An underwater super-amphiphobic polyvinylidene fluoride oil-water separation membrane and a preparation method thereof

Abstract

The application belongs to the field of oil-water separation membranes, and provides an underwater super-amphiphobic polyvinylidene fluoride oil-water separation membrane and a preparation method thereof.The separation membrane is composed of a polyvinylidene fluoride membrane substrate and a functional polymer, the functional polymer is a polyamide polymer, the amount of the functional polymer is 60% to 80% of the mass of the polyvinylidene fluoride membrane substrate, the molecular chains of the functional polymer and the polyvinylidene fluoride membrane substrate are intertwined to form a semi-interpenetrating network structure, the surface of the separation membrane has a plurality of uniformly distributed nano-pillar structures, and the inside of the separation membrane has a membrane hole structure; the separation membrane has underwater oleophobicity and oil-water emulsion separation performance. The application simultaneously improves the super-amphiphobic property and bidirectional oil-water emulsion separation performance of the polyvinylidene fluoride oil-water separation membrane, effectively widens the application scenarios of the polyvinylidene fluoride oil-water separation membrane, and simplifies the preparation process of the underwater super-amphiphobic polyvinylidene fluoride oil-water separation membrane.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of oil-water separation membranes, and relates to an underwater super-dual-scrubbing polyvinylidene fluoride oil-water separation membrane and a preparation method thereof. BACKGROUND

[0002] With the development of modern society, oily wastewater pollution has become a difficult problem that needs to be solved urgently. Traditional oil-water separation methods include centrifugal separation, flotation, adsorption, electrochemistry, oxidation, microwave radiation, biodegradation, and extraction, etc. These methods have problems such as low separation efficiency and precision, high energy consumption, complex process, expensive equipment, and secondary pollution, etc. Membrane separation technology can solve the above problems. Especially for oil-water emulsion systems with droplet diameters less than 20 μm, precise separation can be achieved by adjusting the membrane pore size, and the membrane separation process can be carried out at room temperature and can be completed under gravity, which belongs to a physical process and does not require the addition of chemical reagents, and is simple and convenient to operate.

[0003] Wettability is one of the important basic properties of solid surfaces, which is mainly affected by the chemical composition and multi-level structure of the solid surface. To achieve extremely high wettability of the surface, the chemical composition of the surface must meet certain requirements, and then a rough surface, especially a multi-level micro-nano structure, is constructed on this basis to amplify the inherent wetting behavior of the surface. Restricted by the thermodynamic contradiction, in the solid-oil-water system, according to Young's equation, the sum of the contact angles of water under oil and oil under water is 180° in principle. Therefore, the surface that is oleophobic in water is generally hydrophilic in oil, and the surface that is hydrophobic in oil is generally oleophilic in water, i.e. underwater oleophobicity and oil underwater hydrophobicity are understood as mutually contradictory properties. Moreover, the surface with high surface energy has stronger affinity to water than to oil, and the surface with low surface energy has stronger affinity to oil. This is the reason why underwater super-oleophobicity occurs on high-energy surfaces, while oil underwater super-hydrophobicity occurs on low-energy surfaces. Although there are many surfaces that exhibit either of the above two properties, surfaces that simultaneously exhibit both properties are extremely rare. These two properties are like the "fish and bear's palm cannot be obtained simultaneously" state for oil-water separation membranes, which greatly limits the application scenarios of oil-water separation membranes. In this regard, researchers hope to simultaneously endow the oil-water separation membrane surface with underwater super-hydrophobicity and oil underwater super-hydrophobicity through chemical modification or physical modification, in order to achieve the purpose of bidirectional separation of water-in-oil and oil-in-water emulsions.

[0004] With the advantages of surface modification technology and nanotechnology, "super-hydrophobic-super-oleophilic" and "super-hydrophilic-underwater super-oleophobic" materials have been developed. But these materials can only be applied to one type of oil / water separation. Therefore, it is of great significance to construct "under-liquid super-amphiphobic" surface to deal with various oil / water systems, including immiscible light / heavy oil water mixture, water-in-oil emulsion and oil-in-water emulsion. In order to construct "under-liquid super-amphiphobic" surface, functional materials are usually needed to modify the membrane substrate and construct multi-level micro-nano structure. According to whether chemical bonds are formed between functional materials and membrane substrate, the modification methods can be divided into chemical modification method and physical modification method. In the separation membrane prepared by chemical modification method, functional materials are combined with membrane substrate by covalent bond. For example, first treat the membrane substrate surface with alkali to form carbon-carbon double bond, then combine the monomer or functional polymer of functional materials with the membrane substrate together by covalent bond. For another example, first form active free radicals on the membrane substrate by free radical polymerization, plasma grafting, ozone treatment, gamma ray irradiation grafting, ultraviolet light grafting, atom transfer radical polymerization, reversible addition-fragmentation chain transfer radical polymerization, etc., then covalently combine the monomer or functional polymer of functional materials with the membrane substrate. But due to the complex operation and high requirement for equipment, the application of chemical modification method is limited. Physical modification method is to introduce functional materials into the membrane substrate by surface coating, blending, template method, etc., and the functional materials are combined together with the membrane substrate by physical action such as mixing, chain intertangling and interpenetration. However, the construction of "under-liquid super-amphiphobic" surface by physical modification method mainly faces the problems of needing template, unstable functional layer and needing external stimulus to realize, etc. SUMMARY

[0005] The present application aims to overcome the deficiencies of the prior art, and provide an under-liquid super-amphiphobic polyvinylidene fluoride oil-water separation membrane and a preparation method thereof, so as to improve the super-amphiphobic properties and bidirectional oil-water emulsion separation performance of the polyvinylidene fluoride oil-water separation membrane, and simplify the preparation process of the under-liquid super-amphiphobic polyvinylidene fluoride oil-water separation membrane.

