Progress in Organic Coatings
Volume 69, Issue 4,
December 2010
, Pages 335-343
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Abstract
This paper reports the results of a comparative study of the electropolymerization of phenol, o-methoxyphenol and o-nitrophenol by cyclic voltammetry on gold and carbon steel electrodes. The aim of this work is to synthesize homogeneous and adherent polyphenols film to protect carbon steel material against corrosion. Gold electrodes were used to optimize the experimental parameters such as the initial concentration of the monomer, the pH, the potential scan rate and the anodic potential limit value. Results showed that poly-o-metoxyphenol synthesized on carbon steel using optimized parameters obtained from gold electrode leads to more effective protection. This is probably due to the electron-donating mesomeric effet (+M) of the methoxy group which stabilized the phenoxy radicals obtained during the monoelectronic discharge of o-methoxyphenol. The best electropolymerization conditions involved an aqueous solution of 0.04M o-methoxyphenol at pH 10.7, 5mVs−1 potential scan rate and an anodic potential limit (1.64V/SCE) avoiding the degradation of the polymer film. The application of these conditions on steel electrodes leads to the formation of a stable, adherent and inhomogeneous film of polymer. A polymerization mechanism was proposed with consideration of results from literature.
Introduction
Electropolymerization is a method of synthesis and deposit of thin layers of polymers on conducting substrates. Many researchers have utilized different monomers and substrates to produce electrochemical conducting, redox, and non-conducting polymer films [1], [2], [3].
Pyrrole, thiophene, aniline, benzene and their derivatives and others monomers were used for the synthesis of electrically conductive materials used as modified electrodes, catalysts for the oxidation or reduction of other materials or as elements in electrochromic display devices. The materials can also be incorporated in batteries [4], [5], [6], [7].
Non-conducting polymers can be ideal membranes for biosensors [8] or for surface protection from corrosion [9], [10]. Phenol and its derivatives such as 2-allylphenol, 3-methylphenol, 2,6-dimethylphenol, m-aminophenol and others were mainly used for the deposit of thin layers of insulating polymers intended for the protection of metals against corrosion [11], [12], [13], [14], [15]. The nature and the position of the substituents play an important role in the stability of polymers. Thus, it was shown that polymer film formation is rapid when the para position of the hydroxyl group is free, while it cannot occur when the para and one of the ortho positions are blocked [16].
In general, the polymers were produced by para coupling of phenoxy radicals and their mesomeric form generated at the electrode surface leading to the deposition of a poly(phenylene oxide) film [17], [18]. Since the development of non-conducting polymers is self-limiting, the resulting films are very thin (3–100nm) [19], [20], adherents and present low permeability to different ionic and molecular species [21], [22].
The electropolymerization was carried out using either cyclic voltammetry or by constant current or potential electrolysis. Several metal electrodes were used to produce stable polymer films such as carbon graphite [23], [24], Pt [13], [25], [26], [27] and Au [13], [25], [28], [29].
The choice of reaction medium can be as influential as the monomer. Generally, organic solvents such as alcohols, glycols, DMF, nitrobenzene, acetone, acetonitrile, and dichloromethane are compatible with the process [17], [30]. The use of water as a solvent has clear-cut practical advantages such as the absence of toxicity and fire hazard, apart from the low cost. However, the addition of some methanol (or higher alcohols) may sometimes be necessary in order to achieve sufficient solubility of the monomers when they are substituted phenols such as 2-allylphenol.
The morphology of the films is found to strongly depend on the pH of the electrolyte. Generally, the formation of the best insulating polymer occurs at higher pH values [31]. In alkaline medium (MeOH, H2O, NaOH), the coating films from phenol oxidation, were adherent homogeneous but did not resist to the penetration of the corrosive agents because of the very low thickness of the layer (<1μm).
In this work, cyclic voltammetry and optical microscopy techniques were used to study the effect of operating parameters on the efficiency of o-methoxyphenol electropolymerization on the surface of gold and carbon steel electrodes in acid and alkaline aqueous solutions. For a comparative study, phenol and o-nitrophenol electropolymerization were carried out in the same conditions. The resistance of carbon steel electrodes coated with poly-o-methoxyphenol film against corrosion has been studied after immersion in 30gL−1 NaCl solutions. Additionally a polymerization mechanism was proposed with consideration of results stated in the literature.
Section snippets
Materials and methods
All chemicals used were of analytical grade and the solutions were prepared with double distilled water. Phenol, o-methoxyphenol and o-nitrophenol were purchased from Aldrich. All chemicals were used without purification. H2SO4, NaOH and NaNO3 solutions were used as supporting electrolyte and for pH adjustment.
Voltammetric measurements and electropolymerization were performed in a three-electrode cell (Metrohm 100mL). Cyclic voltammograms were recorded with the aid of a Sefram X-Y recorder.
Cyclic voltammetry on gold electrodes
Fig. 1 illustrates the j/E profiles obtained for the first, second and tenth cycles during phenol, o-methoxyphenol and o-nitrophenol oxidation. Voltammograms of phenol and o-methoxyphenol, obtained between 0.17 and 0.81mV, exhibited only oxidation peaks around 0.67 and 0.58mV, respectively. This indicates an irreversible oxidation reaction of the monomer on the electrode surface. During the successive cycles, the peak current decreased. For the tenth cycle the oxidation peak current was less
Conclusion
The electropolymerization, by cyclic voltammetry, of phenol, o-methoxyphenol and o-nitrophenol on gold and carbon steel materials is a notoriously complex process which depends on the monomer structure, the potential scan rate, the pH, the upper potential limit and the monomer concentration. Comparing the voltammograms from the different monomer compound solutions, it was demonstrated that the polymer film resulting from o-methoxyphenol oxidation leads to the higher surface deactivation degree.
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Citation Excerpt :
Thus, these materials display extended π conjugated system with interesting electric, optic, and magnetic properties [2]. Since their discovery in 1977, numerous applications were reported related to these intrinsically conductive polymers, especially in analytical chemistry, for the manufacture of rechargeable batteries [3], electrochromic devices [4], light emitting diodes, corrosion protection [5], and construction of sensors and biosensors [6] for analytical monitoring [7,8]. Modified electrodes with polymer films have good stability, reproducibility, more active sites, homogeneity in the electrochemical deposition, and strong adhesion to the electrode surface [9–11].
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