Externally Triggered Healing in Covalent Adaptable Networks

Covalently crosslinked polymer networks have a permanent network topology that makes them well suited for many structural applications but also renders them intractable to post-fabrication manipulation—they cannot be remended, remolded or recycled. The incorporation of reversible covalent crosslinks into the polymer network backbone impart new properties and constitute a new classification of crosslinked materials. Under normal conditions these covalent adaptable networks possess the immutable properties of a chemical gel; however, once triggered by an external stimulus, such as light, heat, or force, the covalent bonds become active, temporarily imparting the network with the transient properties associated with a weak physical gel. The reversible breaking and reforming of network strands ultimately enables a damaged materials to be repaired or ‘healed’ post network fabrication.

This talk will highlight two distinct reversible covalent crosslinking mechanisms, namely the reversible addition and reversible exchange mechanisms, leading to distinctly different mechanical responses. The incorporation of reversible addition crosslinks into the network, such as a Diels–Alder (DA) reaction, enables one to explore the properties related to conversion, such percolation near the gel point. Furthermore, external triggering of reversible gelation in a DA–ferromagnetic nanoparticle polymer composite is demonstrated using an AC field. The incorporation of reversible exchange moeities, such as an allyl sulfide, enables light-triggered network connectivity shuffling. Additional functionality within the network provides a mechanism for photo-directed materials strengthening, exhibiting a local increase in the modulus and also enabling simultaneous healing in fractured materials. Finally, the shape of the elastomer can be manipulated and locked into place via irradiation, providing a route to engineer wrinkled topography for lens and sensors to surface functionalization for coatings.