High surface open pore asymmetric membrane and method for making same

Abstract

The application discloses a kind of asymmetric membranes of high surface porosity and preparation method thereof, preparation method includes the following steps: first, uniform polymer solution is prepared, the polymer solution includes polymer raw material, good solvent, non-solvent additive, pore former and non-ionic surfactant;Then, the asymmetric membrane is obtained by the method of multiphase induced phase inversion, the multiphase induced phase inversion includes gas phase induced phase inversion and immersion precipitation phase inversion;Another aspect of the application discloses a kind of asymmetric membranes of high surface porosity, including high-porosity surface layer, dense intermediate layer and loose sublayer;Wherein high-porosity surface layer presents network-like pore structure, porosity is 30-38.25%, and channel penetrability is good;The preparation method of the application can significantly improve the surface porosity of membrane, optimize the morphology and pore size distribution of membrane, and then improve its flux performance, the overall anti-pollution of membrane is more excellent, suitable for processing complex feed liquid.

Description

Technical Field

[0001] This invention relates to the field of membrane materials technology, specifically to an asymmetric membrane with high surface porosity and its preparation method. Background Technology

[0002] In pharmaceutical research and production, the use of filter membranes is indispensable for both the dissolution of solid dosage forms and the filtration and sterilization of liquid dosage forms. Microfiltration membranes are widely used in the separation of biochemical products, frequently involving the filtration of protein-containing solutions, including initial clarification and aseptic filtration. In these operations, proteins must be transferred through the filter membrane while retaining other substances (bacteria, suspended solids, cells, cell debris, and large molecular impurities). Generally speaking, 0.2μm microporous filter membranes can retain 99.99% of bacteria and microorganisms, meeting GMP or pharmacopoeia sterilization requirements. They can also be used to remove extremely fine particles from samples and mobile phases.

[0003] Protein fouling remains a major problem in microfiltration. Due to specific interactions between proteins and the membrane, protein fouling can lead to irreversible degradation of membrane performance. Extensive experimental evidence suggests that protein fouling is primarily influenced by the physical and chemical properties of the microfiltration membrane. Most research focuses on hydrophilic modification of the membrane, as hydrophilic microfiltration membranes exhibit weaker protein adsorption. Regarding membrane structure, studies have confirmed that fouling in membranes with straight pores is caused by pore blockage due to the deposition of large amounts of protein aggregates on the membrane surface; while membranes with interconnected pores exhibit stronger antifouling properties, meaning slower fouling, because fluid can bypass blocked pores through the interconnected pore structure. The flux decline rate of membranes with interconnected pore structures is much smaller than that of membranes with straight pores.

[0004] Therefore, how to improve the water flux of polymer porous membranes while meeting the requirements of membrane pore size and mechanical properties, and how to improve the membrane's ability to treat complex liquids by optimizing membrane morphology, is an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide an asymmetric membrane with high surface porosity and its preparation method. By introducing a nonionic surfactant into the casting solution formulation, and utilizing its amphiphilic structure with hydrophilic ends and hydrophobic chain parts, water is more efficiently induced to enter the casting solution system through the interaction force between the hydrophilic ends and water molecules, thereby playing the role of interfacial opening. This results in the preparation of a porous membrane with higher surface porosity, better pore size distribution, and more complete interconnection between pores.

[0006] Specifically, when a casting solution containing surfactant comes into contact with water, in order to reduce interfacial free energy, the surfactant molecules in the casting solution will form an oriented arrangement at the interface with their hydrophilic ends facing outwards and their hydrophobic ends facing the polymer raw material molecules. At this point, the surfactant acts as an interfacial mediator between water and the casting solution. The hydrophilic ends of the surfactant induce water to enter the casting solution, thus enhancing the diffusion between water and the casting solution. The channels through which water is induced into the casting solution by the hydrophilic ends of the surfactant form the channels of the membrane pores. Therefore, the surface porosity of the resulting membrane is significantly improved.

