A method for preparing a super-hydrophobic film by silane-assisted non-solvent induced phase separation, a super-hydrophobic film and applications thereof

Abstract

The application belongs to the field of membrane separation technology, and discloses a method for preparing a super-hydrophobic membrane by using silane-assisted non-solvent induced phase separation, the super-hydrophobic membrane and application. After a casting solution prepared from a polymer membrane material, a solvent and a pore-forming agent is made into a nascent liquid membrane, the nascent liquid membrane is modified by using silane molecular spraying, and then the nascent liquid membrane is immersed into a coagulation bath to initiate phase separation and silane hydrolysis crosslinking. The silane molecular layer delays the phase separation rate, constructs a fiber-connected open hole high-rough structure on the membrane surface, simultaneously forms long-chain siloxane entangled with the polymer, and realizes synchronous improvement of hydrophobicity and stability. The method is simple in operation and mild in conditions, the obtained membrane has a water contact angle of > 150°, a tensile strength of > 3.4 MPa and a liquid permeation pressure of > 276 kPa, and is suitable for fields such as membrane distillation.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, and relates to a method for preparing superhydrophobic membranes by silane-assisted non-solvent-induced phase separation, the superhydrophobic membranes, and their applications. Background Technology

[0002] The growing demand for water in industry and agriculture, coupled with the relatively lagging development of advanced wastewater treatment technologies, has led to severe water pollution and freshwater shortages. Desalination of seawater and brackish water, as well as the resource utilization of wastewater, are key to solving this problem. Membrane distillation technology, with hydrophobic microporous membranes at its core, has become a research hotspot in the water treatment industry due to its low-grade energy utilization capacity, near-100% desalination capability, and relatively mild operating conditions. However, the fouling and wetting problem faced by hydrophobic membranes remains a major challenge in its application and promotion.

[0003] Traditional hydrophobic membrane materials used in membrane distillation mainly include polytetrafluoroethylene (PTFE), polypropylene (PP), and polyvinylidene fluoride (PVDF). Among them, PVDF has attracted much attention due to its good chemical stability, film-forming properties, and moderate cost. However, PVDF membranes prepared by conventional non-solvent-induced phase separation methods often have insufficient surface hydrophobicity (water contact angle is usually between 90° and 120°), and when in long-term contact with complex feed solutions, the membrane pores are easily wetted by surfactants or low surface tension substances, leading to a sharp decrease in retention rate or even process failure.

[0004] To improve the hydrophobicity and anti-wetting ability of membranes, researchers both domestically and internationally mainly follow two basic principles: first, constructing micro / nanoscale rough structures to increase surface hydrophobicity; and second, introducing low surface energy materials to reduce surface free energy. Currently, the mainstream techniques can be divided into the following four categories:

[0005] (1) Surface Post-Modification Method: Typically, a porous base film is first prepared, and then hydrophobic nanoparticles (such as silica or titanium dioxide) or low surface energy substances (such as fluorosilanes) are loaded onto the film surface through methods such as spraying, dip coating, graft polymerization, or chemical vapor deposition. For example, existing technologies disclose the dispersion of fluorinated silica nanoparticles in ethanol and spray deposition onto the surface of a pre-formed PVDF base film to obtain a superhydrophobic coating. Although this method can significantly improve the contact angle, the hydrophobic layer introduced is mostly physically adsorbed or weakly chemically bonded to the base film. Under long-term hydraulic scouring, high temperature, or strong acid and alkali environments, it is prone to detachment and peeling, leading to performance degradation and insufficient stability. Moreover, the post-processing steps are cumbersome, increasing production costs and time.

[0006] (2) Blending Modification Method: This method involves directly blending hydrophobic nanofillers (such as carbon nanotubes, graphene, organometallic frameworks, or modified inorganic nanoparticles) with polymers in a casting solution. Through a phase inversion process, these nanofillers are partially exposed on the membrane surface or distributed within the membrane, aiming to simultaneously improve the membrane's hydrophobicity and other properties. However, the dispersibility of the nanofillers and their interfacial compatibility with the polymer are key technical challenges. The fillers are prone to agglomeration, which not only affects the uniformity of the membrane structure but may also block some pores, leading to a decrease in flux. More importantly, blending modification has limited ability to finely control the microstructure of the membrane surface, making it difficult to directionally construct an ideal high-roughness structure.

