Physical, mechanical, and conductivity properties of poly(3-hexylthiophene) -montmorillonite clay nanocomposites produced by the solvent casting method

Article abstract of DOI:10.1021/ma0490496

Polymer nanocomposites (PNCs) of poly(3-hexylthiophene) (P3HT) with organically modified montmorillonite (om-MMT) clay have been prepared by the solvent casting method. WAXS and TEM studies indicate exfoliated clay structure for lower clay content, but at higher clay content (5%, w/w) intercalated structures appear. The interchain lamella of P3HT exists in the nanocomposite, and the P3HT crystals become more ordered, showing better X-ray diffraction peaks. The thermal stability of PNCs increases significantly, and 1% clay content PNC exhibits the maximum thermal stability. The glass transition temperature (T_g), β-transition temperature (T_β), the melting point (T_m), and the enthalpy of fusion (ΔT) of the PNCs are increased as compared to those of pure P3HT. The storage modulus (G′) of PNCs showed a dramatic increase from that of pure P3HT, and the increase is larger in the temperature range 20-50 °C. The FTIR study indicates a decrease in Si-O-Si and Si-O stretching frequency for the exfoliated clay structure. The UV-vis study showed a blue shift of the π-π*transition band of P3HT in the PNCs, and they exhibit photoluminescence quenching which increases with increase in clay concentration. The dc conductivity of undoped PNCs remains almost the same as that of pure P3HT, but iodine-doped PNCs, however, exhibit 2.5-3 times greater conductivity than that of iodine-doped P3HT.

Full text of DOI:10.1021/ma0490496

Macromolecules 2004, 37, 8577-8584
8577
Physical, Mechanical, and Conductivity Properties of
Poly(3-hexylthiophene)-Montmorillonite Clay Nanocomposites Produced
by the Solvent Casting Method
Biplab K. Kuila and Arun K. Nandi*
Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700 032, India
Received May 14, 2004; Revised Manuscript Received September 7, 2004
ABSTRACT: Polymer nanocomposites (PNCs) of poly(3-hexylthiophene) (P3HT) with organically modified
montmorillonite (om-MMT) clay have been prepared by the solvent casting method. WAXS and TEM
studies indicate exfoliated clay structure for lower clay content, but at higher clay content (5%, w/w)
intercalated structures appear. The interchain lamella of P3HT exists in the nanocomposite, and the
P3HT crystals become more ordered, showing better X-ray diffraction peaks. The thermal stability of
PNCs increases significantly, and 1% clay content PNC exhibits the maximum thermal stability. The
glass transition temperature (Tg), â-transition temperature (Tâ), the melting point (Tm), and the enthalpy
of fusion (∆H) of the PNCs are increased as compared to those of pure P3HT. The storage modulus (G′)
of PNCs showed a dramatic increase from that of pure P3HT, and the increase is larger in the temperature
range 20-50 °C. The FTIR study indicates a decrease in Si-O-Si and Si-O stretching frequency for the
exfoliated clay structure. The UV-vis study showed a blue shift of the π-π* transition band of P3HT in
the PNCs, and they exhibit photoluminescence quenching which increases with increase in clay
concentration. The dc conductivity of undoped PNCs remains almost the same as that of pure P3HT, but
iodine-doped PNCs, however, exhibit 2.5-3 times greater conductivity than that of iodine-doped P3HT.
Introduction
region there exists Na+ and Ca2+, which can be replaced
by the alkylammonium and alkylphosphonium ions
rendering the clay into organophilic nature.2,3,9 The
organic modifier plays an important role for producing
the nanocomposite. It may either enhance the inter-
action between the clay and the polymer, making it
more suitably mixed, or it may favor the intercalation
of the polymer chain by dictating the gallery spacing.
Pure P3HT is a comblike polymer having a pendent
hexyl group with interchain lamella structure (Scheme
1). Depending on the extent of interdigitation of the
hexyl groups, two types of P3HT crystals are produced.10
Type I crystals are produced by casting the P3HT film
from its m-xylene solution very fast at 60 °C or by melt
quenching, and type II crystals are produced by casting
the film from the same solution slowly (1-2 days).
