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.

International audienceCovalent surface modification of conductive, semiconductive, and insulating substrates with thin organic polymers films induced by redox activation of aryl diazonium salts in the presence of vinyl monomers has been investigated in acidic aqueous media. This new process, called diazonium-induced anchoring process (DIAP), is an efficient technique to impart covalent adhesion of polyvinyl coatings onto raw inorganic or organic surfaces without any conductivity requirement. Typically, aryl diazonium salts are reduced with iron powder to give surface-active aryl radicals leading (i) to the formation of a grafted polyphenylene-like film on the substrate surface and (ii) to the initiation of the radical polymerization of the vinylic monomer in solution. The resulting radical-terminated macromolecular chains formed in solution are then able to react with the polyphenylene primer layer to form a very homogeneous thin organic film on the surface. The final organic thin coating is strongly grafted on materials surfaces, as evidenced by its persistence after a long ultrasonic treatment in a good solvent of the polymer. We speculate this process is supported by the large concentration of aryl and hydrogen radicals formed when iron powder is added in the acidic aqueous solution. The thickness of the polymer film can be controlled as a function of time, typically a few minutes, and was measured between 10 and several hundred nanometers. Infrared reflection–absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and contact angle measurements were used to characterize the surface modification of metals, glass, carbon nanotubes, or polytetrafluoroethylene (PTFE). This very simple and efficient grafting method provides a powerful tool for the covalent coating of organic or inorganic surfaces possessing complex geometrical shapes

Electrografting is a powerful and versatile technique for modifying and decorating conducting surfaces with organic matter. Mainly based on the electro-induced polymerization of dissolved electro-active monomers on metallic or semiconducting surfaces, it finds applications in various fields including biocompatibility, protection against corrosion, lubrication, soldering, functionalization, adhesion, and template chemistry. Starting from experimental observations, this Review highlights the mechanism of the formation of covalent metal-carbon bonds by electro-induced processes, together with major applications such as derivatization of conducting surfaces with biomolecules that can be used in biosensing, lubrication of low-level electrical contacts, reversible trapping of ionic waste on reactive electrografted surfaces as an alternative to ion-exchange resins, and localized modification of conducting surfaces, a one-step process providing submicrometer grafted areas and which is used in microelectronics.

The surface electroinitiated emulsion polymerization (SEEP) protocol, described here for the first time, combines the advantages of the electrografting process, which delivers thin organic coatings chemically bonded to conducting surfaces, with polymerization in emulsion, which allows synthesizing hydrophobic polymers in aqueous solutions. From the mechanistic point of view, SEEP combines the “grafting to” and “grafting from” methods in one fast electrochemical step at room temperature, taking advantage of the ability of diazonium salts to (i) be easily electrografted on conducting surfaces under moderate cathodic conditions and (ii) act as an initiator for the radical polymerization of vinylic monomers in emulsion. This work is a preliminary description of this process applied to various vinylic monomers like acrylic acid (AA), acrylonitrile (AN), and butyl methacrylate (BUMA). Experiments showed that the process works correctly whatever the surfactant used. In all cases, the surface initiation was obtained via the electrochemical reduction of nitrobenzenediazonium tetrafluoroborate salts. The resulting grafted copolymer (poly(nitrophenylene, vinylics)) was obtained and characterized by infrared and photoelectron spectroscopies.

As recently reported, the SEEP process (surface electroinitiated emulsion polymerization) is a new grafting method that provides covalently grafted polymer films on conducting or semiconducting surfaces by radical polymerization in aqueous dispersed media. It relies on cathodic electroinitiation, which creates radical species able to start a radical polymerization. Contrary to the formerly described cathodic electrografting of vinylic polymers (CE), which also delivers submicrometerthick and stable polymer films on conducting substrates but requires strictly anhydrous conditions and organic aprotic solvent, SEEP brings a major improvement in switching from a purely anionic mechanism to a radical one by adding an aryldiazonium salt in the reaction mixture, while retaining the same polymer films characteristics. Moreover, SEEP is not restricted to water-soluble monomers but can be performed even with hydrophobic ones, such as n-butyl methacrylate (BMA). In such cases, a surfactant is necessary to stabilize the monomer in water emulsion. From this one-pot electrografting process performed in water at room temperature, in a few minutes, without restrictions on vinylic monomer water solubility, results a polymer coating strongly grafted to the substrate. This article aims at completing our first one and focuses on mechanistic aspects of SEEP to eventually establish a possible "grafting onto" mechanism. To achieve that goal, we have analyzed grafted polymer films obtained by SEEP on gold substrate from BMA in water as a miniemulsion by IR-ATR, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and atomic force microscopy (AFM).

As recently reported, the Graftfast process is a grafting method that provides covalently grafted polymer films. It relies on the chemical reduction of diazonium salts by reducing agents in absence or presence of a vinylic monomer. Contrary to electroinduced methods delivering strongly grafted and stable polymer films such as cathodic electrografting (CE) of vinylic monomers (which requires drastic experimental conditions) or surface electroinitiated emulsion polymerization (SEEP), the Graftfast process provides strongly grafted polymer films on any type of materials (conductors, semiconductors, and insulators). Moreover, it is a fast one-step reaction occurring at atmospheric pressure, ambient air and room temperature in water, which makes it more suitable for applications than the slower ATRP-based methods. This article aims to complete the first paper on this process by giving preliminary answers to the question: How does the Graftfast process work? To achieve this mechanistic study, dual surface-solution analyses were performed. Both spontaneous and redoxinduced grafting of polynitrophenylene-like (PNP) films and poly(hydroxyethyl) methacrylate (PHEMA) films were analyzed by infrared-attenuated total reflection (IR-ATR) and X-ray photoelectron spectroscopy (XPS) while the corresponding reactive solutions were studied by electronic paramagnetic resonance, EPR (spin-trapping method using 2-methyl-2-nitrosopropane MNP as spin-trapping agent). The EPR spectra and hyperfine structure of MNP adducts provide evidence of aryl radicals production, growing polymer chains radicals formation, and the existence of critical concentration values, leading to favorable grafting kinetics.

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We report for the first time on grafting of poly(n-methyl methacrylate), (PMMA), and polystyrene (PS) brushes by ATRP from the surface of aligned multiwalled carbon nanotubes (MWCNT) which were electrochemically treated with brominated aryl groups based on diazonium salts. The polymer brushes formed amorphous coatings, as evidenced by high-resolution transmission electron microscopy, by comparison to the nanotube structure. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of PS and PMMA by their characteristic C1s and valence band features. Well-aligned MWCNT network allowed us to sheath individual MWCNTs with polymer brushes while keeping the initial MWCNT alignment structure. This method opens up new avenues for the elaboration of polymer/NT hybrids.