Liquid Crystalline Behavior of Mesogens Formed by Anomalous Hydrogen Bonding

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

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

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

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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

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

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

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.

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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,

(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'-

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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.

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 interference of X-rays reflected from

successive planes occurs when the path difference is a multiple of the wavelength.

Figure 3.3: XRD experimental set up: X-ray beam passes perpendicular to the magnetic

field, and through the sample, diffraction pattern is collected by an image plate

detector.

Figure 3.4: Silicon standard used for the calibration of experimental setup: (a) XRD of

the material, (b) plot of 2θ vs. intensity generated from the data.

Figure 3.5: Molecular arrangement and X-ray diffraction patterns from different LC

mesophases.

Figure 3.6: X-ray diffraction pattern from aligned nematic (Δ> 0) phase of a calamitic

liquid crystal 8OCB at 79oC.

Figure 3.7: Illustration of the scans generated with the help of FIT2d software: (a) X-ray

diffraction pattern of the TOPAB in the nematic phase, (b) plot of q vs. intensity,

diffraction pattern.

Figure 3.8: (a) Calamitic liquid crystal with director, dielectric constant. (b) Experimental

set up for capacitance measurement.

Figure 3.9: Voltage dependence of capacitance (scale on left) and dielectric constant

(right) of 5CB.

Figure 3.10: Schematic representation of conoscopy experiment. θp is the tilt angle of

molecules with respect to the axis O, and ro is the radius of the conoscopic image.

Figure 3.11: Conoscopy image of the uniaxial nematic phase of p-pentyl-p'-

cianobiphenyl (5CB) in (a) homeotropic configuration, and (b) nearly

homeotropic state with anti-parallel rubbing.

Figure 3.12: Conoscopy images of uniaxial nematic (left) and the biaxial (right) nematic

phase. Dashed contours represent the isochromes.

Figure 3.13: Observation of optic axis, the arrows present their fast axis.

Figure 3.14: Imaging of the optic axis image in samples (of 5CB) used for results in Fig.

3.11(a) and (b). The dark spot denotes the location of the optic axis being at the

center of the image. The optic axis is parallel to the light propagation for (a),

while the optic axis in (b) is lilted to the right of the center. (Objective lens x50)

Figure 3.15: A schematic comparison of the mechanisms responsible for Raman and IR

spectroscopies.

Figure 3.16: Irradiation of the sample with plane polarized light incident along the y-

direction and its electric field vector (i.e., polarization direction) in the z-direction.

The photons scattered at different angles having polarization perpendicular and

parallel to the incident polarization are detected and analyzed.

Figure 3.17: Representation of the rotation of a molecule with differential polarizabilities

with respect to the laboratory frame XYZ in terms of the Euler angles.

Figure 4.1: Possible forms and approximate dimensions of TOPAB and THPAB: (a)

Molecular (monomer) length scales of TOPAB and THPAB, (b) a linear dimer of

TOPAB through hydrogen bonding, (c) a trimer, and (d) polymer.

Figure 4.2: XRD patterns of TOPAB at different temperatures. Magnetic field is applied

in the horizontal direction: (a) the isotropic phase: two diffused rings, (b) and (c)

the uniaxial nematic phase, and (d) the columnar phase with hexagonal packing.

Figure 4.3: XRD patterns of THPAB at different temperatures. Magnetic field is applied

in the horizontal direction: (a) the isotropic phase: two diffused rings, (b) and (c)

the uniaxial nematic phase: Two pairs of reflections, and (d) the columnar phase

with hexagonal packing.

Figure 4.4: Diffractographs for TOPAB obtained by intensity integration of XRD pattern

over all azimuthal directions, at different temperatures. The columnar phase at

o 93 C (on log scale) has hexagonal packing with ratio d1: d2: d3 = 1 : √ : √ .

Figure 4.5: Intensity profile in q-space of THPAB at different temperatures. The nematic

to columnar transition at 98oC displays (on log scale) hexagonal packing with

ratio d1: d2: d3 = 1: √ : √ . The inset shows the regions inside the ellipse on an

expanded scale.

Figure 4.6: The d-spacing (Å ) and correlation length (Å ) calculated from the first peak of

TOPAB in the meridional direction. The nematic data is connected by solid lines.

The dotted-vertical lines are where the transitions occur. The d-spacing decreases

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slightly, but the correlation length increases with decreasing temperature in the

nematic phase, and increases dramatically from 65 Å in the nematic phase to 1280

Å in the columnar phase.

Figure 4.7: TOPAB: (a) XRD pattern of the nematic phase at 101oC; (b) azimuthal scans

for different peaks; and (c) black curve for the 24.4 Å peak with intensity scale on

left hand side was obtained from integration over  (azimuthal) angles from 45o to

135o wrt the magnetic field (or, between  45o wrt the meridian). The blue curve

for the 18.2 Å peak, with intensity scale on right hand side, was calculated from

integrating peak intensity from  = - 45o to 45o relative to the magnetic field.

Figure 4.8: THPAB: aligned nematic phase at 114oC. (a) XRD pattern, (b) -scans

through different peaks, (c) blue curve: intensity vs. q -integrated from - 45o to

45o with respect to the magnetic field, and black graph, from - 45o to 45o with

respect to the meridian.

Figure 4.9: Evolution of small angle diffraction in the nematic phase of TOPAB. Small

angle region in yellow box is also shown on an expanded scale using intensity

contour lines with 10% intensity difference between lines. (The color bar at the

bottom shows the intensity scale). The shape of peak resembles a kidney bean at

lower temperature.

Figure 4.10: Thermal evolution of χ-scans (a) through the nematic phase for the smallest

angle peak, and (b) fits to Gaussian distribution function at 120C and 101C. At

higher temperature the peaks are fit with two Gaussian distribution peaks over

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360o however, at lower temperature four Gaussian distribution peaks are needed

to fit the peaks. One peak is the summation of two scattered intensity peaks

Figure 4.11: XRD patterns and intensity vs. q graphs in the crystal phase of (a) TOPAB at

89.5oC and (b) THPAB at 70.0oC.

Figure 4.12: Experimental integrated scattered intensity about azimuthal angle for the

wide angle peak in the nematic phase of TOPAB at 116oC. The corresponding

XRD pattern is presented in Fig. 4.2(b). The red solid line is a fit to eq. 4.2.

Figure 4.13: Temperature dependence of orientational order parameters and

in the nematic phase of TOPAB. and increase as

temperature decreases and lie in ranges 0.52 – 0.68 and 0.12 – 0.30, respectively.

Figure 4.14: Estimated dimensions of the oblique dimer for TOPAB and THPAB. The

rows with background light orange and light sky color list the estimated

dimension at 101oC for TOPAB and at 114oC for THPAB, respectively.

Figure 4.15: (a): Schematic depiction of the molecular conformation of oblique dimer

TOPAB or THPAB. The curved red arrow shows that the molecule is degenerated

about x-axis; the direction perpendicular to the director n and parallel to the

magnetic field B. (b): Schematic depiction of the possible molecular conformation

of tetramer TOPAB or THPAB. Two of such dimers of Fig. 4.15(a) can form a

tetramer via H-bonding with twice of width.

Figure 4.16: XRD patterns of TOPAB In the nematic + columnar phase at 96.2oC under

different orientations. (a) Magnetic field (red arrow) aligned sample obtained after

cooling, (b) immediately after rotation of the capillary (sample) by 90o about its

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vertical axis, (c) magnetic field realigned nematic phase after 5 minutes after

rotation, and (d) intensity vs. q graphs for patterns in (a) – (c). The data

accumulation time in each case was 3sec).

Figure 4.17: The XRD patterns of the nematic phase of TOPAB, top panel is obtained by

cooling from the isotropic phase applied magnetic field, direction shown by red

double arrow. Lower panel depicts the diffraction pattern and molecular

orientation immediately after 90o rotation of the sample about capillary axis.

Figure 4.18: POM textures in the four phases in cells made with untreated ITO glass

substrates (Right column: TOPAB, Left column: THPAB).

Figure 4.19: The POM textures obtained for TOPAB in the 4 m thick cells with two

homeotropically aligning polyimides surface coatings. Two and four brush

disclinations are visible in the nematic phase.

Figure 4.20: The POM textures obtained for TOPAB in the 4 m thick cell coated with

homeotropic polyimide under different applied electric fields in the nematic phase

at 115oC and 100oC.

Figure 4.21: The POM textures at variable temperature obtained for the TOPAB in the 4

m thick cells with homogeneous polyimide SE7792.

Figure 4.22: POM textures of (a) homogeneously and (b) homeotropically aligned

nematic phase of TOPAB under different applied electric fields at 100oC. As the

electric field increases, the brightness increases until (a) E = 3.0 Vrms/m, (b) 2.5

Vrms /m. Brightness decreases at higher electric field.

Figure 4.23: (a) POM observation for homogeneously aligned IPS cell under electric field

in the nematic phase between crossed polarizers when the rubbing direction is

parallel to the electrodes, (b) schematic illustration of the appearance of

disclination lines in the center of the non-electrode areas. (R/D: rubbing direction)

Figure 4.24: Electro-optical behavior of a homogeneously aligned IPS cell, the rubbing

direction (red arrow) is perpendicular to the linear electrodes and parallel to the

applied electric field. The red lines appearing with increasing field arise from the

deformed director field caused by fringing-field at the edge of the electrode. (The

direction marked as P and A on in the circular area on RHS represents the

directions of polarization and analyzer axes.)

Figure 4.25: Conoscopic images of TOPAB at 115oC in the nematic phase as a function

of increasing applied electric field in a homogeneous 4 m anti-parallel rubbed

cell. The rubbing direction is parallel to the polarizer’s easy axis and objective

lens magnification was 20 and N.A. = 0.4.

Figure 4.26: Conoscopy patterns in a homogeneous 4 m cell of TOPAB with anti-

parallel rubbing, at different temperatures and at various angles of rotation with

respect to the polarizer direction (objective lens x20, N.A. = 0.4). Here (a) E =

o o 14.6 Vrms/m at 115 C, (b) E = 15.6 Vrms/m at 100 C. Red arrow, and the

directions marked as A and P represent the rubbing, analyzer, and polarizer

directions, respectively.

Figure 4.27: Conoscopy observations in a 4 m thick cell of TOPAB with homeotropic

polymer coating and anti-parallel rubbing. The pictures were taken with while the

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cell was rotated and at two different temperatures and fields (objective lens x20).

o o (a) E = 12.6 Vrms/m at T = 115 C, (b) E = 17.1 Vrms/m at T = 100 C.

Figure 4.28: Effect of applied electric field on conoscopic patterns of TOPAB in a 14 m

cell at 115oC and 100oC in the nematic phase. The cell walls were coated with

homeotropically aligning polymer and rubbed in an anti-parallel manner. The

pictures shown in the bottom panel were taken with Benford plates, and the dark

spots clearly show the existence of two optic axes at high electric fields.

Figure 4.29: At T = 100oC in the nematic phase of TOPAB, (a) conoscopic image (b)

optic axis observations with Benford plate in a homogeneous 14 m thick cell

under different angles of rotations.

Figure 4.30: At T = 100oC in the nematic phase of TOPAB, (a) conoscopic image, and

(b) optic axis observations in a 14 m homeotropic cell at different rotation angles

between crossed polarizers.

Figure 4.31: At T = 100oC in the nematic phase of TOPAB, transmitted intensity as a

function of applied electric field, POM texture, and conoscopic images in a 4 m

thick homogeneous cell with different applied electric field. Appearance of the

dark line in conoscopic image corresponds to maximum transmittance (or highly

birefringent texture) means the optic axis exists close to the cell’s surface normal.

(The red double arrow presents as rubbing direction.)

Figure 4.32: When sufficient high electric field applied to the sample, the director n is

oriented parallel to electric field (OA: optic axis). Optic axes correspond to the

two dark spots in Fig. 4.29 and Fig. 4.30.

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Figure 4.33: The temperature dependence of capacitance and dielectric constant of

TOPAB in homogeneous (up) and homeotropic alignment (down) conditions in a

4 m cell.

Figure 4.34: Capacitance of TOPAB as a function of electric field at three selected

nematic temperatures in a 4 m thick cell with planar alignment. Capacitance

o increases with increasing electric field beyond ~ 0.8 Vrms/m at 115 C, ~ 1.1

o o Vrms/m at 106 C, and ~ 1.4 Vrms/m at 100 C.

Figure 4.35: Raman scattering spectrum obtained from the TOPAB, shows parallel (IHH)

o and perpendicular (IVH) mode at 103 C in the nematic phase. The yellow star

marks C=C ring stretching vibrations and the blue star is above the peak arising

from Ar–N=N–Ar stretching mode of trans-aromatic azo compounds. The purple

star points to the aromatic C-H in-plane deformation vibrations. (The sharp peak

-1 of the graphs IVH at around 1520 cm is noise peak).

Figure 4.36: Normalized Raman spectra obtained from the TOPAB at 118oC in the

nematic phase are shown with the fitted curves.

Figure 4.37: The depolarization ratio profile calculated from the data in Fig. 4.36 and the

fit to eq. (3.6) yield = 0.49, = 0.27, and r = – 0.18.

Figure 4.38: (a) and (b): Normalized intensities IHH and IVH obtained from fits, and (c)

temperature dependence of depolarization ratio.

Figure 4.39: Temperature dependence of the uniaxial order parameters and

in the nematic phase of TOPAB. The solid dark and open pink circle are

and , respectively, from Raman scattering. The red solid line represents a

xviii

power law dependence on reduced temperature resulting in a virtual transition

o temperature, Tc = 120.5 C and order parameter exponent α = 0.16. The solid and

open blue circles are and obtained from X-ray scattering.

Figure 5.1: DSC results of two mixtures (a) 3:1 molar ratio mixture of TOPAB and

DBBA, and (b) 1:1 molar ratio mixture of TOPAB and OOBA.

Figure 5.2: Representative POM textures at different temperatures and in different phases

of mixtures of TOPAB + OOBA (a) 3:1 molar ratio, (b) 1:1 equimolar, (c) 1:3

molar ratio (white double arrow indicates the rubbing direction).

Figure 5.3: Representative POM textures in different phases of mixtures of TOPAB +

DBBA in (a) 3:1 molar ratio, (b) 1:1 equimolar, (c) 1:3 molar ratio (the white

double-arrow indicates the rubbing direction).

Figure 5.4: Plot of phase transition temperatures for mixtures of (a) TOPAB + OOBA,

(b) TOPAB + DBBA at different concentrations.

Figure 5.5: Representative XRD patterns for mixtures TOPAB + DBBA in molar ratios

of: (a) 3:1, (b) 1:1, and (c) 1:3. The white double arrow indicates magnetic field

direction.

Figure 5.6: Representative XRD pattern for the mixtures TOPAB + OOBA in molar

ratios of; (a) 3:1, (b) 1:1, (c) 1:3. The white double arrow indicates magnetic field

direction.

Figure 5.7: Temperature dependence of the largest d-spacing for the mixtures of TOPAB

+ OOBA in the molar ratios indicated on the left hand side. The lines are drawn as

guide to eye.

xix

Figure 5.8: Temperature dependence of the largest d-spacing for the mixtures of TOPAB

+ DBBA in the molar ratios indicated on the left hand side. The lines are drawn as

guide to eye.

Figure 5.9: (a) X-ray diffraction patterns at high and low temperatures in the nematic

phase of 3:1 mixture of TOPAB + OOBA, and (b) χ-scans of small angle peaks at

different temperatures.

Figure 5.10: (a) X-ray diffraction patterns at high and low temperatures in the nematic

phase of 3:1 mixture of TOPAB + DBBA, and (b) χ-scans of small angle peaks at

different temperatures.

Figure 5.11: (a) X-ray diffraction patterns at high and low temperatures in the nematic

phase of 1:3 mixture of TOPAB + OOBA, and (b) χ-scans of small angle peaks at

different temperatures. Δis independent of temperature.

Figure 5.12: In the nematic phase of the 1:3 mixture of TOPAB + DBBA: (a) XRD

pattern at small angles, and (b) χ-scans at different temperatures, Δχ is an angle

between peaks in meridional direction.

Figure 5.13: The 3:1 mixture of TOPAB + DBBA: (a) integrated intensity vs. q in the

nematic phase, (b) the d-spacing (scale on left hand side) and correlation length

(scale on right hand side) of small angle peaks at different temperatures.

Figure 5.14: At 91.0oC, 3:1 mixture of TOPAB and OOBA: (a) XRD pattern of the small

angle region, (b) integrated intensity vs. q, and (c) overlapping large angle peaks,

the green broken curves represent Lorentzian functions fitted to the data.

LIST OF TABLES

Table 1.1: Types of columnar alignment.

Table 2.1: Phase transition temperatures and phase sequence for the azobenzene

derivatives. The numbers in parentheses show the enthalpy change (kJ/mol) at the

transition.

Table 2.2: The description of main characteristic bands’ assignment in Fig. 2.15.

Table 5.1: Molecular structure, chemical formulae and phase sequence of compounds

OOBA: 4-(4-octyloxy)-benzoic acid, DBBA: 4-(4-decyloxybenzoyloxy)-benzoic

acid, HPBA: 4-(4-hexadecyloxy-phenylazo)-benzoic acid.

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ACKNOWLEDGEMENTS

In all honesty, my Ph.D. work started unexpectedly in 2005 with a recommendation from Prof. Satyendra Kumar, who has been my supervisor during my life as a Ph.D. student. I conquered difficult course works, qualifying exams, and various struggles, all the while maintaining my passion for research. Now I have completed the

Ph.D. in physics at Kent State University, and I am pleased to show my gratitude to my supervisor. I heartily thank him for his support and patience from the initial to final stage.

I sincerely respect his prodigious knowledge and passion for physics and the liquid crystal field.

I would like to thank to my previous advisor, Sung-Tae Shin, at Samsung

Electronics in Korea. He always supports me in many ways and gives me advice and encouragement from my college years to the present. Also his wife Young-Shin Lee helped me so much when I came to Kent and found myself on my own. She was like another mother to me. They taught me how to love people. Particular thanks and gratitude go to Dr. Shin-Woong Kang at Chonbuk National University in Korea, who provided guidance regarding research and life. I would like to thank Dr. Veena Prasad of the

Center for Liquid Crystal Research, Banglore, India, who provided the liquid crystal used in this project.

I would like to express my appreciation to Prof. Mike Lee, Prof. John Portman, and Prof. Khandker Quader. They tried to understand me and listen and talk to me when I felt anxious and uneasy about course work and life in Kent. In addition, thank you to

xxii

Prof. David Allender, whose lectures I will never forget. When he said, “You passed the qualifying exam, but it was very close,” that was music to my ears. Another thank you to

Prof. Jim Gleeson, who helped me with the capacitance measurement.

I would like to give special thanks to our group members - Dr. Hyung Guen

Yoon; Dr. Leela Joshi, who is a teacher in Nepal; Dr. Dena Kooijman; and my officemate

Sonal Dey. They have been friendly and cheerful. I would like to thank librarians

Rochelle Gray and Erica Lilly at the Chemistry and Physics Library for making relevant literature easily accessible on campus throughout the years. Special thanks to Rochelle, who always helped me find, scan, and send materials.

Thanks to Alan Baldwin, Wade Aldhizer, Greg Putman, Cindy Miller, and

Loretta Hauser of the Physics Department as well as Prof. R. Twieg and J. Williams of the Chemistry Department for allowing me to do DSC measurement and to Prof. A.

Gericke and S. Woods of the Chemistry Department for the help with FT-IR for this project. I am grateful to Prof. M. Srinivasarao, M. S. Park, and B. J. Yoon at GIT,

Atlanta, GA, for the Raman scattering experiment in this dissertation.

I especially appreciate Prof. Sangkwon Lee of the Sport Administration

Department, who shared with me his valuable time and lots of coffee and good conversation whenever I was troubled. I appreciate Dr. S. H. Hong, who helped me out with course work and gave me the courage to keep going.

I would like to thank my family, my parents, my little sister and her husband in

Korea for their love. I offer my regards and blessings to my dear best friend, whom I love, and all of those who supported me in any respect during my Ph.D. work in Kent.

xxiii

But most of all,

Thanks to God…

Seung Yeon Jeong

April 2011, Kent, Ohio

xxiv CHAPTER 1

INTRODUCTION

1.1 Liquid Crystals

Liquid Crystals are virtually everywhere, occurring in nature in the form of many proteins, cell membranes, and viruses. The use of liquid crystals has become ubiquitous, ranging from cellular phones to notebook computers, navigators, flat-panel TVs, and flexible displays. Liquid crystals have not only led to the development of three- dimensional displays [1], but their properties are also exploited in the design of flexible organic electronics, organic lighting, and the solar cells of the future based on organic photovoltaic semiconductors [2, 3].

A liquid crystal (LC) is a partially ordered anisotropic fluid, thermodynamically located between three-dimensionally ordered crystalline solids and isotropic liquids [4]

(Fig. 1.1). Generally, liquid crystalline materials can be divided into two categories: thermotropic mesophases if the LC phase is achieved via a temperature change, and lyotropic mesophases if it forms at certain concentrations of (typically) surfactant solutions. Fig. 1.2 shows the various micellar shapes in solutions of surfactant molecules.

The micelles organize at high concentrations into ordered phases known as a lyotropic

LCs. In this dissertation research, the focus is mostly on thermotropic liquid crystals.

Many types of liquid crystal phases exist, depending upon the type of order(s) they possess. For brevity’s sake, we will explain mainly three mesophases: the nematic,

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.

Spherical Cylindrical Disc-like

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.

smectic, and columnar phases. All three have orientational order and only partial (in one or two dimensions) positional order [5, 6] as discussed below.

1.2 Liquid Crystal Phases

1.2.1 Nematic Phases

The nematic phase possesses the least order and the highest symmetry among all mesophases. It plays the most important role in liquid crystal displays and electro-optical applications. Molecules in the isotropic (I) phase do not have any macroscopic correlations. This state totally lacks order. No positional or preferential orientational order exists in any direction. In contrast, molecules in the nematic phase are oriented on average parallel to a common direction, while their position in space is not fixed, and they float around just as in the isotropic liquid phase. Because molecules have a high degree of long-range orientational order along a specific direction, the nematic phase is anisotropic in its physical properties, such as index of refraction, viscosity, and dielectric constant. The nematic phase can be aligned by external forces, such as electric, magnetic, and shear fields, primarily because of its anisotropic properties. A simple picture of the relative arrangement of molecules in the nematic phase is shown in Fig. 1.3(a). The long axis of the molecules tends to align along a preferred direction, defined as the director n, which is a pseudo-vector (i.e., n and - n are indistinguishable). The molecules possess rotational symmetry around n [5 - 8].

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 φ 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, 풍 = 풎 × 풏.

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.

