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This is to certify that the
thesis entitled

SYNTHESIS OF LLTRATHIN PllYlVER FILMS BY SURFACE-INITIATED RADICN.
POLYlVERIZATION FRGVI GOLD SURFACES

presented by

SKANTH GANESAN

has been accepted towards fulﬁllment
of the requirements for

IVS Cl-EMISTRY

degree in

 

 

 

Major professor

Date 08/09/01

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

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SYNTHESIS OF ULTRATHIN POLYMER FILMS BY SURFACE-INITIATED
RADICAL POLYMERIZATION FROM GOLD SURFACES

BY

SKANTH GANESAN

AN ABSTRACT OF A THESIS

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Chemistry

2001

Professor Merlin L. Bruening

ABSTRACT

SYNTHESIS OF ULTRATHIN POLYMER FILMS BY SURFACE-INITIATED
RADICAL POLYMERIZATION FROM GOLD SURFACES

By

Skanth Ganesan

This thesis describes a new method for the controlled synthesis of
ultrathin polymer ﬁlms (<1000 A) by grafting from a gold substrate. Such ﬁlms
could eventually be used in sensing, separations, and other application requiring
surface modiﬁcations. Initial attempts to grow polymer ﬁlms using initiators
attached to alkanethiol monolayers were unsuccessful and yielded coatings that
were less than 100 A thick. External reﬂection infrared spectroscopy studies
showed that grafting of polymers to alkanethiol monolayers is likely restricted by
the instability of these monolayers at high temperature and in the presence of
radicals. Problems associated with the stability of SAMs can be overcome with
the use of mercaptopropyltrimethoxysilane (MPS) as an adhesion layer for
initiator attachment. Attachment of the azo-initiator to a crosslinked MPS layer
affords stabilized initiating sites on the surface, allowing successful grafting of
polymer ﬁlms. FTIR spectra and ellipsometry demonstrate the successful grafting
of polystyrene ﬁlms with thicknesses up to 900 A. In a second method,
attachment of atom transfer radical polymerization initiators to layered

polyelectrolyte ﬁlms also allows grafting of polymer ﬁlms.

6166f QJmCé’mrrr'r Lmbgub (godTGwrﬁmélgg amiﬁuarrtb

ACKNOWLEDGMENTS

First and foremost, I would like to thank my family for their blessings and
prayers, and God, for providing determination and success.

I am extremely grateful to my advisors: Dr. Merlin Bruening and Dr. Greg
Baker, for providing an opportunity to work with them and expand my knowledge
during the course of the program. I would like to acknowledge my committee
member: Dr. Simon Garrett, for careful reading of thesis and providing valuable
insight.

Special appreciation to my group members: Anagi, Bo, Dan, Jacque
Jeremy, Jinhua, JB Kim, Kangping, Milind, Sandra, Wenxi and Yingda. It was
wonderful to work with you all. I want to specially thank Dr. Wenxi Huang for his
major help in the success of this project. My friends: Sridhar, Ramabhadran,
Sudar, Srivats, Sunder, Srini, Kamlesh, Abhi, Mahesh, Param, RG lyer, Mukund,
Ustaad, Muthu 8. family, Kasthuri & family for making my stay in Michigan State,
enjoyable and memorable.

My dream of graduate studies in the USA would have been impossible
without the help of Renga and his family. I am extremely grateful to them.

I want to ﬁnally thank the Department of Chemistry, Michigan State
University and the NSF Center for Sensor Materials for giving me a chance to do

graduate research and for funding this project.

TABLE OF CONTENTS

LIST OF TABLES .............................................................................. vii
LIST OF FIGURES ............................................................................ viii
LIST OF ABBREVIATIONS .................................................................. ix
INTRODUCTION ................................................................................ 1
CHAPTER 1
BACKGROUND .................................................................................. 4
1.1 Grafting Techniques .............................................................. 4
1.1.1 “Grafting To” Technique ............................................. 6
1.1.2 “Grafting From” Technique ......................................... 6
1.2 Self-Assembled Monolayers ................................................... 7
1.3 Molecular Imprinting ............................................................. 9
CHAPTER 2
EXPERIMENTAL ............................................................................... 12
2.1 Ellipsometry ........................................................................ 12
2.2 Infrared Spectroscopy .......................................................... 13
2.3 Experimental Materials ......................................................... 14
2.4 Synthesis of a Trimethoxysilane-Substituted Azo lnitiator.............15
2.5 Preparation and Cleaning of Substrates ................................... 16
2.6 Formation of MUA and MUD Monolayers on Au Substrates .......... 16
2.7 Formation of Trimethoxysilane-Substituted Azo-lnitiator Layers on
Alumina Substrates .............................................................. 16
2.8 Formation of Initiator Monolayers on Various Substrates .............. 17
2.9 Grafting of Polystyrene onto Gold Substrates ............................. 17
CHAPTER 3
POLYMERIZATION OF STYRENE FROM GOLD ..................................... 21
3.1 Results and Discussion .......................................................... 22
3.1.1 Stability of SAMS under Polymerization Conditions ........ 22
3.1.2 Stability of SAMS in the Presence of Radicals ............... 25
3.1.3 Effect of Thiols on Polymerization ............................... 26
3.1.4 Preparation of Cross-Linked Azo-lnitiator Layers on Gold
Surfaces ................................................................ 30

CHAPTER 4

POLYMERIZATION ON ALUMINA MEMBRANES .................................... 35

4.1 Experimental ....................................................................... 36

4.1.1 Deposition of Polyelectrolyte ..................................... 36

4.1.2 Anchoring of Initiator ................................................ 36

4.1.3 Polymerization Procedure ......................................... 38

4.2 Results and Discussion .......................................................... 38
CHAPTER 5

CONCLUSIONS ................................................................................ 42

BIBLIOGRAPHY ................................................................................ 43

vi

LIST OF TABLES

3.1 Thickness of surface-grafted polystyrene layers on Al when polymerized
in the presence and absence of alkanethiols ...................................... 27

vii

1.1
1.2
2.1
2.2
3.1

3.2

3.3

3.4

3.5

4.1

4.2

LIST OF FIGURES

Two methods for grafting of polymer ﬁlms to solid surfaces

(335%; but of. 'mgl'ééui .a.r .i mp" nt. ng o f po|ymer s ....................................... 5
massage}; 22°68;in forinit-iation I)? Bai'y'r'riéii'z'étiah """""""""""""""" 10
gig... lavage; iii gears; Baggage 'irB'm" 556:5 'éijér'é'éé """""""""""" 18
FTIR .s. 555i}; .of. .5. MUA; mono layer ................................................... 22 :

Integrated absorbance of the carbonyl peak in reﬂectance FTIR spectra of
MUA monolayers vs time of exposure to toluene at 60 °C (open squares),

6 mM AIBN in toluene at 60 °C (ﬁlled squares), toluene at room temperature
under UV light (open triangles), and 6 mM AIBN in toluene at room
temperature under UV light (ﬁlled triangles)

