Role of Nanoscale Cross-Link Heterogeneity in Epoxy Fracture: Insights from Molecular Dynamics Simulations

Fully atomistic reactive molecular dynamics simulations were used to investigate how nanoscale cross-link heterogeneity affects the mechanical response and fracture behavior of DGEBA/33DDS epoxy networks. Epoxy models containing controlled low-cross-link regions were subjected to dynamic and quasi-static tensile deformation. Young’s modulus was governed primarily by the overall degree of cross-linking and was largely insensitive to nanoscale heterogeneity. In contrast, plastic deformation and fracture were strongly controlled by the spatial distribution of cross-links. Homogeneous and weakly heterogeneous networks exhibited distributed void growth and pronounced strain hardening, whereas larger low-cross-link regions promoted localized void growth and premature fracture. A critical heterogeneity length on the order of 2 nm was identified, comparable to the equilibrium end-to-end length of a DGEBA monomer. Below this length, polymer chains bridged adjacent highly cross-linked domains; above it, continuous weak pathways formed and governed fracture.