Double-layer uniform-hole ultrafiltration membrane with pH stimulation response active layer and preparation method of double-layer uniform-hole ultrafiltration membrane

Abstract

The invention discloses a double-layer uniform-hole ultrafiltration membrane with a pH stimulation response active layer and a preparation method of the double-layer uniform-hole ultrafiltration membrane, and relates to an ultrafiltration membrane and a preparation method of the ultrafiltration membrane. The invention aims to solve the problems of Trade-off effect between permeability and selectivity of the existing ultrafiltration membrane, difficulty in membrane pollution cleaning and low utilization rate of a block copolymer in the preparation process of the homogeneous-pore ultrafiltration membrane. The double-layer homogeneous-pore ultrafiltration membrane with the pH stimulation response active layer comprises a polyvinylidene fluoride supporting layer and an active layer taking a PS-b-PMMA (Polymethyl Methacrylate) block copolymer as a main component. The double-layer homogeneous-pore ultrafiltration membrane is synchronously synthesized through a selective solvent-induced non-solvent-induced phase-conducting separation method, the use amount of BCPs is reduced, and the dual purposes of enhancing the mechanical property and reducing the preparation cost are achieved. The finished membrane has the advantages of good permeability, high selectivity, low membrane preparation cost, convenience in membrane pollution cleaning and the like, and can be applied to process upgrading and water quality upgrading and reconstruction of urban water purification plants.

Description

Technical Field

[0001] The invention relates to an ultrafiltration membrane and a preparation method thereof. Background Art

[0002] In recent years, with the official implementation of the new generation of "National Drinking Water Quality Standard" (GB5749-2022), "two insects" and microcystins have gradually become important control indicators for drinking water hygiene. Due to the limited treatment capacity of traditional water treatment processes (coagulation-sedimentation-filtration three-stage process, etc.), there is an urgent need for a new type of water treatment technology that can efficiently and specifically remove new pollutants such as "two insects" and microcystins.

[0003] Ultrafiltration is a membrane filtration technology based on physical interception. Its interception pore size distribution is between 2-100nm. It can remove almost all pathogenic microorganisms, and can also remove colloids and organic substances with similar particle sizes in water. Thanks to its physical interception characteristics, ultrafiltration can reduce the addition of chemical agents at the front and back ends of the process, which is in line with the environmental protection water purification concept of "green process". Therefore, in recent years, ultrafiltration has gradually become the core of the third-generation urban drinking water purification process concept, and is widely used in the water purification process transformation of urban water purification plants in combination with other processes, effectively improving the water treatment capacity and effluent quality of urban water purification plants.

[0004] Traditional ultrafiltration membranes are mostly prepared using NIPS. The ultrafiltration membranes prepared using this method have a high degree of pore disorder and a wide range of pore size distribution, and are easily restricted by the "upper limit balance" between permeability and selectivity (trade-off effect). In addition, membrane fouling during ultrafiltration membrane operation is one of the main obstacles affecting the widespread application of ultrafiltration membrane technology. The contaminated membrane must be cleaned to restore the membrane's permeability. The commonly used cleaning method is chemical enhanced cleaning (CEC). Although acid washing or alkaline washing can achieve a relatively ideal mitigation effect on membrane fouling, CEC often faces the problem of insufficient cleaning and damage to the membrane structure. Summary of the invention

[0005] The purpose of the present invention is to solve the problems of trade-off effect between permeability and selectivity of existing ultrafiltration membranes, difficulty in cleaning membrane pollution and low utilization rate of block copolymers in the preparation process of uniform pore ultrafiltration membranes, and to provide a double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer and a preparation method thereof.

[0006] The present invention solves the problem of the trade-off effect between the permeability and selectivity of the existing ultrafiltration membrane, and improves the flux recovery ability of the ultrafiltration membrane after pollution. While ensuring the chemical cleaning effect, it avoids the damage of the internal structure of the membrane by the acid agent, prolongs the life of the membrane material, and reduces the economic cost of the ultrafiltration process.

[0007] A double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer, comprising a polyvinylidene fluoride support layer and an active layer with a PS-b-PMMA block copolymer as a main component;

[0008] The thickness of the polyvinylidene fluoride support layer is 180 μm to 190 μm;

[0009] The thickness of the active layer is 10 μm to 20 μm, and the membrane pore size on the surface of the active layer is 40 nm to 60 nm;

[0010] The PS-b-PMMA block copolymer accounts for 6% to 10% of the mass of the active layer casting solution;

[0011] The polyvinylidene fluoride support layer is prepared from polyvinylidene fluoride, polyvinyl pyrrolidone and N,N-dimethylformamide, wherein polyvinylidene fluoride accounts for 16% to 20% of the mass of the polyvinylidene fluoride support layer casting liquid, and polyvinyl pyrrolidone accounts for 4% to 6% of the mass of the polyvinylidene fluoride support layer casting liquid.

[0012] A method for preparing a double-layer uniform-pore ultrafiltration membrane having a pH stimulus-responsive active layer is specifically completed by the following steps:

[0013] 1. Use anhydrous ethanol to clean the glass plate, and then blow dry to get a clean glass plate;

[0014] 2. Adjust the manual scraper until the lower end of the scraper blade is flush with the glass plate, then adjust the knob to first scrape the polyvinylidene fluoride support layer casting liquid on the clean glass plate into a uniform plane, then adjust the knob to raise the scraper, and then scrape the active layer casting liquid into a uniform plane, and the double-layer film scraping is completed;

[0015] 3. Evaporate naturally for 30s to 40s, then immerse the glass plate in the medium for 5min to 8min to obtain an ultrafiltration membrane with a certain rigidity;

[0016] Fourth, transfer the ultrafiltration membrane with a certain rigidity to the medium and soak it, and replace the medium every 6 to 8 hours until the ultrafiltration membrane and the medium cannot be observed to exchange solvents under the light source, then take out the ultrafiltration membrane and dry it to obtain a double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer;

[0017] The medium described in step 3 and step 4 is deionized water;

[0018] The drying temperature in step 4 is 25°C to 30°C.

