ELECTROSPUN FIBERS FROM BOTH SOLUTION AND MELT: PROCESSING, STRUCTURE AND PROPERTY A Dissertation Presented to the Faculty of the Graduate School of Cornell University In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Huajun Zhou January 2007 © 2007 Huajun Zhou ELECTROSPUN FIBERS FROM BOTH SOLUTION AND MELT: PROCESSING, STRUCTURE AND PROPERTY Huajun Zhou, Ph. D. Cornell University 2007 Electrospinning, in which a droplet of polymer liquid is elongated by the action of a strong electrical field, is an effective method to produce submicron scale fibers. The resulting nanofibers are collected as non-woven mats with large surface area to volume ratios which can be used in filtration, catalysis, tissue engineering and reinforced composites. Studies on electrospinning, however, have been limited to relatively simple polymeric systems and researchers have just begun to scratch at the surface of the structure and morphology of various sub-micron scale fibers. To achieve a fundamental understanding of material-processing-structure relation during nanofiber formation, the following three systems have been investigated experimentally. First, the formation of nanofibers from polylactic acid (PLA) and its nanocomposite solutions has been studied to investigate the effects of inclusion of silica nanoclays. It is observed that the inclusion of nanoclays gives rise to strong cold crystallization, formation of beta crystals and structural orientation in PLA fibers. Electrospun PLA nanocomposite fibers also exhibit improved mechanical properties due to the preservation of intercalated structures and further alignment of nanoclays throughout the entire fiber. We further studied the effect of inclusion of polyethylene oxide (PEO) on PLA fibers. The mechanical properties of PLA are greatly affected by the addition of PEO. Electrospun PLA/PEO blend fibers with less than 10wt% of PEO exhibit increased elongational modulus, yield strength and breaking strain. Secondly, we have created a unique route to nanofibers directly from the melt which eliminates the organic solvents present in solution electrospinning. This new solvent-free approach not only allows us to investigate a rich array of experimental studies to develop novel nanofibers directly from polymer melts and composites but also opens the door to theoretical routes to model nanofiber formation without the complications associated with solvent evaporation. Sub-micron PLA fibers have successfully been electrospun from its melt by investigating the effects of a series of processing parameters on fiber diameter. The degradation during melt electrospinning has also been studied. The third system is an extension from melt electrospinning, which is heated solution electrospinning of polyolefin. We explored the possibilities of electrospinning sub-micron polyolefin fibers directly from their solutions and investigated the effects of processing parameters on fiber morphologies and properties. BIOGRAPHICAL SKETCH Huajun Zhou was born in a small town named Huabu which is located in a eastern province, Zhejiang of People’s Republic of China. He finished the elementary education and junior high school education in the town and then moved to Quzhou city for the senior high school education where he was exposed to a more open environment. He has been interested in studying materials since then. It is deeply believed by him that materials play fundamental role in the development of technologies and human society. Then it was 1996 when he entered the famous research-oriented University of Science and Technology of China (USTC) to continue his dream in the material world. He survived from the competitive environment and was constantly a top five student among sixty-three in the class. He completed the study with a thesis titled with ‘Synthesis and photocatalytic properties of TiO2 nanoparticles’. After five years’ study in material physics and a B.S. degree, he felt it was time to dig deeper into the subject by entering world-famous Cornell University. There is an interesting anecdote in his application to department of material science & engineering at Cornell University. He did not apply to Cornell until late March which was beyond the application deadline. He wrote an email asking if he could still apply. He was lucky enough to get a positive response from a secretary of MS&E and an overnight fedex including application materials. His life was then turned to Cornell after the late unprepared inquiry. At Cornell, he was both brave and lucky enough to be supervised by a young professor in school of chemical and biomolecular engineering. His research topic moved from synthesis of inorganic nanoparticles to fabrication of nanofibers from electrospinning. He invented and patented a melt electrospinning apparatus with his advisor. And then he patented another finding in nanocomposite fibers. Both of them are supported and licensed by a filtration company. When studying at Cornell, he was iii closely affiliated with Cornell