Ionic aggregates in partially Zn-neutralized poly(ethylene-ran-methacrylic acid) 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 direct interpretation of the images for the shape, size, and size distribution of the ionic aggregates. Using tilt series and aspect ratios, the aggregates at all neutralization levels (17-78%) and both as-extruded and recrystallized were found to be spherical. The diameter of the Zn-rich aggregates is similar to 2.1 nm and is independent of the neutralization level. The size of the aggregates increased slightly upon recrystallization, and this was more pronounced at higher neutralization levels. The size distribution was described by a size dispersity index and was found to be nearly monodisperse. Finally, at 17% neutralization the ionic aggregates are widely separated relative to the polymer chains. These combined results suggest that the ionic aggregates include segments from the nonionic polymer backbone and that the aggregate morphology is primarily the result of ionic interactions in the system.