J. of Supercritical Fluids 28 (2004) 233–239
Supercritical carbon dioxide aided preparation of conductive
polyurethane–polypyrrole composites
Suresh L. Shenoy c , Daniel Cohen d , R.A. Weiss a , Can Erkey b,∗
b
a Department of Chemical Engineering and Polymer Science Program, University of Connecticut, Storrs, CT 06269-3136, USA
Department of Chemical Engineering, Environmental Engineering Program, University of Connecticut, Storrs, CT 06269-3222, USA
c Department of Chemical Engineering, Virginia Commonwealth University, Richmond, VA 23284-3038, USA
d Rafael, Haifa, Israel
Received 24 October 2001; received in revised form 29 January 2003; accepted 21 February 2003
Abstract
Conductive polyurethane (PU) foams were prepared using a two-step procedure. First, PU foams were impregnated with the
oxidant I2 from a scCO2 solution. Subsequently, the foams were subjected to pyrrole vapor, which polymerized within the foam in
the presence of the oxidant. The reduced form of iodine as a result of the oxidative polymerization of pyrrole functions as a dopant.
The conductivities of the foams ranged from 10−7 to 10−2 S/cm and the foams were flexible and not brittle. The concentration of
conductive polypyrrole, i.e. the PPy–I2 charge-transfer complex, needed to achieve a conductivity of 10−4 –10−3 S/cm was about
22% w/w. The solubility of I2 in scCO2 and the partition coefficients of I2 between PU foam and scCO2 solution were determined.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Supercritical carbon dioxide; Conductive composites; Polyurethane; Polypyrrole; Iodine; Oxidative polymerization
1. Introduction
Conductive polymers have been the focus of considerable research over the past two decades with foreseeable use in products such as rechargeable batteries,
bio- and chemical-sensors, transducers, antistatic coatings, corrosion-inhibiting films and EMI shielding materials [1–6]. Much recent research has concentrated
on polypyrrole (PPy), polyaniline and polythiophene,
because of their superior oxidative stability compared
with other inherently conducting polymers. PPy can
∗ Corresponding author. Tel.: +1-860-486-4601;
fax: +1-860-486-2959.
E-mail address: [email protected] (C. Erkey).
be synthesized either by an electrochemical oxidative
process or by the chemical oxidation of pyrrole [6,7].
Neat PPy is an intractable, brittle solid, but products
with usable mechanical properties can be made by
blending PPy with another polymer [8]. The blends,
which are often referred to as conductive composites, are generally produced by impregnating the host
polymer with an oxidant, e.g. FeCl3 , followed by in
situ polymerization of pyrrole within the host polymer [9]. A deficiency of this process is that large
quantities of volatile organic solvents (VOCs) such
as methanol are used to introduce the oxidant into
the host polymers and to remove unreacted oxidant
and redox by-products. Supercritical carbon dioxide (scCO2 ) offers advantages in this respect since
0896-8446/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0896-8446(03)00047-0
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S.L. Shenoy et al. / J. of Supercritical Fluids 28 (2004) 233–239
it is non-toxic and environmentally acceptable. The
adjustable solvent properties of scCO2 may also be
beneficial in tailoring the properties of the composites by control of sorption and desorption processes.
We recently showed that the VOCs could be eliminated in the in situ polymerization of pyrrole within
a polyurethane (PU) foam by replacing the methanol
with scCO2 [10] or a mixture of scCO2 and ethanol
[11]. That approach, however, necessitated the synthesis of CO2 -philic, fluorinated metal carboxylate
and sulfonate oxidants, and solubility of the oxidants
were generally low.
Our current work is directed towards using organic electron acceptors such as halogens in place of
organometallic oxidants for the in situ polymerization
of pyrrole in PU foam. Electron acceptors have long
been used as dopants for polyacetylene [12] and PPy
films doped with halogens such as Br2 and I2 are stable and exhibit conductivity of the order of 10−5 S/cm
[13]. Kang et al. [14] used halogens in an organic solvent to polymerize and dope PPy. The PPy–halogen
complexes were relatively stable in air and exhibited
conductivity ranging from 10−5 to 5 S/cm, depending
on the solvent used. Those authors concluded that the
presence of an aprotic solvent facilitated the pyrrole
polymerization.
