Acknowledgements
5.2 Self-Healing Polymeric Materials via Reversible Bond Formation
5.2.1 Self-Healing Polymeric Materials via Dynamic Covalent Bonding
due to their high bonding strength, covalent polymers compose the major- ity of polymeric structural engineering materials.8 generally, covalent bonds are irreversible, and traditional covalent polymers are permanently damaged by the formation and propagation of cracks.9 recently, reversible covalent linkages have been exploited to achieve self-healing by reform- ing new bonds around the damaged zone.10 Various self-healing polymeric materials have been synthesized via dynamic covalent bonds,11 including diels–alder (da) reactions, disulfide bonds, reversible radical reactions, and photocycloaddition.
the diels–alder (da) reaction is a convenient route for cross-linking poly- mers through the formation of C–C bonds.12,13 the thermo-reversibility of the da reaction offers opportunity for polymers to achieve repeated heal- ing.14 Wudl et al. developed highly cross-linked polymeric materials with furan (diene) and maleimide (dienophile) via a da reaction.15,16 as shown in Figure 5.1(a), the “intermonomer” linkages could be cleaved (retro-da reac- tion) upon heating or stress, and at lower temperatures the covalent bonds could be reconstructed to repair the crack.17 this process was fully reversible under mild conditions, and the cross-linked polymers were tough solids with mechanical properties comparable to commercial epoxy resins for engineer- ing applications. the concept of a reversible da reaction has been used to develop several self-healing polymers, such as polyepoxides,18 polylamides,19 and polyacrylates.20 Besides the homogeneous bulk polymers, peterson et al.
utilized this concept to build reversible interfaces in composites.21 the da reaction between a furan-functionalized epoxy-amine matrix and a maleimide- functionalized glass fiber was used to afford healing capacity at the polymer–
glass interface, extending the fatigue life of glass fiber-reinforced composites for potential applications.22
disulfide bonds can undergo exchange reactions, where two neighbor- ing s–s bonds are disrupted and reformed at moderate temperatures,23 as depicted in Figure 5.1(B-1), leading to the reconnection of cross-links across
Figure 5.1 self-healing reactions via dynamic covalent bonding. (a) diel–alder reaction.15 (B) disulfide exchange reactions.24,29 ((B-1), reproduced with permission from ref. 24, copyright 2011 american Chemical society;
(B-2), reproduced with permission from ref. 29, copyright 2011 ameri- can Chemical society). (C) reversible radical reaction of alkoxyamine.31 (reproduced with permission from ref. 31, copyright 2011 american Chemical society.) (d) photochemical cycloaddition of tCe.33 (repro- duced with permission from ref. 33, copyright 2004 american Chemical society.)
the damaged area. Canadell et al. incorporated disulfide linkages into epoxy resins to construct a self-healing rubber, which was able to recover the mechanical properties at 60 °C.24 this approach is applicable to other poly- mers with low glass transition temperature (Tg), such as polyurethanes25 and polyesters,26 since chain mobility is essential for the exchange reaction to take place. this approach is also ideal for developing self-healing coatings with corresponding applications of the polymers. as illustrated in Figure 5.1(B-2), previous studies have suggested that s–s bonds can be cleaved by reduction to form two thiol (s–h) groups and reconnected by oxidation.27,28 Yoon and co-workers prepared polymer gels from star polymers with revers- ible disulfide cross-links at the periphery of branches.29 the cross-linked star polymer films showed a rapidly spontaneous self-healing behavior from cuts micromachined with the atomic force microscopy (aFM) tip at room tem- perature, as studied by aFM imaging. the self-healing ability was attributed to the regeneration of s–s bonds via thiol/disulfide exchange reactions and the healing efficiency was strongly dependent on the film thickness and the width of the damaged area.
Free radicals are commonly the most reactive species resulting from bond cleavage.30 as shown in Figure 5.1(C), dynamically reversible C–on bonds in alkoxyamine moieties were employed to cross-link polystyrene (ps) back- bones, where covalent bond dissociation and radical recombination took place synchronously upon heating.31 self-healing of cracks was achieved with efficiencies of 65–76%, without losing integrity and load bearing ability to meet practical application requirements. Moreover, the healing temperature could be modulated within a wide range by tuning molecular structure of alkoxyamines.
in addition to thermally reversible reactions, photo-reversible reac- tions are also important and commonly used to realize self-healing of polymers, which would be more advantageous over other approaches since light is a remote stimulus and can be controlled with great ease of operation.32 photochemical [2 + 2] cycloaddition of cinnamate mono- mer 1,1,1-tris-(cinnamoyloxymethyl)ethane (tCe) was utilized to create self-healing polymers via reversible conversion between cyclobutane and original cinnamoyl groups.33 as shown in Figure 5.1(d), tCe can cross- link to form a hard solid upon ultraviolet (UV) irradiation at λ > 280 nm.
When the polymer suffers an impact, C–C bond cleavage of cyclobu- tane rings results in the formation of cinnamoyl groups, and the heal- ing process occurs due to the recovery of the cross-linked networks upon UV exposure. similarly, coumarin was introduced into polyurethane as cross-links because of its reversible photodimerization and photocleav- age characteristics.34 in this case, the cleavage of coumarin dimers can be achieved by mechanical damage or 254 nm radiation, while reconnection requires UV irradiation at 350 nm.
self-healing covalent polymers are not limited to those quoted above, which are mainly designed for structural materials. to achieve the aim of practical application, further efforts are needed to deal with remaining
challenges, such as extending healing ability to common engineering mate- rials, increasing healing efficiency, and reducing dependency of the healing on external stimuli.11