ULTRA HIGH MOLECULAR WEIGHT
POLYETHYLENE/GRAPHITIC NANOPLATELET
NANOCOMPOSITES FOR ENERGY STORAGE
APPLICATIONS
A Thesis by
Aasish Jaiswal
Bachelor of Science, Wichita State University, 2012
Submitted to the Department of Mechanical Engineering
and the faculty of the Graduate School of
Wichita State University
in partial fulfillment of
the requirements for the degree of
Master of Science
May 2015
© Copyright 2015 by Aasish Jaiswal
All Rights Reserved
ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE/GRAPHITIC
NANOPLATELET NANOCOMPOSITE FOR ENERGY STORAGE
APPLICATIONS
The following faculty members have examined the final copy of this thesis for
form and content, and recommend that it be accepted in partial fulfillment of the
requirement for the degree of Master of Science, with a major in Mechanical
Engineering.
__________________________________
Bin Li, Committee Chair
__________________________________
Hamid Lankarani, Committee Member
__________________________________
Yi Song, Committee Member
iii
DEDICATION
To my parents (Lal Bihari Prasad Jaiswal and Sakila Kumari Chaudhary Jaiswal),
my brother (Amar Jaiswal), and my dear friends
ACKNOWLEDGEMENTS
I am using this opportunity to express my gratitude to everyone who supported me
throughout the course of my M.Sc. I shall always remain indebted to my supervisors Assistant
Professor Dr. Bin Li and Professor Dr. Hamid Lankarani for having accepted me as a graduate
student. I am thankful not only for their aspiring guidance throughout the work but their benign
encouragement, positive attitude, invaluably constructive criticism, and friendly advice that
helped me keep my spirits high even during difficult times.
I am grateful to Associate Professor Dr. Hamid Lankarani, Assistant Professor Dr. Yi Song, my
committee members for providing their advice and guidance.
I would also like to express my deep appreciation for the assistance and helpfulness of my fellow
research students Zheng Zhuoyuan and Nayan Shrestha for their time and valuable suggestions
during the research work.
Lastly, I would like to thank my parents (Mr. Lal Bihari Prasad Jaiswal and Mrs. Shakila Kumari
Chaudhary Jaiswal) and my brother (Mr. Amar Jaiswal) for their understanding and blessings.
iv
ABSTRACT
Graphene and graphene based nanomaterials are one of the most intriguing functional
materials today because of their exceptional properties including unique electronic properties and
high mass to strength ratio, etc. Their unique properties, plus the abundance of naturally existing
graphite, the ease of functionalization and synthesis in mass volume; graphene and graphitic
nanomaterials have shown great potential in multifunctional polymer nanocomposites. Graphite
Nanoplatelets (GNPs) is one of the most desirable graphitic nanomaterials as the reinforcing
component for polymer nanocomposites, due to its low cost and higher specific surface area of
GNPs compared to other graphitic nanomaterials.
In this study, the heat treated GNPs (ht-GNPs) and surface modified ht-GNP (s-GNP)
were used to modify ultrahigh molecular weight polyethylene (UHMWPE). The objective of this
study focused on understanding the effects of the GNP loadings, temperature, as well as electric
field on energy storage capability of UHMWPE/GNP nanocomposites with and without surface
modifications of ht-GNPs. The results suggested that the surface modification could remarkably
improve the dispersion of GNPs and the interfacial interactions between GNPs and UHMWPE,
both factors have shown the positive impacts on the energy storage performances regardless of
testing conditions, compared with ht-GNPs without surface modifications. Meanwhile, at the
elevated temperature, for both UHMWPE and nanocomposites, both energy storage capability
and efficiency increased, probably due to enhanced dipole switching at high temperature and
high electrical field. The results in this research also revealed the sensitivity of the energy
storage of UHMWPE/GNPs nanocomposites to various factors, which provides us great
opportunities to further optimization of energy storage performances, and eventually leads to
high performance energy storage materials based on polymer nanocomposites.
v
TABLE OF CONTENTS
Chapter
Page
1. INTRODUCTION .................................................................................................................. 1
1.1 ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE (UHMWPE) .............. 1
1.1.1. Polyethylene Materials.............................................................................................. 1
1.1.2. Ultra High Molecular Weight Polyethylene (UHMWPE) ........................................ 4
1.1.3 Electrical and dielectric properties of PE and UHMWPE ......................................... 6
1.2 GRAPHITIC NANOMATERIALS ............................................................................... 10
1.2.1 Graphene .................................................................................................................. 10
1.2.2 Graphite Nanoplatelets and Their Applications ....................................................... 13
1.3 UHMWPE NANOCOMPOSITES ............................................................................... 16
1.3.1 Polymer Nanocomposites ........................................................................................ 16
1.3.2 UHMWPE/Graphitic Nanomaterial Nanocomposites ............................................. 19
1.4 RESEARCH OBJECTIVES AND RATIONALE ......................................................... 20
2. FABRICATION AND CHARACTERIZATIONS OF UHMWPE/GNP
NANOCOMPOSITES ......................................................................................................... 24
2.1 MATERIALS .................................................................................................................. 24
2.2 MODIFICATION of GNPs ........................................................................................... 24
2.2.1 Heat Treatment......................................................................................................... 24
2.2.2 Surface Modification of ht-GNPs ............................................................................ 25
2.2.3 Fabrication of UHMWPE/GNP Nanocomposites ................................................... 26
2.3 CHARACTERIZATION of MODIFIED GNPs ........................................................... 27
2.3.1 Fourier Transform Infrared Spectroscopy (FTIR) ................................................... 27
2.3.2 Transmission Electron Microscopy (TEM) ............................................................. 27
2.4 CHARACTERIZATIONS of NANOCOMPOSITES .................................................. 27
2.4.1 Scanning Electron Microscopy (SEM) .................................................................... 27
2.4.1 Energy Storage Analysis .......................................................................................... 28
3. STRUCTURES AND MORPHOLOGIES ......................................................................... 30
vi
TABLE OF CONTENTS (continued)
Chapter
Page
3.1 FOURIER TRANSFORM INFRARED SPECTROSCOPY......................................... 30
3.2 MORPHOLOGIES OF FRACTURE SURFACE OF NANOCOMPOSITES ............ 32
4. ENERGY STORAGE ANALYSIS ....................................................................................... 34
4.1 CURRENT-VOLTAGE CHARACTERISTICS OF NANOCOMPOSITES .............. 34
4.2 HYSTERESIS BEHAVIORS OF NANOCOMPOSITES ............................................ 35
4.3 EFFECTS OF COMPOSITIONS OF NANOCOMPOSITES .................................... 38
4.3.1 Effects of Compositions on Polarization (Pmax) ....................................................... 38
4.3.2 Effect of Compositions on Energy Storage Capability ............................................ 39
4.3.3 Effects of Compositions on Energy Efficiency ....................................................... 40
4.4 EFFECTS OF TEMPERATURE ................................................................................... 41
4.4.1 Effects of Temperature on Polarization (Pmax) ......................................................... 41
4.4.2 Effects of Temperature on Energy Storage Capability ............................................ 42
4.4.3 Effects of Temperature on Energy Efficiency of GNPs .......................................... 44
4.5 DIELECTRIC PERMITTIVITY OF NANOCOMPOSITES ...................................... 45
5. CONCLUSIONS AND FUTURE PLANS.......................................................................... 48
REFERENCES………………………………………………………………………………………………………………. 51
vii
LIST OF FIGURES
Figure
Page
Figure 1 Molecular structure of polyethylene [2] ........................................................................... 1
Figure 2 Schematic molecular mtructures of different type of PE [4] ............................................ 2
Figure 3 A. Chain Folded structure in polymer B. Crystalline structure of PE [10] ..................... 4
Figure 4 Polarization in dielectric material due to electric field [26] ............................................. 8
Figure 5 Effect of crystallinity on the electric strength of HDPE [31] ........................................... 9
Figure 6 Structures of graphene and graphitic nanomaterials [36] ............................................... 11
Figure 7 Scanning electron microscopic image of an isolated graphite nanopaletelet ................. 14
Figure 8 Frequency dependence of dielectric permittivity of PVDF/GNP nanocomposites at
room temperature [75] .................................................................................................................. 17
Figure 9 Dielectric constant of HDPE-Gr nanocomposite as a function of frequency ................. 18
Figure 10 FTIR spectra of both ht-GNP and s-GNP..................................................................... 30
Figure 11 TEM images of (A) ht-GNP and (B) s-GNPs .............................................................. 31
Figure 12 SEM images of (A) UHMWPE/ht-GNP composite (B) UHMWPE/s-GNP composite
at x 5000 magnification; SEM images of (a) UHMWPE/ht-GNP composite and (b) UHMWPE/sGNP composite at x 60,000 magnification ................................................................................... 33
Figure 13 I-V characteristics of UHMWPE/GNP nanocomposites up to 3500 V ........................ 35
Figure 14 Illustration of monopolar hysteresis loop for energy storage analysis ......................... 36
Figure 15 Monopolar hysteresis loops at 100 Hz for UHMWPE and UHMWPE/s-GNP-3.0wt%
at room temperature ...................................................................................................................... 37
viii
LIST OF FIGURES (continued)
Figure
Page
Figure 16 Effects of concentrations on Pmax for both ht-GNP and s-GNP reinforced
nanocomposites at 1200 V ............................................................................................................ 38
Figure 17 Effects of compositions on energy storage capability of ht-GNP and s-GNP reinforced
nanocomposites samples at 3500 V .............................................................................................. 40
Figure 18 Effect of compositions on energy efficiency of ht-GNP and s-GNP reinforced
nanocomposites at 3500V ............................................................................................................. 41
Figure 19 Effect of temperature on polarization of for both ht-GNP and s-GNP reinforced
nanocomposites at 1200V4.4.2 Effects of Temperature on Energy Storage Capability .............. 42
Figure 20 Effect of temperature on polarization of for both ht-GNP and s-GNP reinforced
nanocomposites at 3500V ............................................................................................................. 43
Figure 21 Effect of temperature on polarization of for both ht-GNP and s-GNP reinforced
nanocomposites at 3500V ............................................................................................................. 44
Figure 22 Dielectric permittivity of UHMWPE and UHMWPE/GNP-3.0wt% nanocomposites at
1200 V at room temperature ......................................................................................................... 46
Figure 23 Dielectric permittivity of UHMWPE and UHMWPE/GNP-3.0wt% nanocomposites at
3500 V at room temperature ......................................................................................................... 46
ix
LIST OF EQUATIONS
Equation
Page
1. ε' ∞ (ɸc - ɸ)-s ..................................................................................................................................... 18
2. p = Qd
………………………………………………………………………………………………………………………….
