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Graduate Studies The Vault: Electronic Theses and Dissertations

2014-01-29

Solution Based Synthesis of Copper

Nanowire/Polymer Composites and Their Electrical

Properties

Li, Yan

Li, Y. (2014). Solution Based Synthesis of Copper Nanowire/Polymer Composites and Their

Electrical Properties (Unpublished master's thesis). University of Calgary, Calgary, AB.

doi:10.11575/PRISM/27825

http://hdl.handle.net/11023/1309

master thesis

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UNIVERSITY OF CALGARY

Solution Based Synthesis of Copper Nanowire/Polymer Composites

and Their Electrical Properties

by

Yan Li

A THESIS

SUBMITTED TO THE FACULTY OF GRADUATE STUDIES

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER OF SCIENCE

DEPARTMENT OF CHEMICAL AND PETROLEUM ENGINEERING

CALGARY, ALBERTA

JANUARY, 2014

© Yan Li 2014

ii

Abstract

Conductive polymer composites (CPCs) are intensively researched partly because of their

advantages in acting as conductive materials while maintaining polymers’ properties such as

easy processability, corrosion resistance and light weight. This thesis focuses on using copper

nanowires (CuNW) as filler, with different polymers as matrices, in order to study the influence

of filler loading, the interaction between filler and matrix on the electrical properties and the

electromagnetic interference shielding properties of the composites. CPCs with CuNW as filler

and different polymers as matrices were successfully prepared using miscible solution mixing

and precipitation method. Among these composites, copper nanowire/polypropylene (CuNW/PP)

composite is investigated primarily and compared with carbon nanotube/polypropylene

composite. The conductivity curve of CuNW/PP composite shows a different trend than typical

percolation, where a plateau was found around CuNW concentration between 0.8 vol. % and 1.7

vol. %. Further study regarding synthesis of CuNW is also reported.

iii

Acknowledgements

First of all, I would like to express my sincere appreciation to my supervisor, Dr. Uttandaraman

Sundararaj, for his kind and inspiring guidance in both my research and my life. He leads me to

the world of polymer science and engineering and gives me advice to go through the problems in

the research.

I would like to take this opportunity to thank all my friends in my research group. I would like to

thank Xiaoxiong Luo and Genaro Gelves for their guidance and help on starting the project. I

would like to thank Ivonne Otero and Mohammad Arjmand for their help and many useful

discussions. I thank Kyle Leinweber for his research assistance and also I would like to thank

Maryam Khajehpour, Shadi Jamshidi, Kambiz Chizari, Ali Sarvi and Soheil Sadeghi for their

great company.

I would like to acknowledge Dr. Michael Schoel for operation of SEM and Dr. Tobias

Fürstenhaupt and Dr. Wei-Xiang Dong for TEM images taken.

Finally, I really appreciate the support from my parents and my sister, for their and support and

understanding. I also thank my boyfriend Keegan Stoyles for his patience and encouragement.

iv

Table of Contents

Abstract ........................................................................................................................................... ii

Acknowledgements ........................................................................................................................ iii

Table of Contents ........................................................................................................................... iv

List of Tables ................................................................................................................................. vi

List of Figures ............................................................................................................................... vii

List of Symbols, Abbreviations and Nomenclature ........................................................................ x

CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW .......................................... 1

1.1 Applications and Requirements of Conductive Polymer Composites (CPCs) ..................... 1

1.2 Mechanism of EMI Shielding ............................................................................................... 4

1.3 Conductive Fillers ................................................................................................................. 6

1.4 Conductivity Mechanism and Percolation Theory ............................................................. 10

1.5 Factors Affecting Distribution of Fillers or Conductivity .................................................. 13

1.6 Motivation and Overview of This Project .......................................................................... 15

References ................................................................................................................................. 17

CHAPTER 2: SYNTHESIS OF CUNWS ............................................................................... 21

2.1 Introduction ......................................................................................................................... 21

2.2 Experiment .......................................................................................................................... 22

2.3 Morphology of CuNWs ...................................................................................................... 28

2.4 Discussion: One-step or Two-steps Anodization ................................................................ 30

2.5 Study of the Oxidation in the Processing ............................................................................ 37

2.6 Conclusions ......................................................................................................................... 40

References ................................................................................................................................. 41

CHAPTER 3: POLYPROPYLENE COMPOSITES WITH CUNW OR CNT AS FILLERS 43

3.1 Introduction ......................................................................................................................... 43

3.2. Experimental ...................................................................................................................... 44

3.3 Results and Discussion ....................................................................................................... 51

3.4 Conclusion .......................................................................................................................... 76

References ................................................................................................................................. 78

v

CHAPTER 4: CUNW/POLYMER COMPOSITE: POLYCARBONATE, POLY(METHYL

METHACRYLATE), POLYLACTIC ACID AND POLYSTYRENE ........................................ 81

4.1 Introduction ......................................................................................................................... 81

4.2 Materials and Experimental Procedures ............................................................................. 84

4.3 Results and Discussions ...................................................................................................... 87

4.4 Conclusion .......................................................................................................................... 95

References ................................................................................................................................. 96

CHAPTER 5: COMPOSITES WITH POLYMER BLENDS FOR MATRIX -

LOCALIZATION OF CUNW IN PLA/LDPE BLEND .............................................................. 99

5.1 Introduction ......................................................................................................................... 99

5.2 Materials and Experiment ................................................................................................. 101

5.3 Results and Discussions .................................................................................................... 103

5.4 Conclusion ........................................................................................................................ 107

References ............................................................................................................................... 108

CHAPTER 6: CONCLUSIONS AND FUTURE WORK ..................................................... 110

6.1 Summary and Conclusions ............................................................................................... 110

6.2 Future Work ...................................................................................................................... 112

APPENDIX A: SCALE UP OF THE CUNW SYNTHESIS SYSTEM................................. 114

1.Design of Scale up System .................................................................................................. 114

2.Products Examination .......................................................................................................... 119

3.Conclusion ........................................................................................................................... 124

Reference ................................................................................................................................ 125

vi

List of Tables

Table 1-1: CPC applications with their surface/volume resistivity range [7,8]. ............................. 3

Table 2-1: Dimension comparison of CuNWs made by two-steps and one-step anodization by

measuring the copper nanowires in SEM images ......................................................................... 34

Table 3-1: Non-isothermal crystallization of CuNW/PP composites ........................................... 59

Table 4-1: General information of polymers used in this chapter. ............................................... 84

Table 4-2: Additional information of polymers used in this chapter [33]. ................................... 85

Table 5-1: Concentration percentage of CuNW/PLA/LDPE sample A and sample B ............... 103

Table A-1: Statistical analysis of pore diameters on aluminum templates anodized from small

setup and scaled up system. ........................................................................................................ 120

vii

List of Figures

Figure 1-1: Schematic representation of the two classes of hybrid materials, reproduced from [6].

......................................................................................................................................................... 2

Figure 1-2: Schematic picture of EMI shielding mechanisms ........................................................ 4

Figure 1-3: Common fillers and their dimensions, r is less than 100 nm. .................................... 10

Figure 1-4: Schematic sketch shows electrical conductivity as a function of fillers loading. ...... 11

Figure 1-5: Schematic dispersion of CNTs with high aspect ratio and low aspect ratio at low

loadings (a and a’) and high loadings (b and b’). Reproduced from [40]. .................................... 12

Figure 2-1: Photographs of the anodization tank: front view (a) and top view (b) and sketch of

top view (c). The aluminum plates are connected to the positive terminal with red alligator clips;

the stainless steel plates are connected to the negative terminal with black clips. ....................... 23

Figure 2-2: The stepwise anodization voltage decrease during the barrier layer thinning at the end

of anodization................................................................................................................................ 25

Figure 2-3: Sketch of the AC electrodeposition set-up. ................................................................ 26

Figure 2-4: The continuous sine voltage produced by a Kepco BOP 20–50 Mg power supply for

the AC electrodeposition............................................................................................................... 27

Figure 2-5: SEM image of copper nanowires. .............................................................................. 28

Figure 2-6: TEM images of copper nanowires with whole nanowires (a) and zoom in image on

the crystalline part (b). .................................................................................................................. 29

Figure 2-7: The flow chart for the CuNWs synthesis process ...................................................... 30

Figure 2-8: SEM images of CuNWs: one-step anodization (a) and two-steps anodization (b). ... 31

Figure 2-9: Bar charts of length distribution of CuNWs made by one-step anodization (a) and

two-steps anodization (b) .............................................................................................................. 32

Figure 2-10: Bar charts of diameter distribution of CuNWs made by one-step anodization (a) and

two-steps anodization (b) .............................................................................................................. 33

Figure 2-11: Conductivity vs. CuNW concentration of two-steps and one-step anodization ...... 36

Figure 3-1: The flow chart of processing procedure of CuNW/PP composite. ............................ 46

Figure 3-2: Keithley 6517A electrometer with an 8009 test fixture (a), Loresta GP resistivity

meter with ESP four-pin probe (b) and the illustration of the working mechanism of a four-pin

resistance testing (c). ..................................................................................................................... 49

Figure 3-3: EMI shield equipment: Agilent Vector Network Analyzer (Model E5071C), the wave

guides (a) and the sample holder (b). ............................................................................................ 50

Figure 3-4: Volume conductivity of CuNW/PP composite (a) and CNT/PP composite (b) as a

function of filler (CuNW or CNT) concentration. ........................................................................ 52

viii

Figure 3-5: TEM image of cross-section of CuNW/PP composites at the concentration of 0.7 vol.

% and 3D schematic illustration of composites. Agglomerations can be found, as circled in the

matrix. ........................................................................................................................................... 54

Figure 3-6: TEM images of CuNW/PP composite, (a) 1.0 vol. %, (b) 1.7 vol. %. ...................... 55

Figure 3-7: DSC cooling scan of CuNW/PP composites (after molding) at 10 °C/min. .............. 60

Figure 3-8: DSC Second heating scan of CuNW/PP composite (after molding) at 10 °C/min. ... 60

Figure 3-9: DSC cooling scan of CuNW/PP composite (before molding) at 10 °C/min. ............ 61

Figure 3-10: DSC Second heating scan of CuNW/PP composite (before molding) at 10 °C/min.

....................................................................................................................................................... 61

Figure 3-11: DSC cooling scan of CNT/PP composite 10 °C/min. .............................................. 62

Figure 3-12: DSC second heating scan of CNT/PP composite 10 °C/min. .................................. 63

Figure 3-13: Crystallinity of CuNW/PP composites .................................................................... 64

Figure 3-14: Crystallinity of CNT/PP composite (after molding) ................................................ 65

Figure 3-15: Optical Microscope images of CuNW/PP samples (a) Neat PP, (b) CuNW/PP 0.4

vol. %, (c) CuNW/PP 0.7 vol. %, (d) CuNW/PP 1.7 vol. %. ....................................................... 67

Figure 3-16: Optical microscope images of CNT/PP samples: (a) CNT/PP 0.3 vol. %, (b)

CNT/PP 0.5 vol. %........................................................................................................................ 67

Figure 3-17: Illustration of X-rays diffractions showing the relationship of incident beam, angle

and distance between the diffracting planes. ................................................................................ 68

Figure 3-18: X-ray diffraction profiles of neat PP and CuNW/PP composite with different

CuNW concentrations. .................................................................................................................. 70

Figure 3-19: SEM images of CuNW/PP samples before molding with CuNW concentration 1.0

vol. % (left) and 1.4 vol. % (right). ............................................................................................... 71

Figure 3-20: Overall shielding of CuNW/PP over testing frequency (a) and EMI shielding

effectiveness of CuNW/PP composite as shown in SE by absorption, SE by reflection and overall

SE (b). ........................................................................................................................................... 73

Figure 3-21: Overall SE (a) and Permittivity (b) as a function of CuNW concentration in X-band.

....................................................................................................................................................... 74

Figure 3-22: Shielding effectiveness of CuNW/PP and CNT/PP composite. .............................. 75

Figure 4-1: Dry CuNW/PS composites: (a) 2.0 vol. %, (b) 1.5 vol. %, (c) 1.0 vol. %, (d) 0.85

vol. %, (e) 0.73 vol. %, (f) 0.6 vol. %, (g) 0.5 vol. %, (h) 0.4 vol. %, (i) 0.2 vol. %. .................. 87

Figure 4-2: SEM images of CuNW/PMMA composite (a) 1.4 vol. %, (b) 2.82 vol. %, CuNW/PC

composite (c) and CuNW/PLA composite (d). ............................................................................. 89

Figure 4-3: SEM images of CuNW/PLA composite (1.2 vol. %), cross-section and zoom-in

images. .......................................................................................................................................... 90

Figure 4-4: Resistivity of CuNW/polymer samples: (a) CuNW/PC composite, (b) CuNW/PMMA

composite, (c) CuNW/PLA composite, (d) CuNW/PS composite, (e) CuNW/PP composite. .... 91

ix

Figure 4-5: EMI shielding properties of CuNW/PC, CuNW/PMMA, CuNW/PP and CuNW/PLA

composites..................................................................................................................................... 94

Figure 5-1: Schematic of useful morphologies of polymer blends, adapted from [2] ................ 100

Figure 5-2: Sample of CuNW/PLA master batch (a) and sample of CuNW/PLA/LDPE after

batch mixing (b). ......................................................................................................................... 101

Figure 5-3: Working flow of synthesis of CuNW/PLA/LDPE. .................................................. 102

Figure 5-4: SEM image of pure PLA/LDPE (70/30) blend. ....................................................... 104

Figure 5-5: CuNW/PLA/LDPE (sample A) with CuNW of 1 wt % in composite. .................... 104

Figure 5-6: Higher magnification of CuNW/PLA/LDPE (sample A) composite indicating

different phase and filler. ............................................................................................................ 105

Figure 5-7: SEM images of LDPE/CuNW/PLA – Sample B composite.................................... 106

Figure A-1: Gantt Chart for the CuNWs synthesis process ........................................................ 114

Figure A-2: Apparatus for copper nanowires synthesis with 1g/batch yield [2]. ....................... 115

Figure A-3: Sketch of the small anodization system .................................................................. 116

Figure A-4: The connection design for the big tank: (a) side view, (b) top view. ..................... 116

Figure A-5: Design of the hood for the anodization tank ........................................................... 118

Figure A-6: Scaled up system with side view (a) and top view (b) ............................................ 118

Figure A-7: SEM images for the small setup with high magnification (a) and uniformities with

low magnification (b); scaled up system with high magnification (c) and uniformities with low

magnification (d). ........................................................................................................................ 119

Figure A-8: Diameter distribution of pore diameters in template for small setup and scaled up

setup ............................................................................................................................................ 121

Figure A-9: A bundle of copper nanowires from the small setup and the EDX analysis from the

rectangular section indicated....................................................................................................... 122

Figure A-10: A bundle of copper nanowires from the scaled up setup and the EDX analysis from

the rectangular section indicated. ................................................................................................ 123

x

List of Symbols, Abbreviations and Nomenclature

Abbreviations

1D One-dimensional

2D Two-dimensional

Al Aluminium

ASTM American society for testing and materials

CB Carbon black

CF Carbon fiber

CNF Carbon nanofiber

CNT Carbon nanotubes

Cu Copper

CuNW Copper nanowire

CPC Conductive polymer composite

CVD Chemical Vapor Deposition

dB Decibel

EDX Energy-dispersive X-ray spectroscopy

EM Electromagnetic

EMI Electromagnetic interference

ESD Electrostatic discharge

MSMP Miscible solution mixing and precipitation

MWCNT Multi-walled carbon nanotube

PAO Porous aluminum oxide

PC Polycarbonates

PE Polyethylene

PI Incident power

PLA Polylactic acid

PMMA Poly(methyl methacrylate)

PP Polypropylene

PS Polystyrene

PT Transmitted power

xi

SE Shielding Effectiveness

SEa Shielding by Absorption

SEm Shielding by Multiple-reflections reflection

SEo Overall Shielding

SEr Shielding by Reflection

SWCNT Single-walled carbon nanotubes

TEM Transmission electron microscopy

VGCNF Vapor Grown Carbon Nanofibers

1

CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW

In this thesis, research on the synthesis of copper nanowires (CuNW) and the polymer

nanocomposites using CuNW as filler is presented. The processing control of CuNW synthesis

and the properties of nanocomposites using polypropylene (PP) and other polymers as matrix are

studied. The localization of CuNW in poly lactic acid/low density polyethylene (PLA/LDPE)

blends as matrix is discussed.

This chapter introduces general background related to the research area, including: the

applications and the requirements of conductive polymer composites (CPCs); the mechanism of

electromagnetic interference (EMI) shielding; the classification of conductive fillers; the

conductivity mechanism and percolation theory and the factors affecting conductivity of CPCs.

1.1 Applications and Requirements of Conductive Polymer Composites (CPCs)

With the rapid growth of demand for electronics, people today are exposed to more severe

electromagnetic pollution, and precision instruments are facing increasing danger of interference

from electric devices [1,2,3]. Developing materials with a high EMI shielding property is

therefore becoming popular in science and technology. Polymers are among the most promising

class of new materials because of their low cost, light weight, easy processability and anti-

corrosive properties. However, most polymers are insulative, which makes them inappropriate

for EMI shielding application. Conductive polymers such as polypyrrole, polyaniline,

polythiophene, polyindole etc. are relatively expensive and have poor processing properties [4].

One way to make insulative polymers conductive is to fill the polymer matrices with conductive

fillers and make them into conductive polymer composites (CPCs).

