Imaging of the helical arrangement of cellulose fibrils in wood by synchrotron X-ray microdiffraction

A complete image of the helical arrangement of cellulose fibrils in the S2 layer of adjacent wood cells of Picea abies (Norwegian spruce) was obtained by applying position-resolved synchrotron X-ray microdiffraction on cells in cross section. In contrast to conventional fiber diffraction studies, th e incident beam was parallel to the longitudinal cell axis, resulting in a glancing angle mu far from 90 degrees with respect to the cellulose fibrils . This special choice of diffraction geometry allowed us to take advantage of an asymmetry effect in the two-dimensional diffraction patterns arising from the curvature of the Ewald sphere to obtain information on the local o rientation of the cellulose fibrils. The small size of the beam, smaller th an the thickness of a single cell wall, allowed mesh scans over intact tran sverse sections of adjacent wood cells with a microscopic position resoluti on. The scan yielded a map of diffraction patterns that could readily serve as a microscopic image. Each of the diffraction patterns was then used to evaluate the local orientation of the cellulose fibrils at the actual beam position. The combination of these results gave an image of cellulose fibri ls forming (Z) helices in several adjacent wood cells.