Gradient film based on GO-PAN component continuous distribution and controllable construction method thereof

Abstract

The invention discloses a GO-PAN component continuous distribution-based gradient membrane and a controllable construction method thereof, belongs to the field of gradient membranes, and aims to prepare a gradient membrane with a wider wettability change range. A hydrophobic material PAN and a hydrophilic modified material GO are selected to construct a wettability gradient membrane; pAN has certain hydrophobicity, and hydrophilic modification of a PAN fiber membrane can be realized by adding a small amount of graphene oxide into the polyacrylonitrile fiber; the traction speed change of a PAN base film is controlled on a blade electrode, gradient change of the deposition amount of GO / PAN fibers is achieved on the surface of the PAN film through electrostatic spinning, wettability gradient construction is achieved through the change of chemical components, namely the change of GO, and the change of the contact angle from 0 degree to 133 degrees is achieved. The method has an excellent application prospect in directional spreading of liquid drops on the surface of a gradient film and in a micro-reaction device.

Description

Technical Field

[0001] The invention belongs to the technical field of gradient membranes, and in particular relates to a gradient membrane based on continuous distribution of GO-PAN components and a controllable construction method thereof. Background Art

[0002] Surface wettability is a fundamental property of solid surfaces and has a significant impact on the functional properties and lifetime of materials. Generally speaking, surfaces with a water contact angle (WCA) greater than 150° are considered superhydrophobic, while surfaces with a contact angle between 0° and 10° are considered superhydrophilic. With the discovery of the lotus effect, a major breakthrough has been made in the study of surface wettability. Several single wettability surfaces, including hydrophilic, hydrophobic, superhydrophilic, superhydrophobic, and superoleophobic surfaces, have been developed for various applications related to interface engineering, such as anti-corrosion, oil-water separation, antifouling, deicing, and catalytic systems. However, with the advancement of technology, it has become clear that a single wettability surface may not be sufficient 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 single wettable surfaces. For example, the dorsal structure of desert beetles has irregularly distributed hydrophilic protrusions and hydrophobic grooves, forming a hydrophilic-hydrophobic composite surface, which not only has the advantages of a single wettable surface and can collect water from fog, but also promotes droplet aggregation through the hydrophobic surface. In oil-water separation, single wettable surfaces such as superhydrophilic / underwater superoleophobic surfaces mainly rely on physical screening effects to achieve separation, while Janus membrane interfaces can produce surface energy differences, thereby increasing the driving force for emulsion breakage. In addition, for wettable surfaces with special patterns, the interaction between their surface energy gradients and Laplace pressure gradients can provide a directional driving force for the spontaneous transport of droplets, thereby enabling the construction of droplet arrays and biochips.

[0004] Compared with single wettability and complex wettability surfaces, wettability gradient surfaces refer to surfaces whose surface wettability changes continuously with the position of the surface. Due to its continuously changing wettability characteristics, it has attracted the attention of many scholars. In nature, the surface structure of the mouth edge of Nepenthes has the characteristics of a wettability gradient, and its wettability gradient is achieved through changes in its surface microstructure and hydrophilic and hydrophobic properties. This wettability gradient enables Nepenthes to effectively guide water droplets from the mouth edge to the inside of the insect trap, thereby helping it to capture insects and digest them. In addition to nature, wettability gradient surfaces have many applications in real life. At present, wettability gradient surfaces have been applied in many fields, such as agriculture, clothing industry, energy industry and biomedical research. Specifically, it can be used to study water mist collection, improve heat exchange efficiency, and realize water droplet self-transportation. Therefore, it is of great significance to prepare a surface with a continuously changing wettability gradient. Summary of the invention

[0005] The invention aims to provide a gradient membrane based on continuous distribution of GO-PAN components and a controllable construction method thereof, which belongs to the field of gradient membranes. In order to prepare a gradient membrane with a wider range of wettability changes, a hydrophobic material PAN and a hydrophilic modified material graphene oxide GO are selected to construct a wettability gradient membrane; PAN has certain hydrophobicity, and a small amount of graphene oxide is added to a polyacrylonitrile fiber membrane to achieve hydrophilic modification of the PAN fiber membrane; the traction speed of the PAN substrate membrane is controlled on a blade electrode, and a gradient change in the deposition amount of GO / PAN fibers is achieved on the surface of the PAN membrane through electrostatic spinning, and the wettability gradient is achieved by utilizing the change of chemical components, namely the change of GO, so as to achieve a contact angle change from 0° to 133°, and the invention has excellent application prospects in the directional spreading of droplets on the surface of the gradient membrane and in a micro-reaction device.