2

The interaction between fillers and polymer matrices is classified into two groups, according to

Judeinstein and Sanchez [5,6]. Class One materials have the inorganic and organic component

interact only through hydrogen bonding or van der Waals interaction. Class Two materials have

the organic and inorganic component linked to each other through covalent or ionic bonds.

Figure 1-1 represents two kinds of materials emphasizing the connections. Interactions between

components in Class One materials are usually weak compared to bonding in Class Two

materials. In this study, polymers filled with inorganic nanoparticles (copper nanowire), where

the connections are interpenetrating but not strongly connected, belong to Class One.

Figure 1-1: Schematic representation of the two classes of hybrid materials, reproduced from [6].

The major applications of CPCs, including antistatic protection, electrostatic discharge (ESD)

dissipation and EMI shielding, are determined by the surface or volume resistivity which are

listed in Table 1-1[7,8].

3

Table 1-1: CPC applications with their surface/volume resistivity range [7,8].

Application Surface resistivity (Ω/sq) Volume resistivity (Ω∙cm)

Antistatic protection 1010

- 1012

109 - 10

12

ESD dissipation 106 - 10

9 10

5 - 10

8

EMI shielding 100 - 10

5 10

0 - 10

4

1.1.1 Antistatic Protection

Antistatic protection is needed in applications where relative motion between dissimilar

materials takes place, such as conveyor belts and airplane tires [9]. Static electricity can damage

electrical components and can even ignite flammable liquids and gases. Antistatic protection

reduces the static electricity generated whenever two surfaces contact and separate, and one of

the surfaces has a high resistance to electrical current.

1.1.2 Electrostatic Discharge (ESD) Dissipation

ESD dissipation is another way to release static by transferring electrostatic charges between

bodies at different potentials, induced by the direct contact of the electrostatic fields. The main

objective of ESD is to limit the possible impact of ESD from the triboelectric charge generation,

direct discharge and electrostatic fields.

4

1.1.3 Electromagnetic Interference (EMI) Shielding

EMI occurs when the radiated or conducted energy of certain frequency impairs the performance

of circuits or sensitive electric devices. EMI can interfere with the operation of electric

appliances, ruin data in large-scale computer systems, lead to inaccurate readings and output in

aircraft guidance systems, and interrupt the functioning of precision medical devices. The main

objective of EMI shielding is to prevent the passage of incoming or outgoing electromagnetic

waves. EMI shielding can be used within a wide variety of products such as: GPS devices,

precise analytical instruments, aerospace shuttles and components, personal computers, laptops,

mobile phone housings, automotive components and medical device housings [1,10].

1.2 Mechanism of EMI Shielding

EMI shielding works by reflection, by absorption, or by transferring the electromagnetic

radiation to the ground; that is, all radiation that is not transmitted is considered to be “shielded”.

EMI shielding effectiveness (SE) is typically defined as a measurement of an attenuation of the

electromagnetic power after going through a shield. SE is expressed in dB, which is calculated

Transmitted Power – PT

Incident Power – PI

Reflected Power – PR

PI-R

Absorption

Figure 1-2: Schematic picture of EMI shielding mechanisms.

5

by the ratio of incident power (PI) to the transmitted power (PT) through the shield as given by

the Equation 1-1 [11]

EMI SE = 10log (PI/PT) (Equation 1-1)

The required shielding effectiveness is 30 dB, which corresponds to 99.9% attenuation of the

EMI radiation (i.e. PT = 0.001PI), and this is an adequate level of shielding for many applications

[1,12].

From the mechanism aspect, the shielding effectiveness of a homogeneous conductive material is

the sum of the shielding by reflection (SEr), absorption (SEa) and the shielding by multiple-

reflections (SEm) as shown in Equation 1-2:

SE = SEr + SEa + SEm

(Equation 1-2)

The multiple-reflections can be ignored if the shield is thicker than the skin depth [10]. Skin

depth (δ) is defined in Equation 1-3:

𝛿= (𝜋ƒ𝜇σ)−1/2 (Equation 1-3)

Where ƒ is the frequency, μ is the shield’s magnetic permeability and σ is the shield’s electrical

conductivity.

In this study, we consider only the contribution of shielding by reflection, SEr and by absorption,

SEa. Reflection of radiation by the shield is caused by the mobile charge carriers, either electrons

or holes, which will interact with the electromagnetic fields in the radiation. Accordingly, the

materials with high conductivity tend to have higher shielding effectiveness; however

conductivity is not a necessary criterion for shielding. Absorption depends on the thickness of

6

the shield and is enhanced when the shielding material has electrical or magnetic dipoles which

can interact with the incident power waves. The equations for the shielding effectiveness by

reflection or absorption are given below [10]:

SEr = 39.5 + 10log (σ/(2𝜋ƒ𝜇)) (Equation 1-4)

SEa = 8.7d/𝛿 = 8.7d (𝜋ƒ𝜇 σ)1/2

(Equation 1-5)

Where σ is the volume conductivity of the material, d is the thickness of the sample, 𝛿 is the skin

depth, ƒ is the frequency, and μ is the shield’s magnetic permeability. It can be noted that

shielding effectiveness not only depends on the material’s volume resistivity but also depends on

the sample thickness (d) and the frequency of the incident wave. Both shielding by reflection and

shielding by absorption increase with increasing conductivity of the material. Shielding by

reflection decreases with increasing incident power frequency, while shielding by absorption

increases with increasing power frequency.

1.3 Conductive Fillers

By adding conductive fillers in polymer matrix, conductive polymer composites (CPCs) can

have good ESD and/or EMI shielding properties. For high EMI shielding, it is recommended that

these fillers contain a large amount of mobile charge carriers and high magnetic permeability.

There are two main kinds of fillers that are of most interest in EMI shielding research: fillers

based on carbon and fillers based on metal. Fillers based on carbon include carbon black (CB),

carbon nanotubes (CNT), and carbon fibers (CF). For metallic fillers, there are metallic powders,

metal flakes, metal-coated fibers and metal nanowires. Nanofillers can also be divided into

7

spheres, platelets and fibers according to their shape. The agglomeration or dispersion of the

fillers in the polymer matrix depends on the mixing conditions and the surface treatments [13].

Carbon black (CB) is a form of amorphous carbon that has high surface area. The diameter of

CB ranges from 10 nm to 100 nm and ranges in surface area from 25 to 1500 m2/g [14]. CB has

become one of the most commonly used conductive fillers because of its low cost and

availability. However, studies showed that CB tends to aggregate into grape-shaped clusters due

to its high surface area and high surface tension [15,16]. Additionally, compared with other

carbon fillers, CB is not competitive for properties when considering the aspect ratio.

Carbon nanotubes (CNT) have garnered of extremely high interest in research since they were

discovered by Iijima [17], because CNTs have many superior properties over other types of

carbon fibers, such as high aspect ratio, high electrical and thermal conductivity, low density and

good mechanical properties [18,19,20]. One of the most attractive properties of CNTs is an

extremely low volume percolation threshold – 5.2×10-5

vol. % is achieved in CNT/epoxy

nanocomposites [21]. There are two types of CNTs: single-wall carbon nanotubes (SWCNTs)

and multi-wall carbon nanotubes (MWCNTs). SWCNTs have higher tensile strength (50-500

GPa) than MWCNTs (10-60 GPa) [22] and a smaller diameter (0.6-1.8 nm vs. 5-50 nm) [23], but

the aspect ratio, thermal and electrical conductivity are around the same level [24]. From a

commercial aspect, the applications for both kinds of CNTs are limited by the high price.

SWCNTs are much harder to produce, especially SWCNTs with high purity, and this results in a

higher price.

Carbon Fiber (CF) has been studied for years and is used to enhance interfacial strength by

strong chemical bonding between the double bonds in the polymer and the functional group at

8

the CF surface [25]. CF/epoxy is a typical combination because of high strength and stiffness

(tensile strength 27 MPa and tensile modulus 2590 MPa at 150°C), which is due to the surface

treatment of carbon fiber [26]. However, due to its brittleness, CF is easily broken during

processing and the conductive networks of branches are destroyed. Vapor-grown Carbon

Nanofibers (VGCNFs) are nano-sized conductive fillers with diameter 50 – 200 nm [27]. Lee et

al. [27] found that the EMI SE of VGCNF/PVA film was lower than that of composites made

with carbon black with same thickness, but the SMI SE became higher after heat treatment of the

CNF. The application of VGCNF is limited by its poor tensile strength, because of which the

fiber is easy to break up during processing, and this will result in lower aspect ratio.

Use of metallic powders as fillers became of interest because of unique properties like dielectric

or magnetic properties, as well as good electrical and thermal conductivity. Metallic powders

filled composites share electrical characteristics close to metal and mechanical properties and

processing methods like plastics. Mamunya et al. [28] studied the electrical and thermal

conductivity of metal powder composite based on epoxy resin and poly(vinyl chloride) (PVC)

and found that the packing factor, F, is the key parameter for electrical properties. Later, they

found smaller particles have a lower packing factor because of more irregular shape compared

with larger particles, which can explain that applied pressure will affect F due to the deformation

of the particles [29]. The applications of metal powder are limited because of their high tendency

to agglomerate deriving from the high surface energy. One way to reduce the agglomeration is to

modify the surface of the powders. Sonoda et al. [30] studied the surface modification of

composites using nano-cobalt (nCo) and nano-iron (nFe) powders as fillers and found that

surface modification can increase the real part of permittivity because the surface layer can

reduce the surface energy and therefore allow better particle distribution.

9

Metal nanowires are 1D materials which are gaining more and more attention because of their

high aspect ratio, and excellent electrical and thermal conductivity. There are six different

approaches used to produce 1D nanostructures: 1) template-assisted synthesis, 2) confinement by

a liquid droplet, 3) dictation by the anisotropic crystallographic structure of a solid, 4) kinetic

control with surfactant as a capping agent, 5) self-assembly of 0D nanostructures, 6) size

reduction of 1D microstructure [31]. Among these six methods, template-assisted synthesis is the

best one for producing nanowires with the required diameter and aspect ratio, and it has been

proven that this method can produce nanowires with a smooth surface [32].

Gelves et al. [33] reported the preparation of copper nanowires (CuNWs) and silver nanowires

(AgNWs) with an average diameter of 25 nm and 5 – 10 µm length by AC electrodeposition in

Porous Aluminum Oxide (PAO) templates. Composites of CuNW/Polystyrene showed an EMI

SE of 27 dB at the concentration of 1.3 vol. % with a sample thickness of 210 µm [34] One

challenge for CuNW is to decrease the influence of the copper oxide layer on the surface of

CuNWs, since the oxide layer will make most nanowires insulative and increase the electrical

resistivity dramatically. The oxide layer can be introduced in different ways during the

processing. Lin et al. [35] reported that copper oxide was found in CuNW/PS nanocomposite

made by melt mixing.

Conductive fillers can also be divided into 0D, 1D and 2D nanofillers according to their

dimensions [36,37], including plate-like fillers (2D), which have two dimensions larger than 100

nm and one dimension less than 100 nm, nanofibers or whiskers (1D), which have only one

dimension which is larger than 100 nm and two dimensions less than 100 nm (i.e. diameter less

than 100 nm) and nanoparticles (0D) which have all three dimensions less than 100 nm, as

illustrated in Figure 1-3.

10

Figure 1-3: Common fillers and their dimensions, r is less than 100 nm.

1.4 Conductivity Mechanism and Percolation Theory

Electrical conductivity is the ability of a material to conduct electric current. In metal, Fermi

level lies in the conduction band, and this makes it possible for electrons to dissociate from

parent atoms freely and travel through the atom lattice to conduct electric current. But in

insulators, the Fermi level lies within the valence band and conduction band gap, the forbidden

zone; so when in the electric field, there is no carrier for the current.

Normally the polymer matrix is insulative in conductive polymer composites (CPCs). For CPCs,

there is a critical concentration where the fillers within the polymer matrix form a conductive

network and the composites change from insulative to conductive, and this concentration is

11

known as the percolation threshold. Often, at the percolation threshold, the conductivity of the

composite increases dramatically, as shown in Figure 1-4.

0 1 2 3

1E-17

1E-15

1E-13

1E-11

1E-9

1E-7

1E-5

1E-3

0.1

Co

nd

uct

ivit

y (

S/c

m)

Filler Volume Percentage

Figure 1-4: Schematic sketch shows electrical conductivity as a function of fillers loading.

According to percolation theory, the curve in Figure 1-4 is divided into three regions: the region

before percolation, the region where percolation occurs and the region after percolation. Before

percolation, there is no contact between adjacent filler particles and the polymer is the insulating

gap. When applying an electric field, the insulative parts will be polarized, with one end rich in

protons, while the other end is rich in electrons. In this circumstance, only limited electrons can

get enough energy to go across the forbidden zone [38]. With a higher concentration of fillers,

the gap between fillers is smaller and the chance for electrons to pass the barrier is greater. When

the gap distance comes to a range smaller than 10 nm, tunneling conduction occurs and becomes

dominant [7]. When percolation occurs, the filler particles form an effective network and the

composites become conductive. After percolation, both direct contact and tunneling occur [39],

12

but free electrons acting as carriers are much more dominant. Accordingly, there are two

restrictions that control conductivity: the constriction resistance of contact spots and the

tunneling resistance between particles [39]. So the conductivity increases as more conductive

networks form.

One thing worth mentioning here is the relationship between aspect ratio and percolation. Aspect

ratio is one of the most important factors when studying conductive nanofillers. Fillers with a

higher aspect ratio have a greater chance of forming a more compact percolating network. As

illustrated in Figure 1-5, composites with high aspect ratio fillers more easily form a network,

both those with low filler loading and high filler loading.

Figure 1-5: Schematic dispersion of CNTs with high aspect ratio and low aspect ratio at low

loadings (a and a’) and high loadings (b and b’). Reproduced from [40].

13

1.5 Factors Affecting Distribution of Fillers or Conductivity

To obtain good conductivity at low filler loading or, in other words, a low percolation threshold,

the dispersion and distribution of fillers is one critical issue. Dispersion is how individual fillers

separate from each other instead of bundling up. Distribution is how fillers distribute within the

polymer matrix. As a matter of fact, the percolation threshold concentration decreases with the

increase of aspect ratio [9] and dispersion of fillers, but it does not require good distribution.

Perfect distribution of well-dispersed fibers will increase the gap between the fillers, which will

result in low conductivity. On the contrary, preferential distribution and good dispersion will

enhance the probability for the individual filler to attach to other fillers and form a connective

network.

For CPCs, there are several factors which can be divided into thermodynamic and kinetic effects

that will affect the distribution and, furthermore, affect the conductivity of the composites.

1.5.1 Physical and Thermal Dynamic Factors

For the same filler loading, conductivity of composites with more conductive fillers will be

higher than composites with less conductive fillers. EMI shielding effectiveness will also

increase with the more conductive fillers by increasing the reflection and absorption of EMI

radiation. This can be achieved by using more conductive filler or by coating the filler with

metal. Shui and Chung [41] pointed out that nickel-coated carbon filament/polyether sulfone has

a much higher conductivity and EMI SE than composites using fillers without coating.

Additionally, an increase in the conductivity of fillers can also decrease the contact resistance

between fillers.

14

Polymer surface tension is another factor affecting percolation threshold [42]. The surface

tension between the polymer and the filler will determine the wettability of polymer to the filler.

Wettability is a measure the bonding or adherence of two materials. When the surface energy of

the polymer increases, the polymer-filler interfacial tension will decrease. At low filler-polymer

interfacial tension, the polymer can easily wet the fillers and result in better distribution. As

discussed previously, better distribution of the fillers will increase the percolation threshold.

Also, wettability of polymer to the fillers will result in the presence of a thin layer of polymer

around the fillers, which will increase contact resistance.

1.5.2 Kinetic Factors – Mixing Procedure and Viscosity of Polymers

Researchers have found that processing procedures have an effect on the electrical properties and

EMI shielding properties. Mamunya et al. [43] found copper powder/polymer prepared by

extrusion has three times higher percolation threshold values and lower conductivity values

compared with the same composites made by compression molding. Arjmand et al. [44] reported

compression molded samples have a much higher EMI SE than injection molded samples. These

were explained by the fillers in the compression molded samples having random and segregated

distribution, where they have more chance to connect with each other, whereas injection molding

generated more orientated fillers [44] or the extruder make spatial distribution [43], in which

cases, fillers are separated from each other.

Mixing temperature and mixing time will also affect the dispersion of fillers inside the polymer

matrix. Particles tend to aggregate with increasing mixing temperature, therefore decreasing the

conductivity. After a certain point, increasing mixing time or speed will decrease the

15

conductivity because of breakage of the fillers [12] or uniform distribution of fibers without

agglomeration [45].

Viscosity of polymers has a direct impact on the mixing of the fillers and the polymers. Higher

viscosity of the polymer matrix will make it more difficult for fillers to disperse while mixing.

Also, fillers have a greater chance of breaking in a high viscosity polymer matrix and lose their

high aspect ratio.

1.6 Motivation and Overview of This Project

Conductive polymer composites have advantages in EMI shielding applications over metal-

coated polymer material. Metal-coated polymer materials have several disadvantages in

processing, including a requirement for cleaning before coating to ensure adhesion between the

coating and the housing. Even though improvements have been made in this regard, there are

still some polymers, such as polytetrafluoroethylene, which are extremely difficult to coat with

metal. What is more, the coating is only a thin layer on top of the substrate which is not enough

for low-frequency magnetic fields. CPCs have the advantage of easy processing and have the

mechanism of EMI shielding, based on the volume resistivity instead of surface properties.