[0006] To achieve the above-mentioned application purposes, the technical solutions adopted by the present application are as follows:

[0007] An under-liquid super-amphiphobic polyvinylidene fluoride oil-water separation membrane, which is composed of a polyvinylidene fluoride membrane substrate and a functional polymer. The functional polymer is a polyamide polymer, and the amount of the functional polymer is 60% to 80% of the mass of the polyvinylidene fluoride membrane substrate. The molecular chains of the functional polymer and the polyvinylidene fluoride membrane substrate intertangle to form a semi-interpenetrating network structure. The surface of the separation membrane has a plurality of uniformly distributed nano-pillar structures, and the inside of the separation membrane has a membrane hole structure. The separation membrane has underwater oleophobicity and oil-under hydrophobicity.

[0008] The functional polymer is poly (N-n-propyl acrylamide), poly (N-isopropyl acrylamide) or poly (N-isopropyl methacrylamide).

[0009] The height of the nano-pillar structure on the surface of the separation membrane is 60-150 nm, the diameter is 0.4-0.6 μm, and the distance between adjacent nano-pillar structures is 0.6-0.8 μm. s The solid fraction f s Calculated from formula (1) :

[0010]

[0011] In formula (1), n is the number of nano-pillar structures per unit area, and D is the height (nm) of the nano-pillar structure.

[0012] The underwater oil contact angle of the separation membrane is 131-155°, the oil underwater water contact angle is 146-165°, and the oil underwater water rolling angle is 2.3-5.4°.

[0013] The separation efficiency of the separation membrane for water-in-oil emulsion is at least 96%, and the separation efficiency of the separation membrane for oil-in-water emulsion is at least 90%. Further, the separation efficiency of the separation membrane for water-in-oil emulsion is at least 99%, and the separation efficiency of the separation membrane for oil-in-water emulsion is at least 96%. Still further, the separation efficiency of the separation membrane for water-in-oil emulsion is at least 99%, and the separation efficiency of the separation membrane for oil-in-water emulsion is at least 99%. For example, the separation efficiency of the super-amphiphobic polyvinylidene fluoride oil-water separation membrane prepared in Example 1 of the present application for oil-in-water emulsion and water-in-oil emulsion reaches more than 99.9%.

[0014] The thickness of the separation membrane is determined according to the actual application requirements, and generally, the thickness of the separation membrane is 70-100 μm.

[0015] The present application also provides a preparation method of the above-mentioned super-amphiphobic polyvinylidene fluoride oil-water separation membrane, comprising the following steps:

[0016] (1) Dissolve the functional monomer, polyvinylidene fluoride and crosslinking agent in a solvent to obtain a reaction solution; in the reaction solution, the amount of functional monomer is 60%-80% of the mass of polyvinylidene fluoride, and the amount of crosslinking agent is 1.5%-2.5% of the molar amount of functional monomer;

[0017] (2) under stirring, nitrogen is introduced into the container containing the reaction solution to remove oxygen in the reaction solution and the container, then an initiator is added and the reaction is carried out at 65-85℃ for 6-10h under sealing condition, to obtain a casting solution; during the reaction, the functional monomer is polymerized in situ into a functional polymer, and the molecular chains of the functional polymer and polyvinylidene fluoride are intertwined to form a semi-interpenetrating network structure;

[0018] (3) the casting solution is allowed to stand at 50-70℃ for 12-24h, then defoaming is carried out, and the temperature of the defoamed casting solution is adjusted to 50-70℃;

[0019] (4) the temperature of the support plate of the flat blade coater is adjusted to 50-70℃, the defoamed casting solution with the same temperature as the support plate of the flat blade coater is poured onto the support plate for blade coating, the wet film obtained by blade coating is transferred to a constant temperature and humidity environment with a temperature of 15-30℃ and a relative humidity of 65%-90% for exposure for 15-30s, then immediately transferred to a coagulation bath with a temperature of 15-30℃ for solidification of the wet film, the solidified film is washed with water and dried, to obtain the liquid-substrate super-amphiphobic polyvinylidene fluoride oil-water separation film.

[0020] In the technical scheme of the above preparation method, the functional monomer is N-n-propyl acrylamide, N-isopropyl acrylamide or N-isopropyl methacrylamide.

[0021] In the technical scheme of the above preparation method, the crosslinking agent is a commonly used crosslinking agent in the art, for example, a polyamine, a polyisocyanate, a polyol, etc., and the crosslinking agent can be a small molecule crosslinking agent or a macromolecular crosslinking agent, for example, the small molecule crosslinking agent can be N,N-methylenebisacrylamide or vinyltriethoxysilane (VTES), and the macromolecular crosslinking agent can be a four-arm star-shaped polyethylene glycol acrylamide, etc. The initiator can be an azo initiator or an inorganic peroxide initiator, for example, azobisisobutyronitrile, azobisisoheptyl nitrile (MSDS) or potassium persulfate, etc.

[0022] In the reaction solution of step (1) of the technical scheme of the above preparation method, the concentration of polyvinylidene fluoride is 0.12-0.18g / mL, and the amount of the initiator added in step (2) is 1%-3% of the molar amount of the functional monomer.

[0023] In step (3) of the technical scheme of the above preparation method, defoaming can be carried out by vacuum defoaming or stirring defoaming.

