Charged hydrophilic polymer porous membrane, preparation method thereof and application of charged hydrophilic polymer porous membrane in concentration and purification of polysaccharide in Chinese herbal medicine

Abstract

The invention discloses a charged hydrophilic polymer porous membrane, a preparation method thereof and application of the charged hydrophilic polymer porous membrane in concentration and purification of polysaccharides in Chinese herbal medicines, and belongs to the technical field of porous membranes. Comprising the following steps: synchronously coating a water-soluble coating layer polymer solution on the surface of a separation layer while a film-forming polymer solution is subjected to non-solvent-induced phase separation and the separation layer is formed on the outer surface of a polymer film, so as to obtain a charged hydrophilic polymer porous film; the film-forming polymer solution takes a fluorine-containing polymer as a film-forming base material, a nitrogen-containing small molecule as a first solvent and a high-molecular organic matter as a pore-forming agent. The charged hydrophilic polymer porous membrane is applied to the aspect of concentration and purification of polysaccharide in Chinese herbal medicine, the problem that a membrane prepared by an existing non-solvent induced phase separation technology is low in surface aperture ratio and poor in hydrophilicity and charge electronegativity and is not suitable for concentration and purification of polysaccharide in Chinese herbal medicine is solved, the prepared charged hydrophilic polymer porous membrane is high in aperture ratio, the cutting molecular weight is 10-1000 kDa, and the specific surface area of the membrane is larger than that of the existing non-solvent induced phase separation technology. And membrane forming, surface aperture ratio improvement and surface hydrophilic negatively-charged modification can be realized at the same time.

Description

Technical Field

[0001] This invention belongs to the field of porous membrane technology, and particularly relates to a charged hydrophilic polymer porous membrane, its preparation method, and its application in the concentration and purification of polysaccharides in traditional Chinese medicine. Background Technology

[0002] Membrane separation technology is a technique that enables highly selective separation. Fluoropolymers are currently widely used materials for preparing porous membranes. Porous membranes prepared from fluoropolymers can be used for the concentration and purification of traditional Chinese medicine (TCM) systems containing polysaccharides of varying molecular weights. Common polysaccharides found in TCM include Astragalus polysaccharides, Lentinus edodes polysaccharides, Ganoderma lucidum polysaccharides, Lycium barbarum polysaccharides, Bamboo fungus polysaccharides, Cordyceps polysaccharides, Polygonatum sibiricum polysaccharides, and Codonopsis pilosula polysaccharides, with a wide molecular weight range of 10-1000 kDa. Hot water extraction is a commonly used technique for TCM extraction, but the resulting polysaccharide aqueous solution has a low concentration and needs to be concentrated. While mature evaporation techniques can achieve concentration, they are energy-intensive and may damage the polysaccharides. Membrane technology offers advantages such as deep separation and concentration at room temperature, making it suitable for the concentration and purification of polysaccharides from TCM. Suitable pore size, high flux, and low polysaccharide adsorption are prerequisites for the practical value of polysaccharide concentration membranes from TCM. A suitable pore size allows for efficient retention of polysaccharides in the TCM solution. The lower the filtration resistance, the higher the membrane flux. Good membrane pore permeability and high surface porosity are the main factors in reducing membrane filtration resistance. In addition, endowing the membrane with strong hydrophilicity and strong negative surface charge can significantly reduce the adsorption of polysaccharides (polysaccharides are usually negatively charged and are easily repelled by negatively charged membrane surfaces), and at the same time reduce membrane filtration resistance to a certain extent.

[0003] Solvent-free phase-forming technology is currently the mainstream technology for preparing porous membranes containing fluoropolymers. The film formation principle of this technology is as follows: the polymer, pore-forming agent, and solvent are combined to form a polymer solution at a certain temperature. When the polymer solution is mixed with a coagulation bath (usually water), mass transfer occurs between the solvent and additives and the coagulation bath due to the concentration gradient. When the water content in the polymer solution reaches a certain level and the amount of remaining solvent is insufficient to dissolve the polymer, the polymer solidifies and precipitates. The space previously occupied by the solvent and additives in the polymer solution forms pores.

[0004] The ideal structure for a separation membrane is one with good mechanical properties, high surface porosity, suitable pore size, and a continuous network structure. However, typical ternary solutions formed by polymer / porogen / solvent often experience instantaneous phase separation due to rapid phase separation, ultimately resulting in a thick, dense skin layer and a finger-like pore structure with poor compressive strength. This structure is unsuitable for long-term concentration and purification of polysaccharides from traditional Chinese medicine. Literature studies indicate that reverse thermal phase separation technology can prepare polyvinylidene fluoride porous membranes with a sponge-like pore structure and a thin skin layer. However, this technology struggles to form a stable solution at room temperature, instead creating a metastable dispersion system. This dispersion is highly sensitive to ambient temperature, becoming increasingly unstable with rising temperatures, resulting in poor batch-to-batch reproducibility of the prepared membrane microstructure. Furthermore, the polymer solution concentration prepared using reverse thermal phase separation technology is only 12%, making it difficult to obtain membranes with molecular weight cut below 500 kDa, and the surface porosity is also low.

[0005] Since the membrane fabrication process of non-solvent phase separation technology is relatively simple and the control requirements for process parameters are not very stringent, in addition to exploring convenient and effective surface hydrophilic modification methods, it is particularly necessary to study the simple preparation of polymer porous membranes with good pore permeability, thin skin, high surface porosity, pore size of 10-1000kDa and good hydrophilicity through non-solvent phase separation technology for the concentration and purification of polysaccharides in traditional Chinese medicine.

