chapter 4. pani–pvp soft-templated page no. blend materials...
TRANSCRIPT
55
Chapter 4. PANI–PVP SOFT-TEMPLATED Page No.
BLEND MATERIALS
4.1
Chemical polymerization, AC electrical properties and
conduction mechanism
57
4.1.1 Introduction 57
4.1.2 Synthesis and characterization of polyaniline blend materials 58
4.1.3 FTIR spectral studies 59
4.1.4 UV-visible spectral studies 61
4.1.5 XRD analysis 62
4.1.6 SEM studies 63
4.1.7 AC impedance behavior of fresh polyaniline and its blend materials
65
4.1.8 AC conduction mechanism 73
4.1.9 Repetition of AC impedance analysis on studied samples 76
4.1.10 Conclusions 86
4.2
Polyaniline-PVP and/or SDBS blend materials:
Interfacial polymerization and AC electrical properties 88
4.2.1 Introduction 88
4.2.2 Synthesis and characterization of polyaniline and its blend materials
89
4.2.3 Formation of polyaniline materials 89
4.2.4 FTIR spectral studies 90
4.2.5 UV-visible spectral studies 93
4.2.6 Thermal studies 94
56
4.2.7 SEM studies 96
4.2.8 AC impedance behavior of polyaniline and its blend materials 97
4.2.9 AC electrical properties 101
4.2.10 Conclusions 106
4.2.11 Comparison of properties of chemically and interfacially polymerized PANI blend materials.
107
References 108
57
4.1. Chemical polymerization, AC electrical properties
and conduction mechanism
4.1.1. Introduction
The synthesis of PANI materials is nowadays focused towards blends, composites
and copolymers [1,2] using common insulating polymers with the aim of altering its
properties. Polymer blends of PANI with insulating polymer PVP are of special interest
and are studied for different reasons; for instance, effectiveness in electron transport,
steric stabilizer, co-dopant, effectiveness in dispersion of PANI particles and dictation of
particle structures [3,4]. In our previous study [5] we observed that PVP could act as a
polymeric co-dopant for PANI; PANI with PVP of less than or equal to aniline
concentration exhibits DC conductivity in increasing order, and with excess PVP the
blend has a far-higher conductivity than that of pristine PANI. However, the
AC conductivity and its scaling behavior of PANI-PVP blends have not yet been studied.
PANI is normally prepared by chemical or electrochemical polymerization [6].
In chemical preparation, APS has been mainly used as an oxidant since the report by
MacDiarmid et al [7]. The standard oxidation potential of APS is 1.94 V. Though FeCl3
has a value of 0.77 V which is lower than that of APS and is close to the potential of
electrochemical polymerization (0.8 V vs SCE) [8], it has not been extensively studied as
an oxidant to prepare PANI. The lower potential possibly reduces the side reaction during
polymerization. Furthermore, FeCl3 is soluble in ether and polar organic solvents and
hence it can polymerize aniline in these solvents where APS could not act. Few studies on
chemical polymerization of aniline to PANI with FeCl3 oxidant are known [8-11].
Thus, FeCl3 offers some advantages when compared to the most used APS. Hence, in the
58
present work, FeCl3 has been chosen as the oxidant for the synthesis of PANI and its
blend materials.
PANI-PVP blends were prepared from aniline hydrochloride and PVP with FeCl3
oxidant involving less, equal and higher concentrations of PVP compared to ANI.HCl in
the molar stoichiometric ratios of 1:0.5, 1:1 and 1:3 (designated as PANI-PVP 1,
PANI-PVP 2 and PANI-PVP 3 respectively). PANI chloride salts maintaining monomer:
oxidant ratio of 1:2 and 1:2.5 were also synthesized in the same way without PVP for
comparison purpose, and were labeled as PANI and PANI-O respectively. PANI-O was
also chosen for the study in order to examine the effect of oxidant concentration, if any,
on the properties of PANI.
Since complex impedance spectroscopy (CIS) is a powerful tool for the study of
electrical properties such as bulk resistance (Rb), bulk AC conductivity (σb), bulk
capacitance (Cb) and bulk dielectric constant (εb) particularly of semiconducting
materials, an attempt is made herein to study these electrical properties of PANI and its
blend materials as a function of frequency and temperature. Since conducting polymer
materials have polaron and bipolarons as the charge carriers, they have semiconductivity
and CIS is applied. Also different conduction mechanism models are applied to get an
insight into the conduction process. Interesting results are observed from frequency and
temperature-dependent AC conductivities of the PANI materials with regard to their
morphological and other physico-chemical properties.
4.1.2. Synthesis and characterization of polyaniline materials
PANI-PVP blend materials were prepared with FeCl3 oxidant by in situ chemical
oxidative polymerization at room temperature in aqueous HCl medium (see Ch. 3).
The synthesized polymer materials were characterized by FTIR and UV-visible
59
spectroscopy, XRD analysis and SEM studies. Further they were investigated for AC
electrical properties and conduction mechanism by CIS analysis.
4.1.3. FTIR spectral studies
Figure 4.1 (a-f) illustrates the FTIR spectra of fresh PANI, PANI-PVP blends,
PANI-O and free PVP. The characteristic peaks of PANI (spectrum a, Fig. 4.1) are
assigned as follows: The first broad peak at 3426-3433 cm-1 is due to O-H stretching of
the absorbed/occluded/coordinated water of PANI. The second sharp intense peak at
3230-3232 cm-1 occurs due to C-H and N-H stretching vibrations of PANI [5,12].
The main peaks of PANI appear at 1570 and 1502 cm-1 which correspond to stretching
vibrations of quinoid and benzenoid rings respectively. The latter peak is more intense
than the former. Other peaks at 1292 and 1238 cm-1 correspond to C−N stretching of
imine and secondary amine of PANI backbone respectively. The broad peak at 1143 cm-1
arises from C−H in-plane bending vibration and is formed during protonation. The
832 cm-1 peak is attributed to 1,4-coupling of aromatic ring and peak at 735 cm-1 to C−H
out-of-plane bending. The IR spectrum of PANI is in good agreement with the previously
reported IR spectrum in literature [5,12]. The intense carbonyl peak of free PVP
(spectrum f, Fig. 4.1) observed at 1663 cm-1 gets red-shifted to 1636, 1640 and 1641 cm-1
as a weak peak in PANI-PVP 1, PANI-PVP 2 and PANI-PVP 3 blends respectively
(spectra b, c and d, Fig. 4.1) indicating the inclusion of PVP and its interaction with
PANI. Similarly, the vibrational peaks of PANI are modified both in peak position and
intensity upon incorporation of PVP. For example, the quinoid, benzenoid and amine
peaks of PANI are red-shifted marginally from their positions suggesting the interaction
of these groups with PVP. The charge delocalization peak (C-H in-plane peak) appearing
at 1143 cm-1 in PANI, which has a direct correlation with conductivity [13], appears
60
with an enhanced intensity in
all PANI-PVP blends.
This suggests a greater
exposure/presence of this
group in blends with spectral
allowedness and in turn an
increase in conductivity.
Obviously, all these changes
indicate the structural and
chemical modifications of
PANI in its blends.
In PANI-O (spectrum e,
Fig. 4.1) also, there is a slight
red-shift in the vibrational
peaks of quinoid and
benzenoid groups when
compared to PANI.
A marginal increase in
intensity of benzenoid
relative to quinoid is also
noticeable. These features are
similar to those observed in
PANI-PVP blends and hence
imply the same inference.
Figure 4.1. FTIR spectra of (a) PANI, (b) PANI-PVP 1,
(c) PANI-PVP 2, (d) PANI-PVP 3, (e) PANI-O
and (f) free PVP.
61
4.1.4. UV-visible spectral studies
The UV-visible absorption spectra of PANI and its blends in methanol and NMP
solvents under saturated dissolution condition are shown in Figs. 4.2 A and B
respectively. In methanol solvent (Fig. 4.2A), the general shape of the spectra is identical
for all the polymer materials. The common feature of all the spectra is the presence of
relatively sharp peak in the region 374 to 383 nm followed by a long tail extending to
higher wavelength. This sharp peak in previous studies [14,15], has been attributed to the
π–π* transition of benzenoid units of the polymer chain. The spectral observation thus
confirms that the prepared polymer materials are in emeraldine salt form.
In NMP solvent (Fig. 4.2B), all the spectra of synthesized PANI materials appear
with higher intensity and peak sharpness in the region 375 to 379 nm and are attributed to
the π–π* transition. In addition, a broad weak hump around 600 nm with a long tail is also
seen in all polymer samples indicating the formation of a little amount of emeraldine
base. Since the solvent NMP is of coordinating nature [16], it coordinates to the
protonated imine site after displacing dopant ions and thereby converts the salt into base.
Figure 4.2. UV-visible spectra of (a) PANI, (b) PANI-PVP 1,
(c) PANI-PVP 2, (d) PANI-PVP 3 and (e) PANI-O in
(A) methanol and (B) NMP solvents.
62
This band has been assigned to a benzenoid to quinoid excitonic transition of the
polymer chain [17]. The spectral observation thus confirms that the major portion of the
prepared polymer materials is in conducting form.
4.1.5. XRD analysis
The wide angle powder XRD patterns of PANI-PVP blends together with PANI
and PANI-O are shown in Fig. 4.3 and the data are presented in Table 4.1. Two broad
peaks appear at 2θ = 18.9º (peak 1) and 25.8º (peak 2) for PANI. These positions are in
accordance with the previous report [JCPDS-53-1718]. On matching the observed pattern
with the literature report, the two peaks are assigned with the hkl values of (110) and
(003) respectively. The two broad peaks
apparently indicate the amorphous/poor
crystalline nature of all polymer materials
[18]. From the data in Table 4.1, it can be
seen that there is a shift in d-space of PANI
upon blending with PVP. This shift could
result in an ordered PANI chain
arrangement [19], improved π-electron
delocalization and enhancement in
conductivity [20] of PANI-PVP blends.
