A unidirectional water-guiding asymmetric composite nanofiber membrane with a wetting gradient and a preparation method thereof

Abstract

The application discloses a one-way water-conducting asymmetric composite nanofiber membrane with a wetting gradient and a preparation method thereof. In view of the problems of the existing hydrophilic-hydrophobic composite membrane, such as difficult water permeation caused by the over-thick hydrophobic layer and discontinuous water conduction caused by interface mutation, the polyacrylonitrile hydrophilic layer and the hydrophobic layer arranged in turn along the thickness direction are prepared by electrospinning, and the hydrophobic layer is a thermoplastic polyurethane weak hydrophobic layer and / or a polyvinylidene fluoride hydrophobic layer. When the three-layer structure is adopted, the thermoplastic polyurethane weak hydrophobic transition layer is located between the polyacrylonitrile hydrophilic layer and the polyvinylidene fluoride hydrophobic layer, so as to buffer the wetting mutation. The film thickness is controlled by regulating the spinning time, and the liquid permeation capacity is controlled. The wetting gradient gradually transitions from hydrophilic to hydrophobic without surface chemical modification, and the obtained composite membrane has good one-way water-conducting performance and reverse blocking capacity, and is suitable for wearable devices, humidity management and liquid directional transmission and the like.

Description

Technical Field

[0001] This invention relates to the field of unidirectional hydrophilic functional materials and their preparation, specifically a unidirectional hydrophilic asymmetric composite nanofiber membrane with a wettability gradient and its preparation method. Background Technology

[0002] Inspired by natural phenomena such as cactus spines, pitcher plant margins, and the gradient of lotus leaf edges and veins, researchers have introduced the concept of asymmetric wettability, which utilizes the asymmetry of hydrophilicity and hydrophobicity, as well as structural asymmetry, to achieve directional water transport on composite fiber membranes. Unidirectional hydrophilic materials are functional materials that preferentially transport water in a specific direction and inhibit reverse osmosis, showing broad application prospects in sportswear, separation filtration, and wearable devices.

[0003] The main methods for achieving unidirectional water transport include constructing wettability gradient structures, pore size gradient structures, and fiber morphology gradient structures. Among these, wettability gradient structures can utilize the surface energy difference on both sides of the material to create a driving force for liquid transport, enabling directional liquid migration. In recent years, heterogeneous porous materials have achieved significant research results in the field of directional water transport. Introducing asymmetric wettability structures along the thickness direction of the material to achieve unidirectional water transport has become one of the mainstream design strategies.

[0004] Most existing unidirectional hygroscopic materials construct hydrophilic-phobic asymmetric structures through methods such as surface coating, chemical grafting, plasma treatment, and electrostatic spraying. However, these preparation processes are cumbersome and costly. Furthermore, some hydrophilic-phobic modified layers exhibit poor interlayer adhesion to the base material, leading to decreased wetting performance or even failure after friction and long-term use. In addition, nanofiber membranes prepared by these methods may suffer from water retention, low transport efficiency, and insufficient transport stability during water transport, making them unsuitable for practical applications.

[0005] It is evident that the above preparation methods have many problems. Therefore, there is an urgent need to develop a unidirectional water-conducting material and its preparation method that has a simple preparation process, readily available raw materials, high structural stability, and high water transport efficiency. Summary of the Invention

[0006] To address the aforementioned technical problems, electrospinning is employed. Electrospinning technology can prepare nanofiber membranes with small fiber diameters, high porosity, and large specific surface areas. Furthermore, the membrane structure and wettability can be controlled by altering parameters such as material type and spinning time. This invention provides an asymmetric composite nanofiber membrane with unidirectional hydrophilicity and its preparation method.

[0007] This invention is achieved through the following technical solution:

[0008] A unidirectional hydrophilic asymmetric composite nanofiber membrane with a wettability gradient in this invention comprises a hydrophilic layer and a hydrophobic layer arranged sequentially along the thickness direction;

[0009] The hydrophilic layer is a polyacrylonitrile electrospun nanofiber layer;

[0010] The hydrophobic layer is a thermoplastic polyurethane electrospun weakly hydrophobic nanofiber layer and / or a polyvinylidene fluoride electrospun hydrophobic nanofiber layer.

[0011] Furthermore, when the hydrophobic layer simultaneously comprises a thermoplastic polyurethane electrospun weak hydrophobic nanofiber layer and a polyvinylidene fluoride electrospun hydrophobic nanofiber layer, the thermoplastic polyurethane electrospun weak hydrophobic nanofiber layer is disposed between the hydrophilic layer and the polyvinylidene fluoride electrospun hydrophobic nanofiber layer to form a weak hydrophobic transition layer.

