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

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