................................................................................................ 24
Fate of surface bound radicals on Au: thermal desorption of surface
bound radicals and desorption induced by reaction of a radical with an
Au-S bond (Top), reaction with a chain transfer agent (Bottom left),
initiation of polymerization (Bottom right)
................................................................................................ 29

Reﬂectance FTIR spectra of (a) a MPS monolayer, (b) a hydrolyzed MPS
monolayer, (c) a hydrolyzed MPS monolayer modiﬁed with the azo-initiator,
CH3C(CH3)(CN)N=NC(CH3)(CN)CHzCHzCONHCHzCHzCHzSi(OCH3)3

and (d) a grafted polystyrene layer
................................................................................................ 33
Thicknesses of polystyrene ﬁlms on modiﬁed Au surfaces as a function

of reaction time
................................................................................................ 34
Steps involved in grafting poly (EGDMA) from a gold surface coated with a
layered polyelectrolyte ﬁlm
................................................................................................ 37
Reﬂectance FTIR spectra of (a) a 4.5—bilayer PAH/PSS ﬁlm, (b) a 4.5-bilayer
PAH/PSS ﬁlm reacted with 2-bromopropionylbromide and, (c) a grafted poly
(EGDMA) ﬁlm on a 4.5—bilayer PAH/PSS ﬁlm on gold. The inset shows the
increase in amide peaks before and after the initator attachment
................................................................................................ 40

viii

LIST OF ABBREVIATIONS

SAM Self-Assembled Monolayer

MIP Molecularly Imprinted Polymer

INTRODUCTION

The control of surface properties is vital to numerous technologies ranging
from biotechnology to advanced microelectronics.“2 Attachment of polymeric
brushes to substrates is one of the most active areas of research aimed at
controlling surface properties. Traditionally, polymeric brushes are prepared from
block copolymers where one block is strongly adsorbed to the surface with the
other block forming the brush layer.3 However, the brush is generally non-
covalently attached to the substrate, and desorption can occur. Additionally, the
choice of functional groups in block copolymer structures is limited due to the
demands of block copolymer synthesis. To overcome these problems, an
increasing amount of interest has been devoted to the covalent attachment of
polymer chains to surfaces.3 These tethered polymer chains on surfaces have
potential applications in chemical separations, sensing, stabilization of colloidal
suspensions, control of wetting and adhesion, corrosion resistance,
microelectronics, biocompatibility and fouling resistance."4

The most common target in research with polymeric brushes has been the
design of surfaces that resist biofouling and protein adsorption. Prevention of
protein adsorption is crucial to blood-contacting devices such as dialyzers,
oxygenators, and blood containers. When a material surface is brought into
contact with blood, adsorption of serum proteins initially takes place, and this
subsequently triggers thrombogenesis. Since these unfavorable foreign-body

responses can lead to serious clinical problems, various water-soluble polymeric

brushes have been used to create surfaces that resist fouling. The grafted
surface is not stable for long periods, but it lasts long enough to accomplish the
speciﬁc purpose of some devices. Anti-fouling brushes will be more valuable if
they can last for longer times in situations such as permanent implantation. Such
hydrophilic polymer coatings can be grafted to surfaCes by various methods
including surface graft polymerization, coupling reactions, and surface
segregation.5

Another possible application of ultrathin polymer ﬁlms is the development
of chemical sensors. Imprinting of polymers provides one means of introducing
selectivity into such systems. One of the major problems with bulk imprinted
polymers however, is the longer time required for equilibration with analyte
molecules, and hence a long response time. Growth of ultrathin imprinted
polymer ﬁlms may circumvent these problems by reducing diffusion distances.
These ultrathin polymer ﬁlms could show both high selectivity and short response
times. Some of the possible uses of molecularly imprinted sensor layers are the
detection of polycyclic aromatic hydrocarbons in water,6 ﬁber-optic detection of
ﬂuorescently labeled amino acids,7 and optochemical sensing.8

The main focus of this thesis is development of a method for preparing
ultrathin polymer ﬁlms by thermal initiation at a gold substrate. Such a method
might potentially prove useful in synthesizing ultrathin imprinted polymer ﬁlms if
polymerization in solution could be minimized. Gold is a very convenient

substrate because initiators can be attached to this surface via Au-thiol bonds.

Gold is also inert towards most organic functional groups and is convenient for
electrochemical, ellipsometric, and reﬂectance FTIR spectroscopy studies.

Two general types of grafting methods are used for covalently attaching
polymer chains to surfaces. The “grafting to” technique, which involves reacting
end-functionalized polymers with surface sites, leads to thinner ﬁlms because
reactive groups cannot reach the surface after attachment of a few chains. In
contrast, the “grafting from” technique, which involves growing polymers from
surface-immobilized initiators, leads to thicker polymer ﬁlms with higher graft
densities. Though extensive studies have been performed on polymer grafting
from surfaces like silica10 and carbon black,11 I have found no studies on thermal
polymerization from gold surfaces. The purpose of this research is to grow
polymer ﬁlms from gold substrates using surface bound initiators and to deﬁne
suitable conditions and strategies for this polymerization.

The background for these studies is outlined in chapter 1, and chapter 2
describes experimental techniques and materials used for polymerization.
Chapter 3 deals with the stability of self-assembled monolayers under
polymerization conditions and the use of a silane layer to stabilize the initiator
layer and permit polymerization from this surface. Current work on polymerization
on alumina membranes using atom transfer radical polymerization is discussed

in chapter 4. Lastly, chapter 5 presents the conclusions of this work.

Chapter 1

BACKGROUND

This chapter ﬁrst describes previous methods for grafting polymers to
surfaces to put the present work in context. Because self-assembled monolayer
(SAMs) are attractive for attachment of initiators to gold surfaces, l next include a
section on these materials. Finally, I discuss molecular imprinting as this is one of

our long-term goals for developing ultrathin ﬁlms.

1.1 Grafting Techniques

Any method for preparing thin polymer ﬁlms should ideally fulﬁll several
criteria. First the technique should allow synthesis of ﬁlms with a wide range of
thicknesses. Second the method should be sufﬁciently versatile to permit growth
of ﬁlms with a large variety of functional groups. Finally, ﬁlms should be
sufﬁciently robust for a variety of applications. Physisorption of either block-
copolymers or polymers containing a short segment that interacts with the
surface12 produces only weakly adsorbed ﬁlms. In most cases such ﬁlms are held
on the surface by van der Waals forces or hydrogen bonding. These polymer
ﬁlms desorb easily upon exposure to a good solvent and can be displaced by
other polymers. To overcome these problems, stronger adhesion between
substrate and polymer is required. This can be achieved by covalently binding
the polymer to the surface. Two broadly deﬁned methods are commonly used for

this purpose: the “Grafting to” and “Grafting from” techniques (Fig 1.1).13

5! ‘ll
(3} (I)

It???

