EP1025171A1

EP1025171A1 - Method for coating electrically conductive particles by grafting a polymer layer and products resulting from said method

Info

Publication number: EP1025171A1
Application number: EP98951121A
Authority: EP
Inventors: Robert Jerome; Lucien Martinot; Marc Mertens; Isabelle Bodart; Philippe Dubois
Original assignee: Internationale de Participation et D'investissement Cipari SA
Current assignee: Internationale de Participation et D'investissement Cipari SA
Priority date: 1997-10-22
Filing date: 1998-10-22
Publication date: 2000-08-09
Also published as: US6375821B1; JP2001520317A; WO1999020697A1; KR20010031323A; AU9730698A; BE1011511A6

Abstract

The invention concerns a method for forming a coat on conductive particles by grafting a polymer and/or a copolymer on said particles from a bath containing at least a monomer from which said polymer and/or copolymer is formed. The method consists in producing said grafting by electrochemical reduction of said monomer in an electrolytic solution, wherein are provided at least a cathode and an anode and containing an aprotic solvent, a support-electrolyte and the monomer(s) required for polymerising or copolymerising the coat by suspending the particles and moving said solution so as to produce an intermittent contact between said particles and the cathode, and to form, by polymerisation or copolymerisation, the coat on the particles by applying an appropriate potential.

Description

"Process for the coating of electrically conductive particles by grafting a polymer layer and products resulting from this process"

The present invention relates to a process for the formation of a coating on conductive particles by grafting a polymer and / or copolymer on these particles from a bath containing at least one monomer from which the polymer and / or copolymer is formed. The expression "conductive particle" relates as well to powders, granules, short fibers as to objects of relatively reduced size and any shape.

The preparation of composite materials comprising a polymer matrix in which electrically conductive particles are homogeneously dispersed is currently limited by the instability of the conductive charge / polymer interface. To overcome this drawback, several methods call for a preliminary coating of the charges with an organic component which will ensure, at least partially, the attachment of the particles to the polymer matrix of the composite.

In reality, this approach provides only a very limited solution to the stability of the final composite since the problem of the adhesion of the conductive particle / polymer matrix is transferred to the interface of the conductive particle / coating layer.

An improvement of the known methods was notably described in 1995 by N. Tsubokawa and S. Hayashi (JMS-Pure Appl. Chem., 1995, A32, 525-535) which relates to the chemical grafting of polymers on a carbon powder previously oxidized by nitric acid, so as to form on the surface functional groups OH, COOH and C = O. The grafting of the polymer takes place in two stages: grafting of the functional group then chemical attachment of the polymer. The grafting of the polymer is not controllable since the density of the grafted chains is limited by the surface density of the functional groups which is otherwise random. The stability of the fixing of the polymer is not really ensured since, in certain cases, it is hydrolysable "ester" functions which ensure the graphite / polymer bond. Finally, these authors use either monomers, the polymerization of which is initiated by the grafted functional group, or polymer chains already formed, the size of which limits the accessibility to the substrate.

One of the essential aims of the present invention is to remedy the drawbacks of known coating methods and to present a method making it possible to form on electrically conductive particles a coating having perfect and uniform adhesion so that these particles thus coated or coated materials are particularly suitable for the manufacture of composites loaded with such conductive particles.

The process according to the invention therefore aims to cover the particles with a stable layer of polymer capable of dispersing them in a stable and homogeneous manner in a conductive or non-conductive polymer matrix, provided of course that it is ensured that this matrix and the polymer coating the particles are miscible, even mutually reactive.

To this end, according to the invention, the grafting is carried out by an electrochemical reduction of said monomer in an electrolysis bath, in which at least one cathode and an anode are provided and containing an aprotic solvent, a support electrolyte and or the monomers required to form, by polymerization or copolymerization, the coating by putting the particles in suspension and in movement in the above-mentioned bath in such a way as to create an intermittent contact of said particles with the cathode, and to form, by polymerization or copolymerization , the coating on the particles by the application of a potential or a current bringing the reaction system into a passivation zone corresponding to the peak of inhibition detected by voltammetry, the cathode remaining insensitive to said polymerization or copolymerization during l application of the above potential or current.

Other details and particularities of the invention will emerge from the description given, hereinafter by way of nonlimiting examples, of some particular embodiments of the method according to the invention with reference to the appended drawings. FIG. 1 shows, by way of example, the passivation zone on a typical voltammogram of a cathode of the same kind as that of particles to be covered.