[0006] Currently, the main methods to improve the surface porosity of membranes prepared using non-solvent phase separation technology are as follows: (1) Add template agents (such as nano-calcium carbonate, nano-silica and other inorganic nanoparticles) to the casting solution, and remove the template agents with strong alkali or strong acid solutions after the membrane is formed. However, due to the weak compatibility between inorganic nanoparticles and polymers, the template agents are difficult to disperse evenly in the casting solution system. Therefore, the membrane pore size is not uniform, which significantly reduces the separation accuracy. In addition, a certain amount of waste acid or waste alkali solution is generated during the removal of the template agent; (2) Add a certain proportion of solvents such as N,N-dimethylacetamide (DMAC) containing fluorinated polymers to the coagulation bath water. The presence of the solvent reduces the curing rate of the polymer, thereby slowing down the formation of a dense thick skin and finger-like pore structure due to the rapid curing of the polymer to a certain extent. However, this method can only reduce the skin thickness and increase the surface pore size and porosity to a certain extent, but the effect on improving the surface porosity is not significant; (3) Increase the air humidity. Before the casting solution enters the coagulation bath, water vapor in the air adheres to the surface of the casting film, which can greatly slow down the diffusion rate between water, solvent and additives in the subsequent coagulation bath, thereby reducing the skin thickness, eliminating finger pores in the cross section, and improving the opening rate and pore size of the film surface. However, this process requires the air humidity to reach supersaturation, which not only greatly increases the equipment process, but also requires a high degree of humidity control. The water in the supersaturated air will condense and precipitate, changing the air humidity, making it difficult to meet the requirements for the membrane pore size and the improvement of the opening rate of the membrane surface is relatively limited. (4) Simultaneous coating of polymer solution: The composite membrane is prepared by using a double scraper (from top to bottom, the polymer coating solution and the film-forming polymer solution) on the surface of the plastic film support simultaneously. The result is that the thickness of the bottom film skin becomes thinner and the finger pores are eliminated. However, since the coating solution eventually forms a polymer thin layer and adheres to the polymer film below, the opening rate and pore size of the membrane surface are reduced. In addition, the polymer used in the coating solution is currently a non-water-soluble polymer that is not strongly hydrophilic and does not carry a negative charge.

[0007] However, due to its simplicity, efficiency, and ease of industrialization, the simultaneous surface coating method is an ideal technology for effectively controlling the microstructure of membrane surfaces. Therefore, the key is whether simultaneous coating can significantly improve the porosity, hydrophilicity, and negative charge of the membrane surface. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is that existing non-solvent-induced phase separation techniques result in membranes with low surface porosity, poor hydrophilicity, and poor charge-to-weight ratio, making them unsuitable for the concentration and purification of polysaccharides from traditional Chinese medicine. This invention proposes a charged hydrophilic polymer porous membrane with high porosity, a molecular weight cutoff of 10-1000 kDa, and the ability to simultaneously achieve membrane formation, improve surface porosity, and modify the surface with hydrophilicity and charge-to-weight ratio. The invention also describes its preparation method and its application in the concentration and purification of polysaccharides from traditional Chinese medicine.

[0009] To solve the aforementioned technical problem, the technical solution adopted by the present invention is as follows:

[0010] This invention provides a method for preparing a charged hydrophilic polymer porous membrane, comprising: simultaneously coating a water-soluble coating polymer solution onto the surface of the separation layer while a non-solvent-induced phase separation occurs in the film-forming polymer solution and a separation layer is formed on the outer surface of the polymer membrane, thereby obtaining a charged hydrophilic polymer porous membrane; the film-forming polymer solution uses a fluoropolymer as the film-forming substrate, a nitrogen-containing small molecule as the first solvent, and a high-molecular-weight organic compound as the pore-forming agent; the coating polymer solution is negatively charged and has a viscosity of 800-3000 mPa·s.

[0011] In some embodiments, the coating polymer solution comprises a negatively charged water-soluble polymer containing carboxyl and hydroxyl groups, wherein the negatively charged water-soluble polymer is selected from at least one of poly(hydroxyethyl methacrylate), sodium alginate, and sodium carboxymethyl cellulose.

[0012] In some embodiments, the mass fraction of the negatively charged water-soluble polymer in the coating polymer solution is 1-2.5%.

[0013] In some embodiments, the coating polymer solution includes a negatively charged water-soluble polymer and a second solvent, wherein the second solvent is water or ethanol that can dissolve the negatively charged water-soluble polymer but cannot dissolve the fluorinated polymer.

[0014] In some embodiments, the mass ratio of the fluoropolymer, pore-forming agent, and first solvent in the film-forming polymer solution is 18-26:3-8:66-79; the fluoropolymer is a homopolymer or copolymer of polyvinylidene fluoride with a weight average molecular weight of 700-900 kDa.

[0015] In some embodiments, the second solvent in the film-forming polymer solution is N,N-dimethylacetamide or N-methylpyrrolidone, and the pore-forming agent is polyvinylpyrrolidone with a molecular weight of 30-45 kDa.

[0016] In some embodiments, including:

[0017] S1. After mixing the fluorinated polymer, pore-forming agent and first solvent at 50-70℃, the mixture is degassed under vacuum to obtain the film-forming polymer solution for preparing the polymer film.

[0018] S2. After uniformly mixing the negatively charged water-soluble polymer with the second solvent at 50°C, the mixture is degassed under vacuum to obtain the coating polymer solution for preparing the coating layer.

[0019] S3. Using a double scraper, the coating polymer solution and the film-forming polymer solution are simultaneously flowed out of a liquid tank containing a scraper with a thickness of 0.2 mm at 50-60℃. This allows the film-forming polymer solution to be coated onto a 0.4 mm thick polyethylene terephthalate plastic film substrate, while the coating polymer solution is coated on top of the film-forming polymer solution. After forming a flat film shape, it is cooled and cured to form a film.

[0020] S4. The membrane is soaked and washed with deionized water extractant at 25°C to obtain a charged hydrophilic polymer porous membrane.

[0021] In some embodiments, the air gap length in S3 is 30-60 cm, and the film scraping rate is 2-4 m / min.

[0022] In another aspect, the present invention provides a method for preparing a charged hydrophilic polymer porous membrane according to any of the above technical solutions. The charged hydrophilic polymer porous membrane is a flat sheet membrane with a pore size of 5-100 nm and a molecular weight cutoff of 10-1000 kDa.