Further, incorporation of PVP into PANI
has led to an increase in average crystallite
size in PANI-PVP 1 and PANI-PVP 2
blends.
Figure 4.3. XRD patterns of (a) PANI,
(b) PANI-PVP 1, (c) PANI-PVP 2,
(d) PANI-PVP 3 and (e) PANI-O.
63
4.1.6. SEM studies
Figure 4.4 (a-e) shows the SEM images of PANI-PVP blend materials together
with images of PANI and PANI-O with ×10000 magnification. It is observed that both
PANI and PANI-PVP 3 have more or less the same morphology. Particles are
agglomerated and the materials have particles/grains of a few microns size with irregular
shape and size. A similar type of tendency has been reported on PANI-NaVO3 composites
[21]. However, the other two blend materials (PANI-PVP 1 and PANI-PVP 2) have much
smaller spherical particles with a smooth surface structure. The approximate sizes of the
particles could be deducible on careful view of the same but expanded image and are
about 100 nm. In the case of PANI-O, it is observed to have lesser number of individual
spherical particles. Thus the SEM characterization together with XRD confirms the
existence of submicron/nanoparticles in the PANI materials.
Table 4.1. XRD peak position, d-space and other data of PANI and its
PVP blends
Polymer sample Peak 1 Peak 2 FWHM
(for Peak 1) (10-2 rad)
Crystallite size (nm) 2θ (°) d (Å) 2θ (°) d (Å)
PANI
PANI-PVP 1
PANI-PVP 2
PANI-PVP 3
PANI-R
PANI-PVP 1-R
PANI-A
PANI-O
18.98
18.91
18.46
18.98
19.07
19.00
19.62
18.95
4.67
4.69
4.81
4.67
4.65
4.67
4.63
4.68
25.89
25.95
25.92
25.87
25.81
26.19
25.93
26.20
3.44
3.43
3.43
3.44
3.45
3.40
3.43
3.40
3.13
2.95
1.70
4.23
6.62
3.54
1.01
2.20
4.43
4.76
8.22
3.32
2.12
3.97
13.87
6.37
64
Figure 4.4. SEM images of (a) PANI, (b) PANI-PVP 1, (c) PANI-PVP 2,
(d) PANI-PVP 3 and (e) PANI-O with ×10000 magnification.
65
4.1.7. AC impedance behavior of fresh polyaniline and its blend materials
Bode plots - General principle
Complex impedance spectroscopy (CIS) is one of the studies performed on
polymeric semiconducting materials to analyze their AC electrical properties. CIS is
based on the measurement of impedance over a range of frequencies and analyzing them
in the complex impedance plane (Bode-magnitude type plots) [22-24]. Specially designed
disc-shaped copper electrodes (parallel plate method) were used for the measurement of
impedance (scheme a, Fig. 4.5) of PANI and its blends. The absolute magnitude of the
impedance (Z) can be expressed as |Z| = √(Z')2 +(Z″)2. The real and imaginary parts of the
impedances (Z′ and Z″ that represent the resistive part and the reactance arising due to the
capacitive or inductive nature of the system respectively) are evaluated as a function of
frequency and temperature. Bode plot (scheme c, Fig. 4.5) is constructed with log
frequency (ω) on the x-axis and the absolute value of the impedance (|Z|) on the y-axis
[24,25]. The electrode resistance (RΩ) and the sum of the bulk resistance and the electrode
resistance (Rb + RΩ) can be read from the Bode plot (scheme c, Fig. 4.5). The bulk
resistance Rb of the material pellet can be obtained from the plot, which gives the bulk
electrical conductivity σb from the equation σb = t/(Rb*a), where t is the thickness, and a is
Figure 4.5. Schematic representation of (a) electrode-sample set up (parallel
plate method), (b) equivalent RC circuit and (c) Bode plot.
66
the area of cross-section of the pressed polymer material in disc/pellet form [26].
The process of dielectric relaxation is observed from the Bode plot at intermediate
frequencies. The critical frequency (fc) of the material, independent of the geometrical
parameters, is found at the intermediate frequency of Bode plot and fulfills the condition
2πfcRbCb = 1. From this relation, the bulk capacitance Cb (or geometrical capacitance) of
the material pellet is also calculated and the bulk dielectric constant (εb) is determined
using the relation Cb = (εbεоa)/t, where εо is the permittivity of free space [27]. The
dielectric relaxation time τ is obtained using the relation τ = 1/ωc = 1/(2πfc).
The frequency dependence of AC conductivity
The frequency dependent conductivity plots i.e., log σ(ω) vs log (ω) for PANI,
PANI-PVP blends and PANI-O are shown in Fig. 4.6 (a-e). The conductivity pattern can
categorically be divided into two parts: (i) a plateau in the low frequency region where
σ(ω) remains constant up to fc (critical frequency/hopping frequency) at all temperatures
for all the materials and (ii) an ascending portion at higher frequencies above fc, where a
drastic change in conductivity occurs. The change in slope occurs at critical frequency fc.
The rise of σ(ω) upon increasing the frequency (ω) is a common response for
polymeric and semiconductor materials [26]. The first trend is contributed by free/mobile
charge carriers available in the material system but the second one is due to trapped
charges which are active only at higher frequency region [28]. There is a tremendous
increase in mobility of the trapped charge carriers in polymer materials at higher
frequencies. Addition of PVP in PANI reduces the charge trapping centers, thus allowing
a number of charges participating in the relaxation process and renders an increase of
conductivity. Similar results have been reported by Dutta et al for PANI-PVA [31] and
Afzal et al for PANI-PVC composites [32].
67
Generally, the total conductivity σ(ω) at a given temperature and frequency is
expressed as σ(ω) = σdc(0) + σac(ω), where ω is the angular frequency (= 2πf), σdc(0)
Figure 4.6. Frequency dependence of AC conductivity log σ(ω) vs log (ω)
plots of (a) PANI, (b) PANI-PVP 1, (c) PANI-PVP 2,
(d) PANI-PVP 3 and (e) PANI-O at different temperatures.
68
(ω→0) is the DC conductivity and σac(ω) is the AC conductivity. The AC conductivity in
disordered materials shows two regions separated by a critical frequency, fc = ωc/2π. At
low frequencies, σ(ω) is nearly constant, corresponding to σdc such that σ(ω) = σdc , f ≤ fc.
At higher frequencies, σ(ω) increases with frequency and its frequency dependence is
given by σ(ω) = σac(ω) , f ≥ fc. It is to be noted that fc is not defined by a lone parameter,
but on the basis of many factors such as method of synthesis, chemical structure,
composition of elements, temperature, etc.
σ(ω) is almost constant over the low frequency region from 10 Hz to critical
frequency fc and increases with frequency above fc. The measured value of σ(ω)
(at f = 1 kHz) for PANI is 7.71×10−8 Scm-1; such a low value of conductivity is
attributable to the low level of protonation as indicated by FTIR and also to poor
crystallinity of the material as observed in XRD. It is well known that σ(ω) depends, apart
from frequency and temperature, on degree of protonation, percentage of crystallinity,
crystalline domain size and amorphous regions and it has a relationship with the
delocalization length [33].
AC electrical properties
The impedance spectra i.e., Bode plots obtained at different temperatures (from
300 to 473 K) for polymer samples are shown in Fig. 4.7. The electrical properties of
fresh PANI, PANI-PVP blends and PANI-O derived from Bode Plots are compiled in
Table 4.2a that contains Rb and σb values. Before delving into the effect of temperature, it
is essential to examine the effect of [PVP] and oxidant content on these parameters of
PANI. All the materials have σb in the order of 10-8 Scm-1 but the variation of σb of
different PANI materials within this order is significant and illustrative of the effects
69
under concern. First of all, inclusion of PVP with increasing concentration in PANI-PVP
blends causes a steady increase in conductivity at room temperature and this trend
reaches maximum at PANI-PVP 3. Coinciding roughly with the proportion of [PVP] in
blends (half, equal and three times of ANI.HCl), this trend fulfils the implication and
anticipation of conductivity increase, enunciated from IR and XRD observations.
Figure 4.7. Bode plots of (a) PANI, (b) PANI-PVP 1, (c) PANI-PVP 2,
(d) PANI-PVP 3 and (e) PANI-O at different temperatures.
70
Behaving as a soft-template and steric stabilizer through strong intermolecular hydrogen
bonding (between C=O group of PVP and amine/imine group of PANI), PVP in
PANI-PVP blends could form cross-links with PANI chains, enhance ordered PANI
chains arrangement, improve crystalline domain in amorphous material and substantially
develop electron delocalization and conductivity [4,5]. PANI-O prepared with slightly
higher oxidant input in synthesis also exhibits about two-times higher conductivity.
Perhaps, a greater degree of oxidation yielding more quinoids may be the reason for such
increase in conductivity.
Effect of temperature on conductivity
Results of temperature dependence of AC conductivities of PANI, PANI-PVP
blends and PANI-O are quite interesting. In the case of pristine PANI and PANI-O,
the conductivity decreases up to 373 K (or Rb increases), beyond which it increases with
raise in temperature. In PANI-PVP blends, the conductivity decrease extends up to 423 K
and afterwards it increases with increase in temperature (Table 4.2a). This trend is
observable as jumbled plots in Fig. 4.7. Up to 373 K for PANI and PANI-O and up to
423 K for blends the plots in Fig. 4.7 follow a downward trend but after these
temperatures they take up an upward trend. The quite interesting influence of temperature
has previously been observed on DC conductivity of PANI materials [37] but it is new for
AC conductivity measurement and that too with different transition temperatures for a
similar group of materials. This intricacy could be resolved by a study of temperature
dependence of frequency exponent s and the pertinent conduction mechanism models as
described in following sections.
The variation of σac(ω) at a particular frequency and temperature for a disordered
polymeric semiconductor material obeys the universal power law σac(ω) = Aωs, where A
71
Tem
pera
ture
(K
)
Poly
mer
sa
mpl
e
300
348
373
423
448
473
Rb
(MΩ
)
σb
10-8
(S
cm-1
)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
PAN
I PA
NI-
PVP
1 PA
NI-
PVP
2 PA
NI-
PVP
3 PA
NI-
O
1.
