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CHAPTER 1

POLYMER NANOCOMPOSITES:

A GENERAL INTRODUCTION

1.1 INTRODUCTION

Composites have played an important role since the beginning of life. The first

man-made composite can be dated back to 5000 BC, in the Middle East where pitch was

used as a binder for reeds in boat building. The proliferation of composites that has

occurred during the last one hundred years can be inferred from the development of

various composite materials over the past several thousand years (Table 1.1). The

expression “composite” first appeared early in twentieth century and is now used to

describe the union of two or more diverse materials to attain synergistic or improved

qualities to those exhibited by individual parent materials. The composite materials have

a bulk phase, which is continuous, called the matrix; and a dispersed, non-continuous,

phase called the reinforcement. The properties of composites are a function of the

properties of the constituent phases, their relative amounts, size and shape of dispersed

phase. The primary classifications of composite materials are natural and synthetic, based

on their origin. Further, they are classified based on their assembly, type of matrix

materials and type of geometry (size & shape) of filler materials, as shown in the Figure

1.1.

Based on the matrix material which forms the continuous phase, the composites

are broadly classified into metal matrix (MMC), ceramic matrix (CMC) and polymer

matrix (PMC) composites. Among the different matrix materials, the most common and

advanced composites are polymer matrix composites. PMCs consist of a polymer (resin)

matrix combined with a reinforcing dispersed phase to provide high strength, stiffness,

toughness along with good resistance to corrosion, abrasion and puncture. Their

popularity arises from their ability to be easily formed into complex shapes with simple

fabrication methods and at a low cost. The three types of material systems used for

fabrication of polymer matrix composites are thermoplastics (polycarbonate,

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polyvinylchloride, polystyrene, polypropylene, and polyamides), thermosets (unsaturated

polyesters, epoxies, phenol-formaldehyde) and elastomers (natural rubber, polybutadiene,

neoprene and silicones). The matrix materials are usually embedded with filler materials

such as fibers (carbon, glass, Kevlar® fibers), flakes (glass, mica, stannous sulfide, and

lead selenide flakes) and particulates (silica, titania, zirconia and calcium carbonate).

Table 1.1 Historical developments of polymer composites

A keyword search for “polymer composite” in Scopus and Engineering Village

data bases results in ~ 0.35 million records consisting of patents, journal articles and

conference proceedings (Figure 1.2). The most commonly studied polymers include

polyester, polypropylene, epoxy and polycarbonates, while the most commonly used

fillers are silicates (mica1, talc, fiber glass

2), metal oxides (titania, alumina, fumed silica,

calcium carbonate3) and clays (montmorillonite, wollastonite

4-6, kaolin

7), and carbon

black8-9

.

Until 1980s, most of the polymer composites were based on micrometer–sized

particles / fibers / flakes at a relatively higher loading level (> 30 wt %). In 1990’s, a new

dimension to polymer composites evolved, where-in the composites with similar property

profiles were achieved with a very low loading of nano-sized (<100 nm) fillers. These

new generations of polymer composites are termed as “polymer nanocomposites”.

Year Composites

(c.5000 BC) Papyus/pitch

(c.5000 BC) Wood veneer

1909 Phenolic composite

1928 Urea formaldehyde composite

1938 Melamine formaldehyde composite

1942 Glass reinforced polyester

1946 Epoxy resin composite

1951 Glass reinforced polystyrene

1956 Phenolic-asbestos ablative composite

1964 Carbon fibre reinforced plastics

1965 Boran fibre reinforced plastics

1969 Carbon/glass fibre hybrid composite

1972 Aramid fibre reinforced plastics

1975 Aramid/graphite fibre hybrids

3

Figure 1.1 Typical classifications of composites based on occurrence, type of matrix materials and

geometry of filler materials.

While a keyword search for “polymer nanocomposites” in Scopus database until

1991 returns with no hits, a similar keyword search until 2012 gives > 16779 hits (Figure

1.3), indicating a phenomenal increase in the development of polymeric nanocomposites,

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over the last 20 years. In these polymer nanocomposites, one of the dimensions of filler

materials is at least in the order of nanometer (1-100 nm). Though the final composite is

not necessarily be in nano-scale, but can be micro-or macroscopic in size10

. It is during

the past decade, the developments in nanotechnology across a variety of disciplines from

chemistry to biology and from materials science to electrical engineering made it a truly

interdisciplinary and multidisciplinary field of research. The scientists around the world

are engaged in creating the nano materials and functional nanosystems to bring

nanotechnology out of the research labs to the market place.

Figure 1.2 Polymer composites in the literature

Nanoparticles (1-100 nm) have high surface to volume ratio. The nanocomposites

incorporated with such particles will have much greater interfacial area than

microcomposites. Such enhanced interfacial area leads to a significant volume fraction of

polymer surrounding the particle that is affected by the particle surface and has properties

different from the bulk polymer. Since this interaction zone is much extensive for

nanocomposites than for microcomposites, it can have significant impact on properties11

.

For example, depending upon the strength of the interaction between polymer and

particle, this region can have a higher or lower mobility than the bulk material, and result

in an increase12

or decrease in glass transition temperature13

. The large surface area and

their interaction with polymer matrix play the most crucial role in obtaining the desired

physical properties (electrical, thermal, optical and mechanical)14

.

5

Nanostructured composite materials consisting of organic polymer and inorganic

fillers represent a merger between traditional organic and inorganic materials with

improved properties. Nature has created many such composite materials, such as diatoms,

radiolarian15

and bone16

, from which we can learn a lesson. Organic-inorganic composites

with nanoscale dimensions are of growing interest because of their unique properties,

such as enhancement of conductivity17-18

, toughness19

, optical performance20-21

, catalytic

activity 22

, chemical selectivity23-24

which make them suitable to use in various potential

applications. In these materials, inorganic and organic components are mixed or

hybridized at nanometer scale with virtually any composition leading to the formation of

hybrid/nanocomposite materials 25-26,27, 28.

The improvement in the properties observed

for nanocomposites could be due to several factors such as primary particle size,

aggregate size and structure 29-30

of the dispersed fillers.

Figure 1.3 Publication trend of nanocomposites related articles for last 20 years.

In general, the polymer nanocomposites are also classified based on their

geometries31-33

into particles, layered, and fibrous materials31-32

and based on physical

properties into thermosets, thermoplastics and elastomers. Among the numerous

nanocomposites, polymer/silica composites are most commonly reported in the prior arts.