Figure 2 is a schematic elevational view of an electrochemical cell allowing the implementation of the method according to the invention, in which the electrodes have been omitted.

Figure 3 is a top view of this same cell. Figure 4 is a partial vertical section, on a larger scale of two electrodes mounted in the cell of the previous figures. FIG. 5 is a front view of a plate forming a working cathode in the embodiment according to FIG. 4.

FIG. 6 is a graph showing an endotherm specific to the cyclization of polyacrylonitrile on graphite carbon powder. FIG. 7 is a graph showing the percentage by weight of polymer grafted on carbon black as a function of the electrolysis time.

FIG. 8 is a graph similar to that of FIG. 7 relating to two different types of carbon particles.

In the different figures, the same reference numbers relate to the same elements.

In general, the method according to the invention differs from the prior art by all of the following points:

- direct electrochemical grafting of the polymer onto the conductive substrate, that is to say without an intermediate link;

- grafting takes place only under strict control of the potential applied;

- intermittent contact between the particles to be covered and the working electrode (cathode) which remains inert with respect to grafting.

In fact, the proposed method is characterized by an electrografting of a monomer unit which then acts as the initiator of the polymerization.

More particularly, it has been found, according to the invention, that it is possible, in an unpredictable manner in the presence of the prior art relating to the coating of conductive particles with a polymer, to obtain such a coating, perfectly stable and homogeneous, in an electrolysis bath containing the monomer, an aprotic solvent and a support electrolyte, by an electrochemical reduction of this monomer, induced during intermittent contact of said particles with a cathode arranged in this bath, this provided that a well-determined potential or current is applied to the cathode and that the latter remains insensitive to the polymerization of the monomer during the application of this well-determined potential or current. Indeed, as shown in FIG. 1, before the reduction of the solvent or of the cation of the support electrolyte, two distinct peaks in potential and in intensity are observed, namely a peak I and a peak II. It has been found that the grafting and polymerization of the monomer on the surface of the conductive particles in contact with the cathode occur at the level of peak I. If the potential applied to the cathode does not exceed this value, it is found, in a way unpredictable, that these particles are effectively covered with a thin film of grafted polymer. If the particles are formed of carbon, there will therefore be continuity between the carbon and the polymer.

Thus, the electrolysis of a solution formed of acrylonitrile in an aprotic solvent (acetonitrile) in the presence of a support electrolyte (Et ₄ NCIO ₄ ) leads to the grafting of a polyacrylonitrile film (PAN) on the conductive particles. , formed in particular of carbon. The PAN film formed under the electrolysis conditions defined above is insoluble in dimethylformamide (DMF) which is a good solvent for PAN. On the other hand, at the second peak II, a thick film of PAN, which has formed, is completely soluble in DMF.

The application of the process according to the invention therefore requires refined control of the electrolysis conditions: it is necessary to work only in the region of peak I (inhibition peak) in order to obtain a homogeneous grafting of the polymer on the cathode. In practice, this condition can be achieved using two techniques. Potentiostatic electrolysis where the potential of the cathode is fixed relative to a reference electrode in the area of peak I and galvanostatic electrolysis where the polymerization current is fixed at a value lower than the maximum current (ip) defined in voltammetry . The current density is adjusted as a function of the average diameter of the particles to be coated and the dynamic stirring conditions of so as not to exceed a current corresponding to the first peak I located on the fixed electrode.

The originality of the process is the production of intermittent contacts between conductive particles to be covered and a fixed cathode chosen from a conductive material on which the grafting of a polymer according to the process according to the invention is impossible. Without restriction, zinc, tin or even silicon carbide (SiC) can be used as cathode in the process. Intermittent contact is ensured by temporarily bringing the particles into contact with the cathode either by a usual method of stirring a liquid such as a propeller mixer, magnetic stirrer, ultrasonic field, or by mechanical movement of the cathode, such as rotation, oscillation or vibration. It is also possible to use an enveloping cathode, which can serve as an electrolytic cell, itself set in motion. In dynamic agitation, each particle statistically comes into contact with the working electrode with renewed spatial orientations. At each contact, the monomer is polarized and an electronic transfer with the electrode is transiently established: this step is the necessary and sufficient condition for the polymerization to begin. After this initiation, the stirring brings the particle back to the solution where the polymerization will continue chemically since the monomer is available there. In this polymerization step, the polarization of the cathode no longer occurs at the level of the particle. This was not necessarily covered in its entirety at the first contact with the cathode but it could be during subsequent contacts between the non-covered parts of the particle and the cathode which always remains polarized under the conditions of potential specified above.

