Preparation method of porous polymer membrane

Abstract

The preparation method comprises the following steps: fully mixing a good solvent and a poor solvent of a polymer, adding a film-forming polymer while stirring at room temperature, fully dissolving to form a homogeneous solution, and then coating or spinning the homogeneous solution to form a primary membrane; immersing the primary membrane into a coagulating bath of which the temperature is higher than the low critical solution temperature of the homogeneous solution for phase separation; according to the invention, heat transfer is adopted as driving force, so that a polymer composition change route rapidly passes through a bisection line area, and spiral section line phase splitting is carried out, so that a complex mass transfer rate regulation process is avoided. And meanwhile, by utilizing the characteristic of low critical solution temperature of a polymer system, the inconvenience conditions of high-temperature operation environment, extraction agent recovery and the like in a thermally induced phase separation process are overcome.

Description

Technical Field

[0001] The present invention relates to porous membranes and methods for preparing porous membranes. More particularly, the present invention relates to a method for producing microporous membranes from homogeneous polymer solutions. Background Art

[0002] Microporous membranes are used in a wide range of applications, as separation filters, they remove particles and bacteria from a variety of solutions. Examples include buffers and therapeutic solutions in the pharmaceutical industry; ultrapure water and organic solvents in microelectronic wafer manufacturing processes; removing bacteria from food and beverage products and for pretreatment in water purification processes. In addition, they are used in medical diagnostic devices, where their high porosity results in excellent absorption and hygroscopic properties.

[0003] The production process of porous polymer microporous membranes requires strict control of each parameter, which can be summarized into two parts: formulation and process. The formulation part includes the type and concentration of polymer, the type and concentration of solvent, the type and concentration of additives, the temperature of homogeneous solution, etc. The flat membrane process part includes the temperature and type of coagulation bath, the temperature and humidity of air bath, the thickness and speed of film scraping, etc. The hollow fiber process part also includes the temperature and type of external coagulation bath, the temperature and type of core liquid, the spinning speed and flow rate, the winding speed, the air bath distance, etc. Many complex parameters are added together to finally prepare a microporous membrane with the desired pure water flux, standard pore size, and narrow pore size distribution.

[0004] In general, the preparation of porous polymer membranes is based on the immersion of a formed polymer homogeneous solution into a non-solvent bath, with phase separation preferentially occurring on the surface of the homogeneous solution, and then gradually occurring internal phase separation. This method is called non-solvent induced phase separation (NIPS). Replacing the liquid non-solvent bath with a gaseous non-solvent bath is called vapor induced phase separation (VIPS). Based on the former, the homogeneous solution generally separates according to the nucleation growth mechanism, and the solvent and the non-solvent bath in the homogeneous solution undergo rapid mass transfer exchange, resulting in an instantaneous phase separation process, thereby generating a sub-layer structure of finger-like macropores or cellular macropores and a thinner cortical structure. Based on the latter, the homogeneous solution generally separates according to the nucleation growth mechanism, but the solvent and the non-solvent bath in the homogeneous solution undergo slow mass transfer exchange, resulting in a delayed phase separation process, thereby generating a sub-layer structure of sponge-like pores or finger-like pores and sponge-like pores coexisting; and because of its slow mass transfer exchange, it is easy to form a porous symmetrical membrane structure without a cortex. In particular, by combining the above methods, that is, performing a pre-gelation treatment in a gas phase coagulation bath or atomizing a non-solvent to assist non-solvent induced phase separation before immersing in a liquid phase coagulation bath, a polymer microporous membrane structure with high surface porosity and sub-layer sponge-like pores can be prepared. Alternatively, adding a large amount of solvent to the liquid phase coagulation bath can effectively control the exchange rate between the homogeneous solution and the non-solvent, and also reduce the polymer concentration at the cortex of the homogeneous solution, thereby achieving a polymer microporous membrane structure with high surface porosity and sub-layer sponge-like pores. The process of preparing a microporous membrane structure is a complex issue, and an appropriate mass transfer rate is usually required to control the growth of the polymer-poor phase in the nucleation growth mechanism, thereby producing a higher porosity within the micropore range. The mass transfer rate of the NIPS process is too fast, while the VIPS process has the inherent disadvantage of a low mass transfer rate.