During the preparation of PNCs the pendent hexyl
group may interact with the organic modifier of the clay
and make the clay easily dispersed into it. The P3HT
interchain lamella may or may not change during such
mixing (Scheme 1). Thus, there is a possibility of
hierarchical structure for the presence of P3HT lamella
into the clay lamella. Depending on the penetration of
the polymer matrix into the silicate galleries of the
organically modified clay, various structures from in-
tercalated to exfoliated may be produced. Certainly
these different structures of the composite would yield
newer physical and mechanical properties of this im-
portant conducting polymer. The entrapment of con-
ducting polymer chains within the silicate layers may
have important consequences, and some study has been
recently reported for the polymer polyaniline (PANI)11-16
and polypyrrole (PPY).17 Various methods have been
used to synthesize the PANI-MMT clay nanocomposite,
e.g., sol-gel method, emulsion polymerization, elec-
tropolymerization, etc., and the properties were found
to be dependent on the preparation procedure. Dode-
Poly(3-hexylthiophene) (P3HT) is an important con-
ducting polymer because of its excellent electrical
conductivity, electroluminescence, and nonlinear optical
properties.1 However, its physical, thermal, and me-
chanical properties are not very good and finds difficul-
ties in many applications. Recently, polymer-clay nano-
composites (PNCs) have received significant research
attention for the large-scale improvement in the me-
chanical and physical properties.2-7 The large interfacial
area and the nanoscopic dimension of the clay nano-
particles in the polymer matrix yield such a dramatic
improvement.2 To improve the physical and mechanical
properties of this important conducting polymer, we
attempted to make P3HT-clay nanocomposites, and
here we report the preparation, characterization, and
properties of these nanocomposites. The conductivity
and optical properties of a conducting polymer depend
mainly on the arrangement of its chains. The clay
nanoparticales may influence the arrangement of P3HT
chains, as it may become confined into the clay tactoids.
So some variation of the above properties in the nano-
composites is expected. Certainly, a useful product may
come out by a combination of good mechanical, thermal,
and conductivity properties.
Montmorillonite (MMT) is a clay mineral consisting
of stacked silicate sheets (thickness ∼1 nm, length ∼218
nm) with high aspect ratio (220) and with a platelike
morphology.8 This high aspect ratio (ratio of length to
thickness) plays an important role for the enhancement
of mechanical and physical properties. Chemically,
MMT consists of two fused silicate tetrahedral sheets
sandwiching an edge-shared octahedral sheet of either
magnesium or aluminum hydroxide. In the interlayer
* Corresponding author: e-mail psuakn@mahendra.iacs.res.in.
10.1021/ma0490496 CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/16/2004

8578 Kuila and Nandi
Macromolecules, Vol. 37, No. 23, 2004
Experimental Section
Scheme 1. Schematic Models of Two Types of
Lamellar Crystals of P3HTa
(a) Samples. P3HT (regioirregular) was synthesized from
the monomer hexylthiophene that was prepared from 3-bro-
mothiophene and n-hexyl bromide using a Grignard reaction.
It was purified by passing petroleum ether solution through
a silica gel column.20 The polymer was synthesized in CHCl3
medium using anhydrous FeCl3 as initiator under a nitrogen
atmosphere.21 The polymerization was carried out at 0 °C for
24 h, and then the reaction mixture was poured into methanol
containing 10% (v/v) 12 N HCl. The precipitate was washed
with methanol and dissolved in chloroform. It was then
filtered, and the solution was dried in air and finally in a
vacuum at 60 °C for 3 days. The head-tail (H-T) regioregu-
1
larity of the sample was measured from H NMR spectra in
CDCl321 and was found to be 82 mol %. The molecular weight
(Mw) of the sample was measured by gel permeation chroma-
tography (GPC; Waters) using CHCl3 as solvent and polysty-
rene as standard. The characteristics of the samples are
presented in Table 1.
a Type I has a tilt on the polymer chain but type II polymer
chain is planar with a good interdigitation of hexyl groups.10
The MMT clay (PGV-PV-178-00) is a product of Nanocor,
Arlington Height, IL, was organically modified using cetyl-
trimethylammonium bromide (CTAB).22 The MMT clay was
dispersed in water, and CTAB was added in the ratio 2:1 by
weight. It was then stirred for 12 h. The modified clay was
collected by centrifugation (9000 rpm). Excess CTAB was
removed by repeated centrifugation with water while monitor-
ing the removal of bromide ion with AgNO3 test. Then it was
dried in a vacuum at 60 °C for 3 days. The exchange capacity
of the clay was measured by thermogravimetric analysis (TGA)
of organically modified MMT (om-MMT) clay by heating it from
30 to 800 °C under a nitrogen atmosphere in a Simadzu TGA
instrument (model TGA-50).23 The weight loss by heating Na+-
MMT clay from 30 to 800 °C was used to obtain the amount
of absorbed water and structural water present in the clay.
In our sample the thermogram from 30 to 120 °C indicates a
weight loss of 9.37% while that from 120 to 800 °C indicates
a weight loss of 5.02% (Figure 1 of Supporting Information).
The former is the absorbed moisture, and the latter is the
structural water present in the clay. The weight loss of om-
MMT from 120 to 800 °C was 34.23%, so the weight loss due
to cetyltrimethylammonium (CTA) ion is 29.21% and is equal
to 101.4 mequiv/100 g (equiv wt of CTA ion 288). This is the
exchange capacity of the clay used in the work.
