JP2012082410A - Method of manufacturing proton-conductive polymer electrolyte membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing industrially a proton-conductive polymer electrolyte membrane of excellent characteristic, by graft polymerization.SOLUTION: This method of manufacturing the proton-conductive polymer electrolyte membrane includes a process for irradiating a resin fine particles with a radiation, a process for graft-polymerizing vinyl monomers having a sulfonic acid group precursor on to the resin fine particles in a solid-liquid two-phase system, to obtain a fine particle-like graft polymer containing the resin fine particles and polymeric chains of the vinyl monomers, a casting process for forming a cast membrane of the graft polymer, a drying process for drying the cast membrane at a drying temperature of a melting point or less of the resin fine particles, to obtain a film, and a process for converting the sulfonic acid group precursor contained in the film into a sulfonic acid group. The solid-liquid two-phase system is constituted of a liquid phase containing the vinyl monomers and a solvent therefor, and a solid phase containing the resin fine particles.

Description

  The present invention relates to a method for producing a proton conductive polymer electrolyte membrane that can be used in various applications such as a polymer electrolyte fuel cell, an electrolysis cell, and a humidification module.

  Polymer electrolyte membranes having proton conductivity are used in polymer electrolyte fuel cells (PEFC), alkaline electrolysis cells, humidification modules for gases (for example, air), and the like. In particular, the use of PEFC as an electrolyte membrane has attracted attention. A fuel cell has a high theoretical power generation efficiency and is a clean electric energy supply source using hydrogen or methanol as fuel. The fuel cell is expected as a next generation power generation method, and has been actively developed as various power sources such as a household cogeneration power source, a power source for portable devices, a power source of an electric vehicle, and a simple auxiliary power source.

  The electrolyte membrane of PEFC needs to have a function as an electrolyte that conducts protons and a function as a partition that separates the fuel supplied to the anode and the oxidant supplied to the cathode. For this reason, PEFC electrolyte membranes have high ion exchange capacity, proton conductivity, electrochemical stability and mechanical strength, as well as electrical resistance, fuel (eg, hydrogen) permeability and oxidizing gas (eg, oxygen). Low permeability is required.

  As a proton conductive polymer electrolyte membrane, a perfluoroalkyl ether sulfonic acid polymer (PFSA polymer) represented by Nafion (registered trademark of DuPont) is frequently used. However, due to the complexity of the manufacturing process, PFSA polymers are expensive. In addition, the mechanical strength of the PFSA polymer in a high temperature range of 100 ° C. or higher is low. Furthermore, the PFSA polymer has high methanol permeability and is difficult to use in a direct methanol fuel cell (DMFC) that uses a solution containing methanol as a fuel.

  Polymers to replace PFSA polymers are being investigated. From the viewpoint of electrochemical stability, a polymer having many fluorine atoms bonded thereto is preferable. From the viewpoint of ion exchange capacity and proton conductivity, a polymer into which many proton conductive groups such as sulfonic acid groups and phosphonic acid groups are introduced is preferable.

  In JP 2001-348439 A (Patent Document 1), a film made of long-chain branched polytetrafluoroethylene is irradiated with radiation, and this film is immersed in a solution containing a styrene compound as a monomer to perform graft polymerization. A polymer electrolyte membrane obtained by proceeding and sulfonating a phenyl group contained in a formed graft chain is disclosed. Japanese Patent Application Laid-Open No. 2004-59752 (Patent Document 2) and Japanese Patent No. 3417946 (Patent Document 3) disclose that the above film is made of an ethylene-tetrafluoroethylene copolymer or polyvinylidene fluoride. A polymer electrolyte membrane obtained in the same manner as in the method of Japanese Patent No. 348439 is disclosed.

  The method for producing a polymer electrolyte membrane disclosed in each of the above documents is simple and reasonable for small scale production. However, since the reaction rate of the graft polymerization is low, it is necessary to treat the film-like resin material for a very long time in the above production method. For this reason, the industrial productivity of the manufacturing method is low, and if a polymer electrolyte membrane is to be industrially continuously produced by the manufacturing method, a huge facility for continuously processing a large-sized film is required. When the irradiation dose of radiation is increased in order to increase the reactivity of graft polymerization, side reactions other than graft polymerization proceed in parallel, a homopolymer is formed, and the reaction solution gels in a short time. In order to prevent this gelation, when a polymerization inhibitor coexists in the reaction solution, uneven distribution occurs in the graft chain distribution on the film surface, and sufficient power generation characteristics are obtained for use as an electrolyte membrane of a fuel cell. I can't.

