Patterns and Conformations in Molecularly Thin Films

Home , The Plateau (Fringe)

PATTERNS AND CONFORMATIONS IN MOLECULARLY THIN FILMS

A dissertation submitted

to Kent State University in partial

fulfillment of the requirements for the

degree of Doctor of Philosophy

Prem B. Basnet

May, 2010

Dissertation written by

Prem B. Basnet

B.Sc., Tribhuvan University, 1988

M.Sc., Tribhuvan University, 1991

M.A., Kent State University, 2007

Ph.D., Kent State University, 2010

Approved by

______, Chair, Doctoral Dissertation Committee Dr. Elizabeth K. Mann

______, Members, Doctoral Dissertation Committee Dr. Carmen C. Almasan

______Dr. Robin Selinger ______Dr. John Portman ______Dr. Quan Li

Accepted by

______, Chair, Department of Physics Dr. Bryon D. Anderson ______, Dean, College of Arts and Sciences Dr. Timothy Moerland ii TABLE OF CONTENTS

LIST OF FIGURES ...... ix

LIST OF TABLES ...... xxx

ACKNOWLEDGEMENT ...... xxxi

1 INTRODUCTION ...... 1

1.1 Thin-ﬁlm: A General Introduction ...... 2

1.1.1 Langmuir ﬁlms ...... 4

1.1.2 Surfactants ...... 5

1.2 Phase coexistence in a Langmuir ﬁlm ...... 6

1.3 Deposition of Langmuir ﬁlms on solid substrates ...... 8

1.3.1 Langmuir-Blodgett ﬁlms ...... 9

1.3.2 Self Assembled Monolayers ...... 10

1.4 Summary ...... 11

2 EXPERIMENTAL TECHNIQUES ...... 13

2.1 Chapter outline ...... 13

2.2 Introduction ...... 13

2.2.1 A brief review of some useful experimental techniques ...... 14

2.2.1.1 X-ray diffraction ...... 14

2.2.1.2 X-ray and neutron reﬂectivity ...... 14

2.2.1.3 Ellipsometry ...... 14

2.2.1.4 Imaging Ellipsometry ...... 15

2.2.1.5 Fluorescence microscopy (FM) ...... 15

iii 2.2.1.6 Transmission electron microscopy (TEM) ...... 15

2.2.1.7 Scanning electron microscopy (SEM) ...... 16

2.2.1.8 Scanning tunneling microscopy (STM) ...... 16

2.2.1.9 Scanning probe microscopy (SPM) ...... 16

2.2.1.10 Nuclear Magnetic Resonance (NMR) spectroscopy ...... 17

2.2.1.11 Second Harmonic Generation (SHG) ...... 17

2.2.2 Experimental techniques used in my research work ...... 17

2.3 Why BAM ? ...... 18

2.3.1 Physics of BAM ...... 18

2.3.1.1 Polarization of light ...... 19

2.3.1.2 Brewster Angle ...... 19

2.3.2 Theory behind BAM ...... 22

2.4 BAM setup ...... 27

2.5 Surface Pressure Measurement ...... 29

2.5.1 Surface tension ...... 30

2.5.2 Theory ...... 31

2.5.3 Instruments for the Surface Pressure Measurement ...... 33

2.5.3.1 Langmuir trough ...... 34

2.5.3.2 Barriers ...... 35

2.5.3.3 Wilhelmy Plate ...... 37

2.5.3.4 Calibration of electrobalance ...... 41

2.5.3.5 Temperature sensor ...... 42

2.5.3.6 Water bath ...... 43

2.5.4 Experimental setup ...... 43

2.5.5 Calibration of Trough ...... 44

2.6 Time correlation ...... 47

iv 2.6.1 Cleaning ...... 48

3 PATTERN FORMATIONS IN LANGMUIR FILMS MADEOF CHIRAL LIPID MOLECULES 55

3.1 Lipids: A General Introduction and Motivation ...... 55

3.2 Molecule ...... 61

3.3 Pattern Formation in Thin Films ...... 63

3.3.1 Equilibrium patterns ...... 64

3.3.1.1 Patterns due to elastic buckling ...... 65

3.3.1.2 Patterns due to polymerization ...... 66

3.3.1.3 Patterns due to tilt variations ...... 67

3.3.1.4 Patterns due to chirality ...... 68

3.3.2 Non-equilibrium patterns ...... 72

3.3.2.1 Organized patterns ...... 72

3.3.2.2 Fractal-like dynamic patterns ...... 74

3.3.2.3 Temperature-dependent Labyrinth patterns ...... 74

3.3.3 Materials and Methods ...... 75

3.3.3.1 Substrate ...... 75

3.3.3.2 Sample ...... 76

3.3.3.3 Film preparation ...... 77

3.4 Qualitative discussion of results ...... 78

3.4.1 Tilt variation ...... 84

3.4.2 Buckling out of the interface ...... 86

3.4.3 Solidiﬁcation front ...... 90

3.4.4 Polymerization ...... 91

3.4.4.1 Thin layer chromatography (TLC) ...... 94

3.4.5 Effect of chirality ...... 98

3.4.6 Role of impurity ...... 98

v 3.4.7 Heterogeneous nucleation around a foreign particle ...... 99

3.5 Conclusion ...... 101

4 TRANSITIONFROM SPIRAL STRIPESTO TARGETSIN COMPRESSED LANGMUIR MONO-

LAYERS MADE OF DIACETYLENICLIPIDS ...... 103

4.1 Introduction ...... 103

4.2 Experimental ...... 104

4.3 Results and discussion ...... 104

4.4 A possible model for the formation of giant spiral patterns ...... 108

4.5 Conclusion ...... 117

5 CHARACTERIZATION OF LANGMUIR FILMS MADE OF BENT-COREMOLECULESWITH

HYDROPHILICENDGROUP ...... 120

5.1 Introduction ...... 120

5.1.1 Liquid crystals - a short introduction ...... 121

5.1.2 Formation of Liquid crystals ...... 123

5.2 Lyotropic liquid crystals ...... 124

5.3 Thermotropic liquid crystals ...... 124

5.3.1 Nematic Phase ...... 125

5.3.2 Smectic phase ...... 126

5.4 Bent-core liquid crystals ...... 126

5.5 Langmuir ﬁlms of bent-core molecules ...... 128

5.6 Materials and methods ...... 129

5.6.1 Materials ...... 131

5.6.2 Method ...... 132

5.7 Results and discussion ...... 132

5.7.1 Molecular conﬁgurations and isotherms ...... 132

vi 5.7.2 Thicker ﬁlms ...... 136

5.7.3 Film thickness ...... 138

5.7.4 Molecular packing and surface conﬁgurations ...... 141

5.7.5 Optical anisotropy of the ﬁlm ...... 144

5.8 Conclusion ...... 149

5.9 Future work ...... 150

6 MOLECULAR CONFORMATIONS IN ULTRATHIN POLYMER FILMS BY USING 2HNMR

TECHNIQUES ...... 151

6.1 Polymer properties ...... 153

6.1.1 Properties in bulk ...... 153

6.1.2 Properties at surfaces ...... 154

6.1.3 Polydimethylsiloxane ...... 155

6.2 What is an NMR? ...... 159

6.2.1 DNMR and Quadrupole interaction ...... 163

6.2.2 Free induction decay (FID) and relaxation function ...... 165

6.3 Materials and methods ...... 166

6.3.1 Material and sample preparation ...... 167

6.3.1.1 Anopore membrane ...... 167

6.3.1.2 PDMS ...... 168

6.3.1.3 Sample preparation ...... 169

6.3.1.4 Orientation of sample with respect to the uniform magnetic ﬁeld

in the probe ...... 172

6.3.1.5 Sample cooling ...... 174

6.3.1.6 NMR Spectrometer ...... 175

6.3.1.7 Experimental parameters ...... 176

6.4 Results ...... 179

vii 6.4.1 PDMS deposited on hydrophilic AnoporeRM membranes ...... 180

6.4.2 PDMS deposited on hydrophobic AnoporeTM discs ...... 181

6.5 Conclusion and future prospect ...... 183

7 SUMMARY AND FUTURE PROSPECTS ...... 187

AFOUR-ROLL MILLS ...... 192

A.1 Design and components ...... 192

A.2 Test of ﬂow ﬁeld ...... 192

A.3 Program for the the RP240 controller ...... 196

BMATHEMATICA PROGRAMS FOR REFLECTIVITY CALCULATIONS ...... 199

B.1 Mathematica program for the p-polarize light ...... 199

B.2 Mathematica program for the s-polarize light ...... 201

CTHIN LAYER CHROMATOGRAPHY ...... 204

C.1 Basic Understanding ...... 204

C.1.1 Chromatography ...... 204

C.2 Experiment ...... 205

C.2.1 Sample preparation (Isolation of phospholipid from the solution) ...... 205

C.2.2 Preparation of developing solution ...... 206

BIBLIOGRAPHY 208

viii LIST OF FIGURES

1.1 Molecular chirality and different possible origins. (a) Mirror image cannot be su-

perimposed with each each other by rotation- an illustration of chirality. (b) An

illustration of intrinsic molecular chirality (c) Conformational chirality due to a

preference for a propeller-like or twist conﬁguration of individual molecules. (d)

Achiral molecules show chiral structure due to their arrangement in layers. After [1]. 2

1.2 An illustration of an amphiphillic molecule which can form micelles and bilayers

because of its surface active properties: (a) an amphiphilic molecule (b) a miclelle

and bilayer [2]...... 6

1.3 (a) An illustration of 3d-compression and decompression of a gas at constant tem-

perature and (b) its corresponding isotherm. Diagrams are not to scale...... 7

1.4 An illustration of coexistence of several different phases in a Langmuir monolayer.

(a)A Langmuir trough (top) and a generalized isotherm of a Langmuir monolayer

[3]. Horizontal section of the isotherm are phase coexistence regions at ﬁrst or-

der transitions, and the kink indicates a continuous transitions. (b) Illustrates the

condensed mesophases found in monolayers of fatty acids and lipids [4]...... 8

1.5 A schematic diagram showing the transfer of a ﬂoating monolayer on a solid sub-

strate to form a LB ﬁlm...... 9

1.6 An illustration of the forces in self-assembled monolayer [5]...... 10

ix 2.1 An illustration of polarization after reﬂection. When light strikes at the Brewster

angle, θ, the reﬂected light is plane polarized. The reﬂected beam and the refracted

beam make an angle of 90◦ with each other. I: incident beam, R: reﬂected beam, T:

transmitted beam; θB: Brewster angle; θt: angle of transmission (or refraction); n =

1.5 is the refractive index of glass with respect to air.The solid circles represent the

vibration perpendicular to the plane of the page (i. e., perpendicular to the plane of

incidence), and the double arrows represent the vibration in the plane of page (i. e.,

in the plane of incidence)...... 20

2.2 An illustration of how the incident beam I, the reﬂected beam R, the transmitted (or

refracted) beam T and the normal to the interface N lie on the same plane - the plane

of incidence. In the ﬁgure, θi is the angle of incidence, θr is the angle of reﬂection

and θt is the angle transmission or refraction(Figure is not to scale)...... 20

2.3 An illustration of how the contrast with and without the presence of a monolayer

changes due to incidence of p-polarized light at the Brewster angle. Without mono-

layer, there is zero reﬂection, but with monolayer there is some reﬂection. Figure is

not to scale ...... 23

2.4 Reﬂection and transmission at the interface of two media. Here, I is the incident

beam, R is the reﬂected beam and T is the transmitted beam. Here, Ei, Er, and Et

are the incident, reﬂected and transmitted electric ﬁelds respectively...... 24

2.5 Multiple reﬂections of light within the thin ﬁlm. n1, n2, and n3 are the refractive

indices of air, ﬁlm and water medium respectively. d is the thickness of the ﬁlm. . . 24

2.6 An illustration of how reﬂectivities change with thickness of the ﬁlm at around

Brewster angle for s- and p-polarization: (a) s-polarization (b) p-polarization. . . . 26

2.7 A simpliﬁed sketch of a BAM setup. L on the left side is the collimating lens, I is

the iris, P is the polarizer, L on the right side is the objective lens, A (optional) is

the analyzer, and CCD camera for grabbing movie frames...... 27

x 2.8 BAM setup in the lab. A: laser-arm, B: camera-arm, C: Huber rotation stages, D:

ﬁne-tuning control knobs, E: vibration-isolation table...... 28

2.9 Interaction of molecules at the interface and in the bulk. The solid spheres represent

the liquid molecules and the arrowheads represent the direction of intermolecular

forces...... 30

2.10 An isotropic ﬁlm on top of a pure substrate. The ﬁlm can be stretched in a direction

shown by the outer arrowhead...... 32

2.11 An illustration of the principle of the Langmuir ﬁlm balance. γ is the surface ten-

sion of the ﬁlm-covered surface and γ0 is the surface tension of the pure liquid

surface.The drawing is not to scale...... 32

2.12 Rectangular Langmuir trough made of Teﬂon sitting on an aluminum base with

rectangular hole at the center of the trough...... 34

2.13 A delrin beam dump.(A) right cylinder with thread texture in the inner wall.(B)

Circular base with a pointed cone...... 36

2.14 An illustration of Langmuir ﬁlm balance with movable barriers. Courtesy- KSV

user’s manual...... 36

2.15 An illustration of Wilhelmy plate. A: platinum stem , B: the sand blasted Platinum

plate. The ﬁgure is not to scale...... 38

2.16 A schematics diagram of the Wilhelmy plate immersed into the subphase. The angle

of contact made by the subphase with the plate surface is θ, h is the height of the

plate inside the subphase, w is the width of the plate, L is the height of the plate and

t is the thickness of the plate...... 39

2.17 An isotherm of Phospholipid Diyne PE at 32 ◦ C. σ is the mean molecular area of

the ﬁlm and π is the surface pressure. The shoulder in the graph is the place where

spiral/target structures are seen...... 40

xi 2.18 Calibration of electrobalance. The solid circles are the balance reading, a solid

straight line passing through these points is the linear ﬁt of these balance readings,

circles are the residues of the ﬁt. Note that in this regime they are nearly randomly

distributed, so that the linear ﬁt can be used...... 41

2.19 KSV system for the surface pressure measurement. 1: Teﬂon trough, 2: barriers, 3:

motorized barrier holder with trough platform, 4: side tubes for temperature control,

5: sand-blasted platinum Wilhelmy plate, 6: KSV electrobalance...... 44

2.20 A schematic diagram of of experimental setup. L1: collimating lens, L2: collect-

ing lens, I: iris, PL: polarizer, E: electric ﬁeld vector associated with the polarized

beam, Er: electric ﬁeld vector after reﬂection of the beam from the surface, A: ana-

lyzer, IRC:infrared-cut, CCD:charge coupled device(camera), B: barriers, T: trough,

TM: tensiometer (ﬁlm balance), P: Wilhelmy plate, W: Langmuir ﬁlm on water sub-

strate, W1: computer controlled water bath, T1,T2: circulating tubes...... 45

2.21 An illustration of the proﬁle of the trough with liquid substrate. The proﬁle near the

trough edge can be considered approximated as a quarter of a circle with radius R [6]. 46

2.22 A calibration graph for the Minitrough. The solid squares are the calculated areas

between the barriers with the measured distances between the barriers, the error bars

are due to the errors associated with the trough proﬁle with liquid substrate and also

due the measurements in the barrier distaces, and the solid line is the ﬁt. A and B

are the ﬁt parameters, Y = A + BX is the ﬁt equation...... 47

2.23 A typical graph to establish a correlation between the movie frame time and the

compression time. A and B are the ﬁt parameters, Y = A + BX is the ﬁt equation.

The vertical errors bars correspond to the errors in estimation of surface pressure

time with respect to the recorded barrier positions...... 49

2.24 A schematic of a barrier in KOH solution for barrier cleaning...... 52

2.25 A schematic of vacuum system for emptying and cleaning the trough...... 53

xii 3.1 An illustration of a phospholipid structure [7]. The arrow shows the position of the

phosphate group substitution for one of the fatty acids...... 56

3.2 Another way of looking into the structure of a phospholipid molecule [7]...... 56

3.3 An illustration of different structures of self-assembled lipid molecules in aqueous

environment [7]...... 57

3.4 A schematic diagram of a plasma membrane [7]...... 58

3.5 An electron micrograph of a multilamellar phospholipid vesicle in which each layer

is a lipid bilayer [2]...... 58

3.6 23:2 Diyne PE lipid molecule. (a) Computer model of the 23:2 Diyne PE molecule.

Color codes: red-Oxygen, blue-Nitrogen, green-Carbon, and white-Hydrogen (b)

Molecular structure of 23:2 Diyne PE. [8]...... 62

3.7 23:2 PC Diyne lipid. (a) Computer model of the 23:2 Diyne PC molecule. Color

codes: red-Oxygen, blue-Nitrogen, green-Carbon, and white-Hydrogen, (b) Molec-

ular structure of 23:2 PC Diyne. [8]...... 63

3.8 Spoke pattern formation on dewetting a dilute Langmuir ﬁlm of gold nanoparticles

(gold dots) on hydrophilic SiO2/Si substrate as observed with an optical micro-

scope. At the initial stage, the nanoparticles precipitate to form the spoke tips at

the rim of the ﬁlm, which then propagate inwards as the water front retreats leading

to a spoke pattern such as that shown in here. An optical microscopy image (scale

bar=200 µm) [9]...... 64

3.9 A BAM image of the monolayer of DSPC (400µm ×250µm) at π ∼ 7mN/m.The

buckled layer shows bright and dark stripes, roughly oriented perpendicular to the

compression direction indicated by arrows [10]...... 65

xiii 3.10 Buckling instability occurs when when ethyl heptadecanoate monolayer is heated

to 19◦ C at constant mean molecular area. (a) The monolayer at 19.5◦ C (b) The

monolayer at ∼ 19.5◦ C where the amplitude of the buckling increase. The bars

correspond to 100µm [11]...... 66

3.11 An ESEM image of crumpling of vesicles. The width of the image as shown by the

arrow is 4µm. The polymerization is 40% [12]...... 66

0 3.12 Freeze-fracture electron micrograph of the Pβ phase of dipalmitoylphosphatidyl-

choline (DPPC) of the ripple phase of DPPC showing the top view of the corrugated

surface [13] due to buckling of the bilayer...... 67

3.13 Polarized ﬂuorescence microscope image of stripes from a monolayer of DPA at

7.6 ◦ C as a consequence of tilt-azimuth modulation [14]...... 68

3.14 Deﬁnition of the tilt angle and the tilt-azimuthal angle. Here (x, z) is the plane of

incidence, t is the tilt angle and ϕ is the tilt-azimuthal angle [15]...... 69

3.15 An illustration of stripe pattern due to tilt-azimuth variation on the Langmuir ﬁlm

surface. (a) BAM image with analyzer. The bar represents 50µm. Change of con-

trast across the stripe due to a change in azimuth-angle due the tilt of the molecule

from the surface normal. In plane optical anisotropy arises with one axis parallel

to the direction of tilt. If this axis is not parallel to the polarization of the incom-

ing light, the ploarization of the reﬂected light will rotate towards that axis. (b) An

illustration of molecular tilt with different tilt angle along the ﬁlm surface [15]. . . 69

3.16 Fluorescence micrograph of a DMPA monolayer containing 1 mol% of cholesterol,

pH 11.4, T = 10 ◦ C, and ionic strength 10−3M [16]. The scale bar = 20µm. . . . . 71

3.17 (a) A ﬂuorescence microscope image of spiral patterns in the solid-ﬂuid coexis-

tence region of L-DPPC monolayer containing cholesterol [17].(b) Epiﬂuorescence

microscope photograph showing chiral solid domains in monolayer comprised of

several DPPC’s. The scale bar equals 50µm [18]...... 71

xiv 3.18 An illustration of pattern formation of dynamic origin in the Langmuir monolayer.

In (a) The chemical structure of R-OPOB and an exaggerated diagram of its Lang-

muir monolayer at the air/glycerol interface. The rod-like molecules are tilted from

their surface normal at a constant tilt angle. φ is the molecular azimuth. The pair of

wings attached to the molecules represents the chiral groups. In (b) Pattern forma-

tion in R-OPOB monolayer at the air-glycerol interface. The scale bar corresponds

to 100 µm. The pattern formation is due to the collective precession of molec-

ular around the surface normal, caused by the transmembrane transfer of water

molecules. The images were taken under the reﬂected-type polarizing microscope

and the contrast is coming from the tilt-azimuth change. The stripe width decreases

going away from the center of the patterns. Taken from [19]...... 73

3.19 BAM images of 6-(cholest-5-ene-3-lyoxy)-6-oxohexanoic acid (cholestoric acid)

2 monolayers at the air-water interface at mean molecular area=54 A˚ /molecule, four

minutes after compression to surface pressure=2.4 mN/m. Time between images=2

sec; arrows show rotation sense. The contrast is due to change in tilt-azimuth of the

molecules. Scale bar=335µm [20]...... 73

3.20 Polarized Light Microscope(PLM) images of thin polymer ﬁlm showing the growth

of a percolated network formed by phase separation (the difference in contrast) into

isotropic and anisotropic liquids at 220 C for 100, 300, and 900 s [21]. The scale

bar = 50µm...... 75

3.21 A BAM image of spiral formation in a Langmuir ﬁlm of DIYNE PE upon compres-

◦ sion at 30 C. t0 in the image represents the time after the compression begins. The

image was captured with increased magniﬁcation...... 78

xv 3.22 BAM images of modes pattern formation in a Langmuir ﬁlm of DIYNE PE upon

compression at different temperatures. The scale bars in the images correspond to

1 mm. The rightmost picture at 30 ◦ C in the second row was captured at a larger

magniﬁcation...... 80

3.23 Compression isotherms at (a) 28 ◦ C and (b) 30 ◦ C. The values of the surface pres-

sure and the mean molecular area is the one at which the nucleation begins to end

up with spiral formation at the this temperature...... 81

3.24 Compression isotherms at different temperatures. In the graph, the dashed lines

represents the isotherm at 28 ◦ C, the solid line represents the isotherm at 32 ◦ C, the

dotted-dash line at 34 ◦ C, and the dotted line at 37 ◦ C. Positions of arrows show

the region of isotherms at which visible nucleation begins and the corresponding

values of mean molecular areas and surface pressures are the critical values (σc and

◦ 2 πc) – (i) at 32 C: σc ∼ (0.6 ± 0.05)nm /molecule, πc ∼ (15 ± 0.2)mN/m (ii)

◦ 2 ◦ at 34 C: σc ∼ (0.5 ± 0.06)nm /molecule, πc ∼ (34 ± 0.4)mN/m (iii) at 37 C:

2 σc ∼ (0.6 ± 0.05)nm /molecule, πc ∼ (41 ± 0.2)mN/m ...... 81

3.25 (a) Isotherms of the Lamgmuir monolayer of [N,N-dihexadecyl-3-(1-imidazolyl)-

propyamine] as a function of temperature. The LE-LC coexistence region is char-

acterized by a long plateau. The arrow indicates the collapse of the the monolayer.

The inset is the ﬂuorescence microscopy image of fractal like pattern in the LC

region(the bar on the inset equals 50µm) [22]. (b) Isothererms of diacetylenic phos-

pholipid monolayer. The inset is the ﬂuorescence microscopy image of the curved

and branched domains (' 50 to 100µm) at the condensed phase in the plateau re-

gions above 20 ◦ C [23] ...... 83

xvi 3.26 BAM images of pattern formation when the mean molecular areas of the Langmuir

ﬁlms made of Diyne PE at the air-water interface were decreased at two different

rate. (a) The rate of decrease of mean molecular area ∼ 1.4nm2 molecule−1 s−1,

barrier speed = 10mm/min. (b) The rate of decrease of mean molecular area ∼

2.9nm2 molecule−1 s−1, barrier speed = 20mm/min. Both in (a) and (b), compres-

sions of the ﬁlms were carried out for about 5min from the beginning...... 84

3.27 An illustration of how the arrow, the direction of molecular tilt projected onto the

layer plane, varies along the path [24]...... 84

3.28 BAM images of Langmuir ﬁlm of Diyne PE. (a) With analyzer parallel to the po-

larizer (b) With analyzer crossed with the polarizer. T = 36 ◦ C. The scale bars

correspond to 100 µm...... 86

3.29 BAM images of Langmuir ﬁlm of Diyne PE with improved resolution.(a) Pattern

when the analyzer and the polarizer are parallel to each other (b) Pattern when the

analyzer and the polarizer are crossed. The scale bars in (a) and (b) correspond to

100 µm. The compression temperature: 30◦ C...... 87

3.30 Molecular structure of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) [25]. . 87

3.31 An illustration of how the critical elasticities were found using an isotherm and

◦ its derivative. (a) Compression isotherm at 34 C, where πc is the critical surface

pressure at which the patterns begin forming. (b) Elasticity versus mean molecular

where Ec is the critical elasticity corresponding to the critical mean molecular area

and the critical surface pressure in (a)...... 89

3.32 Change of critical surface pressure πc with temperature T. The circle are the data

points when there is no patterns and the solid circles are when there is well deﬁned

pattern...... 90

xvii 3.33 Change of critical surface pressure πc and critical elasticity Ec with temperature T.

Solid squares: data with no structure; solid circles: data with structures; circles:

critical elasticity. The error bars represent the standard deviation in the measurements. 91

3.34 Dendrite structure in the Langmuir monolayer of N,N-dihexadecyl-3-(1- imidazolyl)-

propyamine at the air-water interface. The bar equals 50 µm [22]...... 92

3.35 A BAM image of pattern formation upon compressing the Langmuir ﬁlm of Diyne

PE at the air-water interface at 30 ◦ C. The wavelength of the laser beam used

= 488nm. The dark colored curve denoted by B is the guide to the claw-shaped

substructures around the circular patterns. The scale bar = 100µm...... 92

3.36 BAM image of compression, decompression and re-compression of lipid Diyne PE

at 32 ◦ C. The the order of cycles follow from left to right...... 93

3.37 A BAM image of pattern formation with claw-like sub-structures upon compressing

the Langmuir ﬁlm made of Diyne PE. The scale bars correspond to 100 µm. The

time sequence of the images from the top left image to the bottom right image is 0

s, 1 s, 2 s, 3 s, 4 s, 10 s, 15 s, 25 s and 40 s. The wavelength of the laser beam used

= 488 nm...... 95

3.38 BAM images of compression and decompression of Diyne PE Langmuir ﬁlm at

36 ◦ C. (a) Compression (b) De-compression (c) Recompression. The bars corre-

spond to 2mm...... 96

3.39 An example of decrease in hysteresis with increase in temperature. In both of the

graphs solid circles represent compression data and the circles represent the de-

compression data. Area between the compression-decompression isotherms (the

dark regions) are: (a) Compression-decompression at 30 ◦ C, 1.12×10−14J/mole,

(b) Compression-decompression at 35 ◦ C, 3.86×10−15J/mole...... 96

xviii 3.40 Images obtained from the TLC experiments. In (a),A: a different PE, B:stock diyne

PE and C: standard PC samples from left to right were used for the comparision.

In (b), A: different PE, B: sample from the Langmuir ﬁlm after pattern formation

taken at 36 ◦ C, C: standard solution of PC. Rates of different oligomers after poly-

merization results in an elongated mark in the middle spot of (b)...... 97

3.41 Compression isotherms of Langmuir ﬁlms made of DC8,9PC at the air-water in-

terface. Bump at the beginning of the coexistence plateau. All the isotherms are

measured at 25 ◦ C and are shifted by 2mN/m and 0.04nm2 for clarity. (a) isotherm

obtained with a fresh sample, exhibiting no bump; (b) and (c) isotherms obtained

with the same solution as in (a), in presence of (b) 0.1% of cholesterol and (c) 5%

cholesterol [23]...... 99

3.42 BAM image: a test of nucleation around a dust particle. The bright spots within the

circles are the dust particles and the nucleating structure is shown within the square

bracket. The contrast is enhance to make the dust particles visible...... 100

4.1 (a) Change of characteristic length with temperature at a constant surface pressure

of ∼21mN/m. The error bars in the data points are the standard deviations in the

measurements. (b) The section between the two arrows is deﬁned as the character-

istic length...... 105

4.2 A BAM image of melting of patterns at high temperatures: (a) patterns at ∼ 39◦ C

(b) patterns at ∼ 40◦ C (c) patterns at ∼ 41◦ C. The bars correspond to 1 mm. . . . 105

xix 4.3 Transition from spiral to target as we increase the temperature from left to right.

The pattern changes from a spira mirabilis to concentric circles (targets) via the

Archimedean spiral with constant width. We can observe defects in the spirals but

not in the targets despite the fact that some of the circles do not close up. Notice

how straight is the line between the domains (shown in the inset of the second ﬁgure

from the left)in the spiral regime. Scale bar in the picture is 1mm and in the inset it

is 200µm. Temperatures of compression are: 28, 32, 34 and 36 ◦ C ...... 106

4.4 An example of target pattern where every stripe is made of smaller logarithmic

spirals or spira mirabiles at a temperature of 35 ◦ C. The scale bar = 1mm...... 108

4.5 An illustration of chiral molecules and chiral pitch in 3-d [26]...... 109

4.6 An illustration splay and bend. K1 and K3 are the elastic constants corresponding

to the splay and bend deformations respectively. Courtesy [27] ...... 110

4.7 Spiral pattens as a function of the boundary conditions of the director. The position

of the director at inﬁnity and at the center is shown as inset at the corner of each

picture [28]...... 110

4.8 Change of free-energy with the order parameter (|~n|)...... 111

4.9 Atomic force microscope (AFM) measurements of Langmuir ﬁlm of DIYNE PE

transferred onto a thin mica sheet. Data recorded at two different scan sizes (a)

AFM image. Scan size = 5.000µm, scan size = 8.000µm (b) AFM image. Scan size

= 8.000µm, scan size = 5.000µm...... 112

4.10 Two possible models of the tilt orientation with respect to the ﬁlm surface in a

Languir ﬁlm: (a) the arrow shows the tilt direction along the buckled surface trying

to keep the tilt with respect to the ﬁlm surface (b) when splay elasticity is very large

the director always tends to splay...... 113

xx 4.11 An illustration of projection of director on the ﬁlm surface. N is the surface normal,

~t is the director, ~n is the projection of the director onto the ﬁlm surface, and θ is the

tilt angle of the director with respect to the surface normal...... 113

4.12 The wavelength of the pattern λ(mm) which is the distance between the stripe of the

same gray value as a function of the surface pressure π(mN/m) corresponding to the

nucleation of the pattern. The solid circles are the measured data and the solid line

is the ﬁt of Equation 4.5. (a) Change of characteristic length with surface pressure

at 34 ◦ C. Solid circles are the measured data and the solid line is the ﬁt of Equation

4.5 (b) Power law ﬁt of change of characteristic length (λ) with surface pressure (π)

at 34 ◦ C. Both x and y axis are the log scale (c) Change of characteristic length

with surface pressure at 31 ◦ C. Solid circles are the measured data and the solid

line is the ﬁt of Equation 4.5 (d) Power law ﬁt of change of characteristic length (λ)

with surface pressure (π) at 31 ◦ C. Both x and y axis are the log scale...... 115

4.13 Gray value proﬁle in Fig.3.29: (a) taken horizontally. A: when the analyzer and

the polarizer are parallel to each other, B: when the analyzer and the polarizer are

crossed (b) taken vertically. C: when the analyzer and the polarizer are parallel to

each other, D: when the analyzer and the polarizer are crossed. Both in (a) and

(b) the gray values are moved up by 100 for clarity. The higher the gray value the

brighter is the region...... 117

4.14 Critical surface pressure at which the pattern begins forming as a function of (Tm −

−0.71 T ). The best ﬁt to the data is represented by a power law Πc ∼ (Tm − T ) .

The solid circles are the measured data, error bars are the estimated errors in the

temperature measurements, and the solid line is the power law ﬁt...... 118

5.1 Orientations and organizations of molecules in various phases: (a) Crystalline solid

(b) Isotropic liquid (c) Plastic crystal (d) Liquid crystal [26]...... 122

xxi 5.2 Schematic of geometrical structures of liquid crystalline molecules [26].(a) A rod-

like molecule (b) A disk-like molecule (c) A lathe-like molecule (d) A bent-core

molecule. Note, without the ﬂexible end chains, they are not liquid enough. . . . . 123

5.3 An illustration of ordering of molecules in nematic phase [29]...... 125

5.4 An illustration of layer formation of molecules in smectic phase [29]. (a) Smectic

A phase (b) Smectic C phase...... 126

5.5 Bent-core molecule with 5 phenyl ring. ‘R’ is the end chain functional group [30] . 127

5.6 Bent-core molecule with 5 phenyl ring with chlorine atoms attached to the central

phenyl ring. ‘R’ is the end chain functional group [30] ...... 128

5.7 Different kinds of bent core molecules: (a) Bent core molecules with varied end

groups and core [31, 32]. (1) The bent-core molecules with hydrocarbon end chains

R1 and core substituent R2. (2,3) Formulas for the siloxane end-chain molecules,

Bc2-SiO and Bc3-SiO, respectively. (b) Bent core molecule with similar hydrocar-

bon end chains and chlorine substituted core, used in reference [32] and simulated

in reference [33] ...... 130

5.8 Molecular structure of bent-core liquid crystal with hydrophilic group at one end

and hydrocarbon chain at the other end (called Z2b by the group that did the syn-

thesis [refer to the footnote]). There are 5 phenyl ring in this bent-core molecule. . 131

xxii 5.9 Surface pressure isotherms of Langmuir ﬁlms made of bent-core molecules with a

hydrophilic end group. σ is the mean molecular area, i.e., the average area available

for each molecule in the ﬁlm, π is the surface pressure and σ0 is the initial surface

concentration of the Langmuir ﬁlm. All the measurements were carried out at the

◦ 2 room temperature (18 C. ). (a) π-σ isotherm with σ0 = 3.5 nm /molecule. The

2 solid line is the guide to the eyes, (b) π-σ isotherm with σ0 = 1.7 nm /molecule,

solid squares correspond to the compression and circles represent decompression

2 (c) π-σ isotherm with σ0 = 0.86 nm /molecule (d) π-σ isotherm with σ0 = 0.40

nm2/molecule. In (c) and (d) ↑ and ↓ indicate the directions of compressions and

decompressions respectively...... 134

5.10 Representative isotherm for Bc-2Cl [32]...... 134

5.11 A BAM image of Langmuir ﬁlm of Z2b molecules showing compression, decom-

pression and recompression cycles. The rightmost and the leftmost pictures are

taken at the mean molecular area of 0.75nm2/molecule. The middle image was

taken when the mean molecular area was increased by 50% from that of the right-

most image. The somewhat pebbly texture of the partial ﬁlm is similar in all cases.

The scale bar corresponds to 1mm...... 135

5.12 Compression isotherms for different initial surface concentrations A: 0.34 nm2/molecule,

B: 0.4 nm2/molecule, C: 0.86 nm2/molecule, D: 1.37 nm2/molecule and E: 1.75

nm2/molecule (not shown in the graph) are the coareas at which at which isotherms

begin to rise. The rise of isotherms is conﬁguration dependent which ultimately

depends on surface concentrations...... 136

xxiii 5.13 Schemata of possible conﬁgurations of Z2b bent core molecules in water surface (a)

Entire hydrophilic portion of the molecules remains in contact with water surface

(b) One end of the molecules in contact with water surface and the other one free to

leave it (c) Upright position of the molecules; the end of the hydrophilic group on

the water surface and the hydrophobic chain above it (d) Molecules lying ﬂat at the

air/water interface. The hydrophobic (red) chain is longer than the the hydrophilic

(blue) chain...... 137

5.14 Phase behaviors of Langmuir monolayers of different kinds of bent-core molecules

at very dilute surface concentrations at air/water interface. (a) Bc-H, σ ∼ 5.5nm2/molecule;

(b) Bc-NO2, σ ∼ 12 nm2/molecule; (c) Bc3-SiO, σ ∼ 4nm2/molecule. Scale bars

correspond to 1 mm. From [31]...... 137

5.15 BAM image of Langmuir ﬁlm at ∼ 7 nm2/molecule of surface concentration. The

scale bar corresponds to 1mm...... 138

5.16 (a) Compression isotherm with initial concentration of 0.86 nm2/molecule. An il-

lustration of ﬁnding mean molecular area for the close-packed conﬁguration of a

Langmuir monolayre. (b) Variation of co-area with initial surface concentration

(σco) with the initial surface concentration (σ0) of the Langmuir ﬁlm. The circles

correspond to compression data and the solid circles correspond to the recompres-

sion data. The vertical error bars represent to the errors in estimating the co-areas

from the isotherms, the horizontal error bars represent ﬂuctuations in surface con-

centrations. (c) Film thickness (h)as function of the initial surface concentration. . 139

5.17 (a)Molecule BC-2X, where X is hydrogen in BC-2H and chlorine in BC-2Cl(b)Annealed

structure of BC-2Cl in vacuo(c)Snapshot from the molecular dynamics (MD) sim-

ulation of the BC-2Cl molecule on the water surface [33]...... 141

xxiv 5.18 Probability P(Z - Z0) of BC-2Cl molecule, whre z - z0 is the height above the water

surface for different parts of the molecule relative to the height of central phenyl

ring. Top frame:results for the inner phenyl ring of the wing, middle frame: outer

phenyl ring, bottom frame: for the terminal carbon atom of the alkyl chain [33] . . 142

5.19 A FFT of a BAM image to investigate the packing of molecules. (a) Original

BAM image of a dilute monolayer (σ ∼ 7 nm2/molecule) made of Z2b bent-core

molecules; the scale bar cor1 mm(b) FFT image of the region of the image in (a)

surrounded by the white rectangle (c) FFT in (b) with a guide to the eye around the

central bright spot. The sizes of all three images have been adjusted to ﬁt in the

page. The bright spots away from the center are due to diffraction fringes in the

illuminating beam. The elongated side of the hexagon is ∼ 50µm...... 143

5.20 A BAM image of 8CB monolayer and its FFT. In (a), the rectangular region indi-

cates the domain whose FFT is to be taken. The scale bar corresponds to 1mm.(b)

FFT of image (a)...... 144

5.21 Change of gray value of the domains with respect to their orientations (φ). The angle

φ is measured in degree. Position of the analyzer with respect to the polarizer is α

= 90◦. The scale bars correspond to 1 mm. Molecule Z2b. Surface concentration:

0.34 nm2/molecule, surface pressure: ∼ 0 mN/m, after full decompression. . . . . 145

5.22 Change of reﬂectivity as a function of domain orientation. The reﬂectivity is in an

arbitrary units...... 146

5.23 For Bc-2Cl:(a) Domain rotation at α = 30◦; an illustration of gray level change with

respect to their orientations in a Bc-2Cl Langmuir ﬁlm at α = 30◦. The scale bar is

equal to 1mm. (b) Change of reﬂectivity as a function of domain orientation φ at α

= 90◦. Solid squares represent the measured data and the solid line represents the

simulated data [32]...... 147

xxv 5.24 A comparison of previous and current results. Solid circles:current work, circles:

previous measured data, solid line: previous simulation. Both set of data were

obtained for α = 90◦...... 148

5.25 (a) Optical model for the uniaxial layer(b) An illustration of reﬂection of plane

electromagnetic wave by an stratiﬁed anisotropic slab sandwiched between air and

water. E is electric ﬁeld, θB is the Brewster angle, n is the director vector, t is the

tilt angle with respect to the surface normal. X-Y plane deﬁnes the ambient/slab

interface, Z axis is in the direction of stratiﬁcation [32]...... 149

6.1 Examples of polymers in all-trans conformation. (a) Polymethylene with with a

carbon backbone (b) PDMS with an Si-O backbone and CH3 pendant groups. The

monomer in both (a) and (b) is indicated by the dashed box. The bond angle are:

◦ θO w 37 with ﬂuctuations dues to low energy barriers [34]...... 154 6.2 Proposed linear PDMS chain conformations (a) Extended caterpillar (b) Extended

helix (c) Approximate Damaschun helix. In these ﬁgures methyl groups are rep-

resented by three balls as hydrogen around the central grey one as carbon [35].

