A hydrophobic porous separation membrane, its preparation method and application
Hydrophobic porous separation membranes were prepared by selective swelling of block copolymers, which solved the problems of unstable performance and high preparation cost of hydrophobic membranes in the prior art. This method achieves high efficiency and high rejection rate, and is suitable for membrane distillation.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-07-26
- Publication Date
- 2026-04-03
AI Technical Summary
Existing methods for preparing hydrophobic porous membranes suffer from problems such as unstable performance, high preparation costs, and the use of harmful solvents. Furthermore, existing hydrophobic membranes are prone to wetting during long-term use, which affects the quality of produced water and the salt rejection rate.
A hydrophobic porous separation membrane was prepared at room temperature using a block copolymer as a raw material via selective swelling. The block copolymer contains a host phase block A and a hydrophobic dispersed phase block B. A non-toxic alkane-based swelling agent was used to form channels on the membrane surface and inside, ensuring that both the membrane surface and the internal channels are hydrophobic, and the stability was improved through chemical bonding.
A porous separation membrane with high porosity, small pore size, and stable hydrophobicity was prepared. It is suitable for membrane distillation, with a rejection rate of over 99.99%, which reduces production costs and is suitable for industrial production.
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Figure CN117482766B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a separation membrane, specifically to a hydrophobic porous membrane material, its preparation method, and its application in membrane distillation. Background Technology
[0002] Membrane distillation (MD) is a membrane separation technology that combines membrane technology with the distillation process. It uses a hydrophobic porous membrane as the separating medium. Due to the hydrophobicity of the membrane surface, only gas molecules can pass through the pores (liquid water cannot). One side of the membrane is in direct contact with the hot solution to be treated (hot side), while the other side is connected to a cooling medium (cold side). Under the vapor pressure difference caused by the temperature difference across the membrane, water vapor generated in the hot-side solution permeates through the membrane pores and condenses on the cold side, thus producing water or concentrating the hot-side solution. Compared with traditional separation methods, membrane distillation has advantages such as high rejection rate and simple operation, and therefore has broad application prospects in areas such as utilizing low-grade heat sources, treating high-salinity wastewater, and seawater desalination.
[0003] Besides ceramic membranes made from metal oxides such as SiO2, Al2O3, and ZrO2, most existing membrane distillation uses relatively inexpensive organic membranes. Currently, there are two main methods for preparing hydrophobic organic porous membranes: First, the cold stretching method, which is mainly suitable for highly crystalline, poorly soluble polymers, has certain limitations on polymer materials, and the resulting porous membranes have poor hydrophobicity and are easily wetted. Second, the phase inversion method. These methods use organic solvents and generate large amounts of wastewater, thus causing pollution and increasing production costs.
[0004] Existing technologies have also reported the preparation of hydrophobic membranes by coating or depositing natural hydrophobic polymers onto porous membrane surfaces, such as depositing or dip-coating polydimethylsiloxane (PDMS) onto porous surfaces. Although this method can prepare hydrophobic porous membranes for membrane distillation, PDMS is a rubbery material with high fluidity, and cannot be fixed by dip-coating or deposition alone. Therefore, an additional crosslinking step is generally required to chemically bond the rubbery PDMS to the porous membrane surface to give the membrane practical performance. Thus, this method of preparing hydrophobic porous membranes is complex and costly, hindering industrial production.
[0005] Therefore, it is necessary to develop a new method for preparing hydrophobic porous membranes to solve the problems of unstable performance, high preparation cost, or easy generation of harmful solvents in the hydrophobic porous membranes prepared by existing technologies. Summary of the Invention
[0006] The primary objective of this invention is to provide a hydrophobic porous separation membrane that possesses a stable structure, good porosity, hydrophobicity, and pressure resistance.
[0007] Another objective of this invention is to provide a method for preparing the hydrophobic porous separation membrane, which uses only non-toxic and harmless swelling agents such as alkanes, and these swelling agents are non-toxic, harmless, and reusable; the swelling process can be carried out at room temperature, with low energy consumption, is green and environmentally friendly, and is suitable for large-scale industrial application.
[0008] Another object of the present invention is to provide the application of the hydrophobic porous separation membrane in membrane distillation, such that the porous separation membrane has a NaCl rejection rate of over 99.99%.
[0009] The above-mentioned objective of this invention is achieved through the following technical solution:
[0010] In a first aspect, the present invention provides a hydrophobic porous separation membrane made of a block copolymer comprising a host phase block A and a hydrophobic dispersed phase block B; wherein the porous separation membrane has a framework formed by the host block A, and the membrane surface and the internal pore surface are both rich in hydrophobic dispersed phase blocks B.