Conductive polymer composites using copper nanowires as conductive filler have been studied

and have been proven to have high EMI shielding properties [33]. Polymer matrices used include

polystyrene and polyethylene. Other widely used polymers like polypropylene, polycarbonate

and PMMA were intensively studied with carbon-based fillers such as carbon nanotube and

carbon black. However, using copper nanowires as fillers is very limited. Also, copper nanowire

has the advantage of having better conductivity over carbon based fillers. Therefore, the study of

conductive polymer composites based on copper nanowires is important.

16

CPCs can be prepared by either melt mixing or solution mixing. Melt mixing of CuNW and

polymer needs to be processed in nitrogen or argon surroundings to prevent the oxidation of

CuNW while miscible solution mixing and precipitation (MSMP) method has been proven to be

effective to make CuNW/PS composite and MSMP requires less for operation conditions.

Because of this limitation of melt mixing, solution based synthesis is preferred and chosen to be

the method for preparing CuNW/polymer composites in this project.

This thesis is organized into seven chapters. The first chapter contains background information

and an introduction, including a general discussion regarding the applications of CPCs, the

mechanism of EMI shielding and percolation theory. The second chapter reports on the synthesis

of copper nanowires by template-assisted AC electrodeposition and discussions of procedures

and factors of oxidation of nanowires. In the third chapter, the copper nanowire/polypropylene

composite is studied and the electrical and EMI shielding properties are investigated and

compared with carbon nanotube/polypropylene composite. Other polymers such as

polycarbonate, polylactic acid and PMMA as matrices are reported in the fourth chapter. The

fifth chapter gives a brief study on the composite using copper nanowire as filler in a PLA/LDPE

blend, and the localization of copper nanowires is studied. Conclusions and future work are

summarized in the last chapter. In Appendix chapter, a report on a scale-up of an anodization

system for synthesizing copper nanowires is discussed.

17

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21

CHAPTER 2: SYNTHESIS OF CUNWS

2.1 Introduction

Copper nanowire (CuNWs) is a 1D structure nanomaterial, i.e. a material with one dimension

less than 100 nm. CuNWs have a diameter range of 16.2 nm to 90 nm [1,2,3,4]. Wiley et al.

reported CuNWs with a diameter of 90 ± 10 nm and a length of 10 ± 3 µm [1]. Zhang et al.

synthesized CuNWs with a diameter around 79 nm via a self-catalytic growth process, using

hexadecylamine and cetyltriamoninum as a liquid-crystalline medium [2]. A diameter around 50

nm CuNWs was reported [3] by controlling the disproportionation of Cu(I) in the organic phase.

Guo et al. presented high aspect ratio CuNWs, with a diameter of 16.2 ± 2 nm and a length up to

40 µm, synthesized in an oleylamine solution through a Ni(acac)2 catalytic formation process [4].

In our group, CuNWs with an average diameter of 25 nm were synthesized using AC

electrodeposition into porous aluminum oxide (PAO) template [5]. Template-assisted synthesis

involves using a template as mold, filling it with material to assume the shape and morphology

of the template. After this, 1D structure materials can be obtained by removing the template

through a chemical method. A template-assisted synthesis of a 1D structure is accepted as simple,

of high purity and a cost effective method yielding high purity nanomaterials [6]. Several

methods can be used to synthesize 1D materials with such anodic PAO templates, including

chemical vapor deposition (CVD), physical vapor deposition (PVD) and electrodeposition.

Electrodeposition has been reported to synthesize metal nanowires in anodic PAO template for

years [7,8,9,10,11]. In electrodeposition, the growth of nanowires starts at the pore tips and

continues from the pore bottom to the pore opening [12]. The micro structure and properties of

22

the nanowires depend on the deposition solution composition, temperature of the solution and the

parameters of AC waves added to the system [13].

Electrodeposition can also be achieved by DC electrodeposition. AC electrodeposition is chosen

because it requires easier process procedures. For AC electrodeposition, copper can be deposited

into the template directly while DC electrodeposition involves barrier layer removal, metallic

layer substrate coating and metallic layer removal steps [14]. In this chapter, the synthesis

process of CuNWs by AC electrodeposition of copper into an anodic PAO template is presented.

2.2 Experiment

2.2.1 Preparation of Porous Aluminum Plate

The aluminum plates, purchased from Alfa Aesar, (99.99+%, 1mm thickness) were cut into 10

cm width, 25 cm length to fit into the anodization tank (see Figure 2-1) and were immersed in

the 1M NaOH solution to remove the oxide layer.

The anodization tank has a 0.75 inch plexiglass wall, with a 22 liter volume capacity. he tan ’s

cooling system contains 0.5 inch diameter stainless steel tubes, with aqueous glycol used as the

cooling fluid. Figure 2-1(a) (b) shows the setup overview and configuration of electrodes and the

electrical connection. The sketch (Figure 2-1 (c)) shows the connections with power supply and

cooler. The cooling system is separated with the anodes and cathodes by an insulative layer

which is made by lexan plastic sheet with big holes punched for heat transfer.

A 0.3 M H2SO4 electrolyte solution is used for the anodization. An agitator controls the

temperature to enhance the heat transfer between the cooling system and the solution, making it

possible to maintain the temperature between 0 – 4 ºC [14].

23

Figure 2-1: Photographs of the anodization tank: front view (a), top view (b) and sketch of top

view (c). The aluminum plates are connected to the positive terminal with red alligator clips; the

stainless steel plates are connected to the negative terminal with black clips.

(a)

(c)

24

2.2.2 Anodization

In the anodization system, the aluminum plates are used as the anode and same size of stainless

steel plates are used as the cathode. The plates are placed in the tank in sequence and together

with the electrolyte solution, make an electrolyser, as shown in the photograph (Figure 2-1).

Alligator clips connect the plates to the power supply, (LAMBDA, TDK). Aluminum plates

connect to the positive terminal (red alligator clips in Figure 2-1); stainless steel plates connect to

the negative terminal (black alligator clips in Figure 2-1).

Before turning on the power supply, the anodes and cathodes should be secured in their specific

slots, with no connections between them, to prevent a short circuit from occurring. The distance

between the aluminum plate (anode) and the stainless steel plate (cathode) is 2 cm.

A 25 V DC power source will be applied on the system for 8 hours, while the cooler helps to

maintain the system at temperature between 0 – 4 ºC. During the anodization, hydrogen is

generated onto the stainless steel plates and the aluminum plates are anodized. Reactions are:

2H+ + 2e

- → 2↑ (Equation 2-1)

2Al + 3O2-

- 6e-→ 2Al2O3 (Equation 2-2)

2.2.3 Barrier Layer Thinning

At the end of the anodization, the DC voltage is decreased from 25 V to 15 V, by 2 volt

reductions per minute, then decreased from 15 V to 9 V by 1 volt per minute, and is finally held

at 9 V for five minutes. In this way, the barrier layer at the bottom of the porous anodized

25

aluminum (PAO) layer is thinned to the thickness which will be suitable for electrodeposition

and, at the same time, maintains the mechanical strength for the substrate.

The decreasing voltage procedure is shown in Figure 2-2.

0 2 4 6 8 10 12 14 16

10

15

20

25

A

no

diz

atio

n V

olt

age

(V)

Time (min)

Figure 2-2: The stepwise anodization voltage decrease during the barrier layer thinning at the

end of anodization.

After the barrier layer thinning, the aluminum plates are taken out of the tank and rinsed with

water, then rinsed with de-ionized water and then dried thoroughly with air flow.

2.2.4 AC Electrodeposition

Before electrodeposition, the edges of the aluminum plates are covered with nail polish to

prevent preferential deposition of copper on the edges. The electrodeposition takes place in a 4 L

beaker; the electrodeposition solution is composed of 0.5 mol/L CuSO4∙5H2O (99%, Alfa Aesar)

and 0.285 mol/L H3BO3 (99.5% min., EMD Millipore). A single aluminum plate is immersed

26

into the solution vertically, with one copper plate (99.999%, Alfa Aesar) parallel at each side.

Copper plates work as counter electrodes and this copper – aluminum – copper structure makes it

possible to deposit copper into the template on both faces of the aluminum plates. The aluminum

plate connects to one terminal and the copper plates connect to the other terminal of the AC

power supply (Kepco BOP 20 – 50 MG). The set-up is illustrated by Figure 2-3.

Figure 2-3: Sketch of the AC electrodeposition set-up.

The aluminum plate is immersed in the electrodeposition solution for 5 minutes to ensure the

holes in the template are totally immersed into the solution and no air bubbles exist as barriers.

Once this is esbalished, 200 Hz of continuous sine AC current of 10 Vrms (Figure 2-4) is applied

to start the electrodeposition. The electrodeposition lasts for 10 minutes, after which both sides

of the aluminum plate will be covered with copper deposition and turn black. Sometimes, after

electrodeposition, the aluminum plate will show some yellowish color, especially on the margins,

due to the over deposition of copper. This can be removed during the liberation. The aluminum

plates covered with copper deposition are fully rinsed with de-ionized water and dried with air

27

flow.

0 5 10 15

-16

-12

-8

-4

0

4

8

12

16

Volt

age

(V)

Time (ms)

Figure 2-4: The continuous sine voltage produced by a Kepco BOP 20–50 Mg power supply for

the AC electrodeposition.

2.2.5 Liberation of CuNWs

The aluminum plates with CuNWs deposition are immersed into 0.6 mol/L H3PO4 solution for 5

minutes at room temperature. The bulk of copper on the surface is removed by rubbing the

surface with cleaning tissue (Kimwipe).

After removing the bulk copper deposition, the aluminum plates with CuNWs deposition are

immersed into 1 mol/L NaOH solution for 10 minutes at room temperature, in a fume hood. The

liberated u Ws, floating on the solution’s surface, are collected by a spatula, transferred into a

flask and cleaned with methanol under vigorous shaking. The mixture of CuNWs and methanol

is filtered, rinsed at least three more times with methanol, and transferred into 150 mL methanol.

28

After this, the CuNWs – methanol solution is placed into an ultrasound bath for 1 hour to

disperse the nanowires into the methanol. If needed, the copper nanowires can be filtered out,

dried in a vacuum and prepared to use in the melting process or disperse in methanol after

grinding and sonication treatment for solution mixing. At this point, the CuNWs are ready to be

used as filler or to be tested.

2.3 Morphology of CuNWs

Scanning electron microscopy (SEM) micrographs were obtained using a Philips XL30 scanning

electron microscope. All samples were treated with gold and palladium before taking the images.

Transmission electron microscopy (TEM) images were taken with a Tecnai F20 field emission

gun transmission electron microscope under 200 kV of accelerating voltage. The copper

nanowires were transferred onto a holey copper TEM grid for observation.

Figure 2-5: SEM image of copper nanowires.

29

Figure 2-6: TEM images of copper nanowires with whole nanowires (a) and zoom in image on

the crystalline part (b).

The SEM image (Figure 2-5) and TEM image show the general dimension and morphology of

the copper nanowires. he copper nanowires have an average length of 2.5 μm, and an average

diameter of 22 nm. From the TEM images, the diameter within a single wire varies from 20 nm

to 27 nm. Inside the copper nanowire, the copper atom does not have a totally disordered

formation; neither is it a perfect crystal. From the TEM image, it can be observed that the copper

nanowire is polycrystalline and comprised of the segments of crystalline and the amorphous

parts (Figure 2-6), which is also proven by selected-area electron diffraction (SAED) pattern of

the single copper nanowire [15]. This phenomenon is related to the electrodeposition process. In

this work, the electrodeposition uses AC deposition and the copper deposits into the template

during catholic half cycle. When the current changes direction, the copper atoms’ stac is

disturbed, causing the amorphous (disorder) segment. During the time interval of deposition

cycle, the copper atom stack will follow the same sequence and form the crystalline part.

However, it is not guaranteed to form a crystalline structure of copper nanowire in the time

(a) (b)

30

Plates cleaning

First anodization for 2 hours

Etching for 30 minutes

Anodization for 8 hours and barrier layer thinning

AC electrodeposition

CuNW liberation and cleaning

interval of the same direction current, as the slightest disturbance, such as agitation in the

solution, may destroy the order of atom deposition.

2.4 Discussion: One-step or Two-steps Anodization

The typical way to synthesize CuNWs is using two-steps, composed of: one-step pre-

anodization, etching, and a post-etching final anodization for 8 hours. The procedure is shown in

Figure 2-7.

Figure 2-7: The flow chart for the CuNWs synthesis process.

Two-steps anodization provides for more ordered pores on the template. It is proven that after the

first anodization, the pores are less self-ordered. The etching will improve the arrangement and

the distribution of the size of the pores, leading to more similarly dimensioned CuNWs after 8

hours anodization[14].

The new aluminum plates were treated with two-steps anodization, since the oxide layer is

normally thicker than those frequently used. After five repetitions of two-steps anodization,

31

however, the pattern of the pores is decided, leading one to wonder if the first anodization and

the etching are necessary. This question is discussed.

SEM images (Figure 2-8) provide a general idea about how the formation of CuNWs made by

one-step anodization is different from those made by two-steps anodization. From the SEM

images, the CuNWs from one-step anodization and two-steps anodization are similar in length,

diameter and configuration. The CuNWs are collected, suspended in methanol solution, spin-

coated on the quartz carrier and then sputter-coated with gold. The tips of the CuNWs are of high

brightness, as a result of the tip effect where the tips gather more electrons and reflect more

signals back to the detector.

Figure 2-8: SEM images of CuNWs: one-step anodization (a) and two-steps anodization (b).

(a) (b)

00

00

00

00

00

00

00

00

00

00

00

00

00

32

1 2 3 4 50

5

10

15

20

25

Nu

mb

er i

n t

he

Inte

rval

(%

)

Length - One step

(a)

1 2 3 4 50

5

10

15

20

25

Num

ber

in t

he

Inte

rval

(%

)

Length - Two steps

(b)

Figure 2-9: Bar charts of length distribution of CuNWs made by one-step anodization (a) and

two-steps anodization (b).

33

16 18 20 22 24 26 28 300

5

10

15

20

25

30

Num

ber

in t

he

Inte

rval

(%

)

Diameter - One Step

(a)

16 18 20 22 24 26 28 300

5

10

15

20

25

30

Num

ber

in t

he

Inte

rval

(%

)

Diameter - Two Steps

(b)

Figure 2-10: Bar charts of diameter distribution of CuNWs made by one-step anodization (a)

and two-steps anodization (b).

34

Table 2-1: Dimension comparison of CuNWs made by two-steps and one-step anodization by

measuring the copper nanowires in SEM images.

Dimension Comparison

Mean Standard

Deviation Maximum Minimum Median

Length (µm) (One-step) 2.56 1.11 5.33 0.73 2.35

Length (µm) (Two-steps) 2.46 1.03 5.49 0.71 2.38

Diameter (nm) (One-step) 22 2.97 30 16 22

Diameter (nm) (Two-steps) 21 2.82 30 15 22

The diameters of nanowires were measured by copper nanowire TEM images and the lengths

were measured from both SEM images and TEM images. There are several factors affect the

accuracy of the diameter measurement through SEM. First, the samples were coated with an

Au/Pd sputter-coating for 3.5 minutes to enhance the conductivity. According to the operation

manual, the coating layer would be less than 10 nm. However, when considering the diameter of

copper nanowire in nanoscale, the error could not be neglected. Second, the contrast in the

images is low and SEM has the limitation in that it generates images by collecting the secondary

elections emitted from the sample surface. Consequently, it is hard to tell if the nanowires are

fused together as one or overlap one other.

35

The length and diameter distribution and comparison are derived from measuring 100 CuNW

nanowires. The software used for measuring is ImageJ, which can analyze various formats of

images, as well as measure distances and angles by calculating the area and pixel value of the

objects. From the results in Table 2-1, the average diameter of one-step anodization – 22 nm is

very close to the average diameter of two-steps anodization, at 21 nm. The average lengths of the

two methods are also close to each other: 2.56 µm and 2.46 µm, respectively. By comparing the

bar charts of Figure 2-9, we can see the majority of CuNW have a length between 1 µm and 3

µm; about 7 % are shorter than 1 µm, and less than 10 % of two-steps anodization synthesized

CuNW is longer than 4 µm. This percentage is slightly greater at 12% for one-step anodization

CuNW. The diameter of CuNW made by two-steps anodization is mainly distributed between 18

nm to 24 nm, while the diameter for one-step is distributed from 18 nm to 26 nm (Figure 2-10).

From this statistical analysis, it can be concluded that skipping the etching process and using

only one-step anodization will not change the dimension very much. However, from the

comparison of standard deviation, both standard deviations of diameter and length of one-step

synthesized CuNW are larger than two-steps CuNWs. This means that a one-step anodization

procedure may produce more larger diameter nanowires, but the CuNW made by two-steps

anodization are more uniform. This is also proven by the function of etching, which can make

the pattern of aluminum oxide more uniform [14].

However, we need to determine whether the dimension of the nanowires is still the dominant

factor affecting conductivity when considering the application of the CuNWs in conductive

composites.

Copper Nanowire/polystyrene (CuNW/PS) composite was synthesized by the miscible mixing

and precipitation (MSMP) method. Polystyrene (Styron 666D) is provided by Americas

36

Styrenics. After the dry composite was obtained, it was molded into 11.5×26×0.88 mm3 size

samples to test the resistivity using either Keithley 6517A or Loresta GP electrometers at a

voltage of 90 V. A more detailed discussion of this method will be given in following chapters.

lectrical resistivity ρ is defined as ρ = R×A/l, where R is the electrical resistance of a uniform

sample of the material (measured in ohms, Ω), A is the cross-section area of the measured

sample (measured in square centimeters, cm2) and l is the thickness of the sample (measured in

centimeters, cm) so electrical resistivity ρ is in the unit of Ω･cm.

