FR3161986A1 - proton exchange polymer membrane and its synthesis process by electron bombardment - Google Patents

Abstract

The invention relates to a method for preparing a proton exchange polymer membrane containing a crosslinked ionomer, which method comprises electron bombardment of an ionomer in the form of a film, the ionomer being a block polymer of formula (A-B)n-A, the symbol A representing a polyvinylaromatic block bearing sulfonic acid functional groups, the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer, n being an integer equal to or greater than 1. The membrane, intended for use in a fuel cell or electrolyzer, exhibits good properties with respect to water uptake and lifespan.

Description

proton exchange polymer membrane and its synthesis process by electron bombardment

The field of the present invention is that of the processes for synthesizing a proton exchange polymer membrane containing a non-fluorinated ionomer and intended for use in an electrolyzer or a fuel cell.

The core of a fuel cell and electrolyzer consists of two electrodes, an anode and a cathode, an electrolytic layer separating them, and a catalyst located at the interfaces between the electrolytic layer and each electrode. Fuel cells and electrolyzers include a membrane that forms the electrolytic layer. One of the membrane's components is the ionomer, a polymer bearing ionic or ionizable groups. Examples of ionomers usable in proton exchange membrane fuel cells include polymers bearing sulfonate or sulfonic acid functional groups, such as fluorinated polymers like Nafion, and non-fluorinated polymers like PEMION, which has a polyphenylene backbone.

To obtain non-fluorinated ionomers for use in proton exchange membranes, it is also known to modify substantially, or even essentially, hydrocarbon block polymers consisting of rigid terminal blocks connected to soft blocks. The modification involves introducing sulfonate or sulfonic acid functional groups into the rigid blocks. For example, reference can be made to US patents 5,468,574 and WO 9532236, which describe the sulfonation of such block polymers, where the rigid blocks are polystyrenes and the soft blocks are hydrogenated homopolymers of 1,3-diene or hydrogenated copolymers of 1,3-diene and styrene.

The introduction of sulfonate or sulfonic acid groups into rigid blocks results in polymers whose mechanical properties decrease when exposed to a humid environment, such as in a fuel cell or electrolyzer. This decrease in mechanical properties also reduces the properties of the membrane containing them. This reduction in mechanical properties is linked to the ionomer's capacity to absorb water, which increases with the number of sulfonate or sulfonic acid groups present in the ionomer. For example, see document WO 2007010039, which describes this phenomenon and suggests modifying flexible blocks instead of rigid ones to mitigate it.

Furthermore, a proton exchange membrane is also exposed to chemical attack attributed to free radical species resulting from the operation of the fuel cell or electrolyzer. This chemical attack leads to a degradation of the membrane's performance over time, consequently reducing the lifespan of the membrane and, as a result, that of the fuel cell or electrolyzer.

Therefore, there is a need to improve the properties of membranes containing non-fluorinated ionomers.

The Applicant has discovered a process that leads to the production of a new membrane containing a non-fluorinated ionomer and exhibiting improved properties with regard to water uptake and lifespan.

Thus, a first object of the invention is a process for preparing a proton exchange polymer membrane containing a crosslinked ionomer, which process comprises electron bombardment of an ionomer in the form of a film, the ionomer being a block polymer of formula (I)
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1.

The invention also relates to a proton exchange polymer membrane containing a crosslinked ionomer, the crosslinked ionomer being a block polymer of formula (I) crosslinked by electron bombardment
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
where n is an integer equal to or greater than 1,
which membrane is capable of being obtained by the process according to the invention.

Another object of the invention is a crosslinked ionomer, block polymer of formula (I) crosslinked by electron bombardment.

Yet another object of the invention is a fuel cell or electrolyzer containing a membrane according to the invention.

Detailed description of the invention

The polymers mentioned in the description can be of fossil origin or bio-based. In the latter case, they can be partially or entirely derived from biomass or obtained from renewable raw materials derived from biomass. Similarly, they can also come from the recycling of previously used materials; that is, they can be partially or entirely produced through a recycling process, or obtained from raw materials themselves derived from a recycling process.

The terms "membranes" and "films" are well known to those skilled in the technical field. It is well understood that a membrane is a structure as defined by IUPAC in "IUPAC Recommendations 1996." Similarly, and in accordance with the definition provided by IUPAC, the term "film" is understood according to the definition given by IUPAC in "IUPAC Recommendations 1996."

The useful ionomer in the process according to the invention is a block polymer bearing sulfonic acid functions of formula (I)
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1.

According to any one of the embodiments of the invention, the block polymer of formula (I) is preferably a linear polymer.