22
3. D = εoE + P ..................................................................................................................................... 22
x
LIST OF TABLES
Table
Page
1. Summary of physical and mechanical properties of HDPE and UHMWPE [12] ...................... 5
2. Dielectric strength of tested polymers [20] ............................................................................... 10
xi
CHAPTER 1
INTRODUCTION
1.1 ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE (UHMWPE)
1.1.1. Polyethylene Materials
Polyethylene (PE) is one of the most common thermoplastic polymers in use today. PE has
been the largest volume synthetic polymer and has been used in different forms [1]. The
chemical structure of PE in Fig 1 shows it is made with carbon and hydrogen atoms in a long
chain [2]; which makes it susceptible to structure alteration to meet the needs and demands of
various applications. Some of the properties of PE include excellent wear resistance, good
fatigue resistance and outstanding chemical resistance. PE is easily distinguished from other
plastics because of its low mass density (~ 0.91~0.97 g/cm3) and good resistance to organic
solvents, electrolytic attack and degreasing agents. In general, its melting point ranges from 115135oC depending upon the molecular weight and branching structures [3].
Figure 1 Molecular structure of polyethylene [2]
1
Polyethylene is characterized on the basis of its density, structural variations (branching)
and molecular weight. In general, PE has been classified as follows: Low Density Polyethylene
(LDPE), Linear Low Density Polyethylene (LLDPE), Medium Density Polyethylene, High
Density Polyethylene (HDPE) and Ultra High Density Polyethylene (UHMWPE) as shown in
Fig 2 [4].
Figure 2 Schematic molecular structures of different type of PE [4]
High Density Polyethylene (HDPE), shown in Fig 2A, has a linear chain structure with
very few branches; thus, it is the densest PE structures giving it very high degree of crystallinity.
Density of HDPE falls within the range of 0.94-0.96 g/cm3 [4]. The extremely low number of
branches compared to other types of polyethylene and its high density makes HDPE the best
choice for heavy duty applications like radiation shielding, self-supporting containers, household
and kitchenware products, etc. HDPE is regarded to have higher strength to density ratio (i.e.
specific strength), making it more useable in packaging, pipe and petroleum tanks, prosthetic
devices (implants), milk and other food bottles, etc. [1, 3-6].
Medium Density Polyethylene (MDPE), as shown in Fig 2B, is the type of polyethylene
that has little more branching than HDPE, with lower mass density of 0.926-0.940 g/cm3. Due to
2
its lower density, MDPE is less ductile and has better stress crack resistance than HDPE [4].
Some applications for MDPE are sacks, gas and pipe fittings, screw closures and carrier bags.
Low-Density Polyethylene (LDPE) is translucent, semi-rigid material with very high
branching due to its less compact molecular structure which results in low density 0.910-0.925
g/cm3 [4]. LDPE qualities are low water absorption, flexibility, toughness and resistance to
chemicals, organic solvents at room temperature and to corrosion resistance. LDPE offers an
advantage of easy extrusion enhancing its manufacturability; hence LDPE are used for
chemically resistant fittings, laboratory equipment, gas and water pipes , insulation for wires and
cables , liquid packaging and high frequency electrical insulations [3]. However, the numerous
branching in LDPE as shown in Fig 2D, results in low density and low crystallinity, and causes
LDPE to have low strength, easy flammability, poor UV resistance and susceptible to
environmental stress cracking [3, 4, 6], which does somewhat limit its applications.
Linear Low Density Polyethylene (LLDPE) is a type of PE that has a large number of
shorter branches compared to LDPE [4]. Since, the branches of LLDPE are shorter, they can
easily slide on top of each other while elongating without being tangled as in LDPE. The general
schematic of LLDPE is shown in Fig 2C. The LLDPEs degree of branching complexity is lower
than that of LDPE and the chemical structure of LLDPE polymers is a compromise between
LDPE and HDPE, hence the name LLDPE. Short branches affect the crystallization often
increasing the density. The resulting density is higher than LDPE but lower than HDPE. The
density range of this polymer is approximately 0.91-0.94 g/cm3 [4, 7]. Some of the basic
applications for LLDPE are packaging, lower thickness sheets compared to LDPE, cable
covering and containers [1, 4, 6] .
3
In addition to various branching structures, PEs are known as semi-crystalline polymers
including both amorphous and crystalline regions within its crystal structures as shown in Fig. 3,
the variation of which bring changes to PEs properties [2, 5, 6, 8]. Crystallinity of PE is a very
important factor that governs its mechanical properties [8], increase in crystallinity increases
hardness, abrasion and wear resistance, modulus strength, creep resistance, barrier properties,
density and shrinkage [7, 9, 10]. However, low crystallinity provides economical melt
processing, better manufacturability and process ability, good transparency and excellent
thermoforming capability [3, 6, 10].
A
B
Figure 3 A. Chain Folded structure in polymer; B. Crystalline structure of PE [10]
1.1.2. Ultra High Molecular Weight Polyethylene (UHMWPE)
UHMWPE is a type of Polyethylene with extremely long chains, giving it higher
molecular weight often 1 to 6 million. The major difference between HDPE and UHMWPE is
the former has 700-1800 monomer units per molecule while the later has 100,000 to 250,000
4
monomer units per molecule [11]. This leads to highly entangled polymer chain structures,
which are responsible for many unique and superior properties in UHMWPE, compared with
conventional PE materials with lower molecular weight. A brief summary of mechanical and
physical properties of HDPE and UHMWPE has been shown in Table 1, where UHMWPE over
shadows HDPE with its high impact strength and ultimate strength. Based on the data in Table 1
[12], it can be noted UHMWPE has higher wear and abrasion resistance than HDPE. A study
conducted to determine the volumetric wear between HDPE and UHMWPE for multidirectional
hip simulator showed that HDPE wears 4.3 times faster than the UHMWPE [12].
Table 1. Summary of physical and mechanical properties of HDPE and UHMWPE [12]
Property
Molecular weight (106 g/mole)
Melting temperature (oC)
Poisson’s Ratio
Specific gravity
Tensile modulus of elasticity* (GPa)
Tensile yield strength* (MPa)
Tensile ultimate strength* (MPa)
Tensile ultimate elongation* (%)
Impact strength, lzod*
(J/m of notch: 3.175 mm thick specimen)
Degree of crystallinity (%)
HDPE
0.05 - 0.25
130 - 137
0.40
0.952 - 0.965
0.4 - 4.0
26-33
22 – 31
10 – 1200
21 – 214
60 – 80
UHMWPE
2-6
125 - 138
0.46
0.932 - 0.945
0.8 – 1.6
21 – 28
39 – 48
350 – 525
>1070
(No Break)
39 - 75
An important application of UHMWPE is in the form of fibers. Due to its low density
(0.97 g/cm3), UHMWPE fibers show excellent specific modulus and specific strength when
compared to other reinforcing fibers like Kevlar fiber and glass fiber [13]. Thus, the UHMWPE
fibers can be used as light weight reinforcing materials. Composite material made with
UHMWPE fibers will retain similar or higher mechanical strength without sacrificing overall
weight compared to higher mass of glass fibers, Kevlar and metal fibers [11-15]. This is why,
5
UHMWPE is one of the most demanded reinforcement material for polymer matrix composites
[13, 15].
