POLYMER-BASED FILM, PREPARATION METHOD THEREFOR, AND USE THEREOF

Abstract

A polymer film has a loofah-like structure. It has a fibrous framework structure formed by three-dimensional interwoven and interconnected polymer fibers and a three-dimensional interconnected network pore structure distributed in the fibrous framework structure. The polymer is an organic polymer and the fibrous framework structure is integrally formed by the polymer. The film has a volume porosity of from 50% to 95%. The film is obtained by means of a combination method for atomization pretreatment and non-solvent phase separation. The film can be used in the fields of gas filtration, liquid filtration, oil-water separation, adsorption materials, catalysis, pharmaceutical sustained release materials, anti-adhesion coatings, oil delivery and oil spill interception.

Claims

1. A polymer-based membrane having a loofah-sponge like structure comprising a fiber skeletal structure formed by three-dimensionally interwoven and interconnected polymer fibers and a three-dimensionally interpenetrating network pore structure distributed in the fiber skeletal structure, wherein the polymer is an organic polymer and the fiber skeletal structure is integrally formed from the polymer, and the volume porosity of the membrane is 50%-95%.

2. The membrane according to claim 1, characterized in that the volume porosity of the membrane is 65% to 95%, preferably 80% to 95%.

3. The membrane according to claim 1, characterized in that the membrane has an average pore size of from 0.01 to 5 μm, preferably from 0.1 to 3 μm.

4. The membrane according to claim 1, characterized in that the average distance between two adjacent connection points in the thickness direction in the fiber skeletal structure of the membrane is smaller than the average distance between two adjacent connection points in the surface direction.

5. The membrane according to claim 1, characterized in that the cross-section of the membrane has network pores distributed along the thickness direction of the membrane and has substantially no other types of pores, such as sponge-like pores and/or finger-like pores.

6. The membrane according to claim 1, characterized in that the cross-sectional diameter of a single polymer fiber between two connection points in the fiber skeletal structure of the membrane is less than or equal to 2 μm, preferably varies irregularly; and/or the length of a single polymer fiber between two connection points in the fiber skeletal structure of the membrane is less than 10 μm.

7. The membrane according to claim 1, characterized in that the polymer fibers have a cavity structure inside them.

8. The membrane according to claim 1, characterized in that the polymer is selected from polymers suitable for membrane formation by non-solvent induced phase separation method, preferably from hydrophilic polymers or lipophilic polymers, more preferably is at least one selected from the group consisting of polyvinylidene fluoride, polysulfone, polyethersulfone, polyacrylonitrile, polyacrylic acid, polylactic acid, polyamide, chitosan, polyimide, modified cellulose (e.g., cellulose acetate), polystyrene, polyolefin, polyester, polychlorotrifluoroethylene, polyvinyl chloride, polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, modified starch, polyvinylamine hydrochloride, polyethyleneimine, poly-N-isopropylacrylamide and their modified polymers (e.g., polyvinylidene fluoride modified by acrylic acid grafting, sulfonated polysulfone, maleic anhydride-grafted polysulfone, sulfonated polyethersulfone, and acrylic acid-grafted polyacrylonitrile).

9. The membrane according to claim 1, characterized in that the surface of the membrane has micro/sub-micron sized recess structures, with loofah sponge-like structures distributed on or around or among the recess structures; preferably, the recess structure has a size of 0.5 to 10 μm, as determined by SEM.

10. The membrane according to claim 1, characterized in that nano-scale protrusions are distributed on the fiber skeletal structure and integrally formed with the skeleton, preferably in the shape of particles; preferably, the size of the protrusions is in the range of from 20 to 400 nm, as determined by SEM; preferably, the average pore size of the membrane is in the range from 0.1 to 5 μm.

11. The membrane according to claim 10, characterized in that the membrane comprises a mixture of at least two polymers, wherein, preferably, the at least two polymers are the polymers that are soluble or miscible in the same good solvent; more preferably, the first polymer is at least one selected from the group consisting of polyvinyl chloride, polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, polyimide, acrylonitrile-styrene copolymers and their modified polymers; the second polymer is at least one selected from the group consisting of chitosan, polyvinylpyrrolidone, polyethylene glycol, polyvinyl alcohol, and polyoxyethylene polyoxypropylene ether block copolymer; more preferably, the weight ratio of the first polymer to the second polymer is 1:(0.01 to 5), preferably 1:(0.1 to 3).

12. The membrane according to claim 10, characterized in that said membrane is a membrane with a micro-nano composite network structure prepared by at least two hydrophilic polymers, which is super-hydrophilic and super-lipophilic in air, and preferably has a contact angle to both water and oil in air of smaller than 10°, more preferably smaller than 5°, and a contact angle to oil under water of greater than 135°, preferably greater than 150°.