According to the invention, several monomers are capable of being grafted to the first peak, but the monomer / solvent pair must be carefully chosen. By way of nonlimiting example, it is necessary to cite the acrylonitrile / acetonitrile, acrylonitrile / dimethylacetamide, acrylonitrile / N, N-dimethylformamide, acrylonitrile / pyridine, ethyl acrylate / N, N-dimethylformamide, ethyl acrylate / pyridine, 2-trimethyl-silyloxyethylmethacrylate / N, N-dimethylformamide systems , methyl methacrylate / N, N-dimethylformamide, tertiary butyl acrylate / N, N- dimethylformamide, n-butyl acrylate / N, N-dimethylformamide, allyl methacrylate / NN-dimethylformamide, glycidyl methacrylate / N, N -dimethylformamide. All these solvents belong to the class of aprotic solvents in which grafting is possible. The choice of the support electrolyte has, in principle, no fundamental influence on the grafting but it is preferable to use a support electrolyte which does not bring residual water of the solvent in the double layer of the coating formed on the particles.

The shape and the volume of the electrolysis cell used for the implementation of the process according to the invention are determined by the nature of the particles to be covered and by the choice of the type of intermittent particle / cathode contact. The electrolysis cell, as shown in FIGS. 2 to 4, comprises an enclosure 1 containing a solution formed by a solvent, a monomer or a mixture of comonomers, an electrolyte and conductive particles in suspension . In the bottom of this enclosure 1 is provided a magnetic bar 2 rotating around a vertical axis to thereby ensure the homogeneity of the solution and the setting in motion of the conductive particles to be coated with a polymer. The upper part of this enclosure 1 has three internally ground conical housings 3, 4 and 5 in which electrodes are suspended, in particular a central working electrode 6, a counter electrode 8 and a reference electrode 8 '.

These electrodes are shown in more detail in FIG. 4. The working electrode 6 comprises at its free end a plate, for example zinc, (50χ50mm, thickness 0.5mm) perforated as shown in detail in Figure 5. The number of holes as well as their diameter is not critical provided that a free circulation of the suspension is ensured and that the particle / zinc contact time is long enough to allow the particles to polarize at the potential applied to the cathode. This plate 7 is suspended by a clamp 9 from a conductor 10. which extends inside, in the axis, of a glass sleeve 11, the upper opening 12 of which is closed by a plug of epoxy adhesive 13. The sleeve 11 has a lapped conical outer wall 14 which fits in a substantially sealed manner in the housing 4 of the enclosure 1.

The reference electrode 8 ′ and the counter electrode 8 are of the same construction and comprise a platinum sheet wound in the form of a cylinder 15 having an area of the order of 10 cm ² . This cylinder 15 is suspended from a conductor 16 extending in the axis of a glass sleeve 17 closed at its upper end 18 by a plug of epoxy adhesive 19.

As for electrode 6, the electrodes 8 and 8 ′ have a conical outer wall 20 which fits tightly in the corresponding housings 3 and 5 of the enclosure 1.

In a variant, the working electrode 7 may for example be formed by a bar of silicon carbide, for example 2 mm in diameter and 2 cm in length, or else by a tin bar.

The counter-electrode ensuring the passage of the current must be inert: it is produced for example from platinum or from vitreous carbon. If electrolysis under potentiostatic conditions is preferred, the reference electrode is either a standard electrode for organic media (Ag / AgCIO _t / solvent) or a pseudo-reference of platinum. The manipulations are carried out in a protected atmosphere, away from oxygen and water with for example a water content of the order of 5 to 50 ppm.

The concrete examples of embodiment given below, which are applied in an electrolysis cell of the type as shown in FIGS. 2 to 5 and described above, make it possible to further illustrate the process according to the invention.

EXAMPLE 1 Grafting of Polyethylacrylate (PAE) on Graphite Carbon Powder In this example, the grafting and electropolymerization of ethyl acrylate (AE) is carried out on the surface of graphite particles. The electrical contact is ensured by a silicon carbide (SiC) electrode. The electrochemical technique used is chronopotentiometry. The electrolysis bath is prepared on the basis of the following constituents:

- 150 ml of DMF dried over P ₂ O ₅ for 72 hours, then distilled under reduced pressure.

