ROOM-TEMPERATURE SELECTIVE SWELLING METHOD OF PORE-FORMING USED FOR PREPARING SEPARATION MEMBRANES

Abstract

The present invention provides a room-temperature selective swelling method of pore-forming used for preparing separation membranes, comprising: treating a dense membrane of an amphiphilic block copolymer by a composite swelling agent at 15-30° C. for 1 min-24 h, removing the residual solvent, then leaving the membrane at room temperature to dry, so as to obtain an amphiphilic block copolymer separation membrane with a bi-continuous porous structure, wherein the composite swelling agent is composed of 60-96% of a first swelling agent and 4-40% of a second swelling agent, the first swelling agent is an alcohol solvent, the second swelling agent is selected from any one or a mixture of two or more of toluene, styrene, tetrahydrofuran, 1,4-dioxane and so on. In the method of the invention, selective swelling can be achieved at room temperature, abating the energy consumption in membrane-forming process. The method has universality and can be widely used in the pore-forming process of various amphiphilic block copolymers. The swelling level and morphology can be controlled by adjusting the composition of the solvent in the swelling agent and the second swelling agent content in the swelling agent.

Claims

1. A room-temperature selective swelling method of pore-forming used for preparing separation membranes, comprising: treating a dense membrane of an amphiphilic block copolymer by a composite swelling agent at 15-30° C. for 1 min-24 h, removing the residual solvent, then leaving the membrane at room temperature to dry, so as to obtain an amphiphilic block copolymer separation membrane with a bi-continuous porous structure, wherein, the composite swelling agent is composed of 60-96% of a first swelling agent and 4-40% of a second swelling agent by a volume fraction, the first swelling agent is an alcohol solvent, the second swelling agent is selected from any one or a mixture of two or more of toluene, o-xylene, styrene, dichloromethane, 1,2-dichloroethane, 1,1,2-trichloroethane, tetrahydrofuran or 1,4-dioxane.

2. The method according to claim 1, wherein the second swelling agent is selected from any one or a mixture of two or more of toluene, styrene, dichloromethane, tetrahydrofuran or 1,4-dioxane.

3. The method according to claim 1, wherein the second swelling agent is 1,4-dioxane.

4. The method according to claim 1, wherein the alcohol solvent is selected from methanol, ethanol, butanol, isopropanol, ethylene glycol or glycerol.

5. The method according to claim 1, wherein the alcohol solvent is ethanol.

6. The method according to claim 1, wherein treating a dense membrane of an amphiphilic block copolymer by a composite swelling agent at 20° C.

7. The method according to claim 1, wherein the treating time is 5-10 min or 4 h-8 h.

8. The method according to claim 1, wherein the treating a dense membrane of an amphiphilic block copolymer by the composite swelling agent is immersing the dense membrane in the composite swelling agent, and in the composite swelling agent, the second swelling agent is 12%-24% by a volume fraction.

9. The method according to claim 8, in the composite swelling agent, the second swelling agent is 12-16%.

10. The method according to claim 8, in the composite swelling agent, the second swelling agent is 12%.

11. The method according to claim 1, wherein the treating a dense membrane of an amphiphilic block copolymer by the composite swelling agent treatment is coating the composite swelling agent onto the dense membrane, and in the composite swelling agent, the second swelling agent is 16%-40% by a volume fraction.

12. The method according to claim 11, in the composite swelling agent, the second swelling agent is 20-40%.

13. The method according to claim 1, wherein the amphiphilic block copolymer is composed of a Block A and a Block B (A-B), the Block A is selected from any one of polystyrene (PS) or polysulfone (PSF), the Block B is selected from any one of poly(2-vinylpyridine) (P2VP), polyethylene oxide (PEO), polyethylene glycol (PEG) or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), the total molecular weight of the amphiphilic block copolymer is 50,000 to 100,000 Daltons.

