Chemistry Reference
In-Depth Information
9.2.3 Polymorphism of Cellulose
Cellulose exhibits polymorphism, as expected from the possibilities of a
variety of networks of hydrogen bonding between the chains. So far, four
major types of polymorphs have been confirmed and denoted as cellulose I,
II, III, and IV. Cellulose I is the polymorph of native cellulose, and occurs in
plants and algae. In plants, polymer chains of cellulose, bio-synthesized, are
self-assembled to form cellulose microfibrils 3-5 nm in width and height
and several mm in length, which have been considered to have a parallel-
packed structure of 6
6 polymer chains of cellulose as shown schematically
in Figure 9.5. In addition, tunitin, which is a constituent forming the tunica
of Ascidiacea, and bacterial cellulose also exhibit cellulose I. Studies from
NMR spectroscopy
11
and electron diffraction
12
have clarified that native
cellulose is not simply a single form of cellulose I but strictly a composite of
two distinct crystalline forms, I
a
and I
b
. The I
a
form of cellulose is dominant
in bacterial and algal cellulose, while the other form is dominant in woods
and Ascidiacea.
10
The crystalline phase I
a
undergoes irreversible transfor-
mation into the phase I
b
,
13
and the latter is considered to be more stable
than the former.
14
Cellulose II is the phase of regenerated cellulose. It is well known that
rayon is a regenerated cellulose produced from wood pulp or cotton linter.
Native cellulose is transformed into the cellulose II phase by treatment with
ca. 20 wt% NaOH solution, i.e., mercerization. Such transformation is also
possible by dissolution in a solution of carbon disulfide or Cu(
II
)-ammonia
complex followed by regeneration in dilute sulfuric acid solution. These
extreme treatments cause a drastic change in morphology of native cellulose,
Β(1,4)-bond
Cellulose molecule
Cellulose crystal
3 - 5 nm
6
6
Cellulose microfibril
Figure 9.5
Schematic drawing of cellulose microfibril.
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