[0007] To achieve the above objectives, the present invention provides a method for preparing an asymmetric membrane with high surface porosity, comprising the following steps: first, preparing a homogeneous polymer solution, wherein the polymer solution includes polymer raw materials, a good solvent, a non-solvent additive, a pore-forming agent, and a nonionic surfactant; and then obtaining the asymmetric membrane by a multiphase induced phase transformation method, wherein the multiphase induced phase transformation includes gas-phase induced phase transformation and immersion precipitation phase transformation.

[0008] The polymer raw material includes at least one of polysulfone, polyethersulfone, modified polysulfone, and modified polyethersulfone.

[0009] In the polymer solution:

[0010] The polymer raw material has a mass percentage content of 13-17%;

[0011] The nonionic surfactant has a mass percentage content of 0.5-4.5%.

[0012] Preferably, before preparing the polymer solution, the method further includes: determining the maximum homogeneous addition amount of the non-solvent additive by changing the amount of the non-solvent additive added to the polymer solution, i.e., the maximum addition amount that maintains the solution system without phase separation (turbidity); when preparing the polymer solution, the amount of the non-solvent additive added is between 70% and 80% of the maximum homogeneous addition amount.

[0013] Preferably, the good solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; the porogen includes at least one of polyethylene glycol, lithium chloride, and polyvinylpyrrolidone; the non-solvent additive includes at least one of water, ethanol, ethylene glycol, 1,2-propanediol, and 1,3-propanediol; and the nonionic surfactant includes at least one of alkyl glucoside (APG), fatty acid glycerides, fatty acid sorbitan (Span 20 / 40 / 80, etc.), polyoxyethylene dehydrated sorbitan fatty acid esters (Tween 20 / 40 / 80, etc.), alkylphenol polyoxyethylene ethers (OP-10, etc.), and fatty alcohol polyoxyethylene ethers (AEO-9, etc.).

[0014] Preferably, in the polymer solution:

[0015] The pore-forming agent has a mass percentage content of 2-10%;

[0016] The non-solvent additive has a mass percentage content of 12-25%;

[0017] The mass percentage of the good solvent is 45-67%.

[0018] Preferably, the preparation method specifically comprises:

[0019] S1: Preparation of a homogeneous polymer solution: The polymer raw material, pore-forming agent and nonionic surfactant are dissolved in a good solvent at 60-80℃, and then a non-solvent additive is added. After mixing evenly, the polymer solution is obtained by vacuum degassing.

[0020] S2: Coating: The polymer solution is coated onto a film-forming substrate to form a thin layer solution.

[0021] S3: Gas-phase induced phase transformation: The polymer solution forming the thin layer is placed in a temperature and humidity chamber and left for a preset time. The temperature in the temperature and humidity chamber is 15-40℃ and the humidity is 50-90%RH. The preset time is 15-60s.

[0022] S4: Immersion Precipitation Phase Transformation: The polymer solution that has undergone gas-phase induced phase transformation is immersed in a coagulation bath. After the phase transformation is complete, the film substrate is peeled off to obtain an asymmetric film. The temperature of the coagulation bath is 40-60℃. Preferably, the coagulation bath is a pure water coagulation bath.

[0023] S5: Dry the asymmetric film by passing it through a hot roller at 100-150℃.

[0024] Another aspect of the present invention provides an asymmetric film with high surface porosity, comprising a highly porous surface layer, a dense trapping intermediate layer, and a porous sublayer.

[0025] The asymmetric membrane is made of at least one of polysulfone, polyethersulfone, modified polysulfone, and modified polyethersulfone; the average pore size of the asymmetric membrane is 0.18-0.23 μm; the highly open-pore surface layer exhibits a network-like pore structure with a surface porosity of 30-38.25% and good pore connectivity; the dense trapping intermediate layer accounts for 25-35% of the overall membrane thickness in cross-section.

[0026] Preferably, the asymmetric film is prepared by the aforementioned preparation method.

