Objective Bamboo is a natural hygroscopic material. The synergistic effect of “pore-sorption sites” of cell wall is a key factor influencing its hygroscopicity. However, whether this synergistic effect influences the differences in static capacity and dynamic moisture sorption between the outer and inner layers remains unclear. By comparing the “cell wall pore-sorption sites-hygroscopicity” between the bamboo outer and inner layers, this study examined the moisture adsorption-desorption behavior of bamboo with a radial gradient structure under both static and dynamic humidity conditions.

Method After removing the pith ring and bark of Phyllostachys edulis, samples from the outer 2 mm and inner 2 mm along the radial direction were defined as the bamboo outer layer and bamboo inner layer, respectively. The investigation progressed from the macroscopic vascular bundle volume fraction to the microscopic “cell wall pore-sorption sites”, ultimately focusing on static and dynamic hygroscopicity. Parameters including moisture adsorption (M_ac), moisture desorption (M_dc), and moisture buffer value (MBV) of the outer and inner layers were analyzed based on dynamic test results. Thus, the influence of “cell wall pore-sorption sites” on the static and dynamic hygroscopicity was compared between the outer and inner layers.

Result The vascular bundle volume fraction of the outer layer ((32.72 ± 2.10)%) was higher than that of the inner layer ((20.36 ± 1.60)%). Thereby, this gradient structure led to differences in radial pore structural parameters and hydroxyl of the cell walls. At the oven-dry state, the specific surface area and pore volume of the outer layer were smaller than those of the inner layer, and its hydroxyl accessibility ((22.76 ± 0.43)%) was also lower than that of the inner layer ((24.77 ± 0.21)%). The microscopic interplay between cell wall pore and sorption sites led to the observed macroscopic result: static capacity and dynamic response of the outer layer were both lower than those of the inner layer. For static tests, there were significant differences in equilibrium moisture content (EMC) between the two sides when the RH exceeded 70%. When the RH reached 95%, the EMC of bamboo outer and inner layers were 19.48% and 22.04%, respectively. Under dynamic conditions of humidity cycle, compared with RH, the moisture content of both the bamboo outer and inner layers showed a similar trend, but lagged in phase. The inner layer was more sensitive to humidity with higher M_ac, M_dc, and MBV.

Conclusion By testing from microscopic structure to macroscopic performance, this study elucidated the mechanism connecting the gradient structure of “cell wall pore-sorption sites” to the static capacity and dynamic moisture sorption of the outer and inner layer, providing a scientific basis for advancing bamboo processing from “empirical control” to dimension stabilization regulated by “pore-site synergy”.