THÔNG TIN TÀI LIỆU
COLLOIDAL BEHAVIOR OF HIGHLY BRANCHED POSS PARTICLES
AND DEVELOPMENT OF ANION EXCHANGE MEMBRANE
AKLIMA AFZAL
NATIONAL UNIVERSITY OF SINGAPORE
2010
COLLOIDAL BEHAVIOR OF HIGHLY BRANCHED POSS
PARTICLES AND DEVELOPMENT OF ANION EXCHANGE
MEMBRANE
AKLIMA AFZAL
(B. Sc. in ChE, Bangladesh University of Engineering and
Technology, Bangladesh)
A THESIS SUBMITTED
FOR THE DEGREE OF MASTERS OF ENGINEERING
DEPARTMENT OF CHEMICAL & BIOMOLECULAR
ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2010
National University of Singapore
ACKNOWLEDGEMENTS
I would like to express my heartfelt gratitude to my supervisor, Associate
Professor Hong Liang and co supervisor, Dr. Liu Zhao Lin for their sincere
cooperation at every stage of my research. A/P Hong Liang’s valuable advice and
patient guidance always guided me to conduct my research smoothly. I am very
much thankful to Dr. Zhang Xinhui and Dr. Tay Siokwei for their inspiration,
advice and suggestion in carrying out the synthesis part.
I owe my deep gratitude to National University of Singapore for providing
research scholarship and Chemical and Biomolecular Engineering Department for
the facilities provided to carry out his project through in its entirety. Thanks are
also given to the department stuff members for providing various types of help
during this work.
I want to take a privilege to convey my thanks and gratitude to my colleagues as
well as friends Guo Bing, Liu Lei, Chen Xinwei and Sun Ming for their support
and encouragement during the study period. It is a pleasure to thank my friends
namely, Rajib and Iftekhar for their extended help and support whenever I
needed. I am greatly indebted to Abu Zayed for his continuous assistance and
inspiration to perform my work. Sincere thanks also go to Md. Mazharul for his
selfless guidance and immense effort during reviewing my thesis.
Finally, I would like to thank my family for their selfless love and full support at
every stage of my life. Without their encouragement and spiritual support, it
would have been difficult for me to stay in abroad and complete this dissertation.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS .................................................................................. i
TABLE OF CONTENTS ..................................................................................... ii
SUMMARY ......................................................................................................... vii
LIST OF TABLES ............................................................................................. viii
LIST OF FIGURES ............................................................................................. ix
NOMENCLATURE ............................................................................................. xi
CHAPTER ONE INTRODUCTION ..................................................................1
1.1. Preamble .......................................................................................................1
1.2. Objective of the Study ..................................................................................3
1.3 Scope of the Study .........................................................................................4
1.4 Organization of this Thesis ............................................................................5
CHAPTER TWO LITERATUR REVIEW .........................................................7
2.1 Introduction ....................................................................................................7
2.2 Polyhedral Oligomeric Silsesquioxane (POSS) .............................................8
2.2.1 Types of Silsesquioxane .........................................................................8
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Table of Contents
2.2.2 Properties of POSS ...............................................................................10
2.2.3 Researches’ on POSS as an Additive....................................................12
2.2.4 Applications of POSS as Non-additive .................................................15
2.3 Dendrimer ....................................................................................................17
2.4 Fuel cells (FC)..............................................................................................19
2.4.1 Fuel cell theory .....................................................................................20
2.4.2 Classification of fuel cells .....................................................................21
2.4.3 Solid Alkaline Fuel Cell........................................................................22
2.4.4 Anion Exchange Membrane (AEM) .....................................................24
2.4.5 Studies on AEM ....................................................................................25
2.4.6 Fuel Electrolyte Assembly for AEMFC ...............................................27
2.5 Summary ......................................................................................................29
CHAPTER THREE MATERIALS AND METHODS.....................................31
3.1 Introduction ..................................................................................................31
3.2 Materials ......................................................................................................31
3.3 Experimental Framework.............................................................................32
3.3.1 Synthesis for colloidal study .................................................................32
3.3.1.1 Synthesis of G0-HEMA with two branches.................................. 33
3.3.1.2 Synthesis of G0-HEMA with Quaternary Amine ......................... 35
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Table of Contents
3.3.1.3 Synthesis of G0-HEMA with three branches................................ 36
3.3.2 Synthesis of Anion Exchange Membrane .............................................37
3.3.2.1 Synthesis of the backbone ............................................................. 38
3.3.2.2 Preparation of the Charge Carrier Group ...................................... 39
3.3.2.3 Grafting the monomer on to the backbone via ATRP .................. 39
3.3.2.4 Membrane casting ......................................................................... 40
3.4 Characterization and Analytical Tools.........................................................41
3.4.1 Fourier Transform Infrared Spectroscopy (FTIR) ................................41
3.4.2 Transmission Electronic Microscopy (TEM) .......................................41
3.4.3 Thermogravimetric Analysis (TGA).....................................................42
3.4.4 Scanning Electric Microscope (SEM) ..................................................42
3.4.5 Dynamic Light Scattering (DLSC) .......................................................42
3.4.6 Auto Lab ...............................................................................................43
3.4.7 Particle Size Analyzer (Zeta Sizer) .......................................................43
3.4.8 Gas Chromatography (GC) ...................................................................43
3.4.9 UV Cross-Linker ...................................................................................44
3.5 Summary ......................................................................................................44
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Table of Contents
CHAPTER FOUR RESULTS AND DISCUSSIONS .......................................45
4.1 Introduction ..................................................................................................45
4.2 Characterization of POSS derivatives ..........................................................46
4.2.1 Characterization of G0-HEMA with two branches ..............................46
4.2.2 Characterization of G0-HEMA with Quaternary Amine ......................49
4.2.3 Characterization of G0-HEMA with three branches ............................50
4.3 Study of the Colloidal Behavior of POSS....................................................50
4.3.1 Dynamic Light Scattering .....................................................................51
4.3.2 Zeta potential ........................................................................................53
4.4 Kinetics study...............................................................................................56
4.4.1 Kinetic study at different hydrophobic solvents ...................................57
4.4.2 Kinetic study at different Relative Humidity ........................................59
4.5 Characterization of the membrane ...............................................................61
4.5.1 FTIR Spectrum of the Membrane .........................................................61
4.5.2 Energy Dispersion X-ray Spectroscopy ................................................63
4.5.3 Thermal Analysis ..................................................................................64
4.5.4 Surface morphology ..............................................................................65
4.6 Performance of the membrane .....................................................................66
4.6.1 Ethylene Diamine (EDA) as a cross-linker ...........................................67
4.6.1.1 Conductivity.................................................................................. 67
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Table of Contents
4.6.1.2 Water Uptake ................................................................................ 69
4.6.1.3 Methanol Crossover ...................................................................... 70
4.6.2 Amine-POSS as a cross-linker ..............................................................70
4.6.2.1 Conductivity.................................................................................. 71
4.6.2.2 Methanol Crossover ...................................................................... 71
4.6.3 Comparison between Membranes using EDA and Amine-POSS as
cross-linker .....................................................................................................72
4.6.3.1 Comparison of Surface Morphology ............................................ 72
4.6.3.2 Comparison of Ion Conductivity .................................................. 73
4.7 Summary ......................................................................................................74
CHAPTER FIVE CONCLUSIONS AND RECOMMENDATIONS..............75
5.1 Research contributions .................................................................................75
5.1.1 Findings from colloidal study ...............................................................75
5.1.2 Findings from Anion Exchange Membrane ..........................................77
5.2 Recommendations ........................................................................................78
REFERENCES .....................................................................................................80
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SUMMARY
This research focuses on the study of the colloidal behaviors of highly branched
polyhedral oligomeric silsesquioxane (POSS) particles synthesized by performing
dendrimerization from the pendant functional group (i.e. propylmethacryl) at the
eight corners of cubic POSS molecule. As the first targeted POSS dendrimer, 2hydroxyethyl methacrylate (HEMA) was employed to crown the
dendrimerization, it bears therefore bulky hydroxyl end groups.
Colloidal properties of this nano-particle have been studied with three structures
of POSS. Results show that G0-HEMA particle with two branches undergoes both
the protonation and deprotonation with the increase in pH. Besides this, the
particle may show finite affinity with anion of boric acid because of the terminal
hydroxyl groups. However, hydrophobic POSS core was found to retard the
responding capability of the pendant hydroxyl groups to the change of pH and
concentration of boric acid. The hydrophilicity of the G0-HEMA was enhanced
via increasing branches of HEMA and converting the tertiary amine groups in the
inner grafting layer of this particle to quaternary ions. These two measures
promote flexibility of the tele-HEMA branches as well as affinity with boric acid
molecules. In addition to the investigation into aqueous colloidal dispersions of
POSS-tele-HEMA by zeta potential, dynamic light scattering (DLS) and
transmission electronic microscopy (TEM) means, thin films fabricated by spincoat the colloidal dispersions on a flat substrate was studied with the aim to
understand the kinetics of conversion between hydrophobic and hydrophilic
surfaces.
Another focus of this research is to develop grafting anion exchange membrane
by means of living radical polymerization. This polymer is employed to form a
hydroxide (OH-) exchange membrane for its application in solid alkaline fuel cell
(SAFC). Recently, anionic exchange membrane in SAFC has received much
attention because it relies on a much cheap electrode catalyst than platinum which
is the solely anodic catalyst in the proton exchange membrane fuel cell (PEMFC).
Hydroxyl conductivity and mechanical test have been tested to find out the
feasibility in the alkaline fuel cell application. A series of features of the obtained
anion exchange membrane (AEM) has been assessed. In the next step, POSSbased cross-linker will be incorporated into this membrane system to revamp the
performance of it in SAFC.
Experimental results yield that the di-block membrane with ethylene diamine
cross-linker has a good conductivity at a high temperature and the membrane is
super resistant to methanol. However membrane with amine-POSS cross-linker
does not show a good conductivity and surface becomes non-homogenous.
In short, this research has successfully developed highly branched POSS and
provides some useful insights into its properties particularly related to oscillating
behaviors, adsorption of borate ion, kinetics, and cross-linking performances.
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LIST OF TABLES
Table 2.1 Classification of Fuel Cell .................................................................... 22
Table 2.2 Different types of Anion Exchange Membrane electrolyte assembly .. 28
Table 4.1 Effect of relative humidity on the surface structure ............................. 60
Table 4.2 Grafting of monomer on different backbone ........................................ 63
Table 4.3 Water uptake of different membranes ................................................. 69
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LIST OF FIGURES
Figure 2.1 Structures of Silsesquioxanes (Li et al., 2001) ...................................... 9
Figure 2.2 Schematic diagram of POSS (3-4nm) ................................................. 11
Figure 2.3 Dendrimeric structure .......................................................................... 18
Figure 2.4 Fuel cell diagram ................................................................................. 21
Figure 2.5 Diagram of Solid alkaline fuel cell ...................................................... 23
Figure 3.1 The scheme to prepare Amine-POSS (G0) ......................................... 34
Figure 3.2 The scheme to synthesize G0-HEMA with two branches ................... 35
Figure 3.3 Scheme for G0-HEMA with Quaternary Amine ................................. 36
Figure 3.4 Scheme for synthesizing G0-HEMA with three branches .................. 37
Figure 3.5 Structure of backbone .......................................................................... 38
Figure 3.6 Synthesis scheme of the cationic monomer......................................... 39
Figure 3.7 Growing Quaternary Amine chain on Backbone ................................ 40
Figure 4.1 FTIR spectra of (a) POSS, (b) POSS-amine (G0) and (c) G0HEMAwith two branches. ............................................................................ 46
Figure 4.2 1H-NMR of G0-HEMA with two branches ......................................... 47
Figure 4.3 Transmission electron micrograph of (a) POSS and (b) G0-HEMA
with two branches ......................................................................................... 48
Figure 4.4 IR spectrum of G0-HEMA with Quaternary Amine ........................... 49
Figure 4.5 TEM image of G0-HEMA with Quaternary Amine............................ 50
Figure 4.6 Size distribution of G0-HEMA with two branches at different pH..... 51
Figure 4.7 Size distribution of G0-HEMA with quaternary amine at different pH
....................................................................................................................... 52
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List of Figures
Figure 4.8 Size distribution of G0-HEMA with three branches at different pH... 53
Figure 4.9 Effect of pH on G0-HEMA with two branches ................................... 54
Figure 4.10 Effect of pH and Boric Acid on G0-HEMA with Quaternary Amine
on zeta potential ............................................................................................ 55
Figure 4.11 Effect of pH and Boric acid on G0-HEMA with three branches on
zeta potential ................................................................................................. 55
Figure 4.12 Surface morphology of the coated layer ............................................ 56
Figure 4.13 Coated glass at different hydrophobic medium. ................................ 57
Figure 4.14 Study the effect of atmosphere on G0-HEMA coated layer.............. 58
Figure 4.15 Effect of different atmospheric condition on the surface orientation 59
Figure 4.16 FTIR spectra of backbone (a), monomer (b) and monomer grafted
backbone (c) .................................................................................................. 62
Figure 4.17 Thermo gravimetric analysis of the backbone (VBC: AN = 1: 79) and
monomer grafted backbone........................................................................... 64
Figure 4.18 FESEM images of membrane with 1:20 ratio of VBC to AN (a) the
surface cross-linked with EDA; (b) & (c) cross section of 1: 20 and 1: 40
ratio of VBC to AN respectively (cross-linked with EDA) and (d) cross
section at higher magnification. .................................................................... 66
Figure 4.19 Conductivity of the EDA cross-linked membrane vs. temperature: (a)
1: 20; (b) 1: 40; (c) 1: 79 of VBC to AN. ..................................................... 67
Figure 4.20 The Arrhenius plot of hydroxyl ion conduction for VBC to AN ratio
of (a) 1: 20; (b) 1: 40; (c) 1: 79. .................................................................... 68
Figure 4.21 Conductivity of Amine-POSS cross-linked membrane vs.
