Cell wall water characterization in <i>Pinus sylvestris</i> and <i>Ulmus rubra</i> with TD-NMR

Abstract This study employed time domain nuclear magnetic resonance by using inversion recovery and Carr–Purcell–Meiboom–Gill pulse sequences to identify the relaxation properties of bound water in elm and pine wood under various relative humidity levels. Results showed that the sizes of micropores and mesopores in elm and pine increased with water accumulation. Furthermore, the sizes of micropores and mesopores in sapwood were larger than those in heartwood of both elm and pine wood. Hydroxyl bound water (OH bound water) molecules were tightly bound to the primary sorption sites of the polymers. More freely bound water occupied the void spaces between microfibrils within the cell wall. Elm wood exhibited higher T 1 /T 2 ratios for bound water than pine, suggesting that elm had less mobility than pine. Moreover, OH bound water exhibited higher T 1 /T 2 ratios than more freely bound water in pine and elm, indicating greater molecular mobility constraints and stronger interactions with the wood polymer. Notably, OH bound water mobility increased significantly with water accumulation.