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Title:
PROCESS FOR MANUFACTURING RADIOGRAFTED MEMBRANES AND USE OF THE MEMBRANES OBTAINED
Document Type and Number:
WIPO Patent Application WO/2006/040310
Kind Code:
A1
Abstract:
Process for producing radiografted membranes having a low degree of surface grafting, in particular of ion-exchange membranes structured in the thickness, in which a membrane is irradiated and is placed in contact with a grafting composition, the grafting composition comprising a polymerization inhibitor having a steric hindrance that limits its penetration into the membrane at a surface zone having a thickness of not more than 10% of the thickness of the membrane.

Inventors:
Brunea, John A. (Rue de l'Anémone 10, Bruxelles, B-1180, BE)
Chapotot, Agnès (Avenue de la Nivéole, 35bte4, Bruxelles, B-1020, BE)
Application Number:
PCT/EP2005/055140
Publication Date:
April 20, 2006
Filing Date:
October 10, 2005
Export Citation:
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Assignee:
SOLVAY (Société Anonyme) (Rue du Prince Albert 33, Brussels, B-1050, BE)
Brunea, John A. (Rue de l'Anémone 10, Bruxelles, B-1180, BE)
Chapotot, Agnès (Avenue de la Nivéole, 35bte4, Bruxelles, B-1020, BE)
International Classes:
B01D61/42; B01D71/32; C08F259/08; C08J7/18; H01M8/10
Foreign References:
US6242123B12001-06-05
US5994426A1999-11-30
US4608393A1986-08-26
Other References:
PATENT ABSTRACTS OF JAPAN vol. 010, no. 107 (C - 341) 22 April 1986 (1986-04-22)
GUPTA B ET AL: "Crosslinked ion exchange membranes by radiation grafting of styrene/divinylbenzene into FEP films", JOURNAL OF MEMBRANE SCIENCE, ELSEVIER SCIENTIFIC PUBL.COMPANY. AMSTERDAM, NL, vol. 118, no. 2, 18 September 1996 (1996-09-18), pages 231 - 238, XP004041804, ISSN: 0376-7388
Attorney, Agent or Firm:
Jacques, Philippe (Solvay, Intellectual Property Department Rue de Ransbee, 310 Brussels, B-1120, BE)
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Claims:
C L A I M S
1. Process for producing radiografted membranes having a low degree of surface grafting, in particular of ionexchange membranes structured in the thickness, in which a membrane is irradiated and is placed in contact with a grafting composition, the grafting composition comprising a polymerization inhibitor having a steric hindrance that limits its penetration into the membrane at a surface zone having a thickness of not more than 10% of the thickness of the membrane.
2. Process according to the preceding claim, in which the grafting composition comprises a curing agent.
3. Process according to either of the preceding claims, in which the polymerization inhibitor has a steric hindrance of at least 300 A3.
4. Process according to any one of the preceding claims, in which the grafting composition comprises a chaintransfer agent.
5. Process according to any one of the preceding claims, in which the base polymer of the membrane comprises a fluorinated polymer.
6. Process according to any one of the preceding claims, in which the membrane is a monolayer membrane.
7. Process according to any one of the preceding claims, in which the irradiated membrane is successively placed in contact with at least two grafting compositions, at least one grafting composition previously placed in contact with the membrane having a content of at least one polymerization additive higher than that of at least one grafting composition subsequently placed in contact, the polymerization additive being a chaintransfer agent and/or an inhibitor and/or a curing agent.
8. Process according to any one of the preceding claims, in which the irradiated membrane is successively placed in contact with at least two different grafting compositions, at least one grafting composition subsequently placed in contact with the membrane comprising a barrier monomer that is absent from at least one composition previously placed in contact.
9. Use in electrodialysis of an ionexchange membrane obtained via a process according to any one of Claims 1 to 8.
10. Use in fuel cells, in particular in fuel cells running on methanol such as DMFCs, of an ionexchange membrane obtained via a process according to any one of Claims 1 to 8.
Description:
Process for manufacturing radiografted membranes and use of the membranes obtained

The invention relates to a process for manufacturing radiografted membranes structured in the thickness, which are particularly suitable for producing ion-exchange membranes.

The term "membrane structured in the thickness" means a membrane that has in its thickness a controlled gradient of properties, for instance its degree of grafting or, in the case of ion-exchange membranes, its ion-exchange capacity, the surface exchange capacity being different from that in the core of the membrane.

