Combined scattering analysis and atomistic simulation of wood-water interactions

Wood and other cellulosic materials are highly sensitive to moisture, which affects their use in many applications. The water interactions of the nanoscale constituents of the plant cell walls are especially important— most notably those of cellulose microfibrils and the closely associated hemicelluloses. Details of the polysaccharide assembly and its moisture response remains an important question, both for an understanding of the natural system and from a technological viewpoint.

X-ray and neutron scattering techniques are an efficient and non-destructive way to study the moisture response of wood. The scattering data carries information on the average nanostructure within a sample: the size, shape and spacing of the microfibrils, and the molecular-level organization of the polysaccharides. Unfortunately, the interpretation is not straightforward, and often requires model fitting that accounts for the basic geometries of the fibril assembly. Stronger evidence in support or against a given interpretation is obtained by comparing the measured patterns against those computed from atomistic models.

We carried out small and wide-angle X-ray scattering (SAXS/WAXS) experiments on spruce wood samples under controlled humidity. We recorded scattering patterns throughout a relative humidity cycle from wet state to dry and back, and performed dynamic vapor sorption measurements to determine the moisture content at the different humidity levels. To assist the data analysis, we constructed a series of atomistic models based on recent literature on the cell wall nanostructure. The models represent microfibril bundles with bound hemicelluloses.

Our measurements display systematic changes in both the crystal structure of the microfibrils and their spacing. We find that the atomistic models can predict the inter-fibril spacing as a function of moisture content. We use the models to study the sensitivity of the WAXS pattern to the shape and size of the microfibrils, the co-crystallization of the bound hemicelluloses, and the twisting of the microfibrils.