  In Japanese translations of PCT publication No. 2005-525682 (Patent Document 4), a resin material powder is irradiated with radiation, and both the powder irradiated with radiation and a vinyl monomer having a sulfonic acid group or a phosphonic acid group are used as a solvent. Disclosed is a method for producing a polymer electrolyte membrane by dissolving and allowing graft polymerization to proceed, casting the resulting polymer solution into a film, and drying it. In this document, it is preferable to dissolve the resin powder irradiated with radiation and the vinyl monomer in a solvent to obtain a homogeneous solution, and as the solvent, dimethylacetamide, N-methylpyrrolidone, dimethylformamide, dimethylsulfoxide ( DMSO) is preferred. No specific examples are given in this document, and therefore it is not easy for a person skilled in the art to read all the details necessary to implement the invention described in that document. Absent. However, according to the study by the present inventors, when a homogeneous solution is formed using a solvent in which both the resin material irradiated with radiation and the monomer are dissolved, radicals formed in the resin material by radiation irradiation are not. Since it rapidly disappears by chain transfer to the solvent, at least it is actually difficult to carry out graft polymerization.

JP 2001-348439 A JP 2004-59752 A Japanese Patent No. 3417946 JP 2005-525682 A

  An object of the present invention is to provide a method capable of industrially producing a proton conductive polymer electrolyte membrane having excellent characteristics by graft polymerization.

  The production method of the present invention includes a step of irradiating resin fine particles with radiation; and a vinyl monomer having a sulfonic acid group precursor is graft-polymerized to the resin fine particles irradiated with the radiation in a solid-liquid two-phase system, A step of obtaining a fine particle-like graft polymer comprising resin fine particles and a polymer chain of the vinyl monomer; a casting step of forming a cast film of the obtained graft polymer; and And a step of converting the sulfonic acid group precursor in the obtained film into a sulfonic acid group. Here, the solid-liquid two-phase system includes a liquid phase containing the vinyl monomer and a solvent for the monomer and a solid phase containing the resin fine particles.

  According to the present invention, a proton conductive polymer electrolyte membrane having excellent characteristics can be industrially produced by graft polymerization.

  Hereinafter, the form for implementing this invention is demonstrated concretely.

[Irradiation process]
The production method of the present invention includes a step of irradiating resin fine particles with radiation (irradiation step). The resin fine particles are made of a resin material that serves as a base material for the polymer electrolyte membrane. The resin material constituting the resin fine particles is not limited as long as it is a material to which radiation graft polymerization can be applied. Since the polymer electrolyte membrane having high electrochemical stability and high mechanical strength can be produced, the resin fine particles are at least selected from aromatic hydrocarbon polymers, olefin polymers and fluorinated olefin polymers. It preferably contains one polymer. The resin fine particles can be made of the at least one polymer.

  Aromatic hydrocarbon polymers include, for example, polystyrene, polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polyether ether ketone, polyether ketone, polysulfone, polyether sulfone, polyphenylene. Sulfide, polyarylate, polyetherimide, aromatic polyimide, and polyamideimide.

  Examples of the olefin polymer include low density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, polypropylene, polybutene, and polymethylpentene.

  Examples of the fluorinated olefin polymer include polyvinylidene fluoride (PVDF), polyvinyl fluoride, ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene copolymer. A polymer, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, polychlorotrifluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer.

  Among them, the resin fine particles are polystyrene, polyether ether ketone, polyether ketone, polysulfone, polyether sulfone, polyphenylene sulfide, polyarylate, polyetherimide, aromatic polyimide, polyamideimide, polyethylene, polypropylene, polyvinylidene fluoride, polyfluoride. It is preferable to include at least one selected from vinyl fluoride, ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer.

  From the viewpoint of chemical stability of the obtained electrolyte membrane, it is preferable that the resin fine particles contain a fluorine-based polymer. From this point of view, the resin fine particles are polyvinylidene fluoride, polyvinyl fluoride, ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer. It is particularly preferable that at least one selected from polymers be included.

  The resin material constituting the resin fine particles may be a copolymer or a mixture of two or more kinds of polymers.

  The average particle diameter of the resin fine particles is preferably 10 μm to 500 μm, more preferably 50 μm to 300 μm. If the average particle size of the resin fine particles is excessively large, the reaction rate of the graft polymerization inside the fine particles may be reduced, and the subsequent graft polymerization step may take a long time. When the average particle diameter of the resin fine particles is excessively small, it becomes difficult to handle the resin material in each step. In this specification, the value measured by the dry sieving method is adopted as the average particle size of the resin fine particles.

