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Electrochemical Preparation and Conductivity of Polydioxythiophenes

Permanent Link: http://ufdc.ufl.edu/UFE0024116/00001

Material Information

Title: Electrochemical Preparation and Conductivity of Polydioxythiophenes
Physical Description: 1 online resource (69 p.)
Language: english
Creator: Moody, Laura
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: conducting, polymers, thiophene
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: To study the conductivity of electrochemically formed films, the monomers 3,4-ethylenedioxythiophene, diethyl 3,4-phenylenedioxythiophene and 3,4-phenylenedioxythiophene, and the dimer of 3,4-phenylenedioxythiophene have been electrochemically polymerized using a variety of conditions, solvents and electrolytes. The most highly conductive films based on 3,4-ethylenedioxythiophene were formed in an electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in propylene carbonate, with room temperature conductivities of the glossy, free-standing films reaching 120 S/cm. The conductivity of poly(3,4-ethylenedioxythiophene) maintained over half of its room temperature conductivity value as the temperature approached 50 K. Films of 3,4-phenylenedioxythiophene and diethyl 3,4-phenylenedioxythiophene were formed from electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in dichloromethane. Scan rate dependence studies (scan rates of 5 to 200 mV/s) of the first scan of polymer deposition confirmed that the cation radical couples slowly for these monomers. Oligomers were also soluble in propylene carbonate, the solvent which, based upon 3,4-ethylenedioxythiophene studies, was most likely to yield glossy, free-standing films. Free-standing, highly conductive films were not obtained, though material did deposit on the surface of a glassy carbon plate electrode and was delaminated using adhesive tape. The most conductive material formed exhibited resistances in the mega-Ohm range. Films formed using the dimer of 3,4-phenylenedioxythiophene were formed from electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in dichloromethane. The dimer of 3,4-phenylenedioxythiophene oxidized at a lower potential than diethyl 3,4-phenylenedioxythiophene and exhibited faster cation radical coupling rates, leading to the successful formation of free-standing small flakes of conducting polymer film with conductivities measured on the order of 10 S/cm. For all polymerization reactions, the optimal conditions for films formation were determined to be at zero degrees Celsius.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Laura Moody.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Reynolds, John R.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0024116:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024116/00001

Material Information

Title: Electrochemical Preparation and Conductivity of Polydioxythiophenes
Physical Description: 1 online resource (69 p.)
Language: english
Creator: Moody, Laura
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: conducting, polymers, thiophene
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: To study the conductivity of electrochemically formed films, the monomers 3,4-ethylenedioxythiophene, diethyl 3,4-phenylenedioxythiophene and 3,4-phenylenedioxythiophene, and the dimer of 3,4-phenylenedioxythiophene have been electrochemically polymerized using a variety of conditions, solvents and electrolytes. The most highly conductive films based on 3,4-ethylenedioxythiophene were formed in an electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in propylene carbonate, with room temperature conductivities of the glossy, free-standing films reaching 120 S/cm. The conductivity of poly(3,4-ethylenedioxythiophene) maintained over half of its room temperature conductivity value as the temperature approached 50 K. Films of 3,4-phenylenedioxythiophene and diethyl 3,4-phenylenedioxythiophene were formed from electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in dichloromethane. Scan rate dependence studies (scan rates of 5 to 200 mV/s) of the first scan of polymer deposition confirmed that the cation radical couples slowly for these monomers. Oligomers were also soluble in propylene carbonate, the solvent which, based upon 3,4-ethylenedioxythiophene studies, was most likely to yield glossy, free-standing films. Free-standing, highly conductive films were not obtained, though material did deposit on the surface of a glassy carbon plate electrode and was delaminated using adhesive tape. The most conductive material formed exhibited resistances in the mega-Ohm range. Films formed using the dimer of 3,4-phenylenedioxythiophene were formed from electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in dichloromethane. The dimer of 3,4-phenylenedioxythiophene oxidized at a lower potential than diethyl 3,4-phenylenedioxythiophene and exhibited faster cation radical coupling rates, leading to the successful formation of free-standing small flakes of conducting polymer film with conductivities measured on the order of 10 S/cm. For all polymerization reactions, the optimal conditions for films formation were determined to be at zero degrees Celsius.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Laura Moody.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Reynolds, John R.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0024116:00001


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1 ELECTROCHEMICAL PREPARATION AND CONDUCTIVITY OF POLYDIOXYTHIOPHENES By LAURA MOODY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008

