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Materials 2012, 5(8), 1508-1527; doi:10.3390/ma5081508
Article

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

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1 Department of Nanostructured and Electronic Materials, Sandia National Laboratories, Albuquerque, NM 87185, USA 2 Department of Nanoscale and Reactive Processes, Sandia National Laboratories, Albuquerque, NM 87185, USA 3 Computational Materials Science and Engineering Department, Sandia National Laboratories, Albuquerque, NM 87185, USA 4 Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA 5 Dept. of Material Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA 6 DuPont Central Research and Development, Wilmington, DE 19880-0302, USA 7 Center for Macromolecular Science and Engineering, Department of Chemistry, University of Florida, Gainesville, FL 32611, USA
* Author to whom correspondence should be addressed.
Received: 29 June 2012 / Revised: 14 August 2012 / Accepted: 17 August 2012 / Published: 23 August 2012
(This article belongs to the Special Issue NMR in Materials Science)
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Abstract

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 1H and 7Li MAS NMR. While the 7Li 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 1H MAS NMR revealed two different carboxylic acid proton environments in these materials. By utilizing 1H-7Li rotational echo double resonance (REDOR) MAS NMR experiments, it was demonstrated that the proton environments correspond to different average 1H-7Li 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 1H-7Li 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.
Keywords: MAS NMR; REDOR; lithium ionomer; precise polymer; ionomer MAS NMR; REDOR; lithium ionomer; precise polymer; ionomer
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).
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Alam, T.M.; Jenkins, J.E.; Bolintineanu, D.S.; Stevens, M.J.; Frischknecht, A.L.; Buitrago, C.F.; Winey, K.I.; Opper, K.L.; Wagener, K.B. Heterogeneous Coordination Environments in Lithium-Neutralized Ionomers Identified Using 1H and 7Li MAS NMR. Materials 2012, 5, 1508-1527.

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