This invention relates to bipolar membranes and to their preparation and use. Ion exchange membranes of various types are known, including cation exchange membranes, anion exchange and bipolar membranes. Cation exchange membranes and anion exchange are generally categorized as such depending on their predominant charge. Cation exchange membranes comprise negatively charged groups that allow the passage of cations but reject anions, while anion exchange membranes comprise positively charged groups that allow the passage of anions but reject cations. A bipolar membrane (BPM) has both a cationic layer (sometimes called an anion exchange layer or “AEL”) and an anionic layer (sometimes called a cation exchange layer or “CEL”). Some BPMs further comprise an interface between the AEL and the CEL. The interface is typically a distinct layer located between the AEL and the CEL comprising a porous polymer comprising ionic groups and, located in the pores, another polymer having ionic groups of polarity opposite to the ionic groups of the porous polymer.

Some ion exchange membranes and BPMs comprise a porous support, which provides mechanical strength. Such membranes are often called “composite membranes” due to the presence of both an ionically-charged polymer which discriminates between oppositely charged ions and the porous support which provides mechanical strength.

Composite membranes are known from, for example, US4,253,900, which describes a BPM containing a monobead layer of ion exchange resin. WO2017/205458 and the article by McClure in ECS Transactions, 2015 69 (18) pages 35-44 describe a BPM containing a junction layer of interpenetrating polymer nanofibers or microfibers of anion exchange polymers and cation exchange polymers.

According to a first aspect of the present invention there is provided a bipolar membrane comprising: a) a first layer comprising a first polymer having cationic groups; b) a second layer comprising a second polymer having anionic groups; and c) a third layer comprising a co-continuous polymeric network of (i) a third polymer having anionic groups; and (ii) a fourth polymer having cationic groups; wherein layer c) is located between layer a) and layer b); and wherein the molar ratio of sulphur/chlorine in the third layer c) is at least 4.

In this document (including its claims), the verb "comprise" and its conjugations is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. In addition, reference to an element by the indefinite article "a" or "an" does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article "a" or "an" thus usually mean "at least one". Also in this specification the first polymer having cationic groups is sometimes abbreviated to “the first polymer”, the second polymer having anionic groups is sometimes abbreviated to “the second polymer”, the third polymer having anionic groups is sometimes abbreviated to “the third polymer”, and the fourth polymer having cationic groups is sometimes abbreviated to “the fourth polymer”. Weight % is often abbreviated herein to “wt%”. In one embodiment the fourth polymer is obtained from a curable composition which is identical to the curable composition used to prepare the first polymer. In this way one may obtain a membrane in which the first polymer has an identical chemical composition to the fourth polymer. In other words, the fourth polymer is then present in the network of pores of the third polymer and in the first layer a).

Preferably the co-continuous polymeric network comprises the third polymer and the fourth polymer, wherein the third polymer provides a network of pores and the fourth polymer is present within (e.g. filling) that network of pores.

Preferably the third polymer comprises a porous first polymeric domain comprising anionic groups and a network of pores. Preferably the fourth polymer comprises a second polymeric domain comprising cationic groups. In this embodiment the second polymeric domain is located in the pores of the first polymeric domain (i.e. in the network of pores of the third polymer).

The co-continuous polymeric network preferably comprises two individual, continuous, polymeric domains, one bearing anionic charges and the other bearing cationic charges. Preferably the third polymer provides a polymeric domain obtained by phase separation of the third polymer from a composition used to prepare the third polymer. Phase separation is particularly useful for providing the third polymer in porous form. The fourth polymer may then fill the pores of the third polymer. Preferably the third and fourth polymers of the third layer (c) are non-mixed, non-encapsulated and preferably they are non-fibrillar. Optionally the third layer contains one or more than one further polymeric domains, each of which bearing an anionic charge or a cationic charge.

The third layer c) preferably comprises at least two continuous intermingled polymeric domains (one domain derived from the third polymer and the other domain derived from the fourth polymer) having a large contact area with each other. This may be achieved by the third polymer comprising a network of pores and the fourth polymer being different to the third polymer and being present within the network of pores of the third polymer. As a result of this large contact area between the two (or more) polymers present in the third layer, when the BPM is used, the amount of water molecules that are dissociated into H ⁺ and OH- per unit of time is increased and thereby the productivity of the BPM is also increased.

The large contact area between the third and fourth polymers present in the third layer is preferably provided by the co-continuous network wherein the two (or more) polymeric domains are derived from the third and fourth polymers (which bear opposite charges). An advantage of the co- continuous network is that newly produced anions (e.g. OH ) and cations (e.g. H ⁺ ) created at the interface between the third and fourth polymers (i.e. the interface of the two polymeric domains) are separated into the individual polymeric domains immediately after their formation, preventing ion recombination. In addition, the adhesion between the third and fourth polymers (i.e. adhesion between the first and second polymeric domains) in the third layer c) is extremely strong as a result of the entanglement of the third and fourth polymers, and the large contact area between the third and fourth polymers. The strong adhesion between the third and fourth polymers prevents/reduces the so-called ballooning effect in which large water-filled blisters can be formed at the interface between positively and the negatively charged polymers of a bipolar membrane, where OH- and H ⁺ might recombine (undesirably) to form water.

In one embodiment the third layer c) comprises a blend morphology of two continuous polymeric domains derived from the third and fourth polymers respectively, of which one domain (derived from the fourth polymer) is located within the other domain (derived from the third polymer), forming the abovementioned co-continuous polymeric network (of the fourth polymer within the network of pores of the third polymer).

Preferably each of the first and second polymeric domains is continuous, and substantially comprises a single covalently linked carbon backbone such that it is interconnected to itself.

Preferably the polymeric domains are not encapsulated, not isolated, not discontinued and are non-fibrillar (e.g. not made by electrospinning).

The BPM of the present invention preferably comprises an interface between the first layer a) and the third layer c) (a first interface) and an interface between the third layer c) and the second layer b) (a second interface) and preferably both the first interface and the second interface are uninterrupted, without any gaps and/or spaces between the first layer a) and the third layer c) and without any gaps and/or spaces between the third layer c) and the second layer b).

In the present invention the third layer c) preferably comprises a porous support. E.g. the third polymer is porous and is present within the porous structure of a porous support. The fourth polymer is then preferably located within the pores of the third polymer (thereby providing the co-continuous polymeric network, e.g. two polymeric domains of which one bears anionic charges and the other cationic charges). The two (or more) polymeric domains (one from the third polymer and another from the fourth polymer present within the network of pores of the third polymer) occupy the pores of the porous support and preferably comprise a seamless (third) interface. Thus the BPM preferably comprises a first interface at the junction of the third layer c) and the first layer a), a second interface at the junction of the third layer c) and the second layer b), and a third interface within the third layer c) at the junction of the third polymer and the fourth polymer. Preferably this third interface is uninterrupted, without any gaps and/or spaces between the third polymer and the fourth polymer. Preferably this third interface is not an interface between a polymer and fused/compressed fibers, beads, or particles.

