enhancement of mechanical, thermal stability, and tribological …/67531/metadc699889/... · shah,...
TRANSCRIPT
APPROVED: Jose Perez, Major Professor Duncan Weathers, Committee Member Usha Philipose, Committee Member Guido Verbeck, Committee Member Christopher Littler, Interim Chair of
the Department of Physics Mark Wardell, Dean of the Toulouse Graduate
School
ENHANCEMENTS OF MECHANICAL, THERMAL STABILITY, AND TRIBOLOGICAL
PROPERTIES BY ADDITION OF FUNCTIONALIZED REDUCED
GRAPHENE OXIDE IN EPOXY
Rakesh K. Shah, M.S.
Dissertation Prepared for the Degree of
DOCTOR OF PHILOSPHY
UNIVERSITY OF NORTH TEXAS
August 2014
Shah, Rakesh K. Enhancement of Mechanical, Thermal Stability, and Tribological
Properties by Addition of Functionalized Reduced Graphene Oxide in Epoxy. Doctor of
Philosophy (Physics), August 2014, 83 pp., 2 tables, 45 figures, 117 numbered references.
The effects of octadecylamine-functionalized reduced graphene oxide (FRGO) on the
frictional and wear properties of diglycidylether of bisphenol-A (DGEBA) epoxy are studied
using a pin-on-disk tribometer. It was observed that the addition of FRGO significantly improves
the tribological, mechanical, and thermal properties of epoxy matrix. Graphene oxide (GO) was
functionalized with octadecylamine (ODA), and then reduction of oxygen-containing functional
groups was carried out using hydrazine monohydrate. The Raman and x-ray photoelectron
spectroscopy studies confirm significant reduction in oxygen-containing functional groups and
formation of ODA functionalized reduced GO. The nanocomposites are prepared by adding 0.1,
0.2, 0.5 and 1.0 wt % of FRGO to the epoxy. The addition of FRGO increases by more than an
order of magnitude the sliding distance during which the dynamic friction is ≤ 0.1. After this
distance, the friction sharply increases to the range of 0.4 - 0.5. We explain the increase in
sliding distance during which the friction is low by formation of a transfer film from the
nanocomposite to the counterface. The wear rates in the low and high friction regimes are
approximately 1.5 x 10-4
mm3/N·m and 5.5 x 10
-4 mm
3/N·m, respectively. The nanocomposites
exhibit a 74 % increase in Young’s modulus with 0.5 wt. % of FRGO, and an increase in glass
transition and thermal degradation temperatures.
Copyright 2014
by
Rakesh K. Shah
ii
iii
ACKNOWLEDGEMENTS
I would like to greatly thank my advisor Dr. Jose Perez for his inspiring way to guide me
to a deeper understanding of research and his valuable comments during my study at UNT.
I would like to thank Dr. Witold Brostow for providing me an opportunity to work in his
lab with his worldwide colleagues. The work would not have been possible without his help and
suggestions. Further, I am highly thankful to my other committee members Dr. Duncan
Weathers, Dr. Usha Philipose, and Dr. Guido Verbeck for being in my dissertation committee
and for their valuable comments and suggestions in preparing this dissertation. I am also thankful
to Dr. Christopher Littler and Dr. David Schultz for their support whenever it was needed from
the Department of Physics.
I am very grateful to all my friends who helped and encouraged me to achieve success in
this work. I am particularly thankful to Dr. Yudong Mo and Dr. Joshua Wahrmund for their help
and time for useful discussions.
I would like to express my heartfelt thanks to my former advisor Dr. Saikat Talapatra for
his invaluable comments and suggestions at different points to make a successful career. I would
also like to express sincere gratitude to my parents for their unconditional love and support
throughout my life. Last, but certainly not least, I like to thank my wife Nitu for her continuous
support and understanding during this work.
iv
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ............................................................................................................ ii
LIST OF TABLES ........................................................................................................................ vi
LIST OF FIGURES ..................................................................................................................... vii
CHAPTER 1 – INTRODUCTION ................................................................................................ 1
1.1 Introduction .................................................................................................................... 1
1.2 Composite Materials ...................................................................................................... 2
1.3 Band Structure of Graphene ........................................................................................... 4
1.4 Graphene Oxide .............................................................................................................. 7
1.5 Functionalization of Graphene and Graphene Oxide ..................................................... 9
1.6 Epoxy Resin ................................................................................................................. 11
1.7 Challenges in Polymer Nanocomposite ....................................................................... 12
1.7.1 Dispersion of Nanomaterials ................................................................................ 12
1.7.2 Cost Effectiveness ................................................................................................ 13
1.7.3 Health and Environmental Issues ......................................................................... 14
1.8 Motivation .................................................................................................................... 14
1.9 Outline of Dissertation ................................................................................................. 16
CHAPTER 2 – EXPERIMENTAL TECHNIQUES ................................................................... 17
2.1 Synthesis of Graphene Oxide ....................................................................................... 17
2.2 Functionalization of Graphene Oxide with Octadecylamine ....................................... 20
2.3 Characterization of Graphene Oxide and Functionalized Graphene Oxide ................. 22
2.3.1 Raman Spectroscopy of Graphene and Graphene Oxide ..................................... 22
v
2.3.2 X-ray Photoelectron Spectroscopy (XPS) of Graphene Oxide ............................ 24
2.4 Synthesis of Functionalized Graphene Oxide-Epoxy Nanocomposites ....................... 25
2.5 Characterizations of Polymer Nanocomposites ........................................................... 28
2.5.1 Mechanical Properties: Tensile Testing ............................................................... 28
2.5.2 Mechanical Properties: Dynamic Mechanical Analysis ....................................... 30
2.6 Thermal Stability Determination: Thermogravimetric Analysis (TGA) ...................... 34
2.7 Tribological Properties of Polymer Nanocomposites .................................................. 35
2.7.1 Friction ................................................................................................................. 36
2.7.2 Wear ..................................................................................................................... 37
2.7.3 Friction and Wear Measurements of Polymeric Materials:
Pin-on-disk Tribometer ........................................................................................ 37
CHAPTER 3 – CHARACTERIZATION OF GRAPHENE OXIDE AND
FUNCTIONALIZED REDUCED GRAPHENE OXIDE ................................ 40
3.1 Characterization of Functionalized Reduced Graphene Oxide Using SEM ............... 40
3.2 Characterization of Functionalized Reduced Graphene Oxide
Using Raman Spectroscopy ........................................................................................ 42
3.3 Characterization of Functionalized Reduced Graphene Oxide Using XPS ................ 43
CHAPTER 4 – MECHANICAL, THERMAL, AND TRIBOLOGICAL
PROPERTIES OF NANOCOMPOSITES ......................................................... 45
4.1 Introduction .................................................................................................................. 45
4.2 Young’s Modulus Measurements of FRGO-Epoxy Nanocomposites ......................... 47
4.3 Estimation of Young’s Modulus of FRGO-Epoxy Nanocomposites
Using Halpin-Tsai Model ............................................................................................ 50
vi
4.4 Dynamic Mechanical Analysis of FRGO-Epoxy Nanocomposites ............................. 55
4.5 Thermogravimetric Analysis of FRGO-Epoxy Nanocomposites ................................ 58
4.6 Tribological Properties of FRGO-Epoxy Nanocomposites ......................................... 60
4.6.1 Friction Results ..................................................................................................... 60
4.6.2 Wear Results ......................................................................................................... 66
4.6.3 Investigation of Wear Mechanism ........................................................................ 69
CHAPTER 5 –CONCLUSIONS ................................................................................................. 72
REFERENCES ............................................................................................................................ 75
vii
LIST OF TABLES
Page
Table 4.1 Mechanical properties of the neat epoxy and the nanocomposites .............................. 49
Table 4.2 The Young’s Modulus of FRGO-Epoxy Nanocomposites
Calculated Using Halpin-Tsai Model .......................................................................... 52
viii
LIST OF FIGURES
Page
Figure 1.1 Representation of hexagonal lattice of graphene sheet. The shaded
region represents the unit cell. ................................................................................... 5
Figure 1.2 First Brillion zone in reciprocal space where, Γ, K, K´, and M are
high symmetry points. ................................................................................................ 5
Figure 1.3 Band structure of graphene derived from tight band approximation. ........................ 6
Figure 1.4 Chemical structure of graphene oxide with different oxygen-containing
functional groups on basal plane and around edges of a graphene layer. .................. 8
Figure 1.5 Chemical structure of diglycidylether of Bisphenol-A (DGEBA),
n=0, for the derivatives, n > 0 .................................................................................. 11
Figure 1.6 Schematic of various levels of dispersion of nanomaterials in a polymer matrix. ... 13
Figure 2.1 A layout of GO synthesis procedure. ....................................................................... 18
Figure 2.2 Photographs during the oxidation process of graphite powder. (a) During the
second stage of oxidation reaction. (b) After the first dilution
with DI water. .......................................................................................................... 19
Figure 2.3 Schematic of a freeze drying system. ....................................................................... 20
Figure 2.4 (a) Schematic of GO functionalized with ODA. (b) Schematic of functionalized
reduced GO. The long chains represent ODA and is attached to the basal plane
of GO. ...................................................................................................................... 21
Figure 2.5 Schematic of the setup used to functionalize GO. .................................................... 22
Figure 2.6 Vibrational modes of (a) G peak and (b) D peak (b). The black circles represent
the carbon atoms and the arrows represent the direction of vibration. ................... 23
Figure 2.7 Ejection of a photoelectron upon irradiation by a mono-energetic X-ray. ............... 25
Figure 2.8 Photograph of the horn sonicator used for the dispersion of FRGO. ....................... 26
Figure 2.9 A schematic representation of FRGO/epoxy nanocomposite preparation. .............. 27
Figure 2.10 Schematic of the tensile testing equipment. ........................................................... 29
Figure 2.11 Schematic of a tensile testing sample. .................................................................... 29
ix
Figure 2.12 Response of a sinusoidal force applied to a polymeric material
at a certain frequency. ............................................................................................. 31
Figure 2.13 Schematic of the driving system of the DMA equipment. ..................................... 33
Figure 2.14 Schematic of the single cantilever (i.e., 2-point bending mode) system
of the DMA equipment. ......................................................................................... 33
Figure 2.15 Photograph of the thermogravimetric analyzer. ..................................................... 35
Figure 2.16 (a) Schematic illustration of a hard body sliding over a polymeric surface.
(b) Enlarged view of the region of polymeric material in contact with the
hard surface. .......................................................................................................... 37
Figure 2.17 Schematic of the pin-on disk tribometer and a polymeric nanocomposite
sample with a wear track.. ....................................................................................... 39
Figure 2.18 Photograph of the pin-on disk tribometer used for friction and wear
measurements. ......................................................................................................... 39
Figure 3.1 (a-c) SEM of FRGO powder at different magnifications. ........................................ 41
Figure 3.2 Raman Spectra of (a) GO (b) FRGO. ....................................................................... 42
Figure 3.3 XPS spectrum of graphene oxide. The black circles represent raw data. The
black line is fitted sum and the colored lines are fitted peaks using the
software OMNIC™ for Almega 7. .......................................................................... 43
Figure 3.4 XPS spectrum of functionalized reduced graphene oxide. The black circles
represent raw data. The black line is fitted sum and the colored lines are
fitted peaks using the software OMNIC™ for Almega 7. ....................................... 44
Figure 4.1 Load versus elongation curves of the neat epoxy, and nanocomposites
containing 0.1, 0.2, 0.5, and 1.0 wt. % FRGO ......................................................... 48
Figure 4.2 Young’s modulus of neat epoxy and nanocomposites containing
0.1, 0.2, 0.5 and 1.0 wt. % FRGO. Error bars are for three samples. ..................... 49
Figure 4.3 Raman spectra of FRGO deposited on a SiO2 substrate. .......................................... 53
Figure 4.4 Comparision of the Young’s modulus estimated using the Halpin-Tsai
model with the experimental Young’s modulus for the nanocomposites
containing 0.1, 0.2, 0.5 and 1.0 wt. % FRGO. ......................................................... 54
Figure 4.5 Plot of tan δ versus temperature for neat epoxy and nanocomposite
x
containing 0.5 wt %. ............................................................................................... 56
Figure 4.6 Tg for neat epoxy and nanocomposites containing 0.1, 0.2, 0.5 and
1.0 wt. % of FRGO. ................................................................................................ 57
Figure 4.7 Thermogravimetric analysis showing mass % versus temperature for
neat epoxy and nanocomposites containing 0.1, 0.2, 0.5 and 1.0 wt. %
of FRGO. ................................................................................................................. 59
Figure 4.8 Expanded view of thermogravimetric analysis in the temperature
range 330-420 oC. ................................................................................................... 59
Figure 4.9 The first derivative of the thermogravimetric curve with respect
to temperature for neat epoxy and nanocomposites containing 0.1,
0.2, 0.5 and 1.0 wt. % of FRGO .............................................................................. 60
Figure 4.10 Plot of the friction versus sliding distance for neat epoxy and
nanocomposites containing 0.1, 0.2, 0.5 and 1.0 wt. % of FRGO. .......................... 62
Figure 4.11 Expanded view of the friction versus sliding distance for neat epoxy. .................. 63
Figure 4.12 Optical microscopy images of the countersurface (i.e., tungustan
carbide ball of diameter 6mm). (a) Clean surface. (b) Surface after
sliding on neat epoxy showing no transfer film. Surface after sliding
in the low friction regime for nanocomposites contaning (c) 0.1, (d) 0.2,
(e) 0.5 and (f) 1.0 wt.% of FRGO. Transfer films are observed. ............................ 64
Figure 4.13 Optical microscopy images of the countersurface (i.e., tungustan carbide)
after sliding in the high friction regime for nanocomposite containing
(a) 0.1, (b) 0.2,(c) 0.5 and (d) 1.0 wt.% of FRGO. ................................................. 65
Figure 4.14 Profilometer cross-sections of wear tracks taken at the end of the friction
measurements shown in Figure 4.7 for the neat epoxy and the
nanocomposites containing various wt. % of FRGO. ............................................ 67
Figure 4.15 Wear rates of the neat epoxy and nanocomposite containing 0.1, 0.2,
0.5 and 1.0 wt. % of FRGO in the high friction regimes and wear
rates of various nanocomposites in low friction regime. Error bars
are for three wear tracks. ......................................................................................... 68
Figure 4.16 A SEM images of wear tracks in the low friction regime of (a) and
(b) Neat epoxy. (c) and (d) Nanocomposites containing 0.1 wt. % of
FRGO. (e) and (f) Nanocomposites containing 0.5 wt. % of FRGO. ..................... 70
Figure 4.17 SEM images of wear tracks in the high friction regime of (a) and
(b) Neat epoxy. (c) and (d) Nanocomposites containing 0.1 wt. %
xi
of FRGO. (e) and (f) Nanocomposites containing 0.5 wt. % of FRGO. ................ 71
1
CHAPTER 1
INTRODUCTION
1.1 Introduction
The field of nanotechnology has attracted much attention over the last few decades due to
increasing applications in many fields such as devices, sensors, and biomedical applications [1-
2]. The development of these areas depends upon fabrication of nanomaterials of various sizes,
shapes, and properties. The development of these nanomaterials has provided an opportunity to
create new, very useful and very diverse materials which society could use for various
applications such as personal protection, equipment for affordable health care and so on [3]. This
can be achieved from suitable combinations of properties of various kinds of materials
depending upon requirements.