[0007] (3) Film Formation Process Control Method: This method does not rely on external fillers, but directly constructs a rough membrane surface by controlling the thermodynamic and kinetic processes of phase separation. Among them, the vapor-induced phase separation method is one of the representative technologies. This method places the nascent liquid film coated by scraping in a high-humidity environment for a period of time, allowing water vapor in the air to slowly penetrate into the membrane surface, inducing the polymer to undergo slow phase separation, thereby forming a unique surface pore structure. There are reports in the existing technology of rapidly constructing superhydrophobic surfaces with open network structures by adding an alcohol / water mixture to the casting solution and combining it with short-term air exposure. However, the VIPS method usually has strict requirements on the humidity, temperature and exposure time of the environment, the process window is narrow, and the reproducibility is challenging. More importantly, this method mainly focuses on constructing a rough structure, and the surface energy of the membrane material itself is not reduced. Its long-term hydrophobic stability still faces challenges and often needs to be combined with low surface energy material coating treatment, and the process flow is not simplified.

[0008] (4) Composite structure method: In order to balance strength and performance, existing technologies have also proposed to prepare reinforced composite hydrophobic membranes, such as using fiber braided tubes as support layers and coating the inside and outside with hydrophilic / hydrophobic functional layers. Although this method improves the mechanical strength of the membrane, the membrane preparation process is complicated (such as requiring composite spinning nozzles), the multilayer structure interface may have bonding problems, and the hydrophobic stability of the functional layers also faces the common problems of the above surface modification methods.

[0009] In summary, existing technologies either focus on the later addition of hydrophobic functions, resulting in weak bonding and poor stability; or they focus on controlling the structure through complex processes, but fail to simultaneously achieve low surface energy modification, and require high process control; or they separate the structure and function, leading to lengthy process flows and low integration. How to achieve the construction of a high-roughness structure and the stable immobilization of low surface energy materials during membrane formation through a simple, efficient, and controllable process, thereby obtaining a membrane for membrane distillation that integrates superhydrophobicity, high mechanical strength, excellent anti-wetting properties, and long-term operational stability, remains a critical technical challenge to be overcome in this field. Summary of the Invention

[0010] In view of the shortcomings of the above-mentioned background technology, the purpose of this invention is to overcome the problems of functional and structural disconnect, complex process, and poor stability in the existing hydrophobic membrane preparation methods. This invention provides a method for preparing superhydrophobic membranes using silane-assisted non-solvent-induced phase separation, the superhydrophobic membrane itself, and its applications. By spraying silane molecules onto the surface of a nascent liquid membrane, the silane molecular layer slows down the phase separation rate to construct a fibrous, open-pore, high-roughness structure. Simultaneously, a coagulation bath induces in-situ hydrolysis and cross-linking of silane to form long-chain siloxanes entangled with polymers, achieving the integrated and simultaneous construction of a high-roughness membrane structure and low surface energy properties. This method is simple, efficient, and integrated.

[0011] The technical solution adopted in this invention is:

[0012] The first aspect of this invention provides a method for preparing superhydrophobic membranes using silane-assisted solvent-free induced phase separation, comprising the following steps:

[0013] (1) Mix the polymer membrane material, solvent and pore-forming agent, stir and dissolve at a constant temperature to obtain a uniform casting solution, and prepare the casting solution as a nascent liquid membrane;

[0014] (2) A nascent liquid film is modified by spraying silane molecules to form a nascent liquid film with a silane molecule layer on its surface. During this process, polyvinylidene fluoride molecules and silane molecules permeate each other between the film surface layer and the silane molecule layer. The silane molecules include one or more of dimethyldimethoxysilane, ethyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, 3,3,3-trifluoropropylmethyldimethoxysilane, and 1H,1H,2H,2H-perfluorooctylmethyldimethoxysilane.