[0023] The present invention also provides an application of the charged hydrophilic polymer porous membrane provided by the above technical solution in the concentration and purification of polysaccharides in traditional Chinese medicine.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] This invention provides a method for preparing a charged hydrophilic polymer porous membrane. Using a fluoropolymer as a substrate, a polymer sheet membrane is prepared using a non-solvent-induced phase separation technique. Simultaneously with the formation of the fluoropolymer film, a water-soluble polymer aqueous solution with a certain viscosity, adhesiveness, and negative charge is coated onto its surface. This yields a polymer sheet membrane with the desired structure. This method utilizes the fact that during the simultaneous coating process, the contact interface between the water-soluble polymer chains and the film-forming polymer chains allows for sufficient contact before curing. Furthermore, its strong adhesiveness ensures that the water-soluble polymer adheres firmly to the surface of the polymer membrane and is not easily detached. Furthermore, the high viscosity of the coating polymer slows down the double diffusion mass transfer between the film-forming polymer solution and the coagulation bath, thereby causing significant delayed phase separation in the film-forming polymer solution system. This increases the membrane surface porosity, increases the membrane pore size, and eliminates the finger-like pore structure. In addition, the high negative charge density on the membrane surface not only significantly improves the binding with water molecules and thus enhances the hydrophilicity of the membrane, but also significantly improves the repulsion of the membrane surface against polysaccharides in negatively charged Chinese herbal medicines. This enhances the anti-fouling and retention of polysaccharides in Chinese herbal medicines. The preparation process is simple, and the prepared charged hydrophilic polymer porous membrane has good pore connectivity, large pore size, and high surface porosity. Attached Figure Description

[0026] Figure 1 This is a surface morphology diagram of the charged hydrophilic polymer porous membrane provided in Embodiment 1 of the present invention;

[0027] Figure 2 This is a surface morphology diagram of the charged hydrophilic polymer porous membrane provided in Comparative Example 1 of the present invention.

[0028] Figure 3 This is a cross-sectional morphology diagram of the charged hydrophilic polymer porous membrane provided in Embodiment 1 of the present invention;

[0029] Figure 4 This is a cross-sectional morphology diagram of the charged hydrophilic polymer porous membrane provided in Comparative Example 1 of the present invention. Detailed Implementation

[0030] The technical solutions in specific embodiments of the present invention will be described in detail and completely below. Obviously, the described embodiments are only some specific implementations of the overall technical solution of the present invention, and not all implementations. Based on the overall concept of the present invention, all other embodiments obtained by those skilled in the art fall within the protection scope of the present invention.

[0031] This invention provides a method for preparing a charged hydrophilic polymer porous membrane, comprising: simultaneously coating a water-soluble coating polymer solution onto the surface of the separation layer while a non-solvent-induced phase separation occurs in the film-forming polymer solution and a separation layer is formed on the outer surface of the polymer membrane, thereby obtaining a charged hydrophilic polymer porous membrane; the film-forming polymer solution uses a fluoropolymer as the film-forming substrate, a nitrogen-containing small molecule as the first solvent, and a high-molecular-weight organic compound as the pore-forming agent; the coating polymer solution is negatively charged and has a viscosity of 800-3000 mPa·s.

[0032] The above preparation method utilizes the simultaneous coating process of the film-forming polymer solution and the coating polymer solution to ensure sufficient contact between the water-soluble polymer chains in the coating layer and the film-forming polymer chains before curing. Due to the concentration gradient between the coating polymer solution and the film-forming polymer solution, the coating polymer, which has an adhesive effect, diffuses towards the film-forming polymer solution, further strengthening the entanglement between the coating polymer chains. Its strong adhesiveness ensures a firm adhesion to the polymer membrane surface, preventing detachment. Furthermore, the high viscosity of the coating polymer slows down the two-diffusion mass transfer between the film-forming polymer solution and the coagulation bath, resulting in significant delayed phase separation in the film-forming polymer solution system. This improves the membrane surface porosity, effectively eliminates finger-like pore structures, and appropriately increases the membrane pore size. In addition, the high negative charge density on the membrane surface not only significantly enhances the binding with water molecules, improving the membrane's hydrophilicity, but also significantly increases the repulsive force of the membrane surface against negatively charged polysaccharides from traditional Chinese medicine, thereby improving the anti-fouling and retention of polysaccharides from traditional Chinese medicine. Ultimately, a polymer porous membrane suitable for the concentration and purification of polysaccharides from traditional Chinese medicine is obtained.

[0033] In conventional ternary solutions formed by polymers, porogens, and solvents, instantaneous phase separation occurs during non-solvent-induced phase separation processes due to the rapid phase separation rate, ultimately resulting in a thick, dense skin layer with a non-compressive finger-like porous structure. The preparation method of this invention overcomes this problem, and its underlying principle is as follows:

[0034] A thin layer (referred to as the coating layer) of a water-soluble polymer solution with a certain viscosity, adhesion, and negative charge, at the same temperature as the film-forming polymer solution, is simultaneously coated onto the film-forming polymer solution. Because the coating layer polymer solution has a certain viscosity and can form a barrier between the coagulation bath and the film-forming polymer solution, it can significantly slow down the double diffusion mass transfer between the coagulation bath and the film-forming polymer solution, thereby greatly slowing down the solidification and film formation of the polymer in the film-forming polymer solution. This results in a porous skin structure instead of a dense, thick skin layer, increases the membrane surface porosity, effectively eliminates finger-like pore structures, and appropriately increases the membrane pore size, allowing the membrane to cut molecular weight in the range of 10-1000 kDa. During the simultaneous coating process, the contact interface between the water-soluble polymer chains in the coating layer and the film-forming polymer chains in the coating layer undergoes sufficient contact before curing. Furthermore, the concentration gradient between the coating layer and film-forming polymer solutions causes the adhesive-bonding polymer to diffuse towards the film-forming polymer solution, further strengthening the entanglement between the coating layer and film-forming polymer chains. This, combined with its strong adhesion, ensures a firm bond to the polymer film surface, preventing detachment. In addition, the high negative charge density on the membrane surface significantly enhances the binding with water molecules, increasing the membrane's hydrophilicity, and also significantly increases the repulsion of negatively charged polysaccharides from traditional Chinese medicine. This improves the membrane's resistance to contamination and retention of polysaccharides from traditional Chinese medicine, making it highly suitable for polysaccharide concentration and purification in the field of traditional Chinese medicine.

