A free radical-grafted quaternized ammonium modified PVDF antibacterial nanofiltration membrane and its preparation method

Abstract

This invention relates to the field of membrane separation technology and discloses a radical-grafted quaternized ammonium-modified PVDF antibacterial nanofiltration membrane and its preparation method. Addressing the problems of cumbersome, multi-step, and easily contaminated PVDF quaternization modification processes, and the inability of existing products to meet the requirements for small molecule separation, this invention dissolves PVDF, allyltrimethylammonium chloride, and CuCl2 in an organic solvent to form a homogeneous solution. A hydrothermal reaction triggers CuCl2 to catalyze the generation of free radicals in PVDF, which then covalently graft quaternary ammonium groups onto monomer double bonds. The resulting membrane is then directly cast. This one-pot process simplifies the process, reduces costs, and produces a nanofiltration membrane with both stable antibacterial properties and excellent small molecule retention. Furthermore, the quaternary ammonium groups are firmly bonded, resulting in stable long-term performance and a wider range of applications.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a free radical grafted quaternized ammonium modified PVDF antibacterial nanofiltration membrane and its preparation method. Background Technology

[0002] Polyvinylidene fluoride (PVDF) nanofiltration membranes are widely used in pharmaceutical separation, wastewater treatment, and food packaging due to their excellent chemical stability, mechanical strength, and fouling resistance. They rely on pore size sieving and charge interaction to achieve efficient separation of small molecules, making them an important research direction in current membrane separation technology. However, the inherent strong hydrophobicity of PVDF molecular chains makes it easy for organic pollutants to be adsorbed on the membrane surface and bacteria to grow, causing membrane fouling problems. This not only causes a rapid decline in membrane separation flux but also reduces its separation performance, limiting the widespread application of the product.

[0003] Existing technologies often employ quaternization modification to address the hydrophobic and fouling issues of PVDF nanofiltration membranes. The core principle involves introducing quaternary ammonium groups into the membrane material through chemical grafting or polymerization. This utilizes the contact bactericidal effect and enhanced hydrophilicity of quaternary ammonium cations to inhibit bacterial growth and reduce pollutant adsorption. For example, CN118184913A describes an antibacterial dual-block functionalized polyvinylidene fluoride material, ultrafiltration membrane, its preparation method, and applications. This involves modifying PVDF through hydroxylation, bromination, and subsequent atom transfer radical polymerization with quaternary ammonium salt monomers to address membrane biofouling. CN118788141A describes a method for preparing a quaternized polyvinylidene fluoride ultrafiltration membrane. This involves blending cellulose acetate with PVDF, alkaline hydrolysis to generate hydroxyl groups, and then quaternizing with 2,3-epoxypropyltrimethylammonium chloride to improve the membrane's hydrophilicity and antifouling properties.

[0004] However, existing technologies still have many technical problems. Quaternization modification generally relies on multi-step reaction processes, requiring pretreatment, introduction of active sites, graft polymerization, and other steps. Each step requires the addition of desolventizing and drying operations, making the process cumbersome and lengthy, which not only increases production costs but also easily generates pollutants. At the same time, the modified nanofiltration membranes prepared by existing technologies are mostly ultrafiltration membranes with large pore sizes, which can only retain large molecules and cannot meet the separation requirements of small molecules such as antibiotics and dyes, thus limiting their applicability. In addition, multi-step reaction or blending modification can easily lead to problems such as uneven membrane structure, fluctuations in mechanical strength, and insufficient modification stability, making it difficult to adapt to the needs of industrial production and complex separation scenarios. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the defects of the existing technology. To this end, a free radical grafted quaternized ammonium modified PVDF antibacterial nanofiltration membrane and its preparation method are proposed.

[0006] To achieve the above objectives, this application adopts the following technical solution: a free radical grafted quaternized modified PVDF antibacterial nanofiltration membrane and its preparation method, using polyvinylidene fluoride as the substrate, wherein quaternary ammonium groups are bonded to the molecular chain of the polyvinylidene fluoride, the quaternary ammonium groups are introduced by a quaternized monomer containing unsaturated double bonds through a free radical grafting reaction, the free radical grafting reaction uses CuCl2 as a catalyst and does not require the addition of an initiator, and the quaternary ammonium groups are connected to the PVDF molecular chain through C-C covalent bonds.

[0007] Preferably, the quaternized monomer containing unsaturated double bonds is allyltrimethylammonium chloride.

[0008] Preferably, the active site of the free radical grafting reaction is the CH bond on the PVDF molecular chain adjacent to the CF2 group.

[0009] Preferably, CuCl2 induces the breaking of CH bonds on the PVDF molecular chain through electron transfer to generate PVDF radicals, and CuCl2 undergoes Cu oxidation during the catalytic process. 2+ To Cu + The transformation of the price state.

[0010] Preferably, the dissolution of PVDF and the CuCl2-catalyzed free radical grafting reaction are carried out in the same organic solvent system, and the casting solution containing PVDF-quaternary ammonium group graft copolymer can be directly formed without separating the reaction intermediates.

[0011] A method for preparing a free radical grafted quaternized modified PVDF antibacterial nanofiltration membrane includes the following steps: S1: PVDF powder, quaternized modified monomer, and catalyst are mixed and added to an organic solvent and stirred to obtain a mixed solution; S2: The mixed solution in S1 is transferred to a closed reaction vessel for hydrothermal reaction; S3: The cooled reaction solution in S2 is cast into a membrane and immersed in water to obtain the finished product.

[0012] Preferably, in S1, the mass fraction of PVDF powder in the mixed solution is 10%-20%, the mass ratio of the quaternized modified monomer to PVDF powder is 1:300-1:30, and the mass ratio of the catalyst to PVDF powder is 0.001:300-1:300.

[0013] Preferably, the organic solvent in S1 is selected from one or more mixtures of N,N-dimethylacetamide, dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0014] Preferably, the temperature of the hydrothermal reaction in S2 is 80℃-120℃, and the time of the hydrothermal reaction is 24h.

[0015] Preferably, the water immersion time in step S3 is 24 hours, and after water immersion, it is soaked in 0.1-0.5 mol / L NaOH solution 2-3 times, 30 minutes each time.

[0016] The technical effects and advantages of this invention are as follows:

[0017] In this invention, quaternization modification of PVDF is achieved through a CuCl2-catalyzed free radical grafting reaction: using PVDF as the substrate and allyltrimethylammonium chloride as the modifying monomer, after mixing in a specific organic solvent, CuCl2 is triggered under hydrothermal conditions to induce the generation of free radicals in the PVDF molecular chain, which then undergo an addition reaction with the unsaturated double bonds of the monomer, allowing the quaternary ammonium groups to be covalently grafted onto the PVDF molecular chain. At the same time, a one-pot method is used to directly prepare the casting solution and form a film, eliminating the need for additional initiators or crosslinking agents. This eliminates the multiple steps of pretreatment and activation in traditional modification, simplifies the production process, reduces solvent evaporation and waste generation, lowers process costs and environmental burden, and the reaction conditions are mild and controllable, making it easy to achieve continuous industrial production.