In a preferred embodiment the third polymer is obtainable by phase-separation of the third polymer from a composition used to prepare the third polymer. In this way one may obtain the third polymer in a form which comprises a network of pores and the pores may be used to receive the fourth polymer (or a curable composition used to prepare the fourth polymer) in order to make the third layer c) and provide a co-continuous polymeric network of (i) the third polymer having anionic groups; and (ii) the fourth polymer having cationic groups and being present within the network of pores of the third polymer.

In one embodiment the third polymer is obtained by a process comprising polymerisation- induced phase separation, more preferably photopolymerization-induced phase separation, e.g. of the third polymer from a composition used to prepare that polymer. In this process, preferably the third polymer is formed by a (photo-)polymerization reaction. Preferably, the third polymer comprises a network of pores which has an average pore diameter of less than 5 μm, more preferably less than 2 μm, especially less than 1 μm. The pores within the third polymer may then be filled with a (fourth) curable composition and that curable composition may then be cured in order to provide the fourth polymer within the third polymer’s network of pores. In a preferred embodiment the third polymer’s network of pores is substantially or completely filled with the fourth polymer. As a consequence, the third layer c) results in which the third polymer comprises a network of pores which are filled with the (oppositely charged) fourth polymer. The third and fourth polymers may therefore provide a co-continuous polymeric network comprising two polymeric domains: one from the third polymer and another from the fourth polymer. In a preferred embodiment this co-continuous polymeric network is free from other polymers (except for any polymer present in the porous support). In one embodiment there are covalent bonds connecting the third and fourth polymers together. In fact the pores present in the third polymer may comprise more than one polymer, e.g. the fourth polymer (derived from a fourth curable composition) and optionally a second polymer (derived from a second curable composition) such that the fourth polymer partly fills the pores of the third polymer and the second polymer is filling the remaining pores. Additionally the pores in the third polymer may comprise one or more further polymers if desired.

The BPMs of the present invention preferably comprise a catalyst. The catalyst may be present in one or more of layers a), b) and c) and is preferably present in layer c) as this can enhance a watersplitting reaction. The catalyst preferably comprises a multivalent metal, usually in the form of a hydroxide or an oxide, in which the metal centre is typically cationic.

In the present invention it is preferred that the third polymer is a porous polymer comprising an anionic charge. The anionic charge in the third polymer improves the stability of the catalyst and reduces leaching-out of the catalyst from the third polymer when the BPM is used. By selecting an appropriate curable composition for preparing the third polymer a high ion exchange capacity can be obtained and hence a high catalyst load. Furthermore, a small average pore size forthe third polymer can be obtained, resulting in a high effective surface area between the third polymer and the fourth polymer (the fourth polymer being located in the pores of the third polymer).

Examples of suitable catalysts include metal salts, metal oxides, organometallic compounds, monomers, polymers or co-polymers. Examples include, but are not limited to, FeCI ₃ , FeCI ₂ , AICI ₃ , MgCI ₂ , RUCI ₃ , CrCI _3j Fe(OH) ₃ , Sn(OH) ₂ , Sn(OH) ₄ , SnCI ₂ , SnCI ₄ , SnO, SnO ₂ , AI ₂ O ₃ , NiO, Zr(HPO ₄ ) ₂ , MOS ₂ , graphene oxide, Fe-polyvinyl alcohol complexes, polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyethyleneimine (PEI), polyacrylic acid (PAA), co-polymer of acrylic acid and maleic anhydride (PAAMA) and hyperbranched aliphatic polyester. Any of these catalysts may be present in a range up to 5wt%, e.g. 0.001 to 5wt% or 1 wt% to 5wt%, of the weight of the membrane.

The BPM preferably comprises one, two or three porous supports, more preferably two or three porous supports. In a preferred embodiment, at least the third layer comprises a porous support (to enhance mechanical stability). When the third layer comprises a porous support typically the fibers of the porous support are fully or substantially covered by the third polymer, in the present invention an anionic polymer. This can be verified in a cross-section of the BPM by EDX mapping of fibers of the porous support that are perpendicular to the probe. The first polymer is preferably obtainable by curing a first curable composition and the second polymer is preferably obtainable by curing a second curable composition. The third polymer is preferably obtainable by curing a third curable composition and the fourth polymer is preferably obtainable by curing a fourth curable composition. In order to provide the anionic groups in the third polymer, the third curable composition comprises a curable monomer comprising anionic groups. In order to provide the cationic groups in the fourth polymer, the fourth curable composition comprises a curable monomer comprising cationic groups There is a desire to provide BPMs having improved properties, e.g. high permselectivity, low electrical resistance, good mechanical strength and stability at extremes of pH. Ideally such BPMs may be produced quickly, efficiently, and cheaply. Preferably, the third polymer is obtainable by a process comprising curing a composition (the third curable composition) comprising a compound of Formula (I): Formula (I) wherein: X is of the formula –OCnH2n+1 or –OCqH2q-1, wherein n has a value of 1 to 6 and q has a value of 5 or 6; and m has a value of 1 or 2. The -SO ₂ X group shown in Formula (I) is convertible to an anionic group and thus is useful for providing the third polymer with anionic groups. The -SO ₂ X group in Formula (I) (wherein X is as hereinbefore defined) is advantageous over analogous compounds wherein X is Cl or OSO2R (wherein R is C1-6- alkyl or C6-12- aryl) due to their lower reactivity with nucleophiles. As a consequence, the compounds of Formula (I) wherein X is as hereinbefore defined have better stability than corresponding compounds wherein X is Cl or OSO ₂ R (wherein R is C1-6- alkyl or C6-12- aryl). Compounds of Formula (I) are known and many such compounds are available commercially. In a preferred embodiment, m has a value of 1 or 2 and n has a value of 2 (i.e. X is ethyloxy) or q has a value of 6 (i.e. X is cyclohexyloxy). The compound of Formula (I) is preferably highly miscible with apolar compounds, for example non-charged aromatic molecules, e.g. with divinylbenzene. Examples of compounds of Formula (I) include the following: wherein X is as hereinbefore defined, for example methoxy, ethoxy, propoxy, tert-butoxy or cyclohexyloxy.

The third curable composition preferably comprises the following ingredients:

(a) a compound of Formula (I) (as hereinbefore defined);

(b) a non-charged compound comprising at least two ethylenically unsaturated groups or thiol groups;

(c) optionally a solvent

(d) optionally a radical initiator;

Preferably the third curable composition comprises: i) 20 to 75wt% of component (a); ii) 0 to 50wt% of component (b); iii) 10 to 85% of component (c); and iv) 0 to 5wt% of component (d).

Preferably the third curable composition comprises 20 to 70wt%, more preferably 30 to 70wt%, e.g. 35 to 60wt% of component (a).

Component (b) of the third curable composition typically functions as a crosslinking agent and can provide the third polymer with a desirably high crosslinking density, especially when component (a) comprises a single ethylenically unsaturated group. A high crosslinking density is preferred to limit swelling of the BPM when in aqueous environments.