[0006] To achieve the above object, the present invention is implemented through the following technical solutions: A method for controllably constructing a gradient membrane based on continuous distribution of GO-PAN components comprises the following steps: Step S1: PAN spinning solution preparation Weigh polyacrylonitrile PAN powder, dissolve it in a solvent, and stir it to form a uniform spinning solution, which is the PAN spinning solution preparation; Step S2: PAN fiber membrane preparation At room temperature, PAN fiber membrane was electrospun and used as the base membrane to receive GO-PAN fiber filaments; Step S3: GO-PAN spinning solution preparation Weighing graphene oxide GO into a solvent, dispersing it by ultrasonication, then weighing PAN powder into the graphene oxide dispersion, stirring to fully dissolve it to form a uniform spinning solution; that is, GO-PAN spinning solution; Step S4: Preparation of gradient membranes with continuous distribution of GO-PAN components The PAN fiber membrane prepared in step S2 is placed on the blade electrode, and the pulling speed of the PAN fiber membrane is set to decrease linearly. Then, electrospinning is performed above the blade electrode using the GO-PAN spinning solution prepared in step S3 to receive the GO / PAN spinning solution to form a gradient membrane with a continuously changing GO-PAN component content on the surface of the PAN fiber membrane base membrane.

[0007] Furthermore, in step S1, the concentration of PAN powder in the solvent is 10%-20% (m / v) by mass volume ratio.

[0008] Furthermore, in step S1, the solvent is DMF, and the stirring time is more than 10 hours.

[0009] Furthermore, the spinning parameters in step S2 are: Under the spinning parameters of high voltage of 10~20 kV, 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.

[0010] Furthermore, in step S3, the concentration of graphene oxide in the solvent is 0.2% to 2% (m / v) by mass volume, and the ultrasonic dispersion is performed for 30 minutes.

[0011] Furthermore, in step S3, the mass ratio of graphene oxide to PAN powder is 2:100 to 25:100.

[0012] Furthermore, in step S3, the solvent is DMF, and the stirring time is more than 10 hours.

[0013] Furthermore, the spinning parameters in step S4 are: Under the conditions of positive voltage of 10-20 kV and negative voltage of -3~-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.

[0014] Furthermore, in step S4, the pulling speed of the PAN fiber membrane is linearly decreased from 2 mm / min to 0.01 mm / min.

[0015] A gradient membrane based on continuous distribution of GO-PAN components is prepared by any of the above preparation methods.