(b) Preparation of Nanocomposite. The nanocomposite
of the organically modified MMT clay was prepared by the
solution casting method. The clay was first dispersed in
chloroform (12.5 × 10-5 g cm-3) by sonication in an ultrasonic
bath (60 W, model AVI0C, Eyela) operating at 28 kHz
frequency for 30 min. P3HT (2.5 × 10-3 g cm-3) was then added
and stirred at 40 °C for 3 h. Chloroform was dried off by
evaporation and was finally dried in a vacuum at 60 °C for 3
days. The nanocomposites are designated as PNC1, PNC2.5,
and PNC5; the number at the right-hand side indicates the
clay percentage (w/w).
(c) Wide-Angle X-ray Scattering. The WAXS experiments
were done on the above solvent cast films. The experiments
were carried out in a Seifert X-ray diffractometer (C 300) in
reflection mode with a parallel beam optics attachment. The
instrument was operated at a 35 kV voltage and a 30 mA
current and was calibrated with a standard silicon sample.
Nickel-filtered copper KR radiation (λ ) 0.154 nm) was used
in the work. The samples were scanned from 2θ ) 2° to 45° at
the step scan mode (step size 0.03°, preset time 2 s), and the
diffraction pattern was recorded using a scintillation counter
detector.
(d) Transmission Electron Microscopy (TEM). The
dispersity of the clay particles into the polymer matrix was
studied using a TEM (Philips C M 200) operated at an
accelerated voltage of 200 kV without staining. A thin section
(∼80 nm) of the above nanocomposite was made by encapsu-
lating it in an epoxy matrix and cutting at 25 °C through a
ultra-cryomicrotome (Ultracut R, Leica) equipped with a glass
knife. It was then placed on copper grid and was observed
through the microscope.
cylbenzenesulfonic acid (DBSA)-doped PANI-clay nano-
composite exhibited lower conductivity than that of
DBSA-doped PANI, the former exhibited the insulating
region while the latter exhibits the metallic region in
the temperature range 10-170 K.14 The low-conductiv-
ity state of the PANI-DBSA/clay system originates from
the resistivity due to insertion of clay layer. The PANI-
MMT composites exhibited better thermal stability,
anticorrosion performance, and larger reduction in gas
permeability than those in conventional PANI.11,12 Clay
platelets hinder the diffusion of oxygen/air within the
composite, causing increased thermal stability and
reduction in gas permeability. Some enhancement in
conductivity and thermal stability is recently achieved
in PNCs of electropolymerized PPY and caprolactum-
modified clay.17 The clay platelets provide an environ-
ment for the initial growth of ordered PPY films, and
the subsequent growth is not intercalative but by simply
coated on the platelet surface, causing higher conductiv-
ity. Both photoluminescence and electroluminescence
efficiency are enhanced in poly[2-methoxy-5-(2′-ethyl-
hexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/clay nano-
composite compared to that of pure MEH-PPV.18 The
enhancement of photoluminescence property has been
attributed for the reduction of interchain interaction and
isolating the conjugated polymer chains within a con-
fined geometry, thereby preventing exitons from finding
the lower energy trap. Therefore, improvement in the
physical properties of conducting polymers has been
achieved by making the PNCs. However, no study is yet
been reported on the important conducting polymer
polythiophene. Polythiophene is insoluble and infusible,
so it is difficult to process.19 Consequently, P3HT has
been chosen, as it is both fusible and soluble.19,20 So both
the processes of solution mixing and melt mixing can
be applied to produce the PNCs, and here we report the
process of solution mixing to make the PNCs. The main
objective of this work is to improve mechanical and
physical properties of P3HT without lowering the
conductivity. So the material may find its uses in
various electronic applications where good mechanical
property and thermal stability are required. Also, the
absorption and photoluminescence spectra of this sys-
tem have been studied to understand any change in
chain conformation and its relation with conductivity.
Here regioirregular P3HT has been used to prepare the
PNCs.

Macromolecules, Vol. 37, No. 23, 2004
Poly(3-hexylthiophene)-Montmorillonite Clay 8579
(e) Thermal Measurement. A Perkin-Elmer DSC-7 fitted
with Intracooler-1 and working under a nitrogen atmosphere
was used in this work. It was calibrated with indium before
each set of experiment. About 3 mg samples were taken in
aluminum pans and were sealed by a quick press. They were
heated at the scan rate of 10 °C/min from -30 to 230 °C. The
melting temperature (peak temperature) and enthalpy of
melting were measured.
The thermogravimetric analysis (TGA) experiments were
made using a Mettler Toledo TGA/(SDTA851e) instrument
under a N2 atmosphere at a heating rate of 10 °C/min.
(f) Dynamical Mechanical Property Measurements.