[0024] In step (3) of the technical scheme of the above preparation method, the casting solution can be defoamed by vacuum defoaming or stirring defoaming.

[0025] In the technical solutions of the above preparation method, the functionality of the separation membrane surface, the phase separation speed of the casting solution, and the migration speed of the functional polymer to the membrane surface can be regulated by the selection of the functional monomer and the regulation of the preparation process conditions, thereby adjusting the surface structure and internal pore structure of the separation membrane, and thus regulating the oil-water separation performance of the separation membrane. For example, by adjusting the types of the functional monomers in the reaction solution of step (1), the ratio relationship between the functional monomers and polyvinylidene fluoride, the ratio relationship between the crosslinking agent and the functional monomers, the reaction temperature and the reaction time in step (2), the temperature of the support plate of the flat film casting machine in step (4), the temperature and the relative humidity of the constant temperature and humidity environment, and the exposure time in the constant temperature and humidity environment, the height, the diameter, the spacing between adjacent nano-pillar structures, the solid surface fraction of the separation membrane surface, and the pore size and the porosity of the separation membrane can be adjusted.

[0026] The formation mechanism and the oil-water separation mechanism of the liquid-substrate super-amphiphilic polyvinylidene fluoride oil-water separation membrane are as follows:

[0027] In the present application, the functional monomer, the crosslinking agent, and the polyvinylidene fluoride membrane substrate are prepared into a reaction solution, and the functional monomer is polymerized in situ to form a functional polymer based on the addition of an initiator. Since it is in-situ polymerization, the molecular chains of the functional polymer and the polyvinylidene fluoride are intertwined to form a semi-interpenetrating network structure, that is, a casting solution containing a semi-interpenetrating network structure is obtained. After casting, when the wet membrane with a semi-interpenetrating network structure is exposed to a specific vapor environment (constant temperature and humidity environment), because the temperature of the wet membrane is high, when the wet membrane contacts with low-temperature vapor, the exchange between the solvent and the non-solvent occurs between the low-temperature vapor and the high-temperature wet membrane, the non-solvent phase gradually enters the wet membrane, delays the phase separation, and the poor phase grows on the membrane surface to form pores. With the extension of the exposure time in the vapor environment, the poor phase and the rich phase gradually grow, the poor phase forms pores, and the rich phase aggregates to form pore walls. In this process, the functional polymer slowly migrates to the membrane surface. When the wet membrane is transferred from the vapor environment to the coagulation bath, the system undergoes instantaneous liquid-liquid phase separation, and the functional polymer carrying the polyvinylidene fluoride molecular chain rapidly migrates to the membrane surface. In the migration process, the functional polymers repel each other, and finally form a plurality of uniformly distributed nano-pillar structures on the membrane surface. In this process, the non-solvent water phase diffuses into the wet membrane to form a poor phase, and the solvent in the rich phase diffuses out of the wet membrane, and the wet membrane is finally solidified. That is, the present application utilizes the migration of the functional polymer to the membrane surface in the non-solvent phase and the mutual repulsion to finally form a plurality of uniformly distributed nano-pillar structures on the membrane surface.

[0028] The semi-interpenetrating network structure formed by the functional polymer and the polyvinylidene fluoride molecular chains plays the following roles in the present application:

[0029] First, the stability between the functional polymer and the membrane substrate is improved. Since the functional polymer and the membrane substrate are intertwined together, the combination between the two is very close. Therefore, compared with the prior art, the combination stability between the functional polymer and the membrane substrate is improved by introducing the functional polymer into the membrane substrate by physical blending; compared with the prior art, the method for introducing the functional polymer is very simple, only one step of in-situ cross-linking polymer is needed, and no complex operation process is needed.

[0030] Second, the semi-interpenetrating network structure makes the rigid polyvinylidene fluoride membrane substrate carried by the functional polymer chain together to the membrane surface, not only forms a number of nanocolumnar structures on the membrane surface, but also gives the nanocolumnar structure on the membrane surface a certain strength support, so that the nanocolumnar structure can stably exist on the membrane surface, and the long-term use stability of the separation membrane is improved.

[0031] The present application matches the chemical composition of the separation membrane surface and the multi-level structure of the separation membrane surface, improves the hydrophilicity of the separation membrane surface by introducing the functional polymer, and amplifies the hydrophilicity and hydrophobicity of the separation membrane itself by constructing the nanocolumnar structure on the separation membrane surface. Therefore, in the process of separating the oil-water emulsion, when the separation membrane surface contacts with the continuous phase in the emulsion, the above characteristics can accelerate the wetting of the continuous phase to the separation membrane surface, that is, the separation membrane surface can be quickly wetted by the continuous phase. Moreover, the nanocolumnar structure on the separation membrane surface can capture a stable continuous phase layer, which can block the dispersed phase droplets. Since the nanocolumnar structure on the separation membrane surface can firmly lock the continuous phase layer, it cannot be replaced by the dispersed phase droplets, which ensures the stable separation performance of the separation membrane. For example, in the separation of oil-in-water emulsion, the continuous phase water can quickly wet the membrane surface of the separation membrane with the aid of the nanocolumnar structure on the separation membrane surface and form a water layer on the membrane surface of the separation membrane. Since water and oil are immiscible, the dispersed phase oil droplets are blocked on one side of the separation membrane, and the continuous phase water can smoothly pass through the separation membrane, thereby realizing the separation of oil-in-water emulsion. For another example, in the separation of water-in-oil emulsion, the continuous phase oil can quickly wet the membrane surface of the separation membrane with the aid of the nanocolumnar structure on the separation membrane surface and form an oil layer on the membrane surface of the separation membrane. Since water and oil are immiscible, the dispersed phase water droplets are blocked on one side of the separation membrane, and the continuous phase oil can smoothly pass through the separation membrane, thereby realizing the separation of water-in-oil emulsion.

