Autonomic Healing of Polymers
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Scott R. White, Mary M. Caruso, and Jeffrey S. Moore Abstract Self-healing polymers have experienced rapid technological advancement over the past seven years. They have moved from a conceptual demonstration to practical application in this time frame and have grown from a single design to a generic paradigm for modern materials development. Potential applications of self-healing polymers are quite broad, including microelectronic substrates and encapsulants, polymeric paints and coatings, structural composites, and biomedical devices. In this article, we focus on polymeric systems that heal in an autonomic fashion, that is, automatically and without human intervention. The types of systems under development and the future of this paradigm in advanced materials are discussed.
Introduction
Microencapsulated Systems
One natural fate of engineered materials is a slow and steady degradation in performance throughout their service life. Degradation is often a complex mechanism involving many environmental factors that accelerates with time. For a new class of self-healing materials, this unrelenting march toward eventual material failure is no longer a certainty. Self-healing polymers exhibit the ability to repair themselves and recover functional performance using the resources inherently available to them. To trigger this repair, they might require some form of external activation (such as heat). A more restrictive class of self-healing materials achieves functional recovery in an autonomic fashion, that is, automatically and without human intervention. The general concept of autonomic healing is depicted in Figure 1a. When damage occurs in the material, a crack forms and eventually propagates until it ruptures a microcapsule that contains a healing agent. The healing agent is then transported to the crack plane and undergoes polymerization, rebonding the crack faces. The entire process is triggered by damage, and recovery occurs under ambient conditions requiring no external source of energy (such as heat). Many different types of materials systems have been developed, but all are predicated on the same underlying concepts of healing through compartmentalization of reactive phases.
Autonomic healing was first demonstrated in 2001 in a structural epoxy containing a microencapsulated healing agent and a suspended solid-phase catalyst.1 This first demonstration utilized polymeric microcapsules containing a liquid monomer (dicyclopentadiene, DCPD) dispersed in a bisphenol A epoxide (Epon 828) cured with diethylenetriamine (DETA) curing agent. A solid-phase ring-opening metathesis polymerization (ROMP) catalyst (first-generation Grubbs catalyst) was dispersed in particulate form within the unreacted polymer matrix together with the microcapsules. (See the article by Williams et al. in this issue for ROMP reaction details.) After curing of the epoxy matrix, fracture experiments were performed for virgin and healed materials, and recovery of up to 70% of the virgin mode-I fracture toughness was demonstrated. This motif
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