[0027] Compared with existing technologies, it has the following beneficial effects:

[0028] 1. The preparation method of this invention involves adding a nonionic surfactant to the casting solution, which acts as an interfacial mediator. The surfactant molecules, through their hydrophilic ends, more efficiently induce water into the casting solution. During the formation of the membrane skin, this more efficiently opens the bidirectional diffusion channels between the good solvent and water in the casting solution, promoting dual diffusion and significantly increasing the surface porosity of the membrane (up to 38.25%). This optimizes the membrane morphology and pore size distribution, thereby improving its flux performance. Simultaneously, the addition of a small amount of ionic surfactant can greatly reduce the amount of pore-forming agent used, effectively lowering costs and providing a price advantage in industrial production.

[0029] 2. In the preparation method of the present invention, by determining the maximum homogeneous addition amount of the non-solvent additive, the addition amount of the non-solvent additive is adjusted to 70-80% of the maximum homogeneous addition amount, and the phase separation tendency of the casting solution is controlled to achieve the best, which is more conducive to the control of process parameters (phase transformation temperature, humidity, time, etc.). The asymmetric film with high surface porosity obtained in the end also has better batch-to-batch stability.

[0030] 3. The asymmetric membrane with high surface porosity of the present invention has a surface porosity of up to 38.25%, presenting a network structure with better membrane pore connectivity and better overall antifouling properties. It can still maintain high flux and precise retention when processing some complex liquids. Attached Figure Description

[0031] Figure 1 The image shown is a scanning electron microscope (SEM) image of the compact surface of the asymmetric film in Example 1, at a magnification of 5.00k.

[0032] Figure 2 The image shown is a longitudinal cross-sectional scanning electron microscope (SEM) image of the asymmetric membrane in Example 1, at a magnification of 500.

[0033] Figure 3 The image shows a scanning electron microscope (SEM) image of the compact surface of the asymmetric film in Comparative Example 1, at a magnification of 5.00 k.

[0034] Figure 4 The image shown is a scanning electron microscope (SEM) image of the compact surface of the asymmetric film in Comparative Example 2, at a magnification of 5.00 kJ.

[0035] Figure 5 This is a schematic diagram of the preparation process according to an embodiment of the present invention. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Example 1

[0038] Figure 5 The preparation process of this embodiment of the invention is shown below.

[0039] S1: Preparation of a homogeneous polymer solution (casting solution): Mix 15 parts polyethersulfone (total of all components of the casting solution is 100 parts by mass), 1.5 parts polyvinylpyrrolidone, 0.5 parts lithium chloride, 1.2 parts Tween 80 and 64.5 parts N-methylpyrrolidone, stir at 60-80℃ until completely dissolved, cool to below 40℃, add 2 parts pure water and 15.3 parts 1,3-propanediol and continue stirring until clear. Obtain a homogeneous and stable casting solution by vacuum degassing.

[0040] S2: Film coating: The casting solution is coated onto the film-forming substrate using a stainless steel doctor blade (320μm feeler gauge).

[0041] S3: Gas-phase induced phase transformation: The casting solution after coating is placed in a temperature and humidity chamber at 25℃ and 80%RH for 45s. During the gas-phase induced phase transformation, water molecules in the gas phase come into contact with and penetrate the casting solution, increasing the proportion of non-solvent additives on the surface of the casting solution and enhancing the longitudinal asymmetry of the film formation.

[0042] S4: Immersion Precipitation Phase Transformation: The casting solution, after gas-phase induced phase transformation, is immersed in a 50°C pure water coagulation bath until the phase transformation is complete. When the casting solution containing surfactant comes into contact with the coagulation bath, in order to reduce the interfacial free energy, the surfactant molecules in the casting solution will form an oriented arrangement at the interface with their hydrophilic ends facing outwards and their hydrophobic ends facing the polymer raw material molecules. At this time, the surfactant acts as an interfacial mediator between water and the casting solution. The hydrophilic ends of the surfactant induce water to enter the casting solution, thus enhancing the diffusion between the coagulation bath and the casting solution. The channels through which water is induced into the casting solution by the hydrophilic ends of the surfactant will form channels for membrane pores. Ionic surfactants have low solubility in the casting solution, resulting in a stronger tendency for phase separation in the casting solution, and are more likely to generate through-pores during the phase transformation process, which is not conducive to improving the antifouling performance of the membrane. Therefore, in this embodiment of the invention, a nonionic surfactant is added to the casting solution to improve the surface porosity of the prepared membrane. After the phase transformation is complete, the film substrate is peeled off.

