Preparation method of a polytetrafluoroethylene-based hollow fiber nanofiltration membrane

Abstract

The present invention relates to the technical field of nanofiltration membrane preparation, and particularly to a preparation method of a hollow fiber nanofiltration membrane based on polytetrafluoroethylene. By treatments such as polydopamine (PDA) coating and plasma etching, the hydrophilicity of the base membrane is improved; and two interfacial polymerization reactions are carried out with amine and acyl chloride substances to construct a dense and defect-free secondary polyamide layer; a double polymerization reaction is adopted during the interfacial polymerization reaction. After the first polymerization reaction, defects are likely to appear in the polyamide thin film layer. After the second polymerization reaction, the defects of the surface separation layer in the first polymerization reaction can be compensated, so that the polyamide thin film layer has a better separation effect. Finally, through post-treatment, the crosslinking degree of the composite membrane is enhanced. At the same time, through the matching of the reaction effects during the secondary polymerization reaction and the etching process, while avoiding waste of reagents, the forming quality of the secondary polyamide layer is ensured. The nanofiltration membrane prepared by the present invention can be applied to the separation of strong organic solvents.

Description

Technical Field

[0001] The present invention relates to the technical field of nanofiltration membrane preparation, and particularly to a method for preparing a hollow fiber nanofiltration membrane based on polytetrafluoroethylene. Background Art

[0002] Due to ecological environment requirements, more efficient and environmentally friendly separation technologies are currently needed to replace traditional methods such as distillation and extraction, which are energy-consuming and may cause secondary pollution. In the field of separation, nanofiltration membranes have attracted much attention due to their unique filtration performance. Nanofiltration membranes can play a role in screening high- and low-valent ions. At the same time, they can retain large-particle organic substances, and can be effectively applied to remove organic substances and color, reduce the TDS concentration, and soften water quality. And in the field of drug synthesis, it is often necessary to process complex organic solutions. Through organic solvent nanofiltration, target molecules can be effectively separated, and the yield and purity can be improved. Organic solvent nanofiltration is a membrane process that uses a membrane with high chemical stability to treat a feed liquid containing an organic solvent. The molecular weight cut-off (MWCO) of the nanofiltration membrane is between 200 and 1000 Da. These organic solvent nanofiltration membranes can retain small molecules, while the solvent can permeate through the nanofiltration membrane to achieve effective molecular-level separation. The organic solvent nanofiltration process can be used for product purification, molecular separation, catalyst recovery, and solvent recovery, opening up remarkable opportunities for process intensification and organic waste management. In the pharmaceutical industry, organic solvent nanofiltration can be used between steps of drug synthesis or in downstream processing to reduce solvent waste, cost, and energy consumption. Moreover, organic solvent nanofiltration has great application prospects in the green and efficient separation and purification of organic solvents.

[0003] However, due to the obvious swelling or dissolution of most polymers in strong solvents (such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)), the application of polymer-based organic solvent nanofiltration membranes in strong solvents is severely restricted. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a hollow fiber nanofiltration membrane based on polytetrafluoroethylene to overcome the above deficiencies. Using chemically resistant polytetrafluoroethylene (PTFE) as a support substrate, and hydrophilically modifying the base membrane by polydopamine coating and plasma etching methods, and then obtaining a hollow fiber nanofiltration membrane capable of withstanding strong solvents through two interfacial polymerization techniques.

[0005] To this end, on the one hand, the present invention provides a method for preparing a hollow fiber nanofiltration membrane based on polytetrafluoroethylene, and the method for preparing the hollow fiber nanofiltration membrane based on polytetrafluoroethylene includes:

[0006] Step S1, placing the hollow fiber membrane support layer in acetone for ultrasonic cleaning, and then placing it in ethanol for ultrasonic cleaning;

[0007] Step S2: Immerse the obtained hollow fiber membrane support layer in a Tris-HCl buffer solution containing dopamine hydrochloride and perform an oscillating reaction to form a PDA coating on the surface of the hollow fiber membrane support layer;

[0008] Step S3: Etch the surface of the PDA coating by using an O2 plasma etching method to obtain a hollow fiber membrane substrate;

[0009] Step S4: Immerse the hollow fiber membrane substrate successively in an amine aqueous solution and an oil phase solution to carry out an interfacial polymerization reaction;

[0010] Step S5: Immerse the hollow fiber membrane substrate obtained after Step S4 successively in an amine aqueous solution and an oil phase solution with reduced concentrations for a secondary interfacial polymerization reaction;

[0011] Step S6: Wash the hollow fiber membrane substrate obtained after the secondary interfacial polymerization reaction in Step S5 with ultrapure water, place it in ultrapure water at 60 °C and soak for 5 - 10 min, then soak it in an aqueous solution containing glycerol for 1 - 2 min, and dry it to obtain the hollow fiber nanofiltration membrane;

[0012] Wherein, the hollow fiber membrane support layer is a polytetrafluoroethylene hollow fiber membrane with an average pore size of 50 nm - 200 nm and a porosity greater than 70%.