Typically, the organic /polymer component can be introduced as (i) precursor, which can

be monomer or oligomer; ii) linear polymer (in molten, solution or emulsion state) or iii)

a polymer network, physically (e.g., semi crystalline linear polymer) or chemically (e.g.,

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thermosets, elastomers) cross-linked. The inorganic part can be introduced as (i) a

precursor (e.g., tetraethyl orthosilicate) or (ii) by direct blending or in situ formation. The

three general methods for the preparation of polymer/silica composites according to the

starting material and processing technique are blending, sol-gel process and in-situ

polymerization.

Among the different polymer matrix materials, the silicone is the most commonly

used polymer due to their unique properties (Table 1.2). Silicones exhibit

Table 1.2 Typical properties of polymer matrix materials.

many important and interesting properties. Although they possess a very low glass

transition temperature, these polymers are able to maintain thermal stability over a wide

temperature range in both inert and oxidizing environments. Furthermore, these materials

are resistant to UV radiation, ozone and atomic oxygen and are known to form a silicate

char under oxidizing conditions at temperatures of 500-700°C. Low surface tension, low

surface energy, physiological inertness, and high gas permeability are only a few of the

Polymer matrix Properties Application Limitation

SiliconesHeat resistance

Excellent dielectric properties

High arc resistance

Low water absorption

Low temperature flexibility

High gas permeability

Retention of mechanical properties

at high temperature

Electrical and

Electronics

Personal care

Consumer

Health care

Construction

Automotive

Formulators

Aerospace

Poor resistance to oil

and solvent

Poor steam resistance

EpoxidesExcellent mechanical properties,

Dimention stability,

Chemical resistance,

Low water absorption

Good abrasion resistance

Board tooling

Filament winding

Require heat curing

for maximum

performance

Phenolic resinsGood acid resistance

Good electrical properties

Heat resistance

Diverse electrical

and mechanical

applications

Dissolve in caustic

envirnomets unless

specially treated

PolyestersMost versatile,

Economical

Chemical resistance to acids

Corrugated sheeting

Boats

Automotive

Aircraft

Degrades by strong

oxidisers,

Aromatic solvents

Concentrated caustics

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many other interesting properties that these polymers exhibit. The simultaneous presence

of “organic” groups attached to “inorganic” backbone gives silicones a combination of

unique properties making possible their use as fluids, emulsions, compounds, resins and

elastomers in numerous applications (Figure 1.4). For example, silicones are common in

aerospace industry, due to their low and high temperature performance. In the electronic

field, they are used in electrical insulation, potting compounds and semiconductor devices

due to their low dielectric properties. Their long term durability has made silicone

sealants, adhesives and waterproof coatings in construction industry. Their excellent

biocompatibility makes many silicones well suited for use in numerous personal care,

health care and medical device applications. These extraordinary features of silicone

prompted us to choose it as a polymer of choice for our research study.

Figure 1.4 Applications of silicones

1.2 POLY(DIMETHYL) SILOXANE (PDMS)

Silicones are synthetic polymer with a backbone containing Si-O repeat units and

organic groups attached to the silicon atom via silicon-carbon bonds.

Poly(dimethylsiloxane) (PDMS) is the most common example of synthetic silicone

polymer.

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Silicon chemistry has a long history dating 19th

century as illustrated by

preparation of chloro and hydrosilanes34

. The earliest publications date back to 1863-

1880 by Friedel and Craft and later by Ladenburg with the synthesis of tetraethyl silanes

using the dialkylzinc with silicon tetrachloride35

. In 1885, Polis synthesized aromatic

derivatives via the Wurtz-Fitting reaction36

. In 1904, Kipping utilized the Grignard

reaction to synthesize R-Si-X compounds from tetrachlorosilane. These compounds

hydrolyzed to silanol compounds, which in turn could undergo polycondensation to form

Si-O-Si cyclic or chain compounds. The cyclic monomer could undergo polymerization

or redistribution reactions in presence of strong acid or basic catalysts. Interestingly,

Kipping overlooked the polymeric byproducts and chose to investigate the monomeric

crystalline compounds37

. The new polymers were first named silicoketones or silicones as

an analogy to ketones due to the structural resemblance between them (R2SiO and R2CO).

Above methods for the synthesis of silicones did not reach the commercialization due to

the usage of expensive reagents and low yields. In 1930s and 1940s Eugene G. Rochow

from General Electric Company developed a direct process for the synthesis of

organosiloxanes by the catalytic reaction of elemental silicon with alkylchlorides and

scaled up38-39

. At nearly same time, Richard Gustav Mueller (Chemische Fabrik v.

Heyden), Hyde (Corning glass works) and Andrianov made similar discoveries24

. Since

that time several polysiloxane derivatives were developed to use in wide varieties of

applications.

The term “polysiloxane” derived from the structure of repeating unit in the

polymer backbone (Si-O-Si) with two organic groups bound to each silicon atom as

shown in Figure 1.5. The organic groups connected to Si may include methyl (CH3),

vinyl (CH=CH2), phenyl (C6H5), hydrogen (H), and trifluoropropyl (CF3CH2CH2) groups.

Poly(dimethylsiloxane), where R=R’=CH3 is of the greatest commercial importance due

to their unique molecular properties as described in the Table 1.3.

Depending on the length of the polymer chain and the degree of crosslinking

between the chains, silicones are categorized into fluids, compounds, lubricants, resins

and rubbers. The skeletal structure of typical silicone polymer is formed of four units as

shown in Figure 1.6. The molecular / bulk properties of PDMS can vary depending on the

organic group bound to the Si. The commonly known unique properties of PDMS

include good thermo-oxidative stability, ozone and UV resistance, low glass transition

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temperatures ( Tg ~ -120 o

C), low surface energy, hydrophobicity, transparency to

visible light, high gas permeability and biological inertness.

Figure 1.5 General structure of poly(dimethylsiloxane) [R = CH3]

Table1.3 Properties of PDMS.

Most of these properties result from the inherent chemical structure of PDMS,

where in silicon atom possesses d-orbital which are able to accept electron density from

other atoms40

. This additional p -d interaction gives the Si-O bond its partial double

bond character, along with its shorter than expected Si-O bond length (1.63 Å ) and the

larger than expected (Si-O-Si) bond angle (130 ). The bond angle for Si-O-Si can vary

between 105 ° and 180 ° in organosiloxanes. This greatly exceeds the expected valence

angle of oxygen in sp3-hybridized bond of 109 °. The Si-O bond length of PDMS of 1.63

Å allows for the wider valence angles around the Si-O-Si bond due to interelectronic

repulsion. This leads to greater flexibility along the polymer backbone and a very low

glass transition temperature (Tg) of ~ -120 °C41

. The Si-O bond in siloxanes also has a

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relatively large ionic character of ~40 – 50 % with bond orders between 1.2–1.5.