[0019] Compared with the prior art, the present invention has the following significant effects:

[0020] 1. The double-layer uniform-pore ultrafiltration membrane with a pH stimulus-responsive active layer prepared by the present invention has the advantages of high permeability, strong anti-pollution performance, and convenience for chemical cleaning. The preparation method thereof has the advantages of short cycle, simple process, economical raw materials, and low membrane production cost. During the operation of the membrane, the water flux can quickly reach a stable flux. In the anti-pollution test, the retention rate is significantly improved compared with the traditional PVDF ultrafiltration membrane.

[0021] Second, the block copolymer (BCPs) used in the active layer of the double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer prepared by the present invention is PS-b-PMMA. Due to the special functional group structure, PS-b-PMMA will show different forms according to the environmental pH. Studies have shown that when the pH is lower than 3.4, the uniform pore structure formed by PS-b-PMMA will be closed, and when the pH rises to above 5.5, the pore structure will gradually open to the maximum. This intelligent response behavior to external pH stimulation gives the PS-b-PMMA uniform pore membrane a unique advantage in the chemical cleaning process: during acid washing, the membrane pores are closed to remove inorganic scale and colloids on the membrane surface; during alkaline washing, the membrane pores are opened to remove organic pollutants accumulated in the membrane pores; while ensuring the chemical cleaning effect, the acid agent is prevented from damaging the internal structure of the membrane, thereby extending the life of the membrane material.

[0022] 3. The present invention uses PVDF with a flat membrane surface as a support layer and BCPs as an active layer, and simultaneously synthesizes a double-layer uniform-pore ultrafiltration membrane through a non-solvent-induced phase separation method induced by a selective solvent, thereby reducing the amount of BCPs used and achieving the dual purposes of strengthening mechanical properties and reducing preparation costs;

[0023] 4. The ultrafiltration membrane of the present invention is an isoporous membrane, which refers to a separation membrane with uniform pore size, consistent pore shape and vertically running through the entire active layer. This structure is beneficial for maintaining high selectivity while ensuring high permeability flux. Compared with disordered pores, straight pores are also beneficial for improving the membrane's resistance to pollution and flux recovery ability after pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a flow chart for preparing a double-layer uniform-pore ultrafiltration membrane having a pH stimulus-responsive active layer in the present invention;

[0025] Figure 2 is a photograph of a double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer prepared in Example 1;

[0026] Figure 3 It is a bar graph of pure water flux and BSA retention capacity of double-layer uniform pore ultrafiltration membranes with pH stimulus responsive active layers prepared with different surface BCPs concentrations in Examples 1, 3 and 4;

[0027] Figure 4 It is a bar graph of pure water flux and BSA retention capacity of double-layer uniform pore ultrafiltration membranes with pH stimulus responsive active layers prepared with different surface PVDF concentrations in Examples 1, 5 and 6;

[0028] Figure 5 1 is a scanning electron microscope image of a double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer prepared in different embodiments and comparative examples, wherein a is 6 wt % BCPs, b1 is 8 wt % BCPs (1), b2 is 8 wt % BCPs (2), c is 10 wt % BCPs, d is comparative example 1, and e is comparative example 2;

[0029] Figure 6 1 is an X-ray diffraction diagram and an X-ray photoelectron spectrum diagram of a double-layer uniform-porous ultrafiltration membrane having a pH stimulus responsive active layer prepared in Example 1, wherein a is an X-ray electron spectrum diagram and b is an O1s analysis spectrum diagram;

[0030] Figure 7 This is an X-ray diffraction scan of the double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer prepared in Example 1. DETAILED DESCRIPTION

[0031] Specific implementation method 1: This implementation method is a double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer, including a polyvinylidene fluoride support layer and an active layer with a PS-b-PMMA block copolymer as a main component;

[0032] The thickness of the polyvinylidene fluoride support layer is 180 μm to 190 μm;

[0033] The thickness of the active layer is 10 μm to 20 μm, and the membrane pore size on the surface of the active layer is 40 nm to 60 nm;

[0034] The PS-b-PMMA block copolymer accounts for 6% to 10% of the mass of the active layer casting solution;

[0035] The polyvinylidene fluoride support layer is prepared from polyvinylidene fluoride, polyvinyl pyrrolidone and N,N-dimethylformamide, wherein polyvinylidene fluoride accounts for 16% to 20% of the mass of the polyvinylidene fluoride support layer casting liquid, and polyvinyl pyrrolidone accounts for 4% to 6% of the mass of the polyvinylidene fluoride support layer casting liquid.

[0036] Specific embodiment 2: This embodiment differs from the specific embodiment 1 in that the PS-b-PMMA block copolymer is prepared by living anionic polymerization, which is specifically completed by the following steps:

[0037] (1) Add tetrahydrofuran to an anhydrous and oxygen-free reactor, connect it to one of the end branches of a Schlenk double-row tube device, then add styrene, and tighten the stopper of the reactor to isolate it from the atmosphere;

[0038] (2) Turn on the vacuum pump, rotate the valve on / off switch on the Schlenk double-row tube device, connect the anhydrous and oxygen-free reactor to the passage of the vacuum pump, evacuate the reactor, ensure that the reactor is in a vacuum environment, rotate the valve on / off switch on the Schlenk double-row tube device again, introduce high-purity nitrogen, repeat the above evacuation-nitrogen replenishment operation three times to exhaust all the oxygen in the reactor, create a vacuum atmosphere, cut off the connection between the anhydrous and oxygen-free reactor and the Schlenk double-row tube device, add it to a constant temperature low-temperature reaction tank, and cool it for a certain period of time;

[0039] (3) After cooling until the temperature inside the bottle drops, take out the anhydrous and oxygen-free reactor and connect it to one end branch of the Schlenk double-row tube device. Add the initiator n-butyl lithium under a nitrogen blowing atmosphere. The color of the solution turns orange-red. Turn on the vacuum pump, create a vacuum atmosphere, and then transfer it to a constant temperature low-temperature reaction tank to continue the reaction for a certain period of time.

