Nuclear magnetic resonance transport mechanisms

The spectroscopic techniques, on the other hand, probe individual species which make up the various regions. Infrared (chapter 8) and nuclear magnetic resonance (chapter 7) address themselves to water and the interactions of water with the various species with which it is in contact. Mossbauer spectroscopy (chapter 9), in addition, provides valuable information on the proximity of the cations and their environment. Mechanical (chapter 6) and transport (chapter 4) properties provide more indirect insight into the structural aspects, which is supplemented by thermodynamic studies (chapters 2 and 5) of the interaction between the polymer and water or other liquids. All these techniques are discussed in the present volume, and from these studies several structural models have emerged (chapter 13). 

The activation volume, obtained from the pressure dependence of the reaction rate, has become a well-established, and often decisive, criterion for the determination of the reaction mechanism, so that variable-pressure NMR has also become an important tool in the understanding of chemical reactions. High gas pressures can be used to produce increased concentrations of a gas in solution, leading to faster reaction rates, advantageous shifts in chemical equilibria, or even entirely new chemical species. High-pressure NMR is an obvious technique for the study of such reactions. Nuclear magnetic resonance has also proven to be a powerful technique in solid-state physics, where variable-pressure experiments can be used to study phase transitions, transport phenomena, and electronic properties. 

Many experimental techniques have been used to examine the detailed structure of perfluorinated polymeric membranes. These include transmission electron microscopy [23], small angle X-ray scattering [24], Infra Red spectroscopy [25,26], neutron diffraction [27], Nuclear Magnetic Resonance [26,28], mechanical and dielectric relaxation [25,29], X-ray diffraction, and transport measurements. All these methods show convincing evidence for the existence of two phases in the perfluorosulfonate and perfluorocarboxylate polymers. One phase has crystallinity and a structure close to that of polytetrafluoroethylene (PTFE), and the other is an aqueous phase containing ionic groups. 

Most of the polymer electrolytes that have been studied are solid solutions of salts in polymers. There is a possibility that both the cation and anion are mobile in such electrolytes. The ionic transport number in polymer electrolytes is a very important parameter in terms of the conduction mechanism of ions in polymers and of their practical application. Cationic transport numbers have been measured in the polymer electrolytes, especially in those of PEO using various methods, including nuclear magnetic resonance (NMR) [49,50], complex impedance measurements [51,52], tracer diffusion... 

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