[0012] The composite nanofiber membrane has a wettability gradient that gradually transitions from hydrophilic to hydrophobic along its thickness direction.

[0013] Preferably, the hydrophilic layer is formed by electrospinning a polyacrylonitrile spinning solution, wherein the polyacrylonitrile spinning solution is prepared by dissolving polyacrylonitrile in N,N-dimethylformamide and stirring at 15~35°C for 12h, wherein the mass fraction of polyacrylonitrile is 12~15wt.%.

[0014] Preferably, the hydrophobic layer comprises a thermoplastic polyurethane electrospun weakly hydrophobic nanofiber layer, wherein the thermoplastic polyurethane is formed by electrospinning a thermoplastic polyurethane spinning solution, and the thermoplastic polyurethane spinning solution is prepared by dissolving thermoplastic polyurethane in N,N-dimethylformamide and stirring at 15~35°C for 12h, wherein the mass fraction of thermoplastic polyurethane is 18~22wt.%.

[0015] Preferably, the hydrophobic layer comprises a polyvinylidene fluoride electrospun hydrophobic nanofiber layer, wherein the polyvinylidene fluoride is formed by electrospinning a polyvinylidene fluoride spinning solution, and the polyvinylidene fluoride spinning solution is prepared by dissolving polyvinylidene fluoride in N,N-dimethylformamide and stirring at 15~35°C for 12h, wherein the mass fraction of polyvinylidene fluoride is 9~11 wt.

[0016] Preferably, the water contact angle of the polyacrylonitrile electrospun nanofiber layer is smaller than that of the thermoplastic polyurethane electrospun weakly hydrophobic nanofiber layer, and the water contact angle of the thermoplastic polyurethane electrospun weakly hydrophobic nanofiber layer is smaller than that of the polyvinylidene fluoride electrospun hydrophobic nanofiber layer.

[0017] The present invention discloses a method for preparing a unidirectional hydrophilic asymmetric composite nanofiber membrane with a wettability gradient, comprising the following steps:

[0018] Polyacrylonitrile spinning solution and hydrophobic spinning solution are prepared separately, wherein the hydrophobic spinning solution includes thermoplastic polyurethane spinning solution and / or polyvinylidene fluoride spinning solution;

[0019] A layer of polyacrylonitrile hydrophilic nanofibers was prepared on a receiving device using an electrospinning method.

[0020] The hydrophobic spinning solution is electrospun on the surface of the polyacrylonitrile hydrophilic nanofiber layer to form a hydrophobic nanofiber layer. When the hydrophobic spinning solution contains both thermoplastic polyurethane spinning solution and polyvinylidene fluoride spinning solution, a weakly hydrophobic transition layer is first formed by electrospinning thermoplastic polyurethane spinning solution on the surface of the polyacrylonitrile hydrophilic nanofiber layer, and then a hydrophobic nanofiber layer is formed by electrospinning polyvinylidene fluoride spinning solution on the surface of the weakly hydrophobic transition layer.

[0021] The obtained composite nanofiber membrane was dried to obtain a unidirectional hydrophilic asymmetric composite nanofiber membrane with a wettability gradient.

[0022] Preferably, when the hydrophobic spinning solution simultaneously comprises a thermoplastic polyurethane spinning solution and a polyvinylidene fluoride spinning solution, the electrospinning time of the thermoplastic polyurethane spinning solution is 15 min to 2 h, and the electrospinning time of the polyvinylidene fluoride spinning solution is 15 min to 2 h.

[0023] Preferably, the thicknesses of the polyacrylonitrile electrospun nanofiber layer, the weakly hydrophobic transition layer, and the hydrophobic nanofiber layer are controlled by adjusting the electrospinning time of the corresponding spinning solutions.

[0024] Preferably, the electrospinning parameters of the polyacrylonitrile hydrophilic nanofiber layer are: spinning voltage 8~9kV, receiving distance 10~15cm, and feed speed 250~400μL / h; the electrospinning parameters of the thermoplastic polyurethane weakly hydrophobic nanofiber layer are: spinning voltage 7.5~9kV, receiving distance 10~15cm, and feed speed 250~400μL / h; and the electrospinning parameters of the polyvinylidene fluoride hydrophobic nanofiber layer are: spinning voltage 7.5~8.5kV, receiving distance 10~15cm, and feed speed 250~400μL / h.

