Electrically Conductive Polymer Composites A Dissertation

Silver- and polyaniline-filled epoxy composites Composites with high electrical conductivity have been formulated from epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, undoped pol

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ELECTRICALLY CONDUCTIVE POLYMER COMPOSITES

A Dissertation Presented to The Graduate Faculty of the University of Akron

In Partial Fulfillment

of the Requirements for the Degree Doctor of Philosophy

Susan M Rhodes December 2007

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ELECTRICALLY CONDUCTIVE POLYMER COMPOSITES

Susan Rhodes

Dissertation

Dr Judit Puskas

Committee Member

Dr Mark Soucek

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ABSTRACT

Carbon nanofiber composites

Hyperbranched polyol carbon nanofiber (CNF) composites were synthesized by the chemical modification of oxidized CNF with glycidol and boron trifluoride diethyl etherate to improve the dispersion of CNF in polymer matrices The resulting polyol CNF were characterized by thermogravimetric analysis, infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy Hydroxyl groups were reacted with heptafluorobutyryl chloride to determine the amount of oxidized groups in the sample The amount of hydroxyl groups increased by 417 % for the polyol CNF compared to the oxidized CNF and an improvement in dispersion was observed

Silver- and polyaniline-filled epoxy composites

Composites with high electrical conductivity have been formulated from epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, undoped polyaniline (PANI), silver particles, and a Brønsted acid initiator to yield an order of magnitude decrease in electrical resistivity (10-5 ohm-cm) compared to the non-PANI containing composite (10-4ohm-cm) Formulations were characterized by scanning electron microscopy, thermogravimetric analysis, solid-state 13C nuclear magnetic resonance spectroscopy and 4-point probe conductivity It was postulated that an interaction between PANI and the silver particle surfactants resulted in improved connectivity of the silver particles

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3,4-Formulations using undoped PANI exhibited higher conductivity than doped PANI, due

to improved dispersion and latent doping from the Brønsted acid and the acidic silver surfactants

Radiation-cured, silver-filled epoxy composites

Silver fillers were investigated to determine the best aspect ratio for ultraviolet (UV) radiation curing A matrix dependency on the ability to cure a Ag-filled composition was revealed, with Ag-filled acrylate compositions providing higher cure than Ag-filled epoxy compositions Photo-differential scanning calorimetry measurements provided information relating UV curability and the connectivity of Ag particles in the composites The addition of PANI reduced the UV curability of these composites

Synthesis of silver nanomaterials

Silver nanowire syntheses have been reported, but incorporation of these materials into polymers to reduce percolation thresholds has not been reported The potential to use silver nanowires as conductive fillers in polymer composites was explored Despite numerous attempts, high quantity synthesis of silver nanowires is still

an unachieved target Additional research is required to understand the nucleation and kinetics of silver nanowire synthesis to enable their scale-up

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ACKNOWLEDGEMENTS

Several individuals actively supported the completion of this dissertation:

• Dr William Brittain for the past three years of encouragement, assistance, dedication and advice

• My graduate committee members, Dr Roderic Quirk, Dr Alexei Sokolov, Dr Gary Hamed, Dr Judit Puskas and Dr Mark Soucek (Polymer Engineering)

• Dr Bernadette Higgins for her background in carbon nanofiber modification and analysis

• Dr Jennifer Cross and Dr Matthew Espe (Chemistry) for their collaboration in solid-state 13C NMR spectroscopy of polyaniline

• Dr Darrell Reneker, Dr Dale Galehouse and Steve Roberts for their collaboration

in DC electrical conductivity measurements

• Jamie Himesson (Polymer Engineering) for her knowledge of photo-DSC experimentation

• Rajesh Ranjan for his expertise in RAFT technology and assistance with NMR characterization

• Richard Wells and Engineered Conductive Materials for their financial support

• Dr Wayne Jennings of Case Western Reserve University and Dr Thomas Wittberg of The University of Dayton for their assistance with XPS analysis

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• Dr Bojie Wang and Jon Page for their assistance with instrumentation and characterization

• Brittain group members for their support for the past three years: Rajesh Ranjan, Kathryn McGinty, Andrew Constable and Crystal Cyrus

• Quirk group members for their support over the last year in preparation for job interviews and my research presentation: Manuela Ocampo, Mike Olechnowicz, John Janowski, and Camilla Garces

• My family for their support throughout all my education and for making this journey possible

• Special thanks to my husband, who has learned more than his share about polymer science I would like to thank him for his patience and understanding Thank you for encouraging me to pursue my educational goals and for supporting

my career goals as well

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TABLE OF CONTENTS

Page

LIST OF TABLES……… xvi

LIST OF FIGURES……… xviii

LIST OF SCHEMES……… xxx

CHAPTER I INTRODUCTION……….1

1.1 Carbon nanofiber composites………1

1.2 Silver- and polyaniline filled epoxy composites………3

1.3 Radiation-cured, silver-filled epoxy composites……… 4

1.4 Synthesis of silver nanomaterials……….……… 5

II HISTORICAL BACKGROUND……… 7

2.1.1 Oxidized CNF synthesized by Applied Sciences, Inc…… 10

2.1.2 Determination of CNF oxides by acid-base titration………11

2.1.2.1 Determination of CNF oxides by infrared spectroscopy……… 12

2.1.2.2 Determination of CNF oxides by X-ray photoelectron spectroscopy……… 13

2.1.2.3 Determination of CNF oxides by Raman spectroscopy……… 15

2.1.3 Dispersion of CNF in polymer matrices……… 19

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2.1.3.1 Solution blending of CNF into polymer

systems……… 20

2.1.3.2 Melt blending of CNF into polymer systems……… 20

2.1.3.3 Surfactants to reduce CNF aggregation……… 21

2.1.3.4 In-situ polymerization……… 22

2.1.3.5 CNF surface modifications……….22

2.1.3.5.1 “Grafting-to” approach………… 22

2.1.3.5.2 “Grafting-from” approach……….24

2.1.3.6 Hyperbranched polymer composites………… 26

2.2 Silver-filled epoxies……….30

2.2.1 Common formulation ingredients in silver-filled epoxies……… 31

2.2.2 Cationic cure mechanism……… 34

2.2.3 Silver particles……… 39

2.2.3.1 Surfactants……… 40

2.2.4 Percolation theory………41

2.2.5 Electrical conductivity……… 43

2.2.5.1 AC electrical conductivity by dielectric spectroscopy……… 45

2.2.6 Inherently conducting polymers……… 49

2.2.6.1 Polyaniline……… 51

2.2.6.1.1 Synthesis of PANI……….51

2.2.6.1.2 PANI doping……… 54 2.2.6.1.3 Electrical conductivity of

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2.2.7 Polyaniline in epoxy adhesives……….61

