1

CHAPTER 2

PREPARATION AND PROPERTIES OF

POLY(DIMETHYL)SILOXANE – AQUEOUS

COLLOIDAL SILICA NANOCOMPOSITES

2.1 INTRODUCTION

Colloidal particles have widely been used in various industrial products such as

inks, paints, coatings, papers, cosmetics, photographic films and rheological fluids.

Though the use of colloidal silica (CS) was well studied by Schwein1 and further by

Griessbach2 in the early twentieth century, the main issue was their large-scale

commercial availability in the form of stable sols. Continued efforts by various groups on

the process developments opened newer applications of sols in materials, chemistry and

biology. In 1951, Bechtold and Snyder3 patented a process which comprised of making

stable sol and removing sodium from sodium silicate solution by ion exchange and

growing particle to a desired size while concentrating the sols by evaporation, to produce

stable transparent sols containing 30 % of silica as uniform particles 10-15 nm in

diameter. Further developments resulted CS with various unique variations such as size

(3 nm to 100 nm), shape (spherical, elongated, pearl, rod), modified (silane, aluminate,

polymer) and unmodified tailorable (cationic, anionic and electrolyte free) surfaces

created new dimension in nanotechnology. This has expanded their application spectrum

according to the purpose the silica serves, as shown in Table 2.14-14

. The refinement in the

process and the constantly changing demands resulted in new types of commercial silica

sols. To get the insight of different grades, some of major commercial manufacturers are

listed in Table 2.2. Though some of these products have been referred in earlier technical

publications, they are no longer be available given the fast change of products to meet

ever changing industrial needs.

Most of these commercial sols are available in either aqueous or organic sols with

varying particle sizes and shapes and with 10- 50 % by weight of silica. The pH of these

sols is usually derived from the type of stabilizers used. Commercial colloidal silica sols

can be broadly classified into different categories depending on surface charge (anionic,

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cationic, silane modified or electrolyte free), pH (acidic or basic) and dispersion (aqueous

and organic). Hence unlike the amorphous silica, colloidal silica has lot of variations in

size, shape, dispersions and tailorable surface, which can give different spectrum of

properties if it is used optimally.

Table 2.1 Applications of CS according to their function and properties.

Though CS is very well explored in ceramics and metals for their adhesive,

binding, stiffening and corrosion resistant properties, their use in polymers as reinforcing

fillers are not explored to their maximum potential. Most relevant literatures in polymers

are focused towards the coating applications as CS brings in improvement of abrasion

Function Properties Applications

Abrasion

Mechanical and chemical properties

(Tribology, scratch & corrosion resistance, adhesion)

Paints

Lacquers

Hard coats

Frictionising

Antiblocking

Mechanical, Chemical and optical properties

(Tribology, hardness, scratch resistance, corrosion

resistance, adhesion, gloss finish, antireflection, transparency,

hydrophobic/hydrophilic)

Paper packaging

Textiles

Floor wax and Polishes

Metal coatings

Plastic films

Silicon wafers

Polymer emulsions

PrintabilityMechanical and Wetting properties

( Adhesion, hydrophilic/hydrophobic)

Printing

Ink Jet

Textiles

Binding

Stiffening

Mechanical, chemical ,thermal and optical properties

(Tribology, hardness,insulation, corrosion resistance,

hydrophobic/hydrophilic, and gloss)

Ceramics

Catalysts carrier

Foundry

Catalyst

Refractory

Metals

FlocculationChemical and wetting properties

(Coagulation, hydrophobic /hydrophilic, anitifoam)

Beverages and

Water treatment

Ceramic fibers

Cross-linking

Flow modifier

Mechanical, rhelogical, optical and chemical properties

(Hardness, tear, transparency, Gels, thickners)

Elastomers

Leather

Latex-foam

Paints

Coating

Density gradient

Mechanical and rheological properties

(Separation, purification, gelling, coagulation)

Heathcare

Centrifuging

Chromotography

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resistant/corrosion resistant/adhesion/flow/binderError! Bookmark not defined.,Error! Bookmark not

defined.. In comparison to the extensive literature on the PDMS composites containing

“treated” hydrophobic amorphous silica, only a limited knowledge exists with respect to

the PDMS composites containing CS.

Table 2.2 Major commercial manufacturers of CS and the characteristics of CS.

