Nonlinear Rheology of Telechelic Ionomers Based on Sodium Sulfonate and Carboxylate

Abstract

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 M n-COONa and M n-SO3Na, with the number-average molecular weight M n 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 M n-SO3Na ionomers than in the M n-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 M n-SO3Na, while the end aggregates are more stable in M n-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, M n-COONa showed a larger strain on the stress overshoot compared to M n-SO3Na. This result suggested that the M n-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 M n-COONa, suggesting that the associated network can be reconstructed more easily for M n-COONa. All these features are attributable to a nonuniform distribution of the salt aggregate size and superbridge length of M n-COONa, which enables more efficient energy dissipation under strong flow through fast rearrangement of the network.

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