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(a)
W2
W3
2.99
2.76
W1
2.60
2.99
2.51
W2
2.51
2.83
W3
2.95
2.43
W1
2.72
b
= 16.97 Å
(b)
W1
W1
W3
W2
2.43
2.95
3.09
2.50
2.75
W3
W2
2.76
2.83
2.91
Figure 1.5
Packing arrangement of hydrated (tendon) chitosan projected along the
a
-axis (a) and along the
c
-axis (b). Filled
circles denote nitrogen atoms. All hydrogen atoms are omitted, and hydrogen bonds are shown as dashed lines.
The letter “w” is the oxygen atom of the water molecule.
As shown in Figure 1.5 [56], each chitosan chain takes an extended twofold helix: in
other words, a zigzag structure. Chitosan chains on the
c
-axis are up chain, whereas those
in the unit cell are down chain. That is, in this crystal, chitosan chains are packed in an
antiparallel manner. Along the
b
-axis, the up chain and the down chain are bonded by
hydrogen bonds to make a sheet structure, and these sheets are stacked along the
a
-axis
(Figure 1.5b). Water molecules are present between these sheets and stabilize this crystal
structure. Because water molecules are included in this crystal, the tendon polymorph is
a hydrated crystal. This polymorph is the most abundant in chitosan samples; that is,
commercially available chitosan samples have this crystal although their crystallinity
is different.
When tendon chitosan was immersed in water and heated at around 200°C, the resultant
chitosan specimen gave another crystal. This crystal was called the annealed polymorph.
This change in chitosan crystal is an irreversible process.
Figure 1.6
shows the molecular
and crystal structure of annealed chitosan [57-59]. The chitosan chain along the
c
-axis is
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