[0043] S5: Drying: The asymmetric film is dried by passing it through a 125℃ hot roller.

[0044] Figure 2 The microstructure of the longitudinal section of the asymmetric membrane of this embodiment is shown, including a highly porous surface layer (i.e., the surface layer containing the compact surface), a dense trapping intermediate layer, and a loose sublayer. The dense trapping intermediate layer has a more compact microstructure and smaller pores, effectively trapping particles; its thickness accounts for approximately 25-35% of the total membrane thickness. The loose sublayer has a more porous microstructure and larger pores, which can prevent larger particles from blocking the pores and reduce pore blockage. For asymmetric membranes prepared by the phase inversion method, the microstructure gradually transitions from the dense trapping intermediate layer to the loose sublayer without a clear boundary.

[0045] Figure 1 The compact surface of the asymmetric film in this embodiment is shown (in Figure 2 The microstructure of the polymer material (corresponding to the upper surface) is constrained by the surfactant molecules before phase separation, which makes the polymer material molecules exhibit a more regular network structure after phase separation. This is beneficial for the asymmetric membrane to improve its surface porosity when it has the same average pore size (effective pore size cut-off), and to prepare porous membranes with higher surface porosity, better pore size distribution, and more sufficient interconnection between pores.

[0046] The asymmetric film prepared in this embodiment can be tested for performance using the test method disclosed in Embodiment 3.

[0047] Example 2

[0048] The differences between the preparation method in this embodiment and that in Example 1 are as follows:

[0049] Step S1 is as follows: Mix 13.5 parts of polyethersulfone, 2 parts of sulfonated polyethersulfone, 5 parts of polyethylene glycol, 2 parts of Span 20 and 61 parts of N,N-dimethylacetamide, stir at 60-80℃ until completely dissolved, cool to below 40℃, add 16.5 parts of ethylene glycol, continue stirring until clear, and obtain a uniform and stable casting solution by vacuum degassing.

[0050] In step S3: the time spent inside the temperature and humidity chamber is 35 seconds.

[0051] The asymmetric film prepared in this embodiment can be tested for performance using the test method disclosed in Embodiment 3.

[0052] Comparative Example 1

[0053] This comparative example demonstrates the role of surfactants by comparing it with Example 1. The differences in the preparation methods between this comparative example and Example 1 are as follows:

[0054] Step S1 is as follows: Mix 15 parts of polyethersulfone, 1.5 parts of polyvinylpyrrolidone, 0.5 parts of lithium chloride and 65.7 parts of N-methylpyrrolidone, stir at 60-80℃ until completely dissolved, cool to below 40℃, add 2 parts of pure water and 15.3 parts of 1,3-propanediol and continue stirring until clear, and obtain a uniform and stable casting solution by vacuum degassing.

[0055] Figure 3 The microstructure of the compact surface of the asymmetric film in this comparative example is shown. Compared to Example 1 ( Figure 1 Without the addition of surfactant, the surface morphology of the tightly packed surfaces is different, and the porosity is low.

[0056] The asymmetric film prepared in this comparative example can be tested for performance using the test methods disclosed in Example 3.

[0057] Comparative Example 2

[0058] This comparative example reveals the effect of the amount of non-solvent additives added by comparing it with Example 1.

[0059] A certain amount of the casting solution obtained in step S1 of Example 1 was taken, and 1,3-propanediol was added to it. When an additional 4% (percentage of the total mass of the original casting solution) of 1,3-propanediol was added, the system became turbid. Therefore, the amount of 1,3-propanediol added in Example 1 was about 80% of the maximum amount added in the homogeneous phase. This comparative example was prepared according to the casting solution formulation when an additional 2% of 1,3-propanediol was added (corresponding to about 90% of the maximum amount added in the homogeneous phase).