  A known method can be applied as a method of irradiating the resin fine particles with radiation. Ionizing radiation such as α-rays, β-rays, γ-rays, electron beams and ultraviolet rays is used as the radiation applied to the resin particles. Gamma rays and electron beams are particularly suitable for graft polymerization. The irradiation dose necessary for graft polymerization is usually 1 to 500 kGy, preferably 10 to 300 kGy. If the irradiation dose to the resin fine particles is less than 1 kGy, the amount of radicals generated may be reduced, and graft polymerization may be difficult. If the irradiation dose is larger than 500 kGy, an excessive crosslinking reaction may proceed in the subsequent graft polymerization step, or the resin material constituting the resin fine particles may be deteriorated.

  The radical polymerization method includes a peroxide method in which radiation and radical reaction are performed in the presence of oxygen, and a polymer radical method in which radiation and radical reaction are performed in the absence of oxygen. In the peroxide method, the graft reaction proceeds from an oxygen radical bonded to the resin material, whereas in the polymer radical method, the graft reaction proceeds from a radical generated in the resin material. In the production method of the present invention, in order to prevent the graft reaction from being inhibited by the presence of oxygen, it is preferable to proceed radical polymerization by a polymer radical method. Therefore, it is preferable to perform irradiation of the resin fine particles in an inert gas atmosphere or in a vacuum. The temperature during irradiation (irradiation temperature) is preferably in the range of -100 to 100 ° C, particularly preferably in the range of -100 to 60 ° C. If the irradiation temperature is too high, the generated radicals are liable to be deactivated. In order to prevent radical deactivation, it is desirable to store the resin fine particles after irradiation at a temperature not higher than the glass transition temperature of the resin material constituting the fine particles.

[Graft polymerization process]
In the production method of the present invention, after the irradiation step, graft polymerization is performed on the fine particles irradiated with radiation (graft polymerization step). Specifically, using a vinyl compound having a sulfonic acid group precursor as a monomer, the vinyl monomer A is graft-polymerized to resin fine particles irradiated with radiation in the irradiation step. As a result, a fine particle graft polymer containing resin fine particles and a polymer chain (graft chain) of vinyl monomer A bonded to the fine particles is obtained. This graft polymerization is carried out in a solid-liquid two-phase system. The solid-liquid two-phase system includes a liquid phase containing vinyl monomer A and a solvent for the monomer, and a solid phase containing resin fine particles. In the liquid phase, the vinyl monomer A is dissolved in the solvent (polymerization solvent). An example of graft polymerization in a solid-liquid two-phase system is performed in a state where resin fine particles irradiated with radiation are dispersed in a monomer solution obtained by dissolving vinyl monomer A in a polymerization solvent (first solvent). As in the irradiation step, the graft polymerization step is preferably carried out in an atmosphere with an oxygen concentration as low as possible in order to suppress reaction inhibition due to the presence of oxygen.

The vinyl monomer A is not limited as long as it is a vinyl compound having a sulfonic acid group precursor, but a vinyl compound represented by the general formula H ₂ C═C (X) R (hereinafter referred to as a vinyl sulfonic acid compound) is preferable. . In the above formula, X is a hydrogen atom, a fluorine atom or a monovalent hydrocarbon group. R is a sulfonic acid group precursor or a monovalent substituent having a sulfonic acid group precursor.

  The sulfonic acid group precursor is preferably a group that can be easily converted into a sulfonic acid group by hydrolysis and / or ion exchange in a conversion step performed after the graft polymerization step. Specific sulfonic acid group precursors include, for example, sulfonic acid fatty esters represented by sulfonic acid alkyl esters such as sulfonic acid methyl ester, sulfonic acid ethyl ester, sulfonic acid propyl ester, sulfonic acid butyl ester, and sulfonic acid cyclohexyl ester. Sulphonic acid aromatic esters such as sulfonic acid phenyl esters; sulfonic acid metal salts typified by sulfonic acid alkali metal salts such as sulfonic acid potassium salt, sulfonic acid sodium salt, and sulfonic acid lithium salt.

  The vinyl sulfonic acid compounds are, for example, vinyl sulfonic acid derivatives, allyl sulfonic acid derivatives, and sulfonic acid derivatives of styrene compounds.

