Toward two-dimensionally ordered substituted polyacetylenes

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Toward two-dimensionally ordered substituted polyacetylenes ordered monomer and preformed polymer approach
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Advincula, Rigoberto C., 1967-
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Thesis (Ph. D.)--University of Florida, 1994.
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Includes bibliographical references (leaves 254-266).
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by Rigoberto C. Advincula.
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Typescript.
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Vita.

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TOWARD TWO-DIMENSIONALLY ORDERED
SUBSTITUTED POLYACETYLENES: ORDERED MONOMER AND
PREFORMED POLYMER APPROACH










By

RIGOBERTO C. ADVINCULA


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE
UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA


1994
























This dissertation is dedicated to my parents,
Atty. Benedicto F. Advincula and Evelina C. Advincula
for their love, support, encouragement, and desire
to see their children live fruitful lives.










ACKNOWLEDGMENTS


So many special people deserve to be acknowledged. My experience
with graduate school would not be the same without them. First and foremost,
let me begin with my advisor, Dr. Randy S. Duran. The guidance, opportunities,
encouragement, and experience he bestowed on me are greatly appreciated.
My thanks also go to the other professors on the "polymer floor," G.
Butler, K. Wagener, and J. Reynolds. Their experience in the science of
polymers as well as encouragement are appreciated. In the organic division, I
wish to thank Dr. J. Zoltewicz for his advice and friendship. In the chemistry
department, I am indebted to Dr. J.Helling. To the other members of my
graduate committee, Professors M. Vala and C. Batich, I wish to thank them for
their availability. My thanks to Dr. D. Shah of the department of chemical
engineering for making the learning of surface science interesting.
My stay in France would not be as fruitful and exciting without the
guidance and encouragement of Dr. J. Le Moigne. The friendship of the
professors, students, and staff of IPCMS-GMO group are greatly appreciated.
Special thanks to L. Oswald, for her friendship and assistance, and to H.
Hilberer, for the availability of his dissertation work.
I wish to acknowledge technical support from Oriel Corporation through
Mr. C. Calling, KSV Instruments, and the analytical support group of the Institut
Charles Sadron, Strasbourg through Mr. M. Keyser.
Special thanks go to the secretarial staff who always had a ready smile
most specially to Mrs. Lorraine Williams, who was always available and willing
to help and "never busy."







I want to thank all my past and present peers at the "polymer floor," J.
Roberts, R. Bodalia, A. Thibodeaux, T. Herod, P. Bernal, W. Sigmund, J. Adams,
W. Rettig, M. Naumann, H. Fadel, H. Zhou, S. Kim, D. Tao, B. Sankaran, F.
Zuluaga, P. Balanda, D. Smith, C. Marmo, J. Portmess, K. Novak, and to the rest
of the past and present groups of Dr. Wagener and Dr. Reynolds. I want to
thank J. Roberts especially for his friendship and the academic experience we
shared as colleagues.
I wish to thank my family in the Philippines for their encouragement and
support for all these years. The most special acknowledgment is to my wife
Carolyn, who is equally deserving of a Ph.D. for her love, encouragement,
support, and prayers for me.










TABLE OF CONTENTS



ACKNOWLEDGMENTS ..............................................................................................iii

ABSTRACT .............................................................................................................ix
CHAPTERS

1 INTRODUCTION ...............................................................................................1
The Outlook of Nonlinear Optical Materials ...............................................1
The Phenomena of Nonlinear Optics............................. ............ 3
Organic Materials for Nonlinear Optics.........................................6
Polymers for Third Order Nonlinear Optical Materials...................10
Substituted Polyacetylenes.......................................................................17
Cyclopolymerization of Unconjugated Diynes..........................................22
The Langmuir-Blodgett Technique........................ ......................................27
The Monolayer Phenomena..........................................................28
Langmuir-Blodgett Apparatus .............................................. ....29
LB Vertical Deposition Technique ........................................ ... 32
Polymers at the Air-Water Interface....................................... ..34
Theory of Polymer Behavior at the Interface..................................39
Comparison with small molecules.......................................41
Polymer relaxation and equilibrium at the interface...........42
Polymers and water.............................................................44
Interactions of polymers at the interface.............................45
Langmuir-Blodgett Films and Materials for Nonlinear optics.......45
Summary and Overview of the Dissertation....................................46

2 EXPERIMENTAL ............................... ................................................ .. 50
M ate rials .............................................................................................................5 0
Monomers and Polymers ................................... .......................................50
Monoacetylenic Liquid Crystalline Monomers..............................50
Polyethynylbenzoate Polymers.....................................................51
Monomer synthesis ................................................... ........52
Typical polymerization .............................................................55
Synthesis of Poly(diethyldipropargylmalonate) ..............................57
Monomer preparation ............................................... .....57
Typical polymerization ............................................... ....58
General Instrumentation ........................................................................... 61
Langmuir-Blodgett Technique.............................. ........................................63
Langmuir Monolayer Film Studies ....................................... ... 63
Langmuir-Blodgett film deposition................................................64
Surface Analytical Techniques at the Air-water Interface .............66







Surface potentiometer ............................................... ....66
UV-vis spectroscopy at the interface...................................68
Brewster angle microscopy and reflectivity........................71

3 APPROACH I
POLYMERIZATION OF ORDERED MONOMERS................................74
Introduction ........................................................................................................74
Liquid Crystalline Monomers at the Interface................................74
Objectives ......................................................................................... 76
Bulk Characteristics of the Monomers............................................77
Langmuir Monolayer Film Studies ..........................................................79
Surface Pressure-Area Isotherms ........................................ ...79
Stability of the Monolayer............................. ....................................81
Brewster Angle Microscopy ..........................................................83
Analysis of Results........................................................................88
LB Film Deposition and Characterization ....................................... ...89
D position ..............................................................................................89
UV Spectra of LB Films......................................................................90
X-ray of LB Films ........................................... ......... ........91
Polymerization of LB Films................................ ............................................92
Polymerization................................................... ............................ 92
Monolayer polymerization
attempt at the air-water interface..................................92
Polymerization of LB Films...............................................................93
Analysis of Results........................................................................ 95
S um m ary ..........................................................................................................9 9

4 APPROACH II A
POLY(ETHYNYLBENZOATE ESTER) POLYMERS..........................1 00
Introduction
Design of Polymers ............................................................................ 00
Synthesis of Monomers and Polymers .......................................................102
Analysis of Characterization Data .....................................................109
Monolayer Studies ........................................................................................... 13
Surface Pressure-Area Isotherm .....................................................113
Surface Potential ................................................................................ 16
Isobaric Creep Stability ..................................................................... 18
Temperature Dependence.............................................................1 19
UV-Vis Measurements ....................................................................... 20
Poly(ethynylbenzoate alkyl esters)
Alkyl Chain Length ............................................................................. 25
Isotherm measurements ....................................................................127
Hysteresis......................................................................................... 129
Temperature Dependence..................................................................131
Surface Potential Measurements ......................................................133
Brewster Angle Microscopy ....................................................................... 40
Deposition of LB films ....................................................................................145
S um m ary ............................................................................................................1 52







5 APPROACH II(B)
POLY(DIETHYLDIPROPARGYLMALONATE) ...................................... 155
Introduction ........................................................................................................ 55
Synthesis of the Polymer.................................................................................158
Thermogravimetric Analysis ............................................................. 66
Langmuir-Blodgett Film Investigations .......................................................168
Langmuir Film Studies
Monolayer structure and orientation ...................................168
Surface-pressure area isotherm..........................................168
Surface potential isotherms....................................................174
UV-vis measurements at the air-water interface...............180
Brewster angle microscopy (BAM) ...................................... 83
Relative reflectivity ..................................................................189
Analysis of Results................................................................................191
The Chemical Structure of the Repeat Unit...................................193
The ester side group ..............................................................194
Methylene hydrogen ............................................................ 97
Hysteresis
Compression -expansion cycles..........................................199
Surface pressure-area isotherm hysteresis behavior........200
Surface potential-area isotherm hysteresis behavior........203
Brewster angle microscopy (BAM) and reflectivity-
hysteresis ..................................... ..........................................206
In situ UV-vis spectroscopy..................................................... 212
Deposition to Substrates ......................................................................................224
Multilayer Deposition of Pure Polymers .........................................224
Polarized ATR-FTIR .......................................................................225
Alternating Multilayers .....................................................................227
S um m ary..................................................................................................... 231

6 CONCLUSION
COMPARISON OF APPROACHES AND
PROCESSABILITY OF SUBSTITUTED POLYACETYLENES.........234
Polymerization of LB films
reactive functional group and initiators.........................234
Poly(ethynylbenzoate)configuration and
lateral alkyl side chain length.........................................235
Poly(diethyldipropargylmalonate)
Molecular weight and alternate layers..........................236

APPENDICES

A NONLINEAR OPTICS
THE THEORETICAL VIEWPOINT......................................................238

B THE CONJUGATED
POLYMER BACKBONE ......................................................................245







C SURFACE POTENTIAL
AND DIPOLE MOMENT.............................................................................252

REFER ENC ES ........................................................................................................ 254

BIOGRAPHICAL SKETCH ....................................................................................267












































viii











Abstract of Dissertation Presented to the Graduate School of the University of
Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of
Philosophy

TOWARD TWO-DIMENSIONALLY ORDERED
SUBSTITUTED POLYACETYLENES: ORDERED MONOMER AND
PREFORMED POLYMER APPROACH


by
Rigoberto C. Advincula

April 1994

Chairman: Professor Randolph S. Duran
Major Department: Chemistry

The Langmuir Blodgett approach was used to enhance ordering of
substituted polyacetylenes. The ordered monomer and preformed polymer
approaches were taken. The monolayer properties of both monomers and
polymers were analyzed by surface pressure-mean molecular area isotherms,
their hysteresis, and stability behavior. In addition, surface potential, Brewster
Angle Microscopy, and UV-vis spectroscopy were used to investigate the film
behavior. Langmuir-Blodgett films were built-up using the vertical deposition
technique.
Three liquid crystalline monoacetylene monomers where investigated.
Greater monolayer stability was observed at lower subphase temperatures and
the use of an interactant. Z and Y type multilayers were formed by deposition
on a hydrophobic substrate. X-ray and UV-vis confirmed the multilayer integrity
of these films. No polymerization was observed at the air-water interface but
polymerization was observed with y-irradiation with some degradation of the

mesogenic substituent.







The use of preformed polymers, was investigated initially with a series of
poly(ethynylbenzoate) polymers. The stiffness and length of the lateral
substituent has an effect on the backbone microstructure. The polymers were
synthesized using the metathesis catalyst, WCI6. The results were discussed in

terms of the configuration and conformation adopted by the polymer in the bulk
and monolayer films. It was found that a better configuration amenable for
planarity is achieved with a head-to-head trans-transoidal configuration. The
length of the alkyl chain is important in facilitating monolayer behavior.
Poly(diethyldipropargylmalonate), a polymer with improved
configuration, was synthesized via cyclopolymerization. The behavior of the
polymer at the air-water interface was studied. Comparison was made of high
and low molecular weight derivatives. The contribution of the ester group and
the backbone on the monolayer behavior was analyzed. Interesting
conformation dynamics of the polymer backbone was observed by UV-vis
spectroscopy. The low molecular weight derivative had an overall better
monolayer behavior than the high molecular weight polymer. Successful
deposition of both polymers was made using the vertical deposition technique.
LB films up to 30 layers on glass and silicon substrates gave an average
transfer ratio of 1.0 0.05. Interesting superlattice structures were also made by
alternating with stearic acid layers.










CHAPTER 1
INTRODUCTION


Two decisive aspects in the field of science are tradition
and innovation. Tradition is the basis for it is the cumulation of
wisdom in the body of knowledge. To know what a subject is all
about and to control it creates self-confidence, thus paving the way
for innovations. Innovation is the adventure, since with the
challenge comes the risk of calling into question (or even losing)
one's own scientific identity, gained through tradition.

Excerpt from a review article by Helmut Ringsdorfl


The Outlook of Nonlinear Optical Materials

The development of mode-locked lasers capable of generating pulses of
sub-nanosecond duration allowed the direct observation of nonlinear optical
phenomena in materials. This was first reported by Franken et al. in 1961, by
generating the second harmonic frequency of the incident light through a quartz
crystal.2 During that decade, most of the fundamental observations and
discoveries were made. This paved the way for the invention of tunable
wavelength dye lasers during the 1970s which utilized nonlinear effects such as
harmonic generation, sum and difference frequency mixing, and stimulated
Raman scattering.6 Most of these processes utilized inorganic semiconductors,
and dielectric crystals as their nonlinear media. The early 1980s saw the
advent of telecommunications using optical fibers, which has been the most
important stimulus for the explosion of effort in nonlinear optics during the last
decade.3 It is now common to find information carried on a laser beam in the
processes of communication, information storage, information retrieval, printing,




2

and sensing. There is increasing effort to achieve even greater data processing
capabilities, by using ultrafast electronic interactions and by harnessing the
properties of laser light for highly parallel manipulation and interconnection of
signals. This is the age of photonics.
It is but natural that most of the early experimental and theoretical
investigations were concerned with inorganic materials since they constituted a
major part of materials in solid state physics at that time. Because of the growth
in new applications, it became apparent that new types of materials were
needed. Attention was turned to organic materials, which gained a wide
reputation during the 1980s as the next generation of "synthetic metals."4 It is
no surprise that a very important property in these materials namely, a
delocalized t-electron orbital system, would result in large nonlinear optical
responses. In many cases, the observed effects are much larger than their
inorganic counterparts.5 In recent years, a parallel can be drawn with the initial
multidisciplinary effort that has been responsible for the growth of inorganics in
integrated electronics technology. Although the field of nonlinear optics has
traditionally been the stronghold of the physicists and electrical engineers,
materials scientists and chemists are now playing an important role in its future
development.
Thin films of organic or polymeric materials with large second and third
order nonlinearities in combination with the semi-conductor based electronic
circuitry offer the possibility of new phenomena and novel devices for laser
modulation and deflection.6 Information control in optical circuitry, light valves,
optical bistability and optical switches are but a few of the possibilities with
organic systems. Other novel processes occurring through third-order
nonlinearity such as degenerate four-wave mixing, phase conjugation, and third
harmonic generation will rely greatly on the new organic systems that are being







developed. Of particular importance for conjugated organic systems is the fact
that the origin of the nonlinear effect is the polarization of the x-electron cloud
as opposed to displacements or rearrangement of nuclear coordinates in
inorganic materials. As electrons are less massive than nuclei, these
displacements, hence nonlinearity, can be intrinsically faster in organic than
inorganics. In other words, the NLO property is limited only by the synthetic
chemistry involved in extending this electron delocalization in conjugated
systems. In addition, the properties of organic or polymeric materials may be
varied to optimize associated properties, e.g. mechanical, thermal stability, and
laser damage threshold, but preserve the electronic interactions responsible for
the nonlinear optical effect.


The Phenomena of Nonlinear Optics

What are nonlinear optics then? Nonlinear optics are concerned with the
nonlinear or anharmonic interaction of electromagnetic fields, generally in the
optical frequency range with a medium, resulting in the alteration of phase,
frequency, or other propagation characteristics of the incident light. In other

words, incident light properties are altered after passing through a material in a
nonlinear manner. This optical frequency range is usually of the order of 1013-
1017 Hz which is above vibrational and rotational modes and below electronic

resonances.

