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Liquid Phase Exfoliation of Graphite to Graphene & its Applications in Polymeric Nanocomposites By Khalid Nawaz School of Chemical and Materials Engineering (SCME) National University of Sciences and Technology (NUST) 2015
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Liquid Phase Exfoliation of Graphite

to Graphene & its Applications in

Polymeric Nanocomposites

By

Khalid Nawaz

School of Chemical and Materials Engineering (SCME)

National University of Sciences and Technology (NUST)

2015

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Liquid Phase Exfoliation of Graphite

to Graphene & its Applications in

Polymeric Nanocomposites

Name Reg. No

Khalid Nawaz 2008-NUST-TfrPhD-Em-E-08

This work is submitted as a PhD thesis in partial fulfillment of the

requirement for the degree of

(PhD in Energetic Materials Engineering)

Supervisor Name: Dr. Noaman Ul Haq

School of Chemical and Materials Engineering (SCME)

National University of Sciences and Technology (NUST)

H-12 Islamabad, Pakistan

September, 2015

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Thesis Submission Certificate It is to certify that work in this thesis has been carried out by Mr. Khalid Nawaz and

completed under my supervision in School of Chemical and Materials Engineering,

National University of Sciences and Technology, H-12, Islamabad, Pakistan.

Supervisor: ______________

Dr. Noaman Ul-Haq

Assistant Professor

National University of Sciences and

Technology, Islamabad

Submitted through

Principal SCME National University of Sciences and Technology, Islamabad

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DEDICATION

Dedicated to all those scientists & engineers who sacrificed their

lives for the welfare of humanity

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ACKNOWLEDGEMENTS First of all I am greatly thankful to my Almighty Allah, the most Beneficent, the most

Merciful, the Supreme Power and Creator of this Universe and Who enabled me to

complete my research work. Thousands of blessings be upon Hazrat Muhammad

(Peace be upon him), who is the reason for the creation of this universe.

I would like to thank my supervisor Dr. Noaman Ul-Haq for his advisement,

encouragement and guidance during the course of this research. I would also like to

thank Dr. Muhammad Mujahid (Principal, SCME), Dr. Arshad Hussain (HoD,

Chemical engineering department) and Dr. M.B. Khan for their guidance and support

during my research work.

I will never forget the great and out of the way cooperation of Professor Dr. Jonathan.

N.Coleman and Dr. Umar Khan of Trinity College Dublin (TCD), Dublin, Ireland.

They helped me to great extent to complete most of my project work at School of

Physics in TCD, Ireland. I feel delight in the company of Peter May, Harshit Porwal,

Sweta Bansali and Amro (of chemistry department). It has been an immense pleasure

and privilege working with everyone and being part of such a diverse and intelligent

group of talented individuals at TCD.

In particular, I would like to express my gratitude to my colleague, Muhammad Ayub

who helped me a lot to complete the thesis writing, formatting & compilation by

sparing his precious time.

I would also like to thank my family members for their encouragement and support

while I was working on this thesis.

Finally, I cannot conclude these acknowledgments without recognizing the financial

support of my department and Higher Education commission (HEC) of Pakistan

through International Support Initiatives Program (IRSIP) without which I will not be

able to complete my research work at TCD, Ireland and NUST.

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Abstract

Nanocomposites are superior to conventional one in terms of its mechanical

performance. Pristine /functionalized graphene sheets (FGS) were incorporated into a

range of model polymers. Solvent aided blending was adopted for better dispersion of

FGS and graphene sheets in these polymers. Graphene was added to selected

polymers like polyurethane (PU), poly (vinyl chloride) (PVC), poly (acrylonitrile)

(PAN), poly (vinyl alcohol) (PVA) and poly (vinyl acetate) (PVAc) in order to

improve the mechanical performance of these materials. Different forms of graphene

nanosheets like pristine/FGS with different lateral dimensions were selected in order

to study its effects on the mechanical performance of selected polymers in terms of

young’s modulus, tensile strength and elongation at break. Graphene nanosheets were

functionalized with octadecylamine and were incorporated in polyurethane and it was

observed that 2.5 vol% is the mechanical percolation level for this polymer as above

this loading there was improvement in the mechanical performance of polyurethane

while at this loading the elongation at break was suffered slightly. Similarly in case of

poly(vinyl chloride) a critical loading(1.5wt%) was observed at which there was

improvement in mechanical properties of these polymers and almost no elongation at

break was observed for this loading and the modulus determined in this case was

superior to calculated from Halpin-Tsai equation. Two type of graphene nanosheets

with different flake size (one and 3.5 micron) were incorporated in poly

(acrylonitrile). Its comparative study was conducted it was observed the big flake

improved the performance of polymers in terms of modulus and UTs while the

response of small flake in terms of elongation at break was better than big flakes.

Large area graphene oxide were synthesized and were introduced to poly(vinyl

alcohol) and the role of these nano fillers were very pronounced in terms of modulus,

UTS and elongation at break was not disturbed but slightly improved.

Graphene flakes were studied through transmission electron microscopy(TEM) and

Raman spectroscopy while dispersion of these flakes were in selected polymers was

confirmed by scanning electron microscopy(SEM) and the mechanical performance of

these nanocomposites were conducted on Zwick-Roell tensile tester. Graphene-based

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polymer nanocomposites can be a new versatile soft material with numerous

advantages.

Graphite was exfoliated to graphene using NMP and water as solvent as well.

63mg/ml concentration was obtained during tip sonication in NMP while in case of

water as media the maximum concentration obtained was 7mg/ml using sodium

cholate as surfactant. The concentration of graphene nanosheets were studied through

UV-visible spectroscopy while quality of flakes was studied through TEM and Raman

spectroscopy.

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Table of Contents

Certificate

i

Dedication ii

Acknowledgement iii

Abstract iv

Table of Contents vi

List of Figures x

List of Tables xv

List of Acronyms xvi

1.1 Overview and history of graphene and graphene-based materials 1

1.2 Graphene-based polymer nanocomposites 5

1.2.1 Overview and historical perspective 7

1.3 Preparation methods of nano-composites 8

1.3.1 Non-covalent dispersion methods: solution and melt mixing 8

1.3.2 Non-covalent/ in- situ polymerization 9

1.3.3 Graphene-based composites with covalent bonds between

matrix and filler

11

1.3.4 Other methods for composite preparation 12

1.4 Graphene synthesis 13

1.4.1 Growth in-situ on a substrate 14

1.4.2 Bottom up methods to synthesize graphene from organic

precursors.

16

1.4.3 Chemical efforts to exfoliate and stabilize graphene sheets in

solution

18

References 23

2.1 Introduction 32

2.2 Materials 32

2.3 Apparatus and Equipments 32

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2.4 Characterization Techniques 33

2.4.1 UV-Vis Absorption Spectroscopy 33

2.4.2 Raman Spectroscopy 34

2.4.3 Tensile testing (TT) 36

2.4.4 Scanning Electron Microscopy 39

2.4.5 Transmission Electron Microscopy 42

References 45

3.1 Objective 47

3.2 Introduction 47

3.3 Experimental Details 49

3.3.1 Concentrated dispersion of graphene. 49

3.3.2 Second Method for extremly high concentration of graphene 51

3.4 Charetarization 52

3.4.1 Transmission Electron Microscopy (TEM) Histogram 52

3.5 Size selection of graphene Flakes according to its lateral dimensions 54

3.5.1 Experimental Details 54

3.6 Results and Discussions 55

3.6.1 Concentration study 55

3.6.2 Raman Spectroscopy 57

3.6.3 Transmission Electron Microscopy 58

Conclusion 61

References 61

4.1 Objective 65

4.2 Introduction 65

4.3 Experimental Procedure 67

4.4 Results and Discussion 68

Conclusion 76

References 76

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5.1 Objective 80

5.2 Introduction 80

5.3 Experimental Procedure 81

5.4 Results and Discussion 83

Conclusion 90

References 91

6.1 Objective 93

6.2 Introduction 93

6.3 Experimental Procedure 95

6.4 Results and Discussion 96

Conclusion 105

References 105

7.1 Objective 108

7.2 Introduction 108

7.3 Experimental Procedure 110

7.4 Characterization of PVC-Graphene Composites

111

7.5 Results and Discussion

111

Conclusion 116

References 116

8.1 Objective 119

8.2 Introduction 119

8.3 Experimental Part 121

8.3.1 Composites Preparation and Characterization 121

8.4 Results & Discussions 122

Conclusion 129

References 129

9.1 Objective 132

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9.2 Introduction 132

9.3 Experimental Section 135

9.4 Characterization 136

9.5 Mechanical characterization 137

9.6 Results and discussion 137

Conclusion 146

References 147

10.1 Summary of main work 153

10.2 Suggestions for future work 156

References 156

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List of Figures

Fig No. Figure Title Page No.

Fig. 1.1 (A) Schematic of graphene on a SiC substrate (B) E-k

diagram of graphene grown on SiC displaying the band

opening at the Dirac point and (C) An ARPES intensity map,

displaying the band gap.

15

Fig. 1.2 Schematic of the roll based transfer of graphene films grown

on Cu foil

16

Fig. 1.3 (A) Structure of the hexabenzocoronene (HBC) (B) Structure

of the largest polyaromatic hydrocarbons synthesized to data containing 222 C atoms

18

Fig. 1.4 GO structure demonstrating the distorted sp2 atomic

arrangement and attached functionalities (left) and vial of GO brown dispersion (right). Its brown colour is attributed to the

absence of π conjugated structure.

19

Fig. 2.1 Raman Shift 36

Fig. 2.2 Typical stress-strain curves. 38

Fig. 2.3 Schematic of SEM showing the electron path 41

Fig. 2.4 Detector positions in Zeiss Ultra / Supra 42

Fig. 2.5 Schematic of TEM showing the electron path 44

Fig. 2.6 TEM images of a 5 layered graphene flake. These TEM

images demonstrate how the lengths, widths and count the number of layers were measured.

45

Fig. 3.1 Absorbance versus wavelength curve for 1st cycle using (A) sonic

bath (B) sonic tip 49

Fig. 3.2 Change in concentration with time for 1st step using (A) sonic tip

and (B) sonic bath 50

Fig. 3.3 Absorbance versus wavelength curve for 2nd cycle using (A) sonic

tip for 6hrs (B) sonic tip for 10hrs (C) sonic bath 50

Fig. 3.4 Change in concentration with time for 2nd cycle using (A) sonic tip

for 6hrs (B) sonic tip for 10hrs (C) sonic bath

51

Fig. 3.5 (A) Change in concentration with sedimentation of 64mg/ml

(B) sample (B) Comparative study of sedimentation of 20,30 and

45mg/ml graphene concentartion dispersions

52

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Fig. 3.6 (A) Comparative study of average length, width and no of layers

for 20mg/mL (B) 30mg/mL (C) 45mg/mL and (D) 63mg/mL

dispersion

53

Fig. 3.7 Schematic diagram of size separation of graphene flakes

55

Fig. 3.8 Concentration of graphene exfoliated by tip sonicator and

centrifuged at various speed (rpm). 56

Fig. 3.9 Concentration of graphene exfoliated by bath sonicator and

centrifuged at various speed (rpm)

56

Fig. 3.10 Change in ID/IG ratio of graphene exfoliated by (A) sonic tip, (B)

sonic bath with speed (rpm) of centrifuge 57

Fig. 3.11 Increase in the Defect peak of normalized Raman spectra with

increasing rpm (A) for sonic tip (B) for sonic bath 59

Fig. 3.12 TEM images of graphene layers observed (a) un-separated

graphene dispersion for sonic tip (b) 500rpm sonic tip (c) 1000rpm

sonic tip (d) 3000rpm sonic tip (e) un separated graphene

dispersion for sonic bath (f) 500rpm sonic bath (g) 1000rpm sonic

bath (h) 3000rpm sonic bath.

60

Fig. 3.13 (A) shows the change in the average values of length, width and

number of layers with change in rpm for sonic tip sample (B) sonic

bath sample

60

Fig. 4.1 Concentration of graphene after centrifugation (500/45) as a

function of sonication time (Cs=5mg/ml). Concentration was calculated using absorption coefficient “α” value equal to 3.62 ml/mg/m

69

Fig. 4.2 Concentration of graphene after centrifugation (500/45) as a

function of sonication time (CSs=10mg/ml). Concentration

was calculated using absorption coefficient “α” value equal to

3.62ml/mg/m.

70

Fig. 4.3 TEM images of graphene flakes deposited from sample having

concentration of 7mg/ml.

71

Fig. 4.4 Histogram showing (A) the number of layers per flake

measured for 96 hours sonication time (B) average length of flakes and (C) the average width of flakes

72

Fig. 4.5 A

& B

(A) SEM images of the flakes present on the interface of the free standing films prepared (B) SEM image showing the fractured interface

73

Fig. 4.6 Increase in Defect peak (D-band) of normalized Raman

spectra as a function of sonication time

74

Fig. 4.7 Change in ID/IG as a function of sonication time 75

Fig. 4.8 Estimated length of graphene flake with D/G values. 76

Fig. 5.1 FTIR spectra of GO (top), ODA (middle) and GO-ODA

(bottom).

83

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Fig. 5.2 SEM images of 40 wt% composite film. 84

Fig. 5.3 Representative stress strain curves. Inset: the low strain

regime.

85

Fig. 5.4 Effect of GO-ODA content on mechanical properties of

composites. (A) Young’s modulus, (B) stress at 3% strain, (C) Ultimate tensile strength, (D) strain at break.

87

Fig. 5.5 The same data as in figure 6.4 but plotted as a function of

GO-ODA volume fraction on a log-log plot. The lines in (A)

and (B) illustrate percolation-like behavior while the line in

(C) illustrates linearity. The vertical arrows illustrate the

percolation threshold while the horizontal arrows show that

value of each property displayed by the polymer.

88

Fig. 6.1 (A) Large numbers of multilayer graphene deposited on a

holey carbon TEM grid. (B) Individual graphene multilayer. (C) Photograph of PVAc−graphene films with mass fractions

of 0%, 0.2%, 0.4%, 0.7%, and 1.5% (volume fractions from

0−0.8%). SEM image of (D) a PVAc and (E) a

PVAc/graphene fracture surface

97

Fig. 6.2 Stress−strain curves for the PVAc/graphene composite film

studied in this work. (inset) Stress−strain curves on a log−log scale. The dotted line represents linearity.

98

Fig. 6.3 Mechanical properties of PVAc films. (A) Young’s modulus,

(B) ultimate tensile strength, and (C) strain at break, as a

function of graphene volume fraction.

99

Fig. 6.4 Measurements of adhesive properties of PVAc/graphene glue.

(A) Photograph of samples used for adhesive testing. (left) Two T-shaped wood pieces glued together for tensile testing.

(right) Two wooden bars, glued together along an overlapping

region (dashed line), for use in shear measurements. (B)

Photograph of T-shaped pieces during a tensile test. (C)

Applied stress plotted as a function of displacement in both

tensile and shear modes for samples glued using homemade

PVAc adhesive. (D) Tensile and shear bond strength and (E)

toughness as a function of graphene content for the

homemade PVAc adhesives. (F) Tensile stress−strain curves

for as-bought commercially available glue and same with 0.7

wt % graphene added. (G) Tensile and shear bond strength

and(H) toughness as a function of graphene content for the

adhesives prepared with commercially available PVAc glue

The dotted lines represent the untreated glue. The data points

represent the glue, diluted and re-concentrated during the

process of graphene addition.

103

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Fig. 7.1 TEM images of graphene nano flakes exfoliated in NMP 112

Fig. 7.2 Effect on Young’s Modulus of PVC after using graphene

nano-flakes

114

Fig. 7.3 Effect on UTS of PVC after using graphene nano-flakes 114

Fig. 7.4 Effect on Elongation at Break of PVC after using graphene

nanoflakes

115

Fig. 7.5 Comparison of Theoretical and Experimental values of

Young’s Modulus

115

Fig. 8.1 Ratio of Raman d-g bands measured on films prepared from

size selected dispersion as a function of final centrifugation rates.

123

Fig. 8.2 Raman spectra of graphene thin film, of selected size flakes

prepared after different centrifugation rate (rpm).

123

Fig. 8.3

(A) & (B)

(A)TEM images of graphene flakes separated by

centrifugation at 500 rpm. (B) TEM images of graphene

flakes separated by centrifugation at

5500 rpm.

124

Fig. 8.4

(A) & (B)

(A) Histograms of flakes length of graphene in DMF peparated at 500 rpm. (B) Histograms of flakes length of graphene in DMF separated at 5500 rpm.

125

Fig. 8.5 Effect of 1 micron () and 3.5 micron (♦) nano-fillers

(graphene) incorporated in PAN polymer on Young’s modulus.

126

Fig. 8.6 Effect of 1 micron () and 3.5 micron (♦) nano-fillers

(graphene) incorporated in PAN polymer on Ultimate Tensile Strength (UTS).

127

Fig. 8.7 Effect of 1 micron (♦) and 3.5 micron () nano-fillers

(graphene) incorporated in PAN polymer on Elongation at break.

128

Fig. 9.1 FTIR spectra of Graphene oxide. 137

Fig. 9.2 TEM Image of graphene oxide. 138

Fig. 9.3 Histogram of Length of graphene oxide nano flakes 138

Fig.9.4 SEM images of LAGO dispersion in PVA 139

Fig. 9.5 Effect of LAGO on modulus of PVA 140

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Fig. 9.6 Effect of LAGO on Tensile strength of PVA 141

Fig. 9.7 Effect of LAGO on elongation at break of PVA 142

Fig. 9.8 Comparison of Theoretical and experimental data of

Young’s modulus

144

Fig.9.9 DSC of PVA and LAGO based nanocomposites (0.35 wt %) 145

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List of Tables

Table No. Table Caption/Title Page No.

Table 1.1 Mechanical Properties of Graphene/Graphite based Polymer

Nano-Composites

6

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List of Acronyms

2D Two Dimension

BSE Back Scattered Electron

CNTs Carbon Nano Tubes

CVD Chemical Vapour Deposition

DMF Dimethyl Formamide

EG Expanded Graphene

EM Electromagnetic

EVA Ethylene Vinyl Acetate

FGS Functionalized Graphene Sheets

FTIR Fourier Transform Infrared Spectroscopy

FTIR Fourier Transform Infrared Spectroscopy

GNR Graphene Nano Ribbons

GNRs Graphene nanoribbons)

GO Graphite/Graphene Oxide

GO-OD Graphene Oxide Octadecylamine

GPa Giga Pascal

HBC Hexabenzo Coronene

HDPE High Density Polyethylene

HeNe Helium Neon

HOPG Highly Oriented Pyrolytic Graphite

In Indium

LAGO Large area graphene oxide

LDH Low Density Hydrocarbon)

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LPE Liquid Phase Exfoliation

MPa Mega Pascal

MWCNTs Multi Walled Carbon Nano Tubes

NMP N-Methyl Pyrolidinone

ODA Octa Decyl Amine

PA6 Poly Amide

PAHs Polyaromatic Hydrocarbons

PAN Poly(acrylonitrile)

PE-g-MA Polyethylene Grafted Maleic Anhydride)

PET Poly(ethylene terepthalate)

PI Polyimide

PMMA Poly(methyl methacrylate)

PP Poly Propylene

PS Polystyrene

PSS Polystyrene Sulfonate

PVA Polyvinyl Alcohol

PVAc Polyvinyl Acetate

PVC Poly Vinyl Chloride

RPM Round Per Minute

SAED Selected Area electron Diffraction

SDS Sodium Dodecyl Sulfate

SEM Scanning Electron Microscopy

SiC Silicon Carbide

STM Scanning Tunneling Microscopy

SWNT Single Walled Carbon Nano Tubes

TEM Transmission Electron Microscopy

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THF Tetrahydrofuran

TPa Tera Pascal

TPU Thermoplastic polyurethane

UHV Ultra High Vacuum

UTS Ultimate Tensile Strength

UV/Visible Ultra Violet/Visible Spectroscopy

Xe Xenon

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Chapter 1

Introduction

1.1. Overview and history of graphene and graphene-based

materials

Graphene was once believed to be an academic material [1] a thermodynamically

unstable one that upon isolation would crumple up on itself. However, this didn’t

stop some scientists investigating how thin they could actually make graphite

planes. The search for graphene started and for the last about forty years the

graphene is under study by scientific community [2-3]. Graphene is a “monolayer of

sp2-hybridized carbon atoms arranged,” in “a two-dimensional lattice”, has

attracted “tremendous attention in recent years owing to its exceptional thermal,

mechanical, and electrical properties” [4-6]. The “in-plane elastic modulus of

pristine, defect-free graphene is approximately 1.1 TPa and is the strongest material

that has ever been measured on a micron length scale [5,6].” Graphene also

demonstrates brittleness [7] readily folds and can be stretched up to 20% more than

any other crystal [8] similarly “GO-derived fillers” can exhibit, “high moduli

(reported values ranging from 208 GPa [9] to over 650 GPa [10]) and can be easily

functionalized to tailor their compatibility with the host polymer.” The refrence

“values of stiffness of GO derived filler materials can be higher than those reported

for nano clays” [11] but “generally lower than those reported for single-walled

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carbon nanotubes (SWNTs)” [12]. However, “the intrinsic mechanical properties of

SWNTs may be comparable to those of pristine graphene” [12,13]. Moreover, “the

two-dimensional platelet geometry” of graphene and graphene based “materials

may offer certain property improvements that SWNTs cannot provide”when

“dispersed in a polymer composite, such as improved gas permeation resistance of

the composite” [14]. The in-plane stiffness for “chemically modified graphene

(CMG) platelets” is lower and decreases with “increasing level of oxidation of the

platelets” [15]. However in a study conducted on CMG platelets using AFM nano

indentation “reported the opposite, the elastic modulus of the platelets evidently

increasing” “with increasing oxidation level, ranging from 250 GPa for reduced

graphene oxide (RGeO) platelets” up to approximately 650 GPa for graphene oxide

(GeO) platelets [10]. These superior mechanical properties with large aspect ratio

of graphene and graphene derived materials made them potential candidate as

reinforcement in polymeric systems [16]. When dispersed in polymer the thin

sheets transform into wavy or wrinkled structure which loses its modulus value, as

wrinkled structure unfold instead of stretching under applied stress [17]. Some time

incomplete exfoliation or restacking of nanosheets also lower modulus due to

decreased aspect ratio [16]. “One of the most promising applications of this

material is in polymer nanocomposites polymer matrix” which incorporates nano

scale fillers as reinforcement. “Nanocomposites with exfoliated silicate fillers have

been investigated in 1950 [17] but significant academic and industrial interest in

nanocomposites came nearly forty years later” when Toyota Motor Corporation

demonstrated significant improvement in mechanical properties of polymer Nylon-

6 by using montmorillonite as filler [18]. Bunnell “proposed the production of

polymer nanocomposites incorporating as thin as possible” GNPs (“derived from

GICs exfoliated either by shear grinding or thermal treatment”) as fillers in a 1991

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[19], where it has been suggested that with “10 vol% inclusion of graphite flakes in

polyethylene or polypropylene, the stiffness of the finished product will approach

that of aluminum.” A detailed report published in 2000 which explains the

chemistry of nanocomposites based on exfoliated graphite with 10 nm of thickness

which was produced during in-situ polymerization of caprolactam [20].

Tremendous properties improvements have been reported “versus conventional

polymer composites based on micron-scale fillers such as untreated flake graphite

or carbon black” (CB) at low loading [21-27].

Nanofiller usually tends to agglomerate which could become factor for aspect ratio

reduction which ultimately diminishes its reinforcing role [28], while some

researcher reported that large scale aggregate may be beneficial for enhancement of

mechanical performance of system [29,30]. For “effective reinforcement Strong

interfacial adhesion between the platelets and polymer matrix” is also responsible

[31-35]. Apart from this dispersion phenomenon, the two phases (filler and

polymer) should be compatible with each other; otherwise it may also become a

factor for modulus reduction of composite due to low interfacial adhesion matrix-

filler interface.[36] In order to have composite with superior mechanical properties

the matrix-filler interface should be intelligently tailored.

Functionalization of graphene and graphene based materials is selected route to

tailor the interface in order to improve adhesion between filler and polymer either

covalently or non-covalently [14]. “Hydrogen bonding between GO-derived fillers”

and “their matrix has been reported” to be responsible factor for the improvement in

“modulus and strength observed in several polymers that can serve as hydrogen

bond acceptors or donors” [22,36-38]. Stress transfer at interface can be improved

by covalent bonding between graphene oxide and matrix [35] Just at 0.1 wt%

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loading of graphene oxide to Nylon-6 nanocomposites the modulus improved 100%

as compared to neat Nylon-6 it may be due to in situ step-growth polymerization

between functional groups of polymer and GeO [39]. Likewise improvement in

mechanical properties of, epoxy, polyurethane, Polystyrene (PS), poly methyl

(methacrylate) (PMMA) and PVDF composites “with GNP, GO-derived fillers and

polymer-grafted CMG respectively, has been reported”. The “formation of covalent

bonds between matrix and filler is suggested to be responsible for this improvement

in mechanical properties”[14,40-47].

Improvements in reinforcement might be unequivocally influenced if host polymer

and polymer grafted to nanofiller have same chemical nature and with relative

molecular weight this behavior is specially studied in polyurethane modulus gain

[14,43,45,46,48-51]At 55 wt% GNP Increases in modulus from “approximately 10

MPa to 1.5 GPa” have been reported in polyurethanes, yet ductility was retained to

the level of rigid thermoplastic (e.g., polycarbonate) [49] . It has been shown by

calculations that randomly oriented graphene nanofiller give better mechanical

performance than randomly oriented nanotubes while in case of aligned nanofiller

the result of CNTs is better than graphene nanosheets [52]. Similarly it has also

been studied that the mechanical properties obtained as a result of exfoliated

graphene/graphene derived materials are better than GNP based system it may be

due to high aspect ratio and high modulus of former materials [53-58]. The

improvement in reinforcement may be due to native properties of filler which was

observed during comparing experimental results with theoretical e.g., “Mori-

Tanaka and Halpin-Tsai models”, one [31,59].

The results calculated by these models have been compared with graphene based

nanocomposites and was observed that reinforcement at low loading surpass the

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predicted values based on these micro-mechanical models [22,60-63]. The apparent

“discrepancy between these results and theory highlights the need to develop

further understanding of the relative” contributions of native “filler properties and

changes in the polymer matrix in regards to the reinforcement of these systems”.

1.2. Graphene-based polymer nanocomposites

The development of a nano-level dispersion of graphene particles in a polymer

matrix has opened a new and interesting area in materials science in recent years

[79]. These Nano-hybrid materials show considerable improvement in properties

that cannot normally be achieved using conventional composites or virgin

polymers. The extent of the improvement is related directly to the degree of

dispersion of the nano-fillers in the polymer matrix. The most important aspect of

these nano-composites is that all these improvements are obtained at very low filler

loadings in the polymer matrix. Different types of nano graphite forms, such as

expanded graphite and exfoliated graphite, have also been used to produce

conducting nano-composites with improved physicochemical properties. There are

many studies on expanded and exfoliated graphite composites based on a range of

polymers, including epoxy, polymethyl methacrylate, polypropylene, low density

polyethylene, high density polyethylene, polystyrene, Nylon, Polyaniline,

phenylethynyl-terminated polyimide, and silicone rubber. Table 1.1 lists the

percentage enhancement in the mechanical characteristics, such as the tensile

strength at break, storage modulus and flexural strength of

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Matrix

Filler

type

Filler

loading

(Wt.%a,

vol.%b)

Process

%

Increase

%

Increase

TS

% Increase

flexural

strength

Epoxy

EG

EG

EG

EG

1a

1a

1a

0.1a

Sonication

Shear

Sonication and Shear

Solution

8

11

15

-20

-7

-6

87

PMMA

EG

GNP

21a

5 a

Solution

Solution

21

133

PP

EG

xGnP-1

xGnP-15

Graphite

3b

3b

3b

2.5 b

Melt

Melt

Melt

SSSP

60

8

26

8

LLDPE

xGnP

Parrafin

coated

xGnP

15a

30

Solution

Solution

200

22

HDPE

EG

UG

3a

3a

Melt

Melt

100

33

4

PPS

EG

S-EG

4a

4a

Melt

Melt

-20

-30

PVA

GO

Graphene

0.7a

1.8b

Solution

Solution

76

150

TPU

Graphene

Sulfonated

Graphene

5.1b

1a

Solution

Solution

200

75

PETI

EG 5a

10a

In-situ

In-situ

39

42

Table1.1 Mechanical Properties of Graphene/Graphite based Polymer nano-Composites

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1.2.1. Overview and historical perspective

Graphene reported Young’s modulus is between 0.5 - 1. TPa with ultimate strength

130 GPa, so, it is thought that graphene is an excellent candidate for mechanical

reinforcement of polymer in the area of nanocomposites. To this end, there is

significant research in which graphene has been added into a variety of polymers to

make nanocomposites, with varying level of success [22,56-57,62,64].

Interestingly, besides the mechanical properties of graphene, there are two

additional stiffening mechanisms for graphene and graphene derivative

nanoparticles to stiffen certain polymer matrices. With hydrogen bonding, graphene

oxide (GO) generally interacts with polar polymers and this leads to apparently

superior mechanical reinforcements due to the change in visco-elasticity of the

polymer matrix, graphene can enhance the degree of crystallinity as a nucleating

agent in semi-crystalline polymers, and therefore stiffens the polymer matrix by

increasing the crystallinity.

Strength and elongation at break of graphene polymer nanocomposites changes due

to stiffness change [36,64] so with good dispersion tensile strength increases while

at the same decrease in elongation at break is also observed [36].

The earliest “reports on polymer composites with exfoliated graphite fillers

emerged from studies on the intercalation chemistry of GICs. Alkali metal-GICs

could initiate the polymerization” of ethylene, styrene, methyl methacrylate, and

isoprene [64-67]. Later on it was also observed that the alkali metal-GICs can also

exfoliate the layers of [68, 69]. Numerous preparing methods “have been accounted

for dispersing both GNP and GO-derived fillers into polymer matrices” in recent

years, which are almost same to those used for other nano fillers [70]. The nature of

the bonding between filler and matrix along with other factors, has profound effect

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on the mechanical performance of nanocomposites. Whereas in some cases

nanocomposites are produced that “are non-covalent assemblies where the polymer

matrix and the filler interact through relatively weak dispersive forces. However,

presently research is focused to develop chemical bonding between graphene” and

“polymer to promote stronger interfacial bonding” for better mechanical

performance of system.