  The sulfonic acid derivative of the styrene compound is, for example, a compound in which a sulfonic acid group precursor is introduced as a substituent into the styrene compound. Styrene compounds include, for example, styrene; methylstyrenes (α-methylstyrene, p-vinyltoluene, m-vinyltoluene, etc.), ethylstyrenes, dimethylstyrenes, trimethylstyrenes, pentamethylstyrenes, diethylstyrenes. Alkyl styrenes such as isopropyl styrenes, and butyl styrenes (3-tert-butyl styrene, 4-tert-butyl styrene, etc.); chlorostyrenes, dichlorostyrenes, trichlorostyrenes, bromostyrenes (2-bromo Halogenated styrenes such as styrene, 3-bromostyrene, 4-bromostyrene, and the like, and fluorostyrenes (2-fluorostyrene, 3-fluorostyrene, 4-fluorostyrene, etc.); methoxystyrenes, methoxymethyl Alkoxy styrenes such as ethylene, dimethoxy styrenes, ethoxy styrenes, vinyl phenyl allyl ethers, and vinyl benzyl alkyl ethers; hydroxy styrene derivatives such as hydroxy styrenes, methoxy hydroxy styrenes and acetoxy styrenes; vinyl benzoes Carboxystyrene derivatives such as acids and formylstyrenes; Nitrostyrenes such as nitrostyrene; Aminostyrene derivatives such as aminostyrenes and dimethylaminostyrenes; Ions such as vinylbenzyl sulfonic acids and styrenesulfonyl fluorides Is a functional styrene derivative.

  The vinyl monomer A is preferably at least one selected from methyl ester, ethyl ester, propyl ester, butyl ester, cyclohexyl ester, phenyl ester, and sodium salt of styrene sulfonic acid.

  In the graft polymerization step, one kind of vinyl monomer A may be graft polymerized alone, or two or more kinds of vinyl monomers A may be graft polymerized. In the latter case, the graft chain is a copolymer chain of the two or more vinyl monomers A. A monomer other than the vinyl monomer A may be copolymerized as a graft chain.

  As the polymerization solvent (first solvent) for dissolving the vinyl monomer A, a solvent that does not readily dissolve the resin material constituting the resin fine particles is selected. This is because the resin fine particles need to maintain a solid phase in the solid-liquid two-phase graft polymerization. As long as this condition is satisfied, the polymerization solvent is not limited. Specific polymerization solvents include, for example, alcohols such as methanol, ethanol, n-propanol, isopropanol and n-butanol; ketones such as acetone and methyl ethyl ketone; hexane, heptane, octane, nonane, decane and cyclohexane. Hydrocarbons; aromatic hydrocarbons such as benzene, toluene and xylene, and aromatic compounds such as phenols such as phenol and cresol. Among these, an aromatic compound is preferable as a polymerization solvent (a solvent contained in a solid-liquid two-phase liquid phase). By using an aromatic compound as a polymerization solvent, the graft polymerization rate is increased. In addition, since the aromatic compound has high solubility in the homopolymer as a by-product, the progress of graft polymerization in the solid-liquid two-phase system becomes more uniform.

  The solubility of the vinyl monomer A and the resin fine particles in the polymerization solvent varies depending on the resin material constituting the resin fine particles and the structure and polarity of the vinyl monomer A. For this reason, a polymerization solvent can be appropriately selected according to the monomer A and resin fine particles used. The polymerization solvent can be a mixture of two or more solvents. However, amide compounds such as dimethylacetamide, N-methylpyrrolidone, dimethylformamide; sulfoxides such as dimethyl sulfoxide (DMSO); phosphoric acid amides such as hexamethylphosphoric triamide; sulfonamide is both vinyl monomer A and resin fine particles. Therefore, it is not suitable as a polymerization solvent.

  In the graft polymerization step, materials other than the resin fine particles and the vinyl monomer A may be added to the polymerization system as necessary. The material is, for example, a compound having a plurality of unsaturated bonds in the molecule. This compound acts as a crosslinking agent between graft chains in the polymerization system. When the polymerization system contains this compound, a cross-linked structure is formed between the graft chains formed by the graft reaction, so that the durability of the polymer electrolyte membrane is improved.

  The material can be a reaction inhibitor.

  The concentration of the vinyl monomer A, other monomers and the crosslinking agent in the liquid phase is preferably 0.2 to 3 mol / L, more preferably 0.5 to 2.5 mol / L. If the concentration is less than 0.2 mol / L, the graft reaction may not proceed sufficiently. If the concentration is higher than 3 mol / L, the amount of homopolymer as a by-product tends to increase due to polymerization of only the monomer that proceeds in parallel with the graft reaction. The homopolymer does not contribute to the grafting reaction. In addition to this, chain transfer due to the generated homopolymer is likely to occur, the termination reaction becomes dominant, and the graft rate decreases. That is, the characteristics of the electrolyte membrane are lowered, and the yield is lowered.