[0015] (3) The nascent liquid membrane with a silane molecular layer on its surface is immersed in a coagulation bath for phase separation and solidification to obtain a superhydrophobic membrane. During this process, the silane molecular layer slows down the liquid-liquid phase separation rate on the membrane surface. At the same time, the silane molecules undergo hydrolysis and cross-linking under the action of the coagulation bath, resulting in chain growth. This allows the interpenetrating silane molecules and the membrane material polymer to become entangled and fixed on the membrane surface, providing the membrane material with long-term stability and superhydrophobic properties.

[0016] Preferably, the polymer membrane material includes one or more of polyvinylidene fluoride, polyvinyl chloride, polyvinylidene fluoride-hexafluoropropylene, and polyvinylidene fluoride-trifluorochloroethylene.

[0017] Preferably, the solvent includes one or more of dimethylacetamide, dihydrol-L-glucanone, dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0018] Preferably, the pore-forming agent includes one or more of water, ethanol, methanol, isopropanol, propylene glycol, and n-butanol.

[0019] Preferably, the casting solution comprises 11wt%-20wt% polymer film material, 55wt%-80wt% solvent, and 9wt%-25wt% pore-forming agent, based on a total mass of 100wt%.

[0020] Preferably, the temperature for constant temperature stirring and dissolving is 60℃-90℃, and the time is 4-12h.

[0021] Preferably, the spray modification time is 20-200s and the spray flow rate is 100-400mL / h.

[0022] Preferably, the coagulation bath is a deionized aqueous solution, and the coagulation bath temperature is 20℃~70℃.

[0023] The second aspect of the present invention provides a superhydrophobic membrane prepared by the above method, wherein the membrane surface has an enhanced network structure in which low surface energy long-chain siloxane and polymer molecular chains are intertwined, and presents a fibrous, open-pore, high-roughness structure.

[0024] Preferably, the prepared superhydrophobic membrane simultaneously possesses a high water contact angle (>150°), high liquid permeation pressure (>276 kPa), high tensile strength (>3.4 MPa), and long-term operational stability, meeting the demanding requirements of membrane distillation applications.

[0025] A third aspect of the present invention provides an application of the above-mentioned superhydrophobic membrane in the preparation of membrane distillation devices, membrane deammoniation devices, or pervaporation membrane devices.

[0026] This invention proposes an innovative method for preparing superhydrophobic membranes using silane-assisted solvent-induced phase separation. The core concept lies in precisely introducing hydrolyzable crosslinkable silane molecules into the critical initial stage of solvent-induced phase separation through a process intervention, allowing them to simultaneously act as a phase separation kinetic regulator and a surface functionalization precursor. On one hand, the spraying of silane molecules forms a multilayer structure of initial membrane-silane molecule layer-water, delaying liquid-liquid phase separation. This constructs a fibrous, open-pore, high-roughness structure suitable for membrane distillation on the membrane surface in situ, preventing the cross-section from developing into a fragile structure with large pores, eliminating the need for subsequent roughening treatment. On the other hand, under the solubilization and dehydration control of a polar solvent (such as N,N-dimethylacetamide), the hydrolysis reaction is uniform and controllable, avoiding instantaneous self-condensation of silanols. Simultaneously, the hydrolysis rate of multiple hydrolyzable groups in the silane molecule decreases sequentially with the increase in the number of hydrolyzed hydroxyl groups, causing hydrolysis and polycondensation to preferentially proceed along the chain extension direction, forming low-branched long-chain siloxanes. Subsequently, the long-chain siloxane migrates into the membrane through interdiffusion between the solvent and non-solvent on the initial membrane surface, where it interpenetrates and entangles with the polymer molecular chains of the membrane material, forming an interpenetrating network structure. This stabilizes the low surface energy long-chain siloxane on the membrane surface, simultaneously improving the hydrophobicity and stability of the membrane material. Through the above-mentioned phase separation kinetics regulation, a membrane with high mechanical strength is obtained while optimizing the membrane bulk structure and avoiding finger-like macropore defects. However, this method of improving membrane-related properties through silane-assisted non-solvent-induced phase separation is rarely mentioned. This invention is an innovation based on this insight.

