Moisture-related changes in the nanostructure of woods studied with X-ray and neutron scattering

Wood and other cellulosic materials are highly sensitive to changes in moisture content, which affects their use in most applications. We investigated the effects of moisture changes on the nanoscale structure of wood using X-ray and neutron scattering, complemented by dynamic vapor sorption. The studied set of samples included tension wood and normal hardwood as well as representatives of two softwood species. Their nanostructure was characterized in wet state before and after the first drying as well as at relative humidities between 15 and 90%. Small-angle neutron scattering revealed changes on the microfibril level during the first drying of wood samples, and the structure was not fully recovered by immersing the samples back in liquid water. Small and wide-angle X-ray scattering measurements from wood samples at various humidity conditions showed moisture-dependent changes in the packing distance and the inner structure of the microfibrils, which were correlated with the actual moisture content of the samples at each condition. In particular, the results implied that the degree of crystalline order in the cellulose microfibrils was higher in the presence of water than in the absence of it. The moisture-related changes observed in the wood nanostructure depended on the type of wood and were discussed in relation to the current knowledge on the plant cell wall structure. Graphic abstract