Electrically conductive polymer composites a dissertation

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 ELECTRICALLY CONDUCTIVE POLYMER COMPOSITES Susan Rhodes Dissertation Approved: Accepted: Advisor Department Chair Dr. Roderic Quirk Dr. Mark Foster Committee Member Dean of the College Dr.

Alexei Sokolov Dr. Stephen Cheng Committee Member Dean of the Graduate School Dr. Gary Hamed Dr. George Newkome Committee Member Date Dr.

Judit Puskas Committee Member Dr. Mark Soucek ii 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 3,4- 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 (10-4 ohm-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.

iii 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. iv 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.

Bernadette Higgins for her background in carbon nanofiber modification and analysis. Jennifer Cross and Dr. Matthew Espe (Chemistry) for their collaboration in solid-state 13C NMR spectroscopy of polyaniline. 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.

Wayne Jennings of Case Western Reserve University and Dr. Thomas Wittberg of The University of Dayton for their assistance with XPS analysis. 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. vi TABLE OF CONTENTS Page LIST OF TABLES…………………………………………………………………….xvi LIST OF FIGURES………………………………………………………………….xviii LIST OF SCHEMES………………………………………………………………….1 Carbon nanofiber composites…………………………………………1 1.2 Silver- and polyaniline filled epoxy composites………………………3 1.3 Radiation-cured, silver-filled epoxy composites…………………….4 Synthesis of silver nanomaterials……….1 Carbon nanofiber composites…………………………………………7 2.1 Oxidized CNF synthesized by Applied Sciences, Inc…….2 Determination of CNF oxides by acid-base titration………11 2.1 Determination of CNF oxides by infrared spectroscopy………………………………….2 Determination of CNF oxides by X-ray photoelectron spectroscopy………………….3 Determination of CNF oxides by Raman spectroscopy………………………….3 Dispersion of CNF in polymer matrices………………….1 Solution blending of CNF into polymer systems……………………………………….2 Melt blending of CNF into polymer systems……………………………………….3 Surfactants to reduce CNF aggregation……….4 In-situ polymerization……………………….5 CNF surface modifications…………………….1 “Grafting-to” approach………….2 “Grafting-from” approach……….6 Hyperbranched polymer composites………….2 Silver-filled epoxies………………………………………………….1 Common formulation ingredients in silver-filled epoxies…………………………………………………….2 Cationic cure mechanism………………………………….1 AC electrical conductivity by dielectric spectroscopy………………………………….6 Inherently conducting polymers………………………….1 Synthesis of PANI……………….3 Electrical conductivity of doped and undoped PANI……….7 Polyaniline in epoxy adhesives…………………………….3 Ultraviolet radiation curing………………………………………….1 Excitation processes by absorption of UV light………….1 Free-radical photoinitiators and photosensitizers……………………………….3 Limitations of UV radiation curing……………………….4 Photopolymerization kinetics and reaction monitoring……84 2.1 UV/VIS absorption spectroscopy…………….3 Real-time infrared spectroscopy……………….5 Ultraviolet light curable electrically conductive composites using silver filler………………………………92 2.4 Synthesis of silver nanomaterials……………………………………93 2.1 The seed mediated “polyol” process………………………93 2.2 The seed mediated wet chemical synthesis of silver nanorods and nanowires……………………………………96 2.3 Seedless, surfactantless wet chemical synthesis of silver nanowires……………………………………………98 2.4 Structural characterization of silver nanowires……………99 III.1 CNF composite materials………………………………………….1 Instrumental methods of characterization for carbon nanofiber materials……………………………………….2 Purification of oxidized carbon nanofibers………………104 3.3 Surface induced polymerization of CNF with glycidol………………………………………………….4 Free glycidol polymerization…………………………….5 Acid-base titration of CNF-OX………………………….6 Esterification of CNF-OX and CNF-polyol with a fluorinated acid chloride………………………………….7 Dispersion study of CNF-OX and CNF-polyol by visual inspection………………………………………….8 Dispersion study of CNF-OX and CNF-polyol by TEM………………………………………………….9 Synthesis of CNF-COCl………………………………….1 Synthesis of extended CNF-OH…………….1 Polymerization of glycidol with extended CNF-OH…………….2 Synthesis of the macroinitiator CNF-Br…….1 Polymerization of aniline……….2 Polymerization of aniline with CNF-Br……………………111 3.2 Silver and polyaniline filled epoxy composites……………………112 