Magnetic susceptibility of polyaniline in solution in non-polar organic solvents and in polyblends in poly(methyl methacrylate)

Abstract

We report electron spin resonance measurements of the magnetic susceptibility (χ) of the protonated conducting form of polyaniline (PANI) in solution in non-polar or weakly polar organic solvents and as thin films cast from such solutions. At room temperature, χ = 3 × 10 −5 e.m.u./mol (two rings) for PANI films with camphor sulfonic acid (CSA) and dodecylsulfonic acid (DBSA) as counter-ions. In both cases, the concentration of localized Curie-Law spins is about 1% (per two rings of PANI). In solution, χ = 2.8 × 10 −5 e.m.u./mol (two rings) for PANI-CSA in m-cresol and χ = 3.8 × 10 −5 e.m.u./mol (two rings) for PANI-DBSA in xylene, and they are found to be temperature independent. In polyblends with PMMA, χ = 3 × 10 −5 e.m.u./mol (two rings) at room temperature, independent of the volume fraction of PANI-CSA. The temperature independence implies a metallic state with Pauli spin susceptibility, χ P, resulting from a density of states at the Fermi level of about 1 state per eV per two rings. The small Curie-law contribution demonstrates that the DBSA and CSA surfactant counter-ions (S −1) lead to higher quality material with reduced disorder. The concentration independence of χ P in blends implies that the PANI complex is phase separated in the PMMA matrix, consistent with a network-like microstructure. Since the Pauli susceptibility indicates the existence of a true metallic state, the PANI-S − complexes are the first examples of metallic polymers in solution, i.e. stable ‘liquid metals’.