JP4854284B2 - Method for producing polymer electrolyte membrane - Google Patents

Description

  The present invention relates to a polymer electrolyte membrane suitable for uses such as a polymer electrolyte fuel cell, an electrolysis diaphragm, and gas humidification, and a method for producing the same. More specifically, the present invention relates to a method for producing a polymer electrolyte membrane by subjecting a polymerizable monomer containing a sulfonic acid group directly to a polymer substrate by radiation graft polymerization.

  Non-porous polymer electrolyte membranes are conventionally used in applications such as solid polymer fuel cells, alkaline electrolysis, and air humidification modules. Among these applications, a polymer electrolyte fuel cell has recently attracted attention. Solid polymer fuel cells are expected as a future power generation method because they can use hydrogen as a recyclable energy source, and are used in a wide range of fields such as household cogeneration power supplies, portable device power supplies, electric vehicle power supplies, and simple auxiliary power supplies. Development in is promoted.

  In this polymer electrolyte fuel cell, the electrolyte membrane functions as an electrolyte for conducting protons, and at the same time, functions as a diaphragm for preventing hydrogen or methanol, which is a fuel, from directly contacting oxygen. The characteristics of such an electrolyte membrane are high ion exchange capacity, high proton conductivity, electrical and chemical stability because current flows for a long time, low electrical resistance, and membrane dynamics. It is required to have high strength and low gas permeability for hydrogen gas, methanol, and oxygen gas as fuel.

  Here, from the viewpoint of high proton conductivity, an electrolyte membrane into which a sulfone group is introduced as an ion exchange group has been studied so far. Specific examples include a perfluorosulfonic acid polymer (PFSA polymer) represented by Nafion (registered trademark of DuPont), and aromatic hydrocarbon polymers such as polyetheretherketone and polyphenylene sulfide. Known are those obtained by bonding, and those obtained by sulfonating a graft membrane obtained by graft-polymerizing a styrene or other monomer to a hydrocarbon-based or fluorine-based polymer substrate.

  Of these, PFSA polymers have excellent chemical stability and have been used for many years as the only material. However, the manufacturing process is complicated and expensive, and the mechanical strength at high temperatures of 100 ° C or higher is inferior. In particular, direct methanol fuel cell (DMFC) that uses methanol as the fuel in the field of fuel cells. When used as an electrolyte membrane, there are also problems such as a high methanol permeation rate.

  Therefore, as an alternative to this PFSA polymer, a polymer electrolyte membrane in which a sulfonic acid group is introduced into a hydrocarbon-based polymer typified by polyether ether ketone generally called engineering plastics, or a fluorinated polymer typified by polytetrafluoroethylene Polymer electrolyte membranes obtained by introducing a sulfonic acid group after radiation-polymerizing a polymerizable monomer such as styrene together with a crosslinking agent on the base material of these substrates have been the subject of recent research and development. Among these, a radiation graft polymerization polymer electrolyte membrane made of a fluorine-based polymer has attracted attention because it has high ionic conductivity and low permeability to hydrogen and methanol.

  In the manufacturing process of this radiation graft polymerization electrolyte membrane, a graft chain is introduced into a chemically relatively stable fluorine-based substrate, and a specific portion of the graft chain, for example, the styrene aromatic ring portion of the polystyrene graft chain is sulfonated. A method of introducing a sulfonic acid group is generally performed well. Examples of the sulfonation agent for introducing a sulfonic acid group include known sulfonating agents such as chlorosulfonic acid, concentrated sulfuric acid, fuming sulfuric acid, 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, chloroform. , Carbon tetrachloride, and 1-chlorobutane are known to use a treatment agent diluted with a chlorinated solvent (see, for example, Patent Document 1). However, the chlorinated solvent is a material with a very high burden on the global environment, and its use is a serious problem.

For example, in the case of a graft polymer electrolyte membrane obtained by graft polymerization of styrene onto a polytetrafluoroethylene substrate, a “—SO ₃ H” group is introduced at the para-position of styrene by a sulfonation reaction for proton conductivity. It is desirable that However, when a sulfonation agent is used, a sulfoxide unit “—SO—”, a sulfone unit “—SO ₂ —”, and the like are also introduced, thereby increasing “S” that does not contribute to proton conductivity and increasing the proton of the membrane. There are also problems such as lowering the conductivity and causing the film to become brittle due to the formation of a crosslinked structure (see, for example, Patent Document 1).