Addapted from [36]...... 156

6.3 Models for linear chain conformations on surfaces:(a),(b) monolayer structures(c),

(d)possible higher layer formation (e) random coil conﬁguration (f) extended helix

conformation on top of a monolayer (g) exteneded helix conformation (h) close-

packed helix. After [36]...... 156

6.4 Compression isotherms of PDMS with several different degrees of polymerizations

[37]. The ﬂat portion (plateau) of the isotherm is the region where gas liquid phase

of the the polymer at the air/water surface coexists...... 157

xxvi 6.5 BAM images of PDMS Langmuir ﬁlms at the air/water interface. (a) PDMS in

the submonolayer regime (concentration < 0.5 mg/m2) with gas liquid phase co-

existance. Dark regions are dense in polymer, bright regions dilute in polymer (b)

PDMS in the collapse regime (concentration ∼ 4 mg/m2). The bright regions are

of higher polymer density than the dark region here. The scale bars correspond to

50µm [38]...... 158

6.6 Proposed conformational changes in PDMS isotherms at the air/water interface:

(A) all Si and O atoms adsorbed onto the interface, (B) some of the Si and O atoms

adsorbed (drawn as only O adsorbed for convenience), (C) helices with their axis

parallel to the surface, and (D) helices oriented more perpendicular to the interface

[39]...... 159

6.7 A classical picture of magnetic moment µ due to spinning positively charged nu-

cleus. Adapted from [40]...... 161

6.8 Schematic of difference in energy levels of two magnetic states with increasing

magnetic ﬁeld Bo. From [40]...... 161

1 6.9 Schemata of magnetic energy levels for (a) with spin 2 and (b) with spin 1. Adapted from [40, 41] ...... 162

6.10 A classical of precessional motion of a nucleus in a magnetic ﬁeld. A: axis of

rotation, Bo: uniform magnetic ﬁeld, N: nucleus, O: circular orbits. Adapted from

[41]...... 162

6.11 Schematic of energy level for a nucleus of spin I = 1. The three energy levels with m

= -1,0, +1 are degenarate in the absence of magnetic ﬁeld. The Zeeman interaction

splits the m = ±1 levels symmetrically resulting in only one symmetric peak at

ν = νL. Further quadrupole interaction causes a perturbation of the split levels

4(Θ) giving rise to two resonant peaks at ν = νL ± 2 [36]...... 164

xxvii 6.12 An illustration of Pake powder spectra. The separation of peaks is expressed in kHz

[36]...... 166

6.13 Micrographs of scanning AnoporeTM membranes obtained from an electron micro-

scope. The micrographs show the cylindrical structures of the conﬁning pores. The

scale bars correspond to 0.3µm [42]...... 167

6.14 Molecular structure of d-PDMS molecule with Mw/Mn < 1.30...... 168

6.15 An illustration of how the DNMR spectra depends on the sample orientation (α)and

2 surface concentration (CS) of the the sample. (a) CS = 1.3 mg/m (b) CS = 0.8

2 mg/m (c) CS = as in (b), but diluted further .The spectra corresponds to the room

temperature (∼ 300K) [36]...... 169

6.16 An illustration of the dramatic change in the quadrupolar splitting of 2HNMR spec-

tra due to change in conformations (a) PDMS melt (b) spin-coated PDMS ﬁlms on

top of polysterine brushes [43] and after [36]...... 170

6.17 DNMR spectra obtained during different stages of sample preparations: (a) solu-

tion concentration 6.2 mg/ml, 30 Anopore discs, 5 minutes in solution, 30-35%

humidity(b) solution concentration 5 mg/ml, 30 Anopore discs, 5 minutes in solu-

tion 40-45% humidity(c) reduced concentration 3 mg/ml; signiﬁcantly diluted later,

30 Anopore discs, 5 minutes in the solution, sample washed in chloroform, 60-65%

humidity (d) solution concentration 2 mg/ml, 30 Anopore discs with about 80%

humidity, 5 minutes in solution...... 172

6.18 Variation of quadrupole splitting (4ν = ν − νL) with surface concentration CS.

Dots represent data from the hydrophobic samples, (4) represent outermost peaks

and (◦) represent intermediate peaks. The shaded vertical region corresponds to the

transition from monolayer to multilayers [36]...... 173

6.19 Schematic of probe orientation with respect to the sample pores. α is the angle

between the normal to the pores of the sample and the applied magnetic ﬁeld B0 [36].174

xxviii 6.20 A block diagram of Fourier transform NMR spectrometer , after [44]...... 175

6.21 An schematic of showing the C3-symmetric axis of rotation of methyl groups in

d-PDMS (after [36])...... 180

6.22 (a) DNMR spectra at low temperature for α = 0◦. Bulk concentration 2 mg/ml,

Anopore discs humidiﬁed to ∼ 80% bofore polymer adsorption. (b) Low tempera-

ture runs for α = 0◦. Bulk concentration 4 mg/ml, surface concentration 0.5mg/m2.

From [36]...... 182

6.23 DNMR spectra of D-PDMS ﬁlms on a salinated (hydrophobic) substrate at α = 0◦.

Bulk concentration 2 mg/ml, Anopore discs humidiﬁed to ∼ 80% bofore polymer

adsorption...... 184

A.1 (a) Schematic side view of the four-roll mill. Dimensions of roller are a = 6.6 mm,

b = 10.5 mm, and d = 7.9 mm. The blowup on the left shows one of the rollers

submersed in the subphase together with the monolayer of amphiphilic molecules.

(b) Top vies of the inside of the gear box. The gears are mitter gears. These gears

transfer motion onto two mutually orthogonal axes. From [45] ...... 193

A.2 Schematic of the four-roll mill rotation proﬁle. From [45] ...... 193

A.3 Flow ﬁeld produced by the rolls which are wobbling from their place. The metter

gears were worn out...... 194

A.4 Improved ﬂow ﬁeld produced by the rolls after replacing the gear box and the worn

out metter gears...... 195

A.5 The correct ﬂow ﬁeld produced by the rollers when the new mitter gears, new rods

and the new roller were placed correctly...... 195

C.1 A schematic diagram of thin layer chromatogram. After [46]...... 204

C.2 A schematic of a separation funnel...... 206

C.3 A schematic of a TLC box. The shaded portion of the tank represents the devoleping

solution...... 207

xxix LIST OF TABLES

3.1 A qualitative comparison beteween 23:2 Diyne PE and 23:2 Diyne PC...... 61

3.2 A qualitative comparison of our experimental methods with that of Wang et.al. . . 82

4.1 Fitting with arbitrary parameters A and B: ...... 114

A.1 Command for the RP240 controller ...... 196

xxx ACKNOWLEDGEMENT

There are so many wonderful people that I have met during my stay in the department who have helped me to get to this point, where I am both ready and well prepared to graduate and move on to something new. I sincerely thank all of you, I will do my best to remember as many of you as I can in the next paragraphs or so, and to anyone that I inadvertently leave out, I apologize in advance.

I would like to start by thanking my advisor Prof. Dr. Elizabeth K. Mann for her support and careful guidance, and most of all for her patience with me over the past few years. I never would have been able to ﬁnish my dissertation without her. Her advice both in pedagogic and in my research will always remain invaluable to me. I am also very thankful to my curriculum advisor Prof. Dr. Mark Manley, whose proper guidance and timely advice ﬁlled in me with a new enthusiasm to carry on my studies and get to this point. Next, I would like to thank my collaborator

Dr. S. Chaieb for the countless times he answered my questions and shared advice with me. I would like to thank Prof. Dr. Dave Allender for those fruitful discussion in our group meeting and the discussion in person. Similarly, I am thankful to all the faculty members for providing me with the quality education. I like to thank Dr. Edgar Koojman for allowing me to use his lab for the TLC experiments and for his valuable suggestion as regards to my phospholipid project. My thanks also goes to Dr. Mahinda Gangoda, Department of Chemistry, Kent State University, for helping me run DNMR experiments in his lab. I also owe thanks to my colleagues Lu Zou and Ji Wang from whom I learned a great deal of experimental techniques during my initial days in Dr. Mann’s lab.

My thanks goes also to Mr. Fanindra Bhatta and Pritam Mandal for their assistance in setting up the heating and cooling system in the BAM experiments and many more.

My special thanks goes to Kim Birkner, Kris Kurtz, Cindy Miller, Hauser Loretta and all the ofﬁce staff, who were an immeasurable help all the time I was here. I would also like to avail myself of this opportunity to thank Wade Aldhizer for his kind help in putting pieces of instruments

xxxi together when I needed.

I would like to thank all the committee members for accepting my request to serve in my dissertation committee.

Finally, and most importantly, I would like to thank my family for their relentless support and love. My parents have been my constant source of inspiration and encouragement all the time in my life. I am always indebted to them for their support, love, and blessings; I cannot express in words how much I miss them being away from them. I would like to thank my wife Dixa and my son Parik for their perennial love and support. Parik, I apologize for my not being able to give you enough time when you needed that, in particular, when I was extremely busy in my lab and my studies. Dixa, it is you who made this work possible for me; without you none of this would have been possible.

Prem B. Basnet

May, 2010.

xxxii CHAPTER 1

INTRODUCTION

This present work is devoted to the study and characterization of ultrathin ﬁlms, which are

ﬁlms less than a few molecules in thickness: between fractions of a nanometer to a few tens of a nanometers in thickness. These ultrathin ﬁlms made on a liquid subphase are also known as

Langmuir films. I studied the Langmuir films made of two different molecules: polymerizable chiral lipid molecules, and bent-core molecules. I also studied ultrathin films of a polymer called polydimethylsiloxane (PDMS) on a solid substrate, which complements earlier work on Langmuir

ﬁlms of the same polymer. Phospolipid molecules in particular, are chiral, both intrinsically and in conformation, and they give rise to chiral pattern. Bent-core molecules are not intrinsically chiral, but conformation and structure can be changed by surface interactions. Similarly, one of the main hypotheses for the for the conformation of PDMS at a surface is a helix. Thus, in all three cases, the molecules are chiral or can take on chiral conformations, and this property plays an intrinsic role in their behavior at a surface. The most novel result is the observation of a new kind of chiral pattern formation in Langmuir ﬁlms of the chiral lipids.

Chirality is an intrinsic property of a class of molecules where the mirror image of these molecules cannot be superimposed with each each other. The situation of two molecules of the same species but opposite chirality can be best compared with our left and and right hand; there is no way that we can make the ﬁngers of both hands overlap each other facing in the same direction.

A carbon atom binding to four different atoms is intrinsically chiral, and thus chirality is common in biological systems. There are situations when even if the molecules do not posses intrinsic chirality in themselves, they begin to behave like the chiral molecules. This could be due to a preference for a propeller-like or twist conﬁguration of individual molecules or due to the arrangement of molecules in layers. The ﬁrst case is known as conformal chirality whereas the second case is known as layer

1 2

Figure 1.1: Molecular chirality and different possible origins. (a) Mirror image cannot be superimposed with each each other by rotation- an illustration of chirality. (b) An illustration of intrinsic molecular chirality (c) Conformational chirality due to a preference for a propeller-like or twist conﬁguration of individual molecules. (d) Achiral molecules show chiral structure due to their arrangement in layers. After [1]. chirality (Figure 1.1 illustrates the different possible origin of chirality). This means even if the molecules are not intrinsically chiral, their behaviors may be like chiral molecules in some situations due to their spontaneous chiral reorganization.

In this chapter, I will first give a short introduction to thin films and a brief history of thin-film technology. Then I will present a historical background of how one of the modern methods of characterization and production of ultra-thin-film originated. I will also try to give a glimpse of a few possible applications in basic research and technology.

1.1 Thin-ﬁlm: A General Introduction

Today, thin films and thin-film technology have found an important place in both basic research and technology. Workshops and seminars are being organized each year around the world about thin-film research, both fundamental and for applications. Several scientific journals give significant space for articles on thin films and their applications. In this context, one can ask a very simple question: what is a thin-film and what is so special about it? 3

A thin film is a thin, usually quite flat layer ranging from fractions of a nanometer to several hundred micrometers in thickness. Thin layers no more than a few molecules thick, with thickness in the range of fractions of nanometer to a few tens of nanometers are called Ultrathin films, and will be the focus of this thesis.

One answer to the second part of the question above, concerning the interest of thin ﬁlms, lies in the fact that thin-ﬁlm microelectronics and optoelectronics industries are among the strongest and quickest growing technologies of our age and have been contributing substantially to the modern economy, a fact manifested by the explosive growth in communications and information processing, storage, and display applications. Fruits of these technologies have fertilized the expanding thin-

ﬁlm uses in the diverse areas, e.g., coating of all kinds (optical, decorative, environmental, and wear resistant), biotechnology, and the generation and conservation of energy. The ability to synthesize organic molecules, almost without limitations, with desired structures and functionality, in conjunction with sophisticated thin ﬁlm deposition technology, enables the production of electrically, optically and biologically active components on a nanometer scale

Thin-ﬁlm technology is one of the newest sciences. However, it is one of the oldest arts [47].

Involvement with thin ﬁlms dates back to the metal ages of antiquity [48] when people would experiment with varieties of ornamentation out of gold. One can ﬁnd historical records [47] of the ancient crafts of gold beating, which has been practiced continuously for at least four mellennia.

Gold is a precious metal which has great malleability, and it is this malleability which enables it to be hammered into a leaf of extraordinary thinness. Leaf samples from Luxor dating to the Eigh- teenth Dynasty (1567-1320 B.C.) measured 0.3 µm [49] in thickness. For comparison, we we can use human hair’s thickness as a reference, which is about 75µm [47]. Perhaps, that was the beginning of the thin-film-related technology. Since then, human-knowledge has made a tremendous progress in all branches of science and technology and the realm of thin-film cannot be an excep- tion. Today, there has been various methods at hand of making thin films out of different material.

Examples of a few methods of making thin ﬁlms are: chemical bath deposition (CBD), physical 4

vapor deposition (PVD), which includes thermal evaporation, electron beam deposition, sputtering, pulsed laser deposition, and cathodic arc deposition, chemical vapor deposition, molecular beam epitaxy, spin coating, etc. It is beyond the scope of this thesis to give the details of each of the techniques here, what follows in this historical perspective on thin films will concentrate on the type of ultrathin films used for most of this thesis, and introduce Langmuir and Langmuir-Blodgett films, a very powerful method of studying and depositing thin-films in various substrates.

The Langmuir ﬁlm can form a good model system for biomembrane research. Thin organic

films of thickness of few nanometers (a monolayer or multilayers) are sources of high expectations as being useful components in many practical and commercial applications, such as sensors, de- tectors, displays and electronic circuit components [50–52]. Very recently, a group of researchers has announced a novel method of storing bits of data on a nanoscale by using silicon nanoparticles on the protein film as the insulator. It was also claimed that they are hoping to invent nanowires with the slight modification of the combination of nanoparticles and protein [53]. In whole process of this novel invention, a Langmuir-Blodgett trough was used to arrange the proteins in a dense honycomb film over a smooth gold surface and the silicon particles.

1.1.1 Langmuir ﬁlms

If a small drop of oil is poured on a water surface, it keeps on spreading, unless it reaches to the other edge of the water surface, making a monomolecularly thin layer of oil on the water surface. The calming effect of oil on the rough and troubled water surface was known to the ancient people. However, it was Benjamin Franklin who first reported the earliest scientific experiments on the effect of oily films on water to British Royal Society as [50]:

“At the length at Clapman where there is, on the common, a large pond, which I ob-

served to be one day very very rough with the wind, I fetched out a cruet of oil, and

dropped a little of it on the water. I saw it spread itself with surprising swiftness upon

the surface ... the oil, though not more than a teaspoonful, produced an instant calm 5

over a space several yards square, which spread amazingly and extended itself grad-

ually until it reached the lee side, making all the quarter of the pond, perhaps half an

acre, as smooth as a looking glass.”

If Franklin had spent some time doing a bit of simple airthematic, he could have shown that the ﬁlm was about 2.5 nm thick [54], which is of molecular dimensions. Lord Rayleigh did the calculation above in the 1880s.

The scientiﬁc community had to wait about a hundred years to recognize the potentiality of

Franklin’s observations. In 1881, for the first time, a German woman named Fraulein¨ Agnes Pockles described a quantitative method to control an oil film on water [54]. She developed a simple surface balance in her kitchen sink in order to determine the surface contamination as a function of surface area for different oils. This very first work towards the systematic study of the thin film at the air/water surface was published in 1891 [55]. This set the foundation for the Langmuir’s quantitative work on fatty acid, ester and alcohol monolayers [54]. By 1917, Irving Langmuir finally developed experimental and theoretical concepts which underline our modern understanding of the behavior of molecules in insoluble monolayer (Langmuir film)[54]. The surface activity of some chemical compounds are responsible for the formation of Langmuir and Langmuir-Blodgett films. Subsection

1.1.2 gives a short introduction of such compounds and the properties of their molecules.

1.1.2 Surfactants

Surface-active agents are usually organic compounds that have two groups present in the molecule: one being hydrophobic in nature and one being hydrophilic in nature as shown in Fig.1.2a. Hy- drophilic means water-liking and hydrophobic means water-hating or, more accurately, oil-liking.

So when you have a surface-active agent with two groups present, you have a chemical compound that has surface-active properties or the ability to affect the interfacial relationship between two dissimilar substances such as oil and water. Surfactants are wetting agents that lower the surface tension of water by adsorbing at the liquid-gas interface. They also reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface. Detergent is an example of a 6

(a) (b)

Figure 1.2: An illustration of an amphiphillic molecule which can form micelles and bilayers because of its surface active properties: (a) an amphiphilic molecule (b) a miclelle and bilayer [2]. surfactant.

Many surfactants can also assemble in bulk solution into aggregates. Examples of such aggregates are micelles bilayers (Fig.1.2b). The concentration at which surfactants begin to form micelles is known as the critical micelle concentration or CMC. When micelles form in water, their tails form a core that can encapsulate an oil droplet, and their (ionic/polar) heads form an outer shell that maintains favorable contact with water. When surfactants assemble in oil, the aggregate is referred to as a reverse micelles [2]. In a reverse micelle, the heads are in the core and the tails maintain favorable contact with oil.

Surfactant systems represent systems between ordered and disordered states of matter. Sur- factant solutions may contain an ordered phase (micelles) and a disordered phase (free surfactant molecules and/or ions in the solution). They have wide range of applications such as detergents, fabric softener, emulsiﬁers and emulsions, paints, adhesives, inks, anti-fogging, soil remediation, dispersants, wetting, laxatives, etc. This shows the importance of the study of surfactants

1.2 Phase coexistence in a Langmuir ﬁlm

The advent of modern method of studying Langmuir layers, beginning with the surface pressure isotherm and conﬁrmed by x-ray diffraction and other methods, has revealed several different types of phases in a single Langmuir ﬁlms as shown in the Figure 1.4. In three dimension, if a gas is con-

ﬁned in cylinder ﬁtted with a movable piston (Fig.1.3a), change of volume and pressure of the gas 7

(a) (b)

Figure 1.3: (a) An illustration of 3d-compression and decompression of a gas at constant temperature and (b) its corresponding isotherm. Diagrams are not to scale. can be carried out by slowly moving the piston in/out at constant temperature. The corresponding

3d-isotherms can be obtained as shown in Fig.1.3b.

In the case of a two dimensional Langmuir film, which is prepared in a Langmuir trough fitted with movable barriers (the top inset of Fig.1.4a), the area and the two dimensional pressure (known as the surface pressure) can be varied by moving the barriers in/out. We go past different phases as we compress/decompress the film. These phases are in direct analogy with three dimensional ones. For example, a low-pressure gas phase, G, condenses to a liquid phase termed as the liquid- disordered phase; early works call this a liquid expanded or LE phase; now that the nature of the phases are better understood L1 is a more appropriate label. A variety of liquid crystalline phases

(LC) exist at higher pressure and lower temperature. L2 is basically a anisotropic liquid-condensed phase, and also identiﬁed as the smectic I, or rotor phase. It has a short-range positional order yet enough cross-sectional area to allow free rotation. L’2 and L”2 are both anisotropic phases.

However, L”2 is more anisotropic than L’2. Something closer to a true two-dimensional crystal with

chains oriented vertically exists at low temperature and high surface pressure (CS phase as shown

in Figure 1.4b). Note that a true 2d crystal is considered impossible without long-range attraction

[3] because ﬂuctuations are always enough to destroy long-range positional order. 8

(a) (b)

Figure 1.4: An illustration of coexistence of several different phases in a Langmuir monolayer. (a)A Langmuir trough (top) and a generalized isotherm of a Langmuir monolayer [3]. Horizontal section of the isotherm are phase coexistence regions at ﬁrst order transitions, and the kink indicates a continuous transitions. (b) Illustrates the condensed mesophases found in monolayers of fatty acids and lipids [4].

1.3 Deposition of Langmuir ﬁlms on solid substrates

An organic thin film can be deposited on a solid substrate by various techniques. There are two schools of thought regarding the production of such films. Some assert that the LB-technique is one of the most promising techniques for producing such thin films as it enables the precise control of the layer thickness, homogeneous deposition of the layer over large areas, and the possibility to make multilayer structures with varying layer composition [56]. A large number of phases, including coexisting phase with different structures, is available in LB films. In addition to these, one can

find LB-technique to be more advantageous in depositing monolayers or multilayers on almost any kind of solid substrate. The main charecteristics of a thin film should have for most applications are surface uniformity (no defects, bare surface) or controlled phase separation, thermal stability, stability over time, and ease of deposition. The control possible with LB films is highly desirable, but defects and stability can be difficulties. Some point out that self-assembled monolayers may have better stability [5], as it is formed in an equilibrium process, as opposed to the intrinsically non-equilibrium process of transferring a monolayer from the liquid to the solid surface. Both 9

Figure 1.5: A schematic diagram showing the transfer of a ﬂoating monolayer on a solid substrate to form a LB ﬁlm.

Langmuir-Blodgett and self-assembled ﬁlms are described brieﬂy below.

1.3.1 Langmuir-Blodgett ﬁlms

Langmuir also reported the transfer of fatty acid molecules from water onto a solid substrate.

However, it was Katherine Blodgett who gave step by step description of monolayer transfer to solid substrate [57]. These built-up assemblies are referred to as Langmuir-Blodgett(LB-) films and the term “Langmuir film” is generally reserved for the floating monolayer. It took several years before people working in the field of thin film and other research areas started taking interest in the great discovery of the Langmuir and LB techniques. Since the first international conference on LB-films held in 1979 [56], the use of these techniques has been increased widely among scientists working on various fields of research.

The Langmuir film balance is used for the preparation of LB-films. LB films are formed by dipping a solid substrate up and down through the monolayer while simultaneously keeping the surface pressure constant by a computer-controlled feedback system between the electrobalance measuring the surface pressure and a barrier moving mechanism changing the area available for the Langmuir area. Consequently the floating monolayer is adsorbed to the solid substrate [56]. By successively repeating this process multilayer structures of hundreds of layers can be produced. These multilayer structures are commonly called Langmuir-Blodgett or simply LB films. The deposition process is schematically shown in Figure 1.5. 10

Figure 1.6: An illustration of the forces in self-assembled monolayer [5].

The LB deposition is usually accomplished in the “solid” phase of a Langmuir film. The surface pressure is then high enough to ensure sufficient cohesion in the monolayer, i.e., the attraction between the molecules in the monolayer is high enough so that the monolayer does not break during transfer to the solid substrate. This also helps the build up of homogeneous multilayers. The required surface pressure for the best resulted is usually governed by the nature of the substrate and the monolayer material. However, amphiphiles can seldom be successfully deposited at surface pressures lower than 10 mN/m, and at surface pressures above 40 mN/m at which the monolayer usually collapses [56]. Very high rigidity of the film often pose problems.

1.3.2 Self Assembled Monolayers

Self assembled monolayers (SAMS) are molecular assemblies that are formed spontaneously by the immersion of an appropriate substrate into a solution of an active surfactant in an organic solvent [5, 58]. Basically, it is an organized layer of amphiphilic molecules in which one end of the molecule, the head group shows a special afﬁnity for a substrate Figure 1.6. From the energetics points of view, a self-assembled surfactant molecule can be divided into three parts as shown in

Fig.1.6. The head group is the one that provides chemisorption1

The very-strong molecular-substrate interaction makes the headgroup pinned to a speciﬁc site on the surface through a chemical bond. The second molecular part is the alkyl chain. The energies associated with its interchain van der Waals interactions are at the order of few (<10 kcal/mol) [5].

1Chemisorption is a classiﬁcation of adsorption marked by a strong interaction between an adsorbate and a substrate surface. The energies associated with the chemisorption are at the order of tens of kcal/mol [59, 60]. 11

The third molecular part is the functional group, or terminal functionality. In case of a simple alkyl chain, methyl group (CH3) is the functional group. These surface groups are thermally disordered at room temperature [61]. The energy associated with this group is usually of the order of 0.7 kcal/mol

[5]. Selecting the type of head group depends on the application of the SAM [62]. Metal substrates for use in SAMs can be produced through physical vapor deposition techniques, electrodeposition or electroless deposition [62].

1.4 Summary

We have seen that Langmuir films can be the basis of films on solid surfaces with important technical applications. Some of our work is in that direction. But Langmuir films are also good model systems for biomembrane research, as well as an excellent approximation to a two dimensional system, with multiple possibilities for exploring intrinsically two-dimensional behavior in terms of transport, phase behavior, and pattern formation.

In Chapter 2, ﬁrst I will present an overview of different experimental techniques used in the characterization of ultrathin ﬁlms and then I will discuss Brewster Angle Microscopy (BAM) and surface pressure isotherm measurements, the major experimental techniques used in my current work.

In Chapter 3, ﬁrst I will present a short review of previous work on pattern formation in Lang- muir ﬁlms of several different kinds of systems. I observed a novel form of pattern formation in monolayers of a chiral, polymerizable lipid analogous to those found in lungs. A qualitative discussion of the data of my work with possible origin of pattern formation will also be presented in this chapter.

Chapter 4 is an extension of Chapter 3 with more quantitative discussion of data where I will also present a possible model of our system, developed by my collaborator Dr. Sahraoui Chaieb2.

In Chapter 5, I will discuss preparation and characterization of Langmuir ﬁlms of a bent-core

2Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana IL, 61801 Currently: 4700 King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia 12

molecule (know as Z2b). The special shape of bent core molecules constrains their packing and leads to very interesting properties including optical anisotropy. The goal of this work, an extension of other recent work in our laboratory, is to explore the orientation and packing of these molecules at an interface. The resulting ﬁlm may also serve to align such molecules, which has proved difﬁcult but essential for many applications.

In Chapter 6, I will discuss preparation and characterization of ultrathin ﬁlms of PDMS on aluminum oxide surfaces. I will discuss how solid state Nuclear Magnetic Resonance (NMR) spectroscopy can be a useful tool to characterize such system.

Finally, in Chapter 7, I will summarize the results of this work and suggest direction for the future. CHAPTER 2

EXPERIMENTAL TECHNIQUES

2.1 Chapter outline

In this chapter, ﬁrst I will present a short review of a few experimental techniques, such as X-ray diffraction, X-ray and neutron reﬂectivity, Ellipsometry, Imaging Ellipsometry, Fluorescence microscopy (FM), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM),

Scanning probe microscopy (SPM), Scanning tunneling microscopy (STEM), Nuclear Magnetic

Resonance (NMR) spectroscopy, and Second Harmonic Generation (SHG), commonly used in characterizing molecularly thin ﬁlms. The rest of this chapter will be devoted in discussing the main experimental techniques used in my research work, including the theory and experimental set-up of

Brewester angle microscopy (BAM), and the surface pressure measurement by the Wilhlmy plate method. Since cleanliness of lab and instruments plays important role in the kinds of experiments that we perform, I will also devote substantial space at the end of the chapter to the discussion of cleaning, as a separate section.

2.2 Introduction

Many different kinds of molecules can form stable thin films on different substrates. Surface inhomogeneity (such as surface roughness, difference in surface densities, molecular orientations, variations in film textures) is one of the important characteristics of these kind of thin films. Pre- vious studies [14, 15, 17, 63, 64] have shown that some thin films can form intriguing patterns; we can find various explanations, for their formation in literatures [24, 65, 66], to be discussed in chapter 3. My research project has considered three such molecules: a polymerizable phospholipid, a set of bent-core molecules, and a polymer. Surface characterization of the films formed by these molecules basically involves recognizing surface and interfacial properties; describing interfacial

13 14

energies, wetting behavior, and surface roughness. For this purpose different kinds of experimental techniques [4], such as Synchrotron X-ray diffraction, Ellipsometry, Imaging ellipsometry, Fluo- rescence Microscopy (FM), Transmission and Scanning Electron Microscopy (TEM and SEM),

Scanning and Tunneling Microscopy (STM), Scanning Probe Microscopy (SPM), Small Angle X- ray Scattering (SAXS), Nuclear Magnetic Resonance (NMR) can be used. A short description of some of these techniques will be presented in Sec.2.2.1 below.

2.2.1 A brief review of some useful experimental techniques

2.2.1.1 X-ray diffraction

Synchrotron X-ray diffraction is used to characterize the thickness and crystallographic in-plane structure of a substrate-anchored thin film [67, 68]. Background minimization is a condition required for X-ray diffraction; it is achieved by fixing the angle of incidence below the grazing angle of total external reflection. The diffraction peaks, which depend on the diffracted intensity normal to the sample plane, are used to investigate the surface structure. Using synchrotron radiation is essential to provide enough signal off of a single monolayer.

2.2.1.2 X-ray and neutron reﬂectivity

By using these techniques, we can probe the surface morphology and structure, although possible damage to the ﬁlm must always be considered. Important parameters, such as electron density and its modulation along the surface normal, ﬁlm thickness, interfacial roughness, and surface morphology can be studied by these techniques [69].

2.2.1.3 Ellipsometry

Ellipsometry is one of the most powerful optical techniques for the investigation of the optical properties such as the complex refractive index of a thin film [70]. Ellipsometry measures the change of polarization state of light reflected off the sample [71]. By a careful analysis of the reflected light off the sample, one can acquire important information about the complex refractive index which can be useful in investigating morphology, chemical composition, or electrical conductivity of the 15

specimen.

2.2.1.4 Imaging Ellipsometry

This ellipsometry is a modified version of the ellipsometry discussed earlier. With a few mod- ifications, ellipsometry is done as an imaging ellipsometry with a CCD (charge coupled device) camera as a detector [72]. Monochromatic laser light is used to capture a real time image cotrast of the specimen being investigated. The image provides information about the film thickness and refractive index the the sample [73]. Early versions of this technique tends to be slow, with rather poor image contrast, but it holds great potential.

2.2.1.5 Fluorescence microscopy (FM)

A fluorescence microscope is a an optical microscope where a fluorescent material is used to obtain the image contrast of the region of interest of a sample [74]. In this microscopy, usually the component of a sample to be studied, is labeled with a fluorescent molecules. The probe molecules absorb photons upon illumination. Photons are emitted at a longer wavelength than the incident photons. The amount of emitted light depends both on the concentration of the probe and, to some extent, on its environment. Both of these depend on the phase within the monolayer, which provides contrast. The emitted light is captured with a CCD camera to produce the image. The contrast is used to acquire information about the surface properties, and in particular the phases, of a thin film of molecular thickness.

2.2.1.6 Transmission electron microscopy (TEM)

TEM is a microscopy technique where a beam of electrons is allowed to transmit through an ultra thin sample. The transmitted electrons interact with the sample as they pass through. As a result, an image is formed; the image is magnified and then focused onto an imaging device, such as a fluorescent screen or a CCD camera [75]. The image contrast is due to the absorption of electrons in the material. Owing to the small de Broglie wavelength electrons, the resolution of an electron microscope is significantly higher than that of an optical microscope [76]. 16

2.2.1.7 Scanning electron microscopy (SEM)

This is another type of electron microscopy which images the sample surface by scanning with a highly energetic beam of electrons in a raster scan patterns [77]. Electrons scattered from the sample are called Back Scattered Electrons (BSE). These BSE are used in imaging the specimen. Since the

BSE are strongly related to the atomic number (Z) of a specimen, the BSE images can provide information about the distribution of different components in the sample [78]. The resolution of

SEM can be as small as a nanometer.

2.2.1.8 Scanning tunneling microscopy (STM)

STEM is a powerful technique for probing the surfaces at the atomic level [79, 80]. The beauty of this technique is that it can be used in air, and other gas or liquid ambient within a wide range of temperatures- from near zero Kelvin to a few hundred degree Celsius [81].

Its working is based on the quantum tunneling; a quantum mechanical phenomenon where a matter wave leaks through a barrier [82]. The microscope is provided with a conducting tip which can be brought very close to a metallic or a semiconducting surface. A bias between the tip and the surface allows electrons to tunnel through the gap between them. The tunneling current at low voltage is a function of the local density of states at the Fermi level, Ef , of the specimen [81]. The change is current as the probe passes over the surface are digitally converted into an image. The image provides information about the surface properties of the specimen.

2.2.1.9 Scanning probe microscopy (SPM)

In this microscopy, related to the STM, specimen surfaces are scanned by using a physical probe.

When the probe mechanically moves along the surface in a raster scan, line by line, an image of the surface is obtained due to the probe-surface interaction as a function of position [83]. There are different variants of SPM; one of them is an atomic force microscopy (AFM). The resolution varies from technique to technique and usually remains within the nanometer range. 17

2.2.1.10 Nuclear Magnetic Resonance (NMR) spectroscopy

All nuclei that contain odd number of nucleons have spin > 0. When these nulei are subjected to a static magnetic ﬁeld, they align themselves either parallel or antiparallel to the static magnetic

ﬁeld; energy levels of these spins split. When an electromagnetic, usually a radio frequency (RF) pulse, is applied to these spins, each spin absorb energy equal to the energy of splitting thereby raising the spins to a higher energy level [84]. The spins cannot remain in higher energy state for ever, rather they begin to release energy through relaxation process which takes place either due to the spin-spin interactions or spin-lattice interactions or both of them. The energy thus released is converted into the NMR signal. The NMR signal gives important information about the structure, dynamics, orientations, and conﬁrmation of a molecule in the specimen.

2.2.1.11 Second Harmonic Generation (SHG)

In this technique, photons of energy hν are coupled to produce a new photon of energy 2hν [85–

87]. It is particularly useful as a surface probe because, SHG is dipole-forbidden in centrosymmetric

media such as water, but allowed at an interface or a surface where the centrosymmetry is broken.

The technique being surface sensitive, it allows us to directly study the surfaces or interfaces of

interest [88, 89].

2.2.2 Experimental techniques used in my research work

All of techniques discussed in Sec.2.2.1 are excellent for surface characterization. However,

most of these techniques are very expensive [90], or only available in large national user facilities

and they cannot be used for direct visualization of thin ﬁlms and phase separation. Additionally,

most of them are only useful for surface characterization of ﬁlms on solid substrate. The basic

goal for my project is not only to characterize the ﬁlms these molecules form at the air/water inter-

face, but also to study their morphological behaviors during their compression at different physical

conditions (e.g., temperature, pressure, humidity, etc.). Brewster angle microscopy (BAM) can be

an ideal technique for this purpose. Study of ﬁlm properties of the ﬁrst two kinds of molecules 18

(ploymerizable phospholipid and bent-core molecules) have thus been carried out mostly by using

(BAM) technique. For the second type of molecules (polymer molecules) , previous work with

BAM is supplemented by the method of Deuterated Nuclear Magnetic Resonance (DNMR) as the experimental tool in this project. In order to supplement the results obtained with BAM, Atomic

Force Microscopy (AFM) and some other experimental methods has also been used. However, this chapter will be devoted to the discussion of BAM coupled with the determination of thermodynamic properties and the DNMR techniques and the rest of the experimental methods will be discussed in the relevant places of this dissertation.

2.3 Why BAM ?

As just listed, there are large varieties of optical techniques being used in surface characterization. The biggest advantage of optical techniques is that they are non-invasive (non-contact) which is important in the situations where the surface structures are either very soft or must not be contaminated or destroyed. Brewster angle microscopy and Fluorescence microscopy are two such optical techniques which can be used in an individual lab, unlike many of the techniques mentioned above.

These two techniques are also advantageous in characterizing both solid and liquid interfaces. BAM

[91] and FM are complementary techniques which are sensitive to different properties of a surface.

Moreover, in comparison to the FM the main advantage of the BAM technique, apart from being non invasive, is that it allows the direct observation of ultra-thin films at the air-water interfaces or on dielectric substrates without using any fluorescent probes in the material being studied. Basically, the BAM was first built to study first-order phase transitions in monolayers and to study the growth of two-dimensional domains without any contaminations of the surfaces by use of probe materials, which might change significantly the behavior of the monolayer.

2.3.1 Physics of BAM

In order to understand physics behind the working of a BAM, we need to know the meaning of different terminology discussed in the following subsections. 19

2.3.1.1 Polarization of light

One of the key factors for BAM is the polarization of light waves. Now, the question is what is polarization of light? It is a well-known fact that sunlight including every other form of illumination are electromagnetic waves. These waves carry electric and magnetic field vectors and travel in such a way that the electric field vector, the magnetic field vectors and the direction of propagation are always mutually perpendicular in an isotropic medium, such as air. These field vectors continuously vibrate in all possible planes that are perpendicular with respect the direction of propagation (Since the effect of magnetic field vector is feeble compared to the effect of electric field vector on most materials, I will only refer to the electric field vector for a light wave henceforth). However, if the unpolarized light reflects from the surface of a dielectric or an insulating material (e. g., glass, plastic and paper sheets, highways and undisturbed water surface), vibrations of electric field vector is partially limited to one plane, in a certain direction (Fig.2.1). The phenomenon of confinement of vibration of electric field vector in a particular plane is called the polarization of light and the light is now referred to as plane or linearly polarized. Similarly, polarization of light also takes place if it refracts through certain kinds of dielectric media (e, g., Iceland Spar Crystal), usually known as polarizer.

Depending upon the orientation of the electric ﬁeld vector E~ after polarization, the polarized light can be either p-polarized or s-polarized. If the if E~ of the polarized light is parallel to to the plane of incidence (see Fig.2.2), polarization is referred to as p-polarization, whereas in s- polarization E~ is perpendicular to the plane of incidence.

2.3.1.2 Brewster Angle

Sir David Brewster, a Scottish physicist, discovered that when light hits a reﬂective surface at a particular angle, the light reﬂected from the surface is found to be completely plane polarized. The critical angle of incidence at which this phenomenon takes place is known as the Brewster’s angle,

θB. In other words, the Brewster angle θB is an angle of incidence at which light with a particular polarization is completely transmitted through a surface with no reﬂection. The molecules of a 20

Figure 2.1: An illustration of polarization after reflection. When light strikes at the Brewster angle, θ, the reflected light is plane polarized. The reflected beam and the refracted beam make an angle ◦ of 90 with each other. I: incident beam, R: reflected beam, T: transmitted beam; θB: Brewster angle; θt: angle of transmission (or refraction); n = 1.5 is the refractive index of glass with respect to air.The solid circles represent the vibration perpendicular to the plane of the page (i. e., perpendicular to the plane of incidence), and the double arrows represent the vibration in the plane of page (i. e., in the plane of incidence).

Figure 2.2: An illustration of how the incident beam I, the reflected beam R, the transmitted (or refracted) beam T and the normal to the interface N lie on the same plane - the plane of incidence. In the figure, θi is the angle of incidence, θr is the angle of reflection and θt is the angle transmission or refraction(Figure is not to scale). 21

dielectric medium can be thought of as electric dipole induced by the electric field and oscillating randomly in all directions. After reflection from the dielectric medium at the Brewster angle, oscil- lation of these induced dipoles are confined in a direction parallel to the direction of propagation of light. The intensity of radiation parallel to these induced dipoles is zero, i.e., there is no radiation.