[0016] The beneficial effects of the present invention are: The invention discloses a gradient membrane based on continuous distribution of GO-PAN components and a controllable construction method thereof, belonging to the field of gradient membranes. In order to prepare a gradient membrane with a wider range of wettability changes, a hydrophobic material PAN and a hydrophilic modified material graphene oxide GO are selected to construct the wettability gradient membrane; PAN has certain hydrophobicity, and a small amount of graphene oxide is added to the polyacrylonitrile fiber membrane to achieve hydrophilic modification of the PAN fiber membrane; the traction speed of the PAN substrate membrane is controlled on a blade electrode, and a gradient change in the deposition amount of GO / PAN fibers is achieved on the surface of the PAN membrane through electrostatic spinning, and the wettability gradient is achieved by utilizing the change of chemical components, namely the change of GO, so as to achieve a contact angle change from 0° to 133°. The invention has excellent application prospects in the directional spreading of droplets on the surface of the gradient membrane and in a micro-reaction device. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1A schematic diagram of the structure of an electrospinning device used in preparing a gradient membrane in the present invention; Figure 2 It is a molecular structure diagram of the material in Example 1 of the present invention; Figure 3 The static water contact angle test results of the non-gradient membrane prepared with the mass ratio of GO to PAN powder being 0:100 to 15:100 in the embodiment of the present invention; Figure 4 The data diagram of the dynamic change of water contact angle of the non-gradient film of GO-PAN with different proportions in the embodiment of the present invention; Figure 5 This is a scanning electron microscope result of a PAN fiber membrane obtained by electrospinning in the absence of a blade electrode in an embodiment of the present invention; Figure 6 This is a scanning electron microscope result of a PAN fiber membrane obtained by electrospinning in the presence of a blade electrode in an embodiment of the present invention; Figure 7 The scanning electron microscope results of the gradient fiber membrane A, B, C, and D prepared in Example 1 of the present invention and the corresponding N and O element energy spectrum surface scanning results are shown; Figure 8 For the present invention Figure 7 The statistical result diagram of the energy spectrum scan corresponding to area A in the middle; Fig. 9 For the present invention Figure 7 The statistical result diagram of the energy spectrum scan corresponding to the B area in the middle; Fig.10 For the present invention Figure 7 The statistical result diagram of the energy spectrum scan corresponding to the C area in the middle; Fig.11 For the present invention Figure 7 The statistical results of the energy spectrum scan corresponding to the four regions in D; Fig.12 This is a photo of the gradient membrane prepared in Example 1 of the present invention; Fig.13 The water contact angle test results from left to right of the gradient membrane surface prepared in Example 1 of the present invention; Fig.14 The test results of water contact angle at different positions on the surface of the gradient membrane under different negative voltages in the present invention are shown; Fig.15 The spreading behavior diagram of the droplets on the gradient membrane and the hydrophilic non-gradient membrane prepared in Example 1 of the present invention; Fig.16 The spreading length diagram of droplets of different volumes on the gradient membrane (GO / PAN) and the hydrophilic non-gradient membrane (GO@PAN) prepared in Example 1 of the present invention; Fig.17 It is a top view of the continuous liquid transmission behavior of the liquid on the gradient membrane surface along the increasing direction of hydrophilicity and along the increasing direction of hydrophobicity in the present invention; Fig.18 It is a side view of the continuous liquid transport behavior of the liquid on the gradient membrane surface along the increasing hydrophilicity and along the increasing hydrophobicity direction in the present invention. DETAILED DESCRIPTION

[0018] 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.

[0019] PAN fiber membrane has high hydrophobicity, and the wettability difference per unit distance of the gradient membrane can be further improved by continuous hydrophilic modification on the surface of PAN fiber membrane. Polyacrylonitrile (PAN) has good thermal stability (317°C) and can maintain its performance at higher temperatures, which is very important for high temperature application environments.

[0020] The edge structure of graphene oxide (GO) contains many active groups, most of which are hydrophilic groups such as carboxyl and hydroxyl, which can be used as hydrophilic modifying substances. The present invention adds a small amount of GO to the PAN fiber membrane to achieve hydrophilic modification of the PAN fiber membrane. After exploring the appropriate GO content, the pulling speed of the PAN base membrane is controlled on the blade electrode, and the deposition amount of GO / PAN fiber is realized on the surface of the PAN membrane by electrospinning. The wettability gradient is constructed by using the change of chemical components, that is, the change of GO.

[0021] Example 1: Gradient membrane preparation like Figure 1 As shown in the figure, the specific experimental steps for preparing GO / PAN gradient membrane are as follows: 1. Weigh 0.01, 0.03, 0.05, 0.07 and 0.09 g of graphene oxide respectively and add them into 5 mL of DMF solvent. Ultrasonic disperse for 30 min. Then weigh 0.4 g of PAN powder into the graphene oxide dispersion and stir it magnetically for more than 10 h to fully dissolve it to form a uniform spinning solution.

[0022] 2. Weigh 0.7 g of PAN powder, dissolve it in 5 mL of DMF solvent, and stir it magnetically for more than 10 h to form a uniform spinning solution.

[0023] 3. Under the spinning parameters of 14 kV high voltage, 60 r / min collection speed, 15 cm collection distance and 1 mL / h pushing speed, the spinning time was 60 min and at room temperature, PAN fiber membrane was electrospun and used as the base membrane for receiving GO@PAN fiber filaments.

