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CHARACTERIZATION OF POLY(METHYL METHACRYLATE) AND THERMOPLASTIC POLYURETHANE-CARBON NANOFIBER COMPOSITES

PRODUCED BY CHAOTIC MIXING

A Dissertation Presented to the

The Graduate Faculty of the University

In Partial Fulfillment

of the Requirements for the Degree Doctor of Philosophy

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CHARACTERIZATION OF POLY(METHYL METHACRYLATE) AND THERMOPLASTIC POLYURETHANE-CARBON NANOFIBER COMPOSITES

PRODUCED BY CHAOTIC MIXING

Guillermo A. Jimenez

Dissertation

Approved: Accepted: Rlawhdeorlawhdeorlawhdorlawheo rlawhdeorlawhdeorlawhdorlawheo

Advisor Department Chair

Dr. Sadhan C. Jana Dr. Sadhan C. Jana

Rlawhdeorlawhdeorlawhdorlawheo rlawhdeorlawhdeorlawhdorlawheo

Committee Member Dean of the College

Dr. Avraam I. Isayev Dr. Frank N. Kelley

Rlawhdeorlawhdeorlawhdorlawheo rlawhdeorlawhdeorlawhdorlawheo

Committee Member Dean of the Graduate School

Dr. Rex D. Ramsier Dr. George R. Newkome

Rlawhdeorlawhdeorlawhdorlawheo rlawhdeorlawhdeorlawhdorlawheo

Committee Member Date

Dr. Kevin A. Cavicchi

Rlawhdeorlawhdeorlawhdorlawheo Committee Member

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ABSTRACT

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ACKNOWLEDGEMENTS

I would like to express my gratitude to my advisor, Dr. Sadhan C. Jana for his guidance, and his support to complete this work. I would like to extend my thanks to my committee, Dr. Avraam Isayev, Dr. Erol Sancaktar, Dr. Rex Ramsier, Dr. Shing-Chung Wong, and Dr. Kevin Cavicchi for their advice. I am deeply thankful to all my group members, past and present, for their suggestions and help throughout all my studies.

Financial assistantship from National Science Foundation in the form of CAREER Award to Dr. Jana is gratefully acknowledged. I am deeply thankful with the following sponsors: Fulbright Program, and LASPAU from the United States, and Universidad Nacional, CONICIT, and MICIT from Costa Rica, for their financial support. I would also like to thank my colleagues and supervisors at POLIUNA in Costa Rica, and the staff and faculty at the Department of Polymer Engineering, and the Office of International Programs at the University of Akron for their support.

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TABLE OF CONTENTS

Page

LIST OF TABLES……….….………x

LIST OF FIGURES……….………..xii

CHAPTER I. INTRODUCTION ... 1

II. LITERATURE REVIEW ... 6

2.1. Chaotic mixing... 7

2.2. Carbon nanofibers... 10

2.2.1. Structure... 11

2.2.2. Synthesis ... 14

2.2.3. Properties ... 14

2.2.4. Surface chemistry... 16

2.2.5. Surface treatment ... 19

2.2.6. Chemical functionalization ... 22

2.3. Thermoplastic polyurethanes ... 24

2.3.1. Chemistry... 24

2.3.2. Morphology... 26

2.3.3. Shape memory polyurethanes ... 28

2.4. Polymer-carbon nanofiber composites ... 30

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2.4.1.1. Tensile properties... 32

2.4.1.2. Thermal management ... 33

2.4.1.3. Electrical conductivity ... 34

2.4.2. Preparation and characterization... 38

2.4.2.1. General polymeric systems... 38

2.4.2.2. Poly(methyl methacrylate)-carbon nanofiber composites ... 43

2.4.2.3. Thermoplastic polyurethane-carbon nanofiber composites... 45

2.4.2.4. Effect of fiber surface modification... 47

2.4.2.5. Shape memory TPU nanocomposites ... 50

III. EXPERIMENTAL... 55

3.1. Materials ... 55

3.1.1. Poly (methyl methacrylate)... 55

3.1.2. Thermoplastic polyurethanes... 57

3.1.3. Carbon nanofibers... 59

3.2. Composites preparation procedures... 61

3.2.1. PMMA-carbon nanofiber composites... 62

3.2.2. TPU-carbon nanofiber composites ... 66

3.3. Characterization techniques ... 68

3.3.1. X-ray photoelectron spectroscopy ... 68

3.3.2. Scanning electron microscopy ... 68

3.3.3. Transmission electron microscopy and ultramicrotoming... 69

3.3.4. Optical microscopy and image analysis... 69

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3.3.6. Differential scanning calorimetry ... 72

3.3.7. Thermogravimetrical analysis... 72

3.3.8. Tensile test ... 73

3.3.9. Dynamic mechanical analysis... 73

3.3.10. Fourier-transform infrared spectroscopy ... 73

3.3.11. Gel permeation chromatography... 75

3.3.12. Shape memory properties ... 75

IV. PREPARATION AND CHARACTERIZATION OF CARBON NANOFIBERS.. 78

4.1. Morphology of CNF and CNFOX ... 78

4.2. Surface chemistry of CNF and CNFOX ... 80

4.3. Preparation and characterization of CNFOL ... 82

4.4. Summary... 85

V. PMMA-CARBON NANOFIBER COMPOSITES ... 86

5.1. CNF composition and processing technique... 86

5.2. Effect of chaotic mixing time ... 98

5.3. Effect of surface chemistry in CNF ... 101

5.4. Summary... 109

VI. TPU-CARBON NANOFIBER COMPOSITES ... 111

6.1. TPU23-carbon nanofiber composites ... 111

6.1.1. Morphological analysis... 112

6.1.2. Thermal degradation ... 114

6.1.3. Molecular weight and molecular weight distribution ... 116

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6.1.5. Electrical conductivity ... 122

6.1.6. Tensile properties... 122

6.1.7. Degree of phase separation ... 124

6.1.8. Thermal transitions ... 127

6.2. TPU33-carbon nanofiber composites ... 130

6.2.1. Morphological analysis... 130

6.2.2. Thermal degradation ... 132

6.2.3. Thermo-mechanical behavior ... 134

6.2.4. Electrical conductivity ... 139

6.2.5. Tensile properties... 141

6.2.6. Hydrogen bonding ... 147

6.2.7. Thermal transitions ... 152

6.3. Summary... 160

VII. SHAPE MEMORY PROPERTIES IN TPU COMPOSITES ... 162

7.1. Thermally-induced shape memory effect ... 162

7.2. Shape memory effect induced by Joule heating ... 165

7.3. Summary... 168

VIII. OVERALL SUMMARY ... 169

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LIST OF TABLES

Table Page

2-1 Properties of VGCFs (micro-, and nano-size) versus other fibers. (Adapted

from Ref. 30)... 15

3-1 Physical and mechanical properties of PMMA.148... 57

3-2 Chemical composition of TPU systems... 57

3-3 Physical and mechanical properties of CNF.74... 59

3-4 Nomenclature of the composite systems. ... 62

4-1 Oxygen/carbon ratio present on the carbon nanofibers. ... 81

5-1 Thermo-oxidative degradation of PMMA-CNF composites prepared in the chaotic mixer... 93

5-2 Elongation at break (%) of PMMA-CNF composites. ... 96

5-3 Thermo-oxidative degradation of PMMA and composites prepared in chaotic mixer with different CNFOX contents. ... 104

5-4 Bulk electrical conductivity of CNF and CNFOX. ... 108

6-1 Storage, E’, and loss, E”, modulus at room temperature for TPU23 composites prepared in the chaotic mixer... 120

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6-3 Tg values obtained from E” and tan δ plots for TPU33-CNF composites

prepared in the chaotic mixer and Brabender Plasticorder. ... 135 6-4 Ratio of the area under the peak of hydrogen-bonded NH to aliphatic CH,

