Mechanisms of Ion Conduction in Polyelectrolyte Multilayers and Complexes

Abstract

This paper reviews the progress made in understanding of the mechanisms of ion conduction in polyelectrolyte multilayers (PEM) and polyelectrolyte complexes (PEC). The basis are experimental conductivity data obtained by impedance spectroscopy as a function of relative humidity and temperature, respectively. Mechanically stable thin films of PEM have interesting perspectives as ion conductors, however, being prepared by self-assembly, their stoichiometry and content of ionic charge carriers is unknown. Therefore PEC act as a model material with a variable stoichiometry and known ion content.

Employing poly(sodium 4-styrene sulfonate) (NaPSS) and poly(diallyldimethyl ammoniumchloride) (PDADMAC), we present conductivity spectra of dried polyelectrolyte complexes of type xNaPSS·(1-x)PDADMAC as a function of temperature and composition, respectively. The dependence of the dc conductivity is discussed along with scaling properties of the spectra. The results show that the conductivity is always determined by the sodium ions, even in PEC with an excess of PDADMAC. The ion dynamics and transport mechanisms are, however, different in PDADMAC-rich than in NaPSS-rich PEC.

PEM of different polyionic compounds are investigated in dependence on relative humidity. A general law of an exponential increase of the dc conductivity with relative humidity is found. Absolute values of the conductivity and the strength of the humidity dependence are different for different polyion materials, however, they do not depend on the type of small counterion employed in layer formation. Therefore, it is concluded that in hydrated PEM, protons are the dominant charge carriers.

For both PEM and PEC we show that the MIGRATION concept developed by Funke and co-workers can be used for describing the experimental spectra over wide ranges in frequency. This implies that forward-backward hopping motions of small ions play a vital role in solid polyelectrolyte materials. Apart from these potentially successful hops, localized motions of charged particles are found to influence the conductivity spectra as well.