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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

1,* , 1
, 2
, 3
, 3
, 4
, 5
, 6
 and 7
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 which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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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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