Electrical conductivity σ is the ability a substance conduct the electric current and it is calculated

as the inverse of resistivity: 1/ρ, and has the unit of /cm.

Figure 2-11 shows the electrical conductivity of the CuNW/PS samples made by the same

method; using the two types of copper nanowires.

-0.5 0.0 0.5 1.0 1.5 2.0 2.510

-16

10-14

10-12

10-10

10-8

10-6

10-4

10-2

100

102

Co

nd

uct

ivit

y (

S/c

m)

Concentration of CuNW (vol. %)

Two Steps One Step

Figure 2-11: Conductivity vs. CuNW concentration of two-steps and one-step anodization. Error

bar represents the standard deviation, some error bars are within the points.

37

By comparing the conductivity, it shows that the composite containing CuNW synthesized by

one-step anodization has slightly higher conductivity than the samples containing CuNW

synthesized by two-steps anodization. This result leads to the conclusions as listed below:

First, using one-step anodized CuNW will not weaken the conductivity of the composites; on the

contrary, it shows slightly improved conductivity.

Second, the dimension of CuNWs is not the controlling factor of the conductivity of the

nanowire samples.

2.5 Study of the Oxidation in the Processing

As introduced in Chapter One, oxidation of metal nanowire is the challenge in this project, which

will limit both conductivity and EMI shielding properties. The oxide layer can be introduced in

different ways during the processing. Lin et al. [15] reported copper oxide was found in

CuNW/PS nanocomposite by made melt mixing. Luo et al. [16], who studied the kinetics of

CuNWs synthesized by AC electrodeposition, found two oxidation stages exist and affect the

final oxidation degree.

Oxidation is not the only issue in the process; the production of copper carbonate (CuCO3) is

another problem which will influence the final conductivity of CuNWs. Copper carbonate is a

blue-green compound which is generated by copper reacting with moist air. Basic copper

carbonate occurs as malachite (Cu2(OH)2CO3) and azurite (Cu3(OH)2(CO3)2). Reactions might

happen as Equation 2-3 and Equation 2-4:

2 Cu (s) + O2 → 2 uO (s) (Equation 2-3)

38

2 Cu (s) + H2O (g) + CO2 + O2 → u(O )2 + CuCO3 (s) (Equation 2-4)

From the experimental practice, factors that may cause oxidation are listed.

2.5.1 Storage of the Plates – Oxidation Starting From the Tips

After the copper is deposited into the PAO template, the CuNW would be relatively stable since

the main part of CuNWs is covered and protected by the template. The tips, however, are not

protected. The first step of the reaction of oxidation is the adsorption of oxygen on the surface of

CuNWs. According to the study of Ma et al. [17], the most stable site for oxygen adsorption is

the long bridge site at the edge of the CuNWs. We can infer that the oxidation starts from the tips

and runs along the nanowires. Therefore, it is suggested to store the CuNWs in vacuum.

2.5.2 Liberation – Oxidation Accelerates When Exposed to Oxygen

Liberation is the process to remove the template using a chemical treatment. In this study, NaOH

was used to dissolve the aluminum oxide template. After the template is dissolved, the CuNWs

float on the surface of the solution and are exposed to oxygen directly, run the risk of being

oxidized. Also, the heat generated from the reaction of sodium hydroxide with aluminum oxide

(Equation 2-5) accelerates the oxidation.

Al2O3 + 2 NaOH + 3 H2O → 2 Na+ + 2 [Al(OH)4]

- (Equation 2-5)

In this procedure, the copper also interacts with CO2 and H2O in the air, causing the reaction in

Equation 2-4. The Cu(OH)2 and CuCO3 are insulative, meaning they may compromise

conductivity.

39

Because of the reactions that could happen, shortening the time of liberation is critical. Setting a

limit on the numbers of plates to be liberated at one time can also be considered; two plates

would be proper, whereas more than two plates would prolong the liberation time.

2.5.3 Time From Sonication and Mixing

After the CuNWs are liberated, they are normally kept in methanol or dried and kept in the

powder phase. When kept in methanol, CuNWs tend to separate to the bottom of the flask, which

can keep the CuNWs from contacting the main oxygen atmosphere. Both oxygen and carbon

dioxide, however, have certain solubility in methanol at room temperature (298.15 K) and

101.325 kPa. The mole fraction of oxygen is 4.15 [18]. At the same time, the solubility of CO2 in

methanol, in the same situation, is 1.683 mol/kg [19]. This amount of O2 and CO2 is enough to

react with CuNWs, which are active due to the large surface area. For this reason, the time for

both sonication and mixing would need to be optimized. For sonication, if the time is not enough,

the dispersion of CuNWs would not be sufficient; CuNWs might be bundled up. For mixing, if

the time is short, the dispersion and the distribution of CuNWs in the polymer matrix would be

poor.

2.5.4 Compression Molding

For CuNWs/polymer samples, compression molding was used to form the samples into different

dimensions. The compression molding temperature would need to be higher than the melting

temperature of the polymer which, for those used in this study, is around 170 ºC. According to

chemical reaction kinetics, for certain reactants (copper, oxygen, carbon dioxide etc.), the

reaction rate is closely related with the concentration of the reactants, the temperature, pressure

and the catalyst. This characteristic makes it critical to decrease the contact time of the sample

40

with oxygen under the molding temperature (200 ºC) and to exercise careful control over the

molding procedure.

2.6 Conclusions

Copper nanowires were synthesized by AC electrodeposition with 10 Vrms of copper on porous

aluminum oxide (PAO) templates for 10 minutess. PAO templates were produced by anodizing

high purity aluminum plates in 0.3 mol/L H2SO4 solution, with 25 V power supply, at a

temperature of 0 – 4 °C, followed by barrier layer thinning with stepwise decrease of DC voltage

from 25 V to 9 V.

The morphology and the dimensions of CuNWs synthesized by two-steps and one-step

anodization were compared and the dimensions of CuNWs are slightly different for each case.

CuNWs made by one-step synthesis have less uniform diameter and length distribution.

Conductivity of CuNW/PS samples using CuNWs synthesized by two-steps and one-step

anodization were tested and compared. CuNWs made by one-step anodization are less uniform,

but the conductivity is actually higher than those made by two-steps anodization. For the

conductivity of CuNW/polymer composites, the dimensions are not found to be the controlling

factors.

The procedures which can introduce side reactions (oxidation and reaction with carbon dioxide)

are studied and discussed. Factors to prevent oxidation are: using vacuum storage rooms for

deposited plates, shortening the liberation time, controlling the sonication and mixing time, and

closely monitoring the compression molding procedure.

41

References

[1] A.R. Rathmell, S.M. Bergin, Y.-L. Hua, Z.-Y. Li, B.J. Wiley, The Growth Mechanism of

Copper Nanowires and Their Properties in Flexible, Transparent Conducting Films, Advanced

Materials 22 (2010) 3558-3563.

[2] D. Zhang, R. Wang, M. Wen, D. Weng, X. Cui, J. Sun, H. Li, Y. Lu, Synthesis of Ultralong

Copper Nanowires for High-Performance Transparent Electrodes, Journal of the American

Chemical Society 134 (2012) 14283-14286.

[3] E. Ye, S.-Y. Zhang, S. Liu, M.-Y. Han, Disproportionation for Growing Copper Nanowires

and their Controlled Self-Assembly Facilitated by Ligand Exchange, Chemistry - A European

Journal 17 (2011) 3074-3077.

[4] H. Guo, N. Lin, Y. Chen, Z. Wang, Q. Xie, T. Zheng, N. Gao, S. Li, J. Kang, D. Cai, D.-L.

Peng, Copper Nanowires as Fully Transparent Conductive Electrodes, Scientific Reports 3

(2013).

[5] G.A. Gelves, M.H. Al-Saleh, U. Sundararaj, Highly electrically conductive and high

performance EMI shielding nanowire/polymer nanocomposites by miscible mixing and

precipitation, Journal of Materials Chemistry 21 (2011) 829.

[6] D. Routkevitch, A.A. Tager, J. Haruyama, D. AlMawlawi, M. Moskovits, J.M. Xu,

Nonlithographic nano-wire arrays: fabrication, physics, and device applications, Electron

Devices, IEEE Transactions on 43 (1996) 1646-1658.

[7] R.L. Fleischer, P.B. Price, R.M. Walker, Method of Forming Fine Holes of Near Atomic

Dimensions, Review of Scientific Instruments 34 (1963) 510.

[8] G.E. Possin, A Method for Forming Very Small Diameter Wires, Review of Scientific

Instruments 41 (1970) 772.

[9] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, One-

Dimensional Nanostructures: Synthesis, Characterization, and Applications, Advanced Materials

15 (2003) 353-389.

[10] C.J. Murphy, A.M. Gole, S.E. Hunyadi, C.J. Orendorff, One-Dimensional Colloidal Gold

and Silver Nanostructures, Inorganic Chemistry 45 (2006) 7544-7554.

[11] T.R. Kline, M. Tian, J. Wang, A. Sen, M.W.H. Chan, T.E. Mallouk, Template-Grown Metal

Nanowires, Inorganic Chemistry 45 (2006) 7555-7565.

42

[12] K. Nielsch, F. Müller, A.P. Li, U. Gösele, Uniform Nickel Deposition into Ordered Alumina

Pores by Pulsed Electrodeposition, Advanced Materials 12 (2000) 582-586.

[13] J. C. Hulteen, C.R. Martin, A general template-based method for the preparation of

nanomaterials, Journal of Materials Chemistry 7 (1997) 1075-1087.

[14] G.A. Gelves, Z.T.M. Murakami, M.J. Krantz, J.A. Haber, Multigram synthesis of copper

nanowires using ac electrodeposition into porous aluminium oxide templates, Journal of

Materials Chemistry 16 (2006) 3075.

[15] B. Lin, G.A. Gelves, J.A. Haber, P. Pötschke, U. Sundararaj, Electrical, Morphological and

Rheological Study of Melt-Mixed Polystyrene/Copper Nanowire Nanocomposites,

Macromolecular Materials and Engineering 293 (2008) 631-640.

[16] X. Luo, U. Sundararaj, J.-L. Luo, Oxidation kinetics of copper nanowires synthesized by

AC electrodeposition of copper into porous aluminum oxide templates, Journal of Materials

Research 27 (2012) 1755-1762.

[17] L. Ma, J. Zhang, K. Xu, First-principles study of the adsorption of oxygen atoms on copper

nanowires, Science China Physics, Mechanics and Astronomy 55 (2012) 413-418.

[18] R. Battino, T.R. Rettich, T. Tominaga, The Solubility of Oxygen and Ozone in Liquids,

Journal of Physical and Chemical Reference Data 12 (1983) 163-178.

[19] I. Urukova, J. Vorholz, G. Maurer, Solubility of Carbon Dioxide in Aqueous Solutions of

Methanol. Predictions by Molecular Simulation and Comparison with Experimental Data, The

Journal of Physical Chemistry B 110 (2006) 14943-14949.

43

CHAPTER 3: POLYPROPYLENE COMPOSITES WITH CUNW OR CNT AS

FILLERS

3.1 Introduction

Conductive polymer composites (CPCs) have been intensively studied due to their many

advantages, including good processability, corrosion resistance, comparatively low weight and

low cost. One way to fabricate CPCs is to embed conductive fillers into a polymer matrix. Fillers

include carbon black, carbon nanotube, graphene, carbon nanofibers and metal fibers.

Studies show that by adding conductive fillers, the polymer composites gain shielding property

for electromagnetic interference [1]. High electromagnetic interference (EMI) shielding

performance gives CPCs the potential to be used widely in applications such as laptops, cell

phones, aircraft electronics, and medical device housings [2]. EMI is closely related to electrical

conductivity. Even though high conductivity does not necessarily lead to high EMI shielding

effectiveness, studies show that for a nanocomposite with conductive fillers, the more conductive

it is, the higher EMI shielding effectiveness it will have [3]. Researchers have tried different

ways to increase the conductivity of the composites. Shen et al. [4] claimed the resistivity

reduced two orders of magnitude by stretching isotactic polypropylene/carbon nanotube

(iPP/CNT) samples through reconnecting adjacent particles and forming a new network. Wen et

al. [5] found that mixing carbon black(CB) and multiwall carbon nanotube (MWCNT), with a

weight ratio of 6:1, and using a stretching extrusion in a PP matrix, can result in a lower

percolation threshold and higher conductivity. Carbon black can aggregate around MWCNTs

and provide a short charge transport distance.

44

Highly conductive copper nanowire/polystyrene (CuNW/PS) nanocomposite, with a high EMI

shielding effectiveness at low CuNW concentration created by miscible mixing and precipitation

(MSMP) method [6], was demonstrated. A percolation threshold at 0.67 vol. % CuNW [6] was

reported. In this study, we are extending our studies to copper nanowire/polypropylene

(CuNW/PP) composites. As one of the most commercially used polymers, PP exhibits excellent

thermal and electric properties and, most importantly, low cost. Therefore, it is a good choice for

the matrix. Several studies on polypropylene mixed with other fillers, such as Carbon black [5]

CNT [7,8,9] can be found, but studies on copper nanowire as fillers are limited. The challenge is

that the solution method might cause foam in the PP and this formation could not be processed

further. Dang et al. [10] came up with a combination of solution and melting methods which

involve solution mixing the materials, with temperatures held in control (100 °C) for solvent

evaporation, followed by melting the composites film to 170 °C, then cooling them down to

room temperature to further fuse the composite. In our study, we found that compression

molding can be a practical method for processing PP, even if foam is formed. Herein, we report

the electrical conductivity and EMI shielding properties of the CuNW/PP composites prepared

using the MSMP method [6]. At the same time, we studied the EMI shielding properties of the

CNT/PP composites made by the same method, as a comparison.

3.2. Experimental

3.2.1 Materials

The preparation of the CuNWs by AC electrodeposition of copper into the anodic porous

aluminum oxide template was described in Chapter 2. The procedure includes using 25 V 8

hours anodized aluminum electrodes (Alfa Aesar, 99.99+ %) as templates, then using AC

45

electrodeposition to deposit copper into the template. Following this process, the CuNW will be

liberated in 0.1 M NaOH solution. The CuNWs are first washed with, and then kept in, methanol

for sonication to disperse. They are then ready to use. These CuNWs have an average diameter

of 22 nm and an average length of 2.5 µm. The concentration of the CuNWs in the methanol

solution is determined by weighing the solid content after evaporating the methanol in a fume

hood for one hour.

Homopolymer Polypropylene 0500 , with a melt flow rate of 5g∙(10min)-1

(ASTM D1238),

was purchased from Flint Hills Resources (Longview, Texas, US). It has a density of 0.90 g/cm3

and a melting temperature of 165 – 170 ºC.

NC7000 multiwall carbon nanotube (MWNT) was obtained from Nanocyl, S.A. NC7000

MWNT has an average length of 1.5 µm, an average diameter of 9.5 nm and a density of 1.73

g/cm3.

3.2.2 Preparation of CuNW/PP Composite

The CuNW/PP composite powder was prepared by the miscible mixing and precipitation

(MSMP) method [6]. Copper nanowires (CuNWs) were first dispersed in methanol. PP was

dissolved into xylene (98.5% min., BDH) at 120 ºC. For PP, xylene is a solvent, while methanol

is a non-solvent. While the PP solution was still hot, different volumes of certain concentrations

of the CuNW/methanol solutions were dripped into the PP solution under sonication.

Concentration of CuNW/methanol solution was determined by weighing the mass of copper

nanowires in 2 ml solution after the methanol fully evaporated. Extra methanol was added until

methanol-xylene ratio was 3:1 to ensure all the polypropylene was precipitated. By adding the

non-solvent methanol, the CuNWs precipitates out with the PP. The resulting mixture was

46

treated with ultrasonication, in an 80 ºC ultrasound bath with 120 W output power for 10 minutes.

The CuNW/PP mixture was filtered out and transferred into an evaporation dish placed in a fume

hood, at ambient environment, to evaporate most of the methanol and xylene. Then the mixture

was further dried in a vacuum oven at 40 ºC for 2 hours to remove residual solvent.

The dry mixture of the CuNW/PP composite was annealed into 0.87×25×11.6 mm3 samples

using a Carver compression molder at 190ºC and 34.5 MPa (5000 psi) for 15 minutes. Aluminum

foil (AL A ™ Aluminum Foilwrap) was used as release films between the mold and

top/bottom stainless steel hot plates. The molded samples remained in the molder while cooling

down to room temperature with circulating tap water for 10 minutes. When the samples were

released from the molder, the top side was marked with paint marker. The processing procedure

is illustrated as Figure 3-1.

Figure 3-1: The flow chart of processing procedure of CuNW/PP composite.

CuNW in Methanol solution

(room temperature)

PP in xylene solution

(120 oC)

Miscible mixing and precipitation (MSMP) method

Evaporation of solvent

Compression molding (190 oC)

Characterization and property test

47

The carbon nanotube/polypropylene (CNT/PP) samples were prepared in the same method to

compare with CuNW/PP composite. The detailed procedure can be obtained by replacing CuNW

with CNT in preparation flow chart Figure 3-1.

3.2.3 Morphological Characterization of the CuNW/PP Nanocomposites

The SEM images were taken by a Philips XL30 scanning electron microscope. The samples were

fractured in liquid nitrogen, then affixed to the sample stage with carbon tape, with a fractured

cross-section placed on the top for images. Carbon paste was added to the bottom to enhance

conductivity. Before the images were taken, the samples were coated with gold and palladium by

cathode sputtering. The accelerating voltage of the operation was 20 kV.