When a beam of light passes from a rare (low density) medium (refractive index ni) to a denser medium (refractive index nr), Snell’s law can be written as:

n sin θ r = i (2.1) ni sin θr

Eq.2.1 can more generally be written as,

ni sin θi = nr sin θr (2.2) or, n sin θ n = r = i (2.3) ni sin θr

where, n is the refractive index of the rare medium, n , n = nr is the refractive index of i r ni the denser medium relative to the rare medium, θi is the angle of incidence, and θr is the angle of refraction.

It is obvious from Fig.2.1 that with the Brewster angle as the angle of incidence,

θi = θB (2.4) and

◦ θr = 90 − θB (2.5)

So,

◦ sin θr = sin(90 − θB) (2.6)

= cos θB 22

sin θ n = B cos θ B (2.7)

= tan θB

By using Eq.2.7, we can ﬁnd the value of Brewster angle for a particular medium. For example, refractive index for water with respect to air is ∼ 1.33. Using this in Eq.2.7, θB, the Brewster angle, will be ∼ 53.12. Hence, the importance of Eq.2.7 lies in the fact that it makes possible for us to correctly calculate the critical angle of incidence for setting up BAM for a particular interface, which is one of the essential condition for BAM.

2.3.2 Theory behind BAM

Basically, working of BAM makes use of the principle of null reflectance. When p-polarized light strikes an air-water or any dielectric interface at the Brewster angle for that interface, the reflectivity for that interface is zero in principle, and this is also referred to as the condition for the null reflectance. However, the reflectivity of a real interface at the Brewster angle is strongly dependent on the interfacial properties, such as the thickness and roughness of the real interface and also the anisotropy of the film on it [91]. As discussed in Sec. 1.1.1, molecularly thin films (mono- or a small number of layers) whose thickness [90] are of the order of 0.3 % of the wavelength of visible light, can be formed on a water surface. The electric field reflected from a water surface has almost no effect on such thin films and under normal conditions they are quite impossible to visualize by any optical techniques. However, if such a film is irradiated with p-polarized light at the Brewster angle, the background being completely dark, a little effect of the monolayer upon compression can be visualized (see Fig.2.3).

For an ideal interface (Fresnel) the reﬂectivities for the p- and s-polarizations are given by the expressions [71]

p Er p n2 cos θ1 − n1 cos θ2 = r12 = (2.8) Ei 12 n2 cos θ1 + n1 cos θ2 23

Figure 2.3: An illustration of how the contrast with and without the presence of a monolayer changes due to incidence of p-polarized light at the Brewster angle. Without monolayer, there is zero reﬂec- tion, but with monolayer there is some reﬂection. Figure is not to scale and

s Er s n1 cos θ1 − n2 cos θ2 = r12 = (2.9) Ei 12 n1 cos θ1 + n2 cos θ2 where the superscripts p and s refer to the parameters for p- and s-polarized lights and the subscripts

1 and 2 (Fig.2.5) refer to the parameters in the incident medium and in the transmitted medium respectively. Similarly, Ei is the incident electric field and Er is the reflected electric field (Fig.2.4).

In the presence of dielectric ﬁlm at the air/water interface the reﬂectivity [71] can be given by

−i2φ r12 + r23 e r = −i2φ (2.10) 1 + r12 r23 e where r12 and r23 are the reflection coefficients for air/film and the film/water respectively. From

Fig.2.5, the phase difference between the second and the ﬁrst reﬂections is,

2π φ = ( Difference of optical path length) (2.11) λ◦ where, the optical path length is deﬁned as the product of the geometric length of the path light follows through the system and the index of refraction of the medium through which it propagates. d is the ﬁlm thickness and λ◦ is the wavelength of the incident light in vacuum.

Using Eqs.(2.2) and (2.11), we have the phase difference [71] as , 24

Figure 2.4: Reflection and transmission at the interface of two media. Here, I is the incident beam, R is the reflected beam and T is the transmitted beam. Here, Ei, Er, and Et are the incident, reflected and transmitted electric fields respectively.

Figure 2.5: Multiple reflections of light within the thin film. n1, n2, and n3 are the refractive indices of air, film and water medium respectively. d is the thickness of the film. 25

2π φ = (n2 d cos θ2) (2.12) λ◦ or,

d 1 φ = 2 π (n2 − n1 sin θ1) 2 (2.13) λ◦

Then the total reﬂectivity of the system is,

2 2 2 r12 + 2 r12 r23 cos φ + r23 R = |r | = 2 2 (2.14) 1 + 2 r12 r23 cos φ + r12 r23 Let us take an example of a DIYNE PE ﬁlm at the air/water interface at room temperature

◦ (∼ 20 C). In this special case, n1 ∼ 1.00 for air, n2 ∼ 1.55 for DIYNE PE film ( estimated from the bulk value for many organic molecules), n3 ∼ 1.33 for pure water, and the wavelength of light used is 668nm. By using Eqs.2.8, 2.9, 2.11, and 2.14, we can calculate the reflectivities for both p- and s-polarization for different film thickness d and then plot as in Fig.2.6 [6].

◦ In the neighborhood of Brewster angle at the air/water interface (θB ∼ 53.12 ), Rp is very sensitive to the film thickness Rs increases monotonously with θ it is very hard to tell the difference of domain thickness. Hence the example is a very strong evidence of the fact that Brewster angle microscopy works only around the Brewster angle with p-polarization. It can also be seen that for p-polarization, there is a substantial degree of contrast between the films of different thickness. This holds true even if the thickness of the film is molecularly small.

It is imperative to mention here that all the derivations and calculations above are made assuming a macroscopic layer of an uniform density. Additionally, assumptions have been made that the interfaces with the gas and liquid phases are perfectly ﬂat. It might seem surprising that this would describe well a molecularly-thin layer. However, for a wavelength of light much larger than than the molecular size and even larger inhomogeneities, the average refractive index for the macroscopic picture has been found to give exactly the same signal as the homogeneous one in the every test case[6]. 26

(a)

(b)

Figure 2.6: An illustration of how reﬂectivities change with thickness of the ﬁlm at around Brewster angle for s- and p-polarization: (a) s-polarization (b) p-polarization. 27

Figure 2.7: A simpliﬁed sketch of a BAM setup. L on the left side is the collimating lens, I is the iris, P is the polarizer, L on the right side is the objective lens, A (optional) is the analyzer, and CCD camera for grabbing movie frames.

2.4 BAM setup

A Brewster angle microscope (BAM) is equally used for both the Brewster angle microscopy and Brewster angle microscope henceforth) is basically constructed by using a coherent light (a laser), a collimator (which is a converging lens of suitable focal length), a polarizer, an objective or a converging lens and a CCD1 camera as shown in Fig.2.7. Additionally, other components, such as an analyzer (which is another polarizer in between the objective and the CCD camera), infrared-cut

(an infrared ﬁlter) can be added in the path of the reﬂected light to enhance the its functionality for probing the optical anisotropy of a system under investigation, and to protect the CCD camera from the harmful infrared radiations.

We can find large variations of BAM designs [92–95] and the choice mainly depends on one’s need and flexibility. The design of the BAM in our lab is the simplest one as shown in Fig.2.7. It consists of the Huber rotation stages (model 5202.5) provided with two arms, which I call the laser- arm and the camera-arm for my convenience. The camera-arm holds a CCD camera, a converging lens as an objective, and the optional parts such as an analyzer, infrared filter. The laser-arm consists

1CCD stands for a charged coupled device 28

] Figure 2.8: BAM setup in the lab. A: laser-arm, B: camera-arm, C: Huber rotation stages, D: ﬁne-tuning control knobs, E: vibration-isolation table. of a laser source, an iris, a collimating lens (this may or may not be needed depending on the type of laser in use), and a polarizer. These arms are attached to the rotation stages via adjustment mounts

(manufactured by J B Manufacturing) (See Fig.2.8). These mounts are made up of the macrobench setup from LINOS and the choice of these was made for its stability and rigidity. Rigidity of these arms is one of the important factors to be taken into special consideration since they have to bear substantial weight of the CCD camera, all the necessary optical components and the additional parts to keep them tilted at the Brewster angle.

As discussed in Sec.2.3.2, ﬁne adjustment of angle of incidence as close to the Brewster angle as possible, is one of the essential conditions for properly working a BAM. By using the knobs provided with the stages, the tilt of the camera-arm and the laser-arm can easily be tuned to the desired angles with a precision of ∼ 0.02◦. The rotation stages are provided with x, y, z stage assembly so that the movement of the BAM can be made without any extra effort along x-, y-, and 29

z-directions with the help of the knobs ﬁtted with them. Movements in these directions can be made with a precision of ∼ 10µm..

The quality of images obtained with the BAM depends to a great extent on how much the BAM setup is free from mechanical vibrations. If there is some kind of mechanical vibrations (e. g. slam- ming the lab doors), the BAM image might get overexposed due to the ripples (if large enough) produced. These ripples bring the working substrate (e. g., water) out of the Brewster angle, to greatly increase its reﬂectivity, which ultimately poses threats in obtaining quality images. Addi- tionally, these vibrations also inﬂuence the extremely sensitive surface pressure measurements (a very important experimental tool, which will be discussed in the succeeding sections). Keeping these in view, the whole assembly(x-,y-,z-stages, rotation stages, adjustment pieces, and the macrobench arms) is placed on a vibration isolation table (from Technical Manufacturing Corporation).

The vibration isolation table helps reduce the effect of low frequency waves mentioned above.

Two different lasers were used in the course of this work. The red laser (SDL 7470-P6, 668nm) is coupled to the laser-arm via a multimode ﬁbre. This laser needs to be collimated carefully, minimizing the beam divergence. A second Argon ion laser (λ = 488nm), which allows better collimation for better image quality, was introduced later. The CCD camera is a Panasonic (GP-MF

602) with image pixels 704×874. The polarizer used in the experiments was Lambrecht MTYE 15.

Any type of the converging lenses can be used in this setup. However, as dictated by the BAM size, we must be aware of the fact that the sum of the image distance and the object distance should not exceed 60cm. The images are captured by the CCD camera at the rate of 30 frames per second and sent to a the computer connected via an interface(Pinacle System GmBH Movie Box USB Rev:1.0).

Studio 9 software is used to communicate between the computer and the camera.

2.5 Surface Pressure Measurement

Surface pressure isotherm is one of the oldest experimental techniques [55, 96, 97] used by researchers to investigate the phase behaviors of quasi-two-dimensional systems on liquid surfaces.

This is one of the major experimental tools used in our lab to study the various phase behaviors of 30

Figure 2.9: Interaction of molecules at the interface and in the bulk. The solid spheres represent the liquid molecules and the arrowheads represent the direction of intermolecular forces.

Langmuir ﬁlms at the air/liquid interface. The basic principles behind the surface pressure measurements and hence the surface pressure isotherms will be discussed in the subsequent sections.

2.5.1 Surface tension

In order to understand the working theory of surface pressure isotherms it is important to understand the deﬁnition of surface tension.

The molecules in a liquid attract each other up to some extent. The cohesion, which is the degree of attraction, depends upon the properties of the substance under consideration. In the bulk of a liquid, the interactions of molecules are balanced equally in all directions by an attractive force.

However, a molecule at the air/water interface has a larger attraction toward the liquid phase than toward the air or gas phase. In another word, the molecules on the surface of a liquid experience an imbalance of forces resulting in a net attractive force toward the bulk. This will make air-water interface minimize its area and contract of its own volition.

The overall effect of this situation is the existence of free energy at the interface. This free 31

energy can be considered as an excess free energy and is usually called surface free energy. The surface free energy can be quantiﬁed as a measurement of energy/area. Surface tension can be thought of as a measurement of cohesive energy present at the interface. As is obvious from this discussion, surface tension can be measured in units of Joule/m2 or erg/cm2. However, the most

practical units are either mN/m or dynes/cm; these units are equivalent to each other. Solids may also have surface free energy at their interfaces, but they are more complex in nature and their direct measurement is not possible through the techniques used for liquids.

Intermolecular interactions depend mainly on the nature of a liquid in question. If the molecules of a liquid are polar, the intermolecular interactions are much stronger compared to the intermolecular interactions of a non-polar molecules. Water is a typical example composed of polar molecules.

The high surface tension is a manifestation of strong intermolecular interactions. Any physical or chemical factor which decreases the strength of these interactions will lower surface tension. For example, an increase in temperature of a system of liquid will result in a decrease of surface tension. Apart from this, if the system is contaminated, especially by surfactant, surface tension will be lowered. However, salts are exceptions. If salts are added to the system, surface tension will increase. Therefore, we must be very cautious about the issue of contamination in our surface pressure measurement experiments.

2.5.2 Theory

Let us consider a pure liquid covered with a film taken in a system fitted with a movable barrier as shown in Fig.2.10. In this isotropic film system, if the barrier is moved through a distance of dx, the area of the film will be increased by dA = l dx, where l is the width of the system of the film.

Then the work done in moving the barrier through a distance dx or stretching the ﬁlm by an area

dA is given by

dW = F · dx = γl dx = γ dA (2.15) where γ is the surface free energy per unit area, as discussed in Sec.2.5.1. 32

Figure 2.10: An isotropic ﬁlm on top of a pure substrate. The ﬁlm can be stretched in a direction shown by the outer arrowhead.

Figure 2.11: An illustration of the principle of the Langmuir ﬁlm balance. γ is the surface tension of the ﬁlm-covered surface and γ0 is the surface tension of the pure liquid surface.The drawing is not to scale.

Gibb’s free energy [4] at the interface can be given by

X X s dG = Vα dP − Sα dT − S dT + γdA (2.16) α α where the thermodynamical quantities with superscript s are the surface excess quantities and the quantities with subscript α are the thermodynamical quantities of the bulk phase. The last two terms in Eq.2.16 are due to the interfacial region.

Note that Eq.2.16 holds good for the both sides of the movable ﬂoat shown in Fig.2.11. For this special situation we can write then 33

s s s dG = −S dT + γ0dA0 + γdA (2.17) where, dA0 = −dA. Then Eq.(2.17) becomes,

s s dG = −S dT + (γ0 − γ)dA (2.18)

The surface pressure, Π, is generally considered to be equal to the reduction of pure-liquid-surface tension by the film and is defined as the change in surface tensions of a pure-liquid and the surface tension of the liquid covered with a film and is given by

Π = γ0 − γ = −∆γ (2.19)

It is important to note here that Π is two-dimensional thermodynamic analogue of three-dimensional pressure P, as can be seen in Eq.2.18.

2.5.3 Instruments for the Surface Pressure Measurement

The first surface tension isotherms were done by Pockles (Sec.1.1.1). She developed a simple surface balance in her kitchen sink in order to determine the surface contamination as a function of area of the surface for different oils; her work was published in 1891 [55]. Irving Langmuir [97] designed a more complex trough to prepare molecularly thin films on a liquid subphase for the purpose of surface pressure measurements. Now-a-days, with the advancement of technology, trough and film balance of different varieties are available according to one’s need. However, the basic working principle is the same for all designs. In my work, I have used the following components, discussed in detail below, for the purpose of surface pressure measurement:

1. Langmuir trough and the beam-dump

2. Barriers

3. Wilhelmy Plate 34

Figure 2.12: Rectangular Langmuir trough made of Teﬂon sitting on an aluminum base with rectangular hole at the center of the trough.

4. Temperature sensor

5. Water bath for heating and cooling

2.5.3.1 Langmuir trough

Langmuir trough is a very simple device used for making molecularly thin ﬁlms (Langmuir

films) on liquid subphases and then make the surface pressure measurements in conjunction of the other components listed above. In addition to the preparation of Langmuir films, these troughs can be used for creating quasi-two-dimensional systems with various measurable and controllable parameters, such as temperature, surface viscosity, film composition, surface density and the composition of the subphase. There are varieties of Langmuir troughs from different manufacturers, but we use the KSV minitrough to serve our experimental purposes. Fig.2.12 shows the prototype of a

KSV minitrough. The dimension of this minitrough is 364mm × 75mm

Basically, it is a shallow rectangular trough made of milled Teflon fixed on top of a rectangular aluminum base. The reason for using Teflon is that it is a hydrophobic material (a) is relatively easy to clean and keep clean and (b) makes pure water (mostly used as a subphase in our experiments) remain within the trough stable even with the surface level higher than the trough edges. Here it is important to note that keeping the water level higher than the edge of the trough helps prevent the leakage of the deposited material on the surface of the subfluid from underneath the barriers

(discussed below). Very often we may need to change the temperature of the subphase and hence the Langmuir ﬁlms, depending upon the demand of one’s experiments. For this, the trough has been 35

provided with a piping system through the rectangular part of the aluminum base to which a suitable pair of plastic tubes can be attached (Fig.2.12). Later on, these tubes can be connected to the water bath when the need be.

There is a rectangular hole, usually known as dipping hole, at the center of this trough; it is usually used for the transfer of films to solid substrates by dipping. As discussed in Sec.2.4, a BAM uses a highly collimated laser beam in order to collect the information of the Langmuir film on the subphase. The laser beam is reflected not only from the surface of the film, but also from the bottom of the subphase. If the scattered light or the beam from second reflection is not managed well it can badly affect the quality of image of the surface and the desired information is lost. Additionally, an unattended beam from the second reflection can cause serious problems for the human eyes. In order to get the best result we place in the dipping hole a beam-dump (details of description can be found in the references [1, 6]) made of black Delrin, a hydrophilic material, and this beam-dump works in water unlike the ones which work in air provided with other designs of Langmuir trough.

Here, the central point of using a beam dump is to collect the stray beam and make it disappear so as not to interfere with the beam reflected from the film surface. Delrin is considered to be fairly stable material from the chemical reaction point of view. It is not easily damaged when treated with strong cleaning reagents and this property makes it very useful to server our purpose since cleaning is an essential part of our experiment. The black color of this material helps to reduce the energy associated with the stray beams by absorption. The beam dump consists of a right cylinder and a circular cone as shown in the Fig.2.13. The thread texture engraved on the inner wall of the cylindrical parts act as multiple reflection spots for the incident beam and they help lock in the beam.

2.5.3.2 Barriers

In the experiments the surface pressure is measured as a function of mean molecular area of the

ﬁlm. In order to change the mean molecular area of the ﬁlm, we need to compress or decompress the

ﬁlm which is achieved by using two rectangular barriers (Fig.2.14). The parts of the barriers which 36

Figure 2.13: A delrin beam dump.(A) right cylinder with thread texture in the inner wall.(B) Circular base with a pointed cone.

Figure 2.14: An illustration of Langmuir ﬁlm balance with movable barriers. Courtesy- KSV user’s manual. 37

remain in contact with the trough edge is made of delrin supported by rectangular steel base. As mentioned in Sec.2.5.3.1, delrin is a hydrophilic material. By pairing the hydrophobic (trough) and the hydrophilic (barriers) materials can provide the best possible contact of subphase (water) with the edge of the trough and the barriers. It has been found by tests with different films [6] that, when carefully machined and maintained, this pair of material is practically leak-proof for the material deposited on a liquid subphase. The barriers are installed in the slots fitted with a carrier belt and can move along the edge of the Langmuir trough lengthwise. The speed and the direction of motion of the barriers are controlled by a computer. The idea of using two movable barriers is to change the mean molecular area of the film as precisely and symmetrically as possible so that the surface pressure due to the film can be recorded as a function of the mean molecular area while an image of the film is taken at the relatively stationary symmetry point.

The barrier position is controlled by a micro step driven stepping motor. The motor moves the barrier holder using a tooth belt. The holder itself is attached to a linear motion system, which is equipped with ball bearings. The barrier driving system is equipped with adjustable safety switches, which stop the barrier immediately when the barrier holder hits the switches. The position of the barrier is measured using an optical encoder. The position is relative in nature. For this reason, the user must zero the position reading when the barrier is in a known position. The control electronics are located inside of the barrier driving unit. The trough and controller are all purchased from KSV

(Finland).

2.5.3.3 Wilhelmy Plate

We use the Wilhelmy plate method for the measurement of surface pressure. The plate is rectangular in shape and its dimension and material of the plate depends on one’s need and choice. In our lab we use a Wilhelmy plate made of Platinum whose dimensions are about 20mm × 10mm. As shown in Fig.2.14, the plate remains hung from the ﬁlm balance (tensiometer). During the surface pressure measurement experiments about half of the the height of the plate should remain dipped the inside the subphse. It is very essential that there should be a very good contact between the 38

Figure 2.15: An illustration of Wilhelmy plate. A: platinum stem , B: the sand blasted Platinum plate. The ﬁgure is not to scale. surface of the plate and the suphase that contact angle is about 0◦. In order to achieve this condition of good contact the plate is well sand-blasted. Fig.2.15 gives an idea of the shape the Wilhelmy plate used in our lab.

In this method of surface pressure measurements, the force acting on the plate when it is hung from the ﬁlm balance and immersed in the suphase is ﬁrst determined. In our case, this force is directly recorded by a sensitive electrobalance. In principle, the net downward force experienced by the plate can be written as (refer to Fig.2.16),

F = ρp g Lw t − ρs gh w t + 2γ(t + w) cos θ

= ρp g L w t − ρs g h w t + 2γ(t + w) cos θ

= Fd − 2Π(t + w) cos θ (2.20)

In Eq.2.20, ρp is the density of the material of the plate, g is the acceleration due to gravity, and ρs 39

Figure 2.16: A schematics diagram of the Wilhelmy plate immersed into the subphase. The angle of contact made by the subphase with the plate surface is θ, h is the height of the plate inside the subphase, w is the width of the plate, L is the height of the plate and t is the thickness of the plate.

is the density of the substrate. Similarly, Fd represents the downward force on the plate if it were

hung in air. So, the force in the second line of Eq.2.20 can be recognize as due to – the ﬁrst term

the gravity, the second term the buoyancy, and the third term the surface tension. Hence, the surface

pressure is given by,

∆F Π = (2.21) 2(t + w) cos θ

Hence, ∆F = Fd − F can be considered as the difference in Force on the plate without the ﬁlm on

the supbhase and the force on the plate with the ﬁlm on the subphase. In an ideal case the plate is

completely wet by the subphase and we can approximate θ to be equal to 0 and hence cos θ = 1.

Then Eq.2.21 is modiﬁed as,

∆F Π = (2.22) 2(t + w) If w t then from Eq.2.19 and Eq.2.22, we can write, 40

Figure 2.17: An isotherm of Phospholipid Diyne PE at 32 ◦ C. σ is the mean molecular area of the ﬁlm and π is the surface pressure. The shoulder in the graph is the place where spiral/target structures are seen.

∆F Π = −∆γ = (2.23) 2w

It is obvious from Eq.2.23 that the thinner the plate the greater the sensitivity of the ﬁlm balance. In other words, by using a very thin plate the sensitivity of the balance can appreciably be increased.

A typical isotherm of Lipid Diyne PE at 32 ◦ C is shown in Fig.2.17.

In the real time experiments in our lab we use a sensitive electrobalance for the measurements of downward force on the plate and hence the surface pressure measurements. This electrobalance is connected with a computer and the KSV software is employed for the data acquisition and real time data plotting. As mentioned in the previous sections, the Wilhelmy plate is hung from the 41

Figure 2.18: Calibration of electrobalance. The solid circles are the balance reading, a solid straight line passing through these points is the linear fit of these balance readings, circles are the residues of the fit. Note that in this regime they are nearly randomly distributed, so that the linear fit can be used. hook of the electrobalance. There is a delicate cantilever connected with a flexible spring inside the box of the balance. The hook is attached to the cantilever in such a way that any change of force applied on the hook generates a corresponding torque on the cantilever. Thus, if the sitting position of the hook is changed, the torque-arm may also change resulting in a difference in reading by the balance. Keeping this in view, every time the hook of the balance is changed, calibration of this type of balance is a must for an accurate measurement of surface pressure.

2.5.3.4 Calibration of electrobalance

Small masses are used in order to calibrate the electrobalance (also known as the tensiometer).

First of all the masses of several small pieces of wires were taken with a sensitive electronic balance

(Toledo) with a precision of 10−5g. These masses were then hung from the hook of the electrobalance in turn turn with several different combinations of these masses to obtain as wide a range of data points as possible. The balance reading (signal) corresponding to each mass (or each combination of masses) is recorded with the KSV software. Then a graph of Balance reading versus the weight is plotted as shown in Fig.2.18. 42

The slope of the ﬁt of the data in this example is Θ = −0.2033 ± 0.0006. Now, if we deﬁne our

calibration coefﬁcient Φ as

Π = ΦΠ0 (2.24) where Π is the actual surface pressure and Π0 is the balance reading for a particular mass. With

∆F = Fd − mg, Eq.2.22 becomes,

mg = Fd − 2Π(t + w)Φ (2.25) we can re-write this equation as

g F Π = − m + d (2.26) 2(t + w)B 2(t + w)Φ

g From Eq.2.26, the slope of the ﬁt (Θ) = − 2(t+w)Φ . Hence,

g Φ = − (2.27) 2(t + w)Θ

The thickness t of the Wilhelmy is practically negligible compared with its width w. By measurements I found w ≈ 19.60mm. Then we can estimate the calibration coefﬁcient to be

Φ ≈ 1.229 ± 0.007. Therefore, now we can write the calibrated surface pressure Π in terms of the balance reading as

Π = (1.229 ± 0.007)Π0 (2.28)

2.5.3.5 Temperature sensor

The KSV Minitrough of this type can be coupled with temperature measuring unit. It uses a thermistor-type sensor. The analog/digital (A/D) converter is calibrated to give direct readings in Celsius degrees. The temperature sensor is covered with a thin special plastic tube and it can be attached to the trough with a special holder. The round port of the Interface Unit connects 43

to a thermistor cable which terminates in a temperature probe. When the probe is connected to the Interface Unit through its cable the Interface Unit as well as KSV controller program on the computer automatically starts reading the temperature of the system on the trough.

2.5.3.6 Water bath

In order to change the temperature of the subphase on the trough and of the Langmuir ﬁlm on top of the subphase we use a Water Bath-Julabo F12. We have options of using different kinds of

ﬂuids with this water bath for getting different range of temperature. In my experiment I often need to work with the substrate about a range of temperature 18 ◦ C − 50 ◦ C and the pure water obtained from the Purelab+, (> 18.2MΩ − cm) works well to achieve this temperature range.

Before using this Water Bath, it must be checked carefully to see that the connection to the correct port of the computer has been well established. Similarly, the level of the fluid used for heating or cooling must also be carefully checked. If the level of the fluid is either below or above the predefined level, it must be adjusted by adding or removing the liquid from to or from the Water

Bath. After this, circulating tubes of the Water Bath are connected to the the side tubes of the

Minitrough. The temperature of this type of water bath can be controlled manually as well as by the connected computer through the KSV control software. The desired value of temperature can be preset before heating or cooling of the system starts. Any change in temperature is monitored by the computer.

2.5.4 Experimental setup

In this type of experiments, cleanliness is a key factor for obtaining the best results. Any impurity of the sample material or contamination of the surface results in a big change of surface pressure, and may in generally affect the phase and other behavior of the layer. If this happens, we cannot accurately characterize the surface properties of a sample material. Keeping these consequences in view, we maintain each and every component of our experiment clean as far as possible before and after each of our experiment. A detailed procedures of cleaning can be found in Sec.2.6.1. As soon 44

Figure 2.19: KSV system for the surface pressure measurement. 1: Teﬂon trough, 2: barriers, 3: motorized barrier holder with trough platform, 4: side tubes for temperature control, 5: sand-blasted platinum Wilhelmy plate, 6: KSV electrobalance. as the trough and barriers along with the beam-dump are cleaned, they are installed in their respective places - trough on its platform and the barrier on the holders. Then the trough is ﬁlled with the clean substrate. Similarly, cleaned Wilhelmy plate is hung from the hook of the electrobalance and its height is adjusted with respect to the substrate level. Adequate time is given to the substrate on the trough to settle down and in the mean time signal reading on the computer is carefully watched to see if the instrument is giving smooth reading with time. Then the deposition of the material is accomplished with the help of a microcyringe. When the trough, barriers and the Wilhelmy plates are placed in their respective positions, the system looks like in Fig.2.19

It is important to note here that most of the time the surface pressure measurements are carried out in conjunction with the Brewster Angle Microscope (BAM) as explained in the previous sections. Fig.2.20 shows the schematics of the system of surface pressure measurement coupled with the BAM.

2.5.5 Calibration of Trough

As mentioned in the previous sections, the surface pressure is measured as a function of mean molecular area and isotherms are plotted for these parameters for a given temperature. For this reason it is very important to know the area of the trough between the barrier(i.e., the effective area) 45

Figure 2.20: A schematic diagram of of experimental setup. L1: collimating lens, L2: collecting lens, I: iris, PL: polarizer, E: electric field vector associated with the polarized beam, Er: electric field vector after reflection of the beam from the surface, A: analyzer, IRC:infrared-cut, CCD:charge coupled device(camera), B: barriers, T: trough, TM: tensiometer (film balance), P: Wilhelmy plate, W: Langmuir film on water substrate, W1: computer controlled water bath, T1,T2: circulating tubes. 46

Figure 2.21: An illustration of the profile of the trough with liquid substrate. The profile near the trough edge can be considered approximated as a quarter of a circle with radius R [6]. as correctly as possible for the precise determination of the mean molecular area available for the molecules. It is the region between the barriers where the Langmuir film of the sample material is preprepared.

In order to achieve the goal of correct effective area corresponding to the barrier positions, first the barriers are moved all way down to the outer edges of the trough and then the barrier position as read by the computer as well as the interface is zeroed. Then the barriers are move toward each other in steps and the area between the two barriers are calculated. This process is repeated for several barrier positions ranging from the outermost positions to the innermost position as much as the available space between the two barriers gives freedom for their movement. It is difficult to achieve an error-free measurement for the distance between the barriers. Additionally, when the trough is filled with a liquid substrate, the surface of the liquid substrate is not a flat rectangle. The profile of the liquid substrate has a meniscus and looks as shown in Fig.2.21. One of the researchers in our group approximated the profile near the edge of the trough to be a quarter of a circle of radius

R (Fig.2.21) and for the water substrate R ∼ 5mm [6]. This adds an error of about 8% in calculating the surface area of the ﬁlm and hence in the actual molecular area. Therefore, this error as well as the error associated with the measurement of distance between the barriers, must be taken into consideration while calibrating the trough.

A graph of “Effective area” is plotted against the barrier positions as shown in Fig.2.22. By knowing the ﬁt parameter A and B, we can use easily calculate the Effective are for a given value of barrier positions and hence the mean molecular area for a Langmuir ﬁlm. This calibration must 47

Figure 2.22: A calibration graph for the Minitrough. The solid squares are the calculated areas between the barriers with the measured distances between the barriers, the error bars are due to the errors associated with the trough profile with liquid substrate and also due the measurements in the barrier distaces, and the solid line is the fit. A and B are the fit parameters, Y = A + BX is the fit equation. be done whenever we notice a shift of barrier position when they are in the outermost edge of the trough.

2.6 Time correlation

In our experiments we use two different type of software in order to automate the data acquisition process. The surface pressure detected by the probe (Wilhelmy plate) is recorded by one computer (see Fig.2.20) with the help of KSV software and the real time graph is displayed on the screen as well the acquired data are stored in the hard-drive for the future use. There is another computer which is connected to the CCD camera. The CCD camera captures frames at the rate of 30 frames/sec and sends this information to the computer through an interface. Thereby, this computer uses a special software (Video Studio 6.0. or Studio 9) to convert the grabbed frame into movie ﬁles. These two operations - starting the surface pressure measurements and the frame grabbing does not take place at the same time and there is no simple way to synchronize them. 48

However, in most of the cases my experimental results demands that we establish somehow a correlation between the image data and the surface pressure measurements data. After several trial and errors, I have found a way to correlate the information between these two data type recorded by two different computers at appreciably two different times. The method is pretty straight forward - when the compression or decompression starts and in progression one needs the ﬂash light toward the CCD camera several times and keep records of the barrier positions for each light ﬂash. Here one has to be careful enough to choose the source of light. White light containing UV component is not suitable for some special type of materials. In my case, it must not be used for with the Diyne

PE, a highly polymerizable phospholipd. So, I used yellow , nearly UV-free light, obtained from a bug-light bulb. After each experiment the movies taken with light ﬂashes are played with suitable software and the time of the frame where the light ﬂash is recorded against the barrier positions.

Then, with the help of the barrier positions, time record for the barrier position and the surface pressure are found from the data as recorded by another computer with KSV software. Then these two times are plotted as shown in Fig.2.23. The ﬁt parameters A and B are used to ﬁnd the correlation between the frame time and the compression/decompression time. Then this correlation is used to extract information as to what move frame was grabbed at what surface pressure, mean molecular area and so on.

The discussion I have made in so far are the main experimental techniques that I have used for my work. However, I had to make slight modiﬁcation for my experiments depending on the type of sample material I was working with. Any modiﬁcation or change of setups will be discussed in detail in the subsequent chapters in the respective materials and method section.

2.6.1 Cleaning

Before concluding the experimental section of my dissertion, I would like to devote some time in discussing cleaning in our lab. Cleanness is a key factor for working in our lab. Even a small percentage of contamination can make a big difference in the surface pressure measurements and the quality of BAM images taken during the experiments, as well as possibly affecting the phase 49

Figure 2.23: A typical graph to establish a correlation between the movie frame time and the compression time. A and B are the ﬁt parameters, Y = A + BX is the ﬁt equation. The vertical errors bars correspond to the errors in estimation of surface pressure time with respect to the recorded barrier positions.

behavior of the materials. So, we must be very careful and prompt in maintaining all the necessary

components clean. For example, round bottom (RB) ﬂask (1000 ml) is frequently before, during and

after the experiments for ﬁlling water in the trough. Though it may sound trivial, but it is important

to keep that ﬂask always clean. So here, I will start with how to clean the RB-ﬂask.

(a) Pure water: Pure water used for cleaning the instrument such as trough, barriers, Wilhemly

plate, etc., is obtained from Purelab/UV system (with > 18.2MΩ-cm). The water to be used

in the experiments must pass the bubble test. This is a very simple test to see if there is

some contamination in the water. For this, we simply half ﬁll the ﬂask with pure water and

shake vigorously by closing the ﬂask with its clean lead. By doing so, one can clearly see

many bubbles coming to the free surface of water and disappearing. If they do not disappear

immediately when they hit the surface, the water contains surfactant.

(b) Flask Cleaning: If the ﬂask is not in use for a long time, ﬁrst of all pour about 50ml of potassium

hydroxide (KOH) solution into the ﬂask and spread it all around the inner surface of the ﬂask 50

by turning and twisting the flask to different angles. Then the flask is flushed out several

times with pure water, obtained from the Purelab+ (with > 18.2MΩ-cm). The KOH solution

is prepared by mixing 135g of pure ethanol, 25g of KOH, and 24g of water.The ﬁnal step of

this cleaning is to do the bubble test as explained in (a). However, if there are some bubbles

appearing on the the free surface of water or if the bubbles do not disappear instantly when

they reach the free surface, that is indicating that the contaminants have not been completely

gotten rid of, and the ﬂask further needs to be cleaned by repeating the process described

above.

such as such as syringes, small ﬂasks (5ml), and the home-made beam dump, an ultrasound

cleanser (Fisher Scientiﬁc, FS20H) is also used. The items are immersed in pure water and

run the cleanser for about 20 minutes.

(d) Cleaning the trough and barriers:

* The surface and hole of the trough is ﬁlled with KOH solution.

* The edges of the trough are cleaned by gently using your ﬁnger covered with nitrile

gloves.

* The KOH solution is returned back in the bottle. Then the trough is rinsed several

times with pure water before it is placed back to its experimental platform. In this

special step of cleaning, preference should be given to the edges, because, these are the

places where dust and dirt can easily hide.

* KOH solution is an enemy of metals. If we spill it in the metallic sink (which I have

in our lab) the solution rusts it. So if we spill the KOH solution by accident, we must

ﬂush it as soon as possible by using running water from the tap.

* Everything in the trough is cleaned together ﬁnally. For example, if we are using the

large trough (the one with a deep hole in the middle), we will also use a black delrin 51

plate and a beam-dump (Fig.2.13) to be placed in the hole of the trough. First each item

is cleaned separately and then they are put together and cleaned together.

* The trough is placed back to the platform of the experimental setup.

* Now the trough is ﬁlled with pure water until we can see a nice meniscus of water at

the trough edges.

* Leveling of the trough is double checked by observing the meniscus height on all sides

of the trough. If the meniscus height is not the same in all sides then the trough needs

to be proper leveling, which can be achieved by turning around the movable screws

attached to the trough platform.

* We move the barrier-holders to the convenient positions in such a way that they can

be seen easily from the front and top of the trough. The barrier position and motion is

controlled by the LB software.

(e) Barrier cleaning:

* For cleaning the barriers, ﬁrst one of them is taken and dipped into a sallow KOH

solution. While doing this, only the delrin part of the barrier must be in contact with

the KOH solution, not the metallic support. For this, only a reasonably small amount

of KOH solution is taken in the bottle with wide mouth, and then the bottle is tilted

almost horizontally before the barrier is introduced into it (Fig.2.24). This ensures that

that delrin portion of the barrier is soaked thoroughly with the KOH solution. Then the

barrier is taken out of the bottle and rinsed throughly with the pure water. It is made sure

that no residue of the KOH solution is left over the barrier. After cleaning, the barrier is

taken to the experimental table and ﬁtted it into its holder. In order to make sure that the

barrier is seated well on the edge of the trough it is pressed gently.

* The same process is repeated for the another barrier too.

(f) Cleaning the Wilhelmy plate: 52

Figure 2.24: A schematic of a barrier in KOH solution for barrier cleaning.

* The plate should be cleaned with the KOH solution, but sometimes we may need to use chloroform and methanol too, if the plate is too dirty and/or it was used in the material which was too sticky and difﬁcult to get rid of it with KOH solution alone.

* The plate needs to be rinsed several times with pure water after it is cleaned with KOH or other chemical mentioned above. Rinsing is very tricky. Since the joint of the stem and the plate is very delicate, the flow of the water from the faucet must be maintain very small. Additionally, the flow of water should fall on the edges, but not on the flat surface of the plate. If the flow of water falls on the flat portion of the plate, a torque is generated and this causes the stem of the plate to break. During this whole process the plate is held with tweezers.

* As soon as the cleaning of the plate is ﬁnished we need to take it to the experimental table and place it in the hook of the tensiometer very carefully. We should avoid any jerk while putting it to the hook, or else the sensitive balance system may get damaged.

As mentioned above, the plate is very fragile and expensive as well. So we may need to take special precautions to carry it from one place to another place. One of them is to place one of our hands underneath the plate while carrying it. This ensures that even if the plate drops from the tweezer, it is prevented from falling directly to the ﬂoor. 53

Figure 2.25: A schematic of vacuum system for emptying and cleaning the trough.

* Once the plate sits properly sits on the hook, we must adjust the depth of the plate into

the water in the trough. It should neither be too shallow nor be completely immersed

into the water. The depth should be adjusted in such a way that the signal should read

approximately −55mN/m.

(g) Flush several times:

* Before we try to get the background image on the television screen/computer monitor,

we need to do several cleaning here too. For this, we vacuum the water out of the trough

by using water vacuum. This vacuum system, as shown in Fig.2.25, is achieved by using

running water, Nalgene Aspirator, a conical ﬂask with a side tube, and a clean pipette.

We try to make the trough empty at least three times, but the more the better.

* While ﬂushing out, the tip of the pipette is kept on the surface of the water as best

as possible so that it would suck the surface only, not the inner mass of the water. This

guarantees the removal of any dust, dirt or any contamination residing on the water

surface.

