Effect of water on internal aggregate structure in zinc-neutralized ionomers

The effect of water on zinc-neutralized ethylene-methacrylic acid (E-MAA) i onomers and sulfonated polystyrene (SPS) ionomers was studied. The change i n weight after soaking in water was quantified as a function of zinc conten t and correlated to changes in spectra from zinc-edge extended X-ray absorp tion fine structure spectroscopy (EXAFS). EXAFS patterns for the fully drie d ionomers were characteristic of zinc surrounded by four oxygen atoms. E-M AA ionomers absorbed much less water per zinc atom than SPS ionomers. Addit ion of water changed the EXAFS pattern in E-MAA ionomers, but the pattern w as still characteristic of zinc coordinated to four oxygen atoms. This chan ge was consistent with water replacing chelating carboxylate anions coordin ated to zinc at the edges of aggregates or coordinated to isolated zinc ion s. A new interpretation of the v(a)(COO-) infrared vibration was developed from this work. In SPS, addition of water caused zinc to become 6-fold coor dinated to oxygen, which is suggestive of zinc totally solvated by water.