Polymerization of aniline in an organized monolayer

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Title:
Polymerization of aniline in an organized monolayer kinetics, thermodynamics, and mechanism
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xv, 175 leaves : ill. ; 29 cm.
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Bodalia, Rajeshkumar R., 1961-
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Thesis (Ph. D.)--University of Florida, 1993.
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Includes bibliographical references (leaves 163-174).
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Typescript.
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Vita.

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POLYMERIZATION OF ANILINE IN AN ORGANIZED
MONOLAYER: KINETICS, THERMODYNAMICS, AND MECHANISM
















BY

RAJESHKUMAR R. BODALIA


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


1993

















This dissertation is dedicated to Prof. G. A. Takacs at the Rochester
Institute of Technology, Rochester, New York. He believed in me and
provided me an opportunity to begin my professional career in this
country.











ACKNOWLEDGMENTS


Where to begin is always difficult. So many special people have
made this all possible. Let me begin with my advisor Dr. R. Duran. His
support, encouragement, guidance, and belief in me is infinitely
appreciated. Without him I would not have had the great opportunity to do
research with Rhone-Poulenc in Europe, nor research at Dow Coming,
Midland, Michigan, and lastly research at UCLA.
Also my thanks are due for the professional advisement of professors
G. Butler, K. Wagener, and J. Reynolds.
Special thanks are extended for the secretarial support of Mrs.
Lorraine Williams and Jane Poppell. They were always available for "last
minute crisis" with the friendliest disposition.
To all my present and past peers on the polymer floor A.
Thibodeaux, G. Advincula, J. Roberts, T. Herod, W. Sigmund, P. Quint, J.
Adams, T. Wills, P. Bernal, F. Zuluaga, J. Konzelman, D. Smith, D. Tao,
K. Novak, J. Linert, L. Engle, C. Marmo, J. Portmess, B. Sankaran, D.
Patwardhan, P. Balanda, and S. Kim thanks are due for providing a
home-like atmosphere. I am also very thankful to Joe Roberts and G.
Advincula for their relentless help in resolving computer problems and
solving mathematical equations.
Finally, I would like to thank my wife Sheela, my children Krystina
and Priyanka, and other members of my family for their endless love,
continuous encouragement, support, and patience.











TABLE OF CONTENTS


Page

ACKNOW LEDGMENTS....................................................................iii

LIST OF FIGURES...........................................................................vii

LIST OF TABLES.............................................................................. xi

LIST OF ABBREVIATIONS............................................................. xiii

ABSTRACT ................................................................................... xiv

CHAPTERS

1 INTRODUCTION.............................................................1

1.1 History of Monolayers............................................1...

1.2 Organized Films...................................................5

1.3 Polymerization Reaction in Organized
Molecular Systems................................................ 8

1.4 History of Electrically Conducting Polyaniline...........15

1.5 Objectives of This Thesis...................................... 28

2 EXPERIMENTAL..........................................................29

2.1 Synthesis of 2-Alkyl Anilines................................ 29

2.2 Isotherm...............................................................32

2.3 Isobaric Stability.................................................. 33

2.4 Polymerization.....................................................34

2.5 Determination of Molecular Weight....................... 36







3 RESULTS AND DISCUSSION.......................................39

3.1 Isotherm ............................................................... 39
3.1.1 Isotherm of 2-Pentadecylaniline
on Different Sub-Phases...............................39

3.1.2 Effects of Temperature................................45

3.1.3 Effect of Alkyl Side Chain
Length on Isotherms....................................47

3.2 Isobaric Stability.................................................. 50

3.2.1 Effect of the Alkyl Side Chain
Length on the Monolayer Stability................50

3.2.2 Effects of Temperature and Surface
Pressure on the Monolayer Stability..............52

3.3 Polym erization...................................................... 54

3.3.1 Calculation of the Rate Constant...................59

3.3.2 Calculation of Activation Energies................63

3.3.3 Activation Enthalpies and Entropies..............82

3.3.4 Effect of Surface Pressure on
the Reaction Rate........................................ 86

3.3.5 The Temperature Dependence
of Activation Area......................................95

3.4 M echanism .......................................................... 99

3.5 Molecular Weight as a Function
of Reaction Tim e................................................ 104

3.6 Effect of Ionic Strength on the
Polymerization Rate............................................ 106

3.7 Polymer Properties as a Function
Reaction Conditions............................................ 120

4 CONCLUSION S............................................................126







APPENDICES
A MONOLAYER POLYMERIZATIONS OF 2-ALKYL
A N ILIN ES................................................................... 130

B KINETICS OF 2-PENTADECYLANILINE
POLYMERIZATIONS IN MONOLAYERS:
RELATIONSHIPS BETWEEN EXPERIMENTAL DATA
AND A NEW THEORETICAL MODEL..........................133

REFEREN CES............................................................................... 163

BIOGRAPHICAL SKETCH ........................................................... 175











LIST OF FIGURES


Page


Figure 1-1.


Figure

Figure

Figure

Figure


Figure

Figure


1-2.

1-3.

1-4.

1-5.


1-6.

1-7.


Figure 1-8.


Figure

Figure

Figure

Figure


2-1.

2-2.

2-3.

3-1.


Figure 3-2.


Figure 3-3.


Monolayer films (a) before compression and (b) after
com pression. ...............................................................7...

Molecular arrangements in Langmuir-Blodgett films. .......8

Fischer projections of different polymer structures. ........10

The structure of the emeraldine base form of polyaniline. .17

Structures of the fully reduced and oxidized forms
of polyaniline. ........................................................... 18

Repeat units of polyaniline and polypyrrole. .................19

Schematic presentation of conductivity versus
electrochemical doping in polyaniline at a given pH. ........19

Semiquinone formation upon protonic acid doping in
polyaniline. ............................................................... 21

The Synthetic route for 2-alkylanilines. ........................31

200 MHz IH NMR of 2-pentadecylaniline in CDC13. .......31

GPC calibration curve of polystyrene standard .............37

Isotherms of 2-pentadecylaniline measured on
a pure water and on sulfuric acid sub-phases
having different pH at 250C. .......................................40

Isotherms of 4-hexadecylaniline measured on water
and on 0.1M sulfuric acid sub-phases at 25C. ...............42

Surface pressure versus area isotherms of
4-hexadecylaniline on 0.001M NaOH, 0.1M HC1,
and on 1.OM HCI sub-phases at 25C. ...........................43







Figure 3-4.


Figure 3-5.



Figure 3-6.


Figure 3-7.





Figure 3-8.


Figure 3-9.


Figure 3-10.



Figure 3-11.




Figure 3-12.



Figure 3-13.


Figure 3-14.


Isotherms of 2-pentadecylaniline on 0.1M
sulfuric acid sub-phase at different temperatures. .......... 46

Isotherms of 2-undecylaniline (C-11), 2-tridecylaniline
(C-13), 2-pentadecylaniline (C-15), and 2-heptadecyl-
aniline (C-17) on 0.1M sulfuric acid sub-phase
at 270C tem perature. .................................................. 48
The arrangement of 2-pentadecylaniline and 4-hexa-
decylaniline at the interface. ........................................ 50

The change in the mean molecular area versus time
plots of monolayers of 2-undecyl (C-11), 2-tridecyl
(C-13), 2-pentadecyl (C-15), and 2-heptedecyl (C-17)
anilines at 270C temperature and at 15 mN/m
applied surface pressure. ............................................ 51

The average barrier speed and the mean molecular area
versus time for the monolayer polymerization of 2-penta-
decylaniline at 250C. and 30 mN/m surface pressure. .......55

The conformation of a monomer and a polymer
molecules at the air / aqueous acid interface. .................56

The fraction of reacted monomer versus time for the
monolayer polymerization of 2-pentadecylaniline at
300C temperature and 5 mN/m applied surface pressure. .57

The average barrier speed versus time plots for the
mono-layer polymerization of 2-pentadecylaniline with
different concentrations at 250C temperature and at
30 mN/m surface pressure. .........................................62

Average Barrier Speed versus time during the Langmuir
film polymerization of 2-pentadecylaniline at 20 mN/m
applied surface pressure and different temperatures. .......64

In k vs 1/T (1/K) plots for monolayer polymerizations
at 20 mN/m applied surface pressure. ..........................66

Log polymerization rate (PR) versus temperature (C)
for monolayer polymerization reactions at 20 mN/m
applied surface pressure. ............................................ 70







Figure 3-15.


Figure 3-16.


Figure 3-17.


Figure 3-18.


Figure 3-19.



Figure 3-20.


Figure 3-21.


Figure 3-22.


Figure

Figure

Figure

Figure


3-23.

3-24.

3-25.

3-26.


Figure 3-27.


Average barrier speed vs time during the Langmuir
film polymerization of 2-pentadecylaniline at 10 mN/m
applied surface pressure and different temperatures. .......72

In k vs Iff (1/K) plots for monolayer polymerizations
at 10 mN/m applied surface pressure. ..........................75

In k vs 1/T (1/K) plots for monolayer polymerizations
at 15 mN/m applied surface pressure. ..........................79

In k versus 1/T (1/K) plot for monolayer poly-
merizations of 2-pentadecylaniline at 30 mN/m applied
surface pressure. ........................................................ 81

Average barrier speed versus time during the Langmuir
film polymerization of 2-pentadecylaniline at 25C and
various applied surface pressures. ................................88

In k vs surface pressure (mN/m) for monolayer
polymerization reactions at 250C temperature. ...............90

A plot of an enthalpy of activation versus an entropy of
activation for monolayer polymerizations of 2-penta-
decylaniline at different applied surface pressures. ..........93

Areas of monomer, activated complex, and polymer at
10 mN/m surface pressure and different temperatures. ....99

A delocalization of a radical over the ring. ..................100

Structures of three different types dimeric species. ........101

Proposed mechanism for aniline polymerization. ..........102

Degree of polymerization vs % conversion plots
usually seen for polymerizations by chain growth
and step growth mechanisms. .....................................105

Weight average molecular weights vs % conversion
of the monomer for monolayer polymerizations of
2-pentadecylaniline at 250C and 20 mN/m applied
surface pressure. ....................................................... 106







Figure 3-28.



Figure 3-29.


Figure 3-30.



Figure 3-31.




Figure 3-32.


Figure 3-33.


Figure 3-34.


Figure 3-35.



Figure A-1.




Figure A-2.


The average barrier speed vs. time for the monolayer
polymerization of 2-pentadecylaniline on sub-phases
having different ionic strength. ...................................109

Log k versus square root of ionic strength for
monolayer polymerization reactions at 250C
temperature and at 20 mN/m applied surface pressure. ..111
Isotherms of 2-pentadecylaniline at 250C on sub-phases
containing 0.1M sulfuric acid and varying concen-
trations of ammonium sulfate. ....................................112

Isotherms of 2-pentadecylaniline at 25 C temperature.
Isotherms were carried out on sub-phases containing
0.1M sulfuric acid and varying concentrations of
amm onium sulfate. .................................................... 114


Average barrier speed vs time plots for monolayer
polymerization reactions of 2-pentadecylaniline on
sub-phases having different ionic strengths at 250C.


.......116


Log k versus square root of ionic strength for
monolayer polymerization reactions at 250C. ...............117

Surface pressure area isotherms of 2-pentadecylaniline
and poly(2-pentadecylaniline) at 350C temperature. .......121

Isotherms of poly(2-pentadecylaniline) prepared at
30 mN/m applied surface pressure and at different
tem peratures. ............................................................ 123

Average Barrier Speed vs, time plots for monolayer
polymerizations of 2-tridecylaniline (C-13), 2-penta-
decylaniline (C-15), and 2-heptadecylaniline (C-17) at
270C and at 30 mN/m applied surface pressure ............131

Average Barrier Speed vs, time plots for monolayer
polymerizations of 2-tridecylaniline (C-13), 2-penta-
decylaniline (C-15), and 2-heptadecylaniline (C-17).
Polymerization reactions were carried out at 27C
and at 20 mN/m applied surface pressure. ....................132











LIST OF TABLES


Table 2-1.


Table 3-1.



Table 3-2.



Table 3-3.


Table 3-4.





Table 3-5.




Table 3-6.



Table 3-7.


Table 3-8.


Molecular weights and retention times of polystyrene
standards. .................................................................... 37

Values of mean molecular areas and surface pressures
at the onsets and the collapse points on different
sub-phases at 25C. ....................................................... 41

Values of the area change per minute of derivatives
of aniline under isobaric conditions at 270C temperature
and at 15 mN/m applied surface pressure. .......................51

Stabilities of monolayers of 2-pentadecylaniline at different
temperatures and at different applied surface pressures. .....53

Values of areas (A2/molecule), barrier speeds at
the maximum positions (mm/min), number of molecules,
and rate constants (min-1) for polymerization reactions
at different temperatures and at 20 mN/m applied surface
pressure. ..................................................................... 64

Values of areas (A2/molecule), barrier speeds at
the maximum positions (mm/min), and rate constants
(min-1) for the reactions at different temperatures and at
10 mN/m applied surface pressure. ................................72


Values of polymerization rates (PR) and rate constants
for monolayer polymerization reactions at 10 mN/m
applied surface pressure and at different temperatures.


......74


A Comparison of activation energies obtained from a
new theoretical model and experimental data. .................78

Values of activation energies and pre-exponential factors
for monolayer polymerizations of 2-pentadecylaniline at
different applied surface pressures. ................................79







Table 3-9.




Table 3-10.



Table 3-11.




Table 3-12.




Table 3-13.



Table 3-14.


Table 3-15.



Table 3-16.




Table 3-17.