[0018] In this invention, the modification process not only endows the membrane with stable antibacterial capabilities by covalently grafted quaternary ammonium groups, effectively inhibiting bacterial growth, but also enhances the retention of negatively charged small molecule pollutants by introducing positive charges and improving hydrophilicity, thereby improving the membrane's separation performance. Simultaneously, the strong bond between the quaternary ammonium groups and the PVDF backbone prevents the loss of antibacterial agents. The modified membrane exhibits gradual performance degradation and good chemical stability during long-term operation, making it suitable for complex separation scenarios. Compared to ultrafiltration membranes prepared by existing multi-step modification processes, this invention can meet the separation requirements of small molecules such as antibiotics and dyes, offering a wider range of applications and greater practicality. Attached Figure Description

[0019] The disclosure of this invention is illustrated with reference to the accompanying drawings. It should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. In the drawings, the same reference numerals are used to refer to the same parts:

[0020] Figure 1 XPS full spectrum scan of the modified PVDF membrane provided by this invention;

[0021] Figure 2 FTIR spectra of the PVDF membrane and the modified PVDF membrane provided by the present invention;

[0022] Figure 3 SEM images of the PVDF membrane and the modified PVDF membrane provided by this invention;

[0023] Figure 4 An elemental diagram of the modified PVDF membrane provided by this invention. Detailed Implementation

[0024] It is readily understood that, based on the technical solution of this invention, those skilled in the art can propose various interchangeable structural methods and implementations without altering the essential spirit of the invention. Therefore, the following detailed embodiments and accompanying drawings are merely illustrative examples of the technical solution of this invention and should not be considered as the entirety of the invention or as limitations or restrictions on the technical solution of this invention.

[0025] This invention provides a technical solution: a free radical grafted quaternized ammonium modified PVDF antibacterial nanofiltration membrane and its preparation method, comprising the following steps:

[0026] S1: Mix PVDF powder, quaternized modified monomer, and catalyst in a specific mass ratio, add to an organic solvent, and stir until a homogeneous solution is formed;

[0027] S2: Transfer the above mixed solution to a closed reaction vessel and carry out a hydrothermal reaction at a set temperature. After the reaction is completed, cool the vessel.

[0028] S3: The cooled reaction solution is cast into a film and then soaked in water for a certain period of time to obtain a free radical grafted quaternized ammonium modified PVDF antibacterial nanofiltration membrane.

[0029] Specifically, regarding the raw material proportioning parameters:

[0030] In some embodiments, the quaternization modified monomer is allyltrimethylammonium chloride, and its mass ratio with PVDF powder is 1:300-1:30;

[0031] The catalyst is CuCl2, and its mass ratio with PVDF powder is 0.001:300-1:300.

[0032] Specifically, regarding the PVDF mass fraction and solvent parameters:

[0033] In some embodiments, the mass fraction of PVDF powder in the mixed solution is 10%-20%;

[0034] The organic solvent is selected from one or more of N,N-dimethylacetamide (DMAc), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP).

[0035] Specifically, regarding the reaction temperature and time parameters:

[0036] In some embodiments, the temperature of the hydrothermal reaction in step S2 is 80-120°C, and the reaction time is 24 hours.

[0037] The film formation is achieved using a non-solvent-induced phase inversion method, with deionized water as the coagulation bath and a immersion time of 24 hours to ensure complete removal of residual impurities.

[0038] The core raw materials of this invention include PVDF substrate, modified monomer allyltrimethylammonium chloride, and catalyst CuCl2. The chemical structure of these three components determines the feasibility of the reaction and the performance of the final membrane, as detailed below:

[0039] The molecular chain of the substrate polyvinylidene fluoride (PVDF) is composed of repeating units -CH2-CF2-, and its simple structural formula is shown in Formula 1.

[0040] (Equation 1); (Equation 2);

[0041] In this structure, the CF bond energy is as high as 485kJ / mol, which endows PVDF with chemical stability against acids, alkalis and organic solvents, as well as high mechanical strength, with a tensile strength ≥30MPa under normal conditions;

[0042] However, the large number of electronegative F atoms in the molecular chain makes its surface extremely hydrophobic. When unmodified, the water contact angle is 96°, which makes it very easy to adsorb organic pollutants and thus breed bacteria. After modification, the water contact angle is 63°, which has strong hydrophilicity.

[0043] It should be noted that the CH bond adjacent to CF2 on the PVDF molecular chain is a weak bond with a bond energy of approximately 413 kJ / mol, which can serve as an active site for grafting reactions.

[0044] The modified monomer was allyltrimethylammonium chloride (ATMAC), whose chemical structure is shown in Formula 2.

[0045] The molecule in this structure contains two key functional groups. One is an unsaturated allyl group (CH2=CH-CH2-), whose C=C double bond can undergo addition reactions with free radicals, providing a chemical linking point for grafting onto the PVDF molecular chain.

[0046] Second, quaternary ammonium cations (-N) + (CH3)3), carrying a positive charge, can both destroy bacterial cell membranes through electrostatic interaction to achieve contact sterilization, and can also generate charge repulsion with negatively charged small molecule pollutants, such as tetracycline hydrochloride, to improve the rejection rate.

[0047] Chloride ions (Cl) - It acts as a balancing ion to ensure the solubility of the monomer in polar solvents.

[0048] The catalyst is CuCl2, which is a brownish-yellow powder in its anhydrous state. It dissociates into Cu upon dissolution in polar solvents such as DMAc. 2+ With Cl - ;

[0049] No additional initiator is required in this invention, Cu 2+ Electron transfer can trigger the breaking of the CH bond in PVDF to generate a free radical, which can then undergo a valence state transformation, i.e., from Cu... 2+ Converted to Cu + It is the core driving force for initiating free radical grafting reactions.

[0050] It should be further explained that this invention achieves free radical grafting of PVDF and ATMAC through a CuCl2-catalyzed synergistic hydrothermal reaction. The reaction process can be divided into three stages: free radical generation, monomer addition, and reaction termination, as detailed below:

[0051] The first stage is the free radical generation stage. After PVDF, ATMAC and CuCl2 are dissolved in a solvent to form a homogeneous solution, they are placed in a closed reactor at 80-120℃ for hydrothermal reaction.

[0052] Under these conditions, Cu 2+ Electrons are extracted from the CH bonds in the PVDF molecular chain, causing the CH bonds to break and generating PVDF radicals (·PVDF). Simultaneously, Cu... 2+ Reduced to Cu + The reaction formula is as follows:

[0053]

[0054] The second stage is the monomer addition grafting stage, where the generated PVDF radicals have high reactivity and immediately attack the C=C double bond of ATMAC and undergo an addition reaction.

[0055] After the double bond opens, it forms a new C-C covalent bond with the PVDF radical, allowing ATMAC to be chemically anchored to the PVDF molecular chain, forming a PVDF-quaternary ammonium salt graft copolymer. The reaction formula is as follows:

[0056]

[0057] As the reaction proceeds, the concentration of free radicals in the system gradually decreases. The remaining PVDF free radicals or grafted free radicals terminate the reaction through coupling or disproportionation, that is, either two free radicals combine to form a C-C bond, or hydrogen atoms are transferred to form a double bond.

[0058] Since the quaternary ammonium group is covalently linked to PVDF, its structure is stable after the reaction is terminated and will not detach.

[0059] It should be further noted that in this invention, PVDF, ATMAC and CuCl2 can be dissolved and reacted in the same solvent system, i.e., one-pot method;

[0060] No step-by-step pretreatment or activation is required, avoiding the solvent removal and drying steps of traditional multi-step processes;

[0061] Furthermore, no initiators such as AIBN are needed during the process; the reaction can be initiated solely by CuCl2 catalysis, making the process simple and environmentally friendly.