  Graft polymerization can be carried out by applying a known method. For example, it is as follows. The vinyl monomer A is dissolved in the polymerization solvent to prepare a monomer solution that is a liquid phase. Next, the prepared monomer solution is accommodated in a glass or stainless steel container. Next, the stored monomer solution is subjected to vacuum degassing and bubbling with an inert gas (for example, nitrogen) to remove dissolved oxygen that inhibits the graft reaction. Next, while stirring the monomer solution, the resin fine particles irradiated with radiation are added, and the graft polymerization proceeds. The reaction time of graft polymerization is, for example, about 10 minutes to 12 hours. The reaction temperature is, for example, 0 to 100 ° C, preferably 40 to 80 ° C. After the grafting reaction is completed, the particulate graft polymer is removed from the polymerization solution by filtration. Next, the taken out graft polymer is washed 3 to 6 times with an appropriate amount of solvent to remove the polymerization solvent, unreacted monomer and homopolymer. The solvent is, for example, toluene, methanol, isopropyl alcohol, or acetone. Thereafter, the particulate graft polymer is dried.

[Casting process, drying process]
In the production method of the present invention, after the graft polymerization step, a cast film of the graft polymer is formed (casting step). In forming the cast film, a film forming method generally called a casting method is applied. For example, the fine particle graft polymer formed in the graft polymerization step is dissolved in a cast solvent (second solvent) to prepare a cast solution. Next, the prepared cast solution is applied to an appropriate support to form a cast film. The formed cast film is dried in a subsequent drying step (specifically, the cast solvent in the cast film is evaporated) to obtain a film.

  As the casting solvent for dissolving the graft polymer, it is desirable to use a compound that can dissolve both the graft chain formed by graft polymerization and the resin fine particles. In the casting step, it is preferable to form a cast film using an aprotic polar solvent as a casting solvent for dissolving the graft polymer. The aprotic polar solvent is, for example, dimethylacetamide, N-methylpyrrolidone, dimethylformamide, dimethylsulfoxide (DMSO). These solvents are highly versatile.

  The concentration of the graft polymer in the cast solution is preferably 5 to 30% by weight, more preferably 10 to 25% by weight. When the said concentration exceeds 30 weight%, the viscosity of a cast solution will become high too much and formation of the cast film | membrane and film which have a uniform film thickness may become difficult. When the concentration is less than 5% by weight, the applied cast film tends to flow, and it may be difficult to form a cast film and a film having a uniform film thickness.

  The support on which the cast solution is applied is, for example, a sheet made of glass, metal, or resin. What is necessary is just to adjust the film thickness of a cast film suitably so that the thickness of the polymer electrolyte film finally obtained may become a desired value. The thickness of the polymer electrolyte membrane is preferably 10 μm to 70 μm.

  A known method can be applied to apply the cast solution to the support.

  The cast film formed in the casting process is dried at a drying temperature equal to or lower than the melting point of the resin fine particles to form a film (drying process). When the drying temperature exceeds the melting point of the resin fine particles (the melting point of the resin material constituting the resin fine particles), the proton conductivity of the finally obtained polymer electrolyte membrane decreases, and the selective permeability of protons to methanol ( (Selectivity) decreases.

  The drying temperature in the case of forming a film having a thickness of 10 μm to 70 μm is preferably equal to or higher than the temperature at which the time required for drying the film (drying time) is 6 hours, and more preferably, the drying time is 2 hours. Above the temperature. The drying temperature is preferably 10 ° C. or lower than the melting point of the resin material constituting the resin fine particles, and more preferably 20 ° C. or lower than the melting point of the resin material. When the drying temperature is excessively high, the proton conductivity of the finally obtained polymer electrolyte membrane is lowered, and the selective permeability (selectivity) of protons to methanol may be lowered. If the drying temperature becomes excessively low, it takes a long time to perform the drying process.

  In the drying process, the cast film formed in the casting process is dried at a drying temperature of not less than a temperature at which drying time of the film is 6 hours and not more than 10 ° C. lower than the melting point of the resin fine particles. It is preferable to obtain a film having a thickness of 10 μm to 70 μm.

  A known film drying method can be applied to the drying step.

[Conversion process]
In the production method of the present invention, after the drying step, the sulfonic acid group precursor in the film obtained by the step is converted into a sulfonic acid group (conversion step).

  The conversion to a sulfonic acid group is performed using, for example, hydrolysis and / or ion exchange. When the sulfonic acid group precursor is an alkyl ester of a sulfonic acid group, the conversion step can be carried out by acid treatment using nitric acid, hydrochloric acid, sulfuric acid or the like, or treatment using an aqueous alcohol solution. When the sulfonic acid group precursor is a salt of a sulfonic acid group, the conversion step can be carried out by acid treatment using nitric acid, hydrochloric acid, sulfuric acid or the like. The acid concentration in the acid treatment is preferably about 1 N. These treatments can be performed under warming conditions as necessary.

