JP4821946B2 - Electrolyte membrane and method for producing the same - Google Patents

Description

  The present invention relates to an electrolyte membrane excellent in methanol permeation resistance and ion conductivity and a method for producing the same.

  As an example of an electrochemical device using a polymer solid electrolyte as an ionic conductor instead of a liquid electrolyte, a water electrolyzer and a fuel cell can be raised. The polymer membrane used for these must be sufficiently stable chemically, thermally, electrochemically and mechanically with proton conductivity as a cation exchange membrane. For this reason, perfluorocarbon sulfonic acid membranes, typically “Nafion (registered trademark)” manufactured by DuPont, have been used as long-term usable products. However, if the Nafion membrane is operated under conditions exceeding 100 ° C., the moisture content of the membrane drops rapidly and the membrane softens significantly. For this reason, in a fuel cell that uses methanol as a fuel, which is expected in the future, the performance is deteriorated due to the permeation of methanol in the membrane, so that sufficient performance cannot be exhibited. Further, even in a fuel cell that is currently studied mainly using hydrogen as a fuel and operated at around 80 ° C., it is pointed out that the cost of the membrane is too high as an obstacle to the establishment of fuel cell technology.

  In order to overcome such drawbacks, various types of electrolyte membranes in which a sulfonic acid group is introduced into a non-fluorinated aromatic ring-containing polymer have been studied. As a polymer skeleton, considering heat resistance and chemical stability, aromatic polyarylene ether compounds such as aromatic polyarylene ether ketones and aromatic polyarylene ether sulfones can be regarded as promising structures, A sulfonated polyarylethersulfone (for example, see Non-Patent Document 1), a sulfonated polyetheretherketone (for example, see Patent Document 1), sulfonated polystyrene, and the like have been reported. However, sulfonic acid groups introduced onto aromatic rings by sulfonation reaction of these polymers generally tend to be easily removed by heat, and as a method for improving this, sulfonic acid groups are introduced onto electron-withdrawing aromatic rings. A sulfonated polyarylether sulfone compound having high thermal stability has been reported by polymerization using a monomer (see, for example, Patent Document 2). In this case, since the reactivity of a monomer is low, the problem which requires superposition | polymerization for a long time in order to obtain a polymer has arisen (for example, refer nonpatent literature 2).

In addition, ionic groups such as sulfonic acid groups have been introduced into these polymers in order to exhibit functions as electrolyte membranes, but when the cation species bound to the ionic groups are so-called salt types such as metals, There is a problem that the conductivity is not sufficiently exhibited.
JP-A-6-93114 (pages 15-17) US Patent Application Publication No. 2002/0091225 (page 1-2) Journal of Membrane Science (Netherlands) 1993, 83, p. 211-220 ACS Polymer Preprints, (USA), 2000, 41 (2), P.C. 1388-1389

  An object of the present invention is to obtain an electrolyte membrane excellent in methanol permeation resistance and ion conductivity by a production method in which the content of a solvent and the cation amount of an ionic group are controlled.

  As a result of intensive studies, the present inventors have found that the above object can be achieved by removing the residual solvent below a certain level and then converting the ion conductive group into an acid form using an acid.

  That is, the present invention is achieved by the following (1) to (6).

  (1) In the electrolyte membrane having ionic conductivity, after the residual solvent ratio is 0.5% by weight or more and 20% by weight or less of the entire membrane, the cation group of the ionic group is converted to at least 85 mol% or more hydrogen atoms by acid. An electrolyte membrane characterized by conversion.

  (2) The electrolyte membrane according to the first invention, wherein the acid is sulfuric acid.

  (3) A methanol permeation rate with respect to a 5 mol / L methanol aqueous solution at 25 ° C. is 10 mmol / m 2 · sec or less, and an ionic conductivity at 80 ° C. and a relative humidity of 95% is 0.02 S / cm or more. An electrolyte membrane produced by the manufacturing method within the scope of the first or second invention.

ただし、Ａｒは２価の芳香族基、Ｙはスルホン基またはケトン基、Ｘは水素または１価のカチオン種を示す。

ただし、Ａｒ’は２価の芳香族基を示す。 (4) The electrolyte within the scope of the first to third inventions, wherein the electrolyte membrane is an ion conductive electrolyte membrane containing a constituent represented by the following general formula (1) together with the following general formula (1) film.

