The morphology of diblock ionomers in the solid state and in solution : a study by small angle X-Ray scattering

The morphology of diblock ionomers consisting of non ionic polystyrene blocks and ionic poly(cesium methacrylate), or poly(cesium acrylate), or poly(4-vinylpyridinium methyl iodides) blocks in solution and in the solid state is investigated by small angle x-ray scattering (SAXS).When the polystyrene blocks are kept short, they self-assemble in bulk to form spherical microdomains. Furthermore, the polystyrene segments are highly stretched in the non ionic microdomains, as was also observed previously for the ionic segments of comparable lengths in the ionic microdomains. These findings support the hypothesis that chain extension in microdomains of diblock ionomers in bulk is the result of the high surface energy and not of the ionic character of one of the blocks. The presence of spherical ionic domains in diblock ionomers with short ionic blocks is confirmed by electron micrographs. An increasing size of the ionic microdomains for diblocks with a high polydispersity in the ionic chains relative to that in samples of low polydispersity is seen only in samples prepared close to thermodynamic equilibrium, such as casting from a solvent which is good for both blocks.When diblock ionomers of short ionic block lengths are dissolved in a solvent selectively good for the polystyrene block, micelles are formed which consist of ionic cores surrounded by a corona of polystyrene segments swollen by the solvent. The core size depends only on the ionic block length, and varies as the 3/5 power of the number of units in the ionic block, as predicted by the star model for block copolymer solutions. The ionic cores have a low polydispersity in sizes. The sizes are independent of the chemical structure, of polymer concentration, of polystyrene block length, and of solvent composition provided it is of low polarity. Water can be solubilized into the ionic cores up to a limit. Aggregation number, surface area per chain and micellar mass can be obtained from the SAXS results. The micelles show a high degree of organization in solution, as evidenced by the presence of a structure peak even at 1% polymer concentration; up to three peaks are seen for solutions above 2% polymer concentration. The possible scattering models that could explain the observed structure peaks are discussed