X-ray photoelectron spectroscopy studies on sterically-stabilized polypyrrole particles

The surface composition of a sterically-stabilized polypyrrole colloid has been examined by X-ray photoelectron spectroscopy (XPS). The sulfur atom of the polyelectrolyte stabilizer poly(potassium 3-sulfopropyl methacrylate) [PKSPM] and the nitrogen atom of the polypyrrole were used as unique elemental markers for each component. The S/N atomic ratio at the particle surface determined using XPS was found to be significantly higher than the corresponding 'macroscopic' S/N atomic ratio calculated from elemental microanalyses. This is the first direct evidence for the presence of a stabilizer-rich layer at the surface of sterically-stabilized conducting polymer particles. Changing to a lower energy X-ray source (Mg K alpha) did not lead to an increase in the surface S/N ratio; thus, the stabilizer layer is probably 'patchy' rather than a continuous coating. XPS also revealed that the surface concentrations of the K+ and Cl- counterions in the colloid were substantially depleted relative to those found in the PKSPM stabilizer and chloride-doped polypyrrole 'bulk powder'. Moreover, the surface dopant level of the polypyrrole particles calculated on the basis of the remaining chloride ions was unusually low (Cl/N = 0.07), More realistic surface dopant levels were obtained if it was assumed that some of the sulfonate groups of the polyelectrolyte stabilizer acted as dopant anions for the conducting polymer component. It was concluded that the PKSPM stabilizer is adsorbed onto the surface of the polypyrrole particles as a polymeric dopant anion, with the concomitant expulsion of KCl. Finally, there is some evidence that this stabilizer is present as a polymeric dopant in the interior of the particles as well.