The degree of alignment of molecules, that is, how well they are aligned parallel to n, is described, assuming that the director n is along the z-axis in the laboratory frame of reference (x, y, z). We can introduce three Euler angles θ, φ, and ψ (see Fig. 1.3(a)), and because the system has complete cylindrical symmetry about n, the distribution of molecules is independent of φ and ψ. The distribution function should depend only on the polar angle θ wrt n and can be expressed as a series expansion in Legendre polynomials:

2푙+1 f(cos θ) = ∑ < P (cos θ) > P (cos θ). 푙=0,even 2 푙 푙

But the distribution function should be an even function to satisfy the symmetry condition f(θ) = f(π − θ) because n = - n. We take the average of P푙(cos θ) to get

1 < 푃 (cos θ) > = P (cos θ) f(cos θ)d(cos θ). 푙 ∫−1 푙

The first multipole giving a non-trivial contribution is the quadrupole, defined as [6, 9]

3 1 S = < P (cos θ) > = (< cos2θ > − ). 2 2 3

This scalar S quantity is the nematic order parameter, and it is a measure of the degree of molecular alignment. Accordingly, when all molecules are perfectly parallel to n, then S

= 1. When the distribution of the long molecular axes is random as in the isotropic phase, then S = 0 because < cos2θ > = 1/3. In particular, a sharp drop of the order parameter to 0 is observed when the system undergoes a first-order phase transition from the nematic phase to isotropic phase [10]. For a typical nematic phase the value of S lies between ~ 0.4 − 0.7 [11].

The nematic phase with two directions of orientation have been predicted and 7

found in both lyotropic [12, 13] and thermotropic [11] systems. The uniaxial nematic phase has a specific average orientation direction along one spatial direction, n, while a biaxial phase has two symmetry axes (usually called n and m). In the biaxial nematic phase, the degree of rotational freedom of the molecules about n is restricted. These phases are schematically as shown in Fig. 1.3 for calamitic and sandic mesogens. Usually calamitic (cylindrically symmetric) molecules exhibit the uniaxial nematic phase.

However, if the molecules are shaped like a matchbox (board shaped, or sandic), they could form the uniaxial phase followed by the biaxial nematic phase at a lower temperature.

The biaxial nematic phase was predicted by Freiser [14] in 1970 and first identified by Yu and Saupe [12] in a lyotropic ternary amphiphilic system composed of potassium laurate, 1-decanol, and water. After their first report of the much sought-after biaxial nematic phase, researchers tried to find biaxiality in the thermotropic liquid crystal system. Fairly vigorous theoretical and experimental investigations have taken place in the past 25 years to obtain this phase. The experimental studies included new mesogens with biaxial molecular shape that will be conducive to the formation of this phase. Many claims of biaxial thermotropic nematic phases were published [15, 16], including with banana-shaped molecules [17]. In 2004, experimental evidence was presented for the existence of biaxiality in the series of rigid-core V-shaped oxadiazole molecules (see Fig. 1.4) by X-ray scattering [18] and solid-state 2H NMR spectroscopy

[19], followed by further confirmation by Raman scattering [20] and electro-optic method

[21]. A report [22] challenging these findings have already been debunked [23]. Recently, 8

many other materials have been designed and reported to be biaxial, such as tetrapodes

[24].

1.2.2 Smectic Phases

Molecules in a smectic phase, which typically appear at temperatures below the nematic phase, acquire a one-dimensional periodicity of their centers of mass in direction parallel to z. They are effectively arranged in parallel layers. The many different classes of smectic phases include smectic-A, -C, -CA, -B, -I; denoted as SmA, SmC, SmCA, SmB, and SmI, respectively. The SmA and SmC (or, SmCA) are the simplest smectic phases that are most often encountered. Other smectic phases possess higher-order in the form of additional positional ordering within the layers differing in the degree (i.e., short or long range) of bond orientational order [6, 25] and positional orders. Fig. 1.5 shows SmA and

SmC phases, which have no positional order except in the direction (z) perpendicular to layers. In the SmA phase, the collective direction of molecular long axis, that is, n, on average is parallel to z, showing uniaxial optical behavior. In the SmC phase, the molecules are (or, n is) tilted away from z at a temperature dependent angle.

Consequently, the SmC phase possesses biaxiality [26]. The smectic density wave can be written as [5, 6, 27],

1 ρ(퐫) = ρ(z) = ρ [1 + |휓| cos(q z − ω)], 0 √2 0 where ρ0 is the mean density, and q0 = 2π/d is the wavevector of the density wave, d is a layer spacing, and  is an arbitrary phase factor. The average density ρ0 is essentially the same in the N and SmA phases, and obviously |휓| = 0 in the N phase. 9

One can express the smectic order parameter as a complex parameter: 휓 = |휓| 푒푖휔, whose magnitude is proportional to the amplitude of density modulations in the layered smectic phase. However, the SmA phase has quasi-long range positional order rather than true long-range order because of thermal fluctuation of layers. By introduction of u(r), the complex order parameter can be written as 휓(퐫) = |휓(퐫)| 푒−푖푞0푢(퐫). The director in the SmC is specified by 휃 and the azimuthal direction of the tilt. As seen in Fig. 1.5(b),

휃 is the tilt angle of the director wrt the layer normal, and the angle 휑 specifies the direction of the tilt. The SmC differs from the SmA by in the tilt of the director n wrt the direction normal to the layers; thus the SmC order can be described by 휃 and 휑 or equivalently by the complex order parameter: 훹 = 휃 푒푖휑 [6, 11].

Figure 1.5: Schematic representation of the SmA and SmC.

1.2.3 Columnar Phase

The molecular arrangement forms columns rather than layers in this phase.

Because of the packing of molecules into columns, the phases are at least two- dimensionally positionally ordered systems with long-range orientational order, S ≈ 0.9

[28] of the columns. Usually disc-like molecules (see Fig. 1.6), like coins, exhibit the columnar phase (see Fig. 1.7(a)) because they can be naturally stacked into columns; their arrangement within an individual column can be either positionally ordered or disordered as shown in Fig. 1.7(b). However, some bent-core (banana shaped) liquid crystals also form the columnar phase [29-31]. The columns in this phase can have hexagonal, rectangular, or oblique packing, as shown in Fig. 1.7(c). Columnar liquid crystals are interesting as they allow transport of charge easily along the column axis because of a considerable overlap of orbitals of adjacent aromatic rings and lead to quasi-one-dimensional conduction [32, 33]. These days, several groups are trying to use them in photovoltaic solar cells [34].

hexa-alkoxy- R=CnH2n+1O hexa-alkanoyloxy benzenes R=CnH2n+1 [37] and hexa-alkanoyloxytriphenylenes

R=CnH2n+1COO [35, 36]

Figure 1.6: The first examples of thermotropic discotic mesomorphism.

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.

Table 1.1: Types of columnar alignment

Colho, Colhd Hexagonal ordered and hexagonal disordered

Colro, Colrd Rectangular ordered and rectangular disordered

Colobd Oblique disordered

Colt Columns are tilted

1.3 Requirement for the Formation of Liquid Crystal Phases

The common structural features of a mesogen are shown in Fig. 1.8. In general, thermotropic liquid crystal molecules consist of two or more aromatic rings A and A', connected by groups X, such as alkene (− CH = CH −), ester (− COO −), azo (−N =

N −), and having alkyl chains R and R' at the two rings [6, 38]. Thus, by building structures that include a relatively rigid part and one or two flexible end-groups, such molecules can acquire a rod-like statistical shape having geometrical anisotropy. The very general rod-like (calamitic) liquid crystals phases are shown in Fig. 1.9. If the molecule looks like a ping-pong ball (i.e., spherical), arranging them with any orientational order will be impossible. Essentially, sufficiently large molecular shape anisotropy is a prerequisite to form liquid crystal phases. Even though some compounds are geometrically suitable, such as the normal alkanes and the homologues of acetic acid, they exhibit no liquid crystalline phases for the simple reason that the attractive forces operating between the molecules are insufficiently strong to affect any order (e.g., parallel arrangement) [39].

Maier-Saupe and Onsager theory

In going from the I phase to the state of orientational order N phase, there must be a loss of entropy and lowered free energy associated with a freedom of the molecule to be oriented in any arbitrary direction [40]. Two approaches yield predictions about the nature of the transition between the I and N states by simple statistical models. The first approach is due to Onsager and ascribes the origin of nematic ordering to the anisotropic shape (rigid and elongated rod-shaped) of the molecules, i.e., to repulsive interaction. The 14

R, R′ : side chain and/or terminal group A, A′ : aromatic and cyclohexane rings

X : linkage group

R X R'

CnH2n+1 − − CH = N − R

CnH2n+1O − − COO − − C ≡ N

CnH2n+1COO − − C ≡ C − − Cl − CH = CH − − F − N = N − Figure 1.8: The basic structure of traditional liquid crystals.

4-pentyl-4'-cyanobiphenyl (Cr – 22.5oC – N – 35oC – Iso)

4-ethoxy-4'-hexyloxy-α-cyanostilbene (Cr – 54oC – N – 80oC – Iso)

Figure 1.9: Examples of calamitic liquid crystals.

second approach, which formulated by Maier-Saupe, states that the nematic ordering essentially originates from the anisotropic attractive interactions [41].

Onsager [42] developed a molecular theory with the following starting assumptions [6]:

1. Cylindrical rigid rods of length L and diameter D are long; L >> D

2. The only force of importance is steric repulsion; the rods cannot interpenetrate each other.

π L D2 3. The volume fraction of such a solution, ϕ = c (c = concentration of rods) is 4

L small, and the I-N transition present when ϕ ~ 4. D

According to Onsager, the excluded volume effect for a system of rod like objects is orientation dependent; as a result, the orientational entropy is reduced driving the system to the phase [43]. The free energy for a hard-rod solution can be expressed as follows [42]:

1 F = F + k T [∫ 푓(휃) log (4π푓(휃)c)dΩ + c ∬ 푓(휃) 푓(휃′) β (휃 휃′) dΩdΩ′] . 0 B 2 1

The second term accounts the loss in entropy associated with molecular alignment. The

′ third term describes the free energy due to the excluded volume effects; β1(휃 휃 ) is the volume excluded by two rods oriented at angles θ and θ′ wrt the orientation direction. For

′ 2 long rods, β1(휃 휃 ) = 2 L D|sin훾|, where 훾 is the angle between rods. The function f(θ) is the probability of the orientation of rods at angle, such that ∫ 푓(휃)dΩ = 1. No exact solution is known because f(θ) is an unknown function. Onsager used a variational

훼 method, based on a trial function of the form 푓(휃) = cosh (α cos휃) that 4π sinh훼 16

satisfies the normalization condition, ∫ 푓(휃)dΩ = 1, where α is a variational parameter that vanishes in the isotropic phase and is large in the N phase: αN ~ 18.84 at the coexistence of N and I phase. At around θ = 0 and π, the function f is more peaked. The order parameter (S) is

1 S = ∫ 푓(휃) (3cos2휃 − 1)sin휃 d휃 ≈ 1 − 3/훼 (α ≫ 1) . 2

Minimization of the free energy with respect to α shows that the transition from the I to the N phase is first order. The volume fraction occupied by the rods in the N and I

4.5D 3.3D phases are ϕ = and ϕ = [6, 42], respectively. N L I L

Maier and Saupe [44] considered an average attractive and orientation-dependent van der Waals interaction between the molecules in the nematic phase in a mean-field approximation by introducing an angular distribution function and self-consistency. The

Maier-Saupe theory predicts temperature dependence of the order parameter, a first-order phase transition at the I-N phase transition [6].

Thermal stability (usually defined by a lack of change in the N-I transition temperature with time) depends on the terminal substituents R and R′, on the central component X, and on the type / position of ring (benzene, cyclo-hexane, or bicycle- octane). During the 1980s a wide range of nonconventional molecular structures – discotic, banana shaped, or star shaped – were synthesized and shown to possess liquid crystallinity [45].

1.4 Properties of Liquid Crystals

The most important property of liquid crystals is the anisotropy of physical properties, i.e., the values of their physical properties depends on spatial directions. Using the director n as a reference, anisotropy of the dielectric and diamagnetic properties of the Nu phase can be described via a second-rank tensor [6]. By choosing an appropriate symmetry axis (like n // z-axis in Fig. 1.3(a)) and considering the symmetry of the nematic phase, one finds just two independent components of any second-rank tensor property χαβ = χ∥ and χ⊥ , εαβ = ε∥ and ε⊥ (where∥ means parallel to n; and ⊥, perpendicular to n). The difference between the two components is defined as follows:

Δε = ε∥ − ε⊥ is the dielectric anisotropy, and Δχ = χ∥ − χ⊥ is the diamagnetic anisotropy. The two anisotropies can be either positive or negative, depending on the structure of molecules and the nature of the nematic phase. The dielectric constants

ε∥ and ε⊥ are frequency and temperature dependent and are related to the indices of refraction of the medium at optical frequencies. With the Fresnel equation derived from

Maxwell equations for the propagation of light through the uniaxial medium [46], the birefringence, Δn = n∥ − n⊥ = ne − no can be obtained (where no = √ε⊥ is the ordinary refractive index and ne = √ε∥ is the extraordinary refractive index). When the extraordinary ray travels at a slower velocity than the ordinary ray, it indicates that the phase has a positive birefringence and vice-versa. Because the optical pathlength for light depends on the director configuration in a cell, this change can be controlled with an applied electric field, leading to the important applications of liquid crystals in display devices. 18

1.5 Characterization of liquid crystals

The nature of the LC phase can be characterized with several techniques. The most conventional one is the use of polarizing optical microscope and differential scanning calorimetry (DSC). Polarizing optical microscopy (POM) results in colorful textures that are characteristic fingerprints of the specific phase formed by the mesogen.

POM also reveals phase transitions through drastic changes in the texture with changes in sample temperature. With the DSC technique, the sample temperature is raised at a predetermined rate and compared to an empty sample holder. The extra thermal energy supplied to the sample to keep it at the same temperature as the reference depends on its specific heat and the latent heat of transitions. This technique reveals the existence of phase transitions in the form of a peak if the phase is first order and a kink or inflexion if the transition is second order. These two methods make a good combination for qualitative characterization. Applying other experimental techniques to make precise determination of the nature of phases is often necessary, so nuclear magnetic resonance

(NMR) and X-ray scattering are often used to determine the exact nature and symmetry of mesophases. With the help of X-ray scattering, one can obtain information about the arrangement of molecules, the layer orientation distribution, and the orientational order parameter, S [23]. Information about liquid crystal structure and dynamics can be obtained using NMR spectroscopy [47]. NMR observes specifically the nuclei of hydrogen or carbon-13 in a strong magnetic field. Because the result depends on its chemical environment, one can estimate the structure of molecule. Usually spin 1/2 materials (e.g., 1H, 13C) are used for NMR, but Deuterium (2H) NMR spectroscopy can 19

also be applied to determine the symmetry of a nematic phase, especially for the biaxial nematic system [15, 48]. The infrared (IR) and Raman are both vibration spectroscopy techniques. One can investigate specific interactions, such as hydrogen bonding using polarized FT-IR and polarized Raman and biaxiality nematic order parameters [49], and the bent-core material, C5-Ph-ODBP-Ph-OC12 [50], by tuning to specific molecular vibrations.

1.6 Motivation and Outline of Thesis

The dissertation research reported here is on hydrogen-bonding mesogens, which are also known [45, 51] as phasmidic mesogens because of a molecular shape. Such mesogens have been known to form LC phases and have been previously investigated to see whether they form the biaxial nematic phase. During preliminary X-ray-scattering experiments, unconventional diffraction patterns from their nematic phase were observed.

The nematic phase of these materials [51] is very interesting. Usually, the X-ray diffraction pattern of a magnetic field aligned nematic phase shows two pairs of crescent- shaped reflections, one at small angle and parallel to magnetic field and the second pair at large angles and perpendicular to the magnetic field. The two pairs of reflections are in mutually orthogonal directions and corresponding to the average molecular length and width, respectively. However, for the nematic phase of these mesogens, the small and large angle peaks are in the same direction and perpendicular to the magnetic field! This is quite puzzling and stimulated our curiosity. Furthermore, the length scales calculated from the diffraction pattern do not correspond directly to the length of the molecules or of a simple linear dimer! They are difficult to explain and suggest that interesting chemical 20

interactions and physical organization of mesogens are taking place. We undertook detailed X-ray investigations of the nematic, smectic, and columnar phases of these mesogens and their mixtures with mesogenic molecules that chemically resemble sections of the phasmidic molecules. We applied X-ray diffraction, POM, DSC, dielectric constant, conoscopy, FT-IR, and electro-optical measurements to understand their phase behavior.

In chapter 2, we will briefly address hydrogen bonding, give an overview of hydrogen bonded liquid crystalline materials and their assemblies, and introduce the materials used in this research. Chapter 3 provides a description of the experimental techniques, namely, X-ray diffraction, polarizing optical microscopy, capacitance measurement, and conoscopy employed in this dissertation research. In chapter 4, we propose possible models of the dimer-formations to explain experimental result in pure compounds. Results of electro-optical experiments are also presented and discussed to elucidate their response of these materials to applied electric field. In chapter 5, we introduce binary mixtures of hydrogen-bonded liquid crystals and discuss their mesophase sequence and structures. Chapter 6 contains a summary of the results and suggestions for future research towards better understanding them.

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MATERIALS and PHASE CHARACTERIZATION

2.1 What’s a Hydrogen bond?

Hydrogen bonding occurs via attractive intermolecular interactions between molecules having partial electric charges of opposite polarity at certain locations. As the term hydrogen bond implies, a hydrogen atom is always involved in this type of bonding.

When a hydrogen (H) atom is attached to a more electronegative hetero atom (perhaps, covalently), then the electron cloud of hydrogen moves slightly toward this electronegative element, leaving the H atom in a partially positively charged state. If an electronegative atom exists in proximity to a second molecule, the positive H forms the

H-bond with it (e.g., see COOH Fig. 2.1). A typical hydrogen bond [1] can be expressed as X − H + Y − Z → X − H ⋯ Y − Z, where the three dots denote the bond. And X, Y

= nitrogen, oxygen or fluorine, and X and Y can be same or different. X − H is the hydrogen bond donor, and Y, or a fragment or a molecule Y − Z is the hydrogen bond acceptor, where Y is boned to Z. The strength of the hydrogen bond depends on the donor and acceptor as well as their environment. H-bond energies, which can range from 60 to over 155 kJ/mol for strong bonds, 10 – 60 kJ/mol for moderately strong bonds, and 1 –

10 kJ/mol for weak bonds, are lower than the energies associated with ionic and covalent bonds but significantly higher than Van der Waals interaction energies 0.1 – 1 kJ/mol [2,

3]. Water molecules are well known for hydrogen bonding with one another. Hydrogen

bonding also plays an important role in the structure and functioning of biological macromolecules, such as proteins and DNA [2].

Figure 2.1: Hydrogen bonding in carboxylic acids, electronegative oxygen atom and partially positive hydrogen atom attract each other.

2.2 Hydrogen bonded liquid crystals

In thermotropic liquid crystals, most of the mesogenic molecules consist only of covalent bonds; however, more complex and new molecular associations [4], such as 27

elongated molecules formed by bridging of two rod-like molecules, are produced. In particular, one of the interactions responsible for intermolecular attractions causing the bridging is hydrogen bonding. The history of hydrogen bonding in liquid crystals started in early 20th century [5]. However, little progress has since been made as demonstrated by the limited number of publications and reviews [6 - 9]. Typical examples are p-n- alkoxybenzoic acids and p-n-alkoxycinnamic acids, Figs. 2.2(a, b). These compounds form dimers through H-bonding and exhibit stable nematic and smectic mesophases [10 -

12]. A number of new studies on compounds displaying liquid crystalline phases induced by hydrogen bonding have appeared in the past 20 years [6, 13].

2.2.1 Molecular Assembly

The association of molecules via H-bonding is a form of self-assembly, since the structure in liquid crystal system can be assembled in variable formations if the hydrogen bond donor and acceptor exist. These associated complex molecules then undergo arrangement into different structures to form the liquid crystal phases. The various liquid crystals formed by H-bonding can be classified as follows: molecular assemblies of identical or different small molecules and polymeric liquid crystals (illustrated in Fig.

2.3). T. Kato and his coworkers have successfully extended the range of H-bonded liquid crystalline materials [11, 12, 14 - 16]. They obtained liquid crystals called supramolecular

H-bonded liquid crystals, which have a greatly enhanced mesomorphic range through the formation of intermolecular H-bonds between two dissimilar mesogens, such as H- bonded complexes resulting from an equimolar binary mixture of trans-p-[(p- ethoxybenzoyl]oxy]-p′-stilbazole and p-alkoxybenzoic acids and 2:1 H-bonded complexes 28

derived from p-hexyloxybenzoic acid (mesogen) and bipyridines (no mesogen).

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. (Here, three dots express the H-bond.)

Rod-like Mesogens

The linear and rigid structure of the complex formed by H-bonding is achieved through the connecting part of a single or a double hydrogen bond. On one hand, the series of benzoic acids and cinnamic acids exhibit mesomorphism resulting from dimerization of the two identical molecules as we noted above, Figs. 2.2(a, b) and Fig.

2.3(a). On the other hand, H-bonding between dissimilar molecules is shown in Fig. 2.2(c) 30

and depicted in Fig. 2.3(b) [15, 17]. This structure is obtained by a 1:1 association of p- butoxybenzoic acid and trans-[p-ethoxy(benzoyl)oxy]-4′-stilbazole. The benzoic acid unit works as the H-bond donor; the pyridyl part works as the H-bond acceptor. The nematic phase of the H-bonded complex appeared from 160 to 238oC. The structure in Fig. 2.2(d) is formed by the complex formation between p-n-alkoxybenzoic acids and p-octyl pyridine [15]. Even though individual components do not show a mesophase, the 1:1 mixture of these compounds, as shown in Fig. 2.2(d), produced the nematic phase with an electro-optical response below 50oC.

Bent-core like Mesogens

Many bent-core or banana or boomerang-like molecules are associated with exhibiting mesomorphism, some of them via H-bonding. Fig. 2.4 shows two examples of bent shaped assembly: One is built through intermolecular hydrogen bonding between phthalic acid and stilbazole derivative in 1 : 2. The phthalic acid itself is nonmesogenic, however, the assembly as in Fig. 2.4(a) shows nematic phase between 100oC and 130oC

[14]. It is interesting that nonlinear structure compounds consisting of covalently bonded material do not show liquid crystallinity [14]. Another bent shaped example is shown in

Fig. 2.4(b), the assemblies were prepared by the use of a V-shaped stilbazole derivative

(non-mesogen) and a benzoic acid derivative, which act as H-bond acceptor and H-bond donor, respectively. This unsymmetrical complex exhibits one mesophase, a smectic polar phase, between 89 – 122°C [18, 19].

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.