In this study, we report the results of our investigation on using I2 as an oxidant for preparation of
conductive PU foams. The foams were prepared by a
two-step process. In the first stage, the foams were impregnated with I2 from scCO2 solutions. In the second
stage, the iodine impregnated foams were subjected to
pyrole vapor at room temperature. After polymerization of the pyrole, the resulting composite foams were
found to be conductive. The factors controlling both
the impregnation and the polymerization process were
investigated.
2. Experimental
PU foam with a density of 0.32 g/cm3 was obtained
from Rogers Corporation (Rogers, CT). Iodine, 99.8%
ACS, from Aldrich Chemical Co. and 99% pyrrole
from Acros Organics (Fisher Chemical Co.) were used
as received. Our previous investigations on PU foams
had indicated that the solubility of the oxidant was an
important factor in understanding and interpreting the
sorption data. Therefore, the solubility of I2 in scCO2
was measured at two conditions using a static method
coupled with gravimetric analysis that was developed
in our laboratory [15]. In each run, a certain amount
of I2 and a small magnetic stir bar (7 mm L × 2 mm
D) were placed in a 5 ml glass vial. The vial was then
capped with a coarse filter paper (Whatman) that was
attached to the vial with a copper wire. A larger magnetic stir bar (12 mm L × 4 mm D) was placed inside a pressure vessel (Moran Tool and Die, Bolton,
CT) and a perforated stainless steel stand was then
slid into the vessel. The vial containing the iodine was
then weighed and placed on the stand inside the vessel. The vessel was sealed, connected to the circulation
bath and heated to the desired test temperature. Once
the test temperature was reached, stirring was initiated and the vessel was slowly filled with CO2 until
the desired pressure was achieved. The test was carried out for 24 h to allow sufficient time for equilibration of the scCO2 /I2 solution. Upon completion of the
test, the vessel was depressurized in a fume hood and
opened. The vial was removed, wiped carefully, and
weighed again. The solubility of I2 in the scCO2 was
calculated from the change in mass of the iodine-filled
vial.
The synthesis of conductive elastomers using I2 and
scCO2 was carried out in two steps: (1) impregnation
of I2 into the foam from an scCO2 solution and (2)
in-situ polymerization of pyrrole within the I2 impregnated foam. The impregnation step was done in a 50
cm3 , custom-manufactured, high-pressure vessel by
Moran Tool and Die. It was fitted with a sapphire window (Sapphire Engineering, Inc.) and PEEK O-rings
(Valco Instruments Inc.) The experimental procedure
consisted of the following steps: (1) inserting a known
amount of iodine and a magnetic stirring bar into the
high-pressure vessel described above, (2) placing a
30 × 21 × 3.2 mm untreated PU sample on a supporting stand (designed to ensure uniform exposure of the
foam sample to the I2 /scCO2 solution while preventing direct contact between the foam and I2 particles)
within the vessel, (3) sealing the vessel and placing it
on a magnetic stirrer, (4) heating the vessel to the desired temperature, (5) filling the vessel with CO2 using
a syringe pump (ISCO, 260D) until the desired pressure was achieved and (6) starting the experiment by
turning the stirrer on. The PU foam came with a paper backing, which was removed and the sample was
S.L. Shenoy et al. / J. of Supercritical Fluids 28 (2004) 233–239
washed with acetone and dried at least 24 h prior to
the experiment. Upon completion of the impregnation
process (times were varied from 2 to 72 h), the CO2
was slowly vented into a fume hood. The impregnation of iodine was found to change the color of the
foam from off-white to orange–brown. The amount of
iodine impregnated was determined gravimetrically.
The temperature and pressure were monitored continuously during each run using a pressure transducer and
recorder (PX-302 and DP 3002-S, respectively, from
OMEGA Engineering, Inc.), and a temperature sensor
and recorder (GTMQSS-062U-6 and MONOGRAM,
respectively, from OMEGA Engineering, Inc.).