The use of silica in hydrophilic state both in case of amorphous and colloidal

silica is not explored in detail. The main challenge in case of amorphous silica (for

example fumed silica) being the non-wettability, which restrict the loading of filler more

than few percentage. The other issue includes difficulty in ensuring stable dispersion at

nanoscale and handling (exposure to fine powder). This opens up a question whether

colloidal silica can be used in place of well-known fumed silica for reinforcing PDMS

both in hydrophilic and hydrophobic states. Hence, a fundamental study to understand the

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reinforcing capability of colloidal silica is undertaken, as such study can potentially open

up a new dimension to the nanotechnology research field.

In this present chapter, commercially available aqueous hydrophilic colloidal

silica of three different particle sizes (20±5 nm, 30±5 nm and 60±5 nm), are used to make

PDMS-CS composites. Prior to cross-linking, the properties of the CS filled PDMS

composites are studied for their thermal and rheological characteristics. Reinforcing

capability of CS is inferred from the tensile properties, tear strength and Shore A

hardness characteristics of cured PDMS-CS composites. Particle dispersion in the

composites is inferred by analyzing scanning electron micrographs (SEM) and

transmission electron micrographs (TEM) and optical properties. Further, the effect of

addition of alcoholic solvents along with aqueous dispersion of CS on the properties of

resulting composites has also been investigated.

2.2 EXPERIMENTAL

2.2.1 Materials and Methods

Aqueous dispersions of colloidal silica of various particle sizes were purchased

from Nalco Chemicals, USA or received as a free sample from Nissan Chemicals, USA.

Details of the CS used in the present study are presented in Table 2.3. Various

components used for making cross-linked PDMS such as vinyl end capped PDMS with

Table 2.3 Characteristics of aqueous dispersion of colloidal silica used in the present study

the average molecular weight of ~ 65000 µ, hydride functionalized PDMS cross-linker

with molecular weight of ~2800 µ, the hydrosilylation catalyst (chloroplatinic acid) and

the inhibitor (ethynylcyclohexanol (ECH)) were procured from Momentive Performance

Materials, Inc. Typical chemical structures of vinyl-PDMS, hydride-PDMS and inhibitor

are depicted in Figure 2.1. Different solvents such as isopropyl alcohol (99.7 %), 1-

Sample IDParticle size

(nm)

SiO2

(wt %)

Viscosity

(mPs.s at 25°C)pH Appearance

CSA 20± 5 30 <3 2 -4 Clear

CSB 30± 5 40 <3 2 -4 Clear

CSC 60± 5 40 <3 2 -4 Opalescent

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methoxy-2-propanol and xylene (> 98.5 %) used during the preparation of composite

were purchased from Aldrich chemicals and used without any further purification.

Figure 2.1 Chemical structures of vinyl-PDMS, hydride-PDMS and inhibitor used in present study

2.2.2 Characterization of colloidal silica

2.2.2.1 Solid content analysis

The percentage of solid analysis was carried out using Mettler Toledo make, HG

53 Halogen Moisture Analyzer. Colloidal silica dispersed in solvent (0.2 g) was placed in

the form of small drops onto the spherical aluminum pan which was subsequently heated

to 150 °C and kept at that temperature until a constant value is attained.

2.2.2.2 Fourier Transform Infrared Spectroscopy (FT-IR)

The IR measurements were performed using KBr technique. The samples were

prepared by mixing finely ground dried CS powder and with analytical grade KBr into a

thin pellet. The pressed pellets were characterized in transmission mode in the range of

4000-400 cm-1

with a spectral resolution of 4 cm-1

and 64 scans using a Perkin Elmer

FTIR spectrometer.

2.2.2.3 Transmission Electron Microscopy (TEM)

The average size and the size distribution of CS particles were inferred via TEM

measurements performed with a Techai G2 instrument. The digital image acquisition was

performed using a Gatan Model 791 side mount camera. In a typical measurement, a drop

of 1 % silica dispersion (after sonication) was placed onto a forward grid and excess

liquid was blotted on a filter paper prior to the measurement.

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2.2.2.4 Thermo-gravimetric Analysis (TGA)

TGA measurements were carried done by using a TGA 2950 from TA

instruments. Thermal stability of CS and PDMS-CS composites were inferred by heating

the samples placed in a platinum pan under nitrogen atmosphere from 25 C to 700 C at

a heating rate of 10 C min-1

.

