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Preparation of Perfluorinated Ionomers

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Preparation of Perfluorinated Ionomers PREPARATION OF PERFLUORINATED IONOMERS FOR FUEL CELL APPLICATIONS by TODD STEPHEN SAYLER JOSEPH S. THRASHER, COMMITTEE CHAIR RICHARD E. FERNANDEZ, Ph.D. ANTHONY J. ARDUENGO, Ph.D. MARTIN G. BAKKER, Ph.D. KEVIN H. SHAUGHNESSY, Ph.D. DARRYL D. DESMARTEAU, Ph.D. A DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of The University of Alabama TUSCALOOSA, ALABAMA 2012 Copyright Todd Stephen Sayler 2012 ALL RIGHTS RESERVED ABSTRACT One of the major issues with the current membrane technology for polymer electrolyte membrane fuel cells is the low conductivity seen at low relative humidity. This dissertation discloses the preparation of perfluorinated polymers with higher densities of acid sites and higher conductivities to overcome this issue. These materials are prepared using a system designed to safely synthesize and polymerize tetrafluoroethylene (TFE) on a hundred gram scale. The copolymerization of TFE and perfluoro-2-(2-fluorosulfonylethoxy) propyl vinyl ether (PSEPVE) to prepare materials with varying ratios of the two monomers was carried out by solution, bulk, and emulsion polymerization techniques. Additionally, the homopolymer of PSEPVE has been prepared and characterized by MALDI-TOF mass spectrometry, which shows the low molecular weight distribution seen in other similar materials in the literature is due to a high rate of β-scission termination. Spectroscopic measurements and thermal analysis were carried out on these samples to obtain better characterization than was currently available. Producing polymers with a higher amount of PSEPVE, and thus higher density of acid sites, leads to the materials becoming water soluble after hydrolysis. However, addition of a curable ter-monomer allows the polymer chains to be crosslinked to regain water insolubility. Using this approach, water insoluble membranes with high densities of acid sites and conductivities up to 5.5 times higher than Nafion® 115, the standard benchmark for fuel cell membranes, have been produced. Preparation of high molecular weight, low EW copolymers of TFE and PSEPVE is difficult due to the reactivity ratios of the two monomers. Literature reactivity ratios for VDF and PSEPVE are more favorable for preparation of high molecular weight, low EW copolymers. Here, alternating copolymers of VDF and PSEPVE are prepared for the first time; where high ii molecular weight samples have been shown to possess low swelling characteristics in water. It has also been found the lower molecular weight samples that are soluble in perfluorohexane can be converted to perfluorinated polymers by direct fluorination with 20% elemental fluorine in nitrogen with 254 nm UV irradiation. The author makes no warranties, expressed or implied, and assumes no liability in connection with any use of the information presented in this dissertation. No one but persons having technical skill in this area of fluorine chemistry should attempt or repeat anything presented herein, and then at their own discretion and risk. iii DEDICATION This dissertation is dedicated to my wife, Franchessa Sayler, for her love, support, and advice during this work. She has enriched my life in more ways than she knows and filled it with purpose and love. I would also like to dedicate this dissertation to my mother, Kathy Sayler, and grandparents, Hunter and Jean Read, for their love through the years and support in helping me to continue my education. iv LIST OF ABBREVIATIONS AND SYMBOLS ˚C degrees Celsius δ chemical shift in parts per million 3M monomer perfluoro-4-(fluorosulfonyl) butyl vinyl ether, (CF2=CFOCF2CF2CF2CF2SO2F) 3P bis(pentafluoropropionyl) peroxide, (CF3CF2C(O)OOC(O)CF2CF3) 4P bis(heptafluorobutyryl) peroxide, (CF3CF2CF2C(O)OOC(O)CF2CF2CF3) 8CNVE perfluoro 8-cyano-5-methyl-3,6-dioxa-1-octene, (CF2=CFOCF(CF3)CF2OCF2CF2CN) ARC® accelerating rate calorimeter ATR attenuated total reflectance βPS HO3SCF2CF2OCF(CF3)CF2- radical or end group b.p. boiling point BPR back pressure regulator CMC critical micelle concentration DHB 2,5-dihydroxybenzoic acid, [C6H3(CO2H)(OH)2] DMSO dimethyl sulfoxide DP HFPO dimer peroxide end group, CF3CF2CF2O(CF3)CF- DuPont E. I. du Pont de Nemours and Co. Dow perfluoro-3-oxa-4-pentene sulfonyl fluoride, (CF2=CFOCF2CF2SO2F) DSC differential scanning calorimetry ePTFE expanded polytetrafluoroethylene, e-(-CF2-CF2-)n ETFE ethylene tetrafluoroethylene copolymer, (-CH2CH2CF2CF2-)n EW equivalent weight, grams of polymer needed to have 1 mole of acid sites v FT-IR Fourier transform infrared spectroscopy FEP fluorinated ethylene propylene, (-CF2CF2)m-(CF2CF(CF3)-)n g grams HFC 43-10 2,3-dihydrodecafluoropentane, (CF3CFHCFHCF2CF3) HFE 7100 methyl