Values of polymerization rates (PR) and rate constants
for monolayer polymerizations of 2-pentadecylaniline at
30 mN/m applied surface pressure and different
tem peratures. ............................................................... 80

Values of rate constants and polymerization rates (PR)
for monolayer polymerizations of 2-pentadecylaniline at
250C and different applied surface pressures. ..................89

Values of activation energy, activation entropy, activation
enthalpy, and the free energy of activation for polymeri-
zations of 2-pentadecylaniline at different surface
pressures. .................................................................... 92

Values of activation area (A*AO) and total changes in the
area (AA) in going from the monomer to the polymer for
monolayer polymerizations of 2-pentadecylaniline at
different temperatures. ................................................. 95

Values of rate constants for the monolayer polymeri-
zations of 2-pentadecylaniline on sub-phases having
different ionic strength. ............................................... 110

Values of surface pressures which correspond to the
area 45.5 A2/molecule on different sub-phases. ..............115

Values of rate constants for monolayer polymerization
reactions of 2-pentadecylaniline on sub-phases having
different ionic strength. ............................................... 116

Values of compressional modulus (dynes/cm) for the
monomer and polymers measured at 350C. Numbers in
the brackets indicate the surface pressure at which the
polymers were prepared. ............................................. 122

Values of compressional modulus for the polymers
prepared at different temperatures. The instantaneous
slope at 15 mN/m surface pressure was used to calculate
these values. ................................................................ 124











LIST OF ABBREVIATIONS


K = Boltzmann's constant
h = Planck's constant
= the fraction of reacted monomer

K = Kelvin temperature in degrees
t = time
AO = the initial area
Aoo = the area of product
A = surface area at a given time during the reaction
k = rate constant
BS = average barrier speed
N = the number of molecules
W = the width of the trough
L = the length of the trough
kc* = equilibrium constant
R = gas constant
T = temperature
Ea = activation energy
A*S = entropy of activation
A*Ho = enthalpy of activation
A*Go = Gibbs free energy of activation
A*Uo = internal energy of activation
A*Vo = activation volume
A*Ao = activation area
Aa = the area of the transition state (activated complex)
Ar = the area of the reactant
Q = the partition function
PR = polymerization rate
ic = surface pressure
I = ionic strength
y = surface tension
K = compressional modulus
ki = initiation rate constant
kp = propagation rate constant
Mn = number average molecular weight
Mw = weight average molecular weight











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


POLYMERIZATION OF ANILINE IN AN ORGANIZED
MONOLAYER: KINETICS, THERMODYNAMICS, AND MECHANISM


By
Rajeshkumar R. Bodalia

December 1993


Chairman: Professor Randolph S. Duran
Major Department: Chemistry

Monolayer polymerization reactions of 2-alkyl anilines were carried
out at the air-aqueous solution interface using the Langmuir film balance
technique. The Langmuir trough was used as a two-dimensional analog of
Dilatometry where the reaction was investigated by monitoring changes in
the mean molecular area and average barrier speed at a constant applied
surface pressure and temperature. Thus this technique allows the study of
reaction kinetics in a real-time mode.
Isotherms of 2-alkyl anilines on 0.1M sulfuric acid showed that the
area occupied by a molecule at any given surface pressure increased with
increasing temperature of the sub-phase and the number of carbon atoms in
the side chain. Isotherms of 2-pentadecylaniline and a homologue the para
isomer, 4-hexadecylaniline on water and an acid subphase showed a
completely different behavior. These differences were due to their







conformation at the interface. The isotherm of 2-undecylaniline was
substantially different from other derivatives of aniline investigated. This
difference accounted for its poor monolayer stability at the interface.
The effects of varying applied surface pressure and temperature
were elucidated by polymerizing a monolayer film of 2-pentadecylaniline
at the interface. Values of the rate constant at different temperatures were
used to calculate the activation energy, standard enthalpy of activation,
standard entropy of activation, and Gibbs free energy of activation. The
activation energy was found to increase with increasing applied surface
pressure. This result was interpreted in terms of intermolecular distances.
The activation entropy had negative sign and decreased with increasing
surface pressure. These results were explained in terms of an
electrostriction effect.
It was also observed that the polymerization rate at the interface
increased with increasing applied surface pressure at a given temperature.
Such experiments allowed facile measurement of the activation area (area
change during the formation of activated complex from reactants) at a
given temperature. Since the area of monomer was known from the
isotherm experiment, the area of the activated complex was determined at a
given temperature. The area of the activated complex decreased with
increasing temperature while that of monomer and polymer increased.
Signs of the activation area and activation entropy were negative,
indicating that the activated complex was bimolecular in nature and there
was an intensification of charges during its formation. It was also found
that the propagation the polymer chain occurred via non-classical chain
growth type mechanism. A reaction mechanism for the oxidative coupling
polymerization of aniline consistent with the above results was discussed.











CHAPTER 1


INTRODUCTION


1.1 History of Monolayers

At length being at Clapham where there is, on the common, a large
pond which I observed to be one day very rough with the wind, I
fetched out a cruet of oil, and dropped a little of it on the water. I
saw it spread itself with surprising swiftness upon the surface...the
oil, though not more than a teaspoonful, produced an instant calm
over the space several yards square, which spread amazingly and
extended itself gradually until it reached the lee side, making all that
quarter of pond, perhaps half an acre, as smooth as a looking glass.1

These are words of Benjamin Franklin describing his experimental
results of spreading of oil films on a pond at Clapham Common to the
Royal Society in 1774. He did not analyze this result in a simple
quantitative basis. It was believed that he must have been too preoccupied
at that time with political affairs (The Treaty of Paris, September 1783).
If he had done so, he might have calculated that upon spreading a
teaspoonful oil over an area of half an acre, resulted in a layer of oil
approximately 1 nm (10 A) thick. In other words, it was one molecule
thick.
During the second half of century, public interest was being shown
in Britain regarding the question of legislating that all ships should carry
supplies of oil for pouring on stormy seas. Shortly afterward, much public
interest in wave damping by oil was aroused by the large-scale experiment
at Peterhead and Aberdeen harbors carried out by John Shields.2 From his







experiments he was convinced that it would be a practical power for saving
life and property at sea. Although Shield's trials were not reported in the
scientific journals of the day, they were widely reported in the daily press.
The first scientist to take over Shield's work was John Aitken. He
devised and constructed an apparatus to test theories of the calming action
of oil.3 However, his results were contradicting the theory of wave-
damping by oil that had long been held.
Almost a century after Franklin's report on wave-damping by oil,
Rayleigh was the first who suggested that the maximum extension of an oil
film on water represents a layer one molecule thick.4 In that interval, the
nature of other surface phenomena began to be understood. Capillary and
surface tension effects were clarified by the work of Young, Laplace,
Plateau, and others.5 In 1878, Gibbs published his thermodynamic analysis
of adsorption and surface tension effects.6 In his paper Rayleigh reported
that the surface tension of water can be lowered by "contamination with a
surface film of insoluble greases or oil, and then permanent changes in
tension accompany changes in area of the surface. He estimated from
measurements with films of olive oil on a water surface, just able to
prevent the movement of camphor, that such films are between 10 and 20
A thick. Rayleigh believed that if the thickness of such an extended layer
could be determined it would give the first direct measurement of the size
of an organic molecule, but he had not found a method of making an exact
measurement.
In 1891, Agnes Pockels, a young German lady, made direct
measurements of molecular size using a simple apparatus. She is credited
with building the first trough (on her kitchen table) that became the model
for what is now known as Langmuir Trough.7,8 In these studies she







described measurements on films of colophonyy" and "mastic" among other
substances. These observations actually predate the concepts of either
polymer or monolayer. She described her use of a rectangular tin trough,
70 cm x 5 cm x 2 cm, filled with water to the brim, with a 1.5 cm wide
strip of tin laid across it, just in contact with the water. Thus by moving
the strip she could very the area of and also completely clean an enclosed
water surface. By this means she examined the variation in the surface
tension of an oil-contaminated water surface, using a balance which
measured the force required to lift a small disk just from a surface. She
was also the first to publish pressure-area diagram so familiar now as
isothermss" in monolayer research.8
In 1899, Rayleigh returned to a quantitative study of oil films on
water.9 In this paper he reported his use of the Pockels method and
described the size of an olive oil molecule in the monolayer on water as
"about 1 nm (10 A)". It is interesting that Franklin's teaspoonful (~ 2.5
ml) of olive oil spread over half an acre of water gives a similar thickness
of oil film.
In the subsequent decade and a half, interest in the properties of the
monomolecular films increased, although some workers did not yet agree
that they were really only one molecule thick.10,11
Finally, in 1917, Irving Langmuir developed the theoretical and
experimental concepts which underlie our modem understanding of the
behavior of molecules in insoluble monolayers.12-14 It was Langmuir
who surmised correctly that the forces were short range and acted only
between molecules in contact. For his measurements of the spreading
pressures of thin films, Langmuir developed a number of new techniques
including the surface film balance with which his name is now is associated.







In this device, a movable float separates a clean water surface from the
area covered with a film; the deflection of the float then provides a direct
measure of the forces involved. Langmuir was the first to explain the
apparent discontinuities in plots of surface pressure versus area isothermss)
as phase transitions, and he was the first to determine the orientation of
molecules at the air-water interface, with the polar functional group
immersed in the water and the long nonpolar chain directed almost
vertically from the surface. He also confirmed that his film had the
thickness of a single molecular layer. His experiments provided strong
support for the existence of short range forces and explained clearly the
basis on which certain molecules did or did not form good monolayer
films. This explanation was expressed independently by Langmuirl5 and
Harkins.16 For his pioneering research in monolayers, Langmuir was
rightly awarded the 1932 Nobel Prize in Chemistry.
By 1919, K. Blodgett, under Langmuir's guidance, had already been
able to transfer fatty acid monolayers from water surface to solid supports
such as glass slides. In 1920, Langmuir reported the transfer of these fatty
acid molecules from water surface onto solid supports such as glass
slides.17 However, the first detailed description of sequential monolayer
transfer was given several years later by Blodgett.18 These built-up
monolayer assemblies are now referred to as Langmuir-Blodgett films; the
term "Langmuir film" is normally reserved for the floating monolayer or
insoluble monolayer.
Within a few years after Langmuir, other workers began to examine
the properties of monolayers. Adam, particularly, examined a wide variety
of substances in the form of monolayers between 1921 and 1930.19 Since
1945, monolayer research has continued throughout the world, but at a






lower level of activity. It is only recently, in the mid-sixties, that Hans
Kuhn began his stimulating experiments on monolayer organization.20
Kuhn and coworkers provided the foundation for today's area of molecular
electronics.21 Since then research and publications in areas of monolayers
and Langmuir-Blodgett films have increased exponentially. Major names
in the area such as Gaines22 (whose book published in 1966 is still
considered mandatory reading for those starting research in the area
monomolecular films), M6bius23 (surface potential measurements),
Mohwald24 (physical properties of monolayer behavior and phase
transitions), Ringsdorf25,26 (liquid crystals and biological membrane
model), Ulman27 (updated overview of Langmuir Blodgett research and
applications), and Wegner28,29 (rigid rod and other polymeric
monolayers) are just a few of the scientists who have contributed large
fractions of information to the field of monolayer research over the past
thirty years.


1.2 Organized Films
Ultrathin organic films can either be prepared by the self-assembly
technique,30,31 or by the Langmuir-Blodgett deposition method.18 In the
self-assembly technique, the adsorption of surfactant molecules from liquid
onto a solid substrate is governed mainly by electrostatic and hydrophobic
interactions (physical adsorption), though in some cases covalent or
chemical bonds are also formed (chemisorption).27,32-34 An important
difference between self-assembly and the LB technique is that the self-
assembly requires a soluble surfactant whereas the LB deposition requires
that the surfactant be insoluble. In the LB technique, amphiphilic
molecules are spread at the air-water interface and a monomolecular layer







is formed. Upon compression of the monolayer, the molecules are
oriented at the interface as shown in Figure 1-1 .
In the oriented state, the monolayer can be transferred onto the
substrate by dipping the substrate perpendicularly through the interface.
Repeating this process allows several layers to be built up on the substrate.
In this way, highly ordered ultra thin films can be obtained, where the
thickness is controlled exactly by the number of dipping cycles. Depending
on the amphipile used, deposition occurs either during both the downstroke
and upstroke, or only during the downstroke or the upstroke. As a
consequence, different molecular orientations are obtained, which are
designated as X-, Y-, or Z-structures as shown below. In Figure 1-2, the o
refers to the polar head groups and the wavy line to the non-polar tail.
Compounds suited for film formation are primarily the classical
amphipiles consisting of hydrophilic head groups and long-chain
hydrophobic substituents. Many of the recent activities on LB films have
been concerned with photochemical or thermal reactivity, or "active"
physical properties, for example, electrical conductivity, pyroelectric
activity, or nonlinear optical properties. Films exhibiting these properties
can be utilized in electronic applications as discussed by Tieke.35
Langmuir-Blodgett films of small molecules are unstable in many
respects. They dissolve easily in many solvents. Furthermore, they have
poor thermal and mechanical stability. These limitations have prevented
LB films of small molecules from scientific investigations or industrial
applications. In order to stabilize LB films, one solution is to polymerize
small molecules after the film formation, preserving the structure of the
initial LB film.36-41 Generally, the polymerization is induced by






(a) LANGMUIR FILM BEFORE COMPRESSION

U--- ELECTRONIC BALANCE


WILHELMY PLATE


BARRIER
I


(b) LANGMUIR FILM AFTER COMPRESSION

rn- ELECTRONIC BALANCE


bbbhli


WILHELMY PLATE
BARRIER


SUBPHASE


Figure 1-1. Monolayer films (a) before compression and (b) after
compression.

irradiation.42 Other means of polymerization such as polycondensations of
films, electrochemical polymerizations of films, etc. have also been







investigated and indeed, the polymerization improves stability towards
mechanical, thermal, and environmental attack.37,43-46

X-film Y-film Z-film

000000000







Figure 1-2. Molecular arrangements in Langmuir-Blodgett films.



1.3 Polymerization Reactions in Organized Molecular Systems
The vast majority of polymerization reactions are generally carried
out in conventional solution or melt-phase polymerizations. Polymers
obtained from these methods normally exhibit completely isotropic
mechanical, electrical and optical properties. This is due to the tendency of
an individual polymer chain to form random coil conformations. However
anisotropic properties in these materials can be achieved by post-synthesis
processing, which induces orientation of chains or domains within the
material.
It has always been a goal of synthetic chemists to have complete
control over a reaction product. This control is of particular importance
to the polymer scientist. This is due to the fact that the properties of
polymers largely depend upon the stereo chemical configuration of the
individual monomer units. For example, a polymerization of a
monosubstituted ethylene (CH2=CHR) leads to polymers in which every







other carbon atom in the polymer chain is a pseudochiral center (chiral-
like but optically inactive). This can be shown as

H

W C*/W

R

where the chirality resides in the carbon atom C* being attached to four
different substituents: H, R, and the two polymer chain segments with
different lengths. Spatial arrangements of R groups (configuration of
pseudochiral center) in the polymer chain determine the overall order of
tacticity of the polymer chain. If the R groups are randomly distributed on
both sides of the polymer backbone, the polymer is not ordered and is
termed atactic. If all the R groups are located on one side of the plane of
the carbon-carbon polymer chain (or each repeating unit in the polymer
chain has the same configuration), then the polymer is called isotactic. A
syndiotactic polymer structure occurs when the configuration of the
pseudochiral centers alternate from one repeating unit to the next with the
R groups located alternately on the opposite sides of the plane of the
polymer chain. These different polymer structures are shown in Figure
1-3.
It must be remembered that the terms configuration and
conformation are not synonymous. Conformation refers to the different
arrangements of atoms and substitutents in a molecule. Conformational
isomers may be inter converted one into another by bond rotations.
Examples of different polymer conformations are the fully extended planar
zigzag, randomly coiled, helical, and folded chain arrangements.
Configurational isomerism involves different arrangements of the atoms







and substitutents in a molecule which can be inter converted only by the
breakage and reformation of primary chemical bonds.


H-
H-
H-
H-
H-
H-
H-
H-
H-
H-
H-


-H
-R
-H
-R
-H
-R
-H
-R
-H
-R
-H


H-
H-
H-
R-
H-
H-
H-
R-
H-
H-
H-


-H
-R
-H
-H
-H
-R
-H
-H
-H
-R
-H


H-
H-
H-
H-
H-
R-
H-
R-
H-
H-
H-


-H
-R
-H
-R
-H
-H
-H
-H
-H
-R
-H


Figure 1-3.


Isotactic Syndiotactic Atactic

Fischer projections of different polymer structures. Vertical
lines in the projections correspond to bonds going behind the
plane of this page; horizontal lines represent bonds coming in
front of the plane.


Depending upon the tacticity in a polymer, it can have different
physical properties. One common example is crystallinity. Crystalline
polymers have high mechanical strength, and great solvent and chemical
resistance. Crystallinity in a polymer depends on the structural order of
the repeating unit, flexibility of the polymer chain, and on secondary

forces. Crystallinity can be varied with the tacticity of the polymer
repeating unit. Atactic polymers have disorder structures and are
generally amorphous, While isotactic and syndiotactic polymers have
regularity in the backbone and are highly crystalline.







