New Nanocomposite Materials Based on Cellulose Fibers and Other Biopolymers - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 158

better thermal stability than the corresponding unfilled chitosan films. The nanocom-
posites prepared with the water soluble chitosan derivative are particularly interesting
for future studies, since they have an attractive combination of properties, including a
high optical transparency.
Globally, the properties of CH/NFC nanocomposite films were better than those
displayed by similar chitosan films reinforced with BC Nano fibrils. This behavior
could be due to the better dispersion of NFC into the chitosan matrices, related to the
individual fiber morphology, contrasting with the tridimensional network fibers struc-
ture of BC, as well as to the higher aspect ratio of the NFC compared with BC.
The prominent properties of these nanocomposite films could be exploited for sev-
eral applications, such as in transparent functional, biodegradable and anti-bacterial
packaging, electronic devices and biomedical applications.
Cellulose-starch Composites
The preparation and characterization of TPS-based composites with different cellu-
lose substrates have been strongly explored, namely commercial regenerated cellulose
fibers (Funke, 1998), vegetable fibers (Alvarez, 2005; Curvelo, 2001; Funke, 1998;),
microcrystalline cellulose (Ma, 2008), microfibrillated cellulose (Dufresne, 2000) and
cellulose nanocrystallites (Weng, 2006). Apart from the enhanced mechanical proper-
ties of these reinforced TPS materials, a significant improvement in water resistance
is also obtained by adding cellulose crystallites (Lu, 2005) or microfibrillated cellu-
lose (Dufresne, 2000). In general, these TPS/fiber composites also displayed improved
thermal stability due to the higher thermal resistance of cellulose fibers (Curvelo,
2001). The preparation and characterization of bacterial cellulose-starch nanocompos-
ites has been also developed (Grande, 2008; Martins, 2009; Orts, 2005).
Plasticized starch/bacterial cellulose nanocomposite sheets have been obtained by
hot-pressing (Grande, 2008). The ensuing films were characterized in terms of their
morphology, whereas other important parameters, such as their mechanical and ther-
mal properties, were not investigated. The incorporation of bacterial cellulose microfi-
brils, obtained by the acid hydrolysis of the cellulose network, into extruded TPS and
starch-pectin blends was also studied (Orts, 2005). However, in this work, the pecu-
liarity of this cellulose substrate was not fully exploited, since the nano- and micro-
fibril three-dimensional network morphology was partially destroyed during the hy-
drolysis step. Indeed, the microfibrils derived from bacterial cellulose did not improve
the modulus to the same extent as cotton or softwood counterparts.
The work developed by Martins et al. (2009) described the preparation and char-
acterization of biocomposite materials obtained by the incorporation of bacterial cel-
lulose into a TPS matrix during the gelatinization process. Similar composite materials
reinforced with vegetable cellulose fibers were also prepared for comparison purposes.
The starch-BC nanocomposites were prepared in a single step with cornstarch by
adding glycerol/water as the plasticizer and bacterial cellulose (1 and 5% w/w) as
the reinforcing in a melting mixer. All nanocomposites showed good dispersion of
the nanofibers and a strong adhesion between the fibers and the matrix ( Figure 5) , as
well as improved thermal stability and mechanical properties. For example, the Young