2.2.2.5 Elemental analysis

Elemental analyzer was used to infer the elemental composition, specifically

carbon content of silica fillers. In a typical measurement carbon element is converted to

its respective oxides by combustion in oxygen atmosphere. The resultant gases were

separated in a gas chromatographic column (Porapak-Q) and detected by thermal

conductivity detector (TCD).

2.2.3 Characterization of PDMS-colloidal silica composites

2.2.3.1 Rheological properties

Rheological properties of uncured PDMS base composites were inferred using an

ARES II strain controlled Rheometer from Rheometric Scientific, in the dynamic

frequency mode. In a typical rheological measurement, the uncured sample was loaded

between two disks (diameter = 25 mm) separated by a distance of 1.5 mm and the

experiments were conducted from an initial frequency of 0.1 rad/sec to a final frequency

of 100 rad/sec with dynamic strain of 2 %, at a fixed temperature of 24 °C.

2.2.3.2 Fourier Transform Infrared Spectroscopy (FT-IR)

The quantitative FTIR measurements of cured sheets (thickness = 2 mm) of

composites were performed by keeping the sheet in the sample holder. The measurements

were done in the range of 4000-400 cm-1

with a spectral resolution of 4 cm-1

and 64 scans

using a Perkin Elmer FTIR spectrometer in the transmission mode.

2.2.3.3 Scanning Electron Microscopy (SEM)

The distribution of CS particles in PDMS matrix was inferred with a Field

Emission Gun (FEG) Environmental Scanning Electron Microscope (ESEM), from

Philips.

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2.2.3.4 Transmission Electron Microscopy (TEM)

The transmission electron microscope (TEM) was used to analyze the

morphological features of PDMS-CS composites. The sample preparation for TEM

analysis involved cutting and blocking of the part of the cured sheet using the scalpel.

The blocked sample was then faced and microtomed at -140 ˚C using a Leica Ultracut

microtome to obtain 100 nm sections. The resultant sections were collected on a copper

grid of 400 mesh and the images parallel to flow direction of representative areas were

taken at different magnifications.

2.2.3.5 X-ray diffraction (XRD):

The change in crystallinity of PDMS with the incorporation of CS was inferred

through powder XRD measurements, performed on a Panalytical X’Pert Pro X-Ray

diffractometer (C Cu Kα, λ = 0.154 nm). Instrument voltage was set to 40/40 KV/mA and

beam size on the sample was fixed during entire scan at 3.00 mm. Data was collected by

Panalytical X’cerelator detector system from angle (2 theta) 2 to 10°. Average d-spacing

was obtained by the peak position of the basal peak in the XRD scan.

2.2.3.6 Mechanical properties

The mechanical properties such as the tensile properties (at 100mm/ min cross

head speed) and the tear strength were measured using Instron instruments, adapting

protocols specified in DIN 53504 and ASTM D624, respectively. The hardness of the

cured PDMS composites was measured by using a Shore-A Durometer, following DIN

53505 method. The results reported are average values obtained with at least five test

specimens.

2.2.3.7 Optical properties

Transmission is the measure of the amount of light that passes through the sample

and haze is the scattering of visible light as it passes through a transparent material. The

BYK Gardner Spectrophotometer was used to measure the percent transmission (% T)

and the haze. After specifying the illuminant, observer angle, and the reference color, the

test sample (2 mm thick sheet) inserted into the specimen holder and the

spectrophotometer reading was recorded. An average of three readings is reported.

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2.2.4 Preparation of PDMS-colloidal silica composites

2.2.4.1 Equipment used for the preparation of PDMS composite base

Dispersion of colloidal silica in PDMS was carried out in a Ross double planetary mixer

(DPM) kettle (Figure 2.2). The Ross DPM is designed to mix a variety of thick heavy and

hard to mix products efficiently with specially designed blades. The Ross DPM used for

the present work possesses in-built design features such as vacuum line, nitrogen gas line

and two ports for charging the reactants.

