Revisiting the strain-induced softening behaviour in hydrogels

TL;DR

This paper demonstrates that strain-induced softening in hydrogels can occur without network rupture, instead arising from energy-related effects and chain-solvent interactions, supported by a coarse-grained model and experimental data.

Contribution

It introduces a model showing softening in hydrogels can result from elastic energy contributions, challenging the traditional rupture-based explanation.

Findings

Softening correlates with elastic energy contributions.

No network rupture is necessary for softening behavior.

Model aligns with experimental data for various hydrogels.

Abstract

Usually, the strain-induced softening behaviour observed in the differential modulus $K(T,\gamma)$ of hydrogels has been attributed to the breakage of internal structures of the network, such as the cross-links that bind together the polymer chains. Here we consider a stress-strain relationship that we have recently derived from a coarse-grained model to demonstrate that no rupture of the network is needed for rubber-like gels to present such behaviour. In particular, we show that, in some cases, the decreasing of $K(T,\gamma)$ as a function of the strain $\gamma$ is closely related to the energy-related contribution to the elastic modulus that has been experimentally observed, e.g., for tetra-PEG hydrogels. Thus, our results suggest that, instead of the breakage of structures, the softening behaviour can be also related to the effective interaction between the chains in the network and their neighbouring solvent molecules. Comparison to experimental data determined for several hydrogels is included to illustrate that behaviour and to validate our approach.