Polysiloxanes do not inherently have good mechanical strength and hence they must be

chemically cross-linked to form elastomers and reinforced with mineral fillers like silica.

A detailed account on one of the widely used reinforcing fillers used in the preparation of

silicone elastomers, namely silica, is provided in the following section.

Figure 1.6 Skeletal structures of silicone polymer.

1.3 PREPARATION AND PROPERTIES OF SILICA

The chemical formula of silica is SiO2 or SiO2•xH2O, where x is the degree of

hydration on the silica surface. Silica has an infinite three dimensional network structure

in which each silicon atom is bonded to four oxygen atoms and each oxygen atom is

bound to two silicon atoms. Geometrically, each silicon is at the center of a regular

tetrahedron of oxygen. Silica is a solid at ambient temperature with a high melting point

(~1700 °C), a density of 2-3 g cm-3

, and a refractive index of ~1.455. The two major

classes of silica, based on its origin, are natural and synthetic. Naturally occurring silica is

mostly crystalline, generally insoluble in water and exists in a highly ordered geometry.

Common examples of crystalline silica include quartz, cristobalite and tridymite.

Synthetic silica has same basic structure as crystalline silica but lacks a highly ordered

geometry, thus making it an amorphous material.

The two classical methods of making silica include sol-gel method and the micro-

emulsion method. Stober et al 42

had reported a simple process to prepare monodispersed

spherical silica particles using sol-gel method in 1968. Though some improvements were

made at a later stage by others, the Stober’s process is still considered to be the simplest

and the most effective route to prepare mono-dispersed spherical silica particles. Another

widely used process of making mono-dispersed nano-size silica particles comprises the

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controlled hydrolysis of tetraethoxysilane (TEOS) in an inverse micro emulsion, as

reported by Osseo-Asare and Arrigada43

. The amorphous nano-silica particles are usually

prepared on a commercial scale via, either by pyrogenic (or thermal) or wet (or solution

precipitation) processes. While the solution precipitated silica can be in the form of

colloidal silica or silica sols, silica gels (hydrogels, xerogels, and aerogels), precipitates

(formed by the precipitation of silicic acid solutions44

), the pyrogenic silica is mostly in

the form of aerosols, arc silica, and plasma silica. Such amorphous silica in the above

forms is used as a desiccant, adsorbent, reinforcing agent, binder, builder for detergents

and as a catalyst. Water glass is also basic material for production of soluble silicates,

silicon and its alloys, silicon carbide, silanes and silicones.

Among the different synthetic amorphous silica particles, fumed silica and

precipitated silica are the most commonly used reinforcing fillers for the polymers45

.

Fumed silica is a fine, white, odorless, and tasteless amorphous powder. It is typically

manufactured via a high-temperature vapor phase process in which SiCl4 is hydrolyzed in

a flame of oxygen-hydrogen46

. Fumed silica has extremely large surface area and smooth

nonporous surface, which promotes strong physical contact between filler and polymer

matrix. Precipitated silica is manufactured by a wet procedure by treating silicates with

mineral acids to obtain fine hydrated silica particles in the course of precipitation47

and

these are commercially available in the form of a sol with water or other organic solvent

as dispersing medium.

1.3.1 Colloidal silica

Colloidal silica is of particular interest due to the ease of synthesis and precise

control of the size and distribution of particles. Colloidal silica is non-toxic, biologically

and chemically inert, and has excellent durability characteristics and can be

functionalized. Typically, colloidal silica is obtained as the suspensions of fine

amorphous, nonporous, spherical or elongated or pearl shaped particles in a liquid phase.

The colloidal silica particles are of size which is sufficiently small ( 1 m) to be affected

by gravitational forces but sufficiently large ( 1 nm) to show deviation from the

properties of true solutions. In this particle size range, the inter-particle interactions are

mostly dominated by short-range forces, such as van der Waals attraction, London forces

and surface forces. The colloidal silica particles can be prepared in the lab scale by either

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an acid catalyzed reaction48-49

or a base catalyzed (Stöber) reaction42

. The colloidal silica

is often prepared on a commercial scale in a multi-step process where an alkali-silicate

solution is partially neutralized, leading to the formation of silica nuclei. The size of

subunits of colloidal silica particles is typically in the range of 1 to 5 nm. Whether or not

these subunits are joined together mostly depends on the conditions of polymerization.

Initial acidification of a water-glass (sodium silicate) solution yields Si (OH)4. If the pH

is reduced below 7 or if salt is added, then the units tend to fuse together in chains. These

products are often called silica gels. If the pH is slightly on the alkaline side, then the

subunits stay separated and they gradually grow. Because of their small size, the surface

area of colloidal silica is very high. The colloidal suspension is stabilized by pH

adjustment and then concentrated, usually by evaporation. The maximum concentration

obtainable depends on the particle size. For example, 50 nm particles can be concentrated

to greater than 50 wt % solids while 10 nm particles can only be concentrated to

approximately 30 wt% solids before the suspension becomes too unstable. The fine

balance of attractive and repulsive forces between particles could lead to either particle

stabilization or aggregation.

1.3.2 Colloidal silica surface

Colloidal silica has a structure of three-dimensional network with the presence of

several different types of silanols. The number of silanol groups present on the surface of

silica is highly dependent on preparation method and the degree of hydration of the

surface. These silanol groups may exist as pairs or isolated groups with singly, doubly, or

triply linked Si tetrahedra as shown in Figure 1.7. The concentration of silanol groups at

the surface can vary from 2-5 nm-2

, depending upon the levels of surface hydration50

.

High temperature treatments above 450 °C, known to form the particles with much lower

silanol groups at the surface (~ 1 silanol/ nm-2

)51

.

Figure 1.7 Varying silanol groups at the surface of colloidal silica

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The reactivity of the colloidal silica surface in wet media is largely determined

by the pH. Below the isoelectric point (acidic solutions, < pH 2), protonation of the

silanol occurs and the surface acquires a positive charge, acting as an ion exchange

material52

. Silica surfaces are mostly deprotonated above the isoelectric point (> pH 3)

adopting a negative charge, and these surfaces can serve as cation adsorption sites53

.

