WWeesstteerrnn UUnniivveerrssiittyy SScchhoollaarrsshhiipp@@WWeesstteerrnn Electronic Thesis and Dissertation Repository August 2012 SSyynntthheessiiss aanndd CChhaarraacctteerriizzaattiioonn ooff GGrraapphheennee--PPoollyymmeerr NNaannooccoommppoossiitteess vviiaa RReevveerrssiibbllee AAddddiittiioonn--FFrraaggmmeennttaattiioonn CChhaaiinn-- TTrraannssffeerr PPoollyymmeerriizzaattiioonn Renpeng Gu, The University of Western Ontario Supervisor: Paul Charpentier, The University of Western Ontario A thesis submitted in partial fulfillment of the requirements for the Master of Engineering Science degree in Chemical and Biochemical Engineering © Renpeng Gu 2012 Follow this and additional works at: https://ir.lib.uwo.ca/etd Part of the Polymer Science Commons RReeccoommmmeennddeedd CCiittaattiioonn Gu, Renpeng, "Synthesis and Characterization of Graphene-Polymer Nanocomposites via Reversible Addition-Fragmentation Chain-Transfer Polymerization" (2012). Electronic Thesis and Dissertation Repository. 651. https://ir.lib.uwo.ca/etd/651 This Dissertation/Thesis is brought to you for free and open access by Scholarship@Western. It has been accepted for inclusion in Electronic Thesis and Dissertation Repository by an authorized administrator of Scholarship@Western. For more information, please contact [email protected]. SYNTHESIS AND CHARACTERIZATION OF GRAPHENE-POLYMER NANOCOMPOSITES VIA REVERSIBLE ADDITION-FRAGMENTATION CHAIN-TRANSFER POLYMERIZATION (Spine title: Synthesis of graphene nanocomposites via RAFT ) (Thesis format: Monograph) by Renpeng Gu Graduate Program in Chemical and Biochemical Engineering A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering Science The School of Graduate and Postdoctoral Studies The University of Western Ontario London, Ontario, Canada © Renpeng Gu 2012 i THE UNIVERSITY OF WESTERN ONTARIO School of Graduate and Postdoctoral Studies CERTIFICATE OF EXAMINATION Supervisor Examiners ______________________________ ______________________________ Dr. Paul Charpentier Dr. James Wisner Supervisory Committee ______________________________ Dr. Jin Zhang ______________________________ Dr. Elizabeth Gillies ______________________________ Dr. Elizabeth Gillies The thesis by Renpeng Gu entitled: Synthesis and Characterization of Graphene-Polymer Nanocomposites via Reversible Addition-Fragmentation Chain-Transfer Polymerization is accepted in partial fulfillment of the requirements for the degree of Master of Engineering Science ______________________ _______________________________ Date Chair of the Thesis Examination Board ii Abstract Graphene has emerged as a subject of tremendous scientific interest due to its exceptional electrical, mechanical and thermal properties. When incorporated into a polymer matrix, these thin carbon sheets can significantly improve the properties of the host polymer at low loading levels. However, the dispersion of pure graphene throughout a polymer matrix is not homogeneous, due to the strong van der Waals interactions between graphene sheets and the difference in surface compatabilities. To prevent agglomeration of these graphene sheets, surface functionalization is required to weaken the π-π stacking. Living free radical polymerization is a powerful tool for the surface functionalization of nanomaterials such as graphene via the “grafting from” approach. Especially, reversible addition-fragmentation chain transfer (RAFT) polymerization has several attractive advantages as a living technique, such as good compatibility with a wide range of monomers, tolerance to solvents and acidic/basic monomers, and simple implementation for controlling nanocomposite structure. The goal of this thesis was to develop a facile approach for growing polymer chains from the surface of graphene sheets. Graphene oxide was synthesized by Hummers method by reacting graphite with a mixture of potassium permanganate (KMnO ) and concentrated 4 sulfuric acid (H SO ). The oxidation and exfoliation of graphite was investigated using 2 4 FTIR, TEM, and AFM studies. For the methodology of growing polymers from graphene surfaces, polydopamine was coated on graphene oxide as a platform for subsequent “grafting from” RAFT polymerization. This was possible as polydopamine has available hydroxyl groups that can react with carboxylic groups of the RAFT agent via ester linkages. During the formation of polydopamine coating on graphene oxide, graphene oxide can be simultaneously reduced by the released electrons generated by self- polymerization of dopamine. The reduction of graphene oxide was determined by FTIR, UV/Vis, and XPS analysis. For growing the polymer chains from the graphene surface, the living radical polymerization methodology, RAFT polymerization, was investigated. The RAFT agent, S-dodecyl-S’-(α,α’-dimethyl-α’’-acetic acid)trithiocarbonate, having an available iii carboxyl group, was chosen to anchor onto the polydopamine coating and then grow chains of PS, PMMA, PNIPAM, and PtBA from this modified surface. The functionalization of polydopamine/reduced graphene oxide (PDA/RGO) was determined by FTIR and TGA studies. The livingness of the