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Producing Melt Blown Nano-/Micro-Fibers with Unique Surface Wetting Properties

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Producing Melt Blown Nano-/Micro-Fibers with Unique Surface Wetting Properties Producing Melt Blown Nano-/Micro-fibers with Unique Surface Wetting Properties A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Zaifei Wang IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Frank S. Bates and Christopher W. Macosko September, 2015 © Zaifei Wang 2015 ALL RIGHTS RESERVED Acknowledgement The past five years at the University of Minnesota have been the most marvelous experience of my growth. There are so many beautiful memories that will never ever fade in all my life. Most importantly, I met many great people who have helped and supported me throughout my doctoral career and personal life. I am truly grateful to these friends, and thanking them here is the least that I can do to express my sincere gratitude. First of all, I would like to thank my advisors, Dr. Frank Bates and Dr. Chris Macosko, for their great help, support, and guidance. Easy does not enter into grown-up life, especially when I came in with a metallurgical background, neither chemistry nor chemical engineering. I could not have reached this important milestone without your encouragement and support. I have been motived by your great enthusiasm to science. I learned from you how to think critically, how to solve issues, and how to have science make difference in the real world. I appreciated this excellent and joyful experience learning and exploring science under your guidance. Also, I would like to thank Dr. Timothy Lodge, Dr. Satish Kumar, and Dr. Xiang Cheng for their willingness to review my thesis, provide comments, and be in my defense. I am grateful to many collaborators at the University of Minnesota. I thank Dr. Dawud Tan and Dr. Feng Zuo for getting me start quickly in the project. I thank Dr. Satish Kumar, Dr. Changkwon Chung, Dr. Leonardo Espín for their great collaboration. I thank Dr. David Giles for his trainings on rheometers. I thank Dr. Conrado Aparicio at the School of Dentistry for offering the device to measure water contact angles. I thank i Wieslaw Suszynski for helping with setting up high-speed camera, Chris Frethem and Fang Zhou for useful discussions on SEM and microtome, and Dr. Bing Luo for training and discussing XPS. I appreciated the hard work from Xiaotun Liu and helpful discussions from Huaiyu Yan. I would also like to express my gratitude to people from Cummins Filtration, Dr. Soondeuk Jeung, Dr. Kan Wang, Dr. Bill Haberkamp, Dr. Peter Herman for their experimental support and helpful discussions. In addition, I thank Dr. Deborah Massouda from DuPont for offering the new perfluorinated copolyesters and useful discussions. I thank Dr. Jeff Galloway for offering the water-dispersible polymers, Dr. Scott George for helpful discussions on the material properties. Life would have been black and white if there were no friends. I thank the following friends for sharing the wonderful life at the University of Minnesota: Zhen Ren, Zhongda Pan, Jie Song, Jing Han, Donglin Tang, Han Zhang, Lian Bai, Yuqiang Qian, Jun Xu, Chris Thurber, Yogesh Dhande, Yunlong Zou, Yoshihasi Takamori, Aaron Hedegaard, Luca Martinetti, Shigeru Aoyama, Alex Mannion, Liangliang Gu, Siddharth Chanpuriya, Karen Haman, Sam Dalsin, Tessie Panthani, Camelo Declet-Perez, Yulong Li, Xi Chen. I am grateful for warm and continuous support from my wife Zihan (Vera) Cheng, my parents, my younger brother, my grandparents, and Debbie Gustafson. It has been a long-distance relationship for five years. I could not think of any word to express my sincere gratitude to my wife. I thank my wife for her extreme support over the past five years. Also, I thank my parents and my younger brother. I could not have been through all the difficult times without their support. In addition, I want to thank Debbie for her great help and support in my life. She has been part of my family in my heart. ii Finally, I would like to thank Cummins Filtration for their financial support. And, parts of this work were carried out in the Institute of Technology Characterization Facility at the University of Minnesota. iii Dedication To my family iv Abstract Melt blowing is a one-step process producing microfibers with an average fiber diameter (dav) of 1 – 5 μm. One application of melt blown fibers is filter media of water- in-diesel engine fuel filtration. In order to meet the increasingly higher requirements on fuel cleanliness, enhancing the filter efficiency has been desired. One attractive strategy is applying nanofibers (dav < 1 μm). The other is modifying the fiber surface wetting properties, such as superhydrophobic. The goal of this work is to produce melt blown nano-/micro-fibers with superhydrophilic and/or