SYNTHESIS AND CHARACTERIZATIONS OF POLYMER ELECTROLYTE MEMBRANES BASED ON ALIPHATIC IONOMERS DAVID JULIUS NATIONAL UNIVERSITY OF SINGAPORE 2011 Thesis Spine SYNTHESIS AND CHARACTERIZATIONS OF POLYMER ELECTROLYTE MEMBRANES BASED ON ALIPHATIC IONOMERS DAVID JULIUS 2011 SYNTHESIS AND CHARACTERIZATIONS OF POLYMER ELECTROLYTE MEMBRANES BASED ON ALIPHATIC IONOMERS DAVID JULIUS (B. Eng., UNPAR; M. Sc., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENT First of all, I would like to express my deepest and greatest gratitude to my main supervisor, Professor Lee Jim Yang, and my co-supervisor, Associate Professor Hong Liang, for their guidance, patience, support and advices throughout my entire PhD study. Prof. Lee has been the maestro conductor in this PhD thesis study. Like a symphony, he helped me to set the tempo, prepared me for the execution, and listened and critiqued my performance. He shaped the direction of the research project and the physical form of the thesis as it is presented today. His inspirational stories and unconventional ideas have always been a source of motivation. Prof. Hong, on the other hand, has been my resourceful chef de cuisine, who is in charge of the details of the scientific investigation. His immense knowledge in polymer chemistry has helped me overcome many synthesis difficulties and rationalized many of the “perplexing” observations encountered in this study. I am grateful to National University of Singapore, in particular the Chemical and Biomolecular Engineering department for their generous scholarship supports that make this study possible. I would also like to thank my close friends and colleagues in our research group with whom I have formed a strong bond: Dr. Nikken Wiradharma, Dr. Deny Hartono, Mr. Usman Oemar, Dr. Natalia Widjojo, Ms. Fang Chunliu, Dr. Fu Rongqiang, Dr. Tay Siok Wei, Dr. Zhang Xinhui, Dr. Pei Haiqing, Dr. Zhou Weijiang, Dr. Zhang Qingbo, Mr. Cheng Chin Hsien, Dr. Deng Da, Dr. Yang Jinhua, Ms. Yu Yue, Ms. Lu Meihua, Ms. Ji Ge, Mr. Ma Yue, Mr. Bao Ji, Mr. Chia Zhi Wen, Mr. Yao Qiaofeng, Mr. Chen Dongyun, Dr. Liu Bo, Dr. Zhang Cao. My indebtedness also goes to all friends that had supported me in many ways during my PhD studies. I also want to i express my gratitude to Mr. Boey Kok Hong, Ms. Lee Cai Keng, Mr. Chia Pai An, Mr. Mogan, Ms. Samantha Fam, Dr. Yuan Ze Liang, and to all laboratory and professional staffs in Chemical and Biomolecular Engineering department for their technical assistance. The support, friendship, and encouragement of these people have helped to make this PhD study a journey of happiness. Thanks are also extended to my PhD examination panel, A/Prof. Loh Kai Chee, A/Prof. Chen Shing Bor, and Asst/Prof. Karl Erik Birgersson, for their valuable assessment and suggestion on this thesis and my future career. I would to express my sincere thank to Professor Peter N. Pintauro, who has been my inspiring teacher in this research study. Last but not least, I would like to thank my family members: my parents, my brothers, and also to my fiancée. Without their support and encouragement, I may not finish writing up this piece of work. ii TABLE OF CONTENTS ACKNOWLEDGEMENT . i SUMMARY . vii NOMENCLATURES x LIST OF FIGURES . xiii LIST OF TABLES . xix INTRODUCTION 1.1 Problem Statement .1 1.2 Objective and Scope of Thesis .4 (a) Design and Synthesis of New Alcohol-Resistant Alternative PEMs based on Aliphatic Random Ionomers .5 (b) Structural Characterizations of Random Aliphatic Ionomers and Investigations of the Effects of Hydrophobic Functional Groups on Phase Separation in Solution .5 (c) Development of New PEMs based on Aliphatic Block Ionomers and Hydrophilic Covalent Cross-links (d) Investigations of a One-pot ATRP Method and the Phase Separation of Aliphatic Block Ionomers in Solution .7 1.3 Organization of Thesis .7 LITERATURE REVIEW .8 2.1 Scope of the Review 2.2 Ionomers for DAFC Applications 2.2.1 DAFCs .8 2.2.2 Membrane Electrode Assembly (MEA) 10 2.2.3 Ionomers for Polymer Electrolyte Membranes (PEMs) 12 2.3 2.2.3.1 Modified Nafion® Membranes .13 2.2.3.2 Hydrocarbon Membranes .18 Ionomers: Synthesis, Structure, and Properties .24 2.3.1 Synthesis of Ionomers: Radical Polymerization 25 2.3.1.1 Synthesis of Random and Block Ionomers 25 2.3.1.2 Atom Transfer Radical Polymerization (ATRP) .29 2.3.2 Phase Separation of Ionomers in Solution and in the Solid State 32 iii 2.3.2.1 Phase Separation of Random and Block Ionomers in Solution .33 2.3.2.2 Phase Separation of Random and Block Ionomers in the Solid State 36 SYNTHESIS AND CHARACTERIZATION OF ACRYLIC RANDOMIONOMER MEMBRANES FOR ROOM TEMPERATURE DIRECT ETHANOL FUEL CELLS 41 3.1 Introduction 41 3.2 Experimental Method .43 3.2.1 Materials 43 3.2.2 Synthesis of Random Ionomers .44 3.2.3 Fabrication of Random-Ionomer Membranes 45 3.2.4 Characterization Methods 46 3.3 3.2.4.1 Proton Conductivity .46 3.2.4.2 Alcohol Permeability .47 3.2.4.3 Ion-Exchange Capacity (IEC) and Water Uptake 48 3.2.4.4 Mechanical Properties 48 Results and Discussion 49 3.3.1 Ternary Random-Ionomer Membranes for DEFCs .49 3.3.1.1 Rational Design and Synthesis of Ethanol-Resistant Random- Ionomer Membranes 49 3.3.1.2 PEM Properties of the Ternary Random-Ionomer Membranes .53 3.3.2 The Benevolent Effects of the Addition of Strongly Hydrophobic TFPM to Form Quaternary Random-Ionomer Membranes 54 3.3.2.1 Proton Conductivity .55 3.3.2.2 Ethanol Permeability 58 3.3.2.3 Mechanical Properties 61 3.3.3 Reflection on the Design Strategy for Aliphatic Ionomer Membranes .64 3.4 Conclusion .65 MITIGATING EARLY PHASE SEPARATION DURING SOLUTION POLYMERIZATION OF ALIPHATIC RANDOM IONOMERS BY HYDROPHOBIC MODIFIER ADDITION 66 4.1 Introduction 66 4.2 Experimental Method .69 4.2.1 Synthesis of Random Ionomers and Membrane Fabrications .69 iv 4.2.2 Additional Characterization Methods 69 4.3 4.2.2.1 Viscosity 69 4.2.2.2 Zeta Potential .70 4.2.2.3 Laser Light Scattering (LLS) .70 4.2.2.4 Transmission Electron Microscopy (TEM) .71 4.2.2.5 Spectroscopic Analyses .71 Results and Discussions .71 4.3.1 Effects of Hydrophobic Modifier Addition on Phase Separation during Copolymerization .71 4.3.2 Influence of Co-monomer Distribution on Solution Behavior 74 4.3.3 Structures of Ternary and Quaternary Random Ionomers .79 4.4 Conclusion .85 DEVELOPMENT OF POLYMER ELECTROLYTE MEMBRANES BASED ON HYDROPHILIC COVALENTLY CROSS-LINKED ALIPHATIC DIBLOCK IONOMERS .86 5.1 Introduction 86 5.2 Experimental 90 5.2.1 Materials 90 5.2.2 Synthesis of Aliphatic Diblock Ionomers by a One-pot Atom Transfer Radical Polymerization (ATRP) Technique 90 5.2.3 Membrane Formation and Pre-treatment .92 5.2.4 Characterization Methods 93 5.2.4.1 Electrochemical Analysis .93 5.2.4.2 Alcohol Permeability .93 5.2.4.3 Ion-Exchange Capacity (IEC), Alcohol and Water Uptake, and Dimensional Stability .93 5.3 5.2.4.4 Examination of Membrane Morphology .94 5.2.4.5 MEA Preparation and DMFC Tests .94 Results and Discussion 95 5.3.1 Design and Synthesis of Diblock Ionomer Membranes with Hydrophilic Covalent Cross-links 95 5.3.2 Proton Conductivity .100 5.3.3 Liquid Uptake and Dimensional Stability 103 5.3.4 Alcohol Permeability .107 v 5.3.5 Membrane Morphologies .111 5.3.6 MEA Fabrication and DMFC Performance .115 5.4 Conclusion .118 ONE-POT SYNTHESIS OF 3-SULFOPROPYL METHACRYLATE-BASED DIBLOCK IONOMERS AND THEIR PHASE SEPARATION BEHAVIOR IN SOLUTION 119 6.1 Introduction 119 6.2 Experimental Section .121 6.2.1 Materials and Synthesis of Block Ionomers 121 6.2.2 Additional Characterization Methods 121 6.3 6.2.2.1 Chromatographic Analysis .121 6.2.2.2 Spectroscopic Analysis 122 