Objective The hygroscopicity of bamboo and the resulting swelling pose significant constraints on its high-value utilization. However, the mechanism underlying its moisture adsorption-swelling response remains unexplained. To address this, the moisture and dimensional responses of bamboo from the bamboo outer layer to the bamboo inner layer were investigated during adsorption process and at equilibrium state under various relative humidity (RH) conditions, aiming to reveal the influence of radial gradient structure in culm wall on their adsorption-swelling behavior.

Method The culm wall of moso bamboo (Phyllostachys edulis) was radially divided into three sections: bamboo outer layer, bamboo middle layer, and bamboo inner layer as the research objects, with fiber volume fraction serving as a quantitative indicator of its gradient structure. Under five RHs, i.e. 11%, 33%, 57%, 84% and 97%, the moisture content and dimensional changes of specimens from each section were recorded during adsorption process as well as at equilibrium state. The sorption isotherms were further analyzed using the Hailwood-Horrobin (H-H) theory, and the sorption kinetics were fitted with the Parallel Exponential Kinetics (PEK) model to compare the differences in the moisture adsorption-swelling behavior among different radial positions.

Result Moso bamboo exhibited a distinct gradient structure along its radial direction, with the fiber volume fraction showing a decreasing trend from the bamboo outer layer ((34.13 ± 0.84)%) to the bamboo inner layer ((15.87 ± 0.12)%). Under the equilibrium state in this work, the equilibrium moisture content (EMC) demonstrated a monotonous characteristic of bamboo outer layer < bamboo middle layer < bamboo inner layer, whereas the swelling rate displayed an opposite tendency of bamboo outer layer > bamboo middle layer > bamboo inner layer. These differences became more pronounced when the RH exceeded 57%. During the moisture adsorption process, at RH ≤ 57%, the bamboo inner layer exhibited a significantly higher sorption rate during the initial stage compared with the bamboo outer layer, accompanied by a correspondingly faster swelling response. In contrast, at RH > 57%, the initial sorption rate across different radial positions tended to converge, while the bamboo outer layer consistently showed the highest swelling rate.

Conclusion The adsorption-swelling relationship of moso bamboo underwent phase transition during adsorption process. This transition originated from the increasing fiber volume fraction from the bamboo inner layer to the bamboo outer layer, which determined the characteristic transition from “strong adsorption and rapid swelling response” to “weak adsorption and high swelling potential” across this gradient structure. This study revealed the dominant mechanism of moso bamboo’s gradient structure in governing its adsorption-swelling coupling behavior. The findings could not only contribute to further exploration in the stabilizing modification for bamboo but also provide effective strategies for the structural optimization of novel functionally graded bamboo composites.