Isotropic membranes and methods for making the same

Abstract

The application discloses a kind of uniform pore membranes and preparation method, preparation method includes: first fluorine-containing polymer with first molecular weight is dissolved in organic solvent, forms first solution, first colloidal microspheres are dispersed in organic solvent, then second fluorine-containing polymer with second molecular weight is added, forms second solution, second colloidal microspheres are dispersed in organic solvent, then third fluorine-containing polymer with third molecular weight is added, forms third solution, first molecular weight < second molecular weight < third molecular weight;PTFE membrane is immersed in first solution and is treated, solidifies after taking out, obtains PTFE base film;Second solution is scraped on PTFE base film and forms first photonic crystal layer;Third solution is applied to the side of first photonic crystal layer away from PTFE base film, induces second colloidal microspheres self-assembly, forms second photonic crystal layer after solidification;First colloidal microspheres and second colloidal microspheres are removed, and obtain uniform pore membrane.The uniform pore membrane prepared in the application has uniform pore size and good mechanical properties.

Description

Technical Field

[0001] This application relates to the field of membrane separation technology, specifically to a uniformly porous membrane and its preparation method. Background Technology

[0002] Membrane separation technology boasts advantages such as ease of operation and high separation efficiency, making it a key common technology for addressing major global challenges including the energy crisis, water crisis, and air pollution. Membrane materials are the core of membrane separation technology, with polymer porous membranes, possessing excellent processability and cost-effectiveness, dominating the field. However, most polymer porous membranes exhibit a trade-off effect of permeability and selectivity, which not only increases the energy consumption required for membrane separation but also limits the separation efficiency, representing a common challenge that urgently needs to be solved in the field of polymer separation membranes.

[0003] For polymer porous membranes that primarily rely on size sieving for separation, their selectivity and permeability are mainly limited by the size of the pores (larger and smaller, respectively). This is the fundamental cause of the trade-off effect, namely the uneven pore size distribution of traditional polymer porous membranes. Therefore, constructing polymer porous membranes with uniform pore size holds promise for solving the problem of simultaneously improving membrane permeability and selectivity. Besides pore size uniformity, porosity is also a crucial parameter for addressing the trade-off effect. Porosity significantly impacts membrane permeability; higher porosity results in greater permeate flux. In conclusion, if uniformly pore membranes can achieve high porosity, the trade-off effect can be completely overcome.

[0004] To prepare high-porosity polymer uniform-pore membranes, researchers have developed representative methods such as nuclear track etching and block copolymer assembly. Nuclear track etching uses high-energy particle radiation to vertically bombard polymer sheets, forming cylindrical pores with a very narrow pore size distribution. Block copolymer self-assembly utilizes the differences in physicochemical properties of different blocks in the copolymer, achieving uniform-pore membranes through microscopic phase separation. While these typical methods can indeed achieve high porosity and uniform pore size in polymer porous membranes, they generally suffer from drawbacks such as cumbersome processing, high energy consumption, and high cost, limiting their widespread application. Furthermore, high porosity often leads to a decrease in the mechanical strength of the separated membrane, necessitating the search for an efficient, simple, and low-cost method to prepare high-porosity polymer uniform-pore membranes with good mechanical properties.

[0005] Therefore, this invention is proposed. Summary of the Invention

[0006] One objective of this application is to provide a method for preparing a uniformly porous membrane with ultra-high porosity based on a PTFE fiber membrane substrate. This method enables precise control of the membrane's microporous structure, and the prepared uniformly porous membrane exhibits advantages such as high porosity, uniform pore size, and good mechanical strength. Furthermore, this method is simple and easy to implement, and can be mass-produced.

[0007] In one aspect of this application, a method for preparing a uniformly porous membrane is provided, comprising the following steps:

[0008] A first fluoropolymer having a first molecular weight is dissolved in an organic solvent to form a first solution; first colloidal microspheres are dispersed in an organic solvent by ultrasonic vibration, and then a second fluoropolymer having a second molecular weight is added to form a second solution; second colloidal microspheres are dispersed in an organic solvent by ultrasonic vibration, and then a third fluoropolymer having a third molecular weight is added to form a third solution; wherein, the first molecular weight is less than the second molecular weight, and the second molecular weight is less than the third molecular weight;

[0009] A PTFE membrane with a porous structure is provided, and the PTFE membrane is immersed in a first solution, removed and cured to obtain a PTFE base membrane.

[0010] The second solution is scraped onto one side of the PTFE base film to form a first photonic crystal layer on the PTFE base film;

[0011] The third solution is applied to the side of the first photonic crystal layer away from the PTFE base film. Then, the second colloidal microspheres in the third solution are induced to self-assemble using vacuum negative pressure. After self-assembly, they are cured to form a second photonic crystal layer on the side of the first photonic crystal layer away from the PTFE base film.

[0012] The first colloidal microsphere in the first photonic crystal layer and the second colloidal microsphere in the second photonic crystal layer are removed to obtain the uniformly porous membrane.

[0013] In some embodiments of this application, the first molecular weight is selected from 5w to 30w, the second molecular weight is selected from 30w to 80w, and the third molecular weight is selected from 80w to 200w.

[0014] In some embodiments of this application, the step of dissolving a first fluoropolymer having a first molecular weight in an organic solvent to form a first solution includes:

[0015] Weigh the first fluoropolymer having a first molecular weight according to the required amount, add the first fluoropolymer to an organic solvent, and stir under heating conditions of 50°C to 80°C until fully dissolved to obtain the first solution; and / or,

[0016] The process of dispersing the first colloidal microspheres in an organic solvent by ultrasonic vibration, followed by adding a second fluoropolymer with a second molecular weight to form a second solution, includes:

[0017] Weigh the first colloidal microspheres according to the required amount, add the first colloidal microspheres to an organic solvent, sonicate for 1 to 5 hours, then add a second fluoropolymer with a second molecular weight, and stir at 50°C to 80°C until fully dissolved to obtain the second solution; and / or,

[0018] The process of dispersing the second colloidal microspheres in an organic solvent by ultrasonic vibration, followed by adding a third fluoropolymer with a third molecular weight to form a third solution, includes:

[0019] Weigh the second colloidal microspheres according to the required amount, add the second colloidal microspheres to an organic solvent, and sonicate for 1 to 5 hours. Then add a third fluoropolymer with a third molecular weight and stir at 50°C to 80°C until fully dissolved to obtain the third solution.

