Cellulose and Chitin as Nanoscopic Biomaterials - The Nanoscience and Technology of Renewable Biomaterials - page 209

are chemically similar, and both serve a natural role in structural reinforcement for
many plant and animal organisms. By mimicking nature, the materials industry might
benefit from enhanced utilization of these feedstocks. Useful properties of chitin and
cellulose are their high stiffness and strength, low density ( 1.35 g/cm 3

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for chitin

1.6 g/cm 3 for cellulose compared to 2.6 g/cm 3 for glass fiber), biodegradability,
renewability, inherent nanoscale architecture, coupled to an abundance of opportunities
for chemical functionalization. The elastic modulus of cellulose whiskers is reported
to approach 145 GPa (10) while that of glass fibers averages around 70 GPa (11),
suggesting that composites reinforced with cellulose nanoparticles may be superior to
conventional glass fiber reinforced composites. The issue of filler density also becomes
very important when trying to maximize the strength-to-weight ratio for lower ship-
ping costs of materials and reduce fuel consumption when the materials are used in
transportation.
For thousands of years cellulose fibers in the form of straw have been used by people
to mechanically reinforce mud or clay to create adobe bricks for the construction of their
dwellings. These early composites were the first precursors of the present day thermo-
plastic cellulose-based nanocomposites. Thermoplastic nanocomposites reinforced with
cellulose or chitin whiskers derived from sources such as wood pulp, straws, bacteria,
and bagasse, for cellulose, and shrimp, crab, or lobster shells, for chitin, have shown
promising results. In a few particular cases, a 2- to 3-order of magnitude improvement
in modulus of the composites was observed at low filler loadings of cellulose or chitin
(3, 4). Much of the work in nanocomposites reinforced with highly crystalline polysac-
charide nanoparticles has made use of synthetic, petroleum-based matrix polymers that
are largely nonbiodegradable after their usable lifetimes.
The design and utilization of green processes and sustainable materials is relevant
to the urgent need to develop technologies that minimize our dependence on petroleum
feedstocks and to concerns regarding the management of excessive amounts of municipal
solid-waste generated by an expanding human population. Animal, plant, and microbial-
based biopolymers, and derived materials, such as biocomposites, are promising alter-
natives to currently employed petroleum-based plastics. To effectively compete with
existing products, this class of materials still faces challenges to their widespread uti-
lization, including their high cost, and poorer performance relative to petroleum-based
plastics. Bioplastics such as polyhydroxyalkanoates, poly(lactic acid), and cellulose
esters reinforced with cellulose or chitin nanoparticles have recently attracted attention
as renewable and sustainable alternatives to current plastic materials (12, 13). For such
composites to replace traditional materials, further work in characterizing the materials,
enhancing compatibility, achieving stable dispersions, and developing faster and cheaper
'green' processing is essential. The ultimate goal is to create nanocomposites that are
entirely bio-based, controllably biodegradable, and match or exceed the performance
properties of synthetic composites reinforced with inorganic fillers. As such, the cre-
ation and study of cellulose or chitin as the structural reinforcing phase is the main focus
of this report.
Many thermoplastics, whether petroleum or bio-based, are hydrophobic in charac-
ter. Conversely, cellulose and chitin are hydrophilic materials, with their abundance of
hydroxyl groups, and thus exhibit poor compatibility with hydrophobic plastics. Due to
the high surface area of cellulose and chitin nanoparticles, which creates a very large

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and