HK1094232B - Method for forming a polymer film on a surface that conducts or semiconducts electricity by means of electrografting, surfaces obtained, and applications thereof - Google Patents

Description

Method for forming a polymer film on a conductive or semiconductive surface by electro-grafting, surface thus obtained and uses thereof

Technical Field

The present invention relates to a method for forming a polymer film on a conductive or semi-conductive surface by electro-grafting using an electrolytic solution containing a bronsted acid, and to a conductive or semi-conductive surface obtained by using such a method.

Background

The preparation of conductive or semiconductive surfaces covered with a polymer film is advantageous in many fields, in particular for the manufacture of electronic components or integrated optical devices, for the preparation of devices which can be used in the biomedical field or in biotechnology (DNA chips, protein chips, etc.), for corrosion protection, and for any improvement in the properties of metal or semiconductor surfaces.

It appears to be accepted at present that the preparation of a grafted polymer film by electro-grafting of an activated vinyl monomer to a conductive surface is carried out in the following manner: the polymerization is initiated electrically from the surface and subsequently proceeds monomer by monomer. The reaction mechanism of the electrografting is described in particular in the following papers: bureau et al, Macromolecules, 1997, 30, 333; bureau and J.Delhallle, Journal Surface Analysis, 1999, 6(2), 159 and C.Bureau et al, Journal of addition, 1996, 58, 101.

For example, the reaction mechanism of the electro-grafting of acrylonitrile by means of cathodic polarization can be represented by the following scheme 1, in which the grafting reaction corresponds to stage 1, in which the growth proceeds from the surface; stage 2 is the main side reaction leading to the production of non-fishplate polymer:

scheme 1

1. Surface chemical reaction, grafting

2. Desorption, solution polymerization

The growth of the grafted chains thus takes place by purely chemical polymerization, that is to say independently of the polarization of the conductive surface which leads to the grafting. This phase is therefore sensitive to (and in particular interrupted by) the presence of the growing chemical inhibitor.

In the above scheme 1, which has considered electro-grafting of acrylonitrile under the action of cathodic polarization, the propagation of the graft chain is carried out by anionic polymerization. This growth is interrupted in particular by protons, and it has been demonstrated that: the proton content is in fact the main parameter controlling the formation of the polymer in solution and the information recovered during the synthesis (in particular the shape of the voltammogram accompanying the synthesis) (see in particular the article by c.

One of the aims sought since the start of research on the electro-grafting of polymers is to obtain thick and uniform films, and therefore the idea of being able to fully combine plastic and metal objects is attractive. This object assumes that a graft polymer chain of high molecular weight can be obtained by electro-grafting as in the manner obtained in conventional polymer chemistry, and thus the chain growth is maintained.

Since this growth is ionic and in particular anionic when the electrografting is carried out in the case of cathodic polarization, it has been accepted that: traces of water, and more generally protic solvents which behave as bronsted acids in the reaction medium and/or labile protons of any compound constitute a proton source which is detrimental to the propagation of the grafted chain. In the context of the present invention, the term "solvent" is understood to mean the complete electrolytic medium in which the electro-grafting is carried out and in particular comprises a liquid which does not substantially participate in the reaction (spectateur), whose dielectric constant is sufficient to be able to dissolve the salt and to ensure electrical conduction in the liquid phase, the supporting electrolyte or the salt and the optional additives (and in particular water in the case of the present invention).

In fact, even before the mechanism of the electro-grafting reaction of vinyl monomers has been understood, this technical view of encumbrance has been clearly confirmed by the person skilled in the art, as evidenced by the details of the various processes studied on the basis of these compounds:

in patent application FR-A-2480314, the authors mention A process for the electrografting of vinyl monomers, which comprises preparing A polymer containing water up to 10^-3A solution of mol/l, and in an advantageous embodiment even with a water content of at most 5X 10^-4mol/l；

In patent application EP-A-0618276, the authors mention ｃA process for the electro-grafting of vinyl monomers using aprotic solvents;

in patent application EP-A-0665275, the authors also mention ｃA process for the electro-grafting of vinyl monomers using aprotic organic solvents. In addition, the specification section of this prior art application states that the water content in the electrolytic bath is preferably less than 10^-3And M. Thus, prior to electrolysis, the bath is degassed by bubbling with an inert gas containing up to 5ppm water and 10ppm oxygen;

in U.S. Pat. No.6180346, the authors employ a process for electropolymerization of molecules containing vinyl substituents. As an example, they mention the use of acetonitrile as solvent and specify that acetonitrile is to be dried before use, which means to the person skilled in the art that the residual water content is about to most ten ppm;

in us patent 5578188, the authors claim a process for depositing composite films on electrically conductive surfaces by electropolymerization, according to which a mixture comprising (a) precursor monomers for polymerizing electrically non-conductive polymers, (b) a dopant-forming substance for being incorporated into such polymers, (c) a supporting electrolyte and (d) an aprotic solvent is discussed in the description section, with the aid of monomers, supporting electrolytes, dopants and solvents, which follow the constraints of the aprotic nature required for the reaction;

finally, in us patents 6325911 and 6375821, the authors describe the use of a composition comprising (a) at least one monomer capable of forming a polymer on a substrate; (b) a conductive mixture of an aprotic solvent and (c) an electrolyte for increasing the conductivity of the mixture, a method of grafting a polymer onto a carbonaceous substrate or onto a particle by electropolymerization of a vinyl monomer.

