Aligned Bamboo Fiber‐Induced Crystallinity Mitigation of Lightweight Polymer Composite Enables Ultrahigh Strength and Unprecedented Toughness

Abstract Lightweight polymer composites promise incredible applications in aerospace, seaprobes, and medical apparatus. However, their performance is generally limited by a trade‐off between mechanical strength and toughness. Herein, a crystallinity mitigating strategy driven by highly aligned bamboo macrofibers embedded in a polycaprolactone polyol (PCL) matrix for producing ultrastrong and tough lightweight polymer composites is proposed. The embedded bamboo macrofibers have oxygen‐containing functional groups on the fiber surface, that can interact with functional groups (ester and hydroxyl groups) in the molecular chains of the PCL in the form of hydrogen bonds, thus preventing the aggregation of molecular chains and the crystallization of PCL, which ultimately leads to unprecedented toughness. Meanwhile, the bamboo macrofibres with intrinsically aligned microstructure, can enable effective stress transfer and dissipation, providing remarkable ultrahigh strength. As a result, the obtained lightweight polymer composite achieves ultrahigh mechanical strength (31.5 MPa) and superior toughness (21.7 MJ m⁻³) at an unprecedented low density (1.07 g cm⁻³), representing the state‐of‐the‐art in reported lightweight polymers. Such lightweight polymer composite has the potential to greatly expedite the practical realization of artificial medical materials, including orthopedic instruments and joint prostheses.