Microvascular strategies for damage restoration

Autonomous repair of synthetic materials has previously been demonstrated by the incorporation of biomimetic microvascular networks within structural polymers and polymer composites. Internal networks of channels act as pathways for mass transport of liquid healing agents to damage regions that intersect the vasculature. Microvascular self-healing has proven effective in restoring fracture toughness and modulus by re-bonding the surfaces of damage-induced microcracks; however, large damage volumes (crack separations > 100 µm) provide additional challenges. Capillary forces (i.e. surface tension) aid in retention of healing agents within small crack separations, but gravity may draw healing agents away from larger separations to prevent repair. Inspired by the ability of natural organisms to regenerate lost appendages, this dissertation explores the interaction between microvascular self-healing materials and large damage volumes with respect to healing agent composition and delivery mechanism. Previously reported healing agent chemistries are either mechanically robust (epoxy-based) or rapid response (gels). Here, a two-stage polymer system is introduced in which a fast gelation (~30 s) is followed by free radical polymerization (>1 hr) to obtain a rapid-response system that subsequently cures into a structural polymer. Viscosity and surface tension are leveraged to optimize delivery and wetting properties. The healing agents allow for the successful recovery of lost mass for through-thickness damage regions spanning up to 11.2 mm in thin epoxy sheets through the careful control of delivery parameters and sample geometry. Additionally, a series of epoxy-based healing agents is characterized to determine response time, viscosity, surface tension, thermal performance, and fracture properties with the goal of improved healing response. In addition to work involving the performance and function of microvascular systems, a technique for automatically tracking crack length in-situ is presented for the double cantilever beam specimens and a method for creating branched vasculature via 3D printing of poly(lactic acid) is introduced. The components of two-part healing agents must successfully mix in the damage region to obtain a healing response. An air-driven flow system is developed to deliver alternating droplets of healing agents into a damage region to achieve high levels of mixing. The final mixing efficiency of the healed region is determined via confocal fluorescent microscopy. A statistical analysis of embedded fluorescent nanospheres reveals the air-driven system greatly exceeds previously reported methods for inducing mixing of healing agents in microvascular systems. In-situ delivery is demonstrated for a simulated delamination event in a double cantilever beam specimen. The damage event ruptures the microchannel and mixed droplets of epoxy-thiol healing agents are delivered to the crack plane. Following fracture, droplet flow in the air-driven system is reinstated by crack closure, which continuously purges healing agents from the channel to prevent blockage. Multiple healing cycles of up to 190% healing efficiency are demonstrated for a microvascular composite using the air-driven approach