13. The membrane according to claim 10, characterized in that said membrane is a membrane with a micro-nano composite network structure prepared by at least two hydrophobic polymers, which is super-hydrophobic in air, and preferably has a contact angle to water in air of greater than 130°, more preferably greater than 150°, and a contact angle to oil under water of smaller than 10°, preferably smaller than 5°.

14. The membrane according to claim 1, characterized in that the membrane further comprises additives, preferably inorganic nanoparticles, such as MnO.sub.2, SiO.sub.2 and ZnO; and/or inorganic salt porogens, such as LiCl, ZnCl.sub.2, MgCl.sub.2 and LiBr.

15. The membrane according to claim 1, characterized in that the membrane is present on a support layer, e.g., on a fabric, preferably a nonwoven fabric.

16. The membrane according to claim 1, characterized in that the membrane is prepared by a non-solvent induced phase separation method in combination with an atomization pretreatment.

17. A method for preparing a membrane according to claim 1, comprising the following step: subjecting a membrane casting solution comprising the polymer to a non-solvent induced phase separation method in combination with an atomization pretreatment to prepare said membrane.

18. The method according to claim 17, characterized in that the method comprises the following steps: 1) dissolving the polymer in a solvent to formulate a polymer solution as the membrane casting solution; 2) applying the polymer solution in the form of a film, for example, blade-coating it into a film, followed by an atomization pretreatment, wherein the atomization pretreatment is staying in an atomized droplet bath; and 3) immersing the product obtained in step 2) in a solidification bath to obtain the membrane.

19. The method according to claim 18, characterized in that in step 1), the concentration of the polymer solution is in the range of from 5 to 30 wt %, preferably in the range of from 5 to 25 wt %, more preferably in the range of from 6 to 20 wt %; preferably, the solvent is selected from the good solvents for the polymer, and is preferably at least one selected from the group consisting of N,N-dimethylformamide, N-methylpyrrolidone, N,N-dimethylacetamide, dimethyl sulfoxide, tetrahydrofuran, dioxane, acetonitrile, acetone, chloroform, toluene, benzene, hexane, octane and tetramethyl sulfoxide.

20. The method according to claim 18, characterized in that in step 2), the polymer solution is uniformly coated on a support layer or a base material to thereby be applied in the form of a film; preferably, the thickness of the film as applied is in the range of from 50 to 500 μm, preferably from 75 to 300 μm.

21. The method according to claim 18, characterized in that in step 2), the size of the droplets in the droplet bath is 1 to 50 μm, preferably 5 to 18 μm; and/or in step 2), the atomization pretreatment time is to 20 min, preferably 5 s to 3 min, more preferably 5 to 60 s; and/or in step 2), the droplets comprise a poor solvent for the polymer, preferably selected from the group consisting of water, ethanol, ethylene glycol, a mixed solvent containing water, and a solution containing a salt, an acid or a base.

22. The method according to claim 18, characterized in that in step 2), the ambient humidity lies in a relatively high humidity condition of greater than or equal to 40%; and preferably, the temperature is room temperature.

23. The method according to claim 18, characterized in that in step 2), the ambient humidity lies in a relatively low humidity condition of less than 40%; and preferably, the temperature is room temperature.

24. The method according to claim 18, characterized in that in step 3), the solidification bath comprises a poor solvent for the polymer, which is the same as or different from the solvent contained in the droplets of the droplet bath.

25. The method according to claim 18, characterized in that when the membrane comprises a mixture of at least two polymers, in step 1), at least the first polymer and the second polymer are dissolved in a good solvent for the first polymer and the second polymer to formulate a polymer solution, preferably, the total concentration of the polymers in the polymer solution is 6 to 30 wt %, more preferably 10 to 25 wt %; and/or in step 2), the droplets in the droplet bath are a poor solvent for the first polymer; and/or, in step 3), the solidification bath comprises a poor solvent for the first polymer.

26. The method according to claim 18, characterized in that in step 2), the polymer solution is uniformly coated on a support layer, e.g., on a fabric, preferably on a nonwoven fabric, and the method is a roll-to-roll continuous process.

27. A method to use a membrane according to claim 1 in the fields of gas filtration, liquid filtration, oil-water separation, adsorption materials, catalysis, pharmaceutical sustained release materials, anti-adhesion coatings, oil delivery or oil spill interception.

28. A functional material comprising a membrane according to claim 1, wherein the functional material is used for the fields of gas filtration, liquid filtration, oil-water separation, adsorption materials, catalysis, pharmaceutical sustained release materials, anti-adhesion coatings, oil delivery or oil spill interception; and the functional material is preferably a separation membrane, more preferably an oil-water separation membrane; or a filter membrane, more preferably a microfiltration membrane.

Description

DESCRIPTION OF THE DRAWINGS

[0094] FIG. 1 is a surface morphological image of the polymer membrane of Example 1.

