 wt%, 5.5 wt%, 9 wt%, 12 wt%, and 15 wt% were prepared, and the hydrophilic modification with different GO contents was investigated. The wettability of 10 μL of deionized water on the fiber membrane is dynamically changing. Therefore, as... Figure 3 As shown, at t=1s, the contact angles of GO / PAN fiber membranes with 0 wt%, 2 wt%, 5.5 wt%, 9 wt%, 12 wt%, and 15 wt% GO content were 131°, 31°, 18°, 7°, 0°, and 0°, respectively. GO can significantly improve the hydrophilicity of PAN fiber membranes. Figure 4 As shown, the water contact angle of fiber membranes with GO contents of 0 wt%, 2 wt%, 5.5 wt%, 9 wt%, 12 wt%, and 15 wt% was analyzed over time. PAN fibers exhibit some hydrophobicity, with the contact angle remaining constant at 131°. After the addition of GO, GO / PAN becomes hydrophilic. At a wetting time of 10 s, the contact angle of GO / PAN fiber membranes with different GO contents all became 0°. GO / PAN fiber membranes with higher GO contents, such as 12 wt% and 15 wt%, achieved a contact angle of 0° at 0.5 s and 0.25 s, respectively, exhibiting superhydrophilic properties. Subsequent examples selected a spinning solution with a GO content of 12 wt% to construct a surface wettability gradient membrane with varying GO content.

[0060] 2. The effect of current collector on the deposition of PAN fibers

[0061] Characterization of microstructure: After preparing samples of different membrane materials, gold sputtering was performed. The surface morphology of the samples was characterized using a Zeiss SUPRA™ 40 scanning electron microscope at different magnifications. Before scanning the cross-section of the membrane material, the samples were frozen and embrittled in liquid nitrogen before gold sputtering. Simultaneously, EDS energy dispersive spectroscopy was performed on the membrane surface to analyze the chemical composition content of the fiber membrane surface.

[0062] The blade electrode belongs to a point-to-point electric field line system, that is, an electric field collection mode from the spinning needle to the blade, which facilitates the controlled deposition of fiber filaments. When the PAN spinning solution concentration is low, the spun fiber filaments are smaller in diameter, and the fibers are easily influenced by the extremely narrow blade electrode, which can result in the orderly arrangement of the fiber filaments. Figure 5As shown, the morphology of fibers collected on a roller in 14% w / v PAN (w / v, 0.7g PAN: 5mL DMF) exhibits a random and disordered arrangement of fibers, resulting in an isotropic fiber membrane. The roller collection device cannot achieve controlled and ordered deposition of fibers, making it difficult to prepare wettability gradient fiber membranes with varying chemical compositions. Figure 6 As shown, the morphology of 8% w / v PAN (w / v, 0.4 g PAN: 5 mL DMF) fibers collected on the blade electrode is an oriented structure arranged along a specific direction. This arrangement can concentrate fibers of different chemical components to be deposited in an orderly manner along the film surface direction, so that the wettability gradient surface with varying chemical composition can be constructed in a controllable manner.

[0063] 3. Surface morphology of gradient film

[0064] To characterize the microstructure and chemical composition changes of the gradient membrane surface, the PAN / GO gradient fiber membrane was divided into four regions: A, B, C, and D, and scanning electron microscopy (SEM) and EDS were performed on each region. Figure 7 As shown, region A represents the surface morphology of the PAN substrate membrane, specifically the morphology of the fiber membrane prepared with a 14% PAN spinning solution concentration. The fiber diameter is mainly concentrated between 200 and 650 nm, with an average diameter of 451 nm. The fiber diameter is smooth and uniform in size. In the nitrogen element diagram, the fiber outline structure is clearly visible. This morphological structure is mostly composed of PAN fibers, with less oxygen and a small amount of GO component.

