i
SYNTHESIS, CHARACTERIZATION AND DIELECTRIC PROPERTIES
OF CADMIUM SULFIDE POLYMER NANOCOMPOSITES
ERIAWAN RISMANA
UNIVERSITI TEKNOLOGI MALAYSIA
i
SYNTHESIS, CHARACTERIZATION AND DIELECTRIC PROPERTIES
OF CADMIUM SULFIDE POLYMER NANOCOMPOSITES
ERIAWAN RISMANA
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Doctor of Philosophy (Chemistry)
Faculty of Science
Universiti Teknologi Malaysia
FEBRUARY 2010
iii
Specially dedicated to:
My mother, father, wife, daughter and sons
iv
ACKNOWLEDGEMENT
In the name of Allah, the most Gracious, the most Merciful
Alhamdulillaahirabbil ‘alamin, Allaahumma sholli ala Muhammad wa ‘ala
alihi wa shohbihi wa sallam. All praise must be to Allah, The supreme Lord of the
world. Peace and blessing to Rasulullaah Muhammad Shollallahu ‘Alaihi Wasallam,
all the prophets, his families, his close friends and all Muslims.
I wish to thank my supervisors, Prof. Dr. Salasiah Endud and Assoc. Prof.
Dr. Hadi Nur for their continual guidance, encouragement and patience. Their
understanding and supervision are very much appreciated. A research grant from the
Ministry of Science Technology and Innovation Malaysia (MOSTI) is gratefully
acknowledged.
I also would like to express my gratitude to all lecturers and researchers of
the Department of Chemistry for their support and Ibnu Sina Institute for
Fundamental Science Studies UTM for the nanocomposites characterizations.
Thanks to all the laboratory assistants and friends in the Department of Chemistry for
their kindness and wonderful cooperation.
Last but not least, I am grateful to my father and mother: H. E. Suparman and
Hjh. E. Reswati, my father and mother in law: Suparmo and Purniasih, my wife:
Endang, my daughter: Fani, my sons: Faruq and Fathian, and my brothers for their
prayer, love, understanding, encouragement and support.
v
ABSTRACT
Semiconductor/polymer nanocomposites are of increasing importance with
their tunable properties being used as dielectric materials. This thesis focused on
cadmium sulfide (CdS)/polymer nanocomposites. CdS has been combined with three
polymer matrices, i.e. poly(styrene-divinylbenzene) [P(S-DVB)], poly(methacrylic
acid-ethyleneglycoldimethacrylic acid) [P(MAA-EGDMA)] and sulfonated
poly(styrene-divinylbenzene) [SO3H-P(S-DVB)]. CdS/P(S-DVB) nanocomposite
was synthesized by in-situ polymerization in a miniemulsion system using monomer
as oil-phase. CdS/P(MAA-EGDMA) nanocomposite has been synthesized by ion
exchange and precipitation processes. While, the CdS/SO3-P(S-DVB) nanocomposite
has been prepared by sulfonation, ion exchange and precipitation. Agglomerated
nanoclusters of CdS were obtained from the above in-situ preparation methods. The
structure-dielectric property relationship of the nanocomposites is elucidated by
various techniques such as UV – Vis, FTIR, UV-Vis DR, TEM, SEM, XRD,
impedance analyzer, AAS, EDX, thermal conductivity analyzer and
thermogravimetric analysis. Dielectric properties of the CdS/polymer
nanocomposites have been studied at frequencies of 0.1 – 1,000 kHz. The decrease in
dielectric constant was found in CdS/SO3H-P(S-DVB) nanocomposite. Considering
that the SO3H-P(S-DVB) has very high dielectric constant due to its proton mobility,
the replacement of the proton of SO3H-P(S-DVB) with the CdS particles caused the
decrease in dielectric constant of the nanocomposite. Interestingly, an increase in the
dielectric constant was also observed in CdS/P(S-DVB) and CdS/P(MAA-EGDMA)
nanocomposites compared to that of CdS nanoparticles or pure polymers. It is also
demonstrated that the unusual enhancement of dielectric constant of CdS/P(MAAEGDMA) depended on the concentration of CdS nanoparticles. The occurrence of
strong interfacial interaction between CdS nanoparticles with P(MAA-EGDMA)
polymer has been proved by FTIR spectra. One explains that an increase in dielectric
constant is due to the increase of interfacial interaction among CdS nanoparticles and
also CdS nanoparticles with polymer. These interactions increase the mobility of
charge carriers and polarizability of electron. Based on the results of this study, it can
be suggested that dielectric properties of CdS/polymer nanocomposites can be
explained by the following unique properties i.e. nanosize of CdS particles,
semiconductor property of CdS, the interfacial interaction between CdS
nanoparticles and polymer and intrinsic properties of polymer. These conclusions lay
the foundation for developing new synthetic strategies for designing new dielectric
materials by varying the size, concentration and distribution of CdS nanoclusters in
various polymer matrices.
vi
ABSTRAK
Semikonduktor/polimer menjadi sangat penting kerana sifat – sifatnya yang
boleh diselaraskan sebagai bahan dielektrik. Tesis ini memfokuskan kepada
nanokomposit kadmium sulfida (CdS)/polimer. CdS telah digabungkan dengan tiga
polimer matriks iaitu poli(stirena-divinilbenzena) [P(S-DVB)], poli(asid metakrilikasid etilenaglikoldimetakrilik) [P(MAA-EGDMA)] dan poli(stirena-divinilbenzena)
tersulfonat [SO3H-P(S-DVB)]. Nanokomposit CdS/P(S-DVB) telah disintesis
melalui kaedah pempolimeran in-situ dalam suatu sistem miniemulsi menggunakan
monomer sebagai fasa minyak. Nanokomposit CdS/P(MAA-EGDMA) telah
disintesis melalui kaedah penukaran ion dan proses pemendakan. Manakala,
nanokomposit CdS/SO3-P(S-DVB) telah dihasilkan secara pengsulfonan, penukaran
ion dan pemendakan. Nanokluster aglomerat CdS telah diperolehi daripada kaedah
penyediaan in-situ. Hubungan struktur-sifat dielektrik nanokomposit tersebut telah
dianalisis menggunakan pelbagai tenik seperti UV – Vis, FTIR, UV – Vis DR, TEM,
SEM, XRD, penganalisis impendans, AAS, EDX, penganalisis kekonduksian terma
dan analisis termogravimetri. Sifat dielektrik nanokomposit CdS/polimer telah diukur
pada frekuensi 0.1 – 1,000 kHz. Pemalar dielektrik bagi nanokomposit CdS/SO3-P(SDVB) didapati telah berkurang. Memandangkan SO3H-P(S-DVB) mempunyai
pemalar dielektrik yang sangat tinggi disebabkan oleh mobiliti protonnya,
penggantian proton pada SO3H-P(S-DVB) dengan partikel CdS telah mengakibatkan
penurunan pemalar dielektrik bagi nanokomposit. Yang lebih menarik, peningkatan
pemalar dielektrik juga telah diperhatikan bagi nanokomposit CdS/P(MAAEGDMA) dan CdS/P(S-DVB) berbanding partikel CdS atau polimer tulen. Telah
ditunjukkan juha bahawa peningkatan yang luar biasa pemalar dielektrik bagi
CdS/P(MAA-EGDMA) bergantung pada kepekatan partikel CdS. Kehadiran
interaksi antaramuka yang kuat antara partikel CdS dengan polimer P(MAAEGDMA) telah dibuktikan dengan spektrum FTIR. Dapat dijelaskan bahawa
peningkatan pemalar dielektrik adalah disebabkan oleh pertambahan interaksi
anataramuka di kalangan partikel – partikel CdS dan juga nanopartikel CdS dengan
polimer. Interaksi tersebut telah meningkatkan mobiliti pembawa cas dan
kebolehpengutuban elektron. Berdasarkan hasil kajian ini, adalah dicadangkan
bahawa sifat dielektrik nanokomposit CdS/polimer boleh diterangkan dengan sifat –
sifat unik berikut iaitu, saiz nano partikel CdS, sifat semikonduktor CdS, interaksi
antaramuka antara nanopartikel CdS dengan polimer dan sifat intrinsik polimer.
Kesimpulan ini menjadi asas kepada strategi sintesis yang baru bagi menghasilkan
bahan dielektrik dengan cara mengubah saiz, kepekatan dan taburan nanokluster CdS
dalam matriks pelbagai polimer.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xi
LIST OF FIGURES
xiii
LIST OF SYMBOL / ABBREVIATIONS
xix
LIST OF APPENDICES
xxii
INTRODUCTION
1
1.1 Background of Study
4
1.2 Design and Strategy of Research
7
1.3 Objective of The Study
9
1.4 Scope of The Study
9
LITERATURE REVIEW
12
2.1 Dielectric Properties and Dielectric Constant
12
2.2 Embedded Capacitor
15
2.3 Design of High Dielectric Property Materials
16
2.3.1 Overview of Dielectric Properties of Polymer
Nanocomposites
19
viii
2.3.2 CdS/polymer Nanocomposites
2.4 Cadmium Sulfide as Semiconductor Material
2.4.1 Methods of Synthesis of Cadmium Sulfide
25
26
29
Nanoparticles
32
2.5 Miniemulsion Polymerization
2.5.1 Introduction
33
2.5.2
Miniemulsion
34
2.5.3
The Miniemulsion Process
35
2.5.4
Preparation of Miniemulsion
36
2.5.4.1 Formulations
37
2.5.4.2 Method of Preparation
38
2.5.4.3 Homogenization Devices
39
2.5.4.4 Particle Nucleation
40
2.5.5 Applications
42
2.5.5.1 Encapsulation of Inorganic Material
3
42
EXPERIMENTAL
46
3.1 Synthesis and Preparation
46
3.1.1 Purification of Monomer
46
3.1.2 Preparation of CdS Nanoparticles by Reverse
46
Micelles Using n-Decane as Oil-phase
3.1.3 Preparation of CdS Nanoparticles by Reverse
47
Micelles Using Monomer as Oil-phase
3.1.4
Preparation of P(MAA-EGDMA)
3.1.5…Preparation
of
48
CdS/P(MAA-EGDMA)
48
Preparation of P(S-DVB), SO3H/P(S- DVB) and
49
Nanocomposites
3.1.6
CdS/SO3H-P(S- DVB)
3.2 Characterization Techniques
50
3.2.1 UV-Vis Spectroscopy
50
3.2.2 X-Ray Diffraction
51
3.2.3 Fourier Transform Infrared Spectroscopy
51
3.2.4 UV-Vis Diffuse Reflectance Spectroscopy
52
ix
4
3.2.5 Field Emission Scanning Electron Microscope
52
3.2.6 Transmission Electron Microscopy
52
3.2.7 Dielectric Constant and Dissipation Factor
53
3.2.8 Ionic Conductivity
53
3.2.9 Thermal Conductivity
53
3.2.10 Thermogravimetric Analysis
54
3.2.11 Atomic Absorption Spectroscopy
54
RESULTS AND DISCUSSION
55
4.1
58
Optimization of Synthesis of CdS Nanoparticles and
CdS/P(S-DVB) Nanocomposites
4.1.1 Structural and Morphological Properties of
67
CdS/P(S-DVB) Nanocomposites
4.1.2 Fourier Transform Infrared and UV – Vis
Diffuse
Reflectance
Spectroscopy
71
of
CdS/P(S-DVB) Nanocomposites
4.1.3 Dielectric
and
Electrical
Properties
of
74
CdS/P(S-DVB)
79
Properties
81
CdS/P(S-DVB) Nanocomposites
4.1.4 Thermal
Properties
of
Nanocomposites
4.2
Synthesis
and
Physicochemical
Cadmium
Sulfide-Poly(methacrylic
of
acid-ethylene
glycol dimethacrylic acid) [CdS/P(MAA-EGDMA)]
Nanocomposites
4.2.1 Synthesis
of
P(MAA-EGDMA)
and
81
CdS/P(MAA-EGDMA) Nanocomposites
4.2.2 Fourier Transform Infrared and UV – Vis
Diffuse
Reflectance
Spectroscopy
of
84
4.2.3 Structural and Morphological Properties of
88
CdS/P(MAA-EGDMA) Nanocomposites
CdS/P(MAA-EGDMA) Nanocomposites
4.2.4 Dielectric
and
Electrical
Properties
of
92
x
CdS/P(MAA-EGDMA) Nanocomposites
4.2.5 Influence of Temperature on Dielectric and
Electrical
Properties
of
102
CdS/P(MAA-
EGDMA) Nanocomposites
4.2.6 Thermal Properties of CdS/P(MAA-EGDMA)
104
Nanocomposites
4.3
Synthesis
and
Physicochemical
Properties
of
107
4.3.1 Synthesis of P(S-DVB), SO3H-P(S-DVB) and
107
CdS/Sulfonated-Poly(styrene-divinylbenzene)
CdS/SO3-P(S-DVB) Nanocomposites
4.3.2 Fourier Transform Infrared and UV – Vis
Diffuse
Reflectance
Spectroscopy
108
of
CdS/SO3-P(S-DVB) Nanocomposites
4.3.3 Structural and Morphological Properties of
113
CdS/SO3-P(S-DVB) Nanocomposites
4.3.4 Dielectric
and
Electrical
Properties
of
118
4.3.5 Thermal Properties of CdS/SO3-P(S-DVB)
127
CdS/SO3-P(S-DVB) Nanocomposites
Nanocomposites
4.4
Comparison of Physicochemical and Electrical
131
Properties of Those CdS/polymer Nanocomposite
5
CONCLUSIONS
136
REFERENCES
138
Appendices A-H
149-156
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
1.1
Outline of results and discussion
10
2.1
Dielectric constant of several materials
17
2.2
Dielectric constant and method of synthesis of several
23
polymer nanocomposites
2.3
Dielectric constant and methods of synthesis of CdS
26
nanoparticle and CdS/polymer nanocomposites
2.4
Several CdS/polymer nanocomposites, synthesis method,
31
applications and property of interest
2.5
Examples of formulation of miniemulsion polymerization
37
2.6
Somes applications of polymerization miniemulsion
42
4.1
The optimum conditions for synthesis of CdS nanoparticle
59
using n-decane as oil-phase
4.2
The optimum conditions for synthesis of CdS/P(S-DVB)
63
nanocomposite using monomer as oil-phase
4.3
The absorption onset wavelength and particle size of CdS
86
nanoparticles in CdS/P(MAA-EGDMA) nanocomposites
with various CdS contents
4.4
Dielectric
constants
of
P(MAA-EGDMA),
CdS
93
nanoparticles, CdS/P(MAA-EGDMA) prepared by physical
mixing and
CdS/P(MAA-EGDMA) nanocomposites at
various frequencies
4.5
Dissipation factors of P(MAA-EGDMA), CdS nanoparticles
100
and CdS/P(MAA-EGDMA) nanocomposites
4.6
The absorption onset wavelength and particle size of
110
xii
CdS/SO3-P(S-DVB) nanocomposites
4.7
Dielectric constants of P(S-DVB), CdS nanoparticles, SO3H-
119
P(S-DVB) and CdS/SO3-P(S-DVB) nanocomposites
4.8
Meq H+/gram of SO3H-P(S-DVB) with degree of sulfonation
120
= 39.10 % and CdS/SO3-P(S-DVB) nanocomposites with
various amount of CdS
4.9
Dissipation factors of P(S-DVB), CdS nanoparticles, SO3H-
122
P(S-DVB) and CdS/SO3-P(S-DVB) nanocomposites
4.10
Electrical conductivities of CdS nanoparticles, P(S-DVB),
125
Cd(SO3)2-P(S-DVB), NaSO3-P(S-DVB), SO3H-P(S-DVB),
and Cd/SO3-P(S-DVB) with various amount of CdS
4.11
Physicochemical properties of CdS/polymer nanocomposites
132
4.12
Electrical properties of CdS/polymer nanocomposites
135
xiii
LIST OF FIGURES
FIGURE NO.
1.1
TITLE
PAGE
Schematic representation of embedded capacitance solution for
2
electronic devices
1.2
Schematic representation of the synthesis of CdS/polymer
8
nanocomposites
2.1
Schematic representation of capacitor
13
2.2
Schematic representation of types of polarizations
14
2.3
Frequency dependence of the several contributions to the
15
polarizability
2.4
Schematic changes in the density of states on going from a bulk
28
crystal to a nanocrystal to molecule/atom
2.5
Principle of miniemulsion polymerization
35
2.6
Schematic for monomer miniemulsion preparation methods
39
2.7
Schematic of sonication process
39
2.8
The principle of the encapsulation of inorganic materials in
43
polymer particles by the miniemulsion process
4.1
Schematic representation of mechanism of CdS nanoparticles
57
attachment and encapsulation on polymer surface
4.2
Schematic representation of the synthesis of CdS nanoparticles
61
by reverse micelles in miniemulsion system and CdS/P(S-DVB)
nanocomposites via in-situ polymerization
4.3
Reaction of styrene and divinylbenzene by free radical
62
polymerization into P(S-DVB)
4.4
Schematic representation of the encapsulated CdS in P(S-DVB)
matrices
62
xiv
4.5
UV – Vis spectra of CdS nanoparticles in miniemulsion system
64
using styrene-divinylbenzene as oil-phase at Po = 32.5 after 5
minutes formation and different Wo values (a) 5.50, (b) 4.0, (c)
2.75, and (d) 1.50
4.6
UV – Vis spectra of CdS nanoparticles in miniemulsion using
65
styrene-divinylbenzene as oil-phase at different Wo values and
Po = 32.5 at different time intervals after formation (a) Wo = 1.5
(b) Wo = 2.75 (c) Wo = 4 and (d) Wo = 5.55
4.7
XRD patterns of pure P(S-DVB), CdS nanoparticles and
68
CdS/P(S-DVB) nanocomposites with various amounts CdS
4.8
SEM images of CdS/P(S-DVB) nanocomposites at (a) 2,500 and
70
(b) 10,000 magnifications
4.9
TEM images of CdS/P(S-DVB) nanocomposites
71
4.10
FTIR spectra of (a) pure P(S-DVB) and (b) CdS/P(S-DVB)
72
nanocomposites
4.11
UV-Vis DR spectra of pure P(S-DVB) and CdS/P(S-DVB)
nanocomposites
prepared
by
in-situ
polymerization
73
in
miniemulsion system at different Wo value (a) Wo = 4, (b) Wo =
5.55, and (c) Wo = 7.5
4.12
Dielectric
constant
for
P(S-DVB)
and
CdS/P(S-DVB)
74
nanocomposites with different amount of CdS as a function of
various frequencies
4.13
Dissipation factor of (a) pure P(S-DVB) and (b) 0.18 %
75
CdS/P(S-DVB) nanocomposites
4.14
Schematic representation of the mechanism of interfacial
76
interaction in CdS/P(S-DVB) nanocomposites
4.15
Influence of temperature on dielectric constant of 0.03 %
77
CdS/P(S-DVB) nanocomposites at various frequencies
4.16
Percentage decrease of dielectric constant of 0.03 % CdS/P(S-
77
DVB) nanocomposites at various frequencies after heat
treatment at 150o C
4.17
Ionic conductivity of CdS/P(S-DVB) nanocomposites at
different amount of CdS
78
xv
4.18
Thermal conductivity of (a) pure P(S-DVB), (b) 0.01 %
79
CdS/P(S-DVB), (c) 0.02 % CdS/P(S-DVB), (d) 0.03 %
CdS/P(S-DVB) and (e) 0.18 % CdS/P(S-DVB) nanocomposites
at room temperature
4.19
Thermogravimetric analyses curves of pure P(S-DVB), and 0.18
79
% CdS/P(S-DVB) nanocomposites
4.20
Schematic representation of synthesis of P(MAA-EGDMA)
82
particles
4.21
Schematic representation of preparation of CdS/P(MAA-
83
EGDMA) nanocomposites
4.22
Reaction of methacrylc acid and ethylene glycol dimethacrylic
84
acid by free radical polymerization into P(MAA-EGDMA)
4.23
UV-Vis DR spectra of (a) P(MAA-EGDMA), (b) 0.81 %
85
CdS/P(MAA-EGDMA), (c) 0.90 % CdS/P(MAA-EGDMA), (d)
1.08 % CdS/P(MAA-EGDMA), (e) 1.23 % CdS/P(MAAEGDMA), (f) 1.70 % CdS/P(MAA-EGDMA) and (g) 2.07 %
CdS/P(MAA-EGDMA)
4.24
FTIR spectra of P(MAA-EGDMA), CdS/P(MAA-EGDMA)
87
nanocomposites and physically mixed CdS/P(MAA-EGDMA)
4.25
SEM images of 2.07 % CdS/P(MAA-EGDMA) nanocomposites
89
at magnification (a) 50,000 X and (b) 100,000 X
4.26
TEM images of (a) agglomerated and (b) fused samples 2.07 %
90
CdS/P(MAA-EGDMA) nanocomposites
4.27
XRD patterns of pure P(MAA-EGDMA) and CdS/P(MAA-
91
EGDMA) nanocomposites at various CdS amount
4.28
Dielectric constant of CdS/P(MAA-EGDMA) nanocomposite
93
with various amounts of CdS measured at different frequencies
4.28
Dielectric
constant
of (a)
P(MAA-EGDMA),
(b)
CdS
nanoparticles, (c) CdS/P(MAA-EGDMA) physical mixing, (d)
11.9 % Na-P(MAA-EGDMA), (e) 1.4 % Cd-P(MAA-EGDMA),
(f) 0.81 % CdS/P(MAA-EGDMA), (g) 0.90 % CdS/P(MAAEGDMA), (h) 1.08 % CdS/P(MAA-EGDMA), (i) 1.23 %
CdS/P(MAA-EGDMA), (j) 1.70 % CdS/P(MAA-EGDMA) and
94
xvi
(k) 2.07 % CdS/P(MAA-EGDMA) at 100 Hz
4.30
FTIR spectra of P(MAA-EGDMA), CdS/P(MAA-EGDMA)
96
physical mixing, CdS nanoparticle and CdS/P(MAA-EGDMA)
nanocomposites with various content of CdS
4.31
Schematic representation of interfacial interaction in the
97
CdS/P(MAA-EGDMA) nanocomposites
4.32
SEM images of CdS/P(MAA-EGDMA) at various amount of
99
CdS (a) 0.81 % , (b) 1.08 % and (c) 2.07 %
4.33
Dissipation factor of P(MAA-EGDMA) and CdS/P(MAAEGDMA)
nanocomposite
at
various
CdS
content
100
and
frequencies (a) P(MAA-EGDMA), (b) 0.81 % CdS/P(MAAEGDMA), (c) 0.90 % CdS/P(MAA-EGDMA), (d) 1.08 %
CdS/P(MAA-EGDMA), (e) 1.23 % CdS/P(MAA-EGDMA), (f)
1.70 % CdS/P(MAA-EGDMA), and (g) 2.07 % CdS/P(MAAEGDMA)
4.34
Ionic
conductivity
of
(a)
P(MAA-EGDMA),
(b)
CdS
101
nanoparticles, (c) 11.9 % Na-P(MAA-EGDMA), (d) 1.4 % CdP(MAA-EGDMA), (e) 0.81 % CdS/P(MAA-EGDMA), (f) 0.90
% CdS/P(MAA-EGDMA), (g) 1.08 % CdS/P(MAA-EGDMA),
(h) 1.23 % CdS/P(MAA-EGDMA), (i) 1.70 % CdS/P(MAAEGDMA) and (j) 2.07 % CdS/P(MAA-EGDMA)
4.35
Influence of temperature on dielectric constant of 1.70 %
103
CdS/P(MAA-EGDMA) nanocomposites at different frequencies
4.36
Percentage decrease of dielectric constant
of 1.70 %
103
CdS/P(MAA-EGDMA) nanocomposites after heat treatment and
at various frequencies
4.37
Influence of temperature on ionic conductivity of CdS/P(MAA-
104
EGDMA) nanocomposites
4.38
Thermal conductivity of (a) P(MAA-EGDMA), (b) CdS
105
nanoparticle, (c) 0.81 % CdS/P(MAA-EGDMA), (d) 0.90 %
CdS/P(MAA-EGDMA) , (e) 1.08 % CdS/P(MAA-EGDMA),
and (f) 2.07 % CdS/P(MAA-EGDMA) nanocomposites
4.39
Thermogravimectric analysis curve of (a) P(MAA-EGDMA),
106
xvii
(b) 0.90 % CdS/P(MAA-EGDMA) (c) 1.23 % CdS/P(MAAEGDMA)
and
(d)
2.07
%
CdS/P(MAA-EGDMA)
nanocomposites
4.40
Reaction of styrene and divinyl benzene by free radical
107
polymerization and sulfonation process to form SO3H/P(S-DVB)
4.41
Schematic representation of the synthesis of CdS/SO3-P(S-DVB)
109
nanocomposites
4.42
UV-Vis DR spectra of (a) Pure P(S-DVB), (b) SO3H-P(S-DVB),
110
(c) 2.56 % CdS/SO3-P(S-DVB), (d) 3.34 % CdS/SO3-P(S-DVB),
(e) 7.64 % CdS/SO3-P(S-DVB), (f) 14.65
% CdS/SO3-P(S-
DVB) (g) 16.56 % CdS/SO3-P(S-DVB) and (h) CdS
nanoparticles
4.43
FTIR spectra of P(S-DVB), SO3H-P(S-DVB) and CdS/SO3-P(S-
112
DVB) nanocomposites
4.44
XRD patterns of (a) CdS nanoparticles and (b) bulk CdS
113
4.45
XRD patterns of SO3H-P(S-DVB) and CdS/SO3-P(S-DVB)
114
nanocomposites at various CdS content
4.46
SEM images of 7.64 % CdS/SO3-P(S-DVB) nanocomposites at
116
two magnifications (a) 2,000 X and (b) 5,000 X
4.47
SEM images of the surface of CdS/SO3-P(S-DVB) at
117
magnification 100,000 X
4.48
TEM images of (a) non agglomerated and (b) agglomerated
118
CdS/SO3-P(S-DVB) nanocomposites
4.49
Dielectric constant of (a) P(S-DVB), (b) CdS nanoparticles, (c)
119
Cd(SO3)2-P(S-DVB), (d) NaSO3-P(S-DVB), (e) SO3H-P(SDVB), (f) 2.56 % CdS/SO3-P(S-DVB), (g) 3.34 % CdS/SO3P(S-DVB), (h) 7.64 % CdS/SO3-P(S-DVB), (i) 14.65 %
CdS/SO3-P(S-DVB) and (j) 16.56 % CdS/SO3-P(S-DVB) at 100
Hz
4.50
A graph of the relationship between amount of meq H+ and
121
dielectric constant of CdS/SO3-P(S-DVB) nanocomposites
4.51
Dissipation factor of (a) SO3H-P(S-DVB) and CdS/SO3-P(SDVB) at various CdS content and frequencies. (b) 2.36 %
122
xviii
CdS/SO3-P(S-DVB), (c) 3.34 % CdS/SO3-P(S-DVB), (d) 7.64 %
CdS/SO3-P(S-DVB), (e) 14.65 % CdS/SO3-P(S-DVB), (f) 16.56
% CdS/SO3-P(S-DVB)
4.52
Influence of temperature on the dielectric constant of 7.64 %
123
CdS/SO3-P(S-DVB) nanocomposites
4.53
Ionic conductivity of (a) Pure P(S-DVB), (b) CdS nanoparticles,
125
(c) Cd(SO3)2-P(S-DVB), (d) NaSO3-P(S-DVB), (e) SO3H-P(SDVB), (f) 2.36 % CdS/SO3-P(S-DVB), (g) 3.34 % CdS/SO3P(S-DVB), (h) 7.64
% CdS/SO3-P(S-DVB), (i) 14.65 %
CdS/SO3-P(S-DVB) and (j) 16.56 % CdS/SO3-P(S-DVB)
4.54
A graph of the relationship between amount of meq H+ and ionic
126
conductivity of CdS/SO3-P(S-DVB) nanocomposites
4.55
Ionic conductivity of CdS/SO3-P(S-DVB) nanocomposites at
126
different temperatures
4.56
Thermal conductivity of P(S-DVB), SO3H-P(S-DVB), CdS
127
nanoparticles, and CdS/SO3-P(S-DVB) nanocomposites
4.57
A graph of the relationship between amount of CdS nanoparticle
and
thermal
conductivity
of
128
CdS/SO3-P(S-DVB)
nanocomposites
4.58
Thermogravimetric analysis curves of (a) CdS nanoparticles, (b)
129
16.56 % CdS/SO3-P(S-DVB), (c) 7.64 % CdS/SO3-P(S-DVB),
(d) 2.36 % CdS/SO3-P(S-DVB) and (e) SO3H-P(S-DVB)
4.59
Schematic representation of mechanism replacing proton with
sodium and CdS nanoparticles attached on surface of polymer
and their properties
130
xix
LIST OF SYMBOL / ABRREVIATIONS
FTIR
- Fourier transform infrared
UV - Vis
- Ultraviolet – visible
DR
- Diffuse reflectance
TEM
- Transmission electron microscopy
SEM
- Scanning electron microscopy
TGA
- Thermogravimetric analysis
AIBN
- 2,2’-Azobisisobutyronitrile
XRD
- X-ray diffraction
P(S-DVB)
- Poly(styrene-divinyl benzene)
P(MAA-EGDMA)
- Poly(methacrylic acid – ethylene glycol dimethacrylic acid)
CdS
- Cadmium sulfide
SO3H-P(S-DVB)
- Sulfonated-poly(styrene-divinyl benzene)
AOT
- Sodium bis(2-ethylhexyl)sulfosuccinate
SDS
- Sodium dodecyl sulfate
CTABr
- Cetyltrimethylammonium bromide
CVDAC
- Cetyl-p-vinyl benzyldimethylammonium chloride
PS
- Polystyrene
Mac
- Maleic acid anhydride
P(MAA-PMMA)
- Poly(methacrylic acid-polymetylmethacrylic acid)
MMA-BMA
- Methyl methacrylate-butyl methacrylate
PVA
- Polyvinyl alcohol
DMF
- Dimethylformamide
PEO
- Polyethylene oxide
mA
- Milli ampere
KV
- Kilovolt
AC
- Alternating current
xx
PCB
- Printed circuit board
PZT
- Lead zirconate titanate
PANI
- Polyaniline
PTT
- Poly(trimethylene terephtalate)
M
- Monomer
AAS
- Atomic absorption spectroscopy
K
- Dielectric constant
εr
- Relative permittivity of a material
εo
- Relative permittivity of vacuum
C
- Capacitance
Q
- Coulomb
A
- Area of the electrical conductor
V
- Volt
PWB
- Printed wiring board
d
- Thickness
IC
- Integrated circuit
PVC
- Polyvinyl chloride
PI
- Polyimide
PVB
- Polyvinyl butyral
PMN
- Lead magnesium niobate
CB
- Circuit board
P(TMPTA)
- Poly(trimethylolpropane triacrylate)
PEN
- Polyarylene ether nitriles
S-SEBS
- Sulfonated styrene-b-(ethylene-ran-butylene)-b-styrene block
copolymers
LDPE
- Low density polyethylene
DS
- Diphenyl sulfoxide
LUMO
- Lowest unoccupied molecular orbital
HOMO
- Highest occupied molecular orbital
CMC
- Critical micelle concentration
HD
- Hexadecane
SPS
- Sodium persulfate
MWD
- Molecular weight distribution
xxi
KPS
- Potassium persulfate
VAc
- Vinyl acetate
EMA
- Ethyl methacrylic acid
PDA
- Personal digital assistant
Hi-Dk RCF
- High dielectric constant resin coated foil
VBTAC
- Vinyl benzyl trimethylammonium chloride
SLS
- Sodium lauryl sulfate
xxii
LIST OF APPENDICES
APPENDIX.
