Influence of inserted anions on the properties of polypyrrole

Selected properties of supported and free-standing polypyrrole films have been studied as a function of the anion, incorporated in the polymer during electrochemical polymerization. The anion was varied in the range: Cl −, ClO − 4, benzenesulfonate (BS), toluenesulfonate (TOS), 1,3-benzenedisulfonate (BDS) and 1,3,5-benzenetrisulfonate (BTS). The degree of oxidation of polypyrrole as determined by elemental analysis, varied from ca 0.50, when Cl − or ClO − 4 is used, to ca 0.30 for the “organic” anions. Electron micrographs of the films show an increasing smoothness of the films with increasing charge of the ions. The electrical conductivity of free-standing films shows the highest value for the BTS-containing films and decreases in the series: BTS > TOS > BS ⋍ BDS > Cl − ⋍ ClO − 4. The reduction and re-oxidation of thin supported films is a slow process for TOS- and BS-containing films. In these cases, the rate of the process is determined by the diffusion of the anions in the film from Q— t 1 2 plots; diffusion coefficients of 4.3 × 10 −11 cm 2 s −1 for TOS and 4.8 × 10 −11 cm 2 s −1 for BS can be calculated. For Cl − and ClO − 4 containing films, the process is faster and determined by the transition of the polymer from the oxidized to the reduced state (or vice versa). From the linear part of the Q— t 1 2 curves a diffusion coefficient of 3.1 × 10 −9 cm 2 s −1 for Cl − and 3 × 10 −9 cm 2 s −1 for ClO − 4 can be estimated. Also for films containing BDS and BTS ions, the process seems to be determined by the transition of the polymer, instead of by the diffusion of the ions. The differences in behaviour between the various organic ions are explained in terms of a more or less polar character of the ions. Ellipsometric experiments, carried out in ClO − 4 and toluenesulfonate solutions, support the observed differences in behaviour during reduction and re-oxidation of the polymer films.