The extremely high molecular weight of UHMWPE also account for its high melt
viscosity compared to HDPE and LDPE. Studies have shown that the increase in molecular
weight and its distribution increases its melt viscosity [6, 16] making it more suitable for
process-ability.
1.1.3 Electrical and dielectric properties of PE and UHMWPE
Polyethylene materials do not have free electrons; therefore they cannot conduct
electricity effectively. Due to the presence of long, essentially straight chains in polyethylene
made with methylene groups, where the carbon-carbon and carbon-hydrogen bonds do not show
high polar characteristic, PE exhibits a high degree of electrical symmetry, making it completely
nonpolar [6]. This excellent electrical property makes PE best suited for insulation of wires and
cables including high-frequency circuits in the field of electronics and telecommunication [3-6].
Riande [17] has discussed the electrical properties of various polymers from the
standpoint of their electrical insulating behaviors. According to his study, polyethylene- a
nonpolar plastic material - was characterized as having the most uniform dielectric properties
over all the frequencies and temperatures. Further study of the electrical properties of
polyethylene suggests PE does not have sole dependence on one factor, but on several, such as
crystallinity, molecular weight, branching and temperature [17-19].
In the recent years, polyethylene-based materials as electrical insulation in power cables
instead of traditional oil-paper based systems have been of keen interest. Because of the
increasing demands of High Voltage Direct Current (HVDC) transmission systems for power
6
grid connections, studies for better and more reliable electrical insulating materials have been
done [20-23]. The electric permittivity of a material is the measure of its electrical inertness to
the applied electric field. Even though HDPE have been used as cable insulation in power cables
and high voltage transmission system for its excellent resistance to abrasion and better dielectric
properties, its derivative UHMWPE that has extremely high average molecular weight, low
dielectric constant around 2.25, high temperature resistance, better abrasion resistance and
impact strength; UHMWPE has been a more desirable material for electrical insulation, high
voltage applications in the power grid and many other wire and cable coating applications [2325].
A dielectric material is a substance that does not conduct electricity efficiently but is an
efficient supporter of electrostatic fields that is: it can be polarized by applying electric field
where its components (atoms or molecules) shift from their original average equilibrium
positions and orient themselves towards and away from the field. This effect is called “dielectric
polarization”[26]. During dielectric polarization, the positive charges of dielectric material orient
towards the field while negative charges shift orient in the opposite direction. Polarization in a
dielectric material has been shown in Fig 4. Alignment of internal charges toward the plate
creates an internal electric field (i.e. smaller than the original Electric field E) reducing the
overall field the dielectric material has to experience. This affects material properties such as
energy storage, polarization and so on.
7
Figure 4 Polarization in dielectric material due to electric field [26]
Polyethylene has semi-crystalline structure which consists of crystalline regions and
amorphous regions. Several studies have shown the dependency of dielectric properties in PE
with its crystalline regions and the degree of crystallinity [27-29]. Hydrocarbon chains are in
order and fold back to form planar crystalline lamellae as shown in Fig 3. These studies also
show that the presence of isotropic disordered chain structures in amorphous region can cause
cross-linked chains and entanglements which can intrude the efficient transport of charges in
different regions of crystalline structure [30]. Research done by Miyanuchi and Yahagi [31]
show the dependency of dielectric strength on the degree of crystallinity was changed by varying
the annealing process on the same HDPE film. The results as shown in Fig 5 suggest that below
80oC, the electric strength increases with the increase in degree of crystallinity; meanwhile above
80oC the effect is reversed. This could be explained by Frohlich’s amorphous breakdown theory
which suggests that the increase in crystal boundaries due to decrease in crystallinity can help
trap electrons [31].
8
Figure 5 Effect of crystallinity on the electric strength of HDPE [31]
Studies have shown that molecular weight, the degree of polymer chain branching, and
temperature can play vital role in determining polymers dielectric properties [20-22, 27, 29-32].
Some studies have shown that the increase in molecular weight of polyethylene can increase the
electric strength (i.e. for an insulating material, the maximum electrical voltage it can withstand
without losing its insulating properties) and the volume resistivity of the polyethylene [20, 32].
M.E. Sepulveda, Martinez and Sanz-Feito [20] studied dielectric strength and space charge
accumulation of UHMWPE compared to other similar polymers like LDPE and XLPE. Their
results showed that the UHMWPE’s AC breakdown field was lower than that of XLPE and
LDPE and, since UHMWPE did not create, higher space charge accumulation (up to 80kV/mm),
linear increase of overall space charge profile was obtained. Based on these results shown in
Table 2, UHMWPE was considered to be one of the best performing polymer material for High
Voltage Direct current (HVDC) applications [24, 25, 33].
9
Holding electrical charges in dielectric material has been used for energy storage in
various electrical devices and electronic applications [24, 25]. With its nonpolar structure,
UHMWPE has low dielectric constant (2.25) which can prohibit its electrical application as
dielectric material that requires higher permittivity [6, 15]. Studies have shown that UHMWPE
when used in fiber form along with specific volume of conductive filler materials such as
nanoparticles, metal oxides and ceramics can remarkably increase dielectric permittivity and
mechanical strength [20, 34, 35].
Table 2. Dielectric strength of tested polymers [20]
Material
LDPE
UHMWPE
XLPE
α (kV)
10.48
5.365
28.36
β
4.452
1.046
6.363
Thickness
(mm)
0.154
0.145
0.443
Eb (kV/mm)
68.05
37.00
64.02
1.2 GRAPHITIC NANOMATERIALS
1.2.1 Graphene
Graphene is a single layer of sp2 carbon-carbon atoms showing a 2D hexagonal structure.
Graphene possesses extraordinary mechanical, thermal and electronic properties, and have been
widely accepted as one of the most promising materials structures for broad applications.
Graphene is considered as one of the strongest material with Young’s modulus of 1 TPa and
tensile stress of 130 GPa which is 100 times stronger than that of steel. It has been studied to
have theoretical specific surface area close to 2630 m 2/g and thermal conductivity of up to 5000
10
W m-1 K-1 [36]. Besides, these graphene is also considered to have very unique electronic
property; the monolayer graphene is a semi-conductor at zero distance apart; its linear energy
dispersion and the amount of charge it carries could be considered massless. These massless
charges travel at a speed of ~106 ms-1 [37] and is a unique behavior that has gained popularity in
the last decade for quantum electrodynamics [36-38].
Figure 6 Structures of graphene and graphitic nanomaterials [36]
Graphene is also considered as the fundamental structure for other graphitic
nanomaterials. Fig 6 shows graphene with monolayer of hexagonal structure followed by
different types of graphitic nanomaterial. Graphite, which naturally exists, consists of multiple
layers of graphene sheets in crumbly structures that are bonded via Van Der Waals force. By
reducing the number of graphene layers in graphite, the graphitic nanoplatelets could be
obtained. The graphene sheet can also be “wrapped up” to form 1-D nanostructures (1D), known
as Carbon Nanotubes (CNTs). Depending on the numbers of the wrapped up graphene sheets,
single-walled CNTs and multi-walled CNTs could be achieved. The 0-D soccer ball shaped
11
molecules known as Bucky Balls or Fullerene is also an important type of graphitic
nanomaterials showing unique properties and functions.
There have been two essential methods in synthesis of graphene:
i)
Top-down approach:
In this approach, heavy graphitic materials like 0-D Fullerenes, 1-D carbon nanotubes
and 3-D graphite are used as source of carbon for the production of graphene. The top-down
approach is one of the most popular method used in the oxidation of exfoliated graphite, through
the Staudemaier, Brodie and Hummers method [39]. Prepared graphite oxides are expanded into
either single layer or a few layered graphene by exfoliation or heat-treatment to form graphene
oxide nanosheets by ultrasonic treatment in aqueous solution [40]. Once the graphene oxides are
prepared, they are reduced to graphene by using several methods solvo-thermal reduction [41],
radiation-induced reduction, hydrogen plasma treatment [42], thermal annealing [43] and
electrochemical reduction [44]. Among all these methods, chemical reduction method using
reductants like metal Fe, hydrazine and alcohols is the most common and frequently used
method [45].
Even though, the exfoliation of graphite oxide and chemical reduction process seemed to
be more feasible in mass-production scale, functionalization and processing of graphene layers,
has its own withdraws. The resulted graphene has been reported to have defects in its framework
due to the formation of oxygen-containing groups, and while doing the ultrasonic treatment,
results in incomplete reduction that eventually deteriorates the electronic and mechanical
properties of reduced graphene oxide [40]. Some researchers have claimed to prepare the
graphene nanosheets by the same exfoliation method but without using chemical reducing agent,
Abdelsayed et al [46] did the synthesis in water without any reductants.
