Porous membranes, methods of making and using the same

Abstract

The application relates to the field of functional polymer film materials, and discloses a porous film and a preparation method and application thereof. Two surfaces of the porous film are a hydrophilic film surface and a hydrophobic film surface; wherein the pore diameter of the porous film is 0.05-20 mu m, the average pore diameter is 2-15 mu m, the porosity is 35-80 vol.%, and the porous film is distributed with micro-protrusion structures. The porous film can effectively reduce water evaporation, the surface energy of the hydrophobic layer is relatively high, is suitable for water or mist collection, has good stability, is easy to recycle, and can avoid water body pollution caused by residual solvents on the film surface.

Description

Technical Field

[0001] This invention relates to the field of functional polymer membrane materials, specifically to a porous membrane, its preparation method, and its applications. Background Technology

[0002] Water is the most basic substance for all living things to survive. A lack of water will have serious consequences. Many parts of the world are facing more frequent and intense extreme heat weather, which seriously affects the allocation and use of water resources, especially in arid regions. Large-scale evaporation is one of the important reasons for the loss of water resources in the world. Water evaporates during the process of open reservoirs, rivers and farmland irrigation, resulting in the loss of a large amount of water. Therefore, effectively reducing water evaporation is a huge challenge for scientists.

[0003] Previous technologies for preventing water evaporation have proposed some good strategies, but problems still exist, such as complicated preparation processes, high costs, long-term use resulting in solvent or monomer residues polluting water sources, and inability to recycle after use. In order to overcome the shortcomings of existing technologies, it is necessary to continue to develop materials that reduce water evaporation.

[0004] Therefore, there is a need to develop a material that can not pollute water bodies, is easy to recycle, can better reduce water evaporation, and can be used for water or fog collection. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned problems in the prior art and provide a porous membrane, its preparation method and application. The porous membrane can effectively reduce water evaporation, and the hydrophobic layer has a high surface energy, making it suitable for water or mist collection. It also has good stability, is easy to recycle, and can avoid water pollution caused by residual solvents on the membrane surface.

[0006] To achieve the above objectives, the first aspect of the present invention provides a porous membrane, wherein the two sides of the porous membrane are a hydrophilic membrane surface and a hydrophobic membrane surface, respectively.

[0007] The porous membrane has a pore size of 0.05-20 μm, an average pore size of 2-15 μm, and a porosity of 35-80% by volume. The porous membrane is distributed with micro-protrusion structures.

[0008] The second aspect of the present invention provides a method for preparing a porous membrane, the method comprising: forming a membrane using a hydrophilic casting solution and a hydrophobic casting solution, with or without a supporting substrate, and performing a droplet bath to form a membrane with one side being a hydrophilic membrane surface and the other side being a hydrophobic membrane surface;

[0009] The conditions for the droplet bath include: a time of 30-400 s, a droplet size of 5-80 μm, and a relative density of 1 m. 2 The porous membrane has a droplet consumption rate of 3-40 g / h.

[0010] A third aspect of the present invention provides a porous membrane prepared by the method described above.

[0011] The fourth aspect of the present invention provides the application of porous membranes as described in the first or third aspect in reducing water evaporation, collecting water or fog.

[0012] Through the above technical solution, the present invention can achieve the following beneficial effects:

[0013] 1. The porous membrane provided by this invention can effectively reduce water evaporation, has a high surface energy of its hydrophobic layer, making it suitable for water or mist collection. It can remain stable for a long time and avoids water pollution caused by residual solvents on the membrane surface. In particular, the micro-protrusion structure on the membrane increases its specific surface area, further improving its water or mist collection performance. The membrane of this invention can maintain close contact with the water surface, avoiding environmental pollution caused by difficult recycling. Even after multiple cycles, the effectiveness in reducing water evaporation and collecting mist remains essentially unchanged, demonstrating relatively stable performance.

[0014] 2. The preparation method of the present invention does not require expensive equipment or complex processes, has strong universality, a wide range of substrate options, and is simple to prepare on a large scale. It has the advantages of being conducive to practical application production, having low production costs, and being widely applicable. Attached Figure Description

[0015] Figure 1 These are scanning tunneling microscope (SEM) images of the porous membrane prepared in Example 1;

[0016] Figure 2 This is a scanning tunneling microscope image of the porous membrane prepared in Comparative Example 1;

[0017] Figure 3 This is a scanning tunneling microscope image of the porous membrane prepared in Comparative Example 2. Detailed Implementation

[0018] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0019] In a first aspect, the present invention provides a porous membrane, wherein the two sides of the porous membrane are a hydrophilic membrane surface and a hydrophobic membrane surface, respectively.

[0020] The porous membrane has a pore size of 0.05-20 μm, an average pore size of 2-15 μm, and a porosity of 35-80% by volume. The porous membrane is distributed with micro-protrusion structures.

[0021] It is understandable that the terms "pore size" and "average pore size" are different concepts. Pore size refers to the distribution range of pore sizes on the membrane, that is, the range from the smallest pore size to the largest pore size; while average pore size refers to the range of pore sizes of more than 90% of the pores on the membrane.