- 1.9 g of Et »NCIO ₄ (electrolyte-support) dried under vacuum at 80 ° C for 24 hours.

- 37.5 ml of AE dried over CaH ₂ for 48 hours, then distilled under reduced pressure.

- 0.35 g of graphite carbon powder.

An SiC bar (cathode), a counter electrode and a pseudo-reference (both made of platinum) plunge into the electrolysis solution. Compared to the pseudo-reference, a constant current of -15 μA is applied to the SiC bar for 90 minutes. The polarization of the carbon is ensured by the agitation of the dispersion which is ensured by a magnetic stirrer. After electrolysis, the carbon is separated by centrifugation, then washed several times with a solvent of polymer so as to dissolve all the polymer which would not be grafted. The presence of grafted PAE is demonstrated macroscopically by the agglomeration of the particles coated with polymer even after multiple extractions with a good solvent for PAE.

EXAMPLE 2 Grafting of Polyethylacrylate (PAE) on Graphite Carbon Powder

In this example, the grafting and electropolymerization of ethyl acrylate (AE) are also carried out on the surface of graphite particles. The electrical contact is ensured by a zinc electrode (Zn). The electrochemical technique used is chronoamperometry. The electrolysis bath is prepared on the basis of the following constituents:

- 1.9 g of Et ₄ NCIO ₄ (support electrolyte) dried under vacuum at 80 ° C for 24 hours.

- 40 ml of AE dried over CaH ₂ for 48 hours, then distilled under reduced pressure. - 0.2 g of graphite carbon powder.

A Zn plate of 5cmn5cm (cathode), a counter electrode as well as a pseudo-reference electrode (both made of platinum) immerse in the electrolysis solution. With respect to the pseudo-reference, a constant potential of -1.7 V is applied to the Zn electrode for 21 hours; this potential corresponds to the potential of the first peak. Carbon polarization is ensured by stirring the solution. The presence of PAE is demonstrated macroscopically, as described in the first example. The percentage by weight of the polymer grafted onto the powder is determined by TGA (thermal gravimetry analysis), and amounts to 28% of the total weight of the composite. EXAMPLE 3 Grafting of Polyacrylonitrile (PAN) on Graphite Carbon Powder

In this example, the grafting and electropolymerization of acrylonitrile (AN) are also carried out on the surface of graphite particles. The electrical contact is ensured by a zinc electrode (Zn) of 50χ50mm pierced with holes of 2 mm in diameter. The electrochemical technique used is chronoamperometry. The electrolysis bath is prepared on the basis of the following constituents:

- 225 ml of DMF dried on PO ₅ for 72 hours, then distilled under reduced pressure.

- 2.8 g of Et ₄ NCIO ₄ (electrolyte-support) dried under vacuum at 80 ° C for 24 hours.

- 35 ml of AN dried over CaH ₂ for 48 hours, then distilled under reduced pressure. - 1.6 g of graphite carbon powder.

With respect to the pseudo-reference, a constant potential of -1.7 V is applied to the Zn electrode for 21 hours. Carbon polarization is ensured by stirring the solution. The carbon is then washed several times with DMF, solvent for the PAN, so as to extract the PAN which would not be grafted. The presence of PAN is then demonstrated by DSC (differential scanning calorimetry), in the form of an endotherm specific to the cyclization of PAN (Figure 6). The percentage by weight of the grafted polymer, determined by TGA (thermal gravimetry analysis), is 15% relative to the total weight of the composite.

Example 4: Grafting of polyacrylonitrile (PAN) and polyethylacrylate (PAE) on carbon black. In this example, the kinetics of PAN and PAE grafting are compared to the surface of carbon black. The electrical contact is ensured by a zinc electrode (Zn) pierced with 2 mm holes diameter. The electrochemical technique used is chrono-amperometry.

The electrolysis baths are prepared on the basis of the following constituents: 1) AN bath

- 100 ml of DMF dried over P ₂ O ₅ for 72 hours, then distilled under reduced pressure.

- 1.15 g of Et ₄ NCIO ₄ (electrolyte-support) dried under vacuum at 80 ° C for 24 hours. - 15 ml of AN dried over CaH ₂ for 48 hours, then distilled under reduced pressure.

