EXAFS spectroscopy studies of cation local environment in sodium-neutralized ethylene copolymer ionomers

Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to s tudy the local environment around sodium in ethylene-meth acrylic acid (E-M AA) and ethylene-acrylic acid (E-AA) copolymer ionomers. Substantial change s occur in the arrangement of atoms around sodium as the neutralization lev el changes from similar to 20% to 80%, unlike zinc ionomers where the arran gement basically does not change over this range of neutralization levels. In E-MAA ionomers, changes in EXAFS patterns were consistent with the envir onment around sodium becoming less uniform with increasing neutralization l evel while the reverse was true for E-AA ionomers. This difference in the s odium ionomers was unexpected and surprising, particularly since the amount of water absorbed per sodium cation did not depend on acid comonomer ident ity. In E-AA ionomers neutralized with both zinc and sodium, the environmen t around sodium becomes substantially more ordered vs the environment of th e equivalent material neutralized only with sodium. This result is consiste nt with the formation of a single type of zinc-sodium mixed aggregate consi stent with EXAFS studies at the zinc edge. In E-MAA mixed-cation ionomers, the environment becomes more disordered upon addition of zinc, also consist ent with studies at the zinc edge.