In situ measurement of tensile elastic moduli of individual component polymers with a 3D assembly mode in wood cell walls

We attempted to measure in situ the tensile elastic moduli of individual component polymers with a three-dimensional (3D) assembly mode in the cell walls of Sugi (Cryptomeria japonica D. Don) without isolating the polymers. To prepare wood tangential slices [50 × 6 × 0.2 mm (L × T × R)] consisting of lignin with a 3D assembly mode in the cell walls, cellulose and hemicellulose were removed using the method of Terashima and Yoshida (2006) to obtain methylated periodate lignin slices. To prepare wood slices consisting of polysaccharide with a 3D assembly mode in the cell walls, lignin was removed using the method of Maekawa and Koshijima (1983) to obtain holocellulose slices. Static tensile test was applied to determine the elastic moduli of 3D lignin and 3D polysaccharide slices. The followings were revealed. The elastic modulus of the 3D lignin slices was 2.8 GPa, regardless of the microfibril angle (MFA) in the slices. The elastic moduli of the 3D polysaccharide slices with MFAs of 14°, 23°, 34°, and 42° were 18, 12, 9, and 4 GPa, respectively. The former shows that the lignin with a 3D assembly mode behaves as an isotropic substance in the cell walls, while the latter suggests that the 3D polysaccharide slice shows marked anisotropic structure in the cell wall. Despite the fact that cellulose content increased after lignin removal, values of substantial elastic modulus of the cell wall slightly decreased regardless of MFA. Following two possible reasons were pointed out for explaining this phenomenon. First, lignin removal caused an artifactual deterioration in the polysaccharide slices at the level of macromolecular aggregate. Second, rigid and fusiform-shaped cellulose crystallites are dispersed in the soft matrix of amorphous polysaccharide, and those are loosely connected to each other by the intermediary of matrix polysaccharide. Those suggest that the rigid cellulose crystallite can optimize its strong mechanical performance in the polysaccharide framework of the wood cell wall in combination with the ligninification.