[0035] In some embodiments, the coating polymer solution comprises a negatively charged water-soluble polymer containing carboxyl and hydroxyl groups, wherein the negatively charged water-soluble polymer is selected from at least one of poly(hydroxyethyl methacrylate), sodium alginate, and sodium carboxymethyl cellulose.

[0036] Experiments revealed that when organic solvents such as glycerol, triethylene glycol, propylene glycol, and polyethylene glycol 400 were used in the coating polymer solution, their viscosity was very low, resulting in poor controllability during synchronous film coating and making it difficult to form a continuous coating layer. Furthermore, when aqueous solutions of polyethylene glycol and polyethylene oxide with molecular weights of 10-100 kDa were used, the adhesion between the solvent and the underlying film-forming polymer was weak, leading to easy detachment. Moreover, the membrane surface could not be negatively charged, failing to achieve the desired hydrophilicity and surface negative charging effect.

[0037] In some embodiments, the mass fraction of the negatively charged water-soluble polymer in the coating polymer solution is 1-2.5%.

[0038] Experiments have determined that the optimal mass fraction of negatively charged water-soluble polymer in the coating polymer solution is 1-2.5%, corresponding to a viscosity range of 800-3000 mPa·s. When the viscosity is below 800 mPa·s, controllability during simultaneous film application is poor, making it difficult to form a continuous coating layer. Conversely, when the viscosity exceeds 3000 mPa·s, the excessively high viscosity worsens the film application properties of the coating polymer solution, resulting in uneven coating. Low polymer concentrations do not significantly clog pores, thus reducing the porosity of the underlying polymer film. Since the coating is a thin layer of water or ethanol solution, the small amount of solvent only causes phase separation on the surface of the underlying polymer solution without further expansion. The fluoropolymer chains that undergo phase separation on the surface further entangle and adhere to the strongly adhesive polymer chains in the coating solution, making them less prone to detachment. It is understandable that the mass fraction of negatively charged water-soluble polymer in the coating polymer solution can also be 1.5%, 2.0%, or any value within this range.

[0039] In some embodiments, the coating polymer solution includes a negatively charged water-soluble polymer and a second solvent, wherein the second solvent is water or ethanol that can dissolve the negatively charged water-soluble polymer but cannot dissolve the fluorinated polymer.

[0040] In some embodiments, the mass ratio of the fluoropolymer, pore-forming agent, and first solvent in the film-forming polymer solution is 18-26:3-8:66-79; the fluoropolymer is a homopolymer or copolymer of polyvinylidene fluoride with a weight average molecular weight of 700-900 kDa.

[0041] If the weight-average molecular weight of the fluoropolymer is too low, or the content of the fluoropolymer in the film-forming polymer solution is too low, the viscosity of the film-forming polymer solution will be low, resulting in poor formability and low strength. Conversely, if the weight-average molecular weight of the fluoropolymer is too high, or the content of the fluoropolymer in the film-forming polymer solution is too high, the viscosity of the film-forming polymer solution will be too high, making it difficult to process, and the pore size will be too small, failing to meet the pore size requirements for polysaccharide concentration and purification in the field of traditional Chinese medicine. It is understandable that the weight-average molecular weight of the fluoropolymer can also be any value within the range of 750 kDa, 800 kDa, 850 kDa, etc.

[0042] In some embodiments, the second solvent in the film-forming polymer solution is N,N-dimethylacetamide or N-methylpyrrolidone, and the pore-forming agent is polyvinylpyrrolidone with a molecular weight of 30-45 kDa. It is understood that the molecular weight of polyvinylpyrrolidone can also be any value within the range of 32 kDa, 34 kDa, 36 kDa, 38 kDa, 40 kDa, 42 kDa, 44 kDa, and so on.

[0043] In some embodiments, including:

[0044] S1. After mixing the fluorinated polymer, pore-forming agent and first solvent at 50-70℃, the mixture is degassed under vacuum to obtain the film-forming polymer solution for preparing the polymer film.

[0045] S2. After uniformly mixing the negatively charged water-soluble polymer with the second solvent at 50°C, the mixture is degassed under vacuum to obtain the coating polymer solution for preparing the coating layer.

[0046] S3. Using a double scraper, the coating polymer solution and the film-forming polymer solution are simultaneously flowed out of a liquid tank containing a scraper with a thickness of 0.2 mm at 50-60℃. This allows the film-forming polymer solution to be coated onto a 0.4 mm thick polyethylene terephthalate plastic film substrate, while the coating polymer solution is coated on top of the film-forming polymer solution. After forming a flat film shape, it is cooled and cured to form a film.

[0047] S4. The membrane is soaked and washed with deionized water extractant at 25°C to obtain a charged hydrophilic polymer porous membrane.

[0048] In the preparation process of the above-mentioned charged hydrophilic polymer porous membrane, the coating polymer solution and the film-forming polymer solution are simultaneously coated, with the former on top of the latter, and the latter coated on top of a plastic film of a certain thickness. This preparation method has the following characteristics:

