Lehigh University
Lehigh Preserve
Theses and Dissertations
1997
Synthesis and characterization of modified
polyanilines
Gang Liu
Lehigh University
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Liu, Gang
Synthesis an'd
Characterization
of ModifiedPolyanilines
..•.
June 1,1997
SYNTHESIS AND CHARACTERIZATION OF MODIFIED
POLYANILINES
By
GangLiu
A Thesis
Presented to the Graduate and Research Committee
of Lehigh University
in Candidacy for the Degree of
Master of Science
III
Chemistry
Lehigh University
May, 1997
ACKNOWLEDGMENTS
I would like to express my deepest appreciation and respect for my advisor, Dr.
Michael S. Freund, for his guidance during my research and his advice, suggestions and
help in the preparation of this thesis.
I would also like to express my gratitude to the following people: to Keith J.
Schray for his careful review of the thesis; to Dah J. Wang for his help in my work with
NMR, MS and FfIR; to Alfred C. Miller for his help in XPS measurements; to Weili Bu,
my wife, for her continuous unconditional support of my work.
The financial support from Polymer Interface Center and Chemistry Department
are gratefully acknowledged.
III
TABLE OF CONTENTS
ACKNOWLEDGEMENTS
III
LIST OF TABLES
vn
LIST OF FIGURES
VITI
LIST OF SCHEMES
:
XII
'~
ABSTRACT
1
CHARPTERS
1
2
INTRODUCTION
3
1.1 History of Polyaniline
1.2 Structure and Properties of Polyaniline
1.3 Synthesis and Molecular Weight of Polyaniline
1.4 Lithographic Processing of Polyaniline
1.5 Chemical Modification of Polyaniline
3
4
5
7
9
EXPERIMENTAL
~
12
2.1 Materials
2.2 Experimental Procedures and Instruments
2.2.1 Electrochemical Techniques
2.2.2 X-ray Photoelectron Microscopy
2.2.3 Fourier Transform Infrared Spectroscopy
2.2.4 UV-vis Spectroscopy
2.2.5 IH Nuclear Magnetic Resonance
2.2.6 Quartz Crystal Microbalance
2.2.7 Gas Chromatography I Mass Spectroscopy
2.2.8 Gel Permeation Chromatography
2.2.9 Scanning Tunneling Microscopy
2.2.10 Scanning Electron Microscopy
2.2.11 Differential Scanning Calorimetry
3
12
13
13
13
14
14
14
15
15
15
15
16
16
SYNTHESIS AND PROPERTIES OF ANILINE-ANILINE FORMALDEHYDE
17
CONDENSATE COPOLYMERS
IV
3.1 Synthesis and Structure of Aniline Formaldehyde Condensate
3.2 Structure of Aniline-APC Copolymer
3.2.1 Mechanism of Polymerization of Aniline
3.2.2 Mechanism of Copolymerization of Aniline and APC
3.2.3 Kinetics of Aniline-APC Copolymerization
3.2.4 Evidence of Copolymer Structure from Spectroscopic Data
3.2.5 Molecular Weight of Aniline-APC Copolymers
3.3 Properties of Aniline-APC Copolymers
3.3.1 Conductivity
3.3.2 Electrochemistry
3.3.3 Adhesion
3.3.4 Thermal Behavior
·3.4 Conclusions
:
4
5
17
18
18
19
20
25
28
30
30
33
34
35
36
LITHOGRAPHY WITH POLYANILINE
37
4.1 Effect of UV Radiation on the Properties of Polyaniline
4.1.1 Conductivity
4.1.2 Electrochemistry
4.1.3 Solubility
4.2 Mechanism of the Photochemical Reaction of Polyaniline upon UV
Irradiation
4.3 Lithographic Processing of Polyaniline
4.4 Conclusions
37
37
39
40
40
42
44
MODIFICATION OF POLYANILINE THIN-FILMS USING SUBSTITUTED
BENZENEDIAZONIUM SALTS
45
5.1 Electrochemical Polymerization of Aniline
5.2 Characterization of Polyaniline Thin Films with STM
5.3 Modification of Polyaniline with Benzenediazonium Salts
5.4 Mechanism of Modification
5.4.1 Mechanistic Investigations with Model Compounds
5.4.2 Mechanistic Investigations Using XPS
5.4.3 Mechanistic Invest~gations Using Reflectance FTIR
5.4.4 Mechanistic Investigations Using QCM
5.5 Kinetics of Modification
5.5.1 Electrochemistry of Polyaniline upon Modification
5.5.2 Depth Analysis of Modified Polyaniline Films Using XPS
5.6 Conclusions
REFERENCES
45
46
48
49
49
51
53
54
59
59
61
62
63
v
BIOGRAPHY
68
VI
LIST OF TABLES
Table 3.1. The yields and starting materials for polyaniline and APC-anillne copolymers.
Table 3.2. Time required for polyaniline and APe-aniline copolymer films to detach
from glass in contact with distilled water.
Table 4.1. The conductivity of a polyaniline film as a function of UV irradiation time.
Table 5.1. Possible reactions between polyaniline and 4-nitrobenezendiaznium salt, and
the ratios of carbon to nitrogen (CIN) and the ratios of oxidized nitrogen (N02) to reduced nitrogen (other nitrogens) (NoxlNred ) in the corresponding
products.
VII
~
LIST OF FIGURES
Figure 1.1. Cyclic voltammogram of a polyaniline film in 1.0 M H2S04 • Polyaniline is
in its leucoemeraldine form, emeraldine salt form and pemigraniline form in
the potential window (a), (b) and (c), respectively.
Figure 3.1. UV-vis spectra of a 30gIL aniline-l.O M HCI solution after adding 20gIL
H202 for (1) 0 min; (2) 2 min; and (3) 5 min.
Figure 3.2. The absorbance at 700 nm of (1) 30gIL aniline; (2) 30gIL aniline + 1.5gIL
AFC; (3) 30gIL aniline + 5gIL AFC; (4) 30gIL aniline + 10 gIL APC; and
(5) 30gIL aniline + 15 gIL AFC solutions as a function of reaction time with
Figure 3.3. The plot of the concentration of AFC as a function of the corresponding
}
reciprocal slope obtained from Figure 3.2.
Figure 3.4. IH NMR spectra of PanilOO and Pani20 in DMSO-d6 •
Figure 3.5. GPC chromatograms of (a) PanilOO; and (b) Pani20.
Figure 3.6. A schematic representation of AFC-aniline copolymer network formed from
the chemical oxidative polymerization of aniline in the presence of AFC.
Figure 3.7. A schematic representation of the device used to measure the conductivities
of polymers.
Figure 3.8. Plot of log (conductivity) as a function of AFC to aniline ratios used for the
synthesis of the copolymers.
vrn
Figure 3.9. Cyclic voltammograms of PanilOO ( __) and other copolymers (
.) in
1.0 M H2S04 , scan rate =100 mV-s- t •
Figure 3.10. DSC curves of (a) PanilOO; (b) Pani20; (c) PaniS; and Pani2.
Figure 4.1. A schematic representation of the device used for relative conductivity
measurement.
Figure 4.2. Cyclic voltammograms of a polyaniline thin-film (casted onto a platinum foil
electrode) in 1.0 M H2S04 as a function of irradiation time. Scan rate = 100
mV-s- t •
Figure 4.3. UV-vis spectrum of a polyaniline thin-film, spin-casted onto a quartz slide,
as a function of UV irradiation time.
Figure 4.4. A schematic representation of lithographing and developing a polyaniline
film.
Figure 4.5. Optical micrograph of (a) the mask; and scanning electron micrographs of
(b), (c), (d) developed polyaniline structures on a silicon wafer.
Figure 5.1. (A) Charact~rization of a polyaniline film by STM. During
characterization, an STM tip (a) moves toward the surface of the thin
film; (b) penetrates the polyaniline film; and (c) finally approaches
the conducting Pt substrate. (B) The resulting current versus
displacement curve showing regimes corresponding to tip position in
(A).
Figure 5.2. STM tip current versus displacement for a polyaniline film deposited by
IX
passing 30 mC-cm-2 charge. The regimes a, band c correspond to the
corresponding regimes in Figure 5.1.
Figure 5.3. STM images (tip bias +200 mY, tunneling current 2 nA) of a polyaniline
film (a) before; and (b) after thickness characterization by an STM tip.
Figure 5.4. Mass spectra of the products from the nucleophilic reaction of 20 mM model
compound diphenylamine with (A) 20 mM 4-nitrobenzenediazonium
tetrafluoroborate; and (B) 20 mM 4-methoxybenzenediazonium
tetrafluoroborate in 1.0 M H2S04 for 0.5 h.
Figure 5.5. High resolution XPS spectrum of polyaniline films (- 80 nm thick) after
modification in 50 mM 4-nitrobenzenediazonium tetrafluoroborate and 1.0
M H2S04 acid solution for'different time periods.