Disk-like Mesogens

Disk-like mesogens can form via assembly of multiple molecules of one compound in a specific manner. Fig. 2.5 shows the self-assembling discotic mesogens that form a dimer of tetrakis(n-alkoxy)-6(5H)-phenanthridinones with variable R = n- octyl, n-decyl, n-dodecyl [20]. These compounds exhibit a columnar phase between 70oC and 115oC. The possibility of trimer formation also exists through hydrogen bonding. The trimer is assembled from three phthalhydrazide derivatives [21] and exhibits columnar 32

mesophase between 100 and 270°C, depending on the length of the alkyl chains. The structural parameters of the columnar arrangement of these molecules is influenced by the length of alkyl chains, a rectangular columnar structure for n = 6 and hexagonal when n = 14. Disk-like molecule based on the 3:1 hydrogen-bonded complex are also prepared by connecting long chain alkoxy-substituted benzoic acids to around a tribasic core, such as a trisimidazoline [22]. The complex thus formed is a simple and flexible liquid crystal molecule with columnar mesophases in 65oC – 240oC range, which depends on the length of the alkoxy chains.

Figure 2.5: Disk-like complex formed by two molecules of tetrakis(n-alkoxy)-6(5H)- phenanthridinone are self-assembled by hydrogen bonding.

Polymer and Polymer Network

The complexes of side-chain or main-chain polymers and networks have been formed with H-bonding [11, 17, 23, 24]. Fig. 2.6(a) is an example of a side-chain polymer created by mixing the biphenyl core based functionalized mesogenic molecule with poly(4-vinylpyridine) so that forms H-bonding between carboxylic acid group and 33

pyridine [25]. This complex shows the SmA phase over temperatures 125 – 140oC. Fig.

2.6(b) shows a liquid crystalline network by the self-assembly of polyacrylate and 4,4'- bipyridine [26]. The SmA phase exists from 95oC to 205oC on cooling and heating resulting from the contribution of hydrogen bonding [5], but the covalently bonded polymeric networks do not participate in the phase transition because of the lack of flexibility of the structure. A liquid crystalline network was also prepared by self- assembly, involving multifunctional components as illustrated in Fig. 2.6(c). This complex was created from a trifunctional H-bonding donor (c)-1 and a bifunctional H- bonding acceptor (c)-2 and exhibits a smectic phase [27].

(Figure is continued next page) 34

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'-bypyridine, (c) a schematic illustration of smectic network formation (c)-3 by hydrogen bonding of trifunctional compound (c)-1 and bifunctional bipyridine (c)-2.

2.2.2 Materials used in this project

Acid functionalized azo compounds with multi alkoxy chains were synthesized by

Dr. Veena Prasad at the Centre for Liquid Crystal Research, Bangalore, India [28]. These compounds are expected to form dimers by intermolecular H-bonding because of the carboxylic acid unit, and lead to liquid crystalline behavior. A series of azo compounds, mainly 4-[2, 3, 4-tri(octyloxy)phenylazo] benzoic acid and 4-[2, 3, 4- tri(heptyloxy)phenylazo] benzoic acid, were used; their molecular structure is shown in

Fig. 2.7. From this point in the text, the abbreviated names shown in Fig. 2.7 will be used for these materials. From a structural view, we call these materials, formed as a dimer, phasmidic mesogens. Phasmidic liquid crystals are generally composed of a rod-like core of aromatic linkages and two half-disk-shaped parts, such as with three terminal flexible substituents at both ends [4].

n Name of compounds Abbreviation

8 4-[2, 3, 4-tri(octyloxy)phenylazo] benzoic acid TOPAB

7 4-[2, 3, 4-tri(heptyloxy)phenylazo] benzoic acid THPAB

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.

2.3 Differential Scanning Calorimetry

2.3.1 Background

Different states and phases of matter have their characteristic symmetries, and when one phase of matter transitions to another phase under some thermodynamic variable (e.g., temperature), one observes anomalies in the heat capacity vs. temperature graphs. This mostly is related to a (sudden) change in the symmetry and/or the incipient order of the two states. According to thermodynamics, a stable state results when the

Helmholtz free energy F = U – T S, where U is the internal energy and S is entropy [31], is at a minimum. Different phases and phase transition result from the competition between the system’s internal energy term, which favors order and entropy, and the entropy term, which favors disorder. At first-order phase transitions, the system exhibits a discontinuity in the first derivative of the free energy (dF) with respect to a thermodynamic variable (temperature, volume, and the number density of molecules). A second-order transition is continuous as is the first derivative of the free energy across the transition, but the second derivative changes discontinuously. First-order transitions have an associated latent heat, also known as the enthalpy of transition. Second-order transitions do not have associated latent heat and are relatively more difficult to investigate with the DSC method.

Landau-de Gennes theory

The nature of phase transitions is related to the symmetry of the two phases and the order parameter that describes the transition. To describe thermophysical properties at 37

the N-I transition, three theories are mainly used: the phenomenological Landau-de

Gennes, Maier-Saupe mean field theory, and molecular statistical theory [32]. The simplest description of the nematic (N) to isotropic (I) transition is given by the Landau- de Gennes theory [32 - 35], in which the free energy density of the N phase (2.1) is expanded in a power series of the nematic tensor order parameter (Q⃡ ) with an assumption that the order parameter is small and invariant under certain symmetry operation.

a(T) b c 2 c′ F (T, Q⃡ ) = F + Tr[Q⃡ 2] − Tr[Q⃡ 3] + Tr[Q⃡ 2] + Tr[Q⃡ 4] + … (2.1) 0 2 3 4 4

δ Q⃡ = Q = S (n n − αβ ) , δ = 1 for α = β and zero otherwise (2.2) αβ α β 3 αβ

Here, F0 is the free energy density of the isotropic phase, a(T) is linearly dependent on temperature, a(T) = A(T-T*), and S is the scalar order parameter mentioned in the previous chapter. The value a, b, c, and c' are numerical coefficients, and T*, a temperature below the N-I transition temperature (TNI), representing the limit of metastability of the isotropic phase. With the requirement of Q⃡ being traceless, equation

(2.1) can be expressed as

A(T−T∗) B C F = F + S2 − S3 + S4 + … . (2.3) 0 2 3 4

Here, A, B, C are temperature-independent positive constants. From minimization of the

∂F free energy density with respect to S, that is, setting ( = 0), two solutions of S and TNI ∂S are derived as follows:

B 1−4A(T−T∗)C S = 0 (the isotropic phase), 푆 = *1 ± 푤 +, 푤 = √1 − . (2.4) ± 2C B2

The value of S is smaller than zero in the case where molecular symmetry axis is oriented 38

perpendicularly to the director. Generally, the solution S > 0 in the positive N phase at

* points T < T . From S+, we can obtain S in the nematic phase:

2B 2B2 S = > 0, T = T∗ + (2.5) 3C NI 9AC

† And also another temperature Tc is achieved by putting w in eq. (2.4) equals zero.

B2 T† = T∗ + (2.6) c 4AC

† Tc is the absolute limit of overheating of the N phase. The symmetry of the N phase requires that B is nonzero, and consequently the transition is first order. We can describe three regions as follows:

† At T > Tc : the isotropic phase is stable, S = 0

† At TNI < T < Tc : the isotropic phase is the thermodynamically stable, S+ develops

* At T < T < TNI: nematic state is stable, S > 0.44

Generally, the I – N and I – Smectic transitions are of first-order [30, 33, 36], whose order parameter, density, and enthalpy differences change discontinuously at the

∂H phase transition; and the specific heat capacity ,Cp = ( ⁄∂T)p] is almost independent of temperature. The NU – NB, N – SmA, and SmA – SmC phase transition are either

(weakly) first order or second order [37]. In this case, the Cp shows either a discontinuous jump or singular behavior at the critical point [30]. The second-order transition includes continuous changes in other materials properties, diverging correlation length (N – Sm), etc. In DSC measurement, one can determine the type of order from the magnitude of the enthalpy change and changes in Cp or density with temperature at the transition point. 39

2.3.2 DSC Experiment

First piece of information that is needed on a new compound is the number and type of phases formed by it. Among the initial measurements that one makes, thermal analysis methods, such as Differential Scanning Calorimetry (DSC), Heat Capacity (Cp),

Thermal Gravimetric Analysis (TGA), and Thermomechanical Analysis (TMA) [28], are very useful. A specific method can be selected based on the physical property that is to be measured as a function of temperature. For examples, TGA analysis yields changes in the weight of a material as its temperature changes; and TMA measures the change in the dimensions (length or volume), or a mechanical property of the sample as a function of temperature. DSC measures the temperature dependence of the heat capacity and the latent heat associated with the phase transitions of a material. DSC is the most commonly used thermal technique in liquid crystals [30]. We can measure a quantitative change (e.g., enthalpy, heat capacity), using the differential scanning calorimetry technique as described below. The schematic of DSC instruments [38] is shown in Fig. 2.8. The sample and reference are heated in different pans (containers) with a constant heating/cooling rate in the same temperature-controlled enclosure. When the sample undergoes a phase transition, it requires additional power (energy) to raise its temperature at the same rate as the reference. By measuring the difference in heat flow between reference and sample, DSC is able to measure the amount of heat absorbed/released by the sample at the transitions during heating/cooling. Nitrogen is preferred as a purge gas because of its inertness. The measured quantity with this method, that is, the differential power needed is a measure of the enthalpy (ΔH) of transitions, which is the sum of latent 40

heat and integral of the heat capacity of the sample across the transition.

For DSC experiment, manufacturer-provided aluminum pans are used to enclose and seal the sample while an empty cell is used as the reference. The DSC equipment (TA

Instruments, 2920 Modulated DSCTM) in the laboratory of Prof. R. Twieg of the

Department of Chemistry was used, and nitrogen was used as the purge gas. Both cells are sealed, using a crimping tool specifically made for this purpose, after placing the lids on pans. The resulting heat flow is plotted against temperature in Fig. 2.9 and Fig. 2.10.

Figure 2.8: The schematic figure of differential scanning calorimeter (heat flux type).

2.3.3 Result of DSC

The samples were scanned by a rate of 5oC/min. Fig. 2.9 shows the results for

TOPAB, and Fig. 2.10 for THPAB. Both samples show two phase transitions on heating and three phase transitions on cooling; phase transition temperatures and enthalpy changes (kJ/mol) on cooling are given below.

TOPAB I 119.8oC (0.99) N 92.8oC (3.2) Col 82.2oC (21.0) Cr

THPAB I 127.5oC (0.89) N 109.7oC (7.7) Col 95.0oC (18.9) Cr I = isotropic, N = nematic, Col = columnar, Cr = crystal phase

Phase identification was done on the basis of their POM textures. Textures of

TOPAB are present in Fig. 2.11. These materials exhibit the N phase above the columnar phase. Actually, we cannot make a determination of the type of phase we have at hand solely on the basis of DSC results. Integration of peak area is the measure of the enthalpy change associated with the corresponding transition. ΔH at the I-N transition is rather small compared to typical melting/crystallization enthalpies because the I-N transition is weakly first order [40]. The DSC graphs show large ΔH at the transitions from the N to

Col and the Col to Cr phase. The results for a series of azobenzoic acid homologs (the molecule with just one flexible chain of different chain length, n, shown in Fig. 2.12) are summarized in Table 2.1. This series of azobenzoic acid forms dimers by H-bonding, and

o the mesophases appear at higher temperatures with TNI = 260 C for n = 8, compare to

TOPAB or THPAB. Definitely, two extra flexible chains strongly influence the transition temperatures and type of phases; the columnar phase appears in TOPAB and THPAB 42

instead of the smectic phase. However, when COOH group in mesogen TOPAB is replaced by COOCH2CH3 (Fig. 2.13), no mesophases form. Obviously, the carboxylic acid unit, COOH, appears to be important for displaying liquid crystal phases. Certainly,

COOH enables the molecules to form multimolecular associations (i.e., dimer, trimer, etc.) by intermolecular hydrogen bonding and exhibit liquid crystal phases. We investigated, for this dissertation research, the role of such associations in mesomorphism.

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).

I at 122oC N at 115oC Col at 90oC Cr at 80oC

Figure 2.11: POM textures of TOPAB from the I to Cr phase.

Figure 2.12: Compounds belonging to the azobenzene series.

Table 2.1: Phase transition temperatures and phase sequence for the azobenzene derivatives [39]. The numbers in parentheses show the enthalpy change (kJ/mol) at the transition. n (number of methylene units Phase sequence in the alkyl chain)

n = 3 Iso 288oC (5.1) N 250oC (20.0) Cr

n = 5 Iso 276.6oC (5.4) N 229.1oC (16.6) Sm2 166.4oC (1.8) Cr

n = 7 Iso 266.1oC (6.3) N 231.8oC Sm1 229.7oC Sm2 153.9oC (5.6) Cr

Iso 248.7oC N 247.9oC Sm1 215.6oC (7.6) Sm4 171.1oC (10.3) n = 12 Sm5 145.3 oC (6.5) Cr

Iso 237.2oC (19.1) Sm3 204.8 oC (7.7) Sm4 146.4 oC (9.8) Sm5 n = 18 138.6 oC (9.7) Sm6 111.0 oC (11.7) Cr

Iso 262.2oC (5.0) N 239.8oC (3.3) Sm1 224.3oC (8.4) Sm2 n = 8 220.2oC (4.6) Sm3 212.2oC (2.2) Sm4 157.1oC (5.4) Cr

Figure 2.13: The molecule, obtained by replacing COOH in TOPBA by COOH2CH3, shows no mesophase.

2.4 Fourier Transform Infrared (FT-IR) Spectroscopy

2.4.1 Background

The vibrational spectrum of a molecule is a unique characteristic of the molecule

[41] much like the fingerprints of a human being. Even unknown chemicals can be identified by an inspection of their vibrational spectra because chemical bonds in different environments will absorb/emit specific intensities at varying frequencies.

Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. In order to absorb IR radiation, a molecular vibration must cause a change in a dipole moment of the molecule or the functional group. And the intensity of an IR absorption band is dependent on the magnitude of the dipole change during the vibration; the larger this change, the stronger the absorption band [41 – 43].

Asymmetric molecular vibrations can be detectable by the IR method. However, for a non-linear molecule like H-O-H (shaped like a V) there is a change in the dipole moment for a symmetric stretch so the vibration is IR active.

IR absorption positions are generally presented either as wavenumbers (cm-1) or 46

wavelengths (cm). Wavenumbers are directly proportional to the frequency and the energy of the IR absorption.

The primary goal of our FT-IR spectroscopic analysis in this project is to determine the presence of hydrogen bond in the mesophase(s) of TOPAB. Hydrogen bonding has important signature in IR range because it influences the bond stiffness and alters the frequency of vibration. The free (non-hydrogen bonded) C=O stretching band of the carboxylic acid group appears around 1750 – 1720cm-1, but hydrogen bonding tends to decrease its frequency below 1700cm-1.

2.4.2 Sample preparation and Experiment

The material TOPAB was filled into sandwiched cells at temperatures above the clearing point. Two CaF2 (Calcium Fluoride, 1 mm thick) plates were used in making the sandwiched cell for IR measurements. The plates were cleaned and spin coated with a conventional homogeneous polyimide, followed by rubbing so that the material was uniformly aligned in the N mesophase. Two plates were assembled with a 4 m gap. The cells were placed inside an Instec® hot-stage (HCS402) controlled by an Instec temperature controller (STC20A). The IR spectroscopic measurements were performed, using a Bruker® Tensor series FT-IR spectrometer in the laboratory of Prof. A. Gericke of the Department of Chemistry at different temperatures. The sample was scanned from

4000 cm-1 to 400 cm-1 (mid IR region) with a resolution of 4 cm-1. We took IR spectra first at room temperature (crystal phase) and at 125oC (isotropic phase). After that, the sample was cooled from 125oC to 80oC through the N and Cr phases since there is no Col phase observed. Later, the sample was heated from 135oC to 240oC. We waited for 3 47

minutes to allow the sample to equilibrate at every temperature before taking IR spectrum.

2.4.3 Result and Discussion

The IR absorption graph of the CaF2 plate used as the substrate is presented in Fig.

2.14. The graph shows the transmission through the CaF2 plate at frequencies above 1050 cm-1. Because it has strong absorption bands at frequencies below 1050 cm-1 and this absorption distorts the vibration bands of the liquid crystal, we have to consider only the region of higher frequencies, i.e. > 1050 cm-1.

Fig. 2.15 shows the IR absorption spectra of TOPAB in the N phase at 115oC.

Main characteristic bands that appear [42, 44 – 47] are as follows: CH2 in the chains asymmetric stretching bands at 2800 – 3000 cm-1, carbonyl C=O stretching bands in hydrogen-bonded COOH dimer at 1680 – 1700 cm-1, O–H (hydrogen bonded) stretching broad bands of hydrogen-bonded COOH at ~ 3000 cm-1, and carboxylic acid monomers with a weak sharp band at 3550cm-1 [42, 44]. Fig. 2.16(a) shows the spectra in the region

1600 – 1800 cm-1 from 30oC in the Cr phase to 240oC in the I phase. Upon cooling, the

C=O (hydrogen bonded) band shifts from 1696 cm-1 to 1694.5 cm-1 at the I to N transition and from 1693 cm-1 to 1689 cm-1 at the columnar to crystal phase. Furthermore, this band gradually shifts and merges with the band at higher (1720.2 cm-1) wavenumbers with increasing temperature, at 230oC. The temperature dependence of the wavenumber is shown in Fig. 2.16(b). The dissociation of hydrogen bond depends on temperature but, occurs nonlinearly as we observed the jump of wavenumber near phase transitions.

However, the stability of hydrogen bond depends on the molecular orientations [48]. We also observe the band at ~ 1720 cm-1 at all temperatures. We can consider this as the free 48

C=O band (monomer or open dimer), that is, a possibility of nonhydrogen-bonded C=O in COOH exists. In addition, a weak broad band at around 3300 - 3350 cm-1 indicates open dimers [46]. As the temperature is increased (Fig. 2.17), this broad O-H band diminishes as do the bands in the 2500 cm-1 to 2700 cm-1 region, indicating the weakening of H-bonds. A very small peak at 3550 cm-1 clearly appears and reflects free

OH.

It is clear from FT-IR results that the functional group carboxylic acids in TOPAB is associated via H-bonded dimers in the crystal, columnar, and nematic phases and above the isotropic phase up to 200oC, and all H-bonds dissociate at 230oC.

-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 – 1000cm-1. Main characteristic bands’ assignment is described in Table 2.2.

Table 2.2: The description of main characteristic bands’ assignment in Fig. 2.15.

Bands’ Description [41, 43 - 46] number ① O-H stretching broad peak ② C-H stretching at aromatic ring ③ C-H stretching of alkoxy chains O-H combination bands of lower frequency vibrations enhanced by ④ Fermi resonance with the broad OH stretching peak ⑤ C=O stretching dimer of COOH ⑥ C=C stretching band aromatic ring ⑦ C-O stretching of COOH ⑧ O-H stretching (weak, from monomer)

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.

The preliminary POM, DSC, and IR experiments summarized above have provided us the information regarding the transition temperature and tentative phase identification for various mesogens. These studies were conducted for most of the materials used in this research. In the following chapters, we discuss more elaborate experimental techniques to understand the phase structures and molecular configurations to gain deeper understanding of these systems.

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CHAPTER 3

EXPERIMENTAL TECHNIQUES

The techniques used in this dissertation project include Polarizing Optical

Microscopy (POM), X-ray diffraction, conoscopy, capacitance measurements, and polarized Raman scattering, which were conducted to characterize the hydrogen-bonding mesomorphic material. This chapter includes a discussion of several of these experimental techniques.

3.1 Polarizing Optical Microscopy

POM, a general technique and an essential tool in the liquid crystal area, is used to identify the LC phases and phase transition temperatures via their characteristic optical textures and temperature-induced changes in them. We can also easily observe the behavior of material under an applied external field (e.g., electric field). Generally, a polarizing microscope has a fixed polarizer between the source of light and the sample and a second rotatable or removable polarizer after the sample. These are called polarizer and analyzer, respectively [1]. In addition, we can insert a specific wavelength filter, a retardation plate, or a Bertrand lens as needed for other studies. We can capture the image with a CCD camera mounted on the microscope and save it in digital form. We investigated optical textures of TOPAB, THPAB, and several of their mixtures) discussed later) as a function of temperature to characterize the phases and to learn how these compounds align on treated surfaces.

3.1.1 Sample Preparation

Sandwich cells of the sample were prepared using glass plates coated with indium tin oxide (ITO) to allow the application of electric field. The glass plates were cleaned in an ultrasonic bath (Branson 5510), using a surfactant (Branson OC-concentrated formula) at 45 oC for 1 hour, then rinsed with deionized water (di-water) followed by a solution of acetone-methanol-isopropanol. After these steps, they were dried in vacuum at 100oC for

1 to 2 hours. Polyimides, SE7792 for homogenous alignment and S60702 and S659 for homeotropic alignment, were spin-coated onto the ITO side of the plates at two continuous coating speeds of 800 rpm for 5 seconds and 3000 rpm for 40 seconds. Spin- coated plates were prebaked at 95oC for 1 minute to evaporate remaining solvent and then hard-baked in a conventional oven at 210oC for 50 minutes. When needed, the plates were rubbed with a velvet cloth; and cells were assembled with antiparallel rubbing directions for homogenous alignment. Four m diameter fiber spacers were mixed in UV curable epoxy and applied to two edges of one plate and covered with the second substrate. The cells were then exposed to a UV source (Electro-Light Corporation, ELC

4001) operated at 400 watts of electrical power for 4 to 5 minutes to irradiate with

125mW/cm2 of 365 nm radiation. The liquid crystal was finally filled into the sandwiched cell by capillary action at a temperature well above the clearing point. For in- plane switching operation, we used cells made at Samsung Electronics in Korea.

3.1.2 Experimental Setup

The experimental setup for texture observations is shown in Fig. 3.1. A polarizing optical microscope (Olympus BX51-P) with crossed polarizer and analyzer was used for 57

texture observations. Sandwiched cells were placed in a Mettler FP82HT hot-stage, controlled by the FP90 central processing unit (Mettler Toledo) and mounted on the rotation stage of the microscope. The SPOT Insight digital camera (Diagnostic

Instruments, Inc., Model 18.2 Color Mosaic) was attached to the top of the polarizing microscope so that textures could be captured using SPOT software, version 4.5. For optical observations, we avoided high intensity because the azobenzene in some of these materials can be transformed from the low energy trans to cis configuration by light exposure and affect phase transition temperatures. A 1 kHz sine wave electric field was applied to the sample, using a function generator (SRS DS345) and amplifier (A.A. LAB system, A-301 HS HV amplifier) to observe the effect of the electric field and measure dielectric tensor components and frequency response.

Figure 3.1: Optical microscopy experimental setup for texture observations.