[0040] (4) In a nitrogen-blown atmosphere, 1,1-diphenylethylene is added to the anhydrous and oxygen-free reactor using a pipette to block the polymerization reaction. At this time, the solution color changes from orange-red to dark red. Then, the vacuum pump is turned on to create a vacuum atmosphere and send it into a constant temperature low-temperature reaction tank. The reaction continues for a certain period of time.

[0041] (5) Adding methyl methacrylate to the anhydrous and oxygen-free reactor under nitrogen blowing atmosphere, then starting the vacuum pump, creating a vacuum atmosphere and sending it into a constant temperature low temperature reaction tank to carry out the polymerization reaction of the second monomer of the block copolymer, and taking it out after continuing the reaction for a certain period of time to obtain a mixed solution;

[0042] (6) introducing the mixed solution into a methanol-water mixed solution for precipitation, then separating the precipitate, re-dissolving it in tetrahydrofuran, and then slowly introducing the mixed solution dissolved in tetrahydrofuran into a new methanol-water mixed solution for precipitation. After precipitation for a certain period of time, the precipitate is taken out and secondary precipitation is carried out with deionized water. After the precipitation is complete, the water is drained to obtain a block copolymer solid;

[0043] (7) The precipitated block copolymer solid is subjected to vacuum drying to expand. After drying for 0.5h to 1h, the vent valve of the vacuum drying oven is opened to release the vacuum. After the solvent in the solid is completely volatilized, the vent valve of the vacuum drying oven is closed again, and the connecting valve between the vacuum drying oven and the vacuum pump is opened to evacuate the vacuum. Thereafter, the above operation is repeated in the vacuum drying oven to release the vacuum and evacuate the residual solvent to dissipate. After ensuring that the block copolymer no longer contains water, it is made into powder to obtain the PS-b-PMMA block copolymer for standby use. The other steps are the same as those in the first embodiment.

[0044] Specific embodiment 3: This embodiment differs from specific embodiment 1 or 2 in that: the constant pressure of high-purity nitrogen introduced into the anhydrous and oxygen-free reactor in step (2) is 0.02MPa-0.04MPa; the volume ratio of tetrahydrofuran to styrene in step (1) is (34-57):1; the purity of high-purity nitrogen in step (2) is greater than 99.999%; the cooling time in step (2) is 0.25h-0.4h; the continuous reaction time in step (3) is 0.5h-1h; the volume ratio of 1,1-diphenylethylene to styrene in step (4) is (0.12-0.2):1; the continuous reaction time in step (4) is 0.05h-0.2h. The other steps are the same as those in specific embodiment 1 or 2.

[0045] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that: the duration of the reaction described in step (5) is 1 h to 1.5 h; the volume ratio of methanol to water in the methanol-water mixed solution described in step (6) is 1:1; in step (6), the mixed solution dissolved in tetrahydrofuran is slowly introduced into a new methanol-water mixed solution for precipitation, and the precipitate is taken out after precipitation for 0.3 h to 0.5 h. The other steps are the same as those of specific embodiments 1 to 3.

[0046] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that: the volume ratio of styrene to methyl methacrylate is (5-9):1; the volume ratio of styrene to initiator n-butyl lithium is (9-10):1; the medium in the constant temperature low temperature reaction tank in steps (3), (4) and (5) is anhydrous ethanol, and the temperature of the constant temperature low temperature reaction tank is -80°C; the temperature of the vacuum drying oven in step (7) is 60°C to 80°C. The other steps are the same as those in specific embodiments 1 to 4.

[0047] Specific embodiment 6: This embodiment differs from specific embodiments 1 to 5 in that the preparation method of the active layer casting solution is specifically completed according to the following steps:

[0048] (1) adding polyvinylidene fluoride to a solvent to obtain a suspension; heating and stirring the suspension in a constant temperature magnetic stirrer until the polyvinylidene fluoride is completely dissolved in the solvent to obtain a polyvinylidene fluoride solution;

[0049] (2) Adding PS-b-PMMA block copolymer and anhydrous copper acetate to the polyvinylidene fluoride solution, and then placing the mixed solution in a constant temperature magnetic stirrer for heating and stirring for a period of time to obtain a blue-green active layer casting solution with a certain viscosity;

[0050] The PS-b-PMMA block copolymer accounts for 6% to 10% of the mass of the active layer casting solution; the polyvinylidene fluoride accounts for 2% to 4% of the mass of the active layer casting solution; and the anhydrous copper acetate accounts for 0.2% to 0.3% of the mass of the active layer casting solution. The other steps are the same as those of the first to fifth embodiments.

[0051] Specific embodiment 7: This embodiment differs from specific embodiments 1 to 6 in that: the solvent described in step (1) is a mixed solution of DMF and THF, wherein the mass ratio of DMF to THF is 7:3 or 8:2; the temperature of heating and stirring described in step (1) is 60°C to 70°C, and the time of heating and stirring is 2h to 4h; the temperature of heating and stirring described in step (2) is 60°C to 65°C, and the time of heating and stirring is 15min to 30min. The other steps are the same as those of specific embodiments 1 to 6.

[0052] Specific implementation eight: This implementation differs from specific implementations one to seven in that: the method for preparing the polyvinylidene fluoride support layer casting solution is specifically completed according to the following steps: adding polyvinyl pyrrolidone to N,N-dimethylformamide to obtain a suspension; subjecting the suspension to ultrasonic treatment for a period of time, then adding polyvinylidene fluoride, shaking well, so that the polyvinylidene fluoride and the mixed solution in the container are completely mixed, and then the container is sealed; placing the sealed container in a water bath for stirring and heating until the polyvinylidene fluoride is completely dissolved in the solution, and then standing to degas, to obtain a light yellow polyvinylidene fluoride support layer casting solution with high viscosity. The other steps are the same as specific implementations one to seven.

[0053] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that: the ultrasonic time is 20 min to 30 min; the stirring and heating temperature is 65°C to 70°C; the static degassing temperature is 55°C to 65°C, and the static degassing time is 12h to 18h. The other steps are the same as those of specific embodiments 1 to 8.