The examples report experimental situations in which the combination of liquid reactants used has been purified, in particular by distillation or by placing on strong drying agents, in order to make the medium aprotic.

According to the teaching of the prior art described above, the very low water content required for the electro-grafting of vinyl monomers is maintained during or before the synthesis by bubbling with dry inert gas (nitrogen, argon and the like) having a water content of about several ppm, even by electrolysis in a closed chamber of the glove box type placed under a controlled atmosphere of argon or nitrogen.

For similar reasons for controlling the content of proton source in the reaction medium, it has been proposed that only aprotic solvents and monomers which are themselves aprotic are used for producing the electrografted organic membranes, that is to say that no functional groups with an acid function (in the bronsted sense) are included in the solvents studied.

In practice, the reduction of the water content in these solvents is at the expense of a lengthy and laborious preparation, for example by placing in a dehydrating compound such as phosphorus pentoxide (P)₂O₅) On a molecular sieve (e.g. with a porosity of 4 a), by distillation under reduced pressure of an inert noble gas (nitrogen, argon, etc.), or by a combination of these methods. It is reported that:

in patent applications FR-A-2480314 and EP-A-0618276, the authors recommend the use of aprotic organic solvents which do not undergo side reactions with the monomers used;

in patent application EP-A-0665275, in addition to mentioning the use of aprotic solvents, the authors propose various forms of monomeric structures that can be used and specify that it is necessary to mask in advance the possible protic functions of one or more monomers.

In practice, the monomers used for the electrosynthesis are distilled before use, in order to remove the various additives and in particular the polymerization inhibitors added by the manufacturer to stabilize the product and prevent it from polymerizing in the bottles under storage conditions.

It is to be noted that only patent application EP-A-0665275 mentions the use of specific inhibitors in order to be able to introduce new functionalities at the growing polymer chain ends. However, it is demonstrated, in particular in the paper of c.bureau et al, 1996 (mentioned above), that the growth of the polymer chains on the surface must be anionic and that it is possible to find free radical inhibitors introduced by the authors in the membrane at the end of the synthesis, because they are adsorbed on the electrode surface and/or reduced (they are generally electroactive), rather than because of their chain-scission growth, as described in patent application EP- ｃA-0665275.

In fact, very advantageous results, in particular in terms of homogeneity, have been obtained in the case of the electro-grafting of polymers onto metals by carrying out the electro-grafting starting from a completely aprotic solution and under a controlled atmosphere.

However, regardless of the source of the reference, these results report only ultra-thin electro-grafted films, typically several nanometers to, at best, tens of nanometers in thickness. The following facts are emphasized importantly: this relates to the thickness of the film actually electro-grafted onto the surface, that is to say the thickness of the film obtained from stage 1 of scheme 1 described above. According to stage 2 of this procedure, the polymer formed in solution and which can be deposited on the surface during the electrosynthesis process is generally easily removed by washing the surface with a solvent for said polymer (optionally under ultrasonic conditions), while the electro-grafted polymer is resistant to this treatment.

Even though these thickness ranges have been advantageous for some applications, at the same time the practical need to improve the synthesis conditions is observed, in order to increase the obtainable thickness, and/or in order to achieve a better control and a better reproducibility of the fine thicknesses, in particular in the range of 10nm to 1 micron, and secondly the need to reach these thickness ranges with synthesis conditions that are less severe than those used so far is to be able to be applied industrially.

Us patent 3759797 reports the preparation of polymer films on conductive surfaces based on formulations comprising inter alia vinyl monomers and short chain thiol or alcohol (especially ethanol) or quinone additives. The authors of this invention mention that these additives can limit the polymerization in solution and accordingly enhance the propagation reaction starting from the surface. Although the examples of this patent show a reduction in the amount of polymer formed in solution, they do not allow to evaluate the properties of the part left on the surface, in particular in terms of thickness, since no characterization of the surface or any measurement of the thickness of the resulting coating is carried out. In addition, this technical data is not sufficient to evaluate the actual state of the water content in the medium.

Disclosure of Invention

However, the present inventors have found that the inclusion of short chain alcohols (especially ethanol) when formulating a reaction bath for electro-grafting is neither possible to increase the thickness of the resulting film nor to control the film. In contrast, the thickness of the resulting membrane was observed to become increasingly thinner with increasing ethanol concentration, consistent with the "conventional" explanation for the effect of protic additives (e.g., ethanol) on anionic polymerization.

Therefore, in order to overcome all these main drawbacks and to provide a method for forming a polymer film on a conductive or semiconductive surface which allows in particular to control the thickness of the resulting film and which is easy to implement from an industrial point of view, the inventors have developed a method which constitutes the subject of the present invention.

The inventors have now developed formulations for reaction baths for electro-grafting, by means of which they have sought to obtain on conductive or semiconductive surfaces electro-grafted organic films having a thickness greater than that obtained under conventional conditions and not achievable under the usual conditions.

Contrary to the prejudice that has hitherto worked in the art, the formulations of the invention all comprise a proton source selected from compounds that are bronsted acids in the electrolytic bath, such as in particular water, in a proportion selected between 50 and 100000ppm with respect to the other components in the reaction bath. In addition, the thickness obtained can be strictly controlled by the choice of the proton sources and by the choice of their concentration range in the reaction bath: this control proved to be novel for "thick" films (thickness greater than 10nm) and better than the control achievable in strictly anhydrous media for ultra-thin films (< 10 nm).

