A durable super-hydrophobic uniform-pore separation membrane and a preparation method and application thereof

Abstract

The application belongs to the technical field of membrane distillation, and discloses a kind of persistent super-hydrophobic uniform pore separation membrane and its preparation method and application, monomer, polymer membrane material, solvent, pore-forming agent, initiator are mixed according to proportion, constant temperature stirring, under nitrogen atmosphere and polymer membrane material conventional dissolution temperature, initiate in-situ copolymerization, form casting solution containing strong hydrophobic crosslinkable copolymer A-B, then the casting solution is made into nascent liquid membrane, through short time spray treatment accelerates liquid membrane surface interface phase separation, then the membrane is immersed in coagulation bath and crosslinked and solidified, to obtain persistent super-hydrophobic uniform pore separation membrane, its water contact angle is greater than 150°, and the tensile strength is greater than 3.5 MPa. The preparation method is simple in operation, and the reaction conditions are mild. In the process of membrane preparation, the membrane material is persistent super-hydrophobic and the pore size is uniform, which promotes the improvement of the membrane anti-wetting ability.

Description

Technical Field

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

[0002] Severe water pollution and freshwater scarcity have become critical issues affecting human survival and development. Desalination from seawater, saline water, and wastewater has become an essential requirement for ecological civilization construction and sustainable economic and social development. Membrane distillation, a novel membrane separation process using hydrophobic microporous membranes as the separation medium and vapor pressure difference as the driving force, can be used to treat high-concentration, highly polluted brine under relatively mild operating conditions (such as pressure close to atmospheric pressure and temperature below the boiling point of water) with near 100% desalination rates, achieving 100% removal of non-volatile solutes and preferential permeation and efficient recovery of volatile components in water. It has been widely used in the treatment of domestic and industrial wastewater, as well as in the pharmaceutical and food processing industries. With the deepening of research and the expansion of applications of membrane distillation technology, membrane fouling and wetting problems during operation have become increasingly prominent, becoming key factors affecting the quality, efficiency, stability, and lifespan of membrane distillation products.

[0003] Research has found that constructing superhydrophobic films with micro / nanoscale rough structures possessing low interfacial energy is key to effectively mitigating problems such as membrane fouling and wetting. Following this principle, researchers have developed various superhydrophobic films through external modification or intrinsic property alteration. External modification involves applying coatings or surface treatments to the surface of existing membrane materials to enhance their superhydrophobic properties. Materials such as fluorinated silica nanoparticles, carbon nanotubes, and metal-organic frameworks (MOFs) have been used to construct layered rough superhydrophobic functional layers on membrane surfaces to improve their antifouling and antiwetting properties. However, the deposition and modification processes are time-consuming and consume significant chemical resources, making large-scale preparation difficult. Furthermore, there is a risk of deposited components detaching and dissolving during operation, which not only compromises the superhydrophobicity but also raises concerns about potential environmental toxicity. Intrinsic modification can improve the membrane surface structure and endow it with superhydrophobic properties by controlling the intrinsic phase separation behavior during membrane formation. Commonly used methods include non-solvent-induced phase separation (NIPS) and vapor-induced phase separation (VIPS). In the NIPS method, surface-bulk superhydrophobic microstructures can be constructed by nucleation and hydrogen bonding to induce molecular chain crystallization and crystal form transformation in the membrane material, or by using soft coagulants (such as alcohols) to delay the phase separation process, promoting molecular chain rearrangement and crystallization, and constructing a microspherical rough membrane structure to improve the membrane's hydrophobicity. In the VIPS method, the hydrophobicity of the membrane is improved by exposing the scraped nascent liquid membrane to a gaseous non-solvent for an extended period, allowing it to absorb vapor and undergo solid-liquid phase separation, forming larger polymer crystals. However, whether it is induced crystallization and delayed phase separation in NIPS or vapor-induced solid-liquid phase separation in VIPS, both increase the crystallinity of the polymer, reduce the supersaturation of polymer crystal growth in the membrane material and the nucleation energy barrier during the crystallization process, and easily lead to rearrangement of the membrane material molecular chain segments due to the crystallization template effect. This results in increased dispersion of polymer micelle size, difficulty in controlling the uniformity of molecular chains and micelle spacing, and an overall structure that tends towards a spherical particle stacking structure, leading to problems such as decreased membrane strength and non-uniform pore size.