Polymers are one of the most commonly used materials due to impressive properties such
as low cost, ease in processing, and recycling capability [3-4]. Further, the polymeric materials
have provided numerous useful applications, ranging from the fabrication of materials used in
daily life to aerospace components [5]. However, the intrinsic properties of these materials are
not enough to meet many applications. Thus, some other kinds of suitable materials with quite
different properties are combined to the polymer matrix in order to fabricate a new kind of
material with improved properties [3]. In this way, a multi-component material is created with
unique functional and physical properties, such as enhanced mechanical, thermal, and electrical
properties [6-7].
Since the discovery of graphene by Geim and his group [8], this material has been tested
for applications in many fields of science and technology due to its many interesting properties.
Further, the development of techniques for the dispersion of graphene in the nanoscale range has
2
opened a new area in material science [9]. One of such area is polymer composite in which
graphene or modified graphene is used as one of the constituent material [2, 9]. The remarkable
properties of graphene such as superior mechanical strength (~130 GPa), very high Young’s
modulus (1 TPa), very high thermal conductivity (~5,000 W/mK), very high electrical
conductivity (~600 S/cm), and high surface area (~2,630 m2/g) make this material suitable as a
filler for the fabrication of nanocomposite materials [10-14].
1.2 Composite Materials
A composite material has quite distinct properties that are not present in any of the
component materials [4]. There are three main constituents in any kind of composite material:
the matrix, the reinforcement (i.e., filler) and interfacial region [4]. The interfacial region is
responsible for the modification or improvement of any properties of the matrix [4]. There are
various kinds of fillers such as one-dimensional fillers (e.g., fibers and nanotubes), two-
dimensional fillers (i.e., clay, graphite and graphene), and three-dimensional fillers (i.e.,
spherical particles) [3-4]. It requires suitable combination of properties of constituent materials
in order to produce composite materials having improved properties instead of just mixing two or
more materials [4]. Further, properties of polymer composite materials improve significantly
with fillers of nano-scale dimensions because the mixing phase occurs in this scale range [3-4].
The composite material fabricated using a nano-filler is called a nanocomposite material. Most of
nanomaterials have a high surface area compared to their volume. The uniform dispersion of this
type of filler provides a very high interfacial area between the filler and matrix, and this
differentiates nanocomposite material from traditional composite material [4]. Moreover, it
requires much less of these nanofillers to fill the entire volume of the matrix [2]. Further, the
3
distance between the fillers is in the nano-scale range. These unique features of the fillers
provide enhanced performance of the nanocomposite material compared to the neat matrix.
There are various different kinds of composite materials such as a polymer matrix,
cement matrix, metal matrix, carbon matrix, and ceramic matrix [15]. The polymer matrix
composite is very common and is useful for light weight production of structural materials,
which has many potential applications in the automotive, aerospace, construction and electronic
industries [9].
The fabrication of polymer nanocomposite materials has been growing rapidly along with
the development of new kinds of nanofillers. Generally, the nanocomposite materials are
fabricated using nanofillers such as layered silicate clays [15], carbon nanofibres [16-17], carbon
nanotubes [18-20], expanded graphite [3,21], and graphene [2,9]. These nanofillers have
different chemical structures, morphologies, and aspect ratios. Among these nanofillers,
graphene and modified graphene (i.e., graphene oxide, reduced graphene oxide, and
functionalized graphene) offer advantages as fillers because of their planar structure that
facilitates the interaction with the polymer matrix at the molecular level [22]. The significant
improvement of properties of graphene-based nanocomposite materials depends upon the level
of homogeneous dispersion of graphene in the polymer matrix. The uniform dispersion of these
fillers into the matrix provides higher efficiency of external load transfer to the filler through
strong interfacial interactions.
1.3 Band Structure of Graphene
Graphene has a 2-dimensional honeycomb lattice with sp2 bonded carbon atoms as shown
in Figure 1.1 [23]. This material is the building block of all other forms of carbon material such
4
as 0-dimensional fullerenes, 1-dimensional nanotubes and 3-dimensional graphite. The unit cell
of graphene consists of two non-equivalent carbon atoms A and B separated by 1.14 Å as shown
in the Figure 1.1. This distance is less than its inter-planar distance, which is 3.35 Å
The electronic properties of graphene can be derived from its band structure. The tight
binding model is generally used in order to determine the band structure of graphene. The unit
cell vectors a1 and a2 is given by
aaaa
2
3,
2
3,
2
3,
2
32a1a
The lattice constant is given by the magnitude of each of the 21 aa ca
The reciprocal lattice vectors b1 and b2 are given by the relations:
32
21
32
13
32
32 2;2;2aaa
aa
aaa
aa
aaa
aa
111
321 bbb
where 3a is a unit vector along a z-axis, which does not play any role in current discussion as the
electronic states in x-y plane is only discussed here.
The above relation gives
a
aa
aa
aa
a 3
2,
3
2,
3
2,
3
2 2bb1 (1.1)
5
Figure 1.1: Representation of hexagonal lattice of graphene sheet. The shaded region represents
the unit cell.
Figure 1.2: First Brillion zone in reciprocal space where, Γ, K, K´, and M are high symmetry
points.
6
This shows that the reciprocal lattice vectors b1 and b2 are rotated by 30° with respect to the real
space vectors 1a and 2a , respectively. Figure 1.2 also shows high symmetry points (i.e., Γ, K, K´
and M) within the first Brillion zone where, K and K´ are the unequivalent Dirac points in the
reciprocal space.
The solution to the tight bending method, considering only nearest neighboring atoms,
gives the following energy dispersion relation for single layer graphene.
2cos4
2cos
2
3cos41),( 2
aKaKaKKKE
yyxyx (1.2)
Here arises from the nearest neighbor contribution and its value is about 0.3 eV.
Figure 1.3: Band structure of graphene derived from tight binding approximation.
7
The positive and negative parts of equation (1.2) lie above and below the Fermi level as
shown in Figure 1.3, respectively. The part above the Fermi level is the conduction band and the
part below is the valance band. The conduction and valance bands touch each other at singularity
points (K and K´). The band structures around these points are linear and this gives that the
charge carriers are massless [24].
1.4 Graphene Oxide
Graphene oxide (GO) has a two-dimensional planar structure similar to graphene but with
disrupted sp2 hybridized carbon atoms due to covalent attachment of various oxygen containing
functional groups, as shown in Figure 1.4. The complete restoration of sp2 hybridization of a
single layer of GO gives a graphene sheet.
Graphene is usually produced by methods such as mechanical exfoliation, chemical vapor
deposition, epitaxial growth, and chemical exfoliation. Among these methods, chemical
exfoliation is the route best suited for mass production of graphene [25]. This method is cheaper,
simpler, more efficient, and better yielding and is suitable for industrial or commercial
applications [25]. The chemical exfoliation of graphene from graphite is an indirect approach
because graphene oxide (GO) is first produced and then reduced to graphene. The oxidation of
crystalline graphite into GO breaks the sp2 hybridized structure of the graphene sheets that are
stacked in graphite [26]. The oxidation also increases the distance between the adjacent graphene
layers from 0.335 nm to 0.68 nm. The increase in separation depends upon amount of water
intercalated between the stacked sheets, and this reduces the Van der Walls interaction between
the sheets [27]. During the oxidation process, the oxygen molecules not only increase the
separation between sheets, but also make the layers hydrophilic in nature. Low power sonication
8
in water is sufficient to disperse the GO into individual sheets from the oxidized graphite. The
GO produced in this way has various oxygen containing functional groups attached to the edge
and the basal plane of the graphene sheets as shown in the Figure 1.4 [2]. The major oxygen-
containing functional groups attached to the edge of graphene sheets are carbonyl and
carboxylic, while the major oxygen-containing functional groups attached to the basal plane are
hydroxyl and epoxide [2].
Figure 1.4: Chemical structure of graphene oxide with different oxygen-containing functional
groups on the basal plane and around the edges of a graphene layers.
GO was used as a filler in a polymer matrix to produce nanocomposites. GO containing
high amount of oxygen functional groups is analogous to two-dimensional clay sheets such as
montnorilllonite. Nanocomposites based on the clay sheets have been investigated extensively
for various applications [28]. The filler clay is in the range of a few microns thick whereas GO
exhibits a much higher surface-to-volume ratio. In addition to the high surface area, GO has a
9
high dispersibility in various aqueous and inorganic solvents. Further, different oxygen
functional groups attached to the basal plane and the edge of GO promote interaction between
the filler and matrix [2]. This allows GO to be used as a unique filler material to produce various
kinds of nanocomposite materials for different applications. The oxygen-containing functional
groups can be reduced significantly before incorporating the material into the matrix.
1.5 Functionalization of Graphene and Graphene Oxide
Pristine graphene flakes do not disperse in many solvents because of their tendency to
aggregate due to Van der Walls interactions between the layers. This creates difficulties in
producing graphene-based nanocomposites with enhanced properties. In order to overcome this
problem, graphene sheets are usually functionalized with suitable organic molecules. There are
various approaches to functionalization of graphene and graphene oxide. The covalent
functionalization of graphene is one approach that involves attaching certain organic molecules
on the surface of graphene [9,111]. This process disrupts the sp2 hybridization of graphene and
forms sp3
hybridized structures [111]. This also helps to tune the band gap of the material that is
very useful in the fabrication of electronic devices [112].
The covalent functionalization of graphene can be carried out in two different ways
[111]. The first approach is to form direct covalent bonds between the sp2 hybridized carbon
structure and organic molecules that are to be attached [111]. The other approach is to attach the
organic molecules covalently with oxygen containing functional groups of GO [111]. Different
organic molecules such as amines, isocyanates, and diisocyanates compounds have been used for
covalent functionalization of graphene and GO [84,112-113]. These molecules reduce the
hydrophilic nature of GO. Stankovich et al. [113] have reported that isocyanate functionalization
10
of GO readily forms a stable dispersion of GO in a variety of organic solvents such as
dimethylformamide (DMF) and N-methylpyrrolidone (NMP). Covalent attachment of
octadecylamine (ODA) to the surface of GO provides a way to introduce long hydrocarbon
chains that makes ODA functionalized GO disperse very well in polar solvents [111]. The ODA
functionalization of GO occurs due to the nucleophilic substitution reaction between the amine
group of ODA and the epoxy group of GO [111]. The ODA functionalization of GO enhances
the surface roughness of GO, and the effect is significant with longer amine chains [53]. The
surface roughness of ODA functionalized GO enhances the dispersion in polymers [53].
The reduction of a stable dispersion of GO using hydrazine monohydrate produces
irreversible aggregation of the reduced graphene sheets. This problem can be solved by suitable
functionalization of GO before reduction. Non-covalent functionalization is another technique
that prevents aggregation of graphene sheets. In this type of functionalization, functional groups
are attached to graphene without disrupting its sp2 hybridized structure [114-115]. Stankovich et
al. [50] have reported that the reduction of exfoliated graphite oxide in the presence of Poly
(styrenesulfonate) produces a stable aqueous dispersion of reduced GO. The reduction of GO
using hydrazine in the presence of single-stranded DNA (ssDNA) forms non-covalent
functionalization of graphene with ssDNA that is dispersible in water up to 2.5 mg/ml [116].
The functionalization of GO with aryl-diazonium salt followed by reduction using
hydrazine monohydrate forms GO wrapped with sodium dodecyl benzene sulfonate (SDBC)
functional groups [117]. This type of functionalization makes the graphene disperse easily in
inorganic solvents such as DMF and NMP up to concentration of 1 mg/ml with significantly less
aggregation [117]. The thermal stability of SDBC functionalized GO is significantly higher than
that of GO [117].
11
The functionalization of graphene and graphene oxide with both covalent and non-
covalent attachment of suitable functional groups provides highly dispersible graphene sheets
with high concentration in a variety of solvents. This makes the material quite useful in many
different applications that include fabrication of electronic devices, energy storage devices, and
nanocomposite materials.