[0013] As a preferred technical solution of the preparation method of the hollow fiber nanofiltration membrane based on polytetrafluoroethylene, in the Step S4, the amine aqueous solution includes 2 wt% - 3 wt% of m-phenylenediamine, 2 wt% - 3 wt% of polyethyleneimine, 1 wt% - 4 wt% of dimethyl sulfoxide, 2 wt% of piperazine solution, and 0.01 wt% - 0.03 wt% of sodium hydroxide by mass percentage concentration, and the rest is water.

[0014] As a preferred technical solution of the preparation method of the hollow fiber nanofiltration membrane based on polytetrafluoroethylene, in the Step S4, the oil phase solution includes 0.1 wt% - 0.2 wt% of trimesoyl chloride and 0.06 wt% - 0.08 wt% of p-nitrobenzoyl chloride by mass percentage concentration, and the rest is n-hexane.

[0015] As a preferred technical solution of the preparation method of the hollow fiber nanofiltration membrane based on polytetrafluoroethylene, in the Step S5, the mass percentage concentrations of the amine aqueous solution and the oil phase solution are respectively 30% - 50% of those in S4.

[0016] As a preferred technical solution of the preparation method of the hollow fiber nanofiltration membrane based on polytetrafluoroethylene, in the Step S3, the duration of etching the surface of the PDA coating by using an O2 plasma etching method is 5 minutes - 10 minutes.

[0017] As a preferred technical solution of the preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane, in the step S3, before etching the surface of the PDA coating by using the O2 plasma etching method, the following steps are performed:

[0018] Step S31, performing plasma etching on the surface of the PDA coating with an inert gas for 0.5 min;

[0019] Step S32, performing plasma etching on the surface of the PDA coating with O2 for 0.5 min;

[0020] Step S33, respectively detecting the mass change of the hollow fiber membrane support layer in the step S31 and the step S32, and taking the ratio of the difference between the mass change amount of the hollow fiber membrane support layer in the step S32 and the mass change amount of the hollow fiber membrane support layer in the step S31 to the mass of the hollow fiber membrane before performing the step S31 as the chemical contribution degree of the O2 plasma etching.

[0021] As a preferred technical solution of the preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane, in the step S5, the mass percentage concentrations of the amine aqueous solution and the oil phase solution are selected based on the chemical contribution degree of the O2 plasma etching;

[0022] Wherein, the mass percentage concentrations of the amine aqueous solution and the oil phase solution in the step S5 are negatively correlated with the chemical contribution degree of the O2 plasma etching.

[0023] As a preferred technical solution of the preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane, in the step S2, the concentration of dopamine hydrochloride is 2 mg / mL to 4 mg / mL, and the pH value of the Tris-HCl buffer solution is 8.5.

[0024] As a preferred technical solution of the preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane, in the step S2, the reaction temperature of the oscillating reaction is 25 °C, and the duration is 6 h to 12 h.

[0025] As a preferred technical solution of the preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane, in the step S4 and the step S5, the soaking duration of the hollow fiber membrane substrate in the amine aqueous solution is 1 min, and after draining the water droplets on the membrane surface, the soaking duration in the oil phase solution is 0.5 min.

[0026] The beneficial effects of the present invention are:

[0027] The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane of the present invention adopts a double polymerization reaction during the interfacial polymerization reaction. After the first polymerization reaction, defects are likely to appear in the polyamide thin film layer. After the second polymerization reaction, the defects of the surface separation layer in the first polymerization reaction can be compensated, enabling the polyamide thin film layer to have a better separation effect.