[0065] In region B, the fiber diameter decreases significantly, mainly concentrated in the range of 50–200 nm, with an average diameter of 129 nm. This is because the spinning solution concentration is relatively low (8% w / v). At lower concentrations, the solution viscosity decreases, making the jet under the electric field easier to stretch. Due to the weakened interaction forces between polymer chains, the electric field force more easily stretches the fluid in the solution into finer fibers, resulting in a significant reduction in fiber diameter. A small number of GO spindle structures are observed between the fibers. The nitrogen element map does not show a clear fiber outline compared to region A. In region C, the fiber diameter ranges from 50–250 nm, with an average diameter of 126 nm. The GO content and oxygen content gradually increase, and the GO spindle outline is clearly visible, corresponding to the spindle structure in the morphology diagram. In region D, the fiber diameter distribution is similar to regions B and C, ranging from 50–250 nm, with an average diameter of 136 nm. The increase in GO content has no significant effect on the PAN fiber diameter. The formation of the GO spindle structure is due to the fact that the GO sheets have a micron-sized structure, which does not match the fiber diameter of PAN, making it easy for PAN fibers to swell under the influence of the GO sheets. Furthermore, GO can interact strongly with PAN fibers, making it easier for PAN to adhere to the GO sheet structure. During electrospinning, PAN is more likely to accumulate near the GO sheets compared to areas without GO, thus forming the spindle structure.

[0066] 4. Elemental analysis of fiber membrane surface

[0067] The changes in elemental composition on the gradient film surface were analyzed using an energy dispersive spectroscopy (EDS) instrument attached to a scanning electron microscope. Nitrogen (N) is a unique element in the PAN molecular structure, oxygen (O) is a unique element in the GO molecular structure, and carbon (C) is present in both the PAN and GO molecular structures. Figure 8 , Figure 9 , Figure 10 and Figure 11 As shown, the contents of C, N, and O elements were measured, and the changes in N and O elements were analyzed. In regions A, B, C, and D, the contents of N element were 21.47 wt%, 19.93 wt%, 18.85 wt%, and 18.2 wt%, respectively, and the contents of O element were 3.2 wt%, 5.18 wt%, 5.43 wt%, and 5.97 wt%, respectively. Figure 13 As shown, the contents of these two elements exhibit opposite increasing and decreasing trends, indicating that the GO loaded on the PAN fibers is increasing, and the GO component content on the fiber membrane surface is becoming increasingly higher. Through elemental content analysis of the membrane surface, a surface with a gradient change in GO content was successfully constructed on the PAN fiber membrane.

[0068] 5. Surface wettability of gradient films

[0069] like Figure 12As shown, the 30 mm long gradient membrane surface gradually changes from dark to light color from left to right, that is, the GO-enriched area gradually transitions to the pure PAN fiber surface area. Figure 13 As shown, the negative voltage of the blade electrode is crucial for the fabrication of a wettability gradient surface. Therefore, surface wettability gradient films with a wettability length of 30 mm were fabricated under negative voltages of 3 kV, 6 kV, 9 kV, and 12 kV, and the water contact angle was measured at 5 mm intervals on the fiber membrane surface. Under a voltage of 3 kV, the surface wettability of the gradient film varied within the range of 0° to 60°, constructing a wettability gradient within the hydrophilic range. The electric field traction force of the -3 kV blade electrode on the airborne fibers was weak, resulting in a relatively large fiber area deposited on the blade. Fibers with different GO contents partially overlapped, leading to a larger GO-containing area on the membrane surface, thus making the entire membrane surface a hydrophilic surface. As the negative voltage increased, such as to -6 kV and -9 kV, the surface wettability gradient achieved a variation within the range of 0° to 130°. The increased electric field force leads to more concentrated fiber deposition, allowing GO / PAN fibers to deposit orderly on the PAN film surface with minimal overlap, thus not affecting the wettability of adjacent areas. However, excessively high negative voltages, such as -12 kV, can cause fibers already deposited on the cutting edge to be attracted away from the PAN substrate surface by the positive electrode (12 kV). These protruding fibers can interfere with the subsequent fiber reception process, affecting fiber deposition and preventing them from settling in adjacent areas. This results in a discontinuous GO content gradient and failure to construct the wettability gradient. Surface wettability varies within the ranges of 112°–98° and 3°–35°, with abrupt transitions from hydrophilic to hydrophobic surfaces, lacking continuity in the wettability gradient. For wettability gradient films prepared with a negative voltage of 9 kV, such as… Figure 14 As shown, the contact angles from the hydrophilic region to the hydrophilic region are 0°, 27°, 51°, 64°, 96°, 125°, and 133°, respectively, indicating an increasing change in hydrophobicity from left to right. Conversely, there is an increasing change in hydrophilicity from right to left. Such a wettability gradient surface is crucial for the directional spreading of liquids.

[0070] 6. Directional spreading of droplets

[0071] Directional spreading of droplets: The directional spreading behavior of 5 μL droplets on the surfaces of homogeneous and gradient films was recorded using a JC2000D1 contact angle meter. Simultaneously, the spreading lengths of droplets of different volumes (5–30 μL) on homogeneous and gradient films were compared.