The first subject of the invention is a process for forming a polymer film by electro-grafting on a conductive or semi-conductive surface, characterized in that it comprises:

a) preparing an electrolytic solution comprising one or more electropolymerizable monomers and at least one proton source selected from compounds which are bronsted acids in said electrolytic solution, said proton source being present in an amount of 50 to 100000ppm relative to the total amount of ingredients in said electrolytic solution; and

b) the solution is electrolyzed in an electrolytic cell by using the conductive or semi-conductive surface to be covered as a working electrode and at least one counter electrode to cause the formation of an electro-grafted polymer film on the surface by electro-reduction or electro-oxidation of the solution.

In the meaning of the present invention, the term "bronsted acid" is understood to mean any substance comprising at least one functional group bearing at least one labile proton (or at least one labile isotope such as deuterium or tritium) in the electrolytic solution used according to the above-mentioned method, and which is partially (weakly acid) or completely (strongly acid) ionized, even dissociated, in said solution, to give the conjugate base of the compound and a solvated proton (deuterium or tritium, respectively). In water, compounds are readily classified as bronsted acids according to their acidity constant or pKa: the compound in acid form that constitutes the pair with a pKa of less than 14 is a bronsted acid (weak acid (partial dissociation) if its pKa is 0-14, and strong acid (complete dissociation) if its pKa is negative). In an organic solvent having a constituent molecule containing a proton (deuterium or tritium, respectively), a compound can be considered a bronsted acid if its pKa in the solvent is less than the proton autodelivery product (produit) of the solvent. For example, in the article by G.Deniau et al (1998, Journal of electroanalytical Chemistry, 451, 145) it is disclosed that 2-butenenitrile is a weak Bronsted acid in acetonitrile. In some advantageous cases, theoretical models enable to establish a correspondence between the pKa scale in water and its corresponding amount in a given organic solvent, whereby literature data can be utilized, since pKa values of many compounds in water are currently available. In addition, based on quantum chemistry, theoretical models also make it possible to calculate the pKa of certain acid/base pairs in various solvents, as described in the paper by g. When the electrolytic solution comprises other molecules, such as supporting electrolytes or electropolymerizable monomers, etc., it is preferred to employ a direct or indirect measurement of the proton content that is generated as a result of the introduction of the putative bronsted acid into the medium. This can be done by making measurements using a conductivity meter (measuring the change in conductivity in the solution) or a Karl-Fischer device. This is also a way in which the properties of the bronsted acid of a compound in a solvent whose molecular structure is aprotic can be determined.

Among the Bronsted acids which can be used according to the process of the invention, mention may in particular be made of water and compounds which are Bronsted acids in water, for example weak acids such as hydrogen fluoride, ammonium fluoride, nitrous acid, molecules bearing carboxylic acid groups (for example acetic acid, citric acid, amino acids and proteins, etc.) or molecules bearing ammonium, amine, pyridinium or phenolic groups, etc., and strong acids (for example sulfuric acid, nitric acid, hydrogen chloride and perchloric acid), molecules bearing sulfate, sulfonate or thiol groups, etc.

According to the process of the invention, the electropolymerizable monomer is preferably chosen from activated vinyl monomers and cyclic molecules cleavable by nucleophilic attack, which correspond respectively to the following formulae (I) and (II):

wherein:

-A、B、R₁and R₂Identical or different, represents a hydrogen atom; c₁-C₄An alkyl group; a nitrile group; an organofunctional group selected from the group consisting of: hydroxyl group, amine: -NH_xWherein x ═ 1 or 2, thiols, carboxylic acids, esters, amides: -C (═ O) NH_yWherein y is 1 or 2, imide, imido ester, acid halide: -C (═ O) X, wherein X represents a halogen atom selected from fluorine, chlorine, bromine and iodine, acid anhydride: -C (═ O) OC (═ O), nitriles, succinimides, phthalimides, isocyanates, epoxides, siloxanes: -Si (OH)_zWherein z is an integer from 1 to 3 inclusive; benzoquinone, carbonyldiimidazole, p-toluenesulfonylP-nitrophenyl chloroformates, alkenyl and vinyl, aromatic compounds and especially toluene, benzene, halogenated benzene, pyridine, pyrimidine, styrene or halogenated styrene and their substituted equivalents; functional groups that can complex cations and especially cations of reducible metals such as copper, iron, and nickel; substituted and/or functionalized molecular structures starting from these functional groups; groups which can be cleaved by thermal or photonic activation, such as diazonium salts, peroxides, nitrosoanilides, alkoxyamines and especially 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO), benzophenones and their derivatives, dithioesters, dithiocarbamates or trithiocarbonates; electroactive groups and in particular precursors of electrically conductive polymers, such as aniline, thiophene, methylthiophene, bithiophene, pyrrole, Ethylenedioxythiophene (EDOT) and the like, and also electrically cleavable groups such as diazonium salts, sulfonium salts, phosphonium salts and iodonium salts; and mixtures of the foregoing monomers and groups;

-n, m and p are the same or different and are integers from 0 to 20 inclusive.

In the above symbol, R₁And R₂Are groups that implicitly depend on the index i (not shown), where i is 0 to n. This means that in the cyclic molecular structure of the formula (II), R₁And R₂The radicals being substantially different (C (R)₁)R₂) May be different from each other, that is, the symbol (C (R) used₁)R₂)_nNot being identical (C (R)₁)R₂) Repetition of a unit, but rather a series of (C (R)₁)R₂) A group of the kind wherein R₁And R₂The radicals form part of the above list.