Figure 5.6. Reflectance FfIR of (1) a polyaniline film; and (2) a polyaniline film after
reaction with 50 mM 4-nitrobenzenediazonium tetrafluoroborate in 1.0 M
H2S04. The polyaniline films were deposited onto Pt foils at constant
current densities of 0.1 mA-cm-2 for 1000 s from a solution containing 0.25
M aniline and 1.0 M H2S04.
Figure 5.7. Cyclic voltammograms of a polyaniline thin film modified by exposure to 50
mM 4-nitrobenzenediazonium tetrafluoroborate in 1.0 M H2S04 acid for (a)
omin; (b) 5 mi; and (c) 15 min.
Scan rate = 10 mV-s- l .
Figure 5.8. Resonance frequency change versus mass change calibration curve of a
quartz crystal microbalance. Cu was deposited from a 0.025 M CUS04-1.0
x
M NaAc solution onto Pt coated quartz crystal at a constant current of 0.5
mA-cm- 2.
Figure 5.9. Plot of frequency change versus anodic peak current, ip,a, of a polyaniline
covered electrochemical quartz crystal electrode in 1.0 M H2S04 after
modified in a 50 4-nitrobenzenediazonium tetrafluoroborate-1.0 M H2S04
solution for different time periods (0 min; 5 min; 15 min; 45 min and 180
min). Scan rate = 100 mY_sol.
Figure 5.10. Plot of anodic peak current, ip,a, versus reaction time for polyaniline thin
films (_,. - - 80 nm, 0,0 - - 50 nm) reacted with (0,-) 50 mM 4nitrobenzenediazonium tetrafluoroborate; and (0,.) 50 mM 4methoxybenzenediazonium tetrafluoroborate; in 1.0 M H2S04 acid. Inset is
the plot of initial anodic peak current (t=O) versus square root of the total
reaction time (determined from X-intercept of ip,a versus t1l2).
Figure 5.11. High resolution XPS spectrum of a polyaniline thin film (a) before
modification; (b) after modification in 50 mM 4nitrobenzenediazonium tetrafluoroborate-1.0 M H2S04 solution for
10 min; (c) after sputtered with 2 KeV Argon ion beam for 10 min
following modification.
XI
LIST OF SCHEMES
Scheme 1.1. Interchange among different forms of polyaniline.
Scheme 1.2. Mechanism for acid generation from triphenylsulfonium salts upon
exposure to UV radiation.
Scheme 1.3. Doping of polyaniline with acid generated from a UV irradiated photoacid
generator.
Scheme 1.4. Polymerization of aniline derivatives.
Scheme 3.1. Condensation polymerization of aniline and formaldehyde.
Scheme 3.2. Mechanism for oxidative polymerization of aniline.
Scheme 3.3. Termination of polymerization of aniline by AFC.
Scheme 3.4. Copolymerization of aniline and AFC.
Scheme 4.1. Photolytic dissociation of amines.
Scheme 4.2. Annihilation of primary amine radicals.
Scheme 5.1. Nucleophilic reactions of (A) diphenylamine; and (B) polyaniline with
diazonium salts in acid solution.
XII
ABSTRACT
The objectives of this work are to modify polyaniline to achieve specific bulk and
surface properties, and to process polyaniline using lithographic methods to achieve
micron-scale polyaniline structures.
Many of the bulk properties of polyaniline are a function of its molecular weight.
A new strategy has been employed to control the cross-linking of polyaniline which
allows the synthesis of high molecular weight conducting copolymers. The new method
utilizes aniline formaldehyde condensate CAFC) as a cross-linking agent to form high
molecular weight copolymers. The copolymers can be obtained by chemically oxiqizing
aniline-APC mixtures. Aniline is oxidized to form polyaniline chains through a cation
radical mechanism, while AFC acts as a terminating reagent, stopping the propagation of
polyaniline chains. As a result, AFC chains bind polyaniline chains to form a larger
molecular weight copolymers. The conductivity, molecular weight and solubility of the
copolymers can be controlled by varying the ratio of APC to aniline prior to chemical
polymerization.
Micron-scale polyaniline structures can be fabricated using lithographic methods
based on the fact that polyaniline becomes insoluble in DMF after UV irradiation. The
change in solubility of polyaniline in DMF after UV irradiation is likely due to crosslinking that occurs among polyaniline chains when exposed to UV radiation. Patterned
polyaniline structures can be obtained when polyaniline films are exposed to UV through
1
a mask followed by rinsing the exposed polymer films with DMF. Structures down to 3
11m can obtained using this method.
Polyaniline thin films can be chemically modified by reacting with 4-substituted
benzenediazonium tetrafluoroborate salts in acidic aqueous solutions. X-ray
photoelectron spectroscopy results suggest that partially modified polyaniline films
exhibit a layered structure with an electroinactive modified outer layer and an
electroactive unmodified inner layer. Cyclic Voltammetric data demonstrates that the
modification is controlled by the diffusion of diazonium ions into polyaniline film and
thereby allows spatial modification on a nanometer scale by controlling modification
time. The modification kinetics were found to be independent on the substituent group
on the 4-subsutituted benzenediazonium salt, thereby allowing different functional groups
be introduced into polyaniline films by simply varying substituent group
2
CHAPTER ONE
INTRODUCTION
1.1
History of Polyaniline
The first reports of the synthesis of polyaniline occured in the early 20th century,
where it was found that the oxidation of aniline with bichromate or chlorate in the
presence of copper or vanadium salts led to the formation of a black solid material,
"aniline black" (Green and Woodhead, 1910; Willstatter and Moore, 1907). The material
was found to be insoluble, infusible and therefore difficult to process. Because of these
properties, it was virtually ignored for the first half of this century. In the late 1960's, it
was shown that polyaniline could be transformed to a conducting form by oxidation or
protonation (Surville, et al., 1968). However, this discovery remained unnoticed for
nearly two decades. Travers (1985) and Huang (1986) rediscovered polyaniline as an
organic conducting polymer in the middle of 1980's, and since that time extensive
research has been done to explore its potential applications in electronics (Parthasarathy,
et aI. 1994; Gustafsson, et aI., 1992) energy storage (Sotomura, et al., 1992; Osaka, et aI.,
1989; Michaelson, et aI., 1993), catalysis (Genies, et aI., 1991; Sykes, et al., 1995; Hable
and Wrighton, 1993), chemical sensing (Miwa, et al., 1994; Boyle, et aI., 1989; Barker, et
aI., 1994), and electrochromic devices (Depaoli, et aI., 1991; Sherman, et aI., 1994;
Hugotlegoff, et aI., 1993).
3
1.2
Structure and Properties of Polyaniline
Polyaniline can exist in four interchangeable forms including: leucoemeraldine
form, emeraldine salt form, emeraldine base form and pemigraniline form, depending on
its oxidation level (Orata and Buttry, 1987; Genies and Lapkowski, 1988). The structure
of the four forms are shown in Scheme 1.1. The leucoemeraldine form of polyaniline can
be obtained by reducing polyaniline chemically or electrochemically. It is slightly yellow
and insulating in this form. When the leucoemeraldine form of polyaniline is oxidized in
the pH < 3 solutions (Huang, et al. 1986; Hirai, et al. 1989), it is converted to the
emeraldine salt form where it is green and conducting. The emeraldine base form can be
obtained by treating emeraldine salt polyaniline with base solutions resulting in blue
insulating material. When emeraldine polyaniline is further oxidized, the pemigraniline
form is obtained. The pemigraniline form of polyaniline is non-conducting and has a
purple color. Figure 1.1 shows the electrochemically controlled changes between the
three forms of polyaniline by scanning potential in the range of -200 mV to 700 mV vs.
SCE in 1.0 M H2S04. When the potential is below 100 mV vs. SCE (region a),
polyaniline is in its leucoemeraldine form. In the potential window of 100 mV to 500
mV vs. SCE (region b), polyaniline is in its emeraldine salt form. When the potential is
scanned to higher than 500 mV vs. SCE (region c), polyaniline changes to its
pemigraniline form.
4
+20
1l
Leucoemeraldiue,
Insulating, Light Yellow
·20
~Q-NH~o-NHt
t
·~01-01~t
~~-oNt
n
n
Emeraldine Base,
Insulating, Blue
+20·, -+4W
Emeraldine Salt,
Conducting, Green
n
R
·20·, -4H+
Pernigraniline,
Insulating, Purple
Scheme 1.1. Interchange between different forms of polyaniline.
0.3
1..:
(a)
(c)
(b)
0.1
5
""""
= -0.1
U
_0.3'------1---1.--....a...--L.--1-----L.--J
-300
-100
100
300
Potential, vs.
500
700
seE
Figure 1.1. Cyclic voltammogram of a polyaniline film in 1.0 M H2S04 . Polyaniline is
in its leucoemeraldine form, emeraldine salt form and pemigraniline form in the potential
window (a), (b) and (c), respectively.