Preferably component (b) of the third curable composition comprises an aromatic group, e.g. a phenyl or phenylene group (e.g. as is found in styrene). Preferred ethylenically unsaturated groups include vinyl groups (e.g. vinyl ether groups, aromatic vinyl compounds, N-vinyl compounds and allyl groups).

Preferred ethylenically unsaturated groups are free from ester groups because this can improve the stability and the pH tolerance of the resultant composition. Ethylenically unsaturated groups which are free from ester groups include vinyl groups.

As examples of compounds which may be used as component (b) of the third curable composition there may be mentioned the following: The above materials which may be used as component (b) of the third curable composition can be obtained for commercial sources, for example from Sigma-Aldrich.

It is especially preferred that component (b) of the third curable composition is or comprises divinylbenzene because this compound is widely available at low cost (often as a mixture of isomers).

Preferably the third curable composition comprises 0 to 50wt%, more preferably 0 to 40wt%, in certain embodiments preferably 4 to 40wt%, especially 5 to 30% of component (b).

In order to maximize the anionic functionality density of the BPMs, it is preferred that the molar ratio of component (a):(b) of the third curable composition is >1 , e.g. in the range 1 to 200, more preferably 1 .2 to 100, especially 1 .5 to 50 and more especially 1.6 to 5. It is also preferred for the third curable composition to comprise a larger amount (wt%) of component (a) than component (b): wt% (a)/wt% (b) > 1 , for example a weight ratio of (a):(b) of 1 .2 to 10.

Component (c) of the third curable composition is preferably inert, i.e. incapable of reacting with any of the other components of the curable composition.

Suitable solvents include water and non-aqueous solvents. As examples of non-aqueous solvents which may be used as component (c) of the third curable composition there may be mentioned alcohol-based solvents, ether-based solvents, amide-based solvents, ketone-based solvents, sulfoxidebased solvents, sulfone-based solvents, nitrile-based solvents and organic phosphorus-based solvents. Examples of alcohol-based solvents which may be used as or in component (c) of the third curable composition include methanol, ethanol, isopropanol, n-butanol, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol and mixtures comprising two or more thereof. In addition, preferred inert, organic solvents which may be used in component (c) of the third curable composition include dimethyl sulfoxide, dimethyl imidazolidinone, sulfolane, N-methyl pyrrolidone, dimethyl formamide, acetonitrile, acetone, 1 ,4-dioxane, 1 ,3-dioxolane, tetramethyl urea, hexamethyl phosphoramide, hexamethyl phosphorotriamide, pyridine, propionitrile, butanone, cyclohexanone, tetrahydrofuran, tetrahydropyran, 2-methyltetrahydrofuran, ethylene glycol diacetate, cyclopentylmethylether, methylethylketone, ethyl acetate, y-butyrolactone and mixtures comprising two or more thereof.

Preferably the third curable composition comprises 10 to 85wt%, more preferably 20 to 80wt%, especially 21 to 75% of component (c).

Component (d) of the third curable composition preferably is or comprises a thermal initiator, a photo initiator or a combination thereof. Most preferably component (d) of the third curable composition is or comprises a thermal initiator.

Examples of thermal initiators include 2,2’-azobis(2-methylpropionitrile) (AIBN), 4,4’-azobis(4- cyanovaleric acid), 2,2’-azobis(2,4-dimethyl valeronitrile), 2,2’-azobis(2-methylbutyronitrile), 1,1’- azobis(cyclohexane-l-carbonitrile), 2,2’-azobis(4-methoxy-2,4-dimethyl valeronitrile), dimethyl 2,2’- azobis(2-methylpropionate), 2,2’-azobis[N-(2-propenyl)-2-methylpropionamide, 1-[(1-cyano-1- methylethyl)azo]formamide, 2,2'-Azobis(N-butyl-2-methylpropionamide), 2,2'-Azobis(N-cyclohexyl-2- methylpropionamide), 2,2'-Azobis(2-methylpropionamidine) dihydrochloride, 2,2'-Azobis[2-(2- imidazolin-2-yl)propane]dihydrochloride, 2,2'-Azobis[2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-Azobis[N-(2-carboxyethyl)-2-methylpropionamidine] hydrate, 2,2'-Azobis{2-[1-(2-hydroxyethyl)-2- imidazolin-2-yl]propane} dihydrochloride, 2,2'-Azobis[2-(2-imidazolin-2-yl)propane], 2,2'-Azobis(1- imino-1-pyrrolidino-2 -ethylpropane) dihydrochloride, 2,2'-Azobis{2-methyl-N-[1 ,1-bis(hydroxymethyl)-2- hydroxyethl]propionamide} and 2,2'-Azobis[2-methyl-N-(2-hydroxyethyl)propionamide].

Preferably the third curable composition is free from component (d) or comprises 0.001 to 5wt%, more preferably 0.1 to 2wt%, especially 0.2 to 1 % of component (d). When the third curable composition is to be cured by electron beam (EB) or gamma radiation then component (d) is not necessary.

Optionally the third curable composition further comprises a small amount of anionic monomers comprising one and only one ethylenically unsaturated group as component (e). Component (e) can be useful for providing the third polymer with a small degree of hydrophilicity which aids to speed up hydrolysis processes.

Examples of anionic monomers comprising one and only one ethylenically unsaturated group which may be used as component (e) of the third curable composition include sulphonated styrene in free acid or salt form, especially in the form of a lithium salt, a sodium salt or a mixed lithium and sodium salt.

Preferably the third curable composition contains 0 to 15 wt%, more preferably less than 10wt%, of component (e).

The third layer c) preferably comprises a cationically charged catalyst, preferably selected from a multivalent metal ion, a cationic oligomer, a cationic polymer and a combination of two or more thereof, especially a multivalent metal ion.

Preferred metal ions include Fe, Al, Ru, Cr, Ni, Zr, Mo, Cu, Zn, Co, Mg, Mn, Rh, Os, V, Ir, Pd, Ti, Sn and combinations comprising two or more thereof wherein the metal is present as cation. The valency of the metal ion is preferably at least two and may be two, three, four or five. Preferred metal ions include Sn ²⁺ , Sn ⁴⁺ , Ti ⁴⁺ , Zr ⁴ *, Ni ²⁺ , Ni ⁴⁺ , Fe ³⁺ , Al ³⁺ and Zn ²⁺ .

Preferably the BPM comprises at least 0.6 mmoles of metal ions (especially multivalent metal ions) /m ² (and less than 80 mmoles of metal ions/m ² ), more preferably at least 0.8 mmoles of metal ions/m ² (and less than 70 mmoles of metal ions/m ² ). Preferably the metal ions are present as an insoluble salt.

Preferably the BPM is such that after the BPM is left in 4M NaOH solution at 40°C for 3 days, the BPM comprises at least 0.8 mmoles of metal (especially multivalent metal) ions/m ² , more preferably at least 1.0 mmol/m ² , especially at least 1.3 mmol/m ² .