Here, Ar represents a divalent aromatic group, Y represents a sulfone group or a ketone group, and X represents hydrogen or a monovalent cation species.

However, Ar ′ represents a divalent aromatic group.

  (5) An electrolyte membrane / electrode assembly using the electrolyte membrane according to any one of the first to fourth inventions.

  (6) A fuel cell using the electrolyte membrane / electrode assembly within the scope of the fifth invention.

  The electrolyte membrane produced by the production method of the present invention can provide a material exhibiting outstanding performance as an electrolyte membrane for a direct methanol fuel cell having excellent methanol permeation resistance and ion conductivity.

Hereinafter, the present invention will be described in detail.
The present invention relates to a method for producing an electrolyte membrane in which the amount of solvent remaining in a membrane obtained from a polymer having ion conductivity is within a specific range, and then the cation species of the ionic group is 85 mol% or more by hydrogen with an acid. Thus, an electrolyte membrane excellent in methanol permeation resistance and ion conductivity is provided.

  Examples of the ion conductive polymer of the present invention include ionomers containing at least one of polystyrene sulfonic acid, poly (trifluorostyrene) sulfonic acid, polyvinyl phosphonic acid, polyvinyl carboxylic acid, and polyvinyl sulfonic acid components. Furthermore, as aromatic polymers, polysulfone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, polyphenylene sulfide sulfone, polyparaphenylene, polyarylene polymer, polyphenylquinoxaline, polyaryl ketone, polyether ketone, polybenzoxazole, Examples thereof include polymers in which at least one of sulfonic acid groups, phosphonic acid groups, carboxyl groups, and derivatives thereof is introduced into a polymer containing at least one component such as polybenzthiazole and polyimide. Polysulfone, polyethersulfone, polyetherketone, and the like referred to here are generic names for polymers having a sulfone bond, an ether bond, and a ketone bond in their molecular chains. Polyetherketoneketone, polyetheretherketone, It includes polyether ether ketone ketone, polyether ketone ether ketone ketone, polyether ketone sulfone and the like, and is not limited to a specific polymer structure.

  The ionic group-containing polymer in the present invention is preferably a polyarylene ether compound such as sulfonic acid group-containing polysulfone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, polyphenylene sulfide sulfone, or polyether ketone polymer.

  Furthermore, among these polyarylene ether compounds, those containing the structural component represented by the general formula (2) together with the following general formula (1) are particularly preferable.

ただし、Ａｒは２価の芳香族基、Ｙはスルホン基またはケトン基、Ｘは水素または１価のカチオン種を示す。

Here, Ar represents a divalent aromatic group, Y represents a sulfone group or a ketone group, and X represents hydrogen or a monovalent cation species.

ただし、Ａｒ’は２価の芳香族基を示す。

However, Ar ′ represents a divalent aromatic group.

  The component represented by the general formula (2) is preferably a component represented by the following general formula (3).

ただし、Ａｒ’は２価の芳香族基を示す。

However, Ar ′ represents a divalent aromatic group.

  As the sulfonic acid group-containing polyarylene ether-based compound in the present invention, those containing a constituent represented by the following general formula (5) together with the following general formula (4) are particularly preferable. By having a biphenylene structure, the film has excellent dimensional stability under high-temperature and high-humidity conditions, and also has high film toughness.

ただし、Ｘは水素または１価のカチオン種を示す。

X represents hydrogen or a monovalent cation species.

  The sulfonic acid group-containing polyarylene ether compound in the present invention preferably has a sulfonic acid group content in the range of 0.3 to 3.5 meq / g. When the amount is less than 0.3 meq / g, there is a tendency that sufficient ion conductivity is not exhibited when used as an electrolyte membrane, and when the amount is more than 3.5 meq / g, the electrolyte membrane is subjected to high temperature and high humidity conditions. In this case, the film swells so much that it is not suitable for use. The sulfonic acid group content can be calculated from the polymer composition. More preferably, it is 1.0-3.0 meq / g.