2.2.7.1 Latent doping……… 68

2.3 Ultraviolet radiation curing……… 68

2.3.1 Excitation processes by absorption of UV light………… 70

2.3.2 Photopolymerization……….74

2.3.2.1 Free-radical photoinitiators and photosensitizers……… 74

2.3.2.2 Cationic photoinitiators……….76

2.3.3 Limitations of UV radiation curing……… 81

2.3.4 Photopolymerization kinetics and reaction monitoring……84

2.3.4.1 UV/VIS absorption spectroscopy……… 85

2.3.4.2 Fluorescence spectroscopy……….85

2.3.4.3 Real-time infrared spectroscopy……….86

2.3.4.4 Differential photocalorimetry (photo-DSC)… 88

2.3.5 Ultraviolet light curable electrically conductive composites using silver filler………92

2.4 Synthesis of silver nanomaterials………93

2.4.1 The seed mediated “polyol” process………93

2.4.2 The seed mediated wet chemical synthesis of silver nanorods and nanowires………96

2.4.3 Seedless, surfactantless wet chemical synthesis of silver nanowires………98

2.4.4 Structural characterization of silver nanowires………99

III EXPERIMENTAL……….102

3.1 CNF composite materials……… 102

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3.1.1 Instrumental methods of characterization for carbon

nanofiber materials……….103

3.1.2 Purification of oxidized carbon nanofibers………104

3.1.3 Surface induced polymerization of CNF with glycidol……… 106

3.1.4 Free glycidol polymerization……….106

3.1.5 Acid-base titration of CNF-OX……… 107

3.1.6 Esterification of CNF-OX and CNF-polyol with a fluorinated acid chloride……….107

3.1.7 Dispersion study of CNF-OX and CNF-polyol by visual inspection……….108

3.1.8 Dispersion study of CNF-OX and CNF-polyol by TEM……… 108

3.1.9 Synthesis of CNF-COCl……….108

3.1.9.1 Synthesis of extended CNF-OH……… 109

3.1.9.1.1 Polymerization of glycidol with extended CNF-OH……… 109

3.1.9.2 Synthesis of the macroinitiator CNF-Br…… 110

3.1.9.2.1 Polymerization of aniline……….110

3.1.9.2.2 Polymerization of aniline with CNF-Br………111

3.2 Silver and polyaniline filled epoxy composites………112

3.2.1 Silver particles………112

3.2.2 Polyaniline……… 114

3.2.3 Formulation of silver-filled epoxy adhesives……….115

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3.2.3.1 Formulation of silver and polyaniline-filled

epoxy adhesives………115

3.2.3.2 Formulation of silver and aniline monomer filled epoxy adhesives……… 116

3.2.4 Polyaniline dispersion study in ECC……… 116

3.2.5 Preparation of adhesive coated glass slides………116

3.2.6 Instrumental methods of characterization of adhesive-coated glass slides……… 117

3.2.6.1 Solid-state 13C NMR analysis……… 117

3.2.6.2 AC electrical conductivity by dielectric spectroscopy……….117

3.2.6.3 DC electrical conductivity by 4-point probe………118

3.3 Radiation-cured silver-filled epoxy composites……… 119

3.3.1 Preparation of adhesive coated slides……….120

3.3.2 Instrumental methods of characterization for photo-curing formulations……… 121

3.4 Silver nanomaterials……… 123

3.4.1 Instrumental methods of characterization of silver nanomaterials……… 123

IV RESULTS AND DISCUSSION……… 124

4.1.1 Synthesis of CNF-polyol………125

4.1.2 Synthesis of esterified CNF-OX and CNF-polyol……… 125

4.1.3 Analysis of free polyglycidol by 1H NMR, 13C NMR and GPC……… 127

4.1.4 Determination of CNF-OX oxidation level by acid- base titration………129

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4.1.5 Characterization of CNF-OX by Raman spectroscopy… 130

4.1.6 Characterization of CNF-OX, CNF-polyol, free

polyol and esterified products by FT-IR……….134

4.1.7 Determination of CNF-OX and CNF-polyol oxidation

level by XPS……… 138

4.1.8 Characterization of CNF-OX, CNF-polyol, free

polyol and esterified products by TGA……… 141

4.1.9 Characterization of CNF-OX-F and CNF-polyol-F by

elemental analysis……… 144

4.1.10 Microscopy images of CNF-OX and CNF-polyol by

SEM and TEM………145

4.1.11 Solubility of CNF-OX in various solvents by UV/VIS

absorption measurements………147 4.1.12 Dispersion studies on CNF-OX and CNF-polyol……… 149 4.1.13 Synthesis of immobilized initiator CNF-Br………153

4.1.13.1 Polymerization of aniline……… 156 4.1.13.2 Polymerization of aniline with CNF-Br…… 157 4.2 Silver and polyaniline-filled epoxy composite materials………… 159

4.2.1 Determination of PANI oxidation state……….161 4.2.2 Dispersion of PANI in ECC……… 162 4.2.3 Thermal stability study of PANI by TGA……….163 4.2.4 Silver- and polyaniline-filled epoxy formulations……….165

4.2.4.1 DC electrical conductivity of silver and

polyaniline-filled epoxy formulations……… 167 4.2.5 Ag and aniline-filled epoxy formulations……… 168

4.2.5.1 DC electrical conductivity of silver and

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4.2.6 SEM image analysis of Ag and Ag-PANI epoxy

formulations………170 4.2.7 Solid-state 13C NMR analysis……….171

4.2.8 DSC analysis: Effect of changing the silver

4.2.8.1 DSC analysis: Effect of changing the silver

type………180 4.2.8.2 DSC analysis: Effect of changing the PANI

concentration……… 182 4.2.8.3 DSC analysis: Effect of changing the PANI

molecular weight……… 183 4.2.9 AC electrical conductivity by dielectric spectroscopy

for silver-filled, uncured epoxy formulations……….185 4.2.10 AC electrical conductivity by dielectric spectroscopy

for PANI-filled, uncured epoxy formulations……….188 4.2.11 AC electrical conductivity by dielectric spectroscopy

for Ag and PANI-filled, uncured epoxy formulations……189 4.3 UV radiation-cured acrylate formulations……….194

4.3.1 UV radiation cured Ag-filled acrylate composites……….196

4.3.1.1 Temperature effects of curing Ag-filled

4.3.1.2 Effect of adding silica filler to filled and unfilled acrylate compositions……… 200 4.3.1.3 Effect of changing the photoinitiator

and photosensitizer package and concentrations……… 202 4.3.2 Effect of initiator concentration in UV-cured

epoxy composites………203 4.3.2.1 UV radiation cured Ag-filled epoxy

composites……….204

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4.3.3 Effect of adding undoped polyaniline to epoxy and

Ag filled epoxy compositions……… 207

4.3.4 Electrical conductivity of UV radiation-cured samples…210 4.3.5 Reaction kinetics of the UV-cured epoxy composites… 210

