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• W2555175341 abstract "Self-healing occurs through a combination of physical (conformational changes) and chemical (bond reformation) phenomena. In this issue of Chem, Harada and coworkers make use of the best of both worlds to create a rapidly self-healing material by combining the mobility of a polyrotaxane with the versatility of dynamic covalent chemistry. Self-healing occurs through a combination of physical (conformational changes) and chemical (bond reformation) phenomena. In this issue of Chem, Harada and coworkers make use of the best of both worlds to create a rapidly self-healing material by combining the mobility of a polyrotaxane with the versatility of dynamic covalent chemistry. Physical damage in a polymeric material induces chain cleavage and/or chain slippage. Chain cleavage usually produces reactive groups that have the potential to recombine to form a new crosslink, provided that no side-reactions take place. At the same time, conformational changes and diffusion of the chains lead to macroscopic network rearrangement. Efficient self-healing can be achieved when these physical changes bring the reactive groups close enough to induce bond formation.1Yang Y. Urban M.W. Chem. Soc. Rev. 2013; 42: 7446-7467Crossref PubMed Scopus (965) Google Scholar So far, most research efforts have focused on the chemical aspect of self-healing. Vascular2Toohey K.S. Sottos N.R. Lewis J.A. Moore J.S. White S.R. Nat. Mater. 2007; 6: 581-585Crossref Scopus (1264) Google Scholar and capsular3White S.R. Sottos N.R. Geubelle P.H. Moore J.S. Kessler M.R. Sriram S.R. Brown E.N. Viswanathan S. Nature. 2001; 409: 794-797Crossref PubMed Scopus (3588) Google Scholar systems that release healing agents upon crack formation have been successfully applied to composite materials, whereas the use of reversible bonds is more prevalent in gels and soft materials. These crosslinks are usually based on non-covalent interactions or dynamic covalent bonds. The former offers relatively fast kinetics of healing, but the resistance of the material is limited by the strength of the non-covalent interaction. The latter offers the strength of a covalent bond but at the price of slower bond reformation.4Wei Z. Yang J.H. Zhou J. Xu F. Zrínyi M. Dussault P.H. Osada Y. Chen Y.M. Chem. Soc. Rev. 2014; 43: 8114-8131Crossref Google Scholar The rapid expansion of dynamic covalent chemistry,5Jin Y. Yu C. Denman R.J. Zhang W. Chem. Soc. Rev. 2013; 42: 6634-6654Crossref Scopus (908) Google Scholar such as (retro)-Diels-Alder reaction, hydrazone or disulfide exchange, and more recently, boronate esters exchange, has provided a variety of reactions for the design of new systems. One of the main advantages of reversible-bond-based systems is that they offer virtually unlimited healing. Nevertheless, because they often require an external stimulus or specific conditions (e.g., heat, light, or pH) to trigger the healing process, they rarely lead to truly autonomic systems. The contribution of physical changes (chain slippage) to self-healing is best illustrated by polyrotaxane6Harada A. Hashidzume A. Yamaguchi H. Takashima Y. Yamaguchi H. Yamaguchi H. Takashima Y. Chem. Rev. 2009; 109: 5974-6023Crossref PubMed Scopus (737) Google Scholar slide-ring materials, which can dissipate the energy of physical damage by utilizing the “pulley effect” provided by figure-eight crosslinks in the polyrotaxane network.7Mayumi K. Ito K. Kato K. Polyrotaxane and Slide-Ring Materials. The Royal Society of Chemistry, 2016Google Scholar The exceptional mobility of polyrotaxane networks accounts for the enhanced rate of conformational changes observed after physical damage. In this issue of Chem, Harada and his team hypothesized that combining physical and chemical self-healing mechanisms would improve the self-healing ability of polymeric materials by creating synergy between the mobility of polyrotaxane and the dynamic properties of boronic esters.8Nakahata M. Mori S. Takashima Y. Yamaguchi H. Harada A. Chem. 2016; 1: 766-775Scopus (99) Google Scholar Boronic acids are known to react with diols to form boronic esters, making cyclodextrin (CD), with its multiple hydroxy groups, an ideal coupling partner. The material is composed of two distinct parts: a CD-based polyrotaxane and a random copolymer of 4-vinylphenylboronic acid and acrylamide. The polyrotaxane used in this study was assembled from β-CD threaded onto a poly(ethylene glycol) chain and stoppered by an adamantane moiety. The CDs were randomly hydroxypropylated to improve water