[0095] FIG. 2 is a cross-sectional morphological image of the polymer membrane of Example 1.

[0096] FIG. 3 is a surface morphological image of the polymer membrane of Example 5.

[0097] FIG. 4 is a cross-sectional morphological image of the polymer membrane of Example 5.

[0098] FIG. 5 is a surface morphological image of the polymer membrane of Example 6.

[0099] FIG. 6 is a cross-sectional morphological image of the polymer membrane of Example 6.

[0100] FIG. 7 is a surface morphological image of the polymer membrane of Comparative Example 1.

[0101] FIG. 8 is a cross-sectional morphological image of the polymer membrane of Comparative Example 1.

[0102] FIG. 9 is a cross-sectional view of a prior art nanofiber membrane.

[0103] FIG. 10 is a photograph of the morphology of the loofah sponge.

[0104] FIG. 11 is a scanning electron microscope photograph of the microfiltration membrane of Example 8.

[0105] FIG. 12 is a scanning electron microscope photograph of a higher magnification of the microfiltration membrane of Example 8.

EXAMPLES

[0106] In the following, the present invention is further illustrated with reference to the examples. Nevertheless, the scope of the present invention is not limited by these examples, but is set forth in the appended claims.

[0107] 1. Measurement Methods

[0108] 1) Surface morphology and cross-sectional morphology of the membrane: The surface morphology and cross-sectional morphology of the membrane were observed using a high-resolution field emission scanning electron microscope (FESEM) of Model S-4800 from Hitachi, Japan, wherein the sample for the characterization of the cross-sectional morphology of the membrane was prepared by the liquid nitrogen freezing brittle facture method.

[0109] 2) Average pore size of the membrane: The average pore size of the membrane was determined by a gas permeation method, using the instrument PSDA-20 microfiltration membrane pore size analyzer. When testing, the sample, with its rough surface upward, was loaded into a test module; before testing, the membrane material needed to be completely wetted using a test solution (surface tension: 16 mN.Math.m.sup.−1), and the test gas was nitrogen.

[0110] 3) Volume porosity of the membrane: The porosity was determined by a gravimetric method, and was calculated according to the following formula,

θ=1−V.sub.a/V.sub.g,

[0111] where θ is the volume porosity of the membrane, V.sub.a is the actual volume of polymer solids in the membrane sample per unit area, and V.sub.g is the geometric volume occupied by the polymer layer of the membrane sample per unit area.

[0112] 4) Recess size and nano-protrusion size: They were measured with a scale in a scanning electron microscope photograph. As the recess size, the diameter of the opening of the recess on the membrane surface in the SEM photograph was measured. As the nano-protrusion size, the diameter of the protrusion particles in the SEM photograph was measured.

[0113] 5) Oil-water separation performance (oil-water flux, retention rate, oil-water flux after cleaning):

[0114] Preparation of oil-in-water emulsion: First, sodium dodecyl sulfate (SDS, with a concentration of 100 mg L.sup.−1), as an emulsifier, was dissolved in water, and then diesel oil was mixed therein according to the volume ratio of diesel oil to water of 1:99. After mixing, the oil-water mixture was subjected to ultrasonic treatment for 1 h to prepare an emulsified milky emulsion. The diesel oil used was purchased from China Petroleum & Chemical Corporation, with the tradename: No. 0.

[0115] Preparation of water-in-oil emulsion: Span 80 (with a concentration of 300 mg L.sup.−1), as an emulsifier, was added to kerosene, and then water was added, wherein the volume of kerosene and water was controlled at 99:1; after mixing, the oil-water mixture was subjected to ultrasonic treatment for 1 h to prepare an emulsified milky emulsion. The kerosene used was purchased from Aladdin Reagents, with the item number: K118401.

[0116] The oil-water separation performance test of the membrane was performed with a cross-flow device. First, the membrane to be tested, with its front upward, was fixed in a cross-flow membrane cell. During the test process, the driving pressure was kept stable. The separation flux of the membrane was calculated by the following formula according to the mass of the filtrate collected within 1 min and the effective separation area:

J=m/(A×t)

[0117] In the formula, J is the separation flux of the membrane (herein, the oil-water flux); m is the liquid mass permeated in t time; and A is the effective separation area of the membrane. The separation efficiency of the membrane is evaluated by testing the total organic carbon content (TOC) in the filtrate, using a total organic carbon analyzer (Multi NC3100). The calculation formula is as follows:

E=(C.sub.0−C)/C.sub.0×100%

[0118] where E is the separation efficiency (herein, the retention rate) (%); C.sub.0 is the TOC value of the emulsion before separation; and C is the TOC value of the filtrate after separation.

