Polyvinylidene fluoride homogeneous porous membrane as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of polymeric membranes, and discloses a polyvinylidene fluoride homogeneous porous membrane and a preparation method and application thereof.The preparation method comprises the following steps that 1, a membrane casting solution is prepared, specifically, the membrane casting solution comprises 8-14 wt.% of polyvinylidene fluoride, 1-10 wt.% of an organic low-molecular additive, 0.5-2 wt.% of porous graphene nanowires and the balance solvent, the sum of the components is 100 wt.%. The organic low-molecular additive is ethanol, ethylene glycol or polyethylene glycol of which the MW is less than or equal to 600; and step 2, coating a glass plate with the membrane casting solution obtained in the step 1, placing the glass plate in a coagulating bath for curing, rinsing and airing the glass plate to obtain the polyvinylidene fluoride homogeneous porous membrane, and the coagulating bath is an aqueous solution of ethanol with the concentration of 70-90 wt.%. And the graphene nanowires with one-dimensional structures are inserted into adjacent pore structures, so that the separation performance of the porous membrane is optimized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high molecular film, in particular to a polyvinylidene fluoride homogeneous porous membrane and a preparation method and application thereof. BACKGROUND

[0002] In the field of contemporary separation science and material engineering, membrane separation technology has been widely used in water treatment, biological medicine, chemical separation and other key fields due to its high efficiency, energy saving, environmental protection and other outstanding advantages, and has become one of the core technologies to solve the problems of resource shortage and environmental pollution.

[0003] Polymer membranes occupy a dominant position in the membrane material market due to their controllable preparation cost, excellent mechanical properties, strong chemical stability and other characteristics. They have become the preferred substrate for preparing high-performance separation membranes due to their excellent chemical corrosion resistance, high temperature resistance, oxidation resistance and good mechanical strength.

[0004] However, the preparation technology of traditional polyvinylidene fluoride membrane still faces many bottlenecks, which restricts its application in high requirement scenarios. The traditional method is easy to form non-homogeneous asymmetric pore structure, the transition between the dense layer and the sublayer of the membrane surface layer is abrupt, the large pore structure is prone to uneven pore size and poor pore connectivity, and the dense and thick skin layer easily increases the mass transfer resistance of the membrane, making it difficult to simultaneously improve the flux and separation efficiency of the membrane.

[0005] With the continuous expansion of the application scenarios of membranes, higher demands are put forward for the pore structure of polyvinylidene fluoride membranes. Homogeneous porous polyvinylidene fluoride membranes have uniform pore size distribution, excellent pore connectivity, and high structural consistency in the thickness direction, which can not only increase the permeation flux of the membrane by increasing the pore size, but also rely on the homogeneous structure to ensure the separation precision and mechanical stability. It has shown irreplaceable application potential in fields such as seawater desalination pretreatment, antibiotic purification, hemodialysis, and tissue engineering scaffolds. For example, in the field of water treatment, large pore structure can effectively reduce the risk of membrane pore blockage, prolong the service life of the membrane, and reduce the operating cost.

[0006] Currently, the preparation of polyvinylidene fluoride homogeneous porous membrane still faces technical challenges: how to realize precise control of large pore size (pore size > 1 μm) while ensuring the mechanical properties of the membrane; how to optimize the preparation process to reduce pore agglomeration and defects during phase separation and improve the pore connectivity of the membrane; therefore, it is of great scientific significance and engineering value to develop an efficient, stable and low-cost polyvinylidene fluoride homogeneous porous membrane preparation technology, which is also a key breakthrough to promote the expansion of membrane separation technology to high-end fields. SUMMARY

[0007] The application aims to solve the technical defects of polyvinylidene fluoride membrane in the prior art, such as small pore size and high mass transfer resistance, and provides a polyvinylidene fluoride homogeneous porous membrane, a preparation method and application thereof.

