Silver nanoparticle –

polymer nanocomposites

A dissertation submitted to the University of

Manchester for the degree of MSc by research in

Chemistry.

Honeih Etemadi

Supervised by Prof. Stephen Yeates

1

Contents

Abbreviations 2

Abstract 3

Chapter 1 4

1.1. Historic Context to Nanoparticles 5

1.2. Effect of particle size on Physical Phenomena 6

1.3. Nanoparticle Applications 10

1.2.1. Nanoparticle applications in biology and medicine 14

1.2.2. Uses of nanoparticles in catalysis 16

1.2.3. Electronics 17

1.3. Nanoparticle Synthesis 18

1.3.1. Top down method 18

1.3.2. Bottom-up method 18

1.3.3. Colloidal Stabilisation 20

1.4. Nanocomposites 21

1.4.1.Nanocomposite synthesis methods 23

Chapter 2 24

2.1. Materials 25

2.2. Infra-red spectroscopy (IR) 25

2.3. UV-Visible Spectroscopy 26

2.4. Nuclear Magnetic Resonance (NMR) 26

2.5. Matrix-Assisted Laser Desorption/Ionisation (MALDI) 27

2.6. Thermogravimetric Analysis (TGA) 27

2.7. Dynamic Light Scattering (DLS) 27

2.8. X-Ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) 28

2.9. Transmission Electron Microscopy (TEM) 29

2.10. Scanning Electron Microscopy (SEM) 30

Chapter 3 31

3.1. Introduction 32

3.2. Synthesis and Characterisation of PEO-SH capping agent 33

3.3. Bromination of PEO 34

3.4. Synthesis and purification of PEO-SH oligomers 37

Chapter 4 42

4.1. Procedure for the synthesis of PEO-SH capped silver nanoparticles 42

4.2. Purification of Ag-PEGSH particles 41

4.3. Particle size analysis 45

4.4. Discussion 47

Chapter 5 50

5.1. Preparation of nanocomposites 50

5.2. Preparation of composite films 50

5.3. Characterisation of composite films 51

5.4. Thermogravimetric Analysis of Nanocomposites 54

2

5.5. Conductivity 56

5.6. Percolation Theory 56

5.7. Nanocomposite Summary 58

Chapter 6 59

6.1. Synthesis of PEO-SH capping agent 59

6.2. Synthesis of nanocomposites 59

6.3. Summary 59

6.4. Further Work 59

7 References 61

3

Abbreviations

CA Contact angle

DCM Dichloromethane

DLS Dynamic light scattering

FTIR Fourier transform infrared spectroscopy

HCl hydrochloric acid

MW molecular weight

NMR nuclear magnetic resonance

NP Nanoparticle

PEO Monomethyl ether polyethylene glycol

PEOBr Brominated monomethyl ether polyethylene glycol

POL Polystyrene

PDI Polydispersity index

PMMA Poly(methyl methacrylate)

RT room temperature

SEM Scanning Electron Microscope

Tg glass transition temperature

TGA thermogravimetric analysis

TEM Transmission Electron Microscope

THF tetrahydrofuran

UV-vis Ultraviolet visible

XRD X-Ray diffraction

4

Abstract

Nanocomposites are defined as multiphase material where at least one of the phases

has a dimension in the nano-scale. There has been enormous interest in the commercialization

of nanocomposites for a variety of applications including medical, electronic and structural.

The overall purpose of this study was on the formation of silver nanoparticles, due to

the current interest surrounding these metals because of their unique properties which are

different from the corresponding bulk material. Silver nanoparticles were capped with thiol

terminated poly(ethyleneoxide) (PEO-SH) to stabilize the silver nanoparticles in solvent.

Over the ranges considered the size of the silver particles formed did vary significantly with

either PEO-SH concentration or temperature change; however some aggregation in the form

of clusters was witnessed which were not as initially expected.

The synthesised silver nanoparticles, along with the composite films were

characterised using UV-Visible spectrometry, X-ray diffraction studies (XRD), transmission

electron microscopy (TEM), scanning electron microscopy (SEM) and dynamic light

scattering (DLS). The Uv-vis absorption and XRD should confirm the formation of silver

particles whereas the XRD, SEM and TEM should indicate the size in nanometre (nm) range.

Varying concentrations of silver nanoparticles were dispersed in a

polymethylmethacrylate, which were then ground down into a fine powder. Subsequent hot

pressing resulted in thin, well dispersed homogenous nanostuctured films at varying

concentrations targeted at above and below the percolation threshold. The reason for carrying

out such an experiment was to observe the changes in electrical and optical properties, and

the interaction parameters of the surface chemistry of the resulting composite at varying

concentrations. Unfortunately the composite forming process led to silver nanoparticle

aggregation and despite w/w loadings of 40 wt-% silver the volume -% was < 5 vol-% below

the percolation threshold and hence all the composite films were insulating.

5

Chapter 1

1.1. Historic Context to Nanoparticles Nanostructure materials have existed, and used for specific applications throughout

history without knowledge of the small size of the particles involved. An example of the use

of nanoparticles can be traced as far back as Roman times, 4th century AD, when the unique

properties of colloidal silver and gold were used to produce an array of colours in the

Lycurgus cup. The glass of the cup is dichroic; in direct light it turns an opaque greenish-

yellow tone resembling jade, and alternatively when light is shown through the glass it turns

to a translucent ruby colour [1].

In 1857, Michael Faraday was the first documented scientist to experiment with

nanoparticles, in particular gold colloids, where he recognized that their optical properties

were different to those of the corresponding bulk gold metal. His acknowledgment of the

connection between the colour and the size of the particles is considered the birth of

nanoscience and thus initiated the field of nanoscience and nanotechnology that we know

today[1, 2].

Later in 1959, nanoscale materials were again raised conceptually by Richard

Feyman. At this now famous meeting of the American Physical Society, Feyman gave his

presentation “There’s plenty of room at the bottom” where he suggested that materials and

devises could one day be fabricated to atomic specification without disregarding the laws of

physics. His aim was to explore the unknown boundaries of micro-technology in order to

develop a new technology capable of producing, assembling and replicating small

components[3].

Throughout the 1970’s and 1980’s, investigations of reactive species coupled with

new techniques and innovations of instrumentation such as mass spectrometry, vacuum

technology and microscopes gave way for nanotechnology to be brought in into many

different fields, including biology[4], chemistry, physics[5], material science, and

engineering.

Alongside the advancement into the various disciplines in the recent years,

nanoparticles have also been the subject of sporadic research interest for many years,

focusing on understanding the origin of these new properties along with the implementation

of new instruments and techniques to observe, measure, manipulate and synthesise high

6

quality nanoparticles with desired morphologies that would offer even more manifold

perspectives, knowledge and insight into the ever increasing miniaturisation and complexity

of technical developments. This means that they cannot be considered to be just ‘small

molecules’ which they were first termed in the 1980’s[6], but must be studied independently

with such new properties determined and new theories established.

1.2. Effect of particle size on Physical Phenomena The term “nanoparticle”, originally named “small particles” was generally known as

particulate matter consisting of at least one dimension 0f 1-100 nanometers in size (1 x 10-7

-

1 x 10-9

m). This definition places nanoparticles in a similar size domain as that of ultrafine

particles, which include air born particulates along with a subset of colloidal particles.

Figure 1: A typical representative of natural colloids and nanoparticles along with their size

domains[6].

The nanometer scale also incorporates collections of atoms and molecules, which do

not possess properties of either individual constituents or that of the bulk. Many of the atoms

can be located on the surface or one layer removed from the surface as opposed to the inner

core therefore giving rise to new properties due to the interface that is not observed in

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individual atoms or of the bulk. Nanoscale semiconductor materials for example typically

show behaviour which is intermediate between that of a macroscopic solid and that of an

atomic or molecular system.

Within inorganic chemistry, there are three general classifications that inorganic

nanoscale materials can be categorised into: (1) material with delocalized electrons, which

includes metals and conductors, (2) materials with localized electrons (insulators) and (3)

new forms of matter or material with new structures that are usually atomically defined due

to their nanostructure (C60 or carbon nanotubes). Semiconductors can be placed somewhere

in between classifications 1 and 2 depending on their band gap.

The main driver in the field of nanotechnology is not only the small size dimension

that can be created with any type of material (metal, ceramic, polymer, glass or composite),

but also the unique characteristics of such material, whose size might be controlled to only a

few atomic layers in thickness, yet display physical properties which can be very different

from the macro scale properties of the same substance. These special and highly fascinating

attributes can be accredited to the shape, spacing, confined sizes[7] and also the exceptionally

large surface area to volume ratio which has been identified as the key reason for the

properties that nanoparticles exhibit, Figure 2.[8]

Figure 2: A simple model of the surface area of nanoparticles[1].

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As can be seen from the diagram of cubes in figure 2, each time the 1cm3 cube is

divided into smaller cubes, the volume of cubes stay constant whilst the surface area

increases dramatically. The smaller cubes with the larger surface area have a significantly

greater proportion of material exposed and therefore would have to participate in chemical

reactions [7, 9], and as a result making it more reactive than matters of larger size. With this

knowledge in mind, it is safe to assume that the surface of any given nanoparticle is a highly

important component of the material and the physiochemical properties are therefore largely

affected by the size of the surface area. An example to confirm this theory is the occurrence

of gold which has a reputation of being very stable with low reactivity; however gold in the

nanoparticle form has shown to possess some catalytic ability especially the smaller the

nanoparticles get. This is because a nanoparticle of gold with a diameter of 5nm will have

31% of its atoms on the surface area which would be more accessible than a 50 nm particle

which has 3.4% of the atoms exposed. Another example of a metal which also possesses such

characteristics, however on a much more reactive level whilst in the nanoparticle form, is

silver. Silver has over the years provided a very exciting research field due to its interesting

optical[10], electronic[11], magnetic[12] and catalytic properties alongside the ability to

exhibit the highest electrical and thermal conductivity among all the metals. For this reason,

silver nanoparticles are the nanoparticles of choice to use throughout this project[13].

Nanoparticles in the spherical shape are known to be the most thermodynamically

stable form, but various other shapes can be obtained depending on the nature of material and

the process of development. With inorganic colloids, there are a lot of different shapes such

as rod-like, cubic, platelet to name a few however with organic colloids this is not the case

and although not completely impossible, organic colloids are normally in a thermodynamic

metastable state, which tend to evolve towards a spherical form [14].

It has also been proven that outer layers of atoms in nanoparticles have different

composition to the rest of the particles. When silica (Si) was investigated by Parazzo et al, it

became clear that the core of the material had a structure of SiO2, however the surface had a

completely different composition, similar to that of Si(O)(2-x)(OH)(2x). It also became apparent

that approximately, 7% of the Si atoms within the particle were on the surface resulting in the

surface chemistry of the particle to have a significant contribution to the overall properties of

the silica[15]. In addition it has to be noted that the surface of the nanoparticle will in all

cases be the first aspect experience by the environment or by an organism and so a crucial

component.

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Furthermore, another theory to explain the tendency of nanoparticles to behave

differently to the corresponding bulk material is linked with the phenomena that their

electronic energy levels of electron are discrete due to their spatial confinement of the

photogenerated charge carriers. These size dependant quantum effects as they are otherwise

known occur due to the transition from an atom or molecule with defined energy levels to the

dispersed bands of collective aggregation of atoms and finally that of the bulk material. A

clear example of this is the shift of the surface plasmon resonance of gold nanoparticles as

first recognised by Faraday in 1857 where gold appears to have a variety of different colours

depending on the shape, size and dielectric constant of the surrounding medium[16]. As

discussed earlier (section 1.1.) an important effect responsible for the change in fundamental

properties of a material on size reduction is the density of electronic state. Metallic band

structure formation requires a minimum number of electronic levels with almost identical

energy to allow electrons within the particle to move with thermal initiation, Figure 4.

