CN107970789B - Hydrophobic membrane with micro-nano structure surface functional layer and preparation method thereof - Google Patents
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Abstract
A hydrophobic membrane with a micro-nano structure surface functional layer and a preparation method thereof are disclosed, the preparation method comprises the following steps: 1) drying crystalline or semi-crystalline polymer powders or granules; 2) mixing 8-20% of polymer, 0-8% of additive and 70-90% of organic solvent by weight percent, stirring at a constant temperature of 20-70 ℃ for a period of time until the mixture is uniform, and defoaming at a constant temperature of 20-70 ℃ to form a uniform membrane casting solution; 3) scraping the obtained casting film liquid into a film by a film scraping device, and staying in air with certain humidity for a period of time at the temperature of 50-80 ℃ to promote partial solid-liquid phase conversion; 4) immersing the primary film obtained in the step 3) in a coagulating bath at 15-50 ℃, and airing after soaking for a period of time. The surface of the prepared hydrophobic membrane has a micro-nano convex structure with a specific size, so that the hydrophobic property of the hydrophobic membrane is improved, and the membrane wetting phenomenon in the hydrophobic membrane process is solved.
Description
Technical Field
The invention belongs to the technical field of hydrophobic membrane preparation, and relates to a hydrophobic membrane with a micro-nano protruding structure surface functional layer and a preparation method thereof.
Background
Membrane separation processes based on hydrophobic membranes have gained widespread attention over nearly a decade, and the development of processes such as membrane distillation, pervaporation, membrane crystallization, etc. put higher demands on the performance of hydrophobic membranes. Vinylidene fluoride is a commonly used membrane-making material for preparing hydrophobic membranes, and has gained wide attention in the aspect of hydrophobic membrane preparation. In recent years, vinylidene fluoride-based copolymers such as vinylidene fluoride-hexafluoroethylene, vinylidene fluoride-chlorotrifluoroethylene and the like are also applied to hydrophobic film preparation and application.
The current phase inversion method is an important method for preparing the hydrophobic membrane of the vinylidene fluoride and the copolymer thereof due to simple operation and easy realization of engineering. However, the film forming process is influenced by various factors, particularly for crystalline or semi-crystalline polymers, solid-liquid phase inversion and liquid-liquid phase inversion occur simultaneously in the phase inversion process, and the competition process finally determines the film morphology and structure. The optimization of the film appearance and structure can be realized through the adjustment of the phase inversion way and the phase inversion speed, and then the film with ideal performance is prepared. Currently, the adjustment of the film-making system such as the selection of the solvent and the addition of the additive/mixed additive; adjustment of environmental conditions such as control of humidity and temperature; the control of the phase inversion process in the film forming process can be realized by optimizing the composition and temperature of the coagulating bath.
However, with hydrophobic membranes, membrane wetting and membrane fouling are still the most significant problems at present, which leads to unstable membrane performance and short lifetime. To solve these problems, hydrophobic modification of membranes is a widely used method. The membrane modification refers to that the surface treatment is carried out on the formed membrane by a physical or chemical method, and the surface treatment comprises the processes of chemical group grafting, ultraviolet light/plasma treatment, in-situ chemical reaction and the like, and the hydrophobic functional group on the surface of the membrane is increased or the surface structure of the membrane is changed to realize the improvement of hydrophobicity. However, the modification process is complex and difficult to control, and the phenomenon of uneven film surface is easy to occur. Meanwhile, membrane modification may affect the original structure of the membrane, and affect the strength and other properties of the membrane.
Disclosure of Invention
In view of the above, the main objective of the present invention is to provide a hydrophobic film with a micro-nano structured surface functional layer and a method for preparing the same, so as to at least partially solve at least one of the above-mentioned technical problems.
