Rheological properties of sulfonated polystyrene ionomers at high-ion contents

Structure and dynamics are examined for unentangled randomly-sulfonated polystyrene ionomer samples with cesium as counterion, using the X-ray scattering, DSC, and linear viscoelasticity (LVE) measurements. The large size of cesium cation leads to low association energy, enabling the dynamics to be examined for the high-ion-content samples, i.e., samples having a fraction of ionized monomers p up to 19%. X-ray scattering data shows an ionic peak, indicating the formation of ion aggregates. DSC reveals the glass transition process that shifts to higher T and broadens significantly when the ion content p is higher than 10%. In accordance with this broadening, the glassy and rubbery regimes seen in the LVE merge into one broad process with a wide relaxation distribution. We propose that the transition occurs when the number of ionic groups per chain becomes comparable to that of Kuhn segments per chain so that the polymer strands between ionic groups become non-flexible.

Automated summary

In samples of unentangled randomly-sulfonated polystyrene ionomers with cesium as counterion, the large size of cesium cation leads to low association energy, enabling the examination of dynamics for high-ion-content samples. X-ray scattering data indicates the formation of ion aggregates, while DSC reveals a glass transition process that shifts to higher temperatures and broadens significantly with increased ion content. The merging of glassy and rubbery regimes in linear viscoelasticity measurements occurs when the number of ionic groups per chain becomes comparable to that of Kuhn segments per chain.