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2 2008 Laura Moody

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3 ACKNOWLEDGMENTS As I finish writing this thesis, I am thankful to have been bl essed with the support of great people from right here at the Un iversity of Florida to quite li terally across the world. I would first like to thank my parents, Jim and Suza nne, who have always unconditionally supported both the choices I have made, and the circumstances I have ended up in. Over a decade after staying up all night to burn plant leaves with a lighter, I have been from fish mazes to views of rocket launch-pads from Maile Mountain, and back. I would like to thank them for visiting when I arrived in Gainesville, and for the chances to see each other since. Maybe the impending Air Force-mandated migration back West will put me in a better position to make it home more often; I think Japan needs an infusion of Laura (a nd Evan). Ill repeat my usual line and reiterate my hope I am continuing to grow to become more like my parents. I would like to thank Dr. Reynolds for graciously receiving me into the group despite the time limits with which I arrived at UF. I will co nfess I joined this group rather blindly, but the ensuing year in the Chemistry Department here has shown me I could not have ended up in a better place. Dr. Reynolds has assembled an incredible se t of people, and it is not lost on any of us that his approach to both people and scienc e multiply through the group and make this a very special place work and learn. I would like to th ank him for the thoughtfulness with which he has guided me and for the opportunity to have been part of this research group. I am filled with gratitude for the friends and mentors I have found at UF. The guidance of the brilliant women in the Faraday Cage, Dr. Sve tlana Vasilyeva, Dr. Aubrey Dyer, (future Dr.) Ece Unur and (future Dr.) Merve Ertas has been patient, gracious, selfle ss and assertively given even when I was not wise or aggressive enough to ask. I would like to thank Aubrey for editing this thesis; her suggestions, guidance, multiple re adings, and help organizing ideas allowed me to pull my ideas together in a way that I found ve ry rewarding. I would like to thank Ece for

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4 holding my hand as I began my e-chem; every e xperiment I was able to do over the last six months was because of what she showed me. She was patient and showed me thoroughly the first time, and anticipated the way data would eventu ally need to be presente d. She is a joy to be around and I dont tell her often enough how much I admire her disposition, energy and outlook. She has the gift of encouraging those around her and I hope she ends up in a job that allows that energy to propagate through everyt hing she does. I would like to thank Merve for pursuing our friendship this spring as she and I morphed from workout buddies to thesis/dissertation writing buddies, she provided such respit e from writing, with everything fr om finally taking me to a football game, to debate-watch ing, to meeting for dinner or an afternoon of shopping for an hour. My road trip co-queen and I have conquered the road to North Carolina; perhaps its on to Florida to New Mexico? I w ould like to thank Svetlana, who is among the most extraordinary people Ive met in my life. Her thought patterns perspectives, experiences and perceptions are worlds apart from any person I have known over three continents and I have been changed by knowing her. I would like to thank her and (and Boris) for the consideration they have shown me; my mother was able to witness that from the other side of the world and turned me in their direction before the first home I ever owne d was conquered by Gainesvilles homeless. I would like to thank David Li u and Eric Shen, the brains behind the synthesis of the PheDOT monomers. Eric has been a pleasure to work with, and fr om what I have witnessed of him in a pseudo-supervisory role, I think supe rvising people (along with an unreal command of extemporaneous technical speech) is among his greatest strengths. It has been great having David there alongside me learning e-chem, tr aveling to Missouri and always providing entertainment and encouragement. Wait, I think I have an answer to twenty questions: is it PheDOT? In that vein, a great deal of gra titude is also due to Sriram Viswanthan, Joseph

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5 Mbugua and others at Crosslink, for help with the temperature dependent conductivity measurements, advice on electrochemical film deposition, and an inform ative visit to JVIC Laboratories in Springfield. Several other me mbers of the polymer floor are owed special acknowledgement: I would like to thank Unsal Koldemir for logi stical and design support and the sixteen ounces of authentic Turkish tahini that sustained me for an entire week in early January. Bora Inci brightens my day with fascinating conversation, a nd there are few other people I know who approach life and work with su ch an inquisitive and active mind. I think his gift of teaching and the genuine concern he ha s for others will make him a great professor someday. Harun Turkcu, though he is far off enj oying Istanbul, I often think of the laughter he brought to Leigh 302 and ask him to remember American Captain is always his friend! I will close with an acknowledgement of fr iends whose support comes from afar; having parents in rural Northern Japan makes all of them seem close by comparison. I would like to thank Dennis for the near-daily emailseven if many of them are vivi d reminders of human idiocy, the concern he (and Dawn) have shown fo r me since I left California keeps me smiling and sane every day. I would like to thank Dave for his friendship; weve both known for a while that it is for life. Our conversati ons brightens the most trying of days and the fact that he feels the same way leaves me that much happier. I would like to thank Aleea for being there for me as Ive made the move hereI could not ask for a better, more encouraging friend. Finally, my thanks go to Carolyn, ten years after she pried me away from my scienc e project to socialize me, I cannot express how glad I am that we are stil l in each others lives. To those unnamed who have been my hugest inspiration through this process, I have no wa y to express the gratitude that fills me. I thank you from the bottom of my hear t and I hope the occasional passive-aggressive remark will be overlooked and my deepest appreciation known!

