Journal of Minerals & Materials Characterization & Engineering, Vol. 3, No.2, pp 73-80, 2004 Printed in the USA. All rights reserved
Electrical And Thermal Coatings From A Single-Walled
Carbon Nanotube (SWCNT)/Polymer Composite
Gerard T. Caneba
and Jay Axland
Department of Chemical Engineering
Michigan Technological University
1400 Twnsend Drive, Houghton, MI 49931
Single-walled carbon nanotubes (SWCNTs) are uniquely suitable as lightweight
electrically and thermally conductive materials. However, due to their tendency to
agglomerate into fiber bundles and relatively weak adhesion to other materials, it has
always been a challenge to process these materials for various applications. Solvents
and additives with relatively strong specific intermolecular forces were proposed to be
required to effect SWCNT dispersion along with the application of ultrasound. This work
involved the use of this hypothesis, in order to disperse SWCNTs with an acid-containing
copolymer that also served as a binder for the solid composite. In particular, HiPco was
dispersed in DMF ant THF using a tapered vinyl acetate-acrylic acid (VA-t-AA) block
copolymer with 6 wt % AA content. Dispersions were cast onto films and resulted in
solid coatings that have good electrical and thermal conductive and capacitive
Keywords: Carbon Nanotubes, Single-walled Carbon Nanotubes, Dispersion,
Electrically Conductive Films, Capacitor Films, Thermal Films
Single-walled carbon nanotubes (SWCNTs) are of interest as lightweight
materials with enhanced mechanical, thermal, and electrical properties. In order to
satisfy these application possibilities, a multifaceted approach is usually needed. The
first thing to consider is the type of carbon nanotubes that will provide needed properties
for a particular application. For the generation of electrically and thermally conductive
coatings, single-walled types with chiral angle of 30
have been shown to exhibit good
electrical conductivity;
and hence, good thermal conductivity in the longitudinal
direction. A good qualitative understanding of dispersibility of SWCNT in solvents and
polymers is also needed for a successful undertaking. Other issues include the
determination of substrates that will be compatible with the resulting SWCNT/Polymer
composite, manner of casting the films, mechanical integrity of the solid film composites,
SWCNTs made by laser ablation
or through the so-called HiPco process
been chemically purified, because reactor products are mixtures of nanotubes, other
Corresponding author
Gerard T. Caneba and Jay AxlandVol. 3, No. 2
carbonaceous materials, metal catalysts, etc. After purification, nanotube fibers still have
a particular length distribution, which depends on the production method and purification
conditions. A more significant condition for raw and purified fibers is that they are
arranged in bundles held together by Van der Waals forces on the lines of contact along
the fiber length. In particular, these intermolecular forces of attraction are based on a π-
bond stacking phenomenon between adjacent nanotubes (or simply called π-stacking
Because there can be at least hundreds of π-stacking sites between two
SWCNTs, intermolecular forces are stronger than those found between two relatively
small hydrocarbon molecules. However, when nanotubes are made to slide along their
length, resistance is lower than what is normally found in entangled polymeric molecules.
Thus, nanotubes are normally in the form of bundles, which have to be processed and/or
functionalized into dispersed materials. This is especially true if the nanotubes are going
to be used to reinforce polymeric materials. If other properties of nanotubes are desired
even with some degradation of the overall mechanical properties, such as their excellent
electrical and enhanced thermal properties,
then a partial level of dispersion might be
good enough.
In order to understand physical dispersion of SWCNTs, the molecular
thermodynamics concepts
have to be considered. The Gibbs energy change per mole,
G, is a quantity that determines whether a material is soluble in a mixture. The G
needs to be less than zero in order to obtain miscibility in a mixture;
the smaller it is
compared to zero, the better. The Gibbs energy change is relative to those of the pure
components (SWCNT and solvent). Since the Gibbs energy change is an abstract
quantity, it is represented as a relation involving the enthalpy and entropy. Thus,
where H and S are the enthalpy and entropy change, respectively. Again, both
enthalpy and entropy changes are relative to their pure components. In order to obtain
miscibility (or dispersibility for carbon nanotubes), H should go down and/or S should
go up. When H is brought down, other molecules that preferentially interact with the
carbon nanotubes have to be present. When S is brought up, a preferentially ordered
structure between the carbon nanotubes and other molecules in the mixture is formed, in
order to break the nanotube-nanotube ordering.
Based on the above-mentioned molecular thermodynamics criteria, it should not
be a surprise that hydrocarbons are not capable of dispersing carbon nanotubes. They
rely mostly on dispersion forces or weak electrostatic forces (dipoles and quadrupoles),
which are not stronger than the totality of those found between carbon nanotube fibers in
a bundle. At the same time, they do not form persistent associated and/or solvated
structures to yield an ordered molecular arrangement of some kind with the nanotubes.
