Preparation method of PVDF (Polyvinylidene Fluoride) modified porous membrane

Abstract

The invention provides a preparation method of a PVDF (Polyvinylidene Fluoride) modified porous membrane, which belongs to the technical field of membrane separation, and provides a recrystallization post-treatment method for the first time: firstly, oxidizing a PVDF membrane to form a large number of disordered molecular chains, subsequently enabling the molecular chains to tend to form a beta crystal phase through an inducer, and finally, annealing the separation membrane at low temperature, and further amplifying the induction effect under thermal activation. Molecular chains are directionally rearranged, so that conversion from an oxidation region to a beta crystal phase crystalline region is realized, and the ratio of the beta crystal phase in the PVDF film is greatly increased. The beta crystalline phase molecular chain is highly oriented and has strong rigidity, and fiber fusion in the PVDF membrane is effectively inhibited, so that the fiber diameter is reduced, the fiber spacing is enlarged, the membrane porosity is improved, the duct tortuosity is reduced, and the membrane flux is greatly improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of membrane separation, and specifically relates to a PVDF modified porous membrane preparation method. BACKGROUND

[0002] The PVDF water treatment membrane has the advantages of high stability, good mechanical strength, strong pollution resistance, etc., can effectively remove macromolecular impurities, bacteria and suspended particles, and is resistant to acid and alkali cleaning, and is widely used in industrial wastewater treatment, seawater desalination and drinking water purification. However, the current commercial PVDF membrane still has the problem of low flux, which leads to the decrease of water treatment efficiency, the increase of cost and the shortening of service life. Therefore, on the basis of the original commercial PVDF membrane, it is the current research focus to develop and prepare high-efficiency PVDF modified membrane with simple preparation and stable performance.

[0003] The current modification method of the membrane is mainly coating, that is, a specific functional material is used to load a functional area on the surface or inside of the original separation membrane to realize flux improvement, but the surface coating method has the problem of lack of strong interaction force between the coating and the membrane surface. In the face of sewage environment containing hard particles, the coating is unstable and easy to fall off. Therefore, adjusting the intrinsic microstructure of the PVDF membrane has more practical significance, and under the premise of avoiding affecting the membrane pore size and mechanical stability, the membrane stability can be greatly improved by specific methods to improve the membrane porosity and reduce the degree of membrane pore channel tortuosity. In order to change the intrinsic microstructure of the PVDF membrane, the commonly used modification technologies are functional material in-situ blending, melt electrospinning and other membrane preparation technologies, however, the current PVDF commercial membrane preparation technology is mature, comprehensive and forms a stable chemical property of the separation membrane. The stability of the separation membrane prepared by these modification technologies is not as good as that of the current PVDF commercial membrane, and it cannot be applied to practical use.

[0004] Therefore, a better modification method is to realize the modification of the intrinsic microstructure of the PVDF commercial membrane to further improve its performance. The modification method that can be used is mainly to improve the proportion of the beta crystal phase in the membrane. By forming a strong and highly oriented beta crystal phase molecular chain, the fusion of fibers in the membrane is effectively inhibited, the fiber diameter is reduced, the fiber spacing is expanded, the porosity of the membrane is improved, the purpose of reducing the tortuosity of the pores is achieved, and finally the membrane flux is improved. The method for improving the proportion of the beta crystal phase in the membrane at the present stage is mechanical stretching method, electrode polarization method, annealing method and high-energy irradiation method. The mechanical stretching method produces beta crystal phase by mechanically stretching the PVDF membrane. However, high stretching ratio may cause the collapse or rupture of the membrane pore structure, reducing the mechanical integrity and separation performance; the electrode polarization method induces the polarity arrangement of molecular chains by electric field without physical deformation, but the uneven distribution of electric field in the porous structure of the membrane is easy to cause breakdown in the gap, and high voltage source and temperature control device are required, which is complex in process and expensive in equipment cost; the annealing method induces the generation of beta crystal phase by high temperature heating and cooling, but too high temperature may partially melt the PVDF material, resulting in the decline of the connectivity of the membrane pores or the fusion of the pore walls, finally leading to the decline of the flux, which is contrary to the purpose of achieving high porosity; the high-energy irradiation method uses gamma rays and heavy ion beams to induce the formation of beta crystal phase at room temperature, but this method requires a special irradiation source, which is limited in industrial application, and high-dose irradiation may cause the breakage of PVDF molecular chains, affecting the service life of the membrane. As can be seen, although the above methods can improve the proportion of the beta crystal phase in the PVDF membrane to varying degrees, there are problems of pore collapse, rupture or pore wall fusion in the membrane structure after modification, and there are problems of decline of material interception or decline of flux in performance. Although some methods can avoid affecting the performance of the membrane, there are also problems of scarcity of raw materials, complexity of membrane preparation process, and high cost of membrane preparation equipment.

[0005] In summary, in order to improve the flux of the PVDF membrane, the proportion of the beta crystal phase in the membrane needs to be controlled to optimize the intrinsic microstructure of the membrane, so as to improve the porosity and reduce the tortuosity of the pore channel. Since the present beta crystal phase control method has problems of poor membrane performance, complex process and expensive equipment, a preparation method with cheap and easily available raw materials, simple process, low cost and single temperature is needed to improve the performance of the membrane. SUMMARY

[0006] The present application provides a PVDF modified porous membrane preparation method, which aims to solve the problems of membrane structure damage and poor membrane performance in the present PVDF membrane modification process. The present application greatly improves the proportion of the beta crystal phase in the PVDF membrane by recrystallization post-treatment method, effectively improves the porosity, reduces the tortuosity of the pore channel, and greatly improves the membrane flux.