[0027] Advantages and beneficial effects of the present invention:

[0028] (1) High process innovation, achieving integrated construction of structure and function: This invention breaks away from the traditional approach of first forming a film and then modifying or simply blending it. By introducing silane spray in the most critical initial liquid film stage, the two originally independent processes of phase separation process regulation and surface chemical modification are cleverly integrated into a synergistic and continuous step. During the film formation process, silane molecules simultaneously complete the transformation from phase separation kinetic regulator to surface functionalization precursor, realizing the in-situ, synchronous, and integrated generation of high roughness micro / nano structures and low surface energy characteristics on the film surface. The process path is fundamentally innovative.

[0029] (2) Excellent and stable hydrophobic membrane surface: Utilizing the in-situ hydrolysis and cross-linking reaction of silane molecules during phase separation, a low-surface-energy long-chain polysiloxane is generated, which is firmly bonded to the membrane substrate through molecular chain entanglement rather than physical adsorption. This endows the membrane surface with extremely low surface energy and true superhydrophobicity (water contact angle greater than 150°). More importantly, this low-surface-energy layer fixed by chemical cross-linking has excellent structural stability and durability, and can resist long-term hydraulic erosion, temperature changes and chemical corrosion, overcoming the fatal defect of easy peeling off of post-modification coatings.

[0030] (3) Significantly improved membrane bulk mechanical strength: The silane molecular layer slows down the phase separation rate, guiding the internal connection of the membrane to a tightly connected, fibrous, open-pore, high-roughness structure, replacing the finger-like macroporous structure with weak mechanical properties commonly found in the traditional NIPS method. This optimized bulk structure significantly improves the tensile strength of the resulting superhydrophobic membrane (greater than 3.4 MPa), meeting the mechanical strength requirements for long-term operation of the membrane module.

[0031] (4) Outstanding anti-wetting performance and long service life: The combined effect of high surface hydrophobicity (high contact angle) and optimized surface pore structure endows the membrane with extremely high liquid permeation pressure (greater than 276 kPa), which means that higher pressure is required to force liquid into the membrane pores. Therefore, the membrane has extremely strong anti-wetting ability. In long-term operation tests of membrane distillation, the membrane prepared by this invention can maintain high flux and nearly 100% salt rejection rate for more than 100 hours, showing excellent operational stability. Attached Figure Description

[0032] Figure 1 The infrared spectra of the superhydrophobic membrane prepared in Example 1 before and after immersion for 30 days are compared with those of the hydrophobic membrane prepared in Comparative Example 1.

[0033] Figure 2 This is a surface electron microscope (SEM) image of the superhydrophobic film prepared in Example 1;

[0034] Figure 3 This is a cross-sectional electron microscope (SEM) image of the superhydrophobic film prepared in Example 1;

[0035] Figure 4 This is a surface electron microscope (SEM) image of the hydrophobic film prepared in Comparative Example 1;

[0036] Figure 5 This is a cross-sectional electron microscope (SEM) image of the hydrophobic film prepared in Comparative Example 1.

[0037] Figure 6 This is a surface electron microscope (SEM) image of the hydrophobic film prepared in Comparative Example 3;

[0038] Figure 7 This is a cross-sectional electron microscope (SEM) image of the hydrophobic film prepared in Comparative Example 3. Detailed Implementation

[0039] To further understand the present invention, specific embodiments are provided below to describe the invention in detail. This is intended to further illustrate the technical features and beneficial effects of the present invention, and not to limit the scope of the patent rights of the present invention.

[0040] Example 1

[0041] A method for preparing superhydrophobic membranes using silane-assisted solvent-free induced phase separation includes the following steps:

[0042] (1) Dissolve 69wt% (34.5g) N,N-dimethylacetamide (solvent), 16wt% (8g) polyvinylidene fluoride (polymer membrane material), and 15wt% (7.5g) propylene glycol at 80℃ for 4 h to form a uniform casting solution. Scrape the casting solution into a nascent liquid film.