ANH/ACH of TPU33-CNF and TPU33-CNFOX composites (±0.05). ... 148

6-5 Effect of the temperature on the ratio of the area under the peak of hydrogen-bonded NH to aliphatic CH, ANH/ACH in neat TPU33 and

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LIST OF FIGURES

Figure Page

2-1 Elliptic and hyperbolic points in blinking vortex flow... 9

2-2 Dimensionality of carbon materials... 11

2-3 Sketch of a single-walled carbon nanotube. ... 12

2-4 Aspect ratio of several carbon materials. (Adapted from Ref. 31)... 13

2-5 Diameter of several carbon materials. (Adapted from Ref. 31) ... 13

2-6 Growth mechanism of a CNF. (Adapted from Ref. 31) ... 14

2-7 Electrical resistivities of various forms of carbons compared to that of copper (HHT: heat treatment temperature). (Adapted from Ref. 32) ... 16

2-8 Functional surface groups containing oxygen in carbon nanofibers. (Adapted from Ref. 34) ... 18

2-9 Reaction mechanism of the DCC-aided condensation of a polyol onto the surface of CNF. ... 23

2-10 Molecular structures of most common industrial isocyanates... 25

2-11 Chemical unit of a typical TPU hard segment... 27

2-12 Shape deformation and recovery of a SMP. ... 29

2-13 Dependence of the electrical conductivity on the filler content. ... 35

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3-2 Chemical structure of PPG. ... 58

3-3 Chemical structure of PCL diol. ... 58

3-4 (a) Set-up of chaotic mixer 1 set-up and (b) chaotic mixing chamber. Scale bar is given to provide an idea about the dimension of the equipment. ... 63

3-5 Mixing head of a Brabender Plasticorder. Scale bar is given to provide an idea about the dimension of the equipment... 63

3-6 Diagrams showing geometry and dimensions of two mixing heads: (a)Chaotic mixer, and (b)Brabender Plasticorder mixer. ... 64

3-7 Sketch of the setup to prepare a TPU prepolymer... 66

3-8 Mixing chamber of chaotic mixer 2. ... 67

3-9 Composite specimen and electrode set up for measurement of volume electrical conductivity in PMMA composites (a) 8-shaped specimen, (b) one half of 8-shaped specimen for conductivity measurement along flow direction, and (c) conductivity along the thickness direction. ... 71

4-1 SEM images of (a) CNF and (b) CNFOX. ... 79

4-2 Dispersion in water of CNF and CNFOX. ... 79

4-3 XPS spectra of both types of carbon nanofibers... 80

4-4 Narrow XPS spectra of the C(1s) region in (a) CNF, and (b) CNFOX... 81

4-5 Weight loss for CNF, CNFOX and CNFOL. ... 82

4-6 Full and narrow (inset) XPS spectra of CNFOL. ... 83

4-7 Morphology of CNFOL as seen by (a) SEM, and (b) TEM... 84

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5-2 Optical micrographs of PMMA-CNF composites in the lower and upper limit of their electrical percolation threshold according the

processing technique. Arrow indicates flow direction... 89 5-3 Particle size distribution for PMMA-CNF composites prepared by two

processing techniques. ... 90 5-4 Fiber length distribution of fibers extracted from a PMMA composite with

4 wt. % of CNF prepared in chaotic and Brabender Plasti-corder... 90 5-5 Optical micrograph showing conductive networks in 4 wt. % CNF

composite prepared in the chaotic mixer. Arrow indicates flow

direction. ... 91 5-6 Thermo-oxidative stability of PMMA-CNF composites prepared by

chaotic mixing... 92 5-7 Storage modulus of PMMA-CNF composites prepared in (a) chaotic

mixer, and (b) Brabender Plasticorder. ... 94 5-8 Variation of tan δ with temperature for PMMA-CNF composites prepared

in (a) chaotic mixer, and (b) Brabender Plasticorder. ... 95 5-9 Maximum values of loss tangent (at glass transition temperature) of

PMMA-CNF composites prepared in chaotic mixer, and Brabender

Plasticorder... 95 5-10 Tensile properties of PMMA-CNF composites (a) Stress at break, and (b)

Young’s modulus. ... 96 5-11 Effect of mixing time on the electrical volume conductivity along flow

direction for composites prepared in the chaotic mixer... 99 5-12 Effect of mixing time on morphological characteristics of PMMA-CNF 2

wt. % prepared in the chaotic mixer... 100 5-13 Effect of mixing time on the storage modulus of PMMA-CNF 2 wt. %

prepared in the chaotic mixer... 100 5-14 Optical micrographs of PMMA 2 wt. % composites with (a) CNF and (b)

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5-15 Particle size distribution histogram for PMMA 2 wt.% composites with

CNF and CNFOX... 102 5-16 SEM images of the fractured surface of PMMA composites having 2

wt. % of (a) CNF and (b) CNFOX. Some nanofibers are circled for

comparison. ... 102 5-17 Thermo-oxidative stability of composites of PMMA-CNFOX... 103 5-18 Dynamic mechanical properties of PMMA-CNFOX composites prepared

by chaotic mixing. (a) Storage modulus and (b) tan δ... 105 5-19 Dynamic mechanical properties of PMMA with 4 wt.% of CNF and

CNFOX prepared in a chaotic mixer (a) Storage modulus, E’, and

(b) tan δ... 106 5-20 Volume electrical conductivity of PMMA-CNF and PMMA-CNFOX

materials prepared by chaotic mixing. ... 107 5-21 Sketch of the carbon nanofibers morphology (a) as-received, (b) after

chaotic mixing, and (c) after Brabender plasticorder mixing. ... 110 6-1 Mixing torque of TPU23 with different types of carbon nanofibers... 112 6-2 TEM images of TPU23 composites with 3 wt. % of (a) CNF, (b) CNFOX,

and (c) CNFOL. Arrows indicate the flow direction. Scale bars are 2

μm in (a) and (b) and 1 μm for (c). ... 113 6-3 TEM image of a cast film of TPU-CNFOX composite with 3 wt.%

nanofibers... 114 6-4 Thermogravimetric behavior in N2 gas of TPU composites with 0.5 wt. %

of CNF, CNFOX and CNFOL (a) Mass loss (b) First derivative of

mass loss. ... 115 6-5 (a) Molecular weight and (b) molecular weight distribution of TPU23 and

its corresponding 0.5 wt. % composites... 116 6-6 Thermo-mechanical properties of TPU23-CNF composites: (a) storage

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6-7 Thermo-mechanical properties of TPU23-CNFOX composites: (a) storage

modulus, (b) loss modulus and (c) tan δ... 118 6-8 Thermo-mechanical properties of TPU23-CNFOL composites: (a) storage

modulus, (b) loss modulus and (c) tan δ... 119 6-9 Tensile properties TPU composites of CNF, CNFOX, and CNFOL (a)

Stress at break (b) strain at break, and (c) Young’s modulus. ... 123 6-10 Typical infrared spectra of a TPU23 carbon nanofiber composite with

principal peak assignments (3 wt. % CNFOL). ... 125 6-11 Deconvolution of the carbonyl peak into free and hydrogen-bonded. ... 125 6-12 (a) Hydrogen bonding index and (b) degree of phase separation of

TPU23 composites. ... 126 6-13 Thermal transitions of TPU composites. ... 127 6-14 Sketch of the morphology of hard domains in TPU23 composites with

surface treated CNF. ... 129 6-15 Optical micrographs of TPU33 composites prepared in chaotic mixer

with 1 wt. % of (a) CNF, and (b) CNFOX... 131 6-16 SEM pictures of (a) CNF5 prepared in chaotic mixer, (b)

