Thermal characterization of doped polyaniline and its composites with CoPc

Article abstract of DOI:10.1103/PhysRevB.69.235201

Thermal diffusivity of the composites of camphor sulphonic acid (CSA) doped polyaniline (PANI) and its composites with cobalt phthalocyanine (CoPc) has been measured using open cell photoacoustic technique. Analysis of the data shows that the effective thermal diffusivity value can be tuned by varying the relative volume fraction of the constituents. It is seen that polaron assisted heat transfer mechanism is dominant in CSA doped PANI and these composites exhibit a thermal diffusivity value which is intermediate to that of CSA doped PANI and CoPc. The results obtained are correlated with the electrical conductivity and hardness measurements carried out on the samples.

Full text of DOI:10.1103/PhysRevB.69.235201

PHYSICAL REVIEW B 69, 235201 (2004)
Thermal characterization of doped polyaniline and its composites with CoPc
Sajan D. George,1 S. Saravanan,2 M. R. Anantharaman,2 S. Venkatachalam,3
P. Radhakrishnan,1 V. P. N. Nampoori,1 and C. P. G. Vallabhan1
1International School of Photonics, Cochin University of Science and Technology, Cochin 682 022, India
2Department of Physics, Cochin University of Science and Technology, Cochin 682 022, India
3V. S. S. C, Thiruvananthapuram 685 022, India
(Received 31 October 2003; revised manuscript received 1 March 2004; published 8 June 2004)
Thermal diffusivity of the composites of camphor sulphonic acid (CSA) doped polyaniline (PANI) and its
composites with cobalt phthalocyanine (CoPc) has been measured using open cell photoacoustic technique.
Analysis of the data shows that the effective thermal diffusivity value can be tuned by varying the relative
volume fraction of the constituents. It is seen that polaron assisted heat transfer mechanism is dominant in CSA
doped PANI and these composites exhibit a thermal diffusivity value which is intermediate to that of CSA
doped PANI and CoPc. The results obtained are correlated with the electrical conductivity and hardness
measurements carried out on the samples.
DOI: 10.1103/PhysRevB.69.235201
PACS number(s): 78.20.Hp, 81.05.Qk, 66.30.Xj, 82.35.Cd
I. INTRODUCTION
chanical properties of the composites of these materials with
CSA doped PANI have great physical and practical signifi-
cance, especially from the device point of view.
The discovery of the fourth generation of polymeric ma-
terials, viz., conducting polymers, has created a new area of
research on the boundary between chemistry and condensed
matter physics.1 The structural, electrical, and optical prop-
erties of these materials can be tuned using various doping
techniques such as chemical, electrochemical, photodoping,
etc.2 This versatility makes them ideal candidates for the
fabrication of several devices such as polymer batteries, elec-
trochemical windows, and light emitting electrochemical
cells.3 Polyaniline (PANI), the conducting polymer available
in metallic form, after postdoping with sulphonic acid, has
emerged as a useful material both for research as well as
industrial purpose due to its high stability, simpler polymer-
ization procedure with high yield, availability of inexpensive
monomer, etc.1,4 Also among the many conducting polymers,
PANI is a good candidate for preparing conducting polymer
composites since it is stable, both thermally and environmen-
tally. Conducting polymer composites based on PANI and
cobalt phthalocyanine (CoPc) is a suitable candidate for
making rechargeable batteries.5
The past two decades have witnessed the emergence of
photoacoustic and related photothermal techniques as effec-
tive analytical tools for the evaluation of transport and opti-
cal properties of materials with considerable accuracy.6 In
this Brief Report the measurement of thermal diffusivity of
camphor sulphonic acid (CSA) doped PANI using open cell
photoacoustic technique is reported for the first time. As the
composite allows tunability in thermal properties of the par-
ent compound, a systematic study of its compositional varia-
tion on effective thermal diffusivity was performed. The ex-
perimentally obtained results are correlated with electrical
conductivity and hardness measurements performed on all
these samples. Analysis of our observations reveals that
phonons play a dominant role in the heat transport mecha-
nism in these materials. CoPc possesses a planar structure
and has the highest electrical conductivity amongst the other
metallic phthalocyanines. Hence the evaluation of the ther-
mal properties and its correlation with electrical and me-
II. SAMPLE PREPARATION
In order to prepare the samples under investigation, ini-
tially the monomer aniline and aqueous percholoric acid
were kept at 4°C and ammonium per sulphate was added
drop by drop to these starting materials. This mixture was
stirred for 2 h and then filtered and washed with water and
methanol. Subsequently, PANI doped with percholorate was
transformed into insulating polyaniline emarldine base using
hydrazine hydrate. The resulting compound was doped with
50% CSA in nitrogen atmosphere to produce an emeraldine
salt form of this material which is then purified and dried in
vacuum.5,7The tetramer of cobalt phthalocyanine was pre-
pared, purified, and characterized by the solution method,
wherein cobalt sulphate, pyrometallic dianhydride, excess
urea, ammonium chloride, and ammonium molybdate were
homogenized well and heated at 180°C in nitrobenzene am-
bient for 12 h. The reaction mixture was then cooled and
washed with methanol repeatedly to remove nitrobenzene.