[0028] Furthermore, the effective ratio of m-phenylenediamine, polyethyleneimine, dimethyl sulfoxide in the aqueous phase and trimesoyl chloride, p-nitrobenzoyl chloride in the oil phase makes the surface contact angle of the prepared nanofiltration membrane smaller, thereby increasing the membrane flux. At the same time, the resistance to inorganic pollution is stronger, and further reducing the problem of flux attenuation during the operation of the membrane.

[0029] Furthermore, by adding a small molecule piperazine monomer, the present invention can reduce the retention molecular weight of the prepared separation membrane to below 300 Da.

[0030] Furthermore, by combining the dynamic concentration adjustment of the chemical contribution degree of O2 preparation for plasma etching, the present invention realizes the matching of the reaction effects during the secondary polymerization reaction and the etching process, ensuring the forming quality of the secondary polyamide layer while avoiding reagent waste.

[0031] Furthermore, due to the use of PTFE as the base membrane, the hollow fiber nanofiltration membrane obtained in the present invention has significantly improved organic solution resistance compared with traditional nanofiltration membranes. Description of the Drawings

[0032] Figure 1 It is a flowchart of the preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane in the embodiment of the present invention;

[0033] Figure 2 It is a flowchart of determining the chemical contribution degree in the embodiment of the present invention;

[0034] Figure 3 It is a bar chart of the contact angle test results in the embodiment of the present invention;

[0035] Figure 4 It is a bar chart of the permeability test results in the embodiment of the present invention;

[0036] Figure 5 It is a bar chart of the rejection rate test results in the embodiment of the present invention;

[0037] Figure 6 It is a bar chart of the rejection rate decay test results in the embodiment of the present invention. Detailed Embodiments

[0038] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0039] The embodiments of the present invention will be described in detail below. The examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals represent the same or similar elements or elements with the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present invention, and should not be construed as a limitation of the present invention.

[0040] As Figure 1 shown, the present invention provides a preparation method of a hollow fiber nanofiltration membrane based on polytetrafluoroethylene. The preparation method of the hollow fiber nanofiltration membrane based on polytetrafluoroethylene includes:

[0041] Pretreatment of the hollow fiber membrane support layer:

[0042] Step S1, placing the hollow fiber membrane support layer in acetone for ultrasonic cleaning, and then in ethanol for ultrasonic cleaning to remove surface impurities;

[0043] Hydrophilic modification:

[0044] Step S2, immersing the hollow fiber membrane support layer obtained in step S1 in a Tris-HCl buffer solution containing dopamine hydrochloride for oscillating reaction to form a PDA coating on the surface of the hollow fiber membrane support layer;

[0045] Step S3, etching the surface of the PDA coating by using an O2 plasma etching method to obtain a hollow fiber membrane substrate, generating a micro-nano rough structure and introducing oxygen-containing groups to improve the hydrophilicity of the PTFE substrate membrane;

[0046] Preparation of the desalination layer:

[0047] Step S4, sequentially immersing the hollow fiber membrane substrate in an amine aqueous solution and an oil phase solution for interfacial polymerization reaction to form a primary polyamide layer;

[0048] Step S5, sequentially immersing the hollow fiber membrane substrate obtained after step S4 in an amine aqueous solution and an oil phase solution with a reduced concentration for a secondary interfacial polymerization reaction to construct a dense and less defective secondary polyamide layer;

[0049] Post-treatment:

[0050] Step S6: Wash the hollow fiber membrane substrate obtained in step S5 after the secondary interfacial polymerization reaction with ultrapure water, soak it in ultrapure water at 60°C for 5 - 10 min, then soak it in an aqueous solution containing glycerol for 1 - 2 min, and dry it to obtain a hollow fiber nanofiltration membrane;

[0051] Among them, the hollow fiber membrane support layer is a polytetrafluoroethylene (PTFE) hollow fiber membrane with an average pore diameter of 50 nm - 200 nm and a porosity greater than 70%.

[0052] In the above embodiment, since PTFE is used as the substrate membrane, compared with traditional nanofiltration, its ability to resist organic solutions is significantly improved. A double polymerization reaction is used in the interfacial polymerization reaction. After the first polymerization reaction, defects are likely to appear in the polyamide thin film layer. After the second polymerization reaction, the defects in the surface separation layer of the first polymerization reaction can be compensated, making the polyamide thin film layer have a better separation effect.

