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This is to certify that the

dissertation entitled
Synthesis of Electrically Conductive Polymer/
Inorganic Composites “'In-Situ Oxidative Polymerization/

Intercalation of Conducting Polymers in Layered Hosts

presented by

Chun-Guey Wu

has been accepted towards fulfillment
of the requirements for

Ph . D . degree in ChemiStry

 

Date May 11, 1992

 

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SYNTHESIS OF ELECTRICALLY CONDUCTIVE
POLYMER/INORGANIC COMPOSITES~ IN-SITU OXIDATIVE
POLYMERIZATION/ INTERCALATION OF CONDUCTING
POLYMERS IN LAYERED HOSTS.

By

Chun-Guey Wu

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Chemistry

1992

676:?“ (J 7‘/C)

ABSTRACT

SYNTHESIS OF ELECTRICALLY CONDUCTIVE
POLYMER/INORGANIC COMPOSITES~ IN-SITU OXIDATIVE
POLYMERIZATION/ INTERCALATION OF CONDUCTING
POLYMERS IN LAYERED HOSTS.

By

Chun-Guey Wu

The redox intercalation of polyaniline, polypyrrole and
polythiophene in V205 xerogel is a topotactic reaction which was
achieved by reacting aniline, pyrrole and 2,2'-bithiophene with V205
xerogel. Upon intercalation, the interlayer spacings expanded from
11.55A to 13.94A, 15.20A and 14.70A for polyaniline/V205
polypyrrole/V205 and polythiophene/V205 respectively. These data
are consistent with containing alternating monolayers of conducting
polymers and V205 xerogel. The room temperature conductivity of
polymer/V205 composites is in the range of 10'4~100 Q'lcm‘1 and
decreases with decreasing temperature. The charge transport
properties of these composites vary significantly from each other.
They can be semiconductors or metal-like, with n-type or p-typc
conductivity depending on the reaction media, polymer/V205 ratio
and polymer molecular weight. Upon standing in air, the magnetic
moment of all polymer/V20 5 composites decreases. This is due to

reoxidation of some of W” centers to V5+ by oxygen. Another effect

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oxygen has on (PANI)xV 20 5nH20 is to cause the intercalated
polyaniline chains to oxidatively polymerize further to form longer
chains. This increases the conductivity.

(PANI)xFe0Cl was obtained by reacting FeOCl with excess
aniline in acetonitrile at room temperature. The resulting products
have interlayer spacing equal to 13.96A which is also consistent with
containing alternating monolayers of Fe0Cl and polyaniline. X-ray
and electron diffraction studies of single crystal (PANDXFeOCl show a
superlattice phenomenon suggesting a regular arrangement of
polyaniline chains in FeOCl layers. The room temperature
conductivity of (PANDXFeOCl is around 10‘3 to 10‘1 Q‘lcm'1 and the
thermoelectric power of all samples shows p-type semiconductor
behavior. Upon sitting in air (aging), two separate processes
occurred: the FeOCl is hydrolyzed to B-FeOOH, and polyaniline chains
continue to oxidatively polymerize forming longer chains of polymer.

Intercalation of polyfuran was achieved by taking advantage of
the lower oxidation potentials of terfuran and quaterfuran. Two
different products were isolated depending on the reaction
conditions. In refluxing MeOH or CH3CN, the resulting product I-
(PFu)xFe0Cl, is a topotactic intercalation. However, the products
isolated from methanothermal synthesis, m-(PFu)xFe0Cly(0Me)1-y in
which the Cl atoms in FeOCl were partially replaced by Me0 groups.
Although the physicochemical properties of both phases are similar,
r-(PFu)xFe0Cl has higher conductivity than m-(PFu)xFe0Cly(0Me)1-y.
Variable temperature thermopower data of both products show

typical p-type semiconductor behavior similar to (PANDXFeOCl.

TO MY FAMILY

ACKNOWLEDGEMENT

The project would not have been successful without the help of
i so many sincere persons. First and foremost thanks to my advisor
Mercouri G. Kanatzidis whose scientific ingenuity, wise guidance and
financial support contribute to my completion of the degree
requirements. Throughout this journey, he made himself available to
me whenever I needed help and advice.
I would like to express my great appreciation to Professor C. R.
Kannewurf and his students, D. DeGroot, H. Marcy and I. Schinler for
performing all the charge transport measurements, Professor E.
LeGoff and M. Benz for providing the terfuran and quaterfuran
monomers, Professor H. Eick for help with X-ray powder diffraction,
Professor K. Kiemparens and S. Flegler for good discussions for
Electron Microscope experiment, Professor T. I. Pinnavaia, Professor J.
L. Dye and Professor 1. E. Jackson for serving in guidance committee.
I would also like to thank all in the Kanatzidis group and my
friend Hai-Yen whose friendship and kindness made my graduate
career smooth and very pleasant. I give special thanks to Sandeep
Dhingra who has helped me through all my difficult times in the past
four years.
Financial support from NSF, (CHE-8958451, DMR-89l7805),
CFMR, MSU, the Department of Chemistry and a Dow Summer
Fellowship are gratefully acknowledged.

Finally, thanks to my parents who support me in every

endeavor I choose.

 

TABLE OF CONTENTS
Page
LIST OF FIGURES ...................................................................................................... .xiii
LIST OF SCHEMES ................................................................................................... xxiv
ABBREVIATIONS .................................................................................................... xxvi
CHAPTER I. Introduction ....................................................................................... 1
List of References .......................................................................................... 33

CHAPTERZ. Novel Low Dimensional Conducting Polymer]
Metal Oxide Composites ~ In-situ Oxidative

Polymerization/ Intercalation of Aniline in V205

Xerogel ................................................................................................ 47

Abstract. ............................................................................................................ 48

I. Introduction ........................................................................................... 50

II. Experimental Section .......................................................................... 51

Physicochemical Methods ................................................................. 51

III. Results and Discussion ....................................................................... 59
A. Reaction of Aniline with V205 Xerogel and Product

Characterization ................................................................................. 59

B. The Role of Ambient Oxygen ...................................................... 69

C Gel Permeation Chromatography (GPC) Analysis ................ 75

vi

page
D Thermogravimetric Analysis (TGA) and Differential

Scanning Calorimetry (DSC) Studies .......................................... 78
E Scanning Electron Microscopy (SEM) and
Transmission Electron Microscopy (TEM) .............................. 81
F. Electron Paramagnetic Resonance (EPR)

Specuoscopy ....................................................................................... 85
G Magnetic Susceptibility studies .................................................. 87
H. Charge Transport Properties ........................................................ 97

I. Polyaniline Prepared from Chemically Polymerized
versus Extracted from (PANDXV205nH20 ............................ 110
IV. Conclusions ........................................................................................... 112
List of References ....................................................................................... 113

CHAPTER 3. Conductive Polypyrrole/ Vanadium Pentaoxide

Molecular Nanocomposites ~ Intercalation of

Polypyrrole in V205 Xerogel ................................................... 118

Abstract. ......................................................................................................... 119
I. Introduction ......................................................................................... 120
II. Experimental Section ....................................................................... 121
III. Results and Discussion ..................................................................... 123
A. Synthesis and Characterization ................................................ 123
B. Oxygen Effects .................................................................................. 135

C 'I’hennogravimetric Analysis (TGA) and Differential
Scanning Calorimetry (DSC) Studies ....................................... 139

D. Electron Paramagnetic Resonance (EPR)

Spectroscopy ..................................................................................... 141

vii

page

E. Magnetic Susceptibility Studies ............................................... 143
F. Charge Transport Properties ..................................................... 150
IV. Conclusions ........................................................................................... 162
List of References ....................................................................................... 163

CHAPTER4. Synthesis and Characterization of Conducting

Polythiophene/V205 Bronze ~ In-situ Intercalation

of Polythiophene in V205 Xerogel ........................................ 167
Abstract .......................................................................................................... 168
I. Introduction ......................................................................................... 169
II. Experimental Section ....................................................................... 170
III. Results and Discussion ..................................................................... 173

A. Preparation and Characterization ............................................ 173

B. Thermogravimetric Analysis (TGA) and Differential

Scanning Calorimetric (DSC) Studies ....................................... 182
C Scanning Electron Microscopy (SEM) and Selected
Area Electron Diffraction (SAED) ............................................. 184
D. Electron Paramagnetic Resonance (EPR)
Spectroscopy ..................................................................................... 186
E. Magnetic Susceptibility Studies ............................................... 188
F. Charge Transport Properties ..................................................... 194
IV . Conclusions ........................................................................................... 202
List of References ....................................................................................... 203

viii

P38c

CHAPTER 5. Formation of Conducting Polyaniline in a

Crystalline Layered Host ~ In-situ Oxidative

Intercalation of Aniline in FeOCl ........................................... 206
Abstract. ......................................................................................................... 207
I. Introduction ........................................................................................ 209
II . Experimental Section ....................................................................... 211
I I I. Results and Discussion ..................................................................... 213

A. Syntheses: The Reaction of Aniline with FeOCl .................. 214
B. Infrared Spectroscopy and X-ray Diffraction Studies....216
C Insertion of Polyaniline in FeOCl Single Crystals and
Crystallographic Studies .............................................................. 222
D. Aerial Oxidation : a-(I) and the Nature of "[3" Phase ....... 230
E PANI Extracted from ct-(I) and "B"-(I) versus Bulk
PANI ..................................................................................................... 233
F. Gel Permeation Chromatography (GPC) Analysis ............. 238
G. Scanning Electron Microscopy (SEM) and Selected
Area Electron Diffraction (SAED) ............................................. 240
H. Thermogravimetric Analysis (TGA) ....................................... 240
1. Electron Paramagnetie Resonance (EPR)
Spectroscopy ..................................................................................... 244
1. Magnetic Susceptibility Studies ............................................... 246
K. Charge Transport Properties ..................................................... 249
IV. Conclusions ........................................................................................... 261
List of References ....................................................................................... 263

ix

P518e
CHAPTER 6. A New Route to Chemical Polymerization of

Polyfuran ~ Intercalation of Terfuran in FeOCl .............. 267
Abstract. .......................................................................................................... 268
I. Introduction .......................................................................................... 269
II. Experimental Section ......................................................................... 270
III . Results and Discussion ...................................................................... 274

A. Preparation and Characterization of Polyfuran .................. 274
B. Intercalation of Polyfuran in FeOCl ......................................... 275
C Infrared Spectroscopy and X-ray Diffraction Studies....277
D. Scanning Electron Microscopy (SEM) and Selected
Area Electron Diffraction (SAED) ............................................. 285
E Thermogravimetric Analysis (TGA) and Differential
Scanning Calorimetry (DSC) Studies ....................................... 288
F. Electron Paramagnetic Resonance (EPR)
Spectroscopy ..................................................................................... 291
G Magnetic Susceptibility Studies ............................................... 293
H. Charge Transport Properties ..................................................... 300
IV. Conclusions ........................................................................................... 307
List of References ....................................................................................... 308
Concluding Remarks ........................................................................................ 311

1.1
2.1

2.2

2.3

2.4

2.5

2.6

2.7
3.1

3.2

3.3

3.4

LIST OF TABLES

page
The Conductivity of Some Prototype Conductive Polymers .......... 8
IR Vibration Energies of Bulk Polyaniline,
(PANI)xV205nH20 and Extracted Polyaniline ................................. 63
Crystallographic Data of (PANI)xV205nH20 and V205
Xerogel ............................................................................................................... 64
Molecular Weight of Bulk Polyaniline, a-polyaniline and
B-polyaniline ................................................................................................... 76

Room Temperature Magnetic Moment and Temperature
Independent Paramagnetism of (PANI)XV205nH20 versus x....94

Room Temperature Electrical Conductivity (0) versus x of

(PANDxV205nI-I20 ....................................................................................... 100
Room Temperature Thermoelectric Power (S) versus x of

(PANI)xV205nH20 ..................................................................................... 107
The Comparison of Extracted and Bulk Polyaniline ..................... 111

The Interlayer Spacing ((1) versus x of (Ppy)xV205nH2O
Prepared from Acetonitrile Solution .................................................. 127
The Interlayer Spacing ((1) versus x of (Ppy)xV205nH2O
Prepared from Aqueous Solution ........................................................ 127
Room Temperature Magnetic Moment and Temperature
Independent Paramagnetism of (Ppy)xV205nH20 xersus x ..... 148

Room Temperature Electrical Conductivity (0) versus x of

(PPY)xV205nH2O Prepared from Acetonitrile Solution ............... 154

xi

3.5

4.1

4.2

5.1

5.2

5.3

5.4

6.1

6.2

6.3

6.4

PM6
Room Temperature Electrical Conductivity (0) versus x of
(Ppy)xV205nH20 Prepared from Aqueous Solution ..................... 154
Infared Vibration Modes of Bulk Polythiophene,
(Pth)o,15V2050.58H20 and Extracted Polythiophene ................... 176
Room Temperature Magnetic Moment and Temperature
Independent Paramagnetism of (Pth)xV205.nH20 versus x ..... 192
The Major Vibration Modes of a-, "B"-(PANI)xFe0Cl and
Extracted Polyaniline Compared with Bulk Polyaniline ............ 236
Room temperature Effective Spin-only Magnetic Moment
(quf) versus x of a- and "B"-(PANI)XFe0Cl ..................................... 248
Room Temperature Electrical Conductivity (o) of a- and
"B"-(PANI)xFeOCl ......................................................................................... 252
The Interlayer Spacing (d) and Room Temperature

Electrical Conductivity (o) of Several Intercalation

Compounds of FeOCl .................................................................................. 255
Major IR Vibration Modes of r-(PFu)xFe0Cl, (PFu)xFeC14

and Extracted Polyfuran .......................................................................... 281
Room Temperature Effective Spin-only Magnetic Moment

(ucff) versus x of (PFu)xFeOCl ............................................................... 295
Room Temperature Electrical Conductivity (0) versus x of
r'(PFu)xFe0Cl and m—(PFu)xFe0Cly(0Me)1-y ................................... 302

The Interlayer Spacing (d) and Room Temperature

Electrical Conductivity (o) of Several Intercalation

Compounds of FeOCl .................................................................................. 304

xii

1.1

1.2

1.3

1.4

1.5

2.1

2.2

2.3

2.4

LIST OF FIGURES

page
The band structure of polythiophene upon doping (A)
Neutral state (B) Less than 1% doping (C) Few percent

doping (D) Heavy doping ............................................................................. 6
The conductivity of conjugated polymers relative to

some well known materials ........................................................................ 9
The structure of crystalline V205 (A) ac plane (B) ab plane ...... 28
The crystal structure of FeOCl ................................................................. 30

Projection of one slab of FeOCl in ac plane (looking down

baxis) ................................................................................................................ .31
Calibration curve: Molecular weight versus retention time

of polystyrene standard ............................................................................. 53
FT-IR spectra (KBr pellets) of (A) (PANI)XV205nH20 (B)

Bulk polyaniline (emeraldine salt) (C) Polyaniline

extracted from (PANI)xV2O5nH20 .......................................................... 62
Reflection-mode X-ray diffraction patterns of films of (A)
V205 xerogel (B) (PANI)0,44V2050.7H20 ............................................. 65

Transmission-mode X—ray diffraction patterns of films of
(A) V205 xerogel (B) (PANI)o,44V2050.7H20 (In this
experiment, the incident X-ray beam was perpendicular

to the film. The detector is moving along the 29 while the

sample remains stationary) ...................................................................... 68

xiii

2.5

2.6

2.7

2.8

2.9

2.10
2.11

2.12
2.13

Page
Oxygen concentration versus reaction time during the
intercalation of aniline (aqueous solution saturated with
oxygen) in V205 xerogel (The 100% level indicates

saturation and the aniline to V205 ratio in this reaction

was 1) ................................................................................................................. 72
Oxygen consumption versus aniline/V20 5 ratio (At 0%

consumption, the aqueous solution is saturated with

oxygen) .............................................................................................................. 73
GPC diagrams of polyaniline extracted from (A) on-

(PANI)XV205nH20 (B) B-(PANI)XV205nH20 (C) Bulk
polyaniline. (The solvent was NMP and the polyanilines are

in their emeraldine base form) ............................................................... 77
TGA curves (under oxygen flow) of (A) a-
(PANI)o,43V2050.71H20 (B) B-(PANI)0,43V2050.71H20

(The difference between the two curves is small but

significant.) ...................................................................................................... 79
TGA curves (under N2 flow) of polyaniline extracted from

(A) a-(PANI)0,50V205O.33H20 (B) B-(PANI)0,50V2050.33H20...80
SEM micrographs of (PANI)o,44V2050.5H20 ....................................... 82
Selected area electron diffraction patterns of (A)

(PANI)o,44V2050.7H20 (B) V205 xerogel ............................................. 83

TEM micrograph of V205 gel .................................................................... 84
Room temperature EPR spectra of (A) Bulk polyaniline

(emeraldine salt) (B) (PANI)0,44V2050.7H20 (C)
Nao,4oV205 (D) Extracted polyaniline .................................................. 86

xiv

Page
2.14 Variable temperature magnetic susceptibility (xm) data
of (PANI)o,24V2050.71H20 at 5000 gauss applied field ................ 89
2.15 Variable temperature magnetic susceptibility data
(llxm) of (PANI)0,24V2050.71H2O (A) Total magnetic
susceptibility (B) Curie-Weiss susceptibility (C)
Temperature independent paramagnetism ....................................... 92

2.16 Variation of effective spin-only magnetic moment as a

function of temperature of a—(PANI)0,24V2050.71H2O ................. 93
2.17 Variable temperature magnetic susceptibility (l/Xm) data

(5000 gauss field) of (A)a-(PANI)0,43V205O.33H20

(B) B-(PANI)0,43V2050.33H20 ................................................................... 96
2.18 Four-probe variable temperature electrical conductivity

data of films of (A) a-(PANI)0,44V205nH20 (B) Pristine

V205 xerogel .................................................................................................... 98
2.19 Four-probe variable temperature electrical conductivity

data of films of a-(PANI)o,29V2050.56H20 prepared in

(A) Air (B) N2 atmosphere ....................................................................... 99
2.20 Four-probe variable temperature electrical conductivity

data of (A) ct-(PANI)0,30V2051.3H20 (film)

(B) or-(PANI)0,72V2050.33H20 (pressed pellet) .............................. 102
2.21 Four-probe variable temperature electrical conductivity

data of pressed pellets of (PANI)o,43V 20 50.33H20 (A)

Freshly prepared (B) Aging for 16 months (C) Aging for

31 months ...................................................................................................... 103
2.22 Variable temperature thermoelectric power data of

(A) a-(PANI)0_30V2051.3H20 (film)

XV

2.23

2.24

3.1

3.2

3.3

3.4

3.5
3.6

Page
(B) a—(PANI)o,72V2050.33H20 (pressed pellet) .............................. 106
Variable temperature thermoelectric power data of
pressed pellets of (A) a-(PANI)0,50V2050.33H20
(B) B—(PANI)o,5oV2050.33H2O ................................................................ 108
Four-probe variable temperature electrical conductivity
data of pressed pellets of polyaniline extracted from (A)
a-(PANI)0_50V2050.33H2O (B) B-(PANI)0,50V2050.33H20

(The low value of the conductivities is due to the low

degree of protonation in the samples.) ............................................. 109
FT-IR spectra (KBr pellets) of (A) (Ppy)o,56V205nH20
(B) Extracted polypyrrole (C) Bulk polypyrrole ........................... 125

X-ray diffraction patterns of (Ppy)xV205nH20 prepared

from acetonitrile solution (compressed powder) of (A)
(Ppy)o,13V205nH20 (B) (Ppy)o,73V205nH2O ................................... 128
X-ray diffraction patterns of (Ppy)xV205nH2O prepared

from aqueous solution (compressed powder) of (A)
(PPY)O.30V2O5nH2O (B) (Ppy)o_56V205nH20 ................................... 130
Transmission-mode X-ray diffraction patterns of (A)
(Ppy)xV205nH2O (B) V205 xerogel (In this experiment,

the incident X-ray beam was perpendicular to the film.

The detector is moving along 29 angle while the sample

remains stationary) ................................................................................... 132
SEM micrographs of (Ppy)xV205nH20 ............................................... 133
Selected area electron diffraction patterns of

(A) (Ppy)xV205nH20 (B) V205 xerogel .............................................. 134

xvi

3.7

3.8

3.9

3.10

3.11

3.12

3.13

3.14

Page
Oxygen concentration versus reaction time during the
intercalation of pyrrole in V205 xerogel (The oxygen
concentration of pyrrole aqueous solution before adding
V205 xerogel, is set at 100%. In this reaction the pyrrole
to V205 ration in this reaction was 1) ............................................... 136
Oxygen consumption versus pyrrole/V205 ratio of
intercalation of pyrrole in V205 xerogel (At 0%

consumption, the aqueous solution is saturated

with oxygen) ................................................................................................. 137
TGA curves (under Oxygen flow) of (A) Fresh
(Ppy)o,33V205nH20 (B) Aged (Ppy)o,33V205nHzO ....................... 140
Room temperature EPR spectra of (A) (Ppy)o,55V205nH20

(B) Nao,4V205nH20 (C) Extracted polypyrrole .............................. 142
Variable temperature magnetic susceptibility (Xm) data of

(Ppy)o,73V205nH2O at 5000 gauss applied field ........................... 144
Variable temperature magnetic susceptibility (l/xm) data

of (Ppy)o,73V205nH20 at 5000 gauss applied field (A)
Total magnetic susceptibility (B) Curie Magnetic
susceptibility (C) Temperature independent

paramagnetism ............................................................................................ 145
Variation of effective spin-only magnetic moment as a

function of temperature for (Ppy)o,73V205nH20at 5000

gauss applied field ..................................................................................... 146
Variation of effective spin-only magnetic moment as a

function of temperature for (Ppy)o,73V205nH20 (A) Fresh
prepared (B) Aged .................................................................................... 149

xvii

Page

3.15 Four-probe variable temperature electrical conductivity

data of films of (A) (Ppy)o,25V205nH2O film (B) Pristine

V205 xerogel ................................................................................................. 151
3.16 Four-probe variable temperature electrical conductivity

data of films of (A) (Ppy)o,25V205nH2O prepared from

aqueous solution (B) (Ppy)o,32V205nH20 prepared from

acetonitrile solution ................................................................................... 152
3.17 Four-probe variable temperature electrical conductivity

data for (Ppy)o,73V205nH2O (A) Fresh prepared (B) Aged.

(The anomaly at ~270K is due to instrumental instability

and no physical significant) ................................................................... 156
3.18 Variation of room temperature thermoelectric power as a

function of x for (Ppy)xV205nH20 prepared from CH3C N

solution .......................................................................................................... 157
3.19 Variable temperature thermoelectric power data (pressed

pellets) of (A) (Ppy)o,25V205nH20 prepared in water (B)

(Ppy)o,32V205nH20 prepared in acetonitrile .................................. 158
3.20 Variable temperature thermoelectric power data (pressed

pellets) of (Ppy)xV205nH2O prepared in H2O solution

(A) (Ppy)o.56V2Oan20 (B) (Ppym.42vzoan20

C) (Ppy)o.17V205nH20 (D) (Ppy)o.25 V205nH20 ............................. 160
3.21 Variable temperature thermoelectric power data (pressed

pellets) for (Ppy)o,73V205nH20 (A) Fresh prepared

(B) Aged ....................................................................................................... 161
4.1 FT-IR spectra (KBr pellets) Of (A) (Pth)o,45V2050.93H20
(B) Extracted polythiophene (C) Bulk polythiophene ................ 175

xviii

4.2

4.3

4.4

4.5
4.6

4.7

4.8

4.9
4.10

4.11

Page

Reflection-mode X-ray diffraction patterns of (A) V205

xerogel (film) (B) (Pth)o,g7V2050.54H20 (pressed pellet) ........ 178
Transmission-mode X-ray diffraction patterns of (A)
(Pth)o,37V2050.54H20 (B) V205 xerogel (In this
experiment, the incident X-ray beam was perpendicular

to the film. The detector is moving along 26 angle while

the sample remains stationary) ........................................................... 181
TGA curves (under oxygen flow) of (Pth)o,72V2050.52H20

(A) Fresh prepared (B) Aged ............................................................... 183
SEM micrographs of (Pth)o,72V205nH20 ............................................ 185
Room temperature EPR spectra of (A) (Pth)xV205nH20

(B) Nao,4oV205nH20 (C) Extracted polythiophene ........................ 187

Variable temperature magnetic susceptibility data (Xm)
for (Pth)o,72V2050.52H20 ........................................................................ 189
Variable temperature magnetic susceptibility data (llxm)

of (Pth)o,72V2050.52H20 (A) Total magnetic susceptibility

(B) Curie susceptibility (C) Temperature independent

paramagnetism. .......................................................................................... 190
Variation of effective spin—only magnetic moment as a

function of temperature for (Pth)o.72V2050.52H20 ................... 191
Variable temperature magnetic susceptibility data (l/Xm)

of (Pth)o,72V2050.52H20 (A) Fresh prepared (B) aged ............ 193
Four-probe variable temperature electrical conductivity

data of (A) (Pth)o.72V205nH20 (B) (Pth)o.58V205nH20 (C)
(Pth)o.57V20an20 (D) (Pth)o.4sV205nH20
(E) (Pth)o.30V205nH20 ............................................................................... 195

xix

Page

4.12 Four-probe variable temperature electrical conductivity

4.13

4.14

5.1

5.2

5.3
5.4

5.5
5.6

5.7

data of (Pth)o,72V205nH2O (A) Fresh prepared (B) Aged ....... 198
Variable temperature thermoelectric power data of (A)
(Pth)o.72V205nH20 (B) (Pth)o.58V205nH20

(C) (Pth)o.57V205nH20 (D) (Pth)o.4sV205nH20

(E) (Pth)o,3oV2O5nH2O ............................................................................... 199
Variable temperature thermoelectric power data of
(Pth)o,72V205nH20 (A) Fresh prepared (B) Aged ....................... 201

FT-IR spectra (KBr pellets) of (A) a-(PANI)xFeOC1 (B)

Bulk polyaniline (emeraldine salt) (C) Extracted

polyaniline ..................................................................................................... 217
Reflection-mode X-ray powder diffraction patterns of (A)

Pristine FeOCl (B) a-(PANI)xFe0Cl ..................................................... 219
SEM micrographs of single crystal a-(PANI)0,2gFe0Cl ................ 224
Single crystal X-ray random orientation rotation

photograph (upper left) and axial oscillation photographs

along the three crystallographic axes of ct-(PANI)0,23Fe0Cl

(White arrows indicated superlattice reflections

responsible for the doubling of the unit cell in the a- and

c- direction.) ................................................................................................. 225
The ac plane of FeOCl (looking down b axis) .................................. 228

Possible relative position of polyaniline chains and Fe0C1

layers ............................................................................................................... 229
Reflection-mode X—ray powder diffraction patterns of (A)

a-(PANI)xFe0Cl (B) Partially decomposed phase (C) "[3"-
(PANDXFeOCl (D) B—FeOOH ..................................................................... 234

XX

nil-

D‘s

I‘

e‘v

I‘o

Page

5.8. FI-IR spectra (KBr pellets) of (A) a-(PANI)xFeOC1

5.9

5.10

5.11

5.12

5.13

5.14

5.15

5.16

5.17

5.18

(B) "B"‘(PANI)xFeOC1 ................................................................................. 235

TGA curves (under nitrogen flow) of polyaniline extracted
form (A) a-(PANI)xFeOCl (B) "B"-(PANI)xFeOC1 (C) Bulk

polyaniline (The heating rate is 5°C/min.) ...................................... 237
GPC diagrams of polyaniline extracted from (A) a-
(PANI)xFeOCl (B) "B"-(PANI)xFeOCl (C) Bulk polyaniline ........ 239

SEM micrographs of microcrystalline a- and "B"-
(PANDXFCOCI ................................................................................................ 241
left: a-(PANI)xFe0Cl ; right: "B"-(PANI)xFe0Cl.

Selected area electron diffraction pattern of a-

(PAN1)o.20Fe0Cl ........................................................................................... 242
TGA curves (under nitrogen flow) of (A) ct-(PANI)xFe0C1

(B) "B"-(PANI)xFe0Cl (The heating rate was 5°C/min.) .............. 243
Room temperature EPR spectra of (A) a-(PANI)xFe0C1 (B)
"B"-(PANI)XFeOCl (C) Extracted polyaniline ................................... 245
Variable temperature magnetic susceptibility data (llxm)

of (A) a-(PANI)o,2oFeOC1 (B) "B"-(PANI)0,2oFe0Cl ....................... 247

Variable temperature effective spin-only magnetic
moment of (A) a-(PANI)o,2oFe0Cl (B) "B"-(PANI)0,20FeOCl ..... 250

Four-probe variable temperature electrical conductivity
data of a-(PANI)o_23Fe0Cl in (A) Pressed pellet (B) Single

crystal form .................................................................................................. 253
Four-probe variable temperature electrical conductivity

data of pressed pellets of (A) a-(PANI)0,17Fe0Cl (B) "B"-

(PANI)o,17FeOC1 ........................................................................................... 257

xxi

Page

5.19 Four-probe variable temperature electrical conductivity

data of pressed pellets of polyaniline extracted from (A)

a-(PANI)o,17FeOCl (B) "B"-(PANI)o,17FeOC1 ..................................... 258
5.20 Variable temperature thermoelectric power data of (A)

a-(PANI)0,17FeOC1 (B) "B"-(PANI)0,17FeOCl ..................................... 260
6.1 FT-IR spectra (KBr pellets) of (A) r-(PFu)xFe0Cl (B)

(PFu)xFeCl4 (C) Extracted polyfuran .................................................. 278
6.2 FT ~IR spectra (KBr pellets) of (A) m-(PFu)xFe0Cl (B) r-

(PFu)xFe0Cly(0Me)1.y ............................................................................... 280

6.3 Reflection-mode X-ray powder diffraction patterns of (A)
m-(PFu)o,47FeOCly(OMe)1-}. (B) r-(PFu)o,27Fe0Cl (C) FeOCl....282
6.4 SEM micrographs of r-(PFu)xFe0Cl and

m(PFu)xFe0Cly(OMe)1-y ......................................................................... 286
left: r-(PFu)xFe0Cl. right: m-(PFu),‘FeOC1y(0Me)1.y

6.5 Selected area electron diffraction (with electron beam
perpendicular to the FeOCl layers) pattern of
r(PFu)xFeOCl ................................................................................................ 287

6.6 TGA curves (under oxygen flow) of (A) r-(PFu)o,27Fe0Cl
(B) m-(PFu)o,54Fe0Clo,5(0Me)o,5 (The heating rate was
5°C/min) ......................................................................................................... 289

6.7 TGA curves (under nitrogen flow) of (A) (PFu)xFeCl4 (B)
Extracted polyfuran (The heating rate was 5°C/min.) ............... 290

6.8 Room temperature EPR spectra of (A) r-(PFu)o,27Fe0C1 (B)

m-(PFu)o,51Fe0Clo,6(0Me)o,4 (C) (PFu)xFeC14 (D) (PFu)xC1 ....... 292

6.9 Variable temperature magnetic susceptibility data (IIXm)

of (A) r-(PFu)o_13FeOCl (B) m-(PFu)o,51FeOClo,5(OMe)o,4 .......... 2.94

xxii

6.10

6.11

6.12

6.13

6.14

6.15

Page

Variable temperature effective spin-only magnetic

moment of (A) r-(PFu)o,13FeOC1 (B)

m-(PFu)o,51FeOC10_5(0Me)o,4 ................................................................. 297
Variable temperature magnetic susceptibility data
(I/xm)of (A) (PFu)xCl (B) Extracted polyfuran ............................. 298

Variable temperature effective spin-only magnetic
moment of (A) (PFu)xCl (B) Extracted polyfuran ........................ 299
Four-probe variable temperature electrical conductivity

data of pressed pellets of (A) r-(PFu)o,27Fe0Cl
(B) m-(PFu)o,51FeOClo,6(OMe)o,4 ........................................................... 301

Four-probe variable temperature electrical conductivity

data of r-(PFu)o,27Fe0Cl in (A) Pressed pellet (B) Single

crystal form .................................................................................................. 303
Variable temperature thermoelectric power data of r-

(PFu)o,27Fe0Cl in (A) Pressed pellet (B) Single crystal ............. 306

xxiii

1.1
1.2

1.3

2.1
2.2

2.3

2.4

3.1

4.1

5.1
5.2

5.3
5.4

6.1

LIST OF SCHEMES

P38 e
Four different phases of polyaniline .................................................... l9

Polymerization of pyrrole via 0- and rt-radical coupling [70]...21

Schematic representation of V205 layers. Proposed by
Livage et a1 [99] ............................................................................................. 27
Proposed arrangement of polyaniline in V205 layers .................. 66

Proposed arrangement of poly-2,6-dimethyl aniline in

V205 layers ...................................................................................................... 67
Head to tail oxidative coupling of aniline oligomers ...................... 74
Simplified orbital diagrams of V“ in distorted square

pyramid ligand field (A is the energy separation between

dxz and dyz orbitals.) ................................................................................... 91
Proposed arrangement of polypyrrole chains in V205

layers. (A) perpendicular (B) parallel to the layers ................... 131
Proposed arrangement of polythiophene chains in V205
layers ............................................................................................................... 179
Proposed orientation of polyaniline chain in FeOCl layers ....... 220
The pillar effect of anilinium in (PANDXFeOCl .............................. 222
Proposed structure of polyaniline ....................................................... 226

Proposed hydrogen bonding between polyaniline chains

and FeOCl framework ............................................................................... 227
Proposed arrangement of polyfuran chains in FeOCl
layers ............................................................................................................... 283

xxiv

page
6.2 Proposed structure of (PFu),‘FeOCly(OMe)1.y ........ 234

XXV

ABBREVIATIONS

DSC: Differential Scanning Calorimetry.
EPR: Electron Pararnagnetic Resonance.
GPC: Gel Permeation Chromatography.
PANI: Polyaniline.

PFu: Polyfuran.

Ppy: Polypyrrole.

Pth: Polythiophene.

SAED: Selected Area Electron Diffraction.
SEM: Scanning Electron Microscopy.
TEM: Transmission Electron Microscopy.

TGA: Thermogravimetric Analysis.

xxvi

CHAPTER 1

INTRODUCTION

The field of "Low Dimensional Conductors" has attracted
considerable attention recently. Both chemists and physicists are
actively investigating the synthesis and properties of this class of
materials. The activity was initially stimulated by the hypothesis
that a one-dimensional organic compound can be designed that
would exhibit room temperature superconductivity [1]. The
magnitude of conductivity in these materials is dictated at least in
part by structural constraints [2]. Organic metals beautifully
illustrate this point. Various systems which have successfully
produced low dimensional conductors include the following several

categories [3]

1. Linear chain of transition metals complexes, such as
K2Pt(CN)4Bro,3.3.OH2O.

Stack of donors and acceptors molecules, (TTF-TCNQ).
Intercalated graphites, (CgK).

Intercalated 2-D chalcogenides, (MxTasz).
Langmuir-Blodgett (LB) multilayers.

OLA-huh)

Linear conjugated polymers, such as polyacetylene,

polypyrrole and polythiophene.

Especially intriguing among these classes of materials are the
linear conjugated organic conductive polymers. Since the first
electrically conductive polymer, polyacetylene, was discovered by H.

Shirakawa and coworkers 15 years ago [4], conducting polymers

have become a new class of materials under rapid expansion [5].
Several prototype conductive polymers, such as polythiophene (Pth)
and its derivatives, poly-p-phenylene (PPP), polyfuran (PFu),
polypyrrole (Ppy) and polyaniline (PANI), have attracted attention
because of their interesting electronic properties. Rapid progress has
demonstrated that the potential exists for the discovery of new
concepts and phenomena as well as the development of new
technology. The wide-spread fascination is clearly motivated by
their perceived technological potential. In fact, electrically
conducting or semiconducting applications are the last great
challenge for polymeric materials.

These polymers are usually composed of highly conjugated
linear chains and their electronic structure is based upon the overlap
of the p-orbitals of the backbone carbon atoms [6]. They can be
oxidized or reduced more easily and reversibly than traditional non-
conjugated polymers. Except for polyacetylene, almost all conducting
polymers can be prepared by either electrochemical or chemical
oxidative polymerization of the corresponding monomers as shown

below:

Electrochemical polymerization [7]:

S c1130; S
A, C10 1' '
w L n-Bu4NC104 ' W 4)x}n + 2H + (2+X)e

 

 

A-

d:

Chemical polymerization [8]:

2..ng (~11szon '60-“th1' 2804 + NH4 + 2H
HC ltnq)

 

 

In the neutral state, these conjugated polymers are either insulators
or semiconductors. Upon doping (partial oxidation or reduction),
they become electrically conducting and display nearly metal-like
behavior. At the same time, the color and physicochemical properties
also change. This d0ping-undoping process is reversible. The doping
of these conjugated polymers induces charge transfer along the
chains which leads to significant local relaxations. Therefore, the
equilibrium geometry in the ionized state is different from that in
the ground state. This means that the electronic structure is affected
by the localized electronic states in the band gap which modify the 1:
system.

The electrical properties of any materials are determined by
their electronic structure. The theory that most reasonably explains
the electronic structure of materials is band theory [9]. Conductive
polymers are peculiar in that their electrical conductivity cannot be
explained by simple band theory. For example, simple band theory
cannot explain why the charge carriers in polyacetylene and
polythiophene are spinless. Extensive studies of these polymers with
various spectroscopic methods such as EPR [10] and optical [11]
absorption as well as theoretical works [12] point to the formation of

polarons (single charge paramagnetic state) and bipolarons (doubly

charged spinless bound states of two holes). Therefore, conduction
by polarons and bipolarons is now thought to be the dominant
mechanism of charge transport in polymers with non-degenerate
(non equivalent resonance form) ground states. The degenerate
ground state always refer to two geometric structures corresponding
to the same total energy. The evolution of the band structure of
polythiophene on doping [13] is shown in Figure 1.1. The band
formation of polypyrrole upon doping is similar to that of
polythiophene [5,12]. In the neutral state, the band gap of
polythiophene is equal to 2.2 eV which is a semiconductor. At a
doping level of less than 1%, the formation of polarons is evident. As
the doping level is increased to a few percent, polarons start to bind
in pairs to form diamagnetic bipolaron states. When the doping level
becomes heavy (up to 30%), the bipolaron states overlap to form two
bands in the gap. Consequently, the band gap decreases and
becomes as small as 0.14 eV and the polymer shows high
conductivity.

