Chemistry Reference
In-Depth Information
Polymer nanocomposites are reinforced by nano-sized particles with high surface
area to volume ratio including nano-particles, nano-platelet, nano-fi bers, and CNTs.
Nowadays, CNTs have been considered as highly potential fi llers for improving of
the physical and mechanical properties of biopolymers [31]. Following these reports,
researcher assessed the effect of CNTs fi llers in CS matrix. Results of these research
studies showed appropriate properties of CNTs/CS nano-biocomposites with high po-
tential of biomedical science.
CARBON NANOTUBES
The CNT, a tubular form of Buckminster fullerene, was discovered by Iijima in 1991
[32]. These are straight segments of tube with arrangements of carbon hexagonal units
[33, 34]. Scientists have been greatly attracted to CNTs because of the superior electri-
cal, mechanical, and thermal properties [35]. CNTs can be classified as single walled
carbon nanotubes (SWNTs) formed by a single graphene sheet, and multi walled car-
bon nanotubes (MWNTs) formed by several graphene sheets wrap around the tube
core [36]. The typical range of diameters of CNTs are a few nanometers (~0.8-2 nm at
SWNTs [37, 38] and ~10-400 nm at MWNTs [39]), and their lengths are up to several
micrometers [40].There are three significant methods for synthesizing CNTs includ-
ing arc-discharge [41], laser ablation [42], and chemical vapor deposition (CVD) [43].
The production of CNTs also can be realized by other synthesis techniques such as, the
substrate [44] the sol-gel [45], and gas phase metal catalyst [46].
The C−C covalent bond between the carbons atom are similar to graphite sheets
formed by sp
2
hybridization. As the result of this structure, CNTs exhibit a high spe-
cifi c surface area (about 10
3
m
2
/gr) [47] and thus a high tensile strength (more than
200 GPa) and elastic modulus (typically 1-5 TPa) [48]. CNTs have also very high
thermal and electrical conductivity. However, these properties are different according
to employed synthesis methods, defects, chirality, the degree of graphitization, and
diameter [49]. For instance, the CNT can be metallic or semiconducting, depending
on the chirality [50].
Preparation of CNT solution is a challenging area due to strong van der waals
interaction between several nanotubes leads led to nanotube aggregation into bundle
and ropes [51]. Therefore, the various chemical and physical modifi cation strategies
is necessary for improving their chemical affi nity [52]. There are two approaches to
the surface modifi cation of CNTs including the covalent (grafting) and non-covalent
bonding (wrapping) of polymer molecule onto the surface of CNTs [53]. In addition,
the reported cytotoxic effects of CNTs
in vitro
may be mitigated by chemical surface
modifi cation [54]. On the other hand, studies show that the end-caps on nanotubes are
more reactive than sidewalls. Hence, adsorption of polymers onto surface of CNTs
can be utilized together with functionalization of defects and associated carbons [55].
The chemical modifi cation of CNTs by covalent bonding is one of the impor-
tant methods for improving their surface characteristics. Because of the extended
π-network of the sp
2
-hybridized nanotubes, CNTs have a tendency for covalent at-
tachment which introduces the sp
3
-hibrydized C atoms [56]. These functional groups
can be attached to termini of tubes by surface-bound carboxylic acids (grafting to), or
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