3.3 Formulation of silver-filled epoxy adhesives…………….1 Formulation of silver and polyaniline-filled epoxy adhesives………………………………115 3.2 Formulation of silver and aniline monomer filled epoxy adhesives……………………….4 Polyaniline dispersion study in ECC…………………….5 Preparation of adhesive coated glass slides………………116 3.6 Instrumental methods of characterization of adhesive-coated glass slides…………………………….1 Solid-state 13C NMR analysis……………….2 AC electrical conductivity by dielectric spectroscopy……………………….3 DC electrical conductivity by 4-point probe…………………………………118 3.3 Radiation-cured silver-filled epoxy composites………………….1 Preparation of adhesive coated slides…………………….2 Instrumental methods of characterization for photo-curing formulations……………….1 Instrumental methods of characterization of silver nanomaterials………………. RESULTS AND DISCUSSION………………………………………….1 Carbon nanofiber composites………………………………………124 4.1 Synthesis of CNF-polyol…………………………………125 4.2 Synthesis of esterified CNF-OX and CNF-polyol……….3 Analysis of free polyglycidol by 1H NMR, 13C NMR and GPC………………………………………………….4 Determination of CNF-OX oxidation level by acid- base titration………………………………………………129 xi 4.5 Characterization of CNF-OX by Raman spectroscopy….6 Characterization of CNF-OX, CNF-polyol, free polyol and esterified products by FT-IR………………….7 Determination of CNF-OX and CNF-polyol oxidation level by XPS…………………………………………….8 Characterization of CNF-OX, CNF-polyol, free polyol and esterified products by TGA………………….9 Characterization of CNF-OX-F and CNF-polyol-F by elemental analysis……………………………………….10 Microscopy images of CNF-OX and CNF-polyol by SEM and TEM……………………………………………145 4.11 Solubility of CNF-OX in various solvents by UV/VIS absorption measurements…………………………………147 4.12 Dispersion studies on CNF-OX and CNF-polyol……….13 Synthesis of immobilized initiator CNF-Br………………153 4.1 Polymerization of aniline…………………….2 Polymerization of aniline with CNF-Br…….2 Silver and polyaniline-filled epoxy composite materials………….1 Determination of PANI oxidation state………………….2 Dispersion of PANI in ECC…………………………….3 Thermal stability study of PANI by TGA……………….4 Silver- and polyaniline-filled epoxy formulations……….1 DC electrical conductivity of silver and polyaniline-filled epoxy formulations……….5 Ag and aniline-filled epoxy formulations……………….1 DC electrical conductivity of silver and aniline-filled epoxy formulations…………….6 SEM image analysis of Ag and Ag-PANI epoxy formulations………………………………………………170 4.7 Solid-state 13C NMR analysis…………………………….8 DSC analysis: Effect of changing the silver concentration…………………………………………….1 DSC analysis: Effect of changing the silver type……………………………………………180 4.2 DSC analysis: Effect of changing the PANI concentration……………………………….3 DSC analysis: Effect of changing the PANI molecular weight…………………………….9 AC electrical conductivity by dielectric spectroscopy for silver-filled, uncured epoxy formulations…………….10 AC electrical conductivity by dielectric spectroscopy for PANI-filled, uncured epoxy formulations…………….11 AC electrical conductivity by dielectric spectroscopy for Ag and PANI-filled, uncured epoxy formulations……189 4.3 UV radiation-cured acrylate formulations………………………….1 UV radiation cured Ag-filled acrylate composites……….1 Temperature effects of curing Ag-filled acrylate compositions…………………………199 4.2 Effect of adding silica filler to filled and unfilled acrylate compositions……………….3 Effect of changing the photoinitiator and photosensitizer package and concentrations……………………………….2 Effect of initiator concentration in UV-cured epoxy composites…………………………………………203 4.1 UV radiation cured Ag-filled epoxy composites…………………………………….3 Effect of adding undoped polyaniline to epoxy and Ag filled epoxy compositions………………………….4 Electrical conductivity of UV radiation-cured samples…210 4.5 Reaction kinetics of the UV-cured epoxy composites….4 Synthesis of silver nanomaterials………………………………….1 The seed mediated “polyol” process…………………….2 Seed mediated wet chemical synthesis of Ag nanowires…………………………………………………217 4.3 Seedless, surfactantless wet chemical synthesis of silver nanowires………………………………………….4 UV/VIS absorption spectroscopy of Ag particles………. SUMMARY AND CONCLUSIONS………………………………………227 5.1 Carbon nanofiber composites………………………………………227 5.2 Silver- and polyaniline-filled epoxy composites………………….3 Radiation-cured, silver-filled epoxy composites………………….4 Synthesis of silver nanomaterials………………………………….

