PREPARATION AND CHARACTERIZATION OF CONDUCTING
POLYANILINE AND POLYANILINE-TITANIUM(IV) OXIDE
COMPOSITE BLENDED WITH POLY(VINYL ALCOHOL)
CHAN YEN NEE
UNIVERSITI TEKNOLOGI MALAYSIA
PSZ 19:16 (Pind. 1/97)
Universiti Teknologi Malaysia
BORANG PENGESAHAN STATUS TESIS♦
JUDUL :
PREPARATION AND CHARACTERIZATION OF CONDUCTING
POLYANILINE AND POLYANILINE-TITANIUM(IV) OXIDE
COMPOSITE BLENDED WITH POLY(VINYL ALCOHOL)
SESI PENGAJIAN : 2004 / 2005
CHAN YEN NEE
Saya
(HURUF BESAR)
mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah)* ini disimpan di Perpustakaan
Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut:
1.
2.
Tesis adalah hakmilik Universiti Teknologi Malaysia.
Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan
pengajian sahaja.
Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara
institusi pengajian tinggi.
** Sila tanda ( √ )
3.
4.
√
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau
kepentingan Malaysia seperti yang termaktud di dalam AKTA
RAHSIA RASMI 1972)
TERHAD
(Mengandungi maklumat yang TERHAD yang telah ditentukan
oleh organisasi/badan di mana penyelidikan dijalankan)
TIDAK TERHAD
Disahkan oleh
(TANDATANGAN PENULIS)
Alamat Tetap:
87, Taman Rasa Sayang,
06000 Jitra,
Kedah.
Tarikh:
22 JULY 2005
(TANDATANGAN PENYELIA)
PROF. DR. RAMLI BIN HITAM
Nama Penyelia
Tarikh:
22 JULY 2005
CATATAN: * Potong yang tidak berkenaan.
** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi
berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai
SULIT atau TERHAD.
Tesis dimaksudkan sebagai tesis bagi ijazah Doktor Falsafah dan Sarjana secara penyelidikan,
atau disertai bagi pengajian secara kerja kursus dan penyelidikan, atau Laporan Projek
Sarjana Muda (PSM).
“I/We* hereby declare that I/we* have read this thesis and in my/our*
opinion this thesis is sufficient in terms of scope and quality for the
award of the degree of Master of Science (Chemistry)”
Signature
Name of Supervisor I
: ……………………………………
DR. RAMLI BIN HITAM
: PROF.
……………………………………
Date
22 JULY 2005
: ……………………………………
Signature
Name of Supervisor II
: ……………………………………
DR. SATAPAH BIN AHMAD
: PM.
……………………………………
Date
22 JULY 2005
: ……………………………………
* Delete as necessary
BAHAGIAN A - Pengesahan Kerjasama*
Adalah disahkan bahawa projek penyelidikan tesis ini telah dilaksanakan melalui
kerjasama antara _______________________ dengan __________________________
Disahkan oleh:
Tandatangan : .………………………………………………
Nama
: ………………………………………..
Jawatan
: ………………………………………..
Tarikh:
…………
(Cop rasmi)
*Jika penyediaan tesis/projek melibatkan kerjasama.
BAHAGIAN B – Untuk Kegunaan Pejabat Sekolah Pengajian Siswazah
Tesis ini telah diperiksa dan diakui oleh:
Nama dan Alamat
Pemeriksa Luar
Nama dan Alamat
Pemeriksa Dalam I
:
Prof. Madya Dr. Mohd Zaki B Abd Rahman
Departmet of Chemistry
Faculty of Science & Environmental Studies
Universiti Putra Malaysia
43400 UPM, Serdang
Selangor Darul Ehsan
: Prof. Madya Dr. Madzlan B Aziz
Fakulti Sains
UTM, Skudai
Pemeriksa Dalam II
:
Nama Penyelia lain
(jika ada)
:
Disahkan oleh Penolong Pendaftar di Sekolah Pengajian Siswazah (SPS):
Tandatangan : ………………………………………………
Nama
GANESAN A/L ANDIMUTHU
: ………………………………………………
Tarikh : ………...
PREPARATION AND CHARACTERIZATION OF CONDUCTING
POLYANILINE AND POLYANILINE-TITANIUM(IV) OXIDE
COMPOSITE BLENDED WITH POLY(VINYL ALCOHOL)
CHAN YEN NEE
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
JULY 2005
ii
I declare that this thesis entitled “Preparation And Characterization Of Conducting
Polyaniline And Polyaniline-titanium(IV) Oxide Composite Blended With
Poly(vinyl alcohol)” is the results of my own research except as cited in the
references. The thesis has not been accepted for any degree and is not concurrently
submitted in candidature of any other degree.
Signature
:
Name
:
CHAN YEN NEE
Date
:
22 JULY 2005
iii
Specially dedicated to
my parents and siblings with love and
to Leong Mun Hon, my best friend
for their patience and encouragement
iv
ACKNOWLEDGEMENT
First of all, I wish to express my sincere appreciation to my supervisor
Professor Dr. Ramli Bin Hitam for his supervision and encouragement. I am also
gratefully acknowledge my co-supervisor Associate Professor Dr. Satapah Bin
Ahmad for his guidance and valuable comments.
I am also thankful to Assoc. Prof. Dr. Karim Bin Deraman from the
Department of Physic, UTM for his advice and helps in the four probe resistivity
and Hall Effect measurement. I am also grateful to the Universiti Kebangsaan
Malaysia, especially Dr. Muhammad Azmi Abdul Hamid for allowing us to use the
TEM facility.
I also thank all academic and technical staffs of the Department of
Chemistry, UTM for their advice and assistance. The financial support from Short
Term Grant is earnestly and greatly acknowledged.
Finally, I would like to express the utmost thanks to my parents and my
family members for their faithful love and support throughout the entire tenure of
my studies. To all of them, I extend my gratitude and thanks.
v
ABSTRACT
Conductive polyaniline (PAni) and polyaniline-titanium(IV) oxide (PAniTiO2) composites were prepared by chemical oxidative polymerization of aniline in
the presence of dodecylbenzene sulfonic acid (DBSA) in HCl medium, which
played both the role as dopant and surfactant. Such processable conductive PAni
and its composite were blended with poly(vinyl alcohol) (PVA) in water, which
was then cast into film by solution casting, resulting a flexible, free-standing and
conductive blend films. The morphology of the PAni/PVA and PAni-TiO2/PVA
blends was confirmed by using scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). Generally, the thermogravimetric
analysis (TGA) curves of the blends showed gradual weight loss due to absorbed
moisture and solvent upon initial heating up to around 100 oC, followed by a slow
weight loss until around 225 oC, which could be attributed to the elimination of
dopant. The final degradation of the polymer occurs from around 227 to 900 oC.
The presence of a single Tg as revealed by differential scanning calorimetry (DSC)
and its shifts to higher value with increasing PAni and PAni-TiO2 content, revealing
the miscibility between PAni and PAni-TiO2 with PVA through hydrogen bonding
as shown by FTIR. The X-ray diffraction pattern of the blends revealed that the
degree of crystallinity of PAni-TiO2/PVA blends was lower than that of PVA and
TiO2, showing that the amorphous nature of PAni may inhibit crystallization of
TiO2 and PVA. The electrical conductivity of the PAni-TiO2/PVA blends increase
with the increase of TiO2/aniline weight ratio and reaches a saturation value at
weight ratio of 0.13. All the blends samples exhibit similar pattern, i. e. the
conductivity increases with temperature from 30 oC to 50 oC, following with
decreasing conductivity, and reach the maximum at 140 oC, then decrease with
further increasing temperature. PAni/PVA and PAni-TiO2 (I)/PVA show maximum
conductivity at 40 oC, 1.69 and 1.78 S/cm, respectively. The blends films exhibited
good conductivity even at low weight fraction of conductive components, with
conductivity value around 10−5 S/cm. The electrical conductivity of the films
increases with increasing content of conducting PAni and PAni-TiO2 content in the
PVA matrix; indicating the dependence of the blended film conductivity upon the
PAni and PAni-TiO2 content. This was due to the growing of continuous network
formation, which is confirmed by TEM. The percolation threshold was about 2.0
wt. % for both PAni/PVA and PAni-TiO2/PVA blends. From the Hall effect studies,
the conductivity and carrier mobility are linearly related while the carrier mobility
are inversed of the carrier density. At room temperature, PAni-TiO2 (I)/PVA blend
(40 wt. %) shows the highest carrier mobility (4878 cm2 volt−1 sec−1) among the
samples. Finally, the conductivity of the blends decreases as the temperature is
increased and deviates strongly from variable range hopping equation above 300 K.
vi
ABSTRAK
Polyaniline (PAni) dan polyaniline-titanium(IV) oxida (PAni-TiO2) yang
bersifat mengkonduksi telah disediakan secara pempolimeran pengoksidaan kimia
dengan kehadiran asid dodecylbenzene sulfonik (DBSA) dalam medium HCl. PAni
dan PAni-TiO2 yang bersifat mengkonduksi dicampurkan dengan poly(vinyl
alcohol) (PVA). Dengan menggunakan kaedah “solution casting”, filem yang
berciri terlenturan, “free-standing” dan mengkonduksi telah dihasilkan daripada
larutan “blend”. Morfologi PAni/PVA dan PAni-TiO2/PVA telah dipastikan dengan
menggunakan mikroskop electron pengimbas (SEM) dan mikroskop electron
penghantar (TEM). Secara umum, corak analisis termogravimetrik (TGA) ‘blends’
tersebut menunjukkan kehilangan berat yang perlahan akibat air dan pelarut yang
terserap semasa dipanaskan hingga kira-kira 100 oC, diikuti dengan kehilangan
berat hingga kira-kira 225 oC, mungkin disebabkan oleh penghapusan dopan.
Penguraian terakhir polimer berlaku kira-kira dari 227 hingga 900 oC. Kehadiran Tg
tunggal seperti yang ditunjukkan oleh differential scanning calorimetry (DSC), dan
didapati menganjak ke nilai yang lebih tinggi dengan penambahan kandungan PAni
dan PAni-TiO2 dalam ‘blends’ menunjukkan keterlarutcampuran antara PAni dan
PAni-TiO2 dengan PVA. Corak pembelauan sinar X menunjukkan bahawa darjah
kehabluran ‘blends’ PAni-TiO2/PVA adalah lebih rendah berbanding dengan PVA
dan TiO2. Sifat amorfus semula jadi PAni mungkin mengurangkan darjah
kehabluran PVA dan TiO2. Kekonduksian elektrik PAni-TiO2/PVA meningkat
dengan peningkatan nisbah berat TiO2/aniline hingga suatu tahap (0.13). Kesemua
sampel menunjukkan corak yang hampir sama, iaitu kekonduksian elektrik
meningkat dengan peningkatan suhu dari 30 oC hingga 50 oC, diikuti dengan
pengurangan kekonduksian elektrik, mencapai maksimum pada suhu 140 oC,
kemudian menurun dengan peningkatan suhu seterusnya. PAni/PVA and PAniTiO2 (I)/PVA menunjukkan kekonduksian elektrik maksimum pada 40 oC, masingmasing 1.69 and 1.78 S/cm. Filem-filem itu menunjukkan kekonduksian elektrik
yang bagus dengan nilai sebanyak 10−5 S/cm walaupun pada komposisi komponen
bersifat mengkonduksi yang rendah. Kekonduksian elektrik filem-filem meningkat
dengan peningkatan kandungan PAni dan PAni-TiO2. Ini menunjukkan
kekonduksian filem bergantung kepada kandungan PAni dan PAni-TiO2 akibat
pertumbuhan jaringan yang terbentuk secara berterusan dan telah dipastikan dengan
menggunakan TEM. ‘Percolation threshold’ untuk kedua-dua PAni/PVA and PAniTiO2/PVA adalah sebanyak 2.0 % berat. Daripada kajian kesan Hall, kekonduksian
dan mobility cas berkaitan secara linear, manakala mobility cas dan ketumpatan cas
adalah berkaitan secara songsang. Pada suhu bilik, PAni-TiO2 (I)/PVA (40 wt. %)
menunjukkan mobility cas yang tertinggi (4878 cm2 volt−1 sec−1). Kekonduksian
elektrik menurun dengan peningkatan suhu dan tidak mematuhi persamaan model
lompatan pelbagai-jarak (variable-range hopping) pada suhu melebihi 300 K.
vii
TABLE OF CONTENTS
CHAPTER
1
TITLE
PAGE
TITLE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
CONTENT
vii
LIST OF TABLE
xi
LIST OF FIGURE
xiii
LIST OF ABBREVIATIONS
xvii
LIST OF APPENDICES
xx
INTRODUCTION
1
1.1 Percolation theory
2
1.2 Conducting blends
7
1.2.1 Blends with water-soluble polymers
8
9
1.3 Materials
1.3.1 Electrically conductive polymer
1.3.1.1 Electronic properties of conductive
9
11
polymer
1.3.1.2 Polyaniline (PAni)
13
1.3.1.3 Conduction mechanism in PAni
17
viii
1.3.2 Water-soluble polymer – Poly(vinyl alcohol)
28
(PVA)
1.3.3 Metal oxide
1.3.3.1 Titanium(IV) oxide (TiO2)
1.4 Characterization of polymer blends
2
30
31
32
1.4.1 Four-probe method
33
1.4.2 Hall Effect measurement
33
1.5 Research background and challenges
34
1.6 Research scope
36
1.7 Research objective
36
EXPERIMENT
38
2.1 Chemicals
38
2.2 Instrumentation
38
2.2.1 Vibrational spectra
39
2.2.2 Electronic spectra
39
2.2.3 Themogravimetry analysis (TGA)
39
2.2.4 Differential scanning calorimetry (DSC)
40
2.2.5 X-ray diffraction (XRD)
40
2.2.6 Scanning electron microscopy (SEM)
41
2.2.7 Transmission electron microscopy (TEM)
41
2.2.8 Four-probe resistivity measurement setup
41
2.2.9 Hall Effect measurement setup
43
2.3 Synthesis
2.3.1 Synthesis of DBSA doped PAni (PAni-
44
44
DBSA)
2.3.2 Synthesis of PAni doped with DBSA in HCl
46
medium (PAni)
2.3.3 Synthesis of PAni-TiO2 composites
48
2.3.4 Preparation of conducting blends
50
2.3.5 Preparation of free-standing films
51
2.4 XRD diffraction analysis
52
ix
2.4.1 Bragg’s law
2.5 Preparation of test samples for four-probe resistivity
52
56
measurement and Hall effect measurement
3
2.6 Four-probe resistivity measurement
56
2.7 Hall effect measurement
59
RESULT AND DISCUSSION
61
3.1 Synthesis of PAni and PAni-TiO2
61
3.2 Vibrational spectroscopic characterization of PAni
63
and its composites
3.2.1 Infrared spectra of acids doped PAni
69
3.2.2 Infrared spectra of PAni composites
71
3.2.3 PAni and its composite blends
71
3.2.3.1 Interaction between PAni and PVA
71
3.2.3.2 TiO2 incorporation in PAni/PVA
73
3.3 Electronic spectra
73
3.4 Thermal profile of PAni and its composites
75
3.4.1 Thermogravimetry analysis (TGA)
76
3.4.2 Differential scanning calorimetry (DSC)
80
3.5 Structural analysis
84
3.5.1 X-ray diffractogram (XRD)
84
3.5.2 Scanning electron microscopy (SEM)
90
3.5.3 Transmission electron microscopy (TEM)
94
3.6 Electrical properties of PAni and PAni-TiO2 blends
97
with PVA
3.6.1 Effect of TiO2/aniline weight ratio
101
3.6.2 Effect of temperature
104
3.6.3 Effect of weight fractions
107
3.6.4 Percolation threshold
110
3.7 The Hall effect
115
x
4
CONCLUSION AND SUGGESTION
124
4.1 Conclusion
124
4.2 Suggestions
126
REFERENCE
127
APPENDIX A
136
APPENDIX B
137
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
1.1
Chemical structure of some conjugated polymers
10
2.1
Different composition of polyaniline-titanium(IV) oxide
composites
50
3.1
Percentage yield of PAni and its composites
63
3.2
Observed characteristic infrared absorption bands (cm─1)
of TiO2, PVA, PAni, PAni-TiO2 (I), PAni/PVA, PAniTiO2 (I)/PVA
66
3.3
Observed electronic absorption of PAni, its composite
and their blends
74
3.4
Weight loss (%) of PAni, its composites and their blends
79
3.5
Observed glass transition temperature and melting
temperature of PVA and different content of PAni and
PAni-TiO2 (I)
84
3.6
Position of peaks of PAni, TiO2 and PAni composite
blend in Figure 3.9 and 3.10
88
3.7
The XRD data and conductivity data of PAni and PAniTiO2 blends
90
3.8
Conductivity of PAni and PAni-TiO2 composites at
various temperatures
98
3.9
Conductivity of blends films (40 wt. %) at various
temperatures
99
3.10
Conductivity of the blends films (40 wt. %) with various
weight ratio of TiO2/aniline at room temperature
101
3.11
Detailed room-temperature conductivity data of the blend
films with different PAni and PAni-TiO2 (I) composite
110
xii
loading for both blends, PAni/PVA and PAni-TiO2
(I)/PVA
3.12
The conductivity at room temperature σRT, Hall
coefficient R, carrier density n and carrier mobility µ
117
3.13
The conductivity at room temperature σRT, Hall
coefficient R, carrier density n and carrier mobility µ for
PAni-TiO2 (I)/PVA blend with various weight fractions
118
3.14
Carrier density, carrier mobility and conductivity of
iodine doped poly[(6-N-pyrrolylhexyl)hexylsilane]
(PSiPy)
123
xiii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
1.1
Square array percolation problem. (a) First is the empty
array (b) Then, squares are randomly filled in (c) The
cluster of squares that creates a complete path across the
lattice is called the spanning or infinite cluster (d)
Parallel paths are created (Adapted from reference [7])
4
1.2
Random circle problem − demonstrate continuum
percolation (Adapted from reference [6])
4
1.3
Electrically conductive polymer blend (Adapted from
reference [8])
6
1.4
Typical percolation theory plot (Adapted from reference
[8])
6
1.5
Schematic diagram shows the energy level scheme and
optical transition for the positively charged polaron,
bipolaron and the neutral polaron-exciton (Adapted from
reference [18])
13
1.6
Various states of oxidation and protonation of
polyaniline (Adapted from reference [17, 22])
14
1.7
Protonic acid doping of PAni (emeraldine base) to PAni
(emeraldine salt) (Adapted from reference [17])
16
1.8
Chemical structures of (a) camphorsulfonic acid, CSA
and (b) dodecylbenzenesulfonic acids, DBSA (Adapted
from reference [21])
16
1.9
Defects in conjugated chains: a “physical − chemical
dictionary” (Adapted from reference [23])
19
1.10:
Polaron and bipolaron lattice. (a) Emeraldine salt in
bipolar form. (b) Dissociation of the bipolarons into two
polarons. (c) Rearrangement of the charges into a
20
xiv
‘polaron lattice’ (Adapted from reference [24, 25])
1.11
Spin-charge inversion of a conjugational defect. Charged
solitons are spinless; neutral solitons carry a magnetic
moment [26]
20
1.12
Scheme of the protonation process leading to formation
of polaron and bipolaron in doped PAni (a) Emeraldine
salt in bipolar form (b) Dissociation of the bipolarons
into polarons (c) Rearrangement of the charges into a
“polaron lattice” (Adapted from reference [21, 27 – 28])
22
1.13
Propagation of polaron through a conjugated polymer
chain by shifting of double bonds (alternation) that give
rise to electrical conduction (Adapted from reference
[30])
25
1.14
Energy band diagrams and defect levels for polarons and
bipolarons in undoped, lightly doped and heavily doped
conducting polymers (Adapted from reference [30])
25
1.15
(a) Hopping transport: a man crossing the river by
jumping from stone to stone and (b) Electronic level
scheme of disordered PAni to demonstrate the hopping
conductivity (CB = conduction band, VB valence band,
EF = Fermi energy, W = energetic distance between
states, R = local distance between states, Eg = energy
gap) [26]
26
1.16
Conducting network of a conducting polymer with A
indicating intrachain transport of charge, B indicating
interchain transport, C indicating interparticle transport
and arrows showing path of charge carrier migrating
through the material [26]
28
1.17
Structure of PVA
29
2.1
Diagram of the Four-Probe Set-Up
42
2.2
Diagram of the Hall Effect Set-Up
43
2.3
Synthesis of PAni-DBSA
45
2.4
Synthesis of PAni
47
2.5
Synthesis of PAni-TiO2
49
2.6
Preparation of PAni/PVA and PAni-TiO2/PVA blends
into conducting films
51
2.7
Deriving Bragg’s Law using the reflection geometry and
applying trigonometry. The lower beam must travel the
54
xv
extra distance (PQ + QR) to continue traveling parallel
and adjacent to the top beam
2.8
Model for the four probe resistivity measurements
57
2.9
Circuit used for resistivity measurements (Four Probe
Set-up)
58
2.10
Schematic diagram of sample placement in constant
magnetic field (H) to measure the Hall voltage as a
function of current (I)
60
3.1
Structure of acid doped PAni (Adapted from reference
[21])
62
3.2
FTIR spectra of (a) PAni (b) PAni-TiO2 (I) (c) TiO2
64
3.3
FTIR spectra of (a) PVA (b) PAni-TiO2 (I)/PVA (6 wt.
%) (c) PAni/PVA (6 wt. %)
65
3.4
Interaction between PAni and PVA (intermolecular Hbonding) (Adapted from reference [70])
72
3.5
Absorption bands of (a) PVA, (b) PAni/PVA and (c)
PAni-TiO2(I)/PVA
74
3.6
TGA curves of (a) PAni (
(
)
77
3.7
TGA curves (a) PVA (
), (b) PAni/PVA (40 wt. %)
(
) and (c) PAni-TiO2 (I) /PVA (40 wt. %) (
)
78
3.8
DSC curves of (a) PAni-TiO2 (I)/PVA (6 wt. %) (b)
PAni/PVA (0.4 wt. %) (c) PAni-TiO2 (I)/PVA (0.4 wt.
%) (d) PVA
81
3.9
XRD patterns of (a) TiO2 (b) PAni (powder form) (c)
PVA (d) glass
86
3.10
XRD patterns of (a) PAni-TiO2 (IV)/PVA (6 wt. %) (b)
PAni-TiO2 (I) /PVA (6 wt. %) (c) PAni-TiO2 (I)/PVA
(0.4 wt. %)
87
3.11
SEM micrographs of (a) PAni-TiO2 (I)/PVA (10 wt. %)
(b) PAni-TiO2 (I)/PVA (40 wt. %) (c) PAni/PVA (40 wt.
