Small-angle X-ray scattering investigation of noncrystalline poly(ethylene-co-methacrylic acid) ionomers

The structure of ionic aggregates in commercially important ethylene ionomers was studied by small-angle X-ray scattering (SAXS). The samples examined were sodium and zinc salts of poly(ethylene-co-13.3 mol % methacrylic acid) (EMAA) ionomers, which were found to be noncrystalline unlike commercially available EMAA ionomers with lower MAA contents. Not only the model fittings but also the correlation function analyses can be employed on those ionomers because of their noncrystallinity. The results clearly showed that the structure of ionic aggregate in both sodium and zinc salt ionomers can be described by Yarusso and Cooper's liquidlike model. The number of ions per aggregate was estimated on the basis of simple space-filling discussion using the data of low-molecular-mass model compounds, which reveals that the ionic core of the ionic aggregate contains anionic side groups as well as ions, and in the partially neutralized sodium salts, unneutralized COOH groups are also incorporated into the ionic aggregate. Comparisons with other EMAA ionomers with lower MAA contents than 13.3 mol % and with other ionomer systems were also discussed.