JPS5833247B2 - Aeon Kokanki O Yusuru Fukugoumaku Oyobi Sono Seizouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a composite membrane having ion exchange groups and a method for producing the same.

One of the important uses of membranes with ion-exchange groups is electrodialysis, and the power consumption of electrodialysis cells is mostly due to Joule heat loss due to the electrical resistance of the membrane and the electrical resistance of the liquid in each liquid chamber. It is known.

Therefore, by reducing the electrical resistance of the membrane, not only can the power consumption be reduced, but also the lower electrical resistance means that the time for ions to pass through the membrane is shorter. This means that it can be shortened.

An effective way to reduce the electrical resistance of a membrane is to increase the concentration of ion exchange groups in the membrane, that is, to increase the exchange capacity, or to decrease the membrane thickness, but the former inevitably increases the swelling pressure. Therefore, it has its limitations, and the latter results in a decrease in mechanical strength, but it is possible to compensate for this with appropriate reinforcing measures.

Dialysis is known as another important use of membranes with ion exchange groups.

Even in this case, if the membrane thickness is reduced, the time for ions to pass through the membrane becomes shorter, and the dialysis time is shortened.

It has also been discovered that this membrane having ion exchange groups can be used as a reverse osmosis membrane or an ultrafiltration membrane.

In these membrane separation methods, the membrane is required to have permselectivity and a large permeation flux.

It is known that the permselectivity is not related to the thickness of the membrane, so by reducing the thickness of the membrane, the permeation flux can be reduced without impairing the permselectivity.

For this reason, it has been proposed to make the membrane structure asymmetric and to reduce the effective thickness of the membrane.

(Industrial, Engineering, Chemistry, Vol. 61, No. 11, pp. 62-88, 196
9) However, as mentioned above, there is a limit to the reduction in the thickness of a membrane having ion exchange groups unless appropriate reinforcing means are adopted.

There are also restrictions on making the membrane structure asymmetric, and it is particularly difficult to manufacture a membrane with an asymmetric structure using a rubbery polymer substance with a low glass transition temperature.

In recent years, pressure dialysis has become known as a new application for membranes with ion exchange groups.

(The Office of Sarin, Water No. 452, 1969) This utilizes the property that concentrated seawater permeates a membrane that has two types of ion exchange groups with different signs when it comes into contact with saltwater under pressure. This is the method.

However, the permeation flux of conventional membranes is extremely small, making them practically unusable for this purpose.

Even in this case, the permeation flux can be increased by reducing the substantial membrane thickness while reinforcing the mechanical strength by appropriate means.

As mentioned above, if the mechanical strength of a membrane with ion-exchange groups is reinforced by appropriate means and the thickness is reduced, a high-performance membrane that can be used for a wide variety of purposes can be obtained. Such a film has not yet been obtained.

With conventional methods, it is difficult to produce a membrane with ion exchange groups that has practical mechanical strength and a thickness of several microns or less, and has not been realized yet.

For example, one of the typical manufacturing methods for electrodialysis membranes is
5-13009, in which corrosion-resistant stamens are used as reinforcing materials, and the thickness of the membrane thus obtained is approximately 150-300 microns.

On the other hand, if the membrane itself is not used, the membrane itself must have sufficient mechanical strength, so the thickness is 300-6
It can reach up to 0.00 microns.

Such membranes cannot be used for other purposes, such as reverse osmosis, because their permeation flux is extremely low.

An example of reducing the thickness of a conventional membrane with ion-exchange groups and using it for reverse osmosis is reported by Lamaze et al. in the Journal of
Applied, Polymer, Science Vol. 15, 1665
-1677 pages, reported in 1971.

In this example, a 15 micron thick low density polyethylene film is impregnated with a monomer capable of introducing ion exchange groups,
Membranes are manufactured by introducing ion exchange groups after graft polymerization or by impregnating monomers having dissociable groups and fixing the dissociable groups by graft polymerization.

This report does not explain the degree of change in membrane shape due to swelling or mechanical properties such as tensile strength of the membrane.