TPU33-CNFOX5 prepared in chaotic mixer and (c) TPU33-CNF5 prepared

in Brabender Plasticorder... 132 6-17 Thermal degradation stability of TPU33-CNF and TPU33-CNFOX

composites (a) T1, and (b) T2 values... 133

6-18 Thermo-mechanical properties of TPU33-CNF composites prepared in

chaotic mixer: (a) storage modulus, (b) loss modulus and (c) tan δ... 136 6-19 Thermo-mechanical properties of TPU33-CNF composites prepared in

Brabender mixer: (a) storage modulus, (b) loss modulus and (c) tan

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6-20 Thermo-mechanical properties of TPU33-CNFOX composites prepared in chaotic mixer: (a) storage modulus, (b) loss modulus and (c) tan

δ... 138 6-21 Electrical conductivity of TPU33-CNF composites prepared in (a)

chaotic mixer and (b) Brabender Plasticorder and (c) TPU-CNFOX

composites prepared in chaotic mixer... 140 6-22 Tensile properties at room temperature of CNF and

TPU33-CNFOX composites: (a) stress at break, (b) strain at break, and (c)

Young’s modulus. ... 143 6-23 Typical stress-strain curves at 60 °C for (a) CNF and (b)

TPU33-CNFOX composites prepared in a chaotic mixer. ... 145 6-24 Tensile properties at 60 °C for TPU33-CNF composites prepared in

chaotic mixer and Brabender Plasticorder, and TPU33-CNFOX in

chaotic mixer. (a) Maximum stress and (b) Young’s modulus... 146 6-25 Typical FTIR spectra of TPU33-CNF5 prepared in Brabender

Plasticorder with principal peak assignments. ... 147 6-26 Values of AHCO/ACO in TPU33 composites according to (a) mixing

method, and (b) fiber oxidation... 149 6-27 FT-IR spectra of C=O stretching region at different temperatures for (a)

pristine TPU33 and (b) TPU33-CNF 7 wt. % prepared in the chaotic

mixer. ... 151 6-28 Values of AHCO/ACO of neat TPU33 and its 7 wt. % composites at several

temperatures. ... 152 6-29 Full DSC heating scan of TPU33-CNF composites prepared ... 153 6-30 (a) First and (b) second heating scan of specimens of TPU33-CNF

composites mixed in a chaotic mixer... 154 6-31 (a) First and (b) second heating scan of specimens of TPU33-CNF

composites mixed in the Brabender mixer... 155 6-32 Percentage of crystallinity of TPU33-CNF prepared by two processing

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6-33 Glass transition temperature, Tg, of TPU33 materials synthesized in two

different mixers (from first DSC scan). ... 156 6-34 (a) First and (b) second heating scan of specimens of TPU33-CNFOX

composites mixed in a chaotic mixer... 157 6-35 Percentage of crystallinity of TPU33-CNFOX prepared by chaotic

mixing, (a) first and (b) second DSC scan. ... 158 6-36 Heating and cooling DSC scan of TPU-CNF 5 wt. % prepared in (a)

Brabender and (b) chaotic mixer... 159 6-37 Crystallization temperature of TPU33-CNF composites... 159 7-1 Percent of shape retention or fixity of TPU33-CNF and TPU33-CNFOX

composites after 50 % strain at 60 °C. ... 163 7-2 Stress recovery in (a) TPU33-CNF and (b) TPU33-CNFOX composites

prepared in a chaotic mixer... 164 7-3 Recovery ratio of TPU33-CNF and TPU33-CNFOX composites. ... 165 7-4 Increase of temperature with applied voltage for (a) TPU33-CNF 5 and (b)

TPU33-CNF 7 composites. ... 166 7-5 Shape recovery of TPU33-CNF 5 triggered by Joule heating... 167 7-6 Change with time of the deflection angle, θ, in a specimen bar of

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CHAPTER I

INTRODUCTION

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In many instances, tensile modulus is dramatically increased by the reinforcing filler, e.g., clay, talc, carbon and glass fibers, or organic ones like Kevlar, while elongation at break might be sacrificed. Consequently, the composite industry struggles with various trade-off issues.

Nanoscience and nanotechnology have flourished in the last decade because of new methods of production of nanomaterials, and novel characterization and manipulation techniques that are currently available.2 These nanomateriales form the basis of a new class of composites that have been known as nanocomposites. Nanocomposites have revolutionized the composite world since it was first observed that by dispersing very small amounts of nanoclay, mechanical and barrier properties of nylon 6 can be significantly enhanced.3-5 Such improvements were attributed to the increase of the interfacial area between polymer matrix and the filler when the clay platelets were individually dispersed.

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General applications of carbon fibers can be found in fields like sporting goods, aerospace, telecommunications, electronics, and structural reinforcement.8 Carbon nanotubes and carbon nanofibers have brought to the carbon fiber industry a new impetus. The nano-scale dispersion of these nano-fibers in a polymer matrix may produce composite materials with outstanding properties never seen before in their micro-counterparts.

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In this research project, we utilized chaotic advection as an alternative mixing scheme to obtain polymer-carbon nanofiber composites. Chaotic stirring or mixing is a low-shear mixing process, whose fundamentals were first laid down by Aref in 1984.9

Its mixing efficiency is derived from the rapid interface generation capabilities, thereby exposing fibers to the polymer matrix more efficiently. The rapid interface generation feature was capitalized in this study by promoting fiber-matrix interactions, especially, via functional groups chemically placed on the fiber surface. These functional groups produced favorable, energetic interactions with the polymeric matrix via generation of large interfacial areas aided by chaotic mixing.

The relationship between morphology and properties was evaluated in terms of how a structured dispersion is formed in the chaotic mixer. This structured dispersion is not possible in conventional mixing techniques, e.g. in internal mixer, in screw extrusion, or in injection molding.

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CHAPTER II

LITERATURE REVIEW

Tuning the dispersion quality allows us to broaden the scope of applications of polymer-carbon nanofiber composites. Chaotic mixing is a novel technique that can aid such tuning with additional features such as self-similar morphology, alignment, and low-shear mixing. In order to have control on the level of dispersion of the carbon nanofibers in a polymer matrix, we need to know the nature of such nanofibers in terms of structure, morphology, texture, and chemistry.

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composites. A basic understanding of the chaotic mixing approach is given, as well as its usage in dealing with polymer composites especially those having carbon fillers is discussed.

2.1. Chaotic mixing

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In chaotic advection, adjacent fluid elements separate from each other exponentially with time. This has a tremendous potential in polymer mixing as the domain of one polymer will be exponentially dispersed in another polymer to produce a series of morphological signatures. The operating Reynolds number and shear rates in low shear processing by chaotic mixing may be small. Nevertheless, the interfacial area between the fluids can be created at exponential rates due to repeated alignment, stretching, and folding of the interfaces by the action of some low order hyperbolic periodic points as reported by a series of research studies by Ottino and co-workers.11-13 Repeated folding of the interfaces creates self-similar local microstructures, which are retained with the progress of mixing as finer and finer scales are added to them. This is contrary to turbulent mixing, where mixing occurs primarily through randomization of local microstructures. Also, turbulent mixing is much less energy efficient than chaotic mixing.14 Chaotic mixing possesses tremendous potential for applications in polymerization, reactive functionalization, reactive and non-reactive compatibilization, blending, and mixing of pigments, and fillers with matrix polymers.

Chaotic mixing is usually generated by time-dependent velocity fields where particle trajectories lose regularity that characterizes steady two-dimensional flows. Therefore, better mixing is obtained under chaotic advection. A system can be classified as chaotic if it satisfies any of the following criteria:15

a. The flow produces either transverse homoclinic or transverse heteroclinic inter-sections, or

b. the flow has positive Liapunov exponents, or

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Figure 2-1 presents the typical form of the streamlines for the case of blinking vortex. It is seen that elliptic points in the neighborhood of the fluid just circulates around, while around hyperbolic points the fluid moves towards it in one direction and away from it in the other. A transverse homoclinic point is defined as the point where the inflow and outflow of a single hyperbolic point intersects. On the other hand, when the crossing takes place from flows of two different hyperbolic points, it is a transverse heteroclinic point. A positive Liapunov exponent indicates that adjacent fluid elements separate exponentially from each other, which is one of the most remarkable and attractive features of chaotic mixing. Horseshoe maps are formed when a flow is able to stretch and fold the fluid elements and return to its initial location.

