Bamboo as a substitute for plastic: Effects of moisture content on the flexibility and flexural toughness of bamboo with cellulose fibers at multiple scales.

To develop curved bamboo-based products for promoting "bamboo as a substitute for plastic" by deep-molding and high-curvature winding technology, it is urgent to fully understand the effects of moisture content (MC) on flexural properties of bamboo with cellulose fibers at multiple scales. Here, we tested the 3-point flexural behavior of bamboo with cellulose fibers at different MC (0, 5 %, 10 %, 25 % and 50 %), in conjunction with in situ nanoindentation (NI) and environmental scanning electron microscopy (ESEM) to investigate its flexibility and toughness mechanisms. The results showed that gradient distribution of cellulose fibers embedded in soft parenchyma cells adapted to external stresses resulting in bamboo's excellent flexibility and toughness. Low MC (5 %, 10 %) made bamboo more flexible and tougher. The flexibility mechanism acting at the molecular scale was moisture-promoted softening of the lignin-carbohydrate (LCC) complex in the cell wall, and interface slip between the cellulose/LCC caused by water molecules aggregating verified by in situ NI and FTIR spectra, whereas toughness mechanisms originated from the coupling of transwall fracture and fibrils-exposed cellwall tearing, as well as tortuous crack propagation. High MC (25 % and 50 %) impaired flexibility and toughness via intercellular and fibrils-LCC interfacial debonding at multiple scales.