[0011] In this invention, the bulk phase block A of the block copolymer can be selected from any one of polysulfone (PSF), polyethersulfone (PES), and polystyrene (PS), preferably PSF or PS; the hydrophobic dispersed phase block B has a water contact angle greater than 90° and a specific surface energy γ less than 30 mN·m -1 The preferred block B has a water contact angle greater than 100° and a specific surface energy γ less than 25 mN·m -1 Polymer segments possessing these properties must exhibit both suitable mobility and hydrophobicity, which is crucial for ensuring both pore formation and hydrophobicity of the membrane. In a preferred embodiment of the present invention, block B may specifically be polydimethylsiloxane (PDMS) or polymethylphenylsiloxane.
[0012] The block copolymer can be an AB diblock copolymer or an ABA or BAB triblock copolymer.
[0013] In this invention, the total molecular weight of the block copolymer is 5-500 kDa. Through experimentation, this invention has found that when the mass ratio of the hydrophobic dispersed phase block B in the block copolymer is too high, the rigid structure cannot be maintained. Therefore, it is preferable that the mass ratio of block A in the copolymer exceeds 50%, while the mass ratio of block B in the copolymer is not higher than 50%; more preferably, the mass ratio of block B in the copolymer is not higher than 40%.
[0014] In a preferred embodiment of the present invention, the block copolymer is a PS-PDMS-PS block copolymer, wherein the host phase block A of the block copolymer is PS, and the hydrophobic dispersed phase block B is PDMS. When such a block copolymer is prepared into a separation membrane, the glassy PS blocks at room temperature form the membrane skeleton, while the highly mobile rubbery PDMS segments can move freely, ensuring that both the surface of the porous membrane skeleton and the surface of the internal pores are hydrophobic due to the coating of PDMS segments.
[0015] In a further preferred embodiment of the present invention, the PDMS content in the block copolymer is 10wt%-50wt%, more preferably 15wt%-40wt%.
[0016] In a further preferred embodiment of the present invention, the molecular weight of the PDMS segment in the block copolymer is preferably 5kDa-25kDa.
[0017] Secondly, the present invention also provides a method for preparing a hydrophobic porous separation membrane, comprising: preparing an initial dense membrane using a block copolymer as a raw material, wherein the block copolymer structure comprises a host phase block A and a hydrophobic dispersed phase block B, wherein the dispersed phase block B has a water contact angle greater than 90° (preferably greater than 100°) and a specific surface energy γ PDMS Below 30 mN·m -1 (Preferably below 25 mN·m) -1 The initial dense membrane is immersed in a selective solvent to treat it, causing the hydrophobic dispersed phase block B to swell and further form pores after the solvent evaporates, thus obtaining a hydrophobic porous separation membrane.
[0018] During the swelling process of the preparation method described in this invention, the hydrophobic dispersed phase block B absorbs the solvent and undergoes vigorous expansion. After the swelling ends and the swelling agent evaporates, the volume occupied by the expanded block B is transformed into pores, and the hydrophobic block B adheres to the inside of the pores and the membrane surface. During the above-mentioned swelling process, while the block B expands vigorously, the bulk phase block A also moves accordingly. However, the movement of each block is difficult to be completely synchronized, resulting in an increase in membrane surface roughness. Therefore, the longer the swelling time, the more vigorous the movement of each block, and the higher the surface roughness. As the swelling time prolongs, the membrane surface roughness continuously increases during the swelling process. In addition, a large amount of the hydrophobic dispersed phase block B has migrated to the surface, ultimately forming a porous separation membrane with hydrophobic surface and internal pores.
[0019] In a preferred embodiment of the preparation method of the present invention, the interaction parameter between the selective solvent and the dispersed phase block B is less than 0.5, while the interaction parameter with the host phase block A is greater than 0.5. In a more preferred embodiment of the preparation method of the present invention, the interaction parameter between the selective solvent and the dispersed phase block B is between 0.3 and 0.45, while the interaction parameter with the host phase block is between 0.9 and 1.3.
[0020] In one embodiment of the present invention, the dispersed phase block B and the host phase block A are PDMS and PS, respectively, and the selective solvent is an alkane, preferably any one or a mixture of two or more of n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, undecane, dodecane, tridecane, tetradecane or petroleum ether.
[0021] In the preparation method described in this invention, there are various methods for preparing the initial dense film. For example, the dense film can be prepared by melting and pressing, casting, extrusion, etc., of the solid block copolymer. Alternatively, the dense film can be obtained by dissolving the block copolymer in a solvent and then spin-coating, spraying, immersing, or allowing the solvent to evaporate naturally.
[0022] In the preferred preparation method of the present invention, the initial dense film is immersed in a selective solvent at 0-90°C for 1 min-24 h; more preferably, it is immersed at room temperature for 1-4 h, followed by washing off the solvent or allowing the solvent to evaporate naturally, and then further drying.
[0023] Thirdly, the present invention also provides a composite separation membrane with a hydrophobic surface, comprising a porous substrate layer and a hydrophobic porous surface layer; the surface layer is the hydrophobic porous separation membrane described in the first aspect of the present invention; the composite separation membrane has through-holes throughout, and the surface layer has uniformly distributed surface pores, the pore size of which is between 10-100 nm; preferably, the pore size is between 10-50 nm. The composite separation membrane of the present invention has a hydrophobic surface, preventing liquid water from passing through it.
