Electropolymerization electrochemical mechanism

The second problem associated with electrochemical synthesis of polypyrroles seems to be related to die difficulties found in correlating the polymer s properties with the conditions of synthesis. This has been derived from the widely spread idea of an overall understanding of the electrochemical mechanism. However, even the most simple electrochemical process of pyrrole electropolymerization involves different experimental variables in order to optimize polymer properties. These variables can be chemical, such as solvent or reactants (monomer and dopant salt), or physical, such as temperature, nature and shape of die electrodes, cell geometry or electrical conditions during synthesis. In addition, commonly the effects of all these variables are interdependent. 

The faster polymer generation at rising proton concentrations in aqueous solutions can be explained from the initial protonation of pyrrole in the a position, as a previous step in the general mechanism of electropolymerization [142]. As is well known, pyrrole can be protonated in all the possible positions (a, (i and N) resulting in three kinds of cationic species (Figure 10.14) [21,136], although the more stable position for protonation is the a carbon. A general feature of protonated pyrrole molecules is that they are not aromatic and hence they are more easily oxidable. In this way we can propose an electrochemical mechan-... 

Chain length is another factor closely related to the structural characterization of conducting polymers. The importance of this parameter lies in its considerable influence on the electric as well as the electrochemical properties of conducting polymers. However, the molecular weight techniques normally used in polymer chemistry cannot be employed on account of the extreme insolubility of the materials. A comparison between spectroscopic findings (XPS, UPS, EES) for PPy and model calculations has led some researchers to conclude that 10 is the minimum number of monomeric units in a PPy chain, with the maximum within one order of magnitude n9- 27,i28) mechanical qualities of the electropolymerized films,... 

The mechanical properties of PAn differ considerably between the electrochemi-cally prepared polymer and that produced from solvent casting. As described earlier, electropolymerized emeraldine salts are highly porous and, consequently, have low mechanical strength. Freestanding films may be prepared electrochemically, but their poor mechanical properties limit their usefulness. In contrast, the polymers made from solution are much less porous and are widely used as freestanding films and fibers. The effect of polymer structures and morphology on PAn mechanical properties are described in the following text. 

As with polyanilines, polythiophenes can either be prepared directly by electropolymerization, or by casting from solutions (for alkyl-substituted thiophenes). Most interest has focused on the latter because of their improved mechanical properties compared with those of electrochemically prepared films. The factors influencing the mechanical properties of PTh s are reviewed in this section. 

In a third procedure, an adhesion promoter of a 3-(ethyl-phosphonic-acid)thiop-hene was applied to the metal surface followed by an electrochemical film preparation [65]. The following procedure was used in this case Mild steel was mechanically polished, then the specimen were treated for 60 minutes in a solution of 3-(ethyl-phosphonic-acid)thiophene. Afterwards, a layer of poly(3-methylthiophene) was formed by electropolymerization in an electrolyte consisting of 0.1 mol 1 3-methylthiophene, 0.1 moll tetrabutyla-mmonium-hexafluorophosphate (N(Bu)4 PFg) in dichloromethane (CH2CI2). 

The electrochemical polymerization of PVK in LiClOVacetonitrile solution on an SWNT electrode was studied by cyclic voltammetry [194,195], The mechanism of the electropolymerization reaction of VK on the SWNT film was characterized by three stages, chemical-electrochemical-chemical [194]. The main difference between the mechanism of electropolymerization of VK on a Pt electrode only, and an electrode covered with a SWNT film consists in the fact that during the first stage, the formation of a charge-transfer complex results in the formation of VK radical cations and the SWNT radical anions. 

Novel conductive composite films have been developed by Kaplin and Qutubuddin [101] using a two-step process microemulsion polymerization to form a porous conductive coating on an electrode followed by electropolymerization of an electroactive monomer such as pyrrole. The porous matrix was prepared by polymerizing an SDS microemulsion containing two monomers, acrylamide and styrene [102], The electropolymerization of pyrrole was performed in an aqueous perchlorate or toluenesulfonate solution. The effects of polymerization potential on the electropolymerization, morphology, and electrochemical properties were reported [101]. The copolymer matrix improves the mechanical behavior of the polypyrrole composite film. 

Poly(pyrrole), known for over 50 years, is one of the most studied poly-mers. First synthesized by electrochemical polymerization in 1968, poly(pyrrole) is electrically conductive, stable, and insoluble when doped. The mechanism of its electrosynthesis was studied by Genies et Thin films of electropolymerized poly(pyrrole) (< 0.1 fim) are electroactive, and they can be switched between the neutral and the oxidized state at +0.1 V versus SCE. This particular property of polypyrrole can be applied in constructing conductivity... 

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