Among the functional groups of the activated vinyl monomers of formula (I) above, which can complex cations, mention may be made in particular of amides, ethers, carbonyls, carboxyls and carboxylate groups, phosphines, phosphine oxides, thioethers, disulfides, ureas, crown ethers, aza crown compounds, thia crown compounds, cryptands, sepulracates, polydentate ligands, porphyrins, calixarenes, bipyridines, terpyridines, quinolines, o-phenanthroline compounds, naphthols, isonaphthols, thioureas, siderophores, antibiotics, glycols and cyclodextrins.

Among the activated vinyl monomers of formula (I) above, mention may be made in particular of acrylonitrile, methacrylonitrile, methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, glycidyl methacrylate, acrylamide and in particular the methacrylamides of aminoethyl, aminopropyl, aminobutyl, aminopentyl and aminohexyl, cyanoacrylates, diacrylates or dimethacrylates, triacrylates or trimethacrylates, tetraacrylates or tetramethacrylates (for example pentaerythritol tetramethacrylate), acrylic acid, methacrylic acid, styrene and its derivatives, p-chlorostyrene, pentafluorostyrene, N-vinylpyrrolidone, 4-vinylpyridine, 2-vinylpyridine, methyl methacrylate, ethyl methacrylate, butyl methacrylate, aminopropyl and aminohexyl methacrylate, cyanoacrylates, diacrylates or dimethacrylates, triacrylates or trimethacrylates, tetraacrylates, for example, Vinyl halide, acrylic halide, methacrylic halide, Divinylbenzene (DVB), and more generally vinyl crosslinkers or crosslinkers based on acrylates, methacrylates, and derivatives thereof.

Among the cleavable cyclic molecules of formula (II) above, mention may be made in particular of epoxides, lactones and in particular butyrolactone, epsilon-caprolactone and its derivatives, lactic acid, glycolic acid, alkylene oxides, and mixtures and derivatives thereof.

The concentration of electropolymerizable monomers in the electrolytic solution of the process of the present invention may vary from monomer to monomer. However, this concentration is preferably 0.1 to 10mol/l, more preferably 0.1 to 5 mol/l.

According to a particular embodiment of the process according to the invention, the electrolytic solution may additionally comprise at least one additional liquid (solvent) which does not substantially participate in the reaction, that is to say does not participate in the electropolymerization reaction, which serves to dissolve the electropolymerizable monomer or monomers which are insoluble or only sparingly soluble in water, with the aim of moving them into contact. Nevertheless, it is important to point out that the presence of such a liquid is not always necessary, since the following are foreseen: the monomer or monomers used are used in pure form, or some of the monomers in the monomer mixture act as a solvent, or all of the monomers in the monomer mixture are in a miscible ratio.

When these solvents are used, they are preferably chosen from dimethylformamide, ethyl acetate, acetonitrile, tetrahydrofuran, dichloroethane and, more generally, chlorinated solvents.

The process of the invention exhibits the advantage that these solvents can be used directly without the need to subject them to preliminary distillation to remove the water present therein or to strictly control the water content in the atmosphere above the reaction medium. The process of the invention can therefore be carried out easily on an industrial scale.

Likewise, according to another embodiment of the method of the present invention, the electrolytic solution may further comprise at least one supporting electrolyte to ensure and/or improve the flow of electrical current in the electrolytic solution. However, the use of a supporting electrolyte is not essential, for example in the case where the electropolymerizable monomer used itself comprises an ionic group (for example ammonium chloride of aminocyclohexyl methacrylate), which ensures that the resistance voltage drop of the current is maintained at an acceptable value.

When a supporting electrolyte is used, the supporting electrolyte is preferably selected from quaternary ammonium salts, such as quaternary ammonium salts of perchloric acid, toluenesulfonic acid, tetrafluoroboric acid, hexafluorophosphoric acid or quaternary ammonium halides, sodium nitrate and sodium chloride.

Among these quaternary ammonium salts, mention may be made, in particular, by way of example, of tetraethylammonium perchlorate (TEAP), tetrabutylammonium perchlorate (TBAP), tetrapropylammonium perchlorate (TPAP) or benzyltrimethylammonium perchlorate (BTMAP).

As mentioned above, the electrolytic solution used in the method according to the invention is distinguished by: the proportion of the Bronsted acid is 50 to 100000ppm with respect to the total amount of the respective components in the electrolytic solution. The selected concentration of bronsted acid is preferably determined experimentally, as this concentration generally depends on the following factors: the chemical nature of the electropolymerizable monomer or monomers used, the nature of the conductive or semiconductive surface on which the electrografting is to be carried out, the nature of the optional supporting electrolyte, the optional liquids which do not participate in the reaction and the relative concentrations of these various compounds in the reaction mixture.

Good starting points may be based on the type of typical procedure for electro-grafting in an aprotic medium. This is because the inventors have been able to observe, in a completely unexpected and surprising manner, that the thickness of the electro-grafted film obtained by this procedure can be significantly thicker at higher water contents. Even cumulative effects are often observed, as a result of which it is possible to use the best operating procedures studied under aprotic conditions, and their ability to produce membranes with high and controllable thickness can be further improved by optimizing the water content.

In a very preferred manner, this content of Bronsted acid is from 50 to 10000 ppm.