[0024] In the preferred composite separation membrane of this invention, the porous substrate layer can be a macroporous membrane, nonwoven fabric, or paper. The material of the macroporous membrane can be further selected from polyvinylidene fluoride (PVDF), polysulfone (PSF), polyacrylonitrile (PAN), polyterephthalic acid (PET), or polypropylene (PP); preferably PVDF or PSF.
[0025] Fourthly, the present invention also provides a method for preparing the aforementioned composite separation membrane, comprising: coating a block copolymer onto the surface of a macroporous substrate membrane to obtain a dense, non-porous composite membrane, wherein the block copolymer comprises a host phase block A and a hydrophobic dispersed phase block B, wherein the water contact angle of the dispersed phase block B is greater than 90° (preferably greater than 100°) and its specific surface energy γ PDMS Below 30 mN·m -1 (Preferably below 25 mN·m) -1 Then, the dense, non-porous composite membrane is immersed in a selective solvent to swell block B of the block copolymer. After removing the solvent, a hydrophobic composite separation membrane is obtained. The interaction parameter between the selective solvent and the dispersed phase block B is less than 0.5, while the interaction parameter with the host phase block A is greater than 0.5. In a more preferred preparation method of the present invention, the interaction parameter between the selective solvent and the dispersed phase block B is between 0.3 and 0.45, while the interaction parameter with the host phase block is between 0.9 and 1.3.
[0026] The method for preparing composite separation membranes described in this invention can rapidly modify existing porous membrane materials (especially macroporous membranes). The macroporous substrate membrane can be either hydrophilic or hydrophobic; that is, this invention can rapidly transform any macroporous substrate into a hydrophobic membrane with surface pores of tens of nanometers using the aforementioned method. For example, some polyethylene and polypropylene macroporous substrate membranes, even if they do not exhibit hydrophilicity, can have their surface hydrophobicity increased after being prepared as the composite separation membrane described in this invention.
[0027] Fifthly, the present invention also provides the application of the hydrophobic porous separation membrane or the composite separation membrane with a hydrophobic surface in membrane distillation.
[0028] In existing technologies, although many hydrophobic membranes have been prepared and used in membrane distillation, membrane wetting inevitably occurs during long-term use, affecting the quality of produced water and salt rejection rate. Therefore, preventing wetting of hydrophobic membranes is a key problem that urgently needs to be solved in the field of membrane distillation. Furthermore, current methods for preparing hydrophobic membranes mainly include melt stretching, phase inversion, and electrospinning. Melt stretching typically produces hydrophobic membranes with pore sizes ranging from several hundred nanometers to several micrometers, and their hydrophobicity is generally low, resulting in poor long-term stability. Phase inversion requires the use of large amounts of organic solvents, generating organic wastewater and causing significant pollution. Studies have shown that high-flux, highly hydrophobic PVDF microporous hydrophobic membranes can be prepared using electrospinning technology; however, the application of electrospinning for preparing hydrophobic membranes is very limited, and large-scale production is currently difficult. In addition, research has been conducted on preparing composite separation membranes using hydrophobic polymer materials (such as PDMS) and other polymer materials. However, given the high fluidity of hydrophobic materials such as PDMS, it is often difficult to obtain structurally stable separation membranes, requiring additional curing or cross-linking steps. Moreover, the preparation process of this type of composite membrane is mostly complex, demanding, and highly polluting.
[0029] This invention aims to develop a stable hydrophobic membrane suitable for large-scale industrial production. Starting with the selection of raw materials, the inventors used block copolymers containing hydrophobic blocks as the film-forming raw material. Because the hydrophobic structure in the separation membrane of this invention exists in the form of dispersed phase blocks on the surface and internal structure of the membrane, and is chemically bonded to the framework structure formed by the block copolymer rather than physically mixed or laminated, compared with existing hydrophobic membranes containing PDMS, this invention not only makes the membrane surface hydrophobic, but also makes the channels formed inside the membrane hydrophobic. More importantly, the chemical bonding ensures the stability of the connection between the hydrophobic structure and the framework, preventing the hydrophobic components of the membrane from being lost over time. Experiments have shown that the hydrophobic separation membrane or composite membrane of this invention retains good hydrophobicity even after long-term use. Furthermore, to achieve ideal hydrophobicity on both the surface and internal channels of the separation membrane, the hydrophobic blocks of the block copolymer need to be abundantly distributed both inside and on the surface of the membrane. The inventors fully considered the influence of the properties of hydrophobic blocks on the hydrophobicity of the membrane surface when selecting film-forming raw materials. They chose hydrophobic segments with suitable flowability (e.g., rubber-like segments) and utilized their flowability to achieve the movement of hydrophobic segments to the membrane surface, ensuring a large distribution of hydrophobic structures on the membrane surface. Based on the above selection of film-forming raw materials, the inventors also fully utilized the difference between the solvent and polymer block interactions, using a solvent selective for hydrophobic blocks for swelling treatment. As swelling proceeds, the roughness of the membrane surface continuously increases. Combined with the fact that the rubber-like dispersed phase blocks have already moved to the membrane surface, the resulting porous membrane surface is hydrophobic. After the solvent evaporates, the dispersed phase blocks adhere to the pore surface, which also becomes hydrophobic. In this invention, separation membranes with different performance characteristics can also be obtained by adjusting multiple influencing factors. For example, during the swelling process, the degree of swelling of the membrane can be adjusted by selecting the chain length of the solvent, the swelling temperature, and the swelling time, thereby adjusting the overall porosity and surface hydrophobicity of the hydrophobic porous membrane. As another example, in terms of raw material selection, the surface pore size and surface porosity of the hydrophobic porous membrane can be controlled by changing the block ratio of the block copolymer and the molecular weight of the dispersed phase, thereby adjusting the membrane distillation performance of the hydrophobic membrane.