1.6 Epoxy Resin
Epoxy resin is a highly cross-linked thermosetting plastic. Epoxy resin is widely used in a
variety of applications such as adhesive, paint, coating, sealant, medical implants, and electrical
devices [29-30]. Epoxy is also used as the matrix for the fabrication of polymer nanocomposites,
which are used in aerospace and wind-turbine industries [5]. The three dimensional structure of
cured epoxy resin provides excellent physical properties [31]. The main component of the epoxy
resin is diglycidal ether of bisphenol A (DGEBA) as shown in Figure 1.5. Bisphenol A is
produced from the reaction of two phenols with one acetone [31]. After adding the hardener,
oxygen atoms from the glycidyl groups of the epoxy resin react with the amine hydrogen atoms
of the hardener in order to produce cured epoxy resin [31]. The most common hardener of epoxy
resin are the polyamines, which are organic materials with two or more amine groups. The amine
group consists of nitrogen atoms with one or two hydrogen atoms attached to it. The epoxy resin
is referred to as Part A whereas the hardener is referred to as Part B.
Figure 1.5: Chemical structure of diglycidylether of Bisphenol-A (DGEBA), n = 0, for the
derivatives, n > 0.
12
1.7 Challenges in Polymer Nanocomposites
There are many challenges in order to prepare polymer-nanocomposite (PN) materials
that utilize the advantages of the filler materials. This section discusses a few of the challenges
such as dispersion of nanomaterials, cost effectiveness, and health and environment issue.
1.7.1 Dispersion of Nanomaterials
The uniform dispersion of nanomaterials in the matrix is one of the challenges to produce
nanocomposite materials with improved performance [2]. The difference in surface charge
between the filler and polymer and Van der Walls interactions between the nanomaterials often
causes agglomeration of nanomaterials in the matrix [32]. Figure 1.6 shows three different types
of composites according to the level of dispersion. The phase separated polymer composite
material is formed when a polymer matrix is unable to penetrate in between the layers of filler
material [5, 33]. In this situation, the actual potential of the filler material is not completely
utilized. The properties of the composite material improve slightly or can become worse than the
neat polymer [34]. In the case of intercalated composite materials, as shown in Figure 1.6, the
polymeric chains are intercalated in between the layers of nanomaterials, but not the latter are
dispersed fully [3]. This increases the performance of the composite material compared to phase
separated materials. The third type of dispersion is exfoliated. In this case, the layered fillers
such as graphene or reduced graphene are completely and uniformly dispersed in the polymer
matrix [3]. This allows significant improvement in the properties of the composite material.
Further, one can modify the properties of composite materials by optimizing the interfacial
bonding between the filler and polymer [5].
13
Figure 1.6: Schematic of various levels of dispersion of nanomaterials in a polymer matrix.
1.7.2 Cost Effectiveness
Nanomaterials having a high aspect ratio, such as carbon nanotubes and graphene, are
highly desirable for the fabrication of nanocomposites with enhanced mechanical, electrical, and
thermal properties. Specifically, carbon nanotubes are quite expensive to produce. So, it is
important to reduce the cost of nanofillers so that they can be utilized in the manufacturing of
nanocomposite. This can often be achieved by choosing a filler without sacrificing the properties
of the nanocomposite.
14
1.7.3 Health and Environment Issues
The use of nanocomposite materials is increasing yearly due to their enhanced
performance compared to the neat material. As a result of increased use, there is more concern
about the impact of these materials on health and the environment [3]. Nanomaterials can easily
reach inside the body through inhalation, skin contact or by ingestion. As most of the
nanomaterials have a high surface area, the atoms of nanomaterials can react easily with the
atoms of tissue of the human body [3]. This may cause severe health effects. For example,
carbon materials, which have a significant impact on health, are widely used for the fabrication
of nanocomposites. The carbon materials cause skin diseases and respiratory problems [35].
Some of the nanoparticles can be inhaled easily and reach deep in lung tissue. Similarly,
nanoparticles formed during the combustion process such as forest fires and industrial wastes
have severe effects on health [36]. Therefore, a better understanding of toxicity of nanomaterials
and nanocomposite materials is essential to develop materials having little impact on health and
on environment.
1.8 Motivation
Epoxy resin is an important class of thermoset materials used in a wide variety of
applications such as structural applications [31]. The wide variety of applications comes from its
characteristics, including high chemical and corrosion resistance, good mechanical, thermal and
electrical properties, and easy processability [29-30]. However, these materials have a high
friction coefficient and low wear resistance [37]. Thus, it is very important to reduce the friction
and wear rate of the epoxy as these properties are associated with durability. The reduction in
friction and wear rate results from the reduction of adhesion to the counterface and the
15
enhancement of mechanical properties such as Young’s modulus, tensile strength, and hardness
of the nanocomposites [3]. This improvement can be accomplished by the addition of certain
kind of fillers into the epoxy matrix. Previously, fillers containing fluorine atoms such as
fluoropolymers were used to enhance the tribological properties of epoxy [38]. Unfortunately,
this filler reduces wear resistance [38]. Fillers such as carbon fibers and carbon nanotubes have
been used to enhance the tribological properties [39-40]. Dong et al., have observed that
incorporation of 1.5 wt. % multi-walled carbon nanotubes into the epoxy matrix reduces the
friction coefficient from ~0.32 to ~ 0.2 and the wear rate from 2.7×10-5
to 6.0×10-6
mm3/Nm
[39]. Although fillers like carbon nanotubes have the capability to improve tribological
properties, they also have a high tendency to agglomerate in the epoxy matrix. This makes it
difficult to produce a nanocomposite with enhanced properties. Furthermore, the high cost of
fabrication of carbon nanotubes limits their use in the fabrication of nanocomposite materials. On
the other hand, chemically derived graphene is very suitable for mass production and potentially
very useful for the fabrication of nanocomposites. Previous studies have shown that the addition
of graphene in various polymers results in significant improvement in mechanical, thermal,
electrical, and tribological properties [9, 32, 41-45]. Recently, Kanudanur et al., reported that the
addition of graphene in polytetrafluoroethylene reduced the wear rate significantly [44]. Further,
Pan et al., have also reported improvement in the tribological properties of nylon matrix with the
addition of very low concentrations of octadecylamine functionalized graphene oxide [45]. These
results on chemically derived graphene have led me to study the mechanical, thermal, and
tribological properties of graphene-epoxy resin nanocomposites.
16
1.9 Outline of Dissertation
This dissertation concerns the fabrication, characterization, and properties of epoxy
nanocomposite reinforced with various weight percentages of functionalized reduced graphene
oxide (FRGO).
Chapter 1 deals with a description of polymer nanocomposite materials, epoxy resin, the
band structure of graphene, graphene oxide, challenges in polymer nanocomposites, and the
motivation for the research. Chapter 2 discusses the experimental techniques for the synthesis of
graphene oxide, functionalization of graphene oxide, and synthesis of FRGO-epoxy
nanocomposite materials. This chapter also discusses the methods used to characterize FRGO
and the nanocomposite materials. In Chapter 3, the characterization of FRGO using scanning
electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy is presented.
Chapter 4 discusses the enhancement of Young’s modulus, tensile strength, glass transition
temperatures, friction, and wear properties of nanocomposite materials fabricated with various
weight percentages of FRGO filler. Chapter 5 gives conclusions about the properties of FRGO
and the FRGO-epoxy nanocomposite materials.
17
CHAPTER 2
EXPERIMENTAL TECHNIQUES
2.1 Synthesis of Graphene Oxide
The general method to produce large quantities of graphene is to begin with the oxidation
of graphite to graphite oxide (GO). The procedures for the synthesis of GO were developed a
few decades ago by Brodie [46], Staudenmeire [47], and Hummers et al.,[48], and these
procedures are still in use, with and without modifications. The first two methods are very time
consuming and generate ClO2, which must be handled with caution due to high toxicity and
tendency to decompose in air and explode [46, 49]. Thus, modified Hummer’s method was used
for the preparation of GO, since it is less hazardous compared to the other two methods. In this
method, GO preparation starts from bulk graphite or graphite powder. The graphite powder is
oxidized in a water-free medium by treating with concentrated sulfuric acid (H2SO4), sodium
nitrate (NaNO3), and potassium permanganate (KMnO4). The active oxidizing agent is
dimanganese heptoxide (Mn2O7), which is formed from the reaction of KMnO4 with H2SO4 as
shown in reaction 2.1.
7243
423424
OMnMnOMnO
3HSOOHMnOKSO3HKMnO
(2.1)
The oxidation of graphite can be accomplished using various commercially available
graphite powders. However, graphite flake is widely used for making oxidized graphite for
various applications. The GO synthesized using this process not only contains oxidized graphite,
but also non-oxidized heavy graphite particles. The non-oxidized graphite particles can be
18
removed using centrifugation and sedimentation. The prolonged dialysis removes salt and ion
impurities from the oxidation process.
A layout of synthesis of GO is shown in Figure 2.1.
Figure 2.1: A layout of GO synthesis procedure.
The entire reaction process takes less than two hours and is carried out below 45°C with
comparatively little danger. In our preparation of GO, 5 g of graphite powder (325 mesh,
Southwestern Graphite) and 2.5 g of NaNO3 were added to 115 mL of concentrated H2SO4 at
room temperature and stirred for 15 minutes. The mixture was transferred to an ice bath and 15 g
of KMnO4 was added while stirring vigorously. The mixture was then transferred to a water bath
to maintain the temperature within the range of 35-40 °C for half an hour as shown in Figure 2.2
(a). After half an hour, 235 mL of deionized (DI) water was added to slow the reaction and the
19
mixture was stirred for another 15 minutes. At this stage, the color of the sample becomes dark
brown as shown in Figure 2.2 (b). An additional 830 mL of water was added followed by the
slow addition of H2O2 (30%). The mixture was then repeatedly filtered and washed with HCl
(1:10) aqueous solution. The filtered material was dispersed in water using horn sonication, and
centrifuged at 3500 rpm in order to remove any non-oxidized graphite. Residual acids and salt
impurities were removed using dialysis for about 10 days using cellulose permeable membrane.
Finally, the suspension was freeze dried using the setup shown schematically in Figure 2.3 to get
a powdered GO. In the freeze drying process, the water from the GO dispersion sublimates at
low pressure. The water-vapor produced during this process is condensed to water and is trapped
by a refrigerator.
Figure 2.2: Photographs during the oxidation process of graphite powder. (a) During the second
stage of the oxidation reaction. (b) After the first dilution with DI water.
20
Figure 2.3: Schematic of a freeze drying system.
2.2 Functionalization of Graphene Oxide with Octadecylamine
Pristine graphene samples are not suitable for dispersion in polymers because of their
tendency to agglomerate due to π-π interactions, which is difficult to undo using sonication [50].
The agglomeration of sheets can be reduced significantly by functionalization before reduction
[9]. The functional group attached to the graphene sheets can be small molecules of polymer
chains. The chemical functionalization of graphene is very useful because it increases the
solubility in organic polymers with enhanced interactions [51-52]. In this study GO was
functionalized using octadecylamine (ODA). The octadecylamine functionalization not only
facilitates uniform dispersion of graphene in the epoxy, but also prevents the re-aggregation of
graphene sheets [9, 51-52]. The ODA functionalized GO exhibits increased surface roughness
with a higher hydrophobicity [53].
The ODA functionalization was carried out by dispersing 300 mg of GO powder in 300
mL of ethanol by sonication for 2 h and then adding 450 mg of ODA in 45 mL of ethanol. The
mixture was refluxed for 24 h at 90 °C using the setup shown schematically in Figure 2.5, and
then repeatedly filtered and rinsed with ethanol to remove excess ODA. ODA functionalization
21
occurs by nucleophilic substitution reactions between the amine groups of ODA and the epoxide
groups of GO [54]. Therefore, ODA functionalization partially reduces GO; the presence of
other oxygen-containing functional groups, such as hydroxyl and carbonyl groups, remains as
shown in Figure 2.4 (a). These groups can be reduced by reaction with hydrazine monohydrate
without significant effects on ODA functionalization [55]. For reduction, the functionalized GO
powder was dispersed in ethanol, and hydrazine monohydrate was then added. The mixture was
refluxed at 90°C for 24 h, and the final product was repeatedly filtered and washed with ethanol
and then dried in a vacuum oven. The chemical structure of functionalized reduced GO is shown
in Figure 2.4 (b).
Figure 2.4: (a) Schematic of GO functionalized with ODA. (b) Schematic of functionalized
reduced GO. The long chains represent ODA and is attached to the basal plane of GO.
22
Figure 2.5: Schematic of the setup used to functionalize GO.
2.3 Characterization of Graphene Oxide and Functionalized Reduced Graphene Oxide
2.3.1 Raman Spectroscopy
Raman spectroscopy is one of the most important tools for the study of carbon based
nanostructures due to its non-destructive nature and fast acquisition of data [56]. More
importantly, Raman analysis provides structural and electronic information about the carbon
structure [56]. Raman spectroscopy gives detailed information on the crystal structures of
graphene oxide, reduced graphene oxide, and functionalized graphene oxide. Raman
spectroscopy also provides an indication of the degree of oxidation of GO and FRGO.
23
Raman spectra of graphene consists G and 2D peaks at 1580 cm-1
and 2700 cm-1
,
respectively. The G peak is due to in-plane bond stretching of pairs of sp2 carbon atoms, as
shown in Figure 2.6 (a). The 2D peak is due to second order zone boundary phonons [57]. In the
Raman spectra of defected graphene there is a D peak located at about 1350 cm-1
due to first
order zone boundary phonons, which is absent in the case of defect free graphene [56]. The D
peak is similar to a breathing mode which requires out-of-plane translational motion induced by
a sp3 hybridized structure, as shown in Figure 2.6 (b).
Figure 2.6: Vibrational modes of (a) G peak and (b) D peak. The black circles represent the
carbon atoms and the arrows represent the direction of vibration.