According to the invention, the conductive or semiconductive surface is preferably a surface made of stainless steel, iron, copper, nickel, cobalt, niobium, aluminum (especially when it has just been brushed on), silver, titanium, silicon (doped or undoped), titanium nitride, tungsten nitride, tantalum or tantalum nitride, or a noble metal surface selected from gold, platinum, iridium or iridium-platinum surfaces; among which gold surfaces are particularly preferred according to the invention.

The process of the invention is particularly useful for producing grafted polymer films of very precisely controlled thickness: electrografted membranes are of great benefit in the manufacture of DNA chips because they are inherently electrical insulators. In the case of using them to link oligonucleotides whose hybridization is intended to be detected by optical methods (fluorescence), their thickness is to be precisely controlled: it must be on the order of one hundred to several hundred nanometers and must be adjusted with an accuracy of plus or minus 5 nanometers in order to optimize the recovery of fluorescence intensity and minimize the light absorption by the substrate. The electrografted membrane may also be used as a molecule "velcro" to connect thicker layers by various types of bonds: the "reservoir" layers of pharmaceutical molecules are connected for controlled release (e.g. for vascular implants or stents), the layers are connected by chemical or electrochemical compression strips (marcottage), especially inorganic layers (especially for mineralisation of the surface of implants such as bone implants, or surface metallisation in microelectronics, production of "seed layers" for damascene processes of copper interconnects), polymer layers are connected to electro-grafted layers by thermal fusion (for low temperature bonding of polymers to metals), etc. In these cases, chemical reactivity, interdigital length or glass transition temperature, respectively, are parameters by which good attachment can be obtained by the electro-grafted membrane. In fact, in all these application examples, these parameters are related to the thickness of the electrografted membrane acting as "velcro" and achieve advantageous values for membranes with a thickness generally greater than 100 nm.

According to the method of the present invention, electrolysis of the electrolytic solution can be achieved by performing polarization under potentiostatic or galvanostatic voltammetric conditions.

Grafting and growth of the membrane occurs on the cathode when the absolute value of the membrane potential is greater than the electrolytic reduction potential of the electropolymerizable monomer or monomers used.

The subject of the invention is also a conductive or semiconductive surface obtained by using the method described above, at least one face of which is covered with an electrografted polymer film.

Typically, this coating has a thickness of 10 nanometers to 10 micrometers. Very surprisingly and as demonstrated in the examples below, these thicknesses are significantly greater than those of the electro-grafted films obtained by using the same electropolymerizable monomer according to an electropolymerization process carried out under aprotic or anhydrous conditions.

According to a preferred embodiment of the invention, this coating has a thickness of 100 nm to 10 μm.

In addition to the foregoing, the present invention includes other matters appearing in the following description which relate to an example of forming a polymethacrylonitrile film on the surface of a gold electrode in the presence of water of various concentrations, as compared with a non-inventive method using an ethanol-containing electrolytic solution; a second embodiment involving the formation of a polymethacrylonitrile film on the surface of a gold electrode in the presence of water; to examples for describing the effect of methacrylonitrile monomer content in the presence of water on the thickness of the formed film; to examples of studies for describing the influence of water content during the formation of a polymethacrylonitrile film on the surface of a gold electrode in the presence of water; examples relating to the effect of the concentration of the supporting electrolyte in the presence of water on the thickness of the polymethacrylonitrile film obtained on the gold electrode; and to the accompanying drawings, in which:

drawings

FIG. 1 represents a device for covering a gold electrode with a polymethacrylonitrile film in the presence of water. The device comprises a sealed electrolytic cell (1) provided with a lid (2) through which a vent (3) passes, said sealed electrolytic cell (1) comprising an electrolytic solution (4) and a gold working electrode (5), Ag⁺a/Ag reference electrode (6) and a platinum counter electrode (7). Continuously bubbling an electrolytic solution with argon (8), wherein the argon (8) is pre-flowed into a protective device (9) containing the electrolytic solution (10) and a molecular sieve (11), said protective device (9) itself being bubbled with argon (12, 13);

FIG. 2 represents the thickness (in nanometers) of a polymethacrylonitrile film obtained by electropolymerization of methacrylonitrile monomer on a gold sheet as a function of the water content (ppm);

FIG. 3 represents the transmission (%) of the infrared absorption band of the nitrile functions in a polymethacrylonitrile film obtained by electropolymerization of methacrylonitrile monomer on a gold electrode as a function of the ethanol content (ppm);

FIG. 4 represents the thickness (Angstrom) of a polymethacrylonitrile film obtained by electropolymerization on a gold sheet at different concentrations of methacrylonitrile monomer (diamonds: 0.1 mol/l; squares: 1 mol/l; triangles: 2.5mol/l and circles: 9.54mol/l) as a function of the water content (ppm);

FIG. 5 represents the thickness (Angstrom) of a polymethacrylonitrile film obtained by electropolymerization on a gold plaque as a function of the water content (ppm) for different concentrations of supporting electrolyte (TEAP) in the electrolytic solution.

Detailed Description

Example 1: compared to processes using ethanol, polymethacrylene is formed in the presence of different concentrations of water Nitrile (PMAN) films

This example illustrates the preparation of an electro-grafted film having a thickness greater than the thickness under anhydrous conditions for water contents greater than 50ppm, and even 400nm for water contents of 800 to 1000ppm, which is not achievable under anhydrous conditions. This example also shows that this embodiment can greatly simplify the technical environment of the preparation, since the membrane in this example is obtained outside the glove box.

