Enhanced Water Retention Maintains Energy Dissipation in Dehydrated Metal-Coordinate Polymer Networks: Another Role for Fe-Catechol Cross-Links?

The change from wet and soft to dry and hard is a viscoelastic to solid material transition widely displayed in nature, in particular in materials rich in metal-coordinate cross-linking. How metal-coordinate cross-link dynamics contribute to macromolecular material mechanics upon solidification by dehydration remains an open question. Using mussel-inspired Fe-catechol cross-linked polymer hydrogels, we address this question. In addition to a nearly 2-fold increase in stiffness, we find that the presence of Fe-catechol coordination bonds in a dehydrated polymer gel also provides the bulk network with a significantly increased energy dissipation with over three times higher loss factor. We present evidence to suggest that small amounts (similar to 4 wt %) of locally bound water maintain the dynamic nature of Fe-catechol coordinate cross-links in a dehydrated polymer network. The dehydration-induced polymer material mechanics presented here may provide deeper insights on the biological utilization of metal-coordinate cross-link dynamics as well as inspire new ideas on sustainable materials engineering.