Variation of cellulose microfibril angles in softwoods and hardwoods - A possible strategy of mechanical optimization

Position-resolved small-angle X-ray scattering was used to investigate the nanostructure of the wood cell wall in two softwood species (Norwegian spru ce and Scots pine) and two hardwood species (pedunculate oak and copper bee ch). The tilt angle of the cellulose fibrils in the wood cell wall versus t he longitudinal cell axis (microfibril angle) was systematically studied ov er a wide range of annual rings in each tree. The measured angles were corr elated with the distance from the pith and the results were compared. The m icrofibril angle was found to decrease from pith to bark in all four trees, but was generally higher in the softwood than in the hardwood. In Norwegia n spruce, the microfibril angles were higher in late wood than in early woo d; in Scots pine the opposite was observed. In pedunculate oak and copper b eech, low angles were found in the major part of the stem, except for the v ery first annual rings in pedunculate oak. The results are interpreted in t erms of mechanical optimization. An attempt was made to give a quantitative estimation for the mechanical constraints imposed on a tree of given dimen sions and to establish a model that could. explain the general decrease of microfibril angles from pith to bark. (C) 1999 Academic Press.