Self-healing Polymers and Composites - Sottos Research Group

ample, we have looked at thin bladderswhere a microcapsule healing systemwas sandwiched between two epoxycoatednylon layers. The bladders werepunctured with hypodermic needlesand then allowed to heal for 24 hours.The healing efficiency, measured by theeffectiveness of sealing the holes, rangedfrom 40 to 100 percent, depending onmicrocapsule size and concentration.Finally, there are corrosion-resistantcoatings, critical protection for metalparts that operate in wet or salty environments.Barrier coatings lose their protectiveabilities once they are scratchedor abraded, making them obvious targetsfor self-healing systems. Samplesare tested by scoring the coating andthen subjecting the material to salt waterand, sometimes, to ultraviolet light.Recovery is determined by either visiblecorrosion in the cut or by the electricalconductance of the surface; an intactcoating should have more impedancethan bared metal.Several groups have the tested corrosionresistance of capsule-based coatingsystems containing linseed oil, camphoroil and tung oil, among others.In our labs we have shown remarkableperformance for epoxy and vinyl-basedcoatings with capsule systems, uponexposure to salt water. Intrinsic healingsystems are particularly well suited forbarrier protection, as they emphasizethe healing of small defects, and severalgroups have reported complete corrosionsuppression with such systems.Increasing LongevityThe nascent field of self-healing materialsresearch has made great strides overthe past decade, but many technical challengesstill exist. Continuing progress inthe field will lead to new healing chemistriesthat possess greater stability, fasterhealing rates and higher reactivity. Buthow such materials will perform whensubjected to long-term exposure in harshenvironments remains an open question.The ultimate usefulness of self-healingsystems will be in combating fatigue andperiodic damage events, but the vast majorityof research thus far has focused onperformance in response to static fractures,not on the dynamic aspects of selfhealingunder fatigue conditions.It is likely that large-scale applicationswill not incorporate fully distributedself-healing systems, but rather willemploy targeted and locally patternedself-healing components, in order to optimizecost, efficiency and detrimentaleffects to the overall properties of thematerial. For the field to gain traction,it is critical to have a near-term successfulcommercial demonstration of selfhealing.Most likely, this milestone willhappen first for coatings, as they areprevalent across many industrial applicationsand they require only modestmechanical performance compared tostructural components.But there are other material propertiesbesides mechanical ones that maybe good targets for self-healing. Forinstance, restoring conductivity couldbe highly beneficial for applications inenergy storage and microelectronics.Damaged computer chips might be ableto repair themselves on site, instead ofrequiring replacement. Our group andothers have synthesized organometallicpolymers with semiconductor-level conductivitythat can self-heal with appliedheat, and we have recently demonstratedautomatic restoration of conductivityvia the delivery of highly conductivematerials from microcapsules.Restoration of optical properties mayalso be a fruitful path for self-healingresearch. Cracks have a different refractiveindex from the rest of a material, sothey scatter light and disrupt transparency.The ability of a vascular or capsule-basedsystem to deliver an indexmatchedpolymer to a site of damagecould enable autonomous mitigation ofthis problematic effect.Wherever materials are used, thereis hope that self-healing may lead to increasedsafety and utility, and decreasedcost over the lifetime of the material.Safer self-healing batteries, self-repairingautomobile coatings, resealing tires,fade-resistant fabrics and antitamperelectronics are all potential applications.But could the possibilities extendbeyond healing? Looking again to biologicalsystems for a road map, materialssuch as bone regenerate and remodel inresponse to stress and other stimuli. Itis possible that in the future, syntheticmaterials that currently can heal themselvesin response to damage may alsobe able to respond in a more complexfashion, regenerating and remodelingthemselves over their lifetimes.BibliographyBond, I. P., R. S. Trask and H. R. Williams.2008. Self-healing fiber-reinforced polymercomposites. MRS Bulletin 33(8):770–74.Bergman, S. D., and F. Wudl. 2008. Mendablepolymers. Journal of Materials Chemistry18(1):41–62.Blaiszik, B. J., M. Baginska, S. R. White andN. R. Sottos. 2010. Autonomic recovery offiber/matrix interfacial bond strength in amodel composite. Advanced Functional Materials20(20):3547–3554.Chen, X. X., et al. 2002. A thermally remendablecross-linked polymeric material. Science295(5560):1698–702.Cordier, P., F. Tournilhac, C. Soulie-Ziakovicand L. Leibler. 2008. Self-healing and thermoreversiblerubber from supramolecularassembly. Nature 451(7181):977–80.Hansen, C. J., et al. 2009. Self-healing materialswith interpenetrating microvascular networks.Advanced Materials 21(41):4143–47.Jones, A. S., et al. 2007. Life extension of selfhealingpolymers with rapidly growingfatigue cracks. Journal of the Royal SocietyInterface 4(13):395–403.Kalista, S. J., T. C. Ward and Z. Oyetunji. 2007.Self-healing of poly(ethylene-comethacrylicacid) copolymers following projectile puncture.Mechanics of Advanced Material andStructures 14(5):391–97.Kessler, M.R., N. R. Sottos and S. R. White.2003. Self-healing structural composite materials.Composites Part A: Applied Science andManufacturing 34(8):743–53.Patel, A. J., N. R. Sottos, E. D. Wetzel and S.R. White. 2010. Autonomic healing of lowvelocityimpact damage in fiber-reinforcedcomposites. Composites Part A: Applied Scienceand Manufacturing 41(3):360–68.Sauvant-Moynot, V., S. Gonzalez and J. Kittel.2008. Self-healing coatings: An alternativeroute for anticorrosion protection. Progressin Organic Coatings 63(3):307–15.Toohey, K. S., et al. 2007. Self-healing materialswith microvascular networks. Nature Materials6(8):581–85.Trask, R. S., H. R. Williams and I. P. Bond.2007. Self-healing polymer composites:Mimicking nature to enhance performance.Bioinspiration & Biomimetics 2(1):1–9.van der Zwaag, S. 2007. Self Healing Materials:An Alternative Approach to 20 Centuries ofMaterials Science. Dordrecht: Springer-Verlag.White, S. R., M. M. Caruso and J. S. Moore.2008. Autonomic healing of polymers. MRSBulletin 33(8):766–69.White, S. R., et al. 2001. Autonomic healing ofpolymer composites. Nature 409(6822):794–97.Williams, K. A., D. R. Dreyer and C. W. Bielawski.2008. The underlying chemistry of self-healingmaterials. MRS Bulletin 33(8):759–65.Wool, R. P. 2008. Self-healing materials: A review.Soft Matter 4(3):400–18.Youngblood, J. P., N. R. Sottos and C. Extrand.2008. Bioinspired materials for self-cleaningand self-healing. MRS Bulletin 33(8):732–41.Yuan, Y. C., et al. 2008. Self-healing polymericmaterials using epoxy/mercaptan as thehealant. Macromolecules 41(14):5197–202.For relevant Web links, consult thisissue of American Scientist Online:http://www.americanscientist.org/issues/id.92/past.aspxwww.americanscientist.org© 2011 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact perms@amsci.org.2011 September–October 399