Liquid Crystalline Behavior of Mesogens Formed by Anomalous Hydrogen Bonding
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LIQUID CRYSTALLINE BEHAVIOR OF MESOGENS FORMED BY ANOMALOUS HYDROGEN BONDING A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy by SEUNG YEON JEONG August, 2011 Dissertation written by SEUNG YEON JEONG B.S., Korea University, Korea, 2001 M.S., Korea University, Korea, 2004 Ph.D., Kent State University, 2011 Approved by _______________ Prof. Satyendra Kumar , Chair, Doctoral Dissertation Committee _______________ Prof. Shin-Woong Kang , Members, Doctoral Dissertation Committee _______________ Prof. John Portman , _______________ Prof. Hiroshi Yokoyama , _______________ Prof. Robert Twieg , Accepted by _______________ Prof. James T. Gleeson , Chair, Department of Physics _______________ Dr. Timothy Moerland , Dean, College of Arts and Sciences ii TABLE OF CONTENTS LIST OF FIGURES …………….……………………………………………………vii LIST OF TABLES ………………………………………………………………….xxi ACKNOWLEDGEMENTS ………………………………….………………………xxii CHAPTER 1 INTRODUCTION ……………………………………………………1 1.1 Liquid Crystals ……………………………….……………………………………1 1.2 Liquid Crystal Phases ……………………………….……………………………4 1.2.1 Nematic Phase ………………………………………………………………4 1.2.2 Smectic Phase …………………………………….………………………8 1.2.3 Columnar Phase …………………………………….…..…………………10 1.3 Requirement for the Formation of Liquid Crystal Phases ………………………13 1.4 Properties of Liquid Crystals …………………….……………..…………………17 1.5 Characterization of Liquid Crystals …………………….…………………………18 1.6 Motivation and Outline of Thesis……………………………………………19 REFERENCES CHAPTER 2 MATERIALS and PHASE CHARACTERIZATION………….……25 2.1. What’s Hydrogen bond? …………………………………………………….……25 2.2. Hydrogen bonded liquid crystals …………………………………………………26 2.2.1. Molecular Assembly ………………………………………………………26 2.2.2. Materials used in this project ………………………………………………35 2.3. Differential Scanning Calorimetry ………………………………………………36 iii 2.3.1. Background ………………………………………………………………36 2.3.2. DSC Experiment …………………………………………………………39 2.3.3. Result of DSC ……………………………………………………………41 2.4. Fourier Transform Infrared (FT-IR) Spectroscopy ………………………………45 2.4.1. Background ……………………………………………………….………45 2.4.2. Sample preparation and Experiment ………………………………………46 2.4.3. Result and Discussion ……………………………………………………47 REFERENCES CHAPTER 3 EXPERIMENTAL TECHNIQUES …………………………………55 3.1. Polarizing optical microscopy ……………………………………………………55 3.1.1. Sample Preparation …………………………………………………….…56 3.1.2. Experimental Setup ………………………………………………………56 3.2. X-ray Diffraction …………………………………………………….……………59 3.2.1. Background …………………………………………………….…………59 3.2.2. Sample Preparation and Experimental Setup ……………………………62 3.2.3. X-ray Diffraction Patterns of Liquid Crystal Phases ……………………65 3.2.4. Data Analysis …………………………………………………………68 3.3. Capacitance Measurement ……..………………………………………………70 3.3.1. Dielectric Constant ……..………………………………………………70 3.3.2. Experimental Setup ……..………………………………………………71 3.4. Conoscopy …………….………………………………………………………75 3.4.1. Background ………………………………………………………………75 iv 3.5. Raman Scattering and IR Spectroscopy …………………………………………81 3.5.1. Background …………………………………….…………………………81 3.5.2. Sample Preparation and Experimental Setup ……………………………86 REFERENCES CHAPTER 4 SINGLE COMEPOENT H-BONDING MESOGENS ……………91 4.1. X-ray Diffraction Measurements …………………………………………………91 4.1.1. Molecular formation ………………………………………………………91 4.1.2. The Isotropic Phase ………………………………………………………92 4.1.3. The Nematic Phase ………………………………………………………100 4.1.4. The Columnar Phase ……………………………………………………107 4.1.5. Orientational Order Parameters ………………………………………….110 4.1.6. Discussion ………………………………………………………………113 4.2. Polarizing Optical Microscopy …………………………………………………121 4.3. Conoscopy and Optic axis ………………………………………………………134 4.3.1. Conoscopy ………………………………………………………………134 4.3.2. Conoscopy and Optic axes in thick cell ………………………………….138 4.4. Capacitance Measurements ……………………………………………………145 4.4.1 Temperature Dependence of Capacitance ……………………………145 4.4.2. Electric Field Dependence of Capacitance ………………………………146 4.5. Raman Scattering ………………………………………………………………149 4.6. Conclusion .……………………………………………………………………157 REFERENCES v CHAPTER 5 BINARY MIXTURES OF HYDROGEN BONDING MESOGENS …………………………………………………………………………………………161 5.1. Introduction ……………………………………………………………………161 5.2. Preparation of Binary Mixtures …………………………………………………163 5.3. Results and Discussions …………………………………………………………164 5.3.1. DSC Results …………………………………………………………….164 5.3.2. Polarizing Optical Microscopy and LC Textures ………………………166 5.3.3. X-ray Diffraction Results ………………………………………………172 5.4. Summary ……………………………………………………………….…….