Bio-Based Composites Reinforced with Surface-Modified Cellulose Nanofiber Scaffolds: Influence of Interfacial Compatibility on Surface Mechanical Properties

In the context of sustainability, bio-based polymer composites have attracted considerable attention. Cellulose nanofibers (CNFs) are promising eco-friendly reinforcing fillers; however, their compatibility with conventional hydrophobic polymers is limited by their hydrophilic surface. Herein, we fabricated bio-based polymer composites through a two-step process―surface modification of porous CNF scaffolds followed by impregnation with epoxidized soybean oil (ESO)―to investigate the influence of interfacial compatibility between the filler and the matrix. Vapor-phase silane coupling reactions were successfully applied to the CNF scaffolds without compromising their porous architectures. The resulting surface-modified scaffolds exhibited distinct characteristics depending on the functional groups introduced by the silane coupling agents. Subsequently, the composites based on CNFs and ESO were fabricated via impregnation, and their structures were characterized using scanning electron microscopy. Fractured surface observations revealed that the surface modifications significantly improved the interfacial compatibility between the CNF scaffolds and the ESO thermoset. Surface mechanical properties, including hardness and friction behavior, were also evaluated to clarify the role of interfacial compatibility. These results indicate that surface mechanical properties are strongly influenced by interfacial compatibility, but less affected by the specific interfacial chemical bonding.