[0032] Compared with the prior art, the technical scheme provided by the present application has the following beneficial technical effects:

[0033] 1.The present application provides an underwater super-amphiphobic polyvinylidene fluoride oil-water separation membrane, which is composed of a polyvinylidene fluoride membrane substrate and a functional polymer. The functional polymer and the molecular chains of the polyvinylidene fluoride membrane substrate intertwine to form a semi-interpenetrating network structure. The surface of the separation membrane has a number of uniformly distributed nano-pillar structures. The interior of the separation membrane has a membrane pore structure. The separation membrane has underwater oleophobicity and oil underwater hydrophobicity. The separation membrane has high separation efficiency for both water-in-oil emulsion and oil-in-water emulsion. The present application solves the problem that existing separation membranes cannot simultaneously have underwater super-oleophobicity and oil underwater super-hydrophobicity, and can only be used for separating one type of oil-water system. The present application effectively broadens the application scenarios of existing separation membranes.

[0034] 2.The present application proves through experiments that the underwater super-amphiphobic polyvinylidene fluoride oil-water separation membrane provided by the present application has a separation efficiency of at least 96% for water-in-oil emulsion and at least 90% for oil-in-water emulsion. Further, through optimization of the preparation conditions, the separation efficiency of the separation membrane for water-in-oil emulsion can be improved to more than 99%, and the separation efficiency for oil-in-water emulsion can be improved to more than 96%. In the optimal embodiment, the separation efficiency of the super-amphiphobic polyvinylidene fluoride oil-water separation membrane of the present application for both water-in-oil emulsion and oil-in-water emulsion is more than 99.9%.

[0035] 3.The present application also provides a method for preparing the above-mentioned underwater super-amphiphobic polyvinylidene fluoride oil-water separation membrane. The method innovatively combines the semi-interpenetrating network structure formed by in-situ crosslinking polymerization and the characteristic that functional polymers migrate to the membrane surface. By taking advantage of the different migration speeds of functional polymers and membrane substrate molecular chains during phase separation, as well as the mutual repulsion between functional polymers, the method successfully constructs a uniformly distributed nano-pillar structure on the membrane surface. The present application uses in-situ crosslinking polymerization and phase separation method to construct a membrane surface with special wettability, which solves the problems of existing chemical modification methods, such as complex operation, high production cost, and difficulty in large-scale preparation and application. It also solves the problems of existing physical modification methods, such as the need for templates, unstable combination of functional polymers and membrane substrates, and difficulty in long-term stable use in complex environments.

[0036] 4.The method of the present application can adjust the hydrophilicity of the separation membrane surface, the phase separation speed of the casting solution, and the migration speed of the functional polymer to the membrane surface by selecting the type of functional monomer and adjusting the preparation process conditions, thereby adjusting the surface structure and internal pore structure of the separation membrane, and adjusting the oil-water separation performance of the separation membrane. Therefore, in practical applications, the oil-water separation performance of the separation membrane can be flexibly adjusted according to the separation requirements of different oil-water emulsion systems, which can better adapt to different oil-water separation scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1is an infrared spectrogram of the film prepared in Examples 1 to 3 and Comparative Example 1.

[0038] Figure 2 is a scanning electron micrograph of the surface of the film prepared in Examples 1 to 3 and Comparative Example 1.

[0039] Figure 3 is an atomic force micrograph of the surface of the film prepared in Examples 1 to 3 and Comparative Example 1.

[0040] Figure 4 is a scanning electron micrograph of the cross section of the film prepared in Examples 1 to 3 and Comparative Example 1.

[0041] Figure 5 is an infrared spectrogram of the film prepared in Examples 1, 4 to 8.

[0042] Figure 6 is a scanning electron micrograph of the surface of the film prepared in Examples 1, 4 to 8.

[0043] Figure 7 is an atomic force micrograph of the surface of the film prepared in Examples 1, 4 to 8.

[0044] Figure 8 is a scanning electron micrograph of the cross section of the film prepared in Examples 1, 4 to 8.

[0045] Figure 9 is an infrared spectrogram of the film prepared in Examples 1, 9 to 16.

[0046] Figure 10 is a scanning electron micrograph of the surface of the film prepared in Examples 1, 9 to 16.

[0047] Figure 11 is an atomic force micrograph of the surface of the film prepared in Examples 1, 9 to 12.

[0048] Figure 12 is a scanning electron micrograph of the cross section of the film prepared in Examples 1, 9 to 16.

[0049] Figure 13 is an infrared spectrogram of the film prepared in Examples 1, 17 to 22.

[0050] Figure 14 is a scanning electron micrograph of the surface of the film prepared in Examples 1, 17 to 22.

[0051] Figure 15 is an atomic force micrograph of the surface of the film prepared in Examples 1, 17 to 22.

[0052] Figure 16 is a scanning electron micrograph of the cross section of the film prepared in Examples 1, 17 to 22.

[0053] Figure 17Height, diameter, pitch, solid fraction of the nanocolumnar structures on the surface of the membranes prepared in Examples 1, 4-8.

[0054] Figure 18 Height, diameter, pitch, solid fraction of the nanocolumnar structures on the surface of the membranes prepared in Examples 1, 9-12.

