Heterogeneous Coordination Environments in Lithium-Neutralized Ionomers Identified Using H-1 and Li-7 MAS NMR
TM Alam and JE Jenkins and DS Bolintineanu and MJ Stevens and AL Frischknecht and CF Buitrago and KI Winey and KL Opper and KB Wagener, MATERIALS, 5, 1508-1527 (2012).
The carboxylic acid proton and the lithium coordination environments for precise and random Li-neutralized polyethylene acrylic acid P(E-AA) ionomers were explored using high speed solid-state H-1 and Li-7 MAS NMR. While the Li-7 NMR revealed only a single Li coordination environment, the chemical shift temperature variation was dependent on the precise or random nature of the P(E-AA) ionomer. The H-1 MAS NMR revealed two different carboxylic acid proton environments in these materials. By utilizing H-1-Li-7 rotational echo double resonance (REDOR) MAS NMR experiments, it was demonstrated that the proton environments correspond to different average H-1-Li-7 distances, with the majority of the protonated carboxylic acids having a close through space contact with the Li. Molecular dynamics simulations suggest that the shortest H-1-Li-7 distance corresponds to un-neutralized carboxylic acids directly involved in the coordination environment of Li clusters. These solid-state NMR results show that heterogeneous structural motifs need to be included when developing descriptions of these ionomer materials.
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