Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 258

and cellulose was further demonstrated. Yet pectin appeared to be rigidified in presence
of the cellulose network as it aggregated around the rigid cellulose microfibrils (60).
The small deformation rheology of BC/pectin nanocomposites was very similar to
that previously observed with BC/xyloglucan nanocomposites. That is, the cellulose
network dominated the small deformation properties of the nanocomposite. For uniaxial
tensile loading the pectin had a much more dramatic effect than xyloglucans, increasing
the extensibility very significantly while decreasing the composite stiffness and strength
(Figure 9.20). As a result the nanocomposite toughness was much higher than that of the
pure components. While the performance of BC/pectin nanocomposites is reminiscent of
that of xyloglucan/BC nanocomposites, the change in the property was entirely ascribed
to the different cellulose structure that developed in presence of the pectin network
rather than to the pectin itself or to the cellulose/pectin interactions (60). Local isotropic
arrangement of the cellulose fibers into the pectin network caused cellulose fibers to have
less contact between each other's allowing for greater slippage and alignment of fibers
upon tensile loading and therefore higher extension. In fact, in composites comprising
20% pectin and conditioned at different relative humidity, 2D FTIR spectroscopy con-
firmed that the cellulose and pectin networks did not display any connected motion and
behaved independently as interpenetrating networks (58).
A 39% pectin content composite was further characterized for biaxial tensile load-
ing and creep. In biaxial tensile loading, the BC/pectin nanocomposite although it was
weaker and also had a slightly lower maximum elongation than the cellulose alone,
was found to behave as a linear elastic material similarly to cellulose (59). Again the
nanocomposite weakening was ascribed to the modified cellulose network that resulted
from cellulose deposition within a pectin gel. Similarly to cellulose, the BC/pectin
nanocomposites exhibited no creep i.e. time-dependent behavior. This was in sharp
contrast to the behavior of BC/xyloglucan nanocomposites which were prone to creep
(Figure 9.20). Differences in biaxial tensile behavior and in creep reflected the mor-
phological differences between the BC/pectin and BC/xyloglucan nanocomposites. That
is, the cellulose fibers deposited in a pectin gel possibly experienced less interfibrillar
contacts and were more able to align resulting in higher extensibility (58, 59). In these
nanocomposites cellulose was clearly the load bearing component while deposition of
cellulose within a network of pectin could help improve the extensibility and toughness
of the nanocomposites.

9.5.4 BC/Xyoglucan/Pectin Nanocomposites

To further shed light on the morphology and behavior of the primary cell wall, Gidley's
group also manufactured ternary nanocomposites comprising 63% BC, 22% pectin and
15% xyloglucan (59). The mechanical behavior of the BC/pectin/xyloglucan nanocom-
posites was characterized in uni-axial and bi-axial tensile loading and in creep (59).
The ternary nanocomposite combined the structural and morphological properties of the
BC/xyloglucan and BC/pectin nanocomposites. That is, the typical BC/xyloglucan mor-
phology was observed with intimately bound xyloglucans and crosslinks and a pectin
layer appeared to cover the tethered BC/xyloglucan structures (Figure 9.25). As in other
BC nanocomposites, BC was found to be the loadbearing component while xyloglucans
and pectin increased the material compliance, inducing higher extensibility and lower