Tailoring Interlaminar Toughness of CF/EP Laminates via PU/PI-Induced Gradient Modulus Interphase

Abstract The inherent brittleness of the epoxy resin in carbon fiber/epoxy resin (CF/EP) laminates makes them prone to delamination, which severely limits their structural reliability. Herein, a molecularly tailored polyurethane/polyimide (PU/PI) copolymer film was proposed as an interleaf to construct a gradient-modulus interphase for addressing this issue. SEM and AFM results revealed that the introduction of PU significantly improved compatibility with EP, as evidenced by the elimination of the interface of the pure PI film. The film thickness was identified as a critical parameter that governed the interpenetration behavior between EP and PU/PI: insufficient thickness led to a steep modulus mismatch and weak interfacial bonding, whereas excessive thickness induced internal defects and interlaminar failure. Only at an optimal thickness could a broad-gradient interpenetrating network be formed, enabling a smooth modulus transition across the interlaminar region. Mechanical test results showed that f-[PU/PI]35 exhibited improved performance. Compared with the untoughened CF/EP laminate, the values of GIC-PROP and GIIC increased by 100% and 221.3%, respectively. More importantly, the interlaminar shear strength of f-[PU/PI]35 also increased by 19.1%, demonstrating a successful synergy between toughness and strength. This work provides a paradigm for the design of high-performance composite materials.