  In addition, as the main polymerization component other than styrene, when alkyl vinyl ethers such as ethyl vinyl ether, propyl vinyl ether, isopropyl vinyl ether, vinyl 2-ethylhexyl ether, alkyl vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, etc. are used. It is considered that any part such as carbon at the terminal of the main chain or side chain of the graft chain is sulfonated, and the identification of the structure itself is difficult but also difficult to control.

  Up to now, attempts have been made to graft polymerize relatively inexpensive styrene sulfonic acid sodium salt directly onto a polymer substrate, but styrene sulfonic acid sodium salt has a very slow polymerization rate, and the reaction hardly proceeds by itself. Therefore, it is necessary to copolymerize an easily graft-polymerized monomer such as acrylic acid, and the copolymer monomer used here has a problem that the heat resistance and oxidation resistance of the polymer electrolyte membrane are impaired.

  As a method for solving such a problem, “a polymerizable monomer having a benzene ring and having a sulfonic acid group in the form of a vinyl group and an ammonium salt or a lithium salt on the benzene ring is used as an organic polymer substrate. And a method for producing an organic polymer material having a sulfonic acid group, which is characterized by being graft-polymerized on the substrate (for example, refer to claim 1 of Patent Document 2). In the present invention, it is shown that an organic polymer material in which sulfonic acid groups are introduced in a predesigned pure form is obtained. However, as “preferably a woven fabric, non-woven fabric or porous membrane” is shown as a preferred substrate (Patent Document 2, Claim 4), in such a method, the rate at which the graft reaction proceeds is extremely slow, and the woven fabric, non-woven fabric or porous In order to obtain a graft ratio of about 5 to 10% on the surface of each fiber structure of the membrane, a reaction time of about 3 to 6 hours is required.

Accordingly, when a non-porous organic polymer material substrate is used, in order to obtain a practically sufficient proton conductivity in the direction of the substrate thickness (usually about 15 to 200 μm), it is usually 10 to 20% or more. Since a graft ratio of about a certain degree is required, it takes more than 10 hours for the graft polymerization reaction, and it is extremely difficult to carry out industrial production in which a film of a polymer material base material is continuously processed. There was a problem of being.
JP 2005-89608 A JP 2005-8855 A

  The present invention solves the above-mentioned problems of the prior art, speeds up the graft polymerization to such an extent that industrial continuous production is possible, has a sulfonic acid group in a pure form at a pre-designed position, and has proton conductivity. It is an object of the present invention to provide an excellent nonporous polymer electrolyte membrane and a method for producing the same.

As a result of intensive studies in view of the above problems, the inventors of the present invention have a polymerizability having a sulfonic acid group in a pure form (—SO ₃ H) at a predesigned position in radiation graft polymerization onto a polymer substrate film. In order to uniformly graft polymerize the monomer in the thickness direction in a short time, do not consume radicals that are the base point of graft polymerization present in the film, and increase the diffusion permeability of the graft monomer into the film. I thought it was important. Therefore, the inventors have conceived of using a supercritical fluid or a subcritical fluid as a graft polymerization solvent, and found that the polymerization rate can be greatly increased. Further, a specific polymerizable monomer having no sulfonic acid group is obtained. The present inventors have found that the polymerization reaction is further promoted by copolymerization and completed the present invention.

  That is, the present invention is a method for producing a polymer electrolyte membrane, in which a non-porous polymer film substrate is irradiated with ionizing radiation, and one vinyl group and lithium salt or ammonium salt form are formed in the molecule. Or a polymerizable monomer as a main polymerization component having a sulfonic acid group in the form of an ester, and a polymerizable monomer as a crosslinking agent having two or more vinyl groups in the molecule but having no sulfonic acid group The polymer film substrate is immersed in a supercritical or subcritical fluid containing a body to graft polymerize the polymerizable monomer to the polymer film substrate, and then the salt or It is characterized by including making the sulfonic acid group of the ester form into a sulfonic acid type.

