JP2835342B2 - Separation membrane and separation method - Google Patents

Description

The present invention relates to a separation membrane and a separation method for an organic solvent mixture, and particularly to an organic solvent having a high affinity for an acrylic polymer.
The present invention relates to a separation membrane that can be suitably used for separating an organic solvent having a low affinity for an acrylic polymer, and a separation method using the separation membrane.

[Problems to be solved by conventional technology and invention]

In recent years, a membrane separation method for separating various mixtures using a membrane having pores has become increasingly popular, and the technique is being applied in various fields. In addition, the separation target in the membrane separation method is not only a solid-liquid mixture, but also a liquid-liquid, a gas-gas, and a gas-liquid mixture. I have.

Separation of organic solvents by a membrane separation method is also one of the fields that have attracted attention. Mixtures that could not be separated by conventional simple methods (for example, mixtures having close boiling points and difficult to separate by distillation, azeotropic mixtures) , Mixtures containing heat-sensitive substances, etc.). The pervaporation method and the reverse osmosis method are suitable for the membrane separation method of a mixture of organic solvents.

By the way, since the accuracy and efficiency of separation in membrane separation methods including the pervaporation method and reverse osmosis method depend on the performance of the membrane itself, it is necessary to develop a membrane with excellent strength, durability and separation selectivity. Is important, and various polymer films have been proposed so far.

For example, JP-A-50-98568 discloses a permeable membrane in which a polymerizable monomer is graft-polymerized on the inner surface of a pore of a polymer film having pores. This permeable membrane uses a polymer polymer film having pores such as polyethylene, which has excellent durability, heat resistance and chemical resistance, as a base material. Graft polymerized on the inner surface. However, this separation membrane is suitable for separating an aqueous mixture by a reverse osmosis method, but its performance is not sufficient in separating an organic solvent-based mixture such as a benzene / cyclosexane mixture.

JP-A-52-122279 discloses a separation membrane comprising an aliphatic olefin polymer containing an acid group derived from an unsaturated carboxylic acid or the like. In this separation membrane, an unsaturated carboxylic acid or the like is polymerized on the surface of a membrane substrate made of an aliphatic olefin polymer or the like by a radical reaction, a crosslinking reaction by light irradiation or electron beam irradiation. This separation membrane can be used for a pervaporation method, and in particular, an unsaturated compound can be relatively easily separated from a mixture of organic solvents. However, the performance of the membrane is not yet sufficient such that the membrane is practically used on a porous support.

In order to obtain an excellent separation membrane, it is basically necessary to selectively increase the affinity with the compound to be separated. However, if the entire separation membrane is made of a material having an affinity for the target component, the separation membrane swells, and high separation selectivity cannot be obtained due to the plasticizing effect. In addition, there is a problem that the mechanical strength and durability of the separation membrane are reduced.

Accordingly, an object of the present invention is to provide a separation membrane which can separate a mixture composed of an organic solvent such as a benzene / cyclohexane mixed liquid with high selectivity, and is excellent in strength and durability.

Another object of the present invention is to provide a separation method having high selectivity to an organic solvent mixture.

[Means for solving the problem]

As a result of intensive research in view of the above object, the present inventors have used a microporous film of ultra-high molecular weight polyethylene having a specific porosity and average pore size, and plasma graft polymerizing an acrylic monomer on the surface of this film, If the separation membrane has pores substantially closed with an acrylic graft polymer, it selectively permeates an organic solvent having an affinity for the acrylic graft polymer, and has excellent strength and durability. The inventors have discovered that the present invention can be performed and completed the present invention.

That is, the separation membrane of the present invention used for separating the organic solvent mixture has a thickness of 0.1 to 50 μm, a porosity of 30 to 95%, and an average pore diameter.
An acrylic monomer is plasma-grafted onto a microporous membrane made of ultrahigh molecular weight polyethylene having a weight-average molecular weight of 5 × 10 ⁵ or more with a strength of 0.005 to 1 μm and a breaking strength of 200 kg / cm ² or more, thereby forming pores of the microporous membrane. It is characterized by being substantially blocked by an acrylic graft polymer.