The oxidation state of the polymers will affect their electrical,
optical and mechanical properties as well as thermal stability. For
example, the charge transport properties are especially sensitive to
the synthesis conditions and degree of reduction or oxidation.
Polyaniline also shows sensitivity to the degree of protonation. The
wide diversity of electrical, electrochromic and Optical properties
make conducting polymers attractive for several technological
applications. These include rechargeable batteries [14], electronic
devices [15], electrochromic display devices [16], sensors (pH, gas)

[17]. gas transport [18], anticorrosion protection of electrochemical

L1,... . IE!

2.2 V "
e ‘11"— }o.61 eV

 

 

   

neutral < 1% doping 5% doping heavy doping
(3) (b) (C) ((1)

Figure 1.1. The band structure of polythiophene upon
doping. (a) Neutral state (b) Less than 1 % doping
(c) Few percent doping (d) Heavy doping.

photovoltaic devices [19] and photoelectrolysis cells [18], information
storage [22] and electromagnetic shielding materials [21]. Recently,
biomedical applications have also been mentioned [23]. Table 1.1
lists some prototype conductive polymers and the highest
conductivity achieved to date. The room temperature conductivity of
these polymers falls in a very wide range from 200 to 105 Q’lcm'l.
The conductivity of iodine-doped polyacetylene [24] is comparable to
that of copper metal. In order to get a better idea of the magnitude
of conductivity, Figure 1.2 shows the conductivities of some well
known materials.

Conducting polymers could potentially combine, in one
material, the performance and processing advantages of organic
polymers and the electrical propertieS' of semiconductors and metals.
However, the intense effort to understand the chemistry and physics
of these polymers has been hampered by their amorphous nature
and the insolubility in common organic solvents. Therefore,
structure/property relationships are not totally understood. One
principal goal of this field is to achieve a fundamental understanding
of the relationship between chemical structure and electronic
properties of the conjugated polymers. For example, it has been
known that the orientation of conductive polymers will affect the
conductivity [25]. Conductivity is the product of two important
factors: the number of carriers and carrier mobility. The mobility of
the carriers in conducting polymers will be determined by several
effects, including chain perfection and crystallinity, both of which are
a function of chain alignment. Furthermore, regardless of

electrochemical or chemical syntheses, these polymers always

 

.5333 2:3: 53.. “1210*. 22.59:...—
eoEote 3:»:— ..:::t NAG-Y. o:o§:o:n.n..2em
x
32.3... 2.3... .887 7.09. 28:...2?2::_§

 

 

32.2.3 2:»:— 63—1 MEL: ceoaaemfrem
m

3.5.... .2»... 33-..: z ..m\\ 3 29:22....
:2

3.5.... 2:»... .1: x58 2.2.3323.—

Aafimv 5:233:80 253:5 Loam—em

 

 

.meoEEom 2:33:30 0933?..— chm we 35:33:30 2:. .né 03.;

  

 

10‘ ——-wCopper
Platinum
10‘ ——-*Bismuth
2 Graphite
Highly oriented a lo '
polyacetylene 10o --‘*
10'2 —-—*Germanium
.4 _____.
10 Silicon
to" -—-
10 8 —"—l'
Polyethylene
10'1” —-———J.
10.12 ._.___1.
[0'14 —-—-Diamond
10‘“ ——i'
10-18 ———-quartz
Q'lcm'l

* Note: Vertical scale shows room

temperature conductivity in fl'lcm'l

Figure 1.2. The conductivity of conjugated polymers
relative to some well known materials.

”v.4

u

a"

1
z

’-

10

contain some structural defects such as double bond hydrogenation,
crosslinking etc. The variation of conductivity with preparation
methods also suggests that the low conductivities of conductive
polymers are not an intrinsic property. They may result from the
higher degree of structural defects and low crystallinity, which
decrease the mobility of the carriers. The future of this field of
research relies on creative synthesis of new systems with completely
aligned chains containing fewer structural defects which would
achieve higher conductivity and facilitate structure characterization.
Several methods have been used to align the polymer chains
and thus achieve higher conductivity. Shirakawa and coworkers [26]
have demonstrated that polymerization of polyacetylene on specific
substrates produces highly oriented film with conductivities of 104
Q’Icm'l after doping. Aldissi et al. [27] have produced highly
conducting (~104 Orlcm-l) water soluble polythiophene film by
passing a solution of polythiophene through a magnetic field while
simultaneously removing the solvent. Several groups have prepared
well oriented polymers by stretching polymer films or fibers [28].
One particular way to prepare more oriented conductive
polymers may be by intercalating these polymers in structurally
restricted hosts. By using layered materials, the intercalated
polymers can, in principle, be forced to orient themselves within the
host structure. Intercalation chemistry may prove to be a new route
to prepare well oriented polymers. The resulting products would
also represent a new class of organic/inorganic hybrid materials with

interesting electrical properties.

nu

1.]

iLI

In

nl.

(I!

I
M;
.l,

1!
In:

.,.
\T
IN

13::

‘5.
'U’u

r") {—7

11

Intercalation/Insertion Chemistry:

Intercalation can be viewed as the reversible insertion of a
species into a matrix with the identity of both the guest and host
being preserved. However, the term is usually used loosely even for
irreversible insertion reactions. Intercalation compounds are of
interest not only as catalytic materials [29] but also as low-
dimensional conductors [30]. The intercalated guests can be
inorganic or organic molecules, neutral or ionic species. The hosts
can be two or three dimensional compounds. Most generally,
intercalation refers to insertion of guest species in a layered
structure followed by expansion of one crystallographic axis.
Intercalation chemistry can be regarded basically as the result of
competition between the cost in strain energy needed to open the
van der Waals gap of a two dimensional structure and the gain in
electronic energy after intercalation. There are three most common

driving forces for intercalation reactions.

1. Ion-Exchange:

 

 

 

E Smectite clay L Smectite C131 1
.2 W M' + 2Et4N+ r::==> Et4N+Et4N+ + 21W

L Smcctite clay l L Smeetite clay —]

 

 

 

IT] I 7 l

 

12

The Et4N+ ion can be intercalated inside the smectite clay layers by

ion-exchange. In this case, Et4N+ is a guest and the smectite clay is a

host [31].

2. Acid/Base or Coordination Interaction:

 

 

 

The vanadium atoms in VOPO4 have empty coordination (or Lewis

acid) sites. Some good bases such as pyridine can be intercalated

inside VOPO4 host by acid/base (or coordination) interaction [32].

3. Redox Reaction:

 

 

 

 

 

 

 

 

 

 

 

M003 M003-

+ 2KBH, ._ t» K“ K“
Moo .
3 M003

 

 

 

 

 

 

A class of layered compounds which have reducible metal centers.

Upon reacting with reducing agents like BH4', the host layers are

reduced and cations insert inside the host to balance the resultant

negative charge [33]. In the example listed above the K+ ion is able

on

.1-

not

A:-
at,

t“.

Pea.

13

to be inserted in M003 host by redox reaction. In another type of
redox intercalation, the guest species itself acts as the reducing agent.
The intercalation involves charge transfer between the guest
molecules and the layered host, as observed in TTF intercalated FeOCl
[34]. Mossbauer studies of intercalated FeOCl show a reduction of
Fe3+ to Fe“ in agreement with the charge transfer from guest to
host. In such a delocalized system the extent of intercalation
depends on three factors: the stability of Fe3+ oxidation state, the
ionization potential of guest, and the rigidity of the host lattice.

Large molecular weight polymers can not be easily intercalated
inside the hosts because of slow kinetics and insolubility in common
organic solvents. However, the fact that conducting polymers are
prepared by oxidative polymerization of the corresponding
monomers, leads to the intriguing idea of using monomer instead of
polymer. Thus by choosing the right hosts (good oxidants), and the
right guest monomers (good polymerizable reductant) the polymer
may be formed and at the same time inserted in the host via redox
intercalation. Here the host acts as an oxidant to oxidize the
monomer to polymer and further partially oxidize the polymer. The
host itself is reduced and therefore can be regarded as a dopant.
This idea was first proved by Kanatzidis and coworkers in 1987 by
in-situ oxidative polymerization/ intercalation of polypyrrole and
polythiophene in layered FeOCl [35]. The resulting conductive
organic polymer/inorganic nanocomposites showed high electrical
conductivity.

In order to extend this field of research, we sought other host

materials, such as V 205 xerogel. Layered V20 5 xerogel has been

14

known for more than a century. It is a semiconducting material with
diverse intercalation chemistry. Furthermore, it is a good oxidant
and the intercalation of conducting polymers in this host can be
achieved by in-situ oxidative polymerization] intercalation of
respective monomers. We have proved that some well known
conductive polymers such as polypyrrole, polythiophene,
polyaniline were able to intercalate in V205 xerogel by redox
intercalation. The resulting products can be viewed as conducting
polymers-derived molecular scale nanocomposites with interesting
charge transport properties.

To extend the intercalation chemistry of conducting polymers
in FeOCl, two other polymers, polyaniline and polyfuran, were
chosen. The mechanism of charge transport in polyaniline is
different from that in polypyrrole and polythiophene [36] and the
detailed structural knowledge in this polymer is highly desirable.
We decided to examine the polyaniline/FeOCl system in the hope of
obtaining crystalline samples of intercalated polyaniline. This work
also contradicts the previous report [37] which claimed that aniline
does not intercalate in FeOCl.

Polyfuran is a known but not well studied conducting polymer.
The furan ring is relatively unstable compared to other heterocyclic
conducting polymers such as polypyrrole and polythiophene.
Polyfuran prepared from furan and its derivatives always contains a
high number of structural defects and shows low electrical
conductivity. We investigated polyfuran/FeOCl system hoping that
the polyfuran formed in the layers of crystalline host would have

fewer structural defects and a well defined structure.

15

Work centered on the idea of polymer inclusion in fixed
inorganic hosts is rare and the little that has been reported has been
recent after the Kanatzidis et al report [35]. Soma and coworkers
first reported the polymerization of benzene [38] and thiophene [39]
in the interlayers of clays by reacting benzene, thiophene and their
derivatives with Cu2+-montmorillonite and Fe3+—montmorillonite.
The formation of polymer cations was proved by resonance Raman
spectroscopy. However, there is no firm evidence showing that the
polymers were formed inside the clay layers.

Hiroshi, Inoue and coworkers have prepared polyaniline chains
inside montmorillonite clay by absorbing aniline molecules into a
clay modify electrode followed by electrochemical oxidative
polymerization [40]. This is the first report of electrochemical
polymerization of intercalated organic molecules. The oxidation
process is very slow and the interlayer spacing of
aniline/montmorillonite is changed from 15.6A to 13.0A after
polymerization. It is therefore, suggested that before polymerization,
the aniline molecules are incorporated in clay with their phenyl
groups perpendicular to the clay layers. Upon polymerization the
arrangement changed into a planar one in the clay sheets. However,
there are no complete structural spectroscopic or charge transport
data on this material.

Thomas Bein [41] and coworkers first inserted aniline in proton
exchanged zeolites which they subsequently oxidatively polymerized
by an external oxidant, such as (NH4)2S203. At the same time, they
also used Cu2+ and Fe3+ ion exchanged zeolites as hosts, and in-situ

oxidatively polymerized polypyrrole and polythiophene inside the

- 0

‘HM
My”

16

zeolites [41]. These intrazeolite Cu2+ and Fe3+ ions serve as oxidants
for the oxidative polymerization reaction of pyrrole and thiophene.
The formation of polymers was demonstrated by FTIR, EPR and
electronic absorption data. The reaction rate of this system is slow
and the resulting products are essentially insulators since the hosts
themselves are electrically insulating materials.

Giannelis and coworkers used Cu2+ exchanged layered mica-
type silicates as hosts and achieved in-situ oxidative
polymerization/intercalation of polyaniline in clay layers [42]. The
polyaniline formed inside the clay is the emeraldine base form which
can be protonated by exposing to HCl vapor. The "as prepared"
polyaniline intercalated clay shows excellent thermal stability up to
700°C. The room temperature conductivity of HCl exposed samples is
0.05 Orlcmrl, an increase by five orders of magnitude with respect to
Cu“ exchanged host. The conductivity measurements perpendicular
to the clay layers affirm the highly anisotropic nature of the
intercalated materials.

Diaz, Galleja and coworkers have succeeded in intercalating
polypyrrole in [(Me3Sn)3Fe3+(CN)5]x by reacting the latter with
pyrrole at room temperature [43]. The resulting black product is a
diamagnetic material with room temperature conductivity equal to
7.0x10'5 Q-lcm“l which is 105 times higher than the alkali metal ion
intercalated compound, [Lio,3(Me3$n)3Fe3+(CN)5]x.

Recently, Caspar and coworkers 1 at E. I. du Pont Inc. were able
to introduce thiophene oligomers (n<10) in zeolites by reacting short
Chain thiophene oligomers (n= 2, 3, 4, 6) with thermally activated

NaZSM-5 and Na-B zeolites [44]. These zeolites were used as

.3"
II“

‘54
.‘t t

4::

bub

w;
in.»

17

supporting matrices in which short chain oligomers of polythiophene
can be prepared, doped to the conducting state, stabilized and
spectroscopically characterized. The electronic absorption band
energies of these materials were found to be a linear function of
inverse chain length. These results can be used to predict the
position of the electronic transition of bulk polythiophene. An
interesting discovery in this study is that the radicals of thiophene
oligomers (stable up to one month in the zeolite channels) couple
gradually to form longer chains of oligomers upon heating
(40~140°C) or standing for a longer time. All of the above work
involved insulating materials as hosts and the resulting products,
except (PANI)Cu-clay [42], have very low conductivity. Actually, in

most cases no charge transport data were reported.

1. Guest Molecules ~ Conducting Polymers:

1. Polyaniline.

Polyaniline has been known for more than a century [45] as
"aniline-black" and recently has attracted renewed interest [46]. The
term "polyaniline" refers to a class of polymers consisting of up to
1000 or more (ring-N-) repeat units. Polyaniline is unique among
conducting polymers by virtue of its electrical properties which can
be reversibly controlled both by changing of its oxidation state and
degree of protonation [47]. The substantial differences between

POIyaniline and other heterocyclic conducting polymers in the

18

physical phenomena are based on the key role of the ring torsion
angle as well as the overlap of alternating nitrogen atoms and C5
rings [48]. There are four different types [49] of polyaniline as
showed in Scheme 1.1. Only the emeraldine salt is conducting.
Polyaniline exhibits excellent environmental stability and good
conductivity [50]. It can be used in organic batteries [51],
electrochromic displays [52], microelectronic devices and sensors
[53], as corrosion inhibitor [54], nonlinear optical active media [55]
and acid/base indicators [56]. Polyaniline is usually prepared by
electrochemical [7,57] or chemical oxidation of aniline [58]. Its
structure and charge transport properties depend on the synthetic
conditions [59], such as oxidation potential, pH value of reaction
medium and reaction temperature. There has been extensive
investigation of its chemical structure by IR and Raman
spectroscopies [60], 13C and 15N NMR spectroscopies [61], TG/DTA
[62], X-ray photoelectron spectroscopy (XPS) [63], X-ray powder
diffraction [64] and the study of model compounds [65]. However,
our understanding of the structure is still fragmentary due to the
amorphous or poor crystalline nature and the insolubility in common
organic solvents, as well as complicated structural problems
associated with single chains and their macromolecular aggregation.
Therefore, the systematic control and modification of its physical,
structural, and electronic properties is an active area of research [66].
One of the goals in this line of research is to optimize the
charge transport prOperties and understand the relationship between
electronic and lattice structure. This has been approached by

Preparing oriented or crystalline polymers. Progress in this area has

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been made recently by MacDiarmid, Epstein [64] and coworkers who
studied a microcrystalline form of polyaniline and proposed a crystal
structure similar to polyethylene-sulfide based on X-ray powder
diffraction data. The X-ray coherence length of polycrystalline
polyaniline is still small (~ 50A perpendicular to the chain and 150A
in the chain direction) and larger crystallites are not available.
Aniline is a cheap starting material and under certain
conditions polyaniline can be fabricated into thin films [67] A
moderately high electrical conductivity can be achieved by doping
with non—oxidizing Bronsted acid. These desirable properties have
made it the most extensively studied conductive polymer. In the

past 5 years, more than a thousand papers have been published.

2. Polypyrrole.

Pyrrole is known to polymerize to a black powder referred as
"pyrrole black" [68]. Polypyrrole is one of the most widely studied
heteroaromatic conducting polymers because of its highly conducting
properties and, in its doped state, it has better chemical and thermal
stability than polyacetylene. Polymerization has been shown by 13C
NMR and IR techniques to occur mainly by a-carbon substitution
[69]. However, the detail mechanism is still unclear. Two possible
mechanisms have been proposed: via o-radicals [7] and via n-radicals
[70] as shown in Scheme 1.2. The charge transport mechanism has
been rationalized by the polaron and bipolaron model [12d]. The
conductivity of this polymer depends on the reaction solvent,

reaction time and concentration of oxidant [71].

H
[OH

21

2H

NH

[1

2

NH ,
9/ \2H
n-radical 2 / \ o-radical
coupling NH coupling

/\ NH

NH j~m

NH
[NH \/

Scheme 1.2. Polymerization of pyrrole via o-and 1t-
radical coupling [70].

22

‘ When the polymerization takes place in solvents possessing OH
groups, high conductivity products can be obtained. Furthermore, the
physical and charge transport properties of polypyrrole largely
depend on the counter anions [72]. For instance, the mechanical
properties are significantly improved by the incorporation of toluene
sulfonate as an anion [72a]. The polymerization of polypyrrole can
take place electrochemically [73] as well as chemically [74].
However, chemically prepared films are of poor quality; in some
cases they are not even conducting [71]. Thus a chemical preparation
of polypyrrole films remains a desirable goal. Intercalation of
polypyrrole in layered hosts offers a good opportunity to prepare
better oriented polymer through chemical polymerization as well as
to study its intercalation with novel dopants such as electrically

active metal oxides.

3. Polythiophene.

Polythiophene is also one of the prototype conducting polymers
with good environmental stability. In fact, polythiophene and its
derivatives are the first examples of conducting polymers that are
stable toward oxygen and moisture both in their doped and undoped
states. It can be prepared by either electrochemical [7,75] or
chemical [76] polymerization of thiophene or 2,2'-bithiophene
respectively. A chemically synthesized polymer, from the catalytic
coupling of the Grignard reagent of 2,5-dibromothiophene by nickel

salt, is produced in its undoped semiconducting state and can be

23

doped chemically to its conducting state [77]. The electrochemically
synthesized polymer is obtained in the oxidized (doped) conducting
state which can be chemically "compensated" or electrochemically
undoped to its semi-conducting state [78]. Neutral polythiophene is a
semiconductor with band gap (Eg) = 2.2 eV and is red colored. Upon
doping, the electrical conductivity of the polymer film can be varied
over 10 orders of magnitude and the color becomes black. The
charge transport properties change from semiconducting to metal-
like. The conductivity of polythiophene is not determined by the
overall chain length, but by the abundance of long conjugated
segments [79]. In other words, extension of the monomer's planarity
will positively affect the conductivity [80]. Although detailed
structural data on polythiophene are not yet available, the excellent
optical and electrical properties would seem to imply a face-to-face
interchain stacking. This face to face arrangement facilitates
interchain electron transfer leading to extensive delocalization. It
has been known that chemically polymerized polythiophene always
contains some impurities and structural defects, such as cross-
linkage, reticulation and B-carbon coupling [78,81]. However, a
highly regular polymer backbone can be obtained by varying the
nature of dopant and the structure of monomer [78]. In these
respects, polythiophene appears to be a good model for the study of
the parameters which control its physicochemical and electrical

properties.

24

4. Polyfuran.

Polyfuran is a known but not well studied conductive polymer
[82]. In the literature, up to hexafuran has been reported. The
oligomers are prepared in low yield from the couping of shorter
chain oligomers or monomer and involve very complicated organic
synthesis [82]. Similar to polypyrrole and polythiophene, bulk
polyfuran has been reported to be formed from furan by either
electrochemical [83] or chemical [84] polymerization. However, due
to the lower aromaticity of furan, as compared to pyrrole and
thiophene, the conjugation of polyfuran is easily destroyed during or
after polymerization [85]. The resulting polymer sometimes shows
brown color with several structure defects, such as hydrogenation
and ring opening [86]. Therefore, polyfuran is the most unstable and
the least studied amongst the heterocycle ring-based conducting
polymers. There are no detailed studies of the structure and
physicochemical properties of this polymer. In fact there is no hard
evidence that the reported material is actually polyfuran. Its
electrical conductivity is very low. We have synthesized polyfuran
from furan oligomers, such as terfuran and quaterfuran in which ring
opening seems to be minimized [86]. The lower oxidation potential
and better structural regularity of trimer and tetramer, compared to
the furan monomer, may enable us to prepare polyfuran with better

and more stable structure.

M
N

25

II. Layered Hosts:

1. V2051.6H20 Xerogel.

One of the most versatile hosts for intercalation chemistry is
vanadium pentoxide xerogel, V2051.6H20. V20 5 gel has been known
for more than one hundred years [87]. It received renewed attention
recently by Livage and coworkers who have shown that these gels
exhibit semiconducting properties [88]. V20 5 gel can be prepared by
pouring molten crystalline V205 into cold water [89], by
polycondensation of metavanadic acid [90] in water, by thermal
hydrolysis of aqueous solution of VOC13 [91], by hydrolysis and
condensation of vanadium alkoxides, VO(OR)3 [92], or even
hydrothermal method [93]. During preparation, some v5+ sites are
reduced to W” to form a mixed valence compound. The
semiconducting properties, which depend on the method of
preparation, arise from the hopping of small polarons between V4+
and V5 + centers. Upon standing in air, the water in the gel gradually
evaporates and finally a dry gel is obtained with certain water
content, called a xerogel. The gel also can be easily fabricated into
thin films and coatings by simply pouring it on different substrates
like glass or plastics. Some potential applications have been proposed
such as electrochromic [94] and switching devices [95].

The structure of V205 xerogel has been widely studied by X-
ray [93,96], electron [97] and neutron [98] diffraction, electron
microscopy [99], extended X-ray absorption fine structure (EXAFS)
[100] and small angle X-ray scattering (SAXS) [101]. It has been

26

proposed that it is composed of corrugated rigid ribbons with
average planar dimensions of the order of 1000A x 100A. The
ribbons are formed by connecting several fibrils in the zig-zag
fashion by sharing H20 molecules [102]. The periodic structure of
fibrils have repeat dimensions of 27A x 3.6.4 as shown in Scheme 1.3
[98] . The corrugation of the ribbon layers is equal to 2.8.3. with one
layer of H20 inside the layer gallery. The interlayer spacing is
11.55A. The intraribbon structure is thought to be closely related to
crystalline orthorhombic V205 as shown in Figure 1.3 The vanadium
atoms in the V205 slab are 5 coordinate with a square pyramidal
geometry. Four V-O single bonds are in the equatorial position and
one V=O double bond points out of the plane as shown in Figure 1.3b.
V205 xerogel can be regarded as a hydrous oxide or composite of
trapped H20 and oxide networks. It contains three types of water
[93]: free water (about 1 mole per mole of V20 5) which is reversibly
adsorbed between the V20 5 layers and accounts for most of the H20
content; a more strongly bonded H20 (around 0.5 mole per mole of
V20 5) and a very small amount of H20 chemically bonded to
vanadium atoms.

V20 5 xerogel is a porous and very reactive material. It has a
turbostratic structure, in which the interlayer distances are the same
but the flat ribbons stack at random orientation. It shows n-type
semiconducting properties with room temperature conductivity
around 10'5 Q-lcm-1[104]. The conductivity arises from the hopping
of small polarons [105] which are localized but mobile electrons.
V205 xerogel can be dissolved in HCl or alkali and ammonium

hydroxide. The intercalation reactions may involve cation-exchange

Sch
Pro

27

 

 

 

 

 

100.2002

Scheme 1.3. Schematic representation of V205 layers.
Proposed by Livage et al. [99].

28

 

%3'

.0009. q ? .2.
I. '3'39‘”
”3......3? ..

 

 

Figure 1.3. The structure of crystalline V205 (A) ac plane
(B) ab plane.

5“.
M

29

[106], acid-base interaction [107] and redox reaction [108]. Several
organic solvents [109], alkylammonium ions [109], ammonia and
pyridine [107] have been reported to intercalate into V205 xerogel.
Most interestingly, the xerogel is a strong oxidizing reagent. For
example, it can be reduced by KBr at room temperature. Some
molecules such as tetrathiafulvalene, benzidine [108] and ferrocene
[110] have been known to intercalate in V205 xerogel via redox
reaction. The resulting intercalates, called "molecular bronzes", show

interesting electrical properties.

2. FeOCl.

FeOCl is a well defined crystalline material. It is a layered
compound belonging to the orthorhombic space group Pmnm with
two formulas per unit cell [111]. The unit cell parameters are a =
3.8A, b = 7.92A and c = 3.311. The layers consist of double sheets of
cis-FeCl204 octahedra linked by sharing edges within the ac plane,
which are held together by van der Waals interactions between the
chlorine planes. The coordination of Fe is a distorted octahedron
with four Fe-O bonds in the layer plane and two Fe-Cl bonds lining
the van der Waals gap between the layers as shown in Figure 1.4.
The interlayer spacing is 7.9215. and the interlayer C1-C1 distance is
3.68A. The b-axis is perpendicular to the layers. Figure 1.5 shows
the arrangement of the ac-plane by looking down the b-axis.

FeOCl is a quasi-one-dimensional antiferromagnetic system

with interesting but not well understood magnetic properties [112].

30

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Figure 1.4.

31

.Cl

IIIIIIII.

Projection of one slab of FeOCl in ac plane

(looking down b axis).

Figure 1.5.

1m

DIVE

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u

L 1‘-

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mill

32

In order to understand the magnetic behavior, several
physicochemical studies, such as magnetic susceptibility [113],
Mossbauer spectroscopy [114], powder neutron diffraction [115] and
X-ray diffraction [116], have been employed. The potential
application of FeOCl as a cathode material for a primary lithium
batteries has been investigated [117].

The redox intercalation chemistry of FeOCl has been known
since the early seventies [118]. Being a Fe3+ compound, its oxidation
potential is significant. During intercalation, the guest species is
oxidized and the FeOCl framework is reduced and the interlayer
distance expands to accommodate the guest molecules. The
intercalation reactions of FeOCl with a variety of organic compounds
such as amine [119], pyridine [120], organometallic compounds [121],
tetraselenafulvalene [122] and tetrathiofulvalene [123] are also well
known. Non reducing cations also can be introduced by reduction
with their BH4' [33] or I' salts [124]. The good oxidizing properties of
Fe3+ and the weak interaction (van der Waals force) between the

layers make FeOCl a good candidate for redox intercalation.

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33

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CHAPTER 2

NOVEL LOW-DIMENSIONAL CONDUCTING POLYMER/METAL

OXIDE COMPOSITES: IN-SITU OXIDATIVE POLYMERIZATION/
INT ERCALATION OF ANILINE IN V205 XEROGEL

47

no
on

fra;

inil

mol

W)

L.»-
fh)‘

deer

48

ABSTRACT

Polyaniline can be intercalated in the layered V205nH20 xerogel
by in-situ oxidative polymerization/ intercalation of aniline in air.
The reaction is facile and topotactic in nature, in which the layer
structure remains intact after intercalation. The observed 2.39A net
interlayer expansion corresponds to expulsion of one layer of water
and insertion of a monolayer of polyaniline. The polyaniline is
formed as the emeraldine salt. Evidence is presented that oxygen
acts as an electron acceptor during the reaction. This explains the
lower spin number of (PANI),.V205nH20, calculated from magnetic
moment. than expected assuming all the electrons obtained from the
oxidative polymerization of aniline are transferred to the V205
framework. All (PANI)xV205nHzO samples are paramagnetic with a
Curie-Weiss and a temperature independent Van Vleck contribution.

The room temperature conductivity of (PANI)xV205nH20 is in
the range of 10'4 to 100 Q'lcm'1 depending on the degree of
polymerization of polyaniline inside the layers. Room temperature
thermoelectric power is negative (-30~-300 uV/K) depending on the
polymer content and the degree of polymerization. Upon standing in
air (aging). two reactions occur independently: the reoxidization of
the reduced V205 framework and further oxidative coupling of
aniline oligomers inside the V205 layers lead to longer chain
molecules. These observations are supported by several
physicochemical data. The magnetic moment of (PANI)xV205nH20

decreases gradually upon exposure to air but it does not change

49

when the sample is stored in vacuum. Gel permeation

chromatography (GPC) analysis results show that. although both
samples show a mutli-modal molecular weight distribution, the

molecular weight of polyaniline extracted from aged

(PANI)xV205nH20 is larger than that extracted from the fresh

samples. The thermal stability of polyaniline extracted from aged

(PANI)XV205nH20 is better than that extracted from fresh sample.

The conductivity of aged samples increases 100 times. The Seebeck

coefficient for freshly prepared samples is negative and independent
of temperature whereas for aged samples it is relatively small,

negative and increases with raising temperature, characteristic of a

n-type conductor.

50

I. Introduction:

Currently the investigation of electrically conducting organic
polymers is an active area of research [1]. Among them. polyaniline
and its derivatives are probably the most rapidly growing class as
can be seen from the publications of the last several years.
Polyaniline has been known since the last century as aniline black [2]
and it attracted renewed attention by the discovery of its promising
electrical properties which can be reversibly controlled by its
oxidation state and degree of protonation [3]. It is usually prepared
by electrochemical [4,5] or chemical oxidation [6] of aniline. The
resulting products are of poor crystallinity and insoluble in common
organic solvents. There have been extensive studies of its chemical
structure by various spectroscopic methods [7.8]. However, due to
the lack of precise structure information on single chains as well as
interchain organization, a good understanding of the conducting
mechanisms and significant improvement in electrical properties
have not yet been achieved.

Based on the rationale given in chapter 1, we describe the
preparation of a novel material in which polyaniline chains are
sandwiched (intercalated) between layers of V205 xerogel. The
Spectroscopic and physicochemical characterization of this material,

along with its charge transport properties are described.

51

II. Experimental Section:

Reagents: NaOH, HCl, NaVO3, 2,6 dimethyl-aniline, (NH4)2SzOs,
Dimethyl sulfoxide (DMSO), N-methylformamide (NMF), l,4-bis[2-(5-
phenyloxazolyl)]benzene (POPOP). N,N'-bis(p-methybenzylidene)-
a,a-bi-p-toluidine (BMBT) and HPLC graded N-methyl-pyrrolidinone
(NMP). tetrahydrofuran (THF) were purchased from commercial
sources and used without further purification. CH3CN and C5H 5N Hz
were dried under CaHz and were distilled under vacuum or in the

atmosphere prior to use.
Physicochemical Methods.

Elemental analyses were done by Galbraith Laboratories.
Knoxville. TN and Oneida Research Services. INC, Whitesboro, N.Y.
Fourier Transform Infrared (FTIR) spectra were recorded from
pressed KBr pellets using a Nicolet 740 FTIR spectrometer. X-ray
powder diffraction studies were carried out with a Phillips XRG-3000
instrument using Ni-filtered Cu Kat radiation. The samples were run
as either films or powders because pressed pellets show significant
preferential orientation. The interlayer spacing was further
confirmed by using a Rigaku rotating anode X-ray powder
diffractometer, Rigaku-Denki/RW400F2 (Rotaflex), at 45KV and
100mA. Transmission-mode diffraction patterns and accurate d-
sI’acings were obtained with this instrument. Oxygen consumption
was detected by an YSI Model 53 oxygen monitor with a closed cell.

The oxygen concentration measured at an empty cell is defined as 0

52

% and the oxygen concentration of distilled water saturated with
oxygen at room temperature is defined as 100 %. The amount of
oxygen consumed is the decrease of oxygen concentration after
reaction of monomer with V205 xerogel for 20 minutes.

Gel Permeation Chromatography (GPC) results were obtained
by using a Perkin Elmer 250 HPLC with binary LC pump and LC-235
diode array uv/vis detector. The column used was a PLgel 10 mm
mixed B column which has a column efficiency of more than 35,000
plates per meter. In a typical procedure, 5 mg of emeraldine base
was dissolved in 1.5 ml NMP. The mixture was then filtered through
4500A pore size filter which was purchased from Phenomenex Inc.,
Torrance, CA. The column and detector were maintained at room
temperature (18°C). The injection volume was 25 pi and the flow
rate was 1 ml/min, using N-methyl-Z-pyrrolidinone as an eluent.
Monodispersed polystyrene standard (purchased from Polymer
Laboratories Inc., Amherst, MA.) with molecular weight ranging from
580 to 7,100,000 was used for constructing the calibration curve
shown in Figure 2.1. The molecular weight of polyaniline was
obtained from a comparison of the retention times of the polymer
with polystyrene standard at the same flow rate and eluent. Two
model compounds: 1.4-bis[2-(5-phenyloxazolyl)]benzene (POPOP)
and N,N'-bis(p-methybenzylidene)-a.a-bi-p-toluidine (BMBT) with
structures more closely resembling that of aniline oligomers. were
used to further correct the molecular weight obtained with the
polystyrene calibration.

Thermogravimetric Analysis (TGA) and Differential Scanning

Calorimetry (DSC) were performed with Shimadzu TGA-50 and DSC-

53

 

LOG MOLECULAR WEIGHT
01
l
O

 

 

6 7 8 9 1 0 1 1 1 2
RETENTION TIME (min.)

Figure 2.1. Calibration curve: Molecular weight
versus retention time of polystyrene standard.

54

50 thermal analysis system using dry oxygen or nitrogen as carrier
gas at flow rate of 72 ml per minute. The TGA experiments were run
from room temperature to 1000°C (or 800°C) at a linear heating rate
of 10°C (or 5°C) per minute. DSC experiments were run at
temperatures between 20°C to 500°C under nitrogen with heating
rate of 5°C per minute. Electron Paramagnetic Resonance (EPR)
spectra were recorded on a Varian E-4 EPR spectrometer at room
temperature or liquid nitrogen temperature. The solid samples were
scanned from 2900 to 3900 gauss at 8~32 gauss field modulation
with 0.1 second time constant. The field was calibrated with 2.2-
diphenyl-l-picrylhydrazyl (DPPH, g = 2.0036). Scanning Electron
Microscopy (SEM) equipped with a TRACOR Northern Energy
Dispersive Spectroscopy (EDS) and Transmission Electron Microscopy
(TEM) with Selective Area Electron Diffraction (SAED) studies were
done with JEOL-ISM 35CF at 20 KV and a JEOL-100 CX(II) at 100 KV
respectively. The samples for SEM imaging were mounted on Al
stubs with graphite paint without further coating. Samples for
electron diffraction studies were dispersed in acetone, sonicated for
10 minutes then deposited in holey film-coated copper grids. For
preparing V205 gel. a drop of very diluted V205 wet gel was
deposited on the copper grid and dried in air. A thin layer of xerogel
was formed on the grid which was used for SAED studies. The d-
Spacings calculated from diffraction patterns (Bragg rings) were
further calibrated by aluminum standard after each measurement.
Pyrolysis mass spectra were obtained with a JEOL 1MS-AX505H Mass

Spectrometer with temperature programming probe.

55

Variable temperature magnetic susceptibility data were
collected on a Quantum Design SQUID system at various magnetic
fields (500~1500 gauss). A known quantity of material was placed in
a plastic bag and purged with Ar gas before closing. The resulting
magnetic data were further corrected for the diamagnetic
contribution of the container. Measurements were made with an
ascending temperature ramp from 5K to 300K. Direct-current
electrical conductivity and thermopower measurements were made
on compactions of powder in pellet form, free standing films or single
crystals. Conductivity measurements were performed in the usual
four-probe geometry with 60mm and 25mm gold wires used for the
current and voltage electrodes, respectively. Measurements of the
pellet cross-sectional area and voltage probe separation were made
with a calibrated binocular microscope. Conductivity data were
obtained with the computer-automated system described elsewhere
[9]. Thermoelectric power measurements were made by using a slow
ac technique [10] with 60mm gold wires serving to support and
conduct heat to the sample, as well as to measure the voltage across
the sample resulting from the applied temperature gradient. In both
measurements. the gold electrodes were held in place on the sample
with a conductive gold paste.

All charge transport measurements were done by Carl R.
Kannewurf and coworkers in Electrical Engineering Department,
Northwestern University. The protocol for charge transport

measurements has been reports by professor Kannewurf et al.
[11,12].

56

Preparation of V205nH20 Xerogel.

A HV03 solution was obtained by dissolving 4.0 g (32.8 mmol)
of NaVOg in 250 ml distilled H20. and passing the solution through a
HHon-exchange column packed with 30 g of Dowex-50X2-100 resin.
Upon standing, the yellow HV03(aq) polymerized within 24 to 96
hours to a V205 sol which thickens to become a V205 gel via a sol-gel
transition. The gel was then poured on glass substrates and the
excess water was allowed to evaporate upon standing in air. The
dark red film formed on the surface of glass has the chemical
formula V205nH20 (n~1.6 depending on ambient humidity). The dry
film can be peeled off as a free standing film or it can be ground into

powder for subsequent use.

