Toughening diene elastomers by strong hydrogen bond interactions

Introducing reversible hydrogen bonds as sacrificial bonds is an emerging strategy to improve the toughness of elastomers. However, binary hydrogen bonds are not strong enough and highly dynamic, and thus have only a moderate toughening effect. Here we demonstrate that quadruple hydrogen bonds have a remarkable toughening effect for diene elastomers. To fabricate the quadruple hydrogen bonds toughened elastomer, we graft 2-ureido-4[1H]-pyrimidinone (UPy) groups onto the backbone of polyisoprene (PI). The UPy groups dimerize to form strong hydrogen bonds which have higher bond energy than binary hydrogen bonds. Compared with weak hydrogen bonds with the same mole fraction, the strong hydrogen bonds lead to higher reversible crosslinking density and slower chain mobility of the elastomer; this enables higher energy dissipation as the elastomer is subjected to deformation. As a result, the introduction of UPy significantly increases both the toughness and the tensile strength of the elastomer. Moreover, unlike covalent sacrificial bonds, the hydrogen bonds of UPy are dynamic and show the re-association of sacrificial bonds at room temperature, as evidenced by recovery of hysteresis loop during cyclic tensile tests. This work will not only greatly extend our understanding on the different toughening effects of weak and strong hydrogen bonds, but also help us to rationally design tougher elastomers.