Hydrogels are hydrophilic, cross-linked polymers that form three-dimensional, porous networks. Accordingly, hydrogels often exhibit uniform swelling upon exposure to water, and this process is tunable through synthetic handles such as network cross-link density. Although some direct methods to manipulate gel swelling have been developed, the theories behind hydrogel performance can be limited to qualitative predictions, and typically only capture equilibrium behavior. Classical Flory-Rehner (FR) theory postulates that osmotic pressure develops during swelling due to competition between attractive forces and resistive, elastic forces in the network. However, this theory does not fully capture transient swelling behavior (e.g., the period before reaching an equilibrium, stress-free state), during which we have observed intriguing, spontaneous rupture and surface wrinkling in poly(ethylene glycol)-based hydrogels. Here we present how the rate of osmotic pressure decay during swelling, as estimated by FR theory, can be correlated to macroscopic, experimentally observed instability. A thorough investigation of both the rupture regime and the wrinkling regime of instability revealed strict competition between the two. Ultimately, the most significant factor impacting rupture versus wrinkling behavior was the rate of swelling. Examining correlations between cross-link density, type of rupture, and wrinkle size revealed a complicated relationship between network structure and relaxation dynamics during swelling. This approach of combining experimental observations of hydrogel instability with FR osmotic pressure modeling allows for a greater understanding of unsteady-state, and transient swelling behavior. Knowledge of this behavior and the associated internal stresses can inform material design, particularly for rupture control and mitigation. We anticipate our findings to be a starting point for more sophisticated engineering of surface roughness through <i>a priori</i>, engineered wrinkling of soft materials.

Rupture and wrinkling: Probing two regimes of instability in PEG-based hydrogels

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.