Using Dynamic Bonds to Enhance the Mechanical Performance: From Microscopic Molecular Interactions to Macroscopic Properties

Abstract

Polymeric materials combining good mechanical performances with self-healing ability and malleability have attracted dramatic attention, but it presently remains a challenge for the facile fabrication of such high-performance materials, not to mention the atomic-level characterization for understanding the molecular origin of the macroscopic properties. Herein, we proposed a facile strategy to fabricate a dual-cross-linked poly(n-butyl acrylate) polymer material, in which the self-complementary quadruple hydrogen bonding interactions between 2-ureido-4[1H]-pyrimidinone (UPy) dimers were utilized as the dynamic sacrificial cross-linkages, and thus to enhance the mechanical strength and toughness. The hydrogen bonding interactions between UPy dimers in such synthetic cross-linked polymer material were revealed in detail by selective saturation double-quantum (DQ) solid-state NMR spectroscopy under ultrafast magic-angle-spinning beyond 60 kHz. In the meantime, the self-healing capability and recyclability were achieved by utilizing dynamic fast boronic ester transesterification at an elevated temperature. A novel symmetrical diboronic ester cross-linker was developed and employed to enhance the probability of bornoic ester transesterification at an elevated temperature. The boronic ester transesterification was verified on a small molecular model and polymer materials by solution 1H NMR spectroscopy and swelling experiments, respectively, and the cross-linking structure of polymer materials was addressed by low-field proton multiple-quantum NMR spectroscopy and T2 relaxometry. Overall, it is well demonstrated that a combination of diboronic ester bonds and UPy dimers as the chemical and physical cross-linkage, respectively, can impart the rubbery materials with enhanced mechanical stiffness and toughness, good healing and recycling efficiency, and elucidation of the structure–property relationship here can further provide piercing insights into the development of high-performance polymeric materials.