Chemistry Reference
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due to a crucial change in alignment of the chains from a parallel type to an
anti-parallel one, as described below. The transformation from the phase I
into the phase II is irreversible, because of the much greater stability of the
latter phase than the former. Another phase, phase III, is generated by
treatment of native cellulose with liquid NH
3
, without an extreme change in
morphology. Phases I and II are transformed into III
I
and III
II
, respectively.
The structure of phase IV remains unclear. Figure 9.6 describes a simplified
relation between the polymorphs of cellulose with respect to phase trans-
formation and alignment of the chains; further understanding of cellulose
requires crystallographic characterization, in particular of cellulose I and II.
The ordered packing of cellulose chains has been steadily revealed by
intensive studies for nearly 100 years, and some important results are con-
cisely described here. Table 9.1 lists crystallographic data of cellulose I
b
10
and II.
15
With the c axis as the unique axis, the space group of P2
1
requires
two types of twofold screw axes parallel to the c axis: one through the origin
and the other through the center point on the ab plane of a unit cell.
16
Ac-
cordingly, the symmetry operations generate two independent chains at the
origin and the center of the cell, both parallel to the c axis; and origin and
center chains can form two relevant sheets parallel to the bc plane, in-
dependent of each other. The directions of the reducing ends of the origin
chains are arranged to be consistent with that of the c axis.
For cellulose I
b
, center chains are aligned in a sheet with the same dir-
ections as that of the origin chains,
10
with a shift of ca. 1/4c as compared
with the origin chains, so as to avoid repulsive interactions between the
origin and center sheets. Cellulose chains in an origin and a center sheet
are not equivalent, having slightly different values of F and C (Table 9.1),
with both holding a flat ribbon shape. In addition, all the hydroxymethyl
groups are in a tg conformation, and each can readily form a hydrogen bond
with the hydroxyl group at the second place, O2-H2, in the adjacent residue
of the same chain. According to Langan et al.,
10
interestingly, the hydrogen
atoms linked to O2 and O6 take more than one location, in contrast to
that linked to O3, which occupies a well-defined localized position, causing
two distinct networks of hydrogen bonding in each sheet: I
p
and I
d
for
the principal and disordered positions of both H2 and H6, respectively.
Cellulose I
Cellulose I
α
β
Parallel
Cellulose III
I
Cellulose III
II
Cellulose II
Antiparallel
Cellulose IV
Figure 9.6 Cellulose polymorphs and their transformations.
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