1.3. Preparation Methods of Nano-composites

1.3.1. “Non-covalent dispersion methods: solution and melt mixing”

This method involves the mixing of colloidal suspensions of “graphene-based

materials with the desired polymer in same solvent to have molecular level”

interaction between nanofiller and polymer [70]. Due to “ease of processing of

graphene nanosheets in aqueous media as well as in organic solvents solution

mixing has been widely reported in the literature [70]. This approach has been used

for incorporating pristine/functionalized graphene” fillers into different polymers,

like: polystyrene (PS) [41,71]. Polycarbonate [72]. Polyacrylamide [73] polyimide

[74] and “poly (methyl methacrylate) (PMMA)” [75,79]. “The facile production of

aqueous GO platelet suspensions via sonication makes this technique particularly

appealing for water-soluble polymers such as poly (vinyl alcohol) (PVA) [36,75,77-

78] and poly (ally amine),” “composites of which can be produced via simple

filtration”[78,80] a broad range of composite films with different loadings of

GO/PVA and GO/PMMA has been prepared [81] having a “layered morphology

comparable” to that of ‘graphene oxide paper’ [82]. The “dispersion of graphene

nanosheets in composite is usually controlled by level of exfoliation” before mixing

or during mixing in solution mixing methods. Thus, solution mixing offers a

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potentially simple route to dispersing single-layer CMG platelets into a polymer

matrix [83].

In “melt mixing”, “a polymer melt and filler (in a dried powder form) are mixed

under high shear conditions. Relative to solution mixing, melt mixing is often

considered more economical (because no solvent is used) and is more compatible

with many current industrial practices [31].” To date, studies suggest that, such

“methods do not provide the same level of dispersion of the filler as solvent mixing

or in situ polymerization methods” [14]. A thermoplastic polymer is mixed

mechanically “with graphite or graphene or modified graphene at elevated

temperatures using conventional methods, such as extrusion and injection molding.

The polymer chains are then intercalated or exfoliated to form nano-composites.

This is a popular method for preparing thermoplastic nano-composites. Polymers,

which are unsuitable for adsorption or in-situ polymerization,” can be processed

using this technique. A wide range of polymer nanocomposites, such as PP/EG,

HDPE/EG, PPS/EG, PA6/EG, etc., have been prepared using this method. Notably,

“no means of dispersing single- or few-layer GO-derived fillers via melt mixing

without prior exfoliation have been reported akin to layered silicate fillers” [84].

1.3.2. Non-covalent/ in- situ polymerization

This method generally involves “mixing of filler in neat monomer (or multiple

monomers), or a solution of monomer, followed by polymerization in the presence

of the dispersed filler resultantly with precipitation/extraction or solution casting to

generate samples for testing. Covalent linkages between matrix and filler have been

reported in this particular method, however”, “non-covalent composites of a variety

of polymers, such as poly (ethylene) [85] PMMA [86] and poly (pyrrole) [87-88] in

situ polymerization has also been reported”. In situ polymerization high level of

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dispersion of nano fillers obtained without a prior exfoliation step [84]. The

“intercalation polymerization has been widely investigated for nano clay/polymer

composites-monomer is intercalated between the layers of graphite or GO, followed

by polymerization to separate the layers, [84] which has been also applied to GNP

and GO-derived polymer composites. Graphite, GICs and EG can be exfoliated by

an alkali metal or monomer (e.g., isoprene or styrene), to generate dispersions of

GNPs in the matrix followed by polymerization initiated by the negatively charged

graphene sheets [89]. Anyhow isolation of monolayer graphene sheets yet to be

achieved through this method [24,69,90,91]. In a recent study, an attempt was made

to grow PE chains between the graphitic layers in the presence of graphene

nanosheets via polymerization” of poly (ethylene). Although polymerization

“further exfoliated the GNPs, but monolayer graphene platelets were not observed”

which was confirmed by TEM [85]. Monomers and polymers easily intercalates

into galleries of GO due to larger “interlayer spacing (between about 0.6 and 0.8

nm depending on relative humidity) compared to graphite” (0.34 nm) [92].

Promotion of direct intercalation of hydrophilic molecules takes place due to the

polar nature of graphene oxide, with the enlarged interlayer spacing [93]. “In situ

polymerization has been presented for various GO composite systems, including

poly (vinyl acetate) [94] and poly (aniline) (PANI) [95]”.

Intercalated morphology of these systems was confirmed by X-ray diffraction

studies where in polymer the graphene oxide sheets remain loosely stacked. Study

on GO/PMMA composite confirmed an enlarged interlayer spacing of GO (from

0.64 nm to 0.8 nm) which suggests intercalated morphology of system [86].

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1.3.3. “Graphene-based composites with covalent bonds between matrix and

filler”

A covalent linkage “between the polymer matrix and pure carbon materials

surfaces (when used as composite filler) is challenging”. So, graphene is oxidized

to graphene oxide in order to produce functional groups on its surface for

introducing chemical bonding between polymer and nanofiller. Both “grafting-from

and grafting-to approaches” have been used for this purpose to have attachment of

nano filler and polymers. “Functionalized GeO platelets were introduced into

different polymers like of surface-attached poly (styrene), poly (methyl

methacrylate), or poly (butylacrylate) and improvement in mechanical and thermal

properties versus the neat matrix polymer were observed” [41,42,96-98]. In

grafting-to approaches azide-terminated poly(styrene) (PS) chains are grafted of to

alkynes-functionalized GeO platelets by using a cupric iodide(CuI-catalyzed) as

catalyst [99]. Similarly PVA is grafted to the surface of GeO platelets via

carbodiimide-activated esterefication [100]. The “grafting density of chains to the

platelet surface” [101] and its affect on dispersion of these polymer-grafted platelets

are the selection criteria for grafting to or grafting from approach if dispersed into a

polymer matrix [29] For certain polymers, prior functionalization is not required

because “covalent bonding between the matrix and GeO platelets may form during

polymerization (on reaction with the functional groups of GeO)”. For example in

case of an GeO/epoxy composite, GeO gets incorporated into cross linked network

when amine hardener is used as curator, [102] polyamide brushes are grafted to

GeO platelets during ring opening polymerization of caprolactam “via condensation

“reactions” between the amine containing monomer and the carboxylic acid groups”

of the GeO platelets [103].

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1.3.4. Other methods for composite preparation

Several other methods like emulsion polymerizations, lyophilization methods [104]

or phase transfer techniques [105,106] may “offer general approaches to disperse

GeO platelets, CMG platelets and RGeO platelets as filler in a polymer matrix

[107-109] in addition to those mentioned above and which has potential use for

composite fabrication.” Non covalent grafting is one of such approaches of “well-

defined polymers to reduced graphene oxide (RGeO) platelets via pep interactions”.

For instance, the “attachment of pyrene-terminated poly(N-isopropylacrylamide) to

RGeO was recently reported; the composite was stated to retain the thermo-

responsive properties of the neat polymer [110]. This non-covalent grafting opened

a new horizon for the production of graphene based composite [111].” Moreover,

such “non-covalent composites may better preserve the conjugated structure of

graphene-based materials as compared with covalent functionalization or grafting

approaches”. “Attempts to exfoliate graphite directly via conventional melt mixing

techniques have not been successful to date [61]. However, solid state shear

pulverization, which uses a twin screw extruder to blend solid materials using

shear, was reported to exfoliate and disperse unmodified graphite directly into

polypropylene, yielding nanocomposites with platelets having thicknesses of

approximately 10 nm or less [112].” Other “production methods, such as layer-by-

layer assembly of polymer composite films [113] and backfilling of GO platelet

aero gel structures” with polymer may “provide means to produce nanocomposites

with defined morphologies” [114-115].

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1.4. Graphene synthesis

Graphene was once believed to be an academic material [116], a

thermodynamically unstable material that upon isolation would crumple up on

itself. However, this didn’t stop some scientists investigating how thin they could

actually make graphite planes. The search for graphene started.

Many early attempts to make graphene involved intercalation. The technique

involved wedging the carbon planes apart and inserting various molecules between

them [117]. The end product usually consisted of thin graphitic chucks, or graphene

fragments rather than graphene monolayer. It wasn’t until 2004 when Geim and his

collaborators in Manchester [2], refined the micromechanical cleavage technique to

peel 10 µm sized, two-dimensional graphene from highly oriented pyrolytic

graphite (HOPG). Their research apparatus consisted of HOPG and scotch tape.

This tape was stuck on the graphite and peeled off repeatedly. The graphene were

detached from the scotch tape and pinned (by van der Waals forces) to a Si wafer

with a 300nm oxide layer specifically grown. The graphene was then imaged

optically showing visible contrast on the colourful oxide surface. Next the substrate

was etched to minimize induced effects, and graphene’s novel intrinsic properties

were probed. The results attracted the attentions of scientific community.

This original method to produce graphene is delicate and time consuming and is not

suitable for large scale applications like at industrial level. Developments of many

alternative syntheses have since become known. They can be broken into three

categories, i) growth in-situ on a substrate ii) bottom up methods to synthesize

graphene from organic precursors and iii) top down methods of liquid phase

exfoliation of graphite [118].

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1.4.1. Growth in-situ on a substrate

Graphene mono- and multi- layers have been grown on single crystal silicon

carbide (SiC). This process involves heating the SiC to temperatures greater than

1000oC, in ultra high vacuum (UHV) conditions. Si desorbs carbon above this

temperature and thus small islands of graphitized carbon form. A significant

advantage of this technique is that SiC substrates offer an insulating supporting

medium. Few layer graphene that is produced this way can be patterned using

standard lithography techniques. However, it is challenging to achieve large

graphene domains with uniform thicknesses. Emtsev et al. have tried to overcome

this issue by an ex-situ graphitization of Si terminated SiC(0001) [119]. This

method produces undisturbed monolayer graphene terraces that are up to 3 µm wide

and ≥50µm in length.

SiC as a supporting medium has also been shown to have appreciable influence on

graphene’s electrical properties and so must not be made comparable to

mechanically cleaved graphene [120]. Zhou et al. found that the interaction between

the substrate and the epitaxially grown graphene results in gaps appearing at the

Dirac points. This can be exploited to induce a band gap, as seen in Figure 1.2 [121-

122]. Band-gap engineering is very encouraging for carbon based electronics. The

growth of mono- and few layer graphene on transition metals is also well

documented and has established itself as a promising means of producing graphene.

The procedure involves exposing the transition metal to a hydrocarbon gas, under

pressure.

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Fig. 1.1 A) Schematic of graphene on a SiC substrate[122] B) E-k diagram of

graphene grown on SiC displaying the band opening at the Dirac point and C) An

ARPES intensity map, displaying the band gap.

This has been demonstrated on Pt [123], Ir [124], Ru [125], Cu [126] and on both

Ni single [127] and poly- crystalline [128-129] transition metals. There are lot of

requirements for the processing options, for example, high temperatures (~700-

1000oC) and UHV conditions, not to mention variables like cooling rates and gas

phase kinetics. The main issues with chemical vapour deposition (CVD) growth is

grain size limitations, which can result in grain boundaries (i.e. defects) and the

presence of multilayer that are not necessarily AB stacked. CVD growth is favored

for some electronic applications and may eventually lead to integrating graphene

into circuits. Standard lithographic technique can also be employed to pattern the

graphene grown films [129].

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Another advantage of this processing method is the ability to transfer graphene to a

variety of substrates. CVD grown graphene, transferred onto SiO2, has been shown

to exhibit high electron mobility and even the half integer quantum Hall effect,

indicating that the quality can be as high as mechanically cleaved graphene [129].

At present the largest sheet (30 inch diagonal to diagonal) of CVD grown graphene

has been demonstrated by Bae et al [130]. This unique method involves using a 7.5

inch wide quartz tube wrapped in copper foils that is inserted into the 8 inch wide

furnace. After oven processing the graphene is transferred to an adhesive polymer

support and the copper is etched. The graphene films are then transferred from the

polymer support onto a target substrate by removing the adhesive forces (Figure

1.3). The resulting graphene films have set the bar for transparent conductive

electrodes with a sheet resistance of ~40Ω sq-1 and transparency (550nm) ~90%.

Fig. 1.2 Schematic of the roll based transfer of graphene films grown on Cu foil

[130].

1.4.2. Bottom up methods to synthesize graphene from organic

precursors

Bottom up synthetic approaches for benzene-based macromolecules have been

known for some time [131-132]. They are referred as polyaromatic hydrocarbons

(PAHs) and they lie between molecule and macromolecule structures. The

arrangement of the benzene ring is very similar to the 2-D chicken wire structure of

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graphene and has thus attracted attention as a possible route for controlled growth

of graphene on substrates. PAHs are also attractive due to their high versatility,

clean processing and the multitude of aliphatic chains that can be attached to

modify their solubility [133]. These routes have been largely explored by Mullen

and co- workers who have produced a number of graphene precursors [133]. The

main disadvantage of increasing the molecular weight of these planar structures is

that their solubility in common solvents decreases, complicating their process

ability [132]. The core molecule in molecular graphene is the hexabenzocoronene

(HBC), which consists of 13 fused hexagon rings (Figure 1.4). This molecule

became the building block along with other hexaphenyl benzene derivatives. The

largest graphene molecules arranged to date has 222 carbon atoms in its core [134].

Further advances came in 2008, when Yang et al [135], demonstrated total

synthesis of graphene nano ribbons (GNRs) with controlled edge configuration. The

electrical properties of these GNRs were characterized by scanning tunneling

microscopy (STM), and thin films were prepared showing liquid crystal properties.

Furthermore, organic synthesis of graphene offers an alternative route to

synthesizing graphene with defined shape, size and edge structure, factors that are

quite important for applications in the field of electronics that require a finite band

gap and edges that allow spin transport.

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Fig. 1.3 (A) Structure of the hexabenzocoronene (HBC) (B) Structure of the largest

polyaromatic hydrocarbons synthesized to data containing 222 C atoms

1.4.3. Chemical efforts to exfoliate and stabilize graphene sheets in

solution

Dispersing graphene in solution requires overcoming the cohesive energy of the

graphite planes [136]. To overcome this energy barrier, two main methods have

emerged. The first requires the chemical functionalization of graphite which aims to

weaken interlayer interactions [137] and the second involves the sonication of

untreated graphite in solvent [138] or surfactant systems [139].

The first approach results in graphite oxide (GO). This is a product from the

oxidation of graphite which retains the original layered structure of graphite [140].

The principle method to oxidize graphite is the Hummers method [137], and it

involves dispersing graphite in concentrated sulphuric acid, sodium nitrate and

potassium permanganate at 45oC for a few hours. The resulting graphite

intercalation compounds are then rapidly annealed, generating a CO2 over-pressure

that causes the graphite to split. Further ultrasonication results in individual GO

sheets separation. These GO sheets contain large quantities of hydroxyl, carboxyl,

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carbonyl and epoxides functional groups which are attached to the edge or basal

planes [141]. Undesirably during the oxidation processing, the carbon atom is

transformed from planar sp2 hybridized geometry to distorted sp3 hybridized

geometry, thus losing its electrical properties to become electrically insulating as

shown in Figure 1.5. Hydrazine or hydrogen plasma reduction is used to restore the

electrical conductivity of graphene.

Fig. 1.4 GO structure demonstrating the distorted sp2 atomic arrangement and

attached functionalities (left) and vial of GO brown dispersion (right). Its brown

colour is attributed to the absence of π conjugated structure.

GO is its strongly hydrophilic due to the presence of various functional groups like

hydroxyl, carbonyl etc. and can be readily exfoliated in water to form stable

colloidal dispersions at 2 mg/ml [142]. Further dispersion analysis confirmed that

single layer GO sheets, up to hundreds of microns in size, have been dispersed in a

variety of organic solvents at concentrations higher than ~1.5mg/ml [143-145].

Ang et al. [146] obtained stable dispersions with 90% monolayer yield and mean

sheet areas of 330 ± 10μm2. They explained that intercalated GO sediments formed

after oxidation, via a modified Hummers method, result in oxidized outer layers of

the large sized GO aggregates, but the inner layers consist of mildly oxidized

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(mainly at the edge planes) graphene sheets. These sediments were then intercalated

using tetra butyl ammonium hydroxide, (40%TBA water solution) under reflux

conditions for two days. After two days the color changes from pale yellow to black

indicating an increase in UV-Vis absorption region due to the presence of extended

conjugate π structure [147]. Then they are dispersed in dimethyl formamide (DMF)

and spin coated onto SiO2. Their XPS data suggests that less that 10% of the carbon

remains oxidized and a conductivity of 15,000 S m-1 was achieved. Dikin et al.

prepared free standing (1 to 30 µm thick) GO paper showing a mean Young’s

modulus of 32GPa and ultimate tensile strength of 60MPa [148]. These results are

greater than most of the reported nanotubes bucky papers [149].

Despite increased process ability of graphene oxide, it retains significant amounts

of oxygen functionalities even after severe reduction processes and can contain

irreversible lattice defects [150]. In comparison to pristine graphene derived from

expanded graphite, it fails to meet the high electrical conductivities due to distorted

sp2 structure and contains many lattice defects [151-153].

The second approach to overcome the forces that bind graphene layers together is

liquid phase exfoliation of pristine graphite in solvent and surfactant systems.

Solvent exfoliation of graphite has been demonstrated by Hernandez et al. [138] at

concentrations of up to 0.01mg/ml. Other groups have demonstrated concentrations

of between 0.05 - 0.1mg/ml [154-156]. Surfactant exfoliated graphene have also

reached concentrations of 0.04mg/ml [139]. Thin films were prepared from these

graphene/surfactant dispersions, shows conductivity of 1.5 x 104S m-1 with a

transparency of ~70% after annealing [157].

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Liquid phase exfoliation has many advantages including a straightforward approach

that is readily accessible with a low cost. Successful dispersions can be directly

used for mixing or blending with polymers, spin or dip coating, spraying or even

post functionalization. They are also easily analyzed by TEM, can be cast by

filtration or can be made into large thin films by Langmuir–Blodgett assembly in a

layer-by-layer manner [158]. The main drawback, however, is the lack of control

over the exfoliation of the dispersion which can vary considerably from starting

graphite to the method used to exfoliate them [143]. This can result in poly-disperse

dispersions, with flakes of many thicknesses and sizes. It is well understood that

graphene sheets consisting of 10 or less layers, possess electronic structure distinct

from bulk graphite [5,114]. Graphene’s properties also vary as a function of layer

number. As mentioned earlier mono- layer graphene is a zero gap semiconductor,

with linear energy dispersion. Bilayer graphene is also a zero gap semiconductor

but its electrons follow a parabolic energy spectrum [5,158]. Trilayer graphene’s

electronic spectrum becomes even more complicated as several charges appear and

the bands overlap [2, 11, 4, 1, 16]. Thus polydispersity of flakes within dispersions

can result in unpredictable behaviors. To improve this, a post sonication

centrifugation step results in larger graphite pieces sediment to the bottom of the

centrifuge tube. The top percentage of the dispersion is decanted and used for

further analysis. Ultracentrifugation in a density gradient medium has also been

demonstrated [159]. This separates graphene sheets according to their buoyant

density and has produced mono-disperse graphene dispersions.

Solvent exfoliation of graphene is not completely understood. Coleman et al.

explains why solvents exfoliate carbon nanotubes [160-161]. The main factor in

exfoliating nanotubes is the strength of the solvent-nanotubes sidewall interaction.

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One wants to match the surface energy of the solute (graphene) to the surface

energy of the solvent. This results in minimal energetic stress between the two

species and is the basis of the chemistry rule, “like dissolves like”. Specific solvents

that result in favorable interactions exfoliate and stabilize materials more easily.

Work done by Hernandez et al. investigated if this rule is also true for graphene and

thus the solubility parameters for graphene were determined [162]. The multi

component solubility parameters are numerical values that indicate the relative

solvency behavior of a specific solvent. When graphene concentration was plotted

as a function of these multi component solubility parameters it confirmed that

successful solvents show a sharp dependence on surface tension. The dispersibility

of graphene in 40 solvents, (28 of them previously unreported) was measured. It

was found that good solvents for graphene are characterised by a Hildebrand

solubility parameter T~23MPa1/2. Specific physical interactions between the

solvent and graphene were subsequently investigated using Hansen parameters.

These can be related to the Hildebrand parameter, T, through T2 = 2

D + 2P + 2

H

(where D, P and H refer to the dispersive, polar and H-bonding Hansen

components). The effectiveness of the studied solvents was shown to scale with

proximity and to the calculated Hansen solubility parameters of graphene (

D~18MPa1/2, P~9.3MPa1/2 and H~7.3MPa1/2) [160-162]. TEM analysis was used

to show that the graphene is well exfoliated in all cases. Even in relatively poor

solvents, >63% of observed flakes have <5 layers.

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23

Refrences

[1] E. Fradkin, Physical Review B 33 (1986) 3263.

[2] A. Geim and P. Kim, Carbon wonderland. Scientific American 298 (2008) 90.

[3]M.I. Katsnelson and K.S. Novoselov, Solid State Communications 143 (2007) 3

[4] Zhu Y, Murali S, Cai W, Li X, Suk JW, Potts JR, et al. Adv Mater 22 (2010)

3906.

[5] Geim AK, Novoselov KS. Nat Mater 6 (2007) 183.

[6] Compton OC, Nguyen SBT. Small 6 (2010)711.

[7] T. Booth, et al, Nano Letters 8 (2008) 2442.

[8]. C. Lee, et al, Science 321 (2008) 385.

[9] Suk JW, Piner RD, An J, Ruoff RS. ACS Nano; 2010. doi:10.1021/nl101902k.

[10] Gómez-Navarro C, Burghard M, Kern K. Nano Lett 8(2008) 2045.

[11] Alexandre M, Dubois P. Mater Sci Eng R Rep 28(2000)1.

[12] Thostenson ET, Li CY, Chou TW. Compos Sci Technol 65(2005)491.

[13] Li D, Kaner RB. Science 320(2008)1170

[14] Kim H, Miura Y, Macosko CW. Chem Mater 22 (2010)3441

[15] Paci JT, Belytschko T, Schatz GC. J Phys Chem C 111(2007) 18099

[16] Fornes TD, Paul DR. Polymer 44(2003)4993

[17] Carter LW, Hendricks JG, Bolley DS. 2531396, National Lead Company;

1950

[18] Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al.

J Mater Res 8 (1993)1179

[19] Bunnell LR. 5186919, Battelle Memorial Institute; 1993.

Page 44: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

24

[20] Pan YX, Yu ZZ, Ou YC, Hu GH. J Polym Sci Part B Polym Phys 38

(2000)1626

[21] Winey KI, Vaia RA. MRS Bull 32(2007)314.

[22] Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera-Alonso M,

Piner RD, et al. Nat Nanotechnol 3 (2008)327.

.[23] Chen G, Zhao W,editors. Nano- and biocomposites. CRCPress; 2009. p. 79.

[24] Chen G, Weng W, Wu D, Wu C. Eur Polym J 39 (2003)2329

[25] Chen G, Wu C, Weng W, Wu D, Yan W. Polymer 44 (2003)1781.

[26] Zheng W, Wong S-C. Compos Sci Technol 63 (2003)225

[27] Zheng W, Wong S-C, Sue H-J. Polymer 43: (2002) 6767

[28] Schaefer DW, Justice RS. Macromolecules 40 (2007)8501

[29] Akcora P, Kumar SK, Moll J, Lewis S, Schadler LS, Li Y, et al.

Macromolecules 43 (2010)1003

[30] Akcora P, Liu H, Kumar SK, Moll J, Li Y, Benicewicz BC, et al. Nat Mater

8 (2009)354

[31] Paul DR, Robeson LM. Polymer 49 (2008)3187

[32] Kluppel M, editor. Advances in Polymer Science 164 2003) 1.

[33] C. Harper, Handbook of Plastics, Elastomer, and Composites, McGraw-Hill, Inc, New

York (2002).

[34] Lv C, Xue Q, Xia D, Ma M, Xie J, Chen H. J Phys Chem C 114 (2010)6588.

[35] Wagner HD, Vaia RA. Mater Today 7 (2004)38

[36] Liang JJ, Huang Y, Zhang L, Wang Y, Ma YF, Guo TY, et al. Adv Funct

Mater

19 (2009) 2297

[37] Jiang L, Shen XP, Wu JL, Shen KC. J Appl Polym Sci 118(2010) 275.

[38] Yang XM, Tu YF, Li LA, Shang SM, Tao XM. ACS Appl Mater Interfaces

Page 45: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

25

2 (2010); 1707

[39] Xu Z, Gao C. Macromolecules 43 (2010) 6716

[40] Miller SG, Bauer JL, Maryanski MJ, Heimann PJ, Barlow JP, Gosau JM, et al,

Adherent Technologies,Inc. Composite Science and Technology 2010.

[41] Fang M, Wang KG, Lu HB, Yang YL, Nutt S. J Mater Chem 19(2009) 7098

[42] Goncalves G, Marques PAAP, Barros-Timmons A, Bdkin I, Singh MK,

Emami N, et al. J Mater Chem 20 (2010) 9927

[43] Lee YR, Raghu AV, Jeong HM, Kim BK. Macromol Chem Phys 210 (2009);

1247

[44] Pramoda KP, Hussain H, Koh HM, Tan HR, He CB. J Polym Sci Part A Polym

Chem 48 (2010)4262.

[45] Cai DY, Yusoh K, Song M. Nanotechnology 20 (2009) 085712.

[45] Lee HB, Raghu AV, Yoon KS, Jeong HM. J Macromol Sci Part B Phys

49 (2010) 802

[46] Nguyen DA, Lee YR, Raghu AV, Jeong HM, Shin CM, Kim BK. Polym Int

58 (2009)412.

[47] Raghu AV, Lee YR, Jeong HM, Shin CM. Macromol Chem Phys 209 (2008);

2487

[48] Bansal A, Yang H, Li C, Benicewicz BC, Kumar SK, Schadler LS. J Polym

Sci Part B Polym Phys 44 (2006) 2944.

[49] Khan U, May P, O’Neill A, Coleman JN. Carbon 48 (2010)4035.

[50] Quan H, Zhang B-Q, Zhao Q, Yuen RKK, Li RKY. Compos Part A Appl Sci

Manuf 40 (2009)1506

[51] NguyenDA, RaghuAV,Choi JT, JeongHM. PolymPolymCompos18 (2010)351

[52] Liu H, Brinson LC. Compos Sci Technol 68(2008)1502.

[53] Kim S, Drzal LT. J Adhes Sci Technol 23 (2009)1623

Page 46: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

26

[54] Kalaitzidou K, Fukushima H, Drzal LT. Carbon 45 (2007)1446

[55] Kim S, Do I, Drzal LT. Polym Compos 31 (2010)755

[56] Kalaitzidou K, Fukushima H, Drzal LT. Compos Part A Appl Sci Manuf 38

(2007)1675

[57] Kalaitzidou K, Fukushima H, Drzal LT. Compos Sci Technol 67(2007)2045

[58] Ramanathan T, Stankovich S, Dikin DA, Liu H, Shen H, Nguyen ST, et al.

J Polym Sci Part B Polym Phys 45 (2007)2097

[59] Hirata M, Gotou T, Horiuchi S, Fujiwara M, Ohba M. Carbon42( 2004)2929

[60] Zhao X, Zhang QH, Chen DJ, Lu P. Macromolecules43 (2010)2357

[61] Kim H, Macosko CW. Macromolecules 41(2008) 3317

[62] Kim H, Macosko CW. Polymer 50 (2009)3797

[63] Li Q, Li ZJ, Chen MR, Fang Y. Nano Lett 9 ( 2009);9:2129

[64] Rafiee MA, Rafiee J, Wang Z, Song HH, Yu ZZ, Koratkar N. ACS Nano 3

(2009) 3884

[65] Podall H, Foster WE, Giraitis AP. J Org Chem 30 (1958)82

[66] Panayotov IM, Rashkov IB. J Polym Sci Part A Polym Chem 11(1973)2615.

[67] Parrod J, Beinert G. J Polym Sci 53 (1961) 99.

[68] Shioyama H. Synth Met 114 (2000)1

[69] Shioyama H. Carbon 35(1997)1664.

[70] Moniruzzaman M, Winey KI. Macromolecules39( 2006)5194.

[71] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach

EA, et al. Nature 442(2006)282

[72] Higginbotham AL, Lomeda JR, Morgan AB, Tour JM. ACS Appl Mater

Interfaces 1(2009)2256

[73] Pandey R, Awasthi K, Tiwari RS, Srivastava O.N. Polymer 52 (2011) 5.

[74] Chen D, Zhu H, Liu T. ACS Appl. Mater. Interfaces; 2 (2010) 3702.

Page 47: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

27

[75] Das B, Prasad KE, Ramamurty U, Rao CNR. Nanotechnology

20(2009)125705.

[76] Stankovich S, Piner RD, Nguyen ST, Ruoff RS. Carbon 44 (2006)3342

[77] Yang X, Li L, Shang S, Tao X. Polymer 51(2010)3431.

[78] Xu YX, Hong WJ, Bai H, Li C, Shi GQ. Carbon 47(2009)3538

[80] Satti A, Larpent P, Gun’ko Y. Carbon 48(2010):3376

[81] Putz KW, Compton OC, Palmeri MJ, Nguyen SBT, Brinson LC. Adv Funct

Mater 20(2010)3322

[82] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko

G, et al. Nature448( 2007) 457

[83] Cao Y, Feng J, Wu P. Carbon48( 2010)3834

[84] Sinha Ray S, Okamoto M. Prog Polym Sci 28(2003)1539

[85] Fim FC, Guterres JM, Basso NRS, Galland GB. J Polym Sci Part A Polym

Chem 48 (2010):692

[86] Jang JY, Kim MS, Jeong HM, Shin CM. Compos Sci Technol69( 2009)186.

[87] Gu Z, Zhang L, Li C. J Macromol Sci Part B Phys 48 (2009):1093

[88] Gu ZM, Li CZ, Wang GC, Zhang L, Li XH, Wang WD, et al. J Polym Sci Part

B Polym Phys 48 (2010):1329

[89] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al.