[0062] To further illustrate the embodiments of the present invention, verify the influence of different combinations of parameters such as raw material ratio, solvent selection, and reaction temperature on the preparation process and final performance of free radical grafted quaternized ammonium modified PVDF antibacterial nanofiltration membrane, and pinpoint the optimal process conditions, the following examples provide a detailed description of the specific preparation and performance of the present invention.

[0063] It should be noted that these embodiments are merely illustrative and not intended to limit the invention.

[0064] Example 1

[0065] 5.1g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until the solution turned yellowish-brown. The mixture was then transferred to a reaction vessel and reacted at 110℃ for 24h. After cooling, it was directly poured onto a glass plate and soaked in deionized water for 24h to form a film.

[0066] This embodiment verifies the feasibility of free radical grafting within the system and confirms the process stability at a reaction temperature of 110°C.

[0067] It is important to note that the solution state needs to be continuously monitored during stirring. If local clumping occurs, 0.5-1 mL of DMAc should be added to adjust the solution and ensure the homogeneity of the system.

[0068] Example 2

[0069] 4.5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 25.5g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0070] This embodiment studies the effect of reducing the PVDF mass fraction to 15% on the fluidity of the casting solution, the film formation rate, and the grafting uniformity.

[0071] It should be noted that the mass fraction in this embodiment is 2% lower than that in Example 1. It is necessary to record the leveling speed of the film liquid during casting and observe whether ripples or bubbles appear on the film surface after film formation. If so, it is necessary to optimize by adjusting the casting speed.

[0072] Example 3

[0073] 2g of PVDF powder, 0.04g of allyltrimethylammonium chloride and 0.002g of CuCl2 were dissolved in 18g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0074] This embodiment verifies the feasibility of film formation when the PVDF mass fraction is 10%, and studies the continuity and mechanical strength of the film at low concentrations.

[0075] It should be noted that, in this embodiment, due to the low viscosity of the casting liquid, a horizontal scraper was used to control the film thickness during casting, and after film formation, the cross-section of the film was observed to be free of voids by SEM.

[0076] Example 4

[0077] 3g of PVDF powder, 0.06g of allyltrimethylammonium chloride and 0.003g of CuCl2 were dissolved in 17g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0078] This embodiment verifies the sufficiency of the reaction when the PVDF mass fraction is 15%, eliminating interference from other parameters;

[0079] It should be noted that in this embodiment, the change in the mass ratio of solid substances to the total mass of the reaction system before and after the reaction is monitored simultaneously. The coagulation bath water temperature is controlled at 25±1℃ during film formation to avoid temperature fluctuations affecting the phase transformation rate.

[0080] Example 5

[0081] 6g of PVDF powder, 0.12g of allyltrimethylammonium chloride and 0.006g of CuCl2 were dissolved in 24g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0082] This embodiment verifies the dissolution integrity and reaction mass transfer efficiency when the PVDF mass fraction is 20%.

[0083] It should be noted that in this embodiment, PVDF was dissolved using a 60°C water bath with stirring at a speed of 300 r / min, and the homogeneity of the solution was tested after the reaction.

[0084] Example 6

[0085] 5g of PVDF powder, 0.017g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.98g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0086] This embodiment verifies the grafting effectiveness when the mass ratio of allyltrimethylammonium chloride to PVDF is 1:300;

[0087] It should be noted that in this embodiment, a combination of low-speed premixing and high-speed dispersion is used during stirring. First, the mixture is stirred at a rate of 100 r / min for 1 hour, and then at a rate of 250 r / min for 23 hours to avoid local aggregation of monomers.

[0088] Example 7

[0089] 5g of PVDF powder, 0.05g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.95g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0090] This embodiment verifies the grafting efficiency and performance balance when the mass ratio of allyltrimethylammonium chloride to PVDF is 1:100.

[0091] It should be noted that the monomer dissolution time recorded in this embodiment is less than 30 minutes. If the dissolution is slow, 1-2 drops of deionized water can be added to promote dissociation. The pressure of the reactor should be controlled at 0.1-0.12 MPa to avoid excessive pressure that could lead to solvent leakage.

[0092] Example 8

[0093] 5g of PVDF powder, 0.167g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.83g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0094] This embodiment verifies the catalytic effectiveness when the mass ratio of allyltrimethylammonium chloride to PVDF is 1:30;

[0095] It should be noted that after film formation in this embodiment, a tensile strength test is required. The tensile strength should be ≥25MPa. At the same time, the uniformity of hydrophilicity on the film surface should be tested using a contact angle meter. The contact angle deviation should be ≤3°, which meets the standard.

[0096] Example 9

[0097] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.000017g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0098] This embodiment verifies the catalytic effectiveness when the mass ratio of CuCl2 to PVDF is 0.001:300;

[0099] It should be noted that in this embodiment, the free radical signal in the reaction solution needs to be detected by electron paramagnetic resonance spectroscopy. A g-factor of 2.003 indicates that the data is valid. If the free radical signal is weak, the stirring time needs to be extended to 30 minutes.

[0100] Example 10

[0101] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0083g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0102] This embodiment verifies the catalytic efficiency when the mass ratio of CuCl2 to PVDF is 0.5:300;

[0103] It should be noted that in this embodiment, the reactor is cooled to room temperature after the reaction is completed before being opened to prevent the membrane liquid from precipitating out due to a sudden drop in temperature.

[0104] Example 11

[0105] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0167g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1. During water washing, the solution was soaked in 0.1mol / L NaOH solution twice, 30min each time. After soaking, the solution was rinsed with deionized water until the pH was neutral.

[0106] This embodiment verifies the catalytic efficiency when the mass ratio of CuCl2 to PVDF is 1:300;

[0107] It should be noted that in this embodiment, the film needs to be soaked twice in a 0.1 mol / L NaOH solution after formation. Under alkaline conditions, Cu... 2+ The hydrolysis reaction produces a flocculent precipitate of Cu(OH)₂, which can be easily removed by subsequent water washing, effectively reducing the residual Cu content. Specifically, observe the membrane surface for blue-green spots, which indicate Cu... 2+ If any residual characteristics are present, the NaOH soaking time needs to be extended.

[0108] Example 12

[0109] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until the solution turned yellowish-brown. The solution was then transferred to a reaction vessel and reacted at 80°C for 24h. The subsequent film formation steps were the same as in Example 1.

[0110] This embodiment verifies the sufficiency of free radical generation and grafting reaction at 80°C;

[0111] It should be noted that the reaction time in this embodiment can be extended to 30 hours. During the reaction, the pressure inside the reactor must be kept stable at 0.05-0.06 MPa to avoid gas leakage that could cause temperature fluctuations.

[0112] Example 13

[0113] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until the solution turned yellowish-brown. The solution was then transferred to a reaction vessel and reacted at 100°C for 24h. The subsequent film formation steps were the same as in Example 1.

[0114] This embodiment verifies the balance between reaction efficiency and energy consumption at 100°C;

[0115] It should be noted that in this embodiment, the temperature should be rapidly cooled to below 50°C after the reaction is completed before casting to avoid solvent evaporation due to high temperature.

[0116] Example 14

[0117] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until the solution turned yellowish-brown. The solution was then transferred to a reaction vessel and reacted at 120°C for 24h. The subsequent film formation steps were the same as in Example 1.

[0118] This embodiment verifies the stability of the PVDF backbone and the retention rate of quaternary ammonium groups at 120℃, defines the upper limit of the reaction temperature, and avoids the degradation of membrane materials caused by high temperature.

[0119] It should be noted that the reactor in this embodiment must use polytetrafluoroethylene as the inner lining to prevent the membrane material from degrading due to high temperature, thereby generating metal ion contamination.

