Biomedical Engineering Reference
In-Depth Information
the polymer matrix (specifically the melt viscosity) also affects the degree of dispersion.
Unfortunately, the shear forces generated in most mixing equipment are not large enough to break
and disperse the CNT in the polymer matrix efficiently. Special mixers where shear rates are an
order of magnitude higher than obtained in a typical screw extruder are often used leading to better
dispersion and improved properties. However, it also has the potential for degrading the polymer
(particularly aliphatic polyesters used in tissue engineering) and the CNT.
3.2.2
Solution processing of CNT composites
Processing of composites using solution casting remains the most popular method of producing
composites, particularly on a laboratory scale. As the name suggests, solvent casting involves the
agitation of CNTs in a polymer dissolved in a solvent before casting in a mold and evaporating the
solvent. The lower viscosity of the polymer in solution (as opposed to a melt) coupled with agita-
tion by mechanical stirrer or ultrasonication aids in the dispersion of the CNTs. The choice of
solvent is determined by the solubility of the polymer. Since it is difficult to disperse pristine nano-
tubes, a surfactant is added to aid in the dispersion before adding to the polymer solution. In
general, this leads to good wetting of the CNT surface by the polymer. Also, studies with high
CNT loading (
50 wt%) have been reported
[56]
. An important issue in the solution casting
system is the speed at which solvent is removed as nanotubes often reaggregate particularly at high
concentrations
[70]
in a low-viscosity liquid.
.
3.2.3
In situ polymerization technique
A variety of CNT
polymer composite has been prepared using in situ polymerization. This tech-
nique can be used to produce both thermoset and thermoplastic materials. Free radical initiator
AIBN (2,2
0
-azobisisobutyronitrile) led to the formation of strong interface between CNT and
PMMA matrices
[51]
. The tensile strength increased up to 7% by weight addition of CNT after
which it decreased. This decrease in properties at higher fractions due to nanotube agglomeration
is evidence that dispersion affects mechanical properties. Majority of the thermoset-CNT studies
have focused on CNT
thermosetting polyimide composites
[68,72,73]
.
The mechanical properties of epoxy/CNT composites with and without CNT functionalization
have been the focus of several studies
[12]
. Here, the nanotubes are dispersed in the monomer
which is then polymerized. Dispersants may be added to assist in the deagglomeration of the nano-
tubes
[74]
. Alternately, functionalization
[75,76]
or polymer adsorption
[77]
techniques have been
used to aid in dispersion. Polymerization is initiated by increasing the temperature, adding chemi-
cal that initiates the reaction or by mixing two monomers. Since nanotubes are microwave absorb-
ing causing an increase in temperature, microwaves have been used to induce polymerization
[78,79]
. One of the advantages of this technique is that it allows the grafting of polymer molecules
on to the walls of the tube. The technique is useful in making CNT composites with polymers that
are insoluble in most common solvents or are thermally unstable (thereby making melt processing
difficult). Some of the composites developed include polyethylene
[80]
,PP
[81]
,PMMA
[82]
,
polyurethane
[83,84]
, polycaprolactone (PCL)
[85,86]
, and polylactide
[87]
. One potential problem
in making CNT
epoxy
[69,71]
and CNT
aliphatic polyester composites using this method is the ability to obtain