[0030] In summary, this invention provides a simple and selective swelling method for preparing hydrophobic porous separation membranes using a single raw material. The selection of raw materials and the preparation method of this invention complement each other. Compared with other existing methods, the preparation method of this invention not only uses simple raw materials and requires no auxiliary components, but also uses inexpensive and readily available straight-chain alkanes as swelling agents. Swelling and pore formation can be completed under mild conditions (some preferred methods even at room temperature). The process does not require large amounts of solvent, and the solvent can be reused. The resulting separation membrane has high porosity, small pore size, and narrow pore size distribution. It is an environmentally friendly and convenient membrane preparation method that can significantly reduce the overall production cost of hydrophobic membranes and can be used for large-scale industrial production of hydrophobic porous separation membranes. Attached Figure Description
[0031] Figure 1 This is a surface SEM image of the hydrophobic porous membrane obtained in Example 2.
[0032] Figure 2 This is a magnified SEM image of the surface of the hydrophobic porous membrane obtained in Example 2.
[0033] Figure 3 This is a water contact angle diagram of the hydrophobic porous membrane obtained in Example 2.
[0034] Figure 4 This is a surface SEM image of the hydrophobic porous membrane obtained in Example 3.
[0035] Figure 5 This is a cross-sectional SEM image of the hydrophobic porous membrane obtained in Example 3.
[0036] Figure 6 This is a surface SEM image of the hydrophobic porous membrane obtained in Example 4.
[0037] Figure 7 This is a membrane distillation performance diagram of the hydrophobic composite membrane obtained in Example 5.
[0038] Figure 8 This is a surface SEM image of the hydrophobic porous membrane obtained in Example 6.
[0039] Figure 9 This is a membrane distillation performance diagram of the hydrophobic composite membrane obtained in Example 6.
[0040] Figure 10 This is a surface SEM image of the hydrophobic porous membrane obtained in Example 7.
[0041] Figure 11 This is a membrane distillation performance diagram of the hydrophobic composite membrane obtained in Example 7.
[0042] Figure 12 This is a surface SEM image of the hydrophobic porous membrane obtained in Example 8.
[0043] Figure 13 This is a membrane distillation performance diagram of the hydrophobic composite membrane obtained in Example 8. Detailed Implementation
[0044] The hydrophobic porous separation membrane provided by the present invention is made of a block copolymer, wherein the block copolymer comprises a host phase block A and a hydrophobic dispersed phase block B; in the porous separation membrane, the skeleton is formed by the host block A, and the membrane surface and the internal pore surface are both rich in hydrophobic dispersed phase blocks B.
[0045] The block copolymer can be an AB diblock copolymer or an ABA or BAB triblock copolymer, wherein the host phase block A is selected from any one of polysulfone (PSF), polyethersulfone (PES), and polystyrene (PS), and the block B has a water contact angle greater than 90° and a specific surface energy γ less than 30 mN·m -1 Preferably, the water contact angle is greater than 100° and the specific surface energy γ is less than 25 mN·m. -1 Further preferably, block B is polydimethylsiloxane (PDMS) or polymethylphenylsiloxane. The total molecular weight of the block copolymer is 5-500 kDa.
[0046] In a preferred embodiment, the host phase block A is PS and the dispersed phase block B is PDMS. In a further preferred embodiment, the block copolymer has a molecular weight of PS. 20k -PDMS 5k PS 10k -PDMS 5k PS 8k -PDMS 5k PS 40k -PDMS 10k PS 30k -PDMS 10k PS 20k -PDMS 10k PS 40k -PDMS 25k PS 30k -PDMS 25k PS 20k -PDMS 10k -PS 20k PS 15k -PDMS 10k -PS 15k PS 10k -PDMS 10k -PS 10k PS 10k -PDMS 5k -PS10k PS 25k -PDMS 25k -PS 25k PS 20k -PDMS 25k -PS 20k PS 15k -PDMS 25k -PS 15k PS 28k -PDMS 10k -PS 28k PS 18k -PDMS 10k -PS 18k PS 9k -PDMS 5k -PS 9k The unit is kilodaltons.
