Supramolecular structure and performance of wood cellulose

Wood is the renewable material that most freely available to engineers. One example of its potential to substitute for less sustainable materials is the rapid expansion of timber-frame housing in the UK. Wood and wood cellulose are also the raw materials for paper and packaging materials and for the manufacture of a new generation of biofibre-based composite materials. The biggest problem with timber as a raw material is its extreme variability in mechanical performance. There is worldwide interest in modelling the mechanical properties of timber so that we can see how this variability arises during the growth of trees, and how we can best grow, select and saw timber to control the mechanical performance of timber products. Most of the strength and stiffness of wood comes from cellulose fibres or microfibrils, which comprises about half its mass. Paper contains an even higher proportion of cellulose. Obviously we need to know how strong and stiff cellulose itself is before we can make any predictions about the materials that it strengthens. We know this for some very highly crystalline forms of cellulose but not for wood cellulose.Cellulose microfibrils in wood are very long and thin, only about 30 molecules thick. About one-third of these chain molecules make up the crystalline core of each microfibril. The rest are distributed round the outside in a less organised way that is not well understood. These surface cellulose chains determine how cellulose microfibrils stick to one another and interact with other wood components and with water. Working with cellulose from non-woody plants like vegetables we have found ways of using spectroscopy to determine the structures of microfibrils, including their relatively disorganised surface regions. We will apply these methods to cellulose isolated from Sitka spruce wood, carefully chosen so that the cellulose molecules run almost exactly straight along the grain. We are also developing a new spectroscopy method in which fibre materials are stretched under an infrared microscope. This allows us to see how each kind of chemical bond holding the fibre structure together is distorted under load, and what contribution it makes to the stiffness of the fibre as a whole. Applying this method to cellulose will tell us, amongst other things, how much of the load is carried by the crystalline core of the microfibrils and how much is carried by the more disorganised outer chains. From this information we can calculate the mechanical constants that we need for wood cellulose.There are ways of processing cellulose that alter the proportions of crystalline and non-crystalline chains in the microfibrils. The information that we get from these experiments may give us insights into how the mechanical performance of cellulose might be tailored by appropriate processing.