[0020] In some embodiments of this application, the mass fraction of the first fluoropolymer in the first solution, the mass fraction of the second fluoropolymer in the second solution, and the mass fraction of the third fluoropolymer in the third solution are all 1% to 30%; and / or,

[0021] The mass ratio of the second fluoropolymer to the first colloidal microsphere in the second solution and the mass ratio of the third fluoropolymer to the second colloidal microsphere in the third solution are both 1:(1.5~5).

[0022] In some embodiments of this application, the first fluoropolymer, the second fluoropolymer, and the third fluoropolymer are each independently at least one selected from polyvinylidene fluoride, poly(vinylidene fluoride-cotrifluoroethylene), and polyvinylidene fluoride-hexafluoropropylene; and / or,

[0023] The organic solvent is at least one selected from N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; and / or,

[0024] The first colloidal microsphere and the second colloidal microsphere are each independently selected from at least one of silica microspheres, zirconium dioxide, and calcium carbonate, and the first colloidal microsphere and the second colloidal microsphere have the same particle size.

[0025] In some embodiments of this application, the step of immersing the PTFE membrane in the first solution, removing it, and then curing it to obtain a PTFE base film includes:

[0026] The PTFE membrane is immersed in the first solution under a vacuum negative pressure of 0.03MPa to 0.08MPa. After full immersion, it is taken out and vacuum dried at 60℃ to 120℃ to obtain the PTFE base membrane.

[0027] In some embodiments of this application, in the step of coating the second solution onto one side of the PTFE base film to form a first photonic crystal layer on the PTFE base film, the coating thickness is 50 μm to 150 μm and the coating speed is 2 to 10 cm / s.

[0028] In some embodiments of this application, the third solution is applied to the side of the first photonic crystal layer away from the PTFE base film, and then the second colloidal microspheres in the third solution are induced to self-assemble using vacuum negative pressure. After self-assembly, curing is performed to form a second photonic crystal layer on the side of the first photonic crystal layer away from the PTFE base film. In this step, the vacuum negative pressure is 0.02MPa to 0.08MPa, the vacuum negative pressure induction time is 10min to 30min, and the curing temperature is 60℃ to 120℃.

[0029] In some embodiments of this application, the step of removing the first colloidal microspheres in the first photonic crystal layer and the second colloidal microspheres in the second photonic crystal layer to obtain the uniformly porous film includes:

[0030] The PTFE base film with the first photonic crystal layer and the second photonic crystal layer formed thereon is immersed in an HF solution with a mass concentration of 2% to 6% for 4 to 12 hours. After being taken out, it is washed and dried in sequence to obtain the uniformly porous film.

[0031] In another aspect of this application, a uniformly porous membrane is provided, which is prepared by the preparation method described above.

[0032] This application includes the following beneficial effects:

[0033] This application employs impregnation, coating, and vacuum negative pressure assisted induction methods to construct a uniformly porous structure on a PTFE substrate using three fluoropolymers of different molecular weights, thereby obtaining a composite uniformly porous membrane material with excellent performance. The PTFE membrane substrate possesses excellent mechanical properties, significantly improving the mechanical strength of the uniformly porous membrane. Specifically, removing the first colloidal microspheres in the first photonic crystal layer and the second colloidal microspheres in the second photonic crystal layer allows for the formation of an inverse opal-type uniformly porous structure on the PTFE substrate, resulting in a membrane with uniform pore size and easily controllable pore size and distribution. Furthermore, by using fluoropolymers with increasing molecular weights, the uniformly porous structure undergoes simultaneous composite behavior with the PTFE membrane substrate during preparation. This avoids the impact on pore size caused by secondary composite formation of the upper inverse opal-type uniformly porous structure with the substrate, while simultaneously ensuring the complete construction of the uniformly porous structure, promoting its bonding with the substrate, and enhancing the strength of the uniformly porous membrane.

[0034] This application utilizes a photonic crystal template with a regular arrangement to obtain an inverse opal-type uniform porous structure. This results in a porous separation membrane with a regular microporous structure, high porosity, controllable pore size and distribution, and excellent mechanical properties. By adjusting the controllable porosity and pore size of the colloidal microspheres, the membrane achieves both high permeability and high selectivity. Furthermore, the method provided in this application is simple and easy to implement, allowing for large-scale production and significantly improving the efficiency of preparing high-porosity uniform porous membranes. Attached Figure Description

[0035] The above and other objectives, features and advantages of this application will become more apparent from a detailed description of exemplary embodiments thereof with reference to the accompanying drawings.

[0036] Figure 1 This is a flowchart of a method for preparing a uniformly porous membrane according to an embodiment of this application.

[0037] Figure 2 This is a schematic diagram of a uniformly porous membrane structure according to an embodiment of this application.

[0038] Figure 3 This is a pore structure diagram of a uniformly porous membrane according to an embodiment of this application; wherein, surface pores are pores on the upper surface of the membrane, and their pore size is close to the particle size of microspheres; while effective pores are pore necks in the pore structure, which ultimately play the role of interception and filtration.

[0039] Figure 4 This is a diagram showing the pore size and pore size distribution of a uniformly porous membrane according to an embodiment of this application.

[0040] Figure 5 This is an example of an embodiment of the present application showing the absorbance curves and physical images of a uniformly porous membrane before and after filtration of a 150nm microsphere emulsion. Detailed Implementation

[0041] The present application will now be described in further detail with reference to the embodiments. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit the invention.

[0042] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0043] It should be noted that 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.

[0044] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0045] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0046] like Figure 1 As shown, a method for preparing a uniformly porous membrane according to this application includes the following steps:

[0047] S101: A first fluoropolymer having a first molecular weight is dissolved in an organic solvent to form a first solution; a first colloidal microsphere is dispersed in an organic solvent by ultrasonic vibration, and then a second fluoropolymer having a second molecular weight is added to form a second solution; the second colloidal microsphere is dispersed in an organic solvent by ultrasonic vibration, and then a third fluoropolymer having a third molecular weight is added to form a third solution; wherein, the first molecular weight is less than the second molecular weight, and the second molecular weight is less than the third molecular weight.