Further, the method of the present invention for separating an organic solvent mixture comprises a thickness of 0.1 to 50 μm, a porosity of 30 to 95%, and an average pore diameter of 0.005 to 1 μm.
m, breaking strength 200 kg / cm ² or more, weight average molecular weight 5 × 10 ⁵
An acrylic monomer is plasma-grafted onto the microporous membrane made of the above ultrahigh molecular weight polyethylene, and a separation membrane having pores of the microporous membrane substantially closed by an acrylic graft polymer is used. An organic solvent having an affinity for the acrylic graft polymer is selectively separated by a filtration method.

 Hereinafter, the present invention will be described in detail.

 First, the separation membrane of the present invention will be described.

The separation membrane of the present invention is based on a microporous ultrahigh molecular weight polyethylene membrane. The ultrahigh molecular weight polyethylene used as the material is a crystalline linear ultrahigh molecular weight polyethylene comprising a homopolymer of ethylene or a copolymer of ethylene and 10 mol% or less of α-olefin. Its molecular weight is such that the weight average molecular weight is 5 × 10 ⁵ or more, preferably 1 × 10 ⁶ to 1 ×.
10 ⁷ The weight average molecular weight of the ultra high molecular weight polyethylene affects the mechanical strength of the obtained separation membrane. If the weight average molecular weight is less than 5 × 10 ⁵ , a very thin and high strength separation membrane cannot be obtained. On the other hand, the upper limit of the weight average molecular weight is not particularly limited. However, if the weight average molecular weight exceeds 1 × 10 ⁷ , it is not preferable because it is difficult to form a thin film by stretching.

As the ultrahigh molecular weight polyethylene microporous membrane, a film obtained by blending another relatively low molecular weight polyethylene with ultrahigh molecular weight polyethylene can be used. In this case, it is preferable to use a polyethylene composition containing 1% by weight or more of an ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more, and having a weight average molecular weight / number average molecular weight of 10 to 300.

The weight average molecular weight / number average molecular weight of the polyethylene composition is 10 to 300, preferably 12 to 250. If the weight-average molecular weight / number-average molecular weight is less than 10, the average molecular chain length is large and the entanglement density of the molecular chains during dissolution is high, so that it is difficult to prepare a high-concentration solution. On the other hand, if it exceeds 300, the low molecular weight component is broken at the time of stretching, and the strength of the entire film is reduced.

The weight average molecular weight / number average molecular weight is used as a measure of the molecular weight distribution, and the width of the molecular weight distribution increases as the ratio of the molecular weights increases. That is, in the composition composed of polyethylene having different weight average molecular weights, the larger the ratio of the molecular weight of the composition is, the larger the difference in the weight average molecular weight of the polyethylene to be blended is, and the smaller the difference is, the smaller the difference in the weight average molecular weight is. I have.

The content of the ultrahigh molecular weight polyethylene in the polyethylene composition is 1% by weight or more, based on 100% by weight of the entire polyethylene composition. When the content of the ultrahigh molecular weight polyethylene is less than 1% by weight, the entanglement of the molecular chain of the ultrahigh molecular weight polyethylene which contributes to the improvement of the stretchability is hardly formed, and a high strength microporous membrane cannot be obtained. On the other hand, the upper limit is not particularly limited, but if it exceeds 90% by weight, it will be difficult to achieve the desired high concentration of the polyethylene solution.

The polyethylene other than the ultrahigh molecular weight polyethylene in the polyethylene composition has a weight average molecular weight of less than 7 × 10 ⁵ , but the lower limit of the molecular weight is preferably 1 × 10 ⁴ or more. When polyethylene having a weight average molecular weight of less than 1 × 10 ⁴ is used, breakage is likely to occur during stretching,
It is not preferable because a desired microporous membrane cannot be obtained. In particular, it is preferable that a polyethylene having a weight average molecular weight of 1 × 10 ⁵ or more and less than 7 × 10 ⁵ is blended with the ultrahigh molecular weight polyethylene.

Examples of such polyethylene include those of the same kind as the ultrahigh molecular weight polyethylene described above, and high density polyethylene is particularly preferred.

The ultra-high molecular weight polyethylene microporous membrane, whether used alone or in the case of a composition, may optionally contain an antioxidant, an ultraviolet absorber, a lubricant, an antiblocking agent, a pigment, a dye, an inorganic Various additives such as fillers can be added within a range that does not impair the purpose of the present invention.

Next, a method for producing a microporous ultrahigh molecular weight polyethylene membrane will be described.