temperature for VBC to AN ratio of (a) 1: 20; (b) 1: 40 and (c) 1: 79. ........ 71
Figure 4.22 FESEM pictures of the membrane surface (1: 20) cross-linked with
(a) EDA and (b) amine-POSS and cross section cross-linked with (c) EDA
and (d) amine-POSS. .................................................................................... 73
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NOMENCLATURE
POSS
Polyhedral Oligomeric Silsesquioxane
HEMA
2-hydroxyethyl methacrylate
HEM
Hydroxide (OH-) Exchange Membrane
SAFC
Solid Alkaline Fuel Cell
PEMFC
Proton Exchange Membrane Fuel Cell
DMFC
Direct Methanol Fuel Cell
SOFC
Solid Oxide Fuel Cell
AFC
Alkaline Fuel Cell
MCFC
Molten Carbonate Fuel Cell
MEA
Membrane Electrode Assembly
EDA
Ethylene diamine
GMA
Glycidyl Methacrylate
AN
Acrylonitrile
VBC
Vinyl Benzyl Chloride
DMF
N, N-dimethylformamide
DMPA
N-(2, 3-dimercaptopropyl)-phthalamidic acid
ATRP
Atom Transfer Radical Polymerization
Bpy
2, 2-Bipyridine
FTIR
Fourier Transform Infrared Spectroscopy
TTAB
Tetradecyl trimethylammonium bromine
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Nomenclature
ANS
1-anilinonaphtalene-8-sulfonate
TEM
Transmission Electronic Microscopy
TGA
Thermo-gravimetric Analysis
SEM
Scanning Electric Microscope
DLSC
Dynamic Light Scattering
GC
Gas chromatography
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CHAPTER ONE
INTRODUCTION
1.1. Preamble
Polyhedral oligomeric silsesquioxanes (POSS) has attracted a wide attention from
both academics and industries due to its hybrid structure of organic and inorganic
materials (Wahab et al., 2008). This criterion makes POSS molecule particularly
useful to a wide range of applications. POSS can be effectively incorporated into
the polymer (Zhiyong et al., 2008) via different approaches like blending, grafting
and copolymerization. POSS containing polymers and their derivatives can
improve heat, oxidation resistance, and several mechanical properties (Zhiyong et
al., 2008) and have been broadly studied due to its various applications (Xu et al.,
2007) such as liquid crystals, nanocomposites, coatings and photo resists in
lithographic technologies.
A number of recent studies have mainly been focused to explore the properties of
POSS. For example, Zucchi et al. (2009) have found that mono-functional POSS
when cross-linked with polymer decreases the surface energy which could be a
way to obtain hydrophobic coating without using fluorinated monomers. SanchezSoto et al. (2009) have reported that poly carbonate POSS nanocomposites
enhance the tensile stress and storage module without changing the thermal
behavior. Incorporating POSS polymer with different functional groups enhances
mechanical properties (e.g., Lin et al., 2009; Ying-Ling et al., 2008), reduces the
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Chapter One: Introduction
value of dielectric constant to ultra low value, and results a high transparency
(Ying-Ling et al., 2008).
POSS contents in the nano-composite are limited due to the incompatibility
between POSS and polymer. Well defined block polymers are synthesized from
POSS by adopting Controlled living polymerization (Xu et al., 2007). As a
polymer modifier, control in phase separation is required due to thermodynamic
incompatibility between POSS-rich and polymer-rich domain. This is because
thermal and mechanical characteristics of nanocomposite system are significantly
influenced by cross-linking density and dispersion state (e.g., Yen-Zen et al.,
2007).
Many studies have been focused to improve the thermo mechanical properties of
the polymer by incorporating POSS molecule as an additive. Literature has
showed that POSS derivatives influence both thermal and rheological properties
of different polymeric materials such as polycarbonate (Sánchez-Soto et al.,
2009), polypropylene (Lin et al., 2009; Zhiyong et al., 2008), polyimide (Seckin
and Koytepe, 2008; Ying-Ling et al., 2008), poly amic acid (Wahab et al., 2008),
polystyrene (Lei et al., 2007), epoxy (dell’Erba and Williams, 2008), and poly
amide (Yen-Zen et al., 2007). While substantial work has been conducted on the
property of different polymer modified by POSS, very few studies (e.g., Pyun et
al., 2001) have been focused on the structure and colloidal behaviors of POSS
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Chapter One: Introduction
molecules. Moreover the use of POSS as cross-linkers in anion exchange
membrane has also not been found in the literature.
1.2. Objective of the Study
As shown in the previous section most of the studies related to POSS were mainly
focused on the thermo-mechanical behaviors of POSS. Despite of being a nano
structured hybrid material; meager studies have been conducted on colloidal
behaviors of highly branched POSS structures and its application as a cross-linker
on di-block or anion exchange membrane. The objective of this study is to
investigate colloidal behaviors of highly branched POSS structures in aqueous
medium and to evaluate performances of anion exchange membrane while using
ethylene diamine and amine-POSS as cross-linkers. In order to achieve this, the
following research steps have been conducted:
1) Study the colloidal behavior of branched POSS synthesized by adopting
the dendritic approach. Characterization of these particles has been done
by using different analytic tools such as fourier transform infrared
spectroscopy (FTIR) and nuclear magnetic resonance (NMR). And
colloidal behaviors on different medium have been studied by using some
basic analytical tools such as size distribution and zeta potential.
2) Preparation and performance evaluation of hybrid membranes that consist
of both hydrophobic and hydrophilic part.
While the hydrophobic
backbone has been synthesized by polymerized acryl nitrile (AN), vinyl
benzyl chloride (VBC), and glycidyl methacrylete (GMA), the hydrophilic
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Chapter One: Introduction
quaternary amine has been grafted onto hydrophobic backbone by
adopting the atom transfer radical polymerization (ATRP) approach. The
performance of those membranes is studied by measuring methanol
crossover and ionic conductivity.
3) Evaluate the effect of amine-POSS as a cross-linker in hybrid membranes.
Generally, amine group is used to open the epoxy group (if any) present
on the polymeric backbone. In this study, amine-POSS is used for this
purpose and performances of amine-POSS have been compared with other
type of cross-linker like ethylene diamine (EDA).
1.3 Scope of the Study
Colloidal behaviors of POSS have been studied for highly branched structure of
POSS. Hence three structures of POSS have been synthesized for this purpose.
Effects of pH on the size and surface charge have been studied at room
temperature. To observe the surface behavior of 2-hydroxyethyl methacrylate at
zero generation of POSS (G0-HEMA), kinetic study on different hydrophobic
medium like toluene, cyclohexane, and ethyl acetate; and on a wide range (10% 85%) of humid conditions has been conducted.
Anion exchange membrane (AEM) has been prepared by grafting cationic amine
group on polymeric backbone. Amine group was attached to the polymer through
ATRP reaction. EDA and amine-POSS have been used as cross-linkers to open up
the epoxy group.
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Chapter One: Introduction
1.4 Organization of this Thesis
This thesis is organized into five chapters which explicitly explain the steps taken
to achieve the objectives of this study. The structure of this thesis is briefly
discussed below:
Chapter 1 introduces the research background of POSS, identifies research gaps,
states the objective and finally provides the structure of this thesis.
Chapter 2 provides a brief description of POSS, dendrimeric structures, and fuel
cell along with a critical literature review on POSS and Anion Exchange
Membrane. Starting with the properties of POSS, it figures out the major field of
studies related to POSS and identifies research gaps related to POSS behaviors.
For anion exchange membrane, it states the benefit of using this type of
membrane over proton exchange membrane (PEM). Studies related to AEM have
been critically reviewed and its application in different electrolyte membrane has
been discussed.
Chapter 3 focuses on the experimental framework needed to achieve the aim of
the study. It includes the development of POSS particles and anion exchange
membrane as well as the description of the analytical tools used in this study to
characterize and analyze these particles and membrane.
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Chapter One: Introduction
Chapter 4 presents the results obtained from the colloidal and membrane study. It
illustrates the colloidal behavior of particles and evaluates the performance of
anion exchange membrane. It also presents a comparative study between
membranes using two types of cross-linker.
Finally, Chapter 5 summarizes the conclusions derived from this research. Some
recommendations for future research are included.
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CHAPTER TWO
LITERATUR REVIEW
2.1 Introduction
Polyhedral Oligomeric Silsesquioxane (POSS) is relatively a new compound in
the field of research. POSS has some unique characteristics like hybrid structure,
dual property of organic and inorganic performance, cage like shape. Those
unique characteristics of POSS make the researchers more interested in studying
and understanding the behavior of POSS.
This chapter presents a critical review on studies related to POSS and its use in
different purposes. Starting with a basic understanding of POSS, this chapter
describes the impressive features of POSS, incorporation methods into other
polymers and potential features achieved while using as additives and as well as
in other areas such as medical application, modification of electrolyte etc. This
chapter also describes some literature on the modification of anion exchange
membrane (AEM). Literature reviews on anion exchange membrane has
suggested a means to modify AEM by incorporating different POSS
functionalities.
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Chapter Two: Literature Review
2.2 Polyhedral Oligomeric Silsesquioxane (POSS)
Polyhedral Oligomeric Silsesquioxane’s are nano sized core cage like structures
constitute by an inorganic silica Rn(SiO1.5)n where n is the number of silicon
atoms in cage (n= 8,10,12). The inner diameter of POSS is approximately about
1.5 nm. The core structure can be either reactive or non-reactive whereas the
organic part contains reactive functionalities. The unique feature of POSS is that
it has a hybrid chemical composition: (RSiO1.5), which is an intermediate between
silica (SiO2) and silicone (R2SiO). Here, the functional group R can be hydrogen
or any alkyl, alkylene, aryl, arylene groups or organo-functional derivatives of
them.
2.2.1 Types of Silsesquioxane
Polyhedral Oligomeric Silsesquioxane is one of the most popular branched of
silsesquioxane groups. Silsesquioxane’s are large branch of organic-inorganic
silicon containing compounds with the empirical formula R(SiO1.5). There are
mainly three - types of silsesquioxane: random structures, ladder-like structures
and cage like structures (Hany et al., 2005; Lei et al., 2007) as illustrated in figure
2.1. Last group also covers the partially cage like structures.
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Chapter Two: Literature Review
Figure 2.1 Structures of Silsesquioxanes (Li et al., 2001)
First oligomeric organo-silsesquioxane, (CH3SiO1.5)n , were isolated by Scott in
1946 through thermal analysis of polymeric products. Even after half a century,
the interest in this field is still increasing due to the dual property of organic and
inorganic behavior. The polymers from ladder like silsesquioxane have
outstanding thermal stability and resistance from oxidative environment even at
higher temperature. At the same time ladder-like silsesquioxane polymers have
good insulating properties and gas permeability’s. The main application areas of
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Chapter Two: Literature Review
these ladder-like oligomeric structures are in electronic and optical devices as
photo resist coating, as protective coating film in semiconductors, in gas
separation membranes and as a binder for ceramics and carcinostatic drugs (Li et
al., 2001).
Cage structured silsesquioxane (see figure 2.1, structures from c to f) is attracting
much attention from the last 15 years. This cage structured polyhedral oligomeric
silsesquioxane is designated as POSS. The inner structure of the cage is inorganic
in nature and is externally covered by the organic substituent. Total number of
silicon atom in this structure can be eight, ten or twelve.
2.2.2 Properties of POSS
POSS molecules are physically large (3-4nm) with respect to polymer
dimensions. However, it can be thought of as the smallest particle of silica
possible. They can be viewed as molecular silica. Unlike silica, it contains
reactive functionalities that are covalently bonded with the molecule. These
functionalities can undergo polymerization or grafting to the polymer chains and
make POSS compatible with others like polymers, biological systems and
surfaces. Mono-dispersed size, low density, thermal stability and controlled
functionality are some key features that have made POSS as one of the most
utilized nano building blocks in constructing materials (Ying-Ling et al., 2008).
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Chapter Two: Literature Review
R
O
R
O
O
R
Si
O
OR
O
Si
Si
R
Si
O
Si
R
O
Si
O
O
Si
O
O
Si
R
R
Figure 2.2 Schematic diagram of POSS (3-4nm)
POSS chemicals do not release any organic compounds, so they are odorless and
environment friendly. The eight polymeric groups (R) at the eight corners can be
of similar or different functionality. Moreover, all the corners may not contain
reactive functional groups. This means that the groups can be specially designed
to be reactive or nonreactive. Properties of POSS largely depend on these
functional groups. Depending on the organic group’s reactive functionality, POSS
can be classified as non-functional, mono-functional and poly functional. Each
POSS molecule contains reactive-nonreactive organic functionalities for solubility
and compatibility of the POSS segments with the various polymer systems.
POSS has the ability to control the motions of the chains while maintaining the
process and mechanical properties of the base resin when used as additives. The
integration of POSS derivatives as an additive in polymeric materials dramatically
improve the properties like thermal and oxidative resistance, mechanical
properties, surface hardening as well as reduce flammability, heat flux and
viscosity during processing.
