Polyethyleneimine‐Modified Cellulose Nanofibers for Toughening Acid‐Cured Epoxy Resins

Weak interfacial bonding between the two phases limited the toughening efficacy of carboxylated cellulose nanofibers (CNF‐C) on epoxy resin. To address this interfacial incompatibility, surface modification of CNF‐C was implemented through polyethyleneimine (PEI) functionalization. The toughening mechanism of PEI‐modified CNF‐C was systematically investigated through Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA). Optimal composite performance was achieved at a CNF‐C:PEI mass ratio of 1:1, where the impact strength of the epoxy resin increased significantly from 15.7 to 37.2 kJ/m 2 , representing a 136.9% enhancement. At the same time, the effect of using high molecular weight PEI is significantly higher than that of low molecular weight PEI. The toughening mechanism analysis revealed that stress dispersion and transfer were significantly enhanced by the long‐chain PEI structure, while the amine groups on PEI were found to participate in the epoxy‐curing reaction. Through these dual mechanisms, the interfacial bonding strength of CNF‐C was effectively reinforced. This systematic investigation provided both theoretical validation and experimental evidence to support the application of CNF‐C in composite materials.