Viscoelasticity of wood under humidity variation - modelling the effect of transient hydrogen bonding Created by Roger Vauthier on 22/02/2011 07:59:24 Mechanosorption is a complex phenomenon and its underlying mechanisms remain poorly understood. In an attempt to provide insight into these mechanisms, we have modelled numerically the effect of continuous breaking and reforming of hydrogen bonds at the molecular level, based on a concept proposed by Gibson (1966). In the model, the structure of the wood is idealized as a periodic fibre-reinforced composite, whose reinforcing elements are extended chains of crystalline cellulose embedded in matrix of amorphous cellulose. The super cell used in the simulation was constructed from 24 unit cells of cellulose and an equivalent number of "amorphous" sequences, which were lightly cross-linked to prevent crystallization during subsequent relaxation. Dynamics simulations were carried out using the Cerius 2 simulation software package (Molecular Simulations Inc.). Periodic boundary conditions were applied and the structure minimised and relaxed at 350 K and at constant pressure for 100 ps prior to application of a stress. Where required, the water (5-10 wt% overall) could be made to diffuse in and out of the super-cell by placing a sink at either end of the amorphous layers. However, even given the reduced size of the model, this required too long a computation time for a systematic parametric study of the coupling between diffusion (moisture variation), and the mechanical response of the system to be possible. The investigation was restricted to a temperature of 350 K and to relatively large shear stresses acting parallel to the chain direction in the plane of the layers, such that the effective slip planes were perpendicular to the hydrogen bonding direction in the crystallites. Time constraints limited not only the size of the model but also the effective duration of the dynamics simulations, so that realistic simulations of creep measurements under the usual experimental conditions were excluded. Therefore, whilst the results suggested that the transient nature of the hydrogen bonds and the slow relaxation rate of the polymer can promote slip between the crystalline and amorphous cellulose layers in a unidirectional composite, this needs to be confirmed using more realistic models.