These syntheses were carried out starting from a solution in Dimethylformamide (DMF) distilled under argon, said solution comprising 10^-2TEAP in mol/l and Methacrylonitrile (MAN) distilled under argon in 2.5mol/l, with a working electrode (sheet with a gold layer obtained by spraying onto a glass sheet), a platinum counter electrode and a counter electrode based on Ag⁺The reference electrode of the/Ag pair is immersed in the solution. Several syntheses were carried out starting from the same bath and the cell was repeatedly opened in order to introduce a new sheet to be coated. A sample of the electrolyte was taken after each synthesis and the water content of the sample was measured using a Karl-Fischer device. The initial water content of the synthesis solution was 35 ppm; this water content is naturally present in the commercial DMF used. The apparatus used to perform the synthesis is shown in FIG. 1. In this figure, a sealed electrolytic cell filled with electrolytic solution and comprising a working electrode (working), a reference electrode (Ag) is continuously bubbled with argon⁺Ag reference) and a platinum counter electrode (Pt counter electrode), argonThe gas was previously flowed into a guard containing 4 angstrom molecular sieves (anhydrous zeolite) which had been activated by holding in an oven at 350 c for 1 week.

6 gold sheets which have not been in contact with the controlled atmosphere after their preparation are introduced. This introduction is carried out by opening the cover of the cell, gripping the sheet with alligator clips and then closing the cover. Each run lasted about 30 seconds, during which bubbling of argon was not interrupted.

The water content of the electrolytic solution varied from 35ppm at the beginning of the experiment to about 1600ppm after about 2 hours of the experiment.

By means of the equilibrium potential of the electrolytic solution and-2.8V/(Ag)⁺Ag), 10 voltammetric scans at 100mV/s were performed to synthesize. The sheet was removed from the cell, washed with water under ultrasound for 5 minutes, then acetone under ultrasound for 5 minutes, and then dried under a stream of argon.

The film thickness was then measured by profilometry.

The results obtained are given in FIG. 2, which shows the thickness (in nm) of the films obtained as a function of the water content (in ppm).

These results demonstrate the success in obtaining a PMAN film with a thickness of 400nm on gold (the reflection infrared spectrum of the resulting film indicates that a spectrum consistent with PMAN in every respect is indeed obtained), which cannot be obtained by merely varying parameters of the synthesis scheme other than the water content (such as monomer concentration, electrode potential, number of pulses or scanning speed). Of course, other of these parameters may be readjusted on their own to optionally further increase the thickness, but it is observed that adjusting the moisture content alone on its own makes much more dramatic improvements available.

By comparison, the concentration of the water in the electrolytic solution was 5X 10 in the case of replacing the water in the electrolytic solution with a variable amount of anhydrous ethanol^-2The same experiment was carried out with 4mol/l of MAN in anhydrous DMF in the presence of mol/l of TEAP. DMF andthe ethanol was dehydrated beforehand by standing for a long time on a molecular sieve with a pore diameter of 4 angstrom, which was previously conditioned by heating in an oven at 350 ℃ for 1 week, followed by distillation in a glove box under reduced pressure of argon. The resulting water content in DMF and in ethanol (measured using a Karl-Fischer apparatus) was 33ppm in DMF and less than 10ppm in ethanol. The treatment operation was carried out in a glove box under dry argon, wherein the water content in the atmosphere was less than 15 ppm. At a potential from equilibrium (at-0.7V/(Ag)⁺In the range of/Ag) to-2.6V/(Ag)⁺Ag) was electrolyzed under voltammetric conditions by scanning 10 times at 100 mV/s. The sheets were then washed with acetone and then dried under a stream of argon before analysis.

For each ethanol content, the measurement was at about 2270cm^-1Percent transmittance of the infrared absorption band of the nitrile functionality. The obtained results are given in FIG. 3, in which the transmittance (%) is expressed as a function of the content of ethanol (unit: ppm).

These results indicate that there is no effect similar to that observed with the addition of water to the electrolytic solution: the addition of ethanol causes the electrografting membrane to disappear, even at low concentrations.

A surprising effect has been observed under the effect of the addition of water to the medium, which the inventors attribute to the fact that: the water in the reaction medium is a bronsted acid. Water contents that are far from those of aprotic and anhydrous conditions make it possible to facilitate the preparation of electro-grafted membranes having a thickness greater than that obtained under aprotic or anhydrous conditions. It has in fact been observed that for excessively high water contents, the electro-grafted films tend to disappear, in accordance with what is known in the literature. The surprising effect is that: the curve of fig. 2 (which gives the film thickness as a function of moisture content) passes through a maximum value (for an intermediate moisture content) before dropping.

The results presented in fig. 3 also indicate that: the effect of adding short chain alcohol or thiol based additives, such as described in us patent 3759797, is not the same as the effect described in the present invention, since the effect obtained in figure 3 is strictly opposite to the effect described in us patent 3759797. Without owning to all the necessary information and wishing to be bound by any one theory, the inventors believe that the characteristics of the reaction medium used in us patent 3759797 may promote the formation of polymers by free radical polymerization, and that the additives under consideration may be good active site transfer agents and may contribute to the crosslinking of the formed polymer and/or promote termination reactions.

Example 2: formation of Polymethacrylonitrile (PMAN) films in the presence of water

Example 1 above illustrates: for a given monomer concentration and a given process sequence, the thickness range can be determined by adjusting the water content of the medium. It may be noted that it was observed that the water content of the anhydrous solution (e.g. a solution of DMF distilled under argon in a glove box) re-exposed to air changed to its saturation value within minutes. In example 1 given above, argon dehydrated beforehand in a protective device containing activated molecular sieves in a DMF solution was bubbled into the electrolytic cell so that the adjustment time of the water content could be extended to 2 hours.