[0004] Chinese patent CN113578062A proposes a method for preparing a durable hydrophilic uniform-pore ultrafiltration membrane. This method employs a cross-linking synergistic phase separation approach to improve the durable hydrophilicity of the membrane material without affecting its mechanical properties. It also allows for precise control of polymer chain migration, reaction, and solidification, resulting in improved pore size uniformity of the separation membrane. However, introducing strongly hydrophobic functional segments into the membrane material's molecular chains reduces the hydrophilicity of the polymer. This leads to a decrease in the affinity between the casting solution and the coagulation bath (water), slowing down the exchange rate between solvent and non-solvent during phase separation. This, in turn, promotes the formation of a dense skin structure on the membrane surface, resulting in decreased membrane surface roughness and insufficient hydrophobicity, making it difficult to improve the membrane's anti-wetting properties.

[0005] Therefore, there is an urgent need to develop a simple, efficient preparation method that does not affect the performance of the membrane itself to achieve long-lasting superhydrophobicity and uniform pore size of the membrane material, thereby improving the anti-wetting performance of the hydrophobic membrane. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a durable superhydrophobic uniformly porous separation membrane, its preparation method and application. It combines in-situ polymerization, water spray-assisted phase separation and cross-linking curing to construct an open-pore wrinkled micro / nano rough structure on the membrane surface. At the same time, the arrangement of polymer molecular chains is regulated by cross-linking network, which realizes the durability of the membrane's superhydrophobic function and the uniformity of the membrane pore structure, thereby significantly improving the membrane's anti-wetting properties, separation flux and long-term operational stability.

[0007] The objective of this invention is achieved by the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a durable superhydrophobic uniformly porous separation membrane, comprising the following steps:

[0009] (1) The comonomer, polymer membrane material, solvent, pore-forming agent and initiator are mixed in proportion, stirred at a constant temperature, and an in-situ copolymerization reaction is initiated under a nitrogen atmosphere and at the conventional dissolution temperature of the polymer membrane material to form a casting solution containing a strongly hydrophobic crosslinkable copolymer AB; the comonomer includes a strongly hydrophobic molecule A and a crosslinkable molecule B, wherein the strongly hydrophobic molecule A is a molecule that simultaneously contains unsaturated bonds and polyfluoroalkyl groups; and the crosslinkable molecule B is a molecule that simultaneously contains unsaturated bonds and siloxane groups;

[0010] (2) The casting liquid is scraped into a primary ecological liquid film, and the primary ecological liquid film is placed under a spray of 200mL / h-500mL / h to obtain a surface-pre-cured primary ecological liquid film.

[0011] (3) The surface-precured nascent liquid film is placed in a coagulation bath for phase separation and cross-linking curing to obtain a durable superhydrophobic uniform porous separation membrane with an open pore structure, a wrinkled high-roughness micro / nano structure.

[0012] Preferably, the strongly hydrophobic molecule A comprises one or more of the following: 1H,1H,2H,2H-perfluorodecyl acrylate, 1H,1H,2H-perfluoro-1-dodecene, 1H,1H,2H-perfluoro-1-decene, 1H,1H,2H-perfluoro-1-hexene, perfluorohexylethyl acrylate, 2-perfluorooctylethyl methacrylate, 2-perfluorooctylethyl acrylate, and 2-perfluorobutylethyl methacrylate.

[0013] Preferably, the crosslinkable molecule B includes one or more of vinyl dimethyl ethoxysilane, vinyl methyl diethoxysilane, vinyl trimethoxysilane, dimethyl ethoxyformyl silane, 3-methacryloyloxypropylmethyl dimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, and 3-acryloyloxypropyltrimethoxysilane.

[0014] Preferably, the initiator includes one or more of azobisisobutyronitrile, azobisisovalerate, azobisisoheptanenitrile, azobiscyclohexylformitrile, and dimethyl azobisisobutyrate.

[0015] Preferably, in the casting solution, the polymer membrane material comprises 11%-19% by mass, the solvent comprises 45.90%-77.99% by mass, the pore-forming agent comprises 10%-25% by mass, the comonomer comprises 1%-10% by mass, and the initiator comprises 0.01%-0.1% by mass.

[0016] Preferably, the molar ratio of the strongly hydrophobic molecule A to the crosslinkable molecule B is 1:1 to 10:1.

