Repeated Intrinsic Self-Healing of Wider Cracks in Polymer via Dynamic Reversible Covalent Bonding Molecularly Combined with a Two-Way Shape Memory Effect.

To enable repeated intrinsic self-healing of wider cracks in polymers, a proof-of-concept approach is verified in the present work. It operates through two-way shape memory effect (SME)-aided intrinsic self-healing. Accordingly, a reversible C-ON bond is introduced into the main chain of crosslinked polyurethane (PU) containing an elastomeric dispersed phase (styrene-butadiene-styrene block copolymer, SBS). The PU/SBS blend was developed by the authors recently, and proved to possess an external stress-free two-way SME after programming. As a result, the thermal retractility offered by the SME coupled with the reversible C-ON bonds can be used for successive crack closure and remending based on synchronous fission/radical recombination of C-ON bonds. Moreover, multiwalled carbon nanotubes are incorporated to impart electrical conductivity to the insulating polymer. Repeated autonomic healing of wider cracks is thus achieved through narrowing of cracks followed by chemical rebonding under self-regulating Joule heating. No additional programming is needed after each healing event, which is superior to one-way SME-assisted self-healing. The outcomes set an example of integrating different stimuli-responsivities into single materials.