The G and D peaks of GO are broad compared to graphite because of disorder due to
extensive oxidation. Further, the G peak shifts towards higher frequency, which is attributed to
isolated double bonds that resonate at a frequency higher than that of graphite [56]. The upward
shift of the G peak is due to a reduction in size of sp2
hybridized carbon [58]. Moreover, the full
width at half maximum of the 2D peak of GO is very wide (i.e., about 200 cm-1
) compared to
24
that of mechanically exfoliated graphene (i.e., 30 cm-1
). The overall Raman peak intensities are
reduced after reduction, which suggests the loss of carbon during the reduction process [59].
Reduced GO has a structure that is different from that of GO due to the removal of
oxygen-containing functional groups and carbon atoms. The area under the D and G peaks of the
Raman spectrum is a measure of the size of the sp2 clusters in a networks of sp
2 and sp
3 carbon.
According to the Tuinstra-Koenig relation, the intensity ratio of G and D peaks (i.e., G/D) is
proportional to the average size of in-plane sp2
hybridized carbon [58].
2.3.2 X-ray Photoelectron Spectroscopy (XPS) of Graphene and Graphene Oxide
XPS was used to characterize graphene oxide, reduced graphene oxide, and
functionalized graphene oxide in order to understand the structural and electronic properties of
these materials. Since GO consists of many oxygen-containing functional groups, it is important
to know the concentrations and types of functional groups.
In XPS, the electrons are ejected from the sample when the sample is irradiated with
mono-energetic soft x-rays as shown in Figure 2.7. The identification of the sample can be
accomplished by analyzing kinetic energies of the ejected photoelectrons according to equation
2.2. Similarly, one can determine the relative concentration of the elements from the intensities
of the ejected photoelectrons.
The kinetic energy of ejected electrons can be calculated from the energy conservation
equation:
)( BEhKE EEE (2.2)
25
where, KEE is the kinetic energy of the photoelectron, hE is the energy of the incoming X-ray
(e.g., 1486.7 eV for Al, Kα X-rays). Similarly, BEE is the binding energy and is the work
function of the spectrometer.
Figure 2.7: Ejection of a photoelectron upon irradiation by a mono-energetic X-ray.
2.4 Synthesis of Functionalized Reduced Graphene-Epoxy Nanocomposites
Graphene-polymer nanocomposites are commonly prepared using techniques such as
solution-blending, melt mixing, and in-situ polymerization. The functionalized reduced
graphene-epoxy nanocomposites were prepared using a solution-blending method. This is the
most popular technique in which a solvent for both fillers and epoxy is used. Initially, the filler
material is dispersed in a solvent and then the mixture is added to the polymer in the same
solvent. Later, after mixing the mixture of fillers and polymer, the solvent is removed by
evaporation.
26
The process of synthesis of FRGO-epoxy nanocomposites is shown schematically in
Figure 2.9. First, FRGO was dispersed in acetone (100 mg of FRGO in 100 mL of acetone) using
horn-sonication (shown in Figure 2.8) for 2 hours in an ice bath in order to prevent any increase
in temperature. Varying amounts of epoxy resin (System Three Resin, Inc.) were then added and
the mixture was sonicated for 1 hour. The acetone was evaporated by heating the mixture at 70
°C using a hot-plate. Residual acetone was removed by placing the mixture in a vacuum oven for
12 hours at 70 °C. After cooling to room temperature, a low viscosity slow curing agent (System
Three Hardener Part B, Number 3) was added. The mixture was poured into silicone molds of
different shapes to make samples for various types of characterizations. The samples were cured
for 24 h at room temperature. All the samples with varying concentration of FRGO were
fabricated in a similar way. The characterization of all samples was carried out after a week of
curing.
Figure 2.8: Photograph of the horn sonicator used for the dispersion of FRGO.
27
Figure 2.9: A schematic representation of FRGO-epoxy nanocomposite preparation.
28
2.5 Characterizations of Polymer Composite Materials
2.5.1 Mechanical Properties: Tensile Testing
The study of the mechanical properties of polymer composite materials is one of the most
important studies in the field of material science and engineering. This gives insight into the
ability of the material to withstand various load levels [60]. It is important to know the
viscoelastic properties of the material in order to understand its mechanical properties [61]. The
viscoelastic properties depend both on internal and external conditions. The internal conditions
refer to the elastic, viscous or a combination of both properties of the material, whereas the
external conditions refer to the temperature and pressure [60]. The viscous properties can be
observed when a force is applied to a polymeric material for an extended period of a time at a
lower shear rate. During this time, the polymeric chains respond to the force, and the material
flows along the direction of the force [60]. When the force is applied for a short duration of time,
then the molecular chains do not have sufficient time to flow along the direction of the force
[60]. In this case, the cross-linking and entanglements of the polymers are responsible for the
elastic properties [60]. There are various kinds of tests used to study the mechanical properties of
polymeric materials. The tensile test and the dynamic mechanical analysis are examples of static
and dynamic mechanical tests, respectively.
Tensile testing is a widely used method used to determine the Young’s modulus, tensile
strength, Poisson’s ratio, and strength at break. This consists of a constant speed movement
crosshead and a fixed crosshead, both of which have threaded sample grips, as shown in Figure
2.10. The movable crosshead moves at a constant speed (i.e., tensile rate) with respect to the
fixed crosshead, and this produces elongation or compression in the sample. The tensile rate can
be varied from 1 mm/min to 500 mm/min depending on the type of material. The samples are
29
usually in the form of a dog-bone shape as shown in Figure 2.11. In the tensile testing
measurement, stress at increasing strain is measured at a constant tensile rate until the sample
breaks due to application of the load.
Figure 2.10: Schematic of the tensile testing equipment.
Figure 2.11: Schematic of a tensile testing sample.
30
Stress is defined as
0A
F (2.3)
where is the tensile stress, F is the applied force, and 0A is the initial cross-sectional area of
the sample. The maximum value of the stress is called tensile strength of the material.
The strain is given by the relation
00
0 )(
L
L
L
LL
(2.4)
where 0L is the initial length of the sample and L is the length after a certain amount of strain
(i.e., the current length).
The Young’s modulus or elastic modulus can be calculated using the equation
E (2.5)
E is calculated within the elastic limit i.e., the initial linear portion of the stress versus elongation
curve.
2.5.2 Mechanical Properties: Dynamic Mechanical Analysis
Dynamic mechanical analysis (DMA) is used to characterize the viscoelastic behavior of
polymeric materials [61]. The viscoelastic property is a combination of properties of elastic
solids and Newtonian fluids. In DMA, a periodic force is applied to the material, and its response
31
is measured in terms of strain at various temperatures. The frequency of the periodic force can be
varied depending on the type of material and range of measurement [61]. The strain of a
viscoelastic material is out phase with respect to the applied stress by an angle , as shown in
Figure 2.12. The phase change occurs due to the excess time necessary for molecular motions
and relaxations to occur. The amplitude of deformation at the peak of sine wave and the lag
between the stress and strain give quantities such as the modulus, viscosity, and damping.
Figure 2.12: Response of a sinusoidal force applied to a polymeric material at a certain
frequency.
The dynamic stress and the strain are given by the relations
)(sin0 t (2.6)
)(sin0 t (2.7)
where is the angular frequency. Now, from equations (2.6) the stress can be expressed as
32
)(sin)(cos)(cos)(sin 00 tt (2.8)
The above equation can be expressed as
)(cos)(sin "
0
'
0 tEtE (2.9)
where
cos
0
0' E and
sin
0
0" E .
The complex modulus is given by the relation
"'
0
0
0
0* )sin(cos iEEieE i
(2.10)
The above equation shows that the complex modulus obtained from a DMA consists of
real and imaginary parts. The real part of equation (2.10) is the storage modulus i.e., the ability
of the material to store and release potential energy upon deformation. The imaginary part of
equation (2.10) is the loss modulus, and is associated with energy dissipation in the form of heat
energy.
The phase angle is given by
'
"
tanE
E (2.11)
The glass transition temperature is another interesting parameter which can be obtained from the
DMA measurements. The glass transition temperature is obtained from the peak position of
tan versus temperature curve.
33
The DMA was carried using a PerkinElmer DMA8000 apparatus. A schematic of the
equipment is shown in Figure 2.13. The measurements were performed in the single cantilever
bending mode as shown in Figure 2.14.
Figure 2.13: Schematic of the driving system of the DMA equipment.
Figure 2.14: Schematic of the single cantilever (i.e., 2-point bending mode) system of the DMA
equipment.
34
2.6 Thermal Stability Determination: Thermogravimetric Analysis (TGA)
Thermogravimetric analysis (TGA) of polymeric materials is carried out by measuring
mass as a function of increasing temperature at a constant rate in a controlled environment [62].
The controlled environment can be an inert or reactive gas. The TGA measurement gives
information about the physical and chemical properties by recording changes in mass with
increasing temperature [63]. Most polymers exhibit mass loss with increasing temperature due to
the removal of unstable components such as moisture, residual solvents or low molecular mass
additives or oligomers that generally evaporate between room temperature and 300°C [62].
Another source of mass change may be due to absorption, adsorption, desorption, dehydration,
decomposition, oxidation, and reduction [63].
A TGA system consists of a sample pan made from either ceramic or platinum that is
connected to a precision micro-balance. The chamber is purged with a gas in which the sample is
to be analyzed. In most cases, inert gases such as nitrogen, argon or helium are used to study the
thermal degradation behavior. Figure 2.15 shows the TGA system used in our studies.
35
Figure 2.15: Photograph of the thermogravimetric analyzer.
2.7 Tribological Properties of Polymer Nanocomposites
Tribological properties of polymers refer to the friction coefficient and wear resistance of
the materials. The tribological properties are not only a material property as they depend on the
environment as well [64]. It is important to study fiction and wear properties of epoxy as they are
related to durability and performance. Further, epoxy resin has high friction and wear rate [37].
Thus, such studies provide a measure of improvements in friction and wear resistance.
36
2.7.1 Friction
Friction is a force that always resists the motion of an object when it slides or rolls on the
surface. This force always acts tangentially along the direction opposite to the motion, as shown
in Figure 2.16 (a). There are two types of frictional forces: static and dynamic friction. The static
friction acts before the beginning of motion of an object, while the dynamic friction acts when
the object is in motion. These forces are measured from the ratio of the applied force F to the
weight W of the object.
The frictional property is not completely a material property [64]. It depends on the
condition such as temperature, pressure, atmosphere (i.e., air or nitrogen or vacuum), material
roughness, roughness and cleanness of the counterface, lubrication, and speed. Further, the
friction of polymeric materials is the result of the interfacial and the cohesive work done on the
surface [3] as shown in Figure 2.16 (b). It is assumed that the counterface is hard compared to
the polymeric material, and undergoes little or no elastic deformation [3]. The interfacial work
depends on the adhesive interaction between the counterface and the polymeric material and that
varies with factors such as hardness, surface roughness, glass transition temperature, and
electrostatic-chemical interaction between polymer and counterface [3]. The other factor
affecting the interfacial work is the cohesive interaction within the polymeric material, and it
results in plowing of the polymer by the counterface [3]. Further, the energy required for the
plowing depends upon factors like the Young’s modulus, tensile strength, and geometry of the
counterface [3].
37
(a) (b)
Figure 2.16: (a) Schematic illustration of a hard body sliding over a polymeric surface. (b)
Enlarged view of the region of polymeric material in contact with the hard surface.
2.7.2 Wear
Wear is an outcome of friction when an object slides over the surface of another [85].
This is also not completely a material property. It depends on operating conditions such as speed,
contact pressure, and surface roughness of the counterface. Most of materials having high
friction also have high wear rate. There are various kinds of wear mechanisms for polymeric
materials such as interfacial, cohesive, abrasive, and adhesive [3]. The factors affecting the wear
rate are elastic modulus, tensile strength, and hardness of the polymer. Generally, polymers
having higher elastic modulus have higher wear resistance [3].
2.7.3 Friction and Wear Measurements of Polymeric Materials: Pin-on-disc Tribometer
There are various methods for determining the friction and wear of polymeric materials.
The pin-on-disk method is one of the widely used methods for simultaneous determination of
friction and wear. A schematic of a pin-on-disk tribometer is shown in Figure 2.17, and a
38
photograph of the equipment used for measurements is shown in Figure 2.18. In this type of
measurement, the pin (i.e., the counterface) slides over the surface of the polymeric material in a
circular path, and it penetrates the sample as measurement are made. The depth through which
the pin penetrates into the sample depends upon wear resistance. The depth through which the
pin penetrates is measured using a linear differential transduction device (LVDT). The sliding
speed, radius of circular track, and type of counterface (i.e., pin) can be varied depending upon
choice of measurement.
The wear of a composite material using the pin-on-disk method can be measured by
determining the dimension of the groove produced by the pin. The dimension of the groove can
be measured using a profilometer. If A is the cross-sectional area of a worn track and R is the
radius of the track, then the volume V of the groove is given by
ARV 2 (2.12)
The wear rate is given by
xN
VW
(2.13)
where N is the normal load and x is the sliding distance. The rate is usually measured in units of
mm3/N·m. The wear mechanism is generally studied using optical and scanning electron
microscopy (SEM) of the surface, debris formed during abrasion, and film transferred from the
sample to the counterface.
39
Figure 2.17: Schematic of the pin-on disk tribometer and a polymeric nanocomposite sample
with a wear track.
Figure 2.18: Photograph of the pin-on disk tribometer used for friction and wear measurements.
40
CHAPTER 3
CHARACTERIZATION OF GRAPHENE OXIDE AND FUNCTIONALIZED REDUCED
GRAPHENE OXIDE
This chapter deals with the characterization of functionalized reduced graphene using
scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron
spectroscopy.
3.1 Characterization of Functionalized Reduced Graphene Oxide Using SEM
The surface morphology of FRGO powder was characterized using an FEI Nova 200
NanoLab SEM. Figure 3.1 shows SEM images at various magnifications. The higher
magnifications images (i.e., Figure 3.1 (b) and (c)) of FRGO powder show that most of the layers
are separated. Further, Figures 3.1 (b) and (c) show that surface and structure of graphene sheets
are not damaged by chemical modification. The planar structure of graphene is completely
preserved even after the chemical reaction. Furthermore, it can be observed from the SEM
images that that the lateral dimensions of the graphene flakes are in the range of a few microns.