First, in the case of a microporous membrane made of ultrahigh molecular weight polyethylene alone, it can be produced, for example, by the method described in JP-A-60-242035.

Next, in the case of a microporous membrane made of a polyethylene composition obtained by blending relatively low molecular weight polyethylene with ultrahigh molecular weight polyethylene, it can be produced by the following method.

First, a high-concentration solution is prepared by heating and dissolving the above-described polyethylene composition in a solvent. The solvent is not particularly limited as long as it can sufficiently dissolve the polyethylene composition, and may be the same as that described in JP-A-60-242035. The heating dissolution is carried out with stirring at a temperature at which the polyethylene composition completely dissolves in the solvent. The temperature varies depending on the polymer and solvent used, but is
C. is preferred. The concentration of the polyethylene composition solution is 10 to 50% by weight, preferably 10 to 40% by weight.

Next, the heated solution of the polyethylene composition is extruded from a die and molded. As the die, a sheet die having a rectangular base shape is usually used, but a double cylindrical hollow die, an inflation die and the like can also be used. The die gap when using a sheet die is usually 0.1 to 5 mm, and is heated to 140 to 250 ° C. during extrusion molding. At this time, the extrusion speed is usually 20 to 30 cm / min to 2 to 3 m / min.

The solution extruded from the die in this way is formed into a gel by cooling. Cooling is preferably performed at a rate of 50 ° C./min or more at least until the gelling temperature or less.

Next, this gel-like molded product is stretched. The stretching is performed by heating the gel-like molded product and at a predetermined magnification by a usual tenter method, a roll method, an inflation method, a rolling method or a combination of these methods, as described above. Biaxial stretching is preferred, and either vertical or horizontal simultaneous stretching or sequential stretching may be used, but simultaneous biaxial stretching is particularly preferred.

The stretching temperature is the melting point of the polyethylene composition + 10 ° C. or less,
It is preferably in the range from the crystal dispersion temperature to less than the crystal melting point. For example, 90 to 140 ° C, more preferably 100 to 130 ° C
Range.

The thickness of the ultrahigh molecular weight polyethylene microporous membrane serving as the base material of the separation membrane of the present invention thus obtained is 0.1 to 50 μm.
m, preferably 0.2 to 25 μm. When the thickness is less than 0.1 μm, the mechanical strength of the film is small, and it is difficult to put the film to practical use. On the other hand, if it exceeds 50 μm, the thickness is too large and the transmission performance is reduced, which is not preferable.

The porosity of the microporous membrane is in the range of 30-95%, preferably 50-90%. If the porosity is less than 30%, the permeability of the object to be separated becomes insufficient, while if it exceeds 95%, the mechanical strength of the membrane becomes small and the practicability is poor.

The average pore size is in the range from 0.005 to 1 μm. When the average pore size is less than 0.005 μm, the permeability of the separation target becomes insufficient, and when the average pore size exceeds 1 μm, the separation performance is reduced.

Further, by setting the breaking strength to 200 kg / cm ² or more, deformation resistance against swelling when the solvent is dissolved in the acrylate graft polymer becomes sufficient.

The separation membrane of the present invention has a structure in which an acrylic monomer is graft-polymerized on at least the inner surface of the pores of the microporous ultrahigh molecular weight polyethylene membrane described above, and the acrylic polymer substantially closes the pores. The graft polymerization of the acrylic monomer is performed by a plasma graft polymerization method as described later. As the acrylic monomer, acrylic acid, methacrylic acid, and esters thereof can be used, and are appropriately selected depending on the separation target.

To form a separation membrane of a benzene / cyclohexane mixture, methyl acrylate is graft-polymerized. Polymethyl acrylate has an affinity for benzene and tends to swell when benzene dissolves, but plasticization of cyclohexane, which has no affinity, because deformation is suppressed by the ultra-high molecular weight polyethylene microporous membrane. Dissolution diffusion due to the effect is suppressed. Therefore, only benzene passes through the pores, and the separation membrane selectively permeates benzene from the benzene / cyclohexane mixture. In the pores, the swelling of polymethyl acrylate due to benzene is suppressed, the entire film is hardly deformed, and the film strength does not decrease.

