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Electro-Mechanical Couplings in Liquid Crystals

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Electro-Mechanical Couplings in Liquid Crystals Electro-Mechanical Couplings in Liquid Crystals A dissertation submitted to Kent State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy by John Harden Jr. May 2009 Dissertation written by John Harden Jr. B.S., Kent State University, 1991 M.A., Kent State University, 1995 M.A.T., Kent State University, 1995 Ph.D., Kent State University, 2009 Approved by , Chair, Doctoral Dissertation Committee Dr. Antal Jàkli , Member, Doctoral Dissertation Committee Dr. Jonathan Selinger , Member, Doctoral Dissertation Committee Dr. Chanjoong Kim , Member, Doctoral Dissertation Committee Dr. James Gleeson , Member, Doctoral Dissertation Committee Dr. Scott Bunge Accepted By , Chair, Department of Chemical Physics Dr. Oleg Lavrentovich , Dean, College of Arts and Sciences Dr. Timothy Moerland ii TABLE OF CONTENTS LIST OF FIGURES………………………………………………………………………vi LIST OF TABLES……………………………………………………………………...xiii AKNOWLEDGEMENTS………………………………………………………………xiv CHAPTER 1 INTRODUCTION ....................................................................................1 1.1 MOTIVATION AND OPENING REMARKS ...................................................1 1.2 THERMOTROPIC LIQUID CRYSTAL PHASES OF CALAMITIC MOLECULES..................................................................................................................2 1.3 THERMOTROPIC LIQUID CRYSTAL PHASES OF BENT-CORE MOLECULES..................................................................................................................5 1.4 LYOTROPIC LIQUID CRYSTAL PHASES ...................................................18 1.5 ONWARD TO THE EXPERIMENTS..............................................................20 1.6 REFERENCES ..................................................................................................21 CHAPTER 2 GIANT FLEXOELECTRICITY OF BENT-CORE NEMATIC LIQUID CRYSTALS .......................................................................................................................26 2.1 INTRODUCTION .............................................................................................26 2.2 MEASUREMENT OF FLEXOELECTRICITY...............................................30 2.3 EXPERIMENTAL SET-UP FOR DIRECT FLEXING TECHNIQUE ............32 2.4 DETERMINATION OF THE BEND FLEXOELECTRIC COEFFICIENT ....35 2.5 EXPERIMENTAL RESULTS...........................................................................35 2.6 IMPROVED EXPERIMENTAL SET-UP ........................................................42 iii 2.7 CONCLUSIONS................................................................................................49 2.8 REFERENCES ..................................................................................................52 CHAPTER 3 CONVERSE FLEXOELECTRIC EFFECT IN A BENT-CORE NEMATIC LIQUID CRYSTAL .......................................................................................54 3.1 INTRODUCTION ............................................................................................ 54 3.2 EXPERIMENT SET UP................................................................................... 56 3.3 RESULTS AND DISCUSSION....................................................................... 59 3.4 CONCLUSIONS............................................................................................... 69 3.5 REFERENCES ................................................................................................. 70 CHAPTER 4 PIEZOELECTRICITY OF PHOSPHOLIPIDS .....................................72 4.1 INTRODUCTION ............................................................................................ 72 4.2 EXPERIMENT ................................................................................................. 74 4.3 EXPERIMENTAL RESULTS..........................................................................76 4.4 SUMMARY...................................................................................................... 82 4.5 REFERENCES ................................................................................................. 83 CHAPTER 5 MAGNETO-ELECTRICITY MEDIATED BY FERROFLUIDS DISPERSED IN PIEZOELECTRIC PHOSPHOLIPIDS..................................................85 5.1 INTRODUCTION ............................................................................................ 85 5.2 EXPERIMENTAL SET UP.............................................................................. 87 5.3 EXPERIMENTAL RESULTS..........................................................................89 5.4 SUMMARY...................................................................................................... 92 5.5 REFERENCES………...