4.4 Synthesis of silver nanomaterials……… 211

4.4.1 The seed mediated “polyol” process……… 212

4.4.2 Seed mediated wet chemical synthesis of Ag nanowires………217

4.4.3 Seedless, surfactantless wet chemical synthesis of silver nanowires……… 224

4.4.4 UV/VIS absorption spectroscopy of Ag particles……… 225

V SUMMARY AND CONCLUSIONS………227

5.2 Silver- and polyaniline-filled epoxy composites……… 228

5.3 Radiation-cured, silver-filled epoxy composites……… 229

5.4 Synthesis of silver nanomaterials……… 229

REFERENCES………230

APPENDICES……….242

APPENDIX A CALCULATION FOR THEORETICAL FLUORINE CONTENT IN CNF-OX-F……….243

APPENDIX B CALCULATION FOR THEORETICAL FLUORINE CONTENT IN CNF-POLYOL-F……… 245

APPENDIX C CALCULATION FOR THEORETICAL FLUORINE CONTENT IN CNF-POLYOL-F……… 247

APPENDIX D CALCULATION FOR MOLES OF ACID AND NUMBER OF ACID GROUPS IN CNF-OX………….249

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APPENDIX E CALCULATION FOR MOLES OF ACID AND

NUMBER OF ACID GROUPS IN CNF-POLYOL……250

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LIST OF TABLES

2.1 Hu et al [A51] acidity results from the nitric acid oxidation of SWNT

(Reprinted from Ref [A51] “Determination of the acidic sites of purified

with permission from Elsevier)

http://www.sciencedirect.com/science/journal/00092614 ……… 12

2.2 CNF IR absorption frequencies and peak assignments (Reprinted from Ref

[A53] “Surface oxidation of carbon nanofibers” Copyright Wiley-VCH

Verlag GmbH & Co KGaA Reproduced with permission)……… 13

2.3 CNF IR absorption frequencies after oxidative treatment (Reprinted from

Ref [A53] “Surface oxidation of carbon nanofibers” Copyright Wiley-VCH Verlag GmbH & Co KGaA Reproduced with permission)……….13 2.4 Description of SWNT Raman vibrational modes [A55]………16

2.5 Effect of various oxidizing agents on polyaniline properties (Reprinted

from Ref [B41] “Influence of chemical polymerization conditions on the

Limited) http://www.sciencedirect.com/science/journal/00323861………… 53 2.6 DSC data for the curing reaction of varying ratios of doped

PANI:DGEBA(Reprinted from Ref [B54] “Polyaniline as a curing agent

For epoxy resin: Cure kinetics by differential scanning calorimetry”

Copyright Wiley-VCH Verlag GmbH & Co KGaA Reproduced with

permission)………62 2.7 Characteristics of epoxy/PANI-DBSA blends mixed with different

hardeners (Springer and the original publisher Ref [B58] “Hardener type

as critical parameter for the electrical properties of epoxy resin/polyaniline

blends”, original copyright notice is given to the publication in which the

material was originally published, by adding; with kind permission from

Springer Science and Business Media)……… 68 2.8 Photo-polymerizable monomers and mechanistic processes……….74

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2.9 Absorption of UV light (λmax in nm) with respect to substitution

on aromatic rings for aryliodonium salts [B20]………80

3.1 Properties of Ferro silver flake Ag85HV (Reprinted with permission from Ferro Corporation, Ref [B26], Mary Abood, 2007)……… 114

4.1 Description of CNF Raman vibrational modes………133

4.2 XPS analysis comparison for various CNF types and modifications (atomic %)………140

4.3 Determination of CNF-OX and CNF-polyol hydroxyl groups by esterification……….145

4.4 Solvent solubility of CNF-OX as measured by UV-VIS absorbance (AU)……148

4.5 Elemental analysis for macroinitiator CNF-Br and precursors (weight %)……153

4.6 XPS analysis for macroinitiator CNF-Br and precursors (atomic %)………….154

4.7 Summary of IR data results of calculating the oxidation level of PANI……….162

4.8 Example of Ag and undoped PANI epoxy formulations……….167

4.9 Aniline containing formulations……… 169

4.10 Property summary for Ag particles……… 187

4.11 Acrylate formulations SR-B1-77 series……… 195

4.12 Photo-DSC sample data for the Ag filled compositions……… 198

4.13 Density of acrylate monomers compared to silver flake……….199

4.14 Photo-DSC experiments comparing the photoinitiator concentration………….203

4.15 Summary of silver samples used……….205

4.16 Design of experiment for the synthesis of Ag seeds………219

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LIST OF FIGURES

2.1 Schematic of SWNT [A31], MWNT [A32] and CNF [A40] ([A31]:

Repreinted with permission from Robert Wong, SES Research, 2007)

([A32]: Reprinted with permission from Patrick Collins, Hyperion

Catalysis, 2007) ([A40]: Reprinted with permission from Dave Burton,

Applied Sciences, Inc., 2007)……… 8

2.2 Typical methods for synthesizing CNT and CNF [A33]: (a) electric

arc-discharge, (b) laser ablation, (c) thermal CVD and (d) plasma

enhanced CVD……….9

2.3 TEM image of CNF with Fe catalytic particle at the tip [A40] (Reprinted

with permission from Dave Burton, Applied Sciences, Inc., 2007) ………10

2.4 Peak assignments for CNF-OX by Lakshminarayanan et al [A42] for

deconvoluted XPS spectra……….14

2.5 Atomic displacements associated with the RBM and G-band normal mode

vibrations [A65] (Reprinted, with permission, from the Annual Review of

2.6 Room temperature Raman spectra for as-received (a) DWNT and

(b) SWNT, at an excitation wavelength of 633 nm (Reprinted with

after oxidation for (a) DWNT and (b) SWNT (Reprinted with permission

2.8 Room temperature Raman spectra after oxidation (top) in air and

(bottom) with hydrochloric acid (Reprinted with permission from

2.9 Typical surfactant molecules used to make CNF dispersions:

SDS = sodium dodecyl sulfate, CTAB = cetyltrimethylammonium bromide, SDBS = sodium dodecylbenzene sulfonate……… 21

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2.10 Examples of some epoxy monomes (top) cyclohexyl diepoxide, (middle)

diglycidyl ether bisphenol A epoxide and (bottom) siloxane oligomer

diglycidyl epoxide……… 32 2.11 Chemical structure of a polyamide………34 2.12 SEM images of Ag particles; NanoDynamics: 40 nm Ag nanoparticles,

80 nm Ag powder (S2-80), 60 nm x 600 nm Ag platelets (S3-500) and

Ferro: 8 µm (ave.) Ag flake #85HV [B25,B26] ([B25]: Images courtesy of

NanoDynamics, with permission from Alan Rae, 2007) ([B26]: Image

courtesy of Ferro Corporation, with permission from Mary Abood, 2007) 40

2.13 Example surfactants used on Ag particles (a) oleic acid, (b) isostearic acid

and (c) palmitic acid……… 41

2.14 Schematic of a percolation curve for polymers filled with electrically

conductive particles……… 42

2.15 Tunnel resistivity for thin films of TiO2 on Ti as a function of film

thickness (Reprinted with permission from Ref [B30] “Resistivities of

2.16 Example of frequency dependence on total conductivity (σ) at different

temperatures (Reprinted from Ref [B34] “Dielectric spectroscopy of some

http://www.sciencedirect.com/science/journal/13861425 46 2.17 Frequency dependence on conductivity at different temperatures for PTFE

with permission from Ref [B36] “Electrical properties of epoxy/silver

between 0 and 11.1 wt % (Reprinted with permission from Ref [B36]