[0120] Example 15

[0121] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.9g of DMF and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0122] This embodiment verifies the applicability of DMF as a single solvent and compares the solubility and reaction effects of DMAc.

[0123] It should be noted that the boiling point of DMF in this embodiment is relatively low. When reacting at 110°C, the pressure inside the reactor needs to be controlled to be ≤0.15MPa. After film formation, the water immersion time is extended to 30h to ensure that the residual amount of DMF is ≤0.1mg / kg.

[0124] Example 16

[0125] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.9g of DMSO and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0126] This embodiment verifies the applicability of DMSO as a single solvent, studies the promoting effect of its high polarity on the reaction, and examines the strong solubility of DMSO, making it suitable for the preparation of high-concentration PVDF systems.

[0127] It should be noted that DMSO has strong hygroscopicity in this embodiment, and the preparation process must be carried out in a dry glove box. When forming the film, the coagulation bath should use deionized water with a conductivity of ≤5μS / cm to avoid impurities reacting with DMSO.

[0128] Example 17

[0129] 5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in 24.9g of NMP and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0130] This embodiment verifies the applicability of NMP as a single solvent and studies the effect of its high boiling point on improving reaction stability.

[0131] It should be noted that the NMP in this embodiment has a high viscosity, so an anchor-type stirring paddle should be used during stirring at a speed of 200 r / min. After film formation, it needs to be treated in a 60℃ forced-air drying oven for 1 hour to remove residual NMP.

[0132] Example 18

[0133] 4.5g of PVDF powder, 0.1g of allyltrimethylammonium chloride and 0.0048g of CuCl2 were dissolved in a mixed solvent of 10g of DMAc and 15.5g of NMP and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0134] This embodiment verifies the PVDF solubility of a mixed solvent of DMAc and NMP.

[0135] It should be noted that in this embodiment, NMP needs to be heated to 40°C to improve its solubility before being mixed with DMAc.

[0136] Example 19

[0137] 5.5g of PVDF powder, 0.11g of allyltrimethylammonium chloride and 0.0055g of CuCl2 were dissolved in 24.5g of DMAc and stirred for 24h until the solution turned yellowish-brown. The subsequent reaction and film formation steps were the same as in Example 1.

[0138] This embodiment integrates the optimal values ​​of various parameters to verify the industrialization potential of the process.

[0139] To further highlight the outstanding advantages of this invention in terms of process simplification, catalytic efficiency and performance improvement, the following comparative analysis is conducted by setting up comparative examples and embodiments;

[0140] It should be noted that the comparative examples are for illustrative purposes only and are not intended to limit the scope of protection of this invention.

[0141] Comparative Example 1

[0142] 5.1g of PVDF powder was dissolved in 24.9g of DMAc and stirred for 24h until it became colorless and transparent. The solution was then directly poured onto a glass plate and soaked in deionized water for 24h to form a film. There were no hydrothermal steps in the reactor or the addition of quaternary ammonium salts or catalysts.

[0143] This comparative example aims to verify the necessity of quaternization modification and CuCl2 catalysis for grafting reaction, antibacterial properties, and separation performance.

[0144] Comparative Example 2

[0145] 4.5g of PVDF powder was dissolved in 25.5g of DMAc and stirred for 24h until it became colorless and transparent. The mixture was then directly poured onto a glass plate and soaked in deionized water for 24h to form a film. There were no hydrothermal steps in the reactor or the addition of quaternary ammonium salts or catalysts.

[0146] This comparative example aims to further verify the necessity of modification at low PVDF concentrations.

[0147] Comparative Example 3

[0148] 5.1g of PVDF powder, 0.1g of allyltrimethylammonium chloride, and 0.0048g of FeCl2 were dissolved in 24.9g of DMAc and stirred for 24h until light gray color appeared. The mixture was then transferred to a reactor and reacted at 110°C for 24h. The casting process was the same as in Example 1.

[0149] This comparative example aims to verify whether FeCl2 can replace CuCl2 to achieve catalytic activity.

[0150] Comparative Example 4

[0151] 5.1g of PVDF powder, 0.1g of allyltrimethylammonium chloride, and 0.01g of Ce(NH4)2(NO3)6 were dissolved in 24.9g of DMAc and stirred for 24h until pale yellow. The mixture was then transferred to a reactor and reacted at 110°C for 24h. The casting process was the same as in Example 1.

[0152] This comparative example aims to verify the suitability of rare earth catalysts.

[0153] Comparative Example 5

[0154] 5.1g of PVDF powder and 0.1g of allyltrimethylammonium chloride were dissolved in 24.9g of DMAc and stirred for 24h until colorless and transparent. The mixture was then transferred to a reactor and reacted at 110°C for 24h. The casting process was the same as in Example 1.

[0155] This comparative example aims to verify the necessity of a catalyst for the reaction.

[0156] Comparative Example 6

[0157] This comparative example uses an existing multi-step process. 5.1 g of PVDF was dissolved in 40 mL of 7.5 mol / L NaOH-ethanol solution and stirred at 70 °C for 2 h. After washing with ethanol, it was reacted with 10% H₂O₂ for 12 h to obtain PVDF-g-OH. Subsequently, 1.5 g of PVDF-g-OH was reacted with 2.6 mL of triethylamine and 1.9 mL of bromoisobutyryl bromide in dichloromethane for 18 h to obtain PVDF-g-Br. PVDF-g-Br was then reacted with 0.1 g of DMAEMA-C... 12 After reacting 0.1 g of SBMA under CuBr / bipyridine catalysis for 24 h, the product was dissolved in 24.9 g of DMAc, and the casting process was the same as in Example 1.

[0158] This comparative example aims to verify the simplification advantages of the one-pot method of the present invention compared with existing multi-step processes and the differences in product performance.

[0159] Comparative Example 7

[0160] 5.1g of PVDF powder and 0.0048g of CuCl2 were dissolved in 24.9g of DMAc and stirred for 24h until a light green color appeared. The mixture was then transferred to a reactor and reacted at 110℃ for 24h. The casting process was the same as in Example 1.

[0161] This comparative example aims to verify the necessity of quaternary ammonium salt monomer grafting and performance.

[0162] Comparative Example 8

[0163] Dissolve 5.1g of PVDF powder, 0.1g of allyltrimethylammonium chloride, and 0.0048g of CuCl2 in 24.9g of DMAc, stir for 24h until yellowish-brown, and let stand in a sealed container at room temperature (25°C) for 24h. The casting process is the same as in Example 1.

[0164] This comparative example aims to verify the effect of hydrothermal temperature on the reaction.

[0165] To further quantify and verify the core indicators of the modified membrane, such as antibacterial performance, separation performance, and stability, and to corroborate the authenticity and scientific nature of the free radical grafting reaction through structural characterization, the membrane samples prepared in the examples and comparative examples were systematically tested and analyzed using unified testing standards.

[0166] The aim is to form a complete chain of evidence, from process to structure to performance, to provide authoritative data support for the inventiveness and practicality of the technical solution of this invention.

[0167] Test Example 1

[0168] This test case aims to verify the effect of quaternization-modified PVDF membrane on improving antibacterial performance and to clarify the influence of parameters such as monomer ratio, catalyst dosage, and solvent type on the antibacterial rate.

[0169] The test subjects include Examples 1, 6, 7, 8, 9, 11, 15, 16, 17, 19, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 5, and Comparative Example 7.

