Polyaniline autocatalytic growth

Figure 12.8 (d) Frequency response indicating autocatalytic growth of polyaniline from 55th second. [87],... 

However, Trivedi et al. [95] have argued that when there is a possibility of autocatalytic growth of polyaniline, the lower electrochemical potential does not favour the formation of such a crosslinked moiety, unless one crosses the threshold limit of 0.8 V versus SCE. Thus, according to these authors, the middle peak is due to quinone/hydroquinone, whose formation is possible because under acidic conditions the cation radical can be hydrolysed to yield benzoquinone. 

It has been observed [100] that with H2SO4 as an electrolyte there is a continuous rise in current after an initial decay, suggesting parallel nucleation along with autocatalytic growth of polyaniline. 

Kinetic studies by Shim and Park [132] determined the autocatalytic rate for polyaniline growth. Other reaction kinetic studies and comparison of the properties of the chemically synthesized emeraldine to the electrochemical product led Mohilner et al. [131] to conclude that the oxidation of aniline occurs via a free radical mechanism that produces the emeraldine octamer as the primary product. Emeraldine is a blue-violet base form of polyaniline that forms dark green salts as described by Green and Woodhead [133]. Emeraldine loses two protons upon ox-... 

The growth of the polymer film can be followed by cyclic voltammetry since the current peaks related to the polymer redox transformations increase as more and more polymer is deposited. The increase of the surface mass can be detected by using an -> electrochemical quartz microbalance. Such an example is shown in the Figure. In this experiment the positive potential limit of cycling was gradually decreased in order to avoid overoxidation of the polyaniline (PANl) formed. It does not affect the rate of polymerization, i.e., the film growth, since the electrooxidation of aniline is an autocatalytic process. 

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