12
ii)
Bottom-up approach:
Contrary to top-down approach where graphene is obtained from larger forms of carbon
like graphite, bottom-up approach uses the synthesis of graphene in mass volume using small;
organic molecules excluding particles that exclusively have graphene. For instance,
decomposition of hydrocarbons into graphitic materials through chemical vapor decomposition
(CVD) using metal surfaces [40, 47]. Another method for synthesis of graphene is epitaxial
growth of graphene on SiC water surfaces. In this process, SiC is first decomposed then
desorption of Si is done [48]. Although this bottom-up approach results in graphene with fewer
defects as compared to that of top-down approach, its processing and operating costs are
relatively higher than the other approach. Thus, since both of these methods have their own pros
and cons, newer improvised methods are still to be approached to achieve better quality graphene
nanomaterials.
1.2.2 Graphite Nanoplatelets and Their Applications
Graphite Nanoplatelets (GNPs) are very thin nanoparticles with an average thickness of
3-10 nanometers and are usually comprised of small stacks of platelet-shaped graphene sheets
[49-51]. These nanoplatelets have diameter ranging up to 100 micrometers and are comprised of
the same material as in carbon nanotubes. Fig 7 shows the morphology of an isolated GNP
revealed by scanning electron microscope.
Therefore, graphite nanoplatelets have similar
properties as those of CNTs but in a plain form [51]. Due to their unique size and morphology
[51-57], GNPs have enhanced barrier properties and improved mechanical properties, such as:
surface hardness, strength and stiffness. GNPs also have excellent electrical and thermal
conductivity due to the presence of pure graphitic composition [51, 55, 58-61].
13
Figure 7 Scanning electron microscopic image of an isolated graphite nanoplatelets
The unique nanoscale size, shape, structure and material composition of GNP makes it
vital particle in enhancing almost all the barrier properties of the composite where it is used [36,
38, 57-60, 62-64]. GNPs can be used to increase electrical and thermal conductivity and reduce
component mass while maintaining integrity. It can also be used to increase stiffness and
toughness (impact strength) of other matrix materials. Some of the most common and potential
applications of GNP include:
i)
Ultracapacitor electrodes [52]
ii)
Anode materials for lithium-ion batteries and conductive additive for battery
electrodes [38]
iii)
Electrically conductive inks, thermally conductive films and coatings [40, 65]
iv)
Additives for lightweight and metal-matrix composites [40]
14
v)
Radiation and electromagnetic shielding, antistatic, shrinkage and corrosion resistant
coatings [66-68]
vi)
Barrier material for packaging and films or coatings for EMI shielding [28]
vii)
Substrate for chemical and biochemical sensors
Together with other types of graphitic nanomaterials, GNPs have been used to fabricate
high performance and multifunctional polymer nanocomposites. Although CNT’s have been
extensively studied as an ideal nanomaterial to improve the mechanical [61, 65], electrical and
thermal properties [36, 61, 62] of polymer materials, there are several advantages of using GNPs
instead of CNTs, besides the lower cost of GNPs. They are:
a) GNPs can be dispersed more easily than CNTs, which results in even distribution of
reinforcement throughout the polymer composite [49, 51, 54, 65].
b) GNPs have larger surface area compared to CNTs, so a greater area can be covered using
GNPs, enhancing the bonding property where interface can be achieved. This accounts
for homogeneous distribution of load and escalation in strength of the composite [63, 65].
To achieve similar results, a higher aspect ratio of CNT would be required than graphene,
which negatively affects the mechanical property by increasing the volume concentration
of the nanofiller.
c) One of the greatest advantages also results from the 2D graphene layer in GNPs: better
lubrication during any frictional movement causing less wear, debris generation and
longer life-time of the material [11, 62, 63, 65].
15
1.3 UHMWPE NANOCOMPOSITES
1.3.1 Polymer Nanocomposites
Polymeric materials are inexorably limited in applications due to their intrinsic
properties, such as their characteristic high electrical resistivity, low thermal conductivity,
stability and ductile nature as mechanical properties [69-72]. Despite these properties, polymers
have some impressive characteristics such as; they are cheap, easy to process and have higher
strength to weight ratios. If nanometric additives are used in polymers, it improves several
negatives in its properties and increases it functionality and manufacturability [72].
In addition to achieving better strength, modulus and toughness in a polymer
nanocomposite, it could have excellent electrical and thermal properties to have variety of
applications. For example, the graphitic nanomaterials have been frequently used to fabricate
polymer
nanocomposites
with
enhanced
electrical
properties.
Meanwhile,
polymer
nanocomposites have behaved better with thermal properties when used as substrate and
packaging material in miniaturization and advancement of electronic industry. With proper
control of the network of these nanomaterials in these polymer nanocomposites, they could
withstand higher temperature and dissipate thermal energy efficiently without breakage [15, 58,
73, 74].
Regarding dielectric properties of polymer nanocomposites, according to MaxwellWagner-Sillars polarization (MWS), higher dielectric constant can be achieved by introducing
conductive fillers in to the polymer matrix through interfacial polarization phenomenon. In this
study, since GNPs have relatively higher surface area as compared to any other filler materials,
these have more interfacial area that can hold larger amount of charges at its interfaces. So, in
16
general higher dielectric constant can be expected for GNP-filled polymer nanocomposites. In
the research done by Fuan He [75], one of the highest dielectric constant has been reported for
poly (vinylidene fluoride) with GNP nanocomposites (PVDF/GNP). This nanocomposite was
prepared by solution-cast and compression molding method, and recorded dielectric constant was
108 at relatively lower filler loading % (@ 2.34 vol.%) at high frequencies around 1000 Hz and
higher. The result from this research has been shown in Fig 8.
Figure 8 Frequency dependence of dielectric permittivity of PVDF/GNP nanocomposites at
room temperature [75]
In addition, the dielectric constant of a material usually follows percolation threshold
behavior and can be predicted the power law as shown in Eqn. 1 [76]. Percolation threshold is
the fraction of the filler content at which the inclusions form a continuous path instead of electric
resistivity. This kind of continuous network (path) often renders the composite an electrical
conduction, resulting dielectric breakdown [28].
17
ε' ∞ (ɸc - ɸ)-s
Equation 1
where ɸc is the critical filler volume fraction at which percolation in ε' takes place and is called
percolation threshold; s is the critical exponent of ε'.
A study was done with high density polyethylene-graphite (HDPE-Gr) nanocomposites to
determine the variation of dielectric constant as the function of filler loading wt.%. The result as
shown in Fig 9 recommended that when the filler concentration is below 0.039 wt.%, dielectric
property is less dependent on frequency and showed minimal ε'.
Figure 9 Dielectric constant of HDPE-Gr nanocomposite as a function of frequency
This pioneering idea of using nanometric additives into polymers to make composites
have also opened multiple ways in advanced technologies including redox capacitors, catalysis,
electrochemical displays, printed circuit boards and sensors [72, 77]. Besides, graphene-polymer
based nanocomposites could also be used as high temperature conducting materials such as
18
bipolar plates for polymer electrolyte membrane fuel cells and high temperature conductive
adhesives [64, 72, 78, 79]. According to research done by Debelak and Lafdi [72], carbon
(graphene) based nanofibers have been anticipated to be used into polymers instead of nanotubes
in many applications due to its cheaper price and easier processing techniques.
1.3.2 UHMWPE/Graphitic Nanomaterial Nanocomposites
Since, UHMWPE is a semi crystalline polymer, and its usage in different applications
demanded better strength and toughness for better functionality. Delgado and Addiego [80],
developed a new processing method to homogeneously disperse graphite nanoplatelets (GNPs)
into the UHMWPE matrix. They characterized the prepared nanocomposite to study its
microstructural aspects and toughness to make it more suitable for application in electronic
devices and equipment. Their results suggest that, when GNPs are blended with UHMWPE to
make Nanocomposites, the in situ UHMWPE toughness and impact energy had increased from
17.5 J/cm2 to 19.5 J/cm2. Thus, Delgado and Addiego proved the addition of graphene
nanoplatelets (GNPs) increases the strength and toughness of UHMWPE.
Mujibur Khan and Hassan Mahfuz [34] also conducted a research in reinforcing
UHMWPE filaments with 2.wt% of Single-walled carbon nanotube (SWCNTs). According to
their results, when the modified UHMWPE-SWCNT Nanocomposites was tested with multiple
cycles of loading and unloading, it’s Ultimate Tensile Strength and Young’s modulus had
increased by 120 and 463% respectively. In short, the yield strength (0.2%) had increased by
500% with the addition of carbon based nanomaterials to polymer. This suggests that UHMWPE
fiber’s mechanical strength and modulus can be increased drastically by using graphene
nanoplatelets in controlled manner.