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Chapter Two: Literature Review
2.2.3 Researches’ on POSS as an Additive
The inclusion of POSS into other polymers has created the opportunity to build up
high performance materials that unite many striking properties of both organic
and inorganic components. Mostly, POSS has been used as additives -and can be
successfully incorporate into the polymer by blending, copolymerization or
grafting. The derivatives of POSS exhibit wide chemical versatility and good
compatibility with organic materials as they possess organic substituents.
The main reason of adding POSS as an additive is to improve the thermal and
mechanical properties. As an additive, it can also act as viscosity modifier,
dielectric constant modifier, cross-linking agent, flammability and fire retardants.
Seckin and Koytepe (2008) have reported that amino functionalized octafunctional POSS was introduced into star polyimide by in situ curing to achieve
low dielectric constant. This POSS-NH2 restricts the rotation by multiple point
attachment to polyimide backbone. It is reported to lower down dielectric constant
by introducing free volume into the film. Jieh-Ming et al. (2009) have studied the
thermal property of poly benzoxazine/POSS hybrid nanocomposite and found that
both glass transition and thermal degradation temperature have raised higher than
the pristine poly benzoxazine. They have concluded that POSS cage effectively
hinder the polymeric motion that results higher thermal stability. POSS
derivatives also have several advantages over conventional fillers, including
mono-dispersity, low density, high thermal stability and controlled functionality.
Moreover, POSS monomers can be directly blended or copolymerized with other
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Chapter Two: Literature Review
monomers to form polymers or nanocomposites. And thereby it is very simple
and easy to study the effect of adding POSS derivatives. Reactive POSS can be
introduced into the polymers via copolymerization, grafting and blending
processes (Janowski and Pielichowski, 2008).
Mostly, POSS is used to synthesize nano-composite materials with better
performance. Nano-composite materials are attracting much attention as they
possess enhance mechanical, thermal, optical and controlled dialectical properties
which are usually achieved by the method of nano-reinforcement, the nanointerface, the synthetic procedure, the structural effects and the introduced
interaction between polymeric organic and inorganic phase. Wahab et al. (2008)
have suggested hydroxyl terminated POSS as a nanoscale building material to
improve thermo-mechanical and dielectrical properties of polyimide due to its
molecular scale and homogeneous distribution. For synthesizing hybrid
nanocomposite by POSS, the judicious choice of functional group may change
dispersion behavior of POSS in polymer matrix, enhance the processing
parameters and impart desire thermo and mechanical properties of the hybrid
materials.
Usually the polymer-POSS nanocomposite materials are synthesized by
copolymerization method. Copolymerization is the most common approach for
enhancing the flammability, crystallinity and thermal, and polymer mechanical
stability. Zucchi et al. (2009) have emphasized on the phase separation behavior
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Chapter Two: Literature Review
after incorporating POSS into isobutyl methacrylate. Due to thermodynamic
incompatibility, POSS-rich and polymer-rich domain have been formed which is
undesirable. POSS nano cages can disperse at molecular level and act like micro
dispersed filler which reduces the phase separation and enhances the thermo
mechanical properties. Thermo mechanical properties are highly related to the
microstructure which in turn depends to the interaction between the molecules.
Polymer modifier requires control over phase separation in order to stop
formation of additive rich and polymer rich domain. This is mainly controlled by
the amount of additives used. In the study of the group Sanchezat-Soto et al.
(2009), they have showed that good dispersion achieved up to 0-5wt% POSS
derivatives as additives. For higher loading, phase separation usually occurs easily
that degrades the thermo-mechanical properties. Significant decrease in surface
energy took place for both linear and cross-linked polymer when POSS domain
enriches on the surface.
POSS is also used to synthesize the di-block polymer by controlled or living
radical polymerization (Pyun and Matyjaszewski, 2001) as it allows greater
controlled over molecular weight, topology and composition. Various approaches
were taken to prepare hybrid copolymers, nano-particles, polymer brushed from
POSS. POSS is also used as plastic material in medical, packing and coating,
electronics, optical plastics etc. Another use of POSS is as a pre-ceramics in
ablative materials, cladding and electronics coating and precursors to ceramic
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Chapter Two: Literature Review
materials. With biodegradable and biocompatible functional group, POSS can
also be used in medical application including release systems, implant, scaffold
and tissue engineering (Hany et al., 2005).
2.2.4 Applications of POSS as Non-additive
Other than using as an additive, researches on POSS are now spreading in
different fields like simulation study on molecular POSS, medical applications,
improvement of electrolyte properties for both fuel cell and lithium batteries and
even as a space survival material.
Polyimides such as Kapton® are used in spacecraft thermal blankets, solar arrays
and space inflammable structures. One of the major problems with these
Polyimides is the degradation due to the presence of oxygen. Use of silica
enhances the oxidative resistance. However, imperfect coating of silica leads to
Kapton® erosion. Several studies have (e.g., Tomczak, 2004; Tomczak et al.,
2006) reported that POSS/ Kapton® polyimide enhance the oxidative resistance
without costing other properties of Kapton®. There are a few studies on POSS
related to the effects on the properties of dental composites such as mechanical
strength, flexural strength, modulus and tensile strength. Wheeler et al. (2006)
have studied the thermo mechanical properties and found that with 20 to 30%
resign, significant improvement in the mechanical property achieved. Striolo et
al.(2007) has studied the aggregation behavior of POSS monomer in liquid
hexane with the help of molecular simulation. By using simulator they have
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Chapter Two: Literature Review
obtained effective pair potential at dilute condition, short range effective
repulsion, and side radial distribution in hexadecane. However, no experimental
data’s are currently available to validate these results.
Another recent use of POSS is to improve the properties of different membranes
used as electrolyte. Incorporation of POSS derivatives into chitosan enhances the
diffusivity of all amino acids and this property reduces with the longer tether of
POSS derivatives (Tishchenko and Bleha, 2005). Another group (Zhang et al.,
2007) studied the effect of POSS in the solid electrolyte for lithium batteries.
They blend poly (ethylene oxide) derivative of POSS with high molecular weight
poly (ethylene oxide) as solid polymer electrolyte and found that addition of
POSS increase the conductivity at low temperature and it is limited to certain
amount of POSS. However, the improvement is not appreciable above Tm.
Polymer electrolyte membrane is used for proton exchange membrane (PEM) in
fuel cell. Recent studies have also been focused on the improvement of this
membrane while using POSS to improve the stability. Mostly Nafion is one of the
polymeric membrane usually use in fuel cells. However, it has some drawbacks
like methanol permeability which greatly reduces fuel cell’s performance.
Derivatives of POSS are incorporated into Nafion and the effects are being
studied by several researchers. Some studies (e.g., Zhang et al., 2009) have
showed that POSS derivatives restructure the proton conducting channels inside
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Chapter Two: Literature Review
the Nafion. The restructuring reduces methanol permeation that results higher
power generation than pristine Nafion.
Another drawback with Nafion is that it performs poorly in high temperature and
low humid condition. Decker et al. (2009) have synthesized a multilayer PEM
with sulfonated POSS and showed that this membrane performs well even at low
humid condition. In their work, PEM is consists of an outer layer of sulfonated
polyphenyl sulfone (S-PPSU) and the inner layer blend of octa-sulfonated
octaphenyl POSS and S-PPSU. This membrane has excellent physical properties
when compared with Nafion and have outstanding conductivity at 25% RH at
90°C. Yen et al.(2010) have prepared a new type cross-linked composite
membrane from sulfonated poly (ether ether ketone) and POSS moieties. The
membrane with sulfonated POSS moiety is reported to have higher selectivity,
lower methanol permeability and relatively higher proton conductivity. Some
other works have also been reported about the incorporation of POSS moiety in
different PEM that greatly improve the thermo mechanical property as well as
proton conductivity.
2.3 Dendrimer
Dendrimer synthesis is relatively a new field of polymer chemistry. Dendrimers
are highly branched, mono-dispersed macromolecules. They are defined by
regular, highly branched monomers leading to a mono-disperse tree-like or
generational structure. Synthesizing mono-disperse polymers demands a high
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Chapter Two: Literature Review
level of synthetic control which is achieved through stepwise reactions, building
the dendrimer up one monomer layer, or "generation," at a time. Dendrimers are
nanostructures that carries molecule encapsulate to the interior void space or
attached to the surface. Dendrimers are constructed through a set of repeating
chemical synthesis procedure and are characterized by their structural perfection
in terms of both uniformity and polydispersity. The size of dendrimer is about 1 to
10 nm.
Figure 2.3 Dendrimeric structure
The dendrimeric structure consists of a core, branching points and the terminal
groups. Dendrimers of lower generations (0, 1 or 2) have highly asymmetric
shape and possess more open space compared to the higher generations. In higher
generation (generation 4 or more) the dendrimer adopt globular structure and
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Chapter Two: Literature Review
become densely packed as they extent out to the periphery. The dendrimer
diameter increases linearly whereas the number of surface groups increases
geometrically.
Dendrimer can be synthesized in both convergent and divergent method. In
divergent method, the molecule is constructed from core to the periphery; whereas
for convergent method, the dendrimer is synthesizes from outside and terminated
at the core. Due to its structure, dendrimers have two chemical environments: the
surface chemistry due to the functional group which surface to the dendritic
sphere and the spheres interior which is mainly shielded from the external
environment due to the dendrimer structure. The existence of those two distinct
environments implies many possible applications for dendrimer.
POSS structure can be considered as a dendrimeric core from where branches
with desired functionality can be grown. The benefit of this approach is that
higher number of desired functional groups (terminal groups) can be achieved that
can drastically change the phase behavior.
2.4 Fuel cells (FC)
Fuel cells are considered as one of the most promising technology for the power
source of the future. Though the concept of fuel cell was first introduced in 1839
by Sir William Grove, it has emerged as a potential field in recent decades as an
attractive electrical generation technology with least pollution. A fuel cell is an
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Chapter Two: Literature Review
electrochemical energy conversion device. It directly converts the chemical
energy to the electrical energy with high efficiency. Fuel is charged in anodic side
and oxygen in the cathode side. The electrolyte acts as separator between fuel and
oxygen to prevent direct combustion via missing. Therefore, electrons are
conducted to the load via the external circuit.
2.4.1 Fuel cell theory
A fuel cell consists of an electrolyte as a conductor of the charged particle, an
anode and a cathode. In a hydrogen fuel cell, when the cell is activated by
catalyst, the hydrogen gas separates into proton and electron. Electrons are
conducted to the load through the wire whereas, proton pass through the
electrolyte to the cathode to combine with oxygen to form water as waste.
Two main electrochemical reactions occur in a fuel cell at the anode and cathode
respectively.
Anode half reaction: H 2 = 2 H + + 2e −
Cathode half reaction: 1 O2 + 2 H + + 2e − = H 2 O
2
Overall reaction: H 2 + 1 O2 = H 2 O; ∆G = −273KJ / mol
2
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Chapter Two: Literature Review
Load
Electrons folw round
the external circuit
Fuel
Cathode
Electrolyte
Anode
Oxygen
Water
Figure 2.4 Fuel cell diagram
where ∆G is the change in Gibbs free energy of formation. The product of this
reaction is water released at cathode or anode depending on the type of the fuel
cell. The theoretical voltage E0 for an ideal H2/ O2 fuel cell at standard condition
is 1.23 V.
2.4.2 Classification of fuel cells
The fuel cells are classified according to the electrolyte as the electrolyte is
defines the key properties of a fuel cell, particularly the operating temperature.
Generally, there are six different types of fuel cell namely hydrogen fuel cell
(HFC), direct methanol fuel cell (DMFC), phosphoric acid fuel cell (PAFC),
alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), and solid oxide fuel
cell (SOFC). Since HFC and DMFC use polymeric proton exchange membrane as
electrolyte, those two are usually considered as proton exchange membrane fuel
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Chapter Two: Literature Review
cell (PEMFC). Table 2.1 shows some important properties of different fuel cell
types.
Table 2.1 Classification of Fuel Cell
Fuel cell type
PEMFC
PAFC
AFC
MCFC
SOFC
Electrolyte
Polymer
membrane
Liquid
H3PO4
Liquid KOH
Molten
carbonate
Ceramic
Charge carrier
H+
H+
OH-
CO32-
O2-
Operating Temperature
50-120° C
200° C
60-200° C
650° C
600-1000° C
Catalyst
Platinum
Platinum
Platinum
Nickel
Perovskites
Cell components
Carbon
based
Carbon based
Carbon based
Stainless
based
Ceramic
based
Fuel compatibility
H 2,
methanol
H2
H2
H2, CH4
H2, CH4, CO
Among those fuel cell options, PEMFC is the most promising option due to the
high power density, rapid response to varying load and low operating
temperature. However, some limitations like CO poisoning of the catalyst,
expensive novel metal catalyst and methanol crossover make it necessary to look
for suitable solution. Solid alkaline fuel cell (SAFC) could be another promising
alternative for power generation.
2.4.3 Solid Alkaline Fuel Cell
The primary component of a SAFC is similar with the PEMFC: an ion conducting
electrolyte membrane, a cathode and an anode. The basic cell consists of a
hydroxyl conducting membrane sandwiches between two platinum impregnated
porous carbon electrodes. Together these three are often referred as membrane
electrode assembly (MEA). A fuel such as hydrogen is charged into the anode
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Chapter Two: Literature Review
compartment and an oxidant typically oxygen is placed into the cathode
compartment. In contrast to the PEMFC, hydroxyl ion passes through the
membrane form cathode to anode chamber.