3.2 X-ray Diffraction

3.2.1 Background

X-ray scattering is a powerful structural analysis technique to determine the type of phase and the structural changes that occur at phase transitions [2, 3]. When an X-ray beam strikes electrons in an atom, the electrons oscillate with the frequency of the incident X-rays and reradiate energy at the same frequency with spherical wavefronts.

The reemitted radiation propagates over space and undergoes interference to either constructively or destructively, producing a diffraction pattern depending on the periodicity (or, structure) of the phase.

The basic rule of X-ray scattering is the Bragg’s law, expressed as 2d sinθ = n λ

[3]; here d is the separation of the lattice planes or atoms, θ is the angle of incidence (i.e., between the incident wavevector and the scattering lattice planes), n is an integer that indicates the order of the reflection, and λ is the X-ray wavelength, Fig. 3.2. To observe a particular diffraction peak, the Bragg condition must be satisfied; thus the planes need be oriented at the correct angle with respect to the incident beam [2]. The angle between incident and scattered beam is the diffraction angle 2θ. The momentum transfer or scattering vector q⃗ represents the change in the wavevector of the X-ray beam;

⃗ ⃗ ⃗ ⃗ q⃗ = ks − ki, where ks, ki are the wavevectors of scattered and incident radiation,

2π 4π respectively. Its magnitude is expressed as |q⃗ | = = sinθ, and the magnitudes of d λ

2π the two wavevectors are equal ( |k⃗ | = |k⃗ | = ) for elastic scattering. We denote i s λ

|q⃗ | = q in this thesis. Values of q are reciprocally related to the separations between 60

lattice planes; for example, q = 2π/a when a is a distance of certain periodicity; so the smaller the spacing, the larger the diffraction angle 2θ. We can observe the order of q, that is, qn= n (2π/a); if we have a very well-aligned sample (i.e., a perfect crystal), the value of n can be any number. The sharpness of the peaks (characterized via its half width) yields the distance (correlation length) over which the periodicity extends; the sharper the peaks, the larger the correlation length.

Figure 3.2: The Bragg condition: Constructive interference of X-rays reflected from successive planes occurs when the path difference is a multiple of the wavelength.

Bragg’s law predicts the direction of any diffracted ray and d-spacing but not the intensities and quantitative shape of the peaks. X-ray scattering is related to spatial correlations of the electron density within the sample. Because of the periodicity of the electron density distribution, Fourier transform of the structure [3, 6] determines the distribution of scattered X-ray intensity. In general, the scattered intensity (퐼(q⃗ )) is expressed by the product 퐹(q⃗ ) x 푆(q⃗ ) [7]. Here, 푆(q⃗ ) is the structure factor, which is related to the ensemble of molecules; and 퐹(q⃗ ) is the form factor, 퐹(q⃗ ) = < |푓(q⃗⃗⃗ )|2 >, which corresponds to the average of the modulus square of fourier transform of the electron density distribution within a single molecule or the basic unit forming the structure. Assuming a rod-shaped molecule of length L and radius R with uniform electron density is aligned in the z direction, the function 푓(q⃗ ) can be expressed with a use of cylindrical coordinates system as [8]

iq L e z −1 퐽1(q⊥R) 푓(q⃗ ) = 푓(q⃗ z) × 푓(q⃗ ⊥) = ρ × , iqzL q⊥R where, J1(x) is a Bessel function of the first kind (cylindrical) of order 1, q is the

2 2 wavevector, and q⊥ = √qx + qy . For the distribution of the molecules along the director n in the nematic phase, by introduction of a Maier-Saupe-like distribution function, f (θ), the form factor is expressed by

π 퐹(q⃗ ) = 2 |푓(q⃗ )|2 푓(θ) sin θ dθ , ∫0 where θ is the orientation angle of one molecule with respect to the director n. The form factor 퐹(q⃗ ) depends on the distribution of the molecular charge. Thus, if molecules are randomly oriented, 퐹(q⃗ ) is spherically symmetric; however, if molecules are aligned in 62

a certain direction, 퐹(q⃗ ) shows strongly anisotropic [8]. The structure factor in the vicinity of the small angle reflection for an aligned nematic can be approximated by a

Lorentzian and can be expressed using three positional correlation lengths: ξx, ξy, and ξz;

1 S(q⃗ ) ∝ 2 2 2 2 2 2 , 1 + ξz (qz − q0) + ξx qx + ξy qy

2π where q = and corresponds to the average molecular length. 0 d

3.2.2 Sample Preparation and Experimental Set up

Our X-ray experiments were carried out at station 6-IDB of the Midwestern

Universities Collaborative Access Team [9] at the Advanced Photon Source of Argonne

National Laboratory. The incident X-ray beam energies in the range from 30 to 130 keV are available at this station; we used the energy 16.2 keV [λ = 0.7653 Å ] in our measurement. Monochromatic X-rays are incident along the direction parallel to the x- axis as shown in Fig. 3.3. Intensity of the incident beam was controlled using a bank of

Cu and Al attenuators. X-rays passed through samples placed in sample chamber with an in-situ magnetic field to align the director. The diffraction patterns were recorded at different temperatures during cooling from the I phase, using a two dimensional high- resolution image plate detector, MAR345, placed at a distance of 500 – 520 mm, depending on the experimental cycle. A beam stop was mounted in front of the image plate detector to block the direct X-ray beam and protect the detector from overexposure and possible damage to the detector.

Figure 3.3: XRD experimental set up: X-ray beam passes perpendicular to the magnetic field, and through the sample, diffraction pattern is collected by an image plate detector.

(a) X-ray diffraction pattern of silicon (b) Plot of 2θ vs. intensity

Figure 3.4: Silicon standard used for the calibration of experimental setup: (a) XRD of the material, (b) plot of 2θ vs. intensity generated from the data.

In order to perform X-ray experiments on a bulk sample, one typically uses a

Lindéman capillary with a diameter of 1 – 1.5 mm and 10 m thick wall. In order to obtain a uniformly aligned nematic phase, the magnetic field of strength of 2.5 kG was produced at the sample by placing a pair of rare-earth magnets across the sample. The magnetic field direction was perpendicular to the direction of propagation of the incident

X-ray beam as shown in Fig. 3.3. The sample was placed in a modified Instec hot-stage with a temperature accuracy of ±0.1oC, controlled by a STC200 controller connected to a computer. We attached thin Kapton film on the outside of the openings in the oven to avoid heat loss and temperature gradients caused by convection. Actually, the broad peak 65

around 15.5 Å in some of the scans arose from the Kapton film. The sample was first heated to the isotropic state (e.g., above 120oC for TOPAB) and then slowly cooled into the nematic phase in the presence of the field.

The data were calibrated against a silicon standard (NIST 640C), d = 3.135Å (Fig.

3.4). The 2D diffraction patterns were analyzed using the software package FIT2D developed by A. P. Hammersley [10] of the European Synchrotron Radiation Facility.

FIT2D allows us to integrate 2D X-ray diffraction pattern and to yield the conventional intensity vs. 2θ, q, or azimuthal angle scans.

3.2.3 X-ray Diffraction Patterns of Liquid Crystal Phases

Typical X-ray diffraction (XRD) patterns of phases ranging from the isotropic to smectic phase are shown in Fig. 3.5. Usually, X-ray patterns of the I phase and unaligned nematic phase are difficult to distinguish because they both lack long-range positional order. The sample in the N phase is easily aligned by an external magnetic field because of diamagnetic susceptibility anisotropy, Δχ. In most cases a magnetic field is used for spatially orienting the director along a specific direction. In the I phase, the distribution of molecules is random, which means no long-range orientational or positional order. Its

XRD pattern shows only two diffused rings, one at a small angle and the other at large angle. These correspond to the statistical length and diameter of (calamitic) molecules under the prevailing thermodynamic conditions. An aligned N phase generally exhibits two pair of arcs (Fig. 3.5, N), one at a small angle and other at a large angle. These pairs of arcs are in perpendicular directions. Fig 3.6 shows the XRD pattern of p-octyloxy-p′- cyanobiphenyl (8OCB) in the nematic phase, a typical calamitic LC material. The d- 66

spacing corresponding to the small- and large-angle peaks are 29.10 Å and 4.45 Å, respectively. These correspond approximately to an average molecular length (lo) and diameter (or, width) (wo), respectively. The diffraction pattern displays the N phase anisotropy and correlations in two spatial directions in the plane perpendicular to the incident beam. When Δχ > 0, the director orients parallel to the magnetic field (B). In materials with Δχ < 0, the director orients perpendicular to magnetic field.

Phase Ordering of molecules XRD

Isotropic (I)

Nematic (N)

Smectic A (SmA)

Smectic C (SmC)

Figure 3.5: Molecular arrangement and X-ray diffraction patterns from different LC mesophases.

Figure 3.6: X-ray diffraction pattern from aligned nematic (Δ> 0) phase of a o calamitic liquid crystal 8OCB at 79 C.

For q  n (outer peak), the XRD pattern is very liquid-like in the N, SmA, and SmC phases as illustrated in Fig. 3.5. In the SmA phase, two pairs of reflections remain orthogonal to each other just like the N phase. The X-ray diffraction patterns of the N and smectic phases differ mainly in the sharpness of the small angle peaks. The small angle peak in these smectic phases becomes sharper because of the development of long-range positional order in the direction parallel to the director [4]. The director is normal to the smectic layers, which have a thickness (d) of typically one molecular length (d ≈ lo). The first peak at small angle region positions at q = 2/d. Also we can observe 1~ 3 harmonics of this peak (qn = q1, q2, q3), depending on how well aligned the layers in a smectic phase are and how many terms are required to write the series expansion of the smectic order parameter. The large-angle diffuse maximum on the equator (Fig. 3.5, SmA) 68

remains essentially unchanged on passing from the N to the SmA phase, indicating that the in-plane structure remains liquid-like [6]. The XRD pattern of the SmC phase (Fig.

3.5, SmC), in which the director is tilted with respect to the layer normal, causes the layer spacing to decrease by a factor of cos,  being the tilt angle. Usually, when the director tilts with respect to the layer normal, the arc at the large-angle region is tilted; but when the layers tilt, the peak at the small-angle region is tilted by the same angle wrt the direction of q∥.

3.2.4 Data Analysis

Fit2d was used to produce data in the form of two-dimensional diffraction patterns and extract data in various forms. First, we calibrated the distance (sample to detector), coordinates of direct beam center, and the two tilt angles of the detector plane with respect to the incident beam, which specify the deviation of the detector plane from being perpendicular to the beam, using the silicon standard. After that, the image data of the sample (file type: mar345) was input and integrated by the calibrated parameter values. The calibrated 2D diffraction pattern can be converted into various plots. We have a choice of plot axes: scattering vector q (nm-1), scattering angle 2 (o), d-spacing (Å ), and an azimuthal angle (o). As an example, the X-ray diffraction pattern of the TOPAB sample and its developed scan data in q-space (nm-1), are shown in Fig. 3.7.

(a) X-ray diffraction pattern

(b) Plot of q vs. intensity (c) Plot of azimuthal angle vs. intensity

Figure 3.7: Illustration of the scans generated with the help of FIT2d software: (a) X- ray diffraction pattern of the TOPAB in the nematic phase, (b) plot of q vs. intensity, (c) plot of azimuthal angle vs. intensity distribution for large angle area of the diffraction pattern.

3.3 Capacitance Measurement

3.3.1 Dielectric Constant

Dielectric constants and dielectric anisotropy are important physical properties of liquid crystals and their values depend on the orientational order parameter, S. In liquid crystal displays, dielectric anisotropy (Δε) and the birefringence Δn (i.e., the difference in two indices of refraction) of the LC are important factors. The operating voltage of a nematic display depends on Δε, and the level of light transmission is determined by Δn

1/2 along with the material’s elastic constants, for example, Vth = π(k11/Δε) [12]. Dielectric constant (ε) and capacitance (C) are related via C = εoε 퐴 /d, where d is the separation between electrode plates, and A is electrode area. Here, εo is the permittivity of free space

-12 2 2 is 8.85 x 10 C /N m [F/m]. The capacitance measurements yield values of ε∥ when the director n is normal to the cell boundaries (or, the electrodes), and  when the director n is parallel to the plates as shown in Fig. 3.8(a). The dielectric anisotropy for calamitic liquid crystal, ∆ε = ε∥ − ε⊥, can be determined using one or two cells with different (homeotropic and planar) anchoring. The value of ε∥ is obtained using a homeotropic cell while ε is obtained from planar cells. In the one-cell method, a planar aligned cell is used if the material has positive dielectric anisotropy, ∆ε > 0. With the application of a small electric field (or, voltage, V < Vth where Vth is threshold voltage at which the director begins to reorient), no effect is observed on the degree of orientational order of the nematic LC; then one can measure ε⊥, and by applying sufficient voltage in a 71

direction normal to the plates, ε∥ is measured. We measure the capacitance of TOPAB as a temperature from the I to columnar phase and as a voltage in the nematic phase.

3.3.2 Experimental Set up

The experimental setup used to measure the capacitance at different field strengths is shown in Fig. 3.8(b). It employs an LCR meter (QuadTech 1920 Precision with maximum voltage of 1 Vrms) [13, 14]. We used cells made of 2 cm x 2 cm ITO glass plates and 4 m cell gap. The substrates were spin-coated with the same polyimides

(SE7792) as for POM observation. A function generator supplies sinusoidal voltage signal, Vrms at 1 kHz, which is amplified (x40) and applied to the cell. LC’s electrical resistance and capacitance are measured by detecting current (I) through the cell caused

dq dV by the voltage V, which changes with time (퐼 = = C ). Because the changes in c dt dt current I are small, a low-noise current pre-amplifier (Stanford Research Systems, SR570) was used. The pre-amplifier output signal is then measured with a lock-in amplifier

(EG&G Princeton Applied Research 5210). The lock-in amplifier simultaneously measures both the in-phase (Vr) and out-of-phase (Vc) components of the voltage at the signal frequency of f =1 kHz, allowing direct determination of the resistance and the capacitance. Total current flowing in the cell is expressed as a sum of two terms, as follows:

V 퐴 V 퐴 π 퐼 = 퐼 + 퐼 = ( rms,in ) σ sin(ωt) + ( rms,in ) ε ω sin (ωt + ), (3.1) r C d d 2 where, σ is electrical conductivity of the sample, ω is angular frequency of the applied waveform, ω = 2πf, and Vrms, in is the applied voltage. The first and the second term of eq. 72

(3.1) represent the resistive and capacitive currents, respectively. From the second term, the capacitance (Cp) and dielectric constant (ε) can be calculated,

VLIA × sensitivity [A/V] d Cp = , ε = Cp × ( ), (3.2.a) 푉rms,in × 2π 푓 εo퐴

The conductivity is also calculated from the first term,

V × sensitivity [A/V] d σ = LIA × (3.2.b) 푉rms,in 퐴

To ensure that the experimental set up worked properly, it was first tested on an empty cell whose capacitance was measured, first using an LCR meter and then the experimental setup, and checking how close to 1 the measured values of the dielectric constant of air was. In addition, we measured the capacitance of planar cells filled with a common calamitic liquid crystal, 5CB, to see whether it yields the expected response as a function of applied voltage. Fig. 3.9 shows a plot of measured capacitance and dielectric constant as functions of applied voltage. This result ensured us that the experiment set up shown in Fig. 3.8(b) was functioning properly.

Before the cell was filled with TOPAB, we measure the capacitance of empty cell

(Cempty), and then TOPAB was filled at a temperature above the clearing point. The cell was placed in a hot-stage (Instec® HCS402) connected to its controller. First the sample’s capacitance was measured as a function of temperature with the LCR meter, and the dielectric constant calculated.

(a)

(b)

Figure 3.8: (a) Calamitic liquid crystal with director, dielectric constant. (b) Experimental set up for capacitance measurement.

Figure 3.9: Voltage dependence of capacitance (scale on left) and dielectric constant

(right) of 5CB. 74

The cell was connected to measure its capacitance in the nematic phase as we described above [Fig. 3.8(b)]. Two precision capacitors (510 F KS16742L5) were connected in parallel and used as reference. Their combined capacitance was 10.22 x 10-10 F at 1 kHz as determined by the LCR meter. Before making measurements, the cell was replaced by a pair of capacitors, and the phase on the lock-in amplifier adjusted to zero in-phase current component. Then, the cell was reconnected and reading of the out-of-phase component (Vc) recorded to calculate capacitance at the applied voltage.

3.4 Conoscopy

3.4.1 Background

Conoscopy is a polarizing microscope based technique to examine the transmission property of an optical medium in various directions of incidence simultaneously. Information concerning the orientation of the optic axis of optically transparent matter may be obtained by conoscopy [15]. By virtue of the anisotropic nature, the transmission through an anisotropic sample placed between crossed polarizers depends on the sample orientation as well as the angle of incidence. Thus, the pattern of interference fringes and dark extinction brushes, or isogyres, is characteristic of the type of birefringence belonging to the material and its orientation to the microscope axis (O axis in Fig. 3.10).

Uniaxial and Biaxial material

Fig. 3.11 shows conoscopic images of the uniaxial nematic phase of 5CB. To obtain these conoscopic textures, 5CB was aligned by spin-coating two glass plates with homeotropically aligning polyimide S60702 and assembling a cell using 24 m mylar films as spacers. Fig. 3.11(a) is for the case of as-formed PI-coating. Fig. 3.11(b) was obtained with a cell in which the polyimide layer was rubbed before assembling in anti- parallel fashion and filling with LC. Case (a) shows the dark cross (extinction lines) at the center of the field of view with their arms parallel to the easy direction of the polarizer and analyzer. The interference pattern does not change as the sample is rotated about the microscope axis between the two crossed polarizers. Invariance of the isogyre pattern 76

under rotation occurs as the optic axis of the LC remains parallel to the direction of light propagation at all times. One observes the same pattern from the SmA phase; however, in the case of the cell made with rubbed surfaces, Fig. 3.11(b), the cross is shifted from the center of the field of view as the optic axis of the LC is inclined away the surface normal due to rubbing. The center of cross rotates with the sample but without changing the shape of the isogyre pattern. The tilt angle of molecules can be estimated by equation [15]

(see Fig. 3.10);

r 1 sin θp = 푠푖푛 휃. ro no

Here, sinθ is the numerical aperture of the lens, no is the ordinary refractive index of the

LC, and (2/π - θp) is the pretilt angle of the LC.

For biaxial samples like the SmC phase, the conoscopic image deviates from the above two cases. When the sample is rotated by 45, one observes a set of hyperbolic lines, Fig. 3.12. This indicates the existence of two optic axes of the medium because among three principal components in the biaxial system, two directors (the director n, m) are aligned. The difference in the refractive indices along l and m can be determined from conoscopy [17]. The three directors n, l, and m are orthogonal to each other and represent the axes of the biaxial system. Thus, the conoscopy technique can be used to distinguish between uniaxial and biaxial phases.

Figure 3.10: Schematic representation of conoscopy experiment. θp is the tilt angle of molecules with respect to the axis O, and ro is the radius of the conoscopic image.

(a) (b)

Figure 3.11: Conoscopy image of the uniaxial nematic phase of p-pentyl-p'- cianobiphenyl (5CB) in (a) homeotropic configuration, and (b) nearly homeotropic state with anti-parallel rubbing.

Uniaxial Biaxial

Figure 3.12: Conoscopy images of uniaxial nematic (left) and the biaxial (right) nematic phase [16]. Dashed contours represent the isochromes.

Optic axis observation

To visualize the optic axes more clearly, Benford plates are used in a conoscopic setup. Benford plates are two quarter-wave plates [18], the first one inserted between the 79

condenser and the sample and the second one between the analyzer and the objective lens.

The second plate is oriented perpendicularly to the first.

We used a polarizing optical microscope (Olympus BX51) with a Bertrand lens placed above the analyzer. Conventional polyimides S60702 and SE7792 were spin- coated onto the ITO glass plates and prepared in the manner as described above. Two cells of 4 m and 15 m gap were used. The 4 m gap cell was the same cell as used for

POM observation. We also attempted to observe the optic axis/axes by introducing two quarter-wave plates into the microscope. The thicker (15 m gap) cell was used because it was difficult to observe the optic axis in the thin cell due to its small total retardation.

The two quarter-wave plates were introduced during observations and oriented with their slow axes perpendicular to each other as shown in Fig. 3.13. The second one was positioned between analyzer and objective lens at an angle of 45o to the polarizers and could be inserted/removed at will. However, the first one to be placed on the condenser is controlled by our handling; therefore, Fig. 3.12 (a) and (b) show blurred dark spots but enough to recognize the position of the poles of optic axis. Fig. 3.14(a) and (b) show the example of the optic axis visualization under the same conditions as used for conoscopic results shown in Fig. 3.11(a) and (b), respectively. With the addition of Benford plates, the isogyres of conoscopic image disappeared; and the dark spot present at the center of the image for the non-rubbed case, Fig. 3.14(a). The dark spot moves to the right side from the center because of the rubbing direction, which caused the optic axis (director) to tilt away from the vertical.

Liquid crystal cells filled with TOPAB were placed in the hot-stage. We have 80

observed conoscopic images and optic axes from the aligned nematic phase at 115oC and

100oC with and without applying an electric field to find how the optic axis/axes are oriented.

Figure 3.13: Observation of optic axis, the arrows present their fast axis.

(a) (b)

Figure 3.14: Imaging of the optic axis image in samples (of 5CB) used for results in Fig. 3.11(a) and (b). The dark spot denotes the location of the optic axis being at the center of the image. The optic axis is parallel to the light propagation for (a), while the optic axis in (b) is lilted to the right of the center. (Objective lens 50x)

3.5. Raman Scattering and IR Spectroscopy

3.5.1 Background

Raman scattering is a vibrational spectroscopy as is the IR spectroscopy. As shown in Ch.2, the principles underlying the two techniques differ. IR is an absorption spectroscopy, and one measures the absorption of the IR beam by the sample as a function of frequency. The Raman method (Fig 3.15) is a scattering spectroscopy in which the scattered beam consists of two different frequencies, including the frequency of the incident beam (ν0) from elastic Rayleigh scattering and frequencies ν0 ± νm (energy gain or loss), where νm is a molecular vibrational frequency. The latter is an inelastic process referred to as Raman scattering. Raman spectroscopy measures vibrational 82

frequencies (νm) as shifts from the incident frequency (ν0). To be Raman-active, the polarizability (α) must change when the electric field (E) of light interacts with a particular vibration mode of a molecule. The incident radiation induces an electric dipole moment μ in the molecule, expressed in the matrix form as follows:

μx αxx αxy αxz Ex [μy] = [αyx αyy αyz] [Ey]. (3.3) α α α μz zx zy zz Ez

The first matrix on the right-hand side is called the polarizability tensor, and it is symmetric: αij= αji, i, j = x, y, z. Raman activity is determined by differential polarizability (dα/dq)0 , where q is a displacement or periodic distortion in polarizability from the equilibrium position [26], so the Raman scattered intensity can be described thus:

∂α 2 퐼 ∝ *( )qk=0+ . ∂qk

Consider a molecule situated at the origin, irradiated from the y-direction with plane polarized light, whose electric vector oscillates in the yz-plane (Ez), illustrated in Fig.

