Gradient film based on continuous distribution of PVDF-HFP-silicon dioxide electronic injection microspheres and construction method of gradient film

Abstract

The invention discloses a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electronic injection microspheres and a construction method of the gradient membrane, and belongs to the field of gradient membranes, an improved electrostatic spinning and electrostatic spraying technology is utilized, a PVDF-HFP fiber membrane is prepared through the electrostatic spinning technology, PVDF-HFP / SiO2 composite microspheres are electrostatically sprayed, and a collector receiving device is combined, so that the gradient membrane is obtained. The traction speed change of the substrate film is controlled on the blade electrode, and a wettability gradient film with the continuously-changed surface appearance is constructed; on the surface of the prepared fiber membrane, the distribution form of the polyvinylidene fluoride-hexafluoropropylene / silicon dioxide composite microspheres on the surface of the membrane is in gradient change, a microstructure with continuous change is constructed, the contact angle of the fiber membrane is increased from 128 degrees to 153 degrees along with the change of the surface position, and a gradient surface gradually changing from hydrophobicity to super-hydrophobicity is obtained. And the preparation method has an excellent application prospect in the field of gradient films, bubble transmission and bubble micro-reaction.

Description

Technical Field

[0001] The invention belongs to the technical field of gradient membranes, and particularly relates to a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres and a construction method thereof. Background Art

[0002] Inspired by biological surfaces in nature, the study of wettability gradient surfaces has received increasing attention and can be applied to fields such as liquid delivery, bubble transport, and microfluidics. A wettability gradient surface refers to a special gradient surface whose surface wettability changes continuously with the change of surface position. Wettability gradient surfaces are mainly divided into chemical group classification and surface morphology gradient surfaces. Common preparation methods include immersion method, etching method, and vapor deposition method. However, these methods have problems such as complex process and difficult to control parameters. Electrospinning technology has the advantages of simple operation, adjustable parameters, and a wide range of applicable materials. Therefore, electrospinning has great potential in constructing wettability gradient surfaces.

[0003] Surface micromorphology refers to the tiny structures and features on the surface of a solid, including nano- and micro-scale roughness, textures, and pores. This micromorphology significantly affects the wettability of the material. By changing the microstructure of the surface, the wetting state of the material surface can be effectively adjusted to achieve superhydrophilic and superhydrophobic preparation. For example, Chen et al. deposited submicron silicon particles of different particle sizes on cotton cloth, and then performed hydrophobic modification of polydimethylsiloxane (PDMS), successfully constructing a rough surface similar to a lotus leaf on the cotton cloth surface. The surface exhibited superhydrophobicity, with a contact angle of 161° and a rolling angle of only 2.4°. McCarthy et al. used photolithography to transfer the mask pattern to a silicon wafer, and hydrophobized the silicon wafer surface through silanization treatment, thereby obtaining a superhydrophobic surface. Therefore, a micromorphology with a gradient change can be prepared on the solid surface, and a surface with a wettability gradient can be constructed.

[0004] Polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) is a polymer material with good mechanical properties, chemical stability and hydrophobicity. PVDF-HFP fiber membranes prepared by electrospinning technology have been used in many fields such as filtration, piezoelectricity, oil-water separation and super-hydrophobicity. Electrospinning technology uses electric field force to spray liquid solutions into tiny droplets, depositing electrosprayed microspheres on solid substrates, thereby changing the surface morphology of solid substrates.

[0005] Based on this, the present invention utilizes the advantages of simple electrospinning operation, adjustable parameters and wide range of applicable materials to control the precise distribution of different materials on the surface, thereby obtaining a wettability gradient membrane with continuously changing chemical composition or surface morphology. First, using improved electrospinning and electrostatic spraying technology, combined with a programmable collector receiving device, a wettability gradient membrane with continuously changing surface morphology is constructed. On the surface of the prepared fiber membrane, polyvinylidene fluoride-hexafluoropropylene / silicon dioxide (PVDF-HFP / SiO 2 ) The amount of composite microspheres is distributed in a gradient, and the contact angle of the fiber membrane increases from 128° to 153° as the surface position changes, obtaining a gradient surface from hydrophobicity to super-hydrophobicity. The gradient membrane was used to study the unidirectional transmission performance of underwater bubbles. The results showed that on the wettability gradient surface with a length of 20-60 mm, the bubble transmission rate increased with the decrease of length, and the maximum could reach about 85 mm / s. In addition, the experimental demonstration of controllable collection and micro-reaction of underwater bubbles was used to demonstrate the application potential of the gradient membrane. Summary of the invention