[0054] Specific embodiment 10: This embodiment is a method for preparing a double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer, which is specifically completed according to the following steps:

[0055] 1. Use anhydrous ethanol to clean the glass plate, and then blow dry to get a clean glass plate;

[0056] 2. Adjust the manual scraper until the lower end of the scraper blade is flush with the glass plate, then adjust the knob to first scrape the polyvinylidene fluoride support layer casting liquid on the clean glass plate into a uniform plane, then adjust the knob to raise the scraper, and then scrape the active layer casting liquid into a uniform plane, and the double-layer film scraping is completed;

[0057] 3. Evaporate naturally for 30s to 40s, then immerse the glass plate for 5min to 8min to obtain an ultrafiltration membrane with a certain rigidity;

[0058] Fourth, transfer the ultrafiltration membrane with a certain rigidity to the medium and soak it, and replace the medium every 6 to 8 hours until the ultrafiltration membrane and the medium cannot be observed to exchange solvents under the light source, then take out the ultrafiltration membrane and dry it to obtain a double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer;

[0059] The medium described in step 4 is deionized water;

[0060] The drying temperature in step 4 is 25°C to 30°C.

[0061] The following examples are used to verify the beneficial effects of the present invention:

[0062] Example 1: A method for preparing a double-layer uniform-porous ultrafiltration membrane having a pH stimulus-responsive active layer, which is specifically completed by the following steps:

[0063] 1. Preparation of PS-b-PMMA block copolymer:

[0064] (1) Add 170 mL of tetrahydrofuran (THF) to an anhydrous and oxygen-free reactor, connect it to one of the end branches of a Schlenk double-row tube device, then add 25 mL of styrene (St), and tighten the stopper of the reactor to isolate it from the atmosphere;

[0065] (2) Turn on the vacuum pump, rotate the valve on / off switch on the Schlenk double-row tube device, connect the anhydrous and oxygen-free reactor to the passage of the vacuum pump, evacuate the reactor to ensure that the reactor is in a vacuum environment, rotate the valve on / off switch on the Schlenk double-row tube device again, introduce high-purity nitrogen, repeat the above evacuation-nitrogen replenishment operation three times to exhaust all the oxygen in the reactor, create a vacuum atmosphere, cut off the connection between the anhydrous and oxygen-free reactor and the Schlenk double-row tube device, and add the reactor to a -80°C constant temperature low-temperature reaction tank for cooling for 20 minutes;

[0066] The purity of the high-purity nitrogen in step 1 (2) is greater than 99.999%;

[0067] In step 1 (2), the constant pressure of high-purity nitrogen introduced into the anhydrous and oxygen-free reactor is 0.03 MPa;

[0068] (3) After cooling until the temperature inside the bottle drops, take out the anhydrous and oxygen-free reactor and connect it to one end branch of the Schlenk double-row tube device. Add 2.7 mL of initiator n-butyl lithium under nitrogen blowing atmosphere. The color of the solution turns orange-red. Turn on the vacuum pump, create a vacuum atmosphere, and transfer to a constant temperature low-temperature reaction tank at -80°C to continue the reaction for 45 minutes.

[0069] (4) In a nitrogen-blown atmosphere, use a pipette to add 0.6 mL of 1,1-diphenylethylene into the anhydrous and oxygen-free reactor to block the polymerization reaction. At this time, the color of the solution changes from orange-red to dark red. Then turn on the vacuum pump, create a vacuum atmosphere, and send it into a constant temperature low-temperature reaction tank at -80°C for continuous reaction for 10 minutes.

[0070] (5) In a nitrogen-blown atmosphere, add 5 mL of methyl methacrylate (MMA) to the anhydrous and oxygen-free reactor, then shape the atmosphere and send it into a constant temperature low-temperature reaction tank to polymerize the second monomer of the block copolymer. Then, start the vacuum pump, shape the vacuum atmosphere and send it into a constant temperature low-temperature reaction tank at -80°C to polymerize the second monomer of the block copolymer. Continue the reaction for 1 hour to obtain a mixed solution.

[0071] (6) The mixed solution was introduced into a 500 mL methanol-water mixed solution for precipitation, and then the precipitate was separated and redissolved in tetrahydrofuran. The mixed solution dissolved in 100 mL tetrahydrofuran (THF) was slowly introduced into a new 500 mL methanol-water mixed solution for precipitation. The precipitate was taken out after precipitation for 20 minutes, and secondary precipitation was carried out with deionized water for 20 minutes. After the precipitation was complete, the water was drained to obtain a block copolymer solid;

[0072] The volume ratio of methanol to water in the methanol-water mixed solution described in step 1 (6) is 1:1;

[0073] (7) vacuum drying the precipitated block copolymer solid to expand it. After drying for 0.5 h, the vent valve of the vacuum drying oven is opened to release the vacuum. After the solvent in the solid is completely volatilized, the vent valve of the vacuum drying oven is closed again, and the connecting valve between the vacuum drying oven and the vacuum pump is opened to evacuate the vacuum. Thereafter, the above operation is repeated in the vacuum drying oven to release the vacuum and evacuate the residual solvent to dissipate the volatilized residual solvent. After ensuring that the block copolymer no longer contains water, it is made into powder to obtain a PS-b-PMMA block copolymer for standby use;

[0074] The medium in the constant temperature low temperature reaction tank described in step 1 (3), (4) and (5) is anhydrous ethanol;

[0075] 2. Preparation of active layer casting solution:

[0076] (1) adding 0.3 g of polyvinylidene fluoride (PVDF) to 8.87 g of solvent to obtain a suspension; heating and stirring the suspension in a constant temperature magnetic stirrer at 60° C. for 2 h until the polyvinylidene fluoride is completely dissolved in the solvent to obtain a polyvinylidene fluoride solution;

[0077] The solvent in step 2 (1) is a mixed solution of DMF and THF, wherein the mass ratio of DMF to THF is 7:3;