[0053] Specifically, in step S4, the amine aqueous solution includes 2 wt% - 3 wt% of m-phenylenediamine, 2 wt% - 3 wt% of polyethyleneimine, 1 wt% - 4 wt% of dimethyl sulfoxide, 2 wt% of piperazine solution, and 0.01 wt% - 0.03 wt% of sodium hydroxide by mass percentage concentration, and the rest is water. The oil phase solution includes 0.1 wt% - 0.2 wt% of trimesoyl chloride and 0.06 wt% - 0.08 wt% of p-nitrobenzoyl chloride by mass percentage concentration, and the rest is n-hexane. The effective ratio of m-phenylenediamine, polyethyleneimine, and dimethyl sulfoxide in the aqueous phase and trimesoyl chloride and p-nitrobenzoyl chloride in the oil phase in the above scheme makes the surface contact angle of the prepared nanofiltration membrane smaller, so that the flux of the membrane is larger. At the same time, it has stronger resistance to inorganic pollution, thereby reducing the flux attenuation problem of the membrane during operation. By adding a small molecule piperazine monomer, the retention molecular weight of the prepared separation membrane can be reduced to below 300 Da.

[0054] Specifically, in step S5, the mass percentage concentrations of the amine aqueous solution and the oil phase solution are respectively 30% - 50% of those in S4.

[0055] Specifically, in step S3, the duration of etching the PDA coating surface by the O2 plasma etching method is 5 - 10 minutes.

[0056] Specifically, in step S2, the concentration of dopamine hydrochloride is 2 mg / mL - 4 mg / mL, and the pH value of the Tris-HCl buffer solution is 8.5.

[0057] Specifically, in step S2, the reaction temperature of the oscillation reaction is 25°C, and the duration is 6 h - 12 h.

[0058] Specifically, in steps S4 and S5, the immersion duration of the hollow fiber membrane substrate in the amine aqueous solution is 1 min for both. After draining the water droplets on the membrane surface, the immersion duration in the oil phase solution is 0.5 min for both.

[0059] As Figure 2 shown, in step S3, before etching the surface of the PDA coating using the O2 plasma etching method, the following steps are performed:

[0060] Step S31, perform plasma etching on the surface of the PDA coating with an inert gas for 0.5 min;

[0061] Step S32, perform plasma etching on the surface of the PDA coating with O2 for 0.5 min;

[0062] Step S33, respectively detect the mass change of the hollow fiber membrane support layer in steps S31 and S32. Take the ratio of the difference between the mass change amount of the hollow fiber membrane support layer in step S32 and the mass change amount of the hollow fiber membrane support layer in step S31 to the mass of the hollow fiber membrane before performing step S31 as the chemical contribution degree of the O2 plasma etching.

[0063] Specifically, in step S5, the mass percentage concentrations of the amine aqueous solution and the oil phase solution are selected based on the chemical contribution degree of the O2 plasma etching;

[0064] Among them, the mass percentage concentrations of the amine aqueous solution and the oil phase solution in step S5 are negatively correlated with the chemical contribution degree of the O2 plasma etching.

[0065] In detail, the operation process of the O2 plasma etching is as follows: Turn on the power supply, increase the power of the plasma etching machine to the set value (80 W in this embodiment), set the processing time. During this period, the active components (O + radicals, O - radicals) in the plasma react with the surface of the PDA coating, including: (1) Physical sputtering: High-energy ions impact the surface to remove weakly bound impurities; (2) Chemical etching: Oxygen radicals react with the C-F bonds in PTFE to generate volatile products such as CO and CO2, and at the same time introduce oxygen-containing groups (-COOH, -OH).

[0066] The purpose of determining the chemical contribution is to exclude physical factors and evaluate the effect of O2 plasma etching on the PDA coating at the chemical etching level. During implementation, the theoretical range of the chemical contribution can be determined through a limited number of experiments. On the premise of meeting the negative correlation, a mapping relationship between the chemical contribution and the concentration range of 30% - 50% is established, and selection can be made based on this mapping relationship during implementation. By combining the dynamic concentration adjustment of the chemical contribution of O2 plasma etching, the matching of the secondary polymerization reaction and the reaction effect during the etching process is achieved, ensuring the forming quality of the secondary polyamide layer while avoiding reagent waste.

[0067] Example 1: (1) Pretreatment of the PTFE hollow fiber membrane support layer: Take a PTFE hollow fiber membrane (average pore size 100 nm, porosity > 70%), and ultrasonically clean it with acetone and ethanol for 30 minutes in sequence.

