11 THE WATER VAPOUR SORPTION PROPERTIES OF CELLULOSE

摘要

1 INTRODUCTION The water sorption behaviour of cellulose is discussed in this chapter, both from the point of view of the equilibrium conditions that constitute the sorption isotherm and also the sorption kinetic behaviour. A model for explaining sorption hysteresis is presented and a possible link between sorption hysteresis and kinetic properties is examined. Although the specific example of cellulose is given here, the concepts presented herein have a very wide range of applicability to many natural materials that change dimensions in response to changes in atmospheric humidity. The water sorption behaviour of cellulose is dependent upon the presence of hydroxyl groups within the structure, but this is not the only factor. A critical consideration is the mechanical response of the material. It is well known that water molecules are not able to penetrate the microfibrils and that they are only able to associate with their surface and within the matrix that exists between the microfibrils, a region often referred to the 'amorphous' or 'paracrystalline' region of cellulose. Sorbed water acts as a plasticiser for the matrix macromolecules, which consequently affects the mechanical behaviour of the material. Furthermore, the mechanical properties of the materials can affect both the sorption kinetic behaviour and the sorption isotherms.