[0017] Preferably, the polymer membrane material includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-trifluorochloroethylene, and polyvinylidene fluoride-hexafluoropropylene.

[0018] Preferably, the solvent includes one or more of dimethylacetamide, dimethylformamide, and N-methylpyrrolidone.

[0019] Preferably, the pore-forming agent includes one or more of methanol, ethanol, propylene glycol, and n-butanol.

[0020] Preferably, the temperature for constant temperature stirring in step (1) is 60℃-90℃, and the constant temperature dissolution time is 4-24h.

[0021] Preferably, the placement time in the spray is 30s-200s. The spray-assisted acceleration of the phase separation rate at the polymer-water interface of the nascent liquid film enables local phase separation on the membrane surface, forming a wrinkled, high-roughness micro / nano structure with open pores to enhance the hydrophobic properties of the membrane.

[0022] Preferably, the coagulation bath is a deionized water coagulation bath, and the temperature of the coagulation bath is controlled between 20℃ and 70℃.

[0023] Secondly, the present invention provides a durable superhydrophobic uniformly porous separation membrane obtained by the above preparation method. The membrane body has a strong hydrophobic cross-linked interpenetrating network structure, and the membrane surface has a wrinkled, high-roughness micro / nano structure with an open pore structure.

[0024] Thirdly, the present invention provides the application of the above-mentioned durable superhydrophobic uniformly porous separation membrane in membrane distillation.

[0025] The introduction of strongly hydrophobic molecules in this invention effectively improves the hydrophobicity of the membrane surface and reduces the interaction force between the membrane and pollutants. Short-duration water spray treatment accelerates the phase separation rate at the polymer-water interface of the nascent liquid membrane, achieving local phase separation on the membrane surface and increasing surface roughness. This facilitates the formation of a wrinkled, highly rough micro / nanostructure with open pores during phase transformation, further enhancing hydrophobic properties. This avoids the problems of decreased membrane strength and non-uniform pore size caused by prolonged exposure of the nascent liquid membrane to a gaseous non-solvent environment, leading to the formation of a polymer crystal structure. By utilizing a coagulation bath to initiate a cross-linking reaction between polymers, hydrophobic groups are fixed within the membrane as a strongly hydrophobic molecular chain cross-linked interpenetrating network of polymer molecular chains. This provides long-term stability and superhydrophobic properties while also facilitating the adjustment of the molecular chain spacing and aggregate volume during phase separation, achieving a uniform pore size structure throughout the membrane. Through the synergistic effect of strongly hydrophobic group cross-linking and the construction of a rough surface structure, a superhydrophobic uniform-pore separation membrane with superhydrophobic properties and long-term stability is obtained. The method of improving membrane-related properties through in-situ polymerization and spray-assisted crosslinking phase separation is rarely mentioned.

[0026] Advantages and beneficial effects of the present invention:

[0027] (1) This invention introduces strongly hydrophobic crosslinkable molecules into the casting solution and utilizes the synergistic phase separation and crosslinking process of spray-coagulation bath to simultaneously achieve micro / nano roughening of the membrane surface and network locking of the bulk structure. Among them, the highly rough micro / nano structure formed on the surface provides a superhydrophobic basis, while avoiding the problems of decreased membrane strength and non-uniform pore size caused by the formation of polymer crystal structure in the nascent liquid membrane due to long-term exposure to gaseous non-solvents; while the strongly hydrophobic crosslinked interpenetrating network formed in the bulk ensures the long-term stability of hydrophobic groups and overall mechanical strength, ultimately making the water contact angle of the membrane greater than 150° and the tensile strength greater than 3.5 MPa.

[0028] (2) By forming a cross-linked interpenetrating network structure, the present invention regulates the formation, growth and curing of polymer molecular chain micelles during the phase separation process, thereby avoiding the possibility of rearrangement due to the crystallization template effect, which would make it difficult to control the uniformity of polymer molecular chain and micelle spacing and the overall structure would transform into a spherical particle stacking structure. This invention achieves uniform control of membrane pore structure without damage.

[0029] (3) The preparation method of the present invention can achieve the construction of a rough structure on the membrane surface, uniform pore size and overall long-lasting hydrophobic properties without damage, and simultaneously improve the membrane distillation flux, separation performance and anti-wetting ability.