Preparation of (PANI)o,44V2050.7H20 Film.

In a typical reaction, 2.0 g (21.5 mmol) of aniline was mixed
with 30 ml distilled H20 (the mixture is not necessarily a
homogeneous solution). To this was added 0.5 g (2.37 mmol) of V205
xerogel film. The mixture was allowed to stand at room temperature
for 24 hours in air. The black pieces of film were isolated by
filtration. washed with acetone and dried in vacuum. Elemental
analysis: Calcd. for (C5H4NH)0,44V2050.7H20: C. 13.51%; H, 1.58%; N.
2.63%; V, 43.47%. Found: C, 12.92%: H, 1.53%; N, 2.53%; V, 41.69%.

57

Preparation of (PANI)o,19V2050.6H20 Film.

0.1 g (1.1 mmol) of aniline was dissolved in 20 ml distilled H20,
followed by addition of 0.6 g (2.84 mmol) of V205 xerogel film. The
mixture was allowed to stand at room temperature for 5 hours in air.
The product was isolated by filtration, washed with acetone and
dried in vacuum. Elemental analysis: Calcd. for
(C5H4NH)0,19V2050.6H20; C. 6.50%; H, 1.02%; N, 1.26%; V, 48.46%.
Found: C, 6.37%; H, 1.16%; N, 1.33%; V. 47.08%.

Preparation of (PANI)o,77V2050.26H20 Powder.

0.5 g (2.38 mmol) of V205 xerogel (fine powder) was added in
85 ml 3.5 wt% of aniline aqueous solution. The mixture was stirred
at room temperature for 16 hours in air. The black powder was
isolated by filtration, washed with acetone and dried in vacuum.
Anal. Calcd. for (C6H4NH)0,77V2050.26H20: C, 21.59%; H. 1.70%; N,
4.20%; V, 39.73%. Found: C. 21.59%; H. 2.02%; N, 4.07%; V. 39.51%.
Changing the aniline/V20 5 ratio and solvent amount, different
stoichiometric products can be obtained. The interlayer spacings of

all samples calculated from X-ray powder diffraction are equal to
13.94A.

Preparation of (poly-2,6-dimethylaniline)o_4V 20 5.51120 .

0.1 g (0.83 mmol) of 2.6-dimethylaniline was mixed with 50 ml
H20. followed by addition of 0.35 g (1.66 mmol) of film of V20 5

58

xerogel. The mixture was allowed to stand at room temperature
without stirring for 14 hours. The black film was isolated by
filtration, washed with acetone and dried in air. Elemental analysis
Calcd. for (C3H3NH)0,4V2050.5H20: C: 16.13%, H: 1.91%; N: 2.35%, V:
42.85%. Found: C: 15.13%, H; 2.40%, N: 2.36%, V: 40.68%. The
interlayer spacing calculated from X-ray diffraction is equal to

14.5711.

Synthesis of Bulk Polyaniline.

103 g (452 mmol) of (NH4)28203 was dissolved in 150 ml of
water and then added to a solution containing 20.0 g (215 mmol) of
aniline with 300 ml 1M HCl(aq). The mixture was stirred in air at
0°C for 1 hour. then at room temperature for 3 hours. The black
precipitate was isolated by filtration, washed with water then
acetone and dried in air. The product was identified by its

characteristic infrared spectrum [7a].

Extraction of Polyaniline from (PANI)o,sV2050.3H20

in Basic Solution.

0.15 g (0.64 mmol) of (PANI)o,5oV2050.33H20 was mixed with
100 m1 2 wt% NaOH(aq) and stirred at room temperature for 17
hours. The black residue was isolated by filtration, washed copiously
with H20, 1.0 M HCl(aq). then H20 again and acetone, dried in
vacuum. The product was identified to be the emeraldine salt of

P01yaniline by infrared spectroscopy.

II]

1C

1Cl

59

Extraction of Polyaniline from (PANI)o,5 V 2 O 5 0.33 H 2 O

in Acidic Solution.

0.17 g (0.73 mmol) of (PANI)o,50V2050.33H20 was mixed with
100 ml 2M HCl(aq) and stirred at room temperature for 2 days. The
black residue was isolated by filtration, washed with H20 then
acetone and dried in vacuum. This residue was identified to be the

emeraldine salt of polyaniline by its infrared spectrum.

III. Results and Discussion:

A. Reaction of Aniline with V205 Xerogel and

Product Characterization.

The intercalation of aniline in V205 xerogel is a redox reaction
in which aniline is oxidatively polymerized and V205 is reduced as

represented in equations 2.1, 2.2 and 2.3.

xcfiufiufl2 —————» (05H4NH)x + 2.5 xe' + 2th . . . .eq (2.1)

 

Vaosnflzo + 2_5xe' ’. V205'2'5an20 """ eq (2'2)

xcfiupm2 , V205nH20——>(C6H4NH)xH2xV205nH20 --eq (23)

60

This redox intercalation is a facile reaction, which occurs instantly
upon contact of the xerogel with aniline (aqueous solution), as
evidenced by the immediate and dramatic color change to dark blue.
However, the reaction takes several hours to go to completion
because of the slow diffusion of the guest species into the inner
layers of the host. Generally, if a solvent is able to swell the xerogel.
the intercalation reaction occurs fast. In fact, V205 xerogel did not
react with neat aniline presumably because it cannot be swelled by
aniline. When solvents which swell the xerogel such as DMSO, NMF
were used, we found significant amount of solvent was intercalated,
while polymer formation was decreased. Thus far, water seems to be
the most suitable solvent for this redox intercalation.

The black film isolated from the reaction of V20 5 xerogel film
with aniline aqueous solution showed good crystallinity along the
layer stacking direction but the homogeneity of the product was
difficult to control. Reducing the particle size of the xerogel increases
the reaction rate and avoids the homogeneity problem but at the
same times decreases the crystallinity of the products, as judged by
the XRD pattern. When a wet gel (preswollen) of V205 was used, the
reaction was complete within minutes but the final product was
amorphous and sometimes the V205 framework was destroyed as
1ndged by the dramatic changes in the IR spectrum. Acetonitrile
does not swell the V20 5 xerogel at room temperature and we
observed no reaction between V20 5 film and aniline in this solvent at
room temperature. Nevertheless, when a fine powder of V205 was
used or the reaction was carried out at reflux temperature, the

intercalation did occur. However, the products had relatively poor

61

crystallinity. Since CH3CN does not swell the V205 xerogel. and all
reactions were run in air, atmospheric H20 may have been absorbed
by the V205. causing some swelling of the xerogel, thus rendering it
reactive towards aniline. Acetonitrile here is simply an inert solvent.

Strong evidence of formation of the emeraldine salt derives
from the FTIR spectra shown in Figure 2.2. The FTIR spectrum of
(PANI),;VzOanzo shows the characteristic IR pattern of emeraldine
salt [7a] occurring between 1000 cm'1 and 1600 cm‘l. The three
strong peaks at wavenumber below 1000 cm:1 belong to the
vibrations of V205 framework. The polyaniline inside can be easily
extracted by digesting the V20 5 matrix with 2M HCl(aq) or 2%
NaOH(aq). The IR spectrum of extracted polyaniline is similar to that
of chemically polymerized bulk material (see Figure 2.2). Table 2.1
compares the most important absorption peaks of bulk polyaniline,
(PANI)xV205nH20 and extracted polyaniline.

The direct evidence for the formation of intercalated compound
comes from the X-ray powder diffraction (XRD) data listed in Table
2.2. Upon intercalation, the interlayer spacing of V205 xerogel
expands from 11.55A to 13.94A. Only several (001) reflections are
observed as shown in Figure 2.3 which is a typical pattern for quasi-
crystalline layered materials. The crystallite sizes (calculated from
the Scherrer formula [13]) of V205 xerogel and (PANI)xV205nH20 in
the b-direction (ac plane) have the same order of magnitude (70A
and 80A for V205 and (PANI),.VszHzO respectively). This indicates
that the intralayer structure of V205 is preserved after intercalating
polyaniline. The 2.39A interlayer expansion is the result of

removing one layer of H20 (approximately 2.8A) and inserting a

VIy VZ<.-;-.IF.£.IMZ<:.I-

TRANSMITTANCE

62

 

 

 

f (B)

l (C)

 

 

2000 1850 1340 17:60 12:80 1100 520 73:0 530 500

WAVENUMBER (cm-l)

Figure 2.2. FT-IR spectra (KBr pellets) of (A)
(PANI)xV205nH20 (B) Bulk polyaniline
(emeraldine salt) (C) Polyaniline extracted from
(PANI),;V205nH20.

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65

 

 

(A)

INTENSITY

 

 

(B)

 

 

 

0.0 10.0 20.0 30.0 ' 40.0 ' 50.0 ' 60.0

29 (DEGREE)

Figure 2.3. Reflection-mode X-ray diffraction patterns
of films of(A) V205 xerogel (B) (PANI)o,44V2050.7H20.

n
.fl'
\UV‘

 

66

monolayer of polyaniline. Therefore, the net expansion due to
intercalated monolayer of polyaniline corresponds to 5.2A. This
value is comparable to that of (pyridine)o,5Ta82 in which the
monolayer of pyridine and pyridinium ions are intercalated with
their C2 axis parallel to the Ta82 slabs [14]. The same model can be
proposed in (PANI),;V205nH20 where the two-fold axis bisecting the
C-N-C angle is perpendicular to the V20 5 slabs as shown in Scheme
2.1.

 

M5 j‘

L
,QNH‘OfiHOfi-HO J>l3.941°\

V205

’QNKOIOKEHO

v2 05

 

 

 

 

 

 

 

 

 

Scheme 2.1. Proposed arrangement of polyaniline in V205 layers

This model is further supported by insertion of 2,6-dimethyl aniline
in V20 5. The resulting product (poly-2,6-dimethyl-
aniline)xV205nH20 has an interlayer spacing of 14.57A. This larger
d-spacing compared to (PANI)XV205nH20 is due to the two methyl
STOUps in 2,6 position of the benzene ring consistent with the

Structure shown in Scheme 2.2.

Eel

67

 

 

 

 

CH3

J
OCH3 H3
H-gjjjc =§:>=N :0 14.5711
CH3

V2005
CH3

:93}

 

 

 

 

ESQ
9:0

 

 

 

 

Scheme 2.2. Proposed arrangement of poly-2,6-dimethyl

aniline in V20 5 layers.

The X-ray diffraction patterns of (PANI)XV205nH20 and V205
with the X-ray beam perpendicular to the layers are shown in Figure
2.4. This diffraction geometry allows us to examine the structural
integrity and intra-layer atomic organization of the V20 5 slabs. The
two diffraction patterns are similar indicating that the intralayer
framework of V205 is structurally preserved after intercalating
polyaniline. This is consistent with the IR data in the range of
380~1000 cm'1 which also show little change before and after
intercalation. This proves that redox intercalation of aniline in V20 5
is topotactic. Based on the reflections observed in the two diffraction
geometries, we have tentatively indexed them based on the
orthorhombic unit cell. However, due the broadness of the peaks in
transmission diffraction the reliable indices cannot be obtained.

When the reactions were carried out in high aniline/V205

molar ratio using a fine powder of xerogel, the resulting products

68

(A)

INTENSITY

(B)

 

Jvarv W
T W‘mv’nvr‘vavvin.v‘vvvfln'émivv'7;"":"V':"":V"V§ "V: l A 1
V 'v'V' fl VVVVVVVVVV

2 10 20 30 4o 50 so

29 (DEGREE)

Figure 2.4. Transmission-mode X-ray diffraction patterns

of films of (A) V205 xerogel (B) (PANI)o,44V2050.7H20.
(In this experiment, the incident X-ray beam was
perpendicular to the film. The detector is moving along

the 29 angle while the sample remains stationary.)

if.

fa]

69

sometimes absorbed anilinium molecules (in addition to polyaniline)
which were identified in the infrared spectra. These anilinium ions
cannot be removed by washing with acetone or pumping under
vacuum at room temperature. However, they disappear upon aging
(by staying in air for several months). This phenomenon was also
observed in (PANI)xFeOC1 (see page 231). Elemental analysis of the
aged sample shows the same carbon content as those of freshly
prepared sample, suggesting that the anilinium was either
polymerized or oxidatively coupled to existing polymer chains. The
anilinium molecules can be removed by heating the sample at 185°C
for several days as judged by the lower carbon content of the heated
samples. Interestingly, when the heating was performed in an
evacuated sealed tube for 3 days, the polyaniline IR peaks did not
change but the intensity of the peak at around 740 cm'l, which is
due to the V-O vibration of V205 host, decreased dramatically. This
suggests that charge transfer from the guest polymer to the V20 5

framework may occur upon heating.

B. The Role of Ambient Oxygen.

At first, we did not expect the aniline/V205 reaction to be
affected by ambient oxygen.. Further observations however
suggested that the products obtained from reactions carried out
under nitrogen and under air were not the same. For example under
air we observed that the reaction completion times were shorter.
Further indications about the divergence of the two type of products

came from the magnetic properties which showed a higher spin

Ell

70

number in the nitrogen prepared samples, and from the electrical
properties, which showed that the samples prepared in air were
more conductive. This is consistent with the aforementioned
intriguing observation that trapped anilinium ions polymerize in air.
Since samples prepared under air had better conductivities, we
concentrated our efforts on those. The nitrogen based samples will
be studied separately [15]. The detailed reaction steps of aniline and
V205 in air are very complicated. Several reactions may be involved:
the oxidative polymerization of aniline, the recombination of H‘l' with
V20 5, the acid-base interaction of H+ and aniline, the reduction of
V20 5, the reaction between V205 and 02 and the reaction between
aniline oligomers and 02. A more accurate overall reaction can be

rewritten as equation (2.4).

xcuufinu2 + v20" (ax-ammo2 ——->(CofltNH)..,(CoH4NHa),V2°an20

+ (2x-3y)/2H,o,(or H20) .. .eq (2-4)

In some reactions, the anilinium molecules were not observed
in the products (then y=0). In equation (2.4), when the
polyaniline/anilinium ratio equals 0.5 (2x=3y), no oxygen is needed
since all protons react with aniline to form anilinium. Attempts to
determine the polyaniline] anilinium ratio by ion exchange with Na+
or K+ according to equation 2.5 were not successful. EDS analysis did

not show observable sodium and potassium concentration.

(CgflgNH),.y(C.H4NH3)'V205n|'|20 + K‘. _'_>(c6H4NH)x-y(K)yv205nH20
(or Na") ... y CeH4NH-f- ..... eq (2.5)

as

ani
on
its.
reaz
:alt
This

l"e

71

However, this ratio was roughly estimated by isolating the polymer
from the V205 matrix to be greater than 75% of the total organic
component (polyaniline + anilinium).

Oxygen participation in the reaction was proven by using an
oxygen monitor. The solvent was saturated with oxygen and its
concentration was monitored as the aniline/V20 5 reaction proceeded
as shown in Figure 2.5. The decreasing oxygen concentration, during
the reaction indicates that oxygen is consumed. In a controlled
experiment, V20 5 xerogel was reacted with alkali metal iodide, such
as KI, and the oxygen concentration was monitored. No oxygen was
consumed during the entire reaction period (30 minutes). Therefore,
the consumption of oxygen must be related to the formation of
polymer. The consumption of oxygen also depends on the
aniline/V205 molar ratio. High aniline/V205 ratio consumed more
oxygen as showed in Figure 2.6. Experiments have also been
designed to quantify the amount of oxygen consumed during the
reaction. Surprisingly, the results are much less than those
calculated from the equation (2.4) and the magnetic data (vide infra).
This implies that the reaction steps are more complicated than what
we proposed in equation (2.4).

Oxygen is not only involved during the reaction but also
changes the properties of the resulting products after isolation.

Aging the sample in the presence of 02, causes two separate reactions

72

 

 

 

 

120
g 100 .
5 4 '
a o
:5 804 .
*z‘ o
8 ‘ 0
Z 0
8 60" .
z c
m . o
c: 0.
Q 401 .0
o ..
20 v 1 r 1 f r j 1 T
0 5 10 15 20 25

RETENTION TIME (min.)

Figure 2.5, Oxygen concentration versus reaction time
during the intercalation of aniline (aqueous solution
saturated with oxygen) in V205 xerogel. (The 100% level
indicated saturation and the aniline to V205 ratio in this
reaction was 1)

Fig
{ll
0X]

73

60

 

sod ’
4o-w
30-1

ZOJ o

OXYGEN CONSUMPTION (96)

10-

 

 

 

0 T 1 j 1 T r W l 1—
0 1 2 3 4 5
MOLE RATIO or ANILINE/Vzos

Figure 2.6. Oxygen consumption versus aniline/V205 ratio.
(At 0% consumption, the aqueous solution is saturated with
oxygen)

 

s<

Th.

dis

[10!

um

74

to occur: first the reduced V20 5 framework is reoxidized and second

polyaniline chains inside the V20 5 gallery oxidatively couple to form

longer chains (see Scheme 2.3).

$01010” fiHONHz + @NHOyfiHONH)!

02

H20 or H202

 

ammfiowowmflbm

Scheme 2.3. Head to tail oxidative coupling of aniline oligomers.

The aged sample from now on will be called the B-phase in order to
distinguish it from the fresh sample which is named the a-phase.
Pyrolysis mass spectroscopy studies of both on and [3 phases did
not show any volatile low molecular weight species up to 350°C
under vacuum. The polyaniline extracted from B-(PANI)xV 20 5nH20
in its base form, is less soluble in organic solvents, such as
tetrahydrofuran, dimethyl-formamide and acetone, than polyaniline
extracted from the a-phase. The anilinium peaks in the IR spectra

disappeared in the B-phase which, as mentioned above, has the same

org

Elli

=3

-L
at

 

m

an.

Sit

75

organic content as a-phase. Furthermore, the thermal stability,
magnetic susceptibility and charge transport properties are also

changed significantly upon aging (vide infra). More direct evidence

that PANI chains are longer in B-phase comes from the GPC results.

C. Gel Permeation Chromatography (GPC) Analysis.

The GPC technique is a common tool for the determination of
molecular weights of polymers [16]. Recently, it has been applied to
analyze the molecular weight of polyaniline [17]. Unfortunately, GPC
studies on polyaniline are rare and a general consensus on its
molecular weight (MW) has not yet been reached. This makes
relevant MW comparisons difficult. Nevertheless, on a relative basis
such comparisons are still useful especially when made under
identical experimental conditions.

The polyaniline extracted from a-, fi-(PANI)XV205nH20 and
bulk polyaniline were first converted to emeraldine base by treating
with 1% NaOH(aq). The room temperature THE-extracted portion for
all samples shows very low molecular weights (<600). However, the
molecular weight of the NMP soluble portion was very high, as
shown in Table 2.3. All samples show multi modal molecular weight
distributions in GPC chromatograms as shown in Figure 2.7. As
expected, the molecular weight of extracted polyaniline is lower than
bulk polyaniline since it is formed in a structurally restricted
environment which considerably slows down the polymerization
kinetics. More interestingly, the highest molecular weight of

polyaniline extracted from a—(PANI)xV205nH20, a-polyaniline, is

76

 

 

 

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77

M

 

 

 

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"l

 

 

fl
7 If r j
0 5 10 15

RETENTION TIME (min.)

Figure 2.7 GPC diagrams of polyaniline extracted from (A)
ot-(PANI)xV205nH20 (B) B-(PANI)xV205nH20 (C) Bulk
polyaniline. (The solvent was NMP and the polyanilines are
in their emeraldine base form.)

gr0‘
fou:
pm
W
and

Th:

78

853,000 with a good portion of the sample at molecular weight

around 1,300. On the other hand, polyaniline extracted from B-
(PANI)xV205nHzO, B-polyaniline, has molecular weights up to
1,040,000 and is a major portion of the extracted polymer. This is
consistent with the proposal that the polyaniline chains continue to
grow upon aging long after the intercalation reaction is over. We
found that polyanilines extracted from various (PANI)xV205nH20
products had different MW distributions. It should be noted that the
MW numbers reported here should be used only on a relative basis
and we do not believe they represent the true MW of the polymers.
The reason for the discrepancy lies in the (most probably false)
assumption that polystyrene in NMP is a good model polymer for

polyaniline in N MP.

D. Thermogravimetric Analysis (TGA) and
Differential Scanning Calorimetry (DSC) Studies.

Typical TGA curves of a and B-(PANI)XV205nH20 under oxygen
flow are shown in Figure 2.8. A small weight loss at temperatures
below 100°C is probably due to absorbed moisture. This is followed
by a continuous weight loss up to 700°C for the a-phase and 600°C
for the B-phase. The weight loss between 100°C and 700°C is due to
the decomposition of polymer and the loss of trace amounts of
structural water from V205. Since the degree of polymerization
varies somewhat from sample to sample as suggested by GPC
analysis, the weight loss patterns in the TGA diagrams vary slightly
as well. The thermal stability of the a phase is better than that of B

79

 

 

 

 

 

 

5Q ..
‘5
2 J
g .
J
b 4
50 P d
l 1 L 1 4 L _L 1 L Ln L L n a 1 l n
0 250 500 750

TEMPERATURE (0C)

Figure 2.8. TGA curves (under oxygen flow) of (A) a-
(PANI)0,43V2050.71H20 (B) B-(PANI)o,4gV2050.7lH20.
(The difference between the two curves is small but
significant.)

80

 

 

 

 

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p d
p
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0 250 500 750 1000

TEMPERATURE (°C )

Figure 2.9. TGA curves (under nitrogen flow) of
polyaniline extracted from (A) a-(PANI)o,soV2050.33H20
(B) B-(PANI)o,soV2050.33H20.

ph.

be
de
Ih:
pc

81

phase under either nitrogen or oxygen flow. At first, this might seem
counter intuitive given the higher average MW in B-phase. As will
became clear below, the aging process to the B-form results in a
decreasing number of V4+ centers. The larger number of V5+ sites in
the framework increase the oxidizing power which then degrades the
polymer chains more easily. Under nitrogen flow the thermal
stability of both a and B phases is better than under oxygen flow as
expected.

The thermal stability of polyaniline extracted from B-
(PANI)xV205nH20 under nitrogen flow is higher than that extracted
from the ct-form as shown in Figure 2.9 This is expected based on

the higher average MW of the B-PAN I.

E. Scanning Electron Microscopy (SEM) and

Transmission Electron Microscopy (TEM).

Electron microscopy was used to study the morphology of these
materials. Under the electron microscope, the surfaces of a
(PANI)xV205nH20 films are continuous and relatively homogeneous.
The layered nature can be observed clearly in Figure 2.10. This is
consistent with a typical topotactic intercalation in which the layer
structure and morphology is preserved after reaction. The topotactic
nature of the reaction is further supported by TEM Selected Area
Electron Diffraction (SAED) studies. Although the SAED patterns of
(PANI)XV205nH20 did not show diffraction spots, several Bragg rings
were observed. The d-spacings calculated from the observed Bragg

rings are in good agreement with the d-spacings obtained by

82

 

Figure 2.10. SEM micrographs of (PANI)0,44V2050.5H20.

igl

83

 

Figure 2.11. Selected area electron diffraction patterns of
(A) (PANI)0,44V2050.7H20 (B) V205 xerogel

84

 

Figure 2.12. TEM micrograph of V205 gel.

85

transmission-mode X-ray diffraction (see page 69). The SAED
pattern of (PANI),,V205nH20 is similar to that of pristine V20 5
xerogel or gel as shown in Figure 2.11. The TEM image of V205
xerogel and (PANI)XV205nH20 studied in our laboratory did not
always show the fiber nature as seen by Livage and coworkers [18].
Most of the time the image obtained could be interpreted as folded
sheets. This fiber morphology becomes very obvious in V20 5 wet gel

as shown in Figure 2.12.

F. Electron Paramagnetic Resonance (EPR)

Spectroscopy.

Since both emeraldine salt and reduced V20 5 are paramagnetic
materials, we used EPR spectrosc0py to study the nature of free spins
in (PANI)XV205nH20. A representative EPR spectrum of
(PANI)XV205nHzO is shown in Figure 2.13b. It features an intense,
broad signal at g=1.975 with Apr in the range of 75~ 100 gauss.
This signal comes from the reduced V205 framework. It is
qualitatively similar to that observed in AxV205nH20 (A = Na, K, Cs)
(Figure 2.13c) which is prepared from the reduction of the V205
xerogel with alkali metal iodides [15]. The original hyperfine
splitting arising from the 51V (I=7/2) nucleus disappeared after
intercalation due to dipolar broadening. Interestingly, the typical
strong and sharp EPR resonance arising from. the massive polarons of
bulk polyaniline was not observed [19]. This suggests that the
polaron spins on polyaniline are in close proximity to the

paramagnetic V4+ centers, so that magnetic exchange interaction

 

(IX)

 

 

(13)

DPPH
(C) ilk
M: “‘ '

2700'8'3000 3300 3700

 

 

 

 

 

MAGNETIC FIELD (Gauss)

Figure 2.13. Room temperature EPR spectra of (A) Bulk
polyaniline (emeraldine salt) (B) (PANI)o,44V 20 50.7 H20
(C) Nao,4oV 20 5nH20 (D) Extracted polyaniline.

87

between the two materials is significant. Therefore, in
(PANI)xV205nH20 the mixing of polyaniline and V205 is at the
molecular level and not simply a physical mixture. The characteristic
EPR signal of polyaniline is observed after isolation by digesting
away the V20 5 matrix (Figure 2.13d). It is similar to that of bulk
polyaniline (Figure 2.13a). The same phenomenon has also been
observed in polyaniline intercalated in zeolite [20], in Ni(CN)2N H3 [21]
and in FeOCl (see page 245).

Lowering the temperature slightly enhanced the EPR peak
intensity of (PANI)xV205nH20 but not the peak shape and width.
The aged B-phase shows a similar but smaller EPR signal suggesting
that the free spins on V20 5 decreased due to reoxidation of the V205
host. However, the g value and Apr did not change. Nevertheless,
the reoxidation of V205 is not extensive enough to reach pre-
intercalation levels since the hyperfine EPR signal is not observed
even after two years. This is expected because the degree of
reduction of V20 5 should be enough to balance the charge of
protonated polyaniline. Part of the line broadening could arise from
strong spin-spin interactions between polyaniline and the V205

framework despite the decrease of V” concentration.

G. Magnetic Susceptibility Studies.

Consistent with the EPR data, variable temperature magnetic
susceptibility measurements from both fresh and aged
(PANDXV 205nH20 show paramagnetic behavior. The magnetic

susceptibility (Xm) decreases with rising temperature as shown in

88

Figure 2.14. The paramagnetism from polyaniline is relatively small
[22] and therefore the net magnetic moment comes mostly from V4+
species in V205 which probably are involved in some type of
magnetic exchange interaction with the spins on polyaniline. We
found that the best way to interpret the magnetic susceptibility data
was to separate it into two components: Curie—Weiss paramagnetism
and temperature independent paramagnetism (TIP) as represented

in equation 2.6.

Xm = X Curle-Welss 4' XTIP -------------------- 9G (2-5)

0

X CurIe-Welss

 

0 : Weiss constant

The TIP is found in transition metal ions where the interacting d
levels have been split by the ligand field [23]. This TIP susceptibility
is rather small and is usually observed at high temperature where
the Curie paramagnetism is relatively small. X-np depends on the
energy gap between the HOMO and LUMO orbitals as shown in
equation 6.3 [23b].

2
amp: _1§_£.B__ ..................... eq (2.7)

N : Avogadro's number, B : Bohr Magneton, A = E" -E0

The coordination of V in V205 xerogel is assumed to be a distorted

square pyramid geometry of approximate C2,, symmetry [18]. The

89

 

0.02

0.01 n

0.00 r.‘.v.9?4—19-94—-w

0 50 100 150 200 250 300 350
TEMPERATURE (K)

O (D000 0

SUSCEI’TIBILITY (EMU/Mole)

 

 

 

Figure 2.14. Variable temperature magnetic susceptibility

(Xm) data of (PANI)o,24V2050.71H20 at 5000 gauss applied
field.

9O

molecular orbital diagram of W” in square pyramidal ligand field can
be expressed as Scheme 2.4. The A is very small in this case,
therefore the xnp can be observed at high temperature. This
temperature independent paramagnetism was also observed in
reduced V205 such as MxV205nH20 (M = K, CS) [24] and was found to
depend on the degree of reduction of V205 [24]. Figure 2.15 shows
the variation of llxm as a function of temperature of
(PANI)o,24V205nH20. The Weiss constant of Curie-Weiss
susceptibility is -1.35. Variable temperature effective spin-only
magnetic moment of (PANI)0.24V205nH20 is basically constant except
at very low temperature (<15K) as shown in Figure 2.16. This may
be due to the paramagnetic impurity or there is an antiferromagnetic
coupling between the V4+ centers as seen in other V4+ compounds
[24,25]. The room temperature magnetic moments, neff, of
(PANI)xV205nH20 with various x values are shown in Table 2.4. Two
observations become immediately obvious from this Table. First,
there is no direct relationship between the polymer content and
magnetic moment (both Curie-Weiss neff and total neff). Second, the
total neff is always smaller than the theoretically expected value
assuming all the electrons released from aniline during oxidative
polymerization, were transferred to V205. The lower neff values are
consistent with our findings presented above that part of the
released electrons are transferred to oxygen and not to V205. The
complicated interactions between oxygen (or polyaniline) and
reduced V205 makes it is difficult to quantitate the degree of
reduction of V205 in all (PANI)xV205nH20 simply using lieff values.

91

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9.. 1
g I l '(A)
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a:
%

 

 

 

 

I r t 1 1 j t

0 50 100 150 200 250 300 350

TEMPERATURE (K)

Figure 2.15. Variable temperature magnetic data (l/xm) of
a-(PANI)0,24V2050.71H20 (A) Total magnetic susceptibility
(B) Curie-Weiss susceptibility (C) Temperature
independent paramagfnetism.

MAGNETIC MOMENT (BM)

 

 

 

93
1.2
1.0 -
humuunoooooo 000
0.8-

 

 

 

0.6 I | I l ' l ' l ' l ' l
0 50 100 150 200 250 30

TEMPERATURE (K)

Figure 2.16. Variation of effective spin-only
magnetic moment (Curie-Weiss) as a function of
temperature of a-(PANI)0_24V2050.71H20.

O 350

94

Table 2.4. Room Temperature Magnetic Moment
and Temperature Independent Paramagnetism of
(PANI),.‘VzoanzO versus x.

 

 

 

x Total *Curie-Weiss} 7m x 104

nemBM) uddBM) (TIP)
0.21 0.97 0.88 0.75
0.24 1.21 0.98 2.0
0.25 1.17 0.87 2.5
0.29 1.02 0.90 1.0
0.44 0.87 0.61 1.6
0.48 1.25 0.78 4.0
0.72 0.92 0.75 1.2

 

 

 

 

*: Corrected for TIP Van Vleck paramagnetism.

95

We have attempted to quantitate the degree of reduction of

V205 host by X-ray photoelectron spectroscopy (XPS). The V5+ peak

of (PANI)xV205nH20 has a broad shoulder which is not observed in
V205 xerogel. This shoulder is due to the overlapping of V5+ and V4+

However the change of the bonding energy, the peak shape of
poses

peaks.
v4+ and significant overlap between V5+ and V4+ peaks,

serious difficulties in determining a reliable V4+/V5+ ratio, the

exactly quantity of V4+ can not be performed.
Upon aging the samples show decreased magnetic susceptibility

as shown in Figure 2.17. The decreasing of the magnetic

susceptibility is probably due to the reoxidation of some V4+ centers

to V5+. The reoxidation process is slow and the degree of V20 5

reduction reaches a limiting value. As mentioned before, the aging

process not only reoxidizes the host but also causes oxidative

coupling of aniline oligomers inside the V20 5 layers to form longer

chain of polymers. However, when the sample was stored in

vacuum, the magnetic moment remains constant suggesting no

further internal redox reaction between polyaniline and V20 5 at
room temperature. The 11‘“: of (PANI)xV205nH20 also decreases

upon aging (0.00025 and 0.0001 EMU/mole for fresh prepared
(PANI)o,43V205nH20 and its aged samples respectively).

96

 

 

 

5000
a 4000-
...: ‘A(B)
E 3000- A
e ‘A
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E A
53 2000- “
" n‘ o(A)
g A‘ 0000000
g: 1000- 0°
m

r , - 1 a

   

 

O 50 100 150 200 250 300 350
TEMPERATURE (K)

Figure 2.17. Variable temperature magnetic susceptibility
data (5000 gauss field) of (A) a-(PANI)o,43V2050.33H20 (B)

B-(PANI)0,43V2O 50.331120 .

97

H. Charge Transport Properties.

(1) Electrical Conductivity.

intercalation of polyaniline, the charge transport

Upon
The electrical

properties of V20 5 xerogel changed significantly.
conductivity of (PANI)XV205nH20 is 104 times higher than pristine

V20 5 as shown in Figure 2.18. In order to explore the effect of

oxygen on the conductivity of PANI/V205 systems, the reactions

were carried out both in air and nitrogen atmosphere. In the

preliminary test, we found that for the similar polyaniline/V20 5

ratio, the material prepared in air has higher conductivity than that
Therefore, most of

prepared under nitrogen as shown in Figure 2.19.
The

our attention was focused on the materials prepared in air.
materials prepared under nitrogen will be reported separately [15].
The room temperature conductivity of a-(PANI)XV205nH20 is in the
range of 10'4 to 100 Q'lcm'l as listed in Table 2.5. Generally, there

is no direct relationship between the conductivity and

polyaniline/V20 5 ratios. This is probably due to the fact that the

conjugation or chain length of polyaniline inside the V205 host most

be taken into account. Thus short oligomers will give rise to lower

conductivities than long ones. The highest conductivities will be

expected in samples in which both the polymer content and polymer

chain-length is high.
the conductivity increases with increasing

This behavior

In all samples
temperature revealing a thermally activated behavior.

is also observed in other intercalated compounds and conjugated

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TEMPERATURE (K)

Figure 2.18, Four-probe variable temperature
electrical conductivity data of films of (A) a-
(PANI)o,21V2050.561-I20 (B) PristineVzos xerogel.

 

 

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TEMPERATURE (K)

variable temperature

of films of a-
(B) N2

Figure 2.19, Four-probe
electrical conductivity data
(PANI)0,29V2050.56H20 prepared in (A) Air

atmosphere.

100

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101

polymers [26]. Figure 2.20 shows the typical variable temperature
conductivity of a-(PANI)XV205nH20 in film and pressed pellet forms.
Not surprising, the conductivities of film samples are 10 to 100 times
higher than that of pressed pellet powder (see Table 2.5) because of
better interparticle contact. The conductivity parallel to the layers is
at least 10 times higher than perpendicular to the layers.

Upon aging for several months the conductivity of
(PANI)XV205nH20 increased as shown in Figure 2.21. The increase in
the conductivity with aging is slow and levels off to a certain value.
The magnetic data showed that when (PANI)XV205nH20 is aged, its
magnetic susceptibility decreases suggesting that V4+ centers are
reoxidized to V5+. The reoxidation of V20 5 framework will not
increase the conductivity in the V20 5 layers since it results in a
decrease of carriers [26]. Thus the observed increase should be due
to the enhancements in the polymer conductivity. The GPC results
show that polyaniline extracted from B-phase has higher molecular
weight than from a-phase (see page 75). Therefore, the increased
conductivity with aging most probably comes from the formation of
longer chains of polyaniline via oxidative coupling after exposure to
air. This is in contrast with conventional bulk conducting polymers
which show a lower conductivity with aging. Apparently, the
oxidative coupling of polyaniline continues to a certain level and then
stops presumably when kinetic movement of the polymer chains

comes to a halt.

C0

102

 

 

 

 

 

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O
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0 50 1 00 1 50 200 250 300 350
TEMPERATURE (K)

Figure 2.20. Four-probe variable temperature electrical
conductivity data of (A) a-(PANI)0,3oV2051.3H20 (film) (B)

0-(PANI)0,72V 20 50.33H20 (pressed pellet).

103

 

 

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1LlLLLJLLLLLLi1J4i¥Ll_14lgl_41L1_Lll

0 50 100 1 50 200 250 300 350

 

 

 

L.
m

TEMPERATURE (K)

Figure 2.21. Four-probe variable temperature electrical
conductivity data of pressed pellets of
(PANI)o_43V2050.33H20 (A) Freshly prepared (B) Aging
for 16 months (C) Aging for 31 months.

104

(2) Thermoelectric Power.

Unlike electrical conductivity, thermoelectric power (T?) which
was measured as a function of temperature is not affected as much
by interparticle contact resistance. It can provide valuable
information as to the true nature of charge transport within the
material. Typical variable temperature thermoelectric power of a-
(PANI)XV205nH20 in pressed pellet and film forms are shown in
Figure 2.22. The thermoelectric power, TP (or called Seebeck
coefficient, S), of pressed powder samples is large, negative and
almost temperature independent from room temperature down to
250K. Below 250K, the exact Seebeck coefficient cannot be measured
due to the high sample resistance. This is a n-type semiconductor
behavior. The thermoelectric power of reduced V20 5 is independent
of temperature but depends on the degree of reduction. This
behavior was observed both in crystalline V205 [25] and V20 5 xerogel
[27]. Therefore, the charge transport properties of powder on-
(PANI),;V205nH20 are dominated by the reduced V205 framework.
This is consistent with the formation of short polymer chains which
inhibit facile chain to chain charge transport. The charge transport
appears more favorable through V20 5 layers although assistance via
the polymer chains particularly in the direction perpendicular to the
layers may be operative. Surprisingly, except for very low
polyaniline/V205 ratio, the room temperature thermoelectric power
of powder samples are independent on polymer content. As
mentioned above oxygen was involved during the intercalation, and

therefore the degree of reduction of V205 framework is not directly

105

relative to polymer/V205 ratio. Since the reduced V205 dominates
the charge transport in these materials, the thermoelectric power is
also independent on polymer/V20 5 ratio.