Center of Material Research (CCMR) where he participated in various CCMR outreach programs such as Research Experience for Undergraduate (REU) and Research Experience for Teachers (RET). He also participated in the image contest of CCMR and got an honorable mention award. Besides the intensive researches, he enjoyed greatly the life in Ithaca. He frequently went wine-tasting in fingerlake wineries and acquired considerable knowledge on wines. He also benefited from the active participation of soccer games organized by CSSA dragon club and badminton exercises in Helen Newman Hall. Through the long-run in the academia, he has extended both his research interests and personal interests. At this point, he will be finishing up his Ph.D. study and pursue an application-oriented industrial job. iv I dedicate my dissertation work to my beloved parents: Mr. Meixian Zhou and Ms. Rongxiang Qiu. Although they lack of strong educational background, they are so supportive to my studies. In many moments when I need to make a critical decision, my parents always stand by my side and offer their experiential suggestion. Their open minds are a big source of inspiration to me. Without their support and understand, this work is hard to come to a completion. This work is also dedicated to my younger brother: Guojun Zhou and many of my friends. Their encouragement and joyful accompany during my study pulls me through the dark night and come to the completion of this work. v ACKNOWLEDGMENTS I would like to gratefully and sincerely thank Dr. Yong Lak Joo for his guidance, understanding, patience, and most importantly, his friendship during my graduate studies at Cornell. As one of his first two Ph.D. students, I received the most patient and insightful instructions on my researches. He was willing to share his life experiences as an international student studying in the US. His optimistic view on problems and shining ideas always inspired and propelled me towards more progress. He encouraged me to not only grow as an experimentalist but also as an instructor and an independent thinker. It was his encouragement and inspiration that led me into the wonderful polymer world. With his help and the assistance of CCMR at Cornell, I was able to have the opportunity to work with several bright undergraduate students and develop my mentoring skills. I would like to thank my undergraduate assistants: Tania Berges from Univ. Puerto Rico, Kathleen Baron from MIT, Debra Audus at Cornell, Geoffrey C.H. Tang at Cornell, Derrick Tang at Cornell for their diligent and productive work. Debra has been working with me on the heated solution electrospinning of polyolefin for over one year and I benefited from her organizational skills. Derrick Tang and Geoffrey Tang worked with me on the electrospinning of PLA-PEO blend system and had been really thoughtful on the topic. I would also like to thank Choo-won Kim and Timur Ivanikov for giving me humor and entertainment in what could have otherwise been a somewhat stressful laboratory environment. I would also like to thank Colman Carroll and Edward Zhmayev for providing me their unique understanding on the theory of electrospinning. All other Joo research group members are also appreciated for their help in either experiments or technical solutions. vi Due to the inherent interdisciplinary attributes of our research, we have many inner and outer collaborators. I would like to thank Dr. Emmanuel Giannelis and Dr. Kyoung-woo Kim for their support in my study of polymer nanocomposite. And I would also like to thank a number of CCMR facility managers: Dr. Maura Wheathers for XRD characterizations, Anthony Condo for characterization of thermal properties, John Hunt for SEM assistance, John Grazul for TEM assistance, John Sinnott for compression molding support and Jonathan Shu for AFM assistance. I would also like to thank the National Science Foundation for funding this work through a CAREER Award, Grant no. CTS-0448270 to Dr. Joo. This work was supported in part by Clarcor Corporation. We would also like to thank E.I. du Pont de Nemours and Company for funding through a DuPont Young Professor Grant to Dr. Joo. Their generous funding is critical to the completion of this work. vii TABLE OF CONTENTS Biographical Sketch iii Dedication v Acknowledgments vi Table of Contents viii List of Figures xii List of Tables xvi 1. Introduction To Electrospinning 1 1.1. History of Electrospinning 1 1.2. Electrospinning Process 4 1.3. Electrospinning of Functional Materials and Hybrid 10 1.4. Post Processing and Functionalization 13 1.5. Structures of Electrospun Fibers 14 1.6. Applications 17 1.6.1. Nanofiber-Reinforced Composites 17 1.6.2. Filtration Application 18 1.6.3. Scaffold for Tissue Engineering 20 1.6.4. Other Applications 21 1.7. Comparison of Solution Electrospinning and Melt Electrospinning 21 2. Experimental Setup and Method 24 2.1. Design and Features of the Melt Electrospinning Setup 24 2.1.1. Design of the Shielded Heating Unit 25 2.1.2.