%) (d) PAni-TiO2 (IV)/PVA (10 wt. %) with
magnification of 10,000 x
91
3.12
SEM micrographs of particles (a) PAni (b) PAni-TiO2 (I)
(c) PAni-TiO2 (IV)
93
3.13
TEM micrographs of PAni/PVA blends at (a) 0.2 wt. %
(b) 0 4 wt %
95
) and (b) PAni-TiO2 (I)
xvi
(b) 0.4 wt. %
3.14
Conductivity of films (40 wt. %) at various temperatures
100
3.15
Variation of room temperature conductivity of films
102
(40 wt. %) with the weight ratio of TiO2/aniline
3.16
Conductivity of blends as a function of 1/T1/2
106
3.17
Conductivity of PAni/PVA and PAni-TiO2 (I)/PVA
blends at room temperature with various weight fractions
109
3.18
(a) Plot of electrical conductivity of PAni vs weight
fractions in PAni/PVA films, (b) Plot of log
(conductivity) vs log (f – fc)
113
3.19
(a) Plot of electrical conductivity of PAni-TiO2 (I) vs
weight fractions in PAni-TiO2 (I)/PVA films, (b) Plot of
log (conductivity) vs log (f – fc)
114
3.20
Hall Voltage, VH OF (a) PAni-DBSA/PVA (b)
PAni/PVA (c) PAni-TiO2 (I)/PVA (d) PAni-TiO2
(II)/PVA (e) PAni-TiO2 (III)/PVA (f) PAni-TiO2
(IV)/PVA
116
3.21
The samples (40 wt. %) plotted against (a) conductivity
and carrier density, (b) carrier mobility and density and
(c) conductivity and carrier mobility of the samples
120
3.22
The samples with various weight fractions plotted
against (a) conductivity and carrier density, (b) carrier
mobility and density and (c) conductivity and carrier
mobility of the samples
122
xvii
LIST OF ABBREVIATIONS
PA
-
Polyacetylene
PAni
-
Polyaniline
PEO
-
Poly(ethylene oxide)
PT
-
Polythiophene
PPY
-
Polypyrrole
PPV
-
Poly(phenylenevinylene)
PPS
-
Poly(phenylene sulfide)
PPP
-
Poly(p-phenylene)
Eg
-
Energy gap
eV
-
Elektron volt
LEB
-
Leucoemeraldine
PNB
-
Pernigraniline
EB
-
Emeraldine base
ES
-
Emeraldine salt
Cl−
-
Ion chloride
CSA
-
Camphorsulfonic acid
DBSA
-
Dodecylbenzene sulfonic acid
APS
-
Ammonium persulphate
S/cm
-
Siemen per cm
HCl
-
Acid hidrochloric
Na
-
Sodium
K
-
Kalium
Li
-
Lithium
Ca
-
Calcium
xviii
BF4
-
Boron tetrafluoride
PVA
-
Poly(vinyl alcohol)
TiO2
-
Titanium(IV) oxide
UV
-
Ultra violet
SEM
-
Scanning electron microscopy
TEM
-
Transmission electron microscopy
AFM
-
Atomic force microscopy
FTIR
-
Fourier transform infrared
TGA
-
Thermal gravimetry analysia
DSC
-
Differential scanning calorimetry
XRD
-
X-ray diffraction
Ge
-
Germanium
ρ
-
Resistivity
σ
-
Dc conductivity
W
-
Sample thickness
S
-
Probe distance
DC
-
Direct current
I
-
Current
V
-
Voltage
VH
-
Hall voltage
RH
-
Hall coefficient
n
-
Carrier density
µ
-
Carrier mobility
H
-
Magnetic field
q
-
Electron charge
PAni-TiO2
-
Polyaniline-titanium(IV) oxide composite
PAni-TiO2/PVA
-
Polyaniline-titanium(IV) oxide composite blend
with poly(vinyl alcohol)
PAni/PVA
-
Polyaniline blend with poly(vinyl alcohol)
Å
-
Angstrom
2θ
-
Angle of incidence of X-rays diffracting planes
CB
-
Conduction band
VB
-
Valence band
xix
EF
-
Fermi energy
Tg
-
Glass transition temperature
Tm
-
Melting temperature
σdc
-
Conductivity of direct current
Wt.
-
Weight
VRH
-
Variable range hopping
I2
-
Iodine
PSiPy
-
Poly[(6-N-pyrrolylhexyl)hexylsilane]
xx
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A
Equations and calculations for the resistivity and
conductivity of PAni/PVA (40 wt. %, 30 oC) by using
four-probe method
136
B
Sample calculation of Hall Effect measurement for
PAni/PVA (40 wt. %, 30 oC) by using Hall Effect
measurement
137
CHAPTER 1
INTRODUCTION
Plastics, fibers, elastomers, coatings, adhesives, rubber, protein and
cellulose − are all common terms in our modern vocabulary, and all part of the
fascinating world of polymer chemistry. Plastic materials have displaced traditional
materials such as natural polymers (e. g. wood), metals, ceramics and glass in many
applications owing to their physical and mechanical properties (light weight
combined with physical strength) and ease of processability (the ability to mold the
shape of plastic materials or extrude into sheet and rod through a die).
The combination of conventional polymers with conductive polymers or
fillers is an important alternative to obtain new polymeric materials with designed
properties. In such blends, the insulating polymer provides good mechanical
properties and processability while the conducting polymer would provide electrical
conductivity. In addition, through blending, the brittleness and lack of
processability that are the main drawbacks hindering conducting polymers better
utilization could be overcomed by new polymeric materials with improved
processability, flexibility and controllable conductivity [1–2].
An essential requirement for the commercial breakthrough of conducting
polymers in blend applications requires that conductivity is achieved at a small
2
weight fraction of the conducting polymer. Further, there is increasing demand for
polymeric materials whose electrical conductivity can be tailored for a given
application, and that have attractive combined mechanical and other properties.
Thus, this have been the driving force for many of the researches in conductive
blends to obtain a wide range of conductivity, which is controllable with varied
weight fraction of the conducting polymer for various potential applications [3–4].
Consequently, there are many approaches towards preparation of such
blends with desirable properties and are finding a growing number of applications
in
commercial
market,
including
antistatic
(microelectronic
packaging),
electrostatic dissipation (ESD), static discharge and electromagnetic interference
shielding (EMI) [4–5]. For ESD and EMI, the required conductivity levels are
approximately 10─5  10─9 S/cm and > 1 S/cm, respectively [4–5]. At present,
conducting plastics in these applications are prepared by mixing conductive solid
fillers, such as special carbon black or metal fibers into the matrix, or coating the
material with a conducting layer. Percolation takes place at volume fraction of
approximately 16 % vol. spherical rigid fillers used. Though mixing of solid fillers
stiffens the material, it may cause undue brittleness and processing difficulties at
the same time.
1.1
Percolation Theory
Percolation theory deals with the effects of varying, in a random system, the
richness of interconnections present [6]. It can be used to model many things
including flow of liquid through a porous medium, spread of disease in a
population, polymer gelation, and conductor-insulator composites.
3
From the perspective of condensed-matter physicists (who have been the
main ones to adopt this mathematical subject for use in their own discipline), the
single most seductive aspect of the percolation model is the presence of a sharp
phase transition at which long-range connectivity suddenly appears. This
percolation transition, which occurs with increasing connectedness or density or
occupation or concentration makes percolation a natural model for a diversity of
phenomena.
The classic example of a percolation theory problem is an array of wires
connecting one communication station with another. The communication network,
represented by a large square-lattice network of interconnections, is attacked by a
crazed saboteur who, armed with wire cutters, proceeds to cut the connecting links
at random. Thus, what fraction of the wires must be cut to sabotage the
communications array? This fraction, which is 0.5, is the percolation threshold [6].
When half of the bonds are broken, the communications array fails to work.
There are three major categories of percolation − bond, site, and continuum.
Bond and site are used when talking about arrays. Site percolation occurs when
there is a connected path of sites from one side of the array to the other. Bond
percolation is utilized when there is a connected path of bonds across the array.
Continuum percolation is used for system where an array model is inappropriate.
Imagine an array of squares (Figure 1.1). Now randomly shaded in one
square at a time. A group of touching shaded squares is called a cluster. When
enough squares are shaded to make a path across the array, the cluster to which the
path belongs is called the spanning cluster [7]. Now, as this cluster grows, the path
across the array becomes less tortuous. At the same time, other paths may be
forming − parallel paths. This concept of parallel paths explains the increase in
conductivity of a system even after the percolation threshold has been passed. The
same thinking exercise above can be done by throwing random circles on a sheet of
4
paper (Figure 1.2). These circles can overlap. This is an example of continuum
percolation [6].
Figure 1.1: Square array percolation problem. (a) First is the empty array (b) Then,
squares are randomly filled in (c) The cluster of squares that creates a complete path
across the lattice is called the spanning or infinite cluster (d) Parallel paths are
created (Adapted from reference [7])
Figure 1.2: Random circle problem − demonstrate continuum percolation (Adapted
from reference [6])
5
Before addressing the specific issues associated with suspension-based
conductive composites, it is important to understand the basic concepts of these
types of materials in general. Figure 1.3 illustrate the key features of a generic
polymer-based conductive blend and Figure 1.4 is a generalized loading curve that
represents the changes in electrical conductivity that occur as a function of
conducting polymer loading. As conductive filler, such as conducting polymer, is
added to an insulating polymer matrix a network begins to form and begins to span
large distances. Once this conductive network reaches a critical size, on the order of
the composite sample size, the two-component material makes a transition from
insulator to conductor. The critical amount of filler (usually expressed as a volume
fraction or percent) required to cause this insulator-to-conductor transition is known
as the percolation threshold. Composite electrical conductivity typically obeys a
power law as a function of conducting polymer concentration [6]:
σ = C (f − fc)t
(1.1)
where σ is the property of interest, C is a proportionality constant related to the
conductivity of the filler (e. g. conducting polymer), f is the probability of a site (or
bond) being filled or in other words the volume fraction of conductive filler, fc is
the critical volume fraction of filler associated with the percolation threshold and t
is the power law exponent (typically 1.6  2.0 in 3−dimension) [6, 7]. This is
graphed in Figure 1.4. In continuum percolation, f and fc can be thought of in terms
of volume fractions. In an insulating matrix composite with conductor inclusions
added, f would be the volume fraction of the conductor at any time, and fc would be
the volume fraction of conductor it took to make a complete path across the
material.
One problem with percolation is that it never allows for a leveling out of the
property. The property in question keeps increasing as the second phase is added.
Since properties tend to level out away from the percolation threshold, a balance
6
Polymer film
Conducting polymer particle
Stage I (f < fc)
Film exhibits insulating
behavior here
Substrate
Film jumps from insulating to
conducting here
Stage II (f = fc)
Substrate
Stage III (f > fc)
“Percolation threshold”
reached when one
conductive pathway created
Polymer modulus,
crystallinity and surface
tension will influence
loading at which
pathway formed
Electrical conductivity
plateaus here
Substrate
Figure 1.3: Electrically conductive polymer blend (Adapted from reference [8])
Percolation Zone
(Stage II)
Insulation
Zone
(Stage I)
Conducting
Zone
(Stage III)
Volume Fraction of Conductor
Figure 1.4: Typical percolation theory plot (Adapted from reference [8])
7
between percolation theory and effective media theory is a good way of modeling
material behavior.
1.2
Conducting Blends
In a limited way, all polymers can be considered blends since their diversity
in molecular weight and microstructure makes it unlikely that two adjacent
macromolecules are identical. However, the term “blend” is usually reserved for a
mixture of two or more polymers with noticeable differences in an average
chemical composition or microstructure. It is not necessary for both polymers to
mix at a molecular level. However, mixing at the molecular level occurs if the
polymers are miscible. The great majority of polymer pairs are immiscible but this
does not preclude their effective use. However, the mixing and subsequent
fabrication procedures are crucial to performance since they determine the final
morphology of the composite.
Polyaniline has been categorized as an intractable material. Nonetheless, it
is possible to process polyaniline from its solution in concentrated acid such as HCl,
p-phenolsulfonic acid (PSA), camphorsulphonic acid (CSA) and other acids with a
polymer concentration ranging from extremely dilute to more than 20 % (wt/wt) [9–
11].
8
1.2.1 Blends With Water-Soluble Polymers
The strong affinity of polyaniline for water has motivated many groups to
investigate the compatibility of polyaniline with water-soluble polymer such as
polyvinyl alcohol and carboxy methyl cellulose. A. Mirmohseni [12] reported the
preparation of a homogeneously dispersed polyaniline by chemical polymerization
of aniline in a media containing 10 % polyvinyl alcohol, which can be cast to form
a mechanically robust film. The uniform composite films of nanostructured
polyaniline (e. g. nanotubes or nanorods with 60 − 80 nm in diameter) were
fabricated by blending with PVA as a matrix [13]. It was found that the electrical,
thermal and mechanical properties of the composite films were affected by the
nanostructured PAni-β-NSA content in the PVA matrix. The composite film with
16 % PAni-β-NSA showed the following physical properties: room temperature
conductivity is in the range of 10─2 S/cm, tensile strength ~ 603 kg/cm2, tensile
modulus ~ 4.36 x 105 kg/cm2. Pallab Banerjee [14] had reported conductive
polyaniline composite films formed by chemical oxidative polymerization of
aniline inside carboxymethylcellulose matrix films, exhibiting extremely low
percolation threshold (fc ~ 1.12 x 10─3).
Manisara Peesan et al. [15] have reported the preparation of blend films of
β-chitin and PVA by solution casting from corresponding solutions of β-chitin and
PVA in concentrated acid. The glass transition temperature of the blend films was
found to increase slightly with an increase in the β-chitin content.
9
1.3
Materials
1.3.1 Electrically Conductive Polymer
From the initial discovery of nearly 12 orders of magnitude of enhancement
in conductivity in the first intrinsic electrically conducting organic polymer, doped
polyacetylene, in 1977 [16], spurring interest in “conducting polymers”. These
polymer systems containing highly loosely held electrons in their backbones,
usually referred to as π-bonded or conjugated polymers or conducting polymers,
with a wide range of electrical and magnetic properties, are a field of increasing
scientific and technical interest. Inspired by by polyacetylene, many new
conductive polymers were developed, including polypyrrole (PPy), polythiophene
(PT), polyaniline (PAni), poly(p-phenylene) (PPP), poly(phenylene vinylene)
(PPV) and etc (Table 1.1).
Later generations of these polymers were processable into powders, films,
and fibers from a wide variety of solvents, and also air stable although these
intrinsically conducting polymers were neither soluble nor air stable initially. Some
forms of these intrinsically conducting polymers can be blended into traditional
polymers to form electrically conductive blends. The conductivities of these
polymers spans a very wide range from that typical insulators (< 10─10 S/cm) to that
typical of semiconductors such as silicon (~ 10─5 S/cm) to greater than 104 S/cm
(nearly that of a good metal such as copper, 5 x 105 S/cm), depending on doping
[17].
The principal interests of researchers on conductive polymers are the
potential applications of conductive polymer in the electronic industry. They could
10
Table 1.1: Chemical structure of some conjugated polymers
(
)n
(
)
O
(
(
S
N
H
n
(a) Polyacetylene
(PA)
(b) Poly(ethylene oxide)
(PEO)
(c) Polythiophene
(PT)
)
n
(d) Polypyrrole
(PPy)
)n
(
)n
(
S )n
(f) Poly(phenylene sulfide)
(PPS)
(
N )n
(g) Polyaniline
(PAni)
(
)n
(e) Poly(phenylenevinylene)
(PPV)
(h) Poly(p-phenylene)
(PPP)
be applied in antistatic and electromagnetic shielding protection, as capacitors,
electrode in polymer batteries, sensors and actuators, protective coating materials,
light-emitting polymer for electronic display devices (such as polymer-based LED,
monitor or large area display) and much more. The advantages of conductive
11
polymer over conventional materials are the relative ease of processing, cost
effectiveness by mass production and fabrication of smaller electronic devices. But
conductive polymers not remained as an “ideal” material as long as some basic
problems regarding its properties unresolved. Two main problems that greatly
affect the performance of conductive polymers include environment instability
(stability against thermal heating, water vapour and sunlight irradiation) and
difficult of processing (where most conductive polymers are in- or weakly soluble
in organic or inorganic solvents). Technological uses depend crucially on the
reproducible control of the molecular and supramolecular architecture of the
macromolecule via a simple methodology of organic synthesis.
1.3.1.1 Electronic Properties of Conductive Polymer
The essential feature of the conjugated polymer is that it provides bands of
delocalised molecular orbitals, the π bands, within which the full range of
semiconductor and metal behaviour can be achieved through control of the degree
of filling. At the same time, the integrity of the chain is preserved by the strong sp2
π bonds which are unaffected by the presence of the excitations within the πelectron manifold [18]. From the point of view of the solid-state physicists, what
distinguishes these semiconductors from inorganic materials, which are well used in
technology, is the strong anisotropy of the lattice and of the electronic excitations.
This has the effect of allowing very strong local interactions between the geometry
of the polymer chain and electronic excitations, such as injected charges or
excitons. This coupling between lattice and electrons is an important example of a
non-linear system, and there has been great interest in the characterization of
resulting excitations.
12
Conjugated polymers in the undoped state possess one pz electron per site,
thus giving occupation of one half of the molecular orbitals within the manifold. All
these polymers show an energy gap (Eg) between filled, π states (for bonding) and
empty, π* states (for antibonding) so that semiconducting behaviour is observed.
The semiconducting gap or energy gap (Eg), which is the energy separation between
π and π* states, ranges from around 1 eV for poly(isothionaphthene), 1.5 eV for
polyacetylene, to 3 eV for poly(p-phenylene) [18]. The size of this gap is directly
related to the magnitude of the alternation of bond lengths along the chain.
The conducting properties of conjugated polymers are seen when band
filling is altered away from the semiconducting ground state. This is accomplished
by chemical doping, photoexcitation of electrons and holes, or by charge injection
to form regions of space or surface charge density, and each of these methods for
introducing excitations is considered in the following sections. Chemical doping,
through formation of charge transfer complexes, can either remove electrons from
the π valence band (oxidative doping) or add electrons to the π* conduction band
(reductive doping), and in the usual band models, should then give metallic
properties. However, the coupling of the π band structure to the size of the local
geometry of the polymer chain (in the form of bond alternation amplitude) gives a
range of novel non-linear excited states.
All conjugated polymers, such as polyaniline, poly(thienylene), polypyrrole,
poly(p-phenylene) and etc except polyacetylene, show a preferred sense of bond
alternation. For these polymers the ground-state geometry is the so-called aromatic
configuration, with long bonds between rings, and an aromatic structure within the
ring. The other sense of bond alternation gives the quinoidal configuration, with
shortened bonds between rings, and a quinoidal structure in the ring. The electronic
excited states are described as polarons [19–20] and the excited-state geometry of
the chain is shifted towards the quinoidal structure; this geometrical change pulls a
pair of states away from the band edges into the gap. Polaron-like excitations can
exist in different charge states, as is shown in Figure 1.5.
13
For
conjugated
polymers
which
preserve
electron-hole
symmetry
(hydrocarbon polymers such as poly(phenylenevinylene), polyaniline and
poly(phenylene), the polaron has associated with it two gap states pulled away
symmetrically from the band edges, as illustrated in Figure 1.5. The presence of the
gap stated allows several new sub-gap optical transitions, as indicated in Figure 1.5;
for the charged polaron and bipolaron these are commonly detected through
induced sub-gap optical absorption, and for the neutral singlet polaron-exciton
transition
from
upper
to
lower
gap
state
can
be
detected
through
photoluminescence.
Conduction
Band
P2
P3
Eg
P4
P1
BP1
BP2
Valence
Band
Polaron
Bipolaron
Polaron-Exciton
Figure 1.5: Schematic diagram shows the energy level scheme and optical
transition for the positively charged polaron, bipolaron and the neutral polaronexciton (Adapted from reference [18])
1.3.1.2 Polyaniline (PAni)
Electrically conducting polymers in their pristine and doped states have
been the materials of great interest for their applications in modern technologies.
Among all conducting polymers polyaniline (PAni) has a special representation,
probably due to the fact that new applications of PAni in several fields of
14
technology are expected. The first report on the production of “aniline black” dated
back to 1862 when Letheby used a platinum electrode during the anodic oxidation
of aniline in a solution containing sulfuric acid and obtained a dark-green
precipitate [21]. This green powdery material soon became known as ‘aniline
black’. The interest in this material retained almost academic for more than a
century since “aniline black” was a powdering, intractable material, a mixture of
several products which is quite difficult to investigate. Green and Woodhead [21]
performed the first organic synthesis and classification of intermediate products in
the “aniline black” formation and five different aniline octamers were identified and
named as leucoemeraldine base, protoemeraldine, emeraldine, nigraniline and
pernigraniline. These names are still used, indicating various oxidation states of
PAni (Figure 1.6).
(a)
Polyaniline (PAni)
N
H
(b)
N
H
N
N
y
(1-y)
Leucoemeraldine
N
H
(c)
N
H
N
H
N
H
N
N
n
Pernigraniline
N
N
n
(d)
Emeraldine base (EB)
N
H
N
H
N
N
n
Figure 1.6: Various states of oxidation and protonation of polyaniline (Adapted
from reference [17, 22])
x
15
Polyaniline is a typical phenylene-based polymer having a chemically
flexible —NH— group in a polymer chain flanked either side by a phenylene ring.
It is a unique polymer because it can exist in a variety of structures depending on
the value of (1–y) in the general formula of the polymer shown in Figure 1.6 (a)
[17, 22]. The electronic properties of PAni can be reversibly controlled by
protonation as well as by redox doping. Therefore, PAni could be visualized as a
mixed oxidation state polymer composed of reduced {–NH–B–NH–} and oxidized
{–N=Q=N–} repeat units where –B– and =Q= denote a benzenoid and a quinoid
unit respectively forming the polymer chain (Figure 1.6 (a)), the average oxidation
state is given by 1–y. Depending upon the oxidation state of nitrogen atoms which
exist as amine or imine configuration, PAni can adopt various structures in several
oxidation states, ranging from the completely reduced leucoemeraldine base state
(LEB) (Figure1.6 (b)), y–1 = 0, to the fully oxidized pernigraniline base state
(PNB) (Figure 1.6 (c)), where 1–y = 1. The “half” oxidized (1–y = 0.5) emeraldine
base state (EB) (Figure1.6 (d)) is a semiconductor and is composed of an alternating
sequence of two benzenoid units and a quinoid unit. The protonated form is the
conducting emeraldine salt (ES).