6.2.2.3 Zeta Potential Measurements .123 6.2.2.4 Laser Light Scattering (LLS) Measurements .123 6.2.2.5 Transmission Electron Microscopy (TEM) .124 Results and Discussion 124 6.3.1 Synthesis and Structure of P(AN-co-GMA)-b-SPM Diblock Ionomers 124 6.3.1.1 Feed Composition 126 6.3.1.2 Processing Conditions 126 6.3.2 Phase Separation Behavior of P(AN-co-GMA)-b-SPM Diblock Ionomers in DMSO/Water Mixtures 132 6.4 Conclusion .141 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK 142 7.1 Conclusions 142 7.2 Recommendations for Future Work .145 7.2.1 Other Applications of Hydrophilic Covalently Cross-linked Diblock Ionomer Membranes 145 7.2.2 Development of Hydrophilic Covalently Cross-linked Diblock Ionomer Membranes for Medium-Temperature DAFCs .146 PUBLICATIONS AND CONFERENCES 148 Patents 148 Publications 148 Conferences 148 REFERENCES .150 vi SUMMARY Ionomers, one of the many important classes of functional polymers, are able to undergo phase separation either in solution or in the solid state. This unique property facilitates the formation of a continuous hydrophilic network for ionic transport in ionomer membranes and is the basis for the design of polymer electrolyte membranes (PEMs). Many of the PEMs in use today for fuel cells are based on the perfluorosulfonate polymers, as exemplified by the highly successful commercial ionomer Nafion®. Despite their popularity in hydrogen fuel cells, Nafion® membranes are expensive and weak against alcohol permeation, rendering them less suitable for the direct alcohol fuel cells (DAFCs). Such material issues prompted the development of lower cost alcohol resistant alternative ionomers with the desired properties for DAFC applications (high proton conductivity, low alcohol crossover, and good mechanical properties). Among them the aliphatic ionomers are low cost and can be designed to bear organic functional groups that are not solvated by alcohol molecules and hence contribute to alcohol-blocking properties. This is also the approach taken by this PhD thesis study which focused on the design and synthesis of two forms of aliphatic ionomers and investigated the properties of the membranes fabricated from them. Two different ionomer structures, namely random and block ionomers consisting of hydrophobic acrylic; and hydrophobic and hydrophilic acrylate repeating units, were synthesized by free radical polymerization (FRP) and atom transfer radical polymerization (ATRP) respectively. In-situ cross-linking was also used to inhibit alcohol permeation and to strengthen the membrane structure. vii Chapter APPENDIX PUBLICATIONS AND CONFERENCES Patents 1. D. Julius; L. Hong; J.-Y. Lee, “A Nonfluorinated Cation-Exchange Membrane based on the Self-Assembly of an Amphiphilic Diblock Oligomer and In-Situ Cross-Linking”, US provisional patent (Appl. 61/316,886), Filed on 24 March 2010. Publications 1. R.-Q. Fu; D. Julius; L. Hong; J.Y. 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Journal of Electroanalytical Chemistry, 445(1-2), 39-45. 163 [...]... will focus on perfluorosulfonate membranes such as Nafion® and their modifications; as well as hydrocarbon membranes based on aromatic and aliphatic ionomers The second half of this chapter looks at the ionomers for PEM applications in greater detail, in particular the synthesis and phase separation of random and blocks ionomers in solution and in the solid state It is noteworthy to mention that phase... trade-off between conductivity and mechanical properties The benefits of the TFPM modification on PEM properties were investigated through a series of microstructure characterizations (b) Structural Characterizations of Random Aliphatic Ionomers and Investigations of the Effects of Hydrophobic Functional Groups on Phase Separation in Solution The inclusion of hydrophobic