3.16. During the experiment, one measures the scattered beam intensities with polarization vectors in the (parallel) y- and (perpendicular) z-direction using an analyzer.

The Raman band intensity ratios, given by the perpendicular polarization intensity I⊥,

divided by the parallel polarization intensity I∥, is known as the depolarization ratio,

I R = ⊥. The depolarization ratio of specific bands at different scattering vectors provides I∥ valuable dynamic information. 83

Raman

Figure 3.15: A schematic comparison of the mechanisms responsible for Raman and IR spectroscopies.

Figure 3.16: Irradiation of the sample with plane polarized light incident along the y- direction and its electric field vector (i.e., polarization direction) in the z-direction. The photons scattered at different angles having polarization perpendicular and parallel to the incident polarization are detected and analyzed.

The polarized Raman scattering method has been used to determine molecular orientational order in the uniaxial [19 − 22] and biaxial [23, 24] nematic and smectic phases [19(c), 25]. Recently, the biaxiality of the nematic phase has been investigated in bent-core materials [23, 24]. If we consider a molecule with a differential polarizability,

∂α ′ ≡ αji, described above, and the Euler angles (α, β, γ) indicating the orientational ∂qk degrees of freedom of a molecule with respect to a laboratory frame, XYZ [22, 24] (Fig.

3.17), it is possible to express the scattering as a function of α, β, γ and the incident polarization angle θ. By introducing an orientational distribution function (ODF), f (α, β,

γ), scattered intensity for an aligned sample can be expressed as an integral of the square of the electric field and f (α, β, γ) over all possible orientations:

2 I (θ) = I 푓 (α, β, γ)[E (α, β, γ)] dα dβ dγ. (3.4) ij 0 ∫α,β,γ ij

The ODF is generally expressed as [27]

2L+1 ∗ f(α, β, γ) = ∑∞ ∑L 〈DL 〉DL (α, β, γ), (3.5) L=0 m,n=−L 8π2 mn mn

L −imα L −inγ L where Dmn(α, β, γ) = e dmn (β) e , that is, the Wigner function. < Dmn (β) >

L expresses the statistical average of Dmn(α, β, γ). The indices L, m, and n are the symmetry conditions under rotations β, α, and γ, respectively. By applying the symmetries of the sample (e.g., mirror symmetry of director n) to the Wigner function, the ODF can be written as follows: 85

1 5 푓 (α, β) = [1 + 〈P 〉(3 cos2β − 1)] 8π2 2 200

30 + 〈P 〉(1 − cos2β) cos2α 2 220 9 + 〈P 〉(3 − 30 cos2β + 35cos4β) (3.6) 8 400

540 + 〈P 〉 (−1 + 8 cos2β − 7cos4β) cos2α 8 420 630 + 〈P 〉 (1 − 2 cos2β + cos4β) cos4α , 8 440 where, the generalized Legendre polynomials PL, m, n (α, β) are

1 P = (3 cos2β − 1) 200 2

1 P = (1 − cos2β) cos2α 220 4

1 P = (3 − 30 cos2β + 35cos4β) 400 8

1 P = (−1 + 8 cos2β − 7cos4β) cos2α 420 24

1 P = (1 − 2 cos2β + cos4β) cos4α . 440 16

By substituting the modified ODF (eq. 3.6) into the intensity function eq. 3.4, we can derive the depolarization ratio R(θ) [24]. The r in eq. 3.7 is the differential polarizability

′ ′ ratio, 푟 = αyy/αzz. Here, P200 and P400 are uniaxial orientational order parameters, and the remaining P’s are biaxiality order parameters.

−40〈P200〉 − 240〈P220〉 + (105 cos 4휃 − 9)〈P400〉 2 2 −(1260 cos 4휃 − 228)〈P420〉 + 210sin 2휃〈P440〉 푅(휃) = (푟 − 1) 2 2 . 40(4r − r − 3)[(1 + 3cos2휃)〈P200〉 + 12 sin 휃 〈P220〉] 2 (3.7) −3 (r − 1) 〈P400〉(35 cos4휃 + 20 cos2휃 + 9) 2 2 −1200 (r − 1) 〈P420〉 sin 휃 (3 + 7 cos2휃) 2 2 −5607 sin 휃 〈P440〉 − (448 r + 224 r + 168)

Figure 3.17: Representation of the rotation of a molecule with differential polarizabilities with respect to the laboratory frame XYZ in terms of the Euler angles.

3.5.2 Sample preparation and Experimental Set up

The samples for Raman scattering were prepared with ITO coated cover slips of glass (SPI Supplies, 0.13 - 0.17 mm thick) because the ordinary soda-lime glass with thickness of 1.2 mm (e.g., ITO glass used for POM observation) gave significant background fluorescence. With the use of thicker glass, the fluorescence of the glass produces a large slope for the background line and produces significantly less accurate peak values. We chose the ITO-coated substrates so that electric field could be applied.

The cover slips were cleaned by ultrasonication for 1 hour in deionized water. The 87

conventional planar orienting polyimide (PI) SE7792 was spin-coated at two different speeds, at 650 rpm for 5 seconds and at 3000 rpm for 35 seconds. The PI film was then baked for 1 minute at 90oC on the hot-plate and then 50 minutes at 220oC in the oven.

The plates were rubbed very gently to create uniform alignment across the surface. A cell thickness of 14 m was achieved using mylar films. UV-curable epoxy suitable for high temperatures was used to seal the cells. The sample TOPAB was filled in the isotropic phase at 130oC. We confirmed that the cell was aligned well in the nematic phase from

POM observation.

The Raman scattering was performed in the laboratory of Prof. M. Srinivasarao

(School of Polymer, Textile and Fiber Engineering at Georgia Institute of Technology,

Atlanta, Georgia). The cell was placed in a hot-stage (Instec® HCS402) and mounted on the rotation stage of the polarizing optical microscope (Olympus Optical Co. Ltd.,

BX60F). A laser beam of 785 nm wavelength and at a power of 100 mW was focused with an 20x objective lens (about 200 m dia., Kaiser Optical System, Inc.) and made incident normal to the sample plane. The range of observable Raman shift was 0 – 2000 cm-1. An exposure time of 7 seconds was used, and the data was accumulated 10 times.

The sample was cooled very slowly (0.2oC/min) from the isotropic phase into the nematic phase. Once the temperature reached a desired point, it was allowed to equilibrate for 5 minutes. Raman intensity measurements were made in the nematic phase from 118oC to

97oC at an interval of 3oC. At each temperature, the scattered intensity of the two modes

(HH and VH) was measured at different values of angle θ, at 10o intervals from 0 to 180o rotation. The HH mode and VH mode mean the analyzer set to parallel and perpendicular 88

to the polarization of the incident beam, respectively. First, we made measurements in the

HH geometry without an applied electric field and then with an applied electric field. The same process was repeated for the VH mode. From the HH and VH modes, we obtained the intensity I∥(θ) and I⊥(θ), respectively. The acquired data were analyzed using software GRAMS/AI (7.02 version, Thermo Galactic) and generated the Raman scattering graphs. Unfortunately, a technical difficulty occurred with the data under the applied electric field. Electric leads had melted and shorted. Thus, we will present only the results without the electric field. The software GRAMS/AI allows us to fit the peaks with the combination of Lorentzian and Gaussian function and provides values of various fitting parameters, peak center position (frequency), height, width, and peak area. With the values of these parameters, we plotted the data and fitted them to equation (3.7) to calculate the order parameters of the nematic phase. The results are discussed in detail in the next chapter.

References

[1]. Ingo Dierking, Textures of Liquid crystals, Wiley-VCH GmbH & Co. KGaA (2003).

[2]. S. Kumar, Experimental Study of Physical Properties and Phase Transitions, Cambridge University Press, New York (2001).

[3]. C. Kittel, Introduction to Solid State Physics, John Wiley & Sons, Inc (1996).

[4]. A. J. Leadbetter, The molecular physics of liquid crystals edited by G. R. Luckhurst and G. W. Gray, (1979).

[5]. J. Als-Nielsen and D. McMorrow, Elements of Modern X-ray Physics, John Wiley & Sons Ltd (2001).

[6]. J. M. Seddon, Handbook of Liquid Crystals, Vol.1, edited by D. Demus, D. Goodby, J. W. Gray, G. W. Spiess, and H. W. Vill, Wiley-VCH (1998).

[7]. W. L. McMillan, Phys. Rev. A, 7, 1673 (1973).

[8]. B. R. Acharya, S.-W. Kang, V. Prasad, and S. Kumar, J. Phys. Chem. B 113, 3845 (2009).

[9]. The MUCAT Web Site, http://6id.xor.aps.anl.gov/mucat/index.html.

[10]. The FIT2D Home page, http://www.esrf.eu/computing/scientific/FIT2D/ (2004).

[11]. D. Meyerhofer, J. Appl. Phys. 46, 5084 (1975).

[12]. E. Lueder, Liquid Crystal Displays, John Wiley & Sons, Ltd (2000).

[13]. J. T. Gleeson, N. Gheorghiu, and E. Plaut, Eur. Phys. J. B 26, 515 (2002).

[14]. D. Meyerhofer, J. Appl. Phys. 46, 5084 (1975).

[15]. T. Uchida and H. Seki, Liquid Crystals – Applications and uses Vol.3, edited by B. Bahadur, World Scientific (1992).

[16]. G. R. Luckhurst, Thin Solid Film, 393, 40 (2001).

[17]. Y. Galerne, Mol. Cryst. Liq. Cryst. 323, 211 (1998).

[18]. Website: http://jm-derochette.be/Conoscopy/Uniaxial_minerals_4.htm

[19]. (a) S. Jen, N. A. Clark, P. S. Pershan, and E. B. Priestley, Phys. Rev. Lett. 31, 90

1552 (1973); (b) S. Jen, N. A. Clark, P. S. Pershan, and E. B. Priestley, J. Chem. Phys., 66, 4635 (1977); (c) K. Miyano, Phys. Lett. 63A, 37 (1977).

[20]. M. Sidir and R. Farhi, Liq. Cryst. 19, 573 (1995).

[21]. (a) W. Jeremy, D. K. Thomas, D. W. Thomas, and G. Williams, J. Mol. Struct. 614, 75 (2002); (b) W. J. Jones, D. K. Thomas, D. W. Thomas, and G. Williams, J. Mol. Struct., 708, 145 (2004).

[22]. C. D. Southern and H. F. Gleeson, Eur. Phys. J. E 24, 119 (2007).

[23]. C. D. Southern, P. D. Brimicombe, S. D. Siemianowski, S. Jaradat, N. Roberts, V. Görtz, J. W. Goodby, and H. F. Gleeson, EPL. 82, 56001 (2008).

[24]. N. Hayashi, T. Kato, A. Fukuda, J. K. Vij, Y. P. Panarin, J. Naciri, R. Shashidhar, S. Kawada, and S. Kondoh, Phys. Rev. E 71, 041705 (2005).

[25]. M. S. Park, B.-J. Yoon, J. O. Park, V. Prasad, S. Kumar, and M. Srinivasarao, Phys. Rev. Lett. 105, 027801 (2010).

[26]. J. R. Ferraro and K. Nakamoto, Introductory Raman Spectroscopy, Academic Press (1994).

[27]. M. van Gurp, Colloid. Polym. Sci. 273, 607 (1995).

Chapter 4

Single Component H-Bonding Mesogens

As discussed in previous chapters, we have investigated mesomorphism of two pure hydrogen bonding mesogens and their mixtures with other mesogens by different techniques. The goal has been to characterize and understand their phase behavior as completely as possible. In this chapter, we discuss the results of X-ray diffraction, polarizing optical microscopy, conoscopy, dielectric and electro-optical properties, and

Raman spectroscopy on the two pure hydrogen-bonding mesogens, namely TOPAB and

THPAB.

4.1 X-ray Diffraction Measurements

4.1.1 Molecular formations

The materials TOPAB and THPAB, which are expected to form a dimer via hydrogen bonding, were investigated with X-ray scattering experiment. Fig. 4.1(a) shows the structures of molecules and estimated length scales. The molecular structure was drawn in software ChemDraw (CambridgeSoft, version 12). The length was calculated with a help of Mercury (Cambridge Crystallographic Data Centre) after geometry optimization by HyperChem (Hypercube, Inc) software packages. Monomer TOPAB’s length (lm) is about 24.0 Å and THPAB’s length (lm) is about 22.3 Å in the fully extended form. One expects these molecules to form linear dimers (see Fig. 4.1(b)) via hydrogen

bonding between carboxylic groups and behave like calamitic liquid crystal materials.

Then, the estimated molecular length of TOPAB and THPAB are expected to be about ~

48 Å and ~ 45 Å , respectively. It should be pointed out that there are more complex assemblies such as, tetramer, trimer, or even polymer-like structures possible. The latter two are illustrated in Figs. 4.1(c) and (d).

4.1.2 The Isotropic Phase

For X-ray experiments, the samples were filled in the capillary (diameter 1 – 1.5 mm and 10 m thick glass) and heated above the clearing temperature, i.e., 130oC for

TOPAB and 135oC for THPAB. The XRD patterns were obtained on cooling from the isotropic temperature in the presence of ~ 2.5 kG of magnetic field applied perpendicular to the capillary and the X-ray beam directions. Figs. 4.2 and 4.3 show the dffraction patterns in the isotropic, nematic, and columnar phases of TOPAB and THPAB, respectively. In the isotropic phase at 125oC, XRD patterns for TOPAB show two sets of diffuse rings, one at large angle and the other at small angle corresponding to length scales of 4.5 Å and 25.9 Å , respectively. At 130oC for THPAB, d-spacings calculated from the two diffuse rings correspond to 4.5 Å and 24.5 Å . X-ray intensity in these rings is uniformly azimuthally distributed with respect to the center of XRD patterns. The ring

(peak) nearest to the center corresponds to the effective molecular length while the outer ring, corresponding to a d-spacing of ~ 4.5 Å, arises from the intermolecular separation along their diameter which is the average width (diameter) of molecules. However, the length obtained from XRD pattern does not necessarily match with calculated molecular

lm wm TOPAB 24.0 Å 15.9 Å THPAB 22.3 Å 14.3 Å

Figure 4.1 Possible forms and approximate dimensions of TOPAB and THPAB: (a) Molecular (monomer) length scales of TOPAB and THPAB, (b) a linear dimer of TOPAB through hydrogen bonding, (c) a trimer, and (d) polymer.

dimensions because of the thermal motion, molecular flexibility, and different molecular conformations at higher temperature. Here, we notice that a value of ~ 26 Å for a linear shape of TOPAB and THPAB at the isotropic temperature is somewhat smaller than the calculated molecular lengths.

(a) Isotropic T = 125oC (b) Nematic T = 116oC

Figure 4.2: XRD patterns of TOPAB at different temperatures. Magnetic field is applied in the horizontal direction: (a) the isotropic phase: two diffused rings, (b) and (c) the uniaxial nematic phase, and (d) the columnar phase with hexagonal packing.

Isotropic T = 130oC Nematic T = 127oC

Nematic T = 114oC Nematic → Columnar T = 98oC

Figure 4.3: XRD patterns of THPAB at different temperatures. Magnetic field is applied in the horizontal direction: (a) the isotropic phase: two diffused rings, (b) and (c) the uniaxial nematic phase: Two pairs of reflections, and (d) the columnar phase with hexagonal packing.

Figure 4.4: Diffractographs for TOPAB obtained by intensity integration of XRD pattern over all azimuthal directions, at different temperatures. The columnar phase at

o 93 C (on log scale) has hexagonal packing with ratio d1: d2: d3 = 1 : √3 : √4.

Figure 4.5: Intensity profile in q-space of THPAB at different temperatures. The nematic to columnar transition at 98oC displays (on log scale) hexagonal packing with ratio d1: d2: d3 = 1: √3: √4. The inset shows the regions inside the ellipse on an expanded scale.

Figure 4.6: The d-spacing (Å ) and correlation length (Å ) calculated from the first peak of TOPAB in the meridional direction. The nematic data is connected by solid lines. The dotted-vertical lines are where the transitions occur. The d-spacing decreases slightly, but the correlation length increases with decreasing temperature in the nematic phase, and increases dramatically from 65 Å in the nematic phase to 1280 Å in the columnar phase.

100

4.1.3 The Nematic Phase

Figs. 4.2 and 4.3 show two XRD patterns each for TOPAB and THPAB at higher and lower temperatures in the nematic phase. A graph of the integrated intensity vs. q

(nm-1) at various temperatures is shown in Fig. 4.4 and 4.5, for the two compounds. The d-spacing and correlation length vs. temperature plots for the first peak of TOPAB in the meridional direction, Fig. 4.6, were calculated from the relationship d = 2π/q and correlation length = 2π/Δq, the value of q and full width at half maximum, FWHM (= Δq), were obtained from Lorentzian fits. Generally, the pair of small angle peaks is orthogonal to the pair at large angles as they measure the length and width of the elongated molecules (see Ch.3 XRD section). For a field aligned nematic, one of the two sets of peaks is parallel and the other perpendicular to the direction of the applied magnetic field.

However, XRD patterns of the nematic phase at hand are quite different from conventional nematic phase. In case of TOPAB and THPAB, both, the small and large angle peaks lie in the same (i.e., meridional) direction), perpendicular to the magnetic field. Both are diffuse indicating liquid-like short-range positional order, have well aligned director, and long-range orientational order. This is clear confirmation of the nematic phase.

The strongest peak has a value that is close to the molecular length. However, as we can see from the plot of d-spacing against temperature (Fig. 4.6), it is comparable to

o the monomer length (lm), ~24 Å over entire nematic range. At 114 C, d-spacing of

THPAB is 23.2 Å which is also close to its monomer length. These results raise an

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Figure 4.7 TOPAB: (a) XRD pattern of the nematic phase at 101oC; (b) azimuthal scans for different peaks; and (c) black curve for the 24.4 Å peak with intensity scale on left hand side was obtained from integration over  (azimuthal) angles from 45o to 135o wrt the magnetic field (or, between  45o wrt the meridian). The blue curve for the 18.2 Å peak, with intensity scale on right hand side, was calculated from integrating peak intensity from  = -45o to 45o relative to the magnetic field.

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Figure 4.8 THPAB: aligned nematic phase at 114oC. (a) XRD pattern, (b) -scans through different peaks, (c) blue curve: intensity vs. q -integrated from -45o to 45o with respect to the magnetic field, and black graph, from - 45o to 45o with respect to the meridian.

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Figure 4.9: Evolution of small angle diffraction in the nematic phase of TOPAB. Small angle region in yellow box is also shown on an expanded scale using intensity contour lines with 10% intensity difference between lines. (The color bar at the bottom shows the intensity scale). The shape of peak resembles a kidney bean at lower temperature.

104

Figure 4.10: Thermal evolution of χ-scans (a) through the nematic phase for the smallest angle peak, and (b) fits to Gaussian distribution function at 120C and 101C. At higher temperature the peaks are fit with two Gaussian distribution peaks over 360o however, at lower temperature four Gaussian distribution peaks are needed to fit the peaks. One peak is the summation of two scattered intensity peaks.

105

important question: Do the molecules remain as monomers? If so, then it contradicts the

FT-IR results discussed in Ch. 2 which show that TOPAB undergoes intermolecular hydrogen bonding in the mesophase.

The d-spacing in the nematic phase decreases slightly at low temperatures, while the correlation length increases. The measured value of d-spacing is too short for molecular assemblies to be a linear dimer. The trimer or polymer configurations are also not suitable since then the largest d-spacings should be at least more than the monomer length. Fig. 4.7(a) and 4.8(a) show representative XRD patterns of the nematic phase at

101oC for TOPAB and at 114oC for THPAB. There are two pairs of peak in meridional direction at 24.4 Å and 4.5 Å , and at 23.2 Å and 4.5 Å for TOPAB and THPAB, respectively. Additionally, in the equatorial direction, there are relatively weak and broad pairs of peaks at 18.2 Å and 7.1 Å for TOPAB and at 17.7 Å and 6.9 Å for THPAB with additional peaks at 3.6 Å , and 2.8 Å for both TOPAB and THPAB. It is hard to observe these peaks in the plot Fig. 4.7(c) and 4.8(c) and have been marked with arrows. The peak at 18.2 Å had less anisotropy, and was diffused at higher temperature, but the scattering intensity is mostly concentrated in the magnetic field’s direction at lower nematic temperatures. Its d-spacing lies between 18.2 Å and 18.5 Å while the correlation length increased from 35.5 Å (twice of d-spacing) to 55.4 Å (three times of d-spacing) as the temperature decreases in the nematic phase. Small angle XRD patterns in the nematic phase are shown in Fig. 4.9 in the form of contour plots, adjacent contours are separated by 10% of the maximum intensity. Upon lowering the temperature, the shape of the pattern in meridional direction becomes more like a kidney bean rather than just an arc or

106

a crescent shape. As seen in Fig. 3.6 in Ch.3, generally, the calamitic liquid crystal in the nematic phase shows a crescent shape, this means the scattered intensity concentrates mostly in the center of the peak. Interestingly, the shape of the peak for TOPAB and

THPAB differs from this and becomes more like the kidney bean at lower temperature.

This possibly arises from biaxial fluctuations in the nematic phase increasing the off- center scattered intensity. The χ-scans were obtained by the integration of diffracted X- ray intensity over a finite q-range at different azimuthal angle, the starting angle 0o is as shown in Figs. 4.7(a) and 4.8(a). Fig. 4.10(a) shows χ-scans for the first peak in the small angle region at different nematic temperature. At lower temperature, the peak intensity becomes higher, and the width of the peaks changes from 70o at 120oC to 55o at 109oC showing an increasing value of the orientational order parameter. At higher nematic temperatures, it was possible to fit the χ-scans to data very well with two Gaussian functions with their centers 180o apart over the entire range of χ. But, at lower temperatures, two Gaussian distribution functions for each peak were needed because the top of peaks becomes flat, as seen in Fig. 4.10(a) and (b), and the appearance of the peaks becomes kidney shaped, Fig. 4.9. The peaks’ flat top does not arise from experimental artifacts, such as saturation of the image plate. We conclude that there are two unresolved peaks comprising these χ-scans. The counter map at 96oC in Fig. 4.9 begins to reveal signs of splitting in a manner similar to that seen in the biaxial nematic phase of bent-core mesogens [1].

The other very broad and weak peak at d = 7.1 Å for TOPAB and d = 6.9 Å for

THPAB originates from the average length of hydrocarbon chains which will be reduced

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due to thermal motion at nematic temperature. There are two very weak peaks for both

TOPAB and THPAB on the equator at d-spacings of 3.6 Å and 2.8 Å . We tentatively assign them to  –  stacking [2] and O – H ··· O distance [3], respectively.