[0048] S102: Provide a PTFE membrane with a porous structure, immerse the PTFE membrane in the first solution, remove it and cure it to obtain a PTFE base membrane;

[0049] S103: The second solution is coated onto one side of the PTFE base film to form a first photonic crystal layer on the PTFE base film;

[0050] S104: The third solution is applied to the side of the first photonic crystal layer away from the PTFE base film, and then the second colloidal microspheres in the third solution are induced to self-assemble using vacuum negative pressure. After self-assembly, the microspheres are cured to form a second photonic crystal layer on the side of the first photonic crystal layer away from the PTFE base film.

[0051] S105: Remove the first colloidal microspheres from the first photonic crystal layer and the second colloidal microspheres from the second photonic crystal layer to obtain the uniformly porous membrane.

[0052] For example, the components of the first solution include an organic solvent and a first fluoropolymer with a molecular weight of 5w to 30w, the components of the second solution include monodisperse first colloidal microspheres, a second fluoropolymer with a molecular weight of 30w to 80w and an organic solvent, and the components of the third solution include monodisperse second colloidal microspheres, a third fluoropolymer with a molecular weight of 80w to 200w and an organic solvent.

[0053] In this process, the first fluoropolymer in the first solution serves as the pore size control material for the PTFE membrane, and also acts as an interfacial adhesive between the first photonic crystal layer and the PTFE membrane when the first photonic crystal layer is formed in the second solution. The colloidal microspheres and fluoropolymer in the second and third solutions are the building blocks of the composite photonic crystal. After removing the colloidal microspheres, a high-porosity uniform-pore membrane based on the PTFE membrane substrate is obtained. The pore size of this uniform-pore membrane can be adjusted by adjusting the particle size of the microspheres in the solution, thus enabling the prepared uniform-pore membrane to have convenient and precise pore size adjustment and uniform pore size distribution.

[0054] In detail, a PTFE membrane is impregnated in a first solution containing a fluoropolymer with a low molecular weight. The first fluoropolymer permeates into the interior of the PTFE membrane, and its molecular chains are uniformly distributed throughout the pore structure of the PTFE membrane, acting as a pore-blocking agent. By controlling the impregnation time and the concentration of the first solution, a PTFE base membrane with a specific pore size can be obtained after heat curing (i.e., as shown in the diagram). Figure 2 (PTFE substrate shown).

[0055] More specifically, in step S103, when the second solution is coated onto one side of the PTFE base film, the shear force of the scraper on the second solution induces the first colloidal microspheres therein to pre-assemble, thereby forming a first photonic crystal layer on the PTFE base film, resulting in a semi-wettable composite film. Since the PTFE base film undergoes heat curing during preparation, it is essentially dry and solvent-free. However, the second solvent is not dried during the coating process to form the first photonic crystal layer, resulting in a wet state for the PTFE base film with the first photonic crystal layer. This state ensures the smooth progress of the subsequent vacuum-induced self-assembly of the second colloidal microspheres, as wet liquids can move under negative pressure, while cured liquids cannot effectively generate negative pressure.

[0056] Furthermore, since the PTFE base film contains a first fluoropolymer with a first molecular weight, and the first photonic crystal layer contains a second fluoropolymer with a second molecular weight, the medium molecular weight fluoropolymer exhibits good adhesion to the PTFE film impregnated with the first solution. Moreover, because the fluoropolymer is a high molecular weight polymer with viscoelasticity and high rheological properties, it does not react immediately under shear stress, exhibiting a certain degree of lag. This allows the first colloidal microspheres to preferentially and directly assemble at the interface under shear force, thereby preventing polymer blockage of the base film pores through the pre-assembly of the colloidal microspheres at the interface. Compared to first forming the first photonic crystal layer and then combining it with the PTFE base film using adhesives, electrospinning, or centrifugal spinning (i.e., a secondary composite method), this method is not only more efficient but also avoids clogging of the uniform pores by adhesives, reducing the difficulty of preparing a uniformly porous structure. Therefore, this application constructs an inverse opal structure on a PTFE film substrate, achieving the generation of a uniformly porous structure and its composite with the substrate.

[0057] Furthermore, since the organic solvent in the second solution is not removed after it is coated onto the PTFE base film, the first fluoropolymer on the PTFE base film will partially dissolve, resulting in stronger interfacial adhesion and improved bonding strength between the first photonic crystal layer and the PTFE base film.

[0058] The thickness of the first photonic crystal layer can be controlled by adjusting the height and speed of the coating process.

[0059] Among these, fluorinated polymers possess unique performance advantages compared to ordinary polymers, such as excellent chemical stability, non-adhesion, weather resistance, fatigue resistance, oxidation resistance, and lubricity, resulting in superior performance of the prepared uniformly porous membranes. Furthermore, since PTFE membranes are themselves fluorinated polymers, using both a first and a second fluorinated polymer enhances the affinity and bonding strength between molecular chains, significantly improving the strength of the uniformly porous membrane.

[0060] In detail, in step S104, the second colloidal microspheres in the third solution are assembled on the side of the first photonic crystal layer away from the PTFE base film using vacuum negative pressure, and then cured to form the second photonic crystal layer, thus obtaining the composite film base.

[0061] For example, the second colloidal microspheres in the third solution are assembled from bottom to top on a semi-wettable composite substrate under vacuum induction. After a certain period of time, such as 10 to 20 minutes, they are heated and cured to form a composite substrate. The thickness and surface pore size distribution of the second photonic crystal layer can be controlled by controlling the vacuum induction time and pressure, as well as the volume of the applied solution.