  The sulfonic acid group-containing polyarylene ether compound in the present invention can be used as a simple substance, but can also be used as a resin composition in combination with other polymers. Examples of these polymers include polyesters such as polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate, polyamides such as nylon 6, nylon 6,6, nylon 6,10 and nylon 12, polymethyl methacrylate and polymethacrylate. Acrylate resins such as polymethyl acrylate, polyacrylic acid esters, polyacrylic acid resins, polymethacrylic acid resins, polyethylene, polypropylene, various polyolefins including polystyrene and diene polymers, polyurethane resins, cellulose acetate, Cellulosic resins such as ethyl cellulose, polyarylate, aramid, polycarbonate, polyphenylene sulfide, polyphenylene oxide, polysulfone, polyether Heat curing of aromatic polymers such as sulfone, polyether ether ketone, polyether imide, polyimide, polyamide imide, polybenzimidazole, polybenzoxazole, polybenzthiazole, epoxy resin, phenol resin, novolac resin, benzoxazine resin There are no particular restrictions on the conductive resin. A resin composition with a basic polymer such as polybenzimidazole or polyvinylpyridine can be said to be a preferable combination for improving the polymer dimensionality, and when a sulfonic acid group is further introduced into these basic polymers, The processability of the composition becomes more preferable. When used as these resin compositions, the sulfonic acid group-containing polyarylene ether compound of the present invention is preferably contained in an amount of 50% by mass or more and less than 100% by mass of the entire resin composition. More preferably, it is 70 mass% or more and less than 100 mass%. When the content of the sulfonic acid group-containing polyarylene ether compound of the present invention is less than 50% by weight of the entire resin composition, the sulfonic acid group concentration of the electrolyte membrane containing the resin composition is lowered and good ion conduction is achieved. Tend to be not obtained, and the unit containing a sulfonic acid group becomes a discontinuous phase and tends to lower the mobility of ions to be conducted. In addition, the composition of the present invention, if necessary, for example, an antioxidant, a heat stabilizer, a lubricant, a tackifier, a plasticizer, a crosslinking agent, a viscosity modifier, an antistatic agent, an antibacterial agent, an antifoaming agent, Various additives such as a dispersant and a polymerization inhibitor may be included.

  The resin composition in the present invention can be formed into a molded body such as a fiber or a film by any method such as extrusion, spinning, rolling or casting. Among these, it is preferable to mold from a solution dissolved in an appropriate solvent. Examples of the solvent include aprotic polar solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone and hexamethylphosphonamide, and alcohols such as methanol and ethanol. An appropriate one can be selected, but is not limited thereto. A plurality of these solvents may be used as a mixture within a possible range. The compound concentration in the solution is preferably in the range of 0.1 to 50% by weight. If the concentration of the compound in the solution is less than 0.1% by weight, it tends to be difficult to obtain a good molded product, and if it exceeds 50% by weight, the workability tends to deteriorate. A method of obtaining a molded body from a solution can be performed using a conventionally known method. For example, the molded product can be obtained by removing the solvent by heating, drying under reduced pressure, immersion in a compound non-solvent that can be mixed with a solvent that dissolves the compound, or the like. When the solvent is an organic solvent, the solvent is preferably distilled off by heating or drying under reduced pressure. At this time, it can be formed into various shapes such as a fiber shape, a film shape, a pellet shape, a plate shape, a rod shape, a pipe shape, a ball shape, and a block shape in a composite form with other compounds as necessary. . When combined with a compound having a similar dissolution behavior, it is preferable in that good molding can be achieved.