A
TITLE
Comparison of stabilization time of CdS nanoparticles in
PAGE
149
miniemulsion at Wo = 5 with different Po values
B
UV – Vis spectra of CdS nanoparticles in miniemulsion at
150
Wo = 5, Po = 65 and concentration of cadmium and sulfide
ion of 3.0 x 10-4 M at different time after formation
C
UV - Vis spectra of CdS nanoparticles in miniemulsion
151
system at Po = 65 with Wo values = 5 and 10 at different
time after formation
D
UV - Vis spectra of CdS nanoparticles in miniemulsion at
152
Wo = 5 and Po = 65 at different concentration of cadmium
and sulfide ion
E
UV – Vis spectra of CdS nanoparticles in miniemulsion
153
system before and after addition styrene-divinyl benzene at
different times after formation, (a) and (d) without
monomer at 5 min and 24 h, respectively, (b) and (c) after
mixing with monomer at 5 min and 24 h, respectively
F
Quantitative standard calibration plot of cadmium element
154
by using Perkin Elmer AA400 AAS
G
Quantitative standard calibration plot of sodium element
155
by using Perkin Elmer AA400 AAS
H
List of publications
156
1
CHAPTER 1
INTRODUCTION
High dielectric materials have been actively explored and used, because the
material has potential for application in microwave communication devices, artificial
muscles, and embedded capacitor for micro-electromechanical system. Dielectric
materials that can store large electric energy are highly desirable for many electronic
and electric systems for energy pulse and power conditioning applications. Ceramic
materials usually have large dielectric constant, but they are limited by their relative
small breakdown strength. On the other hand, polymers usually enjoy higher
breakdown strength but suffer from much lower dielectric constant.
Nanotechnology has been the subject of increasing interest in recent years
due to the optical, dielectric, electric, magnetic, biological, pharmaceutics and
catalytic properties that are present in the metallic and inorganic nanoparticles. These
properties are used in the development of different nanodevices, including
microelectronic uses. The ever escalating speed, functionality and portability
requirement for microelectronic products exert tremendous pressure interest for
researchers and manufacturers to meet with the rapid growing demand for the
miniaturization and performance. The miniaturization and high performance in
electronic devices has driven research and development activities to produce
embedded passive. Passives are non-active electrical elements and can be divided
into resistive, capacitive and inductive components. In a typical electronic product
today, more than 80 % of the surface area of the printed circuit board (PCB) is
occupied by passive component (Lu et al., 2006).
2
By eliminating surface mount components and embedding it into the
substrate board, embedded passive component offer various advantages over
traditional discrete ones, such as higher component density, increased functionality,
improved electrical performance, increased design flexibility, improved reliability
and reduced unit cost. The architecture of passive component is one area with room
for improvement due to the large and growing number of passive component in
today’s increasingly functional devices. Discrete passives, especially capacitors, have
already become the major barrier of the electronic systems miniaturization.
Therefore, the development of embedded passives is desired, if not required. Among
passives component, the development of embedded capacitor has been an area of
significant activity because the capacitor use in multiple functions, such as
decoupling, by-passing, filtering, and timing capacitors (Rao and Wong, 2004; Lu et
al., 2007). Figure 1.1 shows a schematic of embedded capacitance solution for
electronic devices.
High speed computing boards
(Servers, router, super computers)
Power distribution improvement
Ultra-thin substrate
(used as embedded capacitor)
Copper Foil
8 – 24 micron
Polymer dielectric
Module board
Cell phones, PDA, note book
Miniaturization/HDI
Hi-Dk RCF
(used as embedded capacitor)
Copper Foil
16 micron polymer dielectric
with Hi-Dk filler
Figure 1.1: Schematic representation of embedded capacitance solution for
electronic devices
3
Studies
polymer/ceramic
and
syntheses
nanocomposites,
of
polymer
nanocomposites,
polymer/inorganic
including
nanocomposites
and
polymer/metal nanocomposites with high dielectric constant have been actively
explored, with the hope to substantially enhance the electric energy density of the
resulting nanocomposite. The utilized polymer nanocomposites for electronic
application have many advantages such as light weight, shape-flexibility, cost
effectiveness, and good process ability of the material.
Many studies were developed to produce polymer nanocomposites with high
dielectric constant. Polymer nanocomposites are appealing for two reasons. First of
all, they possess large interfacial exchange coupling through a dipolar interface layer
and leading to enhancement in polarization and polarizability in polymer matrix near
the interface. As a result, enhanced dielectric constant can be expected in the
polymer matrix near the interfaces. Secondly, the nanoscale particles also make it
possible to reduce the thickness of polymer matrix film to nano range, and thus
increase its already high breakdown strength even further by avoiding avalanche
effect.
Nanoparticles are generally categorized as the class of materials that fall
between the molecular and bulk solid limits, with an average size between 1 – 100
nm. Semiconductor nanoparticles, referred to as quantum dots, with dimensions in
the order of nanometers have been the subject of intense research in the past two
decades, due to their unique optical, electronic, physical and chemical properties
(Alivisatos, 1996; Wang and Herron, 1991). Inorganic materials on nano-sized
metals and semiconductors provide a potential solution to meet present and future
technological demand in virtue of the novel properties and unique property
combination of both metal and semiconductor nanoparticles. On the other hand,
uniform dispersion of nanoparticles in the nanocomposites is required because
clumps of particles inside the polymer matrix will not lead to desirable electrical or
dielectric properties. However, uniformly dispersed ultrafine particles in polymer
matrix may not be easily achieved by incorporating pre-made nano-size particles into
a polymer. This is caused by the easy agglomeration of nanoparticles and high
viscosity of polymers. The most promising way to decrease an obstacle of these
4
factors on the dielectric properties of nanocomposites is the in-situ or direct
formation of nanoparticles in polymer matrix.
The focus of this study is to synthesize CdS/polymer nanocomposites and
characterization of the physicochemical and electrical properties. The materials were
synthesize by in-situ polymerization and ion exchange-precipitation method. The
nanocomposites were carried out by several techniques such as UV – Vis
spectroscopy, UV - Vis Diffuse Reflectance (UV – Vis DR) spectroscopy, Fourier
Transform Infrared (FTIR) Spectroscopy, X-ray Diffraction (XRD), Atomic
Absorption Spectroscopy (AAS), Thermogravimetry Analysis (TGA), Transmission
Electron Microscopy (TEM), Field Emmision Scanning Electron Microscopy
(FESEM), AC Impedance Analyzer, and Thermal Conductivity Analyzer.
The used of CdS nanoparticles as nanofiller have received great attention
because of their unique electrical and optical properties. The selected approach of
synthesis methods and the intrinsic of chemical properties of selected polymers will
be produced on nanoscale CdS by encapsulated within polymer and attached on
polymer matrices. The combination of CdS nanoparticles and the polymer have
developed a CdS/polymer nanocomposite with a high dielectric constant and
improved physicochemical properties.
1.1
Background of Study
Many researchers have developed methods for synthesis of high dielectric
properties polymer nanocomposites. The first approach process for enhancing the
dielectric constant of a polymer nanocomposites is to disperse a high dielectric
constant insulating ceramic (ferroelectric materials), namely nanoceramic using
barium titanate (BaTiO3) (Devaraju et al., 2005; Pant et al., 2006), lead titanate
(PbTiO3), lead zirconate titanate (PZT) (Dong et al., 2006), strontium titanate
(SrTiO3) (Prijamboedi et al., 2005), into polymers. In order to obtain a high value of
5
dielectric constant, large amount of the filler has to be loaded into polymer matrix,
resulting in loss of flexibility and inhomogeneous nanocomposites.
The second approach to obtain high dielectric constant
polymer
nanocomposites was achieved by dispersing conductive fillers into polymers. The
common conductive materials used to produce the polymer nanocomposites are
carbon nano-fibers, metals such as silver (Ag) nanoparticles (Lu et al., 2006),
aluminium (Al) and nanoparticle aluminium (Xu and Wong, 2007), cadmium oxide
(CdO) (Pant et al., 2006), zink sulfide (ZnS) (Ghosh et al., 2005b), polymers such as
polyaniline (PANI) (Lu et al., 2007), and organic acids such as sulfamic acid
(Ameen et al., 2007) and copper pthalocyanine. The polymer nanocomposites have
successfully increased the dielectric constant of the polymer. The increased dielectric
constant observed in such composites arises from conducting particles isolated by
very thin dielectric layers to form micro-capacitors. However, the dielectric loss is
very high and difficult to control, because the particles can easily form a conductive
path in the composite as the filler concentration nears the percolation threshold.
The third approach to increase dielectric properties of polymer was to utilize
an inorganic and organic materials such as, titanium oxide (TiO2) (Li et al., 2006;
Yang and Kofinas, 2007; Mo et al., 2008; Dey et al., 2004), aluminium oxide
(Al2O3) (Li et al., 2007), clay (Zhang et al., 2005), zinc oxide (ZnO2) (Hong et al.,
2005) and also other polymers such as poly(trimethylene terephtalate) (PTT)
(Kalakkunnath and Kalika, 2006). The results showed that the dielectric constant of
the polymer did not increase significantly.
The challenge to produce dielectric polymer nanocomposites relies on the
ability to manipulate the fraction, characteristic length, and arrangement of dielectric
component inside the engineered nanocomposites. No systematic studies to date have
been published on the influence of effective volume fraction, characteristic length,
and arrangement of dielectric components on the effective dielectric constant of the
composites. This is due the intrinsic incompatibility between inorganic particles and
organic matrices. Most of the previous studies have utilized the conventional method
of blending a high dielectric constant material into polymer, which has no real
6
control on particles size and distribution within the polymer matrix (Yang and
Kofinas, 2007).
The study, synthesis and characterization of semiconductor nanoparticles
have attracted intense research lately owing to the unique chemical and physical
properties of the nanoparticles and also the vast potential for practical application of
the composite system incorporating the nanoparticles. Semiconductor nanoparticles
have physical and chemical properties that may differ significantly from those of the
bulk material. Such deviations are attributed to the small particle size and the
accompanying surface structure effects. By controlling the particle size and surface
structures of the semiconductor materials, electronic, optical, magnetic, mechanical,
and chemical properties can be modified to suit a wide range of device application in
various fields (Zhao et al., 2001).
Semiconductor nanoparticles have interesting applications in electronic,
optical, electro-optical devices, catalysis (Yanagida et al., 1990; Graetzel and
Graetzel, 1979), and optics (Wang, 1991). More particularly, cadmium sulfide (CdS)
nanoparticles have made a great impact in applications including pigments, battery,
optoelectronic devices, photocatalyst (Yanagida et al., 1990; Graetzel et al., 1979;
Hirai et al., 2002a, 2001, Hirai and Bando, 2005 and Yang, 2005), photosensitive
matrices, and resistor for light detecting laser. Dielectric properties of CdS
nanoparticles and bulk CdS have also been studied and reported (Zhou, 2003; Tiwari
and Tiwari, 2006).
Polymers as matrices have several advantages for producing CdS
nanoparticles and CdS/polymer nanocomposites. On the other hand, polymers with a
type of spatial conformation can be used as template to make nanoparticles in
solution with narrow size distribution and uniform confinement throughout their
periodic micro-domain. Besides that, polymers are able to achieve surface
passivation, prevent particles from agglomeration and maintain the particles degree
of dispersion of particle, which are well-known for their ability in controlling the
particle size and size distribution effectively (Yang et al., 2003).
7
Based on these above explanations and reasons, studies on the synthesis,
characterization, and elucidation of the dielectric and electrical properties of
CdS/polymer nanocomposites are undertaken to develop a high dielectric constant
material. The CdS/polymer nanocomposites were synthesized by combining several
methods i.e. in-situ polymerization, ion exchange and precipitation. The polymer
matrices employed are poly(styrene-divinylbenzene) [P(S-DVB)], poly(methacrylic
acid-ethyleneglycol dimethacrylic
acid)
[P(MAA-EGDMA)]
and
sulfonated
poly(styrene-divinylbenzene) [SO3H-P(S-DVB)].
1.2
Design and Strategy of Research
The synthesis strategy for preparation of high dielectric constant
CdS/polymer
nanocomposites
with
selected
polymer
matrices
is
shown
schematically in Figure 1.2. In this study, the CdS/polymer nanocomposites were
synthesized using three kinds of polymer i.e. P(S-DVB), P(MAA-EGDMA) and
SO3H-P(S-DVB).
Polystyrene and polymethacrylic acid are common polymers used in
preparation of encapsulated metal or metal oxide and solid phase extraction,
respectively. The choice of the polymers is based on their different properties and
degree of polarity. Among the polymers employed in the present study polystyrene
has no functional group, polymethacrylic acid has a weak acid functional group (COOH), while sulfonated-polystyrene has a strong acid functional group (-SO3H).
Based on the different chemical properties of the polymers, hence different
procedure of synthesis was applied in order to disperse CdS nanoparticles on the
polymer surface or encapsulated within the polymer matrix.
8
Nano/micron polymer matrix
CdS nanoparticles
Intramolecular interaction
Intermolecular interaction
CdS/polymer nanocomposites with high dielectric constant
Figure
1.2:
Schematic
representation
of the
synthesis
of CdS/polymer
nanocomposites
In
this
study,
CdS/poly(styrene-divinylbenzene)
[CdS/P(S-DVB)]
nanocomposites were synthesized by in-situ polymerization in miniemulsion system.
First, the CdS nanoparticles were prepared by reverse micelles in miniemulsion
system using CTABr as the surfactant, styrene-divinyl benzene as the oil-phase, 2propanol as the co-stabilizer, and also water. While, the CdS/poly(methacrylic acidethyleneglycol dimethacrylic acid) [CdS/P(MAA-EGDMA)] nanocomposites were
synthesized by combination of three methods i.e. in-situ polymerization to produce
P(MAA-EGDMA) nanoparticles, ion exchange and precipitation process to produce
CdS/P(MAA-EGDMA)
poly(styrene-divinyl
nanocomposites, respectively. The CdS/sulfonatedbenzene)
[CdS/SO3-P(S-DVB)]
nanocomposite
were
synthesized by combining four methods i.e. in-situ polymerization to produce P(SDVB) nanoparticles, sulfonation reaction with fuming sulfuric acid (H2S2O7) to
9
produce SO3H-P(S-DVB), ion exchange and precipitation processes to produce
CdS/SO3-P(S-DVB) nanocomposites, respectively.
1.3
Objective of The Study
The main goals of this research are to synthesize, characterize and evaluate the
dielectric and electrical properties of the CdS/polymer nanocomposites.
The objectives of the research are:
1.
To synthesize and characterize CdS nanoparticles prepared by reverse
micelles in miniemulsion system using monomer as the oil-phase
2.
To synthesize and characterize pure P(S-DVB) and P(MAAEGDMA) prepared by in-situ polymerization in miniemulsion system.
3.
To synthesize and characterize CdS/P(S-DVB) nanocomposites
prepared by in-situ polymerization in miniemulsion system using
monomer as the oil-phase
4.
To
synthesize
and
characterize
CdS/P(MAA-EGDMA)
nanocomposites prepared by ion exchange and precipitation
processes.
5.
To synthesize and characterize CdS/SO3-P(S-DVB) nanocomposites
prepared by sulfonation reaction, ion exchange and precipitation
processes.
6.
To evaluate and to compare the dielectric properties, electrical
properties, thermal properties and thermal conductivity of these
CdS/polymer nanocomposites.
1.4
Scope of The Study
The scope of the study is focused on the following aspects i.e. synthesis of
CdS nanoparticles, synthesis of pure P(S-DVB) and P(MAA-EGDMA), preparation
10
of
CdS/P(S-DVB)
nanocomposites,
preparation
of
CdS/P(MAA-EGDMA)
nanocomposites, preparation of CdS/SO3-P(S-DVB) nanocomposites, and evaluation
of dielectric properties, electrical properties, thermal properties and thermal
conductivity.
Table 1.1: Outline of results and discussion
Outline of results and discussion
CdS/P(S-DVB) nanocomposites
•
Section in chapter
4.1
Optimization of CdS synthesis and synthesis of
CdS/P(S-DVB)
•
Physicochemical properties of CdS/P(S-DVB)
•
Dielectric properties of CdS/P(S-DVB)
CdS/P(MAA-EGDMA) nanocomposites
•
Physicochemical properties of CdS/P(MAA-EGDMA)
•
Dielectric properties of CdS/P(MAA-EGDMA)
CdS/SO3-P(S-DVB) nanocomposites
•
Physicochemical properties of CdS/SO3HP(S-DVB)
•
Dielectric properties of CdS/SO3-P(S-DVB)
Comparison of physicochemical and electrical
4.2
4.3
4.4
properties of CdS/polymer nanocomposites
This research involves the synthesis of CdS nanoparticles by reverse micelles
in miniemulsion using styrene-divinyl benzene as oil-phase, followed by the
synthesis and characterization of CdS/P(S-DVB) nanocomposites via in-situ
polymerization in the miniemulsion system. The miniemulsion system is prepared
using CTABr as surfactant, monomer as oil-phase, 2-propanol as co-stabilizer and
water. The formation, stabilization and particle size of CdS nanoparticles in the
miniemulsion system were characterized using UV – Vis spectroscopy. Outcomes of
this work are reported in Chapter 4. Chapter 4 also reports the synthesis and
characterization of dielectric properties of CdS/P(MAA-EGDMA) nanocomposites
11
and observation on the preparation and characterization of dielectric properties of
CdS/SO3-P(S-DVB) nanocomposites. The outline of the results and discussions
section are shown in Table. 1.1.
Chapter 5 presents the conclusions of the synthesis, characterization and
dielectric properties of all the nanocomposites of the prepared CdS/polymer
nanocomposites.
12
CHAPTER 2
LITERATURE REVIEW
2.1
Dielectric Properties and Dielectric Constant
Dielectric constant (K), also known as relative permittivity (εr), is the ratio of
the permittivity of a material (εr) to that of vacuum (εo). The relationship between the
capacitance (C) and dielectric constant is given by Equation 2.1:
C = Q/V (coulomb/volt) = εr εo A/d (Farad)
Equation (2.1)
Where εo is the dielectric constant of the free space (8.854 x 10-12 F/m); A, the area
of the electrical conductor; d, the thickness of material; and εr, the dielectric constant
of material layer. In general, all materials with dielectric constant higher than 4.0 (or
the dielectric constant of SiO2) are called high dielectric, while those with dielectric
constant lower than 4.0 are called low dielectric and those with dielectric below 2.0
are called ultra low dielectric.
Any material that contains polar components is represented as dipoles (e.g.
polar chemical bond). The dipoles align with an external electric field, adding the
electric field of every dipole of the external field. As a result, a capacitor with a
dielectric medium other than vacuum will hold more electric charge at certain
applied voltage. In other words, its capacitance and dielectric constant will be higher.
13
Plate area A
+ + + + + + + + + + + + + +
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ d : thickness
Figure 2.1: Schematic representation of capacitor
There are several molecular mechanism associated with these polarizations:
(a) Electronic polarization (displacement of electron). The polarization is the result
of dipole moments induced by electric field.
(b) Atomic polarization or distortion polarization (displacement of ions or atoms).
This polarization is the result from an unsymmetrical sharing of electrons when
two different atoms combine to form a molecule.
(c) Dipole or orientation polarization (displacement or reorientation) of polar
molecules. Some molecules have a permanent dipole that can react with an
external electric field and for dipole or orientation polarization.
(d) Space – charge polarization (long range charge migration). The polarization is
different from the previous three types of polarization because it is not due to the
charges that are locally bound. Instead, it is produced by charge carriers, such as
free ions, that can migrate for some distance throughout the dielectric. Space
charge transfer polarization is extrinsic to any crystal lattice and arises as a result
of charges on the surface, grain and phase boundaries. The polarization produces
the highest dielectric constant value.
The schematic representations of these various polarizations are shown in Figure 2.2
14
Insulator
(a) Electronic polarization
occurs in all insulators
Atomic
(b) Molecular polarization occurs
in all insulating molecules,
such as oils, polymer, water
Dipole
(c) Ionic polarization occurs in all ionic
(d) Space charge polarization
solid, such as NaCl, MgO
Figure 2.2: Schematic representation of types of polarizations
The schematic representations of these frequencies dependence contributing
to the polarizability are shown at Figure 2.3.
15
Molecular Polarization
Ionic Polarization
Electronic Polarization
Figure 2.3: Frequency dependence of several contributions to the polarizability
2.2
Embedded Capacitor
Electronic technologies that allow for a reduction in size, shape, weight, and
cost while improving functionality and performance are highly desired for military
and commercial applications, which includes telecommunications, network systems,
automotives, and computer electronic devices. Electronic systems are composed of
active and passive components. Passive components include resistor, capacitor and
inductor. The architecture of passive component is one with room for improvement
due to the large and growing number of passive components in today’s increasingly
functional devices. Today, more than 80 % of the area of printed wiring board
(PWB) is occupied by passive components. Discrete passives, especially capacitors,
have already become the major barrier of the electronic system miniaturization.
Therefore, the development of embedded capacitor has been an area of significant
activities because capacitors are used as multiple functions, such as decoupling, bypassing, filtering and timing capacitor (Rao and Wong, 2004).
16
Therefore, integration of passive component inside the substrate, so namely
embedded passives, is the key for the next generation electronics. Embedded
passives offer many advantages over discrete component. Discrete surface mounted
passives require large board area and can introduce additional parasitic into the
system, which may limit system performance. With integral embedded passives, the
size of the board can be reduced, especially if integrated circuit (IC) chips can be
place above the embedded passives component. Also, embedding these devices leave
free board space for mounting component that cannot be integrated. Integral passives
are defined as functional elements either embedded in or incorporated on the surface
of an interconnecting substrate. With increased production, emphasis towards
efficient electronic packaging, integral embedded passives technology may satisfy
such demand. The several main advantages of embedded passives component are (a)
no separate interconnect to the substrate, (b) improved electrical performance, (c)
lower cost and (d) ease of processing. For producing the embedded passives
component a material with high dielectric constant are required because of the
development of advanced decoupling capacitor in order to reduce the power
distribution (Rao and Wong, 2004).
2.3
Design of High Dielectric Property Materials
Table 2.1 shows the dielectric constant of several substances including
polymer, ceramic, metal, metal oxide, etc. Polymers generally have low dielectric
constant and it is difficult for them to meet the capacitor requirements. On the other
hand, ceramic or metal have high dielectric constant but requires high processing
temperatures. It is already known that polymer has a low dielectric properties
compared to those of inorganic materials. Due to the processibility of polymer, it is
of interest to increase its dielectric properties by combining with inorganic materials
by an appropriate approach to obtain polymer nanocomposites.
Nanomaterials combine one or more separate components in order to obtain
the best properties of each component (composite). In nanocomposite, nanoparticles
17
act as fillers in a matrix, usually polymer matrix. Polymer nanocomposites combine
these two concepts, i.e., composites and nanometer-size materials.
Table 2.1: Dielectric constant of several materials (Sears et al., 1982)
Material
Inorganic material
Polymer
Dielectric constant
Potassium tantalate niobate
6000
Barium titanate
4000
Potassium niobate
700
Rochelle salt
170
TiO2 rutile
50.0
Silicon
11.8
GaAs
10.9
Magnesium titanate
10.0
Magnesium oxide
9.0
Epoxy
4.0
Fused silica
4.0
Nylon 6,6
4.0
PVC
3.5
Polystyrene
2.5
Polyethylene
2.3
Polymer nanocomposites (PNCs) are materials composed of a polymeric host
in which particles (buckyballs, nanotubes, semiconductor of metallic nanocrystals,
clays) of nanoscale dimensions are incorporated. Because the particles and the
polymers posses different functionalities, the range of applications is diverse.
Applications include membranes, coatings, solar cells, electronic devices, sensing,
medicine and structural applications, such as automobile parts. PNCs which contain
layered silicates (clays) have been successfully used for different thermal and
mechanical applications. Conjugated polymers mixed with various semiconducting
particles have received considerable attention because of applications involving solar
cells. PNCs are especially interesting because they exhibit properties that are
superior to conventional composites with relatively low concentrations of the
18
inorganic component. The challenges are associated with controlling the composition
and spatial distribution of the inorganic component within the polymer. Prediction of
the properties is often challenging because the result of a range of complex and
collective intermolecular interactions.
There has been enormous interest in the commercialization of polymer
nanocomposites for a variety of applications, and several of these applications will be
successful in the near future. Mineral fillers, metals, and fibers have been added to
polymer material for decades to form composites. Compared to neat resins, these
composites have a number of improved properties including tensile strength, heat
distortion, temperature, and modulus. Thus, for structural applications, composites
have become very popular and are sold in billion pounds quantities. These filled
thermoplastics are sold in even larger volumes than neat thermoplastics.
Furthermore, the volume of fillers sold is roughly equal to the volume of
thermoplastic resin sold. Clearly, the idea of adding fillers to thermoplastics and
thermosets to improve properties, and in some cases decrease costs, has been very
successful for many years. More recently, with advances in synthetic techniques and
the ability to readily characterize materials on an atomic scale has lead to interest in
nanometer-size materials. Since nanometer-size grains, fibers and plates have
dramatically increased surface area compared to their conventional-size materials,
the chemistry of these nanosized materials is altered compared to conventional
materials.
Polymer nanocomposites were intensively studied and synthesized to produce
high dielectric constant materials. The materials combine the high dielectric constant
of ceramics, metals, metal oxides, semiconductors, inorganic and organic substances
and the ease to process ability and flexibility of polymers.
One of the main advantages of polymer nanocomposites is that their dielectric
properties can be changed over a wide range by choosing the shape, size and
fractions of the filler into polymer matrices. Dispersion of inorganic phases into
polymer matrix is an issue of both scientific and technological interest. The unique
properties and processing flexibility of these polymer nanocomposites have found a
19
number of technological applications. Many researchers have studied and developed
methods for the synthesis of high dielectric constant polymer nanocomposites. (Dang
et al., 2007; Devaraju et al. 2005).