In order to achieve the purpose, the technical scheme of the invention is as follows:
as one aspect of the present invention, there is provided a method for preparing a hydrophobic membrane having a micro-nano protruding structure surface functional layer, the method being a phase inversion method, comprising the steps of:
1) drying crystalline or semi-crystalline polymer powders or granules to remove the effect of moisture on the thermodynamic properties of the casting solution;
2) mixing 8-20% of polymer, 0-8% of additive and 70-90% of organic solvent by weight percentage, stirring at constant temperature of 20-70 ℃ for a period of time until the mixture is uniform, and defoaming at constant temperature of 20-70 ℃ to form uniform casting solution;
3) scraping the casting solution in the step 2) into a film by a film scraping device, and staying in air with certain humidity at the temperature of 50-80 ℃ for a period of time to promote partial solid-liquid phase conversion to form a primary film;
4) immersing the primary membrane obtained in the step 3) in a coagulating bath at 15-50 ℃, and airing after soaking for a period of time to obtain the hydrophobic membrane.
Preferably, in step 1), the crystalline or semi-crystalline polymer is polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyvinylidene fluoride-chlorotrifluoroethylene copolymer (PVDF-CTFE), or polyvinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE).
Preferably, in the step 2), the constant-temperature stirring time is 5-24 hours, the constant-temperature defoaming time is more than 12 hours, and the defoaming mode comprises standing and degassing and vacuum degassing.
Preferably, in the step 2), the organic solvent is selected from one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, N-methylpyrrolidone, dimethylsulfoxide, diethylacetamide and acetone;
the additive is selected from one or more of different film forming additives such as inorganic non-solvent, inorganic salt, micromolecular organic matter, macromolecular organic matter and the like, and preferably comprises one or more of water, polyethylene glycol, acetone, ethanol, glycerol, trimethyl phosphate, oxalic acid, lithium chloride, lithium perchlorate, lithium bromide, sodium chloride and phosphoric acid.
Preferably, the solvent evaporation environment in the step 3) is 30-80 ℃, the solvent evaporation time is 5-1800s, preferably 30-600s, and the evaporation environment humidity is 0-80%.
Preferably, in step 4), the coagulation bath mainly contains non-solvent water, and further comprises one or more organic solvents selected from ethanol, dimethylformamide, dimethylacetamide, triethyl phosphate, N-methylpyrrolidone, dimethyl sulfoxide, diethylacetamide and acetone.
Preferably, in step 4), the soaking time is 2 hours or more.
As another aspect of the present invention, there is provided a hydrophobic film having a micro-nano protruding structure surface functional layer prepared by the above preparation method, wherein a micro-nano crystalline structure is formed on the surface of the hydrophobic film, a contact angle is 85 to 150 degrees, a surface roughness is 20 to 150nm, and a maximum peak-to-valley distance is 40 to 400 nm.
Compared with the prior art, the invention has the following beneficial effects:
1. the hydrophobic membrane realizes the construction of a membrane surface micro-nano structure by a simple phase inversion method, and controls the size of a protruding structure by controlling the time of an exposure stage (a solvent evaporation stage);
2. the hydrophobic membrane provided by the invention has the advantages that the hydrophobic performance of the membrane is obviously improved, and the membrane wetting phenomenon in the hydrophobic membrane process is slowed down;
3. the hydrophobic membrane can increase the turbulence on the surface of the membrane due to the construction of the surface protruding structure, thereby reducing the membrane pollution phenomenon to a certain extent;
4. the hydrophobic membrane of the invention can be applied to various hydrophobic membrane separation processes, including membrane distillation, gas separation membranes, pervaporation and the like.
Drawings
FIG. 1 is an SEM photograph of the surface of the polymer film obtained in example 1;
FIG. 2 is an atomic force microscope photograph of the surface of the polymer film obtained in example 1;
FIG. 3 is an SEM photograph of the surface of the polymer film obtained in example 2;
FIG. 4 is an atomic force microscope photograph of the surface of the polymer film obtained in example 2;
FIG. 5 is an SEM photograph of the surface of the polymer film obtained in example 3;
FIG. 6 is an atomic force microscope photograph of the surface of the polymer film obtained in example 3.
Detailed Description
In order that the objects, technical solutions and advantages of the present invention will become more apparent, the present invention will be further described in detail with reference to the accompanying drawings in conjunction with the following specific embodiments.
