Cornelia Cramer

A conductivity study and calorimetric analysis of dried poly(sodium 4-styrene sulfonate)/poly(diallyldimethylammonium chloride) polyelectrolyte complexes

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.

Tracer diffusion in sodium–rubidium borate glasses: An unconventional mixed-alkali effect?

Abstract In mixed sodium–rubidium borate glasses with a constant total content of 30 mol% alkali oxide and various relative concentrations of sodium and rubidium diffusion of 22 Na and 86 Rb was studied at 673 K using the radiotracer serial sectioning method. The diffusion coefficients of sodium and rubidium show a concentration dependence which is at variance with ‘classical’ expectations for mixed-alkali glasses. No cross-over of the diffusivities is observed since Na diffusion is always faster than Rb diffusion. Na diffusion passes through a minimum near 60% Na 2 O content whereas Rb diffusion shows a maximum near 50% Na 2 O content. The ionic conductivity deduced thereof displays a minimum as a function of the relative composition of sodium and rubidium like in ‘classical’ mixed-alkali glasses.

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.

Ion Dynamics in Solid Polyelectrolyte Materials

Abstract Frequency-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.

Ion dynamics in mixed alkali borate glasses

Abstract Frequency-dependent conductivities are a valuable tool for studying the ion dynamics on different time scales, the latter being determined by the inverse of the experimental frequency. Therefore, wide range conductivity spectra probe the transition from elementary steps of the ionic movement to macroscopic transport. We have studied the ion dynamics in mixed alkali borate glasses where we have systematically varied the total and the relative ion concentration. The glass system under investigation is y [x Li2O · (1−x) Na2O] · (1−y) B2O3 with x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 and y = 0.1, 0.2, 0.3. The conductivity spectra can be formally divided into a low-frequency and a high-frequency regime. In the low-frequency regime the conductivities of all glasses show a transition from their dc values into a dispersive regime where the conductivity is found to increase continuously with frequency, tending towards a linear frequency dependence at sufficiently low temperatures. The conductivity spectra can be described by the MIGRATION concept developed by Funke. We discuss the spectral shape of the conductivity spectra and its implications for the ion transport. In addition, we report on a new mixed alkali effect occurring in the high-frequency regime of the ac conductivity. From our results we conclude that apart from a low-frequency contribution to the conductivity which is linked to ion hopping sequencess, there is an additional contribution involving localized motions of both the mobile ions and the glassy network.

Ion Conduction in Solid Polyelectrolyte Complex Materials

This chapter describes the progress made in understanding the mechanisms of ion conduction in polyelectrolyte complexes (PEC). Understanding of ion dynamics is based on frequency-dependent conductivity data obtained by impedance spectroscopy as a function of temperature, hydration, and composition. In most of the work, strong polyelectrolytes such as poly(alkali 4-styrene sulfonate) (AlkaliPSS) and poly(diallyldimethyl ammoniumchloride) (PDADMAC) are employed, forming complexes of type xAlkaliPSS · (1 − x) PDADMAC. The dc conductivity is always determined by the alkali ions, which exhibit a size-dependent mobility. This holds even in PEC with an excess of PDADMAC. The ion dynamics and transport mechanisms are different in PDADMAC-rich and in NaPSS-rich PEC. We review the treatment of the frequency-dependent shape of conductivity spectra by scaling concepts and by models involving forward–backward hopping motions of small ions as well as localized motions of charges. Thus, many quantitative concepts established in other disordered ion conductors can be transferred to PEC. In addition to the well-known time–temperature superposition principle (TTSP), the novel concept of time–humidity superposition (THSP) was established for PEC and describes the dependence of ion dynamics on water content.