' grafting to ’ ' grafting from ’

 

 

77/

Figure 1.1: Two methods for grafting of polymer ﬁlms onto solid surfaces

1.1.1 “ Grafting To ” Technique

The “grafting to" method involves the reaction of end-functionalized
polymers with appropriate surface sites.‘°"1"”18 The reactive sites on the substrate
serve as anchors and form bonds with the functional end groups on the polymer.
The problem with this technique is that once the surface becomes covered with
attached chains, additional chains have to diffuse through the grafted ﬁlm to
reach reactive sites. As the grafting density increases, chains have to
increasingly stretch from their random coil conﬁrmation to allow further grafting,
and this results in a decrease in the grafting rate.19 Using this method, only very
small amounts of polymer, less than 5 mg/mz, can be grafted to the surface.

This usually limits ﬁlm thickness to 10 - 50 A.

1.1.2 “ Grafting From ” Technique

A more promising approach for the preparation of ultrathin ﬁlms is the
“grafting from” technique. In this strategy, a surface-anchored reactive group is
capable of initiating polymerization, and the polymer chain grows from the
surface. There is little steric hindrance to ﬁlm formation as only a small monomer
has to reach the growing chain ends during ﬁlm growth. This increases the
number of reaction sites to which the monomer can attach when the substrate is
placed in solution. This approach leads to higher grafting densities and ﬁlm
thicknesses than the “grafting to” technique. The polymerization can be carried
out with a variety of monomers20 and thicknesses of polymer ﬁlms can be
adjusted over a wide range of values. Although dense polymer brushes can be

prepared with this method, there are disadvantages associated with the

technique. Poor immobilization of initiator on the surface can in some instances
lead to low densities of initiators and polymers. Side reactions are also possible
in the initiator immobilization step, and these can result in undesired structures
on the surface.13 Riihe et al. reported a strategy to overcome this problem with
the use of silane monolayers of initiators on silica surfaces.13 In the following

chapters, we report the grafting of polymers from gold using a similar approach.

1.2 Self-Assembled Monolayers

This section discusses the formation of self-assembled monolayers on
gold as these material provide a potentially attractive method for attaching
initiators to surfaces. Organic monolayer ﬁlms can generally be prepared by two

21.22

distinct methods: the Langmuir-Blodgett technique which involves the transfer

of a ﬁlm assembled at an air-water interface to a solid substrate, and self-
assembly, which is based on spontaneous adsorption of dissolved molecules.23'24
The Langmuir-Blodgett technique is also adaptable for multilayer construction
and allows for the incorporation of a broad range of Iipophilic materials in mono
or multilayers. The layers are only physically linked to the surface, however, and
therefore quite unstable.25

Self-assembly is much more convenient than the Langmuir-Blodgett
technique and yield more stable ﬁlms. In many cases, ﬁlms form a covalent bond
to the substrate. SAMs can be formed using chlorosilanes on silicon, carboxylic
acids on metal oxides and organosulfur compounds on a variety of surfaces.26

Organosulfur derivatives coordinate strongly to many transition metal and

semiconductor surfaces to form monolayer ﬁlms.27 This versatility of sulfur-

surface interactions allows preparation of oriented organic monolayer ﬁlms
containing many functional groups of biological and chemical interest. SAMs of
alkanethiols on gold are especially well-ordered and consist of densely packed
alkanethiolate chains on the surface. These ﬁlms are easy to prepare and fairly
stable. A variety of SAMs on gold are of interest in ﬁelds such as corrosion
inhibition,28 sensing,29 electrochemistry,30 wetting,25 biofouling,25 and catalysis.25
Thiol or disulﬁde—containing SAMs are potentially useful systems for depositing
polymerization initiatiors on surfaces, but since Au-thiol bonds can be broken by
hot organic solvents,31 there are only a few reports describing surface
polymerization on gold surfaces. Ulman et al. reported cationic ring-opening
polymerization of 2-ethyloxazoline from self-assembled monolayers of initiators
on gold, but ﬁlms were only 100 A thick.32 Shah et al. reported surface-initiated
atom-transfer radical polymerization (ATRP) of various vinyl monomers on gold
substrates.33 Self-assembly of initiators was done at elevated temperature to
prevent desorption during polymerization. The use of ATRP also keeps radical
concentration low, which may help to stabilize the system. The use of room-
temperature for atom-transfer radical polymerization of methyl methacrylate on
gold surfaces was reported by Kim et al.34 Niwa et al. described
photopolymerization of methacrylic acid using self-assembled monolayer of
xanthates.35 However, there are no reports of thermally initiated radical
polymerization from gold surfaces. We report the thermal free radical

polymerization of styrene using azo-bound initiators attached to a gold surface.

The use of a stabilized monolayer is necessary to make these techniques

successful.

1.3 Molecular Imprinting

One of the possible uses of ultrathin polymer ﬁlms is in sensing applications.
This section describes one possible means of incorporating selectivity into
ultrathin ﬁlms. For many years scientists have been trying to exploit and mimic
antibody-antigen interactions. Biosensor technology based on these interactions
is highly developed and widely applied in the speciﬁc detection and
measurement of suitable analytes.11 Because antibodies are not available for a
large number of applications, a parallel technique for development of synthetic
antibodies through “ molecular imprinting “ has emerged. Molecular imprinting
involves polymerization of a monomer-template complex in the presence of
excess cross-linking agent as shown in Figure 1.2. Removal of the template
through rinsing, hydrolysis, or acidiﬁcation leaves behind a selective cavity. The
concept of using molecularly imprinted polymers (MlPs) for chemical recognition
started from Linus Pauling’s template and cast theory36 proposed in the 1940s.
Pauling suggested that when antigen is injected into an animal, antigen
molecules are captured and held by antibodies. An antibody to this antigen is a
molecule with a conﬁguration complementary to that of a portion of the antigen
molecule. The antibodies that surround the antigen bond together rendering the

antigen ineffective. Pauling expected the antigen to dissociate eventually from

 

. w

Figure 1.2: Concept of molecular imprinting of polymers (adapted from Wulff,G.;
Angew. Chem. Int. Ed. Engl., 1995, 34, 1812-1832)

the antibody, thus leaving behind a 3-dimensional cavity. When the antigen is
re- introduced, it can be trapped again by this antibody structure. Depending on
the binding strength of antigen-antibody interactions, the antigen can free itself or
permanently bind with the antibody. Molecular imprinting employs a similar
concept (Fig 1.2). A template molecule in solution is ﬁrst allowed to interact
reversibly with a suitably functionalized monomer in solution. A Cross-linking
agent is present in solution and upon initiation, polymerization occurs around the
template. After the removal of the template, polymer with well—deﬁned cavities is
left behind. The structure of these cavities is pre-determined by the template
molecule. When the template molecule is re-introduced, it binds selectively to the
imprinted site. For example molecularly imprinted polyurethanes have been used
in detecting polycyclic aromatic hydrocarbons in water.37 The selectivity can be
tailored for a distinct analyte by altering the template molecule from anthracene
to 1,12— benzoperylene. The use of imprinted ultrathin ﬁlms should greatly reduce
the equilibration time with the analyte molecule, and grafting of ﬁlms from
surfaces should provide a convenient means for synthesizing ultrathin ﬁlms.
Since the ﬁrst covalent imprinting of organic polymers by Wulff and
coworkers38 in the early 19703, researchers such as Mosbach39 and Shea40 have
extended this technique by imprinting polymers using non-covalent interactions.
Several advances have been made in this area and now 'MlPs have been used
as stationary phases for chromatography and immunoassays, recognition

elements in sensors, and enzyme mimics in catalysis.41

ll

Chapter 2

EXPERIMENTAL

This chapter ﬁrst discusses the techniques used for characterizing grafted
polymer ﬁlms. This is followed by a listing of the materials and chemicals used,

as well as the synthesis of necessary chemicals.

