Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fiber-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. We have developed a structural polymeric material with the ability to autonomically heal cracks.
Engineering this self-healing composite involves the challenge of combining polymer science, experimental and analytical mechanics, and composites processing principles. Autonomic healing is accomplished by incorporating a microencapsulated healing agent and a catalytic chemical trigger within an epoxy matrix. An approaching crack ruptures embedded microcapsules, releasing healing agent into the crack plane through capillary action.
Polymerization of the healing agent is triggered by contact with the embedded catalyst, bonding the crack faces. The damage-induced triggering mechanism provides site-specific autonomic control of repair. An additional unique feature of our healing concept is the utilization of living polymerization (that is, having unterminated chain-ends) catalysts, thus enabling multiple healing events. Our fracture experiments yield more than 90% recovery in toughness, and we expect that our approach will be applicable to other brittle materials systems (including ceramics and glasses).