00
0.
41
0.
16
0.
04
0.
53
7.87
20.9
49.7
197.
0
17.0
1.
06
0.
71
0.
32
0.
18
1.
13
7.
43
12
.1
24
.7
43
.9
8.
0
1.
43
0.
80
0.
55
0.
35
2.
22
5.
51
10
.8
14
.4
22
.6
4.
1
1.
20
1.
86
0.
76
0.
88
1.
81
6.
56
4.
64
10
.4
8.
97
5.
0
0.
95
1.
65
0.
23
0.
83
1.
26
8.
28
5.
23
34
.5
9.
51
7.
2
0.
84
1.
36
0.
11
0.
27
1.
10
9.
37
6.
34
72
.4
29
.2
8.
2
Tab
le 4
.2a.
AC
ele
ctric
al
prop
erti
es (
Rb a
nd
σb)
of
fresh
PA
NI
an
d i
ts b
len
ds
72
is a constant and s is the frequency exponent (or power law index), generally less than or
equal to unity (0 ≤ s ≤ 1) [38]. The dimensionless frequency exponent s, values of fresh
PANI, PANI-PVP blends and PANI-O materials for various temperatures were
determined from the slope of linear ascending portion of the plots in Fig. 4.6 and the
values are listed in Table 4.2b. Figure 4.8 shows the temperature dependence of s for
fresh PANI and its blend materials. For pristine PANI and PANI-O, s increases up to
373 K and then decreases. For PANI-PVP blends the increase is upto 423 K and then a
decrease. The trend in s values is similar to that of bulk resistance Rb which initially
increases upto 373/423 K and then decreases (inverse to σb trend). The critical frequency
(fc), bulk capacitance (Cb) and bulk dielectric constant (εb) of fresh PANI and its blend
materials are listed in Table 4.2c.
Table 4.2b. The frequency exponent (s) values of PANI, PANI-PVP 1,
PANI-PVP 2, PANI-PVP 3 and PANI-O
Temperature (K)
Polymer
sample 300 348 373 423 448 473
PANI 0.90 0.92 0.94 0.88 0.85 0.84
PANI-PVP 1 0.87 0.9 0.92 0.96 0.94 0.93
PANI-PVP 2 0.85 0.88 0.90 0.92 0.92 0.88
PANI-PVP 3 0.81 0.90 0.92 0.93 0.91 0.88
PANI-O 0.82 0.85 0.87 0.79 0.73 0.71
73
4.1.8. AC conduction mechanism
Different theoretical models, such as quantum mechanical tunneling (QMT),
non-overlapping small polaron tunneling (SPT), overlapping large polaron tunneling
(OLPT) and correlated barrier hopping (CBH) have been proposed to explain the
mechanism of AC conduction [39-44]. Mott proposed the QMT model [39] for
amorphous semiconductors. The QMT model assumes that the carrier motion occurs
through quantum mechanical tunneling between localized states near the Fermi level. In
the QMT model the frequency exponent, s, is independent of temperature but dependent
on frequency, ω. SPT is usually associated with an increase in s with increasing
temperature [40]. In the OLPT model [41], s decreases with increase of temperature up to
a particular value and increases thereafter. Pike and Elliot proposed the CBH model in
which s decreases as temperature increases [42,43]. The CBH model considers hopping of
bipolarons, i.e., two electrons hopping between oppositely charged defect centers D+ and
D− under the influence of an external electric field over a barrier WB separating them,
rather than tunneling through it.
Figure 4.8. Variation of frequency exponent ‘s’ with temperature for (a) PANI,
(b) PANI-PVP 1, (c) PANI-PVP 2, (d) PANI-PVP 3 and (e) PANI-O.
74
Tab
le 4
.2c.
T
he
crit
ical
freq
uen
cy (
f c),
bu
lk c
ap
acit
an
ce (
Cb)
an
d b
ulk
die
lectr
ic c
on
stan
t (ε
b)
of
fres
h P
AN
I an
d i
ts b
len
ds
Tem
pera
ture
(K
)
Poly
mer
sa
mpl
e
300
348
373
423
448
473
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
(kH
z)
(pF)
(k
Hz)
(p
F)
(kH
z)
(pF)
(k
Hz)
(p
F)
(kH
z)
(pF)
(k
Hz)
(p
F)
PAN
I 1.
0 15
3 1.
36
1.0
152
1.35
0.
9 12
5 1.
11
0.8
167
1.49
1.
0 17
9 1.
60
1.5
136
1.21
PAN
I-PV
P 1
2.0
187
1.83
1.
5 15
1 1.
44
1.5
130
1.27
1.
0 10
1 0.
99
1.0
103
1.00
1.
0 13
2 1.
29
PAN
I-PV
P 2
6.0
151
1.34
4.
0 11
6 1.
03
3.0
91
0.82
2.
0 10
4 0.
93
4.0
172
1.54
7.
0 20
6 1.
83
PAN
I-PV
P 3
15.0
17
2 1.
54
7.0
106
0.94
5.
0 85
0.
76
3.0
59
0.53
3.
0 62
0.
55
4.0
121
1.09
PAN
I-O
1.
7 17
2 1.
75
1.0
141
1.44
0.
7 10
2 1.
04
0.6
147
1.49
0.
8 15
8 1.
61
0.9
161
1.64
75
Among the various models, only SPT and CBH are found to be applicable to the
present systems as detailed below. In the SPT model the frequency exponent s increases
with increasing temperature [40] as given by equation 4.1.
--- 4.1
where ω is the angular frequency, τ0 is the characteristic relaxation time, WH is the
polaron hopping energy and k is Boltzmann constant. On the other hand, CBH shows a
decrease in s with increasing temperature, which indicates the thermally activated
behavior of charge carrier hopping over the barrier between two sites [43,46], as given by
equation 4.2.
--- 4.2
where Wm is the maximum barrier height. By correlating the SPT and CBH models to our
observation on s, it is plausible and logical to arrive at the inference that for pristine
PANI, SPT conduction mechanism is operative upto its transition temperature 373 K and
for PANI-PVP blends upto their transition temperature, 423 K; CBH mechanism is
operative beyond these temperatures for both the groups of materials. The decrease of s
is, therefore, attributed to the correlated barrier hopping mechanism of the charge carriers
across the defect states D+ and D−.
At high temperature, the bipolaron states (D+ and D−) are converted into single
polaron states (Do) according to the relation 4.3 [43].
[D+] + [D−] → 2[Do] --- 4.3
[D+, D−] is associated with high values of WB. The conversion of [D+, D−] into [Do] leads
to a decrease in barrier height WB at high temperature. In the present case, the low
76
temperature AC conductivities can be explained by considering bipolaron hopping
between D+ and D− centers. But at higher temperature, the behavior is apparently due to
thermally activated single electron hopping.
Since conductivity is primarily dependent on ordered arrangement of PANI chains
in the crystalline domain of amorphous region (See, Ch. 1, Fig. 1.6) [18,19], any factor
that disrupts such arrangement can also influence the conductivity. Presence of
intergallery occluded water in PANI matrices plays an important role in crystalline
arrangement. Though all the materials were dried at 120 °C before study, there would be
definite presence of occluded water [47]. Pron et al [48] and Inoune et al [49] have
reported that both base and salt forms of PANI are hydrated and the water cannot be
eliminated even by vacuum drying. Therefore, in the present case also there is a
possibility that on heating the pellet and maintaining it at different higher temperatures in
AC conductivity measurement, water elimination can gradually take place, which would
affect directly the crystalline arrangement of PANI chains and indirectly the conductivity.
In order to examine this possibility for its key play under temperature influence,
systematic studies were carried out and are discussed in the following section.
4.1.9. Repetition of AC impedance analysis on studied samples
CIS studies were repeated on the already-studied pristine PANI and PANI-PVP 1
(representative for PANI-PVP blends) materials which were given the notations PANI-R
and PANI-PVP 1-R respectively. In order to confirm unequivocally the occurrence of
temperature effect through water elimination, a new material was prepared and
designated. Fresh PANI or pristine PANI was annealed at 463 K continuously for 4 h
(labeled as PANI-A) for thorough elimination of water and was also subjected to CIS
analysis together with PANI-R and PANI-PVP 1-R. Figure 4.9 shows the Bode plots in
which the magnitude of Z decreases continuously and Table 4.3a presents Rb and σb
77
values. A steady decrease in magnitudes is observed in Bode plots (Fig. 4.9).
The remarkable feature of the impedance behaviors of PANI-R, PANI-PVP 1-R and
PANI-A is the uniformity in trend with temperature, i.e., a continuous decrease in Rb or
the continuous increase in σb values of the samples (Table 4.3a) with raise in temperature
from ambient. Evidently all the three materials exhibit CBH mechanism from room
temperature onwards unlike the fresh PANI and its blend materials that show both
SPT and CBH mechanisms. With raise in temperature the charge carriers are thermally
activated, gain more kinetic energy and begin to move with greater degree of hopping
between trapped sites [50]. The very fact that the repeated samples PANI-R and
PANI-PVP 1-R show impedance behaviors identical to that of fresh but annealed PANI-A
Figure 4.9. Bode plots of (a) PANI-R, (b) PANI-A, (c) PANI-PVP1-R
and (d) PANI-PVP 1-RR at different temperatures.
78
Ta
ble
4.3
a.
AC
ele
ctric
al
prop
erti
es (
Rb a
nd
σb)
of
PA
NI-
R,
PA
NI-
A, P
AN
I-P
VP
1-R
, an
d P
AN
I-P
VP
1-R
R
Tem
pera
ture
(K
)
Poly
mer
sa
mpl
e
300
373
42
3
448
47
3
493
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
Rb
(MΩ
) σ
b 10
-8
(Scm
-1)
PAN
I-R
9.
81
0.80
9.
54
0.83
8.
92
0.88
8.
48
0.93
7.
38
1.07
6.