At relatively low light intensities that normally occur in nature, the optical
properties of materials are quite independent of the intensity of illumination. If
light waves are able to penetrate and pass through a medium, this occurs
without any interaction between the waves. However, if the illumination is made
sufficiently intense, the optical properties begin to depend on the intensity and







other characteristics of light. A typical activation energy (internal field), Ea of
3 x1010 V/m is common, which is the energy that binds most electrons and ions.
The light waves may then interact with each other as well as with the medium.
The intensities necessary to observe these effects can only be obtained by
using the output from a coherent light source such as a laser (light with incident
intensity in the range of 1014 W/cm2). This is usually achieved by focusing
powerful picosecond-duration pulses which are obtainable from mode-locked
lasers. Another effect of light on matter is that it can sometimes induce changes
in the chemical composition. This is in the area of photochemistry, which is
differentiated from NLO phenomena in that the medium is chemically altered
instead of just the incident light.


SHG

__ NLO | > 2 c
CO medium



Figure 1-1. Schematic Illustration of the second harmonic generation (SHG), in
which the incident light frequency (co) is doubled (wavelength halved)
upon passing through the nonlinear optically active medium.


The equation for describing the nonlinear phenomena can find some
analogy in the classical force equation for springs. In a spring, displacement, x,
is proportional to an external force, F. With increasing force, F, the spring
stretches rapidly with a nonlinear relationship between x and F, such that at
higher F, the nonlinear terms become important (in physics, the linear
dependence of one physical quantity on another is almost always an
approximation, which is valid only on a limited range of values). This can be
described with a power series:




5

x = aF +bF2 + cF3 +... (1.1)

Nonlinear optical phenomena can be similarly described by replacing x with
polarization p and F with electric field E. When E is small, p is proportional to E
(linear). With increasing E, the nonlinear contribution, E2 and E3, terms
become dominant:

p = aE + PE2 + E3 +... (1.2)

This equation shows the dependence of polarization at higher electric fields
with constants representing the susceptibilities (magnitude of polarizability
analogous to spring constant). A linear term a and second and third order

nonlinear susceptibilities, p and y, describe the microscopic susceptibility. This
equation applies at the molecular level. At the macroscopic level the equation
is given by

P= eoX(1)E + EOX(2) E2 + EOX(3)E3+... (1.3)


where the macroscopic nonlinear susceptibilities are given by the second and
third order nonlinear susceptibilities, X(2) and X(3), respectively, and Eo is

permittivity of a vacuum. When the intermolecular interaction is weak, X(2) is
approximately the sum of the ,As of molecules in a unit volume. Similarly, X(3) is
the sum of rs. Simple additive relationships between the microscopic and
macroscopic coefficients is often not the case, for example in an isotropic or
centrosymmetric media, X(2) = 0. Dispersion, degrees of freedom, spatial
distributions, relationship of driving force frequencies, and variable magnetic
and electric fields all affect the overall predictability between the microscopic
and macroscopic terms. Detailed mathematical treatments of these effects can
be found elsewhere; they are described somewhat further in Appendix A.







On the basis of the above discussion, a direct comparison between the
polarizabilities of electron clouds in organic and nuclear displacements in
inorganic materials can be made. In dielectric materials such as LiNbOs,
electronic polarization is not large because electrons are localized near atoms
(analogous to stretching a strong spring). In semiconductors, electron clouds
are widely spread, their displacement can be regarded like stretching a weak
spring. Organic materials, especially long-chain conjugated molecules, have
electrons delocalized one-dimensionally, i.e. they are regarded as natural
quantum wires. The one dimensional characteristics lead to efficient electronic
polarization induced by an electric field along the direction of the chain.
Therefore compared to dielectric materials, large electronic polarizations are
attainable.


Organic Materials for Nonlinear Optics

In the simplest form, t-electron cloud delocalization in organic materials
is a consequence of the X-X* transition between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The
extension of this delocalization promotes a lowering of the energy of mixing
between wavefunctions. In materials for nonlinear optics, other parameters are
involved. Much theoretical research is based on determining these optimum
parameters for hyperpolarizability. Among the most important are7

(i) the susceptibility, (n) (magnitude, phase, and sign),
(ii) the recovery time, T,
(iii) the real index of refraction, n,
(iv) the absorption coefficient, a,
(v) the relevant frequency domain, Am.







A qualitative guideline based on the analysis of the wavefunction shape
shows that simply extending the wave function overlap does not increase the
inherent nonlinear susceptibility.8 For second order susceptibility, where
noncentrosymmetry is important, P is found to be proportional to the product of
the oscillator strength, f, and the dipole moment difference (Ag) between ground
and excited state, Pe-Pg,

P f(Pe-Pg) (1.4)

For third order nonlinearity (y) one must also promote wavefunction overlap by
increasing the oscillator strength, f, and the wave function differences between
the ground and excited states given by the wave function distribution, Le-Lg

7 Z f (Le-Lg) (1.5)

For both second and third order nonlinearity, note that f and Pe-Pg or f and Le-
Lg are not independent, thus tradeoffs are involved. Therefore a balance of the
wavefunction overlap and wave function separation (or difference) must be
considered simultaneously in optimizing nonlinear effects. A detailed treatment
of the analysis is given in Appendix A.
Materials with second order nonlinear optical properties have received
considerable effort among organic chemists. This is in part due to the large
body of knowledge in the field of organic synthesis, where the synthesis of
asymmetric systems is well known. To date, the largest X(2) values have been
measured from organic molecular crystals.
The design of crystal structures with predictable symmetry is a main
concern among theoreticians, and to this respect ordered films for X(2)
consisting of amphiphiles or polymers have gained much attention. Theoretical
structure-property relationships are important not only in translating microscopic







properties to macroscopic observations but also as a basis for the future design
of these materials. Although numerous theoretical investigations have been
made in formulating the fundamental guidelines for second order nonlinear
optical materials, the following points are specific for third order nonlinear X(3)
optical materials:
1. There is a power law dependence of y on the number of carbon atoms
in a conjugated chain, where y is the molecular hyperpolarizability coefficient.
The length, L, is defined as the distance along the x-direction between two
carbon end sites:9

7' L 4.6 + 0.2 (1.6)
Recent experimental verification suggest that large values of y and
correspondingly of X(3) require only chains of intermediate length of order 100-
200 A, e.g. oligomers, to low molecular weight range.190, 9 That is in long
chains, conformational freedom can result in defects and twists which disrupt
the conjugation. Therefore, the infinite length conjugated polymer chain may
not be required for third order nonlinear optics. Reducing the width of the
absorption band also enhances the X(3).8 Free electron and Huckel
phenomenological models predict %(3) dependence for the polarizability on the
conjugation length.
2. One dimensional systems possess the highest odd order nonlinearity
per valence electron in one direction.7 This implies no actual advantage in two
or three dimensional systems. Qualitatively, this is seen in comparison with Ge
and GaAs semiconductors. Though the intrinsic value of y is independent of
symmetry, the lowering of symmetry in a conjugated chain by the incorporation
of a donor and acceptor group at specific intervals has resulted in a one
dimensional quantum well structure.8.9







3. Specific functionalities in a conjugated system are evaluated as
follows:10 Benzene, due to its aromatic character disrupts the conjugation and
has an adverse effect on the polarizability tensor. The quinoidal form is more
amenable to polarization but is not as easy to form. Triple bonds,
notwithstanding the fact that they are i-electron richer than the double bonds,
are geometrically more concentrated and form a less efficient delocalized
structure.11 The polyene system, on a length basis, is still the best system for
electron delocalization. Recent hyperpolarizabilities have also been observed
for sigma conjugated systems.12
4. Intermolecular forces of attraction such as H-bonding, ionic
interactions, etc. can have the capacity to stabilize the twists in long conjugated
chains and thus control the conformational defects.10 Thus they have value for
structural organization at the molecular level. However, they do not have any
beneficial effect on polarizability.
5. It is important to have some inherent form anisotropy in the
molecules.13 In oriented trans polyacetylene, the tensor component parallel to
the chain is higher than that perpendicular. This demonstrates that the
nonlinear optical properties are entirely associated with the nonlinear
polarizability of the K-electrons in the conjugated backbone.13
It is important that such theoretical guidelines, be validated with
experimental work. Theoretical prediction and experimentation go hand in
hand in testing their validity. The approach has been the building-up of large
databases of actual systems and measuring intrinsic susceptibilities based on
common parameters of measurement.14
Other than chemical modifications by synthesis, the control in molecular
organization is deemed to be the most important aspect of nonlinear optical
properties. Unlike electrical conductivity, molecular organization is critical,







more so in second-order nonlinear effects. It is necessary to create thin film
structures especially for constructing quantum well structures. Well defined
thickness, refractive index, and anisotropy are important for measurements in
the waveguide geometry.15 Real device criteria include: uniform birefringence,
minimized scattering losses, transparency, stability in ambient and operating
environments, dimensional stability, and thickness control


Polymers for Third Order Nonlinear Optical Materials

The history of organic materials for NLO goes back about two decades.
Urea was one of the early organic systems to demonstrate the potential of
achieving large optical nonlinearities in organic crystals. Although many new
interesting organic materials for second order effects have been investigated,
the primary emphasis of this section will be with conjugated organic polymer
systems for third order nonlinear optical properties with the exception of the
poly(silanes).
Polymers have come a long way in being recognized as an important

class of materials for electronic and optical applications. The development of
polymers as structural materials has shown that they offer the flexibility of
modifying molecular structure, conformation, order, and morphology. Therefore,
they can be tailored to suit a specific application. In conjugated polymers for
electrical conducting properties, this has not been much of a consideration.
Conductivity is a bulk property which is heavily influenced by intrachain as well
as interchain charge transport. In contrast, the current status of understanding
of third order optical nonlinearity in conjugated polymers indicates the NLO
behavior to be primarily a microscopic property. This poses specific challenges
for developing conjugated polymers with these properties. Likewise, the lack of







processibility of these conjugated polymeric structures by conventional
techniques is a consideration. Many conjugated polymers are insoluble in
common organic solvents. Understanding of the microstructure and the role of
solvent interaction have played an important role in building on the parent
structure to improve solubility. Despite these seemingly difficult hurdles,
conjugated polymers remain the best superior candidates to replace the
inorganic GaAs system, both in sensitivity as well as speed of response.
Several examples of conjugated polymers with third order nonlinear
optical properties are described briefly below:

Polydiacetylenes have an alternating double and triple bond backbone,
making them relatively rigid compared to polyacetylenes. The most popular in
the class of substituted polydiacetylenes has been the poly(dibutyl 4,19-dioxo-
5,18-dioxa-3,20-diaza-10,12-docosadiynedioate) or poly(-n-BCMU), where n =
3,4 representing the number of methylene carbons between the urethane and
the backbone.16 The presence of the urethane substituent facilitates coil-to-
rigid rod stabilization of planar sequences. Multilayer LB films have been
fabricated for waveguide applications.17 Polymers are formed in bulk or in situ
at the air-water interface prior to deposition. Epitaxial polymerization of other
diacetylene derivatives gives highly oriented crystals.18 Others (different R
groups) that have been studied include PTS {poly[bis(p-toluenesulfonate)-2,4-
hexadiyne-1,6-diol]}, pTDCU {poly[bis-(phenyl-urethane)-5,7-dodecadiyne-1,2-
diol]},19 pDVDA1 {poly-[bis1,8-(p-methylphenyl)-octa-1,7-diene-3,5-diyne]},20
168pDA {poly{1-hexadecyl-6-(cadmiumoctanoate)-2,4-hexadiyne]},21 and
pDCH {poly[bis1,6-(9carbazoyl)2,4-hexadiyne]}.22







R R R


R R R x

Figure 1-2. Polydiacetylene backbone with R, representing different
substituents that can influence the backbone conformation. For instance
in PTS, R = p-toluenesulfonate.


Polyaniline (PANI)23 and polythiopenes (PT)24 belong to a class of
processible polymers made by oxidative chemical or electrochemical methods.
They are well known for their electrical conducting properties. Different
oxidation states of the emeraldine base forms of polyaniline have an effect on
the NLO response. Doping results in the formation of more quinoidal structures.
For polythiopenes, processibility has been improved with alkyl derivatization
into poly(3-alkylthiophenes). Degenerate four wave mixing (DFWM)
measurements showed sub-picosecond response time and spectral behavior,
that could be explained by an inhomogeneously broadened absorption line.25


R

SPT


N0 N N8( 1- yx

H H PANI

Figure 1-3. Polyaniline (PANI) and polythiophene (PT) polymers. R
represents different alkyl chain lengths. Polyaniline is in the emeraldine
base form.


Poly(p-phenylenevinyline), PPV, is well known from the precursor
technique that has made it processible before the actual conjugated species is







produced.26 High nonresonant values have been obtained from films of good
optical and mechanical quality as prepared by film casting. This results from the
interesting combination of phenyl and polyenic moieties e.g. poly(p-
naphthalenevinyline) PNV.27 Polyazomethines (PAM),28 are isoelectronic and
heteroatom containing analogues of poly(p-phenylenevinylene).




\ / \ CH O CH

nn n

PPV PNV PAM
Figure 1-4. Chemical structures of Poly(phenylene) PPV, poly(napthylene
vinylene) PNV, poly(azomethine) PAM.


Poly(p-phenylene-benzobisthiazole) PBT,29 polyquinolines,30 and
polyphenylene are rigid rod polymers processible in the melt form or by
molecular composites with another polymer.





PP PBT
R H 0 0
NN

H R N ""
n

pDEAVQL BBB

Figure 1-5. Chemical structures of Poly(phenylene) PP, Poly(benzothiazole)
PBT, {poly[di(ethylaminovinyl) quinoxaline]} pDEAVQL, poly(benzo-
imidazophenantroline) BBB.







Polyacenes are the theoretical ladder type of polymers and many have
been synthesized.31 Interesting routes to heteroaromatic ladder polymer
structures make full use of the flexibility of organic synthesis. Examples of these
heteroaromatic structures are pDPVQL {poly[di(piperidinovinyl)quinoxaline]},
pDEAVQL {poly[di(ethylamino-vinyl)quinoxaline]},32 and BBL or BBB
poly(benzoimidazo-phenantroline).33
Poly(silane)s belong to a unique class of sigma conjugated polymers.34
These are the only polymers among the group which do not contain the
n-conjugated carbon backbone. The particular feature of polysilanes is the
delocalization of sigma orbitals along the Si atom backbone. Examples are:
pMPSi poly(methyl-phenylsilane),35 pDHSi poly(di-n-hexylsilane),36 and
pDBPSi poly(di-p-butoxyphenylsilane).12



R pDMPSi R = CH3 R'=

I n pDHSi R = R'= (CH2)5-CH3
R'
pDBPSi R = R'= O-C4H9

Figure 1-6. Chemical structures of pMPSi, poly(methyl-phenylsilane),
pol(dihexylsilane) pDHSi, and poly(di-p-butoxyphenylsilane) pDBPSi.


Some x(3) properties for the above polymers and their measurement
techniques are summarized in the Table 1-1.37 From the table, it is seen that
the highest third-order nonlinear optical values measured to date are with the
polyacetylenes (CH)x.38,39







Table 1-1. Conjugated polymers and their respective X(3) values as measured
by THG or DFWM. In general non-resonant measurements are
considered to be more important.