American Institute of Physics)……… 48 2.20 pz-orbital overlap leads to π conjugation and electrical conductivity

[B38,B39]……… 49 2.21 Structures of various conducting polymers………50

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2.22 Conductivity of electrically conductive polymers (Reprinted Figure 1 with

Society) http://link.aps.org/abstract/RMP/v73/p701……… 50 2.23 Repeat structure of polyaniline……… 52 2.24 The oxidation states of polyaniline: LEB = leucoemeraldine base,

EB = emeraldine base and PB = pernigraniline base……….52

2.25 Chemical structure of common PANI doping agents (left) DBSA and

(right) CSA……….55

2.26 The reduced formed of PANI (doped) to form a bipolaron, polaron and a

soliton pair……….56

2.27 Arrhenius plot of undoped (x = 0) and lightly doped (x = 0.08) PANI:

DC electrical conductivity vs T-1 (Reprinted Figure 2a with permission

2.28 AC conductivity vs frequency for (left) undoped PANI and (right) lightly

2.29 Electrical conductivity vs T-1/2 for doped PANI (x = 0.30 and x = 0.50)

(DC and microwave conductivity values are represented by dashed lines

and symbols, respectively) (Reprinted Figure 2 with permission from Ref

2.30 Comparison of conductivity vs temperature for PANI-CSA (circles)

and PANI-Cl (diamonds) (Reprinted Figure 2 with permission from Ref

http://prola.aps.org/abstract/PRB/v47/i4/p1758_1……… 59 2.31 Variation of measured conductivity plotted as a function of (left) reciprocal

temperature and (right) frequency (Reprinted from Ref [B53] “Dielectric

http://www.sciencedirect.com/science/journal/03796779……… 60

2.32 DSC scans for varying ratios of doped PANI:DGEBA (Reprinted from

Ref [B54] “Polyaniline as a curing agent for epoxy resin: Cure kinetics

by differential scanning calorimetry” Copyright Wiley-VCH Verlag GmbH

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2.33 Electrical resistivity of various blends of epoxy and PANI (Reprinted

from Ref [B55] “Electrically conductive composites based on epoxy

from Elsevier Limited)

http://www.sciencedirect.com/science/journal/03796779……… 63 2.34 DSC thermograms of (a) epoxy/PANI powder (10 wt% PANI),

(b) epoxy/PANI paste (10 wt% PANI) and (c) epoxy/PANI paste

(60 wt% PANI) composites with an anhydride hardener (epoxy:

hardener = 10:9) (Reprinted from Ref [B55] “Electrically conductive

composites based on epoxy resin with polyaniline-DBSA fillers”,

http://www.sciencedirect.com/science/journal/03796779………… 63 2.35 DSC thermograms of epoxy/PANI powder (10 wt% PANI) composite

using anhydride as a hardener with the accelerator content at (a) the regular amount, (b) two times, and (c) four times the regular amount (Reprinted

from Ref [B55] “Electrically conductive composites based on epoxy resin

Elsevier Limited)

2.36 Effect of PANI-DBSA content on the electrical conductivity of

epoxy/PANI-DBSA blends (Reprinted from Ref [B56] “Thermal,

mechanical, and electrical properties of epoxy

http://www.sciencedirect.com/science/journal/03796779……….65 2.37 DSC thermograms of uncured epoxy/PANI-DBSA blends (Reprinted

from Ref [B56] “Thermal, mechanical, and electrical properties of epoxy

with permission from Elsevier Limited)

2.38 DSC thermograms of cured epoxy/PANI-DBSA blends (Reprinted from

Ref [B56] “Thermal, mechanical, and electrical properties of epoxy

with permission from Elsevier Limited)

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2.39 TGA thermograms of PANI-DBSA, cured epoxy and cured

epoxy/PANI-DBSA blends (Reprinted from Ref [B56] “Thermal,

mechanical, and electrical properties of epoxy

http://www.sciencedirect.com/science/journal/03796779……….67

2.40 The electromagnetic spectrum of UV radiation [C2]………70 2.41 Wavelength output of two common bulbs used for UV radiation curing

(top) mercury bulb and (bottom) iron-doped mercury bulb (Spectral

output portion of image was provided courtesy of Nordson, Ref [C4],

permission from Ed Mcghee, 2007)……… 71

2.42 The Perrin-Jablonski diagram describing processes that occur in an

electronically excited molecule [C6]……….72 2.43 Examples of free radical photoinitiators………76

2.44 Absorption spectra for (blue) a photoinitiator (Ciba® Irgacure® 907,

an α-aminoketone) and (black) a photosensitizer (Ciba®

Darocur® ITX,

an isothioxanthone), showing how the spectral absorbance range

can be extended by using both in a formulation………77 2.45 Examples of photosensitizers………78 2.46 Graphical relationship between UV intensity and coating thickness………83 2.47 Illustration of “surface cure” and “through cure” [C14]………84

2.48 RT-IR experimental setup (Drawing reused with author’s permission,

Ref [C13], Mark Soucek, 2007)……… 87

2.49 Kinetic plot of epoxide conversion vs time based on RT-IR spectroscopy,

solid line R = 4, dashed line R = 6, where R = (epoxide equivalents/hydroxyl equivalents) (Image reused with author’s permission, Ref [C19], Dean

Webster, 2007)……… 87 2.50 Effect of relative humidity on the conversion of a cyclohexyl epoxide

formulation (left) time = 0 – 30 sec and (right) overall, time = 600 sec

(Image reused with author’s permission, Ref [C13], Mark Soucek, 2007)…… 88 2.51 A typical photo-DSC curve………89

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2.53 Comparison of Cho et al [C20] experimental data to model (Reprinted

from Ref [C20] “Photo-curing kinetics for the UV-initiated cationic

polymerization of a cycloaliphatic diepoxide system photosensitized

http://www.sciencedirect.com/science/journal/00143057 91

2.54 Schematic illustration of the growth process of Ag nanorods & nanowires

(Reprinted from Ref [D16] with permission from Cewen Nan, Tsinghua

Science & Technology, 2005)………95 2.55 (A) SEM image of silver nanowires and (B) TEM image of a single

nanowire (Reprinted from Ref [D7] “Shape-controlled synthesis of

metal nanostructures: The case of silver” Copyright Wiley-VCH Verlag

GmbH & Co KGaA Reproduced with permission)………96 2.56 TEM image of shape-separated Ag nanowires (Reproduced from Ref