* After thorough cleaning of the trough and the barriers by using the techniques men-

tioned above, the trough is again ﬁlled with the pure water. This time the barriers are 54

made to move inward with a moderate speed and at the same time taking water off the

surface from several different location of the surface. This process is called the clean

run and this makes the system cleaner. It also provides a test for cleanliness: the surface

tension should be independent of the position of the barriers, as long as the meniscus

from the barriers does not interfere with the meniscus from the wilhelmy plate.

Finally, before the sample is deposited on the substrate, a clean run compression/decompression with substrate only on the trough is carries out to be certain whether or not there is any systematic change in signal during the compression/decompression cycles. Systematic change in signal is also checked over a period of time by letting the substrate as it is at room temperatures for about an hour. I did all of the surface pressure measurements after making certain that there was not any such changes in signals as discussed here.

As discussed in Sec.2.1, there are several experimental techniques available to characterize the molecularly thin ﬁlms. However, the BAM coupled with the surface isotherms discussed in this chapter is the main experimental techniques used in my research work. Use of this technique in the studies of Langmuir ﬁlms formed by a highly polymerizable and highly chiral diacetylinic phospholipid will be discussed in detail in chapter 3. CHAPTER 3

PATTERN FORMATIONS IN LANGMUIR FILMS MADEOF CHIRAL LIPID MOLECULES

I observed fascinating patterns when the Langmuir films made of a strongly chiral, polymerized lipid are compressed. I will present a discussion of observed qualitative features, which we will describe as quantitatively as possible in this chapter. In the next chapter, we will consider possible models for the behavior described here. But, I will first present a short introduction of lipids, followed by a brief review of previous work on different types of pattern formation in molecularly thin films.

3.1 Lipids: A General Introduction and Motivation

Lipids are substances of biological origin that are soluble in organic solvents such as chloroform and methanol, but are only sparingly soluble, if at all, in water [2]. Fats, oils, certain vitamins

(A, D, E and K), hormones, and most nonprotein membrane components are lipids [98]. Lipids are primarily made of two parts: hydrocarbons (C-H chains) and the head-groups (Fig3.2). The world is abundant in lipid varieties including the phospholipids. From the chemical point of view, phospholipids are very similar to triglycerides1, but with one important difference. A phosphate functional group is substituted for one of the three fatty acids as shown in Fig.3.1 or Fig.3.2.

The most important feature of a phospholipid structure is that the hydrocarbon chain“tails” are non-polar while the phosphate “head” is very polar. This leads to a chemically confused molecule when exposed to an aqueous environment. Energy is reduced in aqueous solutions by forming unique assemblies including micelles and bilayers, which limit exposure of the non-polar part of molecules to the water, as shown in Fig.3.3.

1A triglyceride, also known as the neutral fat, is a non-polar, water-insoluble substance. The molecular structure of a typical triglyceride is shown in the middle portion of Fig.3.1. Triglycerides are present in our blood plasma which you may be most aware of through blood tests: too much is considered bad.

55 56

Figure 3.1: An illustration of a phospholipid structure [7]. The arrow shows the position of the phosphate group substitution for one of the fatty acids.

Figure 3.2: Another way of looking into the structure of a phospholipid molecule [7]. 57

Figure 3.3: An illustration of different structures of self-assembled lipid molecules in aqueous environment [7].

Phospholipids are one of the most interesting groups of organic compounds because of their indispensable role in the human biology. They carry out an extremely important function in our bodies. With other lipids, they form the cell: we can think of each cell as being surrounded by a

ﬂuid fence, generally known as the cell membrane (Fig.3.4) or the plasma membrane. We can see in Fig.3.4 how a spontaneously formed lipid bilayer can form the basis of these kinds of plasma membrane, also the basis of our lives.

These bilayers are basically two weakly coupled monolayers [3] existing together. Phopholipid molecules can form interesting monolayers since they possess more than one chain (Fig.3.1) per head group and the natural curvature is different than that with one chain [99]; the study of Langmuir

ﬁlms of this kind of lipid may provide valuable information regarding their phase behaviors and structural organization. Phospholipid and other lipid monolayers have been used as a model system

[100] to understand the mechanism of the biomembranes like the ones discussed above. 58

Figure 3.4: A schematic diagram of a plasma membrane [7].

Figure 3.5: An electron micrograph of a multilamellar phospholipid vesicle in which each layer is a lipid bilayer [2]. 59

One of the very important and interesting structures constituted of lipid molecules is the lipo- some or unilamillar vesicle. A suspension of phospholipids in water form multilamilar vesicles that have an onionlike arrangement of lipid bilayers (3.5). Sonication (i.e., agitation by ultrasonic vibrations) is one of the methods to produce liposomes. Upon sonication, these structures rearrange to form unilamillar vesicles or liposomes-closed, self-sealing, solvent-ﬁlled vesicles of colloidal dimensions (20nm to 10µm, small enough that Brownian motion is important) that are bounded by

only a single bilayer [101]. Due to their structure, chemical composition and colloidal size, all of

which can be well controlled by preparation methods, liposomes exhibit several properties which

may be useful in various applications [102]. Their applications can be categorized into two thrusts:

in basic science, and in technology (including medical applications).

Liposomes or vesicles can be used as model systems in order to understand the topology, shape

ﬂuctuations, phase behavior, permeability, ﬁssion and fusion of biological membranes [103, 104].

The properties of liposomes make them very useful model system in many fundamental studies from biophysics, photophysics and photochemistry, colloid interactions, cell function, and many others

ﬁelds [103, 105, 106]. One of the rich applications of liposomes in basic science is in biochemical investigations of conformation and function of membrane protein. For example, the glucose transport proteins can be reconstituted in their active form into vesicles and then studied [107]. This type of research is important to broaden the understanding of protein and cells [108].

Some researchers believe that physio-chemical studies of cholesterol and other sterols-contained membranes may provide some clues on the evolution of life. According to these researchers [109], the presence of cholesterol, which could have been synthesised only in a certain geological time when the atmosphere became rich of oxygen, caused the membrane to be more cohesive with increased bilayer ﬂuidity. This ﬂuidity may triggered cytosis processes bringing about the develop- ment of multi-organelle cells from the ones which contained no cholesterol and also no internal organelles. These are a few examples of liposomes in basic science 60

Phospholipids are multi-talented entities. Due to a broad variety of different functions, phospholipids play a decisive role in health and nutrition. In recent years, by virtue of their vast potentiality in the drug delivery, studies of phospholipids have become an increasing trend [110–119].The special properties inherited in liposomes (viz.,colloidal size, bilayer phase behavior,mechanical properties, permeability, charge density, etc.) make them potential candidates in varieties of technical applications. They can be good solubalizers for difﬁcult-to-dissolve substances, dispersent, sustained release system, delivery system for the encapsulated substances, stabilizers, protective agents, and so forth[102, 120].

Another application of phospholipids involves understanding the mechanism of lung surfactants and the role of lipids [110, 111]. Human lung surfactant is a complex mixture of lipids and proteins that forms a monolayer at the alveolar liquid/air interface. This monolayer modulates the surface tension of the lung, lowering the normal air/water surface tension of approximately 70dyne/cm to near zero on expiration [121]. The variation in surface tension stabilizes alveoli against completely emptying of air and ﬁlling with ﬂuid during expiration and minimizes the work of expanding the alveolar surface during inhalation [122–125]. A lack of effective surfactant in premature infants results in neonatal respiratory distress syndrome (NRDS), a potentially fatal disorder characterized by reduced lung compliance(ease of breathing) and gas exchange [122–124].The monolayers of these kinds consists of over 90% of lipids [126] around 80% of which are phospholipids [127, 128].

It is in this context that my collaborator Dr. Sahraoui Chaieb2 has been studying a special kind of phospholipid [1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine], which may ﬁnd applications in lung surfactants and other drug delivery system [129]. Dr. Chaieb’s group has found that crumpling of vesicles formed by these lipid molecule takes place with a structure that depends on the degree of polymerization [12]. Vesicles are basically two weakly coupled monolayers, and

Langmuir ﬁlms of these molecules could form an interesting model system for the studies of these kind of vesicles. Study of Langmuir ﬁlms have both advantages and disadvantages. One of the

2Department of Mechanical Science and Engineering,University of Illinois at Urbana-Champaign, Urbana IL, 61801 61

Table 3.1: A qualitative comparison beteween 23:2 Diyne PE and 23:2 Diyne PC.

23:2 Diyne PE 23:2 Diyne PC # of C in chain = 23 # of C in chanin = 23 # of triple-bonds = 4 # of triple-bonds = 4 Triple-bond/chain = 2 Triple bond/chain = 2 Tiriple-bond position = 10,12 Triple-bond position = 10,12 Three H with N Three CH3 with N Mol.wt.= 872.204 Mol.wt. = 914.300 Formula:C51H86NO8P Formula:C54H92NO8P Chiral Chiral Polymerization:UV light (253nm) Polymerization:UV light (253nm) Sensitive to oxygen Sensitive to oxygen Soluble in chloroform Soluble in Chloroform Insoluble in water and acetone Insoluble in water and acetone disadvantages is that surface tension tends to keep the surface ﬂat, so curvature effects cannot be studied directly. However, good control over the area per molecule of the ﬁlm and the control over the surface pressure are advantages that compensates the limitation imposed by the disadvantages.

It is worth mentioning here that the system of molecules they were using and that we used are not exactly the same for reasons of avaibility, so that this work should be considered as inspired by that earlier work rather than as an extension of it. However, the lipids are similar in many respects. Table

3.1 summarizes the similarities and dissimilarities between the two. This comparison is based on the qualitative observation of the structures of these two types of molecules.

3.2 Molecule

Because of the immediate availability of the material at the time we began the project, the phospholipid used in this project is [1,2-bis(10,12-Tricosadiynoyl)-sn-Glycero-3-Phosphoethanolamine], prepared from the synthesis of diacetylenic acids [25, 130] obtained from Avanti Polar Lipids. The lipid is also known as 23:2 Diyne PE. The presence of triple bonds in the hydrocarbon chain of phospholipids of diacetylenic origin is a marked feature. People have done detailed studies of polymerization of Langmuir monolayers of these type of molecules [23, 131, 132]. Not going into the 62

(a) (b)

Figure 3.6: 23:2 Diyne PE lipid molecule. (a) Computer model of the 23:2 Diyne PE molecule. Color codes: red-Oxygen, blue-Nitrogen, green-Carbon, and white-Hydrogen (b) Molecular structure of 23:2 Diyne PE. [8]. details of the chemical nomenclatures (which does not fall into the scope of this work) the information that we gather from the second representation (Fig.3.6b) is that the lipid is a glycerol-containing phospholipid which has two unsaturated (triple) bonds at the 10th and 12th position of its carbon atoms counting from the carboxylic group carbon atom. The numbers in the ratio 23:2 used in this representation tell us that the total number of carbon atoms (counting from the carboxylic group carbon)in each of the hydrocarbon chain of this molecule are 23 and there are two unsaturated (triple) bonds in each hydrocarbon chain. The molecular weight of this molecule is 872.204 and its molecular formula is C51H86NO8P [25]. According to the information available in www.avantilipids.com, this lipid is insoluble in water and acetone, but readily soluble in chloroform. The Fig.3.6 shows the structures of the molecule.

One of the distinguishing features of this molecule is the presence of kinks at the position of the triple bond. These kinks may play a signiﬁcant role in its packing within the Langmuir

ﬁlms. Further, the presence of triple bonds in its hydrocarbon chain makes the molecule chemically highly unstable; it will spontaneously polymerize in solution [25]. Thus the neat powder form of this lipid is preferred to its solution form for the storage purpose. Additionally, this lipid is highly polymerizable in the oxygen environment and also with the exposure to UV light (∼ 252 nm). Chirality is another important feature of this molecule. For the comparison sake, here I have included the [23:2 Diyne PC] molecule (Fig.3.7), used by my collaborator Dr. Chaieb in his research work [12, 129]. 63

(a) (b)

Figure 3.7: 23:2 PC Diyne lipid. (a) Computer model of the 23:2 Diyne PC molecule. Color codes: red-Oxygen, blue-Nitrogen, green-Carbon, and white-Hydrogen, (b) Molecular structure of 23:2 PC Diyne. [8].

3.3 Pattern Formation in Thin Films

The term “pattern” can be defined as structures of regular repetitions with some kind of characteristic length (which can be loosely defined as a convenient length, usually a constant, to describe a given configuration).

The complex patterns that appear often in nature have been a matter of wonder and fascination for us. Beautiful patterns on the backs of turtles, leopards, and giraffes are spectacular example of cellular patterns. Similarly, stripe patterns on zebras and the patterns in a peacock’s feathers are examples of stripe and spiral patterns that nature has gifted to these animals. People have long been puzzled about how complex snowflakes can form, out of thin air. These are just few examples of the wonders of nature regarding the pattern formation and it is beyond the scope my work to discuss all of them in detail. Here my discussion will be limited mostly to the two-dimensional films, and in particular to stripe-like patterns in such films. However, our 2D world is no less a wonder in this regard.

Now, the question arises, what might be the possible origin of different kinds of patterns? There is not just one direct answer to this question. However, we can think of several different kinds of interaction going on in the micro-level of a system leading to pattern formation. Examples of these could be short range repulsive interactions, middle range van der Waal interactions, and long range dipole-dipole interaction. Depending on the system, either two or more, or all of these interactions can be competing with one another. This situation does not allow a unique ground state and forces the system to have some kind of best compromise or frustrated state [66]. The resulting 64

Figure 3.8: Spoke pattern formation on dewetting a dilute Langmuir ﬁlm of gold nanoparticles (gold dots) on hydrophilic SiO2/Si substrate as observed with an optical microscope. At the initial stage, the nanoparticles precipitate to form the spoke tips at the rim of the ﬁlm, which then propagate inwards as the water front retreats leading to a spoke pattern such as that shown in here. An optical microscopy image (scale bar=200 µm) [9]. compromise is often the formation of stripe patterns of the system with some characteristic length scale. The boundary conditions and elasticity together with these interaction also play role in pattern formation. These are typically equilibrium patterns and several examples will be given below.

3.3.1 Equilibrium patterns

Liquid crystals have been known for forming beautiful pattens and textures under suitable conditions. Many of these patterns can be characterized as stripes with a characteristic thickness. These patterns remain over a period of time. Freely suspended liquid-crystalline film of Smectic-I phase have been found to display distinctive stripe texture [64, 133]. Similarly, when a three-layer tilted hexatic film undergoes a phase transition from C? to I? the disclination lines develop a beautiful spiral structure at T=74.6 ◦C [134]. Many other systems can also show patterns. For example, polymer coated gold and silver nanoparticles have shown their capability to form beautiful spoke-like pattern (Fig.3.8) on dewetting their monolayer film on SiO2/Si by using a dilute solution of these

particles [9]. 65

Figure 3.9: A BAM image of the monolayer of DSPC (400µm ×250µm) at π ∼ 7mN/m.The buckled layer shows bright and dark stripes, roughly oriented perpendicular to the compression direction indicated by arrows [10].

3.3.1.1 Patterns due to elastic buckling

When stable Langmuir ﬁlms made of amphiphilic molecules at the air-water interface are compressed beyond a certain concentration, they become unstable to a collapse transition, usually accompanied by the formation of either multilayer islands on the surface or dissolution in subphase or both [54, 135–138]. Under certain conditions this collapse transition is preceded by a mechanical instability called the buckling that leads to formation of regular striped pattern being determined by the mechanical properties of the ﬁlms [10, 23, 139–144] and in particular the balance of the layer elasticity.

Fig.3.9, taken from the work of Saint-Jalmes and et.al. [10], is an example of buckling of

Langmuir monolayer under compression. The authors prepared monolayer of 1,2-distearoyl-sn-

glycero-3-phosphocholine, usually known as DSPC, in pure formamide. The authors claim that

by using pure formide as the substrate, they were able to observe buckling of the ﬁlm at a surface

pressure as low as 7mN/m.

In a monolayer, when a network formed in a gas-liquid-expanded (LE) two-phase region is

heated, a buckling instability occurs. The instability arises because of constraints on the monolayer

imposed by the network and by the interaction betweens domains of LE phase. The buckling is

shown in Fig.3.10 66

Figure 3.10: Buckling instability occurs when when ethyl heptadecanoate monolayer is heated to 19◦ C at constant mean molecular area. (a) The monolayer at 19.5◦ C (b) The monolayer at ∼ 19.5◦ C where the amplitude of the buckling increase. The bars correspond to 100µm [11].

Figure 3.11: An ESEM image of crumpling of vesicles. The width of the image as shown by the arrow is 4µm. The polymerization is 40% [12].

3.3.1.2 Patterns due to polymerization

Phospolipid monolayers and bilayers (vesicles) can polymerize when exposed to UV light [23,

23, 23, 88, 130, 145–147] including vesicles of diacetlynic orgin [146, 148]. My collaborator Dr.

Chaieb has seen vesicles crumpling [12] in polymerizing such lipids upon UV irradiation resulting in a formation of disordered patterns with a characteristic length scale as shown in Fig.3.11.

The formation of circular pattens has been reported by the process of molding and embossing of spin-coated polymer ﬁlm at the silicon substrate[149]. A combination of x-ray diffraction and 67

0 Figure 3.12: Freeze-fracture electron micrograph of the Pβ phase of dipalmitoylphosphatidyl- choline (DPPC) of the ripple phase of DPPC showing the top view of the corrugated surface [13] due to buckling of the bilayer. freeze-fracture electron microscopy demonstrates that cooling lecithin bilayers from the liquid crystalline (Lα) state to a temperature below main transition temperature, form a rippled bilayer phase

0 denoted by Pβ resulting in the formation of nice stripe patterns [13, 150] as shown in Fig.3.12.

3.3.1.3 Patterns due to tilt variations

The variation of ~n is related to the tilt of the molecules with respect to the surface normal. The strong head-tail asymmetry of the molecules forces them to tilt in order to make the effective area occupied by the hydrocarbon tails to be equal to the effective area of the head-group. A BAM image gives a qualitative estimation of the tilt of the molecules in a Langmuir ﬁlm by observing the change of gray value in the image at different analyzer positions.

The modulation of the tilt azimuth of the molecules [14, 151, 152] is another prominent origin of pattern formation (Fig.3.13). Pattern formation has also been observed [153–155] in Langmuir

ﬁlms due to phase change mechanism.

Riviere` and et.al. [15] have shown that the anisotropy arising from the tilt of the molecules from the surface normal, can play a role in forming stripe patterns in Langmuir ﬁlm of achiral molecules.

First they prepared a Langmuir ﬁlm of sodium myristate by the adsorption of its molecule in pure water substrate and then continuously monitored with BAM. The beauty of a BAM is that it allows 68

Figure 3.13: Polarized fluorescence microscope image of stripes from a monolayer of DPA at 7.6 ◦ C as a consequence of tilt-azimuth modulation [14]. direct visualization of phase coexistences in monolayers (two different phases have different reflectivities). For a molecule with tilt angle t (which is supposed to be constant all over the film [156])

and a tilt-azimuthal direction ϕ from the plane of incidence as shown in Fig.3.14, the polarization

of the reﬂected light depends on ϕ. This implies that when another polarizer (called ‘analyzer’) is

placed in the path of the reflected light with fixed direction, the films appears more or less bright

depending on the tilt-azimuth direction. They report that across each stripe (Fig.3.15a) the tilt-

azimuthal direction turns continuously by 95◦ ± 10◦. At the defect line, which separates a stripe from its next neighbor, the tilt-azimuthal direction reverts by about 95◦ back and forth giving rise to

repeat of stripes with alternating contrast.

3.3.1.4 Patterns due to chirality

Stripe patterns can also form due to a chiral symmetry breaking mechanism [157] of the achiral

molecules that form Langmuir ﬁlms. There could be several possible mechanisms for the chiral

symmetry breaking. If the monolayer is in tilted hexatic phase [63], the tilt direction could be 69

Figure 3.14: Deﬁnition of the tilt angle and the tilt-azimuthal angle. Here (x, z) is the plane of incidence, t is the tilt angle and ϕ is the tilt-azimuthal angle [15].

(a) (b)

Figure 3.15: An illustration of stripe pattern due to tilt-azimuth variation on the Langmuir film surface. (a) BAM image with analyzer. The bar represents 50µm. Change of contrast across the stripe due to a change in azimuth-angle due the tilt of the molecule from the surface normal. In plane optical anisotropy arises with one axis parallel to the direction of tilt. If this axis is not parallel to the polarization of the incoming light, the ploarization of the reflected light will rotate towards that axis. (b) An illustration of molecular tilt with different tilt angle along the film surface [15]. 70

locked at an angle between 0◦ and 30◦ from one of the bond directions. This relation between tilt- order and bond-orientational order breaks chiral symmetry3. Even if the monolayer is not in a tilted hexatic phase, the molecules might pack on the 2D surface in two inequivalent ways that are mirror images of each other and the chiral parameter would be the difference in the fractional coverage of the two packings4. And the other possibility is, if the monolayer is composed of a recemic mixtures of two opposite enantiomers, the recemic mixtures can separate to form chiral domains and in this case the chiral parameter would be the difference in densities of the two enantiomers [157].

Stripes may be arranged in different ways, including spirals. We can ﬁnd in the literature [16–

18, 159] that chirality plays an important role in spiral-like domain formation in Langmuir ﬁlms. R.

Heckl and et.al. [16] have observed that Langmuir films made of DMPA with 1 mole% cholesterol can form spiral like domains. According to them, a change in the ionic strength also changes the effective area of the choline head group. This, along with the chirality associated with the molecules, is responsible for a given domain shape. Using fluorscence microscope with monolayer at the air/water interface they found spiral domains at high ionic strength on film compression (Fig.3.16).

They claim that head groups with high ionic strength can have an electrostatically induced chain tilt that, in conjunction with an in-plane dipole moment, causes a ferroelectric state. Ultimately, this allows for domain aggregation and orientation originating in elongated domains. These elongated domains are bent because of the chirality of the molecules. The presence of cholesterol plays a role in modulating the line tension in the domain boundaries through its anisotropic edge activity [17]

. This stabilizes and further elongates the domain giving rise to their spiral shape (Fig.3.17a). The work due to Weis and et.al. [18] discusses the role of chiral molecules with a mixture of DPPC’s to form fanlike chiral solid domains (Fig.3.17b) in the Langmuir monolayer in which long range orientational order has also vital role to play. A theoretical work on the stability and the domain shape formation of these kinds of patterns can be found in the work of H. M. McConnell [160].

3The chiral parameter is deﬁned as ψ(r) = sin{6[φ(r) − θ(r)]}, where φ is the tilt azimuth and θ is the bond orientation [158]. 4 ψ(r) = ρR(r) − ρL(r) is the difference in densities of right- and left-handed enantiomers [24] 71

Figure 3.16: Fluorescence micrograph of a DMPA monolayer containing 1 mol% of cholesterol, pH 11.4, T = 10 ◦ C, and ionic strength 10−3M [16]. The scale bar = 20µm.

(a) (b)

Figure 3.17: (a) A fluorescence microscope image of spiral patterns in the solid-fluid coexistence region of L-DPPC monolayer containing cholesterol [17].(b) Epifluorescence microscope photograph showing chiral solid domains in monolayer comprised of several DPPC’s. The scale bar equals 50µm [18]. 72

3.3.2 Non-equilibrium patterns

Non-equilibrium effects include the kinetic trapping of patterns formed by dynamic processes during crystallization and other changes in the monolayer, but also patterns which continuously change in time due to a maintained gradient in material, temperature, or in principle ﬁeld.

3.3.2.1 Organized patterns

Langmuir films are the vivid example of the model system that people have long used for making and modeling of patterns of non-equilibrium and equilibrium origins. For example, if there is a small gradient of molecules across a Langmuir monolayer, i.e., their concentrations in the liquid subphase and the gas superphase are different, sometimes this produces a flow of such molecules through the molecular layer. Such trans-membrane (through the monolayer) flow gives rise to a coherent collective precession of molecular rotor and gives rise to the formation of spiral and target like patterns depending on whether the precession is uniform or not [19, 65]. The precessing molecules

(Fig.3.18a) are regarded as synthetic molecular motors of biological importance. If the boundary conditions force the tilt orientation to be conﬁned along a domain wall, the collective precession induces the formation of stripe in this orientation, which aggregate against the wall. Then the chirality associated with theses molecules bends the stripes to give target-like structures. These new type of pattern evolve with time (Fig.3.18b). Similar results have been reported by Gupta and et.al. [20]. In their work they also used chiral mesogenic molecules to prepare Langmuir monolayers at the air/water interface and found four phases: a gas phase, a low density liquid-like phase with varying tilt azimuth of the molecules, a relatively high density liquid phase with tilted molecules and a condensed phase with untilted molecules. They also revealed with BAM images that patterns of linear stripes, concentric circular stripe, and spirals that rotate with time (Fig.3.19).

They found a decrease in the rotation rate with decreasing rate of evaporation. This is an example of concentration-driven pattern formation process. 73

(a) (b)

Figure 3.18: An illustration of pattern formation of dynamic origin in the Langmuir monolayer. In (a) The chemical structure of R-OPOB and an exaggerated diagram of its Langmuir monolayer at the air/glycerol interface. The rod-like molecules are tilted from their surface normal at a constant tilt angle. φ is the molecular azimuth. The pair of wings attached to the molecules represents the chiral groups. In (b) Pattern formation in R-OPOB monolayer at the air-glycerol interface. The scale bar corresponds to 100 µm. The pattern formation is due to the collective precession of molecular around the surface normal, caused by the transmembrane transfer of water molecules. The images were taken under the reﬂected-type polarizing microscope and the contrast is coming from the tilt- azimuth change. The stripe width decreases going away from the center of the patterns. Taken from [19].

Figure 3.19: BAM images of 6-(cholest-5-ene-3-lyoxy)-6-oxohexanoic acid (cholestoric acid) 2 monolayers at the air-water interface at mean molecular area=54 A˚ /molecule, four minutes after compression to surface pressure=2.4 mN/m. Time between images=2 sec; arrows show rotation sense. The contrast is due to change in tilt-azimuth of the molecules. Scale bar=335µm [20]. 74

3.3.2.2 Fractal-like dynamic patterns

Several authors [161, 162] report a purely light-driven spatiotemporal pattern formation mechanism. They report that purely light-driven pattern spatiotemporal pattern formation takes place in liquid-crystalline Langmuir monolayer consisting of an amphiphilic ozobenzene derivative undergoing the cis↔trans photoisomerization. The Langmuir monolayer is in the Smectic-C-like liquid- crystalline phase, whose two-dimensional orientation is easily perturbed by by slight conformation change in the constituent molecules. On illumination with linearly polarized light, a collective and global in-plane reorientation of the azobenzene is induced over an existing static stripe textures, which finally leads to a polarization dependent steady-state orientational pattern, and prolonged photoexcitaion generates sustained traveling and solitary waves associated with variation in molecular tilt directions. Similar result were published by Mu Wang and et.al., [22]. In their work they formed a lipid monolayer film with fluorescence probe in it and the domains formed on the film were continuously illuminated by the microscope light visualizing the fluorescence probe. Authors say that illumination triggered the growth of the domains in the coexistance of Liquid ordered and the Liquid disordered phases. The continuous growth of the domains led to the fractal-like patterns to develop into dendrites. They report that the growth depends on the surface tension gradient and decomposition of the fluorescence molecules upon continuous illumination, which ultimately changes the chemical potential in the illuminated region.

3.3.2.3 Temperature-dependent Labyrinth patterns

Sometimes, non-equilibrium dynamic systems can also trigger pattern formation process. For example, thin polymer ﬁlm deposited on a glass substrate, when heated to ∼ 300 ◦ C, separates

into two phases of different densities and chain rigidities. More rigid molecules tend to form a

separated anisotropic phase and more ﬂexible molecules tend to form an isotropic phase. This

thermodynamically unstable two phase system gives rise to a process of formation of patterns and

[21]. The time evolution of these patterns is shown in Fig.3.20.

This concludes a short review of different types of stripes and possible mechanisms behind their 75

Figure 3.20: Polarized Light Microscope(PLM) images of thin polymer ﬁlm showing the growth of a percolated network formed by phase separation (the difference in contrast) into isotropic and anisotropic liquids at 220 C for 100, 300, and 900 s [21]. The scale bar = 50µm. formation. We will compare our current results to see if any of the suggested mechanisms can explain the reason behind the pattern formation that we saw in our case.

3.3.3 Materials and Methods

In this section I will discuss important aspects of the methods and material used in the present work.

3.3.3.1 Substrate

As discussed in Chap.2, my main experimental techniques involve the application of a BAM coupled with surface pressure measurements. Cleanliness plays a vital role in our lab. So, cleaning is carried out carefully and thoroughly as explained in Sec.2.6.1 in the very beginning of an experiment. Then, the different components of the instruments are put together, including the trough and the barriers, as shown in Fig.2.20. For all of my experiments I used pure water obtained from the

Purelab+ (>18MΩ-cm) system, and in my subsequent discussion the words pure water will mean this unless otherwise stated.

For the substrate, we ﬁrst tried with pure water at room temperature. That worked for experiments at and a little above room temperature (30 ◦ C). However, our collaborator Dr. Chaieb suggested that, based on this work on vesicles, temperature near and above the chain melting temperature (38 ◦ C) of the sample material should be our target. Preparation of Langmuir ﬁlm at or above 76

this temperature was difﬁcult because of bubble formation as well as the increased rate of evaporation. At these higher temperatures, air bubbles would form on the bottom part of the trough and would grow, come to the surface and then burst. This bursting would create an annoying mechanical disturbance for the formation of Langmuir ﬁlms and would also affect the accurate measurements of surface pressure, which is one of the crucial parameters in our experiments. Additionally, with a pure water substrate, the high rate of evaporation would decrease the level of sub-phase during the experiment and again the values of the surface pressure measurements would be far from accurate.

The high rate of evaporation would also bring about unwanted convection, mechanically disturbing the system mechanically disturbed. An increase in substrate viscosity to closer to that of honey would help to overcome this difﬁculty. Purity and the neutrality toward the chemical reaction with the deposited material must be considered in this regard.

Glycerol is commonly [163, 164] used to satisfy these concerns. The advantages of using glycerol are: it is a chemically neutral organic compound [165] that forms a nearly ideal solution with water and does not chemically react with the material deposited on it; it reduces the evaporation of water by changing its chemical potential without itself evaporating; the viscosity of glycerol is much higher than the viscosity of pure water - this would help reduce formation of air bubbles. The third reason relates to the purity. Compared to honey, which might have similar effect, glycerol could be obtained in the much more purified form (∼ 99% pure) from the suppliers. Now, to make the long story about the substrate preparation short, a mixture of reagent grade of glycerol purchased from the Sigma-Aldrich and the pure water was prepared in a round-bottom flask. The ratio was 20% glycerol and 80% water by weight. This mixture reduced the formation of bubbles and the substrate evaporation sufficiently to make the measurements possible.

3.3.3.2 Sample

The sample material, [1,2-bis(10,12-Tricosadiynoyl)-sn-Glycero-3-Phosphoethanolamine], also known as Diyne PE (99% pure), was obtained from the Avanti and was used without further puriﬁ- cation for making solution. HPLC grade chloroform, purchased from Sigma-Aldrich, was used as 77

the solvant. Solution of concentration ∼ 0.8mg/ml was prepared (the solution concentration varied slightly, but still still remained within about 20% of 0.8mg/ml) in a small (5ml) round-bottom ﬂask and stored in a freezer at −20 ◦ C. The solution is taken out of the freezer at least half an hour before the ﬁlm preparation. This help keep the volume of the solution constant at room temperature.

3.3.3.3 Film preparation

The ﬁlms were prepared in a 364mm×75mm Minitrough (KSV, Finland) with movable barriers

(Fig.2.19). Material was deposited on the substrate with a micro syringe and observed through

π − σ isotherms using a platinum Wilhelmy plate (Sec.2.5.3.3) and a home made BAM, mainly with a red laser (λ = 668 mm) and a CCD camera working at 30 frames/second (for the details refer to Sec.2.4) with ﬁeld of view ∼ 11 mm × 10 mm. Later experiments were done with green and blue lasers, allowing better resolution because of better beam quality. Another reason to use different laser beams is that different wavelength of light interact differently with the lipid. By using different wavelengths, we are controlling for any such interaction. These experiments will be described later on wherever it is appropriate.

Before the material was deposited the substrate was heated to ∼ 50 ◦ C and the spreading solution of the sample was deposited to give the initial concentration of ∼ 1 nm2/molecule. The system cooled down to the working temperatures,∼ 18 ◦ C – 45 ◦ C in about 45 minutes. The heating and cooling of the system was carried out by the circulation of water underneath the trough by an automated temperature controller (Julabo F12, Fig.2.20). The Langmuir ﬁlm was then compressed at a constant temperature and at a constant barrier speed of 10mm/min (of course, a few of the experiments were done at different compression rates for the sake of comparision), monitoring the surface continuously with both BAM and the surface pressure. Experiments were performed under humid ambient atmosphere (relative humidity ∼ 50%); an argon or other inert atmosphere would be preferrable, but this was difﬁcult to set up with our BAM and since experiments appeared to be reversible, this was not pursued. The molecular concentration just before compression was

∼ 1 nm2/molecule. 78

Figure 3.21: A BAM image of spiral formation in a Langmuir ﬁlm of DIYNE PE upon compression ◦ at 30 C. t0 in the image represents the time after the compression begins. The image was captured with increased magniﬁcation.

3.4 Qualitative discussion of results

Here in this section I will present mostly qualitative descriptions of the pattern formation. A more quantitative discussion will be presented in the next chapter with a possible model for the pattern formation that we saw in our experiments.

In experiments carried out at a temperature of 30 ◦ C as discussed in Sec.3.3.3, we observed a spiral like pattern as shown in the Fig.3.21. This spectacular process of spiral formation intrigued us and tried to check if we can still see these kinds of patterns at a temperature above or below

30 ◦ C. The Fig.3.22 summarizes the modes of patterns that we saw at different temperatures. From

Fig.3.22 it is obvious that we did not see any patterns formation at a lower temperatures. At about

28 ◦ C some kind of less defined structures appear. Similarly, above 37 ◦ C the trend of well-defined pattern formation seems to disappear. I would also like to pinpoint the disappearance of well- defined patterns between 37 ◦ C and 39 ◦ C, since the chain melting temperature is expected to be in that region, but within the limitations of our temperature controller, it was not practically possible 79

to obtain data in that narrow a temperature range. The chain melting temperature has been observed at 38 ◦ C [166].

In order to monitor the compression (or decompression) process BAM was supplemented with the surface pressure measurement (Sec.3.3.3). Fig.3.23 shows two representative isotherms at the compression temperatures of 28 ◦ C and 30 ◦ C.

Langmuir ﬁlms undergo various stages of phase transition. In our case, it was difﬁcult to pin down what phase or phases we were seeing. Before compression, the system showed gas-liquid phase coexistence. Upon compression, the gas disappears. The pattern nucleates compressing ∼

10% more. The brighter phase that develops into the patterns may be we saw a liquid crystalline phase within a more ﬂuid one although the 10% compression of the ﬂuid phase may have induced more order.

Fig.3.23 taken at 28 ◦ C a is a standard compression isotherm for a stiff monolayer and we can see nothing interesting in it from the prospective of pattern formation. However, in Fig.3.23b, which is a compression isotherm at 30 ◦ C, we gathered some interesting information in regards to pattern formation. We can see a clear plateau in this isotherm around which the surface pressure

(π) almost remains constant even when we are decreasing the mean molecular area of the system.

By carefully observing the video taken during compression and also monitoring the time recorded during compression we found within ±10% accuracy that the nucleation starts around the values shown in the isotherm (π ∼ 13 mN/m, σ ∼ 0.6 nm2/molecule). At higher temperatures (but below the chain melting temperatuer, 38 ◦ C), we can still see plateau but they shift upward with increase in temperature (Fig.3.24).

We can ﬁnd a similar trend of plateau shift in the isotherms with rise in temperature in many other types of lipid Langmuir ﬁlms [22, 23, 167]. Wang and et.al. have reported in their work

[22] that they observed fractal-like patterns in the LC phase during compression (inset of Fig.3.25a) in the plateau regions of the isotherms. In their experiments, they prepared lipid Langmuir films in pure water. In order to employ fluorescence microscopy, they introduced fluorescent dye in the 80

Figure 3.22: BAM images of modes pattern formation in a Langmuir ﬁlm of DIYNE PE upon compression at different temperatures. The scale bars in the images correspond to 1 mm. The rightmost picture at 30 ◦ C in the second row was captured at a larger magniﬁcation. 81

Figure 3.23: Compression isotherms at (a) 28 ◦ C and (b) 30 ◦ C. The values of the surface pressure and the mean molecular area is the one at which the nucleation begins to end up with spiral formation at the this temperature.

Figure 3.24: Compression isotherms at different temperatures. In the graph, the dashed lines represents the isotherm at 28 ◦ C, the solid line represents the isotherm at 32 ◦ C, the dotted-dash line at 34 ◦ C, and the dotted line at 37 ◦ C. Positions of arrows show the region of isotherms at which visible nucleation begins and the corresponding values of mean molecular areas and sur- ◦ 2 face pressures are the critical values (σc and πc) – (i) at 32 C: σc ∼ (0.6 ± 0.05)nm /molecule, ◦ 2 πc ∼ (15 ± 0.2)mN/m (ii) at 34 C: σc ∼ (0.5 ± 0.06)nm /molecule, πc ∼ (34 ± 0.4)mN/m (iii) ◦ 2 at 37 C: σc ∼ (0.6 ± 0.05)nm /molecule, πc ∼ (41 ± 0.2)mN/m 82

sample solution dissolved in chloroform. Table3.2 shows a brief qualitative difference in experimental methods between ours and their. In our case, it is hard to say in which phase we are seeing the pattens. In their work they have described that there is significant role of fluorescence probe in the formation of patterns of different shapes, whereas, we did not use any such probes in our experiments. So it is very difficult to say that there is any similarity of pattern formation mechanism in these two cases. However, despite all the difference we can see a striking similarity in the isotherms at different temperatures in a way or so, suggesting similarity. Fig.3.25b has been taken from the work of Bourdieu and et.al. [23]. They used highly polymerizable diacetylenic phospholipid (this lipid has very similar branched chain with triple bonds as that of ours, with a different head group, chain length and exact placement of triple bonds) to prepare Langmuir films in their experiment. Note that the patterns are superficially similar to ours, but that they appear to represent phase coexistence, with the more fluid phase observed as brighter regions throughout the spirals.

Table 3.2: A qualitative comparison of our experimental methods with that of Wang et.al.

Ours Fig.3.25b, Wang et.al Lipid: phosphoethaloamine Lipid: phosphocholine Triple bonds in the hydrocarbon chains Triple bonds in the hydrocarbon chains Molecule: C51H86NO8P Molecule: DC8,9PC Substrate: 20% glycerol solution pure water Microscope: BAM Fluorescence microscope Fluorescence probe: No Yes Film prepared at 50 ◦ C Film prepared at 20 ◦ C Temp. decreased to working tem. Increased to working temp Compression: Yes Yes

The data presented so far are the consequence of compression of the Langmuir ﬁlms of Diyne

PE at a moderate rate (10mm/min). However, compressions of the ﬁlms were also carried out at a faster rate than those discussed above. Fig.3.26 shows an effect of compression rate on the rate of nucleation of structure. The ﬁlm as shown in Fig.3.26a was compressed at a barrier speed of

10mm/min whereas the ﬁlm shown in Fig.3.26b was compressed at a barrier speed of 20mm/min. 83

(a) (b)

Figure 3.25: (a) Isotherms of the Lamgmuir monolayer of [N,N-dihexadecyl-3-(1-imidazolyl)- propyamine] as a function of temperature. The LE-LC coexistence region is characterized by a long plateau. The arrow indicates the collapse of the the monolayer. The inset is the ﬂuorescence microscopy image of fractal like pattern in the LC region(the bar on the inset equals 50µm) [22]. (b) Isothererms of diacetylenic phospholipid monolayer. The inset is the ﬂuorescence microscopy image of the curved and branched domains (' 50 to 100µm) at the condensed phase in the plateau regions above 20 ◦ C [23] .