In this example, the molecular sieve was introduced directly into the electro-grafting cell. It has been shown to be effective to adjust the resulting water content by preparing the synthesis solution directly from commercial products (without distillation or dehydration).

The operating conditions, in particular the synthesis procedure and the solution, are the same as in example 1 above, except that the reactants are not distilled. The water content in commercial DMF and monomer was measured to be about 150 ppm. An approximately 2cm thick pad of 5 angstrom molecular sieve preactivated for 1 week at 200 ℃ was introduced into the electrochemical cell followed by the synthesis solution. The water content measurement was carried out on the sample extracted after several minutes, resulting in a water content of 30 ppm. The solution was stirred with a magnetic stir bar throughout the synthesis.

The sample taken after 4 hours showed a water content of 328 ppm. Electro-grafted PMAN films were produced after 4 hours of electrolysis: the thickness of the film obtained was 125nm, which is in good agreement with the results obtained in example 1. As in example 1, no structural defects were observed in the IRRAS spectrum of the PMAN film thus obtained.

Example 3: study of the Effect of MAN monomer content on PMAN film formation in the Presence of Water

The purpose of this example is to demonstrate the following facts: the thickness range achievable by varying the water content also depends on the monomer content, but the same trend is observed regardless of the monomer concentration.

Electro-grafted PMAN membranes were produced under the same operating conditions as set out in example 2 above, except that a starting monomer concentration of 5mol/l and 5X 10 in DMF were used^-2TEAP content in mol/l. The water content was adjusted to 300, 500 and 1000ppm by adding distilled water to the reaction medium at this point. Then at equilibrium potential (at-1V/(Ag)⁺In the range of/Ag) to-3.2V/(Ag)⁺Ag), a film was produced by scanning 10 times at 100mV/s underlying. The thus treated sheet was then washed under the same conditions as above.

The thicknesses measured on the profilometer and compared to the thickness of the film obtained in example 1 above are given in table I below.

TABLE I

By comparison, a PMAN film obtained under the same operating conditions but in an anhydrous medium had a thickness of about 20 to 50 nm.

Example 4: study of the Effect of Water content during formation of PMAN film

The purpose of this example is to illustrate the following fact: the curve giving the thickness as the moisture content changes is a curve passing through a maximum, above which the curve falls slowly. This therefore means that on the one hand, the addition of water makes it possible to increase the maximum thickness obtained at a given operating procedure and a given concentration, and on the other hand, better control of the grafted membrane is achieved, even if control is effected with regard to the water content at concentrations greater than the maximum concentration.

To this end, an electro-grafted PMAN film was produced on gold sheet under the same operating conditions as those listed in example 2 above, except that: at-0.7V/(Ag)⁺Ag to-2.6V/(Ag)⁺Ag), 3 voltammetric scans were performed at 200 mV/s. The water content was adjusted to a content varying between 0 and 2200ppm, and this was for different monomer concentrations: 0.1, 1, 2.5 and 9.54 mol/l.

The results obtained are given in FIG. 4, in which the thickness (in. ANG.) obtained at each monomer concentration (diamonds: 0.1 mol/l; squares: 1 mol/l; triangles: 2.5mol/l and circles: 9.54mol/l) is expressed as a function of the water content (in ppm).

These results indicate that in all cases a water content of more than 50ppm is present, for which the thickness obtained is greater than that achievable under anhydrous conditions. It was also observed that the absolute value of the slope of the curve above this concentration was lower than below this concentration: controlling the water content above this concentration rather than below this concentration can therefore enable good control over the resulting film thickness.

The same experiment performed on nickel sheets resulted in the same observations.

Example 5: study of the Effect of supporting electrolyte concentration on PMAN film thickness

The purpose of this example is to illustrate the fact that: the maximum location of the thickness/moisture content curve also depends on the presence or absence of the supporting electrolyte and its content in the electrolytic solution. It was observed in particular that this maximum value shifts towards higher water contents when the content of supporting electrolyte increases. This would foresee a better control of the water content and therefore of the film thickness, which is achieved by producing formulations with a greater supporting electrolyte concentration: the hygroscopicity of the solution increases as its water content decreases. In the case where a given content of supporting electrolyte has been selected, the water content in the medium can be adjusted to the maximum of the thickness/water content curve, and as a result of this increase in water content, solutions with reduced hygroscopicity and therefore improved stability can be obtained.

Electro-grafted PMAN films were produced on gold by using a 2.5mol/l solution of (non-distilled) MAN in (non-distilled) DMF. TEAP was used as supporting electrolyte. The solution was prepared from anhydrous TEAP and the water content was adjusted by adding distilled water, which was measured using a Karl-Fischer device.

At 5X 10^-3To 5X 10^-1Different electrografted membranes were produced with a TEAP content of mol/l and a water content of 16 to 2400 ppm. The thickness is estimated by the strength of the CN label measured by IRRAS. The results are given in fig. 5, where fig. 5 shows the thickness (in angstroms) of the resulting film as a function of water content (in ppm) for each TEAP concentration. In this figure, the solid diamonds correspond to the squares 5 × 10^-3Curves obtained for mol/l TEAP concentration, filled squares corresponding to 1X 10^-2Concentration of mol/l, filled triangles corresponding to 5X 10^-2Concentration of mol/l, the lower open square corresponds to 1X 10^-1Concentration of mol/l, while the open triangle corresponds to 5X 10^-1Concentration of mol/l.

