Biomedical Engineering Reference
In-Depth Information
chemical groups along the CNT sidewalls. These chemical functions can be
used as anchoring sites for polymer chains (Hirsch and Vostrowsky, 2005)
and to improve the interaction with the polymer matrix (Thostenson et al.,
2005). The covalent bonding between CNTs and the polymer chains ensures
an optimal interfacial strength and thus a better load transfer. Nevertheless,
the covalent functionalization can influence the CNTs' properties, depend-
ing on the nature and density of the functional sites. Indeed, the
functionalization can take place on already existing CNT structural defects,
or active sites can be created by grafting chemical functions. In the latter
case, functionalization can degrade the CNTs' physical properties if the
density of the active site is significant, leading to damaged CNT structure
(Fu et al., 2001). The majority of developed covalent sidewall functionaliza-
tions are carried out in organic solvent. Nevertheless, as the CNT surface is
largely inert, rather harsh conditions are needed for wet chemical
functionalization of CNTs, which rarely results in a controlled covalent
functionalization process (Hirsch and Vostrowsky, 2005).
To overcome these drawbacks, 'dry' processes are being developed. For
example, the ball-milling of CNTs in a reactive atmosphere was shown to
functionalize nanotubes with different chemical groups, such as amides,
amines, carbonyls and thiols, for instance, depending on the reactant gas
(Konya et al., 2002). Using this method, CNTs are broken, and dangling
bonds are formed on their newly created extremities that can then react with
a selected gas. A relatively long period of time, a minimum of 24 hours, is
usually necessary to graft a significant amount of functional groups onto
CNTs. However, the average length of the CNTs decreases as the treatment
time increases and some amorphous carbon was also observed after the
breaking process (Ma et al., 2008). Another drawback of this technique is
the presence of functional groups exclusively at the ends of cup-stacked
carbon nanotubes.
Alternative 'dry' approaches to chemical modification of CNTs using
plasma discharges have been also proposed and developed. Considered the
fourth state of matter, plasma is an ionized gas that constitutes a highly
unusual and highly reactive chemical environment. Plasma treatments have
evolved into a valuable technique to modify material surface properties
without altering the bulk properties, allowing their use in various
applications such as plasma cleaning, plasma sterilization or biomedical
applications (D'Agostino et al., 2005). Recently, plasmas were also applied
to modify the surface properties of CNTs. Characterized by a relatively low
reaction time, surface plasma treatment is an environmentally friendly
process without any use of organic solvents. The plasma technique allows
the grafting of a wide range of different functional groups on the CNT
surface depending on plasma parameters such as power applied to the
discharge, nature of the gas used, duration of treatment and gas pressure.