Figure 2-1 Elliptic and hyperbolic points in blinking vortex flow.

In chaotic mixing, even with low Reynolds numbers, the repeated alignment, stretching and folding generates interfacial area at exponential rates. This fact has been proved to enhance the rates of transport of mass and energy, as well as chemical reaction rates in non-polymeric systems.16 Additionally, chaotic mixing produces more ordered structures than random mixing, reducing the percolation concentration in conductive

Streamline

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blends.17 The principles of chaotic mixing have also been applied to improve the mixing in single screw extruders.18,19

Chaotic flows are likely to generate self-similar mixing microstructures. This feature has been exploited by Zumbrunnen and co-workers17,20-22 and Jana and co-workers 23-27 in the production of conductive polymer composites 17,20,21,27 and lamellar and fibrillar morphological forms in immiscible polymer systems.22-26,28 Danescu and Zumbrunnen used two-,21 and three-dimensional17 batch chaotic mixers to produce conductive composites of polystyrene and observed electrical conductivity at much reduced carbon black loadings. Similar observations were made in the mixing of pre-concentrate of polyethylene with the host polymer in a continuous chaotic mixer.20 In all cases, conductivity of composites decreased with prolonged mixing, purportedly due to better dispersion of carbon black particles at long times, which in turn adversely affected conductive networks.17,20,21 Dharaiya et al.27 exploited the fibrillar morphology of polypropylene phase to induce double percolation networks in a composite of 1 wt. % carbon black, and a blend of polypropylene with polyamide-6 produced by chaotic mixing. Although these studies provided basic understanding of how chaotic mixing can be used to produce conductive networks of carbon black in polymer blends, no studies exist on chaotic mixing on high aspect ratio conductive fillers, such as carbon nanofibers.

2.2. Carbon nanofibers

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2.2.1. Structure

In order to have a clear idea about the structure of carbon nanofibers, a review of the allotropes of carbon is presented below. Before 1985, only two allotropes of carbon were acknowledged; graphite and diamond. Graphite shows a 2D hexagonal arrangement of sp2 hybridization, while diamond is a 3D material (sp3 bonding) with isotropic properties. Recently, two new forms of carbon joined graphite and diamond; fullerenes (a zero-dimensional, 0D), and carbon nanotubes, and carbon nanofibers (1D forms).29 Figure 2-2 shows the dimensionality of the carbon materials mentioned above.

Figure 2-2 Dimensionality of carbon materials.

A carbon nanotube (CNT) can be seen as a cylinder made up a graphene layer, capped by hemispheres of fullerenes (Figure 2-3). The curvature in the graphene layers increases the energy of the tubes per carbon atom. However, the lack of dangling bonds at the edges of the graphene layers lowers the total energy.

Diamond Graphite Fibers and tubes Fullerene

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Figure 2-3 Sketch of a single-walled carbon nanotube.

Carbon fibers on the other hand, represent an important class of graphite-related materials. There are several precursors that can be used to synthesize carbon fibers, each producing fibers with different morphologies. The mechanical strength of these fibers is based on the fiber axes lying close to the in-plane direction of a graphene layer.30

Typical diameters for individual commercial fibers are ~10 μm. VGCF can be prepared over a wide range of diameters; from less than 1000 Å (nano fibers) to more than 100 μm (micro fibers). VGCFs have hollow cores similar to carbon nanotubes. A comparison of the aspect ratio and diameter among all the carbon fibers is given in Figures 2-4 and 2-5 respectively. Carbon nanotubes have aspect ratios comparable to VGCFs, while fullerenes have aspect ratios close to unity similar to carbon blacks. Carbon nanotubes possess much smaller diameters than VGCF, and consequently showed a reduced quantity of graphene cylinders. A multi-walled nanotube for example, shows several concentric graphene tubules.

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0 2000 4000 6000 8000 10000

Aspect ratio

(L/D)

Carbon black VGCF Carbon fibers

Figure 2-4 Aspect ratio of several carbon materials. (Adapted from Ref. 31)

1 10 100 1000 10000

Diameter (n

m)

Fullerenes CNT VGCF Carbon fiber

Figure 2-5 Diameter of several carbon materials. (Adapted from Ref. 31)

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2.2.2. Synthesis

Vapor-grown carbon fibers are synthesized based on the decomposition of hydrocarbons at temperatures ranging between 700 to 2500 °C. They may grow on substrates (fixed catalyst method), or without a substrate (floating catalyst method). The growth mechanism of VGCFs occurring via a fixed catalysis dehydrogenation reaction of a hydrocarbon can be divided into several steps as presented in Figure 2-6. The diameter of the fibers is related to the diameter of the catalyst particles. There are several mechanisms leading to VGCF growth resulting in different structural morphologies. Growing fibers with the catalyst fixed normally yields micro and continuous fibers, while by using the floating catalyst method yields nano-size discontinuous filaments.31

Figure 2-6 Growth mechanism of a CNF. (Adapted from Ref. 31)

2.2.3. Properties

The physical properties of VGCF in some instances can approach those of single-crystal graphite. Properties of vapor-grown carbon fibers produced by the fixed catalyst and floating method are summarized in Table 2-1 together with a comparison with some

Metal Metal Metal

Activation Initial

growth

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macro-fibers produced from other precursors.30 This table shows how approaching to the single-crystal graphite morphology, VGCFs can have the highest tensile properties and the lowest electrical resistivity among all the carbon fibers.

Table 2-1 Properties of VGCFs (micro-, and nano-size) versus other fibers. (Adapted from Ref. 30)

Property

Isotropic

pitch-based

fiber

PAN

fiber

VGCF

(micro)

VGCF

(nano)

Diameter, μm 14.5 10 5-8 0.05

Density, g/cm3 1.57 2.0 2.0 2.1

Tensile strength, GPa 0.6 2.1 4 12

Young’s modulus, GPa 30 520 300 600

Resistivity, μΩ-cm (single fiber)

5000 500 50 20

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The electrical conductivity of carbon materials is very sensitive to lattice perfection; the higher the crystal perfection, the higher the conductivity. This feature can be observed in Figure 2-7 which depicts a comparison in terms of electrical resistivity, ρ, of various carbonaceous materials against copper. It can also be observed that thermal treatment can have a significant effect on the electrical resistivity by improving the graphitic character of a carbon fiber.32

5.0E-07 5.5E-06 1.1E-05 1.6E-05 2.1E-05 2.6E-05 3.1E-05 3.6E-05 4.1E-05 4.6E-05 5.1E-05 E le ct ri ca l r es ist iv it y a t 25 °C ( Ω m ) Copper Intercalated graphite Graphite PAN carbon fibers (HTT) VGCF (HTT)

Figure 2-7 Electrical resistivities of various forms of carbons compared to that of copper (HHT: heat treatment temperature). (Adapted from Ref. 32)

2.2.4. Surface chemistry

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the ‘solid state chemistry’ approach which considers the defects as ‘active sites’ on the carbon fiber surface. The other approach is the ‘chemical functionality’ concept which deals with functional groups containing mainly oxygen and other atoms like nitrogen or sulfur.

There are two categories of functional groups or surface oxides: acid and basic surface groups. Acidic and basic groups are simultaneously present on carbons, however the first ones are the most commonly found on the fiber surface. Among the most important oxides with acidic character are the carboxyl and phenol groups. From these two structures, a few other groups may be found at the carbon surface: anhydrides, lactones, and lactols.34 Titration methods are generally used for the measurement of acidic groups. Since the acidity constants of the various groups (carboxyls, phenols, and lactones) differ by several orders of magnitude, an estimate of their relative amounts can be obtained by titration with bases of different strength.35 Carbonyl groups are also present on the carbon surface as isolated or conjugated structures like quinones. Potentiometric techniques and cyclic voltammetry are suitable for the determination of weak acidic groups and quinones. Figure 2-8 shows a scheme of typical functional groups having oxygen in their structures that are possible to find on the surface of carbon nanofibers.