  The film that has undergone the conversion process is washed, for example, purely to obtain a proton conductive polymer electrolyte membrane.

  The production method of the present invention can include steps other than those described above as long as the effects of the present invention are obtained.

  Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to the following examples.

Example 1
50 g of particulate polyvinylidene fluoride (PVDF: manufactured by Kureha, KF polymer W # 1100, average particle diameter of about 200 μm) was put into an oxygen-blocking bag having a barrier film. An oxygen scavenger (manufactured by Mitsubishi Gas Chemical Co., Ltd., Ageless) was introduced into the bag, and the bag was heat sealed and sealed. By storing this overnight, the oxygen in the bag was sufficiently adsorbed by the oxygen scavenger. In this manner, deoxygenation before irradiation with radiation was performed.

  After deoxygenation, this bag containing PVDF fine particles was irradiated with γ rays using cobalt 60 as a radiation source at an irradiation dose of 30 kGy. Next, the bag after irradiation was stored while keeping a low temperature state with dry ice.

  Separately, 150 g of styrene sulfonic acid ethyl ester (EtSS) as vinyl monomer A and 100 g of m-cresol as a polymerization solvent were charged into a separable flask. While stirring this at a temperature of 70 ° C., nitrogen was bubbled for 30 minutes to sufficiently perform deoxygenation. In this way, a monomer solution was obtained.

  Next, the PVDF fine particles irradiated with γ rays were taken out of the bag and put into the monomer solution. Subsequently, nitrogen bubbling and stirring were continuously carried out to advance the graft polymerization. The reaction temperature was 70 ° C. and the reaction time was 20 minutes.

  After completion of the polymerization, the solution in the flask was filtered to take out fine particles of the graft polymer. Next, the operation of immersing the extracted fine particles in acetone (room temperature, 60 minutes) and further filtering to extract the fine particles was repeated five times. The fine particles taken out by the last filtration were dried overnight in a drier maintained at 60 ° C. to obtain a fine particle graft polymer. The graft ratio of this graft polymer was measured by the method described later.

  Next, 4 g of N-methyl-2-pyrrolidone as a cast solvent is added to 1 g of the graft polymer, and vigorously vibrated at 70 ° C. for 2 hours or more to obtain a cast solution in which the graft polymer is dissolved in the cast solvent. It was. Next, the obtained cast solution was applied on a glass plate as a support with a gap thickness of 480 μm to form a cast film.

  Next, the formed cast film was dried by heating at 150 ° C. for 30 minutes to obtain a film.

  Next, the sulfonic acid group precursor contained in the obtained film was converted into a sulfonic acid group. Conversion to the sulfonic acid group was performed by immersing the film in an aqueous ethanol solution having a concentration of 50% by weight at 70 ° C. for 12 hours. Thereafter, the film was taken out from the ethanol aqueous solution, washed several times with pure water, and then dried. In this way, a proton conductive polymer electrolyte was obtained.

(Example 2)
In the same manner as in Example 1, a fine particle graft polymer was obtained. The graft ratio of this graft polymer was measured by the method described later. Next, using the obtained graft polymer, a proton conductive polymer electrolyte membrane was obtained in the same manner as in Example 1 except that the gap thickness when applying the cast solution was 240 μm.

(Example 3)
In the same manner as in Example 1, a fine particle graft polymer was obtained. The graft ratio of this graft polymer was measured by the method described later. Next, the obtained graft polymer was used, the gap thickness when applying the cast solution was 240 μm, and the conditions for drying the cast film were 80 ° C. for 2 hours, the same as in Example 1. Thus, a proton conductive polymer electrolyte membrane was obtained.

(Comparative Example 1)
A commercially available ion exchange membrane (manufactured by DuPont, Nafion 112) was used as Comparative Example 1.

(Comparative Example 2)
A commercially available ion exchange membrane (manufactured by DuPont, Nafion 115) was used as Comparative Example 2.

(Comparative Example 3)
A commercially available ion exchange membrane (manufactured by DuPont, Nafion 117) was used as Comparative Example 3.

(Comparative Example 4)
In the same manner as in Example 1, a fine particle graft polymer was obtained. The graft ratio of this graft polymer was measured by the method described later. Next, proton conductivity was obtained in the same manner as in Example 1 except that the obtained graft polymer was used and the conditions for drying the cast membrane were set at 200 ° C., which is a temperature exceeding the melting point of PVDF, for 15 minutes. A polymer electrolyte membrane was obtained.