Water does not disperse carbon nanotubes, because it does not interact well with the
nanotube surface even though it might be able to form ordered liquid structures due to its
relatively strong hydrogen bonding capabilities. In order for a molecule to physically
interact with the nanotube surface, it should contain π bonds to form π stacks
and/or it
should be able to form a molecular complex (also called a π-complex
) with the electron-
Vo.3, No2. Electrical and Thermal Coatings from SWCNT/Polymer Composite
rich nanotube surface. The latter situation happens when Lewis and protic acids are used.
In terms of the formation of an ordered structure with carbon nanotubes, small molecules
would have to be able to form associated structures (i.e., between like molecules) based
at least on hydrogen bonding. If these associations between solvent molecules are based
on anything less (such as dispersion or van der Waals forces), then the nanotubes can
easily break them up. This also means that the more association sites per small molecule
the better.
When additives were used successfully, they normally interact strongly with the
nanotube surfaces. For example, a pyrene succinimidyl ester was used successfully
because the pyrene group was cited to π-stack onto the nanotube surfaces. Polymeric and
oligomeric materials have also been proposed to aid in nanotube dispersion and
suspension. An example is poly(phenylenevinylene) or PPV, which contains aromatic
groups along the backbone that interact with the carbon nanotubes through π-stacking.
With sonication, wrapping of PPV has been declared to occur.
Another class of
longchain molecules that have been found to aid the dispersibility of carbon nanotubes in
water are amphiphilic surfactants, such as Sodium Dodecylsulfate (SDS), Sodium
Dodecylbenzyl Sulfate (SDDBS), Sodium Octylbenzyl Sulfate, Sodium Octylbenzyl
Sulfonate, Sodium Butylbenzyl Sulfonate, Sodium Benzoate, Triton X-100,
Dodecyltrimethylammonium bromide (DTAB), Dextrin, and poly(styrene)-poly(ethylene
oxide) (PS-PEO) diblock copolymer.
Actual surfactant structures around carbon
nanotubes are still unknown, but again nanotube dispersion should occur based on the
strong interactions between surfactant-surfactant, surfactant-solvent and solvent-solvent
molecules, as well as interactions between surfactant and solvent molecules with the
nanotube fibers and/or bundles. It is worth noting for the same hydrophilic group, the
more effective surfactant contained phenyl groups in the hydrophobic tail.
Exposure alone of carbon nanotubes, especially the SWCNT types, to
thermodynamically favored compounds do not necessarily result in a dispersed carbon
nanotube system. What happens is that dispersing agent molecules will interact with the
outer fibers of the nanotube bundles, and no further dispersion can occur. The fibers
have to be separated from one another even for a short period of time, allowing the
dispersing agent molecules to form a superstructure around the fibers. Sonication has
been shown to provide this kind of mechanical action at such a small area between
carbon nanotube fibers.
Sonication involves the use of sound waves in the frequency range of 20 kHz – 10
It causes the fluid to cavitate locally, followed by the collapse of cavitated
bubbles. Energies involved in this phenomenon are so high that they have been found to
be equivalent to a 2000-5000 K rise in local fluid temperatures and pressures up to 1800
atm inside collapsing cavities.
What might be significant for nanotube dispersion is the
observation that when cavitated bubbles collapse from a solid surface, a jet of fluid from
the opposite side of the bubble impinges onto the surface.
If the solid surface is made
up of nanotube bundles, then this jet can force fluid molecules, additives, and nanotubes
themselves to lodge between the fibers. For small molecules with weak intermolecular
forces, the effect is should be marginal at best. Alternately, the same effect happens if
Gerard T. Caneba and Jay AxlandVol. 3, No. 2
the solvent molecules do not interact with the nanotube surface. However, if solvent
molecules have strong interactions from hydrogen bonding sites and they interact with
the nanotube surface, then the fluid jet forms a strong wedge between fibers to separate
them. If the fluid contains polymers that are strongly interacting with its segments, with
other polymeric molecules, and with the nanotube surface, then an even more stable
wedge is formed. If fluid cavitation occurs behind the polymeric wedge, then the leading
edge of the polymer wedge will be pulled back or around the fibers either in a clockwise
or counterclockwise direction. This is probably an explanation for why polymers are
found wrapped around nanotubes and/or nanotube bundles.
If the above-mentioned effects of highly interacting polymers with the aid of
sonication in nanotube bundles are believed to be true, then polymeric wrapping can be
understood more clearly. First of all, complete polymeric wrapping of single nanotube
fibers can be possible if the fibers are separated in the first place. This might happen if
the fiber concentration is very dilute (in the 1 mg/L level). If the fibers are bundled up in
a more concentrated solution, then, the chainlength of the polymer also plays a significant
role. If the polymer is relatively long compared to the nanotube diameter, then it will
wrap around entire bundles. At least the wrapped bundles are going to be separated from
the main bundles, and wrapped bundles can be suspended in the fluid. If the polymer size
is optimized to wrap around only a single fiber, then it is possible to separate out single
fibers from solution if fiber entanglements are nonexistent.
