Ionic transport in 0.2[XNa 2 OÁ(1ÀX)Rb 2 O]Á0.8B 2 O 3 mixed-alkali glasses A ´rpa ´d W. Imre, Stephan Voss and Helmut Mehrer 3225 Diffusion of calcium and barium in alkali alkaline-earth silicate glasses

We present temperature-dependent conductivity spectra of sodium borophosphate glasses with a varying borate/phosphate ratio but a constant sodium oxide content which can be mapped into time-dependent mean square displacements. For the first time, we show that characteristic lengths of ion transport derived thereof are directly linked to features of network structure, viz., the number of boron oxide tetrahedra. Our results also shed light on the mixed network former effect.

We analyze the scaling properties of pH-dependent shear modulus spectra of complex coacervates made of weak polyanions and strong polycations. For the first time, we report on a "time-pH superposition principle". This principle implies that the charge density in complex coacervates made of not fully charged polyions only influences the time scale of the relaxation dynamics, but not the mechanisms of the underlying dynamics.

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.

Ionically cross-linked polyelectrolyte complexes (PECs) of anionic poly(sodium 4-styrene sulfonate) (PSS) and cationic poly(diallyldimethylammonium chloride) (PDADMAC), xPSS.(1-x)PDADMAC, with molar fractions x ranging from 0.30 to 0.70, were prepared and subsequently dried. The PEC samples were analyzed by differential scanning calorimetry, and the ionic conductivity sigmadc of the samples was measured as a function of temperature by means of impedance spectroscopy. The thermograms display an endothermic peak in the temperature range of 90-143 degrees C, which is attributed to a glass transition of the PEC. The glass transition temperature Tg has a symmetric x dependence with a minimum at x=0.50. The temperature dependence of sigmadcT is not affected by the glass transition. The ionic conductivity of the samples before drying is three orders of magnitude larger than sigmadc after drying; nevertheless, their activation enthalpies are identical. Arrhenius parameters obtained from the systematic study of several PEC compositions are discussed. The ionic conductivity of the PSS-rich samples is significantly higher than sigmadc of PDADMAC-rich samples. This implies a relatively high Na+ mobility as compared to Cl(-) mobility in PEC. In contrast to the symmetric x dependence of Tg, the conductivity of PEC increases and the activation enthalpy decreases with increasing x in the investigated composition range. A strong x dependence of sigmadc is observed for PSS-rich PEC, which is attributed to a significant variation in the mobility of the charge carriers.

Zusammenfassung We discuss the origins of mixed-former effects in borophosphate glasses of general composition: Li0.5BO1.75:(1−x)LiPO3 and 0.4 Na2O:0.6 [xB2O3:(1−x)P2O5], and show how these effects relate to changing ion-network interactions. In particular for the latter system, we analyse the permittivity spectra using the MIGRATION concept. The results show that the increase in the calculated “cage length” (spatial extent of localised ion motion) in the mixed former region correlates well with the increased abundance of the BO4 tetrahedra. We propose that more local cation sites are created and fast electronic bond-switching processes open up these sites to facilitate macroscopic transport. This picture of ion transport independent of network dynamics accounts for all favourable effects observed in mixed former glasses such as increase in ion mobility, rise in T g and improvement in chemical durability.

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To study the ion dynamics in an amorphous mixed alkali system, frequency-dependent conductivities of 0.3[xLi 2 OÁ(1 À x)Na 2 O]Á0.7B 2 O 3 glasses have been measured by impedance spectroscopy in a wide temperature range. The conductivities show a transition from their dc values into a dispersive regime where they increase continuously with frequency, tending towards a linear frequency dependence at sufficiently low temperatures. In addition to the '' classical '' mixed alkali effect, i.e. the occurrence of a minimum in the dc conductivity, we also observe the following new mixed alkali effect. In contrast to conductivity spectra of single cation glasses which follow the time-temperature superposition principle, featuring a temperature-invariant shape, the shapes of the conductivity spectra of the mixed alkali glasses studied here are found to change with temperature. To explain the effect, we suggest differently activated mobilities of the two different ionic species. The spectra are discussed in the framework of the concept of mismatch and relaxation (CMR).