- 2.25 g of carbon black type XE2 (Degussa). 2) AE bath

- 1.15 g of Et ₄ NCIO ₄ (electrolyte-support) dried under vacuum at 80 ° C for 24 hours.

- 27 ml of AE dried over CaH ₂ for 48 hours, then distilled under reduced pressure. - 2 g of carbon black type XE2 (Degussa).

A Zn plate of 5cmχ5cm pierced over its entire surface with 2 mm diameter holes (cathode), a counter electrode as well as a pseudo-reference electrode (both made of platinum) plunge into the electrolysis solutions. With respect to the pseudo-reference, a constant potential of -2.2 V (first peak) is applied to the Zn electrode for 15, 30, 60 and 100 minutes in the AN bath, and a potential of -1.7 V ( first peak) for 15, 30, 60 and 120 minutes in the AE bath. Carbon polarization is ensured by stirring the solution. The various carbon samples are then washed several times with DMF, solvent for PAN and PAE, so as to extract any non-grafted polymer. The grafted PAE sample is also freed of ungrafted PAE by extraction with THF in a Soxhlet apparatus. The percentage by weight of the grafted polymers is determined by TGA (thermal gravimetry analysis). FIG. 7 shows the percentage by weight of polymer grafted on carbon black as a function of the electrolysis time.

Example 5: Grafting of polyethyl acrylate (PAE) on carbon black and on graphite carbon powder. In this example, the kinetics of PAE grafting are compared on the surface of carbon black and graphite carbon powder. The electrical contact is ensured by a zinc electrode (Zn) pierced with 2 mm diameter holes. The electrochemical technique used is chronoamperometry. The electrolysis baths are prepared on the basis of the following constituents:

1) Carbon black bath

- 1.15 g of Et NCIO ₄ (electrolyte-support) dried under vacuum at 80 ° C for 24 hours.

- 27 ml of AE dried over CaH ₂ for 48 hours, then distilled under reduced pressure.

- 2 g of carbon black type XE2 (Degussa).

2) Graphite carbon powder bath - 110 ml of DMF dried over P ₂ O ₅ for 72 hours, then distilled under reduced pressure.

- 1.27 g of Et4NCIO (electrolyte-support) dried under vacuum at 80 ° C for 24 hours.

- 30 ml of AE dried over CaH ₂ for 48 hours, then distilled under reduced pressure. - 5.2 g of graphite carbon powder

A 5cmχ5cm Zn plate pierced over its entire surface with 2mm diameter holes (cathode), a counter electrode as well as a pseudo-reference electrode (both made of platinum). immerse in the electrolysis solutions. With respect to the pseudoreference, a constant potential of -1.7 V (first peak) is applied to the Zn electrode for 15, 30, 60 and 120 minutes in each of the electrolysis baths. The polarization of the two types of carbon particles is ensured by the agitation of the solutions. The different carbon samples are then washed several times with DMF and THF, PAE solvents, so as to extract the polymer which would not be grafted. The percentage by weight of the grafted polymers is determined by TGA (thermal gravimetry analysis). FIG. 8 shows the percentage by weight of polymer grafted onto carbon black and onto the carbon powder as a function of the electrolysis time. Note the influence of the nature of the carbon particle on the grafting rate.

Example 6: Grafting of polyacrylonitrile (PAN) on metallic powder. In this example, the PAN is grafted onto the surface of metallic nickel powder. The electrical contact is ensured by a zinc electrode (Zn) pierced with 2 mm diameter holes. The electrochemical technique used is chronoamperometry.

The electrolysis bath is prepared on the basis of the following constituents:

- 108 ml of DMF dried over P ₂ Os for 72 hours, then distilled under reduced pressure.

- 1.24 g of Et ₄ NCIO ₄ (electrolyte-support) dried under vacuum at 80 ° C for 24 hours. - 17 ml of AN dried over CaH ₂ for 48 hours, then distilled under reduced pressure.

- 5.12 g of nickel particles (INCO, type 110).

Relative to the pseudo-reference, a constant potential of -1.7 V (first peak potential) is applied to the electrode of Zn for 30 minutes. The metal particles are polarized by stirring the solution. The carbon is then washed several times with DMF, solvent for the PAN, so as to extract the ungrafted PAN. The percentage by weight of the grafted polymer, determined by TGA (thermal gravimetry analysis), amounts to 10%.

