Tailoring the Toughness of Elastomers by Incorporating Ionic Cross-Linking

We investigated the morphology, topology, and mechanical characteristics of a loosely cross-linked epoxy network as a function of the varying content of catechol moieties capable of forming reversible, ionic iron-catecholate cross-links. The primary epoxy network structure was kept fixed by a constant mole ratio of difunctional poly(ethylene glycol), monofunctional diluent, and diamine cross-linking agent in all samples. We then systematically replaced the catechol monoepoxide diluent by methyl glycidyl ether, which is incapable of participating in ionic complex formation. This allows the effects of the catechol content on network properties to be isolated and analyzed. Our results support a model in which increasing the concentration of catechol moieties promotes the formation of closely spaced iron-catecholate complex sites. This enables cooperative interactions between netpoints and produces a dramatic improvement in tensile properties. Such ionic interactions are thus a promising approach to creating stiff, strong, and tough load-bearing polymer networks.