[0043] (2) Spray 300 mL / h of silane molecules (dimethyldimethoxysilane) to modify the nascent liquid film for 120 s to form a nascent liquid film with a silane molecule layer on the surface;

[0044] (3) The nascent liquid film with a silane molecular layer on its surface was placed in a 45°C deionized water coagulation bath for phase separation and solidification to obtain a superhydrophobic film.

[0045] Example 2

[0046] The only difference from Example 1 is that the silane molecule is 3,3,3-trifluoropropylmethyldimethoxysilane.

[0047] Example 3

[0048] The only difference from Example 1 is that the silane molecule is 1H,1H,2H,2H-perfluorooctylmethyldimethoxysilane.

[0049] Example 4

[0050] The only difference from Example 1 is that the silane molecule is ethyltrimethoxysilane.

[0051] Example 5

[0052] The only difference from Example 1 is that the silane molecule is methyltrimethoxysilane.

[0053] Comparative Example 1

[0054] The only difference from Example 1 is that silane molecule spraying modification is not performed, step (2) is omitted, and step (3) is performed directly using the nascent liquid membrane.

[0055] Comparative Example 2

[0056] The only difference from Example 1 is that the silane spray liquid is replaced with an equal amount of anhydrous ethanol for spraying. That is, it only simulates the physical process of silane spraying (solvent evaporation, possible instantaneous impact on the surface), but does not contain crosslinkable silane molecules.

[0057] The superhydrophobic membranes prepared in Examples 1-5 and Comparative Examples 1-2 were tested for membrane performance. The test results are shown in Table 1.

[0058] The water contact angle was tested according to the method specified in GB / T 30447-2013 "Method for Measurement of Contact Angle of Nanofilms". After the sample was completely dried, it was pasted onto a glass slide, and 5 μL of deionized water was dropped onto the sample surface. The water contact angle was obtained automatically by computer using the Young-Laplace equation. 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 long strips of fixed size (10 mm wide and 150 mm long), and mechanical loading was applied at a tensile speed of 50 mm / min. The tensile strength was calculated according to equation (1):

[0059] (1)

[0060] In the formula, σ is the tensile strength, MPa; F is the maximum tensile force that the specimen can withstand before tensile fracture, N; and S is the original cross-sectional area of ​​the specimen, mm. 2 .

[0061] The liquid permeation pressure was tested according to the method specified in GB / T 42270-2022 "Test Method for Hydrophobic Properties of Porous Hydrophobic Membranes". The membrane sample was fixed in the membrane cell, deionized water was added to the surface side and the pressure was gradually increased. The initial pressure was fixed at 1.0 MPa, and the pressure was increased by 0.02 MPa each time. The pressure was maintained for 10 seconds, and the pressure value when the first drop of liquid passed through the membrane sample was observed and recorded as the liquid permeation pressure.

[0062] The MD flux and duration were tested according to the method specified in GB / T 37215-2018 "Hollow Fiber Hydrophobic Membranes for Membrane Distillation". The feed solution (70℃, 3.5% NaCl solution) was pumped into the membrane distillation test cell, allowing its vapor to permeate through the membrane pores under the pressure difference. The distillate, after being condensed by deionized water, was collected in the permeate. The weight of the permeate was measured in real time using an electronic balance, and the conductivity of the product water was measured in real time using a conductivity meter to confirm that no wetting or leakage occurred in the hydrophobic membrane. The MD flux was calculated according to equation (2):

[0063] (2)

[0064] 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.

[0065] Table 1. Effects of different types of silane molecular spray modification on the performance of hydrophobic microporous membranes.

[0066]

[0067] In Table 1, Comparative Examples 1-2 serve as the control group, and Examples 1-5 serve as the experimental group. Based on the results of Table 1 showing the effects of different silane molecules spray-modifying the nascent liquid membrane on the performance of the hydrophobic microporous membrane, it can be seen that in the process of preparing the hydrophobic microporous membrane, spray-modifying the nascent liquid membrane with different silane molecules, such as dimethyldimethoxysilane, 3,3,3-trifluoropropylmethyldimethoxysilane, ethyltrimethoxysilane, and methyltrimethoxysilane, can all endow the separation membrane with excellent hydrophobicity, anti-wetting properties, MD flux, and operational stability.