[0049] (1) The preparation process is simple. Only by adding a scraper, using the existing commercial non-solvent phase separation technology, adding a metering pump, and a heatable vessel for holding the coating material, a flat sheet membrane with the desired structure can be prepared. (2) The effect of improving the membrane surface porosity, hydrophilicity, and negative charge is significant and the method is simple: by simultaneously coating the surface and forming the polymer film, a water-soluble polymer solution with a certain viscosity, adhesion, and negative charge is simultaneously coated on the surface of the fluoropolymer film. During the simultaneous coating process, the contact interface between the water-soluble polymer chain and the film-forming polymer chain of the film-forming polymer solution is in sufficient contact before curing. In addition, its strong adhesion makes it adhere firmly to the surface of the polymer membrane and not easy to fall off. Furthermore, the high viscosity of the coating polymer slows down the double diffusion mass transfer between the film-forming polymer solution and the coagulation bath, thereby causing significant delayed phase separation in the film-forming polymer solution system, thus improving the membrane surface porosity, increasing the membrane pore size, and eliminating the finger pore structure. In addition, the high negative charge density on the membrane surface not only significantly improves the binding with water molecules and thus enhances the hydrophilicity of the membrane, but also significantly enhances the repulsive force of the membrane surface against polysaccharides in negatively charged Chinese herbal medicines, thereby improving the anti-fouling and retention of polysaccharides in Chinese herbal medicines. (3) Regulation of membrane pore size. By adjusting the composition of the coating layer and the composition of the film-forming polymer layer, as well as the membrane forming process parameters, the film forming process can be effectively regulated, ultimately improving the membrane pore permeability, effectively reducing the skin thickness, increasing the membrane surface porosity, and achieving hydrophilic modification and negative charge, thereby achieving regulation of the membrane microstructure, hydrophilicity, and negative charge, and effectively separating various polysaccharides in the extract of Chinese herbal medicines.

[0050] In some embodiments, the air gap length in S3 is 30-60 cm, and the film scraping rate is 2-4 m / min.

[0051] Before the coating polymer solution and the film-forming polymer solution enter the coagulation bath together, two-diffusion mass transfer first occurs between the coating polymer solution and the coagulation bath, and between the coating polymer solution and the film-forming polymer solution, respectively. Therefore, it is necessary to complete chain entanglement and integration at the contact interface between the coating polymer and the film-forming polymer during the air gap time before the coating polymer enters the coagulation bath, and to ensure that the coating polymer fully adheres to the surface of the film-forming polymer under the influence of the concentration difference, making it less prone to detachment. In addition, the high viscosity of the coating polymer solution effectively hinders the two-diffusion between water and the film-forming polymer solution in the coagulation bath after the film-forming polymer enters the coagulation bath. The significant delayed phase separation of the film-forming polymer solution further increases the pore size of the membrane surface and eliminates finger pores, ultimately forming a membrane surface with pore size suitable for the concentration and purification of polysaccharides in traditional Chinese medicine and with hydrophilic (hydroxyl) and negatively charged (carboxyl) groups. The time of the polymer solution in the air is closely related to the air gap length, the scraping rate, and the membrane thickness. Experimental tests show that for a 0.2mm thick flat sheet membrane, a doctor blade thickness of 0.2mm, an air gap length of 30-60cm, and a doctor blade speed of 2-4m / min are sufficient to achieve the above objectives. If the air gap length is too short, the coating polymer solution thickness is too thin, or the doctor blade speed is too fast, it is difficult to ensure the effective diffusion of the two-component pore-forming agent in the coating polymer solution layer towards the film-forming polymer solution layer, thus failing to obtain the required pore size and a complete coating layer; conversely, the membrane pore size will be too large and the strength will be reduced.

[0052] It is understandable that the air gap length can be any value within the range of 35cm, 40cm, 45cm, 50cm, 55cm, and the film scraping rate can be any value within the range of 2.5m / min, 3.0m / min, 3.5m / min, and so on.

[0053] The above preparation process of the present invention is as follows: (1) For the polymer membrane layer, a fluoropolymer is used as the substrate, and a non-solvent-induced phase separation technique is used to prepare a polymer porous membrane with a sponge-like cross-section and a high surface porosity; (2) For the coating layer, a water-soluble polymer solution with a certain viscosity, adhesion and negative charge is simultaneously coated on the upper surface of the fluoropolymer membrane to obtain a polymer flat sheet membrane with the desired structure; (3) The membrane formation, surface coating, hydrophilic modification and negative charge are carried out simultaneously. Specifically, the following steps are included:

[0054] 1) Preparation of polymer solution:

[0055] Preparation of film-forming polymer solution: According to the mass fraction, 18-26 parts of dried fluoropolymer with a weight average molecular weight of 700-900kDa, 3-8 parts of dried pore-forming agent, and 66-79 parts of first solvent are mixed evenly in a stirrer at 50-70℃ for 10 hours to form a polymer solution. After vacuum degassing for 1 hour, the film-forming polymer solution for preparing the polymer film is obtained.

[0056] Preparation of the coating polymer solution: According to the mass parts, a water-soluble polymer with strong adhesion, high viscosity and negative charge, and a second solvent are mixed in a batch stirrer at 50°C for 6 hours to form a homogeneous polymer solution. After vacuum degassing for 1 hour, the coating polymer solution is obtained. The viscosity range of the coating polymer solution is 800-3000 mPa·s.

[0057] 2) Preparation of flat sheet membranes:

[0058] Using a double-scraper process, the coating polymer solution and the film-forming polymer solution obtained in step 1) are simultaneously fed through a feed tank containing a 0.2mm thick scraper at 50-60℃. This allows the film-forming polymer solution to coat a 0.4mm thick polyethylene terephthalate (PET) plastic film substrate, while the coating polymer solution is coated on top of the film-forming polymer solution, forming a flat sheet membrane. The coating polymer solution and the film-forming polymer solution pass through an air gap of 30-60cm and then enter a coagulation bath composed of deionized water at room temperature. After cooling with deionized water at a specific temperature, the resulting nascent membrane is extracted with deionized water at 25℃ to remove the solvent and pore-forming agent. After two extractions, the membrane is air-dried to obtain the finished flat sheet membrane. The membrane's molecular weight cut is between 10-1000kDa, meeting the application requirements. Among these:

[0059] The solvent in the film-forming polymer solution in step 1) is N,N-dimethylacetamide and N-methylpyrrolidone; the pore-forming agent is polyvinylpyrrolidone with a molecular weight of 30-45 kDa.

[0060] The solvent in the polymer solution for the coating layer in step 1) is ethanol or water;

[0061] In step 2), the winding speed during the spinning process is 2-4 m / min.