10
1.29
PAN
I-A
9.
86
0.88
9.
61
0.90
9.
39
0.92
9.
07
0.95
8.
69
1.00
8.
24
1.05
PAN
I-PV
P 1-
R
9.49
0.
91
8.81
0.
98
8.25
1.
05
7.71
1.
12
6.24
1.
39
3.34
2.
59
PAN
I-PV
P 1-
RR
9.
68
0.89
9.
37
0.92
9.
07
0.95
8.
85
0.98
7.
73
1.12
4.
56
1.90
79
sample, clearly and beyond doubt points out that it is the occluded/absorbed water that is
responsible for the initial conductivity decrease with fresh PANI, its blends and PANI-O
materials. Once the water elimination is complete, the crystalline domain might get
disrupted and changed into a new one. The process is irreversible because PANI-R and
PANI-PVP 1-R show only conductivity increase at all temperatures on third or fourth
time repetition of impedance study with intermittent time gap of a few days/weeks.
To confirm further the irreversibility nature of the change, the CIS analysis was repeated
for PANI-PVP1-R (denoted as PANI-PVP 1-RR) after a lapse of 16 months
(sufficient for moisture re-absorption). The conductivity trends in PANI-PVP 1-R and
PANI-PVP 1-RR are the same (plots c and d, Fig. 4.9 and Table 4.3a). It reveals the
non-inclusion of water molecule back into the system.
The different transition temperatures i.e., 373 K for PANI and 423 K for
PANI-PVP blends arise from the difference in their chemical compositions. PVP, the
additional component in PANI-PVP blends, is a water-soluble polymer and has greater
affinity for water through hydrogen bonding [51]. Hence, when included in PANI blend,
PVP tends to retain water even at a temperature higher than its boiling point (373 K).
PANI completely eliminates occluded water at 373 K while PANI-PVP blends do so at
423 K, at which the materials transform into a new crystalline domain.
The log σ(ω) vs log (ω) plots for PANI-R, PANI-PVP 1-R, PANI-A and
PANI-PVP 1-RR blends are shown in Fig. 4.10 (a-d). In the low frequency region
σ(ω) remains constant and independent of the frequency at all temperatures. However, as
the frequency increases the conductivity elevates and becomes frequency-dependent as
observed in fresh PANI-PVP blend materials (Fig. 4.6). The frequency exponent s values
were determined from Fig. 4.10 and are presented in Table 4.3b. The critical frequency fc,
bulk capacitance (Cb) and bulk dielectric constant (εb) values are listed in Table 4.3c.
80
Figure 4.10. Frequency dependence of AC conductivity log σ(ω) vs log (ω) plots of
(a) PANI-R, (b) PANI-PVP 1-R, (c) PANI-A and (d) PANI-PVP 1-RR
at different temperatures.
Table 4.3b. The frequency exponent (s) values of PANI-R, PANI-PVP 1-R,
PANI-A and PANI-PVP 1-RR
Temperature (K)
Polymer sample
300 373 423 448 473 493
PANI-R 0.96 0.95 0.94 0.93 0.91 0.90
PANI-PVP 1-R 0.94 0.93 0.93 0.92 0.89 0.87
PANI-A 0.92 0.90 0.89 0.89 0.88 0.86
PANI-PVP 1-RR 0.93 0.93 0.92 0.91 0.88 0.85
81
Tab
le 4
.3c.
Th
e c
riti
cal
freq
uen
cy (
f c),
bu
lk c
ap
aci
tan
ce (
Cb)
an
d b
ulk
die
lect
ric
con
stan
t (ε
b)
of
PA
NI-
R,
PA
NI-
PV
P1-R
, P
AN
I-A
an
d P
AN
I-P
VP
1-R
R
Tem
pera
ture
(K
)
Poly
mer
sa
mpl
e
300
373
423
448
473
493
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
f c
Cb
ε b
(F/c
m)
(kH
z)
(pF)
(k
Hz)
(p
F)
(kH
z)
(pF)
(k
Hz)
(p
F)
(kH
z)
(pF)
(k
Hz)
(p
F)
PAN
I-R
0.
20
81
0.
73
0.
2 83
0.24
0.2
89
0.
80
0.
2 94
0.84
0.2
108
0.
96
0.
2 13
0
1.16
PAN
I-PV
P 1-
R
0.30
56
0.50
0.3
60
0.
54
0.
3 64
0.57
0.3
69
0.
61
0.
3 85
0.76
0.4
119
1.
06
PAN
I-A
0.
25
65
0.
64
0.
2 83
0.81
0.2
85
0.
83
0.
2 86
0.84
0.2
91
0.
90
0.
2 97
0.94
PAN
I-PV
P 1-
RR
0.
30
55
0.
51
0.
3 57
0.53
0.3
58
0.
55
0.
3 60
0.56
0.3
69
0.
64
0.
3 11
6
1.09
82
The s value decreases with increase of temperature (Fig. 4.11) and there is only a uniform
trend. We find that for all the studied materials on repetition of CIS analyses s lies
between 0.96 and 0.85 and obeys power law as the values lie in the range 0 ≤ s ≤ 1.
The s values decrease gradually with increase of temperature in PANI-PVP blends also.
Continuous decrease in s with temperature suggests that CBH is the most plausible
conduction mechanism for all the materials under repeated study.
Water elimination from the fresh materials could not be a discrete physical event.
Being present as hydrogen bonded to amine, imine groups of PANI and to C=O group of
PVP, water when eliminated could not only disturb PANI chain arrangement but also
modify the physico-chemical characteristics of the materials. With the objective of
investigation of such modifications, XRD, SEM, FTIR and UV-visible spectral
characterizations of PANI-R, PANI-PVP 1-R and PANI-A were done and are shown in
Figs. 4.12, 4.13, 4.14 and 4.15 respectively. Since they also contain characterization
figures of their fresh counterparts, convenient comparison of repeated/annealed and fresh
samples is quite possible.
Figure 4.11. Variation of frequency exponent ‘s’ values with temperature for
(a) PANI-R, (b) PANI-PVP 1-R, (c) PANI-A and (d) PANI-PVP 1-RR.
83
The XRD patterns of PANI-R (Fig. 4.12b) and PANI-A (Fig. 4.12e) when
compared with those of fresh PANI (Fig. 4.12a), are modified and d-values are displaced
to lower side. Similar but less pronounced effect occurs with PANI-PVP 1-R (Fig. 4.12d)
on comparison with fresh PANI-PVP 1 (Fig. 4.12c). Quantification of the effect is
realizable in Table 4.1 in terms of FWHM and crystallite size. PANI-R has almost double
the FWHM and half the crystallite size of PANI; PANI-PVP 1-R has somewhat lesser
crystallite size than that of PANI-PVP 1. The lesser crystallite size specifies the reduction
in size of crystalline domain (see Ch.1, Fig. 1.6). d-space values provide clue to
understand this observation. PANI-R, PANI-PVP 1-R and PANI-A all have lesser
d-space at peak 1 than their counterparts (Table 4.1), indicating a denser close-packing of
atoms along the chain [53,54]. The space left by intergallery water on its elimination
during heating is now occupied/adjusted by PANI chain movements through further
close-packing, involving simultaneously structural and conformational reorganization.
Crystallite size decrease observed in XRD is reflected in particle sizes also. SEM images
comparison of fresh PANI with PANI-R and PANI-A (Figs. 4.4a and 4.13a and c)
explicitly shows that PANI-R and PANI-A have considerably smaller particles than does
pristine PANI; in fact, submicron size pristine PANI particles are morphologically
converted into nanoparticles. Similar inference is also drawable from a comparison of
PANI-PVP 1-R (Fig. 4.13b) and PANI-PVP 1 (Fig. 4.4b).
Comparison of FTIR spectra of fresh PANI and PANI-PVP 1 (spectra a and c,
Fig. 4.14) materials with those of repeated PANI-R and PANI-PVP 1-R and annealed
PANI-A samples (spectra b, d and e, Fig. 4.14) points out clearly that the repeated
samples have considerably less intensity at first peak (O-H stretching peak) showing the
elimination of occluded/absorbed water. PANI-A sample has nil intensity at this peak,
84
which indicates the complete elimination of water in the annealed sample. Also the
repeated and annealed samples have less intense, almost flattened and blue-shifted bands
at about 1176 cm-1 relative to the high intense and lower energy peaks for pristine
Figure 4.12. XRD patterns of (a) PANI,
(b) PANI-R (c) PANI-PVP 1,
(d) PANI-PVP 1-R and (e) PANI-A.
Figure 4.13. SEM images of (a) PANI-R,
(b) PANI-PVP 1-R and (c) PANI-A with
×10000 magnification.
85
samples at about 1143 cm-1 substantiating the conductivity decrease in studied samples
(Table 4.3a). At 300 K, PANI-R has about 9.4 times less σb value while PANI-PVP 1-R
has about 22.3 times less value than their respective fresh samples. At higher
temperatures, because of the raise in conductivity of PANI-R and PANI-PVP 1-R by
semiconductor behavior, this conductivity gap gets decreased but continues to persist.
Besides FTIR, the UV-visible spectral features (Figs. 4.15 A and B) also reveal
the marked deviation in characteristics of PANI-R and PANI-PVP 1-R from those of
fresh samples. In methanol solvent, against the almost horizontal curves up to 450 nm and
then relatively sharp signals at 383 and 374 nm of PANI and PANI-PVP 1 (spectra a and
c, Fig. 4.15A), PANI-R, PANI-PVP 1-R and PANI-A (spectra b, d and e, Fig. 4.15A)
materials show gradually rising spectral curves from 900 and 700 nm and then broad
peaks with slightly lower λmax at 380 and 373 nm respectively. This is similar to the
spectral features exhibited by the annealed sample PANI-A (spectrum e, Fig. 4.15A).
