Bio-composites using thermoplastic resin as the base material and cellulose nanofibers (CNFs) extracted from wood and other plants as reinforcing fibers are gaining attention as environmentally friendly materials aligned with the Sustainable Development Goals (SDGs). Among these, the fiber/matrix interface is a critical factor influencing mechanical properties. To form strong interfacial bonds and prevent fiber agglomeration, various surface treatments of fibers and modifications to the base polymer have been implemented, with their effects on mechanical properties requiring thorough investigation.
In this study, first-principles calculations based on density functional theory were employed to analyze the interfacial properties between the base polymer and cellulose fibers. Alongside silane coupling agents, which have been widely recognized as effective interfacial treatments, we focused on maleic anhydride modification and evaluated its effects on polypropylene (PP) and polyethylene (PE). Furthermore, to elucidate the macroscopic mechanical properties of cellulose composites, multiscale nonlinear finite element analyses based on homogenization theory were conducted. Specifically, the macroscopic mechanical properties of cellulose composites were clarified through a two-step homogenization process, considering three structural scales: the microstructure with an interfacial phase around the fibers, the meso-structure with dispersed fibers, and the macrostructure under external loading. First-principles calculations revealed atomistically that both silane coupling agents and maleic anhydride are effective in improving the interfacial bond strength of CNFs. In the multiscale finite element analysis using the silane coupling agent case as an example, the interfacial properties obtained from the first-principles calculations were successfully introduced, and the results were qualitatively consistent with the experimental results.