[0007] Technical scheme: A PVDF modified porous membrane preparation method, comprising the following steps:

[0008] Step 1, after the PVDF membrane is immersed in solution A containing an oxidizing agent for a period of time, a PVDF oxidized membrane is obtained;

[0009] Step 2, after the PVDF oxidized membrane is immersed in solution B containing a reducing agent for a period of time, a PVDF oxidized blocked membrane is obtained;

[0010] Step 3, the PVDF oxidized blocked membrane is immersed in solution C containing a β crystal phase inducer for a period of time, and a β crystal phase induced membrane is obtained;

[0011] Step 4, the β crystal phase induced membrane is placed in a steam environment for a period of time, and a PVDF modified porous membrane is obtained.

[0012] Further, in step 1, the pore size of the PVDF membrane is in the range of 10-500 nm, and the pure water flux is in the range of 20-500 L·m -2 ·h -1 ·bar -1 .

[0013] Further, in step 1, solution A is obtained by dissolving an oxidizing agent and an oxidation aid in solvent D; the content of the oxidizing agent in solution A is 1wt%-5wt%, and the content of the oxidation aid is 0.5wt%-2wt%; the oxidizing agent is one of potassium permanganate, sodium permanganate, permanganate, and ammonium permanganate; the oxidation aid is one of sodium hydroxide, potassium hydroxide, and ammonia water;

[0014] In step 2, solution B is obtained by dissolving a reducing agent and a reducing aid in solvent D; the content of the reducing agent in solution B is 0.5wt%-5wt%, and the content of the reducing aid is 0.5wt%-5wt%; the reducing agent is one of sodium sulfite, sodium bisulfite, and sulfur dioxide; the reducing aid is one of sulfuric acid and hydrochloric acid;

[0015] In step 3, solution C is obtained by dissolving a β crystal phase inducer in solvent D; the β crystal phase inducer is one of polyethyleneimine (PEI), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and polymethyl methacrylate (PMMA);

[0016] Solvent D is one of water, ethanol, and acetone.

[0017] Further, the specific operation of step 1 is as follows:

[0018] Step 1.1, dissolve the oxidizing agent and the oxidation aid in solvent D at 35-80℃ to obtain solution A;

[0019] Step 1.2, immerse the PVDF membrane in solution A under the condition of a 35-80℃ water bath for 1-60min, and then take it out;

[0020] Step 1.3, immerse the PVDF membrane into solvent D as the flushing liquid and flush and replace the solvent D for multiple times until the solvent D is colorless, and then immerse the PVDF membrane in the solvent D and naturally cool the solvent D to 15-35℃, and the obtained PVDF membrane is the PVDF oxidation membrane;

[0021] In step 1.1, the stirring rate is 50-200 rpm. In step 1.1, the dissolution temperature, the reaction temperature in step 1.2 and the temperature of the flushing liquid in step 1.3 are all 35-80℃, which can prevent the change of the membrane performance caused by rapid cooling and ensure the controllability of the experiment.

[0022] Further, the specific operation of step 2 is as follows:

[0023] Step 2.1, dissolve the reducing agent and the reducing auxiliary in solvent D at 15-35℃ to obtain solution B;

[0024] Step 2.2, immerse the PVDF oxidation membrane in solution B at 15-35℃ water bath for 5-120 min and then take it out;

[0025] Step 1.3, use solvent D at 15-35℃ as the flushing liquid to flush the PVDF oxidation membrane until the pH of the flushing liquid is 6.5-8.5, and then obtain the PVDF oxidation blocking membrane.

[0026] In step 2.1, the stirring rate is 50-200 rpm. In step 2.1, the dissolution temperature and the reaction temperature in step 2.2 are consistent, and the temperature is set consistently, so that it is not necessary to continue to adjust the temperature after dissolution for reaction, and the operation steps are simplified.

[0027] Further, the specific operation of step 3 is as follows:

[0028] Step 3.1, dissolve the β crystal phase inducer in solvent D at 40-90℃ to obtain solution C;

[0029] Step 3.2, immerse the PVDF oxidation blocking membrane in solution C at 40-90℃ water bath for 5-120 min and then take it out;

[0030] Step 3.3, place the taken-out PVDF oxidation blocking membrane into solvent D at 15-35℃ and naturally cool it at the constant ambient temperature of 15-35℃;

[0031] Finally, ultrasonically shake the PVDF oxidation blocking membrane in solvent D for a certain time to obtain the β crystal phase induced membrane; the frequency of the ultrasonic shaking is 120-180 W, and the ultrasonic time is 2-5 min.

[0032] In step 3.1, the stirring rate is 50-200 rpm. The reaction temperature range in step 3.2 is just in the range of the β crystal phase inducer dissolution temperature in step 3.1, and the temperature setting is consistent, which can simplify the operation.

[0033] Further, the specific operation of step 4 is:

[0034] Step 4.1, place the β crystal phase induction film in the steam environment of solvent D at 80-100℃, and take it out after 5-120min;

[0035] Step 4.2, the removed β crystal phase induction film is naturally cooled at a constant environment temperature of 15-35℃, and a high-performance PVDF modified porous membrane is obtained, which is stored in solvent D for standby.

