Local motions of counter-ions in polyelectrolyte solutions without added salts. A neutron quasielastic scattering study

Abstract

Local motions of tetramethylammonium counter-ions in semidilute polyelectrolyte solutions without added salts have been investigated by neutron incoherent quasielastic scattering in the energy range 0.05–2.0 meV and Q range 0.4–2.0 Å^–1. A small Debye–Waller factor and negligible inelastic scattering suggest that vibrational motions in the present frequency region are damped by thermal agitations. Measured elastic incoherent structure factors (EISF) were analysed in terms of three models: (a) circular-jump model; (b) tetrahedral-jump model; (c) linear-jump model, each with damped vibrational motion. It is found that the circular-jump model best describes our results. The so-called condensed counter-ion bound on a charged polyion site due to electrostatic attractive forces does not change its binding site but rotates with a three-fold symmetry.