Self-Healing Polymers

Self-healing polymers are a new class of smart materials that have the capability to repair themselves when they are damaged without the need for detection or repair by manual intervention of any kind. Increasing demand for petroleum feed stocks used to produce polymer and the need for polymeric materials with improved performance in challenging applications continue to drive the need for materials with extended lifetimes. One way to extend the lifetime of a material is to mitigate the mechanism leading to failure. In brittle polymers, failure occurs through crack formation and propagation(1,2) and the ability to repair these cracks when they are still very small will prevent further propagation thus extending the lifetime of the material. Emerging self-healing technologies designed to give polymeric materials the capability to arrest crack propagation at an early stage thereby preventing catastrophic failures will go a long way in helping to increase the scope of applications of these materials. Crack repair in polymeric materials by manual intervention has been well studied(3–9) and provides a good foundation for the discussion of self-healing polymers. Experiments performed using thermoplastics such as poly(methyl methacrylate) (PMMA) and polystyrene have shown that cracks can be healed by promoting the entanglement of polymer chains from each side of the crack face. The chain mobility required for this type of manually-induced healing is achieved either by heating the polymeric material above its Tg (3–9), or using solvents such as methanol and ethanol to promote depression of the effective Tg to below room temperature (10–14). However, as successful as these healing techniques were in laboratory studies, the scope of potential applications is limited to applications in which damage to a material can be easily observed and accessed for manual application of solvent or heating. With the need for autonomic repair of materials without external intervention thus evident, more recent research has focused on developing fully selfhealing systems. One approach to the design of such systems employs the compartmentalization of a reactive healing agent, which is then incorporated into a composite material. Thus, when a crack propagates through the material, it causes the release of the healing agent from the compartment in which it is stored into the crack plane where it solidifies and repairs the material. The first basic application of this approach consisted of an epoxy matrix with suspended glass capillaries filled with either cyanoacrylate or a two-part epoxy resin (15). When a crack propagated through the cured epoxy matrix, the glass