Collective Action and Entanglement of Magnetically Active Liquid Crystal Elastomer Ribbons

Interactions between active individuals in animal collectives lead to emergent responses that remain elusive in synthetic soft matter. Shape-morphing polymers are used to create bio-inspired transient solids that self-assemble, modulate their mechanical properties, and disassemble on demand. The solids are composed of aggregates of many magnetic, heat-responsive liquid crystal elastomer ribbons. Dilute-suspensions of curved and moving ribbons mechanically interlock, inducing reversible aggregation. The degree of bend and twist of the ribbon and the motion of the ribbon in a rotating external field control how ribbons interact with one another. Aggregation was favored for ribbons with moderate curvature at 25C above crosslinking temperature as compared to flat ribbons or higher curvature ribbons at higher temperatures. The ribbon suspensions reversibly transition between fluid- and solid-like states, exhibiting up to 6 orders-of-magnitude increase in the storage moduli of the entangled aggregates compared with the liquid dispersions. Subsequent heating by additional 25C resulted in a 2-fold increase in both stiffness and yield stress. Controlled dissociation is induced by imparting kinetic energy to the individual ribbons at high magnetic field rotation speeds. Ribbon shape and the medium in which dissociation occurs were shown to govern disassembly. Imparting dynamic collective behaviors into synthetic systems may enable a range of potential applications from autonomous bio-inspired soft robotics to injectable biomaterials.