Science 306 (2004)666

[90] Gabriel P, Cipriano LG, Ana JM. Polym Compos 20 (1990)804.

[91] Liu P, Gong K. Carbon 37(1999)701

[92] Jang BZ, Zhamu A. J Mater Sci43 (2008)5092

[93] Matsuo Y, Hatase K, Sugie Y. Chem Mater 10(1998)2266

[94] Liu P, Gong K, Xiao P, Xiao M. J Mater Chem 10(2000)933.

[95] Kyotani T, Moriyama H, Tomita A. Carbon 35(1997)1185

Page 48: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

28

[96] Lee SH, Dreyer DR, An JH, Velamakanni A, Piner RD, Park S, et al.

Macromol Rapid Commun 31(2010)281

[97] Layek RK, Samanta S, Chatterjee DP, Nandi AK. Polymer51 (2010)5846.

[98] Fang M, Wang KG, Lu HB, Yang YL, Nutt S. J Mater Chem 20(2010)1982

[99] Sun ST, Cao YW, Feng JC, Wu PY. J Mater Chem 20 (2010) 5605.

[100] Veca LM, Lu FS, Meziani MJ, Cao L, Zhang PY, Qi G, et al. Chem

Commun;(2009):2565

[101] Coleman J, Khan U, Gun’ko Y. Adv Mater18 (2006)689

[102] Yang H, Shan C, Li F, Zhang Q, Han D, Niu L. J Mater Chem 19(2009)8856

[103] Zhang M, Parajuli RR, Mastrogiovanni D, Dai B, Lo P, Cheung W, et al.

Small 6 (2010) 1100

[104] Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko

G, et al. Nature448( 2007) 457

[105] Choi EY, Han TH, Hong J, Kim JE, Lee SH, Kim HW, et al. J Mater Chem

20 (2010) 1907.

[106] Wei T, Luo GL, Fan ZJ, Zheng C, Yan J, Yao CZ, et al. Carbon47(2009)

2296

[107] Hu HT, Wang XB, Wang JC, Wan L, Liu FM, Zheng H, et al. Chem Phys

Lett 484 (2010)247

[108] Zheming G, Ling Z, Chunzhong L. J Macromol Sci Part B Phys48( 2009)226

[109] Tkalya E, Ghislandi M, Alekseev A, Koning C, Loos J. J Mater Chem 20

(2010)3035

[110] Liu JQ, Yang WR, Tao L, Li D, Boyer C, Davis TP. J Polym Sci Part A

Polym Chem 48 (2010)425

[111] Liu JQ, Tao L, Yang WR, Li D, Boyer C, Wuhrer R, et al. Langmuir26

(2010) 10068

Page 49: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

29

[112] Vickery JL, Patil AJ, Mann S. Adv Mater 21(2009)2180

[113] Wu JH, Tang QW, Sun H, Lin JM, Ao HY, Huang ML, et al. Langmuir24

(2008) 4800

[114] Wang J, Ellsworth MW. ECS Trans 19(2009)241

[115] B. Partoens and F. Peeters, Physical Review B 74 (2006) 75404.

[116] E. Fradkin, Physical Review B 33 (1986) 3263.

[117] M. Dresselhaus and G. Dresselhaus., Advances in Physics 30 (1981) 139.

[118] M. Allen, V. Tung and R. Kaner, Chemical Reviews 110 (2010) 132.

[119] Emtsev, K.V., et al., Nature Materials 8 (2009) 203.

[120] W. de Heer, et al, Solid State Communications 143 (2007) 92.

[121] K. Novoselov, Nature materials 6 (2007) 720.

[122] S. Zhou, G. Gweon and A. Fedorov, Nature materials 6 (2007) 770.

[123] H. Ueta, et al, Surface Science 560 (2004) 183.

[124] C. Busse, et al, New Journal of Physics 11 (2009) 22.

[125] P. Sutter, J. Flege and E. Sutter, Nature materials 7 (2008) 406.

[126] X. Li, et al, Science 324 (2009) 1312.

[127] S. Kumar, et al, Chemical communications 46 (2010) 1422.

[128] A. Reina, et al, Nano Letters 9 (2008) 30.

[129] K.S. Kim, et al, Nature 457 (2009) 706.

[130] S. Bae, et al, Arxiv preprint arXiv: 0912 (2009) 5485.

[131] I. Gutman and S. Cyvin, Introduction to the theory of benzenoid hydrocarbons.

Springer (1989)

[132] E. Clar, The aromatic sextet. John Wiley & Sons (1972).

[133] J. Wu, W. Pisula and K. Mullen, Chemical Reviews 107 (2007) 718.

[134] C. Simpson, et al, Chemistry -Weinheim-European journal 8 (2002) 1424.

[135] X. Yang, et al, Journal of the American Chemical Society 130 (2008) 4216.

[136] S. Niyogi, et al, Journal of the American Chemical Society 128 (2006) 7720.

Page 50: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

30

[137] W. Hummers Jr and R. Offeman., Journal of the American Chemical Society 80

(1958) 1339.

[138] Y. Hernandez, et al, Nature Nanotechnology 3 (2008) 563.

[139] M. Lotya, et al, Journal of the American Chemical Society 131 (2009) 3611.

[140] H. He, et al, Journal of Physical Chemistry 100 (1996) 19954.

[141] Y. Geng, S. Wang and J. Kim, Journal of Colloid And Interface Science 336

(2009) 592.

[142] Y. Si and E. Samulski, Nano Letters 8 (2008) 1679.

[143] A. Green and M. Hersam, The Journal of Physical Chemistry Letters 1 (2010) 544.

[144] V. Tung, et al, Nature Nanotechnology 4 (2009) 25.

[145] S. Park, et al, Nano Letters 9 (2009) 1593.

[146] P. Ang, et al, ACS Nano 3 (2009) 3587.

[147] D. Li, et al, Nature Nanotechnology 3 (2008) 101.

[148] D.A. Dikin, et al, Nature 448 (2007) 457.

[149] M.F. Yu, et al, Physical review letters (Copyright (C) 2010 The American

Physical Society) 84 (2000) 5552.

[150] S. Park and R. Ruoff, Nature Nanotechnology 4 (2009) 217.

[151] S. Stankovich, et al, Carbon 45 (2007) 1558.

[152] S. Stankovich, et al, Journal of Materials Chemistry 16 (2006) 155.

[153] R. Hao, et al, Chemical Communications 48 (2008) 6576.

[154] P. Blake, et al, Nano Letters 8 (2008) 1704.

[155] Bourlinos, A.B., et al., Small 5 (2009) 1841.

[156] S. De, et al, Small 6 (2010) 458.

[157] X. Li, et al, Nature Nanotechnology 3 (2008) 538.

[158] E. McCann, D.S.L. Abergel and V.I. Fal'ko, Solid State Communications 143

(2007) 110.

[159] A.A. Green and M.C. Hersam, Nano Letters 9 (2009) 4031.

Page 51: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

31

[160] S. Bergin, et al, ACS Nano 3 (2009) 787.

[161] J.N. Coleman, Advanced Functional Materials 19 (2009) 3680.

[162] Y. Hernandez, et al, Langmuir 26 (2010) 3208.

Page 52: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

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Chapter 2

Materials and Characterization

Techniques

2.1 Introduction

This chapter discusses the materials used and outlines of the characterization

techniques.

2.2 Materials Graphite (Sigma-Aldrich), 1-Methyl-2-Pyrrolidinone (Fluka), N,N,di-methyl

formamide (Sigma-Aldrich), Tetrahydrofuran (Sigma-Aldrich), Poly (vinyl acetate)

(Sigma-Aldrich), Poly (vinyl alcohol) (Sigma-Aldrich), Poly (vinyl chloride)

(Sigma-Aldrich), Poly (acrylonitrile) (Sigma-Aldrich), Poly(urethane) (Hauntsman).

Octdecyl amine (Sigma-Aldrich). Sulfuric acid (Sigma-Aldrich), Hydrochloric acid

(Sigma-Aldrich). Sodium nitrite (Sigma-Aldrich), Potassium permangate (Fluka).

Hydrogen peroxide (Sigma-Aldrich).

These materials were used as received without further purification

2.3 Apparatus and Equipments Ultra Sonication Tip (Make- Vibra Cell- VCX 500, Power- 500 watt, Frequency-

20KHz), Sonication Bath (Make- Bransonic- 1510E MT), Centrifugation Machine

(Make- Hettich Zentrifugen – Mikro 220R, D-78532), Vaccum Pump (Make- Buchi

Switzerland, V-700). Magnetic Stirrer (Make- Heidolph, MR-3002), Oven (Make-

MTI Corporation), Teflon Trays (length x breadth x height, 4x4x1 cm)

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2.4 Characterization Techniques

2.4.1 UV-vis Absorption Spectroscopy

UV-Vis absorption spectroscopy involves exciting a sample with electromagnetic

radiation (EM radiation) of a certain wavelength and measuring the proportion of

radiation that is absorbed by the material. When EM radiation is pointed on a

material, the radiation excites a bonding electron in an atom or molecule into an

unfilled non-bonding orbital (or promoting the electron from the ground state into an

excited state). The change in energy acquired by the electron relates to a line in the

absorption spectrum which occurs at a characteristic wavelength (or energy). As

each electronic transition has associated rotational and vibrational transitions, the

line is broadened to become a peak centered on the characteristic wavelength. The

intensity of the absorption at the wavelength is related to how much energy is

absorbed by the molecule.

UV-Vis spectrometers generally use a broad excitation source such a xenon (Xe)

lamp along with a mono chromator in order to illuminate the sample across a range

of excitation wavelengths to measure the absorbed light as a function of the

wavelength. In this work a Cary 6000i spectrophotometer was used which can

measure absorbance from 350 nm to 850 nm (i.e. in the UV-Visble region). The

spectrometer is run in dual beam mode which means that the exciting radiation is

split into two equal intensity beams using a half-mirror so that simultaneous

measurement can be made on a sample and a background or reference sample for

accurate background subtraction. The intensities of the reference and sample

radiation are measured as I0 and I respectively. The ratio of I to I0 is called the

transmittance, T. The Beer-Lambert law empirically relates T to the length, l of the

sample and the concentration, c of the absorbing species as follows:

T I 10

cl (2.1)

I 0

Where ε is known as the molar extinction coefficient and is unique for different

materials.

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In UV-Vis absorption spectroscopy, the Absorbance, A is usually the parameter used

instead of Transmittance. The Absorbance is defined as:

A = log10 Io/I = -log10 T = αCl, (2.2)

Where C is concentration, l is the path length and α is the absorption coefficient.

For liquid samples, if the path length and absorption coefficient of a sample is

known then the concentration can be calculated from the measured absorbance.

2.4.2 Raman Spectroscopy

Raman Spectroscopy is a form of vibrational spectroscopy and is a measure of the

inelastic scattering of light by molecules. The Raman Effect was first observed

experimentally by Raman and Krishnan in 1928 when they used sunlight and a

narrow band filter to pass monochromatic light through a number of liquids. When a

crossed filter was used to block the wavelength of the incident light after passing

through the liquid, light of a different frequency passed through the filter [1]. This

effect had been predicted previously but this paper was the first observations of what

became known as the Raman Effect. Over the next number of years, further

investigations into this effect continued and as the quality of the light sources

available increased from sunlight to mercury arc lamps and then onto lasers, the

Raman effect became more widely used as a spectroscopic tool to help identify

chemical bonds and molecules [2].

When light is shone onto a sample, the photons interact with the molecules of the

sample. The photons may be reflected, absorbed or scattered. The vast majority of

scattered photons are scattered elastically, i.e. the scattered photons have the same

wavelength and hence energy as the incident photons. This type of scattering is

known as Raleigh scattering. However, a small number of photons (generally less

than 1 in 10-7

) is scattered in elastically with a wavelength that differs to that of the

incident photons. This occurs when the incident photons interact with the electron

cloud and bonds of the molecule. In the Raman Effect, the incident photon excites

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the molecule into a virtual excited state. The molecule then relaxes into a different

rotational or vibrational energy state by emitting a photon of a different energy to

that of the incident photon. This difference in energy between the incident photon

and the emitted photon is the Raman shift and is usually expressed as a change in

frequency in wave numbers [3-4].

As the overall energy of the system must remain constant, if the final energy state of

the molecule is higher than the initial state, then the Raman scattered photons must

have a lower energy (and hence lower frequency) than the incident light. This shift in

frequency is known as a Stokes shift. Similarly if the molecule’s final energy is

lower than its initial energy, then the Raman scattered photons have a higher energy

than the incident photons and the change in frequency is known as an anti-Stokes

shift. At room temperature, the majority of the molecules are likely to be in their

ground state energy levels and as such, Stokes Raman scattering is much larger than

anti-Stokes Raman scattering.

Raman spectroscopy is similar to infrared spectroscopy in that they both probe the

vibrational energies of molecules – in other words, the nature of the bonds between

the atoms of the molecules. However, whereas IR spectroscopy can only measure

vibrations which cause the dipole moment of the molecule to change, for a transition

to be Raman active, the polarisability of the molecule must be changed by the

transition. In this way we can say that IR and Raman spectroscopy are

complementary to each other.

In this research, Raman spectroscopy is used to characterize graphene nanosheets.

Horiba Jobin Yvon Lab Ram HR spectrometer is utilized by using a 633 nm HeNe

laser with laser powers up to 12 mW to excite the samples. A long working distance

100x objective lens and a diffraction grating of 600 lines mm-1

give spatial

resolutions of 3 – 5 cm and a quoted frequency resolution of 0.3 cm-1

. The 633 nm

light interacts well with electronic transitions of the materials investigated (i.e. C-C,

C-H bonds etc.). For this reason, Raman spectroscopy like this is also known as

resonance Raman spectroscopy.

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The Raman spectrum of graphene is dominated by three main features,G, D, and 2D-

Raman modes each having different physical origins. The peak at 1580 cm-1 (G

band), arising from emission of zone-centre optical phonons, corresponds to the

doubly degenerate E2g mode of graphite related to the vibration of sp2 bonded carbon

atoms. The disorder-induced D (1350 cm-1) band and its symmetry –allowed 2-D

overtone band (2700 cm-1) involve preferential coupling to transverse zone-boundary

optical phonons. The “D band gives evidence of the presence of defects, that is,

either edges or topological defects in the population. We can quantify the defect level

by D-to-G- band intensity ratio, (ID/IG). As shown in the inset of Figure 2.1, ID/IG

increases gradually from the powder value with increasing” “sonication time. In

addition, we found that (ID/IG) increase smoothly with rotation rate”. [5]

Figure. 2.1 Raman shift

2.4.3 Tensile testing (TT)

Tensile testing is the most widely used tool to investigate the mechanical properties

of materials. Mechanical properties were measured using “Zwick Roell tensile tester”

with 100 N load cell. In this thesis we have monitored stress-strain behavior of, PVC

(polyvinylchloride), polyurethane (Morethane), polyvinyl alcohol (PVA), poly

(acrylonitrile) (PAN), polyvinyl acetate (PVAc) and graphene based polymeric

composites. A sample is clamped in the jaws/head of a tensile tester. One of the jaws

moves continuously against a static jaw to stretch the sample (until break). Applied

force/stress is plotted as a function of strain. Such a plot is termed as stress-strain

curve. Strain (ε) is given by equations 2.3 and 2.4.

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L1 L0

(2.3)

L0

Or

L

(2.4)

L0

Where, L L1 L0

Where L0 is the initial length while L1 is the final length of the sample.

Stress (σ) at a point is defined as the applied force per cross sectional area and given

by equation 2.5:

Force

A where A is cross sectional area and in case of a film sample, is given

as A W Tk

W is width of sample and Tk is thickness of sample.

(2.5)

(2.6)

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Fig. 2.2 Typical stress-strain curves.

Various regions in the stress-strain plot have been shown in Fig. 2.2 which are the characteristic of various materials.

1. The initial part of stress-strain curve is usually linear (see Fig 2.2). This part

of the curve is generally known as the elastic or proportional region. During

stretching of a sample the inter-atomic/molecular bonding distance slightly

increases elastically. Therefore, the Young’s modulus is a measure of inter

atomic/molecular forces. The linearity in a stress-strain curve also represents

the degree of order in a material which is why crystalline solids have linear

elastic regions compared to amorphous solids. In some cases, the initial

elastic part of a stress-strain curve may not be linear because of lower inter

atomic/molecular attractive forces and lesser or no order. For this behavior

the secant modulus is usually used. A secant is drawn from the origin to some

point of stress-strain curve and the slope is taken as secant modulus. In the

elastic region, stress is increasing proportionally with increased strain or vice

versa. This is because of constant strain of the polymer chain and filler in this

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part. The deformation in this region is reversible. The slope of the linear

region is called Young’s modulus. This is a parameter for measuring a

material’s stiffness. The area under the elastic region of the stress strain curve

is usually termed as the resilience.

2. The point at which stress-strain curve no longer remains linear and an

increase in strain occurs without an increase in stress is called the yield point

and the stress at that point is called the yield point stress or yield stress [5].

3. As stress is further increased beyond the elastic limit, the material starts to

deform irreversibly. The region can be relatively flat (depending on the

material). This part of the curve is called the viscous or plastic part. It

continues until the material breaks. The strain at which the material breaks is

termed as the strain at break (εB ) and stress at that point is known as the

strength at break and denoted as (σB.). The highest stress value in stress-strain

curve is called the ultimate tensile strength (UTS). Strength at break (εB ) and

UTS may or may not be the same. If a material breaks at the UTS point both

UTS and σB will be the same, while if a material breaks at a lower stress

value than UTS the UTS and σB will be different (Fig 2.1). The area under

the stress-strain curve is called strain energy or material toughness.

2.4.4 Scanning Electron Microscopy Scanning electron microscopy (SEM) is a powerful tool used to help image the

surface of samples using a beam of electrons instead of light. First used in the 1930s

it has since become a well known, well established technique both in scientific

research and in industry. In SEM an accelerated beam of electrons is focused onto

the surface of a sample under vacuum using a series of electromagnetic lenses. The

beam is then rastered across the surface using a series of coils. The surface is imaged

using a range of different detectors depending on how the electron beam interacts

with the sample surface. Figure 2.3 shows a schematic of the electron beam through

the SEM before it interacts with the sample.

When the beam hits the sample, several different interactions take place depending

on the energy of the electron beam and the nature of the sample substrate. When the

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beam (consisting of primary electrons) hits the surface, secondary and backscattered

electrons are dislodged from the surface of the sample. They are collected by

detectors consisting of positively charged grids, converted to digital signals and

converted to an image. The primary electrons interact with the sample in a teardrop

shape that extends from between 100 nm to 5 nm into the surface depending on the

beam energy and surface state. Secondary electrons are generated by inelastic

scattering of primary electrons on the atomic core or inner core electrons of atoms on

the surface of the sample and used in the most common imaging mode which uses

the In Lens detector whose position in the chamber is shown in Figure 2.3 Secondary

electrons detectable with the In Lens detector have a low penetration depth and

images formed are very surface sensitive. Electrons that are given off with larger

energies are more commonly back scattered electrons or electrons that have

undergone more interactions with the substrate and have undergone several

scattering events with the surface. They have travelled deeper into Backscattered

electrons carry information on chemical composition as materials with higher atomic

number are better scatterer and hence appear brighter images. the sample and are

detected using either backscattered electron (BSE) detector or an SE2 detector as

shown in Figure 2.4. In the research discussed in the course of this thesis, one of

three different SEMs manufactured by Carl Zeiss Ltd have been used: a Zeiss Supra

variable pressure FE SEM; a Zeiss Ultra Plus FE SEM and a Zeiss Auriga Focused

Ion Beam SEM. Each of these SEMs has InLens detectors and SE2 detectors along

with a range of other detectors.

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Fig. 2.3 Schematic diagram of SEM showing the electron path [6]

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Fig. 2.4 Detector positions in Zeiss Ultra / Supra [7]

2.4.5 Transmission Electron Microscopy

TEM working is similar to a slide projector. A “projector shines a beam of light

through (transmits) the slide, as the light passes through it is affected” by the

structures and objects on the slide. “These effects result in only certain parts of

the light beam being transmitted through certain parts of the slide”.”This

transmitted beam is then projected onto the viewing screen, forming an enlarged

image of the” slide.

TEMs work the “same way except that they shine a beam of electrons (like the

light) through the specimen (like the slide). Whatever part is transmitted is

projected onto a phosphor screen for the user to see”. Here is another more

scientific explanation of TEM and its working:

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1. The "Virtual Source" at the top represents the electron gun, producing a

stream of monochromatic electrons. 2. This stream is focused to a small, thin, coherent beam by the use of

condenser lenses 1 and 2. The first lens (usually controlled by the "spot size

knob") largely determines the "spot size"; the general size range of the final

spot that strikes the sample. The second lens (usually controlled by the

"intensity or brightness knob" actually changes the size of the spot on the

sample; changing it from a wide dispersed spot to a pinpoint beam. 3. The “beam is restricted by the condenser aperture (usually user selectable),

knocking out high angle electrons (those far from the optic axis, the dotted

line down the center)”. 4. The “beam strikes the specimen and parts of it are” transmitted. 5. This “transmitted portion is focused by the objective lens into an image”. 6. Optional “Objective and Selected Area metal apertures can restrict the

beam”; the Objective aperture enhancing contrast by blocking out high-angle

diffracted electrons, the Selected Area aperture enabling the user to examine

the periodic diffraction of electrons by ordered arrangements of atoms in the

sample. 7. The “image is passed down the column through the intermediate and

projector lenses, being enlarged all the” way. 8. The “image strikes the phosphor image screen and light is generated”,

allowing the user to see the image. The darker areas of the image represent

those areas of the sample that fewer electrons were transmitted through (they

are thicker or denser). The lighter areas of the image represent those areas of

the sample that more electrons were transmitted through (they are thinner or

less dense) as shown in Figure 2.5.

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Fig. 2.5 Schematic diagram of TEM showing the electron path TEM of the dispersions was performed on a Jeol 2100, operated at 200kV. Both

bright field and selected area electron diffraction (SAED) imaging modes were used.

Sample preparation involved dropping the graphene dispersions onto a holey carbon

grid (400 mesh). This type of TEM grid allowed flakes to be captured while the

solvent was free to percolate through the membrane. The volume dropped depended

on the concentration of the dispersion and in most cases the dispersion was diluted

by a factor of 10, or even 20. The grid was then either dried in a vacuum oven or in

the lab overnight. The bright field images taken on this TEM were used for

determining the level of exfoliation and for statistical analysis of dimensions and

thicknesses.

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A) B) C)

Fig. 2.6 TEM images of a 5 layered graphene flake. These TEM images demonstrate

how the lengths, widths and count the number of layers were measured.

As one can see from Figure 2.6 A, Lateral dimensions were measured by

approximating the longest axis as its length, L, and the dimension perpendicular to

the long axis as its width, w. The number of layers, N, was estimated by zooming in

on the edge of a flake and identifying the strata (Figure 2.6 C). Additional high

resolution electron diffraction patterns were obtained on the Titan Zeiss TEM. The

resulting spot diffraction patterns correspond to electrons that have been diffracted

from a specific region in single crystal graphene. Such patterns can be used for

identification of mono- , bi- or multi- layered graphene [8-10].

References [1] C.V. Raman and K.S. Krishnan., Nature 121 (1998) 501.

[2] M.J. Pelletier., Oxford: Blackwell Science vii (1999) 60.

[3] Horiba Scientific. Raman Spectroscopy.

http://www.horiba.com/scientific/products/raman-spectroscopy/

[4] J. Javier. An Introduction to Raman Spectroscopy,

http://www.spectroscopynow.com/coi/cda/detail.cda?id=1882&type=EducationFeat

ure&chId=6&page=1

[5] C. Harper, Handbook of Plastics, Elastomer, and Composites, McGraw-Hill, Inc,

New York (2002).

[6] SEM Schematic. 14 October 2010

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http://www.mse.iastate.edu/microscopy/path2.html.

[7] J. Ackermann, Manual for the SUPRA (VP) and ULTRA Scanning Electron

Microscopes (SmartSEM V 05.00). 2005, Carl Zeiss Ltd.

[8] J.C. Meyer, et al, Nature 446 (2007) 60.

[9] S. Horiuchi, et al, Applied Physics Letters 84 (2004) 2403.

[10] J. Meyer, et al, Solid State Communications 143 (2007) 101.

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Chapter 3

Concentrated Dispersion of Graphene&

Size Selection via Centrifugation

3.1 Objective

1.8mg/ml concentration of of graphene dispersion was reported before the

completion of this work which was not enough for use in the preparation of

nanocomposites,because production of nanocomposites requires the concentrated

dispersion of graphene. Highly concentrated dispersion of graphene was obtained in

this work using organic solvent. Similarly different flake sizes have different effects

on the mechanical performance of nanocomposites in terms of modulus, ultimate

tensile strength and elongation at break (dL at break), so graphene sheets were

separated according to its lateral dimensions for use as reinforcement in selected

polymer.

3.2. Introduction

It has been known for some years that graphite can exfoliated in the liquid phase to

give graphene [1]. There are two main ways to do “this oxidation of graphite

followed by exfoliation in water to give graphene oxide” [1-7] One “advantage of

GO based dispersion is that the flakes tend to be predominately monolayer”

However, the “oxidization process tends to introduce large quantities of structural

defects which shift the physical properties away from pristine graphene”[2-5].

While another procedure is exfoliation of graphite in solvents or surfactant solutions

to give dispersed pristine graphene [8-25]. “Solvent or surfactant exfoliated graphene

gives defect-free flakes but with relatively low monolayer content”. “Each method

results in dispersions with concentrations of up to a few mg/ml produced” in up to

litre batches [1].

Although advances in this field have been rapid, a number of outstanding problems

remain. Of these, probably the most important is the relatively low concentration of

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dispersed graphene. For example, graphene oxide has been “dispersed in some

organic solvents at concentrations of up to 1 mg/mL” [6,26-28] and in water having

concentrations of around 7 mg/mL [4]. Similarly, graphene was initially dispersed in

solvents at extremely low concentrations of ~10-2 mg/mL [16,17]. Recently, it was

shown that this could be increased to 1 mg/mL [18]. In contrast, surfactant-dispersed

graphene has not been achieved at concentrations above 1 mg/mL [20,29,30].

Although these concentrations are now in the appropriate range for a number of

applications,yet they are not high enough for applications like nanocomposite

production. For example, solution-phase polymer/graphene composite formation

[1,30,31] would be much simpler if well exfoliated graphene dispersions were

available at high concentrations. In addition, the deposition of thin films by vacuum

filtration followed by membrane dissolution [12] requires dilution with large

quantities of water before filtration. In addition, graphene flakes can be selected by

size or thickness by chromatography or density gradient centrifugation [13]. In both

cases, the amount separated is limited by the starting concentration. In these and

many other areas, a significant barrier to progress is the lack of high-concentration

dispersions.

Two different approaches were followed by using N-methyl-2-pyrrolidinone (NMP)

as solvent for the extremly high concentration of graphene.

First method gives concentrations of 17 mg/mL and second one about 63mg/mL.

Liquid “exfoliation of graphite is usually considered as a method to produce

graphene in large quantities for applications such as in composites materials.

However, many of these applications require flakes” “which are considerably larger

than those currently available”. Gong et al. recently “showed that in order to produce

effectively reinforced graphene–poly (methyl methacrylate) composites, the flake

length would have to be a few microns” or greater [27]. Currently available

“exfoliated graphene is usually significantly smaller than this which partly explains

why most graphene composite papers describe reinforcement values much lower

than the theoretical limit” [28] of dY/dVf ~1 TPa where Y is the composite modulus

and Vf is the graphene volume fraction [29–38]. Thus, there is a real need to

“increase the size of dispersed flakes. Ideally, we would tune the

dispersion/exfoliation process to give larger flakes”. However, “while some progress

has been made in this area, it is “worth exploring methods to post-treat existing

dispersions to select flakes by size””. While a number of methods have been

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demonstrated to separate GO flasks by lateral size [39–43], to our knowledge, lateral

size “selection has not been demonstrated for defect free graphene. Here, we describe

a method to take an existing dispersion of graphene in solvent and separate flakes by

size using controlled centrifugation”. We have produced a set of “dispersions with

mean flake lengths varying from 1 to 3.5 microns. This method is versatile and could

easily be applied to surfactant stabilized graphene” [19, 20, 22] or indeed any

exfoliated layered compounds [44].

3.3 Experimental Details

3.3.1 Concentrated dispersion of graphene.

Two samples of graphite dispersion in NMP with 100 mg/mL concentration

(dispersing 10gms graphite in 100 mL NMP) were sonicated by sonic tip and bath

sonicator indenpendetly.The small aliquots were taken from sonication system and

were periodically analyzed by UV-Vis spectroscopy and concentration of the

dispersion was measured at 660 nm peak for all the samples with the alpha

coefficient value to be 3.62 [18] as shown in Fig.3.1

For the 1st step of exfoliation maximum concentration of 0.545mg/mL and 2.01

mg/ml was observed for sonic bath and sonic tip samples respectively as shown Fig

3.2

Fig. 3.1 Absorbance versus wavelength curve for 1st cycle using (A) sonic bath (B) sonic tip

Time (hrs) Time (hrs)

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Fig. 3.2 Change in concentration with time for 1st step using (A) sonic tip and (B) sonic bath

After drop in concentration, fresh NMP was added to the already sonicated sample

after filteration and was again sonicated for six and ten hours respectivelyand

independently.A difference in the concentration was observed with time for both the

samples as shown in UV-Vis spectra Fig. 3.3

Fig. 3.3 Absorbance versus wavelength curve for 2nd cycle using (A) sonic tip for 6hrs (B)

sonic tip for 10hrs (C) sonic bath

A

A

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For sonic tip samples which was sonicated for 6 hrs continuously a maximum

concentration of 12 mg/mL was observed.while other sample which was pre-

sonicated for ten hours continiously maximum concentration of 17.8 mg/mL was

observed after 28 hrs sonication for it.(Figure 3.4 A and B).

The concentration obtained in second case is better than first one which suggests

that concentration of dispersion in 2nd cycle depends on the time for which it was

sonicated in 1st cycle.

