Effect of drying and rewetting of wood on cellulose molecular packing

Drying and rewetting of Pinus radiata sapwood latewood was studied by synchrotron wide angle X-ray scattering in transmission mode. Scattering from cellulose was interpreted in terms of chains distributed on a monoclinic lattice. Drying wood material to below the fibre saturation point resulted in decreased spacing between adjacent cellulose polymers within the hydrogen-bonded sheets of chains, and also decreased the monoclinic angle. The changes were partly reversed when the dried wood was rewet, but accumulated through multiple cycles of oven-drying and rewetting. No changes were observed in the fibre repeat distance, thus the distortion of the crystal lattice was not attributed to mechanical stresses associated with drying. Instead, the changes were attributed to formation of intrachain hydrogen bonds at dehydrated crystallite surfaces, causing conformational changes within the cellulose chains and increasing the density of packing. The results help account for the hysteresis observed in moisture desorption-adsorption isotherms during wood material drying and rehydration.