The electronic structure and excitations of these three insulating forms
(LEB, PNB, EB) are contrasted. However, the LEB form can be p-doped
(oxidatively doped), the EB form can be protonic acid doped and the PNB form can
be n-doped (reductively doped) to form conducting ES systems. The EB,
intermediate forms of PAni can be non-redox when doped with acids to yield the
conductive emeraldine salt state of PAni as demonstrated in Figure 1.7. It can be
rendered conductive by protonating (proton doping) the imine nitrogen, formally
creating radical cations on these sites. This doping introduces a counterion (e.g. Cl─
if HCl was used as the dopant), and recently, the counterion was affixed to the
parent polymer by partially sulfonating the benzene rings in the polymer, resulting
in a so-called “self-doped” polymer. Both organic acids such as HCSA (camphor
sulfonic acid), and inorganic acids, such as HCl, are effective, with the organic
sulfonic acids leading to solubility in a wide variety of organic solvents, such as
chloroform and m-cresol. The protonic acid may also be covalently bound to the
16
PAni backbone, as has been achieved in the water-soluble sulfonated PAni (Figure
1.7). Similar electronic behavior has been observed for the other nondegenerate
N
H
N
H
N
N
n
2 HA
A
+
N
H
-
A
·
+
N
H
-
·
N
H
N
H
n
Figure 1.7: Protonic acid doping of PAni (emeraldine base) to PAni (emeraldine
salt) (Adapted from reference [17])
a.
O
O
S
O
O
H
b.
O
S
O
H
O
Figure 1.8: Chemical structures of (a) camphorsulfonic acid, CSA and (b)
dodecylbenzenesulfonic acids, DBSA (Adapted from reference [21])
17
ground state systems as for protonic acid doped PAni. That is, polarons are
important at low doping levels. For doping to the highly conducting state, a polaron
lattice (partially filled energy band) forms. In less ordered regions of doped
polymers, polaron pairs or bipolarons are formed.
During doping all the hetero atoms in polymer, namely the imine nitrogen
atoms of the polymer become protonated to give a polaronic form where both spin
and charge are delocalized along the entire polymer backbone. The conductive
emeraldine salt state can be converted back to the insulating emeraldine base state
through treatment with a base, indicating that this process is reversible.
This protonated emeraldine salt form is electronically conducting, the
magnitude of increase in its conductivity varies with proton (H+ ion) doping level
(protonic acid doping) as well as functionalities present in the dopant [22]. In the
doping acid, the functional group that present, its structure and orientation can
influence the solubility of a conducting form of PAni or for obtaining aqueous
dispersion and compatibility with other polymers. The chemical structures of two
organic acids, which are recently used in doping PAni are shown in Figure 1.8.
Thus, PAni owns advantages over other conducting polymers owing to its
moderate synthesis route, superior environmental stability and undergoes simple
doping by protonic acids easily.
1.3.1.3 Conduction Mechanism in PAni
Before the details of conduction in polyaniline are discussed, three
frequently used physical terms in describing conduction in solid have to be
18
understood; namely soliton, polaron and bipolaron as shown in Figure 1.9 [23].
Soliton, sometime called as conjugational defect, is lone electron created in the
polymer backbone during the synthesis of conductive polymer, in very low
concentration. Conjugational defect is a misfit in the bond alternation so that two
single bonds will touch. Soliton can be generated in pairs, as soliton and antisoliton. Three methods were used to generate additional solitons − chemical doping,
photogeneration and charge injection. An electron will be accepted by the dopant
anion to form a carbocation (positive charge) and a free radical during the chemical
doping (oxidation) of the polymer chain, known to organic chemists as radical
cation or polaron to physicists. Both the soliton and polaron can be neutral or
charged (positively or negatively) as shown in Figure 1.9.
The conductivities of PAni can be transformed from insulating to
conducting through doping. Both n-type (electron donating, such as Na, K, Li, Ca)
and p-type (electron accepting, such as I2, BF4, Cl) dopants have been utilized to
induce an insulator-to-conductor transition in electronic polymers. The common
dopants for PAni are hydrochloric acid, sulfuric acids and sulfonic acids. For the
degenerate ground state polymers, the charges added to the backbone at low doping
levels are stored in charged soliton and polaron states for degenerate polymers, and
as charged polarons or bipolarons for nondegenerate systems. Such a situation is
also encountered in PAni, which do not have two degenerate ground states. That is,
the ground state is non-degenerate due to the non-availability of two energetically
equal Kekule structures. Therefore there cannot be a link to connect them. In the
doping process, the heteroatoms − nitrogen will be protonated and become a bipolar
form (Figure 1.10). The motionless charged states are known as carbonium (+ve)
and carbanion (-ve) radicals by organic chemist. The conventional distortion of
molecular lattice can create a localized electronic state, thereby lattice distortion is
self-consistently stabilized (Figure 1.10).
Thus, the charge coupled to the surrounding (induced) lattice distortion to
lower the total electronic energy is known as polaron (i. e. an ordinary radical ion)
19
Undisturbed
conjugation
Vacuum state
·
Neutral Soliton
Free radical
+
Carbocation
(Carbenium-Ion)
Positive Soliton
··
Negative Soliton
Carbanion
+
Positive Polaron
··
Negative Polaron
Positive Bi-Soliton
(Bipolaron)
Negative Bi-Soliton
(Bi-Polaron)
·
Radical Cation
·
Radical Anion
+
Carbodication
+
··
··
Carbodianion
Figure 1.9: Defects in conjugated chains: a “physical − chemical dictionary”
(Adapted from reference [23])
20
H
N
+
N
(a)
+
N
H
(N
H
H
N
(b)
+.
N
H
(N
H
(c)
(N
H
N
+.
)
H
.N+
)
H
.N+
n
)
N
H
H
n
n
Figure 1.10: Polaron and bipolaron lattice. (a) Emeraldine salt in bipolar form. (b)
Dissociation of the bipolarons into two polarons. (c) Rearrangement of the charges
into a ‘polaron lattice’ (Adapted from reference [24, 25])
.
.
Charge, Q = +e
Spin, S = 0
+
..
Q=0
Q = -e
S = 1/2
S=0
.
...
Figure 1.11: Spin-charge inversion of a conjugational defect. Charged solitons are
spinless; neutral solitons carry a magnetic moment [26]
21
with a unit charge and spin = ½. A bipolaron consist of two coupled polarons with
charge = 2e and spin = 0. The polaron and bipolaron have a unique property called
“spin-charge inversion”: whenever soliton bears charge, it has no spin and vice
versa (Figure 1.11) [26]. Bipolarons are not created directly but must form by the
coupling of pre-existing polarons or possibly by addition of charge to pre-existing
polarons (Figure 1.12) [21, 27–28].
At the molecular level a polymer is an ordered sequence of monomer units.
The degree of unsaturation and conjugation influence charge transport via the
orbital overlap within a molecular chain. The charge transport becomes obscured by
the intervention of chain folds and other structural defects. The connectivity of the
transport network is also influenced by the structure of the dopant molecule. The
dopant not only generates a charge carrier by reorganizing the structure (chemical
modification) it also provides intermolecular links and sets up a microfield pattern
affecting charge transport. Any disturbance of the periodicity of the potential along
the polymer chain induces a localized energy state. Localization also arises in the
neighbourhood of the ionized dopant molecule due to the coulomb field.
The conductivity of various conducting polymer tends to be relatively
insensitive to the identity of the doping agent. The most important requirement for a
dopant is sufficient oxidizing or reducing power to ionize the polymer. For
example, though I2 is a relatively weak electron acceptor, it induces conductivity in
polyacetylene (550 S/cm), which is comparable to that achieved on doping with
AsF5, a strong electron acceptor (1100 S/cm) [29].
It is well known that polyaniline with conjugated π-electron backbones can
be oxidized or reduced more easily and more reversibly than conventional
polymers. Charge-transfer agents (dopants) effect this oxidation or reduction and in
doing so convert an insulating polymer to a conducting polymer with near metallic
conductivity in many cases. Since PAni behave like amorphous solids and the
22
··
··
··
N
H
N
··
N
H
··
N
H
N
··
N
H
N
··
N
··
x
(4x) H+AH
N
+
A-
(a)
··
N
H
H
H
N
··
A-
N
+
··
N
H
N
H
··
N
H
A-
+
A-
+
N
x
H
(2 Bipolarons)
Internal redox
H
N
+·
(b)
··
N
H
A
-
A-
H
··
N
H
+·
N
H
N
+·
··
N
H
A-
A-
··
N
H
+·
N
H
x
(4 Polarons)
(Polarons migration)
(c)
H
N
+·
··
N
H
A-
··
N
H
H
N
+·
H
N
+·
-
A-
A
··
N
H
H
N
+·
··
N
H
Ax
Figure 1.12: Scheme of the protonation process leading to formation of polaron and
bipolaron in doped PAni (a) Emeraldine salt in bipolar form (b) Dissociation of the
bipolarons into polarons (c) Rearrangement of the charges into a “polaron lattice”.
(Adapted from references [21, 27–28])
23
simple band theory fails to explain the conduction of electricity in polymers, we
have to assume the validity of the band theory of solids for polymers to the nearest
approximation to explain the conduction in polymers [30]. Further, we have to look
at the basis of doping effects on the PAni in order to wholly understand the
conduction mechanism in PAni.
During the synthesis of PAni, the polymer backbone can have inherited a
certain concentration of conjugational defects, which is also known as solitons
(Figure 1.9). On doping of PAni with oxidizing DBSA (p-doping) in present study,
protonation is realized in which DBSA accept the electrons, conferring positive
charges on the PAni chain and the positive charges of the cationic PAni chains are
balanced by the anionic part of the acids [31]. When electron is removed from the
top of the valence band (by oxidation) of PAni, conjugational defects are generated
from the distorted PAni’s backbone lattice as PAni is a one-dimensional material,
resulting in a vacancy (a radical cation or called positive polaron, which composed
of neutral and positively charged solitons) equivalent to a hole is created [30, 32]. A
polaron give rise to two levels (polaron bands) in the band gap due to the formation
of polaron lattice defect state. The dopant, DBSA itself on the other hand, would be
incorporated into the PAni and sits as counter-ions somewhere between the polymer
chains. If the concentration of defects on the PAni backbone is high enough, the
wave functions of the individual defects will overlap and the Peierls distortion† will
be suppressed not only locally but also globally, leading to the disappearance of
energy gap and PAni chain will becomes metallic [23]. Figure 1.12 indicates the
formation of polaron and bipolaron states in polyaniline.
Nevertheless, before the wave functions of the polarons (combination of
neutral and charged solitons) overlap are suggested, the electrostatic interaction
between the charge of the polarons and that of the counterions has to be taken into
account. One may proposes that the charged solitons themselves might move and
†
Peierls distortion: The typical one-dimensional phenomena for metal-to-insulator transitions
described by Bloch wave where a gap opens between the electronic density of states (Ref [23]).
24
thus contribute to the conduction mechanism. Yet, this is not very likely, because
the charged solitons will be electrostatically bound to the counter-ions and therefore
they are not expected to be mobile [23, 26]. Thus, the remaining unpaired electron
on the PAni chain contributes to the propagation of polaron through a conjugated
polymer chain by shifting of double bonds alternation that give rise to electrical
conductivity (Figure 1.13) [30].
Such polaron is not delocalize completely, but is delocalized only a few
monomeric units deforming the polymeric structure. The energy associated with
this polaron represents a destabilized bonding orbital. It has a higher energy than
the valence band, (to the nearest band theory approximation) and lies in the band
gap. If another electron is subsequently removed from the already oxidized
polymer, another polaron or a bipolaron will be created. Low doping level gives
rise to polarons and with increase in the doping level more and more polarons
interact to form bipolarons [30, 33]. The energy levels in polymers as a result of
doping are indicated in Figure 1.14.
Moreover, the midgap states could acts as hopping centers and the electrical
transport mechanism could well be variable-range hopping (VRH) for PAni, as has
been proposed in amorphous semiconductors [23, 26, 29].
In general, doping will not be homogeneous and a sample doped moderately
will consist of lightly and heavily doped regions. Owing to the local anisotropy of
the samples caused by the polymer chains (one-dimensionality), the percolation
behavior (formation of connected paths of highly doped regions) is very difficult
to predict and the observed conductivity will very often be a combination of
variable-range hopping in lightly doped regions and tunneling between more
heavily doped domains in the PAni and its composites.
25
Unpaired Electron
·
+
·
+
·
+
Figure 1.13: Propagation of polaron through a conjugated polymer chain by
shifting of double bonds (alternation) that give rise to electrical conduction
(Adapted from reference [30])
Conduction Band
Polaron
Levels
Bipolaron Levels
Valence Band
Undoped
Low
Doping
High Doping
Figure 1.14: Energy band diagrams and defect levels for polarons and bipolarons in
undoped, lightly doped and heavily doped conducting polymers (Adapted from
reference [30])
26
In a VRH model [26], the conduction of charge carriers could be explained
by the hopping mechanism (Figure 1.15). Hopping is like a man crossing a river by
jumping from stone to stone, where the stones are spread out at random, as
illustrated in Figure 1.15 (a). It is obvious that the more stones available, the higher
the conductivity, σdc because he can jump effortlessly from stone to a nearer stones
compared to hopping from stone to a far apart stone.
(a)
(b)
CB
Eg
R
EF
... ... ..... .. .. .
W
VB
= electron
Figure 1.15: (a) Hopping transport: a man crossing the river by jumping from stone
to stone and (b) Electronic level scheme of disordered PAni to demonstrate the
hopping conductivity (CB = conduction band, VB valence band, EF = Fermi energy,
W = energetic distance between states, R = local distance between states, Eg =
energy gap) [26]
27
To discuss the temperature dependence of hopping conductivity, it is more
reasonable to look at the band structure represented in Figure 1.15 (b). There are
localized states in the energy gap (Eg), distributed randomly in space as well as in
energy. The Fermi energy level (EF) is about at the center of the Eg with the states
below EF is occupied and the states above empty (except for thermal excitations).
Electrons will hop (tunnel) from occupied to empty states. Most of the hops will
have to be upward in energy. There are many phonons available at high
temperature, which can assist in upward hopping. As these phonons freeze the
electron has to look further and further to find an energetically accessible state. As a
result, the average hopping distance will decrease as the temperature decreases –
hence the name “variable range hopping”. Since the tunneling probability decreases
exponentially with the distance, the conductivity also decreases.
It is well known that doping process produces a generous supply of potential
carriers, but to contribute to conductivity they must be mobile. And, it is found that
the hopping conduction of conducting polymer (including PAni) may be hampered
by three elements contributing to the carrier mobility [29], namely single chain or
intramolecular transport, interchain transport and interparticle contacts. These three
elements comprise a complicated resistive network (illustrated in Figure 1.16),
which determines the effective mobility of the carriers. The polarons and bipolarons
are mobile and under the influence of electric field, can move along the polymer
chain, from one chain to another and from one granule to another. At higher
temperature, softening process will eventually alter the macroscopic and
microscopic properties of PAni.
However, there are many other factors that also influence such conduction
mechanism, such as doping level, method of preparation [32] and temperature [34].
28
B
C A A
A
B
B
C
C
A
A
B
A
B
A
Figure 1.16: Conducting network of a conducting polymer with A indicating
intrachain transport of charge, B indicating interchain transport, C indicating
interparticle transport and arrows showing path of charge carrier migrating through
the material [26]
1.3.2 Water-soluble Polymer  Poly(vinyl alcohol) (PVA)
Water-soluble polymer provide the best matrix for a model blend because
polyaniline can be incorporated into a blend system with the presence of
dodecylbenzene sulfonic acid (DBSA) which helps in the form of dispersing.
Poly(vinyl alcohol), a polyhydroxy polymer, is the largest volume,
synthetic, water-soluble resin produced in the world. With the gradual reduction in
cost, various other end uses began to be exploited. Figure 1.17 shows the chemical
structure of PVA.
29
CH2
CH
n
OH
Figure 1.17: Structure of PVA
The biodegradable and non-toxcity PVA is highly soluble in highly polar
and hydrophilic solvents, such as water, acetamide and glycols. The solvent of
choice is water and the solubility in water depends on the degree of polymerization
and hydrolysis. The polymer is an excellent adhesive for corrugated board, paper
and paper board and in general purpose adhesives for bonding paper, textile and
porous ceramic surfaces. It possesses solvent, oil and grease resistance matched by
few other polymers.
The excellent chemical resistance and physical properties of PVA resins
have led to broad industrial use [35]. PVA forms tough, clean films, which have
high tensile strength and abrasion resistance, oxygen-barrier properties under dry
conditions are superior to those of any known polymer. Furthermore, the
unsupported films cast from aqueous solutions of PVA and plasticiser have
resistance towards organic solvents. The use of PVA film in the cold water-soluble
packaging of materials such as detergent, bleach, for pesticides, herbicides,
fertilizers and other materials which pose health or safety risks, enable dissolving in
water without the removal of the package. Owing to low surface tension,
emulsification and protective colloid properties are excellent. Moreover, it has high
electrical resistivity (3.1 − 3.8) x 107, leading to the extremely excellent antistatic
properties of the film [35–36].
The main uses of PVA are in fibers, adhesives, emulsion polymerization,
production of poly(vinyl butyral) and paper and textile sizing. Furthermore,
30
significant volumes are also used in joint cements for building construction. In
addition, it is used in water-soluble films for hospital laundry bags, temporary
protective films and other applications.
In this study, water-soluble polyvinyl alcohol (PVA) was selected to blend
with the electrically conducting polyaniline (PAni) and polyaniline-titanium(IV)
oxide (PAni-TiO2) composite to form a free-standing conducting film and
investigate the electrical properties of the films.
1.3.3 Metal Oxide
Nowadays, the physical and chemical requirement of a material has been
even greater than ever. This demand cannot be fully met neither a handful of
elements
having
semiconducting
properties nor some relevant chemical
compounds, which are already understood. Therefore, increasing attention is being
paid to studies on less known chemical compounds able to act as semiconductors to
meet the need of the future technology. One of the most important aspects of a
“new” material is most probably the electrical property. This is because the
development of the technology has put electronic and electrical appliances in our
daily life.
Among them, semiconductor oxides may be considered as most promising.
Owing to their electrical properties like conductivity, magnetic, ferroelectric, piezoelectric, electroluminescence and optical property, some of the metal oxides have
been successfully applied in electronic and microelectronic for the production of
electronic components such as transistor, capacitor, resistor, microchip and others.
31
In this study, titanium(IV) oxide (TiO2) was utilized to form composite with
PAni and then blended with water-soluble PVA.
1.3.3.1 Titanium(IV) Oxide (TiO2)
Titanium(IV) oxide, occurring in three polymorphic forms − rutile, anatase,
brookite, is a very common pigment utilized in various industries. Rutile and
anatase are produced commercially in large quantity for the use as pigments,
catalysts and in the production of ceramic and electronic materials. Their electrical
conductance is between 1.1 x 10─5 − 3.4 x 10─3 S/cm [37]. TiO2 is widely used in
welding-rod coatings, specific paints, inks, acid-resistant vitreous enamel, etc.
owing to its high refractive index, durability, dispersion, tinting, strength,
chemically inert nature and non-toxicity.
However, below a critical size, TiO2 clusters can absorb the energy of
ultraviolet (UV) to release electron and radical by oxidation under irradiation of
UV. Thus, they are also used as protectant against external irridiation and sunlight.
Thus, it is used as a sunblock in suncreams because it reflects, absorbs, scatters
light, does not irritate skin and it is water-resistant. In plastic industries, TiO2 is
incorporated on the package of fat-containing food to prevent UV radiation. It is
also utilized in the production of anti-static plastic and as electrically conducting
materials.
Further, the absorbed organic compounds on TiO2 clusters can be
decomposed by oxidation owing to the presence of a radical released by irradiation.
So, TiO2 is known as photocatalyst [31]. Anatase TiO2 have high potential for
32
application in diverse areas of environmental purification, such as purification of
water and air due to the unique properties.
The polyaniline-TiO2 composite had been investigated on its electrical
property, effect of thermal treatment and TiO2 content [31]. According to Somani
and co-workers [38], this composite exhibits high piezosentivity and being
maximum at certain PAni-TiO2 composition.
1.4
Characterization of Polymer Blends
There are several methods could be employed to evaluate a material or
composite, whether they are in the category of nano- or submicron-sized materials.
The most common methods used are Scanning Electron Microscopy (SEM),
Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM).
For the SEM, the resolution ranges from sub-milimeter to sub-micron while the
TEM and AFM could go up to nanometer. Both SEM and TEM are very useful to
determine the morphology and particle sizes of samples where direct imaging of the
material structures can be perform. The AFM is suitable for scanning the 3dimension surface morphology and surface imaging of the samples.
Apart from the morphology and particle sizes of the conducting polymer
blends, we have to look also into another important physical properties, i.e.
electrical properties of the blends. The electrical properties of a conducting polymer
blends involves the charge transfer within the nanocomposite matrix whereby is a
complex and difficult to study. Two of the measurements used in this research are
described below.
33
1.4.1
Four-probe Method
Resistivity measurements are generally made on desired materials to
determine their suitability as materials for electronic components. The resistivity
must be measured accurately since its value is critical in many devices.
Many conventional methods for measuring the resistivity are unsatisfactory
for semiconductors because metal-semiconductor contacts are naturally rectifying.
An excess concentration of minority carriers will affect the potential of other
contacts and hence modulate the resistance of the material.
The four-probe method overcomes the difficulties mentioned above and also
offers other advantages. It permits measurements of resistivity in samples having a
wide variety of shapes, including the resistivity of small volume within bigger
pieces of semiconductor. This method of measurement is also applicable to silicon
and other semiconductor materials such as conducting polymer [10, 39–42].
1.4.2 Hall Effect Measurement
Conductivity measurements could not distinguish the types of carriers
present. But Hall effect measurement, which is basic tools for determination of
mobilities could. This method was discovered by Hall in 1879 [43] when he was
attempting to prove the magnetic field effect on a current. With this method,
parameters such as Hall coefficient, carrier density, mobility and other values could
be derived from the measured resistivity data.
34
1.5
Research Background and Challenges
The principal problem encountered with the potential utilization of
conducting polymers is their poor processability and environmental stability.
Attempts have been made earlier to incorporate plastics or rubber with conducting
polymer in order to improve the processability of the said mixed material without
losing the mechanical properties [14].
Currently, polyaniline (PAni) is one of the most investigated intrinsically
conducting polymers due to its easy synthesis and good conductivity. Significant
progress in the preparation of processible forms of polyaniline has been reported in
recent years. The use of selected sulphonic acids [42], phosphonic acids [11] and
hydrochloric acid [9, 44] as protonating agents has led to the solubilisation of the
conductive polyaniline; enabling its processing from solution. Furthermore, some of
the polyaniline dopants plasticize it upon ptotonation which, in turn, facililitates
thermal processing of PAni. For all these reasons, polyaniline is considered as one
of the most promising candidates for the fabrication of conductive blends with
industrially important classical polymers.