However, invoking coordinating role of NMP to the spectra of Fig. 4.15B, it is inferable
that PANI-R (spectrum b, Fig. 4.15B) and PANI-PVP 1-R (spectrum d, Fig. 4.15B)
undergo partial conversion of emeraldine salt to base, while PANI-A (spectrum e,
Fig. 4.15B) does not. The modified spectral features obviously indicate the disturbance of
electronic energy levels in PANI-R, PANI-PVP 1-R and PANI-A alike because of heating
and the consequent water elimination. The energy gap between π and π* levels in these
samples is not sharply fixed but is probably blurred with interlying spectral states
originating from molecular interactions in nanoscale particles [5]. Altogether the
structural and spectral characterizations of the repeated samples in conjunction with those
of PANI-A demonstrate (1) denser close-packing of PANI chains, (2) morphological
reduction of particle dimension from submicron to nano, (3) deprotonation of imine
86
groups and a concomitant decrease in conductivity of all the materials and
(iv) modification of π orbital energy levels.
Figure 4.14. FTIR spectra of
(a) PANI, (b) PANI-R, (c) PANI-PVP 1,
(d) PANI-PVP 1-R and (e) PANI-A. Peak
numbers and their corresponding peak
positions (in cm-1) are given in the inset.
Figure 4.15. UV-visible spectra of
(a) PANI, (b) PANI-R, (c) PANI-PVP 1,
(d) PANI-PVP 1-R and (e) PANI-A
in (A) methanol and (B) NMP solvents.
4.1.10. Conclusions
CIS analyses of all the synthesized materials demonstrate that PANI-PVP blends
have greater AC bulk conductivities than PANI in proportion to the amount of PVP
87
incorporated. Characterization by FTIR, UV-visible, XRD and SEM methods reveal the
constructive role played by PVP through improvement in ordered PANI chain
arrangement and π-electron delocalization. All the fresh materials have constant
conductivity at low frequency and a cross-over to a frequency-dependent conductivity at
higher frequencies (>fc). The cross-over/critical frequency fc is dependent on PVP
concentration and gets increased with increase of PVP concentration. CIS as a function of
temperature explicitly shows an initial conductivity decrease and a high temperature
increase for fresh PANI and its blend materials, taking place at different transition
temperatures through heating-induced water elimination and a change of conduction
mechanism. Thus, water elimination is very crucial and causes dimensional changes at
particles and physico-chemical changes at the molecular level, leading to transition in
conduction behavior and conduction mechanism from SPT to CBH in all the fresh PANI
and its blend materials. The electrical properties of blends get improved substantially with
higher concentration of PVP. This phenomenon can render them as potential candidates
for use as capacitor, gas sensing, humidity sensing, etc.
88
4.2. PANI-PVP and/or SDBS blend materials:
Interfacial polymerization and AC electrical properties
4.2.1. Introduction
Nanostructures of PANI are of great interest since they combine the advantages of
low-dimensional organic conductors with those of high surface area materials. They
therefore create interesting physico-chemical properties and potentially useful
applications [55]. There are many methods to synthesize nanostructured PANI, including
chemical, electrochemical and interfacial methods [56-58]. Among them,
aqueous/organic interfacial polymerization is a powerful method. Interfacial
polymerization (IP) involves a step polymerization of a reactive monomer with oxidizing
agent, which are dissolved in two immiscible phases (usually organic and aqueous) and
the reaction takes place at the interface of the two liquids [59]. IP is commonly performed
with a volatile organic solvent such as dichloromethane [58], chloroform [59], benzene
[60] and toluene [61] as the organic phase. The major advantages of IP, compared to the
conventional polymerization in aqueous solution, are the slow reaction rate resulting in
nanostructures with a narrow size distribution and the ability to add various surfactants,
either organophilic or hydrophilic to the aqueous or the organic phase. PANI
nanostructures can be prepared by introducing ‘structural directors’ into the interfacial
polymerization method. These structural directors include ‘soft-templates’ such as neutral
polymers, organic dopants or surfactants [62] that assist in the self-assembly of PANI
nano-structures.
In the present work the neutral polymer PVP or the surfactant SDBS or both have
been chosen as the structural directors and soft templates. So, the present work is an
investigation of the role of single/double soft templates in the synthesis and
characteristics of PANI materials. Herein the oxidant FeCl3 was not used for
89
polymerization, as it is soluble in organic solvents. Instead APS that is soluble only in
aqueous phase was used. Further, as demonstrated by Gospodinovo et al [63], with APS
aniline polymerization can proceed even without any acid. This finding also prompted us
to use APS in the present work without any acid for interfacial polymerization of aniline.
PANI-PVP, PANI-SDBS, PANI-PVP-SDBS and pristine PANI were synthesized and
studied by CIS analysis.
4.2.2. Synthesis and characterization of polyaniline and its blend materials
PANI and its blend materials were synthesized by interfacial polymerization at
room temperature (see Ch. 3). The synthesized PANI materials were characterized by
1. FTIR and UV-visible spectroscopy
2. Thermal studies
3. SEM studies and
4. AC electrical properties as a function of frequency and temperature by CIS.
4.2.3. Formation of polyaniline materials
The polymerization progress is illustrated in Fig. 4.16, representatively for pristine
PANI. During the early stages of reaction, a gradual darkening was observed at the
interface. The change in color of the organic phase within 30 min indicated the growth of
oligomers at the interface. After 1 h, a dark thin layer was observed at the interface which
gradually diffused into aqueous phase and this process continued for several hours.
After 3 h, the aqueous phase contained a large number of dark-green particles and the
organic phase became orange in color. The polymerization process was, however,
continued up to 48 h. After 48 h, the entire aqueous phase was filled homogeneously with
the dark-green PANI material, while the organic phase showed a deep color of orange.
Since the color change at the end of the process, i.e., at 48 h is almost the same as that at
3 h, the process display is shown in Fig. 4.16 only up to 3 h. In this controlled interfacial
90
polymerization, APS provided the necessary counterions (SO42-) to develop the charged
PANI. Liu et al argued that the interface between the immiscible aqueous/organic phase
acts as a template that preferentially organizes the aniline monomer prior to
polymerization [64]. Following the addition of surfactant SDBS and APS to the aqueous
solution, the interfacial reaction initiated itself and in a few hours the PANI that was
formed at the interface rapidly migrated into the aqueous phase.
4.2.4. FTIR spectral studies
Figures 4.17 A and B represent the FTIR spectra of PANI and its blend materials
together with the spectra of free PVP and SDBS. The main characteristic peaks of PANI
(spectrum a, Fig. 4.17A) are as follows: The sharp intense peaks at 1576 and 1484 cm-1
are attributed to stretching vibrations of quinoid and benzenoid rings in the polymer chain
respectively [65]. Imine and secondary aromatic amine peaks are observed at 1290 and
1232 cm-1 respectively. The C−H in-plane bending vibration peak that occurs at
1112 cm-1 is an ‘electronic-like’ band considered to be a measure of the degree of
Figure 4.16 Progress (from left to right) of the chemical oxidative interfacial
polymerization of aniline using aqueous APS as the upper phase
and aniline in CHCl3 as the lower phase.
91
π-electron delocalization and is formed during protonation [66]. The peak at 814 cm-1
corresponds to the out-of-plane deformation of C−H in the 1,4-disubstituted benzene ring
which indicates that the polymerization proceeded via head-to-tail linkage [67]. These
vibrational peaks are modified both in intensity and peak position upon incorporation of
PVP and/or SDBS.
In the FTIR spectra of PANI-PVP (spectrum b, Fig. 4.17A), the incorporation of
PVP is witnessed by the appearance and blue-shift of its characteristic carbonyl peak
Figure 4.17. FTIR spectra in (A) of PANI (a), PANI-PVP (b) and free PVP (c)
and in (B) of PANI-SDBS (a), PANI-PVP-SDBS (b) and
free SDBS (c).
92
occurring at 1663 cm-1 (spectrum c, Fig. 4.17A) to 1677 cm-1. In PANI-SDBS sample
(spectrum a, Fig. 4.17B), the characteristic peaks of pristine SDBS (spectrum c,
Fig. 4.17B) observed at 1461 and 1405 cm-1 appear substantially red-shifted to 1449 and
1379 cm-1 respectively. This feature confirms the inclusion of SDBS and also its
interaction with PANI in the PANI-SDBS blend material. Further, both in PANI-PVP and
PANI-SDBS materials, the characteristic peaks of PVP and SDBS appear weak and only
with a little height. This is because their mole fractions are very low compared to PANI
(0.09 and 0.02 respectively) and hence when aniline polymerization occurred around PVP
or SDBS chains, their characteristic groups became buried within the PANI matrix and
their exposure to IR rays (in the spectroscopic study) became feeble. It is also observable
in the IR spectrum of PANI-SDBS that the intensities of the quinoid and benzenoid peaks
are enhanced. This means the microenvironment of quinoid and benzenoid groups of
PANI in PANI-SDBS was influenced. From a theoretical point of view, the interaction
between N−H of PANI and SDBS is found to be responsible for polaron creation and
hence to the degree of conductivity [68]. This literature finding is applicable to the
present PANI-SDBS material also.
In PANI-PVP-SDBS material, where the two soft-templates are included, the
characteristic intense carbonyl peak of free PVP and the characteristic peak of free SDBS
appear red-shifted as small weak peaks at 1640 and 1445 cm-1 respectively suggesting
their inclusion (spectrum b, Fig. 4.17B). In all the blend materials, the peaks occurring at
1019-1050 cm-1 and 650-700 cm-1
are ascribed to the S=O and S−O stretching vibrations
of the sulfate group (dopant) [64]. The spectral peak which pinpoints the role of PVP and
SDBS in increasing the conductivity of PANI is the C−H in-plane bending vibration
appearing at 1112 cm-1 in PANI. This vibrational band is observed to be narrow for the
emeraldine salt of PANI in many studies and reveals the greater π-electron delocalization
93
in PANI chain and the consequent higher conductivity [65,13]. A comparison of this peak
in PANI and its blend materials clearly shows its occurrence both as narrow and with
greater intensity in all the blend materials and hence it could explain their higher
conductivity (Table 4.5b), which is discussed below in the last section.