[0078] (2) Add 0.8 g of PS-b-PMMA block copolymer and 0.03 g of anhydrous copper acetate to the polyvinylidene fluoride solution, and then place the mixed solution in a constant temperature magnetic stirrer at 60° C. and heat and stir for 15 min to obtain 10 g of a blue-green active layer casting solution with a certain viscosity;

[0079] 3. Preparation of polyvinylidene fluoride support layer casting solution:

[0080] 6 g of polyvinyl pyrrolidone (PVP) was added to 78 g of N,N-dimethylformamide (DMF) to obtain a suspension; the suspension was subjected to ultrasonic treatment for 20 minutes, and then 16 g of polyvinylidene fluoride (PVDF) was added and shaken to completely mix the polyvinylidene fluoride and the mixed solution in the container, and then the container was sealed; the sealed container was placed in a 60° C. water bath for stirring and heating until the polyvinylidene fluoride was completely dissolved in the solution, and then allowed to stand at 60° C. for degassing for 12 hours to obtain a light yellow polyvinylidene fluoride support layer casting solution with high viscosity;

[0081] 4. Prepare a 50cm×50cm glass plate for film making, clean the glass plate with anhydrous ethanol, and then use an air gun to blow dry the water to obtain a clean glass plate;

[0082] 5. Adjust the manual scraper until the lower end of the scraper blade is flush with the glass plate, then adjust the knob to raise the scraper to 180μm away from the glass plate, first scrape the polyvinylidene fluoride support layer casting liquid on the clean glass plate into a uniform plane, then adjust the knob to raise the scraper to 200μm, and then scrape the active layer casting liquid into a uniform plane, and the double-layer film scraping is completed;

[0083] 6. Evaporate naturally for 40 seconds, then immerse the glass plate in the medium for 5 minutes to obtain an ultrafiltration membrane with a certain rigidity;

[0084] 7. Transferring an ultrafiltration membrane with a certain rigidity to a medium for immersion, replacing the medium every 8 hours in the next 24 hours until no solvent exchange between the ultrafiltration membrane and the medium is observed under a light source, and obtaining a final finished ultrafiltration membrane after replacing the medium three times. The ultrafiltration membrane is then taken out and dried at 25° C. for 24 hours to obtain a double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer (denoted as 8wt% BCPs (1) or 3% PVDF);

[0085] The medium described in step 6 and step 7 is deionized water.

[0086] Example 2: This example differs from Example 1 in that the solvent in step 2 (1) is tetrahydrofuran (THF); the double-layer homogeneous ultrafiltration membrane with a pH stimulus responsive active layer obtained in step 7 of Example 2 (denoted as 8 wt % BCPs (2)). The other steps and parameters are the same as those in Example 1.

[0087] Embodiment 3: The difference between this embodiment and embodiment 1 is:

[0088] 2. Preparation of active layer casting solution:

[0089] (1) adding 0.3 g of polyvinylidene fluoride (PVDF) to 9.07 g of solvent to obtain a suspension; heating and stirring the suspension in a constant temperature magnetic stirrer at 60° C. for 2 h until the polyvinylidene fluoride is completely dissolved in the solvent to obtain a polyvinylidene fluoride solution;

[0090] The solvent in step 2 (1) is a mixed solution of DMF and THF, wherein the mass ratio of DMF to THF is 7:3;

[0091] (2) Add 0.6 g of PS-b-PMMA block copolymer and 0.03 g of anhydrous copper acetate to the polyvinylidene fluoride solution, and then place the mixed solution in a constant temperature magnetic stirrer at 60° C. and heat and stir for 15 min to obtain 10 g of a blue-green active layer casting solution with a certain viscosity;

[0092] The double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer (referred to as 6 wt % BCPs) obtained in step 7 of Example 3. The other steps and parameters are the same as those in Example 1.

[0093] Embodiment 4: The difference between this embodiment and embodiment 1 is:

[0094] 2. Preparation of active layer casting solution:

[0095] (1) adding 0.3 g of polyvinylidene fluoride (PVDF) to 8.67 g of solvent to obtain a suspension; heating and stirring the suspension in a constant temperature magnetic stirrer at 60° C. for 2 h until the polyvinylidene fluoride is completely dissolved in the solvent to obtain a polyvinylidene fluoride solution;

[0096] The solvent in step 2 (1) is a mixed solution of DMF and THF, wherein the mass ratio of DMF to THF is 7:3;

[0097] (2) Add 1.0 g of PS-b-PMMA block copolymer and 0.03 g of anhydrous copper acetate to the polyvinylidene fluoride solution, and then place the mixed solution in a constant temperature magnetic stirrer at 60° C. and heat and stir for 15 min to obtain 10 g of a blue-green active layer casting solution with a certain viscosity;

[0098] The double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer (represented as 10 wt % BCPs) obtained in step 7 of Example 4. The other steps and parameters are the same as those in Example 1.

[0099] Embodiment 5: The difference between this embodiment and embodiment 1 is:

[0100] 2. Preparation of active layer casting solution:

[0101] (1) adding 0.2 g of polyvinylidene fluoride (PVDF) to 8.97 g of solvent to obtain a suspension; heating and stirring the suspension in a constant temperature magnetic stirrer at 60° C. for 2 h until the polyvinylidene fluoride is completely dissolved in the solvent to obtain a polyvinylidene fluoride solution;

[0102] The solvent in step 2 (1) is a mixed solution of DMF and THF, wherein the mass ratio of DMF to THF is 7:3;

[0103] (2) Add 0.8 g of PS-b-PMMA block copolymer and 0.03 g of anhydrous copper acetate to the polyvinylidene fluoride solution, and then place the mixed solution in a constant temperature magnetic stirrer at 60° C. and heat and stir for 15 min to obtain 10 g of a blue-green active layer casting solution with a certain viscosity;

[0104] The double-layer uniform pore ultrafiltration membrane (denoted as 2% PVDF) with a pH stimulus responsive active layer obtained in step 7 of Example 5. The other steps and parameters are the same as those in Example 1.