Fig. 3.4 Change in concentration with time for 2nd cycle using (A) sonic tip for 6hrs (B)

sonic tip for 10hrs (C) sonic bath

3.3.2 Second Method for extremly high concentration of graphene

100 mg/ml dispersion was made by dispersing 10 gms of graphite in 100 mL of NMP

and was bath sonicated continuously for 11days. After 11days dispersion was

filtered, washed two times with fresh NMP and than re- dispersed in fresh NMP and

was again sonicated for 24 hours.Concentration was recorded initially after one

hour,then after every three hours and finally after every 24 hours using UV

spectroscopy.

3.3.2.1 Dispersions having,20 30,45 and 60mg/ml concentration.

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The prepared dispersion was centrifuged at 500 rpm for 45 minutes the supernatant

was collected and was filtered on filter membranne. Then the collected cake was re-

dispersed in 8 mL of NMP to make 20, 30,45 and 60 mg/mL concentrated

dispersions separately.Then its sedimentation study was conducted in order to study

its settling behavoiur with passage of time and these samples remained under study

for about 192 hrs. Its concentration was recorded after every 24 hrs.It was observed

that concentration remain stable at about 35 mg/mL for 63mg/mL and 45 mg/mL

sample even after the lapse of about 200 hrs.

Fig. 3.5 (A) Change in concentration with sedimentation of 63mg/mL sample (B)

Comparative study of sedimentation of 20,30 and 45mg/mL graphene concentartion

dispersions

3.4 Charetarization

3.4.1 Transmission Electron Microscopy (TEM) Histogram

Re--centrifuged

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Average flake size and number of layers in graphene flakes were found decreased

with sedimentation process of high concentrated dispersions. This result suggests that

bigger and heavier flakes settle with the sedimentation process and we are left with

thinner and lighter flakes after sedimentation (Figure 3.6,A,B,C,D). This study was

confirmed by conducting TEM statistics on about eighty flakes for high concentrated

dispersions of graphene

Fig 3.6 (A) Comparative study of average length, width and no of layers for 20mg/mL (B)

30mg/mL (C) 45mg/mL and (D) 63mg/mL dispersion

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3.5 Size selection of graphene Flakes according to its lateral

dimensions.

3.5.1 Experimental Details

100 mg/mL dispersions were made and were sonicated by sonic tip and sonic bath.

Tip sonic sample was sonicated for ten hours while that of sonic bath sample was

sonicated for ten days later on both sample were centrifuged at 500 rpm for 45

minutes in order to separate the exfoliated graphene from un-exfoliated graphite

flakes.

The supernatant were collected and filtered through nylon membrane. The filtered

cake was again re-dispersed in fresh NMP and both samples were centrifuged

according to given schematic diagram. For example sample was centrifuged at 5500

rpm for 45 minutes the supernatant was kept aside then again to the same sediments

fresh 1bout 20 mL NMP was added and again centrifuged. The process was

continued till 500 rpm and all the supernatants were kept aside for further studies.

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Fig 3.7. Schematic diagram of size separation of graphene flakes [45]

3.6 Results and Discussion

3.6.1 Concentration study

UV-Visible Measurements were carried out on the samples to understand the change

in concentration of sample for various rpms.

A very interesting phenomenon was observed for both sonic bath and sonic tip

exfoliation process. The concentration of graphene contents was gradually decreased

with decreasing the speed of centrifuge in terms of revolutions per minute (rpm)

while for sonic tip process the concentration of graphene increases with decreasing

centrifuge speed (rpm). It is very clear from Fig.3.8 & 3.9. The concentration at

various rpms tells us that we may have different concentration of different flake size

distribution in our initial dispersion.

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Fig. 3.8 Concentration of graphene exfoliated by tip sonicator and centrifuged at

various speed (rpm).

Fig. 3.9 Concentration of graphene exfoliated by bath sonicator and centrifuged at various

speed (rpm)

0 1000 2000 3000 4000 5000 6000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

co

nce

ntr

atio

n a

fte

r d

ilutio

n (

mg

/ml)

rpm

0 500 1000 1500 2000 2500 3000 3500 4000 4500

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

con

cen

tra

tion

(m

g/m

l)

rpm

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3.6.2 Raman Spectroscopy

Free standing films were prepared of the samples, based on alumina membranes,

(Whatman Anodisc 47mm with pore size of 0.02µm) for Raman spectroscopy.

Raman spectroscopy gives us information about the size of graphene flakes. If size of

flake is small then it will have high ratio of ID/ IG (“D” stands for Defect and”G”

graphite peak) and vice versa. An increase in the ID/IG ratio was observed with

increasing speed of centrifuge (rpm) indicating the increase in defect peak. It is

believed that the increasing ID/IG is dominated by new edge formation rather than by

basal plane defects (Figure 3.10). So it is expected that for small size graphene flakes

we will have more number of edges thus higher Defect peak as a result high value of

ID/ IG. This is confirmed by Raman spectra however, Raman spectroscopy is used in

tandem with TEM for better understanding of flake size.

Fig. 3.10 Change in ID/IG ratio of graphene exfoliated by (A) sonic tip, (B) sonic bath with

speed (rpm) of centrifuge

ID/IG ratio increases as number of defects increases in the sample as shown Figure

3.11 which is not due to the structural defects but due to the defects that are formed

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with more number of edges which means small flake size separated at high speed

(rpm) [18].

3.6.3 Transmission Electron Microscopy

As we discussed for the study of graphene flake size TEM is also used in

combination with Raman spectroscopy. TEM analysis performed on selected

samples which were also analyzed by Raman spectroscopy. The result of TEM

analysis confirmed that those flakes which were separated on high rpm having high

value of ID/IG ratio has small sizes as compared to those which were separated at low

rpm. In short flake size of graphene decreases with increasing speed (rpm) of

centrifuge and vice versa as shown in Figs.3.12 and 3.13.

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Fig. 3.11 Increase in the Defect peak of normalized Raman spectra with increasing rpm (A)

for sonic tip (B) for sonic bath

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Fig. 3.12 TEM images of graphene layers observed (a) un-separated graphene dispersion for

sonic tip (b) 500rpm sonic tip (c) 1000rpm sonic tip (d) 3000rpm sonic tip (e) un separated

graphene dispersion for sonic bath (f) 500rpm sonic bath (g) 1000rpm sonic bath (h)

3000rpm sonic bath.

Fig.3.13 (A) shows the change in the average values of length, width and number of

layers with change in rpm for sonic tip sample (B) sonic bath sample

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The above results become clearer when we plot the average of length, width and no

of layers with various rpms. It was found that there was a big difference in the flakes

size from the start to the end rpm in sonic bath sample but for sonic tip sample size

distribution of 500 and 1000rpm were almost same (Fig 3.10). As there was a good

difference in ID /IG ratio for sonic tip sample in Raman spectroscopy we assume that

there is a chance of error in calculating flake size distribution from TEM because in

this method we are just taking the average of 80 flakes so there is a big chance of

error which can justify the error in our graph.

Conclusion

Through this study we not only got highly concentrated dispersions of graphene via

two approaches but also separated it according to its lateral dimensions by controlled

centrifugation in order to study the effects of different flake size on the mechanical

properties of polymer based nanocomposites.

References

[1] S. Park, R.S. Ruoff, Nat Nanotechnol 4 (2009) 217.

[2] S. Park, J.H. An, I.W. Jung, R.D. Piner, S.J. An, X.S. Li, et al. Nano Lett. 9 (2009)

1593.

[3] S. Park, J.H. An, R.D. Piner, I. Jung, D.X. Yang, A. Velamakanni, et al, Chem.

Mater. 20 (2008) 6592.

[4] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A.

Stach, et al, Nature 442 (2006) 282.

[5] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, et

al, Carbon 45 (2007) 1558.

[6] S. Stankovich, R.D. Piner, X.Q. Chen, N.Q. Wu, S.T. Nguyen, R.S. Ruoff, J Mater

Chem. 16 (2006) 155.

[7] P. Blake, P.D. Brimicombe, R.R. Nair, T.J. Booth, D. Jiang, F. Schedin, et al, Nano

Lett. 8 (2008) 1704.

Page 82: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

62

[8] A.B. Bourlinos, V. Georgakilas, R. Zboril, T.A. Steriotis, A.K. Stubos. Small 5

(2009) 1841.

[9] A.B. Bourlinos, V. Georgakilas, R. Zboril, T.A. Steriotis, A.K. Stubos, C. Trapalis,

Solid State Communication 149 (2009) 2172.

[10] J.N. Colemanm, Adv Funct Mater. 19 (2009) 3680.

[11] S. De, P.J. King, M. Lotya, A. O’Neill, E.M. Doherty, Y. Hernandez, et al, Small 6

(2009) 458.

[12] A.A. Green, M.C. Hersam, Nano Lett. 9 (2009) 4031.

[13] C.E. Hamilton, J.R Lomeda, Z.Z. Sun, Nano Lett. 9 (2009) 3460.

[14] R. Hao, W. Qian, L.H. Zhang, Y.L. Hou, Chem Commun. 48 (2008) 6576.

[15] T. Hasan, F. Torrisi, Z. Sun, D. Popa, V. Nicolosi, G. Privitera, et al, Phys Status

Solidi B-Basic Solid State Phy. 247 (2010) 2953.

[16] Y. Hernandez, M. Lotya, D. Rickard, S.D. Bergin, J.N. Coleman, Langmuir 26

(2010) 3208.

[17] Y. Hernandez, V. Nicolosi, M. Lotya, F.M. Blighe, Z.Y. Sun, S. De, et al, Nat

Nanotechnol. 3 (2008) 563.

[18] U. Khan, A. O’Neill, M. Lotya, S. De, J.N. Coleman. Small 6 (2010) 864.

[19] M. Lotya, Y. Hernandez, P.J. King, R.J. Smith, V. Nicolosi, L.S. Karlsson, et al, J

Am Chem Soc. 131 (2009) 3611.

[20] M. Lotya, P.J. King, U. Khan, S. De, J.N. Coleman, ACS Nano 4 (2010) 3155.

[21] A. O’Neill, U. Khan, P.N. Nirmalraj, J.J. Boland, J.N. Coleman, J Phys Chem C.

115 (2011) 5422.

[22] R.J. Smith, M. Lotya, J.N. Coleman, New J Phys. 12 (2010) 125008.

[23] S. Vadukumpully, J. Paul, S. Valiyaveettil, Carbon 47 (2009) 3288.

[24] V. Alzari, D. Nuvoli, S. Scognamillo, M. Piccinini, E. Gioffredi, G. Malucelli, et al,

J Mater Chem. 21 (2011) 8727.

[25] A. Catheline, C. Valles, C. Drummond, L. Ortolani, V. Morandi, M. Marcaccio, et

al, Chem Commun. 47 (2011) 5470.

Page 83: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

63

[26] D. Nuvoli, L. Valentini, V. Alzari, S. Scognamillo, S.B. Bon, M. Piccinini, et al, J

Mater Chem. 21 (2011) 3428.

[27] L. Gong, I.A. Kinloch, R.J. Young, I. Riaz, R. Jalil, K.S. Novoselov, Adv Mater. 22

(2010) 2694.

[28] G.E. Padawer, N. Beecher, Polym Eng Sci. 10 (1970) 185.

[29] H.W. Hu, G.H. Chen, Polym Compos. 31(2010):1770.

[30] L. Jiang, X.P. Shen, J.L. Wu, K.C. Shen, J Appl Polym Sci. 118 (2010) 275.

[31] I.H. Kim, Y.G. Jeong, J Polym Sci Pol Phys. 48 (2010) 850.

[32] J.J. Liang, Y. Huang, L. Zhang, Y. Wang, Y.F. Ma, T.Y. Guo, et al, Adv Funct

Mater, 19 (2009) 2297.

[33] S.G. Miller, J.L. Bauer, M.J. Maryanski, P.J. Heimann, J.P. Barlow, J.M. Gosau, et

al, Compos Sci Technol. 70 (2010) 1120.

[34] K.W. Putz, O.C. Compton, M.J. Palmeri, S.T. Nguyen, L.C. Brinson, Adv Funct

Mater, 20 (2010) 3322.

[35] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera- Alonso, R.D.

Piner, et al, Nat Nanotechnol, 3 (2008) 327.

[36] P. Steurer, R. Wissert, R. Thomann, R. Mulhaupt, Macromol Rapid Commun. 30

(2009) 316.

[37] X.M. Yang, L.A. Li, S.M. Shang, X.M. Tao, Polymer 51 (2010) 3431.

[38] X. Zhao, Q.H. Zhang, D.J. Chen, P. Lu, Macromolecules 43 (2010) 2357.

[39] P.N. Nirmalraj, T. Lutz, S. Kumar, G.S. Duesberg, J.J. Boland, Nano Lett. 11 (2011)

16.

[40] P.E. Lyons, S. De, F. Blighe, V. Nicolosi, L.F.C. Pereira, M.S. Ferreira, et al, J Appl

Phys. 104 (2008) 044302.

[41] G. Eda, M. Chhowalla, Nano Lett. 9 (2009) 814.

[42] A.A. Green, M.C. Hersam, J Phys Chem Lett. 1 (2010) 544.

[43] X.M. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, et al, Nano Res.

1 (2008) 203.

Page 84: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

64

[44] J.N. Coleman, M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, et al, Science

331 (2011) 568.

[45] U. Khan, A. O'Neill, H. Porwal, P. May, K. Nawaz, J.N. Coleman, Carbon 50

(2012) 470.

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Chapter 4

Effect of Surfactant Concentration on

the Exfoliation of Graphite to

Graphene in Aqueous Media

4.1 Objective Graphite was exfoliated to graphene by tip sonic using sodium cholate as surfactant

in the presence of Millipore water as medium. Use of water as solvent in this study

for exfoliation purpose is very important due to its environment friendly nature and

almost no cost contrary to organic media. Two different concentration ratios of

surfactants are used. Graphene dispersions with two different concentrations of

5mg/ml and about 7 mg/mL respectively were obtained in aqueous media. It was

observed that optimum concentration of surfactant has effective role on exfoliation of

graphite to graphene. Concentrations of graphene dispersions were studied through

UV spectroscopy while, Raman spectroscopy, Scanning electron Microscopy (SEM)

and Transmission Electron Microscopy (TEM) were used to study the quality of

exfoliated graphene flakes.

4.2 Introduction Graphene is a nearly transparent, two-dimensional semimetal consisting of a single

atomic lattice of hexagonally arranged sp2 hybridized carbon atoms [1]. Since the

isolation of graphene and the discovery of its unique properties there have been

unprecedented levels of research on and related to the remarkable material. The work

carried out by Geim and Novoselov in 2004 was a simple exfoliation method in

which protrusions of highly-oriented pyrolytic graphite (HOPG) were embedded in

photo resist and adhesive tape was used to successively peel off layers of graphene

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[2]. Although this method is tedious and cannot be scaled up to industrial level yet

open new horizons of research in this specific field. This so called scotched tape

method is simple and does not require any modification to environmental parameters

such as temperature and pressure. In addition this method provides high quality (high

mobility and low defect) single and few layer graphene sheets with large areas as

high as 100μm [3]. Usually strong acids are used for the oxidation of graphite to

graphene oxide (GO) which results in stable aqueous solution of GO [4]. Then this

dispersion of GO can be reduced by aqueous hydrazine as reducing agent [5,6] or by

thermal reduction under a reducing atmosphere [7-9]. Graphene growth by chemical

vapor deposition (CVD) is typically carried out under ultra-high vacuum and at high

temperatures [10]. In this process volatile or gas phase carbon precursor is flowed

over a metallic substrate which acts as a catalyst and nucleation site for graphene

growth [11]. Graphene produced by CVD was first reported by Somani and co-

workers in 2006 using nickel foil and camphor for the metallic substrate and carbon

precursor respectively [12].

Liquid exfoliation of graphite to graphene, also referred to as solution based

graphene exfoliation, was first carried out by the Coleman group [13] in 2008 via

sonication of graphite flakes in organic solvents such as N-methyl-Pyrrolidinone

(NMP) and dimethyl formamide (DMF). Coleman’s work stemmed from previous

research involving dispersion of carbon nanotubes (CNTs) in organic solvents which

was concerned with matching the surface energies associated with CNTs and the

solvent [14]. The use of surfactants in liquid exfoliation is also carried out to create

aqueous dispersion of graphene help mitigate colloidal aggregation of graphene in

solution [15]. Other less common but noteworthy liquid exfoliation methods include

intercalation of graphite with alkaline [16] or halogen salts [17] to form graphite

intercalation compounds (GICs). The GIC’s can be either directly dispersed or exfoliated in solution by

sonication [16]. Likewise these can also be thermally expanded at high temperatures

in which the intercalating compounds volatilize to form expanded graphite (EG)

[18]. In next step these expanded graphite is subsequently exfoliated in solution via

sonication [17]. Liquid exfoliation of graphite to graphene is advantageous method

as compared to other methods such as CVD growth and mechanical exfoliation due

to the simplicity of the process [13]. This does not require high vacuum and high

temperatures as well as the low cost of the starting materials. There are presently

number of commercially available surfactants that have been used in the literature for

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solution processing of graphene by various methods and solvents [16,17]. Surfactant

assisted exfoliation has permitted the use of water as a solvent for solution

processing which is attractive from an environmental standpoint as well as for

applications which cannot tolerate organic solvents. The three main classes of

surfactants include cationic, anionic and nonionic surfactants. This also includes

small molecular surfactants such as sodium dodecyl sulfate (SDS) and sodium

cholate which consists of a hydrophobic tail and a polar head group. Similarly,

cationic, anionic, and non-ionic Pluronic and Tetronic block copolymer surfactants

have been used to form aqueous dispersions of graphene [19]. Likewise it has been

shown that graphene oxide can be dispersed in some organic solvents at

concentration up to 1mg/ml [6,20-22] and in water this concentration raised to

7mg/ml [23]. Similarly graphene concentration increased to about 1mg/ml in organic

solvents [24]. While surfactant based graphene dispersion in aqueous medium above

1mg/ml is not reported [25-27].

In this work we used two different concentrations of surfactants in order to study its

effects on exfoliation of graphite to graphene. Very reasonable concentrations of

graphene dispersions in water are obtained which is not previously reported.

Moreover, it is very interesting to see that low concentration of sodium cholate as

surfactant has very promising role on exfoliation and got high concentration of

graphene dispersion in water for low concentration of surfactant.

4.3 Experimental Procedure Graphite powder and Sodium cholate (surfactant) were purchased from Sigma –

Aldrich and were used as supplied. Sonication was performed by using sonic tip

(GEX600, 48W, 24kHz, flat head probe) running at 25% of maximum power and

sonic bath (Branson 1510E-MT). Centrifugation was performed using a Hettich

Mikro22R typically at 500 rpm for 45 minutes. After centrifugation the 70 % of top

portion of dispersed solution was removed and concentration was determined by

UV-Vis-IR absorption spectroscopy Varian Cary 6000i (with 1mm cuvettes). TEM

was done using a Joel 2100 and holey carbon grids (400 mesh). Thin film was made

using porous alumina membrane (“Whatman Anodisc 47 mm, pore size = 0.02

micron”). “Raman spectra (633 nm) were recorded on a Horiba Jobin Yvon

LabRAM-HR. Scanning Electron Microscopy” (SEM) was performed in a Hitachi S-

4300 field emission.

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4.4 Results and Discussion Graphite was exfoliated to graphene by using sonic tip and Millipore water (water

purified by Millipore technology) was used as solvent. During exfoliation sodium

cholate was used as surfactant to ease the exfoliation of graphite to graphene. We

used two different concentrations of the surfactant, i.e. 5 mg/ml and 10mg/ml

(concentration of surfactant CS=5mg/ml and CS=10mg/ml). Initially 10 grams of

graphite was added to Millipore water. Then surfactant was added in different

amounts to the graphite dispersions. Both of these dispersions were sonicated under

same conditions and small samples were taken from these dispersions hourly. The

samples were bath sonicated for 15 minutes using sonic bath. Finally these bath

sonicated samples were centrifuged at 500 rpm for 45 minutes. The 70 % of the top

portion of centrifuged sample is taken for absorbance study using UV

spectrophotometer, to know the concentration of graphene in these samples. This

process was continued for 96 hours and samples were taken after required interval of

time. The concentration was studied through UV spectroscopy using Lambert-Beer

law eq.(1) [24]

A= α Cl (4.1)

Where the absorption coefficient α is related to the absorbance A, C is concentration

and is l” the path length. We have selected α value equal to 3.62 ml/mg/mm [24].

The concentration was measured by recording the absorbance at 660 nm and

transformed this into the concentration using eq. (4.1) [24]. It is reported that the

exfoliation was conducted using organic solvent N-methyl Pyrrolidinone (NMP). The

NMP used to spoil after 6 hours which might be due to the oxidative degradation

[28]. But this phenomenon was not observed in our study and graphene concentration

tends to rise after every hour. Although the rate of exfoliation was not as fast and

high, as it was observed in NMP [28] but after 96 hours we got 5 mg/mL and 7

mg/mL concentrations of graphene for CS =10 mg/mL and CS = 5 mg/mL of

surfactant respectively.

In this work it has been observed that optimum concentration of surfactant has an

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effective role on the exfoliation of graphite to graphene. After 96 hours the

concentration of graphene exfoliated was 5 mg/mL in the presence of CS=10 mg/mL

of surfactant. While in case of CS= 5 mg/mL (concentration of sodium cholate) the

graphite was exfoliated to 7mg/ml under same conditions and time. It is very clear

from UV graph (Figure 4.1) that there is rapid increase in graphene concentration at

low concentration of surfactant (CS=5 mg/mL) compared to high (CS=10 mg/mL).

Latoya et. al. exfoliated graphite to graphene and obtained very low concentration in

presence of surfactant in aqueous medium [29].

Fig. 4.1 Concentration of graphene after centrifugation (500/45) as a function of sonication

time (Cs=5mg/ml). Concentration was calculated using absorption coefficient ―α value

equal to 3.62 mL/mg/mm

The TEM analysis were performed on flakes of graphene obtained after 96 hours of

sonication using surfactant with the concentration value CS=5 mg/mL deposited on

holey carbon grid. It is apparent that the exfoliated graphene flakes were in few

layers of graphene.

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Fig. 4.2 Concentration of graphene after centrifugation (500/45) as a function of sonication

time (CS=10mg/ml). Concentration was calculated using absorption coefficient ―α value

equal to 3.62mL/mg/mm.

TEM images are presented in Figure 4.3. It was revealed that large numbers of

graphene flakes of various types with different size are present as shown Figure 4.4

histogram-- showing number of layers of graphene after 96 hours of sonication along

with its length and thickness.

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Fig. 4.3 TEM images of graphene flakes deposited from sample having concentration of

7mg/mL

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Fig. 4.4 Histogram showing (Bottom) the number of layers per flake measured for 96

hours sonication time (centre) average length of flakes and (top) the average width of

flakes

It is very clear from this histogram that most of the population consists of few layer

graphene (less than five layers) and the length of flake is about 1.0 micron while the

width of the maximum population of graphene flakes is about 0.6 micron. It confirms

that graphite is fully exfoliated. Likewise Scanning Electron Microscopy (SEM) of

thin film (the segment of the film used for SEM was coated with 10-20 nm of

gold/palladium) also reveals that the exfoliated graphene consist of few layer as

shown in (Figure 4.5 A and B).

This study revealed about size of graphene flakes and defects in it. Spectrum of

graphite materials can be characterized by certain and specific bands like D-band

(1350cm-1

), G-band (1582cm-1

), and 2D band (2700cm-1

) [30]. D-band shows the

evidence of the presence of topological defects in sheets or edges of nanosheets. [31].

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Fig. 4.5 (A) SEM images of the flakes present on the interface of the free standing films

prepared

Fig. 4.5 (B) SEM image showing the fractured interface

A typical Raman spectrum was measured on film prepared from the sample having

7mg/ml concentration. The film was deposited on alumina membrane by filtering the

aqueous graphene dispersion under vacuum. This film was thoroughly washed with

plenty of de-ionized water to make it free from surfactant. For solvent exfoliated

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graphene the D band is associated with presence of flakes edges and can be linked to

flake length by relation in eq. 2.

ID/IG-(ID/IG) powder = k/L (4.2)

Where k is constant [34, 35], so increase in ID/IG value shows decrease in flake size

and vice versa.

The value of k is reported to be =0.26, this also gave (ID/IG) powder = 0.037 [24,32].

It is clear from Figure 4.6, ID/IG value of exfoliated graphene as function of

sonication time increases. The ID/IG of graphene sonicated for 30 hours has low

value of ID/IG while as sonication proceeds on the ID/IG increases and reaches its

maximum value at 96 hours.

Fig. 4.6 Increase in Defect peak (D-band) of normalized Raman spectra as a function of

sonication time

The data is also presented in Figure 4.7 just to show the values of ID/IG with passage

1600 2400 3200 0.00 0.31 0.62 0.93 0.00 0.38 0.76 1.14 0.00 0.38 0.76 1.14 0.00 0.38 0.76 1.14 0.00 0.38 0.76 1.14 0.00 0.38 0.76 1.14

1000 2000 3000

Wave number (cm-1)

24 hrs

36 hrs

48 hrs

64 hrs

24 hrs

80 hrs

96 hrs

Norm

aliz

ed

avera

ge

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of time. The ID/IG value continuously increases from 0.113 at 30hrs to 0.317 at 96

hrs, while (ID/IG) powder for graphite powder is 0.037 [24,32]. This suggests that

with sonication time there is increase in ID/IG value suggesting decrease in flake

size. Which suggests that sonication creates number of defects in graphene flakes by

cutting the size of graphite creating new edges [33-34]. Equation (2) can be used to

estimate the flake length. By putting obtained different values of ID/IG in eq. (4.2).

Fig. 4.7 Change in ID/IG as a function of sonication time

It is shown in Figure 4.8 that flake length decreases from 3.42 micron to 1 micron

after sonication for 96 hours. Similarly the Raman data of estimated flake length of

graphene nanosheets on the basis of ID/IG value is in close agreements with TEM

study. The histogram shown in Figure 4.4 indicates that most of the flakes consist of

less than five layers and having flake length between 1-1.5 microns with average

width of 0.6 micron.

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Fig. 4.8 Estimated length of graphene flake with D/G values.

Conclusion Although high concentration of exfoliated graphene is earlier reported in organic

solvent, yet due to the use of organic solvent environmental issues can be raised. So

in this work water was used as solvent. It has been observed that reasonable

concentration of graphene in aqueous media in short time can also be obtained by

using sonic tip in the presence of surfactant. The flakes length obtained through this

procedure is about one micron with few layer thicknesses. Likewise the remaining

part of graphite crystallites/un-exfoliated graphite can further be used for further

exfoliation after filtration for better results in terms of high concentration.

Concentration of graphene dispersion can be increased from 7mg/mL by just finding

some other suitable surfactant for this purpose.

References

[1] R. Ruoff, Nature Nanotechnology 3 (2008) 10.

[2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V.

Grigorieva, A.A. Firsov, Science 306 (2004) 666.

[3] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

D/G Values of Raman Spectrum

Fla

ke length

of

Gra

phene (

mic

ron)

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77

Phys. 81 (2009) 109.

[4] S. Some, Y. Kim, E. Hwang, H. Yoo, H. Lee, Chem. Commun. 48 (2012) 7732.

[5] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Nature Nanotechnology 3

(2008) 101.

[6] V.C. Tung, M.J. Allen, Y. Yang, R.B. Kaner, Nature Nanotechnology 4 (2009) 25.

[7] X.F. Gao, J. Jang, S. Nagase, J. Phys. Chem. C 114 (2010) 832.

[8] N.W. Pu, C.A. Wang, Y.M. Liu, Y. Sung, D.S. Wang, M.D. Ger, J. Taiwan Inst.

Chem. Eng. 43 (2012) 140.

[9] D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R.D. Piner, S.

Stankovich, I. Jung, D.A. Field, C.A. Ventrice, R.S. Ruoff, Carbon 47 (2009) 145.

[10] W. Choi, I. Lahiri, R. Seelaboyina, Y.S. Kang, Critical Reviews in Solid State and

Materials Sciences 35 (2010) 52.

[11] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Progress in Materials

Science 56 (2011) 1178.

[12] P.R. Somani, S.P. Somani, M. Umeno, Chemical Physics Letters 430 (2006) 56.

[13] Y. Hernandez, V. Nicolosi, M. Lotya, F.M. Blighe, Z. Sun, S. De, I.T. McGovern, B.

Holland, M. Byrne, Y.K. Gun'ko, J.J. Boland, P. Niraj, G. Duesberg, S.

Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A.C. Ferrari, J.N. Coleman,

Nature Nanotechnology 3 (2008) 563.

[14] S.D. Bergin, V. Nicolosi, P.V. Streich, S. Giordani, Z. Sun, A.H. Windle, P. Ryan,

N.P.P. Niraj, Z.-T.T. Wang, L. Carpenter, W.J. Blau, J.J. Boland, J.P. Hamilton, J.N.

Coleman, Advanced Materials 20 (2008) 1876.

[15] A.A. Green, M.C. Hersam, Nano Letters 9 (2009) 4031.

[16] K.H. Park, B.H. Kim, S.H. Song, J. Kwon, B.S. Kong, K. Kang, S. Jeon, Nano

Letters 12 (2012) 2871.

[17] S. Lin, C.-J. Shih, M.S. Strano, D. Blankschtein, Journal of the American Chemical

Society 133 (2011) 12810.

[18] Y.A. Nikitin, M.L. Pyatkovskii, Powder Metallurgy and Metal Ceramics 36 (1997)

Page 98: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

78

41.

[19] J.-W.T. Seo, A.A. Green, A.L. Antaris, M.C. Hersam, Journal of Physical Chemistry

Letters 2 (2011) 1004.

[20] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y.

Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558.

[21] J.R. Lomeda, C.D. Doyle, D.V. Kosynkin, W.-F. Hwang, J.M. Tour, Journal of the

American Chemical Society 130 (2008) 16201.

[22] G. Williams, B. Seger, P.V. Kamat, Acs Nano 2 (2008) 1487.

[24] S. Park, J. An, R.D. Piner, I. Jung, D. Yang, A. Velamakanni, S.T. Nguyen, R.S.