Electron delocalisation in the conduction band is feasible considering the dimension of the

metal particle is a multiple of their As with the case of smaller molecules where the electrons

are localized between atomic nuclei; resulting in the disappearance of the band structure and

discrete energy levels which then become dominant. DeBroglie wavelength λ=h/mv where λ=

electronic wavelength, h=Planck’s constant, m = mass of electron and v= speed of electron.

Metal Semiconductor

En

erg

y

Density of states

NP Bulk Bulk Atom Atom NP

(a) (b)

Ef

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Figure 3: Diagram demonstrating the energy levels for the bulk, a nanoparticle and an atom

of (a) metal and (b) semiconductor material. The black areas show the occupied state and the

white the unoccupied states. The Fermi level (Ef) is also illustrated.

1.3. Nanoparticle Applications Nanotechnology and nanoscience are multidisciplinary fields linking chemistry which

deals with atoms, molecules and condensed matter, to the science of physics which deals with

solids of essentially an infinite array of bound atoms or molecules, and the prospects don’t

just stop there as recently biomedical nanotechnology combining inorganic and biological

materials is pushing to the forefront of this rapidly advancing field of science.

As already mentioned, every known substance and others yet to be discovered will

yield a new set of properties which are highly dependent on size. Optical properties, magnetic

properties, melting points, boiling points and crystal morphologies can all be influenced by

this size dependant property, and it is nanomaterials which serve as the bridge between the

molecular and condensed phases. These thousands of substances which are solids under

normal temperatures and pressures can be subdivided into metals, ceramic, semiconductors,

composites, and polymers. These can be further subdivided into biomaterials, catalytic

materials, coatings, glasses, and magnetic and electronic materials. All of these solid

substances, with their widely variable properties, take on another subset of new properties

when produced in nanoparticle form. Not only is it possible to miniaturize such material to

capture these properties, but it is now possible to manipulate nanoparticles using the top

down method (section 1.3.1) to create larger structures with superior properties. The

continuous increasing demand for materials that can offer improved properties over

traditional material have led to the development of advanced material and over the past

decade, the field of nanostructure science and nanotechnology has grown to be a broad and

rapidly growing area of worldwide research, receiving considerable attention and

representing real widespread possibilities for interesting, fundamental science and useful

technologies. The possibilities are endless however the synthesis of the nanomaterials is the

first prerequisite[1, 4].

As research into nanotech has intensified and new uses discovered, nanotechnology

has expanded to become a huge market. In 2005, $30 billion worth of products were

produced with use of nanotechnology; that will continue to grow with projections from $85

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billion to $3 trillion by 2015[17]. Nanotechnology is projected to require at least 6 million

workers to support the worldwide expansion by the end of the decade[18]. Table 1 highlights

some of the industry sectors with market data for current/future nanotechnology applications.

As it shows, nanoparticles and nanofilms are very important for the growth and development

of some of the key industries in today’s economy.

Market Volume Industry Sectors (market size 2007, $ mln.),

(market size 2007), (estimated market size 2015, $ mln.)

(market size 2015)

Aerospace and 1. Nanocomposites 2. Electronics 3. Nanofilms 4. Wires and

defence (28), (420) (585), (182) (165), (1880) fuel additives

(3235), (3768) 5. Adaptable Materials (28), (420) (45), (376)

Information 1. Carbon 2. Nanowires 3. Nano memory 4. Chipsets

technology nanotubes (45), (800) (30), (900) (250), (21000) (150), (12000)

(585), (41420) 5. Nano-electro-mechanical devices 6. Spintronics 7. Quantum dots

(10), (520) (50), (6000) (50), (650)

Energy generation 1. Photoelectric 2. Fuel 3. Thermoelectric 4. Aerogels

(90), (3615) films cells and batteries materials (25), (760)

(30), (760) (30), (1650) (5), (445)

Medicine and 1. Biosensors 2. Nanofilms on 3. Precise drug

biology and surfaces and delivery

imaging implants (75), (2650)

(20), (1220) (50), (1800)

Construction 1.Sensors and 2. Nanocomposites 3. Nanofilms 4. Cement additives

(66), (1672) materials (1), (212) (5), (375) (50), (750) (10), (335)

Automobile 1. Nano films 2.Composite 3. Catalysis 4. Fuel elements

(404), (7134) (181), (2451) fillers (150), (2106) (69), (1740) (25), (450)

Textile 1. Films 2. Adaptable Materials 3. Nanotubes and

industry (120), (1850) and sensors nanostructures

(122), (2170) (1), (125) (2), (195)

Ecology 1. Membranes 2. Chemical and bio 3.Nanoparticles 4. Nanofilms

(86), (3885) (41), (975) sensors (1), (125) (29), (2000) (11), (420)

Food 1. Nanosensors 2. Encapsulation 3. Nanofilms 4. Nanocomposites

processing (2), (360) (3), (320) (40), (495) (180), (1580)

(265), (3210)

Retail 1. Nanocomposites 2. Nanofilms 3. Nanoparticles

(188), (6225) (67), (1248) (70), (1500) (51), (3477)

Copyright 1.Nanofilms 2. Nanoparticles

(30), (2650) (10), (1000) (20), (1650)

Marine 1. Electronics and 2. Nanofilms 3. Nanocomposites

4. Catalysis, fuel

additives

(357), (4295) sensors (25), (970) (180), (1850) (100), (1100) lubricants (52), (375)

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Table 1: Nanoparticle applications across industry sectors with estimates of current/future

market value in millions USD [19][20].

As can be seen from Table 2, nanoparticles have a range of properties that allow for

different applications, such as improvements to cosmetics, using titanium dioxide

nanoparticles in sunscreen lotion [21], and application of polyethylene glycol (PEG) capped

silver NPs as contrast agents for the detection of cancer cells [22]. Another good example is

the use of titanium oxide self-cleaning coatings which have significant environmental impact

[23] as illustrated by Figure 4.

Figure 4. Photocatalytic(a) and self-cleaning(b) effect of TiO2 coatings on surfaces

Nanotechnology has also enabled researchers to build a better understanding of the

relationship between macroscopic properties and molecular structure in biological materials

of animals and plants.

Within agriculture and the food industry, nanotechnology has revolutionized the many

areas such as processing, pathogen detection, quality control and the development of new

technology to enable plants to absorb nutrients [24]. More recent studies into the

nanoencapsulation of active compounds such as vitamins, minerals, flavours, colourings and

antimicrobial have begun which allows the ability to mask odours or tastes.

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Table 2: Showing the wide range of properties that nanoparticles exhibit with some examples

of the various applications associated with these size dependant properties.

Property Application

Optical: Anti-reflection coatings.

Tailored refractive index of surfaces.

Light based sensors for cancer diagnosis .

Magnetic: Increased density storage media.

Nanomagnetic particles to create improved detail and contrast in MRI images.

Thermal: Enhance heat transfer from solar collectors to storage tanks.

Improve efficiency of coolants in transformers .

Mechanical: Improved wear resistance.

New anti-corrosion properties.

New structural materials, composites, stronger and lighter.

Electronic: High performance and smaller components, e,g, capacitors for small consumer devices such as

mobile phones.

Displays that are cheaper, larger, brighter, and more efficient.

High conductivity materials.

Energy: High energy density and more durable batteries.

Hydrogen storage applications using metal nanoclusters.

Electrocatalysts for high efficiency fuel cells.

Renewable energy, ultra high performance solar cells.

Catalysts for combustion engines to improve efficiency, hence economy.

Biomedical: Antibacterial silver coatings on wound dressings.

Sensors for disease detection (quantum dots).

Programmed release drug delivery systems.

“Interactive” food and beverages that change colour, flavour or nutrients depending on a

diner’s taste or health.

Environmental: Clean up of soil contamination and pollution, e.g. oil.

Biodegradable polymers.

Aids for germination.

Treatment of industrial emissions.

More efficient and effective water filtration.

Surfaces: Highly size dependant dissolution rates of materials.

Catalytic activity.

Self cleaning surface coatings.

Personal care: Effective clear sunscreen.

Use within cosmetics.

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1.2.1 Nanoparticle applications in biology and medicine Nanotechnology has over the years opened up new ways to fight and prevent disease

using atomic scale tailoring of materials. Due to the similarity in size between

nanomaterial’s and most biological molecules and structures, nanoparticles can be highly

beneficial to use both in vivo and vitro biomedical research and applications [25].

To prevent epidemics and loss of lives, new accurate and sensitive methods to identify

trace amounts of infectious pathogens have been in constant demand. Early detection of

pathogens, infectious viruses and bacteria to prevent and treat is crucial.

Bio-imaging technology with the use of nanoparticles available today has led to

significant advances in diagnosis and therapy. Fluorescent nanoparticles mainly in the form

of metal (Au, Ag), semiconductor nanocrytals (e.g. quantum dots and magnetic quantum

dots) and metal oxides (Fe3O4), are seen to be the new class of highly sensitive and

photostable fluorescent tags for labelling biological specimens. Fluorescent tags are

revolutionary tools in assisting biologists to perform real time, sensitive bioimaging and

sensing studies [26]. Due to the fact that nanoparticles exist in a similar sized domain as cell

parts and proteins which is also where most disease processes commence, it makes them

highly for this application opening the opportunity to observe the cellular system closely

without causing too much interference[27].

Silver Np’s make good tags for bioimaging, because they efficiently interact with

light via the excitation of Plasmon resonances that are the collective oscillations of the free

electron density, Figuer 5 [28].

Figure 5: A diagram of plasmon oscillation for a sphere, showing the displacement of the

conduction electron charge cloud relative to the nuclei. [5]

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Polystyrene nanoparticles loaded with Au(III) in particular are currently used as

fluorescent reporters.

Aside from this, silver has also long been recognized for its broad spectrum anti-

microbial properties and has been used as an antiseptic agent for around a thousand years and

especially today within disinfectant devices and home appliances to water treatment with the

acknowledgement that some bacteria are resistant to antibiotics and therefore it is necessary

to use silver nanoparticles for controlled surfaces dissemination as a replacement [29]. Aside

from antibacterial abilities, research has revealed that silver ions can destroy viruses, fungi,

protozoa and even cancer cells [30]. The antibacterial abilities are highly dependent on

surface properties in addition to particle size and shape.

An interesting advance in technology has been to trap silver ions within zeolites, and

to apply these compounds to various materials in order to utilise as long lasting antimicrobial

treatments.[31, 32] It has however been proven that only partial, surface oxidized silver

nanoparticles exhibit antibacterial properties, whereas zero valent nanoparticles do not. This

is due to the levels of chemiabsorbed Ag+ ions which forms on the surface of the particle

during oxidation and reduction are comparable with the observed changes in the surface

Plasmon absorption of antibacterial activities. Also it has been known that the size of the

silver nanoparticles plays a major role in its antibacterial activities as smaller sized particles

have higher activities based on the equivalent silver mass content [33].

In the revolutionary study of nanoparticles with viruses carried out by Elechiguerra et

al, it was shown that Silver nanoparticles undergo a size dependant interaction with HIV-1,

and attach to the virus which inhibits the transmission of the virus. The nanoparticles attached

were in the size region of 1-10 nm with three varying capping agents. Following incubation

with the HIV-1 virus for three hours at 37°C with each of the different capping agents, the

silver particles managed to entirely eradicate the virus each time[34]. There are many studies

that have been carried out that suggest that nanoparticles impede on the nature of the bacterial

membrane by means of producing reactive oxygen species[35]. Although further research is

required into this study, scientists are optimistic that silver nanoparticles are the new

innovative way of killing virtually all viruses. Another novel method recently developed

which utilises the antibacterial properties of silver nanotechnology is the production of both

natural and artificial silver containing fibres that prevent bacterial and fungal growth with

lifetime lasting effects.

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Although a lot of research is currently been carried out on the antibacterial benefits of

silver, some research has also been required due to the increase of bacterial resistance to

antibiotics caused by their overuse [36].