………………………………………………..……..93 iv CHAPTER 6 CHIRALITY OF LIPIDS MAKES FLUID LAMELLAR PHASES PIEZOELECTRIC……………………………….………………………………………94 6.1 INTRODUCTION ............................................................................................ 94 6.2 EXPERIMENTAL APPROACH......................................................................96 6.3 RESULTS AND DISCUSSION..................................................................... 101 6.4 CONCLUSIONS............................................................................................. 107 6.5 REFERENCES ............................................................................................... 109 CHAPTER 7 SUMMARY AND FUTURE OUTLOOK ...........................................111 7.1 GIANT FLEXOELECTRICITY OF BENT-CORE NEMATIC LIQUID CRYSTALS ................................................................................................................ 112 7.2 CONVERSE FLEXOELECTRIC EFFECT IN A BENT-CORE NEMATIC LIQUID CRYSTAL.................................................................................................... 114 7.3 ONGOING WORK WITH THE DIRECT FLEXING TECHNIQUE........... 116 7.4 PIEZOELECTRICITY OF PHOSPHOLIPIDS.............................................. 118 7.5 FUTURE PROBE OF PIEZOELECTRICITY IN MODEL AND REAL CELL MEMBRANES ........................................................................................................... 120 7.6 PIEZOELECTRICITY OF A TETRAHEDRATIC PHASE.......................... 121 7.7 BIO-INSPIRED LIGHT-HARVESTING LIQUID CRYSTALLINE PORPHYRINS FOR ORGANIC NANOSTRUCTURED PHOTOVOLTAICS....... 123 7.8 FINAL WORDS............................................................................................. 126 7.9 REFERENCES ............................................................................................... 127 v LIST OF FIGURES Figure 1.1: Illustration of p-pentyl-p'-Cyanobiphenyl (5CB). A popular calamitic shaped liquid crystal. Cyanobiphenyls are commonly used in liquid crystal displays due to their large dielectric and optical anisotropy……………………………………………………3 Figure 1.2: Illustrations of the liquid crystal phases. The nematic, SmA and SmC all have orientational order with the director pointing to the top of the page. The SmA and SmC also have positional order. The molecules are free to move within their layer but cannot move into other layers forming a 2D fluid………………………………………………...4 Figure 1.3: Sketch of hypothetical nematic phase of bent-core molecules………………..7 Figure 1.4: Schematic structure of the ferroelectric SmAPS and antiferroelectric SmAPA phases……………………………………………………………………………………...9 Figure 1.5: Possible single tilted bent-core smectic structures. Top row: Illustration of the fluid in plane order of the layer polarization, and 3 dimensional explanation of tilt. Middle row: 2 dimensional illustration of the 4 possible situations when only the molecular plane is tilted with respect to the layer normal; Bottom row: 2-dimensional illustration of the 4 possible situations when only the long axis is tilted (leaned) with respect to the layer normal……………………………………………………………….10 vi Figure 1.6: (a) Possible double - tilted bent-core smectic structures. Top row: Illustration of the fluid in plane orders of the layer polarization (either uniform or modulated), and 3 - dimensional explanation of tilt. Middle and bottom rows: 2 dimensional illustrations of the eight possible situations; (b) Graphical explanation of the SmCLP structures in term of two rotation angles θ and φ……………………………………………………………12 Figure 1.7: Schematic representation of the intercalated layer structures……………….13 Figure 1.8: 2-dimensional periodic structures formed by local disc like aggregation of bent-core molecules (a) or due to antiferroelectric in-plane ordering (b)……………..…14 Figure 1.9: Illustration of typical structures formed by lyotropic liquid crystals. a) micelles forming cubic structure. b) normal hexagonal structure. c) bicontinuous cubic phase d) lamellar phase…………………………………………………………………..19 Figure 2.1: Illustration of flexoelectricity based on dipole orientation model proposed by R. Meyer. In a curved system, pear shaped molecules tend to splay and bent-core molecules tend to bend. Both will produce a flexoelectric polarization…………..……..27 Figure 2.2 Molecular structure of the bent-core material and its simplified geometrical model. The molecule shown is 4-chloro-1,3-phenylene bis 4-[4'-(9-decenyloxy) benzoyloxy] benzoate (ClPbis10BB)…………………………………………………….31 Figure 2.3: Schematics of the speaker driven experimental setup……………………….33 Figure 2.4: Model of the sample geometry and deformation of the flexible cell during the periodic vibration driven by the speaker. (a) The geometry of the cell in the xy plane (b) The cell structure during the deformation………………………………………………..34
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