Fracture Mechanism of Epoxy Resins with Different Network Structures

Epoxy resins are thermosetting cross-linked network polymers used in various applications owing to their excellent mechanical and adhesive properties. Achieving controlled mechanical toughness in epoxy networks requires an understanding of how their fracture behavior is governed by the underlying microscopic network structures. In this study, we investigated the fracture behavior of cross-linked epoxy resins with different network homogeneities and mesh sizes by varying the curing agent concentration and monomer length. Stress–strain curves up to rupture were obtained under stretching, and microscopic structural changes during stretching were examined using small-angle X-ray scattering and scanning electron microscopy. Epoxy resins prepared at stoichiometric curing agent concentrations exhibit a homogeneous network structure with fewer dangling chains. In these homogeneous epoxy resins, microcrack formation under stretching is significantly suppressed. At the yielding strain, they undergo necking deformation, and the local strain in the necked region increases with an increasing global strain until rupture in the necking regime. The maximum local strain is nearly identical to the breaking strain of samples in the rubbery state at higher temperatures and increases with the strand length between cross-linking points. This indicates that the fracture behavior of homogeneous epoxy resins is dominated by the mesh size, which is determined by the monomer’s length. As the curing agent concentration decreases from the stoichiometric value, the resulting networks contain a greater number of dangling chains. This inhomogeneous network structure results in craze-fibril formation during stretching, which causes stress concentration in the necking region. Consequently, inhomogeneous epoxy resins exhibit a brittle fracture at lower strains.