Polymer interphase engineering in CF/EP composites: Strength–toughness synergy via dual-alternant interphase

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Interphase engineering dismantles the long-standing strength–toughness trade-off, unlocking a new generation of carbon fiber/epoxy composites. This study introduces a biodegradable poly(2-phenyl-2-oxazoline)-based (PPhOZ-PEI) multifunctional polymer reinforcement to regulate interphase architecture via two distinct incorporation pathways: fiber surface coating and epoxy matrix blending. By maintaining identical polymer chemistry while varying localization, the role of interphase formation sequence on composite performance is systematically decoupled. Results show that matrix-first integration produces a chemically integrated interphase through covalent crosslinking between amine groups and epoxy, combined with π–π interactions with carbon fibers. This leads to simultaneous enhancements in flexural strength (+15%), flexural modulus (+9%), interlaminar shear strength (+49%), and Mode-II fracture toughness (+11%). In contrast, fiber-first deposition yields a localized and mechanically brittle interphase, improving ILSS (∼+29%) but reducing resistance to delamination and fracture propagation. Fractographic analysis confirms a transition from smooth, delamination-dominated failure in unmodified and fiber-coated systems to rough, energy-dissipative surfaces owing to intrinsic and extrinsic toughening mechanisms. These findings demonstrate that interphase formation pathway, rather than polymer chemistry alone, governs mechanical performance. The proposed strategy provides a scalable route for designing sustainable, high-performance CF/EP composites with tailored strength–toughness synergy.