In recent years, the preparation of biomimetic membranes has attracted attention as one of the methods for preparing hydrophobic and superhydrophobic membranes, based on the inspiration of lotus leaves, rose petals and aquatic organisms. One of the salient features of these biological surfaces is the presence of protruding structures on the micro-or nano-scale, which, when in contact with water, increase the contact angle according to the Cassie-Baxter equation, thus exhibiting hydrophobic properties. Under the development of the biological structure, the construction of the micro-nano protruding structure on the surface of the membrane becomes an important research direction for preparing the hydrophobic membrane at present. At present, the construction of the bionic structure is mainly realized by electrostatic spinning, nano material addition and other modes, and more researches are carried out and better effects are obtained.
The method is based on the characteristic that the formation of polymer crystals can be realized by controlling certain conditions in the film forming process of the vinylidene fluoride and the copolymer thereof, and the method for preparing the hydrophobic film by combining solvent-guided phase inversion and immersed phase inversion is considered. In the solvent volatilization stage, the polymer crystallization process begins to occur on the surface layer of the primary membrane, and solid-liquid phase conversion is conducted. And with the increase of the exposure process, the surface layer of the film is formed, the crystal structure starts to agglomerate and grow, a nano-scale or even micron-scale structure is formed, and the hydrophobic property of the film is improved. When the primary membrane is immersed in the coagulation bath, the liquid-liquid phase separation process begins immediately due to the rapid exchange of solvent with non-solvent, forming a polymer-poor phase and a polymer-rich phase, and the polymer begins to solidify. However, the crystallization process is still in progress due to the increased viscosity of the solution and the formation of crystals as a result of the evaporation phase of the solvent, which hinders the exchange of the solvent with the non-solvent. Therefore, the construction of the surface micro-nano structure and the guarantee of the mechanical performance of the film can be realized by combining the solvent evaporation induced phase inversion with the immersion phase inversion, and the regulation and control of the size of the protruding structure of the surface functional layer can be realized by controlling the two-step phase inversion process.
The invention relates to a preparation method of a hydrophobic membrane with a micro-nano protruding structure surface functional layer, which comprises the following steps:
step 1) drying crystalline or semi-crystalline polymer powders or granules to remove the effect of moisture (non-solvent) on the thermodynamic properties of the casting solution;
the crystalline or semi-crystalline polymers include, but are not limited to PVDF, PVDF-HFP, PVDF-CTFE, PVDF-TrFE, and the like.
Step 2) mixing 8-20% of polymer, 0-8% of additive and 70-90% of organic solvent according to weight percentage, stirring and mixing uniformly at the constant temperature of 20-70 ℃ and the speed of 50-1400 rpm, and defoaming at the constant temperature of 20-70 ℃ for more than 12 hours to form uniform membrane casting solution;
wherein the organic solvent includes, but is not limited to, one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, N-methylpyrrolidone, dimethylsulfoxide, diethylacetamide, and acetone;
the additive includes but is not limited to inorganic non-solvent, inorganic salt, micromolecular organic matter, macromolecule organic matter and other film forming additives, preferably, one or more of water, polyethylene glycol, acetone, ethanol, glycerol, trimethyl phosphate, oxalic acid, lithium chloride, lithium perchlorate and phosphoric acid;
the defoaming mode comprises standing deaeration and vacuum deaeration.
Step 3) scraping the casting solution obtained in the step 2) into a film by a film scraping device, and placing the film in an oven at 50-80 ℃ for 5-1800s to realize partial phase inversion, preferably placing the film in an oven for 30-600s, wherein the ambient humidity is 0-80%, and the film scraping device is conventional equipment for preparing polymer films in the field, and therefore, the details are not repeated herein;
in addition, except the oven, the solvent evaporation can also be carried out in other devices which can ensure corresponding constant temperature.
