Variation of microfibril angles and chemical composition: Implication for functional properties

Abstract

Wood cell walls are a composite material consisting of cellulose, hemicellulose, lignin and other minor components, such as pectin and extractives. Two main factors, cellulose microfibril angle (MFA) in S2 layer (thickest layer) and chemical composition, govern functional properties of the cell wall such as Young’s modulus and growth-stress. Recently it was suggested that a change of MFAs resulted in the change of required mechanical properties such as stiffness or flexibility [1–3]. The relationship between growth-stress and MFA and chemical composition has also been discussed [4, 5]. Reaction wood is one of the adaptations required for mechanical balance in wood, which forms in leaning stems or branches. The reaction wood in softwood, compression wood, is found on the lower side of leaning stems or branches. It has a higher MFA and more abundant lignin than normal wood. On the other hand in hardwood, tension wood forms on the upper side. Typical tension wood has cellulose-rich G-layer, which consists of highly longitudinally orientated microfibrils. A branch is often displaced downward by its own weight or snow and ice. The function of the reaction wood in the branch is to maintain it in its proper orientation. In this study, change of MFA and chemical composition were investigated by small angle X-ray scattering (SAXS) and FT-IR. Samples used for SAXS and FT-IR were taken from a branch of Cryptomeria japonica (softwood) and Liriodendron tulipifera (hardwood) growing at the Wood Research Institute, Kyoto University. The softwood branch was cut into four disks, 30–60 cm distance from each other. The largest softwood sample had 10 annual rings, the second 8, the third 4 and the smallest 3. A hardwood branch was cut into 8 disks 40–60 cm from each other. 14 annual rings were found in largest disk. From the second to the eighth one, the number of annual ring was 12, 10, 9, 7, 4, 3, 2, 2, respectively. In both cases areas less affected by knots were chosen. 1 mm thick plates in the radial direction were prepared from