Cellulose nanofiber orientation-induced crystallinity evolution: Enabling ultrahigh strength and toughness in polymer composites

Fiber-reinforced polymer composites (FRPs) are indispensable in modern structural engineering owing to their exceptional specific strength and ultralow density. However, their advancement remains constrained by an intrinsic trade-off between strength and toughness. Herein, an orientation-induced crystallization strategy is proposed to construct lightweight FRPs that simultaneously achieve high strength and toughness. This approach couples bidirectional freezing with hot-press processing to create a cellulose nanofiber (CNF) oriented polyvinyl alcohol (PVA) composite architecture. During bidirectional freezing, CNFs align along the ice crystal growth direction, guiding the ordered arrangement of PVA chains. Subsequent hot pressing enables the oriented CNFs to act as crystallization templates, promoting controlled recrystallization of the PVA matrix and significantly enhancing its molecular alignment and crystallinity. Moreover, robust interfacial hydrogen bonding between CNFs and PVA ensures efficient stress transfer, mitigates local stress concentrations, and facilitates superior energy dissipation during deformation. As a result, the resulting PC-orientation composite exhibits a remarkable combination of high tensile strength (44.42 MPa) and impressive toughness (34.74 MJ·m^-3) while maintaining a ultralow density (0.75 g·cm^-3), representing the state-of-the-art in reported FRPs. This work provides a universal pathway to break the long-standing strength-toughness trade-off in FRPs and highlights a promising design paradigm for lightweight structural components.