[0005] In summary, it is difficult to control the membrane pore structure by mass transfer rate because the composition of the polymer homogeneous solution changes after being immersed in the coagulation bath. The change paths are diverse. Figure 1 . When the polymer composition enters the inner region of the binodal, a porous structure is formed. NIPS or VIPS undergoes a mass transfer process, usually entering the binodal region from above or below the critical point to undergo nucleation growth mechanism phase separation; a few enter the spinodal region through the critical point to undergo spinodal mechanism phase separation. The former undergoes instantaneous phase separation or delayed phase separation to generate cellular pores, finger-like pores or sponge-like pores; the latter generates sponge-like pores. Therefore, changing the polymer composition to enter the spinodal region will result in an ideal porous polymer membrane structure. The mass transfer process is obviously difficult to achieve, and the thermally induced phase separation method (TIPS) uses the heat transfer process as the driving force for phase separation, which can make the casting liquid composition quickly pass through the binodal metastable region when the temperature changes, enter the spinodal region, and obtain an ideal membrane pore structure. However, the TIPS method uses the upper critical solution temperature (UCST) of the polymer solution, which is usually high in value, which is easy to cause solvent volatilization, and high-temperature operation has safety hazards. Summary of the invention

[0006] The object of the present invention is to provide a method for producing a microporous membrane from a homogeneous polymer solution, so as to solve the difficulties and shortcomings in the above-mentioned preparation methods.

[0007] The objective of the present invention is achieved through the following technical solutions:

[0008] The gist of the present invention is to prepare a homogeneous solution of a polymer, including a polymer, a good solvent, and a poor solvent, wherein the homogeneous solution has thermodynamic properties. The critical temperature has the lowest point on the polymer composition temperature diagram, which is called the lower critical solution temperature (LCST), see the attached Figure 2 . When the temperature of the homogeneous solution is lower than the LCST, they are miscible and appear as a transparent solution; when the temperature of the homogeneous solution is higher than the LCST, they are immiscible and appear turbid. Prepare a polymer homogeneous solution below the LCST and form it by coating or spinning. Immerse the formed homogeneous solution in a coagulation bath with a coagulation bath temperature higher than the LCST, and quickly enter the spinodal region with heat transfer as the driving force, and the spinodal phase separation process occurs. Wash the formed primary polymer film in deionized water, and then soak it in a 50% glycerol aqueous solution, rinse and dry it naturally.

[0009] A method for preparing a porous polymer membrane, the specific steps of which are as follows:

[0010] Step 1: After fully mixing one or several good solvents with one or several poor solvents, add one or several film-forming polymers thereto, stir evenly to form a homogeneous solution, which is transparent at room temperature, and then keep it at 25°C to degas.

[0011] In step 1, the good solvent is a nitrogen-containing solvent, such as N-methyl-2-pyrrolidone, dimethylformamide, and dimethylacetamide; the good solvent can also be one or more of dimethyl sulfoxide, tetrahydrofuran, γ-butyrolactone, and ε-caprolactam.

[0012] In step 1, the poor solvent is an oligomeric alkylene oxide, specifically comprising one or more of ethylene glycol, diethylene glycol, triethylene glycol, tripropylene glycol ether, polyethylene glycol, dipropylene glycol, tripropylene glycol, tetraethylene glycol or tetraethylene glycol dimethyl ether.

[0013] In step 1, the film-forming polymer is one or more of polyvinylidene fluoride, cellulose acetate, cellulose nitrate, polyamide, polysulfone, polyethersulfone, polyacrylonitrile, polyimide, and polyolefin.

[0014] Additives may also be added to the homogeneous solution in step 1, and the additives include one or more of polyvinyl pyrrolidone, polyethylene glycol, poly 2-ethyl-2-oxazoline, hydroxy polyether sulfone, and sulfonated polyether sulfone.