[0105] Embodiment 6: The difference between this embodiment and embodiment 1 is:

[0106] 2. Preparation of active layer casting solution:

[0107] (1) adding 0.4 g of polyvinylidene fluoride (PVDF) to 8.77 g of solvent to obtain a suspension; heating and stirring the suspension in a constant temperature magnetic stirrer at 60° C. for 2 h until the polyvinylidene fluoride is completely dissolved in the solvent to obtain a polyvinylidene fluoride solution;

[0108] The solvent in step 2 (1) is a mixed solution of DMF and THF, wherein the mass ratio of DMF to THF is 7:3;

[0109] (2) Add 0.8 g of PS-b-PMMA block copolymer and 0.03 g of anhydrous copper acetate to the polyvinylidene fluoride solution, and then place the mixed solution in a constant temperature magnetic stirrer at 60° C. and heat and stir for 15 min to obtain 10 g of a blue-green active layer casting solution with a certain viscosity;

[0110] The double-layer uniform pore ultrafiltration membrane (denoted as 4% PVDF) with a pH stimulus responsive active layer obtained in step 7 of Example 6. The other steps and parameters are the same as those in Example 1.

[0111] Comparative Example 1:

[0112] Purchase the common finished PVDF commercial membrane on the market, do not make any modification, and only use deionized water for cleaning.

[0113] Comparative Example 2: A method for preparing a polyvinylidene fluoride support layer membrane, specifically prepared according to the following steps:

[0114] 1. Preparation of polyvinylidene fluoride support layer casting solution:

[0115] 6 g of polyvinyl pyrrolidone (PVP) was added to 78 g of N,N-dimethylformamide (DMF) to obtain a suspension; the suspension was subjected to ultrasonic treatment for 20 minutes, and then 16 g of polyvinylidene fluoride (PVDF) was added and shaken to completely mix the polyvinylidene fluoride and the mixed solution in the container, and then the container was sealed; the sealed container was placed in a 60° C. water bath for stirring and heating until the polyvinylidene fluoride was completely dissolved in the solution, and then allowed to stand at 60° C. for degassing for 12 hours to obtain a light yellow polyvinylidene fluoride support layer casting solution with high viscosity;

[0116] 2. Prepare a 50cm×50cm glass plate for film making, clean the glass plate with anhydrous ethanol, and then use an air gun to blow dry the water to obtain a clean glass plate;

[0117] 3. Adjust the manual scraper until the lower end of the scraper blade is flush with the glass plate, then adjust the knob to raise the scraper to 200μm away from the glass plate, and scrape the polyvinylidene fluoride support layer casting liquid on the clean glass plate into a uniform plane. The scraping is completed;

[0118] Fourth, evaporate naturally for 40 seconds, and then immerse the glass plate in the medium for 5 minutes to obtain an ultrafiltration membrane with a certain rigidity;

[0119] 5. Transfer the ultrafiltration membrane with a certain rigidity to the medium and soak it. In the next 24 hours, replace the medium every 8 hours until the solvent exchange between the ultrafiltration membrane and the medium is not observed under the light source. After replacing the medium three times, the final finished ultrafiltration membrane is obtained. Then, the ultrafiltration membrane is taken out and dried at 25°C for 12 hours to obtain a polyvinylidene fluoride support layer membrane;

[0120] The medium described in step 4 and step 5 is deionized water.

[0121] The ultrafiltration membranes prepared in the above four embodiments and two comparative examples were subjected to the following tests:

[0122] 1. Pure water flux test of ultrafiltration membrane:

[0123] The prepared ultrafiltration membrane is placed in the ultrafiltration membrane filter cup, pure water is added, and the air tightness is checked after sealing; the pressure valve is controlled by a program to start the pure water measurement experiment; the water quality out of the ultrafiltration cup is measured by an electronic balance, and the instantaneous water output can be read from the computer to calculate the pure water flux of the membrane;

[0124] 2. Test of bovine serum albumin (BSA) retention rate of ultrafiltration membrane:

[0125] (1) Accurately weigh 1.000 g of bovine serum albumin and place it in a 50 mL beaker, add deionized water to dissolve it, transfer the solution to a 1000 mL volumetric flask, add deionized water to the scale, shake well, and prepare a bovine serum albumin solution with a mass concentration of 1000 mg / L; transfer 2.0 mL of the bovine serum albumin solution prepared in the previous step, add water to the scale, shake well, and prepare a bovine serum albumin (BSA) standard solution with a mass concentration of 20 mg / L;

[0126] (2) Using the BSA standard solution prepared in step (i) to filter pure water, the effluent at three time points, 10 mL, 50 mL, and 90 mL, was measured using deionized water as a reference. The absorbance values ​​of the three samples were measured at a wavelength of 280 nm using a 1 cm cuvette on a UV-visible spectrophotometer. The BSA retention rates at the three time points were calculated based on the BSA standard curve.

[0127] Figure 3 It is a bar graph of pure water flux and BSA retention capacity of double-layer uniform pore ultrafiltration membranes with pH stimulus responsive active layers prepared with different surface BCPs concentrations in Examples 1, 3 and 4;

[0128] from Figure 3It can be seen that by comprehensively comparing the pure water flux and BSA retention rate of the membranes with three BCPs concentrations, it can be seen that with the increase of block copolymer BCPs, the flux shows a gradually increasing trend. It is preliminarily speculated that the more block copolymers there are in the active layer, the easier it is for the interface self-assembly behavior to automatically form a spreading regular structure on the membrane surface to promote the permeation of pure water. At the same time, because of these regular structures, the overall retention rate of the membrane is reduced. The permeability and retention rate conform to the "trade off" effect. Therefore, a certain permeability performance needs to be sacrificed to increase the retention rate of the double-layer membrane, but the overall performance is still better than that of traditional PVDF membranes.