4.1.4 The Columnar Phase

On further cooling, a more ordered liquid crystal phase was formed. In this phase, the sample is not affected by the external force, e.g. magnetic or electric fields. The XRD pattern (Fig. 4.2) shows three quasi-bragg reflections in the small angle area and one diffused ring at large angles at 94 oC. Its diffractograph on q-scale is shown in Fig. 4.3.

The first peak is very sharp which indicates a phase with longer range positional order than the nematic phase. The diffuse peak at 4.5 Å confirms that this phase still disordered

(or, in liquid-like form) in other spatial directions. From the plot of d-spacing and correlation lengths presented in Fig. 4.6, it is clear that the d-spacing does not change significantly but the correlation length increases dramatically from 64 Å (2.6 times of d- spacing) to 1250 Å (50.8 times of d-spacing).

In the case of THPAB, the pattern remains anisotropic at large angle at all temperatures in the nematic phase and after transition to the columnar phase. Nonetheless, as seen from XRD pattern in Fig. 4.3 and intensity plot in Fig. 4.5, the small angle peak becomes the most intense peak and additional quasi-bragg peaks appear in the columnar phase. The d-spacing of various peaks obtained from integrated intensities in diffraction patterns are;

TOPAB: d1 = 24.7 Å, d2 = 14.1 Å, d3 = 12.4 Å, with d1 ∶ d2 ∶ d3 ≅ 1 ∶ √3 ∶ √4

THPAB: d1 = 23.6 Å, d2 = 13.5 Å, d3 = 11.8 Å, with d1 ∶ d2 ∶ d3 ≅ 1 ∶ √3 ∶ √4

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The ratios of d-spacings represented here correspond to hexagonal packing [4] in planes perpendicular to the column axis, which is typical packing in disk-like molecules and phasmids [5]. The three flexible chains on each molecule may be playing an important role in the formation of two-dimensional columnar structure. On further cooling below the columnar phase, a crystal phase was obtained. The XRD patterns and integrated intensity plots of TOPAB and THPAB in the crystal phase are shown in Fig. 4.11.

Based on the XRD pattern, we can summarize few peculiarities;

1. The largest d-spacing in the isotropic and nematic phases is too short to be a

linear coplanar dimer.

2. Positions of the two main peaks (at small and large angles) are perpendicular to

the magnetic field.

3. The small angle peaks becomes significantly sharp and additional peaks appear in

the columnar phase.

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Figure 4.11: XRD patterns and intensity vs. q graphs in the crystal phase of (a) TOPAB at 89.5oC and (b) THPAB at 70.0oC.

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4.1.5. Orientational Order Parameters

In order to determine the orientation order parameters in the nematic phase of

TOPAB, the approach suggested by Leadbetter et al. [6] and Davidson et al. [7] is used.

Leadbetter and Norris derived an expression which allows one to calculate the intensity vs. azimuthal angle () directly from the orientational distribution function (ODF), f()

(ODF was discussed in Ch.1, and  is an angle between the long axis of a molecule at any moment and n). This method has been applied to many mesogenic compounds [8]. But the calculation was rather difficult; hence Davidson, et al. developed a simpler method to evaluate ODF in terms of cos2nfunctions as [7, 9];

∞ 2n 푓(훽) = ∑푛=0 푓2푛 cos 훽 . (4.1)

And the intensity vs. azimuthal angle () can be expressed as a series of cos2n functions with the coefficients of the ODF, f2n, as shown by Davidson, et al.

2푛 푛! 퐼(휒) = ∑∞ 푓 cos2n 휒 . (4.2) 푛=0 2푛 (2푛+1)‼

By fitting the experimental plot of intensity vs. azimuthal angle () with eq. (4.2), one obtains f2n. Then, f() is calculated by inserting f2n into eq. (4.1). The orientational order parameters and can be determined because the orientational order parameters are expressed with f() as;

π/2 ( ) ∫0 P2푛 f β sin(β)dβ < P2푛 >= π/2 ; 푛 = 1, 2. (4.3) ( ) ∫0 f β sin(β)dβ

The plot of integrated intensity I() of the wide angle X-ray peak of the nematic phase of

TOPAB at 116oC is shown in Fig. 4.12. I() is fitted to eq. 4.2 and the fitted curve is presented as the red continuous line. The parameters and are determined

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from eqns. 4.1 and 4.3. Temperature dependences of orientational order parameters

and in nematic phase of TOPAB are shown in Fig. 4.13. and increase with decreasing temperature; = 0.52 – 0.68 and = 0.12 – 0.30.

Figure 4.12: Experimental integrated scattered intensity about azimuthal angle for the wide angle peak in the nematic phase of TOPAB at 116oC. The corresponding XRD pattern is presented in Fig. 4.2(b). The red solid line is a fit to eq. 4.2.

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Figure 4.13: Temperature dependence of orientational order parameters and

in the nematic phase of TOPAB. and increase as temperature decreases and lie in ranges 0.52 – 0.68 and 0.12 – 0.30, respectively.

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4.1.6 Discussions

Based on these observations, we conclude that these materials form unconventional dimers (or, multimers) that are quite different from traditional coplanar linear dimers. To explain the results of X-ray experiments, we tentatively propose that these phasmidic materials form oblique dimers at an angle 2α that resemble the overall shape of bent-core mesogens also termed as banana liquid crystals and schematically illustrated in Figs. 4.14 and 4.15(a). This oblique H-bonding was also tested with several other techniques and is discussed later in this chapter. The dimer dimensions and values of XRD d-spacings were estimated for such oblique configuration. The director of the nematic phase naturally lies along the overall length l of the dimer, i.e., perpendicularly to the line dividing the angle 2α between molecules. In this case, the long axis of individual monomer is tilted at an angle 90-α with respect to the director n. Then the first peak on equatorial direction (the magnetic field direction) in Fig. 4.7 for TOPAB and 4.8 for THPAB comes from the width (w) of dimer. Because we have the estimated molecular length (lm) ~24 Å and 24.4 Å of the d-spacing value which is considered as a dimer

o 푙 length (l) at 101 C for TOPAB, by inserting those numbers into 푙푚 sin α = ⁄2 (see Fig.

4.14) the angle 2α and width w are obtained as 61o and 20.8 Å , respectively. In the same manner, 2α and w are 63o and 18.8 Å at 114oC for THPAB, respectively. However, the molecular length changes with temperature so the actual molecular length can become shorter at high temperatures by increased thermal motion. The calculated value of width

20.8 Å is rather large compared to the d-spacing of 18.2 Å obtained by XRD at 101oC for

TOPAB (see Fig. 4.7). The situation is the same for THPAB. With these values of the

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molecular length 22 Å for TOPAB and 21 Å for THPAB, the value of width arrives close to the measured d spacing. The angle at apex is approximately 67° at 101°C in the nematic phase for TOPAB and at 114oC for THPAB. Because the most intense small angle peak corresponds to the average length of effective molecules and the diffuse peak at large angle arises from the average width of hydrocarbon chains, this way in Fig.

4.15(a), the average orientation of aliphatic chains is parallel to the magnetic field and consequently the scattering peak corresponding to the interchain separation of 4.5 Å appears on the orthogonal (meridional) direction. Also a tetramer is another possible formation of TOPAB and THPAB. The two such dimers in Fig. 4.15(a) can form a tetramer through the H-bonding with twice of molecular width as shown in Fig. 4.15(b).

Normally, the O − H ⋯ O angle is linear and the closer the angle is to 180o [10]. This leads to the coplanar linear dimer formation for our compounds generally. However, our suggested formations deviate from the normal H-bond so our compounds are formed by anomalous hydrogen bond.

The diffraction patterns of the nematic phase at 96.2°C were taken first with the field applied perpendicular to the axis of capillary and the X-ray beam. The diffraction patterns in Fig. 4.16(a) at 96.2oC and Fig. 4.7(a) at 101oC show the same feature except one more peak appeared at 24.8 Å at 96.2oC. This is one of the d spacings of the columnar phase and appears in addition to the peak at 23.7 Å of the nematic phase. The capillary was then rotated about its axis by ~90o to make the direction of alignment parallel to the X-ray beam to probe structure in the orthogonal plane containing the capillary axis. At this low nematic temperature, the viscosity is expected to be high and

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reorientation process slow. We were able to quickly acquire a diffraction pattern, shown in Fig. 4.16(b), before the sample could reorient to a significant degree. This pattern reveals two uniform rings at 23.7 Å and 4.5 Å which correspond to the length of a dimer and width of a hydrocarbon chains, respectively, and one non-uniform peak at 24.8 Å which was present in Fig. 4.16(a). Thus, the X-ray beam propagating along the “magnetic field” was scattered by the dimer ensemble. Nearly uniform azimuthal distribution of the two peaks proves that the main axis (and the plane) of dimer remains without any preferred orientation in the plane perpendicular to the magnetic field. The extra equatorial peaks disappeared in this geometry which can be understandable because the X-ray beam was not able to probe the width of dimers in this situation, where the x-axis of dimer is lying along the direction of the X-ray beam as illustrated in Fig. 4.17. This result supports our proposed oblique dimerization with a negative diamagnetic anisotropy [∆χ = χ∥ −

χ⊥ < 0] and thus, the director n is oriented perpendicular to magnetic field.

The sample gradually realigned parallel to the magnetic field in approximately 5 minutes and yielded the diffraction pattern shown in Fig. 4.16(c). This pattern is the same as the initial pattern of Fig. 4.16(a).

The first X-ray diffraction report from the H-bonded phasmid-like compounds was on 2, 3, 4 – (trialkoxy)cinnamic acids exhibiting only the nematic mesophase on heating [11]. The XRD pattern of these materials was similar to our result; positions of small and large peaks in the aligned nematic phase. However, not enough discussion of

XRD patterns and molecular structure was offered.

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Estimated dimer dimensions for TOPAB and THPAB.

l 2α m l w (monomer (bending (d-spacing) (d-spacing) length) angle) 24 Å 61 o 24.4 Å 20.8 Å 23 Å 64 o 24.4 Å 19.7 Å 22 Å 67 o 24.4 Å 18.5 Å 22 Å 63 o 23.2 Å 18.8 Å 21 Å 67 o 23.2 Å 17.7 Å 20 Å 70 o 23.2 Å 16.5 Å

Figure 4.14: Estimated dimensions of the oblique dimer for TOPAB and THPAB. The rows with background light orange and light sky color list the estimated dimension at 101oC for TOPAB in the N phase and at 114oC for THPAB in the N phase, respectively.

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Figure 4.15(a): Schematic depiction of the molecular conformation of an oblique dimer TOPAB or THPAB. The curved red arrow shows that the dimer is degenerated about x-axis; the direction perpendicular to the director n and parallel to the magnetic field B.

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H- bonding

Figure 4.15(b): Schematic depiction of the possible molecular conformation of tetramer TOPAB or THPAB. Two of such dimers of Fig. 4.15(a) can form a tetramer via H-bonding with twice of width in the N phase.

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(d)

Figure 4.16: XRD patterns of TOPAB In the nematic + columnar phase at 96.2oC under different orientations. (a) Magnetic field (red arrow) aligned sample obtained after cooling, (b) immediately after rotation of the capillary (sample) by 90o about its vertical axis, (c) magnetic field realigned nematic phase after

5 minutes after rotation, and (d) intensity vs. q graphs for patterns in (a) – (c). The data accumulation time in each case was 3sec).

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Figure 4.17: The XRD patterns of the nematic phase of TOPAB, top panel is obtained by cooling from the isotropic phase applied magnetic field, direction shown by red double arrow. Lower panel depicts the diffraction pattern and molecular orientation immediately after 90o rotation of the sample about capillary axis.

To identify the phase visually and to further test our oblique H-bonding model of dimerization, we performed electro-optical, capacitance, polarized optical microscopy, and conoscopy measurements.

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4.2 Polarizing Optical Microscopy

Since POM textures are one of the simplest and reliable way of identifying known liquid crystal phases, we investigated textures of TOPAB and THPAB using cells made with untreated (no polyimide and mechanical treatment) ITO-coated glass plates with a 4

m cell gap. We focused on changes in textures with temperature to pinpoint the transitions from one phase to another. Samples were placed near the edge of the cell and heated above the clearing point. As the sample melted, it was driven into the cell by capillary action. POM textures were observed and recorded using a CCD camera upon cooling from the isotropic temperature of 125oC for TOPAB and 130oC for THPAB. As seen in Fig. 4.18, the phase sequence of both materials is clearly isotropic → nematic → columnar → crystal phase. The dark state of both samples that remained unchanged under sample rotation between crossed polarizers is the isotropic phase at 123oC for

TOPAB and 128oC for THPAB. At lower temperatures, the appearance of Schlieren and mosaic textures confirmed the nematic and the columnar phases, respectively. On further cooling, the non-uniform texture of the crystalline phase was obtained.

We also performed texture studies in cells with homeotropic boundary conditions induced using polyimides S659 and S60702. Both polyimides are good for obtaining homeotropic state for calamitic liquid crystals. The liquid crystals TOPAB and THPAB were filled into the cells at 125oC and 130oC, respectively and annealed for ~1 hour to remove defects and remanence of flow alignment. On cooling the samples, small droplets started appearing at 120oC and 127oC, respectively for the two compounds. These droplets mark the appearance of the nematic phase. As shown in Fig. 4.19, typical non-

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uniform birefringent Schlieren texture is observed that is characteristic of the nematic phase. One can observe, both, the 2 and 4 brush disclinations in both cells made with the two polyimides. Generally, the uniaxial liquid crystals show 4-brush defects only, while the biaxial liquid crystals are expected to show both 2- and 4-brush defects [12]. A real homeotropic state of TOPAB could not be achieved by these polymers. To observe the effect of cell gap on the alignment obtained, wedge cells were prepared. On one side of the cell, a 22 m thick Mylar sheet was used while the other side was separated by a 50

m Mylar sheet. However, the real homeotropic state was not obtained for the nematic phase of TOPAB at any cell gap. Only Schlieren textures with 2- and 4-brush disclinations were observed. The same was true for THPAB. If the microscopic entities of this system were calamitic, we practically expected to achieve a homeotropic state; dark view at any rotation of the sample with respect to the crossed polarizers since the director aligns perpendicular to the surface. In addition to the two polyimides, S659 and S60702, a lecithin layer which has traditionally been used to align the nematic phase homeotropically [13] was employed for TOPAB. However, the only Schlieren texture was obtained, same as with the two polyimides. One class of the materials, which behave differently from calamitic mesogens, is the bent-core mesogens system. Some of them exhibit an opposite behavior [14]. In the case of TOPAB and THPAB, steric intermolecular interactions of different types than those at play in normal calamitic mesogens must exist to explain the observed behavior.

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Isotropic Isotropic

T = 123oC T = 128oC

Nematic Nematic

T = 115oC T = 110oC

Columnar Columnar

T = 88oC T = 105oC

Crystal Crystal

T = 83oC T = 88oC

TOPAB THPAB

Figure 4.18: POM textures in the four phases in cells made with untreated ITO glass substrates (Right column: TOPAB, Left column: THPAB).

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S659 S60702

At transition from Isotropic to Nematic

Nematic T = 115oC

Nematic T = 100oC

Columnar T = 88oC

Figure 4.19: The POM textures obtained for TOPAB in the 4 m thick cells with two homeotropically aligning polyimides surface coatings. Two and four brush disclinations are visible in the nematic phase.

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In the next stage of experiments, mechanical rubbing was added to the homeotropic and homogeneous polyimide coated surfaces. Fig. 4.20 shows a homogeneously aligned state. Maximum brightness was obtained when the rubbing direction orients at 45o to the crossed polarizers. When the rubbing direction is along either polarizer or analyzer direction, minimum brightness is obtained. Upon rotating the sample, brightness varied as a sinusoidal function of the rotation angle. This indicates that the optic axis (or, director n) is parallel to the rubbing direction but with an unknown pretilt angle, i.e., the angle with respect to the substrate’s surface.

When an electric field is gradually applied perpendicular to the cell plane, the cell alignment on unrubbed homeotropic, rubbed homeotropic, and rubbed homogeneous polymers switch to a brighter state than the initial (E = 0) state at electric field strengths of 2-3 Vrms/m. This can be explainable by the director becoming more parallel to the field and far away from the substrate surface and producing comparatively larger value of optical retardation (= Δnt, Δn = birefringence and t = the cell thickness), because the transmitted intensity for a nematic liquid crystal cell between the two crossed polarizers

2 is written as It = I0 sin (π∆n푡/λ), where λ is the wavelength of light. Alternatively, the low applied field may be aligning the second director of the biaxial phase.

At sufficiently high electric fields (7 – 17 Vrms/m), the effective optical retardation of the cell becomes less at all nematic temperatures (see Fig. 4.20 and Fig.

4.22). Other effects, such as electro-hydrodynamic instabilities, were not observed in these samples. When a high electric field was applied to the sample with unrubbed homeotropic polymer film, defect lines were observed. These defect lines persisted even

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after the removal of the electric field at 115oC as shown in Fig. 4.21. These lines still existed at lower temperature down to 100oC. Normally, the director n reoriented by applied electric field returns to the initial state after the removal of field. Therefore this may arise from the deformation of one of the other directors (say, m of the biaxial nematic phase) the applied field which could not revert back to the original state. From these observations, we conclude that overall less birefringent state achieved under the applied high electric field was caused by n’s reorientation, parallel to the electric field which is the normal to the surface plane. We can confirm this from the conoscopy observation and capacitance measurement.

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Figure 4.20: The POM textures at variable temperature obtained for the TOPAB in the 4 m thick cells with homogeneous polyimide SE7792. (The double arrow represents the rubbing direction.)

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T = 115oC

T = 100oC

Figure 4.21: The POM textures obtained for TOPAB in the 4 m thick cell coated with homeotropic polyimide under different applied electric fields in the nematic phase at 115oC and 100oC.

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(a) T = 100oC with planar alignment

(b) T = 100oC with homeotropic alignment

Figure 4.22: POM textures of (a) homogeneously and (b) homeotropically aligned nematic phase of TOPAB under different applied electric fields at 100oC. As the

electric field increases, the brightness increases until (a) E = 3.0 Vrms/m, (b) 2.5 Vrms /m. Brightness decreases at higher electric field.

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In order to observe the behavior with electric field in the plane of the cell, cells fabricated at Samsung Electronics Co. were used because the process of fabrication is rather hard; width of electrodes are in unit of m. The linear electrodes were patterned on transparent glass plates, the width of electrodes was 4 m and the distance between adjacent electrodes 20 m. Non-transparent Al/Mo electrodes (height of metal layer on the glass plate ~150 nm) were used to block the light, instead of the transparent ITO electrodes because for POM observation it is hard to determine which area is active if the electrodes are transparent. The area under electrodes appeared dark. Electric field is applied between adjacent electrodes. These in-plane switching (IPS) cells were used to observe the behavior of nematic phase compounds.

The polymer layers in IPS cells were prepared in two configurations. In one case, the rubbing direction was parallel to the electrodes while in the second case it was perpendicular to electrodes. Figs. 4.23 and 4.24 show the results obtained in the two cases.

Rubbing direction in Fig. 4.23(b) is parallel to the x-axis and normal to the electric field direction. When the applied electric field was high, parallel to the substrate surface, and normal to the rubbing direction and electrodes, optical retardation increases and the disclination lines appear in the regions between the electrodes. These disclination lines appear as the director n cannot be uniformly aligned, as shown in Fig. 4.23(b). The director n is initially parallel to the x-axis. With an applied electric field in y-axis the director tries to reorient with the field and becomes parallel to the y-axis. During reorientation, however, the director at the center cannot move because the molecules from both sides push to an opposite direction (one is clockwise, the other one is

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counterclockwise direction) to become aligned parallel to the electric field thus a dark line is created in the middle which is the same as the case of no electric field. Since the electric field lines are not parallel to the plane of substrate near the edge of active area

(near electrodes), optical retardation is different at different distances from the electrode.

The birefringence changes are larger in the former case. In the latter case in Fig. 4.24, birefringence changes are smaller than the case when the electric field and the director initially are in the same direction. This behavior on the IPS cells is the same as a liquid crystal with positive dielectric anisotropy, ∆ε > 0.

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(a)

(b)

Figure 4.23: (a) POM observation for homogeneously aligned IPS cell under electric field in the nematic phase between crossed polarizers when the rubbing direction is parallel to the electrodes, and insects are the magnified section of the center. (b) schematic illustration of the appearance of disclination lines in the center of the non- electrode areas. (R/D: rubbing direction)

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Figure 4.24: Electro-optical behavior of a homogeneously aligned IPS cell, the rubbing direction (red arrow) is perpendicular to the linear electrodes and parallel to the applied electric field. The red lines appearing with increasing field arise from the deformed director field caused by fringing-field at the edge of the electrode. (The direction marked as P and A on in the circular area on RHS represents the directions of polarization and analyzer axes.)

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4.3 Conoscopy and Optic axis

4.3.1 Conoscopy

Conoscopy provides a map of optical variations and direction of optic axes in a medium. It was employed to study the nematic phase of TOPAB in cells with antiparallel rubbing of two different thicknesses, 4 and 14 m, under an applied the electric field. For conoscopic observations, the iris before the condenser was fully opened which increases the optical energy incident on the sample. Since the material TOPAB has an azo group, exposure to bright light can transform molecules from trans to cis conformation. The intensity of light was kept at a minimum workable level by using long camera exposure times and high sensitivity. The conoscopic images shown in Figs. 4.25 and 4.26 were taken using the same 4 m cell as used in POM studies. Fig. 4.25 shows the changes in conoscopic image in a cell with the rubbing direction parallel to the polarizer’s easy axis and for different electric fields while the sample temperature was kept at 115oC. The electric field was applied normal to the cell. At E = 0 Vrms/m, only the brightness changes when it is rotated about the microscope axis (i.e., the direction of light’s propagation). The maximum/minimum brightness is observed when rubbing direction is at 45o/0o with respect to the polarizer/analyzer axis. It can be said that the optic axis is at a large angle wrt the cell normal. Conoscopic pattern gradually changes when applied electric field is increased and it affects the orientation of the optic axis or the director of the nematic phase. A faint dark line started appearing in the field of view at around 1.2

Vrms/m. At this point, the director began to reorient. At ~ 2.4 Vrms/m, two crossed dark

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line, called isogyres, appear suggesting that the optic axis is essentially parallel to the cell normal. At this field, the sample shows the maximum transmittance under POM observation when the rubbing direction is at 45o to the polarizers.

Figure 4.25: Conoscopic images of TOPAB at 115oC in the nematic phase as a function of increasing applied electric field in a homogeneous 4 m anti-parallel rubbed cell. The rubbing direction is parallel to the polarizer’s easy axis and objective lens magnification was 20 and N.A. = 0.4.