1.3
Synthesis and Molecular Weight of Polyaniline
Polyaniline is typically synthesized by oxidizing aniline monomer either
electrochemically (Diazo and Logan, 1980; Huang, et aI., 1986; Genies, et aI., 1985; Paul,
5
et aI., 1985; Paul, et aI., 1985; Noufi, et aI., 1982) or chemically (Hand and Nelson, 1978;
Wudl, et al., 1987; MacDiarmid, et al., 1985; Armes and Miller, 1988; Yasuda and
Shimidzu, 1993; Abe, et aI., 1989). Electrochemically synthesized polyaniline is difficult
to process due to its insolubility, while the chemically synthesized polyaniline suffers
from low molecular weight, which results in weak mechanical strength (Abe, et aI., .
1989). In recent years, considerable effort has been devoted to developing new methods
of synthesizing polyaniline in order to achieve higher molecular weights while
maintaining high conductivity. The most common strategy has been to change the
oxidizing agent (Abe, et aI., 1989; MacDiarmid, et aI., 1985; Pron, et aI., 1988). It has
been shown that when (Nl4hSzOs or KZCrZ07 is used, high molecular weights can be
achieved by lowering polymerization temperature to below DOC (Cao, et aI., 1989). The
highest weight-average molecular weights obtained using this method were in the range
of 1 x 105 g-mor l .
The adhesion of polyaniline is another important factor which may limit its utility
in areas, such as in antistatic coatings (Trivedi and Dhawan, 1993; Ohtani, et aI., 1993).
Low molecular weight polyaniline films casted from its N-methyl-2-pyrrolidone (NMP),
dimethylsulfoxide (DMSO) or dimethylformamide (DMF) solutions adhere strongly to
glass in dry atmospheres, while adhesio~ failure occurs quickly when immersed in water.
For example, polyaniline thin films detach from glass surfaces quickly when immersed in
water.
In chapter three, a method is introduced that utilizes APC to cross-link polyaniline
thereby forming a aniline-APC copolymer. The copolymer can be synthesized by
6
oxidizing AFC-aniline mixtures at room temperature. A wide range of molecular weight
polymers can obtained by simply adjusting the ratio of AFC to aniline prior to
polymerization. The conductivity and solubility of the copolymer are also a function of
the ratio of AFC to aniline and can be varied systematically. The copolymers also show
much better adhesion on glass as compared to polyaniline in water.
1.4
Lithographic Processing of Polyaniline
There is considerable interest in using conducting polymers in electronic
applications because of their light weight, flexibility, and low cost. As a result
considerable effort has been directed toward developing methods that will allow their use
in lithographic applications (Venugopal, et al., 1995; Angelopoulos, et aI., 1993).
Among the conducting polymers, polyaniline is one of the most attractive candidates
because its environmental stability (MacDiarmid, et aI., 1985).
One of the methods used to lithograph polyaniline (Venugopal, et al., 1995) takes
advantage of its unique solubility properties. The base form of polyaniline dissolves in
NMP, DMF, and DMSO. However, when polyaniline is doped with acid and converted
to its emeraldine form, it becomes insoluble (Zheng, et aI., 1994). A polymer film cast
from a solution containing polyaniline and a photo acid generator can be lithographed by
exposure the film to UV radiation through a UV transparent mask. Acid is generated at
the area exposed to UV radiation (Scheme 1.2) (Reichmanis, et aI., 1989), in tum dopes
polyaniline rendering it insoluble (Scheme 1.3). The polyaniline in the unexposed area
7
remains in its base form and can be washed away by solvents (NMP, DMSO, or DMF).
Therefore, a polyaniline pattern can be obtained after the washing process.
~
o-~
'f ~
-
I 'f
X" -
~
-hv
.-.
o-~s~t
_
...
X"
~I
~
--+-
o-~s~~
-
-
~
+ HX
~I
Scheme 1.2. Mechanism for acid generation from triphenylsulfonium salts upon
exposure to UV radiation.
Emeraldine Base,
Soluble in DMF
+2HX .......: --_hv
1Photoacid Generator
I
Emeraldine Salt,
Insoluble in DMF
Scheme 1.3. Doping of polyaniline with acid generated from a UV irradiated photoacid
generator.
Another approach (Angelopoulos, et aI., 1993) is to introduce reactive side chain
to the polymer by polymerization of monomer with side chain (such as, -CH3). After spin
coating, the polymer film is exposed to an electron beam. The side chains exposed to
electron beam cross-link the polymer, rendering polymer insoluble. After washing the
polymer film with the proper solvent, the unexposed portions of film dissolves, and a
polymer pattern is obtained. Other methods to fabricate fine polyaniline structures
include: UV-directed blocking of an electrode surface followed by electropolymerization
8
of aniline (Rozsnyai and Wrighton, 1995); and light-directed photoelectrochemical
polymerization of aniline on Ti02 (Kuwabata, et al., 1993).
In chapter four, we demonstrate that it is possible to cross-link polyaniline under
ambient conditions without the use of photo acid generators, the chemical modification of
the polymer backbone, or the use of special substrates. Micron-scale polyaniline
structures can be obtained using standard lithographic methods.
1.5
Chemical Modification of Polyaniline
Recent research in the area of conducting polymers has placed considerable
emphasis on controlling both chemical and electronic properties in the bulk polymer
(Salmon, et al., 1989; Genies and Noel, 1991; Wolf and Wrighton, 1994; McCoy, et al.,
1995) and at its interface (Torres and Fox, 1992). To achieve control over the solubility,
chemical and electrical properties of polyaniline, a common approach has been to
functionalize the polyaniline backbone.. One strategy to control polymer properties is to
introduce a suitable side chain to amine group to form a N-substituted aniline derivative
(Waltanable, et al., 1989; Sindhimeshram and Gupta, 1995; Ye, et al., 1995). By either
polymerization of the monomer or copolymerization of the monomer with aniline,
polymers with variable properties can be obtained. Another method is to functionalize
benzene ring of aniline (Genies and Noel, 1991; Sykes et al., 1995). The polymer
obtained by polymerization of such a monomer or copolymerization of the monomer with
aniline shows varied conductivities and solubilities. In some cases, soluble, polyaniline-
9
based polymers with high conductivities were obtained (Chan, et aI., 1995). The general
reaction of the two approaches is summarized in Scheme 1.4.
n
H
-2n e" •
-2n H+
--~
{S'
" +.
_In
it
4
51
Scheme 1.4. Polymerization of aniline derivatives.
Another approach used to alter the properties of conducting polymers involves
chemical modification of the polymer following polymerization (Yue and Epstein, 1990;
Yue, et aI., 1991). It has been previously shown that polyaniline can act as a nucleophile,
and that the rate of nucleophilic reactions can be controlled by changing the
electrochemical potential of the polymer (McCoy, et aI., 1995). These results
demonstrate that bulk chemical modification of polyaniline can be achieved in a
controlled and, in some cases, reversible fashion. However, the ability to introduce a
wide variety of functional groups and to control their spatial distribution within
conducting polymers has yet to be demonstrated.
In chapter five, we demonstrate a new method of chemical modification of
polyaniline that allows spatial control of modification on a nanometer scale in the
dimension perpendicular to the polymer surface. Our approach utilizes the diffusion
controlled nucleophilic reaction of amine groups in polyaniline with 4-substituted
benzenediazonium salts. Although the product, poly(N-4-substituted phenyl)aniline, is
electroinactive, the spatial control of chemical modification allows the formation of a
10
layered structure containing regions of chemically modified less conducting polymer and
regions of unmodified highly conductive polyaniline.
11
CHAPTER TWO
EXPERIMENTAL
·2.1
Materials
All the chemicals were used as received. Aniline, acetonitrile, formaldehyde,
DMF, THF were obtained from Baker. DMSO, K3Fe(CN)6,
~e(CN)6
and 4-
methoxybenzenediazonium tetrafluoroborate were obtained from Aldrich. Concentrated
H 2S04, concentrated HCI and concentrated N~OH were obtained from EM Sciences.
DMSO-~
was obtained from Isotec. Platinum foils and wires were obtained from ESPI.
4-Nitrobenzenediazonium tetrafluoroborate was prepared according to a
procedure described by Venkataraman (1952). 4-Nitroaniline (0.277g, 0.0020 mol) was
added to 4.0 mL 48% HBF4 (0.031 mol). A minimum amount of water was added to
.
dissolve the suspension, and the solution was then cooled down to
oec in ice bath.
Then
0.145g (0.0021 mol) of NaN0 2, dissolved in the minimum amount of cooled water, was
added dropwisely while stirring. The mixture solution was kept cool in ice bath for 5 min
and slight yellow precipitation was formed. The precipitation was cold filtered and
washed with copious cooled 5% HBF4, water, ethanol, and ether sequentially. White 4nitrobenzenediazonium tetrafluoroborate crystals were obtained. The product was stored
at oec and used within 2 days.
12
All the aqueous solutions were made with 18 M,Q distilled deionized water. All
the measurements were made at 25 ± 2 0c.
2.2
2.2.1
Experimental Procedures and Instruments
Electrochemical Techniques
Electrochemical measurements ~ere carried out with a Bioanalytical Systems
electrochemical analyzer, BAS-lOa. Working electrodes were polyaniline coated
platinum foils with areas of 0.75 x 2 cm2 if not otherwise specified. For modification of
polyaniline, films were deposited on platinum foils electrochemically at a constant current
of 0.1 mA-cm-2. For all the other measurements, polyaniline coated electrodes were
obtained by spin-coating chemically synthesized polyaniline from DMF or DMSO
solutions on platinum foils at a rotation rate of 300 rpm, followed by drying in vacuum
for at least 12 h. A platinum wire was used as the auxiliary electrode, and the reference
electrode was a Ag/AgCI electrode. All the potentials were reported versus this electrode
unless otherwise specified.