The TP of film samples is relative small compared to powder
samples, see Figure 2.22a, and became more negative with rising
temperature. This is a n-type conductor behavior although the S
value is higher than typical metal. The TP of bulk (emeraldine salt)
polyaniline shows metal-like behavior and its TP can be negative or
positive depending on the degree of protonation [28]. Therefore, the
polyaniline inside the V20 5 is probably responsible for the charge
transport pr0perties for the film samples. The difference of
thermopower behavior between powder and film samples is not
surprising. Since in film samples, the V205 have better continuity.
The polyaniline chains inside the layers may have better
arrangement, thus facile the chain to chain hoping, dominates the
charge transport. Furthermore, the intercalation of polyaniline in
V20 5 film is slower than in powder. The polyaniline in film sample
may have less structure defects. Table 2.6 summarizes the room
temperature TP of several a-(PANI)XV205nH20 with various x.

Interestingly, the TP value of power samples of B-phase is
relatively small, negative and becomes more negative as
temperature increases revealing a n-type metal-like behavior as
shown in Figure 2.23b. The transition form n-type semiconductor to
n-type metal upon aging is a surprise. The polyaniline inside V205
layers did not degrade upon contact with air, instead it continued to
polymerize forming longer polymer chains which changes

dramatically the charge transport properties. These TP data are

106

 

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3
E
2
E
g
[-
12 4 ._ . l_h ;. 4 J 444 l__l _L .1
100 150 200 250 300 350

TEMPERATURE (K)

Figure 2.22. Variable temperature thermoelectric power
data of (A) a-(PANI)0,30V2051.3H20 (film) (B) a-
(PANI)o,72V20 50.331120 (pressed pellet)

107

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data of pressed

(B) B-(PANI)o.so

TEMPERATURE (K)

Variable temperature thermoelectric power
pellets of (A) a-(PANI)0,50V2050.33H20

V2050.33H20.

 

109

 

 

 

 

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TEMPERATURE (K)

Figure 2.24. Four-probe variable temperature electrical
conductivity data of pressed pellets of polyaniline
extracted from (A) a-(PANI)0.50V2050.33H20 (B) B-
(PANI)o,soV2050.33l-l20,(The low value of the
conductivities is due to the low degree of protonation in
the samples.)

 

110

consistent with our finding that higher molecular weight polyaniline
is present in B-(PANI)xV205nH20. Longer polymer chains reduce the
frequency of carrier hopping from chain to chain, thus facilitating
charge transport through the intercalated polymer. However, the
influence of V205 can not be ignored.

As expected, the conductivity of polyaniline extracted from B-
(PANI)xV205nH20 is higher than that extracted from a-
(PANI)XV205nH20 by an order of magnitude as shown in Figure 2.24.
The overall low value of conductivity is due to the low degree of
protonation and less water content [29] of the samples upon pumping

under vacuum.

I. Polyaniline Prepared from Chemically Polymerized

versus Extracted from (PANI)xV205nH20.

The strategies of intercalating conducting polymers in layered
hosts have double purpose: to obtain more oriented polymers and to
prepare new polymer-derived conducting materials. It is interesting
to know the differences between polyaniline extracted from
(PANI)xV205nH20 and bulk polyaniline polymerized with (NH4)23203.
Table 2.7 lists the comparison of some of the physical and
conductivity data of these two materials. Generally, bulk polyaniline
has higher molecular weight and therefore, higher conductivity.
However, the V20 5 acts as a host and an oxidant which offers a more
controllable reaction condition to synthesize conducting polyaniline

by chemical polymerization. Furthermore, since V20 5 xerogel is an

 

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112

electroactive material, interesting polymer-derived conducting

composites can be prepared in this method.
VI. Conclusions:

The redox intercalation of aniline and V20 5 is a facile reaction.
Products with variable PANI/V20 5 ratios can be obtained with good
lamellar order in both film and powder form. All spectroscopic and
physicochemical evidence shows that the conducting form of
polyaniline is formed. The interlayer expansion calculated from XRD
data corresponds to the insertion of a monolayer of polyaniline. The
room temperature conductivity and thermoelectric power of as
prepared samples are in the range of 10""~100 Q‘lcm'l and -30~-3OO
llV/K respectively.

Upon aging, the reduced V205 framework of (PANI)XV205nH20
was partially reoxidized and the polyaniline inside the V205
interlamellar space continued to oxidatively couple forming longer
polymer chains. This results in higher conductivity in the aged
samples. The conductivity and molecular weight of polyaniline
extracted from B-(PANI)XV205nH20 is higher than that from a-
(PANI)XV205nH20. The thermoelectric power suggests that in the
fresh samples the conductivity is dominated by the reduced V205
host whereas in the aged samples the polymer is responsible for the
charge transport properties. The conductivity characteristic change
from n-type semi-conducting to n-type metal-like base on the MW of

the intercalated polymer.

LIST OF REFERENCES

113

LIST OF REFERENCES

(a) Genies, E. M.; Boyle, A.; Lapkowski, M.; Tsintavis, C. Synth.
Me t. l 99 0 , 3_§_, 139-182. (b) Kanatzidis, M. G.
Chemical&Engineering News 1990, 68, 36-54.

Letheby, H. J. Chem. Soc. 1862 15 161-163.

’.——9

MacDiarmid, A. G.; Chiang, J.-C.; Halpern, M.; Huang, W.-S.; Mu, S.
L.; Somasiri, N. L. D.; Wu, W.; Yaniger, S. 1. Mol. Cryst. Liq. Cryst.
1985.121, 173-180.

(a) Oyama, N.; Kirabayashi, K.; Ohsaka, T. Bull. Chem. Soc. Jpn.
1986, 12, 2071-2080. (b) Kitani, J.; Yano, J.; Kunai, A.; Sasaki,
K. J. Electroanal. Chem. 1987, 221, 69-82. (c) Diaz, A. F.; Logan.
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NA

CHAPTER 3
CONDUCTIVE POLYPYRROLE/VANADIUM OXIDE MOLECULAR

NANOCOMPOSITES " INTERCALATION OF POLYPYRROLE IN V205
XEROGEL

118

119

ABSTRACT

Polypyrrole can be successfully intercalated in V205 xerogel
using pyrrole solutions of CH3 CN or H20 at room temperature or
refluxing (in CH3CN). The reactions are facile and the resulting
products. (Ppy)xV205nH20 (I), revealed increased interlayer spacing.
Variable stoichiometries can be achieved based on the reaction
methods and mole ratios of pyrrole to V205. The expansion of
interlayer spacing depends on the polymer content and preparation
method. Transmission X-ray and electron diffraction patterns
suggest the preservation of the layered V205 framework. Electron
paramagnetic resonance (EPR) and variable temperature magnetic
susceptibility data show that these materials are paramagnetic, the
electron spin mostly comes from the reduced V205 framework.
(Ppy)xV205nH20 show room temperature conductivities up to 10° 9“
1cm‘1 depending on the preparation methods. The materials
prepared in aqueous solution have higher conductivity than those
from acetonitrile solution. The thermoelectric power data of these
composites depend not only on the reaction media but also on
polypyrrole/V205 ratio. At low polypyrrole/V205 ratios. the
Seebeck coefficient is large and negative (n-type conductivity).
However, (I) prepared from aqueous solution with high
polypyrrole/V205 ratio. showed a small and positive Seebeck
coefficient (p-type conductivity). The electrical measurements
suggest that charge transport properties in these materials are
dominated by the V205 framework at the low polymer content limit

while polypyrrole is responsible at high polymer concentration.

DO

120

I. Introduction:

Currently, one of the interesting developments in intercalation
chemistry are that conducting polymers. such as polypyrrole.
polythiophene and polyaniline intercalate into the interlayer space of
layered materials, such as FeOCl [l], V205 [2]. zeolites [3] and clay [4].
Taking advantage of the in-situ oxidative intercalation]
polymerization, novel conducting polymer-inorganic hybrid
laminated materials have been synthesized with good conductivity.

Conducting polymers have been attracting a great deal of
attention from the scientific and technological community because of
their potential applications in practical electronic devices [5.6]. Most
of these polymers can be prepared by electrochemical [7,8] as well as
chemical polymerization [9.10] of the corresponding monomers.
There are indications of crosslinking which occurs during the
chemical polymerization, particularly in the synthesis of polypyrrole
and polythiophene [11]. This significantly affects the charge
transport properties of these polymers.

The intercalation of pyridine in Ta82 and FeOCl has been shown
to proceed via oxidative coupling to yield significant amounts of 4,4'-
bipyridine [12]. This selective coupling (regiospecific) is affected by
the structural constraints of the lamellar hosts. Therefore, by using a
polymerizable guest molecules regiospecific polymerization inside
layered compounds could be envisioned to produce highly regular
polymers.

Polypyrrole is one of the most widely studied heterocyclic

polymers because of its high electrical conductivity. Its charge

121

transport mechanism has been rationalized by the bipolaron model
[13]. The conductivity [14] and mechanical [15] properties of
polypyrrole are largely dependent on the counter anion (dopant).
The mechanical properties can be systematically modified by
forming composites with polyvinyl alcohol or styrene [16]. Here we
investigate the intercalation of polypyrrole in metal oxide and using

the host as counter anion.

11. Experimental Section:

Reagents: NaOH, HCl, NH40H(aq), NaVOg, FeCl3. DMSO. NMF,
catechol, pyrrole, 2,5-dimethyl-pyrrole and N-methyl-pyrrole were
purchased from commercial sources and used without further
purification. CH3CN was dried under CaH2 and distilled in air prior to

use. All reaction were carried out in air.

Physicochemical methods.

See chapter 2.

Preparation of V205nH20 Xerogel.
See page 55.

Preparation of (Ppy)o_44V2050.5HzO Film.

0.30 g (1.42 mmol) V20 5 xerogel film was added to 20 ml of

saturated pyrrole aqueous solution. An immediate color change from

red to blue occurred. The mixture was allowed to stand at room

122

temperature for 24 hours. The black film was picked up from the
pyrrole aqueous solution and washed with acetone, then dried under
vacuum. Elemental analysis: calcd. for (C4H2NH)0,44V2050.5H20: C.
9.62%; H, 1.06%; N. 2.89%; V, 46.44%. Found: C, 8.40%; H, 1.24%; N,
2.73%; V, 40.64%. The water content was obtained from thermal
gravimetric analysis under oxygen flow. Different stoichiometric
products are obtained by changing the pyrrole/V20 5 ratios. When
we used powdered V20 5 xerogel (either in H20 or CH3CN). a black
powder was produced and isolated by filtration and washed with

acetone. An example is given below.

Preparation of (Ppy)o,73V2050.90H20 Powder.

0.64 g (9.55 mmol) of pyrrole was dissolved in 30 ml
acetonitrile, and to the solution was added 1.0 g (4.70 mmol) of V20 5
xerogel fine powder. The mixture was stirred at room temperature
for 4 hours. The resulting black powder was isolated by filtration,
washed with acetone and dried in vacuum. Elemental and
thermogravimetric analysis: calcd for (Ppy)o,73V205.0.9H20: C.
14.26%; H, 1.62%; N, 4.12%; V, 41.52%. Found: C. 14.24%; H, 1.34%; N,
3.92%; V, 41.44%. Upon changing the reagents' ratios. the
polypyrrole] V205 can vary from 0.18 to 0.73.

Synthesis of Bulk Polypyrrole.

A literature procedure was followed [17]. To a solution of 50

ml 5.6 wt% of FeC13 aqueous solution was added 0.5 g (7.5 mmol)

123

pyrrole. The mixture was stirred at room temperature for 2 hours
under nitrogen. The black precipitate was isolated by filtration.
washed with acetone and dried in vacuum. Polypyrrole was

identified by its characteristic IR spectrum [18].

Isolation of Polypyrrole from (Ppy)xV 20 5h 11 20 .

0.15 g (0.67 mmol) of (Ppy)o_56V205nHzO was stirred for 22
hours in 50 ml 2 M HCl(aq) in which 0.50 mg (4.5 mmol) of catechol
was dissolved. The black residue was isolated by filtration. washed
with H20 then acetone and dried under vacuum. It was identified to

be polypyrrole by its characteristic IR spectrum [18].

III. Results and Discussion:

A. Synthesis and Characterization.

We made extensive attempts to identify the optimum
preparation conditions of homogeneous, single phase
(Ppy)xV205nH20. Several methods were used to intercalate
polypyrrole into V205 interlayer space: (a) V205 xerogel was first
swelled with a small amount of water and then reacted with pyrrole
at room temperature, (b) pyrrole/water mixture was poured over a
V205 xerogel film and allowed to stand for one day. (c) V205 xerogel
was refluxed with dilute pyrrole/acetonitrile solution and (d) fine
powder of V205 reacted with pyrrole/acetonitrile solution at room

temperature. In all reactions, upon contact of pyrrole with the

124

V205nH20, a dramatic color change of the V205 occurred from red to
a bluish black with metallic luster. When V205 xerogel films were
used, their use in the synthetic procedure yields free standing films
of the (Ppy)xV205nH20 materials. Pyrrole which has a low oxidation
potential is easily oxidatively polymerized by the V5+ centers in the
xerogel with simultaneous generation of W” centers as shown in

equation 3.1.

XC4H4NH + V2051.6H20 ——"’ (C4H2NH)xH2vaO5NH20 ' ' -OQ(3-1)

Reaction condition (a) always resulted in amorphous products
because of the rapid redox reaction. Efforts to reduce the reaction
rate by lowering the reaction temperature were not successful. Even
the reaction carried out at ice water temperature, the black product
was amorphous. Using method (b), (c) and (d) we were able to get
good layered materials in which we can vary the stoichiometry by
controlling the mole ratio of the reagents. Fourier transform infrared
(FTIR) spectra of (Ppy)xV205nH20 clearly showed the characteristic
vibrations of the polypyrrole backbone as shown in Figure 3.1.
When the reactions were carried out in refluxing acetonitrile solution
using V205 film as starting material, in addition to the weak
polypyrrole absorption, we observed new absorptions originating
from intercalated acetamide which forms by acid-hydrolysis of
CH3CN. These acetamide peaks did not appear in the reaction of V205
powder under the same experimental conditions (82°C 4 hours). The

polypyrrole inside the V205 layers can be isolated by digesting the

125

 

TRANSMITTANCE

 

J 1 r 1 F 1 r w *1
4000 3590 3180 2770 2360 1950 1540 1130 720 310

 

 

WAVENUMBER (cm-1)

Figure 3.1. FT-IR spectra (KBr pellets) of (A)
(Ppy)o,55V205nH20 (B) Extracted polypyrrole (C)
Bulk polypyrrole.

126

host layered framework with 2M 11C] or 2% NaOH aqueous solution.
Polypyrrole will degrade gradually in basic and acid medium and
therefore isolating it intact proved to be difficult. To avoid the
polymer degradation in acid we used catechol as a ligand for (VO)3+
which is formed by reacting V20 5 with acid and is a known strong
oxidant. The catechol complex formed is a considerably weaker
oxidant and does not appear to attack the polymer. The IR spectra of
extracted polypyrrole are very similar to bulk polypyrrole. see
Figure 3.lb&3.lc. Although the IR spectra of (Ppy)xV205nH20
prepared from aqueous solution and acetonitrile solution are the
same, we were unable to isolate the intact polypyrrole from the
latter without adding catechol. This is probably because of the
polymer in (Ppy)xV205nH20 prepared from acetonitrile solution has
shorter chains. Therefore the extracted polymer degraded upon
treating with acid or base in the presence of good oxidizing agent.
(VO)3+.

X-ray diffraction patterns (XRD) of these materials showed that
the intercalation is accompanied by a large interlayer spacing (d)
expansion. The increase of the interlayer spacing varies with the
stoichiometry of the products and reaction conditions. as shown in
Table 3.1 and Table 3.2 Generally, the (Ppy)xV205nH20 prepared
from aqueous solution show higher d-spacings and vary slightly from
sample to sample. This is probably due to the random arrangement
of the polypyrrole molecules in V20 5 interlamellar space. caused by
the fast redox intercalation reaction and perhaps due to some co-
intercalated H20. This is suggested by the relatively weak and broad
XRD peaks as shown in Figure 3.2. (Ppy)szOanzO are relatively

127

Table 3.1. The Interlayer Spacing (d) versus x of
(Ppy)xV205nH20 Prepared from Acetonitrile Solution.

 

X 0.18 0.36 0.56 0.73

 

 

 

 

 

Table 3.2. The Interlayer Spacing (d) versus x of
(Ppy)xV205nH20 Prepared from Aqueous Solution.

 

X 0.17 0.25 0.26 0.32 0.42 0.56

 

 

o(A) 13.10 13.33 14.71 14.56 13.14 15.91

 

 

 

128

 

INTENSITY

 

29 (DEGREE)

Figure 3.2. X-ray diffraction patterns of (Ppy)xV205nl-120
prepared from acetonitrile solution (compressed powder)
(A) (Ppy)o.18V205nH20 (B) (Pp?)o.73V20511H20- '

129

porous as they absorb moisture and increase their interlayer spacing.
Therefore, the variation of interlayer spacings may be due to the
presence of differing amounts of water.

For (Ppy)xV205nH20 prepared from acetonitrile solution, there
are two major phases, one with interlayer spacing of 15.20(i20)A for
high polymer content, and another with a spacing of 12.42(:t20)A for
low polymer content, as shown in Figure 3.3. After intercalation of
polymer most of the interlayer water in V20 5 xerogel is lost. The d-
spacing expansions of the two phases due to the insertion of
polypyrrole correspond to 6.425. (15.20-11.55+2.8)A and 3.72A
respectively. These values can be rationalized as intercalation of a
monolayer of polypyrrole with the five member ring perpendicular
to the V20 5 layers for the first phase and parallel to V20 5 slabs for
the second phase as shown in Scheme 3.1. The transmission XRD
patterns of all (Ppy)xV205nH20 films with the X—ray beam
perpendicular to the film are identical to that of pristine V205
xerogels, see Figure 3.4. This confirms our belief that we are dealing
with a topotactic reaction in which the host framework is preserved
after intercalation.

The SEM micrographs of (Ppy)xV205nH20 are very similar to
that of (PANI)XV205nH20 and (Pth)xV205nH20 [19]. Intercalation of
polypyrrole into the V20 5 film created some defects on the surface of
the film. However there was no observable separate phase and the
layered nature was preserved after intercalation, even at this

macroscopic level, as shown in Figure 3.5. Selected Area Electron

lN'l‘FINSl'l‘Y

Figl
We]

130

 

AAW’

INTENSITY

(B)

 

“MM

2 10 20 30 40 50 60

29 (DEGREE)

Figure 3.3. X-ray diffraction patterns of (Ppy)xV205nH20
prepared from aqueous solution (compressed powder) (A)
(PD!)0.30V205nH20 (B) (Ppy)o.56V20an20-

131

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

15.1021
V205
NH NH °
NH NH 12.4.4
V205
NH NH
_ NH NH
I V205
(B)

Scheme 3.1 Proposed arrangement of polypyrrole chains in

V205 layers (A) Perpendicular. (B) Parallel to layers.

INTPJNSI'I‘Y

._.._ _—A— A

..+

 

132

 

 

 

g: (A)
r73
2
[fl
2
(B)
J,I'IIII| MAL“
2 10 20 30 40 50 60

29 (DEGREE)

Figure 3.4. Transmission-mode X-ray diffraction patterns
Of (A) (Ppy)xV205nH20 (B) V205 xerogel. (In this
eXperiment, the incident X-ray beam was perpendicular to
the film. The detector is moving along the 26 angle while

sample remains stationary.)

 

133

 

Figure 3.5. SEM micrographs of (Ppy)xV205nH20.

134

 

Figure 3.6. Selected area electron diffraction patterns of
(A) (Ppy)xV205nH20 (B) V205 xerogel.

Dii

th.
to
St

C0

hi

0X

135

Diffraction (SAED) patterns of (Ppy)xV205nH20 with the electron
beam approximately perpendicular to the layers is shown in Figure
3.6a. It showed several weak rings as seen early in
(PANI)XV205nH20, see page 83. The corresponding spacings are
similar to that of pristine V20 5 xerogel (see Figure 3.6b) and are in
good agreement with the X-ray data. The similar electron diffraction
patterns of V205 xerogel and (Ppy)xV205nH20 further supported that
the V20 5 framework was preserved after the reaction, consistent

with the XRD results.
B. Oxygen Effects.

As we discussed in chapter 2, oxygen acts as an electron
acceptor during the intercalation of polyaniline in V20 5 xerogel.
During the course of our work with the polypyrrole system we
discovered that oxygen plays a similar role. This phenomenon was
monitored by using an oxygen electrode. As the reaction proceeds,
the oxygen concentration in the solvent decreases very rapidly in the
beginning and then levels off as the reaction nears completion. This
is shown in Figure 3.7. The amount of oxygen consumed depends on
the ratio of reagents. At higher pyrrole/V205 ratio (V205 kept
constant), more oxygen was consumed as shown in Figure 3.8.
Surprisingly, higher V205/pyrrole ratio (pyrrole keep constant) also
consumed more oxygen. This implied that V20 5 in this reaction
system acts as an oxidant as well as a catalyst. The oxygen oxidizes
highly reactive (reducing) intermediates generated by the initial

oxidation of the pyrrole itself. Such intermediates could be radical

136

 

 

 

 

120~
1oo '
E
5 80- '
[—
E < o
*2
8 60'1 .
E23 1 0
Z _ O
8 4o ..
32 1 0.
o 20- "0
.......
0 ' t ' l w u r l '
o 5 1o 15 20 25

REACTION TIME (min.)

Figure 3.7. Oxygen concentration versus reaction time
during the intercalation of pyrrole in V205 xerogel. (The

oxygen concentration of pyrrole aqueous solution, before
adding V205 xerogel, is set at 100%. In this reaction the

pyrrole to V205 ratio was 1)

as!» s alas-Linn): PIE/z», v zuiqurthv

Fi!

la‘
C0
0X

137

 

200

180 -

160 1 .

140 J

120'1

OXYGEN CONSUMPTION ( %)

100-

 

 

 

80 - 1 ' I
0 1 2

3

f

I

4 5

MOLE RATIO OF PYRROLE/Vzos

Figure 3.8. Oxygen consumption versus pyrrole/V205
ratio of intercalation of pyrrole in V205 xerogel. (At.0%

consumption, the aqueous

oxygen)

solution

is

saturated with

138

cation oligomeric species. In a controlled experiment, V20 5 xerogel
was reacted with alkali metal iodide, such as KI, and monitored the
oxygen concentration of the system. We found no oxygen was
consumed during the entire reaction period (30 minutes). Therefore,
the consumption of oxygen must related to the formation of
polypyrrole. Therefore, the more accurate reaction for this redox

intercalation can be represented in equation 3.2.

XC4H‘NH + V2051.6H20 '0' X‘Y/202 ——>(C4H2NH),‘HYV205nH20

+ x-y/2H202 ..... eq(3.2)
(0' H20)

Oxygen is not only involved during the reaction but also affects
the products after isolation. Upon standing in air (aging) for several
months, the magnetic moment of (Ppy)xV205nH20 decreases as
observed in (PANI)XV205nHzO, see page 87. We found that
polypyrrole in (Ppy)xV205nH20 degrades gradually upon aging. This
can be clearly detected by the small but significant change in the
FTIR spectra. No change in the XRD pattern is observed. The
samples are stable in sealed evacuated tubes, as judged by the IR
and magnetic susceptibility data. Unlike (PANDXVzOanzO, oxygen
here did not cause continuing oxidative polymerization of pyrrole
oligomers in the interlamellar space of V205 but instead it reacted
the V205 and perhaps with polypyrrole and destroyed its conjugating
structure. Another cause for the degradation of polypyrrole could be

the V205 framework after it has been reoxidized. Pyrolysis mass

139

spectra of both aged and fresh samples showed no volatile species at
temperature up to 350°C under vacuum. This suggested that the
degradation did not seriously reduce the polymer chain length. This
polymer degradation results in a decrease in electrical conductivity
(vide infra).

The oxidative degradation is accelerated at higher temperature.
When (Ppy)xV205nH20 was heated at 175°C in air for 3 days, IR
spectra showed changes consistent with degradation of polypyrrole.
No change was observed in the V205 framework. However, when the
sample was heated in vacuum at 175°C, we observed IR spectra
changes associated with changes both in the polymer as well as V205
structures. The V205 is reduced and Ppy is oxidized consistent with

internal redox chemistry between the two species.

C. Thermogravimetric Analysis (TGA) and
Differential Scanning Calorimetry (DSC) Studies.

Typical thermogravimetric diagrams of fresh and aged
(Ppy)xV205nH20 under oxygen flow are shown in Figure 3.9. A less
than 2% weight loss is observed at temperatures below 100°C
probably due to absorbed and intercalated water. A continuous
weight loss was observed between 100°C to 470°C and stabilized
thereafter. The decreasing weight between 100°C and 470°C (differs
slightly from sample to sample) is due to the decomposition of the
polymer and loss of trace amount of residual structural water. The
thermal stability of fresh samples is better than that of aged

samples. This is because the aged samples have more V5+ sites in

WEIGHT %

140

 

100-

 

 

 

 

b

)-

i- 1

» 1

- +

)- «1
so 1 A 1 J L J_ 1 L J J J _1 J 1 A L ‘

0 250 500 750

TEMPERATURE (°C)

Figure 3.9. TGA curves (under oxygen flow) of (A)
Fresh (Ppy)o.9V205nflzo (B) Aged (Ppy)o.9V205nH20.

141

V205 layers which in turn are more oxidizing. The same reason
explains why the material under nitrogen flow is more stable than
that under oxygen flow. The thermal behavior of (Ppy)xV205nHzO is
similar to that of (PANI)XV205nH20 and (Pth)xV205nH20 systems,
see page 78 and Page 182.

The DSC diagram of (Ppy)xV205nH20 was featureless.
Compared to that of pristine V20 5 xerogel which showed a strong,
broad endothermic peak at temperature around 150°C which is due
to the loss of the interlayer water [19]. The DSC result was consiSIent
with the XRD data which showed that after intercalation, most of the
interlayer water was expelled. When (Ppy)xV205nH20 was placed in
air for several months (aged), it absorbed moisture as judged the
increase in interlayer spacing. The DSC diagram of aged sample

showed a weak endothermic peak at temperature around 150°C.

D. Electron Paramagnetic Resonance (EPR)

Spectroscopy.

The EPR spectra of all (Ppy)xV205nH20 showed very broad
signals ( Apr = 100~200 G) centered at g = 1.9637 which was
similar to that of reduced V20 5, such as NaxV205nH20 [20]. This
signal is due to the presence of V41“, (11 centers [21]. The typical
narrow EPR peak arising from static defects of conducting
polypyrrole [22] was not observed even at liquid nitrogen
temperature. This suggests that either the unpaired spins on the
static defects of polypyrrole are magnetically coupled to the electron

spins in the reduced V20 5 framework or the number of such defects

142

 

(A)

(B)

(C)

 

DPPII '

 

 

2900

3300

MAGNETIC FIELD (Gauss)

3900

Figure 3.10. Room temperature EPR spectra of (A)

(Ppy)o.56V205nH20
polypyrrole.

(B) Nao.4V205nH20

(C) Extracted

143

in these materials considerably smaller than typical bulk Ppy or the
defects are diamagnetic. The latter is unlikely since the narrow EPR
signals of polypyrrole can be observed after extracting the polymer
from the host matrix by acidic solution as shown in Figure 3.10.
These results are consistent with the absence of any separate Ppy
impurity phase. The EPR data show that the magnetic behavior of
these polypyrrole "bronzes" is dominated by the V20 5 network.
Efforts to relate the spin quantity derived from the EPR signal
to the degree of reduction of V20 5 framework were not pursued, as
they could be unreliable, due to the strong spin-spin interaction
between polypyrrole and V20 5 host and the large peak width of the

signal.

E. Magnetic Susceptibility Studies.

Consistent with the EPR results, variable temperature magnetic
susceptibility data showed (Ppy)xV2Oan20 to be a paramagnetic
material. The susceptibility decreases with rising temperature as
shown in Figure 3.11. The magnetic moment of polypyrrole is
relatively small [20] and thus the magnetic moment of
(Ppy)xV205nH20 mostly comes from the reduced V205 host. Similar
to (PANI)XV205nH20 (see page 87), the susceptibility of
(Ppy)xV205nH20 can be separated into two components: Curie
paramagnetism and Van Vleck paramagnetism (or temperature
independent paramagnetism) as shown in Figure 3.12. For a
discussion of TIP see chapter 2 above. Figure 3.13 shows the

variable temperature effective spin-only (Curie) magnetic moment,

144

 

 

 

 

 

0.020
.4
32:7 0.0155
g o
D
E 18
a
E 0.010~O
1.]
g .8
S:
a 0.0055 O
o
0.000 .j - 1 -OFOPQOJDrOQQ -

0 50 100 150 200 250 300 350
TEMPERATURE (K)

Figure 3.11. Variable temperature magnetic susceptibility
(Xm) data of (Ppy)o,73V205nHzO at 5000 gauss applied
field.

145

 

 

 

12000
s10000—(fiw,, 4+ 4» 0- + e 0- 4- + o- 4 4-(C)
g d
"‘5 8000-
2 .1
8
7 6000-4
E .
:3.
E 4000- a Elc103)
u: ‘ am A
ézoood QQQBBBBEE ()
M
d an Q
OM '— I ' I 1 r F I n T f

 

0 50 100 150 200 250 300 350
TEMPERATURE (K)

Figure 3.12. Variable temperature magnetic susceptibility
data (l/xm) of (Ppy)o,73V205nH20 at 5000 gauss applied
field (A) Total magnetic susceptibility (B) Curie
susceptibility (C) Temperature independent paramagnetism

146

 

1.0

0.9 -

08-4me Ooooooooooo

0.7 -‘

0.6 -1

0.5 -i-~—1—-I—r—I'—I—-I-I-v—r--—r—I-i

0 50 100 150 200 250 300 350

TEMPERATURE (K)

 

MAGNETIC MOMENT (BM)

 

 

Figure 3.13. Variation of effective spin-only magnetic
moment as a function of temperature for
(Ppy)o,73V205nH20 at 5000 gauss applied field.

Heft
enor
pmal
scve
33.

we

oxki
preh
Fun)
nunt
pobn

deca

lere
hnfil
3am;
mfpz
indh

form:

whh
{Con

Olyg{

may

PhOtc

“her

147

ueff, of (Ppy)o.73V205nH20. The ueff is constant, within experimental
error, in all measured temperature (5k~3OOK), consistent with the
paramagnetic behavior. The room temperature magnetic moment for
several (Ppy)xV205nH20 with different x values are listed in Table
3.3. In all samples, the total ucff is lower than the value expected if
we assume that all electrons released from the monomer during
oxidative polymerization are transferred to V20 5 layers. This is
preliminary due to the effect of oxygen as discussed above.
Furthermore, the chain length of the polymer is also related to the
number of electrons transferred to V205 during oxidative
polymerization. For instance, for 20 monomer unit, to form 2
decamers, more electrons are transferred than by the formation of
10 dimers. Due to these complicated interactions the ueff does not
represent fully the degree of reduction of V20 5. Generally, for
similar x value in (Ppy)xV205nH20, the magnetic moments of the
samples prepared from aqueous solution are higher than those
prepared from acetonitrile solution (see Table 3.3). This may
indicate (qualitatively) that the degree of reduction of host in the
former was higher than the latter.

Upon standing in air (aging), the magnetic moment decreased
with time as shown in Figure 3.14. This is probably due to the
reoxidation of V4+ centers to V513 However, in (Ppy)xV205nH20,
oxygen also degrades the polypyrrole inside the V20 5 layers, which
may also effect the magnetic behavior.

We attempted to determine the V4+IV5+ ratio by X-ray
photoelectron spectroscopy (XPS). Although the V+4 peak was

observed, due to the change of V4+ peak position and shape as well

148

Table 3.3. Room Temperature Magnetic Moment
and Temperature Independent Paramagnetism of
(Ppy)xV205nH20 versus x.

 

 

 

 

X HenlBM) ueMBM)‘ xm x 10"
(Total) f (Curle) (TIP)
0.18 0.89 0.74 1.0
0.32 0.73 0.52 1.0
0.36 0.85 0.73 0.8
0.56 0.96 0.91 1.0
0.73 0.93 0.79 1.0

"0.17 1.14 0.91 0.911.6
H0.26 1.50 0.90 5.5
"0.56 1.67 0.93 8.0

 

*:

**: Samples

prepared

Corrected for TIP Van Vleck paramagnetism.
in aqueous solution

 

MAGNETIC MOMENT (BM)

149

 

0.9

 

 

 

 

0.71
AA

AAAAAA‘AAAAAA(B)

0.6lfiffilfifirfjrt'
0 50 100 150 200 250 300 350
TEMPERATURE (K)

Figure 3.14. Variation of effective spin-only
magnetic moment as a function of temperature for

(Ppy)o,73V2Oan20 (A) Fresh prepared (B) Aged.

150

as serious overlap between V4+ and V5+ peaks, we were not able to
do a complete quantitation of each species. Further investigation of

XPS spectra is in progress.

F. Charge Transport Properties.

(1). Electrical conductivity.

The electrical conductivities of (Ppy)xV205nH20 were measured
by the in-line four—probe method as a function of temperature. We
found that the conductivity of film samples prepared in water
increases 5 orders of magnitude compared to pristine V20 5 as shown
in Figure 3.15. AC/DC measurements suggest that the charge-
transport in these materials is electronic in nature. No ionic
contribution was detected. Interestingly, the film samples of
(Ppy)xV205nHzO prepared in H20 show significantly higher
conductivity than those prepared in CH3CN, as shown in Figure 3.16.
For all samples the conductivity increases with rising temperature.
This is a typical thermally activated behavior dominated by
interparticle contact resistance as well as intrinsic effects such as
polymer conjugation length, interchain distance and V4+ to V5+
hOpping barriers. Similar behavior was observed in
(PANI)XV205nH20 and (Pth)xV205nH20 (see page 97 and 194).
Thermally activated behavior is also existent in other intercalated
compounds [22] and conducting polymers [23].

The higher conductivity for the samples prepared by using

water as a solvent may be due to the formation of longer polymer

151

 

(A)

#-

0°

0

1
A

d,
I

O

l
O)

T

1

0'5
7

LOG CONDUCTIVITY (fl'lcm'l)
15 i.
I I

D
0
O
G
O
O
O
o

1
‘6
I

 

 

I

ll] [IJJJ4JP1lLLlJJIJLIIngLlpIJI

 

 

0 40 80 120 1 50 200 240 230 320

TEMPERATURE (K)

Figure 3.15. Four-probe variable temperature electrical
conductivity data of films of (A) (Ppy)o,25V205nH20 (B)
Pristine V205xerogel.

152

 

LOG CONDUCTIVITY (fl'lcm'l)

 

 

 

10 1 11 I L #1 L1 1 144 I_L_L_LLL I I_IJ _L Li 1_I 4 L A_1

0 50 100 150 200 250 300 350

 

TEMPERATURE (K)

Figure 3.16. Four-probe variable temperature electrical
conductivity data of films of (A) (Ppy)o,25V205nHzO
prepared from aqueous solution (B) (Ppy)o,32V205nH20
prepared from acetonitrile solution.

153

chains or polymer with fewer structural defects and thus longer
conjugation length. Also, according to the magnetic data, the V4+/V5+
ratio in the water-prepared samples is higher than the acetonitrile
based, meaning that the number of carriers in the former is greater
than in the latter, and thus contribute to higher conductivity. It has
been known that polypyrrole prepared from solvents having OH
group exhibits higher conductivity [13], but the reason is not
understood. Table 3.4 lists the room temperature conductivities of
(Ppy)xV205nHzO prepared from CH3CN solution with different x
value. It is clear from this table that with exception of very low x
values, the conductivity of film samples are similar to those of
pressed pellets. It is also evident that the magnitude of the
conductivity is more or less consistent. This is surprising since we
would expect that the conductivity would be sensitive to the
polymer/V205 ratio. When current is passed through these
materials, it either goes through the reduced V205 framework or
through the partially oxidized polymer blocks, depending on which
way is more favorable. In low polymer/V205 ratio, the distance
between polymer chains should be large, and the current flow should
be better mediated through the V205 framework. In high
polymer/V205 ratio, the polymer chains should be closer together,
and thus we might except the carriers to travel easier through the
polymer. In our studies, all the (Ppy)xV205nH20 samples prepared
from CH3CN solution showed similar conductivity (10'3~10'2£2'1cm'1)
regardless of the polymer content. A possible explanation is that the

polypyrrole chains inside the V20 5 layers are low molecular weight

154

 

 

Table 3.4. Room Temperature Electrical Conductivity
(0) versus x of (Ppy)xV205nH20 Prepared from
Acetonitrile Solution.
x 63.1.(9'1cm'1) measured form
0.16 < 10“ pellet
0.18 < 10" pellet
0.32 10"3..10'2 film
0.36 10-3~1o-2 pellet
0.55 104.10"2 pellet
0.64 10'2 film
0.73 10'3~1°'2 pellet

 

 

 

 

 

Table 3.5.

Room Temperature Electrical Conductivity

 

 

(0) versus x of (Ppy)xV205nH20 Prepared from
Aqueous Solution.
X 63.1. (Q-lcm-l) measured form
0.17 10'3~1°'2 pellet
0.25 10'2 pellet
0.26 10° film
0.40 100 film
0.42 10'3~1°‘2 pellet
0.56 10'3~1°'2 pellet

 

 

 

 

 

155

polymers, and the charge transport is preliminarily through the
vanadium oxide framework.