It is obvious from the Fig.3.26 that the faster the rate of decrease of mean molecular area, the more is the nucleation rate. We can also see in Fig.3.26b that the overall size of a pattern deceases with increase rate of nucleation.

Now the central question is: what could be the reason behind formation of patterns that we are seeing?

In Sec.3.3, we reviewed a few mechanisms of equilibrium pattern formation in Langmuir ﬁlms.

We will also consider the role of impurity, solidiﬁcation of fronts and heterogeneous nucleation around a foreign particle. I will discuss them one by one below.

1. Tilt variation (Se.3.3.1.3)

2. Elastic buckling out of interface (Se.3.3.1.1)

3. Solidiﬁcation of fronts

4. Polymerization (Sec.3.3.1.2) 84

(a) (b)

Figure 3.26: BAM images of pattern formation when the mean molecular areas of the Langmuir ﬁlms made of Diyne PE at the air-water interface were decreased at two different rate. (a) The rate of decrease of mean molecular area ∼ 1.4nm2 molecule−1 s−1, barrier speed = 10mm/min. (b) The rate of decrease of mean molecular area ∼ 2.9nm2 molecule−1 s−1, barrier speed = 20mm/min. Both in (a) and (b), compressions of the ﬁlms were carried out for about 5min from the beginning.

Figure 3.27: An illustration of how the arrow, the direction of molecular tilt projected onto the layer plane, varies along the path [24].

5. Effect of chirality (Sec.3.3.1.4)

6. Role of impurity

7. Heterogeneous nucleation around a foreign particle

3.4.1 Tilt variation

As discussed in Sec.3.3, people have seen spiral pattern formation due to tilt-azimuth (3.14) variation [15] across the ﬁlm surface. Going along the path of a spiral (refer to Fig.3.15a) one can clearly see a change in contrast arising from the change in tilt along the path. Fig.3.27 help understand the process of how the direction of tilt changes around a curve.

We tried to check this mechanism in our experiments by changing the analyzer position along the 85

path of the reflected beam from the film surface as shown in Fig.3.28. As discussed in Sec.3.3, the polarization of reflected light depends on the tilt-azimuth direction [15]. In plane optical anisotropy arises with one axis parallel to the direction of tilt. If this axis is not parallel to the polarization of the incoming light, the ploarization of the reflected light will rotate towards that axis. This implies that, if there was any change in the tilt along the surface of the film, change of analyzer position would result in a change of contrast of the image too because of a change in the polarization direction. I did not consistently test for analyzer position, but one case where I did, at T = 36 ◦ C, is shown in figure

Fig.3.28. We set the analyzer and the polarizer parallel parallel to each other, we almost did not see any contrast. And when we crossed the analyzer and the polarizer position with each other (i.e., when the analyzer was set to 90◦ out of the direction of polarizer) we saw a good contrast as shown in Fig.3.28b. At ﬁrst sight, it looked like the contrast that we are seeing could be also from the tilt variation. However, gray value estimated (by usnig ImageJ program) around the centerline of circle- like structures was almost constant, whereas in Fig.3.15a one can clearly see that there is sharp change in contrast going around the stripes. Our results on the contrary suggest that the average brightness remains constant as the stripe changes direction, very unlike what is seen in Fig.3.15a.

The director (the average direction of molecular orientations in a system) thus does not seem to be at a ﬁxed angle with respect to the stripes. But, then one needs to ask why the contrast is sensitive to the analyzer. Perhaps the contrast is due to a variation of optical activity (that is rotating of plane polarized light) of the ﬁlm rather than its director orientation. It is worthwhile to note here that behavior may be very different at different temperatures. My collaborator Dr. Chaieb suggests that spirals and targets have very different behavior of the director, just because of boundary conditions

. Some hint of substructure within the stripes is seen, especially during the growth of the targets.

Better resolution may be needed to decide the question of boundary conditions. Improvements in the BAM allow us to see this substructure.

A more quantitative description of the gray value proﬁle will be presented in Chap.4.1. Here

I have considered two images captured at 0 ◦ and 90 ◦ with respect to the polarizer (Fig.3.29) at 86

(a) (b)

Figure 3.28: BAM images of Langmuir ﬁlm of Diyne PE. (a) With analyzer parallel to the polarizer (b) With analyzer crossed with the polarizer. T = 36 ◦ C. The scale bars correspond to 100 µm.

30◦ C. There is a substantial change in the average gray level going from 0 ◦ to 90 ◦ position of the analyzer which is natural for a plane polarized light when the analyzer and the polarizer are crossed to each other. Most importantly, if we compare the two images carefully, we see that the region which was bright at 0 ◦ analyzer position is now dark at 90 ◦ analyzer position and vice-versa. The change of gray value due to tilt variations may follow around the circular substructure (Fig.3.35) in a consistent way, but it needs more analysis to be certain about it, particularly the circles in the claw as shown in Fig.3.35, which I am beginning to analyze.

3.4.2 Buckling out of the interface

Amphiphilic molecules are known to form stable Langmuir ﬁlms at the air-water interface and can sustain fairly high surface pressure [54, 135]. However, on compression beyond a certain concentration, these ﬁlms are known to become unstable to a collapse transition, usually accompanied by the formation of either multilayer islands on the surface or dissolution in subphase or both

[54, 135–138]. Under certain conditions this collapse transition is preceded by a mechanical instability called buckling that leads to formation of a regular striped pattern being determined by the mechanical properties of the ﬁlms [10, 23, 139–144] and in particular the balance of the layer 87

(a) (b)

Figure 3.29: BAM images of Langmuir ﬁlm of Diyne PE with improved resolution.(a) Pattern when the analyzer and the polarizer are parallel to each other (b) Pattern when the analyzer and the polarizer are crossed. The scale bars in (a) and (b) correspond to 100 µm. The compression temperature: 30◦ C.

Figure 3.30: Molecular structure of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) [25]. elasticity with boundary conditions.

Fig.3.9 is an example of buckling of Langmuir monolayer under compression. The authors prepared monolayer of 1,2-distearoyl-sn-glycero-3-phosphocholine (refer to Fig.3.30), usually known as DSPC, in pure formamide. The authors claim that by using pure formide as the substrate, they were able to observe buckling of the ﬁlm at a surface pressure as low as 7mN/m. Despite the fact that there is little resemblance between the molecule used in our system (3.6b) and the one used in this example, it gives a good understanding of how a monolayer under compression can buckle and can give rise to almost periodic stripe patterns.

One of the best-known examples of buckling is the Euler instability of a beam that is compressed axially in an elastic medium [11]. The stability of the system can be determined by the solution to the equation [168] 88

d4X d2X EI + |T | + αX = 0 (3.1) dy4 dy2 where I is the second moment of inertia (i.e. area moment of inertia)5 of the beam and E is its

Young’s modulus, T is the compressive force, which is along the y-axis and α is the elastic modulus

of the medium in which the beam is immersed, and X is the deﬂection of the beam. The ﬁrst term of the equation comes from the change of shear force along the beam length, the second term arises from the curvature due to bending and the third term is due to the response of the elastic medium.

1/2 If the compressive force exceeds its critical value Tcr = 2(αEI) , the beam will buckle with a wavelength given by

1 λ = 2π(EI/α) 2 (3.2)

In the absence of the elastic medium, the wavelength is twice the length of the beam. This example was explicitly used for buckling of thin lamella within a phase separated Langmuir monolayer. In such a case, other forces, notably dipolar repulsion across the lamella, may play the role of the elastic medium [11].

Buckling within a Langmuir monolayer perpendicular to the plane is not directly analogous to this case, since the substrate water can be considered incompressible (α → ∞). However, we would expect to carry over the dependence on the elasticity of the ﬁlm..

In order to see the effect of elasticity on pattern formation in our system we calculated the elasticities at different temperatures. The Gibbs elasticity of a Langmuir ﬁlm is deﬁned as

dπ E = − (3.3) d(lnσ) where π is the surface pressure and σ is the mean molecular area. Eq.(3.3) can now be written as,

5 R 2 Ix = y dA, where Ix is the second moment of inertia about the axis x, dA is an elemental area, and y is the perpendicular distance to the element dA from the x-axis 89

(a) (b)

Figure 3.31: An illustration of how the critical elasticities were found using an isotherm and its ◦ derivative. (a) Compression isotherm at 34 C, where πc is the critical surface pressure at which the patterns begin forming. (b) Elasticity versus mean molecular where Ec is the critical elasticity corresponding to the critical mean molecular area and the critical surface pressure in (a).

dπ E = − d(lnσ) dπ 1 = − 1 dσ σ dπ E = −σ (3.4) dσ

dπ A routine of OriginLab software was used to calculate the derivavtive dσ and then multiplied by the negative of mean molecular area to obtain the corresponding values of elasticities as given by

Eqn.3.4. Then these data were replotted as shown in Fig.3.31.

Critical surface pressures at different temperature, at which pattern formation takes place, are

ﬁrst estimated from the plateau of the isotherms. We estimate that surface pressure corresponding to the beginning of the plateau (shown by the arrow in Fig.3.31a). To get as close to the accurate value of the critical surface pressure we calculated the Gibbs elasticity given by Eq.3.4 then plotted against the mean molecular area. The critical elasticity value is taken as the largest elasticity before the patterns form at the plateau of the isotherm as shown by arrows in the graphs. Then the mean molecular area was found corresponding to the critical elasticity. Now, from the isotherm again, 90

Figure 3.32: Change of critical surface pressure πc with temperature T. The circle are the data points when there is no patterns and the solid circles are when there is well deﬁned pattern. the critical surface pressure was determined against this mean molecular area. The reliability of this technique of ﬁnding the critical surface pressures was checked by observing the video taken during compression/decompression and then surface pressure data. The values of the critical surface pressures determined by this method were found within ±10% accuracy.

The method described above was repeated for all compressions at different temperatures; plotted in Fig.3.32 (critical surface presures) and Fig.3.33 (critical elasticity). From these two ﬁgures it is obvious that our system is shows an increasing trend of critical surface pressures and critical elasticities with temperature where there is nucleation for pattern formation.

3.4.3 Solidiﬁcation front

As discussed in the earlier section, several different groups have seen forming the dendrite- like structures (3.34) because of the solidiﬁcation process in lipid monolayer [22]. Similarly, solid domains of different structure have been reported in the work of Lenne and et.al. [169]. Millers and 91

Figure 3.33: Change of critical surface pressure πc and critical elasticity Ec with temperature T. Solid squares: data with no structure; solid circles: data with structures; circles: critical elasticity. The error bars represent the standard deviation in the measurements. et.al. [167] is another example of pattern formation due to the solidification of monomolecular films at the air-water interface. They found that after step-wise increase of surface pressure, fractal-like single-crystalline domains were formed in the film.

In an experiments performed with better resolution and improved laser power, we saw the claw- like substructures of the spiral/target patterns (Fig.3.35). Though we do not have any concrete evidence to support our statement, it is possible that we are seeing these substructures due to solid- iﬁcation of our Langmuir ﬁlm to some extent.

3.4.4 Polymerization

Many phospolipid bilayers (vesicles) and Langmuir monolayers can undergo polymerization upon UV irradiation [23, 88, 130, 145–147] and bilayers (vesicles) as do including vesicles of diacetlynic orgin [146, 148]. My collaborator Dr. Chaieb has seen crumpling of vesicles[12] in polymerizing such lipids upon UV irradiation. You can see a disordered patterns with a characteristic length scale in such crumpled vesicles (Fig.3.11). When polymerization is induced by UV 92

Figure 3.34: Dendrite structure in the Langmuir monolayer of N,N-dihexadecyl-3-(1- imidazolyl)- propyamine at the air-water interface. The bar equals 50 µm [22].

Figure 3.35: A BAM image of pattern formation upon compressing the Langmuir ﬁlm of Diyne PE at the air-water interface at 30 ◦ C. The wavelength of the laser beam used = 488nm. The dark colored curve denoted by B is the guide to the claw-shaped substructures around the circular patterns. The scale bar = 100µm. 93

Figure 3.36: BAM image of compression, decompression and re-compression of lipid Diyne PE at 32 ◦ C. The the order of cycles follow from left to right. light in diacetylene molecules, this polymerization was found to be effective only if the chains are packed with very speciﬁc orientation and distance, corresponding to a particular pressure and area per molecule [170]. This could explain the sudden onset of nucleation, if it were due to the onset of polymerization.

Our lipid is also of diacetylenic origin and its polymerization is highly sensitive to UV light and ambient oxygen [25, 130]. We tried our best to keep the sample from polymerization before and after our experiments. However, it is difﬁcult to conclude absolutely on the state of polymerization of the sample during these processes. One way to check that is to see if there is any reversibility in our system or not. The system should not behave in the same way before and after polymerization.

Fig.3.36 are the images captured after compression, re-compression and decompression at 32 ◦ C.

The order of compression, decompression and recompression is from left to right in the ﬁgure. After compression we saw structures more like spirals at this temperature. During decompression of the

film, the structure on it seemed to melt with decreasing surface pressures, the structures first looking somewhat gel-like and finally disappearing from the field of view of our BAM as shown in the middle section of Fig.3.36. Upon re-compressing the film we could again see some stripe patterns, as shown the final panel of Fig.3.36. At firt sight, the structures look very different. These look like claws, closed-packed in some places but not others. However, they look like remarkably larger, but much less organized version of the claw-like substructures seen in the higher temperature images 94

(Fig.3.37).

We must ask whether our lipid ﬁlm might have already polymerized to some extent so that it is less reversible at this particular temperature. We repeated the same cycles of compression and recompression at 36 ◦ C and captured the images after compression and re-compression as shown in Fig.3.38. Fig.3.38a shows the structures after compression and Fig.(3.38c) shows the structures after re-compression. Looking at these two pictures one can clearly tell that the structures seen after compression and that after re-compression are not exactly the same. It looks the structures formed after re-compression around some remnant left after decompression. However, unlike at 32 ◦ C, at

36 ◦ C we are again seeing well deﬁned pattern after re-compression. From these results at two different temperatures we can infer that our system exhibit some irreversibly at lower temperature around 30 ◦ C, and shows a much greater reversibility at higher temperatures close to the transition temperatures. It is possible that there is less polymerization at higher temperature, but it is also possible that some underlying structure, not visible in the BAM images, is kinetically trapped, which would be stronger at lower temperatures.

The hysteresis between compression and decompression in the isotherms is larger at low temperature than at high temperature as shown in Fig.3.40. This also shows that the system is more reversible at higher temperatures than at lower temperature.

3.4.4.1 Thin layer chromatography (TLC)

In order to test whether or not the sample was polymerized, I performed thin layer chromatography (TLC) experiments both with the stock solution (sample in the bottle before it was used for making the Langmuir ﬁlm), and the sample collected from the ﬁlms after pattern formation.

With these experiments, the stock solution was found to be unpolymerized, as indicated in Fig.

3.40a and time factor did not have any effect on polymerization (remain in freezer for about a year before this test) whereas the sample collected from the trough after pattern formation showed very low levels of contaminants or of longer (polymerized) molecules) by a very narrow single line

(Fig.3.40b). For a detailed discussion of the TLC experiments, please refer to appendix C.2. 95

Figure 3.37: A BAM image of pattern formation with claw-like sub-structures upon compressing the Langmuir ﬁlm made of Diyne PE. The scale bars correspond to 100 µm. The time sequence of the images from the top left image to the bottom right image is 0 s, 1 s, 2 s, 3 s, 4 s, 10 s, 15 s, 25 s and 40 s. The wavelength of the laser beam used = 488 nm. 96

(a) (b) (c)

Figure 3.38: BAM images of compression and decompression of Diyne PE Langmuir ﬁlm at 36 ◦ C. (a) Compression (b) De-compression (c) Recompression. The bars correspond to 2mm.

(a) (b)

Figure 3.39: An example of decrease in hysteresis with increase in temperature. In both of the graphs solid circles represent compression data and the circles represent the decompression data. Area between the compression-decompression isotherms (the dark regions) are: (a) Compression-decompression at 30 ◦ C, 1.12×10−14J/mole, (b) Compression-decompression at 35 ◦ C, 3.86×10−15J/mole. 97

(a) (b)

Figure 3.40: Images obtained from the TLC experiments. In (a),A: a different PE, B:stock diyne PE and C: standard PC samples from left to right were used for the comparision. In (b), A: different PE, B: sample from the Langmuir ﬁlm after pattern formation taken at 36 ◦ C, C: standard solution of PC. Rates of different oligomers after polymerization results in an elongated mark in the middle spot of (b). 98

3.4.5 Effect of chirality

Chirality is an important microscopic property of a molecule; the absence of symmetry under improper rotations [171]. Langmuir ﬁlms made of such chiral molecules have been reported to form several different kinds of patterns [16, 18, 24, 64–66, 172]. Groups have seen spiral patterns in Langmuir ﬁlms made of chiral molecules [17]; the mechanisms have been discussed in detail in

[172]. Fig.3.17a shows spiral or circle like patterns in a Langmuir film made of chiral molecules in solid-fluid phase as indicated by the difference in contrast within the domains. We see similar kinds of patterns (Fig.3.38a). However, in our case there is no evidence of liquid phase between spiral arms. This suggest that most probably, we are seeing our patterns not in a solid-fluid phase, but something else. However, the structures of these domains are certainly chiral, which may suggest the chirality associated with our system of molecules must be playing a pivotal role in giving rise to the spiral or target like patterns. A closer look at the patterns, with an version of the microscope recently improved to give better resolution, shows even more striking evidence of Chirality (Figures

3.35 and 3.37).

I will discuss chirality and one model leading to such structures in the next chapter.

3.4.6 Role of impurity

We can ﬁnd in literature that impurities can plays a role in spiral, target or fan like pattern formation [16–18], in addition to chirality. One way to check our sample purity is to look the trend shown by our isotherms. Generally, there is a bump in the compression isotherms obtained from an impure sample [23] as shown in Fig.3.41. In order to obtain the isotherms shown in Fig.3.41, the authors prepared Langmuir ﬁlms of DC8,9PC (a diacetylenic phospholipd which has two hydrocarbon chains with two triple bonds in each with a phosphocholine head group) with varying degrees of impurities. The bump in the isotherms b, c, and d are attributed to the cholesterol impurity added to the fresh sample. Looking back at our isotherms at different temperatures (Fig.3.24) no such bumps are obvious. From this qualitative comparison of isotherms, there is no evidence of impurities in our sample. In order to further test the purity of the sample, we carried out Thin Layer Chromatography 99

Figure 3.41: Compression isotherms of Langmuir ﬁlms made of DC8,9PC at the air-water interface. Bump at the beginning of the coexistence plateau. All the isotherms are measured at 25 ◦ C and are shifted by 2mN/m and 0.04nm2 for clarity. (a) isotherm obtained with a fresh sample, exhibiting no bump; (b) and (c) isotherms obtained with the same solution as in (a), in presence of (b) 0.1% of cholesterol and (c) 5% cholesterol [23].

(TLC) tests with our sample. The results of the TLC tests indicated that the sample contains less than 0.1% of contaminants such as cholesterol. Therefore, most probably there is no contribution of impurities in these kinds of pattern formation.

3.4.7 Heterogeneous nucleation around a foreign particle

Molecularly thin ﬁlms on different kinds of substrates can undergo nucleation processes [173–

177] and give rise to pattern formation. Thus one can wonder if there was any role of foreign particles, such as dust in nucleation in our case. In order to ﬁnd the answer to this query, I conducted an experiment with some dust particles on the ﬁlms. During the compression, as soon as the required critical pressure approached, the nucleation started far away from the location of the dust particles as shown in Fi.3.42. Thus, foreign particles play no obvious role in nucleation of the patterns.

BAM and Fluorescence Microscopy from our prospective

Langmuir monolayers of similar lipids were studied previously, to reveal an interesting assort- ment of chiral-patterned domains (Sec.3.3.3), but with a very different character from the patterns 100

Figure 3.42: BAM image: a test of nucleation around a dust particle. The bright spots within the circles are the dust particles and the nucleating structure is shown within the square bracket. The contrast is enhance to make the dust particles visible. shown here with different techniques. A major difference in the techniques is that they utilized fluorescence microscopy. Fluorescence and Brewster angle microscopy are complementary techniques, sensitive to different properties of the layers. The fluorescent probe acts like an impurity in the monolayer, i.e., its solubility in the co-existing phase can be very different. Contrast is primarily due to differences in probe density [91]. Thus, liquid disordered/liquid crystalline phase coexistence may be observed due to the very low solubility of the probe in the more ordered phase, which thus appears dark. Brewster angle microscopy on the other hand is sensitive to surface density, molecular orientation, and the roughness of the surface. It is sensitive to both changes in molecular tilt and to any buckling out of the surface, to which fluorescence microscopy is entirely insensitive.

Fluorescence probes may be almost insoluble in most of the more condensed phases, which again limits contrast in those regimes. Thus, Brewster angle microscopy can reveal entirely new behavior in even well-studied systems.

There are several other things to be wary of with ﬂuorescence microscopy:

1. The probe may affect the phase transition and the growth of the phase domains. For example, 101

the probe must often be expelled from the more condensed phases.

2. Photochemical transformations are possible [178] (which can also be a problem with BAM).

3. The phase transitions that take place at high surface density are difﬁcult to observe.

3.5 Conclusion

People have found several different types of pattern formation in Langmuir ﬁlms. Some of them are of equilibrium origin and some are not. Mostly, people have seen equilibrium patterns in

Langmuir ﬁlms due to the growth of one phase in another, change in the tilt azimuth along the the surface of the ﬁlm, chirality, elastic buckling and so on.

In my case, qualitative results obtained from the BAM images at different analyzer positions with improved resolution suggest some indication of tilt variation. However, it is equallyy possible that the contrast in the ﬁlm that I saw at different analyzer positions could be because of the optical activity caused by the chiral nature of our molecules.

With sudden nucleation of any pattern or phase, one must ask if they started around some foreign nucleation centers such as dust particles. However, I found that dust particles do not play any role for the nucleation in the process of pattern formation.

In the pattern formation process, I found that the nucleation rate increases with increase in temperature. Similarly, the nucleation rate also increases if the rate of decrease of mean molecular area increases (i.e., if the rate of compression is increased).

I also observed different modes of pattern formation. At lower temperatures, close to 30 ◦ C, we found that the patterns are more like spirals but with many defects, while near the transition temperature, the patterns are more like circle and almost defect-free. Wed did not ﬁnd organized patterns below 30 ◦ C and above the chain melting temperature, which is 38 ◦ C.

I tried to test whether elastic buckling has any role to play in our pattern formation by estimating critical elasticity and characteristic lengths at different temperatures. I found that the critical pressure and critical elasticity at which our patterns begin to form, both increase with temperatures. 102

However, I did not see any clear trend of characteristic length with temperature. This makes difﬁcult to say that elastic buckling could be a key player in this game of pattern formation.

As discussed in Sec.3.3.1.4 and Sec.3.4.4, impurities (such as cholesterol) and the polymerization can play important role in pattern formation in a Langmuir ﬁlm. However, from the results of TLC and a test with dust, it seems that impurities have little role to play in the nucleation of structures that we saw. The TLC results showed that there is some degree of polymerization of the

ﬁlm. Therefore, polymerization of the ﬁlm might also have some role to play in this type of pattern formation. Previous results of patterns formation were obtained by mostly using the techniques of

Fluorescence microscope (FM), in which photosensitive chemicals are used as probes. It is possible that they did not see the patterns that we are seeing due to the direct effect by these probe materials on, in surface properties of the ﬁlm. Fluorescence microscope may simply not be sensitive to the variation of surface properties in these patterns.

In short, we still have many more questions than answers about these patterns. Different types of techniques must be brought to bear to answer these questions. One possible technique would be surface light scattering, which is particularly sensitive to the roughness to the ﬁlm.

A more quantitative discussions with one possible model for the observed behavior will be presented in the next chapter CHAPTER 4

TRANSITIONFROM SPIRAL STRIPESTO TARGETSIN COMPRESSED LANGMUIR

MONOLAYERS MADE OF DIACETYLENICLIPIDS

4.1 Introduction

The mechanism behind different types of pattern formation has been discussed in Sec.3.3 in detail. In this section, I will present a short review with some additional points particularly relevant here.

There are varieties of systems where spatial modulation of some order parameter trigger formation of stripe patterns (Sec.3.3). Macroscopic rolls in convection [179, 180], ripples in phospholipid membranes [181, 182], and self-assembled diblock copolymer undergoing phase separation

[183, 184] are a few examples of systems exhibiting such morphology. Depending on the system, competing interactions play vital roles in the formation of domain in general and stripes in particular. The competition can be either between short-range attraction and long-range repulsion, as in ferroﬂuids, or involves coupling between various parameters such as in lipid membranes and liquid crystals [185, 186]. In coexistence of two phases in Langmuir monolayers, such stripes may be due to a competition between long-range dipolar repulsion and line tension [187, 188]. The long-range interactions of these kinds cause deformation of lipid domains when they move toward each other

[15, 63]. As a consequence of the balance between elasticity and boundary conditions, stripes can also represent the modulation of molecular tilt within domains of more ordered phase [15]. Addi- tionally, there are other interactions such as molecular orientation, which are purely geometrical by nature, , can dictate lipid domains in Langmuir monolayer to take exotic shape. Similarly, chiral interactions for example, can give rise to chiral domains [17, 18, 189–191]. Searching in the literature, we could not ﬁnd any systematic study of shape transition within chiral domains as a function of temperature.

103 104

4.2 Experimental

As discussed in Sec.3.3, we used BAM (Please refer to Sec.2 for the detail discussion of BAM) to study a new transition from spiral and stripes to concentric target patterns as the temperature is increased toward and beyond the chain melting temperature of the lipids. If we consider just the structure, similar patterns have been observed (Fig.3.17a) with a mixture of L-DPPC, 2mole% cholesterol, and 2mole% NBD-PC [17]. However, in that case there were two different phasees, liquid and solid within the spirals. In our case, the spirals appear to be modulations within a single phase. Again, the question is: why are we seeing these patterns? In the next section, I will try to answer this question with a possible model based on our quantitative measurements. I leave out most of the details of the materials and methods in this section since that has already been covered in Sec.3.3.3.

The patterns were seen in compression (refer to Sec.3.3.3) of the Langmuir ﬁlm of the DIYNE

PE. Typical isotherms at different temperatures have been shown in Fig.3.24; the significance of these isotherms have been discussed in Sec.3.4. At higher temperatures where many domains nucleate with the field of view of the BAM, the spirals begin forming as the plateau begins. At lower temperatures, nucleation is rare and usually out of the field of view. However, the nucleation seems to be consistent at the beginning of the plateau. While a nucleation hump was not observed, patterns were seen to grow continuously until they filled the whole surface, once they start, no matter the mean molecular area was held fixed in the beginning of the plateau or allowed to change.

4.3 Results and discussion

As discussed in Sec.3.4, formation of patterns was seen within a temperature range of 28 ◦ C

- 38 ◦ C (Sec.3.4), but the patterns below 30 ◦ C and above 36 ◦ C were not well-organized. We observed a transition from spiral to target patterns as we increased the compression temperature.

Let’s remind ourselves of the progression of patterns (Fig.3.22) and look at a few more details, in ﬁgure (Fig.4.3). As shown in Fig.3.24 pattern formation begins at the plateau of the isotherms around the region shown by arrows. The characteristic length of the patterns within a particular 105

(a) (b)

Figure 4.1: (a) Change of characteristic length with temperature at a constant surface pressure of ∼21mN/m. The error bars in the data points are the standard deviations in the measurements. (b) The section between the two arrows is deﬁned as the characteristic length. experiment at ﬁxed temperature remains constant to within 15% and does not vary systematically with temperature as shown in Fig.4.1a.

I also tried forming the spirals at 30 degrees and then increasing the temperature, up through

42◦ C. Up to 39◦ C the patters remained very stable with no obvious change in stripe width.

At 41◦ C, it suddenly started melting, very much like in decompression at constant temperature

(Fig.4.2).

Figure 4.2: A BAM image of melting of patterns at high temperatures: (a) patterns at ∼ 39◦ C (b) patterns at ∼ 40◦ C (c) patterns at ∼ 41◦ C. The bars correspond to 1 mm. 106

Figure 4.3: Transition from spiral to target as we increase the temperature from left to right. The pattern changes from a spira mirabilis to concentric circles (targets) via the Archimedean spiral with constant width. We can observe defects in the spirals but not in the targets despite the fact that some of the circles do not close up. Notice how straight is the line between the domains (shown in the inset of the second ﬁgure from the left)in the spiral regime. Scale bar in the picture is 1mm and in the inset it is 200µm. Temperatures of compression are: 28, 32, 34 and 36 ◦ C

In my experiments, cycles of compression and decompressions were carried out. On decompression, the pattern disappears and re-appears on recompressions, though not well-organized (Fig.3.36 and Fig.3.38).

The pattern we see from 28 ◦ C up until 30 ◦ C was also observed in similar systems [190, 191]. The pattern at 28 ◦ C is made of logarithmic1 spiral as shown in the inset of left of Fig.4.3. As we increase the temperature, these logarithmic spirals become archimedean2 spirals with a constant width. Most of these spirals have been observed to contain edge dislocations with burger vector of the order of stripe width [185, 192]. It is believed [193] that these defects are a signature of dynamic formation of stripes in two-dimensional system. Coexistence of spiral and target patterns were observed at an intermediate temperature range of 32-35 ◦ C. At a temperature range of 36-38 ◦ C, i.e., at the chain melting temperature (Tm) and close to it, target pattern becomes dominant.

It was found that the nucleation rate increased appreciably at higher temperature as shown in the rightmost portion of Fig.4.3 (at about 36 ◦ C). We can see in the ﬁgure that that there are many

1The locus of points given by the equation, r = aebθ , where a and b are two real constants. 2The locus of points given by the equation, r = a + bθ 107

structures in a single image and the domains are smaller by an order of magnitude , corresponding to their increased number.

I have already shown a relation between the critical surface pressure Πc, at which pattern formation takes place, and temperature (Fig.3.32). That picture summarizes how different types of pattern formation takes place at different temperatures (between 28-40 ◦ C).

From Fig.3.32 we see that the archimedean spirals occur in a temperature range of 28-34 ◦ C.

We observed that these domains, unlike the domains in Langmuir monolayers made of fatty acids

[15, 63] do not distort when they are very close to each other, but they keep on moving toward each other until they form somewhat straight grain-boundary instead of coalescing. We observed that at about 34 ◦ C, when the spirals are within a range of several stripes of each other, target domains nucleate between the spirals and the giant spirals coexist with smaller target domains. It is interesting to note in the second picture from the right in Fig.4.3 that when the target patterns nucleate between two spirals, 3 to 4 stripes at the outer boundary of each of the large domains melt away, coarse and then form one wider stripe. Here too, no coalescence occurs and both patterns coexist even when the large domains are touching each other. Domains at higher temperatures but below Tm, are made of target patterns with fewer dislocations. If we take a closer look at the stripes in the intermediate temperature regime, between spirals and targets, the individual stripes are clearly themselves made of smaller spirals similar to the cat’s claw or spira mirabilis. This substructure is clearly displayed in Fig.4.4. Moreover, with the higher laser power and better resolution these substructures look more prominent as shown in Fig.3.37.

The domains beyond Tm are made of branched stripes that originate from the center of the domains. These domains look like a Chinese silk-fan as shown in second picture from right in

Fig.3.22 (at 38.5 ◦ C). We consider this branched pattern is elastically costly since its energy is linear with the size of the domain while the energy of a spiral scales like the logarithm of the domain size [24]. It is also possible that beyond Tm any geometrical interactions are destroyed and the system assume a conﬁguration when it maximize wall-boundaries. Probably this could be the 108

Figure 4.4: An example of target pattern where every stripe is made of smaller logarithmic spirals or spira mirabiles at a temperature of 35 ◦ C. The scale bar = 1mm. reason for low surface pressure (Fig.3.32) at which this pattern nucleate . To answer the question why such giant spirals nucleate, Dr. Chaib has purposed a possible model as discussed in the next section.

4.4 A possible model for the formation of giant spiral patterns

The lipid used in our study is strongly chiral. They are three order of magnitude more chiral than the next most chiral [28, 193] than the widely studied chiral lipid DPPC [18, 194]. When these highly chiral molecules tend to sit together, the situation can be thought of as two screws being forced to align so that the grooves should ﬁt like “hand-in-glove” where they would adjust at an angle relative to each other. This angle of ﬁt is given by the pitch of the grooves. If we squeeze many such objects we obtain a macroscopic chiral object whose pitch (refer to Fig.4.5) is given by the degree of molecular chirality.

We consider that below Tm the lipid molecules are aligned in a smectic C manner. When they are squeezed by the effect of the moving barriers they are forced to close-pack. However, the 109

Figure 4.5: An illustration of chiral molecules and chiral pitch in 3-d [26]. intrinsic nature of this molecules forces to give rise to an edge dislocation as a defect induced by the chirality of the molecules. As in other lipids, when the domains nucleate the boundary conditions given by the shape of the domain forces the stripes to align in the azimuthal direction and a spiral form. At higher temperatures the molecules preferably oriented in a radial fashion and targets are formed. The molecular director in this conﬁguration are pointing radially outward. One other possible explanation is that we might have frustrated buckling of the monolayer. To illustrate the transition of such a system, we used the Frank elastic free energy of liquid crystals (LC) with a chiral term [24, 66] as,

Z 1 1 1 1 F = d 2r − t |~n|2 + u |~n|4 + K (∇~ · ~n)2 + K (∇~ × ~n)2 − h |~n|2 zˆ · ∇~ × ~n 2 4 2 1 2 3 (4.1)

The ﬁrst two terms are the Ginzburg-Landau expansion in powers of ~n. Vector ~n is the molecular tilt projected onto the layer plane. These lipid molecules have strong head-tail asymmetry and may splay when compressed. The third term accounts for the compensation for this splay; K1 is the splay elastic constant. The fourth term is the cost of bending when spirals form with K3 as the 110

Figure 4.6: An illustration splay and bend. K1 and K3 are the elastic constants corresponding to the splay and bend deformations respectively. Courtesy [27]

Figure 4.7: Spiral pattens as a function of the boundary conditions of the director. The position of the director at inﬁnity and at the center is shown as inset at the corner of each picture [28]. bending elastic constant (refer to Fig.4.6).

The last term is a chiral term. In order to account for the head-tail asymmetry that affect the molecular chirality where the term |~n|2 couples variation in the magnitude of ~n with the variation in the

orientation, the last term is written in this fashion. We will assume a spatial periodic modulations of

the director ~n and solve the Euler-Langrange (EL) equations for this free energy [28, 193]. In order

to solve the EL equations, boundary conditions are imposed at the origin and at inﬁnity. Fig.4.7

illustrates the loci of the director in space as a function of the boundary conditions.

In Eq.4.1, if the derivatives of ~n are zero (i.e., if ∇~ · ~n = 0 and ∇~ × ~n = 0), then we can write

|~n| = (t/n)1/2; the change of free-energy with the order parameter (|~n|) is as shown in Fig.4.8.

To understand the selection of the width of the stripes in the spiral, we consider that chiral term

balances the surface pressure so that 111

Figure 4.8: Change of free-energy with the order parameter (|~n|).

h |~n|2 zˆ · ∇~ × ~n ∼ Π

The height modulations between different stripes were measured by using Atomic Force Microscopy

(AFM) and found that the height ζ is of the order of 6-10nm (Fig.4.9).

This allows the use of a long wave approximation to perform the scaling since the stripe width is of the order of hundreds of microns. In order to proceed, the following approximation are made:

n ≈ ∇ζ to n ∼ ζ/λ where λ is the width of the stripe. For small tilt angle (Fig.4.11), similar to the ones at the crest/trough of a buckled surface (Fig.4.10a), we can write,

dy = tan θ dx ≈ sin θ ζ ≈ λ

With these approximations, now we can write,

ζ 2 ζ h ∇ · ∼ π λ λ 112

(a)

(b)

Figure 4.9: Atomic force microscope (AFM) measurements of Langmuir ﬁlm of DIYNE PE transferred onto a thin mica sheet. Data recorded at two different scan sizes (a) AFM image. Scan size = 5.000µm, scan size = 8.000µm (b) AFM image. Scan size = 8.000µm, scan size = 5.000µm. 113

(a) (b)

Figure 4.10: Two possible models of the tilt orientation with respect to the film surface in a Languir film: (a) the arrow shows the tilt direction along the buckled surface trying to keep the tilt with respect to the film surface (b) when splay elasticity is very large the director always tends to splay.

Figure 4.11: An illustration of projection of director on the ﬁlm surface. N is the surface normal, ~t is the director, ~n is the projection of the director onto the ﬁlm surface, and θ is the tilt angle of the director with respect to the surface normal.

π λ4 which implies that h ∼ ζ3 Since we have stripes, we have to take into account the domain walls and the energy cost of their formation. This is given by the balance of the ﬁrst term and the third term of Eq.4.1 which will deﬁne domain-wall width of the form

t2 E ∼ ξ wall u

In order to ﬁnd the stripe width he balanced the energies that account for splay, chirality and domain- wall. That means,

K1 ≈ h λ − Ewall λ (4.2)

With the approximations made above, Eq.4.2 modiﬁes as

π t2 ξ K ≈ λ5 − λ (4.3) 1 ζ3 u We thus see that in this model the surface pressure and the characteristic wavelength are related as

K ζ3 t2 ξ ζ3 1 π ≈ 1 + (4.4) λ5 u λ4 114

If we set

3 K1 ζ = A and t2 ξ ζ3 = B u then Eq.(4.4) becomes,

A B π ≈ + (4.5) λ5 λ4

We tested this equation by ﬁtting the data of stripe width (λ) versus the surface pressure. This model seemed to work (Fig.4.12a) at a particular temperature (34 ◦ C). However, we were not able to see the same trend at other temperatures (refer to Fig.4.12c as an example).

The parameters in the ﬁt should correspond to physical values. However, using the ﬁt parameters we have found that the domain wall width Ewall and K1 to be negative; which is unphysical (Table

4.1).

Table 4.1: Fitting with arbitrary parameters A and B:

◦ 3 2 T( C) T(K) ζ (m) A (Jm ) B(Jm ) K1(J) Ewall(J) 30 303 1.00E-08 -3.04E-20 4.70E-16 -3.04E+04 4.70E+08 30 303 1.00E-08 -2.54E-19 2.70E-15 -2.54E+05 2.70E+09 30 303 1.00E-08 -8.14E-20 5.73E-16 -8.14E+04 5.73E+08 34 307 1.00E-08 1.83E-20 -4.18E-17 1.83E+04 -4.18E+07

In the table 4.1 note that K1 as calculated from A assuming ζ = 10nm tends to be negative, which is not meaningful; this unphysical number is probably due to the very limited range of λ.

Therefore, A and B are highly correlated and are not giving meaningful results. Because of very narrow range of λ, A and B are highly correlated and cannot give meaningful results. So, we tried

−43 setting A at a physically reasonable value K1 = 50 kB T , ζ = 10nm, so that A = 2.09 × 10 then found B in the range from 5 × 10−17 to 2 × 10−15.

