Dynamics of counter-ions in a conducting rigid polymer matrix: the relation with electrical properties

Abstract

Using the quasielastic neutron scattering technique on the 10⁻¹² s time scale, the dynamics of highly conducting free-standing films of doped polyaniline have been studied in detail. The latter are protonated with sulphonated organic acids made of flexible molecules which also play the role of plasticizer of the system. While polymer chains appeared immobile, the study of the dynamics of these counter-ions reveals a glass transition temperature which corresponds to the change in the electronic conductivity of the film. Competing interactions in between the counter-ions and between polymer chains and counter-ions limit the available space for alkyl chains to move around. A description of the dynamics is proposed on the basis of a model where protons undergo a local diffusion inside individual spheres whose radii depend explicitly on their position on the molecule. When the temperature is decreased, only protons attached to the aliphatic tails explore a significant space while the others are progressively frozen. Refinement of the neutron data permit one to access the radii of the sphere and the diffusion constant as functions of temperature.