Spherical and vesicular ionic aggregates in Zn-neutralized sulfonated polystyrene ionomers

Ionic aggregates in a series of Zn-neutralized poly(styrene-co-styrene sulfonate) (SPS) random ionomers have been imaged using scanning transmission electron microscopy. The Zn-rich aggregates were found to have two shapes: solid spheres (Type I) and shells or vesicles (Type II). Type I aggregates range in a maximum diameter from 4 to 10 nm, whereas Type II aggregates range in a maximum diameter from 9 to 55 nm with a vesicle wall thickness of ∼ 3 nm. Lightly neutralized ionomers exhibited only Type I aggregates, whereas higher neutralization levels exhibited both Type I and II aggregates. Lightly neutralized ionomers also showed evidence of macrophase separation at the micron size scale. These direct observations of ionic aggregates contradict previous interpretations of small-angle X-ray scattering data with respect to size, size dispersity, shape, and spatial distribution. In addition, the aggregates observed in SPS differ markedly from the nearly monodisperse ∼ 2-nm spherical aggregates observed in Zn-neutralized poly(ethylene-co-methacrylic acid). The presence of vesicular aggregates encourages a re-examination of the morphologies and properties of styrenic ionomers.