Accordingly, the surface hydroxyls can be characterized either as weak acids or weak

bases, as depicted in the Figure 1.8.

Figure 1.8 Dissociation reactions at the surface of colloidal

silica at different pH - 1) pKa =1.94 and 2) pKa =7

1.3.3 Surface modification

Surface hydroxyl groups may undergo several chemical reactions that ultimately

yield a functionalized surface54

. The reactivity difference on the silica surface is mainly

arisen due to the non-uniform distribution of hydroxyl functionality. For instance, the

reactivity of an isolated OH can vary from that of a group of silanols perturbed by mutual

hydrogen bonding55

. The chemical means of surface modification is carried out either

with modifying agents or grafting polymers. Silane coupling agents are most widely used

as modifying agents. The general structure of coupling agents can be represented as

RSiX3, where X represents hydrolysable groups (chloro, ethoxy / methoxy) and R

represents the variety of organic functionalities (methyl, phenyl, acryloxy). Typical silane

coupling agents used for surface modification listed in Table 1.4. Grafting of polymer

chains onto silica surface is also an effective method to increase the hydrophobicity.

There are two main approaches for grafting, either via covalent attachment of end–

functionalized polymers or via in-situ monomer polymerization.

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Table 1.4 Typical silane-coupling agents used for surface modification of silica nanoparticles

Surface modification based on physical interaction mainly involves the use of

surfactants and macromolecules which absorbed onto the surface of silica particles. The

polar groups of surfactants / macromolecules are preferentially adsorbed onto the surface

of silica particles by electrostatic interaction, thereby promoting the surface

hydrophobicity. Hydrophobic characteristics of silica surface depend on the functionality

of coupling agents. Among the different coupling agents listed in the Table 1.4, the order

of hydrophobicity decreases with the increasing polar functional groups. The silica

particles surface modified with polar groups are typically used for coating applications, in

which cases surface with reactive functionalities (for example, vinyl, glycidoxy) are used

to improve the dispersion within polymer matrices via reactive compatibilization. The

reaction of alcohols (HOR) with the surface silanol groups of colloidal silica is known to

depend on the size of R group. For instance, while methanol readily reacts with both free

and hydrogen bonded silanol groups56

, t-butanol reacts mainly with isolated silanols57

due

to steric reasons.

1.4 SILICONE ELASTOMERS

Silicone elastomers are composed of cross-linked PDMS which consist of

functional groups such as hydroxyl, vinyl, acetoxy and hydride, either along the backbone

15

or at the chain end. Generally, three different cross-linking (cure) reactions are used to

prepare the silicone elastomers and they are free radical or peroxide curing, the

condensation or sol-gel curing and the addition or hydrosilylation curing. The

comparative characteristic features of these three types of cure reactions are summarized

in Table 1.5.

Silicone elastomers, being based on very thermally stable PDMS, require

incorporation of fillers that are also thermally stable so that they do not detract from the

stability of PDMS. They are normally classified as extending fillers (for example, quartz,

zinc oxide, titanium dioxide, calcium carbonate and iron oxide) and reinforcing fillers

(silica and carbon black). Such fillers are typically introduced in two different ways58-59

.

The first method involves a simple mechanical blending of separately prepared fillers into

the functionalized PDMS which is subsequently cross-linked. The second method is

based on the in-situ generation of silica particles through “sol-gel” hydrolysis of

tetraalkoxysilane and its subsequent condensation60-62

.

Table 1.5 Different types of cure chemistries of PDMS.

Commercially, silicone elastomers are classified into three distinct categories

based on their processing; pourable or room temperature vulcanized silicone rubber

(RTV), pumpable or liquid silicone rubber (LSR), and millable or heat cured silicone

Cure type Reactants Advantages Disadvantages

Free radical cureVinyl end capped PDMS +

hydride end capped PDMS +

organic peroxide catalyst

Easily available.

Lower cost.

Effective regardless of

molecular weight.

Effective even at high

temperature.

Selective cross linking limited.

Oxygen sensitive (exception:

benzoylperoxides)

Peroxide decompostion products

Sensitive to acid and bases.

Require post curing.

Condensation

cure

Alcoxy/acetoxy end capped

PDMS + trifunctional or

tetrafunctional silane + organo

metallic reagents/metal salts

(e.g; tin alkonates)

Procesable at room

temperature.

Byproducts formed during the

reaction is non toxic.

Selective cross-linking.

Stoichimetric amount of

cross-linker used.

Sensitive to relative humidity.

Sensitive to filler chemistry.

Small molecule formed during the

reaction need to be escaped.

Difficult to crosslink high

molecular weight gums.

Addition cureVinyl end capped PDMS/ vinyl

or alkyl functional organic

compounds + hydride end

capped PDMS + platinum or

rhodium catalyst.

Very reactive & require very

low levels (10-20 ppm)

Selective cross linking.

No by-products formed.

Effective even in presence of

moisture.

Expensive & excessive speed of

reaction, needs inhibitors to

moderate the reaction.

Sensitive to amines, tin and sulfur

contaning compounds.

Sensitive to acid and bases.

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rubber (HCR). RTV silicones are cured by condensation or hydrosilylation reaction

using polyfunctional silanes / siloxanes and a catalyst. Silicone elastomers made from

solid HCR or LSR have better mechanical properties than RTV components at similar

loadings of treated fumed silica. Silicone elastomers find applications in cable

insulations, seals and tubing for food and processing equipment, keypads, baby nipples,

pacifiers, shaft seals, anode caps, sparkplug boots, membranes, implants, drug release

carriers, electronic encapsulations, photocopier rolls, molds for replication of art or

denture, window or sanitary seals, adhesives, adhesive controlled release coatings for

paper and thermal-shielding.

1.5 REINFORCEMENT OF ELASTOMERS

Reinforcement of elastomers results in improvement of properties such as

abrasion resistance, tensile strength, tear strength and modulus63 -65

. Since, the tensile

strength of neat crosslinked PDMS is very low (~ 0.35 MPa), it requires reinforcement

with the addition of fillers to exhibit mechanical properties suitable to use in many

practical applications.