Such a great demand for flexible, conductive plastic film and the latest
progress on the fabrication of polyaniline and its blends has attracted enormous
industrial interest. The ability to process PAni into tough, free-standing, air stable
films has been a pulling factor for the intense research and great number of
published material related to this promising conducting polymer. Nevertheless, one
of the difficulties associated with the processing of conjugated polymers is their
poor solubility in common volatile organic solvents. PAni in its emeraldine form
(PAni-EB)
is
commonly
processed
by
dissolving
the
polymer
in
N-
methylpyrrolidone (NMP) or m-cresol, both high boiling point solvents, resulting in
cast films containing a non-negligible amount of residual solvent, often associated
with residual water from the polymerization reaction [45–46]. The resulting PAni
film normally contains a considerable amount of N-methylpyrrolidone (NMP),
35
about 18 % by weight due to the high boiling point of NMP (202 oC) and the
presence of the hydrogen bonding interaction of the carbonyl group with the NH
group in PAni. The residual solvent will affect the electrical conductivity and
degree of crystallization and crystalline structure apart from mechanical and
thermal properties [47].
Interactions between the two blended polymers would greatly influence the
conductivity and physical properties of the films. In terms of percolation theory [6],
the conductivity is very small at sufficiently low concentration where there is no
connected path. As the concentration of the conducting polymer is increased, the
conductivity shows a sharp increase at a certain concentration of conducting
polymer, which is called the percolation threshold.
Several works have been reported on the preparation of interpenetrating
composites of polyaniline (PAni) with several matrix polymer such as poly(methyl
methacrylate), poly(vinylalchohol), polycarbonate etc [2, 12, 15, 48–50]. Several
studies also reported conducting polyblends prepared with polyaniline and a
classical polymer (polystyrene, poly(vinyl acetate), polyimide, etc) in organic
solvents, which exhibit good mechanical properties associated with interesting
electrical properties [9–10, 45]. Those polyanilene blends are all in an organic
system. Recently, Shi-Jian Su and Noriyuki Kuramoto had reported that polyaniline
blends in organic system obtain higher conductivity than in aqueous system [46].
The information about the polymer blends in aqueous is much more scarce. In this
respect, in this works we explored and studied polyaniline blends in aqueous
system.
36
1.6
Research Scope
The scope of this research is as listed below:
a)
To synthesis conductive polyaniline (PAni) and polyaniline-titanium(IV)
oxide composites through in situ polymerization method.
b)
To form conductive, free-standing films of PAni/PVA and PAni-TiO2/PVA
by using solution casting method.
c)
To study the effect of TiO2 content on the conductivity behavior of the
blends with varying TiO2/aniline weight ratio, ranging from 0.13 – 0.53.
d)
To study the heating effect on conductivity behavior within temperature
range from 30 oC – 160 oC.
e)
To study the influence of weight fractions of conductive components (PAni
and PAni-TiO2), ranging from 0.1 wt. % – 40 wt. %, on the blends’
conductivity behavior.
1.7
Research Objective:
Polymer blends have been extensively studied in recent years due to their
significant importance in applied as well as basic sciences. They are formulated to
provide a material with an appropriate balance of thermal performance,
processability and toughness among other properties that cannot be met with single
polymers. Even though the metallic conductivity of certain conductive polymer
blends came close to those of metal, the main problems of long-term stability of
charge carrier and its mobility in conductive polymer blend yet to be addressed.
The focus of this research is to synthesis conductive polyaniline (PAni) and
polyaniline-titanium(IV) oxide, and consequently to process PAni and PAni-TiO2
conducting blends into tough, free-standing, flexible and conductive films in
37
aqueous system through blending with water-soluble poly(vinyl alcohol). PAni has
been selected as the conductive polymer candidate to be synthesized and used to
composite with titanium(IV) oxide by in situ polymerization, doped with
dodecylbenzenesulfonic acid in hydrochloric acid medium. The chemical and
physical characterizations of these systems were emphasized on those related to
antistatic films properties, such as electrical conductivity and charge carrier
mobility.
These two activities, namely synthesize and characterization, were carried
out in tight interaction with each other. On one hand, the aim of physical
investigation is to account for the influence of TiO2 content, temperature and
weight fractions of conductive components (PAni and PAni-TiO2) on the
conductivity behavior of the films. Besides, the electrical transport properties such
as electrical conductivity, charge carrier mobility and density are studied and
correlated by using the four-probe and Hall effect measurement. On the other hand,
the objective of the chemical study is to confirm and defined feature of the polymer
and its composite blends and is meant to innovate better synthetic method for better
quality conducting films.
CHAPTER 2
EXPERIMENT
2.1
Chemicals
Aniline (99.5 %, Merck) as the monomer was purified by double distillation
under reduced pressure prior to use. Ammonium persulphate APS (min. 98 %,
Merck) and 4-dodecylbenzenesulfonic acid (DBSA, 90 %, Fluka) were used as
oxidant and dopant, respectively. Polyvinyl alcohol (PVA, Mw 100,000, Fluka) was
used as matrix polymer. Hydrochloric acid (HCl, 37 %, Merck), titanium(IV) oxide
TiO2 (99.99 %, Aldrich) and nitrogen N2 were used as received without further
purification. The deionized water was used in this research.
2.2
Instrumentation
Samples were characterized by using the instruments as listed below.
39
2.2.1 Vibrational Spectra
Vibrational spectra of the samples were recorded with Shidmadzu FTIR8300 Infrared Spectrophotometer in the range of 400 –
4000 cm─1 with 4 cm─1 resolution. All the FTIR-analyzed polymer samples
were free-standing films. The only exception was the PAni and PAni-TiO2
composites single material sample. The PAni and PAni-TiO2 powder was triturated
together with KBr powder to form PAni and PAni-TiO2 KBr pellet. The interaction
between PAni and its composites with PVA was examined.
2.2.2 Electronic Spectra
A Shidmadzu UV-1601 PC UV/VIS spectrophotometer was used to
measure optical absorption of aqueous PAni, its composite and blends suspensions.
The suspensions were diluted 500 times with deionized water and the electronic
absorption spectra were measured using a 0.5 cm quartze cell. The scanning was
obtained within the wavelength range from 200 to 900 nm with the absorption (0 –
2 A) measuring mode.
2.2.3 Thermogravimetry Analysis (TGA)
Thermogravimetry analysis of PAni, its composites and blend films were
investigated by using Mettler TG50 thermogravimetric analyzer with nitrogen as
purge gas. The heating rate was 5 °C per minute from 30 °C to 900 °C.
40
2.2.4 Differential Scanning Calorimetry (DSC)
Differential scanning calorimeter measurement was performed with a Perkin
Elmer DSC System (Pyris 1 DSC) to analyze the thermal properties and determine
Tgs of PAni, its composites and blend films. Temperature readings were firstly
calibrated with an empty pan. Approximately 4 mg sample was weighed and sealed
in an aluminium pan. The samples were equilibrated at 30 oC for 10 min and hold
for 1 min at 30 oC. The samples were subsequently heated from 30 oC to 250 oC at a
heating rate of 10 oC/min. After keeping the temperature at 250 oC for 1 min, the
samples were cooled to 30 oC at the same heating rate. The second heating scans
were run from this temperature to 250 oC to record stable thermograms. The Tg of
the samples was determined from the second heating run.
In this study, the initial inflection of slope in the differential scanning
calorimetry curve was taken as the Tg. All the measurements were performed at a
heating rate of 10 °C/min under N2 purge at 20 mL min─1 from 30 °C to 250 °C.
2.2.5 X-ray Diffraction (XRD)
The X-ray diffraction of PAni and its composite blend in powder and film
form were obtained from Siemen D-5000 X-ray diffractometer using Cu Kα
radiation (λ = 1.5418 Å) operated at 40 kV and 30 mA. The scanning range was
between angle 2 θ of 2 – 60
o
in a step of 0.04
o
s─1. The samples were in both
powder and films form. The XRD patterns for the films were recorded on a glass
slide.
41
2.2.6 Scanning Electron Microscopy (SEM)
The fracture surfaces of the PAni and its composites in powder and their
blends in films form were examined by using FEI Quanta 400 Scanning Electron
Microscopy. Scanning Electron Microscopy (SEM) was used to check the particle
size distribution, surface morphology and dispersal of particle in material matrices.
2.2.7 Transmission Electron Microscopy (TEM)
For transmission electron microscopy (TEM) the blend films were cast
directly on carbon-coated copper grids (400 Mesh Square Grids) from dispersions
containing 0.2 to 0.4 wt. % blends. The lower concentrations were used for blends
with higher PAni and PAni-TiO2 loadings. The details of the conducting doped
PAni and its composite network in the blend were checked and confirmed using a
TEM Model Philips CM12.
2.2.8 Four-probe Resistivity Measurement Setup
The four probe method is one of the standard and most widely used method
to measure the resistivity of semiconductors. The four probes are collinear in its
useful form. The error due to contact resistance, which is especially serious in the
electrical measurement on semiconductors, is avoided by the use of two extra
contacts (probes) between the current contacts. In this arrangement the contact
resistance may all be high compare to the sample resistance, but as long as the
resistance of the sample and contact resistances are small compared with the
42
effective resistance of the voltage measuring device (potentiometer, electrometer or
electronic voltmeter), the measured value will remain unaffected. The arrangement
is also specially useful for quick measurement on different samples or sampling
different parts of the same sample due to the pressure contacts. The schematic
diagram of the Four-Probe Set-Up is shown in the Figure 2.1.
Figure 2.1: Diagram of the Four-Probe Set-Up
The whole four probes set-up Model DFP-02 produced by Scientific
Equipment Roorkee was used to measure the resistivity of the film samples in
various temperatures. The set-up unit consists of three sub-units enclosed in one
cabinet, including a multi range digital voltmeter, constant current generator and
oven power supply. The temperature variation for the small oven to achieve is from
room temperature to 200 °C. This equipment was calibrated by using standard
Germanium (Ge) slice before measuring the samples. The resistivity of the sample
was calculated using equation shown in Appendix A [51–53].
Some assumptions are necessary to be made in order to use this four probes
method in conducting polymer blend films. It is assumed that the resistivity of the
43
material is uniform in the measurement area. The surface on which the probes rest
must be flat with no surface leakage. Instead of this, the four probes for resistivity
measurements must contact the surface at points that lie in a straight line.
Furthermore, the diameter of the contact between the metallic probes and the
material should be small compared to the distance between probes.
2.2.9 Hall effect Measurement Setup
The Hall Effect experiment was carried out by using the whole Hall Effect
set-up (as shown in Figure 2.2) supplied by Scientific Equipment & Services, which
consist of Hall Effect Set-Up (DHE-21), Digital Gaussmeter (DGM-102),
electromagnet (Model EMU-75V), constant current power supply (DPS-175).
Figure 2.2: Diagram of the Hall Effect Set-Up
44
The collected data can be used to calculate hall coefficient (RH), carrier
density (n) and carrier mobility (µ) by using equations shown in Appendix B [52–
55]. A set of data for the calculation was shown in the Appendix B.
2.3
Synthesis
2.3.1 Synthesis of DBSA Doped PAni (PAni-DBSA)
DBSA (0.1611 mol) was stirred mechanically for 2 hours in a four-neck
round bottom reaction flask in ice bath (0 − 5 oC) for the DBSA to dissolve in 111
mL H2O. Aniline (0.1611 mol, stirred in 10 mL deionized water) was added dropwise into the DBSA solution and stirred vigorously for 3 hours to obtain optimum
homogeneity in N2 atmosphere. A solution of ammonium persulphate (0.1933 mol,
cooled prior to use) in 120 mL H2O was then added slowly to the solution with
stirring to initiate the polymerization of anline. After 2 hours of APS addition, the
reaction was allowed to take place with vigorous stirring for 24 hours at room
temperature. The polymer suspension was centrifuged and washed with deionized
water repeatedly until the washing liquid was completely colorless. The resulting
wet product was dried under vacuum at 60 − 80 o C for more than 8 hours until a
constant weight was obtained to calculate its water content and to determine yield.
DBSA solution (half amount of the DBSA used, 220 mL H2O) and the other portion
of wet product were then blended vigorously. The dark green PAni-DBSA solution
was later centrifuged and large particles were removed, resulting the final dark
green PAni-DBSA wet product for subsequently further experimental works. The
steps of synthesis process are shown in Figure 2.3.
45
Aniline (0.16 mol)
• Stirred in 10 mL
•
•
DBSA (0.16 mol)
• Dissolved in 111
mL H2O
Vigorous
stirring
0 − 5 oC
•
•
•
•
•
Added
dropwise
0 − 5 oC
APS (0.19 mol)
• Dissolved in 120 mL
H2 O
•
Vigorous
stirring
0 − 5 oC
Vigorous
stirring
0 − 5 oC, N2
Milky dispersion of
anilinium-SA complex
• Vigorous stirring
• Room temperature
• 24 h
Dark green PAni suspension
•
•
•
•
•
Washed with H2O
repeatedly
Added DBSA
(dissolved in H2O)
Vigorous stirring
Centrifuged
Removed large
PAni-DBSA
(wet product)
Figure 2.3: Synthesis of PAni-DBSA
46
2.3.2 Synthesis of PAni Doped With DBSA In HCl Medium (PAni)
Polymerization was carried out by the chemical oxidation of aniline in the
presence of DBSA and APS in an aqueous HCl (1N) medium, both played the role
as dopant and oxidant respectively. DBSA (0.1611 mol) was dissolved in 111 mL
of HCl in a four-neck round bottom reaction flask under mechanical stirring in ice
bath (0 − 5 oC) for 2 hours. Aniline (0.1611 mol, distilled under reduced pressure
prior to use) was stirred in 10 mL of HCl for 30 minutes. The solution was then
added drop-wised into the DBSA solution with vigorous stirring for 3 hours to
disperse the aniline homogenously, resulting a milky dispersion of particles
anilinium-SA complex. The dispersion was stirred in an ice bath maintained at 0 −
5 oC under N2 atmosphere. A solution of APS (0.1933 mol, cooled prior to use) in
120 mL HCl was latter added drop-wised for 2 hours into the solution to initiate the
aniline polymerization. The temperature of the solution was kept between 0 − 5 oC
under vigorous stirring until the addition of APS solution was completed. The
reaction was later carried out for 24 hours at room temperature with stirring. A dark
green colored PAni suspension was obtained without precipitation. The synthesized
PAni was ontained as finely dispersed particles, which were recovered from the
polymerization mixture by centrifugation and washed with deionized water
repeatedly until the washing liquid was completely colorless. Finally, the polymer
was transferred to a beaker containing 1 N aqueous solution of protonic acid (HCl).
After keeping overnight, the PAni was obtained by centrifugation and excess acid
in PAni washed with deionized water and centrifuged each in sequence for several
times. A portion of the resulting wet product was dried under vacuum at 60 − 80 oC
for more than 8 hours, then its water content could be calculated. Another fraction
of the wet product was then redispersed in DBSA solution (half amount of the
DBSA used, 220 mL H2O) and blended vigorously using a commercial blender for
the PAni to disperse homogeneously into the solution. The dark green PAni
solution was later centrifuged and large particles were removed, resulting the final
dark green PAni wet product for subsequently further experimental works. The
flowchart of the synthesis process was depicted in Figure 2.4.
47
Aniline (0.16 mol)
• Stirred in 10 mL HCl
DBSA (0.16 mol)
• Dissolved in 111 mL HCl
• Vigorous stirring
• 0 − 5 oC
• Vigorous stirring
• 0 − 5 oC
• Vigorous stirring
• 0 − 5 oC, N2 atm
• Added dropwise
• 0 − 5 oC
APS (0.19 mol)
• Dissolved in 120 mL HCl
Milky dispersion of
anilinium-SA complex
• Vigorous stirring
• Room temperature
• 24 h
Dark green PAni suspension
• Washed with H2O, HCl
and H2O repeatedly
• Added DBSA
(dissolved in H2O)
• Vigorous stirring
• Centrifuged
• Removed large particles
PAni (wet product)
Figure 2.4: Synthesis of PAni
48
2.3.3 Synthesis of PAni-TiO2 Composites
The synthesis steps of PAni composite are similar to the synthesis method of
PAni. Various amount of TiO2 was dispersed into the DBSA solution (being stirred
for 1 hour in 111 mL HCl) and stirred for 1 hour prior to the addition of aniline.
Aniline (0.1611 mol) stirred in 10 mL of HCl were added drop-wised into the
DBSA-TiO2 solution and stirred vigorously to form homogeneous dispersion. The
dispersion was stirred in an ice bath maintained at 0-5 °C under N2 atmosphere.
After 3 hours, APS (0.1933 mol, cooled prior to use) in 120 mL HCl was then
added to the dispersion to commence the polymerization reaction. After 2 hours of
APS addition, the reaction was carried out for 24 hours by maintaining the
condition at room temperature under vigorous stirring. The resulting PAni-TiO2
suspension was obtained by washing with deionized water, immersed in HCl
(overnight) and washed again with deionized water each in sequence for several
times. Then, the green PAni-TiO2 wet product was dried under vacuum at 60 − 80
o
C for more than 8 hours until reaching a constant weight, so that its water content
and yield could be calculated. The PAni composite was chemically characterized by
infrared spectroscopy. Another fraction of the wet product was then redispersed in
DBSA solution (half amount of the DBSA used, 220 mL H2O) and blended
vigorously using a commercial blender for the PAni to disperse homogeneously
into the solution. The green PAni-TiO2 solution was later centrifuged and removed
large particles, resulting the final dark green PAni wet product for subsequently
further experimental works. The flowchart of the synthesis process was depicted in
Figure 2.5.
For convenience, polyaniline doped with DBSA is designated as PAniDBSA while PAni doped with DBSA in HCl medium is designated as PAni.
Besides, PAni composites with various TiO2/monomer weight ratio were
recognized as PAni-TiO2. Table 2.1 shows the PAni-TiO2 composites prepared with
varied amount of TiO2 at different TiO2/monomer (TiO2/aniline) weight ratio.
49
TiO2
• Ultrason
icated in
H2O
DBSA (0.16 mol)
• Dissolved in 111
mL HCl
•
•
Aniline (0.16 mol)
• Stirred in 10 mL
HCl
•
•
Added
dropwise
0 − 5 oC
APS (0.19 mol)
• Dissolved in 120 mL
HCl
•
•
Vigorous
stirring
0 − 5 oC
Vigorous stirring
0 − 5 oC, N2 atm
Milky dispersion of
anilinium-SA-metal oxide complex
•
Vigorous
stirring
Room
Temperature
•
Dark green PAni-TiO2
•
•
•
•
•
Washed with
H2O, HCl and
H2O repeatedly
Added DBSA
(dissolved in H2O)
Vigorous stirring
Centrifuged
Removed large
PAni-TiO2
(wet product)
Figure 2.5: Synthesis of PAni-TiO2
50
Table 2.1: Different composition of polyaniline-titanium(IV) oxide composites
Sample
Amount of TiO2 (g)
Weight Ratio of
TiO2/ monomer
PAni-DBSA


PAni


PAni-TiO2 (I)
2
0.13
PAni-TiO2 (II)
4
0.27
PAni-TiO2 (III)
6
0.40
PAni-TiO2 (IV)
8
0.53
2.3.4 Preparation of Conducting Blends
Stock solution of doped conducting polymer or polymer composite (PAni or
PAni-TiO2) and PVA were prepared separately in aqueous system. Different
proportions of conducting PAni and PAni-TiO2 dispersion were mixed with the
insulating matrix PVA in water, respectively, in order to obtain the desired
composition of blends according to the following procedure. First, PVA was fully
dissolved in water at 80 oC and cooled to room temperature. Then, various amount
of PAni and PAni-TiO2 dispersion in aqueous solutions were each added to PVA
aqueous solution, and stirred until each solution was homogeneous under
ultrasonication for 3 h. The steps of conducting blends preparation process are
shown in Figure 2.6. Blends containing various wt. % of PAni and PAni-TiO2 were
prepared and denoted as PAni x % or PAni-TiO2 x %, where x % represents the
percentage of PAni or PAni-TiO2 in the blends.
51
PAni or PAni-TiO2
(wet product)
• Disperse in H2O
PVA (0.16 g/mL)
• Dissolved in H2O
• 80 oC
• Cooled to room temperature
• Vigorous stirring
• Ultrasonicated
PAni/PVA Or PAni-TiO2/PVA
Blends
• Solution casting
Casting on polyethylene petri dish
• Drying at room temperature
Conducting film
Figure 2.6: Preparation of PAni/PVA and PAni-TiO2/PVA blends into conducting
films
2.3.5 Preparation of Free-standing Films
Polyethylene petri dishes were precleaned with deionized water and dried at
room temperature. Free-standing films (40 – 60 µm) were prepared by casting the
conducting blend solution (PAni/PVA and PAni-TiO2/PVA) onto the polyethylene
petri dishes using a solution casting method. The films were thoroughly dried on a
flat surface at room temperature for a period of 1 − 2 days and kept in silica gels
filled desicator before the measurement. For preparation of PVA films, the fully
dissolved PVA aqueous solution was cooled from 80 oC to room temperature, and
52
then cast into films using the same procedures as for the blends. The steps of freestanding films preparation process are shown in Figure 2.6.
2.4
XRD Diffraction Analysis
The most dramatic progress in understanding minerals perhaps came with
the discovery of X-rays. In 1912, German physicist Max von Lane (1879 – 1960)
had provided that minerals posses a regular and repeating internal arrangement of
atoms by demonstrated that crystals would diffract X-rays. Since then, the English
physicist Sir William Henry Bragg (1862 – 1942), pioneered the determination of
crystal structure by X-ray Diffraction methods, for which he and his son Sir
William Lawrence Bragg (1890 – 1971) received the 1915 Nobel Prize in physics
for their work in determining crystal structures beginning with NaCl, ZnS and
diamond.
2.4.1 Bragg’s Law
In common with other types of electromagnetic radiation, interaction
between the electric vector of X-radiation and the electrons of the matter through
which it passes through results in scattering. When X-ray are scattered by the
ordered environment in a crystal, interference takes place among the scattered rays
owing to the same order of magnitude of the distances between the scattering
centers as the wavelength of the radiation. Thus, resulting in diffraction.
53
When X-ray beam strikes a crystal surface at some angle θ, a portion is
scattered by the layer of atoms at the surface. The unscattered portion of the beam
penetrates to the second layer of atoms (Figure 2.7). The cumulative effect of this
scattering from the regularly spaced centers of the crystal is diffraction of the beam.