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ............................................................................................................... 3LIST OF TABLES ...........................................................................................................................8LIST OF FIGURES .........................................................................................................................9LIST OF ACRONYMS .................................................................................................................11ABSTRACT ...................................................................................................................... .............12 CHAP TER 1 INTRODUCTION .................................................................................................................. 14Brief History of Conjugated Polymers ...................................................................................14Electronic Properties of Conjugated Po lymers and the Conductivity Theory ........................ 16Mechanism of Oxidative Electrochemical Polymerization .................................................... 21Poly (3,4-Dioxythiophene) Based Polymers .......................................................................... 23Applications .................................................................................................................. ..........262 EXPERIMENTAL TECHNIQUES ........................................................................................28Chemicals, Materials and Instrumentation ............................................................................. 28Electrochemical Methods ....................................................................................................... 30Cyclic Voltammetry for Polymer Deposition .................................................................. 30Electrochemical Cell Setup ............................................................................................. 31Film Formation ................................................................................................................ 32Conductivity Measurements ...................................................................................................34Standard Two-Probe Conductivity Measurements .......................................................... 34Four-Probe Conductivity Measurements ......................................................................... 34Temperature Dependent Conductivity Measurements .................................................... 363 RESULTS AND DISCUSSION ............................................................................................. 373,4-Ethylenedioxythiophene (EDOT) Studies ........................................................................37Monomer Properties and Electrochemistry on Pt Button Electrode ............................... 38Potentiodynamic deposition ..................................................................................... 38Galvanostatic deposition ..........................................................................................40Potentiostatic deposition ..........................................................................................41Film Formation Experiments ........................................................................................... 43Galvanostatic and potentios tatic film formation ...................................................... 43Solvent and electrolyte effects ................................................................................. 44Polymerization temperature effects .......................................................................... 47

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7 Conductivity Measurements ............................................................................................47Room temperature measurements ............................................................................47Low temperature measurements ............................................................................... 49Diethyl Phenylenedioxythiopene Studies ...............................................................................50Electrochemistry on Pt Button Electrode and Monomer Properties ............................... 50Film Formation ................................................................................................................ 58Room Temperature Conductivity Measurements ............................................................ 59Phenylenedioxythiophene Dimer Studies ............................................................................... 60Monomer Electrochemistry ............................................................................................. 61Film Formation ................................................................................................................ 61Room Temperature Conductivity Measurements ............................................................ 63Conclusions .............................................................................................................................63LIST OF REFERENCES ...............................................................................................................66BIOGRAPHICAL SKETCH .........................................................................................................69

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8 LIST OF TABLES Table page 3-1 Conductivity measurements for PEDOT film s formed using 800 CV cycles. .................. 483-2 Conductivity measurements for Phe DOT and PheDOT-Et2 film formation experiments. .................................................................................................................. .....603-3 Conductivity measurements for biPheDOT film formation experiments. ......................... 63