[0008] The technical scheme adopted to achieve the object of the application is: A preparation method of a polyvinylidene fluoride homogeneous porous membrane, comprising the following steps: Step 1, preparing a casting solution: the casting solution comprises 8-14 wt.% of polyvinylidene fluoride, 1-10 wt.% of an organic low-molecular additive, 0.5-2 wt.% of porous graphene nanowires, and the balance is a solvent, and the sum of the components is 100 wt.%; the organic low-molecular additive is ethanol, ethylene glycol or M W ≤600 of polyethylene glycol; Step 2, coating the casting solution obtained in step 1 on a glass plate, placing it in a coagulation bath for solidification, the coagulation bath is an aqueous solution of ethanol with a concentration of 70-90 wt.%, and then rinsing and air-drying to obtain the polyvinylidene fluoride homogeneous porous membrane.

[0009] In the above technical scheme, in step 1, the weight average molecular weight of the polyvinylidene fluoride is 400000-600000, and the solvent is one or a mixture of more than one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

[0010] In the above technical scheme, in step 1, the preparation step of the casting solution is: first dispersing the porous graphene nanowires in the solvent by ultrasonic, then adding polyvinylidene fluoride and an organic low-molecular additive thereto, mechanically stirring for more than 5h under the condition of a water bath at 30-50℃, and then performing room temperature defoaming treatment after the casting solution is uniformly mixed to prepare the casting solution.

[0011] In the above technical scheme, in step 1, the length of the porous graphene nanowires is 8-10μm, the diameter is 100-200nm, and the aspect ratio is 50-100, and the porous graphene nanowires are prepared by the following steps: Step s11, mixing the graphene oxide dispersion liquid and ammonia water in a volume ratio of (2-4):1, and then performing hydrothermal reaction at 90-120℃ for 4-8h, wherein the ammonia water chemically cuts the graphene oxide during the hydrothermal reaction, and after filtration, reduced pressure distillation and freeze-drying, the amino-functionalized graphene quantum dots are obtained; Step s12, the amino-functionalized graphene quantum dots obtained in step s11 are dispersed in water and the pH value is adjusted to 10-14, and an interfacial polymerization reaction is carried out with a trimesoyl chloride / hexane organic solution, and the product is washed, filtered, centrifuged and freeze-dried to obtain the porous graphene nanowire.

[0012] In the above technical solution, in step 2, the coating method comprises the following steps: placing a clean glass plate on a hot table of a film doctoring machine at 30-50 DEG C, uniformly pouring the casting solution on the glass plate, and then running the film doctoring machine, using a film doctoring rod with a thickness of 500-1000 mu m to uniformly coat the casting solution on the glass plate.

[0013] In the above technical solution, in step 2, the coagulation bath temperature is 20-50 DEG C, and the time for solidification in the coagulation bath is 1-5 min.

[0014] In the above technical solution, in step 2, the rinsing and air-drying steps are as follows: after the film is solidified, the wet film is obtained by taking it off the glass plate, the wet film is transferred to distilled water for rinsing for more than 48 h, then transferred to anhydrous ethanol for soaking for 10-30 min, then the film is covered and fixed on both sides with non-woven fabric, and dried in a clean room at room temperature to obtain a dry polyvinylidene fluoride porous membrane.

[0015] Another aspect of the present application also includes a polyvinylidene fluoride homogeneous porous membrane prepared by the preparation method.

[0016] In the above technical solution, the thickness of the polyvinylidene fluoride homogeneous porous membrane is 90-130 mu m.

[0017] Another aspect of the present application also includes the application of the polyvinylidene fluoride homogeneous porous membrane as a separation membrane in water treatment.

[0018] Compared with the prior art, the present application has the following advantages: 1. The present application uses an interfacial polymerization method to synthesize a porous graphene nanowire, and adds the porous graphene nanowire as an additive to a casting solution system, the porous graphene nanowire has a high aspect ratio, a typical one-dimensional porous structure, and hydrophilic properties, and can promote the extension and growth of the membrane pore structure during the phase inversion of the casting solution into a membrane, and its one-dimensional structure penetrates between adjacent pore structures, serving as a bridge connecting the pores, which is beneficial to improving the transmission rate between adjacent sponge pores.