This crude product was further boiled with 2N sodium hy-
droxide containing sodium chloride and then filtered. The
residue was acidified with hydrochloric acid, washed sever-
ally and then dried to obtain the final product, viz., phthalo-
cyanine tetramer.8 The composites of these materials were
prepared as follows. The powdered PANI doped with CSA
was blended with tetrameric cobalt phthalocyanine by mix-
ing them homogenously in agate mortar. The materials were
finally prepared in the following volume fractions: 90%
PANI:CSA - 10% CoPc, 50% PANI:CSA - 50% CoPc, and
10% PANI:CSA - 90% CoPc. All these samples were pallet-
ized and are black in color.
III. EXPERIMENTAL SETUP AND THEORETICAL
BACKGROUND
In the present investigation, optical radiation at 488 nm
and at 50 mW power from an Argon ion laser is chopped
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SAJAN D. GEORGE et al.
PHYSICAL REVIEW B 69, 235201 (2004)
FIG. 1. The amplitude spectrum of the sample PANI:CSA.
FIG. 2. Phase spectrum of all the samples under
investigation.
(Stanford Research Systems SR 540) before it impinges on
the surface of the sample, which is fixed to the photoacoustic
cell cavity with vacuum grease. The output amplitude and
phase of the PA signal generated in the cavity was detected
using a sensitive electret microphone (Knowles BT 1754)
and measured using a dual phase lock-in amplifier (Stanford
Research Systems SR 830). Details of the experimental setup
are described elsewhere.9 All the samples under investigation
have a thickness of ϳ600 ␮m.
sample and the inverse of thermal diffusivity yields a mea-
sure of the time required to establish thermal equilibrium in
systems for which a transient temperature change has
occurred.6,9 Figure 2 shows the phase spectrum of all the
specimens under investigation. The values obtained for the
best fit between the experimental and the theoretical spec-
trum suggested by Eq. (1) are tabulated in Table I.
From Table I is obvious that CSA doped PANI exhibits
maximum value for thermal diffusivity. This can be under-
stood in terms of carrier assisted heat transport mechanism in
these materials. Recent studies on conducting polyaniline
and other polymers have created a debate on whether its
structure is homogenous or inhomogeneous.13 In general, the
conduction processes in conducting polymers is taking place
through either hopping or tunneling processes.14 It has al-
ready been reported that the conduction processes in the
polyaniline protonated partially with emeraldine can be ex-
plained on the basis of interpolaron hopping mechanism.15
The conduction phenomena in samples protonated with em-
eraldine base can be explained on the basis of granular po-
laron model.16 However, for highly doped polyaniline, as in
the present case, the density of charge carriers is approxi-
mately equal to the density of protonated charge sites. In
such cases, the increase in conductivity (diffusivity) occurs
due to three factors; the increased “metallic” content of the
sample, a reduction in tunneling/hopping distance between
the metallic regions, and the increased charge-carrier
density.4 Polyaniline doped with a sulphonic acid such as
According to the theoretical background suggested by
Rosencwaig and Gersho on the basis of the thermal piston
model,10 at low frequencies, thermal diffusion is the major
mechanism of heat transport in materials. In the low fre-
ͱ
quency range ͑f͒, the thermal diffusion length, ␮= ␣s/␲f
(where ␣s is the thermal diffusivity of the specimen under
investigation) is greater than the sample thickness and the
specimen is said to be thermally thin. It becomes thermally
thick with increase in modulation frequency and is charac-
terized by a critical frequency ͑fc͒in the amplitude spectrum
of the PA signal. Figure 1 shows the PA amplitude spectrum
for CSA doped PANI. By taking into account the thermoelas-
tic bending of the specimen, due to the temperature gradient
existing within the sample, the phase of the PA signal under
heat transmission configuration varies as11
␲
1
⌽el Ϸ + arctan
͑1͒
ͫ ͬ
x − 1
2
where x=ls͑␲f /␣s͒1/2, with ls as the sample thickness. Thus
by using ␣s as the fitting parameter in the PA phase spectrum,
the thermal diffusivity value of the specimen under investi-
gation can be evaluated.