Preferably, n is equal to 1, in which case the block polymer of formula (I) is a triblock. The triblock then has the formula A-B-A in which the symbol A represents a polyvinylaromatic block bearing sulfonic acid functions and the symbol B represents a hydrogenated block of a homopolymer of a 1,3-diene or of a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer.

The polyvinylaromatic block of the ionomer can be a block of a homopolymer of a vinylaromatic monomer or a block of a copolymer of two or more vinylaromatic monomers. The vinylaromatic monomer of the polyvinylaromatic block of the ionomer is also referred to as the "first vinylaromatic." In the present invention, the term vinylaromatic monomer means a monomer of the formula Ar-CH= _CH₂ or Ar-CR= _CH₂ , the symbol Ar representing an aryl group, substituted or unsubstituted, and the symbol R an alkyl group such as methyl. The aryl represented by the symbol Ar is preferably a phenyl or a phenyl substituted with an alkyl group having 1 to 4 carbon atoms. Examples of vinylaromatic monomers useful for the purposes of the invention include styrene, styrene substituted with an alkyl group, in the para, meta, ortho, or alpha positions. The alkyl group is preferably an alkyl group having 1 to 4 carbon atoms. The polyvinyl aromatic block is preferably a polystyrene block, a polyalphamethylstyrene block, or a styrene-alphamethylstyrene copolymer block. The styrene-alphamethylstyrene copolymer block can be stochastic, tapered, or block-type.

The polyvinylaromatic block of the ionomer is also characterized by the presence of sulfonic acid functional groups. The polyvinylaromatic block of the ionomer is a polyvinylaromatic block that typically contains vinylaromatic monomer units in which the aryl group, preferably phenyl, is substituted by one or more sulfonic acid functional groups. In other words, the polyvinylaromatic block bearing sulfonic acid functional groups is a polyvinylaromatic block modified generally by a sulfonation reaction. The sulfonation reaction of a polyvinylaromatic is a well-known reaction. For example, reference can be made to documents EP 1840993B1, US 5239010, US 5468574, and US 20070021569, which describe the selective sulfonation of a polyvinylaromatic block of a polymer of formula (I).

According to the invention, the block represented by the symbol B is a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer. The vinylaromatic monomer or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer is also referred to as "second vinylaromatic".

A hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomeric units of a 1,3-diene and a vinylaromatic monomer is defined as a block in which the 1,3-diene monomer units are reduced to more than 95%, preferably more than 98%, and more preferably more than 99% by mole of the 1,3-diene monomer units. The hydrogenation reaction of a poly(1,3-diene) or a copolymer comprising monomeric units of a 1,3-diene and a vinylaromatic monomer is a well-known reaction. For example, reference can be made to documents EP 1840993B1, US 5239010, US 5468574 and US 20070021569 which describe the selective hydrogenation of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer of a block polymer of formula (I).

The term "a 1,3-diene" refers to one or more 1,3-dienes, that is, at least two 1,3-dienes. The 1,3-diene preferably contains 4 to 8 carbon atoms. The 1,3-diene is preferably 1,3-butadiene, isoprene, or a mixture of 1,3-butadiene and isoprene, and more preferably 1,3-butadiene.

According to a first embodiment, the block represented by the symbol B is a hydrogenated block of poly(1,3-diene). In the present invention, poly(1,3-diene) is understood to be a polymer whose constituent units are the monomeric units of 1,3-diene. Poly(1,3-diene) is preferably a homopolymer of 1,3-diene, in which case the constituent units of poly(1,3-diene) are monomeric units of a single 1,3-diene. Poly(1,3-diene) is more preferably a homopolymer of 1,3-butadiene.

According to a second variant, the block represented by the symbol B is a hydrogenated block of a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer. The copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer may be statistical or gradient.
The vinylaromatic monomer constituting the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer may or may not be the same as the vinylaromatic monomer of the polyvinylaromatic block. The vinylaromatic monomer constituting the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer is preferably styrene. The 1,3-diene constituting the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer is preferably 1,3-butadiene.
The copolymer comprising monomeric units of a 1,3-diene and a vinylaromatic monomer preferably contains monomeric units of a vinylaromatic monomer in which the aryl group, preferably phenyl, is substituted by one or more sulfonic acid groups. Preferably, the 1,3-diene and the vinylaromatic monomer of the copolymer comprising monomeric units of a 1,3-diene and a vinylaromatic monomer are 1,3-butadiene and styrene, respectively. The copolymer comprising monomeric units of a 1,3-diene and a vinylaromatic monomer is advantageously a copolymer of a 1,3-diene and a vinylaromatic monomer, in which some or all of the vinylaromatic monomer units have the aryl group, preferably phenyl, substituted by one or more sulfonic acid groups.