Preferably the effect of leaving the BPM in 4M NaOH solution at 40°C for 3 days is small, i.e. at least 70% of the catalyst remains after this treatment. In this way the ability of the BPM to split water is not significantly reduced as a result of the treatment and indicates that the BPM is robust and has a long lifetime.

In one embodiment the third polymer is fully or partially hydrolysed before and/or after a catalyst is applied thereto. The third polymer may be fully or partially hydrolysed by placing the third polymer in a basic solution at an elevated temperature for a period of time (e.g. up to 24hrs, up to 48 hrs or even up to several days). This is useful for increasing the number of anionic groups present in the third polymer and therefore increasing the number of sites to which a cationic catalyst may bind to ionically. The third polymer preferably has an ion-exchange capacity of 0.5 meq/g to 8.0 meq/g, more preferably 0.5 meq/g to 6.0 meq/g and especially 0.7 meq/g to 4.0 meq/g.

The third polymer is preferably porous, e.g. having an average pore size of 0.3 to 1 ,2μm, as may be determined by a Porolux™ measurement during preparation of the third layer.

The weight ratio of third polymer to fourth polymer in the third layer c) is preferably 0.5 to 0.8, more preferably 0.6 to 0.7.

When the third polymer contains a known amount of sulphur (and the fourth polymer is free from sulphur) and the fourth polymer contains a known amount of chlorine (and the third polymer is free from chlorine) one may determine the weight ratio of the third polymer to fourth polymer in the third layer c) by determining the molar ratio of sulphur to chlorine in the third layer c). The molar ratio of sulphur and chlorine atoms in the third layer c) may be determined as described in the Examples section below. Before the determination, preferably the BPM is allowed to stand in 0.5M NaCI solution for at least 16 hrs at 20°C and is then washed with water before the amounts of sulphur and chlorine are determined as this provides particularly reproducible results.

The molar ratio sulphur/chlorine in the third layer c) is preferably at least 4.5. For good functioning of the BPM in certain applications it is preferred that the molar ratio sulphur/chlorine in the third layer c) is less than 20, more preferably less than 12.

Preferably the third layer c) comprises 1 .6 to 25.6wt% of sulphur, more preferably 1 .6 to 19.2wt% of sulphur and especially 2.2 to 12.8wt% of sulphur, relative to the weight of the third layer c).

Preferably the third layer c) comprises 0.01 to 12.8wt% of chlorine, more preferably 0.01 to 9.6wt% of chlorine and especially 0.02 to 6.4wt% of chlorine, relative to the weight of the third layer c).

The preferred wt% of sulphur and chlorine stated above are based on the dry weight of the third layer c) after equilibration of the BPM in 0.5M NaCI for at least 16 hrs at 20°C (including all components of the third layer, e.g. any porous support, catalyst(s) etc.) and subsequently drying.

One may achieve the required molar ratio of sulphur/chlorine in the third layer c) simply by appropriate selection of the components and their amounts used to make the third and fourth polymer.

In one embodiment, the first curable composition, the second curable composition and/or the fourth curable composition comprises at least 60wt% of curable compounds.

The first curable composition and the fourth curable composition are cationic. Preferred cationic groups are quaternary ammonium groups. The second curable composition is anionic. Examples of curable compounds having an anionic group or a cationic group are provided below.

The curable compositions which may be used to prepare the first, second and fourth polymers preferably further comprise a crosslinking agent, e.g. a curable compound comprising at least two ethylenically unsaturated groups and optionally an ionic group, preferably in an amount of 1 to 88 wt% relative to the total weight of the composition. Examples of curable compounds comprising at least two ethylenically unsaturated groups and optionally an ionic group are provided below.

The curable compositions which may be used to prepare the first, second and fourth polymers preferably further comprise a radical initiator, e.g. 0 to 10 wt% of radical initiator. Examples of suitable radical initiators are provided below. The curable compositions which may be used to prepare the first, second and fourth polymers preferably further comprise a solvent, e.g. 0 to 55wt% of solvent. Examples of suitable solvents are provided below.

Preferably the first polymer is obtainable by a process comprising curing a first curable composition comprising:

(a1) 0 to 60 wt% of a curable compound comprising a cationic group and one (and only one) ethylenically unsaturated group or thiol group;

(b1) 1 to 88 wt% of a curable compound comprising at least two ethylenically unsaturated groups or thiol groups and optionally further comprising a cationic group;

(c1) 0 to 10 wt% of radical initiator; and

(d 1 ) 0 to 55 wt% of solvent.

In a preferred embodiment, the first curable composition and the fourth curable composition are identical. When the first curable composition and the fourth curable composition are identical one may form the first polymer and the fourth polymer in a single step.

Preferably the second polymer is obtainable by a process comprising curing a second curable composition comprising:

(a2) 0 to 60 wt% of a curable compound comprising an anionic group and one (and only one) ethylenically unsaturated group or thiol group;

(b2) 1 to 88 wt% of a curable compound comprising at least two ethylenically unsaturated groups or thiol groups and optionally further comprising an anionic group;

(c2) 0 to 10 wt% of radical initiator; and

(d2) 0 to 55 wt% of solvent.

Preferably the fourth polymer is obtainable by a process comprising curing a fourth curable composition which falls within the definition provided above for the first curable composition. The fourth curable composition may be identical to or different from the first curable composition. Preferably the fourth curable composition comprises a curable compound having one (and only one) ethylenically unsaturated group and a cationic group. Thus, preferably the fourth polymer is obtainable by a process comprising curing a fourth curable composition comprising:

(a4) 0 to 60 wt% of a curable compound comprising a cationic group and one (and only one) ethylenically unsaturated group or thiol group;

(b4) 1 to 88 wt% of a curable compound comprising at least two ethylenically unsaturated groups or thiol groups and optionally further comprising a cationic group;

(c4) 0 to 10 wt% of radical initiator; and

(d4) 0 to 55 wt% of solvent.

The amount of each of components (a1), (a2) and (a4) independently is preferably 0 to 40wt%.

The amount of each of components (b1), (b2) and (b4) independently is preferably 5 to 80wt%, especially 10 to 70wt%.

Preferably the first, second and fourth curable compositions comprise a radical initiator (component (c1), (c2), and (c4)) when it is intended to cure the composition(s) by UV, visible light or thermally. Alternative methods for curing include electron beam and gamma irradiation. These alternative methods do not require a radical initiator. Thus the amount of component (c1), (c2), and (c4) present in the relevant compositions is preferably 0 to 2wt%, more preferably (for curing by UV, visible light or thermally) 0.001 to 2wt%, especially 0.005 to 0.9wt%.

The amount of component (d1), (d2) and (d4) present in the relevant compositions is preferably 20 to 45wt%.

Preferably the solvent(s) used as component (d1), (d2) and (d4) are inert, i.e. they do not react with any of the other components of the relevant curable composition.