[0006] The present invention aims to provide a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres and a construction method thereof, which belongs to the field of gradient membranes. The PVDF-HFP fiber membrane is prepared by electrospinning technology using improved electrospinning and electrostatic spraying technology, and the PVDF-HFP / SiO2 electrostatic spraying is used to prepare the PVDF-HFP fiber membrane. 2 The composite microspheres, combined with the programmable collector receiving device, control the traction speed change of the base membrane on the blade electrode to construct a wettability gradient membrane with continuously changing surface morphology; on the surface of the prepared fiber membrane, the distribution of the polyvinylidene fluoride-hexafluoropropylene / silicon dioxide composite microspheres on the membrane surface presents a gradient change, constructing a continuously changing microscopic morphology, and the fiber membrane contact angle increases from 128° to 153° with the change of surface position, obtaining a gradient surface from hydrophobicity to super-hydrophobicity, realizing the preparation of wettability gradient surface, which has excellent application prospects in the field of gradient membranes, bubble transport and bubble microreaction.

[0007] To achieve the above object, the present invention is implemented through the following technical solutions: A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres comprises the following steps: Step S1: Preparation of PVDF-HFP spinning solution Weigh PVDF-HFP and dissolve it in the compound solvent solution, stir it thoroughly, and prepare a uniformly dissolved PVDF-HFP spinning solution; Step S2: PVDF-HFP fiber membrane preparation Through electrospinning, the drum receives the PVDF-HFP spinning fiber at a constant speed; after the spinning is completed, the obtained PVDF-HFP fiber membrane is placed in a vacuum oven at room temperature for more than 24 hours to remove the residual solvent, thereby obtaining the PVDF-HFP fiber membrane, which is used as a base membrane; Step S3: Preparation of PVDF-HFP-silica spinning solution Weigh the hydrophobic SiO 2 Dispersed in a compound solvent solution, ultrasonically dispersed, to obtain SiO 2 Dispersion; then weigh PVDF-HFP and put it into SiO 2 The dispersion was stirred to fully dissolve to prepare a PVDF-HFP-silica electrospray liquid; Step S4: Preparation of gradient membrane with continuous distribution of PVDF-HFP-silica electrosprayed microspheres The PVDF-HFP fiber membrane attached to the oil paper prepared in step S2 is placed on the blade electrode, the pulling speed is set to decrease linearly, and then the PVDF-HFP-silicon dioxide electrospray liquid prepared in step S3 is electrostatically sprayed on the blade electrode to prepare PVDF-HFP / SiO 2 The composite fiber membrane has microspheres with gradient changes on the surface of the fiber membrane. Finally, the prepared fiber membrane is placed in a vacuum oven at room temperature for more than 24 hours to remove the residual solvent, thereby obtaining a gradient membrane based on continuous distribution of PVDF-HFP-silica electrosprayed microspheres.

[0008] Furthermore, in step S1, the concentration of PVDF-HFP in the compound solvent solution is 8% to 20% (w / v) by mass volume ratio.

[0009] Furthermore, the composite solvent solution in step S1 is a mixture of DMF and acetone, the volume ratio of DMF to acetone is 1:3 to 3:3; and the stirring time is more than 10 hours.

[0010] Furthermore, the specific spinning parameters in step S2 are: The spinning voltage was set to 10~20 kV, the pushing speed of the PVDF-HFP spinning solution injection pump was 0.5~2 mL / h, and the distance between the spinning nozzle and the receiving device was kept at 10~20 cm for electrospinning. During this process, a layer of silicone oil paper was pre-wrapped on the drum to receive the PVDF-HFP spinning fibers at a constant speed of 10~100 r / min. The spinning time was 30~240 min.

[0011] Furthermore, in step S3, the hydrophobic SiO 2 The concentration in the compound solvent solution is 0% to 6% (w / v) by mass volume, and the ultrasonic dispersion is performed for 30 to 60 minutes.

[0012] Furthermore, the composite solvent solution in step S3 is a mixture of DMF and acetone, and the volume ratio of DMF to acetone is 1:3 to 3:3.

[0013] Furthermore, in step S3, PVDF-HFP and hydrophobic SiO 2 The mass ratio is 0:1~1:3, and the stirring time is more than 10 hours.

[0014] Furthermore, the spinning parameters in step S4 are: The positive voltage was 10~20 kV, the PVDF-HFP-silica electrospray spinning solution push rate was 0.2~2 mL / h, the electrospray distance between the spinning needle and the blade electrode was 10~20 cm, and the total spinning time was 10~120 min.

