G. Lécayon

Grafting and growing mechanisms of polymerised organic films onto metallic surfaces

Abstract The build up of a homogeneous polyacrylonitrile film laid down by electropolymerisation on a metallic surface begins by the chemisorption of a monomer monolayer. As the growth of the conducing film proceeds (conduction by hopping) the number of electrons is constant. The electronic properties of the substrate and monolayer (conjugated molecule) are important.

Valence electronic structure of a thin film of polyacrylonitrile and its pyrolized derivatives

Abstract The electronic structure of a very thin film (≈100 A) of electropolymerized polyacrylonitrile is studied by metastable de-excitation spectroscopy and UPS. The study is made before and after pyrolysis of the sample at 200 °C under high vacuum. Before the pyrolysis the electronic structure is dominated by the localized levels of the nitrile group, while after pyrolysis it is graphite-like. We conclude that this thin film is ordered enough to undergo graphitization at only 200 °C and propose to correlate the increase of the conductivity due to pyrolysis, observed by many authors, with the denitrogenation of the film.

Molecular structure of an electropolymerized polyacrylonitrile film and its pyroylzed derivatives

Abstract Polyacrylonitrile films, grafted by electropolymerization on a nickel substrate, have been studied ex-site by specular reflexion infrared spectroscopy. The layer is shown to consist of linear polymer chains exhibiting structural alterations varying in density and nature with the film thickness. Hest trestments under high vacuum conditions produce chemical transformations of the polymer (intra-chain cyclization, inter-chain reticulation, de-hydrogenation and de-nitrogenation) leading to a film having a carbon-fiber type structure for temperatures higher than 600°C.

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.

A NEXAFS Study of Thin Polyacrylonitrile Films Electrochemically Deposited on Ni: The Effect of the Film Thickness and Annealing Treatment

Polyacrylonitrile (PAN) thin films electrochemically deposited on Ni have been studied by near edge x-ray absorption fine structure, as a function of the film thickness and annealing treatment. For 20 {angstrom} thick films, the polymer chains are oriented perpendicular to the surface with the C {equivalent to} N groups parallel to it. Below a few angstroms, no polymerization occurs but molecules are adsorbed perpendicular to the surface. Annealing at 300{degrees}C results in the loss of the majority part of the N content of the film in contrast with the admitted mechanism for bulk PAN.

Electropolymerization of methacrylonitrile and N-vinyl-2-pyrrolidone as probed by an EQCM

The electropolymerization of methacrylonitrile (MAN) and N-vinyl-2-pyrrolidone (NVP) in anhydrous acetonitrile has been followed in situ using an electrochemical quartz crystal microbalance (EQCM). These two monomers essentially differ in that MAN leads to both a grafted polymer and a polymer formed in solution, while NVP only delivers the grafted polymer. For both molecules, our results indicate that the frequency response of the EQCM anticipates the voltammetric response. In the case of MAN, the admittance of the EQCM shows a considerable increase which is synchronized with the voltammetric peak. Alternatively, in the case of NVP, the admittance of the EQCM remains almost constant during the course of the experiment. These results lead us to the proposal that: (i) the eigenfrequency variations of the EQCM may be related to mass changes on the surface of the quartz crystal in the case of the electropolymerization of NVP molecules over the whole potential range, while the same is true only over a few hundreds of mV before the peak in the case of MAN; (ii) poly-NVP films grow directly from the surface of the electrode, while at least two regimes are detectable in the case of MAN.

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 ...

Electrical properties of very thin heat-treated polyacrylonitrile layers electropolymerized on nickel for contact application

Abstract Recent investigations concerning weakly pyrolysed thin films of polyacrylonitrile (PAN) grafted and grown by electropolymerization on a nickel underplate are reported, which clearly attest the potential interest of the obtained material for low-level connector coating application. The electrical characterization of these organic layers in a contact situation requires various precautions, principally the use of surfaces as smooth as possible in order to avoid piercing the film and therefore metal/metal junctions. An original device for static contact resistance measurements is presented, simulating the real case of industrial connector parts by a sphere/plane contact with an adjustable normal load. The tested samples consist of Ni-electroplated plane brass strips covered by PAN layers about 550 A thick and heated at different temperatures in the range 350–500 °C for 10 or 30 s; the pressing sphere is a 3.2 mm diameter smooth golden ball. IRRAS and XPS spectra of raw and thermally treated samples reveal a series of drastic changes in the molecular structure of the polymer and the correlative evolution of the electrical properties of the films is unambiguously observed. The most promising results concern the 400 °C/30 s heated coating, for which stable and reproducible contact resistance values lower than 10 mΩ are found under a 50 gf normal load.

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.