Ion-exchange membranes are well known in the art. Their capacity to be selectively permeable to anions or to cations gives them many uses. The membrane electrolysis of sodium chloride solutions for the joint production of chlorine and sodium hydroxide, fuel cells and, finally, electrodialysis, which itself has numerous applications such as water treatment are examples of technologies using ion-exchange membranes. When ion-exchange membranes are used in electrodialysis for the treatment of fluids comprising substances to be removed, they are quite quickly fouled by these substances ("fouling"), which means that they have to be cleaned or replaced. For practical reasons, it is thus important for them to have sufficient resistance to fouling, for example by means of non-stick properties. Moreover, they must have a sufficient overall ion conductivity so as to maintain an economically advantageous electrical consumption.

It is known practice (US 2003/0024816 Al) to improve the fouling resistance of ion-exchange membranes by attaching polyalkylene glycol chains thereto. However, the fouling resistance of these known membranes is insufficient when the fluid to be treated contains polluting substances of low molecular weight.

The invention is directed towards providing an improved process for obtaining fouling-resistant ion-exchange membranes that have excellent ion conductivity. Consequently, the invention relates to a process for producing radiografted membranes having a low degree of surface grafting, in particular of ion-

exchange membranes structured in the thickness, in which a membrane is irradiated and is placed in contact with a grafting composition, the grafting composition comprising a polymerization inhibitor having a steric hindrance that limits its penetration into the membrane at a surface zone having a thickness of not more than 10% of the thickness of the membrane.

Radiografting is a technique for producing copolymers in which a base polymer (which may itself be a homopolymer or a copolymer) is subjected to high-energy radiation intended to create reactive radicals therein. In combination with the irradiation, the polymer is placed in contact with a composition comprising the monomer that it is desired to graft. This monomer polymerizes on the sites made active by the radiation. The grafted groups may then be converted into ion-exchange groups by means of an additional functionalization treatment (such as sulfonization, phosphonation, amination, carboxylation, etc.). The functionalization yield is usually very high, more than 80% and preferably 90% of the grafted groups being functionalized as exchange groups (also known as exchange sites). Control of the experimental parameters of the radiografting step, such as those relating to the irradiation and to the nature of the composition comprising the monomer, thus has a direct impact on the control of the structuring in the thickness of the exchange capacity of the membranes obtained, since the exchange capacity is directly linked to the number of exchange sites/groups. The radiation used for the irradiation may be electromagnetic, such as X-rays or gamma rays, or may consist of charged particles such as electrons. Beta radiation, consisting of electrons having a sufficient energy, for example at least 0.5 MeV and preferably at least 1 MeV, may be suitable. It is usually preferable for the energy not to exceed 20 MeV, energies of between 1.5 and 10 MeV being recommended. This radiation should be applied to the base polymer for the time required to obtain a sufficient amount of reactive radicals. This results in an irradiation dose, expressed in kGy (kilograys), one gray being equal to 10 4 ergs/gram. The necessary irradiation dose depends on the sensitivity of the polymer to radiation. When the base polymer is ETFE, times corresponding to doses of between 20 and 100 kGy have been seen to be suitable. The irradiation may be performed when the polymer is in contact with the composition comprising the monomer. It may also be performed beforehand. In this case, the irradiated polymer is advantageously kept at low temperature, while awaiting its placing in contact with the composition comprising the monomer. For the production of membranes according to the invention, it is

recommended to irradiate the base polymer, already implemented in membrane form. It is also recommended for the irradiation to be performed before placing the polymer in contact with the composition comprising the monomer.

According to the invention, the use of a polymerization inhibitor in the grafting composition makes it possible to reduce the surface grafting. However, in the general case, the inhibitor also penetrates into the membrane and exerts its effect therein. The overall degree of grafting is then insufficient to be able to obtain ion-exchange membranes that have good ion conductivity. The inventors have observed that the selection of inhibitors with a sufficient steric hindrance prevents their penetration deep into the membrane while at the same time sufficiently inhibiting the grafting of the surface zone. According to the invention, this surface zone represents not more than 10%, advantageously not more than 5% and preferably not more than 1% of the thickness of the membrane. The penetration depth of the inhibitor may be measured by any analytical technique for detecting the presence of the inhibitor, optionally after peeling the surface zone, which is also performed by any suitable known technique. According to the invention, it is considered that the inhibitor is absent, at a certain depth, if its concentration thereat is not more than one tenth, advantageously one twentieth, preferably one fiftieth and particularly advantageously one hundredth of its surface concentration.

