Designing tougher elastomers with ionomers

Elastomers, such as a rubber band, undergo large deformations at low stresses, and when the stress is released, return to their original shape. For a material to have this response, it must have sufficient molecular mobility to allow the polymer conformations to be distorted by the applied stress, and covalent cross-links between the polymers to prevent flow and provide the restoring force. Increasing the stiffness of elastomers, particularly by increasing the cross-link density, typically comes at the expense of toughness (the ability to absorb energy without fracturing), because shorter polymer strands between cross-links have limited conformational flexibility. On page 502 of this issue, Filippidi et al. (1) have devised a versatile synthetic scheme based on the iron-catechol complexes found in mussels and used these ionic interactions to simultaneously strengthen as well as toughen an elastomer. There are many compelling reasons to control the mechanical properties of networks and gels, perhaps none more so than the role of mechanical properties in cell and tissue development (2).