[D1] with permission of The Royal Society of Chemistry)

http://www.rsc.org/Publishing/Journals/CC/article.asp?doi=b100521i 98 2.57 Aqueous solutions of silver nanoparticles showing increased red-shifting

with increased aspect ratio (Reprinted from Ref [D17] “Controlling the

aspect ratio of inorganic nanorods and nanowires” Copyright Wiley-VCH

Verlag GmbH & Co KGaA Reproduced with permission)……… 100

2.58 TEM image of a microtomed Ag nanowire sample (Reprinted from Ref

[D13] “Shape-controlled synthesis of metal nanostructures: The case of

silver” Copyright Wiley-VCH Verlag GmbH & Co KGaA Reproduced

with permission) ……… 101 2.59 Electron diffraction pattern for an individual Ag nanowire obtained by

aligning the electron beam perpendicular to one of the five sides (Reprinted from Ref [D13] “Shape-controlled synthesis of metal nanostructures: The

case of silver” Copyright Wiley-VCH Verlag GmbH & Co KGaA

Reproduced with permission)……… 101 3.1 (a) Various carbon fibers and tubes and (b) TEM of a CNF [A40]

(Reprinted with permission, Ref [A40], from Dave Burton, Applied

Sciences, Inc., 2007) ……… 102 3.2 TGA thermogram of CNF-OX (black) as-received and (red) washed…………105 3.3 Chemical structures of some formulation materials………113 3.4 SEM image of Ferro silver flake Ag85HV (SEM image courtesy of Ferro

Corporation, Ref [B26], with permission from Mary Abood, 2007)………….113

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3.5 (left) High frequency sample holder used in dielectric spectroscopy

measurements and (right) sample setup between gold electrodes……… 118 3.6 Diagram for the 4-point probe conductivity apparatus……… 119 3.7 Photograph of the Fusion UV conveyor system (belt moves right to left)…… 121 3.8 Schematic of sample irradiation……… 121

4.1 1H NMR of (a) glycidol monomer over molecular sieves (CDCl3) and

(b) glycidol free polymer (DMSO) (spectra provided by B Higgins)…………128 4.2 13C NMR and peak assignments for (a) glycidol monomer dried over

molecular sieves and (b) free polyol, in d6-DMSO (spectra provided by

B Higgins)……… 128

4.3 CNF Raman spectra (a) Various types of treated CNF (b) Two batches of

CNF-OX (ASI batch 2140 and 2590)……… 131 4.4 FT-IR spectrum of CNF-OX……… 134 4.5 FT-IR spectra of (a) CNF-OX and CNF-OX-F, and (b) CNF-OX,

CNF-polyol and CNF-polyol-F……… 136 4.6 FT-IR spectra of (a) CNF-polyol, (b) CNF-OX and (c) free polyol………136

4.7 XPS multiplex scans for CNF-OX and their corresponding

deconvolutions (a) C1s and (b) O1s……….139 4.8 TGA scan of CNF-OX (left) weight % and (right) derivative weight % 141 4.9 TGA thermal stability overlay scans of CNF with various treatments…… 142 4.10 TGA weight % and derivative weight % plots of CNF-OX, CNF-polyol…… 143 4.11 TGA derivative weight % plots for two batches of CNF-OX-F……… 144 4.12 SEM of CNF-OX at 2000x magnification, 25 kV voltage, 14 mm working

distance (58 µm across picture)……… 146 4.13 TEM images of CNF-OX (left) bamboo structure, (middle) surface defects

and (right) Fe catalyst particle……….146

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4.14 TEM images of (a) CNF-OX, (b) CNF-polyol, and (c) CNF-polyol with

polymer aggregate………147 4.15 Photograph of sample filtrates used for the solubility study (from left to

right: DMF, o-DCB, H2O, CHCl3, DCM)……… 149

4.16 CNF-OX after four hours aging in (left) H2O and (right) H2O2 (from left

to right: low 0.05 to 10.0 wt%)………150

4.17 Pictures of various CNF dispersed in water (a) CNF-polyol after four weeks

and (b) CNF-OX after two weeks………150

4.18 CNF-polyol deposited from (a) o-DCB and (b) H2O and CNF-OX

deposited from (c) o-DCB and (d) H2O, at a concentration of 10,000/1

(solvent/CNF, w/w)……….152 4.19 TGA weight % overlay plot for each step in the synthesis of CNF-Br……… 155

4.20 TGA derivative weight % overlay plot for each step in the synthesis of

CNF-Br………156 4.21 Polyaniline TGA plots (blue) Panipol PA (undoped, Panipol Ltd.), and (red)

synthesized undoped (a) Weight % plot and (b) Derivative weight % plot……157 4.22 CNF-PANI reaction scheme and TGA weight % plot……….158 4.23 TGA derivative weight % plot for (red) polyaniline (green) CNF-PANI…… 159 4.24 IR spectrum of undoped PANI………162

4.25 Pictures of undoped and doped PANI in ECC at 0.005 wt % (left) Mw =

10,000 g/mol undoped, 50,000 g/mol undoped, 100,000 g/mol undoped,

50,000 g/mol doped with DBSA; listed left to right and (right) coated

slides at Mw = 50,000 g/mol 163 4.26 Overlay thermograms for doped and undoped PANI in N2(g) and air………… 164 4.27 Overlay TGA thermograms for undoped PANI samples for samples

Mw = 10000, 50000 and 100,000 g/mol (a) N2(g) and (b) Air……… 166

4.28 DC electrical conductivity results for Ag-filled formulations containing

PANI………168 4.29 DC electrical conductivity results of silver-filled epoxy formulations

comparing aniline and undoped PANI……….169

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4.30 SEM images of Ag epoxy (reference) and PANI containing formulations…….171

4.31 Solid state 13C NMR spectra of undoped PANI (a) Repeat structure of

PANI, (b) Before heating and (c) After heating……… 172 4.32 Solid state 13C NMR spectrum of PANI doped with DBSA……… 173 4.33 Solid state 13C NMR spectrum of epoxy formula with 2 wt% undoped

PANI………174 4.34 Stack spectra of 6 wt% PANI formula with different initiator

concentrations (bottom) original concentration and (top) increased

concentration………175 4.35 Solid state 13C NMR spectrum of the aryliodonium salt initiator………175 4.36 Solid state 13C NMR stack plot of Ag and non-Ag filled epoxy

formulations……….176

4.37 DSC thermogram overlay of Ag epoxy with increasing concentrations of

Ag flake #85HV (black) 0 wt %, (red) 25 wt %, (blue) 40 wt % and (green)

60 wt % 178 4.38 DSC thermogram overlay of Ag epoxy with increasing concentrations of

Ag flake SF1 (black) 0 wt %, (red) 25 wt %, (blue) 40 wt % and (green)

60 wt % 179 4.39 DSC thermogram overlay of Ag epoxy with increasing concentrations of

Ag flake SF25 (black) 0 wt %, (red) 25 wt %, (blue) 40 wt % and (green)