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9 LIST OF FIGURES Figure page 1-1 Common conjugated polymers .......................................................................................... 151-2 Band diagrams showing differences among metals, semiconductors and insulators. ....... 181-3 Doping mechanism in polythiophene ................................................................................ 201-4 Mechanism of electrochemical polymerization in polythiophene. ....................................211-5 Coupling pathways during thiophene polymerization. ...................................................... 221-6 Poly(3,4-phenylenedioxythiophene) (PPheDOT) ..............................................................242-1 Monomers used in electrochemical studies. ...................................................................... 292-2 Three-electrode electroc hemical cell configuration. .......................................................... 322-3 Electrochemical cell showing setup for film formation experiments. ............................... 332-4 Standard two-probe surface resistivity measurement. ....................................................... 342-5 Setup for a four-wire conductivity measurement. .............................................................. 353-1 First CV scan of EDOT deposition on a platinum button and first through sixth CV scans of EDOT deposition .................................................................................................393-2 Voltammogram of PEDOT film deposited on a Pt button electr ode at scan rates between 10 and 200 mV/s and linear dependenc e of peak current on the scan rate. ......... 403-3 Galvanostatic deposition of PEDOT in 10mM EDOT/0.1M TBAP/ACN on a platinum button (A = 0.02cm2). ......................................................................................... 413-4 Potentiostatic deposition of EDOT on Pt button (A = 0.02 cm2) revealed a potentiostatic regime between 0.82 and 1.25 V vs Fc/Fc+ for 180 seconds in 10mM EDOT/0.1M TBAP/ACN. .................................................................................................423-5 Peak current densities of PEDOT film s deposited potentiostatically between 0.82V and 1.25 V vs Fc/Fc+ for 180 seconds at 10mM monomer concentration ......................... 423-6 Voltammogram showing EDOT deposite d potentiodynamically on the large GC plate electrode. 0.1M TBAP/PC (scan rate of 50mV/s) at 0C .......................................453-7 Voltammogram showing the continued de position of PEDOT on a large GC plate electrode. 0.1M TBAP/PC (scan rate of 50mV/s) at 0C ................................................46

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10 3-8 Voltammogram showing EDOT deposite d potentiodynam ically on the large GC plate electrode. 0.1M TBAPF6/PC (scan rate of 50mV/s) at 0C .................................... 463-9 Temperature dependence of conductivity of PEDOT deposited potentiodynamically in various conditions ..........................................................................................................493-10 First through twentieth CV scans of PheDOT-Et2 deposition (10mM PheDOTEt2/0.1M TBAP/ACN) with a scan rate (Vs) of 50mV/s.on a platinum button (Area = 0.02 cm2) ............................................................................................................................513-11 Scans of PheDOT-Et2 switching in 0.1M TBAP/ACN w ith a scan rate of 50mV/s indicate stability for up to 200 cycl es on a platinum button (Area = 0.02 cm2). ...............523-12 Voltammogram of PheDOT-Et2 film deposited on a Pt button between 50 and 200 mV/s indicate increasing current re sponse at increasing scan rates .................................. 533-13 Detail of PheDOT-Et2 monomer oxidation on the first CV scan at scan rates of 5, 10, 25, 50, 75, and 100 mV/s on a platinum button (Area = 0.02 cm2) ...................................543-14 First through twentieth CV scans of PheDOT-Et2 deposition (10mM PheDOTEt2/0.1M LiBTI/ACN) with Vs = 50mV/s on a platinum button (Area = 0.02cm2) .........553-15 First through twentieth CV scans of PheDOT-Et2 deposition (10mM PheDOTEt2/0.1M EMI-BTI/ACN) with a scan rate (Vs) of 50mV/s.on a platinum button (Area = 0.02cm2) ................................................................................................................553-16 First through tenth CV scans of PheDOT-Et2 deposition (10mM PheDOT-Et2/0.1M TBAPF6/ACN) with a Vs = 50mV/s on a platinum button (A = 0.02cm2) ....................... 563-17 First through twentieth CV scans of PheDOT-Et2 deposition (10mM PheDOTEt2/0.1M TBAPF6/ACN) with a scan rate (Vs) of 50mV/s.on a platinum button (Area = 0.02cm2) ...............................................................................................................573-18 Scans of PheDOT-Et2 switching in 0.1M TBAPF6/ACN indicate film stability for up to 100 cycles on a platin um button (Area = 0.02cm2). ...................................................... 573-19 Voltammogram of PheDOT-Et2 film deposited on a Pt button between 25 and 200 mV/s indicates increasing current response at increasi ng scan rates and linear relationship between scan rate and peak current density. .................................................. 583-20 Voltammogram of PheDOT-Et2 deposition on 2 cm2 glassy carbon plate electrodes in ACN/TBAP at 0C ........................................................................................................593-21 First through twentieth scans of bi-PheDOT deposition (10mM biPheDOT/0.1M TBAP/DCM) with Vs = 50mV/s on a platinum button (A = 0.02 cm2). ........................... 613-22 Voltammogram of bi-PheDOT deposited potentiodynamically on a large GC plate electrode in 60 mMol bi-Phe DOT/0.1M TBAP/DCM (scan rate of 50 mV/s) at 0C ...... 62