[0119] The small oil droplets or small water droplets in the oil-water emulsion (O/W) or (W/O) formulated in the oil-water separation experiment had a particle diameter between 0.3 and 3 μm, and most of the oil droplets were in the sub-micron scale, wherein the size was obtained by data analysis via a laser particle size analyzer (DLS). Hydrophilic membrane was used to filter O/W emulsion, and hydrophobic membrane was used to filter W/O emulsion. The test pressure was adjusted between 1 kPa and 0.2 MPa according to different membranes. After one round of test, the test membrane was taken out, rinsed, and tested for its oil-water flux after recovery, i.e., the oil-water flux after rinsing, so as to evaluate its anti-fouling ability and long-term usability.

[0120] 6) Droplet size in atomized droplet bath: It was determined using a laser particle size analyzer (Model: Bettersize 2000S).

[0121] 7) Contact angle test: The contact angle test was performed at room temperature (about 25° C.) using a contact angle measuring instrument of the model DSA100 from Kruss, Germany. First, the sample membrane to be tested was fixed on a glass slide, and the membrane surface was ensured to be even. When the contact angle of liquid in air was tested, the glass slide attached with the membrane was directly placed on the sample stage of the instrument for testing; and when the contact angle of oil under water was tested, the membrane was wetted and then placed in a quartz water tank filled with water for testing, wherein the oil used in the test was 1,2-dichloroethane. In this experiment, the volume of the test liquid used to test the static contact angle was 3 μL, and the contact angle of the droplet when it was in contact with the membrane surface for 3 s was the value of the contact angle in this test.

[0122] 2. Raw Materials and Equipment Used in Examples and Comparative Examples

[0123] 1) The chemical reagents used were all commercially available products without special purification treatment, unless otherwise specified. [0124] Polyacrylonitrile (PAN): purchased from Shaoxing Gimel Composite Materials Co., Ltd., China, with the tradename P60C. [0125] Nano silica: purchased from Aladdin Reagents, with a particle diameter of 30 nm. [0126] Cellulose acetate (CA): purchased from InnoChem Science & Technology Co., Ltd. [0127] Polyvinylidene fluoride (PVDF): purchased from Solvay S.A., with the tradename 6010. [0128] Polystyrene (PS): purchased from Aladdin Reagents, with the product number P107090. [0129] Lithium chloride: purchased from InnoChem Science & Technology Co., Ltd. [0130] Polyvinylpyrrolidone: purchased from Macklin, K13-18, with an average molecular weight of 10,000. [0131] Sulfonated polyethersulfone: purchased from Klamar, with the item No. 091343. [0132] Polyethylene glycol: purchased from InnoChem Science & Technology Co., Ltd., with an average molecular weight of 20,000. [0133] Polyethersulfone: purchased from BASF under the tradename E201006. [0134] Polyvinyl alcohol: purchased from Aladdin Reagents, with the model 1788. [0135] Pluronic F-127: polyoxyethylene polyoxypropylene ether block copolymer, purchased from Sigma Reagents. [0136] Nonwoven fabric used: purchased from Hirose, Japan, with the model 75AX, polyester.

[0137] 2) Spray equipment: High pressure nozzle: SK508 from Huajue Technology Co., Ltd., Dongguan, China; Ultrasonic humidifier: HQ-JS130H from Haoqi, China. The droplet bath was deionized water. By data analysis with a laser particle size analyzer (DLS), the droplet particle diameter in the atomized droplet bath used for the test was substantially in the range of 0.3-10 μm.

Example 1

[0138] A specific mass of polyacrylonitrile (PAN) was weighed out and dissolved in N-methylpyrrolidone (NMP), and heated to 60° C. with stirring, until the polyacrylonitrile was sufficiently dissolved, to formulate a raw material solution with a concentration of 8 wt %, which was evacuated for defoaming. The formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 200 μm, followed by staying for 30 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier. The above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a low humidity ambient condition of 20%-35%. The average pore size of the membrane was 269 nm. The volume porosity of the membrane was 65.2%. The membrane could be used as a microfiltration membrane. Oil-water separation experiment was performed using the obtained membrane, and the results are listed in Table 1.

[0139] As observed via SEM, the surface morphology of the membrane obtained in Example 1 is shown in FIG. 1, and the cross-sectional morphology is shown in FIG. 2. It could be seen from the SEM photographs of FIGS. 1 and 2 that the polyacrylonitrile membrane of Example 1 exhibited a loofah sponge-like structure, the surface and the interior of the membrane formed an interpenetrating three-dimensional network pore structure, the polymer fibers of the membrane were interwoven and interconnected to form a three-dimensional fiber network, the surface of the obtained membrane exhibited obvious recess structures, which had a size of 0.5 to 4 μm. The cross-section of the membrane had a structure in which polymer fibers and the same type of pores (network pores) were distributed. The average distance between two adjacent connection points in the thickness direction in the fiber skeletal structure of the membrane was smaller than the average distance between two adjacent connection points in the surface direction. It could be seen from FIG. 2 that the polymer fibers of the obtained membrane had a cavity structure inside them.

