Photoplastic Self-Healing Polyurethane Springs and Actuators

Typical cross-linked polymer networks have rubbery elasticity and do not exhibit thermoplasticity such as linear polymers. However, thermodynamically cross-linked polymer networks exhibit certain thermoplasticity that allows for thermal processing. Here, we introduce a new concept of photoplasticity, that is, under light illumination, the covalently cross-linked polymer elastomers become flexible and plastic-like linear polymers. A novel class of photoplastic polyurethane elastomers based on dynamically covalent cross-linker hexaarylbiimidazole (HABI) and permanent cross-linker glycerol is designed and synthesized. Under the dual actions of light and stretching, the photoplastic elastomer exhibits reversible elongation and contraction-like springs. Because of the photoinduced reversible dissociation/recombination of HABI, two cut samples of solvent-free elastomers can be healed under irradiation at room temperature. Under asymmetric illumination, the photoplastic elastomer exhibits the appreciative phototropism, which demonstrates that artificial crucifer actuators are selectively driven by light. Photoplastic elastomers integrate photodriven healing, reversible stretching, and bending deformation, which offers the potential for new applications as optical actuators through optimized design of molecular structure, composition, and geometry.