  The electrolyte membrane in the present invention can also be produced as follows. The most preferable method for forming the electrolyte membrane is casting from a solution, and the electrolyte membrane can be obtained by removing the solvent from the cast solution as described above. Examples of the solution include a solution using an organic solvent such as N-methylpyrrolidone, N, N-dimethylformamide, and dimethyl sulfoxide, and in some cases, an alcohol solvent. The removal of the solvent is preferably by drying from the viewpoint of the uniformity of the electrolyte membrane. Moreover, it is also possible to remove a solvent by immersing in a compound non-solvent that can be mixed with a solvent, and it may be used in combination with drying. Moreover, in order to avoid decomposition | disassembly and alteration of a compound or a solvent, it can also dry at the lowest temperature possible under reduced pressure. Further, when the viscosity of the solution is high, when the substrate or the solution is heated and cast at a high temperature, the viscosity of the solution is lowered and the casting can be easily performed. The thickness of the solution at the time of casting is not particularly limited, but is preferably 10 to 1000 μm. More preferably, it is 50-500 micrometers. If the thickness of the solution is thinner than 10 μm, the shape as an electrolyte membrane tends to be lost, and if it is thicker than 1000 μm, a non-uniform electrolyte membrane tends to be easily formed. As a method for controlling the cast thickness of the solution, a known method can be used. For example, the thickness can be controlled with the amount and concentration of the solution with a constant thickness using an applicator, a doctor blade, or the like, and with a cast area constant using a glass petri dish or the like. The cast solution can obtain a more uniform film by adjusting the solvent removal rate. For example, in the case of heating, the evaporation rate can be reduced by lowering the temperature in the first stage. In addition, when immersed in a non-solvent such as water, the coagulation rate of the compound can be adjusted by leaving the solution in air or an inert gas for an appropriate time. The electrolyte membrane of the present invention can have any thickness depending on the purpose, but is preferably as thin as possible from the viewpoint of ion conductivity. Specifically, it is preferably 5 to 200 μm, more preferably 5 to 50 μm, and most preferably 5 to 20 μm. If the thickness of the electrolyte membrane is less than 5 μm, the handling of the electrolyte membrane becomes difficult and a short circuit or the like tends to occur when a fuel cell is produced. Tend to decrease.

  In the electrolyte membrane thus obtained, the remaining solvent is preferably 0.5% by weight or more and 20% by weight or less of the entire membrane. When the amount is more than 20% by weight, the methanol permeability becomes high. When the amount is less than 0.5% by weight, the conversion to the acid form in the next step is delayed and the ionic conductivity tends to be lowered. In any case, when the electrolyte membrane is used in a direct methanol fuel cell, the performance tends to be adversely affected. ,

  The sulfonic acid group in the electrolyte membrane obtained by reducing the amount of the remaining solvent in this way may partially include a salt form with a cationic species, but 85 mol% or more is a hydrogen atom. It is preferable to use a free sulfonic acid group. If the number of free sulfonic acid groups is less than 85 mol%, the electric resistance value increases, and the power generation performance of the fuel cell decreases.

  As a method for converting the above-described electrolyte membrane into a free sulfonic acid group, an acid can be used. As the acid to be used, sulfuric acid, fuming sulfuric acid, hydrochloric acid and the like can be used, but it is more preferable to use sulfuric acid. The conditions for conversion may be gas phase or liquid phase, and may be room temperature or under heating.

  The ionic conductivity of the electrolyte membrane in the present invention at 80 ° C. and 95% relative humidity is preferably 0.01 S / cm or more. When the ionic conductivity is 0.01 S / cm or more, a good output tends to be obtained in the fuel cell using the electrolyte membrane, and when it is less than 0.01 S / cm, the output of the fuel cell is obtained. A decline tends to occur.

  The permeability of the electrolyte membrane in the present invention at 25 ° C. and 5 mol / L of methanol is preferably 10 mmol / (m 2 · sec) or less. When the methanol permeability is higher than 10 mmol / (m 2 · sec), when the electrolyte membrane is used for a direct methanol fuel cell, the durability tends to be inferior.

  By installing an electrode on the electrolyte membrane of the present invention described above, a joined body of the electrolyte membrane and the electrode of the present invention can be obtained. As a method for producing this joined body, a conventionally known method can be used. For example, an adhesive is applied to the electrode surface and the electrolyte membrane and the electrode are bonded, or the electrolyte membrane and the electrode are heated. There is a method to press. Among these, a method of applying and bonding an adhesive mainly composed of the sulfonic acid group-containing polyarylene ether compound and the resin composition of the present invention to the electrode surface is preferable. This is because the adhesion between the electrolyte membrane and the electrode is improved, and it is considered that the ion conductivity of the electrolyte membrane is less impaired.

  A fuel cell can also be produced using the above-described assembly of the electrolyte membrane and the electrode. The electrolyte membrane of the present invention is excellent in methanol permeation resistance, ion conductivity, heat resistance, workability and dimensional stability, so it can withstand operation at high temperatures, is easy to produce, and has good output. Can be provided. It is also preferable to use it as a fuel cell using methanol as a direct fuel.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely using an Example, this invention is not limited to these Examples. Various measurements were performed as follows.