The molar content of vinylaromatic monomer units and of vinylaromatic monomer units substituted by a sulfonic acid function in the hydrogenated block of a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer varies preferentially in a range of 1% to 25%, more preferentially from 5% to 20% of the repeating motifs constituting the hydrogenated block.

According to any one of the embodiments of the invention, the block represented by the symbol B is preferably a hydrogenated block of a homopolymer of 1,3-butadiene or a hydrogenated block of a copolymer of 1,3-butadiene and styrene, a copolymer in which all or part of the monomer units of styrene have the phenyl group substituted by one or more sulfonic acid functions.

In the block polymer ionomer of formula (I), the block represented by the symbol B preferentially represents 50% to 90% by mass of the total mass of the block polymer ionomer of formula (I), more preferably 60% to 80% by mass of the total mass of the block polymer ionomer of formula (I).

The concentration of sulfonic acid functional groups in the ionomer, a block polymer of formula (I), can vary considerably and is adjusted by those skilled in the art according to the desired performance of the proton exchange membrane, particularly based on the desired trade-off between water uptake and ionic conductivity. It is known that increasing the concentration of sulfonic acid functional groups in an ionomer is beneficial for the ionic conductivity of the membrane containing the ionomer, but also promotes water uptake. Preferably, the concentration of sulfonic acid functional groups in the ionomer is greater than or equal to 1.0 meq/g of ionomer (meq/g, milliequivalents per gram) and less than or equal to 3.0 meq/g of ionomer, the ionomer being the block polymer of formula (I).

The average molar mass of the block polymer ionomer of formula (I) can vary widely and is chosen by those skilled in the art according to the desired performance of the proton exchange membrane, particularly based on the desired compromise between mechanical properties and its ability to be formed into a film. Preferably, the number-average molar mass of the block polymer ionomer of formula (I) is greater than 20,000 g/mol and less than 500,000 g/mol in polystyrene equivalent (values determined by steric chromatography (SEC) coupled with a refractometer).

The average molar mass of the block represented by the symbol A can vary widely and is chosen by those skilled in the art according to the desired performance of the proton exchange membrane, particularly the desired compromise between the membrane's mechanical and electrical resistance properties. It is known that increasing its average molar mass is beneficial for the membrane's mechanical properties, while decreasing it tends to lower the membrane's electrical resistance. Preferably, the number-average molar mass of the block represented by the symbol A is greater than 1000 g/mol and less than 100,000 g/mol in polystyrene equivalent (values determined by steric chromatography (SEC) coupled with a refractometer).

The number-average molar masses of the block polymer ionomer of formula (I) are the number-average molar masses of the block polymer before the introduction of sulfonic acid groups into the block polymer. Similarly, the number-average molar masses of the block represented by the symbol A are the number-average molar masses of the block represented by the symbol A before the introduction of sulfonic acid groups into the block polymer.

The block polymer ionomer of formula (I) can be prepared from a starting block polymer. The number-average molar mass of the ionomer can be determined from the number-average molar mass of this starting block polymer, as previously mentioned. To form the ionomer, the starting block polymer is modified by a selective hydrogenation reaction of the 1,3-diene monomer units of the starting block polymer, followed by a selective sulfonation reaction of the vinylaromatic monomer units. The starting block polymer differs from the block polymer of formula (I) in that it does not contain sulfonic acid or sulfonate groups and the 1,3-diene monomer units are not reduced. Typically, the block polymer is a thermoplastic elastomer, preferably a triblock. According to any one of the embodiments of the invention, the starting block polymer is preferably a linear polymer.

The starting block polymer can be a commercial product, available for example from Kraton or Kuraray, or can be prepared according to any of the known synthesis processes described below:
- a) anionic polymerization of 1,3-diene or its copolymerization with the second vinylaromatic initiated by a dilithiated compound such as the diisopropenylbenzene and sec-butyllithium adduct described for example in document EP1237941 or 1,1,4,4-tetraphenyl-1,4-dilithiobutane, followed by the polymerization of the first vinylaromatic to form a triblock polymer, the synthesis of block polymers containing more than 3 blocks being able to be carried out by successively continuing the polymerization of 1,3-diene and where appropriate of the second vinylaromatic and that of the first vinylaromatic;
- b) anionic polymerization of the first vinylaromatic initiated by a monolithic compound such as butyllithium, followed by the polymerization of 1,3-diene or its copolymerization with the second vinylaromatic, then reaction with a coupling agent, for example a dichlorosilane to form a triblock, the synthesis of block polymers containing more than 3 blocks being able to be carried out by successively carrying out the polymerization of the first vinylaromatic and that of 1,3-diene and where appropriate of the second vinylaromatic before the coupling reaction.

