Tissue-specific variation of softwood nanostructure revealed by scanning wide-angle X-ray scattering and clustering analysis

Abstract Understanding the properties of hierarchical materials such as wood requires combining information representing different length scales. Scanning wide-angle X-ray scattering (WAXS) connects the nanoscale cell wall structure to the scale of the annual growth rings, as demonstrated here by studying radially cut slices of mature softwood (Scots pine and Norway spruce), including the bark. WAXS offers information on the crystalline structure and other properties of the semi-crystalline cellulose microfibrils in the plant cell walls. Scanning-WAXS is data intensive but applicable to even laboratory X-ray sources, making it relatively accessible but underutilized. We present ways of efficiently analyzing the large data sets by robust analysis methods, principal component analysis (PCA) and clustering. We use PCA directly with two-dimensional scattering patterns, making the method highly applicable and easy to use. Scanning over wood samples containing xylem and bark, several different tissues were studied, such as xylem, phloem and outer bark, and scattering patterns for all these tissues are presented and compared. In our experiments, X-ray microtomography information was also used to verify and visualize the observations from the scattering data analysis. In the spruce xylem, the intra-annual variation of mean microfibril angle and crystallinity index was linear with respect to radial distance, whereas the density showed a steeper increase with radial distance, better modeled with a second order function. Scanning wide-angle X-ray scattering was shown to be able to connect different length scales and thus be able provide new information on the multi-scale structure of wood.