The effect of pH on polymerization and volume change in PPy(DBS)

Polymerization and electrochemically induced volume change in polypyrrole doped with the large aromatic surfactant anion dodecylbenzenesulfonate, PPy(DBS), were studied as a function of pH in unbuffered aqueous solutions prepared by mixing DBS acid and NaOH. Volume change was measured during the first, anomalous reduction and in subsequent cycles using a bending bilayer. The results differed completely from those obtained by others for PPy doped with small, mobile anions. Films could be deposited potentiostatically at all nominal pH values between 1 and 11, but growth was fastest at the lowest pH and uneven above pH 10 due to impaired nucleation. However, during potentiodynamic polymerization, deposition and redox were easier at alkaline pH, having a smaller peak separation and greater charge density. At pH less than 3, there were no reduction or oxidation (redox) peaks and the films became insulating. Contrary to expectations, the pH of the growth solution did not affect the performance of the bilayer actuators, but the pH of the cycling solution was important. Bending was fastest in alkaline solutions, although the degree of bending was the same at low and high pH. Except at very low pH, two pairs of redox peaks could be distinguished in the cyclic voltammograms, with volume change restricted to the lower potential pair associated with Na+ movement. There was no volume change associated with the anomalous current peak seen during the first reduction scan, regardless of pH, but the films contracted during re-oxidation. Thereafter, during reduction the films expanded to their original, as-grown volume.