[0129] Figure 4 It is a bar graph of pure water flux and BSA retention capacity of double-layer uniform pore ultrafiltration membranes with pH stimulus responsive active layers prepared with different surface PVDF concentrations in Examples 1, 5 and 6;

[0130] from Figure 4 It can be seen that by comprehensively comparing the membrane pure water flux and BSA retention rate diagrams of three PVDF concentrations, it can be seen that with the increase of PVDF concentration, the flux shows a gradually decreasing trend, and the retention rate gradually increases. At the same time, combined with the subsequent SEM scanning electron microscopy characterization, it can be considered that the higher the PVDF concentration, the uniform pore structure of the active layer of the ultrafiltration membrane will be affected to a certain extent, and the pore size of the membrane will be slightly reduced. At the same time, due to the decrease in the uniformity of the membrane pores and the decrease in the pore size, the retention rate of the double-layer uniform pore ultrafiltration membrane for pollutants such as BSA is improved. Considering the permeability and retention performance of the ultrafiltration membrane, it is finally believed that when the PVDF concentration is 3%, the comprehensive water purification performance of the membrane is optimal.

[0131] 3. Scanning electron microscope (SEM) and energy spectrum analysis (EDS) characterization of ultrafiltration membrane:

[0132] In order to visually observe the surface pore size, porosity and pore size distribution of the ultrafiltration membrane, the surface morphology of the ultrafiltration membrane was observed using a scanning electron microscope (Sigma 500, Zeiss, Germany). It was determined that the double-layer uniform pore ultrafiltration membrane prepared by the formula in Example 1 and Example 2 had a good uniform pore morphology, a pore size between 40-60 nm, and could be used as an active layer to effectively intercept organic pollutants in water. The surface pore structure of the double-layer membrane was significantly optimized and improved compared with the traditional PVDF commercial membrane, and the pore size was in line with the category of ultrafiltration membrane. The results are shown in FIG. Figure 5 .

[0133] 4. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) characterization of double-layer uniform pore ultrafiltration membrane:

[0134] In order to accurately characterize and analyze the chemical elements on the surface of the material, the Thermo Fisher 250Xi X-ray photoelectron spectrometer was used for the experiment. In order to more accurately clarify the elemental composition of the membrane material surface and achieve quantitative analysis, XPS was used for data analysis. From the results, it can be seen that the surface of the double-layer uniform pore ultrafiltration membrane is mainly composed of C, H, N, F and other elements. The nitrogen element comes from the casting liquid solvent DMF used in the material preparation process, and the F element comes from the PVDF component contained in both the active layer and the support layer. Through the analysis of the O1s spectrum, it is found that It was found that the material contained two characteristic functional groups, O=CO and O=CH, which should come from the two monomers of styrene and methyl methacrylate in the block copolymer. Based on this, the successful synthesis of the block copolymer PS-b-PMMA was confirmed. At the same time, X-ray diffraction was used to further reveal the structural characteristics of the material. The XRD spectrum had obvious characteristic peaks at 19.76°, 29.22°, 35.78°, 39.2° and 47.36°. The results showed that the double-layer uniform pore ultrafiltration membrane contained block copolymer PS-b-PMMA and PVDF and other components. The results are as follows Figure 6 and Figure 7 .

[0135] The above are only preferred specific embodiments of the present invention, which are all different implementations based on the overall concept of the present invention, and the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer, characterized in that The double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer comprises a polyvinylidene fluoride support layer and an active layer with a PS-b-PMMA block copolymer as a main component; The thickness of the polyvinylidene fluoride support layer is 180 μm to 190 μm; The thickness of the active layer is 10 μm to 20 μm, and the membrane pore size on the surface of the active layer is 40 nm to 60 nm; The PS-b-PMMA block copolymer accounts for 6% to 10% of the mass of the active layer casting solution; The polyvinylidene fluoride support layer is prepared from polyvinylidene fluoride, polyvinyl pyrrolidone and N,N-dimethylformamide, wherein polyvinylidene fluoride accounts for 16% to 20% of the mass of the polyvinylidene fluoride support layer casting liquid, and polyvinyl pyrrolidone accounts for 4% to 6% of the mass of the polyvinylidene fluoride support layer casting liquid.

2. A double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer according to claim 1, characterized in that The PS-b-PMMA block copolymer is prepared by living anionic polymerization, which is specifically completed by the following steps: (1) Add tetrahydrofuran to an anhydrous and oxygen-free reactor, connect it to one of the end branches of a Schlenk double-row tube device, then add styrene, and tighten the stopper of the reactor to isolate it from the atmosphere; (2) Turn on the vacuum pump, rotate the valve on / off switch on the Schlenk double-row tube device, connect the anhydrous and oxygen-free reactor to the passage of the vacuum pump, evacuate the reactor, ensure that the reactor is in a vacuum environment, rotate the valve on / off switch on the Schlenk double-row tube device again, introduce high-purity nitrogen, repeat the above evacuation-nitrogen replenishment operation three times to exhaust all the oxygen in the reactor, create a vacuum atmosphere, cut off the connection between the anhydrous and oxygen-free reactor and the Schlenk double-row tube device, add it to a constant temperature low-temperature reaction tank, and cool it for a certain period of time; (3) After cooling until the temperature inside the bottle drops, take out the anhydrous and oxygen-free reactor and connect it to one end branch of the Schlenk double-row tube device. Add the initiator n-butyl lithium under a nitrogen blowing atmosphere. The color of the solution turns orange-red. Turn on the vacuum pump, create a vacuum atmosphere, and then transfer it to a constant temperature low-temperature reaction tank to continue the reaction for a certain period of time. (4) In a nitrogen-blown atmosphere, 1,1-diphenylethylene is added to the anhydrous and oxygen-free reactor using a pipette to block the polymerization reaction. At this time, the solution color changes from orange-red to dark red. Then, the vacuum pump is turned on to create a vacuum atmosphere and send it into a constant temperature low-temperature reaction tank. The reaction continues for a certain period of time. (5) Adding methyl methacrylate to the anhydrous and oxygen-free reactor under nitrogen blowing atmosphere, then starting the vacuum pump, creating a vacuum atmosphere and sending it into a constant temperature low temperature reaction tank to carry out the polymerization reaction of the second monomer of the block copolymer, and taking it out after continuing the reaction for a certain period of time to obtain a mixed solution; (6) introducing the mixed solution into a methanol-water mixed solution for precipitation, then separating the precipitate, re-dissolving it in tetrahydrofuran, and then slowly introducing the mixed solution dissolved in tetrahydrofuran into a new methanol-water mixed solution for precipitation. After precipitation for a certain period of time, the precipitate is taken out and secondary precipitation is carried out with deionized water. After the precipitation is complete, the water is drained to obtain a block copolymer solid; (7) The precipitated block copolymer solid is vacuum dried to expand it. After drying for 0.5 h to 1 h, the vent valve of the vacuum drying oven is opened to release the vacuum. After the solvent in the solid is completely evaporated, the vent valve of the vacuum drying oven is closed again, and the connecting valve between the vacuum drying oven and the vacuum pump is opened to evacuate the vacuum. Thereafter, the above operation is repeated in the vacuum drying oven to release the vacuum and evacuate the residual solvent to dissipate. After ensuring that the block copolymer no longer contains water, it is made into powder to obtain the PS-b-PMMA block copolymer for standby use.