In this work, an acid-containing amphiphilic copolymer (vinyl acetate-acrylic acid
tapered block copolymer) was used as a dispersion aid for HiPco in DMF and THF.
Solid films were cast from the dispersion, whereby the copolymer was used as a binder
for the HiPco at high concentration (50 wt % HiPco in the solid). Effectiveness of the
dispersing and binding capabilities of the VA-t-AA copolymer resulted in good electrical
and thermal properties of the composite coatings.
Purified HiPco was dispersed in
both DMF and THF with the aid of a vinyl
acetate-acrylic tapered block (VA-t-AA)
copolymer. The process used to produce
the VA-t-AA copolymer was described
elsewhere [10]. It had an acrylic acid
content of 6 wt %, an average molecular
weight of 30,000-40,000 Daltons, and a
polydispersity index of about 2.76. Figure
1 shows a linear bead model of the VA-t-
AA copolymer material.
Figure 1. Linear bead model of the Vinyl Acetate
(open beads) and Acrylic Acid (filled beads)
segments that make up the tapered block
copolymer. Bead numbers are drawn to scale to
approximate molecular make-up of the VA-t-AA
material with 6 wt % AA segments
Vo.3, No2. Electrical and Thermal Coatings from SWCNT/Polymer Composite
Preparation and Testing of the Electrical Films
For the electrical composite films, a 0.1 wt % HiPco in THF was prepared using a
50-ml closed bottle. The VA-t-AA copolymer was added in such a way that there was a
1:1 wt/wt ratio between the HiPco and the copolymer. The idea is that the VA/AA
copolymer can also function as a binder for the SWCNT onto relatively polar surfaces,
such as wood, paper, PVC, and cellulose. An example formulation was one that
contained 10 mg HiPco, 10 mg VA/AA Copolymer, and 10 ml THF, which was
sonicated for 24 hrs in an ultrasonic bath that was operated at 180 Watts. Films were cast
by pouring the sonicated mixture into a 4”x6” cavity comprising a Teflon™ film that was
bounded by 2-mm thick polypropylene sheets. The solvent from the wet film was
allowed to dry inside a fume hood for at least 24 hrs. Then, the partially dried film was
vacuum-dried for at least 1 hr at 50-60
C. Surface resistivity of the coatings were
measured using a concentric electrode apparatus that was manufactured by Monro
Electronics (Lyndoville, NY), Model 803A. Dry polymer and HiPco loadings per area
were measured by gravimetric means, based on measured weight of the Teflon™ support
per unit area.
Preparation and Testing of the Thermal Films
The procedure used for the preparation of thermal HiPco/VA-t-AA films was
similar to that used in the preparation of electrical films. A difference was that DMF was
used as a solvent. Also, sonication was done using a 240-Watt ultrasonic bath. Finally,
the wet sonicated mixture was cast onto a Saran™ film and kept within a 3-inch-diameter
circular cavity by a short Teflon™ tube. After drying, the film weight per unit area was
obtained by cutting a rectangular piece from the uncoated Saran™ substrate, obtaining its
weight using a Mettler™ balance, and then calculating its area from the length of the
sides (using a digital Vernier calipher). A similar approach was done with the
HiPco/VA-t-AA film on the Saran™ film substrate. The difference between the weight
per area of the HiPco/VA-t-AA film onto the Saran™ substrate and that of the Saran™
substrate was the HiPco/VA-t-AA weight per unit area.
Figure 2 shows the schematic
diagram of the apparatus used to test the
thermal generation properties of the
HiPco/VA-t-AA film. A 20-Volt DC
source was used to pass current across the
film area. Folded Aluminum foils were
used to establish contact between the
alligator clips from the DC source to the
HiPco/VA-t-AA film. The temperature of
the film was measured by a 1/16-inch
Type J thermocouple. The whole
HiPco/VA-t-AA film setup was then
thermally insulated by polyurethane foam
blocks that were at least three inches thick.
12 VDC
##.# C
Power Source
Alligator Clip
Figure 2. Thermal film experimental setup
Gerard T. Caneba and Jay AxlandVol. 3, No. 2
Results and Discussion
Electrical Films
Application of dispersion of HiPco
in DMF and VA-t-AA copolymer onto
Teflon™ resulted in a dry coating that
contained 0.059 mg HiPco per sq cm
(Figure 3). Using the concentric electrode
apparatus, a surface resistivity of 3.5 k-
Ohms/sq was obtained. This was
definitely in the conductive range of
surface resistances up to 100 k-Ohms/sq.