Polymer Orientation X(3) (esu) Technique Ref.
non-resonant and (pim)
Poly(diacetylenes)
PTS 8.5 x10-10 THG 1.89 19
pTDCU 7 X 10-11 THG 1.89 19
16-8pDA LB 8 X 10-11 THG 1.907 21
pVDA1 LC 10-11 THG 20
pDCH 5x10-11 THG 1.907 22
pDCH epitax 2 x 10-10 THG 1.907 22

Aromatic Polymers
PPV 4 x 10-10 DFWM 0.602 26
PNV 5 x 10-12 DFWM 27
Hetero Aromatic Polymers
PT resonant 4.0 x 10-10 DFWM 0.602 40
PANI resonant 3.7 x 10-11 THG 1.83 23
PAM 1.0 x 10-11 THG 0.604 28
PBT 5.4 x 10-12 DFWM 0.604 29
1.4 x 10-11 THG 1.907 29

Ladder Polymers
pDEAVQL 3 x 10-10 DFWM 0.532 32
pDPVQL 2.8 x 10-10 DFWM 0.532 32
BBL 1.5 x 10-12 DFWM 1.064 33
BBB 5.5 x 10-12 DFWM 1.064 33

Polysilanes
pDMPSi 1.5 x 10-12 THG 1.064 35
1.6 x 10-12 THG 1.064 35
pDHSi 1.1 x 10-11 THG 1.064 12
pDHSi LB 4 x 10-12 THG 1.064 12


Polyacetylenes
(CH)x



(CH)x
pPA
Reference: Quartz


Shirakawa 1.3 x 10-9
0.7 x 10-9
1.1 x 10-9
0.1 x10-9
Durham 2.7 x 10-8
5x 10-11
3.8 x 10-14


THG 1.907
1.442
1.362
0.826
THG 1.907
DFWM
THG 1.907







H R


H R

Figure 1-7. Polyacetylene and Substituted polyacetylene. R denotes various
substituents. The backbone is in the trans configuration.


Polyacetylene has been widely studied for its electrical conducting
properties.42 It is characterized by large n-electron delocalization along the
backbone and as a consequence has a high molecular hyperpolarizability.
However, the polymer is unstable to oxidation, insoluble, and difficult to
process. Because of these reasons, much attention has been redirected to the
substituted derivatives of polyacetylene.
Substituted polyacetylenes are characterized by lateral substituents
including heteroatoms that confer new properties on the unsaturated backbone.
A host of potential applications has been observed, other than materials for
nonlinear optics43,44 They are usually soluble, amorphous, or semi-crystalline
and are more stable to oxidation by air than, unsubstituted polyacetylene.45
The effect of the side groups is to confer these new properties but at the same
time they lower the conjugation efficiency. This lower conjugation efficiency can
be associated with defects introduced in the polymer backbone during
polymerization, e.g. saturated defects, conformational defects, etc.46,47
The nonlinear optical properties of several ortho-substituted

polyphenylacetylenes, pPA, have been studied by Wegner et al. using THG and
DFWM techniques.48 The bulkiness of the substituents was varied, which
subsequently affected the conformation of the polymer giving rise to visible
absorption properties.49







Substituted Polvacetylenes

Compared to various vinyl polymers which are manufactured in large
scale, ethynyl polymers are not. One of the reasons is the difficulty in
synthesizing high polymers in good yields. Polymerization has often been
attempted by using radical and ionic initiators in the past.50,51 In most cases,
the products were linear oligomers in which the molecular weights were in the
range of a few thousands. The formation of cyclotrimers was often observed.
Thus it was rather difficult to synthesize polymers whose MWs' were higher than
ten thousand. The polymers are also often composed of a mixture of
configurations and conformations.
Acetylene selectively polymerizes in the presence of Ziegler catalysts,
whose components have low Lewis acidity.40 Cis polyacetylene forms at low
temperature, and trans polyacteylene at high temperature.52 Likewise, Ziegler
catalysts have been the most versatile catalysts for the polymerization of
substituted acetylenes until the advent of Group 5 and 6 metal halide catalysts.
Primary and sec-alkylacetylenes yield high MW polymers in the presence of
Ziegler catalysts such as with a mixture of iron trisacetylacetonate with
triethylaluminum [Fe(acac)3-EtAl(1:3)]. However, aromatic and heteroatom
containing monosubstituted acetylenes produce mostly insoluble polymers
and/or oligomers. Furthermore, no disubstituted acetylenes are known to
polymerize with Ziegler catalysts.
The most widely studied monoacetylene to date has been the
phenylacetylene, an analog of the vinyl monomer, styrene. The monomer
produces only oligomers with number average molecular weight of a few
thousand by use of conventional radical, cationic, or anionic initiators. Ziegler
catalysts provide relatively higher molecular weight oligomers of







phenylacetylene, but a large fraction of the product is insoluble. In 1974, the
first report on the polymerization of phenyl acetylenes using WCI6 and MoCI5
catalysts was published by Masuda and Higashimura.53 Since then, various
derivatives of this catalyst have been made and applied to a large number of
monomers: mono- and disubstituted, hydrocarbon, and heteroatomic. The
result of which is the large number of publications and reviews mainly by
Masuda and Higashimura.54
Group 5 and 6 catalysts can be classified according to their reactivities
and monomer specificity. The first group of catalysts is, simply the respective
metal chlorides, MoCI5 and WCI6 (Class I).53 These catalysts are capable of
polymerizing various monosubstituted acetylenes. In particular, they have been
found to be excellent with phenylacetylenes. NbCI5 and TaCI5 have been
known to produce cylcotrimerized products with phenylacetylene.54 But with
disubstituted acetylenes, they tend to give high MW products. The second type
is equimolar mixtures of WCI6 and MoCI5 with an organometallic co-catalyst
(Class 11).185 These catalysts polymerize not only monosubstituted but also
disubstituted acetylenes. Ziegler catalysts can only polymerize primary or
secondary monoalkylacetylenes. Class II catalysts can polymerize other
structural isomers of a particular monoalkyne. Organometallics containing
Group 4 and 5 main group metals are very effective as cocatalysts, e.g.
tetraphenyltin (Ph4Sn), triethylsilane (Et3SiH), triphenylbismuth (Ph3Bi). The
third group of catalysts (Class III) is obtained by ultraviolet irradiation of a CCI4
solution of metal hexacarbonyls [Mo(CO)6-CCl4-hv, W(CO)6-CCl4-hv].54,187
These catalysts polymerize various monosubstituted acetylenes and
disubstituted acetylenes such as those in which one substituent is chlorine.
This class is generally less active than the above two kinds of catalysts but







tends to provide polymers having higher MWs. Polymerization for this class
proceeds only with UV irradiation and a halogen containing solvent.
Other Mo and W catalysts for polymerization that have been used are
generally less reactive. The Fischer carbene and Casey's carbene have been
found to give good yields.54 Interestingly enough, Schrock's Lewis acid free
molybdenum alkylidine catalyst has been found to polymerize monosubstituted
acetylenes in good yield but not disubstituted acetylenes.55 The reactions with
this catalyst supports a reaction mechanism in which polymerization proceeds
via the metal carbene.
The reaction mechanism involves an active specie: a complex having a
metal-carbon double bond called a metal carbene which forms a ring structure.
It has been shown many times that the metal carbenes mediate various
reactions. Other than polymerization of acetylenes, W and Mo catalysts are also
known to catalyze olefin metathesis. The scission of the C=C bond takes place
with the result that an olefin having the substituents R and R' is converted to two
olefins, one with only an R and the other R'. Another interesting case in
particular, is with polymerizations involving ring opening metathesis
polymerization (ROMP)56 and acyclic diene metathesis (ADMET)
polymerization.57 In fact, a novel route to substituted polyacetylenes which
does not use the monoacetylene monomer is the ROMP of cyclotetraene
monomers. This has been the specialization of Grubbs' group at Caltech where
they attach various substituents to the cyclooctatetraene monomer.58 The use
of open chain monomers to produce substituted polyacetylenes have been
made by K. Wagener and D. Tao to give good yields and interesting
chemistry.55 All of these make use of the metal carbene intermediate as the
active species.







The rationale for the metal carbene mechanism is as follows: First, there
are many catalysts effective in olefin metathesis that are also effective with the
polymerization of acetylenes. Second, acetylenes can be regarded as extreme
members of cycloolefins, i.e. two membered rings in which the aliphatic chain of
a cycloolefin is replaced with a carbon-carbon single bond.




C-CC
2_ CE=C C C

M=Mo, W, Ta, Nb
C L=CI

CEC
Ilt-C-MLnM
L JJ c "C=ML, CC



.n C-M L

C MLn CCML


Cyclotrimerization Polymerization

Figure 1-8. Mechanism of Polymerization and cyclotrimerization. The metal
alkylidenes are the active species which forms a metallacyclobutene ring
intermediate upon coordinating with the monomer.


Katz has demonstrated the validity of this mechanism using 13C NMR
and deuterated polymers.59 In some cases, the metal carbene has been
isolated. The conclusions are that the cleavage of two 7 bonds are involved
with these catalysts whereas only one r bond is cleaved with Ziegler-Natta
catalysts. These findings show that the polymerization of acetylenes by the W
or Mo catalysts proceeds via the metal carbene mechanism, whereas that by
the Ziegler catalyst involves a metal alkyl (insertion) mechanism.







Almost no information on the transfer and termination reactions in the
polymerization of acetylenes by Group 5 and 6 transition metal catalysts has
been obtained. This is important in that if an effective transfer agent can be
found, polymers with controlled MW and well defined ends can be obtained.
The competing reaction between cyclotrimerization and polymerization is
an interesting case in that suppression of cyclotrimerization leads to higher
MWs. An important difference between the two is that for cyclotrimerization to
occur, two acetylene monomers must coordinate simultaneously to the active
species, while only one molecule must coordinate before polymerization should
occur. Hence cyclotrimerization is prone to steric effects. An example is that 1-
hexyne produces a mixture of cyclotrimer and polymer with Mo and W catalysts
whereas t-butyl monoacetylene yields only high polymer.60
The copolymerization ability using this catalyst is dependent on the
coordinating ability of the monomers. In general, acetylenes are more reactive
than olefins in a coordination reaction since the former have a stronger
coordinating ability. For example, in the copolymerizaiton of phenylacetylene
with styrene using WCl6-Ph4Sn essentially only phenylacetylene
polymerizes.61 With various acetylenes using W and Mo based catalysts,
sterically less hindered acetylenes always show higher reactivity. This
suggests that the propagation reaction consists of two stages, i.e. monomer
coordination and reaction of the coordinated monomer with metal carbene, and
that the relative reactivity of monomers in copolymerization is governed by the
competitive coordination of the monomers.
As a result, the versatile Group 5 and 6 catalysts have been widely used
for the polymerization of substituted polyacetylenes. With the view of using
these materials for nonlinear optical properties, Wegner et al. have used
several substituted phenylacetylenes using these catalysts to produce high MW







polymers with good absorption properties.27 X(3) has been measured using
spin cast films, and as outlined earlier, has a high value. Le Moigne et al. have
extended this work to other derivatives, and sometimes with liquid crystalline
properties.62 Their results have been oriented toward maximization of the
conjugation length by controlling several aspects of the synthesis. Various
derivatives are outlined as shown in Figure 1-9. Subsequently, their method of
synthesis was used and studies were made in this dissertation on the film
forming properties of these polymers.


H3C
CH3
H CH3
A ,f- n CHCH3


H

rn

I'


R = H, CH3, C2H5, C8H17, Si(CH3)3


Figure 1-9. Cholesteryl and methoxybiphenyl esters of poly(o-alkynoic esters)
and ortho substituted poly(phenylacetylenes).



Cyclopolymerization of Unconjugated Diynes

Interestingly, Group 5 and 6 catalysts have also been found to be very
effective with cyclopolymerization of unconjugated diynes to produce







substituted polyacetylenes. Cyclopolymerization is a mechanism which favors
intramolecular cyclization first over intermolecular reaction in the propagation
step of the polymer (alternating intra-intermolecular polymerization).
Cyclopolymerization was extensively studied by G. B. Butler et al. of the
University of Florida, who has been cited as the pioneer in
cyclopolymerization.63,64 Most of the work done was on different types of a
and o dienes using a variety of initiators. Statistical and mechanistic arguments
have been put forward to explain the occurrence of such polymerization. It has
been found that the favorable formation of a six-membered ring together with
the smaller decrease in activation entropy leads to a less energetic pathway.65
In 1961, J.K.Stille reported the cyclopolymerization of 1,6-heptadiyne
using Ziegler-Natta type catalysts to form polymers with cyclic recurring units in
an alternating double and single bond backbone :66


Ziegler-Natta
Catalysts
| |------ -) M

Figure 1-10. Cyclopolymerization of nonconjugated diynes by Ziegler-Natta
catalysts.


Other structures were speculated as to the possible products formed with
these reaction conditions.67 In 1983, sparked by the interest to produce highly
conducting polymers, H.W. Gibson et al. synthesized free-standing polymer
films of 1,6-heptadiyne using homogeneous catalysts derived from Ti(OC4H9-
n)4 and AI(C2H5)3.68 A combination of spectroscopic, chemical techniques,
and model compounds proved the structure as that initially reported by Stille.
Other diynes such as 1,7-octadiyne and 1,8-nonadiyne gave predominantly







cross-linked products. Modest electrical conductivity values were obtained with
these films, but they were not stable to oxidation by air for long periods of time.
Recently, a series of papers have come from S.K.Choi and coworkers on
the cyclopolymerization of several diyne derivatives using the metathesis
catalysts MoC15 and WC16 with several cocatalysts. Several derivatives are
shown in Figure 1-11.69,70,71,72,73,74,75


X = O, S, Ge, S=0, SiPhMe, SiPh2, SiMe2
= C(COOCH2CH3)2, C(CO2CH2CF3)2
= {+N[(CH2)nCH3]2 }Br-

= O cCO2(CH2)60 OMe 12

0 0 % 1

H2C ,C02(CH2)60 CN


Figure 1-11. Cyclopolymerized nonconjugated diyne derivatives synthesized
using Group 5 and 6 metal halide catalysts. X is represented by different
groups.


The versatility of these type of polymerizations can be seen in the
number of derivatives that have been polymerized since the methodology was
introduced. Using standard Schlenck techniques under vacuum, high
molecular weight polymers were obtained. These polymers are surprisingly
stable for long periods of times, have high conjugation lengths, are soluble in







common polymer solvents (except for thioether and ether derivatives), and have
high molecular weights of the order of 1x105. The favorable formation of a six-
membered ring allows cyclopolymerization. Propagation with this type of
catalyst is generally thought to proceed via a metal carbene and
metallacyclobutene intermediate similar to olefin metathesis as discussed
earlier. Two types of monomer orientation have been proposed in which a-

addition leads to the possibility of five-membered ring formation.76






n
rotation at every single bond





n
rotation at every other single bond

Figure 1-12. Statistical comparison of rotations in the main chain of substituted
polyacetylenes for a regular polyenic and cyclopolymerized backbone.


Formation of these types of polymers is an important development toward

control of conformation in substituted acetylenes. Alternating six-membered
ring units gives a trans configuration that prevents isomerization to the cis form
in the endo double bond and at the same time limits the conformational
rotations possible for the chain resulting in trans polymers with higher
conjugation efficiency (see Figure 1-12). Thus a more "controlled" backbone is
observed, with longer conjugation lengths exhibited in the UV spectra.71,72










X

a-addition/





X
_J


-IV
X


CM
n.---- X


a-product


SML =C

p-addition


ML C

X


C

M"ttf1X


S p-produc

C I MLn


Figure 1-13. Mechanism of cyclopolymerization with Group 6 metal catalysts
via the metallacyclobutene ring. a and P addition are possible with
P-addition ( six membered ring) commonly observed.