60 wt % 179

4.40 DSC thermogram overlay of Ag epoxy with increasing concentrations of

Ag powder EGED (black) 0 wt %, (red) 25 wt %, (blue) 40 wt % and

(green) 60 wt % 180

4.41 DSC thermogram overlay of Ag epoxy with different Ag shapes and sizes

at 25 wt% Ag (black) 85HV, (red) SF1, (blue) SF25 and (green) EGED…… 181

4.42 DSC thermogram overlay of Ag epoxy with different Ag shapes and sizes

at 40 wt % Ag (black) 85HV, (red) SF1, (blue) SF25 and (green) EGED…… 181 4.43 DSC thermogram overlay of Ag epoxy with different Ag shapes and sizes

at 60 wt % Ag (black) 85HV, (red) SF1, (blue) SF25 and (green) EGED…… 182

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4.44 DSC thermogram overlay of Ag-PANI filled epoxy at different undoped

PANI concentrations with 84 wt % Ag (black) 0 wt %, (red) 3 wt %, (blue)

6 wt % and (green) no PANI or Ag……….183 4.45 DSC overlay thermogram of PANI filled epoxy as a function of PANI

molecular weight (black) no PANI, (red) 10,000 g/mol, (blue) 50,000

g/mol, (green) 100,000 g/mol……… 184

4.46 DSC overlay thermogram of Ag-PANI filled epoxy as a function of PANI

molecular weight, 84 wt % Ag85HV and 6 wt % PANI (black) no PANI,

(red) 10,000 g/mol, (blue) 50,000 g/mol, (green) 100,000 g/mol………185

4.47 AC conductivity vs frequency for uncured Ag-filled epoxy as a function

of Ag filler concentration……….186

4.48 AC conductivity vs Ag filler concentration (wt %) as a function of

frequency……… 187

4.49 AC conductivity vs Ag concentration (wt %) for Ag-filled epoxy as a

function of Ag filler type……….188 4.50 AC conductivity vs frequency as a function of increasing undoped PANI

concentration (wt %)………189 4.51 AC conductivity vs frequency for undoped PANI at 1.5 wt % with

increasing molecular weight PANI; includes 84 wt % Ag85HV (blue)

10,000 g/mol, (red) 50,000 g/mol and (green) 100,000 g/mol………190

4.52 AC conductivity vs frequency for undoped PANI at 3 wt % with increasing

molecular weight PANI; includes 84 wt % Ag85HV (blue) 10,000 g/mol,

(red) 50,000 g/mol and (green) 100,000 g/mol………191

4.53 AC conductivity vs frequency for undoped PANI at 6 wt % with increasing

molecular weight PANI; includes 84 wt % Ag85HV (blue) 10,000 g/mol,

(red) 50,000 g/mol and (green) 100,000 g/mol………191 4.54 AC conductivity vs frequency for undoped PANI at 10,000 g/mol with

increasing PANI concentration (wt %); includes 84 wt % Ag85HV (blue)

1.5 %, (red) 3 % and (green) 6 % 192 4.55 AC conductivity vs frequency for undoped PANI at 50,000 g/mol with

increasing PANI concentration (wt%); includes 84 wt % Ag85HV (blue)

1.5 %, (red) 3 % and (green) 6 % 192

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4.56 AC conductivity vs frequency for undoped PANI at 100,000 g/mol with

increasing PANI concentration (wt %); includes 84 wt % Ag85HV (blue)

1.5 %, (red) 3 % and (green) 6 % 193 4.57 AC conductivity vs frequency for PANI doped with DBSA at increasing

concentrations (wt %); includes 84 wt % Ag85HV (blue) 1.5 %, (red) 3 %

and (green) 6 % 193

4.58 Overlay UV/VIS absorption spectra for photoinitiator Irgacure 907 and

photosensitizer ITX (both from Ciba Specialty Chemicals)……… 195 4.59 Photo-DSC curves for acrylate formulations (black) SR-B1-77A, (red)

SR-B2-77B and (blue) SR-B1-77C; varying ratios of IBOA:GMA………… 196 4.60 (left) Photo-DSC curves and (right) % conversion graph for acrylate

formulation with varying concentrations of Ag flake……….197 4.61 (left) Heat of polymerization vs acrylate monomer concentration and

(right) % conversion per gram acrylate vs silver concentration……….198

4.62 % Conversion vs time graph for unfilled and 50 wt % Ag flake filled

acrylate formulations at (green) 30 °C, (red) 50 °C and (black) 100 °C……….200

4.63 Photo-DSC curves for acrylate formulations containing Ag and silica

(green) unfilled acrylate, (black) acrylate + 5 wt % silica, (blue)

acrylate + 25 wt % Ag, and (red) acrylate + 25 wt % Ag + 5 wt % silica…… 201

4.64 Comparison of photoinitiator/photosensitizer concentration to curability

with UV light by photo-DSC……… 202

4.65 Photo-DSC curves for epoxy formulations with varying photoinitiator

concentrations……… 204

4.66 (left) Heat flow curves and (right) % conversion vs time graph for

Ag-filled epoxy formulations……… 205

4.67 Effect of Ag particles on UV light curing by photo-DSC; comparison of

epoxy -based systems……… 206

4.68 Effect of Ag particles on UV light curing by photo-DSC; comparison of

epoxy and acrylate-based systems……… 206 4.69 UV/VIS absorption spectra for undoped PANI in NMP

(Mw = 50,000 g/mol)………208

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4.71 Heat flow for aniline-filled epoxy compositions………209 4.72 Comparison of data to model for the epoxy system………211 4.73 SEM of Ag particles from “polyol” experiment 6; scattered evidence of Ag

nanorods, but spherical Ag is the major product……….214 4.74 SEM of Ag particles from “polyol” experiment 7; spheres and nanorods…… 214 4.75 SEM of Ag nanowire formed by using a higher reaction temperature…………215 4.76 TEM images of Ag nanowire synthesis……… 215 4.77 Capping action of trisodium citrate in the synthesis of Ag seeds………218 4.78 Pictures of seed solutions from the DOE……….219 4.79 (left) Ag seeds and (right) Ag nanorod with citrate capping agent……… 220

4.80 TEM image of a mixture of Ag particle sizes resulting from a poor match

of raw material stoichiometry……… 221

4.81 Example pictures of Ag particle solutions with aspect ratios of

(A) 1-spheres, (B) 4-rods, (C) 5-rods, (D) 3-rods, (E) squares & rods,

(bottom green) triangles and (bottom blue) platelets……… 223 4.82 TEM images of Ag spheres, ovals, squares and wires……….223 4.83 TEM image of Ag nanowires……… 225 4.84 UV/VIS absorption spectra for (left) three different batches of Ag seeds

(blue) sample F, (red) sample G and (green) sample H (right) various Ag

nanorods……… 226 4.85 UV/VIS absorption spectra for (green) Ag nanorods, (blue) Ag nanowires

and (red) Ag nanowires (longer aspect ratio)……… 226

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LIST OF SCHEMES

2.1 Forward and backward titration of carboxylic acid groups on SWNT

(Reprinted from Ref [A51] “Determination of the acidic sites of purified

with permission from Elsevier)

http://www.sciencedirect.com/science/journal/00092614 ……….11 2.2 Schematic of “grafting-to” and “grafting-from” techniques……… 23 2.3 The two step γ-radiation method for grafting PAA to MWNT [A90]………… 24 2.4 Functionalization of MWNT with GMA by ATRP (Reprinted with