[0022] The inventors of this invention discovered during their research that the porous membrane described above has different hydrophilic and hydrophobic properties on both sides. When the hydrophilic side of the porous membrane is placed on a water surface as the water-near side, the hydrophilic membrane surface has good wettability and can come into close contact with water. The adhesion between the porous membrane and the water surface is stronger, and water can enter the pores with a larger specific surface area, allowing the porous membrane to adsorb more water. However, due to the hydrophobicity of the hydrophobic side, water does not easily pass through the pores and reach the hydrophobic membrane surface, making it difficult for water to evaporate. At the same time, because the hydrophobic membrane surface has a high surface energy, water droplets falling on the hydrophobic membrane surface can form a spherical shape and roll into the water. The close contact between the hydrophilic membrane surface and the water can prevent it from being blown away by the wind, facilitate recycling, and avoid environmental pollution caused by non-recyclable materials. Furthermore, if the membrane is placed perpendicular to the ground, in conditions of high air humidity, the hydrophilic membrane surface, with its good wettability, will allow the droplets to spread on the surface and permeate into the membrane through the pores. However, the hydrophobic surface, with its poor wettability, will restrict further water penetration, causing the droplets to accumulate on the hydrophilic membrane surface. Once they reach a certain weight, they will quickly drip off under gravity, thus being collected and achieving the purpose of water or fog collection. In particular, the micro-protrusion structure on the membrane can further increase the specific surface area of ​​the membrane, further improving the water or fog collection performance. Therefore, the porous membrane described above can be used to cover the surface of lakes, reservoirs, etc., to reduce or prevent water evaporation, and can also be used for water collection (including rain), or for fog collection in high-humidity fog environments.

[0023] According to the present invention, preferably, the porous membrane has a pore size of 0.1-10 μm, an average pore size of 3-10 μm, and a porosity of 40-70% by volume. A pore size and porosity that meet the above conditions can simultaneously provide a stronger adsorption force on water to the hydrophilic membrane surface and a stronger repulsion force on water to the hydrophobic membrane surface. Under the synergistic effect of the hydrophilic and hydrophobic membrane surfaces, a better water collection effect can be obtained, and water evaporation can be further prevented.

[0024] According to the present invention, preferably, the thicknesses of the hydrophilic membrane surface and the hydrophobic membrane surface are each independently 50-160 μm (e.g., 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm), more preferably, the thickness of the hydrophilic membrane surface is 75-130 μm, and the thickness of the hydrophobic membrane surface is 80-135 μm. It is understood that if the thickness of the porous membrane is greater, the weight of the porous membrane is generally also greater, and the membrane's buoyancy is also affected to some extent. Hydrophilic membrane surfaces have a certain adsorption capacity for water, while hydrophobic membrane surfaces have a certain repulsive resistance to water. The inventors of this invention further discovered that when the thicknesses of the hydrophilic and hydrophobic membrane surfaces meet the requirements described above, the hydrophilic membrane surface can more effectively adsorb water, and the hydrophobic membrane surface can more effectively repel water attempting to pass through the porous membrane. The combination of the hydrophilic and hydrophobic membrane surfaces can better reduce water evaporation while ensuring that the porous membrane can float on the water surface. In water or mist collection, if the membrane thickness is too large, excessive mist droplets will penetrate into the membrane, easily leading to a decrease in surface water collection efficiency. The inventors of this invention further discovered that meeting the above-mentioned range can further ensure the membrane's water collection efficiency.

[0025] According to the present invention, preferably, the hydrophilic film surface is formed of a hydrophilic material, and the hydrophobic film surface is formed of a hydrophobic material.

[0026] According to the present invention, preferably, the weight-average molecular weight of the hydrophilic substance is 50,000-200,000 g / mol.

[0027] According to the present invention, preferably, the weight-average molecular weight of the hydrophobic substance is 50,000-800,000 g / mol.

[0028] According to the present invention, preferably, the hydrophilic substance is selected from at least one of cellulose acetate, polyacrylamide, polyacrylonitrile, chitosan, chitosan, polyacrylic acid, polydopamine, polylactic acid and polyvinylpyrrolidone, more preferably at least one of polyacrylonitrile, polydopamine, polyvinylpyrrolidone, cellulose acetate and chitosan.

[0029] According to the present invention, preferably, the hydrophobic substance is selected from at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinyl chloride, silane, polyethersulfone and polystyrene, more preferably at least one of polysulfone, polyvinylidene fluoride, polystyrene, methyltrichlorosilane and phenyltrisilane.

[0030] It is understood that a supporting substrate capable of providing support can also be selected, with a hydrophilic film surface and a hydrophobic film surface formed on both sides of the supporting substrate. The selection of the supporting substrate is based on the principle of not affecting the water adsorption or water repulsion performance of the hydrophilic and hydrophobic film surfaces. Generally, a supporting substrate with larger pores can be selected, which better achieves the purpose of not affecting the performance of the hydrophilic and hydrophobic film surfaces, and thus does not affect the water collection or fog collection performance of the porous membrane. According to the present invention, preferably, there is or is not a supporting substrate between the hydrophilic and hydrophobic film surfaces, and the pore size of the supporting substrate is 30-80 μm; more preferably, the thickness of the supporting substrate is 60-120 μm (for example, it can be 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm), preferably 80-100 μm. It is understood that when a supporting substrate is present, a hydrophilic film surface and a hydrophobic film surface can be formed on both sides of the supporting substrate, respectively; when there is no supporting substrate, one film surface is formed first, and then a film surface with opposite wettability is formed on one side of that film surface. The specific material of the supporting substrate is not particularly limited, as long as it meets the parameters mentioned above, especially the pore size parameters. For example, the supporting substrate can be selected from polypropylene film, polyethylene terephthalate film, and polytetrafluoroethylene film.

[0031] It's understandable that the water contact angle refers to the angle (denoted as θ) formed when a liquid interface contacts a solid surface. If θ < 90°, the solid surface is generally hydrophilic, meaning the liquid easily wets the solid; the smaller the angle, the better the wettability. If θ > 90°, the solid surface is hydrophobic, meaning the liquid does not easily wet the solid; the larger the angle, the worse the wettability. It's also understandable that when a membrane and water are in contact, the contact angle between them may change over time, but it stabilizes after a certain period and no longer changes. Therefore, the angle that no longer changes over time is generally taken as the water contact angle.

[0032] According to the present invention, preferably, the water contact angle of the hydrophilic membrane surface is 0-8° (for example, it can be 0°, 10°, 20°, 30°, 40°, 50°, or 60°), more preferably 0-1°. When the above-described range is met, the better the wettability of the hydrophilic membrane surface, the closer it can adhere to the water surface, preventing water evaporation, and the stronger the adhesion to collect water or mist, thus obtaining a higher amount of water or mist collected.