[0170] The test method refers to the CFU method in GB / T 21510-2008 "Test Method for Antibacterial Properties of Nano-Inorganic Materials":

[0171] Culture Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 6538) to 1×10⁻⁶. 5 CFU / mL;

[0172] Immerse a 1cm×1cm membrane sample in bacterial solution, incubate at 37℃ for 2 hours, and then spread it on a plate to count the number of colonies.

[0173] The antibacterial rate is calculated using the following formula:

[0174] Antibacterial rate (%) = (Number of blank colonies - Number of sample colonies) / Number of blank colonies × 100%

[0175] The specific test results are shown in Table 1.

[0176]

[0177] Table 1. Antibacterial performance test results

[0178] Antibacterial properties are one of the core indicators for evaluating the practicality of quaternized ammonium modified PVDF, and their essence depends on the quaternary ammonium cations (-N) on the membrane surface. + The density and distribution of (CH3)3) quaternary ammonium cations destroy bacterial cell membranes through electrostatic attraction, thereby achieving contact sterilization;

[0179] In this test case, the CFU method was used to quantify the effect of different process parameters on the antibacterial rate, aiming to reveal the intrinsic relationship between process design and grafting efficiency and antibacterial performance.

[0180] First, the effect of quaternary ammonium salt monomer dosage was compared and studied in conjunction with Examples 1, 6, 7, and 8:

[0181] The amount of quaternary ammonium salt monomer directly determines the number of antibacterial active sites that can be grafted onto the membrane surface, which is the core factor affecting the antibacterial rate. Test data show that as the mass ratio of monomer to PVDF increases from 1:300 to 1:30, the antibacterial rate of Escherichia coli increases from 82.3% to 99.2%, and the antibacterial rate of Staphylococcus aureus increases from 79.5% to 98.9%, showing a significant positive correlation.

[0182] Specifically, in Example 6, the amount of monomer used was extremely low. Even under CuCl2 catalysis, the number of monomer molecules that could react with PVDF free radicals was limited, resulting in insufficient grafting rate and an antibacterial rate of only about 83% of that in Example 1.

[0183] In Example 7, the dosage was increased to 0.05g monomer per 5g of PVDF, which increased the number of grafting sites, increased the density of quaternary ammonium cations, and increased the antibacterial rate to about 95%, balancing antibacterial properties and cost.

[0184] In Example 8, the monomer dosage was sufficient, allowing PVDF radicals to fully add to the C=C double bond, resulting in the highest grafting rate and an antibacterial rate close to 100%. However, it should be noted that excessive monomer dosage may lead to an excessively high charge density on the membrane surface, which in turn increases the resistance of water molecules. Therefore, comprehensive optimization based on separation performance is necessary.

[0185] Next, the effect of the catalyst was studied by comparing Examples 1, 9, 11, Comparative Example 3, and Comparative Example 5:

[0186] CuCl2 is the sole driving force for initiating the generation of PVDF free radicals, and its amount and type directly determine the initiation efficiency of the grafting reaction.

[0187] Specifically, in Example 9, the catalyst dosage was only 0.000017 g of CuCl2 per 5 g of PVDF. 2+ If the concentration is too low, it cannot effectively capture electrons from the CH bond of PVDF to generate free radicals, resulting in insufficient grafting reaction and a reduction in the antibacterial rate to about 85%.

[0188] In Example 11, the catalyst dosage was increased to 0.0167 g CuCl2 per 5 g PVDF, although Cu 2+Excessive catalyst dosage can be removed by soaking in 0.1 mol / L NaOH during water washing, without affecting the grafted quaternary ammonium groups. The quaternary ammonium cation density on the membrane surface is not damaged, and the antibacterial rate against Escherichia coli is still 96.5%, which is only slightly different from Example 1. This proves that high catalyst dosage does not impair antibacterial performance and only requires optimization of post-treatment steps.

[0189] In Comparative Example 3, Fe 2+ The electron transfer ability is weaker than Cu 2+ It could not effectively trigger the breaking of the CH bond in PVDF, and the antibacterial rate against Escherichia coli was only 23.6%, which was much lower than that of the CuCl2 catalytic group, confirming the irreplaceable nature of CuCl2.

[0190] In Comparative Example 5, without a catalyst, the hydrothermal conditions of 80-120℃ were insufficient to break the CH bonds of PVDF. Without the generation of free radicals, it could not add to the monomer. The membrane surface had no quaternary ammonium groups, and the antibacterial rate was only 15.2%, which was close to that of the unmodified membrane, further demonstrating the importance of CuCl2.

[0191] Furthermore, the influence of solvent type was compared and studied in conjunction with Examples 1, 15, 16, and 17:

[0192] The core function of the solvent is to dissolve PVDF and monomers to form a homogeneous reaction system. Its polarity and boiling point affect the reaction mass transfer efficiency, but its effect on the antibacterial rate is weaker than that of monomers and catalysts.

[0193] Test data showed that when DMF, DMSO, and NMP were used as solvents, the inhibition rates of Escherichia coli were 97.4%, 97.8%, and 98.1%, respectively, all close to the DMAc baseline group, with differences ≤1.5%.

[0194] Specifically, in Example 15, DMF has moderate polarity and can fully dissolve PVDF, but its boiling point is low. When reacting at 110°C, the pressure needs to be controlled to ≤0.15MPa to avoid volatilization.

[0195] In Example 16, DMSO is highly polar and has hydrogen bond donor capability, which can promote the dissociation of CuCl2 into Cu. 2+ It improves catalytic efficiency, with a grafting rate slightly higher than that of the DMF group;

[0196] In Example 17, NMP has a high boiling point, which ensures that there is no volatilization during the reaction at 110°C, the system is stable, and the viscosity is high, which can alleviate molecular diffusion, avoid local aggregation of monomers, and make the grafting distribution more uniform. The antibacterial rate is closest to that of Example 1.

[0197] Furthermore, by comparing and verifying the impact of missing raw materials with Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 7:

[0198] The lack of raw materials directly leads to the absence of antibacterial groups, which is the root cause of the sharp drop in antibacterial rate. The following analysis will be divided into two cases: unmodified and monomer-free.

[0199] Firstly, both Comparative Examples 1 and 2 had added monomers and catalysts, and the membrane surface was only pure PVDF without quaternary ammonium cations, which could not destroy bacterial cell membranes. The inhibition rate of Escherichia coli was only 11.5%-12.1%, and the inhibition rate of Staphylococcus aureus was only 8.3%-9.2%, and the antibacterial activity was negligible. This confirms that quaternization modification is a necessary means to improve antibacterial performance.

[0200] In Comparative Example 7, a catalyst was added, but no monomer was present; Cu 2+ Although it can generate PVDF free radicals, there is no C=C double bond donor to add to it, and the free radicals eventually terminate through coupling. There are no quaternary ammonium groups on the membrane surface, and the antibacterial rate is close to that of the unmodified membrane, proving that the monomer is the only source of antibacterial active sites.

[0201] Test Example 2

[0202] This test case aims to evaluate the modified membrane's ability to separate tetracycline hydrochloride, clarify the effects of parameters such as PVDF concentration, reaction temperature, solvent, and catalyst on throughput and rejection rate, and compare the performance differences between the present invention and existing processes.

[0203] The test subjects include Example 1, Example 2, Example 3, Example 5, Example 8, Example 12, Example 13, Example 14, Example 15, Example 16, Example 18, Example 19, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 5, Comparative Example 6, and Comparative Example 8.

[0204] The test method refers to GB / T 32360-2015 "Test Methods for Nanofiltration Membranes":

[0205] Cross-flow filtration device, effective membrane area 3.14 cm² 2 The test pressure was 0.6 MPa and the temperature was 30℃.