[0036] In the present application, since step 1 is a material oxidation process, the amount, time and reaction temperature of the oxidizing agent need to be precisely controlled, otherwise the degree of oxidation cannot be controlled, which easily leads to over-oxidation and poor membrane performance, therefore slow cooling method needs to be used. There is no such problem in steps 3 and 4, and ordinary cooling method can be used.

[0037] Beneficial effects: the present application provides a high-performance PVDF modified porous membrane preparation method, which activates the crystalline region in the PVDF commercial membrane by using an oxidizing agent, and then induces the generation of β crystal phase by adding a β crystal phase inducer and low-temperature annealing method (steam treatment), which greatly increases the proportion of β crystal phase in the PVDF membrane. The β crystal phase molecular chain is highly oriented and has high rigidity, which effectively inhibits the fusion of fibers in the membrane, reduces the fiber diameter, expands the fiber spacing, increases the membrane porosity, reduces the tortuosity in the pores, and greatly improves the membrane flux. The membrane prepared by the preparation method of the present application can be used for treating suspended solids in wastewater. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a schematic diagram of the preparation process of the high-performance PVDF modified porous membrane in example 4.

[0039] Figure 2 is a micro-morphology diagram of the membrane surface in the related examples and comparative examples of the present application.

[0040] Figure 3 is the Fourier infrared spectrum (FT-IR) of the membrane in the related examples and comparative example 1 of the present application.

[0041] Figure 4 is the X-ray diffraction pattern (XRD) of the membrane in the related examples and comparative examples of the present application.

[0042] Figure 5 is the XRD data fitting result and the proportion of β crystal phase in all PVDF crystal phases in the related examples and comparative examples of the present application.

[0043] Figure 6 Figure 1 is the cross-sectional morphology and corresponding mercury intrusion test curve of the membrane in Comparative Example 1 and Comparative Example 2 and Example 4 of the present application.

[0044] Figure 7 Figure 2 is the mercury intrusion test result of the membrane in Comparative Example 1 and Comparative Example 2 and Example 4 of the present application.

[0045] Figure 8 Figure 3 is the membrane flux and substance interception data in the relevant examples and comparative examples of the present application. DETAILED DESCRIPTION

[0046] The technical solutions of the present application will be described in detail below through examples, but the protection scope of the present application is not limited to the examples.

[0047] Example 1: Preparation of PVDF oxidized membrane

[0048] The preparation method of the PVDF oxidized membrane in this example is as follows:

[0049] Potassium permanganate and potassium hydroxide were stirred at 100 rpm and dissolved in high-purity water at 55°C to obtain solution A. In solution A, the content of potassium permanganate was 2.81wt%, and the content of potassium hydroxide was 1wt%.

[0050] The PVDF commercial membrane (the manufacturer is Shandong Zhaogin Membrane Technology Co., Ltd., and the membrane filament taken from the membrane module with the type FPA-18-2000-PVDF*P) was immersed in solution A at a water bath temperature of 55°C for 10 min and then taken out. After taking out, the membrane was immersed in high-purity water at 55°C for washing and multiple replacement of high-purity water until the high-purity water was colorless. Subsequently, the high-purity water in which the membrane was immersed was naturally cooled to 25°C to obtain a PVDF oxidized membrane. The membrane was stored in high-purity water.

[0051] Example 2: Preparation of PVDF oxidized rejection membrane

[0052] The preparation method of the PVDF oxidized rejection membrane in this example includes the following steps:

[0053] S101, preparation of the PVDF oxidized membrane:

[0054] Potassium permanganate and potassium hydroxide were stirred at 100 rpm and dissolved in high-purity water at 55°C to obtain solution A. In solution A, the content of potassium permanganate was 2.81wt%, and the content of potassium hydroxide was 1wt%.

[0055] The PVDF commercial membrane (manufacturer: Shandong Zhaogin Membrane Technology Co., Ltd., model: FPA-18-2000-PVDF*P membrane assembly) was immersed in solution A at 55°C water bath temperature for 10 min, then taken out. After taking out, the membrane was immersed in 55°C high-purity water for washing and multiple replacement of high-purity water until the high-purity water was colorless. Subsequently, the high-purity water in which the membrane was immersed was naturally cooled to 25°C, to obtain a PVDF oxidized membrane;

[0056] S102, preparation of a PVDF oxidized blocking membrane:

[0057] Sodium bisulfite and dilute sulfuric acid were stirred in 25°C high-purity water at 100 rpm to obtain solution B. In solution B, the content of sodium bisulfite was 2.81wt%, and the content of H2SO4 was 1.6wt%.

[0058] The PVDF oxidized membrane was immersed in solution B at 25°C for 30 min, then taken out, and the membrane was repeatedly washed with 25°C high-purity water until the washing liquid pH=6.5, to obtain a PVDF oxidized blocking membrane, which was stored in high-purity water for standby.

[0059] Example 3: preparation of a β crystal phase induced membrane

[0060] The preparation method of the β crystal phase induced membrane in this embodiment includes the following steps:

[0061] S101, preparation of a PVDF oxidized membrane:

[0062] Potassium permanganate and potassium hydroxide were stirred in 55°C high-purity water at 100 rpm to obtain solution A. In solution A, the content of potassium permanganate was 2.81wt%, and the content of potassium hydroxide was 1wt%.