The conductivity of (Ppy)xV205nH20 prepared from aqueous
solution varied significantly between film samples and pressed
pellets (011.11; 100 vs 10‘3 Q-lcm'l) as listed in Table 3.5. In this
table we see no obvious correlation between conductivity and
polymer/V205 ratios. The high conductivity of the film samples can
be rationalized by the longer conjugation length of the intercalated
polypyrrole and less structural defects. We emphasize however we
do not have molecular weight information to substantiate this claim.

The Ppy inside the V20 5 degraded gradually upon aging as
observed in FT-IR spectroscope. However, the room temperature
conductivities of the aged samples are similar to those of fresh
prepared as shown in Figure 3.17.

Although the conductivities of (Ppy)xV205nH20 prepared from
acetonitrile solution are approximately the same , the thermoelectric
power (TP) data vary with different x values as shown in Figure
3.18. The Seebeck coefficient becomes steadily less negative as the
Ppy content increases. The large and negative TP indicate that
(Ppy)xV205nH20 is a n-type semiconductor. Typical variable
temperature thermoelectric power data are shown in Figure 3.19.
The TP behavior is similar to alkali metal xerogel bronzes, CsxV205
[24] and suggest that the reduced V205 dominates the charge
transport. This implies that the polymer chains in (Ppy)xV205nHzO
are short, not favorable for current flow. The decreasing of
thermopower may be due to the increasing degree of reduction of

V20 5 framework as the polymer/V205 ratio increases,.

156

 

 

 

 

 

-2 7
-4 _
.5 +
Q -5 L.
E: _
E
r
E -e 4.
a
2 L.
O
o _
CD
3 -10 .- °
. AAA
-12 L 1 . 1 . 1 . 1 E 1 L
50 100 150 200 250 300 350
TEMPERATURE (K)
Figure 3.17. Four-probe variable temperature electrical

conductivity data for (Ppy)o,73V 2 O 5 n H 20 (A) Fresh
prepared (B) Aged. (The anomaly at ~270K is due to
instrumental instability and no physical significant)

157

 

THERMOPOWER (EV/K)
8
o
L L J _l l

 

 

 

-200 T r T 1 j— Iw U I: T I If

. 1 . 1
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
X

Figure 3.18. Variation of room temperature thermoelectric
power as a function of x for (Ppy)xV205nH20 prepared from
CH3CN solution.

158

 

 

 

 

 

 

20 ,
0 I
L
2 5
3 -20 L
g l
g i
3-
g 4%
2
a:
E r
H >-
-60 t—
-80 LA L 1 L I I I L L I l L LA I L _L i i L L I 1
100 150 200 250 300 350
TEMPERATURE (K)
Figure 3.19. Variable temperature thermoelectric power

data (pressed pellets) for (A) (Ppy)o,26V205nH20 prepared
in water (B) (Ppy)o,32V205nH20 prepared in acetonitrile.

159

The TP of (Ppy)xV205nH20 prepared from aqueous solution
show very interesting behavior. The Seebeck coefficients are
relatively small compared to those prepared from CH3CN solution
(with similar polymer content) and become less negative as the
polymer/V20 5 ratio increases as shown in Figure 3.20. At low
polymer/V205 ratio, the Seebeck coefficient is almost temperature
independent as seen in reduced V20 5. This indicates that for the low
x values the reduced V20 5 is responsible for the charge transport
pr0perties. Near the high x limit, the sample shows a n-type to p-
type transition at 175K (see Figure 3.20a). This behavior is not
observed in either reduced V205 or polypyrrole. The p-type
conductivity is not possible for V20 5 alone while n-type conductivity
is not possible for partially oxidized polypyrrole. Furthermore the
slepe of the TP in the polypyrrole-rich samples is opposite to what is
observed in conventional p-doped polymer. Therefore, the charge
transport property of this composite is a hybrid of both components.
In contrast to the CH3CN derived samples, the film samples of
(Ppy)xV205nH20 prepared from aqueous solution show small and
positive TP in all temperature range (4.2K~300K), consistent with p-
type metallic behavior, see Figure 3.19b. The fact that the TP
decreases as temperature increases makes it different from
polypyrrole, and it is another demonstration that we are looking at
hybrid behavior. The charge transport through the polypyrrole is
more favorable despite the fact that the polymer content is not high.
This implies that the polymer chains may be much longer than those
in the samples prepared in CH3CN. This also consistent with the

conductivity data.

160

 

 

 

 

 

 

20
r o
L O
0 ~ "
A -20" '
é .
3 r-
; -40~ (m
g :
g -50L— x o °
g _ X >5 x o o o o 009w)
E- x 2‘ o QM
i" meX%(%%° » fi
'80— )x WI”)
I x x
-10971141J1144111L4 L14 LLJ LL11
00 150 200 250 300 350
TEMPERATURE (K)
Figure 3.20. Variable temperature thermoelectric power

data (pressed pellets) of (Ppy)xV205nH20 prepared in H20
solution (A) (Ppy)o.56V205nH20 (B) (Ppy)o.42V205nH20 (C)
(Ppy)o.17V20an20 (D) (Ppy)0.2sV205nH20-

161

 

 

 

 

 

-50 .
-60 L
g -70 E—
> +-
3 .—
9‘ Z
g ~80 +—
2 ..
o i
5 -90 _.
E :
E" L-
-100 L
-110 h 1 1 4 1 i __I 4 g 1 i L 1 J 1 J L 1 4 L
150 200 250 300 350
TEMPERATURE (K)
Figure 3.21. Variable temperature thermoelectric power

data (pressed pellets) for (Ppy)o,73V205nH20 (A) Fresh
prepared (B) Aged.

162

Not surprising, the TP of aged samples became more negative
and almost can not be measured at temperature below 230K due to
high contact resistance as shown in Figure 3.21. This is due to
reoxidation of the V205 framework and degradation of the

polypyrrole as evidenced by the IR and magnetic data (vide supra).

IV. Conclusions:

The (Ppy)xV205nHzO composites consist of alternating
monolayers of metal oxide and conductive polymer and thus they
may be called molecular nanocomposites. The charge transport
properties of (Ppy)xV205nH20 does not depend on polymer/V205
ratios but rather on the reaction media in which they are prepared.
The charge carriers can switch from electrons to holes, and the
behavior can be changed from semiconducting to metal—like by
varying the reaction conditions. This coupled with the ability to
synthesize different polymer/V20 5 ratios with variable charge
transport properties, suggest possible applications as multi-
functional materials and devices that take advantage of both organic
and inorganic components. ln-situ oxidative intercalation/
polymerization reaction may provide a new strategy to synthesize
interesting conducting polymer-derived electroactive materials in a

relatively controlled way.

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163

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Logan, J. A.; Rabolt, J. F.; Street, G. B. J. Chem. Soc. Chem.
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D.; Grant, P. M.; Street, G. B. Synth. Met. 1979/1980,_l__, 329-
336. (c) Satoh, M.; Kaneto, K.; Yoshino, K. Synth. Met. l986,l_4_,
289-296.

(a) Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1983, 142,
101-113. (b) Letheby, H. J. Chem Soc. 1962,Li, 161-163. (c)
Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980,311, 111-
114.

(a) Dall’Olio, A.; Dascola, Y.; Varacca, V.; Bocchi, C. R. CR Seances,
Acad. Sci. Paris Ser. C 1968,_2__6_7_, 433-435. (b) Salmon, M.;
Kanazawa, K.; Diaz, A. F.; Krounbi, M. J. Polym. Sci. Polym. Lett.
1982,;(1, 187-193.

(a) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Somasiri, N. L.
D.; Epstein, A. J. In Conducting Polymers Alcacer, L. Ed. Reidel
Dordrecht, Holland, 1987 . (b) MacDiarmid, A. G.; Chiang, J. C.;
Halpern, M.; Huang, W. S.; Mu, S. L.; Somasiri, N. L. D.; Wu, W.;
Yaniger, S. I. Mol. Cryst. Liq. Cryst.1985, _1_2_1, 173-180.

(a) Yamamoto, T.; Sanechika, K.; Yamamoto, A. J. Polym. Sci...
Polym. Lett.1980,3, 9-12. (b) Kossmehl, G.; Chatzitheodorou,
G. Mol. Cryst. Liq. Cryst. 1982, 33, 291-296. (c) Lapkowski, M.;
Genies, E. M. Polimery, 1989, 33, 45-52. ((1) MacDiarmid, A. G.;
Asturias, G. B.; Kershner, D. L.; Manohar, S. K.; Ray, A.; Scherr, E.

12.

13.

14.

15.

16.

17.

18.

19.

20.

165

M.; Sun. Y.; Tang, X.; Epstein, A. J. Polym. Prepr. 1989,3011)
147-148.

(a) Whittingham, M. S. Mat. Res. Bull. 1978,13, 775-782. (b)
Ruthard, R.; Schollhom, R.; Weiss, A. Z. Naturforsch 1972, 21,
1275-1276. (c) Ruitz-Hitzky, B.; Casal, B. J. Chem. Soc. Faraday
Trans. 1 1986,32,, 1597-1604.

(a) Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985, 13, 309-315.
(b) Patil, A. 0.; Heeger, A. J.; Wudl, F. Chem Rev. 1988, 33, 183-
200.

Walker, I. A.; Warren, L. F.; Witucki, E. F. J. Polym. Sci. Part A:
Polym. Chem. 1988,23, 1285-1294.

Diaz, A. F.; Hall, B. IBM J. Res. Dev. 1983, 21, 342-347.

(a) Lindsey, S. B.; Street, G. B. Synth. Met. 1985, _1_Q, 67-69. (b)
Nazzal, A.; Street, G. B. J. Chem. Soc. Chem. Commun.1985, 375-
376.

Machida, S.; Miyata, S.; Techagumpuch, A. Synth. Met. 1989,31
31 1-318.

(a) Street, G. B.; Clarhe, T. C.; Krounbi, M.; Kanazawa, K.; Lee, V.;
Pfluger, P.; Scott, J. G; Weiser, G. Mol. Cryst. Liq. Cryst. 1982,33,
253-264. (b) Neugebauer, H.; Sariciftci, N. S. "Low Dimensional
Systems and Molecular Electronics" Ed. R. M. Metzger et al
Plenum Press New York 1991, P401-406.

Aldebert, P.; Baffler, N.; Gharbi, N.; Livage, J. Mat. Res. Bull.
1981,_l_6_, 669-676.

Genoud, F.; Guglielmi, M. Phys. Rev. Lett. 1985, 53, 118-121.

21.

22.

23.

24.

166

"Non-Stoichiometric Compounds, Tungsten Bronzes, Vanadium
Bronzes and Related Compounds" Bevan, D. J.; Hagemuller, P.
Eds. Pergamon Press Oxford 1973.

Babonneau, P. F.; Barboux, P.; Josien, F. A.; Livage, J. J. de Chim.
Phys. 1985, .82“ 761-766.

Maddison, D. S.; Roberts, R. B.; Unsworth, J. Synth. Met. 1989,
31, 281-287.

Kanatzidis, M. G. et a1 work in progress.

CHAPTER 4

SYNTHESIS AND CHARACTERIZATION OF CONDUCTING

POLYTHIOPHENE/Vzos BRONZE ~ In-situ INTERCALATION OF
POLYTHIOPHENE IN V205 XEROGEL

167

168

ABSTRACT

Polythiophene (Pth) can be easily intercalated in V20 5 xerogel
by reacting 2,2'-bithiophene with V20 5 in acetonitrile. The resulting
products, (Pth)xV205nH20, have interlayer spacing of 14.7A. The
polymer formed inside the layers can be extracted by digesting away
the V20 5 framework with the assistance of chelating agent, catechol.
The EPR spectra of (Pth)xV205nH20 show a broad signal (Apr=155
gauss) centered at g=l.9623. The typical sharp radical-based peak of
polythiophene was not observed due to the spin-spin interaction
between polythiophene and V205 host. Variable temperature
magnetic susceptibility revealed Curie paramagnetism mixed with
temperature independent paramagnetism. The conductivity of
(Pth)xV205nH20 increased continuously with increasing x at the same
time the thermopower also changed. The Seebeck coefficient, S, of
high x value samples reveals p-type metal-like behavior. Samples
with low x show a relative large S and negative suggesting n-type
semiconductor behavior. Upon aging, the magnetic moment of
(Pth)xV205nHzO decreased and the room temperature electrical
conductivity slightly decreased compared to the fresh sample. The
thermoelectric power of the aged sample with high polymer content
still remain small and positive, but decreases as temperature

increases which is different from all the fresh samples.

169

I. Introduction:

The redox intercalation of conducting polymers into various
layered hosts is the subject of intense investigation in our laboratory.
The resulting compounds can be viewed as polymer bronzes by
analogy to the alkali metal [1] or molecular [2] bronzes. It has been
mentioned that in molecular scale organic/inorganic composites, the
interaction between the two components can give a new set of
properties which is not possible from either component separately
[3]. Therefore, by inserting polymers in structurally restricted and
electrically active hosts, two goals may be achieved: preparing
oriented polymers and synthesizing new interesting polymer/
inorganic conducting molecular composites.

Polythiophene, (Pth) is one of the prototype conducting
polymers with excellent environmental stability and good
electroactivity [4]. It can be prepared by either electrochemical or
chemical polymerization of thiOphene or 2,2'-bithiophene
respectively. Neutral polythiophene is a red-colored semiconductor
with band gap (Eg) of 2.2 eV [5]. Upon dOping, the electrical
conductivity of the polymer film can be increased over 10 orders of
magnitude, to the metallic state [6]. The color of the doped material
is black. This semiconductor to metal transition is also accompanied
by important modifications of mass [7], volume [8] and chain
geometry [9]. The conductivity of polythiophene is not determined
only by the chain length, but the abundance of long conjugated
segment is required [10]. In other words, the extension of the

monomer ring planarity (without ring opening) will affect the

170

conductivity [11]. It has been known that electrochemically
polymerized polythiophene contains some impurities and structural
defects [12], such as cross-links and C-C coupling through the [3-
positions of the thiophene ring. However, a highly regular polymer
backbone can be obtained by varying the nature of dopants [13]
during polymerization. A good synthetic strategy to prepare well
oriented polythiophene is by in-situ oxidative polymerization/
intercalation of polythiophene in layered hosts. Here the host also

acts as oxidant as well as dopant.

11. Experimental Section:

Reagents: NaOH, HCl, NHaOH(aq), NaVO3, FeClg, DMSO, NMF,
catechol, thiophene and 2,2'-bithiophene were purchased from
commercial sources and used without further purification. CH3CN

was dried under CaH2 and distilled in air prior to use.

Physicochemical Methods.
See chapter 2.

Preparation of V205nH20 Xerogel.
See page 56.

Preparation of (Pth)o,37V2050.54H20 Film.

0.4 g (1.9 mmol) film of V20 5 xerogel was added to 30 ml of

0.04 M acetonitrile solution of 2,2'-bithiophene. The mixture was

171

refluxed for 12 hours. The black film was isolated by filtration,
washed with acetone and dried in vacuum. Elemental analysis: Calcd
for (C4H2$)o,37V2050.54H20: C, 15.87%; H, 1.07%; S, 10.58%; V, 38.77%.
found: C, 14.98%; H, 1.25%; S, 8.12%; V, 36.48%. Water content was
obtained from thermal gravimetric analysis under oxygen flow. The
interlayer spacing calculated from X-ray diffraction is equal to

14.70A.

Preparation of (Pth)o,15V2050.58H20 Film.

0.1 g (0.60 mmol) of 2,2'-bithiophene was dissolved in 20 ml
CH3CN. To the solution, added 2 drops of H20 followed by 0.4 g (1.9
mmol) of film of V205 xerogel. The mixture stood at room
temperature without disturbing for 24 hours. The black film was
isolated by filtration, washed with acetone and dried in vacuum.
Elemental analysis: Calcd for (C4H28)o,15V2050.58HzO: C, 3.74%; H,
0.72%; S, 2.49%; V, 49.62%. Found: C, 3.41%; H, 1.72%; S, 3.04%; V,
44.53%. The water content was obtained from thermal gravimetric
analysis under oxygen flow. The interlayer spacing determined from

X-ray diffraction is equal to 14.50A.

Preparation of (Pth)o,72V2051.101-120 Powder.

To a 0.62 g (3.73 mmol) 2,2'-bithiophene solution in 30 ml
acetonitrile was added 1.0 g (4.74 mmol) of V20 5 xerogel (fine
powder). The mixture was stirred at room temperature for 20 hours.

The resulting black powder was isolated by filtration, washed with

 

172

acetone and dried in vacuum. Anal. Calcd. for (C4H2S)o,72V2051.1H20:
C, 7.47%; H, 1.26%; S, 4.98%; V, 44.09%. Found: C, 13.80%; H, 0.82%; S,
7.31%; V, 40.61%. Changing the 2,2'-bithiophene/V205 ratio, the x
value of (Pth)xV205nH20 can be varied from 0.30 to 0.72.

Synthesis of Bulk Polythiophene.

In an inert atmosphere glove box, 3.0 g (18.50 mmol) of FeCl3
was dissolved in 50 ml acetonitrile. To this was added 1.25 g (7.53
mmol) 2,2'-bithiophene and the mixture was stirred at room
temperature for 4 hours. The black precipitate was isolated by
filtration, washed with acetone, dried in vacuum and characterized

by Infrared spectroscopy [14]-

Extracting Polythiophene from (Pth)o,45V2050.93H20 in

Acidic solution.

0.15 g (0.63 mmol) of (Pth)o,45V205nH20 with 0.50 g (4.5
mmol) catechol was stirred in 100 ml 1M HCl(aq) for 40 hours or in 2
weight % NaOH(aq) for 15 hours. The black residue was isolated by
filtration, washed with acetone, dried under vacuum and identified

by FTIR spectroscopy.

_—__—r'_--— -_._ _.

173

 

 

III. Results and Discussion:
A. Preparation and Characterization.

In our initial attempts for intercalation of polythiophene in
V20 5 xerogel, we used thiophene as monomer. However, thiophene
cannot be oxidatively polymerized by V205 xerogel due to its high
oxidation potential. Although upon contact of V20 5 xerogel with
thiophene acetonitrile solution, a color change to green is observed
FTIR spectroscopy revealed that no organic product was intercalated.
Therefore, we used 2,2'-bithiophene which has lower oxidation
potential.

The intercalation of polythiophene into V20 5 xerogel was
achieved by refluxing 2,2'-bithiophene with V205 xerogel in
acetonitrile. 2,2'-bithiophene was oxidatively polymerized and V20 5
is reduced as showed in equation 4.1 and 4.2. The total reaction is

listed in equation 4.3.

xC,H,S,————-— (0911482)?“ + 2xe'+ 5e' + 2xH+ --------- eq(4.1)

vzosnnzo + 2xe' + de'___.. vzos"2"*“’nuzo -------- eq(4.2)

XCgHsSg + V205nl'120 ———'> (CgH4SZ)‘V2°5nH20 """" OQ(4.3)

 

This reaction is relatively slow compared to that of aniline and
pyrrole with V205. Higher oxidation potential and higher molecular
weight of 2,2'-bithiophene could be the reasons. The solvent used
with 2,2'-bithiophene was acetonitrile. Unlike water, acetonitrile
cannot swell V20 5 xerogel at room temperature. Even though the red
color of V205 turns black immediately upon contacting 2,2'-
bithiophene solution, complete intercalation requires more than 4
hours depending on the reagent ratios. When mixed CH3CN/H20 was
used as a solvent, the reaction rate increased but often the products
were not homogeneous and exhibited amorphous character. The
reaction principles of intercalating polythiophene in V205 xerogel are
similar to that of polyaniline and polypyrrole analog. Use of other
solvents, such as DMF and DMSO yielded products in which solvent
molecules were inserted with very little polymer inside. Primary
alcohols seem unsuitable as solvents because they are oxidized
gradually by V205.

The polythiophene formed between the slabs of V20 5 xerogel
can be detected easily from FTIR spectra as shown in Figure 4.1a.
The three strong peaks at frequencies below 1000 cm;1 belong to
V205 framework which does not change after intercalation. The IR
spectra of polythiophene in V205 interlamellar space show slightly
different frequencies compared to that of bulk polythiophene (see
Table 4.1). The vibration energies are shifted to higher frequency
and the relative intensities are altered. This may be due to a
different degree of polymerization and oxidation or the effect of the
counter anion (in this case ([V205nH201X’). However, the

polythiophene inside the V20 5 layers can be extracted by digesting

TRANSMITTANCE

175

 

 

 

 

 

 

   

I

40.. 3400 28.0

WAVENUMBER (cm-1)

Figure 4.1. FT-IR spectra (KBr pellets) of (A)
(Pth)o,45V2050.93H20 (B) Extracted polythiophene
(C) Bulk polythiophene.

176

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177

away the V205 framework with dilute acid or base (with the
assistance of catechol). Polythiophene is damaged slowly both in
concentrated basic solution and also in acidic solution during
isolation, where the strong oxidizing power of V5+ species, which are
released in solution, react with it. Intact polythiophene was
successfully extracted from (Pth)xV205nH20 from an acidic solution
in which catechol was added. The catechol acts as a chelating ligand
to arrest the aquated V5+ ions and reduce their oxidation potential,
thus rendering them unreactive towards polythiophene. The IR
pattern of extracted intact polythiophene is virtually the same as
that of bulk polythiophene (see Figure 4.1). It has been pr0posed
that the vibration bands of polythiophene are independent of the
doping species [15]. The difference in IR pattern of bulk
polythiophene and polythiOphene inside the V205 host is because of
the latter was probably bound, at least electrostatically, to a
structurally restricting framework.

The XRD pattern of (Pth)xV205nH20 prepared from 2,2'-
bithiophene with V20 5 in refluxing acetonitrile is shown in Figure
4.2. Only several 00] reflections are observed consistent with the
turbostratic nature of the product. Upon intercalation, the 001 peaks
shift to lower Bragg angles. The interlayer spacing calculated from
the average of several 001 reflections are equal to 14.70(i20)A which
corresponds to an interlayer expansion of 5.95A. This interlayer
expansion was smaller (by 0.3A) in the low polymer content samples
(interlayer spacing of (Pth)o,45V2050.93H20 is 14.5011 compared to
14.80A for (Pth)o,37V2050.54H20). When the reactions were carried

out at room temperature using fine powder of V205 xerogel, the

178

 

(A)

 

INTENSITY

 

 

 

26 (DEGREE)

Figure 4.2. Reflection-mode X-ray diffraction patterns
of (A) V20 5 xerogel (film) (B) (Pth)o,37V 20 50. 54 H 20
(pressed pellet).

179

resulting black powder had similar interlayer spacing although the
diffraction peaks are relatively broad and weak. We found that
(Pth)xV205nHzO is a relatively porous materials, which upon sitting
in air absorb moisture and increase their interlayer spacing as
evidenced by corresponding changes in the XRD pattern.

It will be interesting to know the arrangement of
polythiOphene in V20 5 layers. There are two most common proposed
structures of polythiophene, linear chain [12,16] and coil [17]
arrangements. The dimension of coil arrangement is larger than
10A. Therefore, the 5.95A interlayer expansion due to the
intercalation of polymer can be regarded as insertion of a monolayer
of linear polythiophene chain with the five member rings

perpendicular to the V205 slabs as shown in Scheme 4.1.

 

 

 

 

 

 

 

 

 

 

 

 

 

Scheme 4.1. Proposed arrangement of polythiophene
chain in V20 5 layers.

The same arrangement has also been proposed in (Pth)xFeOCl [18],
(PANI)xV205nHzO and (PANI)xFeOCl, see chapter 2&5. The slightly

different interlayer spacing for each sample may be due to the

180

imperfect arrangement of polythiophene chains in (Pth)xV205nH20
layers. To study the alteration of V205 layers after intercalation, x-
ray diffraction with the X-ray beam perpendicular to the
(Pth)xV 20 5nH20 film was performed. The XRD pattern of
(Pth)xV205nH20 is identical to that of pristine V20 5 as shown in
Figure 4.3. This implies that there is a topotactic intercalation in
which the host framework is preserved after the reaction.

During in-situ insertion/ polymerization of aniline and pyrrole
in V20 5, we found that ambient oxygen was intimately involved in
the reaction. In the PANI/V205 system it even continues to
oxidatively polymerize aniline oligomers to longer chain of polymer
long after (PANI)XV205nH20 is isolated. The oxygen effects on
(Pth)xV205nH20 are not as obvious as seen in (PANI)XV205nHzO and
there is no direct evidence showing that significant quantity of
oxygen is consumed during the reaction. It is difficult to document
the role of oxygen because the intercalation is relatively slow
compared to polyaniline and polypyrrole systems. However, if we
look at equation 4.3 carefully, when the polymer/V20 5 ratio is equal
to 0.5, then more than 50% of V5+ were reduced to V442 In this case,
the V205 framework will change as observed by Livage and
coworkers [19]. However, in (Pth)xV205nHzO the IR and XRD data
showed the structural preservation of V205 layers implying that
some oxygen must be involved. The more accurate reaction is

written as equation 4.4.

XC3H552 + VZOS + (X'y/2)02 —-—>(08H482)XHVV2°5

(x- [2 H O ...... 4.4
+ {0311202) eq( )

181

 

 

INTENSITY

 

(A)
Jr
1L
-» (B)
l
, . . A... A. .k
2 10 20 30 40 50

26 (DEGREE)
Figure 4.3. Transmission-mode X-ray diffraction

patterns of (A) (Pth)o,37V2050.54H20 (B) V20 5 xerogel.
(In this experiment, the incident X-ray beam was
perpendicular to the film. The detector is moving
along the 29 angle while the sample remains
stationary)

182

There is also no firm evidence showing that the polythiophene
continues to couple upon sitting in air (aging). The pyrolysis mass of
both fresh and aged samples did not show any volatile species up to
350°C under vacuum. The IR and XRD spectra did not change upon

aging although the conductivity slightly decreased (vide infra).

B. Thermogravimetric Analysis (TGA) and
Differential Scanning Calorimetry (DSC) Studies.

Polythiophene is stable up to 350°C in air and 950°C under
inert atmosphere [13b,4b]. However, the thermal stability of
(Pth)xV205nHzO is similar to that of (PANI)XV205nH20. Typical TGA
diagrams of fresh and aged (Pth)xV205nHzO under oxygen flow are
shown in Figure 4.4. A small amount of weight-loss (1~2%) is
observed below 100°C (belong to the absorbed moisture), followed
by a continuous weight loss up to 500°C. A slight weight-increase
between 500°C and 700°C is observed and stabilized after 700°C.
The weight loss between 100°C and 500°C is primarily due to the
decomposition of polythiophene chains. The slight weight gain is
because during the decomposition of polymer, some oxygen of V205
is lost to form V20 5-x. Oxygen is regained when the residue is heated
at high temperature under oxygen atmosphere.

The molecular weight of polythiophene inside the V20 5
interlamellar space is currently not known. The lower thermal
stability of (Pth)xV205nH20 compared to bulk polythiophene does not
imply that the polymer in (Pth)xV205nH20 has lower molecular

weight. Since the polymer is in intimate contact with a strong

183

 

100 l

WEIGHT %

50

 

 

 

11L141141L4+411

 

d

 

 

250 500 750

TEMPERATURE(°C)

Figure 4.4. TGA curves (under oxygen flow) of
(Pth)o,72V2050.52H20,(A) Fresh prepared (B) Aged

184

oxidant V205, the thermal stability of polythiophene will be affected
by V20 5 even under nitrogen atmosphere. This argument can be
further supported by the fact the thermal stability of fresh sample is
higher than that of the aged sample (see Figure 4.4). In aged
samples, the V5+/V4+ ratio is increased due to the reoxidation of V”
centers by oxygen (vide infra). This increases the oxidizing power of
V20 5 framework which in turn degrades faster the polythiophene
chains.

The DSC diagram of (Pth)xV205nH20 is almost featureless. A
very broad exothermal peak start at room temperature all the way

to 200°C is due to the decomposition of polymer.

C. Scanning Electron Microscopy (SEM) and
Selected Area Electron Diffraction (SAED).

The SEM micrographs of (Pth)xV205nH20 films are similar to
that of (PANI)XVzOan20 and (Ppy)xV205nHzO as shown in Figure
4.5. They reveal the layered nature of the materials and do not show
obvious separate phases. The electron diffraction patterns of
(Pth)xV205nH20 showed only Bragg rings with weak intensities.
Although the pattern of (Pth)xV 20 5nHzO is not as sharp as
(PANI)xV205nH20, the corresponding d-spacings of the Bragg rings
are similar to that of the V205 xerogel. This further proves that this
is a topotactic intercalation reaction. The relatively weak diffraction
may be due to the more random orientation of the polymer inside
the layers and more disordering in the V205 framework. This

disordered structure was also observed in XRD data which also

 

Figure 4.5. SEM micrographs of (Pth)o,7zV2Oan20.

186

showed rather broad peaks and slight variation in the interlayer

spacing for different samples.

D. Electron Paramagnetic Resonance (EPR)

Spectroscopy.

In order to understand the spin-spin interactions between
polythiophene and V205 host, EPR studies were carried out at room
temperature and liquid nitrogen temperature. Typical EPR spectra of
(Pth)xV205nHzO show a broad peak centered at g value equal to
1.9632 with Apr = 125~155 gauss as shown in Figure 4.6a. This
signal comes from the reduced V205 layers as observed in NaxV205
(Figure 4.6b). The sharp peaks which belongs to the resonant spins
which are carried by delocalized charge in partially oxidized
polythiophene did not show up. This is probably due to the exchange
broadening by the strong magnetic interaction between
polythiophene and V20 5. The same phenomenon is also observed in
(PANI)XV205nHzO and (Ppy)xV205nH20. This confirmed the intimate
contact between polythiophene and reduced V205 as expected for
intercalated polymer. However, the sharp polythiophene signal can
be recovered when the V205 framework was digested away by acid
or base solution (see Figure 4.6c). At lower temperature, the EPR
signal of (Pth)xV205nHzO was more intense but the peak shape and

width does not change.

 

 

 

(A)
DPPH +
(B)
_; (C)
29'00 347m 3700

MAGNETIC FIELD (Gauss)

Figure 4.6. Room temperature EPR spectra of (A)
(Pth)xV205nH20 (B) Nao,4oV205nH20 (C) Extracted
polythiophene.

188

E. Magnetic Susceptibility studies.

The magnetic behavior of (Pth)xV205nH20 is similar to those of
(PANI),;VzOanzO and (Ppy)xV205nH20. The magnetic moment in
(Pth)xV205nH20 mostly is due to the reduced V205 layers. The
susceptibility ()5) decreases as the temperature increases, indicates a
paramagnetic behavior as shown in Figure 4.7. The susceptibility is a
summary of two individual components: Curie susceptibility and
temperature independent paramagnetism (TIP) as represented in

equation 4.5.

xm = XCurlo + XTIP ................ BC] (4.5)

Variation of susceptibility as a function of temperature of each
component is shown in Figure 4.8. The effective spin-only magnetic
moment, ucff, is constant from 5K to 300K as shown in Figure 4.9.
This is consistent with Curie paramagnetic behavior. Table 4.2 lists
the room temperature magnetic moment versus x of several
(Pth)xV205nHzO. There was no linear relationship between the
magnetic moment and polymer content. The magnetic moment is
always smaller than theoretical value which assumed that all
electrons released from oxidative polymerization of monomers are
transferred to V205 framework. These may be due to a couple of
reasons: first, oxygen may also be involved in this reaction as
observed in (PANI)XV205nH20 and (Ppy)xV205nHzO, see page 69,

135. Second, the molecular weight of polythiophene in different

SUSCEPTIBILITY (EMU/Mole)

189

 

 

 

0.02 *
0.01 8
. O
0.00 . , °.°°°.°.°,9_2.9.q.2,9_p_9_._

 

0 50 100 150 200 250 300 350

TEMPERATURE (K)

Figure 4.7. Variable temperature magnetic susceptibility
data (cm) for (Pth)o,72V2050.521-I20,

 

190

 

GOOD-M...ee 4- a a a. c a- e- a- a. ,(C)

4000-

. Danna”

2000- a“:::"'.(A)
I

suscmrrmna'rv‘1 (EMU‘l Mole)

 

 

 

. fluni'
OW-lulvl-I'I'

0 50 100 150 200 250 300 350

TEMPERATURE (K)

Figure 4.8. Variable temperature magnetic susceptibility
data (l/xm) of (Pth)o,72V205,0.52H20. (A) Total magnetic
susceptibility (B) Curie susceptibility (C) Temperature
independent paramagnetism. .

MAGNETIC MOMENT (BM)

 

191

 

L0

09'

q

07-

Q6-

08W°°°°°°°°°°

 

 

 

05

i

50

I ' I ' l l ' I "

100 150 200 250 300 350

TEMPERATURE (K)

Figure 4.9. Variation of effective spin-only
magnetic moment as a function of temperature

for (Pth)o,72V205,0.52H20

192

 

 

 

 

 

 

Table 4.2. The Room Temperature Magnetic Moment and
Temperature Independent Paramagnetism of
(Pth)xV205nH20 versus x.
X llerr(BM) [ Herr(BM) 1 9m x 104
(Total) (Curie) (TIP)
0.16 0.87 0.62 1.5
0.30 0.94 0.80 1.0
0.45 0.93 0.77 3.2
0.57 1.20 0.77 3.5
0.58 0.96 0.77 1.3
0.72 1.06 0.84 1.7

193

 

 

  

4000
.4
3 30001
5 . ‘(B)
b - A
5 a ‘
.. O
s. 2000-« ‘ ‘ o 0 (A)
l: O
.4 A O O
E . 0
g: a o
6 “ O O
3 1000'“
U2

 

 

0 50 100 150 200 250 300 350

TEMPERATURE (K)

Figure 4.10. Variable temperature magnetic susceptibility
data (l/xm) of (Pth)o,72V205,0.52HzO. (A) Fresh prepared
(B) Aged.

194

samples could be different, therefore the degree of reduction of V205
xerogel could vary even for samples with the same carbon content.
Therefore the magnetic data alone can not determine the precise
degree of reduction of V205 framework. We note here that carbon
content found from elemental analysis does not necessarily give the
polymer length and thus cannot give the expected V4+/V5+ ratios.
Attempted to quantify the degree of reduction of V20 5 host by X-ray
Photoelectron Spectroscopy (XPS). Although the V” peak is
observed, due to the large change in the chemical shift and shape of
V4+ peak and serious overlap between V4+ and V5+ peaks, the exact
V4+/V5+ ratio could not be obtained.

Upon sitting in air (aging), the magnetic susceptibility of
(Pth)xV205nH20 decreased gradually as shown in Figure 4.10. This is
due to the reoxidation of the v4+ sites to V51" presumably by ambient
oxygen as observed in (PANI)XV205nH20 (page 96) and
(PPY)xV205nH20 (page 149)-

F. Charge Transport Properties.

The charge transport properties of the (Pth)xV205nH20 system
varied widely with polymer/V205 ratio. The room temperature
conductivities vary from 0.2 to 10'4 Q'lcm'1 depending on the
polymer content, with higher polymer content materials exhibiting
higher conductivity. Generally, the conductivities of films are 10 to
100 times higher than those of pressed pellets and the conductivities
measured parallel to the layer are 10 times higher than

perpendicular to the layers. Figure 4.11 shows the variable

LOG CONDUCTIVITY (n'lcm'l)

195

 

 

 

 

 

O

-1 L

-2 L~

-3 r-

-4 f

’5 [A file"

-6 ~

-7 -
l

_ I_J 141—L4_LL1_1'1+._L1 1_1_L_L1 1_1_m111._1
50 100 150 200 250 300 350

TEMPERATURE (K)
Figure 4.11. Four-probe variable temperature

electrical conductivity data of (A) (Pth)o,72V205nH20
(B) (Pth)o.ssV205nH20 (C) (Pth)o.57V205nH20 (D)
(Pth)o.45V205nH20 (E) (Pth)o.3oV205nH20-

196

temperature conductivities of (Pth)xV205nH20 with different x
values. The conductivities increase with rising temperature showing
typical thermally activated behavior, most likely associated with the
interparticle contact resistance, as observed in (PANI)anzO and
(Ppy)xV205nH20 (vise supra).

For similar polymer content, (Pth)xV205nH20 has higher
conductivity than (Ppy)xV205nH20, but has similar magnetic moment.
Since we used 2,2'-bithiophene as monomer, for the same degree of
electron transfer, the chain length of polythiophene will be doubled

the size of that of polypyrrole as showed in equation 4.6, 4.7 and 4.8.

/\ my
(1?

/ \
N x
H
/S\ /S\ ——-—> @x 4' 2X9. '''' eq (4'7)

V205 + 2x; ,. (vzosyfl ............ eq (43)

 

+ 2xe' ----- eq (4.6)

 

On the other hand, the higher oxidation potential and molecular
weight of 2,2'-bithiophene, reduced the reaction rate, compared to
pyrrole, and may have increased selectivity of the more
thermodynamic stable 2,5 coupling. Higher molecular weight and
less crosslinking in intercalated polythiophene may also be

responsible for the higher conductivity.

197

Not surprising, the conductivity of (Pth)xV2Oan20 did not
increase upon aging. On the contrary, it slightly decreased upon
sitting in air as shown in Figure 4.12. This is probably due to the
reoxidation of the V205 framework and some oxidative degradation
of polythiophene Although the IR spectra of aged (Pth)xV205nHzO
did not show any significant difference compared to the fresh
sample, polythiophene may react with oxygen to create structural
defects in low concentration which cannot be detected by IR. Those
structural defects decreases the conjugation length of the polymer,
thus decreasing the conductivity. Possible oxidative degradation
pathways may be the epoxidation of C-C double bonds and the
oxidation of sulfur atoms to a sulfoxide or sulfone.