[0072] On the surface of a fiber membrane with a gradient in GO content, the wetting behavior of droplets exhibits anisotropy. Specifically, droplets can spread directionally under the influence of the surface wettability gradient. Figure 15As shown, the droplet wetting behavior on the surfaces of hydrophobic non-gradient membranes (PAN), hydrophilic non-gradient membranes (GO@PAN, GO to PAN mass ratio of 15:100, prepared by electrospinning), and gradient membranes (GO / PAN) was compared. On the homogeneous GO@PAN membrane, droplets spread uniformly on both sides of the needle tip and diffuse isotropically under capillary force, showing no difference in wetting behavior. Similarly, droplets on the hydrophobic PAN membrane are spherical and stable on the fiber membrane surface, showing no difference in wettability. However, for the gradient membrane with a wettability gradient, droplets adhere to the fiber membrane surface at 1 s and then gradually spread towards the more hydrophilic region under the influence of hydrophilic force. Throughout the wetting behavior, the hydrophilic region on the right side of the droplet leading edge gradually spreads, while the trailing edge of the droplet is pinned to the hydrophobic region on the left.

[0073] To further characterize the effect of the wettability differences between hydrophobic non-gradient membranes (PAN), hydrophilic non-gradient membranes (GO@PAN), and gradient membranes (GO / PAN) on droplet spreading behavior, the spreading length of droplets of different volumes on the fiber membrane surface was measured. Figure 16 As shown, the droplet length on the PAN fiber membrane is equal to the diameter of the droplet volume. Because droplets on the hydrophobic PAN membrane are difficult to spread, even with a droplet volume increased to 30 μL, the length only changes from 1.82 mm to 4.05 mm, failing to exhibit directional wetting behavior. Similarly, on the hydrophilic surface of GO@PAN, droplets can spread uniformly on the membrane surface, and the droplet length represents the size of the spreading and wetting area. The spreading length of a 5 μL droplet is 3.45 mm, increasing to 6.17 mm with a 30 μL droplet volume. The droplet spreading length on the hydrophilic fiber membrane surface increases with increasing droplet volume. However, on surfaces with a wettability gradient, there are significant differences in the spreading length of droplets of different volumes: the spreading length of a 5 μL droplet is 4.22 mm, while that of a 30 μL droplet is 10.65 mm, representing a 2.52-fold increase in length. The wettability gradient on the surface of the gradient membrane enables droplets to spread directionally, with the droplet leading edge diffusing towards the hydrophilic region under the influence of hydrophilic forces. The GO / PAN gradient membrane significantly enhances the liquid spreading behavior; the spreading length of a 15 μL droplet (7.45 mm) is higher than that of a 30 μL droplet (6.17 mm) on the hydrophilic GO@PAN membrane. Therefore, continuous droplet application on the surface of the wettability gradient membrane can achieve directional liquid spreading.

[0074] 7. Liquid spreading of gradient membranes

[0075] Liquid spreading test method: The fiber membrane was cut into strips with a width of 4 mm, pasted onto a glass slide, placed on a flat surface, and then pre-wetted. Liquid droplets were continuously injected into both sides of the fiber membrane using a syringe to examine the liquid transport characteristics. The liquid transport behavior was studied by video playback.

[0076] Similarly, the gradient membrane was pre-wetted to expel air from the pores and form a water film on the membrane surface, which facilitates continuous liquid transport. Then, liquid was continuously added dropwise to the pre-wetted gradient membrane along both the direction of increasing hydrophilicity and the direction of increasing hydrophobicity, and the directional spreading behavior of the liquid was observed.

[0077] like Figure 17 As shown, in the direction of the hydrophobic-to-hydrophilic transition, the liquid can be continuously and rapidly transported to the hydrophilic end, where more deionized water accumulates. The liquid on the entire membrane exhibits a cone shape, meaning the curvature on the left side is greater than that on the right. This change in liquid curvature creates a Laplace pressure difference across the liquid, driving the droplet to be transported towards the side with greater curvature. Figure 18 As shown in the side view, in the direction of increasing hydrophilicity, the droplet is transported to the hydrophilic end at 1 s and continues to be transported. At 8 s, the liquid level on the hydrophilic side is higher than that on the hydrophobic side, and the droplet continuously accumulates at the hydrophilic end. Conversely, in the direction of increasing hydrophobicity, the droplet is difficult to transport to the hydrophilic end. At 3 s, the liquid level is higher on the hydrophilic side than on the hydrophobic side, and more liquid remains at the hydrophilic end and cannot be transported to the hydrophobic side. Only when the static pressure of the droplet exceeds the wettability gradient force at 10 s does partial diffusion to the hydrophobic side occur.