2.1 Ellipsometry
Ellipsometry is a sensitive technique for determining the thickness and/or
refractive index of thin ﬁlms. The method actually measures the change in
polarization of light reﬂected from a surface. This change in polarization is
expressed in psi (Lu) and delta (A) values, and these values are related to Fresnel
reﬂection coefﬁcients for p and s polarized light,42 as shown in equation 1.
RP
p = — =tan (Lu) exp (i A) (1)
Rs
In this equation, Rp and Rs are complex Fresnel reﬂection coefﬁcients for
the p and 5- directions, respectively, tan (llJ) represents the relative amplitudes of
p and s polarized light, and A is the phase difference induced between the two
polarizations. Using a model based on Fresnel reﬂection coefﬁcients, the
thickness (t) and refractive index (n) of a ﬁlm on a surface can be calculated,
provided the optical constants of the substrate are known. The s and p polarized
light components are perpendicular and parallel, respectively, to the plane of

incidence.

12

A J. A. Woollam Co. M-44 rotating analyzer ellipsometer measured the
changes in polarization of reﬂected light and WVASE32 (Variable Angle
Spectroscopic Ellipsometry) software calculated the optical constants of
substrates and thicknesses of thin ﬁlms. In this instrument the white beam of

incident light is generated by a Xenon lamp source and strikes the sample with
an angle of incidence of 75°. The reﬂected light passes through a rotating

analyzer. In the detector unit, the white light beam is dispersed onto an array of
silicon detectors, allowing simultaneous measurement of 44 wavelengths. In our
experiment, a refractive index of 1.5 was assumed for all organic ﬁlms as this
value is typical for organic materials. For very thin ﬁlms, q: and A are relatively

insensitive to changes in refractive index, and this value cannot be calculated.

2.2 Infrared Spectroscopy
Reﬂectance Fourier Transform Infrared (FTIR) spectra were recorded with

a Nicolet Magna 560 FTIR spectrometer containing a PIKE grazing angle (angle
of incidence of 80°) attachment. This instrument contains a tungsten/ceramic

glow bar, a polarizer (p-polarization) and an MCT detector. The instrument was
kept in a plexi-glass box continuously purged with dry nitrogen to keep levels of
moisture and other contaminants low. The MCT detector was cooled using liquid
nitrogen. Spectra were recorded in the range 750-4000 cm'1 using 256 scans
and a spectral resolution of 4 cm". All spectra were calculated in absorbance
units, where absorbance is deﬁned as [- log (R / Ro)]. R and R0 are the reﬂected

intensities from the ﬁlm-covered and bare substrate, respectively.43

13

2.3 Experimental Materials

Gold-coated substrates were prepared by electron beam evaporation of
20 nm of Ti on Si (100) wafers doped with boron, followed by similar deposition
of 200 nm of Au on the Ti. Titanium is used to promote adhesion of gold onto Si.
In some cases, wafers were sputter-coated with 20 nm of Cr and 200 nm of Au.
Gold is used because well-organized monolayers can be readily formed by
spontaneous adsorption of organic thiols and disulﬁdes44 on this relatively
homogeneous surface. Gold is also a convenient electrode for performing
electrochemical studies. Aluminium-coated wafers were prepared by sputter-
coating 20 nm of aluminium on Si (100) wafers.

3-Mercaptopropyltrimethoxysilane (MPS) was used as received from
United Chemical Technologies. 4,4'-Azobis(4-cyanovaleric acid), 11-
mercaptoundecanoic acid (MUA), 11-mercaptoundecanol (MUD), 3-
aminopropyltrimethoxysilane (APS), 1,3—dicyclohexylcarbodiimide (DCC),
ethanol, cystamine, dichloromethane, pyridine, chloroform, poly(allylamine
hydrochloride) (PAH) (Mw=70 000), poly(styrene sulfonate) (PSS) (Mw= 70 000),
sodium chloride (NaCl), copper(l) chloride (99.999% purity), copper(ll) bromide,
2,2’-dipyridyl (BPY), anhydrous N,N’-dimethylformamide (DMF), 2-
bromopropionylbromide (2-BPB), 4,4'-Azobisisobutyronitrile (AIBN), triethylamine
and mercaptopropionic acid (MPA) were purchased from Aldrich Chemicals. The
silane compounds are moisture sensitive and were opened under nitrogen.
Sodium hydroxide pellets and anhydrous sodium sulfate were purchased from

Spectrum. MnClz was purchased from Mallinckrodt.

l4

Styrene monomer (Aldrich) inhibited with p-ten‘butylcatechol was puriﬁed
by washing with 0.25 M sodium hydroxide and Milli-Q water (Millipore, 18M!)
cm). The monomer was then dried over sodium sulfate and passed through a
column of basic alumina (Aldrich, mesh size 80-200). Before the polymerization
step, the monomer was distilled under reduced pressure to remove any
unwanted styrene polymer that was present in the monomer mixture. Toluene
(Aldrich) was ﬁrst reﬂuxed under a nitrogen atmosphere in the presence of
sodium/potassium alloy using benzophenone as indicator until a dark blue colour
was obtained and then distilled. Ethylene glycol dimethacrylate (EGDMA) was
purchased from Aldrich and puriﬁed by passing it through a column of basic

alumina and distilling under reduced pressure.

2.4 Synthesis of Trimethoxysilane-Substituted Azo Initiator

The monocarboxylic acid precursor
CH3C(CH3)(CN)N=NC(CH3)(CN)CH2CH2COOH, was synthesized according to
the method of Riihe.”45 1.11 g of this compound was then coupled with 0.9 g (5
mmol) of 3-aminopropyltrimethoxysilane using 1.24 g (6 mmol) of DCC with
pyridine as a catalyst. The synthesis was carried out in 20 mL of anhydrous
CH2C|2 at room temperature for 12 hrs. After completion of the reaction, urea
byproducts were removed by ﬁltration of the CH2CI2 solution of the product over
anhydrous sodium sulfate, and the solvent was evaporated to afford a light-
orange solid. The product was used without further puriﬁcation because of its

sensitivity to heat and water. Proton-NMR spectra of the product had multiplet

15

peaks at 2.18 and 3.2 ppm, corresponding to two methylene groups on either
side of the amide bond, conﬁrming the formation of azo-initiator. This synthesis

was performed by Dr. Wenxi Huang.