[0024] 4. Under the conditions of positive voltage of 14 kV and negative voltage of -9 kV, the GO-PAN spinning solution pushing speed is 1 mL / h; the PAN fiber membrane prepared in step S2 is placed on the blade electrode, and the PAN fiber membrane pulling speed is set to decrease linearly, and the electrospray distance between the spinning needle and the blade electrode is 15 cm; the pulling speed of the receiving membrane PAN is linearly decreased 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 to form a gradient membrane with varying GO / PAN content on the surface of the base membrane. The spinning time is 30 min.

[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] like Figure 2 As shown in Figure 1, GO / PAN fiber membranes with GO contents of 0 wt%, 2 wt%, 5.5 wt%, 9 wt%, 12 wt% and 15 wt% were prepared to investigate the hydrophilic modification of different GO contents. The wettability behavior of 10 μL of deionized water on the fiber membrane changes dynamically. Therefore, Figure 3 As shown in the figure, at t=1s, the contact angles of GO / PAN fiber membranes with GO contents of 0 wt%, 2 wt%, 5.5 wt%, 9 wt%, 12 wt% and 15 wt% are 131°, 31°, 18°, 7°, 0° and 0° respectively, indicating that GO can significantly improve the hydrophilicity of PAN fiber membranes. Figure 4 As shown in the figure, the change of water contact angle on fiber membranes with GO content of 0 wt%, 2 wt%, 5.5 wt%, 9 wt%, 12 wt% and 15 wt% over time was analyzed. PAN fiber has a certain hydrophobicity, and the contact angle remains unchanged at 131°. After adding GO, GO / PAN has hydrophilicity. When the wetting time is 10s, the contact angle of GO / PAN fiber membranes with different GO contents all becomes 0°. The GO / PAN fiber membranes with high GO content, such as 12wt% and 15wt%, have a contact angle of 0° at 0.5 s and 0.25 s, which has super hydrophilic properties. The subsequent examples selected a spinning solution with a GO content of 12 wt% to construct a surface wettability gradient membrane with a gradient change in GO content.

[0027] 2. Effect of collector on deposition of PAN fibers 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.

[0028] The blade electrode belongs to a point-to-point electric field line system, that is, the electric field collection mode from the spinning needle to the blade, which is conducive to the controlled deposition of fiber filaments. When the concentration of PAN spinning solution is low, the diameter of the spun fiber filaments is small, and the fiber is easily affected by the extremely narrow blade electrode, which can make the fiber filaments arranged in an orderly manner. Figure 5 As shown in the figure, the collection morphology of 14% w / v PAN (w / v, 0.7g PAN: 5mL DMF) fibers on the drum, the fiber filaments in this morphology are randomly and disorderly arranged, the fiber membrane is isotropic, and the drum collection device cannot achieve the controlled and orderly deposition of fibers, making it difficult to prepare a wettability gradient fiber membrane with a gradient change in chemical composition. 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 the fibers of different chemical components and deposit them in an orderly manner along the membrane surface, so that the wettability gradient surface with gradient changes in chemical composition can be controllably constructed.

[0029] 3. Surface morphology of gradient membrane In order to characterize the microscopic morphology 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 and EDS tests were performed on each region. Figure 7 As shown in the figure, region A is the surface morphology of the PAN substrate membrane, that is, the fiber membrane morphology prepared with a 14% PAN spinning solution concentration. The fiber diameter is mainly concentrated in 200~650 nm, with an average diameter of 451 nm. The fiber diameter is smooth and flat, and the size is uniform. In the N element diagram, the outline structure of the fiber is clearly seen. Most of the morphological structure is PAN fiber, with less O elements and a small amount of GO components.

[0030] In the B region, the diameter of the fiber decreases significantly, and the size is mainly concentrated in 50~200 nm, with an average diameter of 129 nm. Because the spinning solution concentration is relatively low (8% w / v), when the spinning solution concentration is low, the viscosity of the solution decreases, causing the jet under the action of the electric field to be more easily stretched. Due to the weakening of the interaction force between the polymer chains, the electric field force makes it easier for the fluid in the solution to be stretched into thinner fibers, and the diameter of the fiber will be significantly reduced. A small number of GO spindle structures are seen between the fibers. The N element map has no obvious fiber outline relative to the A region. In the C region, the fiber diameter is distributed in 50~250nm, with an average diameter of 126 nm. The GO content is gradually increasing, and the O element is increasing. The GO spindle outline can be clearly seen, corresponding to the spindle structure in the morphology map. In the D region, the fiber diameter distribution is the same as that in the B and C regions, distributed in 50~250nm, with an average diameter of 136nm. The increase in GO content has no significant effect on the diameter of PAN fibers. Among them, the reason for the formation of GO spindles is that the GO sheet is a micron-sized structure, which does not match the fiber diameter of PAN, and the PAN fiber is easily swollen by the GO sheet. In addition, GO can interact strongly with PAN fibers, making it easier for PAN to adsorb on the GO sheet structure. During the electrospinning fiber traction process, PAN is more likely to gather near the GO sheet than in other areas without GO, forming a spindle structure.

