EFFECT OF IONIC CHAIN POLYDISPERSITY ON THE SIZE OF SPHERICAL IONIC MICRODOMAINS IN DIBLOCK IONOMERS

The effect of ionic chain polydispersity on the morphology of spherical ionic microdomains in the dry state is studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) for two systems of diblock ionomers, polystyrene-b-poly(4-vinylpyridinium methyl iodide) (PS-b-P4VPMeI) and polystyrene-b-poly(cesium methacrylate) (PS-b-PMACs). The polydispersity of the ionic blocks is controlled by blending diblock copolymers of constant polystyrene length but different ionic block lengths. The samples are mixed in appropriate ratios so that the number-average molecular weight is constant but a wide range of polydispersity in the ionic blocks is obtained. The samples are prepared by three different methods. The first method involves preparation from a solution in which a nonequilibrium state exists. This is carried out either by addition of a nonsolvent to the single-chain polymer solution (as for the PS-b-P4VPMeI series in DMF) or by neutralization in a selective solvent (as for the methacrylic acid copolymers in THF). In the second method, the PS-b-P4VPMeI samples are cast from a good solvent for both blocks. Finally, in the third method, the 4-vinylpyridine diblocks are mixed before quaternization and then cast from THF. Electron micrographs show a regular arrangement of spherical ionic microdomains and ruled out dual morphology. The radii of the ionic microdomains of samples prepared by methods 2 and 3 are found to increase linearly with the ionic chain polydispersity, while those which are prepared by method 1 stay constant. This difference is explained in terms of the thermodynamic conditions prevailing in the solutions at the onset of microphase separation.