Heterogeneous Coordination Environments in Lithium-Neutralized Ionomers Identified Using (1)H and (7)Li MAS NMR

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 (1)H and (7)Li MAS NMR. While the (7)Li 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 (1)H MAS NMR revealed two different carboxylic acid proton environments in these materials. By utilizing (1)H-(7)Li rotational echo double resonance (REDOR) MAS NMR experiments, it was demonstrated that the proton environments correspond to different average (1)H-(7)Li 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 (1)H-(7)Li 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.