Self-healing Polymers and Composites - Sottos Research Group

Self-healing Polymers and CompositesCapsules, circulatory systems and chemistry allow materials to fix themselvesScott R. White, Benjamin J. Blaiszik, Sharlotte L. B. Kramer, Solar C. Olugebefola,Jeffrey S. Moore and Nancy R. SottosNothing lasts forever. Any humanmadematerial, used in anythingfrom toys to bridges, will eventuallyfail, if not maintained and repaired.Traditionally, this problem has been addressedthrough extensive inspectionand expensive replacement of damagedparts. Biological systems, however, haveevolved to include alternative ways torepair internal and external damage viahealing mechanisms. Materials researchersworldwide, including those in ourgroup, have been working on methods tomimic these healing abilities in polymers,composites and other synthetic materials.Such self-healing materials, when triggeredby a crack or tear, can repair themselvesand recover their original functionalityusing only the materials that areinherently available to them. They offer anew means to achieve safer and longerlastingproducts and components, andsignal a shift in the traditional paradigmsof material design and engineering.The guiding principles for syntheticself-healing are seen in biological systems.Damage that causes injury triggersthe first response, inflammationand blood clotting. This initiating stepis followed by cell proliferation at thesite of injury, which deposits a matrixfor the repair. The final stage of healing,matrix remodeling, is the regrowthof new tissue to fill in the wound. Thisprocess can take place over a longerScott R. White is a professor of aerospace engineeringat the University of Illinois. Benjamin J. Blaiszik is apostdoctoral fellow at the Center for Nanoscale Materialsat Argonne National Laboratory. Sharlotte L.B. Kramer and Solar C. Olugebefola are postdoctoralresearchers, Jeffrey S. Moore is a professor of chemistryand Nancy R. Sottos is a professor of materialsscience at the University of Illinois. This article wasadapted from Blaiszik, B. J., et al. 2010. Self-healingPolymers and Composites. Annual Review ofMaterials Research 40:179–211. Address for White:Department of Aerospace Engineering, Universityof Illinois, Urbana-Champaign, Urbana, IL 61801.E-mail: swhite@illinois.eduperiod, months to possibly years, dependingon the severity of the injury.In synthetic systems, there is a similarcascade of events, but it is more simplisticand takes place at a faster rate. At first,damage actuates the start of the process,then new materials are transported tothe site rapidly, and healing occurs asthe material reacts to form an adhesivebond with the damaged area. Most oftenthe healing agent is made of two typesof liquid materials that solidify whenmixed. Finally comes a chemical repairprocess, analogous to matrix remodeling;its timescale varies depending onthe type of healing mechanism, but itoccurs in the range of hours to severaldays at most. The goal of self-healing isto match the rate of repair with that ofthe damage, thereby achieving a state ofstasis in the material. The rate of damageis largely controlled by external factors,such as the material’s strain rate, howfrequently it experiences loading and themagnitude of the loading. However, thehealing rate can be adjusted by, for example,varying temperature or chemicalreactionrates through control of rawmaterialtypes and concentrations.Research on self-healing materials isrelatively new, with most of the progresscoming within the last decade. Althoughin theory any material can self-heal, theresults for polymers and fiber-reinforcedcomposites are relatively more maturein comparison with efforts in ceramics,metals and other materials. Whateverthe class of material, self-healing mechanismscan be broadly classified into threegroups: capsule-based, vascular and intrinsic.Each group differs by the methodused to sequester the material’s healingfunctionality until it is triggered by damage.The groups also vary by the differentamounts of damaged volume thatthey can heal, as well as the repeatabilityof healing in the same location and therate of healing. Thus each approach hasits own challenges and advantages.Capsule-based materials incorporatea healing agent that is held and protectedin discrete spherical shells, which areruptured by damage. The self-healingmechanism is activated by the releaseand reaction of the healing agent at thedamage site. However, after release,the healing agent is depleted, so it onlyworks for a single local healing.Vascular materials carry the healingagent in a network of capillariesor hollow channels, which may besingle tubes, discrete planes of interconnectedtubes, or three-dimensionalnetworks of channels. When damageruptures the vasculature and deliversthe healing agent, the network can berefilled (either from an external sourceor from undamaged, connected channels),so it can support multiple localhealingevents.Intrinsic materials instead have a latentself-healing ability, usually builtinto the chemical network of the polymermaterial, rather than a separate,sequestered healing agent. They rely onrepairs made through molecular-scalemechanisms, such as hydrogen bonding,ionic interactions or polymer-chainmobility and entanglement. Each ofthese mechanisms is reversible, makingmultiple healings possible.Raw MaterialsFor capsule-based materials, there are anumber of techniques for creating polymershell walls that protect the reactivematerials inside them. The most commonmethods involve forming a shellat the interface of droplets in an oil-inwateremulsion. In this case, the resultingshell will be thin and brittle, likethat of an egg, and it will rupture whenforce is applied. Another procedure involvesemulsifying a melted polymerso that it forms droplets, which are thensolidified by a temperature change orby the removal of a solvent, creating athick protective sphere around the core.392 American Scientist, Volume 99 © 2011 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact perms@amsci.org.