The mechanical properties of the polymers were measured
using a dynamic mechanical analyzer (DMA) (TA Instruments,
model Q-800). Films of 25 mm × 5 mm × 0.15 mm dimensions
were made from the composites by solution casting on a die,
and they were installed in the tension clamp of the calibrated
instrument. The samples were heated from -130 to 100 °C at
a heating rate of 10 °C/min. The storage modulus, loss
modulus, and tan δ were measured at constant frequency of
10 Hz with a static force of 0.02 N.
(g) dc Conductivity Measurement. The dc conductivity
of the samples at 30 °C were measured by the standard spring-
loaded pressure contact four-probe method.24 A circular film
of 1.3 cm diameter was used. A constant current (I) was passed
from a constant direct current source electrometer (Keithley
model 617) through two adjacent leads of the four probes, and
the voltage (V) across the other two leads was measured using
a multimeter (Keithley, model 2000). The conductivity (σ) was
calculated using the van der Paw relation
Figure 1. WAXS patterns of MMT-P3HT clay nanocompos-
ites: (a) pure clay, (b) organically modified clay, (c) P3HT, (d)
PNC1, (e) PNC2.5, and (f) PNC5. Inset: enlarged portion of
diffractogram (f) at lower diffraction angle (2-7°).
posites (PNC1 and PNC2.5) this P3HT interchain
lamellar peak occurs at some higher angle (2θ ) 5.24°);
i.e., the interchain lamellar length decreased by 0.05
nm. The absence of lamellar peak at 2.01 nm indicates
that the MMT clay becomes totally exfoliated. In the
PNC5 sample apart from the main peak at 2θ ) 5.09°
there are two humps at 2θ ) 4.4° and 3.5° (judged from
enlarging the X-ray pattern, inset of the figure). These
correspond to d spacings of pure polymer, om-clay (2.01
nm), and a new lamellar thickness (2.52 nm), respec-
tively. The new lamellar structure may be produced due
to expansion of clay lamella for the intercalation of
P3HT lamella (thickness 1.73 nm). In such a case
hierarchical structure (P3HT lamella within clay lamel-
la: i.e., a superstructure within other superstructure)
is produced. The intercalation of P3HT lamella may not
be limited to a single P3HT lamella; multiple P3HT
lamella may also be intercalated. Because of the limita-
tion of our X-ray apparatus to work at lower angles, we
could not ascertain other lower angle peaks.
An interesting observation is that the crystalline
diffraction peak of P3HT (2θ ) 23.2°) becomes more
distinct in the PNCs; probably crystals become more
ordered here. The X-ray diffraction peak positions and
their normalized intensities with respect to their lamel-
lar peak (I/I0) are presented in Table 2. It is apparent
that the peak positions of PNCs (except the lamellar
peak) are the same with that of pure P3HT, indicating
P3HT unit cell structure remains unaffected in the
PNCs. The crystal structure of P3HT is reported by
many workers,10,26 and there exists two types of P3HT
crystals (type 1 and type 2), depending upon the
orientation of the side chain along the main chain.10 The
increased I/I0 values of all the diffraction peaks for the
PNCs in comparison with those of pure P3HT supports
that the P3HT crystals became more ordered in the
PNCs. The X-ray diffraction pattern of P3HT and the
PNCs clearly indicate the formation of type 1 crys-
tals.10,20 Here the overlap (interdigitation) between the
side chains in neighboring stacks is minimal although
there can be ordering of the side chains across the
layer-layer interface.10 With addition of clay some more
σ ) (ln 2/πd)(I/V)
(1)
where d, the thickness of the film, was taken as the average
of four measurements at different places using a screw gauge.
Conductivity of two films of same sample, each with two trials,
was measured, and the average of four such measurements
yields the conductivity value.
(h) Spectroscopy. The UV-vis spectra of the P3HT-clay
nanocomposite films were studied by casting the films on
quartz plates and drying at 60 °C in a vacuum for 2 days. The
spectra were taken against air from 200 to 1000 nm in an UV-
vis spectrophotometer (Hewlett-Packard, model 8453) at 30
°C. The photoluminescence spectra of the samples were
measured in a Perkin-Elmer instrument (LS55 Luminescence
spectrometer) on solvent cast films. The photoexcitation at 490
nm was performed at a 45° angle of the thin film plane, and
the emission was detected at a right angle with respect to the
excitation beam direction. Each spectrum was normalized with
film thickness of one micrometer for comparison purposes. The
FT-IR spectra of the samples were performed from the films
in a Nicolet FT-IR instrument [Magna-1R-750 spectrometer
(series-11)].
Results and Discussion
WAXS Pattern and TEM Observation. Figure 1
shows the WAXS patterns of MMT clay, organically
modified clay, P3HT, and P3HT-MMT composites at
different MMT concentrations. It is clear from the figure
that in organically modified clay (om-MMT) the lamella
peak shifted to a lower 2θ value compared to that of
unmodified clay. This is similar to those observed in
previous works.4,25 Here the gallery spacing of MMT
shifts from 1.33 to 2.01 nm. The increase in gallery
spacing is due to the incorporation of cetyl chains of
CTAB into the gallery replacing the Na+ and Ca2+. This
certainly indicates that the MMT clay became organi-
cally modified. From the P3HT diffractrogram (diffrac-
trogram C) it is clear that P3HT has interchain lamellar
structure with lamellar thickness 1.73 nm.20 The inter-
chain lamellar structure is produced due to the side
chain ordering as shown in Scheme 1. In the nanocom-

8580 Kuila and Nandi
Table 1. Characteristics of the Samples
Macromolecules, Vol. 37, No. 23, 2004
poly(3-hexylthiophene)
(P3HT)
montmorillonite clay data from
Nanocor, Inc. PGV; PV-178-00
product: synthesized
product: Nanocor, Inc. USA
color: white
H-T regioregularity: 82%
cation exchange
Mw: 111200
polydispersity index (PDI): 1.78
melting temperature: 189 °C
capacity (mequiv/100 g): 101.4a
aspect ratio: 150-200
specific gravity: 2.6
a Measured value using TGA.
