Guy Deniau

Spontaneous Grafting of Diazonium Salts: Chemical Mechanism on Metallic Surfaces

The spontaneous reaction of diazonium salts on various substrates has been widely employed since it consists of a simple immersion of the substrate in the diazonium salt solution. As electrochemical processes involving the same diazonium salts, the spontaneous grafting is assumed to give covalently poly(phenylene)-like bonded films. Resistance to solvents and to ultrasonication is commonly accepted as indirect proof of the existence of a covalent bond. However, the most relevant attempts to demonstrate a metal-C interface bond have been obtained by an XPS investigation of spontaneously grafted films on copper. Similarly, our experiments give evidence of such a bond in spontaneously grafted films on nickel substrates in acetonitrile. In the case of gold substrates, the formation of a spontaneous film was unexpected but reported in the literature in parallel to our observations. Even if no interfacial bond was observed, formation of the films was explained by grafting of aryl cations or radicals on the surface arising from dediazoniation, the film growing later by azo coupling, radical addition, or cationic addition on the grafted phenyl layer. Nevertheless, none of these mechanisms fits our experimental results showing the presence of an Au-N bond. In this work, we present a fine spectroscopic analysis of the coatings obtained on gold and nickel substrates that allow us to propose a chemical structure of such films, in particular, their interface with the substrates. After testing the most probable mechanisms, we have concluded in favor of the involvement of two complementary mechanisms which are the direct reaction of diazonium salts with the gold surface that accounts for the observed Au-N interfacial bonds as well as the formation of aryl cations able to graft on the substrate through Au-C linkages.

Study of the polymers obtained by electroreduction of methacrylonitrile

In this work, we studied the electrochemical formation of polymethacrylonitrile by cathodic polarization in an anhydrous organic medium, at two different working potentials either side of the potential of the high concentration reduction pre-peak. In both cases, the polymer formed in solution and the so-called grafted polymer were obtained. A detailed analysis has been performed by infrared reflection absorption spectroscopy for the polymer chemisorbed on the cathode surface, and by size exclusion chromatography, infrared and 13C nuclear magnetic resonance for the polymer obtained in solution. These analyses reveal essentially no difference, in terms of products of reaction, between the synthesis carried out at the two working potentials, and suggest that the grafting reaction is probably disconnected from the occurrence of the pre-peak. On the contrary, it is proposed that this peculiar peak is due to the dynamic formation of the polymer in solution, and that this hides all effects possibly stemming from surface chemical reactions.

Study of the grafting and of the electrochemical polymerization of acrylic monomers on a metallic surface by impedance spectroscopy

Abstract Under cathodic polarization in aprotic media some vinylic monomers like acrylonitrile and methacrylonitrile can be polymerized and strongly bound to a metallic surface (nickel for instance). The bonding of polymer films on metallic electrodes has been studied by electrochemical impedance spectroscopy combined with classical electrochemical methods such as chronoamperometry and linear sweep voltammetry. The growth of the polymer on the surface introduces a change in the double layer capacitance on the metal which can be measured by impedance spectroscopy when the polymer is swelled by the electrolytic solution. The modification of the double layer capacitance is related to the part of the surface occupied by the bonds between the polymer and the metal (grafting ratio). This ratio was found to be dependent on the surface preparation mode and on the concentration of the monomer in the electrolytic solution used for the polymerization process. This result makes clear the differences observed in the thickness and in the morphology of the polymers obtained when using various monomer concentration and different treatments of the metallic surface. On the other hand the impedance results allow us to give a correct interpretation of the usual electrochemical characteristics (current-potential curves and current-time curves) registered during the electropolymerization process in various conditions.