To determine the carrier type and understand the conducting
behavior in detail, thermoelectric power (TP) measurements were
carried out at Professor Kannewurf‘s laboratory. The variable
temperature thermoelectric power of (Pth)szOan20 with different
x values are shown in Figure 4.13. They can be separated into three
categories: (a) when the polymer/V20 5 is very low (<0.3), the
thermoelectric power (TP) is large, negative and becomes more
negative as temperature increases (see Figure 4.13e). This is a n-
type semiconductor with metal-like behavior, presumably due to the
reduced V205, which is responsible for the charge transport
properties of this material; (b) at medium polymer/V205 ratio, the
Seebeck coefficient (S) is positive at temperature below 170°C and
decreased as temperature increased, a typical p-type semiconductor
behavior. Above 170°C the S became negative and kept constant

eventually up to 350°C, as shown in Figure 4.13d. This mixed

198

 

 

 

 

 

0
__.. -1 -
.E _
_U
'9 t
E -2 l—
5 t
’5 _.
2
z '3 *-
O
o _
U 1.
o
4 -4 1—

L. AA
P- AA
-5 1 1 . 1 1 1 . 1 1 J A
50 100 150 200 250 300 350
TEMPERATURE (K)

Figure 4.12. Four-probe variable temperature electrical

conductivity data of (Pth)o,72V205nH20. (A) Fresh prepared
(B) Aged.

199

 

 

 

 

 

10L
;
0?
L x
~ x
"‘ ‘10 l" , X
E ~ ”I
3 . as
a: L X?
g -20: >><<
2 1 X
0 g x
5 -301— :3.
g l X>g<
E" t
-40 r: x
E (E)
_50rtt+41111L14141411m1144441,4,‘
50 100 150 200 250 300 350

TEMPERATURE (K)

Figure 4.13. Variable temperature thermoelectric power
data of (A) (Pth)o,72V205nH20 (B) (Pth)o.sgV 20 5111120 (C)
(Pth)o.s7V 20 511 H 20 (D) (Pth)o.4sV 20 511 H 20 (E)
(Pth)o.30V20 5n H20 .

200

semiconductor and metal behavior did not occur in
(PANI)xV205nH20, (Ppy)xV205nHzO, MXV2Os or bulk polythiophene
[20]. Therefore the charge transport properties of these materials
are the hybrid of reduced V205 host and polythiophene. The
semiconductor-metal switch temperature was also dependent on
polythiophene/V20 5 ratios. (c) the TP of higher polymer content
samples (x>0.57) showed typical p-type metallic behavior. The S was
small and positive (+3 uV/K at room temperature for
(Pth)o,72V205nH20) and decreased smoothly as temperature
decreased (see Figure 4.13a, 4.13b and 4.13e). Since polythiophene
has p-type metallic property [4d,2l], the charge transport pr0perties
of high polymer content samples must be dominated by the
polythiophene inside the V205 layers.

Interestingly, although the conductivities of aged samples did
not change dramatically, the thermopower varied and showed
unusual behavior as shown in Figure 4.14. The S of aged sample was
relatively small and positive but became less positive as the
temperature increases to 200K. The small and positive TP values
indicated p-type conductor with holes being the predominant charge
carriers. However, the slope of the variable temperature
thermoelectric power data suggested a semi-conducting behavior.
This property was also not observed in either polythiophene or V20 5.
Therefore, the charge transport properties of aged (Pth)xV205nHzO
with high polymer content are a hybrid of those of polymer and
reduced V205 but not dominated by polythiophene. This may be
because upon aging the v4+ centers are reoxidized to V51”, and at the

same time, the d0ping level of polythiophene may have decreased.

201

 

I T I I

01
l

THERMOPOWER (11V/K)

 

rT—WI—TITI—TfiIfi r'T CH I7 I I T_I

 

 

 

1 I L¥L414111 ILLIJLIJJ+IJIIIJLI

50 100 1 50 200 250 300 350

TEMPERATURE (K)

Figure 4.14. Variable temperature thermoelectric power
data of (Pth)o,72V205nI-I20. (A) Fresh prepared (B) Aged.

202

Another possibility is the degradation of the polymer with time in

air.

VI. Conclusions.

The in-situ oxidative polymerization/ intercalation of
polythiophene in layered V20 5 host can be achieved by refluxing
2,2'-bithi0phene with V20 5 xerogel in CH3CN. It is a topotactic
reaction with 5.95A interlayer spacing expansion. Both X-ray and
electron diffraction data confirmed the intactness of the layered
framework. EPR spectra of (Pth)xV205nHzO revealed only one broad
signal at g=1.9623 which comes from reduced V20 5 layers. The
typical sharp peak of polythiophene did not appear which indicated
that there is an intimate contact and strong spin-spin interaction
between polymer and host. This magnetic interaction phenomenon
is also observed in the magnetic data. (Pth)xV205nH20 is a
paramagnetic material with magnetic susceptibility followed the
Curie law. The room temperature conductivity of (Pth)xV205nH20
falls in a wide range (10'4~1000'1cm'1) and the thermopower can be
positive or negative. The carrier type and conductivity can be varied
by simply changing the polythiophene/V20 5 ratios under the same
reaction condition. These unique properties open a new avenue for
preparing electroactive materials with controllable charge transport

prOperties.

LIST OF REFERENCES

 

203

LIST OF REFERENCES

"Non-Stoichiometric Compounds, Tungsten Bronzes Vanadium
Bronzes and Related Compounds" Bevan, D. J.; Hagemuller P. Eds
Pergamon Press Oxford 1973, P 569-601.

(a) Alderbert P.; Baffler, N.; Legendre, J. J.; Livage, J. Rev. Chim.
Min. 1982,12, 485-495. (b) Lemordant, D.; Bouhaouss, A.;
Alderbert, P.; Baffier, N. J. Chim. Phys. 1986,83, 105-113. (c)
Ruitz-Hitzky, E.; Casal, B. J. Chem. Soc. Faraday Trans. 1 1986,
82, 1579-1604.

Day, P. Phil. Trans. R. Soc. Lond. A 1985, 314, 145-158.

(a) Yamamoto, T.; Sanachika, K.; Yamamoto, A. Bull Chem. Soc.
Jpn. 1983,16, 1497-1502. (b) Tourillon, G.; Garnier, F. J.
Electrochem. Soc. 1983,130, 2042-2044. (c) Tourillon, G.;
Garnier, F. J. Phys. Chem. 1983, 81, 2289-2292. (d) Kaneto, K.;
Yoshino, K. Solid State Commun. 1983, g, 389-391. (e) Hotta,
S.; Hosaka, T.; Shimotsuma, W. Synth. Met. l983,6, 317-318.
(1) Horowitz, G.; Tourillon, G.; Garnier, F. J. Electrochem. Soc.
1984,1111, 151-156. (g) Alcacer, L. Ed. Proc. Workshop, Sintra,
Portugal, July 28-31 1. "Polymers Special Applications" Reidel,
Dordrecht, 1987.

Glenis, S.; Tourillon, G.; Garnier, F. Thin Solid Films 1984, 1___2_,
9-17.

Ofer, D.; Wrighton, M. S. J Am. Chem. Soc. 1988,1112, 4467-
4468.

Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. Phys. Rev. Lett.
1984,51, 2461-2464.

10.

11.

12.

13.

14.

15.

16.

17.

18.

 

204

Tourillon, G.; Garnier, F. J. Polym. Sci. Polym. Phys. Ed. 1984,
22, 33-39.

Heinze, J.; Meinze, J.; Storzbach, M. Ber. Bunsen-Ges, Phys. Chem.
1986.20. 1043-1048.

(a) Akimoto, M.; Furukawa, Y.; Takeuchi, H.; Harada, 1.; Soma, Y.;
Soma, M. Synth. Met. 1986 15 353-360. (b) Furukawa, Y.;

’—’

Akimoto, M.; Harada, I. Synth. Met. 1987,18, 189-194.

Daniell, R.; Taliani, C.; Zamoni, R.; Giro, G. Synth. Met. 1986, 13,
325-328.

(a) Yamamoto, T.; Sanechika, K.; Yamamoto, A. J. Polym. Sci.
Polym. Lett. Ed. 1980,18, 9-12.

"Handbook of Conducting Polymers" T. A. Skotheim Ed. Marcel
Dekker Inc. New York 1986, Vol, 1 293-350.

(a) Hotta, S.; Hosaka, T.; Saga, M.; Shimotsuma, M. Synth. Met.
l984/1985,11)_, 95-99. (b) Yamamoto, T.; Sanachika, K.;
Yamamoto, A. Bull. Chem. Soc. Jpn. 1983,16, 1497-1502. (c)
Tourillon, G.; Gariner, F. J. Phys. Chem. 1983, 81, 2289-2292.

Hotta, S.; Shimotsuma, W.; Taketani, M. Synth. Met.
1984/1985,_1_Q_, 85-94.

(a) Hotz, C. 2.; Kovacic, P.; Khoury, I. A. J. Polym. Sci. Polym.
Chem. Ed. 1983 21 2617-2628. (b) Tourillon, G.; Garnier, F. J.

’—’

Electroanal. Chem. 1982, 131, 173-178.
Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1984, 161, 51-58.

(a) Kanatzidis, M. G.; Tonge, C. R.; Marks, T. J.; Marcy, H. 0.;
Kannewurf, C. R. J. Am. Chem. Soc. 1987,1112, 3797-3799. (b)
Kanatzidis, M. G.; Marcy, H. O.; McCarthy, W. J.; Kannewurf, C. R.
Marks, T. J. Solid State Ionics 1989, 32133, 594-608.

19.

20.

21.

 

 

205

Babonneau, F.; Barboux, P.; Josien, F. A.; Livage, J. J. De Chim.
Phy. 1985, 82, 761-766.

Kobayashi, M.; Chen, J.; Chung, T.-C.; Morase, F. ; Heeger, A. J.;
Wudl, F. Synth. Met. 1984,2, 77-86.

(a) Bredas, J. L.; Themans, B.; Fripiat, J. G.; Andre, J. A. Phys.
Rev. B 1984, 22, 6761-6773. (b) Kobayashi, M.; Chen, J.; Cheng,
T.-C.; Moraes, F.; Heeger, A. J.; Wudl, F. Synth. Met. 1984, 12,
77-86.

CHAPTER 5

FORMATION OF CONDUCTING POLYANILINE IN A CRYSTALLINE

LAYERED HOST ~ IN-SITU OXIDATIVE INTERCALATION OF
AN ILINE IN FeOCl

206

ABSTRACT

Polyaniline was inserted into FeOCl, to yield a black
microcrystalline product, a-(PANI)xFeOCl, a-(I). The observed 5.94A
interlayer spacing expansion, corresponds to the insertion of one
layer of polyaniline chains inside the intralamellar space of FeOCl.
The reaction rate can be accelerated by the presence of oxygen. The
Infrared spectrum of a-(I) show the characteristic peaks of
conducting polyaniline. The room temperature conductivity of
powder samples (in pressed pellet form) are 10‘3 to 10‘7- Q‘lcm'1
(single crystal samples show conductivities of ~10“1 Q‘lcm'l) with
thermally activated behavior. Variable temperature thermoelectric
power shows typical p-type semiconductor behavior. When
polyaniline was intercalated in single crystal FeOCl, a superlattice
phenomenon was observed indicating the polyaniline chains in FeOCl
layers are oriented.

Upon standing in air, two separated processes were occur: the
FeOCl framework was hydrolyzed to form B-FeOOH and the
polyaniline chains continue to oxidatively couple to longer chains.
The obtained product is a composite of polyaniline and B-FeOOH
which remain intimately mixed. This composite we called the "B"-(I)
phase. The physicochemical and charge transport properties of
decomposition products, "B"-(I) are dramatically different from those
of a-(I). The conductivity of "B"-(I) is 10 times higher and

thermopower measurements show metallic behavior which is

208

entirely due to polyaniline. The room temperature effective spin-

only magnetic moment, neff, of a-(PANI)o,2oFeOC1 is 2.8 B.M. The

observed antiferromagnetic behavior is similar to that of FeOCl.

209

I. Introduction:

Electrically conducting polymers have received a great deal of
attention due to their promising electrical properties and their
potential applications in a variety of devices [1,2]. A prototype
conductive polymer, polyaniline, known for more than a century [3],
has attracted renewed interest during the last decade [4]. Its
conducting form exhibits excellent environmental stability and can
be used in various electronic devices and sensors [5]. There has been
extensive investigation of its chemical structure by various
spectroscopy methods [5]. However, our understanding of the
structure is still fragmentary due to the amorphous or poor
crystalline nature and insolubility in common organic solvents.
Important structural problems associated with single chain and their
macromolecular aggregation remain to be solved. The systematic
control and modification of its physical, structural and electronic
properties is an active area of research [6]. Goals include the
optimization of the charge transport properties and the better
understanding of the relationship between electronic and lattice
structure. This has been approached by preparing oriented or
crystalline polymers. Progress in this area has been made recently
by MacDiarmid, Epstein and coworkers [7] who studied a
microcrystalline form of polyaniline and proposed a crystal structure,
based on X-ray powder diffraction data, similar to that of
polyethylene-sulfide. Further progress in determining structural
details is hampered by the small X-ray coherence length of

polycrystalline polyaniline (~50-150A) as larger crystallites are not

210

available. Also key structural features of acceptor-doped polyaniline
were deduced from the single crystal structure determinations of the
C104‘ and BF4' dication salts of H-(C6H4-NH)4-C5H5, radical cation
salts of shorter oligomers and neutral H5C5-N=C6H4=N-C5H5 [8].

One way to obtain orientation of the polymer chains would be
to grow them inside a structurally organized host framework. The
framework could be either three dimensional with large oriented
tunnels or two dimensional with accessible interlayer space. We
have proved that conductive polymers can be inserted in layered
V205 xerogel, see chapter 2,3&4. Recently, conductive polymers
were also reported to insert in other layered materials such as
montmorillonite [9], three dimensional matrices [10] and zeolites [ll].
FeOCl is another excellent layered system for intercalation reactions
[12]. Growing polymers in FeOCl would result in intercalated
compounds with alternating monolayers of polymer chains and FeOCl
monolayers. These compounds, if crystalline, may enable us to
further characterize the polymer structure by crystallographic and
spectroscopic methods. This work follows previous studies of in-situ
intercalative polymerization of pyrrole [13a] and thiophene [13b] in
FeOCl and complements our studies of polyaniline intercalation in the
layered V205 xerogel. The products obtained in the latter system are
turbostratic, contrary to the FeOCl-derived products which are

crystalline [14].

211

11. Experimental Section:

Reagents. FeClg, Fe203, KPF5, toluene, nitrobenzene, ethanol,
dimethoxy-ethane, acetone and HPLC grade N-methyl-Z-
pyrrolidinone (NMP) were purchased from commercial sources and
used without further purification. Aniline, methanol and acetonitrile
were dried over CaHz and distilled prior to use (aniline was distilled

under vacuum).

Physicochemical Methods.

See chapter 2.

Synthesis of FeOCl.

FeOCl was prepared by mixing 9.69 g (59.7 mmol) FeC13 with
7.20 g (45.1 mmol) Fe203 in a evacuated pyrex tube at 380°C for 40
hours. The product was washed with acetone to remove excess
FeCl3, dried in vacuum and characterized by X-ray powder
diffraction [15]. During the reaction, some large single crystals of
FeOCl were formed on the edge of the tube away from the bulk
material. They were separated carefully from the powder for single

crystal reaction studies.

Preparation of Microcrystalline a-(PANI)xFeOCl.

(a) a-(PANI)0,16Fe0Cl: 60 ml of a 5% aniline solution

(32.3 mmol) in acetonitrile was stirred with 0.50 g (4.66 mmol) FeOCl

212

in air for a week. The black shiny microcrystalline product was
isolated by filtration, washed with acetone and dried in vacuum. The
yield was quantitative.

The intercalation reaction was deemed complete when the
(0k0) peak of the starting FeOCl disappeared from the X-ray powder
diffraction pattern. Elemental analysis: Calcd. for (C6H4NH)o,15FeOC1:
C, 9.45%; H, 0.66%; N, 1.84%; Fe, 45.83%. Found: C, 8.53%; H, 1.75%; N,
1.59; Fe, 40.12%.

(b) a-(PANI)0,zoFe0Cl: 50 ml of a 4% aniline solution
(21.5 mmol) in acetonitrile was mixed with 0.34 g of (3.17 mmol)
FeOCl. The mixture was stirred in air for 8 days. The black product
was isolated in the same way as in (a). Elemental analysis: Calcd. for
(C6H4NH)o,zoFeOCl: C, 11.47%; H, 0.80%; N, 2.23%; Fe, 44.50%. Found
C,ll.04%; H, 1.59%; N, 1.89%; Fe, 42.07%. Other compounds with
different stoichiometries were prepared the same way with different
ratios of the reagents (the particle size of FeOCl also affect the
PANI/FeOC1 ratio of the products). The PANI to FeOCl ratio typically
varied between 0.16 and 0.28.

Intercalation with single crystals of FeOCl.

(a) a-(PANI)o,nge0CI: 0.30 g (2.80 mmol) of single
crystals of FeOCl were mixed with 2.00 g (21.51 mmol) of aniline in
50 ml acetonitrile. Upon standing in air at room temperature without
any disturbance for 30 days black shiny crystals of (PANI)o,23FeOCl

were obtained. This product was isolated by filtration, washed with

213

acetone and dried in vacuum. Elemental analysis: Calcd. for
(C5H4NH)0,23FeOCl: C, 15.18%; H, 1.06%; N, 2.95%; Fe, 41.99%. Found: C,
15.05%; H, 1.66%; N, 4.29%; Fe, 41.50%.

(b) a-(PANI)0,23Fe0Cl: Mixed 0.34 g (3.17 mmol) of single
crystals of FeOCl with 2.00 g (21.51 mmol) of aniline in 50 ml
acetonitrile. The mixture, upon standing in air at room temperature
without disturbance for 60 days, afforded black single crystals of
(PANI)o,23FeOCl. The isolation procedure was the same as above in
(a). Elemental analysis: Calcd. for (C5H4NH)0,23FeOC1: C, 12.91%; H,
0.90%; N, 2.51%; Fe, 43.55%. Found: C, 12.63%; H, 1.61%; N, 2.76%; Fe,
41.70%.

Isolation of polyaniline from a- and "B"-(PANI)xFeOCl

2.50 g of a- or "B"-(PANI)0,23FeOCl were stirred with 400 ml
2M HCl(aq) at room temperature for 3 days. The black solid,
polyaniline, was isolated by filtration, washed with H20 then acetone

and dried in vacuum. It was characterized by infrared spectroscopy

[16].

III. Results and Discussion:

The objective of this study was the detailed characterization of
the redox intercalation reaction of aniline with FeOCl and the
subsequent polymerization chemistry associated with this system.

We begin with a discussion of the synthetic chemistry and polymer

214

characterization, followed by magnetic and charge transport studies
of the products a-(PANI)xFeOCl. We then discuss the nature of the
so-called "B"-phase (which is the decomposition product of a-phase)

and its ultimate characterization.
A. Syntheses: The Reaction of Aniline with FeOCl.

The reaction of aniline with FeOCl can yield poorly defined
products, if not done properly and in one occasion was reported not
to occur [17]. For this reason, we investigated the reaction in detail
in order to determine the optimum synthetic conditions. Several
solvents such as toluene, nitrobenzene, acetone, ethanol, acetonitrile
and even aniline itself were used. No reaction occurred in refluxing
nitrobenzene. When the reactions were carried out in refluxing
toluene, acetone, ethanol and aniline, intercalation was observed
early in the reaction, but the products decomposed to a brown
material before the reaction went to completion. After careful
experimentation, we found acetonitrile to be the most suitable
solvent in this reaction system. Violet crystals of FeOCl react with
excess aniline in CH3CN at room temperature, in air to form black
microcrystalline products of the general formula a-(PANI)xFeOC1.
The reaction is simply represented as equation 5.1 in which the

aniline is oxidatively polymerized and FeOCl is reduced.

CliqCN

csHsNflg-l- FeOCl n.1,,1~2weoii’(CGH4NH)x“2xF°°°'----°Q(5-1)

215

The reaction is typically complete after one to two weeks depends on
aniline to FeOCl ratio and FeOCl particle size. In general, the mole
ratio of aniline to FeOCl must be larger than seven in order to drive
the reaction to completion, without losing too much crystallinity in
the product. Raising the temperature accelerates the reaction, but at
the same time can accelerate decomposition of the products. It is
interesting that reactions performed under air are much faster than
those under nitrogen, indicating that oxygen is involved in the
reaction. It most likely acts as an electron acceptor and reacts with
H+ released from the oxidative polymerization of aniline as observed
in polyaniline/V205 system. Therefore, the H+ content of the product
is much less than what represented in equation 5.1. Not surprising,
when large single crystals of FeOCl are used, the reaction is much
slower, but the resulting products contain similar amounts of PANI
as the microcrystalline samples. It is nevertheless remarkable that
single crystals as long as 0.5 mm can be intercalated completely
under the conditions used. Elemental analyses showed various
stoichiometries (0.16<x<0.30) depending on reaction conditions and
FeOCl particle size. This is consistent with the maximum theoretical
stoichiometry calculated based on the molecular sizes of FeOCl and
polyaniline, (PANI)o,32FeOCl. Regardless of stoichiometry the
products exhibited very similar properties.

Reactions were also carried out in evacuated sealed tubes using
ethanol, methanol, acetone, acetonitrile or dimethoxyethane as a
solvent at 80-110°C. Methanol was found to be the most suitable
solvent, always yielding single phase products. In the other solvents,

the products either decomposed before the reaction went to

216

completion or resulted in multiple phases. The products isolated
from sealed tube reactions were black shiny microcrystalline solids
with a slightly higher interlayer expansion than those isolated from
reactions in open systems. Furthermore, we found that the
methanothermally made materials trapped a lot of solvent inside,
making it difficult to determine the actual polymer content by
elemental analysis. These products are also slightly different
(electronically and compositionally) from those prepared in
acetonitrile, at ambient pressure, most likely differing in the degree
of reduction of the FeOCl layer and the degree of aniline
polymerization. We also believed that the FeOCl framework was
changed by covalent attachment of MeO groups as observed in
polyfuran/FeOCl system [18]. Thus these products should be
considered different compounds from those prepared at ambient
temperature and pressure. The spectroscopic studies were focused
on the products isolated from reactions in air at room temperature

using acetonitrile as a solvent.

B. Infrared Spectroscopy and X-ray Powder

Diffraction Studies.

Unequivocal evidence for the formation of conducting
polyaniline in the interlamellar space of FeOCl is derived from the
FTIR spectra, given in Figure 5.1, which show the characteristic
fingerprint of the emeraldine salt form [19]. This form is positively
charged and requires a counter anion which in this case is the

reduced FeOCl. By comparison, the counterion in bulk emeraldine

217

 

 

 

(A) u

TRANSMITTAN CE

 

 

 

 

I

1130 310

I I—

1 r
4000 2770 1950

WAVENUMBER (cm-1)
Figure 5.1. FT-IR spectra (KBr pellets) of (A) at-

(PANI),.FeOCl (B) Bulk polyaniline (emeraldine
salt) (C) Extracted polyaniline.

218

salt (prepared by aniline oxidation with (NH4)28203 in 0.1M HCl) is
Cl'. The broad rising absorption at 2000-4000 cm-1 which continues
into the near-IR is due to the continuum of overlapping electronic
transitions, originating from both the mixed valence FeOCl [20], and
emeraldine salt [21] components. The peak at around 470 cm*1 is
the vibration of FeOCl framework which did not change after
reaction. The polyaniline can be extracted from a-(PANI)xFeOCl, by
digesting the FeOCl framework with 2 M aqueous HCl solution for 20
hours. The IR spectrum of the extracted polyaniline is identical with
that of an authentic bulk polyaniline sample (see Figure 5.1). It is
interesting to note that the absorptions of intercalated PANI
generally occur at higher energies than of bulk PANI, suggesting that
the former is found in a more rigid environment and/or possesses a
lower average molecular weight than bulk PANI (vide infra).
Consistent with intercalation, the interlayer spacing of the host
lattice expands along the b axis. As the reaction proceeds, the XRD
peak intensities 0k0, hk0, OH and hkl of FeOCl decrease gradually
and new broader peaks appear at lower 29 angles. At the same time
the color of FeOCl turns black. The reaction was judged complete by
the total disappearance of FeOCl 0k0 reflections. During the reaction
we observe basal 0k0 reflections from both the product and the

pristine FeOCl consistent with an inhomogeneous intercalation

mechanism. The XRD patterns of FeOCl and a-(PANI)xFeOCl are
shown in Figure 5.2 for comparison. The interlayer spacing of a-
(PANI),;FeOCl is 13.86A, corresponding to an interlayer expansion of
5.94A, sufficient to accommodate the monolayer of polyaniline

molecules. We found that the magnitude of expansion varies slightly

219

 

 

 

 

 

 

 

E:
g (A)
H 1
E
M
V T V 1 T f j T 7‘ 1 T
o 10 20 30 40 50 60

29 (DEGREE)

Figure 5.2. Reflection-mode X-ray powder diffraction
patterns of (A) Pristine FeOCl (B) ct-(PANI)xFeOCl.

220

(around 0.6A) between single crystal and powder samples (the
interlayer spacing of single crystal is 14.46A). This may be due to
the imperfect arrangement of molecules in the layer gallery which
also can be seen from the broadness of the peaks. The magnitude of
interlayer expansion suggests the presence of a monolayer of
polymer. This is also consistent with the two-fold axis, bisecting the
C-N-C angle, being oriented roughly perpendicular to the FeOCl layers

as shown in Scheme 5.1.

 

 

 

 

I FeOCl
.10 m0 m0 W
L FeOCl I

 

Scheme 5.1. Proposed orientation of polyaniline chain in FeOCl

layers.

During intercalation, a few aniline molecules fail to polymerize
and are trapped inside the interlamellar space as anilinium (AnH+)
ions. The latter can form by reaction of aniline with released H+
during polymerization. The AnH+ are easily identified by their
characteristic vibrations in the FTIR spectra of the products (1492,
745, 687 cm'l). Subjecting the material to vacuum does not remove
the AnH+ ions. However, upon standing in air the AnH+ vibrations

disappear without decrease in the carbon to iron ratio, suggesting

221

that they polymerize or incorporate to existing polymer chains in the
interlamellar space. Oxygen must act as an oxidant and proton
acceptor. The polymerization of AnH+ ions can be accomplished by
heating the a-(PANI)xFeOCl sample at 140°C in air or oxygen for
several days, as judged by the disappearance of the anilinium peaks
from the infrared spectrum while the peaks of FeOCl and polyaniline
remain intact. This phenomenon was also observed in
polyaniline/V205, and is the first example of post-intercalative
oxidative polymerization of a monomer in a layered host induced by
molecular oxygen. This observation raises important implications of
broad applicability to other systems which contain molecular, but
oxidatively polymerizable guests, in host matrices.

In order to determine how much anilinium was present, we

attempted ion exchange with K+ according to equation 5.2.

(AnH*)x(CBH4NH)yFeOCl+nK”—>K,(C6H‘NH)yFeOCI+nAnHf..eq(5.2)

This is based on the assumption that anilinium, being the smallest
species in the interlamellar space, and thus the most mobile, would
ion exchange faster than its less mobile oligomers and polymers.
Based on nearly complete ion-exchange, the amount of K+ in the
product of eq. 5.2 would correspond to the amount of unpolymerized
AnH+ (ignore the trace amount of proton in the a-(PANI)xFeOCl).

The reaction was carried out in CH3CN with KPF6 for 20 hours.
Indeed, ion-exchange is successful as judged by disappearance of the

anilinium derived peaks in the FTIR spectrum. EDS analysis

222

confirmed the presence of 0.017 mole of potassium per mole of
FeOCl. This corresponds to ~8-10% of aniline to be in the form of
AnH+ in freshly prepared a-(PANI)xFeOCl. Interestingly, the
interlayer spacing of the K+ exchanged product is slightly smaller
than that of the starting material by 0.6A. This can be explained via
a structural model in which the anilinium ions are oriented with
their C-N bonds perpendicular to the FeOCl layers acting as pillars
while the PANI chains lie parallel to the layers, as shown in Scheme

5.2.

 

 

 

 

 

FeOCl
. NH3+ . HN‘
vl'azomommmosomo <3;
FeOCl

 

 

 

Scheme 5.2. The pillar effect of anilinium in (PANI),,FeOCl

C. Insertion of Polyaniline in FeOCl Single
Crystals and Crystallographic Studies.

The intercalation of single crystals of many materials at room
temperature is known to be very slow, it is seldom complete, and
usually it destroys the crystals. Nevertheless we still attempted the
reaction of FeOCl with aniline to see if similar difficulties existed in
this system. After several unsuccessful tries, we have been able to
insert polyaniline into single crystals of FeOCl by careful slow

reaction using a CH3CN solution of aniline in air at room temperature.

223

Surprisingly, the resulting compound, (polyaniline)o.2gFeOCl, retains
significant single crystal character, despite the fact that the overall
crystallinity has decreased and most crystals appear to have been
exfoliated. The crystals cleave perpendicular to the [010] direction,
consistent with their lamellar structure. This can be observed clearly
in the SEM photographs of (I) which are shown in Figure 5.3. The
insertion of polyaniline causes an interlayer expansion of 6.54A. The
formation of polyaniline (emeraldine salt) in the interlayer space can
be easily confirmed by FT IR spectroscopy.

Examination of crystals of (I) by X-ray diffraction revealed that
the crystal quality, though inferior to that of starting FeOCl, was
sufficiently good for preliminary X-ray diffraction experiments.
Oscillation photographs of such crystals show broad but intense
diffraction peaks. The broadest peaks are those of the ow class of
reflections. This is to be expected considering that the FeOCl crystals
have undergone a topotactic intercalation reaction. Axial X-ray
photographs from single crystals are shown in Figure 5.4.

Interestingly, the diffraction pattern of the h01 zone reveals a
set of strong reflections associated with the parent 3.30A x 3.78A
cell, and a set of several weak reflections half way between the
strong ones, suggesting that the periodicity along the a- and c- axes
have doubled relative to the original FeOCl unit cell. The new unit
cell is orthorhombic with a=6.60A, b=28.86A, c=7.56A and
V=145 9A3. The origin of this superlattice must be due to substantial

long-range order of polyaniline in FeOCl. Long range order of

224

 

Figure 5.3. SEM micrographs of single crystal
a-(PANI)o,2sFeOCl.

 

225

rotation photograph

c-axis

   

a-axis

 

b-axis (perpedicular to the layers)

Figure 5.4. Single crystal X-ray random orientation
rotation photograph (upper left) and axial oscillation
photographs along the three crystallographic axes of
a-(PANI)o,2sFeOCl. (White arrows indicated
superlattice reflections responsible for the doubling of
the unit cell in the a- and c- direction.)

 

226

polyaniline could be achieved by orientation of the polymer chains
along certain crystallographic directions, such that a doubling of the
periodicity of a- and c- axes is caused by the special arrangement of
polyaniline chains in FeOCl (vide infra).

Although the structure of polyaniline is not yet known in
detail, it has been proposed, based on X-ray diffraction studies on
powders, that it is similar overall to that of poly-phenyleneoxide

[22]. This is shown in Scheme 5.3.

2-fold screw

 

 

L +
I 10.05 A =l- 5.02 A_..-|
I repeating unit r

Scheme 5.3. Proposed structure of polyaniline.

Based on single crystal crystallographic studies of aniline
oligomers, by Baughman et al [22] the repeating unit of polyaniline is
estimated to be 10.05A (ignoring the details in individual phenyl
groups). The highest symmetry possible for the polymer chain
corresponds to a 2-fold screw axis in the chain direction. The step of
the 2-fold screw axis is 10.05/2=5.02A. If we now attempt to orient

such a chain along either the a- or the c-axis of the FeOCl layer, as

 

227

shown in Figure 5.5, we see that the repeating unit of the polymer
and the inorganic layers are not commensurate. A doubling of the a-
or c- axes cannot be achieved. A close examination of the structural
details of FeOCl reveals that the minimum Cl---Cl' distance along the
[101] direction (diagonal to the a-,c- axes) is 5.03A, almost one-half
of the repeating unit of polyaniline. By orienting parallel to the [101]
direction, polyaniline can provide each of his NH units with a Cl
"partner" thus establishing H---Cl hydrogen bonding which can act as
an additional stabilizing force. The main stabilizing force, of course,
being the coulombic attraction of the positively charged polymer to

the negatively charged FeOCl layers, as given in Scheme 5.4.

 

Scheme 5.4. Proposed hydrogen bonding between polyaniline

chains and FeOCl framework.

Figure 5.6 shows that a diagonal orientation of polyaniline produces a

system in which the positions of the NH groups are commensurate

228

MIIIIII.I...
:IIIIII“.
iIIIII.I

iIIIIIII
iIIIIIII
.III.I.III.

4“3.81.1

The ac plane of FeOCl (looking down b axis).

Figure 5.5.

229

/

'I :4III
E/ 'ff'I? fig:
u’IfiIIuI f .
/I.' 353332,. , l 2.
=1 ’2I,I!/§"I
LIQIQ III
'IIIIg’iII'I,’

it

 

~——a'=2a—-

Figure 5.6. Possible relative position of polyaniline
chains and FeOCl layers.

230

with those of the Cl atoms in the layers. This produces a new
orthorhombic unit cell with crystallographic axes a'=2a and c‘=2c. In
this fashion, the polymer chains can stack side by side lying above
every other row of C] atoms with an interchain distance of ~5A.
Because the doubling of the unit cell is caused by the light carbon
and hydrogen atoms, the superlattice reflections would be weak,
consistent with the experimental data. What the experimental data
cannOt tell us is how the polyaniline chains are oriented (i.e. along
the [101] or [-101]) in the interlayer space below and above the one

shown in Figure 5.6. Work along these lines is continuing.

D. Aerial Oxidation: a-(I) and the Nature of the
"[3" Phase.

Early on in this investigation it became evident that oxygen
plays an important role in the intercalation of aniline into FeOCl. The
first noticeable observation was that intercalation is slower under
nitrogen than under air. In fact, reactions carried out under
nitrogen, often resulted in decomposition before they went to
completion. We later determined that decomposition was due to
over-reduction of FeOCl. Based on our own observation and those
from the literature, we now recognize that the reductive capacity of
this host material is 0.1e per Fe. Reduction beyond this value results
in decomposition by virtue of structural change.

During the reaction, aniline is oxidatively polymerized while
FeOCl is reduced. An electron-balanced equation for this redox

intercalation reaction (assuming that aniline and FeOCl are the only

231

reactants) is shown for each component separately in equation 5.3

and equation 5.4. The overall reaction is shown in equation 5.5.

XCQH5NH2 -—-I>[(CGH4NH)(CGH4NH+)10.5x-P (2.5X)O.+ 2XHt...OQ(5.3)

 

Fooc| ... (2,5x)9' :» Feoc|‘2-5")' ...................... eq(5.4)

XCGH5NH2+ FeOCl —>[(CGH4NH)(CGH4NH*)]o_5xH2,FeOCI....eq(5.5)

Equation 5.5 demands that all the protons released from aniline
are retained in the product and that FeOCl accepts all the transferred
electrons. Thus, in a product with 0.20 equivalents of aniline
monomer per FeOCl the oxidation state of Fe should be ~+2.5. In
other words, 50% of the Fe3+ sites should be reduced to Fe2+. This
conclusion is contrary to two experimental facts: the quantitation of
Fe2+/Fe3+ by Mossbauer spectroscopy shows that only 10% of the Fe
atoms are in the Fe2+ state, regardless of stoichiometry; the inability
of FeOCl lattice to sustain more than ~10% of the Fe atoms in +2 state
without structural decomposition [23]. Furthermore attempts to
carry the reaction out in nitrogen resulted in decomposition of FeOCl
before the reaction could go to completion. Given that a-
(PANI)xFeOCl is crystalline and that we have no evidence for excess
H+ ions in the products, it appears that only part of the electrons are
transferred to the host while the rest end up somewhere else. We
believe the latter is molecular oxygen, which acts not only as electron

acceptor but also as proton acceptor, forming either H20 or H202.

232

Oxygen could participate in the reaction by either oxidizing
generated Fe2+, thus increasing the oxidation state of FeOCl, or by
oxidizing directly intermediate aniline oligomers which themselves
might be more reducing than aniline. We have obtained sufficient
evidence that the intercalative oxidation of aniline by V205 xerogel
also consumes ambient oxygen, suggesting a similar intercalation
mechanism.

a-(PANI)xFeOCl is a moisture and air sensitive material, upon
standing in air for several months, it gradually transforms to a
different product. This change is associated with considerable
changes in the physicochemical and electrical properties of the
material. No change is observed when the samples are stored in
evacuated sealed tubes. Here we would like to specially define the
so called a phase and "[3" phase. The material obtained upon
standing in air for several month is defined as the "[3" phase, "B"-(I)
or "B"-(PANI)XFeOCl. At this stage, the product is very weakly
crystalline or X-ray amorphous. Based on the data given below, the
phase change can be rationalized by assuming that the polyaniline
chains in the interlamellar space, couple further to form higher
molecular weight polymer. However, attempts to accelerate the
transformation by heating the sample under oxygen flow at
l40~200°C were not successful presumably due to the slow coupling
of aniline oligomers and the absence of water. At this temperatures
the structure of polyaniline changed, as evidenced by the IR spectra.
At the same time, independent of the polymer oxidation, a hydrolysis
take place at the FeOCl framework. This change is not abrupt but

slow and continuous. Certainly, there are infinite stages of oxidation

 

233

in between, as this is a continuous oxidation/hydrolysis. If the
oxidation is allowed to continue, we observe the slow but steady
appearance of diffraction peaks which correspond to crystalline B-
FeOOH as shown in Figure 5.7. For simplicity, we define the "[3"-(l)
by the complete fading of the IR peak of a—phase at 480 cm'1 which
is due to the Fe-O vibration in FeOCl framework. The infrared
spectra of a-(I) and "[3" phase of (PANI)xFeOCl are shown in Figure
5.8. Table 5.1 lists the IR vibration modes of polyaniline extracted
from a-(I) and "B"-(I), compared with extracted polyaniline and bulk
polyaniline. The term "B" phase here is used with caution being
cognizant of the fact that this material is not a pure compound (vide
infra) and lies between the a-(I) and its oxidative decomposition
product. The full characterization of both the a- and "[3" phases was
accomplished with a variety of experimental techniques, as described

below.

