Manipulate dynamic chemical interactions in renewable polymers for 3D printing tunable, healable, and recyclable metamaterials

Mechanical metamaterials have garnered significant interests due to their unique properties, which arise from the periodic structures of their constituent units. However, traditional mechanical metamaterials suffer from limitations of fixed properties of the constituent units or irreversibility if they are damaged or little-to-no recyclability. Addressing this challenge, herein, a photoresin consisting of a monomer synthesized from a renewable biomass-epoxidized soybean oil (ESO)-citric acid (CA) and other components was developed for light-based 3D printing. It is discovered that the UV irradiation initiates photopolymerized of the highly photoactive epoxidized soybean oil-ethyl acrylamide (ESO-EA) monomer while enabling the epoxy-acid reaction (EAR) of the epoxy groups in ESO-EA with carboxyl groups in CA to create dynamic chemical bonds (DCBs). This formed dynamic crosslinking network endows the printed material with tunable tensile strengths (0.3-10.4 MPa) and shape memory behaviors (transition temperature: 22-48 degrees C) while maintaining high stretchability (fractural strain: 81 %-205 %). Thermal annealing facilitates dynamic transesterification reactions (DTER) to further improve the mechanical properties while making the printed materials healable and recyclable. By integrating these innovations into liquid crystal display (LCD) based 3D printing, we demonstrate origami metamaterials with tunable, multi-stable mechanical behaviors as well as property recovery after self-healing of the damaged structures by both simulations and experiments. Finally, the printed metamaterials can be recycled with the well-preserved properties.