Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 250

group (57-61) and by a few other researchers (30-32, 62, 63) clearly illustrates the
potential of biomimecry for the development of novel biomaterials that are completely
based on natural resources. Indeed Gidley group's has extensively used the in vitro
assembly of BC with other cell wall polymers in order to mimic the biosynthesis of the
primary cell wall (57-61). In so doing a wide range of BC/hemicellulose nanocomposites
have been developed and characterized for morphology, physical and mechanical prop-
erties (57-61). In particular BC nanocomposites have been produced with mannans (64),
xyloglucans (57-59, 61, 65-67), pectins (58-61), xyloglucans in conjugation with pectins
(59) and xylans (30-32). Lignin precursors have also been polymerized in situ in order to
produce BC/lignin nanocomposites (62, 63). The structural, morphological and mechan-
ical features of these bio-based nanocomposites manufactured in vitro are reviewed next.
A similar biomimetic approach has been taken to develop a few nanocomposites with
synthetic polymers and these developments are also reviewed (68-70).

9.5.1 BC/Xyloglucan Nanocomposites

Whitney et al . first demonstrated that the in vitro assembly of cellulose/xyloglucan
networks could result in the formation of model composite materials that closely resem-
bled the primary cell wall of plants (65, 66). Xyloglucans are β -1,4-linked glucose
residues to which α -1-6 linked xylose residues are attached. Some of the xylose residues
are substituted with galactose and focusylgalactose (71). Composites were thus pro-
duced by modifying the incubation medium of acetobacter xylinum with 0.5% tamarind
xyloglucan and growing BC under agitated conditions. The deep-etch, freeze frac-
ture transmission electron microscopy of washed composite materials revealed that the
xyloglucans imparted some degree of lateral order in the BC ribbons which contrasted
with the randomness of ribbon orientation in control BC (Figure 9.18). Besides thin
strands of xyloglucans, 20-70 nm long, bridged the cellulose ribbons, whose widths
(36 nm) were similar to those of BC. Comparison of the cellulose:xyloglucan ratio of
1:0.38 with the measured level of cross-bridges further suggested that the majority of
the xyloglucans (33% of the total 38%) present in the nanocomposites might be inti-
mately associated with the cellulose ribbons, either surface bound or interwoven in
the fibers and ribbons. Furthermore, the combination of cross-polarization (CP/MAS)
and single pulse excitation with dipolar decoupling (SP/MAS) NMR techniques con-
firmed the existence of two types of xyloglucans in the nanocomposites with the evidence
of both rigid and mobile xyloglucan domains. The former domain was ascribed to the
xyloglucans that are intimately associated and oriented parallel to the cellulose microfib-
rils and the latter domain corresponded to the xyloglucans in the cross-bridges (65). In
presence of xyloglucans, BC developed with less crystallinity and with a significantly
lower I α content and higher I β content compared to the control BC, confirming molecu-
lar association of xyloglucans with cellulose (65), a phenomenon that was not observed
in another study (55). Additional analysis of the crystal structures in BC/xyloglucan
nanocomposites by small-angle X-ray diffraction (XRD) and environmental scanning
electron microscopy (ESEM) concluded that during in vitro synthesis of the nanocom-
posites, xyloglucans get entrapped in the less dense shell of the ribbons (57). In contrast,
composites created by association of xyloglucans and bacterial cellulose as an abiotic