John J. Fontanella

Effect of high pressure on electrical relaxation in poly(propylene oxide) and electrical conductivity in poly(propylene oxide) complexed with lithium salts

Audio frequency electrical conductivity and relaxation studies have been carried out on Parel 58 elastomer and Parel 58 elastomer complexed with a variety of lithium salts. The measurements have been carried out in vacuum over the temperature range 5–380 K and at pressures up to 0.65 GPa over the temperature range 230–380 K. Both the electrical conductivity for the complexed material and the electrical relaxation time associated with the α relaxation in the uncomplexed material exhibit VTF or WLF behavior. From a VTF analysis for both the vacuum electrical relaxation time and electrical conductivity, Ea is found to be about 0.09 eV and T0 is found to be about 40 °C below the ‘‘central’’ glass transition temperature. In addition, it is found that the activation volumes for the electrical relaxation time and the electrical conductivity are the same when compared relative to T0. These results imply that the mechanism controlling ionic conductivity is the same as that for the α relaxation, namely large‐scale ...

Low‐frequency dielectric constants of α‐quartz, sapphire, MgF2, and MgO

The 1000‐Hz 300°K dielectric constants were measured for crystalline α‐quartz, sapphire, MgF2, and MgO, and polycrystalline sapphire (Lucalox), magnesium fluoride (IRTRAN 1), and magnesium oxide (IRTRAN 5). The measurements were performed using the method of substitution (two‐fluid method). The results are as follows: single‐crystal α‐quartz, es∥=4.6368±0.001 and es⊥=4.5208±0.001; single‐crystal sapphire, es∥ = 11.589±0.005 and es⊥=9.395±0.005; polycrystalline sapphire (Lucalox), es=10.154±0.007; single‐crystal MgF2, es∥ =4.826±0.01 and es⊥=5.501±0.01; polycrystalline MgF2 (IRTRAN 1), es=5.289±0.001; single‐crystal MgO, es=9.830 ±0.001; and polycrystalline MgO (IRTRAN 5), es=9.833±0.001, where es∥ and es⊥ are the dielectric constants parallel and perpendicular, respectively, to the c axis.

NMR, DSC, DMA, and High Pressure Electrical Conductivity Studies in PPO Complexed with Sodium Perchlorate

Abstract : Audio frequency electrical conductivity, DSC, DMA, and 23 Na nMR measurements have been carried out on Parel 58 elastomer complexed with sodium perchlorate. (As Parel 58 is primarily poly(propylene oxide), it will be referred to as PPO.) The DSC and DMA measurements yield similar values for Tg which are about 72 C higher than the central Tg for uncomplexed PPO. In addition, the DSC studies show that the sodium perchlorate is insoluble above about 140 C. The conductivity measurements have been carried out in vacuum over the temperature range 290-370K. From a VTF analysis Ea is found to be about 0.09 eV and T0 is found to be about 45 C below the central glass transition temperature which is the same behavior observed previously for PPP complexed with lithium salts and for the alpha relaxation in uncomplexed material. In addition, it is found that the vacuum activation volumes for the electrical conductivity and the alpha relaxation are approximately the same when compared relative To. The 23Na NMR measurements reveal the presence of both bound and mobile sodium species, the relative concentrations of which change by about a factor of ten over the temperature range -90 to +90 C. In addition the mobile 23Na resonance becomes motionally narrowed above Tg. The NMR results combined with the conductivity data imply that ion motion is controlled by large scale segmental motions of the polymer chains.

NMR, DSC and high pressure electrical conductivity studies of liquid and hybrid electrolytes

Abstract Electrical conductivity, differential scanning calorimetry (DSC) and 7 Li nuclear magnetic resonance (NMR) studies have been carried out on liquid electrolytes such as ethylene carbonate:propylene carbonate (EC:PC) and EC:dimethyl carbonate (DMC) containing LiPF 6 (and LiCF 3 SO 3 for NMR) and films plasticized using the same liquid electrolytes. The films are based on poly(vinylidene fluoride) (PVdF) copolymerized with hexafluoropropylene and contain fumed silica. All measurements were carried out at atmospheric pressure from room temperature to about −150°C and the electrical conductivity studies were performed at room temperature at pressures up to 0.3 GPa. The liquids and hybrid electrolytes are similar in that the electrical conductivity of the EC:PC-based substances exhibits Vogel–Tammann–Fulcher (VTF) behaviour while that for the EC:DMC-based substances does not. Part of the deviation from VTF behaviour for the EC:DMC-based materials is attributed to crystallization. Further, the glass transition temperatures as determined from NMR, DSC and electrical conductivity measurements are about the same for the liquids and hybrid electrolytes. However, substantial differences are found. The electrical conductivity of the hybrid electrolytes at room temperature is lower than expected and, more importantly, the relative change of conductivity with pressure is larger than for the liquids. In addition, above the glass transition temperature, the NMR T 1 values are smaller and the NMR linewidths are larger for the hybrid electrolytes than for the liquids while at both low and high temperature the NMR linewidths are larger. Consequently, it is concluded that significant solid matrix–lithium ion interactions take place.