[0062] In this process, the second solution is applied using a blade coating method to quickly and thinly form the first photonic crystal layer. This layer primarily serves as a transition layer, preventing the high molecular weight fluoropolymer in the third solution from seeping into the PTFE membrane and clogging its pores. It also enhances the bonding strength between the second photonic crystal layer and the PTFE substrate. Specifically, choosing a fluoropolymer with a lower molecular weight results in weaker mechanical properties, while choosing one with a higher molecular weight leads to easy seepage and clogging of the PTFE membrane pores. Therefore, a rapidly prepared intermediate layer is chosen to achieve uniform pore size, high porosity, and good mechanical strength. More specifically, the first solution aims to control the pore size of the PTFE membrane substrate and increase its strength. Therefore, a low molecular weight fluoropolymer is selected because it disperses more uniformly in organic solvents and easily distributes within the pores of the PTFE membrane substrate, achieving pore size control. The second solution uses a medium-molecular-weight fluoropolymer. This is to prevent the polymer from penetrating and clogging the pores of the substrate, as the higher the molecular weight of the fluoropolymer, the easier it is to penetrate downwards. It also acts as a buffer, ensuring a smooth transition between the second photonic crystal layer and the PTFE base film. This enhances the mechanical properties and reliability of the uniformly porous membrane by strengthening the bonding strength between the layers. The third solution uses a higher molecular weight fluoropolymer because it has good film-forming properties and mechanical strength, which can significantly improve the strength of the uniformly porous membrane and increase its service life and durability.

[0063] The third solution employs a vacuum method because it can both induce assembly to form a photonic crystal structure and promote the bonding between the PTFE base film and its two upper layers. In the presence of a solvent, both the second and third solutions can be assembled under vacuum, allowing the fluoropolymer to flow with the solvent and causing entanglement between polymer layers, thus strengthening the interfacial bonding.

[0064] It can be seen that the uniformly porous membrane of this application has strong bonding between its layers, which gives it high mechanical properties. One reason is that the solvent in the middle wetting layer (i.e., the first photonic crystal layer) can partially dissolve the polymer, thereby achieving interlayer bonding. Another reason is that the subsequent vacuum-induced continuous negative pressure can also make the entire membrane tightly bonded.

[0065] In detail, in step S105, removing the first and second colloidal microspheres from the composite membrane substrate yields a uniformly porous membrane based on a PTFE membrane. This membrane, with its pore size and porosity controlled by the colloidal microspheres, offers advantages such as high porosity and precisely adjustable pore size. The colloidal microspheres in the second and third solutions maintain the same particle size.

[0066] like Figure 2 As shown, the first inverse opal structure layer can be obtained by removing the first colloidal microsphere in the first photonic crystal layer, and the second inverse opal structure layer can be obtained by removing the second colloidal microsphere in the second photonic crystal layer. The first inverse opal structure layer is located between the PTFE substrate and the second inverse opal structure layer.

[0067] As can be seen, the preparation method described in this application uses PTFE fiber membrane as a substrate to construct a uniformly porous membrane structure. This not only allows for precise control of the microporous structure of the separation membrane, but also enables the simple preparation of separation membranes with adjustable pore size and excellent mechanical properties. Compared with traditional preparation methods, it has the advantages of simple operation, regular microporous structure, controllable pore size and distribution, and good mechanical strength.

[0068] Furthermore, in some embodiments of this application, three fluoropolymer casting solutions with different molecular weights are prepared; the three fluoropolymer resins with different molecular weights are dissolved in an organic solvent under heating and mechanical stirring conditions to obtain a first solution, a second solution and a third solution, wherein the second solution and the third solution contain colloidal microspheres, and the colloidal microspheres are dispersed in the solution by ultrasonic vibration.

[0069] Specifically, the steps for forming the first solution include: weighing the first fluoropolymer having a first molecular weight according to the required amount, adding the first fluoropolymer to an organic solvent, and stirring under heating conditions of 50°C to 80°C until fully dissolved to obtain the first solution.

[0070] Specifically, the steps for forming the second solution include: weighing the first colloidal microspheres according to the required amount, adding the first colloidal microspheres to an organic solvent, ultrasonically vibrating for 1 to 5 hours, adding a second fluoropolymer with a second molecular weight, and stirring under heating conditions of 50°C to 80°C until fully dissolved to obtain the second solution.

[0071] Specifically, the steps for forming the third solution include: weighing the second colloidal microspheres according to the required amount, adding the second colloidal microspheres to an organic solvent, ultrasonically vibrating for 1 to 5 hours, adding a third fluoropolymer with a third molecular weight, and stirring under heating conditions of 50°C to 80°C until fully dissolved to obtain the third solution.

[0072] The polymer in the first solution has a molecular weight of 5w to 30w, the polymer in the second solution has a molecular weight of 30w to 80w, and the polymer in the third solution has a molecular weight of 80w to 200w.

[0073] Furthermore, in some embodiments of this application, the first solution is impregnated into the PTFE membrane using a vacuum impregnation method, and then dried and cured in a vacuum oven after impregnation; optionally, the vacuum negative pressure is 0.03MPa~0.08MPa, and the vacuum drying temperature is 60℃~120℃.

[0074] Furthermore, in some embodiments of this application, the second solution is applied to the PTFE base film by a blade coating method; optionally, the coating height parameter is 50μm~150μm, and the coating speed parameter is 2cm / s~10cm / s.

[0075] The coating process allows the first colloidal microspheres in the second solution to pre-assemble on the PTFE base membrane to form a buffer layer. The coated layer should not be too thin, as this results in low mechanical strength; conversely, the coated layer should not be too thick, as this will affect the subsequent vacuum-induced assembly of the second colloidal microspheres. Since the subsequent assembly is induced by vacuum negative pressure, if the coated layer is too thick, the inducing effect of vacuum filtration on the second colloidal microspheres will be weakened.

[0076] Furthermore, in some embodiments of this application, the second colloidal microspheres in the third solution are assembled onto the semi-wetting composite film substrate using vacuum-assisted induction, and after a certain period of negative pressure, they are dried and cured into a film in a vacuum oven.

[0077] Optionally, the vacuum negative pressure is 0.02MPa~0.08MPa, the vacuum negative pressure time is 10min~30min, and the vacuum oven drying temperature is 60℃~120℃.

[0078] Furthermore, in some embodiments of this application, the colloidal microspheres of the composite membrane substrate are removed by acid / alkali solution etching. Specifically, the first and second colloidal microspheres are removed using an HF solution with a mass fraction of 2% to 6%. More specifically, the PTFE-based membrane on which the first and second photonic crystal layers are formed is immersed in an HF solution with a mass concentration of 2% to 6% for 4 to 12 hours, and then removed, followed by washing and drying to obtain the uniformly porous membrane.