The Langmuir-Blodaett Technique

Thin films are required not only for application purposes but also to
implement many fundamental optical measurements that are crucial in
understanding the origin of nonlinearities in organic. Specially for conjugated
polymers, with the absorption coefficients being very large (105 cm-1), many
optical measurements cannot be performed without thin films. For applications
of course, molecular organization is required since the effective nonlinearity of a
material is minimized unless the molecules are organized. Additionally, thin
films are a prerequisite to applications in the guided wave geometry which is
known to be the desired geometry in integrated optics. Because of all the
above reasons, thin film organization of polymers has received extensive
attention.
The Langmuir-Blodgett (LB) method is a technique for assembling
ordered molecular multilayers from monolayers by vertical deposition on a
substrate. Irving Langmuir, whose name is associated with the technique, did
much to make the field into a fully developed science. However, many pioneers
in the past, produced the significant advances that led to this development.
Benjamin Franklin and Lord Rayleigh, are among the few famous men who
contributed.77,78 Agnes Pockels and Katherine Blodgett were women beyond
their time who pioneered the deposition methods that are now used routinely for
LB film fabrication.79,80 Beginning in the late 1920s, significant work was
published by Adam,81 Davies and Rideal,82 Crisp,83, and Harkins,84 together
with Langmuir performed research which quantified the science and its physical
and thermodynamic aspects Gaines' book "Insoluble Monolayers at the Gas-
liquid Interface" first published in 1967, has been the standard text for
researchers for many years.85 He is credited with extending Gibbs'







thermodynamic treatment to monolayers at surfaces. A period of renewed
interest in Langmuir Blodgett films followed in the 1960s with the possible
applications towards electronics miniaturization. Hans Kuhn, who is now being
honored by associating the LB film technique as the Langmuir-Blodgett-Kuhn
(LBK) technique, did much to apply the technique into novel and exciting
application of thin films.86 Langmuir-Blodgett research today is an
interdisciplinary science with many prospects for device applications. Excellent
overviews of this horizon were given by Roberts87 and Ulman.88 Major
contributions in the literature have also been given by: Ringsdorf89, Mobius90,
Mohwald91, Wegner92, and Lando93 just to name a few, who have contributed
to the growth in knowledge and technology of LB films.


The Monolaver Phenomena

The technique starts with an understanding of the delicate balance at the
interface which is responsible for monolayer forming properties. From the three
phases of matter; solid, liquid, gas, five groups of interfaces can be classified.
The interface can be treated as a mathematical plane, in which adsorption of a
third substance gives rise to a surface excess concentration that give it a
definite thickness. The Langmuir-Blodgett Monolayer technique makes use of
the air-water (gas-liquid) interface. Commonly, an amphiphile is a molecule
that has two properties in a single molecule. Translated to the air-water
interface, these are molecules that have both a hydrophobic and a hydrophilic
group. A delicate balance of the hydrophilic group associating with water and
the hydrophobic group pulling away from the subphase, produces the
spreading properties of the amphiphile. This spreading force is often quantified
by measuring the surface tension y. Thermodynamically, it is defined as a







derivative of Gibbs free energy (G) with respect to the surface area (s) at a
constant temperature (T), volume (V) and moles (n). It is given by the equation

7 = (G/8s)T,V,n (1.7)

The various interfaces in an amphiphile contain specific surface energies
defined by the surface tensions. The balance between these forces and the
tendency toward minimization of the free energy, determine the spreading
properties of a material.
A more common experimentally determined parameter is the surface
pressure H which is defined as follows:

1= o (1.8)

The plot of n versus mean molecular area (Mma) or simply Area, measured at
constant temperature is referred to as the surface pressure-area isotherm.
Such experiments are considered as two dimensional analogues of
bulk/pressure volume isotherms with analogous treatment of phase transitions
and compressibility less one dimension.94


Langmuir-Blodgett Apparatus

The amphiphile is usually spread at the interface using a volatile solution
to form a monolayer. A shallow trough, commonly made of teflon, holds a
certain area of water. The purity of the water used is of utmost importance, not
to mention the purity of the amphiphile. Movable barriers are responsible for
controlling the area of the trough. The barrier is either of hydrophobic or
hydrophilic materials, each having certain advantages and disadvantages. This
two dimensional compression is responsible for pressure induced changes and






orientation of amphiphiles at the interface. The surface pressure changes are
measured by a sensitive surface balance. Two types are available: either the
Langmuir or the Wilhelmy plate balance. One is more effective than the other,
depending on the viscosity of the amphiphile monolayer. The Wilhelmy plate
method is the most common, which employs a rectangular plate dipping into the
interface. The plate must have zero contact angle with one of the phases.
Changes in the interfacial tension are measured by the vertical pull of the plate.
The Langmuir Balance makes use of a floating barrier which measures the
force directly exerted by the film, in relation to a clean surface. The result is the
surface pressure-area isotherm, which is the standard measurement for
describing monolayer behavior.


trough
/ Barrier

/ImI


- Surface balance


Before compression


After Compression


Figure 1-14. Schematic diagram of a monolayer compression of amphiphilic
molecules spread at the air-water interface.


Several designs have been used by most researchers. Before the
advent of commercially available apparatus, most of the troughs were "home







made". A circular design, called the Fromherz trough, and the constantly
moving wall trough are variations to the common rectangular troughs with
movable barriers.95,96 A commercial trough produced by KSV Instruments
(Helsinki, Finland) is currently the most popular because of its modular design.
The film balance used is based on the Verger-de Haas trough principle.97
Different measuring devices can be attached to the mainframe and are all
controlled by a software program that allows calibration and control of the
parameters for measurement.


Figure 1-15. KSV 5000 LB Langmuir-Blodgett System with an IBM
computer interface. The modular frame allows attachment of other
surface analytical instruments such as a surface potentiometer.


In addition to the surface pressure-area isotherms, various surface
analytical techniques have been utilized for investigations at the air-water
interface. They range from surface potential measurements to delicate X-ray







synchrotron measurements. The following techniques have been important in
particular for our investigations: surface potential, in situ UV-vis spectroscopy,
Brewster angle microscopy, and relative reflectivity. A detailed treatment of
these various surface analytical techniques will be given at the chapter on
methodology (Chapter 2).


LB Vertical Deposition Technique

One result of being able to control monolayers is to deposit them into
multilayer films by dipping. Two types are possible, the horizontal and the
vertical technique. The horizontal method, developed both by Langmuir and
Schaefer in 1938 is involves transferring the monolayer film to a horizontally
positioned plate and then lifting the film off the subphase.98 This technique is
especially important for transferring very viscous films.
Vertical transfer, commonly called the Langmuir-Blodgett technique, was
developed mainly by Katherine Blodgett. Various solid substrates are used and
are generally of hydrophilic or hydrophobic surface. The most common
treatment for hydrophobization being the use of alkylchlorosilanes. Transfer is
usually achieved when the monolayer has greater attraction for the surface of
the substrate than it has to the subphase. By compressing the monolayer at a
specific surface pressure, the monolayers can be deposited at certain
orientations. During transfer, the pressure is maintained constant by barrier
movement, that is why isobaric stability is an important prerequisite for film
deposition. Deposition data are collected in the form of the transfer ratio. The
value is defined as the change in area of the monolayer at the water surface
calculated by the barrier movement divided by the area of substrate that actually
passes through the monolayer at the surface. Any area change at the film






surface is assumed to result from monolayer transfer onto the surface of the
substrate. Complete transfer should yield a ratio of 1.0.


IITMII IT



Y Type


L M


TllTT??TTlTTl






X Type


Figure 1-16. Different alternations and deposition modes in LB films.


Different deposition modes are expected because of the two way
movement of the substrate in vertical dipping. They are classified as X, Y, Z
deposition. Y-type being the most common, deposits layers in both the
downstroke and upstroke whereas Z type deposits in the upstroke and X type
on the downstroke.
The use of the alternate trough has allowed the formation of more
intricate structures and even control of the deposition behavior of films. The
design involves a two compartment trough with a common subphase area. A
roller mechanism has been developed to deposit alternately, the disadvantage
being the absence of a third compartment by which the substrate can pass
before depositing the other film.99 This has been solved with the addition of a
third compartment incorporated in the KSV LB5000 Alternate Trough.









three compartment trough


it-


upper dipper arm

substrate
lower
dipper arm
__ ,.


ii


Figure 1-17. Schematic diagram of a three compartment trough (KSV
Instruments) used for alternate deposition of layers. An upper arm and
lower arm are utilized to cycle the substrate through the troughs.

Polymers at the Air-Water Interface

Polymers at dissimilar interfaces or polymer interfaces are a growing
research area with numerous scientific, medical, and technological
applications. Adhesives, peptide synthesis, synthetic membranes, flocculation,
bio-materials, nonlinear, and opto-electronic materials, are some of the direct
applications that are benefited by such research. The majority of research in
the field of Langmuir Blodgett monolayers and multilayers however, has been
on low molecular weight amphiphiles. As the inherent thermal and mechanical
instability of LB films from low molecular weight amphiphiles has become a
handicap for applications, current research is now being focused to films of
organic polymers.100 The two dominant approaches in this direction are to
polymerize ordered monomers into their film structures or to spread preformed
polymers.


I







Agnes Pockels was the first to spread polymeric films on water surfaces.
In the 1890s she described measurements on films, colophonyy" and "mastic,"
being just some of the substances that she investigated using a home made
trough.101 In 1903, Devaux reported the spreading of the protein albumin.102
These initial observations predated then the concept of both polymers and
monolayers. The first documentation of synthetic polymeric monolayers was
made by Katz and Samwel in 1928, for poly(vinyl acetate) and
poly(methylmethacrylate).103 Polymers of hydroxydecanoic acid by a
condensation-step process were studied by Harkins, Carman, and Ries in
1935.104 An excellent review was published in 1946 by Crisp, who
established a criteria for the interpretation of surface pressure and potential-
area isotherm measurements.105 He also attempted to categorize the
monolayers based on viscosity and isothermal reversibility. Since then, a
variety of synthetic polymers have been studied at the interface over the years
such as : poly(vinylacetate), poly(acrylates) and poly(methacrylates), poly(vinyl
fluoride), poly(vinylidene fluoride), and poly(dimethylsiloxane) have been
investigated.106,107,108,109,110,111 Their behavior has been characterized
on the basis of surface-pressure and surface potential area isotherms. These
systems are able to form stable monolayer films on pure water, however,
transfer to solid substrates often results in inhomogeneous films containing
numerous defects.
Two classifications can be made based on the character of the X-A
curves: expanded and condensed types. Expanded films are more
compressible and exhibit a reversible collapse. Fowkes112 offered a molecular
interpretation distinguishing the two, suggesting that in expanded films, the
polymer segments are miscible with water molecules in the surface layer, while
in the condensed films the polymer chains are in contact and water is







substantially excluded. Measurements during compression and expansion
cycles suggests categorization of spread polymer films according to their
hysteresis behavior. Gaines used the terms; reversible, reversible collapse,
irreversible collapse, and rearrangements to characterize surface pressure
decay and curve compatibilities.113 A reversible behavior is one in which the
cycle can be stopped with no decay in surface pressure, and the expansion
curve reproduce the initial compression. In cases of reversible collapse, the
collapsed film respreads when the available area is increased, and below the
collapse pressure, the compression and expansion curves are again identical.
In cases of irreversible collapse, once the film has collapsed, the material does
not respread, although subsequent compressions-expansions limited to surface
pressures below collapse are reversible. Lastly, rearrangement behavior is
observed where the initial compression curve, bears little relation to subsequent
compression and expansions.
Proteins are copolymers with a non-regular sequence of amino acids
and are of uniform molecular weight. Interfacial properties are sensitive to the
degree of polymerization (higher molecular weight materials generally become
insoluble in water), which gives proteins an advantage in this respect. The n-A
curves of proteins tend to be similar and are typified by that of bovine serum
albumin.114 For proteins, the approach to segmental equilibrium after

compression or expansion of the monolayer is often sufficiently slow to be
followed experimentally. The comparative slowness reflects the high surface
viscosity in protein films, evidently arising from the need to break interchain H-
bonds for flow to occur. The great stability of protein monolayers despite high
solubility in the subphase, can be rationalized on the basis of this large value
for the free energy of adsorption. However, as a protein monolayer is
compressed to high pressures and increasing numbers of segments are







expelled from the interface, the free energy difference between the molecule in
its adsorbed and solution states is progressively reduced. The probability of
desorption which is negligible at t=0, attains values where its rate can be
detected by the permanent decrease of area of the monolayer. It is also
possible to calculate the free energy of adsorption from the n-A curve of the
protein.120 A consequence of the effect of molecular size in desorption is that,
in systems containing a mixture of proteins, the higher molecular weight
proteins will tend to displace the smaller size proteins from the interface, other
things being equal. Experimental evidence points very strongly to the a-helix or

some closely related conformation being present at the air-water interface.115
The conformation of the polymer backbone is fully defined and its symmetry is
such that it has no significant net dipole moment perpendicular to its axis.
Furthermore, its rodlike nature imposes a high degree of order in the
monolayer.
One way to improve the spreadablity and orientation of these polymers is
to polymerize well known amphiphilic monomeric units. As mentioned earlier,
the first approach involved linking together monomer monolayers by
polymerization at the air-water interface. The most of popular of these are the

poly(diacetylenes), polymerized either at the interface or after subsequent LB
multilayers have been formed.17,92 Due to shrinkage and movement during
polymerization, some structural problems arise in the films. Alkyl derivatives of
Polyanilines have been successfully synthesized and characterized at the
interface by Duran and co-workers.116 Usually, the polymerization reactions
induce structural reorganizations which can induce defects in the multilayers,
e.g. shrinkage. These problems can be minimized by appropriate molecular
design. However, the control of the degree of polymerization, and the removal
of unreacted monomers and side-products is not possible using this approach.







Polymerization research at the air-water interface continues as well as that of
LB films. The spreading of preformed polymers still offers the most expedient
way of producing well characterized polymer monolayers and multilayers free
from structural defects.
The presence of defects in polymerized ordered monomers thus favors
preformed polymers. To improve their ordering, the trend has been to make
them more amphiphilic or orientable. One approach to this is to synthesize
amphiphilic orientable functionalities and incorporate them either as side
groups or at the mainchain of the polymer. Polymers studied usually fall into
one of these categories:
(1) Amphiphilic polymers, such as polymers of fatty acids,
Poly(octadecylmethacrylate), poly(maleic-anhydride), poly(vinyl stearate),
poly(octadecylacrylate);
(2) Liquid crystalline polymers, the most common of which is the side
chain liquid crystalline polymer (SCLP). The mesogenic part is attached to
main chain via a flexible spacer;
(3) Rod-like polymers, such as poly(silanes), poly(alkylglutamates),
which have a rigid polymeric backbone with a flexible hydrophilic or
hydrophobic backbone. The polymers of interest to this research are of the
latter type.
Recently, Wegner et al. have studied a type of this polymer for NLO
studies.12,100 Utilizing a rigid polysilane backbone, monolayers were
deposited with high anisotropy of the polymeric chains. Well-ordered
multilayers are formed with the long alkyl chains acting as bond solvents,
fluidizing the monolayer.







Theory of Polymer Behavior at the Interface

Polymers show the same general types of behavior as small molecules.
The compressibility of condensed films varies markedly with temperature and
can be extremely low at the theta temperature (temperature in which there is
ideal mixing between solvent and polymers). Mixed monolayers of compatible
polymers can be formed as well as mixed monolayers of polymers and simple
molecules.117 It is important to have a main polymer chain with hydrophilic
groups regularly distributed at short intervals along it. Long sections without
such groups, tend to form ill-defined loops clear of the water surface as
described earlier. In general, there is much greater toleration of structural
variation than with monomer systems. Not only completely nonpolar polymers

[poly(ethylene), poly(propylene)] but also some very polar monomer units
[nylon, Poly(acrylonitrile)] could not spread at the air-water interface. This
would result in the formation of biphasic regions or microdomains similar to
defects in the bulk.118 Presumably, the attractive forces between the chain
segments are much larger than the hydration forces, and hence the spread
state is less stable than a three-dimensional polymer aggregate.
In order to understand the behavior of polymers at the interface, it is
necessary to define the free energy (AGtotal) of the polymer at the interface. It
appears that the surface free energy in this case is not an intrinsic characteristic,
but corresponds to a notion of a "potential surface free energy depending on the
environment."119 The conformation taken-up by a long chain flexible polymer
molecule adsorbed at an interface illustrates the competing tendencies of
energy minimization and entropy maximization via the well known Gibbs' free
energy equation:


AG = AH-TAS


(1.11)







AG is a summation of the individual free energy changes associated with the
various segments or structural groupings of the polymer (AGtotal = i AGi). The

behavior of a polymer molecule may be described in terms of a series of
discrete segments, each segment containing on the average a certain number
of monomer units. Three possible situations can occur depending on the
enthalpy of the system as outlined by MacRitchie:120








Case I Case II Case III

Figure 1-18. Various conformations of the polymer depending on the
enthalpic behavior and its effect towards free energy minimization.