Examples of inert solvents which may be present in the first, second and fourth curable compositions include water, alcohol-based solvents, ether-based solvents, amide-based solvents, ketone-based solvents, sulfoxide-based solvents, sulfone-based solvents, nitrile-based solvents and organic phosphorus-based solvents. Examples of alcohol-based solvents which may be used (especially in combination with water) include methanol, ethanol, isopropanol, n-butanol, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol and mixtures comprising two or more thereof. Preferably component (d 1 ), (d2) and (d4) is water.

In addition, preferred inert, organic solvents which may be used in component (d 1 ), (d2) and (d4) include dimethyl sulfoxide, dimethyl imidazolidinone, sulfolane, N-methyl pyrrolidone, dimethyl formamide, acetonitrile, acetone, 1 ,4-dioxane, 1 ,3-dioxolane, tetramethyl urea, hexamethyl phosphoramide, hexamethyl phosphorotriamide, pyridine, propionitrile, butanone, cyclohexanone, tetrahydrofuran, tetrahydropyran, 2-methyltetrahydrofuran, ethylene glycol diacetate, cyclopentylmethylether, methylethylketone, ethyl acetate, y-butyrolactone and mixtures comprising two or more thereof. Dimethyl sulfoxide, N-methyl pyrrolidone, dimethyl formamide, dimethyl imidazolidinone, sulfolane, acetone, cyclopentylmethylether, methylethylketone, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran and mixtures comprising two or more thereof are preferable.

Preferably the first curable composition and the fourth curable composition comprise a curable compound comprising a cationic group and one (and only one) ethylenically unsaturated group or a thiol group and the second curable composition comprises a curable compound comprising an anionic group and one (and only one) ethylenically unsaturated group or a thiol group.

Examples of curable compounds comprising an anionic group or cationic group and having one or more polymerizable (e.g. ethylenically unsaturated) groups include the following compounds of Formula (A), (B), (CL), (SM), (MA), (MB-a), (C), (ACL-A), (ACL-B), (ACL-C), (AM-B), (II) and/or (IV): Formula (A) wherein in Formulae (A) and (B),

R ^A1 to R ^A3 each independently represent a hydrogen atom or an alkyl group;

R ^B1 to R ^B7 each independently represent an alkyl group or an aryl group;

Z ^A1 to Z ^A3 each independently represent -O- or -NRa-, wherein Ra represents a hydrogen atom or an alkyl group;

L ^A1 to L ^A3 each independently represent an alkylene group, an arylene group or a divalent linking group of a combination thereof;

R ^x represents an alkylene group, an alkenylene group, an alkynylene group, an arylene group, or a divalent linking group of a combination thereof; and

X ^A1 to X ^A3 each independently represent an organic or inorganic anion, preferably a halogen ion or an aliphatic or aromatic carboxylic acid ion.

Examples of compounds of Formula (A) or (B) which may be used as component (a1), (b 1 ), (a4) or (b4) include:

Synthesis methods for the above compounds can be found in, for example, US2015/0353721 , US2016/0367980 and US2014/0378561 . wherein in Formulae (CL) and (SM):

L ¹ represents an alkylene group or an alkenylene group;

R ^a , R ^b , R ^c , and R ^d each independently represent a linear or branched alkyl group or an aryl group;

R ^a and R ^b , and/or R ^c and R ^d may form a ring by being bonded to each other; R ¹ , R ² , and R ³ each independently represent a linear or branched alkyl group or an aryl group, R ¹ and

R ² , or R ¹ , R ² and R ³ may form an aliphatic heterocycle by being bonded to each other; n1 , n2 and n3 each independently represent an integer of 1 to 10; and

Xf, X ₂ ‘ and X ₃ ‘ each independently represent an organic or inorganic anion.

Examples of compounds of Formula (CL) or (SM) include:

Synthesis methods for the above compounds can be found in EP3184558 and US2016/0001238.

For the first and fourth curable composition compounds of Formula (CL) and/or (SM) are preferred.

Fcnnuta (MA) Formula (MB-a) wherein in formula (MA) and (MB-a):

R ^A1 represents a hydrogen atom or an alkyl group;

Z ¹ represents -O- or -NRa-, wherein Ra represents a hydrogen atom or an alkyl group;

M ⁺ represents an organic or inorganic cation, preferably a hydrogen ion or an alkali metal ion; R ^A2 represents a hydrogen atom or an alkyl group;

R ^A4 represents an organic group comprising a sulphonic acid group and having no ethylenically unsaturated group; and

Z ² represents -NRa-, wherein Ra represents a hydrogen atom or an alkyl group preferably a hydrogen atom.

Examples of compounds of Formula (MA) or (MB-a) which may be used as component (a2) or (b2) include: Synthesis methods for the above compounds can be found in, for example, US2015/0353696.

Synthesis methods for the above compounds can be found in, for example, US2016/0369017.

Formula (C) wherein in Formula (C): each L ¹ independently represents an alkylene group; n represents an integer of 1 to 3, preferably 1 or 2; m represents an integer of 1 or 2;

L ² represents an n-valent linking group; each R ¹ independently represents a hydrogen atom or an alkyl group; each R ² independently represents -SO ₃ M ⁺ or -SO ₃ R ³ ; each M ⁺ independently represents an inorganic ion or an organic ion (especially a hydrogen ion); and each R ³ independently represents an alkyl group or an aryl group.

Examples of compounds of Formula (C) which may be used as component (a2) or (b2) include:

Synthesis methods for the above compounds can be found in, for example, EP3187516. M-B) wherein in Formulas (ACL-A), (ACL-B), (ACL-C) and (AM-B), each of R and R' independently represents a hydrogen atom or an alkyl group;

LL represents a single bond or a bivalent linking group; each of LL ¹ , LL ¹ ', LL ² , and LL ² ' independently represents a single bond or a bivalent linking group; each of A and A' independently represents a sulfo group in free acid or salt form; and m represents 1 or 2. Examples of compounds of Formula (ACL-A), (ACL-B), (ACL-C) or (AM-B) which may be used as component (a2) or (b2) include: Synthesis methods for the above compounds can be found in, for example, US2016/0362526. Formula (II) wherein: R’ is vinyl, epoxy or C1-3-alkylenethiol: n has a value of 1 or 2; m has a value of 1, 2 or 3; M’ ⁺ is a cation; wherein: (i) when m and n both have a value of 1 then X is vinylphenyl or of Formula (III): Formula (III) wherein in Formula (III): R’’ is vinyl, epoxy or C1-3-alkylenethiol; M’’ ⁺ is a cation; and n in Formula (III) has a value of 1 or 2; (ii) when m has a value of 2 or 3 then X is C1-6-alkylene, - C6-18-arylene, or N(R’’’)(3-m) wherein each R’’’ independently is H or C ₁ - ₄ alkyl; and (iii) when m has a value of 1 and n shown in Formula (II) has a value of 2 then X is of Formula (III) (as defined above) or C1-6-alkyl, C6-18-aryl, or N(R’’’)2 wherein each R’’’ independently is H or C1-4 alkyl. Examples of Formula (II) include: Formula (IV) wherein R is C1-C4 alkyl, NH2, C6-C12 aryl; and M+ is H+, Li+, Na+, K+, NL ₄ + wherein L is H or C ₁ -C ₃ alkyl. Examples of Formula (IV) include:

Other suitable compounds which may be used as component (a2) or (b2) include: For the second and third curable compositions compounds of Formula (I), (ACL-B), (ACL-C), (AM-B), (II) and/or (IV) are preferred.