Table 2. Peak Positions, Lamella Thickness, and I/I0 of
Different XRD Peaks of P3HT in the Nanocompositesa
I/I0 at different
Figure 3. TGA thermograms of P3HT and PNCs under a N2
atmosphere at 10 °C/min heating rate: (a) P3HT, (b) PNC1,
(c) PNC2.5, and (d) PNC5.
peak positions (2θ)
lamellar
distance I0 (height
10.36
15.9
23.25
sample
(nm)
in cm)
(0.853 nm) (0.557 nm) (0.382 nm)
P3HT
PNC1
PNC2.5
PNC5
1.73
1.68
1.68
2.52
1.99
1.73
7.2
5.8
9.6
3.9
0.04
0.16
0.11
0.10
0.04
0.48
0.26
0.73
0.13
0.61
0.49
0.95
distance larger than 10 nm. But in the PNC5 there are
parallel stacks of clay lamella (dark contrast) supporting
intercalated structure. When the micrographs are ex-
amined with a peak scale lupe (7×) lamella distances
of 2.2, 2.8, and 5.6 nm are observed for the black
contrast clay lamella, indicating multiple intercalation
of P3HT lamella into the clay lamella. The P3HT
lamella are also observed through the peak scale lupe
(7×) (gray contrast) or by enlargement of TEM pic-
tures27 (Figure 2, Supporting Information), and single
to three such lamella intercalation was observed be-
tween two clay lamella (inset of Figure 2b). Thus,
hierarchical structure is produced for the PNC5 sample.
In the PNC2.5 sample the P3HT lamella, observed
through the peak scale lupe(7×) or by enlargement, are
not straight and parallel as in PNC5; rather, many folds
within the P3HT strands (e.g., 60° fold, 120° fold,
hairpin fold, etc.) are observed.27 As a result, they are
not in parallel position to the clay tactoids, but they are
mostly at different angles to the tactoids. The average
distance between the clay platelets is about 15 nm, and
the clay platelets are not parallel to each other. So the
clay platelets are exfoliated in PNC2.5, and multiple
clay platelets also coexist with individual silicate lay-
ers.11 The frequent folding of P3HT strands may allow
the hexyl side chains to interdigitate more, giving higher
crystallinity and also a small decrease in lamellar
thickness (cf. DSC and X-ray results).
a I0 is the intensity (height from baseline) of P3HT interchain
lamellar peak in each diffractogram. Figures in parentheses
indicate the d values.
Thermogravimetric Analysis. Figure 3 shows the
typical TGA thermograms of weight loss as a function
of temperature of P3HT and the PNCs as measured
under a N2 atmosphere. It is apparent from the figure
that all the PNCs have greater thermal stability than
that of pure P3HT and the PNC1 has the highest. The
degradation temperature as measured from the inter-
section of the tangent of the initial part and the
inflection part are 428.6, 456, 437.1, and 437.1 °C for
pure P3HT, PNC1, PNC2.5, and PNC5, respectively.
The reason for the dramatic increase in thermal stabil-
ity of PNC1 is not known, but it might be attributed to
the more ordered crystal (cf. DSC results) of P3HT at
this composite composition. It is suggested that in the
PNCs the clay particles act as barriers due to their high
aspect ratio and thereby hinder the degradation process.
Differential Scanning Calorimetry. In Figure 4
the DSC thermograms of pure P3HT and P3HT-MMT
nanocomposites (as prepared) are presented. It is clear
from the thermograms that PNC1 and PNC2.5 exhibit
10 and 5 °C higher melting point (Tm) than that of pure
P3HT, but the PNC5 exhibits almost same melting point
as that of pure P3HT. The enthalpy of fusion (∆H)
Figure 2. TEM micrographs of MMT clay-P3HT nanocom-
posites: (a) PNC2.5 (inset 3 times enlargement of box mark);
(b) PNC5 (inset 3 times enlargement of box mark).
interdigitation takes place, decreasing the interchain
lamellar spacing for PNC1 and PNC2.5 sample (Table
2).
