Biomedical Engineering Reference
In-Depth Information
nanotubes remain randomly dispersed. Attempts have been made to align nanotubes to increase
reinforcement. Alignments techniques include melt drawing
[29]
, polymer stretching
[30
33]
,
alternating-current electric field
[34
36]
, surface acoustic waves
[37]
, direct-current electric field
[35,36,38,39]
and magnetic fields
[40
46]
have shown that in
composites where the nanotubes were aligned, a significant increase in the modulus was obtained
over nonaligned composites. Alignment of nanotubes in the composite also caused anisotropy with
improvement in the perpendicular direction being significantly less
[43]
. The use of magnetic field
as a technique to align nanotubes gave conflicting results on modulus enhancement
[40]
. However,
magnetic field was found to disrupt van der Waals interactions, improving dispersion and electrical
conductivity
[41]
.
42]
. Several studies
[29,33,43
3.2.1
Melt processing of CNT composites
There has been extensive work published in the melt processing of CNT with polymers. This pro-
cess involves heat processing the polymer and the CNT in a mixing equipment (screw extruder or
batch mixer). The mixer imparts shear and elongational stress to the process helping to break apart
the CNT agglomerates and dispersing them uniformly in the polymer matrix. The extruder is much
more versatile where by simply changing the screw configuration (in a twin-screw system) better
control of shear and mixing is obtained. Production rates and material throughputs in a continuous
extrusion process can be high. Another advantage of melt processing is that it does not require the
use of organic solvents during processing. The compounded CNT
polymer composite can be
further processed using other polymer-processing techniques such as injection molding, profile
extrusion, blow molding, and so on. Because of the large number of variables involved (tempera-
ture, screw speed, residence time, shear stress) the mixing process needs to be fine-tuned for
optimal properties.
Most of the work reported in the literature has involved polymers such as low-density polyethyl-
ene
[47]
, high-density polyethylene
[48,49]
, polypropylene (PP)
[50]
, polystyrene (PS)
[50]
, poly
(methyl methylacrylate) (PMMA)
[32,51]
, polyamide
[52]
, polyesters
[53,54]
, and polycarbonate
(PC)
[47]
. In most instances, the mechanical, electrical, and morphological properties were evalu-
ated. There are several reviews that detail the important findings
[4,12,55,56]
. Melt processing has
shown modest improvement in mechanical properties. Jin et al.
[57]
reported a 132% increase in
Young's modulus when 17 wt% multiwalled carbon nanotubes (MWNT) were mixed with PMMA.
MWNT (1% by weight) added to PS increased the modulus by 36
42% and strength by 25%
[58]
.
Significant increases (15
60%) in modulus was obtained in MWNT-polyamide 6 blends
[59]
with
increasing nanotube concentration. Addition of amine-functionalized MWNT made nylon 6 tougher
[60]
. SWNT increased the modulus of PC
[61]
and PP
[62]
by 50% and 28% for nanotube loading of
7.5 and 0.75 wt%, respectively. Results for composites made from different type of tubes show that
the reinforcement scales linearly with the total nanotube surface area in the films, indicating that
low-diameter multiwall nanotubes are the best tube type for reinforcement
[63]
. The properties of
several CNT
polymer composites produced by various techniques are summarized in
Table 3.1
.
The dispersion of nanotubes in polymer matrix is affected by material and processing para-
meters
[53,54]
. Varying methods of synthesis used by different manufacturers of nanotubes lead to
differing characteristics such as agglomerate structure, packing density, length to diameter ratio,
and purity. These variations affect dispersibility of the MWNT in polymeric matrix. In addition,