2.2.2 X-ray Photoelectron Microscopy (XPS)
XPS data were obtained with a SCIENTA ESCA-300 spectrometer with a Mg Ka Xray source (hv = 1253.6 eV). Sensitivity factors used for CIs and Nis were 1.00 and 1.80
respectively (Moulder, 1992). The samples were electrochemically deposited polyaniline on
platinum foils. Prior to introduction into analysis chamber, the samples were doped with 1.0
13
M H2S04 acid and dried in vacuum for at least 12 h. In the depth analysis experiment, the
outer layer of the polymer was sputtered away with an 2 KeV Ar ion beam for 10 min.
2.2.3
Frourier Tranform Infrared Spectroscopy (FfIR)
FTIR spectra were recorded using a Mattson Sirius-l00 FfIR spectrometer. For
surface modified polyaniline thin films on Pt foils, reflectance IR spectra were obtained.
Transmission IR spectra were obtained for all other measurements. The samples were
pressed 99% KBr-l % polymer (w/w) pellets or polymer coated silicon wafers.
2.2.4 UV-vis Spectroscopy (UV-vis)
The UV-vis spectra were obtained on a Shimadzu UV-2101PC UV-vis scanning
spectrophotometer. In all the measurements in Chapter 3, the reaction mixtures were
placed in quartz cuvettes. For other measurements, samples were prepared by spincoating polymer on quartz substrates.
2.2.5
IH Nuclear Magnetic Resonance eH NMR)
IH NMR spectra were collected on a Broker AMX 360 NMR spectrometer,
DMSO-d6 was used as solvent. Before the measurements, the solvent/polymer mixtures
were filtered through 0.4 Jlm filters to remove insoluble precipitation.
2.2.6
Ouartz Crystal Microbalance COCM)
14
QCM measurements were performed on a EG&G QCA 197 microbalance
analyzer coupled to a EG&G 273A potentiostat/galvanostat. The quartz crystals were 8.8
MHz AT-cut (EG&G). The active portions of the quartz were coated with Pt and had
round areas of 0.20 cm2•
2.2.7 Gas Chromatography / Mass Spectroscopy (GC/MS)
GC/MS data were obtained on a Hewlett-Packard 5890 series II plus Gas
Chromatograph coupled with a Hewlett-Packard 5972 series Mass Selective Detector.
The GC inlet temperature was set to 300°C, the column temperature was programmed to
increase from 60°C to 320°C with rates of 10°C-s- 1• After 20 mM diphenylamine and 20
mM substituted benzenediazonium salts were mixed in 1.0 M H2S04 for 0.5 h, 1 /-11 of
reaction mixtures were injected in.
2.2.8 Gel Permeation Chromatography (GPC)
The polymer molecular weights were obtained on a Water 150C gel permeation
chromatograph. The mobile phase was 50% NMP/50% THF (v/v). Polystyrenes were
used as standards.
2.2.9 Scanning Tunneling Microscopy (STM)
STMs were obtained on a Digital Instrument ECM-NS3 Nanoscope Scanning
Probe Microscope. Prior to the measurement, polyaniline films was doped using 1.0 M
H2S04 for 5 min followed by drying in vacuum for at least 2 hours. The tungsten STM
15
tips were prepared by etching tungsten wires in 1.0 M KOH solution electrochemically.
The depth characterization experiments were carried out with biases of 200 mV and
tunneling currents of 100 nA. The surface topography images of polyaniline films were
obtained using biases of 200 mV and tunneling currents of 2 nA. All the STM
experiments were carried out in air.
2.2.10 Scanning Electron Microscopy (SEM)
SEM images were obtained on a JEOL JSM-6300F Scanning Microscope with
accelerating voltages of 1 KV.
2.2.11 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetric data was obtained on the base forms of
polymers in air using a Perkin Elmer DSC 7 Differential Scanning Calorimeter with a
temperature rate of +20°C-min- l .
16
CHAPTER THREE
SYNTHESIS AND PROPERTIES OF ANILINE-ANILINE
FORMALDEHYDE CONDENSATE COPOLYMERS
In this chapter, a new strategy is introduced to control the cross-linking among
polyaniline chains, thereby allowing the synthesis of high molecular weight, conducting
copolymers. The new method utilizes aniline-formaldehyde condensate (AFC) as a crosslinking agent to form high molecular weight copolymers. The conductivity, molecular
weight and solubility of the copolymers can be controlled by varying the ratio of AFC to
aniline prior to chemical polymerization of aniline-AFC mixture. The adhesion of the
copolymers to glass is significantly better than pure polyaniline.
3.1
Synthesis and Structure of Aniline Formaldehyde Condensate
AFC was synthesized according to the method described previously (Kuo, et aI.,
1989). In a 100 ml reaction vessel, 5g of a 37% formaldehyde solution was added to the
mixture of 9.3g aniline-3g of a 37% HCI solution. Condensation was carried at - 100°C
for 2 h (see Scheme 3.1). The temper~ture was then decreased to 60°C and the mixture
was neutralized with 4g of a 30% NaOH solution resulting in an insoluble liquid resin.
The resin was then washed with warm water three times and separated from the aqueous
layer. The resin was kept at 80°C under a reduced pressure of 10 KPa for 1 h to remove
any unreacted aniline and/or formaldehyde.
17
~
nO
+
HO~
nIKHO
f01
n
Scheme 3.1. Condensation polymerization of aniline and formaldehyde.
3.2
3.2.1
Structure of Aniline-AFC Copolymers
Mechanism of Polymerization of Aniline
The mechanism for the polymerization of aniline has been the subject of
considerable research (Mohilner, et aI., 1962; Duic and Mandie, et al., 1992; Duic, et aI.,
1995). A cation radical mechanism has been proposed as shown Scheme 3.2, where the
polymerization can be divided into three steps involving: initiation, propagation and
termination (Mohilner, et aI., 1962; Genies and Tsintavis, 1985; Duic, et aI., 1995). Since
every step involves removing electrons from the system, a proper oxidizing agent or
oxidizing condition (Le., by applying a positive potential), is required for the
polymerization to occur. The molecular weight of polyaniline will be determined by its
kinetic length (Le., the average number of monomers that react with an active center from
its formation until it is terminated). Since the growth of polyaniline follows a radical
mechanism, the kinetic length of polymer is defined to be the ratio of the rate of
polymerization to the rate of initiation (Rudin, 1982), which is equal to the ratio of the
rate of polymerization to the rate of termination under steady conditions. The increasing
termination rate results in shorter chain length.
18
Initiation
HN-~
2
\=.I
_..:;-e.....
H ~_~ ..._ +
\=.I
2
Propagation
Termination
H
N-~HN-~
1 HN=F\.·
+
\=.IL
\.:.II
"="H
2
n
H
2
N-~
\=.I
-e, -2H+. H N-~HN-~l HN~
2
\=.IL
\.:.II ~
n
Scheme 3.2. Mechanism of oxidative polymerization of aniline.
3.2.2 Mechanism of Copolymerization of Aniline and APC
APC has alternating aniline and methylene units. The substituted aniline unit in
APC has a similar structure to that of alkyl substitute aniline in the sense that they both
have bulky groups attaching to the benzene rings. Although previous work (D'Aprano, et
aI., 1993) has demonstrated that 2,5-substituted anilines (Le., 2,5-dialkylanilines) can be
polymerized to form substituted polyaniline using both chemical and electrochemical
methods, attempts in our laboratory to polymerize APC by itself were not successful. The
observed resistance to polymerization is attributed to the strong steric interactions
associated with the substituted aniline units.
Oxidative polymerization of aniline in the presence of AFC results in copolymers
that incorporate APC (see below). The fact that copolymerization occurs quite readily is
19
attributed to the fact that the resulting structures allow AFC chains to be further apart,
thereby reducing the influence of steric interactions. When AFC is added to the reaction
mixture, its role on the rate of polymerization can be estimated by considering the effect
of AFC on each step of the polymerization (Scheme 3.2). For example, AFC will not
likely influence the initiation or propagation step significantly due to the steric hindrance
associated with AFC. However, AFC is expected to act as an effective terminating agent
since the steric effects of AFC will hinder further propagation. Therefore, the most
important role of AFC on the overall polymerization reaction will likely be in the
termination step. The proposed mechanism for AFC termination of the polymerization is
shown in Scheme 3.3.
H
2
N-~HN-~l
+
'=.Tl::
\.:.II HN=F\.·
'="H
-e,-2H+. H
H 2N-P";;
n
2
-
N-~HN-~l
HN-9";;
~
\.:.II
n
Scheme 3.3. Termination of polymerization of aniline by AFC.