1.2.2. Uses of nanoparticles in catalysis Due to their unique properties, such as surface area, nanoparticles have found to be

catalytically active. One of the approaches has been to produce the nano-sized particles or

films of the elements that have already been known to have catalytic activity, such as

transition metals (as Fig 6 illustrates, as palladium catalysis is a well-known area in the

organic chemistry). However, it was shown that due to increased surface area even elements

which are normally unreactive in bulk show catalytic activity when applied on the nano scale,

as was a case with gold nanoparticles in organic synthesis.

Figure 6. Use of Pd-Fe2O3 complex catalysts for treating of absorbable organically bound

halogen waste [37]

Recent research has proven that silver nanomaterials possess good electro-catalytic

activity. In commercial application of fuel cells, it is of high importance that the electro-

cataylitic materials are inexpensive, abundant, and efficient[38]. A series of other studies

have also shown methods to produce cheap silver which can in time replace other noble

metals (Au, Ru, Pt, and Pd) to become the best catalyst in fuel cells.[39]

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1.2.3. Electronics Nanotechnology allows further miniaturisation of the current electronic circuits.

Several attempts have been made to advance the lithography process to the sub-10nm scale,

and at that scale it is the question of single-electron device fabrication that is most important

for development of nanoelectronics. The operating temperature of such devices is directly

determined by the geometrical size of the electron localization. A formation of single-

electron transistor has been reported by Sato et al [40] by deposition of a chain of gold

nanoparticles capped with dithiol molecules on the SiO2 surface which bridged the gap

between source and drain metal electrodes. Such work demonstrates the importance of

capped metal NPs for further development of nanoscale electronics.

Figure 7. (a) SEM-observation of a three-dot gold colloidal chain incorporated into the

system of source, drain and gate electrodes; (b) Schematic of electrode pattern defined by

beam lithography (Sato et al, [40])

A current interest in research on nanostructured materials is the production of

nanocomposite conducting colloid particles consisting of inorganic component and

conducting polymers with differing electrical properties from their individual nanoparticles or

macroscopic equivalent. This s so that they can be effectively applied in the fields of sensors,

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optics [41], electronics and [42] catalysis[43]. Nanocomposites in the form of the

functionalised thin films have applications in printed electronics due to sparing use of

materials as well as further miniaturisation of devices such as visual display units and light-

emitted diods. Conducting polymer-based nanocomposites embedded with metallic

nanoparticles provides exciting systems to investigate with the possibility of designing device

functionality, making flexible circuit elements possible and potentially revolutionising the

industrial processes. Such conduscting nanocomposites would be investigated during this

project.

1.3. Nanoparticle Synthesis Due to the high level of interest in NPs from many different fields of research there

have been numerous different reported synthesis methods by which to produce NPs in both

the biological and chemical disciplines. The two main chemical synthesis approaches to

produce NPs are; the top down approach, and the bottom up approach.[44]

1.3.1 Top down method Top down This approach to the formation of nanoparticles entails breaking, or cutting

down larger pieces of a material to the size of a NP by bombardment with high energy

electrons, etching or milling. Some techniques that employ the top down approach include

wet/dry grinding, reactive grinding, etc. The method offers the ability to create uniform sized

particles, however it has proven to hold some limitations, in particular agglomeration as well

as contamination by the material abraded from the grinding body. The method produces

limited proportion of nanoparticles with diameter <50mn. However, this method is important

industrially as it allows production of large quantity of material in an inexpensive fashion.

Applications for this technique have been found for computer chips, metal oxanes and

precision engineered surfaces [12].

1.3.2 Bottom-up method The other, more popular bottom up approach is where a NP is ‘grown’ from simpler

molecules. This can be done either in the gas phase via methods such as chemical vapour

deposition (CVD) or laser ablation deposition (LAD), or sputtering techniques, or

alternatively in the liquid phase via forced hydrolysis, hydrothermal synthesis, sol-gel process

. Gas-phase methods are continuous processes that produce crystalline nanoparticles with

uncovered surfaces. Due to reaction taking place at elevated temperatures (above 500oC),

19

some nanoparticles would form hard clusters. As reaction in the gas phase is generally

controlled by thermodynamics, the yield of meta-stable NPs in such environment is limited.

The main advantage of using liquid-phase synthesis is that a large quantity of NPs can

be formed, with a greater degree of control over particle size [6]. Almost uniform sized NPs

with small degree of agglomeration could be obtained via bottom-up liquid phase methods if

there is precise control over nucleation and nucleus growth and efficient suppression of the

agglomeration process. Synthesis of NPs in solution could be understood via the LaMer and

Dinegar model which describes nucleation (Figure 8).

Figure 8. LaMer and Dinegar model to describe nucleation and nucleus growth [6].

According to the model, nucleation is an endothermic process. A large amount of

energy is required to break bonds of the starting materials, remove solvation shells and

overcome surface tension of the solvent. On the other hand, formation of infinite solid is and

exothermic process due to release of enthalpy, such as lattice binding energy. Hence

formation of solid is thermodynamically favoured over formation of the nanoparticles;

formation of agglomerate is also energetically favoured as it lowers the surface area and

reduces the number of unoccupied coordination sites.

There are many reported methods to synthesise NPs using the bottom up technique.

Applications of NPs synthesised using the bottom up technique include quantum dots,

nanofilms and fullerenes [13]. This project is focused upon production and investigation of

polymer stabilised silver nanoparticles by bottom up synthesis.

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1.3.3 Colloidal Stabilisation Particles dispersed in a liquid medium, also known as colloids are thermodynamically

unstable. To avoid further growth or irreversible aggregation at various points of handling,

whether it be from when they are first synthesised, storaged, or functionalized within the vast

range of application domains; (either mixed in with other colloids, under shearing, or with

biological fluids which exhibit significant ionic strength) efficient colloidal stability should

be put in place. The two main types of stabilizations which are used to prevent agglomeration

and provide a metastable state for long term periods for nanoparticles in solution are steric

stabilization which involves the self assembly of long-chain organic molecules on the surface

of the metal nanoparticles (e.g. oleyl amine or oleic acid), or charge stabilisation otherwise

known as electrostatic stabilization by adsorbtion of ions such as H+,

-OH, NO3

-, RCOO

-, etc.

[45].

(a) (b)

Figure 9: Two general modes of colloidal stabilization (a) sterically and (b) electrostatically

functionalized with organic molecules.

Steric stabilization involves the absorption of large polymeric molecules which attach

onto the surface (common colloids include P, N, or S donor, and alkyl thiols) and provide

steric repulsion which prevent aggregation.

In the electrostatic mode there is a bilayer of anions (often halides) and a second layer

of cations (eg tetra alkylammonium). The mutual repulsion of these double layers provides

stability in electrostatic stabilisation.

One method of providing this repulsion is coulombic repulsion which requires the net

charge of the solution to be equal, but opposite charge to that of the surface of the particle.

21

1.4 Nanocomposites Nanocomposites are multiphase solid materials usually with two or more physically

distinct components and one or more dimensions less than 100 nm with a recognizable

interface between them, which result in a material with superior properties compared to the

individual constituents. For structural applications, the definition can be restricted to include

those materials that consist of a reinforcing phase such as fibres or particles supported by a

binder or matrix phase. Such materials have a large variety of applications as they can be

designed to combine different properties according to specific requirements. Nanocomposites

represent a new class of materials as alternatives to conventional filled polymers. This newly

formed class of materials which contain nano-sized inorganic filler (on at least one

dimension) are dispersed in the polymer matrix. This allows for significant improvement in

the polymer characteristics due to extremely high surface area to volume ratio and a high

fraction of surface atoms along with different bonding which dramatically changes their

properties when compared with their bulk size equivalents. As a result, composites have new

and improved properties such as higher tensile strength, heat resistance and chemical

resistance [46], as well as glass transition temperature, thermal degradation and viscoelastic

properties[6]. Some composites have even shown to be a 1000 times tougher than their

corresponding bulk component materials[47].

Over the last ten years, several different types of polymer composites have become

the dominant class of multi component polymer systems and a great proportion of worldwide

research, development and commercialization has been devoted to these classes of materials.

Composites are already widely used in such diverse areas as transportation, construction,

electronics and consumer products. They offer unusual combination of stiffness, strength, and

weight that is difficult to attain separately from the individual components[48].

Applications for nanocomposites include:

Thin- film capacitors for computer chips

Food packaging

Gas and oxygen barriers

Car body parts

Bionanotechnology and nanomedicine for use in drug delivery

Sport equipment

Electro chromic display devices and phosphors for high definition televisions

22

Due to their nanometer size dispersion, nanocomposites exhibit markedly improved

properties when compared with the pure polymers or conventional composites. It has been

found that nanoparticulate solids added to polymers as ‘fillers’ cause remarkable changes in

properties, and such changes will be investigated for PMMA polymers within this project.

1.4.1 Nanocomposite synthesis methods Nanocomposites can be synthesised by various methods, and the choice of synthesis

method depends on both the nanoparticle, and the polymer being used. All of these could be

separated into three different approaches. One approach is the in situ preparation of the NPs

in the polymer matrix by reduction of metal salts depositited in the matrix or by heating

metals on polymer surface. Another approach is the polymerisation of the matrix around the

nanoparticles. Third approach is the blending of pre-made NPs with pre-made polymer and it

allows for full synthetic control over both the matrix and the nanoparticles.

Introduction of the stabilized NPs into the polymer matrix is sensitive to the relative

molecular weight of both the capped NPs and polymer matrix. It also depends upon the

solubility of the polymer in water. A range of work has been reported on the introduction of

water-soluble Ag NPs into the functionalised water-soluble matrices such as polyvinyl

alcohol and acrylic acid [49, 50, 51, 52]. However, introduction of silver nanoparticles into

the hydrophobic polymers is much less researched. This is because a favourable interaction of

functional groups with silver nanoparticles is missing, and hence it is necessary to employ

melt technique for formation of nanocomposite with capped NPs. This holds true for ester-

functionalised polymers such as PMMA.

This project would employ melt compounding techniques [27] to investigate

nanocomposites containing silver NPs in PMMA/polystyrene matrix. Nanocomposited would

be formed by mixing of pre-formed polymer/ pre-formed capped NPs and heating those

above glass transition temperature of the polymer. [53]

23

1.5 Aims and Objectives

This project aims to develop new materials in form of nanocomposites containing capped

silver nanoparticles dispersed in both polymethylmethacrylate and polystyrene matrices .

1. Synthesis of PEO-based capping agent. Capping agent to be characterised by a range

of methods including 1H NMR and Infra-red spectroscopy.

2. Synthesise and characterise capped silver nanoparticles.

3. Introduce the capped silver nanoparticles into the polymer matrix and to characterise

resulting nanocomposites using a range of methods.

24

Chapter 2 - Materials and characterisation

Methods

2.1 Materials All the material used in this study was supplied from Sigma-Aldrich.

UV absorption spectra were collected on a Varian Cary 5000 UV-vis absorption

spectrometer. DLS measurements were collected on a Malvern Zetasizer Nano ZS.

Flourescence emission spectra were collected on a Varian Cary Eclipse spectrometer. NMR

spectra were recorded on a Brucker 300 MHz spectrometer.

2.2 Infra-red spectroscopy (IR) IR spectroscopy is one of the most commonly used techniques in chemistry. It allows

the chemist to identify functional groups present in the solid, liquid or gaseous sample. The

sample is positioned in the path of the IR radiation beam and absorbtion measurement of

different IR frequencies is taken. Infra-red region includes wavelengths from 13000-10 cm-1

and is bound by the red region of the visible region at high frequencies and the microwave

region of the spectra at low frequencies. IR radiation can itself be divided into three smaller

regions, Near IR (13000-4000 cm-1

), Mid IR (4000-400 cm-1

), and Far IR (200-10 cm-1

).

Absorbance peaks from IR spectroscopy are sometimes presented in wavelengths λ.

The relationship between wavenumbers (ν, cm-1) and wavelengths (λ , μm) can be

summarised as: ν (cm-1)=1/ λ (μm) x 10

4 .