If we set ζ = 10nm A K = = 2.09 × 10−19J 1 ζ3 115

(a) (b)

Figure 4.12: The wavelength of the pattern λ(mm) which is the distance between the stripe of the same gray value as a function of the surface pressure π(mN/m) corresponding to the nucleation of the pattern. The solid circles are the measured data and the solid line is the fit of Equation 4.5. (a) Change of characteristic length with surface pressure at 34 ◦ C. Solid circles are the measured data and the solid line is the fit of Equation 4.5 (b) Power law fit of change of characteristic length (λ) with surface pressure (π) at 34 ◦ C. Both x and y axis are the log scale (c) Change of characteristic length with surface pressure at 31 ◦ C. Solid circles are the measured data and the solid line is the fit of Equation 4.5 (d) Power law fit of change of characteristic length (λ) with surface pressure (π) at 31 ◦ C. Both x and y axis are the log scale. 116

and B E = = 4.70 × 108J wall ζ3

which is not physical either. We tried one more thing; modeling with the physically reasonable

−2 values ζ = 10nm, K1 = 50kBT and Ewall = 1pN. We found π ∼ 10 N/m, λ ∼ 10nm while

for the experimental λ ∼ 1mm, we found the unphysical π ∼ 10−23N/m. We also tested with

λ ∼ 100nm, which is close to the length of internal substructure. With this value, we found that

that π ∼ 10−7N/m; an unphysical surface pressures within our experimental regime. This behavior

is suggesting that the buckling instability of the Langmuir ﬁlms may have some role to play in the

pattern formation, but not the major one.

As discussed earlier of this section, the variation of ~n is related to the tilt of the molecules with

respect to the surface normal. Because of the strong head-tail asymmetry of the molecules, they

tend to tilt in order to make the effective area occupied by the hydrocarbon tails to be equal to

the effective area of the head-group. With a BAM, we can estimate the tilt of the molecules in a

Langmuir film by measuring the the change of gray value profile in the film at different analyzer

positions with respect to the polarizer as shown in Fig.4.13. These proﬁles are taken from Fig.3.29 in

two mutually perpendicular directions. Fig.4.13a is the gray value proﬁle along horizontal direction

and Fig.4.13b is the gray value proﬁle along vertical direction. Note that the brightest areas in 90 ◦

correspond to darkest in 0 ◦, but almost half as high as brightest. This tells about unusual rotation of

polarization for such a thin ﬁlm: up to almost 90 ◦. Despite the fact that we can see different trend

of gray value proﬁle at 0 ◦ and 90 ◦ analyzer position, it is not obvious that we are seeing the change

of gray value due to the change of molecular tilt.

We also measured a critical pressure (Πc) as a function of the deviation from the melting tem-

perature (Tm-T) and found that they are related by a power (scaling) law as shown in Fig.4.14. This

would be expected of a second order transition. This suggests that Tm may be a critical temper-

ature where topological defects melt away. Remember, however that the pattern formation itself

appears to nucleate at critical pressure for each constant-temperature run. Additionally, patterns do 117

(a) (b)

Figure 4.13: Gray value proﬁle in Fig.3.29: (a) taken horizontally. A: when the analyzer and the polarizer are parallel to each other, B: when the analyzer and the polarizer are crossed (b) taken vertically. C: when the analyzer and the polarizer are parallel to each other, D: when the analyzer and the polarizer are crossed. Both in (a) and (b) the gray values are moved up by 100 for clarity. The higher the gray value the brighter is the region.

not change on heating until they melt above Tm.

4.5 Conclusion

In Langmuir ﬁlms made of kinked and rigid chiral lipid molecules, we discovered that pattern

◦ formation takes place in the temperature range of 28 C to below Tm. However, well-deﬁned patterns were observed between 30-36 ◦ C. We discovered that the patterns undergo a transition from giant spirals to targets to branched stripes. We are not in a position to pin down the exact mechanism behind these pattern formation. However, the formation of spiral, similar to the vortices can be due to the competition among strong chirality of the lipid induced by the appearance of the defects and dislocations, boundary conditions at the center of the defects, and to some extent the buckling instability. Some of the mechanisms, such as role of the boundary conditions, are very subtle to observed. Observable change of contrast across the spiral patterns can indicate toward the strong role of such boundary conditions, but in our experiments we did not observe such strong changes in the contrast across the boundary of the patterns within the limit of our instrument. It is possible that we are missing something here, that might be possible to observe with systematic 118

Figure 4.14: Critical surface pressure at which the pattern begins forming as a function of (Tm −T ). −0.71 The best ﬁt to the data is represented by a power law Πc ∼ (Tm − T ) . The solid circles are the measured data, error bars are the estimated errors in the temperature measurements, and the solid line is the power law ﬁt. studies using the recent big improvement in the resolution of the instrument.

The transition from spirals to targets and to the branched structures could be due to the disappearance of chirality. Similarly, it is certain that the chiralilty of these molecules is playing some role for the formation of these patterns, but we are uncertain about how big a role they are playing in this game of pattern formation. It can be a good future work to perform the same types of experiments with lipid molecules with kink just in one chain and another lipid molecule with no kink at all in both of the chains as opposed to our molecule having kinks in both chains for the comparison sake.

In order to pin down the mechanism of pattern formation more experimental work is needed. As outlined in Sec.3.5, surface light scattering can be a very useful tool to delve into the role of elastic buckling. Work has already been started in this direction3. I have trained students in working with layer. A better understanding of the change in gray values across the boundary of the neighboring substructures is at crux of the theory that we have purposed in Sec.4.4. Further improvement in the

3Prof. Dr. J. A. Mann Jr.’s group at Case Western University, Department of Chemical Engineering 119

BAM resolution and a better way of measuring the boundary gray values can be another important step in this direction. The current resolution of our BAM is of the order of 1 pixel = 1.25 microns, which could be improved to ∼ 0.5 microns with further improvements and very careful optical arrangements. It is possible that we are still missing something here, but might be possible to observe with more improvements in the resolution of the instrument than the current one. The evolving data needs to be analyzed with possibly a new and developed methods of analysis. CHAPTER 5

CHARACTERIZATION OF LANGMUIR FILMS MADE OF BENT-COREMOLECULESWITH

HYDROPHILICENDGROUP

5.1 Introduction

Bent-core liquid crystals, sometimes knows as banana liquid crystals, have drawn increasing attention by the richness in phases that they exhibit. Due to the unique coupling between dipole properties and the packing constraints placed by the bent shape, these molecules are emerging as strong candidates for practical applications [195] such as electromechanical [196] display devices or in different kinds of organic devices [197]. However, most applications require that the molecules be aligned, which has proved difﬁcult. Our group has tested such molecules both as Langmuir layers and, transferred to a solid, as alignment layers with some limited success. However, due to the shape, the relatively low hydrophilicity of the core, and the presence of the two hydrophobic end groups of the molecule, these molecules do not behave well at the air/water interface. They tend to form ill-controlled multilayer structures since attraction with the surfaces is relatively weak. In my present work, I have used a new type of bent-core molecules with hydrophilic group at one end. We expect this molecule to form more stable layers on the surface because of the stronger attraction of the hydrophilic group towards the surface than for the bent-core molecules without the hydrophilic end group.

In this chapter ﬁrst I will present a short introduction of liquid crystals (LC) and then I will give a brief overview of bent-core liquid crystals; also known as banana liquid crystals. Then I will discuss the special type of bent-core liquid crystal studied here, along with the experimental methods used to study its surface properties, followed by the discussion of the results.

120 121

5.1.1 Liquid crystals - a short introduction

We all are cognizant of the fact that there are different states of matter; the most common ones are the solid, liquid, and gas. It is the molecular orientation and organization that differentiates crystals from liquids. The fundamental difference is that the molecules in a crystal are ordered whereas in a liquid they are not. Crystals possess both positional and orientational ordererings. Contrary to this, the molecules in liquids diffuse randomly throughout the sample container with the molecular axes tumbling wildly [26]. The states of matter whose symmetric and mechanical properties lies between those of a crystalline solid and an isotropic liquid are known as liquid crystals (LC)

[198–202].

Some molecular materials, in which the building blocks are anisotropic entities, exhibit more complex phase sequences. If they are heated from the solid phase (Fig.5.1a) at the melting point, three possibilities exist that, either both positional and orientational order disappear simultaneously and a new phase will evolve as a “isotropic liquid” phase as shown in figure 5.1b or only orientational order disappears leaving the positional order unchanged and the phase is called a “plastic crystal” (PC) as shown in figure 5.1c. The third possibility is that positional order either fully or partially disappears while some degree of orientational order is maintained. The new phase thus obtained is called a “liquid crystal” (LC) phase, sometimes also referred to as the mesophase (intermediate phase) [198–202] as shown in figure 5.1d. In this phase, each of the molecules has a tendency to align itself along a specific direction defined by the unit vector nˆ which is known as the

“director”.

The liquid crystal phase possesses hybrid characteristics as far as its positional order and orientational order is concerned. The molecules in liquid crystal phases diffuse much like the molecules in a liquid, but they maintain some degree of orientational order and sometimes some positional order also. The amount of order in a liquid is quite small as compared to a crystal [203].

Generally, liquid crystalline materials may have varieties of molecular structures. However, all of them are anisotropic. In most situations, either their shape is such that one axis is very different 122

(a) (b)

Figure 5.1: Orientations and organizations of molecules in various phases: (a) Crystalline solid (b) Isotropic liquid (c) Plastic crystal (d) Liquid crystal [26]. 123

Figure 5.2: Schematic of geometrical structures of liquid crystalline molecules [26].(a) A rod-like molecule (b) A disk-like molecule (c) A lathe-like molecule (d) A bent-core molecule. Note, without the ﬂexible end chains, they are not liquid enough. from the other two or, in some cases, different parts of the molecules have very different solubility properties. There are several types of liquid crystals depending on their molecular structures. The liquid crystals derived from the rod-like molecules are called “calamitics” (Fig.5.2a). The mesogenic molecules must have some degree of rigidity at least some portion of its length in order to maintain elongated shape for producing interactions that favor alignment. There are liquid crystals which are formed from the disk-shape molecules; these molecules are known as “discotics” [198–

202, 204–206] as shown in ﬁgure 5.2b. Here too, the rigidity in the central part of the molecules is essential. The are molecules which are in between the rod-like and the disk-like molecules which are known as “lathe-like” as shown in Fig.5.2c. Note that the bent-core molecule has 3 independent axes, as does the lathe-like molecule, but that the bent-core puts additional constraints on close packing. Also note that if the bent core molecule could rotate freely along its long axis, it would be essentially equivalent to the rod-like molecule.

5.1.2 Formation of Liquid crystals

Formation of liquid crystals involves phase transitions. As discussed in the earlier section, the intermediate phases between a crystalline solid and an isotropic liquid are the liquid crystalline phases also referred to as mesophases (the molecule is known as the mesogen). Mesophase transitions can be brought about either by the inﬂuence of solvents or by purely thermal process . The liquid crystals obtained by the inﬂuence of solvent are called “lyotropics” and those obtained by the method of thermal process are known as “thermotropics”. 124

5.2 Lyotropic liquid crystals

Lyotropic liquid crystal molecules can be obtained by combining a hydrophobic group at one end and a hydrophylic group at the other end. The molecules thus formed is known as an amphiphiles. Different varieties of phospholipids and soaps are good examples of such molecules.

These molecules are capable of forming ordered structures in both polar and non-polar solvents.

When they are dissolved in a polar solvent such as water, the hydrophobic tails aggregate together to form hydrophilic heads to the solvent. The resulting structure for soap molecules is a micelle and that of phospholipids is vesicle 3.3 as discussed in 3.1. It is usually non-spherical phases that are considered to order in LC phases: cylindrical micelles, lamillar phases, cubic phases, etc.

There are many other forms of liquid crystals. A Langmuir layer can itself be considered a lyoptropic liquid crystal, with many different forms of order in the plane and no, or greatly reduced, order perpendicular to it. Biological membranes and cell membranes are another form of liquid crystals. Many other biological structures exhibit LC behaviour. For instance, the concentrated protein solution that is extruded by a spider to generate silk is, in fact, a liquid crystal phase. The precise ordering of molecules in silk is critical to its renowned strength. DNA and many polypeptides can also form LC phases [202] and this too forms an important part of current academic research. Detail discussion of these liquid crystals is beyoned the scope my present work, but can be found in the cited references if interested.

5.3 Thermotropic liquid crystals

Thermotropic liquid crystals are not diluted by a solvent. Thermotropic phases exist within a certain range of temperatures. When subjected to higher temperature, their cooperative liquid crystal phase (LC) will be destroyed by thermal ﬂuctuations. This will push the material to a conventional isotropic liquid phase. At different ranges of temperatures a gamut of thermotropic liquid crystals exist. For instance, a particular mesogen may demonstrate various smectic and nematic phases as temperature is increased. Again, at considerably higher temperature they will become isotropic 125

Figure 5.3: An illustration of ordering of molecules in nematic phase [29]. liquids. An example of a compound displaying thermotropic LC behavior is 4-octyl[1,1-biphenyl]-

4-carbonitrile (8CB) [207].

Technical applications of liquid crystal materials are greatly influenced by their long-scale molecular alignment in their thin films. Depending on application, we may also want molecules parallel, perpendicular, or at some angle in between in the film. This can be achieved if we can align them to surfaces, since alignment is transmitted over fairly long scales in the bulk. Most of these molecules can be fairly easily aligned at the surface by a combination of physical and chemical interactions.

5.3.1 Nematic Phase

This is one of the most common phases of liquid crystals. In this phase, the molecules do not possess positional ordering, but they do have a long range of orientational order. The molecules can ﬂow with their center of mass randomly distributed as in an isotropic liquid, but within a domain the molecules point in the same direction [29]. Uniaxiality is another salient feature of most nematic liquid crystals, with rotational symmetry around a single axis. This situation can be best approximated as a cylinder.

As mentioned earlier in this section, nematics can flow almost like a isotropic liquids. However, despite their fluidity, they can be oriented in a particular direction without much difficulty by the 126

Figure 5.4: An illustration of layer formation of molecules in smectic phase [29]. (a) Smectic A phase (b) Smectic C phase. application of electric or magnetic ﬁelds. The essence of the nematic lies in the fact that they acquire the optical properties of uniaxial crystals when they are aligned. This has made them useful material in the liquid crystal display devices, although more complex phases are most often utilized.

5.3.2 Smectic phase

When the crystalline order is lost in only two dimensions, rather than all three, one obtains stacks of two-dimensional liquids (Fig. 5.4), which can slide over one another: such systems are called smectics. These phases are found at a lower temperature than the nematic phases and are positionally ordered along one direction.

5.4 Bent-core liquid crystals

Bent core molecules are also referred to as banana-shaped molecules because of their unique shape somewhat similar to a banana. A typical bent core molecule consists of amphiphilic phenyl rings, hydrophobic hydrocarbon end-chain/chains or/and hydrophilic end groups. One of the salient features of thermotropic molecules is that they consist of a substantially rigid core, often due to phenyl rings, and a sufﬁciently long and ﬂexible end chains. This rigid core is often supplied by a series of phenyl rings joined more or less rigidly. The number of phenyl rings can vary from one bent-core molecule to the other (Figures 5.5 and 5.6). The rigid core will contribute to align the 127

Figure 5.5: Bent-core molecule with 5 phenyl ring. ‘R’ is the end chain functional group [30] molecules upon entropy minimization, whereas the long chains tend to fluidize the dense-packed states [31]. In the case of bent-core molecules, the core may take a number of different non-planar chiral configurations, but on average they are planar and bent in isolation [30]. The molecular packing and the mobility of these molecules are largely restricted by their unique molecular structure; these constraints ultimately give rise to a large varieties of phases. These molecules can form many liquid-crystalline [208, 209] including a large number of columnar and smectic phases [30]. These phases have been found to have switchable ferroelectric and anti-ferroelectric structures where the switching time may be as short as a milliseconds. Structures of bent-core molecules remain intriguing in view of their applications as a organic functional materials. Generally, thin films of these material are used as active layers in the devices such as in the scattering switching, in storage devices and liquid crystal display.

As discussed in Sec.5.1, these molecules are not easily aligned by a surface, unlike most other liquid crystalline molecules. The reason for this may be attributed mainly to their complex shape and their inability to form nematic phases above the smectic ones. Conventional alignment layers such as rubbed polyimide [210] are not effective. We can ﬁnd in literature applications such as switching between transparent and scattering [211] where alignment is irrelevant. However, the lack of alignment over large regions limits the practical application of bent core molecules.

Chirality (Sec.1) is another interesting feature associated with the bent-core molecules. It has not been fully understood if the chirality is conformational or structural in origin. The chirality of bent-core molecules may arise from various reasons. In the smectic phase, packing of the bent-core 128

Figure 5.6: Bent-core molecule with 5 phenyl ring with chlorine atoms attached to the central phenyl ring. ‘R’ is the end chain functional group [30] molecules leads to polar ordering, which may lead achiral molecules to behave as a chiral molecules and also giving rise to ferro- or anti-ferroelectricity [212].

In order to utilize their potential fully, studies of growth mechanism, molecular ordering, and the overall ﬁlm morphology are extremely important for device design manufacturing. Theoretical

[213] as well as experimental [214, 215] work has already been done in this direction. The work has revealed the fact that the phases exhibited by the bent-core molecules depend greatly on the bent-core angle, especially at a surface. However, the mystery of bent-core molecules has yet to be solved fully, in particular regarding their structures.

5.5 Langmuir ﬁlms of bent-core molecules

The study of Langmuir films can give first hand information about the molecular packing within layers. A stable Langmuir layer, transferred to a solid interface, may form a natural alignment layer for bent core liquid crystals. We cannot find much work on Langmuir films made of bent core molecules. However, a literature survey suggests that significant progress is being made in this direction. One such work [216] considers bent core molecules with similar hydrocarbon end chains. The reported results revealed the fact that the bent core molecules can form Langmuir films.

The surface pressure isotherms results also suggested that the film thus formed were more than one molecule thick at even the lowest measurable pressures. The other earlier work considers two different types of cores with very short hydrophobic side chains [217, 218]. X-ray reflectivity and the second-harmonic generation results showed that the Langmuir-Blodgett film of these molecules 129

formed simple, well-aligned monolayers.

A fair amount of work on the Langmuir layers made of bent core molecules has already been done in our lab. One such study considers the different possible layer structures at the interface.

In this work both the end chains (siloxane vs hydrocarbon) and the core (more or less amphiphilic) were varied (Fig.5.7a) and then the film was characterized by using the techniques of surface pressure isotherms, Brewster angle microscopy, and surface potential measurements [31]. The layers were reported to be strongly dependent on the end chains of molecule: the molecules with amphiphilic end chains lay quite flat on the surface, while the molecules with hydrophobic end chains constructed multilayer structures. In both cases, the three-dimensional collapse structure was at least partially reversible. The other project considered the Langmuir layers of a symmetric bent-core molecule with hydrocarbon end chains and two chlorine atoms substituted on the central phenyl ring of the bent core [32], as shown in Fig.5.7b. These layers were found to be optically anisotropic, in contrast to Langmuir layers of similar molecules with different substitutions on the core. It was reported that after compression, the orientation of the optical axis was essentially uniform over the film, whereas upon decompression, the film broke into uniform islands or domains.

Optically, the ﬁlms were reported to be strongly anisotropic, and the reported results indicated that near 90◦ tilt angle, the bent-core molecules lie quite ﬂat on the surface. Mesostructures in inversed

Langmuir/Schaefer ﬁlms of each of two contrasting bent-core molecules have also been studied via

AFM [219]. In this study, one of the molecules formed optically isotropic and the other optically anisotropic Langmuir ﬁlms. Both showed mesostructures in the 100 nm range, in spite of appearing optically uniform.

5.6 Materials and methods

In this section ﬁrst I will introduce the material that I have used in my current research work then I will give a brief over view of the experimental techniques. 130

(a)

(b)

Figure 5.7: Different kinds of bent core molecules: (a) Bent core molecules with varied end groups and core [31, 32]. (1) The bent-core molecules with hydrocarbon end chains R1 and core substituent R2. (2,3) Formulas for the siloxane end-chain molecules, Bc2-SiO and Bc3-SiO, respectively. (b) Bent core molecule with similar hydrocarbon end chains and chlorine substituted core, used in reference [32] and simulated in reference [33] 131

Figure 5.8: Molecular structure of bent-core liquid crystal with hydrophilic group at one end and hydrocarbon chain at the other end (called Z2b by the group that did the synthesis [refer to the footnote]). There are 5 phenyl ring in this bent-core molecule.

5.6.1 Materials

The sample material made of bent-core molecules were obtained from Prof. Dr. C. Tschierske1.

The structure of the molecule is as shown in Fig. 5.8.

As we can see in Fig.5.8, one of the contrasting features of this molecule from the previous ones (Fig.5.7)is the different end groups. One of the end groups is a hydrophobic hydrocarbon chain and the other terminates in a hydrophilic COOH group at the end of four spacer CH2 groups.

The chain containing the hydrophobic end group is longer than the one containing the hydrophilic end group, making the molecule asymmetric. The phenyl rings shown in Fig.5.8 are, as with the earlier molecules, weakly amphiphilic. Overall, this molecule should be more strongly amphiphilic, attaching more strongly to the air/water interface because of the additional hydrophilic group. The transition temperature (solid to liquid) of these molecules is 144 ◦ C – 146 ◦ C [220]. The molecular charge, and thus hydrophilicity of these molecules can probably be tuned by changing the pH of the subphase, but I did all experiments at the natural pH of water in the presence of carbon dioxide (∼

5.5). The bent core material was spread in solution with chloroform (Sigma-Aldrich, A.C.S, HPLC grade).

1Department of Chemistry, Institute of Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes- Str 2, D-06120 Halle/Saale, Germany 132

5.6.2 Method

In order to study the properties of Langmuir ﬁlms of these bent core molecules with both hydrophobic and hydrophilic end groups, I prepared the Langmuir ﬁlms at the air/water interface in a Langmuir trough as described in Section 2.5.3.1. The water used as the substrate and for the cleaning purpose was obtained from Purelab Plus UV system (US Filter, resistivity 18.2 MΩ-cm).

Surface pressure measurements coupled with the Brewster angle microscopy (BAM) were used to investigate the molecular and optical properties of this bent-core molecules at the air/water interface. The compression and decompression of the ﬁlm was carried out at constant barrier speed of

5 millimeters per minute. The rate of increase or decrease of average area of the ﬁlm available for each molecule ranges from about 2.0 × 10−3 nm2/molecule to about 2.0 × 10−5 nm2/molecule,

depending upon the initial concentrations of the ﬁlms. Please refer to Chapter 2 for the details of

BAM and the techniques of surface pressure measurements. I also investigated the optical proper-

ties of the Langmuir ﬁlms formed by these bent-core molecule. The techniques involve the in-plane

change of domain orientation. This was accomplished by turning the domains manually by stirring

the subphase with a thin platinum wire, so that any optical axis of the domains rotates with respect

to the polarization of the incoming light and to the analyzer for the reﬂected light.

5.7 Results and discussion

I studied the isotherms of the Langmuir ﬁlms constantly monitored with the Brewster angle

microscope (BAM). A BAM allows visualization of the Langmuir ﬁlm and in particular any phase

separation including three-dimensional collapse. Similarly, qualitative information of phase transi-

tions [3] can also be deduced from a surface pressure area isotherm. In this way, BAM and surface

pressure measurement complement each other.

5.7.1 Molecular conﬁgurations and isotherms

With earlier molecules, with purely hydrophobic end chains, the observed layers were unsta-

ble and depended strongly on the history of layer. No pressure was observed until the surface was 133

covered with multilayers, but thinner layers, at approximated zero pressure, had been produced by starting at lower concentrations. Note that with the current apparatus, the surface concentration can only be reduced by a factor of 5, which means that the whole range of concentrations cannot be attained in one experiment. Keeping this in view, isotherms were obtained for different initial surface concentrations (i.e., the mean molecular area before compression/decompression cycles begin). Ini- tial concentrations ranged very dilute (∼ 7nm2/molecule) to very high (∼ 0.4nm2/molecule). Note that the higher the mean molecular area (nm2/molecule), the lower is the surface concentration. With the initial surface concentration of ∼ 7nm2/molecule, and compressing to a minimum of about 1.3 nm2/molecule, we did not observe any appreciable change in the surface pressure and practically remained zero throughout the compression, decompression and recompression cycles. By considering the minimum thickness of a monolayer to be of the order of the phenyl ring width., we can estimate the minimum surface concentration required to form a continuous monolayer to be ∼ 2.3

nm2/molecule for this type of bent core molecules. So, it is obvious by this fact that the initial sur-

face concentration of 7 nm2/molecule is slightly small to be able to form a continuous monolayer

and hence give rise to any change in surface pressure in compressing/decompressing. However, with

increasing initial surface concentration, substantial increases in surface pressures were observed as

shown in Fig.5.9. We did not observe obvious three-dimensional collapse, indicated by a sudden

drop of surface pressure, even when the layer was compressed to well up to the maximum avail-

able limit of the mean molecular area as shown in Fig. 5.9d. Even after compression stopped, no

such sudden drop in surface pressure was observed, unlike on previous work on other bent core

molecules. This molecule forms a much more stable layer.

Figure 5.10 shows compression and decompression isotherms of bent core molecules with hy-

drophobic end group at both ends and chlorine core substitutions. Comparing with ours (e.g.,

Fig.5.9d), we can see that when starting at concentrations needed for earlier earlier hydrocarbon

chains, the pressure rises almost immediately.

The lower pressures seen with lower initial concentrations approach the limits of the resolution 134

(a) (b)

Figure 5.9: Surface pressure isotherms of Langmuir films made of bent-core molecules with a hydrophilic end group. σ is the mean molecular area, i.e., the average area available for each molecule in the film, π is the surface pressure and σ0 is the initial surface concentration of the Langmuir film. All the measurements were carried out at the room temperature (18 ◦ C. ). (a) π-σ 2 isotherm with σ0 = 3.5 nm /molecule. The solid line is the guide to the eyes, (b) π-σ isotherm 2 with σ0 = 1.7 nm /molecule, solid squares correspond to the compression and circles represent 2 decompression (c) π-σ isotherm with σ0 = 0.86 nm /molecule (d) π-σ isotherm with σ0 = 0.40 nm2/molecule. In (c) and (d) ↑ and ↓ indicate the directions of compressions and decompressions respectively.

Figure 5.10: Representative isotherm for Bc-2Cl [32]. 135

Figure 5.11: A BAM image of Langmuir ﬁlm of Z2b molecules showing compression, decompression and recompression cycles. The rightmost and the leftmost pictures are taken at the mean molecular area of 0.75nm2/molecule. The middle image was taken when the mean molecular area was increased by 50% from that of the rightmost image. The somewhat pebbly texture of the partial ﬁlm is similar in all cases. The scale bar corresponds to 1mm. of our instrument, which is approximately ±0.02 mN/m due to both the electronics and vibrations.

Fluctuations at this level obscure the isotherms trends in this region as shown in Fig. 5.9a. The further we decrease the initial surface concentration, the more difﬁcult it becomes to see a trend of the isotherms because of the limitations of the resolution of our instrument. However, I found almost no hysteresis during compression, decompression and recompression in dilute Langmuir ﬁlms

(Fig. 5.9b and Fig. 5.9c), which suggest that molecular configuration changes in the compression, decompression and recompression cycles are reversible in these dilute films. The hysteresis result is also supported by the BAM images as shown in Fig. 5.11. At higher concentration, however, the reproducibility of the Langmuir films properties in compression/recompression cycles was less and a large hysteresis was also seen in the compression/recompression cycles of isotherms (Fig.

5.9d). A possible reason for the large hysteresis is permanent change in the molecular conﬁguration caused by applied surface pressure. Here it is worthwhile to note that the 1mN/m of surface pressure is equivalent to about 10 Atm of its three-dimensional counter part applied in the longitudinal direction of the ﬁlm.

Figure 5.12 shows all the compression isotherms for different initial surface concentrations in the same scale. The surface concentration decreases going from A to E isotherms. It is obvious from 136

Figure 5.12: Compression isotherms for different initial surface concentrations A: 0.34 nm2/molecule, B: 0.4 nm2/molecule, C: 0.86 nm2/molecule, D: 1.37 nm2/molecule and E: 1.75 nm2/molecule (not shown in the graph) are the coareas at which at which isotherms begin to rise. The rise of isotherms is configuration dependent which ultimately depends on surface concentrations. the graph that the lift-off of the isotherm is dependent on the surface concentration of the films. This also tells us that molecular configuration changes with the initial concentration of the film on the water suphase.

5.7.2 Thicker ﬁlms

The surface pressure isotherms can give first hand idea about the molecular configurations in the film. Our bent-core molecule has a hydrophobic hydrocarbon chain at one end and a hydorphilic group at the other end. So, we expect that our molecule behaves differently in the water surface than the molecules with both end group hydrophobic or amphiphilic [31]. It is our expectation that the hydrophilic end of the molecules remains in contact with the water surface, while the other chain is free to leave the surface. Another possible configuration can be thought of as the entire portion of the molecule containing the amphiphilic groups remaining on the water surface and the hydrocarbon chain on average above it. These possible configurations have been shown in Fig.5.13. 137

(a) (b)

Figure 5.13: Schemata of possible conﬁgurations of Z2b bent core molecules in water surface (a) Entire hydrophilic portion of the molecules remains in contact with water surface (b) One end of the molecules in contact with water surface and the other one free to leave it (c) Upright position of the molecules; the end of the hydrophilic group on the water surface and the hydrophobic chain above it (d) Molecules lying ﬂat at the air/water interface. The hydrophobic (red) chain is longer than the the hydrophilic (blue) chain.

As in 3d, gas/solid-like phase coexistence is possible and often observed in 2-d monolayers, including for hydrophobic bent-core [3, 36, 156, 207, 221, 222] molecules. An example of such a phase coexistence is shown in Fig. 5.14. The BAM images in Fig. 5.14 shows islands of uniform thickness with sharp corners. These solid like island are probably surrounded by a dilute gas.

Fig.5.15 is a BAM image of the dilute ﬁlm (∼ 7nm2/molecule) made of our bent-core molecules.

You can see bright domains with sharp edges in the the dark background. The bright domains with

sharp edges are more like solid whereas the dark region could be a gaseous phase. If we compare

Figure 5.14: Phase behaviors of Langmuir monolayers of different kinds of bent-core molecules at very dilute surface concentrations at air/water interface. (a) Bc-H, σ ∼ 5.5nm2/molecule; (b) Bc-NO2, σ ∼ 12 nm2/molecule; (c) Bc3-SiO, σ ∼ 4nm2/molecule. Scale bars correspond to 1 mm. From [31]. 138

Figure 5.15: BAM image of Langmuir ﬁlm at ∼ 7 nm2/molecule of surface concentration. The scale bar corresponds to 1mm. these features with the Fig.5.14, we can infer a possibility of a solid-gas phase coexistence in the monolayer made of Z2b bent-core molecules.

Phase segregation corresponds to strong attractive interactions giving rise to a close-packed configuration of monolayers. We can make an estimation of mean molecular area in such a close- packed array from the respective isotherms. The area per molecule at which the isotherm begins to lift-off, as indicated by the arrow in Fig.5.16a, corresponds to the co-area of the film for a particular initial surface concentration. We found that the co-area depends on the initial surface concentration as shown in Fig.5.16b and the film thickness for the close-pack configuration depends on the coarea

(Fig.5.16c).

5.7.3 Film thickness

We also estimated the thickness for different conﬁgurations of molecules in Langmuir ﬁlms.

Suppose we deposited m grams of bent core material of bulk density ρ on a water surface to prepare a Langmuir film of thickness t. If the effective area of between the two barriers is A and the volume of the film is V , then the average thickness of the film can be given by 139

(a) (b)

(c)

Figure 5.16: (a) Compression isotherm with initial concentration of 0.86 nm2/molecule. An illustration of finding mean molecular area for the close-packed configuration of a Langmuir monolayre. (b) Variation of co-area with initial surface concentration (σco) with the initial surface concentration (σ0) of the Langmuir film. The circles correspond to compression data and the solid circles correspond to the recompression data. The vertical error bars represent to the errors in estimating the co-areas from the isotherms, the horizontal error bars represent fluctuations in surface concentrations. (c) Film thickness (h)as function of the initial surface concentration. 140

V V/m m t = = = (5.1) A A/m ρA where m is the mass of bent-core deposited on the surface and ρ is the polymer density in the ﬁlm.

Using this formula we estimated the thickness of the thinnest possible continuous Langmuir ﬁlm of the bent-core molecule in our study using ρ = 1 g/cm3, since this is a reasonable estimate for

an organic material. As discussed in Section 5.7.1, I prepared Langmuir ﬁlms of varying surface

concentrations ranging from 7 nm2/molecule to 0.34 nm2/molecule (the larger this value the smaller

is the concentration). However, during compression and decompression only the ﬁlms up to an

initial surface concentrations 1.75nm2/molecule showed appreciable change in the surface pressure.

With more dilute ﬁlm, surface pressure remained practically zero during compression. Until the

ﬁlm is covered with a continuous layer, the surface pressure should stay below the ideal gas level .

Therefore, we consider that the ﬁlm with mean molecular area of 1.75 nm2/molecule is the ﬁlm with

minimum surface concentration at which the surface pressure rises signiﬁcantly above the ideal gas

limit. Equation 5.1 suggests that the minimum ﬁlm thickness of these molecules is ∼ 0.7nm. We

can then compare this thickness to molecular sizes to get a ﬁrst estimate of molecular conformation

at the surface. For the comparison sake, we used the simulation results by Duff et.al.[33] for a

different but similarly-shaped bent-core molecule. For the simulation, a single BC-2Cl molecule

was considered on water surface as shown in Fig.5.17c. In Fig.5.17a and Fig.5.17b you can see the

structure of the molecule and its annealed structure in vacuo for comparisons sake.

Fig.5.18 shows position of different parts of a BC-2Cl molecule in water surface. It shows

that the alkyl chain sits farthest above the water surface whereas the wing phenyl rings sit below

the central phenyl ring. From the graph we ﬁnd the distance between the peak corresponding to

the position of lowest position of the outer phenyl ring and the peak corresponding to the highest

position of the terminal carbon atom of the alkyl chain. This gives an estimate of 6A˚ . This is a

center-to center distance, so we should add about half the thickness of these groups, or about 0.3 nm

for the phenyl ring for total average thickness of 0.9nm. This value is close to the one we estimated 141

Figure 5.17: (a)Molecule BC-2X, where X is hydrogen in BC-2H and chlorine in BC- 2Cl(b)Annealed structure of BC-2Cl in vacuo(c)Snapshot from the molecular dynamics (MD) simulation of the BC-2Cl molecule on the water surface [33]. for the surface concentration of 1.75nm2/molecule. Despite the fact that the simulation result is for different kind of bent-core molecule, the molecules are similar enough that the result tells us that the lowest estimated thickness of the layer is about one molecule thick, with the molecule ﬂat on the surface. This is a strong indication that this type of molecules can form a ﬂat monolayer on the surface, at very low surface pressures.

5.7.4 Molecular packing and surface conﬁgurations

In order to investigate packing of molecules in a monolayer, we took a Fast Fourier Transform

(FFT) of the bright portion of the image as shown in Fig.5.19. In Fig.5.19a, the white rectangle is the portion of the image whose FFT is as shown in Fig.5.19b. The dark region around the central bright spot is very close to a hexagonal structure with a slight distortion.

Sometimes the type of pattern found in Fig.5.19b may arise due to diffraction. One way to check whether the pattern is real or due to diffraction of laser beam, is to get an image of just pure water; then take its fft and if the pattern is still there. A better alternative is, ﬁrst deposit a sample which 142

Figure 5.18: Probability P(Z - Z0) of BC-2Cl molecule, whre z - z0 is the height above the water surface for different parts of the molecule relative to the height of central phenyl ring. Top frame:results for the inner phenyl ring of the wing, middle frame: outer phenyl ring, bottom frame: for the terminal carbon atom of the alkyl chain [33] can form uniform pattern on the water surface (such as 8CB), capture an image of it and then take its fft. We chose the second alternative. We ﬁrst prepared a monolayer of 8CB on a clean water surface whose BAM image is as shown in Fig.5.20a. The FFT of this image did not show any structure that we had seen in the FFT of Fig.5.20b. The BAM images certainly showed mesostructures roughening the ﬁlm; the hexagon may indicate that they are suprisingly regular. Such regularity at a smaller scale of 50 microns was seen in AFM images on BC-NO2 [219], where the lattice constants exactly agreed. This preliminary results might be indicating toward a more ordered mesostructure structure of the Langmuir monolayer made of Z2b bent-core molecules. However, without further investigation using more reliable probes such as x-ray diffraction, it is too early to jump to this conclusion. 143

(a) (b)

(c)

Figure 5.19: A FFT of a BAM image to investigate the packing of molecules. (a) Original BAM image of a dilute monolayer (σ ∼ 7 nm2/molecule) made of Z2b bent-core molecules; the scale bar cor1 mm(b) FFT image of the region of the image in (a) surrounded by the white rectangle (c) FFT in (b) with a guide to the eye around the central bright spot. The sizes of all three images have been adjusted to ﬁt in the page. The bright spots away from the center are due to diffraction fringes in the illuminating beam. The elongated side of the hexagon is ∼ 50µm. 144

(a) (b)

Figure 5.20: A BAM image of 8CB monolayer and its FFT. In (a), the rectangular region indicates the domain whose FFT is to be taken. The scale bar corresponds to 1mm.(b) FFT of image (a).

5.7.5 Optical anisotropy of the ﬁlm

One goal of this work is to produce well-organized ﬁlms capable of aligning bent core molecules parallel to the surface. Optical anisotropy would indicate such ﬁlms, as they did in the case of Bc-

2Cl ([219]), but not in the case of three other bent-core molecules with symmetric hydrocarbon end chains. In an isotropic medium, the permittivity or the dielectric constant (relative permittivity) is the same in all direction. However, in an optically anisotropic medium it does vary with respect to the direction. For optically isotropic dielectric materials the refractive index can be given by √ n = εr, where n is the refractive index, εr = /o is the dielectric constant, 0 is the permittivity of the vacuum, and is the permittivity of the film. Equation 2.14 in Sec.2.3.2 relates the reflectivity of a Langmuir film with its refractive index. For an anisotropic domain, the reflectivity changes with respect to its orientation.

In general, the dielectric constant is a second rank tensor, which can be diagonalized in terms of three principle axes. In a uniaxial system, the dielectric constant in a plane orthogonal to the optical axis is isotropic. The dielectric constant in any other direction shows birefringence and it 145

Figure 5.21: Change of gray value of the domains with respect to their orientations (φ). The angle φ is measured in degree. Position of the analyzer with respect to the polarizer is α = 90◦. The scale bars correspond to 1 mm. Molecule Z2b. Surface concentration: 0.34 nm2/molecule, surface pressure: ∼ 0 mN/m, after full decompression. can have components perpendicular and parallel to the optical axis. In a biaxial system, however, the dielectric constants are different in all thee directions. For a uniaxial medium, the anisotropy can be expressed as the difference between the dielectric constant in parallel and perpendicular directions to the unique optical axis. It can be expressed as

4 = k − ⊥

I tested the optical anisotropy of the Langmuir film made of Z2b bent-core molecules and compared this anisotropy with previous results with the anisotropic films of symmetric bent-core molecules [32]. First of all the Langmuir film as prepared in a Langmuir trough, compression, decompression and recopression (as needed) were carried out together with the BAM as discussed in

Section 5.6. At a particular laser power, polarizer and analyzer positions, change in brightness of the domains were observed when the domains were rotated along the surface (x-y plane) about the z-axis, which immediately implies optical anisotropy in the plane of the layer. Later, the captured video frames were converted into still images; then change of gray level was measured with respect to the angle of rotation (φ). When the domains were rotated, substantial change in gray value was observed as in Fig.5.21.

