Transmission electron microscopy study on correlations between microstructures and mechanical properties of cellulose nanofiber-reinforced polypropylene composites: effect of dispersion of maleic anhydride-grafted polypropylene as a compatibilizer

• High-resolution TEM revealed the key role of CNF dispersion and MAPP coating in determining the mechanical properties of CNF-reinforced PP composites. • Excessive MAPP coating on CNFs led to poor dispersion and reduced tensile properties. • Well-dispersed CNFs (ca. 20 nm in diameter) improved the tensile properties, particularly in terms of the elongation at break. • This study established the first direct correlation between microstructure and mechanical properties using TEM. The development of polymer composites with superior mechanical properties through the addition of fillers has been extensively explored. However, further improvements in mechanical performance require not only a high degree of filler dispersion but also effective interfacial control between the fillers and the polymer matrix, both of which remain significant challenges. In this study, we investigated the correlations between the microstructures and mechanical properties of cellulose nanofiber (CNF)-reinforced polypropylene (PP) composites containing maleic anhydride-grafted PP (MAPP) as a compatibilizer. We employed high-performance transmission electron microscopy (TEM) to analyze the morphology and dispersion of CNF and MAPP within the PP matrix under various processing conditions. It was revealed that, in PP/CNF/MAPP composites with lower tensile properties, MAPP formed aggregates that excessively coated certain CNF microfibrils, observed by TEM. In contrast, PP/CNF/MAPP composites exhibiting superior tensile properties contained well-dispersed CNF microfibrils with diameters of approximately 20 nm. Furthermore, the overall dispersion was significantly improved in the composites with high elongation at break. These results indicate that the mechanical performance of PP/CNF/MAPP composites can be significantly enhanced by achieving high CNF dispersion and an appropriate coating of CNFs with MAPP. This correlation between the microstructural features, such as the distribution of MAPP and its coverage on CNFs, and the mechanical properties was clarified for the first time through high-performance TEM analysis.