Transmission electron microscopy (TEM) images were taken with a Tecnai F20 field emission

gun transmission electron microscope under 200 kV of accelerating voltage. The sample were

microtomed in nitrogen and transferred onto a copper TEM grid for observation.

Optical Microscope images were taken by an Olympus BX60 microscope. The composite

samples were obtained by a Leica RM2265 Microtome.

3.2.4 Electrical Properties Measurements of the CuNW/PP Nanocomposites

The electrical resistivity measurements of the CuNW/PP were conducted using two different

electrometers. For the nanocomposites with resistivity higher than 106 Ω∙cm, the electrical

resistance measurements were performed using a Keithley 6517A electrometer. The Keitheley

6517A electrometer was connected to an 8009 test fixture, with Keithley 6524 high resistance

measurement software conforming to ASTM D991-89 standards (see Figure 3-2 (a)). For

resistivity lower than 106 Ω∙cm, a Loresta GP resistivity meter (MCP-T610 model, Mitsubishi

48

Chemical Co., Japan) was connected to an ESP four-pin probe (MCP-TP08P model, Mitsubishi

Chemical Co., Japan) with inter-pin distance of 5 mm and pin diameter of 2 mm (see Figure 3-2

(b)). This was used to minimize the impact of contact resistance, according to ASTM 257-75

standards.

For the Keithley 6517A electrometer, the volume resistance Rv was converted to resistivity ρv by

the Equation 3-1:

ρv = Rv×A/t (Equation 3-1)

Here, ρv is the resistivity, Rv is the resistance, A is the effective area of the tested samples, and t

is the thickness of the samples. The tested sample was placed in the center of the electrodes of

the fixture, with a PTFE sheet covering the extra area of the electrodes which the sample cannot

cover. In this way, a short circuit of direct connection of the top and bottom electrodes can be

avoided. For the Loresta GP, the four-pin constant current method of measuring is used. The

working mechanism is illustrated in Figure 3-2 (c).

49

Figure 3-2: Keithley 6517A electrometer with an 8009 test fixture (a), Loresta GP resistivity

meter with ESP four-pin probe (b) and the illustration of the working mechanism of a four-pin

resistance testing (c).

3.2.5 Electromagnetic Interference Shielding Properties

The Electromagnetic Interference (EMI), shielding effectiveness (SE) and permittivity

measurements were carried out with an Agilent Vector Network Analyzer (Model E5071C)

(Figure 3-3 (a)) with an X-band frequency range (8.2 GHz – 12.4 GHz). Sample holders of

140mm were placed between two wave guides and connected to separate ports of the analyzer

(Figure 3-3 (b)). The mechanism of EMI shielding was discussed in Chapter 1. Briefly, the

shielding effectiveness was calculated by the power reflected and absorbed compared with the

power of incident wave, where the power of incident wave was sent from the analyzer port 1.The

power reflected was measured with port 1 as well and the transmitted power was collected by the

analyzer port 2 as marked out in Figure 3-3 (a).

50

Figure 3-3: EMI shield equipment: Agilent Vector Network Analyzer (Model E5071C), the

wave guides (a) and the sample holder (b).

3.2.6 Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) was conducted using a DSC Q100 by TA Instruments.

The temperature calibration was done with Indium (Tm=156.6 ° , Δ m =28.4J/g). The analyses

were done in a nitrogen atmosphere, using standard aluminum pans. Samples (4 – 6 mg) were

heated to 250 °C, with a heating rate of 10 °C/min, held for 5 minutes to erase the thermal

history effects, then cooled down to -30 °C, followed by reheating to 250 °C, with a heating rate

of 10 °C/min. The crystallisation temperature Tc was obtained from the peak of the

crystallisation exotherm during the cooling cycle. he heat of crystallisation (Δ c) was

determined by the area under the crystallisation pea . he heat of fusion (Δ m) was obtained

51

from the area of the melting peak. The crystallization onset and peak temperatures were

determined according to ASTM D3418.

3.2.7 X-Ray Diffraction (XRD)

X-Ray Diffraction (XRD) patterns were obtained from a Geigerflex 2173 XRD machine from

Rigaku Corporation, with a vertical goniometer and cobalt as an X-ray source. The machine also

has a scintillation detector with a graphite monochromator. The test ran from 2 degrees to 65

degrees, with 2θ at 1 degree per minute, with a 0.02 degree step size.

3.3 Results and Discussion

3.3.1 Electrical Conductivity

Electrically conductive polymer nanocomposites were made by adding conductive fillers into the

polymer matrix. The composite becomes conductive once the fillers form a continuous network.

The minimum loading of the conductive filler, to make the composites conductive, is known as

the percolation threshold. The aspect ratio, concentration and surface properties of the fillers,

dispersion, distribution and alignment of the filler in the polymer matrix will affect the

percolation threshold and electrical conductivity [11].

52

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

1E-17

1E-15

1E-13

1E-11

1E-9

1E-7

1E-5

1E-3

0.1

10

Co

nd

uct

ivit

y (

S/c

m)

Concentration of CuNW (vol%)

Conductivity of CuNW/PP

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

1E-17

1E-15

1E-13

1E-11

1E-9

1E-7

1E-5

1E-3

0.1

10

Co

nd

uct

ivit

y (

S/c

m)

Concentration of CNT (vol%)

Conductivity of CNT/PP

Figure 3-4: Volume conductivity of CuNW/PP composite (a) and CNT/PP composite (b) as a

function of filler (CuNW or CNT) concentration. Error bar represents the standard deviation.

The electrical conductivity of the CuNW/PP composite increased by about 18 orders of

magnitude compared with pure PP by adding 3 vol. % CuNW, and reached 5 S/cm at a CuNW

(a)

(b)

53

concentration of 3.0 vol. % (Figure 3-4, (a)). A plateau was found on the conductivity curve

around 10-7

S/cm at CuNW concentration from 1.2 vol. % to 1.7 vol. %. For the CNT/PP

composite (Figure 3-4 (b)), the percolation threshold is much lower; it reached conductivity of

10-2

S/cm at 0.8 vol. % and a conductivity of 10-1

S/cm at 3.0 vol. %. A narrower plateau showed

up around 0.4 vol. % - 0.6 vol. % of CNTs also, with the conductivity of 10-7

S/cm. The

conductivity of the CuNW/PP composite is much higher than the CNT/PP composite after

percolation threshold.

This is very different than the typical percolation curve reported in other studies [6], where the

conductivity increased dramatically near the percolation threshold with no plateau behavior. For

CuNW/PP, it only reached high conductivity of 1 S/cm above 1.7 vol. %. The plateau between

0.8 and 1.7 vol. % shows a much wider percolation threshold region. This wider percolation

concentration window offers potential for the composite to be applied in charge storage devices

[11].

3.3.2 Morphology of Composites

To explain the plateau phenomenon, the morphology and the crystallization rate were studied.

The MSMP method is developed to give a perfect dispersion of fillers in the matrix, which is the

case in the CuNW/PS composite [6]. From the TEM image of the CuNW/PP sample (Figure 3-5),

at a CuNW concentration of 0.7 vol. %, agglomeration can be observed.

54

Figure 3-5: TEM image of cross-section of CuNW/PP composites at the concentration of 0.7

vol. % and 3D schematic illustration of composites. Agglomerations can be found, as circled in

the matrix.

As mentioned previously, percolation can be achieved once the conductive fillers form an

effective network. Our guess is that in PP/CuNW composites, the conductivity of the PP

composites is first controlled by the distribution of the CuNW agglomeration, instead of the

CuNW dispersion inside the matrix. When a certain concentration (0.7 vol. %) of filler is reached,

excess fillers will tend to join the clump in the formation, rather than disperse inside the polymer

55

as a single nanowire. When the distributed clumps and dispersed fillers form an effective

network, the composite becomes conductive. Network of clumps of CuNWs can be observed and

some examples are shown in Figure 3-6.

Figure 3-6: TEM images of CuNW/PP composite, (a) 1.0 vol. %, (b) 1.7 vol. %.

There may be several reasons for the agglomeration of the CuNWs in the PP matrix. First, the

cloud point of the PP in xylene is easily reached. Cloud point is a temperature when a phase

separation occurs and there is precipitation of the polymer; it depends on the solvent/non-solvent

ratio [12]. During MSMP, for the PP and CuNW systems, temperature drops quickly since the

ultrasound bath temperature (80 ºC) is lower than the PP solution temperature (120 ºC). At the

same time, a room temperature CuNW/methanol solution is added to the PP solution.

Evaporation of the methanol causes additional heat transfer. Once the cloud point is reached, the

PP starts to precipitate out of the system, without good dispersion of the CuNWs. In addition,

van der Waals forces between the CuNWs are strong and dominant, due to the large surface area,

(a) (b)

56

as compared to weak interfacial attraction between the nanowire surface and polymer chains in

solution. This presents the nanowires from being homogeneously distributed during the polymer

precipitation process. Other possible reasons for the poor dispersion of the CuNW in the PP

would be that the PP (Flint Hills H0500HN) has higher viscosity than the PS (Styron 666D). The

higher the viscosity, the harder for the fillers to disperse in the matrix.

3.3.3 Crystallinity and DSC results

To investigate the relationship between agglomeration and composite structure, crystallinity of

the composites were studied. Crystallinity of a polymer can influence the dispersion of fillers as

well, especially when the filler has a certain tendency to distribute in either an amorphous or

crystalline region [13], and PP is a semi-crystalline polymer. At the same time, the introduction

of fillers would affect the crystalline phase of the PP [14,15,16]. The PP crystalline phase

includes four inds: monoclinic (α), hexagonal (β), orthorhombic (γ) and mesomorphic (semctic)

[14,17,18,19,20]. To study the effect of adding fillers on the crystallinity of polymer, the DSC

results are compared. DSC is a widely used thermal-analysis technique. It measures the heat

needed to increase certain amount of temperature of a sample and reference sample. By

following a certain cooling rate, the crystallization exotherms can be recorded and useful

information can be collected. The information includes: Tp, the peak temperature of

crystallization exotherm; Tc, the temperature at the intercept of the base line and the exotherm, at

the high temperature side; Si, the slope of the initial portion of the exotherm; ∆W, the width at

half-height of the exotherm pea ; ∆ c, the heat of fusion.

During the second heat up and cooling cycle, the peak temperature of crystallization melting (Tm)

and the melting heat of crystallization (∆ m) can be obtained.

57

Tc is the temperature where the sample starts to crystallize during the cooling process; Tp is the

peak temperature of the crystallization. As shown in the Table 3-1 and Figure 3-7, both Tc and Tp

increase with an increasing concentration of the CuNW in the composites. This means the

crystallization of the sample becomes easier by adding copper nanowires. Also, the increase is

relatively large at the first loading of copper nanowires (from 110.39 oC to 118.94

oC, i.e. 8.55

oC difference over 0.4 vol. % filler loading), compared with the higher concentration of CuNW

(within 2 oC when the concentration increases from 2 vol. % to 3 vol. %). Based on these results,

the conclusion is that the CuNWs are acting as nucleating agents to accelerate the crystal growth

at low loading. At higher concentrations of the CuNW, the agent becomes saturated and cannot

promote the heterogeneous nucleation at the same rate.

Tc – Tp is a measure of the overall crystallization rate. The smaller the Tc – Tp, the greater is the

crystallization rate. The slope of the initial portion of the exotherm Si is another parameter of the

crystallization rate. The greater the slope, the faster the nucleation happens. From Table 3-1, all

of the samples with the CuNW have greater Si and smaller Tc – Tp than the pure PP. Essentially,

composites with copper nanowires filler have faster nucleation rates.

The width at half-height of the exotherm pea , ∆W, is a measure of the distribution of crystallite

size, determined after the normalization of the peaks to a constant mass of the samples. The

smaller the width at half-height of the exotherm pea , ∆W, means the narrower the crystallite

size distribution. he u W/PP composite shows smaller ∆W, which means the u W/PP has

a more uniform crystallite size.

Tm is the peak temperature of crystallization melting. The difference in the melting peak and

crystallization peak in temperature (Tm – Tp) is an indicator for the degree of undercooling. As

58

shown in Figure 3-8, Tm changed slightly with the different CuNW concentration, with less than

1 oC difference in the whole loading range.

59

Table 3-1: Non-isothermal crystallization of CuNW/PP composites.

60

Figure 3-7: DSC cooling scan of CuNW/PP composites (after molding) at 10 °C/min.

Figure 3-8: DSC Second heating scan of CuNW/PP composite (after molding) at 10 °C/min.

61

Figure 3-9: DSC cooling scan of CuNW/PP composite (before molding) at 10 °C/min.

Figure 3-10: DSC Second heating scan of CuNW/PP composite (before molding) at 10 °C/min.

62

The CuNW/PP composite before molding was tested as a comparison to study the effect of

compression molding. By comparing the cooling scan of the composite before and after molding

(Figure 3-7 and Figure 3-9), the increase in Tc and Tp of before molding samples are smaller

(9.54 oC compared to 16.5

oC). The Tm shifted slightly as well.

CNT composite behaves similarly to CuNW composite. When the loading of CNT is low, the

CNT acts as a nucleation agent to accelerate the crystallization. As shown in Figure 3-11, during

the cooling scan of the CNT/PP composite, the crystallization peak temperature increased from

110. 39 oC to 128.84

oC, In Figure 3-12, showing the second heating scan of the CNT/PP

composite, the melting peaking increased slightly with increased loading.

Figure 3-11: DSC cooling scan of CNT/PP composite 10 °C/min.

63

Figure 3-12: DSC second heating scan of CNT/PP composite 10 °C/min.

Crystallinity of composites is calculated by the Equation 3-2:

Xc = Δ m/Δ 0 (Equation 3-2)

ere, Δ m is the melting heat of crystallization obtained from the second heating scan and is

listed in Table 5-1, Δ 0 is the melting enthalpy of 100% crystalline PP, and Δ m = 207.1 W/g

[21,22].

For the composite, however, the crystallinity needs to be normalized to the polymer by using the

Equation 3-3 [23]:

Xc = Δ m/Δ 0(1-Wf) (Equation 3-3)

64

Here, Wf is the weight percentage of the filler, Δ m is the melting heat of crystallization and Δ 0

is the melting enthalpy of the 100% crystalline PP.

Figure 3-13: Crystallinity of CuNW/PP composites. Error bar represents the standard deviation,

some error bars are within the points.

The crystallinity of CuNW/PP composite is studied and illustrated in Figure 3-13. The samples

show maximum crystalline fraction at CuNWs concentration of 0.4 vol. %. Crystallinity then

decreases with increasing CuNWs loading. This peak implies that when small amounts of the

CuNWs are added to the PP matrix, the CuNWs act as nucleation agents to promote

heterogeneous nucleation. The mobilization of the PP molecular chains is enhanced because

adding the CuNWs accelerates the nucleation rate. However, when the loading of the CuNWs is

above 2.0 vol. %, the nanowires start to block the mobilization of the PP chains, resulting in a

lower probability of ordered crystal lattice alignment and therefore decreased crystalline fraction.

65

Surprisingly, between concentration of 0.8 – 1.7 vol. %, i.e. in the plateau area, the crystallinity

remains almost same, as do other parameters as listed in Table 5-1. This is compelling evidence

that the number of the dispersed CuNWs did not increase in the matrix, but are actually

agglomerated or bundled up. When the volume fraction of the CuNWs increases, the effect of the

CuNW bundles and the increasing number of dispersed nanowires result in a decreasing trend in

crystallinity.

Figure 3-14: Crystallinity of CNT/PP composite. Error bar represents the standard deviation,

some error bars are within the points.

The CNT/PP composites show a peak of crystallinity of 59 % at a CNT concentration of 0.1 vol. %

(Figure 3-14). This loading is much lower than reported in Kaganj et al. [24], as they reported a

peak at 1 % CNT loading. One reason for the discrepancy would be the different processing

technique, as Kaganj used melt mixing, while in this project, solution-based MSMP is used.

66

Similarly for the CuNWs based composite, there is a crystallinity peak on the CNT/PP curve as

well, which represents the effect of different loading of the fillers. Also, crystallinity remains

same around the percolation zone (0.5 vol. % to 0.7 vol. %) which is a sign of filler bundles.

What can be noticed is that the percentage drop of crystallinity from the CNT samples is not as

obvious as the CuNWs samples at high concentrations (3.0 vol. %). For the CNT/PP composite,

all the composites have a crystalline fraction higher than that of the pure PP, and the percentage

drop of crystallinity is less than 5 percent (59 % to 55 %). On the other hand, the CuNW/PP

composite has a maximum crystallinity of 57 % and drops to 50 % at a CuNW concentration of 2

vol. % and 47 % at a CuNW concentration of 3 vol. %.

To study the crystallinity, optical microscope was used to see how the crystallite’s grow inside

the composites. As shown in Figure 3-15, the size of the lamella decreases with increasing of the

CuNW loading. Also, the CuNWs act as nucleates, but more nucleates actually limit the growth

of crystals. The big agglomerations stop the crystals from growing larger as well.

67

Figure 3-15: Optical Microscope images of CuNW/PP samples (a) Neat PP, (b) CuNW/PP 0.4

vol. %, (c) CuNW/PP 0.7 vol. %, (d) CuNW/PP 1.7 vol. %.

Figure 3-16: Optical microscope images of CNT/PP samples: (a) CNT/PP 0.3 vol. %, (b)

CNT/PP 0.5 vol. %.

