Electropolymerization, Characterization And Ion Exchange Property Of Polypyrrole Films

Polypyrrole (PPy), as a typical conducting polymer, has many advantages such as easy polymerization, harmlessness and good environmental stability. One of its promising behaviors is that it could be doped and dedoped reversiblely by ions under the control of electrical signals. Therefore, PPy composites could be a potential electrically switched ion exchange resin.The electropolymerization and characterization of polypyrrole films doped by para-toluene sulfonate (PPy/pTS) were investigated in this paper. The results proved that the obtained polypyrrole films had good conductivity, electroactivity and long-term electrochemical stability. The polypyrrole film with fibrillar morphology, which had a large specific surface, was obtained at constant current density of 3mA/cm² during 1800s electropolymerization time on a platinum electrode.Ion exchange property of PPy films performed in aqueous solutions was investigated by cyclic voltammetry. The results certified that PPy films doped by large, small, mediate dopant ions held cation, anion and a combination of cation and anion exchange properties, respectively. This paper chose PPy/pTS films as main PPy films under investigation. The results showed that the ion exchange capability of the PPy/pTS films increased with decreased hydrated ion radius and ion valence. Large ions like pTS^- and (C₂H₅)aN⁺ even could not enter the film. Both acid and base have a negative influence on the exchange property of the PPy/pTS film. The anion exchange capacity of the film was suppressed in NaOH solution; the cation exchange capacity decreased in HCl solution as well. The reason should be related to the nature of OH^- and H⁺. During the cyclic voltammetry, the extent of the ion exchange decreased with the increased potential scan rate. Moreover, through the analysis of the redox peaks on the CVs, the ion exchange processes were proved to be electrochemically irreversible and ion diffusion-controlled reaction. The ion exchange capacity of the PPy/pTS film was theoretically calculated to be 2.64 mmol/g and 5.21 mmol/g for cation and anion exchange, respectively.The structure change of the PPy/pTS film during its ion exchange processes was investigated by ananlysing the chronoamperogram in potential step experiments. The results showed that the film swelled or shrank when the ion incorporated into or out of the flim. The swelling process was also called conformational relaxation which could be distinguished from the current shoulder on theâ… -t curves during the potential step experiments. The conformational relaxation happened earlier as the ion size was smaller. The higher electrolyte concentration caused the film a rapidly conformational relaxation. The different electroactivity of the PPy/pTS film could induce various ion transport processes. In addition, theâ…¤-t curves obtained during the constant current polarization showed that the film has two ion exchange processes.The investigation on the overoxidation of PPy/pTS film proved the maximum potential should be controlled under 0.8 V vs.SCE during its electropolymerization and ion exchange processes. Moreover, anion exchange capacity of the film decreased firstly, and then cation exchange capacity also decreased during its overoxidation. Counterions could be released from the film during a long term ion exchange processes without overoxidation, which would decrease the cation exchange capacity but increase the anion exchange capacity, however, its overall ion exchange property kept constant.The analysis of the redox peaks of CVs of PPy/pTS film proved that the anion transport could be divided into two stages, the cathodic peak of CVs at more positive potential only corresponded anion transport; the other peak at more negative potential could be a combination of anion expulsion and cation incorporation.