Example 2

[0140] A specific mass of polyacrylonitrile (PAN) and a specific mass of nano-silica were weighed out, so that the nano-silica was sufficiently dispersed in N,N-dimethylformamide (DMF) in a mass concentration of 2%, and polyacrylonitrile (PAN) was dissolved in N,N-dimethylformamide (DMF) in a mass concentration of 6%; and the mixture was stirred and heated at 60° C. until a homogeneous membrane casting solution was formed, which was evacuated for defoaming. The formulated membrane casting solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 100 μm, followed by staying for 15 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier. The above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a low humidity ambient condition of 30%-38%. The obtained membrane had an average pore size of 314 nm, and a volume porosity of 79.2%. The membrane could be used as a microfiltration membrane.

[0141] As observed via SEM, the membrane had a loofah sponge-like structure, and exhibited obvious recess structures on the surface, which recess structures had a size of 0.5 to 5 μm. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Example 3

[0142] A specific mass of cellulose acetate (CA) was weighed out and dissolved in acetone, and stirred to formulate a raw material solution with a concentration of 8 wt %, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a clean glass plate with a doctor blade, with the thickness of the coating film being controlled at 200 μm, followed by staying for 30 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a low humidity ambient condition of 15%-30%. The average pore size of the obtained membrane was 106 nm. The volume porosity of the membrane was 83.1%. The membrane could be used as a microfiltration membrane.

[0143] As observed via SEM, the membrane had a loofah sponge-like structure, and exhibited obvious recess structures on the surface, which recess structures had a size of 1 to 4 μm. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Example 4

[0144] A specific mass of polyacrylonitrile (PAN) was weighed out and dissolved in DMF, heated to 60° C., and formulated into a raw material solution with a concentration of 8 wt % under stirring, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 200 μm, followed by staying for 35 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a solidification bath of 1 mol/L aqueous solution of sodium hydroxide for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a low humidity ambient condition of 20%-35%. The obtained membrane had an average pore size of 435 nm and a volume porosity of 87.9%. The membrane could be used as a microfiltration membrane.

[0145] As observed via SEM, the obtained membrane had a loofah sponge-like structure, and exhibited obvious recess structures on the surface, which recess structures had a size of 1 to 5 μm. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Example 5

[0146] A specific mass of polyacrylonitrile (PAN) was weighed out and dissolved in N-methylpyrrolidone (NMP), heated to 60° C., and stirred to formulate a raw material solution with a concentration of 8 wt %, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 150 μm, followed by staying for 40 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 50%-80%. The obtained membrane had an average pore size of 437 nm and a volume porosity of 71.3%. The membrane could be used as a microfiltration membrane.

[0147] As observed via SEM, the obtained membrane had no obvious recess structures on the surface. The surface morphology of the obtained membrane is shown in FIG. 3, and the cross-sectional morphology is shown in FIG. 4. It could be seen from the SEM photographs of FIGS. 3 and 4 that the polyacrylonitrile microfiltration membrane of Example 5 exhibited a loofah sponge-like structure, the surface and the interior of the membrane formed an interpenetrating three-dimensional network pore structure, wherein the polymer fibers of the membrane were three-dimensionally interwoven and interconnected to form a three-dimensional fiber network, i.e., the fiber skeletal structure, the cross-section of the membrane had a structure in which polymer fibers and the same type of pores (network pores) were distributed. The average distance between two adjacent connection points in the thickness direction in the fiber skeletal structure of the membrane was smaller than the average distance between two adjacent connection points in the surface direction.

[0148] Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Example 6

[0149] A specific mass of polyvinylidene fluoride (PVDF) was weighed out and dissolved in N-methylpyrrolidone (NMP), and heated to 70° C. to formulate a raw material solution with a concentration of 8 wt % under stirring, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 150 μm, followed by staying for 40 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. The obtained membrane had an average pore size of 487 nm, and a volume porosity of 71.0%. The membrane could be used as a microfiltration membrane. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 50%-80%.

[0150] As observed via SEM, the obtained membrane had no obvious recess structures on the surface. The surface morphology of the membrane obtained in Example 6 is shown in FIG. 5, and the cross-sectional morphology is shown in FIG. 6. It could be seen from the SEM photographs of FIGS. 5 and 6 that the polyvinylidene fluoride microfiltration membrane of Example 6 exhibited a loofah sponge-like structure, the surface and the interior of the microfiltration membrane formed an interpenetrating three-dimensional network pore structure, wherein the polymer fibers of the microfiltration membrane were three-dimensionally interwoven and interconnected to form a three-dimensional fiber network, i.e., the fiber skeletal structure, the cross-section of the microfiltration membrane had a structure in which polymer fibers and the same type of pores (network pores) were distributed. The average distance between two adjacent connection points in the thickness direction in the fiber skeletal structure of the membrane was smaller than the average distance between two adjacent connection points in the surface direction. A single polymer fiber had a length of between 1 and 4 μm, and a fiber diameter of between 50 nm and 0.4 μm.