However, if graft polymerization is carried out uniformly over the entire membrane and ion exchange groups are introduced at a uniform concentration in all directions, it is no different from a normal electrodialysis membrane, so the exchange capacity and membrane thickness are similarly limited. Therefore, even if only the surface of the membrane is graft-polymerized and the ion-exchange groups are concentrated in that area, it may be difficult to create a membrane with a thickness of several microns or less. Since it is no different from a normal polyethylene film, the permeation phenomenon is hindered at that part, so it is clear that the purpose of reducing the film thickness and reducing the permeation flux cannot be achieved.

According to the present invention, the drawbacks of conventional membranes having ion-exchange groups as described above are eliminated, and the membranes are improved to have sufficient mechanical strength and permeability even when the membrane thickness is several microns or less. A composite membrane having ion exchange groups that has a flux is provided.

The composite membrane according to the present invention has a thin film having an electrochemically permeable ion exchange group formed on a physically selectively permeable polymeric microporous layer.

The base of the composite membrane of the present invention is a permselective polymeric film or layer which itself can be used for ultrafiltration or reverse osmosis.

This base has many fine continuous pores, and molecules with a size smaller than the pore size can physically pass through.

Such a permselective polymer film is already known, and its manufacturing method is also known.

The composite membrane having ion-exchange groups according to the present invention is applied onto the above-mentioned microporous polymer layer by dipping, casting or spraying a solution or emulsion of a polymeric substance capable of introducing ion-exchange groups; The solvent in this solution or the dispersion medium of the emulsion is removed by evaporation and drying, or
Alternatively, the coated polymeric substance and the support microporous layer are leached into a solvent that is a non-solvent but is compatible with the solvent or dispersion medium of the coated polymeric substance and removed to form a thin film. Manufactured by introducing ion exchange groups into a thin film.

As mentioned above, in the composite membrane of the present invention, the support is a microporous polymer layer, so when forming a thin film of a polymer material into which ion exchange groups can be introduced, the support is swollen or dissolved. Do not crush the micropores by

Therefore, as a solvent or dispersion medium for coating polymeric substances,
A material having a relatively low affinity for the support is selected.

Normally, if such a solvent or dispersion medium is used to coat another polymer substance on the film surface, a composite film with good adhesion cannot be obtained, but if the film has micropores, it is extremely difficult to obtain a composite film. This creates a strong adhesive force that is virtually impossible to peel off.

As will be shown later, it is surprising that the adhesion was very good even though the pores were extremely small and did not allow even the coated polymer to pass through.

Materials for the polymeric microporous layer that can be used as a support include those that are dissolved in the solvent or dispersion medium of the applied polymeric substance,
Although there are no particular limitations as long as the material does not swell excessively and does not undergo changes during the reaction of introducing ion exchange groups into the coated polymer material, it is easy to form a microporous layer structure. Polyvinyl chloride resin is preferred because it has excellent chemical resistance and chemical resistance.

The present invention aims to reduce the thickness of the membrane having ion exchange groups as much as possible and obtain a composite membrane in which the polymeric microporous layer serves only as a reinforcing layer. The size and shape of the equipment shall be suitable for this purpose.

For example, if the diameter of the pores is too large and allows the coating liquid to penetrate into the pores, that is, the coated polymer substance, the thickness of the final membrane containing ion exchange groups will be almost equal to the thickness of the entire composite membrane. Furthermore, if the diameter of the micropores is too small, the microporous layer will impede the permeation phenomenon, and in this case as well, the substantial film thickness will be almost equal to the thickness of the entire composite film.

Therefore, in order for the thickness of the membrane containing ion exchange groups to be substantially small, the coated polymer must not penetrate into the micropores of the reinforcing layer, and the permeation phenomenon after introducing the ion exchange groups must be prevented. The rate-determining step is only in the membrane part that has ion exchange groups,
It must be ensured that the reinforcement does not interfere with the transmission phenomenon.

A microporous layer having such a pore size can be produced by a method known per se.

However, a wet method is advantageous, and can be prepared, for example, by the method described in Japanese Patent Application No. 47-56266 by the present inventors or based thereon.

According to the method described in the patent application, a reinforcing material manufactured by Nissho et al. is wetted in advance with a non-solvent of the resin in which a microporous layer is to be formed, and a solution of polyvinyl chloride resin is applied while only the surface thereof is dry. A microporous layer with an asymmetrical structure is formed on Nissho soil by applying the resin solution and immersing it in a solvent that is compatible with the solvent in the resin solution and is a non-solvent of the resin.