Society) Image redrawn……….26

2.5 CNT sidewalls functionalized with phenol groups by 1,3-dipolar

cycloaddition (1), derivatized to an ATRP initiator (2), and polymerized

with MMA (3) (Reprinted with permission from Ref [A98] Copyright

2006 American Chemical Society) Image redrawn……… 27 2.6 Activated chain end mechanism of glycidol polymerization [A21]……… 28 2.7 Activated monomer mechanism of glycidol polymerization [A21]……… 28 2.8 Hyperbranched polyol grafted to MWNT by Xu et al (Reprinted with

Society.) Image redrawn……… 29

2.9 ATRP of a hyperbranched polymerized MWNT (Reprinted with

permission from Ref [A24] Copyright American Chemical Society)

Image redrawn……… 30 2.10 The reaction pathway of a primary amine with an epoxy group……… 33 2.11 The reaction of an anhydride with an epoxy group……… 33 2.12 Epoxide polymerization promoted by a tertiary amine……….34

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2.13 Pathway for generating an acid initiator for cationic polymerization………35 2.14 Cationic ring opening polymerization with a Brönsted acid [B21]……… 37 2.15 Activated monomer mechanism [B21]……… 37 2.16 Suggested mechanisms of epoxide polymerization [B22, B23] (Reprinted

from Ref [B23] “Novel substituted epoxide initiators for the carbocationic

polymerization of isobutylene” Copyright Wiley-VCH Verlag GmbH &

Co KGaA Reproduced with permission)……….38 2.17 Isomerization of a substituted epoxide by a Lewis acid [B22-B24]

(Reprinted from Ref [B23] “Novel substituted epoxide initiators for the

carbocationic polymerization of isobutylene” Copyright Wiley-VCH

Verlag GmbH & Co KGaA Reproduced with permission)……….39 2.18 Values for ne or three oxidizing agents [B43]……… 53 2.19 Polyaniline doping scheme………55 2.20 Formation of free radicals by homolytic fragmentation………75 2.21 Formation of free radicals by intermolecular hydrogen abstraction……… 75 2.22 Formation of free radicals by intramolecular γ-hydrogen abstraction………… 75 2.23 Formation of free radicals by electron transfer followed by proton transfer…….76

2.24 Reaction scheme of photosensitization showing that energy is transferred

from a photosensitizer (S) to a photoinitiator (I)……… 77 2.25 Generation of a Lewis acid from a diazonium salt………79 2.26 Generation of an oxonium ion by a Lewis acid……….79 2.27 Protonation of an oxonium ion by a Brönsted acid………79

2.28 Example ferrocenium complex, an organometallic salt capable of

initiating cationic photopolymerization……….80 2.29 Formation of peroxy radical and hydroperoxide (where * is a radical)………….82 2.30 Inhibition of a propagating radical with oxygen (Reprinted from Ref

[C7] “Photopolymerization kinetics of multifunctional monomers”,

http://www.sciencedirect.com/science/journal/00796700 82

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2.31 Illustration of the seed-mediated growth of metal nanorods and nanowires

(Reprinted from Ref [D17] “Controlling the aspect ratio of inorganic

nanorods and nanowires” Copyright Wiley-VCH Verlag GmbH & Co

KGaA Reproduced with permission) ……… 97

4.1 Hyperbranched polyol grafting synthesis on carbon nanofibers

(CNF-polyol)………126 4.2 Heptafluorobutyryl chloride reaction with CNF-OX……… 127 4.3 Synthesis of surface immobilized initiator CNF-Br………153 4.4 Degradation of polyaniline……… 165 4.5 Decomposition of NaBH4 in water……… 217 4.6 Reduction of silver by NaBH4……….218 4.7 Reduction of silver by ascorbic acid [D3] ………221 4.8 Role of NaOH in the reduction of Ag ions……… 222

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CHAPTER I INTRODUCTION

1.1 Carbon nanofiber composites

Carbon nanofiber (CNF) polymer composites have been investigated for enhanced mechanical, thermal and electrical properties of polymers [A1,A2] The main focus of this research was to provide improved electrical properties of polymers through the network formation of CNF at low weight percent (percolation threshold) in polymer composites Electrically conductive or semi-conductive composites could be used for electrostatic dissipation (ESD) and electromagnetic radio frequency interference (EMI/RFI) protection, as well as electronic biological sensing [A3-A15]

To obtain percolation in CNF composites, the CNF must be homogenously dispersed within the polymer matrix [A1] The use of mechanical and/or chemical forces

is required to exfoliate CNF aggregates prior to composite formation [A1,A16] High shear methods have been utilized to effectively disperse CNF; however, this method often results in CNF shortening The decreased aspect ratio leads to ineffective property improvement of the CNF composite [A17] One alternative to this high shear methodology is chemical modification of CNF to improve the interaction between CNF and the polymer matrix [A16,A18] This technique decreases the aggregation of the CNF and the extent of fiber shortening is far less than high shear methods Chemically

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assisted dispersion in polar media was theorized to be possible if a hyperbranched polyol could be polymerized from the surface of the CNF

Glycidol, a hydrophilic trifunctional (AB2) monomer, was used to produce a

hyperbranched polymer on CNF [A19] This monomer can be polymerized via a cationic

or anionic mechanism depending upon the initiator used One common initiator for cationic synthesis is boron trifluoride diethyl etherate (BF3OEt2) [A20,A21] and was used

in this research BF3OEt2 provided a polymer with a number average molecular weight

of 500-3000 g/mol at -20 °C Lower reaction temperature can provide higher molecular weight NMR studies showed monomer addition is a combination of activated monomer and activated chain end mechanisms Activated monomer addition produced primary and secondary alcohols, while activated chain end addition produced only primary alcohols [A21]

Royappa et al [A22] cationically synthesized polyglycidol at room temperature

Xu et al [A23] performed surface induced polymerization of a highly strained

hydrophilic monomer, 3-ethyl-3-(hydroxymethyl) oxetane, with oxidized multi-walled carbon nanotubes (MWNT) with BF3OEt2 Hong et al [A24] prepared hyperbranched polymers on MWNT using ATRP (atom transfer radical polymerization)

An aspect of hyperbranched-CNF research that has not been explored yet is a step, surface-induced synthesis of a hyperbranched polymer on CNF The increased amount of hydroxyl groups on the surface of the CNF will assist dispersion in hydrophilic solvents and polymers (e.g., PVOH, epoxy) To compatibilize CNF with other polymers, such as polystyrene and poly(methyl methacrylate), radical polymerizations could be used to surface modify CNF

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one-The surface-induced polymerization of oxidized carbon nanofibers by glycidol is being reported The resulting polyol modified CNF was characterized by SEM/TEM, FT-IR, and TGA The hydroxyl groups were reacted with heptafluorobutyryl chloride to determine the amount of oxidized groups in the sample The resulting composite was characterized by FT-IR, TGA and elemental analysis Solubility, aggregation, and dispersion studies were performed to determine the compatibility of the oxidized CNF and modified CNF with other materials