To substantiate the X-ray results, we made a TEM
study of the samples, and the TEM micrographs are
presented in Figure 2a,b. It is clear from the micro-
graphs that PNC2.5 has exfoliated structure as there
are no parallel stacks of clay platelets; rather, there are
angular dispositions of clay platelets with average

Macromolecules, Vol. 37, No. 23, 2004
Poly(3-hexylthiophene)-Montmorillonite Clay 8581
Figure 4. DSC thermograms of the P3HT-PNCs: (a) P3HT,
(b) PNC1, (c) PNC2.5, and (d) PNC5.
values are 7.0, 6.1, 5.9, and 4.5 J/g for PNC1, PNC2.5,
PNC5, and pure P3HT, respectively. Thus, in PNC1 and
PNC2.5 both melting point and enthalpy showed an
increase indicating better crystal formation, supporting
the increased intensity of X-ray peaks. The reason for
larger increase of melting point and enthalpy of fusion
of PNC1 and PNC2.5 compared to those of PNC 5 and
P3HT is not clear to us. It may be proposed that 1%
omMMT clay produce good dispersion in the solution,
thereby helping the crystallization of P3HT by nucle-
ation and growth processes to produce more ordered
crystal. At higher percentage of clay it may not be so
highly dispersed. At these compositions also the exfo-
liated clay tactoids are at different angles to the P3HT
lamella. It might be probable that such an arrangement
of dispersed clay tactoids for their cohesive force of
attraction decreases the tilt of the P3HT side chains
(Scheme 1) and helps the side chains to interdigitate
more (Table 2), yielding an increased crystallinity and
melting point. But clay lamellas parallel to the P3HT
strands do not influence to decrease the tilt of side
chains, and as a result the crystallinity, melting point,
and the interchain lamellar distance remain unchanged
for the PNC5 sample.
Mechanical Properties. Figure 5a-c shows the
temperature dependence of storage modulus, loss modu-
lus, and tan δ of P3HT and of three PNCs. The storage
or elastic modulus (G′) relates the ability of the material
to store or return energy when oscillatory force is
applied to the specimen, and the loss modulus (G′′)
relates the ability to lose the energy. From Figure 5a it
is apparent that with increase in temperature storage
modulus decreases, and at about 100 °C it is negligibly
small for all the samples. The storage modulus of all
P3HT-clay nanocomposite increased than that of pure
P3HT. In these thermograms there are two breaks one
at lower temperature (∼-100 °C) and the other at
higher temperature (∼20 °C), indicating two different
types of transitions in the P3HT chains. It is clearer in
Figure 5b where the loss modulus is plotted with
temperature. There are two peaks in every sample; both
the peak temperatures increased with increasing clay
content. The lower temperature peak (Tâ) may arise
from the â transition (i.e., relaxation of hexyl side
chains) while the higher temperature peak may arise
from the R transition (i.e., relaxation of the main
chain).28,29 The latter may be described as the glass
transition temperature (Tg), and both transition tem-
peratures are presented in Table 3. The results show a
remarkable increase in Tg (∼40 °C) for the PNCs, but
the â transition temperature’s increase is much lower
(5-17 °C). So it appears that the effect of clay nano-
Figure 5. Mechanical property variation of P3HT nanocom-
posites with temperature: (a) storage modulus; (b) loss
modulus; (c) tan δ.
particles on the various relaxation processes of P3HT
chain is not quantitatively the same. The induced
rigidity of the P3HT main chain is greater than that of
the side chain. The latter effect is minimal in the PNC1
and PC2.5 (only 5°), but in the PNC5 the effect is quite
large (17°). No definite reason is yet known for this
different behavior, and the relative angular disposition
of clay platelets on the P3HT strands might be a factor.
In tan δ vs temperature plots (Figure 5c) the peak
temperature also varies as the R transition peak in loss
modulus. The difference between the tan δ peak tem-
perature and loss modulus peak temperature is ∼24 °C
for the PNCs, but for the pure P3HT the difference is
larger (36 °C). The reason for large difference between
tan δ peak and loss modulus peak in the case of P3HT
compared to PNCs is not clear. To explain the increase
in Tg in the PNCs, a probable explanation that may be

8582 Kuila and Nandi
Macromolecules, Vol. 37, No. 23, 2004
Table 3. Summary of Mechanical Properties of PNCs Measured by DMA
storage modulus (MPa)
sample
Tâ (°C)a
Tg (°C)a
-50 °C
(%) increase
20 °C
(%) increase
+50 °C
(%) increase
P3HT
PNC1
PNC2.5
PNC5
-102.1
-97.8
-98.3
-84.6
-16.0
25.2
1863
2124
2341
2074
122.6
859.7
1071
29.6
157.8
212.2
194.2
14
25.6
11.3
601.2
773.5
613.8
433.1
616.8
556
28.2
31.7
875.2
a From loss modulus peaks.
proposed is that the clay platelets attract each other in
the dispersed state due to van der Waals attraction
causing the P3HT chains to become more close-packed
structures, decreasing the free volume. As a result,
higher temperature is required to attain the required
free volume fraction (0.025) for the onset of segmental
Brownian motion,30 and hence Tgs in the PNCs are
increased. Additionally, in the case of intercalated
structure the confinement of P3HT chains into the clay
lamella may also yield increased Tg value, and the
highest Tg value of PNC5 (Table 3) may be attributed
for this reason.