Step 4) immersing the primary membrane subjected to certain evaporation process in the step 3) in a 15-50 ℃ coagulating bath for 24 hours, and then airing in an air environment to obtain the novel hydrophobic membrane; the coagulating bath takes non-solvent water as a main body, and also comprises one or more organic solvents at least selected from ethanol, dimethylformamide, dimethylacetamide, triethyl phosphate, N-methylpyrrolidone, dimethyl sulfoxide, diethylacetamide and acetone, and the coagulating bath is mixed with water.
The hydrophobic membrane prepared by the preparation method has a micro-nano crystalline structure on the surface, the contact angle is 85-150 degrees, the surface roughness is 20-150nm, and the maximum peak-valley distance is 40-400 nm.
The following examples are provided to further illustrate the technical solution of the present invention.
Example 1
And mixing the dried PVDF, lithium chloride and an organic solvent DMAc according to the weight ratio of 12: 5: 83 is stirred for 24 hours at the constant temperature of 30 ℃ at the rotating speed of 200 revolutions per minute; then standing and defoaming for 24 hours at the constant temperature of 30 ℃ to obtain a membrane casting solution; scraping the casting film liquid on glass adhered with non-woven fabric by using a self-made scraper in a laboratory to form a primary film; and finally, standing the primary membrane in a 50 ℃ drying oven for 10 seconds, soaking the primary membrane in a 30 ℃ water coagulation bath for 24 hours, taking out the primary membrane, and airing the primary membrane in the air to obtain the hydrophobic membrane. The surface of the membrane forms a certain nano-scale crystal structure (as shown in figure 2), the surface roughness is 112.12nm, the contact angle is 95.0 degrees, the average pore diameter is 0.15 micron, the temperature and the flow rate of the hot side and the cold side are respectively 56 degrees centigrade and 65 liters/hour by taking 3.5 percent NaCl solution as stock solution; the operation is carried out at the temperature of 24 ℃ and under the condition of 50 liters per hour, the retention rate reaches more than 99 percent, and the water production flux is 16.96 kg/(m)2·h)。
FIG. 1 is an SEM photograph of the surface of the film obtained in this example; FIG. 2 is an atomic force microscope photograph of the surface of the film obtained in this example.
Example 2
Mixing the dried PVDF-CTFE powder, lithium chloride and dimethylacetamide according to the mass ratio of 12: 5: 83, and stirring at the constant temperature of 25 ℃ for 36 hours at the rotating speed of 1400 revolutions per minute; then carrying out vacuum defoaming for 8 hours at the constant temperature of 25 ℃ to obtain a membrane casting solution; and scraping the membrane casting solution on glass adhered with non-woven fabric by using a flat membrane scraping machine to form a primary membrane, standing the primary membrane in a 50 ℃ drying oven for 60 seconds, soaking the primary membrane in 25 ℃ tap water coagulation bath for 24 hours, taking out the primary membrane, and airing the primary membrane in the air to obtain the hydrophobic membrane. The surface of the membrane forms an obvious nano-to micron-sized structure (figure 4), the surface roughness is 135.4nm, the contact angle of the membrane is 97.8 ℃, the average pore diameter is 0.1086 microns, the temperature and the flow of a hot side and a cold side are respectively 56 ℃ and 65 liters/small by taking 3.5 percent NaCl solution as a stock solutionWhen the current is over; the operation is carried out at the temperature of 24 ℃ and under the condition of 50 liters per hour, the retention rate reaches more than 99 percent, and the water production flux is 22.78 kg/(m)2·h)。
FIG. 3 is an SEM photograph of the surface of the film obtained in the present example; FIG. 4 is an atomic force microscope photograph of the surface of the film obtained in this example.