At the beginning, various mineral oxide fillers were used to reinforce the silicone

elastomers. However, with the invention of fumed silica and its use as reinforcing filler

in silicone polymer, numerous silicone elastomer composites with varying property

profiles could be made. The extent of reinforcement is usually improved with the increase

in the filler surface area and with the increase of interaction of the PDMS chains with the

filler surface. It is well known that when a mixture of silica and PDMS is blended

together, strong physico-chemical interactions occur at the silica-PDMS interface. These

interactions may be covalent chemical bond resulting from condensation of silanols in

siloxanes with silanols on the silica surface, hydrogen bonds between the silanols in

siloxane and on silica surface, polar and/or van der Waals forces through dipole-dipole

interactions between the silica silanols and the polar Si-O-Si of the siloxane. The use of

precipitated silica along with fumed silica is also widely practiced for making silicone

elastomers. The precipitated silica is usually manufactured as aqueous suspensions and it

is comparatively less expensive and has established potential to be used as secondary

reinforcing filler. Precipitated silica shows slightly less reinforcement compared to fumed

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silica. The precipitated silica based silicone composites exhibit less crepe hardening, have

better compression set, high resilience and low viscosity66

. The literature pertained to

fumed silica and precipitated colloidal silica in vinyl end capped PDMS polymer will be

discussed in detail in the following section. The development of hydrophobic silica via

wet-process has added another dimension to the preparation of silicone elastomers. This

process provides a technology to tailor a silica surface and achieve a specific desirable

elastomeric property profile67-69

. The silicone elastomers with surface modified silica

have been studied for the filler-polymer and filler–filler interactions70-71

. Most widely

used compounds in the present context are alkoxysilanes.

1.6 SILICA AS REINFORCEMENT IN PDMS NETWORK /

SILICONE ELASTOMERS

1.6.1 PDMS-silica composites- A literature overview

The preparation, characterization, properties, and applications of PDMS-silica

composites have been the subject of numerous studies over the last few decades. A

detailed literature analysis was performed using the Sci-finder and engineering village

data bases and with the use of keywords related to silica and polymer nanocomposites.

The keyword search in Sci-finder data base for “silica” resulted in ~ 61888 hits,

consisting of many patents, journal articles and conference proceedings. A further

refinement of above hits in relation to keywords related composites, elastomers and

PDMS led to a segmentation of hits, as shown in the Figure 1.9. The main focus areas of

these prior-arts can be categorized into four sections, as follows:

1. Preparation of silica nanoparticles

2. Various types of surface modification and their characterization

3. Dispersion methods – solution blending and powder mixing

4. Properties of composites and their applications.

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Figure 1.9 Sci-finder search results using different key words.

The main goal of most of above prior arts is to achieve the maximum possible

reinforcement of PDMS with the addition of silica fillers with varying type, size, shape

and surface chemistry. One of the main outcomes from these prior arts is the use of high

surface area amorphous silica particles as reinforcing fillers for making silicone

elastomers. It has also been shown that the effectiveness filler depends on filler

characteristics such as primary particles size, shape, aggregate size, structure, surface

energy, and more significantly, on the strength of polymer-filler interactions72-74

. A

review by Wagner75

on the reinforcing silicas and silicates in organic elastomers provides

a good summary of surface chemistry of silicas. It appears that the process of making of

fumed silica plays critical role in deriving the number of surface hydroxyls. Moreover,

the interaction between silicone elastomers and high surface area hydrophilic silica leads

to high levels of bound polymer and long mill-softening times, often referred as crepe

hardening. The mechanistic investigation of the polymer–filler interaction and the

reinforcement in silicone elastomers reveal the importance of the surface chemistry of

silica as well as the chemical makeup of polymer76-78

. The filler-filler interaction can also

play an important role, including extreme case where the filler particles connect into

network-like arrangements pervading the elastomeric host material79

.

Perusal of the prior arts related to PDMS-composites indicates, however, that

there are only limited studies on the PDMS composites containing colloidal silica as a

reinforcing filler. Most of the prior arts related to PDMS-colloidal silica composites are

patent literatures which describe the use of colloidal silica of varying size, shape and in

different dispersions, to make elastomeric silicone composites used in different

applications such as coating, personal care, emulsions, drug delivery, paints and optical

19

materials. A summary of the key patents related to the use of colloidal silica in patent and

journal literatures are given in the Table 1.6 80-98

and Table 1.799 -106

respectively.

Though the studies on the fumed silica-PDMS composite provide insights on the

property alteration of PDMS composites when the size and the surface chemistry of

fumed silica is varied, these composites exhibit inherent limitations with respect to

handling (fine powder) and processing which include the challenge of uniform dispersion,

high viscosity, and the in-situ surface modification.

Table 1.6 Summary of patent literatures related to PDMS-CS composites

As the colloidal silica with varying particle size, shape is commercially available,

either as aqueous or organic dispersions, it is relatively straightforward to investigate the

structure property relationship exist in PDMS-colloidal silica composites. The other

Technology Properties Application Ref (s)

Siloxane elastomer formation using addition cure chemistry.

Surface modification of colloidal silica using

alkoxy/chlorosilane/hexamethyl disilazane and their dispersion in

alcoholic solvents

Use of spherical silica suspentions in two stage treatement -one to

compatabilize to achieve reinforcement, other with surfactant/

polymers to achieve property of choice (ex: impression)

Different ways of preparation of CS and their dispersion in PDMS

Tensile Strength

High Transparency

Low viscosity

CTE

Tear Strength

Refractive index

Impressions

Optical elements

Optical components

Medical care

Automotive gaskets

Light emitting diodes

81-91

Colloidal silica core modified with polysiloxane contaning

ethylenically unsaturated group used to cure the elastomer with

polyorgano hydrogensiloxane.

Conductive elastomers from electrically conductive fibres in silicone

emulsions-use of CS in water in dispersed state of silcone elastomer

Emulsion process to prepare FCS and dispersion in PDMS

Emulsion resistance

Conductivity

Tensile Strength

Flims

Emulsions

Electronics

Coatings

92-94

Polyorganosiloxane grafting on the collodal silica surface

Transparency Optoelctronic device

Encapsulants 95

The precipitization of aqueous CS by partiall modification and their

dispersion in organic solvent and further dispersion in PDMS to

prepare PDMS-CS composites are disclosed.

Transparency

Tensile Strength

Cathetors

Baby nipples

96

Incorporation of CS into polyorgano siloxane containing nitrogen

compound.

High transparency

High tear strength Personal care 97

Functionalization of CS and their repellency with the water Water repellent Coatings 98

20

practical advantages include the ease of handling and the finite control on the surface

functionalization, as it is available in the form of single discrete particles as against the

aggregates in case of fumed silica. Though basic building blocks of colloidal silica and

fumed silica are similar, the surface chemistry of these two differ significantly, as they

are in different physical states and are prepared by different methods. Hence, it is not

straightforward to extrapolate the properties of fumed silica to that of colloidal silica,

unless proved experimentally.