For the CNT/PP samples, the crystal size is not influenced within percolation ranges. As shown

in Figure 3-16, the CNT loading increases from 0.3 vol. % to 0.5 vol. %; the volume of the CNT

agglomerations increased, while the size of lamella remains almost the same.

68

From the microscope images, the crystalline phase could not be discerned. To verify the

crystalline phase of the CuNW/PP samples, XRD tests were conducted. XRD is a method to

determine the crystal structure by measuring the angles and intensities of diffracted beams

according to ragg’s law:

2dsinθ = nλ (Equation 3-4)

where d is the spacing between diffracting planes, θ is the incident angle, n is any integer and λ is

the wavelength of the beam, as illustrated in Figure 3-17.

Figure 3-17: Illustration of X-rays diffractions showing the relationship of incident beam, angle

and distance between the diffracting planes.

As mentioned earlier, crystalline PP has α, β and γ phase, and different structures depend on the

processing conditions and thermal history. Among these phases, an α phase crystal is monoclinic;

the most stable and most observed phase. he β phase would appear under the β-phase

nucleating agent, like carbonate [25] and silica [26], or under specific conditions, such as

temperature gradient and strain [16,27]. he crystallization of the γ phase is reported to be

obtained under high pressure or shear flow conditions [28,29]. Not only do processing conditions

69

affect crystal nucleation of polypropylene, but the introduction of fillers may change the

crystalline structure. Misra et al. [14] pointed out that the introducing of clay would halt the

morphology of monoclinic and orthorthombic structure and promote the formation of γ phase at

ambient pressure.

For the CuNW/PP composite, crystal phases can be identified by the peaks on the XRD patterns,

as marked out in Figure 3-18. Peaks can be characterized by their 2θ value. ix distinct α phase

peaks are found at 2θ values of 14.2º, 17.0º, 18.7º, 21. º, 21.7º and 25.4º. These peaks

correspond to the (110), (040), (130), (111), (041) and (060) reflections. In the composites

containing 0.7 vol. % and 1.0 vol. % copper nanowires, the pea at 16 º corresponds to the β

phase ( 00) reflection. he γ phase crystallization pea at 20 º, corresponding to the (117)

reflection, is not obvious. he existence of the β phase at 16 º in the u W/PP composites

containing 0.7 and 1.0 vol. % of copper nanowires, indicates that copper nanowires can act as

nucleating agents. However, the peak does not appear in the composite containing 0.4 vol. %

copper nanowire. It becomes smaller in 1.0 vol. % composites as compared with 0.7 vol. % and

disappears entirely in higher concentrations (1.7 vol. %). The assumption is that copper

nanowires are agglomerating during the plateau concentrations (0.7 – 1.7 vol. %). It can be

inferred that only certain sizes of agglomerations can act as β phase nucleating agents.

70

Figure 3-18: X-ray diffraction profiles of neat PP and CuNW/PP composite with different

CuNW concentrations.

The SEM images of the CuNW/PP samples before compression molding (Figure 3-19) show

more detail about the bonding of the CuNW-PP and CuNW-CuNW. The PP forms spherical

particles, by nucleation, in the solution surrounding it. The average diameter of the PP particles

is .4 μm. opper nanowires partly cover the PP particles. Some connect differently to form a

network, while others bundle up to create agglomerate, with agglomerate size range from 1 μm

to 10 μm.

71

Figure 3-19: SEM images of CuNW/PP samples before molding with CuNW concentration 1.0

vol. % (left) and 1.4 vol. % (right).

3.3.4 Electromagnetic Interference (EMI) Shielding Effectiveness (SE) of CuNW/PP Composite

EMI SE is the ability of a material to block or reduce the influence of the incident energy which

is radiated or conducted. EMI may impair the performance of the devices. In this experiment,

EMI SE is the logarithm of the ratio of the incident energy field to the transmitted energy field

and is reported in the unit of dB.

The shielding effectiveness of the CuNW/PP composite is depicted as Figure 3-20. Overall SE is

plotted over the testing frequency as shown in Figure 3-20 (a). The SE for each concentration is

based on one sample. As illustrated, the overall SE increases with increasing loading of copper

nanowire and remains in a small range of vibration for each concentration. Shielding by

reflection and absorption are found to follow same pattern which is remains almost same over all

testing frequency. SE is calculated by averaging the value over all testing frequency, then

calculated the average SE for all samples with same concentration. In this way, SE of CuNW/PP

samples with different concentrations is obtained as shown in Figure 3-20 (b). SEr, SEa, SEo

CuNW/PP 1.0 vol. % CuNW/PP 1.4 vol. %

72

represent for SE by reflection, SE by absorption and overall SE, respectively. As introduced in

Chapter 1, SE by reflection is caused by the mobile charge carriers. These carriers can be

electrons or holes, that can interact with the electromagnetic field in the radiation [30]. Materials

with higher conductivity tend to have a higher SE. SE by absorption depends on both the

thickness of the shield material and the conductivity of the material. SEa will enhance when the

material has electrical or magnetic dipoles which can interact with the incident power wave.

For compression molded CuNW/PP samples, the SE by reflection is dominant. The overall SE

comes from the reflection of the copper nanowires when the loading of the filler is lower than 1.7

vol. %, while the SE by absorption remains around zero. The SEr increase with increasing copper

nanowire loading, demonstrates that the copper nanowire can interact with the electromagnetic

waves regardless of whether the nanowires form a network or not. The SEa did not increase until

CuNW loading of 1.7 vol. %, where the network formed.

73

8.0x109

1.0x1010

1.2x1010

1.4x1010

0

10

20

vol. %

3.0

2.0

1.7

1.4

1.2

1.0

0.7

0.4

0

SE

o (

dB

)

Frequency (Hz)

Shielding Effectiveness of CuNW/PP composite

0 1 2 3

0

10

20

Sh

ield

ing

Eff

ecti

ven

ess

Concentration of CuNW (vol. %)

SEo

SEr

SEa

Shielding Effectiveness of PP composites

Figure 3-20: Overall shielding of CuNW/PP over testing frequency (a) and EMI shielding

effectiveness of CuNW/PP composite as shown in SE by absorption, SE by reflection and overall

SE (b). Error bar represents the standard deviation, some error bars are within the points.

(a)

(b)

Shie

ldin

g E

ffec

tiven

ess

Shielding Effectiveness of CuNW/PP Composite

Shielding Effectiveness of CuNW/PP Composite

74

Figure 3-21: Overall SE (a) and Permittivity (b) as a function of CuNW concentration in X-band.

Error bar represents the standard deviation, some error bars are within the points.

The EMI SE and real/imaginary permittivity of the CuNW/PP composites in the X-band

frequency range are investigated as shown in Figure 3-21 (a) and (b).The SE remains below 5 dB

when the concentration is less than 1.7 vol. %, whereas the CuNW/PS showed seven times more

SE (about 40 dB) [6]. At the same time, real permittivity increases with the increase of the

CuNW concentration, while imaginary permittivity remains around zero before 1.7vol. %. Both

permittivities rise quickly above 1.7 vol. %. Increasing concentration of the CuNWs could lead

to the enhancement of real permittivity because of the increased amount of conductive filler.

This would also occur for imaginary permittivity since the increase in the amount of mobile

charge carriers (Ohmic loss) and the number of nanocapacitors (polarization loss) increase. But

the imaginary permittivity remains unchanged below a concentration of 1.7 vol. %, which can be

related to inferior network formations due to the agglomeration of the CuNWs. The

(a)

(b)

75

agglomeration has two main effects on the imaginary permittivity: a decrease of interfacial loss

and relatively larger insulative gaps. Both of these can result in lower imaginary permittivity.

After 1.7 vol. % of copper nanowire loading, both real permittivity and imaginary permittivity

shot up quickly. This is caused not only by the enhanced conductivity of the composite, but also

by the high magnetic permeability, since copper nanowires formed closed conductive loops.

Once again, results point to network formation being an important parameter: first, the CuNWs

distributing as agglomerations and then forming an effective network after reaching a certain

loading of conductive filler.

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

5

10

15

20 CuNW/PP

Sh

ield

ing

Eff

ecti

ven

ess

Concentration of fillers (vol. %)

0

5

10

15

20

CNT/PP

Figure 3-22: Shielding effectiveness of CuNW/PP and CNT/PP composite. Error bar represents

the standard deviation, some error bars are within the points.

The EMI SE comparison for the CuNW/PP and CNT/PP composites and is laid out in Figure 3-

22. For the CNT/PP composite, the formation of the conductive network started as low as 0.4

76

vol. %. Due to this relatively low percolation threshold, the CNT/PP has a higher SE than the

CuNW/PP composite when the filler volume loading is low and the conductive network in the

CuNW/PP is not formed. However, after the filler loading is sufficient to form the network in the

CuNW/PP composite, the EMI SE of the CuNW/PP composite is higher than the CNT/PP

composite. This is because copper nanowires have relatively higher conductivity than CNT. At

the filler loading of 3.0 vol. %, the CNT/PP composite displayed an EMI SE of 13 dB, while the

SE for the CuNW/PP composite was 18 dB.

3.4 Conclusion

CuNW/PP composites were synthesized by the MSMP method. The conductivity and EMI

shielding properties were studied and compared with the CNT/PP composites prepared by the

same method. The conductivity curves of both the CuNW/PP composites and the CNT/PP

composite experience plateaus during the percolation area. The plateaus are at different

concentration ranges: for the CuNW/PP composite, it is between 0.8 vol. % and 1.7 vol. % on the

conductivity curve, while for the CNT/PP composite, the plateau happens between 0.4 vol. %

and 0.6 vol. %, which is a narrower range. Through DSC and morphology analysis, it can be

concluded that the plateau phenomena is due to the agglomeration of fillers in the matrix. The

expectation when adding fillers would be to have more dispersed single nanowires or nanotubes.

However, the experimental data shows a different story: the extra fillers tend to join the bundles

instead of being individually dispersed in a matrix. This phenomenon is likely due to the high

surface energy of the CuNW and low surface tension between the CuNW and PP matrix. The

same would be true for CNT and PP matrix. The plateau area is the range where the extra filler

joins the bundles, therefore no more effective networks are formed. The DSC results show that

the copper nanowires act as nucleating agents and accelerate the nucleation procedure. Also,

77

both the peak temperatures of crystallization Tp and melting Tm are almost identical around the

plateau concentration range, as is the crystallinity. These results provide evidence that the fillers

agglomerated instead of dispersing in the matrix. Additionally, the crystallinity of the composite

containing copper nanowires with a concentration higher than 2 vol. % is lower than the neat PP,

indicating that the bundles block the nucleation. The EMI shielding results were tested and

compared. Both the CuNW/PP and CNT/PP composites show the increasing trend of the

shielding effectiveness. The CuNW/PP composite has a lower SE when the concentration of

filler is below 2 vol. % and higher than the CNT/PP when the loading is above 2 vol. %, mainly

due to the relative percolation threshold and conductivity of the two fillers.

78

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80

[24] A.B. Kaganj, A.M. Rashidi, R. Arasteh, S. Taghipoor, Crystallisation behaviour and

morphological characteristics of poly(propylene)/multi-walled carbon nanotube nanocomposites,

Journal of Experimental Nanoscience 4 (2009) 21-34.

[25] P.M. McGenity, J.J. Hooper, C.D. Paynter, A.M. Riley, C. Nutbeem, N.J. Elton, J.M.

Adams, Nucleation and crystallization of polypropylene by mineral fillers: relationship to impact

strength, Polymer 33 (1992) 5215-5224.

[26] D.N. Bikiaris, G.Z. Papageorgiou, E. Pavlidou, N. Vouroutzis, P. Palatzoglou, G.P.

Karayannidis, Preparation by melt mixing and characterization of isotactic polypropylene/SiO2

nanocomposites containing untreated and surface-treated nanoparticles, Journal of Applied

Polymer Science 100 (2006) 2684-2696.

[27] J. Varga, β-modification of isotactic polypropylene: preparation, structure, processing,

properties, and application, Journal of Macromolecular Science, Part B 41 (2002) 1121-1171.

[28] S.V. Meille, D.R. Ferro, S. Brückner, Recent results on the polymorphic behavior of

isotactic polypropylene, Macromolecular Symposia 89 (1995) 499-511.

[29] K. Watanabe, T. Suzuki, Y. Masubuchi, T. Taniguchi, J.-i. Takimoto, K. Koyama,

Crystallization kinetics of polypropylene under high pressure and steady shear flow, Polymer 44

(2003) 5843-5849.

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shielding effectiveness of carbon nanofiber/LCP composites, Composites Part A: Applied

Science and Manufacturing 36 (2005) 691-697.

81

CHAPTER 4: CUNW/POLYMER COMPOSITE: POLYCARBONATE,

POLY(METHYL METHACRYLATE), POLYLACTIC ACID AND

POLYSTYRENE

4.1 Introduction

Properties of Conductive Polymer Composites (CPC) are not only related to the filler properties

- the dispersion, distribution and the alignment [1] – but they are also related to the processing

condition and properties of polymer matrix [2,3]. In this thesis, miscible mixing and precipitation

(MSMP) is used for preparing the composites and copper nanowires (CuNWs) are used as the

main filler. In this chapter, composites with different polymers as matrix are prepared to study

the effect of the polymer.

Different polymers have different viscosity, polarity, wettability and crystallinity. Viscosity

affects the mixing process directly since it is harder for the fillers to distribute homogeneously in

polymer with higher viscosity. Also, higher viscosity will cause more severe degradation in the

mixing process, resulting in a lower aspect ratio. Polarity and wettability can influence the

interaction between fillers and polymer. Miyasaka et al. [2] reported a higher polarity of the

polymer matrix and a higher percolation threshold of the CPCs. Crystallinity of polymer can

affect the distribution of the fillers, since some fillers have certain tendency to join certain phases.

For example, carbon black tends to concentrate in the amorphous region. Al-Saleh and

Sundararaj [3] succeeded to localize the carbon black at the immiscible interface of the

polypropylene/polystyrene blend by adding tri-block copolymer styrene-butadiene-styrene.

82

The CPC based on polypropylene (PP) was studied and reported in Chapter 3. In this chapter,

some other thermoplastics, such as polycarbonate (PC), poly(methyl methacrylate) (PMMA),

polystyrene (PS) were also studied, including the biodegrade polymer poly lactic acid (PLA).

PC is of significant commercial importance as a widely used thermoplastic polymer because of

its easy processability, durability and high transparency. PC is, therefore, widely researched for

electrically conductive polymer composite [4,5,6] and thermally conductive polymer composite

[7]. The degradation of PC in solvent [8], reprocessing and aging [9] have also been studied.

Degradation of PC during the processing procedure, either in solution or melt mixing, remains a

key challenge. Jiang et al. [10] reported that the molecular weight and tensile strength decreased

rapidly after mixing potassium titanate whiskers; they proposed that this is likely due to the poor

resistance of the PC to the alkaline component.

PMMA is one of the most popular materials because of its outstanding mechanical, chemical-

physical properties and thermal stability [11]. PMMA is an amorphous thermoplastic with high

transparency, as well as good scratch and chemical resistance (both acidic and alkaline). Most

important, PMMA received further development and application in dielectric films because of its

specific electrical and outstanding dielectric properties [12]. Last but not least, PMMA has the

advantage of wide availability, low cost and easy preparation.

Composites based on the PMMA matrix have been studied intensively. Different fillers were

added to obtain different properties. CNT [13], silver-doped MWCNT [14] and graphite

nanosheets [12,15] were introduced to improve the electrical conductivity. Motaung et al. [16]

introduced titania to the PMMA composite to obtain better thermo-stability. Through

photocopolymerisation of the cluster Zr4O2(OMc)12, Gross et al. [17] created a Zr4O2(OMc)12 –

83

PMMA inorganic – organic film with a dielectric constant of 1.93 at 25 °C and 1 kHz, which can

be applied into field effect transistors.

Research interest in PLA arose because of its biodegradability, renewability and biocompatibility

[18], especially given the background of increasing pollution resulting from man-made plastics

and a lack of petroleum resources [19]. Raquez et al. [20] reviewed PLA nanocomposites and

summarized the clay-based, nanocellulose-based, carbonaceous-based and metal/metallic

hydroxide-based nanocomposites. Villmow et al. [21] prepared the composite with CNT as filler

using extrusion redisperse master batch MWCNT and PLA then added into the PLA matrix and

found that the dispersion of CNT is predominantly controlled by the initial dispersion of the

MWCNTs in the master batch. For MWCNT/PLA composites, the agglomeration of the CNTs is

the main problem. The dispersion and distribution of the CNTs in the PLA matrix is the key

challenge. Researchers tried different methods to improve the dispersion of the MWCNTs.

Grafting methods [22,23] and surface-functionalization [24,25] have been reported effective to

improve the CNT dispersion and properties of the composites.

PS is an amorphous, glassy polymer, widely studied in literature because of easy processability

(easily extruded, injection molded or compression molded), good mechanical properties and

relatively inexpensive. Composites using PS as a matrix have been studied by our group,

including using carbon black [26], CNT [1,27,28,29] and copper nanowires as filler [30,31,32].

The electrical properties, electromagnetic shielding properties and dielectric properties were

studied.

84

4.2 Materials and Experimental Procedures

4.2.1 Materials Information

The general manufacture’s information, listed in Table 4-1, includes density, melt flow index

(MFI) and melting temperature. MFI is the inverse to viscosity proportionally: higher MFI

polymers have lower viscosity. Also, MFI indirectly measures the molecular weight of the

polymer: polymer with a higher MFI has a lower molecular weight.

Table 4-1: General information of polymers used in this chapter.