TABLE I. Thermal diffusivity values of the samples under
investigation.
IV. RESULTS AND DISCUSSION
Sample
Thermal diffusivity ͑cm2s−1
͒
The present experimental setup is calibrated by evaluating
the thermal diffusivity value of aluminium and GaAs. The
measured values (0.98 cm2 s−1 and 0.26 cm2 s−1, respec-
tively, for Al and GaAs) are found to be in agreement with
earlier reported values of these specimens (0.99 cm2 s−1 and
0.26 cm2 s−1, respectively).12 Thermal diffusivity value es-
sentially determines the rate of heat diffusion through the
PANI:CSA
0.760±0.0004
0.404±0.0003
0.356±0.0002
0.289±0.0003
0.241±0.0002
PANI:CSA ͑90%͒ and CoPc ͑10%͒
PANI:CSA ͑50%͒ and CoPc ͑50%͒
PANI:CSA ͑10%͒ and CoPc ͑90%͒
Co-Pc
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THERMAL CHARACTERIZATION OF…
PHYSICAL REVIEW B 69, 235201 (2004)
camphor sulphonic acid can be classified as an inhomog-
enously disordered metal comprising of “metallic islands”
separated by insulating barriers (metallic island model).17
However, doping of polyaniline with CSA at this high level
͑50%͒ results in interchain coupling and consequently an
enhanced ordering between crystalline regions (metallic re-
gions) as well as on the chains bridging the metallic regions.
Such an ordering results in the increase of localization length
of electronic wave function and hence allows the coherent
transport of both heat and electricity. Transport models for
three-dimensional amorphous semiconductors have often
been used to account for the charge delocalization phenom-
ena in conducting polymers, despite fundamental
differences.18 In conducting polymers the dopant ions are
positioned interstitially between chains, whereas in conven-
tional semiconductors they are usually substituted directly
into the host lattice.18 Further, covalent bonding along poly-
mer chains and weak bonding between them result in a
quasi-one-dimensional morphology which has an important
role in the charge delocalization of these systems.19 In a rela-
tively ordered crystalline material, as in the present case,
protonation of polyaniline with sulphonic acid results in a
decrease in hopping distance and consequently enhances the
hopping mechanism (phonon-assisted tunneling between
electronic localized states centered at different positions).20 It
is difficult to distinguish between different conduction
mechanisms such as quasi-1D variable range hopping
tivity as well as thermal diffusivity of its constituents.26 The
thermal properties of all the composites are discontinuous
functions of the location and consequently neither Fourier’s
law nor heat conduction equation can be applied.26 However,
the effective thermal parameters, i.e., the properties of
equivalent homogeneous material that produces the same
physical effects of the specimen under investigation, is of
great significance and has wide applications in device fabri-
cation. Eventhough the effective heat capacity follows the
mixture rule, it has already been reported that the effective
thermal diffusivity of composites depend on the thermal dif-
fusivity of constituents as well as on their relative volume
fraction.26 In the present case also, the composites of CSA
doped PANI and CoPc exhibit a thermal diffusivity value,
which is intermediate to that of CSA doped PANI and CoPc.
This can be ascribed to the existence of interfacial thermal
contact resistance between the different constituent phases in
a composite as well as on their thermal expansion
mismatch.27 The existence of such thermal barriers results in
a lowering of the effective thermal diffusivity of the compos-
ite. It is evident from our observations that introduction of
10% CoPc results in large inhomogeneties in the sample,
which consequently lowered its thermal diffusivity in a sub-
stantial manner. However, further increase in volume fraction
of CoPc does not cause considerable variation in their al-
ready inhomogeneous distribution and is evident in their
thermal diffusivity values. The present analysis shows that
the combination of a good thermal diffuser with a bad dif-
fuser can result in composites of intermediate thermal diffu-
sivity value. The measured thermal diffusivity value of CoPc
falls in the typical range of the thermal diffusivity of
phthalocyanines.28
In order to ensure the effective charge transport mecha-
nism in these materials, both dc and ac electrical conductiv-
ity measurements were carried out by employing the two
probe technique on the specimen placed in a conductivity
cell under high vacuum, ͑10−5 Torr͒ using Keithley Voltage
source. The results obtained are tabulated in Table II. It is
seen from the table that the variation in electrical conductiv-
ity of the samples also follows the thermal diffusivity mea-
surements. However, the order of variation is small in the
case of thermal diffusivity as compared to electrical
conductivity.29 This is due to the fact that thermal energy
transport mechanism in conducting polymers is dominated
by phonon assisted mechanism, whereas electrical conduc-
tion is dominated by the variable range hopping process and