The pyrrole polymerization step was conducted in
a dessicator which had a volume of 3800 ml. The atmosphere in the dessicator was saturated with pyrrole
vapor by volatilization of liquid pyrrole placed at the
bottom of the dessicator. The iodine-impregnated foam
was transferred to the dessicator immediately after the
iodine sorption step, and the in situ polymerization
reaction took place instantaneously upon exposure to
the pyrrole vapor. For most of the experiments, the
polymerization was carried out at 21 ◦ C, the temperature inside the fume hood, in which the dessicator was
placed. In one experiment, however, the polymerization was carried out at 0 ◦ C in a small, 54 cm3 vessel, which was cooled by circulating antifreeze using
a refrigerated circulation bath through copper tubing
wrapped around the vessel. For the polymerizations
at 21 ◦ C, the polymerization times were varied from
24 to 72 h, but except where noted, a time of 48 h
was used. However, at 0 ◦ C, polymerization was conducted for a period of 115 h. When the polymerization was completed, the foam sample was removed
from the dessicator and dried in a fume hood for 4–7
days until all excess monomer/oxidant had evaporated
and the sample weight remained constant. The color
of a typical PU–PPy composite polymer was dark
black.
Electrical conductivity was measured with a
custom-built four-in-line probe. Four copper wires
(lines) lying parallel to the surface of a flat fixture
were pressed onto the surface of the foam. A constant current was applied to the outer two lines by a
Keithley 224 Programmable Current Source and the
voltage drop across the inner two lines was recorded
with a Keithley 197A Autoranging Microvolt DMM
meter.
235
Table 1
Solubility of iodine in scCO2
Temperature
(◦ C)
Pressure
(MPa)
Density
(g/cm3 )
Solubility
(wt.%)
40
50
13.7
13.7
0.762
0.670
0.60
0.83
3. Results and discussion
The solubility of I2 in scCO2 is given in Table 1.
The accuracy of the data is estimated to be ± 10%
based on solubility measurements with naphthalene
and phenantherene [15]. Increasing the temperature
by 10 ◦ C, form 40 to 50 ◦ C, at a constant pressure
increased the solubility by about 40%. For that same
10 ◦ C increase in temperature, the vapor pressure of I2
increases by a factor of 2 [16] and the density of CO2
decreases by 20%. That suggests that the effect of the
decrease in the solvating power of scCO2 due to the
density reduction is compensated by the increase of
the vapor pressure of I2 , and that the solubility of I2
in scCO2 is controlled primarily by the vapor pressure
of iodine at these conditions.
A series of experiments on sorption of I2 into the PU
foam were conducted to determine the time it takes to
reach equilibrium with the experimental arrangement.
The data are presented in Table 2 and indicate that
equilibrium was established in about 4 h. The maximum amount of iodine uptake was ca. 15 wt.% at this
condition. For those studies, an excess of I2 , i.e. more
than could be accommodated by the CO2 phase (based
on the solubility of I2 in scCO2 ) and the PU phase,
was placed in the vessel. Therefore, a separate iodine
phase was observed at the bottom of the vessel at the
Table 2
Variation of iodine uptake of foams with impregnation time conditions: 40 ◦ C, 13.7 MPa, initial amount of I2 in the vessel: 400 mg
Time of impregnation (h)
I2 uptake (wt.%)
2.0
4.0
4.0
4.0
4.0
4.0
6.0
24.0
9.5
14.2
15.2
14.4
15.8
15.4
13.2
15.4
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S.L. Shenoy et al. / J. of Supercritical Fluids 28 (2004) 233–239
end of each experiment. The data at 4 h show the reproducibility achievable in the impregnation process—the
mass uptake at the end of 4 h was 15.0 ± 0.7 wt.%.
Even though quite a few studies on the impregnation
of polymers with a variety of additives from scCO2 solutions have been reported in the literature, both data
and predictive methods on sorption equilibria for such
systems are scarce. In this study, we determined the
sorption equilibria by varying the amount of I2 placed
in the vessel and letting the system to reach equilibrium. After depressurization, I2 uptake by the foam
was determined gravimetrically and the concentration
of I2 in scCO2 at equilibrium was calculated from a
material balance. The sorption isotherms for I2 at 40
and 50 ◦ C are shown in Fig. 1 by solid and dashed lines
which are fits to experimental sorption data. Both of
the isotherms are linear and close to each other. The
higher uptake of iodine at 50 ◦ C may be attributed to
the higher solubility of iodine in the scCO2 phase at
50 ◦ C. The partition coefficients, defined as the ratio of
the concentration of I2 in the PU phase to the concentration of I2 in the scCO2 phase, were obtained from
the slopes of sorption isotherms in Fig. 1. The partition
coefficients were 28.1 ± 1.49 g CO2 /g foam at 40 ◦ C
and 24.9 ± 3.13 g CO2 /g foam at 50 ◦ C. For sorption
of solutes from supercritical fluid solutions by solids,
Fig. 1. Sorption isotherms for iodine–scCO2 –PU foam at 2000
psig.
