Effect of coneutralization on internal aggregrate structure in ethylene-based ionomers

Local aggregate structures in ethylene-methacrylic acid (E-MAA) and ethylen e-acrylic acid (E-AA) ionomers were studied using extended X-ray absorption fine structure spectroscopy (EXAFS). In the case of pure zinc neutralizati on, E-AA and E-MAA ionomers showed very minor differences in EXAFS spectra. These small differences were statistically significant, however, and almos t certainly a result of the structural differences between the acid repeat units. Overneutralization with zinc acetate dihydrate caused significant ch anges in the EXAFS pattern which could not be attributed to a linear combin ation of EXAFS spectra from the ionomer and zinc acetate dihydrate. Coneutr alization with either lithium or sodium also changed the local environment around zinc. Zinc/sodium ratios were systematically varied in both E-AA and E-MAA. In the former, the local environment around zinc changed as the Zn/ Na mole ratio changed from 1:0 to 1:1; at ratios between 1:I and 1:9 the lo cal environments were identical. Therefore, at low Zn/Na ratios only one ty pe of zinc-containing aggregate exists in the system, and this aggregate pr obably contains both sodium and zinc. A partial list of properties for this mixed-cation aggregate was generated, and possible structures were propose d. For E-MAA ionomers, zinc local environment changes continuously as Zn/Na ratio changes. Hence, more than one type of aggregate exists at all Zn/Na ratios, and the relative distribution of these aggregates changes with this ratio. The implications of the differences in aggregate structure on the r heological behavior of these mixed-cation systems are discussed.