3.2.3 Kinetics of Aniline-AFC Copolymerization
The influence of AFC on the rate of the overall polymerization reaction can be
summarized as follows:
Initiation
Propagation
Termination
A+· + e
A
An+· + A
An+· + A
An+· + AFC
An+l +.
An+1 - e
An(AFC) - e
20
where A represents neutral aniline monomer; A+. represents the aniline cation radical;
AFC represents aniline-formaldehyde condensate; and k1, k2 , k3, ~ are corresponding
reaction rate constants. The reaction rate for the above mechanism will then be a function
of rate of generation of A+. in the initiation step,
(3.1)
the rate of consumption of A in propagation step through the reaction with monomer,
(3.2)
and the rate of consumption of An+. in the termination step
(3.3)
Assuming that the rate for generation of An+. equals the rate for consumption of
An+· (Le., under steady state conditions), equations 3.1 and 3.3 will be equal.
(3.4)
Hence
(3.5)
If the rate of polymerization is taken to be the rate of disappearance of monomer,
and the contribution of the initiation and termination steps to the disappearance of
monomer is negligible, the rate of polymerization, R, will be equal to the rate of
propagation (eq. 3.2). Substituting equation 3.5 into equation 3.2.
R =k2[A] [A+·]
=k1k2[Af /{k3[A] + ~[AFC]}
(3.6)
Rearranging equation 3.6
(3.7)
21
it becomes apparent that a plot of AFC concentration versus the reciprocal of the
polymerization rate will yield a straight line with a negative intercept.
Polyaniline is a highly conjugated system, and as a result, has a very different UVvis absorbance spectrum from that of aniline monomer or AFC. Therefore, the amount of
polyaniline formed by oxidizing aniline using H20 2 can be tracked by observing the UVvis spectra as a function of reaction time (Figure 3.1). Before H20 2 is added to the
mixture, there is no absorbance peak in the range of 300 nm to 900 nm, however upon
addition of H20 2, polymerization occurs and two absorbance peaks at 340 nm and 700
nm emerge corresponding to the formation of polyaniline. Similar UV-vis spectra are
obtained for polymerization of AFC-aniline mixtures, where the absorbance peak
positions are independent of the ratio of AFC to aniline in the reaction mixture. The
progress of the polymerization reaction for various AFC-aniline mixtures can be tracked
by observing the absorbance of the polymerization mixture at 700 nm as a function of
polymerization time, as shown in Figure 3.2. The absorbance reflects the concentration
of polymer formed in the reaction mixture, while the slopes of the curves are proportional
to the reaction rates. It is clear from the data shown in Figure 3.2 (constant
concentrations of aniline and H20 2) that the rate of polymerization decreases as the
concentration of AFC increases in the mixtures.
22
4
3
Q)
o
c
m
.c
le
~
«
2
1
o
..
1
01-_ _.....a._ _----1
' - -_ _.......
200
400
600
Wavelength, nm
800
1000
Figure 3.1. UV-vis spectra of a 30gIL aniline/1.0 M HCI solution after adding 20gIL
H2 0 2 for (1) 0 min; (2) 2 min; and (3) 5' min.
4
1
2
3
10
20
4
5
3
Q)
0
c
m
.c
2
l-
ef/)
.c
«
1
30
40
Reaction Time, min
Figure 3.2. The absorbance at 700 nm of (1) 30gIL aniline; (2) 30gIL aniline + 1.SglL
AFC; (3) 30gIL aniline + SglL AFC; (4) 30gIL aniline + 10 gIL AFC; and (5) 30gIL
aniline + 15 gIL AFC solutions as a function of reaction time with 20gIL H20 2.
23
If the slopes of the lines in Figure 3.2 are assumed to be proportional to the
reaction rate at different concentrations of AFC, a plot of AFC concentration versus the
reciprocal of the slope can be constructed as shown in Figure 3.3. The linear relationship
observed supports the assertion that AFC acts as a terminating agent in the
polymerization reaction. Further, as seen in equation 3.7, the y-intercept is proportional
to k3~, which is the ratio of the rate constants for termination by monomer to
termination by AFC. From Figure 3.3 a value of 1/8.5 was determined for k3~
indicating that AFC is 8.5 times more effective as a terminating agent compared to aniline
monomer. These results suggest that the copolymer obtained from the polymerization of
the AFC-aniline mixture has a structure similar to that represented in Scheme 3.4 where a
single AFC polymer chain can bind many polyaniline chains.
16
-
12
«
a
c
0
4
Q)
0
.J
0)
cS
LI-
'0
~
b
c
0
c
0
0
-4 0
2
4
a 10
6
Slope -1 , min
12
Figure 3.3. The plot of the concentration of AFC as a function of the corresponding
reciprocal slope obtained from Figure 3.2.
24
1
Hz
2nk
Q
-&eH,r.
NH
+
(m+2)
Scheme 3.4. Copolymerization of aniline and AFC.
3.2.4 Evidence of Copolymer Structure from Spectroscopic Data
Clearly, the ratio of AFC to aniline in the initial mixture will influence the degree
of cross-linking and the spacing between polyaniline chains. These changes will in turn
alter the structure, molecular weight and conductivity of the resulting copolymers. In
order to investigate these relationships, copolymers were synthesized using different
starting material ratios. The polymerization yields for different copolymers are
summarized in Table 3.1. The yields obtained using this method are higher than those
reported for methods using FeCb (Yasuda and Shimidzu, 1993), (~)S208 (Cao, et aI.,
1989) or O2 (Toshima, et aI., 1994) as oxidizing agents, which were typically less than
40%.
25
Table 3.1. The yields and starting materials for polyaniline and APC-aniline
copolymers. Percent yields were determined by calculating: %Overall Yield =
1000/£oX P
. ht/(APC weIg
. ht + amTme weIg
. h)
t .
rod
uct weIR
Polymers
Aniline (2)
Overall Yield (%)
AFC (sd
3.0
Pani100
0.00
62
3.0
Pani20
0.15
57
2.5
Pani5
0.50
47
2.0
Pani2
1.00
40
It is apparent that the yield of the polymerization reaction decreases with the
increasing relative amounts of APC in the reaction mixture. There are two factors that
are expected to influence the overall yield of the polymerization reaction. First, the
substituted aniline units in APC are not as reactive as aniline monomer due to strong
steric effects as discussed above. As a result, most of AFC may remain unreacted after
the polymerization. These unreacted APC chains would then be removed in the washing
process (AFC is soluble in 1.0 M HCI) thereby decreasing the overall yield. Under these
conditions, the relative amount of APC in the copolymer is expected to be lower than that
in the starting materials. This assertion is supported by IH NMR and FTIR
measurements. Figure 3.4 compares the IH NMR spectra ofPani100 and Pani20 (No
spectra were obtained for Pani5 and Pani2 due to their relatively low solubility). As
shown in Figure 3.4, Pani20 has a spectrum similar to that of pure polyaniline where the
peak at 3.34 ppm corresponds to -NH- and -NH2 protons and the peaks around 7 ppm are
assigned to protons on benzene ring. The peak at 5.23 ppm is assigned to trace amounts
of water. The spectrum of Pani20 shows a minor peak at 3.7 ppm, corresponding to CH2- group in the copolymer. The ratio of -CHr/-NH- in the copolymer can be
calculated from the area ratio of 3.7 ppm peak to 3.34 ppm peak. The value was
26
determined to be approximately 1:150, suggesting that AFC chains in the copolymer
accounts for less than 1% of copolymer mass. In addition, all the polymers had nearly
identical IR spectra with no identifiable peaks associated with methylene groups, further
supporting the assertion that the copolymers have significantly less AFC than in the initial
reaction mixtures.
methylene
protons
Pani20
amIne
protons
aromatic
protons
/
PanilOO
ppm
8
7
6
5
4
Figure 3.4. 1H NMR spectra of Pani100 and Pani20 in DMSO-d6•
The second factor likely to influence the overall yield of the polymerization is that
AFC is a stronger terminating agent than aniline monomer as described above. Since the
27
rate of termination will increase as more AFC is added to the polymerization mixture (see
Figure 3.2), it follows that the kinetic chain length of polyaniline (i.e., the average
number of monomers that react with an active center from its formation until its
termination), which is proportional to the reciprocal of terminating rate of the
polymerization (Rudin, 1982), will be shorter with increasing amounts of AFe. Since
AFC and short chain aniline oligomers are more soluble in acidic aqueous conditions, it is
expected that the percent yield will decrease as observed in Table 1.
3.2.5 Molecular Weight of Aniline-AFC Copolymers
Although polyaniline is not soluble in conventional organic solvents in its acid
doped form, the base form (obtained upon exposure to 1.0 M ~OH) is soluble in NMP,
DMSO and DMF. The copolymers PanilOO, Pani20, Pani5 and Pani2 exhibit
significantly different solubilities in these same solvents. For example, both Pani100 and
Pani20 dissolve in NMP or DMSO up to approximately 5% by weight while Pani5 and
Pani2 are only partially soluble, with Pani2 being the least soluble. These changes in
solubility can only be attributed to changes in molecular weight of the copolymers since
both polyaniline and AFC are soluble in the above solvents on their own. Further, since
AFC is soluble in the 1.0 M HCI used in the washing procedure, any unreacted AFC will
be removed from the final products. The weight-average molecular weight of Pani100
and Pani20 were determined to be 8,200 and 1.2 x 106 g-mor 1 by GPC, respectively. The
corresponding polydispersities were 1.39 and 6.75 respectively (Figure 3.5). Pani5 and
Pani2, on the other hand, were not sufficiently soluble to characterize by this method.