Case I No absorption or emission of heat when the polymer molecule is
introduced to the interface (i.e. AH =0). The enthalpy of interaction
between polymer and subphase is equal to the polymer-polymer
interaction. Molecular configuration is determined exclusively by
entropy. The molecule will therefore adopt the conformation of greatest
disorder, that of a perfectly random coil at the interface.


Case II There is an absorption of heat when the polymer molecule is
spread at the interface (AH=positive). The molecules acquire energy
from the environment and go to a higher potential energy state. The
energy of interaction between polymer and subphase is higher than the
sum of the polymer-polymer and subphase molecule interactions. To
reduce this unfavorable enthalpy change, the polymer molecule "folds-







up" to minimize its interactions with subphase molecules. This signifies
an increase in order of the system, resisted by the universal drive to
increase the disorder or entropy. The conformation taken up by the
polymer molecule is one that, although not completely folded-up, is more
folded than a random coil.


Case III There is an emission of heat when the polymer molecule
enters the interface (AH=negative). Polymer-subphase molecule energy
of interaction is less than the sum of the polymer-polymer and subphase-
subphase molecule energies of interaction. AH is negative and polymer
and subphase molecules decrease their potential energy when they
approach each other. The polymer molecule will stretch out to increase
interactions of its segments with subphase molecules. The conformation
will be one that is more stretched out than the random coil. For a very
large AH (magnitude), polymer molecules adopt a rigid rod configuration.


Temperature influences the entropic term of the free-energy since the
entropy is multiplied by the temperature. As the temperature increases, the
ordering caused by the enthalpy changes assume less significance and the
disrupting effects of the thermal motion become more dominant.


Comparison with small molecules
Usually, monomers of most polymers do not have large non-polar and
strong hydrophilic groups like typical small molecules possessing a long
hydrocarbon chain and one or more strong polar groups. However, because a
given polymer chain contains a large number of repeating monomeric units, its
total free energy of adsorption (nAGmonomer) per polymer chain (n = degree of







polymerization) can be very high even if AGmonomer, the free energy per
monomer unit is relatively small.120 In addition, linear polymers are assumed
to have a conformation in which the main chain backbone lies along the
interface in contrast to the long axes of small molecules, which are normally
assumed to be orthogonal to the surface. In reality, the possibility of chain
entanglements in polymers are dominated by their spreading behavior.121 The
polar segments predominantly interact with the aqueous phase and the non-
polar ones with the non-aqueous phase, consistent with steric requirements. In
this conformation, the chains have a certain flexibility that leads to an effect that
is unique for polymer monolayers. As the monolayer is compressed, segments
of the molecular chain, each consisting of a number of monomer units are
pushed out of the interface into the adjacent bulk phases (see Figure 1-19).


Polymer relaxation and equilibrium at the interface
For each value of r, there is an equilibrium between segments in the
interface and in the adjacent phases. This depends on: (1) the interfacial
pressure, (2) the nature and flexibility of the polymer, and (3) the compression of
the subphase and the temperature.120,105,85 As the monolayer is
compressed, the equilibrium shifts in favor of the displaced segments. As a
result, the n-A curves of polymers tend to show relatively high compressibility.
The n-A curves of polymers are relatively featureless. At low areas, n-A curves
of a sigmoid form are given by many polymers. The high compressibility in the
low pressure regions arises largely from entropic effects associated with the
large number of arrangements (configuration and conformation) formed by the
mixing of flexible polymer segments and solvent molecules.113







loop
train
Air tail

Water _-


Low I Moderate H High n

Figure 1-19. The Loop and Train model for polymer conformation depending
on the surface pressure at various stages of compression at the air-water
interface.

A statistical thermodynamic treatment of this effect for a completely
unfolded monolayer was developed by Singer:122

H = (kT/Ao)ln(l-Ao/A) + (kT/Ao)(t-1/t)(z/2)ln((1-2Ao)/zA) (1.12)
This equation, is based on the lattice theories of Huggins and Flory where tis
the total number of segments per molecule, k is the equilibrium constant for
segment-subphase interaction, A is the area available per segment, Ao is the
limiting area available per segment, and z is the surface coordination number in
the 2-D quasi lattice in the interface. For a completely rigid chain z= 2, and for
a completely random chain z = 4. For very dilute polymer monolayers, the
equation has been found to give good agreement with experimental results.
Other contributions to equations of state for polymer monolayers take into
account surface activity coefficients, partial solution of polymer chains in the
subphase, intermolecular cohesion, and intermolecular repulsion.123 At
moderate interfacial pressures, polymer chains become close packed and the
monolayer exhibits its lowest compressibility in this region. At higher pressures,
the compressibility increases markedly again as segments of molecules are
forced out of the interface (loops and tails), decreasing the proportion of
attached segments (trains). The contributions of the loops and tails to the
surface pressure is negligible compared to that of the trains.







Polymer structure and properties in general are time dependent.124
Because of their relatively large size and high molecular weight, they rarely
achieve true equilibrium, e.g. physical aging can take years. However, at
aqueous interfaces, this equilibrium is sometimes observable within the time
frame of experiment. The polarity of the aqueous phase provides a high
interfacial energy driving force for orientation of polar phases, blocks, segments
of the polymer towards the aqueous phase. In vacuum, air, or other non polar
surfaces, the polymer chain orients its non-polar components towards the
interface minimizing the interfacial energy. Polymer relaxation has been
defined as a time dependent return to equilibrium of the system which has
recently experienced a change in the constraints acting upon it. Each polymer
chain has a particular motion frequency which is dependent on temperature
and inertia of the participating segments. These chain motions are also usually
dependent on the presence of a "free volume" between polymer chains.125
Because the free volume in the polymers increases with temperature, the
relaxation times decrease, in general, with increasing temperature.


Polymers and water
Polymer surface restructuring effects in response to a surrounding liquid
phase are probably more pronounced in aqueous systems due to the unique
hydrogen bonding and acid base characteristics of water. Studies suggest that
the restructuring of a polymer in the presence of water corresponds to two
stages:126 After a first step of macromolecular chain movement, necessary in
order that the hydrophilic part can appear in the surface, there is a second step
of orientation of the polar groups at the interface which is accompanied by a
rapid increase of the polar component of the surface free energy. There is an
asymmetry of the force field at the interface. In contact with air, for instance, the







polymer chain segments orient in order to expose the hydrophobic group
towards the gaseous phase and to bury the polar group inside the polymer. So
the surface appears hydrophobic, in spite of the hydrophilic sites present in the
matrix. In contact with water, we see the inverse: polymer segments reorient to
adapt a conformation achieving minimal interfacial tension by exposing the
hydrophilic parts. The ability to respond to changes depends on the mobility of
the surface groups and segments. This is related to, but not the same as, the
mobility of polymeric chains in the bulk


Interactions of polymers at the interface
Intermolecular forces of polymers at interfaces can be reduced into two
phenomena: London dispersion forces and electron donor-acceptor (acid-base)
interactions. H-Bonding is included in the latter, and dipole phenomena are
usually negligibly small. Polymers that exhibit polar character can often be
divided into acidic or basic, e.g. poly(methylmethacrylate) basic, post-
chlorinated poly(vinylchloride) acidic. The liquid phase in this case is important
in "controlling" the nature of interaction, e.g. a solvent that is more acidic than a
third component (solid) is preferentially bound to a basic polymer.127
Molecular weight studies have also been done to determine the effect of chain
length on these type of interactions.128


Lanqmuir-Blodgett Films and Materials for Nonlinear optics

The requirements of organic materials for nonlinear optics dictate that
high anisotropies and thin film structures are important for maximizing their
properties. In this respect, the LB technique has been widely employed as a
means of fabricating interesting structures. Early work has focused mostly on







small molecule amphiphiles with applications as materials exhibiting second
order effects. The requirements, being a non-centrosymmetric structure, are
largely fulfilled by the orientation of the amphiphiles at the interface and at the
same time the deposition of various configurations preventing centrosymmetry
to the whole multilayer. Since the inherent instability of these small molecule
systems has been discussed previously, the trend now is the use of polymers.
The design of spread polymers incorporates these active materials as side
chain groups appended to an inactive mainchain. Rare still is to find LB films of
polymers with applications for third order effects. Most of the polymers studied
in the past have applications for electrical conductivity. To the author's
knowledge, no one has examined monolayer and multilayer LB thin films of
substituted polyacetylenes. Although these polymers are prime candidates for
such materials, usually the method employed for film preparation has been film
casting27 or epitaxial vacuum deposition.22


Summary and Overview of the Dissertation

The principal area of interest is the air-water interface and its influence in
ordering. The principal polymers of interest are substituted polyacetylenes.
The structural requirements of the monomer and polymers, characterization,
behavior at the interface, and orientation are presented. Indirectly, the results of
these studies should be useful towards improving the properties of polymers
with nonlinear optical properties. The use of the Langmuir-Blodgett technique
is critical as outlined earlier for device requirements and fundamental studies of
such materials. The study of these polymers comprises both the synthetic
aspect in which the configurational and conformational behavior is studied. The
major part of this dissertation though, is in addressing the Langmuir monolayer







film behavior of substituted polyacetylenes as a class of polymers. Once stable

and well characterized films of these polymers are formed, they may be

transferred to solid substrates in order to form highly ordered films with defined

thickness. It is expected that this approach will differ substantially from bulk or

cast films, in the degree of ordering, control, and applicability. This should

result in various new physical properties of polymeric thin films.

The approach to highly ordered substituted polyacetylenes can be

divided into two pathways:


APPROACH I


Monomers III
s


Monolayers and


Multilayers


APPROACH II

Monomersill
R



Polymerization


RRRRRRRRR



Polymerization





Two-dimensionally


R R R R R
RRRRR




Monolayers and Multilaye





Ordered Substituted Polyacetylene


_K K K K~
R. R R R R

R R R R R


Figure 1-20. Schematic diagram of the twofold approach to two-dimensionally
ordered substituted polyacetylenes.







Ordered monomer approach: Monomers will be ordered as films and
polymerized. This can be done by restricting the monomers to two dimensions
using the Langmuir technique. Multilayers will be deposited on a substrate.
Polymerization can be done in situ or with the thin films.
Preformed polymer approach: Polymers will be synthesized in solution
or in bulk. Their behavior at the air-water interface will be studied in order to
control molecular conformation and ordering. Thin films can be subsequently
built-up using the LB technique.
Eventually, the preformed polymer approach is dealt with in detail as the
more promising approach. The other approach will be studied further by
another doctoral student, Mr. M.J. Roberts.
This chapter has introduced the development of materials for nonlinear
optical applications, the important aspects of this type of polymers, the
Langmuir-Blodgett technique, and the behavior of polymers at the interface.
This background and literature review is necessary to develop a complete
appreciation of the science described herein.
Chapter 2 gives details of the synthetic procedure and the experimental

apparatus used in the investigation. An extended description of the various
surface analytical techniques is given.
In Chapter 3, investigations on the behavior and polymerizability of liquid
crystalline monomeric monoacetylene amphiphiles is discussed. LB films of the
monomers are built-up and characterized. Subsequently, these multilayers are
polymerized by y-irradiation and the results are discussed.

In Chapter 4, the second approach, a series of poly(ethynylbenzoate)

polymers are investigated. The configuration and conformational behavior as
well as catalyst effects are discussed. The film behavior of the polymer are then

detailed. An interesting result is the importance of the alkyl chain in fluidizing




49

the monolayer. A variety of monolayer analytical techniques proves its worth in
these investigations.
In Chapter 5, the configurational improvement of the
poly(diethyldipropargylmalonate) is discussed. Then the monolayer film studies
are described in terms of film fluidity. Comparison between high and low MW
derivatives is highlighted in terms of monolayer properties. Interesting LB film
depositions leading to multilayer structures are subsequently made.
Lastly, Chapter 6 contains the important comparisons between the
different approaches and types of polymers.










CHAPTER 2
EXPERIMENTAL


Materials


All materials used for synthesis of monomers and polymers were
obtained from Aldrich Chemical Company unless otherwise specified. Starting
materials of reagent grade purity were used without further purification. TLC
and 1H NMR were employed to verify the purity of compounds. Reagent grade
solvents were used for the reactions. Anhydrous solvents were prepared
usually by distillation with calcium hydride or sodium. The solvents used for the
polymerization were freshly distilled by vacuum to the reaction vessel (Schlenck
tube).

Monomers and Polymers

Monoacetvlenic Liquid Crystalline Monomers

The monoacteylenic monomers used in this study denoted as
compounds I, II, Ill, were obtained from Dr. A. Blumstein and coworkers
(University of Massachusetts at Lowell). These belong to a series of liquid
crystals that have different length alkyl chains between the mesogen and the
acetylene group. Samples with eight carbon long chains were used for this
study. In addition, several amphiphiles (IV and V) used for blend studies were
obtained from Dr. W. Ford and Dr. A. Schuster of Oklahoma State University

and Max Planck Institute in Mainz, respectively. The detailed synthesis of the
liquid crystalline monomers are described elsewhere.143,129 In general, the







monomers were synthesized by the condensation of acetylenic chloride and
mesogenic alcohol. The undecynoic acid was purchased from Farchan
Laboratory. The schematic diagram of the reaction is shown below:

SOC12
HC=C(CH2)8-COOH HC=C(CH2)8-COC I
pyridine
HC-C(CH2)8-COC I + ROH 1 HC=C(CH2)8-COOR


0
ROH = HO'O* OCH3


HONIN- -OCH3



HO- 0 -- OCH3 II


Figure 2-1. Esterification of the undecynoic acid chloride with the mesogenic
alcohols to produce compounds I, II, and III.


Polvethvnylbenzoate Polymers

The polyethynylbenzoate alkyl and phenyl esters were synthesized by
A. Hilberer from Dr. J. Le Moigne's group in Strasbourg, France. These include:
Polymers 1 and 2, and the different alkyl chain derivatives PPAC-n where
n=1,2,4,8,16 carbons. Polymer 1 is denoted alternatively as PPAC-16. Details
of the synthesis are described elsewhere.47 The scheme of the monomer
syntheses is shown below (Figure 2-2). In general, dehydrating condensation
was employed effectively using dicyclohexylcarbodiimide (DCC) and
dimethylaminopyridine (DMAP) to esterify the bromobenzoic acid with the linear
alcohols to high yields. A palladium coupling reaction using







trimethylsilylacetylene (TMSA) and a palladium (II) acetate (PdAc2) -
triphenylphosphine (PPh3) was employed to introduce the acetylene group.
Removal of the trimethylsilyl group was facilitated by
tetrabutylammoniumfluoride (TBAF). 1H NMR, 13C NMR, IR, and elemental
analysis were used to elucidate and verify the structure, with good results.
Polymerization of the monomers proceeded as described in Figure 2-3.


O DCC O
Br-Q C + ROH Br- C
\ \OH DMAP OR

O Pd(II)Ac/PPh3
Br- 0'- + (CH3)3Si- H ----
'OR Et3N

(CH3)3Si-=- CR
aOR

O [CH3(CH2)314N F
(CH3)3Si COR TH
'OR THF



Figure 2-2. Synthetic scheme for the monomers of Polymer 1, 2, and the
PPAC-n series. In general, R is either any of the homologous alkyl
chains or the phenyl.