superhydrophobic surfaces for water-in- diesel filtration. In this thesis, we advanced the new technique of producing nanofibers from melt blown fiber-in-fiber polymer blends by understanding the nanofiber formation and developing water-extractable nanofibers. Firstly, poly(styrene) (PS)/poly(butylene terephthalate) (PBT) and PS/poly(ethylene-co-chlorotrifluoroethylene) (ECTFE) binary blends were melt blown followed by tetrahydrofuran (THF) extraction. The control extrusions demonstrated that the core nanofibers were primarily generated during the fiber blowing process. We speculate that both aerodynamic and fiber-fiber drag influence the nanofiber formation. Varying the minor phase volume fraction revealed the nanofiber breakup. We proposed that both the single-droplet-elongation and dumbbell-formation contribute to the formation of nanofibers. In addition, we hypothesize that the coalescence leads to the formation of non-uniform nano-/micro-fibers. Following the discussion on nanofiber formation, we showed nanofiber fabrication v from water-extractable melt blown fiber-in-fiber polymer blends containing a water- dispersible sulfopolyester (SP). This new method eliminates issues associated with organic solvents. Also, it provides another route to prepare multilayer nano-/micro-fiber composites. Surface wetting modifications of melt blown PBT fibers were achieved by alkaline hydrolysis and subsequent fluorination. Application of alkaline hydrolysis generates sponge-like PBT fibers decorated with hydroxyl and carboxyl functional groups, resulting in a superhydrophilic fiber mat surface. The subsequent fluorination creates a sticky-superhydrophobic surface. An alternative achieving enhanced or even superhydrophobic PBT fiber surfaces is incorporation of a random perfluorinated multiblock copolyester (PFCE). Adding only 5 wt% of PFCE into PBT leads to over 20 wt% of PFCE on PBT fiber surfaces. The blooming of fluorine and the overall mat roughness increased the water contact angle from about 128° to above 150° and lowered the fiber mat surface adhesion energy by 25%. Finally, the appendices show some preliminary works on: 1) synthesis and characterizations of poly(L-lactide) and perfluoropolyether block copolymer; 2) the effects of alkaline hydrolysis on surface topography and mechanical properties of melt blown PBT fiber mats; 3) compatibilized polyamide 6 and poly(ethene-co- tetrafluoroethene) (PETFE) blends, which, however, were not suitable for melt blowing due to high viscosity; 4) collection of individual melt blown nanofibers. vi Table of Contents Acknowledgement……………………………………………………………….……….i Dedication……………………………………………………………………………......iv Abstract…………………………………………………………………………………..v List of Tables……………………………………………………………………………xii List of Figures………………………………………………………………………….xiii Chapter 1 Introduction..................................................................................................... 1 1.1 Nonwoven ................................................................................................................ 1 1.2 Melt blown nanofibers ............................................................................................. 3 1.2.1 Spinneret design .......................................................................................... 5 1.2.2 Air & Polymer flow rates (Qa, Qp) .............................................................. 8 1.2.3 Air & Polymer temperatures (Ta, Tp) ........................................................ 10 1.2.4 Air pressure (Pin) & Air velocity (va) ........................................................ 11 1.2.5 Die-tip-to-collector distance (DCD) ......................................................... 12 1.2.6 Molecular weight & MFI .......................................................................... 14 1.2.7 Viscoelasticity ........................................................................................... 15 1.3 Surface wetting modifications ............................................................................... 17 1.4 Thesis overview ..................................................................................................... 21 Chapter 2 Nanofiber Formation from Melt Blown Fiber-in-Fiber Polymer Blends 23 2.1 Introduction ............................................................................................................ 23 2.2 Experimental .......................................................................................................... 25 2.2.1 Materials ................................................................................................... 25 2.2.2 Preparation of polymer blends .................................................................. 27 2.2.3 Characterization of blend morphology ..................................................... 29 2.2.4 Melt
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