Defects in liquid crystal polymers : their origins and behavior in magnetic and flow fields

Pattern formation and evolution in liquid crystal polymers is studied in both static situation as well in applied magnetic and flow fields. Observations of nucleation and growth of nematic domains in an isotropic matrix under static conditions were used to establish the origins of the most common types of point defects. The patterns describing the director field distribution were revealed using a solidification induced banding method on quenched samples. At a low percent conversion of isotropic to nematic, the defect density is dominated by domain density and by anchoring conditions between the nematic domains and the isotropic matrix. At higher percent conversions, the defect density is controlled by domain-domain interaction events. The defect density rises as the domains nucleate and impinge but decreases after the generation of defects of negative strengths and an annihilation process begins. Larger domains created in the coalescence process can then grow and impinge and continue defect generation via domain interaction. The absence of negative strength defects during the preimpingement phase indicates these defects arise solely due to the domain-domain coalescence during the growth process. The subsequent effects of both magnetic fields and flow fields on defect structure are also examined. With respect to the former, a solution of the director orientation distribution function is presented for N6el inversion walls created in a liquid crystal by application of a magnetic field. The theoretical model is used to simulate director trajectories across walls as a function of both the elastic anisotropy and the orientation of the wall with respect to the applied field. The gradual transition of the defect structures from Neel bend walls to Neel splay walls as the angle between the wall and the magnetic field is varied is also presented. Experimental micrographs obtained via atomic force microscopy on a thermotropic liquid crystalline polymer are used to test and evaluate the theoretical model. Visualization of director textures using the lamellar decoration technique shows the theoretical and experimental results to be in good agreement. The results were used to determine that the sample had an elastic anisotropy of 0.5, implying that k, = 3k 33 The effects of flow on the disclination textures were studied with an optical shearing cell. Flow visualization experiments were used to study curvature driven motion in disclination loops created during flow. Theories of differential geometry were used to predict the way the line curvature should evolve over time. Theoretically the relationship between velocity and local curvature is linear. Experimentally, deviations from the theory sometimes occurred due to forces of attraction between nearby disclinations. Even though disclination evolution in both small molecular and polymer liquid crystals obeyed similar physical laws, several significant differences were observed. Disclinations in the polymer systems generally displayed initially highly contorted contours. In the small molecule liquid crystals however, the loop contours consistently displayed very simple shapes. In the polymer system, the complex line shape reflects the many prior loop-loop coalescence events due to the greater density of loops than in the low molar mass liquid crystal. Moreover, the reduction of regions of high loop curvature is inherently slower in the polymer liquid crystal due to the higher viscosity of the medium. In addition, the motion of the disclination contour may also be affected by a possible reduction in the mobility of disclinations due to the presence of lower molecular weight components at the defect core which themselves must diffuse along with the line defect. Thesis Supervisor: Edwin L. Thomas Title: Morris Cohen Professor of Materials Science and Engineering Acknowledgments: It is hard to believe that my 8+ years at MIT are coming to an end! Before departing there are many people I would like to thank. First I would like to express my appreciation to my advisor Professor Edwin Thomas, for his enthusiasm and support over the years. His excitement for scientific research made an indelible impression on me when I came to him as a sophomore in September of 1989 to inquire about undergraduate research opportunities in his group. Since that time he has taken great care in my professional development, something that has added significantly to my positive experience at MIT. As I reflect on the last 8 years I have worked for him, I am beginning to understand the advice given to me by one of his former students. One person in particular suggested that if I wanted to start research as a undergraduate I should "go work for Ned Thomas". That was it. No "ifs", "ands" or "buts", just "do it". When I look back now, if I had to do it over again I wouldn't have done anything differently. The lessons I have learned from Ned about academics, research, and group management are invaluable and will serve a foundation for some day in the future when I start my own research group. I am also very grateful to Professor Michael Rubner for his support, especially of my desires to become involved with teaching. The first time we worked together was in the Fall of 1992 when I was a grader for his course in Polymer Chemistry. He gave me his full support when I expressed an interest in also running review sessions and holding office hours in addition to my regular duties. I was honored when he asked me to become a fulltime graduate teaching assistant for him in the fall of 1995. The marvelous example he set in the classroom is something to which I aspire. His continued advice over the years is something that I have appreciated a great deal. There are many other people that have been instrumental in my career at MIT. I am very grateful to Professor Robert Rose for his guidance and support over the years. He has been one of the most influential professors I have interacted with and has been instrumental in my decision to become a professor. I would also like to thank the members of my committee: Professors Robert Armstrong, Ken Russell and Chris Scott for their advice and suggestions. Professor Sam Allen was also very instrumental in my research on curvature driven motion of disclination loops. Scott Clingman (Cornell University, Professor Chris Ober's group) was a main driving force in this research through the superb polymers he synthesized for me. I don't know what I would have done without him. I owe you one Scott! I also could not have done this without the continual support of my friends and family. I would like to thank Leslie Lawrence, Marshall Hughes, Dongsik Yoo and Barbara Dirsa for their friendship over the years. A big thank-you is also due to Kevin McGinty, my piano teacher, who was not only a superb teacher but a good friend as well. He was one of the main people who helped me maintain my sanity while finishing my degree. The love and support of my parents, Paul and Lynda Gunther, helped pull me through the difficult moments. To all my brothers and sisters: Jordan, Jenessa, Jilenne, Julia, Jansen, Justus, and Jesse.yes! I am now Dr. Gunther! I am deeply indebted to my brother Jordan who spent nearly one week of his summer vacation helping me finish the final corrections on my thesis. I couldn't have done it without you! Finally, I would like to thank the Lord for his love and blessings and without whom none of this would have been possible.