14. A room-temperature selective swelling method of pore-forming used for preparing separation membranes, comprising the following steps: (1) Preparing a Membrane-Forming Solution An amphiphilic block copolymer is dissolved in chloroform to prepare a membrane-forming solution with a 1-2 wt % concentration, wherein, the amphiphilic block copolymer is composed of a Block A and a Block B (A-B), the Block A is selected from any one of polystyrene (PS) and polysulfone (PSF), the Block B is selected from any one of poly(2-vinylpyridine) (P2VP), polyethylene oxide (PEO), polyethylene glycol (PEG) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), the percentage of the Block B to the total volume of the amphiphilic block copolymer is 10-40%, the total molecular weight of the amphiphilic block copolymer is 50,000 to 100,000 Daltons; (2) Forming a Membrane A certain amount of the membrane-forming solution obtained in step (1) are spin-coated or drop-coated on a silicon wafer substrate, which is next placed in a natural environment for 6 hours, so as to completely volatilize the solvent to btain a dense membrane of an amphiphilic block copolymer; (3) Pore-Forming Process of Room-Temperature Selective Swelling The membrane of an amphiphilic block copolymer obtained in step (2) is soaked in a container containing a composite swelling agent at 20° C., wherein the composite swelling agent is composed of 84-88% of ethanol and 12-16% of 1,4-dioxane by a volume fraction. The membrane of an amphiphilic block copolymer is immersed in the composite swelling agent at 20° C. for 4-8 hours to complete pore-forming, then immediately taken out, and washed three times with alcohol reagents, and left at room temperature for drying, to obtain a bi-continuous porous structure membrane of an amphiphilic block copolymer.

15. A room-temperature selective swelling method of pore-forming used for preparing separation membranes, comprising the following steps: (1) Preparing a Membrane-Forming Liquid An amphiphilic block copolymer is dissolved in chloroform to prepare a membrane-forming solution with a 1-2 wt % concentration, wherein, the amphiphilic block copolymer is composed of Block A and Block B (A-B), the Block A is selected from any one of polystyrene (PS) and polysulfone (PSF), the Block B is selected from any one of poly(2-vinylpyridine) (P2VP), polyethylene oxide (PEO), polyethylene glycol (PEG) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), the percentage of the Block B to the total volume of the amphiphilic block copolymer is 10-40%, the total molecular weight of the amphiphilic block copolymer is 50,000 to 100,000 Daltons; (2) Forming a Membrane A certain amount of the membrane-forming solution obtained in step (1) are spin-coated or drop-coated on a silicon wafer substrate, which is next placed in a natural environment for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of an amphiphilic block copolymer; (3) Selective Swelling Pore-Forming at Room Temperature A composite swelling agent is coated onto the membrane of an amphiphilic block copolymer obtained in step (2), which is placed at 20° C. for 5-10 minutes, until the composite swelling agent is completely volatilized and becomes dry, wherein, the composite swelling agent is composed of 60-84% of ethanol and 16-40% of dichloromethane or tetrahydrofuran by a volume fraction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 is a graph showing the variation of the porosity of the PS-P2VP system in different swelling agent systems following the swelling time in the case of the second swelling agent with a 12% concentration.

[0050] FIG. 2 is a graph showing the variation of the membrane thickness of the PS-P2VP system in 1,4-dioxane/ethanol system following the swelling time in the case of the 12% concentration.

[0051] FIG. 3 is a SEM image of the porous structure of the block copolymer obtained in Example 1.

[0052] FIG. 4 is a SEM image of the porous structure of the block copolymer obtained in Example 2.

[0053] FIG. 5 is a SEM image of the porous structure of the block copolymer obtained in Example 3.

[0054] FIG. 6 is a SEM image of the porous structure of the block copolymer obtained in Example 4.

[0055] FIG. 7 is a SEM image of the porous structure of the block copolymer obtained in Example 4.

[0056] FIG. 8 is a SEM image of the porous structure of the block copolymer obtained in Example 5.

[0057] FIG. 9 is a SEM image of the porous structure of the block copolymer obtained in Example 6.

