Mechanical stable, self-healing and reprocessable multifunctional polymer with dynamic piperazine-hindered urea bonds

In order to address the relative brittleness and non-recyclability defects of traditional cross-linked epoxy resins, a novel self-healing and reprocessable epoxy resin based on dynamic piperazine-hindered urea bonds (PA-HUBs) was synthesized. Piperazine is a cheap cyclic diamine, which can regulate the skeleton stiffness of epoxy network and control the mechanical properties of epoxy materials. The tensile strength of cured epoxy resin reaches 103 MPa. In addition, the piperazine monomer contains two methyl substituents and an asymmetric aliphatic ring structure, which provides sufficient chain mobility to activate dynamic exchange reactions, and imparts highly dynamic properties to the conventional urea bond, allowing the urea bonds to possess both stability and dynamic properties. Consequently, the synthesized epoxy resin exhibits not only the excellent mechanical performance but also the remarkable self-healing, reprocessable, shape memory and shape reconfiguration capabilities. The epoxy samples with scratches can basically achieve self-healing within 10 min, and the mechanical properties recovery rate of welded splines is as high as 80 %. Furthermore, we also regulated the content of dynamic PA-HUBs and the cross-linking density of the resin network to examine their impact on the dynamic properties and mechanical performance. Further analysis showed that the higher dynamic bond content and lower cross-linking density increase the free volume of dynamic bond exchange and the stress relaxation rate, which further improves the exchange efficiency of dynamic bonds and endows the resin with better self-healing ability. This kind of piperazine-hindered urea bond, which is dynamically adjustable and has both mechanical stability and dynamic properties, introduces a novel approach for solving the balance problem between the mechanical and dynamic performance of thermosets and preparing dynamic polymer materials with excellent performance.