Recyclability and reprocessability of permanently cross-linked polymeric materials have received considerable scientific and technological attention in view of the environmental pollution and sustainable development. By introducing dynamic covalent bonds, vitrimers are emerging as a promising attempt to address this pressing challenge. However, there is still a lack of thermodynamic and kinetic understanding of the bond exchange reactions (BERs) of vitrimers at the molecular level. Herein, by employing coarse-grained molecular dynamics simulations, we successfully construct a model vitrimer system composed of a polymer network formed from linear chains, which can rearrange the network topology via BERs. In this study, we examine the effect of the bond swap energy barrier (ΔE sw ) on a variety of mechanical properties. We find that ΔE sw critically controls the dynamics of the linear chains and the reactive beads located on the linear chain. Our results indicate that the best mechanical performance characteristics are achieved at an intermediate value of ΔE sw . Meanwhile, stress relaxations are examined for different ΔE sw systems. By performing a triaxial deformation to induce the cavities, the vitrimer exhibits excellent self-healing capability by decreasing ΔE sw , as well as increasing the self-healing time and temperature. Lastly, extrusion of polymer vitrimer is simulated, and we find that the extrusion rate tends to increase linearly as ΔE sw decreases. In general, our results provide rational guidelines for designing high-performance vitrimers with good mechanical properties, excellent self-healing ability, and good reprocessability.

Sheared granular layers undergoing stick‐slip behavior are broadly employed to study the physics and dynamics of earthquakes. Here a two‐dimensional implementation of the combined finite‐discrete element method (FDEM), which merges the finite element method (FEM) and the discrete element method (DEM), is used to explicitly simulate a sheared granular fault system including both gouge and plate, and to investigate the influence of different normal loads on seismic moment, macroscopic friction coefficient, kinetic energy, gouge layer thickness, and recurrence time between slips. In the FDEM model, the deformation of plates and particles is simulated using the FEM formulation while particle‐particle and particle‐plate interactions are modeled using DEM‐derived techniques. The simulated seismic moment distributions are generally consistent with those obtained from the laboratory experiments. In addition, the simulation results demonstrate that with increasing normal load, (i) the kinetic energy of the granular fault system increases, (ii) the gouge layer thickness shows a decreasing trend, and (iii) the macroscopic friction coefficient does not experience much change. Analyses of the slip events reveal that, as the normal load increases, more slip events with large kinetic energy release and longer recurrence time occur, and the magnitude of gouge layer thickness decrease also tends to be larger; while the macroscopic friction coefficient drop decreases. The simulations not only reveal the influence of normal loads on the dynamics of sheared granular fault gouge but also demonstrate the capabilities of FDEM for studying stick‐slip dynamic behavior of granular fault systems.

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