Therefore, if the material thus prepared is used as a reinforcing material in the present invention, a product with a three-layer structure can be obtained.

When trying to obtain a product with a two-layer structure, it is preferable to apply the above-mentioned polyvinyl chloride resin onto a smooth glass surface and then leaching the solvent into a non-solvent of the resin that is compatible with the polyvinyl chloride resin. A microporous layer having micropores is formed.

Next, a polymeric substance capable of introducing ion exchange groups is applied onto one surface of this microporous layer by a conventional method.

Examples of polymeric substances that can be used for this purpose include those having reactive groups such as benzene nuclei, double bonds, nitrogen groups, nitrile groups, and hydroxyl groups. Examples include styrene-butadiene copolymers, Examples include vinyl chloride/acrylonitrile copolymer, polyvinyl alcohol, etc.

These polymeric substances are applied as a solution or emulsion, and the application method may be any conventional method, such as a dipping method, a casting method using a doctor knife, or a spray method.

However, if the viscosity of the coating solution is low, about 100 centipoise or less, coating by dipping or spraying is preferable, and if a high viscosity solution of about 200 centipoise or more is used, coating by casting is preferable.

As mentioned above, the solvent or dispersion medium of the coating solution should be one that has a low affinity for the material of the microporous layer, and care should be taken not to crush the micropores.

After applying the polymeric substance into which ion exchange groups can be introduced, the solvent or dispersion medium is removed to form a coating film of the polymeric substance.

In this case, the solvent or dispersion medium may be removed by evaporation drying, and is compatible with the solvent or dispersion medium,
It may be removed by leaching into a solvent that is a non-solvent for any of the materials forming the composite membrane.

In the case of volatilization drying, hot air may be used, but in that case care must be taken not to heat the material above the softening temperature of the microporous layer.

Since the thickness of the coated polymeric material film is approximately equal to the thickness of the final product film having ion exchange groups, the thickness can be adjusted by heating the coating amount.

For example, in the case of a casting method, the amount of coating can be adjusted as desired by adjusting the flow rate of the coating liquid or adjusting the feeding speed of the reinforcing material.

When using the dipping method, by changing the concentration of the coating solution,
In the case of spraying, the film thickness can be similarly adjusted by changing the amount of spray from the nozzle, the feeding speed of the microporous layer, and the concentration of the coating liquid.

For example, when a cyclohexane solution containing 2.5% by weight of a styrene-butadiene copolymer is applied by dipping onto a microporous polyvinyl chloride film, a thin film of about 1 micron in thickness is formed.

In addition, commercially available styrene-butadiene copolymer latex (Dow Latex 612, Nissin Kasei Co., Ltd.) was mixed with water for 4 hours.
When diluted twice and applied by dipping onto the same microporous material, a film approximately 5 microns thick is formed.

As the final step, ion exchange groups are introduced into the polymer thin film thus formed.

Various known ion exchange groups can be introduced.

A sulfonic acid group is used as a strong acidic cation exchange group, a quaternary ammonium group is used as a strong basic anion exchange group, a carboxylic acid group is used as a weakly acidic cation exchange group, and a tertiary amine is used as a weakly basic anion exchange group. The group is typical.

The introduction of the ion exchange group is carried out depending on the type of reactive group possessed by the polymer substance that is the material of the thin film.

For example, when a styrene-butadiene copolymer is sulfonated with concentrated sulfuric acid, a sulfonic acid group is introduced into the benzene nucleus.

In the case of a vinyl chloride/acrylon copolymer, the nitrile group is hydrolyzed by about 70% by weight of sulfuric acid to introduce a carboxyl group.

In some cases, after introducing one type of ion exchange group as described above, another ion exchange group having a different sign may be introduced to obtain an amphoteric ion exchange membrane.

For example, if a thin film made of vinyl chloride/acrylonitrile copolymer is hydrolyzed as described above to introduce carboxyl groups, and then immersed in an aqueous solution of ethylenediamine, the chlorine part reacts with ethylenediamine, forming weak base tertiary amine groups. will be introduced.

Generally, when an ion exchange group is introduced into a polymeric substance, it becomes hydrophilic, and strong swelling pressure is generated within the polymeric substance in water or an aqueous solution.

Therefore, unless reinforced by crosslinking or the like, the polymer substance will swell or dissolve due to this force.