[0068] Example 1 describes a hydrophobic membrane prepared using dimethyldimethoxysilane as the modified silane molecule. Silane molecules are sprayed onto the surface of the nascent liquid membrane. After immersion in a coagulation bath, deionized water allows for hydrolytic cross-linking reactions between the silane molecules, forming low-surface-energy long-chain siloxanes fixed on the membrane surface, providing good hydrophobic stability to the membrane material. Figure 1 As shown, the infrared spectrum of this membrane, compared with that of Comparative Example 1, confirms the formation of a cross-linked structure on the membrane surface; the infrared spectra before and after 30 days of immersion show no significant changes, demonstrating the stability of its chemical structure. Simultaneously, the presence of the silane molecular layer hinders the rapid intrusion of deionized water into the membrane surface, slowing down the liquid-liquid phase separation rate, thereby constructing a highly rough morphology on the membrane surface and enhancing its hydrophobic properties. The membrane surface structure and cross-sectional structure of Example 1 are shown below. Figures 2-3 As shown.

[0069] In Comparative Example 1, the hydrophobic membrane prepared by conventional phase separation without silane molecule spray modification exhibits several drawbacks. Firstly, the membrane surface displays a porous skin structure, lacking the high roughness required for superhydrophobicity, resulting in significantly lower hydrophobicity compared to the superhydrophobic membranes prepared in Examples 1-5. Secondly, the absence of a silane molecule layer allows for rapid diffusion of solvent and non-solvent, leading to an asymmetric cross-section composed of finger-like macropores and a sponge-like support layer. The presence of these finger-like macropores negatively impacts membrane strength, resulting in significantly lower tensile strength compared to the superhydrophobic membranes prepared in Examples 1-5. The surface and cross-sectional structures of the membrane in Comparative Example 1 are as follows: Figures 4-5 As shown.

[0070] Comparative Example 2 prepared a hydrophobic membrane by spray modification with anhydrous ethanol. The membrane surface structure underwent a gelation reaction, resulting in a highly rough structure on the liquid membrane surface after immersion in the coagulation bath. The contact angle was improved compared to Comparative Example 1, but it was difficult to achieve superhydrophobicity (>150°), and the liquid permeation pressure and long-term operational stability were far lower than those of the examples.

[0071] Example 6

[0072] A method for preparing superhydrophobic membranes using silane-assisted solvent-free induced phase separation includes the following steps:

[0073] (1) Weigh 69wt% (34.5g) N,N-dimethylacetamide (solvent), 16wt% (8g) polyvinylidene fluoride (polymer membrane material), and 15wt% (7.5g) propylene glycol, and stir at 80℃ for 4 h to form a uniform casting solution. Scrape the casting solution into a nascent liquid film.

[0074] (2) Spray 400 mL / h of dimethyldimethoxysilane to modify the nascent liquid film for 5 s to form a nascent liquid film with a silane molecular layer on the surface;

[0075] (3) The nascent liquid film with a silane molecular layer on its surface is placed in a 50°C deionized water coagulation bath for phase separation and solidification to obtain a superhydrophobic film.

[0076] Example 7

[0077] The only difference from Example 6 is that the spraying time is 30 seconds.

[0078] Example 8

[0079] The only difference from Example 6 is that the spraying time is 60 seconds.

[0080] Example 9

[0081] The only difference from Example 6 is that the spraying time is 90 seconds.

[0082] Example 10

[0083] The only difference from Example 6 is that the spraying time is 120 seconds.

[0084] Comparative Example 3

[0085] The only difference from Example 6 is that the spraying time is 300s.

[0086] The superhydrophobic membranes prepared in Examples 6-10 and Comparative Example 3 were tested for membrane performance. The test results are shown in Table 2.

[0087] Table 2. Effects of different spraying times on hydrophobic film properties.