[0055] Figure 19 Height, diameter, pitch, solid fraction of the nanocolumnar structures on the surface of the membranes prepared in Examples 1, 17-22.

[0056] Figure 20 Results of the separation performance test of the membranes prepared in Examples 1-3 and Comparative Example 1 on oil-in-water emulsion (a) and water-in-oil emulsion (b).

[0057] Figure 21 Optical photos of the filtrate of the separation of oil-in-water emulsion and water-in-oil emulsion by the membranes prepared in Examples 1-3 and Comparative Example 1.

[0058] Figure 22 Results of the separation performance test of the membranes prepared in Examples 1, 4-8 on oil-in-water emulsion (a) and water-in-oil emulsion (b).

[0059] Figure 23 Optical photos of the filtrate of the separation of oil-in-water emulsion and water-in-oil emulsion by the membranes prepared in Examples 1, 4-8.

[0060] Figure 24 Results of the separation performance test of the membranes prepared in Examples 1, 9-12 on oil-in-water emulsion (a) and water-in-oil emulsion (b).

[0061] Figure 25 Optical photos of the filtrate of the separation of oil-in-water emulsion and water-in-oil emulsion by the membranes prepared in Examples 1, 9-12.

[0062] Figure 26 Results of the separation performance test of the membranes prepared in Examples 1, 17-22 on oil-in-water emulsion (a) and water-in-oil emulsion (b).

[0063] Figure 27 Optical photos of the filtrate of the separation of oil-in-water emulsion and water-in-oil emulsion by the membranes prepared in Examples 1, 17-22. DETAILED DESCRIPTION

[0064] The liquid-submerged superamphiphilic polyvinylidene fluoride oil-water separation membrane and the preparation method thereof described in the present application are further illustrated by the following examples. The following examples are only used to further illustrate the present application and cannot be understood as limiting the protection scope of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the specific implementation of the present application according to the above description, which still belongs to the protection scope of the present application.

[0065] In the following examples and comparative examples, the membrane substrate, polyvinylidene fluoride (PVDF), was purchased from Sigma-Aldrich, USA, with a molecular weight M. W The value is 534,000 Da. The deionized water is prepared using the Millipore purified water system.

[0066] Example 1

[0067] In this embodiment, N-isopropylacrylamide (NIPAM) was used as the functional monomer to prepare a liquid-submersible superhydrophobic PVDF oil-water separation membrane. The steps are as follows:

[0068] (1) Preparation of reaction solution

[0069] The functional monomer NIPAM, the membrane substrate PVDF, and the crosslinking agent N,N-methylenebisacrylamide (MBA) were added to N,N-dimethylacetamide (DMAc) and stirred at 80°C until dissolved to obtain a reaction solution. In this reaction solution, the amount of NIPAM was 80% of the mass of PVDF, the amount of MBA was 1.67% of the molar amount of NIPAM, and the concentration of PVDF was 0.15 g / mL.

[0070] (2) In-situ crosslinking polymerization

[0071] Under stirring conditions (1000 r / min), high-purity nitrogen gas was passed through the container holding the reaction solution for 30 min to remove oxygen from the reaction solution and the container. After the nitrogen gas was passed through, the initiator azobisisobutyronitrile (AIBN) was immediately added. The amount of AIBN was 2% of the molar amount of NIPAM. Then the container was quickly sealed and reacted at 80 °C for 8 h under stirring conditions to obtain the casting solution.

[0072] During the reaction, NIPAM polymerizes in situ to form poly(N-isopropylacrylamide) (PNI). While NIPAM is polymerizing, it becomes entangled with the molecular chains of PVDF, and the final PNI and PVDF molecular chains form a semi-interpenetrating network structure.

[0073] (3) Defoaming of casting solution

[0074] The casting solution was allowed to stand at 65°C for 24 hours, and then placed in a vacuum drying oven at 65°C for 2 hours for vacuum degassing.

[0075] (4) Membrane preparation

[0076] The temperature of the support plate of the flat film applicator was adjusted to 65℃, and the gap between the doctor blade and the support plate was set to 200 μm. The defoamed casting solution was taken out of the vacuum drying oven and poured onto the support plate, and the film was cast using the flat film applicator. The wet film obtained by the casting was transferred into a constant temperature and humidity chamber with a temperature of 25℃ and a relative humidity of 70% for exposure for 30 s, and then immediately transferred into a deionized water coagulation bath with a temperature of 25℃ for solidification for 20 min. The solidified film was transferred into deionized water, and the water was changed 3 times per day. The film was washed for 3 days in total. The washed film was naturally dried at room temperature, and a liquid-submerged super-amphiphobic PVDF oil-water separation membrane was obtained. The membrane was stored in a dry cabinet.

[0077] Example 2

[0078] The operation of this example was basically the same as that of Example 1, except that the functional monomer in step (1) was replaced by N-n-propyl acrylamide (NNPAM).

[0079] Example 3

[0080] The operation of this example was basically the same as that of Example 1, except that the functional monomer in step (1) was replaced by N-isopropyl methacrylamide (NIPMAM).

[0081] Comparative Example 1

[0082] In this comparative example, a pure PVDF membrane was prepared, and the operation was basically the same as that of Example 1, except that no functional monomer was added in step (1).