[0047] The method for preparing hydrophobic porous separation membranes provided by this invention is based on selective swelling, and may specifically include the following steps:
[0048] 1) Using the block copolymer as the film-forming material, a dense film is prepared by solution film formation or melt film formation methods;
[0049] The dense film can be prepared either by melting the block copolymer and forming a film through pressing, casting, extrusion, or by dissolving the two blocks in a common good solvent and then spin-coating, spraying, and evaporating the solvent to form a film.
[0050] 2) Immerse the dense membrane obtained in 1) in a selective solvent and treat it in a water bath at 0-90℃ for 1 min-24 h to allow the dispersed phase block B to swell under the action of the solvent. The volume occupied by it forms a pore structure after the solvent evaporates. Dry it to obtain a separation membrane with a porous structure with continuous open pores. The selective solvent can be an alkane, specifically selected from any one or a mixture of two or more of n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, undecane, dodecane, tridecane, tetradecane or petroleum ether.
[0051] In a preferred embodiment of the present invention, step 2) involves immersing the dense membrane obtained in step 1) in a selective solvent and treating it at room temperature for 1 min to 24 h. The selective solvent is n-hexane or n-heptane.
[0052] In another preferred embodiment of the present invention, step 2) involves immersing the dense film obtained in step 1) in a selective solvent and treating it at 35-65°C for 1 min-24 h. The selective solvent is n-octane or n-decane.
[0053] The hydrophobic composite separation membrane provided by this invention comprises a porous substrate layer and a hydrophobic porous surface layer; the porous substrate layer can be a macroporous membrane, nonwoven fabric, or paper; the surface layer is the hydrophobic porous separation membrane described in this invention; the composite separation membrane has through-holes throughout, and the surface layer has uniformly distributed surface pores, the pore size of which is between 10-100 nm; preferably, the pore size is between 10-50 nm. The material of the macroporous membrane can be further selected from polyvinylidene fluoride (PVDF), polysulfone (PSF), polyacrylonitrile (PAN), polyterephthalic acid (PET), or polypropylene (PP); preferably PVDF or PSF.
[0054] The method for preparing a composite separation membrane with a hydrophobic surface as described in this invention is also a method based on selective swelling to rapidly modify the surface properties of a macroporous base membrane. It includes: spin-coating, spraying, or dipping a block copolymer onto the surface of a macroporous base membrane to obtain a dense, non-porous composite membrane, and then subjecting the composite membrane to the same swelling treatment process as described in 2) above to obtain a hydrophobic composite separation membrane with tens of nanometer pores.
[0055] In the preparation of surface hydrophobic composite membranes, pores can be formed in just one step of swelling without any other treatment. Compared with the existing technology that requires cross-linking of the hydrophobic PDMS surface, the operation is simpler, the resulting pores are smaller, and the hydrophobicity of the membrane surface is more stable.
[0056] Regarding the membrane distillation performance of the hydrophobic porous separation membrane described in this invention, unlike the large pore sizes (generally from several hundred nanometers to several micrometers) of existing hydrophobic membranes reported in the literature, the hydrophobic membrane provided by this invention has a pore size between 10-100 nm, or even between 10-50 nm, exhibiting a smaller pore size. This makes it more difficult to wet during use and results in a longer service life. Furthermore, the separation membrane prepared by this invention has a significantly higher salt rejection rate, exceeding 99.99%.
[0057] The present invention will be further explained below with reference to the embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.
[0058] Example 1
[0059] PS 28k -PDMS 10k -PS 28k The solution was prepared, spin-coated onto a silicon wafer, and the wafer was then immersed in n-hexane at 25°C for 1 hour. Immediately after treatment, the sample was removed and dried at room temperature to obtain a hydrophobic porous membrane.
[0060] The hydrophobic porous membrane prepared in this embodiment has a porosity of 41.3%, a surface porosity of 8.7%, an average surface pore size of 23.8 nm, and a water contact angle of 115.8°.
[0061] Example 2
[0062] PS 28k -PDMS 10k -PS 28k The solution was prepared, spin-coated onto a silicon wafer, and the wafer was then immersed in n-hexane at 25°C for 4 hours. Immediately after treatment, the sample was removed and dried at room temperature to obtain a hydrophobic porous membrane.
[0063] The hydrophobic porous membrane prepared in this embodiment has a porosity of 43.3%, a surface porosity of 9.8%, an average surface pore size of 24.3 nm, and a water contact angle of 119.5°.
[0064] Figure 1 , Figure 2 The images shown are SEM images of the surface and magnified surface of the hydrophobic porous membrane prepared in this embodiment. Figure 3 This is a diagram of the water contact angle in this embodiment.
[0065] Example 3
[0066] PS 28k -PDMS 10k -PS 28k The solution was prepared, spin-coated onto a silicon wafer, and the wafer was then immersed in n-octane at 55°C for 1 hour. Immediately after treatment, the sample was removed and dried at room temperature to obtain a hydrophobic porous membrane.