[0068] (2) Hydrophilic modification: Hydrophilically modify the hollow fiber membrane obtained in step (1). Immerse the PTFE membrane in a Tris-HCl buffer solution (pH = 8.5) containing dopamine hydrochloride (2 mg / mL), and react with shaking at 25 °C for 6 hours to form a uniform PDA layer. Use O2 plasma (power 80 W, treatment time 5 minutes) to etch the surface of the PDA coating to generate a micro-nano rough structure and introduce oxygen-containing groups.

[0069] (3) Prepare the amine aqueous solution: Add 2 wt% m-phenylenediamine, 2 wt% polyethyleneimine, 1 wt% dimethyl sulfoxide, 2 wt% piperazine solution, and 0.01 wt% sodium hydroxide to ultrapure water, and stir until completely dissolved to obtain the amine aqueous solution.

[0070] (4) Prepare the oil-phase solution: Dissolve 0.1 wt% trimesoyl chloride and 0.06 wt% p-nitrobenzoyl chloride in n-hexane, and stir until dissolved to obtain the oil-phase solution.

[0071] (5) Prepare the desalination layer: Immerse the hollow fiber-based membrane prepared in step (2) in the amine aqueous solution for 1 minute, then drain the water droplets on the membrane surface. Then immerse it in the oil-phase solution for 0.5 minute for an interfacial reaction.

[0072] (6) Conduct a secondary interfacial reaction, but reduce the monomer concentration. The monomer concentration is 50% of the first addition amount to construct a secondary polyamide layer.

[0073] (7) Post-treatment: Take out the nanofiltration membrane prepared in step (4), wash it with ultrapure water, then immerse it in hot water at 60 °C for heat treatment for 5 minutes, wash it with pure water, then soak it in an aqueous solution containing glycerol for 1 minute, and then dry it to obtain a nanofiltration membrane that can withstand high-concentration organic solvents.

[0074] Example 2:

[0075] (1) Take a PTFE hollow fiber membrane (average pore size 100 nm, porosity > 70%), and ultrasonically clean it with acetone and ethanol for 30 min in sequence.

[0076] (2) Hydrophilically modify the hollow fiber membrane obtained in step (1). Immerse the PTFE membrane in a Tris-HCl buffer solution (pH = 8.5) containing dopamine hydrochloride (3 mg / mL), and react with shaking at 25 °C for 8 h. Etch the surface of the PDA coating with O2 plasma (power 80 W, treatment time 8 min) to generate a micro-nano rough structure and introduce oxygen-containing groups.

[0077] (3) Add 2 wt% m-phenylenediamine, 2 wt% polyethyleneimine, 1 wt% dimethyl sulfoxide, 2 wt% piperazine solution, and 0.01 wt% sodium hydroxide to ultrapure water, and stir to dissolve completely to obtain an amine aqueous solution.

[0078] (4) Dissolve 0.1 wt% trimesoyl chloride and 0.06 wt% p-nitrobenzoyl chloride in n-hexane, and stir to dissolve to obtain an oil-phase solution.

[0079] (5) Immerse the hollow fiber-based membrane prepared in step (2) in the amine aqueous solution for 1 min, then drain the water droplets on the membrane surface. Then immerse it in the oil-phase solution for 0.5 min for an interfacial reaction.

[0080] (6) Conduct a secondary interfacial reaction, but reduce the monomer concentration. The monomer concentration is 50% of the first addition amount to construct a secondary polyamide layer.

[0081] (7) Post-treatment: Take out the nanofiltration membrane prepared in step (4), wash it with ultrapure water, then immerse it in hot water at 60 °C for heat treatment for 5 min, wash it with pure water, then immerse it in an aqueous solution containing glycerol for 1 min, and then dry it.

[0082] Example 3:

[0083] (1) Take a PTFE hollow fiber membrane (average pore size 100 nm, porosity > 70%), and ultrasonically clean it with acetone and ethanol for 30 min in sequence.

[0084] (2) Hydrophilically modify the hollow fiber membrane obtained in step (1). Immerse the PTFE membrane in a Tris-HCl buffer solution (pH = 8.5) containing dopamine hydrochloride (4 mg / mL), and react with shaking at 25 °C for 12 h to form a uniform PDA layer. Etch the surface of the PDA coating with O2 plasma (power 80 W, treatment time 10 min) to generate a micro-nano rough structure and introduce oxygen-containing groups.