The storage modulus (G′) values shown in Table 3
clearly indicate that there is a large mechanical re-
inforcement by the clay particles all over the tempera-
ture range for all the PNCs. The nanodimension clay
particles for its large surface area and its high aspect
ratio show such a large mechanical reinforcement.31 The
enhancement in the modulus appears in different
magnitude at different temperature ranges. Below Tg
the mechanical reinforcement is lower compared to
those at 20 and 50 °C. These results indicate that the
effect of clay on mechanical property reinforcement is
better in rubbery state than that in the glassy state. In
the rubbery state the movement of polymer chain
segments is relatively free so the reinforcement effect
of clay particles is more effective, causing a larger
increase in storage modulus.4,23 It is apparent from
Figure 5a and also from Table 3 that PNC2.5 has the
highest storage modulus value than that of the other
PNCs. This is probably because of the fact that exfoli-
ated clay structure yields better mechanical property,
and the greater the exfoliated clay concentration, the
larger is the storage modulus of the sample. PNC2.5 has
the greatest exfoliated clay concentration studied here
so it showed the highest storage modulus value.
FTIR Spectra. The PNCs are now widely character-
ized using FTIR spectroscopy.11,13,32,33 In Figure 6 the
FTIR spectra of om-MMT, P3HT, and PNCs are pre-
sented. For the om-MMT the bands at 2923, 2851, and
1470 cm-1 are for CH2 asymmetric stretching, sym-
metric stretching, and in-plane scissoring vibrations,
respectively. Silicate-related absorption bands are 1032
cm-1 for Si-O-Si stretching and 516 and 463 cm-1 for
Si-O stretching and Si-O bending, respectively. The
3623 cm-1 peak is the absorption band of lattice water
(O-H stretching).33 For P3HT the absorption band at
3054 cm-1 is for aromatic CH stretching, 2935 and 2856
cm-1 are for aliphatic CH stretching, 1510 and 1457
cm-1 are for ring stretching, 1377 cm-1 is for the methyl
deformation, 820 cm-1 is for the aromatic C-H out-of-
plane, and 725 cm-1 corresponds to the methyl rock-
ing.34 In the PNCs all the characteristic peaks of om-
MMT and P3HT are retained, at lower concentration
of the clay intensity of the corresponding peaks are
lower, and with increasing om-MMT concentration the
intensity of the peaks increases. From the FTIR data it
has been noticed that Si-O-Si frequency of om-MMT
Figure 6. FTIR spectra of P3HT-PNCs: (a) om-MMT, (b)
P3HT, (c) PNC1, (d) PNC2.5, and (e) PNC5.
clay at 1032 cm-1 decreased to 1028 and 1029 cm-1 for
PNC 1 and PNC 2.5 while it increased to 1035 cm-1 for
PNC 5. We propose that in the exfoliated state the Si-
O-Si bonds experience lesser hindrance to vibrate as
they are free compared to that in the lamellar state. The
increase in Si-O-Si vibration frequency for PNC 5 may
be due to the intercalation of P3HT lamella into the clay
lamella making the clay lamella structure more com-
pact, shifting the Si-O-Si frequency to a higher value.
The same explanation follows for the -Si-O- stretch-
ing frequency which occurs at 518, 512, 515, and 518
cm-1 for om MMT, PNC 1 PNC 2.5, and PNC5, respec-
tively. In this respect it may also be compared with Na+-
MMT clay where Si-O-Si stretching vibration occurs
at 1036 cm-1 and Si-O stretching vibration occurs at
520 cm-1 due to more compact nature of the lamella.
Optical Properties: (a) UV-Vis Absorption Spec-
tra. The UV-vis absorption spectra of P3HT and the
PNCs in the solid state are presented in Figure 7. Pure
P3HT shows an electronic transition band at 516 nm
which may be attributed to the π-π* transition of
π-conjugated segments of P3HT. In addition, there is a
hump at 272 nm which is arises for the π-π* transition
of thiophene unit.35 With addition of clay the 516 nm
peak shifts to lower wavelength (blue shift). The blue
shift is maximum at 2.5% clay content, and it is similar
to the blue shift in the UV-vis spectra of cadmium
sulfide (CdS) embedded P3HT nanowire.36 Probably a
different structure of P3HT in the composite than that

Macromolecules, Vol. 37, No. 23, 2004
Poly(3-hexylthiophene)-Montmorillonite Clay 8583
Figure 8. Photoluminescence spectra of P3HT and the PNCs
after excitation by radiation of 490 nm wavelength (normalized
to film thickness of 1 µm): (a) P3HT, (b) PNC1, (c) PNC2.5,
and (d) PNC5.
