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In-Depth Information
H
2
O
O
O
O
O
6
6
H
O
O
O
O
3
O
3
O
O
3
O
3
6
6
H
1
1
1
1
O
O
O
O
H
H
H
H
H
H
H
H
2
2
2
O
O
O
2
O
O
O
O
O
5
5
5
5
3
3
H
6
6
6
3
O
O
O
6
3
O
O
O
O
O
H
1
O1
1
O
1
O
O
H
H
H
H
H
H
H
H
2
2
2
2
O
O
5
O
5
O
5
O
O
O
5
O
H
6
O
O
O
O
O
O
3
3
6
H
O
O
O
O
Figure 9.7
Schematic representation of the principal network of hydrogen bonds
(I
p
) in the origin (left) and center (right) sheets of cellulose Ib. Hydrogen
bonds are represented by dotted lines. Only the oxygen atoms involved in
hydrogen-bonding have been labeled for clarity.
Reproduced from ref. 10 with permission of American Chemical Society.
chains. Langan et al.
15
have proved on the basis of neutron diffraction that
a systematic intramolecular three-center hydrogen bond is formed in both
chains with a three-center mode of O3-H3
O5/O6, that intermolecular
hydrogen bonds are formed between origin chains with a mode of
O2-H2
O6
0
and between center chains with a mode of O6-H6
O2
0
, and
that intermolecular hydrogen bonds are formed even between origin and
center chains with a mode of O2c-H2c
O2o as well as with a four-center
mode of O6o-H6o
O6c/O5c/O3c, where suxes ''c'' and ''o'' denote
''center'' and ''origin'', respectively. Such a network of hydrogen bonding
between chains in a sheet as well as in neighboring sheets substantiates well
the highest stability of cellulose II among the polymorphs.
Native cellulose is, as clarified above, an assembly of cellulose molecules
aligned orderly in sheets, based on a beautiful network of hydrogen bonding
with all the hydroxyl groups. Such hydrogen bonding is the fundamental
cause of mechanical as well as aggregation properties of cellulose as
described later. This also explains the extremely low reactivity of cellulose
because all the active sites are completely involved in a network of hydrogen
bonding.
9.3 Activation of Cellulose by Ball Milling
9.3.1 Micro-pulverization of Native Cellulose
Ball milling is considered to be a mechanical method to pulverize and
activate native cellulose in a solid state, because it is expected that reactive
hydroxyl groups can be liberated by collapse of the network of hydrogen
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