The steric hindrance necessary to prevent the deep penetration of the inhibitor depends on the nature of the base polymer constituting the membrane before its radiografting and on the nature of the grafting composition, including the presence of solvents that penetrate into the membrane and swell it. The determination of the inhibitor having the ideal steric hindrance will thus, in certain cases, be performed by trial and error.

However, in one advantageous embodiment of the process according to the invention, the polymerization inhibitor has a steric hindrance of at least 250 AVmolecule. Inhibitors having a steric hindrance of between 300 and 350 AVmolecule have proven to be particularly advantageous in a large number of circumstances.

Moreover, in general, the use of methylene blue, consisting of tetramethylthionine hydrochloride - (C 16 H 18 SN 3 Cl) as inhibitor has been seen to be extremely advantageous. It is recommended to use this inhibitor in concentrations ranging from 0.05 g/1 and preferably 0.1 g/1 to 0.75 g/1 and preferably 0.5 g/1.

- A -

The presence of polymerization inhibitor in the grafting composition also has the advantage of maintaining the efficacy of the composition up to temperatures that may reach 80°C, advantageously 85°C and ideally 90°C, which ensures highly advantageous grafting kinetics. According to one advantageous variant of the process according to the invention, the grafting composition also comprises a curing agent. Under the action of the curing agent, a three-dimensional network forms in the membrane. This results in an improvement in the mechanical properties, a reduction in the permeability to certain substances, but also better chemical resistance. The nature of the curing agent depends on the chemical composition of the irridiated membrane and on the nature of the grafted monomer. In particular, when the grafted monomer is styrene and the irradiated membrane is based on a fluorinated polymer, it is recommended to use as curing agent divinylbenzene (DVB) or triallyl isocyanate (TAIC), or even a mixture of the two. DVB is preferred. In this variant, since the membrane is at least partially cured, the inhibitor penetrates therein with greater difficulty. All factors being otherwise equal, it is then possible to use an inhibitor of lower steric hindrance. In this variant, it is recommended to use the curing agent in a proportion of at least 0.5% and preferably at least 1% by volume relative to the grafted monomer. Usually, it is recommended not to exceed 15%, values of between 1.5% and 10% being preferred.

According to another advantageous variant of the process according to the invention, the grafting composition also comprises a chain-transfer agent. This additive makes it possible to further improve the control of the grafting in the thickness of the membrane. The use of chain-transfer agents of the thiol iamily gives excellent results. In this family, certain thiols such as hexanethiol diffuse readily and make it possible to obtain a homogeneous grafting profile within the membrane (with the exception of the surface). Other thiols, of higher steric hindrance, such as dodecanethiol, diffuse with greater difficulty and more slowly and give rise to higher grafting gradients, the grafting at the centre of the membrane being greater than that in the outer zones. It is preferable to use the chain-transfer agents in very low amount, of between 0.01%, advantageously 0.02% and 0.5%, advantageously 0.25%, by volume relative to the total volume of the grafting composition. The combined use of a polymerization inhibitor, a curing agent and a chain-transfer agent makes it possible to produce grafted membranes having an

extremely low degree of surface grafting, preferably less than 10%, and a controlled profile inside, also ensuring an overall degree of grafting of greater than 20%, advantageously 30% and preferably 40%, values of between 50% and 60% being especially advantageous. The degree of grafting is the weight gain following the grafting of a weight unit of ungrafted membrane. It may be evaluated locally, in a given layer of the membrane, or globally, for the entire thickness. The term "surface" means a zone having a thickness of not more than 5 μm, preferably not more than 1 μm and more preferentially not more than 0.1 μm. When the grafted membranes according to the invention are functionalized to produce ion-exchange membranes, these membranes have few surface sites, over a depth of a few molecular layers. The extreme thinness of this surface layer and the excellent control of the deep grafting profile, obtained according to the invention, maintain excellent electrical properties and in particular good overall ion conductivity. In the process according to the invention, the diffusing properties of certain specific polymerization additives is exploited. The change in the degree of grafting, from the surface to the core of the membrane is consequently continuous, which presents numerous advantages, in particular when the grafted membranes are functionalized as ion-exchange membranes. In order best to exploit this situation, in one recommended embodiment of the process according to the invention, the membrane is a monolayer membrane. The term "monolayer membrane" means a membrane having no interface in its thickness. Such a membrane is thus distinguished from multilayer membranes, which result from the superposition of two or more thin membranes or from successive covering of a membrane with additional outer layers. A monolayer membrane has a continuous evolution, in the direction of its thickness, of its main material macroscopic parameters, such as its density, its ion conductivity or its mechanical properties. The absence of interfaces and of discontinuities in a monolayer membrane treated by means of the process according to the invention presents many advantages, for instance better ion conductivity, the absence of risk of delamination, less internal tension and thus less deformation on wear. When the membrane is functionalized as an ion-exchange membrane, it may be of the cationic or anionic type, depending on whether it is selectively permeable to cations or to anions, respectively. The ion-exchange sites it comprises may be of various known types, for instance, and depending on the type of membrane, carboxyl, sulfonate or trimethylammonium groups.