Polymers Density

(g/cm3)

Melt Flow

(g/10min)

Melting

Temp (oC)

Manufacturer

PC 1.20 10.5 155 SABIC Innovative Plastics, LEXAN 141

Polycarbonate (North America)

PMMA 1.19 1.60 160 Arkema Group, Plexiglas® V826 Acrylic

PP 0.9 5 130 Flint Hills, H0500 Homopolymer

PLA 1.24 6 156 ature wor s, Ingeo™ iopolymer 40 2D

PS 1.19 8 155 Dow Plastics, Styron 666H

Also, in contrast to using melt mixing, where the interactions are only between the fillers and the

polymer matrix. Solution mixing involves the compatibility between the solvent and the polymer.

To study the interactions between polymer, filler polymer and solution, solubility parameters are

compared. Related information, including the solvent used and structural formula, are listed in

Table 4-2.

85

According to C. M. Hansen [33], three major types of interactions occur, related with the

solubility of polymer with the solvent, including nonpolar interactions, permanent dipole –

permanent dipole interactions and hydrogen bonding. Nonpolar interactions, also known as

dispersion interactions, are the most general type of interactions and they are derived from

atomic forces. The second type of interactions is polar cohesive interactions caused by

permanent dipole – permanent dipole interactions. The third interactions are hydrogen bonding,

also known as an electron exchange parameter.

Table 4-2: Additional information of polymers used in this chapter [33].

Polymers Dispersion Polar Hydrogen

Bonding

Interaction

Radius Solvent Structural Formula

PC 18.10 5.90 6.90 5.50 CH2Cl2

PMMA 18.64 10.52 7.51 8.59 CH2Cl2

PP 18.00 0.00 1.00 6.00 Xylenes

PS 22.28 5.75 4.30 12.68 CH2Cl2

86

4.2.2 Experimental Method

The CPCs were prepared with the MSMP method, using copper nanowires as filler and different

polymers as the matrix. The CuNWs were synthesized by a template-assisted AC

electrodeposition method by anodizing an aluminum plate to form the template and then

inserting electrodeposition copper into the template to obtain nanowires. Polymers were

dissolved in the relevant solvent listed in Table 6-2. Only the PP needs to be dissolved in xylene

at 120 oC; all other polymers use dichloromethane as the solvent and are dissolved at room

temperature. The miscible mixing and precipitation method is used: mixing the CuNW/MeOH

and the polymer/solvent together in a sonication bath. The CuNWs will precipitate out with the

polymer to form the composite. In this method, MeOH acts as a non-solvent.

The electrical resistivity of the CuNW/polymer composites were measured with a Keithley

6517A electrometer and a Loresta GP resistivity meter to measure resistivity higher than 106

Ω∙cm and lower than 106 Ω∙cm, respectively. he Keithley 6517A has an 8009 test fixture and

resistance measurement software to test the resistivity; Loresta GP has the ESP four-pin probe

for connecting and performing the test.

Scanning electron microscopy (SEM) micrographs were collected using a Philips XL30 scanning

electron microscope. The compression molded samples were fractured in liquid nitrogen. All

samples were coated with gold and palladium before taking the images.

87

4.3 Results and Discussions

4.3.1 The Morphology of CuNW/Polymer Composite

Figure 4-1: Dry CuNW/PS composites: (a) 2.0 vol. %, (b) 1.5 vol. %, (c) 1.0 vol. %, (d) 0.85

vol. %, (e) 0.73 vol. %, (f) 0.6 vol. %, (g) 0.5 vol. %, (h) 0.4 vol. %, (i) 0.2 vol. %.

As discussed in Chapter 3, the crystallite size of the CuNW/PP composite decreases with an

increase in the CuNWs concentration. For the polystyrene composite, since the PS is amorphous,

there are not crystallites. However, a similar trend was found for the composite sample after the

88

mixing and drying procedure: the higher the u Ws concentration, the smaller the samples’ size

(before compression molding). As shown in Figure 4-1, the CuNW/PS composite with 2.0 vol. %

CuNWs resembles a powder, while 1.0 vol. % CuNW composite has a larger size and fleck

shape. When the concentration of the CuNW shifts down to 0.73 vol. %, near the point where

percolation happens, the CuNW/PS composite sample exists as big lumps. Samples containing a

lower concentration of the CuNW have a similar morphology as big lumps. This phenomenon

can be explained as follows: before percolation concentration, the CuNWs within the PS could

not form the network, therefore polystyrene chains are still able to tangle and fold together,

maintaining the character of polymer. After percolation concentration, the CuNWs form

networks and precipitate out with polystyrene. This interaction between nanowires and

polystyrene chains disturbs the interaction between the polystyrene-polystyrene chains, resulting

in a weak polymer-polymer bond and smaller phase size of samples.

89

Figure 4-2: SEM images of CuNW/PMMA composite (a) 1.4 vol. %, (b) 2.82 vol. %,

CuNW/PC composite (c) and CuNW/PLA composite (d).

The SEM images (Figure 4-2) indicate the interaction between copper nanowire and the polymer

matrix directly. The CuNW/PMMA composite images reveal that the copper nanowires are

distributed with different orientations, and are present a both individual wires and clusters. This

disordered distribution enhances the chance of copper nanowires form an effective network,

which would decrease the resistivity of the composite. It can be observed that both individual

nanowires and agglomerated nanowires exist in the matrix, forming the network as seen in the

SEM image (b) and (d). Especially in SEM image (d), with the CuNW/PLA sample at low

magnification, the network of copper nanowires is easily recognized.

For the CuNW/PLA composite, the cross-section and the zoom-in images of the boxed-out

regions are shown in Figure 4-3. The copper nanowires are more concentrated near one face than

the other. This result indicates the effect of compression molding on the filler distribution, where

the filler has the tendency to accumulate to one surface.

90

Figure 4-3: SEM images of CuNW/PLA composite (1.2 vol. %), cross-section and zoom-in

images.

91

4.3.2 Electrical Conductivities of the Composites

0 1 2 310

-17

10-15

10-13

10-11

10-9

10-7

10-5

10-3

10-1

101

CuNW/PC

Conduct

ivit

y (

S/c

m)

Concentration of CuNW (vol. %)

0 1 2 310

-17

10-15

10-13

10-11

10-9

10-7

10-5

10-3

10-1

101

CuNW/PMMA

Conduct

ivit

y (

S/c

m)

Concentration of CuNW (vol. %)

-0.5 0.0 0.5 1.0 1.5 2.010

-17

10-15

10-13

10-11

10-9

10-7

10-5

10-3

10-1

101

CuNW/PLA

Conduct

ivit

y (

S/c

m)

Concentration of CuNW (vol. %)

-0.5 0.0 0.5 1.0 1.5 2.010

-17

10-15

10-13

10-11

10-9

10-7

10-5

10-3

10-1

101

CuNW/PS

C

onduct

ivit

y (

S/c

m)

Concentration of CuNW (vol. %)

0 1 2 310

-17

10-15

10-13

10-11

10-9

10-7

10-5

10-3

10-1

101

Co

nd

uct

ivit

y (

S/c

m)

Concentration of CuNW (vol. %)

CuNW/PP

Figure 4-4: Conductivity of CuNW/polymer samples: (a) CuNW/PC composite, (b) CuNW/

PMMA composite, (c) CuNW/PLA composite, (d) CuNW/PS composite, (e) CuNW/PP

composite. Error bar represents the standard deviation, some error bars are within the points.

(a) (b)

(c) (d)

(e)

92

The conductivity results of copper with polymers are compared in Figure 4-4. For all five kinds

of composites, the conductivity of pure polymers (i.e. 0 vol. % CuNW) is around 10-16

S/cm,

then decreases with an increase in the CuNW concentration. For the CuNW/PC composite, the

percolation occurs at 0.9 vol. % of CuNWs, and the composite shifts from insulative (10-15

S/cm)

to conductive (10-5

S/cm). At this point, the copper nanowires form an effective conductive

network - which may not necessarily be the shortest path – where the electrons can pass through

and the composite become conductive. The conductivity continues increasing after percolation to

10-3

S/cm at CuNW concentration of 3.0 vol. %. The increase in conductivity after percolation is

due to more and more conductive networks forming, as shorter passways of electrons are created.

The CuNW/PMMA sample has a similar trend, but the percolation region occurs in a wider

range (0.4 vol. % to 1.5 vol. %). The conductivity of the PMMA samples, however, is not as

good, even at a CuNW concentration of 3.0 vol. %. This is probably because of the good

wettability of the PMMA to the CuNW, which can be seen in the SEM images in Figure 4-2.

When the wettability of polymer on the copper is good, the copper nanowires, whether they are

individual or bundled, are fully covered with an insulative polymer layer. This layer prevents the

copper nanowires from forming an effective network therefore the conductivity of composite is

not as high.

The conductivity curve of the CuNW/PLA samples is unavailable for the samples with a high

CuNW concentration. When the u W concentration is higher than 1.6 vol. %, the sample’s

mechanical strength is so poor that no complete molded samples were obtained. The CuNW/PLA

samples with CuNW concentration lower than 1.6 vol. %, remain insulative until 0.75 vol. %;

conductivity increases after this concentration.

93

CuNW/PS has been studied by Sundararaj and co-workers [30,31], with electrical properties and

EMI shielding properties reported. In this study, the CuNW/PS composite is prepared with the

same method and compared on the basis of electrical conductivity. The conductivity shows the

same trend based on reported results and has the same percolation concentration (0.67 vol. %).

The percolations of the CuNW/PC and CuNW/PMMA composites are higher than the CuNW/PS

composite. By comparing the solubility parameters, the postulate is that the PS has a larger

dispersion interaction energy which makes it easier to disperse in the solvent during the solution

mixing. After adding the CuNW and non-solvent methanol, the PS molecules are harder to

precipitate out at the beginning of mixing. This provides a longer time for the copper nanowires

to disperse among the PS molecules, leading to a better distribution and lower percolation. Also,

the PS macromolecule has a larger interaction radius [33] than the PC and PMMA

macromolecule. Ginzburg [34] predicted the interaction between nanoparticles and polymers and

stated that the particle radius and the polymer chain length affected the mixing of the polymer, as

well as the spinodal curve. As concluded by Fenouillot [35], when the radius of nanoparticles is

smaller than the radius of the polymer chain, the nanoparticles stabilized in the homogeneous

phase by reducing the enthalpy of the blend. If the nanoparticle size becomes larger than the

polymer chain radius, the phase contains more particles that tend to segregate from the system,

even though the concentration of particles might be low. Copper nanowires have a larger radius

than polymer chain length, so it becomes easier that for particle-rich phase to precipitate during

the mixing procedure. Therefore, CuNW/PC and CuNW/PMMA combinations, which have a

smaller polymer chain radius than the CuNW/PS combination, are easier to segregate from the

system. Easier segregating combinations provide a shorter time for the copper nanowires to

disperse and distribute evenly in the polymer matrix, resulting in a higher percolation.

94

4.3.3 EMI Shielding Properties

EMI shielding results of the CuNW/polymer composites over a low filler loading range are

compared in Figure 4-5. The error bars show the range of the SE over X-band frequency (8.2 –

12.4 GHz). Results for the CuNW/PP composites are listed here for comparison.

Figure 4-5: EMI shielding properties of CuNW/PC, CuNW/PMMA, CuNW/PP and CuNW/PLA

composites. Error bar represents the standard deviation, some error bars are within the points.

Even though CuNW/PC and CuNW/PP have competitive conductivities with the CuNW/PMMA

composite, the EMI shielding effectiveness (SE) is lower in the concentration range of 0 vol. %

to 1.6 vol. %. This result indicates that high electrical conductivity is not necessary for a high

EMI SE. However, for individual composites, it is obvious that the EMI SE is higher for higher

95

loading, where the conductivity is higher. For the CuNW/PLA composite, the SE at 1.6 vol. % is

lower than the SE at 1.2 vol. %. This is likely due to the poor distribution of copper nanowires

inside the PLA matrix when the loading is high. For all four kinds of composites, the SE by

absorption is lower than SE by reflection. Additionally, the SE by absorption is around zero

before 1.2 vol. %. These phenomena can be explained as follows: the SEr increases with the

higher loading of conductive fillers, but before the conductive network forms inside the polymer

matrices, there are no closed loops of conductive fillers. These closed loops can generate a

magnetic field and interact with incoming electric energy. This interaction is one of the sources

for SEa. The SEa remains at a low level before the conductive network formation occurs and SEa

could not increase since it depends on existence of the magnetic field caused by the closed loop

of conductive fillers.

4.4 Conclusion

It is possible to prepare the CuNW/polymer composite through the MSMP method with most of

the thermoplastic polymers used in this study. For biodegradable polymer PLA, the mechanical

strength is not strong enough for the sample to have integrity to be molded. In comparing the

morphology and electrical properties, the CuNW/PC samples have the same trend with the

CuNW/PS composite, but have higher percolation. Polymer polarity and macromolecule

interaction radius both play an important role in electrical properties. The addition of copper

nanowires into the polymer matrix would affect the crystalline size, i.e. the structure; even for

the amorphous polymer PS which has no crystallinity, the macro-morphology changes from big

chunks to powder-sized samples.

96

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99

CHAPTER 5: COMPOSITES WITH POLYMER BLENDS FOR MATRIX -

LOCALIZATION OF CUNW IN PLA/LDPE BLEND

5.1 Introduction

Polymer blends became one of the major research interests in polymer science because blends

provide possibility to obtain materials with enhanced overall properties by combining the

preferable properties in each component [1]. Additionally, polymer blending may enhance the

material in some properties which are unobtainable, or less economical, than single

homopolymer or copolymer [2]. Polymer blends are divided into miscible blends and immiscible

blends. In miscible blends, the properties of the blends are generally averaged between those of

the two parent polymers. In immiscible blends with proper control of the interface, it is likely

that the combination of blends exceeds the properties of the individual components. As a result,

immiscible blends are most often used in industry.

Several morphologies can be utilized to meet different requirements, as shown in Figure 5-1 [2].

Toughness of material can be achieved by disperse droplets in the matrix phase [3]. Lamellar

morphology can offer great barrier performance [4]. Co-continuous morphology is suitable for

electrical applications [5].

Adding filler might change the morphology of the blends, giving them special characteristics

such as conductivity and EMI shielding properties [6,7]. Additionally, the localization of fillers

is of high importance to obtain desirable properties. For instance, the filler can be selectively

located in one phase or interface to reduce the loading while maintaining the conductivity

[8,9,10]. Many factors can affect the localization of fillers in polymer blends. The order of

addition of components is of importance since this process has a direct influence in the melting

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of polymers and the distribution of fillers. The most common method is adding all the

components at the same time. When one of the polymers has a melting temperature much lower

than the other one, however, the filler will join the phase with the lower melting point, with less

affinity than would occur with the polymer melting at a higher temperature. During this one-time

mixing, the filler has a chance to immigrate to the favorable phase if there is enough time.

Another option is to melt the polymers first, then add the filler. This order makes it possible for

the fillers to migrate to the preferred phase [11]. The last option is to add the filler with one

polymer, then mix with the other polymer. If the components are mixed into the filler with a low

affinity phase, there is a chance to localize the filler at the interface during the migration from

one phase to the other [12,13].

Figure 5-1: Schematic of useful morphologies of polymer blends, adapted from [2].

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In this chapter, composite of CuNW as filler with PLA/LDPE blend as matrix is reported. This

study was done in collaboration with Dr. Basil Favis and Ebrahim Jalali Dil from Ecole

Polytechnique de Montreal.

5.2 Materials and Experiment

PLA Natureworks 2003D was purchased from Cargill-Dow with density of 1.24 g/cm3, melting

temperature of 156 oC and melt flow rate of 6 g/10 min under the ASTM D1238 test method.

LDPE 133A was purchased from Dow Chemical company with the density of 0.925 g/cm3,

melting temperature of 120 oC and melt flow rate of 0.25 g/10 min under the ASTM D1238 test

method.

Copper nanowires (CuNWs) were prepared by AC electrical deposition of copper into an

anodized aluminum oxide (AAO) template. Copper nanowires made via this method have an

average diameter of 23 nm and an average length of 2.56 µm.

Figure 5-2: Sample of CuNW/PLA master batch (a) and sample of CuNW/PLA/LDPE after

batch mixing (b).

(b) (a)

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Two samples were prepared to study the localization of the copper nanowires. Sample A was

made following the procedure as below: the CuNWs were first mixed with PLA, using the

MSMP method, to make a masterbatch. After the solvent evaporation, the samples, as shown in

Figure 5-2 (a), are packed in vacuum packaging to prevent any further oxidation. For the blends,

an internal batch mixer (Plasti-Corder DDR501, Brabender) is used to mix the master batch and

LDPE. The batch mixer has a total volume of 30 ml and the mixing was taken under 180 oC

mixer temperature for 10 minutes. The sample after melt mixing is shown in Figure 5-2 (b).

Sample B was made with CuNWs first mixing with LDPE as a masterbatch, then melt mixing

with PLA following the same procedure as Sample A. The procedures of preparing Sample A

and Sample B are illustrated in Figure 5-3. Melt mixing was performed in a nitrogen atmosphere

for 50 RPM at 180 oC for 10 minutes mixing time, using the internal batch. Before mixing, both

the masterbatch and the pure polymer PLA and LDPE were dried at 70 oC, under a vacuum. The

sample turned from red-brown after solution mixing to black after melt mixing. After melt

mixing, the sample was microtomed for SEM images to study the morphology.

Figure 5-3: Working flow of synthesis of CuNW/PLA/LDPE.

(a)

(b)

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The percentage of each polymer is listed in Table 5-1.