[0025] Preferably, the drying process is vacuum drying, with a drying temperature of 40~60℃ and a drying time of 6~12h.

[0026] Compared with existing technologies, the present invention has at least the following beneficial effects:

[0027] This invention utilizes the differences in wettability of PAN, TPU, and PVDF to construct composite nanofiber membranes. The operation is simple and does not require surface chemical modification to form a wettability gradient.

[0028] This invention controls the thickness of the TPU and PVDF layers by changing the electrospinning time, thereby controlling the permeability of the weak hydrophobic layer and the hydrophobic layer.

[0029] In the PAN / TPU bilayer membrane, the permeability of the TPU side decreases with increasing thickness, but it can still maintain water permeability within a certain range and form a permeability difference with the PAN side, indicating that the weakly hydrophobic TPU layer can play a role in regulating liquid transport.

[0030] In PAN / PVDF bilayer membranes, the permeability of the PVDF side decreases significantly with increasing thickness, and may even become impermeable. This indicates that the PVDF layer can act as a strong hydrophobic barrier layer, but when directly combined with the PAN layer, the difference in wettability is too great, which may result in excessive forward water ingress resistance.

[0031] The present invention further proposes a three-layer structure of PAN / TPU / PVDF, wherein the weakly hydrophobic TPU layer serves as a transition layer, which can buffer the excessive wettability difference between the hydrophilic PAN layer and the hydrophobic PVDF layer, and is conducive to forming a more gentle hydrophilic-weakly hydrophobic-hydrophobic gradient structure.

[0032] The composite nanofiber membrane obtained by this invention can be evaluated for its unidirectional hydrophobicity by comparing the water permeation time on both sides, and is suitable for wearable devices, humidity management, waterproof and breathable applications, and directional liquid transport. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of the bilayer / trilayer composite nanofiber membrane of the present invention.

[0034] Figure 2 This is a static contact angle diagram of the PAN, TPU, and PVDF single-layer spun fiber membranes in this invention; where a) is a PAN hydrophilic fiber membrane, b) is a TPU weakly hydrophobic fiber membrane, and c) is a PVDF hydrophobic fiber membrane.

[0035] Figure 3 This is a graph showing the diffusion time of water droplets on both sides of the double-layer fiber membrane in Embodiment 1 of the present invention.

[0036] Figure 4 The diagram shows the water droplet diffusion experiment on both sides of the double-layer fiber membrane in Example 2 of the present invention; where a) is a double-layer fiber membrane spun with PAN for 4 hours and PVDF for 15 minutes, b) is a double-layer fiber membrane spun with PAN for 4 hours and PVDF for 30 minutes, c) is a double-layer fiber membrane spun with PAN for 4 hours and PVDF for 1 hour, and d) is a double-layer fiber membrane spun with PAN for 4 hours and PVDF for 2 hours.

[0037] Figure 5The diagram shows the water permeation process of the three-layer fiber membrane in Example 3 of the present invention under the action of water droplets of the same volume; wherein, a) is a three-layer fiber membrane spun with PAN for 4 hours, TPU for 30 minutes and PVDF for 15 minutes, and b) is a three-layer fiber membrane spun with PAN for 4 hours, TPU for 1 hour and PVDF for 15 minutes. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0039] One of the unidirectional hydrophilic asymmetric composite nanofiber membranes with a wettability gradient in this application includes a hydrophilic layer and a hydrophobic layer arranged sequentially along the thickness direction.

[0040] The hydrophilic layer is a polyacrylonitrile (PAN) electrospun nanofiber layer, and the hydrophobic layer can be a thermoplastic polyurethane (TPU) electrospun weakly hydrophobic nanofiber layer (i.e., a weakly hydrophobic layer), or a polyvinylidene fluoride (PVDF) electrospun hydrophobic nanofiber layer (i.e., a hydrophobic layer), or a composite hydrophobic control layer composed of a TPU weakly hydrophobic nanofiber layer and a PVDF hydrophobic nanofiber layer, see [link to relevant documentation]. Figure 1 The layers have strong bonding force and are not easy to fall off. The surface of the fiber membrane has flocculent filaments in its microstructure, which greatly enhances the contact area of ​​the interface. On the other hand, the fibers are interwoven and entangled with each other, so the composite nanofiber membrane has excellent interfacial bonding force.