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and the electron shielding (due to bonding to electronegative or electropositive atoms), every element will produce a set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies. The exact binding energy of an electron therefore depends not only upon the level from which the photoemission is occurring, but also upon the formal oxidation state of the atom and the local chemical and physical environment.

Figure 2-8 Functional surface groups containing oxygen in carbon nanofibers. (Adapted from Ref. 34)

Changes in the last two factors give rise to small shifts in the peak positions in the spectrum called chemical shifts. These chemical shifts energies can be quite small compared to the line width, thus making necessary to perform a deconvolution of the overlapping peaks. Normally carbon materials show two main peaks assigned to carbon and oxygen, C(1s) and O(1s) respectively. Most XPS studies on the surface chemistry of

O

O

O O

O O

O O

O

O O

O

H H

Carboxylic

Hydroxyl Anhydride

Lactone

Carbonyl Quinone

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carbon materials involve C(1s) analysis which shows one graphitic peak at 284.6 eV and four oxide peaks: 286.3 eV assigned to -C-OH or -C-O-C-; 287.7 eV assigned to -C=O; 289.4 eV corresponding to -COOH or -COOR, and 290.6 eV corresponding to –COO- and the π-π* shake-up satellite.35,36,38,40,41

The decomposition of the oxygen peak, O1s, at 528-536 eV in individual peaks is generally more difficult than for the C1s peak. XPS analysis of oxidized carbon fibers confirms that by moderate treatment mainly hydroxyl groups are formed and that after extensive oxidation carboxyl groups are also detected. The amount of oxygenated groups present on carbon fibers may be estimated from the ratio of the intensity of the O1s to the C1s peak.

Other surface characterization techniques that have been applied to carbon fibers are Auger spectroscopy, secondary ion mass spectroscopy (SIMS), infrared spectroscopy (IR), Raman spectroscopy and surface enhanced Raman spectroscopy (SERS), surface energy through wetting experiments, inverse gas chromatography, surface area and pore structure by gas or liquid adsorption, and scanning tunneling spectroscopy to obtain information on the surface roughness.39

2.2.5. Surface treatment

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structure and a pyrolytic phase characterized by irregular graphite layers and amorphous carbon. In order to improve the surface chemical reactivity of the carbon fibers, surface treatment is essential. Surface treatment of carbon fibers is generally done in order to increase the polymer melt infiltration into the fiber bundles, thereby increasing the level of dispersion. This will result in enhancement of most of the mechanical properties. Also, removal of the pyrolytic carbon is important to improve the electric and thermal conductivity of the composite. The effect of the surface treatments can be formation of functional groups and/or creation of steps, pits or defects in the surface.43

There are three main processes for modifying the oxygen surface groups on carbon nanofibers: (i) thermal treatment, (ii) coating, and (iii) mild oxidation.

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potassium permanganate and potassium dichromate.33 Electrolytic (anodic) etching is well adapted for conductive materials like carbon filaments. It allows a continuous treatment process with great flexibility in the operating conditions. In fact, anodic oxidation is largely used for surface treatment of carbon filaments and yarns. Low-pressure plasma treatments employ gases such as nitrogen, argon, oxygen, and air among others. Plasmas contain highly energetic ions and radicals with very high average temperatures. Therefore, chemically inactive basal planes of graphite may be functionalized.

Among all the modification methods cited above, liquid and plasma oxidation are the most used to alter the surface of carbon nanofibers. Nitric acid and oxygen plasma treatments of VGCF can be used to increase the concentration of surface oxygen without changing the morphology of these fibers. By performing an XPS survey of the O1s region, Serp et al. 44 concluded that an oxygen plasma treatment might improve the adhesion of this type of fibers to polymeric matrices. Later on, Figueiredo et al. 45 using Atomic Force Microscopy (AFM) determined that nitric oxidation of the VGCFs did not change the topography of the fiber. However, plasma treatment increased the size of the grains in the granular texture of the carbon nanofibers.

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that an increment of the specific surface area and pore volume upon acid treatment was due to opening of the inner tubes in CNF.

2.2.6. Chemical functionalization

Functionalization allows for the segregation of entangled or bundled CNF for their subsequent alignment. The surface modification of CNF plays an important role in their use in composites, providing strong fiber-matrix bonding and thus improving the mechanical properties of the material.

Two main paths are usually followed for functionalization of CNF; attachment of organic moieties to carboxylic groups that were previously formed by oxidation and direct bonding to the surface double bonds. The covalent bonding can be realized via chemical or electrochemical reactions. The chemical functionalization involves mainly oxidation, fluorination, amidation, and esterification.47

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Therefore, the concentration of carboxyl groups on the surface is very low. This, together with the fact of the low chemical reactivity of the carboxyl groups, makes it necessary to use chemical aids to speed up the reaction and to obtain maximum grafting density. One of those chemicals is thionyl chloride, SOCl2, which transform a carboxylic acid group into a much more reactive acyl chloride. Poly(ethylene oxide) 50, poly(propionylethylenimine-co-ethylenimine)51, and polyurea 52 among many others have been grafted onto multi-walled nanotubes (MWNT) using SOCl2. Few papers have been published to date dealing with chemical derivatization of CNF. In one of those, Wei et al. 53 grafted a poly(ethylene glycol) (PEG) onto the surface of VGCF by using N,N’-dicyclohexylcarbodiimide (DCC) as a condensation agent. Mechanism for this reaction is sketched in Figure 2-9. By using thermogravimetrical analysis, they determined that 11 mol % of grafting was equivalent to 1 mol % of COOH used in the reaction. However, they observed that in the absence of DCC the grafting reaction does not occur appreciably.

Figure 2-9 Reaction mechanism of the DCC-aided condensation of a polyol onto the surface of CNF.

Oxidized carbon nanofiber having carboxylic acid groups on its surface

N C N

O O

H O

O N

N H

OH

O N

N H

H

O

O OH

O-Acylisourea

Dicyclohexylurea

DCC

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2.3. Thermoplastic polyurethanes

Thermoplastic polyurethanes are versatile and to some extent complex materials in that their chemistry, structure and morphology can be tweaked to obtain the desired final properties. Therefore, a detailed review of their characteristics is presented below.

2.3.1. Chemistry

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Figure 2-10 Molecular structures of most common industrial isocyanates.

To obtain improved low-temperature performance with relatively high resistance to hydrolysis, 6-hydroxycaproic acid polyesters, made by the polymerization of ε -caprolactone are also used.

In this specific case, poly(ε-caprolactone) (PCL) – based polyols are produced. Polyether TPU is usually based upon polyethyleneglycols, polypropyleneglycols, polytetramethyleneglycols, or polytetrahydrofurans.

The reaction between isocyanates and polyols is accelerated by the addition of catalysts such as acids, bases (mostly aliphatic tertiary amines) and metal complexes (organo tin compounds).56 Two general types of organotins are used as polyurethane catalysts, tin II (stannous) and tin IV (stannic). The major stannous compound used is stannous 2-ethylhexanoate, more commonly referred to as stannous octanoate. The main tin IV compounds used are dialkyltin dicarboxylates or dialkyltin mercaptides. The

CH3

NCO

NCO

CH3

NCO OCN

OCN NCO

2,4-TDI isomer 2,6-TDI isomer

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proposed mechanism for tin IV catalysts, dialkyltin dicarbonates and dialkyltin dialkylthiolates, is the reaction of the tin with a polyol forming a tin alkoxide, which can then react with the isocyanate to form a complex. Transfer of the alkoxide anion onto the co-ordinated isocyanate affords an N-stannylurethane, which then undergoes alcoholysis to produce the urethane group and the original tin alkoxide

2.3.2. Morphology

Thermoplastic polyurethanes have a combination of high elongation and tensile strength and Young’s modulus, and so form a bridge between rubbery polymers and thermoplastics and in addition their toughness provides excellent abrasion and tear resistance.