2.5 Preparation and Cleaning of Substrates

Substrates were ozone cleaned using a Boekel UV/Oa cleaner (Model
135500) for 12-15 minutes and then rinsed with Milli-Q water and dried with
nitrogen. Cleaned slides were used to determine the optical constants (n and k)
of the particular substrate. These values are necessary to calculate the thickness

of the polymer. The FTIR background was also measured with a cleaned slide.

2.6 Formation of MUA and MUD Monolayers on Au Substrates

After cleaning, gold-coated slides were immersed in 2 mM ethanolic
solutions of MUA or MUD for 12 hrs (2.0 mg of MUD or 2 mg of MUA in 10 mL
EtOH). The slides were then rinsed with EtOH followed by Milli-Q water, and

dried under nitrogen.

2.7 Formation of Trimethoxysilane-Substituted Azo-Initiator Monolayers
(Fig 2.1 C) on Aluminium Substrates

The aluminium substrates were ozone cleaned for 15 min. The initiator
attachment was done in a glove box as the silanes are moisture sensitive.
Substrates were immersed in a 0.2 wt % solution of the initiator,

CH30(CH3)(CN)N=NC(CH3)(CN)CH2CH2CONHCHzCHzCH2$i(OCH3)3, in toluene

l6

in the presence of 20 pL of pyridine (base) at 60 °C for 5 min. The slides were

then cleaned with toluene and dried under nitrogen.

2.8 Formation of Initiator Monolayers on Various Substrates

Monolayer A (Fig 2.1) was prepared by ﬁrst attaching a MUD monolayer
to the Au surface. The monolayer was coupled with the asymmetric azo-
monocarboxylic acid CH3C(CH3)(CN)N=NC(CH3)(CN)CH2CH2COZH, by
immersion of the Au substrate in a solution containing 0.1 g of the azo compound
and 0.14 g of DCC in 10 mL DMF with 20 uL pyridine as catalyst. After two
hours, the substrates were rinsed several times with DMF and dried under
nitrogen.

Initiator monolayer B (Fig 2.1) was attached to the gold substrate using

the same method, but cystamine was ﬁrst attached to gold instead of MUD.

2.9 Grafting of Polystyrene onto Gold Substrates

Cleaned gold substrates were immersed in a 2 mM solution of
mercaptopropyltrimethoxysilane (MPS) (20 uL in 10 mL methanol) containing 2-3
drops of pyridine as a catalyst for 10 - 12 hrs at room temperature in a glove box
(UNILAB 1200, MBraun Inc.). These substrates were then rinsed with methanol.
The MPS-coated slides were then immersed in 0.1M HCI overnight to hydrolyze
the methoxy groups. The hydroxylated surface crosslinked and stabilized the

Iayen

17

 

 

CH3 CN

 

 

 

 

S-C11H22-O\”/\/< 4N CH3
I“ CN

 

 

0 CH3
CH3 CN
N 9N CH3
__S/\/ CN
0 CH3

 

 

 

 

::O:_SI\/\/N FIN: ’N OCH“?

CH3N

 

 

Al203

Figure 2.1: Monolayers employed for initiation of polymerization

18

The modiﬁed gold substrate was further treated with a solution containing

150 uL of trimethoxysilane substituted azo initiator, 1 (Fig 2.2), and 20 pL of
pyridine in 7 mL of toluene at 60 °C for 5 minutes as shown in ﬁg 2.2. The

reactions with silanes were done in a glove box because they are sensitive to
moisture and oxygen. The substrate was then washed with toluene 2-3 times and
dried under nitrogen.

Monomer solutions containing equal volumes of puriﬁed styrene and
toluene were degassed using three freeze-pump-thaw cycles. This procedure
removed traces of oxygen from the monomer mixture. The mixture was opened
in the glove box under nitrogen. A vial containing the initiator-coated gold
substrate was ﬁlled with the monomer solution until the slide was completely
immersed in solution. Polymerization was done in the glove box using an oil bath
maintained at 60 °C for different time periods. After the polymerization, the
samples were taken outside the glove box and rinsed with toluene several times
without allowing the substrate to dry. The substrate was then Soxhlet extracted
with CHCI3 at 60° - 70 °C overnight. Extraction removed physisorbed polymer that
formed due to polymerization in solution. Ellipsometry and FTIR spectra were

taken at each step of the polymerization process.

OCH3 o
SMS/i-OCHa + SMS/i-OH
Au 00113 i. Au i)
S/\/\S OCH3 SMSII-OH
\ OCHa \
OCH3 /o

  

I
’8' N e N CH3
CH3O \/\/ MN CN

0 CH3
\
>3 9 1
s/\v/\$-o—se—
Au O \O H
/ / H CN
SMS{‘O-S\i\/\/N 3 NcN CH3
CN
/0 /O 0 CH3
/
A
\
,0 \9
S/\V/\&-O—SF—
Au 2) \O
sMsi~o-s:/ : CH3 CN
\ \O\/\/ MCHZCHa‘n

Figure 2.2: Steps involved in grafting polystyrene from a modiﬁed gold surface

20

Chapter 3

POLYMERIZATION OF STYRENE FROM GOLD

Self-assembled monolayers (SAMs) on gold are widely used to vary
surface properties. These ﬁlms have been used for corrosion protection,46 control
of wetting,”46 lithographic patterning,47 and selective adhesion of cells.48 SAMs
are more stable than traditional Langmuir-Blodgett ﬁlms49 because gold strongly
interacts with sulfur.” This allows formation of monolayers with many types of
functional groups. By modifying the functional groups in monolayers, different
types of initiators can be attached to the surface.” However, surface-initiated
thermal radical polymerization from SAMs on Au is challenging because of the
lability of Au-S bonds and the use of elevated temperatures in polymerization.
Strong attachment of initiators to a surface is crucial for surface-initiated
polymerization. Initiators tethered to siloxane-based monolayers on oxide

surfaces““""5'51'56

are covalently bound to the substrate, and these systems allow
grafting polymerization to be conducted at high temperatures.

This chapter ﬁrst examines the stability of SAMs on gold under
polymerization conditions and how free radical polymerization from gold is

hindered by the stability of alkanethiol monolayers. Subsequently, we

demonstrate polymerization from stabilized monolayers on gold.

21

3.1 Results and Discussion

3.1.1 Stability of SAMs under Polymerization Conditions
Thermal grafting of polymers from gold surfaces requires that the initiator
remain attached to the surface during polymerization. To evaluate the stability of

alkanethios on gold, MUA monolayers were prepared and exposed to toluene at
60 °C. The absorbances due to the acid carbonyl groups in the monolayers were

studied as a function of time of exposure to heated toluene. The MUA reﬂectance
FTIR spectra contained both hydrocarbon peaks at 2925 and 2851 cm“1 and a
carbonyl stretch at 1724 cm'1 (Fig 3.1). However, we relied on the carbonyl peak
intensity as a measure of monolayer stability because methylene peaks are
subject to increases due to contamination. The normalized integrated
absorbance of the carbonyl peak was obtained by dividing by the absorbance of
the integrated carbonyl peak before exposure. Figure 3.2 (open squares) shows
that the carbonyl absorbance decreases by 40% over the ﬁrst two hours of
exposure to hot toluene and then remains stable for an additional 4 hours of
exposure. This is in agreement with data found in the literature,”33
demonstrating that a signiﬁcant amount of thiol still remains adsorbed under
typical polymerization temperatures.