There are many ways to produce oriented macromolecules such as
the use of Ziegler-Natta catalysts, solid-state polymerizations, polymeri-
zations in the liquid crystalline state, polymerizations in micellar or
vesicular systems, and polymerizations in Langmuir-Blodgett and
Langmuir films, etc.47 In general, polymerization reactions in organized
molecular systems are easy to perform and are capable of producing
polymers of controlled polymer chain length, structure, and shape.
An example of solid state polymerization is the formation of a
cyclobutane containing polymer upon radiation of a monomer containing a
reactive double bond.48 The main objective of the solid-state
polymerization is the solid-state transformation of a crystalline monomer
to a polymer. However, this is not always the case since the formation of
by-products complicates the relationship between the crystalline order in
the monomer and the resulting polymer.
The first example of the polymerizations in the liquid crystalline
state was reported in 1967.49 The main feature of this type of reactions is
to take advantage of low viscosity, higher anisotropy, and an ability of
monomer molecules to reorient in the presence of an external field.
Subsequent treatment (UV radiation or thermal) results in the formation of
polymer that retains the liquid crystalline order of the monomer.
Recently, Labes and coworkers reported an extremely interesting anti-
Arrhenius behavior for chemical reactions in liquid crystalline states.50
Polymerizations in micellar system is another important technique where
one has a control over the destination of products.51 Micelles are formed
in a medium such as water by the aggregation of ionic surfactant molecules
above a critical concentration. The organization present in such system can







be utilized for the polymerization of micelle forming molecules containing
reactive groups.
As discussed above, the polymerization of reactive Langmuir-
Blodgett films leads to polymer structures with controlled thickness and
orientation. However, often the polymerization reactions result in
structural reorganizations which can induced defects in the multilayers, e.g.
by shrinkage. These problems can be minimized by appropriate molecular
design.52 However, the control of the degree of polymerization, and the
removal of unreacted monomers and side-products is not possible using this
approach.
These problems can be solved by spreading pre-formed polymers at
air-water interfaces or by spreading small amphiphilic molecules and
polymerizing them in monolayers. In any case, the resulting polymer
monolayers can be transferred onto solid substrates giving a stable layered
structure. In the former case, polymers can be prepared monomer-free in
macroscopic quantities with defined molecular weight and can be
characterized by standard techniques. Pre-formed polymers can be spread
from solutions in the same way as the monomeric materials. If there are
sufficient hydrophilic groups to allow spreading but insufficient to cause
solubility, then stable monolayers can be obtained. Structural requirements
for polymers to form monolayers are less restrictive compared to
monomers. However, it is important to have hydrophilic groups regularly
distributed at short intervals along a polymer backbone. Polymer
monolayers show the same general types of behavior as monomers; for
example, they form expanded and condensed films but do not show sharp
phase transitions. Several polymers such as polyacrylates, poly(methyl







acrylates), poly(vinyl stearate), poly(vinyl fluoride), silicone copolymers
etc. have been studied for monolayer properties over the years.
Spreading preformed polymers has problems of its own. In a bulk
solution, polymer molecules generally exist as random three-dimensional
coils. Most authors have assumed that in the spread monolayers the
polymer molecules are extended at the interface, with every monomer
segment in the surface layer. The main evidence for this assumption is the
fair agreement between the area requirement in the spread and compressed
film and measurement on the projected area of molecular models of the
monomer segment. How likely is it that the spreading process can produce
such a profound change in average polymer molecule conformation? This
is the utmost question under discussion among monolayer researchers. For
more discussion readers are encouraged to read a classic paper by Gaines
and references therein.53
Lando et al. have carried out dynamic mechanical analysis and other
tests on monolayers of in situ polymerized films and on monolayers of
preformed polymers (prepared in 3-D bulk).54,55 They found that the
monolayer polymerized films have a higher compressional modulus and a
lower area/molecule compared to the polymer prepared in bulk. These
factors and other results led them to the conclusion that monolayer
polymerized films have a better packing order and there is a structural
difference between a three-dimensional bulk polymer and a two-
dimensional monolayer film. Other studies have shown that
poly(octadecylmethacrylate) prepared at an interface showed higher
tacticity than that formed in bulk solution.56 Also restricting monomer
molecules to a planar surface during polymerization prohibits the resulting
polymer chains from overlapping. Thus, polymer films prepared in







monolayers offer distinct advantages over monolayers of preformed
polymers.
Two-dimensional reactions have not been studied to the same extent
as ordinary reactions in 3-D systems. However, their importance has been
recognized in many branches of chemistry, including catalysis and
biological and biomedical chemistry. Many years ago, Carothers suggested
that the biological formation of proteins from amino acids might require
the interfacial orientation of the growing chain to prevent the formation of
cyclic compounds, which are the usual product of such polymerizations in
homogeneous solution.57 However, Gee was the first who showed that
monolayers of long chain derivatives of maleic anhydride can polymerize
spontaneously when spread on aqueous acid solutions.58,59 By measuring
expansion of the monolayer as a function of conversion at constant surface
pressure, he was able to determine the overall rate constant and global
activation energy for the reaction. The global activation energy for the
monolayer reaction was found to be lower than the corresponding value
for gas-phase polymerization.60 Polymerizations of monolayers mainly
fall in two categories. In a first type, amphiphilic molecules contain a
reactive double bond in their tail which can be polymerized by UV-
irradiation. Second, amphiphilic compounds having reactive head groups
are spread as monolayers and initiators dissolved in the sub-phase induce
polymerizations. Numerous examples of both cases can be found in the
literature.61-81
In monolayer reactions, there is a better control over intermolecular
distances between the reacting molecules and their orientations compared to
those in bulk (3-D). Only in insoluble monolayers do we have simple
experimental techniques which enable us to hold molecules in well defined







orientations which can be varied at will. In fact, in some cases
polymerization occurs only in the monolayer and no polymerization was
observed in solution, melts, or crystals.82 Since the molecules are
essentially pre-oriented prior to reaction, it is expected that the resulting
product will have long range positional, conformational and orientational
order and the reaction will occur with a lower activation energy. In a few
cases, evidence has been obtained that molecular orientation can affect the
nature of reaction products significantly.83
Typically, a chemical reaction in a monolayer at liquid/gas interface
is accompanied by changes in molecular area, interfacial pressure,
interfacial potential, electric dipole moment, and interfacial viscosity or
elasticity. The kinetics of reactions may therefore be followed by
measuring changes in any of these parameters. An alternative approach is
to remove samples from the interface at different times and subject them to
physical and chemical analysis to determine the extent of conversion of
reactants to product. This is not as convenient as measuring the kinetics
directly from changes in interfacial parameters.


1.4 History of Electrically Conducting Polyaniline
For most of the history of polymer technology, one of the most
valued properties of synthetic polymers has been their ability to act as
excellent electrical insulators, both at high voltages and at high frequencies.
In spite of this, there has been interest for many years in the possibility of
producing electrically conducting polymers. The obvious attraction is to
combine two sets of properties in one material, the electrical properties of
a metal while retaining the processibility and mechanical properties of a







conventional polymer. These types of organic polymers are known as
electrically conducting polymers or synthetic metals.
Organic polymers can be made electrically conductive either by
adding conductive substituents such as metal particles, ions etc. into a non-
conductive polymer matrix84 or by preparing a polymer in which the
backbone is composed of a conjugated it system. In the latter case, neutral
or undoped polymers are either insulators or semiconductors. These
polymers become electrically conductive via doping. The doping is
generally carried out by the removal (oxidation) or addition (reduction) of
electrons from or to the n system of the polymer backbone.85-87 Doping
can be achieved by P-type (partial oxidation of the backbone), N-type
(partial reduction of the backbone), and/or protonic acid (special case for
polyaniline) type doping process.88-90 Common dopents are AsF5, 12,
LiC104, sodium naphthalide etc. During the doping process, insulating or
semiconducting organic polymers having a conductivity in the range of 10-
10 to 10-5 -l1cm-1 are converted to polymers which are in the "metallic"

conducting region (1 to 104 Q-1cm-1).
Early work in the electrically conducting polymers area was slow
because of their infusibility, insolubility, and extreme intractability.
However, this situation was changed dramatically in 1970s. Following the
successful synthesis of polyacetylene by Shirakawa91 and its subsequent
doping by MacDiarmid,88 electrically conducting polymers have raised
great interest because of their many possible applications. Many review
articles regarding these materials have been published in last
decade.85,86,92-94 Commonly known conducting polymers are
polyacetylene, poly(p-phenylene), poly(p-phenylenevinylene), polypyrrole,
polythiophene, polyaniline, etc.







Among all conducting polymers, polyaniline is the most studied
material. It is also among the oldest known organic polymers. Its
synthesis was first reported by Letheby in 1862.95 In this report he
characterized this polymer as "blue pigment." Almost 40 years after
Letheby's report, Willstatter and coworkers showed interest in this
polymer.96 Following them a series of studies in 1910-1912 established
the existence of different oxidation states for the then so-called "aniline
black" polymer.97,98 During the years to follow, many authors studied
the products of chemical and electrochemical oxidation of aniline.99-105
Until recently, however, this polymer was not considered as an electrically
conductive polymer. Its potential as an electrically conductive polymer
was first recognized by MacDiarmid in the mid-1980s.106-110 Since then,
the polyanilines have probably been the most rapidly growing class of
conducting polymers and the number of publications on the subject of
polyaniline is becoming greater and greater.
The base form of polyaniline having the general formula shown in



P __NH- NH Y N- N IT

A B

Figure 1-4. The structure of the emeraldine base form of polyaniline.


Figure 1-4 contains a reduced repeat unit (A) and an oxidized repeat unit
(B). The ratio of amine to imine nitrogen atoms depends upon the degree
of oxidation. The value of y, which ranges from 0 < y < 1, also depends
upon the oxidation state. The oxidation state of the polymer continuously
increases with decreasing value of y. Polyaniline exists in three different







oxidation states. The fully reduced form (shown in Figure 1-5),
leucoemeraldine [poly(paraphenyleneamine)] corresponds to a value of
y = 1; the fully oxidized form (shown in Figure 1.5), pernigraniline
[(poly(paraphenyleneimine)], corresponds to a value of y = 0; and the half
oxidized form, emeraldine base [poly(paraphenyleneamineimine)] (shown
in Figure 1-4), corresponds to a value of y = 0.5. Each oxidation state can
exist in the form of its base or as its protonated form (salt) by treatment of
the base with a protonic acid.111 They can be interconverted into each
other by redox and/or acid-base reactions. The pernigraniline and
leucoemeraldine form of polyanilines (bases as well as salts) are electrical
insulators. Only the emeraldine base form of polyaniline is conductive
upon protonation with maximum conductivity observed at y = 0.5.
Polyanilines differ substantially from other conducting polymers in
that their electronic structure is based on the overlap of 7r orbitals of
alternating nitrogen atoms and benzene rings. In other words the nitrogen
atoms bridge benzene rings and play an important role in the t structure.


NHn leucoemeraldine


iC 0_N- -- N 4n o- pernigraniline

Figure 1-5. Structures of the fully reduced and oxidized forms of
polyaniline.


This is in contrast with polypyrrole whose nitrogen atoms are, functionally
speaking, pendant groups attached to a polyacetylene-type backbone as
shown in Figure 1-6. Polyaniline is also unique among all conducting







polymers in that its electrical properties can be controlled by both the
oxidation of the main chain and the level of protonation. 112,113 Upon
electrochemical doping at a given pH, the conductivity of polyaniline is
switched "on" at the initial stage of doping and switched "off" at high levels




H
polyaniline polypyrrole

Figure 1-6. Repeat units of polyaniline and polypyrrole.



6



S3


0


Figure 1-7.


-0.3 E/V vs SCE 0.6
Schematic presentation of conductivity versus electrochemical
doping in polyaniline at a given pH. The conductivity is
switched "on" at the initial stage of doping and switched "off"
at high levels of doping. (This figure was taken from the
reference 114)


of doping as depicted in Figure 1-7. This three-state switching is novel and
in contrast with the usual two-state switching of ordinary semiconductors
and other conducting polymers.1 14 The emeraldine base form of







polyaniline can be converted from insulator (a < 10-10 92-lcm-1) to
conducting (a = 101 Q-lcm-1) through protonation. It is believed that
upon protonation, imine nitrogen atoms tend to be preferentially
protonated as compared to the amine nitrogen atoms due to a large driving
force for semiquinone formation (Figure 1-8). The existence of
semiquinone radical cations in the conducting form of polyaniline and its
important role in the electrical conduction in polyaniline were proved by
Harada et al. using vibrational spectroscopy.115 It was remarkable to find
that the conductivity increases by 1010 as the pH is decreased by only
three to four units, i.e., from a pH of -~ 4 to a pH of 0.111,112 Thus,
polyaniline is probably the only conducting polymer in which the number
of electrons remains the same and the number of protons increase after a
doping process. Also, upon protonic acid doping, the formation of an
environmentally stable nitrogen base salt results rather than a carbonium
ion (potentially higher reactive) as in conventional oxidative p-doping of a
polymer.
Besides its electrical conducting properties, polyaniline is a material
of great interest due to its various simple methods of preparation in
aqueous solutions and good chemical and thermal stability in the presence
of oxygen and moisture even when doped.116-119 For example,
conductivity of polyaniline samples were measured as a function of water
vapor pressure. It was observed that the presence of water favors higher
conductivity and the protons exchange between the polymer solid phase and
the mobile water phase.120 It may be noted that the presence of water also
possibly solvates the anion, resulting in reduced localization of the positive
charge on the polymer and in the vicinity of the anion. This reduction in
the electrostatic interaction might be expected to result in greater















2 H+A


internal redox reaction


polaron separation


Figure 1-8. Semiquinone formation upon protonic acid doping in
polyaniline.







