Ultra-tough and high-strength bamboo composites via hyperbranched PEI-POSS core-shell architectures and subnanoscale interfacial modulation

The strength and toughness of thermosetting phenolic formaldehyde (PF) resin are generally mutually exclusive, often leading to stress concentration and interfacial cracking within bamboo-based composites, which severely limits their mechanical performance and service life. The preparation of PF resin that combine high rigidity, high toughness, and strong interfacial adhesion remains a major challenge. Here, we designed a core-shell structured poly(ethylene imine)-polyhedral oligomeric silsesquioxane (PEI-POSS) hybrid to simultaneously enhance the toughness of the PF resin and the bonding interface in bamboo composites. With 2.5 wt% PEI-POSS, the modified PF resin showed increases in flexural modulus and impact toughness by 70.92% and 76.92%, respectively. This improvement is attributed to the fact that the molecular mixture with linear and hyperbranched topological structures forms uniform molecular holes or free volumes. Applied in bamboo composites, the modified resin promoted uniform stress distribution, enhanced interface modulus by 43.08%, and improved dry/wet shear strength by 34.24% and 31.97%, due to improved penetration and chemical bonding with bamboo cell walls. This work resolves the conflict between strength and toughness in PF resin, providing an effective strategy for developing high-performance bamboo composite interfaces.