Hierarchical Reaction‐Induced Phase Separation in Epoxy/Oligomeric Poly(phenylene ether) Blends: Multiscale Structural and Mechanical Heterogeneity

Epoxy/thermoplastic blends often undergo reaction‐induced phase separation (RIPS) during curing, and introducing a deformable thermoplastic component is generally expected to improve the deformation tolerance of brittle epoxy networks. In this study, hydroxyl‐terminated oligomeric poly(phenylene ether) (oPPE) was introduced into an epoxy/amine system to clarify how an initially optically homogeneous formulation evolves into a hierarchically phase‐separated network and how the resulting oPPE‐rich morphology affects tensile ductility. Before curing, no optically detectable macroscopic phase separation was observed in the bisphenol‐A diglycidyl ether (DGEBA)/oPPE mixtures, whereas curing produced a hierarchical phase‐separated morphology consisting of primary oPPE‐rich domains and nanoscale secondary subdomains, which is consistent with a two‐stage RIPS scenario. Synchrotron X‐ray scattering, thermal relaxation, and atomic force microscopy nanomechanics confirmed the multiscale structural and local mechanical heterogeneity. Despite this hierarchical morphology, tensile ductility decreased with increasing oPPE content. Fracture‐surface observations revealed interfacial gaps and cavities around the primary domains, suggesting that primary domain–matrix debonding contributes significantly to the reduced tensile ductility and overall failure process.