A separate portion of this project involved the immobilization of an ATRP initiator onto the surface of oxidized carbon nanofibers Although the initiator modified CNF could be used to polymerize monomers common to ATRP, it was used instead in the oxidative polymerization of aniline In its doped form, polyaniline is electrically conductive Although CNF are less expensive than CNT, they are also less conductive

A CNF-polyaniline composite was prepared

1.2 Silver- and polyaniline-filled epoxy composites

High electrical conductivity compositions are commonly comprised of epoxy monomers and silver particles [B1-B4] An epoxy monomer blend can be loaded with 70-80 wt % silver to achieve electrical resistance values as low as 10-4 ohm-cm, making them suitable for semi-conductor die attachment, conductive via-filling, and some electrical interconnect applications [B5-B7]

Inherently conductive polymers (ICPs) are effectively utilized as conductors only when they are doped Doping a precursor of an electrically conductive polymer promotes the conductivity by as much as 10 orders of magnitude [B8,B9] A material is classified

as an ICP precursor if it is comprised of a highly-conjugated backbone, and/or populated

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with electron rich moieties, such as nitrogen, sulfur, and aromatic rings Doped ICPs are difficult to process, insoluble in dielectric polymers and monomers, and typically exhibit poor physical properties [B10,B11]

Methods have been described for integration of ICPs into dielectric polymers, silver-filled systems, and thermosetting resins [B2,B5,B12,B13] However, we revealed

a synergism between ICPs and silver particle surfactants that can be extended to other acid-base ingredient interactions The achieved electrical resistivity of said systems approaches that of bulk silver (10-6 ohm-cm), with resistivity on the order of 10-5 ohm-cm These improved systems are ideal for printed electronics applications, such as radio frequency identification (RFID) tags

1.3 Radiation-cured, silver-filled epoxy composites

Curing of electrically conductive composites is most commonly achieved by thermal methods In many cases, the composite is a solvent-based system that requires evaporation of the solvent to leave a solid cured product The use of solvents requires special handling and disposal facilities and typically presents hazards to workers using these materials Solvent-free systems (100 % solids) are being explored with great interest, but thermal curing still takes minutes to hours to coat and cure, which can be very costly The long processing time required for thermal methods makes price targets for applications in printed electronics, such as RFID tags, difficult to meet Hence, the introduction of RFID tags into consumer applications has been very slow

UV radiation curing has gained more popularity for unfilled systems over the past two decades, but is still limited in applications requiring high levels of filler In this

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sizes and shapes of silver filler are best suited for UV radiation curing The use of both acrylate and epoxy matrices was explored We revealed a matrix dependency on the ability to cure a silver-filled composition Further, the curability of the coating is inversely related to the connectivity of silver particles and is readily determinable by photo-DSC

The rate of polymerization of a polymer composite is well known to be dependent upon reaction temperature and was shown here for unfilled systems In the case of silver-filled composites, it was found that when higher temperatures were used, percolation of the filler particles was reached at lower concentrations and had a dramatic impact on the ability to be cured with UV light The synergy between silver particles and polyaniline,

as described in the previous project, has been found to have a negative impact on the UV curability of these polymer composites

1.4 Synthesis of silver nanomaterials

It is well known that the use of conductive fillers with higher aspect ratios leads to percolation at lower filler concentration Over the past decade, a few research groups have been investigating the synthesis of metal nanorods and nanowires [D1-D7] Although more difficult to synthesize, silver nanomaterials are of great interest for their high level of conductivity while having a reasonable cost, as compared to gold nanomaterials Silver nanorods have aspect ratios (length-to-width ratio) between 2 and 35; however, values of 10 and 20 are more common They can be synthesized with widths between 10 and 60 nm and lengths between 100 nm and 500 nm Silver nanowires have much higher aspect ratios, with lengths up to 50 microns

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Researchers have reported high yield synthesis of silver nanowires, but there have been no reports of polymer composites incorporating these materials In this research, we explored the potential to use silver nanowires as the conductive filler component in polymer composites The purpose was two-fold First, can higher values of electrical conductivity be achieved when silver particles with higher aspect ratio than silver flake is used? Second, can the UV radiation curability of silver-filled composites be improved?

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CHAPTER II HISTORICAL BACKGROUND

2.1 Carbon nanofiber composites

Carbon nanotubes (CNT) were discovered by Iijima in 1991, as described in

Nature [A25], while studying the carbonaceous deposit from an arc discharge between

graphite electrodes Highly crystallized carbon filaments of only a few nanometers in diameter and a few microns long were formed [A26] CNT are concentrically aligned walls of sp2 hybridized carbon atoms in a fused aromatic carbon structure around a hollow center (Figure 2.1) Nanotubes may be single-walled (SWNT), double-walled (DWNT) or multi-walled (MWNT) SWNT have a typical diameter of 1 nm and a length

of 10-20 µm MWNT have multiple concentric walls of crystalline carbon, diameters in the range of 10-20 nm and lengths comparable to SWNT The high aspect ratio of these CNT provides many desirable physical property characteristics; these include high mechanical strength (11-200 GPa stress at break) [A27-A29], modulus (1-2 TPa) [A30], thermal [A25] and electrical conductivity [A28], chemical inertness and high thermal stability (700-800 °C decomposition temperature)

Carbon nanofibers (CNF) are similar to CNT, Figure 2.1, but contain more walls

of crystalline carbon Their diameters are typically 50-100 nm with lengths upwards of

100 µm Four methods (Figure 2.2) are commonly used to synthesize CNF: electric

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arc-discharge, laser ablation, thermal chemical vapor deposition (CVD) and plasma enhanced CVD [A33]

Figure 2.1: Schematic of SWNT [A31], MWNT [A32] and CNF [A40]

([A31]: Repreinted with permission from Robert Wong, SES Research, 2007)

([A32]: Reprinted with permission from Patrick Collins, Hyperion Catalysis, 2007) ([A40]: Reprinted with permission from Dave Burton, Applied Sciences, Inc., 2007)

In the arc-discharge method, two graphite electrodes are used to produce a dc electric arc-discharge in an inert gas atmosphere (gas mixture of methane and argon) The arc-discharge is generated by running a dc current (typically 95 A at 20 V) between the electrodes Pure powdered metals, such as iron (Fe), nickel (Ni) and cobalt (Co), are used to catalyze the reaction Nanotubes with amorphous or graphitic carbon impurities are formed and the nanotubes must undergo additional processing for removal, such as high heat treatment or reaction with strong acid [A34,A35]

Smalley and coworkers [A36] in 1996 produced high yields of SWNT by laser (Nd:YAG) ablation (vaporization) of graphite rods with small amounts of Ni and Co at

1200 °C in an inert gas atmosphere CNT were deposited onto a water-cooled collector outside the furnace

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