nonafluorobutyl ether, (CH3-O-CF2CF2CF2CF3) HFP hexafluoropropylene, (CF2=CF(CF3)) HFPOdaf hexafluoropropylene oxide dimer acid fluoride, (CF3CF2CF2O(CF3)CFC(O)F) HFPOdp hexafluoropropylene oxide dimer peroxide, [CF3CF2CF2O(CF3)CFC(O)O-]2 K Kelvin k rate constant - + KHP potassium hydrogen phthalate, C6H4(CO2H)(CO2 K ) L liters M molarity M1 monomer 1 M2 monomer 2 Mn number average molecular weight Mw weight average molecular weight MAFS-010 Merck advanced fluorosurfactant 010 - bis(perfluorobutyl) phosphinic acid, (C4F9)2P(O)OH MALDI-TOF matrix assisted laser desorption ionization - time of flight MEA membrane electrode assembly mil milliinch MS mass spectrometry vi mS millisiemens NMR nuclear magnetic resonance N117 Nafion® membrane with equivalent weight of 1100 and thickness of 7 milliinches PDI polydispersity index PE polyethylene, (-CH2CH2-)n PEMFC polymer electrolyte membrane fuel cell PFIB perfluoroisobutylene, [(CF3)2C=CF2] - + PFOA ammonium perfluorooctanoate, (CF3CF2CF2CF2CF2CF2CF2CO2 NH4 ) PFA perfluoroalkoxy resin, (-CF2CF2-)m(-CF2CF(ORf)-)n PFPA pentafluoropropionic acid, (CF3CF2CO2H) ppm parts per million psia pounds per square inch absolute psig pounds per square inch gauge PTFE polytetrafluoroethylene, (-CF2-CF2-)n PVDF polyvinylidene difluoride, (-CF2-CH2-)n R113 1,1,2-trichloro-1,2,2-trifluoroethane, CCl2F-CF2C R22 chlorodifluoromethane, CClF2H Rf perfluoroalkyl RH relative humidity rpm rotations per minute SEC size-exclusion chromatography SEM scanning electron microscopy Td decomposition temperature (5 percent weight loss) vii TFA trifluoroacetic acid, (CF3C(O)OH) TFE tetrafluoroethylene, (CF2=CF2) TGA thermal gravimetric analysis UAXD Copolymer of TFE and Dow (University of Alabama eXperimental Dow resin) UAXDC Terpolymer resin of Dow, TFE, and 8CNVE UAXDC#C Cured terpolymer of Dow, TFE, and 8CNVE UAXDH Dow homopolymer (University of Alabama eXperimental Dow Homopolymer) UAXH PSEPVE homopolymer (University of Alabama eXperimental Homopolymer) UAXH#A Sulfonamide resin of PSEPVE homopolymer UAXH#H Hydrolyzed resin of PSEPVE homopolymer UAXHC Copolymer of PSEPVE and 8CNVE UAXHC#C Cured copolymer of PSEPVE and 8CNVE UAXR Copolymer of TFE and PSEPVE (University of Alabama eXperimental Resin) UAXRA Sulfonamide resin of TFE and PSEPVE copolymer UAXV Copolymer of VDF and PSEPVE VDF vinylidene difluoride, (CH2=CF2) viii ACKNOWLEDGEMENTS I would like to begin by thanking my research advisor, Prof. Joseph S. Thrasher, for his guidance and encouragement during the course of this work. He has had a significant role in both my scientific and personal development during my graduate career. I must also thank Dr. Richard E. Fernandez for his help and advice while acting as a second advisor to me. I also would like to thank Prof. Anthony J. Arduengo, Prof. Martin G. Bakker, Prof. Kevin H. Shaughnessy, and Prof. Darryl D. DesMarteau for serving as my dissertation committee and for their valuable suggestions and comments. I also must thank coworkers on this project Dr. Andrej Matsnev, Dr. Yoichi Hosokawa, Dr. Alfred Waterfeld, and Dr. Alex Kauffman for their help and advice. Dr. Zhiwei Yang of UTC provided support by running the conductivity, fuel cell, and SEM measurements. Additionally, Kevin Tice contributed a large amount of the work presented here and must be thanked for his hard work and extremely valuable ideas and suggestions. Instrumentation assistance was provided to me by Dr. Ken Belmore with NMR spectral characterization and Dr. Qiaoli Liang with MALDI-TOF mass spectra analysis. Mr. Richard Smith, Mr. David Key, Mr. Billy Atkins, and the UA Machine Shop all provided support with the construction and maintenance of experimental apparati. I would also like to thank group members Dr. John Hauptfleisch, Dr. Huizhen Zhu, and Mr. Matt Hennek for their helpful advice and discussions along with teaching me laboratory techniques. I must thank Adam Beg and Anthony Cesnik for their hard work as summer students with EW titrations and fine powder PTFE synthesis, respectively. Funding for this research was provided by a grant through United Technologies Corporation (UTC). ix CONTENTS ABSTRACT ............................................................................................................ ii DEDICATION ....................................................................................................... iv LIST OF ABBREVIATIONS AND SYMBOLS ....................................................v ACKNOWLEDGEMENTS ................................................................................... ix LIST OF TABLES ............................................................................................. xviii LIST OF FIGURES ............................................................................................
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