(Comparative Example 5)
In the same manner as in Example 1, a fine particle graft polymer was obtained. The graft ratio of this graft polymer was measured by the method described later. Next, the obtained graft polymer was melt-kneaded at 220 ° C. for 20 minutes using a Laboplast mill (manufactured by Toyo Seiki Co., Ltd.), and then hot-pressed at 220 ° C. and about 20 MPa to form a film. This film was subjected to a conversion step (hydrolysis treatment) in the same manner as in Example 1 to obtain a proton conductive polymer electrolyte membrane.

  The graft ratios of the fine particle graft polymers prepared in Examples 1 to 3 and Comparative Examples 4 and 5 were evaluated as follows.

[Graft ratio]
The graft ratio G (%) is defined by the following formula (1).

G = (W2−W1) ÷ W1 × 100 (1)
G: Graft ratio [%], W1: Weight of resin material before graft reaction [g], W2: Weight of resin material after graft reaction [g]

  When the measurement object is in the form of fine particles, it is difficult to directly measure W1 and W2. Therefore, in this example, the infrared absorption spectrum of the graft polymer constituting the produced film is measured, and the resin material (PVDF in this example) and the graft chain (EtSS in this example) constituting the resin fine particles are measured. The peak ratio of the characteristic peaks of each chain was determined, and the graft ratio was determined using a calibration curve representing the correlation between the peak ratio and the graft ratio. This calibration curve was prepared in advance as follows.

  First, a plurality of types of fine particle graft polymers having different degrees of grafting were prepared by changing the graft polymerization time. After measuring the infrared spectrum of each produced graft polymer, each polymer was weighed by a predetermined weight W2. For each graft polymer having a dry weight W2, the molar amount “n (acid group) obs” of acid groups was measured by the same method as the ion exchange capacity measurement method described later. The mass W1 of the resin material before the graft reaction is obtained by the following formula (2), where the molecular weight of the EtSS monomer is Mw.

W1 = W2- {Mw × n (acid group) obs / 1000} (2)
Mw: molecular weight of EtSS monomer, n (acid group) obs: molar amount of acid group of sample [mmol]

  By substituting W1 and W2 into the formula (1), the graft ratio G of each graft polymer was calculated, and a calibration curve representing the relationship between the peak ratio of the infrared spectrum and the graft ratio was created.

  About the electrolyte membrane produced in Examples 1-3 and Comparative Examples 1-6, the film thickness, ion exchange capacity (IEC), proton conductivity (sigma), methanol permeation | transmission flow rate Fm, and the selective permeability parameter | index Sindex were evaluated. The evaluation method of each of these characteristics is shown below.

[Film thickness]
The thickness of the electrolyte membrane was measured in an atmosphere of a temperature of 25 ± 2 ° C. and a humidity of 65 ± 20% RH using a dial thickness gauge G-6C (1/1000 mm, probe diameter 5 mm) manufactured by Ozaki Seisakusho.

[Ion exchange capacity (IEC)]
The ion exchange capacity (IEC) of the electrolyte membrane is defined by equation (3).

IEC = n (acid group) obs / Wd (3)
IEC: ion exchange capacity [mmol / g], n (acid group) obs: molar amount of acid group of sample [mmol], Wd: dry weight of sample [g]

  “N (acid group) obs” was measured as follows. First, the sample was immersed in an aqueous sulfuric acid solution having a concentration of 1 mol / L maintained at 50 ° C. for 4 hours to make all the acid groups of the sample an acid type. Next, the immersed sample was washed with ion-exchanged water, and then immersed in an aqueous sodium chloride solution having a concentration of 3 mol / L maintained at 50 ° C. for 4 hours to replace the proton of the acid group with sodium ions. Next, the sodium chloride aqueous solution after the sample was immersed was titrated with an aqueous sodium hydroxide solution, and the amount of ion-exchanged protons was measured to obtain “n (acid group) obs”.

[Proton conductivity σ]
The proton conductivity of the electrolyte membrane was measured using a dedicated membrane resistance measurement cell, potentiostat (Hokuto Denko, HABF-5001) and voltmeter (Hokuto Denko, HE-104). First, a cell equipped with two platinum electrodes was filled with a 1 mol / L sulfuric acid aqueous solution and placed at room temperature. Next, by applying a current to the two platinum electrodes and measuring the potential difference between the electrodes when the applied current value is changed, a sample film having an area Sm is placed between the electrodes. A resistance value R1 and a resistance value R0 when no sample film was placed were obtained. The resistance value Rm of the sample membrane was obtained from the difference between R1 and R0, and the proton conductivity σ of the sample membrane was calculated by the following equation (4).