Table 1.7 Summary of journal literatures related to PDMS-CS composites.

To get a deeper insight into the properties of PDMS-CS composites, the present

research activity is undertaken. This study mainly focuses on developing thorough

understanding on the use of spherical shaped colloidal silica particles, both in hydrophilic

Focus area Key learning Ref (s)

Modification of CS

and their applications

in silicone

elastomers

Modification of aqueous CS with either chlorosilane or disiloxanes in

presence of acid and isopropyl alcohol. The influence of reactive

functionality on reinforcement.

Use of pearl necklaced-shaped CS particles, their modification and

incorporation into PDMS to achieve high mechanicals.

99-100

Modification of CS

and their applications

in coatings

Modification of CS with various different silane coupling agents ( PDMS,

aminosilane, Glycidoxypropyl trimethoxysilane) using various different

methods.

Importance of modification for specific properties ( transparency,

adhesion).

The size and functionality plays critical role in acheiving scrach

resistance properties.

Use of optimal cross links for controlling scach damage.

101-105

Rhelogical behavior

of CS

Influence of aggregation and interfacial modification on the viscosity.

Increase in viscosity with increased CS loading.

CS containing emulsion shows shear-thining behavior with smaller size

and higher concentraction.

106-107

Use of CS for foam

application

Surface characterization and foaming studies were carried out using nine

industrially manufactured, colloidal silica (5-40nm) particles.

Influence of pH, particle size , particle concentration on foamability

charateristics are discussed.

108

Composites without

the modification of

nano -silica

Surfactant free synthesis of polymer-silica nano composites particles has

been achieved in aqueous alcoholic media and in absence of auxiliary

comonomers.

109

21

and hydrophobic states, to make PDMS-CS composites and investigating optical,

rheological and mechanical properties of the resulting PDMS-CS composites.

1.7 OBJECTIVE OF THE PRESENT WORK

The incorporation of silica fillers is known to improve the mechanical properties in

PDMS composites. The main challenges of making such composites are the uniform

dispersion of silica particles within PDMS matrix while adding externally and the control

of particle size when formed in-situ by sol-gel or polymerization methods. The general

objective of the present work is to investigate the various parameters that can potentially

influence the properties of PDMS-CS composites.

The specific objectives are as follows;

1) To investigate the nature and extent of interactions between the hydrophilic

colloidal silica filler and the PDMS with the use of CS of varying size and the

dispersion media.

2) To explore the nature and extent of interaction between the hydrophobic colloidal

silica filler and PDMS with the use of CS of varying size and different surface

functionalization.

3) To arrive at optimal parameters with respect to functionalizing agent, particle size

and dispersion medium, in order to prepare PDMS-CS composites with improved

mechanical and optical properties.

1.8 FUTURE PROSPECTS

The present study is envisaged to further the understanding on the various

parameters critical for making PDMS-CS composites, not only to achieve optimal optical,

mechanical and rheological performances of such composites, but also to develop

fundamental understanding on such nano-composites. By judiciously choosing the

dispersion aid, particle size and surface functional agent, one can potentially overcome

the challenges associated with the uniform dispersion of nano-material within a matrix

material and thereby achieving a significant enhancement in overall performance of the

22

resulting composites. The PDMS-CS composites with diversified performance profiles

find their use in multifarious applications, including automotive (hose, gaskets, boots,

grease, hard coat and tires), aerospace (sealants, coatings and ablatives), consumer

(sealants, tapes, adhesives), health care (nipples, tubing, masks, catheters), electrical and

electronics (connectors, encapsulants, coatings, molding compounds), personal care (hair

treatments, cosmetics) and constructions (glazing, roof and masonry coatings) and marine

(antifouling coating).

1.9 STRUCTURE OF THE THESIS

Chapter-one consisted of a general overview of the prior arts pertaining to PDMS-

silica nanocomposites. The sub-sections within this chapter included the historical

evaluation of polymer composites, an overview of the components involved and the

process of making silicone elastomers and the technology-gap which necessitates a

thorough investigation of PDMS-colloidal silica (CS) composites. The influences of size,

dispersion aids and the surface functionalization of CS on the properties of resulting

PDMS-CS composites are described in the chapters-two to four. The preparation and

evaluation of PDMS-colloidal silica composites based on the optimal parameters thus

identified are detailed in the chapter-five. Specifically, the chapter-two details a study on

the preparation and the characterization of PDMS-colloidal composites with the use of

commercially available aqueous dispersion of “hydrophilic” colloidal silica of different

particle sizes. The chapter-three, describes the use of different dispersing media

employed for dispersing colloidal silica of similar particle size while making PDMS-

colloidal silica composites and their influence on the properties of resulting composites.

The chapter-four deals with the use of different silane functionalizing agents to prepare

the surface functionalized colloidal silica and the preparation and the characterization of

PDMS-functionalized colloidal silica composites. The preparation and the evaluation of

PDMS-colloidal silica composites with optimal performance, based on the research

outputs in terms of particle size, dispersing aid and the surface functionalization from

chapters- two to four, are described in the chapter-five.

23

operations involving the pilot plant and large scale manufacturing of PDMS-CS/FCS

composites. Though the present study is pertained to the dispersion of CS/FCS within

PDMS matrix in an optimal way, the conclusions derived from this study could

judiciously be extended to the preparation of the composites based on various polymers

and fillers.

The results of present study not only provide various parameters that can

influence the mechanical, rheological and optical properties; it also shows as how one can

use CS with and without modification. Most importantly, there is a growing concern on

the usage of nano powders due to their “unknown” effect on environment, especially

human health. The use of nano-particulate dispersion, as investigated in detail in the

present study can be considered as a viable alternative route to the use of nano powder,

for making nano-composites, both in terms of its handling and its impact on the human

health. The PDMS-CS composites of present investigation provide an incredibly large

range of property profiles from which one can judiciously choose, depending on the cost

(aqueous CS, CS in organic solvents without modification), the performance (optical and

high mechanicals using specific organic sols with surface treatments) and the intended

applications which include examples from different segments such as personal care,

health care, construction, consumer, aerospace, automotive, electrical and electronics.

