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Abstract

Extensive studies on a model system (imidazole-terminated ethylene
oxide oligomers doped with small amounts of strong acids) for a
proton-conducting polymer functioning without a liquid phase, but
instead using imidazole tethered to the backbone via flexible spacers
as a proton solvent, are presented and the parameters governing
conductivity and its mechanism are discussed. Temperature-dependent
conductivities are well described by free-volume considerations
(VTF-behavior). Thus, besides a high density of imidazole moieties, a
low T-g is in favor of high proton conductivity, which experimentally
is shown to be predominantly due to structure diffusion. The available
free volume is suggested to correlate with the rate of hydrogen bond
breaking and forming processes within the dynamical hydrogen bond
networks, which generally limit the rate of long-range diffusion of
protonic defects via structure diffusion. The equilibrium constants for
the protonation of imidazole by two different dopants in an
oligo(ethylene oxide) environment are determined by NMR, indicating
complete dissociation of strong acids such as trifluoroacetic acid. The
crystal structure obtained from one of the model compounds is dominated
by hydrogen bond interactions interconnecting the imidazole units and
suggesting an easy proton migration within the imidazole-rich domains.