Ruoff, Chemistry of Materials 20 (2008) 6592.

[25] U. Khan, A. O'Neill, M. Lotya, S. De, J.N. Coleman, Small 6 (2010) 864.

[26] M. Lotya, P.J. King, U. Khan, S. De, J.N. Coleman, Acs Nano 4 (2010) 3155.

[27] L. Guardia, M.J. Fernandez-Merino, J.I. Paredes, P. Solis-Fernandez, S. Villar-

Rodil, A. Martinez-Alonso, J.M.D. Tascon, Carbon 49 (2011) 1653.

[28] Y.T. Liang, M.C. Hersam, Journal of the American Chemical Society 132 (2010)

17661.

[29] U. Khan, H. Porwal, A. O'Neill, K. Nawaz, P. May, J.N. Coleman, Langmuir 27

(2011) 9077.

[30] M. Lotya, Y. Hernandez, P.J. King, R.J. Smith, V. Nicolosi, L.S. Karlsson, F.M.

Blighe, S. De, Z. Wang, I.T. McGovern, G.S. Duesberg, J.N. Coleman, Journal of

the American Chemical Society 131 (2009) 3611.

[31] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S.

Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Physical Review Letters

97 (2006).

[32] C. Casiraghi, A. Hartschuh, H. Qian, S. Piscanec, C. Georgi, A. Fasoli, K.S.

Novoselov, D.M. Basko, A.C. Ferrari, Nano Letters 9 (2009) 1433.

[33] A. O'Neill, U. Khan, P.N. Nirmalraj, J. Boland, J.N. Coleman, J. Phys. Chem. C 115

(2011) 5422.

Page 99: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

79

[34] U. Khan, A. O'Neill, H. Porwal, P. May, K. Nawaz, J.N. Coleman, Carbon 50

(2012) 470.

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Chapter 5

Observation of mechanical percolation

in functionalized graphene oxide –

elastomer composites

5.1 Objective

We have covalently functionalized graphene oxide (GO) with octadecylamine

(ODA) to form GO-ODA. This material can be dispersed in tetrahydrofuran (THF)

and subsequently formed into composites with polymers such as polyurethane.

Prominent rise in stiffness and low-strain stress were observed at loading levels up to

50wt%. However, most interestingly we found no increase in these properties at

loading levels below 2.5vol%. Reinforcement appeared to turn on sharply at this

volume fraction and subsequently increase as a power law with volume fraction.

This behavior is typical of percolation and shows that the low-strain stress cannot be

enhanced until the functionalized graphene flakes form a percolating network.

Slightly different behavior is observed for properties related to material failure. The

ultimate tensile strength increased linearly with graphene content up to the

percolation threshold before subsequently falling off. Similarly the ductility was

constant below the percolation threshold but fell off dramatically above it. This work

shows the importance of network formation in the reinforcement of elastomeric

materials.

5.2. Introduction

Graphene oxide (GO) [1] is an exceptional material which has shown great promise

in a number of application areas, [2-5]. One area where GO is expected to make a big

impact is as a filler in composites. Incorporation of GO into polymers such as

polyvinyl alcohol can result in significant increases in mechanical properties, [6],

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while composite formation with reduced GO can give large increases in electrical

properties [1]. However, due to its polar nature, as-produced GO is not a natural

filler material for non-polar polymers. This can be addressed by the covalent

functionalization of GO with non-polar polymers or molecules. These groups alter

the surface chemistry thus compatiblising the functionalized GO with both non-polar

solvents and polymer matrices.

Of particular interest is the functionalization of graphene oxide with octadecylamine

(ODA). The reason for this is that the amine group of the ODA should reach readily

with epoxides or carboxylic acid groups in the GO [7-9]. Critically, this reaction can

be followed with FTIR [10] allowing researchers to be confident that they have

produced GO-ODA. Because of the presence of covalently attached non-polar

groups, GO-ODA should disperse well in non-polar polymers. This makes it a good

model system for the exploration of the properties of polymer-graphene composites

processed using organic solvents.

One interesting class of composite consists of thermoplastic elastomers filled with

nano-fillers such as carbon nanotubes or graphene [11-14]. These are of great

interest as addition of the nano-filler can result in the scaling of ductility and low-

strain stress all the way from those of an elastomers to a rigid thermoplastic [15, 16].

However, the reinforcement mechanism at work in these systems is unclear. We

suggest that GO-ODA is an ideal model system to investigate the nature of this

mechanism. In this work, we functionalize GO with ODA. FTIR shows the

attachment to be covalent while SEM analysis shows the GO-ODA to be well

dispersed, even at very high loading levels. Mechanical measurements show no

reinforcement at volume fractions below 2.5%. Above this filler content, both

stiffness and low-strain stress increases significantly in line with Percolation theory.

However, we find that once a percolating network is first formed, the ultimate tensile

strength and ductility begin to fall with increasing graphene content.

5.3 Experimental Procedure Graphite powder (Sigma Aldrich) was oxidized using a modified version of the

Hummers method [17]. In brief, “graphite powder was oxidized using NaNO3,

H2SO4 and KMnO4 in an ice bath [17]. The material obtained was centrifuged at

2000 rpm for 10 minutes. The supernatant, containing inorganic salts”, was decanted

and the deposit washed several times with de-ionized water until the pH was neutral.

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This sediment was dried overnight in a vacuum oven at 80°C.

The dried GO was treated with ODA as previously described [8] with slight

modification.Briefly,300 mg of “GO was dispersed in 30 ml of water” while 300 mg

of ODA was dissolved in 30 ml of dimethyl formamide (DMF). Both the dispersions

were mixed together in round bottom flask and refluxed for 48 hours at 100oC under

continuous stirring. The resultant material was filtered through a “nylon membrane

of pore size 0.45 µm” (Sterlitech). The filtered material was thoroughly washed with

THF to remove any un-reacted, free ODA.

In next step the ODA functionalized graphene (GO-ODA) was dispersed in THF by

sonication for 30 minutes in a sonic bath (Branson 1510-MT). The dispersion was

centrifuged at 500 rpm for 45 minutes using a Hettich Mikro 22R to remove large

aggregates or un-functionalized GO. The supernatant was collected and filtered

through a nylon membrane of pore size 0.45 µm (Sterlitech). The membrane

supported GO-ODA was dried in a vacuum oven overnight at 60 oC to facilitate

accurate mass determination. The thermoplastic polyurethane (TPU) used in this

work was Morthane which was obtained from Huntsman Polyurethanes (Morthane

PS455-203 - an aromatic polyester based thermoplastic polyurethane). This polymer

(6 g) was dissolved in 100ml of THF (60 mg/ml) by stirring at 40oC for 24 hours.

GO-ODAwas added to the TPU solution at the required concentration and sonicated

for 30 minutes.

Many composite dispersions were prepared containing various mass fractions of

ODA-GO from 1 to 50 wt %. These dispersions were sonicated for four hours in a

sonic bath and were cast in Teflon trays of dimensions 4×4×2 cm. These cast

samples were dried at room temperature and 900 mbar in a vacuum oven for 24 hrs.

The dried films were then placed in an oven at 60oC for 48 hours to remove trace

THF. All samples were of constant mass (~150 mg). The film thicknesses were in

the range of 50 to 60 microns. A reference sample of TPU was also prepared.

Mechanical “testing was performed using a Zwick-Roell tensile tester with a 100N

load cell at a strain rate of 50mm/min. Scanning electron microscopy (SEM) was

performed using a Carl Zeiss Ultra Plus Field Emission Scanning Electron

Microscope”. FTIR was measured on crushed powder on a glass slide in

transmittance mode using Nexus Nicolet FTIR.

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5.4 Results and Discussion Initially, it is important to ascertain whether the ODA has been covalently

attached to the GO. We expect the reaction to be of the form [8].

R-CO-OH + R-NH2 RCO-NH-R + H2O (5.1) Where the Nitrogen of the ODA bonds to the carbonyl carbon of the GO, although

reaction with the basal plane epoxides has also been suggested,[7-9].To test this

hypothesis, we performed FTIR spectroscopy on the GO, neat ODA and GO-ODA

samples (Figure 5.1). The GO exhibits the characteristic bands at 1720, 1630, 1390,

1220 and 1050 cm-1

, which can be associated with C=O, C=C, C-O (carboxy), COH

and C-O (alkoxy) groups, and a broad absorption band between 3000 and 3500

cm−1

associated with the hydroxyl groups [18-21].

C-O

GO-ODA

ODA

GO

C=O

C-OH

AlkylC-H

N-H

Amide C=O

(alkoxy)

C-O (alkoxy)

C-O (carboxy)

Tra

nsm

issio

n

4000 3000 2000 1000

Wavenumber (cm-1)

Fig. 5.1 FTIR spectra of GO (top), ODA (middle) and GO-ODA (bottom).

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The FTIR spectrum of the neat ODA shows a peak characteristic of alkyl groups at

2850 cm-1

and a peak at 2918 cm-1

due to asymmetric C-H stretching. The peaks

around 1500 cm-1

are due to methylene scissoring deflection and other effects

associated with the alkyl chain [22]. Most importantly, we observe a band at 3333

cm-1

associated with the amine group (N–H) [4, 17]. In the GO-ODA FTIR spectra,

this peak has totally disappeared, strongly suggesting that the nitrogen of the ODA

has covalently bonded to the GO as described above [8]. Further evidence of

functionalization comes from the fact that carbonyl C=O band at 1720 cm-1

in GO

sample is obscured in the GO-ODA spectra and new band has appear at around 1640

cm-1

. This is consistent with amide C=O stretching as would be expected from the

reaction above [18]. Thus, we believe that “FTIR spectra clearly confirm that the

ODA molecules were attached to the graphene nanosheets through chemical

modification”.

Unlike GO, the GO-ODA was readily dispersible in THF. This allows the formation

of composites by blending dispersions of GO-ODA with solutions of TPU in THF.

These composite dispersions can then be formed into composite films. In all cases,

including 50wt% GO-ODA, composites were very uniform to the naked eye with no

appearance of aggregates. Figure 5.2 shows an SEM image of the fracture surface of

a 40 wt% GO-ODA composite. The inset shows a high magnification image showing

a homogeneous dispersion of GO-ODA within the polymer.

Fig. 5.2 SEM images of 40 wt% composite film.

We performed mechanical characterization of GO-ODA/TPU composites for a range

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of GO-ODA mass fractions. Major stress strain curves for the composites are shown

in Figure 5.3.

40

10 4%

8

(MP

a) 30

6

4 1%

2

0 PU

20

0 2 4 6 8 10

10%

Str

ess,

50%

40%

10

0

0 200 400 600 800 1000

Strain, (%)

Fig. 5.3 Representative stress strain curves. Inset: the low strain regime.

It is clear from this graph that the presence of GO-ODA has a significant effect on

the polymer. For low mass fractions the stress at all strains appears to increase while

for higher mass fractions, both stress and ductility fall dramatically. From the stress

strain curves, one can extract four main mechanical properties; the Young’s modulus

or stiffness, E, the stress at low (3%) strain, 3% , the ultimate tensile strength,

B , and the strain at break, B . The average values of these quantities are shown as a

function of the filler mass fraction in Figure 5.3. The Young’s modulus appeared to

increase almost linearly with increasing GO-ODA mass fraction from 9.6 MPa for

the polymer to 335 MPa for the 50wt% composite. Similarly, the stress at 3% strain

increased in an almost linear fashion from 0.3 MPa for the polymer to ~10 MPa for

the 50wt% composite. In contrast, the ultimate tensile strength initially increased

from 27 MPa for the polymer to 38 MPa for the 3 wt% composite before falling

steadily, reaching 10 MPa for the 50wt% composite. Interestingly, the initial increase

was linear with a slope of 330 MPa. Although a linear increase is “predicted by the

rule of mixtures”, this slope is low compared with typical values of GPa usually

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found for composites of thermoplastic polymers filled with nanotubes or graphene

[23, 24]. The strain at break decreased steadily with increasing GO-ODA mass

fraction from ~1000% for the polymer to ~10% for the 50 wt% sample.

Broadly speaking, these results are typical of what is observed for elastomers filled

with high volume fractions of nanotubes [11, 13, 16] or graphene [15]. These results

appear similar to those of Khan et al who mixed pristine solvent-exfoliated graphene

with thermoplastic polyurethane [15]. The starting polyurethane had very similar

properties while their 50wt% sample had values of E~1 GPa, 3% ~10 MPa,

B~30 MPa and B~10%, values almost identical to those found here. This implies

that the ODA functional groups (or indeed the oxides) play no significant role in the

reinforcement but ensure good dispersion.

We can also compare these results with other work on composites of thermoplastic

elastomers filled with other nano materials. Liff et. al. [25] increased the modulus of

polyurethane by a factor of 20 by addition of 20wt% nanoclay. By the inclusion of

functionalized nanotubes in TPU, Koerner et. al. [26] reported a thirteen-fold

increase in modulus and a significant enhancement of the strength. Sahoo et. al. [27],

fabricated functionalized SWNT/ PU composites and achieved an increment of 3

times in modulus at 20% loading. While a seven fold increase in modulus was

reported by Cheng et. al. in PU composites reinforced by functionalized MWCNTs

[28], the ductility of the composites was reduced significantly.

It is worth looking at the dependence of mechanical properties on graphene content

in more detail. To do this we plot the same data as in Figure 5.4 A and B but in

double logarithmic format.

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0.0

0.1

0.2

0.3

0.4

0.5

E (

GP

a)

A

0

2

4

6

8

10

=

3% (

MP

a) B

0

10

20

30

40

50

B (

MP

a)

C

0 10 20 30 40 50

0

400

800

1200

B (

%)

Graphene content (wt%)

D

Fig. 5.4 Effect of GO-ODA content on mechanical properties of composites. (A) Young’s

modulus, (B) stress at 3% strain, (C) Ultimate tensile strength, (D) strain at break.

In addition, we have transformed the mass fraction to volume fraction assuming a

GO-ODA density of 1800 kg/m3 and a TPU density of 1100 kg/m

3. This data is

shown in Figure 5.5 A and B and immediately illustrates some behavior not

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apparent from the linear graphs. It is clear from figures 5.4 A and B that the modulus

and stress at low strain hardly increase at all for graphene contents below ~2.5vol%.

A

E (

GP

a)

0.1

0.01

(MP

a) 10 B

=3

%

1

50

(MP

a) 40

30

20

B

10 C

0

1000

(%) 800

600

B

400

200

D

0

0.01 0.1 0.6

Graphene volume fraction,

Fig. 5.5 The same data as in figure 6.4 but plotted as a function of GO-ODA volume fraction

on a log-log plot. The lines in (A) and (B) illustrate percolation-like behavior while the line

in (C) illustrates linearity. The vertical arrows illustrate the percolation threshold while the

horizontal arrows show that value of each property displayed by the polymer.

However, above this threshold, both properties increase with graphene content as a

power law. Such non-linear increases in mechanical properties of elastomeric

composites are generally described as the Payne effect. This is usually attributed to

either the effects of strong matrix-filler interaction or the formation of a filler

network [29]. Here the fact that essentially no increase in modulus (or 3% ) are

observed below 2.5 vol% suggests the latter mechanism to be at work. Indeed, this

lack of reinforcement at low volume fraction implies that neither polymer-filler stress

transfer nor hydrodynamic effects are important [29]. The increase in E and 3%

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above 2.5 vol% implies that network formation begins at this critical volume

fraction. Network formation and growth can be described by percolation theory [30]

Where the formation of networks of the filler controls the mechanical properties of

composites, it has been shown that certain mechanical properties are described by a

percolation-like scaling law [21, 25]

A AP A0 c t

(5.2)

Here A can represent either E or 3% , is the “filler volume fraction, c is the

percolation threshold, i.e. the critical filler” volume fraction when a network first

forms and AP is the value of the relevant mechanical property at the percolation

threshold. A0 and t are constants. As can be seen from figure 5.5 A and B, both E and

3% are described very well by this expression. In both cases, the percolation

threshold was c=2.5 vol% while the percolation exponent was t=0.8. This can be

compared to percolation thresholds of 1.4vol% and 6wt% and exponents of 1.5 and 2

as reported by Ramorino and Liff respectively using nanoclays as fillers [21, 25].

These results imply that that reinforcement of TPU is not due to standard

mechanisms which rely on stress transfer at the polymer-filler interface or the effect

of the filler particles on polymer flow under stress. Rather, the critical factor is the

formation of a network which mechanically stiffens the material. We interpret this by

assuming that once the network forms, the GO-ODA flakes form a jammed system.

It has been shown that for high concentration carbon black dispersions, such a

system can be described by a shear modulus which obeys a percolation scaling law

[31]. We believe that in the case of a solid state composite, this behavior manifests

itself as the tensile modulus (and low-strain tensile stress) obeying a percolation

scaling law.

The formation of such a network is likely to impact on other mechanical properties.

We note that E and 3% are low strain properties. In figure 5.5 C and D we

consider high strain properties i.e. those associated with fracture; B and B . As

described above, the ultimate tensile strength, B , increases linearly (solid line) at

low strain. However, it is clear from figure 5.5 C that the point where linearity ceases

and B begins to fall off coincides with the percolation threshold. In addition, as

demonstrated in figure 5.5 D, the strain at break is reasonably constant up to the

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percolation threshold, after which it falls dramatically with GO-ODA volume

fraction. It is not clear why this should be the case. However, it may be that polymer

chains in the vicinity of the graphene surface tend to have reduced mobility. This

may reduce the scope of large strain deformation resulting in premature failure.

With this in mind, it is worth considering the nature of the interfacial region.

Previously, it has been shown that ODA functionalized SWNTs selectively interact

with the hard segments of thermoplastic elastomers as indicated by differential

scanning Calorimetry and other techniques [13]. Interestingly, these composites

showed very similar mechanical properties to those studied here; an apparent linear

increase in modulus and a peak in strength close to 2% volume fraction. Also they

observed a slow decrease in strain at break as ODA-SWNTs were added, reaching

~600% for a nanotubes content of 10%, very similar to what was observed here.

Thus it is likely that the ODA functionalized graphene sheets interact predominately

with the PU hard segments. Anchoring of these hard segments at the GO-ODA

surface may immobilize adjacent soft segments, limiting polymer flow and

ultimately causing premature failure.

Conclusion

We have used graphene oxide covalently functionalized with octadecylamine as model

filler in polyurethane based composites. We find no appreciable increases in either

stiffness or low-strain stress for loading levels below 2.5vol%. However above this

threshold, both mechanical quantities increase as a power law. This behavior is

consistent with mechanical percolation. This implies that the graphene oxide platelets

are effectively isolated at low volume fractions but begin to form a network at a volume

fraction of 2.5vol%. This loading level can be thought of as a percolation threshold. As

the loading level is increased, the network becomes more extensive and the stiffness and

low-strain stress increase as described by the percolation scaling law. Interesting the

ultimate tensile strength initially increases but reaches a maximum at the percolation

threshold. Similarly the ductility is invariant with graphene content up to the

percolation threshold, after which it falls steadily. This work shows that the

mechanical properties of elastomers reinforced with graphene can depend on

parameters other than interfacial stress transfer. For example, the formation of a

network of filler particles which acts like a jammed system can dominate the

mechanical properties of the system.

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References

[1] S. Stankovich, D.A. Dikin, G.H.B. Dommett, et al, Nature. 442 (2006) 282.

[2] M.J. Allen, V.C. Tung, R.B. Kaner. Chemical Reviews. 110 (2010) 132.

[3] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, Chemical Society Reviews. 39

(2010) 228.

[4] P. Capkova, M. Pospisil, M. Valaskova, et al, Journal of Colloid and Interface

Science. 300 (2006) 264.

[5] Y.W. Zhu, S. Murali, W.W. Cai, et al, Advanced Materials. 22 (2010) 3906.

[6] K.W. Putz, O.C. Compton, M.J. Palmeri, et al, Advanced Functional Materials. 20

(2010) 3322.

[7] A.B. Bourlinos, D. Gournis, D. Petridis, et al, Langmuir. 19 (2003) 6050.

[8] G.X. Wang, X.P. Shen, B. Wang, et al, Carbon. 47 (2009) 1359..

[9] O.C. Compton, D.A. Dikin, K.W. Putz, et al, Advanced Materials. 22 (2008) 892.

[10] S. Niyogi, E. Bekyarova, M.E. Itkis, et al, Journal of the American Chemical

Society. 128 (2006) 7720.

[11] F.M. Blighe, W.J. Blau, J.N. Coleman, Nanotechnology. 19 (2008) 415709.

[12] D. Cai, K. Yusoh, M. Song, Nanotechnology. 20 (2009) 085712.

[13] U. Khan, F.M. Blighe, J.N. Coleman, The Journal of Physical Chemistry C. 114

(2010) 11401.

[14] A.V. Raghu, Y.R. Lee, H.M. Jeong, C.M. Shin, Macromolecular Chemistry and

Physics. 209 (2008) 2487.

[15] U. Khan, P. May, A. O'Neill, J.N. Coleman, Carbon. 48 (2010) 4035.

Page 112: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

92

[16] U. Khan, P. May, A. O'Neill, J.J. Vilatela, A.H. Windle, J.N. Coleman. Small. 7

(2011) 1579.

[17] W.S. Hummers, R.E. Offeman, Journal of the American Chemical Society. 80

(1958) 1339.

[18] Y.S. Yun, Y. Bae, D.H. Kim, et al, Carbon. 49 (2011) 3553.

[19] S. Park, D.A. Dikin, S.T. Nguyen, R.S. Ruoff, Journal of Physical Chemistry C. 113

(2009) 15801.

[20] J.I. Paredes, S. Villar-Rodil, A. Martinez-Alonso, J.M.D. Tascon, Langmuir. 24

(2008) 10560.

[21] S. Stankovich, R.D. Piner, S.T. Nguyen, R.S. Ruoff. Carbon. 44 (2006) 3342.

[22] J.J. Benitez, M.A. San-Miguel, S. Dominguez-Meister, et al, Journal of Physical

Chemistry C. 115 (2011) 19716.

[23] J.N. Coleman, U. Khan, W.J. Blau, et al, Carbon. 44 (2006) 1624.

[24] T. Kuilla, S. Bhadra, D.H. Yao, et al, Progress in Polymer Science. 35 (2010) 1350.

[25] S.M. Liff, N. Kumar, G.H. McKinley, Nature materials. 6 (2007) 76.

[26] H. Koerner, W. Liu, M. Alexander, et al, Polymer. 46 (2005) 4405.

[27] N.G. Sahoo, Y.C. Jung, H.J. Yoo, J.W. Cho. Macromolecular Chemistry and

Physics. 207 (2006) 1773.

[28] H.K.F. Cheng, N.G. Sahoo, Y. Pan, et al, Journal of Polymer Science Part B:

Polymer Physics. 48 (2010) 1203.

[29] G. Ramorino, F. Bignotti, S. Pandini, et al, Composites Science and Technology. 69

(2009) 1206.

[30] D. Stauffer, A. Aharony, Introduction to Percolation Theory. 2nd ed. London:

Taylor & Francis 1985.

[31] V. Trappe, V. Prasad, L. Cipelletti, et al, Nature. 411 (2001) 772.

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Chapter 6

Improved Adhesive Strength and

Toughness of Polyvinyl Acetate Glue

on Addition of Small Quantities of

Graphene

6.1 Objective Composites of polyvinyl acetate (PVAc) reinforced with solution exfoliated

graphene were prepared. A 50% increase in stiffness and a 100% increase in tensile

strength on addition of 0.1 vol % graphene compared to the pristine polymer was

observed. As “PVAc is commonly used commercially as glue, we have tested such

composites as adhesives”. The “adhesive strength and toughness of the composites

were up to 4 and 7 times higher”, respectively, than the pristine polymer.

6.2 Introduction Adhesives play a critical role of modern manufacturing and are essential in a wide

range of areas from packaging to electronics [1] to aerospace technology [2,3]. While

they come in many forms, possibly the simplest are synthetic thermoplastic

adhesives. Essentially, these are high concentration polymer solutions which can be

spread on the surfaces to be bonded. After the surfaces are brought into contact, the

solvent slowly evaporates to give a solid polymer which forms an effective bond.

In general, adhesives can fail cohesively or adhesively, that is within the bulk of the

adhesive or at the adhesive−surface interface. Many synthetic thermoplastic

adhesives form relatively strong interfacial bonds. In addition, when a porous

material such as wood is bonded, the adhesive can permeate into the pores, resulting

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in mechanical interlocking and an increase in the bonded area [4]. This means that

the limitations of synthetic thermoplastic adhesives can sometimes be associated

with the mechanical properties of the polymer. Amorphous polymers tend to have

limited mechanical strengths which are generally below 50 Mpa [5]. In addition,

many of the thermoplastics commonly used as adhesives have a glass transition

temperature which is close to room temperature [6], resulting in limited thermal

stability of the bond.4 It is common practice to modify the properties of the adhesive

by the addition of additives. While such additives are usually included to alter the

adhesive properties [7−9], some researchers have used additives to improve the

mechanical properties of the adhesive [10,11]. In addition, it is worth noting that in

the last few years a small number of researchers have begun to explore using nano

materials as additives in adhesives [9−12]. One of the most commonly used

thermoplastic adhesives is polyvinyl acetate (PVAc) [4,10,13,14]. We note that this

material is not to be confused with polyvinyl alcohol (PVA), a polymer that has been

much studied as a nano composite matrix [15,16]. Generally found as a water-based

emulsion, PVAc is most often used as an adhesive for porous materials such as wood

and paper. As such, it generally forms a strong adhesive bond, and so, the adhesive

strength tends to be limited by the mechanical properties of the polymer. A number

of papers have described reinforcement [17], of PVAc with nano materials such as

carbon nanotubes [18] cellulose nano fibers [19] or nano clays [20]. Adhesives based

on PVAc loaded with small quantities of nano clays have even exhibited small but

significant increases in adhesive strength [10].

However, the adhesives studied all display some negative aspects. For example,

carbon nanotubes, while very promising as filler due to their extremely high strength

and stiffness [17], are ultimately impractical due to their high cost. At the other

extreme, nano clays are extremely cheap but do not have the superlative mechanical

properties displayed by nanotubes [21,22]. However, recently a new nano material

has become available which combines the high strength of carbon nanotubes with the

low cost of clays. Graphene is a two-dimensional sheet of sp2 bonded carbon which

has become renowned for its superlative properties [23], For example, “pristine

graphene has a modulus and strength” of 1 TPa and 130 GPa, respectively [24].

Originally produced in very small quantities [25], “graphene can now be produced in

large quantities by exfoliation [26], of graphite in solvents” [27], aqueous surfactant

solutions [28], or polymer solutions [29,30]. Already, graphene has displayed

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significant success in reinforcing [31−34], both thermoplastics [35−37] and

elastomers [38,39] in some cases at very low loading level [36,40,41]. With this in

mind, graphene appears to be a promising additive for thermoplastic adhesives.

However, to the best of our knowledge, no work has been done in this area. In this

report, we use solution processing to prepare composites of PVAc and solvent

exfoliated graphene. We show that the addition of <1% graphene can result in a

doubling of the composite strength and stiffness without significant reduction in

ductility. In addition, we find the adhesive properties of the composite to be

significantly better than the neat polymer.

6.3. Experimental Procedure

Graphite powder (10 g, Sigma Aldrich) was exfoliated by sonicating (GEX600, 24

kHz, flat head probe, 25% amplitude) in 100 mL N-methyl pyrrolidinone (NMP)

(100 mg/mL) for 6 h. “The resulting dispersion was centrifuged at 1000 rpm for 45

min” (Hettich Mikro 22R). This results in the sedimentation of un-exfoliated graphite

and large graphene flakes. The sediment was collected and re-dispersed in fresh

NMP by sonicating in a sonic bath (Branson 1510E-MT) for 15 min. This dispersion

was centrifuged at 500 rpm for 45 min to remove the un-exfoliated graphite. The

supernatant, which is expected to contain reasonably large graphene flakes [42], was

retained. This supernatant was filtered through a nylon 0.45 micron membrane and

washed with 200 mL tetrahydrofuran (THF), resulting in a re-aggregated graphene

filter cake. Previous studies have shown that such materials tend to be free of defects

and oxides and consist of flakes of good quality graphene [27,43]. In addition, such

cakes are known to be easily re-dispersed in appropriate solvents [44, 45]. During

this work, it was found that re-aggregated graphene filter cakes could be effectively

re-dispersed, even in poor solvents such as THF. Such a dispersion (5 mg/mL),

prepared by bath sonication (Branson 1510E-MT, for 4 h) was used as a graphene

stock dispersion. While such dispersions are unstable, they can be stabilized by

subsequent addition of a polymer such as PVAc. If carefully chosen, the polymer can

partially bind to the graphene sheets stabilizing them against re-aggregation by the

steric mechanism [30]. Polyvinyl acetate (Sigma Aldrich, Mw = 100 000 g/mol) was

dissolved in THF at two concentrations, 30 and 200 mg/mL. These solutions were

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blended with graphene/THF dispersion (5 mg/mL) in the required ratio to give the

desired graphene/PVAc mass fraction. The resulting mixtures were further bath

sonicated for 4 h to homogenize. These dispersions were stable with no visible

evidence of aggregation in the liquid phase. Dispersions were characterized by

depositing a drop of liquid onto a holey carbon transmission electron microscopy

(TEM) grid and analyzed using a Jeol 2100.

The composite dispersions with PVAc concentration of 30 mg/mL were poured into

Teflon trays and dried at room temperature for 24 h and then at 60 °C for 8 h. They

were cut into strips of thickness ~50 micron and lateral dimensions 2.5 mm × 20 mm

using a die cutter. “Tensile testing was performed with a Zwick Z100 at a strain rate

of 15 mm/ min. The fracture surfaces were imaged using a Zeiss Ultra scanning”

electron microscope (SEM) operating at 2 kV. The mass fractions were converted to

volume fraction assuming mass densities of ńG = 2100 kg/m3 and ńP = 1180 kg/m

3.