[0067] The hydrophobic porous membrane prepared in this embodiment has a porosity of 52.8%, a surface porosity of 27.2%, and an average surface pore size of 39.1 nm.
[0068] Figure 4 and Figure 5 The images shown are SEM images of the surface and cross-section of the hydrophobic porous membrane prepared in this embodiment.
[0069] Compared with Example 1, this embodiment uses high-temperature treatment. Under this temperature condition, the swelling agent interacts more strongly with PS, resulting in higher porosity and larger pore size.
[0070] Example 4
[0071] PS 18k -PDMS 10k -PS 18kThe solution was prepared, spin-coated onto a silicon wafer, and the wafer was then immersed in n-hexane at 25°C for 1 hour. Immediately after treatment, the sample was removed and dried at room temperature to obtain a hydrophobic porous membrane.
[0072] The hydrophobic porous membrane prepared in this embodiment has a porosity of 50.4%, a surface porosity of 17.2%, an average surface pore size of 17.7 nm, and a water contact angle of 119.7°.
[0073] Figure 6 This is a surface SEM image of the hydrophobic porous membrane prepared in this embodiment.
[0074] Compared with Example 2, this embodiment uses a block copolymer with a higher PDMS content, which can achieve a porosity of over 50% and a hydrophobicity of 119.7° at room temperature upon swelling.
[0075] Example 5
[0076] PS 18k -PDMS 10k -PS 18k The solution was prepared and spin-coated onto a hydrophilic PVDF macroporous substrate to form a composite membrane. The composite membrane was then immersed in n-hexane at 25°C for 1 hour. Immediately after treatment, the composite membrane was removed and dried at room temperature to obtain a hydrophobic porous composite membrane.
[0077] The properties of this composite membrane are basically the same as those in Example 4. However, because it is integrated with the macroporous base membrane, the entire membrane has through-pores. Its hydrophobic surface and surface pores of tens of nanometers prevent liquid water from passing through the composite membrane even under pressure, while allowing gas to pass through. Therefore, this membrane is used in membrane distillation. The water flux of the membrane obtained in this example is 13.2 ± 0.9 kg·m³. -2 ·h -1 The salt interception rate is 99.99%.
[0078] Figure 7 The graph shows the membrane distillation performance of the hydrophobic composite membrane prepared in this embodiment. The left vertical axis represents water flux, the right vertical axis represents salt rejection rate, the upper line represents the salt rejection rate test results at each time point, and the lower line represents the water flux test results at each time point. The water flux data are from three sets of parallel experiments.
[0079] Example 6
[0080] PS 28k -PDMS 10k -PS 28kThe solution was prepared and spin-coated onto a hydrophilic PVDF macroporous substrate to form a composite membrane. The composite membrane was then immersed in n-hexane at 25°C for 1 minute. Immediately after treatment, the composite membrane was removed and dried at room temperature to obtain a hydrophobic porous composite membrane.
[0081] The hydrophobic porous composite membrane prepared in this embodiment has a porosity of 34.6%, a surface porosity of 3.6%, and an average surface pore size of 17.7 nm.
[0082] Figure 8 This is a surface SEM image of the hydrophobic porous composite membrane prepared in this embodiment.
[0083] Figure 9 This is a membrane distillation performance graph of the hydrophobic porous composite membrane prepared in this embodiment. The left vertical axis represents water flux, the right vertical axis represents salt rejection rate, the upper broken line represents the salt rejection rate test results at each time point, and the lower broken line represents the water flux test results at each time point. The water flux data are from three sets of parallel experiments.
[0084] In this embodiment, high porosity and uniformly distributed surface pore size were achieved through swelling at room temperature for only 1 minute. The water flux of the composite membrane obtained in this embodiment was 7.6 ± 0.3 kg·m³. -2 ·h -1 The salt interception rate is 99.99%.
[0085] Example 7
[0086] PS 9k -PDMS 5k -PS 9k The solution was prepared and spin-coated onto a hydrophilic PVDF macroporous substrate to form a composite membrane. The composite membrane was then immersed in n-hexane at 25°C for 1 hour. Immediately after treatment, the composite membrane was removed and dried at room temperature to obtain a hydrophobic porous composite membrane.
[0087] The hydrophobic porous composite membrane prepared in this embodiment has a porosity of 40.2%, a surface porosity of 8.9%, and an average surface pore size of 13.9 nm.
[0088] Figure 10 This is a surface SEM image of the hydrophobic porous composite membrane prepared in this embodiment.
[0089] Figure 11 This is a membrane distillation performance graph of the hydrophobic porous composite membrane prepared in this embodiment. The left vertical axis represents water flux, the right vertical axis represents salt rejection rate, the upper broken line represents the salt rejection rate test results at each time point, and the lower broken line represents the water flux test results at each time point. The water flux data are from three sets of parallel experiments.
[0090] Compared to Example 1, this embodiment uses a block copolymer with a lower molecular weight of PDMS, which can achieve higher porosity and smaller surface pore size through swelling at room temperature. The water flux of the composite membrane obtained in this embodiment is 15.96 ± 0.2 kg·m³. -2 ·h -1 The salt interception rate is 99.99%.
