Structure–conductivity relationships in chemical polypyrroles of low, medium and high conductivity

Abstract

Chemically synthesized polypyrroles of low ( σ < 75 S/cm), medium (75 < σ < 200 S/cm) and high ( σ > 200 S/cm) electrical conductivity ( σ) with the same dopant and degree of doping have been investigated by means of Wide Angle X-ray Scattering (WAXS), 13C Cross Polarized Magic Angle Spinning Nuclear Magnetic Resonance ( 13C CP/MAS NMR) spectroscopy and Fourier Transform Infrared (FTIR) Spectroscopy to establish structure–conductivity relationships useful for industrial applications. A similar amorphous structure was found by WAXS even for the higher conducting PPy ( σ = 288 S/cm). WAXS spectra for polypyrroles of medium and high conductivity showed a weak peak at 2 θ = 10–11° due to improved order of the counterions in these materials. The effect of the counterion size in the asymmetry of the PPy main WAXS peak was elucidated by performing ion exchange of the Cl − dopant with counterions of larger size such as BF 4 − and ClO 4 −. From 13C CP/MAS NMR measurements predominantly α–α′ bonding was found in these materials. The main 13C CP/MAS NMR resonance peak of PPy located at 126–128 ppm was broadened upon increasing conductivity. Interestingly, a linear relationship was observed between the half-width at half-height (HWHH) of the 13C CP/MAS NMR peak and conductivity where a doubling of the polypyrrole conductivity leads to an increase of HWHH by 6–7 ppm. FTIR data of these materials were analysed in the framework of the Baughman–Shacklette theory describing the dependence of conductivity on conjugation length. By comparison of model predictions and experimental results, the PPy samples were found to be in the regime of long conjugation lengths, L ≫ K 2/ k B T, where K 2 is a parameter related to the energy change on going from j − 1 to j charges on a conjugated segment of conjugation length L, k B the Boltzman constant and T is the absolute temperature.