The composite dispersions with PVAc concentration of 200 mg/mL were used for

adhesive testing. In all cases, equal masses of the high concentration dispersion were

spread on a wood surface over a well-defined area. An identical piece of wood was

then pressed onto the glue. These assemblies were then placed in a custom built

holder and 0.042 MPa applied for three days at room temperature and further dried

over night at 60 °C. Both tensile and shear adhesive testing was performed. For

tensile tests, the wood pieces were in the shape of the letter T with the glue applied

to the top of the T over an area of 2.5 mm × 27 mm. During testing, the applied

stress was in a direction perpendicular to the glued surface. For shear tests, the wood

was in the shape of a bar with the glue applied to the side of the bar over an area of

10 mm × 14 mm. During testing, the applied stress was in a direction parallel to the

glued surface. In each case the strain rate was 0.1 mm/min. For both shear and

tensile measurements, 3−5 assemblies were tested for both polymer and composite

adhesives

6.4 Results and Discussion High concentration dispersions of graphene in THF (5 mg/ mL) were mixed with

solutions of PVAc in THF (30 mg/mL) to yield hybrid polymer-graphene

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dispersions with graphene volume fractions in the range 0−0.85%. The “exfoliation

state of the graphene in these hybrid dispersions” can be assessed by TEM. Shown in

Figure 6.1 A and B are TEM images of typical exfoliated graphene flakes. They

appear to be of good quality, with no holes or other obvious defects.

Fig. 6.1 (A) Large numbers of multilayer graphene deposited on a holey carbon TEM grid.

(B) Individual graphene multilayer. (C) Photograph of PVAc−graphene films with mass

fractions of 0%, 0.2%, 0.4%, 0.7%, and 1.5% (volume fractions from 0−0.8%). SEM image

of (D) a PVAc and (E) a PVAc/graphene fracture surface

Shown in Figure 6.1 C are free standing films of PVAc and PVAc−graphene

composites (volume fractions of 0−0.84%). It can be seen that while the dispersion

is reasonably good, some aggregation cannot be avoided, even at low volume

fractions. This aggregation probably occurs during film drying due to the increasing

graphene/THF concentration. Figure 6.1 D and E show SEM images of the fracture

surfaces of PVAc and PVAc/ graphene films respectively. While the polymer film

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shows a relatively featureless surface, the presence of graphene greatly alters the

film morphology with numerous graphene sheets observable.

We performed tensile tests on films with a range of mass fractions (Figure 6. 2). For

the polymer, the stress initially increases nonlinearly with strain.

The polymer yields at approximately 5% strain above which the stress falls on. This

behavior is in line with previous reports of the tensile response of PVAc [46],

although it is important to stress that the mechanical response of PVAc at room

temperature is very sensitive to strain rate [19]. The composites stress strain curves

show greater linearity at low strain but otherwise have broadly similar shapes to the

polymer.

Fig. 6.2 Stress−strain curves for the PVAc/graphene composite film studied in this work.

(inset) Stress−strain curves on a log−log scale. The dotted line represents linearity.

From these stress strain curves, we can obtain a number of mechanical parameters.

Shown in Figure 6.3 A is the Young’s modulus, Y, plotted as a function of graphene

volume fraction. The modulus increases linearly with graphene content from 0.75

GPa for the polymer to 1.5 GPa for the 0.1 vol % composite. The initial rate of

increase was dY/dVf = 530 GPa, reasonably close to the maximum value of 1 TPa

set by the graphene sheet modulus and the rule of mixtures [24,47]. It is likely that

this value is lower than 1 TPa because of the finite length of the flakes used in this

study, [36]. This result agrees well with the value of 680 GPa measured for

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graphene/poly (vinyl alcohol) composites [36]. At higher volume fractions, the

modulus falls on before rising again albeit at a slower rate. This behavior may be

indicative of aggregation. We note that the initial increase is competitive with

published data (expressed in terms of filler mass fraction, Mf) for PVAc reinforced

Fig. 6.3 Mechanical properties of PVAc films. (A) Young’s modulus, (B) ultimate tensile

strength, and (C) strain at break, as a function of graphene volume fraction.

with cellulose nano fibers (dY/dMf ≈ 80 GPa) [19] carbon nanotubes (dY/dMf ≈ 200

GPa),[18], and nano clays (dY/dMf ≈ 340 GPa) [20] (The last value was calculated

for only two data points so must be treated with caution. The vast majority of

clay−polymer composites show much lower reinforcement [21].

Very similar behavior was observed for the ultimate tensile strength, óB, which

increased linearly from 21 MPa for the polymer to 38 MPa for the 0.1 vol %

composite with a slope of dóB/dVf = 15 GPa (Fig. 6.3B). Such a large increase at

such a low loading level is impressive and is generally only found for high

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performance nano fillers. For example, this result compares well to the value of

dóB/dVf = 22 GPa measured for graphene/ polyvinyl alcohol composites, [36].

Again, this value is also similar to published data for PVAc reinforced with carbon

nanotubes (dóB/dMf ≈ 10 GPa) [18], but much higher than equivalent data for

cellulose nano fibers (dóB/dMf ≈ 0.2 GPa) [19]. However, the slope is much less

than the value of 130 GPa predicted by the graphene sheet strength and the rule of

mixtures, [24,47]. However, this probably means that the flake length is below the

critical length, [48] (expected to be of order of many micrometers [29,49]). Under

such circumstances, material fracture generally involves failure of the polymer

graphene interface rather than breaking of the flakes [5,36,48]. Under these

circumstances, we can write:

dóB/dVf ≈ ôB[ L + w ]/4 t (6.1)

where ôB is the interfacial strength [36]. Using the flake dimensions given above,

this means ôB ≈ 27 MPa, similar to the value of 29 MPa recently measured for

graphene/PVA composites [36]. Indeed, given the structural similarities between

PVAc and PVA, it is hardly surprising that their interfaces with graphene have

similar shear strength.

We note that both dY/dVf and dóB/dVf values we have measured for PVAc−graphene composites are quite high as discussed above. That the value of

dY/dVf is high implies that the polymer-graphene interfacial stress-transfer is very

effective while the relatively large value of dóB/dVf implies a strong polymer-

graphene interface. Taken together, this suggests a strong interaction between PVAc

and graphene. As described above, a similarly strong interaction is observed for PVA−graphene composites [36]. The detailed nature of these interactions is not well-

understood. However, we suggest that the results described above are consistent with

the hydrogenated parts of the polymer chain binding strongly to the graphene by

dispersive interactions. It is likely that the polar acetate group (or hydroxyl group in

the case of PVA) protrudes outward and so is available to interact with other

polymer chains. However, molecular dynamics simulations are required to test this

hypothesis.

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The strain at break appeared to increase slightly from -100% for the polymer to -

175% for the 0.23vol% composite sample before subsequently falling. This is

slightly unusual as ductile polymers usually display a decrease on strain at break on

the addition of nano fillers such as nanotubes or graphene [39,50−52]. Indeed

previous work on PVAc filled with nanotubes or nano clays showed a reduction in

ductility for all filler contents [18,19]. It is not clear why this should be the case.

However, for polymers which fail by craze formation, if the fibular bridges were

reinforced by the presence of the nanofiller, this might result in an increase in

ductility in the composite.

Because one of the most common applications of PVAc is as an adhesive [10,13,14].

We tested the effect of adding graphene on the adhesive properties of PVAc. We

prepared very high concentration solutions of PVAc in THF (200 mg/mL) both with

and without the presence of various amounts of graphene from 0.2 to 3 wt %. These

viscous liquids were then coated on pieces of wood over a well-defined area as an

adhesive. Identical pieces of wood were then pressed onto the adhesive in geometries

designed to test both the tensile and shear properties of the adhesive (Figure 6.4A).

The glued assemblies were then pulled apart using a tensile tester (Figure 6.4B).

Typical stress-strain curves for polymer and composite adhesives, tested in both

tensile and shear geometries, are shown in Figure 6.4C For both shear and tensile

measurements, the stress strain curve looked very different to the tensile stress strain

curves of the PVAc and PVAc/graphene composites shown in Figure 6.2. Indeed,

this suggests that the mechanical properties of the bond are not controlled solely by

the mechanical properties of the adhesive.

The tensile adhesive strength increased sub linearly from 0.3 MPa for the pure

polymer to 0.75 MPa for the 3 wt % composite. The shear strength increased linearly

from 0.5 MPa for the PVAc to 2.2 MPa for the 4 wt % sample. Interestingly, the

initial rate of increase of both shear and tensile adhesive strength is similar at ~50

MPa. This is considerably lower than the rate of increase of composite tensile

strength with graphene mass fraction again indicating that the bond strength is not

solely limited by the strength of the composite. This suggests that failure may be

adhesive rather than cohesive. We can compare this with Kaboorani et al. [10], who

tested PVAc filled with 4% nano clay. They achieved 25% increase in adhesive

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strength, albeit from a much higher base (the shear strength of their commercial

PVAc adhesive was 19 MPa).

We also measured the area under the stress-displacement curve for each test. This

parameter is equal to the energy cost per unit area of breaking the bond between the

wood pieces and can be considered the adhesive toughness. This data is shown in

Figure 6.4E. For both tensile and shear tests, the toughness increases dramatically

with graphene addition up to 1.5 wt % with some fall on observed for the tensile case

at higher graphene content. However, the tensile adhesive toughness increased by

more than 3-fold for 0.7 wt % graphene addition while the shear toughness had

increased by almost 4-fold for the 3 wt % sample. This is an important result as it

shows that graphene-containing adhesives can absorb significantly more energy

before failure than the polymer adhesive alone. We note that the adhesive strength in

both tensile and shear modes was less than 3 MPa. Commercially available PVAc

glues can have strengths of up to 7 MPa for a range of woods [4,13,14].

However, such glues tend to be complex mixtures of PVAc and a range of additives,

which have been developed over decades. In comparison, our PVAc adhesives were

deposited from simple PVAc solutions. It is important to assess the efficiency of

graphene addition to commercially available PVAc wood glue. To test this, we

purchased Tonic Studio Craft Glue PVAc wood glue. The concentration of solids

(mainly PVAc) in the glue was measured by drying a known volume of glue (1 mL)

at 60°C for 3 days to remove the solvent (water) followed by weighing.

The commercial glue was then mixed with a 5 mg/ mL Graphene/THF stock

solution. Excess solvent was evaporated to bring the glue back to its original

concentration (although now dissolved in a THF/water mixture rather than pure

water). Shear and tensile tests were carried out as before both on samples bonded

with as-purchased glue and those bonded with commercial glue with graphene added

(During the graphene addition process, one sample was prepared with processing

identical to the composites but with no added graphene. This sample is included in

the composite glue data set but with graphene content = 0). Representative stress-

displacement curves are shown in Figure 6.4F and were found to be considerably

different to those measured before, possibly due to the presence of additives in the

commercial glue.

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Fig. 6.4 Measurements of adhesive properties of PVAc/graphene glue. (A) Photograph of

samples used for adhesive testing. (left) Two T-shaped wood pieces glued together for

tensile testing. (right) Two wooden bars, glued together along an overlapping region (dashed

line), for use in shear measurements. (B) Photograph of T-shaped pieces during a tensile test.

(C) Applied stress plotted as a function of displacement in both tensile and shear modes for

samples glued using homemade PVAc adhesive. (D) Tensile and shear bond strength and

(E) toughness as a function of graphene content for the homemade PVAc adhesives. (F)

Tensile stress−strain curves for as-bought commercially available glue and same with 0.7 wt

% graphene added. (G) Tensile and shear bond strength and (H) toughness as a function of

graphene content for the adhesives prepared with commercially available PVAc glue. The

dotted lines represent the untreated glue. The data points represent the glue, diluted and re-

concentrated during the process of graphene addition.

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We found no significant improvement in the adhesive shear strength on addition of

graphene. However, small but significant changes were observed for the tensile

adhesive strength. On addition of graphene, the tensile adhesive strength increased

linearly from 1.25 MPa for the glue reference sample to 1.75 MPa for the sample

containing 0.7 wt % graphene before falling at higher loading levels. Importantly, we

found that the dilution/re-concentration procedure used to add the graphene had no

effect on the tensile adhesive strength of the graphene-free glue; identical values

were found for the pristine PVAc glue and PVAc glue that had been treated

identically to the composites but with no graphene added. This shows that graphene

addition can have a positive effect on commercial PVAc glue.

We also calculated the adhesive toughness for all glues based on the commercial

adhesive. This data is shown in Figure 6.4H. Increases in both tensile and shear

toughness were observed. The tensile adhesive toughness increased from 0.2 kJ/m2

for the as-purchased glue to 1.5 kJ/m2 for the 0.7 wt % sample, a >7-fold increase. It

is worth noting that this increase in toughness is mostly due to increases in

displacement at failure (see Figure 6.4B) on addition of graphene. A much smaller

but still significant increase in the shear toughness was observed.

It is worth considering the mechanism of failure. Under stress, it is known that

cavities begin to form in the adhesive.11

When failure is cohesive these cavities tend

to be wholly contained within the adhesive. Cavity formation tends to first occur

close to the yield stress (i.e., the maximum stress observed in the stress strain curves

in Figures 6.3 and 6.4B) [11]. Once the cavities have formed, the stress is maintained

by fibrils in a manner similar to crazing in polymers [5]. As the displacement is

increased the cavities expand and the fibrils become extended. This process

dissipates considerable amounts of energy, often resulting in high adhesive

toughness. Failure occurs when the last fibril breaks. Such fibrils can be observed in

Figure 6.4B just before failure. The addition of graphene results in increases in

adhesive stress because graphene both stiffens and strengthens the polymer resulting

in cavity formation at higher stress and the fibrils resisting deformation with greater

stress. The increased work of adhesion is largely due to failure occurring at higher

displacements and is due to the reinforcement of the fibrils which delays failure to

higher displacements.

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Conclusion

In conclusion, we have shown that the polymer PVAc can be mechanically

reinforced by addition of solvent exfoliated graphene. Addition of 0.1 vol %

graphene results in the doubling of modulus, strength, and ductility. When used as an

adhesive, addition of 0.7% graphene results in increases in both adhesive strength

and toughness. We believe graphene shows great promise as an additive for

adhesives. It is produced from a precursor, graphite, which is very cheap making it

economically plausible. In addition, the results presented here represent only the first

tentative steps in this area. Further work is likely to see further advances in both

strength and toughness of graphene reinforced adhesives.

References [1] B.G.Yacobi, S. Martin, K. Davis, A.Hudson, M.Hubert, J. Appl. Phys., 91

(2002) 6227.

[2] J. Gegner, Materialwiss. Werkstofftech. 39 (2008) 33.

[3] S. Park, Y.W.J. Choi, H.S. Choi, et al, J. Adhes. 86 (2010) 192.

[4] Y. Hatano, B. Tomita, H. Mizumachi, Holzforschung 40 (1986) 255.

[5] W.D. Callister, Materials Science and Engineering an Introduction, 7th ed.; Wiley:

New York, 2007; p 721.

[6] W.W. Lim, H. Mizumachi, J. Appl. Polym. Sci. 66 (1997) 525.

[7] N. Pastor-Sempere, J.C. Fernandez-Garcia, A.C. Orgiles-Barcelo, et al, J.

M. J. Adhes. 59 (1996) 225.

[8] A. Torro-Palau, J.C. Fernandez-Garcia, A.C. Orgiles-Barcelo, et al,

J. Adhes. 57 (1996) 203.

[9] L. L. Zhai, G. P. Ling, J. Li, Y.W. Wang, Mater. Lett. 60 (2006) 3031.

[10] A. Kaboorani, B. Riedl, Compos. Part A 42 (2011) 1031.

[11] T. Wang, C.H. Lei, A.B. Dalton, C. Creton, et al, Adv. Mater. 18 (2006)

2730.

[12] E.N. Gilbert, B.S. Hayes, J.C. Seferis, Polym. Eng. Sci. 43 (2003) 1096.

Page 126: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

106

[13] E. Burdurlu, Y. Kilic, G.C. Eli’bol, M. Kilic, J. Appl. Polym. Sci. 100 (2006) 4856.

[14] O. Dajbych, D. Herak, A. Sedla ek, G. G rdil, es. Agric. Eng. 56 (2010)

159.

[15] A.B. Dalton, S. Collins, E. Munoz, J.M. Razal, et al, Nature 423 (2003)

703.

[16] B. Vigolo, A. Penicaud, C. Coulon, C. Sauder, et al, Science 290 (2000)

1331.

[17] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Carbon 44 (2006) 1624.

[18] R.D. Maksimov, J. Bitenieks, E. Plume, et al, Mech.Compos.Mater. 46 (2010) 237.

[19] G. Gong, J. Pyo, A.P. Mathew, K. Oksman, Compos. Part A 42 (2011) 1275.

[20] Y. Mansoori, A. Akhtarparast, M.R. Zamanloo, et al,Polym.Compos. 32

(2011)1225.

[21] S. Pavlidou,C.D. Papaspyrides, Prog. Polym. Sci. 33 (2008) 1119.

[22] L.J. Zhu, K.A. Narh, J. Polym. Sci. Part B: Polym. Phys. 42 (2004) 2391.

[23] A.K. Geim, Science 324 (2009) 1530.

[24] C. Lee, X. D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385.

[25] K.S. Novoselov, D. Jiang, F. Schedin, et al, Proc. Natl. Acad. Sci. U.S.A. 102 (2005)

10451.

[26] J.N. Coleman, Acc. Chem. Res. 46 (2013) 14.

[27] Y. Hernandez, V. Nicolosi, M.Lotya, et al, Nat.Nanotechnol. 3 (2008) 563.

[28] M. Lotya, Y. Hernandez, P.J. King, et al, J. Am.Chem. Soc. 131 (2009) 3611.

[29] U. Khan, P. May, A. O’Neill, A.P. Bell, et al, Nanoscale 5 (2012) 581.

[30] P. May, U. Khan, J.M. Hughes, J.N. Coleman, J. Phys. Chem. C 116 (2012) 11393.

[31] H. Kim, A.A. Abdala, C.W. Macosko, Macromolecules 43 (2010) 6515.

[32] T. Kuilla, S. Bhadra, D.H. Yao, N.H. Kim, et al, Prog. Polym. Sci. 35 (2010) 1350.

[33] R. J. Young, I.A. Kinloch, L. Gong, K.S. Novoselov, Compos. Sci.

Technol. 72 (2012) 1459.

Page 127: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

107

[34] J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Polymer 52 (2011) 5.

[35] U. Khan, K. Young, A. O’Neill, J.N. Coleman, J. Mater. Chem. 22 (2012) 12907.

[36] P. May, U. Khan, A. O’Neill, J N. Coleman, J. Mater. Chem. 22 (2011) 1278.

[37] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, et al, Nat.

Nanotechnol. 3 (2008) 327.

[38] O. Menes, M. Cano, A. Benedito, E. Gimenez, P. Castell, et al, Compos.

Sci. Technol. 72 (2012) 1595.

[39] U. Khan, P. May, A. O’Neill, J.N. Coleman, Carbon 48 (2010) 4035.

[40] M.A. Rafiee, J. Rafiee, Z. Wang, et al, ACS Nano 3 (2009) 3884.

[41] S. Morimune, T. Nishino, T. Goto, Polym. J. 44 (2012) 1056.

[42] U. Khan, A. O’Neill, H. Porwal, P. May, K. Nawaz, J.N. Coleman, Carbon 50

(2010) 470.

[43] U. Khan, A. O’Neill, M. Lotya, S. De, J. N. Coleman, Small 6 (2010) 864.

[44] U. Khan, H. Porwal, A. O’Neill, K. Nawaz, P. May, J.N. Coleman, Langmuir 27

(2011) 9077.

[45] A. O’Neill, U. Khan,; P.N. Nirmalraj, J.J. Boland, J.N. Coleman, J. Phys.

Chem. C 115 (2011) 5422.

[46] S. S. Ochigbo, A. S. Luyt, W.W. Focke, J. Mater. Sci. 44 (2009) 3248.

[47] G.E. Padawer, N. Beecher, Polym. Eng. Sci. 10 (1970) 185.

[48] D. Hull, T.W. Clyne, Cambridge University Press: New York, (1996) 344.

[49] L. Gong, I.A. Kinloch, R.J. Young, I. Riaz, et al, Adv. Mater. 22 (2010)

2694.

[50] F.M. Blighe, W.J. Blau, J.N. Coleman, Nanotechnology 19 (2008) 415709.

[51] U. Khan, F.M. Blighe, J.N. Coleman, J. Phys. Chem. C 114 (2010) 11401.

[52] U. Khan, P. May, A. O’Neill, J.J. Vilatela, A.H. Windle, J.N. Coleman,

Small 7 (2011) 1579.

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Chapter 7

The Effect of Graphene nanosheets on

the Mechanical Properties of

Polyvinylchloride

7.1 Objective

Graphite was exfoliated to graphene using sonic tip and N-methyl-2- Pyrrolidinone

(NMP) as solvent and after specific time the exfoliated graphite (unexfoliated

graphite) was centrifuged at 500 rpm for 30 min. The supernatant of centrifuged

material was stored and filtered. Then the filtered graphene nanoflakes were re-

dispersed in tetrahydrofuran (THF) and sonicated in bath sonicator to make

homogenous dispersion. Then this dispersion was used as nanofiller in PVC as

reinforcement. 03 mg/ml concentration of graphene in THF was used as nanofiller to

PVC matrix. Thin film of composites were prepared by using drop casting and

annealing procedure. An excellent improvement in mechanical properties was

observed. At 1.5% loading tremendous improvement in mechanical properties was

noted. Modulus improved from 1.31Gpa to 2.14Gpa (75 % increment) and UTS

improved from70 Mpa to 83.2Mpa. There was no fall in elongation at break along

with these improvements at this loading.

7.2 Introduction

Recently “discovered planar 2D form of carbon known as graphene has become one

of the most exciting materials today because of its unique properties” [1]. “Individual

graphene sheets show high values of thermal conductivity [2], Young’s modulus [3],

large surface area” [4], ballistic transport on submicron scales and mass less “Dirac-

fermion charge carrier abilities [5,6]. These properties make graphene a promising

material for using in many applications such as photovoltaic devices, sensors”,

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transparent electrodes, “super capacitors conducting composites [4,7–12]. At present,

carbon-based reinforcing materials employed in polymer composites are dominated

by carbon nanotubes (CNT). But difficulty in dispersing CNT and high cost of

production limits its widespread use”. Challenge is to find an alternative for CNT for

which graphene can prove itself as a suitable candidate due to remarakable and

outstanding mechanical “properties and ultra large interfacial surface area [3,4].

Incorporation of graphitic nano flakes into elastomeric polymer matrix generates

high performance composites with improved mechanical and functional properties

[12–15]. Other interesting properties such as high dielectric permittivity” and “low

percolation threshold have also been observed in graphene incorporated composites

of poly (vinylidene fluoride) and polystyrene, respectively” [16,17]. Recently,

“graphene oxide (GO) has also been used as a filler in various polymer matrices, due

to its hydrophilicity and ease of formation of stable colloidal suspensions” [13,14].

Besides, “functionalized graphene sheets (FGS) are also employed as they provide

better interactions with the host polymers compared to unmodified CNT or

traditional expanded graphite (EG) [12]. In our current investigation”, “poly(vinyl

chloride) (PVC) is chosen as the host polymer matrix, because of its wide range of

applications, low cost, chemical stability, biocompatibility” [18]. However, “PVC

has low thermal stability, which hinders some of its applications [19]. The present

day challenge is to introduce thermal stability along with high mechanical strength

for PVC with the use of minimum amount of fillers”. “Substantial amount of work

has been carried out in the past few decades towards this goal [20–22]. Fillers such

as clay [23,24], “wood flour” [21], wood fibers [25], agricultural residues [26],

cellulose whiskers [27] and calcium carbonate [28] were used to improve the thermal

and mechanical stability of PVC. In recent times, CNT have also been identified as a

suitable filler material for PVC” [29]. “Kevlar coated CNT used as additives to PVC

resulted in composites with improved mechanical properties demonstrating” up to 50

and 70% increase in “tensile strength and Young’s modulus respectively at very low

CNT loading [30]. Similarly, CNT grafted with styrene-maleic anhydride

copolymers (SMA) was found to enhance the interaction with PVC matrix and both

thermal and mechanical stabilities improved considerably” [31]. But “dispersion of

CNT in organic solvents is a challenge, which is very critical for the preparation of

polymer composites” [13]. In our previous study we have used the soluble graphene

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nano flakes [32] as reinforcing “filler for PVC at a very low loading level. In order to

have efficient reinforcement in polymer composites, it is important to have

molecular level dispersion in the polymer matrix”. In this case, both PVC and

graphene sheets can be readily dispersed in Tetrahydrofuran (THF) for “solution

blending, which will help to achieve molecular level dispersion”. In present study we

have introduced exfoliated graphene nano flakes at different wt % to study its effect

on the mechanical strength of PVC polymer. The loading was from 0.1 wt% to 10

wt%. And the neat PVC sample was taken as reference material.

7.3 Experimental section 100 mg/ml dispersion of graphite flakes (Sigma –Aldrich) were taken in N-Methyl

Pyrrolidinone (NMP) and was sonicated using sonic tip (GEX600,48W,24khz,flat

head probe) continuously for 24 hours keeping temperature between 5oC—10

oC by

using ice bath, so that the heat generated during sonication may not spoil the solvent

(NMP). As a result the obtained exfoliated graphite was separated from un-exfoliated

by centrifugation at 500/45 (rpm/minutes) and supernatant was collected, filtered

through a nylon membrane of pore size 0.45 micron (Sterlitech). The membrane

supported graphene nano flakes were dried at 60oC for accurate mass. Then its

dispersion was made in tetrahydrofuran (THF) and used in thin films casting.

Similarly 60 mg/ml solution of PVC was made in THF by dissolving 6 gm of PVC,

in 100 ml of solvent for about 24 hours.

The composites samples were prepared using different weight percent of graphene to

PVC solution between 0.1 to 13 wt. % and a reference sample was also prepared

using same recipe. The total weight of sample was 150 mg and with constant volume

of 13 ml. These samples were sonicated for four hours in bath sonicatior and were

cast in Teflon trays of dimension 4x4x2. These cast samples were dried in vacuum

oven at room temperature at 900 mbar. Then these samples were shifted to another

oven and kept there for eight hours to remove traces of THF, if any present.

7.4. Characterization of PVC-Graphene Composites

The graphene nano flakes were studied using TEM (Joel 2100, Japan) and hole

carbon grids (400mesh). Mechanical “testing was performed using a Zwick Roell

tensile tester with 100N load cell at a strain rate” of 50mm/min.

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7.5. Results and Discussion

TEM images of graphene nano flakes exfoliated in NMP as solvent under tip

sonicator were taken to study its flakes size and number of layers. It is very clear

from TEM images that most of the graphene flakes are consist of less than three

sheets with a thickness of 1.4 nm, which indicates that it consists of less than three

layers as shown in Figure 7.1.

Fig. 7.1 TEM images of graphene nano flakes exfoliated in NMP

It was expected that the mechanical properties of PVC would be enhanced to a great

extent, although it is improved but not to very high extent. Figures 8.2-4 shows the

effect of graphene nanoflakes on mechanical properties of composites for a range of

loading levels. i.e. Young’s Modulus, UTS and elongation. For blank PVC the

Young’s Modulus is 131Mpa and its Ultimate Tensile Strength (UTS) is 70Mpa. Just

at 1.5% loading, Modulus improved from 1.31 to 2.14 GPa (61% improvement) as

shown Figure 7.2.

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Fig. 7.2 Effect on Young’s Modulus of PVC after using graphene nanoflakes

A comparison is of theoretical and experimental data is also shown in Figure 7.5. At

the same time UTS reached to 83.2 MPa (18% improvement) from 70 MPa for 1.5%

graphene loading Figure 7.3.

Improvement in UTS may be due to good dispersion/interaction of polymer and

graphene nano sheets [33] which is expectable at this low level addition of nanofiller.

While Young’s Modulus enhanced to good extent (2.29 GPa for 8 wt.% loading) and

its Ultimate Tensile Strength (UTS) dropped from 83.2 MPa (for 1.5 wt % loading)

to 72 MPa. This decrease in UTS may be due to agglomeration of nanofiller and/or

improper dispersion in polymeric matrix. Likewise increase in mechanical resistance

properties (Modulus and UTS) results in decrease in ductility in terms of elongation

at break. But in our case at 1.5% loading, we not only get improvement in

mechanical properties in terms of UTS and modulus but there was negligible

decrease in elongation at break from 15.3% to 13% Figure 7.4. But at 8% loading the

elongation at break affected disastrously and dropped from 15.3% to 6.8%.

This fall in elongation at break may be due to interaction of graphene with polymer

chain which restricts the movement of polymeric chain [34]. A comparison is shown

in From these results it is concluded that 1.5% loading is critical loading on which

we get very good mechanical properties in terms of Modulus, UTS and elongation at

break. While above this loading there is downtrend in mechanical properties

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especially in UTS and elongation at break.

The Halpin-Tsai model was used [35-38] in our work for random distribution of

graphene nanofiller in polymer to simulate our graphene/PVC nano composites and

its equation for randomly distribution is given as follow:

In equation 7.1, Ec is the modulus of the nano composites with randomly distributed

graphene nano flakes, where as Vc is volumetric fraction of graphene in polymer.

Likewise Eg and Em are the modulus of graphene nano flakes and polymer (PVC)

which are 1 Tpa [39] and 1.31 Gpa respectively. ξ, 1g and tg are the aspect ratio,

length and thickness of graphene nano flakes While the density of PVC & and of

graphene is1.4gm/mL and 2200kg/m3 respectively` [40]. The statistical average of

the length and thickness of graphene nano flakes were about 1µm and 1.2nm

respectively as determined by TEM. [32]. This gives aspect ratio value of 556.

Putting all these values in above equations we can easily deduce the theoretical

modulus for randomly distributed graphene nano flakes.

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Fig. 7.3 Effect on UTS of PVC after using graphene nanoflakes

Fig. 7.4 Effect on Elongation at Break of PVC after using graphene nanoflakes It is worth mentioning that two tendencies have been observed in our experiments.

One is at low loading up to 1.5% having close resemblance between theoretical data

Halpin-Tsai data and second is experimental one which is clear from Figure 7.5.

By increasing the contents of filler in polymer, there is clear difference between

theoretical and experimental data. It may be due to the dispersion of graphene nano

flakes.