Load
2e
-
2e-
H2
½ O2
A
n
o
d
e
2H2O
2 OH-
C
a
t
h
o
d
e
Anion Exchange Membrane
Figure 2.5 Diagram of Solid alkaline fuel cell
The electrokinetics of oxygen reduction in an alkaline medium is much enhanced
in comparison with an acid medium which yields higher power densities and leads
to higher efficiency for such systems. It also leads to the use of non precious
catalysts. The following cell reaction shows it clearly:
Anode :
H 2 + 2OH → 2 H 2 O + 2e −
Eo= -0.83 V
Cathode : 0.5O2 + H 2 O + 2e − → 2OH −
Eo = 0.40 V
Overall cell reaction: H 2 + 0.5O2 → H 2 O
Eo = 1.23 V
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Chapter Two: Literature Review
2.4.4 Anion Exchange Membrane (AEM)
Anion exchange membrane is one of the possible solutions for power generation
instead of proton exchange membrane. Though proton exchange membrane such
as Nafion has been extensively studied and is promising candidate for portable
power sources, it encounters some serious problems such as: 1) slow electrode
kinetics, 2) CO poisoning at Pt electrode at low temperature, 3) methanol
crossover that reduces the performance, and 4) high cost of the catalyst,
membrane, separator, and so on. High volatility and toxicity of methanol may
cause problems when used as portable power source.
AEM could be a better way out to remove these limitations. In alkaline medium
the electrode kinetics are faster and can be operate with non precious metal
electrode such as silver and perovskite type oxides. These metals are inexpensive
as well as tolerant to methanol. At high pH anionic exchange membranes can
potentially eliminated or reduced the need of platinum based catalysts; thereby
allowing cheaper metal catalysts and also improve the electrochemical kinetics.
However, these hydrocarbon membranes are not as chemically stable as
perfluorinated membrane. At high pH, nucleophilic attack take place (Ogumi et
al., 2005) at the cationic sites which reduces the stability of the hydrocarbon
based solid polymer anion exchange membrane. The atmospheric CO2 helps to
start carbonation resulting decreasing the pH value of the solution and reactivity
of the cell.
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Chapter Two: Literature Review
2.4.5 Studies on AEM
Recent studies have mostly been focused on the preparation and modification of
anion exchange membrane. There are several routes to prepare anion exchange
membrane (Vinodh et al., 2010); (a) polymer blended with alkali, (b) pyridinium
base type polymer, (c) radiation grafting and quaternization of polymer, and (d)
chloromethylation and quaternization of polymer. Among these methods, the last
one is the most advantageous and important one as it provides good physical
stability as well as relatively high chemical stability.
Most of the research on anion exchange membrane is about hydroxyl conducting
membrane. Still there are few works on other type of anion such as carbonate
ions. Unlu et al. (2009a) have compared between hydroxyl and carbonate
conducting ions on fuel cell operation at about room temperature. In their study
they have used poly (arylene ether sulfone) polymer membrane functionalized
with quaternary ammonium cations and found that maximum power density is
achieved with carbonate (4.1mW/ cm2) as conducting ions. Experimental analysis
shows that CO2 is involved to oxygen reduction reaction and transported from
cathode to anode resulting cell performance. However, their study did not
mention about the performance at higher temperature. Another study (Vega and
Mustain, 2010) also confirms that adding CO2 in the cathode stream of an anion
exchange membrane of fuel cell increase the cell performance. A series of crosslinked quarternized poly vinyl alcohol (PVA) membrane was prepared by Xiong
and his group (Xiong et al., 2008).
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These membrane have reasonable
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Chapter Two: Literature Review
conductivity {(2.74- 7.34) × 10-3 S/ cm2} at room temperature. More notably these
membranes have lower methanol crossover compared to Nafion and thus
methanol permeability decreases with increasing concentration. Fung and Shen
(2006) have reported that hydroxyl ion exchange membrane exhibits stability at
higher temperature (below 150°C) with moderate performance.
Recently, anion exchange membrane has been used to enhance the stability of
power generation of single chamber microbial fuel cell (Yinghui et al., 2009). A
decrease in power density is observed in cation exchange microbial fuel cell. To
improve the stability of the cell, anion exchange membrane has been used and
found 29% dropping in power density in 70 days which is 48% smaller than
cation exchange membrane. This is mainly due to the difference of internal
resistance development. In anion exchange membrane, lower amount of salt
precipitates on cathode surface (comparing to cation exchange membrane) and
results in less increase in cathode resistance that holds the stability of the
microbial fuel cell. Jung et al. (2007) have studied the power generation effect in
two chamber microbial fuel cell by using cation, anion and three types of ultra
filtration membranes. Among those, AEM produces maximum power density due
to proton charge transfer facilitated by phosphate ion and lower internal
resistance.
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Chapter Two: Literature Review
2.4.6 Fuel Electrolyte Assembly for AEMFC
Different types of fuels are used in AEMFC. Among those methanol, ethanol and
H2 are mostly used fuels. Literature shows different types of membrane as
electrolytes in AEMFC. Quaternized amine is mostly used as charge carrier in
membrane. Zhang et al. (2008) have reported that aqueous ammonium borate is a
potential fuel for AEMFC and can achieve up to 110 mW/cm2 power at 25°C.
Ogumi et al. (2005) have used four polyhydric alcohol and methanol as fuel (in
different study) in polyolefin chain containing Tetra alkyl ammonium membrane.
The conducting ion was hydroxyl ion that generated about 9.8 mW/cm2 of power
while using ethylene glycol as fuel at 25°C temperature.
Fujiwara et al. (2008; 2009) have prepared hydroxyl conducting membrane
contain quaternary amine. Then the membrane has been tested using ethanol and
D-glucose in KOH solution as fuel. Unlu et al. (2009a) have compared conducting
ion between OH- and CO32- with hydrogen gas as fuel and concluded that
carbonate ion as conducting ion results higher power density as CO2 is related to
oxygen reduction reaction at cathode. Hou et al. (2008) have prepared and studied
alkali doped polybenzyl imidazole membrane using 2M methanol as fuel and
found this membrane has a high thermal stability, acceptable mechanical
properties, and less methanol crossover compared to Nafion. The following table
shows some of the electrode fuel assembly from the literature.
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Chapter Two: Literature Review
Table 2.2 Different types of Anion Exchange Membrane electrolyte assembly
Conducting
ion
Power density
mW/ cm2
Aq. ammonium borane
(NH3BH3)
OH-
~110
Polybenzimidazol doped in KOH
Methanol
OH-
31
polyolefin chain containing Tetra
alkyl ammonium
Four Polyhydric
alcohols and CH3OH
solution
OH-
9.8
Electron-beam grafted ETFE
H2/O2 and methanol
OH-
130
Poly (arylene ether sulfone)
containing quaternary amine
Humidified Hydrogen
gas
OH- / CO32-
2.1 / 4.1
Quaternary amine blend with anion
exchange resin
ethanol
OH-
58
Quaternary ammonia in ionomer
solution
D-glucose in KOH
solution
OH-
20
Membrane
Anion exchange membrane
Fuel
Several studies have reported the performance of the membrane in various
conditions like different types of fuel, concentration of fuels, electrolyte,
membrane electrode assembly, operating conditions and so on. Therefore it is
quite challenging to compare among these membranes.
A novel type of hybrid fuel cell was prepared by using both PEM and AEM in
such way that at least one electrode operates at high pH (Unlu et al., 2009b). The
operation was conducted at steady state condition with a temperature of 65°C and
zero relative humidity. Both the AEM/ PEM junction and fuel cell (AEM cathode/
PEM anode and PEM cathode/ AEM anode) have been evaluated and the
operating behavior at the AEM/ PEM junction has been studied. Regardless of the
configuration of the hybrid cell, the junction potential is about 1.23 Volts. AEM
cathode and PEM anode offers viable configuration compared to PEM fuel cell
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Chapter Two: Literature Review
only. They have also demonstrated that the overall performance of the hybrid cell
is modest comparing to the PEM fuel cell.
Kim et al. (2010) have reported the usage of anion exchange membrane in air
breathing direct methanol fuel cell. Fuel cell designed to breathe air without any
auxiliary pumps or fans can offer several advantages such as simplicity and more
compact design. The electrochemical performance of this membrane strongly
depends on the concentration of methanol in KOH solution. Further work is
necessary to use this membrane effectively.
2.5 Summary
This chapter has presented a brief introduction of POSS and Anion Exchange
Membrane and reviewed literatures on the application of POSS. Attracting
features and general research areas of POSS have been discussed in an ordered
fashion. An extensive description on the property modification of various
polymers has also been presented by adopting different functionality of POSS.
Use of POSS in different fields other than additive has been reviewed as well.
Literature suggests that behaviors of POSS have not been explored well. In
particular, colloidal behaviors of POSS in different medium seem to have a great
potential. Since POSS is a nano structure material, a fuller understanding on its
colloidal behaviors would be helpful to apply such material in numerous
directions. Another potential gap identified by literature reviews is the anion
exchange membrane. Anion exchange membrane is considered to be an
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Chapter Two: Literature Review
alternative solution for Fuel cell to overcome the limitations with Proton
Exchange Membrane. Synthesis of AEM is relatively a new idea and hence will
be interesting and useful to look at. The experimental procedures as well as
materials and equipments used to achieve the aim of this study will be elaborately
discussed in the next chapter.
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CHAPTER THREE
MATERIALS AND METHODS
3.1 Introduction
This chapter describes experimental frameworks and materials that have been
used in this study. Three different structures of POSS have been designed to
synthesize from methacrylate-POSS derivatives and a polymeric membrane has
been developed by using UV polymerization. The name of the chemicals and
suppliers employed in the reaction steps are mentioned. Step by step reaction
procedures are described along with the reaction conditions. At the same time
product purification and collection procedure are also mentioned.
To characterize and analyze the synthesized products, different types of
characterization and analytical tools have been used. This chapter also presents a
brief description of different equipments used in this study along with their
manufacturers and model number.
3.2 Materials
Polyhedral Oligomeric Silsesquine (POSS) was purchased from Sigma-Aldrich.
Ethylene diamine was obtained from Merck Schuchardt. 2-Hydroxyethyl
methacrylate (HEMA, 97%), glycidyl methacrylate (GMA, 97%), acrylonitrile
(AN), vinyl benzyl chloride (VBC, 97%) were obtained from Sigma-Aldrich. N-
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Chapter Three: Materials and Methods
methyl diethanol amine, copper (I) chloride (CuCl), 2, 2-Bipyridine (Bpy) were
purchased from Alfa Aesar.DMPA as UV initiater, allyl bromide, acetonitrile
(J.T. Baker, 99.9%), ethyl acetate (Scientific Fisher, 99.97%), N, Ndimethylformamide (DMF, AR grade from Merck) and methanol (Merck, 99.8%)
analytic grade was used for synthesizing POSS molecules and membranes. Both
Deionized (DI) water and Millipore water have been used in this study.
3.3 Experimental Framework
Experimental procedures and reaction conditions of this study have been
elaborately described in this section. Experimental materials for synthesizing of
colloidal particles and anion exchange membranes have been introduced and
discussed in subsequent sections.
3.3.1 Synthesis for colloidal study
In this section, the reaction procedure for the colloidal part has been described.
Three different structures of POSS have been synthesized and a comparative
study of colloidal behavior has been carried out. All the structures of POSS are
branched via dendrimeric reaction. POSS is considered as a dendrimeric core and
these three structures have been constructed from the amine derivatives of POSS
which can be denoted as generation zero (G0). The first structure is designed to
carry two chains of 2-Hydroxyethyl methacrylate (HEMA) at each branching unit
of G0. For the second structure, it carries the same chains together with a charged
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Chapter Three: Materials and Methods
particle at each corner. For our study, the charged particle is quaternary amine.
And the third structure is designed in such way that it has three branches of
HEMA at each corner. Synthesis procedures of each structure are presented in
following sections subsequently.
3.3.1.1 Synthesis of G0-HEMA with two branches
The first structure is synthesized in two steps. The first step is identical for all the
three structures. Here, the amine group is linked to the POSS to create the
branching points. This structure is considered as G0. In the second step, HEMA is
added to construct G0-HEMA with two branches.
(a) Synthesis of amine-POSS (G0): amine-POSS was synthesized according to the
procedures described to synthesis dendrimeric structure in literature (Tulu et al.,
2009). 0.2 g (0.14 mmol) of methacryl substituted was added into large amount of
methanol and was stirred for about an hour. 50% excess (0.11 ml) EDA was
added in round bottom flux together with 30 ml of methanol. The whole system
was kept in ice bath at 0° C and argon was purged through it. POSS-methanol
solution was added very slowly while the mixture was vigorously stirred. After
half an hour of the addition of the POSS solution, the ice bath was removed and
the reaction was continued at room temperature for 24 h. Then the mixture was
heated at 40° C for 12h. The reaction scheme is shown in figure 3.1.