28
These results are consistent with the structure proposed above where AFC chains bind
polyaniline chains to form larger molecular weight molecules, as illustrated in Scheme
3.4. This mechanism is expected to result in a complex network similar to that shown in
Figure 3.6 where the ratio of APC to polyaniline in the structure will depend on the ratio
of APC to aniline in the reaction mixture.
4
6
8
10
Log(Molecular Weight, g-mol-1)
Figure 3.5. GPC chromatogram of (a) PanilOO; and (b) Pani20.
29
Polyaniline
Figure 3.6. A schematic representation of polyaniline-AFC copolymer network formed
from the chemical oxidative polymerization of aniline in the presence of AFe.
3.3
3.3.1
Properties of Aniline-AFC Copolymers
Conductivity
The conductivities of the polymers were obtained by measuring the resistance of
pressed, 1.0 M HCI doped, polymer pellets using the device shown in Figure 3.7. The
conductivity was calculated using the equation (Dean, 1979)
k = L/(SR)
(3.8)
where k is the conductivity; L is the distance between two Pt foils (2 cm); S is the cross
section area (which is perpendicular to cross section shown in Figure 3.7) and equals to
the thicknesses of polymer pellets, a (-0.05 cm) times the widths of polymer pellets, b (l
cm); R is the resistance measured using a multimeter.
30
Pt foils
iL~
Glass
Polymer
Glass
Multimeter
Multimeter
Cross section view
Up-down view
Figure 3.7. A schematic representation of the device used to measure the conductivities
of polymers.
The conductivity of the polymers as a function of the AFC to aniline ratio in
starting materials is shown in Figure 3.8. The conductivity of the polymer decreases as
the ratio of AFC to aniline increases. However, in all cases, the copolymers have higher
conductivities than the 3.10 x 10-6 n-1_cm- 1 reported for poly(2,5-dialkylanilines)
(D'Aprano, et al., 1993). It has been demonstrated that interchain electron hopping is the
dominant charge transport mechanism in these materials (Genies and Noel, 1990; Lu, et
aI., 1986; Wudl, et aI., 1987). As a result, any factor that inhibits electron hopping
between polyaniline chains will decrease the conductivity of the polymer. The presence
of AFC chains increases the interchain distance between polyaniline chains, which in tum
reduces the probability of interchain electron hopping between polyaniline chains. As a
result, addition of AFC is expected to lower conductivity. In addition, AFC chains in the
copolymer may also induce twisting of the polyaniline chains thereby· decreasing 1t
31
overlap and thereby decreasing the conductivity of the copolymer (Bredas, et aI., 1985;
D'Aprano, et aI., 1995).
-E
~
()
1.0
,
I
6. 0.5
~
:~ 0.0
...,
()
~
'C
c -0.5
o
o
C)
0-1.0
.J
0.0
0.1 0.2 0.3 0.4 0.5
AFC(g)/Aniline(g)
Figure 3.8. Plot of log (conductivity) as a function of AFC to aniline ratios used for the
synthesis of the copolymers.
Based on the above discussion, it is apparent that copolymers with desired
molecular weights can be obtained by carefully controlling the ratio of AFC to aniline in
the reaction mixture. Furthermore, the conductivity and solubility of copolymer can also
be controlled by adjusting the APC to aniline ratio. Our results demonstrate that, in some
cases, copolymers (such as Pani20) with high molecular weights, high conductivities and
good solubilities can be obtained.
32
3.3.2
Electrochemistry
The electrochemistry of the polymers was investigated using cyclic voltammetry.
Figure 3.9 shows the cyclic voltammograms of the polymers in 1.0 M H2S04 • In order to
avoid the degradation of the polymers at high potential, the upper potential limit was set
to 0.6 V. Pani20 has a similar anodic peak potential and similar peak separation to that of
PanilOO indicating that they have similar redox behavior (Kaliji, et al., 1991). The fact
that the peak separation of PaniS is larger than that of Pani100 or Pani20 suggests that
PaniS has a lower conductivity, resulting in a significant iR drop. Previous work (Genies,
et al., 1990) has established that the voltammetric current in the potential window
positive of the anodic peak (between 0.4 V and 0.6 V) is purely capacitive current
associated with the conducting polymer. The current response in this potential window
can in turn be modeled as the charging and discharging of a simple RC circuit, where R is
the resistance of the polymer film, C is the capacity of the double layer of the polymer
coated electrode. When R is small, the cyclic voltammetric response of the circuit
approaches that of a simple capacitor. Under these conditions, the discharging process is
fast, and therefore the change of the sign of voltammetric current from positive to
negative at positive reverse potential (0.6 V) is fast. When the resistance of the polymer
increases, the discharging process becomes slower, and in turn the rate of the change of
the current sign upon reversing the potential decreases. As seen in Figure 3.9, the
discharging behavior of Panil00 and Pani20 is fast, and that of PaniS is slow. These
results are consistent with the conductivity measurements, indicating that the resistance of
PaniS is significant higher than Pani100 or Pani20.
33
1.0
0.5
~
.-.-
Q.
;
;
;
0.0
-1'~200
0
200
400
600
Potential, mV vs. Ag/AgCI
Figure 3.9. Cyclic voltammograms ofPani100 E-) and copolymers (------) in 1.0 M
H2S04, scan rate =100 mV-s- 1•
.
3.3.3 Adhesion
Peel tests and water immersion tests were performed to evaluate the strength of
adhesion of different polymers to glass. Polymer coated glass slides were prepared by
casting their DMSO solutions (Pani100 and Pani20) or its DMSO suspension (paniS)
onto glass slides, followed by drying in vacuum for at least 12 h. Pani2 films could not
be obtained due to its very low solubility. The dried polymer films were found to adhere
well to glass. Although all the films survived peel tests with standard scotch tape, only
the copolymers were able to survive peel tests using duct tape, which contains a stronger
adhesive. The strength of adhesion of the polymers on glass was also compared by
immersing them in water and measuring the time required for the films to detach. Table
34
3.2 shows the time required for detachment for the different polymers. The cross-linked
copolymers have significantly stronger adhesion to glass than PanilOO. This is likely due
to the decreased permeability and increased mechanic strength of the polymer after the
cross-linking.
Table 3.2. Time required for polymer films to detach from glass in
contact with distilled water.
Polymers PanilOO
5 min
Time
3.3.4
Pani20
Pani5
> 2 weeks
>2 weeks
Pani2
---------
Thermal Behavior
The thermal behavior of the copolymers were investigated using differential
scanning calorimetry. Neither the pure polyaniline nor the copolymers showed a glass
transition temperature or decomposition in the range of 20 to 500°C. A gradual decrease
in power was observed between 50 and 500 °C corresponding to loss of absorbed water
(Conklin, et al., 1995).
35
d
c
b
a
o
100 200 300 400 500
o
Temperature, C
Figure 3.10. DSC curves of (a) PanilOO; (b) Pani20; (c) Pani5; and Pani2.
3.4
Conclusions
Aniline-AFC copolymers can be synthesized by oxidizing aniline-AFC mixtures.
AFC acts as a terminating agent in the polymerization process. The molecular weight,
solubility, and conductivity of the copolymers can be varied by controlling the ratio of
AFC to aniline prior to polymerization. In some cases, higher molecular weight
copolymers with similar conductivities and solubilities to that of polyaniline can be
obtained. When the ratio of AFC to aniline is further increased, an insoluble conducting
gel is obtained. The method introduced here represents a new strategy to control the
molecular weight of aniline-AFC copolymers, and hence control their physical properties.
36
CHAPTER 'FOUR
LITHOGRAPHY WITH POLYANILINE
t
In this chapter, a new method is presented for fabricating micron-scale polyaniline
structures. The approach takes advantage of the discovery that polyaniline becomes
insolu~le
in DMF after UV radiation. When a mask covered polyaniline film is exposed
to UV radiation and then developed by rinsing with DMF, the pattern on themask is
transferred to polyaniline film. The unirradiated portions of the polymer film dissolve
into DMF, while the irradiated portions remain on the substrates. Structures on the order
of 3 J..Lm can be obtained using this method. The effect of UV radiation on the
conductivity and redox activity of polyaniline is relatively small.
4.1
4.1.1
Effect of UV Radiation on the Properties of Polyaniline
Conductivity
Polyaniline was synthesized by oxidizing aniline in 1.0 M HCI with H20 2 in the
presence of trace amount of Fe (II) as catalyst (see Chapter 3 for detailed description).