[0030] (4) The preparation method of the present invention integrates the modification, shaping and crosslinking steps into the conventional phase separation process, without the need for complex post-processing or external coating. The conditions are mild and the operation is simple, with good process compatibility and large-scale production potential. Attached Figure Description

[0031] Figure 1 This is a SEM image of the separation membrane surface obtained in Example 1;

[0032] Figure 2 This is a cross-sectional electron microscope (SEM) image of the separation membrane obtained in Example 1;

[0033] Figure 3 This is a SEM image of the separation membrane surface obtained in Comparative Example 1;

[0034] Figure 4 This is a cross-sectional electron microscope (SEM) image of the separation membrane obtained in Comparative Example 1;

[0035] Figure 5 This is a SEM image of the separation membrane surface obtained in Comparative Example 6;

[0036] Figure 6 This is a cross-sectional electron microscope (SEM) image of the separation membrane obtained in Comparative Example 6;

[0037] Figure 7 This is a comparison chart of membrane pore size and pore size distribution under different copolymer addition amounts. Detailed Implementation

[0038] To further understand the present invention, specific embodiments are given below to describe the present invention. These descriptions are only for further illustrating the features and advantages of the present invention and are not intended to limit the claims of the present invention.

[0039] Example 1

[0040] A method for preparing a durable superhydrophobic uniformly porous separation membrane, comprising the following steps:

[0041] (1) Mix 30.45g of N,N-dimethylacetamide, 8g of polyvinylidene fluoride, 7.5g of propylene glycol, 4g of comonomer, and 0.05g of dimethyl azobisisobutyrate. Stir at 80°C for 6 hours. Initiate in-situ copolymerization under nitrogen atmosphere and conventional dissolution temperature of membrane material to form a casting solution containing a strong hydrophobic crosslinkable copolymer AB. The comonomer is composed of a strong hydrophobic molecule A (1H,1H,2H,2H-perfluorodecyl acrylate) and a crosslinkable molecule B (3-acryloyloxypropyltrimethoxysilane) in a molar ratio of 2:1.

[0042] (2) The casting solution is scraped into a nascent liquid film. The nascent liquid film is placed under a water spray of 200 mL / h for 120 s, and then placed in a deionized water coagulation bath at 25 °C for phase separation, cross-linking and curing to form a film, thus obtaining a durable superhydrophobic uniform porous separation membrane with a wrinkled, high-roughness micro / nano structure and an open pore structure.

[0043] Example 2

[0044] The only difference from Example 1 is that the crosslinkable molecule B is 3-methacryloyloxypropylmethyldimethoxysilane.

[0045] Example 3

[0046] The only difference from Example 2 is that the strongly hydrophobic molecule A is 2-perfluorobutylethyl methacrylate.

[0047] Example 4

[0048] The only difference from Example 3 is that the crosslinkable molecule B is γ-methacryloyloxypropyltrimethoxysilane.

[0049] Example 5

[0050] The only difference from Example 4 is that the strongly hydrophobic molecule A is perfluorohexyl ethyl acrylate.

[0051] Comparative Example 1

[0052] The only difference from Example 1 is that the comonomer used is only the strongly hydrophobic molecule A (1H,1H,2H,2H-perfluorodecyl acrylate).

[0053] Comparative Example 2

[0054] The only difference from Example 5 is that the crosslinkable molecule B is ethoxylated trimethylolpropane triacrylate.

[0055] Comparative Example 3

[0056] The only difference from Example 1 is that the comonomer used is only crosslinkable molecule B (3-acryloyloxypropyltrimethoxysilane).

[0057] Comparative Example 4

[0058] The only difference from Example 5 is that the strongly hydrophobic molecule A is ethyl 2-methacrylate, and the crosslinkable molecule B is γ-methacryloyloxypropyltrimethoxysilane.

[0059] Comparative Example 5

[0060] The difference from Example 1 is that the casting solution is prepared by prepolymerization followed by blending, and non-in-situ polymerization, as detailed below:

[0061] (1) Prepolymerization of the strongly hydrophobic crosslinkable copolymer AB: 8g of comonomers, including the strongly hydrophobic molecule A (1H,1H,2H,2H-perfluorodecyl acrylate) and the crosslinkable molecule B (3-acryloyloxypropyltrimethoxysilane) in the comonomers, were dissolved in N,N-dimethylacetamide at the molar ratio of Example 1. 0.1g of dimethyl azobisisobutyrate initiator was added, and the reaction was carried out at 80°C for 4 hours under nitrogen protection to synthesize the strongly hydrophobic crosslinkable copolymer AB. After the reaction was completed, the solvent was removed by rotary evaporation to obtain the solid copolymer AB.