CALCULATION FOR THEORETICAL FLUORINE CONTENT IN CNF-OX-F………………. CALCULATION FOR THEORETICAL FLUORINE CONTENT IN CNF-POLYOL-F………. CALCULATION FOR THEORETICAL FLUORINE CONTENT IN CNF-POLYOL-F………. CALCULATION FOR MOLES OF ACID AND NUMBER OF ACID GROUPS IN CNF-OX………….

CALCULATION FOR MOLES OF ACID AND NUMBER OF ACID GROUPS IN CNF-POLYOL……250 xv LIST OF TABLES Table Page 2.1 Hu et al. [A51] acidity results from the nitric acid oxidation of SWNT (Reprinted from Ref.com/science/journal/00092614.2 CNF IR absorption frequencies and peak assignments (Reprinted from Ref. [A53] “Surface oxidation of carbon nanofibers”. Copyright Wiley-VCH Verlag GmbH & Co.

Reproduced with permission)………………….3 CNF IR absorption frequencies after oxidative treatment (Reprinted from Ref. [A53] “Surface oxidation of carbon nanofibers”. Copyright Wiley-VCH Verlag GmbH & Co. Reproduced with permission)…………………….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 properties of polyaniline”. Copyright 1989, with permission from Elsevier Limited).com/science/journal/00323861………….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.

Reproduced with permission)………………………………………………………………………62 2.7 Characteristics of epoxy/PANI-DBSA blends mixed with different hardeners (Springer and the original publisher Ref.8 Photo-polymerizable monomers and mechanistic processes…………………….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.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…………………………………………………………………….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 %)………….7 Summary of IR data results of calculating the oxidation level of PANI……….8 Example of Ag and undoped PANI epoxy formulations……………………….9 Aniline containing formulations……………………………………………….10 Property summary for Ag particles…………………………………………….11 Acrylate formulations SR-B1-77 series……………………………………….12 Photo-DSC sample data for the Ag filled compositions……………………….13 Density of acrylate monomers compared to silver flake……………………….14 Photo-DSC experiments comparing the photoinitiator concentration………….15 Summary of silver samples used……………………………………………….16 Design of experiment for the synthesis of Ag seeds……………………………219 xvii LIST OF FIGURES Figure Page 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.2 Typical methods for synthesizing CNT and CNF [A33]: (a) electric arc-discharge, (b) laser ablation, (c) thermal CVD and (d) plasma enhanced CVD…………………………………………………………………….3 TEM image of CNF with Fe catalytic particle at the tip [A40] (Reprinted with permission from Dave Burton, Applied Sciences, Inc.4 Peak assignments for CNF-OX by Lakshminarayanan et al. [A42] for deconvoluted XPS spectra……………………………………………………….