polymerization was verified by GPC characterization of cleaved polymer chains. The additional free RAFT agents in the reaction system could not only enhance the control of the polymerization on PDA/RGO surface and in solution, but also narrow the gap between grafted polymer and free polymer produced in solution as measured by GPC. The polymer grafted PDA/RGO nanocomposites showed excellent dispersibility in several organic solvents. The final polymer matrix dispersed of functionalized reduced graphene oxide showed higher maximum decomposition temperature measured by TGA, indicating better thermal stability. Key Words: RAFT polymerization, reduced graphene oxide, polydopamine, grafting from, polystyrene, nanocomposites. iv To my dad and mom For their full love, support, and encouragement From the other side of the earth To all my best friends in Canada and China For the happiness and courage you bring to me v Acknowledgments I would like to extend my heartfelt gratitude to my advisor, Dr. Paul A. Charpentier, for his supervision, insight, advice, and guidance throughout all stages of this research. He provided me enthusiastic support and encouragement in various ways, and he is such a nice supervisor that I am very proud to have had the opportunity to work with him. Many thanks to Prof. Elizabeth Gillies, Prof. Jin Zhang, and Prof. James Wisner, members of the advisory and examiners committee, for their important guidance and suggestions. I would like to express my sincere gratitude to Qasem Alsharari for Raman analysis and to Jenna Allan and Golam Moula for the AFM analysis. Many thanks go to Ross Davidson, Mark Biesinger, and Heather Bloomfield for XPS and EDX elemental analysis and to Hossein Kazemian for XRD analysis. I thank Jeffrey Gribbon for TGA analysis and Richard Gardiner for TEM analysis. I am thankful to Aneta Borecki for GPC measurement. I am also very grateful to Yixing Tang and Qasem Alsharari for their help and suggestions during my research. In addition, I wish to thank the University of Western Ontario and Department of Chemical and Biochemical Engineering. At last, I want to thank all my friends and colleagues for their continuous support, and advices. vi Table of contents CERTIFICATE OF EXAMINATION ........................................................................... ii Abstract .............................................................................................................................. iii Acknowledgments.............................................................................................................. vi Table of contents ............................................................................................................... vii List of Tables ..................................................................................................................... xi List of Figures ................................................................................................................... xii List of Appendices ........................................................................................................... xvi List of abbreviations ....................................................................................................... xvii Chapter 1 ............................................................................................................................. 1 Introduction ......................................................................................................................... 1 1.1 Graphene-Polymer Nanocomposites ......................................................................... 2 1.2 Living Free Radical Polymerization ......................................................................... 4 1.3 RAFT Polymerization ............................................................................................... 4 1.3.1 Mechanism of RAFT Polymerization ................................................................ 6 1.3.2 Choice of RAFT Agents ..................................................................................... 7 1.4 Graphene ................................................................................................................... 9 1.4.1 Polymer Grafting of Graphene via Living/Controlled Free-Radical Polymerization ........................................................................................................... 10 1.4.1.1 The Covalent “Grafting To” Approach...................................................... 10 1.4.1.2 The Covalent “Grafting From” Approach ................................................. 12 1.4.1.2.1 Atom Transfer Radical Polymerization ............................................... 13 1.4.1.2.2 Reversible Addition Fragmentation Chain-Transfer Polymerization . 14 1.4.1.2.3 The One-pot Approach ........................................................................ 15 vii 1.4.1.3 Non-Covalent Functionalization ................................................................ 16 Chapter 2 ........................................................................................................................... 