Based on the above result, we expected that polymerization could still be
initiated by the remaining derivatized thiols on the surface. To test this possibility,

two different monolayers containing azo initiators (Fig 2.1, A and B) were

prepared on gold surfaces. Films of initiator A had an ellipsometric thickness of

22

 

I 0.001

Absorbance

 

 

 

1

3000 2500 2000 1500

Wavelength (cm‘1)

Figure 3.1: Reﬂectance FTIR spectra of a MUA monolayer showing the
crystalline behavior.

23

 

12? A A A

 

 

d)
o A
S
0.75-
9 El
3, A
'3 I A
:2 I A
g 0254 A
' I
3 I I A
z
0 . . u
0 6

2 4
Time (hours)

Figure 3.2. Integrated absorbance of the carbonyl peak in reﬂectance FTIR
spectra of MUA monolayers vs time of exposure to toluene at 60 °C
(open squares), 6 mM AIBN in toluene at 60 °C (ﬁlled squares),
toluene at room temperature under UV light (open triangles), and
6 mM AIBN in toluene at room temperature under UV light (ﬁlled
triangles).

24

17 i 2 A and reﬂectance FTIR spectra of these ﬁlms exhibited ester carbonyl
peaks around 1737 cm“. Films of initiator B had thicknesses of 14 i 2 A and had
amide peaks at 1652 and 1548 cm". These results conﬁrm that the coupling
reactions succeeded and initiators were attached to the surface.

Polymerization was performed by immersing substrates in a 1:1 v/v

styrene/toluene mixture at 60 °C for 12 hr. After polymerization, the slides were

rinsed with toluene several times and cleaned by Soxhlet extraction. Film
thicknesses in the case of both initiators were < 50 A, and FTIR peaks from
polystyrene were not intense. For comparison, we synthesized polystyrene from
a silane attached to Al/Al203 (Fig 2.1, monolayer C). The initiator monolayer on
alumina had a thickness of 18 i 2 A and exhibited amide peaks at 1650 and
1556 cm". Polymerization done using the initiator attached to alumina resulted
in 400 A thick ﬁlms. This showed that for some reason the initiators on gold were
rather ineffective. This might be due to quenching of initiator radicals by gold or

to desorption of the initiator monolayer.

3.1.2 Stability of SAMs in the Presence of Radicals
We also assessed the stability of SAMs in the presence of radicals.
Radicals were generated by either thermal or photo-induced decomposition of

AIBN in solution. For the thermal reaction, MUA monolayers on gold were
exposed to a hot toluene solution (60 °C) containing 6 mM AIBN. Reﬂectance

FTIR spectra were taken after 1-6 hr of immersion in solution. The intensity of the

carbonyl peak in the FTIR spectra revealed that more than 80 % of the MUA

25

desorbed from the surface after 6 hours (Fig 3.2 ﬁlled squares). Experiments
done in the absence of AIBN showed that only 40 % of the SAM desorbed from
surface (Fig 3.2 open squares). We should note that coverage values based on
FTIR spectra are approximate because desorption of SAMs could change the
orientation of carbonyl groups in the monolayer.57

In a second experiment, MUA SAMs in room temperature toluene
solutions were exposed to UV radiation from a mercury lamp. FTIR spectra
showed that > 85% of the SAM remained on the surface during 6 hr of UV
exposure without AIBN (Fig 3.2 open triangle), but only 20% remained when
AIBN was present (Fig 3.2 ﬁlled triangles). These results suggest that the
radicals may attack the Au-S bond thereby accelerating desorption of thiol bound

to surface.

3.1.3 Effect of Thiols on Polymerization

One possible explanation for the lack of polymerization on gold substrates
might be that desorbed thiols inhibit radical polymerization. To test this
possibility, polymerization on aluminium was done in the presence of either free
thiols or SAMs on gold. Reaction conditions were identical to those done with the
alumina wafer alone. Thicknesses of polystyrene ﬁlms on aluminium were 230 i
30 A and 310 i 30 A when polymerization was performed in the presence of
MUD and MUA monolayers, respectively (expts 3, 4 Table 3.1). When compared

to polymerization in the absence of SAMs, these values represent a decrease in

26

Table 3.1: Thickness of surface-grafted polystyrene layers on Al when
polymerized in the presence and absence of alkanethiols.a

 

Thickness of Polystyrene

 

 

 

 

 

 

 

5"“ Additives in the Reaction vessel 0" Alumina (A)

1 Nothing 41 01:30

2 Bare Au slide 40012

3 Mercaptoundecanol in solution 250:1:40
(0.25uM in toluene)

4 Mercaptoundecanol in solution 330120
(0025le in toluene)

5 Mercaptoundecanol monolayer on an 230130

Au slide
6 Mercaptoundecanoic acid monolayer 310130

 

on an Au slide

 

 

 

a Polymerization was performed for 12 hr at 60 °C, and ﬁlms were cleaned
by Soxhlet extraction.

27

E-

thickness of 100 - 180 A. Absorbances in FTIR spectra also decrease when
polymerization occurs in the presence of SAMs. In another experiment, trace
amounts (0.25 (M or 0.025 uM) of MUD were added to polymerization solutions.
In this case, the polystyrene ﬁlm thicknesses were 250 i 40 A and 330 i 20 A for
solutions containing 0.25 uM and 0.025 (M of MUD, respectively. Film
thicknesses were not signiﬁcantly affected when polymerization was done in the
presence of a bare gold slide. The outcome of these experiments, summarized in
Table 3.1, suggests that desorbed thiols inhibit radical polymerization from
surface.

Figure 3.3, depicts the possible fates of surface bound radicals. When
polymerization occurs in solution, there can be radical transfer to solvent or
monomer, but neither of these processes terminates polymerization. When
polymerizing from a substrate, however, thermal desorption of thiols or transfer of
a radical to solvent or a scavenger terminates ﬁlm growth. In the experiments
described in Table 3.1, the ratio of the concentration of monomer to thiol is 106 -
108. This is a huge excess of monomer, and we would thus expect negligible
transfer of radicals to thiols. However, the experimental values indicate
decreases in ﬁlm thickness of 80-100 A even when only 0.25 uM or 0.025 (M of
MUD solution is present.

One possible explanation for inhibition of polymerization by thiols is that
chain-transfer reactions initiate polymerization in solution. This could lead to
physisorption of polymer on the surface, thereby blocking monomers from

reaching reactive sites. To test this explanation, polymer ﬁlm thickness was

28

SH

(g SH
thermal . {8 radical-indu_ced

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

d-—_——> t - ’
esorp ion desorption SM-
l

AU AU AU

M S H M SH . M M .
chain transfer POIYMGT growth
s ~77 <7:
Au Au Au

 

 

 

 

 

 

 

 

 

Figure 3.3. Fate of surface bound radicals on Au. Top: thermal desorption of
surface bound radicals and desorption induced by reaction of a
radical with a gold-sulfur bond. Bottom left: reaction with a chain
transfer reagent. Bottom right: initiation of polymerization.

