Toughening Vitrimers Based on Dioxaborolane Metathesis through Introducing a Reversible Secondary Interaction

Developing a dual-cross-linked network, with weaker cross-links to dissipate energy and stronger ones to sustain integrity, is an effective protocol to overcome the well-known trade-off between hardness and stretchability for achieving high toughness. Although this protocol has been extensively explored in the past two decades, the molecular mechanism underlying the protocol is yet to be better understood. In this study, we copolymerize hexyl methacrylate with hydrogen-bonding n-isopropyl methacrylamide and vitrimeric cross-linkers to prepare dual-cross-linked networks. During the tensile tests, the energy dissipation is greatly enhanced by increasing the density of hydrogen bonds, enabling the initial modulus to increase from ∼1 MPa, which is typical for an elastomer, to 300 MPa, which is closer to the glass. Nevertheless, the breakup of the hydrogen bonds during the elongation significantly dissipates the energy, leading to the softening of the materials, thereby facilitating the stretch to high strain. The softening process, seen as an overshoot in the stress-strain curve, is well captured by the Dobrynin theory modified by Konkolewicz and co-workers upon including a strain rate-dependent element. The deviation at a high content of hydrogen bonds, where the distance between hydrogen bonds becomes smaller than the Kuhn length, is attributed to the coupled motion of the hydrogen bonds.