19 Objectives ......................................................................................................................... 19 Chapter 3 ........................................................................................................................... 21 Functionalization of Reduced Graphene Oxide via Surface “Graft from” RAFT Polymerization .................................................................................................................. 21 3.1 Introduction ............................................................................................................. 23 3.2 Experimental ........................................................................................................... 26 3.2.1 Materials ........................................................................................................... 26 3.2.2 Experimental Setup .......................................................................................... 26 3.2.3 Preparation of Graphene Oxide Sheets (GO) ................................................... 28 3.2.4 Reduction of Graphene Oxide via Self-polymerization of dopamine .............. 29 3.2.5 Synthesis of RAFT agent-bonded graphene sheets .......................................... 29 3.2.6 Surface RAFT Polymerization of Styrene on RGO/PDA ................................ 29 3.3 Characterization ...................................................................................................... 30 3.4 Results and Discussion ............................................................................................ 31 3.4.1 Graphene Oxide ................................................................................................ 31 3.4.2 Dopamine-Induced Reduction of Graphene Oxide .......................................... 34 3.4.2.1 Effect of Temperature on Reduction of Graphene Oxide .......................... 37 3.4.2.2 Effect of Weight Ratio between reactants ................................................. 40 3.4.3. Evidence of Immobilization of RAFT agent ................................................... 41 3.4.5 Evidence of Grafted Polymer and Cleavage of Grafted Polymer .................... 43 3.4.6 Evidence of Grafted Polymer (PMMA, PNIPAM, PtBA) on PDA/RGO ........ 44 3.4.7 Dispersibility of Polymer grafted PDA/RGO ................................................... 46 viii 3.5 Conclusion ............................................................................................................... 49 Chapter 4 ........................................................................................................................... 51 Livingness and Kinetic Study of Surface “Grafting from” RAFT Polymerization on PDA/RGO ......................................................................................................................... 51 4.1 Introduction ............................................................................................................. 53 4.2 Experimental ........................................................................................................... 54 4.2.1 Materials ........................................................................................................... 54 4.2.2 Surface RAFT Polymerization of Styrene on RGO/PDA without Free RAFT agent .......................................................................................................................... 55 4.2.3 Surface RAFT Polymerization of Styrene on RGO/PDA with Free RAFT agent ................................................................................................................................... 55 4.2.4 Preparation of PS Composite Containing Graphene ........................................ 56 4.2.5 Cleaving Grafted Polymer from RGO/PDA88 .................................................. 56 4.2.6 RAFT Polymerization of Styrene ..................................................................... 57 4.2.7 Free Radical Polymerization of Styrene ........................................................... 57 4.3 Characterization ...................................................................................................... 57 4.4 Results and Discussions .......................................................................................... 58 4.4.1 Kinetics of Solution and Surface RAFT Polymerization of Styrene ................ 58 4.4.2 Conventional Free Radical and RAFT Polymerization .................................... 59 4.4.3 Livingness Study of Surface “Grafting from” RAFT Polymerization ............. 63 4.4.4 Kinetic Study of Surface “Grafting from” RAFT Polymerization ................... 69 4.4.5 Improvement of dispersibility of PS-PDA/RGO nanocomposites ................... 72 4.4.6 Improvement of thermal stability of PS-PDA/RGO nanocomposites .............. 74 4.5 Conclusion ............................................................................................................... 75 Chapter 5 ........................................................................................................................... 77 ix