Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 261

compatibilizer between cellulose and DHP. However with solid-state NMR no covalent
bond between lignin and pectin could be determined (63) suggesting that noncovalent
bonds might govern the association of DHP with pectin. At the nanometer scale however
the pectin and the lignin were found to phase separate (63). With thiadoacydolysis, it
was shown that presence of pectin influenced the polymerization of coniferyl alcohol
since lignin had higher amounts of β -O-4 linkages (62).

9.5.6 BC/Synthetic Polymer Nanocomposites

A couple of studies have examined the reinforcing potential of BC for polyethylene
oxide (PEO) in nanocomposites prepared in vitro (68, 70). Takai first reported on incor-
porating PEO into BC fleeces, among other water soluble polymers including cellulose
derivatives, to form nanocomposites for application as membranes (70). Incorporation of
a 50,000 g/mol PEO into the BC fleeces caused a 10% wt gain, lower than that observed
with other cellulose derivatives. The BC/PEO nanocomposite displayed a similar modu-
lus than the neat BC at around 25 GPa, which was not the case of other nanocomposites
with modulus all below 10 GPa. A similar bioengineering approach was taken in another
study in which BC was grown into media enriched with various PEO contents with a view
to tailoring nanocomposite composition, morphology and properties (68). The unpurified
nanocomposites were freeze dried and compression molded into films for characteriza-
tion by thermogravimetric analysis (TGA), atomic force microscopy (AFM), infrared
spectroscopy (FTIR), differential scanning calorimetry (DSC) and dynamic mechani-
cal analysis (DMA). As expected, increasing PEO concentration in the growth medium
resulted in a systematic change in composition with the BC: PEO ratio varying from
59:41 to 15:85. The nanocomposite morphology also changed with addition of PEO. In
compression molded samples, the BC fibers appeared to aggregate in larger bundles as
the PEO content increased (Figure 9.27).
Fine dispersion of the cellulose fibers into the PEO matrix was further demonstrated
by the significant drop in the melting temperature (Tm) and crystallinity of PEO. In
presence of BC fiber, less stable PEO crystals might have nucleated on the fiber surface
and crystal growth might have been impinged by the finely dispersed BC fibers resulting
in a low Tm and crystallinity matrix. This fine mixing of cellulose fibers in PEO was
also consistent with the indication of hydrogen bonding between cellulose hydroxyls
and the PEO oxy group. Finally, selected physical, thermal and mechanical properties
could also be tailored from the nanocomposite composition. The thermal decomposition
temperature of PEO increased by 15 ◦ C in the nanocomposites, and this increase was
ascribed to mutual thermal stabilization with cellulose. DMA in tension mode showed
that BC effectively reinforced the PEO matrix in the glassy and rubbery states and most
significantly above the melting temperature of PEO (Figure 9.28). Surface roughness
also decreased with higher PEO content. Integrating the BC synthesis with the mixing
step with a thermoplastic polymer was thus proposed to be a potent manner to manip-
ulate the properties of BC/PEO nanocomposites. A similar approach was taken with
poly(vinyl alcohol) (PVA), demonstrating the potential to manipulate composition and
selected properties of BC/PVA nanocomposites as well (69).
In an attempt to control the water absorption potential of BC materials for biomedical
applications, Seifert et al . (29) also used in vitro synthesis of BC in the presence of