In the case of a cationic membrane obtained via the process according to the invention, comprising ion-exchange sites of sulfonate type, the low density or, preferably, the absence of exchange sites in the surface zone may be very rapidly and easily demonstrated by placing the surface of the membrane in contact with methylene blue (tetramethylthionine hydrochloride - C 16 H 18 SN 3 Cl). This dye has a high molecular weight, which reduces its penetration into the membrane. In the case of membranes obtained according to the invention, the methylene blue is not fixed on contact with the surface sulfonate groups and gives no blue coloration. In comparison, the membranes obtained without polymerization inhibitor fix methylene blue and take a strong blue colour. A similar test using "Congo red" makes it possible to demonstrate the absence of quaternary ammonium exchange sites on the anion-exchange membranes obtained according to the invention.

In the membrane according to the invention, the ion-exchange sites, for example carboxyl, sulfonate or trimethylammonium groups, are covalently bonded to a base polymer. This polymer should especially ensure the mechanical strength of the membrane and its dimensional stability and should have the necessary chemical resistance suited to the medium in which it is in service. Many base polymers may be used successfully in the process according to the invention. This is thus the case, for example, for:

■ fluorinated polymers such as polytetrafluoroethylene (PTFE), polyvinyldene fluoride (PVDF), perfluorinated poly(ethylene-propylene) (FEP) and ethylene-polytetrafluoroethylene copolymers (ETFE) ;

■ polyolefins (polyethylene and polypropylene); ■ polyamides.

According to one preferred variant of the process according to the invention, the base polymer comprises a fluorinated polymer. It preferably consists of a fluorinated polymer. In this variant, non-stick hydrophobic membranes are obtained, the surface properties of the base polymer being preserved by means of the process according to the invention.

To obtain a membrane the base polymer is used by any means suited to the thickness and to the dimensions of the membrane to be obtained and also to the necessary precision thereof. For small membranes with a large thickness of high precision, injection techniques may be advantageous. Usually, the polymer is used by extrusion or calendring. The ion-exchange sites may be linked to the

base polymer before or after implementing it in membrane form. They are preferably linked to the base polymer after implementing it in membrane form. When the irradiated membrane is placed in contact with the composition comprising the monomer, the grafting proceeds by progression of a front from the surface inwards. Without wishing to be bound by a technical explanation, the inventors consider that the control of the mechanism of progression of the front makes it possible to improve the structuring of the membrane in its thickness and in particular to obtain excellent control of the degree of grafting. Under certain circumstances, the kinetics of progression of the front are such that it is possible to optimize structuring of the membrane by varying over time the composition comprising the monomer.

Consequently, in one particularly advantageous variant of the process according to the invention, the irradiated membrane is successively placed in contact with at least two grafting compositions, at least one grafting composition placed in contact previously with the membrane having a content of at least one polymerization additive higher than that of at least one grafting composition subsequently placed in contact, the polymerization additive being a chain- transfer agent and/or an inhibitor and/or a curing agent. Since the polymerization additive is present in larger amount at the start of the grafting, it preferentially affects the outer layers of the membrane. For example, when the curing agent is present mainly at the start of the grafting, it is possible to obtain a membrane that is more cured in its surface zones than in its inner layers.

