INTERCONVERSION OF POLARONS AND BIPOLARONS OF POLYANILINE DURING THE ELECTROCHEMICAL POLYMERIZATION OF ANILINE

An in situ electron spin resonance (ESR)-electrochemical method was em ployed to study the interconversion of polarons and bipolarons of poly aniline during the electrochemical polymerization of aniline. The ESR signal was hardly observed for the first scan during electrolysis of t he solution containing 20 mM aniline and 1 M HCl when the potential wa s swept from 0 to 1.07 V (VS Saturated calomel electrode). However, an ESR signal with a single line was clearly observed when the potential was swept to about 0.95 V for the first scan during electrolysis of t he solution containing 0.2 M aniline and 1 M HCl. For the second scan, from 0 V in the positive direction at the same potential range, the E SR intensity first increased and then a peak of the ESR signal, i.e. a maximum value of the ESR intensity, occurred at 0.26 V. This was caus ed by polyaniline polymerized on platinum due to the first scan; over the peak, the intensity of the ESR signal decreased quickly with incre asing potential. The change in the ESR intensity with potential shows that a transfer of polarons in polyaniline to bipolarons occurs from 0 .26 to 0.95 V. As the potential increased further, the intensity of th e ESR signal increased again from 0.95 to 1.07 V, although this increm ent is small compared with the intensity change from 0 to 0.26 V. This is caused by the oxidation of aniline, since the intensity of the ESR signal of polyaniline in 1 M HCl solution in the absence of aniline a lways decreased with increasing potential from 0.26 to 1.07 V. After s topping electrolysis at 1.07 V, the intensity of the ESR signal increa sed quickly with decreasing potential of polyaniline and then increase d slowly when the potential of polyaniline reached an equilibrium valu e. This indicates that bipolarons in polyaniline immediately transferr ed to polarons, followed by the establishment of an equilibrium betwee n polarons and bipolarons. The potential range for formation of the ES R signal in the first scan during electrolysis of aniline is extended in the presence of NaCl. (C) 1998 Elsevier Science S.A. All rights res erved.