Monomer synthesis
The monomer for Polymer 3 was obtained by the prior synthesis of
alkoxyphenols from hydroquinone and the corresponding linear alkyl bromide
in cyclohexanone. The alkoxyphenol was then used for esterification with
bromobenzoic acid. A series of even alkyl chain lengths was synthesized in
good yields. The typical synthesis of the monomer is described as follows:







A mixture of the alkylbromide (16 mmol.), the hydroquinone (32-64
mmol.), and potassium carbonate (64 mmol.) was refluxed for sixteen hours in
cyclohexanone. The reaction solution was filtered. The filtrate was
concentrated, and then recrystallized from ethanol, to yield the p-alkoxy-
phenol.130 The esterification of the p-bromobenzoic acid and the p-alkoxy-
phenol was carried by dehydrating condensation using DCC.131 To 8 mmol. of
p-alkoxyphenol in 40 ml. methylene chloride with 100 mg of DMAP was added
8 mmol. of 1,4-bromobenzoic acid. The solution was initially stirred at 0C to
which DCC was slowly added and urea precipitated. The mixture was stirred at
200C for 3 hours and the precipitated urea filtered off. The filtrate was
evaporated under vacuum and the residue was redissolved in methylene
chloride in order to filter the insoluble urea. The organic solution was
successively washed with aqueous solutions of HCI, NaHCO3, and pure water,
and finally dried. The solvent was removed and chromatography of the residue
was done on a SiO2 column with CHC13 as eluent. Crystallization in
methanol/methylene chloride gave white platelets of the esterified product.
The bromo derivative was then reacted with trimethylsilylacetylene
(TMSA) to subsequently produce an ethynylated product.132 To a deaeriated
solution of the (17 mmol) bromo derivative in 50 ml of anhydrous triethylamine,
34 mmol of TMSA was added dropwise. Catalyst, palladium(ll)acetate, (5 mg)
was then added together with 50 mg triphenylphosphine. The mixture was
refluxed for 24 hours, then cooled and filtered to remove the hydrobromide salt.
The orange brown filtrate was concentrated, mixed with 200 ml of aqueous
sodium bicarbonate, and then extracted with dichloromethane (3 x 50 ml). The
organic fractions were combined, dried over magnesium sulfate, and
concentrated to yield an oil. The residue was taken up in methylene chloride
and passed through a column of silica with a mixture of methylene chloride and







pentane. The eluent was concentrated and further purified by column
chromatography. The final product was a yellowish solid.
Different chain lengths from the series were obtained in good yield and
the structure was elucidated using common organic spectroscopic
characterization techniques. The data shown below for the ten carbon alkyl
chain length derivative is taken as typical for the rest of the monomers with
differences only on the alkyl regions of the spectra.
Monomer, p-decanoxyphenyl-p-ethynylbenzoate: Yellowish solid, yield =
78%; m.p. 74.35 C; IR (KBr pellet) 3240 cm-1 (m, sharp, acetylenic C-H
stretching), and 610 cm-1 (m, acetylenic C-H bending), 1740 cm-1 (s, carbonyl
ester); 1H NMR (CDCI3) 3.25 ppm (s, acetylenic proton), 8.2-7.6 ppm (s, 4H
aromatic benzoate), 7.2-6.9 (s, 4H aromatic phenol). 13C NMR (CDCl3) 80 and
88 ppm (acetylenic a and P), 164.88 ppm (m, carbonyl carbon), 157-115 ppm

(m, 8C aromatic); Analysis, Calc. C: 79.4, H: 7.9, 0:12.7. Found. C: 79.1, H: 8.0,
0:12.9.


Table 2-1. Elemental Analysis of the monomers with calculated and actual
values.

Alkyl Chain Length of Calculated Observed
Derivative C%, H%, 0% C%, H%, 0%

8 78.9, 7.4, 13.7 78.6, 7.6, 13.8

10 79.4, 7.9, 12.7 79.1, 8.0, 12.9

12 79.8, 8.4,11.8 79.8, 8.5, 11.7

14 80.2, 8.7, 11.1 79.2, 8.8, 12

16 80.5, 9.1, 10.4 81.1, 12.18, 6.72

18 80.8, 9.4, 9.8 78.9, 9.3,11.8







Elemental analysis of the monomers show good correlation with the
calculated values for the structures with different alkyl chain lengths. The data
are summarized in Table 2-1.
Polymerization was done for these monomers using the Schlenck tube
technique. The apparatus and set-up used was similar to that of the other poly-
ethynylbenzoate derivatives and the description below can be taken as typical.
Figure 2-4 shows a schematic diagram of the polymerization set-up in which the
whole system, under vacuum, was purged with N2 gas. All glass apparatus was
cleaned and flamed or oven dried prior to use. Dry box handling technique was
used to introduce the catalyst and the solid monomer in the reaction vessel.

H H

II n


0 wC1,6 0

0" O Toluene at 35 OC


0 0
OR
OR

Figure 2-3. Polymerization of the monomer with WCl6 in toluene using the
Schlenck tube technique. R = 8, 10, 12 carbons polymerized
successfully.


Typical polymerization
Monomer (250 mg) and catalyst (5.8 mg) were placed in a clean and dry
reaction vessel under argon. Fresh dry solvent (toluene) was introduced to the







reaction vessel by evaporation to make an approximately 1.0M solution. The
polymerization was carried out at 300C for 24 hours. The product was
recovered in methylene chloride and poured into a large excess (150 ml) of
methanol. The precipitant was then redissolved in THF and passed through a
silica column to remove the catalyst. The eluent was then concentrated and
reprecipitated in methanol, then oven dried and weighed.


N2 gas

Vacuum 4 |

gas and vacuum line

two-way valve

one-way valve

rotating stopcock

Schlenck tube

solution
stirrer -
water bath

Figure 2-4. Polymerization set-up under N2 gas and vacuum line. Monomer
and catalyst was introduced to the tube under dry box conditions prior to
polymerization.


Polymer of p-decanoxyphenyl-p-ethynylbenzoate: A reddish brown
product was obtained with 89% yield. IR (KBr) 1600 cm-1 (m, sharp polyenic
double bonds), 1265 cm-1(=C-H in plane deformation), disappearance of 3240
cm-1 peak (acetylenic C-H stretching), 922 and 970 cm-1 (=C-H out of plane







deformation); 1H NMR (CDCl3) disappearance of 3.25 ppm resonance (s,
acetylenic proton), 7.2-6.5 ppm (b, ethylenic protons); 13C NMR disappearance
of 80.4 and 82.7 ppm (acetylenic a and P), 135-125 ppm (b, ethylenic carbons
with phenylene and cyclohexadiene C=C carbons); Analysis, calc; C:79.2, H:8.1
Found; C:79.1 H:8.1.
Only polymers from 10,12, and 14 carbon derivatives were obtained. No
further reactions were run however since the goal of producing a lateral
substituent variant for the series was achieved. The polymer with a 12 carbon
derivative gave similar NMR, IR, and UV spectral observations with the 10
carbon derivative except for the alkyl region. The 13C NMR of the 14-carbon
derivative showed a small resonance at 0.2 ppm signifying the presence of the
trimethylsilyl groups in the polymer. This residue came from the incomplete
conversion of the monomer by the removal of the trimethylsilyl protecting group.
This could account for the discrepancy between the calculated and observed
value in the elemental analysis. Despite drying of the polymer for a long time,
this peak was not removed and therefore is probably associated with the
polymer backbone (trimethylsilane or trimethyl-silylacetylenes are volatile).
This would mean that some disubstituted acetylenes were copolymerized with
the monomer resulting in a more irregular polymer. Some saturated defects are
observed in the 4.1-3.5 ppm region in the 1H NMR and between 60 to 30 in the
13C NMR. This derivative was not used for comparison with the other polymers.


Synthesis of Poly(diethyldipropargylmalonate)

Monomer preparation
The monomer diethylhepta-1,6-diyne-4,4-dicarboxylate, was prepared
according to a literature procedure.74 The following procedure was used:







diethylmalonate (6.75 g = 1 equiv.) was added to previously dried ethanol (100
ml) containing sodium ethoxide in ice bath. The sodium ethoxide was prepared
from 3.0 g of sodium in ethanol. After 5 min., propargyl bromide (10.5 g = 2
equiv.) was slowly added to the stirred suspension, and the mixture heated
under reflux for 6 hours. The reaction was stopped and alcohol was removed
under pressure. The product was diluted in water and petroleum ether was
added to precipitate the product. The product was recovered by filtration, and
dried to get a 75 % yield. m.p. was at 45 OC (lit. 45.5 C). It had the following
spectral properties: IR (KBr) 3300 cm-1 (m, =C-H stretching), 1740 cm-1
(carbonyl ester); 1H NMR (200 Mhz CDCI3) 1.28 ppm (t, 6H), 2.1 ppm(s, 2H), 3

ppm (s, 4H), 4.3 ppm (q, 4 H). 13C NMR: 15 ppm (CH3), 60.8 ppm(CH2 of
CH2C-C), 72 ppm (=C-H), 81.6 ppm (C=C), 171 ppm carbonyll C), Analysis,
calc; C:66, H:6.7,O:27.3 Found; C:65.5 H:6.8, 0: 27 .7


Typical polymerization


EtOOC/ COOEt MoCI EtOOC,, COOEt
M oC I 5 00

11 dioxane 35 C


Figure 2-5. Polymerization of diethyldipropargylmalonate using MoCI5 catalyst
in dioxane solution.


Monomer (250 mg) and catalyst (5.8 mg) were placed in a clean and dry
reaction vessel under argon. Fresh dry solvent (dioxane) was introduced to the
reaction vessel by evaporation to make an approximately 0.1 M solution
(varied). The polymerization was carried out at 250C (35 OC) for 24 hours







(varied). The product was recovered in anhydrous methylene chloride and
poured into a large excess (150 ml) of methanol. The precipitate was then
redissolved in THF and passed through a silica column to remove the catalyst.
The eluent was then concentrated under vacuum and reprecipitated in
methanol, then oven dried. The yield was determined by gravimetry.



Table 2-2. Summary of polymerization yields and conditions. Reaction was
done at 35 C for the high MW polymer derivatives and at 25 C for the
low MW polymers.

Polymer Ct./Mo Conc. Time Total% sol. % insol.%
ratio in [M] rxn. Conver
(mole) 0.02 (hrs) sion

pDDM1H 1:50 0.07 24 88% 100 0

pDDM2H 1:50 0.15 24 63% 95 5

pDDM3H 1:50 0.30 24 70% 86 14

pDDM25 1:25 0.15 24 66% 85 15

pDDM1L 1:50 0.12 1 40% 87 13

pDDM2L 1:50 0.12 3 45% 100 0

pDDM3L 1:50 0.12 12 55% 89 11

pDDM4L 1:50 0.12 18 68% 100 0


Variations on the procedure were made to change the parameters of the
reaction. For different concentrations, the solvent introduced was measured on
graduations in the Schlenck tube within 0.2 M uncertainty. Catalyst ratio was
controlled by weighing the right amount of catalyst at the dry box stage to obtain
a 1/25 and 1/50 catalyst/monomer ratio. The time of polymerization was







controlled simply by stopping the reaction at the desired point and using
separate tubes. Table 2-2 summarizes the yield and parameters of
polymerization.
The following spectral properties were taken as typical for the high MW
polymer and the low MW polymer groups respectively. No noticeable
differences in the spectral properties were observed and the elemental analysis
is tabulated separately.
Poly(diethyldipropargylmalonate), pDDM3H:A black amorphous solid
was obtained with a 70 % yield. UV = 550 nm (purple solution), IR (KBr)
1600cm-1 (m, sharp polyenic double bonds), 1250 cm-1 (=C-H in plane
deformation), 1727 cm-1 carbonyll), disappearance of 3300 cm-1 peak
(acetylenic C-H stretching), 925 and 970 cm-1 (=C-H out of plane deformation);
1H NMR (CDCI3) disappearance of 2.1 ppm resonance (acetylenic proton),
7.0-6.4 ppm (broad, ethylenic proton), 13C NMR 14 ppm (methyl C), 41 ppm
(ring methylene C) disappearance of 72.4 and 81.2 ppm (acetylenic a and 0),
123.2 ppm and 138 ppm(s, polyenic carbons), 171 ppm (s, carbonyl);
Poly(diethyldipropargylmalonate), pDDM1L: A black amorphous solid
was obtained with a 40 % yield. UV= 546 nm (purple solution), IR (KBr)
1610cm-1 (m, sharp polyenic double bonds), 1250 cm-1(=C-H in plane
deformation), 1725 cm-1 carbonyll), disappearance of 3315 cm-1 peak
(acetylenic C-H stretching), 930 and 990 cm-1 (=C-H out of plane deformation);
1H NMR (CDCl3) disappearance of 2.1 ppm resonance (acetylenic proton), 6.7
ppm (sharp, ethylenic protons), 13C NMR 14.8 ppm (methyl C), 41.5 ppm (ring
methylene C) disappearance of 72.4 and 81.2 ppm (acetylenic a and p), 124
ppm and 137.6 ppm (s, polyenic carbons), 172 ppm (s, carbonyl).
Several very small peaks were observed at 41, 57, 123,137 and 172
ppm (usually adjacent to the larger peaks) which were not previously reported.







This may be indicative of small amounts of five membered ring units on the
polymer backbone as reported in the literature.133 No attempt was made
though to systematically study the parameters which control the formation of this
ring. In any case, these are not expected to significantly affect the configuration
as well as the conformation of the conjugated sequences .

Table 2-3.Elemental Analysis of the polymeric products obtained from the
soluble fractions of the polymers.


General Instrumentation


13C NMR and 1H NMR spectra were recorded on a Bruker AC200F

spectrometer system operating at (1H) 200 and (13C) 60 MHz. Deuterated
chloroform (CDC13) was used as solvent and all chemical shifts reported are
internally referenced to this. (13C) spectra were run typically between 12-14 hrs
acquisition time. Infrared Spectra of the powder-pressed pellets in KBr were


Polymer C% H% 0% Total

calculated 66.0 6.7 27.3 100
C13H1104

pDDM1H 65.1 6.8 27.0 98.9

pDDM2H 65.6 5.5 27.3 98.4

pDDM3H 65.5 6.5 27.0 99.0

pDDM25 62.8 7.2 27.0 97.0

pDDM1L 65.6 6.2 27.0 98.8
pDDM2L 64.8 6.7 26.8 98.3
pDDM3L 65.6 6.7 26.5 98.8

pDDM4L 65.0 7.0 26.4 98.4







recorded on a Perkin Elmer 983. UV spectra were recorded on a Perkin-Elmer
UVNIS/NIR Lambda 9 spectrophotometer and/or Shimadzu UV 210-PC UV-vis
scanning spectrophotometer in spectro grade methylene chloride, THF in
solution (10-4- 10-5 M), and on a quartz plate for the cast film.
The reaction set-up for polymerization was attached to a high vacuum
(10-5 mmHg) and N2 gas line, separated by a two way high vacuum valve
(Figure 2-4). Specially constructed Schlenck tubes (50 75 ml) with a high
vacuum valve was used. Monomer and catalyst was weighed and introduced to
the tube in a glove box under Argon.
Differential Scanning Calorimetry (DSC) and Thermogravimetric
Analysis (TGA) were performed on a Perkin Elmer 7 Series Thermal Analysis
System and/or DSC 7 Thermal Analysis System in aluminum pans with a
heating and cooling rate of 10 OC/min. DSC samples (10-20 mg) were
analyzed with liquid nitrogen as coolant under a helium or nitrogen flow rate of
25 mL/min. TGA samples were performed under nitrogen with a flow rate of 50
ml/min. and program heating from 50 to 700 OC at a rate of 10 OC/min. Optical
observations were obtained with a Leitz Orthoplan or Nikon Diaphot optical
polarizing microscope equipped with a Mettler FP52 hot stage and FP80
Central Processor.
Elemental analyses were performed by the analytical services group (Mr.
M. Keyser) of the Institut Charles Sadron in Strasbourg, France.
For molecular weight determination, Gel Permeation Chromatography
(GPC) method was used. Data was collected using a Waters Associate Liquid
Chromatograph apparatus equipped with a U6K injector and differential
refractometer and a Perkin Elmer LC-75 ultraviolet (UV) spectrophotometric
detector. Two Phenomenex 7.8 mm x 30 cm Phenogel 5 consecutive linear
cross-linked polystyrene gel columns were used. The eluting solvent was







HPLC grade tetrahydrofuran (THF) at a flow rate of 1.0 mL/min. Polymer
samples were dissolved in THF (0.5 to .05 % w/v) and filtered (-50 mrn) before
being injected (10-40 g-L) to the sample injection port. The retention times were
calibrated using polystyrene standards (Scientific Polymer Products Inc.). The
following narrow molecular weight polystyrene standards were used with a
polydispersity Mw/Mn s 1.06: Mw = 650000, 142000, 79000, 59500, 47500,
30700, 12200, 7820, 1940. The Millennium 2010 data collection and analysis
software system running on a 386 IBM PC computer was used for calibration
curve calculation and peak analysis.