24

1.10 REFERENCES

1. Lin, J. J.; Wu, Z.; Yang, M. Pu. Macromol Rapid Commun, 2001, 6, 422.

2. Gurland, J. Trans Metall. Soc. AIME .1966, 236, 642.

3. XiaO-ming Xu; Xiao-le Tao; Qiang Zheng. Ce Jour of Pol Sci. 2008,145.

4. Pinnavaia, T.J.; Beall, G. Wiley New York, (2001)

5. Vu, Y.T.; Mark, J. E.; Pham, L. H.; Engelhardt, M. J. Appl. Polym. Sci. 2001, 82,

1391.

6. Schmidt, D. F.; Clement, F.; Giannelis, E. P. Adv. Funct. Mater. 2006,16, 417.

7. Kabiri, K.; Zohuriaan-Mehr, M.J. Polym Adv. Technol. 2003, 14, 438.

8. Davenport, D. E. Polymer News 1982, 8, 134.

9. Shen, P.; Sichel, E. K.; Gittleman. J. I. Phys. Rev. Lett. 1978, 40, 1197.

10. RTO Lecture Series, EN-AVT-129, May 2005.

11. Peukert, W.; Schwarzer, H. C.; Gotzinger, M.; Gunther, L.; Stenger, F. Adv. Power

tech. 2003, 14, 411.

12. Cousin, P.; Smith, P. J. Poly. Sci. 1994, 32. 459.

13. Ash, B. J.; Schadler, L. S.; Siegel, R. W.; Apple, T.; Bemicewicz, B. C.; Roger, D. F.;

Wiegand, C. J. Polymer composites, 2002, 23, 1014.

14. Khalfi, A.; Papirer, E.; Balard, h.; Barthel, H.; Heinemann, M. J. Colloid Interface

Sci. 1996, 6, 27.

15. Volkmer, D. Chemie in unsererZeit. 1999, 33, 6.

16. Porter, D. B. Conference proceedings from Organic -inorganic hybrids conference

Guildford, UK. June (2000).

17. Coronado, E.; Galan-Mascaros, J. R.; Gomez-Garcia, C. J.; Laukhin V. Nature. 2000,

408, 447.

18. Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nature 1998, 394, 456.

25

19. Pinnavaia, T. J. Science 1983, 220, 365.

20. Wang, Y.; Herron, N. Science 1996, 273, 632.

21. Winiarz, J. G.; Zhang, L. M.; Lal, M.; Friend, C.S.; Prasad, P.N. J. Am. Chem. Soc.

1999, 121, 5287.

22. Sidorov, S. N. J. Am. Chem. Soc. 2001, 123, 10502.

23. Merkel, T. C.; Freeman, B. D.; Spontak, R. J.; He, Z.; Pinnau, I.; Meakin, P.; Hill, A.

J. Science. 2002, 296, 519.

24. Joly, C.; Smaihi, M.; Porcar, L.; Noble, R. D. Chem. Mater. 1999, 11, 2331.

25. Hajji, P.; David, L.; Gerard, J. F.; Pascault, J. P.; Vigier, G. J. Polym. Sci. 1999, 37,

3172.

26. Sanchez, C.; LeBeau, B.; Ribot.F. J. Sol-Gel Sci. Tech. 2000, 19, 31.

27. Sanchez, C.; Ribot, F.; Lebeau, B. J. Mater. Chem. 1999, 9, 35.

28. Pomogailo, A. D. Russ. Chem. Rev. 2000, 69, 53.

29. Ma, D.; Akpalu, Y. A.;.Li, Y.; Seigel, R. W.;.Schadler, L.S. J. Poly. Sci Part B ,

Polymer Phy. 2005, 43, 463.

30. Nelson, J. K.; Fothergill, J.C.; Dissado, L.A.; Peasgood, W. IEEE Conf. Electr.

Insul. Dielectr. Phenomena, Mexico, 2002, 295.

31 Ash, B. J.; W.Siegel, R.; Schadler, L. S. J. Polymer Sci. B. 2004, 42, 4371

32. Schmidt, D.; Shah, D.; Giannelis, E. P. Current Opinion in Solid State and Materials

Science, 2002, 6, 205.

33. Thostenson, E. T.; Li, C.; Chou, T. W. J. Compos. Sci. Technol. 2005, 65, 491.

34. Colvin, E. W. Silicone in Organic Synthesis: Butterworths: London, (1981).

35. Frieldel C,Crafts, J. M. Comp. Ren.1863, 56, 592.

36. Noll, W. Chemistry and technology of silicones; Academic Press: New York, (1968).

37. Eaborn, C. Organosilicon Compounds; Butterworth Scientific Publications: London,

(1960).

26

38. Rochow, E. G.; Gilliam, W. F. J. Am. Chem. Soc.1945, 67, 963.

39. Rochow, E. G. USA, 2,380,995, General Electric Company, (1945).

40. Voronkov, M. G. In The Siloxane Bond, Consultant Bureau: New York (1987).

41. ChenX, J. A.; Gardella. Macromolecules, 1994, 27, 2206.

42. Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.

43. Osseo-Asare, K.; Arriagada F. J. Colloids Surf. 1990, 50, 321.

44. Vansant, E. F.; Van Der Voort, P.; Vracken, K. C. In Characterization and Chemical

Modification of the Silica Surface; Elsevier: New York 93, (1995).

45. Levresse, P.; Feke, D. L.; Manas-Zloczower, I. Polymer, 1998, 39, 3919.

46. Vassiliou, A. A .; Papageorgiou, G. Z.; Achilias, D. S.; Bikiaris, D. N. Macromol.

Chem. Phys. 2007, 208, 364.

47. Wu, C. L.; Zhang, M. Q.; Rong, M. Z.; Friedrich, K. Compos. Sci. Technol. 2005,

65, 635.

48. De, G.; Karmakar, B.; Ganguli, D. J. Materials Chemistry. 2000, 10, 2289.

49. Karmakar, B.; De, G.; Kundu, D.; Ganguli, D. J. Non-Cryst. Solids, 1991,135, 29.

50. Zhuravlev, L.T. Langmuir. 1987, 3, 316.

51. Yates, D. E.; Healy, T. W. J. Colloid Interface Sci.1976, 55, 9.

52. Hiemstra, T.; Van Riemsdijk, W. H.; Bolt, G. H. J. Colloid Interface Sci. 1989, 133,

91.

53. Hair, M. L.; Hertl W. J. Phys. Chem , 1970, 74, 91.

54. Rong, M. Z.; Zhang M. Q.; Ruan, W. H. Materials Science and Technology. 2006,

22, 787.