The curable compositions may be cured by any suitable process, including thermal curing, photocuring, electron beam (EB) radiation, gamma radiation, and combinations of the foregoing. However the curable compositions are preferably cured by photocuring, e.g. by irradiating the curable compositions by ultraviolet of visible light and thereby causing the curable components present in the compositions to polymerise.

Examples of suitable thermal initiators which may be included in the curable compositions are as described above.

Examples of suitable photoinitiators which may be included in the curable compositions include aromatic ketones, acylphosphine compounds, aromatic onium salt compounds, organic peroxides, thio compounds, hexaarylbiimidazole compounds, ketoxime ester compounds, borate compounds, azinium compounds, metallocene compounds, active ester compounds, compounds having a carbon halogen bond, and an alkyl amine compounds. Preferred examples of the aromatic ketones, the acylphosphine oxide compound, and the thio-compound include compounds having a benzophenone skeleton or a thioxanthone skeleton described in "RADIATION CURING IN POLYMER SCIENCE AND TECHNOLOGY", pp.77-117 (1993). More preferred examples thereof include an alphathiobenzophenone compound described in JP1972-6416B (JP-S47-6416B), a benzoin ether compound described in JP1972-3981 B (JP-S47-3981 B), an alpha-substituted benzoin compound described in JP1972-22326B (JP-S47-22326B), a benzoin derivative described in JP1972-23664B (JP-S47- 23664B), an aroylphosphonic acid ester described in JP1982-30704A (JP-S57-30704A), dialkoxybenzophenone described in JP1985-26483B (JP-S60-26483B), benzoin ethers described in JP1985-26403B (JP-S60-26403B) and JP1987-81345A (JPS62-81345A), alpha-amino benzophenones described in JP1989-34242B (JP H01-34242B), U.S. Pat. No. 4,318,791A, and EP0284561A1, p- di(dimethylaminobenzoyl)benzene described in JP1990-211452A (JP-H02- 211452A), a thiosubstituted aromatic ketone described in JP1986-194062A (JPS61 -194062 A), an acylphosphine sulfide described in JP1990-9597B (JP-H02- 9597B), an acylphosphine described in JP1990-9596B (JP-H02- 9596B), thioxanthones described in JP1988-61950B (JP-S63-61950B), and coumarins described in JP1984-42864B (JP-S59-42864B). In addition, the photoinitiators described in JP2008-105379A and JP2009-114290A are also preferable. In addition, photoinitiators described in pp. 65 to 148 of "Ultraviolet Curing System" written by Kato Kiyomi (published by Research Center Co., Ltd., 1989) may be used.

Especially preferred photoinitiators include Norrish Type II photoinitiators having an absorption maximum at a wavelength longer than 380nm, when measured in one or more of the following solvents at a temperature of 23°C: water, ethanol and toluene. Examples include a xanthene, flavin, curcumin, porphyrin, anthraquinone, phenoxazine, camphorquinone, phenazine, acridine, phenothiazine, xanthone, thioxanthone, thioxanthene, acridone, flavone, coumarin, fluorenone, quinoline, quinolone, naphtaquinone, quinolinone, arylmethane, azo, benzophenone, carotenoid, cyanine, phtalocyanine, dipyrrin, squarine, stilbene, styryl, triazine or anthocyanin-derived photoinitiator. The curable compositions may be applied continuously to moving supports, preferably by means of a manufacturing unit comprising curable composition application stations, one or more curing stations comprising irradiation source(s) for curing the compositions, a membrane collecting station and a means for moving the supports from the curable composition application stations to the curing station(s) and to the membrane collecting station.

The curable composition application stations may be located at an upstream position relative to the curing station(s) and the curing station(s) is/are located at an upstream position relative to the membrane collecting station.

Examples of application techniques include slot die coating, slide coating, air knife coating, roller coating, screen-printing, and dipping. Depending on the used technique and the desired end specifications, it might be necessary to remove excess coating from the substrate by, for example, roll- to-roll squeeze, roll-to-blade or blade-to-roll squeeze, blade-to-blade squeeze or removal using coating bars. Curing by ultraviolet of visible light can occur at wavelengths between 100 nm and 800 nm using doses between 40 and 20000 mJ/cm ² . Thermal curing preferably takes place in the range between 20°C and 100°C for 1 to 20 h.

In some cases additional drying might be needed for which temperatures between 40°C and 200°C could be employed.

In one embodiment the membrane comprises a further catalyst. The further catalyst or a precursor thereof may be included in one or more of the first curable composition, the second curable composition, the third curable composition and the fourth composition.

It is also possible is to apply the catalyst or a precursor thereof (e.g. as a post-treatment step) to the third polymer i.e. after curing the third curable composition) using for example (but not limited to), dipping, air knife coating, microroller coating, spraying, chemical (vapour) deposition or physical (vapour) deposition. In this embodiment the catalyst is present at the (third) interface between the third and the fourth polymer. In a preferred embodiment the third layer comprises a catalyst.

In one embodiment at least one of the first, second, third and fourth curable compositions comprise a catalyst.

Examples of suitable catalysts are as described above in relation to the layers. Any of these catalysts may be present in a range up to 5wt%, e.g. 0.001 wt% or 1 wt%, of the weight of the relevant curable composition.

According to a second aspect of the present invention there is provided a process for preparing a BPM comprising the following steps:

(i) providing a first and fourth curable composition, each such composition comprising a curable compound having a cationic group;

(ii) providing a second and a third curable composition, each such composition comprising a curable compound having an anionic group;

(iii) impregnating a porous support with the third curable composition;

(iv) curing the third curable composition present within the porous support to give a porous third polymer comprising a first side and a second side opposite to the first side; (v) contacting the first side of the porous third polymer with the fourth curable composition such that at least a part of the fourth curable composition enters into at least a part of the pores of the porous third polymer and optionally provides a layer of the fourth curable composition on the first side of the porous third polymer;

(vi) contacting the second side of the porous third polymer with the second curable composition such that the second curable composition enters into any remaining pores of the porous third polymer and provides a layer of the second curable composition on the second side of the porous third polymer;

(vii) curing the layers of curable composition present on each side of the porous third polymer and present within the pores of the porous third polymer in any order or simultaneously to form: a first layer a) comprising a first polymer having cationic groups, a second layer b) comprising a second polymer having anionic groups, and a third layer c) comprising a third polymer having anionic groups and a fourth polymer having cationic groups present within the pores of the third polymer; wherein layer c) is interposed between layer a) and layer b); wherein the molar ratio sulphur/chlorine in the third layer c) is at least 4.

