Atomistic investigation of fracture mechanisms in phosphorus-functionalized epoxy resins

Modifying epoxy resin molecules using phosphorus-containing functional groups can enhance the liquid oxygen compatibility (LOC) of polymeric matrix composites, but will significantly change the mechanical and fracture properties of the functionalized thermosets due to the increased complexity of the molecular architecture. The underlying mechanisms responsible for these property changes are not well understood. In this work, we unveiled the molecular-scale fracture mechanisms of epoxy resins modified by 10-(2,5-dihydroxyphenyl) -10-hydrogen-9-oxa1-phenanthroline-10-oxide (ODOPB) using molecular dynamics (MD) simulations. We adopted a two-step reaction scheme to prepare the modified cross-linked networks of the epoxy thermosets. Then, a fracture simulation approach was developed based on hybrid use of non-reactive and reactive force field parameters, which enables accurate and efficient bond scission representation during tensile deformation. Efforts were made to explore the impact of varying the P content (proportional to ODOPB amount) on the molecular architectures and mechanical performance. It was found that the chain length distribution was a crucial determinant of the mechanical and fracture properties. More intriguingly, the simulation results showed that at a fixed P content, properties such as the fracture energy could be enhanced by regulating the chain length. This study offers valuable insights into the design and fabrication of high-performance aerospace composites with remarkable tolerance to harsh engineering environments.