[0058] FIG. 10 is a SEM image of the porous structure of the block copolymer obtained in Example 7.

[0059] FIG. 11 is a SEM image of the porous structure of the block copolymer obtained in Example 8.

[0060] FIG. 12 is a SEM image of the porous structure of the block copolymer obtained in Comparison 1.

[0061] FIG. 13 is a SEM image of the porous structure of the block copolymer obtained in Comparison 1.

[0062] FIG. 14 is a SEM image of the porous structure of the block copolymer obtained in Comparison 2.

[0063] FIG. 15 is a SEM image of the porous structure of the block copolymer obtained in Comparison 2.

[0064] FIG. 16 is a SEM image of the porous structure of the block copolymer obtained in Comparison 3.

[0065] FIG. 17 is a SEM image of the porous structure of the block copolymer obtained in Comparison 3.

[0066] FIG. 18 is a SEM image of the porous structure of the block copolymer obtained in Example 10.

[0067] FIG. 19 is a SEM image of the porous structure of the block copolymer obtained in Example 10.

[0068] FIG. 20 is a SEM image of the porous structure of the block copolymer obtained in Example 11.

[0069] FIG. 21 is a SEM image of the porous structure of the block copolymer obtained in Example 11.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

[0070] The invention will be further explained below in combination with the examples. The following examples are only used for describing the invention, rather than limiting the implementation scope of the invention.

EXAMPLE 1

[0071] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and 1,4-dioxane, in which the volume fraction of 1,4-dioxane is 12%, after soaking the membrane at 20° C. for 1 hour, take it out and leave it at room temperature for drying.

[0072] FIG. 3 is a SEM image of the porous structure of the block copolymer obtained in this example. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 12% concentration of 1,4-dioxane for 1 hour at 20° C., the membrane surface presents bi-continuous morphology to the lower extent with a relatively high degree of pore-opening for the membrane, showing that adding the neutral reagent of the dispersed phase and continuous phase of a block copolymer to alcohol reagents can achieve selective swelling pore-forming.

[0073] In addition, after measuring the thickness of the polymer membrane with increase from 238 nm to 333 nm in this example after swelling, the porosity of the membrane prepared under this conditions works out at 28%.

EXAMPLE 2

[0074] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μum pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and 1,4-dioxane, in which the volume fraction of 1,4-dioxane is 12%, after soaking the membrane at 20° C. for 8 hour, take it out and leave it at room temperature for drying.

[0075] FIG. 4 is a SEM image of the porous stmcture of the block copolymer obtained in this example. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 12% concentration of 1,4-dioxane for 8 hours at 20° C., the membrane surface presents bi-continuous morphology to the high extent with a relatively high degree of pore-opening for the membrane, showing that the swelling degree can be improved by extending the swelling time.

[0076] In addition, after measuring the thickness of the polymer membrane with increase from 238 nm to 358 nm in this example after swelling, the porosity of the membrane prepared under this conditions works out at 34%.

EXAMPLE 3

[0077] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and 1,4-dioxane, in which the volume fraction of 1,4-dioxane is 16%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0078] FIG. 5 is a SEM image of the porous stmcture of the block copolymer obtained in this example. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 16% concentration of 1,4-dioxane for 4 hours at 20° C., the membrane surface presents bi-continuous morphology to the high extent with a relatively high degree of pore-opening for the membrane, showing that the swelling can be accelerated by increasing the concentration of 1,4-dioxane in the solvent with an improvement of the swelling degree.

[0079] In addition, after measuring the thickness of the polymer membrane with increase from 245 nm to 406nm in this example after swelling, the porosity of the membrane prepared under this conditions works out at 40%.

EXAMPLE 4

[0080] We add 0.02 g of PS.sub.60-PEO.sub.36 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and dichloromethane, in which the volume fraction of dichloromethane is 16%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0081] FIG. 6 and FIG. 7 are a SEM image of the porous structure of the block copolymer obtained in this example, respectively. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 16% concentration of dichloromethane for 4 hours at 20° C., the membrane surface presents bi-continuous morphology to the lower extent with a relatively lower degree of pore-opening for the membrane surface, but a relatively high degree of pore-opening for the membrane cross section, showing that the mixed solvent of dichloromethane and ethanol acting as a swelling agent for selective swelling pore-forming at room temperature is effective to the block copolymer system.