[0151] Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Example 7

[0152] A specific mass of lithium chloride and a specific mass of polystyrene (PS) were weighed out, so that lithium chloride was sufficiently dispersed in N,N-dimethylformamide (DMF) in a mass concentration of 0.5%, and polystyrene (PS) was dissolved in N,N-dimethylformamide (DMF) in a mass concentration of 6%; the mixture was heated to 50° C., and stirred until a homogeneous membrane casting solution was formed, which was evacuated for defoaming; the formulated membrane casting solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 150 μm, followed by staying for 30 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 60%-80%. The obtained membrane had an average pore size of 1217 nm, and a volume porosity of 90.7%. The membrane could be used as a microfiltration membrane.

[0153] As observed via SEM, the obtained membrane had a loofah sponge-like structure, and had no obvious recess structures on the surface. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Comparative Example 1

[0154] A specific mass of polyacrylonitrile (PAN) was weighed out and dissolved in NMP, heated to 60° C., and stirred to formulate a raw material solution with a concentration of 8 wt %, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 150 μm, followed by immersion in a deionized water solidification bath for complete phase inversion. After washing with water, the membrane was obtained. The membrane had an average pore size of 35 nm, and a volume porosity of 53.7%. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

[0155] The surface morphology of the membrane of Comparative Example 1 is shown in FIG. 7, and the cross-sectional morphology is shown in FIG. 8. It could be seen from the SEM photographs in FIGS. 7 and 8 that the surface of the membrane was the typical surface morphology of an ordinary flat ultrafiltration membrane, that is, the surface was substantially covered by a flat and even polymer layer, with a small number of small pores distributed thereon; its cross-section was such a combination of a sponge pore structure close to the surface layer of the membrane and a finger-like pore structure at the lower part, thus its overall structure was not a loofah sponge-like structure.

Comparative Example 2

[0156] A specific mass of polyacrylonitrile (PAN) was weighed out and dissolved in DMF, heated to 60° C., and stirred to formulate a raw material solution with a concentration of 8 wt %, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 200 μm, followed by staying for 40 s in a constant temperature and humidity box at a room temperature of 25° C. and a humidity of 100%; and the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. The membrane had an average pore size of 40 nm, and a volume porosity of 59.1%.

[0157] As observed via SEM, the obtained membrane did not have a loofah sponge-like structure. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Comparative Example 3

[0158] A specific mass of cellulose acetate was weighed out and dissolved in NMP, heated to 60° C., and stirred to formulate a raw material solution with a concentration of 8 wt %, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 150 μm, followed by immersion in a deionized water solidification bath for complete phase inversion. After washing with water, the membrane was obtained. The membrane had an average pore size of 24 nm, and a volume porosity of 52.8%.

[0159] As observed via SEM, the obtained membrane did not have a loofah sponge-like structure. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

Comparative Example 4

[0160] A specific mass of polyvinylidene fluoride (PVDF) was weighed out and dissolved in N-methylpyrrolidone (NMP), heated to 70° C., and stirred to formulate a raw material solution with a concentration of 8 wt %, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 150 μm; then the above film was immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. The obtained membrane had an average pore size of 48 nm, and a volume porosity of 61.4%.

[0161] As observed via SEM, the obtained membrane did not have a loofah sponge-like structure. Oil-water separation experiment was performed using the membrane, and the results are listed in Table 1.

TABLE-US-00001 TABLE 1 Comparison of membrane performance of Examples 1-7 and Comparative Examples 1-4 Oil-water Oil-water flux separation Oil-water flux Retention after cleaning membrane (L/(m.sup.2 .Math. h)) rate (%) (L/(m.sup.2 .Math. h)) Example 1 998 99.5% 973 Example 2 1119 99.1% 1106 Example 3 836 99.7% 824 Example 4 1372 99.0% 1361 Example 5 1255 99.0% 1221 Example 6 858 98.4% 839 Example 7 1478 98.1% 1459 Comparative 1730 .sup. 30% 755 Example 1 Comparative 1620 .sup. 35% 735 Example 2 Comparative 1496 .sup. 45% 841 Example 3 Comparative 949 .sup. 41% 716 Example 4

[0162] Test pressure: Since the membranes prepared in the inventive examples were microfiltration membranes, the test pressure used for these membranes was 10 kPa; and since the membranes prepared in the comparative examples were ultrafiltration membranes, the test pressure used for these membranes was 0.1 MPa.

