Mechanisms of ion conduction in polyelectrolyte multilayers and complexes

Abstract

This paper reviews the progress made in understanding of the mechanisms of ion conductionin polyelectrolyte multilayers (PEM) and polyelectrolyte complexes (PEC). Thebasis are experimental conductivity data obtained by impedance spectroscopy as a functionof relative humidity and temperature, respectively. Mechanically stable thin films of PEMhave interesting perspectives as ion conductors, however, being prepared by self-assembly,their stoichiometry and content of ionic charge carriers is unknown. Therefore PEC actas 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 polyelectrolytecomplexes of type x NaPSS ̇ (1̇x)PDADMAC as a function of temperature and composition,respectively. The dependence of the dc conductivity is discussed along with scalingproperties of the spectra. The results show that the conductivity is always determinedby the sodium ions, even in PEC with an excess of PDADMAC. The ion dynamics andtransport mechanisms are, however, different in PDADMAC-rich than in NaPSS-rich PEC.PEM of different polyionic compounds are investigated in dependence on relativehumidity. A general law of an exponential increase of the dc conductivity with relativehumidity is found. Absolute values of the conductivity and the strength of the humiditydependence are different for different polyion materials, however, they do not depend onthe type of small counterion employed in layer formation. Therefore, it is concluded thatin hydrated PEM, protons are the dominant charge carriers.For both PEM and PEC we show that the MIGRATION concept developed by Funkeand co-workers can be used for describing the experimental spectra over wide rangesin frequency. This implies that forward-backward hopping motions of small ions playa vital role in solid polyelectrolyte materials. Apart from these potentially successful hops,localized motions of charged particles are found to influence the conductivity spectra aswell. © by Oldenbourg Wissenschaftsverlag, München.