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Fig. 7.5 Comparision of Theoratical and Experimental values of Young’s Modulus

At low level of loading, nano fillers are homogenously dispersed while at high

loading it may form agglomerate. So we can say that solution blending is efficient

for low loading but it may not work for high loading (beyond 1.5 wt.%). It is clear

from data that small addition up to 1.5% gives excellent improvement in tensile

strength and modulus.

Conclusion

Here we used in our experiments graphene nano flakes as nanofiller in Polyvinyl

chloride as polymeric matrix. An interesting phenomenon was observed at 1.5 wt. %

loading both mechanical properties UTS and Young Modulus was improved to a

good extent while elongation at break was slightly affected, and this trend was

prevalent up to 1.5wt % loading but beyond this loading up to 8 wt % no

improvement was observed in terms of UTS and elongation at break. Elongation at

break was disastrously affected. The effect on elongation at break may be due to the

restacking of graphene flakes in polymer which restricts the movement of polymer

chain.

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References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et

al, Science 306 (2004) 666.

[2] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al, Nano

Lett. 8 (2008) 902.

[3] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385.

[4] M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Nano Lett. 8 (2008) 3498.

[5] Y.B. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201.

[6] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson,

I.V. Grigorieva, et al, Nature 438 (2005) 197.

Page 137: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

117

[7] Z.F. Liu, Q. Liu, Y. Huang, Y.F. Ma, S.G. Yin, X.Y. Zhang. Adv. Mater. 20 (2008)

3924.

[8] J.T. Robinson, F.K. Perkins, E.S. Snow, Z.Q. Wei, P.E. Sheehan. Nano Lett. 8

(2008) 3137.

[9] P.K. Ang, W. Chen, A.T.S. Wee, K.P. Loh. J Am. Chem. Soc. 130 (2008) 14392.

[10] J.B. Wu, H.A. Becerril, Z.N. Bao, Z.F. Liu, Y.S. Chen. Appl Phys Lett. 92 (2008)

263302.

[11] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A.

Stach, et al, Nature 442 (2006) 282.

[12] T. Ramanathan, A.A. Abdala, S. Stankovich, et al, Nat Nanotechnol. 3 (2008) 327.

[13] X. Zhao, Q. Zhang, D. Chen, Macromolecules 43 (2010) 2357.

[14] J. Liang, Y. Huang, L. Zhang, Y. Wang, Y.F. Ma, T. Guo, et al, Adv. Funct. Mater.

19 (2009) 2297.

[15] K. Kalaitzidou, H. Fukushima, L.T. Drzal, Compos Part A – Appl Sci. 38 (2007)

1675.

[16] S. Ansari, E.P. Giannelis. J Polym Sci. Part B 47 (2009) 888.

[17] N. Liu, F. Luo, H. Wu, Y. Liu, C. Zhang, J Chen. Adv. Funct. Mater. 18 (2008)

1518.

[18] C.E. Wilkes, J.W. Summers, C.A. Daniels, M.T. Berard, PVC handbook. 1st ed.

Germany: Hanser Verlag; (2005) 414.

[19] B. Iva´n, T. Kelen, F. Tu¨do¨s, Amsterdam, Netherlands: Elsevier Science (1989)

483.

[20] S. Mayeda, N. Tanimoto, H. Niwa, M. Nagata, J Anal. Appl. Pyrol. 33 (1995) 243.

[21] H. Djidjelli, J.J. Martinez-Vega, J. Farenc, D. Benachour, Macromol. Mater. Eng.

287 (2002) 611.

[22] S.Y. Tawfik, J.N. Asaad, M.W. Sabaa. Polym. Degrad. Stabil. 91 (2006) 385.

[23] T. Peprnicek, A. Kalendova, E. Pavlova, et al, Polym. Degrad. Stabil. 91 (2006)

3322.

Page 138: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

118

[24] M.E. Romero-Guzman, A. Romo-Uribe, E. Ovalle-Garcia, R. Olayo, et al, Polym.

Adv. Technol. 19 (2008) 1168.

[25] L.M. Matuana, C.B. Park, J.J. Balatinecz. Polym. Eng. Sci. 38 (1998) 1862.

[26] S.T. Georgopoulos, P.A. Tarantili, E. Avgerinos, et al, Polym. Degrad. Stabil. 90

(2005) 303.

[27] L. Chazeau, J.Y. Cavaille, G. Canova, et al, J Appl. Polym. Sci. 71 (1999) 797.

[28] S.S. Sun, C.Z. Li, L. Zhang, H.L. Du, et al, Polym. Int. 55 (2006) 158.

[29] Y. Mamunya, A. Boudenne, N. Lebovka, L. Ibos, et al, Compos. Sci. Technol. 68

(2008) 1981.

[30] I O’Connor, H Hayden, S O’Connor, JN Coleman, et al, J Mater. Chem. 18 (2008)

5585.

[31] G.J. Wang, Z.H. Qu, L. Liu, Q. Shi, et al, Mat. Sci. Eng: A 472 (2008) 136.

[32] U. Khan, H. Porwal, A. O‖Neill, K. Nwaz, P. May, J.N. Coleman, Langmuir 27

(2011) 9077.

[33] J. Pascual, F. Peris, T. Bronat, O. Fenollar, R. Balart, Polym. Eng. Sci. 52 (2012)

733.

[34] S. Vadukumpully, J. Paul, N. Mahanta, S. Valiyaveettil, Carbon 49 ( 2011 ) 1 9 8.

[35] R.R. Tiwari, K.C. Khilar,U.J. Natarajan, J. Appl. Polym. Sci. 108 (2008) 1818

[36] K. Kalaitidou, H.Fukushima,H. Miyagawa, L.T. Drazal, Polym. Eng. Sci. 47

(2007) 1796

[37] J.C. Halpin,J.L. Kardos, Polym. Eng. Sci. 16 (1976) 344.

[38] F.T. Cerezo,C.M.L. Preston, R.A. Shanks, Macromol. Mater. Eng. 292 (2007) 155

[39] U. Khan, P. May , A. O´Neill, J. N. Coleman, Carbon 48 (2010) 4035.

[40] U. Khan, A. O´Neill, M. Lotya, S de, J.N. Coleman. Small 6 (2010) 864.

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Chapter 8

Effects of selected size of graphene

nanosheets on the mechanical

properties of Polyacrylonitrile (PAN)

Polymer

8.1 Objective Mechanical properties of Poly (acrylonitrile) (PAN) polymer can be remarkably

improved by incorporation of grapheme nanosheets of different sizes. For this

purpose Graphite was exfoliated to graphene using sonic tip in the presence of N-

methyl pyrrolidinone (NMP) as solvent. Exfoliated graphene was separated from un-

exfoliated graphitic crystallites using selected speed (rpm) of centrifuge for specific

time. Then these exfoliated graphene nanosheets were further classified into two

different categories on the basis of flake size, i.e. 1 µm and 3.5 µm on the basis of

different speed of centrifuge. Later on these graphene sheets were incorporated into

PAN to study the effects of these different flake sizes on mechanical properties.

Different mechanical properties such as Young’s modulus, ultimate tensile strength

(UTS) and elongation at break were studied. It was observed that the Young’s

modulus and UTS were improved about 45% & 25% respectively for 3.5 µm

graphene flake size and 40% & 21% for 1 µm graphene sheet.

8.2 Introduction The use of nanofiller/nanoparticles has attracted the attention of scientific

community during the last decade. Due to their large surface area and high aspect

ratio small quantity of these nanofiller may give remarkable changes in desired

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properties (such as mechanical performance, thermal stability, electrical conductivity of polymers) which are highly attractive for industrial applications [1-6]. Carbon

based reinforcing materials used in polymeric matrix are mostly dominated by

carbon nanotubes (CNTs). But its non-homogenous dispersion and tedious methods

of production has restricted its widespread applications. Due to these factors a new

material (graphene) was realized which may have mechanical performance like

carbon nanotubes but can easily be produced at large scale. So, graphene emerged as

promising candidate for CNTs [7-8]. Theoretical and experimental results show that

the single layered two dimensional (2D) graphene sheets are the mechanically

strongest material developed so far [9-10].

Liquid phase exfoliation (LPE) of graphite to pristine graphene [11-13] is an

attractive approach for many applications as filler in composites or hybrid materials

[14]. Graphene produced through LPE is defect free but it produces small flakes

(average size of about 1 micron) [15]. Through this method dispersions of graphene

in solvents with concentration of few mg/ml can be produced [16]. Some

applications require larger flake size for better results [17]. Gong et. al. demonstrated

that, for better mechanical properties the flake length should be of few microns or

greater [17]. Currently available graphene is significantly smaller in size that is why

it does not impart the desired mechanical properties to polymeric matrices. It is

reported that the rotation rate and flake size affects the mechanical properties

particularly young’s modulus [18]. The reinforcement values obtained by the

addition of graphene are much lower than the theoretical values [19] for example

dy/dVf ~1TPa, where Y is the composite modulus and Vf is the graphene volume

fraction [20−29].

PAN is promising polymer having extensive applications in various fields that’s why

PAN was selected in present study. In this work we used two types of graphene

nanosheets having different flake sizes (i.e. 1 micron and 3.5 micron) to study the

effects of flake sizes on the mechanical performance of PAN in terms of young

modulus, tensile strength and elongation at break. These nano-flakes were used in

different weight percentage (wt. %) from 0.25 wt. % to 12 wt. % and pure PAN was

used as reference. Dimethyl formamide (DMF) was used as solvent for solution

blending in order to have molecular level interaction of nanofiller and matrix.

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8.3 Experimental Part

PAN polymer, N-methyl pyrrolidinone (NMP) and Dimethyl formamide (DMF),

Graphite flakes were purchased from sigma-Aldrich. Graphite was exfoliated to

graphene using tip sonicator (GEX600, 48W, 24kHz, flat head probe) in the presence

of NMP as solvent and highly concentrated dispersion of graphene was obtained

[30]. Then these exfoliated graphene sheets were separated according to their lateral

sizes using a centrifuge machine (Hettich Mikro 22R) with 5500 and 500 rpm for 45

minutes in both cases respectively. Two different size of graphene nanosheets were

separated using this procedure.

8.3.1 Composites Preparation and Characterization

PAN based composites thin films were prepared by solution casting method. PAN

was dissolved in DMF and the dispersions of two type of graphene nanosheets based

on their flake sizes were also prepared in DMF. Same solvent was used in order to

have uniform and homogenous mixing of polymer and nanofiller. Different wt. % of

nano-composites were prepared using pure PAN as reference materials. PAN

polymer and graphene nano-sheets as nano-fillers were mixed and sonicated for 30

minutes. Then drop cast into Teflon trays (of 4 x 4 x 2 cm dimensions). These trays

were placed into vacuum oven at 900 mbar for the removal of solvent (i.e. DMF) at

80 oC for eight hours. Then these trays were placed at room temperature to get

constant weight of thin films for 24 hours. The film thickness was in the range of

50—60 micron with lateral dimensions of 2.5 x 20 mm. Raman analysis was

performed on 633 nm, Horiba Jobin Yvon Lab RAM-HR. The peak intensity ratio

was calculated using following equation.

IR = ID / IG .(8.1)

Where IR is peak intensity ratio, ID is peak defect and IG is graphitic peak. TEM

analyses were conducted using a Joel 2100. Mechanical properties were measured

using Zwick-Roell tensile tester using 100N load cell at strain rate of 15 mm/min.

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8.4 Results and discussion Traditional composites structure usually contains high volume (60%) filler in

polymeric materials. While in nanocomposites a significant change in properties can

be obtained by the addition of small quantity of nanofiller. These well-dispersed

nano-fillers at very low loading create the vast interfacial area which can affect the

behavior of surrounding polymer matrix that change its mechanical, thermal and

electrical properties [31,32].

The various flakes were separated at various speeds from 5500 rpm to 500 rpm

separating the supernatant for each cycle, like 5500, 4500, 3000, 2000, 1000 and 500

rpm and 45 minutes were given to each centrifugation cycle [15]. These dispersions

were vacuum filtered using porous alumina membrane (whatman Anodisc 47 mm

with pore size of 0.02 micron) to make thin films for Raman study. These separated

samples were studied using Raman spectroscopy and their peak intensity ratio was

studied. It was observed that at high rpm the flakes separated have high value of IR.

While those separated at low rpm has low value of IR. This indicates that the flakes

separated at high speed has large number of defects (edges) and are free from basal

defects. It was observed that as the rpm of centrifuge is lowered, the IR value

decreases which indicates that the flake size changes with the speed of centrifuge. That’s why we selected two types of flakes (separated at 5500 rpm and 500 rpm) in

this study. We are interested to know the effect of different flake sizes on the

mechanical performance of composites materials in terms of Young’s modulus, Ultimate Tensile Strength (UTS) and elongation at break (dL at break). It is clear from Figure 8.1 – 8.4, that at high centrifuge speed the graphene flakes

have high value of IR (equation. 9.1). This indicates that the graphene flakes

separated at high rates have small flake sizes, while flakes separated at low rpm

(such as 500) has low IR value which indicates that the flakes have large size

compared to previous one (i.e. separated at 5500 rpm). Raman spectroscopy also

reveals that the flakes are free from basal defects and this high value of IR is due to

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the large number of edges which may be due to the small flake size of graphene nanosheets and vice versa [13,33,34]. Fig. 8.1 Ratio of Raman d-g bands measured on films prepared from size selected dispersion

as a function of final centrifugation rates.

1000 1500 2000 2500 3000

0.99 5000-45

0.66

0.33

0.00

0.99 4000-45

0.66

0.33

Avera

ge

0.00

0.99 3000-45

0.66

0.33

Norm

aliz

ed

0.00

0.96 2000-45

0.64

0.32

0.00

0.9 1000-45

0.6

0.3

0.0

0.96 500-45

0.64

0.32

0.00

1000 1500 2000 2500 3000

wavenumber (cm(-1)) Fig. 8.2 Raman spectra of graphene thin film, of selected size flakes prepared after different

centrifugation rate (rpm).

TEM analysis was performed on the selected samples of two types as shown in Figure 8.3 with different flake size separated at 5500 and 500 rpm.

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Fig. 8.3 (A) TEM images of graphene flakes separated by centrifugation at 500 rpm.

Fig. 8.3 (B) TEM images of graphene flakes separated by centrifugation at 5500 rpm.

About 80 flakes were examined using TEM and similar observation was found in TEM

histograms (Figure 8.4 A & B). It is also clear that flakes separated at high rpm having small

size which is about 1 micron while those separated at low speed has flakes size about 3.5 micron.

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Fig. 8.4 (A) Histograms of flakes length of graphene in DMF separated at 500 rpm.

Fig. 8.4 (B) Histograms of flakes length of graphene in DMF separated at 5500 rpm. Earlier it has been shown by some groups that the flake/particle size plays vital role in

improving mechanical properties of composites [17, 35]. In this study an interesting

phenomena was observed, that the selected big flake of graphene nanosheets (about

3.5 micron) has promising effects on the mechanical properties of PAN composites in

terms of Young modulus, and ultimate tensile strength (UTS).

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Different wt. % samples of nanocomposites were prepared starting from 0.25% to

12%. It is shown in Figure 8.5 that the 50% improvement in mechanical properties in

terms of young modulus has been observed for large flake size, it means that the large

flake size has made the composite more stiff as compared to small flake size because

in case of small flake size there is improvement in modulus but not to the level of big

flake size.

Fig. 8.5 Effect of 1 micron () and 3.5 micron (♦) nano-fillers (graphene) incorporated in

PAN polymer on modulus.

It is very much clear from Figure 8.5 that the modulus was increased from 0.558 GPa

for pure PAN to 0.837 GPa for 12 wt % loading (50% increase). In case of small

flake size (1 micron) the modulus improved from 0.558 GPa (pure PAN) to 0.789

GPa (41% increase) and this is also a great improvement. However it was observed

that, at low loading from 0.25% to 1.5% the improvement in mechanical properties in

terms of modulus is more prominent for small flakes size graphene nanosheets. This

may be due to the well dispersion of nano fillers at low loading.

While at high loading, it may agglomerate and improvement is not prominent as

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 2 4 6 8 10 12 14

E [

GP

a]

Weight %Graphene weight %

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observed in large flake size of graphene. The presence of graphene flakes in PAN

matrix may offer resistance to the segmental movement of polymeric chains which

results in enhancement of modulus [36].

Fig. 8.6 Effect of 1micron () and 3.5 micron (♦) nano-fillers (graphene) incorporated in

PAN polymer on UTS.

Likewise it is apparent from Figure 8.6 that UTS also follow the same trend as

observed in case of modulus and also remarkable strength has been observed in

composites for both types of nanofillers of different flakes sizes. But in case of large

flake size this contributions is a little bit more prominent. The UTS improved from

37.8 MPa to 48.1 MPa for 12 wt.% loading (Figure 8.6) which is about 27%

improvement for big flake sizes. While UTS improved from 37.8 MPa (pure PAN) to

46.6 MPa for 12 wt.% loading for small flakes which is more than 23% increase.

Similar results were observed in modulus study. UTS improvement is more

prominent for low level loading of small size graphene nanoflakes; it may be due to

well dispersion of these flakes at low loading.

UTS

(M

Pa)

Graphene weight %

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This significant enhancement in case of big flake size may be due to the proper

dispersion and adhesion of polymeric matrix. While slight inferior response of small

flake may be due to the poor dispersion of small size flakes which may agglomerate

during incorporation to polymer matrix. Similarly in Figure 8.7 an opposite response

has been noticed. The enhancement in mechanical properties, in terms of modulus and UTS of nanocomposites for both type of nanofillers was observed. While

decrease in dL at break was observed for both type of flakes but this trend is very

prominent in big flake size. However, in small flake size this situation comparatively

less than big flake size. At 12 wt% loading dL at break dropped from 18.8% to 5.3%

for big flake size, while, dL at break was affected greatly, and dropped from 18.8% to

8.6% for small flake size. This decrease in dL at break may be attributed to the

interaction of graphene with polymeric material that restricts the movement of

polymeric chains (i.e. elasticity).

Fig. 8.7 Effect of 1 micron (♦) and 3.5 micron () nano-fillers (graphene) incorporated in

PAN polymer on Elongation at break.

The poor mechanical property in terms of young’s modulus and UTS may be due to

the poor dispersion of small flakes that may agglomerate during incorporation to

polymer in the formation of thin films.

Graphene weight %

Elo

nga

tio

n a

t b

reak

(%

)

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Conclusion PAN polymeric thin films were prepared with two different types of graphene

nanosheets as reinforcing agent by drop casting method using DMF as solvent. The

mechanical properties of both type of graphene nanosheets contributed well in

enhancement of mechanical properties to great extent. The large size flake

contributions are more prominent in enhancement of young modulus and UTS. While

in terms of dL at break the small flake size role is better than big flake size. In big

flake size Young’s modulus and UTS improved more than 45% and 25%

respectively. While in small flakes these enhancements in Young’s modulus and UTS are about 40% and 21% respectively.

References

[1] D.L. Burris, B. Boesl, G.R. Bourne and W.G. Sawyer, Macromol. Matter. Eng. 292

(2007) 387.

[2] J.Y. Kim, D.K. Kim, S.H. Kim, Polym. Compos. 30 (2009) 1779.

[3] T. Mahrholz, J. Stangle, M. Sinapius, Compos. A. 40 (2009) 235.

[4] M. Maryniak, N. Guskos, J. Typek, D. Petridis, A. Szymezyk, Polimery 54 (2009)

546.

[5] P. Mavinakuli, S.Y. Wei, Q. Wang, A.B. Karki, S. Dhage, Z. Wang, D.P. Young,

Z.H. Guo, J. Phy. Chem. C, 114 (2010) 3874.

[6] W.H. Ruan, Y.L. Mai, X.H. Wang, M.Z. Rong, M.Q. Zhang, Compos. Sci.

Technol. 67 (2007) 2747.

[7] Lee C, Wei XD, Kysar JW, Hone J. Science 321 (2008) 5887.

[8] M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Nano Letters 8 (2008)

3498. [9] M. J. McAllister, J. L. Li, D. H. Adamson, H. C. Schniepp, A. A. Abdala, et al,

Chem. Mater. 19 (2007) 4396. [10] C.G. Lee, X.D. Wei, J.W. Kysar, J. Hone. science 321 (2008) 385.

Page 150: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

130

[11] Y. Hernandez, M. Lotya, D. Rickard, S.D. Bergin, J.N. Coleman, Langmuir, 26

(2010) 3208.

[12] Y. Hernandez, V. Nicolosi, M. Lotya, F.M. Blighe, Z.Y. Sun, et al, Nat.

Nanotechnol. 3 (2008) 563.

[13] U. Khan, A. O’Neill, M. Lotya, S. De, J.N. Coleman, Small 6 (2010) 864.

[14] S. Stankovich, D.A. Dikin, G. Dommett, et al, Nature, 442 (2006) 282.

[15] U. Khan, A. O’ Neill, H.T. Porwal, P. May, K. Nawaz, J. N.Coleman, Carbon, 50

(2011) 470.

[16] S. Park, R.S. Ruoff, Nat. Nanotechnol. b (2009) 217.

[17] L. Gong, I.A. Kinloch, R.J. Young, I. Riaz, R. Jalil, K.S. Novoselov, Adv. Mater. 22

(2010) 2694.

[18] U. Khan, P. May, A. O’Neill, J.N. Coleman, Carbon 48 (2010) 4035.

[19] G.E. Padawer, N. Beecher, Polym. Eng. Sci. b (1970) 185.

[20] H.W. Hu, G.H. Chen, Polym. Compos. 31 (2010) 1770.

[21] L. Jiang, X.P. Shen, J.L. Wu, K.C. Shen, J. Appl. Polym. Sci. 118 (2010)

275. [22] I.H. Kim, Y.G. Jeong. J. Polym. Sci. Pol. Phys. 48 (2010) 850.

[23] J. J. Liang, Y. Huang, L. Zhang, Y. Wang, Y. Ma, et al, Adv. Funct.Mater. 19

(2009) 2297.

[24] S.G. Miller, J.L. Bauer, M.J. Maryanski, P.J. Heimann, et al, Compos. Sci. Technol.

70 (2010) 1120.

[25] K.W. Putz, O.C. Compton, M.J. Palmeri, S.T.Nguyen, L.C.Brinson, Adv.

Funct. Mater. 20 (2010) 3322.

[26] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. et al, Nanotechnol 3 (2008)

327.

Page 151: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

131

[27] P. Steurer, R. Wissert, R. Thomann, R. Mulhaupt, Macromol Rapid Commun.

30 (2009) 316.

[28] X.M. Yang, L.A. Li, S.M. Shang, X.M. Tao, Polymer 51 (2010) 3431.

[29] X. Zhao, Q.H. Zhang, D.J. Chen, P. Lu, Macromolecules 43 (2010) 2357.

[30] U. Khan, H. Porwal, A. O’Neill, K. Nawaz, P. May, J.N. Coleman, Langmuir

27 (2011) 9077. [31] T. Ramanthan, H. Liu, L.C. Brinson, J. Polym. Phys. 43 (2005) 2269.

[32] A. Bansal, H. Yang, C. Li, K. Cho, B. C. Benicewicz, et al, Nature Materials 4

(2005) 693. [33] M. Lotya, P.J. King, U. Khan, S. De, J.N. Coleman, ACS Nano 4 (2010) 3155.

[34] A. O’Neill, U. Khan, P.N. Nirmalraj, J.J. Boland, J.N. Coleman, J. Physical

Chemistry C 115 (2011) 5422.

[35] M. Conradi, M. Zorko, I. Jerman, B. Orel, I. Verpoest, Polymer Engineering &

Science 53 (2012) 1448. [36] S. Vadukumpully, J. Paul, N. Mahanta, S.Valiyaveettil, Carbon 49 (2011)

198.

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Chapter 9

The Effect of Large Area Graphene

Oxide (LAGO) nanosheets on the

Mechanical Properties of Polyvinyl

Alcohol

9.1 Objective

Large area graphene oxide (LAGO) sheets were synthesized, dispersed in water and

used as nanofiller for mechanical improvement in terms of Young’s Modulus and

Ultimate Tensile Strength (UTS) of polyvinyl alcohol (PVA) at low loading. The

molecular level dispersion and interfacial interactions between the graphene oxides

(GO) and polymeric matrix PVA was the real challenges. An excellent improvement

in mechanical properties at 0.35wt% loading was observed. Modulus improved from

1.58 GPa to 2.72 GPa (~71 % improvement), UTS improved from 120 MPa to 197

MPa (~65% improvement) and in spite of these improvements, interestingly, there

was no fall in elongation at break at this loading.

9.2. Introduction

Planar “2D form of carbon, known as graphene has become one of the most exciting

materials today because of its unique properties [1]. Individual graphene sheets show

high values of thermal conductivity” [1], Young’s modulus [2], large surface area [1],

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“ballistic transport on submicron scales and mass less Dirac fermions charge carrier

abilities [3]. These properties make graphene a promising material in many

applications such as photovoltaic devices”, “sensors, transparent electrodes, super

capacitors conducting composites” [4, 7–12]. “At present, carbon-based reinforcing

materials employed in polymer composites are dominated by carbon nanotubes

(CNT), but difficulty in dispersing CNT and high cost of production, limits its

widespread use”. Challenge of today is to find any alternative to “CNT and for this

purpose graphene proved itself to be a suitable candidate because of its outstanding

mechanical properties and ultra large interfacial surface area” [3, 4]. However, a

detailed comparative study of pristine graphene and multi walled carbon nanotubes

(MWCNTs) vs. silane functionalized graphene and MWCNTs in Poly (L-lactic acid)

polymer has been reported and they observed almost same improvement in

mechanical properties of polymer in terms of tensile strength, elongation at break and

young’s modulus for pristine graphene and MWCNTs [13].

In fact a lot of work has been done [14-16] showing that graphene and graphene

oxide [11, 17] are potentially effective reinforcement materials [18-26].

“Incorporation of graphitic nano flakes into elastomeric polymer matrix generates

high performance composites with improved mechanical and functional properties”

[12, 19, 27]. It has been shown that the fracture of graphene-polymer composites is

due to failure of the polymer graphene interface [26]. This failure may be due to the

short flake length of graphene than required critical length [28, 29]. It means the flake

length should be greater than critical length, for better interfacial strength, resulting

better load transfer to reinforcement [26]. Recently, “graphene oxide (GO) has also

been used as a filler in various polymeric matrix, due to its hydrophilicity and ease of

formation of stable colloidal suspensions” [19,12]. Functionalized graphene sheets

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(FGS) are also employed because they provide much better interactions with the “host

polymers compared to unmodified CNT or traditional expanded graphite (EG)” [12].

Likewise in our previous study of incorporating functionalized graphene in

polyurethane, improvement in mechanical properties in terms of modulus and

ultimate tensile strength were observed [30]. Similarly, we observed interesting

phenomena during incorporation of selected size (big and small flakes) of graphene

nanosheets improvement in mechanical properties in terms of modulus and ultimate

tensile strength were observed for big flakes as compared to smaller ones [31].

Today’s challenge is to attain high mechanical strength with the use of minimum

amount of fillers. In this study, we selected LAGO for improving mechanical

properties of selected polymer at low loading. In our current investigation, “PVA is

chosen as the host polymer matrix, because of its wide range of applications, low

cost, chemical stability, and bio- compatibility” [32].

Oxygen containing functional groups affects the van der Waals forces between the

layers of graphene oxide and imparts desired property of water solubility. These

functional groups have been found to be effective means for the improvement of

dispersion of graphene [18,34-36]. Likewise, additional functional groups improve

its solubility / dispersibility in specific solvents [18, 19, 30]. PVA functionalized

graphene oxide sheets have been used as reinforcement in PVA matrix and 60%

improvement in terms of modulus has been observed [36]. Graphene oxide can be

dispersed at the individual sheet level in water so molecular level dispersion of GO

can be made in water. In present study improvement in mechanical properties

specially in terms of Young’s Modulus, ultimate tensile strength (UTS) has been

studied at very low loading i.e.0.35 wt%, while elongation at break was almost

undisturbed at this loading. The observed improvement in mechanical properties may

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be due to proper dispersion, strong H-bonding of GO and PVA and the interfacial

interaction between the filler and wrapped polymer matrix [33,37-39]. Similarly,

excellent agreement between experimental nanocomposites modulus and theoretical

modulus based on Halpin-Tsai equation has been observed [37,40-42].

9.3. Experimental

A reported procedure for the synthesis of LAGO was followed [43]. In brief, a 2gm

portion of natural graphite (Aldrich), 2gm of NaNO3 (Aldrich) and 96 ml of

concentrated sulfuric acid (Sigma –Aldrich) were mixed at 0 oC. The mixture

obtained was first stirred at 0 oC for 90 minutes and then at 35 oC for 2 hrs. Millipore

water (80ml) was slowly added into the resulting solution in about half hour to dilute

the mixture. Then 200ml of water was added followed by 10ml of hydrogen peroxide

(Aldrich) (H2O2, 30%) and the stirring continued for 10 mins to obtain a graphite

oxide suspension. During this final step, H2O2 reduced the residual permanganate and

manganese dioxide to colorless soluble manganese sulfate. The graphite oxide deposit

was collected from the graphite oxide suspension by high speed centrifugation at

16000 rpm for 10 min, and repeatedly washed with distilled until its pH=7. Then a

mild bath sonication was used to exfoliate the graphite oxide to obtain a graphene

oxide (GO) suspension. Later on a low speed centrifugation at 3000 rpm (3-5 min)

was used to remove thick layer of graphite oxide from exfoliated large area graphene

oxide sheets. The supernatant was further centrifuged at 5000 rpm for 5 min to

separate large flakes (precipitate) from small one (supernatant). Finally the

precipitate was re-dispersed in water to get LAGO sheets suspension and filtered

through a nylon membrane of pore size 0.45 micron (Sterlitech). The membrane

supported graphene nano flakes were dried at 60 oC for accurate mass. Then its

dispersion was made in Millipore water and used in thin films casting. Similarly 60

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mg/ml solution of PVA was made in Millipore water by dissolving 6 gm of PVA, in

100 ml of water for about 24 hours to get PVA solution.