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Chapter Three: Materials and Methods
O
O
R
R
R
RT, 24 h
in methanol
O
R
H3C
R
R
R'
CH2
H 2C
+
H2N
R'
POSS
EDA
NH2
CH3
R'
R
R
NH
O
R'
NH2
R'
CH2
H2C
R'
Amine-POSS (G0)
Figure 3.1 The scheme to prepare Amine-POSS (G0)
The synthesized product was purified by washing in methanol in rotary
evaporator under vacuum to remove the excess amine. This synthesized particle is
considered as zero generation (G0). After purification, this product was used to
carry out the next step to prepare the highly branched nanoparticle.
(b)Synthesis of G0-HEMA with two branches: Same procedure was applied to
graft 2-Hydroxyethyl methacrylate (HEMA) on G0 generation of POSS. G0 was
dissolved in methanol and kept at 0° C in inert medium for half an hour. 0.3 ml
HEMA in methanol solution was added slowly. Then the reaction was continued
for 24h at room temperature followed by heating at 40° C for 12h. Purification
was carried out using rotary evaporator under vacuum. The reaction scheme is
shown in figure 3.2.
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Chapter Three: Materials and Methods
O
R'
R'
H2C
R'
NH
NH2
O
CH3
+
C
H2C
R'
R'
R'
CH2
O
R
C
H2
O C
RT, 24h
in Methanol
H2
C OH
HEMA
CH3
O
'
H
C
H2C
Amine-POSS
O
R''
R''
R
R
CH2
NH
R''
OH
CH3
N
O
R''
''
H2C
''
H2 H2
O C C
C
O
H2C
CH
CH3
C O
H2
C
H2
C OH
CH3
R''
G0-HEMA
O
R' =
H 2C
CH2
NH
NH2
O
CH3
O
H 2C
O
C
H2 H2
O C C OH
CH3
CH2
R''= H2C
H
C
NH
N
O
CH3
H 2C
O
H
C
C
H2 H2
O C C OH
CH3
Figure 3.2 The scheme to synthesize G0-HEMA with two branches
3.3.1.2 Synthesis of G0-HEMA with Quaternary Amine
The quaternary amine was synthesized following the procedure mentioned in a
recently published protocol (e.g., Chatterjee et al., 2002; Das and Das, 2003).
Briefly, first structure and allyl bromide were taken in molar ratio 1:8 in mixed
solvent (30% methanol/ 70% acetonitrile) and refluxed at 90° C for 24h. The
product was obtained by crystallization and was washed in methanol. Figure 3.3
shows the scheme for the synthesis.
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Chapter Three: Materials and Methods
H2C
O
R''
R''
R''
R''
O
H2
C
H2
C OH
H2C
+
O
H
C
H2C
CH3
C
CH3
N
O
R''
R''
CH2 NH
H2C
O
H
C
C
H2
C
O
C
H
H2C
Reflux- 24h at 900 C
Br
30% methanol/
70% acetonitrile
Allyl Bromide
H2
C OH
CH3
R''
O
G0-HEMA
H2C
R'''
R'''
+
CH3
N Br
CH2
NH
H2C
H2C
H
C H2C
CH3
R'''
R'''
R'''
H
C
O
R'''
O
H2C
C
O
H2
C
H2
C OH
O
H2
C
H2
C OH
O
H
C
C
CH3
R'''
G0-HEMA with Quaternary Amine
O
H2C
O
R''=
H 2C
CH2
NH
+
N Br-
O
H2C
CH3
H
C
H2 H2
O C C OH
C
CH3
O
H
C
H2 H2
O C C OH
C
CH3
O
R'''=
H2C
CH2
O
CH3
H2C
H2C
H
C
+
CH3
C
N Br
NH
H2C
C
H
O
H2C
H
C
O
C
H2 H2
O C C OH
H2 H2
O C C OH
CH3
Figure 3.3 Scheme for G0-HEMA with Quaternary Amine
3.3.1.3 Synthesis of G0-HEMA with three branches
Third structure was synthesized similarly as the procedure of the second structure.
Amount of HEMA taken for this reaction is 0.4ml (exactly 1: 3 ratio of POSS
reactive corner to HEMA). The reaction was carried out for 1 day at room
temperature followed by heating at 40°C for 12 hr in inert atmosphere. The
product was crystallized into the reaction medium and was washed with methanol.
Figure 3.4 shows the structure of the particle.
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Chapter Three: Materials and Methods
O
R'
R'
R
H 2C
NH2
O
CH3
H 2C
+
R'
R'
R'
NH
CH2
O
'
R
C
C
H2
C
O
RT, 24h
in Methanol
H2
C OH
HEMA
CH 3
O
'
H2 C
amine-POSS
RIV
R
IV
H2
C
RIV
H2
C
H2
C
O
O
CH3
C
C
H
H2
C
RIV
H3C
R IV
RIV
O
H2
C
H2
C
OH
O
H2
C
H2
C
OH
N
N
H 2C
CH
H
C
O
C
CH 3
O C
Target 3
C
CH 3
CH2
RIV
H
C
O
H2
C
C
H2
OH
O
R'=
CH2
H2 C
NH
NH2
O
CH 3
H2C
O
R'v=
H
C
O
C
O
H2
C
H2
C OH
O
H2
C
H2
C OH
CH3
H2 C
CH2
N
N
CH2
H2C
O
CH3
C
O
H2
C
O
C
CH3
CH CH3
O
H
C
H2
C OH
Figure 3.4 Scheme for synthesizing G0-HEMA with three branches
3.3.2 Synthesis of Anion Exchange Membrane
Anion exchange membrane is also considered as di-block block membrane as it
contains hydrophobic and hydrophilic part. The hydrophobic part provides the
structure of the membrane and can be named as backbone. And, the hydrophilic
part is mainly charged particles that help to pass ions through the membrane. The
preparation of this membrane contains three experimental steps followed by
membrane casting. The steps are described below:
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Chapter Three: Materials and Methods
3.3.2.1 Synthesis of the backbone
The ratio of different monomers (VBC, AN, GMA) was selected to synthesize the
backbone by trying different batch of polymerization reaction and looking at their
membrane properties. Finally, VBC: AN: GMA were taken in 2.5: 92.5: 5 volume
ratios in a glass tube. 5 ml of DMF was added with 2ml of these monomers
together with 20mg of the initiator (DMPA). Argon gas was purged through the
solution for 15 min. Then UV polymerization reaction took place for 2h.
CH3
HC
CH2
+
HC
CH2
+
UV polimerization
DPMA initiator,2h
CH2
C
C
N
H2C Cl
VBC
C
O
O
H2
C
O
C
H
CH2
GMA
AN
CH3
H
C
H2C Cl
H2
C
x
H
C
H2
C
y
H2
C
C
C
C
N
O
z
O
O
H2
C
CH
Backbone
Figure 3.5 Structure of backbone
After UV polymerization the product was collected by precipitation into cold
methanol-water (1:1) solution and was washed to remove the impurities. This
product was considered as hydrophobic backbone for membrane preparation and
stored for the next step after drying.
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CH2
Chapter Three: Materials and Methods
3.3.2.2 Preparation of the Charge Carrier Group
Quaternary amine was used as a charge carrier group in this study. The
preparation procedure has been adopted from a recent work (Das and Das, 2003).
According to the procedure, 1 ml of allyl bromide and excess amount of N-methyl
diethanol amine (5ml) were taken in 30% methanol/ acetonitrile solution (15ml)
and refluxed at 90° C. After refluxing for 24h, the solvent was evaporated using
rotary evaporator and was washed with ethyl acetate and kept inside the fridge to
crystallize the product.
OH
OH
H 2C
H 2C
C
H
+
CH2Br
H 3C
Allyl Bromide
Reflux 24h at 900C
methanol:acetonitrile(3:7)
N
OH
N-methyl diethanol amine
C
H
CH 2
N+ BrH 3C
OH
Quaternary monomer
Figure 3.6 Synthesis scheme of the cationic monomer
The crystallized product was purified by washing with ethyl acetate and dried
under vacuum. This quaternary amine acted as a monomer in ATRP reaction in
the following step.
3.3.2.3 Grafting the monomer on to the backbone via ATRP
Atom Transfer Radical Polymerization (ATRP) was employed to carry out the
grafting of quaternary amine to the VBC group of backbone. A typical procedure
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Chapter Three: Materials and Methods
for this synthesis is as follows: the backbone (0.7g), quaternary amine (36mg) and
Bpy (70.2324 mg) were added into 10ml of DMF and the temperature was
maintained at 40°C. After 30 minute, calculated amount of CuCl (14.815 mg)
were added and the reaction medium was charged with argon gas.
CH3
H2
C
H
C
H2
C
H
C
x
H2
C
y
C
C
C
z
O
O
H2
C
O
N
+
CH
OH
H2C
C
H
N+ Br
CH 2
H2C Cl
Backbone (initiator)
H2
C
H
C
x
H3C
OH
monomer
CuCl/Bpy in DMF
12h, 400C
CH3
H
C
CH2
H2
C
C
y
H2
C
N
C
C
O
z
O
H2
C
O
CH
CH2
H2C
C
H
CH
n
Cl
OH
H 2C
N+ Br
H3C
OH
Figure 3.7 Growing Quaternary Amine chain on Backbone
After 12h, sample was precipitated into methanol water (v:v=1:1) mixture and
washed with methanol to remove the impurities.
3.3.2.4 Membrane casting
A calculated amount of cation grafted polymer obtained from the ATRP was
introduced into DMF (10ml) and mixed uniformly with the aid of ultrasonication.
Required amount of the cross-linker (either EDA or Amine-POSS) was added into
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Chapter Three: Materials and Methods
the solution to open the epoxy ring of GMA. The resulting solution was cast in a
Teflon Petri-dish (d= 8cm). The dish was kept into the furnace at 80° C for 8h to
generate composite membrane. After casting, the membrane was washed with DI
water and stored in hydrated condition for further analysis.
3.4 Characterization and Analytical Tools
After synthesizing the materials, characterization need to be done to confirm the
exact structure of the product. Once the product is confirmed, colloidal behavioral
analyses and membrane performance evaluations have been conducted. This part
gives a brief description of the characterization and analytical tools and the
conditions under which the study has been done.
3.4.1 Fourier Transform Infrared Spectroscopy (FTIR)
FTIR measurements of all samples had been performed using a Shimadzu infrared
spectrometer (Model 400) with KBr as background over the range of 4000-400
-1
cm .
3.4.2 Transmission Electronic Microscopy (TEM)
A bright-field TEM (JEM-2010) was used to measure the size of branched
polymeric particles. The sample was prepared by coating a thin layer of diluted
sample suspended on methanol on a copper grid (200mesh and cover with
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Chapter Three: Materials and Methods
formvar/carbon). The copper film was then dried at 50°C for few hours before the
measurement.
3.4.3 Thermogravimetric Analysis (TGA)
The thermogravimetric analysis was performed on a thermal analysis system
(Model: TA 2050). For TGA measurements, the mass loss of dried sample was
monitored under N2 at temperatures from room temperature to 850°C at a rate of
10°C/min.
3.4.4 Scanning Electric Microscope (SEM)
The morphology of the membrane was analyzed using SEM. The sample was
prepared by coating the suspended sample on the carbon tape of the sample
holder. The sample was then dried at 50°C for few hours and coated with
platinum before measurement.
3.4.5 Dynamic Light Scattering (DLSC)
Size distributions of the colloidal particles were measured using DLSC. Samples
were prepared at different pH using water as the solvent. For each sample, six
runs were conducted and the average was taken as the result.
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Chapter Three: Materials and Methods
3.4.6 Auto Lab
The logarithms of the impedance of membranes at different temperature were
measured by AUTO LAB. The measurement was carried out in potentiostatic
mode over a frequency range from 10Hz to 1MHz with an oscillating voltage of
5mV. The four probe membrane holder was prepared in the laboratory. The
sample was dipped into KOH for 24 hours and heated at 60º C for 30 min before
the measurement. Then the conductivity, σ (S/cm) was calculated by following
equation.
Conductivity (σ) =
l
RA
where l is thickness of the membrane (cm), A is the membrane surface area
exposed to the electric field (cm2), and R is the membrane resistance (Ω).
3.4.7 Particle Size Analyzer (Zeta Sizer)
Malvern instrument of zeta sizer was used to find out the charging behavior of the
colloidal particles at different pH and also in presence of different charging ions.
Mainly water was used as dispersant phase. In some cases, mixer of water and
ethanol (about 1~2%) was used.
3.4.8 Gas Chromatography (GC)
Shimadzu GC was used to measure the crossover of the membrane. The method
used for analysis was made for methanol and ethanol crossover.
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Chapter Three: Materials and Methods
3.4.9 UV Cross-Linker
UV cross-linker (lab made) was used to polymerize the monomers under UV light
for a certain period of time. Polymerization reaction took place for 2 hours and
the number of light source used to undergo the polymerization reaction was 12.
3.5 Summary
This chapter gives an overall idea of materials and methods employed in the
study. The experimental procedures of all the three structures from POSS have
been described extensively. Brief descriptions of analytical tools have also been
presented. Characterization tools such as IR, NMR, and TEM are used to analyze
the structures from POSS. Dynamic light scattering, particle size analyzer are
occupied to study the colloidal behavior of those particles.
Anion exchange membrane was prepared in four steps. A comprehensible
description of these steps has been provided. In the first step, the polymeric
backbone was prepared by UV polymerization that gave the structure of the
membrane. In second step, the charge carrying part (quaternary amine) was
prepared followed by the grafting of the charged part to the backbone via ATRP
reaction in the third step. Finally the membrane was cast using two cross-linkers
in a Teflon dish. Analytical tools were used to characterize and to evaluate the
performance of the membrane. The results are shown and discussed in the
following chapter.