delocalization of charge on the polymer chain, with concomitant increase in
conductivity. The thermal stability of conductive polyaniline was also
studied by a heat-aging test at 150C.1 19 The stability in terms of _
__ conductivity was found to depend upon the sample form as well as on
the environment. Upon aging the disks form in air, the decrease in
conductivity was not significant: initially 17 Q-1cm-1, and 0.17 Q-lcm-1
after aging for 2000 hr. The powder sample aged in vacuum retained its
electrical conductivity at a higher level than that aged in air, the
conductivity being 0.42 Q-lcm-1 even after 2000 hr. of aging.
Thermally the emeraldine base form of polyaniline is a very stable
material. In TGA a major weight loss was observed only at ~ 5000C. A
solution cast film of a chemically prepared polyaniline showed the glass
transition temperature (Tg) 2200C. The storage modules of the same
film was studied by dynamic mechanical analysis and found to be about 200
MPa at 250C.121 It was also shown that the emeraldine base form of
polyaniline can be processed through simultaneous heat treatment and
application of stress to produced oriented, partially crystalline polyaniline
film. Heating the sample at an elevated temperature (T > 1100C) while
under stress leads to an elongation of the polymer film with 1/lo up to 4.5.
After the orientation process, the polymer chains were found to be
significantly oriented parallel to the stretching direction and the resulting
material had enhanced mechanical properties.122 Scanning electron
microscope studies revealed that doped amorphous polyaniline undergoes a
thermal transition to an oriented partly crystalline polymer. 123
The precise molecular structural arrangement of the backbone of
polyaniline is difficult to estimate due to the fact that these structures are
affected by both oxidation and reduction and by concomitant protonation







and deprotonation of nitrogen atoms in the polymer. X-ray studies of
polyanilines in the emeraldine base and salt forms indicated that they
contain amorphous and crystalline (fraction of crystallization up to 50%)
structure, respectively. X-ray studies also showed that many physical
properties of these materials, such as charge transport phenomena, are
controlled by structural parameters, such as fraction of crystallinity, size of
crystalline domains, degree of chain orientation, inter chain separation,
inter chain order, polymer ring torsion angles, intra chain disorders, etc.
In addition, these structural parameters can be modified by the sample
history, preparation and treatment conditions, and various chemical
modification.124,125 From X-ray studies on perchlorate and
tetrafluoroborate salts of polyanilines, Baughman et al. proposed that the
ring positions have a dihedral angle of +15 and -15 (alternately) with
respect to the planar zigzag chain made by the nitrogen atoms.126 Langer
carried out theoretical structural studies and concluded that polyaniline can
not form a planar structure because of the steric hindrance caused by
strong interactions of neighboring six-membered rings.127 Detailed
analysis of IR spectra gave an estimate that the plane of the phenyl rings
makes an angle of 560150 with respect to the plane of the nitrogen atoms,
which was in reasonable agreement with the ring torsion angle of -300
determined by analysis of X-ray diffraction patterns of the emeraldine
base.122
It has also been found that polyaniline exhibits multiple color
changes depending upon both oxidation state and pH.128-132 Watanabe et
al. suggested that the color change from transparent yellow to green is due
to the formation of Wurster-type radical cations by the oxidation at 0.2 V,
and that from green to blue is due to the formation of a diimine structure







and the doped state at potentials higher than 0.3 V.133 As a result, a
number of potential applications have been reported for polyaniline such as
an ion exchange polymer,134,135 an acid-base indicator,134 to protect
metals and semiconductors from corrosion,136,137 a membrane for gas
separation,138 electrochromic displays, 139,140 erasable optical
information storage techniques,141 and perhaps most importantly in
rechargeable batteries.107,142
Polyaniline and its derivatives are generally synthesized by standard
techniques in isotropic media. It can be prepared chemically using different
oxidizing agents. 106,143-149 In general, the reaction is mainly carried out
in an acidic medium, in particular sulfuric acid, at a pH between 0 and 2.
Whilst MacDiarmid et al. use hydrochloric acid at pH 1,106 Genies et al.
use a eutectic mixture of hydrofluoric acid and ammonia for which the pH
is probably less than zero.150 The polymerization of aniline in neutral and
basic media has also been reported.151
Another common method to polymerize aniline in aqueous or non-
aqueous media is the electrochemical polymerization.
103,104,130,148,150,152-155 In this technique, a reaction is generally

carried out in a one compartment cell using a saturated calomel electrode
as the reference electrode and a platinum wire as the working electrode.
However, the use of other working electrodes can be found in
literature.150 The anodic oxidation of aniline is normally performed by
cycling the potential between -0.2 to 0.7-1.2 V (vs. SCE).
It is possible to polymerize aniline in a two-phase system.156 The
polar phase is generally composed of a homogeneous mixture of an oxidant
and an acid and the non-polar phase contains aniline and an organic solvent.
The polymerization reaction occurs at the interface of the two solutions.







Recently, Duran and coworkers have used an electrochemical method
coupled with a two-phase system to polymerize the ortho substituted
aniline.157 Other routes such as vapor-deposition techniques,158 synthesis
via Schiff base chemistry,159 in situ intercalation/polymerization of aniline
in layer V205-nH20 xerogels,160 condensation and decarboxylation
involving polyanilinecarboxylate,161 and the homogeneous polymerization
of anilines involving a monomer-alkyl ligand complex have also been
reported. 162
Aniline can polymerize at both an ortho and the para positions.
Polymerization reaction at both positions leads to a cross-linked polymer.
In addition, the backbone of polyaniline is very stiff due to benzene rings
and conjugated r systems. Because of these facts, polyaniline synthesized
by conventional methods is generally insoluble in common organic solvents
and can not be melted below its range of decomposition. These properties
cause characterization limitation and difficulties in processing this polymer.
A common technique to increase the solubility and lower the melting
point of stiff-chain polymers is the attachment of flexible side chains to the
polymer backbone. It was observed that the attachment of an alkyl group
such as methyl, ethyl, etc. at an ortho position has a strong effect on the
polymerization yield; the yield of the reaction decreases with the bulkiness
of the substituent. These results lead to a conclusion that alkyl substituents
with more than three carbon atoms will result in a negligible reaction
yields by conventional methods. Nonetheless, larger alkyl substituents are
necessary to improve polyaniline's solubility in common organic solvents.
Monolayer polymerization shows superiority over conventional
methods in the polymerization of substituted anilines. In this technique a
long alkyl chain is necessary to form a stable monolayer. Thus a problem







in conventional methods is a blessing in the monolayer reaction. The
substituent on the benzene ring plays two crucial roles in the monolayer
polymerization of aniline. First, it holds the reactive part of the monomer
molecule at the interface and second, it improves the solubility of the
resulting polymer in common organic solvents. In addition, the substituent
at an ortho position blocks one of the reactive centers of aniline. This
should lead to a lower content of ortho coupling, and a more regular head-
to-tail polymer structure is expected.
Recently our group has polymerized some o -substituted anilines at
the air-aqueous interface using the Langmuir film technique.61,163-165 In
this technique known amounts of amphiphilic monomer molecules are
confined to a planar aqueous surface, which defines their orientation and
intermolecular distances during the reaction. Since the surface is fluid, the
molecules can be manipulated in a Langmuir trough, yet are free to
reorient within the monolayer upon reacting. The reaction then occurs
under anisotropic conditions caused by the surface-oriented monomer. The
polymer chains in the resulting monolayer may have enhanced long-range
configurational, orientational, and positional ordering and higher
anisotropic properties than polymers synthesized by conventional methods.
The Langmuir film technique has further advantages: First, the average
distance between reacting molecules can be easily changed by varying the
temperature and applied surface pressure. In addition, constant
intermolecular distances can be maintained through applied surface
pressures. Second, monomer molecules are anchored to a planar surface
during polymerizations. This will prohibit resulting polymer chain from
overlapping. Third, kinetics can be followed in real time by observing the
change in the average surface area occupied by a monomer or repeat unit







of the polymer backbone. Under conditions of constant surface pressure
this is analogous to volume dilatometry in two dimensions, where the mean
molecular area takes the place of the specific volume. 166
Although a large number of papers on the preparation and properties
of polyaniline have been published, the kinetics and polymerization
mechanism are still under discussion. After the formation of a dimer by
coupling of two monomer radical cations, the method by which these
species propagate to polymer is not clearly understood. Several
mechanisms were proposed but none have been
confirmed.103,143,150,153,155,162,167-172 Wei et al. believed that after
the formation of a dimer, it reoxidizes quickly to form either diiminium
dication or nitrenium ion. An attack of a neutral molecule of aniline on
either of these species propagates a polymer chain.173-175 Gregory et al.
has speculated that an oxidized polymer chain oxidizes catalytically a
neutral molecule of aniline in the presence of acid and then adds
together.176
In order to understand this reaction mechanism, it is important to
know the nature of the activated complex (transition state). In classical
synthetic techniques, the volume of activation is an experimentally
accessible quantity which is easier to interpret than the free energy,
enthalpy, or entropy of activation. Mechanistic interpretations of organic
reactions using activation volumes have been reported in the literature. 177-
179 In spite of its importance in the elucidation of reaction mechanisms,

however, only a small amount of work dealing with activation volumes has
been reported. This is in part, because determining activation volumes
requires difficult experiments at high pressures. Here, as already noted,
the LB technique has a distinct advantage because the mechanically applied







surface pressure can be easily changed. As a result, the measurements of
the activation area are greatly simplified.


1.5 Objectives of This Thesis
The aim of this thesis is to understand the kinetics of the monolayer
polymerization and to evaluate the activation energy, entropy of activation,
and activation area of the 2-pentadecylaniline polymerization reaction
occurring at the air-aqueous solution interface. Measurements of activation
energy will provide energetic of this reaction. In conventional methods,
activation energies are measured at one pressure, generally at atmospheric
pressure. In this study activation energy will be determined at different
surface pressures and compared with values obtained by other methods.
Activation energies obtained at different pressures will provide
information regarding energetic of aniline polymerizations in terms of
their intermolecular distances. An entropy of activation is also a useful
parameter to understand a nature of the transition state and reaction
mechanism. Determination of activation entropies is important,
particularly for reactions occurring in a polar solvents and involving
charged species. Its interpretation is generally, based on electrostriction
effect of binding solvent molecules. An activation area also provides
valuable information about a reaction in solution and insight into the
reaction mechanism. For example, it can be used to distinguish the
difference between a unimolecular and bimolecular reaction. Also the
activation area is an easily excessible parameter in monolayer
polymerizations. Values of the above parameters will be used to
understand the polymerization of aniline at an air-water interface and a
reaction mechanism consistent with the results will be proposed.











CHAPTER 2


EXPERIMENTAL


2.1 Synthesis of 2-Alkyl Anilines
All chemicals were purchased from Aldrich Chemical Company, and
used as received after confirming purity. Purity of the compounds was
checked by Thin Layer Chromatography (TLC), 1H and 13C NMR.
Anhydrous ether was prepared by distillation over calcium hydride.
2-Alkyl anilines were prepared in a two step reaction using a
reported literature procedure.180,181 A 500 ml three neck flask
containing 0.15 mole of Mg curlings was dried and purged with nitrogen.
1-Bromoalkane (0.14 mole) in 50 ml of anhydrous ether was then added
slowly to the Mg curlings. The mixture was allowed to reflux, under a
dry nitrogen atmosphere, for 30 min to ensure the Grignard reaction. A
solution of 2-aminobenzonitrile (0.02 mole) in 15 ml anhydrous ether was
added, under vigorous stirring, into the above prepared Grignard reagent.
A yellow solid was obtained. This yellow solid was heated at 900C for 15
hrs. Then it was poured into a solution containing 45 ml water, 10 g
ammonium chloride, and 65 g of ice. A brown-yellow color organic layer
was separated and the aqueous layer was extracted three times with ether.
The combined etheral solution was dried over anhydrous sodium sulfate
and concentrated on a rotavap. To the residue, 10 ml of water and 10 ml
of concentrated HCl was added. The mixture was allowed to reflux for 40
min. The homogeneous solution was cooled and treated with ammonium







hydroxide until basic and then extracted three times with ether. The
combined ether extracts were dried over anhydrous sodium sulfate and an
evaporation of ether gave the crude 2-alkanoylaniline product. This 2-
alkanoylaniline product was purified by vacuum distillation and by column
chromatography (toluene, silica gel).
The above prepared 2-alkanoylaniline compound was reduced to
hydrocarbon by a Huang-Minlon reduction.182 The reduction reaction
was performed in a three neck flask containing product (I) from Figure
2-1, 25 g NaOH, 20 ml hydrazine hydrate (95%), and 100 ml triethylene
glycol. The mixture was heated to 1400C for 70 min. The mixture was
concentrated by distilling off a by-product and then it was refluxed at
180C for 4 hours. After refluxing for four hours, the mixture was
cooled to room temperature and diluted with 125 ml of water. The
product was extracted three times with ether. The combined ether extracts
were dried over anhydrous sodium sulfate and concentrated on a rotavap.
The obtained crude product was purified by a column chromatography and
the yield was less than 30%. For chromatography, silica gel and toluene
were used as an adsorbent and a solvent respectively. An outline of the
method used is shown in Figure 2-1. The products were identified by
NMR and TLC. TLC showed a one spot. NMR spectra were obtained
with a varian XL-200 NMR spectrometer using deuterated chloroform as a
solvent and the TMS as a reference. For example, 1H NMR of 2-
pentadecylaniline is shown in Figure 2-2. 1H NMR of 2-alkyl anilines in
CDC13 shows chemical shifts at a = 0.9 (t, 3 H CH3); 1.3 (m, 30 H (26H,
22H), CH2); 2.5 (t, 2 H, CH2); 3.6 (s, 2 H, ArNH2); 6.65-6.8 (m, 2 H,
ArH); 7.0-7.1 (m, 2 H, ArH). The 1H NMR spectra of all homologous
were similar to that shown in Figure 2-2. 2-Tridecylaniline was a










CH3(CH2)nBr + N ether CH3(CH2)nMgBr


CH3(CH2) nMgBr


NH2
A+ CN 1. Heat, 15hrs.
+ II-
2. HC, reflux
4 hrs.


NH2
-C-(CH2)nCH3


(I)


1. Hydrazine monohydrate
2. Triethylene glycol
3. NaOH
4. Reflux at 180C
for 4 hrs.


NH2

CH2-(CH2).CH3

(LI)
n = 11, 2-Tridecylaniline
n = 13, 2-Pentadecylaniline
n = 15, 2-Heptadecylaniline


Figure 2-1. The Synthetic route for 2-alkyl anilines.


g
NH2d
H2- d c b a
e CH2-CH2-(CH2)12-CH3

f /f


6


5


.................I


4


b---


d
g c



19,21"".1 l ""'"
3 2 I


PPM
Figure 2-2. 200 MHz 1H NMR of 2-pentadecylaniline in CDC13.







yellowish-white liquid at room temperature. While 2-pentadecyl and 2-
heptadecyl anilines were yellow-white solids and had melting points of
34C and 390C, respectively.


2.2 Isotherm
Isotherms, often known as pressure-area curves, provide a
characteristic description of an insoluble monolayer at the interface. They
measure the change in surface tension, and thus surface pressure due to the
change in area of a film forming molecule on a liquid surface.
All isotherm experiments were carried out on a commercially
available rectangular Langmuir trough, LB5000 (KSV Instruments,
Finland). This trough was equipped with a computerized control and one
or two barriers. Surface pressure is the most commonly measured
property of an insoluble monolayer. It was measured using the Wilhelmy
plate film balance method with a platinum plate. The platinum plate was
carefully pre-wetted and zeroed in the clean aqueous surface prior to
measurement. Thus this method allows an absolute measurement of the
force on a plate due to surfactant. The interior trough surfaces and barrier
were made of Teflon. The trough and sub-phase temperature was
controlled by passing water from a constant temperature bath through
channels below the trough. The sub-phase temperature was measured
either by placing a calibrated thermometer or a Teflon coated
thermocouple into the sub-phase; temperature stability was typically +
0.1C.
Spreading solutions were prepared by dissolving the monomer in
chloroform (Fisher, spectranalyzed grade). The spreading solution
concentration ranged from 0.5 mg/ml to 1.0 mg/ml. Sub-phase solutions







were prepared with ACS reagent grade chemicals and highly purified
TM
water (Millipore >18 MK2 resistance). Presence of surface active
impurities in prepared solutions was checked by moving a barrier towards
the Wilhelmy plate and observing the change in surface pressure. The
surface was considered clean if no measurable surface pressure changes
were observed.
Unless otherwise noted, all isotherms were measured on a 0.1M
sulfuric acid sub-phase with no added oxidizing agent. A pressure
detector, Wilhelmy plate, was placed near one end of the trough and a
barrier was placed at the other end. Known amounts of monomer solutions
were spread on the sub-phase using a microliter Hamilton syringe. After
spreading, chloroform was allowed to evaporate from the surface for about
2 minutes. The resulting monolayer was then compressed at a constant rate
of surface area change (9 A2molecule-lmin-1) by displacing the barrier.
Barrier movement in a forward or backward direction changes the average
surface area occupied by a molecule and thus the surface tension. A change
in the surface pressure, which is the difference in a difference in the
surface tension of a bare surface and that with surfactant, versus mean
molecular area was computer recorded. Measurements were repeated at
least three times; isotherm repeatability was better than 1 A2 at a given
surface pressure.