[0083] The membranes prepared in Examples 1-3 and Comparative Example 1 were tested by infrared spectroscopy, and the results are shown in Table 1. Figure 1 As can be seen from Table 1, the membranes prepared in Examples 1-3 contained amide groups on the surface, indicating that the functional polymers were successfully introduced onto the membrane surface. Figure 1 The scanning electron microscope (SEM) and atomic force microscope (AFM) photos of the membranes prepared in Examples 1-3 and Comparative Example 1 are shown in Table 1. Figures 2-3 As can be seen from Table 1, after the introduction of the functional polymers, a number of nanocolumnar structures were successfully constructed on the membrane surface, and with the increase of the hydrophilicity of the monomers, the nanocolumnar structures on the membrane surface gradually became clear and uniform, and the nanocolumnar structures gradually transitioned from short and fat to high and thin. Figures 2-3 The cross-sectional SEM photos of the membranes prepared in Examples 1-3 are shown in Table 1. Figure 4 As can be seen from Table 1, with the increase of the hydrophilicity of the monomers, the membrane pore structure gradually transitioned from cellular pores to finger-like pores. Figure 4 The membranes prepared in Examples 1-3 and Comparative Example 1 were tested for film thickness, porosity, and surface wettability, and the results are shown in Table 1.

[0084]

[0085] ​Example 4

[0086] The operation of this example is basically the same as that of Example 1, except that the amount of NIPAM in the reaction solution of step (1) is 20% of the mass of PVDF.

[0087] Example 5

[0088] The operation of this example is basically the same as that of Example 1, except that the amount of NIPAM in the reaction solution of step (1) is 40% of the mass of PVDF.

[0089] Example 6

[0090] The operation of this example is basically the same as that of Example 1, except that the amount of NIPAM in the reaction solution of step (1) is 60% of the mass of PVDF.

[0091] Example 7

[0092] The operation of this example is basically the same as that of Example 1, except that the amount of NIPAM in the reaction solution of step (1) is 100% of the mass of PVDF.

[0093] Example 8

[0094] The operation of this example is basically the same as that of Example 1, except that the amount of NIPAM in the reaction solution of step (1) is 120% of the mass of PVDF.

[0095] The membranes prepared in Examples 4-8 were tested by infrared spectroscopy, and the results are shown in Table 2. Figure 5 As can be seen from Table 2, the membranes contain amide groups on the surface, indicating that the functional polymer was successfully introduced onto the membrane surface. Figure 5 The SEM and AFM photos of the membranes prepared in Examples 4-8 are shown in Table 3. Figures 6-7 As can be seen from Table 3, with the increase of the amount of functional monomer, the nano-pillar structure on the membrane surface gradually became clear, and then gradually disappeared, and large pores began to appear on the membrane surface. Figures 6-7 The cross-sectional SEM images of the membranes prepared in Examples 4-8 are shown in Table 4. Figure 8 As can be seen from Table 4, with the increase of the amount of monomer, the pore structure of the membrane showed a transition trend from cellular pores to finger-like pores, and then to cellular pores. Figure 8

[0096] The membranes prepared in Examples 4-8 were tested for thickness, porosity, and surface wettability, and the results are shown in Table 1.

[0097] Example 9

[0098] ​The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 0 s.

[0099] Example 10

[0100] The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 5 s.

[0101] Example 11

[0102] The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 15 s.

[0103] Example 12

[0104] The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 60 s.

[0105] Example 13

[0106] The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 120 s.

[0107] Example 14

[0108] The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 180 s.

[0109] Example 15

[0110] The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 240 s.

[0111] Example 16

[0112] The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 300 s.

[0113] The films prepared in Examples 9-16 were tested by infrared spectroscopy, and the results are shown in Table 1, from which it can be seen that the surface of the film contains amide groups, indicating that the functional polymer was successfully introduced onto the surface of the film. Figure 9 The SEM and AFM photographs of the surface of the films prepared in Examples 9-16 are shown in Table 2, from which it can be seen that the surface of the film is smooth. Figure 9 Figures 10-11 The operation of this example is basically the same as that of Example 1, except that the exposure time in the constant temperature and humidity chamber in step (4) is 300 s. Figures 10-11 ​It can be seen that the nano-pillar structure on the membrane surface gradually disappears and large pores appear on the membrane surface with the increase of exposure time in the constant temperature and humidity chamber. Figure 12 It can be seen that the nano-pillar structure on the membrane surface gradually disappears and large pores appear on the membrane surface with the increase of exposure time in the constant temperature and humidity chamber. Figure 12 It can be seen that the nano-pillar structure on the membrane surface gradually disappears and large pores appear on the membrane surface with the increase of exposure time in the constant temperature and humidity chamber.

[0114] The membranes prepared in Examples 9-16 were tested for membrane thickness, porosity, and surface wettability, and the results are shown in Table 1.

[0115] Example 17

[0116] The operation of this example is basically the same as that of Example 1, except that the amount of crosslinking agent MBA in the reaction solution of step (1) is 0% of the molar amount of NIPAM.

[0117] Example 18

[0118] The operation of this example is basically the same as that of Example 1, except that the amount of crosslinking agent MBA in the reaction solution of step (1) is 0.5% of the molar amount of NIPAM.

[0119] Example 19

[0120] The operation of this example is basically the same as that of Example 1, except that the amount of crosslinking agent MBA in the reaction solution of step (1) is 1.0% of the molar amount of NIPAM.

[0121] Example 20

[0122] The operation of this example is basically the same as that of Example 1, except that the amount of crosslinking agent MBA in the reaction solution of step (1) is 1.5% of the molar amount of NIPAM.

[0123] Example 21

[0124] The operation of this example is basically the same as that of Example 1, except that the amount of crosslinking agent MBA in the reaction solution of step (1) is 2.5% of the molar amount of NIPAM.