[0062] (2) Polyvinylidene fluoride, N,N-dimethylacetamide, and propylene glycol were mixed according to the proportions in Example 1 and stirred at 80°C to dissolve. After the PVDF was completely dissolved, the pre-synthesized solid copolymer AB (with the same mass as the total mass of monomers in Example 1) was added to the above polyvinylidene fluoride solution, and stirring was continued for 6 hours to dissolve or disperse it evenly, forming a casting solution. Subsequent processes such as film coating, water spraying, and coagulation bath curing were consistent with those in Example 1.

[0063] Comparative Example 6

[0064] The only difference from Example 1 is that water spraying is omitted. Instead, a conventional coagulation bath is used. After the casting solution is scraped into a nascent liquid film, water spraying is not performed. Instead, the support with the liquid film is directly immersed in the same 25°C deionized water coagulation bath as in Example 1 for conventional non-solvent-induced phase separation (NIPS) curing. Subsequent processing steps are the same.

[0065] Comparative Example 7

[0066] The only difference from Example 1 is that in step (2), the nascent liquid film is placed under a water spray of 200 mL / h for 300 s.

[0067] The membrane performance of the separation membranes prepared in Examples 1-5 and Comparative Examples 1-7 was tested, and the test results are shown in Table 1.

[0068] The water contact angle was tested according to GB / T 30447-2013 "Method for Measuring Contact Angle of Nanofilms". 5 μL of deionized water was precisely dropped onto the film surface using the seated drop method. A high-precision CCD camera system of a fully automatic contact angle measuring instrument was used to capture the droplet morphology changes in real time, and the contact angle was calculated using image analysis software. The tensile strength was tested according to the method specified in GB / T 1040.3-2006 "Determination of Tensile Properties of Plastics Part 3: Test Conditions for Films and Sheets". The sample was cut into strips with a width of 10 mm and a length of 150 mm. A tensile speed of 50 mm / min was set for mechanical loading. The tensile strength was calculated according to equation (1):

[0069]

[0070] In the formula, σ is the tensile strength, MPa; F is the maximum tensile force that the specimen can withstand before tensioning, N; and S is the original cross-sectional area of ​​the specimen, mm. 2 .

[0071] The membrane distillation flux (MD flux) and duration were tested according to the method specified in GB / T 37215-2018 "Hollow Fiber Hydrophobic Membranes for Membrane Distillation". The hot feed solution (3.5% NaCl solution) was heated to 70°C in a constant-temperature water bath and then entered the membrane distillation test cell under the drive of a magnetic pump. The solution evaporated on the membrane surface and returned to the constant-temperature water bath. Water vapor permeated through the membrane pores under the pressure difference across the membrane. The distillate, after being condensed by deionized water, was collected in the permeate. The weight of the distillate was measured using an electronic balance. Data was recorded every 10 minutes, and each test was repeated three times, with the permeate conductivity measured to confirm that the hydrophobic membrane did not wet or leak. The MD flux was calculated according to equation (2):

[0072]

[0073] In the formula, F is the membrane distillation flux, L / (m 2 ·h); Water production (L) is represented by A; effective membrane area (m²) is represented by A. 2 T represents the test time, in hours. The operating time before the concentration and permeate flow on the cold side of the membrane stabilize and before wetting or leakage occurs, as the test device continues to run, is the MD duration.

[0074] Table 1. Types of copolymers and membrane performance test results in Examples 1-5 and Comparative Examples 1-7

[0075]

[0076]

[0077] Table 1 shows that adding different strongly hydrophobic molecules A and crosslinkable molecules B to the casting solution has different effects on the performance of hydrophobic microporous membranes. Comparative Examples 1-7 serve as the control group, and Examples 1-5 serve as the experimental group. It can be seen that adding a certain amount of different strongly hydrophobic molecules A, such as 1H,1H,2H,2H-perfluorodecyl acrylate, perfluorohexyl ethyl acrylate, and 2-perfluorobutyl ethyl methacrylate, and different crosslinkable molecules B, such as 3-methacryloyloxypropylmethyldimethoxysilane, γ-methacryloyloxypropyltrimethoxysilane, and 3-acryloyloxypropyltrimethoxysilane, to the casting solution for preparing hydrophobic microporous membranes can impart excellent hydrophobicity, mechanical properties, MD flux, and operational stability, and also indicates that this method has universality.