The relative conductivity of the polymer was measured using the device shown in Figure
4.1, where a scratch separates the conducti~g regions on an ITO coated glass slide. A
polyaniline film was spin-coated onto the slide from a DMF solution containing the base
form of polyaniline. The thin polyaniline filni acted as a contact between the two
37
conducting regions of ITO. The conductivity of the film was measured in its "acid doped
form obtained by treating the film with 1.0 M Hel for 5 min followed by drying in
vacuum for at least 2 hours. Mter the conductivity measurement, the film was treated
with 1.0 M
N~OH
for 5 min to convert it back to.its base form. The base form
polyaniline film was then subjected to UV radiation from a low-pressure mercury lamp,
..
and the process was repeated. Table 4.1 shows the results obtained from conductivity
measurements. The results suggest that the effect of UV radiation on the conductivity of
polyaniline is relatively small. Even after prolonged radiation, at least 80% of its
\
conductivity is maintained.
!---I-
.
scratch
PANI film---.. . . . .
___.....--iiilIIIII. . . .--....l-r
TO coate
slide
R J------'
Figure 4.1. A schematic representation of the device used for relative conductivity
measurement.
Table 4.1. The conductivity of a polyaniline film as a function of UV
radiation time.
19 .
1
0
4
Radiation Time. h
0.90
1.24
0.84
1.00
Relative Conductivity
38
4.1.2 Electrochemistry
Cyclic voltammograms of a polyaniline film as a function of UV radiation time
are shown in Figure 4.2. It can be seen that the magnitude of the response decreases with
the increasing radiation time. The anodic peak current, ip,a, decreases approximately 20%
after 47 h UV radiation, indicating the electroactivity of the polymer decreases with the
increasing radiation time. However, the peak separation does not change noticeably even
after 47 h of UV radiation, indicating that the resistance of polymer does not change
significantly. This is consistent with results obtained from conductivity measurements.
<C
E
........
C
Q)
s..
s..
~
o
-200
0
200
400
600
Potential, mV vs Ag/AgCI
Figure 4.2. Cyclic voltammograms of a polyaniline thin-film (cast onto a platinum foil
electrode) in 1.0 M H2S04 as a function of radiation time. Scan rate =100 mV_S-I.
39
4.1.3
Solubility
The solubility of polyaniline decreases with the increasing UV radiation time.
The freshly spin-coated polyaniline films can thoroughly dissolve into DMF. However,
upon prolonged UV radiation (Le., more than 47 h), polyaniline becomes insoluble. As a
result, irradiated portions of polyaniline films remain on the substrates even after with
copious DMF.
4.2
Mechanism of the Photochemical Reaction of Polyaniline upon UV Radiation
The mechanism of the photochemical reaction of polyaniline under UV radiation
was investigated by UV-visand FTIR. Since the photochemical reaction was found to be
independent of the presence of O2 and H20 (Le., the reaction occurred under N2), it was
concluded that the reaction must be among or within polyaniline chains. FTIR data
shows that IR spectrum (600 - 4000 em-I) of polyaniline does not change noticeably even
with prolonged radiation times (72 h). This suggests that the polymers, after UV
radiation, has a similar chemical structure and composition as that prior to UV radiation.
It is known that amines can undergo photolytic dissociation reactions as shown in
Scheme 4.1 (Mouseeron-Canet and Mai, 1972). The radicals produced are highly
teactive and can subsequently undergo the reactions shown in Scheme 4.2 (only the
reactions among primary amine radical are shown, others are similar) (Mouseeron-Canet
and Mai, 1972). The·amine groups in polyaniline chains may also undergo similar
40
reactions and form radicals under UV radiation. The radicals can then react with other
radicals in other polyaniline chains resulting in cross-linking. As the degree of crosslinking increases, the polymer will become insoluble.
Prim ary am ines
R-NH
hv. R-NH
2
+
H' ...- -.... R-NH
2
+
H'
Secondary am ines
'T
_
~h__
v ...l~ ....
+
+
H'
Scheme 4.1. Photolytic dissociation of amines.
R.NH
2
R-NH
+ R.NH 2 - - -
NH 2 ·R.R-NH 2
+ k.NH 2- -
R.NH·R-NH 2
Scheme 4.2. Annihilation of primary amine radicals.
The UV-vis spectra of a polyaniline film as a function of radiation time are shown
in Figure 4.3. The decrease in intensity and the accompanying shift to shorter
wavelengths of the absorbance peak at 650 nm indicates a decrease in the average
conjugation length, likely caused by broken double bonds or twisting of the backbone
induced by cross-linking after radiation. This in tum decreases the conductivity and
electroactivity of polyaniline (Bredas, et aI., 1985; D'Aprano, et aI. 1995). These results
are consistent with that obtained from conductivity and cyclic voltammogram
experiments.
41
reduced nitrogen and the 6: 1 ratio for carbon to nitrogen indicates that the mechanism of
chemical modification is nucleophilic displacement (Genies and Noel, 1991). These
results are consistent with those obtained from model compound (Scheme 5.1). For
example, if absorption of the 4-nitrobenzenediazonium ion or azolinkage formation
(Venkataraman, 1952) were responsible for the incorporation of nitro groups into the
film, the ratio of oxidized to reduced nitrogen would be 1:3 and the ratio of carbon to
nitrogen would be 3:1.
modified 60 min
,t
5':10
sc:
modified 10 min
::s
o
o
unmodified
408
404
400
396
Binding Energy (eV)
Figure 5.5. High resolution XPS spectrum of polyaniline films (-80 nm thick) after
modification in 50 mM 4-nitrobenzenediazonium tetrafluoroborate and 1.0 M H2S04 acid
solution for different time periods.
52
Table 5.1. Possible reactions between polyaniline and 4-nitrobenezendiaznium salt, and
the ratios of carbon to nitrogen (C/N) and the ratios of oxidized nitrogen (-N02) to
reduced nitrogen (other nitrogens) (Nox/Nred ) in the corresponding products.
Reactions
Product Formulas
elN
NoxlNred
<1:1
Absorption
~~-J-n
< 6:1
H
N~NO
'~'
Partial Modification
fO-'4EO-il
6:1
<1:1
Azo Formation
3:1
1:3
Nucleophilic
Substitution
6:1
1:1
Y:,
5.4.3 Mechanistic Investigations Using Reflectance FfIR
The bulk properties of modified polyaniline were investigated by RFTIR. Figure
5.6 shows the RFTIR spectra of a polyaniline film and a 4-nitrobenzenediazonium
tetrafluoroborate modified polyaniline film using a Pt foil as a reflective substrate. The
spectrum of unmodified polyaniline is similar with that reported in the literature
(Kanamura, et aI., 1994; Neon, et aI., 1995; Wudl, et aI., 1987), where the absorbance
peaks of the polymer films at 1580 and 1480 cm- 1 are characteristic of the various
vibration modes of C-C in benzene ring (Colthup, et aI., 1975), and the peaks at 1247 and
1301 cm- 1 for polyaniline correspond to the stretching vibration of C-N bonds. The peaks
at 1511 and 1339 cm- 1 (indicated by the arrows in Figure 5.6) for the modified film
correspond to the asymmetrical and symmetrical stretching vibration of nitro group. No
53
absorption peak at 1415 cm- l corresponding to N=N stretching was observed. The results
are therefore in agreement with those obtained from XPS.
Q)
o
c
m
.Q
L-
o
en
.Q
«
1000
1200
1400
1600
1800
Wavenumber, cm- 1
Figure 5.6. Reflectance FfIR of (l) a polyaniline film; and (2) a polyaniline film after
reaction with 50 mM 4-nitrobenzenediazonium tetrafluoroborate for 48 h in 1.0 M
H2S04 • The arrows indicate the vibrations associated with -N02 group. The polyaniline
films were deposited onto Pt foils at constant current densities of 0.1 mA-cm- 2 for 1000 s
from a solution containing 0.25 M aniline and 1.0 M H2S04 •
5.4.4 Mechanistic Investigations Using QeM
Progress of the reaction between polyaniline and substituted benzenediazonium
ions was followed by monitoring the anodic peak current from cyclic voltammetry, ip,a,
for the oxidation of a polyaniline film as a function of time (Figure 5.7). Since the
completely modified polyaniline films were found to be electroinactive within the
potential window shown in Figure 5.7, and since the ip,a of polyaniline did not decrease as
54
a function of time in the absence of the substituted benzenediazonium ion, the anodic
peak current was assumed to be proportional to the amount of unmodified polyaniline. If
the modification proceeds as in Scheme 5.1 (B), the mass of the polymer is expected to
increase proportionally to the amount of modified polyaniline. Therefore, a linear
relation between the mass of the polymer and its peak current should be observed.
600
r---..,.....---T"--,r--~...,
!:
400
<C
:i
..,; 200
s::
Q)
s..
s..
;j
CJ
0
-200
-400 '--~~----_---.l~_"""'-""
-200
0
200
400
600
Potential, mV vs. Ag/AgCI
Figure 5.7. Cyclic voltammograms of a polyaniline thin film modified by exposure to 50
mM 4-nitrobenzenediazonium tetrafluoroborate in 1.0 M H2S04 acid for (a) 0 min; (b) 5
min, and (c) 15 min. Scan rate =10 mV-s· l .
