Fundamental Science and Applications for Biomaterials 43
Figure 2.3A representation of the
unit cell mode of chain packing for
cellulose. (a) The triclinic unit cell (Iα).
(b) The monoclinic unit cell (Iβ).
c
b
a
c
b
γ
a
(a) (b)
has vectors of unequal length, but they form a rectangular prism with a parallelogram – a
base indicating that two vectors are orthogonal, but the third angle,훾,> 90 ∘.^3
2.2.2 Heteropolysaccharides
Akin to cellulose in form, but not in function, are the heteropolysaccharides (or
“hemicelluloses,” a more colloquial, but inexact term for this class of biomaterials)
[4, 5]. This class of biomaterials is likely the second most dominant terrestrial material
available after cellulose. In general, the amount of hemicelluloses is typically 20–30%
of the dry weight of wood. They constitute a variegated class of materials without
the chemical precision and homogeneity of cellulose, yet they are naturally produced
annually in the order of 60 billion tons. This class of biomaterials is almost mandatorily
associated with cellulose in any cellulosic matrix. They act in general to maintain the
physical integrity of the cellulosic microfibrils and likely engage in a covalent complex
(the lignin–carbohydrate complex, or LCC) with the lignin polymer of the plant cell
[6]. They adopt a multiplicity of structural motifs, the most popular form being “xylan,”
which is likely the third most abundant biomaterial on the planet [7]. Figure 2.4 shows
several representations of xylan, a typical and globally dominant heteropolysaccharide.
As shown in Figure 2.4, heteropolysaccharides are very much unlike cellulose in a
structural sense. Cellulose is crystalline, that is, well organized or packed, of a high
molecular weight, and possesses a low polydispersity.^4 However, heteropolysaccharides
tend to be rather amorphous in their structure (often they are branched as shown in
Figure 2.4), of a low molecular weight, and display a low PDI. In fact, the heteropolysac-
charides typically do not have just a single repeating unit (such as xylan) but can often
contain a multiplicity of monomers. For example, the dominant heteropolysaccharide
is xylan, which tends to be fairly localized in angiosperms (hardwoods), whereas
the dominant heteropolysaccharide in gymnosperms (softwoods) is glucomannans.
3 For more information on the theory and technical aspects of X-ray crystallography and crystal lattices, the
reader is encouraged to obtain and inspect a free copy of “Elements of X Ray Diffraction” by B.D. Cullity
(1956) from http://archive.org/details/elementsofxraydi030864mbp.
4 The polydispersity or polydispersity index (PDI) is a measure of the distribution of molecular masses in a
polymer sample. It is calculated by obtaining the quotient of the weight average molecular weight (MW)
divided by the number average molecular weightnMn). It is a reliable metric for the “purity” or uniformity of
the chain length of the polymer in the sample.