[0015] Furthermore, in step S4, the pulling speed is linearly decreased from 2 mm / min to 0 mm / min.

[0016] A gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres is prepared by any one of the above preparation methods.

[0017] The beneficial effects of the present invention are: The invention discloses a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres and a construction method thereof, which belongs to the field of gradient membranes. The invention utilizes improved electrospinning and electrostatic spraying technology to prepare a PVDF-HFP fiber membrane by electrostatic spinning technology, and electrostatically sprays PVDF-HFP / SiO 2 The composite microspheres, combined with the collector receiving device, control the change in the pulling speed of the base membrane on the blade electrode to construct a wettability gradient membrane with a continuously changing surface morphology; on the surface of the prepared fiber membrane, the distribution of the polyvinylidene fluoride-hexafluoropropylene / silicon dioxide composite microspheres on the membrane surface presents a gradient change, constructing a continuously changing microscopic morphology, and the fiber membrane contact angle increases from 128° to 153° with the change of surface position, obtaining a gradient surface from hydrophobicity to super-hydrophobicity, realizing the preparation of a wettability gradient surface, which has excellent application prospects in the field of gradient membranes, bubble transport and bubble microreactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The present invention is a schematic diagram of the process of preparing a wettability gradient membrane with continuously changing surface morphology by electrospinning and electrostatic spraying technology, and realizing directional air film transmission on the obtained gradient membrane; Figure 2 Different SiO 2The water contact angle test results of the non-gradient film formed by electrospraying PVDF with a high content; Figure 3 The PVDF-HFP / SiO 2 The extremely low roll angle test result graph at a scale of 2 / 2; Figure 4 This is a scanning electron microscope image of the PVDF-HFP electrospun fiber membrane of the present invention; Figure 5 This is a scanning electron microscope image of pure PVDF-HFP microspheres electrosprayed on the surface of the PVDF-HFP electrospun fiber membrane of the present invention; Figure 6 This is a scanning electron microscope magnified image of pure PVDF-HFP microspheres electrosprayed on the surface of the PVDF-HFP electrospun fiber membrane of the present invention; Figure 7 The surface of the PVDF-HFP electrospun fiber membrane of the present invention is electrosprayed with PVDF-HFP / SiO 2 Scanning electron microscopy images of microspheres; Figure 8 The surface of the PVDF-HFP electrospun fiber membrane of the present invention is electrosprayed with PVDF-HFP / SiO 2 Scanning electron microscope magnified image of microspheres; Fig. 9 The surface of the PVDF-HFP electrospun fiber membrane of the present invention is electrosprayed with PVDF-HFP / SiO 2 (Scale 2 / 2) Side view of the composite film obtained from microspheres; Fig.10 The PVDF-HFP electrospun fiber membrane and the surface electrosprayed pure PVDF-HFP microspheres, electrosprayed PVDF-HFP / SiO 2 (Scale is 2 / 2) Mechanical tensile test results of the composite film formed by microspheres; Fig.11 For the present invention Fig.10 Statistical results of fracture stress of three composite membranes in mechanical tensile test results; Fig.12 A schematic diagram of a wettability gradient film with a continuously changing surface morphology prepared in Example 1 of the present invention and four region identification diagrams Ⅰ to Ⅳ selected at equal intervals; Fig.13 In Example 1 of the present invention Fig.12 Scanning electron microscope images of the surface of the middle Ⅰ~Ⅳ area; Fig.14 The present invention corresponds to Fig.13 Si element distribution map in the SEM image; Fig.15 The present invention corresponds to Fig.13 F element distribution map in the SEM image; Fig.16 The test photographs and result diagrams of the water contact angle (WCA) and underwater bubble contact angle (BCA) at both ends of the wettability gradient membrane with continuously changing surface morphology prepared in Example 1 of the present invention; Fig.17 A diagram showing the change rules of contact angles at different positions on the wettability gradient film with continuously changing surface morphology prepared in Example 1 of the present invention; Fig.18 Actual photographs of contact angles at different positions of the wettability gradient membrane with continuously changing surface morphology prepared in Example 1 of the present invention; Fig.19 The figures are experimental result diagrams of the bubble unidirectional transmission experiment conducted from left to right and from right to left on the wettability gradient membrane with continuously changing surface morphology according to the present invention; Fig. 20 A side view of continuous bubble transmission on a wettability gradient membrane with continuously changing surface morphology according to the present invention; Fig.21 A top view of continuous bubble transport on a wettability gradient membrane with continuously changing surface morphology according to the present invention; Fig. 22 The images of bubble transmission on the wettability gradient membranes of different lengths of the present invention are shown; Fig.23 Graph showing gas adsorption on wettability gradient membranes of different lengths according to the present invention; Fig.24 is a graph showing the average velocity of bubble transmission on the wettability gradient membranes of different lengths according to the present invention; Fig.25 This is a diagram showing the application of using the wettability gradient membrane with continuously changing surface morphology prepared in Example 1 of the present invention to carry out directional bubble transmission and collection. DETAILED DESCRIPTION

[0019] The present invention is further described in detail below in conjunction with specific embodiments, which are intended to explain the present invention rather than to limit it.