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o (a) T = 115 C applied E =14.6 Vrms/m

o (b) T = 100 C applied E = 15.6 Vrms/m

Figure 4.26: Conoscopy patterns in a homogeneous 4 m cell of TOPAB with anti- parallel rubbing, at different temperatures and at various angles of rotation with respect to the polarizer direction (objective lens 20x, N.A. = 0.4). Here (a) E = o o 14.6 Vrms/m at 115 C, (b) E = 15.6 Vrms/m at 100 C. Red arrow, and the directions marked as A and P represent the rubbing, analyzer, and polarizer directions, respectively.

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o (a) T = 115 C applied E = 12.6 Vrms/m

o (b) T = 100 C applied E = 17.1 Vrms/m

Figure 4.27: Conoscopy observations in a 4 m thick cell of TOPAB with homeotropic polymer coating and anti-parallel rubbing. The pictures were taken with while the cell was rotated and at two different temperatures and fields (objective lens o o 20x). (a) E = 12.6 Vrms/m at T = 115 C, (b) E = 17.1 Vrms/m at T = 100 C.

At a higher electric field, the isogyres appear in the field of view which suggests that the optic axis passes through points in the field of view. Fig. 4.26 shows conoscopic images at different rotation angles with respect to the polarizer’s easy axis at high electric field, at 115oC and 100oC. Splitting of the isogyres was observed as the sample was

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rotated, and the maximum splitting occurred at 45o. This pattern was repeated every 90 o of rotation. Same result was observed for the homeotropic cell, which was used for POM observations (see Fig. 4.27). The splitting of isogyres indicated the existence of phase biaxiality [15]. However, this can also result from tilt surface anchoring conditions in thin cell [16] that will cause the director near substrate’s surface to tilt with respect to the surface normal. Thus, we used cells of 14 m thickness to eliminate this possibility.

4.3.2 Conoscopy and Optic axes in thick cell

In thick cells, clear changes in isogyres were observed. The results at 115oC and

100oC in the presence of a high electric field applied to a homeotropically-aligned cell are shown in Fig. 4.28. As the sample was rotated to different angles, the splitting of isogyres was very clearly observed due to real phase biaxiality and is presented in Figs. 4.29 and

4.30 at 100oC and for two different electric fields. To confirm this further and locate the site where the optic axes crossed the field of view, Benford plates (two λ/4 retarders) were inserted as previously discussed. Fig. 4.28 shows that, at low electric fields, nothing was observed very clearly. At high electric fields, two poles representing the optic axes appeared as the major director was rendered perpendicular to the cell normal. The line joining the two optic axes was roughly perpendicular to the rubbing direction. As shown in Figs. 4.29 and 4.30, the two poles moved in the direction of cell’s rotation.

All cells used in our investigations exhibited the same trend when sufficient high electric field was applied. Based on the results of conoscopy, the maximum transmittance observed at intermediate electric fields discussed previously can now be reconciled. To obtain a quantitative dependence of the brightness change for TOPAB on the applied

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electric field, a homogeneously aligned cell was used to measure transmittance in between crossed polarizers with a laser of  = 633 nm. As shown in Fig. 4.31, from a review of the plot of transmittance vs. applied electric field and changes in POM textures and conoscopic patterns, at applied E = 3.2 Vrms/m, it is clear that the appearance of dark line in conoscopic image is due to the optic axis orienting almost normally to the

o surface Tmax. Thus, as the electric field was increased beyond E = ~1.3 Vrms/m at 100 C, the director began to continuously reorient towards the cell normal and eventually became almost parallel to the electric field. This was also later confirmed by capacitance measurement described in the next section.

All results of conoscopy, observation of the location of two optic axes and 2-brush disclinations, anomalous response to electric field, and appearance of splitting of small angle peak at low temperatures point to a strong possibility of the existence of the nematic biaxiality in these materials as we have tried to illustrate in Fig. 4.32.

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Figure 4.28: Effect of applied electric field on conoscopic patterns of TOPAB in a 14 m cell at 115oC and 100oC in the nematic phase. The cell walls were coated with homeotropically aligning polymer and rubbed in an anti-parallel manner. The pictures shown in the bottom panel were taken with Benford plates, and the dark spots clearly show the existence of two optic axes at high electric fields.

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o T = 100 C applied E = 6.4 Vrms/m (a) Conoscopy (b) Optic axis

Figure 4.29: At T=100oC in the nematic phase of TOPAB, (a) conoscopic image (b) optic axis observations with Benford plate in a homogeneous 14 m thick cell under different angles of rotations.

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o T = 100 C applied E = 7.2 Vrms/m (a) Conoscopy (b) Optic axis

Figure 4.30: At T=100oC in the nematic phase of TOPAB, (a) conoscopic image, and (b) optic axis observations in a 14 m homeotropic cell at different rotation angles between crossed polarizers.

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Figure 4.31: At T = 100oC in the nematic phase of TOPAB, transmitted intensity as a function of applied electric field, POM texture, and conoscopic images in a 4 m thick homogeneous cell with different applied electric field. Appearance of the dark line in conoscopic image corresponds to maximum transmittance (or highly birefringent texture) means the optic axis exists close to the cell’s surface normal. (The red double arrow presents as rubbing direction.)

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Figure 4.32: When sufficient high electric field applied to the sample, the director n is oriented parallel to electric field (OA: optic axis). Optic axes correspond to the two dark spots in Fig. 4.29 and Fig. 4.30.

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4.4 Capacitance Measurements

4.4.1 Temperature Dependence of Capacitance

Capacitance measurements were made using an LCR meter (QuadTech 1920

Precision). The ITO substrates of liquid crystal cells were coated with homogeneously

(SE7792) and homeotropically (S659) aligning polyimides, followed by mechanical

o rubbing. The liquid crystal TOPAB was filled at a temperature above TNI ~ 130 C. The sample was placed inside a hotstage (Instec® HCS402) and its temperature controlled with a precision of  0.1C. Before starting the capacitance measurements, uniformity of alignment was confirmed using polarized microscopy. A 0.2 Vrms signal which is well below the voltage that could begin to perturb the reorientation of n at 1 kHz (sine wave) was applied to the sample and capacitance measured at different temperatures. Fig. 4.33 shows the capacitance measured on 4 m thick planar and homeotropically aligned cells during cooling from the isotropic to the columnar phase. The dielectric constant of

TOPAB was calculated from the ratio of capacitance between TOPAB filled cell and empty cell, εLC = CLC / Cempty. The capacitance or dielectric constant abruptly dropped by ∆ε121℃−118℃ = ε121℃ − ε118℃ = 0.093 at the first order isotropic to nematic phase transition. Dielectric constant in the isotropic phase is the average dielectric constant of the system. Its value does not depend on the type of surface coating and the treatment.

Small value of average dielectric constant ~ 2.97 is close to the expectation on the basis of our oblique H-bonding model of the TOPAB because the molecules in a dimer (or, a tetramer) are symmetrically oriented at an effectively large tilt wrt n. For both cells coated with planar and homeotropic polyimides the dielectric constants in the nematic

146

phase entirely are smaller than the isotropic phase.

4.4.2 Electric Field Dependence of Capacitance

We also measured the capacitance in the nematic phase of TOPAB as a function of electric field strength. For this experiment, the method described in Chapter 3 and illustrated in Fig. 3.7(b) was used. At every change of the input voltage, the in-phase component on the lock-in-amplifier (Vr) was nulled using pure compensating capacitors before connecting to the sample cell. The out-of-phase component (Vc) was recorded and used to calculate the capacitance via eq. 3.2(a). Fig. 4.34 shows a plot of measured capacitance vs. electric field at three selected temperatures in the nematic phase. The capacitance begins to increase significantly beyond a field of approximately 0.8 Vrms/m

o o o at 115 C, 1.1 Vrms/m at 106 C, and 1.4 Vrms/m at 100 C. These values are threshold electric fields (or, voltages) at which that molecules start orienting in response to the applied field. Clearly, this threshold field increases with decreasing temperature due to an enhancement of orientational order parameter [17] and other physical properties at lower temperatures. Dielectric constants were calculated from the capacitance curve when it becomes flat at high electric fields as we mentioned before when the director n becomes perpendicular to the surface. Therefore this value can be considered as ε∥. Whilst we do not know how the molecules of TOPAB are initially aligned on the surface coated with homogeneous polyimide, the value ε⊥ cannot be determined in this experiment. Hence, we cannot calculate Δε at this stage. However, if we put together the results of POM, conoscopy observation, and capacitance, this nematic phase behaves like a liquid crystal with positive dielectric anisotropy (i.e., Δε > 0).

147

Figure 4.33: The temperature dependence of capacitance and dielectric constant of TOPAB in homogeneous (up) and homeotropic alignment (down) conditions in a 4 m cell.

148

Figure 4.34: Capacitance of TOPAB as a function of electric field at three selected nematic temperatures in a 4 m thick cell with planar alignment. Capacitance o increases with increasing electric field beyond ~ 0.8 Vrms/m at 115 C, ~ 1.1 Vrms/m o o at 106 C, and ~ 1.4 Vrms/m at 100 C.

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4.5 Raman Scattering

In order to measure the nematic order parameter in the nematic phase, we performed FT-Raman investigation of TOPAB using the facilities at Georgia Institute of

Technology. For this purpose, the homogeneously aligned and anti-parallel rubbed cells were prepared. The liquid crystal TOPAB was filled into the 14 m cell above the clearing point of ~ 130oC and cooled at a rate of 0.2oC/min to 118oC. The Raman intensity profiles were taken at 3oC decrements starting at 118oC.

o Fig. 4.35 shows the Raman spectrum, IHH and IVH, obtained for TOPAB at 103 C.

Three peaks corresponding to different modes of the molecules are marked with stars of

-1 different color. These peaks detectable in both IHH and IVH spectra were at 1600 cm ,

1442.5 cm-1, and near 1200 cm-1. In the mono-, di-, and tri- substituted benzenes, two quadrant stretch components usually gave rise to bands between 1620 – 1565 cm-1 of intermediate intensity [18]. At and below 1200 cm-1, other bands were found that involve mainly the in-plane C – H bend of aromatic rings [18]. The N = N stretch mode of symmetrically substituted trans isomer is usually IR-forbidden [19], but exhibited a very intense Raman band. The trans azobenzenes (Ar – N = N – Ar) have an N = N stretch frequency between 1463 – 1380 cm-1 whereas the N = N stretch frequency of cis azobenzenes was near 1511 cm-1 [18, 20, 21]. The peaks around 1600 cm-1 were difficult to analyze because of their weak intensity. The peaks in the range of 1200 cm-1 to 1140 cm-1 overlapped with many other peaks. The peak at 1442.5 cm-1 was selected for quantitatively analysis to obtain the orientational order parameter S since the Ar–N=N–

Ar chromophore lies on the axis of the rigid core unit of the TOPAB, and shows strong

150

intensity.

Figure 4.35: Raman scattering spectrum obtained from the TOPAB, shows parallel o (IHH) and perpendicular (IVH) mode at 103 C in the nematic phase. The yellow star marks C=C ring stretching vibrations and the blue star is above the peak arising from Ar–N=N–Ar stretching mode of trans-aromatic azo compounds. The purple star points to the aromatic C-H in-plane deformation vibrations. (The sharp peak of the -1 graphs IVH at around 1520 cm is noise peak).

151

In order to determine the area under the peak, the Raman peaks at 1442.5 cm-1 were fitted to Lorentzian and Gaussian functions. Its integrated intensity was used to determine the average polarizability tensor <αij, αij> at corresponding temperatures, polarization conditions and relative orientations of the sample [9]. A linear baseline was assumed from 1510 to 1370 cm-1 neither the frequency nor the width of the bands was fixed. The best least-squares fit was chosen by χ2 minimization and standard deviations were obtained from the fitting program GRAMS/AI, version 7.02 from Thermo Galactic.

Fig. 4.36 is an example of the experimental data and fitted curves for IHH and IVH mode at

118oC. The data were fit to a combination of cosines functions as the molecule is vibrating or rotating in a periodic manner that can be described by cosine functions. The corresponding molecular electric fields will then be sinusoidal functions of time. Since the scattered intensity by these modes is proportional to the square of electric field, one is left with only the cosine terms because the square of the sinusoidal function of electric field (eq.3.4 in Ch.3) is a complex conjugated multiplication. Thus, the imaginary part (or, the sine terms) [ 2 = ( ei )( ei ) ] drops out [22]. As discussed in the chapter 3, the

Raman scattering depolarization ratio R can be obtained from the ratio of each intensity,

R(θ) = I⊥(θ) / I∥(θ), where θ is the angle of sample orientation with respect to the polarizer. The corresponding depolarization ratio calculated from the data in Fig. 4.36 is shown in Fig. 4.37 along with the fits to eq. (3.7) without the biaxial order parameters,

, , and because the fitting was reasonably good even without them.

o The fitting procedure yielded {, , r} values of {0.49, 0.27, - 0.18} at 118 C.

Fig. 4.38 (a) and (b) show the fits to the normalized Raman intensity profile of IHH and

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IVH at various temperatures. Temperature dependence of the depolarization ratio fitted to eq. (3.7) is shown in Fig. 4.38 (c). From the fits to R, the uniaxial order parameters

and were obtained and are plotted in Fig. 4.39 as functions of temperature along with the parameters obtained from X-ray scattering discussed earlier in Section

4.1.5. The two uniaxial order parameters in the nematic phase increase with decreasing temperature as expected; = 0.48 – 0.75 and = 0.25 – 0.48 with variation of r, the differential polarizability ratio of the N=N stretching band. The red solid line is a fit

T to of the equation of 푆 = [1 − ( )] [23] where, S is a scalar nematic Tc orientational order parameter and which is the same as and Tc is a temperature

o slightly above the clearing point, i.e., TNI = 120 C. The values obtained from

Raman scattering are higher than that obtained from X-ray scattering. This discrepancy can arise from the different mathematical process or poorer alignment of x-ray samples.

There has been a mystery concerning the value and sign of determined by the

Raman technique [9] such as a sign of is negative or positive even for the same material.

Raman scattering experiment allowed the determination of the uniaxial order parameters of the nematic phase from the analysis of N = N stretch vibration band at

1442.5 cm-1 of TOPAB over full nematic range.

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Figure 4.36: Normalized Raman spectra obtained from the TOPAB at 118oC in the nematic phase are shown with the fitted curves.

154

Figure 4.37: The depolarization ratio profile calculated from the data in Fig. 4.36 and the fit to eq. (3.6) yield = 0.49, = 0.27, and r = – 0.18.

155

(a) Normalized intensity profile of IHH (b) Normalized intensity profile of IVH vs. temperature vs. temperature

Figure 4.38: (a) and (b): Normalized intensities IHH and IVH obtained from fits, and (c) temperature dependence of depolarization ratio.

156

Figure 4.39: Temperature dependence of the uniaxial order parameters and

in the nematic phase of TOPAB. The solid dark and open pink circle are

and , respectively, from Raman scattering. The red solid line represents a power law dependence on reduced temperature resulting in a virtual transition o temperature, Tc = 120.5 C and order parameter exponent α = 0.16. The solid and open blue circle are and obtained from X-ray scattering.

157

4.6 Conclusion

In this chapter, the results of X-ray results in the nematic and columnar phases of the H-bonded liquid crystal TOPAB and THPAB presented here include the structure of these phases. In addition several other techniques were employed to obtain and confirm the conformation of molecules and the type of H-bonding that occurs in them. The columnar phase has hexagonal packing as confirmed from the ratio of d-spacings. These compounds also exhibit several distinct characteristics that are different from general calamitic liquid crystal, most remarkably in the direction of the small and large angle peaks. The largest d-spacing (the most intense peak closest to the center of XRD pattern) at all temperature from the isotropic to crystal phases varies from 24 Å to 26 Å and does not agree with simple linear and coplanar H-bonding. Positions of both, small and large angle peaks, in the aligned nematic phase are perpendicular to the magnetic field. Also the shape of the first peak at small angle becomes kidney shaped at low nematic temperature. The top of the -scans peak becomes flatter and needed four Gaussian distribution functions to satisfactorily fit the peak-shape. Each of the peaks is found to includes two unresolved peaks which are consistent with short range biaxial nematic order or the presence of biaxial fluctuation. This splitting is similar to that seen in the biaxial nematic phase of bent-core mesogens. Based on XRD observations, we tentatively propose the formation of oblique dimers or tetramers through hydrogen bonding in these phasmidic materials.

Optical observations under various boundary anchoring conditions were carried out. The director n (or, optic axis) aligns along the rubbing direction on homogeneously

158

aligning surface. However, the homeotropic state could not be achieved with any of the two homeotropic polyimides, S659 and S60702 or with lecithin. This result was independent of the cell thickness. Investigation involving the application of an electric field show that the director aligns parallel to the field, i.e., the nematic phase has a positive dielectric anisotropy. The capacitance/dielectric constant in the nematic phase is found to be smaller than that in the isotropic phase. In the nematic phase, the capacitance increases from about 0.8 – 1.2Vrms/m with increasing electric field. From conoscopic observation one can visualize the appearance of two optic axes upon the introduction of

Benford plate further confirming the presence of biaxial features of the nematic phase.

The uniaxial order parameters and in the nematic phase of TOPAB were obtained with both Raman scattering and X-ray scattering. The order parameters were found to be = 0.52 – 0.68 and = 0.12 – 0.30. As determined from

Raman scattering, their values lie in the ranges: = 0.48 – 0.75 and = 0.25 –

0.48, in rather good agreement with the value of from X-ray measurements.

159

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[4]. J. M. Seddon, Handbook of Liquid Crystals, Vol.1, edited by D. Demus, D. Goodby, J. W. Gray, G. W. Spiess, and H. W. Vill, Wiley-VCH (1998).

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Chapter 5

Binary Mixtures of Hydrogen Bonding Mesogens

5.1 Introduction

The self-assembly process has versatility in dynamically constructing functional liquid crystal molecules using non-covalent H-bonding [1] as is the case with mesogens discussed in the previous chapter. Many liquid crystalline compounds have been prepared and reported to exhibit mesophases formed via the association of identical or dissimilar molecules. The individual hydrogen-bonding molecules may or may not be mesogenic [2] by themselves. When the component molecules associate with each other by H-bonding, the resulting molecular shape is determined by the molar ratio of the components, their chemical properties, molecular shape, etc. One expects interesting concentration dependent mesomorphism in such systems.

The XRD images in the nematic phase of TOPAB shows very different pattern that much deviates from that of typical calamitic nematic liquid crystal. The flexible chains are subjected to interchain attractive interactions [3] and steric and conformation constraints. To learn more about the H-bonding, we blended pure TOPAB with other simpler and linear hydrogen bonding mesogenic materials shown in Table 5.1. When a molecule of these compounds H-bonds with one molecule of TOPAB, one expects the formation of a molecular level entity that resembles a side-ways fork. Such a shape is inherently biaxial. Thus these investigations will also test the possibility of the formation

161

162

of the biaxial nematic phase in these mixtures.

In this chapter, results of investigations of H-bonded mixtures of TOPAB with three cylindrically symmetric molecules will be presented. In addition to equimolar mixtures for stoichiometric match to build heterodimers, mixtures in molar ratios of 1:3 and 3:1 of TOPAB with OOBA and DBBA were also studied with the expectation that these mixtures may display mesophases with formation of two and/or four H-bonded hetero- and homo-dimers. Thermal properties, phase behavior, and phase structures of these mixtures were measured by differential scanning calorimetry, polarized optical microscopy, and X-ray scattering experiments.

Table 5.1 Molecular structure, chemical formulae and phase sequence of compounds OOBA: 4-(4-octyloxy)-benzoic acid, DBBA: 4-(4-decyloxybenzoyloxy)-benzoic acid, HPBA: 4-(4-Hexadecyloxy-phenylazo)-benzoic acid.

OOBA

C15H22O3 Iso 147 oC N 108 oC SmC 101 oC Cr

DBBA

C24H30O5 Iso 228 oC N 184 oC SmC 135 oC Cr

HPBA

C29H42N2O3 o o o o Iso 245 C S1 209 C S2 172 C S3 142 C Cr [6]

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5.2 Preparation of Binary Mixtures

Table 5.1 shows the compounds and their phase transition temperature that were used mixed with our main compound TOPAB in preparing binary mixtures. The compound OOBA (4-(4-octyloxy)-benzoic acid) was purchased from Aldrich Chemical

Co. and used as received, and HPBA (4-(4-hexadecyloxy-phenylazo)-benzoic acid) and

DBBA (4-(4-decyloxybenzoyloxy)-benzoic acid) were synthesized by Dr. Veena Prasad at the Centre for Liquid Crystal Research, Bangalore in India. The material HPBA has several smectic phases without the nematic phase, whereas OOBA and DBBA exhibit the nematic and smectic phases. OOBA has the simplest molecular structure and its monomer length of 17.5 Å . DBBA has COO group instead of N=N of TOPAB and its molecular length is about 26.0 Å . HPBA is chemically compatible with TOPAB as it has just one long flexible chain containing the same core unit as TOPAB. As seen from molecular structures in Table 5.1, the presence of a carboxylic acid (COOH) group on one side of the aromatic core of these materials facilitates H-bonding. Furthermore, OOBA has previously been used in studies of mixture involving hydrogen bonding [4].

To prepare mixtures, the materials in powder form were weighed by a microbalance (Sartorius MC21S) with accuracy 0.001 mg. Binary mixtures of all materials used in this dissertation were prepared by mechanical stirring of the components at a temperature above the clearing point [5] with the use of a minishaker at a speed of 2500 rpm for 40 minutes.

164

5.3 Results and Discussions

5.3.1 DSC Results

DSC measurements were performed on the DSC equipment (TA Instruments,

2920 Modulated DSCTM) in Prof. R. Twieg’s laboratory at a scan rate of 5oC/min. Since we did not have sufficient amount of all compounds, DSC measurements were performed on only two mixtures. We relied on POM and X-ray work for the identification of phases and transition temperatures for other mixtures. Fig. 5.1 shows DSC results from equimolar mixtures of TOPAB and OOBA, and 3:1 molar ratio mixture of TOPAB and

DBBA. The DSC thermograms of the two mixtures show that at least two mesophases exist below the isotropic phase.

In case of the 3:1 molar ratio mixture for TOPAB and DBBA, the values of TNI and TNA are much different than the transitions temperatures of individual components and stand at 139.0oC and 95.7oC, respectively, see Fig. 5.1 (a). The DSC graphs of two different mixtures clearly show the two phase transitions leading us to conclude that these mixtures are stable and no phase separation occurs over the time period of the experiments.

In the case of the equimolar mixture of TOPAB and OOBA, the nematic phase is

o observed at 120.7 C which is nearly same transition point as for pure TOPAB (TNI =

o o 120 C) and much lower than the pure OOBA (TNI = 148 C). The transition to the smectic

o phase appears at around TNA = 85 C. The temperature range of the nematic phase is widened by ~10oC compared to pure TOPAB. The decrease in the (lower) second order nematic-smectic transition temperature may be due to impurity effects. 165

(a) TOPAB : DBBA = 3 : 1

(b) TOPAB : OOBA = 1 : 1

Figure 5.1: DSC results of two mixtures (a) 3:1 molar ratio mixture of TOPAB and DBBA, and (b) 1:1 molar ratio mixture of TOPAB and OOBA.

