Effects of moisture imbibition on cellulose microfibril structures in paper and wood observed by in situ X-ray scattering

Absorption of liquid water is vital for many applications of cellulosic materials. However, studying the effects of water imbibition on the structure of hierarchical porous materials, such as paper and wood, is challenging. We used time-resolved synchrotron X-ray scattering to track water propagation and the associated nanostructural changes in paper and wood samples brought into contact with liquid water. The small-angle and wide-angle X-ray scattering data were fitted, revealing systematic correlations between the nanostructural parameters and moisture content. The time-dependent behavior of the paper samples indicated two wetting fronts with different time constants, assigned to water within fiber walls and between fibers. Changes in the nanostructure were mostly linked to faster intra-fiber water, with characteristic time scale around 1 s. The changes in the wood samples were considerably slower, taking place in the scale of minutes, and more complex to observe due to the heterogeneity of the samples. Systematic differences in wetting behavior of wood samples were observed according to their orientation, origin and drying. Nevertheless, nanostructural changes generally coincided with water absorption. The results elucidate water transport in hierarchical cellulosic materials and highlight the potential of X-ray scattering methods for studying such dynamic phenomena.