[0033] According to the present invention, preferably, the water contact angle of the hydrophobic film surface is 105-150° (for example, it can be 105°, 100°, 110°, 120°, 130°, 140°, or 150°), more preferably 110-140°. When the above conditions are met, the hydrophobic surface has a stronger repulsive force on water, which can better prevent water in the pores from seeping to the upper surface, further reducing the loss of water evaporation. Regarding water collection performance, the hydrophobic surface restricts further water penetration, promotes the aggregation of droplets on the hydrophilic film surface, and after reaching a certain weight, they quickly drip down under the action of gravity, thereby enabling collection.

[0034] According to the present invention, preferably, the time for the water contact angle of the hydrophilic membrane surface to reach 0° is 0-5s. It is understood that if the water contact angle of the hydrophilic membrane surface can reach 0°, it indicates that the hydrophilicity of the hydrophilic membrane surface is good; furthermore, the shorter the time to reach 0°, the better the hydrophilicity of the hydrophilic membrane surface.

[0035] Secondly, the present invention provides a method for preparing a porous membrane, the method comprising: forming a membrane using a hydrophilic casting solution and a hydrophobic casting solution, with or without a supporting substrate, and subjecting the membrane to a droplet bath to form a membrane with one hydrophilic side and the other hydrophobic side; wherein the conditions of the droplet bath include: a time of 30-400 s, a droplet size of 5-80 μm, and a relative density of 1 μm. 2 The porous membrane has a droplet consumption rate of 3-40 g / h.

[0036] The hydrophilic casting solution refers to a solution of a substance capable of forming a hydrophilic film surface, and the hydrophobic casting solution refers to a solution of a substance capable of forming a hydrophobic film surface.

[0037] It is understood that the droplet bath refers to placing the membrane in an atomized droplet environment, allowing smaller droplets to contact the membrane. Furthermore, the droplet bath can generally be achieved using an atomizing device, and the "droplet size" refers to the size of the droplets ejected by the atomizing device. It is understood that the size of the droplets ejected by the atomizing device is not a fixed value, but rather falls within a range. The inventors discovered in their research that, using the above method, pores with suitable pore size and porosity can be formed on the membrane, thereby giving the membrane excellent anti-evaporation and water collection performance. In addition, this allows the prepared membrane to have micro-protrusion structures distributed on it, which can increase the specific surface area of ​​the membrane, further improving its anti-evaporation and water collection performance. Using the method described above, a membrane with opposite wettability on both sides can be prepared, which can be used to reduce or prevent water evaporation, and can also be used for water collection (including rain), fog collection, etc.

[0038] The porous membrane has two sides, both of the same size, and the term "relative to 1m" is used.2 "Porous membrane" "1m 2 The area of ​​"" refers to the area of ​​one side of the porous membrane.

[0039] According to the present invention, the order in which the hydrophilic and hydrophobic film surfaces are formed is not particularly limited; the hydrophilic film surface can be formed first, followed by the hydrophobic film surface, or vice versa. However, preferably, a hydrophilic casting solution is first used to form the film, followed by a first droplet bath to obtain the hydrophilic film surface; then, a hydrophobic casting solution is used to form the film on one side of the hydrophilic film surface, followed by a second droplet bath to obtain the hydrophobic film surface; wherein the first and second droplet baths are each independently 15-200 seconds. After obtaining the hydrophilic film surface, the film can be dried, for example, by placing it in an oven at 60-65°C for 1.5-2.5 hours to dry, or by air-drying it under natural ventilation for 2-3 days, and then forming the hydrophobic film surface on one side of the dried film. It is understandable that when a supporting substrate is present, a hydrophilic film is first formed on one side of the supporting substrate; when no supporting substrate is present, the hydrophilic film is formed first. "One side of the hydrophilic film" can be any side of the hydrophilic film. By using the above method of first forming a hydrophilic film and then forming a hydrophobic film, a porous membrane with better evaporation reduction and fog collection performance can be obtained. Furthermore, in this case, the droplet bath includes a first droplet bath and a second droplet bath.

[0040] According to the present invention, preferably, the concentrations of the hydrophilic casting solution and the hydrophobic casting solution are each independently 4-30% by mass.

[0041] According to the present invention, preferably, the concentration of the hydrophilic material casting solution is 8-18% by mass (for example, it can be 8% by mass, 9% by mass, 10% by mass, 11% by mass, 12% by mass, 13% by mass, 14% by mass, 15% by mass, 16% by mass, 17% by mass, or 18% by mass).

[0042] According to the present invention, preferably, the concentration of the hydrophobic material casting solution is 5-15% by mass (for example, it can be 5% by mass, 6% by mass, 7% by mass, 8% by mass, 9% by mass, 10% by mass, 11% by mass, 12% by mass, 13% by mass, 14% by mass, or 15% by mass).

[0043] The inventors of this invention discovered in their research that when the concentration ranges of the hydrophilic and hydrophobic casting solutions are met, the problems of low solution viscosity and difficulty in film formation can be avoided, while the problems of high concentration causing solution viscosity, difficulty in film scraping, and bubble generation can also be avoided.

[0044] According to the present invention, preferably, the weight-average molecular weight of the hydrophilic substance is 50,000-200,000 g / mol.

[0045] According to the present invention, preferably, the weight-average molecular weight of the hydrophobic substance is 50,000-800,000 g / mol.

[0046] According to the present invention, preferably, the hydrophilic substance is selected from at least one of cellulose acetate, polyacrylamide, polyacrylonitrile, chitosan, chitosan, polyacrylic acid, polydopamine, polylactic acid and polyvinylpyrrolidone, more preferably at least one of polyacrylonitrile, polydopamine, polyvinylpyrrolidone, cellulose acetate and chitosan.