[0062] Compared with the prior art, the present invention uses a negatively charged water-soluble polymer containing hydroxyl and carboxyl groups with strong adhesion and high viscosity as the coating solution. While forming the polymer film, the above-mentioned coating polymer solution is simultaneously coated on the surface of the polymer film to obtain a polymer flat sheet film with the desired structure. By selecting appropriate polymers, solvents and pore-forming agents with appropriate weight-average molecular weight and content, it is relatively easy to obtain or control the formation of a surface hydrophilic negatively charged structure with a thin and porous outer skin layer and a molecular weight cutoff of 10-1000kDa through non-solvent phase-induced film formation technology.

[0063] Compared with existing related technologies, this invention differs in coating method, coating polymer used, coating temperature, film formation effect, obtained film structure, and air gap length. In this invention, the coating polymer solution and the film-forming polymer solution are at the same temperature, which has two main effects: First, it hinders the double diffusion between the coagulation bath and the film-forming polymer solution, thus greatly delaying the phase separation process of the film-forming polymer solution; second, during the simultaneous coating and molding process, the hydrophilic negatively charged polymer with strong adhesion and high viscosity in the coating polymer solution allows for sufficient contact between its chains and the film-forming polymer chains at the interface before curing, ensuring firm adhesion to the polymer film surface and preventing detachment, ultimately imparting strong hydrophilicity and negative charge to the film surface. The non-solvent-induced phase separation of the polymer solution layer during film formation and the simultaneous coating of the polymer film onto the membrane surface are different from the stepwise coating (film formation followed by polymer coating on the membrane surface - the coated polymer is not washed off, this method achieves membrane pore reduction), homogeneous composite (the coating layer and the membrane are the same polymer, the coating layer undergoes non-solvent-induced phase separation, the separation layer undergoes thermally induced phase separation - the coated polymer is not washed off, this method achieves membrane pore reduction), and the effect obtained by simultaneous coating with styrene-maleic anhydride copolymer (styrene-maleic anhydride copolymer has poor film-forming properties, poor adhesion to the polymer membrane, and a more obvious pore reduction effect).

[0064] In this invention, the coating layer uses a polymer with high viscosity, strong adhesion, and soluble in ethanol or water, containing carboxyl and hydroxyl groups. After film formation, it adheres to the surface of the fluoropolymer, thereby improving the film's porosity, hydrophilicity, and negative charge. In contrast, substances such as water-soluble tannic acid and dopamine used as coatings have low viscosity, offering no significant pore-opening effect and exhibiting poor film-scraping performance. Furthermore, the polymerized film surface lacks strong hydrophilicity, resulting in significant pore blockage. The membrane obtained by this invention has a molecular weight cutoff of 10-1000 kDa, while coating techniques reported in literature and patents aim to reduce skin thickness or surface pore size. In summary, this invention is highly adaptable, simple to implement, uses widely available materials, is low-cost, and yields good results. It requires only a small amount of additional equipment, is easy to promote, and has excellent industrialization prospects.

[0065] In another aspect, the present invention provides a method for preparing a charged hydrophilic polymer porous membrane according to any of the above technical solutions. The charged hydrophilic polymer porous membrane is a flat sheet membrane with a pore size of 5-100 nm and a molecular weight cutoff of 10-1000 kDa.

[0066] Understandably, the pore size of the flat sheet membrane can be any value within the range of 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, and so on, and the molecular weight cut can be any value within the range of 100kDa, 200kDa, 300kDa, 400kDa, 500kDa, 600kDa, 700kDa, 800kDa, 900kDa, and so on.

[0067] The present invention also provides an application of the charged hydrophilic polymer porous membrane provided by the above technical solution in the concentration and purification of polysaccharides in traditional Chinese medicine.

[0068] To more clearly and in detail introduce the charged hydrophilic polymer porous membrane provided in the embodiments of the present invention, its preparation method, and its application in the concentration and purification of polysaccharides in traditional Chinese medicine, the following description will be based on specific embodiments.

[0069] Example 1

[0070] 1) Preparation of polymer solution:

[0071] Preparation of the film-forming polymer solution: 18 parts of dried fluoropolymer with a weight average molecular weight of 700 kDa, 3 parts of dried polyvinylpyrrolidone with a molecular weight of 30 kDa, and 79 parts of N,N-dimethylacetamide were mixed evenly in a stirrer at 50°C for 10 h to form a polymer solution. After vacuum degassing for 1 h, the polymer solution for preparing the polymer film was obtained.

[0072] Preparation of the polymer solution for coating: According to the mass fraction, 1 part sodium carboxymethyl cellulose and 99 parts deionized water are mixed evenly in a stirrer at 50°C for 6 hours to form a polymer solution. After vacuum degassing for 1 hour, the polymer solution for preparing the coating is obtained.

[0073] 2) Preparation of flat sheet membrane: Using a double-scraper, the coating polymer solution and the film-forming polymer solution obtained in step 1) are simultaneously flowed out of a feed tank containing a 0.2 mm thick scraper at 50°C. The film-forming polymer solution is coated onto a 0.4 mm thick polyethylene terephthalate plastic film substrate, while the coating polymer solution is coated on top of the film-forming polymer solution, forming a flat sheet membrane. The coating polymer solution and the film-forming polymer solution are simultaneously passed through a 60 cm long air gap at a winding speed of 2 m / min and then enter a coagulation bath composed of deionized water at room temperature. After being cooled by deionized water at a certain temperature, the resulting nascent membrane is extracted with deionized water at 25°C to remove the solvent and pore-forming agent. After extraction twice, it is air-dried to obtain the finished flat sheet membrane.

[0074] Membrane surface morphology such as Figure 1 As shown, the test results (using the test method specified in "Test Methods for Ultrafiltration Membranes" (GB / T32360-2015)) indicate that... Figure 3 As shown, the membrane has a sponge-like cross-section, a molecular weight cutoff of 1000 kDa, and a flux recovery rate of 86.2% (1 g / L bovine serum albumin solution, pH = 7.4); the membrane without coating has a pore size of 78 nm, a finger-like cross-section, and a flux recovery rate of 42.7%.