[0079] Furthermore, in some embodiments of this application, the first fluoropolymer, the second fluoropolymer, and the third fluoropolymer are each independently selected from at least one of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-CTFE), and polyvinylidene fluoride-hexafluoropropylene (PVDF-HEP).

[0080] Optionally, the mass fraction of the first fluoropolymer in the first solution, the mass fraction of the second fluoropolymer in the second solution, and the mass fraction of the third fluoropolymer in the third solution are all 1% to 30%.

[0081] Preferably, the mass fraction of the first fluoropolymer in the first solution is 2-10%; the mass fraction of the second fluoropolymer in the second solution is 10-20%; and the mass fraction of the third fluoropolymer in the third solution is 5-20%.

[0082] Furthermore, in some embodiments of this application, the first colloidal microsphere and the second colloidal microsphere are each independently selected from at least one of silica microspheres, zirconium dioxide, and calcium carbonate;

[0083] Optionally, the colloidal microspheres in the second and third solutions are all silica microspheres with a particle size range of 150–500 nm.

[0084] Optionally, the mass ratio of the second fluoropolymer to the first colloidal microspheres in the second solution is 1:1.5–5, and the mass ratio of the third fluoropolymer to the second colloidal microspheres in the third solution is 1:1.5–5. Controlling these mass ratios can effectively regulate the porosity of the membrane. Specifically, controlling the mass ratio of the second fluoropolymer to the first colloidal microspheres in the second solution to 1:1.5–5 ensures a higher proportion of the first colloidal microspheres, guaranteeing that the first colloidal microspheres are pre-assembled at the interface and preventing the second fluoropolymer from pre-depositing at the interface and clogging the pores of the PTFE base membrane interface.

[0085] The colloidal microspheres in the second and third solutions have the same particle size. In this application, the colloidal microspheres act as a pore-forming agent to control the uniformity of the particle size and obtain a high-porosity fluorinated membrane with uniform pore size distribution.

[0086] Furthermore, in some embodiments of this application, the organic solvent is selected from at least one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO).

[0087] In this application, organic solvents that are good solvents for fluoropolymers are preferred to obtain a uniform casting solution.

[0088] Example 1

[0089] The following steps were used to prepare an ultra-high porosity uniform pore membrane based on a PTFE fiber membrane substrate:

[0090] (1) Preparation of fluoropolymer solutions with different molecular weights: Weigh 4g of PVDF powder with a molecular weight of 10w and add it to 46g of DMF. Stir mechanically in a 60℃ water bath for 12h to obtain the first solution; Weigh 12g of SiO2 powder with a particle size of 300nm and add it to 50g of DMF. Sonicate for 3h and then add 5g of PVDF powder with a molecular weight of 60w. Stir mechanically in a 60℃ water bath for 12h to obtain the second solution; Weigh 10g of SiO2 powder with a particle size of 300nm and add it to 50g of DMF. Sonicate for 3h and then add 4g of PVDF powder with a molecular weight of 120w. Stir mechanically in a 60℃ water bath for 12h to obtain the third solution;

[0091] (2) Preparation of PTFE base film with specific pore size: PTFE film with pore size of 0.22 μm was vacuum impregnated in the first solution obtained in step (1), the vacuum strength was set to 0.05 MPa, and after impregnation for 1 h, it was cured in a vacuum oven at 80 °C to obtain PTFE base film with pore size of 100-120 nm.

[0092] (3) Preparation of semi-wetting composite film base: The second solution obtained in step (1) is rapidly coated onto the PTFE base film obtained in step (2) using a coating process parameter of 100 μm coating height and 4 cm / s coating speed to obtain a semi-wetting composite base film containing a layer of colloidal microspheres / polymer.

[0093] (4) Vacuum-induced assembly to prepare composite base film: Using the semi-wetting composite base film obtained in step (3) as the base film, the third solution in step (1) is applied to its surface to facilitate the assembly of colloidal microspheres in the third solution under vacuum-assisted induction, forming a composite photonic crystal structure. The vacuum negative pressure is selected as 0.06 MPa, and the film is cured in a vacuum oven at 80°C for 15 min under negative pressure driving to obtain a composite film base containing colloidal microspheres.

[0094] (5) Removal of colloidal microspheres: The composite membrane substrate obtained in step (4) is placed in an HF solution with a mass concentration of 4% for 12 hours. After being washed with deionized water and vacuum dried, an ultra-high porosity uniform pore membrane based on PTFE fiber membrane substrate is obtained.

[0095] Testing revealed that the obtained ultra-high porosity uniform porous membrane based on PTFE fiber membrane substrate possesses a unique three-dimensional pore structure. A schematic diagram of the layered structure of the uniform porous membrane is shown below. Figure 2 As shown, the surface morphology of the inverse opal structure of the membrane is shown in [reference needed]. Figure 3Its porosity is as high as 80.5%, and its pure water flux is 898.9 L / m³. -2 h -1 The membrane has a tensile strength of 19.3 MPa, exhibiting good mechanical strength; the membrane's pore size distribution curve is extremely sharp. Figure 4 The membrane has a pore size of 100 nm and can achieve a 100% rejection rate for colloidal microspheres with a particle size of 130 nm. After 5 test cycles, the rejection rate of the filtrate is still 100%, and the flux recovery rate is 96%, which shows good repeatability.

[0096] Example 2

[0097] The following steps were used to prepare an ultra-high porosity uniform pore membrane based on a PTFE fiber membrane substrate:

[0098] (1) Preparation of fluoropolymer solutions with different molecular weights: Weigh 3g of PVDF-HFP powder with a molecular weight of 20w, add it to 47g of DMAc, and mechanically stir in a 70℃ water bath for 12h to obtain the first solution; Weigh 14g of SiO2 powder with a particle size of 500nm, add it to 50g of DMAc, sonicate for 3h, then add 4g of PVDF powder with a molecular weight of 80w, and mechanically stir in a 60℃ water bath for 12h to obtain the second solution; Weigh 10.5g of SiO2 powder with a particle size of 500nm, add it to 50g of DMAc, sonicate for 2.5h, then add 3.5g of PVDF powder with a molecular weight of 150w, and mechanically stir in a 70℃ water bath for 12h to obtain the third solution;

[0099] (2) Preparation of PTFE base film with specific pore size: PTFE film with pore size of 0.22 μm was vacuum impregnated in the first solution obtained in step (1), the vacuum strength was set to 0.07 MPa, and after impregnation for 1.5 h, it was cured in a vacuum oven at 100 °C to obtain PTFE base film with pore size of 130-150 nm.

