Counterion effect on the rheology and morphology of tailored poly(dimethylsiloxane) ionomers

We have carried out a systematic study of the time-dependent rheology and morphology of poly(dimethylsiloxane) (PDMS) ionomers with tailored number of monomers between ions and number of ions per chain as a function of the cation. Small-angle X-ray scattering (SAXS) and scanning transmission electron microscopy (STEM) were used to examine the structure of the samples. The flow behavior of nonequilibrated freshly precipitated ionomers varies from flowing liquids to weak networks. Low mol % zinc and cobalt ionomers that flow do not show aggregates in STEM images whereas gallium, barium, and high mol % zinc ionomers that precipitate as gels show a diverse range of aggregates. The equilibrium state of all ionomers is reached very slowly at room temperature or more rapidly at high temperatures and is a physically cross-linked network irrespective of the cation. Very different morphologies were observed for different cations. Low mol % (similar to 1 mol %) barium ionomers have rod-shaped and spherical ionic aggregates; gallium ionomers have inhomogeneously distributed highly polydisperse spherical aggregates; and low mol % zinc and cobalt ionomers have no aggregates whereas high mol % (similar to 7.4 mol %) zinc ionomers have spherical and rodlike aggregates. The nature of the cation has little influence on the elastic modulus of the equilibrated samples.