[0078] Example 1 describes a separation membrane prepared using polyvinylidene fluoride resin as the membrane material, 1H,1H,2H,2H-perfluorodecyl acrylate as the strongly hydrophobic molecule A, and 3-acryloyloxypropyltrimethoxysilane as the crosslinkable molecule B. The addition of the strongly hydrophobic molecule A and the crosslinkable molecule B accelerates the phase separation rate at the polymer-water interface of the nascent liquid membrane with the assistance of water spray, achieving local phase separation on the membrane surface and constructing a porous and rough surface, giving the membrane superhydrophobic properties. The construction of the crosslinked structure regulates the formation and growth of polymer molecular chain micelles during the phase separation process, giving the membrane cross-section a sponge-like structure and avoiding the formation of large pores. The membrane surface and cross-sectional structures are as follows: Figure 1 and Figure 2 As shown.

[0079] In Comparative Example 1, adding only a certain amount of strongly hydrophobic molecule A failed to accelerate the phase separation rate of the polymer-water interface on the membrane surface under the assistance of water spray. The membrane surface exhibited a dense skin structure, thus the membrane did not possess superhydrophobic properties. Furthermore, the cross-linking structure was missing, and the membrane cross-section exhibited an asymmetric structure composed of finger-like macropores and a sponge-like support layer. The membrane surface and cross-sectional structures are as follows: Figure 3 and Figure 4 As shown, the presence of finger-like macropores causes a decrease in membrane strength. In Comparative Example 3, only crosslinkable molecule B was added. Due to the lack of introduction of strongly hydrophobic segments, the hydrophobicity of the membrane surface was significantly lower than that of the separation membranes prepared in Example 1 and Comparative Example 1.

[0080] Compared to Example 1, Comparative Example 2, using crosslinkable molecule B, employed ethoxylated trimethylolpropane triacrylate. This crosslinking structure of the copolymer was completed before phase separation, causing it to entangle and aggregate with the membrane material polymer. This limited the migration rate of the polymer chains, preventing the membrane from accelerating solvent-to-solvent transfer with the assistance of water spray. Consequently, the membrane surface had low roughness and lacked superhydrophobic properties. Therefore, its surface hydrophobicity was significantly lower than that of the separation membrane prepared in Example 1. Comparative Example 4, using strongly hydrophobic molecule A, employed ethyl 2-methacrylate. This only constructed a copolymer crosslinking structure and lacked strongly hydrophobic groups, failing to effectively improve the hydrophobicity of the membrane material. Therefore, its membrane surface hydrophobicity was significantly lower than that of the separation membrane prepared in Example 1.

[0081] The hydrophobicity and MD (membrane diffusing) time of the membrane obtained by prepolymerization blending modification in Comparative Example 5 were significantly lower than those in Example 1. This is because the copolymerization reaction conditions were mild, and polymerization was still likely to occur during solvent removal after the reaction, causing overpolymerization of the solid copolymer AB. This led to a sharp increase in the viscosity of the casting solution during subsequent blending, preventing it from accelerating the solvent-to-solvent transfer rate with the assistance of water spray. Consequently, the membrane surface roughness was low, and it did not possess superhydrophobic properties. Therefore, its surface hydrophobicity was significantly lower than that of the separation membrane prepared in Example 1. Furthermore, compared to in-situ polymerization, the number of operation steps was significantly increased, and a large amount of solvent resources were wasted during the preparation process.

[0082] In Comparative Example 6, water spray was omitted, and conventional non-solvent-induced phase separation and solidification were used to form the membrane. Due to the limitation of the phase separation rate at the interface between the nascent liquid membrane surface and the coagulation bath water, it was difficult to construct a highly rough micro / nano structure with open pores on the membrane surface. As a result, the hydrophobicity, membrane distillation flux, and operational stability of the separation membrane were significantly lower than those in Example 1. The membrane surface structure and cross-sectional structure are as follows: Figure 5 and Figure 6 As shown.

