Supramolecular structure and performance of wood cellulose

木材是工程师最免费获得的可再生材料。它有潜力取代较难持续的材料的一个例子是英国木结构房屋的迅速扩张。木材和木材纤维素也是制造纸张和包装材料以及制造新一代生物纤维基复合材料的原材料。作为原材料的木材最大的问题是它的机械性能极不稳定。全世界都有兴趣对木材的机械性能进行建模，以便我们可以看到这种变异性是如何在树木的生长过程中产生的，以及我们如何最好地种植、选择和锯木来控制木材产品的机械性能。木材的大部分强度和硬度来自纤维素纤维或微纤维，它们约占木材质量的一半。纸含有更高比例的纤维素。显然，我们需要知道纤维素本身的强度和硬度，然后才能对它增强的材料做出任何预测。我们知道一些高度结晶的纤维素是这样的，但对于木材纤维就不是这样了。木材中的纤维素微纤维又长又细，只有大约30个分子厚。这些链状分子中约有三分之一构成了每个微原纤维的结晶核心。其余的则以一种组织得不太好的方式向外分布，人们对此并不太了解。这些表面纤维素链决定了纤维素微纤维如何相互粘连，以及如何与其他木材成分和水相互作用。通过对蔬菜等非木本植物的纤维素进行研究，我们发现了使用光谱学来确定微原纤维结构的方法，包括它们相对松散的表面区。我们将把这些方法应用于从锡特卡云杉木材中分离出来的纤维素，经过精心挑选，使纤维素分子几乎完全沿着纹理直线运行。我们还在开发一种新的光谱方法，即在红外显微镜下拉伸纤维材料。这使我们能够看到将纤维结构连接在一起的每一种化学键在载荷下是如何扭曲的，以及它对纤维整体硬度的贡献是什么。将这种方法应用于纤维素将告诉我们，除了其他事情外，有多少负载是由微纤维的晶体核心携带的，有多少负载是由更无序的外链携带的。根据这些信息，我们可以计算出木材纤维所需的机械常数。有一些处理纤维素的方法可以改变微纤维中结晶链和非结晶链的比例。我们从这些实验中获得的信息可能会让我们深入了解如何通过适当的加工来调整纤维素的机械性能。

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.