Plasma graft polymerization is used to graft polymerize the acrylic monomer onto the inner surface of the pores of the microporous membrane. In the plasma graft polymerization method, a microporous ultra-high molecular weight polyethylene membrane is irradiated with plasma to generate radicals, and then an acrylic monomer is brought into contact with the microporous membrane to perform graft polymerization. Examples of the plasma graft polymerization include a gas phase polymerization method and a liquid phase polymerization method, and a liquid phase polymerization method is preferable for graft polymerization of an acrylic monomer. By generating radicals and graft-polymerizing the microporous film serving as the base material, instead of the acrylic monomers to be graft-polymerized in this manner, the acrylic monomers can be graft-polymerized to the inner surfaces of the pores. The homopolymer formed at that time is washed away with a solvent. The graft polymer is also formed on the surface other than the inner surface of the pores of the ultrahigh molecular weight polymer microporous membrane. However, since this affects the substantial permeability, it is desirable to minimize the graft polymer.

FIGS. 1 (a), (b-1) and (b-2) conceptually show a step of plasma graft polymerization of acryl monoethylene onto ultra-high molecular weight polyethylene microporous membrane 1 to form a separation membrane of the present invention. It is sectional drawing shown in FIG. The ultrahigh molecular weight polyethylene microporous membrane 1 has a large number of pores 2 penetrating the membrane.
Plasma graft polymerization is performed on the microporous film, and an acrylic monomer is graft-polymerized on its surface. The graft-polymerized acrylic polymer 3 is formed not only on the surface of the microporous membrane but also on the inner surface of the pores. (B-1) shows one embodiment of a membrane in which the pores are substantially filled with the graft polymer 3. In (b-2), the graft polymer 3 is formed on one surface of the microporous membrane. Here, the graft polymer 3 is formed in a part of the pores,
Is closed. The separation membrane of the present invention may have either of these structures.

The homopolyethylene by-produced in the process of plasma graft polymerization is completely washed away using a solvent such as toluene, and only the graft polymer is deposited on the surface of the ultra-high molecular weight polyethylene microporous membrane (inner pore surface and membrane surface). Leave.

The plasma graft polymerization specifically includes the following steps.

(A) Argon, helium, at a pressure of 10 ^{-2 to} 10 mbar;
In the presence of gas such as nitrogen and air, usually frequency 10-30MHZ
z, plasma processing is performed on the microporous film at an output of 1 to 1000 W for 1 to 1000 seconds.

(B) The microporous film subjected to the plasma treatment is immersed in a solution in which 1 to 10% by weight of an acrylic monomer is dissolved or suspended in an inorganic or organic solvent, particularly in an aqueous solution, and bubbling with nitrogen gas, argon gas, or the like is performed. While at 20-100 ° C, 1-6
Perform a graft polymerization reaction for 0 minutes.

(C) The obtained microporous membrane is washed with toluene, xylene or the like for about one hour and dried.

By the plasma graft polymerization method described above, it is possible to obtain a target separation membrane in which the pores of the microporous membrane are substantially closed with the acrylic graft polymer. Since the plasma graft polymerization occurs only on the surface of the microporous ultrahigh molecular weight polyethylene membrane, the membrane base material does not deteriorate. Further, since the graft polymer is chemically bonded to the film substrate, it does not change with time.

In the separation membrane of the present invention, it is necessary that the acrylic graft polymer substantially closes the pores of the ultrahigh molecular weight polyethylene microporous membrane as the membrane base material. The pore-blocked acrylic graft polymer selectively takes up certain components of the organic solvent mixture and permeates them to the other side of the membrane. Since the porosity of the ultrahigh molecular weight polyolefin microporous membrane is high, the amount of the substance permeating (separating) through the acrylic graft polymer in the pores also increases, and efficient separation can be performed. In addition, since the swelling of the acrylic graft polymer is suppressed by the ultrahigh molecular weight polymer microporous membrane, the strength of the entire membrane does not decrease.

Next, a separation method using the above-described separation membrane of the present invention will be described.

In the method of the present invention, the organic solvent mixture is separated by the pervaporation method or the reverse osmosis method using the separation membrane of the present invention described in detail above. The pervaporation method or reverse osmosis method in the method of the present invention is basically the same as the pervaporation or reverse osmosis method of public land except for using the separation membrane of the present invention. The mixed liquid to be separated is supplied to the primary side across the membrane, the secondary side is set to the low pressure side, and one component of the mixed liquid is taken out as a gas or a liquid to the secondary side. When a microporous ultra-high molecular weight polyethylene membrane graft-polymerized with methyl acrylate is used as a separation membrane, for example, when a benzene / cyclohexane mixture is separated, benzene is taken out as a gas or a liquid on the secondary side.