[0083] In Comparative Example 7, the polyvinylidene fluoride membrane obtained by excessive water spraying for 300 seconds tends to exhibit increased dispersion in polymer micelle size, making it difficult to control the uniformity of molecular chains and micelle spacing. The overall membrane structure transforms into a spherical particle packing structure. Although the hydrophobicity is comparable, the membrane strength decreases significantly, and the pore size becomes non-uniform. Therefore, compared to Example 1, the membrane tensile strength and operational stability are significantly reduced.

[0084] Comparative Example 8

[0085] A method for preparing a durable superhydrophobic uniformly porous separation membrane, comprising the following steps:

[0086] Mix 8g of polyvinylidene fluoride, 7.5g of propylene glycol, 64.9g of N,N-dimethylacetamide, and 0.05g of dimethyl azobisisobutyrate, maintaining a total casting solution volume of 100g. Stir at 80℃ for 6 hours to form a homogeneous solution. After coating, place the film in a water spray for 120 seconds, and then place it in a 25℃ deionized water coagulation bath for phase separation and curing to form a film.

[0087] Example 6

[0088] A method for preparing a durable superhydrophobic uniformly porous separation membrane, comprising the following steps:

[0089] (1) Mix 8g of polyvinylidene fluoride, 7.5g of propylene glycol, 33.45g of N,N-dimethylacetamide and 1g of comonomer (wherein, the strong hydrophobic molecule A in the comonomer is 1H,1H,2H,2H-perfluorodecyl acrylate and the crosslinkable molecule B is 3-acryloyloxypropyltrimethoxysilane, with a molar ratio of 2:1), and 0.05g of dimethyl azobisisobutyrate, maintain the total amount of casting solution at 100g, stir at a constant temperature of 80℃ for 6h, and initiate an in-situ copolymerization reaction under a nitrogen atmosphere and at the conventional dissolution temperature of the membrane material to form a casting solution containing a strong hydrophobic crosslinkable copolymer AB;

[0090] (2) The casting solution was scraped to form a nascent liquid membrane. The nascent liquid membrane was placed in a water spray of 200 mL / h for 120 s, and then placed in a deionized water coagulation bath at 25 °C for phase separation, crosslinking and curing to form a membrane. A durable superhydrophobic uniform porous separation membrane with a wrinkled, high-roughness micro / nano structure with an open pore structure was obtained.

[0091] Example 7

[0092] The only difference from Example 6 is that 32.45g of N,N-dimethylacetamide and 2g of comonomer were added to the casting solution.

[0093] Example 8

[0094] The only difference from Example 6 is that 31.45g of N,N-dimethylacetamide and 3g of comonomer were added to the casting solution.

[0095] Example 9

[0096] The only difference from Example 6 is that 29.45g of N,N-dimethylacetamide and 5g of comonomer were added to the casting solution.

[0097] Example 10

[0098] The only difference from Example 6 is that 26.45g of N,N-dimethylacetamide and 5g of comonomer were added to the casting solution.

[0099] Table 2. Effect of different comonomer contents on the hydrophobic properties of polyvinylidene fluoride membranes

[0100]

[0101] Table 2 shows the effect of comonomer content on the properties of polyvinylidene fluoride (PVDF) hydrophobic membranes. It can be seen that within a suitable range (M1-M4), the water contact angle, liquid permeation pressure, and tensile strength of the hydrophobic membrane increase with increasing copolymer content, and the membrane flux and operating time show an increasing trend. When the copolymer content exceeds a certain amount (M5), due to the increase in membrane pore size (e.g., ... Figure 7As shown in the figure, it actually has an adverse effect on the membrane's anti-wetting properties, significantly reducing its liquid permeation pressure and operating time. Therefore, it can be demonstrated that within a suitable range, changing the amount of copolymer added helps improve the membrane's hydrophobicity, anti-wetting performance, and filtration performance; the membrane pore size and pore size distribution under different copolymer addition amounts are shown in the figure. Figure 7 As shown, within a suitable range (M0-M4), the pore size distribution of the membrane narrows. This is because as the content of the comonomer increases, the polymer crosslinking network is continuously improved, and the uniformity of the pore size distribution is continuously enhanced. This indicates that this highly active coupling-induced phase separation method can provide a highly selective membrane with uniform pore size. At the same time, the average pore size gradually increases, which helps to improve the permeation flux of the hydrophobic membrane. When the additive content exceeds the suitable range (M5), the solvent in the casting solution cannot completely dissolve the copolymer, resulting in impurities in the membrane material. The overall uniformity of the membrane pore size decreases, and the pore size further increases, causing the liquid permeation pressure of the membrane material to decrease, leading to a decrease in its air wettability and a reduction in operating time.