In a preferred embodiment the process according to the second aspect of the present invention further comprises the step (viii) of soaking the BPM in an aqueous solution of sodium chloride (e.g. NaCI solution in water at a strength of 0.1 to 1 .0 moles per litre, for example 0.5M), e.g. for a period of over 5 hours (especially 6 to 24 hours, more especially 10 to 20 hours and particularly 14 to 18 hours e.g. for 16 hrs). The soaking is preferably performed at a temperature in the range 5°C to 35°C, especially 10°C to 30°C, more especially 15°C to 25°C and most preferably at room temperature (about 20°C).

In a preferred embodiment of the process according to the second aspect of the present invention the first curable composition and the second curable composition each comprise a porous support when they are cured in step (vii).

In a preferred embodiment of the BPM according to the first aspect of the present invention the first layer a) and in the second layer b) each comprise a porous support.

Optionally the process according to the second aspect of the present invention further comprises step of fully or partially hydrolysing the product of step (iv) before performing step (v), e.g. by contacting the product of step (iv) with aqueous alkali at a temperature above 40°C during a period of time of at least 1 hour (e.g. soaking the product of step (iv) in NaOH or LiOH at a temperature in the range of 80 to 90°C for a period of 16 to 48 hours).

In a preferred embodiment the fully or partially hydrolysed product of step (iv) is contacted with a catalyst (preferably comprising a multivalent metal ion) before step (v) is performed. In this way a BPM may be prepared which comprises a catalyst in layer c) at the junction between the third and fourth polymers.

In a preferred embodiment, step (iii) of the process according to the second aspect of the present invention further comprises placing the porous support impregnated with the third curable composition between transparent foils to give a sandwich of the impregnated porous support and two foils and then squeezing the sandwich, e.g. between rollers or blades, to remove any excess of the third curable composition. After curing step (iv) the transparent foils may be removed before performing step (v). In a further preferred embodiment curing of the third curable composition in step (iv) is performed under an inert atmosphere, e.g. under nitrogen, carbon dioxide or argon gas.

The BPMs of the present invention may be used for various applications, including recovery and production of organic and inorganic acids and bases (e.g. ammonia, ethanolamine, lithium hydroxide, gluconic acid, formic acid, amino acids, sulphuric acid, recycling of HF and HNO ₃ ), production of oligosaccharides and proteins, capture of CO ₂ and SO ₂ from air and flue gases, energy conversion and storage, wastewater treatment and pH-control.

They have good durability in acidic and basic media, low swelling, and may be produced cheaply, quickly and efficiently.

The invention will now be illustrated by the following, non-limiting examples in which all parts and percentages are by weight unless specified otherwise.

Examples

Table 1 : Materials and ingredients Analysis methods

The performance of the BPMs was characterized by means of a current intensity versus voltage, the so-called V-l curve, wherein the current density was monitored as a function of the applied voltage or the other way around. For measuring this V-l curve a six-compartment cell equipped as follows was used. The 1 ^st electrode compartment contained a platinum plate as cathode and was separated from the 2 ^nd compartment by a CEM (CMX from Astom). The electrode compartment was filled with 0.5 M Na ₂ SO ₄ . Between the 2 ^nd and the 3 ^rd compartment a reference BPM (from Fumatech) was present. Both the 2 ^nd and the 3 ^rd compartment contained a 0.5M NaCI solution. Between the 3 ^rd and 4 ^th compartment the BPM to be analysed was placed. Between the 4 ^th and the 5 ^th compartment the same reference BPM was placed (from Fumatech) and between the 5 ^th and the 6 ^th compartment a CEM (CMX from Astom). The 4 ^th and 5 ^th compartments were also filled with a 0.5M NaCI solution. The 6 ^th compartment containing a platinum plate as anode was an electrode compartment and contained 0.5 M Na ₂ SO ₄ . By using the above six-compartment cell the solutions were pumped at a rate of 100 mL/min through the compartments at a temperature of 25°C while applying a constant current density of 1000 A/m ² . The BPMs under test were placed with the AEL side towards the anode and with the CEL side towards the cathode. The voltages were measured by using a Luggin capillary placed at each side at a distance of 4.5 mm of the BPM to be analysed.

Metal Content Analysis

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to quantify the metal content of the BPMs. 1 ,327cm ² of BPM sample was digested in 5ml concentrated nitric acid. The digestion was carried out in an Anton-Paar Multiwave 3000 instrument in conjunction with the 48MF50 rotor. The power was increased from 0 to 1000Watt in 5 minutes and this condition was maintained for 20 minutes to ensure the BPM samples were fully digested. After digestion the samples of BPM were diluted with water to a total volume of 15ml and the metal content of the resultant solutions was measured directly.

Tin (Sn) metal analysis was carried out with the Thermo Scientific™ iCAP™ PRO XP ICP-OES apparatus. A standard concentric nebulizer is used in conjunction with a cyclonic spray chamber. Yttrium was used as internal standard. Two samples of each BPM were analysed and the results provided here are the average of the two tests on each sample. The results are expressed in mg and mmol of metal per m ² of membrane.

Determining the molar ratio of sulphur/chlorine in the third layer c)

Prior to analysis the samples were equilibrated in a 0.5M NaCI solution for (at least) 16 hrs at 20°C, followed by rinsing with water.

The molar ratio of sulphur/chlorine in the third layer c) (within the BPM) was determined using scanning electron microscope-energy dispersive X-Ray analysis (SEM-EDX). Sample preparation for energy dispersive X-Ray analysis (EDX): To improve the quality of the cross section, the BPM samples were wetted before cutting. The cutting was done manually at room temperature using a first razor blade and along a second razor blade as guide from left to right. After removing a thin wet section of the BPM, the remaining block-face was dried again and from this part a thin section (placed on an aluminium stub with carbon tab) was analysed using dispersive X-Ray analysis (EDX), focusing at the ‘wet cut’ cross-section side.

The BPM samples were cleaned with a ZONESEMII™ sample cleaner from Hitachi (UV cleaning of the cross-section in polymer mode), before & after applying a 4nm Pt coating.

The SEM (scanning electron microscope) analyses of layer c) within the BPM were done using a Hitachi S-4800 High Resolution SEM. For the HR-SEM imaging the following settings were applied:

Accelerating voltage 10kV, Current intensity (le): 10pA, SE(U), WD 15mm.

For the EDX-Mapping of the BPM samples an Oxford Instruments X-Max 80mm ² SDD detector (Primary Detector Serial Number: 53905VO) was used.

Acquisition parameters:

Resolution (Width) 1024 pixels

Resolution (Height) 768 pixels

Pixel Size 0.02487 μm

Image Width 25.5μm

Image Height 19.1 μm

Accelerating Voltage 10.00kV

Magnification 5000 x

Working Distance 15.3mm

Stage Tilt Degrees 0.00°

Number of Averaged Frames 1

Dwell Time 20ps

The molar ratio of sulphur/chlorine in layer c) of the BPMs was determined by scanning a 10 μm line within the area corresponding to the interface and determining the amount in mol% of each element in this section as observed by EDX. The abundancy of each element in the observed line scanned in the EDX-MAP was derived from the number of counts corresponding to each element. To minimise the error originated from the signal coming from the surface underneath the layer mapped, several lines were scanned from the same cross-section and the number of counts were averaged until the standard deviation was lower than 10% (2 to 5 lines). The molar ratio of sulphur/chlorine was calculated by dividing the count of S by the count of Cl.

