Ion pairing controls rheological properties of "processionary" polyelectrolyte hydrogels

TL;DR

This study explores how ion pairing influences the rheological properties of polyelectrolyte hydrogels, demonstrating that different ion pairs can modulate gel stiffness and strain over a wide range, supported by a scaling model.

Contribution

It introduces a systematic method to tune hydrogel properties via ion pairing and provides a quantitative model linking ion interactions to rheology.

Findings

Rheological properties vary over three orders of magnitude with different ion pairs.

A scaling model accurately describes the ion pairing effects on gel mechanics.

Counterion condensation can be quantitatively measured through rheology.

Abstract

We demonstrated recently that polyelectrolytes with cationic moieties along the chain and a single anionic head are able to form physical hydrogels due to the reversible nature of the head-to-body ionic bond. Here we generate a variety of such polyelectrolytes with various cationic moieties and counterion combinations starting from a common polymeric platform. We show that the rheological properties (shear modulus, critical strain) of the final hydrogels can be modulated over three orders of magnitude depending on the cation/anion pair. Our data fit remarkably well within a scaling model involving a supramolecular head-to-tail single file between cross-links, akin to the behaviour of pine-processionary caterpillar. This model allows the quantitative measure of the amount of counterion condensation from standard rheology procedure.