In this variant, it is also possible to produce membranes with a low degree of surface grafting, of less than 10% and preferably less than 1%, while at the same time being highly grafted deep down, the maximum degree of grafting thereat exceeding 40% and preferably 50%. To this end, the membrane is first placed in contact with a grafting composition containing a sufficient amount of polymerization inhibitor and of chain-transfer agent to limit the surface grafting. Next, the membrane is placed in contact with a second grafting composition from which these additives are sufficiently absent to obtain substantial grafting when the grafting front reaches the inner layers of the membrane. The precise contents of these additives in the grafting compositions will be determined on a case by case basis by a person skilled in the art as a function of the base polymer and of the monomer to be grafted. Ion-exchange membranes having a fluorinated base polymer, obtained starting from such membranes with very low surface grafting, consequently have

a high concentration of fluorine at the surface while at the same time being highly grafted deep down. They also have an advantageous curing profile, the surface zones being more cured than the inner zones. These membranes are particularly chemically resistant and especially foul very little. They are especially useful in electrodialysis, for example for treating wine.

The monomers included in the grafting compositions may be identical or different. When they are different, the membrane will have in its thickness a structuring of its chemical composition. This spatial structuring in the thickness will depend on the temporal variation of the grafting compositions, the monomer included in the grafting composition subsequently placed in contact with the membrane being grafted in the outermost layers of the membrane relative to the monomer included in the compositions previously placed in contact. When the grafting compositions comprise several monomers, and when the membrane is placed in contact with a larger number of different grafting compositions, it is possible to gradually vary the monomer content of the various grafting compositions and to obtain a membrane that has in the thickness a chemical composition gradient that is proportionately more continuous the more gradually the content of the various grafting compositions is varied.

In one recommended embodiment of the process according to the invention, at least one grafting composition subsequently placed in contact with the membrane comprises a barrier monomer that is absent from at least one composition previously placed in contact. The term "barrier monomer" means a monomer which, when it is present in the grafting composition, gives rise to a grafted membrane that has a permeability less than that which it would have in the absence of the barrier monomer in the grafting composition. In the process according to the invention, the permeability considered is the permeability to the fluid with which the membrane is placed in contact when it is in service.

Although the grafting compositions may be in gaseous form, or even in plasma form, in the variants of the process according to the invention in which the grafting composition is varied over time, it is recommended for the grafting compositions to be in liquid form and for the placing in contact to be performed by immersing the membrane in at least two different baths. The composition of the baths may be constant over time, in which case the membrane is successively placed in contact with at least two different baths. In this case, the placing in contact may be batchwise, all of the amount of membrane produced being at a given moment in one given bath ("batch" mode) or continuous, different parts of

the membrane being in different baths, the membrane being mobile. The composition of the baths may also change over time, by addition or even elimination of certain components. In this case, the membrane may rest in contact with only one bath. In the process according to the invention, any grafting composition suitable for the membrane that it is desired to produce may be used. In particular, when it is desired to obtain ion-exchange membranes, the grafting composition depends on the canonic or anionic nature of the desired membrane. The grafting compositions advantageously comprise chloromethylstyrene (anionic membranes) or styrene (cationic membranes). Good results may also be obtained with optionally substituted fluorostyrenes. Examples of fluorostyrenes that may be mentioned include α-fluorostyrene, α, β-difluorostyrene, α, β, β-trifluoro- styrene and the corresponding fluoronaphthylenes. The term "substituted fluorostyrene" means a fluorostyrene containing a substituent in the aromatic ring.

The process according to the invention is particularly suitable for producing ion-exchange membranes with a low surface density of exchange sites.

These membranes have excellent compromise between their fouling resistance and sufficient ion conductivity.

Consequently, the invention also relates to the use of the ion-exchange membranes obtained via the process according to the invention in electrodialysis, especially for the treatment of fluids such as water or, advantageously, wine. Moreover, it has been observed that, in general, the radiografted membranes obtained via the process according to the invention, probably by virtue of their low surface grafting, have improved impermeability to certain fluids - in particular to alcohols and most particularly to methanol - when compared with similar membranes with greater surface grafting. This improved impermeability makes them advantageous in fuel cells, more especially in cells running directly on methanol, without prior conversion of the methanol into hydrogen. Specifically, in these cells, known as Direct Methanol Fuel Cells - DMFC, the following reactions takes place at the electrodes: At the cathode: 02+4H + +4 e " → 2H 2 O At the anode: CH 3 OH + H 2 O → CO 2 +6H + +6e " The membrane used as separator in such a fuel cell must satisfy specific and strict requirements, since its physicochemical properties have a considerable

influence on the performance qualities of the cell. In particular, important parameters thereof are the proton conductivity and the impermeability to iuel, in the present case methanol.