[0163] As can be seen from the data in Table 1, the membranes having a loofah sponge-like structure prepared in the inventive examples showed a better oil-water separation effect (a higher or equivalent oil-water flux, a higher retention rate, a better or equivalent oil-water flux after cleaning) and had a better anti-fouling ability and long-term usability, compared with the membranes without a loofah sponge-like structure prepared in the non-inventive comparative examples.

[0164] From a comparison between the data of Comparative Example 1 and the data of Example 5, and a comparison between the data of Comparative Example 4 and the data of Example 6, it can be seen that under the circumstance of using the same membrane casting solution formulation to prepare a membrane directly by a non-solvent induced phase separation method without an atomization pretreatment, the obtained membrane had a very poor oil-water separation effect (both the retention rate and the oil-water flux after cleaning were relatively low), and microfiltration membrane could not be obtained.

Example 8

[0165] 8 g of polyacrylonitrile and 8 g of polyvinylpyrrolidone were dissolved in 84 g of N,N-dimethylformamide (DMF); heated and stirred at 50° C. to form a homogeneous solution, which was then evacuated for defoaming; then the resultant was coated on a non-woven fabric, with the thickness of the coating film being controlled at 100 μm, followed by staying for 30 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 60%-80%. The obtained membrane had an average pore size of 0.8 μm, and a volume porosity of 80.3%. The membrane could be used as a microfiltration membrane.

[0166] The surface morphology of the obtained membrane is shown in FIG. 11 and FIG. 12. It could be seen from the SEM photographs of FIGS. 11 and 12 that the membrane had a loofah sponge-like structure, and the fiber skeletal structure had protrusions distributed thereon, which protrusions had a size of 50 to 250 nm. The membrane exhibited a contact angle of 0° for both oil and water in air, and a contact angle of 165° for oil under water.

Example 9

[0167] 12 g of sulfonated polyethersulfone and 1 g of polyethylene glycol were dissolved in 87 g of N-methyl-2-pyrrolidone (NMP); heated and stirred at 60° C. to form a homogeneous solution, which was then evacuated for defoaming; then the resultant was coated on a clean glass plate, with the thickness of the coating film being controlled at 300 μm, followed by staying for 10 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 65%-85%. The obtained membrane had an average pore size of 0.3 μm, and a volume porosity of 86.8%. The membrane could be used as a microfiltration membrane.

[0168] As observed via SEM, the obtained membrane had a loofah sponge-like structure, and the fiber skeletal structure had protrusions distributed thereon, which protrusions had a size of 40 to 300 nm. The membrane exhibited a contact angle of 0° for both oil and water in air, and a contact angle of 158° for oil under water.

Example 10

[0169] 8 g of polyethersulfone and 2 g of polyvinyl alcohol were dissolved in 90 g of dimethyl sulfoxide (DMSO); heated and stirred at 60° C. to form a homogeneous solution, which was then evacuated for defoaming; then the resultant was coated on a non-woven fabric, with the thickness of the coating film being controlled at 150 μm, followed by staying for 2 min in a deionized water droplet bath obtained by atomization of a high-pressure gas flow generated by a high-pressure nozzle; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 70%-90%. The obtained membrane had an average pore size of 4 μm, and a volume porosity of 83.8%. The membrane could be used as a microfiltration membrane.

[0170] As observed via SEM, the obtained membrane had a loofah sponge-like structure, and the fiber skeletal structure had protrusions distributed thereon, which protrusions had a size of 20 to 50 nm. The membrane exhibited a contact angle of 0° for both oil and water in air, and a contact angle of 152° for oil under water.

Example 11

[0171] 6 g of polyethersulfone and 10 g of PluronicF-127 were dissolved in 84 g of NMP solvent; heated and stirred at 70° C. to form a homogeneous solution, which was then evacuated for defoaming; then the resultant was coated on a non-woven fabric, with the thickness of the coating film being controlled at 250 μm, followed by staying for 50 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 70%-99%. The obtained membrane had an average pore size of 4.3 μm, and a volume porosity of 80.9%. The membrane could be used as a microfiltration membrane.

[0172] As observed via SEM, the obtained membrane had a loofah sponge-like structure, and the fiber skeletal structure had protrusions distributed thereon, which protrusions had a size of 20 nm to 40 nm. The membrane exhibited a contact angle of 0° for both oil and water in air, and a contact angle of 154° for oil under water.

Example 12

[0173] 6 g of polyethersulfone and 18 g of polyvinylpyrrolidone were dissolved in 76 g of NMP; heated and stirred at 70° C. to form a homogeneous solution, which was then evacuated for defoaming; then the resultant was coated on a clean glass plate, with the thickness of the coating film being controlled at 100 μm, followed by staying for 20 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 65%-85%. The obtained membrane had an average pore size of 3 μm, and a volume porosity of 91.3%. The membrane could be used as a microfiltration membrane.

