The effect of axial strain on crystalline cellulose in Norway spruce

Abstract

The effect of strain on dry, clear Norway spruce (Picea abies [L.] Karst.) wood was studied by tensile testing along the cell axis and by in situ X-ray diffraction measurements. The mean microfibril angle (MFA) was initially 3–12 degrees and did not decrease due to strain. Based on the positions of the reflections 200 and 004 of crystalline cellulose, cellulose chains elongated and the distance between the hydrogen bonded sheets of chains decreased due to the strain. The elongation of the unit cell parallel to the cellulose chains was twice as high in juvenile wood as in mature wood. The (X-ray) Poisson ratio νca for crystalline cellulose in Norway spruce was calculated from the deformation of the unit cell. The average νca of earlywood was 0.28 ± 0.10 in juvenile wood and 0.38 ± 0.23 in mature wood. In latewood, the average νca was 0.48 ± 0.10 in juvenile wood and 0.82 ± 0.11 in mature wood. The average νca values were not directly correlated to the crystallite dimensions or to the mean MFA in juvenile and mature earlywood and latewood. The results show that the amorphous matrix has a definite effect on the deformation of cellulose crystallites.