Biomedical Engineering Reference
In-Depth Information
sheet of carbon atoms rolled into a seamless one-dimensional cylindrical
shape. Besides their extremely small (nanometric) size, their excellent
electrical and thermal conducting performances, combined with their strong
toughness and transverse flexibility make them promising for use in a wide
variety of applications such as electronic components, chemical and
biological sensors, chemical and genetic probes, field emission tips or
mechanical memories (Collins and Avouris, 2000). Carbon nanotubes are
thus expected to expand their unique properties in multidisciplinary fields
and should play a key role in nanotechnology.
Characterized by both high aspect ratio (length-to-diameter) and low
density, this allotropic variety of carbon is an ideal candidate to be used as
filler materials in composites. In fact, CNTs can provide a three-dimensional
conductive network through the polymer matrix with exceedingly low
percolation thresholds (Kilbride et al., 2002). Furthermore, it has been
suggested that their high thermal conductivity can be exploited to make
thermally conductive composites (Bagchi and Nomura, 2006). In addition,
the mechanical enhancement of polymer materials using CNTs as
reinforcing nanofillers will be exploited in the very near future (Wang
et al., 2008).
A key parameter for producing high quality nanocomposite materials
with improved physical properties is the homogeneous dispersion of the
individual CNTs. Unfortunately, the CNTs tend to form large bundles
thermodynamically stabilized by van der Waals forces and physical
entanglements between the tubes, which occur during their synthesis. The
presence of these aggregates in addition to the CNT's low solubility in water
and organic solvents represent a drawback for the engineering of CNT in
polymer nanocomposites. It has been reported that the homogeneous
dispersion of CNTs is relatively difficult to achieve in the large majority of
polymers (Andrews and Weisenberger, 2004).
The chemical functionalization of the CNT sidewalls, i.e. the CNT
surface-anchoring of functional (reactive) groups, represents a solution for
the tuning of interactions between CNTs and the host polymer matrix,
improving their dispersion ability (Hirsch and Vostrowsky, 2005). This
surface modification can be divided into two main approaches.
First is the non-covalent functionalization, used in different techniques
such as the addition of surfactants (Hirsch, 2002), polymer wrapping (Liu,
2005) or polymerization-filling technique (PFT) (Bonduel et al., 2005),
which allows the unaltered CNTs to preserve their physical properties.
However, the interaction between the wrapping molecules and the
nanotubes remains generally weak, limiting the efficiency of the property
transfer between the nanotubes and the host polymer, in particular, there
may be a low load transfer for mechanical properties.
The other approach relies upon the covalent grafting of functional