[0100] (3) Preparation of semi-wetting composite film base: The second solution obtained in step (1) is rapidly coated onto the PTFE base film obtained in step (2) using a coating process parameter of 80 μm coating height and 3 cm / s coating speed to obtain a semi-wetting composite base film with a layer of colloidal microspheres / polymer.

[0101] (4) Vacuum-induced assembly to prepare composite base film: Using the semi-wetting composite base film obtained in step (3) as the base film, the third solution in step (1) is applied to its surface to facilitate the assembly of colloidal microspheres in the third solution under vacuum-assisted induction, forming a composite photonic crystal structure. The vacuum negative pressure is selected as 0.04 MPa, and the film is cured in a vacuum oven at 100°C for 18 min under negative pressure driving to obtain a composite film base containing colloidal microspheres.

[0102] (5) Removal of colloidal microspheres: The composite membrane substrate obtained in step (4) is placed in a 6% HF solution for 12 hours. After washing with deionized water and vacuum drying, an ultra-high porosity uniform pore membrane based on PTFE fiber membrane substrate is obtained.

[0103] Testing revealed that the obtained ultra-high porosity uniform-pore membrane based on PTFE fiber membrane substrate possesses a unique three-dimensional pore structure, with a porosity as high as 83.2% and a pure water flux of 1003.7 L / m³. -2 h -1 The membrane exhibits a tensile strength of 16.9 MPa, demonstrating excellent mechanical strength. Its pore size distribution curve is extremely sharp, with a pore size of 130 nm. It achieves a 100% rejection rate for colloidal microspheres with a particle size of 150 nm. See the comparison chart of the filtrate before and after filtration. Figure 5 After 5 test cycles, the retention rate of the filtrate was still 100%, and the flux recovery rate was 98%, demonstrating good repeatability.

[0104] Example 3

[0105] The following steps were used to prepare an ultra-high porosity uniform pore membrane based on a PTFE fiber membrane substrate:

[0106] (1) Preparation of fluoropolymer solutions with different molecular weights: Weigh 1.5g of PVDF-CTFE powder with a molecular weight of 20w, add it to 48.5g of NMP, and mechanically stir in a 70℃ water bath for 12h to obtain the first solution; Weigh 23g of SiO2 powder with a particle size of 1μm, add it to 50g of NMP, sonicate for 4h, then add 3.75g of PVDF powder with a molecular weight of 70w, and mechanically stir in a 70℃ water bath for 12h to obtain the second solution; Weigh 10.5g of SiO2 powder with a particle size of 1μm, add it to 50g of NMP, sonicate for 5h, then add 4g of PVDF powder with a molecular weight of 120w, and mechanically stir in a 70℃ water bath for 12h to obtain the third solution;

[0107] (2) Preparation of PTFE base film with specific pore size: PTFE film with pore size of 0.22 μm was vacuum impregnated in the first solution obtained in step (1), the vacuum strength was set to 0.07 MPa, and after impregnation for 1.5 h, it was cured in a vacuum oven at 100 °C to obtain PTFE base film with pore size of 200 nm.

[0108] (3) Preparation of semi-wetting composite film base: The second solution obtained in step (1) is rapidly coated onto the PTFE base film obtained in step (2) using a coating process parameter of 120 μm coating height and 4 cm / s coating speed to obtain a semi-wetting composite base film with a layer of colloidal microspheres / polymer.

[0109] (4) Vacuum-induced assembly to prepare composite base film: Using the semi-wetting composite base film obtained in step (3) as the base film, the third solution in step (1) is applied to its surface to facilitate the assembly of colloidal microspheres in the third solution under vacuum-assisted induction, forming a composite photonic crystal structure. The vacuum negative pressure is selected as 0.06 MPa, and the film is cured in a vacuum oven at 100°C for 14 min under negative pressure driving to obtain a composite film base containing colloidal microspheres.

[0110] (5) Removal of colloidal microspheres: The composite membrane substrate obtained in step (4) is placed in a 5% HF solution for 12 hours. After being washed with deionized water and vacuum dried, an ultra-high porosity uniform pore membrane based on PTFE fiber membrane substrate is obtained.

[0111] Testing revealed that the obtained ultra-high porosity uniform-pore membrane based on PTFE fiber membrane substrate possesses a unique three-dimensional pore structure, with a porosity as high as 83.8% and a pure water flux of 1120.2 L / m³. -2 h -1 The membrane has a mechanical tensile strength of 14.5 MPa, exhibiting good mechanical strength. The membrane's pore size distribution curve is extremely sharp, with a pore size of 170 nm. It can achieve a 100% rejection rate for colloidal microspheres with a particle size of 200 nm. After 5 test cycles, the rejection rate of the filtrate is still 100%, and the flux recovery rate reaches 97%, demonstrating good repeatability.

[0112] Example 4

[0113] The following steps were used to prepare an ultra-high porosity uniform pore membrane based on a PTFE fiber membrane substrate:

[0114] (1) Preparation of fluoropolymer solutions with different molecular weights: Weigh 3g of PVDF-CTFE powder with a molecular weight of 30w, add it to 47g of DMF, and mechanically stir in a 60℃ water bath for 12h to obtain the first solution; Weigh 16.5g of SiO2 powder with a particle size of 450nm, add it to 50g of DMF, sonicate for 3h, then add 6g of PVDF powder with a molecular weight of 50w, and mechanically stir in a 60℃ water bath for 12h to obtain the second solution; Weigh 11.2g of SiO2 powder with a particle size of 450nm, add it to 50g of DMF, sonicate for 4h, then add 4g of PVDF powder with a molecular weight of 120w, and mechanically stir in a 60℃ water bath for 12h to obtain the third solution;

[0115] (2) Preparation of PTFE base film with specific pore size: PTFE film with pore size of 0.22 μm was vacuum impregnated in the first solution obtained in step (1), the vacuum strength was set to 0.04 MPa, and after impregnation for 1 h, it was cured in a vacuum oven at 100 °C to obtain PTFE base film with pore size of 130-140 nm.