[0015] In step 1, the good solvent is 10 to 70 parts, the poor solvent is 20 to 90 parts, the film-forming polymer is 10 to 20 parts, and the additive is 0 to 10 parts.

[0016] In step 2, the homogeneous solution is coated or spun below the LCST to form a primary membrane, and a coagulation bath with a temperature higher than the LCST is used as a heat source. When the homogeneous solution contacts the heat source, the heat transfer process rate is much greater than the mass transfer process, and a heat-driven polymer composition path change occurs, rapidly passing through the binodal region, and spinodal phase separation occurs. The primary membrane undergoes phase separation in the coagulation bath, thereby generating a porous polymer membrane.

[0017] In step 2, the coagulation bath is one or more of water, ethanol, toluene, acetone, and ethyl acetate.

[0018] The coagulation bath in step 2 can also be a mixed solution of a good solvent and a non-solvent, the good solvent includes one or more of N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, tetrahydrofuran, γ-butyrolactone, and ε-caprolactam, and the non-solvent includes one or more of water, ethanol, toluene, acetone, and ethyl acetate.

[0019] In step 2, the primary membrane is treated in an air bath for 5-30 seconds; the coagulation bath temperature is 30-100° C., and the treatment time is 3-300 seconds.

[0020] The method for preparing the porous polymer membrane may further include other steps, such as placing the porous polymer membrane in a water bath for immersion for 6 to 72 hours, with the water temperature being room temperature.

[0021] The method for preparing the porous polymer membrane may further include other steps, such as placing the porous polymer membrane in a 50% glycerol aqueous solution, soaking the porous polymer membrane for 6 to 72 hours at room temperature; and naturally drying the membrane after rinsing.

[0022] Compared with the prior art, the present invention utilizes the LCST characteristics of the polymer, prepares a homogeneous solution below the LCST of the system and forms a primary membrane, and performs a phase separation process dominated by heat transfer in a coagulation bath whose coagulation bath temperature is higher than the LCST, so that the composition of the homogeneous solution system changes rapidly into the spinodal region, and spinodal phase separation occurs, thereby obtaining a porous polymer membrane with a high surface porosity, a narrow pore size distribution, and a sub-layer with a sponge-like pore structure. This technology avoids the influence of the difficult-to-control mass transfer process on the pore size structure in NIPS and VIPS, and at the same time, the lower LCST makes the system temperature relatively mild, avoiding the safety hazards of high temperature operation and solvent volatilization in TIPS. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the NIPS ternary phase diagram.

[0024] Figure 2is a schematic diagram of polymer composition and temperature.

[0025] Figure 3 This is a scanning electron microscope image of the upper surface of the microporous membrane in Example 1.

[0026] Figure 4 This is a scanning electron microscope image of a sublayer of the microporous membrane in Example 1.

[0027] Figure 5 This is a scanning electron microscope image of the lower surface of the microporous membrane in Example 1.

[0028] Figure 6 This is a scanning electron microscope image of the upper surface of the microporous membrane in Comparative Example 1.

[0029] Figure 7 This is a scanning electron microscope image of the sublayer of the microporous membrane in Comparative Example 1.

[0030] Figure 8 This is a scanning electron microscope image of the lower surface of the microporous membrane in Comparative Example 1. Specific embodiments

[0031] The following are preferred specific embodiments of the present invention, but the present invention is not limited to these embodiments.

[0032] Example 1

[0033] 45 parts of diethylene glycol and 45 parts of dimethylacetamide were fully mixed to prepare a mixed solvent of a good solvent and a poor solvent. 16 parts of polyethersulfone (E6020P, BASF) resin were added to the solvent and stirred and dissolved at room temperature for 12 hours to obtain a polyethersulfone homogeneous solution. The homogeneous solution was kept at 25°C for 6 hours for degassing. The homogeneous solution was then coated on a glass plate with a 300μm scraper to obtain a primary membrane. After a 5s air bath time, the primary membrane was transferred to a 60°C aqueous solution for phase separation, and the phase separation time was 20s to obtain a porous polymer membrane. The porous polymer membrane can also be immersed in an aqueous solution for 12 hours at room temperature; then the porous polymer membrane was immersed in a 50% glycerol aqueous solution for 12 hours, rinsed and dried naturally.