____________________________________________________________________________
Note: Most of this chapter is duplicated from the accepted paper with the permission of Maney Publishing: ‘Effects
of Functionalized Reduced Graphene Oxide on Frictional and Wear Properties of Epoxy Resin’, R. Shah, T.
Datashvili, T. Cai, J. Wahrmund, B. Menard, K. P. Menard, W. Brostow, J. Perez, Material Research Innovations,
2014. References and figures numbers are changed to accommodate the dissertation.
41
Figure 3.1: (a-c) SEM images of FRGO powder at different magnifications.
42
3.2 Characterization of Functionalized Reduced Graphene Oxide Using Raman Spectroscopy
Micro-Raman spectroscopy was carried out using a Thermo Electron Almega XR
spectrometer using a green laser (i.e., λ= 532 nm) as the excitation source. Figure 3.2 shows
Raman spectra of the GO and FRGO powders. The spectra are normalized so that the G peaks of
the GO and FRGO have the same height. The G peak is due to first order scattering of E2g
phonons (in plane optical mode) of sp2 hybridized carbon atoms close to the Γ point [65]. As
shown in Figure 3.2 (a), the G peak in GO is at about 1593 cm-1
. The G peak is shifted from its
value in graphite—of 1581 cm-1
—due to oxidation. Carbon materials also exhibit a D peak at
about 1340 cm-1
due to defect-induced zone boundary phonons [66]. The D peak in GO is due to
a reduction in size of sp2 hybridized domains due to oxidation [65]. The Raman spectrum of
FRGO has G and D peaks at 1587 cm-1
and 1346 cm-1
, respectively, as shown in Figure 3.2 (b).
In FRGO, the G peak shifts towards the position of the G peak in graphite due to restoration of
sp2 hybridized domains [67]. The ratio of the intensities of the D and G peaks in FRGO is greater
than that in GO due to a decrease in size of in-plane sp2 hybridized domains in FRGO due to
reduction [66]. The Raman results are in good agreement with previous reports on GO and
FRGO [68].
Figure 3.2: Raman Spectra of (a) GO (b) FRGO.
43
3.3 Characterization of Functionalized Reduced Graphene Oxide Using XPS
The GO and FRGO was further characterized using a VersaProbe X-ray Photoelectron
Spectroscopy (XPS) system from Physical Electronics. The XPS spectra of GO and FRGO are
shown in Figures 3.3 and 3.4, respectively. The spectrum for GO indicates covalently attached
hydroxyl (C-OH), epoxide (-C-O-C-), and carboxylic (-O-C=O) oxygen groups at 286.5 eV,
287.2 eV, and 288.7 eV, respectively, along with sp2 hybridized C-C bonds at 285.0 eV. The
XPS spectrum of FRGO shows that the intensity of oxygen-containing functional groups is
significantly reduced and a new functional group appears at 286.1 eV, corresponding to C-N
bonds that appear due to the functionalization of GO with ODA. These results are also in good
agreement with previous reports [68].
Figure 3.3: XPS spectrum of graphene oxide. The black circles represent raw data. The black line
is fitted sum and the colored lines are fitted peaks using the software OMNIC™ for Almega 7.
44
Figure 3.4: XPS spectrum of functionalized reduced graphene oxide. The black circles represent
raw data. The black line is fitted sum and the colored lines are fitted peaks using the software
OMNIC™ for Almega 7.
45
CHAPTER 4
MECHANICAL, THERMAL, AND TRIBOLOGICAL PROPERTIES OF
NANOCOMPOSITES
4.1 Introduction
Graphene is a material with a two dimensional honeycomb lattice with sp2 bonded carbon
atoms [1, 8, 23, 69-70]. Graphene has high mechanical strength and high electrical and thermal
conductivities making it potentially useful in the fabrication of polymer nanocomposites with
enhanced mechanical, tribological, thermal and electrical properties [10-13, 32, 41]. Earlier
publications dealt with various carbon materials including carbon black, carbon nanofibers,
exfoliated graphite and carbon nanotubes (CNTs) as fillers in polymers to improve these
properties [7, 71-76]. CNTs have been shown to be particularly effective due to their high
mechanical strength, and high electrical and thermal conductivities [7, 77-78]. It has been
reported that the addition of 2 weight percentage (wt. %) of CNTs in epoxies doubles the
Young’s modulus [71]. The addition of 0.1 wt. % of CNTs increases the electrical conductivity
from 10-9
Sm-1
to 10-2
Sm-1
[72], and 1.0 wt. % increases the thermal conductivity by 80% [73].
In addition, CNTs have been reported to increase the glass transition temperature (Tg) of epoxies
[78]. CNTs functionalized with maleic anhydride and amino groups increase Tg by 10°C and
14°C, respectively [79]. CNTs modified with nonionic surfactants increase Tg by 25°C [75]. The
dispersion and adhesion at the molecular level, altering the chain dynamics [80-81]. In contrast,
____________________________________________________________________________
Note: Most of this chapter is duplicated from the accepted paper with the permission of Maney Publishing: ‘Effects
of Functionalized Reduced Graphene Oxide on Frictional and Wear Properties of Epoxy Resin’, R. Shah, T.
Datashvili, T. Cai, J. Wahrmund, B. Menard, K. P. Menard, W. Brostow, J. Perez, Material Research Innovations,
2014. References and figures numbers are changed to accommodate the dissertation.
46
increase in Tg has been attributed to the nano-scale dimensions of CNTs that result in good
macroscopic fillers such as carbon fiber and graphite do not significantly affect Tg [60, 82].
In comparison to CNTs, graphene is less expensive to produce and more miscible due to its large
surface area [76]. It has been reported that the addition of 0.4 wt. % of amine functionalized
graphene to epoxy increases the Young’s modulus by 60% [83]. The addition of 4 wt. % of
graphene and 2.5 wt. % of surface-modified graphene to epoxy increases Tg by ≈ 8°C and ≈
14°C, respectively [81]. The addition of 1 wt. % of functionalized graphene to poly
(acrylonitrile) increases Tg by as much as 40°C [78]. Other studies on graphene-epoxy
nanocomposites have reported the effects of graphene oxide on curing [59], graphene platelets on
fracture properties [81], functionalized graphene oxide on hardness, electrical conductivity and
thermal properties [42], and organosilane functionalized graphene on thermal degradation and
tensile strength [43]. Tribological studies of graphene-polymer nanocomposites have shown that
the wear rate of polytetrafluoroethylene (PTFE) is significantly reduced by the addition of 10 wt.
% of thermally reduced graphene platelets [44]. The friction and wear rate of nylon are lowered
by modified graphene oxide [45].
Epoxy resin is used in various aerospace and automotive applications due to its
moldability and good mechanical and thermal properties [85-87]. It would be of interest to study
the tribological properties graphene-epoxy nanocomposites with the goal of lowering friction and
wear rates to extend their lifetime [64, 88-90]. Here, the effects of octadecylamine-functionalized
reduced graphene oxide (FRGO) on the friction and wear properties of diglycidyl ether of
bisphenol A (DGEBA) epoxy are reported. The effects on the Young’s modulus, Tg and thermal
stability of the epoxy are also reported.
47
4.2 Young’s Modulus Measurements of FRGO-Epoxy Nanocomposites
A MTS system was used to measure Young’s modulus at a tensile rate of 10 mm/min
with the high load limit set at 5000 lbf. Testing was performed at a temperature of approximately
23 °C.
Figure 4.1 shows load versus elongation curves for the neat epoxy and nanocomposites
containing various wt. % of FRGO. The Young’s modulus, tensile strength and strain at break
for the various FRGO concentrations are shown in Table 1. Figure 4.2 shows that the Young’s
modulus increases with the addition of FRGO for concentrations from 0.1 to 0.5 wt. %. For 0.5
wt. % of FRGO, the Young’s modulus increases by 74% from 1.23 to 2.14 GPa, while the tensile
strength increases by 68%. Similar increases in the Young’s modulus and tensile strength of
graphene-epoxy nanocomposites have been reported and attributed to good dispersion of
graphene and the strong interfacial interactions between graphene and the epoxy matrix [41, 68,
91] that results in good transference of applied stress from the matrix to the FRGO [68,80, 91].
With the addition of 1.0 wt. % of FRGO, the Young’s modulus and the tensile strength decrease
by 6.7% and 13 %, respectively, compared to their values at 0.5 wt. %. Such a decrease has been
previously reported [91-93] and attributed to the aggregation of FRGO at high concentrations
that weakens the adhesion of graphene to the matrix. It was explained that the weak adhesion
reduces the stress transfer capability, what ultimately reduces the Young’s modulus and tensile
strength [91-93].
As shown in Table 1, a decrease in strain at break occurs as FRGO is added. A similar
observation at higher concentrations of filler has been reported and attributed to the lower
susceptibility of deformation of graphene compared to the matrix; thus, the filler reinforces the
matrix such that it deforms less [91, 93-94]. The decreased deformation indicates an increase in
48
strength and stiffness, and is consistent with the tensile strength and the Young’s modulus
measurements [94-96]. A comparison of the experimental value of Young’s modulus of
nanocomposites with the theoretical results is presented in the next section.
Figure 4.1: Load versus elongation curves of the neat epoxy, and nanocomposites containing
0.1, 0.2, 0.5, and 1.0 wt. % FRGO.
49
Table 4.1. Mechanical Properties of The Neat Epoxy and The Nanocomposites.
FRGO Content
(wt. %)
Young Modulus
(GPa)
Tensile Strength
(MPa)
Strain at Break
(%)
0.0 1.23 24.1 4.39
0.1 1.43 29.1 4.22
0.2 1.60 36.1 4.03
0.5 2.14 40.6 2.92
1.0 2.00 35.2 3.48
Figure 4.2: Young’s modulus of neat epoxy and nanocomposites containing 0.1, 0.2, 0.5 and 1.0
wt. % FRGO. Error bars are for three samples.
50
4.3 Estimation of Young’s Modulus Using Halpin-Tsai Model
The Young’s modulus of graphene-epoxy nanocomposite can be estimated from the
Halpin-Tsai model, which is developed for fiber reinforced composite materials. The model can
be applied for graphene-epoxy nanocomposites by assuming that graphene sheet is a fiber with
rectangular cross-sectional area of width (w), length (l), and thickness (t). The Halpin-Tsai
equation [96-98] for fiber reinforced composite materials is given by
m
fibeffw
fibeffw
fibeffl
fibeffl
c EV
V
V
VE
,
,
,
,
1
21
8
5
1
1
8
3
(4.1)
where,
m
fibeff
m
fibeff
l
E
E
E
E
,
,1
,
2
1
,
,
m
fibeff
m
fibeff
w
E
E
E
E
, cE is the Young’s modulus of the composite material,
fibeffV , is the volume fraction of fiber, fibeffE , is the Young’s modulus of the fiber, and mE is the
Young’s modulus of the matrix. The constant depends on the geometry of the fiber, and is
given by
t
lw (4.2)
The volume fraction of the fiber is given by
m
fib
fibfib
fib
m
fib
fib
fib
fib
fib
fibeff
mm
m
mm
m
compositeofvolume
fiberofvolumeV
)1()1(,
(4.3)
51
Equations (4.1) and (4.3) can be applied to the graphene-epoxy composite material by
assuming frgofibeff VV , and frgofibeff EE , . In this case, with the values of l and w , equation (4.1)
can be written as
m
frgo
frgo
frgo
frgo
c E
V
V
V
V
E
2
11
2
121
8
5
11
11
8
3
(4.4)
where, m
frgo
E
E is the ratio of the Young’s moduli of FRGO and the epoxy matrix. This
equation can be used to estimate the theoretical Young’s modulus of graphene-based
nanocomposites.
Similarly, the volume fraction of functionalized reduced graphene frgoV can be calculated
using equation (4.2) and is given by
m
frgo
frgofrgo
frgo
frgo
mm
mV
)1(
(4.5)
where, frgom is the mass fraction of FRGO, frgo is the density of FRGO, and m is the density of
epoxy matrix. The density of graphite is 2.25 g/cm3 with an interlayer separation of 0.335 nm.
The interlayer separation for the graphene oxide is~ 0.68 nm. Therefore, frgo =1.1 g/cm3.
From the SEM observations, width (w), length (l), and thickness of FRGO are ~2.5 μm,
~2.0 μm, and ~3.4 nm (for 5 FRGO layers), respectively. The aspect ratio of the material is
~1,180. The number of layers was estimated from the ratio of the intensities of G and Si peaks of
the Raman spectrum of FRGO deposited on a SiO2 substrate shown in Figure 4.3 [99]. Similarly,
52
m =1.07 g/cm3. The theoretical Young’s modulus, calculated using Halpin-Tsai model (i.e.,
equation 4.4), for various concentrations of FRGO-epoxy composite is shown in Figure 4.4. This
shows that the experimental value of Young’s modulus is in agreement with the theoretical value
estimated using the Halpin-Tsai model for FRGO-epoxy nanocomposites up to 0.5 wt. % of
FRGO in the epoxy matrix. The decreases in the experimental value of the Young’s modulus
beyond 0.5 wt. % of FRGO is attributed to agglomeration of FRGO at higher concentrations in
the epoxy matrix [91-93]. Agglomeration is not included in the Halpin-Tsai model.
Table 4.2. The Young’s Modulus of FRGO-Epoxy Nanocomposites Calculated Using Halpin-Tsai
Model.