2.3 Isobaric Stability
Isobaric stability experiments were carried out on a commercially
available rectangular Langmuir trough LB5000 (KSV Instruments,
Finland) equipped with a computerized control and one or two barriers.
The kinetic program software from LB5000 was used for these







measurements. The surface pressure was measured using the Wilhelmy
plate film balance method.
All isobaric stability measurements were made on a 0.1M sulfuric
acid sub-phase with no added oxidizing agent. Solution of sulfuric acid was
TM
prepared using highly purified water (Millipore >18 MQ resistance).
Isobaric stability is defined here as the change in average surface area with
time at a constant applied surface pressure. In these experiments, the
Wilhelmy plate was placed near one end of a trough and a barrier was
placed at the other end of the trough. Solutions of known monomer
concentrations were spread on the sub-phase. After spreading, chloroform
was allowed to evaporate from the surface. The resulting monolayer was
compressed at 9 A2molecule-lmin-1 until the desired surface pressure was
reached. The applied surface pressure was kept constant during the
experiment by displacing the barrier. The change in mean molecular area
with time, after constant surface pressure was reached, was recorded.
Isobaric creep measurements were performed at different sub-phase
temperatures and at different surface pressures. Experiments were
repeated at least three times; area change repeatability at a given time was
better than 5%.


2.4 Polymerization
Polymerization reactions were also carried out on a commercially
available rectangular Langmuir trough LB5000 (KSV Instruments,
Finland) equipped with a computerized control and one or two barriers.
The kinetic program software from LB5000 was used for these
experiments. Once again the surface pressure was measured using the
Wilhelmy plate film balance method.







All polymerization reactions were also carried out at constant
applied surface pressure. In these experiments, monomer was spread on a
sub-phase consisting of a homogeneous mixture of 0.1M sulfuric acid and
0.03M ammonium persulfate. A solution of the above concentration was
TM
prepared using the highly purified water (Millipore >18 MQ resistance).
Ammonium persulfate acts as an oxidizing agent (slightly different than a
conventional polymerization reaction in that it is necessary to start the
reaction, but is also needed-and continually consumed-as the
polymerization proceeds.) and it has been used for chemical polymerization
of aniline and derivatives of aniline.106,110,154,173 After spreading the
monomer solution, chloroform was allowed to evaporate from the
interface for 2 minutes. The resulting monolayer film was then
compressed (90 A2molecule-lmin-1 ) until it reached the desired applied
surface pressure. The surface pressure was kept constant during the entire
reaction by changing the barrier position. The polymerization reaction
was monitored either by measuring the change in mean molecular area or
by measuring the average barrier speed. A reaction time of zero was taken
to be the point at which the surface pressure reached the desired value.
After the polymerization, and between each experiment, the trough was
cleaned first with a mixture of chloroform and ethanol and then, several
times, with purified warm water. Polymerization experiments were
repeated at least three times at surface pressures of 10 and 20 mN/m and
twice at pressures of 15 and 30 mN/m. Typical repeatability of the time
and magnitude of the barrier speed maximum at a given condition was
better than 2%.






2.5 Determination of Molecular Weight
Polymer was synthesized using the procedure described in the
polymerization section. All polymerization reactions were carried out at
250C temperature and at 20 mN/m applied surface pressure.
Polymerization reactions were quenched at different time intervals (=20%,
=40%, =60%, =80%, and =100% conversion of the monomer) using a
quenching solution as prepared below. 47.0 gm Na2S203, 27.0 gm NH4Cl
and 34.0 ml concentrated NH3 were added to a 250.0 ml volumetric flask
and diluted with highly purified water. 30 ml of the resulting solution was
mixed with 12 ml of 10.6M NaOH to make the final quenching solution.
This amount is for a 900 ml sub-phase solution. This solution was also
used by another group to quench the aniline polymerization.173 In order
to quench the polymerization reaction, the barrier was stopped and moved
back rapidly to its original position. Then the above prepared quenching
solution was injected into the subphase with an aid of a syringe. The
resulting sub-phase solution was mixed using a small magnetic Teflon
coated stirring bar. After mixing for few minutes, the barrier was moved
forward and the polymer was collected by sweeping the surface with a
fritted glass attached to an aspirator. The fritted glass containing polymer
was washed with distilled water, vacuum dried in a dessicator under
dynamic vacuum, and kept in the dark. Prior to molecular weight
determination, the polymer was dissolved in HPLC grade tetrahydrofuran.
Molecular weights of polymers were determined by Gel Permeation
Chromatography technique coupled with an ultra violet detector. HPLC
grade tetrahydrofuran was used as the mobile phase. A column containing
stationary phase was purchased from Phenomenex. Specifications for
columns are as follows. Type: 5X103 A (5X104 A), size: 300 X 7.8 mm,







Molecular weights and
standards.


12.5 -


11.5 -


10.5 -


9.5 -


8.5


retention times of polystyrene


1 12 13 14 15 1


Figure 2-3. GPC calibration curve


Vmax [min]
of polystyrene standard.


and s/no: GP/2775 (GP/2776). The Mobile phase was passed through
columns at a speed of 1 mm/min. A calibration curve was prepared (as


Table 2-1.


Mp [g/mole] Mw/Mn In Mp Vmax [min]

7,820 1.06 8.96 15.53

17,500 1.04 9.77 13.93

30,700 1.02 10.33 13.40

59,500 1.03 10.99 12.23

142,000 1.06 11.86 11.12


y = 19.13- 0.66x R = 0.995




38

shown in Figure 2-3) using Polystyrene standards (obtained from Scientific
Polymer Product) having molecular weights ranging from 7000 to 142,000
g/mole. Molecular weight distributions and retention times of these
standards are shown in Table 2-1.











CHAPTER 3


RESULTS AND DISCUSSION


3.1 Isotherm


3.1.1 Isotherms of 2-Pentadecylaniline on Different Sub-phases
The single most important indicator of the monolayer properties of a
material is given by a plot of surface pressure as a function of the surface
area available to each molecule. This is carried out at a constant
temperature and is accordingly known as a surface pressure/area isotherm,
and often abbreviated to isothermm". From this, in principle, the
equilibrium value of a surface pressure can be obtained at any given area,
but it is more common to record a pseudo equilibrium isotherm by
compressing the film at a constant rate while continuously monitoring the
surface pressure.
Figure 3-1 shows isotherms of 2-pentadecylaniline recorded on pure
water and sulfuric acid sub-phases having different pH values at 250C. In
all experiments, no surface pressure was measurable at mean molecular
areas higher than 90 A2/molecule. As the area was decreased by
compression, no pressure was detected until intermolecular forces
increased the surface pressure to a measurable value. We will refer to the
area at which the pressure becomes measurable as the surface pressure
onset.










I40 -
pH=1
S30-
2-
pH 32.5
20

10-
water
0
10 30 50 70 90
Area [A /molecule]
Figure 3-1. Isotherms of 2-pentadecylaniline measured on a pure water
and on sulfuric acid sub-phases having different pH at 250C.


The onsets for 2-pentadecylaniline on water, 0.001M sulfuric acid
(pH 3), 0.00316M sulfuric acid (pH 2.5), and 0.1M sulfuric acid (pH 1) are
shown in Table 3-1. Upon further compression, a monotonic increase in
surface pressure was observed at all conditions investigated. Isotherms
also show that monolayers are in the liquid analogous phase and there is no
apparent phase transition during the compression. The point at which
further increase in surface pressure becomes negligible or the surface
pressure drops considerably upon compression is generally known as the
collapse point. Mean molecular areas and surface pressures at the collapse
point for 2-pentadecylaniline monolayers on different sub-phases at 25C
temperature and at 9 A2molecule-lmin-1 compression speed are shown in
Table 3-1. The isotherm results show that monolayer films are highly
compressible and that the compressibility increases with sub-phase acidity,
while the collapse area is not highly affected. However, mean molecular







area at the surface pressure onset and surface pressure at the collapse point
were found to be highly influenced by the sub-phase acidity. The mean
molecular area at the surface pressure onset as well as at any given
pressure, and the collapse pressure increases with the sub-phase acidity.


Table 3-1.


Values of mean molecular areas and surface pressures at the
onsets and the collapse points on differ C


The first conclusion drawn from these results was that the area
change with the sub-phase acidity was due to the coulombic repulsion
between anilinium head groups. Later, it was found that this was not the
case. This will be explained in the following discussion in terms of
structural changes within the molecule.
Figure 3-2 shows isotherms of a homologue of the para isomer, 4-
hexadecylaniline, on water and 0.1M sulfuric acid at 250C. The para
isomer spread well and gave reproducible isotherms which showed little
hysteresis upon expansion below collapse. The surface pressure onsets on


Sub-phase Area at Onset Area at Collapse Surface Pressure
[A2/molecule] [A2/molecule] at Collapse
[mN/m]

water 47.7 32.5 18.1

0.001M H2SO4 57.0 33.9 22.8

0.00316M H2SO4 63.4 33.8 27.8

0.1M H2SO4 74.6 35.2 39.2









50

40 0.1M sulfuric acid
water
30

20


0

0 10 20 30 40 50
Area [A/molecule]
Figure 3-2. Isotherms of 4-hexadecylaniline measured on water and on
0.1M sulfuric acid sub-phases at 250C.

water and 0.1M sulfuric acid were 24.6 and 24.9 A2/molecule, in that
order. Collapse was observed at areas of 21.6 and 22.8 A2/molecule,
respectively. 4-Tetradecylaniline was investigated and showed similar
surface areas in its condensed region. Isotherms of 4-hexadecylaniline
were also carried out on aqueous hydrochloric acid and sodium hydroxide
sub-phases as shown in Figure 3-3. The surface pressure onsets on water
as well as on 0.1M aqueous HC1 and 0.001 M NaOH sub-phases were very
similar. The 0.1M HCl sub-phase results agreed, within experimental
error, with results previously reported by Adam.183 At 1M HC1
concentration, the results differed from those reported by another
group,184 though at surface pressures higher than 30 mN/m, the
monolayer appeared to show condensed phases in both studies. It should
be mentioned that n-alkyl amines, under acidic conditions, are also known
to have similar surface areas in the condensed region of the isotherm.185










S60


i 40 -
1.OM HCl

S20 1 0.1M HC1
0.001M NaOH

0 ,i
0 20 40 60 80
Area [A 2/molecule]

Figure 3-3. Surface pressure versus area isotherms of 4-hexadecylaniline
on 0.001M NaOH, 0.1M HC1, and on 1.OM HC1 sub-phases at
250C.


Comparing the ortho and para compounds, a difference of more than
20 A2/molecule in the onset areas on water is observed. Additionally the
isotherm of 4-hexadecylaniline shows a substantially lower compressibility
than the ortho substituted monomer. The observed surface areas and low
compressibility implies that the alkyl side chains of the para substituted
monomer are well-aligned and tightly packed near its collapse point. This
type of packing is possible when the chains are in a predominantly trans
conformation. The isotherm of 2-pentadecylaniline indicates that it is
highly compressible and the higher surface areas observed near collapse
indicate that the side chains do not achieve a close-packed conformation.
In addition the surface pressure onset occurred at a larger area than in 4-
hexadecylaniline. As 2-pentadecylaniline and 4-hexadecylaniline are







chemically similar, the difference in the surface pressure onset is due
largely to the substitution position.
A small area increase was observed for the para compound upon
decreasing the sub-phase pH. Nonetheless, in 0.1 M sulfuric acid the
unsubstituted monomer is protonated and exists as an anilinium cation.
This result implies that the surface pressure observed upon compression is
not strongly affected by coulombic repulsion between anilinium ions.
Therefore the increase in the surface pressure onset of the ortho substituted
monomer with sub-phase acidity is unlikely to arise from similar
coulombic repulsion between protonated head groups. If this were the
case, the surface pressure onset for the para substituted monomer should
also shift to substantially higher areas with increasing sub-phase acidity.
The details of molecular arrangements which lead to monolayer
behavior are not well known. It has been shown that in condensed films,
molecules have close-packed, well-aligned conformations with high
positional and orientational order, while in gaseous films they are widely
separated with no long-range orientational or positional order.
Apparently, the molecular conformation in these expanded films is
intermediate between these two extremes.22
In the case of the ortho substituted monomer, it is likely that the
polar amine group is hydrated and disposed toward the aqueous sub-phase,
while the alkyl side chain is likely to be oriented away from the surface
towards the air.163 As the amine and alkyl groups are attached to adjacent
ring carbons, the first few carbons of the alkyl chain probably adopt
gauche conformations to allow them to leave the water surface. Once
protonated by an acidic sub-phase, the conformation of 2-pentadecylaniline
molecules may change due to hydration forces which would tend to pull the







anilinium ion formed further downwards into the sub-phase. The amine
groups should become increasingly hydrated by the sub-phase and more
gauche conformations must be introduced in the alkyl side chain,
increasing the net surface area occupied per monomer.
In the case of the para substituted moiety, the amine group is easily
disposed towards the sub-phase due to its favorable position on the
aromatic ring. For steric reasons, the plane of the aromatic head group is
expected to be perpendicular to the air/water interface instead of lying flat
on the surface. The alkyl side group is able to protrude from the plane of
the interface and pack in a largely trans conformation. Packing of the
para compound in acidic solution is quite similar. This accounts for the
differences between the isotherms of 2- and 4- substituted anilines in
aqueous and acidic sub-phases.


3.1.2 Effects of Temperature
Isotherms of 2-pentadecylaniline measured on a 0.1M sulfuric acid
sub-phase at four different temperatures (100, 250, 350C, and 400C, +
0.10C) are shown in Figure 3-4. In all experiments, no surface pressure
was measurable at mean molecular areas higher than 90 A2/molecule.
Upon further compression (9 A2molecule-lmin-1), a monotonic increase
in the surface pressure was observed at all conditions investigated. The
isotherms show that monolayers are in the liquid analogous phase and free
from any phase transition during the compression. It can also be seen that
the mean molecular area at the surface pressure onset (73.7, 74.6, 77.5,
and 84.4 A2/molecule at 100C, 250C, 350C, and 400C, respectively ) as well
as at any given surface pressure increases as the sub-phase temperature
increases. Otherwise, isotherms at different temperatures look similar







except for the collapse region. The surface pressure at collapse decreases
slightly as the temperature increases. The collapse pressures for 100, 25,
35C, and 400C at 9 A2molecule-lmin-1 compression speed are 41.4, 39.2,
38.2, and 37.6 mN/m, in that order. In addition, isotherms at 250C, 350C,
and 400C have a well defined collapse point while the isotherm at 10C has
a broad collapse region. This may be due to different collapse
mechanisms.




