Low-field NMR method on determining water absorption diversity between bamboo nodes and internodes

Exploring the water status during the water absorption process in different parts of the moso bamboo is crucial to understand the mechanism of water absorption in bamboo. This study used low-field NMR (LF-NMR) to analyze the quantitative spatiotemporal differences in water distribution between the nodes and internodes of moso bamboo. During the water loss process, the internodes' orderly vascular bundles created straight pores, leading to a higher free water relaxation time (T2 = 18 ms) than the nodes (T2 = 10 ms), with distinct separation of bound and free water peaks. The interwoven vascular bundles at the nodes increased pore heterogeneity, caused overlapping of water relaxation peaks, which resulted in a higher proportion of free water. The thermal treatment (200°C, 4h) reduced the free water relaxation peak area in the internodes from 45.61 to 19.37, decreased the moisture content by 20%-26%. Thermal treatment also lowered the moisture content of the bamboo nodes by 11%. This was primarily achieved by reducing hydrophilic hydroxyl groups and promoting lignin cross-linking to enhance hydrophobicity. FTIR spectra showed that after thermal treatment, the intensity of the O-H stretching vibration peak (3400 cm⁻¹) decreased, and the number of C = O and C-O groups reduced, confirming that thermal treatment altered the functional group composition to regulate the water absorption properties of bamboo.