[0091] Example 8
[0092] PS 45k -PDMS 25k -PS 45k The solution was prepared and spin-coated onto a hydrophilic PVDF macroporous substrate to form a composite membrane. The composite membrane was then immersed in n-hexane at 25°C for 1 hour. Immediately after treatment, the composite membrane was removed and dried at room temperature to obtain a hydrophobic porous membrane.
[0093] The hydrophobic porous composite membrane prepared in this embodiment has a porosity of 30.2%, a surface porosity of 4.8%, and an average surface pore size of 35.7 nm.
[0094] Figure 12 This is a surface SEM image of the hydrophobic porous composite membrane prepared in this embodiment.
[0095] Figure 13 This is a membrane distillation performance graph of the hydrophobic porous composite membrane prepared in this embodiment. The left vertical axis represents water flux, the right vertical axis represents salt rejection rate, the upper broken line represents the salt rejection rate test results at each time point, and the lower broken line represents the water flux test results at each time point. The water flux data are from three sets of parallel experiments.
[0096] Compared to Example 1, this embodiment uses a block copolymer with a larger molecular weight of PDMS, resulting in a lower porosity achieved through swelling at room temperature, but with a larger surface pore size. The water flux of the composite membrane obtained in this embodiment is 11.0 ± 0.1 kg·m³. -2 ·h -1 The salt interception rate is 99.99%.
[0097] Comparative Example 1
[0098] The porous membrane was prepared according to the method described in Example 1, using n-hexadecane at room temperature. The specific procedure is as follows:
[0099] PS 28k -PDMS 10k -PS 28k The solution was prepared, spin-coated onto a silicon wafer, and the wafer was then immersed in n-hexadecane at 25°C for 1 hour. Immediately after treatment, the wafer was removed and dried at room temperature.
[0100] Compared with Example 1, although both comparative examples used alkanes for swelling at room temperature, the interaction between hexadecane and the block copolymer was too low, with interaction parameters of 1.80 and 1.05 with PS and PDMS blocks, respectively. Therefore, there was no swelling effect on the film, no pores were formed on the surface of the resulting film, and the film thickness did not increase.
[0101] Comparative Example 2
[0102] The porous membrane was prepared according to the method described in Example 1, and the swelling temperature was increased. The specific method is as follows:
[0103] PS 28k -PDMS 10k -PS 28k The solution was prepared, spin-coated onto a silicon wafer, and the wafer was then immersed in n-hexane at 65°C for 1 hour. Immediately after treatment, the sample was removed and dried at room temperature.
[0104] Compared with Example 1, although both comparative examples used n-hexane for swelling, the interaction between n-hexane and PS was too strong at 65°C. As a result, the PS blocks had high mobility in the swelling agent, resulting in micellization on the surface of the obtained membrane and collapse of the membrane skeleton, making it difficult to obtain a porous membrane with stable pore size and dimensions.
[0105] Comparative Example 3
[0106] The porous membrane was prepared according to the method described in Example 1 to increase the PDMS content. The specific method is as follows:
[0107] PS 5k -PDMS 10k -PS 5k The solution was prepared, spin-coated onto a silicon wafer, and the wafer was then immersed in n-hexane at 25°C for 1 hour. Immediately after treatment, the sample was removed and dried at room temperature.
[0108] Compared with Example 1, although both comparative examples used n-hexane for swelling, the PDMS content was too high. Since n-hexane and PDMS have a strong interaction, the movement of PDMS blocks easily drives the movement of PS blocks. As a result, the swollen membrane experienced skeletal collapse, making it difficult to obtain a porous membrane with stable pore size.
Claims
1. A method for preparing a hydrophobic porous separation membrane, comprising: An initial dense film was prepared using a block copolymer as a raw material. The block copolymer structure comprises a host phase block A and a hydrophobic dispersed phase block B. The host phase block A is selected from any one of polysulfone PSF, polyethersulfone PES, and polystyrene PS. The dispersed phase block B has a water contact angle greater than 90° and a specific surface energy less than 30 mN·m. -1 The initial dense membrane is treated by immersing it in a selective solvent, wherein the selective solvent is an alkane, and the interaction parameter between the selective solvent and the dispersed phase block B is between 0.3 and 0.45, while the interaction parameter between the selective solvent and the host phase block A is between 0.9 and 1.3, so that the hydrophobic dispersed phase block B swells and further forms pores after the solvent evaporates, thus obtaining a hydrophobic porous separation membrane.
2. The method according to claim 1, characterized in that: The main phase block A has a mass ratio of more than 50% in the block copolymer, while the dispersed phase block B has a mass ratio of no more than 50% in the block copolymer.
3. The method according to claim 1, characterized in that: The main phase block A has a mass ratio of more than 50% in the block copolymer, while the dispersed phase block B has a mass ratio of no more than 40% in the block copolymer.