TFPM in the random ionomer... of this thesis study includes the following: (a) Design and Synthesis of New Alcohol-Resistant Alternative PEMs based on Aliphatic Random Ionomers New aliphatic random ionomers consisting of hydrophobic (AN-GMA) and ionic (SPM) units were synthesized by a simple solution polymerization method and cast into freestanding membranes The PEM design was predicated based on the low solubility of acrylic polymers... secondary channels expanded the connectivity of the primary hydrophilic channels and consequently increased the proton conductivity without loss of mechanical integrity and fuel crossover resistance 6 Chapter 1 (d) Investigations of a One-pot ATRP Method and the Phase Separation of Aliphatic Block Ionomers in Solution The synthesis of the aliphatic diblock ionomers in (c) was based on a one-pot ATRP method... polymerization (□), after 6h of polymerization (■), after 6h of polymerization (▲) 80 Figure 4.8 UV-Vis spectra of (SPM-GMA-TFPM) monomer mixture in DMF before polymerization (□), after 1h of polymerization (■), after 6h of polymerization (▲) 81 Figure 4.9 UV-Vis spectra (at the λmax of AN) of the two copolymerization systems before and after 6 h of copolymerization: the monomer mixture S0-10 (○) and. .. initial ionomer concentration was 1 wt.% 78 Figure 4.5 UV-Vis spectra of AN monomer in DMF before polymerization (□), after 1h of polymerization (■), after 6h of polymerization (▲) 79 Figure 4.6 UV-Vis spectra of GMA monomer in DMF before polymerization (□), after 1h of polymerization (■), after 6h of polymerization (▲) 80 Figure 4.7 UV-Vis spectra of TFPM monomer in DMF before polymerization... random ionomers and to deduce the origin of the benevolent effects of TFPM modification (c) Development of New PEMs based on Aliphatic Block Ionomers and Hydrophilic Covalent Cross-links The design of low-cost aliphatic ionomers membranes must successfully address the trade-off between proton conductivity and mechanical strength In this part of the study, a new approach which combines an ordered polymer. .. intensities as a function of H2O addition to the DMSO solution of P(AN-co-GMA)-b-SPM diblock ionomers: A50G4S-10 ionomer/DMSO (A), A100G4S-10 ionomer/DMSO (B), A150G4S-10 ionomer/DMSO (C) 135 Figure 6.7 Size distribution of aggregates formed upon water dilution of the DMSO solution Comparisons between the A50G4 copolymer, blend of the A50G4 copolymer and hydrophilic P(SPM) ionomer, and the A 50G4S-10... the miscibility problem of hydrophobic and ionic monomers due to the lack of a good common solvent The last part of this thesis study examined the details of the one-pot synthesis and the structures of the diblock ionomers Structural characterizations of the ionomers by Fourier-transform-infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM) and zeta-potential and light-scattering measurements... membrane as a function of sulfonation degree (A) DMFC performance benchmarked against a Nafion® 115 membrane (B) (Lee et al., 2008) 20 Figure 2.8 A PEM design based on ionic cross-linking of aliphatic diblock ionomers (A), and its proton conductivity as a function of the weight fraction of SA (B) (Do Kyoung et al., 2008) 23 Figure 2.9 Organization of amphiphilic block ionomers in a selective . Separation of Random and Block Ionomers in Solution 33 2.3.2.2 Phase Separation of Random and Block Ionomers in the Solid State 36 SYNTHESIS AND CHARACTERIZATION OF ACRYLIC RANDOM- IONOMER MEMBRANES. on Aliphatic Random Ionomers 5 (b) Structural Characterizations of Random Aliphatic Ionomers and Investigations of the Effects of Hydrophobic Functional Groups on Phase Separation in Solution. SYNTHESIS AND CHARACTERIZATIONS OF POLYMER ELECTROLYTE MEMBRANES BASED ON ALIPHATIC IONOMERS DAVID JULIUS 2011 SYNTHESIS AND CHARACTERIZATIONS OF