A person skilled in the art understands that the methods of preparing the starting block polymer can lead to mixtures containing the starting block polymer. They understand that the method of preparing the starting block polymer according to process b) can lead to the formation of a mixture containing the starting block polymer, a polyvinylaromatic compound, and a diblock formed in the third, first, and second steps described in the process, respectively. Similarly, they understand that process a) can lead to a mixture containing the starting block polymer and a homopolymer or copolymer of a 1,3-diene formed in the first step.

For the sulfonation reaction, one can, for example, refer to documents US 5239010, EP 1986257 and WO 2019010290 which describe the sulfonation of a hydrogenated block polymer; for the hydrogenation reaction, one can, for example, refer to document WO 03008467 which describes the selective hydrogenation of a block polymer.

Alternatively, the ionomer, a block polymer of formula (I), can be prepared by sulfonation of a commercial product, available for example from the Kraton company, for example under the name "Kraton G", or from the Kuraray company, for example under the trade name "SEPTON", which commercial product is an already hydrogenated block polymer which differs from the ionomer in that it does not carry sulfonic acid or sulfonate functions.

When one of these methods of preparing the starting block polymer results in a mixture containing the starting block polymer, the hydrogenation and sulfonation reactions are generally carried out on the mixture containing the starting block polymer. In cases where the hydrogenation and sulfonation reactions are performed on the mixture containing the starting block polymer, the ionomer is obtained as a mixture and is generally used without further purification.

The ionomer, before its electron beam crosslinking, is in film form. Prior to electron beam crosslinking, the ionomer is formed into a film. The ionomer can be formed into a film by depositing a solution or dispersion containing the ionomer onto the surface of a substrate to cover it, followed by drying, a step involving the evaporation of the solvent in the solution or dispersion.
Typically, a solution or dispersion containing the ionomer is prepared that is suitable for spreading on a flat surface of a support to form a layer of the ionomer by coating the support.
The coating temperature is chosen by a person skilled in the art, taking into account factors such as the viscosity of the solution or dispersion containing the ionomer, and the boiling point of the solvent in the solution or dispersion containing the ionomer. Coating is preferably carried out at a temperature close to ambient temperature, typically 20°C to 25°C, or at a temperature above ambient temperature but below the boiling point of the solvent in the solution or dispersion containing the ionomer.
The concentration of the solution or dispersion containing the ionomer is adjusted by a person skilled in the art, taking into account the solubility of the ionomer in the solvent and the viscosity of the solution or dispersion. The solvent is selected by a person skilled in the art, taking into account the ionomer's solubility in that solvent and its boiling point. The concentration of the solution or dispersion typically ranges from 1 to 15% by mass of solids.
The solvent for the solution or dispersion containing the ionomer preferably has a relatively low boiling point, typically less than or equal to 100°C, so that it can be easily removed from the layer or assembly, particularly by evaporation under vacuum, air currents, or an inert gas such as nitrogen or argon. Suitable solvents include, for example, ethers, mixtures of ethers and alcohols, and halogenated solvents. The solvent is preferably chosen from tetrahydrofuran, mixtures of tetrahydrofuran and an alcohol, or chloroform, with ethanol being the preferred alcohol.
Drying to remove the solvent from the solution or dispersion containing the ionomer is generally carried out under vacuum, with a stream of air, or with an inert gas such as nitrogen or argon, preferably at a temperature ranging from ambient temperature (23°C) to the boiling point of the solvent, preferably lowered by 15°C under a stream of air or an inert gas. The thickness of the layer spread after drying is preferably less than or equal to 110 µm, more preferably less than or equal to 90 µm. It is preferably greater than or equal to 50 µm.

The ionomer is recovered either as a laminate formed by the support and the film, or it is peeled off from its support. The ionomer is then exposed to electron beam bombardment, which crosslinks the ionomer.

During electron bombardment, the crosslinking of the ionomer is caused by the radiochemical effect of the irradiation. Electron bombardment is preferably carried out in the absence of a crosslinking agent (chemical curing agent). The electron bombardment dose can vary widely and is adjusted by a person skilled in the art according to the film thickness and the desired compromise between membrane properties such as lifetime, water absorption, and ionic conductivity. Preferably, the electron bombardment dose is greater than 50 kGy and less than 5000 kGy. More preferably, the electron bombardment dose is greater than 100 kGy and less than 3000 kGy, preferably less than 2000 kGy. Electron bombardment can be carried out under air or under an inert atmosphere, generally nitrogen, preferably under an inert atmosphere, more preferably under nitrogen. Doses can be delivered in a single irradiation sequence or in several consecutive irradiation sequences, depending on the power of the electron bombardment device used. Electron bombardment results in a cross-linked ionomer in the form of a film.