Process for making the BPMs

1) Providing the Third Curable Compositions: Third curable compositions P3-CC1 to P3-CC4 and comparative third curable composition P3-CEx1 were prepared by mixing the ingredients indicated in Tables 2 and 3 below for 10 minutes at room temperature (20°C).

Table 2: Third Curable Compositions to prepare the third polymer in Examples of the Invention

2) Preparing the Third Polymer

A layer of the third curable composition P3-CC1 (as described in Table 2 above) was applied to an aluminium plate of dimensions 148 x 210 mm using an 80 μm Meyer bar. A piece of FO-2223-10 porous support of dimensions 120x180 mm was placed on the layer of third curable composition followed by a PET cover sheet. The aluminum plate carrying the layer of third curable composition, porous support and PET cover sheet was then placed inside a zip-bag and, the third curable composition was thermally cured overnight (16hr) in an oven at 80°C to give a third polymer P3-1 .

The same process was repeated for each of third curable compositions P3-CC2 to P3-CC4 to give third polymers P3-2, P3-3, and P3-4 respectively.

To prepare third polymer cP3-1 the comparative third curable composition P3-CEx1 (as described in Table 3 above) was applied to an aluminium plate of dimensions 148 x 210 mm using a 80 μm Meyer bar. A piece of FO-2223-10 porous support of dimensions 120x180 mm was placed on the layer of third curable composition and the excess solution was removed with a 4 μm Meyer bar. The sample was placed on a conveyor of a Light Hammer® 10 of Fusion UV Systems Inc. and cured under UV-light using a D-bulb at 50% power and 5 m/min conveyor speed and to produce comparative third polymer cP3-1.

3) Functionalisation of the Third Polymers with Catalyst

Catalyst solutions were prepared by dissolving tin chloride in 0.015 M HCI solution as specified in Table 5.

The third polymer P3-1 obtained in step 2) above was immersed in an aqueous solution of 1 M alkali (1 M aqueous NaOH in this case) and the immersed third polymer was placed in an oven at 90°C for 24 hours. The alkali fully hydrolysed the third polymer to produce surface ionic groups in salt form. The hydrolysed third polymer was rinsed with purified water then allowed to air-dry for one hour before it was immersed in catalyst solution CS1 (defined below) for 30 minutes and then in a 0.1 M aqueous solution of NaOH for 10 minutes. After that time, the third polymer which had been treated with CS1 , was rinsed with purified water and dried in air for 1 hour to give a third polymer comprising a catalyst (which is hereinafter referred to as P3-1C).

The same process was repeated for each of the third polymers P3-2, P3-3 and P3-4 using the conditions shown in Table 4 below to give third polymers comprising a catalyst, hereinafter referred to as P3-2C, P3-3C, P3-4C and P3-5C respectively. Comparative third polymer cP3-1 did not require the hydrolysis step, and only was treated with catalyst solution as described above to give comparative third polymers cP3-1Ca and cP3-1Cb. Table 5 below shows the composition of the catalyst solutions (CS1 and CS2) used to functionalise each third polymer. The catalyst (tin) content of the BPMs shown in the final row of Table 4 Sn _in itiai (mg/m ² ) and Sn _in itiai (mmol/m ² ) were determined as described below in section 6.

Table 4: Functionalisation of Third Polymers with Catalyst Table 5: Catalyst Solutions

4) Preparation of Curable Compositions for the First, Second and Fourth Polymers

Curable compositions LCC-1 suitable for making the first polymer and fourth polymer and LCC- 2 suitable for making the second polymer were prepared by mixing the ingredients indicated in Table 6 below in the specified amounts:

Table 6: Curable Compositions (used to prepare first, second and fourth polymers)

5) Preparation of the BPMs

BPMs were prepared as follows from the functionalized third polymers described in Table 4 above (comprising catalyst) and the curable compositions indicated in Table 7 below:

A first curable composition (as indicated in Table 7 below) was coated on top of a PET foil using a 100 μm Meyer bar. A sheet of the porous support FO-2223-10 was applied on top of the first curable composition, followed by a sheet of the third polymer (comprising a catalyst) such that the pores of the porous support and the pores of the third polymer were impregnated with the first curable composition. Thus the first curable composition also acted as the fourth composition because it impregnated the pores of the third polymer. An aluminum plate was placed on the impregnated third polymer, turned upside down and the resulting laminate was then photocured through the PET foil using a Light Hammer® 10 of Fusion UV Systems Inc. equipped with a D-bulb at 50% power and 5 m/min conveyor speed, thereby forming a bilayer of the first layer a) - for Examples Ex1 to Ex5 - comprising a first polymer having cationic groups and the third layer c) comprising a co-continuous polymeric network of (i) the third polymer having anionic groups; and (ii) a fourth polymer having cationic groups (the fourth polymer being identical to the first polymer). For Comparative examples CEx1 and CEx2 the first polymer (and the fourth polymer) had anionic groups and the third polymer had cationic groups.

The second curable composition (as indicated in Table 7 below) was coated on top of a PET foil by using a 100 μm Meyer bar. A sheet of the porous support FO-2223-10 was applied on top of the second curable composition and the excess composition was removed using a 4 μm Meyer bar. The resulting wet, impregnated porous support was placed on top of the bilayer (prepared as described above) and squeezed with a 4 μm Meyer bar. An aluminum plate was placed on the impregnated porous support. The resulting laminate was cured through the PET foil using the same device equipped with a D-bulb working at 100% power and a conveyor speed of 5m/min. After curing, the aluminum plate and the PET foil were removed and the formed BPM was equilibrated overnight (at least 16 hrs) at room temperature (20°C) in 0.5M NaCI solution prior to performing the analyses described below.

Table 7: Components of the BPMs

6. BPM Performance Results

The tin content of the BPMs described in Table 7 were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) by the method described above. This gave the initial tin content of the BPMs (Sninitiai) before ageing.

The BPMs were then immersed in 4M NaOH aqueous solution at 40°C for 3 days. After this period, the BPMs were rinsed with water and their tin content was again measured by (ICP-OES). This gave the tin content of the BPMs after ageing (Snfmai).

The catalyst (tin) loss as a result of ageing was then calculated by the formula and the results are shown in Table 8 below:

Catalyst (tin) loss % = 100% x [(Sn _in itiai - Sn _fin ai)/ Sn _in itiai)

Table 8: Performance Results

Notes:

1) voltage@1000 A/m ² was determined from the V-l curve of each bipolar membrane using the method described above. 2) Molar ratio S/CI was the molar ratio of sulphur/chlorine in the third layer c) of the relevant BPM, determined by SEM-EDX using the method described above.