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

Ionic aggregates in a series of Zn-neutralized poly(styrene-co-styrene sulf onate) (SPS) random ionomers have been imaged using scanning transmission e lectron microscopy. The Zn-rich aggregates were found to have two shapes: s olid spheres (Type I) and shells or vesicles (Type II). Type I aggregates r ange in a maximum diameter from 4 to 10 nm, whereas Type II aggregates rang e in a maximum diameter from 9 to 55 nm with a vesicle wall thickness of si milar to3 nm. Lightly neutralized ionomers exhibited only Type I aggregates , whereas higher neutralization levels exhibited both Type I and II aggrega tes. Lightly neutralized ionomers also showed evidence of macrophase separa tion at the micron size scale. These direct observations of ionic aggregate s 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 mo nodisperse similar to2-nm spherical aggregates observed in Zn-neutralized p oly(ethylene-co-methacrylic acid). The presence of vesicular aggregates enc ourages a re-examination of the morphologies and properties of styrenic ion omers. (C) 2001 John Wiley & Sons, Inc.