Furthermore, even when cross-linked, if ion exchange groups that have a strong affinity for water, such as strong acidic groups or strong basic groups, are introduced in an amount of about 2 milliequivalents or more per dry polymer material, this polymer material becomes extremely brittle. It is known that

According to the experience of the present inventors, if a styrene-butadiene copolymer film with a thickness of about 0.3 mm is crosslinked and then sulfonic acid groups of about 2 milliequivalents or more per dry polymer 11 are introduced, it becomes extremely brittle. It was frequently torn during handling.

According to the method of the present invention, even when not crosslinked, there is no change in shape after the introduction of ion exchange groups, and the microporous layer and ion exchange membrane do not separate or tear, making them easy to handle. It will be very easy.

However, crosslinking may be carried out if desired.

Furthermore, in general, when an ion exchange group is introduced, the chemical properties of the polymer material become completely different from those of the original material, so it is natural that it becomes easier to peel off, but in the case of the present invention, the reason for this is unknown. However, no phenomenon of decrease in adhesive strength was observed.

For example, on the microporous layer of polyvinyl chloride mentioned above,
A composite film with a thin film of micron styrene-butadiene copolymer was mixed with concentrated sulfuric acid and fuming sulfuric acid to a concentration of 9.
The composite membrane did not peel off even after being immersed in sulfuric acid containing 5% or more for 100 hours.

From this, it is clear that the composite membrane of the present invention has excellent advantages over conventional membranes.

The attached photograph shows a layer of commercially available styrene coated on a polyvinyl chloride microporous layer by the method of the present invention described in Example 5 below.
Butadiene copolymer latex (Dow Latex 612
, Nissin Kasei Co., Ltd.) diluted 4 times with water using a dipping method to form a thin film of approximately 5 microns in thickness, and then sulfonated to form a thin film. This is an electron micrograph of a cross section showing its structure.

In this photograph, the black continuous layer at the top is a polymer material layer having ion exchange groups, and the mesh layer at the bottom is a microporous layer.

(The white crack lines on the left and right are scratches caused when cutting the sample and do not exist in the actual product.

) The latex used for this has a particle size of about 2000 angstroms, so a microporous layer with relatively large pores is used.

As is clear from the photograph, the coating liquid has not penetrated into the microporous layer, and the pores in the reinforcing layer are continuous, and the pores have sufficient pore size to prevent the permeation phenomenon of the membrane portion having ion exchange groups. It will now be understood that there is no hindrance.

Therefore, when the composite membrane of the present invention is used in a membrane separation method, the actual membrane thickness, which is related to its performance, should be equal to the thickness of the polymer membrane having ion exchange groups, and this should be as small as several microns or less. As a result, it exhibits superior effects in use that are not found in conventional products.

For example, if this is used for electrodialysis, the electrical resistance will be reduced and the power consumption will be reduced.

In particular, when the composite membrane of the present invention is used for reverse osmosis or pressure dialysis, the effects are remarkable.

For example, a composite membrane obtained by forming a styrene-butadiene copolymer thin film with a thickness of about 1 micron on a polyvinyl chloride microporous layer and sulfonating this (see Example 1 below) has a
When exposed to 0.78% saline at 7°C under a pressure of 50 kg/crA, 67% of the salt was eliminated.

The permeate flux was then approximately 12 gallons/square foot/day.

In addition, a composite membrane obtained by immersing the composite membrane having the sulfonated styrene-butadiene copolymer membrane in an aqueous epichlorohydrin-pyridine solution to introduce quaternary ammonium groups (see Example 2 below) was heated. When used for dialysis,
When 3.5% saline was applied under a pressure of 50 kg/crA, the salt rejection rate decreased to 10%.

The permeate flux was then slightly higher, about 15 gallons/square foot/day.

In addition, 50 kg/crA of 5% sucrose aqueous solution was added to this composite membrane.
80% of the sucrose was rejected and the permeate flux was about 11 gallons/square foot/day.

As far as the present inventors know, a membrane with such performance has not yet been obtained, and by utilizing such characteristics, it is possible to add an aqueous solution containing low-molecular organic substances and inorganic salts to the composite membrane of the present invention. By contacting under pressure, concentration and desalting of low-molecular-weight organic substances are performed simultaneously, and this is used for various purposes as a new application of pressure dialysis.