[0019] 2. The one-dimensional hydrophilic porous graphene nanowire used in the present application penetrates between adjacent porous structures, on the one hand, it can reduce the two-dimensional sheet structure (such as graphene) nanometer additive (the pure water flux is only 193.47 L·m -2 ·h -1 ·bar-1 )The potential barrier effect of the membrane pore channel, on the other hand, this interpenetrating structure can improve its anchoring force to the membrane matrix, can ensure the stability during long-term operation without loss, maintain the hydrophilicity of the membrane and the rapid penetration of the polar solvent.

[0020] 3. The application uses a certain concentration of ethanol aqueous solution as the coagulation bath, which is more conducive to delaying phase separation than using pure distilled water as the coagulation bath (the pure water flux is 110.42 L·m -2 ·h -1 ·bar -1 ), thereby inhibiting the formation of finger-like macro-pores and dense skin layers, combined with the interpenetration of graphene nanowires, which can significantly affect the phase inversion process, thereby greatly improving the mass transfer separation performance of polyvinylidene fluoride porous membranes. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 It is a schematic diagram of the scanning electron microscope photograph of the porous graphene nanowire in Example 1; Figure 2 It is a schematic diagram of the surface and cross-section scanning electron microscope photograph of the polyvinylidene fluoride homogeneous porous membrane prepared in Example 1; Figure 3 It is a schematic diagram of the surface and cross-section scanning electron microscope photograph of the polyvinylidene fluoride homogeneous porous membrane prepared in Example 2; Figure 4 It is a schematic diagram of the surface and cross-section scanning electron microscope photograph of the polyvinylidene fluoride homogeneous porous membrane prepared in Example 3. DETAILED DESCRIPTION

[0022] The application will be further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the application and do not limit the application.

[0023] Example 1 A preparation method of a polyvinylidene fluoride homogeneous porous membrane, comprising the following steps: (1) Preparation of porous graphene nanowires: Preparation of aminated graphene quantum dots: 373.6 mg of graphene oxide was dispersed in 362 mL of distilled water, after ultrasonic treatment for 3 h, 108.93 mL of ammonia water was added, and the mixture was uniformly transferred to a reaction kettle and subjected to hydrothermal reaction at 90℃ for 5 h. The graphene oxide was chemically cut with ammonia water, and after filtration, vacuum distillation and freeze-drying, aminated graphene quantum dot powder was obtained.

[0024] Preparation of porous graphene nanowires: prepare an aqueous solution of aminated graphene quantum dots with a concentration of 0.5 w / v% and adjust the pH to 12.3, and then perform interfacial polymerization with 0.1 w / v% of a trimesoyl chloride / n-hexane organic solution for a certain period of time. The product is obtained after washing, filtering, centrifuging, optimizing, and freeze-drying. The scanning electron microscope image of the product is shown in Figure 1 , which has a size of about 10 μm and a diameter of about 100-200 nm, and a length-diameter ratio of 50-100.

[0025] (2) According to the mass percentage, the casting solution formula is: 12 wt.% of polyvinylidene fluoride powder (M W = 600000), 3 wt.% of polyethylene glycol (M W = 200), 0.5 wt.% of porous graphene nanowires, and 84.5 wt.% of N,N-dimethylacetamide.

[0026] Preparation method of the casting solution: first, disperse the porous graphene nanowires in N,N-dimethylacetamide by ultrasonic dispersion, then add polyvinylidene fluoride and polyethylene glycol thereto, and mechanically stir for more than 5 h at 50°C in a water bath. After the polymers are completely dissolved, degas the casting solution at room temperature until the bubbles in the casting solution completely disappear, and a uniform transparent casting solution is obtained.

[0027] (3) The formula of the coagulation bath is prepared as follows: 80 wt.% of anhydrous ethanol and 20 wt.% of distilled water.