FRGO Content Theoretical Young’s Modulus (GPa)
wt. % vol. % 3 layers 5 layers 7 layers
0.0 0.000 1.23 1.23 1.23
0.1 0.097 1.50 1.46 1.43
0.2 0.180 1.75 1.68 1.62
0.5 0.480 2.57 2.37 2.22
1.0 0.970 3.92 3.51 3.22
53
Figure 4.3: Raman spectra of FRGO deposited on a SiO2 substrate.
54
Figure 4.4: Comparision of the Young’s modulus estimated using the Halpin-Tsai model with
the experimental Young’s modulus for the nanocomposites containing 0.1, 0.2, 0.5 and 1.0 wt. %
FRGO.
55
4.4 Dynamic Mechanical Analysis (DMA) of FRGO-Epoxy Nanocomposites
DMA was carried out using a PerkinElmer DMA8000 apparatus. The measurements were
performed in the single cantilever bending mode. Three frequencies were applied, namely 0.1,
1.0 and 10.0 Hz. The differences between the sets of results for different frequencies were not
significant, hence results for 1.0 Hz are reported below. All deformations were 50 microns so
that strain was well below 1%.
DMA measurements provide the storage modulus E’ (representing the elastic energy, that
is the solid-like behavior) and the loss modulus E” (representing liquid-like behavior). From
these one obtains tan δ = E”/E’. In the glass transition region (i.e., the temperature at which a
substance changes from hard or brittle state to rubbery state), E’ dramatically decreases while E”
shows a maximum. The width of the glass transition region varies—while for convenience that
region is often represented by a single number called the glass transition temperature Tg. It
should be remembered that representation of a region by a single number is a large simplification
[61, 100-102]. The onset of the drop in storage modulus as a function of temperature gives the
best agreement with the peak of tan δ. The peak in the loss modulus is often weak or absent in
certain materials and not in general use [61,101]. The peak of tan δ was used for the location of
Tg. This method is in common use and gives peaks with good visibility, reproducibility and
minimal dependence on the analyst [61, 100-102]. Figure 4.5 shows tan δ for neat epoxy and
epoxy with 0.5 wt. % FRGO. As shown in Figure 4.6, Tg shows a maximum at 0.5 wt. % FRGO.
An increase in Tg implies an increase in interaction between the filler and matrix. The increase in
Tg is consistent with previous reports using graphene platelets as fillers [81], and indicates good
dispersion of FRGO in the epoxy. The decrease in Tg with further addition of FRGO may be due
to the agglomeration of FRGO sheets or a reduction in cross-linking density that results in a less
56
stiff material [103]; a similar conclusion from the Young’s modulus measurements have been
already formulated above.
Figure 4.5: Plot of tan δ versus temperature for neat epoxy and nanocomposite containing 0.5 wt
% of FRGO.
57
Figure 4.6: Tg for neat epoxy and nanocomposites containing 0.1, 0.2, 0.5 and 1.0 wt. % of
FRGO.
58
4.5 Thermogravimetric Analysis of FRGO-Epoxy Nanocomposites
The thermal stability of the nanocomposites was studied using a Perkin Elmer TGA 7
thermogravimetric analyzer (TGA). The measurement was performed in a range from 20 to 600
°C at a heating rate of 20 °C/min under N2 gas environment. Figure 4.7 shows a TGA
thermogram of the neat epoxy and several nanocomposites and Figure 4.8 shows expanded TGA
from 330 ºC to 420 ºC. The first derivatives of the thermogravimetric curves shown in Figure 4.7
with respect to temperature are given in Figure 4.9. The weight loss for the neat epoxy and
nanocomposites appears to occur in two stages as shown in Figure 4.9. In the first stage, which
takes place from 180 to 330°C, both the neat epoxy and nanocomposites lose their weight by
about 12 %. This is attributed to a loss of adsorbed water and oligomers [92]. In the second
stage, which occurs between 330 and 530 °C, a significant amount of weight loss is observed and
attributed to thermal decomposition of the epoxy [42, 104]. In this region, the decomposition
temperature of nanocomposite increases with the addition of FRGO and attain a maximum value
for the nanocomposite containing 0.5 wt. % of FRGO. The onset temperature for decomposition
is about ~16 °C greater for nanocomposites with 0.5 wt. % FRGO than for neat epoxy. The
improvement in thermal degradation temperature attributed to good dispersion of FRGO at
molecular level in the epoxy that result in strong interfacial interaction between FRGO and the
epoxy which is consistent with the observed increases in the Young’s modulus [41, 68, 91].
Wang et al. have reported a similar enhancement in the thermal stability of organosilane-
functionalized graphene-epoxy nanocomposites [43]. The thermal degradation temperature of the
nanocomposite containing 1.0 wt. % of FRGO decreases with respect to that for 0.5 wt. % of
FRGO. The decrease at higher wt. % of FRGO is due to aggregation of FRGO sheets in the
epoxy which weakens the adhesion of FRGO to the epoxy [91,93].
59
Figure 4.7: Thermogravimetric analysis showing mass % versus temperature for neat epoxy and
nanocomposites containing 0.1, 0.2, 0.5 and 1.0 wt. % of FRGO.
Figure 4.8: Expanded view of thermogravimetric analysis in the temperature range 330-420 oC.
60
Figure 4.9: The first derivative of the thermogravimetric curve with respect to temperature for
neat epoxy and nanocomposites containing 0.1, 0.2, 0.5 and 1.0 wt. % of FRGO.
4.6 Tribological Properties of FRGO-Epoxy Nanocomposites
The frictional and wear properties of the nanocomposites were studied using a Nanovea
tribometer from Micro Photonics, Inc. A tungsten carbide ball with a diameter of 6 mm was used
as the counter surface. All measurements were performed in air using a 15 N normal load, 80
rpm rotational speed and a circular track having a radius of 2 mm. Further details are provided
below.
4.6.1 Friction Results
Plots of friction versus sliding distance for the neat epoxy and several nanocomposites
are shown in Figure 4.10. The neat epoxy exhibits an initial friction of less than or approximately
61
equal to 0.1 during the first 1.5 meters of sliding distance, as shown in the expanded view in
Figure 4.11. After this, the friction sharply increases to 0.53. In contrast, the nanocomposites
exhibit sliding distances—during which the friction is, again, less than or approximately equal to
0.1—that are more than an order of magnitude greater. As shown in Figure 4.10 for 0.1 wt. % of
FRGO, the friction is less than or approximately 0.1 for about 44 m before increasing to 0.51.
For 0.2, 0.5 and 1.0 wt. % of FRGO, the friction is less than or approximately 0.1 for about 55,
61 and 93 m, respectively, before increasing to 0.43, 0.44 and 0.45, respectively.
The increase in sliding distance during which the friction is low is attributed to a transfer
film from the nanocomposite to the counter surface. It is well known that transfer films reduce
the friction by providing interfacial sliding between the surface and counter surface [105]. Figure
4.12 (a) shows an optical microscopy image of the counter surface taken using a tungsten light
source before any sliding. Figure 4.12 (b) shows an image of the counter surface after sliding on
the neat epoxy after a distance of 90 m; there was a clean surface with no transfer film. Figures
4.12 (c)-(f) show images of the counter surface after sliding only in the low friction regime of
nanocomposites with 0.1, 0.2, 0.5 and 1.0 wt. % of FRGO, respectively. The images were taken
after sliding a distance equal to approximately 50% of the distance at which the friction sharply
increases to 0.4-0.5. The transfer films was observed, some of which have fringes due to
interference between the incident light and the reflected light from the counter surface. Figure
4.13 (a)-(d) show images of the counter surface after sliding in the high friction regime of
nanocomposites with wt. % FRGO of 0.1, 0.2, 0.5 and 1.0, respectively. These images were
taken after completion of the runs shown in Figure 4.10. Transfer films are also observed in these
cases, although less coverage of the counter surface is seen.
62
Figure 4.10: Plot of the friction versus sliding distance for neat epoxy and nanocomposites
containing 0.1, 0.2, 0.5 and 1.0 wt. % of FRGO.
63
Figure 4.11: Expanded view of the friction versus sliding distance for neat epoxy.
64
Figure 4.12: Optical microscopy images of the countersurface (i.e., tungustan carbide ball of
diameter 6mm). (a) Clean surface. (b) Surface after sliding on neat epoxy showing no transfer
film. Surface after sliding in the low friction regime for nanocomposites containing (c) 0.1, (d)
0.2, (e) 0.5 and (f) 1.0 wt.% of FRGO. Transfer films are observed.
65
Figure 4.13: Optical microscopy images of the countersurface after sliding in the high friction
regime for nanocomposite containing (a) 0.1, (b) 0.2, (c) 0.5 and (d) 1.0 wt.% of FRGO.
66
4.6.2 Wear Results
The wear rates of the nanocomposites in the low and high friction regimes were
calculated by measuring the depth of the wear track using a Veeco Dektak 150 profilometer.
The wear behavior of polymers can be significantly affected by fillers [39, 64, 90, 107-110]. For
the low friction regime, the wear track corresponding to each wt. % of FRGO was measured after
a sliding distance equal to approximately 50 % of the sliding distance at which the friction
sharply increases. For the high friction regime, the wear track was measured at the end of the
run. The depth profiles of worn surface of various FRGO/epoxy nanocomposites are shown in
Figures 4.14. The worn volume V was calculated using relation V =2πR·A, where R is radius of
the wear track and A is the average cross-sectional area of the worn track obtained from the
profilometry measurement. The wear rate W (in mm3/N·m) was calculated using the relation W =
V/(N·x), where N is the normal load and x is the sliding distance. The wear rate as a function of
FRGO concentration in the low and high friction regimes is shown in Figure 4.15. The wear rate
in the low friction regime is about 5 times lower than that in the high friction regime. This is
attributed to the transfer film, which is also known to reduce the wear rate by isolating the
surface from the counter surface and reducing frictional stresses [64, 90]. In the low and high
friction regimes, there is a reduction in wear rate of approximately 33 % and 13 %, respectively,
at 1.0 wt. % of FRGO.
67
Figure 4.14: Profilometer cross-section of wear tracks taken at the end of the friction
measurements shown in Figure 4.7 for neat epoxy and nanocomposites containing various wt. %
of FRGO.
68
Figure 4.15: Wear rates of the neat epoxy and nanocomposite containing 0.1, 0.2, 0.5 and 1.0
wt. % of FRGO in the high friction regimes and wear rates of various nanocomposites in low
friction regime. Error bars are for three wear tracks.
69
4.6.3 Investigation of Wear Mechanism
In order to study the wear in more detail, SEM images of the wear tracks were taken.
Figures 4.16 (a) and (b) show SEM images of the wear track of the neat epoxy near the end of
the low friction regime after a sliding distance of approximately 3 m. Figure 4.16 (a) shows that
there are areas where the surface of the wear track has roughened. Figure 4.16 (b) shows a
magnified view of a roughened area showing the formation of wear particles. Therefore, the
roughening starts near the very beginning for the neat epoxy, and this facilitates the sharp
increase in friction after a very short sliding distance. Figure 4.16 (c) and (d), and (e) and (f)
show SEM images of the wear tracks in the low friction regime for nanocomposites containing
0.1 and 0.5 wt. % of FRGO, respectively; the images were obtained after sliding distances of
approximately 20 and 35 m, respectively. For the nanocomposites, the wear tracks are
significantly smoother than that of the neat epoxy even though the sliding distances are about an
order of magnitude longer and wear particles are not observed. It has been reported that a
transfer film diminishes the formation of wear particles [104]; this is consistent with our
observations.
Figures 4.17 (a) and (b), (c) and (d), and (e) and (f) show wear tracks of the neat epoxy
and nanocomposites with 0.1 and 0.5 wt. % of FGRO, respectively, in the high friction regime at
the end of the run. All of the wear tracks are rough, consistent with the significantly higher
friction and wear rate in this regime. The wear track of the neat epoxy is very rough and
ploughed with loosely bound chunks of debris. Similar observations have been reported for
epoxy [39, 105]. The debris has been attributed to the formation and expansion of surface and
subsurface cracks due to repeated loading [44, 105, 109]. As shown in Figures 4.17 (c)-(f), when
0.1 and 0.5 wt. % of FRGO is added, the wear tracks becomes progressively smoother with less
70
debris. Other studies [105] have reported that the addition of fillers in an epoxy matrix forms a
protective layer that reduces the wear rate. Dang et al. have reported similar observations with
CNTs in an epoxy matrix and proposed that these fillers diminish the adhesion between the
matrix and the counter surface; the ploughing phenomenon is thus reduced, and this results a
relatively higher wear resistance [39].
Figure 4.16: SEM images of wear tracks in the low friction regime of (a) and (b) Neat epoxy. (c)
and (d) Nanocomposites containing 0.1 wt. % of FRGO. (e) and (f) Nanocomposites containing
0.5 wt. % of FRGO.
71
Figure 4.17: SEM images of wear tracks in the high friction regime of (a) and (b) Neat epoxy. (c)
and (d) Nanocomposites containing 0.1 wt. % of FRGO. (e) and (f) Nanocomposites containing
0.5 wt. % of FRGO.
72
CHAPTER 5
CONCLUSIONS
Graphene oxide was successfully prepared using the Hummer’s method. The
functionalization of graphene oxide was carried out using octadecylamine. Then, hydrazine
monohydrate was used to reduce oxygen-containing functional groups that remained after ODA
functionalization. The Raman analysis confirmed the reduction of graphene oxide into
functionalized reduced graphene oxide. The x-ray photoelectron spectroscopy revealed that
oxygen containing functional groups reduced significantly, and the formation of a new functional
group corresponding to ODA functionalization.