2.3.1 Overview of Dielectric Properties of Polymer Nanocomposites
Many researchers have studied and synthesized the polymer/ceramic
nanocomposites using common ceramic materials such as barium titanate (BaTiO3),
lead zirconate titanate (PZT) and lead titanate (PbTiO3) (Dang et al., 2007). Devaraju
et al. (2005) have studied and synthesized BaTiO3/polyimide (BaTiO3/PI)
nanocomposites film by in-situ imidization in n-methyl pyrrolidinone as the solvent.
The result of the study showed high dielectric constant nanocomposites at various
BaTiO3 content (in the range 40 – 90 wt.%) and frequency 1 kHz to 1 MHz. The
single layer capacitors were formed and a dielectric constant as high as 125 have
been attained at a frequency of 1 kHz (about 117 at 1 MHz), with a 90 wt.% loaded
film. The nanocomposites showed high stability dielectric constant in the frequency
range of 1 kHz to 1 MHz. Pant et al. (2006) have investigated BaTiO3/polyaniline
(BaTiO3/PANI) and BaTiO3/maleic acid resin (BaTiO3/Mac). Both nanocomposites
were synthesized by mechanical grinding process. Dielectric properties of both
nanocomposites were investigated for microwave studies in the frequency range of
8.2 – 12.4 GHz. They observed that pure BaTiO3 showed quite high dielectric
constant value in the range of 100 – 125. However, it is found that on mixing with 10
wt.% of PANI the dielectric constant values of BaTiO3/PANI drastically change and
reduce to 22 over the whole frequency range. Different result was obtained for
BaTiO3/Maleic acid resin nanocomposites at the same percentage and frequency in
which the dielectric constant of the nanocomposites was at about 70 – 80.
Dong et al. (2006) have synthesized polyvinyl-butyral/lead zirconate titanates
(PVB/PZT) by a simple mixing method. The highest dielectric constant of 155 can
be achieved at a PVB volume fraction of 0.15 at 100 Hz. Rao et al. (2002)
incorporated lead magnesium niobate – lead titanate/BaTiO3 (PMN-PT/ BaTiO3) into
20
epoxy matrix. The PMN-PT/BaTiO3/epoxy nanocomposites with 70 wt.% ceramic
loading and showed a dielectric constant value of 110 at 1 kHz.
Several researchers have also studied and synthesized the polymer
nanocomposites using many inorganic and organic materials as filler such as, nano
aluminium, conducting polymer (polyaniline) and sulfamic acid. Xu and Wong
(2007)
have
reported
dielectric
properties
of
aluminum/epoxy
and
nanoaluminum/epoxy composite with coupling and non-coupling treatment using γglycidoxypropyl-trimethoxysilane as sylane coupling agent. The polymer matrix
used was diglycidyl ether of bisphenol-A with methyl-hexahydrophtalic anhydride as
hardener agent. The dielectric constant before coupling treatment of 70 wt.%
microaluminum (3 micrometer)/epoxy and 40 wt.% nanoaluminum (100 nm)/epoxy
which were measured at 10 kHz were 82.1 and 52.3 respectively. After the addition
of coupling agent, the dielectric constants of both nanocomposites increased to 96
and 61, respectively. The improvement of dielectric constant came from the better
dispersion of coupling agent treated aluminum particles in the polymer matrix.
Ghosh, et al., (2005b) have studied and synthesized zinc sulfide (ZnS)
nanobelt in pores of polyvinylalcohol (PVA) by chemical bath deposition. The
dielectric constant of the ZnS/PVA obtained lies in the range of 30 – 130 at 100 –
1000 kHz, and higher than the dielectric constant of pure PVA, which was about 28.
Lu et al. (2007) have studied and synthesized conducting polyaniline/epoxy
resin (PANI/epoxy resin). The nanocomposites were prepared via in-situ
polymerization of mixed aniline salt and epoxy (using EPON 828) matrix. Dielectric
properties of PANI/epoxy resin nanocomposites with different PANI contents at 10
kHz have been examined. The dielectric constant at 8 wt.%, 15 wt.%, 20 wt.% and
25 wt.% PANI contents are 10, 192, 916 and 2980, respectively. The results also
suggested that the in-situ polymerization was useful to achieve good dispersion and
high compatibility of PANI with the epoxy matrix in which it was effective to
eliminate agglomeration surrounded by insulating matrix and the formation of fine
network led to enhanced dielectric properties of the nanocomposites.
21
Dielectric properties of polyarylene ether nitriles/titanium oxide (PEN/TiO2)
hybrid films prepared by sol-gel method have been studied by Li et al. (2006). The
results showed that the dielectric constant was slightly increased with increasing
content of TiO2 in the polymer matrix. Dielectric constant of 4 wt.% TiO2/PEN at
temperature of 200 oC and 250 oC and 1 kHz were 3.7 and 4.0, respectively. Yang
and Kofinas (2007) have reported the dielectric properties of TiO2/sulfonated
styrene-b-(ethylene-ran-butylene)-b-styrene block copolymers (TiO2/S-SEBS) by
simple mixing and gradualy hydrolyzed method. The dielectric constant of uncrosslinked pure S-SEBS at 1, 100, 1,000 and 10,000 Hz were approximately
165,000, 18,000, 10,000 and 6,000 respectively. Dielectric properties of the
nanocomposites were decreased with dispersion of TiO2 into polymer matrix and also
the dielectric constant of 0.26 to 6.4 wt.% TiO2/S-SEBS nanocomposites lies in the
range of 2 – 3.2 at 100 Hz.
Li et al. (2007) have reported dielectric properties of nano-aluminum
oxide/polyimide
(nano-Al2O3/PI)
nanocomposites
prepared
by
in-situ
polymerization. The result obtained showed that the dielectric constant of pure
polyimide and 20 wt.% nano-Al2O3/PI nanocomposites at 100 Hz were 2.4 and 3.2,
respectively. This finding was clearly affected by Al2O3 filler because pure alumina
has higher dielectric constant compared to PI. The number of polar group increased
due to the incorporation of filler into the polymer matrix and it grew bigger with the
increase of the filler concentration which caused an extra polarization under an
electric field.
Zhang et al. (2005) have studied the dielectric properties clay/polyimide
(clay/PI) prepared by thermal imidization from the polyimide precursors PMDAODA and organo-modified clay in N-N-dimethylacetamide. The results displayed the
dielectric constant decreased with the increase of the clay content and the film
nanocomposites showed low dielectric constant when the clay content was over 1
wt.%. The dielectric constant of pure PI and 10 wt.% clay/PI nanocomposites at 1
kHz and temperature 50 oC were 3.3 and 3.0, respectively.
22
Hong et al. (2005) have investigated the dielectric properties of micron-zinc
oxide/low density polyethylene (micron-ZnO/LDPE) and nano-zinc oxide (49
nm)/low density polyethylene (nano-ZnO/LDPE) prepared by melt mixing with
either homogeneous or controlled inhomogeneous dispersion of ZnO. The dielectric
constant of homogeneous 14 wt.% micron-ZnO/LDPE and inhomogeneous nanoZnO/LDPE at 10 kHz were 5 and 6 respectively. The result concluded that the small
particles size of the ZnO nanoparticles did not affect the dielectric constant of the
nanocomposites. Therefore, the interfaces between ZnO and LDPE did not appear to
contribute to dielectric properties of the nanocomposites.
Scorcik
et
al.
(2005)
have
reported
dielectric
properties
of
polymethylmethacrylate (PMMA) solution in acetone doped with 20 wt.% of
diphenyl sulfoxide (DS) prepared by three consecutive steps, namely sputtering of
bottom, spinning film and sputtering of upper. The result showed that the presence of
DS dopant will slightly increase the dielectric properties of the nanocomposites. The
dielectric constant of 20 wt.% DS/PMMA and PMMA which were measured at 1
kHz and temperature of 30 oC were 7.0 and 3.0, respectively. Podgrabinski et al.
(2006) also studied dielectric properties of doped polystyrene (PS) and
polymethylmethacrylate (PMMA) films with diphenyl sulfoxide (DS). These
composites were prepared from solutions using spin-coating method. The results
showed that the DS doping leads to an increase of dielectric constant of the PS and
PMMA films. The dielectric constant of 20 wt.% DS/PMMA and DS/PS at 40 oC
and 1 kHz were 5.5 and 2.5, respectively.
Tanwar
et
al.
(2006)
studied
dielectric
properties
of
doped
polymethylmethacrylate (PMMA) with iodine (I2), FeCl3, and benzoic acid prepared
by solution cast method. Dielectric properties of those nanocomposites were
increased by a little with increasing content of doped materials. The dielectric
constant of pure PMMA, 8 wt.% I2/PMMA, 20 wt.% benzoic acid/PMMA, and 10
wt.% FeCl3/PMMA at 8.92 GHz were 2.47, 2.88, 3.00 and 2.50, respectively. The
different results were attributed to the different properties of the dopant materials i.e.
iodine an acceptor dopant, benzoic acid has a phenyl group which can function as an
23
electron withdrawing group and FeCl3 is also an acceptor dopant and selected for the
dielectric studies.
Table 2.2: Dielectric constant and method of synthesis of several polymer
nanocomposites
Polymer/nanocomposites
Dielectric constant
Method of synthesis
(Reference)
90 wt.% BaTiO3/PI
125 (1 kHz)
in situ imidization
(Devaraju et al., 2005)
10 wt.% PANI/BaTiO3
22 (8.2 – 12.4 GHz)
mechanically grinding
70 – 80 (8.2 – 12.4 GHz)
mechanically grinding
155 (100 Hz)
simply mixing method
110 (1 kHz)
simply mixing method
(Pant et al., 2006)
10
wt.%
maleic
resin/BaTiO3
(Pant et al., 2006)
15 wt.% PVB/PZT
(Dong et al., 2006)
70 wt.% PMN-PT/
BaTiO3/epoxy
(Rao et al., 2001)
70 wt.% microaluminum
82.1 (10 kHz)
simply mixing
(3 µm)/epoxy
(Xu and Wong, 2006)
ZnS/PVA
(Ghost et al. 2005b)
15 wt.% PANI/
30 – 130
chemical
(100 – 1000 kHz)
deposition
192 (10 kHz)
in-situ polymerization
epoxy resin
(Lu et al., 2007)
3
wt.% TiO2/PEN
( Li et al.,2006)
4.0 (250 oC, 1 kHz)
bath
sol-gel
24
Table 2.2 (continue): Dielectric constant and method of synthesis of several
polymer nanocomposites
Polymer/nanocomposites
Dielectric constant
Method of synthesis
(Reference)
0.26 to 6.4 wt.%
2 – 3.2 (100 Hz)
simply
TiO2/S-SEBS
mixing
and
gradually hydrolyzed
(Kofinas, 2007)
20 wt.% nano-Al2O3/PI
3.2 (100 Hz)
in-situ polymerization
(Li et al., 2007)
3.0 (1 kHz,50 oC)
10 wt.% clay/PI
thermal imidization
(Zhang et al., 2005)
14 wt.% micron-
5 (10 kHz)
melt mixing
ZnO/LDPE
(Hong et al., 2005)
7.0 (1 kHz, 30 oC)
20 wt.% DS/PMMA
sputtering
(Scorcik et al., 2005)
spinning film
8 wt.% I2/PMMA
2.88 (8.92 GHz)
solution cast
(Tanwar et al., 2005)
20
wt.%
and
solution cast
benzoic
3.00 (8.92 GHz)
solution cast
acid/PMMA
(Tanwar et al., 2005)
10 wt.% Fe Cl3/PMMA
2.50 (8.92 GHz)
(Tanwar et al., 2005)
5.5 (40 oC, 1 kHz)
20 wt.% DS/PMMA
spin-coating method
o
20 wt.% DS/PS
2.5 (40 C, 1 kHz)
spin-coating method
(Podgrabinski et al., 2006)
In previous studies as mentioned above, the polymer/metal or conductive
nanocomposites have demonstrated that the dielectric constant of the polymer
significantly
increased.
One
disadvantage
of
polymer/ceramic
and
polymer/conducting nanocomposites is that in order to obtain a high dielectric
constant, large amount of the filler has to be loaded into polymer matrix, resulting in
loss of flexibility and inhomogeneous nanocomposites. The increased dielectric
25
constant observed in such nanocomposites arises from conducting particles isolated
by very thin dielectric layers to form micro-capacitors. However, the dielectric loss is
very high and difficult to control, because the particles can easily form a conductive
path in the nanocomposites as the filler concentration nears the percolation threshold.
On the other hand, several polymer nanocomposites have demonstrated that the
dielectric constant of the polymers were increased by a little or was not affected at
all. The summary of dielectric constants and methods of synthesis of polymer
nanocomposites are shown in Table 2.2.
2.3.2 CdS/polymer Nanocomposites
The dielectric properties of CdS nanoparticles, bulk CdS, and CdS/polymer
nanocomposites have also been investigated and reported. Zhou (2003) studied
dielectric properties of conventional CdS powder, cubic and hexagonal CdS
nanoparticles at various frequencies. The CdS nanoparticles were prepared by
sonochemistry, while the conventional CdS powder was prepared by ion exchange.
The results showed that the CdS nanoparticles have higher dielectric constant
compared to conventional CdS powder measured at low frequency. The dielectric
constant of cubic CdS nanoparticles and hexagonal conventional CdS powder at 100
Hz are 156.34 and 10.7, respectively. The high dielectric constant of CdS
nanoparticles was suggested to appear from interfaces with a large volume fraction in
the nanoparticles. Also the grain size of the sample has a great influence on the
dielectric behaviour of CdS nanoparticles, which was explained via the theory of
dielectric polarization, including space-charge polarization.
Ghosh et al. (2005a) studied the dielectric properties of CdS nanoparticles
growth at porous poly(vinyl alcohol) (PVA) matrix by chemical bath process. The
dielectric constant of CdS/PVA nanocomposites was examined at various
frequencies and dispersibility. The dielectric constants on dispersibility at 0.8, 0.6,
and 0.3 lies in the range 120 – 250. El-Tantawi et al. (2004) also studied dielectric
properties of CdS/PVA nanoconducting composite prepared by organosol technique.
26
It was shown that the dielectric constant of 20 wt.% and 40 wt.% CdS/PVA at 1 kHz
were 10 and 20, respectively. Mondal et al. (2007) have investigated optical and
dielectric properties of CdS nanocomposites embedded in PVA using simple
chemical deposition bath. In their report, it was suggested that the dielectric
properties of CdS and CdS/PVA were dependent on the particle size of CdS,
interface (inter-intraparticle) interaction, and disperse stability of CdS nanoparticle at
the polymer matrix. The summary of dielectric constant and methods of synthesis of
CdS nanoparticles and CdS/polymer nanocomposites are tabulated in Table 2.3
Table 2.3: Dielectric constant and methods of synthesis of CdS nanoparticle and
CdS/polymer nanocomposites
Dielectric
Method of
constant
synthesis
CdS nanoparticles
156.34 (100 Hz)
sonochemistry
Zhou, 2003
Conventional CdS
10.7 (100 Hz)
ion exchange
Zhou, 2003
120 – 250
chemical bath
Ghost et al., 2005b
Material
CdS/PVA
Reference
process
40 wt.% CdS/PVA
2.4
20 (1 kHz)
organosol
El-Tantawi et al.
technique
(2004)
Cadmium Sulfide as Semiconductor Nanoparticles
Nanoparticles are generally categorized as the class of materials that fall
between the molecular and bulk solid limits, with an average size between 1 – 50 nm.
Semiconductor nanoparticles, referred to as quantum dots, with dimensions on the
order of nanometers have been the subject of intense research in the past two
decades, due to their unique optical, electronic, physical and chemical properties
(Alivisatos, 1996; Wang and Herron, 1991).
27
Compared to corresponding bulk semiconductor material, semiconductor
nanoparticles have two distinguishing characteristics, both related to the size of
individual nanoparticles. First, nanoparticles have a large ratio of surface atoms to
those located in the crystal lattice, which make the physical and chemical properties
of semiconductor nanoparticles particularly sensitive to the surface structure. Second,
nanoparticles have size dependent optical properties, namely “quantum size effect”.
As dimension of the particle decreased the bandgap energy of the nanoparticles
increases. “Quantum size effect”, the bandgap energy of semiconductor nanoparticles
increases as their size decreases. The most notable size effect is the significant blue
shift of electronic absorption and emission spectrum with decrease in size. Brus has
developed a popular effective mass model that relates particle size to the bandgap
energy of semiconductor nanoparticles (Chestnoy et al., 1986; Brus, 1986;
Steigerwald and Brus, 1990).
As illustrated in Figure 2.4, the quantum size effect causes the continuous band
of the crystal to split into discrete, quantized levels and the bandgap of
semiconductor nanoparticles to increase. As the nanoparticles decreases in size, the
bandgap increase, approaching the energy difference between LUMO and HOMO
for the individual molecule. Due to the large percentage of surface atoms for
nanoparticles, even a small amount of surface defects can introduce a high density of
“surface trap”, shown in Figure 2.4. Therefore, the emission of semiconductor
nanoparticles is often used as a sensitive probe to reveal their surface structure.
The bandgap of semiconductor nanoparticles is predicted to be inversely
proportional to the particle size (radius, R2), shown in Equation 2.2 (Brus, 1986;
Brus, 1984).
Eg (quantum dot) = Eg (bulk) + (h2) ( 1 + 1) – 1.8e2
8R2 mc mh 4πεoεR
Equation (2.2)
Where Eg is the bandgap energy of the corresponding bulk crystal, mc is the
effective mass of the electron in the crystal, mh is the effective mass of the hole in the
28
crystal, and is the electric constant of the crystal. The middle term on the right side
on the equation is a particle-in-a-box-like term for the exciton, while the third term
represents the electron-hole pair coloumb attraction, mediated by the crystal.
Equation 2.2 is based on two assumptions. One is that the nanoparticles are in
spherical shapes; the other is that the effective mass of charge carriers and the
dielectric constant of the crystal are constant as a function of particle size.
BULK
NANOCRYSTAL
MOLECULE
Conduction band
LUMO
Energy
bandgap
HOMO
Valence band
Figure 2.4: Schematic changes in the density of states on going from a bulk crystal
to a nanocrystal to molecule/atom
For CdS nanoparticles, Brus equation (Equation 2.3) is used to estimate the
particle size where the bandgap energy is approximately equal to the band edge of
optical absorption spectra. The Brus model works well for estimating the size of
larger nanoparticles, but often shows significant deviation for very small particles.
Eg (R) = 2.43 + 2.446 – 0.3031
R2
R
Equation (2.3)
Nanoparticle semiconductors are currently under intense investigation because
of their enhanced photoreactivity and photocatalytic properties. Nanoparticles of CdS
29
are by far the most studied system among all the semiconductor nanoparticles as has
been used as many applications e.g. photocatalyst (Bard, 1980; Yanagida, et al.,
1990; Graetzel and Graetzel, 1979; Hirai, et al., 2002a, 2002b, 2001, 2006, Hirai and
Bando, 2005; Hirai and Ota, 2006; Yang, et al., 2005), paint industry, battery,
optoelectronic devices, photosensitive matrices, resistor for light detecting, laser,
electrochemical cell (Haram, et al., 2001; Hagfeldt and Graetzel, 1995), fluorescent
or luminescent labelling (Bruchez et al., 1998; Harruf et al., 2003), pharmaceutical
and medical, especially for drug delivery system (Lai et al., 2003), biosensing (Chen
et al., 2005), optics (Wang, 1991; Hines, 1998), and unique electronics (Henglein,
1989; Steigerwald and Brus, 1990; Wang, 1991).
2.4.1 Methods of Synthesis of Cadmium Sulfide Nanoparticles
A wide range of nanomaterials including metals, semiconductors, inorganic
oxides, and polymers have been produced using various methods and nanoreactor
systems. Many methods and nanoreactors have been developed and studied including
wide variety of techniques, such as micelles or reverse micelles (Bunker et al., 2004;
Dutta and Fendler, 2002; Hirai et al., 2000a, 2000b, 2001, 2002a, 2005; Harruf et al.,
2003; Zhao et al., 2001) or salt induced micelles (Zhao et al., 2001), aggregation
(Libert et al., 2003), liquid crystal (Dellinger and Braun, 2001), miniemulsion or
microemulsion (Agostiano et al., 2000; Khiew et al., 2003, 2004), microgels,
amphiphylic polymers or block copolymer (Dellinger and Braun, 2001), liposomes
(Graff et al., 2001), dendrimes (Crooks et al., 2001), thermally evaporated aerosol
(Shankar et al., 2001),
UV or microwave irradiation (Yang et al., 2005),
precipitation (Banarjee et al., 2000) have been used in the preparation of
semiconductor CdS nanoparticles.
Several researchers have used a variety of surfactants to prepare the
miniemulsion systems for preparation of CdS nanoparticles, such as sodium bis (2ethylhexyl) sulfosuccinate (AOT) (Hirai et al., 2001, 2002a), sodium dodecyl sulfate
(SDS), cetyl trimethyl ammonium bromide (CTABr) (Agostiano et al., 2000), tritonX series, nonionic surfactant (Khiew et al., 2004), sucrose ester (Khiew et al., 2003)
30
and polymerizable surfactant such as cetyl-p-vinylbenzyldimethylammonium
chloride (CVDAC) (Hirai et al., 2000c) and didecyl dimethylammonium
methacrylate.
Hirai and Bando (2005) synthesized CdS/aluminosilicate particle (AS-SH)
composites. The CdS nanoparticles were prepared in reverse micelles system using
sodium bis (2-ethylhexyl) sulfosuccinate (AOT), water and isooctane at Wo =
[H2O]/[AOT] = 6. Ni, Ge, and Zhang (2006) have been reported and synthesized of
CdS/polyacrylonitrile nanocomposites in an ethanol solution by γ-irradiation
technique at room temperature and atmospheric pressure. The XRD result showed
that the crystallinity of CdS was affected by several factors such as the amount of the
organic compound in the composites and the dose of irradiation.
Xu and Akins (2004) synthesized of CdS nanoparticles by reverse micellar in
microemulsion system and self-assembly into superlattice. The microemulsion
system was prepared using AOT-water-isooctane with Wo = [H2O]/[AOT] = 9,
Cd(OAT)2 as cadmium ion source and thioacetamide in DMF as H2S source. The
UV-Vis spectra of CdS nanoparticles displayed a blue-shifted band-edge when
compare to bulk CdS (< 5 micron) i.e. 489 nm vs. 540 nm, in line with expectation
associated with formation of quantum dots of semiconductor materials. But, the
baseline of the spectra is high which possibly signal the presence of CdS aggregates.
While, the TEM micrograph showed that the CdS nanoparticles are essentially
monodispersed with particles size about 13 nm and the two dimensional superlattice
hexagonally oriented with interparticles spacing of ca. 3 nm. The result also
concluded that the CdS nanoparticles are capped and surrounded by AOT surfactant.
Zhang et al. (2001) have described a controlled synthesis of CdS nanoparticles
using functionalized MCM-41 (Phe-MCM-41) by ion exchange reaction. The
absorption onset wavelength of CdS nanoparticles was close to 450 nm, which is
significantly blue-shifted relative to that compare bulk CdS. The TEM and XRD
analyses showed that the particles size of CdS nanoparticles were 2.5 and 3.6 nm,
respectively.
31
Hirai et al. (2000c) prepared CdS nanoparticles-polymer composites by direct
reverse
micelle
polymerization
using
cetyl-p-vinylbenzyldimethylammonium
chloride (CVDAC) as polymerize surfactant by photopolymerization method. The
result observed that the CdS nanoparticles were incorporated successfully into
polymer (PCVDAC) and retained their size and quantum size effect. The onset
wavelength UV – Vis spectra of CdS in miniemulsion at Wo = 1.5 and
CdS/PCVDAC in ethanol were observed significantly blue-shifted compared to bulk
CdS with particles size of 4.8 nm and 4.9 nm. The result concluded that the
photopolymerization of CdS with PCVDAC was effective in incorporating the CdS
nanoparticles into PCVDAC and also in maintaining their quantum size effect.
Table 2.4: Several CdS/polymer nanocomposites, synthesis method, applications and
property of interest
Material
Method of synthesis
(Reference)
Applications and
property interest
CdS/PVA
chemical
bath
process, dielectric and
(Ghosh et al.,2005a)
organosol technique
electrical properties
CdS/PVA
organosol technique
conducting properties
(El-Tantawy et al., 2004)
CdS/PEO
simply
mixing, optical properties
(Yang et al., 2003)
precipitation
CdS/PAA
precipitation,
(Chen et al., 2006)
polymerization
determination of protein
CdS/SBA-15
acid base reaction,
optical properties
(Xu et al., 2002)
ion exchange, calcination
CdS/Aluminosilicate
reverse micelles
Photocatalytic
γ-irradiation
optical properties
simply mixing
optical properties
in-situ quantitative
(Hirai and Bando, 2005)
CdS/PAN
(Ni et al., 2006)
CdS/polyphosphazene
(Olshasky and Allcock,
2007)
32
Nanometer-sized semiconductors of lead sulphide (PbS) and CdS prepared by
microemulsion using sucrose monoester (S-1670) as surfactant, 1-butanol as costabilizer, cyclohexane as oil-phase and water were studied by Khiew et al. (2003).
The TEM micrograph exhibited that the particle size of CdS and PbS were in the
range 2 – 7 nm and 3 – 8 nm, respectively. While, the particle size of the CdS and
PbS nanoparticles estimated from the absorpstion onset wavelength were 5.1 nm and
4.5 nm, respectively.
Yang et al. (2005) studied luminescence and photocatalytic properties of CdS
nanoparticles. The CdS nanoparticle was prepared by microwave irradiation from
cadmium chloride (CdCl2) solution and sodium sulfide (Na2S) solution. The particle
size of CdS nanoparticle obtained from TEM was about 10 nm. The red
photoluminescence of CdS nanoparticles also displayed a strong peak at 602 nm,
which may be attributed to the recombination of an electron trapped in a sulfur
vacancy with a hole in the valence band of CdS. The summary of several CdS
nanocomposites materials, method of synthesis, application and property interest
were tabulated in Table 2.4.
2.5
Miniemulsion Polymerization
A conventional emulsion polymerization is one of the most common
techniques employed for encapsulation of inorganic materials, such as titanium
dioxide, magnetite, silicon dioxide, etc. In this method, the principal locus for
particle nucleation is either in the aqueous phase or in the monomer swollen
micelles. The dominant site of particle nucleation depends on the degree of water
solubility of monomer(s) and the amount of surfactant used. The complexity of the
particle nucleation mechanism and the difficulties in controlling the dispersion
stability of inorganic particles in the aqueous phase during emulsification and
encapsulation polymerization are the biggest obstacles to success of the
encapsulation method and need to be investigated.
33
In contrast, the characteristic features of the miniemulsion polymerization
technique may present several advantages as an encapsulation method. Potential
advantages include the ability to control the droplet size, having particles directly
dispersed in the oil phase, the ability to nucleate all the droplets containing the
particles, and a faster rate of polymerization (Erdem et al., 2000a, 2000b).
2.5.1 Introduction
Polymeric dispersions are used in a wide variety of application such as
synthetic rubber, paints, adhesives, binder for non-woven fabrics, additive in paper
and textiles, leather treatment, impact modifiers for plastic matrices, additive for
construction materials and flocculants. They are also used in biomedical and
pharmaceutical applications such diagnostic tests and drug delivery systems. The
rapid increase of this industry is due to environmental concerns and governmental
regulations to substitute solvent-based systems by water borne products, as well as to
the fact that polymeric dispersions have unique properties that meet a wide range of
market needs (Asua, 2002).
Commonly, these products are produced by means of conventional emulsion
polymerization. In this process, monomer is dispersed in an aqueous solution of
surfactant with a concentration exceeding the critical micelle concentration (CMC)
and polymerization is started by means of an initiator system. In principle, polymer
particles can be formed by entry of radicals into the micelles (heterogeneous
nucleation), precipitation of growing oligomers in the aqueous phase (homogeneous
nucleation) and radical entry monomer droplets. However, monomer droplets are
relatively large (1 – 10 micron) compared to the size of monomer-swollen micelles
(10 – 20 nm), and hence the surface area of the micelles is in orders of magnitude
greater than that of the monomers droplets. Consequently, the probability for a
radical to enter into monomer droplets is very low, and most particles are formed by
either homogeneous or heterogeneous nucleation.
34
Once they are nucleated, the polymer particles undergo substantial growth by
polymerization. The monomer required for the polymerization must be transported
from the monomer droplets by diffusion through the aqueous phase. In some cases,
this represents a severe limitation of the conventional emulsion polymerization.
Thus, water resistance of coating prepared from dispersed polymers is significantly
improved if very hydrophobic monomers, e.g. lauryl and stearyl methacrylate are
incorporated into polymer backbone. However, mass transfer of this monomer from
monomers droplets to polymer particles through the aqueous phase is diffusionally
controlled, and hence they cannot be readily incorporated into the polymer in
conventional emulsion polymerization (Asua, 2002).