4.2.5. UV-visible spectral studies
The UV-visible electronic absorption spectra of PANI and its blend materials in
methanol and ethanol solvents are portrayed in Figs. 4.18 A and B respectively. The two
solvents have different polarities (their dielectric constants, ε = 33.0 and 25.3
respectively) [71] and consequently they may interact with and influence the electronic
absorption energy levels in the polymers differently. Figures 4.18 A and B show their
influences clearly. In both the solvents, the spectra show peaks at about 369-390 nm,
attributable to the π-π* transition of benzenoid rings [72]. The peaks in the wavelength
range 527-583 nm are due to the exciton-type transition from HOMO of benzenoid to the
LUMO of quinoid [73]. The two above-said peaks are found to be blue-shifted in the
blends compared to PANI. The very weak π-π* transition band of PANI in both the
Figure 4.18. UV-visible spectra of (a) PANI, (b) PANI-PVP, (c) PANI-SDBS and
(d) PANI-PVP-SDBS in (A) methanol and (B) ethanol solvents.
94
solvents (spectrum a, Figs. 4.18 A and B) appears with greater absorption in blends. For
example, a more intense absorption band at about 370 nm occurs in PANI-PVP-SDBS
material (spectrum d, Figs. 4.18 A and B) that indicates a more compact and regular
polymer chain arrangement in this double soft-templated blend than in PANI or other
blends. This feature also has relevance in explaining the greater conductivity of
PANI-PVP-SDBS (Table 4.5b).
4.2.6. Thermal studies
The TGA and DTG curves of PANI and its blends are illustrated in Fig. 4.19 (a-d)
and the results are summarized in Table 4.4. The thermal behavior shows a four-step
Figure 4.19. TGA and DTG curves of (a) PANI, (b) PANI-PVP,
(c) PANI-SDBS and (d) PANI-PVP-SDBS.
95
weight loss process, as observed rather clearly in the DTG curves. The first weight loss
occurring at 30-188 °C is attributed to the expulsion of loosely-bound water molecules
and low-molecular weight oligomers [74]. The second weight loss taking place in the
range 168-328 °C is caused by the departure of physically interacted acid dopant, SO42-.
The dopants that are bound to the imine centers of PANI are liberated in the temperature
Table 4.4. TGA and DTG data of PANI and its blend materials
Polymer sample
Temperature
range (°C)
weight loss %
Residue %
Inflection points (°C)
Comments
PANI
PANI-PVP
PANI-SDBS
PANI-PVP-SDBS
30-168 168-311 311-439 439-625
30-188 188-307 307-440 440-620
30-188 188-328 328-463 463-620
30-175 175-319 319-471 471-625
11.25 14.77 32.15 41.83
8.00 8.91
32.94 50.15
9.44 13.25 36.75 40.56
7.81 14.06 38.40 39.73
Nil
Nil
Nil
Nil
170, 312, 441
188, 307, 440
188, 328, 463
175, 319, 471
Decomposition is completed by 625 °C with one broad and two small inflexion points Decomposition is completed by 620 °C with one broad and two small inflexion points Decomposition is completed by 620 °C with one broad and two small inflexion points Decomposition is completed by 625 °C with two broad and two small inflexion points
96
range 307-471 °C [75]. The final weight loss, starting at about 440-470 °C, corresponds
to thermal decomposition of PANI backbone chains [56]. Blends have lower weight loss
in the first step compared to PANI (Table 4.4) and this indicates a smaller amount of
absorbed water. The thermal degradation upto the third stage is lower in PANI-PVP
(the fourth step weight loss ~50%), exhibiting higher stability compared to other
materials. However, all the materials decomposed fully at about 625 °C leaving no
residue.
4.2.7. SEM studies
Typical SEM images of the PANI materials together with particle diameter
markings are shown in Fig. 4.20 (a-d) with ×25000 magnification. The distinctive
features of these images are summarized below: (1) PANI (Fig. 4.20a) material has
aggregates of irregularly shaped particles composed of nanoparticles of average diameter
in the range 50-100 nm estimated over a larger cross-section of the image. A similar type
of tendency has been reported in many studies [76,77], (2) PANI-PVP (Fig. 4.20b) blend
shows the presence of smooth-surfaced more or less spherical submicron aggregates of
nanoparticles, whose size lies in the range of 50 - 150 nm. PVP plays the role of a
soft-template inducing the formation of nanosized particles/grains, (3) In PANI-SDBS
(Fig. 4.20c) blend, the particles are extensively agglomerated, smooth surfaced and
irregularly shaped flakes or grains and (4) The SEM image of PANI-PVP-SDBS
(Fig. 4.20d) blend is similar to that of the PANI-SDBS blend. Nevertheless, because of
the presence of two templates, PVP and SDBS, a significant number of relatively smaller
size grains, in addition to larger aggregates, is formed and both are noticeable in the
image. Liu et al have explained that the interface between the immiscible
aqueous/organic phase acts as a platform where the aniline monomer preferentially
organizes before polymerization [64]. Since PVP and SDBS are both
97
amphipathic/amphoteric in nature, in their individual or combined presence, the growth of
polymer chains at the interface is influenced by these soft-templates in such a way to
form smaller grains and then their larger aggregates. The morphological study thus
clearly points out that employment of two soft-templates instead of the usual one brings
about smaller aggregates of nanoparticles.
4.2.8. AC impedance behavior of polyaniline and its blend materials
The frequency dependence of AC conductivity
The variation of AC conductivity as a function of frequency, log σ(ω) vs log (ω)
plots of PANI and its blends is shown in Fig. 4.21. As evident from the plots, two trends
Figure 4.20. SEM images of (a) PANI, (b) PANI-PVP, (c) PANI-SDBS and
(d) PANI-PVP-SDBS with ×25000 magnification. The sizes of the
different particles are labeled in the images. .
98
are observable in the conducting process; the first one is frequency-independent
conductivity (horizontal portion) and the second one is frequency-dependent conductivity
(ascending/rising portion). In the low frequency region σ(ω) remains constant and
independent of frequency for all materials at all temperatures. However, as the frequency
increases, the conductivity elevates and becomes frequency-dependent. The onset
frequency (or critical frequency) fc, at which σ(ω) starts to rise, increases with the
increase of temperature for all materials. For example, the critical frequency fc, observed
at 0.8 kHz at room temperature for PANI, is shifted towards higher frequency with the
increase of temperature. The same trend is observed in all the materials (Table 4.5a).
Figure 4.21. Frequency dependence of AC conductivity log σ(ω) vs log (ω)
plots of (a) PANI, (b) PANI-PVP, (c) PANI-SDBS and
(d) PANI-PVP-SDBS at different temperatures.
99
Po
lym
er
sam
ple
T
empe
ratu
re (
K)
300
34
8
373
39
8
423
44
8
f c
(kH
z)
τ (µs)
s
f c
(kH
z)
τ (µs)
s
f c
(kH
z)
τ (µs)
s
f c
(kH
z)
τ (µs)
s
f c
(kH
z)
τ (µs)
s
f c
(kH
z)
τ (µs)
s
PAN
I
PAN
I-PV
P
PAN
I-SD
BS
PAN
I-PV
P-SD
BS
0.80
0.35
0.70
0.30
199
398
227
531
0.78
0.67
0.54
0.49
1.25
0.40
0.80
0.40
127
398
199
398
0.54
0.51
0.46
0.39
1.75
0.50
0.90
0.50
91
318
177
318
0.44
0.41
0.39
0.35
2.00
0.60
1.25
0.60
80
265
127
265
0.40
0.38
0.24
0.23
2.5
0.7
1.3
0.7
64
227
122
227
0.36
0.34
0.21
0.18
3.5
0.8
1.5
0.8
46
199
100
199
0.21
0.19
0.16
0.12
Tab
le 4
.5a.
Th
e c
riti
cal
freq
uen
cy (
f c),
rel
axati
on
tim
e (τ)
an
d f
req
uen
cy e
xp
on
ent
(s)
of
fresh
PA
NI
an
d i
ts b
len
d m
ate
rials
100
The trend in fc suggests an increase in mobility of charge carriers at higher frequencies
[78]. Also, it is apparent from Fig. 4.21 that, irrespective of temperatures, all the curves
become overlapped with one another at higher frequencies.
The variation of σ(ω) with frequency and temperature for disordered
semiconductor materials obeys the universal power law (see Ch. 4.1, sec. 4.1.7).
The frequency exponent s values for various temperatures have been determined from the
slopes of linear ascending portion of the plots in Fig. 4.21 and are entered in Table 4.5a.
The s values vary from 0.78 to 0.12 for PANI and its blends. The s value decreases with
the increase of temperature and is shown graphically in Fig. 4.22. The decrease in s is,
therefore, attributed to the correlated barrier hopping mechanism of the charge carriers
across the defect states D+ and D−. Altogether CBH is found to be the most plausible
conduction mechanism for the materials under study. Table 4.5a also lists the relaxation
time, τ, which also decreases with temperature.
Figure 4.22. The temperature-dependence of frequency exponent ‘s’ values for (a)
PANI, (b) PANI-PVP, (c) PANI-SDBS and (d) PANI-PVP-SDBS.
101
4.2.9. AC electrical properties
The complex impedance spectra (Bode plots) obtained at different temperatures
(ambient ~300 K to 448 K) for PANI, PANI-PVP, PANI-SDBS and PANI-PVP-SDBS
are displayed in Fig. 4.23. The AC electrical parameters of PANI and its blends are
presented in Table 4.5b that contains the bulk resistance (Rb) and bulk conductivity (σb)
as a function of temperature. First let us examine the individual effect of PVP and SDBS
blending agents at room temperature. All the materials have Rb and σb in the order of MΩ
and 10-8 Scm-1 respectively. With the blends Rb decreases and σb increases. For example,
incorporation of PVP into PANI (PANI-PVP) enhances the conductivity of PANI two-
fold, SDBS (PANI-SDBS) three-fold and their complex PVP-SDBS (PANI-PVP-SDBS)
Figure 4.23. Bode plots (|Z| vs log (ω) plots) of (a) PANI, (b) PANI-PVP,
(c) PANI-SDBS and (d) PANI-PVP-SDBS at different temperatures.