The crosslinked ionomer, another object of the invention, is a block polymer crosslinked by electron bombardment, the block polymer being any one of the block polymers of formula (I) defined in the process relating to the preparation of a proton exchange polymer membrane according to the invention. The crosslinked ionomer constitutes all or part, preferably all, of a membrane intended for use in a fuel cell or electrolyzer. The electron bombardment crosslinked ionomer confers to the membrane containing it, which conforms to the invention, improved properties with respect to water uptake and lifespan while preserving ionic conductivity properties.

The membrane according to the invention and capable of being obtained by the process according to the invention is a proton exchange polymer membrane containing the crosslinked ionomer.

In summary, the invention is advantageously implemented according to any one of the following embodiments 1 to 36:

Mode 1: A process for preparing a proton exchange polymer membrane containing a crosslinked ionomer, which process comprises electron bombardment of an ionomer in the form of a film, the ionomer being a block polymer of formula (I)
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1.

Mode 2: Process according to mode 1 in which the block polymer of formula (I) is linear.

Mode 3: Process according to mode 1 or 2 in which the block polymer of formula (I) is a triblock.

Mode 4: Process according to any one of modes 1 to 3 in which the 1,3-diene is 1,3-butadiene, isoprene or a mixture thereof.

Mode 5: Process according to any one of modes 1 to 4 in which the polyvinylaromatic block is a polystyrene block, a polyalphamethylstyrene block or a styrene-alphamethylstyrene copolymer block.

Mode 6: Process according to any one of modes 1 to 5 in which poly(1,3-diene) is a homopolymer of 1,3-butadiene.

Mode 7: A process according to any one of modes 1 to 6 in which the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer is a copolymer of a 1,3-diene and a vinylaromatic monomer, copolymer in which some or all of the vinylaromatic monomer units have the aryl group substituted by one or more sulfonic acid functions.

Mode 8: Process according to mode 7 in which the aryl group is the phenyl group.

Mode 9: A process according to any one of modes 1 to 8 in which the molar content of vinylaromatic monomer units and of vinylaromatic monomer units substituted by a sulfonic acid function in the hydrogenated block of a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer varies in the range of 1% to 25%, preferably 5% to 20% of the repeating motifs constituting the hydrogenated block.

Mode 10: A process according to any one of modes 1 to 9 in which the 1,3-diene and the vinylaromatic monomer of the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer are respectively 1,3-butadiene and styrene.

Mode 11: Process according to any one of modes 1 to 10 in which the block represented by the symbol B represents from 50% to 90% by mass of the total mass of the block polymer of formula (I), preferably from 60% to 80% by mass of the total mass of the block polymer of formula (I).

Mode 12: A process according to any one of modes 1 to 11 in which the block represented by the symbol B is a hydrogenated block of a homopolymer of 1,3-butadiene or a hydrogenated block of a copolymer of 1,3-butadiene and styrene, a copolymer in which all or part of the monomer units of styrene have the phenyl group substituted by one or more sulfonic acid functions.

Mode 13: A process according to any one of modes 1 to 12 in which the level of sulfonic acid functions in the block polymer of formula (I) is greater than or equal to greater than or equal to 1.0 meq/g of block polymer of formula (I) and less than or equal to 3.0 meq/g of block polymer of formula (I).

Mode 14: A process according to any one of modes 1 to 13 in which the electronic bombardment is carried out in the absence of a crosslinking agent.

Mode 15: A method according to any one of modes 1 to 14 in which the electronic bombardment dose is greater than 50 kGy and less than 5000 kGy.

Mode 16: A method according to any one of modes 1 to 15 in which the electronic bombardment dose is greater than 100 kGy and less than 3000 kGy.

Mode 17: A method according to mode 16 in which the electronic bombardment dose is less than 2000 kGy.

Mode 18: Proton exchange polymer membrane containing a crosslinked ionomer, the crosslinked ionomer being a block polymer of formula (I) crosslinked by electron bombardment
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1.

Mode 19: Membrane according to mode 18 in which the block polymer of formula (I) is linear.

Mode 20: Membrane according to mode 18 or 19 in which the block polymer of formula (I) is a triblock.

Mode 21: Membrane according to any one of modes 18 to 20 in which the 1,3-diene is 1,3-butadiene, isoprene or a mixture thereof.

Mode 22: Membrane according to any one of modes 18 to 21 in which the polyvinylaromatic block is a polystyrene block, a polyalphamethylstyrene block or a styrene-alphamethylstyrene copolymer block.

Mode 23: Membrane according to any one of modes 18 to 22 in which poly(1,3-diene) is a homopolymer of 1,3-butadiene.

