NMR investigations of ionic host polymer systems

Williams, Martin Alexander Keith (1994). NMR investigations of ionic host polymer systems. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000d4a3

Abstract

NMR studies of the archetypal ionic host polymer system, in which polyethylene oxide (PEO) forms the polymer host, and lithium trifluoromethanesulphonate (LiCF3S03) is the guest ionic salt, are reported. The work is primarily concerned with the ability of the polymer host to incorporate the guest salt within its structure. Studies of pure PE~, LiCF3S03 itself, and ionic host polymer systems of O:Li ratios of 3.5: 1 and 9: 1 are reported in detail. A key experimental approach is the use of spin-lattice relaxation time measurements (I H, 7Li and 19F) to act as a probe of structure and dynamics. 19F spin-lattice relaxation time investigations in LiCF3S03 and PEO/LiCF3S03 systems are interpreted in terms of a mechanism in which the hindered reorientation of CF3 groups plays a key role. Crosscorrelations are shown to be responsible for non-exponentiality in nuclear magnetization recoveries. The problem of defining and measuring spinlattice relaxation times in these circumstances is considered in some detail. In the ionic host polymer systems an additional spin-lattice relaxation mechanism is that due to 19F_I H cross-relaxation. A model is developed to simulate the key features that can be observed experimentally for this type of mechanism. 7Li spin-lattice relaxation time studies in LiCF3S03 and PEOILiCF3S03 systems are also reported. In the salt a heteronuclear dipolar coupled 7Li_19F spin-lattice relaxation mechanism is shown to be dominant. T,he predicted relaxation behaviour is calculated in detail from a proposed crystal structure. I H lineshape and spin-lattice relaxation time measurements are reported in PEO and PEOILiCF3S03 systems. Overall, no evidence is found for the existence of the CF3S03- anion in the amorphous phase of the PEO/LiCF3 SO 3 systems studied. Hindered reorientation of the CF3 group also occurs in these systems, with a similar activation energy to that found in the salt itself. However, there is evidence that the 7Li+ cation can move between crystalline and amorphous regions.

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