29

measured before and after Soxhlet extraction on samples prepared with and
without the presence of thiols. We expected that the amount of physisorbed
material would be higher for ﬁlms prepared in the presence of thiols. The
ellipsometric thickness of a polymer ﬁlm prepared in the absence of thiol
changed from 440 i 30 A to 400 i 30 A after Soxhlet extraction. For ﬁlms
prepared in the presence of 0.025 uM MUD, thickness decreased from 340 i 30
A to 300 i 30 A. These data suggest that the amount of physisorbed material is
similar with and without thiols. From our experiments, we clearly see that small
concentrations of thiol affect polymerization on aluminium, but the origin of this
effect is not yet known.

These studies suggest two possible reasons why we are unable to grow
thick polymer ﬁlms on Au surfaces. First, the alkanethiol monolayers on gold
desorb when exposed to high temperatures and radicals. Second, thiols inhibit
polymerization. Thus, the stability of SAMs must be ensured before
polymerization can be reliably performed. This can be done either by using
stabilized SAMs or by carrying out the polymerization at low temperatures using
different radical generation techniques.”58 Below I describe thermal radical
polymerization from Au surfaces using a cross-linked monolayer containing azo-

initiators. The cross-linked surface stabilizes the initiator monolayer.
3.1.4 Preparation of Cross-Linked Azo-lnitiator Layers on Gold Surfaces

Mercaptopropyltrimethoxysilanes (MPSs) were used as an adhesion layer

for initiator attachment to gold. The mercapto group allows monolayer formation

30

and terminal silanes can be cross-linked to ensure the stability of the underlying
monolayer. Gold substrates were immersed in a 2 mM MPS solution prepared in
methanol to form MPS monolayers (Fig 2.2). The reﬂectance FTIR spectrum of
these ﬁlms contained peaks at 2938 cm"1 (overlapping CH3 and CH2 bands),
2846 cm‘1 (CH2 symmetric stretch) and 1114 cm'1 (Si-O-C stretch) (Fig 3.4a).
After hydrolysis, methyl peaks disappeared and the intensity of the peak at 1114
cm'1 decreased, demonstrating hydrolysis of the MPS layer (Fig 3.4b). During
hydrolysis, a three-dimensional poly(siloxane) network formed due to cross-
Iinking of neighboring trimethoxysilane groups. The MPS ﬁlm thickness was 9 i 2
A, in good agreement with values in the literature.”’61 The synthesized azo-
initiator, CH3C(CH3)(CN)N=NC(CH3)(CN)CHzCHZCONHCHZCHzCHZSi(OCH3)3,

was then attached to the hydroxylated MPS monolayer at 60 °C (as shown in Fig

2.2). The reﬂectance FTIR spectrum of this ﬁlm contained amide peaks at 1652
and 1548 cm"1 and the thickness of the layer was 21 i 2 A. The presence of
amide peaks conﬁrms that the initiator is attached to the hydrolyzed MPS layer
(Fig 3.4c). Surface radical polymerization of styrene from this surface was carried
out in a glove box at 60 °C for 12 hr. The reﬂectance FTIR spectrum of the
polymerized ﬁlm agrees with standard polystyrene spectra indicating that
polymerization was successful, and this sample had a thickness of 400 A.
Polymerization was carried out for different periods of time using several initiator-
coated substrates and the graph of thickness versus polymerization time shows a

fairly linear relationship between thickness and reaction time (Fig 3.5). This result

31

shows that polymerization is reproducible, and the thickness data are in good

agreement with the results of Riihe et al. on silica surfaces.62

32

 

W x0.33 AA- ALAAC-‘M

 

 

 

 

d)

U

C

(U

.D

L

O

3 c A
< 4

b
‘M _.__

 

 

 

 

F l 1 l

3000 2500 2000 1 500 1 000

Wavenumbers (cm'1)

Figure 3.4: Reﬂectance FTIR spectra of (a) a MP8 monolayer, (b) a hydrolyzed
MPS monolayer, (c) a hydrolyzed MPS monolayer modiﬁed with
CH30(CH3)(CN)N=NC(CH3)(CN)CHZCHZCONHCHZCHZCHZSI(OCH3)3,
an Azo-initiator, and (d) a grafted polystyrene layer.

33

100 .

N

01

I
l-O-l

25..

Thickness (nm)
0'!
O

 

 

0 5 10 15 20 25
Time (hour)

Figure 3.5: Thicknesses of polystyrene ﬁlms on modiﬁed Au surfaces (refer to
Fig 2.2 for reaction scheme) as a function of reaction time. Error bars
represent the standard deviations of thickness determinations on at
least three different samples.

34

Chapter 4

POLYMERIZATION ON ALUMINA MEMBRANES

The growth of ultrathin polymer ﬁlms via the alternating adsorption of
polyelectrolytes has emerged as a new method of polymer self-assembly?”65
This method overcomes problems associated with conventional techniques such
as LB assembly, where sample geometry is limited to ﬂat surfaces and
preparation requires complicated equipment. In contrast to self assembly, which
is usually a slow process and prone to defects, alternating polyelectrolyte
deposition is fast and defects are minimal.65 This process is capable of producing
ultrathin membranes on porous supports. Such composite membranes may ﬁnd
applications in ion separation, food processing, and sensing.66 Because they are
ultrathin, such membranes allow high ﬂux and can be highly selective. By
choosing different layered polyelectrolytes, one can tailor the selectivity of these
membranes.

This chapter discusses a hybrid technique of atom transfer radical
polymerization (ATRP) at the surface of layered polyelectrolyte membranes.

Such a procedure may greatly expand the variety of available ultrathin

membranes.

35

4.1 Experimental
This procedure can be split into three steps: deposition of polyelectrolyte,
initiator anchoring, and polymerization as shown in Fig 4.1. The exact procedures

for each step are described elsewhere.‘°""”'67

4.1.1 Deposition of Polyelectrolyte

UV/03 cleaned gold-coated substrates were ﬁrst immersed in a 1.5 mM
mercaptopropionic acid (MPA) solution in ethanol for about 30 min. The slides
were rinsed with ethanol and dried under N2. Polyelectrolyte deposition on MPA-
coated gold was performed by alternating immersion of the wafer in a PAH
solution for 5 min and in a PSS solution for 2 min, with a 1 min water rinse
between each immersion. The 0.02 M PAH solution contained 0.5 M NaCl as
supporting electrolyte and its pH was adjusted to 4.5 with 0.01 M HCI and 0.005
M NaOH, while the 0.02 M PSS solution contained 0.5 M MnClz and its pH was
adjusted to 2.3 with 0.01 M HCI and 0.005 M NaOH. Films were dried with N2
after depositing all layers. (Polymer molarities are given with respect to the

repeating unit.)