[0060] The differences between the preparation methods of this comparative example and Example 1 are as follows:

[0061] Step S1 is as follows: Mix 15 parts of polyethersulfone, 1.5 parts of polyvinylpyrrolidone, 0.5 parts of lithium chloride, 1.2 parts of Tween 80 and 64.5 parts of N-methylpyrrolidone, stir at 60-80℃ until completely dissolved, cool to below 40℃, add 2 parts of pure water and 17.3 parts of 1,3-propanediol and continue stirring until clear, and obtain a uniform casting solution by vacuum degassing.

[0062] Figure 4 The microstructure of the compact surface of the asymmetric film in this comparative example is shown. Compared to Example 1 ( Figure 1 The surface morphology is more compact, making the liquid channels more easily blocked. On the other hand, because the casting solution in this comparative example has a stronger tendency to phase separation, it is prone to premature phase separation in the process flow, and fine-tuning of process factors can have a significant impact on the membrane structure, making it difficult to control in actual production.

[0063] The asymmetric film prepared in this comparative example can be tested for performance using the test methods disclosed in Example 3.

[0064] Example 3

[0065] This embodiment discloses a performance testing method for the asymmetric membrane of the present invention. The detailed steps of some of the testing methods can be obtained by those skilled in the art from other related published technical documents, and will not be repeated here.

[0066] (1) Pore size test: The average pore size of the membrane to be tested is obtained by using a pore size analyzer.

[0067] (2) Surface porosity analysis: After freeze-drying the test film, a scanning electron microscope (SEM) image was taken, and the surface porosity was analyzed using ImageJ software.

[0068] (3) Membrane permeation separation performance test (flux test): Place a 47 mm diameter disc of the membrane to be tested (pre-wetted with pure water, with the tightly sealed surface facing down) on a vacuum filter holder. Connect the filter bottle to a vacuum pump and slowly evacuate to a negative pressure of -10 psi. Measure the flow time of 500 ml of water and 100 ml of the test solution (feed solution). Repeat the test at least five times for the same sample membrane and record the average value. The test solution is composed of an aqueous solution of 1 mg / L BSA, 0.5 g / L yeast, 3 g / L sodium bicarbonate, and 10 g / L tryptic soy broth (TSB). Stir the test solution thoroughly before use.

[0069] (4) Sterilization level test: Bacterial retention performance was tested using defective Pseudomonas aeruginosa.

[0070] The test results for the asymmetric membranes of Example 1, Example 2, Comparative Example 1, and Comparative Example 2, and the existing PES membrane (commercial PES membrane) are shown in Table 1.

[0071] Table 1 Comparison of Membrane Performance

[0072]

[0073]

[0074] As shown in Table 1, with an average pore size of 0.22 μm (i.e., comparable effective pore size) and a sterilization level > LRV7, the embodiments of the present invention exhibit higher surface porosity (up to 38.25%) and the lowest pure water and feed liquid flow times, demonstrating superior performance compared to commercial PES membranes. In other embodiments, those skilled in the art can also, as needed, adjust the composition and proportion of the formulation, and process parameters (temperature, humidity, time, etc. during phase transition) to prepare asymmetric membranes with other average pore sizes and flux performance, such as asymmetric membranes with an average pore size in the range of 0.18 μm-0.23 μm.

[0075] A comparison of the test results from Example 1 and Comparative Example 1 shows that the addition of surfactant during the preparation process significantly increased the surface porosity on the compact side, while also reducing the possibility of large molecules such as proteins in the feed solution clogging the pores, thereby reducing the flow time required for the feed solution to pass through. A comparison of the test results from Example 1 and Comparative Example 2 shows that when the amount of non-solvent additive added is between 70% and 80% of the maximum amount added in the homogeneous phase, the surface structure of the asymmetric membrane on the compact side is relatively more porous, resulting in a higher surface porosity on the compact side and better flux performance.