Bragg expressed this in an equation now known as Bragg's Law
nλ = 2d sin θ, for n = 1, 2, 3 etc
(2.1)
to explain why the cleavage faces of crystals appear to reflect X-ray beams at
certain angles of incidence (theta, θ in degree). The variable d is the distance
between atomic layers in a crystal, λ (lambda) is the wavelength of the incident Xray beam, n is the positive integer (the index) and θ is the angle from the crystal
plane (not the usual normal).
Bragg’s Law can easily be derived by considering the conditions necessary
to make the phases of the beams coincide when the incident angle equals and
reflecting angle. The rays of the incident beam are always in phase and parallel up
to the point at which the top beam strikes the top layer at atom S (Figure 2.7). The
penetrating second beam travels down to the next layer where it is scattered by
atom Q, reflects and travels back over the same distance before being back at the
surface. Thus, the path of an X-ray reflected off of the top surface is shorter than the
path of the X-ray reflected off of the plane of atoms inside the crystal. The distance
traveled depends on the separation of the layers and the angle at which the X-ray
entered the material. In other words, the second beam must travel the extra distance
PQ + QR for the two beams to continue traveling adjacent and parallel. For the two
beams to be in same phase, this extra distance must be an integral (n) multiple of
the wavelength (λ):
54
Incident X-rays
Reflected X-rays
Ray 1
Ray 2
S
θ
θ
d
θ
θ
P
R
d
Q
S
d
θ
P
Q
Figure 2.7: Deriving Bragg’s Law using the reflection geometry and applying
trigonometry. The lower beam must travel the extra distance (PQ + QR) to continue
traveling parallel and adjacent to the top beam
55
nλ = PQ + QR
(2.2)
Recognizing d as the hypotenuse of the left triangle PQS, trigonometry can be use
to relate d and θ to the distance (PQ +QR). The distance PQ is opposite θ, thus
PQ = d sin θ
(2.3)
Because PQ = QR Equation (2.3) turn into,
nλ = 2 PQ
(2.4)
Combining equations (2.3) and (2.4) gives,
nλ = 2d sin θ
Note that X-rays appear to be reflected from the crystal only if the angle of
incidence satisfies the conditions that
sin θ =
nλ
2d
At all other angles, destructive interference occurs.
(2.5)
56
Bragg’s Law indicates that diffraction is only observed when a set of planes
makes a very specific angle with the incoming X-ray beam. This angle depends on
the inter-plane spacing d, which itself depends on the size of the molecules which
make up the structure. Thus, the size of the unit cell of the studied crystal can be
derived by the positions of atoms and Bragg’s Law and then intensity maximums
give the atomic positions within the crystal lattice, enabling the structure of crystals
to be investigated.
2.5
Preparation of Test Samples For Four probe Resistivity Measurement
and Hall effect Measurement
The films cast from solutions on the polyethylene petri dishes were peeled
off, cut into pieces and thoroughly dried before the measurements. The thicknesses
of the films were between 40 – 60 µm.
2.6
Four-probe Resistivity Measurement
DC resistivity of samples was measured in a chamber with controlled
temperature. The basic model for all these measurements is indicated in Figure 2.8
[51–53, 56]. The sample was put on the base plate of the four-probe arrangement.
The zinc-coated probes were rested gently on the sample. Then the four-probe
arrangement was placed into the oven and a thermometer was fixed in the oven
through the hole provided.
57
V
I
I
Probes
Sample
S1
S2
S3
S1
S2
S3
Support
Glass slide
Figure 2.8: Model for the four probe resistivity measurements
Then, the ac power supply of Four Probe Set-up was switched on and the
digital panel meter was put to the current measuring mode. The current was
adjusted to a desired value. The digital panel meter mode was subsequently
adjusted to voltage measuring mode and the voltage between the probes can be
read. Lastly the oven was on and the rate of heating can be chosen between low and
high as desired.
The voltage of sample was collected every 5 °C increment in the
temperature range from 30 to 160
o
C. The experimental circuit used for
measurement is illustrated schematically in Figure 2.9. A nominal value of probe
spacing which, has been found satisfactory, is an equal distance of 2.0 mm between
adjacent probes. This permitted the measurement of resistivity of semiconducting
material with reasonable current from 0.001 to 50 ohm cm [51].
58
Galvanometer
Potentiometer
Milliammeter
Direct
current
Millivoltmeter
V
I
I
Probes
Figure 2.9: Circuit used for resistivity measurements (Four Probe Set-up)
(a)
Measurement for low resistivity to medium resistivity sample
The measurement of low resistivity and medium resistivity sample at
various temperatures (30 to 160 oC) was carried out using the whole Four Probe
Set-up (Model DFP-02, Scientific Equipment Roorkee).
(b)
Measurement for high resistivity sample
Low constant current sources are needed when the sample resistance, either
inherently or due to contact resistances, is large. Because of the constant current
provided by the Four Probe Set-up was not sufficient enough for the measurement
of high resistivity sample, some adjustments were made. A programmable current
source (Model 220, Keithley) was connected to the outer probes to provide a low
constant current and a multimeter (FLUKE 189 True RMS) was connected to the
inner probes so that the voltage between the probes could be read. The zinc-coated
probes were rested gently on the sample, which placed on the base plate of the four-
59
probe arrangement. Then the four-probe arrangement were placed into the oven
connected to the Four Probe Set-up and a thermometer was fixed the in the oven
through the hole provided to measure the high resistivity of the sample at various
temperature.
The four-probe set-ups, for both low resistivity to medium resistivity sample
and high resistivity sample were calibrated with a standard germanium chip before
measurements. Both set-ups give similar data for the resistivity measurement of Ge
Chip.
2.7
Hall effect Measurement
In order to do the Hall Effect measurement, sample must be connected to a
constant current power supply and millivoltmeter (as shown in Figure 2.10). As the
set-up was switched on, the current was adjusted to a desired value. The voltage
value was then adjusted to zero (zero field potential). The test sample was placed in
the magnetic field (as shown in Figure 2.10) [52–55] and electromagnet power
supply was switched on. When the magnetic field was applied on the sample,
current was adjusted to a desired value. Then the sample was rotated until it become
perpendicular to the magnetic field and at the same time, hall voltage will be
maximum in this adjustment. Measurement of Hall Voltage was made for both
directions of the current and magnetic field. Lastly, a graph of Hall voltage versus
current and Hall voltage versus magnetic field were plotted.
From the graphs, Hall coefficient (RH), charge carrier density, (n) and
carrier mobility (µ) can be calculated [52, 54].
60
I
H
Milivoltmeter
SS
N
Constant current
power supply
Sample
Figure 2.10: Schematic diagram of sample placement in constant magnetic field
(H) to measure the Hall voltage as a function of current (I)
CHAPTER 3
RESULT AND DISCUSSION
3.1
Synthesis of PAni and PAni-TiO2
A single route ‘in-situ’ deposition polymerization method was employed to
synthesis PAni and PAni-TiO2. In this research, titanium(IV) oxide (TiO2) was
incorporated in PAni doped with dodecylbenzene sulfonic acids (DBSA) in the
presence of hydrochloric acid (HCl).
Initially, during the polymerization of DBSA doped PAni and PAni-TiO2
composite in HCl medium, the acidic aqueous solution was colorless. Nonetheless,
10–20 minutes after the addition of the initiator APS solution into the aqueous
solution, the milky color of the solution turned blue and later green and turned dark
green at the end of the polymerization, indicating the formation of protonated
emeraldine form PAni. For the polymerization of PAni-TiO2 composite in the
presence of TiO2, the PAni was formed preferentially on the oxide particles giving a
higher yield than bare PAni, which is consistent with the previous result [31].
By using organic sulfonic acids as dopants, many advantages could be
enjoyed. Many other functional groups could be attached to the aromatic nucleus of
62
the acid so as to change its property, and thus would modify the physicochemical
properties of the conducting polymer. For example, alkyl chain substituted aromatic
acids in DBSA was known for its surfactant properties. Hence, it may help to
induce solubilization of PAni by micellar action. Figure 3.1 shows the likely
chemical structures of PAni doped with DBSA [21].
n
Figure 3.1: Structure of acid doped PAni (Adapted from reference [21])
Table 3.1 shows the percentage yield of the PAni and its composite (PAniTiO2) calculated by the ratio of the weight of PAni and its composite to the weight
of monomer (aniline) and TiO2 content used. The yield of PAni was high (~ 86 %),
but after the incorporation of TiO2 with the ratio of 0.13, the yield of the PAni
composite was increased (~ 91 %). This could be due to the enhancement of the
polymerization process of the monomer in the presence of particles of TiO2.
Nevertheless, with increasing TiO2 content in the PAni composite, the yields of the
composites were decreased. This could be due to the structural disturbance, which
63
was caused by the overloaded TiO2 content in the composites, as could be observed
in the SEM micrographs of PAni composites particles (Figure 3.12, Section 3. 5. 3).
Table 3.1: Percentage yield of PAni and its composites
Amount of TiO2
Weight Ratio of
Yield
(g)
TiO2/monomer
(Weight %)
PAni-DBSA


88.67
PAni


85.67
PAni-TiO2 (I)
2
0.13
90.94
PAni-TiO2 (II)
4
0.27
79.53
PAni-TiO2 (III)
6
0.40
57.48
PAni-TiO2 (IV)
8
0.53
41.96
Sample
3.2
Vibrational Spectroscopic Characterization of PAni and Its Composites
Infrared absorption spectra are long known and well established in
characterizing the chain structure of polymers. This includes the configurational
and conformational structure of the chains and the identification of defect structures
and end groups. The infrared spectra of PAni, PAni-TiO2 composite and composite
blends obtained from this study are shown in Figures 3.2 and 3.3. The position and
characteristic bands observed (Table 3.2) are in good agreement with those reported
in the literatures.
4000
Transmittance, % T
3000
2000
1500
(b)
Wavenumber, cm─1
1000
Figure 3.2: FTIR spectra of (a) PAni (b) PAni-TiO2 (I) (c) TiO2
(c) TiO2
(b) PAni-TiO2 (I)
(a) PAni
400
64
4000
3000
2000
(c) PAni/PVA (6 wt. %)
1500
Wavenumber, cm─1
(b) PAni-TiO2 (I)/PVA (6 wt. %)
Figure 3.2: FTIR spectra of (a) PAni (b) PAni-TiO2 (I) (c) TiO2
(a) PVA
1000
Figure 3.3: FTIR spectra of (a) PVA (b) PAni-TiO2 (I)/PVA (6 wt. %) (c) PAni/PVA (6 wt. %)
Transmittance, % T
400
65
1553 m
1461 m
1709 w
1658 w
1560 m
1480 m
νs(CH2)
δ(O–H) of OH groups of PVA
C–N
1664 sh
1734 s, shp
2922 w
2925 s
Aromatic CH stretching
2942 s, shp
2930 s
C–H stretching
3355 s, br
3300 s, br
3247 sh
PAni
3230 br
PVA
3453 s, br
TiO2
3450 br
references
From
O–H stretching
>N–H stretching
Characteristic vibration
1461 m
1553 m
2929 m, shp
3232 sh
3439 m, br
(I)
PAni-TiO2
Observed
Wavenumber, cm─1
1443 m
1572 w
2928 m, shp
3343 s, br
PAni/PVA
1441 m
1572 w
2934 m, shp
3335 s, br
(I)/PVA
PAni-TiO2
Table 3.2: Observed characteristic infrared absorption bands (cm─1) of TiO2, PVA, PAni, PAni-TiO2 (I), PAni/PVA, PAni-TiO2 (I)/PVA
66
1230 m, sh
Deprotonation of aromatic amines
O=S=O
C–N+ stretching +C–C
C–N stretching +C–C or
1008 sh
1111 s, shp
1236 m, shp
1295 m, shp
1410 w, sh
PAni
1074 sh
1259 s
1329 w
1375 m, shp
1431 br, s
PVA
1030 sh
TiO2
1172 sh
1120 s, shp
1143 s
1293 m, shp
Aromatic amine
C–O stretching
1330 m, shp
1375 w
1380 w, sh
1429 br, s
references
From
δ(OH) coupled with CH wag
CH2 bending
C–N+
to the δ(CH) wag
δ(OH) In-plane bending coupled
Characteristic vibration
1026 sh
1070 sh
1107 s, shp
1236 m, shp
1292 m, shp
1410 sh
(I)
PAni-TiO2
Wavenumber, cm─1
1004 sh
1034 sh
1144 w, sh
1315 m
1220 w
1236 w
1373 w, sh
PAni/PVA
1002 sh
1033 sh
1115 w, sh
1317 m
1224 w
1233 w
1373 w, sh
(I)/PVA
PAni-TiO2
67
Ti–O
Degenerate bending of SO3-
And H2O deformation
550 br
500 br
m: medium
699 br
TiO2
660 br
570 m, shp
660 m, br
s: strong
Out-of-plane C–OH deformation
703 w
S–O
854 m, shp
C–C–O in-phase stretching
800 m
900 w, sh
Undissociated acids
C–H(op)
1099 s
references
From
ν(CCO) out-of-phase
Characteristic vibration
w: weak
565 w
658 w
613 w
698 w
794 m
br:
934 w, sh
(I)
PAni-TiO2
shp: sharp
573 m, shp
661 m
794 m
875 w, sh
PAni
sh: shoulder
607 m, br
851 m, shp
1095 s
PVA
Wavenumber, cm─1
584 w
602 w
683 w
829 m
916 w, sh
1096 m
PAni/PVA
665 w
584 w
602 w
682 w
829 m
916 w, sh
1095 m
(I)/PVA
PAni-TiO2
68
69
3.2.1 Infrared Spectra of Acids Doped PAni
The doping of PAni leads to the formation of –Q=N+H– groups. The
positive charge on the polymer chain may lead to an increase in the dipole moment
of the molecule, resulting in an increase in the intensity of the infrared bands [57].
The bands related to N–H stretching of an aromatic amine (> NH stretching)
normally appear in the region between 3100 and 3500 cm─1 [41]. Due to the
presence of moisture in the samples, we saw a broad band near 3453 cm─1 and a
shoulder band at 3247 cm─1 with medium intensity. Both bands could be assigned
to the asymmetric and symmetric stretching modes of –NH2+ group, respectively
[57]. It is reported that these bands are absent in the dedoped sample. The –NH+
stretching could have overlapped with the asymmetric NH2 stretching mode and the
–NH2+ group may have participated in a hydrogen bonding formation [57]. The
band around 2922 cm─1 is due to the aromatic CH stretching.
The two bands observed in the 1440 – 1600 cm─1 region are related to the
stretching of the C–N bonds of the benzenic and quinonic rings, respectively and
are present due to the conducting state of the polymer [58–59]. The intensity of
these bands illustrates an idea of the oxidation state of PAni. When they appear in
equal intensities, PAni is in the emeraldine base form [60]. The bands
corresponding to quinoid (N=Q=N) and benzenoid (N–B–N) ring stretching modes
were observed at 1553 cm─1 and 1461 cm─1, respectively.
Another characteristic band in the infrared spectra for the acids doped PAni
is the C–N+ stretching absorption of the QBQ or quinoid (B refers to cis-benzenoid
unit and Q refers to quinoic unit) at about 1380 cm─1, which arises due to
protonation of PAni, by dopant [21, 57]. According to Nayanna C. [61],
leucoemeraldine does not absorb at this frequency in contrast to the emeraldine and
70
pernigraniline form. We observed this frequency as a weak shoulder band at 1410
cm─1.
Peaks in the region close to 1300 cm─1 are attributable to the presence of
aromatic amines in all forms of PAni [58, 60] whereas a shoulder band at 1230
cm─1 has been associated with deprotonation of the aromatic amines of the polymer
[60]. We observed both bands at 1295 and 1236 cm─1, respectively. The existence
of the shoulder peak may also imply that PAni doped with various acids has not
been fully oxidized.
The band characteristic of the conducting polymer due to the delocalization
of electrical changes [21, 60, 62] was observed as a sharp and strong band at around
1111 cm─1.
The absorption peaks of the sulfonic group dopant were observed partially
obscured by the consonation band of C–N or C–N+ + C–C stretchings at 1111 cm─1.
We attributed the two shoulder bands at 1030 and 1008 cm─1 as the asymmetric and
symmetric O=S=O stretching vibrations of sulfonic group, and peaks at 661 cm─1
assigned to S–O [40, 62–63]. Protonic acids, which are used as dopants are never
fully associated, particularly when they are incorporated into a solid matrix such as
PAni. Therefore, the two extra absorption bands observed near 875 cm─1 and 573
cm─1 are due to undissociated acids [21] and the degenerate bending mode of the
SO3─ group, respectively [57].
The evidence of the formation of poly(p-aniline) with 1,4-substituted phenyl
rings could be established by the C–H out-of-plane bending band occurred at
around 800 cm─1 [58, 60, 64, 65]. We observed this characteristic band at 794 cm─1.
71
3.2.2 Infrared Spectra of PAni Composites
The infrared spectrum of the PAni-TiO2 (I) composites is very much similar
to the spectrum of the doped PAni with additional bands at 658 and 565 cm─1
belonging to the TiO2 (Figure 3.2, Table 3.2) [66].
The shear sharpness of the metal-oxides absorption bands and remained at
almost the same frequencies of pure metal oxides strongly suggest that the metal
oxides were well dispersed in the polymer matrices [67]. Slight shifts and changes
in intensities in certain bands among the PAni salts and PAni composite were
observed [38].
3.2.3 PAni and Its Composite Blends
3.2.3.1 Interaction Between PAni and PVA
The characteristic bands of both PAni and PVA [13, 68–69] were observed
in the blend films and the characteristic vibrations are summarized in Table 3.2.
Some peaks have been altered both in intensity and position upon blend formation.
Undoubtedly, these changes indicate structural and chemical alterations of the PAni
framework in the blends.
The band assigned to C–O stretching of PVA and the Caromatic–N stretching
of PAni either diminished or shifted upon blending. The most probable explanation
for this occurrence is by considering the possibility of the formation of hydrogen
72
bonding between hydroxyl groups of PVA and amine and positively charged amine
and imine sites of PAni as illustrated in Figure 3.4 [10, 48, 70]. The formation of
hydrogen bonding between the two components has caused a decrease in
crystallinity of PVA in the blends, as was confirmed by a XRD result [70]. Similar
formation of H-bonding between the H-acceptor hydroxyl groups of PVA and Hdonor imino group of PAni has been related by K. S. Ho et al. [40]. However, the
sulfo-group in the doped PAni, which is another source of H-acceptor, is shielded
by the long alkyl chain of the DBSA dopant and itself could not be free to form Hbonding with H-donating group [40].
C
C
O
O
+·
N
R
SO3-
N
R
+·
N
H
SO3-
O
C
H
H
N
x
H
H
O
C
N
R
H
N
R
+
N
H
SO3-
SO3-
O
H
+
N
H y
C
Figure 3.4: Interaction between PAni and PVA (intermolecular H-bonding)
(Adapted from reference [70])
Earlier than that, Dhawan [62] revealed that the characteristic bands of
nitrogen quinoid and benzenoid show the blue shift from 1571 to 1603 cm─1 and
from 1483 to 1501 cm─1 when dopant was removed from the polymer, indicating
the conversion of the benzenoid ring to quinoid rings in the PAni matrix. Much
more later, a paper [39] reported that some quinoid rings in the emeraldine base of
PAni were transformed into benzenoid through doping with HCl, LiPF6 and LiBF4,
which the C–N stretching suffers a major set-back. The peak at 1461 cm─1 in PAni
shifted to 1443 cm─1 in PAni/PVA blend. The large shift and enhanced intensity in
the blend manifested the conversion of C=N– (quinoid) to C–NH– (benzenoid) and
its promotion. Undoubtedly, this spectral feature arises due to the involment of the
benzenoid –NH in intermolecular hydrogen bonding with oxygen of PVA. The
73
effect on other vibrational groups also leads to the same view, and such interaction
becomes the driving force for the quinoid to benzenoid conversion.
3.2.3.2 TiO2 Incorporation In PAni/PVA
The additional characteristic bands of PAni and PVA in the PAni-TiO2
(I)/PVA blend films confirm the presence of both components in the composite
blend film. Nevertheless, the Ti–O band cannot be observed clearly in the PAniTiO2 (I)/PVA blend film because it appears as a weak band at 665 cm─1.
3.3
Electronic Spectra
The observed electronic absorption bands (Figure 3.5) are consistent with
the emeraldine salt form of PAni [71]. The observed optical absorption frequencies
of the samples are presented in Table 3.3.
The conducting emeraldine salt form in water has two characteristic
absorption bands at 420 and 800 nm owing to the radical cation (polaron) state
formed on the PAni backbone chain [71–72]. Thus, the absorption pairs observed at
430 nm and 846 nm for PAni and 433 nm and 811 nm for PAni-TiO2 (I) composite
are related to polaron band transition after doping and correspond to localization of
electrons. UV absorption bands at 360 nm and 361 nm for PAni and PAni-TiO2 (I)
composite, respectively are attributable to the π π∗ transition on the PAni
backbone [13, 63, 70]. But, the absorption for PAni-TiO2 (I)/PVA is increased due
74
to overlapping of the absorption of TiO2 in this region [31]. As seen from the bands
at around 800 nm, all these spectra exhibit strong evidences for the protonation of
PAni.
(c)
(b)
(a)
Figure 3.5: Absorption bands of (a) PVA, (b) PAni/PVA and (c) PAni-TiO2
(I)/PVA
Table 3.3: Observed electronic absorption of PAni, its composite and their blends
λmax/nm
Transition
PAni-TiO2
PAni
PAni-TiO2 (I)
PAni/PVA
π  π∗
360
361
360
361
Delocalized polaron
430
433
430
432
free-carrier tail
846
811
846
826
absorption
(I)/ PVA
75
The electronic spectra for PAni/PVA and PAni-TiO2 (I) blended with PVA
exhibited similar characteristic bands of both π π∗ transition and polaron
transition (Figure 3.5) albeit with some slight shifts of the wavelengths. PVA does
not have an absorption peak in the region of the interest.
Furthermore, since there is no evidence for the electronic absorption of fully
oxidized pernigraniline (supposed to be at 528 nm and 320 nm [71]) once again
reestablished the oxidation state of the synthesized PAni used in this study is in its
emeraldine salt form.
3.4
Thermal Profile of PAni and Its Composites
TGA is widely used to study all physical process involving the weight
changes, such as to measure the diffusion characteristic and the moisture uptake of
a sample. In addition, it is also employed to investigate the thermal degradation,
phase transition and crystallization of polymers.