Langmuir-Blodgett Technique

Lanamuir Monolayer Film Studies

For the materials studied at the air-water interface, spreading solutions
were prepared using spectro grade chloroform or methylene chloride ( Kodak
or Fisher ACS grade) at 0.2-1.0 mg/ml concentrations. Volumes of 50 to 200
ml were spread using a Hamilton gas tight microsyringe with an uncertainty of
+ 1 %. The solution was delivered dropwise, allowing time for evaporation of
solvent. The water used was of excellent purity (ion resistivity of 18 MQ) and
was purified by deionization (Continental Water Systems), followed by the
Millipore Milli-Q Plus 5-bowl filtering system.
Data was collected using the KSV LB5000 modular Langmuir-Blodgett
System (KSV Instruments Helsinki, Finland) in a clean dust free environment.
The trough is made of teflon with channels underneath to allow for circulation of
cooling/heating liquid from a temperature bath. Temperature was controlled
with a deviation of 0.5 OC using a Neslab refrigerating and heating circulating
bath. A teflon or nylon barrier was used for controlling the surface area by







symmetrical compression. A surface balance using the Wilhelmy plate method
(platinum or paper) was used to record the surface pressure. Isotherm, mean
molecular area- time, surface pressure-time plots were recorded with the
version 4.4 KSV software package installed in an IBM-PC AT computer
interface. The monolayers were compressed with a speed of 1 to 3
A2/[molecule (repeat unit) x min]. and were considered reproducible only if a
range of 1 A2/molecule(repeat unit) in area deviation were observed. All
isotherms were run a minimum of two times. The mean molecular area (Mma)
refers to the average surface area/molecule or area/repeat unit in the case of
polymers. To determine the stability of a monolayer, the following procedures
were used. First, the monolayer was compressed until it reached a given
surface pressure and the barrier was stopped. The surface pressure changes
were then monitored as a function of time. In the second method, the mean
molecular area of the compound or polymer was monitored as a function of time
while keeping the applied surface pressure constant. The applied surface
pressure was kept constant by computer control of the barrier movement.


Langmuir-Blodgett film deposition

A KSV LB5000 dipping trough was used for conventional vertical
deposition. Precise dipping movement was performed by a computer controlled
mechanical dipping arm attached to the mainframe. For alternate dipping, a
three well compartment trough was used together with a 1800 angle moving
mechanical dipper/arm. Films were deposited on specially prepared glass,
quartz plates, and silicon wafers. The dipping speed was maintained at speeds
of 1.0 to 10.0 mm/min. in both directions. The dipping motion was delayed at

the top of its travel for either 5 or 10 minutes to allow time for the previously







deposited layer to dry in the air. The transfer ratio and surface pressure were
monitored and recorded during the dipping process.
For alternate deposition, the KSV 5000 Alternate Deposition System was
used. The dipping procedure is the same as above, with two monolayer films
maintained at constant pressures simultaneously. Delay time, dipping speed,
and cycle sequence (trough 0, 1, and 2) were controlled by programming.
The glass and quartz plates were cleaned using the following procedure:
the plates were immersed in a detergent solution and sonicated for 10 minutes
and then placed in a solution consisting of a 1:1:3 mixture containing NH40H,
H202, and Milli-Q water respectively. The plates in the solution were then
heated for 30 minutes at 80 C. Alternatively, a solution of Chromerge in
concentrated sulfuric acid was prepared and the substrate soaked for several
hours. Hydrophobization of substrates was done using a standard technique
which involves placing the clean substrate sequentially in each of the following
solvents: CH3OH, CHC13 / CH3OH, CHCl3. This is done at 15 minute intervals
in a sonicating bath. The substrate is then placed in a mixture of 70 ml pure
decalin, 10 ml chloroform, 20 ml carbon tetrachloride, and 2 vol %
octadecyltrichlorosilane and sonicated for 2 hours. The plates are then
sonicated for 10 minutes on the reverse order of solvents mentioned above.
UV spectra of the deposited multilayers were obtained using a specially
constructed sample holder on a Perkin-Elmer UV/VIS/NIR Lambda 9
spectrophotometer.
X-ray diffraction of the liquid crystalline monomer LB Films was taken
using a GE XRD-5 diffractometer at 35kV and 30 mA using Cu Ka (1.54 A)

radiation with a scan rate of 0.4 deg./min. Measurements were done with the
assistance of Prof. Blanchard of the Department of Geology, University of
Florida. Low angle X-ray diffraction for the poly(diethyldipropargylmalonate)







and stearic acid films was performed by Dr. Jing Fei Ma of Prof. Nagler's
research group, Department of Physics, University of Florida. A Rigaku 18-kW
rotating anode diffractometer with a Cu Ka line, at X = 1.54 A passed through a

graphite monochromator. The diffracted X-ray beam was passed through a
graphite analyzer before detection.
FTIR-ATR measurements were performed by Mr. Houston Byrd of Prof.
Talham's research group, Department of Chemistry, University of Florida. The
spectra were recorded with a Mattson Instruments Research Series-1 Fourier
Transform infrared (FTIR) spectrometer using a narrow-band mercury cadmium
telluride detector. A Harrick TMP stage was used for the ATR experiments.
Polarized FTIR-ATR spectra was taken with s- and p-polarized light. All spectra
consist of 1000 scans at 2.0 cm-1 resolution.


Surface Analytical techniques at the air-water Interface

In addition to the use of the surface balance for surface pressure-area
isotherm measurements, several analytical techniques were used to investigate
the surface properties of spread monolayers. A brief background is described
for each technique and then the specifications of the apparatus as well as the
measurement procedures are described:

Surface potentiometer
Next to the surface-pressure area isotherm, the most useful tool for
characterizing the air-water interface is the surface potentiometer. The surface
potential is defined as the change in phase boundary potential produced by an
interfacial film. The measured quantity is the difference between the potential at
the air-water interface, with and without a monolayer. The Helmholtz equation
relates the potential to changes in the surface dipole moment, gI :134







Ag /A= CoAV


where A, is the area per molecule, AV the surface potential, and Eo is the

permittivity constant in vacuum. Further discussion is given in Appendix C. Two
methods are generally used, the ionizing electrode method and the vibrating
plate (condensor) method. The first method makes use of an air-electrode
mounted a few millimeters above the surface and incorporates a small
radioactive source.120 This ionizes the gap between the air electrode and the
surface thereby making it conducting.


trough

Figure 2-6. Surface Potentiometer set-up (vibrating plate method) showing the
schematic diagram of the compensating voltage circuitry. Diameter of the
vibrating plate and electrode is 4 cm.


The other method is a capacitance method. An alternating current is
generated by the small amplitude vibration of metal plate at a certain frequency
(about 80-120 Hz).120 This results from the potential difference between the


(2.1)


drive unit


oscillator







plates producing a capacitance that induces AC current into the circuit. The
plate is attached to a voice coil of a loudspeaker and is located as close as
possible to the interface without disturbing it. The alternating current is
amplified and detected by a phase detector. The phase detector gives a DC
current output which charges the following feedback capacitor until the
compensating voltage equals the surface potential and no currents occur. This
potential can be plotted as a function of the mean molecular area to produce the
typical surface potential-area isotherm.
A KSV 5000 SP module with a measuring range of 10 V with a 5mV
precision was used to measure the surface potential at the air-water interface.
Based on the vibrating plate method, an alternating current is generated by the
small amplitude vibration of a metal plate at frequencies of about 80-120 Hz.
Another metal plate electrode is placed underneath the subphase at a distance
of 1-2 cm from the upper plate. Calibration was done using the software
calibration program and an adjustable DC-power supply or battery. The
potential was set to zero for the air/water subphase, just before the monolayer is
spread.


UV-vis spectroscopy at the interface

In situ measurements of UV-visible absorption spectra can be made at
the interface by using the appropriate set-up.87 Simultaneous recording of
surface-absorption-area and pressure-area relationships provide insight into
the orientation and arrangement of molecules at the interface. The presence of
changes in both wavelength and absorption can be measured in situ with
changes in surface area by compression of the monolayer. The result is a three
dimensional plot containing the axes of surface area, wavelength and

absorbance. Wavelength shifts in compression can be used to determine the







presence of molecular aggregations by i-~* interactions as well as changes in
the conjugated sequences of absorbing amphiphiles. As light passes through a
monolayer, it is either reflected, absorbed or transmitted. The portion of light
that is absorbed is detected by a detector which gives the spectral changes.
The set-up, shown below, makes use of optical fibers to direct light as
well as to detect the transmitted light. Light channeled from a Xe arc lamp
source is first passed through the monolayer. This is in turn reflected by a mirror
beneath the water subphase which causes the transmitted light to pass through
the monolayer for the second time. The light is then detected by the other optic
fiber and channeled through a photodiode array detector. The photocurrent
output is in turn converted to a voltage, amplified and converted by an analog to
digital converter into a series of digitized readings that generates a spectra in a
computer interface. To maximize resolution and spectral range, the wavelength
sensitivity can be varied by a holographic or ruled grating to the desired blaze
wavelength with the detector optics.
In this study, in-situ UV-Vis spectroscopy was performed using an Oriel
Instaspec III Spectroscopy system. A modified KSV trough (minitrough) was
used with a special attachment for holding the optical fibers as well as in fixing
the mirror underneath the subphase. A Xe Arc lamp light source with a spectral
range of 190-900 (UV to near IR) was used, coupled to a quartz optical fiber
bundle. The exit plane of the bundle was set horizontally at 3 cm above the
water surface. A piano-convex mirror was placed at 5 mm underneath the
subphase (parallel to the water subphase) to reflect the transmitted beam. A
nitrogen gas and cooling bath cooled the Multispec photodiode array detector
of 1024 elements. Cooling increases the dynamic range of the PDA detector.








photodiode array
detector
\ r


Xe arc lamp /
trough ,


computer
interface



surface balance



I barrier
/


mirror


Figure 2-7. Schematic diagram of an in situ UV-Vis Spectroscopy set-up for
measurements at the air-water interface. The photodiode array detector
is cooled with N2 gas and a circulating cooling bath


Gratings of 2400 and 400 lines/mm were used together with
combinations of blaze wavelength with ranges from 250-750nm. The choice of
the grating/blaze wavelength combination is based on proximity to the
absorption peak of interest. The grating with higher number of lines/mm gives a
smaller spectral range but greater resolution. The grating with the closest blaze
wavelength to the desired absorption peak was chosen. A scanning frequency
and integration time of 1 spectra/10-40 seconds and 1 second, respectively,
were used for the measurements. Exposure time (0.5 1 sec.) was controlled
(to below the saturation level) via an automated shutter mechanism interfaced
to the computer. Correction was made for shot noise (photon noise), electrical
noise, and dark current noise prior to data acquisition. This was done by taking
a reference reading at the the interface without the monolayer (before







spreading) and a background reading with the shutter off prior to the actual
measurements. The data acquisition cycle of the spectrometer was timed to
coincide with events in the monolayer e.g. compression, isobaric creep
measurements, etc. Data is stored in the computer and retrieved at the end of
the cycle. Absorption spectra and three dimensional UV-vis/time or area plots
were generated using the Instaspec III peak analysis software program.


Brewster angle microscopy and reflectivity measurements
Brewster angle microscopy allows the direct observation of monolayer
morphology without the complication of added probe molecules since contrast
is dependent upon differences in the refractive indexes of pure components
rather than on emission by excitation of added chromophores. Brewster angle
microscopy or BAM is based on a simple principle of optics. When p-polarized
light is directed at an interface of two materials, with refractive indices ni and n2
where ni > n2, reflected light intensity is zero at an angle which is called the
Brewster angle and defined as

tan (a) = n2 / nl (2.2)

When a film possessing a refractive index n different from that of n2, is
introduced to the surface, light is reflected. The Brewster angle microscope
images are light reflected from the interface. Such changes in refractive index
at the interface are caused by differences in density, orientation and

aggregation in pure films and phase separation in mixed films are seen as
intensity changes in the BAM image. An advantage of this technique is that
contrast is obtained without the addition of foreign molecules. Therefore the
concern for miscibility of a fluorescent probe and its possible effect on
monolayer behavior is eliminated.

























Figure 2-8. Brewster Angle Microscope set-up showing the path of laser light
reflected through a set of mirrors. A polarizer-analyzer system is used for
polarized microscopy. Images are observed through an external video
monitor. An auxiliary integration unit can be attached.

The Brewster Angle Microscopy(BAM) used was a BAM-1 from

Nanofilm Technologie GmBh (Goettingen, Germany). A 5 mW He-Ne laser light

set at p-polarization was used to illuminate the images. The angle of incidence

was set initially at 530 and then adjusted to minimize the reflected intensity of

the clean water surface prior to the spreading of each film.


no reflection p-pol reflection



nl
n im
2 n2




Figure 2-9. Schematic diagram of the Brewster angle phenomena in optical
media of different refractive indices.


74







The images are observed on a video monitor from which recordings were
made on a VHS video cassette recorder. Frame grabbing of images was done
using a Sun Sparc station with commercially available imaging processor
software.
Relative reflectivity data were obtained from the images in two ways: An
auxiliary video integration unit VIU1 also from Nanofilm Technologie was
attached to the control module of the microscope, in which integration of the
pixels are automatically given as DC output. The plots are recorded in an X-Y
plotter or as voltage changes from a multimeter. Calibration is made between
the air/water interface (zero reflection), and a fully saturated image. The other
method involved integration of the image pixels using a software program in a
SUN Sparc station (VideoPix, Sun Microsystems Inc. 640 x 480 pixels in 256
gray scales). Data are plotted as a ratio of intensity (air-monolayer-water)/
intensity (air-water).