[0041] In this application, the PAN hydrophilic layer is used for absorbing, diffusing, and expelling liquid; its thickness is 100–300 μm; the TPU weakly hydrophobic layer is used to regulate the liquid transport path and resistance in the film thickness direction; its thickness is 100–200 μm; and the PVDF hydrophobic layer is used to provide an external hydrophobic barrier and inhibit liquid backflow; its thickness is 10–50 μm.

[0042] Furthermore, the water contact angle of the PAN hydrophilic layer is smaller than that of the TPU weak hydrophobic layer, and the water contact angle of the TPU weak hydrophobic layer is smaller than that of the PVDF hydrophobic layer.

[0043] When water is dripped in from the PVDF hydrophobic layer side, the water can pass through the PVDF hydrophobic layer and the TPU weak hydrophobic layer in sequence and be discharged from the PAN hydrophilic layer side.

[0044] When water is dripped into the hydrophilic PAN layer, the permeation of water into the hydrophobic PVDF layer is hindered.

[0045] The PAN hydrophilic layer, TPU weak hydrophobic layer and PVDF hydrophobic layer are continuously electrospun and stacked to form an interlayer fiber overlap, fiber interlacing or fiber entanglement structure, thereby forming an integrated composite nanofiber membrane.

[0046] The thicknesses of the PAN hydrophilic layer, the TPU weak hydrophobic layer, and the PVDF hydrophobic layer are controlled by adjusting the electrospinning time of the corresponding spinning solutions.

[0047] In some embodiments, the composite nanofiber membrane has a PAN / TPU bilayer structure. The PAN layer is a hydrophilic layer, and the TPU layer is a weakly hydrophobic layer. The thickness of the TPU layer is adjusted by changing the electrospinning time, thereby regulating the permeability of the TPU side. Experiments show that as the TPU spinning time increases, the TPU layer thickness increases, and the permeability of the TPU side decreases, but the PAN side and the TPU side still exhibit a certain difference in water permeability.

[0048] In some embodiments, the composite nanofiber membrane has a PAN / PVDF bilayer structure. The PAN layer is a hydrophilic layer, and the PVDF layer is a hydrophobic layer. The thickness of the PVDF layer is adjusted by changing the electrospinning time, thereby regulating the water-blocking capacity of the PVDF side. Experiments show that as the PVDF spinning time increases, the PVDF layer thickness increases, and the permeability of the PVDF side decreases significantly, even becoming difficult or impermeable to water.

[0049] In some embodiments, the composite nanofiber membrane is a three-layer structure of PAN / TPU / PVDF, comprising a PAN hydrophilic nanofiber layer, a TPU weakly hydrophobic transition nanofiber layer, and a PVDF hydrophobic nanofiber layer arranged sequentially. The TPU weakly hydrophobic transition nanofiber layer is located between the PAN hydrophilic layer and the PVDF hydrophobic layer, serving to buffer the abrupt change in wettability between them, forming a smooth wettability transition, reducing the excessive resistance to liquid transport caused by the strongly hydrophobic PVDF layer, thereby improving the controllability of liquid transport along the thickness direction. The composite nanofiber membrane forms a hydrophilic-weakly hydrophobic-hydrophobic wettability gradient structure along the direction from the PAN hydrophilic layer, through the TPU weakly hydrophobic layer to the PVDF hydrophobic layer, allowing the liquid to have high permeability from the PVDF hydrophobic layer side to the PAN hydrophilic layer side, while the permeability from the PAN hydrophilic layer side to the PVDF hydrophobic layer side is inhibited.

[0050] This application also provides a method for preparing the unidirectional hydrophilic composite nanofiber membrane with a wettability gradient, comprising the following steps:

[0051] Preparation of PAN spinning solution: Dissolve PAN in the first solvent and stir until completely dissolved to obtain PAN spinning solution;

[0052] Preparation of TPU spinning solution: Dissolve TPU in a second solvent and stir until completely dissolved to obtain TPU spinning solution;

[0053] Preparation of PVDF spinning solution: Dissolve PVDF in a third solvent and stir until completely dissolved to obtain PVDF spinning solution;

[0054] The PAN hydrophilic layer was prepared by electrospinning.

[0055] Electrospinning of TPU spinning solution is continued on the surface of the PAN hydrophilic layer to form a TPU weak hydrophobic layer.

[0056] Electrospinning PVDF spinning solution is continued on the surface of the TPU weak hydrophobic layer to form a PVDF hydrophobic layer.

[0057] The obtained PAN / TPU / PVDF three-layer composite nanofiber membrane was dried to obtain a unidirectional hydrophilic composite nanofiber membrane with a wettability gradient.