Differential Scanning Calorimetry (DSC) has been selected as a tool to
extract more information on the thermal behavior and characterize the miscibility of
polymer blends. The determination of the Tg of the blend compared with the Tgs of
the two components separately is one of the most commonly used methods to
estimate polymer-polymer miscibility. In the case where one component is
crystalline, observation of a melting point depression of this polymer may also be
used as evidence to support the miscibility of the polymer pair, as reported by
Kenshi Miura et al. [73] and others [10, 58].
76
3.4.1 Thermogravimetry Analysis (TGA)
The thermogravimetric profiles of acids doped PAni and PAni-TiO2
composites (Figure 3.6) exhibited a similar pattern, with a small variation in
degradation temperature, even though they were found to have different
degradation kinetics. Generally, the thermograms indicate three major stages of
weight loss. In the first stage, 3 – 4 % weight loss at temperature up to 125 oC is
associated with the loss of water molecules from the polymer matrix [57, 59–60,
74]. The second stage that commences after 125 oC until 225 oC, the weight loss
(about 1 %) is due to the removal of the acid dopant bound to the polyaniline chain
and low-molecular-weight oligomers. A slow and somewhat gradual weight loss
profile observed starting at 225 oC onwards, represents degradation of the skeletal
polyaniline chain structure after the dopant has been removed [57, 59–60]. Above
600 oC, the results obtained are associated with the residues only.
The interaction between the polymers could also be deduced from the
oxidative degradation curves through thermogravimetric analysis studies. TGA
thermograph of PVA, PAni/PVA and its composite blend (PAni-TiO2 (I)/PVA) are
presented in Figure 3.7. For pure PVA, it has been reported that the decomposition
occurred in two stages and it is thermally stable up to 265 oC [75]. However, from
our study, the PVA (curve (a) in Figure 3.7) exhibited a gradual weight loss of
about 8 % upon initial heating up to 225 oC. This weight loss is associated with the
loss of moisture from the sample. Subsequently, rapid weight loss occurred up to
500 oC (almost 96 % weight loss). This followed by a gradual weight loss with a
nearly complete decomposition after 800 oC (weight loss of 98 %). Similar
complete weight loss of thermal degradation of PVA has been observed by other
authors earlier [75–76].
The PAni blends and its composite blends TGA curves (Figure 3.7) showed
gradual weight loss due to absorbed moisture and solvent upon initial heating up to
Weight Loss, % w
0
20
40
60
80
100
120
0
100
125 oC
300
400
500
o
700
(b)
(a)
) and (b) PAni-TiO2 (I) (
600
600 oC
Temperature, C
Figure 3.6: TGA curves of (a) PAni (
200
225 oC
)
800
900
1000
77
0
20
40
60
80
100
0
100
o
100 C
o
200
125 C
o
400
o
Temperature, C
500
), (b) PAni/PVA (40 wt. %) (
300
226 C
Figure 3.7: TGA curves (a) PVA (
Weight Loss, % w
120
700
(a)
800
(c)
900
(b)
) and (c) PAni-TiO2 (I)/PVA (40 wt. %) (
600
o
626 C
)
1000
78
30-126 (3.2)
PAni-TiO2 (I)
30-126 (7.2)
30-75 (6.6)
30-85 (5.8)
PVA
PAni/PVA
PAni-TiO2 (I)/PVA
(b) PVA, PAni and its composite blends
30-125 (4.0)
1 step
st
PAni
(a) PAni and composite
Sample
85-185 (17.7)
75-175 (17.5)
126-226 (7.9)
126-226 (4.1)
125-225 (5.3)
2 step
nd
185
175
226
226
225
3 step
rd
Temperature range (oC) (Weight loss, %)
Table 3.4: Weight loss (%) of PAni, its composites and their blends
22.3
18.7
1.5
21.2
22.4
% residue
79
80
around 100 oC. The PAni and its composite showed a slow weight loss between 125
o
C and 225 oC, which could be attributed to the elimination of dopant. Weight loss
(11–12 %) due to the elimination of acid dopant and the degradation of PVA chain
in the blends have been reported to occur in the temperature range 100 – 227 oC
[10].
Upon heating, the crosslinking reaction occurs through a conversion of
quinoid rings to benzoid rings [70]. Besides, the samples are thermally undoped,
accompanying a loss of majority of the polarons after the thermal treatment, which
is consistent with the concomitant loss of conductivity as discussed later (Section 3.
6. 2). The final degradation of the polymer occurs from around 227 to 900 oC with
the corresponding rapid weight loss of about 60 – 65 %. The overall TGA curves
shape of PAni and its composite blends is similar to a combination curve of its
components, indicating that PAni is a dominant factor affecting the thermal stability
of the blends films. The percentage weight loss for doped PAni, its composites and
their blends are tabulated in Table 3.4.
3.4.2 Differential Scanning Calorimetry (DSC)
PAni and its composite blend films cast from the stable, mixed polymer
solution (PAni/PVA and PAni-TiO2/PVA) showed good homogeneity to the naked
eye over the whole range of the composition. Thus, visual inspection gave no
indication of phase separation in any of the blends. Figure 3.8 compiles the DSC
thermograms of pure PVA, PAni/PVA and PAni-TiO2 (I)/PVA with different
composition.
81
(a)
(b)
82
(c)
(d)
Figure 3.8: DSC curves of (a) PAni-TiO2 (I)/PVA (6 wt. %) (b) PAni/PVA (0.4 wt.
%) (c) PAni-TiO2 (I)/PVA (0.4 wt. %) (d) PVA
83
The PVA homopolymer (curve (d) in Figure 3.8) exhibited a broad
crystalline melting endotherm with a peak at 189 oC and a clear baseline gap
reflecting the glass transition at around 78 oC [35, 69, 73]. Meanwhile, the PAni
does not show glass transition temperature because PAni has a rigid main chain and
degrades before the melting temperature of PAni, resulting in the difficulty in
determination of its Tg [10, 48]. The results reported by S. Palaniappan also
confirmed the absence of any glass transition and melting temperatures for the
polyaniline salt systems [59]. Concerning the blends of PAni and its composites
(PAni-TiO2 (I)) with PVA, a single glass transition was detected for all the
compositions investigated. However, the resulting blend films have different Tgs
from PVA. The Tg values of various blends shift to higher temperatures with
increasing PAni and its composite content as compared to PVA. It implies that the
PAni/PVA and PAni-TiO2 (I)/PVA blends are not a simple blend but have some
intimate interaction among components that makes the Tg higher than that of PVA
itself [48]. This increase with the PAni and PAni-TiO2 content is taken to indicate
the miscibility as reported by Raji K. Paul [58] and others [10, 48]. Furthermore,
our results are in agreement with those reported by Byoung Ho Jeon et al. [48].
Thus, the glass transition behavior and the homogeneous appearance of the blends
indicate miscibility between PAni and its composite with PVA, through the existent
of H-bonding between both component, which is confirmed by our FTIR and DSC
data, and others [10, 48]. Table 3.5 lists Tg and Tm for PVA and blends with
different content of PAni and PAni-TiO2 (I) in the blends.
It can be seen in Figure 3.8 that the melting temperature Tm of PVA
component is depressed systematically to lower temperatures with increasing PAni
and PAni-TiO2 (I) content. Furthermore, the development of crystallinity of PVA
component becomes less prominent with increasing PAni and PAni-TiO2 (I) content
in the blends and finally the Tg and the Tm are difficult to detect. Such effects in
thermal transition behavior observed for the present system, with the compositiondependent shift in Tg of the blends, the depression in Tm and the repression in the
degree of crystallinity of the crystallizable component (PVA), owing to the addition
of second component (PAni and PAni-TiO2 (I)), are common features which are
84
shared with other crystalline/amorphous polymer pairs that are capable of forming a
miscible phase in blends [10, 48, 73]. However, the presence of the third inflection
of slope in the PAni-TiO2 (I)/PVA (0.4 wt. %) DSC curve is unexplainable.
Table 3.5: Observed glass transition temperature and melting temperature of PVA
and blends with different content of PAni and PAni-TiO2 (I)
Sample content (wt. %)
Temperature (oC)
PVA
(100)
Glass transition
temperature, Tg
Melting
temperature, Tm
3.5
PAni-TiO2 (I)/PVA
PAni/PVA
(0.4)
(6)
(0.4)
78.10
72.22
81.91
76.05
189.04
189.00
185.00
182.50
Structural Analyses
3.5.1 X-Ray Diffractogram
Many materials, especially polymers, have a substantial volume fraction
without crystalline order. Though these regions are often termed amorphous, they
frequently have specific local order. Structural information and crystallinity of the
doped PAni and its composites blends are very important in order to understand the
interchain interaction that could affect the electrical conductivity of the polymer
composites and its blends. The variation of the crystallinity has been known to be
85
affected by the synthetic methods, substitution, protonation doping, redox
processes, and solvent casting. The X-ray diffraction analysis is a powerful tool to
determine the structure and crystallization of polymer matrices. The effect of TiO2
addition and PVA blending in the PAni and its composite blends were analyzed
through the same XRD technique. The respective diffraction patterns of pure PVA,
TiO2, PAni and its composite blends with different TiO2/monomer weight ratio and
PAni-TiO2 composite weight fractions are shown in Figures 3.9, 3.10 and
diffraction data are tabulated in Tables 3.6 and 3.7.
Just like the regular rigid polymers that own lots of benzene rings, the XRD
pattern of PAni obtained is very similar to previous reports [13, 77–80] where the
orientation of the polymer has been taken on the basis of a pseudo orthorhombic
cell [77–78]. As reported in most literature, most of the forms of PAni essentially
amorphous and show the presence of broad high-angle asymmetric scattering peaks
stretching from 2θ between 15 – 25o. The main peaks of the synthesized PAni itself
seems to comprise at least three broad separate peaks situated at approximately
4.80o, 11.20o and 25.40o, corresponding to d-spacing of 18.38, 7.89 and 3.50 Å,
indicate a low degree of crystallinity of the polymer and consistent with those
reported by D. Djurado et al. [77] and M. Laridjani et al. [79]. Some peaks located
at 2θ = 23 – 30o are attributed to the periodicity perpendicular to the chain direction
[13].
The pure PVA film exhibits two main peaks at 2θ = 19.60o and 23.60o [75–
76] and that of TiO2 reveals peaks at 2θ = 25.20o, 37.80o, 48.00o, 53.80o and 55.00o,
which corresponding to d = 3.56 Å, 2.38 Å, 1.89 Å, respectively. According to
reference [81], all these three peaks belong to TiO2 anatase. Nevertheless, it is noted
that these peaks become broader and the intensity is reduced in the blends, possibly
owing to the low TiO2 content in PAni composite in the PVA blends and the
amorphous nature of PAni. For PAni-TiO2 (IV)/PVA (0.53 weight ratio of
TiO2/monomer, 6 wt. % of PAni-TiO2) blend, the peaks at 25.60o, 38.00o, 48.80o,
50.80o and 56.60o, which are attributed to TiO2 can hardly be seen and these peaks
86
Intensity
(a)
(b)
(c)
(d)
0.00
10.00
20.00
30.00
40.00
50.00
60.00
2 θ (degrees)
Figure 3.9: XRD patterns of (a) TiO2 (b) PAni (powder form) (c) PVA (d) glass
87
Intensity
(a)
(b)
(c)
0
10
20
30
40
50
60
2 θ (degrees)
Figure 3.10: XRD patterns of (a) PAni-TiO2 (IV)/PVA (6 wt. %) (b) PAni-TiO2 (I)
/PVA (6 wt. %) (c) PAni-TiO2 (I)/PVA (0.4 wt. %)
88
Table 3.6: Position of peaks of PAni, TiO2 and PAni-TiO2/PVA blend in Figure 3.9
and 3.10
2θ (o)
d-spacing (Å)
PAni
4.80
18.38
d-spacing (Å)
11.20
7.89
When 2θ = 4.80o,
25.40
3.50
25.20
3.53
37.80
2.38
48.00
1.89
53.80
1.70
55.00
1.67
PAni-TiO2 (IV)/PVA
5.80
15.22
(0.53 weight ratio of TiO2/monomer,
11.60
7.62
6 wt. %)
25.60
3.48
19.80
4.48
22.60
3.93
38.00
2.37
48.80
1.91
50.80
1.80
56.60
1.62
Sample
d=
nλ
=
2 sinθ
(1)(1.54 x 10
−10
)
4.80
2 sin ( 2 )
= 18.38 Å
TiO2
89
disappear in the PAni-TiO2 (I)/PVA (0.13 weight ratio of TiO2/monomer, 6 wt. %
of PAni-TiO2) blend with lower TiO2 content at the same weight fraction. This can
be due to the highly disordered in the composite blends.
The XRD patterns of the PAni-TiO2/PVA blends do not show sharp peaks
characteristic of crystalline materials and suggest generally an amorphous nature to
all the PAni-TiO2/PVA blends samples. However, the PAni-TiO2/PVA blends
display one broad peak in the region of 2θ = 15o – 25o, with a maximum around
20.0o for PAni-TiO2 (I)/PVA (0.13 weight ratio of TiO2/monomer, 6 wt. %) blend.
For PAni-TiO2 (IV)/PVA (0.53 weight ratio of TiO2/monomer, 6 wt. %) blend with
higher TiO2 content, these peaks, which attributed to PVA becomes sharper and
reduced amorphous scattering region. It can be observed that as the content of TiO2
in the PAni composite blend (PAni-TiO2/PVA) is increased, with weight ratio of
TiO2/monomer ranging from 0.13 to 0.53, there is an increase in crystallinity.
Nevertheless, the degree of crystallinity of PAni composite blends, however is
lower than that of PVA and TiO2, showing that PAni may inhibit crystallization of
TiO2 and PVA. Furthermore, the diffraction pattern of the bare PAni reveals the
amorphous nature and the peaks of PAni can hardly be observed in the PAni
composite blends.
Table 3.7 displays the peak values and the corresponding d-spacing values.
According to R. Murugesan [39], this d-space is the characteristic distance between
the planes of benzene rings in adjacent planes and is the interchain distance or the
close contact distance between two adjacent chains. Further, it is a general
observation that the polymer chain array and the interchain distance are affected by
the size and shape of the interlying dopants which result in an increase in electron
delocalisation length and conductivity on higher d-space. Perhaps, this may be the
prime factor for the lower conductivity of PAni-TiO2 (IV)/PVA (0.53 weight ratio
of TiO2/monomer, 6 wt. %) blends, which has smaller d-space than PAni. However,
the introduction of appropriate amount of TiO2 (0.13 weight ratio of
TiO2/monomer) for PAni-TiO2 (I)/PVA blend slightly improves the d-space and
90
hence the conductivity. Such change in d-space upon blending is analogous to the
trend in d-space of polyaniline-poly (vinyl alcohol) blend with the introduction of
Cu(II) [39].
Table 3.7: The XRD and conductivity data of PAni and PAni-TiO2 blends
XRD data
Sample
PAni
PAni-TiO2 (IV)/PVA (0.53
weight ratio of TiO2/monomer, 6
Conductivity
(S/cm)
2θ (o)
d-spacing (Å)
25.40
3.50
1.03 x 10o
25.60
3.48
6.53 x 10−5
20.00
4.04
4.37 x 10−4
wt. %)
PAni-TiO2 (I)/PVA (0.13 weight
ratio of TiO2/monomer, 6 wt. %)
3.5.2 Scanning Electron Microscopy (SEM)
The surface morphology of PAni and PAni-TiO2 composite in powder form
and their blends in film form were studied with SEM to elucidate the structure
responsible for the electrical conductivity behavior. The micrographs of the blends
films x 10,000 magnification are presented in Figure 3.11.
In the case of PAni-TiO2 (I)/PVA, at the content of 10 wt. %, the apparent
separation of PAni from PVA with agglomeration at place could be observed
(d)
(c)
Figure 3.11: SEM micrographs of (a) PAni-TiO2 (I)/PVA (10 wt. %) (b) PAni-TiO2 (I)/PVA (40 wt. %) (c) PAni/PVA (40 wt. %)
(d) PAni-TiO2 (IV)/PVA (10 wt. %) with magnification of 10,000 x
(b)
(a)
91
92
(Figure 3.11 (a)). The PAni-TiO2 (I) composite appears as white spots and the
others are grey or black in the micrograph. However, at 40 wt. %, the separation of
PAni-TiO2 (I) composite and PVA in the film is less apparent (Figure 3.11 (b)).
Even though without TiO2 in the blend, the PAni is even well distributed in the
PVA matrix polymer (Figure 3.11 (c)). Nevertheless, the surface of the PAni-TiO2
(I)/PVA film appears rougher than the PAni/PVA blend with the same weight
fraction.
Increasing the weight ratio of TiO2/monomer in the PAni-TiO2 (IV)
composite would only cause the formation of agglomerates, were evident, and the
distribution of the agglomerates is fairly evenly distributed in the blend film (Figure
3.11 (d). Raji K. Paul [58] has also reported the distribution of PAni agglomerates
in the polymer matrix for a solution mixing blend. The formation of the
agglomerates would normally interfere the conduction activity and would
eventually lower the conductance of the blend system. Generally, the electrical
conductivity of a film is strongly influenced by the way a blended film was made (i.
e. the process and the mixture composition).
Furthermore, the difference in conductivity of the films probably stems from
the different composition of TiO2 in the samples, affecting the structures of the
particles and hence the conductivity of the blends [82]. It can be observed that PAni
particle has fine surface compared with other samples. If the surface morphology of
PAni-TiO2 (I) and PAni-TiO2 (IV) particles are compared, it is evident that the
PAni-TiO2 (I) synthesized at 0.13 weight ratio of TiO2/monomer, are intimately
associated with TiO2 particles, only little TiO2 particles (brighter region) “glued” on
the particle, implying a homogeneous mixing between PAni and TiO2 particles. For
that reason, the linking between the PAni moieties is improved due to intimate
association of PAni moieties with TiO2 particles in the composite [41] and hence
the conductivity of PAni-TiO2 (I) blend with PVA is higher than bare PAni/PVA
blend. The latter (PAni-TiO2 (IV) with excess TiO2 particles revealing an
aggregative nature surface with many TiO2 particles (brighter region) “glued” on
(b)
(c)
Figure 3.12: SEM micrographs of particles (a) PAni (b) PAni-TiO2 (I) (c) PAni-TiO2 (IV)
(a)
93
94
the surface of the particle. The excess TiO2 content (at 0.53 weight ratio of
TiO2/monomer) in the composite may have disturbed the structure of PAni, leading
to poor linking of PAni particles and agglomerates formation in the PAni-TiO2
(IV)/PVA film, resulting in decrease in conductivity of its blend film. In
comparison with the bare PAni, for which linking between the grains is very poor.
Figure 3.12 shows the micrograph of the particles.
3.5.3 Transmission Electron Microscopy (TEM)
The remarkably good conductivity in the present conducting polyblends
indicates an unusual morphology, with connected pathways existing even at
remarkably very low loading of PAni and Pani-TiO2 composite in the blends, and
the conductivity increases with increasing loading of conducting components (PAni
and its composite). This phenomenon implies the formation of a self-assembled
interpenetrating fibrillar network of PAni during the coarse of liquid-liquid phase
separation [83]. Thus, we have undertaken an investigation of the morphology of
the PAni and its composite in the polyblends with PVA using transmission electron
microscopy (TEM).
The implied networks are directly imaged through TEM studies of the
blends. Through studies of films with different contents of conducting PAni, we
demonstrate the correlation between the morphology, as observed by TEM, and the
electrical conductivity of the blends. Figure 3.13 represents the TEM micrographs
of blends made from 0.2 wt. % and 0.4 wt. % PAni in PVA, respectively. The
details of the PAni network can be seen quite clearly. Here, as well as in the
following micrographs, the brighter regions indicate the PAni, which forms the
interconnected network, and the darker regions are the voids that had been occupied
by PVA. The micrographs resemble the typical scenario imagined for a percolating
95
(a)
(b)
Conductive
network
Host polymer
matrix
5 µm
5 µm
Figure 3.13: TEM micrographs of PAni/PVA blends at (a) 0.2
wt. % (b) 0.4 wt. %
96
medium with “links” (PAni fibrils), “nodes” (crossing points of the links), and
“blobs” (dense, multiply connected regions) [6].
The micrographs are interesting in that the structure is very “stringy” and
tenuous. As the content of PAni in the polyblend is decreased (0.2 wt. %, Figure
3.13 (a)), the interconnected network becomes less dense, even the dispersed PAni
remains organized and appears in a fibrillar morphology indicative of connected
pathways. This indicates that the network is unstable and tends to break up at
fractions below 0.2 wt. % PAni in PVA. From the figure, one can also observe
isolated and weakly connected segments or cluster. At the higher loading of PAni
(0.4 wt. %, Figure 3.13 (b)), the PAni domains appear interconnected and the
connected pathways become denser. There are of course some isolated clusters
remaining. Thus, the TEM results establish the evidence for a continuous network
formation of the conducting phase in blends. The interconnected network becomes
denser with increasing weight fraction of PAni in the blend, which is in good
agreement with the conductive components dependence of conductivity of the
blends that will be discussed later (Section 3. 6. 3).
According to Pallab Banerjee [84], the PAni particles are directly involved
in the formation of the globules of the network and also in forming contacts
between the globules in the network in the polyblend. The network is found to be
considerably empty with nanoparticles pervading through it. It is clear that the
particles involve in making the bridge in the networks.
According to C. Y. Yang et al. [49], the morphology observed for the
solution-processed blends is drastically different from the usual filled polymers in
which conducting particles are dispersed into non-conducting matrix polymers. The
electrical conductivity in filled polymer systems is observed to turn on sharply at a
volume fraction (f) of conducting particles corresponding to the percolation
threshold (f ~ 0.16) predicted for conducting globular particles in an insulating
97
matrix [6]. The electrical conductivity of many polyblend systems, which utilize the
counter- ion-induced processibility of PAni, commences at concentrations that are
at least an order of magnitude below this standard percolation threshold. Although
the driving force for formation of the stringy interconnected microsturcture of PAni
in the blends, as shown by the micrographs in Figure 3.13, remains obscure for the
time being, it provides a basis for understanding this remarkable behavior [49].
Furthermore, the relationship between conductivity and volume fraction of
PAni is critically depend on the nature of mass distribution among the links, nodes,
and blobs in the sample which in turn is determined by various parameters involved
in the sample preparation, such as the molecular weight of both polymers in the
blends, the viscosity of the polyblend solution, the drying temperature, the solvent
and others [83].
3.6
Electrical Properties of PAni and PAni-TiO2 Blends With PVA
Our approach consists of the preparation of blends containing electrically
conducting PAni and PAni-TiO2 composite, which have been preliminarily formed.