Sulfonic acid group content: Weigh the sample dried overnight under a nitrogen atmosphere, and after stirring with aqueous sodium hydroxide, determine the ion exchange capacity (IEC) by back titration with aqueous hydrochloric acid, and compare it with the theoretical value. The amount of more free sulfonic acid groups was determined.

Ion conductivity measurement: A platinum wire (diameter: 0.2 mm) was pressed against the surface of the strip-shaped membrane sample on a self-made measurement probe (manufactured by Teflon (R)), and a constant temperature and humidity oven at 80 ° C. and 95% RH ( The sample was held in Nagano Scientific Machinery Co., Ltd., LH-20-01), and the impedance between the platinum wires was measured by SOLARTRON 1250 FREQUENCY RESPONSE ANALYSER. The measurement was performed while changing the distance between the electrodes, and the conductivity obtained by canceling the contact resistance between the film and the platinum wire was calculated from the gradient obtained by plotting the distance measured between the electrodes and the resistance measurement value estimated from the CC plot.
Conductivity [S / cm] = 1 / film width [cm] × film thickness [cm] × resistance interelectrode gradient [Ω / cm]

Methanol permeation rate: The liquid fuel permeation rate of the ion exchange membrane was measured as the permeation rate of methanol by the following method. An ion exchange membrane immersed in a 5 M (mol / liter) aqueous methanol solution adjusted to 25 ° C. for 24 hours is sandwiched in an H-type cell, 100 ml of 5 M aqueous methanol solution is placed on one side of the cell, and 100 ml of ultrapure water ( 18 MΩ · cm) was injected, and the amount of methanol diffusing into the ultrapure water through the ion exchange membrane was measured using a gas chromatograph while stirring the cells on both sides at 25 ° C. (ion) The area of the exchange membrane is 2.0 cm ² ).

Power generation characteristics: After adding a commercially available 40% Pt catalyst-supported carbon (Tanaka Kikinzoku Co., Ltd. fuel cell catalyst TEC10V40E), a small amount of ultrapure water and isopropanol to a 20% Nafion (trade name) solution manufactured by DuPont, Stir until homogeneous to prepare a catalyst paste. This catalyst paste was uniformly applied to Toray carbon paper TGPH-060 so that the amount of platinum deposited was 0.5 mg / cm ² and dried to prepare a gas diffusion layer with an electrode catalyst layer. By sandwiching the electrolyte membrane between the gas diffusion layers with the electrode catalyst layer so that the electrode catalyst layer is in contact with the membrane, and pressurizing and heating at 130 ° C. and 2 MPa for 3 minutes by a hot press method, An electrode assembly was obtained. This joined body was incorporated into an evaluation fuel cell FC25-02SP manufactured by Electrochem, and power generation characteristics were evaluated. Power generation is performed while supplying high purity oxygen gas (80 ml / min) adjusted to 40 ° C. and 5 or 10 mol / L aqueous methanol solution (1.5 ml / min) to the anode and cathode at a cell temperature of 40 ° C. went. Using the output voltage at a current density of 0.01 A / cm ² , the output decrease when the concentration of the aqueous methanol solution was changed was evaluated.

Example 1
3.683 g (0.0075 mol) of 3,3′-disulfo-4,4′-dichlorodiphenylsulfone disodium salt (abbreviation: S-DCDPS) and 3,6-dichlorobenzonitrile (abbreviation: DCBN) of 3 100 ml four-neck flask containing 0.010 g (0.0175 mol), 4.650 g (0.025 mol) of 4,4′-biphenol, 3.968 g (0.0288 mol) of potassium carbonate, and 2.61 g of molecular sieve And then flushed with nitrogen. After adding 35 ml of NMP and stirring at 150 ° C. for 1 hour, the reaction was continued by raising the reaction temperature to 195-200 ° C. and sufficiently increasing the viscosity of the system (about 5 hours). After standing to cool, the precipitated molecular sieve was removed and the mixture was precipitated in water as a strand. The obtained polymer was washed in boiling water for 1 hour and then dried. The logarithmic viscosity of the polymer was 1.68. The polymer NMP solution with the adjusted concentration was cast on a glass plate on a hot plate while adjusting the thickness, and dried until the remaining NMP amount was 4% with respect to the entire film to prepare a film having an average thickness of 200 μm. The obtained film was immersed in sulfuric acid prepared at 2 mol / l for 1 hour at room temperature, washed with pure water to remove excess sulfuric acid, and then dried to obtain an electrolyte membrane. The IEC of this electrolyte membrane was 1.55, and the hydrogen atom of the sulfonic acid group was 95 mol% with respect to the cationic species. The ionic conductivity of this electrolyte membrane was 0.16 S / cm, indicating good ionic conductivity. Moreover, when methanol permeability was evaluated, it was found that the methanol permeation rate was 0.67 mmol / (m 2 · sec) and the methanol permeation resistance was excellent.