[0058] Furthermore, the first solvent is one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and acetone;

[0059] The second solvent is one or more of DMF, N,N-dimethylacetamide (DMAc), and acetone;

[0060] The third solvent is one or more of DMF, tetrahydrofuran (THF), and ethyl acetate.

[0061] Furthermore, the mass fraction of PAN in the PAN spinning solution is 12-15 wt.%;

[0062] The mass fraction of TPU in the TPU spinning solution is 18–22 wt.%.

[0063] The PVDF spinning solution contains 9–11 wt.% PVDF.

[0064] Furthermore, the electrospinning process parameters are as follows: PAN spinning voltage 8-9kV, TPU spinning voltage 7.5-9kV, PVDF spinning voltage 7.5-8.5kV, receiving distance 10-15cm, spinning solution propulsion speed 250-400μL / h, ambient temperature 15-35℃, and relative humidity 20%-70%.

[0065] Furthermore, by preparing PAN / TPU bilayer composite nanofiber membranes, PAN / PVDF bilayer composite nanofiber membranes, and PAN / TPU / PVDF trilayer composite nanofiber membranes as comparative samples, the effects of the TPU weak hydrophobic layer, the PVDF hydrophobic layer, and the PAN / TPU / PVDF trilayer wettability gradient structure on unidirectional hydrophobic performance were evaluated.

[0066] Furthermore, the PAN / TPU bilayer composite nanofiber membrane is used to characterize the regulatory effect of the TPU weak hydrophobic layer on liquid transport;

[0067] The PAN / PVDF bilayer composite nanofiber membrane is used to characterize the barrier effect of the PVDF hydrophobic layer on liquid transport.

[0068] The PAN / TPU / PVDF three-layer composite nanofiber membrane was used to characterize the effect of the hydrophilic-weathering gradient structure on the unidirectional transport performance of liquids.

[0069] Example 1: Preparation of PAN / TPU bilayer composite nanofiber membrane:

[0070] This embodiment illustrates the bilayer composite nanofiber membrane formed by combining a PAN hydrophilic layer and a TPU weak hydrophobic layer, and its liquid permeability.

[0071] Preparation of PAN spinning solution: Weigh PAN into a sample bottle, then add DMF solvent to the sample bottle, and stir for 12 hours at room temperature (15~35°C) to completely dissolve the PAN, thus obtaining the PAN spinning solution. The preferred mass fraction of PAN in the PAN spinning solution is 12~15 wt.%.

[0072] Preparation of TPU spinning solution: Weigh TPU into a sample bottle, then add DMF solvent to the sample bottle, and stir for 12 hours at room temperature (15~35°C) to completely dissolve the TPU, thus obtaining the TPU spinning solution. The preferred mass fraction of TPU in the TPU spinning solution is 18~22 wt.%.

[0073] Preparation of the PAN hydrophilic layer: The PAN spinning solution was added to a syringe, and the PAN hydrophilic nanofiber layer was prepared on the receiving device by electrospinning. The electrospinning parameters can be: spinning voltage 8~9kV, receiving distance 10~15cm, and feed speed 250~400μL / h.

[0074] Preparation of TPU weak hydrophobic layer: Continue electrospinning TPU spinning solution on the surface of the obtained PAN hydrophilic nanofiber layer. The electrospinning parameters can be: spinning voltage 7.5~9kV, receiving distance 10~15cm, and feed speed 250~400μL / h to form TPU weak hydrophobic nanofiber layer and obtain PAN / TPU bilayer composite nanofiber membrane.

[0075] The obtained PAN / TPU bilayer composite nanofiber membrane was placed in a vacuum drying oven and dried at 40–60°C for 6–12 h to remove residual solvent.

[0076] Because PAN has strong hydrophilicity, its thickness only needs to be kept moderate, and the spinning time of PAN should be controlled at 4 hours. The thickness of the weak hydrophobic layer of TPU can be controlled by adjusting the spinning time of TPU. For example, the TPU spinning time can be set to 15 minutes, 30 minutes, 1 hour, 2 hours, or longer.

[0077] The obtained PAN / TPU bilayer composite nanofiber membrane was subjected to a water droplet permeability test on both sides (the experimental water was methylene blue-stained deionized water). The test results showed that there was a certain difference in permeability between the PAN side and the TPU side; for example... Figure 3 As shown, the permeability of the TPU side decreases with increasing TPU layer thickness. This result indicates that the weakly hydrophobic TPU layer can regulate the liquid transport process and can serve as a wettability transition layer in a three-layer structure.

