Viscoelastic Behavior of Very Lightly Sulfonated Polystyrene Ionomers

Abstract

The linear viscoelastic behavior of sulfonated polystyrene ionomers with as few as one sulfonate group per 1000 styrene repeat units was studied by dynamic mechanical analysis. Interactions of the sulfonate groups shifted the terminal flow region to lower frequency and produced a new low-frequency relaxation that was not present in the unmodified polystyrene. The observation of the low-frequency relaxation at very low sulfonation levels suggests that even for 0.11 mol % sulfonation, ionic clusters formeven though, no SAXS peak has been resolved for ionomers with that low a sulfonation level. The effect of the ionic groups on the viscoelastic properties can be varied for a fixed sulfonation level by weakening the ion-dipole strength of the ion pair. This can be achieved by substituting ammonium (NH4+) cations for metal cations such as Na+.