Site‐Specific Grafting of the Reducing End of Cellulose Nanocrystals on Carbon Fibers via <scp>CuAAC</scp> Click Chemistry for Enhanced Interfacial Properties in Epoxy Resin

ABSTRACT Enhancing the interface performance of carbon fibers (CF) reinforced composite materials has always been a hot topic. Here, polyethylene imine (PEI) and cellulose nanocrystals (CNC) were used in synergy to construct a “flexible‐rigid” interfacial layer on CF via copper(I)‐catalyzed azide‐alkyne cycloaddition (CuAAC) click chemistry to enhance their interfacial performance in epoxy resin. Firstly, the aldimine condensation was proposed to anchor propargylamine (PPA) to the reducing end of CNC. Secondly, PEI was covalently bonded with acyl chloride functionalized CF. The retained amine groups of PEI were subsequently modified by diazotization. Finally, the above two were site‐specifically grafted together via CuAAC click chemistry. The results show that the surface free energy of CF‐PEI (43.2 mJ/m 2 ) increased by 142.70% compared to untreated‐CF (17.8 mJ/m 2 ), while that of CF‐PEI‐Click‐CNC (70.5 mJ/m 2 ) increased by 296.07%. Especially, the interfacial shear strength (IFSS) of CF‐PEI was 54.94 MPa, an increase of 68.84% compared to untreated‐CF (32.54 MPa), while that of CF‐PEI‐Click‐CNC was 77.65 MPa, an increase of 138.63%. This modification strategy would yield CF samples with an enhanced mechanical meshing effect and chemical reactivity. Additionally, the “flexible‐rigid” interfacial layer, alleviating the modulus difference between CF and epoxy resin, would thereby reduce the stress concentration at the interphase.