Tensile Stress Enhances Moisture Sorption by Increasing Cellulose Hydroxyl Accessibility in Wood Cell Walls.

Mechanical loading coupled with moisture sorption causes mechano-sorptive creep in wood, but its molecular origin remains unclear. Here, DMA, in situ tensile FTIR, SAXS/WAXS, and MD/GCMC simulations were combined to examine molecular and nanostructural changes in wood cell walls during sorption under tensile stress. Adsorption under stress produced a higher moisture content than stress-free adsorption, whereas viscoelastic creep at constant relative humidity caused no additional uptake. The enhancement was most evident when the loading was approximately parallel to cellulose microfibrils. FTIR revealed changes in cellulose hydroxyl environments consistent with increased accessibility, while SAXS/WAXS showed enlarged interfibrillar distances with nearly unchanged crystalline lattice spacings. Simulations further indicated reduced polymer-polymer hydrogen bonding, greater solvent-accessible surface area, and pore opening during simultaneous adsorption and loading. Together, these findings link tensile-stress-induced cellulose hydroxyl accessibility to enhanced moisture sorption in the wood cell walls.