55. Shafei, G. M. S. In Silica Surface Chemical Properties; Marcel Dekker: New York,

(2000).

56. Yates, D. J. C.; Dembinski, G. W.; Krell, W. R.; Elliot,J. J. J. Phy. Chem. 1969, 3,

911.

27

57. Azrak, R. G.; Angell, C. L. J. Phy. Chem.1973, 77, 3048.

58. Boonstra, B. B. Polymer. 1979, 20, 691.

59. Rigbi, Z. Adv. Polymer Sci. 1980, 36, 21.

60. Mark, J. E. J. Appl. Polym. Sci., Appl. Polym. Symp. 1992, 50273.

61. Yuan, Q. W.; Mark, J. E. Macromol Chem Phy. 1999, 200, 206.

62. Dewimille, L.; Bresson, B.; Bokobza, L. Polymer. 2005, 46, 4135.

63. Boonstra, B. B.; Cochrane, H.; Danneberg, E. M. Rubber Chem. Technol. 1975, 48,

558.

64. Kohls, D. J.; Beaucage, G. Curr. Opin. Solid St. M. 2002, 6, 183.

65. Heinrich, G.; Kluppel, M.;.Vilgis, T. A. Curr. Opin. Solid St. M. 2002, 6, 195.

66. Okel, T.A.; Waddell, W.H. Proceedings of the 145th ACS Rubber Division Meeting

, Chicago, IL, USA. 44. (1994).

67. Polmanteer, K. E. Rub Chem and Tech. 1981, 54, 1051.

68. Iller, R. K. Polymerization,Colloid and Surface Properties, and Biochemistry, John

Wiley and Sons, New York, USA, 554, ( 1979).

69. Lutz, M. A.; Polmanteer, K. E.; Chapman, H.L. Rubber Chem. Technol. 1985, 58, 5,

939.

70. Southwart, D. W.; Hunt, T. J. Inst. Rubber Ind. 1968, 2, 77.

71 Schatz, M.; Sb Vys.Sk. Chem-Technol, Praz. Org.Technol, 1966, 10, 21.

72. Bokobza, L. Macromol Symp. 2001, 169, 243.

73. Bokobza, L. Macromol Symp, 2001, 171,163.

74. Bokobza, L.; Rapoport, O. J. Appl. Polym Sci. 2002, 85, 2301.

75. Wagner. Rubber Chem. Technol. 1976, 49, 703.

76. Pierce, L. E. S.; Chahal, R. S. J. Polym. Sci. Part C, 1970, 30, 429.

77. K. E. Polmanteer, J. D. Helmer, Rubber Chem. Tech. 1965, 38,123.

28

78. Vondracek, P.; Schatz, M. J. Appl. Polym. Sci. 1977, 21, 3211.

79. Smith, T. L. Rubber Chem. Technol. 1978, 51, 225.

80. Torto, M. D.; Howe, F.; Pouchelon, A.; Pusineri, C. US6762242B1, Rhodia Services,

(2004).

81. Campbell, J. R.; Rubinsztajn, S.; Anostario, J.; Michael. US2004/0138343A, General

Electric Company, (2004).

82. Watanabe, H.; Ige, H.; Yanagase, A. US593947,1 Mitsibishi Rayon Co.,Ltd., (1999).

83. Burns, G. T.; DeGroot, Jr. J. V.; Hahn, J. R.; Harrington, D. A.; Moore, P. A. US

6051672, Dow corning (1998).

84. Yoshitake, K.; Yokoyama, T. US6025455, Nissan Chemicals Industries, Ltd., (2000).

85. Chao, T.; Chi, S. US 2004/0067361, Dow Corning Corporation, (2004)

86. Burns, G. T.; Hahn, J. R.; Degroot, J. V. Jr.; Harrington, D. A.; Moore, P. A. EP

0982268 B1, Dow Corning Corporation, (2003).

87. Sweet, R. P. US4427801. Dow Corning, (1984).

88. Schuster, J.; Muller, H.; Vorbuchner, H.; Maier, A.; Pradl, F. US 5057151, Wacker-

Chemie GmbH, (1991).

89. Cole, W. M.; Tartamella, T. L.; Naik, S. D.; Hergenrother, W. L.; Bohm, Georg G. A.

US 6579929, Bridgestone Corporation, (2003).

90. Takanashi, M.; Matsumoto, M. US5916687, Toshiba Silicone Co.,Ltd

91. Graiver, D.; Kalinowski, R. US4545914A, Dow Corning Corporation

92. Borza, I.; Horvath, C.; Farkas, S.; Nagy, J.; Kolok, S. US2009012118,

93. Chao, T.; Chi, S. US20040068047A1, Dow Corning Corporation

94. Krohberger, H.; Burkhardt, J.; Patzke, J. US4008198, Wacker-Chemie GmbH.

95. Kaijou, A. US5651921, Idimitsu Kosan Company, (1994).

96. Kwan, K.S.; Harrington, D.A.; Moore, P.A.; Hahn, J.R;. Degroot, J.V.; Jr., Burns,

G.T. Rubber Chem Technol. 2001, 74, 630.

29

97. Timothy, C. C.; Debora, F. B. Mat.Res .Soc. Sym Proc. 2003, 778.

98. Castaing, J. C.; Allain, C.; Auroy, P.; Auvray L.; Pouchelon, A. Europhys. Lett.

1996, 36, 153.

99. Douce, J.; Boilot, J.-P.; Biteau, J.; Scodellaro, L.; Jimenez, A. Thin Solid Films

2004, 466, 114.

100. Chu, L.; Daniels, M. W.; Francis, L. F. Chem of Mate. 1997, 9, 2577.

101. Pham, K. N. J. Colloid Interface Sci. 2007, 315, 123.

102. Gellermann, C.; Storch, W.; Wolter. H. J. Sol-gel Sci. Technol. 1997, 8,173.

103. Julian, O.; Abdeslam, E. H.; Geraldine, C.; Jacques, J.; Francois, B. Polymer. 2004,

46, 6695.

104. Huang, H.; Bo, Y.; Shuxue, Z.; Limin, W. J. Colloid Interface Sci. 2007, 310,

105. Irena, B.; Robert J P.; John Van De Pas.; Ian, C. J. Colloid Interface Sci. 2007,

313, 645.

106. Percy, M.J.; Amalvy, J. I.; D.P. Randall, D. P.; Armes,S. P. Langmuir, 2004, 20,

2184