Nonlinear Rheology of Telechelic Ionomers Based on Sodium Sulfonate and Carboxylate
In this study, we examined the structure, linear viscoelastic (LVE) properties, and nonlinear shear and elongational properties of unentangled telechelic ionomers based on either sodium carboxylate or sodium sulfonate groups, coded as Mn-COONa and Mn-SO3Na, with the number-average molecular weight Mn varying from 5 to 16 kg mol–1. A combination of X-ray scattering and LVE data revealed that both types of ionomers form networks sustained by “superbridges” transiently cross-linked by salt aggregates. The aggregates were found to be more stabilized in the Mn-SO3Na ionomers than in the Mn-COONa ionomers. In addition, the X-ray scattering and LVE data suggested that the inner and end aggregates of the superbridge are equally stabilized in Mn-SO3Na, while the end aggregates are more stable in Mn-COONa. This difference in the aggregate stability in the two series of ionomers was found to have a significant effect on nonlinear rheology; namely, under the shear or elongational flow fields with the Weissenberg number Wi > 1, Mn-COONa showed a larger strain on the stress overshoot compared to Mn-SO3Na. This result suggested that the Mn-COONa ionomers can better adjust the network under flow before the overshoot. After the overshoot, the weaker pseudo-yielding and higher fracture strains were attained for Mn-COONa, suggesting that the associated network can be reconstructed more easily for Mn-COONa. All these features are attributable to a nonuniform distribution of the salt aggregate size and superbridge length of Mn-COONa, which enables more efficient energy dissipation under strong flow through fast rearrangement of the network.
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