[0020] Polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) is a polymer material with good mechanical properties, chemical stability and hydrophobicity. PVDF-HFP fiber membranes prepared by electrospinning technology have been used in many fields such as filtration, piezoelectricity, oil-water separation and super-hydrophobicity. Electrospinning technology uses electric field force to spray liquid solutions into tiny droplets, depositing electrosprayed microspheres on solid substrates, thereby changing the surface morphology of solid substrates.

[0021] The present invention prepares PVDF-HFP fiber membrane by electrostatic spinning technology, and electrostatically sprays PVDF-HFP / SiO 2The distribution of composite microspheres on the membrane surface changes in a gradient manner, constructing a continuously changing microscopic morphology and realizing the preparation of a wettability gradient surface. The surface wettability gradient of the fiber membrane was characterized by scanning electron microscopy and contact angle test. Finally, the gradient membrane was used for bubble directional transport applications.

[0022] Example 1: Gradient membrane preparation PVDF-HFP / SiO 2 The preparation process of surface wettability gradient membrane is as follows: Figure 1 The specific experimental steps are as follows: (1) First, weigh 0.7 g of PVDF-HFP and dissolve it in 5 mL of a mixed solvent solution of DMF and acetone (DMF:Acetone=2:3, v:v), and stir it magnetically for more than 10 hours to prepare a uniformly dissolved PVDF-HFP spinning solution. Electrospinning was performed under the conditions that the spinning voltage was set to 15 kV, the injection pump push speed was 1 mL / h, and the distance between the spinning nozzle and the receiving device was kept at 15 cm. During this process, a layer of silicone oil paper was pre-wrapped on the roller to receive the spun fiber at a constant speed of 40 r / min. The spinning time was 60 min. After spinning, the obtained PVDF-HFP fiber membrane was placed in a vacuum oven at room temperature for 24 hours to remove the residual solvent, and the PVDF-HFP fiber membrane was obtained as the base membrane.

[0023] (2) Weigh 0.1 g of hydrophobic SiO 2 Dispersed in 5 mL of DMF and acetone mixed solvent solution (DMF: Acetone = 2: 3, v: v), ultrasonic dispersion for 30 min. Then weigh 0.1 g PVDF-HFP and put it into SiO 2 The dispersion was stirred magnetically for 10 h to fully dissolve and prepare PVDF-HFP / SiO 2 The electrospray spinning solution, in which PVDF-HFP and SiO 2 The weight ratio of PVDF-HFP and SiO 2 The weight ratios of the composite PVDF-HFP / SiO are 0:1, 2:1, 2:3, 2:4, 2:5, and 2:6. 2 Electrospray liquid, electrospray PVDF-HFP / SiO on top of PVDF-HFP fiber membrane 2 Composite microspheres were used to investigate the hydrophobic modification of fiber membranes by composite microspheres in different proportions.

[0024] (3) Place the PVDF-HFP fiber membrane (the fiber membrane is attached to the oil paper) on the blade electrode, set the pulling speed (2 mm / min~0 mm / min), and then electrostatically spray PVDF-HFP / SiO above the blade electrode. 2 Composite microspheres, positive voltage of 12 kV, push rate of 1 mL / h, electrospray distance (between spinning needle and blade electrode) of 15 cm, total spinning time of 30 min. 2 The microspheres were placed on the surface of the fiber membrane in a gradient manner. Finally, the prepared fiber membrane was placed in a vacuum oven at room temperature for 24 h to remove the residual solvent, and PVDF-HFP / SiO 2 Surface wettability gradient membranes.

[0025] The gradient membrane obtained above was evaluated: 1. Wettability selection and regulation Characterization of wetting properties: 10 μL of water was added to the fiber membrane, and the contact angle was measured using a JC2000D1 contact angle meter. Similarly, using a JC2000D1 contact angle meter, the fiber membrane was immersed in water, 5 μL of bubbles were squeezed onto the membrane surface, and the underwater bubble contact angle of the fiber membrane was measured.

