Ball-milling Promoted Chemical Bonding between Cellulose and Plastics - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 210

Hydrogen bonding is considered as a primary interaction even between a
pair of oligosaccharide chains as well as within each chain, in the double
strand, with the same definition of a hydrogen bond described above but
with an extended region of the angle O-H O of 90-1801. First, the intra-
molecular inter-residue hydrogen bonding in the double strands is also
characterized mostly by the mode of O3-O5 0 ; the averaged occupancies are
81% for c4d, 66% for c5d, and 88% for c6d, compared with those for the
single strands of 78-80%. The value of c4d is almost equal to that of c4s,
reflecting the separated state of the chains in c4d. A significant decrease in
the occupancy for c5d is probably due to the bending of a chain between
Ring 2 and Ring 3 (Figure 9.3). The double strand c6d exhibits the greatest
value of 88% among the six systems, as a result of the formation of a nearly
complete aggregation state (Figure 9.3). This means that a flat ribbon surface
in cellulose germinates already in cello-hexamer. It is also recalled that such
a O3-O5 0 mode of intramolecular inter-residue hydrogen bonding possesses
crucial significance for the firm structure of cellulose. 9 Secondary inter-
residue hydrogen bonds are O3-O6 0 and O2-O6 0 modes, with a greater oc-
cupancy for the former than for the latter, differing from native cellulose,
which predominantly has the O2-O6 mode due to the tg conformation y of
O6. Exchange between the tg and the gt conformation y at O6 is probably due
to frequent attacks on O6 atoms by water molecules around them still in the
double strands.
Intermolecular hydrogen-bonding can occur between the chains in the
double strands. The average occupancy of the hydrogen bonds increases
with DP, 0.64 (c4d), 2.93 (c5d), and 4.43% (c6d), which is consistent with
their aggregation state. Notably, almost all of these hydrogen bonds are
formed with O6: 59.0 (c4d), 84.1 (c5d), and 97.2% (c6d). Among the inter-
molecular hydrogen bonds with O6, the rate for a pair of O3 and O6 0 in-
creases the most significantly with DP, for example 10.7% (c4d) to 39.0%
(c6d), in line with the fact that native cellulose possesses dominant inter-
molecular hydrogen bonds between O3 and O6 0 . 10
Molecular dynamics simulation of the double strands of cello-oligo-
saccharide with DP ¼ 4-6 describes clearly their aggregation states with a
critical point at DP ¼ 5. While the double strand of cello-pentamers is in a
half-aggregated state, the double strand of cello-hexamers exhibits a com-
plete aggregation state stabilized through intermolecular hydrogen bonds
between O3 and O6 0 with an almost flat ribbon surface by intramolecular
inter-residue hydrogen bonds of O3-O5 0 . Even the double strand of cello-
hexamers represents fairly well the nature of native cellulose.

y

The tg and gt conformations are related with O6 in a D- glucopyranose. The tg conformation
denotes that O6 is in a trans and a gauche position with respect to O5 and O4, respectively,
around the C5-C6 axis, while for the gt conformation O6 is in a gauche and a trans position for
O5 and O4 in the same manner.