Properties of the cell wall constituents in relation to the longitudinal elasticity of wood - Part 2: Origin of the moisture dependency of the longitudinal elasticity of wood

This study examined the origin of the moisture dependency of the longitudinal Young's modulus of wood (E-L) in relation to the microfibril angle (MFA) of the S2 layer of the secondary wall. Microtomed early wood specimen of sugi (Cryptomeria japonica D.Don) were used for the experiment. The following was revealed: 1.EL tends to decrease as the moisture content increases in the region below the fiber saturation point (FSP). 2.The percentage reduction of EL from the oven-dried state to the FSP is almost constant regardless of the MFA. Subsequently, the relationship between E-L and the moisture content was simulated theoretically using the simplified wood fiber model proposed in our previous paper (Part 1, 2002). The simulation considered the two hypotheses proposed in Part 1 for the origin of the moisture content dependency of E-L. The first is a traditional theory that the reduction of E-L is caused mainly by the moisture dependency of the lignin-hemicellulose matrix. The second assumes that an intermediate domain exists between the rigid crystal and the compliant disordered amorphous regions in wood cellulose microfibril (CMF). It is assumed that such a domain fluctuates between the rigid crystal-like and the compliant amorphous-like states at which the elastic modulus is of the same order as the lignin-hemicellulose matrix in accordance with the moisture sorption. When the first hypothesis is adopted for the simulation, the percentage reduction of E-L from the oven-dried state to the FSP should increase as MFA increases; this was contradicted by the experimental results (2). On the other hand, when the second hypothesis is applied to the simulation, the experimentally obtained results (1) and (2) are simulated reasonably. This suggests that the moisture dependency of E-L is controlled by the second hypothesis.