[0088]

[0089] Table 2 shows the effect of dimethyldimethoxysilane on the performance of the hydrophobic membrane under different spraying times. It can be seen that, within an appropriate spraying time, the water contact angle and liquid permeation pressure in the hydrophobic membrane significantly increase with increasing spraying time, while the membrane flux and operating time also show an increasing trend. Therefore, it can be demonstrated that changing the spraying time helps improve the membrane's hydrophobicity, anti-wetting properties, and filtration performance. Thus, when using silane molecules to spray-modify the membrane surface, the hydrophobicity and permeability of the hydrophobic membrane can be controlled to a certain extent by changing the spraying time.

[0090] In Comparative Example 3, the excessive silane molecule spraying time modification of the hydrophobic membrane led to the transformation of the fibrous open-pore high-roughness structure of the membrane surface and the sponge-like support structure of the membrane cross-section into a spherical aggregate structure. This resulted in increased polymer micelle size dispersion and decreased uniformity of molecular chains and micelle spacing, leading to decreased membrane strength and pore size inhomogeneity. Therefore, compared to Example 1, the membrane tensile strength, liquid permeation pressure, and operational stability were significantly reduced. The membrane surface and cross-sectional structures of Comparative Example 3 are as follows: Figure 6-7 As shown.

[0091] Comparative Example 4: Film Formation Followed by Modification

[0092] A method for preparing a PVDF hydrophobic membrane includes the following steps:

[0093] (1) Dissolve 69wt% (34.5g) N,N-dimethylacetamide (solvent), 16wt% (8g) polyvinylidene fluoride (polymer membrane material), and 15wt% (7.5g) propylene glycol at 80℃ for 4 h to form a uniform casting solution. Scrape the casting solution into a nascent liquid film.

[0094] (2) The nascent liquid membrane was placed in a 45°C deionized water coagulation bath for phase separation and solidification to obtain the base membrane.

[0095] (3) After the cured PVDF film is air-dried, it is placed in an oven at 40°C for 4 hours to dry and remove water. Then, dimethyldimethoxysilane molecules and aqueous solutions with the same composition and concentration as in Example 1 are sprayed onto the surface of the dried base film, and then heat-treated by drying in an oven at 80°C for 2 hours to promote silane crosslinking.

[0096] Comparative Example 5: Direct Blending Modification

[0097] A method for preparing a PVDF hydrophobic membrane, comprising:

[0098] 59wt% (29.5g) N,N-dimethylacetamide (solvent), 16wt% (8g) polyvinylidene fluoride (polymer membrane material), 15wt% (7.5g) propylene glycol, and 10wt% (5g) dimethyldimethoxysilane were stirred and dissolved at 80℃ for 4 h to form a uniform casting solution. After coating, the solution was placed in a 45℃ deionized water coagulation bath for phase separation and curing to form a film.

[0099] Comparative Example 6

[0100] Referring to the core of patent CN112295409A, a casting solution containing alcohol / water additives was prepared: 59wt% (29.5g) N,N-dimethylacetamide (solvent), 16wt% (8g) polyvinylidene fluoride (polymer membrane material), 25wt% (12.5g) a mixture of propylene glycol and water (propylene glycol / water mixture mass ratio of 8:1). The solution was stirred and dissolved at 80℃ for 4 h to form a homogeneous solution. After coating, the solution was placed in a 45℃ deionized water coagulation bath for phase separation and curing to form a film.

[0101] The superhydrophobic membranes prepared in Comparative Examples 4-6 were tested for membrane performance, and the test results are shown in Table 3.

[0102] Table 3. Membrane test results of Comparative Examples 4-6

[0103]

[0104] As shown in Table 3, the PVDF hydrophobic membrane prepared in Comparative Example 4 using a pre-filming and post-modification method resulted in an improved water contact angle compared to Comparative Example 1, due to the presence of only low surface energy materials and the absence of highly rough micro / nano structures on the membrane surface. However, it still failed to achieve superhydrophobicity (>150°), and the membrane structure remained largely unchanged. Consequently, its tensile strength was similar to that of Comparative Example 1, but significantly lower than that of Example 1. Furthermore, the presence of the modified layer may have blocked the pores on the membrane surface, leading to a significant decrease in MD flux, which was far lower than that of the hydrophobic membranes prepared in Examples 1-5.