[0047] According to the present invention, preferably, the hydrophobic substance is selected from at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinyl chloride, silane, polyethersulfone and polystyrene, more preferably at least one of polysulfone, polyvinylidene fluoride, polystyrene, methyltrichlorosilane and phenyltrisilane.

[0048] According to the present invention, the solvents of the hydrophilic and hydrophobic casting solutions are not particularly limited, as long as they have strong solubility for the polymer solute and the change in interaction energy during the mixing of the polymer solute and the solvent is small. Preferably, the solvents of the hydrophilic and hydrophobic casting solutions are each independently selected from at least one of chloroform, dimethyl sulfoxide, tetrahydrofuran, toluene, benzene, hexane, octane, acetone, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and dioxane, more preferably at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and toluene. The interaction parameter χ (also known as the Huggins parameter) between the above solvents and the hydrophilic / hydrophobic substances is less than 0.5. Using the solvents described above results in stronger solubility for the polymer solute and a smaller change in interaction energy during the mixing process. The hydrophilic / hydrophobic substance and the solvent can be mixed separately, and then heated and stirred to obtain a clear solution.

[0049] According to the present invention, the film-forming method can employ methods commonly used in the art, but preferably, the film-forming method is selected from one of the following: casting, casting, and scraping. The film-forming method can be selected according to the production scale and needs. For example, casting and casting can be selected for continuous production in a factory, while scraping can be used in a laboratory. Taking scraping as an example, a doctor blade can be used to form a film on a clean glass plate using a hydrophilic / hydrophobic casting solution.

[0050] According to the present invention, preferably, the film-forming conditions result in the thicknesses of the hydrophilic and hydrophobic film surfaces being independently 50-160 μm; more preferably, the film-forming conditions result in the thickness of the hydrophilic film surface being 75-130 μm and the thickness of the hydrophobic film surface being 80-135 μm. It is understood that in actual operations such as film-coating, the set thickness of the hydrophilic or hydrophobic film surface to be coated will differ slightly from the final thickness of the hydrophilic or hydrophobic film surface on the porous membrane. Generally, the final thickness of the hydrophilic or hydrophobic film surface will be about 5-10 μm less than the thickness set during coating. Therefore, to obtain a suitable thickness, the set thickness can be adjusted to the desired thickness plus 5-10 μm during film formation.

[0051] According to the present invention, although a porous membrane capable of effectively reducing water evaporation and suitable for water or mist collection can be obtained by using the method described above, it is preferable that the times of the first and second droplet baths are each 20-80 s independently. When the conditions described above are met, a membrane with a more suitable pore size and porosity can be obtained. The droplet bath can be operated using equipment conventional in the art, such as pressure atomization equipment (e.g., high-pressure airflow atomization equipment) or acoustic atomization equipment; the droplet size of the droplet bath is not particularly limited and can be a particle size commonly used in atomization operations, such as 1-50 μm. The conditions for forming the hydrophilic film surface and the conditions for forming the hydrophobic film surface of the droplet bath can be the same or different, preferably the same. The humidity of the droplet bath is not particularly limited and can be a relatively low humidity condition with a humidity of less than 40% or a relatively high humidity condition with a humidity of greater than or equal to 40%.

[0052] According to the present invention, preferably, after the droplet bath, the method further includes: performing non-solvent-induced phase separation on the material to remove the solvent introduced by the hydrophilic casting solution and the hydrophobic casting solution. The non-solvent-induced phase separation time is 0.3-10 min. Taking a hydrophilic membrane surface as an example, the non-solvent-induced phase separation refers to the process where, because a hydrophilic casting solution containing solvent is used in film formation, the solvent in the formed membrane is not removed. In this case, the membrane is placed in a reagent (or extractant) with stronger miscibility with the solvent in it to extract the solvent, removing the residual solvent from the membrane and allowing the membrane to solidify. This ensures that residual solvent will not be introduced into the water body during subsequent use, achieving reduced water evaporation or water / fog collection without impacting the environment. Deionized water can be used as the extractant. The inventors of this invention also discovered in their research that solvent removal plays a crucial role in pore size and porosity. Incomplete solvent removal prevents pore formation in areas with residual solvent, resulting in smaller pore size and porosity. By using the method described above, solvent removal can be ensured, resulting in a porous membrane with suitable pore size and porosity, and residual solvent pollution of water bodies can be avoided.

[0053] Thirdly, the present invention provides a porous membrane prepared by the method described above.

[0054] Fourthly, the present invention provides the application of porous membranes as described in the first or third aspect of the present invention in reducing water evaporation, collecting water or fog.

[0055] According to a particularly preferred embodiment of the present invention, a porous membrane is prepared in the following manner:

[0056] 1) Using DMF as a solvent, chitosan and cellulose acetate are added and stirred to prepare a homogeneous, transparent, hydrophilic casting solution with chitosan and cellulose acetate concentrations of 9.5-10.5% by mass and 3.5-4.5% by mass, respectively. This solution is then scraped onto a clean polytetrafluoroethylene (PTFE) support substrate (70-90 μm thick) using a doctor blade (with a thickness of 105-109 μm) to form a film. A first droplet bath is then performed, with the following conditions: droplet size of 5-80 μm, time of 28-32 s, relative to 1 m... 2 The porous membrane was prepared with a droplet consumption rate of 8-13 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 4-6 minutes, yielding a membrane with one hydrophilic side. The resulting membrane was then dried in an oven at 60-62℃ for 1.8-2.2 hours for later use.