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segment to the polyether or polyester soft segment, but also on the choice of glycol, the type and the molecular weight of the polyester or polyether diol, and also upon the manufacturing process and the reaction conditions. Hard segments from MDI and linear aliphatic glycols have the general structure depicted in Figure 2-11.

Segmented thermoplastic elastomers exhibit structural heterogeneity on the molecular domain, and in some cases, on a large scale involving spherulitic texture.

Figure 2-11 Chemical unit of a typical TPU hard segment.

Several morphological models have been published in the past in order to explain most of the features observed on this type of materials. An early model by Estes58 considered the hard-segment domains as an interconnecting network. Both phases were considered to be continuous and interpenetrating. Early X-ray diffraction studies performed by Bonart et al. 59 identified short-range order associated with hydrogen bonding. Wilkes and Yusek 60 studied domain formation in polyesterurethanes using X-ray techniques. They found that the domains were lamellar in shape with an average separation of 100 to 250 Å. They concluded that the hard segments act as crosslinks, inhibiting stress-relaxation and inducing stress-crystallization of the soft segments which results in higher tensile strength. Finally, Blackwell et al. 61,62 were able to interpret wide

-O-C-NH-O

CH2 NH-C-O-(CH2)n -O

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angle X-ray data of heat-set polyurethanes which defined the chain conformation and packing of hard segments in crystalline polyurethane elastomers. The models were based on the structure of MDI-butanediol hard-segment analogs with chain packing. Planar zig-zag –CH2-CH2- sections connect successive diisocyanate units. The chains are linked together in stacks through C=O---H-N hydrogen bonds which involve half of the urethane groups.

2.3.3. Shape memory polyurethanes

Shape-memory polymers (SMP) are stimuli-responsive. They have the capability of changing their shape upon application of an external stimulus. A change in shape can be caused by increasing the temperature of the surroundings around the material or by increasing locally the temperature by generating heat with the application of an electrical voltage. The shape-memory effect is not related to a specific material property of single polymers; it rather results from a combination of the polymer structure and the polymer morphology together with the applied processing and deformation.63 The deformation and recovery of a shape is shown schematically in Figure 2-12. First, the polymer is conventionally processed to receive its permanent shape. Then, the polymer is deformed and the temporary shape is fixed. The original shape is then recovered by application of heat.

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Figure 2-12 Shape deformation and recovery of a SMP.

For this type of TPU, the hard segments act as cross-links and are responsible for the permanent shape. The polymer can be processed above the melting temperature of the hard segment domains. The soft segment phase serves as a molecular switch and enables the fixation of the temporary shape. The transition temperature for the fixation of the switching segments can either be the glass transition temperature, Tg, or the melting

temperature, Tm. Soft blocks for Tg-controlled shape recovery that have been used are

poly(tetramethylene oxide) glycol69,70, poly(ethylene adipate) 66, and poly(tetrahydrofuran) 63 among others. Soft blocks used to provide Tm-control on the

shape memory behavior are mostly based on poly(ε-caprolactone) (PCL) 65,67,72,73. The most common system for this case is PCL/MDI/BDO in which PCL diols with number-average molecular weight (Mn) between 2000 and 8000 form the switching segments.

The switching temperature for the shape-memory effect can oscillate from 44 to 55 °C depending on the composition of the soft phase (between 50 and 90 wt. %) and the molecular weight of the PCL diols. The crystallization of PCL in TPU is restricted because of the connectivity between the soft and the hard phase. As a matter of fact, the

Permanent shape Temporary shape Permanent shape

Deformation

Heat

Recovery

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values for PCL crystallinity may vary between 10 and 40 % when compared to 100 % PCL material.67

2.4. Polymer-carbon nanofiber composites

Reinforcement of polymers is one of the most important applications of all types of carbon fibers. Thermosetting resins and thermoplastic polymers are commonly used as matrices. Epoxy resins have been the primary polymeric matrix material used in carbon-based polymer composites.

A prerequisite for a good polymer-carbon nanofiber composite is to have an adequate interfacial adhesion between the inorganic and the organic material. This occurs when there is a good wetting of the nanofiber by the molten thermoplastic polymer or the liquid precursor of a thermosetting resin. The extent of interfacial contact depends on the wetting behavior (contact angle and viscosity) of the fibrous composite. Consequently physical bonding between fiber and matrix is very critical for producing a high performance composite.33

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One of the most important companies in United States related to the production and commercialization of VGCF is Applied Sciences, Inc, (ASI). In the past few years, ASI has used the fixed catalyst method to fabricate long VGCF (known as Pyrograf I®) This type of fiber can be used in composites for thermal management, high power electronic devices, space power system radiator fins, and high performance applications such as plasma facing components in experimental nuclear fusion reactors. These composites include carbon-carbon composites, polymer matrix composites, and metal matrix composites.

Recently, ASI started marketing short nano-VGCF, Pyrograf III®, produced by the floating catalyst method. It is produced in the vapor phase by decomposing either methane, ethane, other aliphatic hydrocarbons, or coal gas in the presence of a metal catalyst, hydrogen sulfide and ammonia. The additional processing can include pyrolytic stripping to remove tars and other hydrocarbons from the surface of the fiber.74

2.4.1. General properties

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2.4.1.1. Tensile properties

A major stimulus for the development of any low-cost carbon fibers is for their potential applications in the automotive industry. A very high degree of graphitic perfection in the fibers and by inference, a high modulus of elasticity has been determined by x-ray diffraction for selected preparations of floating catalyst VGCF even without subjecting the fiber to any post-growth heat treatment. Based on the presumed high modulus, VGCF can be used to produce thermoplastic- and thermoset-matrix composites with elastic moduli comparable or exceeding that of aluminum, provided that preferential orientation in two dimensions can be obtained.

The tensile strength of an ideal composite (all fibers are aligned in one direction and submitted to an uniaxial tension), σC, can be calculated knowing the fiber strength, σf, the strength of the matrix at ultimate fiber deformation, σmε, and the volume fraction of

the fiber, Vf, by using the rule of mixture:1

) 1

( f

m f f

CV +σ ε −V

σ (1)

The premise that discontinuous short fibers such as floating catalyst VGCF can provide structural reinforcements can be supported by the Cox model 75, and later extended by Baxter.76 This theoretical model predicts that modulus of a composite, Ec,

can be determined from the fiber and matrix moduli, Ef and Em respectively, and the fiber

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) (θ

f d

l g V E V E

EC m m f f

⎠ ⎞ ⎜ ⎝ ⎛ +

= (2)

where Vm is the matrix volume fraction, and functions, f and g, can take values

between 0 and 1. The function g is small for particles having a low aspect ratio, but increases rapidly as the aspect ratio increases. The function f is dependent upon orientation of the fibers, θ, and is greatest for uniaxial alignment. These findings imply that if floating catalyst fibers –which have a very high aspect ratio – can be restricted in orientation in two dimensions, the resulting composite could be several times stiffer than glass-reinforced composites.

2.4.1.2. Thermal management

A significant portion of the development work conducted on VGCF composites has been motivated by the potential of these composites for high performance thermal management applications, such as electronic heat sinks, plasma facing materials, and radiator fins. Both the fixed catalyst and the floating catalyst VGCF have the potential to be economically important for thermal management or higher temperature composites.

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for pistons, brake pads, and heat sink applications and the low cost of fiber synthesis could permit these price-sensitive applications to be developed economically. A random orientation of fibers will give a balance of thermal properties in all axes, which can be important in brake and electronic heat sink applications.