[0026] Electrosprayed microspheres can significantly change the microstructure of the fiber membrane surface, thereby affecting its wettability. Based on this, this embodiment prepared different SiO 2 PVDF-HFP / SiO 2 Electrospray to explore SiO 2 The influence of the content change on the wettability of the fiber membrane surface. 2 The weight ratios of PVDF-HFP / SiO were 0:1, 2:1, 2:3, 2:4, 2:5, and 2:6, respectively. 2 The composite membranes are denoted as FS 0 , FS 1 , FS 2 , FS 3 , FS 4 , FS 5 and FS 6 , and measure its water contact angle. Figure 2 As shown, FS 0 , FS 1 , FS 2 , FS 3 , FS 4 , FS 5 and FS 6The water contact angle of the composite membrane increases first and then decreases. The contact angle of PVDF-HFP is 125°, while that of FS 0 The contact angle of the membrane reached 138°. The results show that the hydrophobicity of the membrane can be effectively improved by modifying the membrane surface with electrosprayed microspheres.

[0027] With SiO 2 As the content of SiO increases gradually, the contact angle of the film surface also increases. 2 When the content reaches 50 wt% (PVDF-HFP / SiO 2 The ratio is 2:2), FS 2 The contact angle of the film reached 153°, showing super hydrophobic properties. This is because the hydrophobic SiO 2 With lower surface energy, it changes the chemical composition and microstructure on the membrane surface, thereby improving the wettability of the membrane from a hydrophobic state to a superhydrophobic state. 2 When the content continues to increase, the hydrophobicity of the membrane surface decreases. 6 The contact angle of the film surface is 142°, which is not SiO 2 The higher the content, the stronger the hydrophobicity of the membrane. 2 During the process of microsphere modification of membrane surface, excessive SiO 2 Nanoparticles agglomerate together, resulting in uneven dispersion of electrosprayed microspheres on the membrane surface, which reduces the hydrophobicity. Therefore, in order to construct a gradient membrane with obvious wettability changes, this embodiment selects SiO 2 The electrospray liquid with a content of 50 wt% was used to prepare a wettability gradient film with a gradient distribution of microspheres on the surface.

[0028] Among all the fiber membranes mentioned above, FS 2 The membrane has super hydrophobic properties and also has an extremely low sliding angle. Figure 3 As shown in Figure 2, when a 10 μL deionized water droplet lands on the fiber membrane surface (which is inclined at about 5°), the water droplet immediately rolls away from its initial contact position.

[0029] 2. Surface morphology of fiber membrane Characterization of microscopic morphology: After preparing samples of different membrane materials, they were sprayed with gold. The Zeiss SUPRA™ 40 scanning electron microscope was used to characterize the surface morphology of the samples at different magnifications. Before scanning and characterizing the cross-section of the membrane material, the sample was frozen with liquid nitrogen to embrittle it and then sprayed with gold. At the same time, the membrane surface was also analyzed by EDS energy spectrum to analyze the chemical component content on the surface of the fiber membrane.

[0030] The SEM images of PVDF-HFP membranes are shown in Figure 2. Figure 4As shown in the figure, the surface morphology is a three-dimensional porous structure, in which the fiber diameter is uniform, the surface is smooth and flat, and has a high specific surface area. The surface of the PVDF-HFP fiber membrane has high hydrophobicity, and its water contact angle is about 125°. FS 0 The surface morphology of the membrane is composed of PVDF-HFP fibers and PVDF-HFP microspheres dispersed on the fibers, such as Figure 5 As shown in Figure 2, the surface of PVDF-HFP microspheres is smooth and evenly distributed. Figure 6 As shown. FS 0 The contact angle of the membrane surface is 137°. The microspheres are distributed on the fibers, increasing the roughness of the membrane surface and forming more micro-nano structures. This structure reduces the contact area between water molecules and the membrane, thereby effectively enhancing the hydrophobicity of the membrane. 2 The addition of nanoparticles, such as Figure 7 As shown in the figure, the surface of the PVDF-HFP membrane has larger and more densely distributed PVDF-HFP / SiO 2 Compared with PVDF-HFP microspheres, composite microspheres are composed of PVDF-HFP and inorganic nanoparticles SiO 2 The introduction of inorganic nanoparticles changes the morphology of the microsphere surface. There are many small protrusions on the surface of the composite microsphere, such as Figure 8 As shown in the figure, these small protrusions have nanometer-scale sizes. Therefore, micrometer-scale microspheres and nanometer-scale protrusions form a composite multi-level structure on the PVDF-HFP membrane, which can further improve the surface roughness of the PVDF-HFP membrane, making the membrane have relatively excellent super-hydrophobic properties, with a contact angle of 153°.