[0057] 2) Using DMF as a solvent, add polystyrene and stir to prepare a uniform, transparent, hydrophobic casting solution with a concentration of 9.5-10.5% by mass. Then, use a doctor blade (set the thickness to 102-108 μm) to coat one side of the dried film. Next, perform a second droplet bath. The conditions for the second droplet bath include: droplet size of 5-80 μm, time of 45-55 s, and relative to 1 m... 2 The porous membrane was subjected to a droplet consumption rate of 8-13 g / h. Finally, it was placed in deionized water for non-solvent-induced phase separation for 4-6 minutes. The resulting membrane had one hydrophilic side and the other hydrophobic side.

[0058] The present invention will be described in detail below through examples. In the following examples, all chemical reagents used are commercially available products and have not undergone special purification treatment, unless otherwise specified.

[0059] Polyacrylonitrile, with a weight-average molecular weight of 58,000 g / mol, was purchased from Shaoxing Jiema Composite Materials Co., Ltd. in China, under the grade P60C.

[0060] Polydopamine, with a weight-average molecular weight of 50,000 g / mol, was purchased from Solvay Group.

[0061] Polysulfone, with a weight-average molecular weight of 22,000 g / mol, was purchased from Innocare Technology Co., Ltd.

[0062] Fiber acetate, with a weight-average molecular weight of 55,000 g / mol, was purchased from Solvay Group.

[0063] Polyvinylidene fluoride, with a weight-average molecular weight of 300,000 g / mol, was purchased from Solvay Group.

[0064] Chitosan, with a weight-average molecular weight of 60,000 g / mol, was purchased from Innocare Technology Co., Ltd.

[0065] Methyltrichlorosilane, purchased from Innocare Technology Co., Ltd.

[0066] Polystyrene, with a weight-average molecular weight of 100,000 g / mol, was purchased from Xilong Chemical Co., Ltd.

[0067] Polyvinylpyrrolidone, with a weight-average molecular weight of 58,000 g / mol, was purchased from Xilong Chemical Co., Ltd.

[0068] Toluene was purchased from Xilong Chemical Co., Ltd.

[0069] N-methylpyrrolidone, NN-dimethylformamide, and NN-dimethylacetamide were all purchased from Xilong Chemical Co., Ltd.

[0070] Example 1

[0071] 1) Using DMF as a solvent, polyacrylonitrile was added and stirred to prepare a homogeneous and transparent hydrophilic casting solution with a concentration of 8% by mass. This solution was then scraped onto a clean glass plate (without a supporting substrate) using a doctor blade (set to a thickness of 82 μm). A first droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 30 s, relative to 1 m... 2 A porous membrane was prepared with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 2 minutes, yielding a porous membrane with one hydrophilic side. The resulting membrane was dried in a 60°C oven for 2 hours before use.

[0072] 2) Using DMAc as a solvent, polysulfone was added and stirred to prepare a homogeneous, transparent, hydrophobic material with a concentration of 10% by mass. This material was then coated onto one side of the dried film using a doctor blade (with a thickness set at 86 μm). A second droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 20 s, relative to 1 m... 2 A porous membrane was prepared, with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 2 minutes. The final result was a membrane with one hydrophilic side and the other hydrophobic side. STM imaging of the membrane was performed, and the results are shown below. Figure 1 .

[0073] Example 2

[0074] 1) Using DMAc as a solvent, polyacrylonitrile and polydopamine were added and stirred to prepare a homogeneous, transparent, hydrophilic casting solution with a concentration of 8% by mass for both polyacrylonitrile and polydopamine. This solution was then scraped onto a clean polypropylene support substrate (90 μm thick) using a doctor blade (set to a thickness of 126 μm) to form a film. A first droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 40 s, relative to 1 m... 2 A porous membrane was prepared, with a droplet consumption rate of 40 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 3 minutes, yielding a membrane with one hydrophilic side. The resulting membrane was then dried in a 60°C oven for 2 hours for later use.

[0075] 2) Using DMF as a solvent, polyvinylidene fluoride was added and stirred to prepare a homogeneous, transparent, hydrophobic casting solution with a concentration of 15% by mass. This solution was then coated onto one side of the dried film using a doctor blade (with a thickness set at 127 μm). A second droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 80 s, and relative to 1 m... 2 A porous membrane was used, with a droplet consumption rate of 40 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 2 minutes. The final result was a membrane with one hydrophilic side and the other hydrophobic side.

[0076] Example 3

[0077] 1) Using NMP as a solvent, chitosan and cellulose acetate were added and stirred to prepare a homogeneous, transparent, hydrophilic casting solution with chitosan and cellulose acetate concentrations of 6% and 8% by mass, respectively. The solution was then scraped onto a clean glass plate (without a support substrate) using a doctor blade (set to a thickness of 120 μm) to form a film. A first droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 40 s, relative to 1 m... 2 A porous membrane was prepared, with a droplet consumption rate of 3 g / h. Finally, it was placed in deionized water for non-solvent-induced phase separation for 5 minutes, yielding a membrane with one hydrophilic side. The resulting membrane was then dried in a 60°C oven for 2 hours for later use.

[0078] 2) Using toluene as a solvent, methyltrichlorosilane was added and stirred to prepare a homogeneous, transparent, hydrophobic casting solution with a concentration of 5% by mass. This solution was then coated onto one side of the dried film using a doctor blade (with a thickness set at 126 μm). A second droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 30 s, and relative to 1 m... 2 A porous membrane was used, with a droplet consumption rate of 3 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 0.3 min. The final result was a membrane with one hydrophilic side and the other hydrophobic side.

[0079] Example 4

[0080] 1) Using DMF as a solvent, chitosan and cellulose acetate were added and stirred to prepare a homogeneous, transparent, hydrophilic casting solution with chitosan and cellulose acetate concentrations of 6% and 8% by mass, respectively. This solution was then scraped onto a clean polyethylene terephthalate (PET) substrate (100 μm thick) using a doctor blade (set to a thickness of 106 μm) for film formation. A first droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 40 s, relative to 1 m... 2 A porous membrane was prepared, with a droplet consumption rate of 20 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 2 minutes, yielding a membrane with one hydrophilic side. The resulting membrane was then dried in a 60°C oven for 2 hours for later use.