2.5.2 Miniemulsion
The formulation and application of polymeric nanoparticles enjoy great
popularity in academia and industry. The techniques of macroemulsion and
microemulsion polymerization which are usually used for the preparation of polymer
particle are based on a kinetic control during preparation, the particles are built from
the centre to the surface, and the particle structure is governed by kinetic factors.
Because of the dictate of kinetics, serious disadvantages such as lack of homogeneity
and restrictions in the accessible composition have to be accepted. The concept of
nanoreactors for the formation of the nanoparticles can take advantage of a potential
thermodynamic control for the design of nanoparticles. This means that the droplets
have to become the primary locus of the nucleation of the polymer reaction. The
polymerization in such nanoreactors should take place in a highly parallel fashion,
i.e. the synthesis is performed in 1018 – 1020 nano-compartments which are separated
from each other by a continuous phase (Asua, 2002). In miniemulsion
polymerization, the principle of small nanoreactor is realized as demonstrated in
Figure 2.5.
35
Surfactant
Water
Monomer
Polymer
Monomer
Water
Miniemulsification (ultrasonification)
Figure 2.5: Principle of miniemulsion polymerization (Asua, 2002)
Miniemulsions are dispersions of critically stabilized oil droplets with a size
between 50 and 500 nm prepared by shearing a system containing oil, water, a
surfactant, and a hydrophobe agent. Polymerization in such miniemulsions, where
carefully prepared, result in latex particles which are about the same size as the initial
droplets. This means that the appropriate formulation of a miniemulsion suppresses
coalescence of droplets or Ostwald ripening. The polymerization of miniemulsions
extends the possibilities of the widely applied emulsion polymerization and provides
advantages with respect to copolymerization reactions of monomers with different
polarity, incorporation of hydrophobic materials or with respect to stability of the
formed latexes.
2.5.3 The Miniemulsion Process
A system where small droplets with high stability in a continuous phase are
created by using high shear is classically called a “miniemulsion”. One of the tricks
for obtaining stability of the droplets is the addition of an agent which dissolves in
36
the dispersed phase, but is insoluble in the continuous phase. The small droplets can
be hardened by either a subsequent polymerization by decreasing the temperature (if
dispersed phase is a low-melting-point material). For a typical oil-in-water
miniemulsion, an oil, a hydrophobic agent (or several), an emulsifier and water are
homogenized by high shear to obtain homogenous and monodisperse droplets in the
size range of 30 – 500 nm.
In the first step of the miniemulsion process, small stable droplets in the size
range between 30 and 500 nm are formed by shearing a system containing the
dispersed phase, the continuous phase, a surfactant, and an osmotic pressure agent. In
a second step, these droplets are polymerized without changing their identity.
On the basis of the principle on miniemulsion, the preparation of new
nanoparticle which could not be prepared in heterophased processes is now possible.
That also includes, in particular, the encapsulation of inorganic material such as CdS
semiconductor. For creating a miniemulsion, the step of homogenization is of great
importance since fairly monodisperse small droplets have to be achieved. The
homogenization
can
be
achieved
by
using
an
ultrasonicator
for
the
miniemulsification of small quantities in a laboratory scale batch process.
2.5.4
Preparation of Miniemulsion
Monomer miniemulsions suitable for miniemulsion polymerization are
submicron monomer-in-water dispersion stabilized against both diffusional
degradation and droplet coagulation by using a water-insoluble low-molecular
weight (co-stabilizer) compound and an efficient surfactant. The key issues in the
preparation of the monomer miniemulsions are the formulation and the method of
preparation.
37
2.5.4.1 Formulations
A typical formulation includes water, a monomer mixture, a co-stabilizer and
the surfactant and initiator system. Table 2.5 shows that miniemulsions of monomers
with a wide range of water solubility have been prepared including vinyl chloride,
vinyl acetate, methyl methacrylic, butyl acrylic, styrene, vinyl hexanoate, dodecyl
methacrylic , and stearyl methacrylic. In addition, multimonomer formulations have
been prepared, including miniemulsions in which some amounts of completely
water-soluble monomers such as acrylic acid and methacrylic acid have been used.
Table 2 5: Examples of formulation of miniemulsion polymerization (Asua, 2002)
Monomer
Costabilizer
Surfactant
Initiator
Vinyl acetate
Hexadecane
Polyvinyl
Sodium persulfate
alcohol
Methyl
Polymethyl
Sodium lauryl
sulfate
Potassium persulfate
methacrylate
metacrylate
Methyl
Hexadecane
Sodium lauryl
sulfate
Ammonium persulfate
Styrene
Hexadecane
Potassium persulfate
Styrene
Polystyrene
Styrene /methyl
Hexadecane
Sodium lauryl
sulfate
Sodium lauryl
sulfate
Sodium lauryl
sulfate
methacrylate
methacrylate
Potassium persulfate
Ammonium persulfate
/sodium hydrogen sulfite
Miniemulsions have been stabilized with long-chain alkanes, alcohol and
polymeric hydrophobic. Hexadecane and cetyl alcohol are the co-stabilizers most
often used in preparation. However, these co-stabilizers remain in the polymer
particles and may have deleterious effects on the properties of the polymer. Alducin
et al. (1994) proposed to minimize these negative effects of the co-stabilizers by
incorporating it into the polymer backbone by means of covalent bounds. A series of
oil-soluble initiators with different water solubility were used. It was found that only
lauroyl peroxide was water-insoluble enough to stabilize styrene miniemulsions
38
droplets. A drawback of this method is that the initiator concentration affects not
only miniemulsion stability, but also polymerization rate and molecular weight
distribution (MWD). In addition, it was found that rather low-molecular weights
were obtained 20,000 – 40,000 g/mol.
Anionic, cationic, non-ionic and mixed anionic/non-ionic, non reactive
surfactants, as well as reactive surfactants have been used. The surfactants useful for
miniemulsion polymerization should meet the same requirements as in conventional
emulsion polymerization:
(i)
it must have a specific structure with polar and non-polar groups
(ii)
it must be more soluble in the aqueous phase so as to be readily available for
adsorption on the oil droplets surface
(iii) it must adsorb strongly and not easily displaced when two droplets collide
(iv) it must reduce the interfacial tension to 5 x 10-3 N/m or less
(v)
it must impart a sufficient electro kinetic potential to the emulsion droplets
(vi) it must work in small concentrations
(vii) it should be relatively inexpensive, non toxic, and safe to handle.
2.5.4.2 Method of Preparation
Figure 2.6 shows the experimental scheme most often used for miniemulsion
preparation. The surfactant system is dissolved in water, the co-stabilizer is dissolved
in the monomers and mixed under stirring. Then, the mixture is subjected to high
efficient homogenization. Other methods have also been reported. Thus, when a long
chain alcohol is used as co-stabilizer, the long chain alcohol is first mixed with water
and surfactant at temperature higher than the melting point of the alcohol, the
mixture is cooled to room temperature and sonicated to break up the gel phase. Then,
monomers are added under stirring and the resulting mixture homogenized to form
the miniemulsion.
39
Dissolve
Dissolve
emulsifier
surfactant
in water
in water
Mixing under
stirring
High efficient
homogenization
Dissolve
costabilizer
in monomer
Figure 2.6: Schematic for monomer miniemulsion preparation methods
2.5.4.3 Homogenization Devices
A variety of equipment is commercially available for emulsification. The
most important are rotor-stator system, sonifiers and high-pressure homogenizers.
Rotor-stator system and other shear devices rely on turbulence to produce the
emulsification. The minimum droplets size that can be achieve with this type of
equipment depends on the size of the smallest turbulent formed, which in turn
depends on the geometry of the rotor-stator system and on the rotation speed.
Sonication tip
Sonication
region
Sonication time
Figure 2.7: Schematic of sonication process
40
The sonifier produces ultrasound waves that cause the molecules to oscillate
about their main position as the waves propagate. During the compression cycle, the
average distances between the molecules decrease, while during rarefaction the
distance increases. The rarefaction result is negative pressure that may cause the
formation of voids or cavities (cavitations bubbles) that may grow in size. In the
succeeding compression cycle of the wave, the bubbles are forced to contract and
may be even disappear totally. The shock waves produced on the total collapse of the
bubbles cause the break up of the surrounding monomer droplets. A problem
associated with the sonifier is that, as shown in Figure 2.7, only a small region of the
fluid around sonifier tip is directly affected by the ultrasound waves. In order to be
broken up, the monomer droplets should pass through this region. Therefore, when
sonication is used to form the miniemulsion, additional stirring must be used to allow
all the fluid to pass through the sonication region. This process makes the
miniemulsion characteristics dependent on the sonication time. Thus, there are
evidence that droplet size decrease with sonication time. The decrease is initially
pronounced and later the droplets size evolves asymptotically towards a value that
depends on both the formulation and the energy input.
2.5.4.4 Particle Nucleation
Droplet nucleation is unique of miniemulsion polymerization. The
characteristics of miniemulsion that is charged into reactor depend on the
formulation, homogenization procedure and storage time. In general, the
miniemulsion is composed of submicron droplets stabilized with a surfactant against
coalescence and with a co-stabilizer to minimize Ostwald ripening. These droplets
are not equal in size. Depending on the amount of surfactant used in the formulation
and on the homogenization procedure, micelles may be present. Assuming that a
water-soluble initiator is used, particle nucleation involves the following seriesparallel processes:
a)
Radical formation in the aqueous phase by initiator decomposition
b)
Polymerization of the radicals in the aqueous phase to give oligomers of
increasing hydrophobicity.
41
c)
Once the oligoradicals are hydrophobic enough, they can enter into monomer
droplets (droplet nucleation) and micelles (micellar nucleation). They may
also grow to a length that makes them insoluble in water and precipitate
(homogeneous nucleation).
d)
In addition to droplet nucleation, monomer droplets may disappear by
coalescence with other droplets and polymer particles, as well as by
diffusional degradation if the co-stabilizer is water-soluble enough to diffuse
from small to large droplets.
For practical reasons, it is important to maximize the fraction of particles
generated by droplet nucleation. This fraction depends on the number of droplets and
micelles and on the relative values of the rate coefficients for entry of radicals into
monomer droplets and micelles, and the propagation rate in the aqueous phase. The
presence of micelles depends on the amount of surfactant and the homogenization
procedure.
In miniemulsion polymerization using hexadecane, Hansen and Ugelstad
(1979a, 1979b) observed that as the intensity of the homogenization increased, the
number of polymer particles initially decreased and after going through a minimum
increased again. At low homogenization intensity, big droplets were formed and free
surfactant formed micelles that gave particles by micellar nucleation. As
homogenization intensity increased, smaller droplets were formed and more
surfactant was required to stabilize these droplets, leaving less free surfactant to form
micelles. The number of particles decreased because micellar nucleation decreased.
At a higher homogenization intensity, micelles disappeared and formation of
particles occurred by droplets nucleation. Under these conditions, increasing
homogenization intensity produced more droplets, and hence the number particles
increased.
Lim and Chen (2000) investigated the miniemulsion polymerization of
styrene using a block copolymer as surfactant and hexadecane as co-stabilizer
finding that a significant fraction of particles were formed by micellar nucleation at
high surfactant concentrations. In the miniemulsion polymerization of vinyl chloride
42
Saethre et al. (1995) found that particles were produced by micellar nucleation when
the concentration of free surfactant (taking into account the amount adsorbed on the
droplets and particles) was above the CMC.
2.5.5 Applications
The applications of polymerization miniemulsion is tabulated in Table 2.6.
Table 2.6: Somes application of polymerization miniemulsion (Asua, 2002)
Application
Production of high solids low viscosity latexes
Continuous polymerization reactors
Controlled radical polymerization in dispersed media
Catalytic polymerization
Encapsulation of inorganic solids
Incorporation of hydrophobic monomers
Hybrid polymer particles,
Polymerization in non-aqueous media
Anionic polymerization
Step polymerization in aqueous dispersed media
Production of low molecular weight polymers in dispersed media
Latexes with special particle morphology
2.5.5.1 Encapsulation of Inorganic Material
Encapsulation of inorganic particles in polymer matrices is of interest in
cosmetics, pharmaceuticals, paint and for reinforcing filler particles for polymer.
Attempts
to
encapsulate
polymer
particles
using
conventional
emulsion
43
polymerization have been reported but it was found difficult to locate the dominant
polymerization at the surface of the inorganic material. Figure 2.8 shows the
principle of the encapsulation of inorganic materials in polymer particles by the
miniemulsion process.
inorganic
material
surfactant 1
surfactant 2
monomer
polymer
monomer
water
addition of water
and surfactant 2
stirring
ultrasound
polymerization
Figure 2.8: The principle of the encapsulation of inorganic materials in polymer
particles by miniemulsion process (Landfester and Ramirez, 2003)
Nunes et al. (2006) synthesized poly(ethylmethacrylate-co-methacrylic acid)
magnetite particle via miniemulsion polymerization. Firstly, the hydrophobization
magnetite was prepared by adding oleic acid into magnetite particles and then the
miniemulsion was prepared using EMA-co-MAA, SDS, and water. The SEM
analysis result showed that the magnetite was dispersed within the polymer matrix
with the linear dimension below 500 nm. Faridi-Majidi et al. (2006) studied
encapsulation of magnetite nanoparticles with polystyrene via emulsifier-free
miniemulsion polymerization. The miniemulsion was prepared using hexadecane as
hydrophobe agent, monomer and water. The TEM result proved the presence of
44
magnetite in polymer particles which appeared to be monodisperse in size
approximately 100 – 300 nm.
Tong and Deng (2007) reported the synthesis of polystyrene encapsulated
nanosaponite composites via miniemulsion polymerization. Firstly, the saponite
nanoparticle was modified by adding vinyl benzyl trimethylammonium chloride
(VBTAC) in water solution. Secondly, the miniemulsion system was formed using
styrene, hexadecane, TX-405 and water. The results showed that the VBTAC
modified nanosaponite could be fully exfoliated and encapsulated inside polystyrene
latex. When the concentration of hexadecane as co-stabilizer was high, the final
composite particles were composed mainly of spherical particles with sizes less than
100 nm and a small number of hemispherical or bowl-structured particles of size 100
– 1000 nm.
Erdem et al. (2000a, 2000b) prepared aqueous miniemulsion of the styrene
containing TiO2 dispersed particles by both sonication and a high-pressure
homogenizer. Sodium lauryl sulfate was used as surfactant and hexadecane plus
polystyrene the costabilizer system. It was found that the presence of TiO2 particles
within the miniemulsion droplets resulted in a significant increase in the droplets size
(measured by the soap titration method). Increasing surfactant concentration did not
result in a significant change in the size of the miniemulsion droplets. An increase in
the loading of the TiO2 led to larger miniemulsion droplets.
It was also found that the use of a more efficient homogenization (high pressure
homogenization vs sonication) could not significantly reduce the droplets size. The
reasons for these results were not provided. The polymerization of the miniemulsion
trying to encapsulate the TiO2 particles was also investigated. It was found that the
extent of the encapsulation strongly depended on the quality of dispersion of the
TiO2 particles in the monomer. The maximum encapsulation efficiencies (83 %
coverage) were obtained with the smallest TiO2 particles (hydrophilic particles
stabilized with 1 % of polybutene-succinimide pentamine). Hydrophobic TiO2 or the
use of fewer stabilizers led to larger TiO2 particles that resulted in lower
encapsulation efficiencies. The failure to achieve 100 % encapsulation was attributed
45
to: (i) formation of new particles by homogenous nucleation; (ii) presence of large
TiO2 aggregates that may not be contained inside the monomer droplets; and (iii) less
than 100 % nucleation of droplets.
A higher carbon black loading was achieved by Tiarks et al. (2001). The
author dispersed the carbon black in an aqueous surfactant solution by using
sonication. A miniemulsion of styrene and a polyurethane co-stabilizer was prepared
using sonication. Then, the miniemulsion was mixed with the aqueous dispersion of
carbon black and the resulting dispersion was sonicated. The resulting dispersion was
polymerized with AIBN obtaining stable latexes that contained larger fractions of
carbon black (40%) with a good coverage. It was found that particle size was not
dependent on surfactant concentration, but the process is sensitive to the type and
amount of co-stabilizer used. Hexadecane was less efficient than polyurethane and
there is an optimum amount of polyurethane, below which coagulation occurs and
above which new particles are formed. The reason for this behaviour remained
unclear. Also an optimum monomer/carbon black fraction was found. Larger
amounts of monomer yielded pure polymer particles and smaller amount resulted in
uncovered carbon black particles.
46
CHAPTER 3
EXPERIMENTAL
3.1
Synthesis and Preparation
3.1.1 Purification of Monomer
Monomer usually contains stabilizer and it can affect the polymerization
process. It is important to purify the styrene and divinyl benzene by removing the
stabilizer with 10 % (w/v) NaOH solution before the polymerization process. These
monomers were washed with 20 mL of 10 % (w/v) NaOH in a separating funnel.
Two layers were formed and the lower layer which contains NaOH solution with
stabilizer was removed. The washing process was repeated 3 times to ensure that all
the stabilizer in the monomer was removed. The monomer was washed with distilled
water until neutral before it was kept in a dry bottle and stored in a refrigerator.
Monomer methacrylic acid, monomer ethylene glycol dimethacrylic acid and all the
other chemicals were used without further purification (Antolini et al., 2005; FaridMajidi et al., 2006; Kuljanin et al., 2006)
3.1.2 Preparation of CdS Nanoparticles by Reverse Micelles Using n-Decane
as Oil-phase
CdS nanoparticles was synthesized by reverse micelles in miniemulsion
system as follows. Aqueous standard solutions of cadmium nitrate (0.1 M) and
47
sodium sulfide (0.1 M) were first prepared. These solutions were then used to
prepare two other solutions : (1) About 45 μL of the Cd(NO3)2 solution was injected
into 5 mL of 0.1 M cetyl trimethyl ammonium bromide (CTABr) solution in
mixtures of 2.5 mL of 2-propanol and 2.5 mL of n-decane, and (2) About 45 μL of
the Na2S solution was injected into 5 mL of 0.1 M CTABr solution in mixtures of 2.5
mL of 2-propanol and 2.5 mL of n-decane. After stirring for 1 h, they were mixed
together for 5 minutes resulting in the formation of CdS nanoparticles in
miniemulsion. For obtaining CdS nanoparticles at different Wo = [water]/[surfactant],
and Po = [2-propanol]/[surfactant] values, different amounts of the aqueous solutions
of 0.1 M Cd(NO3)2 and Na2S were added into 5 mL of 0.1 M CTABr solution
mixtures of various compositions of 2-propanol and n-decane. Stability of the
nanoparticles formed were monitored by UV – Vis spectroscopy and the rate of
precipitation of CdS particle (Khiew et al., 2003, 2004; Agostiano et al., 2000). The
particle size of CdS nanoparticle was also determined by means of UV – Vis
spectroscopy. The absorption onset wavelength (λ), that is calculated from the UV –
Vis spectra was converted into a CdS particle size using Brus equation (Brus, 1984).
3.1.3 Preparation of CdS Nanoparticles by Reverse Micelles Using Monomer
as Oil-phase
CdS nanoparticles were synthesized by reverse micelles in miniemulsion with
the following steps. Aqueous standard solutions of cadmium nitrate (0.1 M) and
sodium sulfide (0.1 M) were foremost prepared. These solutions were then used to
prepare two other solutions: (1) about 45 μL cadmium ion solution was injected into
5 mL of 0.1 M CTABr solution in mixture of 2.5 mL of 2-propanol, 2.0 mL of
styrene, and 0.5 mL divinylbenzene and (2) about 45 μL sulfide ion solution was
injected into the same solution as above. After stirring for 1 h, they were mixed
together for 5 minutes resulting in the formation of CdS nanoparticles. To obtain
CdS nanoparticles by reverse micelles in miniemulsion at different Wo, different
amount of the aqueous solutions of Cd(NO3)2 and Na2S were added into the same
solution as mentioned above. The stabilization and particle size of the CdS
nanoparticles were determined by the same procedure at 3.1.2. The CdS/P(S-DVB)
48
nanocomposites were synthesized via in-situ polymerization in miniemulsion at 70 –
80 oC for 6 h using 2,2’-Azobisisobutyronitrile (AIBN) as the initiator. The
nanocomposites were filtered and then washed with water, ethanol and methanol, to
successively remove excess of monomer, cadmium and sulfide ion.
3.1.4 Preparation of P(MAA-EGDMA)
The P(MAA-EGDMA) polymer was synthesized by in-situ polymerization in
miniemulsion system. The miniemulsion system was prepared using cetyl
triammonium bromide (CTABr) as surfactant, 2-propanol as co-surfactant, water,
methacrylic acid-ethylene glycol dimethacrylic acid (MAA-EGDMA) monomer and
n-decane acted as the oil-phase. The preparation involved stirring and polymerization
of a mixture of 0.3645 g of CTABr, 2.5 mL of 2-propanol, 1.0 g of MAA monomer,
0.5 g of EGDMA monomer, 2.5 mL n-decane and water. After stirring for 1 h, it was
followed by in-situ polymerization at 70 – 80
o
C for 6 h using 2,2’-
Azobisisobutyronitrile (AIBN) as the initiator. The polymer was filtered and then
washed with water, ethanol and methanol, to remove the surfactant and monomer
residue from the external surface of polymer and finally dried for 4 h at 100 oC.
3.1.5 Preparation of CdS/P(MAA-EGDMA) Nanocomposites
The preparation of occluded cadmium inside P(MAA-EGDMA) was carried
out by ion exchange method. This preparation involved stirring a mixture of 0.5 gram
P(MAA-EGDMA) with various mmol of cadmium ion in methanol solution for 24 h.
The solution was then filtered and washed with water, ethanol and methanol, to
successively remove excess cadmium ion from the external surface and then dried
for 4 h at 100o C. The final step was synthesis of CdS/P(MAA-EGDMA) using
precipitation reaction between Cd2+/P(MAA-EGDMA) with sulfide ion solution.
About 0.5 gram Cd2+/P(MAA-EGDMA) was soaked in a solution of various mmol
49
Na2S. 9 H2O in 50:50 H2O: methanol. The mixture was stirred for 24 h before
filtering the CdS/P(MAA-EGDMA) and washed with water, ethanol and methanol,
and dried at 100o C for 4 h.
3.1.6 Preparation of P(S-DVB), SO3H-P(S-DVB) and CdS/SO3-P(S-DVB)
Pure poly(styrene-divinylbenzene) [P(S-DVB)] was synthesized by in-situ
polymerization in miniemulsion system. Meanwhile the miniemulsion system was
prepared using sodium dodecyl sulfate (SDS) as surfactant, 2-propanol as costabilizer, water, and styrene-divinylbenzene as oil-phase. The preparation involved
stirring a mixture of 2.5 mL of 2-propanol, 1.80 gram of styrene, 0.60 gram of
divinylbenzene and 2,2’-azobisisobutyronitrile (AIBN) for 1 h. After stirring for 1 h,
the mixture was added to 20 mL 0.1 M of SDS in water and then ultrasonication was
carried out for 15 min followed by more stirring for 1 h. Then in-situ polymerization
was done at 70 – 80
o
C for 6 h. The polymer was filtered and then washed with
water, ethanol and methanol, to remove surfactant and monomer residues, before
drying for 4 h at 100o C.
The next step is the sulfonation reaction of the polymer with H2SO4/SO3. In
the preparation of SO3H-P(S-DVB), 10.0 mL H2 SO4/SO3 was added slowly to 0.5
gram of P(S-DVB). The mixture was slowly stirred and maintained at 50 oC for 1 h,
followed by careful dilution of the mixture with water. The SO3H-P(S-DVB) was
filtered and then washed with water until neutral pH was obtained and finally dried at
100 oC for 4 h. The preparation of occluded cadmium ion on the surface of SO3HP(S-DVB) was achieved by ion exchange method. This preparation involved stirring
a mixture of 0.5 gram SO3H-P(S-DVB) with various mmol of cadmium ion in
methanol solution for 24 h.
The solution was filtered and then washed with water, ethanol, and methanol,
to effectively remove excess cadmium ion and then dried for 4 h at 100o C. The final
step was the synthesis of CdS/SO3-P(S-DVB) nanocomposites by precipitation
50
reaction of Cd2+/SO3H-P(S-DVB) with sulfide anion solution. About 0.5 gram of
Cd2+/SO3H-P(S-DVB) was soaked in a solution of various mmol Na2S. 9 H2O in
50:50 H2O: methanol and then was stirred for 24 h. The nanocomposites was filtered
and then was washed with water, ethanol and methanol, successively and dried at
100 oC for 4 hours. The sulfonate content (degree of sulfonation) of SO3H-P(S-DVB)
was determined by the following ways.
To perform titration, 0.1 gram of SO3H-P(S-DVB) was dissolved in mixture
of 15 mL of toluene and 2 mL ethanol. The solution was titrated with standardized
methanol solution of potassium hydroxide (KOH), using phenolphthalein as
indicator. The ion exchange capacity of SO3H-P(S-DVB) was determined (with unit
of meq/gram of dry polymer of sulfonated polymer) by measuring concentration of
hydrogen (H+) that was exchanged with sodium cation when acid sulfonated polymer
samples were equilibrated with sodium (Na+) solution. 0.1 gram sample of acid
polymer was placed into 100 mL of 0.2 mL NaCl solution and shaken sporadically
for 24 h, the amount of hydrogen ion released by the polymer was determined by
titration with 0.01 M NaOH.
3.2
Characterization Techniques
The as-synthesized polymer nanocomposites were characterized using X-ray
Diffraction (XRD), Fourier Transformation Infrared (FTIR), Field Emission
Scanning Electron Microscope (FESEM), Transmission Electron Microscopy
(TEM), UV – Visible Diffuse Reflectance (UV- Vis DR), Thermal conductivity and
Thermogravimetric analyses (TGA).
3.2.1 UV-Vis Spectroscopy
The stabilization and particle size of CdS nanoparticles in miniemulsion
system were characterized using UV – Vis spectrocospy. Stability of the
51
nanoparticles formed were monitored by UV – Vis spectroscopy and the rate of
precipitation of CdS particle. The particle size of CdS nanoparticle was also
determined by means of UV – Vis spectroscopy. The absorption onset wavelength
(λ), that is calculated from the UV – Vis spectra was converted into a CdS particle
size using Brus equation (Brus, 1984). The UV – Vis absorption spectra of the
solutions were recorded on a Perkin Elmer Lambda 900 UV/Vis/NIR spectrometer in
the wavelength range of 350 – 700 nm using a 10 mm cuvette (Agostiano et al.,
2000; Khiew et al., 2003, 2004).
3.2.2 X-Ray Diffraction
The structure of the nanocomposites was characterized by XRD using a
Bruker Advance D8 Difractometer with Cu Kα (λ = 1.5405 A) radiation as the
diffracted monochromatic beam at 40 kV and 40 mA and was scanned in the 2θ
range between 20o – 70 o at ambient temperature. The sample was ground to a fine
powder using a mortar and then 1 g of dried sample was mounted on a plastic sample
holder. The sample was then pressed and smoothed on the sample holder using a
glass slide (10×12 mm2) with 2 mm width. The analysis was carried out in the step
interval of 0.05o with counting time of 1 second per step. (Ghosh et al., 2005a, Hirai
and Ota, 2006).
3.2.3 Fourier Transform Infrared Spectroscopy
The pure polymer, sulfonated polymer and CdS/polymer nanocomposites
were characterized by FTIR spectroscopy using a Perkin Elmer Spectrum One FTIR
Spectrometer at room temperature and the spectra were recorded in the region of
4,000 – 400 cm-1. Preparations of samples for FTIR characterization were done using
KBr pellet technique. A small amount of solid sample was mixed with KBr with the
ratio of KBr to sample of 100:1 and was ground well to powder form. Then the
sample was compressed under 8 tons for 1 - 2 minutes, producing transparent pellet
52
with 13 mm in diameter. FTIR spectra were recorded at room temperature with 4
cm-1 resolution (Chen et al., 2006, El-Tantawy et al, 2004).
3.2.4 UV-Vis Diffuse Reflectance Spectroscopy
The structure and particle size of CdS nanoparticles at CdS/polymer
nanocomposites were determined by UV – Vis Diffuse Reflectance (UV – Vis DR)
using Perkin Elmer Lambda 900 UV/Vis/NIR spectrometer. In preparation of the
samples for UV - Vis DR, the samples were finely ground and placed on a sample
holder to be analyzed. The scanned wavelength ranges between 350 – 700 nm
(Zhang et al,. 2001)
3.2.5 Field Emission Scanning Electron Microscope
Field Emission Scanning Electron Microscope (FESEM), Zeiss Supra 35VP
VP Series model was used to determine the morphology of nanocomposite sample.
