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Realizing Enhanced Toughness in Block Copolymer Modified Brittle Plastics

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Realizing Enhanced Toughness in Block Copolymer Modified Brittle Plastics Realizing Enhanced Toughness in Block Copolymer Modified Brittle Plastics A Dissertation SUBMITTED TO THE FACULTY OF UNIVERSITY OF MINNESOTA BY Tuoqi Li IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Frank S. Bates and Lorraine F. Francis August, 2016 © Tuoqi Li 2016 Acknowledgements Il y avait certainement quelqu’un que tu avais dans ta jeunesse, jamais tu ne lui oublierai. On August 13, 2011, it was the beautiful summer sunshine welcoming me when I stepped out from the Minneapolis airport. Exhausted from the long flight but overwhelmingly exited as I was at that time, I expected to have a new chapter of my life in a completely strange environment, barely thinking of the challenges and hardships waiting ahead. Sometimes, we can select which route to go, but life itself sets all the ups and downs on the way, leaving no more choices for us but saying “C’est la vie.” Now, looking back at my five-year pathway, I feel like being blessed, because it is not only what you experienced but also who you encountered that shape you, both physically and mentally. Thanks to all of you, who enriched my Minnesota memories with warmth like sunshine besides the bitter coldness. “He who teaches me for one day may be considered my father-figure for life.” Firstly, I would like to extend my gratitude to my advisors: Prof. Frank Bates and Prof. Lorraine Francis, who act as the “father” and “mother” during this great period of intellectual and personal growth for me. Frank, your passion and enthusiasm can cheer me up like Beethoven’s energetic symphony; Lorraine, your patience and serenity can calm me down like Chopin’s comforting nocturne. You have been a tremendous source of inspiration and encouragement and your efforts guide me to the place where I am. i “Live and learn.” Secondly, I really want to thank Prof. Chris Macosko, who influenced my life on multiple faces. Chris, partially because of you, I decided to join University of Minnesota five years ago. As a world-famous and respectable scholar, you generously offered me lots of help, suggestions and opportunities on my research and job hunting, although you are not my advisor. As my mentor during Bible study, you kindly shared your stories and thoughts, shaping my minds at a profound level. There is always more to learn from you. I will try to live gratefully and devoutly as you do. “We are a team. Whatever you lack, I got you.” Thirdly, it is a great fortune for me to have so many fantastic and helpful people around during the past years. I want to sincerely acknowledge my collaborators on research projects: Dr. Feng Zuo, Dr. Michael Heinzer, Dr. Jiuyang Zhang, Dr. Erica Redline, Debbie Schneiderman, Alex Mannion and Siyao He. You are amazing. I also would like to thank Dr. Athanasios Touris, Dr. Karen Haman and Dr. Jie Lu for teaching me polymer synthesis, Dr. Carmelo Declet- Pérez, Dr. Chris Thurber and Dr. Sangwoo Lee for inspiring suggestions and hands-on help during my experiments, Dr. Kyungtae Kim, Dr. Tim Gillard and Matt Irwin for the help during SAXS experiments at Argonne, Dr. David Giles, Wieslaw Suszynski and Fang Zhou for the training and help on many facilities, Prof. Marc Hillmyer, Prof. Bill Gerberich and Prof. Andres Stein for useful suggestions to my projects, and all current members and alumni in my two wonderful research groups. Whatever I lack, those brilliant persons can always get me, making my school life much easier. Thank you all! “Friendship marks a life even more deeply than love.” Pour Boxin and Yunlong, vous êtes mes surprises fantastiques, des amis qui m’acceptent comme je suis. Toutes les ii fautes futiles, ne dont plus que mes souvenirs à vie. Thank all of you for blending various colors into my plain life: Yutao, Andrew & Jincheng, Weihua, Lian, Liangliang, Yiming, Jun & Yuyang, En & Hongyun, Tianqi, Yan, and Han. Thank you for your accompanying during my Bible study: Bo, Mengen and Yuezhou. Thank you for your encouragement and great help during my job hunting: Jing, Ankit, Jingwen and Sipei. Thank you for our everlasting friendship: Zichao, Huize, Xiangyang and Zixuan! “I do not try to be better than anyone else. I only try to be better than myself.” Lastly but most importantly, I need to thank one person, myself, for trekking through the jungle of self-denial and depression, not being crumpled nor giving up. I finally understand, there is a doomed reason for trudging our selected route in the manner that we did; I finally believe, there is also a destined direction for our path to go. I should always keep one point in mind, to be true to myself and to progress every day. “I suppose in the end, the whole of life becomes an act of letting go, but what