Cathodic electropolymerization of methacrylonitrile studied in situ by quartz crystal microbalance, cyclic voltammetry, and impedance spectroscopy

The cathodic electropolymerization of methacrylonitrile was followed with an electrochemical quartz crystal microbalance used in combination with other classical techniques such as cyclic voltammetry, chronoamperometry and impedance spectroscopy. The quartz microbalance has revealed a large adsorption-desorption process of the growing electropolymer on the cathode during repeated cycling. However, a polymer layer strongly bound to the electrode was obtained after several scans, though it is normally soluble in the electrolytic solution. A visible delay between the monomer reduction current and the quartz frequency response was measured during successive sweeps and was found to be dependent on the scan rate. This delay was attributed to the time needed to form a critical polymer concentration in the solution before adsorption. A progressive change in the polymer conformation on the electrode was found during repeated scans. This reorganization process of the electropolymer on the cathode was found to be comparable with the spontaneous adsorption of special polymer chains bearing reactive groups capable of forming chemical bonds with some substrates. These grafted polymers present well-identified structures, called pancake, mushroom and brush. Finally, the behaviour of a polymethacrylonitrile layer swollen by solvent as well as in the dry state suggests a plausible evolution from a mushroom type conformation to a definitive brush conformation during the growth of the electropolymer on conducting electrodes.

Tunable grafting of functional polymers onto carbon nanotubes using diazonium chemistry in aqueous media

In this work, we present an efficient grafting of functional polymers onto multi-walled carbon nanotubes using a simple and versatile chemical process carried out in open air and in aqueous media. The method involves in situreduction of substituted (–COOH, –NH2, –F…) aryl diazonium salts which yields poly(phenylene)-like coatings covalently grafted on the nanotube surface. Tuning the concentration of in situ generated diazonium was found to be an efficient way to control the total amount of grafted polymer. Functionalized nanotube samples were studied by complementary techniques such as electron microscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy.

Electrochemistry as a Tool to Monitor Lewis Acid-Base Reactions between Methacrylonitrile and Metallic Surfaces: A Theoretical and Experimental Proposal

Abstract This paper presents a tentative extension of the Lewis acid-base concept to the case of an organic molecule interacting with a polarized metallic surface. Towards this aim, we make use of the Density Functional Theory (DFT) viewpoint on Lewis acid-base interactions. This theory has been shown to be relevant to describe adhesion processes at a molecular scale. It allows the introduction of three key parameters, for the molecule as well as for the metallic surface. These are the DFT chemical potential, μ, the absolute hardness, η and the Fukui function, f(inag hear). In the present paper, we show that the DFT chemical potential, μ, of the metallic surface is linearly related to the electrode potential drop, Δe, imposed between this surface and a reference electrode in an electrochemical cell. Thus, while the chemical potential of the molecule is only determined by its chemical structure, that of the metallic surface can be monitored continuously. This means that the Lewis acidic or basic character ...

Study of the polymerization mechanisms of methacrylonitrile under cathodic polarization on a nickel electrode

Abstract Methacrylonitrile was polymerized under cathodic polarization onto nickel electrodes. Compact polymer films strongly bound to the metal were obtained when using strictly controlled organic electrolytic solutions. The polymerization process was found to be highly dependent on the monomer concentration in the electrolytic solution. For a low monomer concentration (less than 30% of the volume) the reduction mechanism was dominated by a diffusion process occurring at a very negative potential ( −2.8 V versus Ag|RiAg + ). No adsorption pre-peak appeared and no visible film was formed on the electrode. For a high monomer concentration and especially for a pure methacrylonitrile—supporting salt solution the electrochemical process was characterized by a well-defined adsorption—reduction prepeak located at a more positive potential (−2.3 V). The polymer film formed on the electrode was found to be insoluble in the monomer but highly soluble in solvents specially in acetonitrile. On the contrary, when using a moderate monomer concentration (between 30% and 90% of the volume) the major part of the electropolymer formed under the adsorption peak was dissolved in the electrolytic solution; however, a thin (600 A) insoluble compact and covering film remained strongly bound to the electrode. In order to explain the complex behaviour of the polymerization process, different polymer arrangements on the electrode depending on the monomer concentration are proposed and discussed.