  In the present invention, the polymerizable monomer as the main polymerization component is composed of ammonium styrene sulfonate, lithium styrene sulfonate, ethyl styrene sulfonate, ammonium vinyl sulfonate, lithium vinyl sulfonate, ethyl vinyl sulfonate. It is preferably at least one monomer selected from the group or a mixture of two or more thereof.

  In the present invention, the polymer film substrate is preferably a fluorine polymer film.

  In the present invention, the fluid in the supercritical state or subcritical state is preferably carbon dioxide.

  In the present invention, the supercritical or subcritical fluid further contains a polymerizable monomer as a copolymer component having one vinyl group but no sulfonic acid group in the molecule. It is preferable to graft copolymerize a polymerizable monomer containing a main polymerization component, a crosslinking agent component and a copolymerization component on a polymer film substrate. The polymerizable monomer as a copolymerization component is preferably at least one selected from the group consisting of styrene, vinyl naphthalene and acrylonitrile, or a mixture of two or more thereof.

  According to the present invention, the rate of graft polymerization is increased to such an extent that industrial continuous production is possible, and a non-porous structure having a sulfonic acid group in a pure form at a predesigned position and having excellent durability. A polymer electrolyte membrane and a method for producing the same are provided.

  Hereinafter, preferred embodiments of the method for producing a polymer electrolyte membrane of the present invention will be described.

In the method for producing a polymer electrolyte membrane of the present invention, a non-porous polymer film substrate is irradiated with ionizing radiation, and one vinyl group and lithium salt or ammonium salt form or ester form is contained in the molecule. Superpolymer containing a polymerizable monomer as a main polymerization component having a sulfonic acid group, and a polymerizable monomer as a crosslinking agent having two or more vinyl groups in the molecule but not having a sulfonic acid group The polymer film substrate is immersed in a critical or subcritical fluid to graft polymerize the polymerizable monomer onto the polymer film substrate, and then the salt or ester form sulfone. It is characterized by including making an acid group into a sulfonic acid type. According to the present invention, the rate of graft polymerization is increased to such an extent that industrial continuous production is possible, the sulfonic acid group is present in a pure form (—SO ₃ H) at a predesigned position, and proton conductivity is achieved. An excellent non-porous polymer electrolyte membrane can be provided. Here, the “predesigned position” means, for example, when the polymerizable monomer as the main polymerization component is an aromatic vinyl compound, any of ortho, meta, and para with respect to the vinyl group of the benzene ring. This means the designed position. In addition, when the polymerizable monomer is an alkyl vinyl ether, vinyl ketone, or the like, a monomer in which a specific site such as a side chain end is previously substituted with a sulfonic acid group may be used.

  For example, when producing a polymer electrolyte membrane in which polystyrene sulfonic acid is graft-polymerized, styrene monomer is graft-polymerized on a polymer film substrate according to the conventional technique, and sulfonated to obtain a sulfonating agent and reaction conditions to be used. Depending on the position, the ortho, meta, and para positions are sulfonated, and there is a possibility of disubstitution, which is difficult to control. However, according to the present invention, the substitution position of the sulfonic acid group can be controlled, for example, by directly graft-polymerizing a para-substituted styrene sulfonic acid ester to the polymer film substrate.

  In the present invention, first, a non-porous polymer film substrate is irradiated with ionizing radiation.

  The polymer film substrate used in the present invention is not particularly limited as long as it is composed of a non-porous polymer film capable of radiation graft polymerization. A porous polymer film cannot be used in the present invention because it cannot block the permeation of gas and ions and cannot function as a diaphragm. The polymer film substrate is preferably a film composed of an aromatic hydrocarbon polymer, an olefin polymer, a fluorinated olefin polymer, or the like from the viewpoints of chemical stability and mechanical strength.

  Non-limiting examples of aromatic hydrocarbon polymer films include polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polyether ether ketone, polyether ketone, polysulfone, polysulfone A film composed of one or two or more copolymers or mixtures of ether sulfone, polyphenylene sulfide, polyarylate, polyetherimide, polyimide, thermoplastic polyimide, and the like can be given.