3. A double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer according to claim 2, characterized in that The constant pressure of high-purity nitrogen introduced into the anhydrous and oxygen-free reactor in step (2) is 0.02MPa-0.04MPa; the volume ratio of tetrahydrofuran to styrene in step (1) is (34-57):1; the purity of the high-purity nitrogen in step (2) is greater than 99.999%; the cooling time in step (2) is 0.25h-0.4h; the continuous reaction time in step (3) is 0.5h-1h; the volume ratio of 1,1-diphenylethylene to styrene in step (4) is (0.12-0.2):1; the continuous reaction time in step (4) is 0.05h-0.2h.

4. A double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer according to claim 2, characterized in that The duration of the continuous reaction in step (5) is 1 h to 1.5 h; the volume ratio of methanol to water in the methanol-water mixed solution in step (6) is 1:1; in step (6), the mixed solution dissolved in tetrahydrofuran is slowly introduced into a new methanol-water mixed solution for precipitation, and the precipitate is taken out after precipitation for 0.3 h to 0.5 h.

5. A double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer according to claim 2, characterized in that The volume ratio of styrene to methyl methacrylate is (5-9):1; the volume ratio of styrene to initiator n-butyl lithium is (9-10):1; the medium in the constant temperature low temperature reaction tank described in steps (3), (4) and (5) is anhydrous ethanol, and the temperature of the constant temperature low temperature reaction tank is -80°C; the temperature of the vacuum drying oven described in step (7) is 60°C to 80°C.

6. A double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer according to claim 1, characterized in that The preparation method of the active layer casting solution is specifically completed according to the following steps: (1) adding polyvinylidene fluoride to a solvent to obtain a suspension; heating and stirring the suspension in a constant temperature magnetic stirrer until the polyvinylidene fluoride is completely dissolved in the solvent to obtain a polyvinylidene fluoride solution; (2) Adding PS-b-PMMA block copolymer and anhydrous copper acetate to the polyvinylidene fluoride solution, and then placing the mixed solution in a constant temperature magnetic stirrer for heating and stirring for a period of time to obtain a blue-green active layer casting solution with a certain viscosity; The PS-b-PMMA block copolymer accounts for 6% to 10% of the mass of the active layer casting liquid; the polyvinylidene fluoride accounts for 2% to 4% of the mass of the active layer casting liquid; and the anhydrous copper acetate accounts for 0.2% to 0.3% of the mass of the active layer casting liquid.

7. A double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer according to claim 6, characterized in that The solvent described in step (1) is a mixed solution of DMF and THF, wherein the mass ratio of DMF to THF is 7:3 or 8:2; the temperature of the heating and stirring described in step (1) is 60°C to 70°C, and the heating and stirring time is 2h to 4h; the temperature of the heating and stirring described in step (2) is 60°C to 65°C, and the heating and stirring time is 15min to 30min.

8. The double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer according to claim 1, characterized in that The preparation method of the polyvinylidene fluoride support layer casting liquid is specifically completed according to the following steps: adding polyvinyl pyrrolidone to N,N-dimethylformamide to obtain a suspension; subjecting the suspension to ultrasonic treatment for a period of time, adding polyvinylidene fluoride, shaking to completely mix the polyvinylidene fluoride and the mixed solution in the container, and then sealing the container; placing the sealed container in a water bath for stirring and heating until the polyvinylidene fluoride is completely dissolved in the solution, and then standing to degas, to obtain a light yellow polyvinylidene fluoride support layer casting liquid with high viscosity.

9. A double-layer uniform pore ultrafiltration membrane having a pH stimulus responsive active layer according to claim 8, characterized in that The ultrasonic time is 20 minutes to 30 minutes; the stirring and heating temperature is 65°C to 70°C; the static degassing temperature is 55°C to 65°C, and the static degassing time is 12h to 18h.

10. The method for preparing a double-layer uniform-porous ultrafiltration membrane having a pH stimulus-responsive active layer according to claim 1, characterized in that The preparation method is specifically completed according to the following steps:

1. Use anhydrous ethanol to clean the glass plate, and then blow dry to get a clean glass plate; 2. Adjust the manual scraper until the lower end of the scraper blade is flush with the glass plate, then adjust the knob to first scrape the polyvinylidene fluoride support layer casting liquid on the clean glass plate into a uniform plane, then adjust the knob to raise the scraper, and then scrape the active layer casting liquid into a uniform plane, and the double-layer film scraping is completed; 3. Evaporate naturally for 30s to 40s, then immerse the glass plate in the medium for 5min to 8min to obtain an ultrafiltration membrane with a certain rigidity; Fourth, transfer the ultrafiltration membrane with a certain rigidity to the medium and soak it, and replace the medium every 6 to 8 hours until the ultrafiltration membrane and the medium cannot be observed to exchange solvents under the light source, then take out the ultrafiltration membrane and dry it to obtain a double-layer uniform pore ultrafiltration membrane with a pH stimulus responsive active layer; The medium described in step 3 and step 4 is deionized water; The drying temperature in step 4 is 25°C to 30°C.

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