Figure 7. UV-vis spectra of P3HT and PNCs in the solid
state at 30 °C.
Table 4. Conductivity Values σ (S/cm) of Undoped and
I2-Doped P3HT and PNCs
of pure P3HT may be the reason for its absorption at
lower wavelength. It may arise due to both intrachain
and interchain mechanisms.37,38 Intrachain mechanism
arises from the backbone conformational change and
side group disorder while the interchain mechanism
arises from crystallization, gelation, induced rigidity,
etc.37 As Tg is increased in the PNCs so induced rigidity
may be an important cause for the short conjugation
length due to coiling of the chain, causing the blue shift.
Also, the increased crystallinity of PNCs makes the
P3HT chains in the compact form which may also be
responsible for the blue shift. At the 5% clay content
the PNC structure is interlamellar; the clay platelets
are not well dispersed, and the crystallinity is lesser
compared to that of the PNC2.5. As a result, a decrease
in blue shift from that of PNC2.5 is observed at the
PNC5.
(b) Photoluminescence Spectra. In Figure 8 the
photoluminescence spectra of P3HT and PNCs are
compared. It is apparent from the figure that there is
photoluminescence quenching in the PNCs, and it
increased with increasing clay concentration in the
composites. This is an interesting observation and is in
sharp contrast to the (MEH-PPV)/clay nanocomposite
system where there was an increase of photolumines-
cence in the PNCs.18 The observed quenching may be
due to rapid charge transfer from the photoexited states.
The singlet exiton being dissociated can no longer decay
radiatively to the ground state. Another probable reason
for the photoluminescence quenching may be due to the
energy transfer by Forster mechanism. The overlap of
the polymer photoluminescence with the absorption can
end the energy transfer with nonradiative decay.
Conductivity. The dc conductivity of both undoped
and I2-doped P3HT and PNC samples is presented in
Table 4. It is clear from the table that the conductivity
values of undoped samples are almost the same as each
other. However, PNC1 has somewhat increased value.
There is a blue shift in the UV-vis spectra of PNCs than
that of P3HT (Figure 7) but almost same conductivity
of them is not in accordance with the spectral observa-
tion. Here it is necessary to mention that in the CdS
sample
undoped
doped
P3HT
6.675 × 10-5
1.17 × 10-4
7.174 × 10-5
7.485 × 10-5
0.34
1.15
1.09
1.04
PNC1
PNC2.5
PNC 5
embedded P3HT nanowire (showing blue shift) a de-
crease in conductivity of the nanowire containing film
by two orders was observed.36 A probable reason for this
anomalous behavior may be proposed is that the blue
shift in UV-vis spectra is mainly due to the intrachain
contribution arising due to induced rigidity which
increases the band gap of P3HT in the PNCs. As a
result, the conductivity should decrease; rather, it
remains the same. Probably, the increased interchain
part of conductivity arising from the increased inter-
chain contacts due to induced rigidity compensates for
the intrachain contribution. The increased interchain
contact may also be responsible for the decrease in
photoluminescence efficiency of the PNCs as it helps for
the rapid charge transfer from the photoexited state.
In the iodine-doped samples, the conductivity of PNCs
are 2.5-3 times larger than that of pure P3HT. It is
really an interesting observation, and no definite reason
can be afforded at present. So it is possible to get PNCs
of P3HT where conductivity is fully retained, and in the
doped system it is improved. The mechanical properties
and thermal stability are highly improved compared to
that in the pure polymer. Such PNCs may therefore find
use in various electronic and optoelectronic applications.
Conclusion. P3HT produces nanocomposite with
organically modified MMT clay when cast from chloro-
form. At lower clay content the clay has exfoliated
structure, but at higher clay content (5%) it has lamellar
structure as evidenced from WAXS and TEM studies.
The interchain lamella of P3HT exists in the nano-
composite, and the P3HT crystals become more ordered,
showing better Xray diffraction peaks. The melting
point and enthalpy of fusion of P3HT are increased in
the PNCs; so also are the â-transition temperature and
glass transition temperature. The storage modulus of

8584 Kuila and Nandi
Macromolecules, Vol. 37, No. 23, 2004
(13) do Nascimento, G. M.; Constantino, V. R. L.; Temperini, M.
L. A. Macromolecules 2002, 35, 7535.
PNCs increased greatly than that of pure polymer. The
FTIR study indicates a decrease in Si-O-Si and Si-O
stretching frequency for the exfoliated clay structure.
UV-vis study shows a blue shift in the spectra of PNCs,
and photoluminescence quenching occurs in the PNCs.
The conductivity values of undoped samples remain
unaffected, but iodine-doped PNCs however exhibit
2.5-3 times increase in conductivity.
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Acknowledgment. We gratefully acknowledge the
Council of Scientific and Industrial Research (Grant 01-
(1919)/04/EMR -II), New Delhi, for financial support.
Also, the financial help from Department of Science and
Technology under Nano Science and Nano Technology
Program is gratefully acknowledged.
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