2.4.1.3. Electrical conductivity

The electrical conductivity of a composite is generally characterized by its dependence on the filler volume fraction. At low filler loadings, the conductivity of the composite is very close to that of the pure, electrically insulating polymer matrix. At some critical loading (i.e., percolation threshold), the conductivity increases by several orders of magnitude with very little increase in the filler amount. After this region of drastic increase, the conductivity levels off and approaches that of the filler material. It is at the percolation threshold that enough filler has been added so that it begins to form a continuous conductive network through the composite (Figure 2-13).77

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aspect ratios and a broader range of them can lower the percolation threshold.77 Most of the models are of the statistical percolation type. These models predict the conductivity based on the probability of particle contacts within the composite.

One of basic statistical models follows a power-law equation as follows,

S C V

V )

(

0 −

σ (3)

where σ is the conductivity of the mixture; σ0 is the conductivity of the filler; V is the volume fraction of the filler; VC is the percolation volume fraction; and the critical

exponent, s, dependent on the dimension of the lattice. More accurate statistical models include polymer gelation, and a general effective media equation which considers resistivities of both components.

Figure 2-13 Dependence of the electrical conductivity on the filler content. 0

-4

-8

-12

-16

-20

Log Conductivity (S/cm)

Carbon Fiber Volume Fraction

Percolation concentration

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Thermodynamic models consider factors like filler and polymer surface energies and polymer melt viscosity among others. At all points above the percolation threshold, the conductivity of a composite has been found to be, 77

k C C C m C

F ⎟⎟

⎞ ⎜⎜ ⎝ ⎛ − − − + = φ φ φ σ σ σ

σ log (log log )

log (4)

and,

(

φ φ

)

0.75 ; γpf

φ B A K K k C

C =

= (5)

where σC is the conductivity at the percolation threshold; σm, the conductivity at F, the maximum packing fraction; φ, the volume fraction; (l/d), the aspect ratio; φc, the

percolation threshold; γpf , the interfacial tension; and A and B, constants. The value k is dependent upon the filler volume fraction, percolation threshold, and interfacial tension as calculated by,77

(

)

0.5

2 p f

f P

pf γ γ γ γ

γ = + − (6)

where γpf is the interfacial tension; γp, the surface energy of the polymer; and γf,

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( ) ( )

l d l d F

/ /

10 75

5

+ +

= (7)

The geometrical percolation model, on the other hand, was intended to predict the conductivity of sintered mixtures of conducting and insulating powders. The main parameters used in determining the conductivity are the diameters of the nonsintered particles or the edge length of the sintered particles. Equations in this model use the diameter of the particles, the probability for the occurrence of long bands of conductive particles, and the arrangement of the conductive particles on the surface of the insulating particles.

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2.4.2. Preparation and characterization

The dispersion of highly entangled materials such as CNF or CNT in polymeric viscous materials is a critical deterrent in transferring all the qualities of the carbon materials to the organic polymer. However, a perfect CNF or CNT dispersion it is not a requirement for all the applications. The level of dispersion can be modulated by the preparation method, and the interface between the polymer and CNF/CNT. Following, we review the principal routes of fabrication of polymer-CNF and polymer-CNT composites.

2.4.2.1. General polymeric systems

One of the first attempts to use the sub-micron floating catalyst form of VGCF to prepare polymer composites was done by Dasch and collaborators.78 They reported the fabrication of thermoplastic composites reinforced with randomly oriented VGCF, up to 30 % of volume fraction, having diameters of 0.08 μm and lengths of 2.5 μm. All the composites exhibited similar flexural strength near 70 MPa, in accordance with Baxter’s theory 76 for 3D short fiber reinforced composites. Also, flexure modulus increased with fiber volume fraction in agreement with calculations based on Cox’s theory for random 3D short fiber reinforcements.8

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observed with the addition of discontinuous nanofibers (0.6 vol. %, and diameter of 0.1-0.2 μm) to an epoxy composite having already 56.5 vol. % of continuous carbon fibers (diameter of 7 μm).

Carneiro et al. 80 prepared polycarbonate (PC) and VGCF composites by co-rotating twin-screw extruder and then injection molded to evaluate their properties. Tensile properties of those nanofiber composites were very similar to those PC composites containing carbon black (CB). Moreover, the PC-CNF composites showed a decreasing tendency of their impact properties with CNF content. This fact was attributed to the lack of good adhesion between the untreated fiber and the polymer matrix. They also used oxygen plasma treated CNFs, with an oxygen concentration of 12.6% (in contrast to a 1% for the untreated carbon nanofibers). PC composites made with plasma-treated CNFs did not have great influence on the tensile behavior. Although impact properties improved, still they were below the performance of the resin. The authors concluded in this case that the difference in surface energy among PC and CNFs played an important role on the results observed. Recently Higgins and Brittain81 reported the preparation of PC-CNF composites by in situ polymerization method. They estimated a 9 wt. % in CNF concentration for the percolation threshold. These authors attributed this to the much less electrical conductivity of CNF when compared to CNT. They also observed less uniform dispersion in their composites compared to high-shear methods.

Tibbets 82 obtained low values of tensile strength with as-received and untreated CNFs combined separately with nylon 6,6 and polypropylene (PP). It was argued that such poor tensile properties were due to the presence of large clumps of fibers (about 500

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employed ball milling on the fibers before mixing them with the thermoplastics into a mini injection molding apparatus. By ball-milling, the initial fiber clumps were reduced, and the polymer composites showed an improvement in the mechanical properties. Additional surface etching of the ball milled-fibers helped in the mechanical properties, unlike the clumped nanofibers. However, they also observed that the fibers aspect ratio reduced dramatically by ball milling.

The difficulty of infiltration of a viscous polymer matrix into carbon nanofiber clumps can be explained using previous models of molten metals being mixed with fibrous materials. The pressure P required to infiltrate a porous material or a clump of fibers, is due to the pressure to overcome capillary effects, Pγ, and the pressure required to overcome viscous drag, Pν:

P = Pγ + Pν (11)

The effective capillary pressure is given by,

Pγ = -Sf εLA cosθ (12)

where Sf is the surface area of the interface of the clump per unit volume of

composite, εLA is the surface energy of the polymer, and θ is the contact angle. Once this pressure is supplied, the gradient dPV /dx generated across the clump by the polymer at

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K dx

dPV =μυ

(13)

where μ is the viscosity of the polymer, and K is the permeability of the clump. Solving this equation, the infiltration depth D is,

) 1 ( 2 0 V KP D − = μ τ (14)

where τ is the duration of the infiltration at an applied pressure P0, and fiber

volume V.

For fibrous materials, the permeability K can be represented by the following equation,82 ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = π V V r

K 1 4

9 2 2 2

(15)

The dependence on r2 means that K for submicron fibers is only 10-4 times the value for conventional 10 μm-diameter fibers. This extremely small value for K leads to very slow infiltration rates in nanofibers.

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nanofibers without and with purification. The purification step was conceived to remove non-nanofiber material (amorphous carbon for example) while opening up the nanofiber network for easy deagglomeration of the nanofibers. They observed a remarkable improvement in the final decomposition temperature analyzed by Thermal Gravimetric Analysis (TGA) for the composite having 30 wt. % of CNF. This behavior was due to restrictions on the mobility of the polymer chains due to the nano-sized VGCF. From the DMA data, the storage modulus, E’ (at room temperature), of the composites was superior to the resin alone. However, the E’ difference between 2 and 20 wt.% composites was not as drastic as the change in fiber composition. Furthermore, the high temperature E’ only started to increase significantly when 60 wt. % PP-CNF composite was analyzed. Tensile properties of the composites did not show any improvement when compared with that of PP.

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the case of carbon nanotubes dispersed in epoxy resin. These authors determined a percolation threshold for their materials between 0.0225 and 0.04 wt.%.

