Solutions and physical properties of conducting polymers: Polypyrrole

Inherent viscosity of polypyrrole (PPy) solution and the electrical conductivity of PPy film cast from the solution indicate that higher molecular weight polymer Is obtained with higher oxidant/monomer ratio. Conductivity and EPR data indicate that soluble PPy consists more of polaronic forms than insoluble PPy does and that the solubility of PPy is determined by the molecular weight, oxidation state, and the degree of crosslinking. It was found that, for PPy, m-cresol was a better solvent than NMP and the film cast from NMP solution was more brittle than that cast from m-cresol, because the polymer chains have more compact structures in a poor solvent. The conductivity data also support this argument: namely PPy film cast from NMP showed very low conductivity (similar to 10(-10) S/cm) while the film cast from m-cresol showed a conductivity of 1.3 x 10(-4) S/cm. From spectroscopic studies, it is concluded that the individual chain of PPy molecule is metallic and the polymer chains tend to have expanded structures as a result of association with the dopant ions. This leads to higher conjugation length and ultimately to higher conductivity of the PPy film cast from m-cresol. In the UV-Vis spectra of the solutions, the sharp peak at 475 nm and the broad peak at 420 nm were assigned to the pi-->pi* transitions of the neutral and doped forms, respectively. The peak absorption at 933 nm (1.33 eV) seems to arise from the transition between defect states. The conducting form is responsible for the broad absorption peaks at 420 nm, 660 nm, and 950 nm.