[0082] In addition, after measuring the thickness of the polymer membrane with increase from 270 nm to 325 nm in this example after swelling, the porosity of the membrane prepared under this conditions works out at 17%.

EXAMPLE 5

[0083] We add 0.02 g of PSF.sub.60-PEG.sub.20 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and toluene, in which the volume fraction of toluene is 24%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0084] FIG. 8 is a SEM image of the porous structure of the block copolymer obtained in this example. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 24% concentration of toluene for 4 hours at 20° C., the membrane surface presents bi-continuous morphology to the high extent, showing that the mixed solvent of toluene and ethanol acting as a swelling agent for selective swelling pore-forming at room temperature is effective to the block copolymer system.

[0085] In addition, after measuring the thickness of the polymer membrane with increase from 229 nm to 305 nm in this example after swelling, the porosity of the membrane prepared under this conditions works out at 25%.

EXAMPLE 6

[0086] We add 0.02 g of PS.sub.70.1-PDMAEMA.sub.21.5 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and 1,4-dioxane, in which the volume fraction of 1,4-dioxane is 16%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0087] FIG. 9 is a SEM image of the porous structure of the block copolymer obtained in this example. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 16% concentration of 1,4-dioxane for 4 hours at 20° C., the membrane surface presents bi-continuous morphology, showing that the mixed solvent of 1,4-dioxane and ethanol acting as a swelling agent for selective swelling pore-forming at room temperature is effective to the block copolymer system.

[0088] In addition, after measuring the thickness of the polymer membrane with increase from 218 nm to 281 nm in this example after swelling, the porosity of the membrane prepared under this conditions works out at 22%.

EXAMPLE 7

[0089] We add 0.02 g of PS53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and tetrahydrofuran, in which the volume fraction of tetrahydrofuran is 12%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0090] FIG. 10 is a SEM image of the porous structure of the block copolymer obtained in this example. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 12% concentration of tetrahydrofuran for 4 hours at 20° C., the membrane surface presents bi-continuous morphology to the high extent with a relatively high degree of pore-opening for the membrane, showing that the ethanol/tetrahydrofuran system can be suitable to selective swelling pore-forming at room temperature.

EXAMPLE 8

[0091] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and styrene, in which the volume fraction of styrene is 12%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0092] FIG. 11 is a SEM image of the porous structure of the block copolymer obtained in this example. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 12% concentration of styrene for 4 hours at 20° C., the membrane surface presents bi-continuous morphology to the high extent with a relatively high degree of pore-opening for the membrane, showing that the ethanol/styrene system can be suitable to selective swelling pore-forming at room temperature.

EXAMPLE 9

[0093] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the macroporous polyvinylidene fluoride substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and 1,4-dioxane, in which the volume fraction of 1,4-dioxane is 12%, after soaking the membrane at 20° C. for 4 hours, take it out and leave it at room temperature for drying.

[0094] The composite membrane of the porous block copolymer obtained in this example can be used to separate the macromolecules in water system. The composite membrane has a pure water permeance of ˜530L/(m.sup.2.Math.h.Math.bar), and a rejection to bovine serum protein of 68%.

Comparison 1

[0095] We add 0.02 g of PS.sub.53-P2VP21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in pure ethanol reagent, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0096] FIG. 12 and FIG. 13 are a SEM image of the porous structure of the block copolymer obtained in this comparison, respectively. It can be seen from the figure that a pure ethanol solvent cannot achieve pore-forming throughout the entire polymer membrane at 20° C., only leaving some shallow pores caused by segregation on the surface, and no pores are formed in the cross section, showing that reagents only selective to the dispersed phases of a block copolymer cannot achieve selective swelling pore-forming.