The invention thus also relates to the use of the ion-exchange membranes obtained via the process according to the invention in fuel cells, in particular in fuel cells running on methanol such as DMFCs.

The examples that follow serve to illustrate the invention.

In these examples, the process was performed in the following manner.

The irradiation of the membranes was performed in the presence of air under an electron beam at a tension of 1.5 MeV and at a dose rate of 10 kGy/s. The dose deposited in the film is from 20 to 100 kGy. The irradiated membranes were stored at a temperature of less than or equal to -18°C until the time of use. At -18°C, the irradiated membranes may be stored for 12 months with very little loss of reactivity. The monomers, and more particularly those containing large amounts of stabilizers, for instance divinylbenzene (DVB) and chloromethylstyrene (CMS), were destabilized by washing in aqueous basic medium 0.1 M NaOH and then rinsed to neutral pH with demineralised water in a separating funnel. The destabilized monomers were stored at -18°C until the time of use. The ion conductivity, the water content and the exchange capacity of the membranes obtained were measured according to French standard AFNOR NF X45-200 December 1995.

The methanol permeability was measured at ambient pressure by introducing the studied membrane into a measuring cell with a cross section of 8.55 cm 2 . The membrane delimited two 10 ml compartments. One of the faces of the membrane was exposed to a molar solution of methanol continually renewed at a constant rate of 24 ml/h. The second compartment was flushed with helium at a flow rate of 400 ml/min. The measuring cell was maintained at 25°C. The entrained vapours were condensed in two consecutive traps containing acetone at 2°C. The analysis of the condensate was performed by gas chromatography. Under the same measuring conditions, the Nafion® 117 membrane, with a dry thickness of 175 μm, used as reference had a methanol permeability of 1215 g/m 2 day and a water permeability of 11 230 g/m 2 day for an electrical resistance (10 g/1 NaCl) of 2.4 ohms. cm 2 and the Nafion® 112 membrane, with a dry thickness of 45 μm, had a methanol permeability of 1770 g/m 2 day for an electrical resistance of 1.0 ohms. cm 2 .

The contact angle was measured with water and with diiodomethane (G2 measuring machine from Kruss).

The carbon and fluorine concentration and/or sulfur concentration profiles were measured by X-ray microanalysis (SEM-EDX) on a section of the membrane obtained by cutting with a cryogenic microtome. The cross section of the sample is obtained by ultramicrotome smoothing at room temperature. It is then covered by cathodic sputtering with a thin conductive layer based on a platinum/palladium alloy. The examination is performed using a field-effect scanning electron microscope (FEG-SEM) of brand LEO 982, equipped with an X ISIS 300 microanalysis system from Oxford Instruments. The electron energy used is 20 keV. To measure a concentration profile, the X-ray signals of the elements to be monitored (sulfur, fluorine, optionally carbon and oxygen), emitted after incidence of the electrons along a chosen line parallel to the thickness of the membrane, are collected point by point, the displacement taking place by deflection of the electron beam using reels used for imaging. The concentration of the elements whose profile is measured is proportional to the intensity of the X-ray signal measured.

The presence and the accessibility of the sulfonate sites at the surface of the membrane was evaluated by immersing the samples for 1 minute in an aqueous solution containing 5 g/1 of methylene blue, followed by measuring the L*a*b* coordinates in transmission with illuminant D65 and an observation angle of 10°.

Figures 1 and 2 show the sulfur and carbon- fluorine profiles in the thickness of the membrane, for the membranes obtained in Examples 1 and 2, respectively. Example 1

A grafting solution containing 30% by volume of non-destabilized styrene and 70% ethanol comprising 0.1 g/1 of methylene blue was prepared. To this solution were added 3.15% by volume of pure divinylbenzene relative to the volume of styrene used and 0.055% by volume of 1 -dodecanethiol relative to the total volume of the grafting solution. An ETFE membrane irradiated at a dose of 60 kGy was introduced into the grafting solution and the assembly was purged with nitrogen until an oxygen concentration of less than 100 ppm was obtained in the headspace of the reactor. The grafting solution was maintained at a temperature of 80°C for

16 hours. The degree of grafting obtained was 47%. The styrene grafted in the

sample was then sulfonated for 12 hours at room temperature in a solution of 1,2-dichloroethane (DCE) containing 6% by weight of chlorosulfonic acid. The membrane was rinsed in DCE and then in ethanol for 1 hour. The sulfonate sites were finally obtained by hydrolysis of the chlorosulfonyl sites in an aqueous 0.1 M solution at 60°C for 16 hours. The profile of sulfur distribution in the thickness of the grafted film showed that the grafting had penetrated down to the core of the film and comprised a less grafted zone at the surface (Figure 1).