40 -




S20 -

10 -


Figure 3-4.


20 40 60 80
2
Area [A /molecule]
Isotherms of 2-pentadecylaniline on 0.1 M sulfuric acid sub-
phase at different temperatures. Curves from the left to the
right correspond to the temperature 100C, 250C, 350C, and
400C, respectively.


The temperature effects cited above are expected when the
monolayer is in a liquid expanded state. At surface areas greater than 90
A2/molecule, molecules at the interface are thought to be lying flat, far
apart from each other, with negligible interaction between them. Brewster
angle microscopy, 186,187 a powerful technique for looking at monolayer
morphology and homogeneity, performed on a 2-pentadecylaniline







monolayer at the air / 0.1 M sulfuric acid interface showed that the film
was homogeneous, not birefringent, and free from domains during
compression from 90 A2/molecule until collapse. At higher surface
pressures the change in surface area with temperature is likely to be due to
increases in the van der Waal repulsion, causing the aniline molecules to
occupy a larger surface area.


3.1.3 Effect of Alkyl Side Chain Length on Isotherms
Isotherms of 2-alkyl anilines, where the number of carbon atoms in
the alkyl chain ranges from eleven to seventeen, were carried out on 0.1M
sulfuric acid at 27C. These amphiphilic molecules formed a one molecule
thick layer upon spreading at the interface. These monolayers were then
compressed from one side with the help of a barrier. Surface pressure
versus mean molecular plots for 2-alkyl anilines on a 0.1M sulfuric acid
are shown in Figure 3-5.
Figure 3-5 indicates that the surface pressure was negligible above
the area of 80 A2/molecule. In this region molecules are thought to be
lying flat and have a negligible interaction between them. As discussed
before, the area at which the interaction between molecules is measurable is
known as the surface pressure onset. Isotherms of 2-alkyl anilines show
that at any given mean molecular area the surface pressure increases as the
number of carbon atoms in the alkyl side chain increases. This is probably
due to the increasingly bulky nature of the side chain. Upon compression a
monotonic increase in the surface pressure was observed in all cases. The
surface pressures at the collapse points were 39.4, 40.3, and 41.0 mN/m
for tridecyl, pentadecyl, and heptadecyl aniline, respectively. The mean
molecular area at the collapse points were 28.6, 33.2, and 33.2







A2/molecule for tridecyl, pentadecyl, and heptadecyl aniline respectively.
The collapse areas for pentadecyl and heptadecyl anilines were about the
same indicating that at the collapse point, alkyl chains are orienting away
from the interface and probably attaining the same conformation. The
collapse area for 2-tridecylaniline is slightly lower than 2-pentadecyl and
2-heptadecyl anilines. This may be due to instability of this monolayer at
higher surface pressures.




40 -. C-11
\ -----C-13
30
i30- ,\ -- C-15

20 \ ------ C-17

S10


Figure 3-5.


0 20 40 60 80 100
Area [A /molecule]
Isotherms of 2-undecylaniline (C-11), 2-tridecylaniline (C-13),
2-pentadecylaniline (C-15), and 2-heptadecylaniline (C-17) on
0.1M sulfuric acid sub-phase at 270C temperature.


Isotherms of 2-alkyl anilines show that monolayers were highly
compressible and exist in a liquid analogous phase. Also, no apparent
phase transition was observed upon compression. It can also be seen that
the isotherm of 2-undecylaniline is substantially different from 2-tridecyl,
2-pentadecyl, and 2-heptadecyl anilines. This difference will be explained
in terms of its isobaric stability on 0.1M sulfuric acid sub-phase.







In conclusion, isotherms results indicate that 2-alkyl anilines are
amphiphilic in nature, spread well and form monolayers at the air/aqueous
solution interface. Under all conditions investigated, the surface pressure
was increased monotonically, below a collapse point, as the average surface
area occupied by a molecule was reduced. Monolayers of 2-alkyl anilines
formed liquid analogous phases and did not undergo any apparent phase
transition upon compression under all conditions investigated. Monolayer
characteristics of ortho substituted anilines did not change greatly with
temperature or with the number of carbon atoms (in the range of thirteen
to seventeen) attached to the ortho position. However, the average surface
area occupied by a molecule was slightly increased with temperature and
alkyl side chain length at any given surface pressure. The collapse pressure
of 2-pentadecylaniline increased with the sub-phase acidity however, the
collapse area remained the same. 2-Pentadecylaniline also showed a large
change in surface area occupied with the sub-phase pH. This was found
mainly due to the substitution position rather than coulombic repulsion
between molecules. 4-Alkyl anilines also spread well and formed
monolayers at the air/aqueous solution interface. Comparing isotherms of
2-pentadecylaniline and 4-hexadecylaniline and arguments presented in the
discussion section, the following picture (Figure 3-6) can be drawn
regarding to the arrangement of molecules at the interface.












H2o \ HS.4s -I ------- -.- ---H
H ----. .. -H 0 H 0 20 H
......-- 3- \ ---- 0 + 0 -- ....H O .-- ---
.0 W: SO4 ----------- 2 ------HO
---- HSO4 H3 H0 ... NH3 HSO -
HS04 -- 3+ 3 HS 4 ------
:::::::::" H20 H .- HSO4 HO ...............
......... H 20 ........................ ...... .........
............. H20 -------- HO --- ---
Figure 3-6. The arrangement of 2-pentadecylaniline (a) and 4-hexadecyl-
aniline (b) at the interface.



3.2 Isobaric Stability


3.2.1 Effect of the Alkyl Side Chain Length on the Monolayer Stability
Isobaric stability experiments of different derivatives of aniline were
carried out on a 0.1M sulfuric acid sub-phase. This sub-phase was selected
because all polymerization reactions would be performed on it. All
stability experiments were performed at 270C and at 15 mN/m applied
surface pressure. These experiments allowed the study of monolayer
stability as a function of number of carbon atoms in the alkyl chain.
Isobaric stability results of 2-undecyl, 2-tridecyl, 2-pentadecyl, and
2-heptadecyl anilines are shown in Figure 3-7. In this figure the change in
the average surface area occupied by a molecule is plotted against time.
The rate of change in the average surface area with the time for 2-
undecylaniline was significantly greater than the other derivatives of
aniline. In other words 2-undecylaniline did not form a stable monolayer
film on 0.1M sulfuric acid sub-phase. This is due to an imbalance of the
























Time [min]


Figure 3-7.







Table 3-2.


The change in the mean molecular area versus time plots of
monolayers of 2-undecyl (C-11), 2-tridecyl (C-13), 2-
pentadecyl (C-15), and 2-heptedecyl (C-17) anilines at 27C
temperature and at 15 mN/m applied surface pressure.



Values of the area change per minute of derivatives of aniline
under isobaric conditions at 270C temperature and at 15 mN/m
applied surface pressure.


Compound Change in Area

[A2molecule-1min-1]

2-Undecylaniline 2.24


2-Tridecylaniline 0.46


2-Pentadecylaniline 0.30


2-Heptadecylaniline 0.30


o7
a.)



<
a.)
0




I-,

a.
QS


0-


-5 -


-10-


-15 -


-20


C-11
----C-13

------ C-15

-------- C-17






hydrophobic and the hydrophilic part in the molecule. The alkyl chain
with eleven carbon atoms is not hydrophobic enough to keep the molecule
at the interface. It was also observed that as the number of carbon atoms in
the alkyl group increases, the isobaric stability of Langmuir films of 2-
alkyl anilines on the sulfuric acid sub-phase increases and then becomes
constant. This behavior is a result of increasing hydrophobic nature of the
compound as the number of carbon atoms in the alkyl group increases.
Table 3-2 shows the calculated values of the change in the average surface
area per time for different derivatives of aniline.


3.2.2 Effects of Temperature and Surface Pressure on the Monolayer
Stability
Isobaric stability as a function of temperature and the surface
pressure was also investigated. In this study, 2-pentadecylaniline was used
and experiments were performed on a 0.1M sulfuric acid sub-phase at
different temperatures and applied surface pressures. Measurements were
started five minutes after the surface pressure reached the desire value to
allow the molecules to relax and equilibrate. Stability values, shown in
Table 3-3, are the changes in the apparent mean molecular area per fifteen
minutes, a time period selected because most of the polymerization
reactions reached their maximum rate before this time. Stability at a given
surface pressure decreases with increasing temperature and increases with
decreasing pressure at a given temperature. Results also indicate that the 2-
pentadecylaniline does not form a stable Langmuir film at 400C and 30
mN/m applied surface pressure. Polymerization of 2-pentadecylaniline
under these conditions was not successful.







Table 3-3.


Stabilities of monolayers of 2-pentadecylaniline at different
temperatures and at different applied surface pressures.

Applied Surface Temperature Change in Area
Pressure [mN/m] [C] (in 15 minutes)
A2/molecule

30 10 0.86

25 1.43

32.5 2.36

40 6.16

10 10 0.23

25 0.44

40 1.9


In conclusion, 2-alkyl anilines form relatively stable monolayers at
the air/aqueous acid interface. The stability of the monolayer increases
with number of carbon atoms in the alkyl side chain and then becomes
constant at a given temperature and surface pressure. 2-Undecylaniline
does not form a stable monolayer due to improper balance of hydrophilic
and hydrophobic regions. The stability of 2-pentadecylaniline was found to
increase with decreasing applied surface pressure at a given temperature.
It was also found to increase with decreasing temperature at a given applied
surface pressure. Under all other conditions, the isobaric creep values
were sufficiently small to make rate constant calculations possible.







3.3 Polymerization

Polymerization reactions of 2-pentadecylaniline were carried out on
a sub-phase containing a homogeneous mixture of 0.1M sulfuric acid and
0.03M ammonium persulfate. Ammonium persulfate acts as an oxidizing
agent and has been used for aniline polymerization by several groups. A
monolayer of 2-pentadecylaniline was compressed from one side only with
the help of a Teflon barrier and all polymerization reactions were carried
out at a constant applied surface pressure. It is a common practice in
monolayer studies to perform a reaction at constant surface pressure as
opposed to constant area. It has been suggested that films are more likely
to be continuous when a constant surface pressure is maintained on the
monolayer throughout the course of the reaction.22,188 The reaction was
monitored by measuring the change in mean molecular area and the
average barrier speed (the rate of barrier displacement needed to maintain
isobaric conditions) with time. Typical results for monolayer
polymerization of ortho substituted aniline are shown in Figure 3-8.
In this figure (Figure 3-8) the mean molecular area and the average
barrier speed are plotted against time. It can be seen that the mean
molecular area decreases monotonically during the reaction and then
becomes constant. The average barrier speed increases from its initial zero
value, reaches a maximum and then decreases to a zero or negligible value.
The monotonic decrease in the area during the polymerization is due to the
replacement of van der Waal's radii by covalent bonds between monomer
molecules, and changes in their conformation. The mean molecular area
after the reaction is in close agreement with the isotherms of para
substituted aniline. It should be noted that almost no induction period was







observed in the beginning of the reaction. When the change in area with
time was negligible or barrier speed dropped to zero the reaction was
considered completed. Thus this technique allows the study of the reaction
kinetics in real time by observing the change in the average surface area
occupied by a monomer or repeat unit of the polymer backbone.


45


- 40
o
( 35


2 30


25


lz
(U
6 &


4)

24


0 10 20 30
Time [min]


Figure 3-8.


The average barrier speed and the mean molecular area versus
time for the monolayer polymerization of 2-pentadecylaniline.
The reaction was carried out at 250C temperature and at 30
mN/m applied surface pressure.


The magnitude of the area change (40% of the initial monomer area
in Figure 3-8, for example) is quite large. Typical volume changes due to
replacement of van der Waals distances by covalent bonds in
polymerization reactions rarely exceed several percent. However, aniline

is most easily reacted into para position, leading to the large conformation
change shown pictorially in Figure 3-9.











3 H, 0

Figure 3-9. The conformation of a monomer and a polymer molecules at
- the air / aqueous acid interface.


This implies that during the reaction a large part of the observed
area change is caused by conformational changes in the monomer which
allow the alkyl side chain to pack with less steric hindrance and in a
manner more analogous to the para monomer.
It should be noted that in monolayer polymerizations of o-substituted
anilines, reactions occurs mainly at the para positions. This is because one
of the ortho position is blocked by the alkyl side chain and the other is not
accessible for reactions. Thus polymerizations of the o-substituted anilines
in monolayers have an advantage over other methods in that one can
control the position of a reaction. It should also be noted that at zero
applied surface pressure no polymerization occurs.
Aniline and substituted anilines are known to undergo
polymerization via an oxidative coupling mechanism. It has also been
reported that dimer, trimer, oligomers, and higher polymers have lower
oxidation potentials than the monomer. So, once oligomers are formed in
the reaction medium they can oxidize faster than the monomer and the
reaction rate accelerates. Thus, when aniline is polymerized by classical
techniques, two phenomena have been observed: an induction period of the
order of 10 to 30 minutes, and then an acceleration in the overall reaction
rate. By adding catalytic amounts of dimer to a reaction medium







containing monomer, Wei and coworkers found that the polymerization
could be carried out below the oxidation potential of aniline and the
induction period decreased. 175


10 20 30 40 50
Time [min]


Figure 3-10. The fraction of reacted monomer versus time for the
monolayer polymerization of 2-pentadecylaniline at 30C
temperature and 5 mN/m applied surface pressure.


The above mentioned increase in the overall polymerization rate
from its initial value has been termed "auto-acceleration".172,174 As
shown in Figure 3-10, auto-acceleration is also seen during the Langmuir
film polymerization of 2-pentadecylaniline. In this figure the fraction of
reacted monomer is plotted against time. The curve is seen to be S-shaped
and is typical of autocatalytic reactions.189 The fraction of reacted
monomer was calculated using the following equation.


N1 (AO- A)
N (A -A A)


(3.1)







where N1 = the number of reacted monomer molecules at any time during
the reaction, N = total number of monomer molecules spread at the
interface, Ao = the initial mean molecular area, A = the mean molecular
area at any time during the reaction, and Aoo = the mean molecular area of
the product.
Equation (3.1) assumes complete conversion of monomer molecules
to the product. However, this might not be the case and it is difficult to
determine the extent of reaction in this study. Nevertheless, the shape of
the curve would remain the same in spite of incomplete conversion.
Our experimental protocol is such that it allows a continuous
measurements of the change in the area with time and thus the fraction of
reacted monomer. If the Langmuir film polymerization of 2-
pentadecylaniline is not autocatalytic then a plot of Ni/N vs. time would be
a straight line. A difference observed between the Langmuir film
polymerization method and classical aniline polymerization, however, is
that there is almost no induction period.
To summarize, polymer of 2-pentadecylaniline having a long range
order in the backbone can be obtained by polymerizing it at the air /
aqueous interface. The extent of reaction can be controlled and monitored
by measuring the change in mean molecular area and the average barrier
speed. Certain experimental signatures of aniline polymerization (e.g.
auto-acceleration effect) observed in other methods were also found in
these monolayer reactions. A smaller than normal induction period was
observed, however.