Figure 2.2: Double planetary mixing system used to prepare the PDMS composites

2.2.4.2 Method for preparation of PDMS-colloidal silica composites

Cured PDMS composites investigated in the present study were prepared using a

two-step process. The first step involved the preparation of composite base and the

second step comprised of curing of composite base with addition of an inhibitor, a cross

linker and a catalyst at elevated temperature. The composite base (100 g) was prepared by

mixing aqueous dispersion of colloidal silica (133 g with 40 g equivalent solid)

dispersion with PDMS fluid (60 g) at room temperature. The CS dispersions were mixed

in portions (30 ml during every 10 min) to ensure homogeneous mixing. Subsequently,

the contents were heated to a temperature of 80 °C with concomitant application of

vacuum for an hour, to remove the water. The temperature of the kettle was further raised

to 110 °C and kept at that temperature for 60 min under vacuum (10 mbar) to remove

trace of water. The contents were cooled to room temperature, prior to the start of second

step. In the second step, the composite base was mixed with required amounts of hydride

functionalized PDMS cross-linker (0.48 g), ethynylcyclohexanol (30 mg) and platinum

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catalyst (10 ppm), in a kitchen blender. It is essential to remove any air bubbles from the

mass with the application of vacuum, prior to curing it in a compression-molding

machine. The curing of the PDMS-CS composite into the sheets (~ 2 mm thick) in the

compression-molding machine was done at 180 °C by applying 90 kN pressure for a

period of 10 min. Using above procedure, PDMS composites consisted of various

loadings (30 wt %, 20 wt % and 10 wt%) of colloidal silica were also prepared (Table

2.4). To understand the influence of the addition of alcoholic solvents, the aqueous

dispersion of CS was mixed with alcoholic solvents such as 1-methoxy 2- propanol and

isopropanol in 1:1 ratio with respect to water present in the dispersion. For the purpose of

comparison a control sample in which the CS dispersion was diluted with water in the

same ratio as alcoholic solvent was also made. The alcohol/water diluted CS dispersions

were subsequently used for preparing PDMS-CS composites. The sample code and the

loading levels of PDSM-CS are provided in Table-2.4.

Table 2.4 Sample codes and the details of composite formulation

2.3 RESULTS AND DISCUSSION

2.3.1 Analysis of commercial aqueous colloidal silica

Prior to the preparation of PDMS-CS composites, the colloidal silica dispersions

were analyzed for their solid content in terms of % solids. The solid content analysis was

Samle

name

PDMS

( wt %)

CS

(wt %)

Neat PDMS 100 0

CSA 10 90 10

CSA 20 80 20

CSA 30 70 30

CSA 40 60 40

CSB 10 90 10

CSB 20 80 20

CSB 30 70 30

CSB 40 60 40

CSC 10 90 10

CSC 20 80 20

CSC 30 70 30

CSC 40 60 40

CSB40 water 90 10

CSB40 IPA 80 20

CSB40 MP 70 30

10

performed by placing 0.3 g of CS dispersions at 150 °C temperature for 3 min and the

amount of solid present in the dispersions were found to be ~ 31 % for CSA and ~ 40 %

for CSB and CSC respectively.

The size and distributions of CS particles was inferred through Transmission

Electron Microscopic (TEM) analysis of diluted CS dispersion (1 %) as shown in Figure

2.3. From TEM analyses, it is evident that all the CSA has particle size ~ 28 nm with

small distribution, whereas CSB and CSC have large distributions with average particle

size of ~ 38 nm and ~ 58 nm, respectively.

Figure 2.3 TEM analyses of commercial aqueous colloidal silica dispersions.

The surface functionalities of CS particles were inferred through FTIR analysis. The

Figure 2.4 shows the FTIR spectral features of CSA, CSB and CSC. All the samples

showed the vibration absorption peaks between 950-1100 cm-1

and ~ 785 cm-1

. While the

peaks in the range of 950-1100 cm-1

are assigned to Si-O-Si asymmetric stretching

modes, the peak at 785 cm-1

can be assigned to both symmetric Si-O-Si stretching and

bending vibrations15

. The broad peak at 3000-3800 cm-1

indicates the -OH stretching

vibrations of Si-OH and possibly residual water present along with CS. The peak at950

cm-1

(which is merged with a broad vibration absorption peak at 950-1100) is assigned

for residual Si-OH of CS particles. However, the area under the curve in the region of

3049-3652 cm-1

which is assigned to OH stretching vibration is found to vary with

varying particle sizes. The area under the curve is found to be in the order of

CSA=CSB>CSC.

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Figure 2.4 FTIR analysis of commercial colloidal silica powder.

