Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 174

state, and hence this chapter will discuss the topochemical and interfacial properties
of lignin.
The structure of lignin is based upon the polymerised phenylpropane unit linked
together through varied covalent bonds. The functions of lignin in the wood cell wall
are numerous with the aromatic rings as well as various substituents such as ether or
hydroxyl groups providing the molecular means for imparting strength and structural
integrity to the wood fibre wall often through covalent or nonspecific interactions with
the polysaccharides present (Sj ostr om 1993). For an analogy from materials science, if
a wood fibre is considered as a composite, the crystalline cellulose microfibrils would
be the load-bearing component of the fibre wall whilst lignin is the matrix material. The
highest proportion of lignin can be found in the secondary cell wall however the greatest
concentration is found in the middle lamella between wood fibres. This is exploited in
the chemical pulping of wood where specific reagents such as hydrogen sulphide and
hydroxide are used to target lignin usually at the ether linkages causing a degradation
of the polymeric structure and hence a liberation of the fibres. This suggests that lignin
aids in binding the fibres together providing strength to not only individual fibres on an
ultra-structural level but also to the wood macrostructure itself by effectively acting as a
glue between fibres. Furthermore, due to its inherent hydrophobicity from its aromatic
structure and low charge in the native state, lignin acts to inhibit the swelling of wood
fibres thereby waterproofing the cell wall and providing an efficient means for the trans-
port of water and nutrients throughout the plant vascular system. Another major function
of lignin is to impart protection to the plant cell wall against microbial attack. Lignin
acts to solidify or compact the cell wall. This makes the penetration of enzymes and
proteins secreted by bacteria and fungi into the cell wall for digestion of the polysaccha-
rides extremely difficult to achieve. Perhaps the best evidence for lignin acting to inhibit
the attack of microbes is the relatively slow degradation of more heavily lignified plant
materials such as wood compared to plants such as grasses which have significantly less
lignin. Indeed, much of the humus of soil is derived from lignin polymer fragments with
components such as humic or fulvic acids present due to oxidation reactions and not the
action of microbes.

7.2 Lignin Synthesis and Structural Aspects

As mentioned above, lignin is a highly branched amorphous biomacromolecule with
variable composition dependent on the plant source. However, lignin can be sim-
ply conceived as the polymerised product of the three basic substituted phenylpropane
repeat units known collectively as 'monolignols': p -coumaryl alcohol, coniferyl alcohol
and sinapyl alcohol as shown in Figure 7.1 (Sarkanen and Ludwig 1971, Freudenberg
and Neish 1968). As seen in Figure 7.1, the structure of the lignin monomers varies
only in the number of substituted methoxy groups on the aromatic ring. Whilst these
three monolignols account for the overwhelming majority of the repeat units mak-
ing up the lignin polymer molecules, other lignols may be present in much smaller
quantities. Furthermore, the proportion of each of these monomers in lignin varies
considerably depending upon type of plant material under consideration as shown in
Table 7.1. It is henceforth convenient to describe lignin in terms of its source. Softwood