Polyelectrolyte multilayers are built up from ionically modified polyphosphazenes by layer-by-layer assembly of a cationic (poly[bis(3-amino-N,N,N-trimethyl-1-propanaminium iodide)phosphazene] (PAZ + ) and an anionic poly[bis(lithium carboxylatophenoxy)phosphazene] (PAZ -). In comparison, multilayers of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) are investigated. Frequency-dependent conductivity spectra are taken in sandwich geometry at controlled relative humidity. Conductivity spectra of ion-conducting materials generally display a dc plateau at low frequencies and a dispersive regime at higher frequencies. In the present case, the dispersive regime shows a frequency dependence, which is deviating from the typical behavior found in most ion-conducting materials. Dc conductivity values, which can be attributed to long-range ionic transport, are on the order of σ dc ) 10 -10 -10 -7 S‚cm -1 and strongly depend on relative humidity. For PAZ + /PAZ -multilayers σ dc is consistently larger by one decade as compared to PSS/ PAH layers, while the humidity dependence is similar, pointing at general mechanisms. A general law of a linear dependence of log(σ dc ) on relative humidity is found over a wide range of humidity and holds for both multilayer systems. This very strong dependence was attributed to variations of the ion mobility with water content, since the water content itself is not drastically dependent on humidity.
A range of different combinations of polyelectrolytes is employed to form multilayers by layer-by-layer assembly, which are investigated by impedance spectroscopy. In particular, the alkali counterion employed in the layer formation is varied. Impedance spectra of different multilayer systems are qualitatively similar, and they are changing in a similar way with relative humidity (RH). From the spectra, the dc conductivity σdc and its dependence on humidity are extracted. The humidity dependence of σdc follows a general law of log(σdc) = aRH + b, which is valid for all systems. Absolute values of σdc and slopes a depend on the type of polyelectrolytes employed but are independent of the type of alkali counterion involved. On the basis of these data, we discuss the contribution of different small ionic species, i.e., anions, alkali cations, or protons, to the conductivity and conclude that the differences between different polymer systems as well as the humidity dependence are consistent with the conduction of protons or hydronium ions, while the contribution of other cations or anions to σdc is negligible.
Ion Dynamics . Solid Polyelectrolyte MaterialsFrequency-dependent conductivities are a valuable tool for studying the ion dynamics on different time scales. We present and analyze conductivity spectra of two kinds of solid polyelectrolyte materials, viz. polylectrolyte multilayers (PEM) and polyelectrolyte complexes (PEC). The PEM spectra are taken as a function of relative humidy at ambient temperature. By contrast, the conductivity of different kinds of dried PEC is studied as a function of temperature. For both kinds of material classes we show that the MIGRATION concept developed by Funke and co-workers can be used to describe the experimental spectra over wide ranges in frequency, indicating 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. Based on the shape of our conductivity spectra and their scaling properties, we arrive at important conclusions about the microscopic ion dynamics in PEM and PEC materials.
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