[0031] Fig. 9 YesFS 2 The composite membrane consists of a PVDF-HFP substrate membrane and a PVDF-HFP / SiO 2 The membrane is composed of a microsphere layer. When viewed from the side, the surface morphology is uneven, similar to the papillary structure on the surface of a lotus leaf. The concave parts between the papillary structures are filled with air, forming a thin air layer close to the membrane surface. When water droplets fall on the surface of the fiber membrane, due to the presence of the composite microspheres, the water droplets can only form a few points of contact with the top of the microspheres, and will not contact the fiber surface. Moreover, the air layer adsorbed on the membrane surface also blocks the contact between the water droplets and the fiber surface. This structure allows the water droplets to form spheres under the action of their own surface tension, making the membrane surface super-hydrophobic.

[0032] 3. Mechanical properties of fiber membrane Mechanical properties: The mechanical properties of the fiber membrane were tested using a universal material testing instrument (CMT6104). During the test, the sample was cut into a standard dumbbell shape (ISO527 standard). The clamping distance of the tester was 40 mm, the tensile speed was 15 mm / min, and the test result was the average value of 3 samples.

[0033] PVDF-HFP, FS 0 , FS 2 The mechanical properties of the composite membranes are Fig.10 and Fig.11 As shown in the figure, the tensile strengths of the three are 4.28 MPa, 4.53 MPa and 5.14 MPa, and the elongation at break are 115.4%, 117.86% and 117.73%, respectively. 0 , FS 2 The tensile strength of the composite membrane is improved, which may be because the microspheres can enhance the bonding between fibers, thereby improving the tensile strength of the composite fiber membrane.

[0034] 4. Surface morphology of gradient membrane PVDF-HFP / SiO was prepared by blade electrode device. 2 Gradient membrane, including PVDF-HFP / SiO 2 The distribution of microspheres on the fiber membrane surface shows a gradient change, which is named GM. Fig.12 As shown in Figure 1, four regions are selected at equal intervals on the GM surface and are marked as I, II, III and IV. Fig.13 As shown in Figure 1, from region I to region IV, the microsphere content gradually increases, and the surface morphology changes from a fiber structure to a microsphere structure. Fig.14 , Fig.15 As shown in Figure 2, elemental analysis shows that with the increase of Si content and the decrease of F content, more PVDF-HFP fibers are covered with SiO 2 Therefore, the surface morphology and chemical composition of GM show a gradient change.

[0035] 5. Wettability of gradient membrane The contact angle change from the hydrophobic end to the superhydrophobic end was tested on the gradient membrane. Fig.16 As shown in the figure, the contact angle increases from 125° to 151°, while the corresponding underwater air bubble contact angles are 16.84° and 0°, respectively, indicating that as the hydrophobicity increases, the aerophilicity also increases. On the surface of the microsphere gradient distribution, the contact angles of water and air bubbles were measured every 10 mm. Fig.17It is shown that the contact angle changes with the change of position: the water contact angle gradually increases from 125° to 151°, while the bubble contact angle decreases from about 16° to 0°. This transition from aerophilicity to superaerophilicity contributes to the unidirectional transport of bubbles at the bottom of the water.

[0036] Bubble transport analysis: The fiber membrane was cut into strips with a width of 4 mm, pasted on a glass sheet, and placed in a 300×100 mm glass tank. Bubbles were squeezed into one side of the fiber membrane with a syringe to examine the transport characteristics of underwater bubbles. The transport behavior of the bubbles was studied by video playback.

[0037] like Fig.18 As shown in the figure, the water contact angle on the surface of the wettability gradient membrane gradually increases, indicating that the hydrophobicity is increasing, and the transition from hydrophobic to superhydrophobic performance is successfully achieved. In the water environment, 50 μL of bubbles were injected into the hydrophobic side and superhydrophobic side of the gradient membrane, respectively, and the unidirectional transmission performance of the bubbles was studied. The experimental results show that Fig.19 As shown in the figure, bubbles can be transported along the direction of increasing aerophilicity, that is, unidirectional transport is achieved. However, in the direction of increasing aerophobicity, bubbles will be fixed in the superaerophilic region and cannot be transported. Such unidirectional transport characteristics are of great significance for the precise control of underwater bubble movement.

