Electron microscopy has been used to examine the morphology of four ionomers. The transfertheory of imaging, which has been overlooked in previous ionomer studies, has been used to interpret theimages of both solvent-cast and microtomed thin films. Solvent casting was found to produce a number ofinteresting artifacts but no useful information about ionic domains. Microtomed sections of a copolymer ofethylene and methacrylic acid, sulfonated polypentenamer, and sulfonated polystyrene contained no preparativeartifacts but showed no distinct domain structure even in the ionomers neutralized with cesium, probablybecause of film thickness problems. Microtomed sections of sulfonated EPDM, however, contain 300-nmphase-separated regions. Osmium tetraoxide staining of these EPDM sections showed domains averagingless than 3 nm in size primarily inside these regions. Unfortunately, the section thickness prohibits an accuratedetermination of the size distribution or the detailed shape of these domains and hence the selection of themost appropriate model of domain structure