4.1.2 Anchoring of Initiator

Initiator was attached to the PAH/PSS ﬁlms by immersion in 2-
bromopropionylbromide (2-BPB) in the presence of triethylamine. The initiator-
attachment solution contained 0.432 g of 2-BPB in 10 mL DMF and a solution of

base contained 0.242 g of triethylamine in 10 mL DMF. Since the anchoring step

36

coo'

coo'

coo‘ n

coo' +
. NH

coo 3

coo‘ I
. (PAH)

coo

coo'

 

coo' T

Repeat to give desired number of layers
BrCOCH(CH3)Br

anhydrous DMF
Et,N

 

_ __l
4 bilayers 4 bilayers

Figure 4.1: Steps involved in grafting poly (EGDMA) from a gold surface coated
with a layered polyelectrolyte ﬁlm.

37

is exothermic, both solutions were cooled in a freezer to 0 0C before the reaction.

The gold slide was ﬁrst immersed in the solution of base, and then the initiator
solution was added all at once. The reaction was stopped after 2 minutes by
transferring the slide to a DMF wash solution. The whole process was done in a
glove box because the acid bromide is moisture sensitive. Rinsing was done
outside the glove box with ethylacetate, ethanol, and Milli-Q water in this order,

followed by drying with N2.

4.1.3 Polymerization Procedure

The monomer mixture contained ethylene glycol dimethacrylate, Milli-Q
water, and DMF in a 3:3:8 ratio by volume. The mixture was degassed using
three freeze-pump-thaw cycles. To 42 mL of this degassed mixture, CuCl (180
mg, 1.8 mmol), CuBr2 (120 mg, 0.54 mmol), and BPY (731 mg, 2.34 mmol) were
added under a nitrogen atmosphere. The mixture was continuously stirred until
the solution became dark brown. This solution was then transferred to several
vials containing initiator-modiﬁed substrates in a glove bag purged with N2, and
polymerization was done at room temperature. After the polymerization,

substrates were rinsed with DMF several times and dried under N2.

4.2 Results and Discussion
Atom tranfer radical polmerization is a living polymerization process. The
initiating sites can be activated or deactivated by introduction of Cu (I) (as shown

in equation 2). This polymerization can be carried out at room temperature if the

38

catalysts are sufﬁciently active, and thus the process can be used with substrates

that are sensitive to high temperature. High ﬁlm thicknesses and negligible

reaction in solution make ATRP from a surface an attractive process.”67

R‘ R‘
I I
Br—C—CO,R’ + cumanapv «S’— CuIIIIBr.IBPY+ «5-001? (2)
1
CH, CH,

Polymerization was tried initially on gold substrates for characterization
purposes. 4.5 bilayers of PAH/PSS were deposited on MPA—coated gold (PAH as
the top layer). The ellipsometric thickness of the ﬁlm was 165 i 10 A. The FTIR
spectra of these ﬁlms had strong peaks at 1219 and 1177 cm'1 characteristic of
sulfonate groups and several peaks characteristic of NH3+ and phenyl grOups
(Fig 4.2a). The initiator was then anchored to the surface via the formation of an
amide bond. FTIR spectra of the initiator-containing ﬁlms looked similar to those
of PAH/PSS except for a small increase in the peak intensity in the amide region,
between 1650-1560 cm'1 (Fig 4.2b). Polymerization from this surface was carried
out for 24 hrs at room temperature, and the ﬁlm thickness increased from 170 A
to 1100 A. Strong carbonyl peaks at 1735 cm'1 in the reﬂectance FTIR spectrum
conﬁrmed the formation of poly(EGDMA) on the surface (Fig 4.2c). C=C
stretching from unpolymerized vinyl groups was also seen at 1637 cm".
Comparison of normalized IR absorbances of vinyl groups in pure monomer and

poly (EGDMA), suggest approximately 50 % crosslinking.

39

 

 

 

 

 

 

 

 

 

 

 

Io.01
I 0.01
b
a:
o
r:
N a
g . .
h 1700 1050 1000 1550
O
m
a M
< M X 0.05
k w
k w
3000 2500 2000 1500 1000
Wavenumbers (cm'1)

Figure 4.2: Reﬂectance FTIR spectra of (a) a 4.5-bilayer PAH/PSS ﬁlm, (b) a
4.5-bilayer PAH/PSS ﬁlm reacted with initiator, and (c) a grafted poly
(EGDMA) ﬁlm on a 4.5-bilayer PAH/PSS ﬁlm on gold. The inset
shows the amide peaks before and after the initiator attachment.

4O

After the successful grafting of poly(EGDMA) on polyelectrolyte ﬁlms on
gold, the procedure was tried on porous alumina supports (Whatman Anodisc
0.02 urn membrane ﬁlters). PSS was deposited ﬁrst on a UV/03 cleaned
membrane because of the positive charge on alumina membranes at pH < 8, and
then 4.5 bilayers of PAH/PSS were deposited. Deposition of the polyelectrolyte
ﬁlms was restricted to the skin side of the membrane with the use of a holder.
Initiator attachment and polymerization were done without a holder. To ensure
the reliability of the synthetic procedure, a similar procedure was done on a gold
substrate using the same batch of solutions. FESEM (Field Emission Scanning
Electron Microscopy) studies show polymer formation on the membranes, but we
are not yet able to conclude whether these ﬁlms can cover underlying pores. In
the future, gas permeability and ion-transport studies will give more information
about the ﬂux and selectivity of these polymer-coated membranes. The results
were promising as ATRP provides a way to graft poly(EGDMA) on polyelectrolyte
ﬁlms on porous supports. By varying the concentration of copper catalyst and
the reaction time, cross-linked ﬁlms of varying thickness can be synthesized at
room temperature. This process opens up a new way to prepare cross-linked
ﬁlms. Unlike other methods for preparing cross-linked ﬁlms, this technique
minimizes polymerization in solution. The low temperature process is also
compatible with substrates that are unstable at elevated temperature, eg. Au-

thiolates.

41

Chapter 5
CONCLUSIONS

A short review of previous grafting methods was initially discussed.
Because of concerns that the instability of alkanethiol SAMs might affect polymer
growth from gold surfaces, we studied the stability of SAMs under thermal
polymerization conditions. These studies showed that SAMs are somewhat
thermally unstable, but are also attacked by radicals. Other results showed that
polymerization is inhibited by the instability of SAMs on gold.

The ability to thermally grow polymers from gold is dependent on
establishing a stable initiation system on the surface. The problems associated
with the stability of underlying SAMs can be overcome by using
mercaptopropyltrimethoxysilane to form an adhesion layer for initiator
attachment. The presence of the cross-linked polysiloxane base layer stabilizes
the initiator-containing surface to polymerization conditions. When the modiﬁed
initiator-containing layer is used for radical polymerization of styrene, polystyrene
ﬁlms with thickness ranging from 100 to 900 A can be obtained.

We conclude that the use of a cross-linked initiator layer provides a simple
platform for thermal polymerization. ATRP at room temperature also provides a
convenient method to graft high density cross-linked polymers on polyelectrolyte

ﬁlm supports.

42

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43

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