Further, the elemental analyses with dried samples of CS provided the information

on the surface functionalization, if any, of CS particles. As seen in the Table 2.5, the

percentage carbon values are mostly below the detection level, indicating that CS

particles used for present work are not surface functionalized. The percentage hydrogen

found to be highest for CSA (0.29 %) followed by CSB (0.23%) and CSC (<0.1%). The

results are in line with FTIR indicating the presence of more hydroxyls on the smaller CS

particles. Similarly, the thermal stability of dried powders of CS of different particles was

inferred using the TGA and DTGA measurements as shown in Figure 2.5. As shown in

the figure, a two-step weight loss is evident. The first degradation occurred between 25

°C to 125 °C and second step weight loss is from 200°C to 700°C. The observed weight

loss can be related to the weight loss of physically adsorbed water and hydrogen bonded

water from the surface of the CS

BDL=below detection levels

Table 2.5 Percentage carbon and hydrogen analysis of dried CS particles.

particles. The varying degree of weight loss indicates the presence of different type of

surface hydroxyls on the CS with different particle sizes.

CSA CSB CSC

% Carbon BDL BDL BDL

% Hydrogen 0.29 0.23 <0.1

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Figure 2.5 TGA and DTGA curves of CS of different particle size.

2.3.2 Effect of particle size on thermal and rheological properties of

uncured composites

2.3.2.1. Thermogravimetric analysis of uncured composites

Figure 2.6 illustrates thermogravimetric analysis (TGA) and differential thermo

gravimetric analysis (DTGA) curves for the uncured CS- PDMS composites containing

two different loadings (20 and 40 wt %) of hydrophilic colloidal silica having different

particle sizes (CSA, CSB and CSC). As shown in the figure, the main thermal

degradation of neat PDMS occurred at ~ 460 C, as evidenced from a sharp weight loss.

The two-step thermal degradation of pure PDMS in inert atmosphere occurs by

depolymerization over the range 400-650°C. The intra and intermolecular

rearrangements, which occur through cyclic transition states16-17

leads to the elimination

of cyclic dimethylsiloxane and shortening of the residual chain length until it is too short

to cyclize. This process can be associated with the first step degradation of the neat

PDMS, with maximum weight loss rate temperature at 400 °C (Tmax). Following this step,

the second weight loss occurs at 610 °C and it can be related to the evaporation of the

shortened residual linear fragments of the PDMS chains, which competes with the

cyclization. Irrespective of type of CS used both the on-set degradation temperature (Td)

and the char residue at elevated temperatures (> 650 C) are found to increase with the

addition of hydrophilic CS to PDMS and such trend is similar to that reported in the

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Figure 2.6 TGA & DTGA curves of uncured PDMS-CS composites.

literature for poly(methylmethacrylate) based hybrid material made with colloidal silica18

.

This behavior can be related to a change in the mechanism of thermal degradation of the

PDMS in the presence of CS. The adsorption of thermal energy19

and barrier effect of the

CS particles on the diffusion of volatiles are reasoned to the shift of the main weight loss

processes to higher temperatures, as compared to the neat PDMS. However, for a given

loading, the increase in Td is more significant for the composite which contained

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colloidal silica of lower particle size. Accordingly, the increase of Td for different

PDMS-CS composites follows the trend: CSA > CSB > CSC. The increased Td in case of

CSA and CSB could be due to the barrier effect and heat sinking capability coming from

the strong hydrogen bonding due to relatively large number of silanols present at the

surface of CSA and CSB particles, as compare to those present on the CSC. The char

residue is found to vary slightly for different PDMS composites and such variations can

be rationalized to a difference in the dispersion of silica in the chosen samples.

2.3.2.2 Rheological characteristics

Figure 2.7 depicts the change in viscosities of uncured PDMS-colloidal silica

composites with varying loading of colloidal silica of three different particle sizes. The

figure shows viscosity of PDMS found to increase with the addition of CS, but magnitude

of increase is found to vary with the size of CS particles20

. An almost similar viscosity

noted for uncured PDMS-CS composites at 10 wt % loading of CS, irrespective of

particle size of CS. However, as the loading of CS increases, the magnitude of viscosity

enhancement found to be very significant in case of composites with smaller particle

(CSA) as compared to ones with larger particle (CSB and CSC). In cases of both CSB

and CSC, though a linear increase of viscosity is observed up to 30 wt. % loadings, the

viscosity is found to increase abruptly at

Figure 2.7 Rheology data of uncured PDMS-CS composites containing CSA, CSB, and CSC.