[0063] The PVDF commercial membrane (manufacturer: Shandong Zhaogin Membrane Technology Co., Ltd., model: FPA-18-2000-PVDF*P membrane assembly) was immersed in solution A at 55°C water bath temperature for 10 min, then taken out. After taking out, the membrane was immersed in 55°C high-purity water for washing and multiple replacement of high-purity water until the high-purity water was colorless. Subsequently, the high-purity water in which the membrane was immersed was naturally cooled to 25°C, to obtain a PVDF oxidized membrane;

[0064] S102, preparation of a PVDF oxidized blocking membrane:

[0065] Sodium bisulfite and dilute sulfuric acid were stirred in 25°C high-purity water at 100 rpm to obtain solution B. In solution B, the content of sodium bisulfite was 2.81wt%, and the content of H2SO4 was 1.6wt%.

[0066] The PVDF oxidized membrane was immersed in solution B at 25℃ for 30 minutes, and then taken out. The membrane was repeatedly washed with high-purity water at 25℃ until the washing liquid pH=6.5, to obtain a PVDF oxidized blocking membrane.

[0067] S103, preparation of a β crystal phase induced membrane:

[0068] Polyvinylpyrrolidone (PVP, K90) was dissolved in high-purity water at 60℃ under stirring at 100 rpm to obtain solution C. In solution C, the content of polyvinylpyrrolidone was 0.5wt%.

[0069] The PVDF oxidized blocking membrane was immersed in solution C at a water bath temperature of 60℃ for 30 minutes. After the reaction was completed, the membrane was placed in high-purity water at 60℃ and naturally cooled at an ambient temperature of 25℃. Finally, the membrane was cleaned with high-purity water under ultrasonic vibration for a certain period of time to obtain a β crystal phase induced membrane, which was stored in high-purity water for later use.

[0070] Example 4: Preparation of high-performance PVDF modified porous membrane

[0071] The preparation method of the high-performance PVDF modified porous membrane in this example comprises the following steps:

[0072] S101, preparation of a PVDF oxidized membrane:

[0073] Potassium permanganate and potassium hydroxide were dissolved in high-purity water at 55℃ under stirring at 100 rpm to obtain solution A. In solution A, the content of potassium permanganate was 2.81wt%, and the content of potassium hydroxide was 1wt%.

[0074] A PVDF commercial membrane (manufactured by Shandong Zhaogin Membrane Technology Co., Ltd., and taken from a membrane filament in a membrane module with a model number of FPA-18-2000-PVDF*P) was immersed in solution A at a water bath temperature of 55℃ for 10 minutes, and then taken out. After taking out, the membrane was immersed in high-purity water at 55℃ for washing and multiple replacement of the high-purity water until the high-purity water was colorless. Subsequently, the high-purity water in which the membrane was immersed was naturally cooled to 25℃, to obtain a PVDF oxidized membrane.

[0075] S102, preparation of a PVDF oxidized blocking membrane:

[0076] Sodium bisulfite and dilute sulfuric acid were dissolved in high-purity water at 25℃ under stirring at 100 rpm to obtain solution B. In solution B, the content of sodium bisulfite was 2.81wt%, and the content of H2SO4 was 1.6wt%.

[0077] The PVDF oxidized membrane was immersed in solution B at 25℃ for 30 minutes, and then taken out. The membrane was repeatedly washed with high-purity water at 25℃ until the washing liquid pH=6.5, to obtain a PVDF oxidized blocking membrane.

[0078] S103, preparation of the β crystal phase inducing membrane:

[0079] Polyvinylpyrrolidone (PVP, K90) was dissolved in high-purity water at 100 rpm and 60°C to obtain solution C. In solution C, the content of polyvinylpyrrolidone was 0.5 wt%.

[0080] The PVDF oxidation blocking membrane was immersed in solution C at a water bath temperature of 60°C for 30 min. After the reaction was completed, it was placed in high-purity water at 60°C and naturally cooled at an ambient temperature of 25°C. Finally, it was cleaned with high-purity water for a certain period of time by ultrasonic oscillation to obtain a β crystal phase inducing membrane.

[0081] S104, preparation of the PVDF modified porous membrane:

[0082] The β crystal phase inducing membrane was placed in a water vapor environment at 95°C, taken out after 30 min, and naturally cooled at an ambient temperature of 25°C to obtain a high-performance PVDF modified porous membrane. The membrane was stored in high-purity water for standby.

[0083] Comparative Example 1: PVDF commercial membrane

[0084] Comparative Example 1 is a PVDF commercial membrane, the manufacturer is Shandong Zhaogin Membrane Tian Co., Ltd., and the membrane filament in the membrane module with the type FPA-18-2000-PVDF*P was taken out.

[0085] Comparative Example 2: PVDF annealed membrane

[0086] The preparation method of the PVDF annealed membrane in Comparative Example 2 includes the following steps:

[0087] S101, preparation of the PVDF oxidation membrane:

[0088] Potassium permanganate and potassium hydroxide were dissolved in high-purity water at 100 rpm and 55°C to obtain solution A. In solution A, the content of potassium permanganate was 2.81 wt%, and the content of potassium hydroxide was 1 wt%. The PVDF commercial membrane (the manufacturer is Shandong Zhaogin Membrane Tian Co., Ltd., and the membrane filament in the membrane module with the type FPA-18-2000-PVDF*P was taken out) was immersed in solution A at a water bath temperature of 55°C for 10 min and then taken out. After taking out, the membrane was immersed in high-purity water at 55°C for washing and multiple replacement of high-purity water until the high-purity water was colorless. Subsequently, the high-purity water in which the immersed membrane was naturally cooled to 25°C to obtain a PVDF oxidation membrane.

