184 Introduction to Renewable Biomaterials
Poplar sawdust Lignocellulose
MeO
MeO
OMe
OMe
HO OH HO
HO
OH
HemicelluloseOH
(xylan)
O O
O
O
O O
OMe
HO
O
O
O
O
O
OH
OH n
Cellulose
HO
HO
OH
OH
HOO O O OH
OHLignin
Lignocellulose
Lignin
Cellulose
Hemicellulose
Figure 6.1General structure of lignocellulosic materials.
H HOH
OH
OH
O
O
O
H H OH
H H
HO
H
O
HO H H
H OHH
OH
HO
H
O
HO H O
HH
OH
OH
OH
HO
OH HO
H H H
H
H
O
O
HO O
H
H
H
Figure 6.2Schematic structure of cellulose molecule.
is comprised of several parallel chains attached to each other (Faulonet al., 1994).
The schematic of the arrangement of the cellulose molecules in parallel chains and the
associated hydrogen bonding is presented in Figure 6.3.
The linear structure of the cellulose chain enables the formation of both intra- and
intermolecular hydrogen bonds resulting in the aggregation of chains into elemen-
tary crystalline. Fibrils of 36 cellulose chains form the structure of the elementary
crystalline fibrils. The structure of cellulose along with the intermolecular hydrogen
bonds increases cellulose tensile strength, makes it insoluble in most solvents, and is
partly responsible for the resistance of cellulose against microbial degradation. The
hydrophobic surface of cellulose leads to the formation of a dense layer of water that
may hinder diffusion of enzymes and degradation products near the surface of cellulose
(Kirk-Otmer, 2001; Jorgensenet al., 2007).
Cellulose is found in crystalline and the noncrystalline or amorphous structures. The
coalescence of several polymer chains leads to the formation of microfibrils, which in
turn are united to form fibers. By this way, cellulose gets a crystalline structure. Figure 6.4
shows the structure and the placement of cellulose in the plant cell wall.
Cellulose absorbs 8–14% water under normal atmospheric conditions (20∘C, 60%
relative humidity), and so it is a relatively hygroscopic material. On the other hand,
it is insoluble in water, where it swells, and it is also insoluble in dilute-acid solutions