Electropolymerization of methacrylonitrile on a rotating disk electrode at high spinning rate

Abstract The electropolymerization of methacrylonitrile has been performed under voltammetric conditions on a rotating disk electrode (RDE) spinning at high angular rotation rate ( ω =10000 rpm) in anhydrous acetonitrile. The analysis of the electrode surface by X-ray photoelectron spectroscopy (XPS) and ellipsometry reveals that a grafted film of polymethacrylonitrile is present on the surface at the end of the synthesis. In addition, the polymer known to result from a polymerization in solution, which is usually (i.e. at ω =0) found on the surface before rinsing, is not present on the electrode. Estimations provided by the Navier–Stokes equation indicate that, at such high rotation rates, considerable convection effects are to be expected at distances from the surface of the order of 1 nm. These observations, combined with the fact that acetonitrile is an excellent solvent of polymethacrylonitrile, lead us to the proposal that the grafted polymer film is initiated directly on the surface, and may not result from an a posteriori precipitation of the polymer formed in solution.

First attempts at an elucidation of the interface structure resulting from the interaction between methacrylonitrile and a platinum anode : an experimental and theoretical (ab initio) study

Abstract The aim of the present paper is to contribute to the elucidation of the molecular structures obtained on a platinum surface as this surface is submitted to an anodic potential (with respect to a silver reference electrode) when dipped into pure 2-methyl 2-propenenitrile (methacrylonitrile). Modified surfaces are examined using X- and UV-photoelectron spectroscopies (UPS and XPS). The results evidence the formation of an ultra-thin (20–40 A) grafted oligomer film, which is not classical polymethacrylonitrile (PMAN), as obtained through a radical or anionic mechanism: spectral characteristics argue in the sense of a cationic polymerization of methacrylonitrile through its nitrile groups, as evidenced by a lowering of the gap as well as by the UPS and XPS (N 1s region) spectra. Molecular models of the reactants and reaction intermediates are proposed for the cationic polymerization of methacrylonitrile, and show that this polymerization is about as feasible as that of acetonitrile, at least on kinetic control grounds. Two different mechanisms are nonetheless possible, leading either to a quasi conjugated poly-imine type -(N  C)n-, or to a poly-cumulene type -(N  C  C)n- network. Theoretical consierations on reactants properties lead us to select the poly-imine way as the most plausible. Along with literature data concerning chemisorbed nitriles on platinum surfaces, a molecular model of the final state of the poly-imine reaction is then designed, comprising a three atom cluster to render the grafting site, and a dimer to render the grafted structure. A full geometry optimization is performed on the organic moiety at the Hartree-Fock (ab initio) level of theory, and a rough evaluation of the spectral footprint of the interface bond in the N 1s region is performed on the basis of Koopmans theorem with calibration on the bulk polymer peak. A preliminary 2.7 eV downward shift is predicted for N 1s interface nitrogens with respect to the polymer peak, which can be compared with the low-energy contribution, found about 2.0 eV below the polymer peak, in the experimental spectrum. The directions in which the molecular model of the interface need be improved are discussed. On the basis of the present results, as well as those obtained previously in the methacrylonitrile/nickel cathode interaction, the conclusion examines the proposition that the structure of the very interface in the final state of electropolymerization reactions is the frozen footprint of the initial stages of the interaction, and alludes to electrochemistry as a tool to monitor molecule/surface interactions.

Coupled chemistry revisited in the tentative cathodic electropolymerization of 2-butenenitrile

2-Methyl 2-propenenitrile (methacrylonitrile) and 2-butenenitrile (crotononitrile) are vinylic molecules which differ only in the position of a methyl group on the double bond. By combining gas chromatography coupled to chemical ionization mass spectrometry, steric exclusion chromatography and infra-red spectroscopy, along with quantum chemistry based calculations of standard Gibbs energy changes of plausible reactions, we demonstrate that the protons of the methyl group in crotononitrile are acidic in anhydrous acetonitrile, while those of methacrylonitrile are not. In this way, we bring a definite explanation to the fact that methacrylonitrile forms homogeneous films of polymethacrylonitrile (PMAN) on a working electrode subjected to a cathodic polarization in anhydrous acetonitrile, while crotononitrile does not.