[0089] S102, preparation of the PVDF oxidation blocking membrane:

[0090] Sodium bisulfite and dilute sulfuric acid were dissolved in high-purity water at 25°C under stirring at 100 rpm to obtain solution B. In solution B, the content of sodium bisulfite was 2.81 wt%, and the content of H2SO4 was 1.6 wt%.

[0091] The PVDF oxidized membrane was immersed in solution B at 25°C for 30 min and then taken out, and the membrane was repeatedly washed with high-purity water at 25°C until the pH of the washing liquid was 6.5 to obtain a PVDF oxidized blocking membrane.

[0092] S103, Preparation of a PVDF annealed membrane:

[0093] The PVDF oxidized blocking membrane was placed in a water vapor environment at 95°C, taken out after 30 min, and naturally cooled at an ambient temperature of 25°C to obtain a PVDF annealed membrane, which was stored in high-purity water for standby.

[0094] Figure 1 is a schematic diagram of the preparation process of a high-performance PVDF modified porous membrane in Example 4. In Example 4, the re-crystallization post-processing method is adopted, and the commercial PVDF membrane is oxidized by adding an oxidizing agent, the continuous oxidation of the PVDF membrane by the oxidizing agent is blocked by adding a reducing agent, the disordered stacking molecular chains in the oxidation zone are induced to form a β crystal phase by adding an inducing agent, and finally the membrane is separated by low-temperature annealing. Under thermal activation, the inducing effect is further amplified, the molecular chains are directionally rearranged, and the oxidation zone is promoted to convert into a crystallization zone containing a large amount of β crystal phase. Since the planar zigzag conformation (TGTG', β crystal phase) molecular chain is easy to arrange along the stretching direction, a highly oriented and rigid fibrous aggregate is formed. The increase of the content of the β crystal phase can effectively inhibit the fusion of the fibers in the membrane, reduce the fiber diameter, and expand the fiber spacing, thereby improving the porosity of the membrane and reducing the tortuosity of the pore channel, forming a PVDF modified porous membrane.

[0095] Figure 2 is a micro-morphology diagram of the membrane surface (scanning electron microscope SEM test, model: ZEISS Sigma 360, Germany) in the examples and comparative examples, wherein a is the micro-morphology diagram of the surface of the PVDF commercial membrane in Comparative Example 1, b is the micro-morphology diagram of the surface of the PVDF oxidized membrane in Example 1, c is the micro-morphology diagram of the surface of the PVDF oxidized blocking membrane in Example 2, d is the micro-morphology diagram of the surface of the β crystal phase induced membrane in Example 3, e is the micro-morphology diagram of the surface of the PVDF modified porous membrane in Example 4, and f is the micro-morphology diagram of the surface of the PVDF annealed membrane in Comparative Example 2. It can be seen from Figure 2 that there are many spherical aggregates on the surface of the PVDF commercial membrane. In addition, under a smaller scale, the membrane surface is covered with fibrous structures, and there are gaps of about 5 nm between the fibrous structures. Figure 2 a in the above table, after oxidation, it is found that the spherical structures on the membrane surface are less and the fibrous structures are blurred Figure 2(b) Upon further reduction, the fibrous structure reappeared, and compared to Comparative Example 1, the fibrous structure was more prominent. Figure 2 (c) After adding the inducing agent, the spherical aggregates on the membrane surface disappeared, but the fibrous structure remained. Additionally, irregular coatings appeared on the membrane surface, which should be a phenomenon caused by the inducing agent being loaded onto the membrane surface. Figure 2 (d). After low-temperature annealing, spherical aggregates and fibrous structures were observed on the film surface simultaneously. More significantly, the fibers became noticeably narrower (the width decreased from approximately 65 nm to approximately 20 nm), and larger gaps formed between the narrowed fibers, creating gaps with a diameter of approximately 100 nm. Figure 2 (e). After forming the oxidation-blocking film, if annealing is performed directly, a PVDF annealed film is formed (Comparative Example 2). It can be observed that spherical aggregates and fibrous structures also form on the surface of the annealed film. Figure 2 Unlike Example 2, after direct annealing, the fiber structure size on the film surface is uneven. In some areas, the fibers are not significantly narrower compared to Example 2, while in other areas the fibers become excessively narrow (the width decreases to about 10 nm), resulting in uneven void sizes. Figure 2 (f)

[0096] Figure 3 These are XRD patterns of the membranes in the examples and comparative examples (testing instrument: Rigaku Ultima IV, Japan). It can be observed that compared to the approximately 57.05% crystallinity of the commercial PVDF membrane, the crystallinity of the PVDF oxide membrane decreased to approximately 22.65%, confirming that the oxidation process destroyed a large number of crystalline structures within the membrane (Example 1). After reduction, the crystallinity of the membrane increased to approximately 48.22%, indicating that the reduction process washed away some of the disordered polymers within the membrane (Example 2). Relevant evidence is as follows... Figure 2 As shown in Figures b and c, after oxidation, the number of spherical structures (α-crystal form) on the membrane surface decreases and the fibrous structures (β-crystal form) become blurred, indicating that a large number of crystals inside the membrane are destroyed and disordered polymers are formed and attached to the membrane surface. After reduction, the structures become clear again, indicating that the disordered polymers detach from the membrane surface after reduction. After adding the inducing agent, the membrane crystallinity decreases again to about 47.64% (Example 3). This is because the inducing agent is an amorphous polymer, which, when loaded on the membrane surface, reduces the overall crystallinity. Figure 2 (d). After low-temperature annealing, the crystallinity increased to approximately 70.63% (Example 4), indicating that the annealing process stacked the disordered polymers in the oxide zone within the film into crystals. Furthermore, for the film formed by directly annealing the PVDF film after oxidation, its crystallinity was approximately 62.30% (Comparative Example 2), which was weaker than that of Example 4, indicating that the crystallinity improvement effect after annealing was poor without the addition of an inducing agent.