The addition of FRGO in the range of 0.1 to 1.0 wt. % into the epoxy significantly
increases the Young’s modulus and tensile strength. For 0.5 wt. % of FRGO, the Young’s
modulus and tensile strength increases by 74% and 68 %, respectively. These observations are
consistent with previous results for nanofillers such as CNTs. The increases are attributed to the
unique structural properties of the fillers; in particular, their nanoscale dimensions that result in
high dispersion and bonding to the epoxy matrix, which increases the efficiency of load transfer
at the interface. The experimental results on the Young’s modulus of the nanocomposites are in
good agreement with the theoretical results, estimated using the Halpin-Tsai model, for FRGO
concentrations up to 0.5 wt. %. However, the experimental value of the Young’s modulus at
higher wt. % of FRGO deviates from the theoretical calculation. The main reason for this
deviation is aggregation that is not included in the theoretical model [91, 93]. The aggregation of
FRGO provides weak interfacial interaction with the epoxy that reduces the stress transfer
capability of nanocomposites.
73
The addition of FRGO in the range of 0.1 to 1.0 wt. % into the epoxy moves up the
thermal degradation temperature as well as the glass transition temperature. The thermal
degradation temperature and the glass transition temperature shifted toward higher temperatures
by ~16 °C and ~7.5 °C for nanocomposite containing 0.5 wt. % of FRGO, respectively. These
improvements are attributed to the good dispersion and adhesion of FRGO at molecular level
that results in strong interfacial bonding with the epoxy. This observation is consistent with our
Young’s modulus measurements. These results show improvement in the thermal stability of the
material that is important in many applications.
The addition of FRGO in epoxy lowers the dynamic friction and wear. Our tribological
studies reveal the new result that there is a significant increase in the low-friction sliding distance
of the nanocomposites by over an order of magnitude. The friction in this regime is < 0.1. The
increase in low-friction sliding distance is attributed to the formation of a thin uniform transfer
film on the counter surface. It is well known that transfer film reduce friction by providing
interfacial sliding between the nanocomposites and the counterface. With continued sliding, the
friction eventually increases to about 0.5. The wear rate in the high-friction regime is
significantly greater than in the low-friction regime. The wear tracks in high friction regime
become progressively smoother with less ploughing. The addition of FRGO in epoxy matrix
diminishes the adhesion between the matrix and the counter surface; the ploughing phenomenon
is thus reduced, and this results a relatively higher wear resistance. Further, the transfer film in
the high-friction regime has thicker clumps and less coverage than in the low-friction regime,
indicating that ploughing is an important mechanism in abrasion.
The increase in sliding distance in the low friction regime increases the wear life time of
the nanocomposite. The improvements in friction and wear are accompanied by increases in the
74
mechanical strength and thermal stability of the material. While epoxies have quite a variety of
applications [86, 110], the present work extends the range of applications further.
75
REFERENCES
1. J. H. Jung, D. S. Cheon, F. Liu, K. B. Lee and T.S. Seo: ‘A graphene oxide based immune-
biosensor for pathogen detection’, Angew. Chem. Int. Ed., 2010, 49, 5708-5711.
2. J. R. Potts, D. R. Dreyer, C. W. Bielawski and R.S. Ruoff: ‘Graphene-based polymer
nanocomposites’, Polym, 2011, 52, 5-25.
3. F. Hussain, M. Hojjati, M. Okamoto and R. E. Gorga: ‘Review article: Polymer-matrix
Nanocomposites, Processing, Manufacturing, and Application: An Overview’, J.Comp.
Mater., 2006, 40, 1511-1575.
4. R.A. Vaia and H. D. Wagner: Mater. Today, 2004, 7, 32-37.
5. G. Marsh: ‘Composites get in deep with new-generation engine’, Reinforced Plastics,
2006, 50, 26-29.
6. M. Alexandra and P. Dubois: ‘Polymer-layered silicate nanocomposites: preparation,
properties and uses of a new class of materials’, Mater. Sci. & Eng., 2000, 28, 1-63.
7. M. Moniruzzaman and K. I. Winey: ‘Polymer Nanocomposites Containing Carbon
Nanotubes’, Macromolecules, 2006, 39, 5194-5205.
8. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V.
Grigorieva, A. A. Firsov: ‘Electric field effect in atomically thin carbon films’, Science,
2004, 306, 666-669.
9. T. Kuilla, S. Bhadra, D. Yao, N. H. Kim, S. Bose and J. H. Lee: ‘Recent advances in
graphene based polymer nanocomposites’, Prog. Polym. Sci., 2010, 35, 1350-1375.
10. C. Lee, X. Wei, J. W. Kysar and J. Hone: ‘Measurement of the elastic properties and
intrinsic Strength of monolayer graphene’, Science, 2008, 321, 385-388.
11. A. A. Balandin, S. Ghosh , W. Bao , I. Calizo , D. Teweldebrhan , F. Miao , C. N. Lau:
‘Superior thermal conductivity of Single-Layer Graphene’, Nano Lett. 2008, 8, 902–907.
12. X. DU, I. Skachko, A. Barker and E. Y. Andrei: ‘Approaching ballistic transport in
suspended graphene’, Nano Lett. 2008, 3, 491-495.
13. M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff: ‘Graphene-based ultracapacitors’,
Nano Lett. 2008, 8, 3498-3502.
14. S. Scharfenberg, D. Z. Rocklin, C. Chialvo, R. L. Weaver, P. M. Goldbart and N. Mason1:
‘Probing the mechanical properties of graphene using a corrugated elastic substrate’, Appl.
Phys. Lett., 2011, 98, 091908.
76
15. D. D. L. Chung: ‘Composite materials: Functional materials for modern technologies’,
Springer. London. 2003.
16. A. Okada and A. Usuki: ‘Twenty Years of Polymer-Clay Nanocomposites’, Macromol.
Mater. Eng., 2006, 291, 1449-1476.
17. G.G. Tibbetts, M.L. Lake, K. L. Strong and B.P. Rice: ‘A review of the fabrication and
properties of vapor-grown carbon nanofiber/polymer composites’, Comp. Sci. & Tech.,
2007, 67, 1709–1718.
18. E. Hammel, X. Tang, M. Trampert, T. Schmitt, K. Mauthner, A. Eder and P. Potschke:
‘Carbon nanofibers for composite applications’, Carbon, 2004, 42, 1153-1158.
19. X.L. Xie, Y. W. Mai and X.P. Zhou: ‘Dispersion and alignment of carbon nanotubes in
polymer matrix: A review’, Mater. Sci. & Eng., 2005, 49, 89-112.
20. J.N. Coleman, U. Khan and Y.K. Gun’ko: ‘Mechanical Reinforcement of Polymers Using
Carbon Nanotubes’, Adv. Mater., 2006, 18, 689-706.
21. D. Cho, S. Lee, G. Yang, H. Fukushima and L. T. Drzal: ‘Dynamic Mechanical and
Thermal Properties of Phenylethynyl-Terminated Polyimide Composites Reinforced With
Expanded Graphite Nanoplatelets’, Macromol. Mater. & Eng., 2005, 290, 179-187.
22. R. Verdejo, M. M. Bernal, L. J. Romasanta and M. A. Lopez-Manchado: ‘Graphene filled
polymer nanocomposites’, J. Mater. Chem., 2011, 21, 3301-3310.
23. A. K. Geim and K. S. Novoselov: ‘The rise of graphene’, Nat. Mater., 2007, 6, 183 – 191.
24. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva,
S. V. Dubonos and A. A. Firsov: ‘Two-dimensional gas of massless Dirac fermions in
graphene’, Nature, 2005, 438, 197-200.
25. S. Park and R. S. Ruoff: ‘Chemical methods for the production of graphenes’, Nat.
Nanotech., 2009, 4, 217 – 224.
26. 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, and I. A. Aksay: ‘Single Sheet
Functionalized Graphene by Oxidation and Thermal Expansion of Graphite’, Chem.
Mater., 2007, 19, 4396–4404.
27. W. Scholz, H. P. Boehm, Untersuchungen am Graphitoxid. VI. Betrachtungen zur Struktur
des Graphitoxids, Anorg. Allg. Chem., 1969, 369, 327-340.
28. S. S. Ray: ‘Clay-Containing Polymer Nanocomposites: From Fundamentals to Real
Applications’ Elsevier, UK, 2013.
77
29. L.H. Lee: ‘Adhesives, sealents and soatings for space and harsh environments’, New
York: Plenum Press, 1988.
30. J.I Distasio: ‘Epoxy resin technology: development since 1979’, New Jersey: Noye Data,
1982.
31. M. A. Boyle, C. J. Martin and J. D. Neuner: ‘Epoxy Resins’, ASM Handbook, 2001, 21,
78-89
32. H. B. Zhang, W. Q. Zheng, Q. Yan, Y. Yang, J. Wang, Z. H. Lu, G. Y. Ji and Z. Z. Yu:
‘Electrically conductive polyethylene terephthalate/graphene composite prepared by melt
compounding’, Polymer, 2010, 51, 1191-1196.
33. R.E. Gorga, and R.E. Cohen: ‘Toughness Enhancements in Poly (methyl methacrylate) by
Addition of Oriented Multiwall Carbon Nanotube’, J. Polym. Sci., Part B: Polym. Phys.,
2004, 42, 2690–2702.
34. E.T. Thostenson, C. Li and T-W. Chou: ‘Nanocomposites in context’, Comp. Sci. &
Techno., 2005, 65, 491–516.
35. D. J. Eedy: Carbon-fiber-induced Airborne Irritant Contact Dermatitis, Contact Dermatitis’,
1996, 35, 362-365.
36. S. Agarwal, E. Tatli, N.N. Clark, and R. Gupta: ‘International Symposium on Polymer
Nanocomposites Science and Technology’, Boucherville, Canada, 2005.
37. W. Brostow, P.E. Cassidy, H.E. Hagg, M. Jaklewicz and P.E. Montemartini:
‘Fluoropolymer addition to an epoxy: phase inversion and tribological properties’, Polymer
2001, 42, 7971-7977.
38. T. Kasemura, Y. Oshibe, H. Uozumi, S. Kawai, Y. Yamada, H. Ohmura and T. Yamamoto:
‘Surface modification of epoxy resin with fluorine-containing methacrylic ester
copolymers’ Appl. Polym. Sci., 1993, 47, 2207-2216.
39. B. Dong, Z. Yang, Y. Huang and H.-L. Li: ‘Study on tribological properties of multi-
walled carbon nanotubes/epoxy resin nanocomposites’, Trib. Lett., 2005, 20, 251-254.
40. Z. Zhang, C. Breidt, L. Chang, F. Haupert and K. Friedrich: ‘Enhancement of the wear
resistance of epoxy: short carbon fibre, graphite, PTFE and nano-TiO2’, Composites: Part
A, 2004, 35, 1385–1392.
41. T. Kuila, S. Bose, A. K. Mishra, P. Khanra, N. H. Kim and J. H. Lee: ‘Effect of
functionalized graphene on the physical properties of linear low density polyethylene
nanocomposite’, Polym. Test., 2012, 31, 31-38.
78
42. C. Bao, Y. Guo, L. Song, Y. Kan, X. Qian and Y. Hu: ‘In situ preparation of functionalized
graphene oxide/epoxy nanocomposites with effective reinforcements’, J. Mater. Chem.,
2011, 21, 13290-13298.
43. X. Wang, W. Xing, P. Zhang, L. Song, H. Yang and Y. Hu: ‘Covalent functionalization of
graphene with organosilane and its use as a reinforcement in epoxy composites’, Compos.
Sci. & Tech., 2012, 72, 737–743.
44. S. S. Kandanur, M. A. Rafiee, F. Yavari, M. Schrameyer, Z-Z Yu, T. A. Blanchet and N.
Koratkar: ‘Supression of wear in graphene polymer composites’, Carbon, 2012, 50, 3178-
3183.
45. B. Pan, S. Zhang, W. Li, J. Zhao, J. Liu, Y. Zhang and Y. Zhang: ‘Tribological and
mechanical investigation of MC nylon reinforced by modified graphene oxide’, Wear,
2012, 294-295, 395-401.
46. B. C. Brodie: ‘On the atomic weight of graphite’, Philos. Trans. R. Soc. London 1859, 149,
249-259.
47. L. Staudenmaier: Ber. Dtsch. Chem. Ges. 1898, 31, 1481-1487.
48. W. S. Hummers and R. E. Offeman: ‘Preparation of Graphitic Oxide’, J. Am. Chem. Soc.,
1958, 80, 1339-1339.
49. F. S. Hyde: ‘Graphitic acid or oxide’, J. Soc. Chem. Ind., 1904, 23, 300-302.
50. S. Stankovich, R. D. Piner, X. Chen, N. Wu, S. T. Nguyen and R. S. Ruoff: ‘Stable
aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide
in the presence of poly(sodium 4-styrenesulfonate)’, J. Mater. Chem., 2006, 16, 155-158.
51. Y. Geng, S. J. Wang and J. K. Kim: ‘Preparation of graphite nanoplatelets and graphene
sheets’, J. Colloid & Interface Sci., 2009, 336, 592–598.
52. T. Wei, G. Luo, Z. Fan, C. Zheng, J. Yan, C. Yao, W. Li and C Zhang: ‘Preparation of
graphene nanosheet/polymer composites using in situ reduction-extractive dispersion’,
Carbon, 2009, 47, 2290–2299.
53. Z. Lin, Y. Liu and C.P. Wong: ‘Facile Fabrication of Superhydrophobic Octadecylamine-
Functionalized Graphene Oxide Film’, Langmuir, 2010, 26, 16110-16114.
54. A. B. Bourlinos, D. Gournis, D. Petridis,T. Szabo, A. Szeri and I. Dekany: ‘Graphite
Oxide: Chemical Reduction to Graphite and Surface Modification with Primary Aliphatic
Amines and Amino Acids’, Langmuir, 2003, 19, 6050-6055.
55. C. Zhu, S. Guo, Y. Fang and S. Dong: ‘Reducing Sugar: ‘New Functional Molecules for
the Green Synthesis of Graphene Nanosheets’, ACS Nano, 2010, 4, 2429–2437.