The application temperature range in the separation method of the present invention is usually 0 to 120.
° C, preferably 10 to 100 ° C. If the temperature exceeds 120 ° C., the heat resistance of the high-molecular-weight polyethylene microporous membrane becomes insufficient, causing a problem in maintaining the shape of the membrane. If the temperature is lower than 0 ° C., the permeation amount per unit film area, film thickness and time is small. It is not preferable.

The pressure range applicable to the separation method of the present invention is 200 k.
g / cm ² or less, preferably 100 kg / cm ² or less. 200kg / cm
^At a pressure exceeding ² , it becomes difficult to maintain the shape of the ultrahigh molecular weight polyethylene microporous membrane.

Organic liquid mixtures that can be separated by the method of the present invention include:
Benzene, toluene, chlorinated hydrocarbons, tetrahydrofuran, which are solvents that dissolve polyacrylate
Aliphatic alcohols having 1 to 4 carbon atoms and non-solvents which do not dissolve the polyacrylate, such as aliphatic hydrocarbons, carbon tetrachloride, aliphatic alcohols (having 5 or more carbon atoms), cyclohexanol, and tetrahydrofurfural For example, in addition to the above-mentioned benzene / cyclohexane, benzene / n-hexane, benzene
/ n-heptane, toluene / cyclohexane, toluene /
Examples include methylcyclohexane, hexane / heptane, toluene / benzene, benzene / water and the like. These mixtures may be not only two-component systems as described above but also multi-component systems of three or more components.

〔Example〕

The present invention will be described in more detail with reference to the following specific examples.

Example 1 Weight average molecular weight 25 × 10 ⁵ , film thickness 10 μm, porosity 70%,
A microporous ultrahigh molecular weight polyethylene membrane (manufactured by Tonen Petrochemical Co., Ltd.) having an average pore diameter of 0.02 μm and a breaking strength of 4700 kg / cm ² was irradiated with plasma using a plasma polymerization apparatus (manufactured by Samco Corporation). Table 1 shows the conditions of the plasma treatment at this time.

Next, the ultra-high molecular weight polyethylene microporous membrane subjected to the plasma treatment was placed in a 4% by volume aqueous methyl acrylate solution for 15 minutes.
Soak for minutes. The temperature of the aqueous solution during immersion was 30 ° C.

After immersion, the ultra-high molecular weight polyethylene microporous membrane was washed in toluene for one day and dried at room temperature. After drying, the weight of the membrane was measured, and the amount of graft polymerization was measured based on the change from the initial membrane weight. The polymerization amount was 2.3 mg / cm ² .

The ATR of the obtained membrane was measured, and it was confirmed that the substance formed on the ultrahigh molecular weight polyethylene microporous membrane was polymethyl acrylate.

Using this separation membrane, a separation experiment of a benzene / cyclohexane mixed solution was performed by pervaporation. Supply liquid (benzene / cyclohexane mixed solution: benzene concentration (weight ratio) 0.5) at 25 ° C, 40 ° C, 50 ° C and 60 ° C
And the permeation flow rate (Q) and the separation coefficient (α) at that time
I asked. The results are shown in FIG.

Example 2 In the same manner as in Example 1, a separation membrane in which methyl acrylate was graft-polymerized on a microporous ultrahigh molecular weight polyethylene membrane was formed.

Separation experiments were performed by the pervaporation method with the temperature of the feed liquid (benzene / cyclohexane mixed solution) set to 50 ° C. and the concentration (weight ratio) of benzene in the mixed solution set to 0.2 to 0.9. The weight ratio of benzene in the gas permeating the separation membrane was determined. The results are shown in FIG. At this time, the permeation flow rate (Q) and the separation coefficient (α) were determined. The results are shown in FIG.

As can be seen from the above, the separation membrane of the present invention has a wide concentration,
And benzene from the benzene / cyclohexane mixed solution selectively over a temperature range. In particular, when the temperature of the supply liquid is 25 ° C., the separation coefficient shows an extremely high value of 14.8.

