Linear and nonlinear rheology of unentangled telechelic ionomers: Effects of cation species

This work aims to investigate the effects of counterion species, including sodium, cesium, and zinc ions, in unentangled telechelic ionomers based on carboxylate groups on linear viscoelasticity and nonlinear rheological responses in both shear and extensional flows. The linear viscoelastic response of the telechelic ionomers reveals the presence of “superbridges” structures composed of linearly associated sequences of ionomer chains. During nonlinear shear response, the sodium-containing ionomer demonstrates a more significant stress decay after the overshoot compared to the prepolymer and the cesium/zinc-containing ionomers. This could be due to the stress-bearing network strand being sufficiently stretched when the ions are pulled out from the strong ionic aggregates of carboxylate sodium, leading to a stronger chain retraction following the strain-induced rupture of aggregates. In addition, compared to prepolymers, the ionomers neutralized by various cations display stronger damping responses, and the damping behavior is highly dependent on the association strength of cation species. In extensional flow, the sodium-containing ionomer shows more significant strain hardening compared to other ionomers as expected, also due to its stronger ionic aggregates. These findings highlight the significant impact of the association strength of cation species on the rheological behavior of these polymers, which offers valuable insights for optimizing the processability of these materials.