Besides its simplicity, this approach allows control of the design of blends with the
host polymer by attaining the controlled design in ‘organized’ systems (polymer
dispersions and networks). The percolation behavior of the resulting films is
affected by the design of the blends. Further increase of the particles (above the
percolation threshold) in the blends result in improvement of the conducting
network and hence enhance the conductivity of the blends, which will be discussed
later (Section 3. 6. 3). Detailed conductivity data of all samples are listed in Table
3.8 and 3.9.
PAni-DBSA
0.84991
0.86156
0.86761
0.86184
0.84428
0.81694
0.76703
0.72291
0.68736
0.65856
0.64178
0.64192
0.652
0.66945
Temperature,
o
C
30
40
50
60
70
80
90
100
110
120
130
140
150
160
0.7368
0.7199
0.7054
0.71332
0.74051
0.77547
0.8327
0.87221
0.9273
0.92293
0.99559
1.06031
1.06.392
1.03251
PAni
0.49951
0.49068
0.48263
0.49688
0.48164
0.49538
0.50128
0.52727
0.55818
0.58041
0.58096
0.61512
0.63924
0.63418
0.81674
0.82143
0.8447
0.87286
0.92087
0.97227
1.05592
1.14321
1.16563
1.18895
1.19857
1.17071
1.17071
1.14801
Conductivity of samples, S/cm
PAni-TiO2
PAni-TiO2
(I)
(II)
0.90028
0.8736
0.89742
0.9129
0.94553
0.99917
1.06729
1.14538
1.23582
1.27704
1.33453
1.3689
1.38343
1.37817
PAni-TiO2
(III)
Table 3.8: Conductivity of PAni and PAni-TiO2 composites at various temperatures
0.628851
0.631477
0.625492
0.631477
0.643839
0.66883
0.708717
0.749901
0.77501
0.803458
0.813683
0.809356
0.79414
0.777146
PAni-TiO2
(IV)
98
0.95283
0.96196
0.97534
0.95593
0.93951
0.88841
0.82889
0.76588
0.72375
0.70705
0.72965
0.76955
0.64397
0.12887
40
50
60
70
80
90
100
110
120
130
140
150
160
PAniDBSA/PVA
30
Temperature,
o
C
0.16785
0.93845
1.20308
1.19722
1.21405
1.27087
1.3554
1.44532
1.53597
1.59716
1.64442
1.66714
1.68649
1.67478
PAni/PVA
0.18071
0.85208
1.19309
1.23087
1.25066
1.2992
1.40065
1.50413
1.62086
1.70929
1.73947
1.76734
1.77534
0.404018
1.040717
1.146397
1.125273
1.129945
1.179466
1.263562
1.386579
1.489159
1.56286
1.604205
1.622961
1.623104
1.616729
PVA
PVA
1.76729
PAni-TiO2 (II)/
PAni-TiO2 (I)/
Conductivity of samples, S/cm
0.20235
0.67269
0.83587
0.81431
0.7928
0.81657
0.87261
0.98002
1.08088
1.13717
1.17597
1.18943
1.19494
1.18634
PVA
PAni-TiO2 (III)/
Table 3.9: Conductivity of blends films (40 wt. %) at various temperatures
0.23565
0.66936
0.82305
0.77526
0.74798
0.75517
0.78503
0.82834
0.87189
0.90193
0.91729
0.92106
0.91953
0.91035
PVA
PAni-TiO2 (IV)/
99
100
2
1.8
1.6
Conductivity, S/cm
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
20
40
PAni-TiO2 (I)/PVA
PAni-TiO2 (II)/PVA
60
80
100
o
Temperature, C
PAni/PVA
PAni-TiO2 (IV)/PVA
120
140
160
180
PAni-TiO2 (III)/PVA
PAni-DBSA/PVA
Figure 3.14: Conductivity of films (40 wt. %) at various temperatures
It is reported that higher acidity of the reaction medium leads to the increase
in the conductivity [85]. The efficient polymerization of aniline is achieved only in
acidic medium, where aniline exists as an anilinium cation [85]. Therefore, the
electrical conductivity of DBSA doped PAni in the HCl medium has higher
conductivity than PAni doped with DBSA without in HCl medium, as indicated in
Figure 3.14.
Overall, it is observed that the electrical conductivity of the PAni/PVA and
PAni-TiO2/PVA blends at 40 wt. % are higher than the pure PAni and PAni-TiO2
composites at all temperatures, despite the fact that the major component of the
blends is nonconducting (Table 3.8, 3.9). This could be due to the barriers to
conductivity in PAni and PAni-TiO2 composite sample are reduced in the blending
process, same as reported result [50]. According to W. Jia et al. [86], the increase in
conductivity of the PAni-DBSA/MMT blend compared with bulk PAni-DBSA is
because of the conducting polymer chains in the interlayer spaces are assumed to
take a more extended conformation than in the bulk form. Thus, the conductivity of
101
the composites is increased due to the free motion of charge carriers. In the present
case, the higher electrical conductivity of the blends compared to the bare PAni and
PAni-TiO2 may be also due to this factor.
3.6.1 Effect of TiO2/aniline Monomer Weight Ratio
PAni and its composite (PAni-TiO2) synthesized with various TiO2/aniline
weight ratio have been blended with insulating PVA and characterized with respect
to their DC electrical conductivity. Conductivity of the samples with various
compositions of TiO2 (at 40 wt. %) at room temperature is shown in Table 3.10.
The electrical conductivity of PAni/PVA and PAni-TiO2/PVA blends at 40 wt. % of
conductive fraction are plotted as a function of weight ratio of TiO2/aniline (Figure
3.15).
Table 3.10: Conductivity of the blends films (40 wt. %) with various weight ratio
of TiO2/aniline at room temperature
Amount of
Weight Ratio of
DC conductivity
TiO2 (g)
TiO2/aniline
(S/cm)
PAni-DBSA/PVA


0.95
PAni/PVA


1.67
PAni-TiO2 (I)/PVA
2
0.13
1.77
PAni-TiO2 (II)/PVA
4
0.27
1.62
PAni-TiO2 (III)/PVA
6
0.40
1.19
PAni-TiO2 (IV)/PVA
8
0.53
0.91
Sample
102
Since PAni and its composites are disordered and in amorphous system, the
micro- and macroscopic conductivity play a vital role on the overall conductivity of
the samples. The samples were prepared in identical conditions. Thus, the
microscopic conductivity, which depends upon the doping level, conjugation length
or chain length etc. are expected to be more or less equal [87]. However, the
physical properties, viz. compactness and molecular orientation, are significantly
varied depending upon the polymer content in the samples. Whereas, the
macroscopic conductivity depends on some external factors, such as compactness
of the sample, orientation of the microparticles etc. Furthermore, the electrical
conductivity of the films is provided by the conductive components (PAni and
PAni-TiO2), not PVA, the bare PAni and PAni-TiO2 composite have major effect
on the conductivity.
2
Conductivity, S/cm
1.8
1.6
1.4
1.2
1
0.8
0
0.1
0.2
0.3
0.4
0.5
0.6
Weight ratio of TiO2/aniline (g/g)
Figure 3.15: Variation of room temperature conductivity of films (40 wt. %) with
the weight ratio of TiO2/aniline
A number of distinct features are indicated by the samples at room
temperature. It is interesting to notice that, despite the insertion of an insulating
103
inorganic oxide, the DC conductivity of the PAni-TiO2 blends at 40 wt. % (with
appropriate weight ratio of TiO2/aniline) is found to be significantly higher than the
bare polymer blend at room temperature (Figure 3.14). The electrical conductivity
of the blends substantially increased with the TiO2 incorporation, reached a
maximum at 0.13 weight ratio of TiO2/aniline, and finally decreased with further
increase of the weight ratio of TiO2 in the blends (Figure 3.14, 3.15). The
conductivity of PAni-TiO2 (I)/PVA is appreciably enhanced compared to the
conductivity of the unmodified PAni/PVA (~ 1.67 to 1.77 S/cm at room
temperature).
Pure PAni itself is a lightweight polymer with poor compactness, thus the
microparticles are very randomly oriented and the linking among the polymer
particles through the grain boundaries is very poor, resulting in relatively lower
conductivity [82]. The higher conductivity of the PAni-TiO2 (I)/PVA blend
compared to bare PAni/PVA blend could be attributed to the grain boundaries or
the localized defects that separate the grains of different orientations in the polymer
and its composite [82]. The comparatively large value of resistivities of the polymer
samples was due to the random orientations of microcrystals and their weak
coupling through the grain boundaries, as reported for the case of conducting
polypyrrole-Zirconium oxide (PPy-ZrO2) nanocomposite [82]. The formation of
composites leads to an improvement on compactness of the material. This apparent
physical change runs in parallel with the improvement in the internal ordering of
the polymer planes as revealed by the XRD studies. Thus, it may be proposed that
the weak links between the grains are improved and coupling through the boundary
becomes stronger with the formation of PAni-TiO2 composite (with appropriate
weight ratio of TiO2/aniline), resulting in the increased conductivity of the PAniTiO2 (I) composites and subsequently increase the conductivity of PAni-TiO2
(I)/PVA blends.
Nevertheless, it is noted that the electrical conductivity of the PAniTiO2/PVA blends increases with the increase of TiO2/aniline weight ratio and
104
reaches a saturation value (Figure 3.14, 3.15). This may imply that the conductivity
value can be increased up to a certain limit by improving the coupling strength
between the grains, which is consistent with previous results [82, 87]. In PPy-ZrO2
system, the large increase in conductivity was attributed to an improved linking
between PPy moieties due to intimate association of PPy moieties with ZrO2
particles in the nanocomposite [82]. According to results reported by Suprakas
Sinha Ray [41], the conductivity of PAni-ZrO2 nanocomposites showed only a
marginal change tending to level off with increasing ZrO2 content in the
composites. In other words, the nanocomposites exhibited increasing conductivity
with increasing PAni loading in the composite.
In addition, there is evidence for a continuous network formation of the
conducting phase in blends [11, 13, 14, 58, 83–84]. The formation of agglomerates
would interferes with the conduction phenomenon, resulting in the disconnected or
weakly connected parts of the fractal network and eventually lowers conductivity
for blend system [58]. Thus, the decreasing conductivity with overloaded TiO2
content (weight ratio of TiO2/aniline more than 0.13) in the composite may be due
to this factor, as indicated in SEM micrographs.
3.6.2
Effect of Temperature
Figure 3.14 shows the variation of conductivity (S/cm) of PAni and its
composite blends films at 40 wt. % with temperature. The temperature dependence
of the conductivity of the films is shown in Table 3.9. It is noted that the
conductivity of the films increases slowly with the increase of temperature until a
temperature of around 50 oC is reached, after which the conductivity decreases
gradually. The increase in conductivity with temperature is a property of
105
semiconductor. Such phenomenon has been reported for polypyrrole and
poly(methylacrylate) or poly(styrene-co-butyl acrylate) composites [88].
The decreasing conductivity above 50 oC could be due to the presence of
absorbed water and its removal may have caused structural changes in the
hygroscopic PAni polymer chains [47, 88], as indicated in the TGA curves shown
in Figure 3.7, which revealed a small weight loss below 125 oC. According to
Byoung Ho Jeon et al. [48], the decreasing conductivity with increasing
temperature is owing to chemical change, its degradation or evaporation of dopant.
Furthermore, it has been reported that a dry sample has lower conductivity than a
hydrated sample and the loss of moisture results in a decrease of conductivity for
polypyrrole
and
poly(methyl
acrylate)
or
poly(styrene-co-butyl
acrylate)
composites [88]. Further, the chain segmental motion of host as well as the
conducting polymer itself is related to the electrical conductivity of the blends [48].
Nevertheless, the samples subsequently show an increase in conductivity at
around 120 oC, reach the maximum at around 140 oC and finally drop rapidly with
increasing temperature. The increase of conductivity until the critical point is
attributed to polymer chain mobility and activation of dopant while the decrease of
conductivity can be attributed to the volatilization of dopant, followed by structural
change [45]. Moreover, decomposition of PVA takes place above 130 oC [35]. One
fact that may pass over is that under mild thermal annealing, PAni also experiences
conversion to be more ordered structure [89]. The change of chain structure by
thermal treatment and interchain interaction between two components has an effect
on the morphological change. In addition, with increasing temperature, the
intermolecular spacing between adjacent chains of the blends is decreased. All
samples exhibit similar pattern, i.e. the conductivity increase with temperature from
30 to 50 oC, following with decreasing conductivity, and reach the maximum at
around 140 oC, then decrease with increasing temperature.
106
According to Show-An Chen and G. W. Hwang [70], the condensation
reactions of –OH groups with the protons of –SO3H groups (which lead to a
crosslinked structure) and acid-catalysed dehydration of PVA occur after thermal
treatment at 150 oC. The crosslinking reaction occurs through a conversion of
quinoid rings to benzoid rings. Therefore, a permanent thermal undoping occurs,
accompanying a loss of majority of the polarons after the thermal treatment,
resulting in decreasing conductivity in the samples, which is consistent with the
concomitant loss of conductivity at the temperature higher than 140 oC, as
discussed above.
0.4
Log Conductivity (S/cm)
0.2
0
0.046
-0.2
0.048
0.05
0.052
0.054
0.056
0.058
-0.4
-0.6
-0.8
-1
-1/2
T
(K
-1/2
)
PAni-TiO2 (IV)/PVA
PAni-TiO2 (II)/PVA
PAni-TiO2 (III)/PVA
PAni-TiO2 (I)/PVA
PAni-DBSA/PVA
PAni/PVA
Figure 3.16: Conductivity of blends as a function of 1/T1/2
Variable range hoping model is generally utilized to describe the
temperature dependence of conductivity for disorder semiconductors. The overall
pattern is illustrated in Figure 3.16, which showing a logarithmic scale conductivity
of the blends over a wide temperature range (high temperature). Normally, the
approximate linearity of the plot reveals that the conductivity over a wide
107
temperature range (but not at high temperatures) is generally consistent with the
usual form [24, 90] for granular metals:
σ
dc
(T ) = σ
0
exp

 T
 − 
 T

0



1
2



3.1
where σο and To are constants.
Nevertheless, as can be seen from the figure, the conductivity of the blends
decreases as the temperature is increased and deviates strongly from equation 3.1
above 300 K. Kaiser et al. attributed this behavior to the contribution of the intrinsic
metallic conductivity of the conducting polymer [50]. This phenomenon is
consistent with previous reported results [9, 50, 91] where the conductivity only fits
the equation 3.1 well at low temperatures (below 250 K). Furthermore, the
temperature dependence of conductivity of conducting polymers is predominantly
of non-metallic sign (i. e. conductivity increases with temperature), but often shows
a change to metallic sign at higher temperatures [50, 91].
3.6.3 Effect of Weight Fractions
Flexible, free standing conducting blends of PAni and PAni-TiO2 with PVA
were prepared as films by solution casting from appropriate weight fractions of both
the conductive components (PAni and PAni-TiO2) and the insulating host polymer
(PVA). The conductivity of the films was measured for several samples in all cases
and the resulting conductivity values were taken as average of the respective
samples values. The conductivity value of the films for each sample was almost the
same order of magnitude. In this study, thickness of the films was in between 40
µm to 60 µm. Blends containing various wt. % of PAni and PAni-TiO2 were
108
prepared and denoted as PAni/PVA x % or PAni-TiO2/PVA x %, where x %
represents the percentage of PAni or PAni-TiO2 in the blends. The PAni/PVA and
PAni-TiO2/PVA blends present green coloured films. As the weight fraction of
PAni and PAni-TiO2 is increased, the green colour becomes denser. Blends with
low conductive component content are flexible while those containing more than 40
wt. % conductive components are brittle and can’t form films, thus these blends
were not further examined.
Figure 3.17 shows the variation of room-temperature conductivity of the
blend films with different PAni and PAni-TiO2 (I) loading for both blends,
PAni/PVA and PAni-TiO2 (I)/PVA. Detailed data of the conductivity is tabulated in
Table 3.11. The sample with 0 wt. %, i.e. PVA only, there was low reading for its
conductivity as expected for a non-conductive plastic. Nevertheless, reference [35]
reported that PVA electrical resistivity is in the range of (3.1 – 3.8) X 107 Ω cm or
electrical conductivity 0.29 x 10−7 S/cm. In the present study, the electrical
conductivity of PVA film is 8.51 x 10−6 S/cm.
It can be noted that the room temperature electrical conductivity depends
strongly on the fraction of PAni and PAni-TiO2 (I) in the blends. The conductivities
of these blends increase with increasing content of conductive PAni and PAni-TiO2
(I), showing the conductive fractions dependence of the conductivity, in agreement
with previously reported results [13–14, 45, 48, 84]. The enhancement of
conductivity of the blends films with increasing PAni and PAni-TiO2 (I) loading is
owing to the conductive paths formation through the blends [13]. At low weight
fractions of the conductive components, the conductivities of the films change
slightly and approach that of PVA. At the region near to percolation threshold, the
conductivity increases rapidly and subsequently steadily with increasing conductive
components content in the blends. Nonetheless, when PAni and PAni-TiO2 content
was higher than 40 wt. %, the films turn cloudy, could not form free standing films,
hence could not be measured.
109
2.0E+00
1.8E+00
Conductivity, S/cm
1.6E+00
PAni/PVA
1.4E+00
1.2E+00
1.0E+00
8.0E-01
6.0E-01
4.0E-01
PAni-TiO2 (I)/PVA
2.0E-01
0.0E+00
0
0.1
0.2
0.3
0.4
0.5
wt. % of PAni or PAni-TiO2 (I)
Figure 3.17: Conductivity of PAni/PVA and PAni-TiO2 (I)/PVA blends at room
temperature with various weight fractions
The PAni and PAni-TiO2 composite can be redispersed in deionized water
in the presence of appropriate amount of DBSA and no aggregation was found after
redispersion. In this aqueous system, DBSA plays as dopant as well as surfactant,
which presents the well dispersion of PAni and PAni-TiO2 and their complex with
DBSA. So, appropriate amount of DBSA can increase the dispersity of PAni and
PAni-TiO2 in aqueous system as well as their stability, resulting in enhanced
conductivity [46]. Hence, the blends exhibit reasonably good conductivity even at
very low PAni and PAni-TiO2 (I) loading. The conductivity is 3.67 x 10−5 S cm−1
and 1.00 x 10−5 S cm−1, at the lowest loading used, namely, 0.1 wt. % for PAni/PVA
and PAni-TiO2 (I)/PVA, respectively.
110
Table 3.11: Detailed room-temperature conductivity data of the blend films with
different PAni and PAni-TiO2 (I) composite loading for both blends, PAni/PVA and
PAni-TiO2 (I)/PVA
Weight Fraction, f
Conductivity (S/cm)
(wt. %)
PAni/PVA
PAni-TiO2 (I)/PVA
0.001 (0.1)
3.67 x 10−5
1.00 x 10−5
0.002 (0.2)
3.68 x 10−5
1.10 x 10−5
0.004 (0.4)
5.02 x 10−5
1.27 x 10−5
0.008 (0.8)
5.69 x 10−5
4.27 x 10−5
0.02 (2)
6.63 x 10−5
6.53 x 10−5
0.03 (3)
1.20 x 10−4
1.38 x 10−4
0.06 (6)
1.69 x 10−3
4.37 x 10−4
0.1 (10)
2.23 x 10−2
1.51 x 10−2
0.2 (20)
4.92 x 10−1
4.78 x 10−1
0.3 (30)
1.03 x 10o
6.94 x 10−1
0.4 (40)
1.67 x 10o
1.77 x 10o
3.6.4 Percolation Threshold
The value of percolation threshold is of a crucial importance in the
preparation of highly conductive blends of polyaniline and classical polymers.
Interactions between the two blended polymers can greatly influence the
conductivity and physical properties of the films [14]. The desired mechanical
111
properties of the insulating host polymer can be retained only in the case of small
admixtures of polyaniline. In addition transparent films can be fabricated only at
extremely low content of conductive phase in the blend. Thus, special morphology
of the self-assembled network type must be created upon casting in order to obtain
low percolation threshold [11]. It is acknowledged that the formation of such
networks is facilitated by the extended chain conformation of protonated
polyaniline. Table 3.11 shows the conductivity data of the samples at various
weight fractions.
Commonly, there is a sharp change in conductivity until the weight fraction
of a conductive component reaches a percolation threshold. The percolation
threshold depends on the shape and the distribution of the conductive particles in
the matrix polymer [48].
Figure 3.18 (a) and 3.19 (a) shows the variation of conductivity with PAni
and PAni-TiO2 (I) loading. The percolation threshold has been determined from the
conductivity data using the following scaling law derived on the basis of the
percolation theory [6]. In our study, we expressed the percolation threshold in a
weight fraction, which is proportional to the volume fraction.
Percolation theory [6] predicted that at concentrations sufficiently dilute,
there are no connected paths and the conductivity is zero. As the concentration of
the conducting polymer is increased, the conductivity shows a sharp increase at a
certain concentration of conducting polymer. This concentration is called the
percolation threshold. The conductivity becomes finite and increases slowly as f is
increased above the percolation threshold (fc) as the connectivity (i. e., the number
of conducting paths) increases. In agreement, the data presented in Figure 3.18 (b)
and 3.19 (b) indicate the well-defined percolation threshold for the systems.
112
Figure 3.18 (b) and 3.19 (b) show the fitting of the conductivity of our
experimental data (the conductivity vs composition data) to the above scaling law.
Thus, the method yields an estimated value of critical percolation threshold (fc) of
0.02 for both PAni/PVA and PAni-TiO2 (I)/PVA blend. These values are much
lower than the theoretically predicted value of fc = 0.16 for spherical conducting
particles dispersed in a nonconducting matrix in three dimensions [6]. The
exceptionally low value of fc can be attributed to the formation of a self-assembled
interpenetrating fibrillar fractal network of PAni [9, 88]. Such a low fc has been
reported by Pallab Banerjee [79] for blends of HCl-doped polyaniline nanoparticles
and poly(vinyl chloride) with extremely low percolation threshold (fc = 4.02 x 10−4
and t = 1.87). The lower percolation threshold is also consistent with the results of
Byoung Ho Jeon [48] for polyaniline-polycarbonate composites by emulsion
polymerization (fc = 0.13). Tarun Kumar Mandal and Broja M. Mandal [88]
reported similar low percolation threshold (fc = 0.023, 0.045 and t = 2.42, 2.26 for
polypyrrole-poly(methyl acrylate) and polypyrrole-poly(styrene-co-butyl acrylate),
respectively). For polyaniline-polyvinyl alcohol blends, P. Dutta et al. [92] reported
a low fc = 3.4 x 10−4 and t = 2.80.