[0078] Example 2: Preparation of PAN / PVDF bilayer composite nanofiber membrane:

[0079] This embodiment illustrates the bilayer composite nanofiber membrane formed by combining a PAN hydrophilic layer and a PVDF hydrophobic layer, and its liquid permeability.

[0080] The preparation of the PAN spinning solution is the same as in Example 1.

[0081] Preparation of PVDF spinning solution: Weigh PVDF into a sample bottle, then add DMF solvent to the sample bottle, and stir for 12 hours at room temperature (15~35°C) to completely dissolve the PVDF, thus obtaining the PVDF spinning solution. The preferred mass fraction of PVDF in the PVDF spinning solution is 9~11 wt.%.

[0082] Preparation of PAN hydrophilic layer: PAN hydrophilic nanofiber layer was prepared by electrospinning.

[0083] Preparation of PVDF hydrophobic layer: PVDF spinning solution is electrospun on the surface of the obtained PAN hydrophilic nanofiber layer. The electrospinning parameters can be: spinning voltage 7.5~8.5kV, receiving distance 10~15cm, and feed speed 250~400μL / h, to form PVDF hydrophobic nanofiber layer and obtain PAN / PVDF bilayer composite nanofiber membrane.

[0084] The obtained PAN / PVDF bilayer composite nanofiber membrane was placed in a vacuum drying oven and dried at 40–60°C for 6–12 h to remove residual solvent.

[0085] The spinning time for PAN is the same as in Example 1. The thickness of the PVDF hydrophobic layer is controlled by adjusting the PVDF spinning time. For example, the PVDF spinning time can be set to 15 min, 30 min, 1 h, 2 h, or longer.

[0086] The obtained PAN / PVDF bilayer composite nanofiber membrane was subjected to a water permeability test on both sides (the experimental water was methylene blue-stained deionized water). The experimental results are as follows: Figure 4 As shown, the results indicate that the permeability of the PVDF side decreases with increasing PVDF hydrophobic layer thickness; when the PVDF hydrophobic layer thickness is too large, the PVDF side may become impermeable or even completely impermeable. This suggests that while the PVDF hydrophobic layer can provide strong reverse barrier properties, when directly combined with the PAN hydrophilic layer, it is prone to affecting liquid transport from the hydrophobic side to the hydrophilic side due to excessively large abrupt changes in wettability.

[0087] Example 3: Preparation of PAN / TPU / PVDF three-layer composite nanofiber membrane:

[0088] This embodiment illustrates a PAN / TPU / PVDF three-layer composite nanofiber membrane with a smooth wettability gradient and its unidirectional hydrophilic properties.

[0089] The PAN spinning solution, TPU spinning solution, and PVDF spinning solution were prepared according to the methods described in Example 1 and Example 2, respectively.

[0090] First, a certain amount of PAN spinning solution was taken with a syringe, and a layer of PAN hydrophilic nanofibers was prepared on the receiving device using an electrospinning method. The spinning time of PAN was the same as in Examples 1 and 2.

[0091] Then, electrospinning of TPU spinning solution is continued on the surface of PAN hydrophilic nanofiber layer to form TPU weakly hydrophobic nanofiber layer.

[0092] Then, electrospinning of PVDF spinning solution continues on the surface of the TPU weakly hydrophobic nanofiber layer to form a PVDF hydrophobic nanofiber layer.

[0093] Finally, the obtained PAN / TPU / PVDF three-layer composite nanofiber membrane was placed in a vacuum drying oven and dried at 40-60℃ for 6-12 hours to remove residual solvent, thus obtaining a unidirectional hydrophilic composite nanofiber membrane with a wettability gradient.

[0094] The resulting composite nanofiber membrane consists of a PAN hydrophilic layer, a TPU weakly hydrophobic layer, and a PVDF hydrophobic layer, arranged sequentially along its thickness. The TPU weakly hydrophobic layer is located between the PAN hydrophilic layer and the PVDF hydrophobic layer, serving to create a smooth wettability gradient that gradually transitions from hydrophilic to hydrophobic.

[0095] Water permeability tests were performed on both sides of the obtained PAN / TPU / PVDF three-layer composite nanofiber membrane (the experimental water was methylene blue-stained deionized water). When a water droplet was applied to the hydrophobic side of PVDF, water could pass through the hydrophobic PVDF layer and the weakly hydrophobic TPU layer under the wettability gradient and capillary action, and exit from the hydrophilic PAN layer side; when a water droplet was applied to the hydrophilic side of PAN, the transport of water to the hydrophobic PVDF layer was inhibited due to the barrier effect of the weakly hydrophobic TPU layer and the hydrophobic PVDF layer, such as... Figure 5 As shown.