4. The method according to claim 1, characterized in that: The main phase segment A is PSF or PS.
5. The method according to claim 1, characterized in that: The dispersed phase block B and the host phase block A are PDMS and PS, respectively; the alkane is any one or a mixture of two or more of n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, undecane, dodecane, tridecane, tetradecane or petroleum ether.
6. The method according to claim 5, characterized in that: The PDMS content in the block copolymer is 10 wt%-50 wt%.
7. The method according to claim 5, characterized in that: The PDMS content in the block copolymer is 15wt%-40wt%.
8. The method according to any one of claims 5-7, characterized in that: The molecular weight of the PDMS is 5 kDa-25 kDa.
9. The method according to claim 1, characterized in that: The alkane mentioned is n-hexane, n-octane, or n-decane.
10. The method according to claim 1, characterized in that: The process of immersing the initial dense film in a selective solvent is described as soaking at 0-90°C for 1 min-24 h.
11. The method according to claim 1, characterized in that: The process of immersing the initial dense film in a selective solvent involves soaking it at room temperature for 1-4 hours, followed by washing off the solvent or allowing the solvent to evaporate naturally, and then further drying it.
12. A hydrophobic porous separation membrane, characterized in that: The membrane is prepared according to any one of claims 1-11; in the porous separation membrane, the framework is formed by the main phase block A, and the membrane surface and the internal pore surface are both rich in the hydrophobic dispersed phase block B.
13. A composite separation membrane with a hydrophobic surface, comprising a porous substrate layer and a hydrophobic porous surface layer; wherein the surface layer is the hydrophobic porous separation membrane of claim 12; wherein the composite separation membrane has through channels as a whole, and the surface has uniformly distributed surface channels, wherein the pore size of the surface channels is between 10-100 nm.
14. The composite separation membrane as described in claim 13, characterized in that: The pore size of the surface channels is between 10-50 nm.
15. The composite separation membrane according to any one of claims 13-14, characterized in that: The porous substrate layer is a macroporous membrane, nonwoven fabric, or paper.
16. The composite separation membrane as described in claim 15, characterized in that: The macroporous base membrane is made of polyvinylidene fluoride (PVDF), polysulfone (PSF), polyacrylonitrile (PAN), polyterephthalic acid (PET), or polypropylene (PP).
17. The composite separation membrane as described in claim 15, characterized in that: The material of the macroporous base membrane is selected from PVDF or PSF.
18. A method for preparing the composite separation membrane of claim 13, comprising: A block copolymer is coated onto the surface of a macroporous substrate membrane to obtain a dense, non-porous composite membrane. The block copolymer comprises a host phase block A and a hydrophobic dispersed phase block B. The host phase block A is selected from any one of polysulfone (PSF), polyethersulfone (PES), and polystyrene (PS), while the dispersed phase block B has a water contact angle greater than 90° and a specific surface energy less than 30 mN·m. -1 The composite membrane is prepared by immersing it in a selective solvent, wherein the selective solvent is an alkane, to swell block B of the block copolymer. After removing the solvent, a hydrophobic composite separation membrane is obtained. The interaction parameter between the selective solvent and the dispersed phase block B is between 0.3 and 0.45, and the interaction parameter between the selective solvent and the host phase block A is between 0.9 and 1.
3.
19. A method for rapidly modifying the hydrophobicity of a macroporous membrane surface, comprising: A block copolymer is coated onto the surface of a macroporous substrate membrane to obtain a dense, non-porous composite membrane. The block copolymer comprises a host phase block A and a hydrophobic dispersed phase block B. The host phase block A is selected from any one of polysulfone (PSF), polyethersulfone (PES), and polystyrene (PS), while the dispersed phase block B has a water contact angle greater than 90° and a specific surface energy less than 30 mN·m. -1 The composite membrane is prepared by immersing it in a selective solvent, wherein the selective solvent is an alkane, to swell block B of the block copolymer. After removing the solvent, a hydrophobic composite separation membrane is obtained. The interaction parameter between the selective solvent and the dispersed phase block B is between 0.3 and 0.45, and the interaction parameter between the selective solvent and the host phase block A is between 0.9 and 1.
3.
20. The method according to any one of claim 18 or 19, characterized in that: The alkane is any one or a mixture of two or more of the following: n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, undecane, dodecane, tridecane, tetradecane, or petroleum ether.
21. The method according to any one of claim 18 or 19, characterized in that: The alkane mentioned is n-hexane, n-octane, or n-decane.
22. The method according to any one of claim 18 or 19, characterized in that: The composite film with a dense and non-porous surface is immersed in a selective solvent for 1 min to 24 h at 0-90 °C.
23. The method according to any one of claim 18 or 19, characterized in that: The composite film with a dense and non-porous surface is immersed in a selective solvent for 1-4 hours at room temperature, followed by washing off the solvent or allowing the solvent to evaporate naturally, and then further drying.
24. The application of the hydrophobic porous separation membrane of claim 12 or the composite separation membrane of any one of claims 13-17 in membrane distillation.