  Non-limiting examples of the olefin polymer film include low density or high density polyethylene, ultrahigh molecular weight polyethylene, polypropylene, polybutene, polymethylpentene, and the like, or two or more copolymers or A fill composed of a mixture may be mentioned.

  Non-limiting examples of the fluorine polymer film include polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, polytetrafluoroethylene, tetrafluoroethylene-perfluoroethylene film. Examples include films composed of fluoroalkyl vinyl ether copolymers, polychlorotrifluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, and mixtures thereof.

  The polymer film substrate is particularly preferably a fluorine-based polymer film from the viewpoint of chemical stability.

Examples of the ionizing radiation that can be used in the present invention include α, β, γ rays, electron beams, ultraviolet rays, etc., and γ rays and electron beams are particularly suitable. The irradiation dose necessary for carrying out the graft polymerization is usually 5 to 500 kGy (kilo gray: 1 gray corresponds to 1 J / kg energy absorption), preferably 20 to 300 kGy. By irradiation of ionizing radiation onto the polymer film substrate, for example, radicals such as —CH _2. , —CF _2. , —CH • —, —CF • — are generated, and this radical is used for the reaction of graft polymerization. It is considered to be a base point. When the irradiation dose is small, a sufficient number of radicals are not generated for graft polymerization, and when the irradiation dose is excessive, the crosslinking reaction or deterioration reaction of the polymer film substrate proceeds excessively, which is disadvantageous. Irradiation with ionizing radiation is preferably performed in an inert gas such as argon or in a vacuum because the graft polymerization reaction is inhibited by oxygen.

  In another aspect of the present invention, ionizing radiation may be irradiated in the presence of oxygen. In this case, a peroxide is generated by oxygen in addition to the radicals described above, and is generated by decomposition of the peroxide. In addition, "-O. Radical" is considered to be a base point for graft polymerization. In either case, graft polymerization is performed by bringing a film irradiated with radiation into contact with a solution in which a monomer and a crosslinking agent are dissolved in a solvent, but during this polymerization reaction, because the polymerization reaction is inhibited by oxygen, The oxygen concentration should be as low as possible.

  Since the irradiation with ionizing radiation is completed in a relatively short time to irradiate to the required irradiation dose, it is not necessary to suppress the heat generation of the polymer film substrate due to the irradiation. However, after irradiation, in order to prevent radical deactivation, the polymer film substrate is preferably stored at a low temperature of room temperature or lower, preferably -60 ° C. or lower.

  Next, in the present invention, the polymer film substrate is immersed in a supercritical or subcritical fluid containing a polymerizable monomer as a main polymerizable component and a polymerizable monomer as a crosslinking agent. Then, these polymerizable monomers are graft-polymerized on the polymer film substrate.

  The polymerizable monomer as the main polymerizable component used in the present invention has one vinyl group and a sulfonic acid group in the form of lithium salt or ammonium salt or ester in the molecule. Thereby, there is an advantage that it is not necessary to sulfonate separately after graft polymerization of the polymerizable monomer. Non-limiting examples of polymerizable monomers as the main polymerizable component include ammonium styrene sulfonate, lithium styrene sulfonate, ethyl styrene sulfonate, ammonium vinyl sulfonate, lithium vinyl sulfonate, ethyl vinyl sulfonate. Examples include esters.

  Moreover, the polymerizable monomer as a crosslinking agent component used in the present invention has two or more vinyl groups in the molecule but does not have a sulfonic acid group. Thereby, there exists an advantage that the oxidation resistance (namely, durability) of the polymer electrolyte membrane manufactured is improved. Non-limiting examples of polymerizable monomers as crosslinking agents include 1,2-bis (p-vinylphenyl), divinylsulfone, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, divinylbenzene. , Cyclohexanedimethanol divinyl ether, phenylacetylene, diphenylacetylene, 2,3-diphenylacetylene, 1,4-diphenyl-1,3-butadiene, diallyl ether, 2,4,6-triallyloxy-1,3,5 -Triazine, triallyl-1,2,4-benzenetricarboxylate, triallyl-1,3,5-triazine-2,4,6-trione, bisvinylphenylethane, butadiene, isobutene, isoprene, cyclopentadiene, ethylide Such as norbornene and the like.