[0031] 4. Element analysis of fiber membrane surface The changes in the elements on the surface of the gradient film were analyzed by the energy dispersive spectrometer attached to the scanning electron microscope. The N element is a unique element in the molecular structure of PAN, the O element is a unique element in the molecular structure of GO, and the C element is an element contained in both the molecular structure of PAN and the molecular structure of GO. Figure 8 , Fig. 9 , Fig.10 and Fig.11 As shown, the contents of C, N, and O elements were measured, and the changes of 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. Fig.13 As shown in the figure, the contents of these two elements show opposite increasing and decreasing trends, indicating that the amount of GO loaded on the PAN fiber is increasing, and the content of GO components on the fiber membrane surface is increasing. By analyzing the element content on the membrane surface, a surface with a gradient change in GO content was successfully constructed on the PAN fiber membrane.

[0032] 5. Surface wettability of gradient membrane like Fig.12As shown in the figure, the surface of the gradient membrane with a length of 30 mm changes from dark to light from left to right, that is, the GO-enriched area gradually transitions to the pure PAN fiber surface area. Fig.13 As shown in the figure, the negative voltage of the blade electrode is crucial to the preparation of the wettability gradient surface. Therefore, the surface wettability gradient membrane with a wettability length of 30 mm was prepared under negative voltages of 3 kV, 6 kV, 9 kV, and 12 kV, and the water contact angle was measured at every 5 mm distance on the fiber membrane surface. Under the voltage of 3 kV, the surface wettability of the gradient membrane varied in the range of 0°~60°, and a wettability change gradient was constructed in the hydrophilic range. The electric field traction of the blade electrode of -3 kV on the fiber filaments in the air was not strong, and the fiber area deposited on the blade was relatively large. The fibers with different GO contents partially overlapped, resulting in more GO-containing areas on the membrane surface, and the wettability of the entire membrane surface was a hydrophilic surface. With the increase of negative voltage, such as -6 kV and -9 kV, the gradient of surface wettability achieved a change in the range of 0°~130°. The increase in electric field force makes the deposition of fibers more concentrated, so that GO / PAN fibers can be deposited on the surface of the PAN membrane in an orderly manner, with a small overlap between them, which will not affect the wettability of adjacent areas. However, for excessively large negative voltages, such as -12 kV, for the fiber filaments that have been deposited on the blade, the fiber part is attracted by the positive electrode (12 kV) to detach from the surface of the PAN matrix. The protruding fibers will affect the subsequent fiber reception process, thereby affecting the deposition order of the fibers, and will not allow the fibers to fall in adjacent areas, resulting in discontinuous construction of the GO content gradient and failure to construct the wettability gradient. The surface wettability varies in the range of 112°~98° and 3°~35°, and the hydrophilic surface suddenly changes to the hydrophobic surface, without the continuity of the wettability gradient change. For the wettability gradient membrane prepared with a negative voltage of 9 kV, such as Fig.14 As shown in the figure, from hydrophilic area to hydrophilic area, the contact angles are 0°, 27°, 51°, 64°, 96°, 125° and 133° respectively, and the wettability has achieved an increasing hydrophobic change from left to right. Conversely, the hydrophilicity increases from right to left. Such a wettability gradient surface is crucial for the directional spreading of liquids.

[0033] 6. Directed spreading of droplets Directed spreading of droplets: The directional spreading behavior of 5 μL droplets on the surfaces of homogeneous and gradient membranes was recorded using a JC2000D1 contact angle meter. At the same time, the spreading lengths of droplets of different volumes (5-30 μL) on homogeneous and gradient membranes were compared.