Example 7: Grafting of polyethyl acrylate (PAE) on short carbon fibers.

In this example, the grafting and the electro-polymerization of ethyl acrylate (AE) are carried out on short carbon fibers, the latter not being directly polarized by permanent contact. The electrical contact is ensured by a zinc electrode (Zn) pierced with 2 mm diameter holes. The electrochemical technique used is chronoamperometry. The electrolysis bath is prepared on the basis of the following constituents: - 200 ml of DMF dried over P ₂ O ₅ for 72 hours, then distilled under reduced pressure.

- 2.3 g of Et4NCIO ₄ (electrolyte-support) dried under vacuum at 80 ° C for 24 hours.

- 60 ml of AE dried over CaH ₂ for 48 hours, then distilled under reduced pressure.

- 0.1 g of short fibers (2 to 3 mm) of carbon type AS-4 (Hercules).

A Zn plate of 5cmn5cm (cathode), a counter electrode as well as a pseudo-reference electrode (both made of platinum) immerse in the electrolysis solution. With respect to the pseudo-reference, a constant potential of -1.7 is applied to the electrode of Zn V (first peak potential) for 21 hours. The polarization of the short carbon fibers is ensured by the stirring of the solution. The presence of PAE is demonstrated macroscopically by the stickiness of the polymer even after repeated extraction with a good solvent for the polymer, and also microscopically by SEM (electron scanning mieroscopy). The percentage by weight of the polymer is determined by TGA ("thermal gravimetry analysis"), 52% is obtained.

It is understood that the invention is not limited to the embodiments described above but that many variants can be envisaged without departing from the scope of the present invention as well as regards the nature of the conductive particles to be coated by a polymer film that the electrolysis cell to use.

Claims

1. Method for forming a coating on conductive particles by grafting a polymer and / or copolymer onto these particles from a bath containing at least one monomer from which the polymer and / or copolymer is formed , characterized in that this grafting is carried out by an electrochemical reduction of said monomer in an electrolysis bath, in which at least one cathode and an anode are provided and containing an aprotic solvent, a support electrolyte and the monomer (s) required to form the coating by polymerization or copolymerization by suspending and moving the particles in the above bath in such a way as to create intermittent contact of said particles with the cathode, and to form, by polymerization or copolymerization, the coating on particles by applying a potential or a current bringing the reaction system into a passivation zone corresponding to the peak of in hibition detected by voltammetry, the cathode remaining insensitive to said polymerization or copolymerization during the application of the above potential or current.

2. Method according to claim 1, characterized in that carbon particles are used which are in the form of granules, powder or fibers.

3. Method according to claim 1, characterized in that metal particles are used which are, for example, in the form of powder, granules or fibers.

4. Method according to any one of claims 1 to

3, characterized in that a cathode is used in a conductive material on which the grafting of the aforementioned polymer or copolymer is impossible, such as zinc, tin or silicon carbide.

5. Method according to any one of claims 1 to 4, characterized in that use is made of a monomer / solvent couple aprotic formed for example from one of the following systems: acrylonitrile / acetonitrile, acrylonitrile / dimethylacetamide, acrylonitrile / N, N-dimethylformamide, acrylonitrile / pyridine, ethyl acrylate / N, N-dimethylformamide, ethyl acrylate / pyridine, 2 - trimethylsilyloxyethylmethacrylate / N, N-dimethylformamide, methyl methacrylate / N, N- dimethylformamide, tertiary butyl acrylate / N, N-dimethylformamide, n-butyl acrylate / N, N-dimethylformamide, allyl methacrylate / NN - dimethylformamide, glycidyl methacrylate / N, N-dimethylformamide.

6. Method according to any one of claims 1 to 5, characterized in that, in order to maintain the conditions of electrolysis in the region of the inhibition peak, the potential of the cathode is fixed and maintained with respect to a reference electrode in the area of the inhibition peak.

7. Method according to any one of claims 1 to 5, characterized in that the polymerization current is fixed at a value lower than that of the maximum current (ip) defined in voltammetry, keeping the current pulse substantially constant lower to that of the polymerization of the first inhibition peak, so as to avoid a second polymerization mechanism.

8. Method according to any one of claims 1 to 7, characterized in that the electrolysis current is applied continuously or discontinuously.