[0206] The feed solution was a 50 mg / L tetracycline hydrochloride aqueous solution, which was stabilized and filtered for 1 hour before sampling.

[0207] The flux is calculated using the following formula:

[0208] J = V / (S × t)

[0209] Where V is the volume of liquid permeated, S is the membrane area, and t is time;

[0210] The HPLC rejection rate is determined using the following formula:

[0211] R = (C f -C p ) / Cf ×100%

[0212] Where C f C represents the feed concentration. p The concentration of the permeate.

[0213] The specific test results are shown in Table 2.

[0214]

[0215] Table 2. Flux and Rejection Rate Test Results

[0216] The separation performance of tetracycline hydrochloride is a core indicator for evaluating the nanofiltration function of modified PVDF membranes. Its essence is determined by both the membrane pore structure and surface charge characteristics. Flux depends on the membrane's porosity and hydrophilicity, while the rejection rate depends on the quaternary ammonium cations (-N...). + (CH3)3) electrostatic repulsion of negatively charged tetracycline hydrochloride, rather than simple pore size sieving;

[0217] This study aims to reveal the intrinsic relationship between preparation parameters and membrane microstructure and separation performance by quantifying the effects of variables such as PVDF concentration, reaction temperature, solvent type, catalyst and process route.

[0218] First, the influence of PVDF quality fraction is verified by comparing Examples 2, 3, 5, Comparative Example 1, and Comparative Example 2:

[0219] The mass fraction of PVDF directly determines the viscosity of the casting solution and the pore structure after film formation, and is the primary parameter affecting flux; while the rejection rate mainly depends on the charge repulsion of quaternary ammonium groups, and has no direct positive correlation with the PVDF concentration, but is only affected by the integrity of the membrane structure.

[0220] Specifically, in Example 2, the casting solution has a moderate viscosity, and a loose and continuous pore structure is formed during the non-solvent-induced phase transformation. The mass transfer resistance of water molecules is small, and the hydrophilicity of the membrane is improved after modification, so the flux is the highest.

[0221] In Example 3, the casting solution had low viscosity, and the solvent-non-solvent exchange was too rapid during phase transition, resulting in a loose and uneven membrane structure, local pore collapse, discontinuous water molecule mass transfer pathways, and a flux of only 2.8 L / m. 2 h・bar, lower than in Example 1;

[0222] In Example 5, the casting solution viscosity was too high, the phase transformation rate was slow, resulting in a dense, asymmetric structure with narrow and tortuous channels, and the flux decreased to 2.9 L / m. 2 h·bar, but with optimal membrane structure integrity;

[0223] Examples 2, 3, and 5, belonging to the modified group, all had retention rates ≥92%, significantly higher than Comparative Examples 1 and 2 in the unmodified group. The core reason is the quaternary ammonium group (-N). + (CH3)3) electrostatic repulsion of tetracycline hydrochloride anions;

[0224] Example 5 showed the highest retention rate, and the sieving effect of the dense pores was significant in addition to charge repulsion.

[0225] The unmodified comparative examples 1 and 2 showed no charge repulsion and relied solely on hydrophobic adsorption or inefficient sieving through loose pores, resulting in significant performance differences.

[0226] Next, the effect of reaction temperature was verified by comparing Examples 1, 12, 13, and 14:

[0227] The reaction temperature indirectly affects the hydrophilicity and charge density of the membrane by regulating the catalytic efficiency of CuCl2 and the stability of the PVDF backbone, thereby changing the separation performance.

[0228] Specifically, in Example 12, 80°C is below the optimal catalytic temperature for CuCl2. 2+ The ability to capture electrons from the CH bonds of PVDF is weakened, the density of quaternary ammonium groups on the membrane surface is insufficient, charge repulsion is weakened, and the tetracycline hydrochloride rejection rate drops to 88.3%. At the same time, the improvement in hydrophilicity is limited, the mass transfer resistance is high, and the flux is only 2.5 L / m. 2 h・bar is the lowest in the modified group;

[0229] In Example 13, Cu at 100°C 2+ The valence state transformation is complete, the amount of free radical generation is moderate and stable, and the pore structure has good permeability due to the regular molecular arrangement, with a flux of 3.5 L / m. 2 h・bar, slightly higher than in Example 1, with a rejection rate of 95.3%, achieving a simultaneous increase in both flux and rejection, is the optimal temperature that balances performance and energy consumption;

[0230] In Example 14, at 120°C, close to the glass transition temperature of PVDF, the main chain underwent slight shrinkage, mass transfer resistance increased, and flux decreased to 3.1 L / m. 2 Despite the high temperature (h・bar), the grafted quaternary ammonium groups remained unaffected, the charge repulsion was stable, and the retention rate was still 94.8%, showing only a slight difference from the baseline group, demonstrating the thermal stability of the modified film.

[0231] Furthermore, the influence of solvent type is verified by comparing and contrasting Examples 1, 15, 16, and 18:

[0232] The polarity and solubility of the solvent determine the dispersion uniformity of PVDF and CuCl2, which in turn affects the grafting uniformity and membrane pore structure.

[0233] Specifically, in Example 16, a strongly polar solvent promotes the dissociation of CuCl2 into Cu. 2+ The catalytic efficiency is improved, and DMSO exhibits good compatibility with water, resulting in more regular pores during phase transformation, optimal membrane hydrophilicity, and a flux of 3.7 L / m³. 2 h・bar, with a retention rate of 95.5%, making it the best performing single solvent;

[0234] In Example 15, the polarity of DMF is slightly lower than that of DMSO, and Cu 2+ The degree of dissociation is slightly lower, the grafting rate is slightly reduced, and the flux is 3.3 L / m. 2 h・bar and retention rate of 93.5% are both close to those of Example 1;

[0235] In Example 18, NMP improves the solubility of PVDF, and DMAc ensures the fluidity of the casting solution. The mixture of the two results in more uniform dissolution, better pore connectivity during phase inversion compared to a single solvent, a loose membrane structure without collapse, and a flux of 4.8 L / m. 2 h・bar, close to Example 2, with a rejection rate of 94.2%, demonstrating that the mixed solvent can optimize separation performance through synergistic effects.

[0236] The influence of the catalyst and process route was verified by comparing and verifying Examples 1, 3, 5, and 6:

[0237] Specifically, in Comparative Example 3, Fe 2+ The standard electrode potential is lower than that of Cu 2+ It cannot effectively capture CH bond electrons from PVDF, and the membrane performance is close to that of the unmodified group, with a flux of 1.2 L / m. 2 h・bar, retention rate 65.2%, solely due to Fe 3+ Trace adsorption was slightly improved;

[0238] In Comparative Example 5, no Cu was present. 2+ At this temperature, hydrothermal treatment at 80-120℃ cannot break the CH bond, no quaternary ammonium groups are introduced, and the flux is 1.1 L / m. 2 h・bar, retention rate 63.5%, consistent with unmodified membrane;

[0239] In Comparative Example 6, due to the pore size being much larger than the size of tetracycline hydrochloride molecules, the rejection rate was only 58%, meeting the requirements for small molecule separation. Although the large pore size resulted in a flux of 4.2 L / m³, the overall performance was still satisfactory. 2 h・bar, but its overall performance is far inferior to the nanofiltration membrane of this invention.

[0240] Test Example 3

[0241] This test case aims to evaluate the modified membrane's flux decay and retention rate maintenance capabilities during continuous filtration, and to verify its long-term industrial application potential.