[0081] 2) Using toluene as a solvent, add methyltrichlorosilane and stir to prepare a homogeneous, transparent hydrophobic casting solution with a concentration of 5% by mass. Then, use a doctor blade (set to a thickness of 117 μm) to coat the back side of the dried hydrophilic film. A second droplet bath is then performed, with the following conditions: droplet size of 5-80 μm, time of 30 s, relative to 1 m... 2 A porous membrane was used, with a droplet consumption rate of 20 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 3 minutes. The final result was a membrane with one hydrophilic side and the other hydrophobic side.

[0082] Example 5

[0083] 1) Using DMF as a solvent, chitosan and cellulose acetate were added and stirred to prepare a homogeneous, transparent, hydrophilic casting solution with chitosan and cellulose acetate concentrations of 10% and 4% by mass, respectively. This solution was then scraped onto a clean PTFE support substrate (80 μm thick) using a doctor blade (set to a thickness of 106 μm) to form a film. A first droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 30 s, relative to 1 m... 2A porous membrane was prepared, with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 5 minutes, yielding a membrane with one hydrophilic side. The resulting membrane was then dried in a 60°C oven for 2 hours for later use.

[0084] 2) Using DMF as a solvent, add polystyrene and stir to prepare a homogeneous, transparent hydrophobic casting solution with a concentration of 10% by mass. Then, use a doctor blade (set to a thickness of 105 μm) to coat the back side of the dried hydrophilic film. Next, perform a second droplet bath. The conditions for the second droplet bath include: droplet size of 5-80 μm, time of 50 s, and relative to 1 m... 2 A porous membrane was used, with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 5 minutes. The final result was a membrane with one hydrophilic side and the other hydrophobic side.

[0085] Example 6

[0086] 1) Using DMF as a solvent, polyvinylpyrrolidone and cellulose acetate were added and stirred to prepare a homogeneous, transparent, hydrophilic casting solution with polyvinylpyrrolidone and cellulose acetate concentrations of 10% by mass and 8% by mass, respectively. The solution was then scraped onto a clean glass plate (without a support substrate) using a doctor blade (set to a thickness of 136 μm) to form a film. A first droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 20 s, relative to 1 m... 2 A porous membrane was prepared, with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 6 minutes, yielding a membrane with one hydrophilic side. The resulting membrane was then dried in a 60°C oven for 2 hours for later use.

[0087] 2) Using DMF as a solvent, polysulfone was added and stirred to prepare a homogeneous, transparent, hydrophobic casting solution with a concentration of 6% by mass. Then, a doctor blade (with a thickness set at 141 μm) was used to coat one side of the dried film, so that one side was a hydrophilic film surface and the other a hydrophobic film surface; then a second droplet bath was performed, the conditions of which included: droplet size of 5-80 μm, time of 30 s, relative to 1 m... 2 A porous membrane was prepared, with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 6 minutes. The final result was a membrane with one hydrophilic side and the other hydrophobic side.

[0088] Example 7

[0089] 1) Using DMAc as a solvent, cellulose acetate and polydopamine were added and stirred to prepare a homogeneous, transparent, hydrophilic casting solution with cellulose acetate and polydopamine concentrations of 8% by mass and 6% by mass, respectively. This solution was then scraped onto a clean glass plate (without a support substrate) using a doctor blade (set to a thickness of 125 μm) to form a film. A first droplet bath was then performed, with the following conditions: droplet size of 5-80 μm, time of 50 s, relative to 1 m... 2 A porous membrane was prepared, with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 8 minutes, yielding a membrane with one hydrophilic side. The resulting membrane was then dried in a 60°C oven for 2 hours for later use.

[0090] 2) Using DMF as a solvent, add polystyrene and stir to prepare a uniform, transparent, hydrophobic casting solution with a concentration of 8% by mass. Then, use a doctor blade (set to a thickness of 125 μm) to coat one side of the dried film, so that one side is a hydrophilic film surface and the other side is a hydrophobic film surface; then, perform a second droplet bath. The conditions for the second droplet bath include: droplet size of 5-80 μm, time of 30 s, relative to 1 m 2 A porous membrane was prepared, with a droplet consumption rate of 10 g / h. It was then placed in deionized water for non-solvent-induced phase separation for 10 min. The final result was a membrane with one hydrophilic side and the other hydrophobic side.

[0091] Example 8

[0092] The membrane was prepared according to the method of Example 6, except that step 2) was performed first, followed by step 1), that is, the hydrophobic membrane surface was prepared first, and then the hydrophilic membrane surface was prepared.

[0093] Example 9

[0094] The membrane was prepared according to the method of Example 7, except that step 2) was performed first, followed by step 1), that is, the hydrophobic membrane surface was prepared first, and then the hydrophilic membrane surface was prepared.

[0095] Example 10

[0096] The membrane was prepared according to the method of Example 2, except that in steps 1) and 2), the concentration of both the hydrophilic and hydrophobic casting solutions was 4% by mass.

[0097] Example 11

[0098] The membrane was prepared according to the method of Example 2, except that in steps 1) and 2), the concentrations of the hydrophilic casting solution and the hydrophobic casting solution were both 30% by mass.

[0099] Example 12

[0100] The membrane was prepared according to the method of Example 2, except that the first droplet bath and the second droplet bath time were both 15 s.

[0101] Example 13

[0102] The membrane was prepared according to the method of Example 2, except that the time for both the first droplet bath and the second droplet bath was 200 s.

[0103] Example 14

[0104] The membrane was prepared according to the method of Example 2, except that the thickness of both the hydrophilic and hydrophobic membrane surfaces was set to 58 μm using a doctor blade.