σ = 1 / (Rm × Sm) = 1 / {(R1−R0) × Sm} (4)
σ: proton conductivity [S / cm ² ], Rm: resistance value of sample film [Ω], Sm: area of sample film [cm ² ]

[Methanol permeation flow rate Fm]
Two glass containers having an opening and an inlet that can be sealed were prepared. The openings of the two containers were connected together so as to sandwich the sample film as a partition wall. The area of the sample film held in the container was defined as D. One container was filled with a 2 mol / L aqueous methanol solution through the container inlet, and the other container was filled with distilled water. This was kept at 60 ° C. with a water bath. The aqueous solution in the container on the distilled water side was sampled at an appropriate elapsed time and analyzed by gas chromatography. Using a calibration curve created from the relationship between the standard aqueous methanol solution and the peak area in the gas chromatogram, the methanol concentration in the sampling solution was determined. Furthermore, the slope t when the methanol concentration in each sampling solution was plotted against the elapsed time of the incubation was obtained. The methanol permeation flow rate Fm was calculated by substituting the slope t of the plot and the area D of the sample membrane into Equation (5).

Fm = t ÷ D (5)
Fm: methanol permeation flow rate [mmol / (cm ² · hr)], t: slope of plot [mmol / hr], D: area of sample film [cm ² ]

[Selective permeability index Sindex]
The selective permeability index Sindex is a numerical value expressed by the equation (6) and obtained by dividing the proton conductivity σ by the methanol permeation flow rate Fm. The higher this value, the greater the degree of selective permeation of protons to methanol, and it can be judged that the performance of the electrolyte membrane is good.

Index = σ ÷ Fm (6)
Sindex: selective permeability index [(S · hr) / mmol], σ: proton conductivity [S / cm ² ], Fm: methanol permeation flow rate [mmol / (cm ² · hr)]

  These evaluation results are shown in Table 1 below.

  The electrolyte membranes of Examples 1 to 3 are proton conductive polymer electrolyte membranes produced by the production method of the present invention, have a high proton conductivity, a low methanol permeation flow rate, and therefore a high permselectivity index.

  Since the electrolyte membranes of Comparative Examples 1 to 3 are Nafion membranes, the permselectivity index is low. The electrolyte membrane of Comparative Example 4 has a low proton conductivity and a low selective permeability index because the drying temperature of the cast membrane is higher than the melting point of PVDF. Since the electrolyte membrane of Comparative Example 5 was produced by melt kneading and pressing instead of the casting method, the proton conductivity was low and the selective permeability index was low.

  The proton conductive polymer electrolyte membrane obtained by the production method of the present invention can be suitably used for an electrolyte membrane of PEFC, particularly DMFC.

Claims (6)

Irradiating resin fine particles with radiation;
In a solid-liquid two-phase system, a vinyl monomer having a sulfonic acid group precursor is graft-polymerized onto the resin fine particles irradiated with the radiation, and the fine particle graft weight containing the resin fine particles and the polymer chain of the vinyl monomer is included. Obtaining a coalescence;
A casting step of forming a cast film of the obtained graft polymer;
Drying the formed cast film at a drying temperature below the melting point of the resin fine particles to obtain a film; and
Converting the sulfonic acid group precursor in the obtained film to a sulfonic acid group,
A method for producing a proton-conductive polymer electrolyte membrane, wherein the solid-liquid two-phase system includes a liquid phase containing the vinyl monomer and a solvent for the monomer and a solid phase containing the resin fine particles.   The method for producing a proton conductive polymer electrolyte membrane according to claim 1, wherein the solvent contained in the liquid phase of the solid-liquid two-phase system is an aromatic compound. In the casting step,
The method for producing a proton conductive polymer electrolyte membrane according to claim 1 or 2, wherein the cast membrane is formed using an aprotic polar solvent as a cast solvent for dissolving the graft polymer. In the drying step,
The film having a thickness of 10 μm to 70 μm is dried by drying the cast film at a drying temperature not lower than a temperature at which drying time of the film is 6 hours and not higher than a melting point of the resin fine particles by 10 ° C. The method for producing a proton conductive polymer electrolyte membrane according to claim 1, wherein:   The resin fine particles are polystyrene, polyether ether ketone, polyether ketone, polysulfone, polyether sulfone, polyphenylene sulfide, polyarylate, polyetherimide, aromatic polyimide, polyamideimide, polyethylene, polypropylene, polyvinylidene fluoride, polyvinyl fluoride. And at least one selected from ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer. A method for producing a proton conductive polymer electrolyte membrane according to any one of the above.   The vinyl monomer is at least one selected from methyl ester, ethyl ester, propyl ester, butyl ester, cyclohexyl ester, phenyl ester, and sodium salt of styrene sulfonic acid. A method for producing a proton conductive polymer electrolyte membrane.