  Examples of the supercritical or subcritical fluid (hereinafter also simply referred to as “supercritical fluid or subcritical fluid”) that can be used in the present invention include carbon dioxide, hydrogen, nitrogen, and argon. The pressure of the supercritical fluid or subcritical fluid is close to the critical point where the substance used as the supercritical fluid or subcritical fluid is in the subcritical state, or higher than the supercritical point of the substance. Similarly, the temperature of the supercritical fluid or subcritical fluid shall be near the critical point at which the substance used as the supercritical fluid or subcritical fluid is in the subcritical state or above the supercritical point of the substance. . However, since the polymerization reaction may proceed outside the inside of the polymer film substrate when the temperature is too high, the temperature of the supercritical fluid or subcritical fluid is preferably 120 ° C. or less, more preferably 100 ° C. or less. It is.

  The temperature of the graft polymerization is 10 to 60 ° C, preferably 20 to 40 ° C, more preferably around room temperature (25 ° C). If the temperature is too high, the generated radicals are deactivated, which is inconvenient. Although the reaction time depends on the thickness of the polymer film substrate and the concentration of the polymerizable monomer, it is about 10 minutes to 2 hours.

  In the present invention, the supercritical or subcritical fluid further contains a polymerizable monomer as a copolymer component having one vinyl group but no sulfonic acid group in the molecule. It may be. In this case, the copolymer component is graft-copolymerized to the polymer film substrate together with the main polymerization component and the crosslinking agent component. Thereby, there exists an advantage that the polymerization rate of the polymerizable monomer as a main polymerization component is improved. Examples of the polymerizable monomer as the copolymerization component include, but are not limited to, styrene, vinyl naphthalene, and acrylonitrile.

  In the present invention, if necessary, the polymerizable monomer is diluted with a solvent to carry out the graft polymerization reaction. Non-limiting examples of suitable solvents include alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, ketones such as acetone and methyl ethyl ketone, hydrocarbon solvents such as hexane, heptane, octane and nonane. In addition, benzene, toluene, cyclohexane, etc. are mentioned.

  The concentration of the polymerizable monomer as the main polymerization component is preferably 0.05 M / L to 3 M / L, more preferably 0.1 M / L to 2 M / L, based on the total fluid volume. When the concentration of the polymerizable monomer (main polymerization component) is too low, the graft polymerization reaction does not proceed sufficiently, and when the concentration of the polymerizable monomer (main polymerization component) is too high, the polymer It is inconvenient because a reaction takes place outside the film base material or a yield is reduced.

  The concentration of the polymerizable monomer as the cross-linking agent is preferably 0.5 to 40%, more preferably 2 to 20% with respect to the concentration of the polymerizable monomer as the main polymerization component. When the concentration of the crosslinking agent is low, the polymer electrolyte membrane obtained without exhibiting the effect of crosslinking is inferior in durability, and when the concentration of the crosslinking agent is high, the polymer film substrate becomes brittle. This is inconvenient.

  The concentration of the polymerizable monomer as the copolymerization component is preferably 0.5 to 100%, more preferably 2 to 50% with respect to the concentration of the polymerizable monomer as the main polymerization component.

  Graft polymerization can be performed while stirring the liquid in a closed container such as stainless steel.

  The degree of progress of the graft polymerization reaction can be evaluated by the graft rate defined below. The graft rate is the rate of increase in the dry weight of the polymer film substrate before and after the graft polymerization reaction.

Graft ratio (G) = (W ₂ −W ₁ ) × 100 / W ₁ (Formula 1)
W ₁ : Dry weight (g) of the polymer film substrate before the graft polymerization reaction
W ₂ : Dry weight (g) of the polymer film substrate after the graft polymerization reaction
The dry weight of the polymer film substrate after the graft polymerization reaction is weighed after sufficiently washed with toluene or alcohol and dried.