40 wt. % loading. The observed results are in accordance with Guth Gold equation

pertained to composites consisted of ideal spheres, in which case the viscosity

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enhancement mostly depends on the particle size and the interaction between the filler

and the polymer. To probe the microstructures and the colloidal interactions, the

composites with 40 wt % CS of three particle sizes are studied further for their elastic

(G´) and viscous modulus (G˝) behavior. As evidenced from the Figure 2.8, for all the

three systems, the viscous modulus (G˝) exceeds the elastic modulus G´ over the entire

frequency range and both moduli depend strongly on the frequency. Such dynamic

rheology response is characteristic of a system showing purely viscous behavior

(structureless sol), consisting of non-flocculated units21

. Interestingly G´ and G˝ in CSA

and CSB shows linear trend and almost overlap at all frequency range, whereas CSC

shows different trend, where in G˝ is more closer to G´ at lower frequency indicating a

trend moving toward the elastic range. The least change in the viscosity at lower

frequencies and the difference between G´ and G˝ in case of CSC at high frequency is an

indication of relatively higher polymer–filler interaction as compare to filler-filler

interaction which typically leads to a higher viscosity22

as observed in case of CSA and

CSB.

Figure 2.8 G´ and G˝ of uncured PDMS-CS containing CSA, CSB and CSC PDMS composites.

16

2.3.3 Effect of particle size on structure and morphological properties

of cured composites

2.3.3.1 FTIR characterization of PDMS composites

To understand the possible interactions of surface groups of hydrophilic CS with the

matrix PDMS, the cured sheets of colloidal silica–PDMS composites are analyzed by

FTIR in the transmission mode. FTIR spectra of PDMS composites containing 40 wt %

of different types of colloidal silica (CSA40, CSB40 and CSC40) are shown in the Figure

2.9 along with the spectrum of neat PDMS. The vibration absorption peaks observed at

2962 and 2872 cm-1

are assigned to the CH asymmetric and symmetric stretching

vibrations of CH3 of PDMS. The broad vibration absorption peak observed at ~ 3500 cm-

1 which is assigned to OH stretching vibrations of silanol groups on the colloidal silica is

found to increase with smaller particle size and such observation can potentially be

rationalized to increased interaction of surface hydroxyl groups of CS with PDMS

backbone, through H-bonding between the –OH group present at the surface of CS and

Si-O-Si of PDMS. A similar peak broadening is reported in cases where there is a change

in the fluctuation of Si-O-Si linkages23

. The observed variation in the splitting pattern of

the peak at 1100 cm-1

is probably rationalized to the strained Si-O-Si linkage due to its

interaction (disorder-induced coupling) with surface –OH groups of the hydrophilic

colloidal silica24-25

.

Figure 2.9 FTIR spectra of PDMS-CS composites with 40 wt % loading of CS of different particle sizes.

17

2.3.3.2 Powder XRD of PDMS composites

Figure 2.10 shows the powder X-ray diffraction (XRD) patterns observed for cured

PDMS composites containing 40 wt % CS of different particle sizes. The XRD pattern

observed for neat cured PDMS is also shown in the figure, for comparison. The broad

diffraction peaks (at 2 ~ 12 ˚ and a broad hump at 2 ~ 23 ˚) observed for neat cured

PDMS is due to the presence of a few crystalline phases, dispersed in mostly amorphous

PDMS matrix. Such crystalline phases are formed due to the ordered packing of small

segments of PDMS chains, as reported earlier26-27

. In comparison, no significant change

in the diffraction pattern is evident for PDMS-colloidal silica composite, except in the

case of composite containing smaller particle size colloidal silica (CSA). The observed

similarity of diffraction pattern between the neat and composite PDMS could indicate that

the addition of 40-wt % of hydrophilic colloidal silica into PDMS matrix did not affect

the crystalline phases of the PDMS.

Figure 2.10 X-ray diffraction patterns of cured PDMS composites containing 40 wt % of CS with different

particle sizes.

2.3.3.3 Microscopic analyses of PDMS-CS composites

2.3.3.3.1 Transmission Electron Microscopic analysis of cured composite

The TEM images of PDMS composites containing 40-wt % of different types of

colloidal silica are shown in Figure 2.11. The analyses of the images are indicative of the

presence of relatively larger aggregates in the case of composites containing smaller

colloidal silica particle (CSA) and relatively a good dispersion in the case of composite

18

(a) (b) (c)

Figure 2.11 TEM micrographs of cured PDMS-colloidal silica composites containing (a) CSA40

(b) CSB 40 and (c) CSC40.