[0078] As can be seen from the above description, by controlling the traction speed of the PAN substrate film on the blade electrode, the deposition amount of GO / PAN fibers on the PAN film surface can be made to vary in a gradient through electrospinning. The wettability gradient can be constructed by utilizing the change in chemical composition, i.e., the change in GO. The contact angle can be changed from 0° to 133°. This method of preparing gradient films by controlling the traction speed of the PAN substrate film on the blade electrode has excellent application prospects.

[0079] In summary, this invention provides a gradient membrane based on the continuous distribution of GO-PAN components and its controllable construction method, belonging to the field of gradient membranes. To prepare a gradient membrane with a wider range of wettability variations, the hydrophobic material PAN and the hydrophilic modifier graphene oxide (GO) are selected to construct the wettability gradient membrane. PAN has a certain degree of hydrophobicity; adding a small amount of graphene oxide to the polyacrylonitrile fiber membrane can achieve hydrophilic modification of the PAN fiber membrane. The traction speed of the PAN substrate membrane is controlled on the blade electrode, and the deposition amount of GO / PAN fibers on the PAN membrane surface is gradient-varied through electrospinning. The wettability gradient is constructed by utilizing the change in chemical composition, i.e., the change in GO, achieving a contact angle variation from 0° to 133°. This invention has excellent application prospects in droplet-oriented spreading and microreactors on the gradient membrane surface.

[0080] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components, characterized in that, The controllable construction method includes the following steps: Step S1: Preparation of PAN spinning solution Weigh out polyacrylonitrile (PAN) powder, dissolve it in a solvent, and stir to form a uniform spinning solution, which is the preparation of PAN spinning solution; Step S2: Preparation of PAN fiber membrane PAN fiber membranes were electrospun at room temperature and used as a base membrane to receive GO-PAN fiber filaments. Step S3: Preparation of GO-PAN spinning solution Weigh out graphene oxide (GO) and add it to a solvent. Disperse it using ultrasound. Then weigh out PAN powder and add it to the graphene oxide dispersion. Stir until fully dissolved to form a uniform spinning solution; this is the GO-PAN spinning solution. Step S4: Preparation of a gradient film with continuous distribution of GO-PAN components The PAN fiber membrane prepared in step S2 is placed on the blade electrode, and the PAN fiber membrane traction speed is set to decrease linearly. Then, electrospinning is performed above the blade electrode using the GO-PAN spinning solution prepared in step S3. The GO / PAN spinning solution is received to form a gradient membrane with continuously varying GO-PAN component content on the surface of the PAN fiber membrane substrate.

2. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, In step S1, the concentration of PAN powder in the solvent is 10%-20% by mass-volume ratio.

3. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, In step S1, the solvent is DMF, and the stirring time is more than 10 hours.

4. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, The spinning parameters in step S2 are as follows: Under spinning parameters of 10-20 kV high voltage, roller collection speed of 10-100 r / min, collection distance of 10-20 cm, and push speed of 0.5-2 mL / h, the spinning time is 30-120 min.

5. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, In step S3, the concentration of graphene oxide in the solvent is 0.2%~2% by mass-volume ratio, and the ultrasonic dispersion is performed for 30 minutes.

6. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, In step S3, the mass ratio of graphene oxide to PAN powder is 2:100 to 25:

100.

7. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, In step S3, the solvent is DMF, and the stirring time is more than 10 hours.

8. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, The spinning parameters in step S4 are as follows: Under positive voltage conditions of 10-20 kV and negative voltage conditions of -3 to -12 kV; the GO-PAN spinning solution push rate is 0.5-2 mL / h; the electrospray distance between the spinning needle and the blade electrode is 5-20 cm; and the spinning time is 0-60 min.

9. The method for controllable construction of a gradient membrane based on continuous distribution of GO-PAN components as described in claim 1, characterized in that, In step S4, the PAN fiber membrane traction speed decreases linearly from 2 mm / min to 0.01 mm / min.

10. A gradient membrane based on a continuous distribution of GO-PAN components, characterized in that, The gradient film is prepared using any one of the preparation methods in claims 1-9.

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