Direct observation of ionic aggregates in sulphonated polystyrene ionomers

Applications of electron microscopy to ionomers have not generally been successful in the past, due to the small size of the ionic aggregates and their irregular distribution throughout the polymer matrix. Ionic domains were observed in samples of Zn²⁺ and Ni²⁺ neutralized sulphonated polystyrene (SPS), at sulphonation levels of 1.68 mol% and 3.37 mol%, using high-voltage (1.0 MeV) electron microscopy (HVEM). The specimens were solvent-cast films which contained suitable 'thin spots' only a few interaggregate dimensions thick. The observed contrast is not due to phase grain artifact, as demonstrated by a through-focus series of micrographs of the Zn²⁺ ionomer and polystyrene homopolymer. Contrast was observed only in the former case. The aggregates are approximately spherical and roughly 3 nm in diameter, and appear identical for both Ni²⁺ and Zn²⁺ materials. However, Cs⁺ neutralized SPS exhibited no ionic aggregates. This is probably due to the rapid rate of solvent evaporation coupled with the weaker coulombic forces in the Cs⁺ neutralized material, both of which hinder the formation of ionic aggregates.