In recent years, other polymer systems such as acrylonitrile-butadiene-styrene 86, poly(ethylene terephthalate) 87, polyvinylesters 88, polyester 89 and liquid crystal polymers90 have been explored to produce composites with carbon nanofibers.

2.4.2.2. Poly(methyl methacrylate)-carbon nanofiber composites

One of the first research works dealing with poly(methyl methacrylate)-carbon nanofiber (PMMA-CNF) composites was performed by Wu et al.91. They noted a 50 % reduction of the percolation threshold after addition of 1 wt.% of high density polyethylene (HDPE) to the PMMA-CNF composites. This phenomenon was interpreted as a result of the architecture of self-assembled conductive network constructed by selective adsorption of HDPE in VGCF/PMMA composites.

Zeng et al.92 fabricated PMMA-CNF composites by means of a counter rotating twin-screw extruder, pelletized, and fed into a spinning system in order to obtain composite fibers. It was observed that the tensile modulus of the composites did not improve significantly using high content of CNF (10 wt. %), but a 50 % increase was observed with half the carbon concentration. By applying a modified equation of the Cox model 75,76 for the composite tensile modulus, they determined that the fibers were randomly distributed in the matrix.

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CNFs and mixed them with PMMA in a chaotic mixer and compared the data with same materials prepared in a commercial mixer. It was determined that the low shear mixing in the chaotic mixing device helped build up of carbon nanofiber networks from the dispersed fibers and the agglomerates. These helped to produce a percolation threshold of 2 wt. % CNF in a chaotic mixer compared to a percolation threshold of 6 wt.% for composites prepared in a commercial mixer. In the latter mixer, fibers experienced much damage. Schueler et al.95 stated that the presence of agglomerates was responsible for low percolation thresholds for carbon black in epoxy, which could not be explained in terms of a percolation theory but based on a theory for colloids.

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nanotubes when aligned – thus aligned composites required more nanotubes to reach the percolation threshold.

2.4.2.3. Thermoplastic polyurethane-carbon nanofiber composites

Scientific reports on thermoplastic polyurethane-carbon composites can be found in conjunction with carbon fibers108-111, carbon black112,113, carbon nanofibers114, multi-walled nanotubes115-122, and single-walled nanotubes 123-125. The onset of thermal degradation of TPU has been reported to increase up to 17 °C for a composite having 30 wt. % of carbon fibers.108 The authors did not observe a major change in the Tg of the

polyester-based TPU, even at high loadings of 10 μm-diameter carbon fibers. On the other hand, the tensile stress at break of TPU experienced a dramatic drop due to addition of carbon fibers, and only after addition of 30 wt. % of fibers that the stress at break of the composites reached that of the resin alone. In the case of carbon black (CB), Segal et al.112 prepared TPU-CB composites by melt mixing in a Brabender Plasticorder and observed that the Tg of TPU decreased with CB content. They argued that CB particles

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In the case of nanocarbon materials, most of the recent research has been focused on carbon nanotubes much more than carbon nanofibers. Heat-treated CNFs were solution mixed with TPU having crystallizable soft segments, by Koerner et al.114, showing a percolation threshold of about 0.010 wt. %. Carbon nanofibers used in that work had a post production process of heating to temperatures up to 3000 °C, creating highly electrically conductive carbon nanofibers.126 One important observation made by these authors is that the crystallinity of the soft segments exerted strong influence on the tensile properties of TPU-CNF composites. They observed an increase in the Young’s modulus, decrease of the stress at the apparent yield point, and an irregular behavior of the values for stress and elongation at rupture with the addition of CNF. From differential scanning calorimetry experiments, Koerner et al.114 observed enhanced initial soft-segment crystallinity with the addition of CNF up to 5 wt.% as determined by the first DSC scan. On the other hand, a second DSC scan demonstrated no enhancement of the crystallinity of soft segments which suggested that the initial increase in crystalline fraction with CNF content was probably derived from the inhomogeneous strain distribution created during solvent removal. Finally, the authors concluded that parameters like nucleation, strain induced crystallization, polymer crystallite orientation, and fiber alignment all played roles in determining the reinforcement effect of CNF in TPU.

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TPU-MWNT – Tg decreased with carbon content due to a higher phase separation of

soft-segment domains from mixtures with hard soft-segments. They also observed that SWNT and MWNT have different reinforcement effects on TPU, i.e., SWNT yielded better tensile strength and elongation at break, while MWNT improved the Young’s modulus of TPU.

It is noted from prior studies that polyurethanes degrade through three mechanisms: (i) dissociation of the urethane bond into its starting components, e.g., polyol and isocyanate groups, (ii) breaking of the urethane bond with the formation of primary amine, carbon dioxide and an olefin, and (iii) breaking of the urethane bond into secondary amine and carbon dioxide.127 It is also possible to distinguish two characteristic stages of thermal degradation of TPU in a thermogravimetrical experiment; an initial first stage related to the hard segments and a second one related to the soft segments in TPU.118,127 Xia et al.118 found out that these degradation reactions occurred respectively at about 332 °C and 393 °C. They noted that the incorporation of either SWNT or MWNT only delayed the second degradation temperature by ~ 5 °C. Therefore, the authors concluded that carbon nanotubes may preferably interact with the soft segment in TPU.

2.4.2.4. Effect of fiber surface modification

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values than composites made with more defective as-grown CNFs.129 Similar findings have been found for epoxy systems containing single-walled nanotubes SWNT131 and multi-walled nanotubes MWNT.132 Tensile properties of epoxy having acid-treated SWNTs were higher than corresponding composites fabricated with untreated SWNTs.131 Conversely, the electrical conductivity of epoxy/acid-treated MWNT composites decreased compared to those with untreated CNT. Decrease of the electrical conductivity was attributed to damage caused on the MWNT by the oxidation treatment.132

Cortes et al.133 treated carbon nanofibers with nitric acid, and copper, and mixed with polypropylene in a Banbury-type mixer. The authors achieved a percolation threshold at about 5 wt. % CNF, which was less than that previously obtained by the same research group (9-18 wt. %). Nitric acid oxidation and the addition of copper did not produce significant changes in the mechanical and rheological properties of the polymer, although a decrease in tensile strength was observed. Xu et al.88, on the other hand, observed a substantial increment on the electrical percolation composition after they treated VGCFs with boiling nitric acid.

Brandl et al.128,134 treated CNFs with plasma oxygen, and combined them with PP. They observed an increment of the surface energy of the fibers after the plasma treatment. Mechanical properties of the composites improved after plasma functionalization. However, the electrical conductivity of these composites was not investigated.

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Also, they found an influence of this treatment on the Izod impact strengths of the plasma-treated carbon filler-reinforced polymer matrix composites.

There are few papers in which surface-treated MWNT were used with PMMA.96,97,136-138 Among those papers, only a few dealt with oxidized MWNT.96,97,138 In all those cases, PMMA-MWNT nanocomposites were fabricated by an in-situ

polymerization procedure. A significant improvement in the tensile strength was observed by Jia et al.96 when acid-treated MWNT were used in comparison with non-treated MWNT. On the other hand, Sung et al.97 observed a dramatic reduction of the electrical conductivity by about 6 orders of magnitude, after performing electrospinning of composites of PMMA with acid-treated MWNT. These authors attributed such reduction to the presence of porosity in the electrospun nanofibers and to the fact that PMMA chains wrapped perfectly around MWNTs which lowered the electrical conductivity.

PMMA composites with oxidized CNF – labeled as CNFOX – were investigated recently by Jimenez and Jana.139 They observed improvement in the quality of dispersion, thermo-oxidative stability (TOS), and dynamic mechanical properties compared to PMMA composites of untreated CNF. However, electrical conductivity of PMMA-CNFOX dropped substantially compared to those of PMMA-CNF.

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