190 Introduction to Renewable Biomaterials
Table 6.2Summary of linkages between the monomer units that form
the individual polymer lignin, cellulose, and hemicellulose, and between
the polymers to form lignocellulosic biomass.
Polymeric intra bonds (bonds within different components)
Ether bond Lignin, (hemi)cellulose
Carbon to carbon bond Lignin
Hydrogen bond Cellulose
Ester bond Hemicellulose
Polymeric inter bonds (bonds connecting different components)
Ether bond Cellulose–lignin, hemicellulose–lignin
Ester bond Hemicellulose–lignin
Hydrogen bond Cellulose–hemicellulose,
cellulose–lignin, hemicellulose–lignin
6.2.1 Intrapolymeric Bonds x Contents
The most important bonds of lignin are C—C, ether, ester, and hydrogen bonds
(Table 6.2). Ether bonds may be formed between allylic carbons, aryl, or carbon atoms
of aryl–aryl and between two carbon atoms of allylic. Ether bonds formed about 70%
of total bonds between monomeric units in lignin. The C—C bonds form 30% of
remaining linkages between units (Kirk-Otmer, 2001).
Cellulose polymer is formed based on two main bonds:
1- Glucosidic bonds constitute the initial chain of a polymer. It isβ-1-4-d-glucosidic
bond that links glucose units together. Glucosidic bond is also considered as an ether
bond. Since it is in fact the connection of two carbon atoms with an elementary
oxygen interfering (Solomon, 1988)
2- Hydrogen bond is responsible for the crystalline structure of cellulose. Cellulosic
chains are long, straight, and parallel; as a result, they can connect together with
hydrogen bond that formed between two hydroxyl groups of different polymer
chains (Faulonet al., 1994). Indeed hydroxyl groups are evenly distributed on both
sides of the glucose monomer, and this leads to the long, straight, and parallel
structure of cellulose polymer.
It has been distinguished that there are carboxyl groups in cellulose (one carboxyl
per 100 or 1000 glucose units) (Krassig and Schurz, 2002). In hemicellulose, most
principal bonds are ether bonds (as glucosidic and fructosidic bonds) and there are
also carboxyl groups. Carboxyl groups can be found as carboxyl, ester, or salt in the
molecule (Kirk-Otmer, 2001). The main difference between cellulose and hemicellulose
isthatthehydrogenbondsareabsentinhemicellulosewhilethereisasignificant
amount of carboxyl groups.
6.2.2 Polymeric Inter Bonds
In order to distinguish and determine the connections and bonds that link different
polymers of lignocellulosic materials, first its structures should be broken and
