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Physically Crosslinked Hydrogels: Impact of Interfaces and Stress on Structure and Properties

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Physically Crosslinked Hydrogels: Impact of Interfaces and Stress on Structure and Properties PHYSICALLY CROSSLINKED HYDROGELS: IMPACT OF INTERFACES AND STRESS ON STRUCTURE AND PROPERTIES A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Clinton G. Wiener May, 2017 PHYSICALLY CROSSLINKED HYDROGELS: IMPACT OF INTERFACES AND STRESS ON STRUCTURE AND PROPERTIES Clinton G. Wiener Dissertation Approved: Accepted: ________________________________ ________________________________ Advisor Department Chair Dr. Bryan D. Vogt Dr. Sadhan Jana ________________________________ ________________________________ Co-Advisor and Committee Member Dean of the College Dr. Robert Weiss Dr. Eric J. Amis ________________________________ ________________________________ Committee Member Dean of the Graduate School Dr. Nicole Zacharia Dr. Chand Midha ________________________________ ________________________________ Committee Chair Date Dr. Matthew Becker ________________________________ Committee Member Dr. Bi-min Zhang Newby ii Adapted from Ref: Wiener, C. G.; Weiss, R. A.; Vogt, B. D. Overcoming confinement limited swelling in hydrogel thin films using supramolecular interactions. Soft Matter 2014, 10 (35), 6705- 6712. with permission from The Royal Society of Chemistry. Copyright [2014] RSC Wiener, C. G.; Tyagi, M.; Liu, Y.; Weiss, R. A.; Vogt, B. D. Supramolecular Hydrophobic Aggregates in Hydrogels Partially Inhibit Ice Formation. Journal of Physical Chemistry B 2016, 120 (24), 5543-5552. This is an unofficial translation of an article that appeared in an ACS publication. ACS has not endorsed the content of this translation or the context of its use. Copyright [2016] Journal of Physical Chemistry Reprinted with permission from Wiener, C. G.; Wang, C.; Liu, Y.; Weiss, R. A.; Vogt, B. D. Nanostructure Evolution during Relaxation from a Large Step Strain in a Supramolecular Copolymer-Based Hydrogel: A SANS Investigation. Macromolecules 2017, 50 (4), 1672-1680 Copyright [2017] Americal Chemical Society ABSTRACT The methods to control hydrogels’ toughness, ultimate stress, and fracture energy have received attention by researchers in recent years. The toughness and fracture energy can be increased by incorporating energy dissipating mechanisms. Physical crosslinks that can break and reform during stressing provide a simple method to modulate hydrogel mechanics. Hydrophobic physical crosslinks are a well suited for increasing toughness and fracture energy. This work investigated a physically crosslinked hydrogel composed of a random copolymer containing hydrophilic segments [N,N-dimethylacrylamide or N- isopropylacrylamide] and hydrophobic segments [2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate (FOSA)] that form a network by the hydrophobic aggregation of FOSA segments into nanodomains. This system provides a model system for tough physically crosslinked hydrogels. How physical crosslinks impact swelling in laterally confined thin films, how the physical hydrogel and nanodomains deform in stress relaxation, and how absorbed water is altered due to confinement between hydrophobic nanodomain crosslinks were studied. Unlike the significantly reduced swelling of chemically crosslink thin film gels, the nature of the physical crosslinks’ allows rearrangements in laterally confined thin films. These rearrangements allow the thin films to obtain an equilibrium swelling ratio similar to bulk. The osmotic pressure of the hydrophilic chain swelling induces these rearrangements. The result of this is the ability of physically crosslinked thin films ability to overcome thin film swelling constraints. An equilibrium water iii content/distribution can be obtained by rearrangement of the network. The route by which physical crosslink domains breakup and reform was identified by performing 1D elongation during small angle neutron scattering with contrast matching (CM-SANS). The stress induced relaxation time was found to span 5 orders of magnitude when fit with seven Maxwell elements. From the CM-SANS measurements, the relaxation times of the physical crosslinks and the interconnecting hydrophilic segments were found to correlate with distinct macroscopic stress relaxation times. The CM-SANS measurements also revealed that the domain breakup occurs primarily by the pullout of the FOSA segments only after the interconnecting swollen segments have become significantly strained. Upon relaxation of the physical crosslinks, the physical domains display a spring-like rebound effect, increasing their spacing, and hydrophilic chain stretching, in the opposite direction in which the network is strained macroscopically. This strain-transverse domain spacing increase only relaxes after the physical crosslinks have reformed. The