Polyaniline reversible acid/base

Polyaniline can undergo a reversible acid/base doping/dedoping process. In the emeraldine salt form, polyaniline is conductive. This form can then be converted to the insulating emeraldine base form of polyaniline, upon exposure to base as in Equation 1, where HX is a protonic acid ... 

Polyaniline and its substituted derivatives, such as poly(o-toluidine), poly(o-anisidine), poly(N-methylaniline), poly(N-ethylaniline), poly(2,3-dimethylaniline), poly(2,5-dimethylaniline) and poly (diphenylamine) have been reported [36] to show measurable responses (sensitivity 60%) for short chain alcohols (viz., methanol, ethanol and propanol) at concentrations up to 3000 ppm. The change (decrease) in resistance of the polymers on exposure to alcohol vapors has been explained based on the vapor-induced change in the crystallinity of the polymer. Polypyrrole (PPy) incorporated with dodecyl benzene sulfonic acid and ammonium persulfate has been reported to show a linear change in resistance when exposed to methanol vapor in the range 87-5000 ppm [37]. The response is rapid and reversible at room temperature. 

The Cu(II) complex with polyaniline (emeraldine base) exhibits a higher catalytic efficiency for the dehydrogenative oxidation of cinnamyl alcohol into cin-namaldehyde. Iron(III) chloride is similarly used instead of copper(II) chloride. The catalytic system is applicable to the decarboxylative dehydrogenation of man-delic acid to give benzaldehyde. The cooperative catalysis of polyaniline and cop-per(II) chloride operates to form a reversible redox cycle under oxygen atmosphere as shown in Scheme 3.4. The copper salt contributes to not only oxidation process but also metallic doping. The reduced phenylenediamine anionic species appear to be stabilized by the metallic dopants. 

There are two weak bases, ammonia and polyaniline (emeraldine base) competing for the acid. When ammonia is present, the reaction moves to the right and polyaniline is dedoped. When the ammonia vapor is turned off (removed), the reaction may move back to the left and polyaniline becomes redoped, releasing ammonia. This process involves a reversible change in the mass of the film [85] in addition to the changes in color and resistance. 

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