3.3.1 Calculation of the Rate Constant
Kinetic data have been used to provide much detailed insight into
reaction mechanisms. The rate of a given reaction can be determined by
following the disappearance of a reactant or the appearance of a product.
The extent of reaction is often measured spectroscopically, since
spectroscopic techniques provide a rapid, continuous means of monitoring
changes in concentration. Numerous other methods such as pH
measurement, conductance measurement etc. have been applied. In
general, any property that can be measured and related to the concentration
of the reactant or product can be used to determine a reaction rate.
In the monolayer polymerization of a substituted aniline, the
technique applied in this study, allows the continuous measurements of the
change in area and the rate of change in area with time normally known as
the average barrier speed. The maximum value of the barrier speed and
the corresponding mean molecular area were chosen to calculate the
polymerization rate and the rate constant. They were chosen because they
can be monitored continuously and reflect concentrations of the reactant
and the product during the course of the reaction. The barrier speed
maximum was used because it is reproducible and gives an estimation of
the overall reaction kinetics at the air/aqueous solution interface. Our
theoretical studies have also shown that the rate constant calculated from
the barrier speed maximum is an average value and supports the above
statement.
The rate constant was derived as follows. The relationship between
the average barrier speed (BS) and the change in the mean molecular area
with time (dA/dt) is differential and given by the following equation.






L dA1
BS = A(3.2)
Ao L dt

where L is the length of the trough and Ao is the initial mean molecular
area.

But L = N (3.3)
AO W

SBS () (dA (3.4)

where N is the number of monomer molecules spread on the sub-phase,
and W is the width of the trough.
According to Gee and Rideal the rate at which a monolayer
polymerizes at an air-aqueous acid interface can be derived as
follows.59,190,191
The fraction (4) of the original material undergoing reaction in time
T, measured from the time of half reaction, is given by

F 1]1
= + e(3.5)

where 0 is a function of temperature and pressure of the form:

0 = pm e-E/RT (3.6)

In equations (3.5), if 0 is replaced by k and T by t tl/2, where tl/2 is the
time of half reaction, we readily find:

C = kL (1 ) (3.7)






Equation (3.7) can be expressed in terms of any parameter X which varies
linearly with 4. If Ao, Aoo, and A are the values of this parameter at the
start of the reaction, at the end, and at time t, it is easily seen that:

dA k (A A)(A A.) (3.8)
dt (Ao-A.)

where k is the rate constant at a given temperature and pressure.
The following assumptions were made in order to derive equation
(3.8). Products remain at the interface during the reaction. Molecular
areas of reactants and products are additive. Coupling monomers, or
monomer to a growing polymer chain, or of one reactive chain to another
results in a constant area change. Reactant concentrations are a constant in
the vicinity of the reacting molecules.
By combining equations (3.4) and (3.8), one obtains:

BS kN [(AO A)(A A.)
W (AO A..)

1014 xWx BS (AO A) (3.10)
SN (A0 A)(A A,,.)

The units of k in equation (3.10) are found to be min-1, which
indicates that the reaction follows first order kinetics. Both chemical and
electrochemical polymerizations of aniline, however, have been found to be
first order in monomer concentration and concentration of polymer
formed. 174,176 In the monolayer polymerization though, the rate constant
is normalized by the monomer concentration and thus it follows pseudo-
first order kinetics.










"a +I.6 4g
u41.8 g 4





0

0 10 20 30
Time [min]

Figure 3-11. The average barrier speed versus time plots for the mono-
layer polymerization of 2-pentadecylaniline with different
concentrations at 250C temperature and at 30 mN/m surface
pressure.


The polymerization rate was found to be first order in the monomer
concentration and is shown in Figure 3-11. The maximum value of the
barrier speed and the corresponding mean molecular area were used to
calculate the rate constant and values obtained were 2.682x1016 and
2.326x1016, molecules-min-1 for 47.8 pg and 41.8 gg, respectively. The
ratio of the amount of monomer spread at the interface (1.14) was found
approximately the same as the ratio of the rate constants (1.15). Thus the
polymerization rate increase with the amount of monomer spread on the
sub-phase and is first order in monomer concentration. This result agrees
with the results obtained from chemical and electrochemical
polymerization of aniline. It should also be noted that the time required to
complete the reaction was found to be independent of the amount spread at






the interface. This indicates that at a given temperature and surface
pressure the amount of monomer spread at the interface only changes the
maximum value of the barrier speed and has no measurable effect on the
time required for completion of the reaction.
In summary, the equation for calculation of the rate constant was
formulated using Gee-Rideal's equation. The calculation of rate constant
requires the maximum value of the barrier speed and the corresponding
mean molecular area. Values of the rate constant were found to be highly
reproducible and typical error in rate constant measurements under given
conditions was better than 5 ppt.


3.3.2 Calculation of Activation Energies
The effect of temperature on the rate and degree of polymerization
is of prime importance in determining the manner of performing a
polymerization. Increasing the reaction temperature usually increases the
polymerization rate. In order to elucidate the effect of temperature on the
monolayer polymerization kinetics, reactions were carried out at different
temperatures and at 20 mN/m surface pressure. The results are shown in
Figure 3-12.
It can be seen from Figure 3-12 that the maximum barrier speed
increases and the time required to complete the reaction decreases with the
sub-phase temperature. The maximum value of the barrier speed and the
corresponding mean molecular area were used in equation (3.10) to
calculate the rate constant as shown in Table 3-4.








20





E 10


10 5

0


50 100


Time [min]


Figure 3-12





Table 3-4.


. Average Barrier Speed versus time during the Langmuir
film polymerization of 2-pentadecylaniline at 20 mN/m
applied surface pressure and different temperatures.



Values of areas (A2/molecule), barrier speeds at the maximum
positions (mm/min), number of molecules, and rate constants
(min-1) for polymerization reactions at different temperatures


and at ZU mN/m ap lied surface pressure.

Temp. Ao A,. AA BSm Am N k In k

10DC 47.42 24.07 23.35 1.53 36.88 8.991x1016 0.0442 -3.119

47.62 25.23 22.39 1.39 33.26 8.298x1016 0.0488 -3.020

47.83 22.85 24.98 1.51 39.14 8.298x1016 0.0482 -3.032

150C 50.38 26.63 23.75 2.29 36.13 8.991x1016 0.0670 -2.703

51.20 26.90 24.30 2.39 36.00 8.991x1016 0.0700 -2.659







Table 3-4 continued..

Temp. Ao Aoo AA BSm Am N k In k


2(0C 45.81 25.45 20.36 3.80 36.09 8.991x1016 0.1248 -2.081


47.69 25.43 22.26 3.48 39.93 8.298x1016 0.1245 -2.083


47.83 25.57 22.26 3.64 39.03 8.298x1016 0.1237 -2.090

48.37 25.57 22.80 3.64 38.91 8.298x1016 0.1189 -2.129


250C 47.62 26.92 20.70 5.67 39.37 8.298x1016 0.2066 -1.577


48.42 27.15 21.27 5.32 39.48 8.298xl016 0.1856 -1.684


48.30 27.04 21.26 5.19 39.48 8.298x1016 0.1818 -1.705


30PC 48.64 28.51 20.13 7.74 41.04 8.298x1016 0.2958 -1.218


48.08 27.94 20.14 7.78 39.14 8.298x1016 0.2829 -1.263


350C 48.08 29.19 18.96 12.35 41.04 8.298x1016 0.5074 -0.678


48.76 29.52 19.24 12.42 41.13 8.298x1016 0.4876 -0.718


48.19 29.07 19.12 12.35 40.95 8.298x1016 0.4963 -0.701


40(C 45.59 27.38 18.21 15.84 37.96 9.520x1016 0.5630 -0.574


46.15 27.83 18.32 16.47 38.14 9.520x1016 0.5757 -0.552


46.83 28.28 18.55 18.01 39.14 9.520x1016 0.6303 -0.462


46.04 27.94 18.10 19.46 38.60 9.806x1016 0.6793 -0.387






According to the Arrhenius law, a rate constant can be expressed as


In k = In A E
(RT)


(3.11)


where k = is the rate constant, A = is the collision frequency factor, Ea =
the activation energy, R = the gas constant, and T = is the Kelvin
temperature. A plot of In k versus 1/T allows the determination of both A
and Ea from the intercept and slope, respectively. The plot of In k vs. T-1
(K-1) for the Langmuir polymerization of 2-pentadecylaniline is shown in
Figure 3-13.


-4 -1-
3.15


3.25 3.35 3.45
l/T (1/K) x 1000


3.55


Figure 3-13.


In k Vs 1/T (1/K) plots for monolayer polymerizations at 20
mN/m applied surface pressure. A linear regression method
was used to draw a straight line from experimental data.


This plot is linear, indicating that the reaction obeys the Arrhenius
law. The activation energy was calculated from the slope according to


y = 24.93 7.931x R = 0.9973







equation (3.11). The value of activation energy at 20 mN/m surface
pressure was 66 kJ/mol ( 2 kJ/mol).
This calculated activation energy is reasonable and falls within the
range obtained by other research groups for the polymerization of
unsubstituted aniline. The activation energy for aniline polymerized
electrochemically and chemically has been found to be 64 and 87 kJ/mol,
respectively.162,174 However, the monomer used in this study was ortho
substituted with an alkyl side chain and thus chemically different. Previous
studies have also shown that the monomer film does not polymerize
measurably without applied surface pressure.165 Furthermore the
Langmuir polymerization of ortho substituted anilines is independent of the
alkyl side chain length in the range of thirteen to seventeen carbon atoms at
applied surface pressures of 10, 20, and 30 mN/m. Thus the steric bulk of
the side chain does not appear to affect the monomer reactivity in the
monolayer in this case. This can be understood as follows. As discussed in
the isotherm section above, for Langmuir monolayers, the applied surface
pressure dehydrates large portions of the alkyl side chain, therefore
removing it from the vicinity of the reaction center. As such, activation
energies similar to unsubstituted anilines are not surprising.
Besides the property of concentrating reactants, another general
effect of the interface that influences reaction rates is that of orienting
molecules. Based on this, one might anticipate a lower activation energy
for aniline monolayer polymerization as compared with a solution since the
monomer is essentially pre-oriented before reaction. This might favor
lower energy pathways to the activated complex. In fact, the
polymerization is completed faster in the monolayers than in solution,
while the activation energies are similar. However, large changes in






reaction rates may also occur because of differences in the pre-exponential
factor. These differences result from losses in degrees of freedom for
reactants and activated complexes in the monolayer compared with the
solution, or from changes in collision frequency. The reactants and
activated complex in the monolayer have one degree of translation freedom
and two degrees of rotational freedom, less than in bulk or solution. Thus
the pre-exponential term is larger for a reaction in two dimensions than for
a reaction in three dimensions as discussed below.192
In order for reaction to occur, reactant molecules need to pass
through a state known as the activated complex (or transition state), which
then decomposes spontaneously to form the reaction products. An
equilibrium is assumed between the reactants and the activated complex.
The reaction rate depends on the concentration of activated complex and
the rate at which it passes through this state. The general expression for
the rate of any chemical reaction is given asl92

Rate = (CACB ***) ( ) QAB (-AE/RT (3.12)
h QAQBV

where C = is the concentration, K = Boltzmann's constant, h = Planck's
constant, Q = the partition function.
Now, let us compare the rates of a bimolecular reaction in bulk
solution with the rates of the same reaction at an interface on the basis of
the above equation. It is assumed that the activation energy is the same for
the bulk reaction as the reaction at the interface. The reaction to be
considered is of the type


A + B <-> AB* -4 C


(3.13)






Let us suppose that the solution contains 1018 molecules/cm3 of both
A and B. The concentrations at the interface are also equal (101 4
molecules/cm2). It will be assumed that the partition functions for one
degree of translation (Qt) and one degree of rotation (Qr) have values of
109 and 10, respectively.192 The rates at the interface (ratei) and in bulk
(rateb) in terms of the pre-exponential factor of the above equation are
given by

K(T Q2Qr
Ratei = (CACB) L (3.14)
Q tQrtr

= (KT x 1014 x 1014 x 10-19 (3.15)
t ( 333
Rateb = (CACB) (- 3h (3.16)
t -r 3,t r

= ( x 1018 x 1018 x 10-30 (3.17)


We now compare the absolute rates for two systems that contain the
same number molecules. The number of molecules in 1 cm2 of interface
(1014) would occupy a volume of 1 cm2 x 10-4 cm. Therefore,

Ratei- [ 1014 x 1014 10301
Rateb 10-4 x 1018 x 1018 1019

= 107

This large increase in the pre-exponential term causes the observed
rate increase according to equation (3.11), since the activation energies for
both cases are similar. Furthermore, systematic studies of other reactions







in Langmuir monolayers of non-polymeric compounds have not shown
differences in activation energies between the monolayer and the bulk.193
The large increase in the monolayer pre-exponential factor may be a
reason for no induction period for the monolayer polymerization reaction.



-0.75

-1 U

W. -1.25

S-1.5 -

-1.75

-2 -
0 10 20 30 40 50
Temperature [C]

Figure 3-14. Log polymerization rate (PR) versus temperature (C) for
monolayer polymerization reactions at 20 mN/m applied
surface pressure.


Figure 3-14 shows a plot of log polymerization rate (PR) versus
temperature [C] for the reaction at 20 mN/m applied surface pressure.
Polymerization rate was calculated from the maximum barrier speeds using
the following equation. 163


PR 1014 x W x BS (3.19)
R = N x AA


where AA is the area change during the reaction in A2/molecule.







The units of PR are found to be min-1. The polymerization rate
increases monotonically with temperature. such behavior is in agreement
with the electrochemical results of Wei et al., and differs from that
predicted by Contractor et al. 174,194
Langmuir film polymerization is one of the few techniques that
allows the distance between reacting molecules to be varied while
maintaining anisotropic orientations. In fact, it can be seen from Figure
3-1 that by changing the applied surface pressure between 1 and 35 mN/m,
the monomer surface concentration is varied between 0.014 and 0.027
molecules/A2 on a 0.1M sulfuric acid sub-phase at 250C. Such
concentration changes can not be achieved easily by varying the pressure in
a three dimensional solution. From Figure 3-1, it can be seen that the
surface concentration can also be varied, under isobaric conditions, by
changing the sub-phase acidity (0.019 and 0.027 molecule/A2 at 10 mN/m
surface pressure on 0.1M sulfuric acid and water at 25C, respectively). In
this study the monomer surface concentration was varied by changing the
surface pressure. To measure the effect of the pressure (or surface
concentration) on the activation energy, the polymerization of 2-
pentadecylaniline was carried out at 10 mN/m, a lower pressure, as
described above.
Figure 3-15 shows the Langmuir film polymerization results for 2-
pentadecylaniline at different temperatures and at 10 mN/m applied surface
pressure. The barrier speed vs. time shows a trend similar to the
monolayer polymerization at 20 mN/m surface pressure. Equations (3.19)
and (3.10) were used to calculate polymerization rates and rate constants
and the results are shown in Tables 3-5 and 3-6.










12
CO
3 9




S3

0


20 40 60


80 100


Time [min]

Figure 3-15. Average barrier speed Vs time during the Langmuir film
polymerization of 2-pentadecylaniline at 10 mN/m applied
surface pressure and different temperatures.


Table 3-5.


Values of areas (A2/molecule), barrier speeds at the maximum
positions (mm/min), and rate constants (min-1) for the
reactions at different temperatures and at 10 mN/m applied


surface pressure.