with larger colloidal silica particle (CSC). It is well known that secondary structures such

as aggregates and agglomerates are formed via non-covalent interaction such as hydrogen

bonding between silanol groups on the silica particles28-30

. The formation of larger

aggregation in the case of composites with smaller CS particles can possibly be

rationalized to the larger number Si-OH groups on the surface resulting in more filler-

filler interactions as compared to particle-polymer interaction. Similarly, the improved

dispersion observed in case of composites with larger CS particles (CSC) can be due to

the lower number of surface silanols resulting mostly in polymer-particle interactions as

compared to particle-particle interactions. As indicated in the prior art, the difference

observed with composites containing CS particles of different sizes can, in general, also

be related to different extent of hydrogen bonding. Hence the difference observed with

composites containing CS particles of different sizes can also be related to different

extent of hydrogen bonding between different types of surface silanol groups (geminal,

vicinal and isolated), as well as different surface curvature of CS particles resulting in

different degree of interfacial interactions.

2.3.3.3.2 Scanning Electron Microscopic analysis of cured PDMS-CS composites

Further to substantiate the observations made by TEM, the cured composites were

analyzed by SEM, as shown in the Figure 2.12. The SEM results are similar to those

obtained from TEM analyses; where in PDMS composites with CSC shows a relatively

good dispersion of CS with less aggregation or agglomeration, as compared ones with

CSA and CSB.

19

Figure 2.12 SEM micrographs of cured PDMS-CS composites containing (a) CSA40 (b) CSB 40 (C) and

CSC 40.

2.3.4 Effect of particle size on optical and mechanical properties of

PDMS-CS composites

2.3.4.1 Optical characteristics of PDMS-CS composites

The optical characteristics (% transmittance) of cured PDMS-CS composites with

varying loading of different CS particles are depicted in Figure 2.13. The %

Figure 2.13. Optical properties of cured PDMS-colloidal silica composites.

transmittance of the neat cured PDMS is also shown in the figure for comparison.

As shown in the figure, irrespective of the type of the filler, % transmittance (%T) is

found to decrease with the increasing loading of CS. The decrease of % T is found to be

relatively more for the composites containing smaller CS particles (CSA) and such

20

observation is attributed to more aggregated structures of CS particles as evidenced from

microscopic analyses of the composites (section 2.3.3.3).

2.3.4.2 Mechanical properties of PDMS-CS composites

The mechanical properties, namely, tensile properties, hardness and tear strength

of cured PDMS-CS composites with varying loading of CS particles of different sizes, are

summarized in Table 2.6. As evidenced from the table, irrespective of the type of CS

particles, an improved mechanical properties (tensile strength, % elongation, hardness

and tear strength) is evident for the CS filled PDMS composites as compared to those

observed for neat PDMS. Among different PDMS composites, the mechanical properties

observed for the composites with smaller CS particles (CSA) are lower, as compared to

those observed with composites containing larger particle sizes (CSB & CSC). The

observed less significant improvement of mechanical properties for the composites with

smaller CS particles (CSA) can possibly be attributed to the more agglomerated structure

of CS, as it is evidenced from microscopic and optical property analysis. A similar

mechanical properties profile observed with CSB and CSC could be due to the similar

Table 2.6 Summary of mechanical properties of CSA, CSB, and CSC filled PDMS composites.

particle curvature of CSB and CSC. As the particle size increases, the curvature of

the spherical particle become more and more wide, due to which the particle starts to

acquire more hydrophobic characteristic, which in turn results in different surface

energies31

.

21

The reinforcement is a surface phenomenon and it results from both

chemisorptions and the physisorption of filler onto the polymer matrix. The physisorption

is due to the interaction of surface –OH groups of colloidal silica with the PDMS chain

segments. A small variation in number of silanol groups, the surface curvature of

particles and the resulting surface energies can lead to different levels of physisorption of

CS onto PDMS in PDMS-CS composites. The ultimate particle-particle and particle–

polymer variation may also get stabilized after certain particle size range, as evident from

only a small difference in mechanical properties of composites with CSB and CSC.

2.3.5 Effect of co-solvent on rheological properties of PDMS composites

Figure 2.14 depicts the change in the viscosities of uncured PDMS-CSB

composites prepared by dispersing CSB with additional alcoholic (isopropanol (IPA and

1-methoxy 2-propanol (MP)) solvents. As compare to the PDMS-CSB composite, the

PDMS-CSB-IPA and PDMS-CSB-MP exhibits a higher viscosity indicating higher

polymer-filler interactions with the use of later. These results appears to indicate that the

addition of solvents improve the interfacial interaction of CS with PDMS matrix. Among

Figure 2.14. Effect of solvent on Rheology of uncured PDMS-CSB 40 composites.

two different alcoholic solvents, the use of MP leads to a higher degree of interaction

which in turn results in a better dispersion and a higher viscosity of the composite. The

observed trend can also be understood in terms of miscibility of alcoholic solvent with

DPMS matrix.