The gray level of an image in the CCD used for capturing the image depends on the reﬂectivity

R of the surface under investigation. Gray level can range from 0 to 255; 0 is assigned for the saturation and 255 for complete darkness (Scion Image; a different image analysis software. ImageJ interprets these values the other way around). The relative gray level Gr is deﬁned as Gr = Gb −G, 146

where Gb is the gray level with respect to background; practically Gb is equivalent to the gray level observed with a pure water surface. G is the gray level with film on it. Gr was found to be linear over a wide range of light intensity on the detector; thus the CCD gray level is linearly related to the reflected light for a fixed incoming beam [31, 32].

In order to quantify the change in gray value with respect to the domain orientation, first we need to define a physical quantity, called the relative reflective intensity R as

Gr Iref ∝ = R (5.2) Iinc Iinc

We plotted the change in relative intensity deﬁned in Equation 5.2 as a function of domain

orientations shown in Fig.5.22.

Figure 5.22: Change of reﬂectivity as a function of domain orientation. The reﬂectivity is in an arbitrary units.

This result can be compared to work done in our lab with a different type of bent-core molecule

[32]. Figure 5.23b is the result from the previous work. Fig.5.23a is included here so that a qualitative comparison can be made with our domain rotations. In my system, shown in Fig.5.22, you cannot see a full range of data as can be seen in the earlier system, Fig.5.23b. The Z2b domains 147

(a) (b)

Figure 5.23: For Bc-2Cl:(a) Domain rotation at α = 30◦; an illustration of gray level change with respect to their orientations in a Bc-2Cl Langmuir ﬁlm at α = 30◦. The scale bar is equal to 1mm. (b) Change of reﬂectivity as a function of domain orientation φ at α = 90◦. Solid squares represent the measured data and the solid line represents the simulated data [32].

could not be fully turned around because of a very strong viscoelastic response from the ﬁlm. Do-

mains would immediately come to a halt when the force causing them to rotate, rotational ﬂow in

the subﬂuid induced by a platinum wire, was stopped.. Despite this fact, we can still see similar

trends in our data.

A better comparison can be made between the results from my present work and from the

previous work if both sets of data can be plotted in the same graph with the same scales as shown

in Fig.5.24.

In a uniaxial model, the relative reﬂective intensity R as a function of domain orientation (φ)

depends on the analyzer angle α, the tilt angle t with respect to the surface normal the difference

φ0 between the orientation of the obvious surface feature used to determine the domain orientation

and the projection of the extraordinary axis on the surface, the ﬁlm permittivity perpendicular to

the the extraordinary axis ⊥, and the anisotropy 4 [32]. In principle, φ0 and t can be determined

independently, but ⊥, anisotropy 4 and the layer thickness h are correlated in the ﬁtting.

A simple optical model let to the simulated data shown by solid line in Fig.5.24. In this model, a uniaxial medium (5.25a) was considered with 4 = k − ⊥ and with arbitrary director orientation t and φ as shown in Fig.5.25b. The optical model, the original work of [223–225] with cumbersome 148

Figure 5.24: A comparison of previous and current results. Solid circles:current work, circles: previous measured data, solid line: previous simulation. Both set of data were obtained for α = 90◦. calculations [1], lead to Eq.5.3

1 2 1 R = cos θB(k0h){cos α 2 × 4 cos θB 2 2 2 ⊥4 sin t cos φ 2 sin θB − ⊥ + 2 cos θB + 1 − 2 + ⊥ + 4 cos t ⊥ + 4 cos t 24 sin t sin φ 2 sin α 2 (cos t sin θB + ⊥ sin t cos φ cos θB)} (5.3) ⊥ + 4 cos t

For t = 90◦ and α = 90◦ the equation reduces to

2 R ∝ sin 2φ (5.4)

For these values the simulated data seems to be perfectly matching with the measured one in the previous work data. This tells us that the molecule is very close to parallel to the film surface. In other words, the molecules lie flat on the surface of water. Comparing the data presented in Fig.5.24, we can clearly see that our minima do not go all way to zero as in the previous work. It could be because of the the background is difficult to determine. If it is not a background effect and the gray level never goes to zero, it could mean that the anisotropy is less uniform compared to the previous 149

Figure 5.25: (a) Optical model for the uniaxial layer(b) An illustration of reflection of plane electromagnetic wave by an stratified anisotropic slab sandwiched between air and water. E is electric field, θB is the Brewster angle, n is the director vector, t is the tilt angle with respect to the surface normal. X-Y plane defines the ambient/slab interface, Z axis is in the direction of stratification [32]. case. The distance between two consequitive minima in the previous result is about 90◦ whereas the the distance between two minima for Z2b is ∼ 130◦. This means that in our case tilt angle might not be exactly equal to 90◦, which again means less uniform film anisotropy and the molecules in our case might be oriented at some angle with respect to the film surface, rather than lying flat as in the case of BC-2Cl.

5.8 Conclusion

One goal of changing the molecule to make it more hydrophilic was attained with the new

Z2b bent-core molecule. Z2b bent-core molecules form relatively stable Langmuir films at the air/water interface at room temperature; especially compared to earlier Langmuir layers of bent core molecules with hydrocarbon end chains. The film was stable well up to the maximum possible compression (∼ 50 mN/m) within the limit of our ability to compress the films. Compression was found to be reversible at low initial surface concentrations, with almost with no hysteresis in the compression/recompression isotherm. However, hysteresis appeared at higher pressures, although less than that observed with symmetric bent cores. Domains in the Langmuir film made of the this type of bent-core molecules have been found to be sitting in a gel-like layer which is highly viscoelastic in nature. The surface viscosity is extremely high in this type of film making domain 150

rotations for the anisotropy test almost impossible. The ﬁlm has been found to possess fair amount of optical anisotropy in our preliminary results, but the tilt of the molecule is probably not parallel to the surface normal as was seen in the previous studies of Bc-2Cl molecules. The absolute minimum thickness of a continuous mononlayer at the air/water interface has been estimated to be ∼

1nm, which is order of one molecule thick with the molecule very flat on the surface. However, isotherms also imply more upright configurations, by either starting at higher concentrations or by compressing the films. The preliminary result obtained from the FFT of the BAM image taken at ∼ 7nm2/molecule has hinted a more ordered mesophase on the water surface, however, more experiments with reliable probes are needed to be certain about it.

5.9 Future work

In order to test the optical anisotropy of the film made of this type of bent-core molecules, a more convenient experimental design is essential, since viscoelastic behavior of the film was so strong it was hard to rotate the domains as such. In this context, it would have been a beautiful experiment to measure its viscoelastic properties. In order to obtain more reliable information about the film thickness, and to better-probe the evident mesostructure at ∼ 1 micron, AFM experiments would be useful. We compared the thickness to realistic atomic simulations on a different molecule; it would be desirable to perform simulations directly on this molecule. It would be particularly interesting to perform molecular dynamic simulation for these molecules on the water surface. Last but not least, it would be interesting to use the film made of these molecules as alignment layers for another set of bent-core liquid crystals. A member of our research group, Wilder Iglesius, has begun to do this in collaboration with Antal Jakli. CHAPTER 6

MOLECULAR CONFORMATIONS IN ULTRATHIN POLYMER FILMS BY USING 2HNMR

TECHNIQUES

Polymers are one of the widely used materials in modern industries. Many of the structural polymers that we use in every-day life have carbon backbones, but polysiloxanes, with an Si-O backbone, are also widely used. One advantage for many applications of PDMS is that its backbone is particularly ﬂexible. Some of the best-known applications of PDMS currently make use of this property. Cross-linked PDMS is viscoelastic and permits rapid prototyping using soft lithography

[226, 227]. The flexibility of PDMS is suitable for the integration of actuators such as microvalves and micropumps. This capability is crucial for increasing the number of functionalities in lab-on-a- chip devices [228] and achieving flow control in microfluidic systems [229].

Other commercial applications of polysiloxanes include lubricants, cosmetics, insulators, mois- ture repellents, antifoams and release agents. Most of these technological applications involve thin

ﬁlms and and surface effects, and typically use linear siloxanes, which are oils or greases at room temperature, depending on the chain length and side groups. One unique property of PDMS oil is that it can spread on both hydrophilic and hydrophobic surfaces and renders hydrophilic surfaces hydrophobic. One reason for this useful behavior is the ﬂexibility of PDMS, which allows multiple conformational possibilities that may be different at different surfaces.

The conformation of PDMS at a surface is thus of industrial importance. This conformation is also a source of long standing controversy. However, making the right choice of experimental tools for investigating PDMS surface properties is a big challenge since there is a very small amount of material on the sample surface. None the less, PDMS surface properties have been extensively studied, both because of its commercial importance and also because of its usefulness as a model system. The backbone ﬂexibility makes this polymer close to the standard polymer model of a

151 152

freely joined chains, near the monomer level.

The earliest studies, began over more than 60 years ago by indirect surface tension isotherms, were consistent with both helical and layered structures at the air/water surface. A variety of groups continue to attack this question by a variety of methods [39, 43, 230, 231]. Svetlana Primak , while a student in our laboratory, showed that 2H NMR, which is also know as DNMR (I will interchangeably use the acronyms 2H NMR and DNMR in the following discussion), a technique previously used to probe conformational properties of this and other polymers, can be sensitive to surface structure of a single monolayer. However, averaging due to molecular motion on the surface at room temperature limited the amount information that could be extracted from the DNMR results. However, she also found that the partially immobilized deuterium-labeled methyl groups in a uniaxially oriented ultrathin PDMS film gives rise to characteristic 2H NMR lineshapes when measured at different sample inclinations [232] with respect to the spectrometer magnetic field. The partial immobilization was achieved by cooling the sample to lower temperatures. Her preliminary results [36, 232] suggests layered structures in the multilayer region. However, we want to confirm this by further lowering the temperature below its glassy state (Tg ∼ 130K), since a glassy state can be regarded as a frozen equilibrium state at higher temperatures, due to a longer relaxation time between different conformations [233]. We also want to extend previous results by studying the difference in behavior of the molecules both on hydrophobic and the hydrophilic surfaces in the glassy state.

In order to determine the degree of segment mobility in polymer chain, the 2H NMR lineshape

due to quadrupole splitting is the most important parameter to be measured. Unoriented samples

have been used to study grafted or adsorbed polymer chain dynamics whereas oriented samples

have usually been used in determining the structural details of the polymer chains by translating the

observed spatial orientation of the side groups into structural information [234, 235]. These types

of studies may help resolve many structural issues such as the conﬁguration of polymer backbone

[236]. 153

In this chapter I discuss the techniques of solid state NMR methods to determine orientation and conformation of PDMS polymer chain in ultrathin films deposited in solid substrates. First, general polymer properties will be discussed in Sec.6.1. Then, in Sec.6.2, a short introduction of NMR will be presented first followed by a theoretical discussion of the quadrupole splitting as a tool of deuterium NMR. In Sec.6.3, I will discuss the sample, and in particular sample preparation, which gave significant difficulties when trying to reproduce previous results. In Sec.6.4, I will present the experimental results and discuss in detail the difficulties we had in making the samples and in cooling them to appropriate temperatures. The present work can best be seen as setting the stage for a more definitive study when an NMR spectrometer with appropriate cooling is identified.

6.1 Polymer properties

Polymers exhibit different properties in different forms. Here I will present brief discussions of these properties of linear polymers in bulk and and as ﬁlms on the surfaces.

6.1.1 Properties in bulk

Polymer chains are made of large number of identical units as shown in Fig.6.1. These units are called monomers. These units are linked together either linearly or cross-linked. Some polymeric materials are such as rubbers and most plastics are isotropic. At low temperatures amorphous polymeric materials may be brittle and large deformations are almost impossible without destroying their original properties. This state of polymer is called the “glass” state of the polymeric material characterized by the glass temperature (Tg). This temperature is material dependent and also depends on the experimental time scale. As we go above (Tg), the polymer metamorphoses into a rubber-like viscoelastic state. Finally at higher temperatures, the polymer turns into a viscous ﬂuid state. This state of polymer is called a “melt”.

Each polymer segment possesses independent degree of freedom due to their ﬂexible nature

[233]. That means, polymeric chains have a definite flexibility and are capable of fluctuating constantly changing its configuration. Higher probability and greater entropy corresponds to a 154

(a) (b)

Figure 6.1: Examples of polymers in all-trans conformation. (a) Polymethylene with with a carbon backbone (b) PDMS with an Si-O backbone and CH3 pendant groups. The monomer in both (a) ◦ and (b) is indicated by the dashed box. The bond angle are: θO w 37 with ﬂuctuations dues to low energy barriers [34]. randomly-coiled chain [34, 237]. As discussed earlier in this section at a temperature below (Tg) molecular ﬂuctuations of the macromolecular structure are frozen resulting in a glassy state.

A viscoelastic polymer is practically incompressible like any normal liquid. Because of the many available conformations, a polymer sample can be deformed greatly without changing its internal energy signiﬁcantly. An analogy of this situation can be made with a system of a gas whose internal energy does not depend on its volume. The elastic force arising from deformation is mostly due to the entropy change rather than the internal energy change. Elastic deformations in solids are accompanied by a change in the potential energy of molecules and atoms whereas isothermal elastic deformation in a polymer is accompanied by a change in conformation.

6.1.2 Properties at surfaces

Since a polymer can interact with a substrate in various ways, its properties at an interface are inevitably different from its bulk properties. Basically there can occur two types of adsorption of a polymer at an interface: chemisorption accompanied with chemical bond formation and physisorp- tion due to physical interactions. The physical interactions can be electrostatics and dispersion or van der Walls interactions. Interactions such as hydrogen bonding can be both chemical and physical depending on the observer’s perspective. Polymer molecules can change their conformation from the equilibrium bulk conformation exposing groups that bond with the interface. In doing 155

this, the interface imposes constraints to the polymer molecules which results in a decrease in the number of available conﬁgurations of the polymer chain. Entropy thus plays an important role in interactions between a polymer and the surface.

6.1.3 Polydimethylsiloxane

Polydimethylsiloxane (PDMS) has been used as a model sysmtem for polymers because of its extraordinarily ﬂexible backbone. Its versatility is considered to be due to its siloxane backbone. The geometry of the PDMS chain is somewhat peculiar as compared with the carbon-based polymers (Fig.6.1). Nonetheless, their conformational-dependent properties may be modeled and interpreted in the similar fashion [238]. As shown in Fig.6.1b, the valence angles at Si and O

◦ ◦ (θO w 37 and θSi w 70 ) are not equal in a PDMS molecular chain, which implies that the all- trans chain is not an extended chain. After about 11 repeating units or 22 chain bonds the polymer deﬁnes a closed polygon [34, 238]. Thus, from this geometrical scenario, one might expect a ﬂat helical chain conformation of PDMS crystal structures with trans bond conformation distorted to keep from polygon closure. The resulting open structure is believed to be the basic reason for the lower barrier for methyl group rotation about the Si-O bond, about 2.5kJmole−1 (∼ RT at room temperature) compared to 17 2.5kJmole−1 [34, 238]. This implies that the PDMS molecular chain has high mobility and a low glass temperature (Tg ∼ 130K) compared to any other polymers

[34, 238]. Substitution of methyl groups by longer alkane side groups or the introduction of para or meta disilphenylene groups into the polymer backbone increases Tg considerably. This kind of behavior suggests that the ﬂexibility of siloxane chains is by and large dependent on side-groups and interchain interactions.

In reality, there is no universal agreement in deﬁning a crystalline structure of PDMS. The school of thought represented by Damaschun purposes a ribbonlike, 2-fold helical conformations for PDMS. These are (a) two extended chains (Fig.6.2b), each with three repeat units within a unit cell or (b) one folded chain (Fig.6.2c) with six repeat units passing through each cell. Both of these conﬁgurations end up with a helical structure as opposed to the extended chain (Fig.6.2a) [35]. 156

(a) (b) (c)

Figure 6.2: Proposed linear PDMS chain conformations (a) Extended caterpillar (b) Extended helix (c) Approximate Damaschun helix. In these ﬁgures methyl groups are represented by three balls as hydrogen around the central grey one as carbon [35]. Addapted from [36].

Figure 6.3: Models for linear chain conformations on surfaces:(a),(b) monolayer structures(c), (d)possible higher layer formation (e) random coil conﬁguration (f) extended helix conformation on top of a monolayer (g) exteneded helix conformation (h) close-packed helix. After [36].

However, in the later years, the model purposed by Damaschun came into controversy when

Schilling et.al [239] published their results obtained from solid state 13C and 29Si NMR. These

ﬁndings did not support the idea put forwarded by Damaschun regarding the helical, ribbonlike conformation proposed for PDMS. Hence, no direct experimental conﬁrmation of a helical crystalline structure of PDMS in crystal can be found in the literature.

As discussed in Sec.6.1.2, the conﬁguration of PDMS molecules on a surface can be different than that in the bulk crystalline form, and may depend on the surface properties. Several different conﬁguration have been shown in Fig.6.3.

The results from surface pressure isotherm [37] show that there is an abrupt increase in surface pressure from very low values (Fig.6.4). At this low value of surface pressure, PDMS molecules 157

Figure 6.4: Compression isotherms of PDMS with several different degrees of polymerizations [37]. The flat portion (plateau) of the isotherm is the region where gas liquid phase of the the polymer at the air/water surface coexists. can be lying flat at about one extended layer. At first, the conformation of the molecule resists any changes in configuration during compression, but with increase in surface pressure there is a plateau.

The first plateau can be interpreted in two different ways: (1) in this region, the configuration either begins to change from extended caterpillar to the extended helix which then slowly compresses, then there is another short increase in surface pressure when at maximum natural helix or (2) in the first plateau a multilayer, either bi-or tri-layer depending on its exact configuration, forms in coexistence with the monolayer, and the second step in pressure occurs when the entire surface is covered with a low-order multilayer, and regions of thicker multilayer begins to form.

Direct evidence of phase coexistences and formation of multilayers have been found in some of the earliest Brewster angle microscopy (BAM) results [38] as shown in Fig.6.5. The ﬁrst Fig.6.5a shows the submonolayer regime with gas liquid phase coexistance. Here, dark regions are dense liquid whereases the bright regions are dilute 2-d gaseous regions1. Fig.6.5b is the BAM image of

1For technical reasons involving the thinness of the PDMS monolayer, only ∼ 0.6nm, the contrast for the PDMS liquid/gas is inversed from the usual case where thicker, denser layers are brighter under Brewster angle microscopy. 158

(a) (b)

Figure 6.5: BAM images of PDMS Langmuir ﬁlms at the air/water interface. (a) PDMS in the submonolayer regime (concentration < 0.5 mg/m2) with gas liquid phase coexistance. Dark regions are dense in polymer, bright regions dilute in polymer (b) PDMS in the collapse regime (concentration ∼ 4 mg/m2). The bright regions are of higher polymer density than the dark region here. The scale bars correspond to 50µm [38].

PDMS at the air/water interface in the collapse regime, that is, in the second plateau. In contrast to

the submonolayer regime, here the bright regions are of higher polymer density due to the multi-

layer formation, subdark regions of thinner layers. These direct results suggest that the multilayer

model is correct, and certainly refutes model (d) and (e) in Fig.6.3: many different multilayers are

observed, so that there must be some mechanism that produces discrete thicknesses. However, no

co-existence was observed in the ﬁrst plateau, which leaves the question of conformation in that

region open.

Very recent work [39] on Langmuir ﬁlms of PDMS at the air/water interface suggests that that methyl group side-chains at the interface are completely disordered in the dilute regime of the surface density. At higher surface densities, however, the two methyl groups on the repeating unit point into the air asymmetrically; one points more normal to the interface, whereas the other lies more parallel to the interface [39] as shown in Fig.6.6.

So far I have presented a brief discussion of surface properties of polymers in general, and that of PDMS in particular on a hydrophilic surfaces. However, in principle, we can expect different 159

Figure 6.6: Proposed conformational changes in PDMS isotherms at the air/water interface: (A) all Si and O atoms adsorbed onto the interface, (B) some of the Si and O atoms adsorbed (drawn as only O adsorbed for convenience), (C) helices with their axis parallel to the surface, and (D) helices oriented more perpendicular to the interface [39]. behaviors of PDMS molecules on hydrophobic surfaces. The concept that the hydrogen bonds with the oxygen on the siloxane backbone tends to stretch the polymer on the hydrophilic surfaces would not apply in the hydrophobic surface despite the fact that the thinnest ﬁlm on both types of surfaces should be nearly the same (one monomer or ∼ 0.7 nm).

There is a controversial explanations as to the difference in the behaviors of hydrophilic and hydrophobic surfaces toward the polymers. One school of thought [240] believes that the adsorbed amount on the hydrophobic substrate is signiﬁcantly less than that on a hydrophilic ones resulting in a difference in the mechanism of molecular attractions. However, these interpretation considers nothing about the alignment of the linear polymer chains. It was shown by the recent results of

PDMS ﬁlms that methyl groups have strong ordering on such substrates [241].

In this work I have considered both hydrophilic and the hydrophobic surfaces to study the behavior of PDMS ultrathin ﬁlms.

6.2 What is an NMR?

In this section I will present a brief discussion of the basics of NMR. Rigorous mathematical details are beyond the scope of the this present work. They can be found in many NMR text books 160

[40, 41, 242].

Nuclear magnetic resonance (NMR) is a phenomenon which is based on the fact that nuclei of certain elements possess a spin angular momentum and magnetic moment. When such nuclei are subjected to a magnetic field, they can acquire one of the available quantized states. Each of these states corresponds to a particular energy level. As such, the orientation with the lowest energy is the one in which the nuclear magnetic moment is most closely aligned with the direction of the magnetic field. Changes in the states of these nuclei with respect to the external magnetic field bring about transitions in the energy levels. These transitions can be induced by the using radio frequency radiation of suitable frequency and can be recorded in the form of NMR signal of the nucleus [40].

In classical terms, one of the fundamental facts of physics is that a spinning charged body gives rise to a magnetic moment. Classically, a nucleus being positively charged, its spinning will result in the rotation of the positive charge which can be thought of as a current in a circle. This would produce a magnetic ﬁeld parallel to the spin axis and the nucleus would have a magnetic moment µ

(in the units of nuclear magneton) as shown in Fig. 6.7. The quantized angular momentum and the magnetic moment are given by equations 6.1 and 6.2 respectively.

h P = I (6.1) 2π and

µ = γ P (6.2) where, h is the Plank’s constant, P is the spin angular momentum, γ is the magnetogyric ratio

−1 −1 1 3 (rad s T ) and I is the spin quantum number which may have values 0, 2 , 1, 2 , etc. If a nucleus has value of I equal to zero, it will not have any spin angular momentum. Hence, the nucleus cannot contribute to give rise to NMR signal.

When a system of nucleus with spin angular momentum I is placed in a uniform magnetic

ﬁeld, the angular momentum will be quantized and and will assume one of (2I + 1) states; the magnitude of energy splitting depends on the magnitude of the applied magnetic ﬁeld Bo as shown 161

Figure 6.7: A classical picture of magnetic moment µ due to spinning positively charged nucleus. Adapted from [40]. in Fig.6.8. The mechanism of splitting of the energy states can be better explained by considering

1 2 1 1 the examples of H and H nuclei. The H has I = 2 ; there will be 2 orientations for the nucleus (Fig.6.9a). Similarly, 2H has I = 1; there will be 3 different orientation (Fig.6.9b).

Figure 6.8: Schematic of difference in energy levels of two magnetic states with increasing magnetic ﬁeld Bo. From [40].

Nuclei having I = 1 (such as 2H or 14N) have 3 different states (Fig.6.9b), each having its

1 characteristic energy. Nuclei with I > 2 have asymmetrical distribution of charge, which results in an electric quadrupole moment Q, which will be discussed in the succeeding section.

From the classical point of view, a spin in a magnetic field behaves like a tiny magnet with the restrictions that it can assume only (2I + 1) orientations as opposed to any orientations that a real magnet can take with respect to an external magnetic field. Since the nucleus is spinning on its axis, the external field causes it to precess it [41], i.e., it undergoes a circular motion . The two ends of the spinning axis trace two opposite circular paths as shown in Fig.6.10. All the NMR experiments are based on the measurements of this precessional motion. 162

(a) (b)

1 Figure 6.9: Schemata of magnetic energy levels for (a) with spin 2 and (b) with spin 1. Adapted from [40, 41]

Figure 6.10: A classical of precessional motion of a nucleus in a magnetic ﬁeld. A: axis of rotation, Bo: uniform magnetic ﬁeld, N: nucleus, O: circular orbits. Adapted from [41]. 163

By the application of electromagnetic radiation of suitable frequency, transitions between the adjacent energy levels can be caused. The frequency of electromagetic radiation for such transitions is given by the equation 6.3. This is the basic equation for NMR. When the value of this electromagnetic radiation reaches the value of the precessional frequency of the nucleus, absorption of energy will take place. It is very important to note here that the frequency of electromagnetic radiation required to induce transitions from one nuclear spin state to the other is exactly equal to the precessional frequency ωo; this condition is called the resonance. The precessional frequency

ωo is directly proportional to the applied magnetic ﬁeld Bo and the gyromagnetic ratio γ as given by equation 6.4.

γ B ν = o (6.3) 2 π

ωo = γ Bo (6.4)

Thus as the value of the applied magnetic ﬁeld is increased, the difference in energy between two spin states (4E) also increases as shown in Fig.6.8. The increase in 4E also increases the precessional frequency. This requires correspondingly higher frequencies of the applied electromagnetic radiation. One can therefore induce resonance in different protons by keeping the applied magnetic ﬁeld constant and then gradually increasing the value of oscillator (mechanism to produce electromagnetic radiations of desired frequencies) frequencies. As soon as the precessional frequencies of the different nuclei are reached, resonance will occur and NMR signal will occur.

6.2.1 DNMR and Quadrupole interaction

In this section I will present a brief review of the basic concept of the deuterium NMR in the anisotropic ﬂuids relevant to our work. More elaborate discussions of these concepts can be found in various references [243–245]. A deuterium nucleus consists of a non-zero electric quadrupolar moment Q and therefore possesses an electrostatic energy in the nonuniform electric ﬁeld of the 164

Figure 6.11: Schematic of energy level for a nucleus of spin I = 1. The three energy levels with m = -1,0, +1 are degenarate in the absence of magnetic ﬁeld. The Zeeman interaction splits the m = ±1 levels symmetrically resulting in only one symmetric peak at ν = νL. Further quadrupole interaction causes a perturbation of the split levels giving rise to two resonant peaks at ν = νL ± 4(Θ) 2 [36].

C-D bond with a ﬁeld gradient tensor ∂Ei . The nuclear quadrupole interaction can be described ∂rj by a second order tensor ﬁxed to the nucleus. The instantaneous interactions can be given by the

Hamiltonian,

3 H = ν P (cos Θ)(3 I − 2) (6.5) q 2 q 2 z

2 where Iz is the spin operator in the z-direction. For a deuterium nucleus, I = 1 and I = 1, we get doublet resonance lines due to the quadrupole interactions as shown in Fig.6.11. The static

2 e QVzz quadrupolar constant νq = h ∼ 175 Hz [43], where Vzz is the largest component of the electric ﬁeld gradient (EFG) at the nucleus, e is the electronic charge, and h is the thickness. The assymetry parameter is given by

V − V η = xx yy (6.6) Vzz which measures the deviation of the electronic environment of the nucleus from the axial symmetry whose values lies betwee (0 ≤ η ≤ 0). Since the electron density in a σ-bonds of aliphatic C-D is approximately uniaxial, the asymmetry parameter of the electric ﬁeld gradient tensor is almost zero and Eq.6.6 yields Vxx w Vyy, cylindrical symmetry. This symmetry causes the unique z-axis of the principal axis system of the electric ﬁeld gradient tensor to coincide the the C-D bond direction, or more precisely, the C-D internuclear vector, about 3◦. Let us consider an instantaneous angle Θ 165

between the C-D bond and the uniform magnetic ﬁeld Bo. For molecular motion, a temporal average over fast segmental reorientation with respect to the inverse spectral width in the rigid lattice limit

(10−6 −10−5 s) [36] must be taken into account in Eq.6.6, giving an average quadrupolar interaction

0 3 4 = ν |P (cos Θ)| (6.7) 2 q 2

where 3cos2Θ − 1 P (cos Θ) = 2 2 is the second order Legendre polynomial. The overbar in Eq.6.7 represents the temporal averages

0 taken over rapid molecular motions. 4 does not depend on the the external uniform magnetic ﬁeld

Bo and experimentally it will be deﬁned as a maximum frequency splitting in a doublet of resonance line.

6.2.2 Free induction decay (FID) and relaxation function

Nuclear spin relaxation is caused by the molecular motion giving rise to ﬂuctuations in the local

fields (couplings) which results in energy transitions. The primary cause of relaxation in deuterium is coupling of the nuclear quadrupole moment to the electric field gradient (EFG) of the bond. The relative field orientation of the EFG (or C-D bond) with respect to the applied uniform magnetic

ﬁeld Bo determines this coupling. Hence, molecular level dynamics can modulate interaction and create ﬂuctuations which brings about relaxation [243]. On this basis, the time evolution transverse magnetization for a deuterium nucleus may be written as

t − 0 T ∗ M(t) = M0 e 2 cos 4 t (6.8)

The NMR signals dictated by Eq.6.8 are referred to as the free induction decay (FID). For the

0 isotropic motion of the C-D bond as in liquids, 4 reduces to zero and the the transverse magneti- − t T ∗ ∗ zation is just M(t) = M0 e 2 with relaxation time T2 . The Fourier transform of this exponential 166

Figure 6.12: An illustration of Pake powder spectra. The separation of peaks is expressed in kHz [36].

∗ −1 decay makes the spectrum a Lorentzian line of line width (π T2 ) , whereas for the anisotropic

0 motions, the ﬂuctuations no longer average out the interactions 4 to zero. As a result of this, a modulation of the transverse relaxation function in the frequency domain results in a doublet of Lor-

0 nentzian lines symmetric to the Larmor frequency characterized by 4 . If we consider a nonuniform

rigid system, there exists one such doublet for each C-D bond with a particular orientation. The re-

sulting spectrum arises because of the superposition of all the of these doublets. A three dimensional

Pake powder spectra (Fig.6.12) is analogous to an isotropic C-D bond distribution [243].

The theory discussed above can also be used for C-D groups mobile on NMR time sclaes. In

this case, the consequent spectrum is a unique doublet whose splitting 4 may be expressed in terms

of frequency units as as in Eq.6.9 [36]:

3 4ν(θ) = ν |P (cos β)| |P (cos θ)| (6.9) 2 q 2 2 where θ is the angle between the symmetry axis and the magnetic ﬁeld Bo and β is the angle between the C-D bond and the symmetry axis.

6.3 Materials and methods

The experimental details involves several importnat steps, which I will discuss in the following subsections. 167

Figure 6.13: Micrographs of scanning AnoporeTM membranes obtained from an electron microscope. The micrographs show the cylindrical structures of the conﬁning pores. The scale bars correspond to 0.3µm [42].

6.3.1 Material and sample preparation

The purpose of my current work is to study the molecular conformation of PDMS polymer on the surface at an equilibrium state. However, due to the molecular ﬂuctuations such studies are practically impossible at higher temperature. Thus we want to freeze these molecular ﬂuctuations by cooling the sample to below the glassy state. A glass state can be thought of as a frozen situation of an equilibrium state at higher temperatures due to a delayed relaxation time between different conformations. Thus, cooling of the sample below Tg and study the DNMR spectra was the main goal of my current work. As such I choose the previous work [36, 232] as a basis for my work to begin.

6.3.1.1 Anopore membrane

The beauty of the solid state NMR studies in our studies lies on the application of oriented porous media as a substrate for the molecularly thin polymer ﬁlm. This techniques use in the sample preparation was able to give measurable NMR signal.

TM Commercial thin Anopore membranes (Al2O3) were used as the substrate for the deposition 168

Figure 6.14: Molecular structure of d-PDMS molecule with Mw/Mn < 1.30. of PDMS thin ﬁlms. According to the AnoporeTM manufacturer, its surface is chemically compati- ble with a wide range of chemical solvents, as well as aqueous solutions. This includes chloroform and siloxane oils. Ideally it does not contain any surfactants or wetting agents, adhesives, plasticiz- ers or monomers [246]. The AnoporeTM membranes are made of an aluminum matrix of highest purity with a nondeformable pore structures as shown in Fig.6.13. These are commerical membranes: they have been used for many things. But Prof. Dr. Daneile Finotello2 had the idea of using them to study the conﬁnement of liquid crystals [247], which is what inspired Svetlana Primak to use these membranes to approach the current problem [232].

The AnoporeTM membrane surface is expected to be a pure aluminum oxide, which is strongly hydrophilic. It may thus have a layer of absorbed water on it. We expect it to be a good model hydrophilic surface. It should not affect the polymer otherwise. In particular, it’s pH should be close to neutral, and depolymerization of the PDMS is almost negligible fore pH < 11 [246].

6.3.1.2 PDMS

Predeuterated PDMS (d-PDMS) polymer was used for the current work. The sample was obtained from Polymer Source Inc.3. The sample was rated at 96-97% pure by the manufacturer and was used without furhter puriﬁcations. The details of its preparation can be found in the literature[248], which is beyond the scope of this work. The weight average molecular weight (Mw) of the sample is 10,000 and its ratio to the number average molecular weight (Mn), or polydispersity,

Mw is Mn < 1.30. Fig.6.14 gives the general idea about its molecular structure.

2Department of Physics, Kent State University, Kent, OH 3Polymer Source Inc., 771 Lajoie, Dorval, Quebec, Canada H9P 1G7 169

Figure 6.15: An illustration of how the DNMR spectra depends on the sample orientation (α)and 2 2 surface concentration (CS) of the the sample. (a) CS = 1.3 mg/m (b) CS = 0.8 mg/m (c) CS = as in (b), but diluted further .The spectra corresponds to the room temperature (∼ 300K) [36].

6.3.1.3 Sample preparation

Sample preparation was one of the most challenging steps for the DNMR experiments. Before we cooled the sample to below Tg, we wanted to see the reproducibility of the DNMR spectra at room temperature as a test of the sample. Then our goal was to cool the sample in order to freeze the molecular ﬂuctuation as as discussed in Sec.6.3.1. Note that one important variable to control is the surface concentration, on which the spectra depend markedly (Fig.6.15).

The quadrupolar splitting of DNMR signal due to PDMS sample is greatly affected by the molecular conformation at surfaces. Fig.6.16 illustrates such variations in quadrupolar splitting.

Samples with hydrophilic or non-silinated surface

Anopore membrane of 60 microns in thickness and 47 mm in diameter with pores 0.2 microns in diameter were cut into circular discs of 8 mm diameter for the sample preparation. They were washed with chloroform in order to ensure that they were free of any contamination before they were used in sample preparation. Spreading solution of d-PDMS was prepared in chloroform as solvent. Solutions of different concentration were used in different sets of experiments as test runs, 170

Figure 6.16: An illustration of the dramatic change in the quadrupolar splitting of 2HNMR spectra due to change in conformations (a) PDMS melt (b) spin-coated PDMS ﬁlms on top of polysterine brushes [43] and after [36]. in order to vary the surface concentration. First we started with the solution of concentration 6.2 mg/ml. Then 30 Anopore discs of 8 mm diameter were soaked in the d-PDMS solution. After 5 minutes the discs were taken out and spread in a wide aluminum foil and allowed to dry them. About

20 minutes later the discs were completely dried in an oven at a temperature of about 60◦C and then

ﬁnally transferred to a 17.8mm long and 8.76 mm diameter ﬂat bottom NMR tube (Norell Inc.).

The spectra obtained with this sample is shown in Fig.6.17a. A 10 mm Bruker HX probe was used

with this sample. This probe uses much energy during data acquisition. Thus, there always remains

a chance of damaging the sample by its use. The spectra obtained with the experimental conditions

just mentioned (Fig.6.17a) do not resemble with the spectra from the previous work (Fig.6.15).

We suspected that the difference in spectra between these two cases could be either due to the

difference in surface concentration of the sample or due to the damage in the sample caused by the

probe, or both. So, ﬁrst we tried with changing the spectrometer probe as well as slightly reducing

the concentration of the d-pdms solution (5 mg/ml). This time the old probe was replaced with a 5

mm TXI 13C Z probe keeping the number of discs and humidity of the Anopore discs the same. The

spectra with the new probe is shown in Fig.6.17b. We can see a slight improvement in the spectra,

but not enough. As a next step, I washed the and then used with the new prob just mentioned. The

spectra was as shown in Fig.6.17c. Here, we can see a big improvement in the spectra compared 171

to the last two trails. The improvement in spectra by washing the sample with chloroform suggests that the surface concentration had something to do with this improvement. We also considered the surface properties of the anodisc pores. Previous work had been done in the summer, with a naturally high humidity, which would lead to an adsorbed layer of water on the pores.

We saw increased improvement in the spectra when increasing the humidity. With 2 mg/ml of solution concentration and 80% of humidity, I was able to obtain the spectra as shown in Fig.6.17d.

Qualitatively, the spectra looks very similar to the one shown in Fig.6.15 except for the central peak. I was able to achieve 60-90% of humidity by conﬁning the Anopore discs in a desiccator containing pure water obtained from Purelab+, > 18.2MΩ-cm. The degree of humidiﬁcation of the

Anopore discs depends on the length of time the discs remain conﬁned in the “desiccator” (here we are not using desiccator for its proper purpose, but for the opposite one). For the rest of my experiments samples were prepared with ∼ 2 mg/ml solution concentration in about 60-90% of humidity. However, I increased the number of Anopore discs from 30 to 50 in order to accomodate more nuclei to improve the quality of the NMR signals.

We can estimate the surface concentrations of the samples used for obtaining the spectra shown in Fig.6.17 by using a graph from the previous work in our lab shown in Fig.6.18. The graph shows the qudrupolar splitting (4ν) as a function of the average surface concentration (CS). Except for

Fig.6.17a, outer peaks in the rest of the spectra in Fig.6.17 are prominent enough to make fair estimates of the outermost peak separations. In Fig.6.17b, the splitting is ∼ 2 kHz. In Fig.6.18, this value corresponds to surface concentrations range of 0.4 – 0.8 mg/m2. Spectra in Fig.6.17c is

∼ 5 kHz and this corresponds in the surface concentration range of 0.7 – 0.8 mg/m2. Similarly,

the splitting in Fig.6.17d is ∼ 6 kHz. In Fig.6.18, this splitting corresponds to 0.1 – 0.6 mg/m2.

This comparisons suggest that our sample concentrations fall in the vicinity of the ones used in the previous work. 172

Figure 6.17: DNMR spectra obtained during different stages of sample preparations: (a) solution concentration 6.2 mg/ml, 30 Anopore discs, 5 minutes in solution, 30-35% humidity(b) solution concentration 5 mg/ml, 30 Anopore discs, 5 minutes in solution 40-45% humidity(c) reduced concentration 3 mg/ml; signiﬁcantly diluted later, 30 Anopore discs, 5 minutes in the solution, sample washed in chloroform, 60-65% humidity (d) solution concentration 2 mg/ml, 30 Anopore discs with about 80% humidity, 5 minutes in solution.

Samples with hydrophobic or silinated surface

First of all, a 5% solution (by weight) of dimethyldichlorosilane (C2H6Cl2Si) was prepared

in 30 ml of choloroform4 by dissolving 2.25g of dimethyldichlorosilane. These conditions are

measurements consistent with those in the previous work. The already circularly cut AnoporeTM membrane discs were soaked in the solution just prepared and allowed to remain in solution for about 5 minutes. These discs were taken out of the solution and thoroughly rinsed in pure water

ﬁrst, then allowed to dry thoroughly in an oven at a temperature of 60◦. Finally, the rest follows the

method discussed in this section for the hydrophilic surface.