Furthermore, it is generally known that the permselectivity of a membrane does not depend on the membrane thickness, so it is possible to make the membrane even thinner than the one described above to further improve other performances without impairing the permselectivity. It is possible.

From the detailed description above, the object, structure, and effects of the present invention are clear, but in order to further facilitate understanding, the present invention will be explained in detail below using Examples.

Example 1 A tube with an outer diameter sealed at the bottom was placed in a solution of 12 parts of polyvinyl chloride resin (S100I, Kanebuchi Kagaku Kogyo Co., Ltd.) with a degree of polymerization of 1000 dissolved in 88 parts of a mixed solution of tetrahydrofuran and dimethylformamide at a weight ratio of 1:7. A glass cylinder of 85 mm was immersed in the solution, pulled up with a mallet at a rate of 1.7 crrL/sec, and then left at room temperature (24° C.) for 30 seconds to evaporate a portion of the solvent.

Thereafter, this was immersed as it was in an 80% by weight aqueous methanol solution for 1 hour, and the remaining solvent was leached out.

When we conducted a separate permeation test of aqueous solutions containing various proteins using the microporous film prepared by this procedure, it showed no resistance to the permeation of substances with a molecular weight of about 1000 or less, but Anything over 5000 was not allowed to pass through.

While this microporous layer was attached to the glass cylinder, it was heated to 45°C.
After drying for 1 hour, styrene-butadiene copolymer (
Turuprene 303 (Japan Gokai Rubber Co., Ltd.) was immersed in a 2.5% by weight cyclohexane solution for several seconds, and then 1.
It was pulled up at a rate of 1 crrL/sec and dried at 45° C. for 1 hour to form a thin film.

Thereafter, this was immersed in 91% sulfuric acid for 1 hour and in 95% sulfuric acid for 24 hours to perform crosslinking and sulfonation. After completion of sulfonation, it was washed with 60% sulfuric acid for 3 minutes, and further washed with water.

The resulting composite membrane was peeled off from the glass cylinder and stored in a dilute aqueous caustic soda solution or saline.

The micropores in the microporous polyvinyl chloride layer were not changed at all by the above operation, and no peeling phenomenon of the film was observed.

Since the membrane having sulfonic acid groups of the composite thus obtained is extremely thin, it is difficult to directly measure its ion exchange capacity.
A membrane with a uniform heap was prepared and sulfonated using the same method, and its ion exchange capacity was measured, and it was found to be about 2.5. ::IJ, it is presumed that the exchange capacity of the composite membrane described above is also comparable to this.

The resulting composite membrane was photographed using an electron microscope, and the thickness of the membrane layer containing ion exchange groups was measured to be approximately 1 micron.
It was also observed that the styrene-butadiene copolymer did not penetrate into the polyvinyl chloride microporous layer.

A test piece with a diameter of 52 plates was cut from this composite membrane and heated at 37°C.
When 0.78% saline is brought into contact with the membrane surface having sulfonic acid groups at a pressure of 50 kg/crit.

The salt was rejected by 67% with a permeate flux of about 12 gallons/square foot/day.

Example 2 After washing the composite membrane preserved with the dilute caustic soda aqueous solution obtained by the operation in Example 1 with water to remove excess caustic soda adhering to it, a mixture of epichlorohydrin and pyridine in a molar ratio of 1:1 was added to the membrane to remove 50% of the composite membrane. An amphoteric ion exchange composite membrane was produced by immersing it in a volume% aqueous solution at 31.5° C. for 2 hours to introduce amino groups.

Even by performing this operation, the microporous layer did not change, and no peeling phenomenon of the film was observed.

Although the mechanism of this amphoteric reaction is not certain, the presence of hydroxyl groups in the sulfonated film has been confirmed by infrared absorption spectra and elemental analysis based on the findings of the present inventors. It is thought that the amino group is introduced by the reaction of the phosphorus/pyridine mixture.

Since the amphoteric membrane portion of this composite membrane is extremely thin, it is difficult to directly measure its ion exchange capacity. When the dried film 1 was treated and aminated using the same procedure as above, and the amino groups were quantified by elemental analysis,
Since the exchange capacity was 2 to 2.5 milliequivalents per unit, it is presumed that the composite membrane obtained in this example also has an anion exchange capacity comparable to this.