[0242] The test subjects include Example 1, Example 19, Comparative Example 1, and Comparative Example 6;

[0243] The test method refers to GB / T 32360-2015 "Test Methods for Nanofiltration Membranes":

[0244] The cross-flow filter device from Test Example 2 was used;

[0245] Filter continuously for 48 hours, and record the flux and rejection rate every 4 hours.

[0246] The 48-hour flux decay rate is calculated using the following formula:

[0247] Attenuation rate (%) = (Initial flux - 48h flux) / Initial flux × 100%

[0248] The specific test results are shown in Table 3.

[0249]

[0250] Table 3. Results of Long-Term Operational Stability Tests

[0251] The long-term operational stability of membranes is a core indicator for evaluating their potential for industrial application, and is directly related to operating costs and separation efficiency. For antibacterial nanofiltration membranes, stability mainly depends on the strength of grafted groups, the hydrophilicity of the membrane surface, and the integrity of the structure. The shedding of quaternary ammonium groups, the adsorption of pollutants on hydrophobic surfaces, or pore blockage can all lead to flux decline and a decrease in rejection rate.

[0252] This test involved continuous filtration of tetracycline hydrochloride solution for 48 hours, comparing the performance changes of the modified membrane of this invention, the unmodified membrane, and existing multi-step process membranes, revealing the intrinsic relationship between preparation parameters and stability.

[0253] Specifically, in Example 1, under CuCl2 catalysis, PVDF radicals undergo an addition reaction with the C=C double bond of the monomer, and the quaternary ammonium group is anchored to the PVDF molecular chain through a C=C covalent bond, rather than through physical adsorption or weak bond bonding. During continuous filtration, no quaternary ammonium group is lost, the charge repulsion effect is stable, and the retention rate hardly decreases.

[0254] After modification, the water contact angle on the membrane surface decreased from 96° to 63°. The improved hydrophilicity enabled water molecules to form a continuous water film on the membrane surface, inhibiting the adsorption of hydrophobic tetracycline hydrochloride.

[0255] In addition, the one-pot homogeneous reaction ensures uniform grafting, no local collapse or excessive looseness in the membrane pore structure, stable mass transfer pathway, and flux decline mainly due to slight pore surface contamination rather than structural damage.

[0256] In contrast, in Comparative Example 1, the unmodified PVDF surface is highly hydrophobic with a contact angle of 96°. It exhibits strong van der Waals forces with the hydrophobic tetracycline hydrochloride molecules. During filtration, pollutants are rapidly adsorbed onto the membrane surface and gradually clog the pores, resulting in a sharp decrease in flux.

[0257] Due to the electrostatic repulsion caused by the lack of quaternary ammonium groups, tetracycline hydrochloride can easily pass through the pores and adsorb onto the pore walls.

[0258] In Comparative Example 6, the quaternary ammonium groups are linked to the monomers via ATRP polymerization of PVDF-g-Br. Some chain segments may form weak bonds such as ester bonds and ether bonds due to uneven polymerization. During long-term operation, these bonds may fall off under the shear force of water flow, resulting in weakened charge repulsion and a decrease in retention rate.

[0259] Furthermore, in the multi-step reaction, each step of the desolventizing and drying process is prone to causing membrane shrinkage or local densification, forming dead pores or macropores. After 48 hours, the degree of contamination in the uneven areas varies significantly, and the overall stability is inferior to the homogeneous modified membrane of the present invention.

[0260] Test Example 4

[0261] This test case aims to evaluate the antibacterial properties and separation performance retention rate of the modified membrane under strong acid and strong alkali environments, verify its chemical stability, and expand its application scenarios.

[0262] The test subjects include Example 1, Example 19, and Comparative Example 1;

[0263] The test method refers to the chemical stability test requirements in GB / T 30300-2013 "Terminology for Membrane Separation Technology":

[0264] The membrane samples were immersed in 1 mol / L HCl and 1 mol / L NaOH solutions, respectively, and allowed to stand at 25°C for 24 h.

[0265] Wash with deionized water until neutral, and measure the antibacterial rate of Escherichia coli / Staphylococcus aureus according to Test Example 1. Measure the flux and rejection rate of tetracycline hydrochloride according to Test Example 2.

[0266] The performance retention rate is calculated using the following formula:

[0267] Retention rate (%) = (Performance after treatment / Performance before treatment) × 100%

[0268] The specific test results are shown in Table 4.

[0269]

[0270] Table 4 Results of acid-base tolerance test

[0271] Acid and alkali tolerance is a key indicator for evaluating the suitability of nanofiltration membranes under complex operating conditions. The acid and alkali stability of the membrane essentially depends on the stability of its chemical structure and the integrity of its physical structure.

[0272] This invention utilizes CuCl2 catalytic free radical grafting of quaternary ammonium salts to covalently anchor quaternary ammonium groups onto the PVDF backbone, which theoretically enhances the chemical stability of the membrane.

[0273] This test quantifies the performance retention difference between modified and unmodified membranes by immersion treatment with 1 mol / L HCl and 1 mol / L NaOH, revealing the intrinsic relationship between the preparation process and acid and alkali tolerance.

[0274] Specifically, in Example 1, the quaternary ammonium group is connected to the PVDF backbone via a C-C covalent bond, rather than through physical adsorption or ionic bonding as in traditional coating methods. The quaternary ammonium structure of allyltrimethylammonium chloride (-N...) + (CH3)3) has a large steric hindrance. After soaking in 1 mol / L HCl or 1 mol / L NaOH for 24 hours, only a very small amount of protonation and deprotonation occur, with no obvious hydrolysis and breakage. The charge repulsion effect is basically maintained, so the antibacterial rate and retention rate are high.

[0275] Furthermore, the one-pot homogeneous reaction ensures that quaternary ammonium groups are uniformly distributed within the PVDF molecular chain, resulting in an 8% increase in crystallinity of the modified membrane compared to the unmodified membrane, and a more compact molecular chain packing. In acidic and alkaline environments, the CF bonds in the PVDF backbone exhibit strong corrosion resistance, and the membrane's swelling rate is only 3%-4%, significantly lower than the 12%-15% of the unmodified membrane. The pore structure shows no obvious deformation, thus resulting in a gradual flux decline.

[0276] The modified membrane has a water contact angle of 63°, and its hydrophilic surface does not readily adsorb H₂ in acidic or alkaline solutions. + OH - Or impurity ions, to avoid performance fluctuations caused by surface charge disorder, and further ensure stable rejection rate;

[0277] In contrast, in Comparative Example 1, the unmodified PVDF molecular chains lacked the cross-linking support of quaternary ammonium groups, making them prone to segmental swelling in 1 mol / L HCl or 1 mol / L NaOH. The fluorine atoms of PVDF interacted with H... + OH - The formation of weak hydrogen bonds leads to an increase in the intermolecular distance, increasing the membrane porosity from the initial 45% to 60%, disrupting the sieving effect, resulting in large flux fluctuations and a sharp drop in retention rate.

[0278] Furthermore, the unmodified membrane has a water contact angle of 96°, and its strongly hydrophobic surface readily adsorbs Cl in acidic and alkaline solutions. - Na + Hydrolyzed impurities form a contaminant layer that blocks the pores, while also accelerating bacterial growth on the surface, resulting in an antibacterial retention rate of less than 53%.

[0279] Unmodified membranes lack the charge stabilizing effect of quaternary ammonium groups, resulting in disordered surface charges in acidic and alkaline environments. Consequently, they cannot suppress solute adsorption through electrostatic repulsion, further exacerbating performance degradation.

