Mary C. Wintersgill

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 ...

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.

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.

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.

NMR studies of Na+-anion association effects in polymer electrolytes

Abstract 23 Na nuclear magnetic resonance (NMR) measurements on poly(propylene oxide) (PPO) and siloxane based polymer electrolytes containing various sodium salts at a single nominal concentration are reported. In addition, differential scanning calorimetry (DSC) and electrical conductivity studies were carried out on the PPO materials. The NMR-determined mobile Na + concentrations and DSC results provide evidence for ionic aggregation effects which, for some samples, result in salt precipitation at elevated temperatures. 23 Na chemical shifts observed in solid state NMR due to mobile Na + ions are obtainable without the use of high resolution techniques, and exhibit strong dependences on anion and temperature. These results indicate that Na + -anion interactions influence ionic transport as well as the number of available carriers.

Ionic conductivity in solid, crosslinked dimethylsiloxane‐ethylene oxide copolymer networks containing sodium

The preparation of an ion‐conducting elastomeric solid based on a dimethylsiloxane‐ethylene oxide copolymer complexed with a sodium salt is described. 23Na nuclear magnetic resonance measurements reveal the presence of both bound and mobile sodium species throughout the temperature range −120 to 100 °C. Electrical conductivity measurements over a similar temperature range are found to be consistent with the configurational entropy model for transport, with a T0 parameter about 50 °C below the ‘‘central’’ glass transition temperature Tg.

Ion implantation in polymers

Abstract An introductory overview will be given of the effects of ion implantation on polymers, and certain areas will be examined in more detail. Radiation effects in general and ion implantation in particular, in the field of polymers, present a number of contrasts with those in ionic crystals, the most obvious difference being that the chemical effects of both the implanted species and the energy transfer to the host may profoundly change the nature of the target material. Common effects include crosslinking and scission of polymer chains, gas evolution, double bond formation and the formation of additional free radicals. Research has spanned the chemical processes involved, including polymerization reactions achievable only with the use of radiation, to applied research dealing both with the effects of radiation on polymers already in commercial use and the tailoring of new materials to specific applications. Polymers are commonly divided into two groups, in describing their behavior under irradiation. Group I includes materials which form crosslinks between molecules, whereas Group II materials tend to degrade. In basic research, interest has centered on Group I materials and of these polyethylene has been studied most intensively. Applied materials research has investigated a variety of polymers, particularly those used in cable insulation, and those utilized in ion beam lithography of etch masks. Currently there is also great interest in enhancing the conducting properties of polymers, and these uses would tend to involve the doping capabilities of ion implantation, rather than the energy deposition.