[0038] 6. Bubble transport in gradient membranes The test method is: cut the fiber membrane into strips with a width of 4 mm, paste them on a glass sheet, place them in a 300×100 mm glass tank, use a syringe to squeeze bubbles into one side of the fiber membrane, examine the transmission characteristics of underwater bubbles, and study the transmission behavior of bubbles by video playback. By rationally regulating the surface wettability, effective capture, directional transmission and collection of underwater bubbles can be achieved. The gradient membrane prepared by the present invention has a certain hydrophobicity and can form a stable air film in water, such as Fig. 20 and Fig.21 As shown, it helps to effectively capture and continuously transmit underwater bubbles. Observing the wettability gradient membrane with a length of 60 mm, bubbles can be continuously transmitted from the right side (aerophilic side) of the membrane to the left side (superaerophilic side). Slow motion playback shows that at 0 seconds, a layer of air film has been attached to the surface of the gradient membrane, and the left side of the air film is thicker than the right side, forming a surface curvature difference. After the bubble contacts the membrane surface, due to the Laplace pressure difference and the wettability gradient force, it moves to the side with greater curvature (i.e., higher aerophilicity), achieving directional transmission along the wettability gradient. At 0.25 s, the third bubble is transmitted toward the superaerophilic end. When the eighth bubble is continuously transmitted (1.175 s), the large bubble formed detaches from the membrane surface under the action of buoyancy, completing the unidirectional transmission of underwater bubbles. Similarly, in the top view, the continuous bubbles are captured on the right side, and after a period of displacement on the underwater membrane surface, they detach from the left side, achieving the transmission of a single large bubble.

[0039] On the wettability gradient membranes with lengths of 20 mm, 30 mm, 40 mm, 50 mm, and 60 mm, bubbles can be effectively transported over long distances of 20 mm to 60 mm. Fig. 22 However, when the length exceeds 60 mm, due to the discontinuity of the wettability gradient, the bubbles cannot sustain the transport and detach from the membrane surface during the process, resulting in transport failure.

[0040] The wettability gradient membrane was fixed on a transport track with a width of 3 mm and a length of 20 mm to 60 mm, and the amount of gas adsorption was measured to determine the critical value of gas detachment from the membrane surface. Fig.23 As shown in the figure, the amount of gas adsorption increases with the increase of the gradient membrane length, from 100 mL to about 180 mL, but the amount of gas adsorption per unit length shows a downward trend. This is because the bubble contact angle varies between 0° and 16°. As the transmission length increases, the wettability difference between adjacent areas on the gradient membrane is smaller, and the affinity for air per unit length decreases, thereby reducing the volume of adsorbed gas. In the continuous bubble transmission experiment, the average transmission speed of a single bubble (volume of about 5 μL) was calculated. Fig.24 As shown in the figure, when the wettability gradient length is 20 mm, the bubble transmission speed is the fastest, about 85 mm / s. As the transmission track length increases, the average transmission speed decreases, and when the wettability length is 60 mm, the transmission speed is about 35 mm / s. This shows that on the surface with a shorter wettability gradient, due to the large difference in wettability between adjacent regions, the curvature difference on both sides of the bubble is also large, which is driven by a larger Laplace pressure difference, making the transmission speed faster.

[0041] 7. Collection of Bubble Transmission The present invention designs a highly efficient underwater bubble capture, transportation and collection device, such as Fig.25 As shown. The device uses the properties of the wettability gradient to capture continuously injected bubbles on one side. Driven by the wettability gradient, the bubbles are continuously transported along the gradient membrane to the superhydrophobic area until the maximum gas adsorption capacity of the gradient membrane is reached. Subsequently, under the action of buoyancy, the bubbles detach from the membrane surface. In order to effectively collect these detached bubbles, a sponge with superhydrophobic / superaerophilic properties is placed above the bubble detachment. The sponge can capture the transmitted gas and introduce it into a special collection device through a catheter to achieve bubble collection. The whole process takes only 1.4 s, reflecting the high efficiency of gas collection. Such devices have potential application value in the future exploitation, transportation and collection of submarine natural gas.

[0042] From the above description, it can be seen that by controlling the change in the pulling speed of the base membrane on the blade electrode, the distribution of polyvinylidene fluoride-hexafluoropropylene / silica composite microspheres on the membrane surface is gradiently changed through electrospinning, and a continuously changing microscopic morphology is constructed, thereby obtaining a gradient surface from hydrophobicity to super-hydrophobicity. This method of preparing a gradient membrane by controlling the change in the pulling speed of the base membrane on the blade electrode has excellent application prospects.

