BLOCK IONOMER MICELLES IN SOLUTION .1. CHARACTERIZATION OF IONIC CORES BY SMALL-ANGLE X-RAY-SCATTERING

Diblock ionomers of polystyrene-b-poly(cesium acrylate) and polystyrene-b-poly(cesium methacrylate) in toluene, a solvent selective for the polystyrene blocks, were investigated by small-angle X-ray scattering (SAXS). The insoluble poly(cesium acrylate) and poly(cesium methacrylate) blocks are found to be assembled in spherical microdomains, surrounded by a styrene corona, a morphology analogous to that of reverse micelles. In the range of polymer concentrations from 0.01 to 0.1 g/mL, the polymeric micelles show an increasing degree of organization in the solution, as evidenced by up to three peaks in the SAXS profiles. Since the electron density of the ionic core is much larger than those of either the corona or the solvent, the X-ray scattering comes from the core only. Furthermore, the sizes of the cores are much smaller than the distances between them; thus, the features corresponding to the form factor and the structure factor appear in different regions of the scattering profile, leading to an unambiguous interpretation. The core radius is found to vary as the 3/5 power of the number of the ionic units, N-B, as predicted by Halperin's star model for polymeric micelles. The aggregation number varies as N-B(4/5) while the surface area per chain scales as N-B(2/5). The polydispersity in the core radius of these micelles is of the order of 1.03, much smaller than the distribution in the length of the individual ionic blocks.