  In the present invention, the graft ratio is preferably 10 to 150%, more preferably 15 to 100%. In order to evaluate the degree of progress of the graft polymerization reaction, it may be possible to examine the number of moles of the polymerizable monomer introduced into the polymer film substrate by graft polymerization. Can be derived directly from the weight of the polymer film substrate, and a reaction rate is also required. Therefore, the graft ratio is a simple and convenient evaluation parameter that does not depend on the type of polymerizable monomer.

  In the present invention, after the graft polymerization reaction, a sulfonic acid group in the form of a salt or an ester is converted to a sulfonic acid type.

  After the graft polymerization reaction, the polymer film substrate is taken out of the supercritical fluid or subcritical fluid by lowering the pressure and temperature.

  When the polymerizable monomer (main polymerization component) is a sulfonic acid ester, an alkali treatment is performed to hydrolyze the ester bond. In the alkali treatment, lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide and the like are mixed in a water / alcohol mixed solvent (ratio of about 1 / 0.5 to 1/20) having a concentration of about 0.5 to 10 N. What is necessary is just to perform about 5 minutes-about 20 hours at the temperature of 40-100 degreeC. By appropriately selecting the water / alcohol mixing ratio and reaction temperature, the reaction time can be shortened.

  When the polymerizable monomer (main polymerization component) is a lithium salt and an ammonium salt, and when the polymerizable monomer (main polymerization component) is subjected to the above alkaline hydrolysis treatment with a sulfonic acid ester, The sulfonate in the polymer film substrate is converted to the acid form. Specifically, the polymer film substrate is treated in an aqueous solution of hydrochloric acid, sulfuric acid, nitric acid or the like having a concentration of about 0.5 to 10 N at a temperature of 30 to 100 ° C. for about 5 minutes to 10 hours. Next, in order to remove excess acid, washing is performed in pure water at a temperature of 30 to 100 ° C. for about 5 minutes to 10 hours. In order to perform such a series of treatments continuously on the line, it is desirable that each treatment time is at most 1 to 2 hours, preferably within several tens of minutes, from the viewpoint of productivity.

  Hereinafter, although the present invention is explained in detail, referring to an example and a comparative example, the present invention is not limited to these.