[0105] Example 15

[0106] The membrane was prepared according to the method of Example 2, except that the thickness of both the hydrophilic and hydrophobic membrane surfaces was set to 155 μm using a doctor blade.

[0107] Comparative Example 1

[0108] The membrane was prepared according to the method of Example 2, except that the first droplet bath and the second droplet bath were not performed.

[0109] Comparative Example 2

[0110] The membrane was prepared according to the method of Example 2, except that a droplet bath was not performed. Instead, a pore-forming agent, polyethylene glycol, was added when preparing the hydrophilic and hydrophobic casting solutions, and the concentration of polyethylene glycol in both solutions was 10 g / L.

[0111] Test case

[0112] Membranes prepared in Examples 1-15 and Comparative Examples 1-2 were used. The thickness of the hydrophilic membrane surface, the thickness of the hydrophobic membrane surface, the pore size, the average pore size, the porosity, the water contact angle of the hydrophilic membrane surface, and the water contact angle of the hydrophobic membrane surface were measured. Initial water evaporation rate tests and initial fog collection tests were also performed. For membranes with a water contact angle of 0° on the hydrophilic membrane surface, the time required for the water contact angle to reach 0° was measured. The results are shown in Table 1-2.

[0113] Furthermore, the membranes prepared in Examples 1-15 and Comparative Examples 1-2 were imaged using a scanning tunneling microscope (SEM). Images of the membranes prepared in Example 1 and Comparative Examples 1-2 are shown below. Figure 1-3 .

[0114] The thickness of the hydrophilic and hydrophobic film surfaces was measured using a Q / ILBN2-2006 benchtop thin film thickness gauge purchased from Shanghai Precision Instruments Co., Ltd.

[0115] The pore size and average pore size of the membrane were measured using a PSMA-30 instrument purchased from Nanjing Gaoqian Functional Materials Technology Co., Ltd.

[0116] The method for determining porosity is as follows: First, weigh a membrane sample of a certain volume. This weight is the dry weight. Then, immerse the membrane completely in water and weigh it again. This weight is the wet weight of the wet membrane. The difference between the wet weight and the dry weight is the weight of the water retained in the pores. Convert the weight into the volume of water, which is the volume of the pores. Then, divide the volume of the pores by the volume of the membrane sample to obtain the porosity.

[0117] Water contact angle test: A German Kruss DSA100 contact angle meter was used at room temperature. First, the sample to be tested was fixed onto a glass slide, ensuring the membrane surface was flat. When testing the contact angle of liquids in air, the glass slide with the membrane attached was placed directly on the instrument's sample stage. Distilled water was used as the test liquid. The contact angle measured 3 seconds after the droplet touches the membrane surface is the contact angle value for this test.

[0118] The test method for water evaporation rate is as follows: Cut the sample to be tested into a circular piece with a diameter of 5 cm, place it in a small beaker with a diameter of 5 cm containing 50 g of distilled water, and put the small beaker in a constant temperature oven at 50℃ for 20 hours. Test the change in the mass of water before and after 20 hours, and then calculate the water evaporation rate Q. The calculation formula is as follows:

[0119] Q = (m1 - m2) × 100% / m1

[0120] Where m1 and m2 are the weights of water in the container before and after evaporation, respectively. Furthermore, while measuring the water evaporation rate of the membranes prepared in the examples and comparative examples, the water evaporation rate in the membrane-free state was also measured without any membrane covering.

[0121] The method for testing fog collection capacity is as follows: cut the prepared membrane sample into 2×2cm pieces. 2 A square sample was fixed to a support, with a container for collecting water droplets placed underneath. The sample was positioned 20 cm from the atomizing nozzle for mist collection testing. After 60 minutes of collection, the weight of the collected water was measured.

[0122] The membranes prepared using 1-15 and Comparative Example 1-2 were subjected to 5 water evaporation rate tests and 5 fog collection tests, respectively. The water evaporation rate and fog collection amount of each membrane were recorded at the 5th test. The results are shown in Table 3.

[0123] Table 1

[0124]

[0125]

[0126] Table 2

[0127]

[0128] Table 3

[0129]

[0130]

[0131] As can be seen from the results in Tables 1-2, Examples 1-15 using the technical solution of this invention can obtain porous membranes with more suitable pore size and porosity. Furthermore, the hydrophilic membrane surface of these porous membranes has a smaller water contact angle (better wettability in water and closer contact with water), while the hydrophobic membrane surface has a larger water contact angle, resulting in a more significant difference in wettability between the two surfaces. When these porous membranes are used to cover water surfaces, they can better reduce water evaporation; during fog collection, the amount of fog collected per unit area per unit hour is greater, and the fog collection effect is better, with Examples 1-7 showing even better results.

[0132] As can be seen from the data in Table 3, the membranes prepared using the technical solutions of this invention in Examples 1-15 show that the effect of reducing water evaporation and the effect of collecting fog remain basically unchanged after multiple cycles of use, and the effect is relatively stable.

[0133] Furthermore, from Figure 1-3 It can be seen that the method provided by this invention can prepare structures with distributed micro-protrusions. Figure 1 The porous membrane with a granular structure (i.e., micro-protrusions) can further increase the specific surface area of ​​the membrane and further improve its water collection or mist collection performance. Without a droplet bath or a pore-forming agent, the obtained membrane does not have a distributed micro-protrusion structure, and its water collection or mist collection performance is significantly poor. SEM imaging also revealed that the membranes prepared in Examples 2-15 also have a micro-protrusion structure similar to that of Example 1 (the membranes of Examples 2-15 are not shown).

[0134] Furthermore, the porous membrane provided by this invention can avoid water pollution caused by residual solvents on the membrane surface, and because it can have closer contact with the water surface, the membrane is easier to recycle. Moreover, the method of this invention for preparing the membrane does not require expensive equipment or complex processes; the preparation process is highly universal, simple, and easy to scale up, showing good prospects for industrialization.