[0097] Figure 4This section presents the XRD data fitting results of the films in the examples and comparative examples, as well as the resulting proportions of the β-phase and PVDF phases among all PVDF phases. Specifically, the fitting was performed using MDI Jade 6 software. Figure 3 XRD data were used to obtain the characteristic peaks of each PVDF phase, and then the ratio of the β-phase peak area to the characteristic peak areas of all PVDF phases was calculated to obtain the proportion of the β-phase to all PVDF phases. Similarly, the proportion of PVDF phases was obtained by calculating the characteristic peak areas of the PVDF phase to the characteristic peak areas of all phases. It can be found that commercial PVDF films (Comparative Example 1) Figure 4 In Example a), the β-phase content was 1.17%, and the PVDF phase content was 41.03%; after oxidation, the β-phase content in the film was 13.92%, and the PVDF phase content was 96.39% (Example 1). Figure 4 (b) After reduction, the β-phase content in the film was 15.21%, and the PVDF phase content was 99.60% (Example 2). Figure 4 (c). The above results indicate that commercial PVDF membranes contain additives. Through a redox process, most of the additives were washed away, and Example 2 became a pure PVDF membrane. The β-phase proportion in the pure PVDF membrane was approximately 15%. After adding an inducing agent, the PVDF phase proportion decreased to 72.03% (Example 3). Figure 4 (d) This indicates that although the inducing agent is an amorphous polymer, it still exhibits a corresponding XRD diffraction peak, which reduces the crystalline phase ratio of PVDF. Furthermore, the inducing agent can effectively increase the β-crystalline phase ratio within the film to 25.60%. Supporting evidence is as follows: Figure 2 As shown in Figure d: After adding the inducing agent, the spherical aggregates (aggregates formed by the α phase) on the membrane surface are difficult to detect, but the fibrous structures (aggregates formed by the β phase) still exist. After low-temperature annealing (Example 4), Figure 4 In the middle (e), the proportion of β-phase further increased to 47.41%, while after direct annealing (Comparative Example 2), Figure 4 In the middle (f), the proportion of β-phase only slightly increased to 27.15%. These results indicate that direct annealing at low temperatures does not significantly increase the β-phase content within the film. Therefore, the addition of an inducing agent is a necessary prerequisite for achieving an increase in β-phase content after low-temperature annealing. Relevant supporting evidence is as follows: Figure 2 As shown in Figure ef: the induced annealing process reduces the fiber width uniformly, while the direct annealing process results in varying degrees of fiber width reduction. A large number of fibers have the same diameter as those in Example 2 and show no significant change.

[0098] Figure 5 These are the FT-IR images of the membranes in Examples 1 and Comparative Example 1 (instrument model: Thermo Fisher Scientific Nicolet iS20, USA). The data in the images show that in Comparative Example 1, at 1110 cm⁻¹…-1 and 1040 cm -1 The absorption peaks (marked by solid lines) are characteristic of the stretching vibration of C-O bond on the polysaccharide skeleton. The two absorption peaks do not appear in Examples 1-4, indicating that there is an additive in the PVDF commercial film, and after subsequent treatment, the relevant additive has been mostly washed away. This phenomenon confirms the conclusion in a-c in the middle. Figure 4 763 cm -1 and 614 cm -1 is the α phase exclusive peak; since the γ phase exclusive peak (1234 cm -1 ) is not obvious, 1275 cm -1 , 840 and 510 cm -1 are β phase exclusive peaks. It can be found that the β phase exclusive peak of the PVDF modified porous membrane (Example 4) is obviously enhanced compared with the PVDF commercial film (Comparative Example 1), and the α phase exclusive peak is obviously weakened, indicating that after modification, the content of β crystal phase is obviously increased. The reason why the content of β crystal phase is obviously increased can be explained by the absorption peaks of each film at 1644~1654 cm -1 . The absorption peaks at 1644~1654 cm -1 are the -C=O stretching vibration peaks of the amide carbonyl in the inducing agent. It can be found that this peak exists in all films, indicating that there is also a modifier containing amide carbonyl in the PVDF commercial film. When the inducing agent is added (Example 3), the vibration peak red shifts to 1647 cm -1 , indicating that after oxidation treatment, the -CH2- group in the disordered molecular chain in the film forms a stronger hydrogen bond with the -C=O group in the inducing agent, and the formation of the hydrogen bond limits the vibration of the -C=O bond, resulting in red shift of the absorption frequency. More obviously, after low-temperature annealing (Example 4), the vibration peak continues to red shift to 1654 cm -1 , indicating that the hydrogen bond formed by the addition of the inducing agent is further strengthened after low-temperature annealing. In fact, this hydrogen bond action can effectively limit the folding and packing of the PVDF segment, slow down the crystallization speed, and the slowed down crystallization speed is conducive to the relaxation of the chain segment to form the trans conformation (i.e., the β crystal phase). The modification scheme of the present application is to first oxidize the PVDF commercial film to form a large number of disordered molecular chains, then add the inducing agent to make the molecular chains tend to form the β crystal phase, and finally low-temperature annealing to separate the film, which further amplifies the inducing effect under thermal activation and directional rearrangement of the molecular chain, so as to realize the transformation of the oxidation zone to the β crystal phase. The above characterization confirms the theoretical correctness of the modification scheme of the present application.