The origin of IR spectra lies in the molecular vibrations. At temperatures above

absolute zero, all atoms in the molecule are vibrating relative to each other. When the

frequency of this vibration corresponds to the IR radiation directed on the molecule, the

molecule absorbs radiation. Two main types of vibration are stretching and bending, and

when IR radiation is absorbed, the energy is converted into those types of vibrations.

However, vibrational motion is usually accompanied by rotation, thus leading to appearance

of distinct peaks, instead of sharp lines.

Each atom in the polyatomic molecule has three degrees of freedom corresponding to

the motions along the three Cartesian coordinates (x, y, z). Polyatomic molecule of n atoms

has 3n-6 degrees of freedom as 3 d.o.f. correspond to rotation and 3 for translation. Total

25

number of vibrations observed on the spectra is different from the fundamental estimate.

This is because not all modes of vibration are IR active, as well as because single frequency

can cause more than one vibration to occur. Overtones and interactions between fundamental

vibrations also influence number of peaks observed.

In Fourier Transform spectrometers, all frequencies in range are examined

simultaneously.

2.3. UV-Visible Spectroscopy The method of the UV-Vis spectroscopy is in many ways similar to the IR

spectroscopy, the main difference being the use of photons in the UV- visible region of the

spectra (200-800 nm). The energy of the uv-vis radiation is sufficient to excite the outer

electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied

molecular orbital (LUMO), resulting with a molecule in the excited state.

When sample molecules are exposed to light having an energy that matches a

possible electronic transition within the molecule, some of the light energy will be absorbed

as the electron is promoted to a higher energy orbital. An optical spectrometer records the

wavelengths at which absorption occurs, together with the degree of absorption at each

wavelength. The resulting spectrum is presented as a graph of absorbance (A) versus

wavelength. Because the absorbance of a sample will be proportional to the number of

absorbing molecules in the spectrometer light beam (e.g. their molar concentration in the

sample tube), it is necessary to correct the absorbance value for this and other operational

factors if the spectra of different compounds are to be compared in a meaningful way.

2.4. Nuclear Magnetic Resonance (NMR) NMR is a valuable tool for characterisation of atom environments as long as the

nuclei has spin angular momentum I=1/2 (e.g. 1H,

13C,

15N,

19F,

31P). The basis of NMR is

that the magnetic dipoles of nuclei generated when the spins of protons and neutrons are not

paired interacts with external magnetic field in 2I+1 ways, either reinforcing or opposing the

direction of the applied field. The orientation of the dipole with the external field is more

energetically favourable (spin +1/2) and orientation of the dipole against the field is less so

(spin -1/2). Hence when the external field is applied the population of nuclei with spin +1/2

would be higher than population with spin -1/2 level, and if sufficient quanta of energy is

supplied the nuclei can undergo transition from spin +1/2 to spin -1/2 level. NMR is about

interpreting such transitions to obtain information about environment the nuclei is in. The

26

sensitivity of the technique depends on the strength of the external magnetic field, as well as

the temperature of the sample.

2.5. Matrix-Assisted Laser Desorption/Ionisation (MALDI) Mass spectrometry is a method of measuring the molecular mass of the sample. It can

also be used to confirm the structure of the organic molecule by analysing the fragments

produced. MALDI is a further development of the LD (laser desorption) spectroscopy

developed in the 60s. In a laser desorption spectroscopy low mass organic sample is

irradiated with a high-intensity laser beam to produce ions that can be analysed. MALDI

expands this method by introducing a matrix of solid solvent (a small organic molecule) in

which the sample is irradiated. This technique allowed tocircumvent the mass cut-off of the

LD mass spectroscopy at 5-10 kDa.

2.6. Thermogravimetric Analysis (TGA) Thermogravimetric analysis (TGA) is a technique in which the mass of a specimen is

subjected to a controlled temperature program in a controlled atmosphere to determine the

moisture content, structure-property relationships and thermal stability of composite material

by monitoring the in-situ weight change on heating under an air or in an inert atmosphere

such as nitrogen or argon. The analysis relies on high degree of precision in three

measurements; weight, temperature and temperature change. Isothermic mode involves

holding a sample at a constant temperature and measuring the weight loss with time. Non-

isothermal mode involves measuring the weight loss as a function of temperature.

The TGA instrument consists of a platinum or ceramic sample pan which is

supported by a sensitive balance. The pan is located in a furnace and is heated or cooled

during the experiment.

2.7 Dynamic Light Scattering (DLS) Characterisation and manipulation of individual nanostructures requires not only

extreme sensitivity and accuracy, but also atomic level resolution. The development of novel

instruments allowing for characterisation of nanomaterials has been one of the biggest

challenges in nanotechnology and this section will expand on the numerous measurement

techniques that are commonly used to identify nanoparticles and measure any interactions

and reactions that occur.

27

The surface analysis of a material is of great importance, particularly in studies of

nanocomposites which exhibit varying properties dependant on size and structural

composition. Most methods of surface analysis involve some degree of irradiation with an

energy source of photons, electrons, or ions. The irradiation must contain sufficient energy to

penetrate the sample and induce a transition that will result in emission from the surface of

the energy beam which is then analysed.

Surface analysis does exhibit a few setbacks especially as with regards to insulators

whereby an electrical charge can develop from emitted electrons or ions from the surface to

produce spectral distortion. [54]

Dymanic light scattering is a technique which is commonly used for fast and accurate

measurement of nanoparticle size. It is also heavily used in protein chemistry for

measurement of hydrodynamic sizes, polydispersion and aggregation effects of protein

samples. DLS is also known as quasi elastic light scattering (Quels), or photon correlation

spectroscopy (PCS) in some sources.

DLS measures the light scattered from a sample of dissolved macromolecules or

suspended particles. Because molecules in a sample would be moving due to Brownian

motion, the scattering intensity would fluctuate. DLS measures the light scattering in very

short time intervals and correlates the data; as larger molecules move slower than the smaller

ones, a defined correlation function results. On fitting the data produces diffusion

information and hydrodynamic radius of particles and molecules.

DLS does not require any sample information; only solvent viscosity and refractive

index of the solvent must be known.

2.8 X-Ray Diffraction (XRD) and X-ray Photoelectron

Spectroscopy (XPS) There are also several analysis techniques which use X-rays as the energy source

with XRD and XPS being two popular examples. Each of these two techniques measure

different physical characteristics of samples and can thus be used in conjunction with other

NP characterisation techniques.

XRD is a straight forward method of estimating particle size. It operates by

measuring the angle and intensity of a beam of X-rays that has been diffracted through a

crystalline sample. When the radiation of X-rays is directed onto a sample, the X-rays are

scattered (“diffracted”) by the electrons present in the material. The diffraction which occurs

28

as a result of the X-ray radiation causes each electron in a periodic array to scatter

coherently, producing a correspondent interference at specific angles. Throughout the course

of a measurement, the angles at which incidents occur are detected and scanned.

If the material is crystalline, therefore containing atoms arranged in a regular ordered

structure, then this scattering results in maxima and minima in the diffracted intensity. The

signal maxima follows Bragg’s law, n = 2dsin where n is an integer, is the X-ray

wavelength, d is the distance between crystal lattice planes and is the diffraction angle.

The sample preparation for XRD analysis is relatively easy and analysis can be

performed within a few hours. The potential of XRD however is limited since all of the data

produced is averaged over the whole sample.

XPS however can provide information about the bonding at the surface of a sample.

This is achieved by measuring the kinetic energy of electrons released from a nucleus once it

has been radiated by a monoenergetic X-ray beam; this kinetic energy is equal to the binding

energy between an electron and it’s nucleus. Each element has a specific binding energy

meaning that elemental analysis of a sample can be determined. However, XPS is limited to

providing data regarding surface chemistry because the X-rays cannot penetrate into the core

of a sample. An example of an XPS spectrum of nanopartiulate silver is shown in Figure 10.

Figure 10. An example of XPS spectrum from Gao et al [13].

2.9 Transmission Electron Microscopy (TEM) The use of TEM is arguably one of the most popular and powerful technique in

nanochemistry. The principles of TEM are that an electron beam with a well-defined de

Broglie wavelength is directed at a small sample by a magnetic field which focuses the

29

beam. The electrons go through the sample are detected. Electrons have a shorter wavelength

than photons, and can therefore provide a higher resolution that the conventional light

microscope. TEM also has a higher resolution than SEM, because the resolution of SEM is

limited to the minimum beam width of the SEM. TEM provides the cornerstone of the

analysis of the nanostructure of the composite materials produced in this work because it has

sufficient resolution to directly image the nanoparticles. TEM can provide two separate kinds

of information about a specimen – a magnified image and a diffraction pattern. Electrons that

are transmitted through the bulk material generate a form of contrast that enables observation

of the internal structure. Electrons pass more easily through thinner, lower density, or lower

atomic mass material. This means that because fewer electrons pass through the thicker,

denser or higher atomic mass regions of the sample, these regions appear darker. Electrons

that are diffracted produce a diffraction pattern which gives information on the crystallinity

of the sample and enables identification of the specimen by comparison to corresponding d

spacing’s of known phases. In this work, TEM has enabled visualisation of the trapped

nanoparticles throughout the different impregnated matrices. The dispersion and

homogeneity of the nanocomposites has been assessed and quantitative particle size analysis

achieved.18

Information that can be collected from TEM images include particle size and

identifying whether nanoparticles are binding to larger surfaces.

The great advantage of TEM is its capability to observe, by adjusting the electron

lenses, both electron microscope images (information in real space) and diffraction patterns

(information in reciprocal space) for the same region. This allows for even the inner

diameters of a cluster of material to be determined. A diffraction pattern for an area as small

as 100nm in diameter can be produced.

2.10 Scanning Electron Microscopy (SEM) The essential features of SEM are that the image is formed point by point, by

scanning the surface of a sample to build a very detailed three dimensional image of the

specimen. Three essential signals from the specimen are back scattered electrons, secondary

electrons, and X-rays. Back-scattered electrons are primary beam electrons which have been

elastically scattered by nuclei in the sample and escape from the surface. Using detectors the

topographic contrast can be obtained, but at low resolution. Secondary electrons produced by

the primary beam produce high resolution topography images. Other sources of secondary

electrons may degrade the image.

30

Usually SEM is used together with other surface analysis techniques to help the

interpretation of data received.

31

Chapter 3 - Synthesis and Characterisation of PEG capping

agent

3.1 Introduction Capping agents are molecules which have the ability to bind to and surround the

interface of other materials to form a protective shell. For the capping agent to sustain this

protective shell, the liquid/ solid interface of the colloidal nanocrystals must be soluble and

not aggregate.

There are several types of capping agents available such as polymers, organic and

inorganic molecules which are commonly used in the production of nanoparticles for various

reasons. [14]. The fundamental reason behind the use of capping agents is to prevent

nanoparticles from aggregating together and forming colloids or other material larger than

100nm in size[15]. Nanoparticles naturally tend to aggregate because the acting repulsive

forces are not strong enough to counteract the van der Waals attraction [14]. Capping agents

however achieve this by ensuring that no two nanoparticulates can get close enough together

in order to aggregate. However for this to work well, each Nanoparticle has to be completely

surrounded by the capping agent to prevent any aggregation.

Capping agents can also directly affect the properties of nanoparticles. For example it

is possible to form nanoparticles that act like micelles if the capping agent is hydrophobic

such as polystyrene [14]. Also interactions are not just expected to take place on the surface

of the core of the molecule but will also potentially occur with the capping agent. Therefore,

when designing a nanoparticle the choice of capping agent needs to be carefully considered to

meet the requirements of the researcher.

Figure 11: A diagrammatic representation of two NPs unable to aggregate due to the

protectionprovided by the capping agent.