102
Ta
ble
4.5
b.
AC
ele
ctr
ical
pro
per
ties
(Rb a
nd
σb)
an
d a
ctiv
ati
on
en
ergy (
Ea)
of
fresh
PA
NI
an
d i
ts b
len
ds
Poly
mer
sa
mpl
e
Tem
pera
ture
(K
)
300
348
373
398
423
448
Act
ivat
ion
ener
gy
Ea
(eV
)
Rb
(MΩ
)
σb
10-8
(S
cm-1
)
Rb
(MΩ
)
σb
10-8
(S
cm-1
)
Rb
(MΩ
)
σb
10-8
(S
cm-1
)
Rb
MΩ
)
σb
10-8
(S
cm-1
)
Rb
(MΩ
)
σb
10-8
(S
cm-1
)
Rb
(MΩ
)
σb
10-8
(S
cm-1
)
PAN
I
PAN
I-PV
P
PAN
I-SD
BS
PAN
I-PV
P-SD
BS
1.95
1.03
0.71
0.47
5.78
11.0
0
15.9
0
20.0
0
1.00
0.75
0.57
0.32
11.3
15.0
19.8
29.4
0.68
0.60
0.51
0.26
16.6
18.8
22.1
36.1
0.60
0.53
0.36
0.19
18.8
21.3
31.3
49.5
0.49
0.46
0.31
0.14
23.0
24.5
36.4
67.1
0.34
0.30
0.29
0.08
33.2
37.6
38.9
117.
0
0.13
4
0.08
7
0.07
5
0.08
9
103
four-fold. The electrical parameters fc, and τ also display a uniform trend at room
temperature but in a slightly different series; fc shows a decreasing trend whereas τ has an
increasing trend in the series, PANI, PANI-SDBS, PANI-PVP and PANI-PVP-SDBS
(Table 4.5a). The bulk capacitance (Cb) and bulk dielectric constant (εb) are listed in
Table 4.5c. The variation in electrical parameters’ values of the blends can be explained
by the incorporation of PVP, SDBS or PVP-SDBS and their single/double soft-templates
roles. PVP and/or SDBS in PANI leads to creation of large number of polarons and
bipolarons [79]. In conjugated polymers, the existence of polarons and bipolarons is
responsible for the conductivity as they serve as hopping sites for the charge carriers.
With the application of an electric field the localized charge carriers can hop to
neighboring sites, which form a continuous network permitting the charges to travel
through the entire physical dimensions of the material pellet resulting in electrical
conduction [80]. In the absence of strong charge-trapping centers, the charge hopping
could extend throughout the material leading to a continuous current at low frequencies
[29]. Behavior of PVP as a steric stabilizer and SDBS as a dopant/surfactant could
possibly enhance the ordered PANI chains arrangement (as inferred from spectral studies
- see previous sections), reduce the number of charge trapping centers and substantially
facilitate the π-electron delocalization and hence conductivity. These features allow a
number of charges to participate in hopping and an increase of conductivity.
Now returning to the effect of temperature on conductivity, it is observable in
Table 4.5b that there is generally a gradual increase in conductivity in all the materials
with the temperature. However, PANI-PVP-SDBS material undergoes a dramatic effect at
448 K (Table 4.5b). The inclusion of PVP or SDBS in PANI increases respectively the σb
of PANI-PVP and PANI-SDBS materials at 448 K by 1.53 and 1.06 times only. However,
in the case of PANI-PVP-SDBS material, a dramatic raise in σb occurs at 448 K
104
Tab
le 4
.5c.
T
he b
ulk
cap
aci
tan
ce (
Cb)
an
d b
ulk
die
lect
ric
con
stan
t (ε
b)
of
fres
h P
AN
I an
d i
ts b
len
ds
Poly
mer
sa
mpl
e
Tem
pera
ture
(K
)
300
348
373
398
423
448
Cb
(pF)
ε b
(F/c
m)
Cb
(pF)
ε b
(F/c
m)
Cb
(pF)
ε b
(F/c
m)
Cb
(pF)
ε b
(F/c
m)
Cb
(pF)
ε b
(F/c
m)
Cb
(pF)
ε b
(F/c
m)
PAN
I
PAN
I-PV
P
PAN
I-SD
BS
PAN
I-PV
P-SD
BS
102
385
320
1130
1.30
4.93
4.08
11.9
9
127
531
349
1240
1.62
6.76
4.45
13.2
1
134
531
347
1220
1.70
6.76
4.42
13.0
0
133
501
354
1400
1.69
6.38
4.50
14.8
3
130
495
395
1620
1.66
6.30
5.03
17.2
5
134
663
366
2490
1.70
8.46
4.66
26.4
2
105
(1.74 times). Also at 448 K there is a jump in conductivity-raise in PANI-PVP-SDBS
relative to its conductivity increases in previous temperature steps (1.2 – 1.4 times). The
individual effects of PVP/SDBS incorporation in PANI become synchronized, and hence
further enhanced, with PVP-SDBS incorporation in PANI (PANI-PVP-SDBS) leading to
a larger increase in conductivity at 448 K and larger variation in other electrical
properties. A possible explanation for the raise in conductivity of all the materials with
temperature is as follows, which has been proposed for many amorphous semiconductor
materials [38,40]. Conversion of bipolaron state into single polaron states occurs and
there is thermal activation of charge carriers at higher temperatures (see Ch. 4.1,
sec. 4.1.8). Both the processes are responsible for the larger degree of charge carriers’
movement and hence a greater level of conductivity at higher temperatures. The bulk
conductivity (σb) is a thermally activated process and obeys Arrhenius law (see Ch.3,
eqn. 3.7) [81-83]. Figure 4.24 exhibits the variation of ln (σb) against 103/T.
Figure 4.24. Arrhenius plots ln (σb) Vs 1000/T for (a) PANI, (b) PANI-PVP,
(c) PANI-SDBS and (d) PANI-PVP-SDBS.
106
All are straight line plots and they indicate a negative temperature coefficient of
resistance behavior (semiconducting nature of materials) for PANI and its blends.
The activation energy (Ea) values were calculated from the slope of the plots and are
listed in Table 4.5b. Ea varies from 0.075 to 0.134 eV for different PANI materials. These
values are comparable to those for ferric chloride doped poly(3-methyl thiophene)
(0.176 eV) [84] and PANI-tetrafluoroborate (0.082 eV) [85]. Ea is actually a barrier for
charge carriers to move. The higher Ea value of the pristine PANI became reduced by the
addition of soft-templates PVP and/or SDBS. Hence a favorable conductive situation was
developed in the blends.
4.2.10. Conclusions
Different-sized and featured PANI blend materials were successfully synthesized
by interfacial polymerization using PVP as a soft-template or SDBS as a surfactant or
using their combination. FTIR spectroscopy confirms the chemical interaction of PVP,
SDBS and PVP-SDBS with PANI and also their incorporation into PANI. The four
materials PANI, PANI-SDBS, PANI-PVP and PANI-PVP-SDBS exhibit
characteristically different particles/grains morphology. In AC impedance analysis all the
materials exhibit a single conduction mechanism (CBH) involving constant AC
conductivity at low frequency and a cross-over to a frequency-dependent regime at high
frequencies. The room temperature AC electrical properties show a variation in the blend
materials compared to pristine PANI. The AC conductivity increases with the increase of
temperature. This trend obeys the universal power law as s is in the range 0.12 < s < 0.78.
This obeyance and the s values suggest that the CBH mechanism is the most appropriate
one for the present system. The trends in electrical properties of the blend materials have
their origin in the role of single/double soft-templates which promote π electron
delocalization and hopping process.
107
4.2.11. Comparison of properties of chemically and interfacially polymerized PANI
blend materials.
PANI-PVP and PANI-PVP and/or SDBS blend materials were polymerized
chemically and interfacially respectively. It is of our interest to know how the method of
synthesis affects the characteristics of resulting PANI materials. Hence a comparison of
the properties of the two types of blend materials in Ch. 4.1 and 4.2 was made and the
following inferences emerge. Morphological studies show the presence of
submicron/nano particles in PANI-PVP blend materials. However, in the interfacially
synthesized blend materials, addition of PVP and/or SDBS to PANI reduces the grains’
size because of the presence of two templates. With regard to AC conductivity, the order
is 10-8 for both the type of blend materials. For chemically synthesized blend materials
the AC conductivity is a function of PVP concentration but for the interfacially
synthesized materials, both PVP and SDBS influence the conductivity of PANI materials
either individually or in complex form. Water molecules in the inter-gallery of the chains
of chemically synthesized PANI-PVP blend materials are responsible for the change of
AC conduction mechanism from SPT to CBH with temperature. Contrarily, the
interfacially polymerized materials exhibit only one conduction mechanism i.e., the CBH
mechanism throught the temperature range of study.
108
References
[1] M. Xu, T. Zhang, B. Gu, J. Wu, Q. Chen, Macromolecules, 39, 3540, (2006).
[2] S. Ameen, V. Ali, M. Zulfequar, M.M. Haq, M. Husain, Physica B: Condens.
Matter., 403, 2861, (2008).
[3] J. Stejskal. P. Kratochvil, M. Helmstedt, Langmuir, 12, 3389, (1996).
[4] R. Murugesan, G. Anitha, E. Subramanian, Mater. Chem. Phy., 85, 184, (2004).
[5] E. Subramanian, G. Anitha, N. Vijayakumar, J. Appl. Polym. Sci., 106, 673,
(2007).
[6] E.M. Genies, A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met., 36, 139,
(1990).
[7] A.G. MacDiarmid, J.C. Chiang, M. Halpern, W.S. Huang, S.L. Mu,
L.D. Nanaxttara, S.W. Wu, S.I. Yaniger, Mol. Cryst. Liq. Crys., 121, 173,
(1985).