22

2.3.6 Effect of co-solvent on morphological properties of PDMS

composites

2.3.6.1 TEM analysis

TEM analyses of cured sheets of PDMS-CSB-MP and PDMS-CSB-IPA, as

shown in the Figure 2.15 indicate an improved dispersion of CSB with the use of IPA or

MP as a co-dispersant. This can possibly due to the surface passivation of the colloidal

silica via reaction of surface Si-OH of silica with IPA and MP, which in turn leads to

improved compatibility with PDMS during the processing. Among the two alcoholic

solvents, the use of MP seems to lead to better dispersion, possibly due to the closer

solubility parameter (17.6) with PDMS (14.6) as compare to IPA (23.4).

Figure 2.15 TEM micrographs of cured PDMS-colloidal silica composites containing (a) CSB-40, (b)

CSB-40-MP and (c) CSB-40-IPA.

2.3.7 Effect of co-solvent on optical and mechanical properties of

PDMS composites

2.3.7.1 Influence of co-solvent on the optical properties of PDMS composites

Similar to the trend observed in case of morphological characteristics, an

improved optical properties (%T) (Figure 2.16) are evident for the composites obtained

by dispersing CS along with alcoholic solvents (CSB-40 IPA and CSB-40- MP). Though

the magnitude of increase is arguably minimal, it still underlines the importance of

dispersing CS along with suitable co-dispersion aid such as alcoholic solvents.

23

Figure 2.16 Optical properties of different PDMS-CSB-40 composites.

2.3.7.2 Influence of co-solvent on mechanical properties

The cured sheets of CSB-40-IPA and CSB-40-MP possessed improved tensile

strength and % elongation, as compared to neat PDMS composites with CSB-40 (Figure

2.17). The magnitude of increase is found to be different for two different alcoholic co-

dispersants. The CSB-40-IPA composites exhibits ~ 60% increase in tensile strength,

while CSB-40-MP showed ~87% increase in tensile strength, thus indicating an enhanced

polymer-filler interaction which in turn result in better reinforcing ability as compare to

the composites prepared with the use of neat aqueous dispersion of CS. The scanning

electron microscopic images of tensile fractured surfaces are shown in Figure 2.18. Large

Figure 2.17 Mechanical properties of PDMS-CSB-40, PDMS-CSB-40-IPA and PDMS-CSB-40-MP

composites.

aggregate pull-out failure in case of neat CSB40 indicates poor dispersion of CS particles.

On the other hand, in both CSB40-IPA and CSB40-MP samples, a uniform dispersion of

24

CS particles and a thin coating of PDMS on to the CS particles indicating good polymer-

filler interaction.

Figure 2.18 SEM micrographs of tensile fracture surface of PDMS-CSB-40, PDMS-CSB-40-IPA and

PDMS-CSB-40-MP composites.

2.4 CONCLUSIONS

Poly(dimethyl)siloxane (PDMS)-colloidal silica (CS) composites are prepared,

using aqueous dispersions of colloidal silica (CS) of different sizes. The resultant

composite are investigated for their rheological, optical and mechanical properties. The

results indicate a significant aggregation of CS particle via strong filler-filler interactions.

The extent of filler-filler interactions seems to vary with varying particle sizes. The use of

larger CS, in general, leads to the composites with better bulk properties as compared to

ones with smaller CS particles. As evidenced from the present study, it is possible to

prepare PDMS–hydrophilic CS composites even with 40 wt % loadings, with a

reasonably good mechanical and optical properties. It has also been shown that the

performance of PDMS-CS composites can be improved with the use of co-dispersant

such as alcoholic solvents, while dispersing CS into PDMS. The present results underline

the importance of choice of dispersing medium while preparing PDMS-CS composites

without ex-situ or in-situ filler treatment.

Having understood the importance of choice of dispersing medium from the

present study, it has been decided to investigate the influence of dispersing medium in

determining the properties of resulting PDMS-CS composites in detail. The result of such

investigation is given in the chapter three.

25

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