Samples were mounted on stubs using double-sided tape and then coated with gold
using instrument model BIO-RAD Polaron Divison SEM Coating System machine at
10-1 mbar with 30 mA for 75 minutes. Gold coating is needed to prevent charge
build-up on the sample surface, besides increasing secondary electron emission.
Then, the stubs were put in field emission scanning electron microscope model Philip
XL40 and with pressure of 5 bars. Electron source was attained from tungsten
filament. The FESEM micrograph was recorded with resolution of 10 kV for certain
times of magnification.
3.2.6 Transmission Electron Microscopy
A JEOL JEM-2100 Transmission Electron Microscope (TEM) was used for
analyzing the morphology and particle size of CdS polymer nanocomposites. In
53
preparation of the samples for TEM analysis, the nanocomposite was dispersed in
ethanol by ultrasonic stirring. Then a drop of the suspension was placed on the grid
with Formvar thin film, before carrying out the TEM analysis.
3.2.7 Dielectric Constant and Dissipation Factor
The capacitance and dissipation factor of nanocomposite were determined
using Impedance Analyzer. Pellets of 2–4 mm thick were prepared by placing
sufficient amount of sample (~50 mg) in a steel die measuring 13 mm in diameter,
and a pressure of 5 tons were applied and held for 30 seconds. The dielectric
constants (εr) of the pellet prepared were calculated from the measured capacitance at
various frequencies (0.1 kHz to 1,000 kHz) using AC Impedance Analyzer using the
equation: C = εr εo A/d, where εo is the dielectric constant of the free space (8.854 x
10-12 F/m); “A” the area of the electrical conductor; and “d” the thickness of material.
3.2.8 Ionic Conductivity
The ionic conductivity was studied using AC Impedance Analyzer with
frequency response analyzer (Auto Lab POST AT 30). The resistances of the
specimens were measured at frequency of 0.1 kHz – 1,000 kHz using AC Impedance
Analyzer. The ionic conductivity, σ, of the polymer nanocomposites were calculated
from the resistance data.
3.2.9 Thermal Conductivity
The thermal conductivity was measured by Thermal Conductivity Analyzer .
Model Therm Test Mathis TC30. Before using, the Thermal Conductivity Analyzer
was calibrated using High Density Polyethylene (HDPE) 0.5771 W/mK and Lexan
54
0.25409 W/mK and then, the thermal conductivity of the samples were measured at
room temperature.
3.2.10 Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was conducted on Mettler Toledo
TGA/SDTA instrument. The sample was fine ground and place in a 70 μL pan. The
procedures included setting 10-15 mg of sample to be heated from 25 oC up to 1000
o
C, with heating rate of 10 oC/min in oxygen atmosphere.
3.2.11 Atomic Absorption Spectroscopy
The cadmium (Cd) and sodium (Na) content of the polymer nanocomposite
samples were determined using Perkin Elmer AA400 Atomic Absorption
Spectrometer equipped with flame atomizer and EDL lamp for Cd and Na. The
samples were prepared by decomposing of 50 mg sample in the furnace and then
added with 5.0 mL of concentrated HCl solution. Samples solution was diluted by
adding in double deionized water to the mark, and homogenized by shaking.
Cadmium and sodium in samples were calculated by using standard calibration plot
shown in Appendix F and Appendix G.
55
CHAPTER 4
RESULTS AND DISCUSSION
This chapter describes the synthesis, characterization, electrical properties and
the correlation between the structural and physicochemical properties of three
cadmium sulfide (CdS)/polymer composites. This chapter is divided into four
sections:
•
The synthesis, characterization and properties of cadmium sulfide nanoparticles
encapsulated in poly(styrene-divinyl benzene) [P(S-DVB)] to produce CdS/P(SDVB) by in-situ polymerization in a miniemulsion system. (Section 4.1)
•
The
synthesis,
characterization
and
properties
of
cadmium
sulfide/poly(methacrylic acid-ethylene glycol dimethacrylic acid) [CdS/P(MAAEGDMA)] nanocomposites. (Section 4.2)
•
The synthesis, characterization and properties of cadmium sulfide/sulfonatedpoly(styrene-divinyl benzene) [CdS/SO3-P(S-DVB)]. (Section 4.3)
•
Comparison of physicochemical and electrical properties of those CdS/polymer
nanocomposites. (Section 4.4)
Firstly, the synthesis of cadmium sulfide (CdS) nanoparticles encapsulated in
poly(styrene-divinyl benzene) [P(S-DVB)] by in-situ polymerization in a
miniemulsion system is described. In this chapter, the effect of Wo =
[water]/[surfactant], Po = [2-propanol]/[surfactant], bulk concentration of cadmium
or sulfide ion, and monomer on the stability and particle size of CdS nanoparticle in
reverse micelles system using n-decane as oil-phase was studied. The stability of
56
CdS nanoparticle in reverse micelles system was analized semi-quantitatively by
observing the initial time when CdS aggregation occurred and by means of UV – Vis
spectra of the miniemulsion. The effect of Wo = [water]/[surfactant] at Po = [2propanol]/[surfactant] = 32.5 on the stability and particle size of CdS in reverse
micelles system using monomer as oil-phase is also investigated. Following this,
synthesis methods and properties of CdS/P(S-DVB) nanocomposites i.e. by in-situ
polymerization in two miniemulsion system using n-decane and monomer as oilphase in the presence of CdS nanoparticles dispersed in reverse micelles system are
compared and discussed.
Secondly, the synthesis, characterization and properties of cadmium
sulfide/poly(methacrylic acid-ethylene glycol dimethacrylic acid) [CdS/P(MAAEGDMA)] nanocomposites are detailed. This section is devoted to the synthesis of
P(MAA-EGDMA) by in-situ polymerization in miniemulsion system and
CdS/P(MAA-EGDMA) by post synthesize route (by ion exchange and precipitation
with various concentrations of cadmium and sulfide ions).
Subsequent sections discuss the synthesis, characterization and properties of
cadmium
sulfide/sulfonated-poly(styrene-divynilbenzene)
[CdS/SO3-P(S-DVB)]
nanocomposites including the synthesis P(S-DVB) as polymer host by in-situ
polymerization, the synthesis SO3H-P(S-DVB) by sulfonation process with H2S2O7
and the preparation of CdS/SO3-P(S-DVB) nanocomposites by post synthesize with
ion exchange and precipitation processes with various concentration of cadmium and
sulfide ion. The schematic representation of results of the studies is outlined in
Figure 4.1.
To understand the structural and physicochemical-electrical properties,
appropriate techniques are used in the characterization, including UV – Vis
spectroscopy for determination of the optical properties, stability and particle size of
CdS nanoparticles in the miniemulsion. Meanwhile, the UV – Vis DR spectra was to
estimate the particles size of CdS in the polymer nanocomposites, along with FTIR
spectroscopy for the polymer characterization and verification of the interaction
between polymer and CdS nanoparticles. The CdS nanoparticles and the polymer
57
nanocomposites prepared in this study was subsequently characterized by XRD to
determine structure, SEM and TEM to determine the morphology, particles size of
polymer and average crystallite size of CdS, while AAS analysis was performed to
determine concentration of cadmium ion. In addition, results of studies on dielectric
properties (dielectric constant and dissipation factor), ionic conductivity, thermal
conductivity, and thermal properties were also detailed and discussed in this chapter.
CdS
P(S-DVB)
In-situ polymerization
COO
P(MAA-EGDMA)
–
CdS
Ion exchange,
COO
–
precipitation
SO3H-P(S-DVB)
CdS
Ion exchange,
precipitation
SO3Na HSO3 HSO3
SO3Na
HSO3
NaSO3
HSO3
SO3Na
NaSO3
HSO3
HSO3
SO3Na
SO3Na
HSO3
NaSO3
HSO3
SO3Na
HSO3
SO3Na HSO3
Figure 4.1: Schematic representation of mechanism of CdS nanoparticles attachment
and encapsulation on polymer surface
58
4.1
Optimization of Synthesis of CdS Nanoparticles and CdS/P(S-DVB)
Nanocomposites
A prerequisite for the in-situ polymerization in miniemulsion system is the
stability of the initial miniemulsion and stability during the polymerization process.
The stability of the miniemulsion has been shown to be strongly affected by
parameters such as Wo = [water]/[surfactant], Po = [2-propanol]/[surfactant], bulk
concentration of ion, temperature, the number and properties of all the components in
the reaction mixture (Agostiano, et al. 2000).
In this research, we attempted to synthesize CdS nanoparticles by reverse
micelles in a miniemulsion system using cetyltrimethylammonium bromide (CTABr)
as surfactant, 2-propanol as co-stabilizer, n-decane as oil phase, and water. A typical
CTABr surfactant has low solubility in oil-alcohol system and the solubility usually
increases with addition of water. In a miniemulsion system, the stability of CdS
nanoparticles in reverse micelles was found to be dependent on Wo =
[water]/[surfactant], Po = [2-propanol]/[surfactant] and bulk concentration of
cadmium ion. For investigation of the influence of Po value, Wo, concentration of
Cd2+ and S2- solutions, and effect of monomer on the stabilization time of CdS
nanoparticles, various miniemulsion systems were examined under different
conditions. Table 4.1 summarizes the optimum parameters that were obtained in this
research including the absorption onset wavelength, and mean diameters of CdS
nanoparticles.
Under the optimum conditions, stabilization of CdS nanoparticles in the
miniemulsion occurs at Wo = 5, [Cd2+] = [S2-] = 3 x 10-4 M and Po = 65. In the
present study the stabilization time increased when the Po value increased from 16 to
65, then decreased beginning at Po = 78. Stability of the nanoparticles formed were
monitored by UV – Vis spectroscopy and the rate of precipitation of CdS particle
(see Appendix A and B). The particle size of CdS nanoparticle was also determined
by means of UV – Vis spectroscopy. The absorption onset wavelength (λ), that is
calculated from the UV – Vis spectra was converted into a CdS particle size using
Brus equation (Brus, 1984). The absorption onset wavelength and calculated particle
59
size of CdS nanoparticles at the optimum condition were 480 nm and 6.3 nm,
respectively (see Appendix C).
Table 4.1: The optimum conditions for synthesis of CdS nanoparticle using n-decane
as oil-phase a
Parameter
Values
Remarks
Po b
65 d
Onset λ = 480 nm
Particle size = 6.3 nm
Wo c
Concentration of Cd2+ and S2-
a
5e
f
3 x 10-4 M
Amount of styrene
2.0 mL
Increase stability of
Amount of divinyl benzene
0.5 mL
miniemulsion, see appendix E
The stability of CdS nanoparticles in miniemulsion was monitored by UV – Vis spectroscopy (see
appendices A and B)
b
c
Wo = [water]/[surfactant]
d
e
Po = [2-propanol]/[surfactant]
The experiment was carried out in the range 16 – 105 at Wo = 5, C = 3 x 10-4 M (see appendix B)
The experiment was carried out in the range 5 and 10 at Po = 65 (see appendix C)
f
The experiment was carried out in the range 3 x 10-4 M and 6 x 10-4 M (see appendix D)
For investigation of the influence of Wo the miniemulsion system at Wo = 5
and 10 with concentration of cadmium and sulfide ions of 3.0 x 10-4 M was observed
at different time interval after formation (see Appendix C). The red-shift suggested
that at lower Wo CdS nanoparticles in miniemulsion are more stable compared to that
at a higher Wo value. The UV – Vis spectra indicate that the larger the values of Wo,
the larger the micelle core and the farther red-shifted was the CdS nanoparticle
samples relative to those of the bulk samples.
For investigation of the influence of concentration of cadmium and sulfide
ions on nanoparticles size, the miniemulsion systems at Wo = 5 and Po = 65 with Cd2+
and S2- concentrations ions of 3.0 x 10-4 M and 6.0 x 10-4 M, respectively, were
investigated (see Appendix D). From the UV – Vis spectra, it can be seen that as the
60
quantity of Cd2+ and S2- ion decreases, the absorption onset wavelength becomes
more red-shifted suggesting that CdS nanoparticles in miniemulsion are more stable
at lower ion concentration than at higher concentration, which leads to aggregation of
CdS nanoparticles with larger diameters. This phenomenon was an indication that
the smaller droplets of the miniemulsion with concentration of 3.0 x 10-4 M Cd2+ and
S2- could have resulted from the higher stability and low flocculation rate of the CdS
nanoparticles.
The effect of addition of monomer namely, styrene and divinyl benzene on
the size of CdS nanoparticles in miniemulsion system was investigated (see
Appendix E). The results showed that addition of monomer to the miniemulsion
system increased the stabilization of reverse micelles. The absorption onset
wavelengths after addition of monomer were significantly blue-shifted compared to
bulk CdS. It is concluded that the monomer could be used to replace n-decane as oilphase in the present miniemulsion system.
CdS nanoparticles has also been synthesized by reverse micelles in
miniemulsion using cetyltrimethylammonium bromide (CTABr) as surfactant, 2propanol as co-surfactant, styrene-divinyl benzene as oil-phase and water phase.
Schematic representation of the synthesis of CdS nanoparticles and CdS/P(S-DVB)
nanocomposites is illustrated in Figure 4.2. While, the schematic reaction of styrene
with divinyl benzene and representation of the encapsulated CdS in P(S-DVB)
matrices are shown in Figures 4.3 and 4.4 respectively
61
CTABr, monomer (M)
2-propanol, Cd2+/H2O
M
M
M M M
M
M
M
M
M
M
+
M
Cd2+
M
M M
Divinyl benzene
M
M
M
M
M
M M
M
M
CdS
M
M
M
M
M
M
M
M
M
M
M M
M
M
M
2-
M
S2-
M
M
Styrene
M
M
M M
M
M
M
Surfactant
S
M
M
M
M
M
M
M
M
M
M
Monomer
M
M
Cd2+
M
M
CTABr, monomer (M), 2propanol, S2-/H2O
M
•
M
M
M
M
M
M
M
CdS nanoparticle in miniemulsion
Polymerization
Polymer P(S-DVB)
CdS
Figure 4.2: Schematic representation of the synthesis of CdS nanoparticles by
reverse micelles in miniemulsion system and CdS/P(S-DVB) nanocomposites via insitu polymerization
62
CH
CH2
CH
CH2
CH
CH2
AIBN,
70oC
+
Styrene
in miniemulsion
system
Divinyl benzene
Figure 4.3: Reaction of styrene and divinylbenzene by free radical polymerization
into P(S-DVB)
CdS
Polymer chain
Figure 4.4: Schematic representation of the encapsulated CdS in P(S-DVB) matrices
n
63
The optimum parameters for the synthesis of CdS nanoparticles in
miniemulsion are shown in Table 4.2.
Table 4.2: The optimum conditions for synthesis of CdS/P(S-DVB) nanocomposite
using monomer as oil-phase a
a
Parameter
Values
Results
Po b
32.5 d
Onset λ = 484, particle size = 5.1
Wo c
1.5 – 7.0 e
Concentration of Cd2+ and S2-
3 x 10 -4 M
Monomer styrene
2 mL
Monomer divinyl benzene
0.5 mL
The optimum condition miniemulsion for synthesis of CdS nanoparticles was determined by UV –
Vis spectroscopy (see Figure 4.4 – 4.5)
b
c
Wo = [water]/[surfactant]
d
e
Po = [2-propanol]/[surfactant]
The experiment was carried out at Wo = 5.55
The experiment was carried out at Po = 32.5
The stability and particle size of the CdS nanoparticles in miniemulsion
system were monitored by UV –Vis spectroscopy. The stability of CdS nanoparticles
could be observed from the absorption onset wavelength of the UV – Vis spectra.
Meanwhile, the particle size of the CdS nanoparticles was determined from the
absorption onset wavelength and calculated using Brus equation (Equation 4.1)
(Brus, 1984).
Brus equation : E (R) = 2.43 + 2.446/R2 – 0.3031/R
(Equation 4.1)
with Ebulk CdS = 2.43 eV, E = hc/λ and R = particle size
Figure 4.5 shows the effect of the Wo value of the reverse micelles solution
on UV – Vis spectra of CdS nanoparticles. The results show that the absorption onset
wavelength and the size of the CdS nanoparticles are successfully controlled by
64
changing the Wo values. The results indicate that the reverse micelles in
miniemulsion can produce CdS nanoparticles with increased stabilization and their
quantum size effect were also maintained as shown by the blue shifting of the
absorption onset wavelength. The absorption onset wavelength and particle size of
CdS nanoparticles in miniemulsion at Wo values = 1.5, 2.75, 4.0 and 5.55 determined
from the UV – Vis spectra are 445 nm (3.2 nm), 472 nm (4.5 nm), 480 nm (4.9 nm)
and 484 nm (5.1 nm), respectively. These results indicated that the particle size of
CdS nanoparticles changed as a function of water/surfactant molar ratio.
Relative intensity (a.u)
(a)
(b)
(c)
(d)
390
420
460
500
Wavelength (nm)
540
580
600
Figure 4.5: UV – Vis spectra of CdS nanoparticles in miniemulsion system using
styrene-divinylbenzene as oil-phase at Po = 32.5 after 5 minutes formation and
different Wo values (a) 5.50, (b) 4.0, (c) 2.75, and (d) 1.50
The stabilization of CdS nanoparticles in miniemulsion at different Wo values
and various time intervals after formation was characterized by UV – Vis
spectroscopy as shown in Figure 4.6.
65
(a)
Relative intensity (a.u)
after 5 min
after 1 h
a fter 24 h
380
420
460
500
540
580
600
540
580
600
Wavelength (nm)
(b)
after 5 min
Relative intensity (a.u)
after 1 h
after 24 h
390
420
460
500
Wavelength (nm)
Figure 4.6: UV – Vis spectra of CdS nanoparticles in miniemulsion using styrenedivinylbenzene as oil-phase at different Wo values and Po = 32.5 at different time
intervals after formation (a) Wo = 1.5 (b) Wo = 2.75 (c) Wo = 4 and (d) Wo = 5.55
66
( c)
Relative intensity (a.u)
after 5 min
after 1 h
after 24 h
390
440
520
480
560
600
Wavelength (nm)
Relative intensity (a.u)
(d)
after 5 min
after 24 h
after 1 h
390
420
460
500
540
580
600
Wavelength (nm)
Figure 4.6 (continue): UV – Vis spectra of CdS nanoparticles in miniemulsion using
styrene-divinylbenzene as oil-phase at different Wo values and Po = 32.5 at different
time intervals after formation (a) Wo = 1.5 (b) Wo = 2.75 (c) Wo = 4 and (d) Wo =
5.55
67
The UV – Vis spectra showed similar behavior in which the intensity of
absorption of CdS nanoparticles in miniemulsion 5 to 60 minutes after formation is
almost unchanged. But, 24 hours after formation, the absorption intensity of CdS
nanoparticles decreased gradually showing that the stability of CdS nanoparticles in
miniemulsion is also decreasing. These results are different from those reported by
Agostiano et al. (2000) who synthesized CdS nanoparticles by reverse micelles in
quaternary “water-in-oil” miniemulsion comprising CTABr/pentanol/hexane/water
and CTABr/2-propanol/n-decane/water, respectively. They observed a significant
decrease in optical intensity of the solution following precipitation of CdS
nanoparticles in the miniemulsion. The present study demonstrated that
miniemulsion using styrene-divinylbenzene as oil-phase could increase stability and
favour the formation of CdS nanoparticles.
4.1.1 Structural
and
Morphological
Properties
of
CdS/P(S-DVB)
Nanocomposites
Figure 4.7 shows the XRD patterns of pure P(S-DVB), CdS nanoparticles and
CdS/P(S-DVB) nanocomposites with various CdS contents. The XRD pattern of the
CdS sample exhibit broad peaks centred at 2θ values of 26.4o, 43.7o, 52o and a
shoulder at~ 31.5o which could be indexed to scattering from 111, 220, 311 and 200
planes, respectively, of cubic CdS (Wang, et al. 2005). The XRD patterns of the
CdS/P(S-DVB) nanocomposites are identical and they exhibit broad peaks centered
at 2θ value of 26.4o. Broad XRD peaks are attributed to the absence of long-range
order in the samples and reflect the small crystallite size (less than 10 nm as
estimated from the Brus equation). This result agrees with the presence of
nanocrystalline CdS, for which quantum size effect are expected.
68
26.4o
43.7o
52.0o
CdS/P(S-DVB) 0.18 %
Relative intensity (a.u)
CdS/P(S-DVB) 0.03%
CdS/P(S-DVB) 0.02%
CdS nanoparticles
Pure P(S-DVB)
20
30
40
2 Theta degree
50
60
70
Figure 4.7: XRD patterns of pure P(S-DVB), CdS nanoparticles and CdS/P(S-DVB)
nanocomposites with various amounts of CdS
69
Comparing the XRD pattern of CdS nanoparticle with CdS/P(S-DVB)
nanocomposites, one can see that the diffraction peak at 2θ = 26.4o is attributed to
CdS nanoparticles. The weak reflection peaks around 26.4o should belong to CdS by
comparison with the XRD pattern of CdS nanoparticles. to the amount of CdS
nanoparticles is very small compared to polymer matrix. The low intensity peaks
indicated that the amount CdS nanoparticle is very small. The polymer particles
which are amorphous and present in large amount also contribute to the peak
broadness. However the size of CdS nanoparticles could not determined by XRD due
to the amount of CdS nanoparticles is very small compared to polymer matrix.
The morphology of CdS/P(S-DVB) nanocomposites were characterized by
TEM and SEM. Figure 4.8 shows the typical SEM images of CdS/P(S-DVB)
nanocomposites. SEM showed small spherical particles with diameter 0.2 – 2 μm
and some particles appeared to be in agglomerated form. These spherical particles
should be ascribed to the aggregation of small CdS nanoparticles during the
formation of the nanocomposites. The low tendency of aggregation in the CdS/P(SDVB) nanocomposite as shown in the SEM image suggested that stability of the
miniemulsion system was enhanced by using monomer as oil-phase during the
polymerization process. In particular, the smooth surface of the spheres which is
indicative of polymer particles and the obvious absence of CdS crystals dispersed on
the polymer surface, suggested that CdS nanoparticles were encapsulated in P(SDVB). The encapsulation of CdS in P(S-DVB) was also identified from the
corresponding change in colour of the products, in which the CdS/P(S-DVB)
nanocomposites showed a yellow colour, instead of the white appeareance of the
P(S-DVB).
The shape and size of the nanocomposite can be seen in the TEM micrograph
of Figure 4.9, where the micrograph also shown the dark center and pale edge of the
spheres. The dark surface observed in the TEM image indicates that it originated
from CdS nanoparticles. Cadmium as heavy element developed a darker color
compared to carbon and hydrogen in polymer structure. Of interest, TEM
observations showed that cloud-like products were the main morphology, indicating
that CdS nanoparticles were very small and homogeneously dispersed in P(S-DVB)
70
matrix. As can be seen, the spheres exhibit a diameter distribution in the range 200 –
2,000 nm and average diameter of ~500 nm. However, it is difficult to observe the
particle size and size distribution of CdS nanoparticles by visual inspection of the
TEM images, due to the TEM image was obtained by a low accelerating voltage (70
keV), hence the a statistical description of the size distribution of CdS nanoparticles
cannot be carried out.
(a)
(b)
Figure 4.8: SEM images of CdS/P(S-DVB) nanocomposites at (a) 2,500 X and (b)
10,000 X
71
(a)
(b)
Figure 4.9: TEM images of CdS/P(S-DVB) nanocomposites
4.1.2 Fourier Transform Infrared and UV – Vis Diffuse Reflectance
Spectroscopy of CdS/P(S-DVB) Nanocomposites
The pure P(S-DVB) and CdS/P(S-DVB) nanocomposites were characterized
by FTIR spectroscopy. Figures 4.10(a) and 4.10(b) show both of the FTIR spectra
were identical with respect to the main peaks of the polystyrene component. Pure
polymer and the CdS/P(S-DVB) nanocomposites exhibit peaks assigned to the
following groups: OH (at 3,400 – 3,200 cm-1), unsaturated aromatic C-H stretching
vibrations (at 3,025 cm-1), CH2 bending vibration (at 2,920 cm-1 – 2,840 cm-1),
aromatic ring (at 1,510 cm-1), CH2 (at 1,475 cm-1), and peaks associated with various
substitutions of benzene ring between 900 cm-1 and 700 cm-1. Therefore, the
chemical composition of the P(S-DVB) matrix was not changed by the presence of
the CdS nanoparticles as filler, indicating no chemical interaction was involved but
only weak physical interaction between two phases (Chen, M., et al. 2000).
(a)
72
- OH
(b)
Transmittance (%)
C-H2 bending
C-H streching
C-H aromatic
- OH
C-H2 bending
C-H streching
C-H aromatic
4000
3200
2400
1800
1400
1000
600
400
Wavenumbers (cm-1)
Figure 4.10: FTIR spectra of (a) pure P(S-DVB) and (b) CdS/P(S-DVB)
nanocomposites
73
The
CdS/P(S-DVB)
nanocomposites
at
different
Wo
values
were
characterized by UV – Vis DR spectroscopy and the spectra are shown in Figure
4.11. The absorption onset wavelength of CdS nanoparticles determined from the
absorption UV – Vis DR spectra was significantly blue-shifted compared to bulk
CdS. The absorption onset wavelength and particle sizes at Wo = 4, 5.55 and 7.5
calculated using Brus equation are 425 nm (3.1 nm), 462 nm (4.1 nm), 483 nm (5.0
nm), respectively. The blue shifting is indicative of quantum confinement effect due
to decreasing particle size and dependence of the particle size of CdS/P(S-DVB)
nanocomposites on Wo value. Quantum size effects were observed also through an
obvious difference in colours of bulk (orange) and nanoparticle samples (yellow).
Absorbance (a.u)
(c) Wo = 7.5 (5.0 nm)
(b) Wo = 5.55 (4.1 nm)
(a) Wo = 4 (3.1 nm)
P(S - DVB)
380
420
460
500
540
580 600
Wavelength (nm)
Figure 4.11: UV-Vis DR spectra of pure P(S-DVB) and CdS/P(S-DVB)
nanocomposites prepared by in-situ polymerization in miniemulsion system at
different Wo value (a) Wo = 4, (b) Wo = 5.55, and (c) Wo = 7.5
74
A comparison of particles size of CdS/P(S-DVB) nanocomposite at Wo = 4,
5.55 and 7.5 showed a correlation between particle size. Thus, the particle size is
expected to increase with increasing water content. This in turn allows us to obtain
desired size of nanoparticles by controlling the amount of water in the miniemulsion
system. The CdS particles with Wo = 7.5 exhibit larger size than those of particle
with Wo = 4 and 5.55.
4.1.3 Dielectric and Electrical Properties of CdS/P(S-DVB) Nanocomposites
Figure 4.12 shows the dielectric constant of pure P(S-DVB) and CdS/P(SDVB) nanocomposites over a frequency range of 100 Hz – 1 MHz. Dielectric
properties of the nanocomposites are generally enhanced with increasing amount of
CdS as compared to the pure P(S-DVB).
Dielectric constant
250
200
a.
b.
c.
d.
e.
150
P(S-DVB)
CdS/P(S-DVB) 0.01 %
CdS/P(S-DVB) 0.02 %
CdS/P(S-DVB) 0.03 %
CdS/P(S-DVB) 0.18 %
100
50
e
d
c
0
b
2
3
a
4
5
Log frequency (Hz)
6
Figure 4.12: Dielectric constant for P(S-DVB) and CdS/P(S-DVB) nanocomposites
with different amount of CdS as a function of various frequencies
75
Meanwhile the dissipation factors of the nanocomposites were higher than
those of P(S-DVB) as shown in Figure 4.13. The result suggests the presence of
charge carrier with strong mobility and polarizability of the electron attributed
largerly to excess polarity of the CdS nanoparticles. According to Ghosh et al. (2005)
and El-Tantawy et al. (2004), the possible reasons to explain the enhancement of
dielectric properties, especially at low frequencies are, firstly, due to increasing
interfacial interaction, including intermolecular and intramolecular, between CdS
nanoparticles–CdS nanoparticles and also polymer particle–CdS nanoparticles.
Secondly, increasing mobility of charge carrier and polarizability of electron to a
larger excess with increasing CdS nanoparticle content. Finally, the increased value
of the dielectric constant of the nanocomposites is due to the fact that the CdS
nanoparticle and polymer are disconnected from each other, and under the
application of voltage, these nanoparticles act as nano-dipoles. As CdS particle size
is in the order of nanometers, the number of particles per unit volume increases,
hence dipole moment per unit volume also increases, therefore dielectric constant
also increases.