[0116] (3) Preparation of semi-wetting composite film base: The second solution obtained in step (1) is rapidly coated onto the PTFE base film obtained in step (2) using a coating process parameter of 100 μm coating height and 5 cm / s coating speed to obtain a semi-wetting composite base film with a layer of colloidal microspheres / polymer.

[0117] (4) Vacuum-induced assembly to prepare composite base film: Using the semi-wetting composite base film obtained in step (3) as the base film, the third solution in step (1) is applied to its surface to facilitate the assembly of colloidal microspheres in the third solution under vacuum-assisted induction, forming a composite photonic crystal structure. The vacuum negative pressure is selected as 0.05 MPa, and the film is cured in a vacuum oven at 80°C for 15 min under negative pressure driving to obtain a composite film base containing colloidal microspheres.

[0118] (5) Removal of colloidal microspheres: The composite membrane substrate obtained in step (4) is placed in a 3% HF solution for 12 hours. After being washed with deionized water and vacuum dried, an ultra-high porosity uniform pore membrane based on PTFE fiber membrane substrate is obtained.

[0119] Testing revealed that the obtained ultra-high porosity uniform-pore membrane based on PTFE fiber membrane substrate possesses a unique three-dimensional pore structure, with a porosity as high as 79.8% and a pure water flux of 953.7 L / m³. -2 h -1 The membrane has a tensile strength of 16.4 MPa, exhibiting good mechanical strength. The membrane's pore size distribution curve is extremely sharp, with a pore size of 130 nm. It can achieve a 100% rejection rate for colloidal microspheres with a particle size of 150 nm. After 5 test cycles, the rejection rate of the filtrate is still 100%, and the flux recovery rate is 99%, demonstrating good repeatability.

[0120] Comparative Example 1

[0121] The following steps were used to prepare an ultra-high porosity uniform pore membrane based on a PTFE fiber membrane substrate:

[0122] (1) Preparation of fluoropolymer solutions with different molecular weights: Weigh 1.5g of PVDF-CTFE powder with a molecular weight of 20w, add it to 48.5g of NMP, and mechanically stir in a water bath at 70℃ for 12h to obtain the first solution; Weigh 10.5g of SiO2 powder with a particle size of 1μm, add it to 50g of NMP, sonicate for 5h, add 4g of PVDF powder with a molecular weight of 120w, and mechanically stir in a water bath at 70℃ for 12h to obtain the third solution;

[0123] (2) Preparation of PTFE base film with specific pore size: PTFE film with pore size of 0.22 μm was vacuum impregnated in the first solution obtained in step (1), the vacuum strength was set to 0.07 MPa, and after impregnation for 1.5 h, it was cured in a vacuum oven at 100 °C to obtain PTFE base film with pore size of 200 nm.

[0124] (3) Vacuum-induced assembly to prepare composite base film: Using the PTFE base film obtained in step (2) as the base film, the third solution in step (1) is applied to its surface to facilitate the assembly of colloidal microspheres in the third solution under vacuum-assisted induction, forming a composite photonic crystal structure. The vacuum negative pressure is selected as 0.06 MPa, and the film is cured in a vacuum oven at 100°C for 14 min under negative pressure driving to obtain a composite film base containing colloidal microspheres.

[0125] (4) Removal of colloidal microspheres: The composite membrane substrate obtained in step (3) is placed in a 5% HF solution for 12 hours. After washing with deionized water and vacuum drying, an ultra-high porosity uniform pore membrane based on PTFE fiber membrane substrate is obtained.

[0126] Testing revealed that the obtained inverse opal-type membrane based on PTFE fiber substrate possesses a unique three-dimensional pore structure; however, the PTFE substrate exhibited significant clogging. The membrane porosity was 81.2%, and the pure water flux was 433.7 Lm. -2 h -1 The mechanical tensile strength of the membrane was 11.1 MPa, which was a decrease in mechanical strength. The pore size distribution of the membrane was uneven, with an average pore size of 140 nm. The rejection rate for colloidal microspheres with a particle size of 150 nm was 72%. After 5 test cycles, the rejection rate for the filtrate decreased to 53%, and the flux recovery rate was 71%, which reduced the reusability.

[0127] Comparative Example 2

[0128] The following steps were used to prepare an ultra-high porosity uniform pore membrane based on a PTFE fiber membrane substrate:

[0129] (1) Preparation of fluoropolymer solutions with different molecular weights: Weigh 23g of SiO2 powder with a particle size of 1μm and add it to 50g of NMP. After sonication for 4h, add 3.75g of PVDF powder with a molecular weight of 70w. Stir mechanically in a 70℃ water bath for 12h to obtain the second solution; Weigh 10.5g of SiO2 powder with a particle size of 1μm and add it to 50g of NMP. After sonication for 5h, add 4g of PVDF powder with a molecular weight of 120w. Stir mechanically in a 70℃ water bath for 12h to obtain the third solution;

[0130] (2) Preparation of semi-wetting composite film base: The second solution obtained in step (1) is rapidly coated onto a PTFE membrane with a pore size of 0.22 μm using a coating process parameter of 120 μm coating height and 4 cm / s coating speed to obtain a semi-wetting composite base membrane with a layer of colloidal microspheres / polymer.

[0131] (3) Vacuum-induced assembly to prepare composite base film: Using the semi-wetting composite base film obtained in step (2) as the base film, the third solution in step (1) is applied to its surface to facilitate the assembly of colloidal microspheres in the third solution under vacuum-assisted induction, forming a composite photonic crystal structure. The vacuum negative pressure is selected as 0.06 MPa, and the film is cured in a vacuum oven at 100°C for 14 min under negative pressure driving to obtain a composite film base containing colloidal microspheres.