[0096] Comparative Example 1: Comparison between the three-layer structure and the PAN / TPU two-layer structure:

[0097] A PAN / TPU bilayer composite nanofiber membrane was prepared and compared with a PAN / TPU / PVDF trilayer composite nanofiber membrane.

[0098] In the PAN / TPU bilayer structure, the TPU side has a certain degree of weak hydrophobicity. When water droplets are dripped from the PAN side and the TPU side, there is a certain difference in permeability. Water droplets on the PAN side absorb water quickly, while on the TPU side, as the thickness of the TPU increases, the water permeation time is longer and the permeability is weaker. However, due to the lack of a strong hydrophobic PVDF layer, its reverse barrier ability is relatively limited.

[0099] In the PAN / TPU / PVDF three-layer structure, the PVDF layer provides stronger hydrophobic barrier, the TPU layer provides transition regulation, and the PAN layer provides water absorption and drainage. Therefore, the three-layer structure can take into account both unidirectional water conduction and reverse barrier.

[0100] Comparative Example 2: Comparison between the three-layer structure and the PAN / PVDF two-layer structure:

[0101] A PAN / PVDF bilayer composite nanofiber membrane was prepared and compared with a PAN / TPU / PVDF trilayer composite nanofiber membrane.

[0102] In the PAN / PVDF bilayer structure, the PVDF layer is highly hydrophobic. As the thickness of the PVDF layer increases, the PVDF side permeability decreases, and even impermeability may occur. However, due to the lack of a weakly hydrophobic TPU layer, the wettability difference between PAN and PVDF is too large, and the hydrophobic PVDF layer will affect the hydrophilicity of the PAN hydrophilic layer.

[0103] In the PAN / TPU / PVDF three-layer structure, the weakly hydrophobic TPU layer is placed between the PAN layer and the PVDF layer, which creates a smoother wettability transition between the two layers. This helps to reduce the abrupt interface change between the hydrophilic layer and the strongly hydrophobic layer, and improves the continuity of water transport from the PVDF side to the PAN side.

[0104] Performance testing methods:

[0105] The performance of the composite nanofiber membrane of the present invention can be evaluated using the following methods:

[0106] Water contact angle test: The static water contact angles of the PAN side, TPU side, and PVDF side are measured separately to characterize the differences in wettability of each layer. For example... Figure 2 As shown, the initial static contact angles of PAN, TPU, and PVDF are 32.1°, 108.6°, and 130.9°, respectively.

[0107] Single-drop permeation test: Add equal volumes of water to both sides of the membrane and record the time required for complete permeation, or observe whether the water droplet permeates within a certain time period. Figures 3-5 As shown, when water droplets are placed on the PVDF surface of the composite nanofiber membrane, they can pass through or exit from the PAN side within a certain time. However, when water droplets are placed on the PAN surface, water cannot pass through or exit from the PVDF side. Instead, the water is rapidly absorbed by the PAN layer, but due to the barrier effect of the hydrophobic layer, it is eventually retained in the PAN layer and at its interface with the hydrophobic layer. This phenomenon indicates that the composite nanofiber membrane has good unidirectional water conduction properties.

[0108] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A unidirectional hydrophilic asymmetric composite nanofiber membrane with a wettability gradient, characterized in that: It includes a hydrophilic layer and a hydrophobic layer arranged sequentially along the thickness direction; The hydrophilic layer is a polyacrylonitrile electrospun nanofiber layer; The hydrophobic layer is a thermoplastic polyurethane electrospun weakly hydrophobic nanofiber layer and / or a polyvinylidene fluoride electrospun hydrophobic nanofiber layer. Furthermore, when the hydrophobic layer simultaneously comprises a thermoplastic polyurethane electrospun weak hydrophobic nanofiber layer and a polyvinylidene fluoride electrospun hydrophobic nanofiber layer, the thermoplastic polyurethane electrospun weak hydrophobic nanofiber layer is disposed between the hydrophilic layer and the polyvinylidene fluoride electrospun hydrophobic nanofiber layer to form a weak hydrophobic transition layer. The composite nanofiber membrane has a wettability gradient that gradually transitions from hydrophilic to hydrophobic along its thickness direction.