[0034] On the fiber membrane surface with a gradient of GO content, the wetting behavior of the droplets is anisotropic. Specifically, the droplets can spread in a directional manner under the action of the surface wettability gradient. Fig.15As shown in the figure, the wetting behaviors of droplets on the surfaces of hydrophobic non-gradient membrane (PAN), hydrophilic non-gradient membrane (GO@PAN, the mass ratio of GO to PAN is 15:100, prepared by electrospinning) and gradient membrane GO / PAN are compared. On the homogeneous membrane GO@PAN, the droplets can spread evenly on both sides of the needle, and under the action of capillary force, the droplets can spread isotropically around, and there is no difference in wetting behavior. Similarly, the droplets on the hydrophobic membrane PAN are spherical and stable on the fiber membrane surface without any wettability difference. However, for the gradient membrane with a gradient change in wettability, the droplets adhere to the fiber membrane surface at 1 s, and then gradually spread to the more hydrophilic area under the action of hydrophilic force. In the entire wetting behavior, it can be seen that the hydrophilic area on the right side of the droplet front gradually spreads, and the trailing edge of the droplet is pinned in the hydrophobic area on the left.

[0035] In order to further characterize the effect of the wettability difference between the hydrophobic non-gradient membrane PAN, the hydrophilic non-gradient membrane GO@PAN and the gradient membrane GO / PAN on the droplet spreading behavior, the spreading length of droplets of different volumes on the fiber membrane surface was measured. Fig.16 As shown in the figure, the droplet length on the PAN fiber membrane is the diameter of the droplet volume, because the droplet on the hydrophobic membrane PAN is difficult to spread. Even if the droplet increases to 30 μL, the length only changes from 1.82 mm to 4.05 mm, and directional wetting behavior cannot occur. Similarly, on the hydrophilic surface of GO@PAN, the droplet can spread evenly on the membrane surface. The length of the droplet is the size of the spreading wetting area. The spreading length of the 5 μL droplet volume is 3.45 mm, and the spreading length increases to 6.17 mm when the droplet volume is 30 μL. The droplet spreading length on the hydrophilic fiber membrane surface increases with the increase of the droplet volume. However, on the surface with a wettability gradient, the droplet spreading length of different volumes has obvious differences. The spreading length of the 5 μL droplet is 4.22 mm, and the spreading length of the 30 μL droplet is 10.65 mm, which increases by 2.52 times. The wettability gradient on the gradient membrane surface enables the droplets to spread in a directional manner, and the front of the droplets diffuses toward the hydrophilic area under the action of the hydrophilic force. The GO / PAN gradient membrane can significantly enhance the spreading behavior of the liquid, and the spreading length of a 15 μL droplet (7.45 mm) is higher than the spreading length of a 30 μL droplet on the hydrophilic membrane GO@PAN (6.17 mm). Therefore, continuously dripping liquid on the surface of the wettability gradient membrane can achieve directional spreading of the liquid.

[0036] 7. Liquid spreading of gradient membranes Liquid spreading test method: The fiber membrane is cut into strips with a width of 4 mm, pasted on a glass sheet, placed on a flat surface, and then pre-wetted. Liquid droplets are continuously squeezed into both sides of the fiber membrane with a syringe to examine the transmission characteristics of the liquid, and the transmission behavior of the liquid is studied by video playback.

[0037] Similarly, the gradient membrane is pre-wetted to expel the air in the membrane pores and form a water film on the membrane surface, which helps the continuous transmission of the liquid. Then, on the pre-wetted gradient membrane, liquid is continuously dripped in the direction of increasing hydrophilicity and increasing hydrophobicity to observe the directional spreading behavior of the liquid.