Mode 24: Membrane according to any one of modes 18 to 23 in which the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer is a copolymer of a 1,3-diene and a vinylaromatic monomer, copolymer in which some or all of the vinylaromatic monomer units have the aryl group substituted by one or more sulfonic acid functions.

Mode 25: Membrane according to any one of modes 18 to 24 in which the aryl group is the phenyl group.

Mode 26: Membrane according to any one of modes 18 to 25 in which the molar content of vinylaromatic monomer units and of vinylaromatic monomer units substituted by a sulfonic acid function in the hydrogenated block of a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer varies in the range of 1% to 25%, preferably 5% to 20% of the repeating motifs constituting the hydrogenated block.

Mode 27: Membrane according to any one of modes 18 to 26 in which the 1,3-diene and the vinylaromatic monomer of the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer are respectively 1,3-butadiene and styrene.

Mode 28: Membrane according to any one of modes 18 to 27 in which the block represented by the symbol B represents from 50% to 90% by mass of the total mass of the block polymer of formula (I), preferably from 60% to 80% by mass of the total mass of the block polymer of formula (I).

Mode 29: Membrane according to any one of modes 18 to 28 in which the block represented by the symbol B is a hydrogenated block of a homopolymer of 1,3-butadiene or a hydrogenated block of a copolymer of 1,3-butadiene and styrene, copolymer in which all or part of the monomer units of styrene have the phenyl group substituted by one or more sulfonic acid functions.

Mode 30: Membrane according to any one of modes 18 to 29 in which the rate of sulfonic acid functions in the block polymer of formula (I) is greater than or equal to greater than or equal to 1.0 meq/g of block polymer of formula (I) and less than or equal to 3.0 meq/g of block polymer of formula (I).

Mode 31: Membrane according to any of the modes 18 to 30 in which the electron bombardment is carried out in the absence of a crosslinking agent.

Mode 32: Membrane according to any of the modes 18 to 31 in which the electron bombardment dose is greater than 50 kGy and less than 5000 kGy.

Mode 33: Membrane according to any of the modes 18 to 32 in which the electron bombardment dose is greater than 100 kGy and less than 3000 kGy.

Mode 34: Membrane according to any of the modes 18 to 33 in which the electron bombardment dose is less than 2000 kGy.

Mode 35: Crosslinked ionomer, block polymer of formula (I) crosslinked by electron bombardment.
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1.

Mode 36: Fuel cell or electrolyzer containing a membrane defined according to any one of modes 18 to 34 or a membrane capable of being obtained by the process defined in any one of modes 1 to 17.

The aforementioned features of the present invention, as well as others, will be better understood upon reading the following description of several examples of embodiments of the invention, given by way of illustration.

Examples

Molar composition of polymers:
It is determined by nuclear magnetic resonance (NMR) analysis.

Ion exchange capacity (IEC):
It is calculated from proton NMR analysis data by identifying sulfonated styrene motifs in the polymer, which has been previously solubilized in deuterated tetrahydrofuran. Indeed, as the sulfonation reaction progresses, the resulting proton spectrum exhibits a characteristic signal of protons in the α position of the _-SO3H group, which increases while the signal attributed to protons in the unsulfonated styrene block decreases. The proton allocation in the sulfonated polymer, as illustrated in the formula representing a sulfonated SEBS (where m, n, o, and p represent the number of each of the constituent motifs of the sulfonated SEBS), is used to quantify the mass percentage of sulfonated styrene motifs in the ionomer. The latter is calculated by integrating the signal from the two protons marked "1" characteristic of the α protons of the -SO ₃ H function. The IEC is then determined by dividing the mass percentage of sulfonated styrenes by the molar mass of a sulfonated styrene motif (184 g/mol).

Membrane lifespan:
It is determined by the Fenton reaction, a method commonly used as an ex-situ technique to study the chemical degradation of membranes. The Fenton reaction-based degradation protocol involves immersing membrane samples in an aqueous solution of hydrogen peroxide containing trace amounts of ferrous ions to initiate radical attacks, and nitric acid. More specifically, the hydrogen peroxide concentration is 3% by volume, the ferrous ion concentration ( ^Fe²⁺) is 5 ppm, and the nitric acid concentration is 5% by volume. The solution is heated to approximately 80°C and changed every 24 hours.

This method provides an initial indication of the lifespan of a membrane, even though the rate of chemical degradation reaction is much higher than that in a fuel cell.

Water absorption of the membranes:
It is determined by the mass difference between the membrane immersed for 24 hours in deionized water (resistivity on the order of ^MΩ.cm⁻¹ ) and the membrane dried for 24 hours at 40°C and 100 mb of pressure. The mass percentage of water absorbed by the membrane (water uptake) is determined using the following calculation:
%WU=(m _submerged -m _dried )/m _dried x 100.