The composites samples were prepared using different weight percent of LAGO to

PVA (average Mw 89,000-98,000, Aldrich) solution ranging between 0.15 wt% to 3.0

wt. % and a reference sample of PVA was also prepared. These samples were

sonicated for one hour in bath sonicatior for homogenous mixing and were drop cast

in Teflon trays of dimension 4x4x2 cm. These cast samples were dried in vacuum

oven at 900 mbar. Then these samples were shifted to another oven and kept there for

eight hours to remove traces of water, if any present. The weight of each sample was

~ 150 mg. The film thickness of each film was in the range of ~ 80-90 microns.

9.4. Characterization

FT-IR was conducted on crushed powder on a glass slide in transmittance mode using

Nexus Nicolet FTIR. Transmission electron microscopy (TEM) was performed by

dropping small quantity of LAGO containing dispersions on holey carbon grids using

a Jeol 2100, operated at 200 kV. “Scanning Electron Microscopy (SEM) was

performed using a Carl Zeiss Ultra Plus Field Emission Scanning Electron

Microscope”.

Perkin-Elmer DSC 7 was used for Differential Scanning Calorimetry (DSC) under

inert atmosphere with 10 Co/min.

9.4.1. Mechanical characterization

Zwick Roell tensile tester was use for mechanical testing with 100N load cell at a

strain rate of 15mm/min.

9.5. Results and discussion.

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Graphite oxide (supernatant) was separated from un-oxidized graphite (precipitant)

which is not dispersible in aqueous media by adding water and centrifuged at 500 rpm

for 45 minutes to separate these both entities from each other. Then the suspension

was filtered and dried and once again its aqueous suspension was made from dried

powder and separated its large flakes from smaller one at different centrifuge speed.

The synthesis of GO was confirmed through FTIR spectroscopy as shown in

Figure.9.1.

The GO exhibits the specific peaks at 1720,1630,1390,1220, and 1050 cm-1 which

can be linked with the presence of C=O, C=C, C-O (carboxy), COH, and C-O (alkoxy

group), and broad band absorption between 3000 and 3500 cm-1 associated with

hydroxyl group [30,44-47].

Figure. 9.1 FTIR spectra of Graphene oxide

Tran

smit

tan

ce

Wave number cm-1

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Figure.9.2 TEM images of LAGO flakes deposited from dispersion

Figure.9.3 Data for LAGO in aqueous media (A) Number of layers per flake of

LAGO (B) Length of LAGO nanosheets ~5.0 micron (C). Width of LAGO

nanosheets ~0.8 micron.

Figure 9.2 and 9.3 shows the TEM image and histogram of dimensions of the flakes

of LAGO. These dimensions in terms of number of layers per flake, length and width

have been measured [48-50]. It is clear from histogram (Figure.3) that the LAGO

flakes consist of about 3 layers in average with width of 0.8 µm and length of ~5.0

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µm. These flakes were incorporated in PVA at different wt% for study of mechanical

properties in terms of Modulus, UTS and elongation at break.

Fig.9.4 SEM images of LAGO in PVA. (A) 3% (B) 0.35%

Figure.9.4 represents SEM images of dispersion of LAGO in polymer for different %

weight showing homogenous distribution of LAGO at low concentration. Similarly,

dispersion of LAGO in polymer at various loading can be seen in Fig.4 showing

homogenous distribution of LAGO at low concentration. Similarly, mechanical

properties in terms of modulus,UTS , elongation at break and comparative study of

predicted and observed modulus are presented in Figures 9.5- 9.8 respectively.

A B

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Figure.9.5 Effect of LAGO on modulus of PVA

It was observed that the mechanical properties of PVA (at 0.35 wt%), in terms of

modulus and UTS improved about 71% and 65% as shown in Figures 9.5 and 9.6

respectively. The Young’s Modulus is 1.58 GPa and the Ultimate Tensile Strength

(UTS) is 120 MPa for neat sample of PVA. At 0.35 wt% loading Modulus improved

from 1.58 to 2.72 GPa (71% improvement) as shown Figure.9.5

% weight of LAGO

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Figure. 9.6 Effect of LAGO on Tensile strength of PVA

while UTS increased from 120 MPa to 197 MPa (~ 65% increase) as shown in

Figure 9.6. This improvement in modulus and UTS may be due to good

dispersion/interaction of polymer and graphene nano sheets at this loading [48].

Beyond this loading (0.35 wt %) improvement in modulus was not linear and the

value of UTS dropped drastically. This non linear behavior in modulus and decrease

in UTS may be due to agglomeration of nanofiller / not proper dispersion in

polymeric matrix [51, 52].

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Figure. 9.7 Effect of LAGO on elongation at break of PVA

Likewise increase in mechanical resistance properties (Modulus and UTS) results in

decrease in ductility in terms of elongation at break [52]. But in our case at 0.35 wt%

loading, we get not only improvement in mechanical properties in terms of UTS and

modulus but elongation at break is not affected but improved slightly, e.g. from

32.1% to 32.6%, but at 3.0 wt% loading the elongation at break affected disastrously,

dropping from 32.1% to 8.5% as shown in Figure.9.7. The fall in elongation at break

may be due to interaction of graphene with polymer chain which restricts the

movement of polymeric chains [52].

From these results we understand that 0.35 wt% loading is critical loading on which

very good mechanical properties in terms of Modulus, UTS and elongation at break

can be obtained, while above this loading there is downtrend in mechanical properties

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specially in UTS and elongation at break. The non-linear increases in mechanical

properties of elastomeric composites are generally due to either the effects of strong

polymer-filler interaction or the formation of filler network [53]. In the present study

for all loadings, increase in modulus was observed, so it is believed that there exist

the phenomena of matrix-filler interaction [53]

The Halpin-Tsai model [41-43, 55] for random distribution of filler in polymeric

matrix was used to simulate our obtained results in terms of modulus based on

LAGO/PVA nano composites. Its equation for randomly distribution is given as

follows,

In this equation No. (9.1) Ec shows the modulus of the nano composites with

randomly distributed LAGO nano flakes. Similarly Vc is the volumetric fraction of

LAGO in polymer. Likewise Eg and Em are the modulus of graphene oxide nano

flakes and polymer (PVA) in eq.9.2 and 9.3 which are 0.25 TPa (Tera Pascal) [55]

(9.1)

(9.2)

(9.4)

(9.3)

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and 1.88 GPa respectively. ξ, 1g, ,tg are the aspect ratio, length and thickness of

graphene nano flakes respectively as presented in eq.9.4. The density of PVA and

graphene oxide is 1.3 g/cm-3 and 2.200 g/cm3respectively [19]. The statistical average

of the length and thickness of LAGO nano flakes were about 5.0µm and 0.8nm

respectively as determined by TEM histogram in Figure.9.3. Now we look in detail

the dependence of mechanical properties on the content of graphene oxide. Putting all

these values in Eq. (9.1) it becomes apparent from Figure.9.8, that the

experimental results are better than theoretical one for low loading, i.e. below 0.7

wt%. This may be explained on the basis of proper dispersion of LAGO nano flakes

in polymer at low concentration, effective load transfer due to H-bonding between the

oxygen containing groups of LAGO sheets and PVA chains, and high aspect ratio of

LAGO [54].

Figure. 9.8 Comparison of Theoretical and experimental data of Young’s modulus

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

LAGO Content (wt%)

Mo

du

lus

(G

Pa

)

Modulus(experimental)

Modulus(Theoratical)

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These results can be compared with already published work [19] on molecular level

dispersion of graphene into PVA in which for 0.7% loading 62% improvement in

modulus and 76% increase in UTS was reported, while elongation at break was

drastically decreased as compared to neat PVA. However in our study for 0.35 wt%

loading the improvement in terms of modulus is ~ 71% and enhancement in the value

of UTS is about ~ 65% while elongation at break was not affected but improved

slightly. The elongation at break appeared to increase slightly from ~32.1% for the

polymer to ~32.6% for the 0.35wt% composite sample before subsequently falling

off. This is slightly unusual as ductile polymers usually display a decrease on strain at

break on the addition of nanofiller such as nanotubes or graphene [56-59]. Indeed

previous work on PVA filled with functionalized and pristine graphene showed a

reduction in ductility for all filler contents [19, 36]. However, in our previous study of

polyvinyl acetate as polymeric matrix the mechanical properties in terms of modulus

and UTS were improved and elongation at break was not affected [60].At low level of

loading, nano fillers are homogenously dispersed while at high loading it may form

agglomerates. So, we can say that solution blending is efficient for low loading but it

may not work for high loading.

Figure.9.9 DSC of PVA and LAGO based nano composites (0.35 wt%)

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PVA is semi crystalline polymer and its mechanical properties depends on its degree

of crystallinity.DSC was conducted in order to see whether these improvement in

mechanical performance is due to change in crystallinity. Enthalpy of pure PVA and

sample containing 0.35 wt% graphene nanofiller was conducted. Both melt curves

has been shown in Figure.9.The melting peaks has similar pattern and both are in the

range of 160-220C0.This indicates that both samples have same crystallinity.

Crystallinity () can be determined as

Xc=ΔHm/ΔH0 (9.5)

In equation (9.5) Hm and H0 are enthalpy measured by DSC and enthalpy of pure

PVA crystallization respectively, which is 138.6 J/gm [59,60]. As there is no distinct

difference between these two melt curves as shown in Fig.9.9 so we can say that the

improvement in mechanical performance cannot be linked with change in

crystallinity.

Conclusion

LAGO flakes as nanofiller have been used in polyvinyl alcohol as polymeric matrix.

An interesting phenomenon was observed at 0.35 wt. % loading both mechanical

properties UTS and Young Modulus were improved to a good extent while elongation

at break was slightly improved. Improvement in terms of modulus is very prominent.

But beyond this loading mechanical properties in term of elongation at break and

UTS affected disastrously. The effect on elongation at break may be due to the

restacking of graphene flakes in polymer which restricts the movement of polymer

chain. Simple method of drop casting was followed using water as processing solvent

from environment stand point. The obtained results were better than as predicted by

Halpin –Tsai equation. It may be due to strong interaction of hydrogen bonding of

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GO and PVA polymer and homogenous dispersion of GO at low loading i.e. less than

1%.

References

[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V.

Dubonos, I.V. Grigorieva, A.A. Firsov, Science, 306, 666 (2004).

[2] AA Balandin, S Ghosh, W Bao, I Calizo, D Teweldebrhan, F Miao, CN Lau.

Nano Lett., 8, 902 (2008).

[3] C. Lee, X.D. Wei, J.W. Kysar and J. Hone, Science, 321, 385 (2008).

[4] M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Nano Lett., 8, 3498

(2008).

[5] Y. Zhang, Y.W. Tan, H. L. Stormer and P. Kim, Nature, 438, 201 (2005).

[6] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I.

V. Grigorieva, S. V. Dubonos & A. A. Firsov, Nature, 438, 197 (2005).

[7] Z.F. Liu, Q. Liu, Y. Huang, Y.F. Ma, S.G. Yin, X.Y. Zhang, J. Adv. Mater.,

20 , 3924 (2008).

[8] J.T.Robinson, F.K. Perkins, E.S. Snow, Z.Q. Wei, P.E. Sheehan, Nano

Letters, 8, 3137 (2008).

[9] P.K. Ang, W. Chen, A.T.S. Wee, K.P. Loh, J. Am. Chem. Soc., 130, 14392

(2008).

[10] J.B. Wu, H.A. Becerril, Z.N. Bao, Z.F. Liu, Y.S. Chen, Appl. Phys. Lett., 92,

263302 (2008).

Page 168: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

148

[11] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney,

E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature, 442, 282 (2006).

[12] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso,

R.D. Piner, D.H. Adamson, H.C.Schniepp, X.Chen, R.S. Ruoff, S.T. Nguyen ,

I.A. Aksay, R.K. Prud'Homme , L.C. Brinson, Nat. Nanotechnol., 3 , 327

(2008).

[13] L. Wenxiao, S. Chengbo, S. Mingling, G. Qiwei, X. Zhiwei,W. Zhen, Y.

Caiyun, M. Wei , N. Jiarong, J. Appl. Polym. Sci., 130, 1194 (2013).

[14] Y.G. Yang, C.M. Chen, W. YF, Q.H. Yang, M.Z. Wang, New Carbon

Mater., 23, 345 (2008).

[15] R. Verdejo, M.M. Bernal, L.J. Romasanta, M.A. Lopez-Manchado, J. Mater.

Chem., 21, 3301 (2011).

[16] T. Kuilla, S. Bhadra, D. Yao, N.H. Kim, S. Bose, J.H. Lee, Prog. Polym. Sci.,

35, 1350 (2010).

[17] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y.

Jia, Y.Wu, S. T. Nguyen and R. S. Ruoff, Carbon , 45, 1558 (2007).

[18] J.J. Liang, Y. Huang, L. Zhang, Y. Wang, Y.F. Ma, T.Y. Guo, Y.S. Chen,

Adv. Funct. Mater., 19, 2297 (2009).

[19] K.W. Putz, O.C. Compton, M.J Palmeri, S.T. Nguyen, L.C. Brinson, Adv.

Funct. Mater., 20, 3322 (2010).

[20] S. G. Miller, J. L. Bauer, M. J. Maryanski, P. J. Heimann, J. P. Barlow, J. M.

Gosau and R. E. Allred, Compos. Sci. Technol., 70, 1120 (2010).

Page 169: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

149

[21] L. Jiang, X-P. Shen, J-L. Wu, K-C. Shen. J. Appl. Polym. Sci., 118, 275

(2010).

[22] I-H. Kim, Y.G. Jeong, J. Polym. Sci. Pol. Phys., 48, 850 (2010).

[23] P. Steurer, R. Wissert, R. Thomann, R. Mu¨lhaupt, Macromol. Rapid Comm.,

30, 316 (2009).

[24] X. Zhao, Q. Zhang, D. Chen, P. Lu, Macromolecules, 43, 2357 (2010).

[25] X. Yang, L. Li, S. Shang, X-M. Tao, Polymer, 51, 3431 (2010).

[26] P. May, U. Khan, A. O’Neill, J.N. Coleman, J. Mater. Chem., 22, 1278

(2012).

[27] K. Kalaitzidou, H. Fukushima, L.T Drzal, Compos. Part A–Appl. S., 38, 1675

(2007).

[28] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Carbon, 44, 1624 (2006).

[29] G.E. Padawer, N. Beecher, Polym. Eng. Sci., 10, 185 (1970).

[30] K. Nawaz, U. Khan, N. Ul-Haq, P. May, A. O’Neill, J. N. Coleman. Carbon,

50, 4489 (2012).

[31] K. Nawaz, M. Ayub, N. Ul- Haq, M.B.K. Niazi, A. Hussain, Fibers and

Polymers, 15, 2040 (2014).

[32] L.Q.Liu, A.H.Barber, S.Nuriel, H.D.Wagner, Adv. Funct. Mater., 15, 975

(2005).

[33] M.J.McAllister, J.L.Li, D.H.adamson, H.C.schniepp, A.A.Abdala, J.Liu,

M.Herrera Alonso, D.L.Milius, R.Car, R.K.Prud’homme, I.A.Aksay,

ChemMatters – ACS, 19, 4396 (2007).

Page 170: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

150

[34] H.C.Schniepp, J.L.Li, .J.McAllister, H.Sai, M.Herrera-Alonso, D.H.adamson,

R.K.Prud’homme, R.Car, D.A.Saville, I.A. Aksay, J. Phy. Chem. B., 110,

8535 (2006).

[35] Y.C.Si, E.T.Samulski, Nano Lett., 8, 1679 (2008).

[36] M. Cano, U. Khan, T.Sainsbury, A. O’Neil, Z. Wang, I.T.Mgovern,

W.K.Maser, A.M. Benito, J.N. Coleman. Carbon, 52, 363 (2013).

[37] D.Qian, E.C.Dickey, R.Andrews,T.Rantell, Appl. Phys. Lett., 76, 2868

(2000).

[38] J.B.Gao, M.E.Itkis, A.P.Yu, E.Bekyarova, B.Zhao, R.C.Haddon, J. Am.

Chem. Soc., 127, 3847 (2005).

[39] S.Jeong, J.S.Moon, S.Y.Jeon, J.H.Park, P.S.Alegaonkar, J.B.Yoo, Thin Solid

Films, 515, 5136 (2007).

[40] R.R.Tiwari, K.C.khilar, U.J.Natarajan, J. Appl. Polym. Sci., 108, 1818 (2008).

[41] K. Kalaitzidou, H. Fukushima, H. Miyagawa, L.T. Drzal, Polym. Eng. Sci.,

47, 1796 (2007).

[42] D.W.Schaefer, R.S.Justice, Macromolecules, 40, 8501 (2007).

[43] J. Zhao, S. Pei, W. Ren, L. Gao and H-M. Cheng, ACS NANO, 4, 5245

(2010).

[44] Y.S. Yun, Y. Bae, D.H. Kim, J.Y. Lee, I.J. Chin, H.J. Jin, Carbon, 49, 2942

(2011).

[45] S. Park, D.A. Dikin, S.T. Nguyen, R.S.Ruoff, J. Phys. Chem. C, 113, 15801

(2009).

Page 171: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

151

[46] J.I. Paredes, S. Villar-Rodil, A. Martinez-Alonso, Langmuir, 24, 10560

(2008).

[47] S. Stankovich, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Carbon, 44, 3342 (2006).

[48] U. Khan, A. O’Neill, M, Lotya, S. De, J.N. Coleman, Small, 6, 864 (2010)

[49] U. Khan, H, Porwal, K, Nawaz, P, May, J.N. Coleman, Langmuir, 27, 9077

(2011).

[50] U. Khan, A. O'Neill, H. Porwal, P. May, K. Nawaz and J. N. Coleman,

Carbon, 50, 470 (2012).

[51] J.pascual, F.peris, T.Bronat, O.Fenollar, R.Balart, Polym. Eng. Sci., 52, 733

(2012).

[52] S. Vadukumpully, J. Paul, N. Mahanta, S. Valiyaveettil, Carbon, 49, 198

(2011).

[53] D. Stauffer and A. Aharony, Introduction to Percolation Theory. Taylor &

Francis, 2nd ed. London, 1992.

[54] J.C.Halpin, J.L.Kardos, Polym. Eng. Sci., 16, 344 (1976).

[55] C.Gomez-Navarro, M.Burghad, K.Kern, Nano Lett., 8, 2045 (2008).

[56] U. Khan, P. May, A, O’Neill, J. N. Coleman, Carbon, 48, 4035 (2010).

[57] F. M. Blighe, W. J. Blau, J. N. Coleman, Journal of Nanotechnology, 19,

415709 (2008).

[58] U. Khan, F. M. Blighe, J. N. Coleman, J. Phys. Chem. C., 114, 11395

(2010).

Page 172: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

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[59] U. Khan, P. May, A. O’Neill, J. J. Vilatela, A. H. Windle, J. N. Coleman,

Small, 7, 1579 (2011).

[60] U. Khan, P. May, H. Porwal, K. Nawaz, J.N. Coleman, ACS-Applied

Materials & Interfaces, 5, 1423 (2013).

B

A C B A

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Chapter 10

Summary and Future Suggestions

10.1. Summary of main work

Nanocomposites are superior to conventional composites in the sense that in former

case small amount (less than 5%) of nano-fillers are added to polymeric matrix which

ultimately produces big impact in the mechanical performance of these selected

polymers. Superior mechanical, electrical and thermal properties and potential high

aspect ratio make graphene versatile polymer reinforcement [1-6]. Graphene can be

modified with organic functional groups via liquid-phase reaction for better

interaction with polymeric materials to have improved mechanical properties [7].

In this study various polymers were selected for the study of mechanical response in

terms of modulus, UTS and dL at break. Graphene nanosheets in various forms and

sizes were selected as nano-fillers for this study.

Graphene was functionalized with organic entity like octadecylamine (ODA) and its

functionalization was confirmed through FT-IR spectroscopy. The functionalized

graphene and poly (urethane) were readily dispersible/soluble in THF solvent which

helped in molecular level interaction of these both entities. It was observed that no

appreciable increases in either stiffness or low-strain stress for loading levels below

2.5vol%. However above this threshold, both mechanical quantities increase as a

power law. This behavior is consistent with mechanical percolation. This implies that

the graphene oxide platelets are effectively isolated at low volume fractions but begin

to form a network at a volume fraction of 2.5vol%. This loading level can be thought

of as a percolation threshold. As the loading level is increased, the network becomes

more extensive and the stiffness and low-strain stress increase as described by the

percolation scaling law, interestingly the ultimate tensile strength initially increases

but reaches a maximum at the percolation threshold. Similarly the ductility is invariant

with graphene content up to the percolation threshold, after which it falls steadily.

The Young’s modulus appeared to increase almost linearly with increasing GO-ODA

mass fraction from 9.6 MPa for the polymer to 335 MPa for the 50wt% composite.

the ultimate tensile strength initially increased from 27 MPa for the polymer to 38

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MPa for the 3 wt% composite before falling steadily, reaching 10 MPa for the 50wt%

composite . The strain at break decreased steadily with increasing GO-ODA mass

fraction from ~1000% for the polymer to ~10% for the 50 wt% sample.

This work shows that the mechanical properties of elastomers reinforced with

graphene can depend on parameters other than interfacial stress transfer.

Similarly in case of PVC polymer a critical loading (1.5 wt %) was observed at which

the mechanical performance was enhanced and interestingly there was very slight fall

in dL at break. At 1.5wt% loading, modulus improved 63% (from 1.31 to 2.14 GPa),

UTS improved about 19% from (70 MPa to 83.2 MPa), and negligible effect on

elongation at break of PVC (from 15.3% to 13 %) was observed. On other hand, at 10

wt% loading, modulus enhanced 75 % (1.31 to 2.29 GPa) UTS dropped about 22%

(from 70 MPa to 55 MPa), dL at break was disastrously affected (15.3% to 4.4 %).

Modulus determined in this study was compared to that calculated from Halpin-Tsai

equation. During this comparative study it was observed that the response of modulus

was superior to theoretical one based on Halpin-Tsai model at low loading( at or

below 1.5 wt %).I understand that at low loading of nanofiller the solution blending

works effectively and homogenous distribution can be obtained which results in better

mechanical performance of nanocomposites.

Likewise I also incorporated two different flake sizes of graphene nanosheets

(1 µm and 3.5 µm) to PAN polymer. At 12 wt. % loading the modulus increased 50%

(from 0.558 GPa to 0.837 GPa) while the UTS improved about 27% (from 37.8 MPa

to 48.1 MPa). However, in case of small flake size (1 µm) the modulus improved 41%

(from 0.558 GPa to 0.789 GPa). While improvement in UTS is 23 % (from 37.8 MPa

to 46.6 MPa) for 12 wt. % loading for small flakes was observed.

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dL at break dropped from 18.8% to 5.3% and 8.6% for big and small flake size at 12

wt% loading respectively.

Similarly large area graphene oxides (LAGO) were synthesized and were introduced

in PVA. Water was used as solvent for both polymer and nanofiller in order to have

molecular level interaction. The performance of LAGO as nanofiller was very

impressive in terms of modulus, UTS and dL at break. At 0.35% loading Modulus

improved from 1.88 to 2.64 GPa (71% improvement) UTS increased from 130 MPa to

195 MPa (~ 67% increase) and elongation at break is not affected but improved

slightly, e.g. from 32.1% to 32.6%. But beyond this loading, improvement in modulus

was persistent but UTS and the elongation at break affected disastrously. dL at break

dropped from 32.1% to 8.5% drastically.

I understand that 0.35% loading is critical loading on which we get very good

mechanical properties in terms of modulus, UTS and elongation at break.

While above this loading there is downtrend in mechanical properties especially in

UTS and elongation at break. In this study, for all loading increase in modulus was

observed so it is believed that there exist the phenomena of matrix-filler interaction.

Similarly in case of PVAc 50% increase in stiffness, 100% increase in “tensile

strength on addition of 0.1 vol % graphene compared to the pristine polymer”. The

adhesive “strength and toughness of the composites were up to 4 and 7 times higher,

than the pristine polymer”.

Graphite was exfoliated to graphene in organic and aqueous media, having 63 and 7

mg/mL concentrations in these both media respectively.

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In case of water as exfoliating media sodium cholate was used as surfactant in order to

ease exfoliation. In the process of this exfoliation sonic tip was used as exfoliating

instrument. Later on the exfoliated graphene was separated via controlled

centrifugation according to its lateral dimensions.

10.2 Future suggestions

Although graphene based nanocomposites are technologically prominent

development to emerge from the interface between graphene and polymeric materials

[8,9]. But there are some certain challenges which must be resolved in order to fully

exploit the mechanical performance of graphene. For example, graphite and GO is

usually exfoliated through sonication which ultimately reduces its size in terms of

lateral dimension as result it negatively affect the mechanical properties of

composites [10-12]. Likewise, Defects and wrinkles in platelets are also one of the

reasons for poor reinforcing capabilities. Composites properties can further be

improved if alignment and spatial organization of graphene are addressed [13-15].

In spite of these problems nanocomposites has very bright future in commercial

activities. Graphene –based composites materials can become commercial reality due

to its extraordinary mechanical properties and low cast raw material like graphite if

its production methods are improved.

References

[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V.

Dubonos, et al, Science 306 (2004) 666.

[2] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et

al, Nano Lett. 8 (2008) 902.

[3] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385.

[4] M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Nano Lett. 8 (2008)

3498.

Page 177: Liquid Phase Exfoliation of Graphite to Graphene & …prr.hec.gov.pk/jspui/bitstream/123456789/6685/1/Khalid...Liquid Phase Exfoliation of Graphite to Graphene & its Applications in

157

[5] Y.B. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201.

[6] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson,

I.V. Grigorieva, et al, Nature 438 (2005).

[7] Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Carbon , 44

(2006) 3342.

[8] Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chem Soc Rev 39

(2010) 228.

[9] Schniepp HC, Li JL, McAllister MJ, Sai H, Herrera-Alonso M,

Adamson DH, et al. J Phys Chem B 110 (2006) 8535.

[10] U. Khan, A. O’ Neill, H.T. Porwal, P. May, K. Nawaz, J. N.Coleman,

Carbon, 50 (2011) 470

[11] Kalaitzidou, H. Fukushima, L.T. Drzal, Journal of Composites Part A –

Applied Science Manufacturing 38 (2007) 7.

[12] U. Khan, H. Porwal, A. O’Neill, K. Nawaz, P. May, J.N. Coleman,

Langmuir 27 (2011) 9077

[13] Xie XL, Mai YW, Zhou XP. Mater Sci Eng R Rep 2005;49:89-112.

[14] Hussain F, Hojjati M,Okamoto M, Gorga RE. J. Compos Mater 40

(2006) 1511.

[15] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun Z, Slesarev

A, et al. ACS Nano, 4 (2010) 4806.

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Author’s List of Publications (as Ist author)

[1] K. Nawaz, U. Khan, N. Ul-Haq, P. May, A. O’Neill, J.N. Coleman.

“Observation of mechanical percolation in functionalized graphene oxide

/elastomers composites.” Carbon 50 (2012) 12.

[2] Khalid Nawaz, Muhammad Ayub , Noaman Ul haq ,M.B.Khan,

Muhammad Bilal khan Niazi,Arshad Hussain “Effects of selected size of

graphene on the mechanical properties of poly (acrylonitrile) (PAN).”

Fibers and Polymers. 15 (2014) 2040.

[3] Khalid Nawaz, Muhammad Ayub, Noaman Ul haq, M.B.Khan,

Muhammad Bilal Khan Niazi, Arshad Hussain,: “Effects of

graphene nanosheets on the mechanical properties of polyvinyl chloride

(PVC)” published on line in Journal of Polymer composites.

[4] Khalid Nawaz, Noaman Ul Haq,Muhammad Ayub, M.B.Khan,

Muhammad Bilal Khan Niazi,Arshad Hussain. “Effects of Large

Area Graphene Oxide (LAGO) on the mechanical properties of Poly

(vinyl alcohol) (PVA).” Published on line in Journal of Polymer

Engineering and Science.

[5] Khalid Nawaz,Muhammad Ayub,M.B.Khan,Arshad Hussain,Abdul

Qadeer Malik, Muhammad Bilal khan Niazi, Noaman Ul Haq “Effect of

surfactant concentration on the exfoliation of graphite to graphene in

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159

aqueous media.” Under review in J. NanoMaterials and Nano

Technologies, Manuscript number.2015.0108R1.

[6] Khalid Nawaz, ,Muhammad Ayub, M.B.Khan,Arshad Hussain, Noaman Ul

Haq “Effect of Thermal Treatment on the Exfoliation of Graphite to Graphene

in Acetonitrile (ACN).” Submitted in the journal of “Asia-Pacific Journal of

Chemical Engineering”.

Publications of author as Co-author

[1] U. Khan, H. Porwal, A. O'Neill, K. Nawaz, P. May, J.N. Coleman, “Solvent-

Exfoliated Graphene at Extremely High Concentration.” Langmuir 27 (2011)

9077.

[2] U. Khan, A. O’ Neill, H.T. Porwal, P. May, K. Nawaz, J. N.Coleman, “Size

selection of dispersed, exfoliated graphene flakes by controlled

centrifugation.” Carbon, 50 (2012) 470.

[3] Umar Khan, Peter May, Harshit Porwal, Khalid Nawaz, and Jonathan N.

Coleman. “Improved Adhesive Strength and Toughness of Polyvinyl acetate

Glue on Addition of Small Quantities of Graphene.” ACS Applied Materials

and Interfaces 2013.

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Papers presented in International conferences.

[1] Khalid Nawaz, Noaman Ul Haq, Muhammad Ayub, M.B.Khan. Paper

presented in 10th International Bhurban Conference of Science and

technology (IBCAST) January 2013. Titled “Effects of graphene

nanosheets on the mechanical properties of polyvinyl chloride (PVC)”

[2] Khalid Nawaz, Noaman Ul Haq,Muhammad Ayub, M.B.Khan Poster

presented in 11th International Bhurban Conference of Science and

technology (IBCAST) January 2014.titled “Effect of Large Area

graphene oxide on the mechanical properties of Polyvinyl alcohol

(PVA).

[3] Khalid Nawaz, Noaman Ul Haq, Muhammad Ayub, M.B.Khan.

“Effect of Surfactant concentration on the exfoliation of graphite to

graphene in aqueous media”. Paper presented in 12th IBCAST 2015.


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