Reconciling STEM and X-ray Scattering Data To Determine the Nanoscale Ionic Aggregate Morphology in Sulfonated Polystyrene Ionomers

This paper reports the first quantitative reconciliation of imaging and scattering data for poly(styrene-ran-styrenesulfonate) (P(S-SS x )) ionomers. We examined the morphology of solvent-cast and spin-cast P(S-SS0.019)-M ionomers using the combination of scanning transmission electron microscopy (STEM) and X-ray scattering, where the scattering data were fit with a liquidlike hard-sphere model. Both the ionic aggregate sizes (R 1) and the sample volume per ionic aggregate (V P) as measured by both techniques were in good agreement. In addition, STEM found that P(S-SS0.019) ionomers prepared by spin-casting exhibit nanometer spherical ionic aggregates that are indistinguishable in size, shape, and spatial distribution from the bulk solvent-cast ionomers. Six P(S-SS0.019)-M ionomers fully neutralized with various cations have ionic aggregate compositions that are predominately ionic, and the ionic aggregate radius (R 1) increases as the cation radii increases. Finally, the influence of copolymer type was studied by comparing P(S-SS0.070) and P(S-MAA0.072) ionomers. The ionic aggregates in P(S-SS0.070)-Cu are surrounded by a thicker region of limited mobility and are more ionic as compared with P(S-MAA0.072) ionomers. Although STEM and X-ray scattering have been reconciled for these P(S-SS x ) ionomers, a broad application of the liquidlike hard-sphere scattering model is not recommended. However, when STEM and X-ray scattering are reconciled, detailed morphological information can be extracted from the scattering data, particularly regarding the composition of the ionic aggregates, which is important for understanding the mechanisms of ion transport.