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330 Biomimetics: Biologically Inspired Technologies

catalyst

microcapsule
crack

healing agent

polymerized
healing agent

Figure 12.25 Healing concept of an autonomic healing polymer (From White, S.R., Sottos, N.R., Guebelle, P.H.,
Moore, J.S., Kessler, M.R., Sriram, S.R., Brown E.N. and Viswanathan S. Nature 2001: 409(6822), 794–797. With
permission.).

could be repaired without the use of additional monomers or surface treatment to repair a cracked
interface. Chen et al. accomplished this by synthesizing a polymer based on a thermally reversible
Diels–Alder (DA) and retro-DA cycloaddition. The Diels–Alder cycloaddition is a widely used
reaction in organic synthesis. Many polymers involving the DA cycloaddition have been synthe-
sized, though in many cases the retro-DA reaction is not observed if the diene and dieneophile are
not sufficiently stable on their own. Those polymers with suitable monomer combinations to exhibit
the retro-DA reaction have incorporated the DA adduct into the backbone of the polymer (Chujo
et al., 1990; Engle and Wagener, 1993; Imai et al., 2000) In contrast, the unique aspect of the
polymer created by Chen et al. is that all of the monomer linkages, or cross-links, are formed by DA
cycloaddition and furthermore exhibit the retro-DA reaction.

12.2.3.3 Healing Experiments

The weakest bond in the polymer structure is the polymerization or cross-linking bond of the DA
adduct. While strong in comparison to other types of noncovalent chemical bonds, this is the
first bond to break when the material is loaded to failure or heated above its transition temperature.
However, because this bond is reversible, this is also the bond that reforms when the material
is cooled below the transition temperature. To test the healing ability of this bond, quantitative
testing of the fracture toughness was performed (Chen et al., 2003). Compact tension samples were
notched with a razor blade and loaded in a direction perpendicular to the pre-crack. To arrest crack
propagation in these tests, a hole was drilled into the middle of the specimen. In this way the cracks
were arrested before fracturing the material into two halves and allowed more accurate alignment of
the fracture surfaces during the healing treatment. Healing was carried out at 1158C for about 30 min
with pressure applied by a clamp. Averaging over three tests, the material was able to recover 81% of
its original fracture load. Furthermore, when the same healing procedure was applied a second time,