[0132] (4) Removal of colloidal microspheres: The composite membrane substrate obtained in step (3) is placed in a 5% HF solution for 12 hours. After washing with deionized water and vacuum drying, an ultra-high porosity uniform pore membrane based on PTFE fiber membrane substrate is obtained.

[0133] Testing revealed that the obtained inverse opal-type membrane based on PTFE fiber membrane substrate possesses a unique three-dimensional pore structure with a porosity as high as 84.2% and a pure water flux of 1048.7 L / m³. -2 h -1 The mechanical tensile strength of the membrane is 10.4 MPa, indicating a decrease in mechanical properties. The pore size distribution of the membrane is relatively uneven, with a pore size of 190 nm. The rejection rate for colloidal microspheres with a particle size of 200 nm is 82%. After 5 test cycles, the rejection rate for the filtrate is 69%, the flux recovery rate is 78%, and the reusability is reduced.

[0134] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of disclosure in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the foregoing disclosed concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A method for preparing a uniformly porous membrane, characterized in that, Includes the following steps: A first fluoropolymer having a first molecular weight is dissolved in an organic solvent to form a first solution; first colloidal microspheres are dispersed in an organic solvent by ultrasonic vibration, and then a second fluoropolymer having a second molecular weight is added to form a second solution; second colloidal microspheres are dispersed in an organic solvent by ultrasonic vibration, and then a third fluoropolymer having a third molecular weight is added to form a third solution; wherein, the first molecular weight is less than the second molecular weight, and the second molecular weight is less than the third molecular weight; A PTFE membrane with a porous structure is provided, and the PTFE membrane is immersed in a first solution, removed and cured to obtain a PTFE base membrane. The second solution is scraped onto one side of the PTFE base film to form a first photonic crystal layer on the PTFE base film; The third solution is applied to the side of the first photonic crystal layer away from the PTFE base film. Then, the second colloidal microspheres in the third solution are induced to self-assemble using vacuum negative pressure. After self-assembly, they are cured to form a second photonic crystal layer on the side of the first photonic crystal layer away from the PTFE base film. The first colloidal microspheres in the first photonic crystal layer and the second colloidal microspheres in the second photonic crystal layer are removed to obtain the uniformly porous film; The first molecular weight is selected from 5w to 30w, the second molecular weight is selected from 30w to 80w, and the third molecular weight is selected from 80w to 200w.

2. The preparation method according to claim 1, characterized in that, The step of dissolving a first fluoropolymer having a first molecular weight in an organic solvent to form a first solution includes: Weigh the first fluoropolymer having a first molecular weight according to the required amount, add the first fluoropolymer to an organic solvent, and stir under heating conditions of 50°C to 80°C until fully dissolved to obtain the first solution; and / or, The process of dispersing the first colloidal microspheres in an organic solvent by ultrasonic vibration, followed by adding a second fluoropolymer with a second molecular weight to form a second solution, includes: Weigh the first colloidal microspheres according to the required amount, add the first colloidal microspheres to an organic solvent, sonicate for 1 to 5 hours, then add a second fluoropolymer with a second molecular weight, and stir at 50°C to 80°C until fully dissolved to obtain the second solution; and / or, The process of dispersing the second colloidal microspheres in an organic solvent by ultrasonic vibration, followed by adding a third fluoropolymer with a third molecular weight to form a third solution, includes: Weigh the second colloidal microspheres according to the required amount, add the second colloidal microspheres to an organic solvent, and sonicate for 1 to 5 hours. Then add a third fluoropolymer with a third molecular weight and stir at 50°C to 80°C until fully dissolved to obtain the third solution.

3. The preparation method according to claim 1, characterized in that, The mass fraction of the first fluoropolymer in the first solution, the mass fraction of the second fluoropolymer in the second solution, and the mass fraction of the third fluoropolymer in the third solution are all 1% to 30%; and / or, The mass ratio of the second fluoropolymer to the first colloidal microsphere in the second solution and the mass ratio of the third fluoropolymer to the second colloidal microsphere in the third solution are both 1:(1.5~5).

4. The preparation method according to claim 1, characterized in that, The first fluoropolymer, the second fluoropolymer, and the third fluoropolymer are each independently at least one selected from polyvinylidene fluoride, poly(vinylidene fluoride-cochlorotrifluoroethylene), and polyvinylidene fluoride-hexafluoropropylene; and / or, The organic solvent is at least one selected from N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; and / or, The first colloidal microsphere and the second colloidal microsphere are each independently selected from at least one of silica microspheres, zirconium dioxide, and calcium carbonate, and the first colloidal microsphere and the second colloidal microsphere have the same particle size.

5. The preparation method according to claim 1, characterized in that, The steps of immersing the PTFE membrane in the first solution, removing it, and then curing it to obtain a PTFE base film include: The PTFE membrane is immersed in the first solution under a vacuum negative pressure of 0.03MPa~0.08MPa. After full immersion, it is taken out and vacuum dried at 60℃~120℃ to obtain the PTFE base membrane.

6. The preparation method according to claim 1, characterized in that, In the step of forming a first photonic crystal layer on one side of the PTFE base film by scraping the second solution, the scraping thickness is 50μm~150μm and the scraping speed is 2~10cm / s.

7. The preparation method according to claim 1, characterized in that, In the step of applying the third solution to the side of the first photonic crystal layer away from the PTFE base film, and then inducing the second colloidal microspheres in the third solution to self-assemble using vacuum negative pressure, followed by curing after self-assembly to form a second photonic crystal layer on the side of the first photonic crystal layer away from the PTFE base film, the vacuum negative pressure is 0.02MPa~0.08MPa, the vacuum negative pressure induction time is 10min~30min, and the curing temperature is 60℃~120℃.

8. The preparation method according to claim 1, characterized in that, The step of removing the first colloidal microspheres from the first photonic crystal layer and the second colloidal microspheres from the second photonic crystal layer to obtain the uniformly porous film includes: The PTFE base film with the first photonic crystal layer and the second photonic crystal layer formed thereon is immersed in an HF solution with a mass concentration of 2% to 6% for 4 to 12 hours. After being taken out, it is washed and dried in sequence to obtain the uniformly porous film.

9. A uniformly porous membrane, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

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