[0099] Figure 6are cross-sectional morphologies (left side of each figure) and corresponding mercury intrusion test curves (right side of each figure, mercury intrusion tester model: Micromeritics AutoPore IV 9510, USA) of the membranes in Comparative Example 1, Comparative Example 2 and Example 4, wherein a corresponds to the data of Comparative Example 1, b corresponds to the data of Comparative Example 2, and c corresponds to the data of Example 4. It can be found from the cross-sectional images of the membranes that the PVDF commercial membrane (Comparative Example 1) is denser, and no obvious pore morphology is found near the membrane surface; the PVDF annealed membrane (Comparative Example 2) and the modified porous membrane (Example 4) have obvious pore structures near the membrane surface. Through the mercury intrusion test, it can be found that the bulk density of the PVDF commercial membrane is 0.69 g·mL -1 , the bulk density of the PVDF annealed membrane is 0.63 g·mL -1 , and the bulk density of the PVDF modified porous membrane is 0.53 g·mL -1 (The bulk density refers to the ratio of the total mass of the material to its apparent volume. The "apparent volume" includes the three-dimensional skeleton volume of the material, the internal opening volume of the material, and the interstitial volume between the materials during stacking. The lower the bulk density, the looser the material). The above results show that after modification, the three-dimensional skeleton volume in the membrane structure decreases, and the interstitial volume increases, that is, the number of pores in the membrane may increase, or the pore volume may increase. In addition, it can be found that the pore promotion effect of the modified porous membrane is stronger than that of the annealed membrane.

[0100] Figure 7 are the mercury intrusion test results of the membranes in Comparative Example 1, Comparative Example 2 and Example 4, wherein a is the porosity test result, b is the average pore size test result, c is the pore tortuosity test result, and d is the permeation capacity test result. The related results show that the porosity, pore tortuosity, theoretical flux and pure water flux of the modified membrane are greatly improved compared with the commercial membrane, which indicates that the increase of the proportion of β crystal phase can effectively reduce the fiber diameter and expand the fiber spacing, thereby achieving the purpose of increasing the porosity of the membrane, reducing the pore tortuosity, and greatly improving the flux of the membrane. Secondly, it can be found that the average pore size of the annealed membrane (Comparative Example 2) is larger than that of the other membranes, which indicates that there are large pore structures in the annealed membrane, which may be caused by the non-uniform formation of β crystal phase, resulting in a wide pore size distribution. It can be seen that the addition of the inducer can optimize the pore size distribution of the membrane. Thirdly, in addition to the average pore size, the remaining parameters of the annealed membrane (Comparative Example 2) are smaller than those of the porous membrane (Example 4), which indicates that the addition of the inducer can effectively increase the proportion of β crystal phase in the membrane, thereby increasing the porosity and pore tortuosity, and increasing the flux of the membrane. Finally, the pore size of the porous membrane is the smallest compared with the other membranes, and the increase in flux does not cause a decrease in retention rate, which indicates that the PVP in the modified membrane is loaded inside the membrane pores through hydrogen bonds, causing the pore size to decrease, and the retention rate does not change significantly (Table 1).

[0101] Figure 8Figure 1 is the data of membrane flux (5 bar transmembrane pressure difference, a) and substance retention (retention of 1 micron polystyrene microspheres, b) in the related embodiments and comparative examples of the present application. It can be found that after redox (Example 2), the membrane flux is slightly increased by 11.69% compared with the commercial membrane (Comparative Example 1). After adding the inducer, the membrane flux is greatly decreased by 45.17% (Example 3), which may be due to the inducer loaded on the membrane surface blocking the membrane channel, resulting in a smaller membrane flux. After low-temperature annealing, the membrane flux is greatly increased by 405.92% compared with the induced membrane (Example 3), indicating that the inducer loading in Example 3 is unstable, and a large amount of inducer is removed during the annealing process, so that the membrane flux is restored, and the proportion of β crystal phase is greatly increased, causing a great increase in the membrane flux. For the PVDF annealing membrane (Comparative Example 2), the flux is decreased by 24.78% compared with the modified membrane (Example 4), which is due to the fact that the structural parameters of the annealing membrane are all weaker than those of the modified membrane, resulting in a certain degree of decline in the membrane performance. In addition, from the retention rate data of each membrane, it can be seen that except for the PVDF annealing membrane (Comparative Example 2), the retention rate of all membranes to 1 micron polystyrene microspheres is greater than 99.4%, indicating that the microstructure of the membrane does not appear to be defective during the membrane preparation process. The retention rate of the PVDF annealing membrane (Comparative Example 2) is 99.1%, which is slightly lower than the average level, indicating that the large pore structure in the annealing membrane slightly affects the retention rate of the membrane.

[0102] The membrane parameter comparison data of Comparative Example 1, Comparative Example 2 and Example 4 are shown in Table 1, and the membrane performance of Comparative Example 1, Comparative Example 2 and Examples 1 to 4 is shown in Table 2.

