Polymer blends as an effective strategy to designing self-healing epoxy resin

• Epoxy blends with EMAA and PMMA were studied for miscibility, compatibility and self-healing ability. • EMAA formed immiscible domains but enabled healing through melt flow and interfacial chemistry. • PMMA showed partial miscibility but lacked reactive groups, resulting in no healing response. • EMAA blends achieved up to 45 % repair efficiency after catastrophic tensile failure. • Results highlight the role of polarity and functional groups in thermosetting healing. The role of miscibility in the design of self-healing thermosets remains debated. While miscibility is often associated with better dispersion and mechanical reinforcement, effective healing requires not only phase integration but also thermoplastic mobility and interfacial reactivity. This study critically examines that assumption by comparing two thermoplastic agents, poly(ethylene-co-methacrylic acid) (EMAA), an immiscible but chemically reactive copolymer, and poly(methyl methacrylate) (PMMA), a partially miscible yet inert polymer, blended into epoxy systems. Using thermal analysis, spectroscopy, and mechanical testing, we assess each system’s ability to recover mechanical performance following catastrophic fracture. PMMA, despite its homogeneous dispersion and minimal disruption to matrix stiffness, failed to promote healing due to its limited mobility and lack of interfacial reactivity. In contrast, EMAA enabled up to 45 % recovery in tensile strength through a combination of melt flow and covalent bond formation at the interface, as evidenced by in-situ FTIR. These findings demonstrate that miscibility alone does not determine healing capability. Instead, effective repair depends on a thermoplastic’s ability to mobilize under thermal stimulus and chemically interact with the matrix. This work provides a functional framework for designing self-healing thermoset systems based on reactivity and mobility rather than miscibility alone. lmanchado@ictp.csic.es