Charles A. Edmondson

Complex impedance studies of S-SEBS block polymer proton-conducting membranes

Water uptake, swelling, 1 H pulsed gradient spin-echo nuclear magnetic resonance (NMR) and variable temperature and pressure complex impedance electrical conductivity studies have been carried out on sulfonated styrene/ethylenebutylene/styrene (S-SEBS) triblock polymer proton conducting membranes. At the highest water contents, the activation volume calculated from the effect of pressure on the electrical conductivity is negative. Previously reported results for Nafion 117 show the same behavior. In addition, above about 10 wt% water, the diffusion coefficients, D from NMR and Do calculated from conductivity data, are similar for S-SEBS. The same result is obtained for Nafion 117. The conclusion is that proton transport at high water content is by molecular diffusion for both materials. For low water contents, however, the materials are significantly different. For low water content S-SEBS, D and D a are different while they are the same for Nafion 117. In addition, the variation of the conductivity with temperature for S-SEBS is Arrhenius while that for Nafion 117 is not. Finally, the variation of the electrical conductivity with pressure gives rise to activation volumes on the order of 14 cm 3 /mol for S-SEBS while those for Nation 117 are about four times larger. These results indicate that proton transport in low water content S-SEBS occurs via a thermally activated process (ion motion via energy barriers) that is consistent with the more rigid side chains in that material.

Free volume and percolation in S-SEBS and fluorocarbon proton conducting membranes

Abstract Electrical conductivity results at a variety of pressures, temperatures and water contents are evaluated for sulfonated styrene/ethylene–butylene/styrene (S-SEBS) triblock polymer, Nafion 117, and Dow 800 proton conducting membranes. In addition, room temperature and atmospheric pressure diffusion coefficients determined from conductivity and 1 H pulsed gradient spin-echo nuclear magnetic resonance (NMR) studies are considered. While the S-SEBS and fluorocarbons exhibit a percolation threshold at 10 and 4 wt.%, respectively, all materials exhibit this phenomenon at a volume water fraction of C ≈0.05. Above the threshold the conductivity exhibits a power law behavior. When the volume of the hydrophobic portion of the membrane is subtracted the threshold occurs at the adjusted volume fraction of C A ≈0.12 which approaches that expected for continuum percolation. The activation volume results are shown to be consistent with free volume considerations.

Electrical Conductivity and NMR Studies of Methanol/Water Mixtures in Nafion Membranes.

Abstract Complex impedance studies have been carried out in acid form Nafion 117 treated with various amounts of methanol and methanol–water mixtures. At room temperature and atmospheric pressure the conductivity for Nafion treated with “pure” methanol is about a factor of ten less than for Nafion which contains the same wt.% of water. In samples treated with the water–methanol mixtures, the conductivity is lower than for samples having the same total wt.% of water. However, for low mixed fluid wt.% the conductivity is significantly higher than for samples with the same amount of water, only, as was in the mix. This enhancement of conductivity over that for the corresponding water uptake is attributed to a plasticizing effect of the methanol facilitating the segmental motion of the polymer. At higher water concentrations, the conductivity is generally lower in the mixed solution-treated samples than in samples treated with the corresponding amount of water. This is to be expected since in this regime, proton conduction occurs in fluid-rich regions, which in the solution case includes a large fraction of methanol. For a 40 wt.% 1.4:1 molar ratio film, the studies were carried out at pressures up to 0.3 GPa. It is found that the electrical conductivity decreases with increasing pressure. Both the electrical conductivity and the activation volume are similar to the result for Nafion containing the same amount of water only. Deuteron NMR spin-lattice relaxation measurements of isotopically enriched methanol/water mixtures in Nafion 117 at elevated pressure demonstrate greater molecular-level interactions between methanol and Nafion than between water and Nafion. This is consistent with the plasticizing effect observed in the conductivity results.

Complex Impedance Studies Of Proton-Conducting Membranes

Abstract Complex impedance studies have been carried out on Dow 800, Dow 1000 and Nafion 117 membranes at various water contents and a variety of temperatures and hydrostatic pressures. At room temperature and pressure the usual gradual decrease in electrical conductivity with decreasing water content is observed. For very low water content materials the variation of the conductivity with pressure from 0 to 0.2 GPa (2 kbar) is large and gives rise to apparent activation volumes, Δ V , as large as 54 cm 3 /mol. In addition, for low water content materials, there is a tendency for smaller equivalent weights (same side chains) or larger side chains to have larger activation volumes. At high water content, Δ V is relatively independent of the host polymer and negative values are observed at the highest water contents. These results provide support for the model where proton transport in high water content sulfonated fluorocarbons is similar to that for liquid water. All results are explained qualitatively via free volume. Ambient-pressure, variable-temperature 2 H T 1 and linewidth measurements imply a heterogeneous environment of the water molecules. Proton pulsed field gradient NMR studies in saturated Dow membranes verify the expectation that ionic conductivity is determined primarily by diffusion of water molecules.