Ionic conductivity of membranes:
It is determined by measuring the electrochemical impedance across the plane of the membrane at 30°C and 30% relative humidity, the measurement parameters being an amplitude variation of 50 mV, an applied potential of 0V.

Preparation of an ionomer of formula (I):

The ionomer is prepared by sulfonation reaction from a commercial product according to the following procedure:

The commercial product, "A1536H" from Kraton, is a SEBS, a hydrogenated block polymer of a triblock whose central block is a copolymer of 1,3-butadiene and styrene, and whose terminal blocks are polystyrenes, with the hydrogenation level of the butadiene units exceeding 99%. Table 1 gives the composition of the commercial product, its macrostructure (number-average molar mass Mn measured by size-exclusion chromatography, PS calibration).

Table 1: Styrene
(% mol) Ethylene
(%mol) Butylene
(%mol) Mn
(g/mol) A1536H 20.3 59.9 19.8 133,821

Preparation of a SEBS polymer solution (TPE/DCM solution) : SEBS A1536H polymer is solubilized in dichloromethane (DCM) in a bottle, the polymer concentration being 5% by weight per unit volume (w/v), i.e. 5 g of polymer in 85 mL of solvent.

Preparation of an acetyl sulfate solution : in another bottle, a solution of acetic anhydride in dichloromethane is made in an inert medium (the volume of DCM is 5 equivalents DCM / acetic anhydride).

The acetic anhydride solution is maintained at 0°C for 10 minutes.
Sulfuric acid is added to the acetic anhydride/DCM solution.
The medium is maintained at 0°C for 10 minutes to form acetyl sulfate (clear solution).
The sulfuric acid/acetic anhydride/DCM solution is poured onto the TPE/DCM solution (pink/purple solution). The bottle is then inert with nitrogen (3 bar).
The resulting TPE/acetyl sulfate/DCM solution is then stirred and maintained at 40°C for 3 hours.
Isopropanol is added to the reaction medium to stop the reaction (1.1 equivalent / sulfuric acid).
The resulting solution is coagulated in distilled water (1 volume of water / total volume of solution).
The coagulum is then washed with distilled water until a rinsing water of pH 6/7 (neutral) is obtained.
The recovered product is dried in an oven at 30°C under vacuum for 48 hours.

The levels in the ionomer of the motifs ethylene, butylene, styrene and styrene substituted by a sulfonic acid function (designated as "sulfonated styrene" in Table 2), are measured by NMR analysis of the ionomer and are shown in Table 2, along with the ion exchange capacity (IEC) expressed in moles per kilogram of ionomer.

Table 2: Sulfonated styrene
(%mol) Styrene
(%mol) Ethylene
(%mol) Butylene
(%mol) IEC
(mol/kg) Ionomer 11.8 8.5 59.9 19.8 1.67

Formation of films containing the ionomer by coating:

Six films F1 to F6 are prepared according to the following procedure:
a) A solution containing the ionomer is prepared in a THF/ethanol solvent mixture, the proportions of which are indicated in Table 3.
b) The solution is poured onto a flat polytetrafluoroethylene (PTFE) support framed by two shims 1200 µm high to form a 1200 µm thick layer of solution by passing a scraper,
c) The solvent is evaporated at room temperature for 120 minutes under air sweep to dry the layer
d) We retrieve the film from its medium.

The films have a thickness of 90 to 110 µm.

According to the same procedure, but replacing the ionomer of formula (I) with a commercial ionomer, "PEMION" from Ionomr Innovations Inc., and the cutting of THF/ethanol solvents with methanol, 5 films, F7 to F11, are also prepared. The "PEMION" of formula (II) has as its basic skeleton a polyphenylene substituted by phenyls substituted by sulfonic acid functions and is not a block copolymer of formula (I) useful for the purposes of the invention.

For each of the films formed, Table 3 indicates the solvent used and the mass concentration of ionomer in the solution.

Table 3: Movie Solvents Ionomer mass concentration F1-F6 THF/ethanol – 8/2 10% F7-F11 methanol 10%

Electronic bombardment of films:

With the exception of films F1 and F7, the films on their mounts are subjected to electron bombardment (using the EBLab200 device from manufacturer Skan Stein AG). For each film, the absorbed radiation doses are listed in Table 4. Doses are expressed in kilogray (kGy). Since the device can deliver a maximum of 300 kGy per pass, for doses exceeding 300 kGy, the films are exposed in multiple passes.
1 dose of 300 kGy and a dose of 200 kGy for dose 2;
2 doses of 300 kGy and one dose of 100 kGy for dose 3;
3 doses of 300 kGy for dose 4;
4 doses of 300 kGy and one dose of 200 kGy for dose 5.