32

Other than the requirement for nanoparticles to be water soluble, they are also required to be

stable so that they do not aggregate on drying. A solution of thiolated PEG carrying a

methoxyfunctionality on one end (HS-(PEG)-OCCH3. The PEG acts as a NP stabilizer and

reduces the possibility of ligand exchange among the NP dimmers. [46]

PEO-SH was the polymer of choice as the steric stabilizer of the silver colloidal

particles. The reason for this is that is that polyethylene glycol polymers are soluble in water,

as well as organic solvents such as Dicholoromethane because they possess a terminal

thiolate group that strongly chemisorbs to the Ag surface.

Figure 12: Structure of thiolated polyethylene glycol (PEGSH)

3.2 Synthesis and Characterisation of PEO-SH capping agent The PEO-SH capping agent was produced in a two step process. As mentioned

previously, this step is crucial as without the presence of capping agent the particles would

aggregate to form bulk silver metals. The capping agent also assists in stabilising the

particles; either by means of electrostatic or steric surface bound capping agent (section

1.4.3). After some investigation into the best capping agents for the formation of silver

nanoparticles, it was decided to go for steric stabilisation. The reason behind choosing

polyethylene glycol polymers as the type of steric stabiliser was its versatility of solubility

both in water and organic solvents such as DCM. Also the use of this polymer makes it

feasble to convert the alcohol end group polyethylene glycol methyl ether, initially into a

bromine group, followed by thiol group. A single end functionalised polymer opens up the

33

opportunity to graft the polymer chain onto the surface of the silver, and as result creating a

robust stable silver nanoparticle.

Scheme 1. The overall synthesis of Ag-PEGSH nanoparticles

3.3 Bromination of PEO The first step in the synthesis is known as an Appel reaction and occurs when the

hydroxyl group in the mPEG is converted into primary alkyl bromides and is an overall

reaction which resembles a nucleophilic susbstitution. In this reaction the formation of

triphenylphosphine oxide is the driving force of the reaction. The reaction was carried out

via reflux for 24 hours before removing impurities.

Scheme 2. Bromination of polyethylene glycol methyl ether.

Monomethyl ether polyethylene glycol 750 Mw (50 g, 63 mmol) was dissolved in

dichloromethane (100 ml). The solution was treated with carbon tetrabromide (25.03 g, 75

mmol) and the mixture was stirred until the carbon tetrabromide was dissolved. The solution

was then cooled to 0 ºC, and then degassed under high vacuum. The system was placed under

nitrogen, and triphenyl phosphate (19.74, 75mmol) was added slowly. The mixture was left

to stir for 24 hrs.

Water (100 ml) along with the solution were decanted into a 500ml separating funnel

and shaken. The brominated polyethylene glycol (PEO-Br), was extracted with DCM (3x50

ml). The DCM layer containing the PEO-Br was then washed with water (75 ml). The DCM

34

layer was evaporated off, and the solution was filtered by Buchner filtration at 50 ºC. Water

was added to the solution, a white precipitate was formed on addition, and this dissolved on

stirring. More water was added until the precipitate could no longer be dissolved upon

stirring. White crystals were allowed to develop over 24-48 hrs. Crystals were filtered by

Buchner filteration. The PEO-Br solution was evaporated down, and placed on high vacuum

for 24 hrs. A white solid was produced (41 g, % yield of 77 %).

The reason for first brominating the PEG flocculating agent was due to the fact that

bromine is a better nucleophilic leaving group than alcohol meaning the probability of a

successful thiolation reaction is higher. The purified brominated PEG was analysed by IR, 1N

NMR, matrix assisted laser desorption/ionisation (MALDI), and elemental analysis.

Figure 13. IR spectra of PEO-BR

Infra-red spectroscopy shows the bromination of the PEO has taken place. The peak at

2863.92 cm-1

is the C-H stretch. The presence of the terminal bromine is confirmed by the

peak at 542.69 cm-1

(C-Br stretch) and signal at 947.97 cm-1

(terminal –CH2X wag).

The resulting NMR spectrum (figure 14) shows the presence of PEO at 3.6 ppm and

also another peak at 4.7 ppm which corresponds to the deuterium (D20). The peak at 3.2 ppm

is assigned to the end methyl group.

It is not possible to assess whether the hydroxyl peaks have been substituted with

bromine from an NMR spectrum however. The main reason behind this is that bromine is not

35

1H NMR active and so not detected and also because the concentration of hydroxyl groups of

the PEG starting reagent is too small to be identified (Figure 14).

Figure 14. NMR spectrum of brominated polyethylene glycol (PEO-Br)

The MALDI mass spectroscopy results demonstrate that the bromination of the PEG

was successful with each peak corresponding to a different sized chain within the distribution

matched to the molecular weight of CH3(OCH2CH2)nBr with the most significant peak at 821

daltons correlating to 16 repeat polyethylene glycol units. Each of the peaks are also in

duplicates with a 2Mw difference between them which coincides with the isotope of bromine

79 and 81, which have a natural abundance of roughly 50/50.

Figure 15. MALDI spectrum of brominated polyethylene glycol (PEOBr)

36

The results from microanalysis further confirm the formation of brominated

polyethylene glycol which reveals there to be 9.49% bromine, which is very close to the

expected result of 9.48 %. The other results are as follows;

Table 3: Microanalysis results of brominated polyethylene glycol

Element

Expected

-%

Found

-%

Calculated %

BrPEG

C 49.8 49.65 99.70

H 8.58 8.58 100.00

Br 9.47 9.49 100.21

3.4 Synthesis and purification of PEO-SH oligomers

Scheme 3. Thiolation of the brominated polyethylene glycol.

Brominated polyethylene glycol (32.2 g, 40 mmol) was dissolved in ethanol (10

0ml). Thiourea (4.8 g, 63 mmol) was then dissolved into the polymer solution which was was

then stirred under reflux at 80ºC for 24hrs. Sodium hydroxide (2.2 g, 60 mmol) was dissolved

in ethanol (20ml), the NaOH solution was added to the PEO-Br solution and refluxed for a

further 2 hrs.

The mixture was evaporated down to red/orange oil. Water (25 ml) was added, white

precipitate was formed which dissolved upon stirring. More water (20 ml) was added until

the white precipitate did no longer dissolve. The mixture was left for 24 hrs, and the white

solid formed was removed with a Buchner filter.

The pH of the solution was lowered to pH 1 with the addition of HCL (0.1 M). The

solution was then extracted with DCM (3x75 ml). The DCM layer was washed with an

aqueous saturated sodium bicarbonate solution (NaHCO3) (100 ml). The DCM was rotary

evaporated off to peach coloured oil. This oil was then dissolved in water (75 ml), and

cationic ion exchange resin 3 (~30 g) was poured in and allowed to stir for 24 hrs. This stage

37

removed the peach colour in the oil solution. The water within the solution was rotary

evaporated off and the remaining solution left under high vacuum for a few hours until an off

white solid was produced (15 g, % yield of 45 %).

In an attempt to avoid the coupling of two thiol groups to form a disulfide bridge,

(Figure 16, 19), the thiolated PEG was dissolved in propan-2-ol (50ml). Zinc dust (0.67 g, 10

mmol) was added and the mixture heated to 70 ◦C whilst stirring for 20 minutes. The mixture

was than rotary evaporated until all of the propan2ol was removed, then water (50ml) was

added and the mixture was stirred for a further 20 minutes. A white precipitate was formed

which was filtered off then the solution extracted with DCM (3 x 50 ml). The DCM layer was

washed with water (3 x 25 ml) then evaporated off under high vacuum over night.

Scheme 4. Reaction mechanism showing formation of PEGSH (1) and disulfide (2)

The elemental results of the capping agent showed the absence of nitrogen which is an

indication that thiourea had successfully converted into the thiol. There was however trace

amount of phosphorus found which reveals the possibility that not all reagents were removed

in the bromination step.

38

Table 4: Microanalysis results of thiolated polyethylene glycol

Element Expected (%) Found (%) Calculated (%BrPEG) %

C 52.63 50.97 96.85

H 9.11 9.07 99.56

N 0 0 0.00

S 4.25 3.77 88.71

P 0 0.68 0.00

Figure 16. MALDI spectra for the PEO-SH capping agent.

The MALDI mass spectroscopy results (Figure 16) reveal that the thiolation of the

PEOBr was successful as each of the peaks can be allocated to CH3(OCH2CH2)nSH where the

most intense peak is refers to 17 repeat units of ethylene glycol. The smaller peak region

towards the higher end of the MALDI mass spectra was produced due to disulphide

formation.

NMR does show the -S-H peak at about 2.2 ppm (Figure 17). The peak at

approximately 4.6ppm is the -O-CH2-CH2- and the peak at approximately 3.6 ppm is the -O-

CH3 protons which are not coupled to any other protons.

39

Figure 17. NMR spectrum of PEO-SH capping agent.

NMR spectra provides most concise evidence for presence of the thiolate. It shows

the proton environments of both thiolate (left) and the disulfide (right). We expect to see

three distinct proton environments for thiolate, in form of oliphatic backbone (b, c), singlet

for -O-CH3 (a), and a singlet for -S-H proton (d). Three peaks on the NMR spectra (Fig. 17)

perfectly fit, with peak integration for 2.2 ppm about three times smaller than the peak

integration for methyl peak at 3.6 ppm (one thiolate proton vs. three methyl protons).

Figure 18. Proton environments of thiolate and disulfide.

If, on the other hand, purely disulfide was present, then we would see only two peaks

on the NMR spectra: one for the coupled protons from the backbone (f , g) and one for the

methyl group (c) as the molecule is symmetrical over -S-S- bond. Even if we assume that the

40

peak at 2.2 is the proton environment f then that would be split into a 1-2-1 triplet by

protons g. However the peak at 2.2 is a singlet, hence showing that thiol is present.

Figure 19. IR spectra of PEG-S-S-PEG

IR shows the presence of a weak absorption at 2550-2600 cm-1

which can be assigned

to S-H. The peak at 532 cm-1

shows the presence of disulfide.

The evidence is therefore that both PEGSH and PEG disulphide are both present but it

was not possible to quantify the relative ratios. This material was then taken on and used to

synthesize silver nanoparticles.

41

Chapter 4 - Synthesis and Characterisation of PEGSH capped

silver nanoparticles

4.1. Procedure for the synthesis of PEO-SH capped silver

nanoparticles AgNO3 (0.5 g, 0.003 mol) and PEOSH (7.7g, 0.006 mol) were dissolved in MeOH

(350ml) by sonification. In a separate beaker, NaBH4 (1.1 g, 0.026mol) was dissolved in

MeOH (100 ml). The NaBH4 solution was added to the Ag/PEOSH solution under vigorous

stirring, forming an instant dark brown solution. The solution was left to stir for 1 hour to

allow the nanoparticles to grow.

The synthesis of nanoparticles was repeated, with two batches being produced-one

with 3nm nanoparticles (Batch B) and one with 36 nm particles (Batch A) formed.

4.2. Purification of Ag-PEGSH particles Purification of silver nanoparticles was carried out in 3 steps as follows;

Step 1- Removal of the excess capping agent:

Diethyl ether (450 ml) was added to the AgPEGSH that was dispersed in MeOH (50

ml). This resulted in the solution turning cloudy which is the result of the particles crashing

out due to the addition of a non solvent (diethyl ether). The solution was centrifuged at 3000

rpm for 5 minutes. The slightly brown solution on top, containing the excess capping agent)

was decanted off and the dark brown precipitate left at the bottom of the centrifuge tube was

then air dried.

Step 2- Removal of borates

The particles at the bottom of the centrifuge tube were re-dispersed in DCM (100 ml).

The solution was then filtered until only white solid was left on the filter paper. The DCM in

which the AgPEGSH particles were dispersed in was removed on the rotary evaporator (40

°C, 30 mins).

Step 3- Removal of ion impurities

There were two different approaches used for the removal of ions such as sodium

from the silver/citrate nanoparticle aqueous dispersion. Initially dialysis was used to assist the

removal of ions however this method was unsuccessful as the silver nanoparticles aggregated

inside the dialysis bag.