Claims (18)

1. A method of forming a polymer film on a conductive or semi-conductive surface by electro-grafting, comprising:

a) preparing an electrolytic solution comprising one or more electropolymerizable monomers and at least one proton source selected from compounds which are bronsted acids in said electrolytic solution, said proton source being present in an amount of 50 to 100000ppm relative to the total amount of ingredients in said electrolytic solution; and

b) the solution is electrolyzed in an electrolytic cell by using the conductive or semi-conductive surface to be covered as a working electrode and at least one counter electrode to cause the formation of an electro-grafted polymer film on the surface by electro-reduction or electro-oxidation of the solution.

2. The process according to claim 1, characterized in that the Bronsted acid is selected from the group consisting of water; hydrogen fluoride; ammonium fluoride; nitrous acid; molecules bearing carboxylic acid groups or ammonium, amine, pyridinium or phenolic groups; sulfuric acid; nitric acid; hydrogen chloride; perchloric acid and molecules carrying sulfate, sulfonate or thiol groups.

3. Process according to claim 1 or 2, characterized in that the electropolymerizable monomer is chosen from activated vinyl monomers and cyclic molecules cleavable by nucleophilic attack, corresponding respectively to the following formulae (I) and (II):

wherein:

-A、B、R₁and R₂Identical or different, represents a hydrogen atom; c₁-C₄An alkyl group; a nitrile group; an organofunctional group selected from the group consisting of: hydroxyl group, amine: -NH_xWherein x ═ 1 or 2, thiol, carboxylic acid, ester, imidoester, acid halide: -C (═ O) X, wherein X represents a halogen atom selected from fluorine, chlorine, bromine and iodine, acid anhydride: -C (═ O) OC (═ O), nitriles, isocyanates, epoxides, siloxanes: -Si (OH)_zWherein z is an integer from 1 to 3 inclusive; benzoquinone, carbonyldiimidazole, p-toluenesulfonyl, p-nitrophenylchloroformate, alkenyl, aromatic compound; a functional group capable of complexing a cation; a branched and/or functionalized molecular structure starting from these functional groups; groups cleavable by thermal or photon activation; an electroactive group; an electrically cleavable group; and mixtures of the foregoing monomers and groups;

-n, m and p are the same or different and are integers from 0 to 20 inclusive.

4. A process according to claim 3, characterised in that the activated vinyl monomer of formula (I) is selected from vinyl crosslinkers or crosslinkers based on acrylates and methacrylates.

5. The process according to claim 3, characterized in that the cleavable cyclic molecule of formula (II) is selected from the group consisting of epoxides, lactones, lactic acid, glycolic acid, and mixtures thereof.

6. A method according to claim 1 or 2, characterized in that the concentration of electropolymerizable monomer in the electrolytic solution is 0.1 to 10 mol/l.

7. The method according to claim 1 or 2, characterized in that the electrolytic solution additionally comprises at least one solvent selected from the group consisting of dimethylformamide, ethyl acetate, acetonitrile, tetrahydrofuran and chlorinated solvents.

8. A method according to claim 1 or 2, characterized in that the electrolytic solution comprises at least one supporting electrolyte.

9. A process according to claim 1 or 2, characterized in that the content of bronsted acid is 50 to 10000 ppm.

10. A method according to claim 1 or 2, characterized in that the electrically conducting or semiconducting surface is a surface made of steel, iron, copper, nickel, cobalt, niobium, aluminium, silver, titanium, silicon, titanium nitride, tungsten nitride, tantalum or tantalum nitride, or a noble metal surface selected from the group consisting of gold, platinum, iridium or iridium platinum surfaces.

11. A method according to claim 1 or 2, characterized in that the electrolysis of the electrolytic solution is effected by performing the polarization under potentiostatic or galvanostatic voltammetric conditions.

12. The method of claim 3, characterized by A, B, R₁And R₂Identical or different, represents an amide function: -C (═ O) NH_yWhere y is 1 or 2, an imide function, a vinyl function, a toluene, benzene, halogenated benzene, pyridine, pyrimidine, styrene or halogenated styrene group.

13. The method of claim 12, characterized in that the imide functional group is selected from a succinimide group or a phthalimide group.

14. A process according to claim 3, characterized in that the activated vinyl monomer of formula (I) is selected from the group consisting of acrylonitrile, methacrylonitrile, methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, glycidyl methacrylate, acrylamide, cyanoacrylate, diacrylate or dimethacrylate, triacrylate or trimethacrylate, tetraacrylate or tetramethacrylate, acrylic acid, methacrylic acid, styrene, p-chlorostyrene, pentafluorostyrene, N-vinylpyrrolidone, 4-vinylpyridine, 2-vinylpyridine, vinyl halide, acrylic halide, methacrylic halide.

15. The process according to claim 5, characterized in that the cleavable epoxide cyclic molecule of formula (II) is an alkylene oxide.

16. A method according to claim 10, characterized in that the conductive or semi-conductive surface is a surface made of stainless steel.

17. Conductive or semiconductive surface, characterized in that it is obtainable by using a process according to any one of claims 1 to 16 and has at least one side covered with an electro-grafted polymer film.

18. The surface according to claim 17, characterized in that the thickness of the polymer film is 10nm to 10 μm.