19
A Bhattacharyya and S. Chen [81] tried to disperse graphene oxide or graphene in
UHMWPE using two different methods to prepare unique nanocomposite films. The first
method, being the pre-reduction method, graphite oxide (GO) was exfoliated and dispersed into
organic solvents and then reduced to graphene before the addition of UHMWPE. In the second
method, the reduction of graphene oxide was done after the UHMWPE was added to conduct
reduction method. The results were surprising because the nanocomposite that was prepared by
the pre-reduction method had higher crystallinity than the in situ method (still comparable to that
of pure film). Both of these processes increased the modulus from 864 to 1236 MPa, increased
the strength from 12.6 to 22.2 MPa and significantly reduced creep resistance from 50% to 9%
when compared to pure film. The results suggest that the pre-reduction method definitely
increases the tensile strength and creep resistance of the polymer nanocomposite while in the situ
method made the nanocomposite material more ductile thus making suitable for other application
where ductility is more important.
1.4 RESEARCH OBJECTIVES AND RATIONALE
This
research
aims
at
development
of
UHMWPE/graphitic
nanomaterials
nanocomposites for high intensity energy storage applications via understanding the roles of
surface modification of nanomaterials as well as the dispersion of nanomaterials in
nanocomposites for energy storage applications. In particular, in this study, the dielectric
properties at high electrical field will be investigated.
There have been thousands of experimental studies using various nanomaterials to
achieve high performance polymer nanocomposites with increased mechanical, electrical and
thermal properties, etc. However, these studies barely reproduce the success of theoretical
20
predictions of polymer nanocomposites. Among many factors, dispersion of nanomaterials as
well as the interface between nanomaterials and polymer matrix is widely considered as the two
most challenging structural factors showing remarkable effects on the properties of polymer
nanocomposites. The high specific surface areas of nanomaterials lead to their high surface
energy . To minimize the high surface energy, nanomaterials tend to aggregate and form large
agglomerates. These agglomerates typically act as the structural defects showing negative
impacts on the performances of the polymer nanocomposites. Thus, how to individualize the
nanomaterial particles and achieve uniform dispersion remains the fundamental issue in polymer
nanocomposites. Currently melt compounding, ultrasonication and surface modification of
nanomaterials are most commonly used and most effective approaches to achieve uniform
dispersion. To Meanwhile, the interfacial structures also play a significant role in polymer
nanocomposites. An appropriate interface could not only benefit mechanical properties by
effectively enhancing stress transfer, but also benefit various physical properties, such as
dielectric properties, in which, the interfacial polarization is essential. Usually, to improve
effective interfacial interactions, surfactants, which could interact/react with both nanomaterials
and polymers, are applied in polymer nanocomposites.
PE materials have been commercially used in energy storage devices, and their dielectric
properties have been frequently studied. However, for UHMWPE, in spite of its extraordinary
mechanical and thermal properties, its applications in electronics and energy storage devices are
still very limited. Thus, UHMWPE was studied as the matrix material for the composites.
Meanwhile, the low cost GNPs were chosen to modify the dielectric properties of the
nanocomposites. Besides the excellent mechanical and physical properties, similar to CNTs, the
commercial GNP products also possess reactive groups on the particle surface, because of the
21
oxidation process in GNP manufacturing. These reactive groups, such as hydroxyl and carbonyl
groups, could be used to react with surfactants, leading to surface modified GNPs. Thus, the
improved dispersion of GNPs and stronger interface would be expected.
As introduced in 1.3.1, in the presence of electric field, the positive and negative charges
within the dielectrics are displaced from equilibrium position and each of them faces towards the
edges of the capacitor plate. This displacement of charges creates a local electric dipole within
the dielectric material; this process is called polarization of dielectrics. Since, the applied electric
field still exists, it distorts the aligned charges; this phenomenon is explained by the principle of
superposition. According to this principle, the distorted charge distribution is equivalent to the
original distribution plus a dipole thus created whose moment is given by Eq. 2 [82].
p = Qd
Equation 2
where d is the distance vector from –Q to +Q of the dipole and p is the intensity of polarization.
For the dielectric material, the electric flux density D related to its polarization is given by Eqn 4.
D = εoE + P
Equation 3
where D is the electric flux Density, εo is the permittivity of free space (= 8.854 x 10 -12) and E is
the applied electric field.
Thus, it is obvious that the dielectric properties and energy storage capability are directly
related to the eternal electrical field. The dielectric relaxation behaviors at low electrical field
have been extensively studied. However, the understanding on the dielectric relaxation of
polymers and polymer composites is still very limited [83]. In particular, for the high density
electric charge energy storage applications, the dielectric property at high electric field is of great
importance.
In this study, the UHMWPE/GNP nanocomposites were fabricated with and without
surfactant. The silane agent was used as the surfactant to modify the surface properties of the
22
GNPs. The use of this surfactant was not only intended to improve the dispersion quality of
GNPs in the UHMWPE matrix, but also to improve the interfacial interactions between GNPs
and UHMWPE. The nanocomposites with different loading of GNPs were fabricated using melt
compounding method to study the effect of the compositions. At last, since the operation of
energy storage devices and electronic frequently deals with elevated temperatures, thus the effect
of the temperature on the high field dielectric properties and energy storage capabilities were
also investigated.
23
CHAPTER 2
FABRICATION AND CHARACTERIZATIONS OF UHMWPE/GNP
NANOCOMPOSITES
2.1 MATERIALS
The matrix polymer used in this work is the commercial grade UHMWPE powder (GUR,
1020 O) purchased from TICONA. The GNPs with an average thickness of 6nm and an average
diameter of 5μm were supplied by XG Science. Octadecyltrimethoxysilane (ODMS) used as
surfactant was purchased from Sigma Aldrich. The 95% ethanol purchased from Sigma Aldrich
will be used for the surface modification of GNPs.
2.2 MODIFICATION OF GNPs
2.2.1 Heat Treatment
The GNP received from the manufacturer (TICONA) was in powder form. These GNP
materials still have some acid molecules within the layers of GNP. If these acid molecules are
left as is, they could affect the further processing of GNP nanocomposite and its properties. The
received GNP was thermally treated at 800 oC in air for 30 seconds to decompose and eliminate
the acidic residuals from within the layers of GNP. Afterwards, the GNP was allowed to cool at
room temperature before further processing. The heat treated GNP is designated (ht-GNP) and
was used to make nanocomposites with different compositions. Due to the expansion of the
gaseous product of the acidic residuals, the ht-GNPs were further exfoliated.
24
2.2.2 Surface Modification of ht-GNPs
The ht-GNPs obtained in 2.2.1, were also used for preparation of surface modification
GNPs (s-GNP). The chemical modification of ht-GNP has been done using alkoxysilane
coupling agents; that is, ODMS, with two major advantages:

at one end of the silane structure, they bear alkyl groups that are capable of reacting with
non-polar UHMWPE matrix, due to their same chemical structures

at the other hand, they have functional groups which can establish covalent bonding with
the reactive groups on the surface of GNP particles, thus leading to enhanced interfacial
interactions [84].
Scheme 1 Surface modified GNPs (s-GNP)
In this study, the ht-GNPs were chemically treated with ODMS in boiling 95% ethanol
for 5 hours. The ratio between ht-GNPs and silane agent was fixed 1 gram/4 ml. During this
time, the constant turbulence of boiling ethanol ensured even mixture to promote chemical. The
ht-GNPs/ethanol suspension was obtained by low intensity sonication in water bath (Branson
25
2510). The suspension was then heated to allow ethanol to reflux. Liquid ODMS was slowly
added to the boiling ht-GNP/ethanol suspension. The reaction was left to reflux for 5 hours.
After cooling to room temperature, the mixture was filtered, and the solutes were thoroughly
rinsed using both ethanol and acetone to remove any unreacted ODMS, followed by vacuum
drying overnight. The dried black power was designated as s-GNP for fabrication of
nanocomposites. The schematic illustration of the s-GNP is given in Scheme 1.
2.2.3 Fabrication of UHMWPE/GNP Nanocomposites
Both ht-GNPs and s-GNPs were used to fabricate nanocomposites with UHMWPE. The
concentrations of GNPs in the nanocomposites are 0.5wt%, 1.0wt% and 3.0wt % respectively.
The UHMWPE power was melted at 180 OC in a Brabender compounder, and then different
amount of GNPs were added to the compounder with a rotation speed of 70 rpm. After mixing
for 15 mins, the UHMWPE/GNP nanocomposites were obtained. The nanocomposites were then
compression molded at 180oC to thin sheets with an average thickness of ~300μm using a Carver
hydraulic hot press. It should be noted here, although thin films around 10-20 μm thick are more
ideal for high field study, the thin sheets were prepared for characterization, because of the high
viscosity of UHMWPE and its nanocomposites. The high viscosity does not favor the fabrication
of thin film samples without providing very high pressure. The specimens with a diameter 1 cm
were cut off the thin sheet for characterization of high field dielectric properties.