always hurts the most is not taking a moment to say goodbye.” So, at this special moment for myself, sincerely, I just want to say, Merci, merci pour vos présences dans ma vie! iii Dedication Pour mes parents et moi-même iv Abstract The great commercial importance of several brittle plastics continuously drives research efforts to be devoted to fabricating well defined structures in these materials for effectively toughening them. Amphiphilic block copolymers can be appropriately designed to generate nanometer scaled structures in a brittle plastic matrix at relatively low loadings (< 5% by weight). The resultant nanostructured plastics exhibit significant toughness enhancement without sacrificing other desirable properties such as transparency, stiffness and use temperature. The goal of this dissertation is to understand the nanostructure formation of block copolymers and the consequent toughening effect under various conditions. In this work we designed different types of block copolymer modifiers in concert with several commercially important brittle plastics, including epoxy thermosets and poly(lactide) (PLA) thermoplastics. The block copolymer toughening strategy was first established in bulk epoxies as well as in epoxy coatings through a model system study with the Jeffamine resin. Two distinct types of diblock copolymers formed spherical micelles in cured bulk epoxies and 15 m thick coatings, but the process of solvent- casting affected the micelle size and distribution in the coating. The toughness enhancement observed in bulk epoxies (up to 5-fold increase in the critical strain energy release rate GIc) successfully translated to coatings, as evidenced by the over 40% increase in the coating abrasive wear resistance with only 5 wt.% of modifiers. Transmission electron microscopy (TEM) revealed that similar toughening mechanisms as those in bulk epoxies (micelle cavitation and matrix shear yielding) still held in thin v coatings. Moreover, the hardness, modulus, transparency and glass transition temperature (Tg) of modified coatings were not appreciably affected compared to unmodified ones. Based on this model system study, we proceeded to investigate the commercially viable Cardolite resin system that is more complex thermodynamically but industrially relevant. A series of poly(ethylene oxide)-b-poly(butylene oxide) (PEO-PBO) diblock copolymers were synthesized at fixed composition (31% PEO by volume) and varying molecular weight expanding on a commercial product under the tradename Fortegra™ 100. Direct application of this product resulted in little improvement of the poor fracture toughness of the cured material. Modification of the resin formulation and curing protocol led to the development of well-defined spherical and branched wormlike micelles in cured resins. Thermodynamic interactions and the curing reaction together controlled the micelle formation as evidenced by small angle x-ray scattering (SAXS) measurements. A 9-fold increase in GIc over the neat bulk epoxy, and an over 30% improvement in the coating abrasive wear resistance over the unmodified coating were achieved at 5 wt.% loading of wormlike micelles. We then took one step further to explore the toughening efficacy of block copolymer micelles in hybrid composite systems in the presence of a second type of modifier, rigid graphene fillers with amine-functionalization. Both types of modifiers were well dispersed in cured epoxies with no observable interactions under TEM. The crosslink density of the epoxy network strongly affected the toughening effect. In the matrix with the lowest crosslink density, the combination of micelles and graphene drastically enhanced the GIc value to 19 times that of the neat material with no reduction vi in the elastic modulus and Tg. Additionally, hybrid ternary composites exhibited a synergistic toughening effect, revealing some positive mutual interference to the toughening mechanisms noted for micelles and graphene particles. Lastly, we extended the block copolymer toughening strategy to the PLA thermoplastic matrix. A low molar mass PEO-PBO diblock copolymer was uniformly dispersed as short cylindrical micelles in a commercial high molecular weight glassy PLLA plastic. This structure formation resulted from the negative Flory-Huggins interaction parameter () between PEO and PLLA. Those micelles could effectively toughen the matrix through concurrent cavitation, crazing and shear yielding. At only 5 wt.% of loading, micelles led to a greater than 10-fold increase in the tensile toughness and notched Izod impact strength over the neat PLLA in the glassy state. This toughening effect was retained in plastic films prepared with modified blends via a film blowing process. vii Table of Contents List of Tables ...................................................................................................................
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