the variation of the partition coefficient, K, with temperature at a constant pressure is given by [17]:
∂ln K
∂(1/T)
=
P
−H
R
where T is the temperature; H, the heat of sorption and R is the gas constant. The experimental partition coefficient data at the two temperatures were used
to calculate H. As expected, heat of sorption was
exothermic, H = −2.4 kcal/mol, and the low value
indicates physical sorption of iodine to the PU foam.
Conductive PU–PPy composites were obtained
by subsequent exposure of the iodine impregnated
foams to pyrrole vapor. The foams were non-marking
and looked macroscopically homogeneous. The PPy
concentration in the foams was determined gravimetrically by assuming that all of the iodine remained
in the foam as the dopant. The reproducibility of the
process was evaluated by running three replicate impregnation experiments for 4 h at 40 ◦ C and 13.7 MPa,
followed by a polymerization period of 48 h. The reproducibility of the I2 sorption, PPy production data
and conductivity achieved was good as shown by the
data in Table 3. The spread in the conductivity data is
understandable since conductivity in these materials
is a percolation process and at the conductivity in
question, very small differences in PPy concentration
produce large variations in the conductivity. However,
there did appear to be a bias in the conductivity of
the two sides of the specimens. Side 1, which was the
side facing the pyrrole at the bottom of the dessicator,
consistently had higher conductivity. While that result
suggests that there may have been some heterogeneity of the PPy distribution in the foam, it is not clear
whether this was a consequence of a slight gradient of
the iodine concentration from the impregnation step
or an effect arising from the diffusion of pyrrole into
the foam. In either case, the effect was minor in the
context of the study reported herein; the difference in
the averages for the conductivity of the two sides of
all the specimens, see Table 3, can be accounted for
by ca. a 3% difference in the PPy–I2 concentration.
The data given in Table 3 also show that the polymerization time had little effect on the amount of PPy
produced or on the resulting conductivity. That result
is consistent with previous work that showed that the
iodine incorporated in the iodine-impregnation step
S.L. Shenoy et al. / J. of Supercritical Fluids 28 (2004) 233–239
237
Table 3
Reproducibility tests and the effect of polymerization time (impregnation conditions: 40 ◦ C, 13.7 MPa, 4 h)
Sample
number
Polymerization
time (h)
I2 uptake
(wt.%)
PPy produced
(wt.%)
Conductivity (10−4 S/cm)
Side 1
Side 2
PU-IC-4
PU-IC-4/B
PU-IC-4/C
Average
PU-IC-4/D
PU-IC-4/A
Average of all samples
48
48
48
48
24
66
14.2
14.4
15.8
14.8 ± 0.9
15.4
15.2
15.0 ± 0.7
16.5
18.4
21.2
18.7 ± 2.4
16.5
20.0
18.5 ± 2.1
5.6
3.5
8.3
5.8 ± 2.4
4.9
6.2
5.7 ± 1.8
2.0
1.9
7.7
3.9 ± 3.3
3.0
4.1
3.7 ± 2.4
was the key factor in determining the amount of PPy
produced and the conductivity [18].
The variation of conductivity with PPy–I2 content
is shown in Fig. 2 for all the samples produced. The
conductivity dropped sharply for concentrations less
than 20 wt.% which is typical of a percolation phenomenon; in this case, the threshold concentration for
percolation of the PPy–I2 complex (taken here as the
concentration at which the conductivity achieved was
10−4 S/cm was ca. 22 wt.%. The data below the percolation region is denoted by a solid line and shows
the rapid increase in conductivity with small changes
in uptake. Increasing the PPy–I2 concentration beyond
ca. 22 wt.% had a much smaller effect on the conductivity. The dashed line in Fig. 2 is the conductivity
obtained by averaging the conductivity values for samples with PPy–I2 concentrations greater than 20 wt.%.
