Acknowledgements
5.2 Self-Healing Polymeric Materials via Reversible Bond Formation
5.2.2 Self-Healing Polymeric Materials via Supramolecular Chemistry
5.2.2.1 Hydrogen Bonding
challenges, such as extending healing ability to common engineering mate- rials, increasing healing efficiency, and reducing dependency of the healing on external stimuli.11
5.2.2 Self-Healing Polymeric Materials via Supramolecular
fractured surfaces tend to link with each other when brought into contact.
the investigation of self-healing kinetics indicates that self-healing reac- tions compete with external conditions.50,51 When freshly cut specimens were exposed to heat or moisture, the self-healing efficiency was signifi- cantly diminished due to the redistribution of active groups on each surface.
Quadruple hydrogen bonding unit 2-ureido-4[1H]-pyrimidinone (Upy) has become a preeminent motif in supramolecular chemistry since it was intro- duced by Meijer and his co-workers,52 because it is self-complementary with high dimerization constant and readily accessible in a one-step reaction from inexpensive starting materials. as the dimerization of Upy groups is revers- ible and highly thermally responsive,53,54 a number of Upy-based healable materials have been reported either by applying a heat trigger or contact pres- sure trigger.55 the functional hydrogen bonding groups can be employed as telechelic ends, chain-extended segments, or pedant groups to form diverse Figure 5.2 examples of supramolecular hydrogen bonding groups and their asso-
ciation constants in CdCl3 previously reported by (a) Binder et al.46 (reproduced with permission from ref. 46, copyright 2005 WiLeY-VCh), (B) djurdjevic et al.47 (reproduced with permission from ref. 47, copy- right 2007 american Chemical society), (C) Meijer et al.48 (reproduced with permission from ref. 48, copyright 1998 WiLeY-VCh), (d) Zimmer- man et al.49 (reproduced with permission from ref. 49, copyright 2005 american Chemical society).
Figure 5.3 the hydrogen bonding network components of a self-healing rubber reported by Leibler et al.35 reproduced with permission from ref. 35, copyright 2008 nature publishing group.
supramolecular architectures.56 the introduction of Upy groups to a series of polymers has led to corresponding thermally healable materials manu- factured by suprapolix, including polysiloxane, polyethers, and polyesters.57 at elevated temperatures, rearrangement of hydrogen bonding interactions could be achieved due to reduced viscosity and enhanced chain mobility of polymers, while autonomous self-healing at room temperature could occur for low Tg materials, which showed higher level of dynamics of the polymer chains.57 taking advantage of high segmental mobility of polymer matrix, Upy groups were incorporated as side chains into soft poly(n-butyl acrylate).
they showed dynamic dissociation/dimerization at room temperature, and the lifetime was about 1.2 seconds.58
to construct materials with high modulus, guan and co-workers produced multiphase supramolecular thermoplastic elastomers with a hydrogen bond- ing brush polymer consisting of a polystyrene backbone (d.p. ≈ 114) with approximately 11 polyacrylic amide side chains (d.p. ≈ 186).59 these polymers could self-assemble into a two-phase nanostructure due to the immiscibility of polystyrene and polar side groups. the polystyrene backbones collapsed as discrete “hard” domains, which were connected by “soft” polyacrylic amid pendant chains with multiple hydrogen bonding sites. the low Tg (≈ 5 °C) of the polymer matrix facilitated the regeneration of hydrogen bonds at room temperature with healing efficiency of 92% after 24 hours. another heterogeneous strategy to achieve self-healing was designed by hentschel with poly(n-butyl acrylate)-b-polystyrene (pBa-ps) end-functionalized with Upy groups.60 these multiphase supramolecular polymers retained the hard (ps)/soft (pBa) two-phase morphology with reversible hydrogen bonding moieties in polymer matrix, which combined stiffness of the thermoplastic elastomers with dynamic healing capabilities. this concept can be applica- ble to a wide range of multi-phase polymers and various types of supramo- lecular motifs for the design of stiff, strong, and tough self-healing polymers.
plasticizing generally reduces the glass transition temperature of a poly- mer and increases its chain mobility.35 hydrogel systems can be viewed as highly plasticized systems with water being the plasticizer, and have the potential to show autonomous self-healing at room temperature. phadke reported permanently cross-linked hydrogels with self-healing proper- ties in an aqueous environment.61 the feature was achieved by arming the hydrogel with acryloyl-6-aminocaproic acid (pa6aCa) side chains to form hydrogen bonds across the two hydrogel interfaces through the amide and carboxylic functional groups. the healing process could be switched on and off by changing ph due to protonation/deprotonation of carboxyl groups, correlating with association/dissociation of hydrogen bonds. as illustrated in Figure 5.4(a), when ph < 3, two separated pa6aCa hydrogel pieces could heal rapidly and the healed hydrogel was strong enough to sustain repeated stretch. however, the healed pieces could be separated again when exposed to high ph (ph > 9) solution, preventing the formation of hydrogen bonds by elec- trical repulsion. Cui et al. reported a thermally regulated self-healing random copolymer comprising 2-(dimethylamino)-ethyl methacrylate (dMaeMa)
and Upy-based monomers, as shown in Figure 5.4(B).62 the reversibility of multivalent Upy groups imparted self-healing ability to the hydrogel.
the gelation of the polymer solution was induced when the ph increased to 8, due to the dimerization of the Upy units. self-healing of this sample occurred automatically within 5 minutes and could be controlled by the thermal response of the polymer matrix. above low critical solution tem- perature (LCst), the aggregation of pdMaeMa restricted the diffusion of polymer chains and undermined the healing process. however, below LCst, the self-healing ability was regained. similar self-healing systems could be obtained by using different monomers copolymerized with Upy moieties, such as 2-hydroxyethyl methacrylate (heMa), 2-(2-methoxyethoxy) ethyl methacrylate (Meo2Ma) or N-isopropyl-acrylamide (nipaam).63,64 a tough stimuli-responsive supramolecular hydrogel was obtained by adding Upy groups into a poly(ethylene glycol) (peg) main chain.65 as depicted in Figure 5.4(C), the bulk materials contain nanoscopic physical cross-links composed of Upy–Upy dimers. the hydrogen bonding groups were embedded in seg- regated hydrophobic domains dispersed within the peg matrix both in the dry state and in water-swollen hydrogels. as Upy groups were shielded by alkyl spacers, they formed strong hydrogen bonded arrays offering the gel elasticity and recovery of mechanical properties after deformation. Up to now, some applications of self-healing gels include coatings and sealants in industrial fields, as well as tissue adhesives, drug delivery, and cell therapy in Figure 5.4 hydrogen bond-based self-healing hydrogels. (a) ph-regulated self- healing mechanism of pa6aCa hydrogels61 (reproduced from ref. 61, copyright 2012 national academy of sciences, Usa), (B) chemical structure of poly(dMaeMa-co-Upy) and the dynamic dimerization of Upy moiety62 (reproduced with permission from ref. 62, copyright 2012 royal society of Chemistry). (C) Chemical structural of peg–Upy chain-extended polymers and illustrative depiction of hydrogel mor- phology65 (reproduced with permission from ref. 65, copyright 2014 american Chemical society).
the biomedical field.66 however, most self-healing gels have not found practi- cal applications, mainly due to their relatively poor mechanical properties.67 developing tough self-healing hydrogels to address this problem is desper- ately needed. it is also promising to develop self-healing gels combining with multifunctional systems, which would be potentially applied for artificial electronic skin, soft actuator, and artificial muscle.68