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Polymer Carbon Nanofiber Composites Prepared By 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 Guillermo A. Jimenez May, 2007 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 Dr. Shing-Chung Wong ii ABSTRACT Chaotic mixing is a novel mixing technique offering high mixing efficiency even under mild shearing conditions. In this work, chaotic mixing was used to prepare composites of carbon nanofibers and two thermoplastic polymers – poly (methyl methacrylate) (PMMA) and thermoplastic polyurethanes (TPU) – and their electrical, mechanical, and thermal properties were evaluated. The TPU systems were based on the reaction products of 4,4’-diphenylmethane diisocyanate, (MDI), soft segment polyol, and 1,4-butanediol as chain extender. Soft segment polyols in the form of poly(propylene glycol) (PPG), and poly(ε-caprolactone)diol (PCL) were used to obtain respectively amorphous and crystalline soft segments. Of these, the TPU system based on crystalline soft segment exhibited shape memory effects. Both, as-received untreated carbon nanofibers (CNF) with a very low amount of atomic oxygen on the surface, and oxidized carbon nanofibers (CNFOX) were used. CNFOX was also modified by esterifying with PPG to produce a third type of carbon nanofiber named CNFOL. These carbon nanofibers were examined by X-ray photoelectron spectroscopy to determine the elemental composition of the surface, and by scanning electron microscopy and transmission electron microscopy to determine the surface morphology. iii The as-received CNF and CNFOX fibers were highly entangled and showed regions of amorphous carbon which acted as a deterrent to dispersion. The presence of higher content of oxygen-containing functional groups in CNFOX improved their dispersion in PMMA and TPU, but produced electrical percolation at much higher concentrations except in the case of TPU based on crystalline soft segment. Thermal stability and thermo-oxidative properties of the polymers were improved in the presence of carbon nanofibers. It was found that the aspect ratio of carbon nanofibers was preserved and that carbon nanofibers were aligned along the flow direction during chaotic mixing. This led to electrical percolation at much lower nanofiber content, compared to materials produced in a commercial internal mixer. It was also found that crystallinity and phase separation in TPU materials in a chaotic mixer was affected much less by the presence of carbon nanofibers, while TPU materials produced in the commercial mixer showed more phase mixing. Carbon nanofibers interacted with both soft and hard phases in TPU. Composites with CNFOX showed higher crystallinity which resulted in high stress recovery, high fixity, and recovery ratio. Both CNF and CNFOX improved the tensile properties of TPU when measured above the melting temperature of PCL. iv 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. This dissertation is dedicated to those who gave me their ultimate support and patience: my wife, Giovanna, and my kids, Gabriel and Laura. v 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 2.4.1. General properties...................................................................................... 31 vi 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 3.3.5. Electrical conductivity ............................................................................... 70 vii 3.3.6. Differential scanning calorimetry .............................................................. 72 3.3.7. Thermogravimetrical analysis.................................................................... 72 3.3.8. Tensile test ................................................................................................
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