1.18
1.6
0.75
0.49
0.2
0.2
0.38
0.09
0.2
0.4
Pure P(S-DVB)
0.18 % CdS/P(S-DVB)
0.2
0.8
0.39
Dissipation factor
1.2
0
2
3
4
5
Log frequency (Hz)
6
Figure 4.13: Dissipation factor of (a) pure P(S-DVB) and (b) 0.18 % CdS/P(S-DVB)
nanocomposites
76
The schematic illustration of the mechanism of interfacial interactions in
CdS/P(S-DVB) nanocomposites is shown in Figure 4.14.
Intermolecular interaction
CdS nanoparticle – CdS nanoparticle
Intramolecular interaction
CdS nanoparticle – CdS nanoparticle
Intramolecular interaction
CdS nanoparticle – polymer
Increase dielectric constant
Figure 4.14: Schematic representation of the mechanism of interfacial interaction in
CdS/P(S-DVB) nanocomposites
The variation of dielectric constant of the CdS/P(S-DVB) nanocomposites
with temperature is shown in Figure 4.15. The nanocomposites displayed a
decreasing trend in dielectric constant with increasing temperature. This
phenomenon is explained to correspond to the occurrence of two kinds of behaviors,
which would yield converse effect on dielectric constant of the nanocomposites by
changing the temperature. First, increasing temperature would improve the segmental
mobility at the polymer matrix, facilitate the polarization of polar fillers and increase
dielectric constant consequently. Second, the obvious difference in thermal
expansion coefficient of polymer and CdS nanoparticles would disturb the
aggregations of polar component, and thus reduce the dielectric constant. Since
polystyrene is a of kind polymer with glass transition (Tg) temperature at 95 oC, the
second behavior is the main effect (Wikes et al., 2005). It is believed that the
disruption of the CdS nanoparticles cluster that have caused the dielectric constant to
decrease with increase temperature.
77
120
Dielectric constant
100
100 Hz
80
1000 Hz
60
10 KHz
100 KHz
40
1 MHz
20
0
75
50
25
150
100
Temperature (oC)
Figure 4.15: Influence of temperature on dielectric constant of 0.03 % CdS/P(SDVB) nanocomposites at various frequencies.
90
Percentage decrease (%)
80
80
75
70
60
50
38
40
30
20
37
20
10
0
2
3
4
5
6
Log frequency (Hz)
Figure 4.16: Percentage decrease of dielectric constant of 0.03 % CdS/P(S-DVB)
nanocomposites at various frequencies after heat treatment at 150o C
78
The result was further supported by the lower content of CdS and the lower
dielectric constants of nanocomposites measured after heat treatment, where
dielectric constant abruptly decreased. Figure 4.16 shows the percentage decrease of
dielectric constant of 0.03 % CdS/P(S-DVB) nanocomposites measured after heat
treatment relative to that before heat treatment (at room temperature) at various
frequencies.
Figure 4.17 shows the ionic conductivity of CdS/P(S-DVB) nanocomposite at
different amount of CdS The ionic conductivity of the CdS/P(S-DVB)
nanocomposite are increase when compared with pure P(S-DVB). This result
suggested the CdS nanoparticles play a role in increasing the ionic conductivity due
to the mobility of electron between CdS nanoparticles – CdS nanoparticles and also
CdS nanoparticles – polymer particles.
3.00E-09
Ionic conductivity (S/ohm x 10-10)
2.50E-09
2.00E-09
1.50E-09
y = 1E-08x
R2 = 0.9998
1.00E-09
5.00E-10
0.00E+00
0
0.05
0.1
0.15
0.2
CdS amount (%)
Figure 4.17: Ionic conductivity of CdS/P(S-DVB) nanocomposites at different
amount of CdS
79
4.1.4 Thermal Properties of CdS/P(S-DVB) Nanocomposites
Figure
4.18
shows
the
thermal
conductivity
of
CdS/P(S-DVB)
nanocomposites, CdS nanoparticles and pure P(S-DVB) at room temperature. The
thermal conductivity of the CdS/P(S-DVB) nanocomposites is slightly enhanced
compared to pure polymer. Two possible reasons are suggested to explain the results.
Firstly, the conductive path was enhanced as the consequence of increasing amount
of CdS nanoparticles at the CdS/P(S-DVB) nanocomposites, because the CdS
nanoparticles are capable to conduction transfer through the specimen sample
(polymer matrix). Secondly, the interfacial thermal resistance among CdS
nanoparticles between polymer matrix is reduced, leading to the increase in thermal
conductivity. From the Figure 4.18 is clear that the addition of CdS improves
thermal conductivity to an almost constant value
Thermal conductivity (W m-1K-1)
0.2
0.15
0.1
0.05
0
1a
2b
3c
d4
e5
Figure 4.18: Thermal conductivity of (a) pure P(S-DVB), (b) 0.01 % CdS/P(SDVB), (c) 0.02 % CdS/P(S-DVB), (d) 0.03 % CdS/P(S-DVB) and (e) 0.18 %
CdS/P(S-DVB) nanocomposites at room temperature.
80
Figure 4.19 shows the TGA curve of the pure P(S-DVB) and 0.18 %
CdS/P(S-DVB) nanocomposites carried out under an oxygen atmosphere in the
temperature range of 40 – 1000 oC. The weight loss of around 310 - 400 oC in the
TGA is contributed by the degradation of P(S-DVB). A slight shift in the TGA
profile toward a higher temperature indicated a small but significant increase in
thermal stability of the CdS/P(S-DVB) nanocomposites. The possible reason for the
increased thermal stability of the nanocomposite is the reduced molecular mobility of
the polymer chain due to the presence of CdS nanoparticles as filler. This
observation was supported by the residue of the CdS/P(S-DVB) nanocomposites that
still remained at temperature above 400 oC which was less than 1 %, whereas the
pure polymer was completely decomposed at these temperatures.
100
(a) Pure P(S-DVB)
Weight loss (%)
80
(b) 0.18 % CdS/P(S-DVB)
nanocomposite
60
40
20
0
100
200
300
400
500
600
700
800
900
1000
Temperature (oC)
Figure 4.19: Thermogravimetric analyses curves of (a) pure P(S-DVB), and (b) 0.18
% CdS/P(S-DVB) nanocomposites
81
4.2
Synthesis
and
Physicochemical
Sulfide/Poly(methacrylic
acid-ethylene
Properties
glycol
of
Cadmium
dimethacrylic
acid)
[CdS/P(MAA-EGDMA)] Nanocomposites
4.2.1 Synthesis
of
P(MAA-EGDMA)
and
CdS/P(MAA-EGDMA)
Nanocomposites
The
P(MAA-EGDMA)
particles
has
been
synthesized
by
in-situ
polymerization in a miniemulsion system. The miniemulsion system was composed
of CTABr as surfactant, 2-propanol as co-surfactant, MAA-EGDMA monomers and
n-decane as oil-phase and water. Schematic representation of the synthesis of
P(MAA-EGDMA) is illustrated in Figure 4.20.
As previously discussed in section 4.1 the in-situ polymerization in a
miniemulsion system has produced particle polymer. The result was supported by the
SEM and TEM studies in which those images showed of the polymer with particles
size in the range 100 – 300 nm.
The CdS/P(MAA-EGDMA) nanocomposites were synthesized from P(MAAEGDMA) by ion exchange with various concentration of cadmium ion and
precipitation with various concentration of sulfide ion. The mechanism of ion
exchange and precipitation process as is proposed to include the replacing of
hydrogen ion at carboxylic acid groups (-COOH) with cadmium ion and then
followed by precipitation reaction with sulfide ion to produce CdS nanoparticles on
the surface of polymer matrix. The schematic diagram for the synthetic scheme and
reaction of polymerization are illustrated in Figure 4.21 and 4.22, respectively. The
carboxylic acids as functional groups of the polymer were suggested to control
particle size and in-situ formation of CdS nanoparticles on the surface of polymer
matrix and to produce the yellow nanocomposites. These results were supported by
UV – Vis DR spectra in which the spectra showed significantly blue shifted onset
wavelength compared to bulk CdS.
82
Monomer
Co-stabilizer
n-decane
Surfactant
Water
Stirring,
Ultrasonication
Surfactant
M
M
M
Monomer in miniemulsion
Polymerization
Polymer particles
Figure 4.20: Schematic representation of synthesis of P(MAA-EGDMA) particles
83
O
H3C
H3C
O
+
H2C
O
H2C
OH
CH2
O
O
In-situ polymerization
CH3
O
O
OH
O
O
O
OH
O
OH
OH
O
Cd2+
O
O
O
OH
O
OCd2
O+
O
OCd2+
O
O
OH
O
OH
S2-
O
O
O
O CdS
O
O
O
O
Cd
OS
OH
O
Figure 4.21: Schematic representation of preparation of CdS/P(MAA-EGDMA)
nanocomposites
84
H3C
O
H3C
O
+
H2C
OH
O
H2C
AIBN, 70oC
CH2
O
O
in-situ polymerization
CH3
in miniemulsion system
Figure 4.22: Reaction of methacrylic acid and ethylene glycol dimethacrylic acid by
free radical polymerization into P(MAA-EGDMA).
4.2.2 Fourier Transform Infrared and UV – Vis Diffuse Reflectance
Spectroscopy of CdS/P(MAA-EGDMA) Nanocomposites
It is known that the CdS semiconductor nanoparticles have unique sizedependent chemical and physical properties. As the size of CdS semiconductor
particles decreases to the nanoscale, the band gap of the CdS semiconductor
increases, causing a blue shift in the UV – Vis wavelength. The bulk CdS materials
exhibit an onset wavelength in the range of 515 – 520 nm. If the onset wavelength of
the obtained CdS sample appeared blue-shifted compared with that of bulk CdS, it
could indicate the presence of CdS nanosize.
85
The
P(MAA-EGDMA),
CdS/P(MAA-EGDMA)
nanocomposites
with
different CdS content, CdS nanoparticles and bulk CdS were characterized by UV–
Vis DR spectroscopy (see Fig. 4.23). The spectrum of pure polymer did not exhibit
absorption onset wavelength as it did not absorb in the UV – Vis range. The spectra
of CdS/P(MAA-EGDMA) nanocomposites were significantly blue-shifted compared
to CdS nanoparticle and bulk CdS, in agreement with quantum confinement effect
due to decreasing particle size.
Bulk CdS
Absorbance (a.u )
f
CdS nanoparticles
e
d
c
b
a
P(MAA - EGDMA)
380
420
460
500
540
580
Wavelength (nm)
Figure 4.23: UV-Vis DR spectra of (a) P(MAA-EGDMA), (b) 0.81 % CdS/P(MAAEGDMA), (c) 0.90 % CdS/P(MAA-EGDMA), (d) 1.08 % CdS/P(MAA-EGDMA),
(e) 1.23 % CdS/P(MAA-EGDMA), (f) 1.70 % CdS/P(MAA-EGDMA) and (g) 2.07
% CdS/P(MAA-EGDMA)
Considering that the absorption of CdS particles in the composites increased
with the CdS content, only a rough estimation of the particle size could be made for
CdS particles in nanocomposites by UV-Vis DR spectrometry. Hence, absorption
onset wavelength value was utilized to calculate the particle size using the Brus
equation. According to the calculation, the sizes of CdS particles is about 7.1 – 8.0
86
nm (see Table 4.3). This result revealed that ion-exchange and precipitation
processes with various concentration of cadmium and sulfide ion could control the
growth, nucleation and formation of CdS nanoparticles on the surface of P(MAAEGDMA).
Table 4.3: The absorption onset wavelength and particle size of CdS nanoparticles in
CdS/P(MAA-EGDMA) nanocomposites with various CdS contents
Nanocomposites sample
Onset wavelength (nm)
Particle size (nm)
0.81 % CdS/P(MAA-EGDMA)
504
7.1
0.90 % CdS/P(MAA-EGDMA)
507
7.3
1.08 % CdS/P(MAA-EGDMA)
508
7.3
1.23 % CdS/P(MAA-EGDMA)
512
7.6
1.70 % CdS/P(MAA-EGDMA)
515
7.8
2.07 % CdS/P(MAA-EGDMA)
520
8
The FTIR spectra of the series of P(MAA-EGDMA), CdS/P(MAA-EGDMA)
nanocomposites and CdS/P(MAA-EGDMA) physical mixing are shown in Figure
4.24. These spectra show the main peaks due to the P(MAA-EGDMA) component
that are at 3,250 cm-1 (C-H alkene), 2,920 cm-1 and 2,850 cm-1 (C-H alkane), 1,730
cm-1 (C=O carboxylic acid), 1,600 cm-1 and 1,580 cm-1 (C=C alkene), and 1,160
cm-1 (C-O carboxylic acid). Two new peaks appeared in the spectra of CdS/P(MAAEGDMA) nanocomposites at 1,550 cm-1 and 1,340 cm-1 which are the consequence
of the ionization of carboxylic acid groups by cadmium ion. The ionization leads to
the equilibration of the two oxygen atoms attached to carbon, resulting in the
appearance of two peaks (in the 1,610 cm-1 – 1,550 cm-1 and 1,420 cm-1 – 1,300
cm-1), which correspond to the symmetrical and asymmetrical vibrations of
carboxylic ion. Similar results were obtained in the case of CdS/P(S-Mac)
nanocomposites (Nair et al., 2005). Meanwhile, different results were obtained for
CdS/P(MAA-EGDMA) prepared by physical mixing as there were no appearance of
both of these peaks. The FTIR results revealed that the chemical bonding between
CdS nanoparticles with carboxylic group at polymer matrix occurred by ion
exchange and precipitation process.
87
P(MAA-EGDMA)
C=O carboxylic acid
-C-O
carboxylic acid
-CH alkane
Transmittent (a.u)
-CH alkene
New peak
CdS/P(MAA-EGDMA)
nanocomposites
Physically mixed
CdS/P(MAA-EGDMA)
4000
3200
2400
1800
1400
1000
600
400
Wavenumbers (cm-1)
Figure
4.24:
FTIR
spectra of P(MAA-EGDMA),
CdS/P(MAA-EGDMA)
nanocomposites and physically mixed CdS/P(MAA-EGDMA)
88
4.2.3 Structural and Morphological Properties of CdS/P(MAA-EGDMA)
Nanocomposites
The morphologies of the CdS/P(MAA-EGDMA) nanocomposites prepared
by ion-exchange and precipitation were examined by SEM and TEM (see Figure
4.25 and Figure 4.26). According to SEM micrograph, non-agglomerated (fused) and
some agglomerated particles were disperses throughout the nanocomposite. But the
fraction of non-agglomerate particles was smaller. From SEM images it is clear that
the agglomerated CdS nanoparticles were attached on the surface of P(MAAEGDMA) with sizes below 20 nm. The Energy Dispersive X-ray (EDX) analyses
were utilized to confirm the existence of cadmium and sulfur in the CdS/P(MAAEGDMA) nanocomposites. However, the smaller the CdS nanoparticles are the
larger is their internal surface and hence their tendency to agglomerate rather than to
disperse homogenously in the polymer matrix. The resulting materials may be seen
also as filled polymers since there is no or little interaction between two mixed
components. The inorganic, relatively small cadmium ion is charged against more
polymer (organic) molecules. The exchange reaction has two consequences: First,
the polymer swells and the gaps between polymer chains are widened, enabling CdS
nanoparticles to more in between them and secondly, the surface property of the
polymer is changed from hydrophobic to hydrophilic.
The TEM images also show little agglomeration tendency of the
CdS/P(MAA-EGDMA) nanocomposites. The dark shadow around the surface of the
polymer at TEM images again suggested it came from CdS nanoparticles. This
results was supported by the filled surface of P(MAA-EGDMA) that has been filled
with CdS nanoparticles. However, it is difficult to observe the particle size and size
distribution of CdS nanoparticles by visual inspection of the TEM images, due to the
TEM image was obtained by a low accelerating voltage (70 keV), hence the particle
size and distribution of CdS nanoparticle cannot be determined. The SEM and TEM
images also showed the particles sizes of P(MAA-EGDMA) as host matrix in the
nanosize range of 100 - 300 nm.
89
(a)
(b)
Figure 4.25: SEM images of 2.07 % CdS/P(MAA-EGDMA) nanocomposites at
magnification (a) 50,000 X and (b) 100,000 X
90
(a)
200 nm
(b)
Figure 4.26: TEM images of (a) agglomerated and (b) fused samples 2.07 %
CdS/P(MAA-EGDMA) nanocomposites
The
structure
of
P(MAA-EGDMA)
and
CdS/P(MAA-EGDMA)
nanocomposites were investigated by XRD. Figure 4.27 show the XRD pattern of
P(MAA-EGDMA) and CdS/P(MAA-EGDMA) nanocomposites with different
amounts of CdS nanoparticles.
91
2.07 % CdS/P(MAA-EGDMA)
Relative intensity (a.u)
1.70 % CdS/P(MAA-EGDMA)
1.23 % CdS/P(MAA-EGDMA)
1.08 % CdS/P(MAA-EGDMA)
0.90 % CdS/P(MAA-EGDMA)
0.81 % CdS/P(MAA-EGDMA)
Pure P(MAA-EGDMA)
20
30
40
50
60
70
2-Theta degree
Figure 4.27: XRD patterns of pure P(MAA-EGDMA) and CdS/P(MAA-EGDMA)
nanocomposites at various CdS amount
92
The XRD pattern of the nanocomposites exhibit broad peaks corresponding
to 111, 220, and 311 (plane) indicating that cubic CdS nanoparticles was mixtured in
polymer matrix. Broad XRD peaks also are attributed to the absence of long-range
order in the samples and reflect the small particles size (less than 10 nm as estimated
from the Brus equation). The polymer particles which are amorphous and present in
large amount also contribute to the peak broadness This result agrees with the
presence of nanocrystalline CdS, for which quantum size effect are expected. The
weak reflection peaks should belong to CdS by comparison with the XRD pattern of
CdS nanoparticles. The low intensity peaks indicated low contents of the CdS
nanoparticle in polymer matrix.
4.2.4 Dielectric
and
Electrical
Properties
of
CdS/P(MAA-EGDMA)
Nanocomposites
Figure 4.28 and Table 4.4 shows of the data on dielectric constant of P(MAAEGDMA), CdS nanoparticles, physically mixed CdS/P(MAA-EGDMA) and
CdS/P(MAA-EGDMA) nanocomposites at various frequencies. The frequency
dependency of the data on dielectric constant of CdS/P(MAA-EGDMA)
nanocomposites is presented in Figure 4.28. In all cases of the CdS/P(MAAEGDMA) nanocomposites, the dielectric constant showed a very steep decrease from
its initial high values. The decrease in dielectric constant is very obvious for the 1.08
% CdS/P(MAA-EGDMA) sample. At high frequencies above 105 Hz, the dielectric
constants remained nearly constant. Interestingly, at 100 Hz, the dielectric constant
for the 1.08 % CdS/P(MAA-EGDMA) sample was about 5,200. This value
decreased to about 320 for the composite samples having 2.07 wt % of CdS. The
result showed a higher dielectric constant as compared to the dielectric constant of
CdS/PVA nanocomposites with average dispersibility in the range of 0.3 – 0.8
(dielectric constant about 120 – 250 over the range of frequency 10 – 40 kHz) (Zhou,
2003), CdS/PVA 20 % (dielectric constant about 10 at frequency 1 kHz) (Ghosh et
al., 2005) and TiO2/sulfonated styrene-b-(ethylene-ran-butylene)-b-styrene (S-SEBS)
block copolymer 6.4 % (dielectric constant about 3 - 3.2 over the frequency range of
1 – 10,000 Hz) (Yang and Kofinas, 2007).
93
6000
5000
Dielectric constant
CdS/P(MAA-EGDMA) 0.81 %
4000
CdS/P(MAA-EGDMA) 0.90 %
3000
CdS/P(MAA-EGDMA) 1.08 %
CdS/P(MAA-EGDMA) 1.23 %
2000
CdS/P(MAA-EGDMA) 1.70 %
1000
CdS/P(MAA-EGDMA) 2.07 %
0
2
3
4
5
Log frequency (Hz)
6
Figure 4.28: Dielectric constant of CdS/P(MAA-EGDMA) nanocomposites with
various amounts of CdS measured at different frequencies
Table 4.4: Dielectric constants of P(MAA-EGDMA), CdS nanoparticles,
CdS/P(MAA-EGDMA) prepared by physical mixing and CdS/P(MAA-EGDMA)
nanocomposites at various frequencies
Log frequency (Hertz)
6
5
4
3
2
P(MAA-EGDMA)
1.6
3.8
4.0
4.9
9.2
CdS nanoparticles
4.6
16.2
28.9
49.8
100
1 % CdS/P(MAA-EGDMA) phys. mix.
0.8
5.2
9.5
22.0
60.6
11.9 % Na/P(MAA-EGDMA)
2.3
6.8
7.0
13.0
42.0
1.4 % Cd/P(MAA-EGDMA)
2.1
5.8
5.9
8.0
15.3
0.81 % CdS/P(MAA-EGDMA)
1.7
5.0
7.7
16.6
73
0.90 % CdS/P(MAA-EGDMA)
3.3
8.7
15.6
59.4
213.8
1.08 % CdS/P(MAA-EGDMA)
22.4
159.9
803.7
2,321
5,245
1.23 % CdS/P(MAA-EGDMA)
3.9
10.7
29.4
143.5
732.2
1.70 % CdS/P(MAA-EGDMA)
1.5
7.6
23.9
150.7
455.1
2.07 % CdS/P(MAA-EGDMA)
3.3
10.6
36.0
157.5
320.6
94
Figure 4.29. shows the histogram of the dielectric constants between
CdS/P(MAA-EGDMA) nanocomposites with CdS nanoparticles, pure P(MAAEGDMA) and 1.00 % CdS/P(MAA-EGDMA) (physical mixing) measured at 100
Hz. It was found that the maximum value of 5245.6 was achieved for at the 1.08 %
concentration of CdS, which is far higher (ca. 500 times) than the value of obtained
for pure P(MAA-EGDMA). On the other hand, the value of the dielectric constant of
pure P(MAA-EGDMA), CdS nanoparticle, 1 % CdS/P(MAA-EGDMA), 11.9 %
Na/P(MAA-EGDMA) and 1.4 % Cd/P(MAA-EGDMA) at this frequency are about
10, 100, 60, 42 and 15, respectively. The enormous enhancement of properties at the
nanocomposite containing 1.08 % CdS may be related to the effect of adding CdS
nanoparticles.
5245.6
6000
4000
g
i
j
320.6
f
455.1
213.8
d
73
42
c
15.3
60.6
1000
100
2000
732.2
3000
9.2
Dielectric constant
5000
0
a
b
e
h
k
Figure 4.29: Dielectric constant of (a) P(MAA-EGDMA), (b) CdS nanoparticles, (c)
CdS/P(MAA-EGDMA) physical mixing, (d) 11.9 % Na-P(MAA-EGDMA), (e) 1.4
% Cd-P(MAA-EGDMA), (f) 0.81 % CdS/P(MAA-EGDMA), (g) 0.90 %
CdS/P(MAA-EGDMA),
(h)
1.08
%
CdS/P(MAA-EGDMA),
(i)
CdS/P(MAA-EGDMA),
(j) 1.70 % CdS/P(MAA-EGDMA) and (k)
1.23
%
2.07 %
CdS/P(MAA-EGDMA) at 100 Hz.
To better understand the above phenomenon, FTIR measurements were
carried out on CdS nanoparticles, pure P(MAA-EGDMA), CdS/P(MAA-EGDMA)
1.00 % (physical mixing) and CdS/P(MAA-EGDMA) with various concentrations of
95
CdS. The broad feature between 3,500 cm-1 and 2,500 cm-1 was undoubtedly due to
the O-H stretch of the carboxylic acid. Two bands at 2,920 cm-1 and 2,850 cm-1,
which were superimposed on the O-H stretch, were attributed to the asymmetric CH2
stretch and the symmetric CH2 stretch, respectively (Smith, 1999). The intense peak
at 1,710 cm-1 was derived from the existence of the C=O stretch and the band at
1,282 cm-1 exhibited the presence of the C-O stretch (Smith, 1999). As shown in Fig.
4.30, it is worth noting that the C=O stretch band of the carboxyl group, which is
present at 1,710 cm-1 in the FTIR spectrum of pure P(MAA-EGDMA), is lowered in
the spectra of the CdS/P(MAA-EGDMA) nanocomposites. Instead, a new band at
1,350 cm-1 appeared. This peak is characteristic of the symmetric (COO-) stretch.
This reveals that CdS nanoparticels are chemisorbed onto carboxylate functional
groups of P(MAA-EGDMA).
To be more comprehensive, Figure 4.31 shows the intensity of the IR peak at
1,350 cm-1 increasing with decreasing intensity of IR peak at 1,710 cm-1, which can
be associated to the degree of the interfacial interaction between CdS and polymer.
There was no detectable interfacial interaction in 1.00 % CdS/P(MAA-EGDMA)
(physical mixing) and CdS/P(MAA-EGDMA) when the concentration of CdS was
lower than 1.08 wt % since the peak at 1,350 cm-1 was not observed in the FTIR
spectra. The origin of the high dielectric constant of 1.08 % CdS/P(MAA-EGDMA)
is still uncertain and not fully understood yet. Because the nanocomposite consists of
CdS attached on the surface of polymer, it likely that the dielectric constant may
have increased due to interfacial and space charge polarization over a frequency
range of 100 Hz to 106 Hz, which is associated with an inhomogeneous dielectric
medium containing two layers of materials with different permittivity and
conductivity (Kremer and Schonhals, 2002). A very steep decrease in dielectric
constant with increasing frequency in the 1.08 % CdS/PMAA-EGDMA sample
suggested the occurrence of a strong space charge polarization effect in this
nanocomposite. As described above, 1.08 % CdS/P(MAA-EGDMA), having the
highest dielectric constant of 5,200, showed a high intensity of IR peak at 1350 cm-1
which is attributed to the interfacial interaction between CdS nanoparticles and
P(MAA-EGDMA) (see Figure 4.31).
96
C dS
nanopa rticles
P (MAA- EGDM A)
CdS /P( MAA-EGD MA)
(physical mixing) 1.00 %
Transmittance (%)
CdS/P( MA A-EGD MA)
0.81 %
C dS/P(M AA-EGDMA)
0.9 %
CdS/P( MA A-EGD MA)
1.08 %
CdS /P( MAA-EGD M A)
1.23 %
CdS /P( MA A-EGD M A)
1.07 %
CdS /P( MA A-EGD M A)
2.07 %
2850
2920
1350
1710
4000
3600
3200 2800
2400
20 00
1800
1600
1160
1282
1400
Wa venumber / cm
1200
10 00
800
600
-1
Figure 4.30: FTIR spectra of P(MAA-EGDMA), CdS/P(MAA-EGDMA) physical
mixing, CdS nanoparticle and CdS/P(MAA-EGDMA) nanocomposites with
various content of CdS
97
Intensity of IR peak at 1710 cm -1 / a.u.
Intensity of IR peak at 1350 cm-1 / a.u.
P(MAA-ED MA)
P(MAA-EGDMA)
CO O – CdS
CO O–
11.= P(MAA-EGDMA)
P(MA A- EDMA)
P(MAA-EGDMA)
22.= CdS/
Cd S-P(
MAA-ED MA)1.00
1.0%
0%
Phys
mix l mix ing)
(ph ysica
P(MAA-EGDMA)
33.= CdS/
Cd S-P(
MAA-ED MA)0.81
0.8%
1%
4.
CdS/
P(MAA-EGDMA)
0.90
4 = Cd S-P( MAA-ED MA) 0.9%
0%
P(MAA-EGDMA)
55.= CdS/
Cd S-P(
MAA-ED MA)1.08
1.0%
8%
P(MAA-EGDMA)
66.= CdS/
Cd S-P(
MAA-ED MA)1.23
1.2%
3%
P(MAA-EGDMA)
77.= CdS/
Cd S-P(
MAA-ED MA)1.70
1.7%
0%
P(MAA-EGDMA)
88.= CdS/
Cd S-P(
MAA-ED MA)2.07
2.0%
7%
1
P(MAA-EGDMA)
P(MAA-EDM A)
2
3
4
interphase
5
6
7
8
CdS
Figure 4.31: Schematic representation of interfacial interaction in the CdS/P(MAAEGDMA) nanocomposites
Even though CdS/P(MAA-EGDMA) nanocomposites with CdS nanoparticles
content higher than 1.08 % showed a similar degree of the interfacial interaction,
their dielectric constant is very much lower than those of 1.08 % CdS/P(MAAEGDMA). The presence of multilayer CdS nanoparticles on the surface of P(MAAEGDMA) at a high concentration could be the reason of the low dielectric constant
of the CdS/P(MAA-EGDMA) sample containing CdS nanoparticles higher than 1.08
98
%. On the basis of this consideration, we model a dielectric nanocomposite as a
three-phase material, consisting of a polymer, an interfacial phase, and CdS
nanoparticles, schematically shown in Figure. 4.31.
