Ionic aggregates in partially Zn-neutralized poly(ethylene-ran-methacrylicacid) ionomers: Shape, size, and size distribution

The ionic aggregates in a series of partially Zn-neutralized poly(ethylene- ran-methacrylic acid) ionomers have been imaged using scanning transmission electron microscopy. The profound advantage of this technique is the direc t interpretation of the images for the shape, size, and size distribution o f the ionic aggregates. Using tilt series and aspect ratios, the aggregates at all neutralization levels (17-78%) and both as-extruded and recrystalli zed were found to be spherical. The diameter of the Zn-rich aggregates is s imilar to 2.1 nm and is independent of the neutralization level. The size o f the aggregates increased slightly upon recrystallization, and this was mo re pronounced at higher neutralization levels. The size distribution was de scribed by a size dispersity index and was found to be nearly monodisperse. Finally, at 17% neutralization the ionic aggregates are widely separated r elative to the polymer chains. These combined results suggest that the ioni c aggregates include segments from the nonionic polymer backbone and that t he aggregate morphology is primarily the result of ionic interactions in th e system.