Temp. Ao Ao AA Am BSm N k

150C 53.37 25.68 27.69 36.67 2.97 8.5113x1016 0.0788

53.91 26.63 27.28 35.86 2.88 8.5113x1016 0.0829

53.78 27.04 26.74 37.76 2.86 8.5113x1016 0.0783

200C 54.03 27.78 26.25 41.00 3.69 8.5113x1016 0.0989

53.64 27.44 26.20 38.44 3.85 8.5113x1016 0.1061








Table 3-5. continued..

Temp. Ao Aoo AA Am BSm N k

20SC 54.46 28.39 26.07 36.27 3.69 8.5113x1016 0.1179


53.37 27.85 25.52 36.81 3.69 8.5113x1016 0.1116


250C 54.05 29.21 24.84 40.61 5.24 8.5113x1016 0.1493

54.04 29.21 24.83 41.83 5.40 8.5113x1016 0.1530


53.91 29.21 24.70 42.51 5.29 8.5113x1016 0.1515


54.05 28.94 25.11 42.78 5.24 8.5113x1016 0.1483


30YC 54.43 30.57 23.86 42.10 7.31 7.9439x1016 0.2310

54.55 30.57 23.98 44.37 7.80 8.5113x1016 0.2341


54.57 30.41 24.16 44.66 7.86 8.5113x1016 0.2364


350C 55.11 32.31 22.80 43.44 10.59 8.5113x1016 0.3267


54.71 31.90 22.81 44.39 10.45 8.5113x1016 0.3251


54.71 31.63 23.08 43.17 10.40 8.5113x1016 0.3169


54.84 31.90 22.94 45.20 10.37 8.5113x1016 0.3262


40'C 54.32 32.26 22.06 44.48 12.34 7.9439x1016 0.4263


53.87 32.94 20.93 44.14 13.27 8.5113x1016 0.4481








Table 3-5. continued..

Temp. Ao A. AA Am BSm N k

4(PC 54.66 33.17 21.49 44.37 11.62 7.9439x1016 0.4080

54.10 32.83 21.27 46.86 13.07 8.5113x1016 0.4811


Table 3-6.


Values of polymerization rates (PR) and rate constants for
monolayer polymerization reactions at 10 mN/m applied
us rface pressure and at d different temperatures.


It was observed that the polymerization rate increases with
temperature. A plot of In k vs. T-l[K-1] for the polymerization of 2-
pentadecylaniline at 10 mN/m surface pressure is also a straight line giving
an activation energy of 52.6 kJ/mol (1.7 kJ/mol) according to equation
(3.11). The plot is shown in Figure 3-16.
The activation energy thus decreased by = 13 kJ/mol when the
applied surface pressure was reduced by 10 mN/m (or the surface


Temp. [C] PR [min-1] 1/T [K-l] In k

15 0.0188 0.00347 -2.544

20 0.0253 0.00341 -2.249

25 0.0375 0.00336 -1.894

30 0.0576 0.00330 -1.453

35 0.0804 0.00325 -1.121

40 0.1069 0.00319 -0.850







concentration reduced by a factor of 0.85). Reductions in activation
energies with decrease in interfacial pressure have also been reported for
other reactions, though their interpretation remains unclear.193,195 In
this case three possible explanations can be given as follows.



-0.70
y = 19.41 6.33x R = 0.9980

-1.40


-2.10



-2.80
3.15 3.25 3.35 3.45
l/T [1/K] x 1000

Figure 3-16. In k Vs 1/T (1/K) plots for monolayer polymerizations at 10
mN/m applied surface pressure. A linear regression method
was used to draw a straight line from experimental data.


Figure 3-1 shows that decreasing surface pressure at a given
temperature increases the average distance between reacting molecules (For
example, surface areas at 250C. are 37.04 and 71.43 A2/molecule at 35 and
1 mN/m surface pressure, respectively). Thus molecules have more
freedom to change their orientation at lower pressures, and they can
reorient to lower energy conformations not possible at higher pressures.
Second, an increase in the average distance between reacting molecules
increases the probability of a persulfate ion diffusing to the interface and
thus being able to oxidize monomer or growing polymer chain ends.







Third, changes in the surface pressure may change the nature of the
hydration shell around a monomer or growing chain end, thus modifying
the reaction pathway by effectively changing the nature of the "solvent"
around the aniline head group.
A decrease in the activation energy generally results in an increased
reaction rate. In this study, however, the opposite effect is observed; the
polymerization rate was found to increase with an increase in the applied
surface pressure at a given temperature. This indicates that the
polymerization rate is dominated by the pre-exponential term in equation
(3.11) and not by the activation energy.
The pre-exponential term has two components: a steric factor (r),
and a collision frequency. The steric factor represents the fraction of the
total number of collisions that are effective from the orientation point of
view. This term has a value of one if all collisions lead to the reaction. A
value less than one reflects the property that, in bimolecular reactions,
colliding molecules having the energy necessary for activation do not
necessarily react unless suitably oriented.
At low surface pressure, molecules at the interface have more
freedom to reorient to a sterically favorable position. This change in the
steric factor should lead to an increase in the reaction rate. Yet, the overall
reaction rate was found to increase with surface pressure, even though the
activation energy increased. This indicates that the collision frequency is
the dominant component in the pre-exponential term and the governing
factor in determining the reaction rate.
An increase in the polymerization rate with the pressure can be
explained in terms of the collision frequency as follows. The collision
frequency depends upon the population of molecules in a given area. The







increase in the surface pressure at a given temperature increases the
number of molecules per area, and thus the collision frequency. Pre-
exponential terms for the monolayer polymerization of 2-pentadecylaniline
at 20 and 10 mN/m surface pressures were calculated from the intercepts
of In k vs. T-1 (K-1) plots. Values of the pre-exponential term for
reactions at 20 and 10 mN/m pressures were 6.5 x 1010 and 2.7 x 108,
respectively. For a gas phase reaction the pre-exponential term is used to
measure the collision frequencies. In this study, the monolayer
polymerization reaction occurs in a liquid expanded state, not a gas. For
such a case, the pre-exponential term can not be used for a quantitative
measurement of the collision frequency. Nonetheless, qualitative
comparison can be made. For the monolayer polymerization at 20 mN/m,
the pre-exponential term is 240 times greater than the reaction at 10 mN/m
surface pressure. This argument can be used as an explanation for the
increase in reaction rate with surface pressure at a given temperature.
Our new theoretical model shows that (see Appendix B) values of
activation energies reported above are the average values of Eai + Eap.
Eai is the activation energy for the initiation step and Eap is the activation
energy for propagation steps. Activation energies obtained from a new
theory and experimental data are shown in Table 3-7.
The results in Table 3-7 indicate that activation energies calculated
using the modified Gee and Rideal equation were average values. A
striking feature, however, is the pressure dependence of the two activation
energies. The activation energy for initiation actually decreases slightly
with pressure, while that for propagation increases substantially. Since, at
a given pressure, persulfate diffusion is likely to be similar near either a
propagating chain end or two colliding monomers, it is not likely to be the






major cause of the pressure dependence. The number of conformations a
chain end can assume, however, is expected to be much different than that
for a protonated monomer molecule. The hydration shell around a
growing chain end may also be different from that of a protonated
monomer. The above considerations suggest that the applied surface
pressure primarily affects collisions between monomers and chain ends, by
changing the hydration shells and/or their conformations. This
interpretation also explains why no reaction is observed at zero pressure as
the Eai can be too high.



Table 3-7. A Comparison of activation energies obtained from a new
theoretical model and experimental data.

Surface Pressure Eai Eap Average Ea Ea (using Gee
[mN/m] [kJ/mol] [kJ/mol] [kJ/mol] Rideal eq.)
[kJ/mol]

10 97 9 53 53 1.5

20 84 50 67 66 3


The rationale presented above that the pre-exponential factor
dominates the reaction rate was further checked by carrying out the
reaction at an intermediate surface pressure, 15 mN/m. Results are shown
in the following Figure 3-17.
The activation energy was calculated from the slope using equation
(3.11) and found to be 57 (2) kJ/mol. The value of pre-exponential factor
was also calculated to be 2.1 x 109. These values fall in between the values







obtained for the reaction at 10 and 20 mN/m surface pressures and they are
summarized in Table 3-8. Thus these results provide further support for
the argument presented in the above paragraph.


-1.1 -



-1.8


-2.5 1
3.2000


3.3000 3.4000


3.5000


l/T [1/K] x 1000


Figure 3-17





Table 3-8.


. In k Vs 1/T (1/K) plots for monolayer polymerizations at 15
mN/m applied surface pressure. A linear regression method
was used to draw a straight line from experimental data.



Values of activation energies and pre-exponential factors for
monolayer polymerizations of 2-pentadecylaniline at different
arolied surface pressures.


M, y = 21.46 6.88x R = 0.9973


Surface Pressure Ea Pre-exponential
[mN/m] [kJ/mol]

10 53 2.7 x 108

15 57 2.1 x 109

20 66 6.5 x 1010







The polymerization reaction was also carried out at 30 mN/m, a
higher surface pressure. Results showed that the polymerization rate
increases with temperature as was observed at other surface pressures.
Values of rate constants and polymerization rates at different temperatures
for the reaction at 30 mN/m are shown in the following Table 3-9.



Table 3-9. Values of polymerization rates (PR) and rate constants for
monolayer polymerizations of 2-pentadecylaniline at 30 mN/m
applied surface pressure and different temperatures.
Temperature Polymerization Rate In k
[q [min-1]
10 -1.585 -2.0581
15 -1.456 -1.7784
20 -1.276 -1.4389
25 -1.092 -1.0413
30 -0.921 -0.6044


A plot of In k vs. T-1 [K-1] for the reactions at 30 mN/m surface
pressure is shown in Figure 3-18. It did not give a straight line indicating
that the reaction at 30 mN/m surface pressure did not follow the Arrhenius
law. Instead, a concave upward deviation from the straight line was
observed. This indicates that at 30 mN/m the activation energy and the
pre-exponential factor are temperature dependent and these values can not
be directly compared with those at other conditions.
Another probable reason for the observed deviation from Arrhenius
plot might be a solubility of the monomer into the sub-phase. Isobaric
stabilities results of the monomer showed that the monolayer films are not







stable at a higher temperature and surface pressure. One could eventually
try and estimate a solubility correction, but it will be difficult since the
monomer, oligomers and a polymer have different isobaric stabilities and
their populations are not explicitly known.


-0.70-


-1.20-


-1.70-


-2.20 -


Figure 3-18.


3.25 3.35 3.45 3.55
l/T [l/K] x 1000

In k versus 1/T (1/K) plot for monolayer polymerizations of
2-pentadecylaniline at 30 mN/m applied surface pressure. A
curve fitting programme was used to draw a line from
experimental data.


In summary, activation energies at different applied surface
pressures were measured. The value of the activation energy increased
with increasing applied surface pressures. Also the polymerization rate
was increased with the applied surface pressure at a given temperature.
This indicates that the polymerization rate of 2-pentadecylaniline
monolayer was dominated by the pre-exponential term in Arrhenius
equation and not by the activation energy term.







3.3.3 Activation Enthalpies and Entropies
From the activation energy, other thermodynamic parameters such
as activation enthalpy (A*Ho), activation entropy (A*So), and the Gibbs free
energy of activation (A*Go) can be calculated. The magnitude of A*H and
A*So reflect transition state structure. Atomic positions in the transition
state do not correspond to their equilibrium position in the ground
state.196 The result is a higher internal energy of the activated complex
than of the reactants, and this higher energy is reflected in the enthalpy of
activation. The entropy of activation is a measure of the degree of order
or disorder produced in formation of the activated complex. If any
degrees of freedom translationall, rotational, or vibrational) are lost in
going to the transition state, there will be a decrease in the total entropy of
the system. Conversely, the gaining of translational, vibrational, or
rotational degrees of freedom is associated with a positive entropy of
activation.
Of the two principle activation parameters, enthalpy and entropy, it
is the entropy that can be used as a probe of important mechanistic
details.197 Enthalpies and entropies of activation are determined in
completely different ways, although the entropy of activation is calculated
only after a value of the activation enthalpy is known. Whereas activation
enthalpy is calculated from the temperature dependence of the reaction
rate, the entropy is deduced from the absolute value of the rate constant
once the enthalpy is known.
The Arrhenius theory leads to a considerable improvement in our
understanding of the reaction process, but it is still a very qualitative
theory in that it does not show how the pre-exponential factor depends on
the molecular properties of the reaction system. The transition-state theory






focuses attention on the species in the reaction process that corresponds to
the maximum-energy stage in the reaction. In this theory, this species,
called the "activated complex", or "transition state", is treated formally as a
molecule in spite of its ill-defined nature and transitory existence. More
specifically, the theory assumes that this species can be treated as a
thermodynamic entity.
Let us consider a bimolecular elementary reaction that leads to
products. According to the transition state theory, reactants A and B
establish an equilibrium with the activated complex and than this activated
complex decomposes further to form products. Thus

A + B c ) (AB)* products (3.20)

The rate of reaction depends on two factors: the concentration of the
activated complex and the rate with which it breaks up to give products.
The following relationship has been established between the rate and
equilibrium (kc*) constants for the formation of an activated complex from
reactants. 198

k = kc (3.21)


This expression becomes of value when kc* is given a thermodynamic
interpretation. If kc* is expressed in terms of A*Go, the change in
standard Gibbs energy when the activated complex are formed from the
reactants, equation (3.21) becomes

k = T) e-*GO/RT (3.22)
h






The quantity A*GO is known as the standard Gibbs energy of activation.
For a reaction at a given temperature the free energy of activation can be
interpreted in terms of an enthalpy of activation and an entropy of
activation as written below.

A*G = A*H TA*SO (3.23)

where A*H is the standard enthalpy of activation and A*SO is the standard
entropy of activation. The transition-state theory interpretation of the rate
constant is now obtained by substituting the value of A*Go in equation
(3.22). Thus

k = [( e-*So/R e-AHo/RT (3.24)
h

With the recognition that the temperature variation of a rate constant
depends primarily on the exponential term of equation (3.24), this equation
agrees in form with the empirical Arrhenius expression. It will be useful
to recast equation (3.24) into a form that involves the Arrhenius activation
energy and derive the relationship between the activation energy and the
enthalpy of activation. The Arrhenius activation energy can be expressed
as198

dln k) Ea
dT ) RT2 (3.25)

Since kc* is a concentration equilibrium constant, its variation with
temperature is given by the equation

dln kc A* Uo
dn = RT2 (3.26)
dT I-RT2






where A*U0 is the standard change in the internal energy in passing from
the initial to the activated state. Differentiation of the logarithmic form of
equation (3.21) gives

(dlnkk d(In T + Ink)
C dT L dT
JT (3.27)

Cdlnk (1> dln k
..9- 1 + r (3.28)
dT T dT )

On substituting the value of kc* from equation (3.26), equation (3.28)
becomes

ddin k) I + A*U = RT(3.29)
dT T RT2 RT2

Comparison of equations (3.29) and (3.25) gives

Ea = RT + A*Uo (3.30)

Since the enthalpy is defined as U + PV, the general relationship
between A*H and A*U0 is, for a process at constant pressure,

A*Ho = A*Uo + PA*Vo (3.31)

The term A*V0, is the change in volume in going from the initial to the
activated state and is known as the standard volume of activation. Thus the
relationship between the activation energy and the standard enthalpy of
activation can be obtained by comparing equations (3.30) and (3.31).


Ea = A*H PA*V + RT


(3.32)