The interfacial phase is between the polymer and CdS nanoparticles. It is
reasonable to assume that the interfacial area has fixed surface area, since the IR
peak at 1,350 cm-1, attributed to the interfacial interaction between polymer and CdS
nanoparticle was unchanged. The interfacial areas in the nanocomposite could
promote interfacial exchange coupling through a dipolar interface layer leading to
enhanced polarization and polarizability in polymer phase near the interface (Zhang
et al. 2002, Li, 2003). As a result, enhanced permittivity can be expected in the
polymer phase near the interfaces. The schematic model was also supported by the
SEM images of 0.81 %, 1.08 % and 2.07 % CdS/P(MAA-EGDMA) nanocomposites
of CdS with magnification of 100,000 and 150,000 X (see Figure 4.32). The SEM
images of 0.81 % and 1.08 % CdS/P(MAA-EGDMA) nanocomposites show smaller
particle size as compared to that at 2.07 %. There are two kinds of interfacial
interactions in the proposed model shown in Fig. 4.31: (i) intermolecular interaction
between CdS nanoparticles – P(MAA-EGDMA) and (ii) intermolecular interaction
between CdS nanoparticles – CdS nanoparticles.
Only the first layer of CdS nanoparticles contributed to the interfacial
interaction of CdS – polymer and hence to the interfacial space charge polarization or
polarization Maxwell-Wagner. The high electrical conductivity of CdS/P(MAAEGDMA) at the same amount of CdS supported the occurance of polarization
Maxwell-Wagner. The next layer of CdS nanoparticles lowered the space charge
polarization caused by interaction between CdS nanoparticles – CdS nanoparticles.
One explains that an external electric field, inducing an electric current along the
material, produces mobility of charge carrier across the interface between CdS
nanoparticles. Therefore, the decrease in the dielectric constant of CdS/P(MAAEGDMA) nanocomposites with CdS content higher than 1.08 % is observeb.
99
(a)
(b)
(c)
Figure 4.32: SEM images of CdS/P(MAA-EGDMA) at various amount of CdS (a)
0.81 % , (b) 1.08 % and (c) 2.07 %
The dissipation factors of CdS/P(MAA-EGDMA) nanocomposites at various
contents of CdS nanoparticles and frequencies over the range of 0.10 – 1,000 KHz
are shown in Figure 4.33 and Table 4.5.
100
Figure 4.33: Dissipation factor of P(MAA-EGDMA) and CdS/P(MAA-EGDMA)
nanocomposites at various CdS contents and frequencies (a) P(MAA-EGDMA), (b)
0.81 % CdS/P(MAA-EGDMA), (c) 0.90 % CdS/P(MAA-EGDMA), (d) 1.08 %
CdS/P(MAA-EGDMA),
(e)
1.23
%
CdS/P(MAA-EGDMA),
(f)
1.70
%
CdS/P(MAA-EGDMA), and (g) 2.07 % CdS/P(MAA-EGDMA)
Table 4.5: Dissipation factors of P(MAA-EGDMA), CdS nanoparticles and
CdS/P(MAA-EGDMA) nanocomposites
Sample
Log frequency ( Hz)
6
5
4
3
2
P(MAA-EGDMA)
0.01
0.02
0.01
0.04
0.24
CdS nanoparticles
0.20
0.20
0.20
0.50
1.60
0.81 % CdS/P(MAA-EGDMA)
0.02
0.03
0.10
0.30
1.30
0.90 % CdS/P(MAA-EGDMA)
0.06
0.04
0.27
0.53
0.48
1.08 % CdS/P(MAA-EGDMA)
0.87
2.48
1.11
1.35
0.53
1.23 % CdS/P(MAA-EGDMA)
0.02
0.13
0.67
0.35
0.10
1.70 % CdS/P(MAA-EGDMA)
0.34
0.25
0.83
0.35
0.30
2.07 % CdS/P(MAA-EGDMA)
0.86
0.85
0.31
0.29
0.31
101
Most of the dissipation factors was higher in values in comparison to those of
P(MAA-EGDMA). The result suggests the appearance of charge carrier with strong
mobility and polarizability of electron to a larger excess polarity for the CdS
nanoparticles.
Figure 4.34 shows the ionic conductivity of P(MAA-EGDMA), CdS
nanoparticles and CdS/P(MAA-EGDMA) nanocomposites. The ionic conductivity
values of the nanocomposites at various CdS content seem to be following the trend
for dielectric properties of CdS/P(MAA-EGDMA) nanocomposites. The highest
ionic was found in 1.08 % CdS/P(MAA-EGDMA). The enormous enhancement in
ionic conductivity again might be cause by the same reasons specified for the
dielectric properties.
2.9E-06
3.0E-06
2.0E-06
6.4E-08
2.1E-07
2.3E-07
7.3E-08
b
4.1E-08
a
5.0E-09
5.0E-07
4.0E-08
1.0E-06
6.2E-08
1.5E-06
2.9E-09
Ionic conductivity
2.5E-06
0.0E+00
c
d
e
f
g
h
i
j
Sample
Figure 4.34: Ionic conductivity of (a) P(MAA-EGDMA), (b) CdS nanoparticles, (c)
11.9 % Na-P(MAA-EGDMA), (d) 1.4 % Cd-P(MAA-EGDMA), (e) 0.81 %
CdS/P(MAA-EGDMA),
(f)
0.90
%
CdS/P(MAA-EGDMA),
CdS/P(MAA-EGDMA), (h) 1.23 % CdS/P(MAA-EGDMA),
CdS/P(MAA-EGDMA) and (j) 2.07 % CdS/P(MAA-EGDMA)
(g)
1.08
%
(i) 1.70 %
102
4.2.5 Influence of Temperature on Dielectric and Electrical Properties of
CdS/P(MAA-EGDMA) Nanocomposites
The temperature dependence of dielectric constant in nanocomposites is
important for any potential use in electronic devices, because one expects that there
is the dependence of the orientation polarizability of the nanocomposites on
temperature. Figure 4.35 shows the dielectric constant of the CdS/P(MAA-EGDMA)
nanocomposites at different temperatures. The dielectric properties of the
nanocomposites display an upward trend over the frequency range of 0.1 – 1,000
KHz as the temperature increased from 25 to 75 oC, but abruptly decreased after 75
o
C. This phenomenon is explained by the occurrence of two kinds of behaviors,
which would yield converse effect on dielectric constant of the nanocomposites by
changing of temperature. First, increasing temperature would improve the segmental
mobility of polymer and CdS nanoparticles, facilitate the polarization of polar CdS
fillers and increase dielectric constant consequently. Second, the obvious differential
in thermal expansion coefficient between polymer and CdS nanoparticles would
disturb the aggregations of polar components, and thus reduced the dielectric
constant. At temperature range of 25 – 75 oC for the CdS/P(MAA-EGDMA)
nanocomposites, the first behavior seems to be the main effect. On the other hand, at
temperatures above 75 oC, the second behavior is more dominant. It should be the
disruption of the CdS nanoparticles cluster that caused the decreasing dielectric
constant with increasing temperature. The results were also supported by the
dielectric constant of nanocomposites measured after heat treatment, showing that
the dielectric constants were slightly decreased.
Figure 4.36 shows the percentage decrease of dielectric constant of 1.70 %
CdS/P(MAA-EGDMA) nanocomposites measured after heat preparation at 150 oC in
comparison to those measured at room temperature before treatment at various
frequencies.
103
2500
100 Hz
2000
.
Dielectric constant
1500
1 kHz
10 kHz
1000
100 kHz
500
0
1000 kHz
0
50
100
o
Temperature C
150
Figure 4.35: Influence of temperature on dielectric constant of 1.70 % CdS/P(MAAEGDMA) nanocomposites at different frequencies
14
12
12
Percentage decrease (%)
.
10
8
6
4
2
1
0.5
0.5
0.5
0
2
3
4
5
Log frequency (Hz)
6
Figure 4.36: Percentage decrease of dielectric constant of 1.70 % CdS/P(MAAEGDMA) nanocomposites after heat treatment and at various frequencies
104
The ionic conductivity of the CdS/P(MAA-EGDMA) nanocomposites at
different temperatures is shown in Figure 4.37. The nanocomposites display a
decreasing trend in dielectric constant with increasing temperature. The reason for
this is the destruction of percolation of CdS nanoparticles as result of the thermal
expansion of polymer at higher temperatures
Electrical conductivity (S/ohm)
1.20E-05
1.00E-05
8.00E-06
6.00E-06
4.00E-06
2.00E-06
0.00E+00
30
40
50
60
70
80
90
100
Temperature (oC)
Figure 4.37: Influence of temperature on ionic conductivity of CdS/P(MAAEGDMA) nanocomposites
4.2.6 Thermal Properties of CdS/P(MAA-EGDMA) Nanocomposites
Figure 4.38 shows the thermal conductivities for P(MAA-EGDMA), CdS
nanoparticle and CdS/P(MAA-EGDMA) nanocomposites. The thermal conductivity
of nanocomposites was enhanced with increasing content of CdS nanoparticle
compared to the pure polymer employed. Two possible reasons are put forward to
explain the results. Firstly, the conductive path at the nanocomposites was enhanced
partly as a consequence of the incorporated CdS nanoparticle. This is because the
105
CdS nanoparticles are more effective in transfering heat through the specimen. The
aggregates are more effective in forming heat conduction in the polymer matrix as
host of the nanocomposites to transfer heat through the specimen and thereby
thermal conductivity enhances with increasing filler content. Secondly, the interfacial
thermal resistance among CdS nanoparticles/polymer is reduced, leading to the
increase in thermal conductivity. The result clearly concluded that the thermal
conductivity of nanocomposite was mostly dependent on the content and properties
of the CdS nanoparticles. Similar results and reasons were reported in the case of
CdS/PVA nanocomposites (Ghosh et al., 2005).
0.22
0.25
Thermal conductivity (W/mK)
0.2
0.15
0.0907
0.105
0.1493
0.1539
e
f
0.1063
0.1
0.05
0
a
b
c
d
Figure 4.38: Thermal conductivity of (a) P(MAA-EGDMA), (b) CdS nanoparticle,
(c) 0.81 % CdS/P(MAA-EGDMA), (d) 0.90 % CdS/P(MAA-EGDMA) , (e) 1.08 %
CdS/P(MAA-EGDMA), and (f) 2.07 % CdS/P(MAA-EGDMA) nanocomposites
106
Figure 4.39 shows the thermogravimetric analysis (TGA) curves of P(MAAEGDMA) and CdS/P(MAA-EGDMA)
nanocomposites with different
CdS
nanoparticles content. The TGA curves indicated that the losses of weight occurred
around three temperature periods, ranging from 50 – 100 oC, 250 – 400 oC and 450 –
780 oC. The first stage of mass loss was mainly contributed by the elimination of
impurities and water. This stage of mass loss of the CdS/P(MAA-EGDMA)
nanocomposites showed higher mass loss as compared to pure polymer, indicating
that the surfaces of the nanocomposites are more hydrophilic. The second and third
stages of mass losses were contributed by the thermal degradation of the backbone
structure of polymer. The thermal stability of the nanocomposites improved
significantly at higher decomposition temperatures in the range of 700 – 750 oC. The
result indicated that the addition of CdS nanoparticles to polymer made the polymer
more stable thermally.
100
a
Weight loss (%)
80
b
60
40
c
d
20
0
100
200
300
400
500
600
700
800
900
1000
Temperature (°C)
Figure 4.39: Thermogravimetric analysis curve of (a) P(MAA-EGDMA), (b) 0.90 %
CdS/P(MAA-EGDMA), (c) 1.23 % CdS/P(MAA-EGDMA) and (d) 2.07 %
CdS/P(MAA-EGDMA) nanocomposites
107
The latter was related to the combination of CdS nanoparticles with the
polymer matrix, which as result from the interaction between CdS nanoparticles and
the carboxylic acid groups of the polymer. It can also be seen that the remaining
residue obtained after the thermal decomposition at 750 oC is about 1 – 2 % whereas
the P(MAA-EGDMA) was completely decomposed at these temperatures and left no
residue. Similar thermal behaviour was obtained in the cases of CdS/PVA (ElTantawy et al., 2005), CdS/PS-co-Mac (Nair et al., 2005), CdS/PS (Kuljanin et al.,
2002) and CdS/PMMA (Kuljanin et al., 2006).
4.3
Synthesis
and
Physicochemical
Properties
of
CdS/Sulfonated-
Poly(styrene-divinylbenzene)
4.3.1 Synthesis of P(S-DVB), SO3H-P(S-DVB) and CdS/SO3-P(S-DVB)
Nanocomposites
Schematic representation of the reaction of polymerization is illustrated in
Figure 4.40. The in-situ polymerization in miniemulsion was produced polymer
particles with a dimension in the micrometer range.
CH
o
CH2
CH
CH2
AIBN, 70 C
+
Polymerization
Styrene
Divinyl benzene
CH2
CH
Sulfonation
CH
CH2
CH
n
CH2
CH2
CH
SO3H
n
Figure 4.40: Reaction of styrene and divinyl benzene by free radical polymerization
and sulfonation process to form SO3H/P(S-DVB)
108
The SO3H-P(S-DVB) was synthesized by sulfonation reaction of P(S-DVB)
with fuming sulfuric acid (H2SO4/SO3) at 50 oC for 1 hour. The degree of sulfonation
of SO3H-P(S-DVB) which was determined by titration method was 39.10 %.
Meanwhile, the CdS/SO3-P(S-DVB) nanocomposites were synthesized from SO3HP(S-DVB) by ion exchange and precipitation process with various concentrations of
cadmium ion, followed by reaction with sulfide ion. The mechanism of ion exchange
and precipitation process occurred with replacement of hydrogen ion at sulfonic acid
groups by cadmium ion with various concentrations which was then followed by
precipitation by sulfide ion at various concentrations and producing CdS
nanoparticles on the surface of polymer matrix. The ion exchange, precipitation
process and functional groups of the polymer helped to control particle size and
formation of CdS nanoparticles on the surface of polymer matrix and produced the
yellow nanocomposites. Schematic representation of the synthesis of CdS/SO3-P(SDVB) nanocomposites is illustrated in Figure 4.41.
4.3.2 Fourier-Transform Infrared and UV – Vis Diffuse Reflectance
Spectroscopy of CdS/SO3-P(S-DVB) Nanocomposites
The CdS/SO3-P(S-DVB) nanocomposites with various amounts of CdS were
characterized by UV – Vis DR spectroscopy (see Figure 4.42). The absorption onset
wavelength of CdS nanoparticles in the nanocomposites was determined using the
obtained UV – Vis DR spectra and it was found to be significantly blue-shifted
compared to bulk CdS. The direction of the blue shifting is in agreement with
quantum confinement effect due to decreasing particle size. The results also
suggested that the ion exchange-precipitation processes could control the growth,
nucleation and formation of CdS nanoparticles on the polymer surface. Based on the
absorption onset wavelength value, the particle size of CdS was calculated using the
Brus equation (see Table 4.6).
109
SO3H
Sulfonation
H2SO4/SO3
50o C, 1 h
SO3H
SO3H
SO3H
SO3H
SO3H
HSO3
SO3H
HSO3
Ion exchange with
various mmol of Cd 2+
SO3H
SO3H
SO3
CdS
SO3
Precipitation with
various
mmol Na2S
SO3
Cd2+
SO3
SO3H
SO3H
SO3Na
SO3H
HSO3
SO3Na
HSO
Residue
Residue
HSO3
SO3Na
CdS
SO3
Cd 2+
HSO
SO3
Polymer particle
Figure 4.41: Schematic representation of the synthesis CdS/SO3-P(S-DVB)
nanocomposites
110
Relative intensity (a.u)
h
g
c
f
b
d
e
a
380
420
460
500
540
580
600
Wavelength (nm)
Figure 4.42: UV-Vis DR spectra of (a) Pure P(S-DVB), (b) SO3H-P(S-DVB), (c)
2.56 % CdS/SO3-P(S-DVB), (d) 3.34 % CdS/SO3-P(S-DVB), (e) 7.64 % CdS/SO3P(S-DVB), (f) 14.65 % CdS/SO3-P(S-DVB). (g) 16.56 % CdS/SO3-P(S-DVB) and
(h) CdS nanoparticles
Table 4.6: The absorption onset wavelength and particle size of CdS/SO3-P(S-DVB)
nanocomposites
Nanocomposite sample
Onset wavelength (nm)
Particle size (nm) *)
2.56 % CdS/SO3-P(S-DVB)
Not observed
Nd
3.34 % CdS/SO3-P(S-DVB)
Not observed
Nd
7.64 % CdS/SO3-P(S-DVB)
500
7.0
14.65 % CdS/SO3-P(S-DVB)
507
7.3
16.56 % CdS/SO3-P(S-DVB)
550
>8
Nd : not determined, *) determined using Brus equation
111
The FTIR spectra of the series of P(S-DVB), SO3H-P(S-DVB) and CdS/SO3P(S-DVB) nanocomposites are shown in Figure 4.43. These spectra show the main
peaks due to the polystyrene component. The P(S-DVB), SO3H-P(S-DVB) and
CdS/SO3-P(S-DVB) nanocomposites show peaks corresponding to the following
groups: - OH (at 3,400 cm-1 – 3,200 cm-1), unsaturated aromatic C-H stretching
vibrations (at 3,025 cm-1), CH2 bending vibration (at 2,920 cm-1 – 2,840 cm-1),
aromatic ring (at 1,510 cm-1), CH2 (at 1,475 cm-1), various substitution of benzene
ring between 900 cm-1 and 700 cm-1. Martins et al. (2003) have studied the
characteristic infrared spectra of SO3H-P(S-DVB) and suggested that the bonding of
the sulfonic groups to the aromatic ring of polystyrene (out of plane deformation
bands assigned to substituted aromatic ring δ (Car-H) occurs at wavenumbers of
approximately 830 cm-1 to 850 cm-1).
Several new peaks were found in the spectra of SO3H-P(S-DVB) and
CdS/SO3-P(S-DVB) at 560 cm-1, 615 cm-1, 1,000 cm-1, 1,030 cm-1, 1,110 cm-1, 1,180
cm-1 and 1,230 cm-1. The absorption peak at 1,030 cm-1 was attributed to the
symmetric stretching vibration of sulfonic acid groups (-SO3H) and the band at 1110
cm-1 was due to a sulfonate anion attached to phenyl ring. The υassym (S-O) vibration
at 1,180 cm-1 appears as broad band at approximately 1,200 cm-1 to 1,300 cm-1. An
increasing peak at 1,110 cm-1 and a decreasing peak at 1,230 cm-1 were observed in
the spectrum of CdS/SO3-P(S-DVB) are the consequence of the ionization of
sulfonic acid groups by cadmium ion. The phenomena clearly showed that the
chemical bonding between CdS nanoparticles with sulfonated polymer matrix was
quite strong.
112
Transsmitent (a.u)
CdS/SO3H-P(S-DVB)
(SO3)2-Cd/P(S-DVB)
SO3Na-P(S-DVB)
SO3H-P(S-DVB)
CdS nanoparticles
4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 450
Wavelength (cm-1)
Figure 4.43: FTIR spectra of P(S-DVB), SO3H-P(S-DVB) and CdS/SO3-P(S-DVB)
nanocomposites
113
4.3.3 Structural
and
Morphological
Properties
of
CdS/SO3-P(S-DVB)
Nanocomposites
The structure and morphology of SO3H-P(S-DVB) and CdS/SO3-P(S-DVB)
nanocomposites were investigated by XRD, SEM and TEM techniques. Figures
4.44(a) – 4.44(b) and 4.45 show the XRD patterns of CdS nanoparticles, bulk CdS,
SO3H-P(S-DVB) and CdS/SO3-P(S-DVB) nanocomposites with various CdS content
respectively.
Intensity (a.u)
(a)
20
30
40
50
60
70
2-Theta
Intensity (a.u)
(b)
20
30
40
50
60
70
80
2-Theta
Figure 4.44: XRD patterns of (a) CdS nanoparticles and (b) bulk CdS
90
114
26.40
43.75
52.00
Relative intensity (a.u)
16.56 % CdS/SO3-P(S-DVB)
14.65 % CdS/SO3-P(S-DVB)
7.64 % CdS/SO3-P(S-DVB)
3.34 % CdS/SO3-P(S-DVB)
2.56 % CdS/SO3-P(S-DVB)
SO3H-P(S-DVB)
20
30
40
50
60
70
2-Theta degree
Figure 4.45: XRD patterns of SO3H-P(S-DVB) and the various CdS/SO3-P(S-DVB)
nanocomposites
115
The XRD patterns of the nanocomposites are identical with the XRD patterns
of the CdS nanoparticle and bulk CdS. The XRD patterns of CdS/SO3-P(S-DVB)
nanocomposites exhibit broad peaks corresponding to 111, 220, and 311 (plane)
indicating that CdS nanoparticles are in the cubic phase. Broad XRD peaks are
attributed to the absence of long-range order in the samples and reflect the small
particle sizes (less than 10 nm as estimated from the Brus equation). This result
agrees with the presence of nanocrystalline CdS, for which quantum size effect are
expected. The weak reflection peaks should belong to CdS by comparison with the
XRD pattern of CdS nanoparticles. The low intensity peaks indicated that the
dimension of CdS nanoparticles was homogeneously dispersed in polymer matrix.
The polymer particles which are amorphous and present in large amount also
contribute to the peak broadness. However the size of CdS nanoparticle could not be
obtained from the XRD pattern using Scherrer formula owing to poor crystallinity.
The morphology of CdS/SO3-P(S-DVB) nanocomposites were characterized
by SEM and TEM. Figures 4.46(a) and 4.46(b) show the typical SEM images of
CdS/SO3-P(S-DVB) nanocomposites with different magnification under the optimum
condition. SEM showed small spherical particles with diameter 300 nm – 2
micrometer and some particles appeared to be in agglomerated form. These spherical
particles should be ascribed to the aggregation of polymer particles during
polymerization process. The low tendency of aggregation in the polymer matrix as
shown in the SEM image suggested that the stability of the miniemulsion system was
under control by the use of monomer as oil-phase during the polymerization process.
SEM images showed the presence of CdS particles on the surface of SO3HP(S-DVB), suggesting that the CdS nanoparticles were small and dispersed on
polymer surface. The dispersion of CdS on the surface of polymer was also identified
from the corresponding change in colour of products. The Energy dispersive X-ray
(EDX) analyses were also done and confirmed the existence of the elements of
cadmium and sulfur in CdS/SO3-P(S-DVB) nanocomposite.
116
(a)
(b)
Figure 4.46: SEM images of 7.64 % CdS/SO3-P(S-DVB) nanocomposites at
magnifications (a) 2,000 X and (b) 5,000 X
117
Figure 4.47: SEM images on the surface of CdS/SO3-P(S-DVB) at magnification
100,000 X
The shape and size of the nanocomposite can be seen in the TEM
micrographs of Figure 4.48, where the micrograph is shown the dark center and pale
edge of the spheres. The dark domain observed in the TEM image indicates that it
originated from CdS nanoparticles. Cadmium as heavy element developed a darker
color compared to carbon and hydrogen in the polymer structure. Of interest, TEM
observations showed cloud-like products were the main morphology, indicating that
CdS nanoparticles were small and homogeneously dispersed in the polymer matrix.
As can be seen, the spheres exhibit a diameter distribution in the range of 200 nm – 2
micrometer and average diameter of ~500 nm. However, it is difficult to observe the
particle size and size distribution of CdS nanoparticles by visual inspection of the
TEM images, due to the TEM image was obtained by a low accelerating voltage (70
keV), hence the a statistical description of the size distribution of CdS nanoparticles
cannot be carried out.
118
(a)
(b)
Figure 4.48: TEM images of (a) non agglomerated and (b) agglomerated of
CdS/SO3-P(S-DVB) nanocomposites
4.3.4 Dielectric
and
Electrical
Properties
of
CdS/SO3-P(S-DVB)
Nanocomposites
Table 4.7 shows the dielectric constants of SO3 H-P(S-DVB) and CdS/SO3P(S-DVB) nanocomposites at various contents of CdS nanoparticles and at various
frequencies. Over the frequencies range of 0.1 – 1,000 KHz the dielectric constant of
CdS/SO3-P(S-DVB) nanocomposites, shows steep decrease from its initial high
values. Figure 4.49 shows the dielectric constants measured at frequency of 100 Hz
of SO3H-P(S-DVB) and CdS/SO3-P(S-DVB) nanocomposites at various contents of
CdS. The dielectric constants of CdS/SO3-P(S-DVB) nanocomposites decreased with
increasing CdS nanoparticles content, as compared to SO3H-P(S-DVB). On the other
hand, the dielectric constants of the CdS/SO3-P(S-DVB) nanocomposites showed
higher values than P(S-DVB) (~6 measured at 100 Hz) and CdS nanoparticles (100
measured at 100 Hz).
119
Table 4.7: Dielectric constants of P(S-DVB), CdS nanoparticles, SO3H-P(S-DVB)
and CdS/SO3-P(S-DVB) nanocomposites
Sample
Log frequency ( Hz)
6
5
4
3
2
P(S-DVB)
1.4
2.9
2.9
3.1
5.9
CdS nanoparticles
4.6
16.2
28.9
49.8
100.0
Cd(SO3)2-P(S-DVB)
4.2
39.3
294.7
2145.1
9097.8
NaSO3-P(S-DVB)
8.6
55
359
1286.8
2016.6
2,338.0 10,947.0 25,345.0 61,634.0 236,190.0
2.56 % CdS/SO3-P(S-DVB)
1,989.0
7,425.0 13,931.0 30,069.0
87,118.0
3.34 % CdS/SO3-P(S-DVB)
1,281.0
7.702.0 14,933.0 26,988.0
74,394.0
7.64 % CdS/SO3-P(S-DVB)
144.0
923.0
2,862.0
6,326.0
22,861.0
14.65 % CdS/SO3-P(S-DVB)
46.0
297.0
1,090.0
3,662.0
15,683.0
16.56 % CdS/SO3-P(S-DVB)
18.4
39.5
145.0
391.0
1,309.0
236,190
SO3H-P(S-DVB)
250000
74,394
87,118
15,683
22,861
2016.6
5.9
50000
9097.8
100000
1,309
150000
100
Dielectric constant
200000
0
a
b
c
d
e
f
g
h
i
j
Figure 4.49: Dielectric constant of (a) P(S-DVB), (b) CdS nanoparticles, (c)
Cd(SO3)2-P(S-DVB), (d) NaSO3-P(S-DVB), (e) SO3H-P(S-DVB), (f) 2.56 %
CdS/SO3-P(S-DVB), (g) 3.34 % CdS/SO3-P(S-DVB), (h) 7.64 % CdS/SO3-P(SDVB), (i) 14.65 % CdS/SO3-P(S-DVB) and (j) 16.56 % CdS/SO3-P(S-DVB) at 100
Hz.
120
The possible explanation for this result is the higher strength of electrical
properties of proton and polarity of SO3H-P(S-DVB) as a host matrix. The protons at
the sulfonic acid functional groups are very easily moved and when parts of the
protons were replaced with CdS, the amount of proton was decreased and the
dielectric properties of the nanocomposites decreased correspondly. This
phenomenon was supported by the decreasing meq H+/gram of SO3H-P(S-DVB)
with increasing amounts of CdS nanoparticles amount in the CdS/SO3-P(S-DVB)
(see Table 4.8). The low dielectric constants of Cd(SO3)2-P(S-DVB) and NaSO3-P(SDVB) also supported the above explanation. Nevertheless, this result showed
substantially higher dielectric constant as compared to the dielectric constant of 20 %
CdS/PVA (dielectric constant ~10) (El-Tantawy et al., 2004), CdS/PVA with
average dispersibility in the range of 0.3 – 0.8 (dielectric constant about 120 – 250 at
in over the range frequency 10 – 40 khz) (Ghosh et al., 2005) and TiO2/sulfonated
styrene-b-(ethylene-ran-butylene)-b-styrene (S-SEBS) block copolymers (Yang and
Kofinas, 2007). The dielectric constants of 0.26 % and 6.4 % of TiO2/S-SEBS
nanocomposites at 10 kHz were around 2.15 and 3.2, respectively and the values
were low when compared to un-crosslinked S-SEBS (dielectric constant of about
5,258 at 10 kHz and 160,000 at 1 Hz).
Table 4.8. Meq H+/gram of SO3H-P(S-DVB) with degree of sulfonation = 39.10 %
and CdS/SO3-P(S-DVB) nanocomposites with various amount of CdS
Sample
Meq H+/gram
SO3H-P(S-DVB)
3.50
2.56 % CdS/SO3-P(S-DVB)
2.64
3.34 % CdS/SO3-P(S-DVB)
2.35
7.64 % CdS/SO3-P(S-DVB)
1.53
14.65 % CdS/SO3-P(S-DVB)
0.66
16.56 % CdS/SO3-P(S-DVB)
0.24
121
Figure 4.50 shows the graph of decreasing meq H+ in the CdS/SO3-P(S-DVB)
nanocomposite versus dielectric constant at 100 Hz. The curve shows a good
correlation with the coefficient correlation of 0.9171.