When measured in an aqueous 10 g/1 NaCl solution, the membrane has a resistance of 1.8 Ω.cm 2 . The water content is between 34.4% and 36% and the exchange capacity between 2.17 and 2.19 meq/g. The results of the methylene blue test indicate a low concentration of surface sites (L: 74.5; a: 41.9; b: 26.5) and the contact angle (86° with water and 57° with diiodomethane) indicate low wettability. Example 2 A grafting solution containing 20% by volume of non-destabilized styrene and 80% of ethanol containing 0.3 g/1 of methylene blue was prepared. 4% by volume of pure divinylbenzene relative to the volume of styrene used and 0.050% by volume of 1-dodecanethiol relative to the total volume of the grafting solution were added to this solution. An ETFE membrane irradiated with a dose of 60 kGy was introduced into the grafting solution and the assembly was purged with nitrogen until an oxygen concentration of < 100 ppm in the headspace of the reactor was obtained. The grafting solution was maintained at a temperature of 80°C for 6 hours. The degree of grafting obtained is 35% to 40%. The membrane was then treated as in Example 1. The profile on the thickness of the membrane, of the signal for carbon and fluorine and sulfur showed that the grafting had penetrated down to the core of the film and comprised a gradient of grafting at depth of the film (Figure 2).

When measured in an aqueous 10 g/1 NaCl solution, the membrane has a resistance of 3.5 Ω.cm 2 . The water content is 30% and the exchange capacity is 1.8 meq/g. The methylene blue test indicated a low concentration of surface sites. The methanol permeability was 566 g/m 2 day and the water permeability was 14 370 g/m 2 day. Example 3

A 100 μm membrane irradiated with 100 kGy was immersed in a grafting solution containing 20% by volume of destabilized CMS and 80% of ethanol containing 0.3 g/1 of methylene blue. To this solution was added a volume of

pure DVB corresponding to 2.4% of the volume of the CMS. After deoxygenation, the grafting reactor was maintained at 75°C for 16 hours. The degree of grafting obtained was 48.8%. After amination in a solution of trimethylamine at 45% in water, followed by equilibration in a 10 g/1 NaCl solution for 24 hours, the resistance of the membrane was between 3.6 and 4.7 Ω.cm 2 . The membrane immersed for one minute in an aqueous solution of Congo red did not decolour, confirming the absence of surface quaternary amine sites. Example 4 A grafting solution containing 30% by volume of non-destabilized styrene and 70% of ethanol comprising 0.1 g/1 of methylene blue was prepared. To this solution were added 3.15% by volume of pure divinylbenzene relative to the volume of styrene used and 0.055% by volume of 1-dodecanethiol relative to the total volume of the grafting solution. An ETFE membrane irradiated with a dose of 60 kGy was introduced into the grafting solution and the assembly was purged with nitrogen until an oxygen concentration of less than 100 ppm in the headspace of the reactor was obtained.

The grafting solution was maintained at a temperature of 80°C for 15 hours. The degree of grafting obtained was 39%. The styrene grafted into the sample was then sulfonated for 12 hours at room temperature in a solution of 1,2-dichloroethane (DCE) containing 6% by weight of chlorosulfonic acid. The membrane was rinsed in DCE and then in methanol for 1 hour. The sulfonate sites were finally obtained by hydrolysis of the chlorosulfonyl sites in an aqueous 0.1 M solution at 60°C for 16 hours. When measured in an aqueous 10 g/1 NaCl solution, the membrane has a resistance of 1.4 Ω.cm 2 . The water content is 35% and the exchange capacity is 1.19 meq/g. The results of the methylene blue test indicate a low concentration of surface sites. This membrane has a dry thickness of 139 μm. The methanol permeability measured in a pervaporation cell is 682 g/m 2 .day at 25°C.