[0102] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A method for preparing a durable superhydrophobic uniformly porous separation membrane, characterized in that: Includes the following steps: (1) The comonomer, polymer membrane material, solvent, pore-forming agent, and initiator are mixed in proportion and stirred at a constant temperature. Under a nitrogen atmosphere and at the conventional dissolution temperature of the polymer membrane material, an in-situ copolymerization reaction is initiated to form a casting solution containing a strongly hydrophobic crosslinkable copolymer AB. The comonomer includes a strongly hydrophobic molecule A and a crosslinkable molecule B. The strongly hydrophobic molecule A is a molecule that simultaneously contains unsaturated bonds and polyfluoroalkyl groups. The crosslinkable molecule B is a molecule that simultaneously contains unsaturated bonds and siloxane groups. The strongly hydrophobic molecule A includes 1H,1H,2H,2H-perfluorodecyl acrylate, 1H,1H,2H-perfluoro-1-dodecene, 1H,1H,2H-perfluoro-1-decene, and 1H,1H,2H-perfluoro-1-hexene. The crosslinkable molecule B comprises one or more of the following: perfluorohexyl ethyl acrylate, 2-perfluorooctyl ethyl methacrylate, 2-perfluorooctyl ethyl acrylate, and 2-perfluorobutyl ethyl methacrylate; the crosslinkable molecule B comprises one or more of the following: vinyl dimethyl ethoxysilane, vinyl methyl diethoxysilane, vinyl trimethoxysilane, dimethyl ethoxyformyl silane, 3-methacryloyloxypropyl methyl dimethoxysilane, γ-methacryloyloxypropyl trimethoxysilane, and 3-acryloyloxypropyl trimethoxysilane; the initiator comprises one or more of the following: azobisisobutyronitrile, azobisisovalerate, azobisisoheptanenitrile, azobiscyclohexylformitrile, and dimethyl azobisisobutyrate. (2) The casting liquid is prepared into a nascent liquid film. The nascent liquid film is placed in a spray environment for 30s-200s. The short-time water spray treatment accelerates the phase separation rate of the polymer-water interface of the nascent liquid film, realizes local phase separation on the film surface, increases the roughness of the film surface, and forms a wrinkled high roughness micro / nano structure with open pore structure during the phase transformation process, thus obtaining a surface pre-cured nascent liquid film. (3) The surface pre-cured nascent liquid film is immersed in a coagulation bath for cross-linking and curing to obtain a durable superhydrophobic uniformly porous separation membrane.

2. The preparation method according to claim 1, characterized in that: The molar ratio of the strongly hydrophobic molecule A to the crosslinkable molecule B is 1:1 to 10:

1.

3. The preparation method according to claim 1, characterized in that: The polymer membrane material includes one or more of polyvinylidene fluoride, polyvinylidene fluoride-trifluorochloroethylene, and polyvinylidene fluoride-hexafluoropropylene; the solvent includes one or more of dimethylacetamide, dimethylformamide, and N-methylpyrrolidone; and the pore-forming agent includes one or more of methanol, ethanol, propylene glycol, and n-butanol.

4. The preparation method according to claim 1, characterized in that: In the casting solution, based on a total weight of 100%, the polymer membrane material accounts for 11%-19% by mass, the solvent accounts for 45.90%-77.99% by mass, the pore-forming agent accounts for 10%-25% by mass, the comonomer accounts for 1%-10% by mass, and the initiator accounts for 0.01%-0.1% by mass.

5. The preparation method according to claim 1, characterized in that: Step (1) The temperature for constant temperature stirring is 60℃-90℃, and the constant temperature dissolution time is 4-24h.

6. The preparation method according to claim 1, characterized in that: The spray flow rate is 200 mL / h-500 mL / h; the coagulation bath is deionized water with a temperature of 20℃-70℃.

7. A durable superhydrophobic uniformly porous separation membrane obtained by the preparation method according to any one of claims 1-6, characterized in that: The membrane body has a strong hydrophobic cross-linked interpenetrating network structure, and the membrane surface has a wrinkled, highly rough micro / nano structure with an open pore structure.

8. The application of a durable superhydrophobic uniformly porous separation membrane as described in claim 7 in membrane distillation.

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