[0028] (4) Place a clean glass plate on the hot stage of an automatic film drawdown machine at 30°C, pour the casting solution obtained in step (2) uniformly on the glass plate, then run the film drawdown machine, use a film drawdown bar with a thickness of 500 μm to quickly and uniformly coat the casting solution on the glass plate, immediately place the glass plate into the coagulation bath of step (3) at a steady speed, and stand still in the coagulation bath for 1 min. After the film is solidified, transfer it to distilled water for rinsing for more than 48 h to remove residual solvents, stabilize the pore structure of the film, and dry to obtain the polyvinylidene fluoride homogeneous porous film.

[0029] Figure 2 The surface and cross-section electron microscope images of the polyvinylidene fluoride homogeneous porous film prepared for this example are shown in the figures. The surface of the film is a bicontinuous macroporous structure without a skin layer, and the cross-section electron microscope image shows that there is no finger-like pore structure. The pure water flux of the obtained film is measured to be 2623.0 L·m -2 ·h -1 ·bar -1 under a pressure of 0.1 MPa, and the film thickness is 111.8 μm.

[0030] Example 2 A method for preparing a polyvinylidene fluoride homogeneous porous membrane, comprising the following steps: (1) The same as step (1) of Example 1.

[0031] (2) According to the mass percentage, the casting solution formula is: 12wt.% polyvinylidene fluoride powder (M W =600000), 10wt.% polyethylene glycol (M W =200), 0.5wt.% porous graphene nanowire, 77.5wt.% N,N-dimethylacetamide.

[0032] The method for preparing the casting solution is: first, disperse the porous graphene nanowire in N,N-dimethylacetamide by ultrasonic, then add polyvinylidene fluoride and polyethylene glycol to it, mechanically stir for more than 5h under the condition of 50℃ water bath, after the polymer is completely dissolved, deaerate the casting solution at room temperature until the bubbles in the casting solution completely disappear, and obtain a uniform transparent casting solution.

[0033] (3) The formula of the coagulation bath is: 75wt.% anhydrous ethanol, 25wt.% distilled water.

[0034] (4) Place a clean glass plate on the hot table of the automatic film scraper at 40℃, uniformly pour the casting solution obtained in step (1) on the glass plate, then run the film scraper, use a film scraper rod with a thickness of 500μm to quickly and uniformly coat the casting solution on the glass plate, immediately place the glass plate into the coagulation bath of step (3) at a steady speed, stand in the coagulation bath for 1min, after the film is solidified, transfer it to distilled water for rinsing for more than 48h to remove residual solvents, stabilize the film pore structure, and dry to obtain the polyvinylidene fluoride homogeneous porous membrane.

[0035] Figure 3 The surface and cross-section electron microscope images of the polyvinylidene fluoride homogeneous porous membrane prepared for this example are shown in Figure 3 It can be seen that the film surface is a bicontinuous macroporous structure. The pure water flux of the obtained film is 2539.0L·m -2 ·h -1 ·bar -1 under the pressure condition of 0.1MPa, and the film thickness is 119.0μm.

[0036] Example 3 A method for preparing a polyvinylidene fluoride homogeneous porous membrane, comprising the following steps: (1) The same as step (1) of Example 1.

[0037] (2) According to the mass percentage, the casting solution formula is: 12wt.% polyvinylidene fluoride powder (M W=600000), 10 wt.% of anhydrous ethanol, 1 wt.% of porous graphene nanowires, 77 wt.% of N,N-dimethylacetamide.

[0038] The method for preparing the casting solution is as follows: the porous graphene nanowires are dispersed in N,N-dimethylacetamide by ultrasonic, then polyvinylidene fluoride and anhydrous ethanol are added, and mechanical stirring is performed at 50°C for more than 5 hours until the polymer is completely dissolved. Then, the casting solution is degassed at room temperature until the bubbles in the casting solution completely disappear, and a uniform transparent casting solution is prepared.

[0039] (3) The formula of the coagulation bath is as follows: 75 wt.% of anhydrous ethanol and 25 wt.% of distilled water.