42

The second, more effective method of removing ions was to re-disperse the particles

from the round bottom flask which had been on the rotary evaporated with water (100 ml).

Dowex MRX-3 ion exchange resin (50g) was added to the solution and left to stir for 24 hrs.

The solution was then filtered and left on the rotary evaporator (70 °C, 2 hrs) to remove

water. This step was more successful than the first and was apparent in the elemental results

as prior to the addition of Dowex ion exchange, traces of sodium were present in the solution

however following the addition of ion exchange resin, and there were no traces of ions left in

the sample.

The synthesis of nanoparticles was repeated, with two batches being produced-one

with 3nm nanoparticles (Batch B) and one with 36 nm particles (Batch A) formed. The NMR,

IR, MALDI results were identical for both batches so only one set of data is displayed below.

The resulting Ag-PEOSH nanoparticles were characterised by DLS, XRD, TEM,

MALDI, TGA, 1H NMR, Fourier Transform Infra-red and UV-Vis spectroscopy.

Figure 20. UV-Vis spectra of capped silver nanoparticles (same for both batches)

The UV-Vis spectra (Figure 20) of the PEO-SH capped Ag Nanoparticles shows the

typical surface Plasmon resonance peak at approximately 420 nm.

1H NMR of the capped nanoparticles shows the three distinct peaks corresponding to

the three proton environments of the PEO-SH molecule. The weak peak at 5.4 ppm is the -SH

proton, which gives a very weak signal due to possible proton exchange. The singlet at 4.7

43

ppm corresponds to the -O-CH3 protons. Peak at 3.6 ppm is the -CH2- oligomer backbone

(Figure 21). It is somewhat broader at the bottom due to possible coupling.

Figure 21. 1H NMR of Ag-PEOSH nanoparticles (same for both batches)

MALDI-the spectra (Figure 22) shows a range of extra peaks around the 234 region

compared to the spectra of the pure PEO-SH capping agent. These peaks correspond to the

fragmentation of the capping agent and Ag-SH-(CH2)n+ residues.

Figure 22. MALDI spectra of silver nanoparticles (same for both batches)

Infra-red spectroscopy of the silver capped nanoparticles was also performed (Fig.

23). The chart shows extra peaks in the 1300-800 cm-1

region. While the peak at 2881cm-1

is

44

the CH2- stretch, peaks at 1342 cm-1

, 1279 cm-1

correspond to C-O vibrations. Hence it is

clear that interaction of capping agent with silver nanoparticles causes redistribution of

electron density and hence a different set of infra-red active vibrational and rotational modes.

Figure 23. IR spectroscopy of Ag-PEOSH nanoparticles (same for both batches)

4.3 Particle Size Analysis TEM spectroscopy found that nanoparticles produced were varied in size from batch

to batch, with range variation between 3.5 nm and 35 nm, Figure 24 and 25. Several

explanations of this effect are possible and would be discussed later.

Figure 24. Nanoparticle size distribution by TEM with 35nm mean value, (Batch A).

% f

req

ue

ncy

Size - nm

20 nm

Average width 36.93 nm

Standard deviation 8.45 nm

45

3.0 + 0.5 nm

Figure 25. Nanoparticle size distribution by TEM with 3 nm mean value (Batch B).

To explain the histogram (Batch B): out of population of 120 nanoparticles, 45 would

be 3nm size, 35 would be 2.5nm, 27 would be 3.5nm, 5 particles 2nm, 6 particles 4nm, 1

particle 1.5nm and 1 particle 6nm.

Dynamic Light Scattering (DLS) has also showed variation in the size of

nanoparticles produced from batch to batch. According to the DLS, there was a range of

nanoparticles present, with diameters from about 8-50. There was also a small proportion of

aggregates present, at 409 nm (0.1 % volume) and 4860 nm (0.1 % volume) (Figure 26).

Figure 26. DLS size distribution of nanoparticles Batch B.

0

5

10

15

20

25

30

35

40

45

50

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

% F

req

ue

ncy

nm

46

Figure 27. XRD of capped silver nanoparticles similar to that of Batch B

XRD of capped silver nanoparticles show the characteristic peak at approximately 38

degrees (2θ). This corresponds to the 111 lattice plane of silver. From the chart the

nanoparticle size was calculated using Sherrer equation. XRD data gave nanoparticle size as

4.9nm. This is quite close to the size of NPs from batch B according to TEM results

(considering the effect that polymer sheath can have on TEM results, TEM and XRD results

agree with good accuracy).

The Sherrer equation allows calculation of particle size L using the peak width B,

diffraction angle θ and wavelength of radiation λ. In this case, K=0.94 (spherical particles

FWHM), λ=370 Å, B=2, θ=19o.

4.4 Discussion Silver nanoparticles were synthesised by reduction of AgNO3 with NaBH4 in

methanol with presence of capping agent PEOSH and further purification. The successful

cos2

L

KB

47

formation of capped nanoparticles was confirmed by the UV-Visible spectroscopy which

demonstrated the characteristic Plasmon peak at approximately 420 mn (Figure 20).

The mechanism of nanoparticle formation in solution is best described using the

Lamer and Dinegar model of nucleation (Figure 8). According to the model, when the

concentration of metallic Ag formed from reduction of AgNO3 becomes supersaturated, silver

nucleates out from the solution thus forming a new phase. The particles then grow via

addition, which acts to reduce the super-saturation. When the concentration drops below the

critical level, nucleation stops and already formed particles continue to grow via molecular

addition. According to the theory, for uniform size distribution a short nucleation period is

necessary (to generate all of the particles seen at the end) followed by the growth process

when smaller particles grow faster than the large ones. Alternative mechanism to increase

particle size is aggregation with other particles, but negligible amounts of aggregates were

present according to DLS analysis (Figure 26). The aggregation model is usually important if

the initial nuclei formed are unstable, but because the reaction takes place in the presence of

the capping agent which stabilises nanoparticles, aggregation input into overall mechanism is

small.

Two different methods were used for the removal of ions such as sodium from the

silver-citrate aqueous nanoparticle dispersion. The first method was dialysis; however this

method was unsuccessful due to the aggregation of silver nanoparticles inside the dialysis

bag. The second more effective method of removing impurities was the addition of Dowex

MRX-3 ion exchange resin to the silver-citrate aqueous nanoparticle dispersion followed by

stirring for 24 hours. This was apparent in the elemental results as prior to the addition of

Dowex ion exchange; traces of sodium were present in the solution.

Silver NPs formed were analysed by TEM (Figures 24, 25) and DLS (Figure 26)

which both demonstrated that spherical nanoparticles of approximately 3-45 nm were formed.

DLS for Batch B run gave broad peaks with maxima at 8 nm, 14nm, 18 nm for different runs.

Such size difference could be due to either difference in shape or sedimentation of the sample

(different settling of the sample in the apparatus). From DLS results, maximum nanoparticle

size was approximately 50 nm. However, as DLS showed significant variation of results

from one batch (B), it was unreliable and XRD and TEM gave a more accurate estimate of

particle size.

48

Table 5. Nanoparticle size estimates

Analytical Method Silver Nanoparticle Size Range, nm

TEM (Batch A) 17-69

TEM (Batch B) 1.5-6

DLS (Batch B) first run 7-50

DLS (Batch B) 10-52

One of the possible reasons for the difference in size of nanoparticles produced could

be attributed to high concentration of sodium borohydrate used (0.026mol vs 0.003mol of

silver precursor). If we assume instantaneous injection of sodium borohydrate, that would

create a very high local concentration of borohydrate and hence high concentration of

reduced silver atoms. Because of this high local concentration, nucleation would occur in

this area of high concentration-i.e. super-saturation according to Lamer and Dinegar model .

Stirring would equilibrate the concentration of sodium borohydrate/atomic silver with the rest

of the reaction volume, thus lowering it below the critical concentration and resulting in

molecular addition, but no nucleation taking place. Hence that would give fewer

nanoparticles which are larger in size.

If, on the other hand, we assume that on introduction sodium borohydrate was

instantaneously dispersed in the whole volume of reaction mixture, this would result in the

instantaneous nucleation over the whole volume. The concentration would quickly drop from

the supersaturated level and a large number of relatively small NPs would be produced.

The spherical shape of the nanoparticles was reinforced by smooth shape of the UV-

Vis Plasmon peak, as it has a tendency of shifting to longer wavelengths with formation of

characteristic “shoulder” on formation of faceted nanoparticles

49

Chapter 5. Nanocomposites

5.1 Preparation of nanocomposites There is significant amount of nanocomposite research reported in the literature [55,

56, 57]. The introduction of silver nanoparticles into water-soluble matrices such as

polyvinylalcohol, acrylic acid, and polyvinyl pyrolidone is easy due to stabilisation of

nanoparticles by the polymer functional groups and thus thermodynamically favourable[49,

58, 59].

There are few reports on introducing silver nanoparticles into hydrophobic polymers.

Lim and Ast reveal the introduction of silver NPs into polystyrene by using preformed NPs

[60]. An introduction of silicate NPs into PMMA by suspension polymerisation was also

reported with improved thermal properties [61], as was introduction of silver into polyvinyl

fluoride matrices [62]. These synthesis were successful due to interaction of functional

groups (e.g. CF2 dipole) with surface charges of nanoparticles.

Introduction of nanoparticles into polymers with ester functionality is more difficult

because ester groups weakly interact with NPs by dipole interactions [63, 64]. Hence it is

necessary to introduce capping agent to allow the formation of nanocomposites. PMMA is

widely used in aviation/construction where its mechanical properties are important. It is also

used in medicine due to biocomparability. Because of all of these reasons PMMA was chosen

as a polymer matrix. PMMA (Mw = 750kDa, Tg = 105 oC) was used throughout.

The PMMA pellets were dispersed in THF and stirred until fully dissolved. Capped

AgNP’s (Batch B; ps = 3 nm mean by TEM) were then added to the THF/PMMA mixture

and stirred for a further hour in order to suspend them. When the entire mixture (THF,

PMMA and MPEG capped AgNP’s) was thoroughly mixed, the mixture was poured into

methanol, where the PMMA immediately precipitated with the suspended capped Ag NP’s

trapped within its network. The composites were then dried, grounded into a fine powder and

pressed into thin films, ready for characterisation and analysis. A range of nanocomposites

was investigated, from 1% silver to 35% silver by weight (0.1% to 3.5% volume silver NPs).

5.2 Preparation of composite films The dried, finely grounded PMMA and capped silver nanoparticle mixture was placed

in a glass sandwich mould as seen in Figure 28.

50

Figure 28. Representation of the compression mould with solid metal as template and two

glass plates to hold composite together during hot, high pressured pressing.

Approximately 1 g of composite was placed within the mould; this amount ensured

that there was enough mixture to give a uniform plaque. Once the mould was filled with

composite mixture, it was heated to 150oC for 5 minutes to ensure that any residual solvent

was evaporated to avoid solvent bubbles within the surface. The composite was then pressed

at 1800 psi at the same temperature of 150oC for 10 minutes. The heating was then stopped

and the composite left to cool for 30 minutes under pressure, and then taken out of the press

and left to stand until fully cooled to room temperature.

Initially thin films of tin foil were used instead of glass to sandwich the composite

however the foil did not produce a consistently smooth surface. Next, pouring the mixture of

solvent, PMMA and nanoparticle over a sheet of acetate and leaving in fume cupboard to dry

naturally was tried. This method, however, produced some solvent regions and hence uneven

surface. Hence the compression mould with glass plates was used (Fig. 28).

Solid metal template

Glass slides

Composite area

51

5.3 Characterisation of composite films Several techniques were used in assessing the composite properties, SEM, UV-Vis

and IR. Conductance was also measured.

Figure 29. Scanning electron microscope image of Ag NPs PMMA composite (top image-

cluster in 10% weight AgNp composite; middle images-close up of silver aggregates in 15%

weight Ag composite; bottom images-left small silver NP cluster/right silver clusters in 20%

weight Ag nanocomposite.