Complex impedance measurements on Nafion

Abstract The need to develop an electrolytic membrane for an efficient, environmentally sound fuel cell has led to intense interest in proton conducting polymers in general and Nafion in particular. While it does not appear very likely that Nafion itself will ultimately prove to be the best choice of material, it may be considered as a prototype membrane material. Initial interest focused on Nafion’s potential use in a hydrogen fuel cell, in which case its conductivity in the presence of water is important, and so extensive studies of the electrical properties of Nafion at various levels of humidity were carried out. Two distinct regimes were identified, one at lower water contents and the other at high water contents. The possible conduction mechanisms associated with these regimes will be discussed. In addition, studies carried out at high pressure yielded activation volumes which provide further clues as to the conduction mechanisms involved. More recently, interest in Nafion as a membrane material in methanol fuel cells has prompted investigation of its electrical properties in the presence of methanol alone and of methanol/water mixtures. It is clear that not only is Nafion an excellent proton conductor but it also exhibits significant methanol transport. This represents a serious crossover problem for fuel cell applications and it is important to be able to characterize the mechanisms involved.

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.

Electrical impedance studies of acid form NAFION® membranes

Electrical conductivity/dielectric relaxation studies of acid form NAFION-117 have been carried out at frequencies from 10 to 108 Hz. By direct measurement, it is shown that when “standard” two terminal measurements are made across the thickness of a 0.18 mm film, it is necessary to use frequencies in excess of 107 Hz in order to observe the bulk conductivity of the sample. As a consequence, previous reports of a power law dependence for the electrical conductivity are not associated with the bulk electrical conductivity but rather are due to electrode effects and space charge. As confirmation, it is shown that by changing the geometry of the electrodes, the low frequency electrical response of the material is significantly changed.

Charge Transport and Water Molecular Motion in Variable Molecular Weight NAFION Membranes. High Pressure Electrical Conductivity and NMR.

Abstract Measurements of the electrical conductivity and deuteron NMR spin-lattice relaxation times ( T 1 ) in three different molecular weights of acid form NAFION conditioned at various levels of relative humidity have been carried out. Complex impedance studies were made along the plane of the film at frequencies from 10-10 8 Hz at room temperature and pressures up to 0.3 GPa. The high pressure electrical data were only obtained for water contents less than 8 wt%. The NMR measurements were also made at room temperature and pressures up to 0.25 GPa. The NMR data are primarily for water contents greater than 6 wt%. The calculated activation volume exhibits a large decrease (from 16 to 3 cm 3 /mol) as the water content is increased from 2.4–8 wt%. In addition, the activation volumes are larger and the electrical conductivity is smaller for the higher molecular weight material. These results represent further evidence that the transport mechanism in low water content materials is dominated by segmental motions of the polymer chain and that proton transport and water molecular rotation are correlated. The activation volumes extracted from the NMR data show only a small further decrease as the water content is increased from 6–22 wt%. Possible explanations for the high water content NMR pressure results are given.

Studies of Water in Nafion Membranes Using Deuteron and Oxygen‐17 Nuclear Magnetic Resonance, and Dielectric Relaxation Techniques

Deuteron and oxygen-17 nuclear magnetic resonance measurements and dielectric relaxation studies of Nafion-117 membranes with variable water content (approximately 5-18% by weight) have been carried out. Glassy behavior of the water domains at low temperature, below ca. 200 K, is evidenced by the specific nature of the 1 H NMR line shapes. Activation energies extracted from 1 H spin-lattice relaxation data on the high temperature side of the T 1 minimum exhibit a steady increase with increasing water content. In spite of a high degree of molecular mobility, angular-dependent spectra in both as-received and stretched samples reflect considerable anisotropy of the host polymer

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.