Time for relaxation: Stress dissipation mechanisms of PEG gels during swelling

The three-dimensional, cross-linked network of polymer gels allows for the imbibement of solvent which typically results in uniform swelling and isotropic expansion. This process introduces internal stresses into the network, the scale of which is influenced by both network constraints (e.g., cross-linking) and diffusive pressure (e.g., solvent interaction). Most hydrogels can withstand the mechanical deformation associated with swelling repeatedly, making them desirable for use in drug delivery systems or smart membranes. However, there are some cases where the stresses associated with swelling lead to instability, such as surface buckling and spontaneous rupture. Limited research investigates the unsteady-state swelling regime prior to achieving equilibrium, during which these instabilities occur. Here we show that analysis of unsteady-state swelling reveals the mechanisms by which internal stresses are dissipated in a polymer network. By manipulating both the cross-link density and solvent during swelling of poly(ethylene glycol) gels, we reveal that the ability of a network to rearrange determines instability behavior. Dissipating stress through surface buckling requires quick relaxation, while the networks that exhibit critical levels of constraint, and therefore limited rearrangement, exhibit bulk rupture. These findings serve as a foundation from which dynamic deformation can be improved, with potential application in smart anti-fouling devices and sensors.