The relaxation process of pristine polypyrrole/dodecyl sulfate (PPy/DS) films at open circuit, immediately after electropolymerization, was studied by in situ spectroelectrochemical, alternating-current conductance, electrochemical quartz crystal microbalance, and pH-metric techniques. A new phenomenon, called irregular reduction, involving apparent redoping and resulting in a polymer with a lower conjugation length is observed, which may occur if the pristine film is reduced without a waiting period. During this waiting period at open circuit, the film is deprotonated and partially desolvated with concurrent stacking of the polymer chains. The length of the waiting period allowed for the newly made polymer is of utmost importance, and ignorance of the relaxation of the film may explain the well-known irreproducibility in the fabrication of conducting polymers.
IntroductionThe realization of the electronically conducting state of polymeric films has been described already in the early 1980s by the polaron/bipolaron model. 1 Since that time, many efforts have been made to reveal the details of the process and to provide a chemical picture formulating the transformation into the conducting state. 2-10 Previous studies on polythiophene films gave direct evidence for the involvement of nonfaradaic steps in the undoubtedly electrochemically induced transformation by demonstrating the conductance increase under currentless circumstances. 11 These studies were initiated by the observation of an increase in the in situ conductance of the pristine film after electropolymerization at open circuit (OC). Recently, an irregular behavior of pristine polypyrrole/dodecyl sulfate (PPy/ DS) films during their electrochemical reduction was reported. 12 It was observed that the time spent at OC before the reduction greatly influences the properties of the polymer.This observation is considered very important in the context of the large variation in the reported properties of the films made essentially under similar conditions. The importance of proper washing of the pristine PPy film by a fresh electrolyte solution was already suggested in the early "history" of conducting polymers, 13-15 and the effect of the composition of the polymerization solution on the behavior of the film was presented. 3 However, the later appearance of such different or even contradictory reports on the "same" polymer has to indicate that the films were not identical. They were only thought to be identical because they were prepared under the same conditions by applying the same electrochemical parameters, using the same solvent, and using the supporting electrolyte in the same concentration. This paper demonstrates that allowing a properly long waiting period for the pristine film to relax in the solution before its first reduction is a necessary and crucial factor to receive a polymer which behaves regularly, that is, according
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