The interfacial mass change was followed by a QCM. The relation between
frequency change and mass change of a QCM can be described in Sauerbrey's equation
(Sauerbrey, 1959; Hillman, et al., 1991),
M=
-2f;&n
Apiezo (Pq~q )112
55
(5.2)
where M is the resonance frequency change; fo is the original resonance of the quartz
crystal; &n is the mass change on the surface of the quartz crystal; Apiezo is the
piezoelectrically active area; pq is the density of quartz; and ~ is the shear modulus.
Since fo, Apiezo. pq and /lq are constants, the Sauerbrey's equation can be simplified to:
M=-K&n
(5.3)
where K is a constant. In order to determine the K value, the AT-cut crystal was
calibrated by depositing Cu from 0.025 M CuSOJI.O M NaAc solution on Pt coated
quartz crystal at a constant current of 0.5 mA-cm-2. Based on Faraday's law, the mass of
Cu deposited on quartz crystal can be calculated using
QM
m=-2F
(5.4)
where m is the mass of deposited Cu; Q is the charge passed for Cu deposition; M is the
molar mass of Cu, 63.57 g-mor l , F is the Faraday's constant, 96485 C-mor l . Figure 5.8
shows the calibration curve of a quartz crystal.
56
10 - - - - - - - - - . . . . - - - - - -
8
N
::I: 6
~
~.4
2
1
2
~m,
3
4
119
Figure 5.8. Resonance frequency change versus mass change calibration curve of a
quartz crystal microbalance. Cu was deposited from a 0.025 M CUS04-1.0 M NaAc
solution onto Pt coated quartz crystal at a constant current of 0.5 mA-cm- z.
From Figure 5.8, the Sauerbrey relation for the system was calculated to be:
M(KHz) =-2.583 Lill(llg)
(5.5)
The frequency change versus anodic peak current plot of a polyaniline film after
different modification times is shown in Figure 5.9. A linear relation between peak
current and frequency change was calculated to be:
ip,a(!!A) =930 + 200 M(Hz)
(5.6)
Substituting equation 5.5 into equation 5.6 yields
ip,a(!!A) =930 + 517 LlM(J.lg)
57
(5.7)
9.5
----w
r - - , - - - - . . , r - - - - - r - -__
9.0
~
E
ft
~
8.5
Q.
8.0
7.5
o
-500
~f,
-1000
Hz
Figure 5.8. Plot of frequency change versus anodic peak current, ip,a, of a polyaniline
covered electrochemical quartz crystal electrode in 1.0 M H2S04 after modified in a 50 4nitrobenzenediazonium tetrafluoroborate-1.0 M H2S04 solution for different time periods
(0 min; 5 min; 15 min; 45 min and 180 min). Scan rate = 100 mV-s- l .
By setting ip,a =0 for the fully modified polyaniline, the mass increase for fully
modified polyaniline is calculated to be 1.80 Jlg from equation 5.7. The mass of
polyaniline film prior to the modification was calculated to be 1.44 Jlg from equation 5.5
by tracking the frequency change of deposition of polyaniline from the solution. If the
modification proceeds through the mechanism in Scheme 5.1 (B), the monomeric unit
weight of the polymer should increase from 91 g-mor 1 to 212 g-mor 1 after modification,
corresponding to a mass increase of 1.911lg. This value is very close to the 1.80 Ilg
obtained from QCM measurement. If azo linkages are formed after modification, the
formula weight of monomeric unit of modified polymer would be 240, corresponding to a
58
2.36 J.Lg mass increase. This mass change is significantly larger than the experimental
value of 1.80 J.Lg.
5.5
5.5.1
Kinetics of Modification
Electrochemistry of Polyaniline upon Modification
The kinetics of the modification of polyaniline with substituted benzenediazonium
salts was investigated using cyclic voltammetry (Figure 5.7). Linear plots of anodic peak
current as a function of the square root of the reaction time (Figure 5.10) indicate that the
modification of polyaniline with both 4-nitrobenzenediazonium and 4methoxybenzenediazonium ions is diffusion-limited for films of different thicknesses
(i.e., film with different initial ip,a's). The similar slopes for both diazonium ions suggest
that their diffusivities are similar. The fact that the reaction of 4methoxybenzenediazoniumion with polyaniline is diffusion limited (Figure 5.10), and
yet it is reported to have the slowest kinetics for homogeneous nucleophilic displacement
reactions in acidic solutions for a series of 4-substituted benzenediazonium ions (R = -H,
-CH3, -C2Hs, -C(CH3)3, -C6Hs, -OH, -OCH3, -Cl, -Br, -N02) (Swain, et aI., 1975),
suggests that the reactions of polyaniline with any of the 4-substituted benzenediazonium
ions in the series will be diffusion-controlled. An average apparent diffusion coefficient
for the 4-substitued benzenediazonium ions within the polyaniline films can be estimated
from the inset plot in Figure 5.10 by assuming a film thickness of 84 nm (see Figure 5.2)
corresponds to an initial ip,a of 6.1 rnA (grown by passing 30 mC-cm-2) and that the
59
diffusion coefficient for the 4-substituted benzenediazonium molecule is the same in the
modified and unmodified polymer. Using the reaction time required to completely
modify the film from Figure 5.10 and the equation for the root-mean-square displacement
of a diffusing species (Bard and Faulkner, 1980):
(5.8)
where I!J. is the average diffusion distance (em); D is the diffusion coefficient (cm2_s- I );
and t is time (s), an average apparent diffusion coefficient of 1.5x10- 15 cm2_s- 1 is
obtained. Such a small apparent diffusion coefficient indicates that these species will
diffuse over very small distances as a function of time, thereby allowing a great deal of
control over modification process at the interface.
«
E
6
8
as
...siG
ni
;24
C
«
-2
4
E
5
10 15 20
25
30
ill
._r:i.
2
o0
5
10
t1/2 , min 1/2
15
20
Figure 5.10. Plot of anodic peak current, ip,a, versus reaction time for polyaniline thin
films (_,e - - 80 nm, 0,0 - - 50 Jl111) reacted with (0,-) 50 mM 4nitrobenzenediazonium tetrafluoroborate; and (O,e) 50 mM 4methoxybenzenediazonium tetrafluoroborate; in 1.0 M H2S04 acid. Inset is the plot of
initial anodic peak current (t=O) versus square root of the total reaction time (determined
from X-intercept of ip,a versus t1l2).
60
5.5.2 Depth Analysis of Modified Polyaniline Films Using XPS
Since the modification is controlled by diffusion of benzenediazonium ions into
polyaniline, and their apparent diffusion coefficients are so small, one would expect that
the modification proceeds from outside in, resulting in a layered structure. It also follows
that the thickness of the layered structure will vary as a function of time. This was
demonstrated to be the case using XPS depth analysis. After a modified polyaniline
surface was sputtered with a 2 KeV Ar ion beam for 10 min to remove the upper fraction
of the surface, the surface showed a similar XPS spectrum to that of unmodified
polyaniline (Figure 5.11). These results suggests that modified polyaniline exhibits a
layered structure with a modified outer layer covering unmodified polyaniline inner layer.
_
sputtered
(c)
T
5000
_mOdified
.L
_
(a)
408
404
unmodified
396
400
Binding Energy, eV
Figure 5.11. High resolution XPS spectrum of a polyaniline thin film (a) before
modification; (b) after modification in 50 mM 4-nitrobenzenediazonium
tetrafluoroborate-l.0 M H2S04 solution for 10 min; (c) after sputtered with 2 KeV
Argon ion beam for 10 min following modification.
61
5.6
Conclusions
Thin polyaniline films can be modified by reacting with 4-substituted
benzenediazonium salt via a nucleophilic reaction mechanism, and resulting a layered
structure with an electroinactive poly(N-(4-substituted phenyl)aniline) outer layer and an
.electroactive polyaniline inner layer. The modification is controlled by the diffusion of 4substituted benzenediazonium ions intothe polyaniline film and thereby allows spatial
control over modification of polyaniline films on a nanometer scale by controlling
modification time. The modification kinetics were found to be independent on the
substituent group in 4-subsutituted benzenediazonium salt, thereby allowing different
functional groups be introduced into polyaniline film by simply varying substitution
group. These results demonstrate that nucleophilic reactions of polyaniline with
substituted benzenediazonium ions can be used to introduce a wide range of functional
groups into polyaniline films. This control allows optimization of the chemistry of the
polymer interface while maintaining the conductivity of the bulk polymer.
62
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67
BIOGRAPHY
NAME:
GANG LID
DATE OF BIRTH:
JANUARY 22, 1970
EDUCATION:
MS, ANAL. CHEM., LEHIGH UNIV., MAY, 1997.
BS, CHEM. FUDAN UNIV., JULY, 1992.
EXPERIENCE:
RESEARCH ASSISTANT, LEHIGHUNIV. MAY, 1995MAY, 1997.
RESEARCH ASSISTANT, FUDAN UNIV., JANUARY.,
1993 - APRIL, 1995.
68
END
OF
TITLE,