[0103] Table 1. Comparison of PVDF commercial membrane and modified membrane parameters

[0104] Table 2. Membrane performance

[0105] The porosity of the PVDF porous modified membrane prepared by the method is increased from 48.21% to 58.18%, the pore tortuosity is decreased from 16.47 to 3.85, the average pore size is decreased from 335.59 nm to 258.73 nm, and the membrane permeability is increased from 417.93 mdarcy to 5770.37 mdarcy.

[0106] The high-performance PVDF modified porous membrane prepared by the method has a pure water flux of 1233.74±11 L·m -2 ·h -1 to 3821.22±16 L·m -2 ·h -1 under the condition of 5 bar transmembrane pressure difference in actual use, and the retention rate to 1 micron polystyrene microspheres is greater than 99.5%.

[0107] In summary, the preparation method of the application adopts a cheap and easily available modifier, and a simple, cheap and mild process is used to prepare a PVDF modified porous membrane. The microstructure in the membrane is controllable, the membrane flux is greatly improved, and the problems of membrane structure damage, poor membrane performance, complex process and expensive equipment in the process of PVDF membrane modification at the present stage are solved.

[0108] As described above, although the application has been indicated and expressed with reference to specific preferred embodiments, it should not be interpreted as a limitation of the application itself. Various changes can be made to the form and details thereof without departing from the spirit and scope of the application.

Claims

1. A method for preparing a PVDF modified porous membrane, characterized in that, The method comprises the following steps: Step 1, after immersing the PVDF membrane in solution A containing an oxidizing agent for a period of time, a PVDF oxidized membrane is obtained; Step 2, after immersing the PVDF oxidized membrane in solution B containing a reducing agent for a period of time, a PVDF oxidized and blocked membrane is obtained; Step 3, after immersing the PVDF oxidized and blocked membrane in solution C containing a beta crystal phase inducer for a period of time, a beta crystal phase induced membrane is obtained; Step 4, after placing the beta crystal phase induced membrane in a steam environment for a period of time, a PVDF modified porous membrane is obtained.

2. The production method according to claim 1, characterized by, In step one, the pore size of the PVDF membrane is in the range of 10-500 nm, and the pure water flux is in the range of 20-500 L·m -2 ·h -1 ·bar -1 .

3. The preparation method according to claim 1, wherein in step 1, solution A is obtained by dissolving an oxidizing agent and an oxidation aid in solvent D; the content of the oxidizing agent in solution A is 1wt%-5wt%, and the content of the oxidation aid is 0.5wt%-2wt%; the oxidizing agent is one of potassium permanganate, sodium permanganate, permanganate, and ammonium permanganate; and the oxidation aid is one of sodium hydroxide, potassium hydroxide, and ammonia water; in step 2, solution B is obtained by dissolving a reducing agent and a reduction aid in solvent D; the content of the reducing agent in solution B is 0.5wt%-5wt%, and the content of the reduction aid is 0.5wt%-5wt%; the reducing agent is one of sodium sulfite, sodium bisulfite, and sulfur dioxide; and the reduction aid is one of sulfuric acid and hydrochloric acid; in step 3, solution C is obtained by dissolving a beta crystal phase inducer in solvent D; the beta crystal phase inducer is one of polyethyleneimine, polyvinyl alcohol, polyvinylpyrrolidone, and polymethyl methacrylate; solvent D is one of water, ethanol, and acetone. The specific operation of step 1 is as follows:

4. The preparation method according to claim 3, characterized in that, Step 1.1, dissolve the oxidizing agent and the oxidation aid in solvent D at 35-80℃ to obtain solution A; Step 1.2, immerse the PVDF membrane in solution A under the condition of a 35-80℃ water bath for 1-60min, and then take it out; Step 1.3, use 35-80℃ solvent D as a flushing liquid, immerse the PVDF membrane in solvent D for flushing and multiple solvent D replacement until the solvent D is colorless, then naturally cool the immersed PVDF membrane to 15-35℃, and obtain the PVDF oxidized membrane. The specific operation of step 2 is as follows:

5. The preparation method according to claim 3, characterized in that, Step 2.1, dissolve the reducing agent and the reduction aid in solvent D at 15-35℃ to obtain solution B; Step 2.2, immerse the PVDF oxidized membrane in solution B under the condition of a 15-35℃ water bath for 5-120min, and then take it out; Step 1.3, use 15-35℃ solvent D as a flushing liquid, flush the PVDF oxidized membrane until the flushing liquid PH is 6.5-8.5, and then obtain the PVDF oxidized and blocked membrane. The specific operation of step 3 is as follows:

6. The preparation method according to claim 3, characterized in that, Step 3.1, dissolve the beta crystal phase inducer in solvent D at 40-90℃ to obtain solution C; Step 3.2, immerse the PVDF oxidized and blocked membrane in solution C under the condition of a 40-90℃ water bath for 5-120min, and then take it out; ​ Step 3.3, the removed PVDF oxidation blocking membrane is placed in solvent D at 15-35℃, and is naturally cooled at a constant environment temperature of 15-35℃; finally, the solvent D is used for ultrasonic cleaning for a period of time to obtain a β crystal phase induced membrane.

7. The preparation method according to claim 3, characterized in that, The specific operation of step 4 is as follows: Step 4.1, the β crystal phase induced membrane is placed in a solvent D steam environment at 80-100℃, and is taken out after 5-120 min; Step 4.2, the removed β crystal phase induced membrane is naturally cooled at a constant environment temperature of 15-35℃ to obtain a high-performance PVDF modified porous membrane.

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