[0105] In Comparative Example 5, the modified molecules were directly added to the casting solution to prepare a PVDF hydrophobic membrane. Due to the low surface concentration of silane distributed throughout the membrane matrix, the improvement in surface hydrophobicity was limited. The water contact angle was improved compared to Comparative Example 1, but it was difficult to achieve superhydrophobicity (>150°). On the other hand, the absence of the silane molecular layer allowed for rapid diffusion of solvent and non-solvent, resulting in a porous skin structure on the membrane surface, which significantly improved the membrane's hydrophobicity. The membrane cross-section exhibited an asymmetric structure composed of finger-like macropores and a sponge-like support layer. Therefore, the tensile strength was similar to that of Comparative Example 1, but much lower than that of Example 1.

[0106] Comparative Example 6 prepared a PVDF hydrophobic membrane according to patent CN112295409A. Due to the synergistic effect of the non-solvent in the casting solution and water vapor in the air, the membrane surface structure was fixed before immersion in the coagulation bath, thus constructing a superhydrophobic membrane during phase separation. However, the membrane as a whole exhibited a loose spherical aggregate structure similar to the surface, which significantly affected its mechanical properties, making them far inferior to those of Comparative Example 1 and the modified membrane prepared by the method of this invention. Furthermore, due to the lack of chemical cross-linking fixation of low surface energy substances on the surface, its anti-wetting properties and long-term operational stability were far lower than those of the examples.

[0107] 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 superhydrophobic membranes using silane-assisted solvent-inducing phase separation, characterized in that: Includes the following steps: (1) Mix the polymer membrane material, solvent and pore-forming agent, stir and dissolve at a constant temperature to obtain a uniform casting liquid, and scrape the casting liquid into a nascent liquid film; (2) Modify the nascent liquid film by spraying silane molecules to form a nascent liquid film with a silane molecule layer on the surface; the silane molecules include one or more of dimethyldimethoxysilane, ethyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, 3,3,3-trifluoropropylmethyldimethoxysilane, and 1H,1H,2H,2H-perfluorooctylmethyldimethoxysilane; (3) The nascent liquid film with a silane molecular layer on its surface is immersed in a coagulation bath for phase separation and solidification to obtain a superhydrophobic film.

2. The method according to claim 1, wherein the polymer membrane material comprises one or more of polyvinylidene fluoride, polyvinyl chloride, polyvinylidene fluoride-hexafluoropropylene, and polyvinylidene fluoride-trifluorochloroethylene.

3. The method according to claim 1, characterized in that: The solvent includes one or more of dimethylacetamide, dihydrol-L-glucanone, dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone.

4. The method according to claim 1, characterized in that: The pore-forming agent includes one or more of water, ethanol, methanol, isopropanol, propylene glycol, and n-butanol.

5. The method according to claim 1, characterized in that: The casting solution comprises 11wt%-20wt% polymer film material, 55wt%-80wt% solvent, and 9wt%-25wt% pore-forming agent, based on a total mass of 100wt%.

6. The method according to claim 1, characterized in that: The spray modification time of the silane molecules is 20s-200s, and the spray flow rate is 100-400mL / h.

7. The method according to claim 1, characterized in that: The constant temperature stirring and dissolving temperature is 60℃-90℃, and the time is 4-12h.

8. The method according to claim 1, characterized in that: The coagulation bath is a deionized aqueous solution, and the coagulation bath temperature is 20℃~70℃.

9. A superhydrophobic membrane, characterized in that, The superhydrophobic membrane obtained by the method according to any one of claims 1-8 has a reinforced network structure on its surface in which low surface energy long-chain siloxanes and polymer molecular chains are intertwined, and exhibits a fibrous, open-pore, high-roughness structure.

10. The application of the superhydrophobic membrane as described in claim 9 in the preparation of membrane distillation devices, membrane deammoniation devices, or pervaporation membrane devices.

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