26
2.3 CHARACTERIZATION of MODIFIED GNPs
2.3.1 Fourier Transform Infrared Spectroscopy (FTIR)
FTIR is a powerful tool to characterize organic chemical structures. Thus, it was applied
to examine if the surface modification in 2.2 was successful. The FTIR analysis was conducted
on a Nexus 670 ESP analyzer. To conduct FTIR analysis, 0.2 mg of GNPs were manually
ground and mixed with 1 g potassium bromide (KBr) as diluent and background. After the
uniform fine power of the mixture of GNPs and KBr were obtained, a hydraulic press was used
to press the power into a disk sample for FTIR analysis.
2.3.2 Transmission Electron Microscopy (TEM)
TEM has high resolution and magnification, and it has been frequently used to
characterize the structures and morphologies of nanomaterials. The GNPs were studied using
Transmission Electron Microscopy (TEM, JOEL-EX). To prepare the GNP samples for TEM
study, very small amount of GNPs was dispersed in deionized water via low intensity sonication
in water bath for 15mins. The resulting suspension of GNPs in deionized water was deposited
onto the carbon-coated copper grids by drop-casting, followed by vacuum drying before TEM
observation.
2.4 CHARACTERIZATIONS of NANOCOMPOSITES
2.4.1 Scanning Electron Microscopy (SEM)
The dispersion states of GNPs with and without surface modification in the polymer
matrix, as well as the interfacial structures were examined using a scanning electron microscopy
27
(SEM, FEI 200). The nanocomposite thin sheets were immersed in liquid nitrogen for 15 mins
before fractured. The thin sheet was completely frozen and became more brittle, thus, during
fracture; the morphologies of the nanocomposite would be reserved. The fracture surface of the
nanocomposites was sputter coated with gold before SEM observations.
2.4.1 Energy Storage Analysis
For the measurement of energy storage capability, Precision LC II Ferroelectric Tester
made by Radiant Technologies Inc. was used. This tester came with Vision Software that could
calculate different ferroelectric properties of samples like current-voltage characteristics and
ferroelectric hysteresis, which are the interest of this study. The samples were tested in the
following steps:
I.
The samples were cut into smaller rectangular pieces and cleaned using Acetone
to remove any dirt, oil or debris off the samples.
II.
The thickness of the sample pieces were measured using ABSOLUTE Digimatic
6” Digital Caliper manufactured by Mitutoyo (Model # 500-196-20).
III.
Each piece of sample was then placed one by one in the testing electrode chamber
which has two copper electrodes connected to the tester. The sample material was
placed in between these electrodes and sealed inside the chamber.
IV.
Then, Precision LC II tester was turned on along with Precision 10kV HVI-SC
High Voltage Amplifier (made by Radiant Technologies Inc., model no# PHVi210KSC). The testing frequency was 100 Hz. Besides room temperature (23
o
C), the characterization was also conducted at 80oC. To conduct high
temperature testing, the sample along with the electrode chamber was placed
28
inside the temperature chamber (made by Delta Design, model no # 9023).10
mins after the chamber temperature reached desired temperatures (80oC), Steps V
& VI was conducted.
29
CHAPTER 3
STRUCTURES AND MORPHOLOGIES
3.1 FOURIER TRANSFORM INFRARED SPECTROSCOPY
As a result of the surface modfication, the Si-O-C (GNP) covalent bond would be
expected to exist in the s-GNP, due to reactions between silane agents and hydroxyl and carbonyl
groups on the surface of ht-GNPs. The FTIR spectrum for both ht-GNPs and s-GNP have been
shown in Fig 10. The broad peak aroung 3400 cm-1 was assigned to the hybroxyl groups on the
sruface of GNPs, suggesting that the reactive surface of ht-GNP for furmodification.
Figure 10 FTIR spectra of both ht-GNP and s-GNP
The absorption peak at 925 cm-1 in s-GNP suggests the successful surface modification of
ht-GNP by ODMS. Besides, Si-O-GNP there are few other composition from the result of
30
reaction with coupling agent such as Si-O-Si and Si-O-CH3, and the existence of these chemical
structures do not have substantial impacts on the improvement of interfaces.
ht-GNP
s-GNP
Figure 11 TEM images of (A) ht-GNP and (B) s-GNPs
31
The TEM morphologies of both ht-GNP and s-GNP in Fig.11 further proved the success
of surface modification. It can be seen in the Fig. 11A that the ht-GNPs are clearer and more
transparent compared to s-GNPs. This is because, due to the successful surface modification, the
surface of GNPs was coated by a layer of ODMS. Meanwhile, since the ODMS is covalently
bonded to the surface of GNPs, the filtration, rinse as well as sonication could not remove this
coating layer from the GNP surfaces.
3.2 MORPHOLOGIES OF FRACTURE SURFACE OF NANOCOMPOSITES
The successful surface modifications could also be proved by the morphologies of the
fracture surfaces of the nanocomposites, as shown in Fig 12. In UHMWPE/ht-GNP
nanocomposites (Fig. 12A), few agglomerates (encircled regions in image) were found which
suggested the dispersion of ht-GNP in UHMWPE still needed further improvement, whereas the
dispersion of s-GNP (Fig. 12B) showed more uniform dispersion when viewed at x5,000
magnification. It is easy to understand that for ht-GNPs, due to the incompatibility between htGNP and UHMWPE, as well as the very high melt viscosity, the dispersion of ht-GNPs were
more difficult. When the images were magnified and viewed at x60,000 magnification (Figs 12a
and 12b), the detail interfacial interactions for both nanocomposites were seen. Obviously, sGNPs led to good interfacial interaction without gaps as observed in UHMWPE/s-GNP
compared to UHMWPE/ht-GNP. With proper surface modification by ODMS, the s-GNP could
effectively interact with UHMWPE matrix. Thus, despite high viscosity, this interaction could
favor the individualization of GNP agglomerates, leading to an enhanced dispersion. This
interaction also created stronger interfacial bonding between s-GNPs and the matrix.
32
UHMWPE/ht-GNP
UHMWPE/s-GNP
Figure 12 SEM images of (A) UHMWPE/ht-GNP composite (B) UHMWPE/s-GNP
composite at x 5000 magnification; SEM images of (a) UHMWPE/ht-GNP composite and
(b) UHMWPE/s-GNP composite at x 60,000 magnification
33
CHAPTER 4
ENERGY STORAGE ANALYSIS
4.1 CURRENT-VOLTAGE CHARACTERISTICS OF NANOCOMPOSITES
The current-voltage characteristics (I-V curves, Fig. 13) of samples were studied between
0 volt and 3500 volts. The reason for studying I-V characteristics of different nanocomposites is
to determine the testing voltage/electrical of hysteresis study. It is known that high current
leakage is not favorable in energy storage applications, in particular, at high electric field. In this
study, the conductive GNPs were used as the nanofiller to modify the properties of UHMWPE,
which might lead to high current leakage, particularly, when the loading level of GNPs is high.
Except UHMWPE-ht-GNP-3wt% nanocomposites, all other nanocomposites showed the similar
I-V characteristic to pure UHMWPE, suggesting their insulating nature and great potential for
high density energy storage application. In contrast, the sudden increase of current was found in
UHMWPE-htGNP-3wt% nanocomposites starting at 500V. This should be a result of
agglomerates and poor interfacial interactions in the nanocomposites observed in the SEM
images (Fig. 12). These defects usually lead to large current leakage and consequently reduce
their electrical breakdown strengths. The nanocomposites would be shorted at a low voltage, and
they are not ideal for high field energy storage applications. In other word, the I-V studies also
revealed the positive contribution of surface modification and resulting uniform dispersion of sGNPs to the dielectric properties. Thus, the analysis at 3500 V in later sections would not include
UHMWPE-ht-GNP-3wt% nanocomposites.