[0040] (4) A clean glass plate is placed on a hot table of an automatic film casting machine at 40°C, the casting solution obtained in step (1) is uniformly poured on the glass plate, and then the film casting machine is operated. A film casting rod with a thickness of 500 μm is used to quickly and uniformly coat the casting solution on the glass plate. Then, the glass plate is immediately placed into the coagulation bath of step (3) at a steady speed, and is kept in the coagulation bath for 1 minute. After the film is solidified, it is transferred to distilled water for rinsing for more than 48 hours to remove residual solvents and stabilize the pore structure of the film, thereby obtaining the polyvinylidene fluoride homogeneous porous film.

[0041] Figure 4 The surface and cross-section electron microscope images of the polyvinylidene fluoride homogeneous porous film prepared in this example show that the pore size of the film is obviously smaller than that of the films obtained in examples 1 and 2. The pure water flux is 1215.0 L·m -2 ·h -1 ·bar -1 ·bar at a pressure of 0.1 MPa, and the film thickness is 100.2 μm.

[0042] Example 4 A method for preparing a polyvinylidene fluoride homogeneous porous film, comprising the following steps: (1) The same as step (1) of example 1.

[0043] (2) According to the mass percentage, the formula of the casting solution is as follows: 13 wt.% of polyvinylidene fluoride powder (M W =600000), 1 wt.% of polyethylene glycol (M W =200), 0.5 wt.% of porous graphene nanowires, and 85.5 wt.% of N,N-dimethylacetamide.

[0044] The method for preparing the casting solution is as follows: the porous graphene nanowires are dispersed in N,N-dimethylacetamide by ultrasonic, then polyvinylidene fluoride and polyethylene glycol are added, and the mixture is mechanically stirred at 50°C for more than 5 hours until the polymers are completely dissolved. Then the casting solution is degassed at room temperature until the bubbles in the casting solution completely disappear, and a uniform transparent casting solution is prepared.

[0045] (3) The formula of the coagulation bath is prepared as follows: 85 wt.% of anhydrous ethanol and 15 wt.% of distilled water.

[0046] (3) The clean glass plate is placed on the hot table of the automatic film scraper at 30°C, the casting solution obtained in step (1) is uniformly poured on the glass plate, then the film scraper is operated, the film scraper bar with a thickness of 500 μm is used to quickly and uniformly coat the casting solution on the glass plate, and then the glass plate is immediately placed in the coagulation bath of step (3) at a steady speed, and is kept in the coagulation bath for 5 minutes. After the film is solidified, it is transferred to distilled water for rinsing for more than 48 hours to remove the residual solvent and stabilize the pore structure of the film, and the polyvinylidene fluoride homogeneous porous film is obtained.

[0047] The water flux of the obtained film is measured to be 2920.0 L·m -2 ·h -1 ·bar -1 under the pressure of 0.1 MPa, and the thickness of the film is 116.4 μm.

[0048] Example 5 (1) The same as step (1) of Example 1.

[0049] (2) According to the mass percentage, the formula of the casting solution is as follows: 14 wt.% of polyvinylidene fluoride powder (M W = 600000), 1 wt.% of polyethylene glycol (M W = 200), 2 wt.% of porous graphene nanowires, and 83 wt.% of N,N-dimethylacetamide.

[0050] The method for preparing the casting solution is as follows: the porous graphene nanowires are dispersed in N,N-dimethylacetamide by ultrasonic, then polyvinylidene fluoride and polyethylene glycol are added, and the mixture is mechanically stirred at 50°C for more than 5 hours until the polymers are completely dissolved. Then the casting solution is degassed at room temperature until the bubbles in the casting solution completely disappear, and a uniform transparent casting solution is prepared.

[0051] (3) The formula of the coagulation bath is prepared as follows: 75 wt.% of anhydrous ethanol and 25 wt.% of distilled water.

[0052] (4) Put the clean glass plate on the hot table of the automatic film doctor machine at 30℃, pour the casting solution obtained in step (1) uniformly on the glass plate, then run the film doctor machine, use the film doctor bar with the thickness of 500 μm to coat the casting solution on the glass plate rapidly and uniformly, immediately put the glass plate into the coagulation bath of step (3) at a steady speed, stand in the coagulation bath for 1 min, after the film is solidified, transfer to distilled water for rinsing for more than 48 h to remove the residual solvent, stabilize the film pore structure, dry, and the polyvinylidene fluoride homogeneous porous membrane is obtained.