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CHAPTER FOUR
RESULTS AND DISCUSSIONS
4.1 Introduction
This chapter presents the characterization techniques used to analyze the structure
of the desired products. For the structures of POSS derivatives, NMR and FTIR
were done in order to have a rough idea about the structure. TEM pictures were
taken to show the differences between the three structures. The structure of the
membrane was analyzed using IR, TGA and EDX.
Effects of pH on particle size and surface charge were studied for three structures
of POSS while suspending into water medium. Studies were also carried out for
boron adsorption at higher pH level. The kinetic study of G0-HEMA with two
branches was conducted by measuring the contact angle with water at different
hydrophobic medium and relative humidity to understand the surface behavior
when coated onto a glass surface. Polymeric membranes were constructed using
two types of cross-linker in the same membrane materials. In the first case amine
was used to open the epoxy groups whereas in the second case amine-POSS was
used. Water uptake, conductivity and methanol crossover were measured to
evaluate the performance of each membrane and a comparative study was done
between the membranes using two separate cross-linkers.
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Chapter Four: Results and Discussions
4.2 Characterization of POSS derivatives
Three derivatives of POSS were developed in this study. Characterizations of
those POSS derivatives are shown in the following section. To confirm the
structural configuration FTIR and 1H-NMR were conducted. TEM pictures were
taken to taken to understand the agglomerating nature of the particles.
4.2.1 Characterization of G0-HEMA with two branches
The reaction pathway was verified by FTIR studies. Figure 4.1 shows the FTIR
spectra of POSS, ethylene diamine grafted POSS (G0) and HEMA grafted G0.
c
b
OH & NH stretching
1731
a
NH stretching
2983
Si-o-Si stretching
4000
3600
3200
2800
2400
2000
1600
1200
800
400
cm-1
Figure 4.1 FTIR spectra of (a) POSS, (b) POSS-amine (G0) and (c) G0HEMAwith two branches.
The functional group of the POSS used in this research - was the propylmethacryl
group. The characteristic IR peak for siloxane bond is in the region of 1100-1000
cm-1. The strong peak in 1720 cm-1 region is due to the carbonyl group in POSS.
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Chapter Four: Results and Discussions
The change in intensity (1731 cm-1) of carbonyl peak amine-POSS (G0) confirms
the breaking of the C=C double bond due to the reaction as the conjugated double
bond appears at the lower energy (higher wave number) region. The peak at 2983
cm-1 in POSS is due to C-H stretching vibration of alkene group (C=C) which is
reduced in both POSS-amine (G0) and G0-HEMA with two branches. This
confirms the arms of POSS undergo reaction with amine. The broad peak
appeared at 3200 cm-1 to 3400 cm-1 is due to the presence of secondary amine in
the sample. For G0-HEMA with two branches, slightly broader peak has been
found for hydroxyl group and secondary amine around 3200 cm-1 to 3400 cm-1
region. From the results of FTIR, the dendrimeric structure of POSS has been
confirmed.
a
R''
R''
Si O
O
Si
O
O
O R''
Si
O
Si
O
R''
O
CH2
Si H2C
O
R''
Si
O
b
O
R''
NH
H2 C
f
O
C
H2
O C
H2
C OH
H2
O C
H2
C OH
CH3
d
H3C
H
C
N
H2C
c
O
Si
O O
Si
e
H
C
O
C
g
CH3
R''
e, f
c
d, g
b
a
Figure 4.2 1H-NMR of G0-HEMA with two branches
In order to figure out the number of HEMA branched grafted on the POSS, 1HNMR has been done. The peaks for the corresponding H are shown in figure 4.2.
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Chapter Four: Results and Discussions
The presence of d, e, f, g peaks on the spectrum confirms the grafting of HEMA.
However, the number of branches is not the same as expected. The ratio between
the hydrogen with oxygen and nitrogen molecule (peak g and d) and the hydrogen
with carbon molecule (peak b) is about 1. This suggests that roughly one chain
has been grafted at each corner of POSS.
a
10-20nm
~50 nm
b
Figure 4.3 Transmission electron micrograph of (a) POSS and (b) G0-HEMA
with two branches
TEM pictures have also been carefully observed to identify the difference
between POSS and G0-HEMA structure. Typical TEM micrographs of POSS and
the dendritic structure are shown in figure 4.3. However, it is quite difficult to
trace individual particles. Therefore from TEM images, the smallest size has been
taken as an individual particle. It shows that the particle size of POSS is about 10
to 20 nm whereas the corresponding size for G0-HEMA increases to 50 nm. The
increase in size of particles is due to the presence of HEMA chain in POSS
structure.
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Chapter Four: Results and Discussions
4.2.2 Characterization of G0-HEMA with Quaternary Amine
The formation of the quaternary amine is not possible to confirm by using the IR
spectrum as there is no characteristic peak for quaternary salt (Socrates, 2001).
For this reason the IR spectrum of G0-HEMA with quaternary amine carries less
importance. Hence the spectrum has been shown in figure 4.4.
3900
3400
2900
2400
1900
1400
900
400
cm -1
Figure 4.4 IR spectrum of G0-HEMA with Quaternary Amine
Like TEM images of first structure, individual particles of G0-HEMA with
quaternary amine are also difficult observe from images. Compare to the first
structure, these particles have a higher degree of agglomeration. The first
structure is branched POSS particles with OH terminated group (G0-HEMA)
while second one contains quaternary amine group with previous structure. The
counter ion is chloride ion which acts as a bridge to bring the particles closer. In
other words, due to the presence of charge, intra-particle attraction took place that
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Chapter Four: Results and Discussions
results in strong aggregation. The TEM picture of the solid particle is shown in
figure 4.5.
Figure 4.5 TEM image of G0-HEMA with Quaternary Amine
4.2.3 Characterization of G0-HEMA with three branches
The IR spectrum of this particle is exactly the same with the G0-HEMA with two
branches (i.e., first structure). This is quite natural as both of these particles have
same functional groups. Moreover, from 1H-NMR it is evident that grafting of
HEMA branches is hampered by the static hindrance of POSS. As a result, there
are no significant differences between G0-HEMA with two and three branches.
4.3 Study of the Colloidal Behavior of POSS
The size changes of three structures of POSS have been studied over a pH range
from 2 to 11. This study has been done using dynamic light scattering (DLSC)
method. Moreover the particle charge at different pH level has been estimated by
measuring zeta potential. Adsorption of boric acid has also been measured at the
same time.
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Chapter Four: Results and Discussions
4.3.1 Dynamic Light Scattering
In this study, the pH effect on the particle size has been studied using DLSC
keeping the concentration of particles on water medium as 0.37 mg/ml. Results
show that particles of G0-HEMA with two branches exhibits oscillating behaviors
with the change in pH range (see figure 4.6). While the particle size is about 574
nm at pH of about 2, the corresponding size reduces to 280 nm with an increase of
pH to about 3. This is because this particle undergoes protonation at low pH and
starts deprotonation with the increase in pH. Interestingly the particles have the
highest size at pH about 8 due to the adsorption of hydroxyl (OH) group.
Moreover further increase of the pH in the basic region, the particle size reduces
due to compression of sodium ion. This phenomenon is also called ionic
compression.
Hydrodynamic behavior of G0-HEMA
1200
zero surface charge
d(nm)
1000
800
protonation
600
ionic compression
400
deprotonation
200
0
2
4
6
8
10
12
pH
Figure 4.6 Size distribution of G0-HEMA with two branches at different pH
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Chapter Four: Results and Discussions
Size distribution of G0-HEMA with quaternary amine has been studied in water
medium with a concentration of 0.1875 mg/ ml. Size distribution of G0-HEMA
with quaternary amine at different pH level shows single-peaked distribution.
Since this particle contains quaternary amine group, significant adsorption of
proton does not take place at low pH due to its positive charge. While particle size
is larger at about pH 5.5 for the adsorption of hydroxyl group, the size reduces at
higher pH because of ionic compression.
Hydrodynamic behavior of G0-HEMA with
Quaternary Amine
1400
size (nm)
1200
1000
800
600
400
200
2
4
6
8
10
pH
Figure 4.7 Size distribution of G0-HEMA with quaternary amine at different
pH
As observed from figure 4.8, G0-HEMA with three branches shows similar
behaviors as if for those with two branches. However a slight shift of pH on
hydroxyl ion adsorption has been noticed between those two. This may be due to
the presence of slightly higher number of HEMA chain in the structure.
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Chapter Four: Results and Discussions
G0-HEMA with three branches
adsorption of
OH groups
8200
7200
protonation
size (nm)
6200
5200
4200
3200
2200
ionic compression
deprotonation
1200
200
2
4
6
8
10
12
14
pH
Figure 4.8 Size distribution of G0-HEMA with three branches at different
pH
Size of the particles of those three POSS structures has also been tested by adding
different salts at different pH level. Experimental results show that the swelling
behavior is quite stable for all three structures when different metal salts like
CuSO4, and CuCl2 have been added at a concentration level of 15µg/L.
4.3.2 Zeta potential
The effect of pH on zeta potential has been studied for G0-HEMA with
quaternary amine and branches using water medium and 1-2% ethanol water
solution respectively. The concentration of POSS particles used for this part is
same as for size distribution study. As all the particles contain terminal hydroxyl
groups, effect of adding Boric acid has also been studied. Previous studies showed
that boric acid starts to dissociate into borate ion at pH > 9.2 and reaches to 100%
borate ion at pH 12 (Glueckstern and Priel, 2007; Koseoglu et al., 2008; Nadav,
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Chapter Four: Results and Discussions
1999). At pH higher than 9.2, it can be removed by adsorbing on the surface
containing hydroxyl groups (Geffen et al., 2006; Senkal and Bicak, 2003).
H 3 BO3 → H 2 BO3
−
Effect of pH and Boric acid on G0-HEMA with two
branches
with out Boric acid
80
Z (mV)
with Boric acid
40
0
2
3
4
5
6
7
8
9
10
11
12
-40
-80
pH
Figure 4.9 Effect of pH on G0-HEMA with two branches
For G0-HEMA with two branches, particles have iso-electric point at about pH of
7.5. As observed from figure 4.9, adsorption of Boric acid does not take place.
Similar experiment has been conducted for G0-HEMA with quaternary amine.
This particle exhibits iso-electric point at pH of about 5.5 and shows a higher zeta
potential in water medium that reflects its stability. Including Boric acid into the
system reduces the value of zeta potential. At pH greater than 9.5 (see figure
4.10), there is a significant drop in zeta potential which indicates sufficient
amount of borate ion has adsorbed on the surface and the value of zeta potential
drops due to the low charge density of borate ion. Earlier researches (e.g., Jacob,
2007) have also reported that presence of quaternary amine enhance the borate
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Chapter Four: Results and Discussions
ion adsorption. The surface adsorption may be expressed by the following
OH
relation:
O
B-
+ H 3 BO3
O
OH
Effect of pH and Boric acid on Quaternized POSS
30
without Boric acid
Z (mV)
wiith 30 ppm of Boric acid
10
with 50 ppm Boric acid
-10 2
3
4
5
6
7
8
9
10
11
12
-30
-50
pH
Figure 4.10 Effect of pH and Boric Acid on G0-HEMA with Quaternary
Amine on zeta potential
Z (mV)
Effect of pH and Boric Acid on G0-HEMA with three
branches
15
without Boric Acid
10
with Boric Acid
5
0
-5
2
4
6
8
10
12
14
-10
-15
pH
Figure 4.11 Effect of pH and Boric acid on G0-HEMA with three branches
on zeta potential
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Chapter Four: Results and Discussions
G0-HEMA with three branches shows almost same behaviors when there were
two branches. Figure 4.11 shows the zeta potential of third structure when using
only water as disperse medium which results the smaller potential of the particle.
Adding Boric acid almost has no effect on the surface charge of the particle.
4.4 Kinetics study
A thin layer of G0-HEMA with two branches in methanol solution was coated on
a glass substrate (22mm × 22mm) using spin coater at 2000 rpm for 60 sec. After
that, methanol was evaporated at 35°C using vacuum. The kinetic behavior of
coated layer on glass surface was studied at different solvent and relative humid
condition. Figure 4.12 shows the optical image of the coated surface. The surface
can be considered as uniformly coated though there is some uncoated area in
micro range.
Figure 4.12 Surface morphology of the coated layer
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Chapter Four: Results and Discussions
4.4.1 Kinetic study at different hydrophobic solvents
After coating, the coated glass was immersed into three different hydrophobic
solvents (Toluene, Cyclohexane, Ethyl acetate) for 30 minutes and dried under
vacuum inside the desicator. After that the effect of moisture was studied when
exposed to the atmospheric condition by measuring the contact angle with water.
Figure 4.13 describes the preparation steps of the coated glass on a typical
solvent.
Figure 4.13 Coated glass at different hydrophobic medium.
The study of the kinetic behavior was conducted by measuring the contact angle
with time and at different conditions. Initially the hydrophobic part of the particle
(cage structure) was oriented upside and assigned very high contact angle. After
the coated substrate was exposed to the hydrophilic atmosphere with moisture,
flip over of the hydrophobic part took place as the contact angle become smaller.
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Chapter Four: Results and Discussions
Effect of hydrophobicity on kinetic behavior
RH~ 50-60%
90
50
80
Contact angle
Contact Angle
100
70
60
50
Cyclohexane
Ethyl Acetate
Toluene
48
46
44
42
40
0
0
10
20
30
40
2
4
50
6
8
10
Time (min)
60
Time (min)
Figure 4.14 Study the effect of atmosphere on G0-HEMA coated layer.
