Electron paramagnetic resonance study of spin centers related to charge transport in metallic polyaniline

Film-like conducting form of polyaniline (PANI), doped with 10-camphorsulfonic acid (CSA) and 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPSA) has been studied by multifrequency EPR spectroscopy at wide temperature range. Two types of paramagnetic centers with different magnetic resonance and mobility parameters were detected to be stabilized in metal-like domains and amorphous matrix of the samples. Sum spin susceptibility of the samples was shown to consist of the terms originated from paramagnetic centers localized in amorphous polymer backbone as well as spin charge carriers delocalized in embedded metal-like domains. The part of electron spins strongly interacts in the samples that resulting antiferromagnetic term in paramagnetic susceptibility. The delocalized paramagnetic centers characterizing by a stronger Pauli component in spin susceptibility are directly involved in the charge transfer, as compared to the localized ones. Their linewidth is governed by spin precession frequency and dipole–dipole interaction modulated by macromolecular torsional oscillations. The charge was described to be transferred in terms of the Mott quasi-one-dimensional variable range hopping of localized carriers through amorphous bridges between crystalline metal-like domains with mobile carriers interacting with the lattice phonons. The latter process is shown to be accelerated as AMPSA counterion is replaced by CSA one. The activation energy of spin dynamics in crystalline phase inversely correlates with the effective energy separation between localized states. This indicates a major role of dopant structure in local molecular-scale ordering and charge transport properties in doped PANI.