[0085] (3) Add 2 wt% m-phenylenediamine, 2 wt% polyethyleneimine, 1 wt% dimethyl sulfoxide, 2 wt% piperazine solution, and 0.01 wt% sodium hydroxide to ultrapure water, and stir until completely dissolved to obtain an amine aqueous solution.

[0086] (4) Dissolve 0.1 wt% trimesoyl chloride and 0.06 wt% p-nitrobenzoyl chloride in n-hexane, and stir until dissolved to obtain an oil-phase solution.

[0087] (5) Immerse the hollow fiber-based membrane prepared in step (2) in the amine aqueous solution for 1 min, then drain the water droplets on the membrane surface; then immerse it in the oil-phase solution for 0.5 min for an interfacial reaction.

[0088] (6) Perform a secondary interfacial reaction, but reduce the monomer concentration, and the monomer concentration is 50% of the first addition amount to construct a secondary polyamide layer;

[0089] (7) Post-treatment: Take out the nanofiltration membrane prepared in step (4), wash it with ultrapure water, then immerse it in hot water at 60 °C for heat treatment for 5 min, wash it with pure water, then immerse it in an aqueous solution containing glycerol for 1 min, and then dry it.

[0090] Example 4:

[0091] (1) Take a PTFE hollow fiber membrane (average pore size 100 nm, porosity > 70%), and ultrasonically clean it with acetone and ethanol for 30 min in sequence;

[0092] (2) Perform hydrophilic modification on the hollow fiber membrane obtained in step (1). Immerse the PTFE membrane in a Tris-HCl buffer solution (pH = 8.5) containing dopamine hydrochloride (4 mg / mL), and react with shaking at 25 °C for 12 h to form a uniform PDA layer; use O2 plasma (power 80 W, treatment time 10 min) to etch the surface of the PDA coating to generate a micro-nano rough structure and introduce oxygen-containing groups;

[0093] (3) Add 3 wt% m-phenylenediamine, 3 wt% polyethyleneimine, 4 wt% dimethyl sulfoxide, 2 wt% piperazine solution, and 0.03 wt% sodium hydroxide to ultrapure water, and stir until completely dissolved to obtain an amine aqueous solution.

[0094] (4) Prepare an oil-phase solution: Dissolve 0.2 wt% trimesoyl chloride and 0.08 wt% p-nitrobenzoyl chloride in n-hexane, and stir until dissolved to obtain an oil-phase solution.

[0095] (5) Immerse the hollow fiber-based membrane prepared in step (2) in the amine aqueous solution for 1 min, then drain the water droplets on the membrane surface; then immerse it in the oil-phase solution for 0.5 min for an interfacial reaction.

[0096] (6) Perform a secondary interfacial reaction, but reduce the monomer concentration. The monomer concentration is 50% of the first addition amount to construct a secondary polyamide layer.

[0097] (7) Post-treatment: Take out the nanofiltration membrane prepared in step (4), wash it with ultrapure water, then immerse it in hot water at 60 °C for heat treatment for 5 min, wash it with pure water, then soak it in an aqueous solution containing glycerol for 1 min, and then dry it to obtain a nanofiltration membrane that can withstand high-concentration organic solvents.

[0098] For Examples 1-3 in the above embodiments, measure the change in their contact angles. The test results are as Figure 3 shown. After the modification treatment, the contact angle of the PTFE-based membrane is significantly reduced, and it has good hydrophilicity after modification.

[0099] For Example 4 in the above embodiments, measure the permeabilities of methanol, acetonitrile, and dimethylformamide. At the same time, measure its rejection rate for Congo red (MW = 697 Da). The specific results are as Figure 4 and Figure 5 shown. The synthesized nanofiltration membrane has very high permeabilities for methanol, acetonitrile, and dimethylformamide, which are 15, 8.2, and 4.1 (unit: Lm -2 h -1 bar -1 ), respectively. At the same time, the rejection rates for Congo red (MW = 697 Da) are 95%, 93%, and 92%, respectively, all reaching over 90%.

[0100] Further, immerse the hollow fiber nanofiltration membrane prepared in Example 4 in a DMF organic solution, and measure the rejection rate of the nanofiltration membrane for Congo red once a day to observe the attenuation of its rejection rate. The measurement results are as Figure 6 shown (the abscissa is time, and the ordinate is the rejection rate). When the nanofiltration membrane is immersed in the DMF organic solution for 7 days, its rejection rate for Congo red decreases from 92% to 85%, and the attenuation rate is 7.6%, indicating that it has strong resistance to organic solvents.