Fig. 3 shows how the concentration of the resultant
conductive PPy, i.e. the PPy–I2 complex, varied with
concentration of iodine in the PU foam following
the impregnation step. The amount of PPy–I2 formed
varied linearly with the amount of I2 incorporated
in the foam during the impregnation step. The data
for all the different experimental conditions fell on
the same curve, which indicates that reactivity to
pyrrole of the PU–I2 composite was not affected by
the impregnation conditions. The equation for the
line is (ccomplex = −1.50+2.27cI 2 ). The dashed lines
represent the 95% confidence intervals. The PPy
Fig. 2. Effect of PPy–iodine uptake on conductivity.
Fig. 3. Variation of PPy–iodine uptake with iodine uptake.
238
S.L. Shenoy et al. / J. of Supercritical Fluids 28 (2004) 233–239
ca. 20% and the conductivity by at least, an order of
magnitude. The reason for the increase in the PPy
formed is not clear. However, lower polymerization
temperatures are expected to produce fewer defects
in the conjugation length of the PPy, which would
at least partially explain the higher conductivity.
Although caution should be exercised formulating
conclusions regarding the polymerization temperature
based on only this single experiment, it would appear
that further study of the polymerization temperature is
justified.
4. Conclusions
Fig. 4. Variation of PPy uptake with iodine uptake.
concentration was estimated by a mass balance as described earlier in this paper and is plotted against the
initial iodine concentration in Fig. 4. The fit is linear
(cPPy =−1.50+cI 2 ) and the molar ratio of iodine to
pyrrole shows that there were about 4.5 pyrrole units
for each iodine molecule in this PPy–Iodine complex.
That ratio is similar to the 4:1 ratio reported by Kang
et al. [14] for the chemical synthesis of conductive
PPy using acetonitrile as a solvent.
The effect of polymerization temperature was studied by lowering the temperature from 21 to 0 ◦ C. This
was done in a small, 54 ml vessel instead of the much
larger dessicator, because this was the most readily
available set up for controlling the polymerization
temperature. The polymerization was carried out for
about 5 days (115 h), in order to ensure complete
polymerization of the pyrrole at the lower temperature
(the polymerization at 21 ◦ C was complete within 48
h). The results are shown in Table 4. The polymerization temperature influenced the PPy production
and the conductivity of the elastomer. Lowering the
temperature to 0 ◦ C increased the PPy produced by
A two-stage process for synthesizing conductive
composite polymers, comprised of a non-conductive
polymer, a PU foam, and conductive polypyrrole was
investigated. Iodine was an effective oxidant for the in
situ polymerization of pyrrole within the PU foams.
The reduced form of iodine produced as a result of the
oxidative polymerization of pyrrole functioned as the
dopant for the conductive polypyrrole. The impregnation of iodine was conducted from scCO2 solution.
Subsequent in situ polymerization of pyrrole resulted
in foams with conductivities ranging from 10−7 to
10−2 S/cm, depending on the amount of iodine initially incorporated. The foams had good mechanical
properties. A percolation threshold concentration for
conductivity of the PPy–I2 complex, chosen as the
concentration that produced a conductivity of 10−4
S/cm was about 22 wt.%.
The sorption behavior and equilibria of I2 during
the scCO2 impregnation process was studied, and the
distribution of I2 between the PU foam and the scCO2
solution was determined. Linear sorption isotherms
were obtained and the dependence of the partition coefficients on temperature was weak. The pyrrole polymerization temperature seemed to have an effect on
Table 4
Effect of polymerization temperature on PPy–PU composite foams (impregnation conditions: temperature, 50 ◦ C; pressure, 13.7 MPa; time,
4 h; for 0 ◦ C, polymerization container, 54 cm3 ; polymerization time, 115 h
Polymerization temperature (◦ C)
21
0
I2 uptake (%)
18.7
19.8
PPy produced (%)
29.5
35.1
Conductivity (S/cm)
Side I
Side II
1.0 × 10−3
3.0 × 10−2
3.0 × 10−4
2.9 × 10−2
S.L. Shenoy et al. / J. of Supercritical Fluids 28 (2004) 233–239
the PPy produced and on the conductivity of PPy–PU
foams. Lowering the temperature from 21 to 0 ◦ C increased the PPy production by almost 20% and the
conductivity by at least an order of magnitude.
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