[0280] Test Example 5

[0281] This test case aims to analyze the elemental composition and chemical state of the modified membrane surface, to corroborate the successful grafting of quaternary ammonium groups, and to verify the accuracy of the free radical grafting reaction;

[0282] Example 1 was selected as the test subject;

[0283] Refer to GB / T 28898-2012 "General Rules for X-ray Photoelectron Spectroscopy in Surface Chemical Analysis":

[0284] An Al Kα ray source of 1486.6 eV was used, with a full spectrum scanning range of 0-1200 eV, and high-resolution scanning of C1s, F1s, N1s, and O1s was performed.

[0285] The chemical states and atomic percentages of each element were analyzed by peak fitting using XPS Peak 4.1 software.

[0286] For detailed test results, please see [link / details]. Figure 1 As shown.

[0287] according to Figure 1 XPS full-spectrum scanning results showed that four characteristic peaks, C1s, F1s, N1s and O1s, existed on the surface of the modified film.

[0288] Since the theoretical chemical composition of PVDF is (C2H2F2)n, which contains only C, H and F elements, the characteristic peaks of N and O elements are direct indicators of quaternary ammonium group grafting.

[0289] Specifically, the strong F1s peak at approximately 688 eV is a characteristic signal of the CF bond in the PVDF matrix, proving that the chemical structure of the PVDF membrane substrate was not destroyed by the modification reaction, thus ensuring the basic mechanical and separation performance of the membrane.

[0290] The combination of the N1s peak at approximately 400 eV corresponds to the quaternary ammonium cation (-N + The characteristic binding energy of (CH3)3) is that the typical binding energy range of N in quaternary ammonium salt is 399-401 eV. Since PVDF itself does not contain N, the appearance of this peak directly proves that the quaternary ammonium group has been successfully grafted onto the PVDF molecular chain through covalent bonds.

[0291] The O1s peak originates from the oxygen-containing functional group in the quaternary ammonium salt monomer; the C1s peak, in addition to the CC or CH bond corresponding to PVDF, also contains the CN bond formed after grafting, with a binding energy slightly higher than that of the pure CC bond, further supporting the free radical addition reaction to covalently bond the quaternary ammonium group with PVDF.

[0292] Test Example 6

[0293] This test case aims to analyze the types of membrane functional groups and changes in characteristic peaks, to corroborate the grafting reaction of quaternary ammonium groups, and to verify the regulation of the chemical structure of PVDF by the modification process.

[0294] Example 1 was selected as the test subject;

[0295] Refer to GB / T 6040-2002 "General Rules for Infrared Spectroscopic Analysis Methods":

[0296] KBr tablet compression method was used, with a scanning range of 400-4000 cm⁻¹. -1 4cm resolution -1 A total of 32 scans were performed;

[0297] Test results are available Figure 2 As shown.

[0298] according to Figure 2 FTIR spectral analysis revealed 761, 796, and 975 cm⁻¹ in the PVDF film spectrum. -1 Characteristic peaks, which are attributed to the α-phase of PVDF;

[0299] However, in the spectrum of the QPVDF film, the characteristic peaks of these α phases completely disappeared, indicating that the modification process may have destroyed or changed the structure of the α crystal phase.

[0300] A new absorption peak appeared in the spectrum of the QPVDF film, with a peak position of 1430 cm⁻¹. -1 This is attributed to the absorption of quaternary amine salts during CH bending vibrations.

[0301] The appearance of this peak indicates that the quaternary amine group of allyltrimethylammonium chloride has been successfully grafted onto the PVDF molecular chain.

[0302] Test Example 7

[0303] This test case aims to observe the surface morphology and cross-sectional microstructure of the modified membrane;

[0304] Example 1 was selected as the test subject;

[0305] Refer to GB / T 16594-2008 "General Rules for Scanning Electron Microscopy Measurement of Lengths in the Micrometer Scale";

[0306] Test results are available Figure 3 and Figure 4 As shown.

[0307] Figure 3The images show the surface morphology of PVDF membranes and modified PVDF membranes, where a and b represent the membranes before modification, and c and d represent the membranes after modification. The surface morphology of the membranes before and after modification does not change significantly.

[0308] Figure 4 This is an elemental diagram of C, N, and F elements in a selected region of the modified PVDF membrane. The uniformly distributed nitrogen element appears in the surface scan of the modified membrane, further indicating successful modification.

[0309] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A radical-grafted quaternized modified PVDF antibacterial nanofiltration membrane, characterized in that, The quaternary ammonium group is introduced into the molecular chain of polyvinylidene fluoride by a radical grafting reaction under hydrothermal conditions at 80-120 DEG C through a reaction system containing CuCl2 catalyst and quaternary ammonium monomer containing unsaturated double bond, with polyvinylidene fluoride as base material; The active site of the reaction is the CH bond adjacent to the CF2 group on the PVDF molecular chain. CuCl2 induces the CH bond to break through electron transfer to generate PVDF free radicals, and CuCl2 reacts during the catalytic process. 2+ To Cu + The valence state transformation occurs, and the quaternary ammonium group is ultimately connected to the PVDF molecular chain through a C-C covalent bond, so the reaction does not require the addition of an initiator. 2.The radical grafting quaternary ammonium modified PVDF antibacterial nanofiltration membrane according to claim 1, characterized in that: The quaternary ammonium monomer containing unsaturated double bond is allyl trimethyl ammonium chloride. 3.The radical grafting quaternary ammonium modified PVDF antibacterial nanofiltration membrane according to claim 1, characterized in that: The dissolution of PVDF and the CuCl2 catalyzed radical grafting reaction are both carried out in the same organic solvent system, and the casting solution containing PVDF-quaternary ammonium group graft copolymer can be directly formed without separating the intermediate product.

4. A method for preparing a radical-grafted quaternized modified PVDF antibacterial nanofiltration membrane, for preparing the radical-grafted quaternized modified PVDF antibacterial nanofiltration membrane according to any one of claims 1-3, characterized in that, The method comprises the following steps: S1: mixing PVDF powder, quaternary ammonium modified monomer and catalyst, then adding into organic solvent and stirring to obtain a mixed solution; S2: transferring the mixed solution in S1 into a closed reaction kettle and carrying out hydrothermal reaction; S3: pouring the cooled reaction liquid in S2 into a film, and obtaining the finished product after water immersion.

5. The method for preparing a radical grafted quaternized modified PVDF antibacterial nanofiltration membrane according to claim 4, characterized in that: The mass fraction of PVDF powder in the mixed solution in S1 is 10-20%, the mass ratio of quaternary ammonium modified monomer to PVDF powder is 1:300-1:30, and the mass ratio of catalyst to PVDF powder is 0.001:300-1:

300.

6. The method for preparing a radical grafted quaternized modified PVDF antibacterial nanofiltration membrane according to claim 4, characterized in that: The organic solvent in S1 is selected from one or more mixtures of N,N-dimethylacetamide, dimethylformamide, dimethyl sulfoxide and N-methyl pyrrolidone.

7. The method of claim 4, wherein the method further comprises the step of: adding a radical initiator to the solution of the PVDF and the quaternary ammonium salt. The hydrothermal reaction time in S2 is 24 h.

8. The method for preparing a radical grafted quaternized modified PVDF antibacterial nanofiltration membrane according to claim 4, characterized in that: The water immersion time in S3 is 24 h, and the finished product is soaked in 0.1-0.5 mol / L NaOH solution for 2-3 times, each time for 30 min.

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