[0034] Example 2

[0035] 54 parts of triethylene glycol and 36 parts of dimethylacetamide were fully mixed to prepare a mixed solvent of a good solvent and a poor solvent. 13 parts of polyethersulfone (E6020P, BASF) resin and 2 parts of poly-2-ethyl-2-oxazoline were added to the solvent, and the mixture was stirred and dissolved at room temperature for 12 hours to obtain a polyethersulfone homogeneous solution. The homogeneous solution was kept at 25°C for 6 hours for degassing. The homogeneous solution was then coated on a glass plate with a 300μm scraper to obtain a primary membrane. After a 5s air bath time, the primary membrane was transferred to a 60°C aqueous solution for phase separation, and the phase separation time was 10s, thereby obtaining a porous polymer membrane. The porous polymer membrane can also be immersed in an aqueous solution for 12 hours at room temperature; the porous polymer membrane is then immersed in a 50% glycerol aqueous solution for 12 hours, rinsed and dried naturally.

[0036] Example 3

[0037] 65 parts of polyethylene glycol and 25 parts of dimethylacetamide were fully mixed to prepare a mixed solvent of a good solvent and a poor solvent. 14 parts of polyethersulfone (E6020P, BASF) resin were added to the solvent and stirred and dissolved at room temperature for 12 hours to obtain a polyethersulfone homogeneous solution. The homogeneous solution was kept at 25°C for 6 hours for degassing. The homogeneous solution was then coated on a glass plate with a 300μm scraper to obtain a primary membrane. After a 5s air bath time, the primary membrane was transferred to an 85°C aqueous solution for phase separation, and the phase separation time was 30s, thereby obtaining a porous polymer membrane. The porous polymer membrane can also be immersed in an aqueous solution for 12 hours at room temperature; the porous polymer membrane is then immersed in a 50% glycerol aqueous solution for 12 hours, rinsed and dried naturally.

[0038] Example 4

[0039] 40 parts of diethylene glycol and 50 parts of N-methyl-2-pyrrolidone were fully mixed to prepare a mixed solvent of a good solvent and a poor solvent. 16 parts of polyethersulfone (3000P, Solvay) resin were added to the solvent and stirred and dissolved at room temperature for 12 hours to obtain a polyethersulfone homogeneous solution. The homogeneous solution was kept at 25°C for 6 hours for degassing. The homogeneous solution was then coated on a glass plate with a 200μm scraper to obtain a primary membrane. After a 5s air bath time, the primary membrane was transferred to a 60°C aqueous solution for phase separation, and the phase separation time was 3s, thereby obtaining a porous polymer membrane. The porous polymer membrane can also be immersed in an aqueous solution for 12 hours at room temperature; then the porous polymer membrane was immersed in a 50% glycerol aqueous solution for 12 hours, rinsed and dried naturally.

[0040] Example 5

[0041] 45 parts of diethylene glycol and 45 parts of dimethylacetamide were fully mixed to prepare a mixed solvent of a good solvent and a poor solvent. 13 parts of polyethersulfone (E6020P, BASF) resin and 4 parts of polyvinylpyrrolidone were added to the solvent, and the mixture was stirred and dissolved at room temperature for 12 hours to obtain a polyethersulfone homogeneous solution. The homogeneous solution was kept at 25°C for 6 hours to degas. The homogeneous solution was then coated on a glass plate with a 200μm scraper to obtain a primary membrane. After a 5s air bath time, the primary membrane was transferred to a 60°C aqueous solution for phase separation, and the phase separation time was 20s, thereby obtaining a porous polymer membrane. The porous polymer membrane can also be immersed in an aqueous solution for 12 hours at room temperature; the porous polymer membrane is then immersed in a 50% glycerol aqueous solution for 12 hours, rinsed and dried naturally.