2. The composite nanofiber membrane according to claim 1, characterized in that, The hydrophilic layer is formed by electrospinning a polyacrylonitrile spinning solution. The polyacrylonitrile spinning solution is prepared by dissolving polyacrylonitrile in N,N-dimethylformamide and stirring at 15~35°C for 12h, wherein the mass fraction of polyacrylonitrile is 12~15wt.%.

3. The composite nanofiber membrane according to claim 1 or 2, characterized in that, The hydrophobic layer comprises a thermoplastic polyurethane electrospun weakly hydrophobic nanofiber layer. The thermoplastic polyurethane is formed by electrospinning a thermoplastic polyurethane spinning solution. The thermoplastic polyurethane spinning solution is prepared by dissolving thermoplastic polyurethane in N,N-dimethylformamide and stirring at 15~35°C for 12h, wherein the mass fraction of thermoplastic polyurethane is 18~22wt.%.

4. The composite nanofiber membrane according to claim 1 or 2, characterized in that, The hydrophobic layer comprises a polyvinylidene fluoride electrospun hydrophobic nanofiber layer. The polyvinylidene fluoride is formed by electrospinning a polyvinylidene fluoride spinning solution. The polyvinylidene fluoride spinning solution is prepared by dissolving polyvinylidene fluoride in N,N-dimethylformamide and stirring at 15~35°C for 12h, wherein the mass fraction of polyvinylidene fluoride is 9~11 wt.

5. The composite nanofiber membrane according to claim 1, characterized in that, The water contact angle of the polyacrylonitrile electrospun nanofiber layer is smaller than that of the thermoplastic polyurethane electrospun weak hydrophobic nanofiber layer, and the water contact angle of the thermoplastic polyurethane electrospun weak hydrophobic nanofiber layer is smaller than that of the polyvinylidene fluoride electrospun hydrophobic nanofiber layer.

6. A method for preparing a unidirectional hydrophilic asymmetric composite nanofiber membrane with a wettability gradient, characterized in that, Includes the following steps: Polyacrylonitrile spinning solution and hydrophobic spinning solution are prepared separately, wherein the hydrophobic spinning solution includes thermoplastic polyurethane spinning solution and / or polyvinylidene fluoride spinning solution; A layer of polyacrylonitrile hydrophilic nanofibers was prepared on a receiving device using an electrospinning method. The hydrophobic spinning solution is electrospun on the surface of the polyacrylonitrile hydrophilic nanofiber layer to form a hydrophobic nanofiber layer. When the hydrophobic spinning solution contains both thermoplastic polyurethane spinning solution and polyvinylidene fluoride spinning solution, a weakly hydrophobic transition layer is first formed by electrospinning thermoplastic polyurethane spinning solution on the surface of the polyacrylonitrile hydrophilic nanofiber layer, and then a hydrophobic nanofiber layer is formed by electrospinning polyvinylidene fluoride spinning solution on the surface of the weakly hydrophobic transition layer. The obtained composite nanofiber membrane was dried to obtain a unidirectional hydrophilic asymmetric composite nanofiber membrane with a wettability gradient.

7. The preparation method according to claim 6, characterized in that, When the hydrophobic spinning solution contains both a thermoplastic polyurethane spinning solution and a polyvinylidene fluoride spinning solution, the electrospinning time of the thermoplastic polyurethane spinning solution is 15 min to 2 h, and the electrospinning time of the polyvinylidene fluoride spinning solution is 15 min to 2 h.

8. The preparation method according to claim 6 or 7, characterized in that, The thicknesses of the polyacrylonitrile electrospun nanofiber layer, the weakly hydrophobic transition layer, and the hydrophobic nanofiber layer are controlled by adjusting the electrospinning time of the corresponding spinning solutions.

9. The preparation method according to claim 6 or 7, characterized in that, The electrospinning parameters for the polyacrylonitrile hydrophilic nanofiber layer are: spinning voltage 8~9kV, receiving distance 10~15cm, and feed speed 250~400μL / h; the electrospinning parameters for the thermoplastic polyurethane weakly hydrophobic nanofiber layer are: spinning voltage 7.5~9kV, receiving distance 10~15cm, and feed speed 250~400μL / h; and the electrospinning parameters for the polyvinylidene fluoride hydrophobic nanofiber layer are: spinning voltage 7.5~8.5kV, receiving distance 10~15cm, and feed speed 250~400μL / h.

10. The preparation method according to claim 9, characterized in that, The drying process is vacuum drying, with a drying temperature of 40~60℃ and a drying time of 6~12h.

Images