Example 3
Firstly, mixing dried PVDF-CTFE copolymer powder, lithium chloride, polyethylene glycol and dimethylacetamide according to the mass ratio of 12: 4: 80, and stirring at the constant temperature of 25 ℃ for 36 hours at the rotating speed of 1400 revolutions per minute; then standing and defoaming for 24 hours at the constant temperature of 25 ℃ to obtain a membrane casting solution; and scraping the membrane casting solution on glass adhered with non-woven fabrics by using a flat membrane scraping machine to form a primary membrane, standing the primary membrane in a 60 ℃ drying oven for 300 seconds, soaking the primary membrane in a 25 ℃ coagulating bath (water: ethanol is 60: 40) for 24 hours, taking out the primary membrane, and airing the primary membrane in the air to obtain the hydrophobic membrane. The contact angle of the film is 103.7 degrees, the surface roughness is 40.87nm, the average pore diameter is 0.1266 microns, the temperature and the flow rate of the hot side and the cold side are respectively 56 degrees centigrade and 65 liters/hour by taking 3.5 percent NaCl solution as stock solution; the operation is carried out at the temperature of 24 ℃ and under the condition of 50 liters per hour, the retention rate reaches more than 99 percent, and the water production flux is 24.78 kg/(m)2·h)。
FIG. 5 is an SEM photograph of the surface of the film obtained in the present example; FIG. 6 is an atomic force microscope photograph of the surface of the film obtained in this example.
Examples 4 to 7 and comparative example 1
Examples 4-7 the preparation process described in this patent was used, and the kinds and compositions of the polymer material, additives and organic solvent were changed, and the conditions of the evaporation stage and the immersion stage of the solvent were changed, and the comparative example was a PVDF film obtained by using a conventional wet preparation process, i.e., immersion directly in a coagulation bath without an evaporation process. The specific parameters and experimental results are shown in table 1 below.
TABLE 1 TABLE of test parameters and test results for examples 4-7
In conclusion, the hydrophobic membrane prepared by the preparation method of the bionic hydrophobic membrane with the micro-nano structure surface functional layer has good membrane permeation and mass transfer performance and good hydrophobic performance.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. A preparation method of a hydrophobic membrane with a micro-nano protruding structure surface functional layer is characterized in that the preparation method is a two-step phase inversion method and comprises the following steps:
1) drying crystalline or semi-crystalline polymer powders or granules to remove the effect of moisture on the thermodynamic properties of the casting solution;
2) mixing 8-20% of polymer, 0-8% of additive and 70-90% of organic solvent by weight percentage, stirring at constant temperature of 20-70 ℃ for a period of time until the mixture is uniform, and defoaming at constant temperature of 20-70 ℃ to form uniform casting solution;
3) scraping the membrane casting solution in the step 2) into a membrane by a membrane scraping device, and staying in air with the humidity of 0-80% for 180-600s at the temperature of 50-80 ℃ to promote partial solid-liquid phase conversion;
4) immersing the primary membrane obtained in the step 3) in a coagulating bath at 15-50 ℃, and airing after soaking for a period of time to obtain the hydrophobic membrane.
2. The method of claim 1, wherein in step 1), the crystalline or semi-crystalline polymer is polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-chlorotrifluoroethylene copolymer, or polyvinylidene fluoride-trifluoroethylene copolymer.
3. The preparation method according to claim 1, wherein in the step 2), the stirring time at constant temperature is 5-24 hours, the defoaming time at constant temperature is more than 12 hours, and the defoaming mode comprises standing deaeration and vacuum deaeration.
4. The method according to claim 1, wherein in the step 2), the organic solvent is selected from one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, N-methylpyrrolidone, dimethylsulfoxide, diethylacetamide and acetone;
the additive is one or more of film forming additives selected from inorganic non-solvents, inorganic salts, small molecular organic matters and large molecular organic matters.
5. The method according to claim 4, wherein the additive is one or more selected from the group consisting of water, polyethylene glycol, acetone, ethanol, glycerol, trimethyl phosphate, oxalic acid, lithium chloride, lithium perchlorate, lithium bromide, sodium chloride, and phosphoric acid.
6. The method according to claim 1, wherein in step 4), the coagulation bath is mainly composed of water as a non-solvent, and further comprises one or more organic solvents selected from the group consisting of ethanol, dimethylformamide, dimethylacetamide, triethyl phosphate, N-methylpyrrolidone, dimethylsulfoxide, diethylacetamide, and acetone.
7. The method according to claim 1, wherein the soaking time in the step 4) is 2 hours or more.
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