6.3.1.4 Orientation of sample with respect to the uniform magnetic ﬁeld in the probe

In the previous experiments [232], it was shown that the line shape of the 2HNMR spectra

depends on the orientation of the pores of the Anopore discs with respect to the external magnetic

ﬁeld B0. So we wanted to test several possible orientation of the sample with respect to the magnetic

4Note that the density of chlorofomm is ∼ 1.5 mg/cc 173

Figure 6.18: Variation of quadrupole splitting (4ν = ν − νL) with surface concentration CS. Dots represent data from the hydrophobic samples, (4) represent outermost peaks and (◦) represent intermediate peaks. The shaded vertical region corresponds to the transition from monolayer to multilayers [36]. 174

Figure 6.19: Schematic of probe orientation with respect to the sample pores. α is the angle between the normal to the pores of the sample and the applied magnetic ﬁeld B0 [36].

field of the spectrometer, but I started with α = 0◦, where α is the angle between the normal to the pore and the magnetic field B0. For this, the sample was placed in the spectrometer such a way that the steady magnetic field B0 of the spectrometer would be perpendicular to the surface of the

Anopore discs as shown in Fig.6.19.

6.3.1.5 Sample cooling

The sample in the spectrometer was cooled by using cold nitrogen gas where liquid nitrogen

(T = 77K) was used as the source of this gas by passing it from the liquid nitrogen cylinder to the spectrometer through a pipe. We were able to cool the sample to about 155K, but the ﬂuctuation of the temperature as such was in the order of 15K. Therefore we believe that the real temperature of the sample in the spectrometer may be higher than the temperature shown by temperature sensor.

Keeping in view of the safety of the spectrometer, that was the lowest attainable temperature that we were able to attain. The NMR spectrometer is not designed to work in such a low temperatures range. Given the temperature ﬂuctuations in the spectrometer, the temperature were able to attain was much higher than we wanted (we wanted to cool below Tm = 133K), as mentioned in Sec.6.3.1.

However, the data we have obtained in these experiments are worth comparing with the results from

the previous work. I will discuss them in Sec.6.4. 175

Figure 6.20: A block diagram of Fourier transform NMR spectrometer , after [44].

6.3.1.6 NMR Spectrometer

The NMR spectrometer setup used in the experiment is shown in Fig.6.20. In order to obtain the NMR signal, sample was placed in a sample probe, which is basically a coil wound around the sample tube as shown in Fig.6.20. A timing device called the “pulse programmer” sends out a precisely timed digital signal. The sequence of pulses is determined by a computer and sent to the pulse programmer. Then the radio frequency transmitter, “RF transmitter” in Fig.6.20, mixes a signal of the required frequency with the digital pulse train, which is amplified and sent to the sample probe. This process excites transitions between the nuclear spin states as discussed in Sec.6.2 and then these nuclei undergo a free induction decay (FID) reestablishing equilibrium. The steady magnetic field is provided by superconducting magnet, which maintains a homogeneous and stable magnetic field.

The spectrometer probe consists of a tuned radio frequency circuit. The inductor or coil is wound and positioned in such a way that the steady magnetic ﬁled B0 and the oscillating electromagnetic ﬁled are always mutually perpendicular to each other. The sample for the NMR spectroscopy is placed in the probe coil for the measurements. 176

The resonance condition or the tuned conditions of the spectroscope is obtained when the condition given by Eq.6.11 is obtained.

1 ωL = (6.10) ωC where C is the capacitance of the capacitor used in the circuit and L is the inductance of the coil.

The resonance frequency can then be written as

r 1 ω = (6.11) LC

In principle, the resonance frequency can be changed either varying L or varying C. However, in practice, it is usually easier to change the capacitance of the circuit.

The NMR signals as such are very feeble. In order to have useful signals, they must be amplified as soon as they are produced. The amplification of the signal is carried out by the amplifier as shown in the block diagram. The amplified radio frequency is converted into two audio signals, which are out of phase by 90◦. These signals are converted into a digital signal by an analog to digital (A-to-D) convert and then the signal are detected as the FID.

There are several things about which one needs to be careful for the better sensitivity and resolution of a NMR experiment. The quality of a NMR signal is mostly determined by the homogeneity of the static magnetic ﬁeld, the spectrometer probe and the electronics behind the process. However, the role of a sample is also very important. The more nuclei in the experimental sample, the better is the quality of the signal. Normally, most spectrometers are sensitive if 1017 or more nuclei are present in a particular sample.

6.3.1.7 Experimental parameters

Our results in this section are based on the low temperature experiments. The NMR spectra were

2 collected using magnetic ﬁeld B0 = 9.4 T at the H Larmor frequency of νL = 61.42 MHz using a

Bruker DMX-400 bore spectrometer. During data acquisition, a “solid echo” pulse sequence of the 177

π π (( 2 )x − τ − ( 2 )y) [249, 250] was used. The magnetic ﬁeld and the pulse sequences are similar to the one used in the previous work in our lab at low temperatures. A Bruker 5 mm TXI 13C Z was used in our experiments as opposed to the 10 mm HX probe in the previous work. For the ﬁrst couple of trial experiments at room temperature I also used the 10 mm HX probe. But, this probes takes much more energy and produces more heat compared with the 5 mm TXI 13C Z probe. After we realized that the 10 mm HX probe was damaging our sample, I discontinued its use. The number of scans were varied from 500 to 1200 depending on the temperature of the sample while cooling.

The interscan delays was 1.5 - 2.0s.

The next important step is to analyzed the data after acquiring them. The NMR data analysis is a somewhat tricky process. The acquired data cannot be directly used as they are in plotting

NMR spectrum. First they need to be processed. Usually, the acquired data are processed by using commercial softwares such as NutsLite. In my case, acquired data could not be backed up from the work station and later on they were lost, I had to rely on the scanned copy of the printed out data from the work station. However, a proper comparison with previous work and an evaluation of the problems still to be overcome will be possible with that data, were already processed on the spot by using a program called “XWinNMR”. The steps taken for this process are discussed below.

Apodization or Linebroadening

FID’s (free induction decay) are composed of coherent signal due to the resonance of the nuclei in our sample, and random noise arising from the electronics. Generally, want to maximize the intensity of the signals that we record relative to the baseline noise. One way of achieving this is to simply record data for a longer period of time, but there is an obvious time cost to doing this. For example, to double the signal-to-noise ratio, or S/N, we must record four times as many scans. To quadruple S/N, we need to increase the acquisition time sixteen-fold, and to achieve a

10x increase in S/N, the acquisition time must be lengthened 100-fold. Obviously, increased signal quickly becomes very costly in terms of time.

Another approach to increasing S/N, and one that can be used after data is collected, is called 178

apodization, or window processing or line broadening. The most commonly used window is a

Lorentzian curve. In a properly recorded FID, the resonances due to the sample are found in the earlier parts of the signal. When we multiply the FID by a window function, we suppress the data at the end of the acquisition period, which is (or should be) essentially all noise.

The spectrum that results from an apodized FID is slightly broader than that from the original data, but the advantages of improved S/N may outweigh the slight increase in line width. Thus, apodization functions are sometimes called line-broadening functions. We used ∼ 500 Hz for the line broadening

The Fourier transform

An NMR signal, or free-induction decay (FID), is composed of one or more damped oscillations.

(in rare instances, it can simply look like a decaying exponential). The signal is a plot of the voltage

(y-axis) induced in the spectrometer’s receiver versus time (x-axis). For this reason, an NMR signal is said to be in the time domain. The FID is composed of several different components of varying frequencies, intensities, and decay constants, giving rise to very complex signals that cannot be interpreted by visual inspection alone. In such cases, the Fourier Transform is used to sort out the components. The fourier trasform of the apodized FID was achieved by using the Fourier transform routine of the XWinNMR program.

Baseline correction

Baseline distortions in 1d NMR spectra are mainly caused by the corruption of the ﬁrst few data points in FID (free induction decay). These corrupted data points add low frequency modulations in the Fourier-transformed spectrum, and thus formed the distorted baseline. Correction of these distortions is a necessary step in NMR spectra data processing because they offset the intensity values and result in inaccuracy in peak assignment and quantiﬁcation. These errors could be critical in interpretation of NMR results [251]. The baseline correction routine of XWinNMR was used to correct the base line. 179

Phase correction

The next important step in the NMR data processing is the “phase correction”. As said earlier in this section, a FID is composed of many exponentially decaying NMR signal and even a single scan of data contains many such exponential decaying signals. Not all these signals are in the same phase. When these signals are transformed from time domain to the frequency domain spectrum, there might be asymmetry in the peaks of the NMR signal and they will lie on the opposite side of the frequency (usually horizontal axis) axis. Before we begin plotting the NMR spectrum, phase correction routine must be applied.

6.4 Results

Deuterium lineshapes are dominated by the orientation dependent quadrupole interaction of I =

1 spin. When the rapid motion of the molecules are anisotropic, the nuclear interactions are only partially time averaged as opposed to the isotropic molecular motions in a liquid sample where the spectra is a sharp peak. In the case of d-PDMS, the quadrupole coupling is preaveraged by fast methyl group rotation around the C3-symmetry axis as shown in Fig.6.21. Hence, the DNMR basically probes the mean orientation of the C3-symmetry axis of each CD3 group. The line shapes

of the spectra are dictated by Eq.6.9, where |S| ≡ |P2(cos β)| averages vq for fast CD3 group

motion with angle β around the symmetry axis. S can also be deﬁned as the order parameter for

the orientational order of CD3 groups and also the anisotropy in the chain segment reorientational

motion with a value of 0 ≤ S ≤ 1.

My current work on PDMS is limited to the low temperature experiments where the polymer

chains undergo a phase transition to a glass state passing through a melting point, Tm as mentioned

in Sec.6.1.1. I will present a qualitative comparison of my results with previous results obtained by

one of the researchers in our lab with same polyer. I will divide the discussion into two parts: (a)

PDMS deposited on a non-silinated AnoporeRM membranes and (b) PDMS deposited on a silinated

AnoporeRM membranes for the reasons discussed in Sec.6.1.3. 180

Figure 6.21: An schematic of showing the C3-symmetric axis of rotation of methyl groups in d- PDMS (after [36]).

6.4.1 PDMS deposited on hydrophilic AnoporeRM membranes

For this section of work, clean AnoporeRM membranes were cut into circular discs of 8 mm diameter and then humidiﬁed for the sample preparation. A solution of bulk concentration ∼ 2.0

mg/ml was used for the sample preparation. Liquid nitrogen was used as the source of cooling the

sample in the NMR spectrometer as discussed in Sec.6.3.1.5. The experimental parameter used in

the data acquisition are the same as discussed in Sec.6.3.1.7.

Fig.6.22a summarizes the the results that I was able to obtain at low temperature experiments.

The results from the previous work at low temperature shows that it has three quadrupole doublets

with 6 kHz outer peaks separation (Fig.6.22b). Qualitatively, our results resembles the previous

results. Let us begin wiht the spectra at room temperature. There are three peaks in both the spectra,

but with less prominent central peak in our case. The separation of central peak in our case is ∼ 5.5

kHz whereas in the previous work it is ∼ 6 kHz, which is comparable. Surface concentrations of

the samples in these two experiments would be a nice comparison since the quality of spectrum also

depends on the its value. However, determining the surface concentration of our sample requires

very careful calibration with a known PDMS sample and very careful integration of the dNMR

peaks, which was not possible here because of the loss of some data. Despite this fact, we can still 181

estimate our sample concentration from Fig.6.18. As discussed earlier in this section, in our case at room temperature, the splitting of the outer most peaks is ∼ 5.5 kHz (Fig.6.22a). This value corresponds to 0.65 – 0.80 mg/m2.

Below room temperatures at 243K, our spectra look very similar to the the previous one, at least qualitatively. If we make a qualitative comparison between the two spectra at ∼ 200K, certainly our spectra is showing the trend toward what we can see in the previous work: the peak narrows as structure is lost. The base of our spectra is not as flat as that of the previous work spectra. I could not find any information about the temperature error bars in the previous experiments, but because of the large fluctuation in temperature in our experiments, the temperatures are only approximately the same. As the temperature decreases, much wider structure, at ± 25kHz, develops. . In previous work, the spectra at T = 163K is somewhat like the 3D isotropic distribution (powder) of the CD3- rotors (a real Pake powder spectra should havea broad shoulder with maximum splitting of about

90 kHz [36]). In our case, we do not see such distinct peaks in the spectrum, but it is not clear that we have reached the same real temperature. In fact, our interest was to cool the sample below the bulk glass transition temperature to see the behavior of molecules at interface, but this was difﬁcult because of the reason discussed in Sec.6.3.1.5.

6.4.2 PDMS deposited on hydrophobic AnoporeTM discs

The motivation of this study is to compare PDMS behavior on both hydrophilic and the hydrophobic surfaces. As discussed in Sec.6.1.3, there is a contradicting explanations for the difference in the behaviors of hydrophilic and hydrophobic surfaces toward PDMS. It is believed [240] that the adsorbed amount on the hydrophobic substrate is signiﬁcantly less than that on a hydrophilic ones resulting in a difference in the mechanism of molecular attractions. Recent studies [241] indicate that the alignment of the linear polymer chains might also have some role to play in this scenario. Surface potential, which is related to the average dipole moment density in the monolayer, has also a signiﬁcant role to play in the molecular conformation [36].

Some work was done in the previous research with this type of sample at room temperature, but 182

(a) (b)

Figure 6.22: (a) DNMR spectra at low temperature for α = 0◦. Bulk concentration 2 mg/ml, Anopore discs humidiﬁed to ∼ 80% bofore polymer adsorption. (b) Low temperature runs for α = 0◦. Bulk concentration 4 mg/ml, surface concentration 0.5mg/m2. From [36]. 183

not at the lower temperatures. In this section, I will present a discussion of the DNMR results that I have obtained using DNMR techniques at low temperatures, which supplements the previous work with hydrophobic surfaces at room temperature

The previous results obtained in our lab [36] showed that the spectra at room temperature obtained with hydrophilic and hydrophobic substrates were strikingly similar at α = 90◦. Here it is worthwhile to mention that I started cooling the sample with α = 0◦ keeping in mind that if we succeeded we would also test for α = 90◦. But, since we could not cool the sample to our target temperatures, we dropped the idea of further testing at α = 90◦. Therefore, I do not have enough data to make a vis-a-vis comparison of my results with the previous ones at the sample orientation of α = 90◦ with respect to the magnetic ﬁeld of the spectrometer. However, looking at my own results at low temperatures for both hydrophilic and hydrophobic surface at α = 0◦, at least qualitatively they did not seem to be much different (Fig.6.23 for the hydrophobic substrate). At room temperature, both hydrophilic and hydrophobic spectra have three similar peaks. The separation of outer peaks in both cases are ∼ 5.5 kHz and the samples were prepared under the similar conditions except for the surface silinization for hydrophobic case. The bulk concentration of the solution used was also the same. At lower temperatures, note particularly the spectra at 243K in Fig.6.22a and in

Fig.6.23. The similarity in spectra suggests similar symmetry for the CD3 group motion. This is perhaps surprising for a very different interaction model.

6.5 Conclusion and future prospect

In order to study the molecular conformation of PDMS polymer on the surface at an equilibrium state in ultrathin films, DNMR is a useful technique. Previous work on PDMS using the DNMR method [232] has shown that the surfaces on which the polymer films are deposited, induces orientational order in the polymer chains for films less than about five monomer thick on average. Different molecular dynamics corresponding to the amount of polymer on the surfaces were investigated and the results presented substantial grounds to assume that a flattened caterpillar-like structure was the conformations for one monolayer whereas multilayer of flat is most likely for the thicker films. 184

Figure 6.23: DNMR spectra of D-PDMS ﬁlms on a salinated (hydrophobic) substrate at α = 0◦. Bulk concentration 2 mg/ml, Anopore discs humidiﬁed to ∼ 80% bofore polymer adsorption. 185

We were able to cool the sample to about 155K , but with large temperature ﬂuctuations (about

±15◦). For the reasons discussed in Sec.6.3.1.5 the sample could not be cooled to the targeted temperature and freeze the molecular ﬂuctuation on the surfaces. We compared qualitatively our results with the results from the previous work carried out in our lab [36, 232]. The spectra in these two cases were not exactly similar to each other, but ours did follow the trend of the spectra in the previous work (refer to Sec.6.4.1).

Our results with hydrophilic substrates looked qualitatively very similar to the ones with hydrophobic surfaces (Sec.6.4.2), which implies that that the CD3 group motion in both case is the same. These results were obtained at α = 0◦. The same type of similarity was reported in the previous work, but at room temperature. From the both sets of results we can infer that the CD3

group motion does not change with the degree of hydrophobicity of the surface.

As mentioned in Sec.6.3, sample preparation was one of the difﬁcult challenges that I had to

face. One of the reasons for the dissimilarities in the spectra between results from the previous

work and my present work could be due to unavoidable differences in the sample parameters. The

surface concentration on the sample affects the NMR spectra substantially. Similarly, total number

of NMR-active nucleus in the sample greatly affects the quality of NMR spectra. The probes used

are also different and this also might have contributed to the dissimilarity in spectra, because of

damage due to sample heating . We tried to adopt all possible sorts of precautions before and during

the experiments, but there always remained some possibility of sample contamination.

The work reported here can best be considered as setting the stage for future work, but below

Tg. I have overcome many, if not all, of the difﬁculties in sample preparation, but needs access to

an NMR spectrometer adapted to lower temperatures with a more modern DNMR probe. Circular

dichroism [231] can another useful experimental technique for distinguishing polymer conforma-

tions. The qualitative information given by DNMR spectra presented here and in the previous work

[36] are not sufﬁcient to understand the chain motion in polymer ﬁlms. A thorough understanding 186

of the geometry of the packing of macromolecular chain incorporated with the pendant group motion is a must for such information. Two dimensional exchange NMR can be a useful tool for slow motion scenario such as this [249]. CHAPTER 7

SUMMARY AND FUTURE PROSPECTS

This dissertation reports my studies in ultrathin films made of a polymerizable phospholipid, a bent-core molecule, and a polymer. Two of these films were studied as Langmuir layers, at the air/water interface. Our major tools for studying these films were Brewster Angle Microscopy

(BAM) coupled with the thermodynamic information that can be deduced from surface pressure isotherms. A complementary technique, solid state deuterated nuclear magnetic resonance (DNMR), probed the molecular conformation of ultrathin ﬁlms of the PDMS polymer on a solid substrate.

Chirality plays an important role in two of the ﬁlms. Phospholipid molecules are particularly chiral, both intrinsically and in conformation, and they gave rise to chiral patterns. Conformational and structural chirality, and the ability to control these, are a major reason for the interest in bent core molecules, both fundamentally and in their potential applications.

Langmuir films can be transferred to solid surfaces, through Langmuir/Blodgett and similar techniques, with important technical applications. Some of our work is in that direction. In other cases, the water surface provides a model surface where density and other properties can be readily controlled without interference from lateral surface structure. Langmuir layers can thus be used to better understand the important properties of such molecules at any surfaces. The phospholipid that we studied can find medical applications in drug delevery systems. Similarly, the bent-core molecules used in our study can find application as alignment layers for the bent-core liquid crystal molecules. The well-aligned bent-core molecules are ultimately used in switching and display devices. Additionally, Langmuir films are also good model systems for biomembrane research, as well as an excellent approximation to a two dimensional system, with multiple possibilities for exploring intrinsically two-dimensional behavior in terms of transport, phase behavior, and pattern formation.

187 188

I observed novel pattern formation in Langmuir ﬁlms made of kinked and rigid chiral lipid molecules. I discovered that such pattern formation takes below chain melting temperature Tm

(38 ◦ C). However, well-deﬁned patterns with deﬁnite characteristic length, were observed between

30-36 ◦ C. I discovered that the patterns undergo a transition from giant spirals to targets to branched stripes. At lower temperatures, close to 30 ◦ C, I found that the patterns are more like spirals but with many defects, while near the transition temperature, the patterns are more like circles and almost defect-free.

I tried to search for analogies of this pattern formation with previous examples both in terms of the types of patterns and their origins. I was was able to nearly eliminate certain possibilities of the origin of our pattern formation, although some of these mechanisms might contribute to the effect.

(i) No two different phases coexist, as seen at liquid disorded/liquid ordered phase coexistence in less chiral lipids. In that case, the characteristic stripe width would be determined by either growth kinetics or the competition of line tension and long-range forces, and chirality of the molecules or of impurities would lead to a characteristic spiral in domain arms. The domains we see are compact and circular once developed, with internal stripes, with no evidence of electrostatic interaction between domains. (ii) These patterns are probably not the common internal stripes seen in tilted monolayers that are due to the competition between elastic energies and boundary conditions at the domain edge.

The pattern of optical anisotropy is very unlike in those cases. (iii) There is no obvious nucleation barrier, which would be seen as a hump in the isotherms, and the patterns do not start near what could be nucleation centers, such as a dust particle. (iv) Elastic buckling would be expected to lead to characteristic lengths dependant on the elasticity of the layer just before buckling; no such dependence was observed.

We are not in a position to pin down the exact mechanism behind these pattern formation so far, but some mechanisms must be seriously considered. 189

(a) Polymerization of the Langmuir ﬁlm

When a Langmuir ﬁlm is polymerized it can undergo buckling with characteristic length scales

[23]. Langmuir films of similar lipids are known to polymerize, but only when neighboring molecules have the exactly correct orientation and spacing [170]. The temperature dependence of the transition pressure could reveal the conditions for such spacing. If this were true, our argument against buckling, that the length scale was independent of the elasticity of the underlying layer, would be irrelevant: it would be the elasticity of the polymerized layer, not the layer just before polymerization started, that is relevant. Our thin layer chromotography (TLC) results suggested that there may be some degree of polymerization of the film. However, the sample remained in the freezer for a prolonged time (about a year) before the TLC analysis could be performed. The TLC results say that the sample was polymerized at some point, but cannot confirm whether the sample polymerized when it was on the water surface or later in the freezer. The possibility of the role of polymerization cannot be rejected, since our lipid itself is highly polymerizable when exposed to UV radiation and oxygen-containing environments. Prompt testing for polymerization should be a priority for the future; I have shown that TLC is an effective method for that test. If polymerization is at the origin of the patterns I observe, then buckling seems a likely cause. However, the very regular patterns in this system have never been observed in such buckled patterns, and would still need to be explained.

(b) Tilt variation

With the more improved resolution of our BAM, we were able to see the claw-like substructures in the pattern domains. The growth of these claws suggested the growth of a more ordered phase into a disordered one. There is some evidence for tilt variation across individual claws, in the polarization of reﬂected light with the curve of the claws. This should be further analyzed. It is possible that the tilt mechanism, of a balance of elasticities and boundary conditions, as mentioned in (ii) above, is after all appropriate to our patterns, but within the individual claws making up the the stripes rather than the stripes themselves. One would still have to explain the stripes, perhaps merely a case of steric hindrance when between individual claw-like structures. 190

Chirality should be considered in our pattern formation since the molecules are strongly chiral by nature, and claw-like substructure is clearly chiral in nature. The transition from spirals to targets could be due to the disappearance of chiral inﬂuence. One model for the inﬂuence of the chirality, discussed in Sec.4.4, suggests that the boundary conditions change between the spiral and circular stripes; this could be tested. However, the place of the observed substructure in this model is not clear.

Different types of techniques must be brought to bear to answer these questions. Some of the mechanisms, such as role of the boundary conditions, have been very difﬁcult to quantify in this system with the implementation of the Brewster angle microscope that I used for most of this work.

Recent improvements in the instrument resolution show substructures in the one case tested, at

30◦ C, which were only hinted at in the lower-resolution experiments. We need to explore further in this direction, over a larger temperature range.

As discussed earlier in this section, we also studied Langmuir layers of a of bent-core molecule.

The bent core molecules have greater hydroplhilicity compared to those used in previous studies in our lab. The goal of forming more stable Langmuir ﬁlms by changing the molecule to make it more hydrophilic was attained with the new Z2b bent-core molecule. No hysteresis was observed at moderate pressures, and even at higher pressures, th hysteresis was much less that for the earlier bent core molecules. Decompression leads to the ﬁlm respreading to a very viscoelastic layer, whereas the other bent-core molecules in the previous work just broke, leaving at most a 2-d gas around multilayers.

Not only did these molecules form stable monolayers; their relative stability allowed one to compress the films and observe different configurations at the surface: quite flat on the surface, or more upright. Optical anisotropy suggests that these layers would be useful to explore as in-plane alignment layers for bent-core liquid crystals, and that the surface pressure would be an interesting control parameter by allowing one to explore the effect of different configurations in the alignment 191

layer.

In order to investigate the properties of ultrathin ﬁlms of PDMS polymer, I employed deutarated nuclear magnetic resonance (DNMR). As a basis of comparison, I used previous work on PDMS using the DNMR method [232]. Sample preparation was a difﬁcult challenge in my DNMR experiments both with hydrophilic and hydrophobic AnoporeTM supports. In later experiments I found that that the humidity is important in the hydrophilic case.

Our preliminary results indicate that the polymer conformation are likely to be similar to the conformation on water, but might be trapped on a solid without a water layer. In order to ascertain the conformations, more experiments need to be done by freezing the molecular fluctuations by sufficiently cooling the sample. Cooling the sample sufficiently low temperatures, below the glass state, was particularly difficult in the NMR spectrometer. I tried to cool the sample and reached almost , but not quite, to the desired temperature range. Thus in this project I have mostly set the stage to do definitive experiments if an NMR machine capable of handling lower temperature well can be identified.

I have thus worked with three different molecularly thin ﬁlms. In all cases, it was necessary to consider carefully the correlation of molecular conformation and interactions and macroscopic behavior. Despite the fact that better control over several different experimental parameters and the interpretation of results were sometimes frustrating, the reward of learning different techniques was invaluable. To put the whole thing in a nutshell, perhaps what shines most clearly through this work is the manifold challenge of this project. APPENDIX A

FOUR-ROLL MILLS

The four-roll mill is a device which finds applications in the study of two-dimensional fluid dynamics [252], drop deformation [253, 254], birefringece in polymer solutions [255], and flow induced crystallizations in polymer melts [256]. Origially, it was invented by G.I Taylor [257] to study drop deformation and break up of a fluid drop in a suspended fluid.

As one of the summer projects in 2003 in Dr. E. K. Mann’s lab I worked with the four-roll mill. My project included to ﬁx the existing mechanical problems and then test its workability for stretching multilayer domains of 8CB liquid crystal at the air/water interface. Additional, I also wrote a program to better control the motion of four-roll mill in different experimental condition. A detail work on it was already been done by a researchers in our lab [45]. Here, in this chapter I will just present my main contributions.

A.1 Design and components

A four-roll mill consists of a gear box with Mitter gears ﬁtted with two rods (Fig.A.1), a step- motor (in Fig.A.1 it is referred to as ‘Motor and gear’), four rollers made of Delrin. The motion of these rollers are such that two roller in one diagonal move in the clockwise direction and the other pair of rollers in the other diagonal move in the counterclockwise direction.

The bottom view of the rollers is shown in Fig.A.2.

A.2 Test of ﬂow ﬁeld

When the four-rolls are in motion submerged in a subphase, they produce flow field in the subphase. The quality of the flow field depends mainly on the correct placement of mitter gears and four rolls. It also dipends on the correct size of four rolls. If they are of different size and thikness, their position will not the same with respect to the level of the subphase surface. This will create

192 193

Figure A.1: (a) Schematic side view of the four-roll mill. Dimensions of roller are a = 6.6 mm, b = 10.5 mm, and d = 7.9 mm. The blowup on the left shows one of the rollers submersed in the subphase together with the monolayer of amphiphilic molecules. (b) Top vies of the inside of the gear box. The gears are mitter gears. These gears transfer motion onto two mutually orthogonal axes. From [45]

Figure A.2: Schematic of the four-roll mill rotation proﬁle. From [45] 194

incorrect ﬂow ﬁeld.

When I undertook the project and tested the flow field, I found that it was far from the correct shape. The test was carried out by using Reagent grade glycerol and food color taken in a flat glass pan. The four-roll mill and the test material were placed in a properly leveled table. It looked very asymmetric with respect to the center of the field of flow (Fig.A.3). I examined the instrument carefully and spent some time working on it. Later on I discovered that the mitter gears were worn out and the rods of both in the gear box and that of rolls were not in the proper place. This type of mechanical problems are normal since the metallic rods and gears can wear out after they are in prolonged use.

Figure A.3: Flow ﬁeld produced by the rolls which are wobbling from their place. The metter gears were worn out.

In the process of taking corrective measures, ﬁrst I tried with tightening the wobbling rods and the realigning the metter gears. This did not solve the problems, rather it worsened further. Then with the help then the machinist in our physics department Mr. Josh, I redesigned the gear box, replaced the old metter gears with the new ones. That alone did not solved the problems. Then, I also replaced the rods of the mills and the rods of the gear box. This showed improvement, but not the in the way that it should be (Fig.A.4).

The next step I took was to change the derelin rolls as well. That help get much better result.

Fig.A.5 is the final version of the flow field with the corrected four-roll mill.

The four-roll mill was then directly used to stretch domains produced in the 8CB multilayers at 195

Figure A.4: Improved ﬂow ﬁeld produced by the rolls after replacing the gear box and the worn out metter gears.

Figure A.5: The correct ﬂow ﬁeld produced by the rollers when the new mitter gears, new rods and the new roller were placed correctly. 196

the air/water interface.

A.3 Program for the the RP240 controller

In order to run the four-roll mill smoothly with the facility of running for a set time, stop for the desired interval of time and then reversed the motion without quitting the program I wrote the following program named “ exampl1”. The table A.1 summarizes the command used in the program.

Table A.1: Command for the RP240 controller

Command Description DCLEAR Clear the display DJOG Enable/Disable RP240 Jog Mode DLED Turn RP240 LEDs on/off DPASS Change RP240 Password DPCUR Position Curser DREAD Read RP240 Data from Numeric Keypad DREADF Read RP240 Function keys DVAR Display Variable on RP240 LCD DWRITE Write Text to RP240 LCD

exmpl1 program

del exmpl1

def exmpl1

drive1

mc0

ma0

lh3

dpcur1,0

dwrite ”acl”

dpcur1,2 197

var1=a dvar1,2,,0 dpcur1,2 a(dread) dpcur2,0 dwrite ”decl” dpcur2,2 var2=ad dvar2,2,,0 dpcur2,2 ad(dread) dpcur2,5 dwrite ”vel” dpcur2,8 var3=v dvar3,2,,0 dpcur2,8 v(dread) dpcur1,12 dwrite ”dist” dpcur1,15 var4=d dvar4,2,,0 dpcur1,15 d(dread) go 198

end APPENDIX B

MATHEMATICA PROGRAMS FOR REFLECTIVITY CALCULATIONS

B.1 Mathematica program for the p-polarize light

This program is used for the calculation of theoretical values of reﬂetivities of s-polarized light with change in the angle of incidence, generate numerical data, and then export them to a text ﬁle.

Written by Prem Basnet, Department of Physics, Kent State University, Kent OH, USA.

(* This program calculates reﬂectivity of a thin ﬁlm at the air/water interface for p- polarized light.*)

(* Here I use the following sign conventions: θ1 = x; θ2 = y; θ3 = z; r12 = r1; r23 = r2;

φ = z1 *)

n2 ∗ Cos[x] − n1 ∗ Cos[y] r1 = n2 ∗ Cos[x] + n1 ∗ Cos[y]

(* The results due to the command above will show up here.*)

n3 ∗ Cos[y] − n2 ∗ Cos[z] r2 = n3 ∗ Cos[y] + n2 ∗ Cos[z]

(* The the following command deﬁnes the angle y as described in the beginning of this program.*)

n1 y = ArcSin[( ) ∗ Sin[x]] n2

(* The the following command deﬁnes the angle y as described in the beginning of this program.*)

n2 n1 z = ArcSin[( ) ∗ Sin[ArcSin[( ) ∗ Sin[x]]]] n1 n2

(* The the following command clears preassigned value of d if there is any.*)

199 200

Clear[d]

(* The the following command assigns a new value 0 to d.*)

d = 0

(* The the following command deﬁnes z1.*)

2 ∗ π ∗ n2 ∗ d ∗ Cos[x] z1 = λ

(* The the following command assigns values for n1, n2, n3, λ, and x.*)

n1 = 1.000

n2 = 1.550

n3 = 1.333

λ = 668

x1∗π x = 180

(* The the following command produces a large array of unformatted data (not shown here) for the the reﬂectivity for the p-polarized light.*)

r12 + r22 + 2 ∗ r1 ∗ r2 ∗ Cos[z1] R = p 1 + r12 + r22 + 2 ∗ r1 ∗ r2 ∗ Cos[z1]

(* The the following command produces converts the data in the numerical format and arranges in

the tabular form in two columns. data0 is a new variable name of the tabulated data. The range of

data for x1 is from 52.00 degree to 54.00 degree at an interval of 0.005 degree. *)

data0 = Table[{x1, N[Rp] }, x1, 52.00, 54.00, 0.005]

(* The following command creates a new directory ‘p-polarize’ in the Desktop. *) 201

SetDirectory[“ C:\\Users \\Prem Basnet \\Desktop \\reﬂectivity \\p-polarize ”]

(* The following command exports the reﬂectivity data as a data ﬁle named ‘trial0.dat’ to the folder

‘p-polarized’ created above. *)

Export[“trial0.dat”, data0, “Table”]

B.2 Mathematica program for the s-polarize light

(* This program calculates reﬂectivity of a thin ﬁlm at the air/water interface for s-polarized light.*)

(* Here I use the following sign conventions: θ1 = x; θ2 = y; θ3 = z; r12 = r1; r23 = r2;

φ = z1 *)

n1 ∗ Cos[x] − n2 ∗ Cos[y] r1 = n1 ∗ Cos[x] + n2 ∗ Cos[y]

(* The results due to the command above will show up here.*)

n2 ∗ Cos[y] − n3 ∗ Cos[z] r2 = n2 ∗ Cos[y] + n3 ∗ Cos[z]

(* The the following command deﬁnes the angle y as described in the beginning of this program.*)

n1 y = ArcSin[( ) ∗ Sin[x]] n2

(* The the following command deﬁnes the angle y as described in the beginning of this program.*)

n2 n1 z = ArcSin[( ) ∗ Sin[ArcSin[( ) ∗ Sin[x]]]] n3 n2

(* The the following command clears preassigned value of d if there is any.*)

Clear[d] 202

(* The the following command assigns a new value 0 to d.*)

d = 0

(* The the following command deﬁnes z1.*)

2 ∗ π ∗ n2 ∗ d ∗ Cos[x] z1 = λ

(* The the following command assigns values for n1, n2, n3, λ, and x.*)

n1 = 1.000

n2 = 1.550

n3 = 1.333

λ = 668

x1∗π x = 180

(* The the following command produces a large array of unformatted data (not shown here) for the the reﬂectivity for the p-polarized light.*)

r12 + r22 + 2 ∗ r1 ∗ r2 ∗ Cos[z1] R = s 1 + r12 + r22 + 2 ∗ r1 ∗ r2 ∗ Cos[z1]

(* The the following command produces converts the data in the numerical format and arranges in

the tabular form in two columns. data1 is a new variable name of the tabulated data. The range of

data for x1 is from 52.00 degree to 54.00 degree at an interval of 0.005 degree. *)

data1 = Table[{x1, N[Rs] }, x1, 52.00, 54.00, 0.005]

(* The following command creates a new directory ‘s-polarize’ in the Desktop. *)

SetDirectory[“ C:\\Users \\Prem Basnet \\Desktop \\reﬂectivity \\s-polarize ”] 203

(* The following command exports the reﬂectivity data as a data ﬁle named ‘trial1.dat’ to the folder

‘s-polarized’ created above. *)

Export[“trial1.dat”, data1, “Table”] APPENDIX C

THIN LAYER CHROMATOGRAPHY

C.1 Basic Understanding

Chromatography is an experimental technique used to separate mixtures of substances into their constituents. Basic principle of all forms of chromatography is the same [46].

A chromatography needs a stationary phase, which can be a solid, or a liquid supported on a solid. It also needs a mobile phase. The mobile phase, as the name suggests, ﬂows through the stationary phase carrying the components of mixtures with it. In the compound, different components move at different rates.

In thin layer chromatography a thin uniform layer of silica gel or alumina coated into a piece of glass, metal or a rigid plastic is used. The silica or alumina layer acts as the stationary phase.

C.1.1 Chromatography

As shown in Fig.C.1, a pencil line is drawn near the bottom of the plate and a small drop of solution of the mixture of different compounds is placed on it. A reference labelling on the plate must also be drawn with a pencil.

Figure C.1: A schematic diagram of thin layer chromatogram. After [46].

When the spot of the mixture is dry, the placed is placed in the shallow layer of solvent in a covered thin layer chromatography box made of glass. It is important that the spot must be above

204 205

the solvent level. The box must be covered so that it is saturated with solvent vapor.

As the solvent slowly travels up the plate, the different components of the mixture travel at different rates and the mixture is separated into different spots. The solvent is allowed to move up to the reference line drawn on the plate with a pencil. Further details of the principles of thin layer chromatography is beyond the scope of my current work.

C.2 Experiment

The thin layer chromatography (TLC) experiments involves several different steps. These steps will be discussed in the following subsection.

C.2.1 Sample preparation (Isolation of phospholipid from the solution)

In a TLC experiment, isolation of the sample material from a mixture or a solution is one of the important steps which requires the following [258],

Reagents

(a) chloroform

(b) methanol

Procedure

One volume of sample suspension (or solution), two volumes of methanol and one volume of chloroform are mixed together and shacked well for about ﬁve minutes. If a one phase system is still not obtained, then 0.2 volume of methanol, 1 volume of aqua dest and 1 volume of chloroform is added further, shaken for a few times and waited until the phase separation is completed. The chloroform phase is collected in a ﬂask and the methanol/water phase is washed twice with 1 volume of chloroform. Chloroform phase is collected again and washed with 1 vol. of aqua dest (or buffer).

In order to collect the separated phases, a phase separating funnel is used as shown in Fig.C.2.

Finally, chloroform is evaporated, preferably in nitrogen environment. 206

Figure C.2: A schematic of a separation funnel.

C.2.2 Preparation of developing solution

In a thin layer chromatography (TLC) experiment, developing solution has very important role to play. A developing solution is prepared in a standard TLC (Fig. C.3) box made of glass. The following are the steps of the preparation of a developing solution.

• Enough chloroform is taken in the TLC box

• Methanol is added

• Water is added

• It was check if there are air bubbles

• If there are air bubbles, more methanol is needed to dissolve them

Those were the cleaning steps. The solution prepared above is thrown away. Now 65 ml of chloroform, 25ml of chloroform, 25 ml of methanol and 4 ml of water is taken in the TLC box. The 207

Figure C.3: A schematic of a TLC box. The shaded portion of the tank represents the devoleping solution. mixture is shaken well by covering the top of the the TLC box. The bubbles are checked again.

Finally, the solution is allowed to saturate for an hour.

The collected phospholipid sample, as described in Section C.2.1 is now used to perform the

TLC experiments by using TLC plates. A TLC plate is basically a thin glass pate coated with a very thin layer of silica on the one side of it. In a TLC plate, a small drop of sample, which is to be tested, is placed along side the dots of the other standard samples. A horizontal line is drawn above the positions of the dots to which the front and the sample is allowed to move up. Name of the each sample is also marked with a pencil above the horizontal line. When these drops dry, the plate is exposed to iodine environment taken in a glass jar. The plate is left for a while until the positions of the drops reaches the horizontal pencil marks. If the spots move toward the standard horizontal line almost at the same rate with no other marks close to them as shown in Fig.3.40a. On the other hand if the same spot splits into different components and move with the different rate toward the standard line with different rates, the sample is considered to contain different oligomers depending upon the degree of polymerization as shown in Fig.3.40b. BIBLIOGRAPHY

[1] J. Wang: Langmuir and Langmuir/Schaefer Films For Bent-Core Molecules, Ph.D. thesis,

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