〔The invention's effect〕

The separation membrane of the present invention uses a microporous ultrahigh molecular weight polyethylene membrane as a substrate, has good swelling resistance in an organic solvent, and has excellent mechanical strength and durability.

Further, in the separation membrane of the present invention, since the acrylic polymer substantially closes the pores of the microporous membrane, by using a pervaporation method or a reverse osmosis method,
Organic substances having an affinity for the graft polymer can be separated with high selectivity. Particularly, when a separation membrane obtained by graft polymerization of a methyl acrylate monomer is used, a solvent dissolving polyacrylic acid such as benzene, toluene, chlorinated hydrocarbon, and tetrahydrofuran, an aliphatic hydrocarbon,
The solvent can be selectively decomposed from a mixed solution with a non-solvent that does not dissolve polyacrylic acid such as carbon tetrachloride, and in particular, benzene can be selectively separated from a benzene / cyclohexane mixed solution.

The separation membrane of the present invention is preferable because a microporous membrane of ultrahigh molecular weight polyethylene is used as a base material, so that a uniform and reproducible separation membrane can be easily obtained.

Such a separation membrane is suitably used for separating an organic solvent mixture by a pervaporation method or a reverse osmosis method, and is particularly useful for separating a benzene / cyclohexane mixture.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view conceptually showing a step of plasma-grafting an acrylic monomer onto a microporous ultrahigh molecular weight polyethylene membrane, wherein (a) shows a microporous ultrahigh molecular weight polyethylene membrane and (b-1) And (b-2) each show a microporous ultrahigh molecular weight polyethylene membrane graft-polymerized, and FIG. 2 shows a benzene / cyclohexane mixed solution by a pervaporation method using a separation membrane according to one embodiment of the present invention. FIG. 3 is a graph showing the relationship between the temperature of a benzene / cyclohexane mixed solution, the permeation flow rate (Q), and the separation coefficient (α) when benzene is separated from the benzene / cyclohexane mixed solution. FIG. 4 is a graph showing the relationship between the concentration of benzene and the weight ratio of benzene in the substances permeated through the separation membrane. And the concentration of benzene Rohekisan mixed solution is a graph showing the relationship between the permeation rate (Q) and separation factor (alpha). 1 ... ultra-high molecular weight polyethylene microporous membrane 2 ... pore 3 ... graft polymer

Continuation of the front page (51) Int.Cl. ⁶ Identification code FI C07C 15/04 C07C 15/04 (56) References JP-A-62-262705 (JP, A) JP-A-64-30602 (JP, A) JP-A-59-160505 (JP, A) JP-A-60-242035 (JP, A) JP-A-3-65225 (JP, A) JP-A-47-11661 (JP, A) (58) (Int.Cl. ⁶ , DB name) B01D 71/78 B01D 71/26 B01D 61/36 C07C 7/144

Claims (4)

(57) [Claims] (1) A fine powder of ultrahigh molecular weight polyethylene having a thickness of 0.1 to 50 μm, a porosity of 30 to 95%, an average pore diameter of 0.005 to 1 μm, a breaking strength of 200 kg / cm ² or more, and a weight average molecular weight of 5 × 10 ⁵ or more. A separation membrane for an organic solvent mixture, wherein an acrylic monomer is plasma-grafted onto a porous membrane, whereby pores of the microporous membrane are substantially closed by an acrylic graft polymer. 2. The separation membrane for an organic solvent mixture according to claim 1, wherein the graft polymer for closing the pores is polymethyl acrylate. 3. A fine powder of ultrahigh molecular weight polyethylene having a thickness of 0.1 to 50 μm, a porosity of 30 to 95%, an average pore diameter of 0.005 to 1 μm, a breaking strength of 200 kg / cm ² or more, and a weight average molecular weight of 5 × 10 ⁵ or more. An acrylic monomer is plasma-grafted onto a porous membrane, and a separation membrane is used in which the pores of the microporous membrane are substantially closed by an acrylic graft polymer, and the acrylic graft is obtained by a pervaporation method or a reverse osmosis method. A method for separating an organic solvent mixture, comprising selectively separating an organic solvent having an affinity for a polymer. 4. The method according to claim 3, wherein the graft polymer for closing the pores is polymethyl acrylate, and benzene is separated from a mixture of organic solvents containing benzene and cyclohexane. A method for separating a solvent mixture.