[8] K. Gurunathan, A.V. Murugan, R. Marimuthu, U.P. Mulik, D.P. Amalnerkar,
Mater. Chem. Phys., 6, 173, (1999).
[9] Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer, 30, 2305, (1989).
[10] A. Yasuda, T. Shimidzu, Synth. Met., 61, 239, (1993).
[11] M.M. Ayad, M. Whdan, Colloid J., 70, 549, (2008).
[12] M.I. Boyer, S. Quillard, E. Rebourt, G. Louarn, J.P. Buisson, A. Monkman,
S. Lefrant, J. Phys. Chem. B, 102, 7382, (1998).
[13] B. Zhao, K.G. Neoh , E.T. Kang, K L. Tan, Chem. Mater., 12, 1800, (2000).
[14] R.C. Patil, S.F. Patil, I.S. Mulla, K. Vijayamohanan, Polym. Int., 49, 189, (2000).
[15] L. Zheng, L. Xiong, C. Liu, L. Jin, Europ. Polym. J., 42, 2328, (2006).
[16] D.C. Sidhimeshram, M.C. Gupta, Indian J. Chem., A34, 260, (1995).
[17] Y. Wei, K.F. Hsueh, G.W. Jang, Macromolecules, 27, 518, (1994).
[18] A. Wolter, P. Rannou, J.P. Travers, B. Gilles, D. Djurado, Phys. Rev. B, 58,
7637, (1998).
[19] J.P. Pouget, C.H. Hsu, A.G. MacDiarmid, A.J. Epstein, Synth. Met., 69, 119,
(1995).
109
[20] D.C. Trivedi, Indian J. Chem., A33, 552, (1994).
[21] T. Machappa, M.V.N. Ambika Prasad, Physica B, 404, 4168, (2009).
[22] A. Adhikari, P. Claesson, J. Pan, C. Leygraf, A. Deidenaitei, E. Blomberg,
Electrochim. Acta, 53, 4239, (2008).
[23] C.P. Fonseca, E.M.J.A. Pallone, S. Neves, Solid State Sci., 6, 1353, (2008).
[24] Y. Li, B. Ying, L. Hong, M. Yang, Synth. Met., 160, 455, (2010).
[25] M. Palencia, B.L. Rivas, E. Pereira, A. Arrieta, Polym. Bull., 65, 145, (2010).
[26] P. Dutta, S. Biswas, M. Ghosh, S.K. De, S. Chatterjee, Synth. Met., 122, 455,
(2001).
[27] S. Lanfredi, A.C.M. Rodrigues, J. Appl. Phys., 86, 2215, (1999).
[28] H.P. De Oliveira, M.V.B. Dos Santos, C.G. Dos Santos, C. P. De Melo,
Synth. Met., 135, 447, (2003).
[29] M. Nadeem, M.J. Akhtar, A.Y. Khan, Solid State Commun., 134, 431, (2005).
[30] H.M. Zaki, Physica B, 363, 232, (2005).
[31] S. Dutta, R.N.P. Choudhary, P.K. Sinha, A.K. Thakur, J. Appl. Phys., 96, 1607,
(2004).
[32] A.B. Afzal, M.J. Akhtar, M. Nadeem, M.M. Hassan, Curr. Appl. Phys., 10, 601,
(2010).
[33] J. Joo, Z. Oblakowaski, G. Du, J.P. Pouget, E.J. Oh, J.M. Wiesinger, Y. Min,
A.G. MacDiarmid, Phys. Rev. B, 49, 2977, (1994).
[34] T. Sulimenko, T.J. Stejskal, I. Krivka, J. Prokes, Eur. Polym. J., 37, 219, (2001).
[35] J. Stejskal, I. Sapurina, J. Colloid Interf. Sci., 274, 489, (2004).
[36] P.R. Somani, Mater. Chem. Phy., 77, 81, (2002).
[37] A.D. Boarkar, M.C. Gupta, Indian J. Chem., A29, 631, (1990).
[38] P. Extance, S.R. Elliot, E.A. Davis, Phy. Rev. B, 32, 8148, (1985).
[39] N.F. Mott, E.A. Davis, Electronic process in Non-Crystalline Materials, 2nd Edn.,
Oxford University Press, London, (1979).
[40] S.R. Elliot, Adv. Phys., 36, 135, (1987).
110
[41] A.R. Long, Adv. Phy., 31, 533, (2001).
[42] G.E. Pike, Phys. Rev. B, 6, 1572, (1972).
[43] S.R. Elliot, Philos. Mag., 36, 1291, (1977).
[44] P. Pollak, G.E. Pike, Phys. Rev. Lett., 25, 1449, (1972).
[45] S.A. El-Hussan, M. Hammad, Phys. Stat. Sol., 185, 413, (2001).
[46] S.R. Elliot, Philos. Mag. B, 37, 553, (1978).
[47] A.A. Syed, M. K. Dinesan, K.N. Somasekaran, Indian J. Chem., A27, 279,
(1988).
[48] A. Pron, F. Genoud, C. Menardo, M. Nechtschein, Synth. Met., 24, 193, (1988).
[49] M. Inoue, R.E. Navarro, M.B. Inoue, Synth. Met., 30, 199, (1989).
[50] Y. Tominaga, S. Asai, M. Sumita, S. Panero, B. Scrosati, J. Power Sources, 146,
402, (2005).
[51] P. Molyneux, The physical chemistry and pharmaceutical applications of
Polyvinylpyrrolidone, in Proceedings of the international symposium, Povidons,
C. A. Digenis and J. Ansell (ed), Lexington, Kentucky, USA, p. 1, (1983).
[52] H.H.S. Javadi, K.R. Cromack, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B, 39 ,
3579, (1989).
[53] Shumaila, G.B.V.S. Lakshmi, M. Alam, A.M. Siddiqui, M. Sulfequar,
M. Hussain, Curr. Appl. Phys., 11, 217, (2011).
[54] Y.B. Moon, Y. Cao, P. Smith, A.J. Heeger, J. Polym. Commun., 30, 196, (1989).
[55] J. Jiang, L. Hong AI, J. Macromol. Sci. B: Phys., 50, 26, (2011).
[56] T. Chen, C. Dong, X. Li, J. Gao, Polym. Degrad. Stab., 94, 1788, (2009).
[57] S. Roy, K. Kargupta, S. Chakraborty, S. Ganguly, Mater. Let., 62, 2535, (2008).
[58] M. Jain, S. Annapoorni, Synth. Met., 160, 1727, (2010).
[59] X. Wang, X. Wang, Y. Wu, L. Bao, H. Wang, Mater. Lett., 64, 1865, (2010).
[60] L.Y. Chu, S.H. Park, T. Yamaguchi, S.I. Nakao, Langumir, 18, 1856, (2000).
[61] Q. Sun, Y. Deng, Mater. Lett., 62, 1831, (2008).
111
[62] H. Qiu, M. Wan, B. Matthews, L. Dai, Macromolecules, 34, 675, (2001).
[63] N. Gospodinovo, P. Mokreva, L. Terlemezyan, Polymer, 34, 2438, (1993).
[64] W. Liu, A.L. Cholli, R. Nagarajan, J. Kumar, S. Tripathy, F.F. Bruno,
L. Samuelson, J. Am. Chem. Soci., 121, 11345, (1999).
[65] K. Gupta, G. Charaborty, P.C. Jana, A.K. Meikap, Solid State Commun., 151,
573, (2011).
[66] M.I. Boyer, S. Quillard, E. Rebourt, G. Louarn, J.P. Buisson, A. Monkman,
S. Lefrant, J. Phys. Chem. B, 102, 7382, (1998).
[67] A. Choudhury, Sens. Actuators B, 138, 318, (2009).
[68] P. Tsotra, K.G. Gryshchuk, K. Friedrich, J. Mater. Sci., 40, 569, (2005).
[69] H. Liu, X.B. Hu, J.Y. Wang, R.I. Boughton, Macromolecules, 35, 9414, (2002).
[70] A.A. Athwale, B.A. Deore, M.V. Kulkarni, Mater. Chem. Phys., 60, 262, (1999).
[71] D.R. Lide, CRC Handbook of Chemistry and Physics, CRC Press, Taylor and
Francis, New York, 9th Edn. p. 13, (2009-2010).
[72] L. Ren, K. Li, X. Chen, Polym. Bull., 63, 15, (2009).
[73] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nkajima, T. Kawagoe,
Macromolecules, 21, 1297, (1988).
[74] G. Song, J. Han, J. Bo, R. Guo, J. Mater. Sci., 44, 715, (2009).
[75] F.F. Fang, J.H. Sung, H.J. Choi, J. Macromol. Sci.: Part B Phys., 45, 923,
(2006).
[76] H. Qiu, S. Qi, D. Wang, J. Wang, X. Wu, Synth. Met., 160, 1179, (2010).
[77] J. Wang, J. Wang, Z. Yang, Z. Wang, F. Zhang, S. Wang, React. Funct. Polym.,
68, 1435, (2008).
[78] A. Fattoum, M. Arous, F. Gmati, W. Dhaoui, J. Phys. D: Appl. Phys., 40, 4347,
(2007).
[79] N.J. Pinto, A.A. Acosta, G.P. Sinha, F.M. Aliev, Synth. Met., 113, 77, (2000).
[80] A.K. Jonscher, Dielectric relaxation in solids, Chelesa dielectric press, London,
(1983).
[81] S. Ebrahim, A.H. Kashyout, M. Soliman, Curr. Appl. Phys., 9, 448, (2009).
112
[82] M. Nadeem, M.J. Akhtar, A.Y. Khan, Solid State Commun., 134, 431, (2005).
[83] J.P. Rao, K.E. Geckler, Prog. Polym. Sci., 36, 887, (2011).
[84] R. Singh, A. Kaur, K.L. Yadav, D. Bhattacharya, Curr. Appl. Phys., 3, 235,
(2003).
[85] K.M. Choi, K.H. Kim, J.S. Choi, J. Phys. Chem. Solids, 50, 283, (1989).