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5.3.2 Polarizing Optical Microscopy and LC Textures

For identification of liquid crystal textures of phases and transition temperatures, cells with 2 m Mylar film spacers were used. Thin film of polyimide SE7792 was spin- coated on to clean ITO-coated substrates, subjected to soft and hard-bake cycle, and then mechanically rubbed unidirectionally. Cells were filled with mixtures through capillarity in their isotropic phase. The cells were then examined between crossed polarizers and the textures were recorded while rotating the cell with respect to the easy axis of polarizers.

The planar director orientation obtained in these cells is excellent for checking the phase behavior of unknown materials.

In case of TOPAB : HPBA equimolar stoichiometric 1 : 1 mixtures, a phase separation apparently of dissimilar concentrations (or, even pure components) was observed by naked eye that was also evident in their POM textures. So we made an attempt to mix them first in chloroform. Any traces that might be dissolved in chloroform were extracted with the help of molecular sieves. The molecular sieves were first dried in an oven to remove water obtained from storage. After cooling in a dessicator, they were placed into a bottle with chloroform for 1 hour, and then the chloroform was used.

Equimolar amounts of the two components were placed in a container and a few drops of chloroform added and mixed with the use of a minishaker at room temperature. Solvent was allowed to slowly evaporate. The two components still remained separated, e.g., in some regions the smectic phase appeared at higher temperature than the isotropic state in other regions, and the nematic phase appeared at lower temperature in yet another region.

The apparent immiscibility of the two components of this binary system might arise from 167

mismatched physical sizes and chemical nature of the two types of molecules [7], causing us to abandon the investigation of this binary system.

Figs. 5.2 and 5.3 show the POM observation of the mixtures TOPAB + OOBA and TOPAB + DBBA with varying molar ratio, respectively. The dependence of the phase transition temperatures on the mole ratio of two components in two mixtures is shown in

Fig. 5.4. All mixtures display the nematic phase on cooling from the isotropic phase and one more mesophase at lower temperatures, eventually ending in a crystal state.

Interestingly, in TOPAB + OOBA binary mixtures, the nematic transition from the isotropic phase occurs at around TNI of TOPAB, whereas TOPAB + DBBA binary systems display distinguishable trend; decreasing TNI with increasing molar ratio of

TOPAB in the mixture. In the TOPAB + DBBA binary mixtures, a coexistence region of isotropic and nematic phases, ΔTNI, is rather wide. Here, ΔTNI is the temperature range from the first appearance of the nematic phase to complete disappearance of the isotropic

o regions. ΔTNI increases with increasing concentration of DBBA and approaches 12 C for the 1:3 mixture of TOPAB : DBBA. This is expected because for pure DBBA, ΔTNI is

~15oC as shown in Fig. 5.4 (b). All samples uniformly align in the nematic phase when a rubbed polymer alignment layer is used. The director lies along with the rubbing direction as determined from maximum brightness of transmitted light observed when the rubbing direction is at 45o with respect to the crossed polarizers. The dark state was obtained when the rubbing direction became parallel to the easy axis of one of the polarizers. Total temperature range of all mesophases was enhanced for all mixtures relative to the pure OOBA and DBBA. Specially, the mixtures of TOPAB : OOBA = 1:3 168

and of TOPAB : DBBA = 1:3 exhibit mesophases over ~ 77oC and 106oC, respectively.

On further cooling from the nematic phase, the clear focal-conic fan textures characteristic of smectic-C phase which was observed in most mixtures. In TOPAB :

OOBA = 3:1 mixture however, it is difficult to distinguish whether it is a smectic or the columnar phase. To address this question better, we carried out X-ray scattering investigations.

169

(a) 3:1 (b) 1:1 (c) 1:3

Iso → N 115.1oC Iso → N 119.3oC Iso → N 119.6oC

N 100.0oC N 107.0oC N 110.0oC

Sm or Col 80.0oC SmC 86.2oC SmC 70.0oC

Figure 5.2: Representative POM textures at different temperatures and in different phases of mixtures of TOPAB + OOBA (a) 3:1 molar ratio, (b) 1:1 equimolar, (c) 1:3 molar ratio (white double arrow indicates the rubbing direction).

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(a) 3:1 (b) 1:1 (c) 1:3

Iso → N 137.0oC Iso → N 163.8oC Iso → N 186.0oC

N 128.0oC N 149.0oC N 170.0oC

SmC 89.0oC SmC 76.0oC SmC 120.0oC

Figure 5.3: Representative POM textures in different phases of mixtures of TOPAB + DBBA in (a) 3:1 molar ratio, (b) 1:1 equimolar, (c) 1:3 molar ratio (the white double-arrow indicates the rubbing direction).

171

(a)

(b)

Figure 5.4: Plot of phase transition temperatures for mixtures of (a) TOPAB + OOBA, (b) TOPAB + DBBA at different concentrations.

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5.3.3 X-ray Diffraction Results

All X-ray experiments were performed at station 6-IDB of the Midwestern

Collaborative Access Team at the Advanced Photon Source of Argonne National

Laboratory. The experimental set up was discussed in the Chapter 3. The samples were filled into capillaries and placed in a 2.5kG magnetic field to align the nematic phase.

XRD patterns of mixtures were taken while lowering the sample temperature from temperatures well above the isotropic phase. Figs. 5.5 and 5.6 show representative XRD patterns in different phases of the mixtures of TOPAB + DBBA and TOPAB + OOBA at the three molar ratios, also the values of d-spacing corresponding to each peak are denoted on these patterns. The white double arrows indicate the direction of the magnetic field (i.e., defining the equator). The d-spacing obtained by Lorentzian fits to the data is plotted against temperature in Figs. 5.7 and 5.8 for TOPAB + OOBA and TOPAB +

DBBA mixtures. It should be noted that d-spacing values for all mixtures are much smaller than expected and gradually decrease on cooling as seen from the plots. For example, one can imagine the average shape of a tetramer to be disk-like in 1:3 or 3:1 mixtures. Its d-spacing should be the sum of each component’s length, which for TOPAB

+ DBBA is about 50.5 Å , however, the d-spacings are obtained between 33.2 - 36.0 Å in the nematic phase.

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(a) 3:1 (b) 1:1 (c) 1:3

4.5Å 4.6Å 4.7Å

29.0Å 31.4Å 35.8Å

7.3Å 18.9Å 19.1Å 19.8Å

N 137.0oC N 165.0oC N 180.0oC

4.5Å 4.6Å 4.6Å

26.2Å 29.7Å 33.5Å

7.4Å 19.2Å 19.3Å 20.3Å

N 103.5oC N 133.0oC N 158.0oC

4.4Å 4.5Å 4.5Å

27.9Å

d =26.3Å 14.0Å 1 d1=29.9Å d =13.1Å d =14.9Å 2 2 SmC 82.0oC N → SmC 114.5oC SmC 145.0oC

Figure 5.5: Representative XRD patterns for mixtures TOPAB + DBBA in molar ratios of: (a) 3:1, (b) 1:1, and (c) 1:3. The white double arrow indicates magnetic field direction.

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(a) 3:1 (b) 1:1 (c) 1:3

4.5Å 4.5Å 4.4Å

25.8Å 27.6Å 27.8Å

18.2Å 19.1Å 7.0Å 7.0Å 18.8Å 8.2Å

N 117.0oC N 120.0oC N 116.0oC

4.5Å 4.5Å 4.5Å

24.6Å 26.3Å 26.5Å

7.2Å 7.0Å 7.8Å 18.5Å 19.3Å 17.8Å

N 95.0oC N 96.0oC N 92.0oC

4.5Å 4.5Å 4.5Å d3=8.7Å 14.3Å

d=25.0 & d =26.1Å d=26.0Å d=15.4Å 24.0Å 1 d =13.1Å 2 N → Col 91.0oC SmC 76.0oC SmC 87.5oC

Figure 5.6: Representative XRD pattern for the mixtures TOPAB + OOBA in molar ratios of; (a) 3:1, (b) 1:1, (c) 1:3. The white double arrow indicates magnetic field direction.

175

3:1

1:1

1:3

Figure 5.7: Temperature dependence of the largest d-spacing for the mixtures of TOPAB + OOBA in the molar ratios indicated on the left hand side. The lines are drawn as guide to eye.

176

3:1

1:1

1:3

Figure 5.8: Temperature dependence of the largest d-spacing for the mixtures of TOPAB + DBBA in the molar ratios indicated on the left hand side. The lines are drawn as guide to eye.

177

All three mixtures showed two diffuse rings, one at small and the other at large angles at their isotropic phase temperatures. As the sample temperature was lowered, diffraction pattern confirmed an aligned nematic phase as expected on the basis of optical textures. As shown in Figs. 5.5 (a, b) and 5.6 (a, b), at higher temperature in the nematic phase, the 3:1 and 1:1 mixtures displayed patterns very similar to the XRD patterns of the pure TOPAB. The first peak at small angle was oriented in the meridional direction or, perpendicular to the magnetic field. This is the most intense peak which originated from the average length of molecules or of molecular associations. In the same (i.e., meridional) direction, second diffuse (liquid-like) peak corresponding to a d-spacing of 4.5 - 4.8 Å appeared at large angles. This arises from average lateral separation between molecules or hydrocarbon segments of molecules. There also are relatively weaker peaks along the equator corresponding to a temperature and concentration dependent d spacing lying between ~18 and 20 Å . Interestingly, the XRD patterns of 3:1 and 1:1 mixtures at higher temperature in the nematic phase are similar to those from the pure TOPAB. These are essentially patterns from the conventional uniaxial nematic phase.

However, the meridional small angle peaks begin to split azimuthally into two peaks at lower temperatures. Fig. 5.9 and 5.10 show magnified XRD patterns in small angle region and χ-scans generated from XRD patterns at several different temperatures in the nematic phase of the two 3:1 mixtures. It is clear that the peaks are weak and broad at high temperatures and they become brighter and split as the temperature is lowered.

This splitting again is indicative of the presence of biaxial fluctuations. In mixtures with

1:3 ratio of OOBA (Fig. 5.11) and DBBA (Fig. 5.12), the splitting of the small angle 178

peaks is apparent even at high temperatures. This suggests that the addition of rod-like molecules to a system where there is a tendency to form oblique multimers, increases the propensity to form biaxial nematic phase.

It should be pointed out that the splitting observed here is not due to the formation of cybotactic groups, which are pretransitional smectic-C fluctuations in the nematic phase. This conclusion is based on the following observations: (i) The azimuthal splitting of small angle peak in the 1:3 mixture of TOPAB + OOBA shown in Fig 5.11(b), and TOPAB + DBBA systems at, both 3:1 and 1:3 concentrations, shown in Figs. 5.10(b) and 5.12(b), remains unchanged with changes in temperature. In a system with small

SmC like clusters the splitting would be temperature dependent and increase with increasing molecular tilt angle with decreasing temperature. (ii) This becomes a strong argument in view of the fact the d spacing is decreasing with temperature. (iii) The constant peak widths show that the correlation lengths remain liquid-like. Fig. 5.13 shows a representative intensity profile and correlation length at different temperature for the

3:1 mixture of TOPAB + DBBA. If cybotactic groups were present, the correlation length would diverge rapidly with a power law and we do not have any indication that it is so.

(iv) Finally, if the nematic phase was infested with cybotactic groups, it will result into the same pattern, i.e. four small angle peaks, in the SmC phase also. However, as we will discuss below for the 1:1 mixture of TOPAB and OOBA, we obtained only two small angle peaks from a single domain of the SmC phase!

179

(a)

(b)

Figure 5.9: (a) X-ray diffraction patterns at high and low temperatures in the nematic phase of 3:1 mixture of TOPAB + OOBA, and (b) χ-scans of small angle peaks at different temperatures.

180

(a)

(b)

Figure 5.10: (a) X-ray diffraction patterns at high and low temperatures in the nematic phase of 3:1 mixture of TOPAB + DBBA, and (b) χ-scans of small angle peaks at different temperatures.

181

(a)

(b)

Figure 5.11: (a) X-ray diffraction patterns at high and low temperatures in the nematic phase of 1:3 mixture of TOPAB + OOBA, and (b) χ-scans of small angle peaks at different temperatures. Δis independent of temperature.

182

(a)

(b)

Figure 5.12: In the nematic phase of the 1:3 mixture of TOPAB + DBBA: (a) XRD pattern at small angles, and (b) χ-scans at different temperatures, Δχ is an angle between peaks in meridional direction.

183

(a)

(b)

Figure 5.13: The 3:1 mixture of TOPAB + DBBA: (a) integrated intensity vs. q in the nematic phase, (b) the d-spacing (scale on left hand side) and correlation length (scale on right hand side) of small angle peaks at different temperatures.

184

In the case of 1:3 mixtures, two pairs of peaks at small angle region already are separated although those are broad at higher nematic temperature (see Fig. 5.11). In 1:1 mixture of TOPAB and OOBA, the nematic phase exhibiting four small angle peaks clearly goes into a single domain SmC phase at 76.0oC with three sharp harmonic reflections [d1 : d2 : d3 = 1 : 2 : 3 in Fig. 5.6 (b)] which means that the layer normal direction is almost parallel to the equatorial direction but molecules are tilted away about

64o from the layer normal which is indicated by the oblique direction of the large angle peaks.

Fig. 5.14 (a) shows the XRD pattern of the 3:1 mixture of TOPAB + OOBA at

91.0oC that is a more ordered phase below the nematic phase. There are some weak peaks appearing as shown in Fig. 5.14 (b) with the d-spacing ratios of 1 : ~1/√3 : 2. The peak ratios are indicative of the hexagonal order. Furthermore, the shape of the peaks at large angle is very different compare to that in other cases. It looks as if the two peaks those are not resolved. Fig. 5.14 (c) shows a plot of the peak intensity vs. q in meridional direction which was fit to a Lorentzian function. The calculated d-spacings of two fitted peaks are

4.4 Å , and 3.7 Å with correlation lengths of 11.8 Å and 18.3 Å , respectively. The peak at

~4.4 Å originates from alkoxy chains while the 3.7 Å peak at higher q is attributed to the separation between aromatic core with significant π - π overlap of aromatic cores within a column. Thus, the 3:1 mixture TOPAB + OOBA displays a considerable hexagonal arrangement of the columnar phase.

185

(a) (b)

(c)

Figure 5.14: At 91.0oC, 3:1 mixture of TOPAB and OOBA: (a) XRD pattern of the small angle region, (b) integrated intensity vs. q, and (c) overlapping large angle peaks, the green broken curves represent Lorentzian functions fitted to the data.

186

5.4 Summary

In this study, we investigated mixtures of TOPAB with two different mesogenic acids: DBBA containing COO between phenyl rings and with one alkoxy chain (OC10H21) attached to the aromatic ring, and OOBA containing just one aromatic ring with the same tail length as TOPAB and having somewhat smaller molecular length. The binary mixtures had molar ratios of 3:1, 1:1, and 1:3 of TOPAB to OOBA/DBBA.

All mixtures display one or more of the N and SmC, or Col phases. All mixtures show extended mesophase range compared to the each individual compounds. The transition temperature TNI in mixtures of TOPAB + OOBA is comparable to the pure

TOPAB. But the mixtures of TOPAB and DBBA reveal the TNI’s dependence on concentration, i.e., larger concentration of DBBA yields higher TNI. All mixtures were uniformly aligned on surfaces with homogeneous anchoring conditions in the nematic phase and the optic axis lies in the rubbing direction.

From X-ray diffraction pattern the oriented samples, we conclude that 3:1 and 1:1 mixtures at higher temperature in the nematic phase are similar to the nematic phase of the pure TOPAB: the small and large angle peaks appear in its structure and response to the magnetic field. However, the small angle peak in meridional direction clearly begins to split azimuthally into two peaks as temperature is lowered. This splitting is a sign of the presence of biaxial fluctuations. At lowered temperature from the nematic phase, most of mixtures form SmC phase except the 3:1 mixture of TOPAB and OOBA which forms the columnar. Due to a hindrance of the many flexible tails of the TOPAB, especially the molecule with shorter molecular length is smeared by TOPAB so the columnar phase 187

forms for the 3:1 mixture TOPAB + OOBA.

The conclusion that can be drawn from these investigations is that in the nematic phase, all mixtures prefer to follow TOPAB’s tendency, i.e., the molecular groups

(multimers) are a mixture of monomers and (hetero- or homo- and closed or open) dimers.

The splitting of small angle peaks clearly was not seen in the pure TOPAB. However, we could fit the small angle peak to two Gaussians in the nematic phase (see Fig. 4.10).

Upon the addition of two acids compounds to the pure TOPAB, the splitting of the small angle peak was observed. This suggests that the mixtures have a tendency to form oblique multimers, which increases the likelihood of formation of the biaxial nematic phase.

However, since there is a possibility of the existence of monomers and homo- and hetero- multimers (or, other molecular associations) in all mixtures, further studies such as FT-IR spectroscopy and NMR are necessary for more information.

188

References

[1]. G. A. Jeffery, An introduction to hydrogen bonding, Oxford University Press, Oxford (1997).

[2]. T. Kato, Structure and Bonding, 96, 95 (2000), and references therein.

[3]. (a) E. Shapiro and S. Ohki, J. Col. Inter. Sci. 47, 38 (1974); (b) F. Jahnig, Biophys. J. 71, 1348 (1996).

[4]. (a) K. N. Koh, K. Araki, T. Komori, S. Shinkai, Tetrahedron Letters 36, 5191 (1995); (b) T. Kato, J. M. J. Fréchet, J. Am. Chem. Soc. 111, 8533 (1989); (c) T. Kato, J. M. J. Fréchet, Liq. Cryst. 33, 1429 (2006).

[5]. (a) S. K. Kang, E. T. Samulski, Liq. Cryst. 27, 371 (2000); (b) S. K. Kang, E. T. Samulski, P. S. Kang, J. B. Choo, Liq. Cryst. 27, 377 (2000).

[6]. (a) M. Sano and T. Kunitake, Langmuir, 8, 320 (1992); (b) M. Sano, D. Y. Sasaki, M. Isayama, T. Kunitake, Langmuir 8, 1893 (1992).

[7]. R. P. Tuffin, K. J. Toyne, J. W. Goodby, J. Mater. Chem. 6, 1271 (1996). Chapter 6

Summary

We have characterized phasmidic-like mesogen 4-[2, 3, 4-tri(octyloxy) phenylazo] benzoic acid, TOPAB, which forms hydrogen-bonded dimer through the carboxylic acid functionality. Molecular shape of TOPAB does not have cylindrical symmetry due to three alkoxy tails in the 2-, 3-, and 4- positions of the peripheral aromatic rings. TOPAB exhibits the nematic and columnar phases in the temperature range from 120oC to 84oC.

Its homologous compound THPAB also shows the nematic and columnar phases between

127oC ~ 95oC.

The FT-IR spectra confirmed that the carboxylic acid group exists in H-bonded form at relevant temperatures as judged from the shift of frequencies corresponding to

(C=O) and (O-H) bonds of the function group COOH. Also the dissociation energy of hydrogen bonding was calculated from the temperature at which dissociation occurred,

o above 230 C, which is well above the clearing point, TNI.

Very interesting and unusual X-ray diffraction patterns were observed from the magnetic field aligned nematic phase of TOPAB and THPAB. In contrast to calamitic nematogens, small and large angle peaks corresponding to the length and width of mesogens of this system were in the same direction, i.e., perpendicular to the field. The results lead us to conclude that these mesogens form somewhat complex oblique (bent- core-like) H-bonded association. The angle between two single mesogens is estimated to

189

190

be 67 at 101oC for TOPAB and at 114oC for THPAB. While the aromatic core of each mesogen tries to become parallel to the field, the long axis of the complex molecule becomes perpendicular to field. The conclusion of oblique association is also consistent with results of electro-optical response and capacitance measurement studies.

X-ray scattering results at low temperatures revealed the presence of biaxial nematic fluctuations which were tested and supported by results obtained from conoscopy and polarizing microscopy textures in the nematic phase. The possibility of this effect arising from cybotactic clusters can be ruled out, as they do not have an underlying smectic-C phase. With the help of Benford plates in conoscopy measurements, we succeeded in visualizing the two optic axes in the low temperature region of the nematic phase. However, more investigations of nematic biaxiality in these systems are needed to fully understand and establish the biaxial nature.

For TOPAB and THPAB homeotropic alignment could not be obtained by conventional methods, however, a uniformly aligned nematic state was obtained and used for textural observations. A relatively high electric field was needed to influence the director in the nematic phase of TOPAB, and its response to an applied electric field was similar to that of a positive liquid crystal, i.e., the birefringence became less at

o sufficiently high field strengths, exceeding 3.2 Vrms/m at 100 C. The conoscopic and optic axis visualization study revealed (a field induced) biaxiality as evidenced by the changes in isogyres and two optic axes at suitable high electric field. Furthermore, two independent methods, X-ray and FT-Raman scattering measurements, were used to calculate the uniaxial order parameters and . and were 191

obtained from the vibration mode associated with N=N stretching for Raman and the integrated intensity of the diffused peak around wide angle for X-ray scattering.

To gain further insight into the nature of the nematic phase and H-bonding in

TOPAB system, its mixtures with two simpler and truly calamitic mesogenic molecules,

OOBA and DBBA, were prepared in three concentrations and investigated by means of

DSC, POM, and X-ray scattering. At least two mesophases in addition to an extended nematic temperature range were obtained. The phase was uniformly aligned with conventional substrate-surface treatments for all mixtures. The XRD patterns of the mixtures were similar to those of TOPAB. Specially, the 3 : 1 = TOPAB : OOBA mixture exhibited a considerable hexatic order of columnar phase while a smectic order was favored in mixtures of different concentrations.

In future, proton-NMR investigations could be carried out to detect open and closed dimers of pure TOPAB at different temperatures [1]. Scanning tunneling microscopy (STM) on a graphite surface and XRD of the single crystalline of TOPAB can allow us to predict assembly of molecules and bulk structure of TOPAB [2]. XRD scattering and FT-IR of a binary mixture with a material that has either an H-bonding acceptor or a donor instead of the carboxylic acid group might be valuable. XRD investigation of cell under a sufficiently high applied electric field may provide a definitive answer regarding the biaxiality of the nematic phase.

192

Reference

[1]. S. I. Torgova and A. Strigazzi, Mol. Cryst. Liq. Cryst. 336, 229 (1999).

[2]. Masahito Sano, Darryl Y. Sasaki, Munetoshi Isayama, and Toyoki Kunitake,

Langmuir 8, 1893 (1992).