Reducing value of contact angle has been observed for both ethyl acetate and
cyclohexane and presented in figure 4.14. Results indicate that the surface
becomes more hydrophilic with time. The surface of the coated layer was initially
hydrophobic and with the contact of the atmospheric moisture it turns out
hydrophilic. Though cyclohexane is more hydrophobic than ethyl acetate, there is
not much difference in the kinetic behavior. The flip over of the surface from
hydrophobic to hydrophilic atmosphere took place within the first few minutes.
Contact angle drops from 50º to 44º within first 10 minutes and for the later part
the change is negligible. After two days it reaches 40°. Figure 4.14 shows how the
surface changes with the effect of atmosphere.
The surface behavior can further be explained by figure 4.15. Coating of G0HEMA makes glass surface hydrophilic. However the orientation of dendritic
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Chapter Four: Results and Discussions
structure changes to a higher contact angle in the extreme hydrophobic condition
because the hydrophobic core of the structures comes to the upper surface.
Moreover exposure to the atmospheric condition leads the hydrophilic branches to
come upside resulting in lower contact angle.
HO
OH
OH
OH
HO
HO
in Hydrophobic
In hydrophobic
In EA
condition
solvent
OH
HO
OH
HO
HO
OH
OH
HO
HO
OH OH
HO
OH
OH
at atm
condition
RH=50-60
HO
HO HO
OH
OH
Figure 4.15 Effect of different atmospheric condition on the surface
orientation
However toluene does not show such trend even after a longer exposure (48 hrs)
on the atmospheric condition. It can be deduced that contact with toluene build a
meta-stable structure that resist any change in surface condition. This may be due
to the longer contact duration with Toluene residue (due to high boiling point)
that interacts with the layer and keeps it unaffected with further changes.
4.4.2 Kinetic study at different Relative Humidity
Different relative humidity (RH) was created by using saturated salt solutions and
the coated glass (after subjected to different hydrophobic solvents) was kept
inside of that saturated environment for 24 hrs. The saturated salt solutions used
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Chapter Four: Results and Discussions
to reach different relative humidity are Lithium Chloride (RH=11.3%),
Magnesium Chloride (RH= 32.8%), Potassium Carbonate (RH= 43.1%),
Magnesium Nitrate (RH= 53%), and Potassium Chloride (RH= 84.3%).
When a coated glass was placed at different humid condition consecutively, no
difference was noticed because of the surface rigidity for the longer time effect.
To overcome this, a single coated substrate was placed in each relative humid
medium. The contact angle was measured before and after placement into each
humid medium. Table 4.1 shows the effect of different RH on the coated surface.
Table 4.1 Effect of relative humidity on the surface structure
Lithium Chloride, 11.3%
In Ethyl acetate
Before
After
35.77±0.21
37.13±1.90
In Cyclohexane
Before
After
36.63±0.32
37.82±0.81
Magnesium Chloride, 32.8%
35.87±0.57
38.33±1.90
33.63±0.32
34.37±0.38
Potassium Carbonate, 43.1%
31.30±1.21
26.10±0.72
35.8±0.40
35.43±0.85
Magnesium Nitrate, 53%
38.87±0.91
33.4±1.90
35.93±0.41
29.52±0.41
Relative humidity
From the table it can be said that the surface structure does not show much
changes in lower humid condition. However at higher humid condition (like
43~53% RH), the contact angle become smaller due to the change in surface
orientation. The kinetic study is very sensitive to the surrounding conditions. The
results varies with the processing steps, time duration, presence of foreign
particles, room temperature etc.
Even higher relative humidity (saturated KCl salt, 84.3%) was also studied for
both solvents. Results show an uneven track with some points shows higher
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Chapter Four: Results and Discussions
hydrophobicity whereas some shows hydrophilicity. This may be due to the
sudden effect of higher humidity that does not allow sufficient time for the
surface orientation.
The kinetic study of branched POSS reveals that this kind of structure undergoes
flip over when it is subjected to hydrophilic atmosphere from hydrophobic one.
This is mainly due to the orientation of the OH-tele branches of the POSS.
Moreover this structure shows oscillating behavior of its size at different pH range
due to the adsorption and desorption of proton on its surface.
4.5 Characterization of the membrane
The structure of the membrane was analyzed using FTIR spectrum. TGA and
EDX were employed to calculate the amount of charge containing group grafted
in the polymeric backbone in mass and mole percentage respectively. The surface
morphology of the membrane was observed using SEM images.
4.5.1 FTIR Spectrum of the Membrane
The grafting of the monomer on the backbone has been confirmed by IR
spectrum. Figure 4.16 shows the IR spectra of the polymeric backbone,
quaternary amine (monomer) and the monomer grafted backbone (VBC: AN= 1:
79 in backbone).
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Chapter Four: Results and Discussions
amine & OH group
c
a
VCN = 2245
C=O
b
quternary amine OH group
3900
3400
2900
2400
1900
1400
900
400
cm-1
Figure 4.16 FTIR spectra of backbone (a), monomer (b) and monomer
grafted backbone (c)
The typical features of the monomer are the strong and broad adsorption band of OH group at 3680-3150 cm-1 region. The peak at 1734 cm-1 in backbone is due to
the carbonyl group stretching present in the backbone. Again, the peak at about
2245 cm-1 is due to nitrile group. These two peaks are also reflected in the
monomer grafted backbone. However, the broad peak for hydroxyl group is not
strong enough which may indicate the poor grafting of the monomer on the
backbone. The low loading is also confirmed by EDX and TGA which will be
discussed in next two sections.
Low loading of monomer can be due to the lesser number of initiators in the
backbone. Increasing the number of initiators may result in a higher number of
monomer grafted. In order to increase the loading, backbone at three different
ratios (VBC: AN= 1: 20; 1: 40; 1: 79) of initiator to backbone was prepared and
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Chapter Four: Results and Discussions
further studies was carried out. EDX and TGA confirm the comparisons among
higher loading in these cases.
4.5.2 Energy Dispersion X-ray Spectroscopy
SEM-EDX analysis was done to study the number of monomers grafted on the
backbone. From a preliminary study (four different ratios of monomer to initiator)
it was found that increasing monomer ratio does not help to obtain longer chain in
ATRP reaction. The number of monomer that can be attached to the initiator is
limited by one. This may be due to the charge of the monomer that repeals other
monomer to be grafted. To overcome this limitation, three different ratios of
initiator (VBC) to backbone (mainly AN) was prepared and ATRP reaction was
carried out at the same condition. The ratio of chlorine (from initiator) and
bromine (from monomer) for different design condition is shown in table 4.2.
From the table it can be seen that number of attached monomer increases with the
increase of the initiator present on the backbone. However, not all the initiator
group present on the backbone carries the monomer.
Table 4.2 Grafting of monomer on different backbone
VBC: AN
Br (from monomer)
Cl (from initiator)
Cl : Br
1:20
0.157
0.93
6:1
1:40
0.063
0.647
10:1
1:79
0.02
0.30
15:1
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Chapter Four: Results and Discussions
4.5.3 Thermal Analysis
Figure 4.17 shows the thermo gravimetric curves for backbone without and with
monomer grafted under a ratio of VBC:AN = 1:79. The weight loss up to 0 to
200° C is considered due to the surface water and thus has not been included in
the figure. TGA curve of the monomer confirms the degradation temperature of
the monomer which is around 250 to 280° C.
TGA curves for the monomer grafted backbone shows three steps weight loss
where the backbone without monomer shows two steps weight loss. This is due to
the monomer decomposition at about 250-280° C temperature. Degradation of the
backbone takes place mostly around at 350 to 450° C which demonstrates good
thermal stability and higher degradation temperature.
Grafting of monomer on the backbone
0.8
monomer degradation
backbone
atrp
weight %
80
0.6
backbone degradation
60
0.4
40
0.2
20
0
200
300
400
500
600
700
800
weigth derrivative
100
900
Temperature
Figure 4.17 Thermo gravimetric analysis of the backbone (VBC: AN = 1: 79)
and monomer grafted backbone.
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Chapter Four: Results and Discussions
TGA curves for the other two backbones (VBC: AN= 1: 20; 1: 40) are similar as
if shown in figure 4.17 and therefore have not repeated. The amount of monomer
grafted on the backbone was roughly calculated from the weight loss measures.
From the curves, it is found that the amount of monomer attached to the backbone
is about 0.5-3.5 wt% which reflects the limitation of the grafting of the monomer.
In summary a higher amount of initiators on the backbone leads to a higher
amount of grafting.
4.5.4 Surface morphology
The picture of the surface and the cross section of the polymeric membrane were
taken using Field Emission Scanning Electron Microscope (FESEM). The
membrane is quite hydrophobic due to the high loading of the hydrophobic
backbone. From the FESEM pictures, it can be seen that phase separation between
the hydrophobic and hydrophilic part takes place. This is not desirable as it
destroy the homogeneity of the membrane. There is slight difference on the
pictures of cross section of different membrane at lower magnification due to
phase separation. However at higher magnification, no major difference has been
observed.
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Chapter Four: Results and Discussions
(a)
(c)
(b)
(d)
Figure 4.18 FESEM images of membrane with 1:20 ratio of VBC to AN (a)
the surface cross-linked with EDA; (b) & (c) cross section of 1: 20 and 1: 40
ratio of VBC to AN respectively (cross-linked with EDA) and (d) cross
section at higher magnification.
4.6 Performance of the membrane
Membranes were casted using two types of cross-linker: 1) Ethylene Diamine
(EDA), and 2) Amine-POSS. A comparative study and the performance measure
of these membranes were carried out.
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Chapter Four: Results and Discussions
4.6.1 Ethylene Diamine (EDA) as a cross-linker
Membranes with three different ratios of VBC to AN were casted using EDA as a
cross-linker. The purpose of using cross-linker was to open up the epoxy ring
present in GMA to smoothen the membrane. Hydroxyl ion conductivity, water
uptake and methanol permeability were measured to evaluate the performance of
these membranes.
4.6.1.1 Conductivity
Figure 4.19 shows the ionic conductivity of different membrane as a function of
temperature. Conductivity was measured at 100% hydrated condition (in deionized water) at a temperature range from room temperature to 80° C. Results
show that
generally conductivity
increase gradually with the raise of
temperature.
Effect of temperature on hydroxyl ion conduction
Conductivity (S/cm)
0.014
0.012
c
0.01
0.008
b
0.006
a
0.004
0.002
0
20
30
40
50
60
70
80
Temperature (C)
Figure 4.19 Conductivity of the EDA cross-linked membrane vs.
temperature: (a) 1: 20; (b) 1: 40; (c) 1: 79 of VBC to AN.
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Chapter Four: Results and Discussions
From the figure, it can be observed that the conductivity is almost same for all
these three membrane at low temperature. However, membrane with lesser
amount of monomer shows higher conductivity at a high temperature like 80° C.
This can be due to the random arrangement of the hydroxyl conducting channel
established inside the membrane. With the higher number of monomer, the
formation rate of channels is very high which may cause disordered arrangement
of the channels resulting lower conductivity due to steric hindrance.
-1.8
for a; y = -0.543x - 0.7117
for b; y = -0.9789x + 0.7481
c
-2
b
-2.2
log σ
for c; y = -1.0023x + 0.9247
19.19 kJ/mol
18.74 kJ/mol
a
10.397 kJ/mol
-2.4
-2.6
-2.8
2.8
2.9
3
3.1
3.2
3.3
3.4
T-1 x 103
Figure 4.20 The Arrhenius plot of hydroxyl ion conduction for VBC to AN
ratio of (a) 1: 20; (b) 1: 40; (c) 1: 79.
The effect of monomer on the hydroxyl conducting channel has been examined by
comparing their hydroxyl ion conducting activation energies obtained from the
Arrhenius plot [log σ (T) = log σ0 – Ea / 2.3RT].
Activation energies are
presented in figure 4.20 which shows that activation energy is lowest for the
membrane having higher number of monomer. This is due to the quaternary
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Chapter Four: Results and Discussions
amine group that contents water and makes the movement of the ion easier. As a
result activation energy reduces.
4.6.1.2 Water Uptake
The membrane was washed, dried at 80° C for overnight and the weight was
recorded. The membrane was then immersed into DI water for two days. After
that, the surface water of the membrane was immediately wiped out and weight
was noted. Water uptake was calculated by following formula:
Water uptake (%) =
Wwet − Wdry
Wdry
× 100
Table 4.3 Water uptake of different membranes
VBC: AN
1:20
1:40
1:79
Water uptake
16.09%
14.60%
11.98%
The water uptake of three types of membranes is reported in table 4.3. Results
show that the water uptake of those is relatively low. Among them, the maximum
water content of about 16% has been found for 1:20 type VBC:AN membrane.
This is because the presence of quaternary amine group in this membrane helps to
retain higher water content. A high water uptake forms continuous transferring
channels that ease the ion movement. And hence the activation energy associated
with ion transport is small for high water content inside the membrane with the
VBC and AN ratio of 1:20 as reported in figure 4.20.