Comparison 2

[0097] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and N,N-dimethylformamide, in which the volume fraction of N,N-dimethylformamide is 12%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0098] FIG. 14 and FIG. 15 are a SEM image of the porous structure of the block copolymer obtained in this comparison, respectively. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 12% concentration of N,N-dimethylformamide for 4 hours at 20° C., only leaving some shallow pores caused by segregation on the surface, and no pores are formed in the cross section, showing that adding reagents only selective to the dispersed phases of a block copolymer to alcohol reagents at room temperature cannot achieve selective swelling pore-forming.

[0099] In addition, after measuring the thickness of the polymer membrane with increase from 237 nm to 247 nm in this example after swelling, the porosity of the membrane prepared under this conditions works out at 5%.

Comparison 3

[0100] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer; immerse the membrane in a mixed solution containing ethanol and cyclohexane, in which the volume fraction of cyclohexane is 12%, after soaking the membrane at 20° C. for 4 hour, take it out and leave it at room temperature for drying.

[0101] FIG. 16 and FIG. 17 are a SEM image of the porous structure of the block copolymer obtained in this comparison, respectively. It can be seen from the figure that if we treat the membrane by the composite swelling agent with a 12% concentration of cyclohexane for 4 hours at 20° C., only leaving some shallow pores caused by segregation on the surface, and no pores are formed in the cross section, showing that adding reagents strong selective to the continuous phases of a block copolymer to alcohol reagents at room temperature also cannot achieve selective swelling pore-forming.

EXAMPLE 10

[0102] We add 0.02 g of PS.sub.53-P2VP.sub.21 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 200 μL of the above solution, then spin-coat it on the 1.5 cm×1.5 cm silicon wafer substrate, which is next placed at room temperature to naturally become dry for 12 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer. We use a mixed solution of dichloromethane with a volume fraction of 16% and ethanol with a volume fraction of 84% as a swelling agent, and evenly apply 50 μl of the agent on the membrane surface, then treat it at 20° C. until it naturally evaporates and becomes dry. As ethanol and dichloromethane are highly volatile, and the swelling agent is completely volatilized in only 5 minutes.

[0103] FIG. 18 and FIG. 19 are a SEM image of the porous structure of the block copolymer obtained in this example, respectively. It can be seen from the figure that when the dichloromethane content is high, only using a small amount of swelling agents can achieve selective swelling pore-forming in a short time, presenting a bi-continuous structure on both the surface and the cross section, showing that the swelling time can be significantly decreased by increasing the second agent content in the swelling agent, while the swelling is undergoing at room temperature without heating, which make it feasible to apply only a small amount of swelling agents on the membrane surface for selective swelling.

EXAMPLE 11

[0104] We add 0.02 g of PS.sub.60-P2VP.sub.20 block copolymers to 2 g of chloroform solutions and fully stir the solution to dissolve it, then filter the prepared polymer solution with a 200 μm pore-size filter to remove large particles of impurities; take 100 μL of the above solution, then spin-coat it on the silicon wafer substrate at 2000 rpm for 30 seconds, which is next placed at room temperature to naturally become dry for 6 hours, so as to completely volatilize the solvent to obtain a dense membrane of the block copolymer. We use a mixed solution of tetrahydrofuran with a volume fraction of 40% and ethanol with a volume fraction of 60% as a swelling agent, and evenly apply 50 ul of the agent on the membrane surface, then treat it at 20° C. until it naturally evaporates and becomes dry.

[0105] FIG. 20 and FIG. 21 are a SEM image of the porous structure of the block copolymer obtained in this example, respectively. It can be seen from the figure that when the tetrahydrofuran content is high, only using a small amount of swelling agents can achieve selective swelling pore-forming of the block copolymer, presenting a bi-continuous structure on both the surface and the cross section.

[0106] In addition, after measuring the thickness of the polymer membrane obtain in this example after swelling, the thickness increases from 230 nm to 331 mn, and the porosity of the membrane prepared under this conditions works out at 31%.