Example 1
A 50 μm-thick ETFE film formed by the melt extrusion method was irradiated with an electron beam in a dose of 60 kGy in air. Next, this film was put in a pressure-resistant container having an internal volume of 100 ml, 42.4 g of ethyl styrenesulfonate (Mw = 212.27) as a main polymerization component previously deaerated, and divinylbenzene (Cross-linking agent) 1.3 g of Mw = 130.19) was added. After sealing the container, carbon dioxide was further introduced, and the mixture was stirred at a temperature of 70 ° C. and a pressure of 25 MPa for 2 hours for graft polymerization. The film after the reaction was immersed in toluene at 60 ° C. for 30 minutes, then dried and weighed. The graft ratio was calculated to be 65%. Next, the film was hydrolyzed by treating it with a solution of 1N NaOH dissolved in pure water: methanol = 1: 1 (volume) at 80 ° C. for 30 minutes, and further with 1N hydrochloric acid at 80 ° C. for 30 minutes. Subsidiary acid substitution was carried out, followed by washing in pure water at 80 ° C. for 15 minutes to obtain a sulfonated electrolyte membrane.
(Example 2)
A PVdF film with a thickness of 25 μm was irradiated with an electron beam in air at a dose of 100 kGy. Next, this film was put in a pressure-resistant container having an internal volume of 100 ml, and lithium vinyl sulfonate (Mw = 114.05) as a main polymerization component previously deaerated was 22.89, divinylbenzene (Cross-linking agent) Mw = 130.19) was added at 1.39. After sealing the container, carbon dioxide was introduced, and the mixture was stirred at a temperature of 70 ° C. and a pressure of 25 MPa for 30 minutes for graft polymerization. The film after the reaction was immersed in toluene at 60 ° C. for 30 minutes, then dried and weighed. The graft ratio was calculated to be 18%. After that, acid substitution was performed with 1N hydrochloric acid at 80 ° C. for 30 minutes, and then washed in pure water at 80 ° C. for 15 minutes to obtain a sulfonated electrolyte membrane.
(Example 3)
A PVdF film with a thickness of 25 μm was irradiated with an electron beam in air at a dose of 100 kGy. Next, this film was put in a pressure-resistant container having an internal volume of 100 ml, 22.8 g of lithium vinyl sulfonate that had been degassed in advance, 4.2 g of styrene (Mw = 104.15) as a copolymerization component, and crosslinked. 1.3 g of divinylbenzene (Mw = 130.19) as an agent was added. After sealing the container, carbon dioxide was introduced and the mixture was stirred at a temperature of 70 ° C. and a pressure of 25 MPa for 45 minutes for graft polymerization. The film after the reaction was immersed in toluene at 60 ° C. for 30 minutes, then dried and weighed. The graft ratio was calculated to be 27%. Thereafter, the acid was replaced with 1N hydrochloric acid at 80 ° C. for 30 minutes, and further washed in pure water at 80 ° C. for 15 minutes to obtain a sulfonated electrolyte membrane.
(Comparative Example 1)
In the same manner as in Example 1, an ETFE film having a thickness of 50 μm was irradiated with an electron beam in air at a dose of 60 kGy. Subsequently, this film was put in a container having an internal volume of 100 ml, and 42.4 g of ethyl styrenesulfonate (Mw = 212.27) as a main polymerization component sufficiently deaerated beforehand and divinylbenzene (Cross-linking agent) Mw = 130.19) was added to 1.3 g, and toluene was added as a solvent so that the total volume became 100 ml. In order to prevent oxygen inhibition of the reaction, nitrogen was continuously fed as an inert gas to the solution at a temperature of 70 Graft polymerization was performed for 15 hours with stirring at ° C. The film after the reaction was immersed in toluene at 60 ° C. for 30 minutes, then dried and weighed. The graft ratio was calculated to be 58%. Next, this film was hydrolyzed by treating it with a solution of 1N NaOH dissolved in pure water: methanol = 1: 1 (volume) at 80 ° C. for 30 minutes, and further with 1N hydrochloric acid at 80 ° C. for 30 minutes. After substituting with acid, it was washed in pure water at 80 ° C. for 15 minutes to obtain a sulfonated electrolyte membrane.
(Comparative Example 2)
In the same manner as in Example 1, an ETFE film having a thickness of 50 μm was irradiated with an electron beam in air at a dose of 60 kGy. Next, this film was put in a pressure-resistant container having an internal volume of 100 ml, 41.2 g of sodium styrenesulfonate (Mw = 206.19) as a main polymerization component previously deaerated and divinylbenzene as a crosslinking agent. 1.3 g of (Mw = 130.19) was added. After sealing the container, carbon dioxide was introduced and the mixture was stirred at a temperature of 70 ° C. and a pressure of 25 MPa for 1 hour for graft polymerization. The film after the reaction was immersed in toluene at 60 ° C. for 30 minutes, then dried and weighed. The graft ratio was calculated to be 30%. Next, the film was subjected to acid substitution with 1N hydrochloric acid at 80 ° C. for 30 minutes and washed in pure water at 80 ° C. for 15 minutes to obtain a sulfonated electrolyte membrane.
(Evaluation of characteristics of electrolyte membrane)
About the electrolyte membrane obtained in the said Example and comparative example, according to the following procedures, the characteristic of ion exchange capacity and electrical conductivity was evaluated.
(1) Ion exchange capacity (IEC)
The ion exchange capacity IEC of the electrolyte membrane is expressed by Equation 2.

IEC = n (acid group) _obs / W _d (Formula 2)
n (acid group) _obs : Molar amount of acid group of electrolyte membrane (mM)
W _d : Dry weight of electrolyte membrane (g)
The measurement of n (acid group) _obs was performed according to the following procedure. That is, first, the electrolyte membrane was immersed in a 1 M (1 molar concentration) sulfuric acid solution at 50 ° C. for 4 hours to obtain a complete acid type. Next, the membrane was washed with ion-exchanged water, immersed in 3 M NaCl aqueous solution at 50 ° C. for 4 hours to form —SO ₃ Na type, and substituted protons (H ⁺ ) were titrated with NaOH aqueous solution to form acid group moles. The amount was determined.
(2) Electrical conductivity (κ)
The electrical conductivity of the electrolyte membrane is measured by an alternating current method (New Experimental Chemistry Course 19, Polymer Chemistry <II>, p992, Maruzen). A normal membrane resistance measuring cell and an LCR meter (E-4925A; manufactured by Hewlett Packard) Was used to measure the membrane resistance (R _m ). A 1M sulfuric acid aqueous solution was filled in the cell, the difference in resistance between the platinum electrodes (distance 5 mm) with and without the electrolyte membrane was measured, and the electrical conductivity (specific conductivity) of the membrane was calculated using Equation 3.