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Chapter Four: Results and Discussions
4.6.1.3 Methanol Crossover
Methanol crossover was measured by following the procedure described by Xiong
et al. (2008). According to this procedure the membrane was clamped between
two chambers. One of the chambers contains 50 ml of 2M methanol and the other
contains 50 ml Millipore water. A magnetic stirrer bar was placed in the water
side and penetrated methanol concentration was measured by GC analysis after a
fixed interval of time. The membrane was treated with KOH before testing the
conductivity in order to open the Hydroxyl ion conducting channel. Tests were
run for 6 hrs and samples were drawn from water side at every hour to measure
methanol concentration from GC.
The result from the methanol crossover is very convincing. The membrane is
totally resistant to methanol. The analysis found no trace of methanol on the water
side.
4.6.2 Amine-POSS as a cross-linker
Membranes with the same ratios of VBC to AN as shown in the previous section
were casted using Amine-POSS as a cross-linker. Hydroxyl ion conductivity and
methanol permeability were measured to evaluate the performance of these
membranes.
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Chapter Four: Results and Discussions
4.6.2.1 Conductivity
Figure 4.21 shows the conductivity of the three membranes while cross-linked
with amine-POSS. Conductivity of these membranes slightly increases with the
increase of temperature. However, the distinction between the membranes is not
significant in terms of ionic conductivity. The smaller value of conductivity may
be due to the presence of core structure that blocks ion conducting channels.
Effect of Temperature
conductivity (S/cm)
0.006
c
b
a
0.005
0.004
0.003
0.002
0.001
20
30
40
50
60
70
80
Temperature
Figure 4.21 Conductivity of Amine-POSS cross-linked membrane vs.
temperature for VBC to AN ratio of (a) 1: 20; (b) 1: 40 and (c) 1: 79.
4.6.2.2 Methanol Crossover
Methanol crossover was performed using the same procedure described for EDA
cross-linked membrane. The data obtained from GC analysis shows that the
membranes with amine-POSS cross-linker do not allow methanol to pass through.
That is membranes are totally resistant to methanol.
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Chapter Four: Results and Discussions
4.6.3 Comparison between Membranes using EDA and Amine-POSS as
cross-linker
It has been noticed that the use of two different cross-linkers do not change the
methanol permeability of the membranes. However, a slight change in water
uptake has been observed. Most importantly, properties like surface morphology
and conductivity show a significant difference between EDA and amine-POSS as
a cross-linker on membranes. A comparison among two cross-linkers has been
presented in next sub-sections.
4.6.3.1 Comparison of Surface Morphology
Surface morphology of membranes with EDA and amine-POSS cross-linker has
been presented in figure 4.22. The phase separation is a significant issue when
amine-POSS is used as a cross-linker. Organic and inorganic rich domains are
observed both in surface and cross sectional view of the membrane. For the
membrane using EDA as a cross-linker, phase separation is not that so significant.
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Chapter Four: Results and Discussions
Phase
Separation
a
c
b
d
Figure 4.22 FESEM pictures of the membrane surface (1: 20) cross-linked
with (a) EDA and (b) amine-POSS and cross section cross-linked with (c)
EDA and (d) amine-POSS.
The mechanical property of the membrane degrades when amine-POSS is used.
This is due to the higher percentage of inorganic phase present in amine-POSS.
This property of the membrane is likely to reduce its lifetime during operation.
4.6.3.2 Comparison of Ion Conductivity
The ion conductivity of the membrane drastically dropped while using aminePOSS compared to EDA cross-linker. Moreover the value of conductivity for
amine-POSS cross-linker has not been changed considerably over the temperature
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Chapter Four: Results and Discussions
range. The core structure of amine-POSS may block the ion conducting channels
resulting smaller conductivity.
4.7 Summary
The analysis of this chapter attempts to ensure the structure of the molecules by
using various characterization tools and evaluate the performances. This chapter
has discussed the experimental results for POSS derivatives and anion exchange
membranes.
The characterization tool FTIR and 1H NMR confirmed the presence of hydroxyl
group on the POSS derivative structures. From the NMR, it’s evident that each
corner of POSS roughly accommodated one hydroxyl group both for two and
three branches of G0-HEMA. This may be due to the static hindrance of POSS
structure. The colloidal behavior of three designed structures was studied at
different pH medium. G0-HEMA with two/ three branches showed an oscillating
behavior on varying pH medium.
Kinetic studies for G0-HEMA with two
branches also conducted at different hydrophobic medium and relative humidity.
Anion exchange membrane was prepared using two different cross-linkers.
Amount of quaternary amine grafted in the membrane were determined using
EDX and TGA. The performance of both types of membrane (cross-linked with
EDA and amine-POSS) such as conductivity, methanol permeability were carried
out. A comparison among performances of membranes with EDA and aminePOSS cross linkers has also been made.
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CHAPTER FIVE
CONCLUSIONS AND RECOMMENDATIONS
5.1 Research contributions
The objective of this study is to investigate colloidal behaviors of highly branched
POSS structures in aqueous medium and to evaluate performances of anion
exchange membrane while using ethylene diamine and amine-POSS as crosslinkers. This chapter summarizes the results of analyses, highlights significant
findings, and finally shows directions to future researchers.
5.1.1 Findings from colloidal study
For the colloidal study, branched POSS was formed by adopting dendrimeric
approach. Three different structures were synthesized and there characteristic
behaviors were studied. The designed difference between structure 1 and 3 was
the presence of hydroxyl group in different number, whereas structure 2 contained
quaternary amine (charged particle) group with the similar structure.
1
H-NMR for G0-HEMA with two branches confirms the restriction of growing
hydroxyl group containing branches on POSS. Despite of designing two different
structures by controlling the stoichiometric ratio of the reaction, there is not much
difference between G0-HEMA with two and three branches (structure 1 and
structure 3). This is due to the static hindrance of POSS that limits the number of
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Chapter Five: Conclusions and Recommendations
branching units from each corner. This results the similar colloidal behavior of
these two structures. G0-HEMA with quaternary amine (i.e., second structure)
agglomerates highly due to the presence of charged amine.
G0-HEMA (with two and three braches) shows oscillating behavior of its size at
different pH range. At low pH, the size of the particle is very small due the
adsorption of proton that interacts with oxygen molecule present in the particle.
The smaller particle size results due to the high attracting force acting on the
particle. With the increase of pH, hydroxyl molecule replaces those proton ions
and increases the overall size. The size reaches to its maximum value near pH 8 as
it is the isoelectric point. Afterwards a slight decrease in size is observed because
of the ionic compression of sodium ion at higher pH. In comparison to previous
structures, G0-HEMA with quaternary amine (i.e., second structure) shows higher
zeta potential. The charge contain makes this particle stable in water medium.
Adsorption of borate ion reduces zeta potential at higher pH which may be useful
in striping the boron from sea water.
The kinetic study of G0-HEMA reveals that this kind of structure flips over when
it is subjected to hydrophilic atmosphere from hydrophobic one. In hydrophobic
condition, the core structure resides outwards keeping the hydrophilic branches
inside. While the atmospheric condition changes from hydrophobic to
hydrophilic, the branches come outside. The reverse phenomenon is not evident
due to the rigid structure of the core.
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Chapter Five: Conclusions and Recommendations
5.1.2 Findings from Anion Exchange Membrane
Anion exchange membrane was synthesized in which quaternary amine act as
charge transferring group. The polymeric structure (backbone) of the membrane
was prepared by UV polymerization of three different monomers among which
one of them can be acted as an initiator for ATRP reaction. Quaternary amine was
grafted to the initiator by ATPR reaction. The membrane was casted using two
different cross-linkers: EDA and amine-POSS. The membrane properties such as
water uptake, conductivity, and methanol crossover were evaluated and results
have been discussed in the previous chapter.
It has been found that properties like water uptake, methanol crossover do not
change significantly for membranes with EDA and amine-POSS cross-linker.
However the surface morphology and ion conductivity show a reasonable change
while using amine-POSS as a cross-linker. Phase separation dominates due to the
POSS rich and polymer rich domain. This reduces the mechanical strength of the
membrane and shortens up the lifetime. It may because of the higher percentage
of rigid structure (inorganic) in amine-POSS that makes the membrane hard.
Applying long branches amine-POSS will increase the polymeric ratio in the
cross-linker and may help to solve this problem. On the other hand, the
conductivity falls drastically when using amine-POSS as a cross-linker. Presence
of POSS may block the ion conducting channels that results lower conductivity.
However the conductivity of the di-block membrane with EDA cross-linker is
quite reasonable (0.0055-0.012 S/cm) at higher temperature and the membrane is
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Chapter Five: Conclusions and Recommendations
super resistant to methanol. EDA membranes can be considered as a promising
alternative anion membranes for alkaline direct methanol fuel cells.
5.2 Recommendations
This study has been focused on investigating colloidal behaviors of highly
branched POSS particles and successfully developed several anion exchange
membranes. The research can be further extended in numerous directions. Some
recommendations for future work are proposed here
The functionality of POSS mainly depends on the functional group it possesses.
While keeping the original features of POSS, other properties can be added with
judicious choice functional moiety. Colloidal behaviors of POSS with other
functional groups will be interesting to study to explore the application of POSS
in different fields.
Branched POSS with considerable number of quaternary group is supposed to
have some distinctive colloidal behaviors due to the presence of high charge on
the structure. By adopting suitable reaction mechanism, an ionic ball from POSS
can be synthesized and studied which may reveal a new direction of POSS.
Experimental results have showed that presence of quaternary amine group
enhances the performance of the membrane. However it has been found that
copolymerization method allows only a limited number of quaternary amine
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Chapter Five: Conclusions and Recommendations
grafted on the backbone. This could be improved in several ways. One of the
possible ways is to graft tertiary amine to the membrane backbone first and then
transform it to quaternary amine.
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[...]... studies have been conducted on colloidal behaviors of highly branched POSS structures and its application as a cross-linker on di-block or anion exchange membrane The objective of this study is to investigate colloidal behaviors of highly branched POSS structures in aqueous medium and to evaluate performances of anion exchange membrane while using ethylene diamine and amine -POSS as cross-linkers In order... POSS, dendrimeric structures, and fuel cell along with a critical literature review on POSS and Anion Exchange Membrane Starting with the properties of POSS, it figures out the major field of studies related to POSS and identifies research gaps related to POSS behaviors For anion exchange membrane, it states the benefit of using this type of membrane over proton exchange membrane (PEM) Studies related... ethylene diamine (EDA) 1.3 Scope of the Study Colloidal behaviors of POSS have been studied for highly branched structure of POSS Hence three structures of POSS have been synthesized for this purpose Effects of pH on the size and surface charge have been studied at room temperature To observe the surface behavior of 2-hydroxyethyl methacrylate at zero generation of POSS (G0-HEMA), kinetic study on... critically reviewed and its application in different electrolyte membrane has been discussed Chapter 3 focuses on the experimental framework needed to achieve the aim of the study It includes the development of POSS particles and anion exchange membrane as well as the description of the analytical tools used in this study to characterize and analyze these particles and membrane National University of Singapore... obtained from the colloidal and membrane study It illustrates the colloidal behavior of particles and evaluates the performance of anion exchange membrane It also presents a comparative study between membranes using two types of cross-linker Finally, Chapter 5 summarizes the conclusions derived from this research Some recommendations for future research are included National University of Singapore 6... understanding of POSS, this chapter describes the impressive features of POSS, incorporation methods into other polymers and potential features achieved while using as additives and as well as in other areas such as medical application, modification of electrolyte etc This chapter also describes some literature on the modification of anion exchange membrane (AEM) Literature reviews on anion exchange membrane. .. Silsesquioxane (POSS) is relatively a new compound in the field of research POSS has some unique characteristics like hybrid structure, dual property of organic and inorganic performance, cage like shape Those unique characteristics of POSS make the researchers more interested in studying and understanding the behavior of POSS This chapter presents a critical review on studies related to POSS and its use... 68 Figure 4.21 Conductivity of Amine -POSS cross-linked membrane vs temperature for VBC to AN ratio of (a) 1: 20; (b) 1: 40 and (c) 1: 79 71 Figure 4.22 FESEM pictures of the membrane surface (1: 20) cross-linked with (a) EDA and (b) amine -POSS and cross section cross-linked with (c) EDA and (d) amine -POSS 73 National University of Singapore x NOMENCLATURE POSS Polyhedral Oligomeric Silsesquioxane... behaviors of POSS National University of Singapore 2 Chapter One: Introduction molecules Moreover the use of POSS as cross-linkers in anion exchange membrane has also not been found in the literature 1.2 Objective of the Study As shown in the previous section most of the studies related to POSS were mainly focused on the thermo-mechanical behaviors of POSS Despite of being a nano structured hybrid material;... electrolyte and found that addition of POSS increase the conductivity at low temperature and it is limited to certain amount of POSS However, the improvement is not appreciable above Tm Polymer electrolyte membrane is used for proton exchange membrane (PEM) in fuel cell Recent studies have also been focused on the improvement of this membrane while using POSS to improve the stability Mostly Nafion is one of .. .COLLOIDAL BEHAVIOR OF HIGHLY BRANCHED POSS PARTICLES AND DEVELOPMENT OF ANION EXCHANGE MEMBRANE AKLIMA AFZAL (B Sc in ChE, Bangladesh University of Engineering and Technology,... anion exchange membrane The objective of this study is to investigate colloidal behaviors of highly branched POSS structures in aqueous medium and to evaluate performances of anion exchange membrane. .. University of Singapore 23 Chapter Two: Literature Review 2.4.4 Anion Exchange Membrane (AEM) Anion exchange membrane is one of the possible solutions for power generation instead of proton exchange membrane
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