Ball-milling Promoted Chemical Bonding between Cellulose and Plastics - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 207

malto-oligosaccharides are determined primarily through different modes of
intramolecular inter-residue hydrogen bonding, supported by relevant
hydrations around glycosidic linkages with double hydrogen-bonded water
molecules. Thus, cello-oligosaccharides have a relatively flat conformation
with more rigidity, in contrast to malto-oligosaccharides, which have a more
helical conformation with more flexibility. Considering a higher solubility
of amylopectin primarily due to the flexibility of conformation at a(1 6)

linkage points, 7 cello-oligosaccharides with the rigid confirmation may
exhibit self-aggregation.

-

9.2.2 Aggregations of Cello-oligosaccharides
Cello-oligosaccharides are less soluble in water than malto-oligosaccharides,
and almost insoluble with DP Z6, indicating a ready aggregation of cello-
oligosaccharides through intermolecular hydrogen bonding. Umemura
et al. 8 examined intermolecular interactions between cellotetraoses (DP ¼ 4),
cellopentaoses (DP ¼ 5), or cellohexaoses (DP ¼ 6) on the basis of molecular
dynamics simulation of each double strand with 5000 TIP3P water molecules
for 1 ns. Each double strand consists of a pair of cello-oligosaccharide chains
initially in parallel, on the basis of the crystal structure of cellulose I. The
double strands are denoted as c4d, c5d, and c6d for cellotetraose, cello-
pentaose, and cellohexaose, respectively. The chain is numbered from the
reducing glucose residue to the nonreducing glucose residue, like Ring 1 for
the residue at the reducing end to Ring 6 for the residue at the nonreducing
end in cellohexaose. Figure 9.3 shows the average structures of the three
double strands in the initial and final 100 ps, compared with those of the
three single strands, c4s, c5s, and c6s, in the last 100 ps, respectively, which
are identical to relevant conformations in Figure 9.2. Close examination
reveals that the conformations of chains in the double strands are closer to
that in cellulose I than are those in the single strands. Notably, the double
strands are separated for cellotetraose, in a half-aggregation state for cello-
pentaose, and in a complete aggregation state for cellohexaose. From the
time evolution of the distances between a pair of corresponding rings in the
double strands, the aggregation states of c5d as well as c6d are maintained
over 1 ns (apart from the initial 100 ps), while the chains of c4d separate
from each other finally by more than 2.0 nm. In c5d, the distances are
relatively unstable, particularly at Ring 1, with fluctuations of 0.48-1.0 nm.
Of these three double strands, c6d exhibits the most stable state of aggre-
gation with the smallest distances around 0.48 nm (except Ring 6). A specific
separation from Ring 1 in c5d remains unresolved, and requires further
considerations on the simulation. Nevertheless, the simulation results show
clearly that the aggregation of cello-oligosaccharides in water depends on
the degree of polymerization, with a critical point around DP ¼ 5, which is
very consistent with experiments.
The diffusivity of an isolated solute in water depends on interactions of
the solute with water molecules around it. The self-diffusion coecient D