[0174] As observed via SEM, the obtained membrane had a loofah sponge-like structure, and the fiber skeletal structure had protrusions distributed thereon, which protrusions had a size of 40 to 200 nm. The membrane exhibited a contact angle of 0° for both oil and water in air, and a contact angle of 155° for oil under water.

Example 13

[0175] 12 g of cellulose acetate and 10 g of polyethylene glycol were dissolved in 78 g of acetone; heated and stirred at 70° C. to form a homogeneous solution, which was then evacuated for defoaming; then the resultant was coated on a non-woven fabric, with the thickness of the coating film being controlled at 100 μm, followed by staying for 20 s in an atomized droplet bath generated by atomizing deionized water by ultrasonic wave using an ultrasonic humidifier; the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. During the membrane preparation process, the humidity was controlled at a relatively high humidity ambient condition of 60%-80%. The obtained membrane had an average pore size of 1.6 μm, and a volume porosity of 89.6%. The membrane could be used as a microfiltration membrane.

[0176] As observed via SEM, the obtained membrane had a loofah sponge-like structure, and the fiber skeletal structure had protrusions distributed thereon, which protrusions had a size of 50 to 300 nm. The membrane exhibited a contact angle of 0° for both oil and water in air, and a contact angle of 158° for oil under water.

Comparative Example 5

[0177] 8 g of polyacrylonitrile and 8 g of PVP were dissolved in 84 g of NMP, heated to 60° C. and stirred to form a homogeneous solution, which was evacuated for defoaming; the formulated solution was uniformly blade-coated on a non-woven fabric with a doctor blade, with the thickness of the coating film being controlled at 100 μm, followed by immersion in a deionized water solidification bath for complete phase inversion. After washing with water, the membrane was obtained. The obtained membrane had an average pore size of 53 nm, and a volume porosity of 66.0%.

[0178] As observed via SEM, the obtained membrane did not have a loofah sponge-like structure, and moreover, the membrane structure had no protrusions thereon. The membrane exhibited contact angles of 40° and 28° for oil and water in air respectively, and a contact angle of 126° for oil under water.

Comparative Example 6

[0179] 12 g of sulfonated polyethersulfone and 1 g of polyethylene glycol were dissolved in 87 g of NMP; heated and stirred at 60° C. to form a homogeneous solution, which was then evacuated for defoaming; the resultant was then coated on a clean glass plate, with the thickness of the coating film being controlled at 300 μm, followed by staying for 30 s in a constant temperature and humidity box at a temperature of 25° C. and a humidity of 100%; and the above film was then immersed in a deionized water solidification bath for complete phase separation. After washing with water, the membrane was obtained. The obtained membrane had an average pore size of 69 nm, and a volume porosity of 58.2%.

[0180] As observed via SEM, the obtained membrane did not have a loofah sponge-like structure, and moreover, the membrane structure had no protrusions thereon. The membrane had a volume porosity of 58%. The membrane exhibited contact angles of 48° and 32° for oil and water in air respectively, and a contact angle of 117° for oil under water.

[0181] For the membranes obtained in Examples 8-13 and Comparative Examples 5 and 6, the oil-water separation flux and retention rate were measured, and the results are shown in Table 2.

TABLE-US-00002 TABLE 2 Comparison of membrane performance of Examples 8-13 and Comparative Examples 5 and 6 Oil-water Oil-water flux separation Oil-water flux Retention after cleaning membrane (L/(m.sup.2 .Math. h)) rate (%) (L/(m.sup.2 .Math. h)) Example 8 1538 99.1% 1529 Example 9 1366 99.0% 1370 Example 10 1768 99.0% 1759 Example 11 1495 99.2% 1487 Example 12 1495 99.3% 1497 Example 13 1299 99.4% 1290 Comparative 1620 .sup. 65% 735 Example 5 Comparative 2490 .sup. 30% 1063 Example 6

[0182] (Test pressure of Examples 8-13: 10 kPa; and test pressure of Comparative Examples 5 and 6: 0.1 MPa)

[0183] From a comparison of the data of Comparative Example 5 and Example 8, it can be seen that under the circumstance of using the same membrane casting solution formulation to prepare a membrane directly by a non-solvent induced phase separation method without an atomization pretreatment, the obtained membrane had a very poor oil-water separation effect, and microfiltration membrane could not be obtained. From a comparison of the data of Comparative Example 6 and Example 9, it can be seen that under the circumstance of first treatment with vapor induced phase separation under a high humidity followed by non-solvent phase inversion, the obtained membrane had a very poor oil-water separation effect, and microfiltration membrane could not be obtained. It can be seen from a comparison of the data of Example 8 and Example 1 that compared with the microfiltration membrane prepared by the method of the present invention using a single polymer, the microfiltration membrane having a micro-nano composite structure prepared by the method of the present invention using a mixture of the two polymers had a better oil-water separation effect.