[0135] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a porous membrane for reducing water evaporation, collecting water, or collecting mist, characterized in that, The method includes: with or without a supporting substrate, first forming a film using a hydrophilic casting solution, then performing a first droplet bath to obtain a hydrophilic film surface; then forming a film using a hydrophobic casting solution on one side of the hydrophilic film surface, then performing a second droplet bath to obtain a hydrophobic film surface; The conditions for the droplet bath include: droplet size of 5-80 μm, relative to 1 m 2 The porous membrane has a droplet consumption rate of 3-40 g / h; The film-forming conditions result in a hydrophilic film thickness of 75-130 μm and a hydrophobic film thickness of 80-135 μm. The duration of the first droplet bath and the second droplet bath are each 20-80 seconds, respectively. The porous membrane has a pore size of 0.05-20 μm, an average pore size of 2-15 μm, and a porosity of 35-80% by volume. The porous membrane is distributed with micro-protrusion structures.

2. The method according to claim 1, wherein, The concentrations of the hydrophilic casting solution and the hydrophobic casting solution are each 4-30% by mass.

3. The method according to claim 2, wherein, The concentration of the hydrophilic casting solution is 8-18 by mass.

4. The method according to claim 2, wherein, The concentration of the hydrophobic casting solution is 5-15% by mass.

5. The method according to claim 1, wherein, The weight-average molecular weight of the hydrophilic substance is 50,000-200,000 g / mol.

6. The method according to claim 1, wherein, The weight-average molecular weight of the hydrophobic substance is 50,000-800,000 g / mol.

7. The method according to claim 1, wherein, The hydrophilic substance is selected from at least one of cellulose acetate, polyacrylamide, polyacrylonitrile, chitin, chitosan, polyacrylic acid, polydopamine, polylactic acid, and polyvinylpyrrolidone.

8. The method according to claim 7, wherein, The hydrophilic substance is selected from at least one of polyacrylonitrile, polydopamine, polyvinylpyrrolidone, cellulose acetate, and chitosan.

9. The method according to claim 1, wherein, The hydrophobic substance is selected from at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinyl chloride, silane, polyethersulfone, and polystyrene.

10. The method according to claim 9, wherein, The hydrophobic substance is selected from at least one of polysulfone, polyvinylidene fluoride, polystyrene, methyltrichlorosilane, and phenyltrisilane.

11. The method according to claim 1, wherein, The solvents of the hydrophilic casting solution and the hydrophobic casting solution are each independently selected from at least one of chloroform, dimethyl sulfoxide, tetrahydrofuran, toluene, benzene, hexane, octane, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dioxane.

12. The method according to claim 11, wherein, The solvents for the hydrophilic casting solution and the hydrophobic casting solution are each independently selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and toluene.

13. The method according to claim 1, wherein, The film-forming method is selected from one of the following: casting, casting, and scraping.

14. The method according to claim 1, wherein, After the droplet bath, the method further includes: performing non-solvent phase separation on the material to remove the solvent introduced by the hydrophilic casting solution and the hydrophobic casting solution; The time for the non-solvent-induced phase separation is 0.3-10 min.

15. The porous membrane prepared by the method according to any one of claims 1-14.

16. The porous membrane according to claim 15, wherein, The porous membrane has a hydrophilic membrane surface and a hydrophobic membrane surface on two sides, respectively. The porous membrane has a pore size of 0.05-20 μm, an average pore size of 2-15 μm, and a porosity of 35-80% by volume. The porous membrane is distributed with micro-protrusion structures. The thickness of the hydrophilic membrane is 75-130 μm, and the thickness of the hydrophobic membrane is 80-135 μm. The water contact angle of the hydrophilic membrane surface is 0-1°; The water contact angle of the hydrophobic film surface is 110-140°. The time for the water contact angle of the hydrophilic membrane surface to reach 0° is 0-5s.

17. The porous membrane according to claim 16, wherein, The porous membrane has a pore size of 0.1-10 μm, an average pore size of 3-10 μm, and a porosity of 40-70% by volume.

18. The porous membrane according to claim 16, wherein, The hydrophilic membrane surface is formed of a hydrophilic material, and the hydrophobic membrane surface is formed of a hydrophobic material.

19. The porous membrane according to claim 18, wherein, The weight-average molecular weight of the hydrophilic substance is 50,000-200,000 g / mol.

20. The porous membrane according to claim 18, wherein, The weight-average molecular weight of the hydrophobic substance is 50,000-800,000 g / mol.

21. The porous membrane according to claim 18, wherein, The hydrophilic substance is selected from at least one of cellulose acetate, polyacrylamide, polyacrylonitrile, chitin, chitosan, polyacrylic acid, polydopamine, polylactic acid, and polyvinylpyrrolidone.

22. The porous membrane according to claim 21, wherein, The hydrophilic substance is selected from at least one of polyacrylonitrile, polydopamine, polyvinylpyrrolidone, cellulose acetate, and chitosan.

23. The porous membrane according to claim 18, wherein, The hydrophobic substance is selected from at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinyl chloride, silane, polyethersulfone, and polystyrene.

24. The porous membrane according to claim 23, wherein, The hydrophobic substance is selected from at least one of polysulfone, polyvinylidene fluoride, polystyrene, methyltrichlorosilane, and phenyltrisilane.

25. The porous membrane according to claim 16, wherein, There may or may not be a supporting substrate between the hydrophilic and hydrophobic film surfaces.

26. The porous membrane according to claim 25, wherein, The aperture of the supporting substrate is 30-80 μm.

27. The porous membrane according to claim 25, wherein, The thickness of the supporting substrate is 60-120 μm.

28. The porous membrane according to claim 27, wherein, The thickness of the supporting substrate is 80-100 μm.

29. The use of the porous membrane according to any one of claims 15-28 in reducing water evaporation, collecting water or mist.

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