Table 4: Dose kGy Ionomer of formula (I) PEMION Dose 1 150 F2 F8 Dose 2 500 F3 9 Dose 3 700 F4 - Dose 4 900 F5 F10 Dose 5 1400 F6 F11

Films F2 to F6 and F8 to F11, as well as films F1 and F6, were subjected to the Fenton reaction to evaluate their lifespan as proton exchange membranes. Their water absorption and proton conductivity were also measured. The results are shown in Table 5.

Table 5: Movie Dose
(kGy) Lifetime
(mn) Water intake
(% mass) Ionic Conductivity
(mS/cm) F1 0.0 75 196 31.0 F2 150.0 120 125 28.0 F3 500.0 150 107 23.0 F4 700.0 180 112 19.0 F5 900.0 210 81 15.0 F6 1400.0 240 81 12.0 F7 0.0 180 378 30.0 F8 150.0 270 354 38.0 9 500.0 210 318 25.0 F10 900.0 180 334 18.0 F11 1400.0 240 333 15.0

Films F2 to F6 are membranes according to the invention and are prepared according to a process according to the invention. Films F8 to F11 are not proton exchange membranes according to the invention, nor are they prepared according to a process according to the invention, as the ionomer constituting the membrane is not of formula (I). Film F1 is not a membrane according to the invention because film F1 has not been exposed to electron bombardment. Film F7 is also not a membrane according to the invention for the following two reasons: the ionomer is not of formula (I) and the film has not been exposed to electron bombardment.

Membranes F2 to F6 exhibit a significantly improved lifespan compared to membrane F1, as well as considerably lower water absorption. This result is achieved while maintaining ionic conductivity properties. As for membranes F8 to F11, neither a significant improvement in lifespan nor a reduction in water absorption is observed compared to membrane F7.

Claims (15)

A process for preparing a proton exchange polymer membrane containing a crosslinked ionomer, which process comprises electron bombardment of an ionomer in the form of a film, the ionomer being a block polymer of formula (I)
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1. Method according to claim 1 wherein the block polymer of formula (I) is linear. A process according to claim 1 or 2 wherein the block polymer of formula (I) is a triblock. A method according to any one of claims 1 to 3 wherein the polyvinylaromatic block is a polystyrene block, a polyalphamethylstyrene block or a styrene-alphamethylstyrene copolymer block. A process according to any one of claims 1 to 4 wherein poly(1,3-diene) is a homopolymer of 1,3-butadiene. A process according to any one of claims 1 to 5 wherein the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer is a copolymer of a 1,3-diene and a vinylaromatic monomer, copolymer wherein some or all of the vinylaromatic monomer units have the aryl group substituted by one or more sulfonic acid functions. A process according to any one of claims 1 to 6 wherein the molar content of vinylaromatic monomer units and of vinylaromatic monomer units substituted by a sulfonic acid function in the hydrogenated block of a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer varies in the range of 1% to 25%, preferably from 5% to 20% of the repeating motifs constituting the hydrogenated block. A process according to any one of claims 1 to 7 wherein the 1,3-diene and the vinylaromatic monomer of the copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer are respectively 1,3-butadiene and styrene. A method according to any one of claims 1 to 8 wherein the block represented by the symbol B represents from 50% to 90% by mass of the total mass of the block polymer of formula (I), preferably from 60% to 80% by mass of the total mass of the block polymer of formula (I). A process according to any one of claims 1 to 9 wherein the level of sulfonic acid functions in the block polymer of formula (I) is greater than or equal to greater than or equal to 1.0 meq/g of block polymer of formula (I) and less than or equal to 3.0 meq/g of block polymer of formula (I). A method according to any one of claims 1 to 10 in which the electronic bombardment is carried out in the absence of a crosslinking agent. Proton exchange polymer membrane containing a crosslinked ionomer, the crosslinked ionomer being a block polymer of formula (I) crosslinked by electron bombardment
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1
which membrane is capable of being obtained by the process defined in any one of claims 1 to 11. Proton exchange polymer membrane in which the block polymer of formula (I) is defined in any one of claims 2 to 10. Crosslinked ionomer, block polymer of formula (I) crosslinked by electron bombardment
(AB) _n -A (I),
the symbol A representing a polyvinylaromatic block bearing sulfonic acid functions,
the symbol B representing a hydrogenated block of a poly(1,3-diene) or a copolymer comprising monomer units of a 1,3-diene and a vinylaromatic monomer,
n being an integer equal to or greater than 1. Fuel cell or electrolyzer containing a membrane defined in claim 12 or 13 or a membrane that can be obtained by the process defined in any one of claims 1 to 11.