Morphology of zinc-neutralized maleated ethylene-propylene copolymer ionomers : structure of ionic aggregates as studied by X-ray absorption spectroscopy

X-ray absorption spectroscopy (EXAFS) was used to characterize zinc-neutralized ionomers made from ethylene-propylene copolymers with grafted maleic anhydride units (Zn-MAn-g-EPM). The structure of the zinc moieties in the dry system consisted of two chelating carboxylate groups around one zinc atomsa structure much different than if the carboxylate anions come from acrylic or methacrylic acid. These self-contained zinc-two chelating carboxylate structures assemble into a larger superstructure as shown in a previous SAXS study, which indicated that the microphase-separated ionic aggregates are approx. 2 nm. This superstructure must therefore contain significant amounts of nonionic material, consistentwith the observation that the superstructure also forms in the unneutralized maleic anhydride precursor.After addition of water, zinc atoms in the sample with 10% of the carboxylate groups neutralized were fully solvated; i.e., zinc was surrounded by six water molecules. Zinc atoms in samples with higher neutralization levels had two carboxylate oxygens replaced by water oxygens, which in turn maintainedthe zinc coordination number at four.