Physical Characterization of Electrochemically and Chemically Synthesized Polypyrroles

Abstract

The results of temperature-dependent dc conductivity, EPR magnetic susceptibility, and X-ray photoelectron spectroscopy (XPS) experiments are compared for electrochemically and chemically synthesized polypyrrole (PPy) samples. For chemically synthesized PPy samples soluble in organic solvent with large size dopants such as dodecylbenzenesulfonic acid (DBSA) or naphthalenesulfonic acid (NSA), dc conductivity (σdc) is ≤0.1 S/cm at room temperature, and its temperature dependence [σdc(T)] shows strong localization behavior, while σdc(T) of the electrochemically synthesized PPy samples doped with hexafluorophosphate (PF6) is in the critical or the metallic regime. The density of states of chemically prepared PPy−DBSA samples is less than one-fourth that of electrochemically synthesized PPy−PF6 samples. The g values and temperature dependence of the line width obtained from EPR experiments show that the paramagnetic signals in both electrochemically and chemically synthesized PPy samples are mainly due to the polarons in π-conjugated polymer chains. We observe the existence of one dopant per three pyrrole rings in PPy−DBSA and PPy−PF6 samples. From the XPS experiments, one-fifth of the pyrrole rings of chemically prepared PPy−DBSA are incorporated into interchain links or side chains, while for electrochemically prepared PPy−PF6, one-third of the pyrrole units are in side chains or cross-links. We analyze that the side chains or cross-links of chemically synthesized PPy samples are relatively reduced, and subsequently the interchain interaction weakens. For chemically synthesized PPy samples, the synthesis method using large size dopants is one of the important roles for the reduction of side chains or cross-links and for the increase of solubility. The weakness of the interchain interaction and the reduction of side chains or interchain links for PPy systems play an important role for charge transport.