High-Modulus Composite Films Fabricated via Adsorption between Delaminated Cellulose Nanofibers and Graphene Nanoplatelets

Lightweight and environmentally friendly cellulose nanofiber (CNF)-graphene composites have attracted increasing attention as promising structural materials. However, the commonly used TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) oxidized CNF (TO-CNF), typically exfoliated to the nanofibril scale (∼3 nm), possesses limited hydrophobic surfaces and exhibits strong hydrophilicity, which hinders hydrophobic interactions with graphene. In this study, delaminated CNF (dCNF) with enhanced hydrophobic surface exposure was prepared via lithium bromide trihydrate (LBTH) pretreatment followed by TEMPO oxidation and high-pressure homogenization. LBTH treatment induced amorphization and disrupted hydrogen bonding within the nanofibrils, particularly breaking internal hydrogen bonds in the fibril bundles, thereby enabling exfoliation below the nanofibril level. The resulting dCNF had an average width of 1.36 ± 0.51 nm and exhibited improved hydrophobicity, as confirmed by Pickering emulsions and water contact angle analysis. These hydrophobic surfaces enabled physical adsorption onto graphene nanoplatelets (GnP) without the need for chemical modification. Composite films fabricated from dCNF and GnP showed improved filler dispersion, and a Young's modulus of 26 GPa was achieved at 9.7 μm thickness, representing a 297% increase over pure CNF films and outperforming nearly all previously reported CNF-graphene systems. This study provides an effective and sustainable strategy for enhancing CNF-graphene interfacial interactions in high-performance nanocomposites.