[0076] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. For example, the polymer raw material may also be polysulfone and its modified polymers; the good solvent for the casting solution may be N,N-dimethylformamide, etc.; the non-solvent additive may also be ethanol, 1,2-propanediol, etc.; and the nonionic surfactant may be a conventionally commercially available nonionic surfactant, such as alkyl glucoside (APG), fatty acid glycerides, fatty acid sorbitan (Span 20 / 40 / 80, etc.), polyoxyethylene dehydrated sorbitan fatty acid esters (Tween 20 / 40 / 80, etc.), alkylphenol polyoxyethylene ether (OP-10, etc.), and fatty alcohol polyoxyethylene ether (AEO-9, etc.), all of which include at least one alkyl hydrophobic end and at least one hydrophilic end (such as polyol, polyoxyethylene ether, etc. structural units) in their molecular structure, thereby producing corresponding beneficial effects in the preparation method of the present invention through the aforementioned principles.

[0077] Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0078] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing an asymmetric film with high surface porosity, characterized in that, The process includes the following steps: preparing a homogeneous polymer solution, the polymer solution comprising polymer raw materials, a good solvent, a non-solvent additive, a pore-forming agent, and a nonionic surfactant; and then obtaining the asymmetric membrane through a multiphase induced phase transformation, the multiphase induced phase transformation comprising a gas-phase induced phase transformation and an immersion precipitation phase transformation. The polymer raw material includes at least one of polysulfone, polyethersulfone, modified polysulfone, and modified polyethersulfone; the nonionic surfactant includes at least one of alkyl glucoside, fatty acid glycerides, fatty acid sorbitan, polyoxyethylene dehydrated sorbitan fatty acid ester, alkylphenol polyoxyethylene ether, and fatty alcohol polyoxyethylene ether. In the polymer solution: The polymer raw material has a mass percentage content of 13-17%; The nonionic surfactant has a mass percentage content of 0.5-4.5%; The porogen has a mass percentage content of 2-10%; The mass percentage of the good solvent is 45-67%; When preparing the polymer solution, the amount of the non-solvent additive added is between 70% and 80% of the maximum amount added in the homogeneous phase; The method for determining the maximum homogeneous addition amount is as follows: while keeping the addition amount of other components in the polymer solution, except for the solvent additive, the addition amount of the non-solvent additive is changed to determine the maximum homogeneous addition amount.

2. The preparation method according to claim 1, characterized in that, The good solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; The pore-forming agent includes at least one of polyethylene glycol, lithium chloride, and polyvinylpyrrolidone. The non-solvent additive includes at least one of water, ethanol, ethylene glycol, 1,2-propanediol, and 1,3-propanediol.

3. The preparation method according to claim 1, characterized in that, The preparation of the homogeneous polymer solution specifically involves: The polymer raw material, pore-forming agent, and nonionic surfactant are dissolved in a good solvent at 60-80°C, and then a non-solvent additive is added. After mixing evenly, the polymer solution is obtained by vacuum degassing.

4. The preparation method according to claim 1, characterized in that, Prior to the multiphase induced phase transformation, the method further includes the step of: scraping the polymer solution onto a film-forming substrate; After the multiphase induced phase transformation, the following step is also included: drying the asymmetric membrane by passing it through a hot roller at 100-150°C.

5. The preparation method according to claim 1, characterized in that, The gas-phase induced phase transformation specifically involves placing the polymer solution that forms a thin layer in a temperature and humidity chamber for a preset time, wherein the temperature in the chamber is 15-40°C and the humidity is 50-90%RH, and the preset time is 15-60 seconds. The immersion precipitation phase transformation specifically involves immersing the polymer solution, which has undergone gas-phase induced phase transformation, into a coagulation bath at a temperature of 40-60°C.

6. The preparation method according to claim 5, characterized in that, The coagulation bath is a pure water coagulation bath.

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