[0038] like Fig.17 As shown in the figure, in the direction of transition from hydrophobic to hydrophilic, the liquid can be continuously and quickly transferred to the hydrophilic end, where more deionized water is enriched. The liquid on the entire membrane is in a cone shape, that is, the curvature on the left is greater than the curvature on the right. Such a change in liquid curvature will cause a Laplace pressure difference on both sides of the liquid, driving the droplets to be transported to the side with a larger curvature. Fig.18 As shown in the figure, in the direction of increasing hydrophilicity in the side view, the droplet is transferred to the hydrophilic end at 1 s and is continuously transferred. At 8 s, it can be seen that the liquid level on the hydrophilic side is higher than that on the hydrophobic side, and the droplet is continuously enriched on the hydrophilic end. On the contrary, in the direction of increasing hydrophobicity, the droplet is difficult to be transferred to the hydrophilic end. At 3 s, the liquid level on the hydrophilic side is higher than that on the hydrophobic side, and more liquid stays on the hydrophilic end and cannot be transferred to the hydrophobic side. It is not until 10 s that the static pressure of the droplet is greater than the wettability gradient force, and then the droplet partially diffuses to the hydrophobic side.

[0039] From the above description, it can be seen that by controlling the change in the pulling speed of the PAN substrate membrane on the blade electrode, the deposition amount of GO / PAN fibers on the PAN membrane surface is changed in a gradient manner through electrospinning. The wettability gradient is constructed by changing the chemical composition, i.e., the change in GO, and the contact angle can be changed from 0° to 133°. This method of preparing a gradient membrane by controlling the change in the pulling speed of the PAN substrate membrane on the blade electrode has excellent application prospects.

[0040] In summary, the present invention provides a gradient membrane based on continuous distribution of GO-PAN components and a controllable construction method thereof, which belongs to the field of gradient membranes. In order to prepare a gradient membrane with a wider range of wettability changes; a hydrophobic material PAN and a hydrophilic modified material graphene oxide GO are selected to construct a wettability gradient membrane; PAN has a certain hydrophobicity, and a small amount of graphene oxide is added to the polyacrylonitrile fiber membrane to 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 is realized on the surface of the PAN membrane through electrospinning. The wettability gradient is constructed by utilizing the change of chemical components, namely the change of GO, and the contact angle is realized from 0° to 133°. The present invention has excellent application prospects in the directional spreading of droplets on the surface of the gradient membrane and in micro-reaction devices.

[0041] 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 controllable construction method of gradient membrane based on continuous distribution of GO-PAN components, characterized in that: The controllable construction method comprises the following steps: Step S1: PAN spinning solution preparation Weigh polyacrylonitrile PAN powder, dissolve it in a solvent, and stir it to form a uniform spinning solution, which is the PAN spinning solution preparation; Step S2: PAN fiber membrane preparation At room temperature, PAN fiber membrane was electrospun and used as the base membrane to receive GO-PAN fiber filaments; Step S3: GO-PAN spinning solution preparation Weighing graphene oxide GO into a solvent, dispersing it by ultrasonication, then weighing PAN powder into the graphene oxide dispersion, stirring to fully dissolve it to form a uniform spinning solution; that is, GO-PAN spinning solution; Step S4: Preparation of gradient membranes with continuous distribution of GO-PAN components The PAN fiber membrane prepared in step S2 is placed on the blade electrode, and the pulling speed of the PAN fiber membrane is set to decrease linearly. Then, electrospinning is performed above the blade electrode using the GO-PAN spinning solution prepared in step S3 to receive the GO / PAN spinning solution to form a gradient membrane with a continuously changing GO-PAN component content on the surface of the PAN fiber membrane base membrane.

2. A method for controllably constructing a gradient membrane based on continuous distribution of GO-PAN components as claimed in claim 1, characterized in that: In step S1, the concentration of PAN powder in the solvent is 10%-20% by mass volume.

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

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

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

6. A method for controllably constructing a gradient membrane based on continuous distribution of GO-PAN components as claimed 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. A method for controllably constructing a gradient membrane based on continuous distribution of GO-PAN components as claimed in claim 1, characterized in that: In step S3, the solvent is DMF, and the stirring time is more than 10 hours.

8. A method for controllably constructing a gradient membrane based on continuous distribution of GO-PAN components as claimed in claim 1, characterized in that: The spinning parameters in step S4 are: Under the conditions of positive voltage of 10-20 kV and negative voltage of -3~-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. A method for controllably constructing a gradient membrane based on continuous distribution of GO-PAN components as claimed in claim 1, characterized in that: In step S4, the pulling speed of the PAN fiber membrane is linearly decreased from 2 mm / min to 0.01 mm / min.

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

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