CHAPTER 3
APPROACH I: POLYMERIZATION OF ORDERED MONOMERS


Introduction


Liquid Crystalline Monomers at the Interface

Liquid crystals are extensively used in both passive and active display
devices. The function of such devices comes from the ability of the liquid
crystalline molecules to self organize at a number of intermediary states
between the isotropic liquid and the organized solid state.135 Liquid crystals
are generally classified as either thermotropic or lyotropic in behavior.
Thermotropic liquid crystals have thermodynamically stable, temperature
dependent intermediary states that are called mesophases, the most common
of which is the nematic phase (N). Others such as the cholesteric (Ch) and the
smectic phases (S) are characterized by higher dimensional order and
sometimes chirality.136 Control of these phases is brought about by systematic
variations in the shape and interactions between the molecules. The classical
shape is that of a stiff rod-like moiety (called the mesogen) with a flexible chain
attached to one or both ends of the moiety giving it a high aspect ratio
(length:width).
The use of liquid crystalline structures in making Langmuir-Blodgett films
is a step towards better morphological control. Considering the fact that many
liquid crystalline compounds are amphiphilic in nature, it would seem inevitable

that these self-organizing systems would be investigated confined to a semi-







two-dimensional water surface. Ringsdorf et al. have synthesized, spread, and
transferred a variety of liquid crystalline molecules and polymers starting two
decades ago.137 Much research since then, has been explored with a wide
range of fields covering both investigation techniques and materials research.
The number of articles appearing in journals such as Langmuir and Thin Solid
Films attests to this growth. In principle, the presence of polar groups
especially in the mesogenic moiety and the hydrophobic nature of the flexible
chain orients the molecule. The presence of x-n interactions between phenyl
rings increases intralayer interactions.138 Interesting multilayer architectures
have been built up with liquid crystalline amphiphiles. However, there is a lack
of device applicability at present due to intrinsic instability. The disadvantage, is
the subsequent rearrangements brought about by mechanical or thermal stress
on the multilayers with time. This lack of temporal stability within LB films of
liquid crystalline amphiphiles points towards polymerization as a means of
retaining this order. It has been shown that polymerization of monomer crystals
proceeds with high rates and low overall activation energies.139 In 1964,
G.M.J. Schmidt and collaborators, enunciated the topochemical principle of
reactions in the solid state.140 This concept emphasized that groups
undergoing reaction in the solid state should be close to 4-4.5 A in distance and
that reactivity is greater in the ordered lattice than at defects. The field has
become immensely popular with the solid state polymerizations of LB films of
diacetylenic monomers by G. Wegner and collaborators beginning in
1969.141,92 The advantages of polymerization of ordered monomers lies not
only in the topological control but also in stabilizing the ordered structures in
multilayers. These often result in enhanced mechanical and thermal stability of
multilayers.










To date, while much study has been confined to polymerizations of
diacetylenic amphiphiles, little work has been done on liquid crystalline
monoacetylenes. Previous work with liquid crystalline monoacetylenic
amphiphiles has been done most notably by three research groups: Le Moigne
and coworkers have synthesized a number of liquid crystalline substituted
acetylenes (including compound III) and have succeeded in polymerizing a few
of these acetylenes in the bulk using metathesis Group 6 catalysts and y-
irradiation in the bulk (albeit to low yield).142 Blumstein and coworkers, who
have synthesized the liquid crystalline acetylenes used in this study, have found
low bulk polymerization yields using y-irradiation.143 Also Ogawa has
attempted to polymerize amphiphilic alkyl substituted acetylene monomers in
LB films using X-ray and KrF excimer laser light.144 Each of these workers
reported low yields for polymerization of these types of monomers. Recently,
liquid crystalline monoacetylenes have also been polymerized successfully with
Ziegler-Natta and metathesis catalysts to high yields by H. Shirakawa and
coworkers.145
In this study, three substituted acetylene compounds, which are known to
form nematic liquid crystalline phases in the bulk (Table 3-1), were chosen to
see if they would form Langmuir films (insoluble monolayers at the air-water
interface). It is hoped that greater order will be induced by the mesogenic
groups when coupled with amphiphilic interactions at the water surface. In
principle, the increased intralayer ordering should position the acetylene group
ideally for polymerization. The liquid crystallinity restricts the orientation of the
monomer normal to the surface while the Langmuir-Blodgett technique

constrains the monomer to the surface. Thus, the interest in Langmuir film







formation lies in the possibility of polymerizing a highly oriented monomer as
multilayers deposited on a substrate (Langmuir-Blodgett film). This chapter will
mainly focus on polymerization of these ordered monomers and constitutes the
Approach I route for highly ordered substituted polyacetylenes.


Bulk Characteristics of the Monomers

The monoacteylenic monomers (I, II, III) used in this study were obtained
from Blumstein and co-workers (University of Massachusetts at Lowell). In
addition, several amphiphiles used for blend studies (IV and V) were obtained
from W. Ford and A. Schuster (Oklahoma State University and Max Planck
Institute in Mainz).



0 0
0 0



HC=C(CH2)8-CO -. N--. JOCH3


0
HC C(CH2)8- COOCH



CH3(CHal),-0-O -- OH IV


II / \ -N(CH3)2 V
HO-(CH2)1o-CO / -

Figure 3-1. Chemical structure of Compounds I, II, III and the interactants
IV and V.







The study of compound III was mainly done by another doctoral thesis
candidate M.J. Roberts and references to this work will be confined to parts

which are relevant for understanding the overall behavior of this class of

monomers.

The detailed synthesis of the monomers is described elsewhere.143 In
general, the monomers were synthesized by the condensation of acetylenic
chloride and mesogenic alcohol. The schematic diagram of the reaction is
shown below:


SOC12
HC-=C(CH2)a-COOH 3W


HCEC(CH2)-COC I


HC=C(CH2)8-COC I


pyridine
+ ROH HCEC(CH2)8-COOR


Figure 3-2. General synthetic route for the monomers. Shorter alkynoic acid
derivatives have also been synthesized by Blumstein et al.



Table 3-1. Thermotropic behavior of the liquid crystalline monomers in bulk as
determined by Blumstein and coworkers.



Compound Heating Cooling

I C 38.38 N I 67.32 N
N 71.931 N 42.19 C

II C 96.23 N 197.24 N

N 102.97 1 N 75.83 C

III C 76.71 I I 65.56 N
N 52.01 C







Table 3-1 shows the transition temperatures of the compounds upon
heating and cooling as observed by optical microscopy and DSC. From the
table, it can be seen that both compounds I and II show enantiotropic behavior
while III shows a monotropic behavior. All mesophases obtained with these
monomers are nematic with characteristic "schlieren" textures, as observed by
optical microscopy. Small transition enthalpies AHIN (typically 0.2 0.4 cal/g)
and entropies ASNI of 0.3-0.4 show "classical," i.e. noncybotactic nematics.

Based on these results, the following order in promoting liquid crystallinity can
be assigned:

II > I > III


Langmuir Monolayer Film Studies

Surface Pressure-Area Isotherms

Plots of surface pressure versus mean molecular area as shown in
Figure 3-3, were done for both compounds under study. For compounds I and
II, at ambient temperature, the surface pressure onset occurred at mean
molecular areas (Mma) of 16 A2 and the apparent collapse pressures were very
low. Two transitions were observed for compound I, at I = 8 and 20 mN/m. All
the isotherms were reproducible within 1 A2 molecule in area.
The monolayers were observed to be viscous as evidenced by the
deflection of the Wilhelmy plate especially on a one barrier trough compression.
Even though the isotherms were very reproducible, consideration of the
chemical structure of the monomers suggests that this onset is too small to
correspond to the formation of a true two dimensional monolayer. Both
monomers consequently showed poor hysteresis behavior in monolayer

compression and expansion cycles.









45
40
. 35
z
E 30
N 25
S20
S15

(i 10


0 10 20 30
Area [ A 2/molecule]


Figure 3-3. Isotherms of pure compounds I and II at 25 OC and compression
speed of 3.5 A2/(molecule x min). Note the two inflection points observed
for the isotherm of compound I.


45
40

I' 35
E 30
S25
2 20
a
| 15
10


II I


0 10 20 30
Area [ A 2 /molecule]


Figure 3-4. Monolayer behavior at lower temperature
and II. Note the change in area (Mma) and the
points for compound I


(16 OC) for compounds I
shift of the inflection







Figure 3-4 shows isotherms recorded at lower temperatures for the two
monomers. It is apparent that at lower temperatures, greater onset Mma (higher
than 20 A2) values are obtained and the collapse pressure increased
substantially. This exothermic behavior is accompanied by an increase in the II
for both transitions of compound I which shifts from the shoulders at 8 and 20
mN/m to distinct inflection points at 25 and 30 mN/m respectively at 16 OC. The
onset Mma values are more reasonable in view of the chemical structure of the
monomers.


Stability of the monolaver.

For good deposition of the liquid crystal on a substrate, the formation of a
stable monolayer is essential. A monolayer is considered stable when it
displays a steady Mma at constant surface pressure over a large time scale.
Stability studies versus time showed that the films of compounds I and II are not
stable over long periods of time (>30% loss in 10 minutes) at 16 OC and 15
mN/m, although they show better hysteresis as compared to higher
temperatures and are reproducible. In general, the lower temperature runs
were characterized by more reasonable Mma values in the condensed phase
region and more stable monolayers, but not stable enough for multilayer
depositions.
In order to improve the stability of compounds I and II, two approaches
were explored: the effect of repeated expansion and compression, and the
effect of mixing other compounds which could act as "stabilizers".
After repeated expansion and compression of the films, an increase in
the stability over time is observed for compounds I and II. One drawback of this
approach though, is the apparent loss of material due to dissolution,







evaporation, or changes in ordering (e.g. crystallization, local bilayer formation,
etc.) over an extended period of time. It takes five cycles of expansion and
compression to produce a reasonably stable film even at low temperatures.
Another drawback is the decrease in Mma and collapse pressure values after
expanding and compressing.
Several experiments were also performed on mixtures of the compounds.
Mixing the monomers with either compound IV or V often increased the stability
of the resulting monolayer film. However, since polymerization of the acetylenic
compounds is desired, the amount of the "monolayer stabilizer" has to be
minimized. Isobaric creep measurements showed that blending of compound I
or II with 9 mol% of compound V was sufficient to stabilize the films. The
isotherms did not follow the additivity rule for the above compositions, which is
indicative of miscibility.85 Initially, compound IV was used for both I and II.
Even at 50 mol % of the stabilizing compound the resulting film was not stable
and apparently no real mixing occurred. When compound V was used as a
stabilizer with I and II, a minimum of only 9 mol % was necessary to produce a
stable film.
The combination of I or II and V produces a stable film. This behavior
might be due to a greater degree of intermolecular i-orbital overlap between
the aromatic rings of the mesogenic groups.146,138,147 In general, much
higher Mma values were obtained for the condensed phases of the compounds
in blends with compound IV and V. The stability of compounds I and II with
compound V at constant Mma was greater than that found for pure I and II.
A similar study using stearic acid with compounds I and II gave
analogous results to blends with IV. In general, by varying the mole ratio of the
mixtures and careful choice of "interactant" one can have the stability
characteristic of a more stable compound. No detailed mixing studies were







done, in particular to higher ratios (results in less reactive functional groups per
area) as the use of the interactant was minimized to that of stabilizing the
monolayer.



Brewster Angle Microscopy

Brewster angle microscopy of compounds I and II were done at both
ambient and low temperatures. As has been observed from the isotherm
measurements, no true monolayers were observed with the pure compounds.
At high areas, before the onset pressure rise to the condensed phase of the
isotherm, small domains or aggregates were observed "floating" within the field
of view of the microscope. This is a clear indication of the nonhomogeneous
nature of the monolayer at high areas. As spread, the liquid crystalline
materials tend to form local aggregates which form a very viscous film upon
compression. The aggregates were observed to be birefringent by the use of an
analyzer rotated at 100/min. The nonuniform intensity within the domains signify
the different local tilt angles of the molecules within the domain. Both

Compounds I and II exhibited this behavior but the formation of larger and more
uniform domains is stronger with compound II. At lower areas, the domains are
observed to fuse together forming a homogeneous film, again more evident with
compound II. Compression beyond the collapse pressure results in a
heterophase film for compound I. This phenomenon was similar in both
ambient and low temperatures. No pictures were taken of the mixed Langmuir
film due to lack of material.























(a)














(b)
100 um
Figure 3-5. Brewster Angle Microscope pictures of Compound I at 11C
(a) 40 A2, (b) 30 A2, (c) 25 A2, (d) 23A2, (e) 22 A2 /molecule
(refer to isotherm in Figure 3-4)


















(c) H
100 Lm











(d)












(e)
Figure 3-5 (continued)






















(a)
100 um














(b)
100 um

Figure 3-6. Brewster Angle Microscope pictures of Compound II at 11C
(a) 40 A2, (b) 30 A2, (c) 20 A2, (d) 15 A2, (e) 12 A2 /molecule areas.
(refer to isotherm in Figure 3-4)


















(c) -
10 m











(d)












(e)
Figure 3-6 (continued)







Analysis of Results

The amphiphilic nature of the monomer is in competition with the
interaction of the monomers with each other. This serves as a driving force
towards the formation of domains of the monomer as initially spread at the
interface (positive AH). It was necessary to break up this domain formation in

order to orient the reactive acetylene group for polymerization. The nature of
the amphiphile based on the results is that of a stronger van der Waals
interaction between the molecules themselves. Some mesogenic moieties can
be presumed to predominantly lay flat at the interface with the alkyl part free to
rotate as long as the acetylene group (which is slightly polar due to the electron
rich x-cloud) is not hydrated by water molecules.147,138 Some molecules
would also be not adsorbed at all. The observed mean molecular area supports
this observation since a lower apparent area indicates less molecules are
adsorbed per Mma than calculated. This indicates that some of the molecules
are laying flat on each other or are actually forming three dimensional domains.
In this case, the effect of lowering the temperature seems to result in forming
more order within the domains with preference towards orthogonal orientation.
This is consistent with the higher mean molecular area observed at lower

temperatures. Nevertheless the film still remained unstable for deposition. The
Brewster Angle Microscopy (BAM) pictures verified a nonfluid ordered
birefringent layer for the pure compound as spread in both ambient and low
temperatures. Larger, but more homogeneous domains are observed to form at
lower temperatures. It is possible that the difference in size and increased
ordering at lower temperatures is brought about by the spreading solution
(chloroform), remaining longer at the interface. This would then increase local







ordering and allow the formation of much larger domains due to slow
evaporation of solvent. Thus, greater ordering is induced at lower temperatures.
The problem of stability was partially solved by blending with a minimal
amount of amphiphilic interactant. In this case, the surfactant properties of the
interactant amphiphile are enough to induce stability in the monolayer. This is
also reflected in the non-additivity of the isotherms.85 The resulting film had
better Langmuir monolayer film properties in which LB films were subsequently
built. No detailed mixture studies were done, since the only interest was to
allow the deposition of the monolayer to substrates. And since only a minimal
amount was used, it was regarded as not enough to effectively hinder
polymerization.


LB Film Deposition and Characterization

Deposition

Multilayers of stable blends of both compounds were successfully
deposited on quartz substrates at low temperatures. Transfer of the monolayers
was done at an applied surface pressure of 15 mN/m for the blends I + V and II +
V since peeling off is observed at lower surface pressures and the film is not
adequately stable at higher surface pressures. During the depositions, the
transfer ratio was recorded. The transfer ratio (TR) gives an indication of the
quality of the dipping process and ideally should be unity. It is found that Z-type
films upstrokee) were formed for the blends of all three compounds using both
hydrophilic and hydrophobic quartz substrates with the exception of II + V blend.
A maximum of 28 layers was deposited for the II + V blend and 32 layers for the I
+ V blend. The blend II + V tends toward a Y-type deposition after the first 3 Z-
type layers are deposited on a hydrophilic substrate. The blend I + V begins as





90


a Y-type and trends toward a Z-type deposition after the first two layers on a

hydrophobic substrate. Two deposition attempts were made for both and the

observations were confirmed.



UV Spectra of LB Films


No absorption bands were observed below 200 nm due to the absorption

of air at this range so that the x-i* transition of the acetylenic moiety is not

observed by UV (normally at 173 nm). Absorptions between 240 to 260 nm for

the forbidden transitions of the benzene rings in the mesogenic group are

observed. The band observed at the 320 nm region for compound II is due to

the azoxy chromophore conjugated to two benzene rings.


t5.

S0.15

0.1
e'


----- 14 layers

---- 10 layers

6 layers


If


220


280


340


400


Wavelength [nm]


Figure 3-7 .UV of I + V LB Film showing additive effect of putting more layers
on the substrate.


No significant difference was observed between the spectra of the pure

compounds and the blends since only a minimal amount of the "stabilizer" was