52

The SEM images (Figure 29), shows the distribution of PEO-SH capped silver NP’s

ranges from 1 µm down to values below 50 nm in the PMMA matrix. Aggregate formation

was the same for all ratios of silver nanoparticles to PMMA in composites, as could be seen

from the scans of various nanocomposites (15% weight silver, 20% weight silver).

Nanocomposites with various proportions of silver nanoparticles were also analysed

using UV-Visible spectroscopy (Figure 30). The characteristic Plasmon peak at 420 nm was

observed, with more intense absorption as the concentration of silver nanoparticles increased.

The peak was also broader as the concentration of silver NPs increased. Plasmon resonance

results from the collective oscillations of free electrons in nanoparticles in response to the

external electric field. The position of the peak with respect to Ag. NPs depend mostly on

particle size and shape, with peak shifting to longer wavelengths as particle size and shape

(from spherical to faceted nanoparticles, e.g. cubic or pentagonal). The peak at approximately

420 nm is consistent with spherical nanoparticles up to 50 nm. The pattern of increasing

absorption with increasing concentration has not been reported previously in the

literature[65].

Figure 30. UV-Vis spectra of nanocomposites at variable Ag NPs concentrations. (lowest

peak-2% weight silver, then 4%, 6%, 8%, 10%, 12%)

The PMMA/Ag NP composites were also analysed by FT IR spectroscopy, Figure 31.

A range of characteristic PMMA peaks was observed, including ester C=O stretch (1720

cm-1

), C-O stretch (1240 cm-1

, 1141 cm-1

), -CH3, -CH2- stretch (2993 cm-1

, 2852cm-1

), and

53

CH2 bends ( 1434 cm-1

, 1482 cm-1

).

Figure 31. IR spectra of PMMA/AgPEO-SH nanocomposite (5% weight silver NPs)

5.4 Thermogravimetric Analysis of Nanocomposites

Figure 32. Thermogravimentric analysis of nanocomposites (Line 1-pure PMMA ; Lines 2,

3, 4, Batch A-3%, 3%, 2.5% w/AgNPs; Line 5 Batch B-, 5% w/AgNPs;)

TGA was carried out to investigate the effect of nanoparticle addition on the thermal

stability of the material. The samples were heated under nitrogen, as the use of oxygen can

oxidise the samples. The samples were heated to 800oC from 25

oC over an hour.

54

The first weight loss around 300oC indicates how much silver there was in the sample.

Line 5 corresponds to Batch B. The maximum loading of silver NPs into composite was 35%

by weight and 3.5% by volume.

An improvement in thermal stability of nanocomposite over pure PMMA was

demonstrated, particularly with nanoparticles of smaller size (below 10 nm), Figure 32.

PMMA first degrades a 250oC however this is not the case with Ag nanocomposites. Cluster

formation has been observed for a range of concentrations of silver nanoparticles. This means

that formation of aggregates did not depend on the loading of silver in the nanocomposites

but on some other factors. Cluster formation has occurred due to the hot pressing method of

nanocomposite preparation and the thermodynamics driven by bond enthalpies. The

suggested mechanism for cluster formation was that on heating the thiolate group becomes

unstable and the interaction between the thiol group and the nanoparticle surface were

disrupted. Then the ester groups displaced the thiol as the capping agent Such displacement

was more favourable thermodynamically as the oxygen is more electronegative element and

oxygen-silver NP interaction would be stronger than sulphur-silver NP interaction. Similar

aggregate formation with thiolate capping agent was reported for the Au/PSt nanocomposites

in the literature where on introduction of capped NPs into the polystyrene matrix a wide

distribution of nanoparticle sizes was observed [65].

Interactions between silver NPs and PMMA helped to stabilize the structure and

hence thermal degradation of nanocomposite occurs at higher temperature. This hasn’t been

observed before and is believed to be due to interactions between ester groups in PMMA and

the silver nanoparticle surface.

This effect has both negative and positive sides for the stability of silver/PMMA

nanocomposites and potential applications. The displacement of capping agent by PMMA

during hot pressing lead to aggregation of nanoparticles together.

On the other hand, such increased interactions between PMMA and silver core of the

nanoparticles lead to higher thermal stability of nanocomposite compared to pure PMMA.

5.5 Conductivity

Pure silver is a good conductor. It is metallic and its conductivity on the Pauling scale

is 15.87 nΩ·m at 25 oC [66]. Pure dry un-sintered PEO-PS capped Ag-NP’s, as prepared in

Chapter 4 were non-conducting, rising to 12-13.5 Ω at 1cm space under composite forming

temperatures. Conductivity was lower than bulk silver because dry capped nanoparticles

55

could be considered as a nanocomposite consisting of silver NPs and the polymer capping

agent. All PMMA – Ag NP nanocomposites formed were not conducting.

According to the percolation theory, the volume loading necessary for conducting

nanocomposite is 16 vol-% for the random distribution of conducting spheres in the non-

conducting medium [67]. However, due to the difference in density of silver (10.5 g/cm3)

versus PMMA (1.18 g/cm3) the 35 wt-% loading of silver by weight equates to 3.5 vol-%

loading by volume. To produce conducting silver NP/ PPMA nanocomposite, 16vol-%

loading of silver NPs would be needed. So for 1cm3 of conducting composite 16 vol-%

volume has to be silver (0.16cm3*10.5g/cm

3=1.68g silver NPs) in 84wt-% volume PMMA

(0.84cm3*1.18g/cm

3=0.99g). To produce the conducting composite, weight loading of silver

has to equal or be greater than (1.68/(1.68+0.99)*100%) = 62.9 wt-%

Hence in the nanocomposites formed there was a high proportion of silver by weight,

but a fairly low proportion of silver NPs by volume. For the percolation to occur, the volume

occupied by the conducting phase in the composite is most important. This was the reason

why all of the nanocomposites produced were not conducting - the loading of silver NPs by

volume was below the percolation threshold.

5.6 Percolation Theory To determine the percolation threshold [68] value for polymer insulator/conductor

filler composite, the percolation theory is used.

Figure 33. Dependence of electrical conductivity σ (logarithm) on the conducting particle

content F.

56

When a polymer matrix of conductivity σp if filled with filler of conductivity σf, the

composite prepared gains a conductivity value of σ. When the loading of filler is increased so

that volume filler fraction φ reaches critical value φc, an infinite cluster is formed and the

composite becomes conducting [69]. Due to the presence of a conduction or percolation path

across the entire sample, a change from an insulator to a semiconductor occurs. As the filler

concentration increases to the filling limit F, the value of r increases rapidly over several

orders of magnitude, from the value rc at the percolation threshold to the maximal value σm

(Fig. 33). Above the percolation threshold, the electrical conductivity is related to the content

of conducting filler by the following power law:

Where is the composite conductivity, is the conductivity of the conductive

filler, volume fraction of conductive filler, percolation threshold, and t critical

exponent.

Below the percolation threshold, the conductivity change is negligible and the

conductivity of the composite is equal to the polymer conductivity rp or slightly higher. The

typical dependence of the logarithm of conductivity on the filler volume fraction is shown in

Another important parameter to consider is filling packing factor F which sets the

limit on the loading of the system and depends on the shape of the conducting particles and

the probability of skeleton or chained structure formation [70]. F = Vf/(Vf+Vp) where Vf is

the volume occupied by filler particles and Vp is the volume occupied by polymer. For

statistically monodispersed particles of spherical shape, F=0.64 [71]. If particle shape

deviates from spherical, F decreases. Real fillers tend to have F lower than 0.64 [70, 72].

Hence F takes into account particle shape, fractional size, and distribution of the particles,

and could be used to characterise filler phase topology.

F is linked to the percolation volume of conducting states φc=XcF where Xc is the

critical parameter (probability of site percolation). For any lattice type, Xc and F have such

values, that their product equals approximately 0.16. According to the Scher and Zallen [73]

model, the change in conductivity occurs at a stricit value of volume of conductive states φc.

The value of the critical parameter Xc is determined for lattice problems and for

models having only one uc value. For example, for random distribution of conductive spheres

in a non-conductive medium φc = 0.16, Fc= 0.64, Xc=0.25 [67].

57

From the data obtained and non-conducting nanocomposites it is possible to conclude

that nanocomposites formed were below the percolation threshold (16% volume).

5.7 Nanocomposite Summary A range of nanocomposites has been prepared and investigated via SEM, TGA and

other analytical techniques. Formation of silver clusters has been observed in nanocomposites

which was potential reason for improvement in PMMA thermal stability. The cluster

formation occurred because of the displacement of thiolate during the hot pressing process by

the ester groups of PMMA.

Range of nanocomposites was investigated for electrical conductivity, however they

were found to be non-conductive due to loading of silver being below percolation threshold.

58

Chapter 6 –Conclusions and Further Work

6.1 Synthesis of PEO-SH capping agent Synthesis of the capping agent has worked quite well, with good purity and yield

across two stages. The bromination step resulted in 77% yield and 97% purity, and the

thiolation produced 50% yield and 95% purity (Figures 18 and 22).The thiolation have also

yielded some amount of disulphide, although that was a minor product (Figures 16, 17, 18).

The synthesis of capping agent was repeated across several batches with similar yields and

purity, thus pointing to good reproducibility of results. These were used in subsequent

studies.

6.2 Ag nanoparticle synthesis Two batches of capped silver nanoparticles were produced: Batch A and Batch B.

Nanoparticle size was investigated via a variety of methods (Chapter 4, 4.3, p.45) which

showed that there was a size variation between Batch A(mean TEM particle size 36nm) and

Batch B (mean TEM particle size 3.0nm). This was due to the synthetic method used, in

particular effect of high concentration of sodium borohydrate.

6.3 Synthesis of nanocomposites Pre-formed nanoparticles were introduced into PMMA matrices. The successful

formation of silver nanocomposite was confirmed by presence of the Plasmon peak at approx.

420 nm on the UV-Visible spectra (Fig. 33). UV-Visible results across a range of different

concentrations of silver nanoparticles were broadly similar, with identical chart shape and as

expected variation of intensity with increasing concentration.

This result suggests that nanoparticles have not aggregated during composite

formation and were uniformly distributed across the PMMA matrix. However, SEM data

demonstrated presence of aggregates and not uniform distribution of silver NPs (Fig. 31). The

conflicting evidence from SEM needs to be further investigated.

6.4 Further Work

Further work has three main avenues of research.

First of all, the method reported results in production of nanoparticles with size

distribution from 3 to 50nm. A range of potential synthetic techniques could be explored to

59

narrow the size distribution, such as various hot injection techniques (for reducing agent) and

variation of relative concentration of silver ions in solution. Investigation of using excess of

capping agent would also be necessary.

Secondly, nanocomposites formed were non-conducting, which means that

concentration of silver particles was below the percolation threshold. A wider range of silver

loading by weight could be investigated to produce conducting nanocomposites which then

could be investigated for a range of nano-electronic applications. An issue of how to stop

aggregate formation within nanocomposite also needs to be investigated.

Finally due to the fact that the use of PMMA caused aggregation of the nanoparticles,

it would be useful and interesting to carry out the same research but instead of using PMMA

to use other types of polymers for example Polystyrene and see if it has the same

destabilisation and aggregation effect.

60

7. References

1. Goesmann, H. and C. Feldmann, Nanoparticulate Functional Materials. Angewandte

Chemie International Edition, 2010. 49(8): p. 1362-1395.

2. Thompson, D.T., Michael Farady's Recognition of Ruby Gold: the Birth of Modern

Nanotechnology. Gold Bulletin, 2008. 40(4).

3. Feymann, R., Plenty of room at the bottom, in Annual meeting of the american

Physical Society. 1959: Californian Institute of Technology.

4. Janardhanan, R., et al., Synthesis and surface chemistry of nano silver particles.

Polyhedron, 2009. 28(12): p. 2522-2530.

