Biomedical Engineering Reference
In-Depth Information
region is called a microcrystal (also called a micel or micella). Since free hydroxyls
at position 2, 3, and 6 of glucosyl in cellulose microcrystal regions have formed
hydrogen bonds, only amorphous regions contain some free hydroxyls.
2.3.3
Physicochemical Properties of Cellulose
2.3.3.1
Chemical Properties of Cellulose
Every glucosyl ring of cellulose has three active hydroxyls: one primary hydroxyl
group and two secondary hydroxyl groups. Thus, cellulose may have a series
of chemical reactions related to hydroxyl. However, these hydroxyl groups also
can form hydrogen bonds between molecules, which has a profound influence on
the morphology and reactivity of cellulose chains, especially the intermolecular
hydrogen bond formed by oxhydryl at C3 and oxygen at an adjacent molecule ring.
These hydrogen bonds not only can enforce the linear integrity and rigidity of the
cellulose molecule but also can make molecule chains range closely to form a highly
ordered crystalline region [
10
]. The accessibility of cellulose refers to the difficulty
reagents have in arriving at the cellulose hydroxyl. In heterogeneous reactions,
the accessibility is mainly affected by the ratio of the cellulose crystalline regions
to the amorphous regions. The reactivity of cellulose is the reactive capability of
the primary hydroxyl and the secondary hydroxyl at the cellulose ring. Generally,
because of the smallest steric hindrance, the reactivity of the primary hydroxyl
groups is higher than for the secondary hydroxyl groups, so the reactivity of
hydroxyl at C6 with a bulky substituting group is higher. For example, esterification
of toluenesulfonyl chloride chiefly occurs in the primary hydroxyl. The reversible
reaction occurs mainly in the hydroxyl group at C6, and an irreversible reaction
always occurs in the hydroxyl group at C2. Thus, for the esterification of the
cellulose, the reactivity of the hydroxyl group at C6 is the highest, but for the
etherification, C2 is the highest [
10
].
The degradation of cellulose is an important reaction that can be used to
produce cellulose products. Acid degradation, microbial degradation, and alkaline
degradation are mainly to break the glycosidic bonds between two adjacent glucose
molecules; an alkali peeling reaction and oxidation-reduction reaction of cellulose
usually act on reducing ends of celluloses, and the oxidative degradation of the
cellulose occurs mainly in dissociating hydroxyls at C2, C3, and C6 of the glucosyl
ring. Cellulose molecule chains will form carbonyls at C2 when oxidized to some
degree and then be degraded in the following alkali treatment process by the
elimination reaction of
-alkoxy. After disconnecting the glycosidic bond, the
reaction product is formed and then degraded to a series of organic acids [
9
].
Esterification and etherification reactions of cellulose act on three alcoholic
hydroxyls of cellulose molecule monomer. They can greatly change the properties
of cellulose, thereby producing many valuable derivatives of cellulose, such as
sulfonic ester, cellulose acetate, cellulose nitrate, and cellulose ether (carboxyl
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