[0101] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limiting the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A preparation method of a polytetrafluoroethylene-based hollow fiber nanofiltration membrane, characterized in that Including: Step S1: Place the hollow fiber membrane support layer in acetone for ultrasonic cleaning, and then place it in ethanol for ultrasonic cleaning; Step S2: Immerse the hollow fiber membrane support layer obtained in Step S1 in a Tris-HCl buffer solution containing dopamine hydrochloride and perform an oscillating reaction to form a PDA coating on the surface of the hollow fiber membrane support layer; Step S3: Use the O2 plasma etching method to etch the surface of the PDA coating to obtain a hollow fiber membrane substrate; In Step S3, before using the O2 plasma etching method to etch the surface of the PDA coating, perform the following steps: Step S31: Use an inert gas to perform plasma etching on the surface of the PDA coating for 0.5 min; Step S32: Use O2 to perform plasma etching on the surface of the PDA coating for 0.5 min; Step S33: Detect the mass changes of the hollow fiber membrane support layer in Step S31 and Step S32 respectively. Take the difference between the mass change amount of the hollow fiber membrane support layer in Step S32 and the mass change amount of the hollow fiber membrane support layer in Step S31, and divide it by the mass of the hollow fiber membrane before Step S31 as the chemical contribution degree of O2 plasma etching; Step S4: Immerse the hollow fiber membrane substrate in an amine aqueous solution and an oil phase solution in sequence to perform an interfacial polymerization reaction; Step S5: Immerse the hollow fiber membrane substrate obtained after Step S4 in an amine aqueous solution and an oil phase solution with reduced concentrations in sequence to perform a secondary interfacial polymerization reaction; In Step S5, the mass percentage concentrations of the amine aqueous solution and the oil phase solution are selected based on the chemical contribution degree of the O2 plasma etching; Among them, the mass percentage concentrations of the amine aqueous solution and the oil phase solution in Step S5 are negatively correlated with the chemical contribution degree of the O2 plasma etching; Step S6: Wash the hollow fiber membrane substrate obtained after the secondary interfacial polymerization reaction in Step S5 with ultrapure water, place it in ultrapure water at 60 °C and soak for 5-10 min, then soak it in an aqueous solution containing glycerol for 1-2 min, and dry it to obtain the hollow fiber nanofiltration membrane; Among them, the hollow fiber membrane support layer is a polytetrafluoroethylene hollow fiber membrane with an average pore diameter of 50 nm to 200 nm and a porosity greater than 70%; 2. The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane according to claim 1, wherein, In Step S4, the amine aqueous solution includes 2 wt% to 3 wt% of m-phenylenediamine, 2 wt% to 3 wt% of polyethyleneimine, 1 wt% to 4 wt% of dimethyl sulfoxide, 2 wt% of piperazine solution, and 0.01 wt% to 0.03 wt% of sodium hydroxide by mass percentage concentration, and the rest is water; 3. The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane according to claim 2, characterized in that In Step S4, the oil phase solution includes 0.1 wt% to 0.2 wt% of trimesoyl chloride and 0.06 wt% to 0.08 wt% of p-nitrobenzoyl chloride by mass percentage concentration, and the rest is n-hexane; 4. The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane according to claim 3, characterized in that, In Step S5, the mass percentage concentrations of the amine aqueous solution and the oil phase solution are 30% to 50% of those in S4 respectively.

5. The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane according to claim 3, characterized in that, In the step S3, the duration of etching the surface of the PDA coating by using the O2 plasma etching method is 5 minutes to 10 minutes.

6. The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane according to claim 1, wherein, In the step S2, the concentration of dopamine hydrochloride is 2 mg / mL to 4 mg / mL, and the pH value of the Tris-HCl buffer solution is 8.

5.

7. The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane according to claim 6, wherein In the step S2, the reaction temperature of the oscillating reaction is 25 °C, and the duration is 6 h to 12 h.

8. The preparation method of the polytetrafluoroethylene-based hollow fiber nanofiltration membrane according to claim 1, wherein, In the step S4 and the step S5, the soaking duration of the hollow fiber membrane substrate in the amine aqueous solution is 1 min for both. After draining the water droplets on the membrane surface, the soaking duration in the oil phase solution is 0.5 min for both.

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