[0042] Comparative Example 1

[0043] 65 parts of polyethylene glycol and 25 parts of dimethylacetamide were fully mixed to prepare a mixed solvent of a good solvent and a poor solvent. 14 parts of polyethersulfone (E6020P, BASF) resin were added to the solvent and stirred and dissolved at room temperature for 12 hours to obtain a polyethersulfone homogeneous solution. The homogeneous solution was kept at 25°C for 6 hours to degas. The homogeneous solution was then coated on a glass plate with a 300μm scraper to obtain a primary membrane. After a 5s air bath time, the primary membrane was transferred to a 25°C aqueous solution for phase separation, and the phase separation time was 20s, thereby obtaining a porous polymer membrane. The porous polymer membrane can also be immersed in an aqueous solution for 12 hours at room temperature; the porous polymer membrane is then immersed in a 50% glycerol aqueous solution for 12 hours, rinsed and dried naturally.

Claims

1. A method for preparing a porous polymer membrane, characterized in that: The specific steps are as follows: Step 1: a solution system contains at least one film-forming polymer, the solution is composed of at least two solvent components, and a homogeneous solution is obtained after sufficient stirring; one solvent in the solution is a good solvent for the polymer, and the other solvent is a poor solvent for the polymer; the poor solvent includes an oligomeric alkylene oxide; Step 2: The homogeneous solution is coated or spun to form the desired primary membrane, and the primary membrane is placed in a coagulation bath and fully heated to separate its phases and become turbid, thereby obtaining a porous polymer membrane; the lower critical solution temperature of the homogeneous solution is lower than the coagulation bath temperature.

2. The method for preparing a porous polymer membrane according to claim 1, characterized in that: The film-forming polymer in step 1 includes one or more of polyvinylidene fluoride, cellulose acetate, cellulose nitrate, polyamide, polysulfone, polyethersulfone, polyacrylonitrile, polyimide, and polyolefin.

3. The method for preparing a porous polymer membrane according to claim 2, characterized in that: The good solvent for the polymer in step 1 includes one or more of N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, tetrahydrofuran, γ-butyrolactone, and ε-caprolactam.

4. The method for preparing a porous polymer membrane according to claim 3, characterized in that: The oligomeric alkylene oxide in step 1 includes one or more of ethylene glycol, diethylene glycol, triethylene glycol, tripropylene glycol ether, polyethylene glycol, dipropylene glycol, tripropylene glycol, tetraethylene glycol or tetraethylene glycol dimethyl ether.

5. The method for preparing a porous polymer membrane according to claim 4, characterized in that: In step 2, the primary membrane is in the form of a flat plate or a hollow fiber.

6. The method for preparing a porous polymer membrane according to claim 5, characterized in that: In step 2, the primary membrane is treated in an air bath for 5-30 seconds; the coagulation bath temperature is 30-100° C., and the primary membrane is immersed in the coagulation bath for 3-300 seconds.

7. The method for preparing a porous polymer membrane according to claim 6, characterized in that: The coagulation bath in step 2 includes one or more of water, ethanol, toluene, acetone, and ethyl acetate.

8. The method for preparing a porous polymer membrane according to claim 6, characterized in that: The coagulation bath in step 2 is a mixed solution of a good solvent and a non-solvent, the good solvent includes one or more of N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, tetrahydrofuran, γ-butyrolactone, and ε-caprolactam, and the non-solvent includes one or more of water, ethanol, toluene, acetone, and ethyl acetate.

9. The method for preparing a porous polymer membrane according to claim 1, characterized in that: Additives may also be added to the homogeneous solution, including one or more of polyvinyl pyrrolidone, polyethylene glycol, poly 2-ethyl-2-oxazoline, hydroxy polyether sulfone, and sulfonated polyether sulfone; the homogeneous solution contains 10 to 70 parts of good solvent, 20 to 90 parts of poor solvent, 10 to 20 parts of film-forming polymer, and 0 to 10 parts of additives.

10. The method for preparing a porous polymer membrane according to claim 1, characterized in that: In step 2, the porous polymer membrane can also be immersed in an aqueous solution for 6 to 72 hours; then immersed in a 5-50% glycerol aqueous solution for 6 to 72 hours, rinsed and naturally dried.

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