Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 255

galactose depleted BC/xyloglucan nanocomposites while their stiffness was similar to
that of cellulose alone. Thus galactose substitution appeared to be a major determinant
of the nanocomposite structure and performance. Besides the lower molecular mass led
to lower xyloglucan incorporation and absence of cross-bridges in the composites. A
secondary role of the degree of fucolysation was also noted where fucosylation appeared
to promote the formation of a crosslinked BC/xyloglucan network. While the effect of
fucose substitution was not as important on the composite formation as the galactose
content, a modulating role was proposed (67). These studies thus clearly demonstrate
the power of biomimetic approaches for tailoring the morphology and performance of
BC nanocomposites when a host polymer capable of interacting closely with cellulose
is selected.

9.5.2 BC/Mannan Nanocomposites

Only a few studies have evaluated the effect of galactomannans and glucomannans on
the in vitro synthesis of BC/mannans nanocomposites, mainly focusing on their mor-
phology rather than their performance (50, 64). A wide range of morphologies has been
observed with mannans, glucomannans leading to more pronounced changes on BC
than galactomannans (64). With glucomannans highly heterogeneous nanocomposites
have been observed comprising (1) regions where cellulose and the glucomannans could
not be distinguished and (2) regions where cellulose ribbons were easily discerned and
crosslinked by thin strands of glucomannan chains and regions where a glucomannan
network formed within a cellulose network (Figure 9.22). Besides the presence of gluco-
mannans in the growth medium was found to drastically decrease cellulose crystallinity
while slightly increasing the relative abundance of I β form suggesting that glucoman-
nans may be present in the cellulose microfibrils. Detection of the glucomannans by
CP/MAS NMR supported the hypothesis that glucomannans exist in a rigid form or in an
extended 'cellulosic' conformation. Similarly to xyloglucans, glucomannans can asso-
ciate with cellulose microfibrils leading to loose microfibrils bundles having a lower I α
content (50).
With galactomannans a wide range of structures and morphologies has also been
observed depending on the galactose-mannose ratio; lower galactose content resulting in
higher incorporation of mannans into the nanocomposites. Crosslinking, self-associations
into gellike structures, cellulose ribbon alignment and self-association of galactomannans
close to cellulose structures and combinations of these different morphologies have been
observed (Figure 9.23). In contrast to glucomannans, galactomannans were not found
to disrupt cellulose crystallinity. Besides when added in large amounts to BC, galac-
tomannans were found to be rather mobile from SP/MAS and therefore considered to be
simply trapped rather than intimately associated in the cellulose fibers as glucomannans.
The lower galactose content galactomannans resulted in more self-association of galac-
tomannans. Cellulose fibers coalesced and the galactomannans formed a gel. Compared
to the xyloglucans the crosslinks were also more random in size thus explaining that
the long-range alignment of BC fibrils was not pronounced in these nanocomposites
compared to BC/xyloglucan nanocomposites. Altogether mannans led to less ordered

network structures compared to xyloglucans.