Effect of Different Network Topologies on Swelling and Mechanical Properties of Polyelectrolyte Hydrogels

We investigate the properties of polyelectrolyte hydrogels with various network heterogeneities, particularly their swelling capacity, elastic modulus and salt partitioning.
We use coarse-grained molecular dynamics simulations with explicit ions and charged bead-spring-polymers. We benchmark the scaling predictions for elastic modulus and equilibrium swelling ratio of the hydrogels under various salinity conditions, verifying the theoretically predicted close link. Decoupling these two quantities is imperative, as it enables, for example, an increase in the swelling ratio while maintaining mechanical strength. To this end, we explore mechanical, structural and thermodynamic properties of the hydrogels with varying topologies, namely gels with dangling ends, gels with floating chains, and bottle-brush gels in counterpart to a reference regular gel. We observe that incorporating dangling ends changed swelling ratio and bulk modulus inline with the scaling predictions, whereas bottle-brush and floating-chain gels deviate from the predictions. Specifically, floating chains resulted in higher moduli and higher swelling ratio, while bottle-brush gels resulted in lower moduli and lower swelling ratios than the regular counterpart, each maintaining the same swelling ratio and modulus.

Concomitantly, a clear distinction of salt partitioning for various hydrogel architectures was observed. The new swelling-mechanical relations allow us to treat them in
a decoupled manner via the topology variation, which turns out to be of paramount relevance in the optimization and on-demand design of hydrogels.