Moisture-mediated freeze-thaw degradation in bamboo from cellulose hydration to macroscale fracture

Bamboo, a sustainable material with an excellent strength-to-weight ratio, faces durability challenges in cold climates due to freeze-thaw damage induced by moisture-phase transitions. This study aims to characterize moisture-dependent structural and mechanical evolutions in bamboo fibers across air-dried (DryB), fiber saturation point (FSP-B), and water-saturated (WS-B) states under ultra-low temperatures. Integrated SEM, SAXS, and mechanical analyses reveal that free water in WS-B generates interfacial stresses via ice crystal expansion, causing macroscale cracks along vascular bundles, while nanoconfined ice crystallization of bound water in FSP-B at -45 °C increases microfibril porosity by 15 %. Cyclic freeze-thaw treatments induce hemicellulose hydrolysis and microfibril disorientation, reducing crystallinity from 85.6 % to 66.8 % and tensile strength by 51 %. Below the FSP, cryogenic strengthening occurs with a 4 % bending strength increase per 5 % moisture gain due to ice-reinforced lumens, whereas post-FSP saturation accelerates damage, with 30-cycle strength retention of 86.5 % (FSP-B) versus 83.5 % (WS-B). Identifying bound water as a nanoscale fibril regulator and free water as a macroscopic fracture initiator, this work suggests moisture control below the FSP to mitigate freeze-thaw damage and leverage ice-mediated reinforcement for frost-resistant bamboo composites and ice-templated bio composites in cold-climate applications, providing a framework for optimizing bio-based material durability in cryogenic environments.