2. Methods
The method of preparation can encompass dissolving a polymer in a
solvent while maintaining the temperature of the solvent at a level high
enough to prevent precipitation of the polymer out of the solvent;
sonicating a plurality of carbon nanotubes in a solvent; mixing the
dissolved polymer and the sonicated carbon nanotubes; and sonicating the
mixture for a sufficient period of time to disperse the carbon nanotubes
throughout the polymer to produce a subject nanocomposite in solvent.
The composite is optionally further processed by spin coating the
nanocomposite and solvent onto a substrate. The spin coating step
evaporates the solvent so that the nanocomposite is deposited as a
coating on the substrate. The solvent can also be removed by heating the
nanocomposite under a vacuum, thereby removing the solvent. The
nanocomposite can then be molded into a device or structure. Optionally,
the solvents used to dissolve the polymer and to sonicate the carbon
nanotubes are the same. Preferably, the solvent is cyclohexyl chloride.
Stable suspensions of carbon nanotubes are achieved in water with the
use of surfactants, and non-covalent and covalent attachment of
polymers. Covalent functionalization is a valuable approach to the
preparation of carbon nanotube materials, as controlled compositions and
reproducible properties may be obtained. The incorporation of carbon
nanotubes into polymer matrices results in composites that exhibit
increased thermal stability, modulus, strength, and electrical and
optical properties. The polymer is dissolved in cyclohexyl chloride at
110 °C to make a solution. Carbon nanotubes are sonicated in
N,N-dimethylformamide using a Branson Sonifer for one hour. The
dispersion is placed in a vacuum oven at 80 °C to remove the solvent.
The N,N-dimethylformamide treated carbon nanotubes are then dispersed in
the polymer solution via sonication for 6 hours. The carbon
nanotube-polymer mixture is placed in a warm beaker lined with TEFLON
film, the solvent is allowed to evaporate at room temperature for 12
hours, and the composite is then placed in a vacuum oven at 80 °C to
remove any residual solvent. The dried composite with carbon nanotubes
is compression molded for analysis. Pieces are placed between KAPTON
film and stainless-steel plates and pressed for 6 minutes at 6000 pounds
of pressure at a temperature of 246 °C. A neat polymer is prepared in
the same manner. After processing, the melt temperature for the neat and
composite sample is measured.
The composites comprise carbon nanotubes that are incorporated into the
matrix of a polymer. Advantageously, the carbon nanotubes can be
single-walled, multi-walled, or a combination thereof. Advantageously,
carbon nanotubes are 100 times stronger than steel, exhibit excellent
electrical and mechanical strength, and are light in weight. Due to
their weight, carbon nanotubes are thought to be ideal fillers in a
polymer matrix in order to produce a composite with improved thermal
properties, as well as with enhanced electrical and mechanical
properties. The polymer utilized in the nanocomposites comprises a
plurality of repeating hydrocarbon units that exhibits solubility in
organic solvents. Preferably, the solvents are cyclohexane, cyclohexyl
chloride, and cyclohexene. More preferably, the solvent is cyclohexyl
chloride. The polymer composites are then characterized by measuring
their physical, melt rheological, and electrical properties. Carbon
nanotubes have a powerful effect on the melt rheology, increasing the
low shear viscosity dramatically. Escalating the carbon nanotube
concentration also increased the flexural and tensile moduli, decreased
the elongation, and increased the electrical conductivity.
Polymerization is generally preferred as the method of dispersing the
carbon nanotubes. The electrical conductivity is in some cases quite
high, approaching that of metal strips. In addition, the electrical
conductivity is pressure sensitive.
Such acid derivatized carbon nanotubes can be added to and subsequently
copolymerized with precursors of polymers including but not limited to
monomeric precursors to polyamides, polyesters, polyimides, or
polyurethanes [61, 62]. For example, the acid derivatized carbon
nanotubes can be contacted with a diacid and a diamine and the resultant
pre-polymer product polymerized to form a carbon nanotube-polymer
composite, or the acid derivatized carbon nanotubes can be contacted
with a diacid or a diester, and a diol and the resultant pre-polymer
product polymerized to form a carbon nanotube-polymer composite [63,
64]. Following contact of the acid derivatized carbon nanotubes with
the polymer precursors, the pre-polymer product of such contact may be
filtered, washed, and dried [65, 66]. For example, this procedure
would be appropriate for treatment of the salt precipitate when using
polyamide precursors [67, 68]. In addition, for the formation of
other pre-polymer products, water or alcohol may be removed such as in
the formation of pre-polymers.
One of the problems with blending any filler into a molten polymer is
the difficulty of dispersing the individual particles into the polymer
matrix. High shear mixing is usually employed for this purpose, but
certain fillers, such as carbon nanotubes, offer a special problem
because they require dispersal at the angstrom or nanometer level, and
they have a particularly strong Van der-Waals affinity for each other.
These characteristics make it especially difficult to effectively
disperse clusters of carbon nanotubes, and then keep them dispersed in
the polymer matrix. Indeed, the quality of dispersion is usually the
limiting factor when engineering these composites. To measure the
electrical resistance across these composites, a circular disk of
0.6-inch diameter and 0.06-inch thickness is first compression molded.
The measurement is carried out by heating the polymer to 180 °C in a 2
inch by 2 inches by 0.05-inch mold, applying about 600 pounds per square
inch pressure to the mold, waiting 6 minutes, then cooling under
pressure for another 8 minutes to 60 °C, where the hardened plaque is
removed. The circular disk is then cut from this larger plaque. To each
side of this disk is then smeared a small amount of micronized silver
paste. The brass disk is then pressed against the silver paste on each
side of the disk, and the two metal plates with sample sandwiched
between are then pressed together by means of a strong spring-loaded
wood-gluing clamp. These two brass plates, which are soldered to a wire,
are then connected to a voltmeter.
Electrical resistance, measured in ohms, is then taken as the two metal
disks are being pressed together. Sometimes the force is varied to
obtain maximum contact, and duplicates are obtained to achieve a
consistent result. A blank determination is also made in which the
composite is omitted, to determine the electrical resistance of the
other parts of the circuit. This electrical resistance is small, usually
0.2 ohms at most, compared to sample measurements of many orders of
magnitude larger. The electrical resistance measured in this way is then
converted into a standard resistivity by multiplying by the area of the
disk and dividing by the path length. Electrical conductivity as a
function of pressure is determined by placing a sample of the composite
between two metal plates, which are then inserted in a hydraulic press
between insulating barriers. Electrical resistance between the plates is
then monitored as the force exerted by the press is increased.
Scanning electron microscopy characterization is performed on a JEOL
7401-F with energies greater than approximately 6 keV in secondary
electron imaging mode with a working distance of 2-8 mm. Electrical
conductivity is measured using the four-probe method with metal
electrodes attached to the ends of cylindrical samples. The amount of
current transmitted through the sample during measurement is 80 mA, and
the voltage drop along the sample is measured over distances of 2 to 6
mm. Seven or more measurements are taken on each sample, and results are
averaged. Mechanical properties are studied by indentation in an MTS XP
Nanoindenter with a Berkovich diamond tip. A series of both continuous
and partial load-unload indents is carried out in laboratory air at room
temperature. The loading rate is continuously adjusted to keep a
constant representative strain rate. For every cycle, the unloading rate
is kept constant and equal to the maximum loading rate of the cycle. The
Oliver-Pharr method is used to analyze partial load-unload data in order
to calculate the indentation elastic modulus as a function of the
indenter penetration.