Diblock ionomers consisting of a short polystyrene block and a long io
nic segment of poly-(4-vinylpyridinium methyl iodide) or poly(cesium m
ethacrylate) have been investigated by small-angle X-ray scattering in
the solid state. The size of the spherical microdomains formed by the
polystyrene blocks in the continuous ionic matrix has been measured a
nd compared to that of the mirror system consisting of an ionic core i
n a nonionic matrix, previously investigated. Similar chain stretching
of the core-forming block is observed for both systems, and it is con
cluded that the chain extension in the microdomains is due to the high
surface energy between the ionic and the nonionic phases rather than
to specific ionic interactions. In addition, the core radii of the two
mirror systems were fitted to the predictions of the star model for b
lock copolymers, R proportional to aN(B)(3/5), where N-B is the length
of the core-forming block.