Influence of doping level on the structure and electrical properties of conducting polymer

Abstract

Poly(pyrrole) and poly( aniline) are prepared on to electrode surface from an aqueous solution by electrochemical oxidative polymerization of pyrrole and aniline, respectively. Optimal electrolytic conditions for the electrosynthesis of the studied polymers are also determined. Film of these polymers on platinum when mounted in electrolytic solution without monomers can be electrochemically switch between the oxidized (doped / conducting) form and the neutral (dedoped / insulating) form. Thus prepared polymer samples employed XRD and IR spectroscopy for their structural analysis. The d.c conductivity and UV-Vis spectroscopic measurements are performed to have an insight about the electrical and optical properties of the studied polymeric materials. Effect of ambient atmosphere and heat on the structure and electrical properties of the poly( aniline) samples are also studied. Optimization of the electrolytic solution concentration as 0.5 M pyrrole and 0.1 M tetraethylammonium tetrafluoroborate for the electrosynthesis of poly(pyrrole) and 0.5 M aniline and 0.8 M perchloric acid as for poly(aniline) resulted a massive and free standing polymer on the electrode substrate. Cyclic voltammograms both for poly(pyrrole) and poly(aniline) indicate that the polymers can be oxidized by doping with an anions and can be neutralize by dedoping the inserted anions. Doping of the studied samples is controlled potentiostatically for poly(pyrrole) and galvanostatically for poly(aniline) samples. From IR spectra the bands, characteristics of pyrrole and aniline confirming the presence of these monomer rings in the polymers. Absorption bands for tetrafluoroborate and perchlorate ions are also found in the doped spectra of poly(pyrrole) and poly(aniline), respectively. XRD spectra showed the diffused scattering for doped and dedoped poly(pyrrole) samples indicating the amorphous nature ofpoly(pyrrole). In contrast, crystalline peaks are observed in the doped spectra of poly(aniline), although its dedoped state exhibits amorphous character. 1 The d.c conductivity measurements both for poly(pyrrole) and poly(aniline) show a general trend of doping characteristics i.e., conductivity rises rapidly at the beginning of the doping process and then shows little change even though the polymer films become more highly doped. The conductivity of poly(pyrrole) and poly(aniline) reached a maximum value of 3.40 x 10-1 and 45.00 x 10-1 Scm-I, respectively, which ate about three orders of magnitude higher than their respective dedoped states. UV-Vis spectra of poly(pyrrole) and poly(aniline) show the strong absorption maxima in the ultra-violet region which are associated with the interband trasition. Absorption in the visible region also indicated two other transitions within the gap region such as : transition from valence band to upper bipolaron antibonding level and trnnsition from valance band to bipolarOll bonding state. Optical studies also provide the elucidation of band model for P9Iy(pyrrole) and poly(aniline) assigning a band gap of 4.96 and 3.70 eV, respectively.