[0125] Example 22

[0126] The operation of this example is basically the same as that of Example 1, except that the amount of crosslinking agent MBA in the reaction solution of step (1) is 3.0% of the molar amount of NIPAM.

[0127] The membranes prepared in Examples 17-22 were tested by infrared spectroscopy, and the results are shown in Table 2. Figure 13 The membranes prepared in Examples 17-22 were tested by infrared spectroscopy, and the results are shown in Table 2. Figure 13It can be seen that the film surface contains amide groups, indicating that the functional polymer is successfully introduced onto the film surface. The SEM and AFM images of the film surfaces prepared in Examples 17-22 are shown in Figures 14-15 It can be seen that the film surface contains amide groups, indicating that the functional polymer is successfully introduced onto the film surface. The SEM and AFM images of the film surfaces prepared in Examples 17-22 are shown in Figures 14-15 It can be seen that the film surface contains amide groups, indicating that the functional polymer is successfully introduced onto the film surface. The SEM and AFM images of the film surfaces prepared in Examples 17-22 are shown in Figure 16 It can be seen that the film surface contains amide groups, indicating that the functional polymer is successfully introduced onto the film surface. The SEM and AFM images of the film surfaces prepared in Examples 17-22 are shown in Figure 16 It can be seen that the film surface contains amide groups, indicating that the functional polymer is successfully introduced onto the film surface. The SEM and AFM images of the film surfaces prepared in Examples 17-22 are shown in

[0128] The film thickness, porosity, and surface wettability of the films prepared in Examples 17-22 were tested, and the results are shown in Table 1.

[0129] Table 1 Film thickness, porosity, and surface wettability test results of the films prepared in Examples 1-22 and Comparative Example 1

[0130]

[0131]

[0132] Example 23: Measurement of structural parameters of the nanocolumnar structure on the film surface

[0133] In this example, the three-dimensional microstructure of the film surface was characterized using AFM, and the height, diameter, and spacing of the nanocolumnar structure on the film surface were measured. The solid fraction f of the film surface was calculated from the above data according to formula (1) s .

[0134]

[0135] In formula (1), n is the number of nanocolumnar structures per unit area, and D is the height (nm) of the nanocolumnar structure.

[0136] The height, diameter, and spacing of the nanocolumnar structure on the film surface prepared in Examples 1, 4-8 are shown in Figures a-c of Figure 17 , and the solid fraction is shown in Figure d of Figure 17 . The height, diameter, and spacing of the nanocolumnar structure on the film surface prepared in Examples 1, 9-12 are shown in Figures a-c of Figure 18 , and the solid fraction is shown in Figure d of Figure 18 . The height, diameter, and spacing of the nanocolumnar structure on the film surface prepared in Examples 1, 17-22 are shown in Figures a-c of Figure 19 , and the solid fraction is shown in Figure d of Figure 19 .

[0137] Example 24: Test of oil-water separation performance of the film

[0138] In this example, the separation performance of the membranes prepared in the above examples and comparative examples to oil-in-water (O / W) emulsion and water-in-oil (W / O) emulsion was tested.

[0139] (1) Preparation of O / W emulsion

[0140] 0.1 g Tween 80 was dissolved in 100 mL pure water, and stirred at a speed of 1000 r / min for 1 h. 2 mL of benzyl benzoate dyed with Sudan red was taken and added to the above solution, and stirred at a speed of 1000 r / min for 3 h to obtain a benzyl benzoate-in-water emulsion, denoted as BB / W.

[0141] 0.1 g Tween 80 was dissolved in 100 mL pure water, and stirred at a speed of 1000 r / min for 1 h. 2 mL of soybean oil dyed with Sudan red was taken and added to the above solution, and stirred at a speed of 1000 r / min for 3 h to obtain a soybean oil-in-water emulsion, denoted as Soybean / W.

[0142] (2) Preparation of W / O emulsion

[0143] 0.1 g Span 80 was dissolved in 100 mL pure benzyl benzoate, and stirred at a speed of 1000 r / min for 1 h. 2 mL of pure water dyed with methylene blue was taken and added to the above solution, and stirred at a speed of 1000 r / min for 3 h to obtain a water-in-benzyl benzoate emulsion, denoted as W / BB.

[0144] (3) Test of separation performance of emulsion

[0145] The membranes prepared in the above examples and comparative examples were cut into a circular shape with a diameter of 1 cm, and clamped in a glass sand core filter device with the front face upward. An emulsion with a liquid surface height of 10 cm was added to the filter cup, and gravity natural filtration was carried out at room temperature. The filtrate was collected, the permeation rate was calculated by formula (2), the water content in the filtrate was measured by a water titrator (Karl-Fischer water titrator), and the separation efficiency was calculated by formula (3).

[0146]

[0147] In formula (2), V is the volume of the filtrate (L), A is the effective membrane area (m 2 ), t is the filtration time (h), and Δp is the pressure difference (bar).

[0148]

[0149] In formula (3), C0and C f are the water contents in the original solution and the filtrate, respectively.