[0043] In summary, the present invention provides a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres and a construction method thereof, which belongs to the field of gradient membranes. The PVDF-HFP fiber membrane is prepared by electrospinning technology using improved electrospinning and electrostatic spraying technology, and the PVDF-HFP / SiO2 electrostatic spraying is used to prepare the PVDF-HFP fiber membrane. 2 The composite microspheres, combined with the programmable collector receiving device, control the traction speed change of the base membrane on the blade electrode to construct a wettability gradient membrane with continuously changing surface morphology; on the surface of the prepared fiber membrane, the distribution of the polyvinylidene fluoride-hexafluoropropylene / silicon dioxide composite microspheres on the membrane surface presents a gradient change, constructing a continuously changing microscopic morphology, and the fiber membrane contact angle increases from 128° to 153° with the change of surface position, obtaining a gradient surface from hydrophobicity to super-hydrophobicity, realizing the preparation of wettability gradient surface, which has excellent application prospects in the field of gradient membranes, bubble transport and bubble microreaction.

[0044] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one 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 present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres, characterized in that: The construction method comprises the following steps: Step S1: Preparation of PVDF-HFP spinning solution Weigh PVDF-HFP and dissolve it in the compound solvent solution, stir it thoroughly, and prepare a uniformly dissolved PVDF-HFP spinning solution; Step S2: PVDF-HFP fiber membrane preparation Through electrospinning, the drum receives the PVDF-HFP spinning fiber at a constant speed; after the spinning is completed, the obtained PVDF-HFP fiber membrane is placed in a vacuum oven at room temperature for more than 24 hours to remove the residual solvent, thereby obtaining the PVDF-HFP fiber membrane, which is used as a base membrane; Step S3: Preparation of PVDF-HFP-silica spinning solution Weigh hydrophobic SiO2 and disperse it in a compound solvent solution, and perform ultrasonic dispersion to obtain a SiO2 dispersion liquid; then weigh PVDF-HFP and put it into the SiO2 dispersion liquid, and stir to fully dissolve it to prepare a PVDF-HFP-silicon dioxide electrospray liquid; Step S4: Preparation of gradient membrane with continuous distribution of PVDF-HFP-silica electrosprayed microspheres The PVDF-HFP fiber membrane attached to the oil paper prepared in step S2 is placed on the blade electrode, the pulling speed is set to decrease linearly, and then the PVDF-HFP-silicon dioxide electrospray liquid prepared in step S3 is used for electrostatic spraying above the blade electrode to prepare a composite fiber membrane with a gradient change of PVDF-HFP / SiO2 microspheres on the fiber membrane surface. Finally, the prepared fiber membrane is placed in a vacuum oven at room temperature for more than 24 hours to remove the residual solvent, so as to obtain a gradient membrane based on the continuous distribution of PVDF-HFP-silicon dioxide electrospray microspheres.

2. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: In step S1, the concentration of PVDF-HFP in the compound solvent solution is 8% to 20% by mass volume.

3. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: The composite solvent solution in step S1 is a mixture of DMF and acetone, the volume ratio of DMF to acetone is 1:3 to 3:3; and the stirring time is more than 10 hours.

4. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: The specific spinning parameters in step S2 are: The spinning voltage was set to 10~20 kV, the pushing speed of the PVDF-HFP spinning solution injection pump was 0.5~2 mL / h, and the distance between the spinning nozzle and the receiving device was kept at 10~20 cm for electrospinning. During this process, a layer of silicone oil paper was pre-wrapped on the drum to receive the PVDF-HFP spinning fibers at a constant speed of 10~100 r / min. The spinning time was 30~240 min.

5. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: In the step S3, the concentration of hydrophobic SiO2 in the compound solvent solution is 0% to 6% by mass volume, and the ultrasonic dispersion is performed for 30 to 60 min.

6. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: The composite solvent solution in step S3 is a mixture of DMF and acetone, and the volume ratio of DMF to acetone is 1:3 to 3:

3.

7. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: In step S3, the mass ratio of PVDF-HFP to hydrophobic SiO2 is 0:1 to 1:3, and the stirring time is more than 10 hours.

8. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: The spinning parameters in step S4 are: The positive voltage was 10~20 kV, the PVDF-HFP-silica electrospray spinning solution push rate was 0.2~2 mL / h, the electrospray distance between the spinning needle and the blade electrode was 10~20 cm, and the total spinning time was 10~120 min.

9. A method for constructing a gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres according to claim 1, characterized in that: In step S4, the pulling speed is linearly reduced from 2 mm / min to 0 mm / min.

10. A gradient membrane based on continuous distribution of PVDF-HFP-silicon dioxide electrosprayed microspheres, characterized in that: The gradient membrane is prepared by any one of the preparation methods of claims 1-9.

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