The critical exponent t can be simulated by plotting log σ = log C + t log (f
– fc) above the percolation threshold fc of PAni/PVA and PAni-TiO2/PVA blends,
as shown in Figure 3.18 (b) and 3.19 (b). From the slope of the straight lines
(Figure 3.18 (b), 3.19 (b)), the values of the critical exponent, t, are determined to
be 2.81 and 2.86 for PAni/PVA and PAni-TiO2 (I)/PVA films, respectively. It is
noted that t in present systems are different from a universal value of tun = 2.0,
which is the most widely accepted universal value is in three dimensions. In the
past years, it was believed that simulation and real continuum media both belonged
to the same universal class, and that t only depended on dimension. Nevertheless,
many experiments have indicated that t may be more than 2.0, or even larger in a
number of continuum system until recently [88, 93–94]. Moreover, small values of
the critical exponent t has been already reported for polyaniline and polyanilinepolystyrene blends, which is attributed to a thermally induced hopping transport
between disconnected or weakly connected parts of the fractal network [9, 83]. A
113
1.E+01
(a)
1.E+00
Log Conductivity
0
0.1
0.2
0.3
0.4
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
f
1
(b)
0.5
0
-2
-1.5
-1
-0.5
Log Conductivity
-2.5
-0.5 0
-1
-1.5
-2
-2.5
fc = 0.02
t = 2.81
-3
-3.5
-4
-4.5
Log (f – f c )
Figure 3.18: (a) Plot of electrical conductivity of PAni vs weight fractions in
PAni/PVA films, (b) Plot of log (conductivity) vs log (f – fc)
114
1.E+01
(a)
1.E+00
Log Conductivity
0
0.1
0.2
0.3
0.4
0.5
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
f
1
(b)
0
-2
-1.5
-1
-0.5
Log Conductivity
-2.5
0
-1
-2
-3
fc = 0.02
t = 2.86
-4
-5
Log (f – f c )
Figure 3.19: (a) Plot of electrical conductivity of PAni-TiO2 (I) vs weight fractions
in PAni-TiO2 (I)/PVA films, (b) Plot of log (conductivity) vs log (f – fc)
115
higher value of t indicates that the cluster of conducting particles becomes
incorporated into the network at a faster rate around the critical concentration
region [88].
As a conclusion, the percolation threshold at which a good electrical
conductivity is measurable is associated with the formation of a continuous network
structure. The correlation between the continuous networks and the electrical
conductivity is observed in the films.
3.7
The Hall Effect
Hall effect is a mutually perpendicular force on the current-carrying sample
directed perpendicular to a magnetic field, resulting in a generated voltage in a
direction perpendicular to both the current and the magnetic field, called Hall
Voltage. This observation, known as Hall effect, arises from the deflection of
charge carriers to one side of the conductor as a result of the magnetic force they
experience. It provides valuable information regarding the sign of the charge
carriers, Hall Voltage, Hall Coefficient (RH), carrier density (n) and carrier mobility
(µ) of the sample. Hall effect measurement is the basic tool for the determination of
charge carrier mobilities. In this study, the PAni and its composite were doped
chemically by protonic acid, DBSA, a known p-type dopant or electron acceptor
that have been widely used.
By measuring the Hall Effect of PAni and its composites, it is noted
that generally there is a linear relationship between the Hall voltage and the applied
current under constant magnetic field (Figure 3.20). The calculated DC conductivity
at room temperature (σRT), Hall coefficient (R), carrier density (n) and carrier
116
(a) PAni-DBSA/PVA
9
30
7
Hall Voltage, Hv (mV)
Hall Voltage, Hv (mV)
(b) PAni/PVA
35
8
y = 2.123x - 0.0319
6
5
4
3
2
25
y = 5.8805x - 0.0792
20
15
10
5
1
0
0
0
1
2
3
4
0
2
Current, I (mA)
(c) PAni-TiO2 (I)/PVA
20
9
16
y = 3.1989x - 0.0588
Hall Voltage, Hv (mV)
Hall Voltage, Hv (mV)
6
(d) PAni-TiO2 (II)/PVA
10
18
14
12
10
8
6
4
2
8
y = 2.9733x + 0.105
7
6
5
4
3
2
1
0
0
0
2
4
6
Current, I (mA)
8
0
(e) PAni-TiO2 (III)/PVA
9
1
2
Current, I (mA)
3
4
(f) PAni-TiO2 (IV)/PVA
4
8
3.5
Hall Voltage, Hv (mV)
Hall Voltage, Hv (mV)
4
Current, I (mA)
7
6
y = 2.342x - 0.3786
5
4
3
2
3
y = 0.6227x - 0.0313
2.5
2
1.5
1
0.5
1
0
0
0
1
2
Current, I (mA)
3
4
0
2
4
6
Current , I (mA)
Figure 3.20: Hall Voltage, VH OF (a) PAni-DBSA/PVA (b) PAni/PVA (c) PAniTiO2 (I)/PVA (d) PAni-TiO2 (II)/PVA (e) PAni-TiO2 (III)/PVA (f) PAni-TiO2
(IV)/PVA
1.67
PAni/PVA
4878.48
2961.89
2806.47
352.19
2.27
3.57
4.35
16.53
1763.39
2525.41
379.03
1.63
1.19
0.91
PAni- TiO2 (II)/PVA
PAni- TiO2 (III)/PVA
PAni- TiO2 (IV)/PVA
3311.94
1174.57
(cm2 volt−1 sec−1)
Carrier Mobility, µ
2748.09
3.04
5.77
(x 1015 cm−3)
Carrier Density, n
1.77
2057.28
1213.78
(cm3 coulomb−1)
Hall Coefficient, R
PAni- TiO2 (I)/PVA
PAni composite/PVA:
0.95
(S/cm)
Conductivity, σ
PAni-DBSA-PVA
Films (40 wt. %)
Table 3.12: The conductivity at room temperature σRT, Hall coefficient R, carrier density n and carrier mobility µ
117
PAni-TiO2 (I)/PVA
Films
0.48
0.02
10
0.69
30
20
1.77
(S/cm)
fractions
40
Conductivity, σ
Wt. % of conductive
with various weight fractions
226462.08
2354.86
5627.75
0.03
2.65
1.11
2.27
(x 1015 cm−3)
(cm3 coulomb−1)
2748.09
Carrier Density, n
Hall Coefficient, R
2883.46
1128.03
3721.00
4878.48
(cm2 volt−1 sec−1)
Carrier Mobility, µ
Table 3.13: The conductivity at room temperature σRT, Hall coefficient R, carrier density n and carrier mobility µ for PAni-TiO2 (I)/PVA blend
118
119
mobility (µ) of PAni and its composite films are summarized in Table 3.12, 3.13
and consequently plotted into graphs in Figure 3.21 to 3.22.
From the results of the present measurements, the Hall coefficient of PAni
and its composites are positive since all the samples were doped by p-type doping.
Hence, it is suggested that the hole acts as a major carrier in the PAni and its
composites, not electron, which is consistent with previous results [95–96]. It is
noted that the electrical conductivity of PAni/PVA was increased almost two times
upon polymerization with the presence of HCl (1.67 S/cm), compared to the PAniDBSA/PVA (0.95 S/cm), which was synthesized with the absence of HCl.
Moreover, PAni/PVA possesses higher carrier mobility (3311.94 cm2 volt−1 sec−1)
and lower carrier density (3.04 x 1015 cm−3) than PAni-DBSA/PVA (carrier
mobility, 1174.57 cm2 volt−1 sec−1, carrier density, 5.77 x 1015 cm−3).
As can be seen (Figure 3.21), the conductivity of all the samples is inversed
of the carrier density, showing that the carrier density is not the main criterion for
high conductivity. The plots of carrier mobility against the carrier density tell us
that the samples with higher carrier density possess lower carrier mobility and vice
versa. This result is very much consistent with the previous report [95].
Furthermore, the electrical conductivity of the samples has a linear relation with
carrier mobility. PAni and its composite blends with higher carrier mobility
exhibited higher electrical conductivity. Thus, it can be concluded that the main
limiting factor of the DC conductivity of the sample is the carrier mobility with a
little effect from the carrier density. According to Kohshin Takahashi et al. [63],
the conductivity values increased about three orders in magnitude when the
concentration of free radical is doubled. Further, the carrier mobility is dependent
on the concentration of free radical, indicating that the conductivity is limited by the
carrier mobility. This means that holes as a carrier rapidly becomes more mobile as
the polaron band is enhanced with increase of free radical in the conjugation system
of polyaniline.
Conductivity (S/cm)
2
18
16
14
12
10
8
6
4
2
0
1.5
1
0.5
0
PAniDBSA/PVA
PAni/PVA
PAni-TiO2
(I)/PVA
PAni-TiO2
(II)/PVA
PAni-TiO2
(III)/PVA
(x 1015 cm-3)
Conductivity
Carrier Density (n)
(a)
Carrier Density, n
120
PAni-TiO2
(IV)/PVA
3000
2000
1000
0
Conductivity
Carrier Mobility
2
Conductivity (S/cm)
PAni-TiO2
(IV)/PVA
6000
5000
1.5
4000
1
3000
2000
0.5
1000
0
(cm volt sec )
(c)
PAni-TiO2
(III)/PVA
-1
PAni-TiO2
PAni-TiO2
(I)/PVA
(II)/PVA )
Sample (40 wt. %)
-1
PAni/PVA
2
PAniDBSA/PVA
cm )
4000
(x 10
5000
-3
18
16
14
12
10
8
6
4
2
0
Carrier Mobility,
(cm2 volt-1 sec-1)
Carrier Mobility,
6000
15
Carrier Mobility
Carrier Density (n)
(b)
Carrier Density, n
Sample (40 wt. %)
0
PAniDBSA/PVA
PAni/PVA
PAni-TiO2
(I)/PVA
PAni-TiO2
(II)/PVA
PAni-TiO2
(III)/PVA
PAni-TiO2
(IV)/PVA
Sample (40 wt. %)
Figure 3.21: The samples (40 wt. %) plotted against (a) conductivity and carrier
density, (b) carrier mobility and density and (c) conductivity and carrier mobility of
the samples
121
The carrier density of PAni/PVA is higher than its composites (PAni-TiO2
(I)/PVA) because of the presence of metal oxides between the polymer chains may
have interrupted the orientation of the chains and hence causing the spacing
between the chains to expand. Since the carrier density is expressed in unit per cm3,
so the carrier density of PAni-TiO2 (I)/PVA becomes lower compared to the
polymer blend itself as a result of volume expansion in the PAni-TiO2/PVA.
Nevertheless, it can be clearly seen that the carrier density increased with the
increasing weight ratio of TiO2 in the PAni composite blends and it shows the
highest value of carrier density among the samples, 16.53 x 1015 cm−3 for PAniTiO2 (IV)/PVA blend with overloaded TiO2 content. This may imply that the
overloaded TiO2 content in the PAni composite leading to a more jam-packed
condition, resulting in a higher carrier density and narrower d-space (as can be seen
in XRD results) and will even lower the carrier mobility and electrical conductivity.
In other words, the encapsulation of the PAni on TiO2 during the composite
formation simply increases the carrier mobility and conductivity of the PAni-TiO2
blends, but to certain TiO2 content.
For PAni-TiO2 (I)/PVA composite blends with various weight fractions of
conductive component (Figure 3.22), generally there are no distinct relation
between the conductivity and the carrier density. The same inverse pattern is also
found for the composite blends with 20 to 40 wt. % between the carrier mobility
and density. The conductivity is directly proportional to carrier mobility.
Nevertheless, for PAni-TiO2 (I)/PVA with 10 wt. % having the lowest conductivity
among the samples, is unexpectedly having relatively high carrier mobility. This
phenomenon may be owing to the low carrier density in the sample, leading to an
increase in carrier mobility.
Compared to iodine doped poly[(6-N-pyrrolylhexyl)hexylsilane] (PSiPy) in
Table 3.14 [96], the carrier density of PAni/PVA and PAni-TiO2/PVA blends
(shown in Table 3.12) with higher conductivity are almost the same as the reported
carrier density value, which support the earlier discussion that carrier density is not
122
Conductivity
3
Carrier Density (n)
2.5
2
1.5
1
0.5
(x 1015 cm-3)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Carrier Density, n
Conductivity (S/cm)
(a)
0
40
30
20
10
Wt. % of PAni-TiO2 (I)/PVA
Carrier Mobility
Carrier Density
(x 1015 cm-3)
2.5
Carrier Density, n
(cm2 volt-1 sec-1)
Carrier Mobility
5000
3
Carrier Mobility
6000
(cm2 volt-1 sec-1)
(b)
4000
2
3000
1.5
2000
1
1000
0.5
0
0
40
30
20
10
Wt. % of PAni-TiO2 (I)/PVA
(c)
2
Conductivity
Carrier Mobility
1.8
Conductivity (S/cm)
1.6
1.4
6000
5000
4000
1.2
3000
1
0.8
2000
0.6
0.4
1000
0.2
0
0
40
30
20
Wt. % of PAni-TiO2 (I)/PVA
10
Figure 3.22: The samples with various weight fractions plotted against (a)
conductivity and carrier density, (b) carrier mobility and density and (c)
conductivity and carrier mobility of the samples
123
the main factor limiting the electrical conductivity. On the other hand, the
PAni/PVA and PAni-TiO2/PVA blends with higher carrier mobility explain the
higher electrical conductivity than the PSiPy. The carrier mobility seems to be the
factor determining the conductivity of the samples.
Table 3.14: Carrier density, carrier mobility and conductivity of iodine doped
poly[(6-N-pyrrolylhexyl)hexylsilane] (PSiPy)
Sample
Hall
Carrier
Carrier
Coefficient, R
Density, n
Mobility, µ
3
I2-PSiPy
−1
−3
2
−1 −1
Conductivity,
σ (S cm-1)
(cm C )
(cm )
(cm V s )
6.09 x 104
1.03 x 1014
1.07 x 103
1.75 x 10−2
4.45 x 103
1.40 x 1015
5.70 x 10
1.28 x 10−2
3.63 x 103
1.72 x 1015
4.61 x 10
1.27 x 10−2
2.06 x 103
3.03 x 1015
1.91 x 10
9.26 x 10−3
CHAPTER 4
CONCLUSION AND SUGGESTION
4.1
Conclusion
In situ chemical polymerization has successfully produced conducting PAni
and PAni-TiO2, resulted in a homogeneous dark green polymerization mixture,
which showed the presence of emeraldine salt, as confirmed by UV results.
We have improved the process in making a conductive film out of
inherently conductive PAni and its composites, which were known impossible to
cast or fabricate, especially in aqueous system. Highly conducting free-standing
films were also successfully prepared by blending with water-soluble PVA as a
matrix and subsequently formed through solution casting method, resulting from a
fractual network structure of the conducting phase. The blending of electrically
conductive PAni and its composite with a water-soluble polymer, PVA, enabled the
formation of free–standing, flexible and conductive film with ease.
The presence of a single Tg, which shifted to higher value with increasing
PAni and PAni-TiO2 content because of the hydrogen bonding between PAni and
PAni-TiO2 with PVA as shown by FTIR, revealing the miscibility between PAni
125
and PAni-TiO2 with PVA. The X-ray diffraction pattern of the blends revealed that
the degree of crystallinity of PAni composite blends was lower than that of PVA
and TiO2, showing that the amorphous nature of PAni may inhibit crystallization of
TiO2 and PVA. Furthermore, the peaks of PAni could hardly be observed in PAni
composite blends.
The electrical conductivity of the PAni-TiO2/PVA blends increase with the
increase of TiO2/aniline weight ratio and reaches a saturation value. All the blends
samples exhibit similar pattern, i. e. the conductivity with temperature from 30 oC
to 50 oC, following with decreasing conductivity, and reach the maximum at 140
o
C, then decrease with further increasing temperature.
It was found that the protonated PAni and PAni-TiO2 with PVA exhibited
low percolation threshold in the relation of the electrical conductivity as a function
of the PAni and PAni-TiO2 loading in the blends, which is characteristic of the
formation of a self-assembled interpenetrating fibrillar fractal network of PAni. The
increasing conductivity of the films by the increasing content of conductive
components due to the growing of continuous network formation is confirmed by
transmission electron microscopy (TEM). The percolation threshold was about 2.0
wt. % for both PAni/PVA and PAni-TiO2/PVA blends.
The carrier density and mobility of the PAni and its composites blends
shared the same trend as semiconducting materials (Ge, Si etc.) where the higher
the carrier density, the lower the carrier mobility, and vice versa. The transportation
between holes, which is major carrier in the system dominated the conducting
mechanism. Finally, the charge transport mechanism of PAni and its composites
blends are compatible to the variable-range hopping model, where charge carriers
migrate through intrachain, interchain and interparticles transport, mainly from
hopping mechanism between polaronic clusters in the PAni network.
126
It can be concluded that the wt. % of conductive fractions, TiO2 content,
temperature, charge mobility and the conductivity behavior of the films are in tight
interaction with each other. The blending procedures could lead to innovation in
synthetic method for better conducting films.
4.2
Suggestions
To further understand the PAni and PAni-metal oxide composites blended
with water-soluble polymer in aqueous system, future effort should focus on the
study of how the carrier density and mobility of the samples are affected by the
preparation method; namely the synthesis temperature, the duration of the
polymerization, the choice of dopant, metal oxides and water-soluble polymers used
and finally the way of film preparation method. In addition, the thermal heating and
ageing effect on the microscopic and macroscopic level of the sample could be
carried out to investigate the conduction and transport properties. The investigation
on the low temperature conductivity of the films is suggested.
This research could be extended by varies the choice of water-soluble
polymer used, such as polyvinylpyrrolidone (PVP), polymethacrylamide (PMAAm)
and etc. The chemical reactivity of the PAni and PAni-metal oxide blending with
different water-soluble polymer casting on different substrates should also be
explored to expand the potential application of PAni films in the future.
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APPENDIX A
Calculation of resistivity and conductivity of PAni/PVA (40 wt. %, 30 oC)
by using four-probe method:
To be corrected Resistivity of PAni/PVA (40 wt. %, 30 oC) (in film form),
V
)x2πS
I
= 13.4796 x 2 x π x 0.2
= 16.9411 Ohm.cm
ρo = (
Resistivity of the PAni/PVA (40 wt. %, 30 oC) film
ρ = ρo / G7 (W/S)
= (16.9411) / 28.2917
= 0.5988 Ohm.cm
where function G7 (W/S) =
Conductivity, σ
2S
log 2
e
W
= 1/ρ
= 1/0.5988
= 1.67 S/cm
Thus resistivity, ρ and conductivity, σ can be calculated at various temperatures.
ρo = To be corrected resistivity
σ = Conductivity (S/cm)
W = Film Thickness, cm
G7= Correction Divisor
I = Current
ρ = Resistivity (Ohm.cm)
S = Probe Distance, cm
T = Temperature (Kelvin)
V = Voltage
Appendix A: Equations and calculations for the resistivity and conductivity of
PAni/PVA (40 wt. %, 30 oC) by using four-probe method.
137
APPENDIX B
Calculation of carrier density and mobility of PAni/PVA (40 wt. %, 30 oC)
by using Hall Effect measurement:
Sample Detail
Sample
Gauss
Thickness (z)
Resistivity (ρ)
onductivity (σ):
:
PAni/PVA
Magnetic Field : 1000
:
:
6.0 x 10-3 cm
5.99 x 10-1 Ohm.cm
1.67 S/cm
4 sets of the values of voltage and current for 2 position (ABCD and BCDA) of
the sample in 2 directions (Coil 1 and 2) of the magnetic field will be recorded.
These data were used to calculate the Hall Coefficient, carrier density and
mobility for each position, respectively. The average value of the Hall
Coefficient, carrier density and mobility are then calculated.
V
Position ABCD: Coil 1 (The value of H = 5.8805)
I
Carrier Density (n)
Hall Coefficient (RH)
1
V
z
n =
= ( H )( )
RH
R .q
H
I
1
0.006
=
= (5.8805)(
)
3
(3.528 x 10 ) (1.6 x 10 −19 )
1000
= 3.528 x 10-5 volt cm amp-1G-1
= 1.77 x 1015 cm-3
3
3
-1
= 3.528 x 10 cm coulomb
q = Charge of electron
Carried Mobility (µ)
µ
= RH x σ
= (3.528 x 103)(1.67)
= 5.89 x 103 cm-2 volt-1 sec-1
Position ABCD: Coil 2 (The value of
Hall Coefficient (RH)
V
z
RH = ( H )( )
I
H
0.006
)
= (5.8691)(
1000
= 3.521 x 10-5 volt cm amp-1G-1
= 3.521 x 103cm3 coulomb-1
VH
= 5.8691)
I Carrier Density (n)
n =
=
1
R .q
1
(3.521 x 10 ) (1.6 x 10 −19 )
3
= 1.77 x 1015 cm-3
(continued)
Appendix B: Sample calculation of Hall Effect measurement for PAni/PVA
(40 wt. %, 30 oC) by using Hall Effect measurement
138
Appendix B (Continued)
Carried Mobility (µ)
µ
= RH x σ
= (3.521 x 103)(1.67)
= 5.88 x 103 cm-2 volt-1 sec-1
Position BCDA: Coil 1 (The value of
Hall Coefficient (RH)
V
z
RH = ( H )( )
H
I
0.006
= (5.8713)(
)
1000
= 3.522 x 10-5 volt cm amp-1G-1
= 3.522 x 103cm3 coulomb-1
VH
= 5.8713)
I
Carrier Density (n)
n =
1
R .q
1
(3.522 x 10 ) (1.6 x 10 −19 )
= 1.77 x 1015 cm-3
=
3
Carried Mobility (µ)
µ
= RH x σ
= (3.522 x 103)(104)
= 5.883 x 103 cm-2 volt-1 sec-1
Position BCDA: Coil 2 (The value of
Hall Coefficient (RH)
V
z
RH = ( H )( )
I
H
0.006
)
= (5.8909)(
1000
= 3.5345 x 10-5 volt cm amp-1G-1
= 3.534 x 103cm3 coulomb-1
VH
= 5.8909)
I
Carrier Density (n)
n =
1
R .q
1
(3.534 x 10 ) (1.6 x 10 −19 )
= 1.77 x 1015 cm-3
=
3
Carried Mobility (µ)
µ
= RH x σ
= (3.534 x 103)(104)
= 5.902 x 103 cm-2 volt-1 sec-1
Average Hall Coefficient, RH = 2.06 x 103 cm3 coulomb-1
Average Carrier Densisty, n = 3.04 x 1015 cm-3
Average Carrier Mobility, µ = 3.31 x 103 cm-2 volt-1 sec-1
Appendix B: Sample calculation of Hall Effect measurement for PAni/PVA
(40 wt. %, 30 oC) by using Hall Effect measurement