E. PANI Extracted from a-(I) and "B"-(I) versus
Bulk PANI.

The PANI extracted from "B"-(I) differs from bulk PANI in
several respects. First, in its base form, the sample is more soluble in
tetrahydrofuran (THF) and dimethylformamide (DMF) than the
corresponding bulk PANI. Second, the salt form has room
temperature conductivity of 0.1 Q‘lcm'1 compared to 1-5 Q'lcm'1 of
the corresponding bulk emeraldine salt. These data suggest that the
intercalated polyaniline contains significantly shorter chains than

bulk PANI and thus correspondingly smaller average MW. This is

 

234

 

f“?

INTENSITY

 

 

20 (DEGREE)

Figure 5.7. Reflection-mode X-ray powder diffraction
patterns of (A) a-(PANI)xFeOCl (B) Partially decomposed

phase (C) "B"-(PANI)xFeOCl (D) fl-FeOOH.

TRANSMITTANCE

235

 

(B)

(A)

 

 

 

 

 

 

L

 

t r v I 1 f
4000 3590 3180 2770 2360 1950 1540

WAVENUMBER (cm-1)

1130

r
720 310

Figure 5.8. FT-IR spectra (KBr pellets) of (A) a-

(PANI)xFeOCl (B) "B" -(PANI)xFeOCl.

 

Table 5.1.

236

The Major Vibration Modes of a-, "5" .

(PANI),!‘eOCl and Extracted Polyaniline Compared

with Bulk Polyaniline.

 

 

 

Mode on "B" Bulk Extracted
=Q=N 1556cm' 1557cm‘ 1569cm‘ 1563cm"
benzene ring 1478cm‘ 1479cm‘ 1478cm‘ 148Scm"
ON in QBQ‘, QBB, 1302cm' 1301cm‘ 1295crn' 129Scm"
BBQ
ON in BBB 1239cm" 1241cm" 1246cm-1 1239cm"
C-H bending 1140cm'1 1129cm" 1140cm'1 lll9cm"
C." 0“ 1,2,4. 8 79cm .1
subst. benzene
p-substituted 809cm" 822crn'1 809cm'l 802cm"
benzene ring
on on 1,2 subs. 702cm" 696cm" 700cm-l
benzene ring
FeOCl or B-FeOOH 478cm" 675cm"

 

 

Subs.: substitution,

*: B: benzene; Q: quinone

 

237

 

100

WEIGHT %
(II
9

 

 

 

 

TEMPERATURE (°C)

Figure 5.9. TGA curves (under nitrogen flow) of polyaniline
extracted form (A) a-(PANI)xFeOCl (B) "B"-(PANI)xFeOCl
(C) Bulk polyaniline. (The heating rate was 5°C/min)

238

rationalized by the slower- kinetics of polymerization of aniline in
FeOCl due to the constrained environment in which it must occur,
thus yielding smaller chains. Efforts are under way to obtain more
quantitative data regarding the MW distribution of PANI chains in a-
(I) and "B"-(I) using gel permeation chromatography (see below for
preliminary data). The thermal stability of the three PANI's was
compared by examining the TGA diagrams under oxygen, shown in
Figure 5.9. We found that the extracted PANI from a-(I) decomposes
at a lower temperature than from "[3" -(I) and bulk polyaniline,
consistent with the smaller MW of PANI in a-(I). The direct
evidence of formation of longer polyaniline chain in "[3"-(I) comes

from the gel permeation chromatography analyses.

F. Gel Permeation Chromatography (GPC) Analysis.

GPC graphs were obtained by using N-methyl-Z-pyrrolidinone
as an eluent at room temperature. The molecular weight distribution
observed in the GP chromatographies of bulk polyaniline and
polyaniline extracted from both a- and "B"-(I) (a- and B-polyaniline)
were multi-modal, similar to those of extracted PANI from
(PANI);V 20 5 (vide supra). Interestingly, the polyaniline extracted
from "B"-(I) and bulk polyaniline have similar molecular weight. As
expected, the molecular weight of polyaniline extracted from "B"-(I)

is higher than that from a-(I) as shown in Figure 5.10.

239

 

 

 

 

 

°1

UI"
H
O
H
(II

RETENTION TIME (min.)

Figure 5.10. GPC diagrams of polyaniline extracted
from (A) a-(PANI)xFeOCl (B) "B"-(PANI)xFeOCl (C) Bulk
polyaniline.

240

G. Scanning Electron Microscopy (SEM) and Selected
Area Electron Diffraction (SAED).

SEM microscopy of a-(PANI)xFeOCl reveals a clearly lamellar
nature and is consistent with a topotactic intercalation reaction in
which the host framework remains intact. We found no
morphological differences between the a- and "B" phases as shown in
Figure 5.11. The topochemical reaction can be further confirmed by
TEM SAED patterns. The SAED patterns of a-(PANI)xFeOCl with the
electron-beam perpendicular to the [Ok0] planes showed the same
(h01) diffraction patterns as that of pristine FeOCl as shown in Figure
5.12. a-(PANI)xFeOCl is a very beam sensitive material. However,
electron diffraction studies on single crystals occasionally also
showed interesting superlattice phenomena which quickly fades due
to beam damage. Further investigations in this direction are in

progress.

H. Thermogravimetric Analysis (TGA).

The thermal stabilities of a-, "B"-(PANI)xFeOCl and FeOCl are
different from one another. TGA diagrams of both a- and "B"-
(PANI)xFeOCl, under nitrogen, show two step decomposition patterns.
A slow weight loss from 50°C to 625°C and a sharp weight loss at
around 625°C are observed. In contrast, FeOCl is stable up to 450°C
before it decomposes to Fe203 and FeC13 in one step.

Unlike FeOCl, the thermal stability of a-(PANI)xFeOCl decreases

significantly when oxygen is used as a carrier gas. Under oxygen the

241

 

Figure 5.11. SEM micrographs of microcrystalline a-
and "B"-(PANI)xFeOCl.
left: a-(PANI)xFeOCl ; right: "B"-(PANI)xFeOCl

 

242

 

Figure 5.12. Selected area electron diffraction pattern
of a-(PANI)o,2oFeOCl.

 

 

243

 

 

 

 

 

 

 

V 7 r U I I t I 1 I y t I r I
(A)
100 i- -
" (B) .
b
52 q
[-
<3 ' ‘
50 '- 1
b C ‘-
v‘ WA
0 b q
1 1 1 1 l 1 1 1 1 l J 1 1 1 i 1
0 250 500 750

TEMPERATURE (°C)

Figure 5.13. TGA curves (under nitrogen flow) of (A) a-
(PANI)xFeOCl (B) "B"-(PANI)xFeOCl. (The heating rate was
5°C/min.)

244

a-(PANI)xFeOCl loses weight slowly from 50°C to 350°C while a faster
weight loss is observed from 350°C to 500°C. On the other hand, the
"B" phase shows continuous weight loss from 50°C to 500°C as shown
in Figure 5.13. Pyrolysis mass-spectroscopy of both a- and "B"-
(PAN I)xFeOCl did not show any aniline or aniline oligomers at
temperatures up to 350°C under vacuum. The weight loss at low
temperature probably is due to the absorbed moisture or solvent and

polymer backbone at high temperature.

1. Electron Paramagnetic Resonance (EPR)

Spectroscopy.

The EPR spectra of a- and "B"-(PANI)xFeOCl are very broad and
complex to be of real characterization value. We examined these
spectra in order to see whether we could observe the typical EPR
S=l/2 singlet of polyaniline (emeraldine salt). Interestingly, the
characteristic sharp EPR polaron signal did not show up in either a-
or "B"- phase as shown in Figure 5.14, presumably due to exchange
broadening with the paramagnetic FeOCl layers. In a control
experiment, finely divided mixtures of (PANI)Cl/FeOCl and
(PANI)Cl/a-(PANI)xFeOCl showed the narrow characteristic S=1/2
signal of PANI at g=2.0023. The absence of PANI signal from "B"-(I)
indicates that the polymer is very close to (<~15 A) the ferromagnetic
centers, Fe atoms. This indicates that "IV-(I) is a composite with
intimate mixed polyaniline and B-FeOOH, although it is not a single

phase.

245

 

(A)
T ppm (B)

(C)

 

 

F I fl
2900 3400 3900

MAGNETIC FIELD (Gauss)

Figure 5.14. Room temperature EPR spectra of (A) a-
(PANI)xFeOCl (B) "B"-(PANI)xFeOCl (C) Extracted
polyaniline.

246

J. Magnetic Susceptibility Studies.

FeOCl is a quasi-two-dimensional antiferromagnetic compound.
The temperature dependence of the magnetic susceptibility of
microcrystalline FeOCl has been studied extensively by several
groups [24], yet it remains ill-understood. Bizette and Adam [23]
reported a characteristic Curie-Weiss antiferromagnetic behavior,
with a large anomaly at 22 K. However, in the investigation by
Halbert [23,25] et al., the susceptibility displayed a small cusp at 15
K and a broad maximum at approximately 350 K. A recent study on
single crystals by Herber and coworkers [19], reported a slow
increase in susceptibility to a broad maximum at 310i10 K and a
discontinuity in the d(xT)/dT curve at 84i1 K. We note that to date
no fundamental understanding of the magnetic behavior of pristine
FeOCl exists which complicates further the understanding of the
magnetism of its intercalation compounds.

In the present work, the susceptibility data were recorded
from 5 K to 300 K in a constant field (5000 gauss). Variable
temperature magnetic susceptibility data of the a-(I) and "b"-(I) are
shown in Figure 5.15. The magnetic behavior of a-(I) is similar to
that of FeOCl except for a low temperature Curie tail. The similar
susceptibilities suggest that the short range magnetic interactions
observed in FeOCl are also present in a-(I). The spin-only magnetic
moment (peff) of the a-(I) is 2.8 BM, at room temperature, which is
slightly higher than that of FeOCl, but much lower than the
theoretical high-spin Fe3+ (5.9 BM) or Fe2+ (4.9 BM) value. Table 5.2
lists the room temperature ucff of (PANI)xFeOCl with various x.

247

 

 

 

 

400
O

A 300- 0 (A)
% 0°
2 O
.. 1 000
E O A A(B)
a 000 A
— 2001 0° A A
>1
5 4 A
=.‘ , A
E 65?? . .
m A
g 100 AAAA
v: A

O t ' rfi 1* r I T l ' i ' l r“

O 50 100 150 200 250 300 350

Figure 5.15.

data(l/xm) of (A) a-(PANI)o,2FeOCl

TEMPERATURE (K)

Variable temperature magnetic susceptibility
(B) "B"-(PANI)o_2FeOCl.

248

Table 5.2. Room Temperature Spin-only Magnetic
Moment (ueff) versus x of a- and "B"-(PANI),FeOC|

 

 

C°mP°WdS 1 llama. r.) (BM)
a-(PANI)0,16FeOCl 2-77
" B" -(PANI)o,16FeOC| 3 . l 9
"B"-(PANI)0,17FeOCl 3.05
a-(PANI)o,zoFeOCl 2.77
" B" -(PANI)o,2oFeOCl 3 . 2 l
a-(PANI)o,steOCl 2-37

 

 

 

 

249

The "[3" phase shows paramagnetic behavior, with the
susceptibility decreasing with increasing temperature with a slight
deviation from Curie-Weiss law, Figure 5.15b. The room
temperature ueff is equal to 3.2 BM. The long range magnetic order
is observed in a wide temperature range (5.300 K). The ucff for
both phases varies with temperature as shown in Figure 5.16. The
steady increase of ucff with temperature indicates antiferromagnetic
coupling of the magnetic spins located in polyaniline and in FeOCl. In
general the "[3" phase has higher magnetic moment than a-(I). The
magnetic behavior of a-(I) is similar to FeOCl (antiferromagnetic)
while the behavior of the "[3" phase is closer to polyaniline
(paramagnetic). It is known that protonated polyaniline is a
paramagnetic material due to the proton induced spin generation
(polarons) and its susceptibility depends on the degree of
protonation [24] with highest Pauli susceptibility (xPaUIi) equal to
100 x 10'6 (emu/mole 2 rings). This xPaUIi is relatively small
compare to the susceptibility of "B"-(I). Therefore, the magnetic

moments of both a- and "B"-(PANI)FeOCl mostly comes from FeOCl.
K. Charge Transport Properties.
(3). Electrical Conductivity Studies.

a-(PANI)xFeOCl: Samples of a-(I) were studied in a pressed
pellet and single crystal form by ac and dc electrical conductivity

measurements using the four-probe geometry. Room temperature

conductivities for pressed pellets and single crystals for several (1-

250

 

4
3 A AAAA(B)
‘1 AA
5 A...“ 000(A)
E ‘ AAA 000
g AA6“ 000
52“ D000
.— A
E: d
I: g 00
ED
film
0 Ir'jrlfirfiT'jT

 

 

 

0 50 100 150 200 250 300 350
TEMPERATURE (K)

Figure 5.16. Variable temperature effective spin-only
magnetic moment of (A) a-(PANI)o_2oFeOCl (B) "B"-
(PANI)o,15FeOCl.

251

and "B"- (PANI)xFeOCl samples are given in Table 5.3. We found, by
X-ray diffraction, that in pressed pellets significant orientation
effects are present which cause the crystallites to orient with their
crystallographic ac-planes perpendicular to the pressing axis. Thus,
the measurements were carried out parallel to the ac-plane.
Interestingly, the observed conductivities of the pressed pellets were
not dramatically lower than those obtained from single crystal
samples. Figure 5.17 shows the temperature dependence of the
electrical conductivity of a-(I) in pressed pellet and single crystal
form. The latter is only 10 times greater, 7.5x10'2 (I'lcm'1 at room
temperature, than the polycrystalline value even though at lower
temperatures this difference becomes greater. A significant feature
of the single crystal data is the thermally activated temperature
dependence of the conductivity, suggesting that the material is a
semiconductor. The powder-like conductivity behavior of a—(I) may
be due to several factors, including domain boundaries in the crystals
due to fracture upon intercalation and most likely the finite length of
polyaniline chains in the interlamellar space which may generate
large activation barriers for carrier hopping from chain end to chain
end. Overall, the electrical conductivity of a-(I) is lower than that of
bulk polyaniline by approximately two orders of magnitude. This
disparity probably reflects the differences in morphology between
the two samples (films of polyaniline are smoother and more
continuous) and the higher carrier density per unit volume in bulk
polyaniline compared to a-(I) which is only ~15% polyaniline by
weight. Furthermore, the molecular weight of the polyaniline chains

in a-(I) is expected to be lower and thus more carrier hopping

252

Table 5.3. Room Temperature Electrical Conductivity (o) of
a- and "B"-(PANI)xFeOCI.

 

 

 

Samples Conductivity (Q'lcm-l)18ample form
a-(PANI)o,zoFeOCl 10"3~10"2 powder
a-(PANI)0.23FeOCl 10'3“»10'2 powder
a-(PANI)o,3zFeOCl 10'3~10-'2 powder
a-(PANI)0,24FeOCl 10"2~10'l single

crystal
oc-(PANI)0,23FeOCl 10'2~l(l'l single
crystal
" B"-(PANI)o,15FeOCl l 0'2~1 0' 1 Powder
"B"-(PANI)0.17FeOC| 10-2~10-1 powder
"B"-(PANI)0,2oFeOCl l 0 ' 1 powder

 

 

 

253

 

 

 

 

 

 

 

_1 F- (B) 4—-—r

-2 ,__.
.5 '3 r
E '4 r—
5 ‘5 *— .
E3 -5 +- .
2 o
z -7 F- n
O 0
U -8 L.- o
t:
o o
1-3 -_9 +—— on

O A
’10 r 0 “A
-11 L_..
ll_1_Ll1_L_111_LJLL1_1LL1_LILLLIJL114411
0 4o 30 120 150 200 240 280 320
TEMPERATURE (K)
Figure 5.17. Four-probe variable temperature electrical

conductivity data of a-(PANI)o,23FeOCI in (A) Pressed pellet
(B) Single crystal form.

254

barriers are present resulting in lower conductivity. Another reason
for the lower conductivity may be the lower intrinsic conductivity of
FeOCl layers themselves which in single crystal intercalated form
exhibit only ~10'3 Q'lcm'1 at room temperature. The reduced FeOCl
layers could act as barriers for electron hopping between polyaniline
chains. a-(I) has lower conductivity than (Ppy)o,34FeOCl [12] and
(Pth)o,24FeOCl [13] by an order of magnitude. This parallels the trend
observed in the corresponding bulk samples which show that
polyaniline is always less conductive than polypyrrole and
polythiophene. However, a-(I) is significantly more conductive than
other FeOCl intercalation compounds containing molecular guests as
listed in Table 5.4. In considering the electrical properties of a-(I),
the conductivity of the reduced FeOCl framework must be
considered. A good representation of the ability of reduced FeOCl to
transport electrical charge comes from the electrical properties of
molecular intercalates of this host. For example, samples of
(pyridine)o.33FeOCl [26], (TTF)0,125FeOCl [26] and
(ferrocenium)o,5FeOCl [27] display maximum conductivities at room
temperature of 10'3 Q'lcm'l. The electrical conductivity of FeOCl is
due to the small but finite mobility of 3d electrons in Fe2+ and Fe3+
in FeOCl slabs [28]. The pristine FeOCl (all Fe“) is an insulator. Since
FeOCl layers alternate with PANI in a-(I) the overall conductivity
will be limited by the ability of the carriers to transport in a
direction perpendicular to the layers. This ability is higher parallel
to the layers, which makes a-(I) a very anisotropic material. The
true conductivity of the confined PANI chains is difficult to assess

but it is expected to be much higher than the observed values.

255

Table 5.4. The Interlayer Spacing (d) and Room
Temperature Electrical Conductivity (o) of Several

Intercalation Compounds of FeOCl.

 

 

 

Compounds d (A) G (fl‘lcm’l) Reference
(Tl'F)o,1FeOCl 13.63 3.5X10’3 2 6
(TIT)0,14FCOC1 15.22 2.2X10'4 2 6
(TTN)0.5FCOC1 15.31 5.5X10‘4 26

(TM'ITF)0,03FCOCI 1 1 . 32 1.6X10' 2 3 l
(Ppy)o.34Fcoc1 13.21 1.5x10- 1 1 3
(Pth)o,28FeOCl 13.3 1 5.0x100 1 4

(Ferrocene)o,17FeOCl 14.92 1.0X10‘4 2 7
(Perylene)o,11FeOCl 1 6.69 2.3X10'2 3 2
(TSF)0.12FeOCl 13.52 5.0X10'3 3 3

(TMTSF)0,1oFeOCl 13.82 5.4X10'3 3 3

(ET)0.25FCOC1 22.18 2.6X10‘3 3 4
(PANI)o,2o-o,23FeOC1 l 3 . 92 10'3~10' 1 this work

 

 

 

256

Attempts to extract the PANI chains from single crystal a-(I)
and still keep them oriented failed, giving rise to an amorphous
polymer. The room temperature conductivity of extracted polymer
is 8.5x10'1Q’1cm'1. The conductivity of polyaniline extracted from
both a- and "B"-(I) is lower than that extracted from single crystal at-
(I), imply that the latter may contain longer conjugated segments.
More work is needed to confirmed this assertion. It would be
interesting to know whether the electrical properties of these
materials originate from either of the individual components (is
FeOCl, PANI) or are the result of a combined, cooperative effect.
Thermoelectric power measurements may provide a better insight
into this question (vide infra).

"B"-(PANI)XFeOCl: The conductivity of ”B"-(I) is ten times higher
than that of a-(I) as shown in Figure 5.18. This suggests the
presence of longer chains of polyaniline in the oxidized product. The
IR and GPC data support this hypothesis (vide supra). Direct
evidence comes from the comparison of the conductivity of
polyaniline extracted from a-(I) and "[3" phase as shown in Figure
5.19. The polymer extracted from "[3" phase is 100 times more
conductive than that from the corresponding a-phase, consistent

with longer chain lengths of polymer in "[3" phase.
(b).Thermoelectric Power (Seebeck coefficient) Studies.
The conductivity measurements alone can not unequivocally

characterize the electrical behavior of a-(I). A complementary probe

to address this issue is thermoelectric power (TP) measurements as a

257

 

 

 

 

 

0
-2F—
'35 :
Hg -4:
L‘ ~ 0
5 '6;- O
E L o
D p 0
g BF 0 O
8 ‘ g o
(D » O
O -
A ‘10r— Cl C
P D
- D
-12L 1 L 1 1 . 1 1 . '1 1:;
O 20 40 60 80 100
1000/T (K-l)
Figure 5.18. Four-probe variable temperature

electrical conductivity data of pressed pellets of (A) a-
(PANI)o,17FeOCl (B) "B"-(PANI)0.17FeOCl.

258

 

 

 

 

 

 

0
. l
-2 +—
2- -4 e
"9 >_
g L.
-5 L_
E _
8 L.
t. A
8 _ 0C9
'4 v-% $0 0
-12 @591 J l 1 4 l 4_ 1 Am 1 l
0 50 100 150 200 250 300

TEMPERATURE (K)

Figure 5.19. Four-probe variable temperature electrical
conductivity data of pressed pellets of polyaniline
extracted from (A) a-(PANI)0,17FeOCl (B) "B"-
(PANI)o,17Fe0Cl.

259

function of temperature. TP measurements are typically far less
susceptible to artifacts arising from the resistive domain boundaries
in the material because they are essentially zero-current
measurements. The reason for this is because temperature drops
across such boundaries are much less significant than voltage drops.
Figure 5.20. shows typical TP data of a-(I) and "[3" phase as a
function of temperature. Accurate measurements at low
temperatures were hindered by the very large sample resistivities.
The room temperature TP of a-(I) is slightly positive (close to zero)
and increases steadily with decreasing temperature, reaching the
value of 100~400 uV/K at 150 K. The single crystal of a-
(PANI)xFeOCl shows the same behavior. This behavior is
characteristic of a p-type semiconductor, similar to that observed for
(ferrocenium)o,5oFeOCl [27], but different from (Ppy)o,34FeOCl [12]
and (Pth)o,24FeOCl [13] which show p-type metallic character. Since
the emeraldine salt of polyaniline shows metal-like TP behavior [32],
we can conclude that in a-(PANI)xFeOCl, the reduced FeOCl
framework dominates the charge transport properties. This can be
explained by the formation of short polymer chains of polyaniline in
the interlayer space which obstructs the chain to chain charge
transport by introducing too many hopping barriers thus making
charge transport through FeOCl layers more favorable.

Interestingly, the TP behavior of the "[5" phase is dramatically
different. The Seebeck coefficient is negative in this case indicating a
conductivity change from p-type to n-type with typical value of
around O~2 uV/K , similar to the conducting form of polyaniline

[29,30]. Unfortunately, there is little information available on the

260

 

100 ~—
90 -
80 *- .. (A)
70 r— A

50 — “8

50 r— 06:.

40r- a
30— “E3

THERMOPOWER (uV/K)

(B)

0 _ wmmmmm—

-10

 

 

+——
i l J 1 gL L4 1 l 1 #1 1_l 1 #LJ41

100 150 200 250 300

 

TEMPERATURE (K)

Figure 5.20. Variable temperature thermoelectric power
data of (A) a-(PANI)o,17FeOCl (B) "B"-(PANI)o,17FeOCl.

261

electrical properties of the yellow colored B-FeOOH. However, we
expect the material to be an insulator. This is in agreement with our
other experimental evidence that "[3" phase is actually a mixture of

PANI and B-FeOOH.

VI. Conclusions:

Conducting polyaniline (emeraldine salt) was formed inside
FeOCl by an in-situ oxidative polymerization/ intercalation reaction
between aniline and FeOCl. The rate of reaction under air is faster
than under nitrogen because molecular oxygen acts as an electron
and proton acceptor. The resulting crystalline compound. a-
(PANI)xFeOCl, consists of alternating monolayers of conducting
polymer and inorganic framework FeOCl. The room temperature
conductivity of a-(PANI)xFeOCl is 10'29'1cm'1 with typical p-type
semiconductor behavior. The polyaniline chain-length inside the
FeOCl layer continues to grow with time in the presence of ambient
oxygen and moisture. At the same time hydrolysis of the FeOCl
framework occurs to yield [S-FeOOH. The conductivity of this
intimate mixture is 10 times higher than a-(PANI)xFeOCl and the
thermopower is similar to the conducting polyaniline.

When single crystal FeOCl was used, polyaniline can be
intercalated inside FeOCl without losing the crystallinity of the
material. X-ray and electron diffraction studies show a superlattice
formation in the product, confirming the regular arrangement of

polyaniline chains in the FeOCl layers. Using crystalline host,

262

oriented conducting polymers can be obtained by in-situ oxidative

polymerization] intercalation reaction.

 

LIST OF REFERENCES

10.

263

LIST OF REFERENCES

(a) Ito, T.; Shirakawa, H.; Ikeda, S. J. Polym. Chem. Ed. 1974, 1;,
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Kanatzidis, M. G. Chemical&Engineering News "Special Report"
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Letherby, H. J. Chem. Soc. 1862,11, 161-163.

(a) MacDiarmid, A. G.; Epstein, A. J. Faraday Disseuss Chem. Soc.
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310-315.

Jozefowicz, M. E.; Epstein, A. J.; Pouget, J. P.; Masters, J. G.; Sun, A.
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Baughman, R. H.; Wolf, J. F.; Eckhardt, H.; Shacklette, L. W. Synth.
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Inoue, H.; Yoneyama, H. J. Electroanal. Chem. 1987,2321, 291-
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Brandt, P.; Fisher, R. D.; Martinez, E. S.; Calleja, R. D. Angew.
Chem. Int. Ed. Engl. 1989, 28.. 1265-1266.

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15.

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18.

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264

(a) Enzel, P.; Bein, T. J. Phys. Chem. 1989,13, 6270-6272. (b)
Bein, T.; Enzel, P. Synth. Met. 1989, 22, E163-E168. (c) Enzel, P.;
Bein, T. J. Chem. Soc. Chem. Commun. 1989, 1326-1327. (d)
Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. J. Am. Chem. Soc.
1991.113. 600-610.

Lind, M. D. Acta. Cryst. 1970, BZQ, 1058-1062.

(a) Kanatzidis, M. G.; Tonge, C. R.; Marks, T. J.; Marcy, H. 0.;
Kannewurf, C. R. J. Am. Chem. Soc. 1987,1512, 3797-3799. (b)
Kanatzidis, M. G.; Marcy, H. O.; McCarthy, W. J.; Kannewurf, C. R.;
Marks, T. J. Solid State Ionics 1989, 32133, 594-608.

Kanatzidis, M. G.; Wu, C. G.; Marcy, H. O.; DeGroot, D. C.;
Kannewurf, C. R. Adv. Mater. 1990, 2, 364-366.

Kauzlarich, S. M.; Teo, B. K.; Averill, B. A. Inorg. Chem. 1986, 21,
1209-1215.

Cao, Y.; Li, S.; Xue, Z.; Guo, D. Synth. Met. 1986, 1_6_, 305-315.

Macda, Y.; Yamashita, M.; Ohshio, H.; Tsutsumi, N.; Takashima, Y.
Bull. Chem. Soc. Jpn. l982,_5_5_, 3138-3143.

See chapter 6.

(a) Wheland, R. C.; Gillson, I. L. J. Am. Chem. Soc. 1976, 28, 3916-
3925. (b) Palvadeau, P.; Coic, L.; Rouxel, J. Mater. Res. Bull. 1981,

16, 1055-1065.

Bizette, H.; Adam, A. C. R. Acad. Sci. Paris Ser. B 1972, 213, 911-
913.

Halbert, T. R.; Johnston, D.C.; McCandish, L. B.; Thompson. L. J.;
Scanion, J. C.; Dumesic, J. A. Physica B, 1980, 22, 128-132.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

265

McCall, R. P.; Ginder, J. M.; Leng, J. M.; Coplin, K. A.; Ye, H. J.;
Epstein, A. J.; Asturias, G. B.; Manohar, S. K.; Masters, J. C.;
Scherr, E. M.; Sun, Y.; MacDiarmid, A. G. Synth. Met. 1991,41;
43, 1329-1332

Phillips, J. B.; Herber, R. H. Inorg. Chem. 1986, 23, 3081-3088.

(a) Epstein, A. J.; Ginder, J. M.; Zuo, F.; Bigelow, R. W.; Woo, H. S.;
Tanner, D. B.; Richter, A. F.; Huang, W. S.; MacDiarmid, A. G. Synth.
Met. 1987, L3, 303-309. (b) Ginder, J. M.; Richter, A. F.;
MacDiarmid, A. G.; Epstein, A. J. Solid State Commun. 1987, 63,
97-101.

Halbert, T. R.; Johnston, D. C.; McCaxdish, L. B.; Thompson, L. J.;
Scanion, J. C.; Dumesic, J. A. Physica B, 1980, 22, 128-132.

Averill, B. A.; Zauzlarich, S. M.; Antonio, M. R. J. De Phys. 1983,
C3-1373-C3-1376.

Villeneuve, G.; Dordor, P.; Palvadeau, P.; Venien, J. P. Mater. Res.
Bull. 1982,11, 1407-1412.

Malhotra, B. D.; Kumar, N.; Chandra, S. Prog. Polym. Sci. 1986, 12,
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(a) Park, Y. W.; Lee, Y. S.; Park, C. Solid State Commun. 1987, 63,
1063-1066. (b) Zuo, F; Angelopoulos, M.; MacDiarmid, A. G.;
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Kauzlarich, S. M.; Ellena, J. F.; Ttupik, P. D.; Reiff, W. M.; Averill, B.
A. J. Am. Chem. Soc. 1987,102, 4561-4570.

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266

Bringley, J. F.; Averill, B. A. J. Chem. Soc. Chem. Comm. 1981,
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Bringley, J. F.; Averill, B. A. Mol. Cryst. Liq. Cryst. 1988, 1:29,
215-222.

Bringley, J. F.; Fabre, J. M.; Averill, B. A. J. Am. Chem. Soc. 1990.
L11, 4577-4579.

CHAPTER 6

A NEW ROUTE TO CHEMICAL POLYMERIZATION OF POLYFURAN
~ INTERCALATION OF TERFURAN IN F eOCl

267

268

ABSTRACT

Intercalation of polyfuran in FeOCl can be achieved by i n-situ
oxidative polymerization] intercalation of terfuran and quaterfuran.
Two products were isolated using different preparation procedures.
In one, the organic monomer is refluxed with FeOCl in CH3CN or
MeOH, to produce r-(PFu)xFeOC1. In the other, the reaction is carried
out methanothermally at 100°C to produce m-(PFu)xFeOCly(OMe)1-y.
The interlayer spacing of the two different products are 13.60A and
15.62A respectively. Although the two furan polymers formed
inside both products are identical, the Cl atoms in FeOCl of the
methanothermally prepared phase were partially replaced by
methoxy groups. The physicochemical properties of the two products
are very similar, except for their charge transport properties. The

room temperature conductivity of r-(PFu)xFeOCl is in the range of
10‘3 to 100 Q'lcm'l while m-(PFu)xFeOCly(OMe)1-y is in the range of
10'4 to 10‘2 Q’lcm‘l. The conductivity of both materials is
independent of the polymer content and it is thermally activated.
Variable temperature thermoelectric power of both phases reveal p-
type semiconducting behavior. It is likely that in these materials the
charge transport properties are dominated by the reduced FeOCl
framework. Polyfuran extracted from (PFu)xFeOCl has smaller
magnetic moment than bulk polyfuran with the same counterion, Cl',

and it is essentially insulating.

269

I. Introduction:

The redox intercalation of conducting polymers, such as
polypyrrole, polythiophene and polyaniline in layered hosts has been
extensively studied. Our general goal is: to intercalate conducting
polymers into structurally restricted hosts to obtain oriented
polymers. These polymer derived composites may be characterized
by crystallographic methods, as well as investigated a new class of
materials with anisotropic electronic properties. To expand this
interesting field of research, we investigated another conducting
polymer, polyfuran [1]. In the literature, oligomers up to hexafuran
have been known [2]. They are prepared, in low yields, from the
couping of shorter chain oligomers or monomers and involve
complicated organic synthesis [2]. There are no detailed studies of
the structure and physicochemical properties of this polymer.

Similar to polypyrrole and polythiophene, polyfuran synthesis
has been attempted by chemical [3] or electrochemical [1,4]
polymerization of furan and its derivatives. However, due to the
lower aromaticity of furan compared to pyrrole and thiophene, the
conjugation of furan was easily destroyed during or after
polymerization [4,5]. The resulting insulating polymer sometimes
showed brown color with several structural defects, such as
hydrogenation and ring opening. As a result studies on polyfuran
have not been aggressively pursued. However, if oligomers of furan,
such as bifuran, terfuran and tetrafuran, are used as starting
materials, polymer with less structural defects may be obtained, due

to two reasons: first, the oligomers have longer effective conjugation

270

length; second, their lower oxidation potentials compared to furan
itself offer a relatively mild polymerization condition which may
reduce the formation of structural defects in the polymer. Recently,
we established a new route of synthesizing polyfuran by in-situ
oxidative polymerization] intercalation of terfuran and tetrafuran in
FeOCl interlamellar space [6].

The redox intercalation of FeOCl with various organic molecules
is well known [7] during which the organic species are oxidized and
the FeOCl framework is reduced. The lower oxidative potential of
terfuran and tetrafuran enable this redox intercalation reaction. This
chapter describes in detail the redox intercalation of terfuran in
FeOCl and the physicochemical properties of the resulting products.
At the same time, due to the lack of available information in the
literature for comparison, we also prepared bulk polyfuran by
chemically ploymerizing terfuran with FeClg in nitrogen atmosphere
using acetonitrile as a solvent at room temperature (the dopant ion
in this material is C1' or FeCl4'). Some physicochemical data of bulk
polyfuran are used to compare with polyfuran extracted from

(PFu)xFeOCl.

11. Experimental Section:

Reagents: FeC13, Fe203, iso-propanol (i-PrOH), iso-butanol (i-
BuOH) and 100% ethanol (EtOH) were purchased from commercial
sources and were used without further purification. CH3CN, MeOH
and dimethylformamide (DMF) were dried over CaH2 and distilled

prior to use. Terfuran and quaterfuran were provided by M. Benz

271

and Professor LeGoff in the Department of Chemistry at Michigan

State University.

Physicochemical Methods.
See chapter 2.

Synthesis of FeOCl.
See page 211.

Synthesis of Bulk Polyfuran, (PFu)xFeCl4.

In a nitrogen glove box, 0.2 g (1.0 mmol) of terfuran was
dissolved in 30 ml CH3CN. To this solution was added 30 ml 3.33 wt%
of FeCl3 acetonitrile solution. The mixture was stirred at room
temperature for 9 hours. The black solid was isolated by filtration,
washed with CH3CN then ether and dried in vacuum. It was
characterized by FTIR spectroscopy [8]. Elemental analysis: Calcd for
(C4H20)(FeCl4)o,063: C, 61.19%; H, 2.55%; Fe, 4.48%; Cl, 11.39%. Found:
C, 58.54%; H, 3.14%; Fe, 4.32%; Cl, 11.36%.

Synthesis of Bulk Polyfuran, (PFu),;Cl.

Under nitrogen, 0.3 g (1.5 mmol) of terfuran was dissolved in
50 ml CH3CN. To this solution was then added 1.5 g (9.2 mmol) of
FeCl3. The mixture was stirred at room temperature for 9 hours.
The black solid was isolated by filtration in air, washed with acetone

and dried in vacuum. It was identified by FTIR spectroscopy.

272

Elemental analysis, Calcd for (C4H20)Clo,23: C, 64.73%; H, 2.70%; CI,
11.0%. Found: C, 49.49%; H, 3.22%; Cl, 8.59%; Fe, 1.52%. The trace
amount of Fe could be the impurity comes from the oxidant, FeCl3.
Slightly changing the preparation procedure, polymers with different

counterion were obtained.

Synthesis of Microcrystalline r-(PFu)o,1sFeOCl.

0.20 g (1.0 mmol) of terfuran was dissolved in 50 ml MeOH,
and then 0.30 g (2.8 mmol) of FeOCl was added. The mixture was
refluxed in air for 6 days. The black microcrystalline product was
isolated by filtration, washed with acetone and dried in vacuum.
Elemental analysis, Calcd for (C4H20)o,13FeOCl: C, 7.35%; H, 0.27%; Fe,
46.86%; Cl, 29.74%. Found: C, 7.45%; H, 0.98%; Fe, 46.38%; Cl, 29.44%.

Synthesis of r-(PFu)o,2sFeOCl Using FeOCl Single
Crystals.

0.24 g (1.2 mmol) of terfuran was dissolved in a mixed solvent
of 60 ml CH3CN] 50 ml MeOH. To this solution, was added 0.28 g
(2.61 mmol) of FeOCl single crystals. The mixture was allowed to
stand in air at room temperature for 3 months. The shining black
crystals were isolated by filtration, washed with acetone and dried in
vacuum. Elemental analysis, Calcd for (C4H20)o.23FeOCl: C, 10.69%; H,
0.44%; Fe, 44.40%; Cl, 28.18%. Found: C, 10.49%; H, 1.27%; Fe, 44.11%;
C1, 28.0%.

 

273

Synthesis of m-(PFu)o,47FeOC10,61(OMe)o,39.

