Molecular dynamics in conducting polyaniline protonated by camphor sulfonic acid as seen by quasielastic neutron scattering

Abstract

Using incoherent quasielastic neutron scattering techniques, the molecular motions were investigated in fully hydrogenated and partially deuterated polyaniline protonated by camphor sulfonic acid (CSA) conducting samples. The obtained results show that on the ${10}^{\ensuremath{-}9}--{10}^{\ensuremath{-}12}\mathrm{s}$ time scale the polymer chains do not exhibit any diffusive motions: the whole observed quasielastic scattering has accordingly to be attributed to motions of CSA ions. From our measurements two molecular movements could be differentiated. A rapid one has been attributed to the three-site rotation of methyl groups present on camphor moieties of CSA and a slower one that has been modeled as a rigid body motion of the whole CSA molecule. Due to the disordered character of the system, the methyl rotors appeared to be dynamically nonequivalent. Their dynamics was then described in terms of a log gaussian distribution of correlation times. This description allowed a good fitting of experimental data and gave an activation energy of 12.5 kJ ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$. However, two different regimes in temperature could be distinguished. At high temperatures $(Tg280\mathrm{K})$ the width of the distribution is nearly zero and thus, the methyl rotors are dynamically equivalent while it turned larger and larger when temperature is decreased below 250 K revealing that the rotors are more and more sensitive to their local environment. In the conducting samples the slowest motion clearly exists in the 280--330 K temperature range and is blocked at temperatures inferior to 250 K. This transition occurs in the temperature range in which the metal-insulator transition also happens.