Table 5-1: Concentration percentage of CuNW/PLA/LDPE sample A and sample B.

5.3 Results and Discussions

An SEM image of pure PLA/LDPE blend is shown in Figure 5-4. PLA is the major phase and

LDPE is the minor phase. The microtomed sample of Sample A, which is CuNW first mixed

with PLA as a master batch, then melt mixing with LDPE, is pictured in Figure 5-5 and Figure 5-

6, with high magnification.

By comparing the morphology of a pure blend and the blend sample A with CuNWs as fillers, it

is interesting to see the phase size almost doubled. What is expected is that by adding CuNWs,

the viscosity of the matrix (PLA) would increase. This will enhance the droplet breakup and

suppress the droplet coalescence. However, the result revealed that the phase size doubled, which

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might be explained as follows: by adding CuNW, the mixing is less efficient due to great

resistance in the system.

Figure 5-4: SEM image of pure PLA/LDPE (70/30) blend.

Figure 5-5: CuNW/PLA/LDPE (sample A) with CuNW of 1 wt % in composite.

LDPE

PLA

LDPE

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Figure 5-6: Higher magnification of CuNW/PLA/LDPE (sample A) composite indicating

different phase and filler.

Also, the localization of CuNWs in the blends, Sample A, still remains in the PLA phase;

migration from PLA to LDPE is rarely found. A few CuNW particles found in PE are due to the

trapping of particles at the interface between colliding droplets during droplet coalescence,

which is observed in the circle at Figure 5-6. Additionally, phase separation is obvious in the

SEM images.

To compare the localization of CuNWs in a CuNW/PLA/LDPE composite, Sample B, which is

made by mixing a masterbatch of LDPE with CuNW and then mixing with PLA, is also

microtomed. SEM images were taken and shown in Figure 5-7.

LDPE

PLA

CuNWs

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Figure 5-7: SEM images of LDPE/CuNW/PLA – Sample B composite.

CuNW

(a)

(b)

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Contrary to ample A’s result, some copper nanowires in ample migrated from the LDP

phase to the PLA phase, while others still remained within the LDPE phase.

Differences between the melting temperatures of PLA (156 oC) and LDPE (120

oC) result in

phase inversion during the initial melt mixing process and this would be the main reason for

migration of CuNWs in Sample B. When mixing PLA and LDPE at the same time, LDPE melts

first and acts as matrix before PLA fully melts. With the increase in temperature, PLA melts and

becomes a major phase and phase inversion occurs. Since PLA is the favorable phase, CuNWs

migrate from LDPE to PLA during phase inversion and further mixing in Sample B, whereas in

Sample A CuNWs remain in the favorable PLA phase.

5.4 Conclusion

Polymer blend PLA/LDPE, with copper nanowire as filler, is prepared using different sequences.

The localization of CuNWs is compared and studied by SEM analysis. Sample A, with CuNW

first mixed with PLA, using solution mixing, and then mixed with LDPE using melt mixing,

shows no copper nanowire appearing in the LDPE phase. In Sample B, which is CuNW solution

mixed with LDPE, then melt mixed with PLA, copper nanowire appears in both the PLA and the

LDPE phase. The migration of copper nanowire from the LDPE phase to the PLA phase in

Sample B indicates the stronger affinity between CuNWs and PLA. The results also indicate that

sequence of mixing fillers and blends is able to change the localization of fillers in the blends.

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References

[1] L.M. Robeson, Polymer Blends: A Comprehensive Review, Hanser, 2007.

[2] C.W. Macosko, Morphology development and control in immiscible polymer blends,

Macromolecular Symposia 149 (2000) 171-184.

[3] H. Li, U. Sundararaj, Morphology Development of Polymer Blends in Extruder: The Effects

of Compatibilization and Rotation Rate, Macromolecular Chemistry and Physics 210 (2009)

852-863.

[4] P.M. Subramanian, V. Mehra, Laminar morphology in polymer blends: Structure and

properties, Polymer Engineering & Science 27 (1987) 663-668.

[5] P. Van Puyvelde, A. Vananroye, R. Cardinaels, P. Moldenaers, Review on morphology

development of immiscible blends in confined shear flow, Polymer 49 (2008) 5363-5372.

[6] M.H. Al-Saleh, U. Sundararaj, Electromagnetic Interference (EMI) Shielding Effectiveness

of PP/PS Polymer Blends Containing High Structure Carbon Black, Macromolecular Materials

and Engineering 293 (2008) 789-789.

[7] M.H. Al-Saleh, U. Sundararaj, Mechanical properties of carbon black-filled

polypropylene/polystyrene blends containing styrene-butadiene-styrene copolymer, Polymer

Engineering & Science 49 (2009) 693-702.

[8] M. Sumita, K. Sakata, Y. Hayakawa, S. Asai, K. Miyasaka, M. Tanemura, Double

percolation effect on the electrical conductivity of conductive particles filled polymer blends,

Colloid and Polymer Science 270 (1992) 134-139.

[9] D. Wu, Y. Zhang, M. Zhang, W. Yu, Selective Localization of Multiwalled Carbon

anotubes in Poly(ε-caprolactone)/Polylactide Blend, Biomacromolecules 10 (2009) 417-424.

[10] A. G ldel, A. armur, G.R. Kasaliwal, P. P tsch e, G. einrich, hape-Dependent

Localization of Carbon Nanotubes and Carbon Black in an Immiscible Polymer Blend during

Melt Mixing, Macromolecules 44 (2011) 6094-6102.

[11] F. Fenouillot, P. Cassagnau, J.C. Majesté, Uneven distribution of nanoparticles in

immiscible fluids: Morphology development in polymer blends, Polymer 50 (2009) 1333-1350.

[12] A.E. Zaikin, R.R. Karimov, V.P. Arkhireev, A Study of the Redistribution Conditions of

Carbon Black Particles from the Bulk to the Interface in Heterogeneous Polymer Blends, Colloid

Journal 63 (2001) 53-59.

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[13] A.E. Zaikin, E.A. Zharinova, R.S. Bikmullin, Specifics of localization of carbon black at the

interface between polymeric phases, Polymer Science Series A 49 (2007) 328-336.

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CHAPTER 6 : CONCLUSIONS AND FUTURE WORK

6.1 Summary and Conclusions

The goal of this project was to synthesize copper nanowire-based composites with different

polymers as matrices, using the Miscible Solution Mixing and Precipitation (MSMP) method and

to study the electrical conductivity and EMI shielding of these composites.

Copper nanowires (CuNW) were prepared in-house using the template-assisted AC

electrodeposition method, which has an average diameter of 22 nm and an average length of 2.5

µm. The anodization condition effect on the copper nanowires was studied.

Copper nanowire-based composites were prepared and tested. Polymer matrices included PP, PC,

PMMA, PLA and PS, which are widely used thermoplastic polymers. A comparative study of

CuNW/PP and CNT/PP composites was carried out using the same preparation method and

properties were compared.

An anodization system of CuNW synthesis was scaled up and was able to produce three times’

more copper nanowires per batch, which significantly shortened the synthesis time per mass.

Through the literature review and the experimental results, the following conclusions can be

drawn:

1. The anodization procedure using one-step anodization, compared with two-steps

anodization, has only slight influence on the morphology, diameter and length of copper

nanowires. When it comes to electrical properties, the influence is not significant, i.e. the

conductivity for the one-step CuNW and two-steps CuNW are in the same range for

CuNW/PS samples.

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2. CuNW/PP composites were successfully prepared using the MSMP method.

Conductivity and EMI shielding properties were tested and compared with CNT/PP

composites prepared using the same method. The conductivity curve of both composites

showed a plateau region, while the CNT/PP composite has a lower plateau concentration.

The CuNW/PP composite shows lower EMI shielding effectiveness than the CNT/PP

composite when the filler concentration is lower than 1.7 vol. %, and it has a high SE

after 2 vol. % of filler loading.

3. The plateaus on the conductivity curves are believed to be a result of agglomeration of

fillers inside the polymer matrix, which was proved by DSC and microscope results. The

agglomeration limited the fillers from forming a conductive network; therefore, a wider

percolation region was found.

4. The agglomeration also affects the crystallinity of PP inside the composites. The

crystallinity first increases, then decreases as the concentration of fillers increases. The

CuNW or CNT acts first as a nucleation agent when the loading is low; however, when

the loading is high, the fillers begin to block the mobilization of the PP macromolecules

and, therefore, the crystallinity decreases. Also, u W is acting as β phase nucleating

agents at concentration of 0.7 vol. % and 1.0 vol. %.

5. The successful preparation of CuNW with other polymers as matrices indicated that

MSMP is a possible method for preparing conductive polymer composites.

6. The scaled up anodization system was proved successful in improving the quantity of

CuNW made in one cycle of synthesis without changing the physical properties like

uniformity of the pores in the template.

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6.2 Future Work

1. The mixing process caused some agglomeration, as discovered in different copper

nanowire/polymer composites. This indicates the dispersion of fillers is not sufficient.

However, if the sonication time is too long, the copper nanowires would break and lose

the advantage of high aspect ratio. Therefore, optimal time for sonication and mixing

need to be determined for different starting aspect ratios.

2. The oxidation problem of CuNWs needs to be overcome. Various methods could be

tested, including adding some anti-oxidant into the non-solvent where the CuNWs are

dispersed and operating inside an inert glove box.

3. Utilizing the MSMP method for preparing CuNW/polymer composites is possible, but

almost no research work has been done on making copper nanowire-based composites

using polymer blends as a matrix. As polymers have different solubility in solvent, when

adding the non-solvent in the MSMP method, one phase would precipitate out with

copper nanowires, with the other phase remains in the solution. This would make it

possible for the copper nanowires to localize in one phase, or interphase, of the blends

and obtain higher conductivity at lower filler loading.

4. Scale-up of the deposition system for the synthesis of copper nanowires will be beneficial

for preparing copper nanowire will be beneficial time-wise in preparing CuNWs. Similar

to the anodization system, the deposition system requires the aluminum and copper plates

to be in parallel to use AC electrodeposition, which makes it possible to deposit several

aluminum templates at one time.

5. Scale up of the system is one way to achieve larger CuNW production, the other

promising way would be to increase the yield on the template. In order to do this, the

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deposition mechanism need to be further studied, as well as the effect of different

anodization conditions on the template formation.

6. Continuous processing for synthesizing CuNW is an important goal for industrial

production. It involves significantly different design for the anodization, deposition and

liberation process. Some challenges are the process transitions between different sections

and the residence time for each. Since different units require different power supply and

electrolyte, the aluminum might be divided into different sections as well and insulative

materials can be used to separate the sections. To optimize the time for each section, units

with longer operation time might use additional parallel same units to balance the

processing time. Another option would be to develop a semi-continuous process first.

This semi-continuous process would provide better residence time control for the total

process.

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APPENDIX A: SCALE UP OF THE CUNW SYNTHESIS SYSTEM

1. Design of Scale up System

A Gantt chart is commonly used to illustrate the start and finish of terminal and summary

elements. These two elements comprise the work breakdown structure of the project [1]. Figure

A-1 is the Gantt chart of the synthesis process of the CuNWs. The orange sections indicate the

occupancy time for procedures occuring in an anodization tank and the blue sections are the

occupancy time for procedures occurring in beaker or filter. The purple sections represent the

time needed for the transfer of the materials between different units.

Figure A-1: Gantt Chart for the CuNWs synthesis process.

Based on the Gantt chart, the anodization time represents a time bottleneck; anodization is the

dominate portion of the process, considering it is a time-consuming procedure. For a batch

process, productivity can be increased by removing bottlenecks with regard to time and volume.

Methods to remove the bottlenecks may include changing the operation time or operating

volume of single tasks. Therefore, to maintain or increase production, de-bottlenecking would

involve scaling up anodization to shorten processing time.

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The system we have, as shown in Chapter 2, can only produce 3~5 g CuNWs in each cycle. Our

goal is to upgrade the system to achieve three times higher production.

The system with 3g production was scaled up from a smaller set-up of 1 g per batch, as shown in

Figure A-2 [2].

Figure A-2: Apparatus for copper nanowires synthesis with 1g/batch yield [2]- Reproduced by

permission of The Royal Society of Chemistry.

A smaller scale set-up is proper for an experimental investigation, such as the synthesis of new

nanowires, specifically nickel and silver. However, when considering using nanowires for

preparing composites, the amount is not enough. Fresh multigrams of nanowires are required to

undertake the experiments. The procedures for making nanowires for both the setups are very

similar. For this chapter’s discussion, the setup will be scaled up to obtain 12~16 g u Ws,

under the same procedures. The limitation of CuNWs production is the limit of repeating the

composites’ samples. overing all the ranges of different concentration samples is challenging to

manage. A small amount of the CuNWs will establish the limit to compare the properties

between different matrixes.

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The set-up, as shown in Chapter 2, appears in the sketch below:

Figure A-3: Sketch of the small anodization system.

The final aim to test the synthesis of CuNW is to prepare the scale up for the system, to increase

the production three-fold.

Main Issue One:

As we can see, if there are three times more plates in the tank, the lead connection will be more

complicated. Leads hanging over the tank may have a chance to short circuit, thereby creating a

safety hazard.

Figure A-4: The connection design for the big tank: (a) side view, (b) top view.

(a) (b)

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In the new design (Figure A-4), the cathodes (stainless steel plates) are connected directly

instead of connected using a clip, then using the lead. The two main benefits are: first, the

connections on top of the system will be cleared out, resulting in a safe and easily-managed

working system; second, fewer connections will decrease the connection resistance.

Main Issue Two:

For the full setup, the hydrogen generation can be calculated from the electric charge. The charge

transferred over time - t - can be obtained by the Equation A-1:

Q = I×t, (Equation A-1)

Where Q is the electric charge transferred, using the unit of coulomb (C), I is the current, in units

of ampere (A), and t is the time in second (s).

For anodization in one hour, Q = I×t = 20 A × 1 ×60×60 s × 2 = 144000 C

Q = ne, where e is the elementary charge e = 1.602×10−19,

C is the charge carried by an electron

and n is the number of electrons. The mole of hydrogen would be: M = (n/2)/NA, NA is the

Avogadro constant and NA = 6.02214129 ×1023

mol−1

. Applying this equation, the result reveals

M = 0.746 mol. At room temperature, the molar volume of hydrogen is 22.4 L, which means the

hydrogen generation would be V = 22.4 L/mol ×0.746 mol = 16.7 L (per hour).

To ensure safety, the hydrogen generated would be collected into the exhaust system; the hood

for the anodization tank is designed accordingly (Figure A-5).

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Figure A-5: Design of the hood for the anodization tank.

Figure A-6: Scaled up system with side view (a) and top view (b).

(a)

(b)

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2. Products Examination

To examine the function of the big tank, the templates and the copper nanowires from the scaled

up system were compared with those in the small tank by SEM images (Figure A-7).

Figure A-7: SEM images for the small setup with high magnification (a) and uniformities with

low magnification (b); scaled up system with high magnification (c) and uniformities with low

magnification (d).

(a)

(d) (c)

(b)

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From the SEM images, the scaled up system has the same uniform pattern as the small setup.

The image with high magnification shows the charges effecting and shifting when taking the

images. The pores, consequently, appear to be not as round as the pores in the image for the

small setup. The template has good uniformity, as shown in the low magnification image. To

compare the pore details, such as the diameter, a statistical analysis was carried out, as listed in

Table A-1. The histograms of the distribution of the diameters of the pores are shown in Figure

A-8.

Table A-1: Statistical analysis of pore diameters on aluminum templates anodized from small

setup and scaled up system.

Mean Standard Deviation

Hole diameter – small setup (nm) 23 6.01

Hole diameter – scaled up system (nm) 23 5.20

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10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

0

50

100

150

200

250

300

350

Num

ber

in t

he

Inte

rval

Diameter

Big setup

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

0

50

100

150

200

250

300

350N

um

ber

in t

he

Inte

rval

Small setup

Figure A-8: Diameter distribution of pore diameters in template for small setup and scaled up

setup.

From the histograms in Figure 6-8, the scaled up system has a better concentrated distribution,

where more than half of the pores have a diameter of 20-26 nm.

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Energy-dispersive X-ray spectroscopy (EDX) analysis of copper nanowires was also compared

and listed in Figure A-9 and Figure A-10.

Figure A-9: A bundle of copper nanowires from the small setup and the EDX analysis from the

rectangular section indicated.

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Figure A-10: A bundle of copper nanowires from the scaled up setup and the EDX analysis from

the rectangular section indicated.

By comparing the dimensions of copper nanowires synthesized using different anodization

systems, it can be concluded that the copper nanowires are of a diameter and length at an

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acceptable difference range. Also, the EDX images indicate the copper nanowires are rarely

oxidized after liberation.

3. Conclusion

The scale-up system of copper nanowire anodization equipment was achieved. Safety issues,

such as the connection between plates and the power supply, and hydrogen generation, were

taken into consideration. The quantity of copper nanowires made in a single batch was three

times more than the small setup production. The quality of copper nanowires and the aluminum

templates were compared by performing statistical analysis on the holes of the templates from

SEM images and the EDX of copper nanowires. The templates prepared by scale-up system can

provide same quality for distribution of pour size as the small setup.

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Reference

[1] J.M. Wilson, Gantt charts: A centenary appreciation, European Journal of Operational

Research 149 (2003) 430-437.

[2] G.A. Gelves, Z.T.M. Murakami, M.J. Krantz, J.A. Haber, Multigram synthesis of copper

nanowires using ac electrodeposition into porous aluminium oxide templates, Journal of

Materials Chemistry 16 (2006) 3075.