In a pyrex tube 0.30 g (2.8 mmol) of FeOCl and 0.40 g (2.0
mmol) of terfuran were mixed with 2.0 m1 MeOH. The mixture was
frozen with liquid nitrogen and it was flame-sealed under vacuum.
The tube was sonicated for 4 minute and heated at 100°C for 6 days.
The black shiny solid was isolated by filtration in air, washed with
acetone and dried in vacuum. Elemental analysis: Calcd for
(C4H20)0,47FeOClo,51(OMe)o,39: C, 19.94%; H, 1.54%; Fe, 40.89%; Cl,
15.83%. Found: C, 20.02%; H, 1.71%; Fe, 37.60%; C1, 14.53%. By
varying the reagents ratios, under the same reaction conditions,
different PFu/FeOCl stoichiometries can be obtained (the particle size
of FeOCl sometimes also affect the PFulFeOCl ratio of the product).

Following is one example.

Synthesis of m-(PFu)o,61FeOClo,49(OMe)o,51.

In a pyrex tube 0.30 g (2.8 mmol) of FeOCl and 0.30 g (1.5
mmol) of terfuran were mixed with 1.5 ml MeOH. The mixture was
frozen with liquid nitrogen and it was flame-sealed under vacuum.
The tube was heated at 100°C for 6 days. The black shiny solid was
isolated by filtration in air, washed with acetone and dried in
vacuum Elemental analysis, Calcd for (C4H20)o,61FeOClo,49(OMe)o,51:
C, 24.36%; H, 1.89%; Fe, 38.44%; Cl, 12.44%. Found: C, 24.42%; H,
2.00%; Fe, 35.95%; Cl, 11.12%. Similar products can be obtained by

using tetrafuran instead of terfuran under the same reaction

274

conditions. However, for the same mole ratio of reagents, the

tetrafuran intercalated FeOCl needs longer time to go completion.

Extracting Polyfuran from r-(PFu)xFeOCl and m-
(PFu)xFeOCly(OME)l.yo

0.3 g of r-(PFu)xFe0Cl (or m-(PFu)xFeOCly(OMe)1-y) was stirred
with 100 ml 2M HCl(aq), at room temperature for 24 hours. The
black residue was isolated by filtration, washed with water then
acetone and dried in vacuum. It was characterized by FTIR
spectroscopy. Elemental analysis, Calcd for (C4H20)Clo,19: C, 65.99%;
H, 2.75%; Cl, 9.26%. Found: C, 63.3%; H, 2.88%; Fe, 0.68%; Cl, 8.72%.
The trace amount of Fe was also detected as impurity from

decomposed FeOCl framework.
111. Results and Discussion:

A. Preparation and Characterization of Polyfuran.

The ideal oxidative polymerization of polyfuran is represented

in equation 6.1, the furan rings are intact after polymerization.

+8

0 'X+e' O/ O a
UM... \/ O\/ Ox ...... q(5,1)

275

However, in the literature, the chemically polymerized polyfuran is
not well characterized species [9], and it is thought to contain
fragments such as -CH=CH-O-, ~CH==(CH3)-O-, ~C=O ...etc. In one case
the furan ring was claimed to be totally absent [10]. The five-
membered hetero-aromatic ring is .relatively reactive due to its high
oxidation potential and lower aromaticity. Taking advantage of the
lower oxidation potential of terfuran and quaterfuran, we are able to
prepare polyfuran with preservation of the furan ring. We prepared
polyfuran by chemical polymerization of terfuran using FeCl3 as an

oxidant in nitrogen atmosphere as shown in equation 6.2.

0 O O o _
\ / O\ \ / 736%. \ / /o\ \ / Fec'“)"---9¢l(¢5-2)

The resulting black powder preserves the structure of furan ring as

evidenced in FTIR spectra (vide infra).
B. Intercalation of Polyfuran in FeOCl.

The insertion of polyfuran in FeOCl, the monomer furan and
several linear oligomers of furan, such as bifuran, terfuran and
tetrafuran were used. As expected, furan itself can not be
intercalated in FeOCl within the experimental conditions because of
its high oxidation potential. When bifuran was used, under the
methanothermal reaction at 80°C for 10 days or 100°C for 7 days, the

resulting brown solids showed an interlayer spacing of 9.6A. The

276

FTIR spectra of this compound showed several vibration energies
between 1600 cm'1 and 400 cm‘1 which are different from bifuran
and (PFu)xFeCl4 indicating significant structure degradation of the
polymer backbone. Therefore, no further characterization was
pursued for these brown products.

Both terfuran and tetrafuran were able to be intercalated in
FeOCl by either reflux in CH3CN or methanol or methanothermal
synthesis at 80-100°C. When reactions were carried out in refluxing
CH3CN and methanol, the complete intercalation products, r-
(PFu)xFeOCl can be obtained, although sometimes parallel
decomposition of the FeOCl framework was observed. Other alcohols,
such as ethanol, i-PrOH and i-BuOH were also used as solvents, but no
intercalation reaction was observed after refluxing FeOCl with
terfuran for 5 days. In refluxing ethanol intercalation occurred in
the beginning but the material decomposed before reaction went to
completion. The reactions carried out in air are faster than those in
nitrogen as observed in other conducting polymer intercalated V205
xerogel and FeOCl systems. The role of oxygen in this reaction was
not investigated.

In general, during methanothermal synthesis, the terfuran (or
quaterfuran)]FeOCl ratio should be larger than 0.25 in order to drive
the reaction to completion. The reaction time is affected by the
temperature, for example, at 80°C it takes 3 weeks for the reaction to
go completion, while at 100°C it needs less than 1 week. However,
the polymer content is independent of terfuran/FeOCl ratio and
reaction temperature. The reaction rate of terfuran and tetrafuran in

FeOCl is slower than that of pyrrole and 2,2'-bithiophene [11]

277

presumably due to the larger size and lower solubility of the furan
oligomers in CH3CN and MeOH. Similarly, the reaction rate of
quaterfuran is slower than that of terfuran at the same reaction
conditions. Nevertheless, regardless of using terfuran or quaterfuran
as monomer, the resulting products have similar physicochemical
properties. Some solvents such as CH3CN, EtOH and DMF have also
been used for the solvothermal reaction. Unfortunately, all afforded

decomposition products (FeOCl decomposed to Fe203)

C. Infrared Spectroscopy and X-ray Diffraction Studies.

The evidence of forming polyfuran comes from the fourier
transform infrared spectra. The r-(PFu)xFeOCl and (PFu)xFeCl4 have
comparable vibration peaks between 500 cm'1 and 1600 cm'1 as
shows in Figure 6.1. Both spectra show some peaks between 1600
cm'1 and 1800 cm'1 which is due to the defects, such as ring opening,
of the polymer backbones. Mostly probably they represent the

formation of carbonyl groups as shown in equation 6.3.

Q "20 7* fl ............ eq (6.3)

00

 

However, these defects are present in small concentrations than that
in (PFu)xFeC14, as judged by relative intensities of the peaks in 1600-
1800 cm'1 region of the IR spectra. The polyfuran in (PFu)xFeOCl can
be extracted by decomposing the FeOCl framework with HCl aqueous

278

 

 

 

(A) U

TRANSMITTANCE

 

 

 

 

‘Trjt—TTIjfiVV—YYV

4000 3000 2000 1000 400

WAVENUMBER (cm-1)

Figure 6.1. FT-IR spectra (KBr pellets) of (A) r-(PFu)xFeOCl
(B) (PFu)xFeCl4 (C) Extracted polyfuran.

279

solution. The IR spectrum of extracted polyfuran shows more serious
structural disruption (i.e. carbonyl formation) compared to that in
FeOCl layers, although it has similar vibration energies to those of
(PFu)xFeCl4 (see Figure 6.1).

The FTIR spectra of m-(PFu)xFeOClx(OMe)1-y prepared from
methanothermal synthesis show an extra peak at around 1040 cm'1
which is not observed in the samples prepared from refluxing MeOH
or CH3CN as shown in Figure 6.2. The intensity of this peak increases
with reaction time and temperature. We attribute this absorption
peak to methoxide groups which partially replaced the Cl atoms in
FeOCl to form FeOCly(OMe)1-y. For convenience, (PFu)xFeOCl
prepared from refluxing MeOH or CH3CN was called r-(PFu)xFeOCl
and the samples prepared from methanothermal synthesis we called
m-(PFu)xFeOCly(OMe)1-y. Polyfuran present in and extracted from r-
(PFu)xFeOC1 and m-(PFu)xFeOCly(OMe)1-y have identical IR patterns.
Table 6.1 lists some major vibration modes of r-(PFu)xFeOCl, m-
(PFu)xFeOCly(OMe)1-y, extracted polyfuran and (PFu),;Cl.

Direct evidence of formation of intercalated compounds comes
from X-ray powder diffraction studies as shown in Figure 6.3. Two
different intercalated products have been obtained. When the
reactions were carried out in refluxing CH3CH or MeOH, the resulting
black products have an interlayer spacing of 13.60A. However, the
interlayer spacing, expansions vary by 0.5A from batch to batch,
depending on the particle size of FeOCl, the ratios of reagents, the
solvent and reaction time. This may due to the imperfect orientation
of polyfuran inside the host, which is also suggested by the

broadness of XRD peaks. The 5.68A interlayer spacing expansion of

TRANSMITTANCE

280

 

 

 

 

(Al)

 

 

(13)

 

 

 

V I I V

7 1 1 I r u I v y '
4000 so'oo 2000 1000 400

WAVENUMBER (cm-1)

Figure 6.2. FT-IR spectra (KBr pellets) of (A) m-
(PFu)xFeOCly(OMe)1.y (B) r-(PFu)xFeOCl.

Table 6.1.

281

(PFu)xFeCl4 and Extracted Polyfuran.

Major IR Vibration Modes of r-(PFu)xFeOCl,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

r-(PFu)xFeOCl (PFu)xFeCl4 Extracted Modes
polyfuran
m 1527 cm'1 m 1527 cm'1 m 1534 cm:1 C=C str., C-H def
In 1492 cm'1 In 1499 cm‘1 s 1492 cm'1 ring def.
C=C str., C-I‘I
s 1436 cm'1 s 1450 cm'1 s 1429 cm'1 def., C-O str.
C-H def., ring
s 1238 cm'1 s 1238 cm'1 s 1231 cm'1 def.
C-H def., C-O
s 1175 cm'1 s 1182 curl s 1175 cm“1 str.
C-H str., C-O
w 1084 cm'1 w 1084 cm1 w 1084 cm'1 str.
C-O str., C-H
w 1048 cur1 w 1055 cm1 w 1049 cm°l str.
5 999 cm’1 vs 1006 cm'1 s 999 cm'1 ----
ring def., C=C
m 929 cm'1 s 939 cm'1 s 922 cm'1 str.
ring def., C-O
s 879cm'l s 872cm'1 5 879 cm'1 str.
w 823 cm’1 vw 818 cm’1 ----
s 781 cm'1 vs 774 cm1 s 774 cm‘1 ----
w 731 cm'1 w 682 cm'1 w 682 cm°l ----
vs 464 cm'1 Fe-O
*vs: very strong; 5: strong; m: medium; w: weak; vw:
very weak. "str.: streching; def.: deformation.

INTENSITY

 

282

 

(A)

(B)

(C)

 

 

 

29 (DEGREE)

Figure 6.3. Reflection-mode X-ray powder diffraction
patterns of (A) m-(PFu)o,47FeOClo_61(OMe)o,39 (B) r-
(PFu)o,27FeOC1 (C) FeOCl.

 

283

r-(PFu)xFeOCl corresponds to intercalated monolayer of polyfuran
inside the FeOCl layers with the five member rings roughly
perpendicular to the FeOCl slabs as showed in Scheme 6.1. This
arrangement is similar to its polypyrrole and polythiophene

analogues [1 l].

 

| FeOCl 1

O / \ O / \ O

\/ o \/ o\/x ““603
FeOCl

0 /\ 0 /\ 0
lo \/ o \/x

| FeOCl 1

 

 

 

 

 

 

 

Scheme 1. Proposed arrangement of polyfuran

chains in FeOCl layers.

The interlayer spacing of m-(PFu)xFeOCly(OMe)1.y is
significantly larger than r-(PFu)xFeOCl, at 15.62A (see Figure 6.3c).
The higher expansion (7.7A) of m-(PFu)xFeOCly(OMe)1-y is due to the
replacement of the Cl' groups in the FeOCl layers by the larger MeO'
groups. FeO(OMe) is a known layered compound with interlayer
Spacing spacing of 9.91A. This is larger than in FeOCl representing
the larger size of MeO’ groups. The replacement of C1' by CH3O' in

this redox intercalation reaction was not expected, but it is not

284

surprising. It was observed in the reaction of (4-Ap)FeOCl (4-AP: 4-
aminopyridine) with MeOH which gives FeO(OMe) [12]. Since
terfuran in this case inserts (maybe also polymerize) in FeOCl layers,
it expands the layer gallery and at the same time appears to act as a
base helping the deprotonation of MeOH. The net layer expansion of
m-(PFu)xFeOCly(OMe)1-y due to the insertion of polyfuran is equal to
5.7111 (15.62-9.91A) which is similar to r-(PFu)xFeOCl and
corresponds to a monolayer of polyfuran with the five member ring

roughly perpendicular to the host layers as showed in Scheme 6.2.

 

 

7 F90 _ l \

OMe (III 131 OMe 1Cl
0 \ o / \ o .
/ o \ / o \ / ,. HM“

fMe fl (IJMe fl OMe
_ - Fee - I J
(EMe LMe Cl 131 gMe
I O I O \ x
1”“ 1' 1”“ f" 13”“
0 1

Fe

 

Scheme 6.2. Proposed structure of m-(PFu)xFeOCly(OMe)1-y

285

In general solvent trapping in the host layers is a common
phenomenon [l4] and it can be removed by heating at moderate
temperature under vacuum. The present case is a rare example in
which the solvent not only in co-intercalates but also reacts with the
host framework. As observed by IR spectroscopy of the products
during the intermediate stages of the reaction, the polyfuran formed
first, followed by the incorporation of the methoxy group. The
insertion of polyfuran opens up the FeOCl layers, so that the
interlamellar space of FeOCl becomes accessible by MeOH molecules
which under methanothermal conditions apparently have enough
energy to nucleophilically replace Cl atoms in FeOCl. r-(PFu)xFeOCl
did not react with methanol at 110°C in sealed tube (in absence of
terfuran) and neither did MeOH react with FeOCl under the same
reaction conditions. Surprisingly, when terfuran and FeOCl were

refluxed in methanol no ligand replacement was observed.

D. Scanning Electron Microscopy (SEM) and Selected
Area Electron Diffraction (SAED).

The SEM micrographs of both r-(PFu)xFeOCl and m-
(PFu)xFeOCly(OMe)1-y are shown in Figure 6.4. They reveal the
crystalline and layered nature of these composites. Both single
crystal and powder samples show smooth, clean surfaces and no
impurity or amorphous phase. Selected area electron diffraction
with the electron beam perpendicular to the slabs (ac plane) is
shown in Figure 6.5. The diffraction spots can be indexed according

to the original a and c axes of FeOCl lattice as expected from the

286

 

Figure 6.4. SEM micrographs of r-(PFu)xFeOCl and
m-(PFU)xFeOC1y(OMe)l.y
left: r-(PFu)xFeOCl; right: m-(PFu)xFeOCly(OMe)1.y

287

 

Figure 6.5. Selected area electron diffraction (with electron
beam perpendicular to the FeOCl layers) pattern of r-
(PFu)xFeOCl.

288

topotactic intercalation reaction. Similar observation were made in
m-(PFu)xFeOCly(OMe)1-y, since the periodicity in the ac plane does
not change significantly. This kind of intercalation can be ‘ regarded
as pseudo-topotactic reaction and thus the similarity of the electron

diffraction patterns between r and m-phase is expected.

E. Thermogravimetric Analysis (TGA) and Differential
Scanning Calorimetry (DSC) Studies.

The thermal stabilities of both r-(PFu)xFeOCl and m-
(PFu)xFeOCly(OMe)1-y are lower than that of pristine FeOCl under
oxygen or nitrogen flow. The TGA diagram of r-(PFu)xFeOCl shows a
continued weight loss from room temperature to 400°C which is due
to the decomposition of polymer and FeOCl framework. The weight
stabilizes after 400°C as shown in Figure 6.6a. The wide range of
decomposition temperature indicate that polyfuran chains inside the
layers are inhomogeneous. m-(PFu)xFeOCly(OMe)1-y is stable up to
300°C, see Figure 6.6b, fellow by two steps weight loss between
300°C and 550°C. This is due to the decomposition polymers and the
FeOCl framework as seen in FeO(OMe) [12] and MxFeOCl [15] (M = K,
Cs). Figure 6.7 shows the TGA diagrams of both extracted polyfuran
and (PFu),;FeCl4. The polyfuran extracted from (PFu)xFeOCl has lower
thermal stability than the chemically polymerized. This result is not
surprising since the former is formed in structurally restricted host
therefore it is expected to have shorter chain length. This tendency

was also observed in polyaniline intercalated V20 5 and FeOCl, see

 

 

 

 

 

 

 

r T V f 1 r I 'f 1 j I I r r 1
100 L .-
l- . .1
p- .
L (A) (B) 1
l- q
:3 .. 1
I
s: » -
g
r d
. 1: _ 4.
50 .- ..
1 l g k 1 4 1 4 _L 1 1 1 _1 1 1 1
0 250 500 750

TEMPERATURE (°C)

Figure 6.6. TGA curves (under oxygen flow) of (A) r-
(PFu)o,27FeOCl (B) m-(PFu)o,54FeOClo,5(OMe)o.5. (The
heating rate was 5°C/min.)

290

 

 

 

 

1 v T ‘7 f T r r I “V T 1— V V j 1 V 7 W '
100“- 4
)- J
—- 1
)- 4
a? '- 1
[—
5 50)- d
E - (A) J
)- \
P (B)\:
o b L 1
0 250 500 750 1000

Figure 6.7.

(PFu)xFeCl4
5°C/min.)

rate was

TEMPERATURE (°C)

TGA curves (under nitrogen flow)of (A)

(B) Extracted polyfuran. (The heating

291

chapter 2&5. The chain length of polyfuran in these systems is still
unknown at this moment.

The DSC diagrams of both r-(PFu)FeOCl and m-
(PFu)xFeOCly(OMe)1-y under nitrogen flow show an exothermic peak
at 320°C and 280°C respectively. These peaks are due to the
decomposition of FeOCl framework. In comparison with the pristine
FeOCl which decomposed at around 460°C, the lower thermal stability

of polyfuran/FeOCl composites was expected.

F. Electron Paramagnetic Resonance (EPR)

Spectroscopy.

The EPR spectra of intercalated FeOCl compounds are broad and
complicated, due to the antiferromagnetic nature of Fe atoms. As
expected for intercalated compounds, the EPR of both r- and m-
phases did not show any signal, due to the spin-spin interaction
between polyfuran and Fe atoms through the intimate contact as
shown in Figure 6.8a, 6.8b. The polyfuran extracted from
(PFu),;FeOCl showed a sharp signal at g value of 2.0023 with peak
width (Apr) of 20 gauss which is similar to polyaniline. The EPR
spectra of as prepared (PFu)xFeCl4 showed a broad peak (Apr = 600
gauss) at g equal to 2.0579 (see Figure 6.8c). However, when this
polymer was stirred with 2 M HCl(aq) for 2 days, the acid treated
(PFu)xFeCl4 showed a sharp signal, similar to (PFu),;Cl and extracted
polyfuran, at g equal to 2.0023 and the intensity of the broad peak
decreased dramatically, see Figure 6.8d. This can be rationalized by

the removal of the paramagnetic counter ion FeCl4' in (PFu)xFeCl4

292

 

 

 

 

 

DPPH

(C)

 

R (D)

 

 

 

2900 3430 T900

MAGNETIC FIELD (Gauss)

Figure 6.8. Room temperature EPR spectra of (A) r-
(PFu)o.27Fe0Cl (B) m-(PFu)o.51Fe0C10.6(0Me)o.4 (C)
(PFu)xFeCl4 (D) (PFu)xCl.

293

and exchange with the Cl‘ ion. This was confirmed by EDS analysis.
The as-prepared (PFu)xFeCl4 contains significant amount of Fe,
However, extracted polyfuran and acid treated (PFu)xFeCl4 contain
almost no Fe. Further studies on the physical and chemical
properties of conducting polyfuran prepared from electrochemical or

chemical polymerization are in progress.

G. Magnetic Susceptibility Studies.

The characteristic Curie-Weiss antiferromagnetic behavior of
FeOCl was reported by Bizette and Adam [16]. A similar behavior
was also observed in (PANI)xFeOCl, see page 247. The magnetic
susceptibilities of both r-(PFu)xFeOCl and m-(PFu)xFeOCly(OMe)1-y
are field independent at magnetic fields between 200 and 15000
gauss at 300 K. Therefore, all susceptibility data were recorded from
5 K to 300 K at constant magnetic field (5000 gauss). Typical
variable temperature magnetic susceptibility of r-(PFu)xFeOCl and
m-(PFu)xFeOCly(OMe)1-y are shown in Figure 6.9. The magnetic
behavior of (PFu)xFeOCl is similar to that of (PANDXFeOCl implying
that the short range magnetic order observed in FeOCl and
(PANI)xFeOCl is also present in polyfuran/FeOCl. The room
temperature effective spin-only magnetic moment, ueff, of
(PFu)xFeOCl with various x are listed in Table 6.2. These values are
well below 5.9 B.M., had the material been simply paramagnetic
(high spin), due to the antiferromagnetic coupling between the Fe
atoms. This also had been observed in pristine FeOCl and its

intercalates [17]. We found no simple relationship between the

SUSCEPTIBILITY" (EMUl Mole)

294

 

400

300 d (B)
Ann‘sggeeeeeo

AA 0
A 000°(A)
40

200‘ A

$9

100 -1

 

 

 

OIfiTfiT ful'r—w

0 50 100 150 200 250 300 350
TEMPERATURE (K)

Figure 6.9. Variable temperature magnetic
susceptibility data (l/xm) of (A) r-(PFu)o,1sFeOCl
(B) m-(PFu)o.51FeOC10.6(0Me)o.4

295

Table 6.2. Room Temperature Effective Spin-only
Magnetic Moment (neff) versus x of (PFu)xFeOCl.

 

 

 

 

Compounds [ ueff(B.M.)
m-(l’Fu)o,¢;1FeOCly(OMe)1.y 2.87
m-(PFu)o,53FeOCly(OMe)1-, 2.90
m-(PFu)o,56FeOCly(OMe)1-, 2.66
m-(PFu)o,53Fe0Cly(OMe)1-3, 2.88
m-(PFu)o,47FeOCly(OMe)1-y 2.91

r-(PFu)o,23FeOCl 3.39
3.06

r-(PFu)o,27FeOCl

r-(PFQmFeOCl 2.93

 

 

296

magnetic moment and polymer content. Figure 6.10 shows the
variable temperature ucff of r-(PFu)xFeOCl and m-
(PFu)xFe0Cly(OMe)1-y. For both samples the pen increases as
temperature increases, further proving that antiferromagnetic
coupling exists in these materials.

The magnetic properties of polyfuran in (PFu)xFeOCl cannot be
assessed due to the complicated magnetic behavior of the FeOCl
layers. However, polyfuran can be extracted and studied separately

by dissolving the FeOCl framework in HCl(aq). The magnetic
susceptibility, x, of both extracted and (PFu)xFeCl4 are similar to that

of polyaniline which is paramagnetic. The )5 increases with rising
temperature as shown in Figure 6.11, slightly deviated from Curie-
Weiss law. This probably is due to the contribution of Puli
paramagnetic which is relatively small and only observed at high
temperature when the normal paramagnetism is small. The room
temperature effective spin-only magnetic moment, ueff of (PFu)xCl is
equal to 0.74 BM and basically temperature independent, consistent
with Curie-Weiss paramagnetic behavior, as shown in Figure 6.12.
Polyfuran extracted from (PFu)xFeOCl has room temperature ucff of
0.52 B.M. and also temperature independent within experiment error
(see Figure 6.12b). Interestingly, the magnetic moment of (PFu)xCl is
larger than extracted PFu. It had been suggested that the
paramagnetism of conducting polyheterocycles may come from the
defects of the polymer chain [18]. The smaller ucff of extracted
polyfuran indicate that polyfuran extracted from (PFu)xFeOC1 have

fewer paramagnetism and may have less structural defects.

MAGNETIC MOMENT (BM)

297

 

 

0 50 100 150 200 250

TEMPERATURE (K)

 

300

Figure 6.10 Variable temperature effective spin-

Only magnetic moment of (A) r-(PFu)xFeOCl
(PFU)xFCOCIy (0M9)1.y.

(B) m-

 

350

298

 

 

 

 

10000

,
A 8000 a
'3 O A
_2 y o ( )
D O o
g 6000 a O
...V O
E , o O
E 4000 - o g l ‘( B)
a o ‘ a
’5: '1 ‘ ‘
8 A
a. 2000 - 00°00 . A
a g .5

'1 “‘

0 I 1 U T I I j T r l j 1 1k

0 50 100 150 200 250 300 350

TEMPERATURE (K)

Figure 6.11. Variable temperature magnetic susceptibility
data (l/xm) of (A) (PFu)xCl (B) Extracted polyfuran.

299

 

 

 

 

1.0
J
g 0.84
E WA‘AAAAAAAAAAAAA(A)
g a..-
5 MOOOOOOOOOCNB)
E 0.41
U .
E
0.2-
0-0 'I‘ITFfiI'I'T'
0 50 100 150 200 250 300 350
TEMPERATURE (K)
Figure 6.12. Variable temperature effective spin-only

magnetic moment of (A) (PFu)xCl (B) Extracted
polyfuran.

300

H. Charge Transport Properties.

Typical four-probe variable temperature electrical conductivity
data on pressed pellets of r-(PFu)FeOCl and m-(PFu)xFeOCly(OMe)1-y
are shown in Figure 6.13. Both conductivities increase as
temperature increases indicating thermally activated charge
transport behavior which is dominated by the interparticle contact
resistance. This is the common behavior of intercalated FeOCl
compounds [11]. Generally, the conductivity of r-(PFu)xFeOCl is
higher than m-(PFu)xFeOCly(OMe)1-y. The conductivity of m-
(PFu)xFeOCly(OMe)1-y does not depend on x but relates to the
intensity of the peak at 1050 cm'1 which comes from the MeO
vibration of m-phase. The weaker the intensity, the higher
conductivity. The room temperature conductivity of pressed pellet
samples falls in the range of 10'4 ~100 fl'lcm'l as lists in Table 6.3.
Surprisingly, for r-(PFu)xFeOCl the conductivity of single crystals is
slightly lower than that of pressed pellet powder as shown in Figure
6.14. This may due to the poor crystallinity of the sample after
intercalation as evidenced by the broad peaks in the X-ray
diffraction pattern. Table 6.4 lists the room temperature
conductivity of several intercalation compounds of FeOCl for
comparison. The conductivity of r-(PFu)xFeOCl is comparable to its
polypyrrole and polythiophene analogs. The lower conductivity of
m-(PFu)xFeOCly(OMe)1-y may be due either to the present of shorter
conjugated polymer segments inside the host or the chemical change

of the FeOCl layers (i. e. formation of FeOCly(OMe)1-y) or due to both.

301

O .

 

(A) j

I
N
4 ) "r jfirfir‘fi"

(B)

4':

 

-10

 

 

l l l 1 g i l I 1 l J

4 1
8 12 16 20

1 J l

r-

-12

LOG connucrmrv (fl'lcm'l)
do
0 firrjw TwTrr—rT‘T‘T‘r‘T-fi

L
4

1000/T (K-l)

Figure 6.13. Four-probe variable temperature electrical
conductivity data of pressed pellets of (A) r-(PFu)o,z7FeOCl
(B) m-(PFu)o.51Fe0Clo.6(0Me)o.4

302

Table 6.3. Room Temperature Electrical Conductivity (0)
versus x of r-(PFu)xFeOCl and m-(PFu)xFeOCly(OMe)1-y.

 

 

 

Compounds 6(R.T.)(Q-lcm-l)
m-(PFu)o,53FeOCly(OMe)1.y 10-5~10-4
m-(PFu)o,5.5FeOCly(OMe)1.y 10‘5~10'4
m-(PFu)o,53FeOCly(OMc)1., 10‘3‘v10'2
m-(PFu)o.47FeOCly(OMe)1-, 10-4~1o-3

’r-(PFu)o,23FeOCl 10'1~10°
r-(PFu)o,27FeOCl 10'2~10'1
r-(PFu)o,1gFeOCl 10"1‘v100

 

 

 

* single crystal r-(PFu)xFeOCl.

303

 

 

 

 

 

O
1.
-1 1—
'75 L
'1 -2 L
9 r
E _
._ -3
s f
D 1.
5 4
C - L—
u 0
ca [ O
S 5 O
1‘ o
“ o
L.
-5 1 1 1 1 1 L 1 1 1 1 L
50 100 150 200 250 300 350
TEMPERATURE (K)
Figure 6.14. Four-probe variable temperature electrical

conductivity data of r-(PFu)o,27FeOCl in (A) Pressed pellet
(B) Single crystal form.

304

Table 6.4. The Interlayer Spacing
Temperature Electrical

Conductivity

Intercalation Compounds of FeOCl.

(d) and Room
(a) of Several

 

 

 

 

 

 

Compounds d (A) 0 (Q‘lcm' Reference
1)

(TTF)o,1oFeOCl 13.63 3.5)(10'3 1 5
(TTT)0,14FeOCI 15.22 2.2)(10'4 l 5
(TTN)o,5oFeOCl 15.31 5.5!(10'4 1 5
(TMTTF)o,osFeOCl l 1.32 1.6)(10'2 18c
(Ppy)o,34FeOCl 13.21 1.5x10-1 1 2
(Pth)o,23FeOCl 13.31 5.0 1 2
(Ferrocene)o,17FeOCl 14.92 1.0x10'4 2 0
(Perylene)o,11FeOCl 16.769 2.3x10'2 18b
(TSF)o,12FeOCI 13.52 5.0x10'3 18a
(TMTSF)o,1oFeOCl 13.82 5.4x10'4 18a
(ET)o,25FeOCl 22.18 2.6x10' 1 2 2

(PANI)o,zo-o,2sFeOCl 13.92 10'3~10'l Chapter 5

(PFu)o,1g.o,23FeOCl 13.60 10'2“'100 This work

 

 

305

The thermoelectric power of m-(PFu)xFe0Cly(OMe)1-y is
unmeasurable due to very high contact resistance of the compound.
Variable temperature thermoelectric power (TP) of r-(PFu)FeOCl in
single crystals and pressed powders are shown in Figure 6.15. The
TP of the single crystals is large, positive and increases with rising
temperature, while the TP of pressed powder is smaller than that of
single crystal as reveals the higher conductivity. It reaches the value
of 4S uV/K at 230 K below which, the exact Seebeck coefficient
cannot be obtained due to the high resistivity of the materials.
However, both sets of thermoelectric power data exhibited typical p-
type semiconductor behavior, similar to (PANI),;FeOCl and other
intercalated FeOCl compounds. This is different from its polypyrrole
and polythiophene analogs which show p-type metallic character [9].
The different charge transport properties between polyfuran/FeOCl
and polypyrrole, polythiophene/FeOCl systems are hard to explain
but it may reflect differences in the length of polymer chains and in
the structural integrity of the polymer backbone in the interlamellar
space of FeOCl. Therefore, the reduced FeOCl framework seems to be
responsible for charge transport properties in the PFu/FeOCl system.
This also explains why the conductivity of r- phase is higher than m-
phase. Since the conductivity of (PFu)xFeOC1 was dominated by FeOCl
framework, the replacement of C1 by MeO may inhibit the charge
transport by acting as disordered defects, thus decreasing the
conductivity. The formation of the shorter conjugated segments of
polyfuran in FeOCl may be due to two reasons: first (PFu)xFeOCl was
prepared from larger molecular monomer (i.e. terfuran), the slow

kinetic of large molecule will prohibit the extensive polymerization

 

306

 

250 j
200
150
100

50

THERMOPOWER (uV/K)

 

rWTTTrrrrrIIITITTTITwTrjIIll

 

 

 

 

 

 

 

 

 

 

 

TEMPERATURE (K)

Figure 6.15. Variable temperature thermoelectric power
data of r-(PFu)o,27FeOCl in (A) Pressed pellet (B) Single
crystal.

307

of monomers inside the FeOCl gallery; second, when the reaction
were carried out in refluxing MeOH, the longer reaction time cause
some structure defects on the polyfuran backbone (e.g. ring-opening)
therefore reducing the conjugation length.

Unfortunately, polyfuran appears to be sensitive to acid, and
the polyfuran extracted from both r-(PFu)xFeOCl and m-
(PFu)xFeOC1y(OMe)1-y is essentially an insulator.

VI. Conclusions:

Polyfuran can be intercalated in FeOCl by in-situ oxidative
polymerization/ intercalation of terfuran or tetrafuran in refluxing
MeOH, CH3CN or by methanothermal synthesis at 100°C. This
polymer is of higher quality showing less ring-opening than what has
been prepared in the literature. Two phases of (PFu)xFeOCl with
different interlayer spacing were obtained depending on the reaction
conditions. The compound prepared from methanothermal synthesis
is a pure product, although it has lower conductivity and the reaction
is not really topotactic. The C1 atoms in the FeOCl framework were
partially replaced by (OMe) groups to form FeOCly(OMe)1-y. r-
(PFu)xFeOCl prepared from refluxing in MeOH and CH3CN has room
temperature conductivity up to 100 (I‘lcm'1 higher than m-phase.
The p-type semiconductor behavior of both phases suggest that the
charge transport properties are dominated by the reduced FeOCl
framework. The redox intercalation of terfuran in FeOCl may offer a
new avenue to synthesize better defined polyfuran through chemical

polymerization method.

LIST OF REFERENCES

308

LIST OF REFERENCES

(a) Zotti, G.; Schiavon, G.; Comisso, N.; Berlin, A.; Pagani, G. Synth.
Met. 1990, fl, 337-351. (b) Tourillon, G.; Garnier, F. J.
Electroanal. Chem. 1982,13; 173-178.

(a) Leung, W.-Y. Thesis Dissertation, Michigan State University
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3673. (c) El-Hajj, T.; Martin, J.-C.; Descotes, G. J. Heterocyclic
Chem. 1983, 20,, 233-235.

Kang, I. T.; Neoh, K. G. Eur. Polym. J. 1987, 23, 719-922.

Cook, M. J.; Hatritzky, A. R.; Linda, P. Adv. Hetercyclic Chem.
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(a) Ohsawa, T.; Kaneto, K.; Yoshino, K. Jpn. J. Appl. Phys. 1984,
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Coic, L.; Spiesser, M.; Palvadeau, P.; Rouxel, J. Mat. Res. Bull.
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Hernandez, V.; Lopez Navarrete, J. T.; Marcos, J. I. Synth. Met.
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(a) Yoshino, K.; Hayashi, S.; Sugimsto, R. Jpn. J. Appl. Phys. 1984,
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Ishigak, A.; Shono, T.; Hachihama, Y. Kogyo Kagaku Zasshi 1963,
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(a) Kanatzidis, M. G.; Tonge, C. R.; Marks, T. J.; Marcy, H. 0.;
Kannewurf, C. R. J. Am. Chem. Soc. 1987,102, 3797-3799. (b)
Kanatzidis, M. G.; Marcy, H. 0.; McCarthy, W. J.; Kannewurf, C. R.;
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Kikkawa, S.; Kanamaru, F.; Koizumi, M. Inorg. Chem. 1976,11,
2195-2197.

Son, S.; Kikkawa, S.; Kanamaru, F.; Koizumi, M. Inorg. Chem.
1980.12. 262-264.

Averill, B. A.; Kauzlarich, S. M.; Antonio, M. R. J. De Phys. 1983,
C3-1373-C3-1376.

Kanatzidis, M. G. et a1 work in progress.

Bizette, H.; Adam, A. C. R. Acad. Sci. Paris Ser. B 1972, 2:25, 911-
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(a) Antonio, M. R.; Averill, B. A. Mol. Cryst. Liq. Cryst. 1988, _l_7_Q,
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(a) Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985,1_8_, 309-315.
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310

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311

CONCLUDING REMARKS

The research of conducting polymers will still be active in this
decade. Some issues, such as the structure, the properties and the
new possible applications as well as discovery of new conducting
polymers and their composites (or copolymers) still attract scientists.
Approach of these issues needs intriguing ideas and correct
directions. The in-situ oxidative polymerization] intercalation of
these polymers in layered hosts is a very recent development. It
gives a promising solution to solve some remaining puzzles in
conducting polymers. From the extensive study in our laboratory,
three interesting discoveries have been made.

First: We were able to prepare organic polymer/inorganic
composites with controllable charge transport properties ~ ranging
from semiconductor to metal-like and with p-type carrier to n-type
carrier. These rare and unique properties may offer some potential
applications in electronic devices, such as p-n junction. Other
applications may be discovered by cooperation of imaginative
chemists and electrical engineers. This field of research can also be
extended to other electrically active hosts to improve the
physicochemical and mechanical properties of the resulting
intercalation compounds.

Second: by using crystalline hosts, oriented polymer can be
prepared. Even though we were not able, as yet, to solve the
polyaniline structure inside (PANI)xFeOCl, this discovery reveals an
encouraging direction for further work. If we are able to find good

crystalline hosts, conducting polymers may orient inside the hosts

312

and preserve the crystallinity of the entire system. The intercalation
compound may enable us to characterize it by single crystal X-ray
diffraction study.

Third: the results of intercalating polyfuran in FeOCl provide
an alternative road to prepare a new polymer with more regular
backbone by this special chemical polymerization/ intercalative
method described in this thesis. Using this technique, we may obtain

other known polymers with better structures.

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