Molecular Insights into the Topological Transition, Fracture, and Self-Healing Behavior of Vitrimer Composites with Exchangeable Interfaces

It is a significant challenge to prepare polymer materials with both high mechanical properties and recyclability. By introducing dynamic covalent bonds (DCBs), the nanorod-filled vitrimer composite (NVC) with exchangeable interfaces is a promising route to solve it, but the molecular mechanism is unclear. In this work, a coarse-grained model coupled with bond exchange reactions is developed to investigate the topological transition, fracture, and self-healing behavior of NVCs with exchangeable interfaces. First, the topology freezing transition temperature rises with the decreasing density (ρ_DCB) of the DCB or the increasing bond swap energy barrier (ΔE_sw), while the glass transition temperature is nearly unchanged. Based on a continuum mechanics model, the fracture energy of the NVC is then analyzed by adopting a triaxial deformation, which rises with the increasing ρ_DCB and ΔE_sw due to the enhanced network via the DCB. The stress decomposition, stretch ratio of network strands, von Mises strain, and local bead stress are characterized to understand the microstructural evolution, which is consistent with the fracture energy. Following it, the volume fraction and number of voids are recorded to analyze the nucleation positions, growth, and coalescence process of voids. The voids initially appear in the polymer matrix at the low strain, which owns a low local elastic modulus, while most of the voids are nucleated in the positions of the extended DCB at the large strain. The introduced DCB can inhibit the growth and coalescence rate of voids, which improves the fracture energy. Finally, the simulated self-healing efficiency is highest at the intermediate ρ_DCB, while it is reduced with the increasing ΔE_sw. The relationship among the self-healing efficiency, healing temperature, and healing time follows the time-temperature superposition relationship. In summary, our work provides an in-depth theoretical analysis of the topological transition, fracture, and self-healing property of NVCs.