34
I-V CURVE
UHMWPE
UHMWPE-htGNP-0.5wt%
UHMWPE-htGNP-1wt%
UHMWPE-htGNP-3wt%
CURRENT (I) (AMP)
1.0E-01
UHMWPE-sGNP-0.5wt%
UHMWPE-sGNP-1wt%
UHMWPE-sGNP-3wt%
1.0E-03
1.0E-05
1.0E-07
1.0E-09
1.0E-11
0
500
1000
1500
2000
2500
VOLTAGE (V) (VOLTS)
3000
3500
4000
Figure 13 I-V characteristics of UHMWPE/GNP nanocomposites up to 3500 V
4.2 HYSTERESIS BEHAVIORS OF NANOCOMPOSITES
Hysteresis behaviors of both UHMWPE and its nanocomposites were recorded at 1200 V
and 3500V, respectively. The energy storage capability was examined using the Monopolar
hysteresis loop obtained from Vision Software. The high field dielectric properties can be studied
using hysteresis loop. In this study, four parameters are our research interests:
1) Pmax----Polarization at highest electric field/voltage
2) Stored Energy---- Area under (A+B) as shown in Fig 14
3) Energy Efficiency---- A/(A+B) as shown in Fig 14
4) Dielectric permittivity ---- Derivative of discharging curve at high electric field/voltage
In which,
A = Energy Recovered Integration Area
35
B = Energy Lost Integration Area
A+B = Energy Stored Integration Area
Energy Efficiency = A/ (A+B) = Energy Recovered / (Energy Recovered + Energy Lost)
Figure 14 Illustration of monopolar hysteresis loop for energy storage analysis
For an ideal energy storage application, high values for A and A+B, small value for B
would be anticipated.
In Fig. 15, the representative monopolar hysteresis loops of UHMWPE and UHMWPE/sGNP-3.0wt% nanocomposites are given. Both pure polymer and nanocomposites showed near
linear behaviors, which mean low energy loss, suggesting good potential for energy storage
applications. Since the hysteresis loops of other nanocomposites at both testing temperatures are
much alike, and they are not shown here. The hysteresis loops were analyzed using four
parameters introduced above.
36
0.175
0.150
P o l a ri z a ti o n (µ C /c m 2 )
0.125
0.100
0.075
0.050
0.025
-0.000
0
10
20
30
40
50
Field (kV/cm)
60
70
80
UHMWPE
0.225
0.200
P o l a ri z a ti o n (µ C /c m 2 )
0.175
0.150
0.125
0.100
0.075
0.050
0.025
0.000
0
25
50
75
Field (kV/cm)
100
125
UHMWPE/s-GNP-3.0wt%
Figure 15 Monopolar hysteresis loops at 100 Hz for UHMWPE and UHMWPE/s-GNP3.0wt% at room temperature
37
4.3 EFFECTS OF COMPOSITIONS OF NANOCOMPOSITES
4.3.1 Effects of Compositions on Polarization (Pmax)
The properties of polymer nanocomposites always show distinct dependence on the
compositions of nanocomposites, thus, it is critical to understand the effects of compositions on
energy storage capabilities of resulting nanocomposites. In this study, the composition include
both concentration of GNPs as well as the surface modification. Dielectric polarization is the
fundamental mechanism, and a good energy storage capability requires high polarization of
materials. The comparison chart of Pmax at 1200 V has been shown below in Fig 16.
ht-GNP
s-GNP
0.07
0.06
0.06
0.05
0.05
Pmax (µC/cm2)
Pmax (µC/cm2)
0.07
0.04
0.04
0.03
0.03
0.02
0.02
0.01
0.01
0.00
UHMWPE
0.5
1
0.00
3
UHMWPE
GNP Loading wt.%
0.5
1
GNP loading wt.%
3
Figure 16 Effects of concentrations on Pmax for both ht-GNP and s-GNP reinforced
nanocomposites at 1200 V
According to the I-V curves (Fig.14), it was found that the UHMWPE/ht-GNP-3.0wt%
nanocomposite has the highest current leakage at the lowest testing voltage (500V). This
suggests that the high loading of ht-GNP might not be suitable for the energy storage
38
applications. The fact is the UHMWPE/ht-GNP-3.0wt% nanocomposite thin sheet was shorted
when the testing voltage was up to 3500V. Thus, to understand the effects of compositions, a
lower testing voltage (1200 V) was applied to the nanocomposite samples. It is obvious that the
lowest Pmax was also found in UHMWPE/ht-GNP-3.0wt% nanocomposites. This result further
suggests the non-uniform dispersion of ht-GNPs in the polymer matrix and poor interfacial
interaction could sacrifice the polarization phenomenon.
The addition of ht-GNP slightly lowered the polarization, however, the surface
modification via ODMS seemed to be able to amend the polarization. In particular, the P max for
UHMWPE/s-GNP-1.0wt% nanocomposite is even slightly higher than the pure polymer,
suggesting the contributions of uniform dispersion and stronger interfacial interactions to the
polarization in the nancomposite.
4.3.2 Effect of Compositions on Energy Storage Capability
Based on the discussion in 4.1 and 4.3.1, we could conclude that with addition of 3.0wt%
ht-GNPs, the nanocomposite has the lowest energy storage capability, thus, the further
discussion on the energy storage capability will exclude this nanocomposite, and the energy
storage capability will be discussed at a higher voltage (3500 V), at which the UHMWPE/htGNP nanocomposite was shorted.
Obviously, the surface modification once again showed positive contributions to the
energy storage capabilities of the nanocomposites, due to the enhanced dispersion as well as
stronger interface. In particular, the UHMWPE/s-GNP-3.0wt% nanocomposite showed the
highest energy storage capability, in contrast to the UHMWPE/ht-GNP-3.0wt% nanocomposite
which was easily shorted at this voltage level. Meanwhile, it could be observed that for the ht-
39
GNP reinforced nanocomposite, a larger deviation among different testing samples is obvious.
This might also prove that without proper surface modification, the quality of UHMWPE/htGNP nanocomposites was not very consistent, due to the existence of agglomerates as well as the
poor interfaces.
ht-GNP
s-GNP
400
350
Energy Storage (Charging Area)
Energy Storage (charging Area)
400
300
250
200
150
100
50
0
350
300
250
200
150
100
50
0
UHMWPE
0.5
1
GNP loading wt.%
3
UHMWPE
0.5
1
GNP loading wt.%
3
Figure 17 Effects of compositions on energy storage capability of ht-GNP and s-GNP
reinforced nanocomposites samples at 3500 V
4.3.3 Effects of Compositions on Energy Efficiency
In high efficiency energy applications, the energy efficiency is as important as energy
storage. High energy efficiency indicates more energy could be released during discharging
process, leading to low energy loss. For pure UHMWPE, it shows a high energy efficiency
around 90%. The addition of both ht-GNP and s-GNP could further improve the energy
efficiency close to 100%. It seems that the dispersion quality as well as interface between GNPs
and UHMWPE did not exert greater effects to the energy release of the nanocomposites during
40
discharging processing. However, combined the results on both energy storage and energy
efficiency, the s-GNPs proved to a more effective additive to improve comprehensive energy
storage properties at room temperature.
1.20
1.10
1.10
1.00
1.00
Energy Efficiency
Energy Efficiency
s-GNP
ht-GNP
1.20
0.90
0.80
0.70
0.60
0.90
0.80
0.70
0.60
0.50
0.50
0.40
0.40
UHMWPE
0.5
1
GNP loading wt.%
UHMWPE
3
0.5
1
3
GNP loading wt.%
Figure 18 Effect of compositions on energy efficiency of ht-GNP and s-GNP reinforced
nanocomposites at 3500V
4.4 EFFECTS OF TEMPERATURE
4.4.1 Effects of Temperature on Polarization (Pmax)
The operation of energy storage devices and electronic devices frequently faces the
elevated temperature, thus it is also important to understand the energy storage performances at
elevated temperature. In this study, since PE materials typically start to slowly melt around 100
°C, thus the high temperature performance was evaluated at 80 °C. Effects of temperature on
41
polarization has been shown in Fig 19. Since 1.0wt% nanocomposites showed the highest
polarization at room temperature, thus, only 1.0wt% nanocomposites are discussed here.
Effects of Temperature on Pmax
0.08
23 °C
0.07
80 °C
Pmax (µC/cm2)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
UHMWPE
ht-GNP-1wt.%
s-GNP-1wt.%
Figure 19 Effect of temperature on polarization of for both ht-GNP and sGNP reinforced nanocomposites at 1200V4.4.2 Effects of Temperature on
Energy Storage Capability
Obviously, the increase of the temperature leads to weakened polarization in pure
UHMWPE, probably due to the thermal motions of the polymer structures at elevated
temperature. It is worthy to note that the addition of ht-GNPs and s-GNPs could enhance the
polarization at higher temperature, in contrast to pure UHMWPE material. This should be
resulted from the hindered thermal motions because of the interactions between GNPs and
polymer matrix as well enhanced interfacial polarization mechanisms. Furthermore, the
UHMWPE/s-GNP-1.0wt% showed the highest polarization, which is 50% higher than pure
42
UHMWPE at 80 °C.
Once again, this suggests the positivity of using ODMS to improve the
dispersion and interfacial interaction for energy storage applications.