[0075] Example 2

[0076] 1) Preparation of polymer solution:

[0077] Preparation of the film-forming polymer solution: 26 parts of dried fluoropolymer with a weight average molecular weight of 900 kDa, 8 parts of dried polyvinylpyrrolidone with a molecular weight of 45 kDa and 66 parts of N,N-dimethylacetamide were mixed evenly in a stirred tank at 70°C for 10 h to form a polymer solution. After vacuum degassing for 1 h, the polymer solution for preparing the polymer film was obtained.

[0078] Preparation of the polymer solution for coating: According to the mass fraction, 2.5 parts of sodium carboxymethyl cellulose and 97.5 parts of deionized water are mixed evenly in a stirrer at 50°C for 6 hours to form a polymer solution. Then, the mixture is degassed under vacuum for 1 hour to obtain the polymer solution for preparing the coating.

[0079] 2) Preparation of flat sheet membrane: Using a double scraper, the coating polymer solution and the film-forming polymer solution obtained in step 1) are simultaneously flowed out of a feed tank containing a scraper with a thickness of 0.2 mm at 60°C. The film-forming polymer solution is coated onto a polyethylene terephthalate plastic film substrate with a thickness of 0.4 mm, while the coating polymer solution is coated on top of the film-forming polymer solution, forming a flat sheet membrane. The coating polymer solution and the film-forming polymer solution are simultaneously passed through an air gap of 30 cm at a winding speed of 4 m / min and then enter a coagulation bath composed of deionized water at room temperature. After being cooled by deionized water at a certain temperature, the resulting nascent membrane is extracted with deionized water at 25°C to remove the solvent and pore-forming agent. After extraction twice, it is air-dried to obtain the finished flat sheet membrane.

[0080] According to the test method (GB / T 32360-2015 "Test Methods for Ultrafiltration Membranes"), the membrane cross-section is sponge pore, the molecular weight cut is 10kDa, and the flux recovery rate is 97.2% (1g / L bovine serum albumin solution, pH=7.4); the membrane without coating has a pore size of 47nm, a finger-like pore cross-section, and a flux recovery rate of 44.8%.

[0081] Example 3

[0082] Preparation of the film-forming polymer solution: 20 parts by mass of dried fluoropolymer with a weight average molecular weight of 700 kDa, 5 parts by dried polyvinylpyrrolidone with a molecular weight of 30 kDa, and 75 parts by mass of N,N-dimethylacetamide were mixed evenly in a stirred tank at 50°C for 10 h to form a polymer solution. After vacuum degassing for 1 h, the polymer solution for preparing the polymer film was obtained.

[0083] Preparation of the polymer solution for coating: According to the mass fraction, 2 parts sodium alginate and 98 parts deionized water are mixed evenly in a stirrer at 50°C for 6 hours to form a polymer solution. After vacuum degassing for 1 hour, the polymer solution for preparing the coating is obtained.

[0084] 2) Preparation of flat sheet membrane: Using a double scraper, the coating polymer solution and the film-forming polymer solution obtained in step 1) are simultaneously flowed out of a feed tank containing a scraper with a thickness of 0.2 mm at 50°C. The film-forming polymer solution is coated onto a polyethylene terephthalate plastic film substrate with a thickness of 0.4 mm, while the coating polymer solution is coated on top of the film-forming polymer solution, forming a flat sheet membrane. The coating polymer solution and the film-forming polymer solution are simultaneously passed through an air gap of 60 cm at a winding speed of 3 m / min and then enter a coagulation bath composed of deionized water at room temperature. After being cooled by deionized water at a certain temperature, the resulting nascent membrane is extracted with deionized water at 25°C to remove the solvent and pore-forming agent. After extraction twice, it is air-dried to obtain the finished flat sheet membrane.

[0085] According to the test method (GB / T 32360-2015 "Test Methods for Ultrafiltration Membranes"), the membrane cross-section is sponge-like, the molecular weight cut is 220kDa, and the flux recovery rate is 89.2% (1g / L bovine serum albumin solution, pH=7.4); the membrane without coating has a pore size of 69nm, a finger-like cross-section, and a flux recovery rate of 43.5%.

[0086] Example 4

[0087] Preparation of the film-forming polymer solution: 20 parts of dried fluoropolymer with a weight average molecular weight of 700 kDa, 5 parts of dried polyvinylpyrrolidone with a molecular weight of 30 kDa, and 75 parts of N-methylpyrrolidone were mixed evenly in a stirred tank at 50°C for 10 h to form a polymer solution. After vacuum degassing for 1 h, the polymer solution for preparing the polymer film was obtained.

[0088] Preparation of the polymer solution for coating: According to the mass fraction, 2 parts of hydroxyethyl methacrylate and 98 parts of ethanol are mixed evenly in a stirrer at 50°C for 6 hours to form a polymer solution. Then, the mixture is degassed under vacuum for 1 hour to obtain the polymer solution for preparing the coating.

[0089] 2) Preparation of flat sheet membrane: Using a double scraper, the coating polymer solution and the film-forming polymer solution obtained in step 1) are simultaneously flowed out of a feed tank containing a scraper with a thickness of 0.2 mm at 50°C. The film-forming polymer solution is coated onto a polyethylene terephthalate plastic film substrate with a thickness of 0.4 mm, while the coating polymer solution is coated on top of the film-forming polymer solution, forming a flat sheet membrane. The coating polymer solution and the film-forming polymer solution are simultaneously passed through an air gap of 50 cm at a winding speed of 3 m / min and then enter a coagulation bath composed of deionized water at room temperature. After being cooled by deionized water at a certain temperature, the resulting nascent membrane is extracted with deionized water at 25°C to remove the solvent and pore-forming agent. After extraction twice, it is air-dried to obtain the finished flat sheet membrane.

