Cellulose and Chitin as Nanoscopic Biomaterials - The Nanoscience and Technology of Renewable Biomaterials - page 208

greatest advantage with thermoset resins is the low viscosity of the materials prior to
cross-linking, allowing for low-pressure processing conditions and good particle disper-
sions. The main disadvantages with such materials are that: (1) the thermoset resins are
very intractable and inherently brittle, (2) they cannot be recycled or biodegraded after
their useful lifetime, (3) almost all thermosetting resins are derived from diminishing
petroleum resources, and (4) the incorporated inorganic filler phases are very dense,
adding to the overall weight and shipping costs of the materials. Recently, improved
polymer processing technology and the demand for materials with more diverse ther-
mal and mechanical performance properties have placed thermoplastic composites in
strong competition with conventional thermoset materials. The high melt-viscosity of
thermoplastics has made it difficult to create adequate particle dispersions in thermo-
plastic matrixes. High shear rates and elongational flow patterns generated in modern
melt-processing equipment (i.e. compounders and extruders) have facilitated the sepa-
ration of particle aggregates in high viscosity systems. Generally, once aggregates are
disrupted, the high viscosity of the thermoplastic melt prevents reaggregation, producing
fairly uniform, high performance composites.
Most reinforcement phases in conventional composites are macroscopic particles, such
as glass or aramid fibers. Developments in analytical techniques, such as microscopy,
and increased understanding of composite behavior, have stimulated massive amounts
of research, which recently has moved into the nanostructure of composite materials.
The term nanotechnology is pervasive in modern scientific research and, as defined by
the National Nanotechnology Initiative (1), implies three essential characteristics. At
least one of the objects in the system must be on a nanoscale, meaning that it must
have at least one dimension less than 100 nm. The second trait is that the work must
involve the creation or use of structures, devices, or systems having novel properties
consequential to their size. The third trait is the element of control or manipulation at
the nanoscale (i.e. the molecular-to-atomic scale). Polymer nanocomposites embody
all three of these traits. It is generally recognized that the gains in surface area as
a consequence of reducing particle sizes to nanometer dimensions (1 nm

10 − 9 m)
can lead to outstanding properties in composites reinforced with these particles. Such
materials have been heavily researched in the last few decades. Since the seminal
work published by researchers at Toyota on Nylon 6,6 reinforced with montmorillonite
nanoclay, the considerable potential of polymer nanocomposites has been realized (2).
The materials showed remarkable improvements in tensile modulus, tensile strength, and
heat resistance at very low filler loadings (2). In much of the subsequent nanocomposite
research, the filler materials studied have been inorganic materials such as nanoparticles
of silica, boron, clay, calcium carbonate, or carbon nanotubes, to name a few. Several
research groups, particularly those in Europe and Japan, as well as a few in North
America, including our own laboratories, have investigated natural polysaccharide based
nanoparticle composites containing cellulose and chitin (3-5).
Cellulose and chitin are the two most abundant biopolymers. It is estimated that the
combined worldwide annual production of cellulose and chitin by nature is nearly 2
×

=

10 12 metric tons (6, 7). The seafood industry alone generates annually some 10 5
metric tons of chitin waste for industrial use (8, 9), with the availability of cellulose
far exceeding this number. Both polymers represent underutilized, readily available,
sustainable feedstock alternatives to petroleum-based materials. Cellulose and chitin