At higher electrical field, the higher field strength seem to be able to conquer the random
thermal motion, and led to the higher energy storage capability. Similar increased energy storage
capability was also found in the nanocomposites. However, the increase was not as significant as
that in pure UHMWPE materials. The different between 1200 V and 3500 V suggests the effects
of the testing voltages on the energy storage capability. Obviously, the effects of surface
modification on the temperature dependence of energy storage were more significant at 1200V.
The simultaneous high voltage (3500 V) and high temperature might have new mechanisms on
the energy storage performances of the nanocomposites. Since the understanding of dielectric
relaxation at higher field is still very limited, future studies will be needed to gain better insights.
600
Effect of Temperature on Energy Storage
23 °C
80°C
Energy Storage
500
400
300
200
100
0
UHMWPE
ht-GNP-1wt%
s-GNP-1wt%
Figure 20 Effect of temperature on polarization of for both ht-GNP and s-GNP reinforced
nanocomposites at 3500V
43
4.4.3 Effects of Temperature on Energy Efficiency of GNPs
Effect of Temperature on energy efficiency was studied (Fig 21), which suggests increase
in energy efficiency for both UHMWPE and nanocomposites when the temperature increased to
80 oC. Similar to the results of polarization and energy storage, the efficiency further increased to
close to 1, due to enhanced thermal motion of dielectric dipoles. This linear charging-discharging
behavior of UHMWPE and its nanocomposites makes them ideal energy storage materials at
high temperature, showing very low energy loss.
According to the results on the effects of temperature, it is obvious that the increase of
temperature have positive impacts on the energy storage capability of UHMWPE and its GNP
nanocomposites. However, at higher temperature, the effects of surface modification of GNPs on
energy storage were not obvious at higher electrical field.
Effects of Temperature vs Energy Efficiency
23 °C
Energy Efficiency
1.20
80°C
1.00
0.80
0.60
0.40
UHMWPE
ht-GNP-1wt%
s-GNP-1wt%
Figure 21 Effect of temperature on polarization of for both ht-GNP and s-GNP reinforced
nanocomposites at 3500V
44
4.5 DIELECTRIC PERMITTIVITY OF NANOCOMPOSITES
Due to the difficulty in dipole switching at high frequency at high electrical field, the
hysteresis testing is a more commonly used approach to analyze the dielectric relaxation
phenomenon at high fields, instead of conventional dielectric spectroscopy. Dielectric
permittivity has been frequently used to analyze the low field (< 5 V) dielectric relaxation
properties. In this research, although our primary focus was on the hysteresis behaviors of the
nanocomposites, the high field dielectric permittivity of the nanocomposites was investigated at
frequency range no higher than 1000 Hz. The high field dielectric permittivity was determined
by derivatives of discharging curve on P-E hysteresis loops. As can be seen in Fig 22, for both
UHMWPE and nanoocmposites, as the frequency increases there is an increase in the dielectric
permittivity at 1200V up until 100 Hz and after that frequency, the dielectric constant drops
significantly. The same frequency frequency of pure polymer and nanocomposites suggest the
dominant roles of the polymer matrix in high field dielectric relaxation. Meanwhile, the highest
dielectric permittivity at 100 Hz indicates the frequency vicinity of dipole switching at this
electrical field. Compared with both types of UHMWPE/ 3.0wt% GNP nanocomposites, the pure
polymer UHMWPE shows the higher dielectric permittivity. The addition of 3.0wt% ht-GNPs
could remarkably reduce the dielectric permittivity, while the surface modification could greatly
amend this reduction. However, the dielectric permittivity of UHMWPE/s-GNP-3.0wt%
nanocomposites is still lower than that of the pure polymer. Also, the surface modification did
not affect the frequency dependence of the dielectric permittivity. The results in Fig. 22 also
reveal the different dielectric relaxation at high electrical field. Normally, the addition of
conductive fillers in polymer matrix could lead to high dielectric permittivity at low electrical
field, such as the results in Figs.8 and 9, when the loading level is reaching a threshold.
45
Dielectric permittivity @ 1200 volts
35000
UHMWPE
Dielectric permittivity ε'
30000
UHMWPE-htGNP3wt%
UHMWPE-sGNP3wt%
25000
20000
15000
10000
5000
0
1
-5000
10
100
1000
Frequency (Hz)
Figure 22 Dielectric permittivity of UHMWPE and UHMWPE/GNP-3.0wt%
nanocomposites at 1200 V at room temperature
Dielectric permittivity @ 3500 volts
35000
UHMWPE
Dielectric permittivity ε'
30000
UHMWPE-sGNP-3wt%
25000
20000
15000
10000
5000
0
-5000
1
10
100
1000
Frequency (Hz)
Figure 23 Dielectric permittivity of UHMWPE and UHMWPE/GNP-3.0wt%
nanocomposites at 3500 V at room temperature
46
Furthermore, UHMWPE with s-GNP showed some higher dielectric constant compared
to ht-GNP this suggested that the surfactant modified GNP had better interfacial polarization,
due to the uniform dispersion as well as better interfacial interactions. The increase of the testing
voltage up to 3500V (Fig.23) did not affect the dielectric permittivity very much. The dielectric
permittivity still shows the same frequency dependence, and the value of dielectric permittivity is
at the same level as that at 1200 V. It is also noticed that, the fast drop of dielectric permittivity
when the frequency increased to 1000 Hz, led to very low dielectric permittivity. Even negative
value was observed. This result further suggests the limitation of investigating dielectric
permittivity at high field and high frequency. This is also why 100 Hz was chosen for studying
hysteresis behaviors of the nanocomposites.
47
CHAPTER 5
CONCLUSIONS AND FUTURE PLANS
On the basis of literature review and results gathered during this research study, it can be
noted that metals are quite scarce and hence are polymers to manufacture due to their own
morphology and properties. However, the advancing demands advancement in various regions
including electronic devices, mechanical appliances, data storage and communications and
biomedical. This urge for reliable, low cost and advanced mechanics of dielectric materials for
different applications have given rise to the interest in manufacturing composite materials
including two or more types of materials that could meet or even surpass these expectations. In
this pursuance, polymer based nanocomposites have better advantages over costly and less
process ability materials like metals and ceramics owing to the properties of both polymer and
filler materials for nanocomposite.
In sum, this study suggested that the successful surface modification of GNPs could lead
to uniform dispersion of s-GNPs and good interfacial interactions with UHMWPE as shown in
SEM images, which substantially affect the energy storage capability of the resulting
nanocomposites.. Also, the results showed the addition of conductive GNPs did not lead to
dramatic current leakage at high field when proper surface modification was applied, suggesting
the potential for energy storage of UHMWPE/GNP nanocomposites.
The agglomerates and poor interface in ht-GNP nanocomposites resulted in high current
leakage and lower breakdown strength, which was seen in the results of ht-GNPs at lower
voltage when the samples short-circuited. Moreover, the results showed the addition of GNPs
and surface modification impacted on energy storage properties at higher temperature. Even at
48
high operation temperature, the UHMWPE/s-GNP nanocomposites still showed satisfactory
energy storage capability. However, the increase in temperature did not reduce the polarization
intensity in nanocomposites, while the energy efficiency was further improved to near 100%,
unlike other reported polymers and polymer nanocomposites for energy storage applications. The
surface modified GNP with UHMWPE polymer at 1 wt.% showed best feature as expected
compared to heat-treated GNP with same polymer at same loading and higher loadings included.
The elevation of temperature further improved the energy storage as well as energy efficiency for
the nanocomposites. These positive effects of temperature should be a result of the enhanced
dipole switching at elevated temperature.
Although the addition of GNPs as well as surface modification showed significant
impacts on the energy storage performances of the UHMPWE nanocomposites, the results are
still below the expectation. To achieve better energy storage performances, following tasks will
be conducted:

Nanocomposite Processing Optimization: UHMWPE gained its reputation for its very
high melt viscosity. Furthermore, the addition of nanomaterials further increases the melt
viscosity. This lead to great difficulty in nanocomposite processing, which also affect the
structures of properties of resulting nanocomposites. In the future, the processing
temperature, compounding time as well as various dispersion technologies will be
investigated to find out the optimal processing conditions. Meanwhile: to deepen the
understanding of the roles of interfaces, different surfactants will be applied in surface
modification process to adjust the interfacial structures and properties in nanocomposites.

Comprehensive Structure Analysis: UHMWPE is a semicrystalline polymer, and it is
believed that the crystal structures will also remarkably impact the energy storage
49
properties, besides the dispersion and interfaces. The nanomaterials have also proven
their capability of changing the crystal structures of the polymer matrix, thus, the crystal
structures and morphologies will be thoroughly studied. In addition, thermal analysis will
be conducted to study the glass transition of the polymer matrix, which could affect the
dielectric relaxation phenomenon.

Other graphitic nanomaterials will be incorporated into UHMWPE to fabricate
nanocomposites with different nanostructures, and the effects of different graphitic
architectures on energy storage applications will be studied.
50
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