κ = 1 / R _m · d / S (Ω ⁻¹ cm ⁻¹ ) (Formula 3)
S: Film area (cm ² )
d: film thickness (cm)
The thickness of the film was measured using a dial thickness gauge G-6C (1/1000 mm, probe diameter φ5 mm) manufactured by Ozaki Seisakusho.
(Evaluation results)
About the electrolyte membrane obtained in the said Example and comparative example, a graft rate, an ion exchange capacity | capacitance, and an electrical conductivity are shown at the following table.

  About Example 1, 2, 3 and the comparative example 1, electrical conductivity was expressed and it was confirmed that it is useful as an electrolyte membrane. However, looking at the time required for the reaction in Example 1 and Comparative Example 1, it can be seen that the speed is accelerated 10 times or more when the supercritical fluid is used.

  Further, from Comparative Example 2, sodium styrene sulfonate has a lower IEC value than that of Example 1 even when treated at the same concentration, temperature, and time as in Example 1, and has a very low electrical conductivity. It was not obtained.

  The method for producing a polymer electrolyte membrane of the present invention relates to a graft polymerization method using a non-porous polymer film as a base material and using a supercritical fluid. According to the method of the present invention, a polymer electrolyte membrane having a sulfonic acid group at a previously designed position can be obtained without using a chlorinated solvent that has a large impact on the global environment.

Claims (4)

Non-porous , polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, polychlorotrifluoro Irradiating ionizing radiation to a polymer film substrate selected from the group consisting of ethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer, and mixtures thereof ,
Ammonium styrene sulfonate, lithium styrene sulfonate, ethyl styrene sulfonate, ammonium vinyl sulfonate, vinyl having one vinyl group and a sulfonic acid group in the form of lithium salt or ammonium salt or ester in the molecule A polymerizable monomer as a main polymerization component selected from the group consisting of lithium sulfonate and ethyl vinyl sulfonate , and
1,2-bis (p-vinylphenyl), divinylsulfone, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, divinyl having two or more vinyl groups in the molecule but no sulfonic acid group Benzene, cyclohexanedimethanol divinyl ether, phenylacetylene, diphenylacetylene, 2,3-diphenylacetylene, 1,4-diphenyl-1,3-butadiene, diallyl ether, 2,4,6-triallyloxy-1,3 5-triazine, triallyl-1,2,4-benzenetricarboxylate, triallyl-1,3,5-triazine-2,4,6-trione, bisvinylphenylethane, butadiene, isobutene, isoprene, cyclopentadiene And polymerizing the polymer film substrate by immersing the polymer film substrate in a supercritical or subcritical fluid containing a polymerizable monomer as a crosslinking agent selected from the group consisting of ethylene and norbornene Graft polymerize the monomer to the polymeric film substrate;
A method for producing a polymer electrolyte membrane, comprising making a sulfonic acid group in the form of a salt or ester into a sulfonic acid type. The method for producing a polymer electrolyte membrane according to claim 1 , wherein the fluid in a supercritical state or a subcritical state is carbon dioxide. Furthermore, a supercritical or subcritical fluid contains a polymerizable monomer as a copolymerization component having one vinyl group but no sulfonic acid group in the molecule, and a main polymerization component, a crosslinking agent The method for producing a polymer electrolyte membrane according to claim 1 or 2 , wherein a polymerizable monomer containing a component and a copolymerization component is graft-copolymerized on a polymer film substrate. Polymerizable monomer as a copolymer component is styrene, and wherein the at least one or a mixture of two or more of these selected from the group consisting of vinyl naphthalene and acrylonitrile, high of claim 3, wherein A method for producing a molecular electrolyte membrane.