effect of the hydrophobic moieties, which are impermeable to water, on the absorbed water behavior was studied. The fraction of supercooled water measured at 200K increases with increasing the copolymer FOSA fraction as found by differential scanning calorimetry. Hydrogels’ primarily supercool water by binding to hydrophilic segments, but here the hydrophilic fraction decreases with increasing FOSA content. The cause of this was found to be the result of water nanoconfinement. Probing the dynamics of the absorbed water with neutron scattering revealed that as the FOSA fraction is increased, water mobility below 240K is higher than in bulk supercooled water. This demonstrates that the nanoconfinements increase the supercooled water mobility. These physical crosslinks provide a route to supercool water in soft tissue-like hydrogels. iv ACKNOWLEDGEMENTS During my 5 years at the University of Akron many individuals have helped to make this dissertation possible. Special thanks to my tireless supervisor and friend, Dr. Bryan Vogt. His mentoring gave me a strong start in the right direction when I began 5 years ago. His ability to let me find my own way, while also insuring I stayed on the right path, has been critical to what I have accomplished and allowing me to achieve success. His knowledge of neutron scattering allowed me first chance at studying the microstructure and behavior of materials. This unknowingly sparked my interest in the powerful technique which has allowed me to discover many great things during my study and fostered my interest in the molecular interactions of materials. I would also like to thank my co-advisor Dr. Bob Weiss. His support in my studies and insightful questions of my work and results has continued to morph the way I think and understand my research. I would like to especially thank him for being there not only as an advisor, but a friend when research was getting tough. He sought to not only see my work be successful, but for me to also be happy and seek enjoyment in everything I did. I wish to thank Dr. Yun Liu, Dr. Madhusudan Tyagi, and Dr. Paul Butler. These scientists were critical in working through the experiments and results from my neutron scattering measurements. They offered insight that could not be found in papers or books. Their input was a key part in my published work. I would also like to thank the beam line staff at NIST, especially Cedric Gagnon and Jeff Kryzon. Their help in experimental setup and troubleshooting was unmatched in assisting with the various neutron scattering v experiments. Even when we brought setups crafted by hand, they worked to insure the best possible setup and suggested modifications to improve. A special thanks also goes out to all the friends I have made at my many trips to the NIST Center for Neutron Research. The discussions have affected my dissertation and life in many subtle ways. During my stay in UA, I made many great friends and colleagues. I would like to especially thank Chao Wang, for the many fruitful discussions on interpreting and understanding scattering results. Through these discussions, some getting quite heated, we were able to fine tune our perception of the behavior of the system and in the course he became a great colleague and friend. Special thanks also to Dr. Jeongwoo Lee and Dr. Sarang Bhaway. Their guidance and insights as senior members of the research group were enormously helpful in my research and my success in my PhD. I would like to extend a thank you to all of my group members, come and gone, over my 5 years in UA. Whether you helped me, or I helped you, both have shaped into the scientist I am today. Lastly, I would like to thank my family, for their continued support in my ever increasing desire to become a scientist. To my brothers and sisters who endlessly (this could go on for some time I recall) had to answer my questions about why and what if. I want to finally and with special emphasis thank my parents, Alphonse and Anne, for nurturing of my scientific mind and continuing to support me in my increasing levels of study. If it were not for their persistence, this long journey of education I have taken could have never been achieved. Sometimes you may have not understood why I continued with my schooling, but you knew I wanted it, so you supported me in my choice to continue on to obtaining my PhD. vi TABLE OF CONTENTS Page ABSTRACT ....................................................................................................................... iii ACKNOWLEDGEMENTS ................................................................................................ v LIST OF FIGURES ............................................................................................................ x LIST OF TABLES ........................................................................................................
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