[0053] The pure water flux of the obtained membrane is 3246.0 L·m -2 ·h -1 ·bar -1 at the pressure of 0.1 MPa, and the film thickness is 122.0 μm.

[0054] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, some improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for preparing a homogeneous porous membrane of polyvinylidene fluoride, characterized in that, Includes the following steps: Step 1, preparing the casting solution: The casting solution comprises 8-14 wt.% polyvinylidene fluoride, 1-10 wt.% organic low-molecular-weight additives, 0.5-2 wt.% porous graphene nanowires, with the balance being solvent, and the sum of all components being 100 wt.%; the organic low-molecular-weight additives are ethanol, ethylene glycol, or M... W ≤600% polyethylene glycol; Step 2: The casting solution obtained in Step 1 is coated onto a glass plate and placed in a coagulation bath for curing. The coagulation bath is an aqueous solution of ethanol with a concentration of 70-90 wt.%. After rinsing and drying, the homogeneous porous polyvinylidene fluoride membrane is obtained.

2. The preparation method according to claim 1, characterized in that, In step 1, the weight-average molecular weight of polyvinylidene fluoride is 400,000 to 600,000, and the solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

3. The preparation method according to claim 1, characterized in that, In step 1, the preparation steps of the casting solution are as follows: first, porous graphene nanowires are ultrasonically dispersed in the solvent, then polyvinylidene fluoride and organic low molecular weight additives are added to it, and mechanical stirring is carried out for more than 5 hours under water bath conditions of 30~50℃. After the casting solution is evenly mixed, it is degassed at room temperature to obtain the casting solution.

4. The preparation method according to claim 1, characterized in that, In step 1, the porous graphene nanowires have a length of 8-10 μm, a diameter of 100-200 nm, and an aspect ratio of 50-100. The porous graphene nanowires are prepared through the following steps: Step s11: After mixing the graphene oxide dispersion and ammonia water at a volume ratio of (2~4):1, the mixture is subjected to a hydrothermal reaction at 90~120℃ for 4~8h. During the hydrothermal reaction, the ammonia water chemically cuts the graphene oxide. After filtration, vacuum distillation and freeze drying, aminated graphene quantum dots are obtained. In step s12, the aminated graphene quantum dots obtained in step s11 are dispersed in water and the pH value is adjusted to 10-14. They are then subjected to interfacial polymerization reaction with a pyromellitic trimethylol chloride / n-hexane organic solution. After washing, filtering, centrifuging, and freeze-drying, the porous graphene nanowires are obtained.

5. The preparation method according to claim 1, characterized in that, In step 2, the coating method includes the following steps: placing a clean glass plate on the hot table of a film scraper at 30~50℃, pouring the casting liquid evenly onto the glass plate, and then running the film scraper to evenly coat the casting liquid onto the glass plate using a film scraper with a thickness of 500~1000μm.

6. The preparation method according to claim 1, characterized in that, In step 2, the coagulation bath temperature is 20~50℃, and the solidification time in the coagulation bath is 1~5min.

7. The preparation method according to claim 1, characterized in that, In step 2, the rinsing and drying steps are as follows: after the membrane has solidified, it is removed from the glass plate to obtain a wet membrane. The wet membrane is then transferred to distilled water for rinsing for more than 48 hours, and then transferred to anhydrous ethanol for soaking for 10-30 minutes. The membrane is then covered and fixed with non-woven fabric on both sides and dried in a clean space at room temperature to obtain a dry polyvinylidene fluoride porous membrane.

8. A homogeneous porous polyvinylidene fluoride membrane prepared by any one of the preparation methods described in claims 1 to 7.

9. The polyvinylidene fluoride homogeneous porous membrane as described in claim 8, characterized in that, The thickness is 90~130μm.

10. The application of the polyvinylidene fluoride homogeneous porous membrane as described in claim 8 as a separation membrane in water treatment.

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