Biomedical Engineering Reference
In-Depth Information
into the multilayer composite, as long as alternating charge is maintained. The technique is cheap
and easy to assemble. Functionalized CNTs (f-CNT) can be incorporated into the multilayer and by
varying the number of layers, the properties of the films can be controlled.
It is widely recognized that the excellent properties of nanotubes have yet to be realized. Little
of the data reported achieve the reinforcement predicted by the rule of mixtures especially at
concentrations of 10 vol% of CNT
[14]
. This is because efficient load transfer depends on the inter-
facial bonding between the polymer matrix and the CNT. Because of the strong van der Waals
forces and electrostatic interactions, CNTs tend to aggregate in solvents. Although van der Waals
forces are considered to be weak intermolecular forces, they become significant at the nanoscale
due to the large surface area per unit mass of the material. This diminishes the interfacial bonding
(probably because of the smooth grapheme-like surface of nanotubes) leading to lower than
predicted properties. Agglomerated nanotubes form ropes that slip when stressed due to their poor
adhesion to the polymer matrix, affecting their elastic properties
[15]
. Nanotube bundles act as stress
concentration points within the polymer matrix and can, in some cases, reduce the mechanical prop-
erties of the original polymer. The reduced aspect ratio due to agglomeration of nanotubes also leads
to a reduction in reinforcement. Thus, nanotube dispersion is critical to efficient reinforcement.
Reinforcement of materials using fillers are affected by four parameters—aspect ratio, extent of
dispersion in the matrix, alignment, and interfacial stress transfer
[4]
. Large aspect ratio maximizes
load transfer to the nanotubes. Nanotubes must be well dispersed in the matrix to the point of indi-
vidual tubes coated by the polymer. Good dispersion helps achieve good load transfer to the
nanotube network resulting in more uniform stress distribution. Alignment while important is not
critical. While alignment maximizes modulus and strength, it makes the composite anisotropic.
Bonding between the nanotube and the polymer is essential to allow the external stress applied to
the composite to be transferred to the nanotubes, enabling them to bear most of the applied load.
Hence, CNTs are functionalized to improve their dispersibility and enable their interactions with
polymers.
There are several reviews available on the functionalization of CNTs
[16,17]
. Chemical functio-
nalization of nanotubes was discovered in an attempt to purify single-walled carbon nanotubes
(SWNT) with acids. Depending on the method of nanotube production, CNTs are often mixed with
impurities such as metal catalysts, amorphous carbon, and soot. Strong acids are utilized to eat
away the impurities leaving behind pure CNTs. The acid reacts with the nanotube caps, which are
reactive because of their high degree of curvature. This method of functionalization creates bonds
that are progressively oxidized, depending on the intensity of treatment to hydroxyl (
a
OH), car-
bonyl (
COOH) groups
[18]
. The carboxylic acid groups are employed as
anchoring sites for functional groups that make the nanotubes soluble in organic solvents
[19
C
Q
O) and carboxyl (
a
.
21]
.
Other oxygenated functionalities include anhydrides, quinines, and esters
[17]
. Such functionalities
can also be introduced by treatment with ozone
[22]
. The formation of carboxylic acid sites has the
potential for the attachment of various moieties through amidization and esterification reactions.
Various materials have been electrostatically
[23]
, hydrophobically
[24]
, or covalently
[25,26]
attached to the surface of CNTs. There are examples in the literature of DNA and protein functio-
nalized CNTs
[27]
which can be exploited in biological application.
In order to ensure a strong bond at the interface, a molecular level entanglement between the
polymer and nanotube is essential
[28]
. This is one of the reasons behind the use of functionalized
nanotubes in composites. Even with improved adhesion and dispersion in the polymer matrix, the