An SEM picture of the dry surface onto an
Aluminum pan (Figure 4(a)) indicated the
formation of well-defined ropes of
SWCNT bundles, compared to the
structure for pure HiPco that was
dispersed in THF only (Figure 4(b)).
Better definition of the structure from
SEM was based on its higher electrical
conductivity. Because of poor dispersion
a satisfactory 4”X6” coating from the
HiPco/THF mixture was not obtained.
Based on these results, with the right
additives, it is feasible to generate
effective SWCNT-containing composites
and fibers from films and coatings.
In Table 1, the effect of HiPco
loading on the surface resistivity is shown.
It appears that an asymptote in surface
resistivity is obtained as the HiPco loading
is increased. This is due to the fact that
the measurement is meaningful only on
the surface of the composite.
Figure 3. Picture of a dry coating of 1:1 wt/wt
HiPco/VA-AA copolymer onto a Teflon™ film,
which was found to have a surface resistivity of
3.5 k-Ohms/sq
Table 1. Surface resistivity of HiPco/VA-t-AA
copolymers films at different HiPco loadings
HiPco Loading,
Surface Resistivity,
Figure 4. Scanning electron micrographs of surfaces of
HiPco coatings dispersed in THF with: (a) the aid of a VA-AA
copolymer dispersing agent/binder, and (b) without the aid of
a polymer dispersing agent/binder. The better definition of
the rope structure in (a) indicates a higher electrical
conductivity on the nanotube
Vo.3, No2. Electrical and Thermal Coatings from SWCNT/Polymer Composite
The measurement of surface resistance of SWCNT-copolymer coatings (1:1
wt/wt) indicated readings that roughly followed an exponential decay with time. Usually,
it took around 15 minutes to obtain the lowest resistance reading. For the lowest
resistance reading of about 350 Ohms (corresponding to the film with 0.059 mg
), an initial resistance (time = 0) of about 1 k-Ohm was obtained. The lowest
resistance was used to calculate surface resistivity values. In this case, the surface
resistance reading of 350 Ohms was multiplied by the geometric factor of 10 to yield the
surface resistivity of 3.5 k-Ohms/sq.
Based on basic electrical circuit theory, the exponential decay of resistance
readings occurs when a resistor is in series with a capacitor. The time constant of the
decay reading is the product of the resistance and the capacitance. For a decay reading
time of 15 minutes, the time constant was about 1/5
of that, or 3 minutes (180 s). This
means that the capacitance was 180 s/350 Ohms or 0.51 Farad. This was a relatively
large capacitance for an SWCNT loading of 0.059 mg/sq cm in a test film area between
concentric electrodes; the outer electrode was about 3.5 inches in diameter while the
inner electrode was 1.9 inches in diameter. Such a large capacitance value is not
surprising since SWCNTs are known to exhibit supercapacitance behavior.
Thermal Film
Figure 5 shows the temperature
history of the thermal film during the
charging phase with 20 Volts DC and
discharging phase. The HiPco loading for
this film was determined to be equal to
0.33 mg/cm
. One can see from the
charging phase that the temperature went
up from 22
C to somewhere above 45
C in
15 minutes. The discharge phase also took
about the same time. This time-dependent
thermal behavior follows the behavior
seen in the electrical films. This means
that the films also act as thermal
The use of an amphiphilic copolymer containing carboxy groups facilitated the
adequate dispersion of HiPco in such a way that nanoscale super-ropes were formed in
the solid films. Also, the VA portion of the copolymer acted as a binder for the HiPco,
even at high loadings in the solid (50 wt % HiPco). The wider implication of this work
demonstrates the possibility of using acid-containing hydrophilic-hydrophobic block or
tapered block copolymers as dispersants/binders for single-walled carbon nanotubes.
Time, min
Film Temperature, C
Charge Temperature
Discharge Temperature
Figure 5. Temperature history of the
HiPco/VA-t-AA copolymer film
Gerard T. Caneba and Jay AxlandVol. 3, No. 2
We have therefore shown that a vinyl acetate (VA)-acrylic acid (AA) tapered
block copolymer (with 6 wt % AA content) is capable of adequate dispersion of HiPco in
THF and DMF, and form composite films that show good electrical and thermal
properties. Such a result offers the possibility of using acid-containing hydrophilic-
hydrophobic block and tapered copolymers as dispersant/binder for carbon nanotubes.
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We acknowledge NASA-Johnson Space Center (Houston, Texas) for the summer
fellowship for G. Caneba, which provided the facilities for the application of the vinyl
acetate-acrylic acid tapered block copolymer onto single-walled carbon nanotubes
(Summer, 2003). This work was also partially funded by the National Science
Foundation, through the Nanoscale Undergraduate Education (NUE) Program (NSF