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ABSTRACT VAID, RADHIKA Chemical and Thermal Degradation of Fiber Forming Aliphatic Polyesters

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ABSTRACT VAID, RADHIKA Chemical and Thermal Degradation of Fiber Forming Aliphatic Polyesters ABSTRACT VAID, RADHIKA Chemical and Thermal Degradation of Fiber Forming Aliphatic Polyesters. (Under the direction of Dr. Martin W. King and Dr. Melissa A. Pasquinelli). Bioresorbable polymers are employed for many applications in the biotechnology, medical device, and pharmaceutical industries on account of their biocompatibility and biodegradability. This makes them viable candidates for treating acute injuries and chronic diseases through tissue engineering scaffolds, wound dressings, resorbable surgical sutures and drug delivery systems. Thus, it becomes important to ensure that when polymers have been processed or fabricated into a particular shape, size and form that they have the desired degradation profile for the application they are designed for. The polymer's processing conditions play a significant role in determining the degradation mechanism which is controlled by the structure property relationship and hence the bioresorbable properties. In addition, the in vivo environmental factors such as pH and enzymes have a significant impact on the degradation profile and clinical performance of the polymer when implanted in the body as a resorbable device. In the past there have been many studies evaluating resorbable polymers, like polylactic acid (PLA), to understand the degradation behavior for biomedical applications. But gaps still exist in our understanding of the degradation process and the reaction mechanism of a wide range of established resorbable polymers. In addition, there is limited scientific literature on the degradation behavior of other newly developed bioresorbable polymers, like poly-4-hydroxybutyrate (P4HB), and how they compare with established polymers. For my doctoral research study, I have evaluated different bioresorbable polymers in the category of aliphatic polyesters, including polycaprolactone (PCL) and polyglycolic acid (PGA) in addition to PLA and P4HB, with the goal of understanding their degradation mechanism, their resorption profile and any implications on their properties critical to their polymer performance. To accomplish this, I have used both experiments and computational techniques and have attempted to establish correlations between these two approaches so that the methods developed are applicable to other polymers. Sometimes in experiments, details at the molecular level critical to polymer degradation and their use are omitted or missed, such as reaction mechanisms and the production of by-products. In the initial studies, I have evaluated the degradation properties as a function of pH for both PLA and P4HB. P4HB is an attractive candidate for use as a surgical suture because it is an elastic thermoplastic that is biodegradable, biocompatible, insoluble in water and nontoxic. It also has a high degree of polymerization and thus has potential for high strength applications. For the PLA and P4HB degradation studies using both experimental and simulation techniques, the degradation conditions were selected so as to mimic normal body temperature and the pH conditions present at various locations in the human body. Then, in order to explore the details of the hydrolytic degradation mechanism at the molecular-level, molecular dynamics (MD) simulations were performed using reaxFF and density functional theory (DFT) calculations. In a second set of studies, I also performed experiments to analyze the effect of the proteinase K enzyme under different pH conditions on the degradation behavior of P4HB. Finally, thermal degradation was studied for four aliphatic polyesters, using both experiments and simulations. The results were correlated in terms of the activations energies and thermal degradation profiles. The findings for hydrolytic degradation from both experiments and simulations illustrated that a faster rate of degradation occurred for PLA in alkaline conditions, whereas for P4HB degradation was enhanced under acidic conditions. Furthermore, similar behavior, but to lesser extent, was observed for the enzymatic degradation of P4HB in the presence of proteinase K. Lastly, thermal degradation using both experimental and simulation techniques revealed similar degradation profiles and activation energies for all four aliphatic polyesters. In summary, it is important to note that this study is unique and one of a kind. It puts forward a methodology where the simulations can be used as a tool to study polymer degradation in combination with experiments, and by taking this approach it is possible to clarify our understanding of the degradation process, and now we can shortlist polymer candidates based on the results of simulations before proceeding with experimental work and material processing. © Copyright 2020 by Radhika Vaid All Rights Reserved Chemical and Thermal Degradation of Fiber Forming Aliphatic Polyesters by Radhika Vaid A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Fiber and Polymer Science and Materials Science and Engineering