solubility. The desired gel (1) was assembled by terpolymerization of 4-vinylphenylboronic acid and acrylamide in the presence of the polyrotaxane (Figure 1A ). Dynamic viscoelastic measurements demonstrated the importance of the interaction between the boronic acid and the hydroxyl groups for the formation of a self-standing gel. The authors demonstrated the self-healing properties of gel 1 by cutting a cuboid piece of gel into two pieces and reassembling them by putting the newly exposed faces in contact with each other. Interestingly, no self-healing occurred when an aqueous solution of boric acid (B(OH)3) was applied on the cut surfaces. By binding to the hydroxyl groups of the CDs, the boric acid inhibits the association between the polyrotaxane and the boronic acid pendent groups of the polyacrylamide. It follows that the self-healing behavior is due to the reformation of boronic esters between the polyrotaxane’s CDs and the polyacrylamide’s boronic acids. Harada and colleagues then investigated the self-healing efficiency by comparing the rupture forces of a healed sample (Fx) with the rupture force of a virgin sample (F0). The recovery ratio (Fx/F0) increased with healing time, and values of 75%–100% could be reached after 15 min. The relative importance of the two key features of this new self-healing approach was then probed against two reference samples: a gel (2) lacking the interlocked architecture (where the polyrotaxane was replaced with pullulan, a linear polysaccharide; Figure 1B) and a gel (3) devoid of dynamic bonds (where the polyrotaxane was linked to the polyacrylamide via non-reversible carbamate linkages). The gel with no dynamic bonds did not heal, whereas the gel lacking the mobility of the polyrotaxane recovered only 20% of its original strength after 60 min. The authors further demonstrated the applicability of their hybrid gel by forming a scratch-healable coating on a glass surface. They converted a thin gel, formed on the glass substrate after photopolymerization of the gel precursors, into a thin film by drying the coated glass in vacuo overnight. Scratching the coating with a razor blade left a groove on the surface that progressively filled up upon healing. Measurements of the groove cross-section by laser microscopy allowed the authors to estimate the extent of self-healing over time. The process proved to be sluggish in comparison to the gel state, and the films had to be incubated at 60°C in a humid atmosphere to reach a healing rate comparable to those observed in the gel state. Here again, system 1 showed the fastest healing with complete recovery in less than 30 min. The mobility of the polyrotaxane network provided fast relief, but the recovery plateaued around 60% when no dynamic boronic esters were present (gel 3). On the other hand, a film relying only on dynamic bonds showed a higher but slower recovery (80% after 3 hr). The dramatic rate differences observed in both gel and semi-dry states give additional insight into the physical aspects of the healing process. In gel 2, bond reformation can occur only through conformational changes of the chains in the network (Figure 1D), whereas in gel 1, a combination of conformational changes and diffusion of the reactive group (CD diols) along the polymer backbone contribute to the rate of bond reformation (Figure 1C). In this paper, Harada and co-workers have demonstrated the importance of addressing the physical aspects of self-healing in designing new materials. The idea of combining mobile and dynamic bonds proved to be a winning strategy and should pave the way to a new range of hybrid self-healing materials combining physical and chemical healing mechanisms. Self-Healing Materials Formed by Cross-Linked Polyrotaxanes with Reversible BondsNakahata et al.ChemNovember 10, 2016In BriefMaintenance-free materials with a self-healing ability are highly desirable in extending the lifespan of materials. Harada and colleagues report on self-healing materials based on polyrotaxanes and vinyl polymers cross-linked by reversible bonds. Compared with conventional types, the interplay of movable cross-links along the axle polymer and reversible bonds realized efficient self-healing in both wet and semi-dry states. This research opens up a new design principle of self-healing materials for a sustainable society. Full-Text PDF Open Archive" @default.
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• W2555175341 title "Combining Mobile and Dynamic Bonds for Rapid and Efficient Self-Healing Materials" @default.
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