[0150] The separation performance of the membranes prepared in Examples 1 to 3 and Comparative Example 1 against O / W emulsion and W / O emulsion are shown in Figs. a, b of Figure 20 , respectively, and the optical photograph of the filtrate is shown in Figs. c, d of Figure 21 , respectively. The separation performance of the membranes prepared in Examples 1, 4 to 8 against O / W emulsion and W / O emulsion are shown in Figs. a, b of Figure 22 , respectively, and the optical photograph of the filtrate is shown in Figs. c, d of Figure 23 , respectively. The separation performance of the membranes prepared in Examples 1, 9 to 12 against O / W emulsion and W / O emulsion are shown in Figs. a, b of Figure 24 , respectively, and the optical photograph of the filtrate is shown in Figs. c, d of Figure 25 , respectively. The separation performance of the membranes prepared in Examples 1, 17 to 22 against O / W emulsion and W / O emulsion are shown in Figs. a, b of Figure 26 , respectively, and the optical photograph of the filtrate is shown in Figs. c, d of Figure 27 , respectively.

Claims

1. A liquid-submerged superamphiphobic polyvinylidene fluoride oil-water separation membrane, characterized by, The separation membrane is composed of a polyvinylidene fluoride membrane substrate and a functional polymer, the functional polymer is a polyamide polymer, the amount of the functional polymer is 60-80% of the mass of the polyvinylidene fluoride membrane substrate, the molecular chains of the functional polymer and the polyvinylidene fluoride membrane substrate are intertwined to form a semi-interpenetrating network structure, the surface of the separation membrane has a plurality of uniformly distributed nano-pillar structures, and the inside of the separation membrane has a membrane hole structure; the separation membrane has underwater oil-repellency and oil-under water hydrophobicity; the functional polymer is poly (N-ethyl acrylamide), poly (N-propyl acrylamide), poly (N-isopropyl acrylamide) or poly (N-isopropyl methacrylamide). N - N-propyl acrylamide), poly (N-isopropyl acrylamide) or poly (N-isopropyl methacrylamide). N - N-propyl acrylamide), poly (N-isopropyl acrylamide) or poly (N-isopropyl methacrylamide). N - N-propyl acrylamide), poly (N-isopropyl acrylamide) or poly (N-isopropyl methacrylamide).

2. The underwater super-amphiphobic polyvinylidene fluoride oil-water separation membrane according to claim 1, characterized in that, The surface of the separation membrane has a nano-pillar structure with a height of 60-150 nm and a diameter of 0.4-0.6 µm, and a distance between adjacent nano-pillar structures of 0.6-0.8 µm.

3. The underwater superomniphobic polyvinylidene fluoride oil-water separation membrane according to claim 2, characterized in that, The solid fraction of the surface of the separation membrane f s The solid fraction is 0.3 to 0.7 f s The solid fraction is calculated from equation (1): （1） In formula (1), n is the number of the nano-pillar structures per unit area, D is the height of the nano-pillar structure, and the unit of the height of the nano-pillar structure is nm.

4. The underwater superamphiphobic polyvinylidene fluoride oil-water separation membrane according to any one of claims 1 to 3, characterized in that, The separation membrane has an underwater oil contact angle of 131°-155°, an oil-underwater contact angle of 146°-165°, and an oil-underwater rolling angle of 2.3°-5.4°.

5. The underwater superamphiphobic polyvinylidene fluoride oil-water separation membrane according to any one of claims 1 to 3, characterized in that, The separation efficiency of the separation membrane for water-in-oil emulsion is at least 96%, and the separation efficiency of the separation membrane for oil-in-water emulsion is at least 90%.

6. The underwater superamphiphobic polyvinylidene fluoride oil-water separation membrane according to any one of claims 1 to 3, wherein The thickness of the separation membrane is 70-100 µm.

7. The method for preparing the under-liquid super-amphiphobic polyvinylidene fluoride oil-water separation membrane according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: (1) dissolving a functional monomer, polyvinylidene fluoride, and a crosslinking agent in a solvent to obtain a reaction solution; in the reaction solution, the amount of the functional monomer is 60%-80% of the mass of the polyvinylidene fluoride, and the amount of the crosslinking agent is 1.5%-2.5% of the molar amount of the functional monomer; (2) under stirring, nitrogen is introduced into a container containing the reaction solution to remove oxygen in the reaction solution and the container, then an initiator is added, and the reaction is carried out at 65-85 ℃ under sealing conditions for 6-10 h to obtain a casting solution; during the reaction, the functional monomer is polymerized in situ into a functional polymer, and the molecular chains of the functional polymer and the polyvinylidene fluoride are intertwined to form a semi-interpenetrating network structure; (3) the casting solution is placed at 50-70 ℃ for 12-24 h, then defoaming is performed, and the temperature of the defoamed casting solution is adjusted to 50-70 ℃; (4) the temperature of the support plate of a flat blade coater is adjusted to 50-70 ℃, the defoamed casting solution with a temperature consistent with the temperature of the support plate of the flat blade coater is poured onto the support plate for film coating, the wet film obtained by film coating is transferred to a constant-temperature and constant-humidity environment with a temperature of 15-30 ℃ and a relative humidity of 65-90% for exposure for 15-30 s, then immediately transferred to a coagulation bath with a temperature of 15-30 ℃ for solidification of the wet film, the solidified film is washed with water and dried, thereby obtaining an under-liquid super-amphiphilic polyvinylidene fluoride oil-water separation membrane.

8. The method for preparing the underwater superhydrophobic polyvinylidene fluoride oil-water separation membrane according to claim 7, characterized in that, The functional monomer is N - n-propyl acrylamide, N - isopropyl acrylamide or N - isopropyl methacrylamide.

9. The method for preparing the PVDF oil-water separation membrane according to claim 7 or 8, wherein the method is characterized by, In the reaction solution of step (1), the concentration of the polyvinylidene fluoride is 0.12-0.18 g / mL, and the amount of the initiator added in step (2) is 1%-3% of the molar amount of the functional monomer.

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