Example 2
A membrane-electrode assembly was prepared using the electrolyte membrane obtained in Example 1 and evaluated for power generation. As a result, when 5 mol / l aqueous methanol solution was used, the output voltage was 0.462 V, 10 mol / l. The output voltage when using 1 methanol aqueous solution was 0.445V. From the results, it can be said that the output decrease due to the increase in methanol concentration is 0.017 V, the output voltage is high, and the electrolyte membrane has little permeation of high concentration methanol.

Comparative Example 1
In Example 1, the amount of residual NMP at the time of film formation was set to 22% by weight of the entire film, and the others were obtained in the same manner. The IEC of the electrolyte membrane was 1.39, and the hydrogen atom of the sulfonic acid group was 85 mol% with respect to the cationic species. The ionic conductivity of this electrolyte membrane was 0.10 S / cm, but the methanol permeation rate was 12 mmol / (m 2 · sec), and the methanol permeation amount was large, so that it could be used as an electrolyte membrane for direct methanol fuel cells. There wasn't.

Comparative Example 2
In Example 1, the amount of NMP remaining at the time of film formation was set to 0.3% by weight of the whole film, and the others were obtained in the same manner. The IEC of the electrolyte membrane was 1.28, and the hydrogen atom of the sulfonic acid group was 79 mol% with respect to the cationic species. The ionic conductivity of this electrolyte membrane was as low as 0.008 S / cm, which was insufficient performance as an electrolyte membrane.

  As can be seen from the above example, by changing the residual solvent amount of the membrane within a specific range before converting the sulfonic acid group from the salt type to the acid type, an electrolyte membrane having good methanol permeability resistance and ion conductivity can be obtained. Obtainable.

  With the electrolyte membrane produced by the production method of the present invention, an electrolyte membrane excellent in methanol permeability resistance and ionic conductivity can be provided. This electrolyte membrane can be used in fuel cells and water electrolyzers that use hydrogen or methanol as raw materials, but it is also expected to be used as various battery electrolytes, display elements, sensors, binders, additives, etc. .

Claims (6)

A solution in which a polymer having a sulfonic acid group that forms a salt with a cationic species was dissolved in an organic solvent was cast, the solvent was removed, and the residual solvent ratio was 0.5 wt% to 20 wt% of the entire membrane. after manufacturing method of the electrolyte membrane and converting the cationic species of the salt sulfonic acid groups and cationic species is formed by acid to at least 85 mol% hydrogen. 2. The method for producing an electrolyte membrane according to claim 1, wherein the acid is sulfuric acid. It is manufactured by the method according to claim 1, and has a methanol permeation rate of 10 mmol / (m 2 · sec) or less with respect to a 5 mol / L methanol aqueous solution at 25 ° C., and an ionic conductivity at 80 ° C. and a relative humidity of 95%. Is an electrolyte membrane characterized by being 0.01 S / cm or more. 4. The electrolyte membrane according to claim 3 , wherein the electrolyte membrane is an ion conductive electrolyte membrane containing a constituent represented by the following general formula (1) and the general formula (2).

Here, Ar represents a divalent aromatic group, Y represents a sulfone group or a ketone group, and X represents hydrogen or a monovalent cation species.

However, Ar ′ represents a divalent aromatic group. An electrolyte membrane / electrode assembly using the electrolyte membrane according to claim 3 or 4 . A fuel cell using the electrolyte membrane / electrode assembly according to claim 5 .