100,000
y = -23033x + 109372
Dielectric constant
80,000
2
R = 0.9171
60,000
40,000
20,000
0
0
1
2.64
2
2.35
3
1.53
4
0.66
5
0.24
6
Meq H+
Figure 4.50: A graph of the relationship between amount of meq H+ and dielectric
constant of CdS/SO3-P(S-DVB) nanocomposites
Table 4.9 and Figure 4.51 show the dissipation factors of CdS/SO3-P(S-DVB)
nanocomposites at various amounts of CdS and frequencies. The dissipation factor
values of the nanocomposites decreased as compared to the dissipation factor values
of SO3H-P(S-DVB). These results suggested that part of the proton in the sulfonic
functional groups were being replaced by sodium ions and CdS as filler and the
mobility of proton in the polymer was also hindered by the CdS nanoparticles on the
surface.
122
Table 4.9: Dissipation factors of P(S-DVB), CdS nanoparticles, SO3H-P(S-DVB)
and CdS/SO3-P(S-DVB) nanocomposites
Sample
Log frequency ( Hz)
6
5
4
3
2
P(S-DVB)
0.40
0.10
0.20
0.20
0.50
CdS nanoparticles
0.20
0.20
0.20
0.50
1.60
SO3H-P(S-DVB)
4.13
0.37
0.22
0.38
1.47
2.36 % CdS/SO3-P(S-DVB)
1.92
0.17
0.16
0.35
0.67
3.34 % CdS/SO3-P(S-DVB)
0.45
0.49
0.10
0.16
0.52
7.64 % CdS/SO3-P(S-DVB)
4.25
0.84
0.24
0.32
1.00
14.65 % CdS/SO3-P(S-DVB)
1.77
1.13
0.79
0.92
1.98
16.56 % CdS/SO3-P(S-DVB)
0.56
0.52
0.45
0.33
1.48
4.50
4.00
3.00
2.50
2.00
1.50
1.00
0.50
0.00
3
4
a
5
b
c
d
6
e
f
Log frequency (Hz)
Dielectric constant
3.50
2
Figure 4.51: Dissipation factor of (a) SO3H-P(S-DVB) and CdS/SO3-P(S-DVB) at
various CdS content and frequencies. (b) 2.36 % CdS/SO3-P(S-DVB), (c) 3.34 %
CdS/SO3-P(S-DVB), (d) 7.64 % CdS/SO3-P(S-DVB), (e) 14.65 % CdS/SO3-P(SDVB), (f) 16.56 % CdS/SO3-P(S-DVB)
123
The
variation
on
dielectric
constant
of
the
CdS/SO3-P(S-DVB)
nanocomposites with different temperature is shown in Figure 4.52. The
nanocomposites display an increasing tendency in dielectric constant at frequencies
0.1 – 1,000 KHz with increasing of temperature in the range of 25 - 75 oC and then
abruptly decreased after 75 oC.
140000
120000
100000
Dielectric constant
80000
60000
0.1 KHz
Temperature oC
1 KHz
40000
20000
10 KHz
100 KHz
1000 KHz
0
0
25
50
75
100
125
150
200
Temperature oC
Figure 4.52: Influence of temperature on the dielectric constant of 7.64 % CdS/SO3P(S-DVB) nanocomposites
124
This phenomenon is explained by the occurrence of two kinds of behaviors,
which would yield converse effect on the dielectric constant of the nanocomposites
by changing the temperature. First, increasing temperature would improve the
segmental mobility of polymer and CdS nanoparticles, facilitate the polarization of
polar fillers and consequently increase dielectric constant. Second, the obvious
differential in thermal expansion coefficient between the polymer and CdS
nanoparticles would disturb the aggregations of polar components (CdS
naoparticles), thus reducing the dielectric constant. Over the temperature range of 25
– 75 oC for the CdS/SO3-P(S-DVB) nanocomposites showed that the first behavior
was the main effect. On the other hand, at temperatures above 75 oC, the second
behavior more dominant. It was likely that disruption of the CdS nanoparticles
cluster was the cause dor the dielectric constant to decrease with rising temperature.
The result was further supported by the glass transition temperature (Tg) obtained
SO3H-P(S-DVB) of around 100 to 160 oC, depended on degree of sulfonated
(Wallace, 1971; Yang et al., 2003). The result consistent with the dielectric constant
of the
nanocomposites measured after heat treatment which showed that the
dielectric constant was slightly decreased, compared with that before heat treatment.
Table 4.10 and Figure 4.53 show the ionic conductivities of P(S-DVB),
SO3H-P(S-DVB), CdS nanoparticles and CdS/SO3-P(S-DVB) nanocomposites. The
ionic conductivity values of CdS/SO3-P(S-DVB) nanocomposites decreased with
increasing CdS nanoparticle content in the nanocomposites compared to SO3H-P(SDVB). The same reason which explained the dielectric properties can also be applied
to explain this ionic conductivity property. When some the protons in sulfonic acid
were replaced with sodium and CdS nanoparticles attached on the surface of
polymer, the mobility of the proton residue was hindered. The result was also
evidenced by the decrease of meq H+ in the nanocomposite with increasing amount
of CdS (see Table 4.8)
Figure 4.54 shows the relationship between meq H+ in the nanocomposite
with ionic conductivity of CdS/SO3-P(S-DVB) nanocomposites. The curve shows a
good correlation between meq H+ and decreasing the ionic conductivity of proton
with coefficient correlation of 0.9913.
125
Table 4.10: Ionic conductivities of CdS nanoparticles, P(S-DVB), Cd(SO3)2-P(SDVB), NaSO3-P(S-DVB), SO3H-P(S-DVB), and Cd/SO3-P(S-DVB) with various
amount of CdS
Sample
Ionic Conductivity
S/ohm
1.6 E-10
CdS nanoparticles
6.2 E-08
Cd(SO3)2-P(S-DVB)
6.0-E-07
NaSO3-P(S-DVB)
2.0 E-07
SO3H-P(S-DVB)
1.1 E-04
2.36 % CdS/SO3-P(S-DVB)
1.7 E-05
3.34 % CdS/SO3-P(S-DVB)
1.7 E-05
7.64 % CdS/SO3-P(S-DVB)
1.2 E-05
14.65 % CdS/SO3-P(S-DVB)
6.4 E-06
16.56 % CdS/SO3-P(S-DVB)
1.8 E-06
1.10E-04
P(S-DVB)
1.20E-04
8.00E-05
d
f
g
1.80E-06
c
6.40E-06
b
1.20E-05
2.00E-07
a
1.70E-05
6.00E-07
2.00E-05
6.20E-08
4.00E-05
2.00E-05
6.00E-05
1.60E-10
Ionic conductivity (S/ohm)
1.00E-04
0.00E+00
e
h
i
j
Figure 4.53: Ionic conductivity of (a) Pure P(S-DVB), (b) CdS nanoparticles, (c)
Cd(SO3)2-P(S-DVB), (d) NaSO3-P(S-DVB), (e) SO3H-P(S-DVB), (f) 2.36 %
CdS/SO3-P(S-DVB), (g) 3.34 % CdS/SO3-P(S-DVB), (h) 7.64 % CdS/SO3-P(SDVB), (i) 14.65 % CdS/SO3-P(S-DVB) and (j) 16.56 % CdS/SO3-P(S-DVB)
126
Ionic conductivity (S/ohm)
2.50E-05
y = -5E-06x + 3E-05
R2 = 0.9913
2.00E-05
1.50E-05
1.00E-05
5.00E-06
0.00E+00
0
1
2.64
2
2.35
3
1.53
Meq H+
4
0.66
5
0.24
6
Figure 4.54: A graph of the relationship between amount of meq H+ and ionic
conductivity of CdS/SO3-P(S-DVB) nanocomposites
The ionic conductivity of the 7.46 % CdS/SO3-P(S-DVB) nanocomposites at
various temperatures is shown in Figure 4.55. Similar behavior and reasons
explained for dielectric properties result of the nanocomposites were observed in the
ionic conductivity of the nanocomposites.
Ionic conductivity (S/ohm)
2.50E-04
2.00E-04
1.50E-04
1.00E-04
5.00E-05
0.00E+00
30
40
50
60
70
80
90
100
Temperature oC
Figure 4.55: Ionic conductivity of 7.64 % CdS/SO3-P(S-DVB) nanocomposite at
different temperatures
127
4.3.5 Thermal Properties of CdS/SO3-P(S-DVB) Nanocomposites
Figure 4.56 shows the thermal conductivity of P(S-DVB), SO3H-P(S-DVB),
CdS
nanoparticles
and
CdS/SO3-P(S-DVB)
nanocomposites.
The
thermal
conductivity of the nanocomposites increased with increasing CdS nanoparticles
content. Two possible reasons is suggested to explain the results. Firstly, with
increasing CdS nanoparticle content, the conductive path of the nanocomposites was
enhanced as consequence of higher amount of CdS nanoparticles. CdS nanoparticles
are more effective in heat conduction through the polymer matrix as host of the
nanocomposites to transfer heat through the specimen. Secondly, the interfacial
thermal resistance among CdS nanoparticles-polymer is reduced, leading the increase
thermal conductivity. The result concluded that the thermal conductivity of
nanocomposites was mostly dependent on the content and properties of the CdS
nanoparticles. Similar results and reasons were reported in the case of CdS/PVA
nanocomposites (El-Tantawy, 2004).
16.56 % CdS/SO3-P(S-DVB)
14.65 % CdS/SO3-P(S-DVB)
7.64 % CdS/SO3-P(S-DVB)
3.34 % CdS/SO3-P(S-DVB)
2.36 % CdS/SO3-P(S-DVB)
CdS nanoparticles
SO3H-P(S-DVB)
Pure P(S-DVB)
0.24
0.23
0.22
0.20
0.19
0.22
0.14
0.09
Thermal conductivity (W/mK)
Figure 4.56: Thermal conductivity of P(S-DVB), SO3H-P(S-DVB), CdS
nanoparticles, and CdS/SO3-P(S-DVB) nanocomposites
128
Figure 4.57 show the relationship between amount of CdS nanoparticle in the
nanocomposite with thermal conductivity of CdS/SO3-P(S-DVB) nanocomposites.
The curve shows a good correlation between the increasing amount of CdS with
thermal conductivity with coefficient correlation of 0.9910.
Thermal conductivity (W/mK)
0.3
y = 0.0137x + 0.1809
R2 = 0.991
0.25
0.2
0.15
0.1
0.05
2.56
3.34
7.64
14.65
16.56
0
0
1
2.56
2
3.34
3
7.64
4
14.65
516.56
6
% CdS
Figure 4.57: A graph of the relationship between amount of CdS nanoparticle and
thermal conductivity of CdS/SO3-P(S-DVB) nanocomposites
Figure 4.58 shows the TGA curves of the CdS nanoparticles, SO3H-P(SDVB) and CdS/SO3-P(S-DVB) nanocomposites with different CdS nanoparticles
contents. Thermogravimetric analyses were carried out under oxygen atmosphere in
the temperature range of 40 – 1000 oC. The TGA curves for SO3H-P(S-DVB) and
CdS/SO3-P(S-DVB) nanocomposites show a two stage weight losses i.e. from 50 –
200 oC and 300 – 600 oC. The mass loss at 50 – 200 oC in SO3H-P(S-DVB) and
CdS/SO3-P(S-DVB) nanocomposites is due to the loss of physically and chemically
bounded water. The observed mass loss between 310 - 600 oC in both samples may
be attributed to the decomposition of the sulfonic acid group, which on cleavage will
eliminate SO3. The mass loss in the range 310 – 600 oC therefore correlated with the
amount of sulfonic acid groups on the basis of the assumption that it represents the
decomposition of the – SO3H groups only. An improved thermal stability of the
129
CdS/SO3-P(S-DVB) nanocomposites was observed as compared to the SO3H-P(SDVB).
The possible reason for the increased in thermal stability of the
nanocomposites is reduced molecular mobility of polymer chains due to presence of
the CdS nanoparticles. This was supported by the TGA curve of CdS nanoparticles in
which it showed a high thermal stability. It can also be seen that the residue of the
nanocomposites after the temperature of 600 oC is about 2 - 12 %, while the SO3HP(S-DVB) was completely decomposed at these temperature and left no residue.
Similar thermal behaviour was obtained in the cases of CdS/PVA (El-Tantawy,
2004), CdS/P(S-co-Mac) (Nair et al., 2005), CdS/PS (Kuljanin et al., 2002) and
CdS/PMMA (Kuljanin et al., 2006).
a
Weight loss (%)
100
80
c
60
d
40
b
20
0
e
100
200
300
400
500
600
700
800
900
1000
Temperature (oC)
Figure 4.58: Thermogravimetric analysis curves of (a) CdS nanoparticles, (b) 16.56
% CdS/SO3-P(S-DVB), (c) 7.64 % CdS/SO3-P(S-DVB), (d) 2.36 % CdS/SO3-P(SDVB) and (e) SO3H-P(S-DVB)
Figure 4.59 shows the summary in schematic representation to explain the
dielectric properties, electrical conductivity and thermal conductivity of CdS/SO3P(S-DVB) nanocomposites
130
SO3Na HSO3 HSO3
HSO3
HSO3
HSO3
HSO3
SO3Na
HSO3
HSO3
HSO
CdS
Polymer particle
3
HSO3
SO3Na
HSO3
HSO3
HSO3
HSO3
HSO3HSO HSO3
3
Increase of amount and particles size of CdS
SO3Na HSO3 HSO3
HSO3
HSO3
HSO3
HSO3
SO3Na
HSO3
HSO3
HSO
3
SO3Na
HSO3
HSO3
SO3Na
SO3Na
HSO3
HSO3
HSO
3
HSO
Decrease of meq proton
Decrease of dielectric constant
Decrease of ionic conductivity
3
Increase of thermal conductivity
SO3Na HSO3 HSO3
SO3Na
HSO3
NaSO3
HSO3
SO3Na
NaSO3
HSO3
HSO
3
SO3Na
SO3Na
HSO3
HSO3
NaSO3
SO3Na
HSO
3
SO3Na HSO3
Figure 4.59: Schematic representation of mechanism replacing proton with sodium
and CdS nanoparticles attached on surface of polymer and their properties
131
4.4
Comparison
of
Physicochemical
and
Electrical
Properties
of
CdS/polymer Nanocomposites
A summary of the results obtained on the physicochemical and electrical
properties of CdS/polymer nanocomposites are presented in Tables 4.11 and 4.12.
The different procedures of synthesis were applied in order to disperse CdS
nanoparticles on the polymer surface or to be encapsulated within the polymer
matrices. Since the chemical properties of polymers are different. Three different
methods were applied to synthesize the nanocomposites. The first type, CdS/P(SDVB) nanocomposite was prepared by in-situ polymerization in miniemulsion
system using styrene-divinyl benzene monomers (entry 1 in Table 4.11). The second
type, CdS/P(MAA-EGDMA) nanocomposite was prepared with three steps
procedures i.e., in-situ polymerization in miniemulsion system to produce P(MAAEGDMA), ion exchange and precipitation process to produce the nanocomposite
(entry 2 in Table 4.11). Lastly, the CdS/SO3-P(S-DVB) nanocomposite was prepared
with four steps procedure i.e., in-situ polymerization in miniemulsion system to
produce P(S-DVB) particles, sulfonation reaction with fuming sulfuric acid (H2S2O7)
to produce SO3H-P(S-DVB), ion exchange and precipitation process to produce the
nanocomposites (entry 3 in Table 4.11).
The morphology and particles size of the polymers were observed using SEM
and TEM techniques. The SEM and TEM studies showed that the particles size of
P(S-DVB) and SO3H-P(S-DVB) (entry 1 and 3 in Table 4.11) is in the range of 0.2 –
2 μm, while the particle size of P(MAA-EGDMA) is in the range of 200 – 300 nm
(entry 2 in Table 4.11). The particles size of CdS nanoparticles in the miniemulsion
and these nanocomposites was examined using UV – Vis and UV - Vis DR
spectroscopy and it showed a blue shifted onset wavelength when compared to bulk
CdS, indicating a quantum confinement effect associated with the nanometer
particles. There is no significant difference in size of CdS nanoparticles in these
polymer nanocomposites.
CdS/SO3-P(S-DVB) c, j
3
using styrene-divinyl benzene as
CdS/P(MAA-EGDMA) b, i
2
h
at CdS 0.18 %, at 1.08 %.
i
Post synthesize
d
b
High
Medium
0.3 – 2 μm
e
using Brus equation,
f
7 – (>8)
7.1 – 8.0
3–5
>500 – 600 oC
Increased
>350 – 400 oC
Increased
>350 – 400 oC
Increased
stabilityf,g
Thermal
g
as compared to pure polymer,
polymer was prepared by in-situ
TGA analysis,
c
200 – 300 nm
0.2 – 2 μm
of CdS (nm)e
of polymerd
of polymer
Low
Particle size
Particles size
Relative polarity
Physicochemical properties
polymer was prepared by in-situ polymerization,
from SEM and TEM images,
oil-phase,
( ion exchange-precipitation)
Post synthesize
(ion exchange-preciptitation)
polymerization and sulfonation reaction,
a
In-situ polymerization
CdS/P(S-DVB) a, h
1
in miniemulsion system
Preparation methods
Nanocomposite materials
Entry
Table 4.11: Physicochemical properties of cadmium sulfide/polymer nanocomposites
132
133
The SEM study clearly showed that the CdS nanoparticles were attached on
the surface of P(MAA-EGDMA) and SO3H-P(S-DVB). Meanwhile, the CdS
nanoparticles were encapsulated in P(S-DVB) and not observed in the SEM image.
These results suggested that the in-situ formation by in-situ polymerization in a
miniemulsion system and ion exchange-precipitation in these polymer matrices could
prevent agglomeration and control the growth of CdS nanoparticles.
As tabulated in Table 4.12, the dielectric properties of three types of
CdS/polymer nanocomposite displayed significant difference in the dielectric
constant values and low dissipation factor. The CdS/SO3-P(S-DVB) (entry 3 in Table
4.12) had higher dielectric constant value as compared to CdS/P(MAA-EGDMA)
(entry 2 in Table 4.12) and CdS/P(S-DVB) (entry 1 in Table 4.12), which is in the
following order: CdS/SO3-P(S-DVB) > CdS/P(MAA-EGDMA) > CdS/P(S-DVB).
One demonstrates that the difference in dielectric properties of these nanocomposites
is affected by the intrinsic polymer properties, amount and particles size of CdS
nanoparticles and also the interfacial interaction between CdS nanoparticle and
polymer. However, when bulk CdS was used in polymer nanocomposite, the
dielectric properties is low, suggesting that the dielectric properties of
nanocomposites were also dependent on the size of CdS particles. In conclusion, the
change of dielectric constant of polymers can be considered by the following factors,
i.e., nanosize of CdS and the interfacial interaction between CdS and polymers.
As shown in Table 4.12, there is a correlation between the electrical
conductivity and dielectric constant. The dielectric constant of P(S-DVB) and
P(MAA-EGDMA) polymers (entry 1 and 2 in Table 4.12) increases with the addition
of CdS nanoparticles. This phenomenon is also applicable for electrical conductivity.
However, in case of SO3H-P(S-DVB), the electrical conductivity and dielectric
constant of the polymer decreased with the addition of CdS nanoparticles. The
simple explanation for the decrease of the electrical properties (electrical
conductivity and dielectric constant) is that, the attachment of CdS nanoparticle on
the surface of SO3H-P(S-DVB) caused by the replacement of the proton with the
CdS nanoparticles. It is generally accepted that very high dielectric constant and
electrical conductivity are due to the proton mobilty.
134
The thermal conductivity (entry 1, 2, and 3 in Table 4.12) and thermal
stability (entry 1, 2, and 3 in Table 4.11) of these nanocomposites have also been
observed. CdS/SO3-P(S-DVB) nanocomposite had higher thermal conductivity and
thermal stability as compared to CdS/P(MAA-EGDMA) and CdS/P(S-DVB). One
demonstrates that the higher electrical conductivity and thermal stability of CdS/SO3P(S-DVB) nanocomposite is affected by the intrinsic polymer properties, especially
from sulfonated functional groups
a
e
(1.47)
(1.6)
d
236,190 g
(0.24)
(1.6)
100
9.2
(0.37)
(1.6)
100
5.9
100
(0.67)
87,118
(0.53)
5245.6
(1.2)
230.4
Composite
CdS
6.2 x 10-8
6.2 x 10-8
6.2 x 10-8
1.14 x 10-4 g
2.90 x 10-9
1.63 x 10-10
Polymer
1.73 x 10-5
2.94 x 10-6
2.62 x 10-9
Composite
at CdS 0.18 %, at CdS 1.08 %, at CdS 2.07 %, at 100 Hz, at room temperature, fat room temperature, gSO3H-P(S-DVB)
CdS/SO3-P(S-DVB) c
3
c
CdS/P(MAA-EGDMA) b
2
b
CdS/P(S-DVB) a
Polymer
(S ohm-1) e
(Dissipation factor) d
Materials
CdS
Ionic conductivity
Dielectric constant
0.22
0.22
0.22
CdS
0.1490 g
0.0907
0.0950
Polymer
0.1940
0.1493
0.1400
Composite
(W m-1K-1) f
Thermal conductivity
Electrical properties
Nanocomposite
1
Entry
Table 4.12: Electrical properties of cadmium sulfide/polymer nanocomposites
135
136
CHAPTER 5
CONCLUDING REMARKS
In this thesis, the physicochemical and electrical properties of CdS/polymer
nanocomposites have been studied. The results from the study have proven that
CdS/polymer nanocomposites possesses high dielectric properties compared to those
of silica. The dielectric properties of CdS polymer nanocomposites changed due to
the presence of CdS nanoparticles in polymer matrices. Three types of CdS/polymer
nanocomposites with high dielectric constant have been successfully designed and
synthesized. The first type, CdS/P(S-DVB) nanocomposite was prepared by in-situ
polymerization in miniemulsion system using styrene-divinyl benzene as a monomer.
The second type, CdS/P(MAA-EGDMA) nanocomposite, was prepared by in-situ
polymerization of P(MAA-EGDMA), followed by ion exchange and then
precipitation process. Lastly, the CdS/SO3-P(S-DVB) nanocomposite was prepared
by in-situ polymerization of P(S-DVB), sulfonation to produce SO3H-P(S-DVB), ion
exchange and then precipitation process.
The dielectrical property on P(MAA-EDMA) with nanosized CdS particles
attached was studied. The attachment of CdS nanoparticles alters the dielectric
property of CdS/P(MAA-EDMA) samples. By addition of a certain amount of CdS
nanoparticles, the dielectric constant at room temperature and 100 Hz of the
CdS/P(MAA-EDMA) is enhanced by ca. 500 times, which is attributed to the
interfacial interaction of CdS nanoparticles with polymer surface. The large value of
dielectric constant is due to interfacial space charge polarization. The same
phenomenon was also exhibited in CdS/P(S-DVB) nanocomposite.
137
The dielectric of constant of CdS/SO3-P(S-DVB) nanocomposite was decrease
as compare to pure sulfonated polymer. As SO3H-P(S-DVB) contains mobile proton
which is responsible for high dielectric constant, the attachment of CdS reduced the
dielectric constant due to the replacement of proton with CdS.
The dielectric properties of three types of CdS/polymer nanocomposite
displayed significant difference in the dielectric constant value. The CdS/SO3-P(SDVB) had higher dielectric constant value as compared to CdS/P(MAA-EGDMA)
and CdS/P(S-DVB), which is in the following order: CdS/SO3-P(S-DVB) >
CdS/P(MAA-EGDMA) > CdS/P(S-DVB).
Based on the above results one can suggests that the enhancement of dielectric
constant of CdS/polymer nanocomposites can be considered by the following unique
properties of CdS/polymer nanocomposite, i.e., nanosize of CdS and the interfacial
interaction between CdS and polymer. A highly dielectric constant CdS/polymer
nanocomposite may be prepared by ion-exchange of cadmium and followed by
reaction with sulfide on the surface of polymer and cannot be achieved by
mechanical mixing of CdS and polymer. It is demonstrated that the above
phenomena can be applied to CdS-P(MAA-EDMA) nanocomposite prepared by insitu polymerization of P(MAA-EGDMA), followed by ion exchange and then
precipitation process.
Based on the above results, one concludes that the attachment of CdS
nanoparticles on the surface of P(S-DVB), P(MAA-EGDMA) and SO3H-P(S-DVB)
polymers can control the dielectrical properties of CdS/polymer nanocomposites.
138
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APPENDIX A
Comparison of stabilization time of CdS nanoparticles in miniemulsion at Wo = 5
with different Po values
Po value
Stability of the CdS in miniemulsion
16
Precipitate out after < 1
26
Precipitate out after 2 min
39
Precipitate out after 3 min
53
Precipitate out after 10 min
65
Some portion precipitate after 60 min
78
Some portion precipitate after 10 min
105
Some portion precipitate after 10 min
150
APPENDIX B
UV – Vis spectra of CdS nanoparticles in miniemulsion at Wo = 5, Po = 65 and
concentration of cadmium and sulfide ion of 3.0 x 10-4 M at different time after
Absorbance ( a.u)
formation
t = 24 h
t = 5 min - 60 min
385
450
500
550
Wavelength (nm)
600
650
700
151
APPENDIX C
UV - Vis spectra of CdS nanoparticles in miniemulsion system at Po = 65 with Wo
values = 5 and 10 at different time after formation
t = 5 min, Wo = 10
Absorbance ( a.u)
t = 5 min, W o = 5
t = 60 min Wo = 5
t = 60 min Wo = = 10
385
400
500
550
600
Wavelength (nm)
650
700
152
APPENDIX D
UV - Vis spectra of CdS nanoparticles in miniemulsion at Wo = 5 and Po = 65 at
different concentration of cadmium and sulfide ion
[Cd2+] = 6 x 10-4 M,
t = 5 min
Absorbance ( a.u)
(a)
[Cd2+] = 6 x 10-4 M,
t = 60 min
[Cd2+] = 3 x 10-4 M,
t = 5 min and 60 min
(b)
385
450
500
550
600
Wavelength (nm)
650
700
153
APPENDIX E
UV – Vis spectra of CdS nanoparticles in miniemulsion system before and after
addition of styrene-divinyl benzene at different times after formation, (a) and (d)
without monomer at 5 min and 24 h, respectively, (b) and (c) after mixing with
monomer at 5 min and 24 h, respectively
Absorbance (a.u)
(a)
(b)
(c)
(d)
385
450
500
550
600
Wavelength (nm)
650
700
154
APPENDIX F
Quantitative standard calibration plot of cadmium element by using Perkin Elmer
AA400 AAS
1.2
1
Absorbance
0.8
0.6
y = 0.9992x
0.4
2
R = 0.9977
0.2
0
0
0.2
0.4
0.6
Concentration of Cd (mg/L)
0.8
1
155
APPENDIX G
Quantitative standard calibration plot of sodium element by using Perkin Elmer
AA400 AAS
0.6
0.5
Absorbance
0.4
0.3
0.2
y = 1.0056x
2
R = 0.9967
0.1
0
0
0.1
0.2
0.3
Concentration of Na (mg/L)
0.4
0.5
156
APPENDIX H
LIST OF PUBLICATIONS
Conference and National Publications:
Publications :
1. Eriawan Rismana, Salasiah Endud,
Hadi Nur. (2005). Synthesis of CdS
nanoparticles in HDTMAB/2-Propanol/Water/n-Decane Miniemulsion System.
Buletin Kimia, 21 - 55-65.
2. Eriawan Rismana, Salasiah Endud and Hadi Nur. Synthesis of Different Sized
Cadmium Sulfide Nanoparticles Inside Polymer and Mesoporous Al-MCM-41 by
Direct
Miniemulsion
Polymerization
and
Ion
Exchange
Techniques”,
Proceedings Annual Fundamental Science Seminar 2006 (AFSS 2006) Institut
Ibnu Sina, 6-7 June 2006.
Conferences :
1. Eriawan Rismana, Salasiah Endud,
Hadi Nur. (2005). Synthesis of CdS
nanoparticles in HDTMAB/2-Propanol/Water/n-Decane Miniemulsion System.
Annual Fundamental Science Seminar 2005 (AFSS 2005) Institut Ibnu Sina, June
2005.
2. Hadi Nur, Eriawan Rismana and Salasiah Endud. (2008). Nanosize Effect of
Cadmium
Sulfide
dimethacrylate)
on
Attached
on
Dielectrical
Poly(methacrylic
Property.
acid-ethylene
International
glycol
Conference
on
Nanoscience and Nanotechnology (ICONN 2008). 25 – 29 February 2008.
Melbourne Exhibition and Convention Centre, Victoria, Australia.