[0090] According to the test method (GB / T 32360-2015 "Test Methods for Ultrafiltration Membranes"), the membrane cross-section is sponge-like, the molecular weight cut is 310kDa, and the flux recovery rate is 92.5% (1g / L bovine serum albumin solution, pH=7.4); the membrane without coating has a pore size of 71nm, a finger-like cross-section, and a flux recovery rate of 42.7%.

[0091] Comparative Example 1

[0092] Same as Example 1, except that synchronous coating is not performed, as detailed below:

[0093] 1) Preparation of polymer solution:

[0094] Preparation of the film-forming polymer solution: 26 parts of dried fluoropolymer with a weight average molecular weight of 900 kDa, 8 parts of dried polyvinylpyrrolidone with a molecular weight of 45 kDa and 66 parts of N,N-dimethylacetamide were mixed evenly in a stirred tank at 70°C for 10 h to form a polymer solution. After vacuum degassing for 1 h, the polymer solution for preparing the polymer film was obtained.

[0095] 2) Preparation of flat sheet membrane: Using a single doctor blade, the film-forming polymer solution obtained in step 1) is fed through a feed tank containing a doctor blade with a thickness of 0.2 mm at 60°C, so that the film-forming polymer solution is coated on a polyethylene terephthalate plastic film substrate with a thickness of 0.4 mm to form a flat sheet membrane. The film-forming polymer solution is wound at a speed of 4 m / min, passes through an air gap of 30 cm in length, and then enters a coagulation bath composed of deionized water at room temperature. After being cooled by deionized water at a certain temperature, the resulting nascent membrane is extracted with deionized water at 25°C to remove the solvent and pore-forming agent. After extraction twice, it is air-dried to obtain the finished flat sheet membrane.

[0096] Membrane surface morphology such as Figure 2 As shown, the pore size of the uncoated film is 47 nm. Figure 4 As shown, the cross-section is a finger-shaped orifice, and the flux recovery rate is 44.8%.

[0097] It is particularly important to note that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the spirit, scope, and content of this invention. Examples include changing the type of coating material, the type of polymer or diluent, the spinning temperature, the type and temperature of the quenching bath, the temperature and length of the air gap, different membrane shapes such as spiral wound, hollow fiber, and tubular, and composite membranes scraped onto high-temperature resistant nonwoven fabrics.

[0098] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the content of the present invention specification, or any direct or indirect application of other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing a charged hydrophilic polymer porous membrane, characterized in that, include: While the film-forming polymer solution undergoes non-solvent-induced phase separation and forms a separation layer on the outer surface of the polymer membrane, a water-soluble coating polymer solution is simultaneously coated on the surface of the separation layer to obtain a charged hydrophilic polymer porous membrane. The film-forming polymer solution uses a fluoropolymer as the film-forming substrate, a nitrogen-containing small molecule as the first solvent, and a high-molecular-weight organic compound as the pore-forming agent. The coating polymer solution is negatively charged and has a viscosity of 800-3000 mPa·s.

2. The method for preparing a charged hydrophilic polymer porous membrane according to claim 1, characterized in that, The coating polymer solution comprises a negatively charged water-soluble polymer containing carboxyl and hydroxyl groups, wherein the negatively charged water-soluble polymer is selected from at least one of poly(hydroxyethyl methacrylate), sodium alginate, and sodium carboxymethyl cellulose.

3. The method for preparing a charged hydrophilic polymer porous membrane according to claim 2, characterized in that, The mass fraction of the negatively charged water-soluble polymer in the polymer solution of the coating layer is 1-2.5%.

4. The method for preparing a charged hydrophilic polymer porous membrane according to claim 2, characterized in that, The coating polymer solution includes the negatively charged water-soluble polymer and a second solvent, wherein the second solvent is water or ethanol that can dissolve the negatively charged water-soluble polymer but cannot dissolve the fluorinated polymer.

5. The method for preparing a charged hydrophilic polymer porous membrane according to claim 1, characterized in that, The mass ratio of the fluoropolymer, the porogen, and the first solvent in the film-forming polymer solution is 18-26:3-8:66-79; the fluoropolymer is a homopolymer or copolymer of polyvinylidene fluoride with a weight average molecular weight of 700-900 kDa.

6. The method for preparing a charged hydrophilic polymer porous membrane according to claim 1, characterized in that, The second solvent in the film-forming polymer solution is N,N-dimethylacetamide or N-methylpyrrolidone, and the pore-forming agent is polyvinylpyrrolidone with a molecular weight of 30-45 kDa.

7. The method for preparing a charged hydrophilic polymer porous membrane according to claim 2, characterized in that, include: S1. The fluorinated polymer, the pore-forming agent and the first solvent are mixed evenly at 50-70°C and then degassed under vacuum to obtain the film-forming polymer solution for preparing the polymer film. S2. The negatively charged water-soluble polymer and the second solvent are mixed evenly at 50°C, and then vacuum degassed to obtain the coating polymer solution for preparing the coating layer. S3. Using a double scraper, the coating polymer solution and the film-forming polymer solution are simultaneously flowed out of a feed tank containing a scraper with a thickness of 0.2 mm at 50-60°C. This allows the film-forming polymer solution to be coated onto a polyethylene terephthalate plastic film substrate with a thickness of 0.4 mm, while the coating polymer solution is coated on top of the film-forming polymer solution. After forming a flat film shape, the film is cooled and cured to form a film. S4. The membrane is soaked and washed with a deionized water extractant at 25°C to obtain the charged hydrophilic polymer porous membrane.

8. The method for preparing a charged hydrophilic polymer porous membrane according to claim 7, characterized in that, The air gap length in S3 is 30-60cm, and the film scraping rate is 2-4m / min.

9. The charged hydrophilic polymer porous membrane prepared by the method according to any one of claims 1-8 is characterized in that, The charged hydrophilic polymer porous membrane is a flat sheet membrane with a pore size of 5-100 nm and a molecular weight cutoff of 10-1000 kDa.

10. The application of the charged hydrophilic polymer porous membrane according to claim 9 in the concentration and purification of polysaccharides in traditional Chinese medicine.

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