79
56. A. C. Ferrari and J. Robertson: ‘Interpretation of Raman spectra of disordered and
amorphous carbon’, Phys. Rev. B, 2000, 61, 14095-14107.
57. Y. Zhu , S. Murali , W. Cai , X. Li , J. W. Suk , J. R. Potts and R. S. Ruoff: ‘Graphene and
Graphene Oxide: Synthesis, Properties,and Applications’, Adv. Mater., 2010, 22, 3906–
3924.
58. F. Tuinstra and J. L. Koenig, ‘Raman Spectrum of Graphite’, J. Chem. Phys., 1970, 53,
1126-1130.
59. Z. Wei, D. E. Barlow and P. E. Sheehan, ‘The assembly of Single-Layer Graphene Oxide
and Graphene Using Molecular Templates’, Nano Lette., 2008, 8, 3141-3145.
60. R. F. Landel and L. E. Nielsen: ‘Mechanical Properties of Polymers and Composites’,
1974, New York, Marcel Dekker.
61. K. P. Menard: ‘Dynamic mechanical analysis: a practical introduction’, 2nd edn, p. 103,
2008, Boca Raton, FL, CRC Press.
62. Wikipedia: Thermogravimetric Analysis.
63. B. Briscoe: ‘Wear of polymers: an essay on fundamental aspects’, Tribo. International,
1981, 231-243.
64. P. J. Blau: ‘Fifty years of research on wear of metals’, Trib. Int., 1997, 30, 321-331.
65. K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prud’homme, I. A. Aksay and R. Car:
‘Raman spectra of graphite oxide and functionalized graphene sheets’, Nano Lett., 2008, 8,
36-41.
66. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu,
S. T. Nguyen and R. S. Ruoff: ‘Synthesis of graphene-based nanosheets via chemical
reduction of exfoliated graphite oxide’, Carbon, 2007, 45, 1558-1565.
67. W. Li, X-Z. Tang, H-B. Zhang, Z-G. Jiang, Z-Z. Yu, X-S. Du and Y-W. Mai:
‘Simultaneous surface functionalization and reduction of graphene oxide with
octadecylamine for electrically conductive polystyrene composites’, Carbon, 2011, 49,
4724-4730.
68. M. A. Rafiee, J. Rafiee, Z. Wang, H. Song, Z.-Z. Yu and Nikhil Koratkar: ‘Enhanced
Mechanical Properties of Nanocomposites at Low Graphene Content’, ACS Nano, 2009, 3,
3884–3890.
69. K. P. Loh, Q. Bao, P. K. Ang and J. Yang: ‘The chemistry of graphene’, J. Mater. Chem.
2010, 20, 2277-2289.
80
70. A. K. Geim: ‘Graphene: status and prospects’, Science, 2009, 324, 1530-1534.
71. J.-M. Park, D.-S. Kim, J.-R. Lee and T.-W. Kim: ‘Nondestructive damage sensitivity and
reinforcing effect of carbon nanotube/epoxy composites using electro-micromechanical
technique’, Mater. Sci. Eng. C, 2003, 23, 971–975.
72. J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte and A.H. Windle:
‘Development of a dispersion process for carbon nanotubes in an epoxy matrix and the
resulting electrical properties’, Polymer, 1999, 40, 5967–5971.
73. M. Russ, S. S. Rahatekar, K. Koziol, B. Farmer and H. Peng: ‘Length-dependent electrical
and thermal properties of carbon nanotubeloaded epoxy nanocomposites’, Compos. Sci. &
Tech., 2013, 81, 42-47.
74. W. Brostow, M. Keselman, I. Mironi-Harpaz, M. Narkis and R. Peirce: ‘Effects of carbon
black on tribology of blends of poly (vinylidene fluoride) with irradiated and non-irradiated
ultrahigh molecular weight polyethylene’, Polymer, 2005, 46, 5058.
75. X. Gong, J. Liu, S. Baskaran, R. D. Voise and J. S. Young: ‘Surfactant-Assisted Processing
of Carbon Nanotube/Polymer Composites’, Chem. Mater., 2000, 12, 1049-1052.
76. A. Nogales, G. Broza, Z. Roslaniec, K. Schulte, I. Sics, B.S. Hsiao, A. Sanz, M.C. Garcia
Gutierrez, D.R. Rueda, C. Domingo and T.A. Ezquerra, Low Percolation Threshold in
Nanocomposites Based on Oxidized Single Wall Carbon Nanotubes and Poly(butylene
terephthalate), Macromolecules, 2004, 37, 7669.
77. Z. Spitalsky, D. Tasis, K. Papagelis and C. Galiotis: ‘Carbon nanotube–polymer
composites: Chemistry, processing, mechanical and electrical properties’, Prog. Polym.
Sci., 2010, 35, 357-401.
78. A. Allaoui and N. El Bounia: ‘How carbon nanotubes affect the cure kinetics and glass
transition temperature of their epoxy composites – A review’, Express Polym. Lett., 2009,
3, 588-594.
79. F. H. Gojny and K. Schulte: ‘Functionalisation effect on the thermo-mechanical behavior
of multi-wall carbon nanotube/epoxy-composites’, Compos. Sci. & Tech., 2004, 64, 2303-
2308.
80. 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 and L. C. Brinson: ‘Functionalized graphene sheets for polymer
nanocomposites’, Nat. Nanotech., 2008, 3, 327 – 331.
81
81. I. Zaman, T. T. Phan, H-C Kuan, Q. Meng, L. T. B. La, L. Luong, O. Youssf and J. Ma:
‘Epoxy/graphene platelets nanocomposites with two level of interface strength’, Polymer,
2011, 52, 1603-1611.
82. A. Yasmin and I. M. Danial: ‘Mechanical and thermal properties of graphite platelet and
epoxy composite’, Polymer, 2004, 45, 8211-8219.
83. M. Fang, Z. Zhang, J. Li, H. Zhang, H. Lu and Y. Yang: ‘Constructing hierarchically
structured interphases for strong and tough epoxy nanocomposites by amine-rich graphene
surfaces’, J. Mater. Chem., 2010, 20, 9635–9643.
84. S. Niyogi, E. Bekyarova, M.E. Itkis, J. L. McWilliams, M.A. Hamon, R.C. Haddon:
‘Solution properties of graphite and graphene’, J. Am. Chem. Soc., 2006,128, 7720-7721.
85. S. L. Qiu, C. S. Wang, Y. T. Wang, C. G. Liu, X. Y. Chen, H. F. Xie1, Y. A. Huang and R.
S. Cheng: ‘Effects of graphene oxides on the cure behaviors of a tetrafunctional epoxy
resin’, Express Polym. Lett., 2011, 9, 809–818.
86. B. Bilyeu, W. Brostow and K. P. Menard: ‘Epoxy thermosets and their applications. I.
Chemical structures and applications’, J. Mater. Ed., 1999, 21, 28-286.
87. B. Bilyeu, W. Brostow and K. P. Menard: ‘Epoxy thermosets and their applications. II.
Thermal analysis’, J. Mater. Ed., 2000, 22, 107-129.
88. L. F. Giraldo, W. Brostow, E. Devaux, B. L. López, and L. D. Pérez: ‘Scratch and Wear
Resistance of Polyamide 6 Reinforced with Multiwall Carbon Nanotubes’, J. Nanosci. &
Nanotechnol., 2008, 8, 1–8.
89. A. Arribas, M.-D. Bermudez, W. Brostow, F.-J. Carrion-Vilches and O. Olea-Mejia:
‘Scratch resistance of a polycarbonate + organoclay nanohybrid’, Express Polym. Lett.,
2009, 3, 621-629
90. W. Brostow, V. Kovačević, D. Vrsaljko and J. Whitworth: ‘Tribology of polymers and
polymer-based composites’, J. Mater. Ed., 2010, 32, 273-290.
91. M. A. Rafiee, W. Lu, A. V. Thomas, A. Zandiatashbar, J. Rafiee, J. M. Tour, and Nikhil A.
Koratkar: ‘Graphene Nanoribbon Composites’, ACS Nano, 2010, 4, 7415-7420.
92. S. Kim, I. Do and L.T. Drzal: ‘Multifunctional xGnP/LLDPE Nanocomposites prepared by
solution compounding using various screw rotating systems’, Macromol. Mater.& Eng.,
2009, 294, 196-205.
93. T. Kuila, P. Khanra, A. K. Mishra, N. H. Kim and J. H. Lee: ‘Functionalized-
graphene/ethylene vinyl acetate co-polymer composites for improved mechanical and
thermal properties’, Polym. Test., 2012, 31, 282–289.
82
94. A. Romisuhani, H. Salmah and A. Akmal: ‘Tensile properties of low density polypropylene
(LDPE)/palm kernel shell (PKS) bio-composites: The effect of acrylic acid (AA)’, Mater.
Sci. Eng., 2010, 11, 1-7.
95. K. Oksman and C. Clemons: ‘Mechanical properties of polypropylene-wood and
morphology of impact modified floor composites’, J. Appl. Polym. Sci., 1998, 67, 1503-
1513.
96. P.K. Mallick: ‘Fiber-Reinforced Composites’, Dekker, New York, 1993.
97. E.T. Thostenson and T-W. Chou: ‘On the elastic properties of carbon nanotube-based
composites: modelling and characterization’, J. Phys. D: Appl. Phys., 2003, 36, 573.
98. J.C. Halpin and S.W . Tsai: ‘Environmental Factors in Composite Material Design, US
Air Force technical reports, 1967, AFML-TR-67-423.
99. Y. K. Koh, M. H. Bae, D. G. Cahill and E. Po: Reliably counting atomic planes of
few-layer graphene (n > 4). ACS Nano, 2011, 5, 269-274.
100. R. J. Seyler: ‘Assignment of the glass transition’, 1994, American Society for Testing and
Materials, Baltimore, MD.
101. I.M. Kalogeras and H.E. Hagg Lobland: ‘The nature of the glassy state: Structure and
transitions’, J. Mater. Ed., 2012, 34, 69-94.
102. J. Reiger: ‘The glass transition temperature Tg of polymers—Comparison of the values
from differential thermal analysis (DTA, DSC) and dynamic mechanical measurements
(torsion pendulum)’, Polym. Testing, 2001, 20, 199-204.
103. H. Kim, A. Abdala, and C. Macosko: ‘Graphene/Polymer Nanocomposites in Graphite,
Graphene, and their Polymer Nanocomposites’, 2012, Boca Raton, FL, CRC Press.
104. B. Bilyeu, W. Brostow and K. P. Menard: ‘Epoxy thermosets and their applications. III.
Kinetic equations and models’, J. Mater. Ed., 2001, 23, 189.
105. W. Brostow, W. Chonkaew, K. P. Menard and T. W. Scharf: ‘Modification of an epoxy
resin with a fluoroepoxy oligomer for improved mechanical and tribological properties’,
Mater. Sci. Eng. A, 2009, 507, 241-251.
106. A. de la Isla, W. Brostow, B. Bujard, M. Estevez, R. Rodriguez, S. Vargas and V.M.
Castaño: ‘Nanohybrid scratch resistant coatings for teeth and bone viscoelasticity
manifested in tribology’, Mater. Res. Innovat., 2003, 7, 110-114.
107. M. Estevez, S. Vargas, A. de la Isla, W. Brostow, V.M. Castaño and J.R. Rodriguez:
‘Anovel dental material with high scratch resistance’, Mater. Res. Innovat., 2005, 9, 80-81.
83
108. B. Bilyeu, W. Brostow, L. Chudej, M. Estevez, H.E. Hagg Lobland, J.R. Rodriguez and S.
Vargas: ‘Scratch resistance of different silica filled resins for obturation materials’, Mater.
Res. Innovat., 2007, 11, 181-184.
109. M.C. Romanes, N.A. D’Souza, D. Coutinho, K.J. Balkus Jr. and T. W. Scharf: ‘Surface and
subsurface characterization of epoxy-mesoporous silica composites to clarify tribological
properties’, Wear, 2008, 265, 88–96.
110. W. Brostow, S.H. Goodman and J. Wahrmund: ‘Epoxies’, Ch. 8 in in Handbook of
Thermoset Plastics, 3rd
edn., edited by H. Dodiuk and S.H. Goodman, Elsevier, Oxford –
Waltham MA 2014.
111. V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza,
R. Zboril and K. S. Kim: ‘Functionalization of graphene: ‘Covalent and Non-Covalent
Approaches, Derivatives and Applications’, Chem. Rev., 2012, 112, 6156-6214.
112. K.A. Worsley, P. Ramesh, S. K. Mandal, S. Niyogi, M. E. Itkis, R. C. Haddon: ‘Soluble
graphene derived from graphite fluoride’, Chem. Phys. Lett., 2007, 445, 51-56.
113. S. Stankovich, R. D. Piner, S. T. Nguyen and R. S. Ruoff: ‘synthesis and Exfoliation of
Isocyanate-treated graphene oxide nanoplatelets’, Carbon, 2006, 44, 3342-3347.
114. N. Karousis and N. Tagmatarchis and D. Tasis: ‘Current Progress on the Chemical
Modification of Carbon Nanotubes’, Chem. Rev., 2010, 110, 5366–5397.
115. D. Tasis, N. Tagmatarchis, A. Bianco and M. Prato: ‘Chemistry of Carbon Nanotubes’,
Chem. Rev., 2006, 106, 1105–1136.
116. Y.Y. Liang, D.Q. Wu, X.L. Feng, K. Mullen: ‘Dispersion of Graphene Sheets in Organic
Solvent Supported by Ionic Interactions’, Adv. Mater., 2009, 21, 1679-1683.
117. J. R. Lomeda, C.D. Doyle, D.V. Kosynkin, W.F. Hwang, J. M. Tour: ‘Diazonium
functionalization of surfactant-wrapped chemically converted graphene sheets’, J. Am.
Chem. Soc., 2008, 130, 16201-16206.