5. Kelly, K.L., et al., The Optical Properties of Metal Nanoparticles:  The Influence of

Size, Shape, and Dielectric Environment. The Journal of Physical Chemistry B, 2002.

107(3): p. 668-677.

6. Christian P, K.F.e.a., Nanoparticles: Structure, properties, preperation and

behaviourin environmnetal media. 2008. 17.

7. Nuffer, J.H. and R.W. Siegel, Nanostructure-Biomolecule Interactions: Implications

for Tissue Regeneration and Nanomedicine. Tissue Engineering Part A, 2010. 16(2):

p. 423-430.

8. Sau, T.K., et al., Properties and Applications of Colloidal Nonspherical Noble Metal

Nanoparticles. Advanced Materials, 2010. 22(16): p. 1805-1825.

9. Burda, C., et al., Chemistry and Properties of Nanocrystals of Different Shapes.

Chemical Reviews (Washington, DC, United States), 2005. 105: p. 1025 - 1102.

10. Giorgetti, E., et al., In Situ Production of Polymer-Capped Silver Nanoparticles for

Optical Biosensing. Macromolecular Symposia, 2009. 283–284(1): p. 167-173.

11. Natsuki, J. and T. Abe, Synthesis of pure colloidal silver nanoparticles with high

electroconductivity for printed electronic circuits: The effect of amines on their

formation in aqueous media. J. Colloid Interface Sci., 2011. 359: p. 19-23.

61

12. Gao, J., H. Gu, and B. Xu, Multifunctional Magnetic Nanoparticles: Design,

Synthesis, and Biomedical Applications. Accounts of Chemical Research, 2009.

42(8): p. 1097-1107.

13. Sun, Y. and Y. Xia, Large-scale synthesis of uniform silver nanowires through a soft,

self-seeding, polyol process. Advanced Materials, 2002. 14(11): p. 833-837.

14. Ken-Tye, Y., Colloidal Nanoparticles in Biotechnology (Elaissari, A.; 2008) [Book

reviews]. Engineering in Medicine and Biology Magazine, IEEE, 2009. 28(5): p. 67-

67.

15. Paparazzo, E., et al., Evidence of Si-OH bonding species at the surface of aged silica.

J. Vac. Sci. Technol., A, 1992. 10: p. 2892-6.

16. Eustis, S. and M.A. El-Sayed, Why gold nanoparticles are more precious than pretty

gold: Noble metal surface plasmon resonance and its enhancement of the radiative

and nonradiative properties of nanocrystals of different shapes. Chem. Soc. Rev.,

2006. 35: p. 209-217.

17. Toth, E. Buying the nano-market. 2009; Available from:

http://www.hesa.etuirehs.org/uk/newsletter/files/HESAmag_1_UK_14-17.

18. M.C. Roco, C.A.M., M.C. Hersam. Nanotechnology Research Directions for Societal

Needs in 2020: Retrospective and Outlook, World Technology Evaluation Center

(WTEC) and the National Science Foundation(NSF), Springer, 2010. 2010;

Available from: http://www.wtec.org/nano2/Nanotechnology_Research_

19. Mini Innovations and Growth Team, Nanotechnology: A UK Industry View, 2009

20. Technology Strategy Board , Nanoscale Technologies Strategy 2009-12, 2009

21. Diegoli, S., et al., Interaction between manufactured gold nanoparticles and naturally

occurring organic macromolecules. Science of The Total Environment, 2008. 402(1):

p. 51-61.

22. Na, H.B., I.C. Song, and T. Hyeon, Inorganic Nanoparticles for MRI Contrast Agents.

Advanced Materials, 2009. 21(21): p. 2133-2148.

62

23. Brockmann, T., Fontana, P., Meng, B., Muller, U., Beton-Stahlbetonbau, 2008,103,

446

24. Timothy V, D., Applications of nanotechnology in food packaging and food safety:

Barrier materials, antimicrobials and sensors. Journal of Colloid and Interface

Science, 2011. 363(1): p. 1-24.

25. Mritunjai Singh, S.S., S. Prasad, I. S.Gambhir, Nanotechnology in Medicine and

Antibacterial effect of Silver Nanoparticles. Nanomaterials and Biostructures 2008.

3(3): p. 115-122.

26. Tallury, P., et al., Nanobioimaging and sensing of infectious diseases. Advanced Drug

Delivery Reviews, 2010. 62(4-5): p. 424-437.

27. Salata, O.V., Applications of nanoparticles in biology and medicine. J.

Nanobiotechnol., 2004. 2: No pp. given.

28. Balan, L., R. Schneider, and D.J. Lougnot, A new and convenient route to

polyacrylate/silver nanocomposites by light-induced cross-linking polymerization.

Progress in Organic Coatings, 2008. 62(3): p. 351-357.

29. Kramer, J., et al., Silver nanoparticle toxicity and biocides: Need for chemical

speciation. Integrated Environmental Assessment and Management, 2009. 5(4): p.

720-722.

30. Jeong, S., Hwang, Y, and Yi, S, Antibacterial properties of padded PP/PE non

wovvens incorporating nano-sized silver colloids. 2005. 40

31. Quintavalla, S. and L. Vicini, Antimicrobial food packaging in meat industry. Meat

Science, 2002. 62(3): p. 373-380.

32. Takai, K.O., Tokuko ; Senda, Yoshiko ; Nakao, Miyuki ; Yamamoto, Kouichiro ;

Matsuoka, Junji ; Hirai, Yoshikazu, Antibacterial properties of antimicrobial-finished

textile products Microbiology and immunology, 2002. 46(2): p. 75-81.

33. Lok, C.-N., et al., Silver nanoparticles: partial oxidation and antibacterial activities.

JBIC Journal of Biological Inorganic Chemistry, 2007. 12(4): p. 527-534.

63

34. Elechiguerra, J., et al., Interaction of silver nanoparticles with HIV-1. Journal of

Nanobiotechnology, 2005. 3(1): p. 6.

35. Neal, A.L., What can be inferred from bacterium–nanoparticle interactions about the

potential consequences of environmental exposure to nanoparticles? Ecotoxicology,

2008. 17(5): p. 362-371.

36. Pan ek, A., et al., Silver Colloid Nanoparticles:  Synthesis, Characterization, and

Their Antibacterial Activity. The Journal of Physical Chemistry B, 2006. 110(33): p.

16248-16253.

37. Hilderbrand, H., Mackenzie, K., Kopinke, F.D., Chem. Ing. Tech. 2007, 79, 1461

38. Geng, J., Y. Bi, and G. Lu, Morphology-dependent activity of silver nanostructures

towards the electro-oxidation of formaldehyde. Electrochemistry Communications,

2009. 11(6): p. 1255-1258.

39. Wang, L., et al., Self-assembled silver nanoparticle films at an air-liquid interface and

their applications in SERS and electrochemistry. Applied Surface Science, 2011.

257(16): p. 7150-7155.

40. Sato, T., et al., Single-electron transistor using a molecularly linked gold colloidal

particle chain. J. Appl. Phys., 1997. 82: p. 696-701.

41. Sharma, A.K. and B.D. Gupta, Fiber optic sensor based on surface plasmon resonance

with nanoparticle films. Photonics and Nanostructures - Fundamentals and

Applications, 2005. 3(1): p. 30-37.

42. Tang, Q., et al., Preparation and electrical conductivity of

SiO&lt;sub&gt;2&lt;/sub&gt;/polypyrrole nanocomposite. Journal of Materials

Science, 2009. 44(3): p. 849-854.

43. Graham J, H., Catalysis by gold. Catalysis Today, 2005. 100(1-2): p. 55-61.

44. Linkov, I., et al., Emerging methods and tools for environmental risk assessment,

decision-making, and policy for nanomaterials: summary of NATO Advanced

Research Workshop. Journal of nanoparticle research : an interdisciplinary forum for

nanoscale science and technology, 2009. 11(3): p. 513-527.

64

45. Belloni, J., Metal nanocolloids. Current Opinion in Colloid & Interface Science, 1996.

1(2): p. 184-196.

46. Chou, T.-W., et al., An assessment of the science and technology of carbon nanotube-

based fibers and composites. Composites Science and Technology, 2010. 70(1): p. 1-

19.

47. Thostenson, E.T., C. Li, and T.-W. Chou, Nanocomposites in context. Composites

Science and Technology, 2005. 65(3-4): p. 491-516.

48. Giannelis, E.P., Polymer Layered Silicate Nanocomposites. Advanced Materials

1996. 8(1): p. 7.

49. Mbhele, Z.H.,Salemane,M.G.,van Sittert.G.C.E.,Nedeljkovic,J.M.,Luyt,A.S.,Chem.

Mater. 2003, 15, 5019

50. Yen, C.C.,Cang, T.C.,Kakinoki, H., J. Appli. Polym. Sci., 1990, 40, 53

51. Hussain, I., Brust, M., Papworth, A.J., Cooper, A.I., Langmuir, 1990, 19, 4831

52 Mayer, A.B.R., Mark, J.E., Eur. Polym. J., 1998, 34, 103

53. Stevens, M.P., Polymer Chemistry, An introduction. 1999, New York: Oxford

University Press.

54. Fabris, L., et al., Aptatag-Based Multiplexed Assay for Protein Detection by Surface-

Enhanced Raman Spectroscopy. Small, 2010. 6(14): p. 1550-1557.

55. Chan K. L., Mariatti M., Lockman Z., Sim L. C., Effects of the Size and Filler

Loading on the Properties of Copper- and Silver-Nanoparticle-Filled Epoxy

Composites, Wiley Online Library, 04/2011

56. Sohi N.J.S., Sambhu Bhadra, Khastgir D., The effect of different carbon fillers on the

electrical conductivity of ethylene vinyl acetate copolymer-based composites and the

applicability of different conductivity models, 2010 Elsevier

57. Khairul Anuar S., Mariatti M., Azizan A., Chee Mang N., Tham W. T., Effect of

different types of silver and epoxy systems on the properties of silver/epoxy

conductive adhesives, J. Mater. Sci: Mater Electron, 2011, 22:757–764

65

58. Kumar R.V., Koltypin Y., Cohen Y.S., Aurbach D., Palchik O., Felner I, Gedanken

A., J. Mater. Chem., 10, 1125 (2000).

59. Strawhecker K.E., Manias E., Chem. Mater., 2000, 12, 2943.

60. Lim , M.H., Ast, D.G., Adv. Mater., 2001, 13, 718.

61. Huang, X. , Brittain W.J., Macromolecules, 2001, 34, 3255.

62. Manna S., Batabyal , S.K., Nandi , A.K.. , J. Phys. Chem. B, 2006, 110, 12318

63. Jewrajka S.K , Chatterjee U., J. Polym. Sci. Polym. Chem., 2006, 44, 1841

64. Chatterjee U. , Jewrajka S.K., J. Col. Int. Sci., 2007, 313, 717

65. Corbierre M.K., Cameron N.S., Sutton M., Mochrie S.G.J., Lurio L.B., Ru¨hm A.,

Lennox R.B., J. Am. Chem. Soc., 2001, 123, 10411.

66. http://www.chemicool.com/elements/silver.html

67. Lux F., J. Mater. Sci., 1993; 28:285–301

68. Xu, P. and Z.-Y. Li, Effect of particle shape on the effective dielectric response of

nanocomposite close to the percolation threshold. Physica B: Condensed Matter,

2004. 348(1-4): p. 101-107.

69. Stauffer D.,Introduction to percolation theory, London: Taylor and Francis; 1985.

70. Mamunya E.P., Davydenko VV., Lebedev E.V., Kompoz Polym Mater , 1991, 50,

37–47.

71. McGeary R.K., J. Am. Ceram. Soc., 1961, 44, 513–22.

72. Katz H.S., Milewski J.V., editors. Handbook of fillers and reinforcements for plastics,

New York: Van Nostrand Reinhold; 1978.

73. Scher H., Zallen R. , J. Chem. Phys., 1970, 53, 3759–61.