High pressure NMR and electrical conductivity studies of gel electrolytes based on poly(acrylonitrile)

Abstract The effect of high pressure on electrical conductivity and NMR in gels prepared from lithium or sodium perchlorate, ethylene carbonate, dimethyl carbonate and poly(acrylonitrile) (PAN) has been determined. The corresponding liquids were also studied. Complex impedance studies at frequencies from 10 to 10 8 Hz and NMR measurements of T 1 were carried out as a function of pressure up to 0.25 GPa. Activation volumes for NMR relaxation and ionic conductivity were calculated from the variable pressure data. Both activation volumes were found to be approximately the same in the liquids. For the gels, however, the NMR activation volumes are the same or lower than for the liquid while the electrical conductivity pressure dependences are larger. The implications of these results are discussed in terms of possible effects of the PAN on the ionic solvation shell.

Water-associated dielectric relaxation in oxide nanoparticles

Audio frequency dielectric relaxation studies were carried out over the temperature range 120 to 390K on nanoscale mesoporous silica coated with polystyrene and nanoscale γ alumina. Two features are observed, both of which are attributed to water. A relaxation is observed at about 150K and 1000Hz that is thermally activated. The activation enthalpy is relatively high and the pre-exponential appears to be unusually low and to depend upon the water concentration. A model is presented that accounts for the apparently anomalous behaviour. This relaxation is attributed to water molecules bound at surface hydroxyl groups. A feature is found in the vicinity of 250K and 1000Hz that is associated with conductivity that varies as ω n . This response is attributed to hopping conductivity in loosely bound surface water.

Electrical Conductivity in Glass-Forming Solid Electrolytes: Theory and Experiment

Abstract A theory of the ionic conductivity in polymer electrolytes based on defect diffusion is evaluated using previously published data. Those data include the pressure and temperature variation of the electrical conductivity for poly(dimethylsiloxane-ethylene-oxide) complexed with NaCF 3 COO. In the defect diffusion model, ion transport is controlled by defects and ion motion occurs when the ion is encountered by a single defect. As temperature is lowered or pressure increases, the number of single defects decreases, thus decreasing the ionic conductivity. Further, there exists a pressure-dependent critical temperature, T c , below which single defects do not exist. It is shown how the pressure dependence of the conductivity is controlled by the pressure dependence of T c . The theory is used to predict the variation with temperature of both the apparent activation volume and curvature in plots of the logarithm of the conductivity with pressure.

Effect of nanoparticles on the dielectric properties of polyimide

As part of a search for a better dielectric for use in high energy density capacitors, polyimide (PI) films containing a layered material, zirconium orthophosphate, ZrO(H2PO4)2?xH2O (?-ZrP), were fabricated. PI has the advantage that it can be used to very high temperatures. To characterize the materials, x-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dielectric measurements (permittivity, loss and breakdown strength) were made. The permittivity and loss studies were also carried out on both neat PI films and ?-ZrP. The XRD, DSC and TGA results are consistent with amorphous composites. The effects of water or other impurities were observed in all three kinds of dielectric studies on all three types of material. For example, the relative permittivity of the composites decreased strongly when water was removed from the materials. Nonetheless, some increase in the relative permittivity of the dry nanocomposites was observed. Impurity or water-associated loss peaks were observed in all three types of material. The frequency and temperature dependences of the loss peaks made it possible to identify which were true relaxations. The effect of water is to decrease the dielectric strength of the composites. However, in both the wet and dry materials, the dielectric strength exhibits a maximum at a loading of about 5?wt% ?-ZrP.

Studies in Mesoporous Silicates: Impedance Measurements on SBA-15 Loaded with Sulfonated-Polystyrene and Nafion with SBA-15 Filler

An organic-inorganic composite was formed by cross-linking polystyrene (PS) into a SBA-15-type mesoporous silica (SBA/PS). The SBA-15/PS reacted with sulfuric acid according to published methods to sulfonate the benzene rings of the styrene (SBA-15/PS/SO3). These two latter solids were investigated by TGA/MS to determine stability and decomposition mechanisms. Complex impedance measurements at 10-105 Hz were taken on (SBA-15/PS/SO3) and Nafion cast films loaded with SBA-15 at a level of 1-10%, and similar films with SBA-15/PS/SO3 in hopes that the mesoporous solids might impart desirable properties on the Nafion.