INTRINSIC ASYMMETRY, HYSTERESIS, AND CONFORMATIONAL RELAXATION DURINGREDOX SWITCHING IN POLYPYRROLE - A COULOVOLTAMETRIC STUDY

Theoretical coulovoltammograms associated with the evolution of anodic charges when a polypyrrole film, which was previously compacted at a strong negative potential in a 0.1 M LiClO4 propylene carbonate soluti on, was submitted to anodic potential sweeps were compared with experi mental responses. The electrochemically stimulated conformational rela xation (ESCR) model provides a good description of either the influenc e of the negative potential limit, the sweep rate, the temperature, or the electrolyte concentration on the rate of electrochemical oxidatio n, this being initiated through conformational relaxation processes an d completed under counterions diffusion control. An analysis of both a nodic and cathodic experimental branches, together with theoretical co nformational relaxation curves, allows to detect four different struct ural rearrangement processes affecting electrochemical responses: the well-known conformational relaxation effect observed only in anodic br anches, a hysteresis effect on cathodic curves at potentials lower tha n -600 mV vs SCE, an intrinsic asymmetry between anodic and cathodic b ranches, and a like-capacitive effect at the beginning of the anodic s weep. The combination of both hysteresis effects in cathodic branches and relaxation effects in anodic ones gives rise to a hysteresis cycle , whose integration yields the electrochemical energy required to expa nd the compacted polymer structure. On the other hand, the intrinsic a symmetry between anodic and cathodic branches has been related to the different values of coefficients z(r) and z(c) which, according to the ESCR interpretation, are associated with the electric charge required to open or close a mole of polymeric segments, respectively. This fac t opens the possibility for a complete theoretical description of elec trochemical responses of conducting polymers as a function of their st ructure.