metallic diffusion of electrons.30
mechanism,21
3D
hopping
with
electron-electron
interaction,22 tunneling between mesoscopic metallic islands
23
or correlated hopping between polaronic clusters in
polyaniline.24 Nevertheless, experimental investigations
show that results can be explained very well in terms of the
metallic content as well as in terms of dominant hopping/
tunneling mechanism. In addition to that, previous x-ray
analysis shows that there is a significant degree of crystallin-
ity in PANI:CSA.25 In these crystalline regions of the poly-
mer, a precise phase order exists between adjacent polymer
chains and this is expected to allow coherent carrier transport
along and between individual chains. This means that carrier
delocalization can occur in more than one dimension, on a
scale larger than the average interchain separation.4 There-
fore, the mean-free-path is limited not by scattering at inter-
chain transfer events but by phonon scattering due to thermal
motion of the crystal lattice, or by molecular vibrational
modes and it become apparent in bulk conductivity of the
specimen at room temperature.4 Therefore, 50% CSA doped
polyaniline can be considered as a heterogeneous conductor
in which two transport mechanisms such as metallic diffu-
sion within the crystalline regions and temperature activated
transport in the disordered region contribute to the conduc-
tion mechanism. Hence the increase in mobility of charge
carriers with protonation of polyaniline with sulphonic acid
results in strong interaction between electrons and phonons
due to lattice vibration, especially at room temperature,
which in turn results in coherent transport of thermal energy
via electron-phonon interaction (polarons). As a result, 50%
CSA doped PANI, exhibits highest value for thermal diffu-
sivity.
The effective thermal parameters of composites depend
on the thermal properties of its constituents as well as on the
microstructural parameters such as volume fraction of each
phase, shape, size, and distribution of the particles. Hence a
study on the correlation between thermal diffusivity and
hardness of the material was carried out, wherein, the surface
hardness of all the specimens under investigation were mea-
sured using the indentation technique (Shore D hardness
technique). The values obtained for the hardness of the
samples under investigation are given in Table II. It is seen
from the Tables I and II that there exists an inverse relation
between thermal diffusivity and hardness of the specimen, as
observed by other researchers.31 The increase in relative vol-
The effective thermal diffusivity value of composite ma-
terials has been reported to depend on the thermal conduc-
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SAJAN D. GEORGE et al.
PHYSICAL REVIEW B 69, 235201 (2004)
TABLE II. Electrical conductivity and hardness of the samples under investigation.
ac conductivity
͑ϫ102S/m͒
100 kHz & 300 K
dc conductivity
͑ϫ102S/m͒
Shore D
hardness
Sample
PANI:CSA
1.3565
1.0125ϫ10−1
27.892
20.484
10
15
90% PANI:CSA: 10%
CoPc
50% PANI:CSA: 50%
CoPc
5.4238ϫ10−2
2.519ϫ10−2
1.115ϫ10−2
9.6763
18
22
25
10% PANI:CSA: 90%
CoPc
5.9871ϫ10−1
4.1109ϫ10−3
CoPc
ume fraction of CoPc increases the hardness of the material
while the increase in interface thermal resistance and thermal
barrier resistance causes a decrease in the effective thermal
diffusivity value. The present investigation also shows that
the variation in volume fraction of the composites allows
tunability in their mechanical properties and a correlation of
various thermophysical properties of such heterogeneous
systems is possible.
tivity measurements substantiate the thermal diffusivity ex-
periment and suggest a decrease in effective carriers for
transport of thermal energy with the increase in relative frac-
tion of CoPc in the composite.
ACKNOWLEDGMENTS
This work is supported by Netherlands University Federa-
tion For International Collaboration (NUFFIC), The Nether-
lands under the MHO assistance to International School of
Photonics. S. D. G. acknowledges the Council of Scientific
and Industrial Research, New Delhi for providing financial
assistance. He also wishes to acknowledge the useful discus-
sion with Dr. A. Deepthy and Dilna S. during the preparation
of manuscript. V. P. N. N. also acknowledges University
Grant Commission for financial assistance through a research
award project. M. R. A. and S. S. acknowledge the ISRO-
RESPOND (Grant No. 10/3/354 dated 23.02.1999) for finan-
cial assistance received in the form of a project.
V. CONCLUSIONS
In conclusion, a study on the measurement of the thermal
diffusivity value of CSA doped PANI and its composites
with CoPc using open cell photoacoustic technique are pre-
sented. From the present investigation it is clear that, with
proper choice of the volume fraction of specimens having
different thermal diffusivity, we can modify the effective
thermal parameters of the composites. The electrical conduc-
Electronic mail: sajan@cusat.ac.in
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