EXAFS Spectroscopy Studies of Cation Local Environment in Sodium-Neutralized Ethylene Copolymer Ionomers

• Published in: Macromolecules 2001-09, Vol.34 (20), p.7108-7112
• Main Authors: Farrell, V, Grady, P
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma002081p

Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to study the local environment around sodium in ethylene−methacrylic acid (E−MAA) and ethylene−acrylic acid (E−AA) copolymer ionomers. Substantial changes occur in the arrangement of atoms around sodium as the neutralization level changes from ∼20% to 80%, unlike zinc ionomers where the arrangement basically does not change over this range of neutralization levels. In E−MAA ionomers, changes in EXAFS patterns were consistent with the environment around sodium becoming less uniform with increasing neutralization level while the reverse was true for E−AA ionomers. This difference in the sodium ionomers was unexpected and surprising, particularly since the amount of water absorbed per sodium cation did not depend on acid comonomer identity. In E−AA ionomers neutralized with both zinc and sodium, the environment around sodium becomes substantially more ordered vs the environment of the equivalent material neutralized only with sodium. This result is consistent with the formation of a single type of zinc−sodium mixed aggregate consistent with EXAFS studies at the zinc edge. In E−MAA mixed-cation ionomers, the environment becomes more disordered upon addition of zinc, also consistent with studies at the zinc edge.