….186 REFERENCES CHAPTER 6 SUMMARY…………………………………………………………… 189 REFERENCES vi LIST OF FIGURES Figure 1.1: Schematic illustration of the solid, liquid crystal, and liquid phases. The basic transition is solid -> liquid -> and/or gas with temperature increasing, but the materials have solid state (high order and low symmetry) at low temperature. As temperature increases, a liquid crystal state exists before disordered (the molecules are randomly distributed) liquids at higher temperature. Here, the nematic phase serves as an example of the liquid crystal phase. Figure 1.2: Lyotropic liquid crystal phases form via micellar aggregates in surfactant solutions. As the concentration in a system increases, the molecules aggregate to form micelles. The shape of the micelles in a system is determined by molecular shape, size, and concentration. Figure 1.3: Distribution of rod and board like molecules in the uniaxial (Nu) and biaxial nematic (Nb) phases. In the Nu phase, there is one preferred direction, n. Here, θ is the angle which determines the deviation of the long molecular axis from n. The angle describes a rotation around the z axis. In the Nb phase, two orthogonal directors n and m are defined to describe orientational ordering of different symmetry axes of the molecules, here, . Figure 1.4: The bent-core liquid crystal with terminal chains, R, of C7H15 and C12H25, exhibits the biaxial nematic phase at lower temperature than the Nu phase. Figure 1.5: Schematic representation of the SmA and SmC. Figure 1.6: The first examples of thermotropic discotic mesomorphism. vii Figure 1.7: Illustration of the columnar phase: (a) disordered hexagonal packing, (b) three types of stacking within a column, and (c) three types of 2D lattices found in columnar phases. Figure 1.8: The basic structure of traditional liquid crystals. Figure 1.9: Examples of calamitic liquid crystals. Figure 2.1: Hydrogen bonding in Carboxylic acids, electronegative oxygen atom and partially positive hydrogen atom attract each other. Figure 2.2: Examples of calamitic complex formation by hydrogen bonding: (a) p-n-alkoxybenzoic acid by double hydrogen bonding, (b) p-n-alkoxycinnamic aicd by double hydrogen bonding, (c) p-butoxybenzoic acid and trans-[p-ethoxy(benzoyl)oxy]-4'-stilbazole, and (d) p-hexyloxybenzoic acid and p-octyl pyridine. Figure 2.3: Hydrogen bonded liquid crystals formed by (a) identical components, (b) dissimilar components, and (c) in polymers between identical and different components. Figure 2.4: Example of bent-core complex by hydrogen bonding: (a) phthalic acid : trans- 4-alkoxy-4’-stilbazole (n=7, 8, 10) = 1 : 2, and (b) p-tetradecloxy benzoic aicd : 4’-stilbazole derivative = 1 : 1. Figure 2.5: Disk-like complex formed by two molecule of tetrakis(n-alkoxy)-6(5H)- phenanthridinoneby are self-assembled by hydrogen bonding. Figure 2.6: Examples of polymer and network formation via hydrogen bonding: (a) Side chain polymer formed between poly(4-vinylpyridine) and H-bonding side chain, (b) liquid crystalline network by self-assembly of polyacrylate and 4,4'- viii bypyridine, (c) a schematic illustration of smectic network formation (c)-3 by hydrogen bonding of trifunctional compound (c)-1 and bifunctional bipyridine (c)-2. Figure 2.7: Figure 2.7: Molecular structure of 4-[2, 3, 4-tri(octyloxy)phenylazo] benzoic acid and 4-[2, 3, 4-tri(heptyloxy)phenylazo] benzoic acid. Figure 2.8: The schematic figure of differential scanning calorimeter (heat flux type). Figure 2.9: DSC thermograph for TOPAB shows the isotropic-nematic-columnar-crystal phase sequence. The blue (upper) graph represents the heating scan, and the pink (lower) curve is obtained upon cooling; both at a rate of 5oC/min (I: isotropic, N: nematic, Col: columnar, Cr: crystal phase). Figure 2.10: DSC thermographs for THPAB show the I-N-Col-crystal phase transitions. The blue line is the heating scan; and the pink line is the cooling scan at a scan rate of 5oC/min (I: isotropic, N: nematic, Col: columnar, Cr: crystal phase). Figure 2.11: POM textures of TOPAB from the I to Cr phase. Figure 2.12: Compounds belonging to the azobenzene series. Figure 2.13: The molecule, obtained by replacing COOH in TOPBA by COOH2CH3, shows no mesophase. -1 Figure 2.14: IR spectra of the CaF2 substrates from 4000-400 cm . The plate strongly absorbs IR below about 1050 cm-1. Figure 2.15: IR spectra of TOPAB in the nematic phase at 115oC in the range 3800 – 1000 cm-1. Main characteristic bands’ assignment is described in Table 2.2. ix Figure 2.16: (a) IR spectra of TOPAB in range 1800 – 1600 cm-1 from 30oC to 240oC. (b) Plots of the wavenumber of the C=O band as a function of the temperature. Figure 2.17: IR spectrum of TOPAB in the range of 2400 – 3750 cm-1 from 30oC to 240oC; 2500 – 2700 cm-1 and OH broad band disappear with higher temperature, but the broad band around 3300 cm-1 and weak sharp 3550 cm-1 band appear. Figure 3.1: Optical microscopy experimental setup for texture observations. Figure 3.2: The Bragg condition: Constructive