Raleigh, North Carolina 2020 APPROVED BY: _______________________________ _______________________________ Dr. Martin W. King Dr. Melissa A. Pasquinelli Committee Co-Chair Committee Co-Chair _______________________________ _______________________________ Dr. Ericka Ford Dr. Yaroslava Yingling _______________________________ _______________________________ Dr. Albena Ivanisevic Dr. Nathalie Lavoine DEDICATION Dedicated to my parents, Neelam Vaid and Pradeep Kumar Vaid, and my sister Pooja (a special gift of God to us) ii BIOGRAPHY Radhika Vaid, was born in Himachal Pradesh, India on September 1st 1988. Having been guided by her elder sister, Dr. Rajni Vaid Puri, she pursued her studies in the less popular field of textiles engineering. During her bachelor's degree she was amazed by exploring a multitude of applications of textiles, so after her bachelors from Jawahar Lal Nehru Government Engineering College, Sundernagar, H.P, she continued to pursue and obtain her master of technology degree in Fiber Science and Technology from the Indian Institute of Technology, Delhi. Thereafter, she worked as a fiber and polymer scientist at Aditya Birla Science and Technology Company Pvt. Ltd., India. After working there for three years she came to North Carolina State University to study for a doctoral degree and to follow her dream to be a polymer scientist. At present, Radhika is enrolled as a candidate for the Ph.D. degree in Fiber and Polymer Science with a co-major in Materials Science and Engineering. Radhika has an inclination and enthusiasm for polymers; to study them, unfurl them and apply them to novel applications. iii ACKNOWLEDGMENTS This journey towards the accomplishment of my Ph.D. is the result of support and continuous motivation from many incredible people in my life. First and foremost, I would like to thank my advisors, Dr. Melissa A. Pasquinelli and Dr. Martin W. King, for their unconditional support and constant push throughout the past four and half years. These two have guided me both technically and emotionally through the storm of failure and winds of success. If it wasn’t for these two, I am sure I would have been lost and disheartened in this journey of earning a doctoral degree. Dr. King, my academic father, as I always address him, has virtually been there at my back whenever I turned around in a moment of despair, telling me with a sparkling smile, "Keep going Radhika. Hang in there. I know you can do it". At times this has pumped and motivated me to have confidence in myself and my research. Dr. King with scientific experience of almost double my age has relentlessly ensured that I attend conferences, learn from others and apply the new knowledge to improve my research. On the other hand, Dr. Pasquinelli has always been a female icon for me and has guided me and challenged me to do my best. She has been there for me to make life easier and overcome whatever barriers I have faced technically and administratively to undertake and obtain a co-major within my Ph.D program. I would rather say that she has been my guiding light throughout my Ph.D. and has helped me to improve my presentation and communication skills. Now when I look back, these two pillars have always shown their confidence and trust in me, and have fostered this journey for achieving my best. Furthermore, I would like to extend my gratitude to the laboratory staff at the Wilson College of Textiles, the Analytical Instrumentation Facility (AIF), the Department of Material Science & Engineering (MSE) and at Donghua University for their support to accomplish the technical objectives of my Ph.D. research project. The people who have assisted me have been iv more than just staff to me. They have been prepared to go out of their way and help me whenever I have asked. I am especially thankful to Birgit Andersen and Joyce Cole for being kind and helpful to me whenever I have needed. Thank you for lifting my morale at times and keeping me motivated. In addition, I would like to extend my appreciation to my committee members for their role in giving me time and making sure that gaps in my research are addressed. I would personally like to thank Dr. Ericka Ford for her patience whenever I arrived uninvited in her office to discuss my research. Her insights in the analysis for DSC and TGA have been particularly helpful in understanding the property changes during degradation of the polymers I studied. In addition, Dr. Yarasolava Yingling have rendered her expertise to understand the degradation of polymers using simulations and how to extract meaningful information from the analysis. Further, I would like to thank Dr. Albena Ivanisevic for being resourceful and directing me to the right people for any technical support and information needed for my research. Lastly, I would like to also acknowledge the role of my Graduate School representative (GSR), initially Dr. Sophia Kathariou for being understanding, kind and supportive. I am sorry for the loss she has incurred and sorry for the fact that she cannot be with us for my final oral examination. To replace her, I appreciate Dr. Nathalie Lavoine agreeing to be my committee member and GSR at such short notice.
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