A test piece of 52 diameters was cut from this composite membrane and heated at 30°C.
When 3.5% saline is brought into contact with the membrane surface having amphoteric ion exchange groups at a pressure of 50 kg/crA, 10% of the salt is removed, and the permeation flux increases slightly over time to about 15 gallons/crA. The square foot was 7 days old.

Furthermore, when exposed to a 5% sucrose aqueous solution at 30°C at a pressure of 50 kg/crrt, 80% of the sucrose was eliminated, and the permeation flux decreased slightly over time to approximately 11 gallons/sq ft/day. there were.

Example 3 Vinyl chloride/acrylonitrile copolymer (Kanekalon Resin, Kanekabuchi Chemical Co., Ltd.) was dissolved in an ethanol saturated solution of zinc bromide to a concentration of 1.0% by weight, and the product prepared in Example 1 was dissolved in this solution. The resulting polyvinyl chloride microporous layer was immersed in the glass cylinder for 10 minutes while remaining attached to the glass cylinder, pulled up at a speed of 1.11/sec, and dried at 50° C. for 2 hours.

Wash this with water for 30 minutes to remove zinc bromide, and then
The sample was immersed in 26N sulfuric acid at 0.degree. C. for 2 hours to hydrolyze the nitrile groups and introduce carboxyl groups.

After that, formalin was added to 1001 rL of 26N sulfuric acid.
The hydrolyzed composite membrane was immersed in a solution added at a ratio of 5rrLl at 0°C for 30 minutes, and crosslinked using the remaining NidoIJru groups.

Even with these operations, the microporous layer did not change, and no peeling of the membrane was observed.

The exchange capacity of the membrane obtained in the same manner as in Example 1 was estimated by an indirect method to be 3 milliequivalents per 11 of the dry vinyl chloride/acrylonitrile copolymer resin.

Example 4 After washing the composite membrane prepared in Example 3 with water for 30 minutes, it was immersed in a 20% by weight aqueous solution of ethylenediamine at 50°C for 3 hours to cause the chlorine in the copolymer to react with ethylenediamine. A tertiary amine group was introduced.

Even with this operation, the microporous layer did not change, and no peeling of the membrane was observed.

When the exchange capacity of tertiary amine was estimated using an indirect method similar to Example 3, it was estimated to be 1.6 milliequivalents per 11 dry copolymers.

Further, the thickness of the membrane portion having ion exchange groups was approximately 5000 angstroms according to electron microscopic observation, and it was found that the resin layer hardly penetrated into the pores of the microporous layer.

A test piece with a diameter of 52 mm was cut from this composite membrane, and
When 3.5% saline at °C is brought into contact with the membrane surface at a pressure of 50 kg/cr7i, 18% of the salt is removed and the permeation flux is approximately 5.
5 gallons/square foot/day.

When a 5% sucrose aqueous solution was applied at 30° C. under a pressure of 50 kg/crrt, 78% of the sucrose was removed, and the permeation flux was 47 gallons/square foot/day.

Example 5 A glass cylinder with an outer diameter of 85 mm with a sealed bottom was immersed in a solution of 10 parts of polyvinyl chloride resin (8,1001) with a degree of polymerization of 1000 dissolved in 90 parts of dimethylformamide, and then heated at a rate of 1.2 Crn/sec. After being pulled up at high speed, it was immediately immersed in water and left to stand for 30 minutes to remove dimethylformamide.

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[Brief explanation of drawings]

The drawing is an electric micrograph showing a cross section of an example Q-) of an unexploded metal film.

Claims (1)

[Scope of Claims] 1. A polymeric microporous layer having pores that allow a protein with a molecular weight of about 1,000 to pass through and prevent a protein with a molecular weight of about 5,000 from passing through; A composite membrane having ion exchange groups formed on the surface thereof and a polymer layer having a thickness of several microns or less having ion exchange groups. 2. A solution or emulsion of a polymer substance capable of introducing an ion exchange group is applied in a thin film form onto the surface of a support consisting of a polymeric microporous layer that itself has physical permselectivity. After removing the solvent or dispersion medium of the substance by evaporation drying or by leaching it into a solvent that is a non-solvent for the support and coated polymeric substance and is compatible with the solvent or dispersion medium, ion exchange is performed. 1. A method for producing a composite membrane having an ion exchange group, the method comprising introducing a group into a thin film of the polymeric substance.