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11 LIST OF ACRONYMS ACN Acetonitrile Bi-PheDOT Bis-phenyl enedioxythiophene BTI Bis(trifluoromethylsulfonyl)imide DCM Dichloromethane EMI 1-ethyl-3-methyl-1-H-imidazolium Fc/Fc+ Ferrocene ITO Indium Tin Oxide PC Propylene carbonate TBAP Tetrabutylammonium perchlorate

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12 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science ELECTROCHEMICAL PREPARATION AND CONDUCTIVITY OF POLYDIOXYTHIOPHENES By Laura Moody December 2008 Chair: John Reynolds Major: Chemistry To study the conductivity of electrochemically formed films, the monomers 3,4ethylenedioxythiophene, diethyl 3,4-phenylenedioxythiophene and 3,4phenylenedioxythiophene, and the dimer of 3,4-phenylenedioxythiophene have been electrochemically polymerized us ing a variety of conditions, solv ents and electrolytes. The most highly conductive films based on 3,4-et hylenedioxythiophene were formed in an electrolyte-solvent combination of 0.1 M tetr abutylammonium perchlorate in propylene carbonate, with room temperature conductivities of the glossy, free-standing films reaching 120 S/cm. The conductivity of poly( 3,4-ethylenedioxythiophene) maintain ed over half of its room temperature conductivity value as the temperature approached 50 K. Films of 3,4-phenylenedioxyt hiophene and diethyl 3,4-phenylenedioxythiophene were formed from electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in dichloromethane. Scan rate depe ndence studies (scan rates of 5 to 200 mV/s) of the first scan of polymer deposition confirmed that the cation ra dical couples slowly for these monomers. Oligomers were also soluble in propylen e carbonate, the solvent which, based upon 3,4ethylenedioxythiophene studies, wa s most likely to yield gloss y, free-standing films. Freestanding, highly conductive films were not obtain ed, though material did deposit on the surface

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13 of a glassy carbon plate electrode and was de laminated using adhesive tape. The most conductive material formed exhibited re sistances in the mega-Ohm range. Films formed using the dimer of 3,4-phe nylenedioxythiophene were formed from electrolyte-solvent combination of 0.1 M tetrabutylammonium perchlorate in dichloromethane. The dimer of 3,4-phenylenedioxythiophene oxidized at a lower poten tial than diethyl 3,4phenylenedioxythiophene and exhibited faster cation radical coup ling rates, leading to the successful formation of free-standing sma ll flakes of conducting polymer film with conductivities measured on the order of 10 S/cm. For all polymerization reactions, the optimal conditions for films formation were determin ed to be at zero de grees Celsius.

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14 CHAPTER 1 INTRODUCTION Brief History of Conjugated Polymers Polym er chemistry has played a seminal role in the modernization of all aspects of life that has taken place in the last century. Polymers ha ve not only been used as light-weight, more durable, and customizable replacements for func tional materials such as woods, metals, or ceramics, but also to enable revolutionar y new technologies. C onducting (or conjugated) polymers are a unique and relatively new class of polymeric materials, whose electroactive and optical properties make them useful in technolog ies that only decades ago could never have been realized. While the first polymerization of acetylene to form polyacetylene was reported by Natta and coworkers in 1958,1 little interest was taken in the discov ery at the time, as the material was a poorly characterized, insoluble and infusible powder. Over a decade later, Shirakawa and coworkers discovered a method for prepari ng strong, flexible, fr ee-standing films of polyacetylene through the accidental additi on of 1,000 times more catalyst during the polymerization reaction.2 The 1977 discovery that doping these films with iodine vapors led to a twelve orders of magnitude in crease in electr ical conductivity3 initiated an exponential increase in conjugated polymer research and development. Although polyacetylene s high conductivity4 initially generated interest for use as a replacement for dense metals, its ap plications, especially in air and space industries, were limited by poor environmental stability5 and insolubility. These limitati ons challenged organic chemists to begin synthesizing alternative conjugated polymers, with a focus on improving solubility, stability and processability while maintaining conductivity. Numerous materials have been developed, with polyheterocyles receiving a great deal of attention for their electron-rich nature,

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15 which contributes to stability in the doped state, and the ease with which they can be structurally modified. As the only st ructural requirement is extended conjugation through pz orbitals, the field attracted numerous research groups inte rested in fine-tuning polymer properties through structural modifications of monomers, allowing precise ta iloring of physical, electronic, and optical properties. The resulti ng plastics are ultimately much mo re easily processed than metals and, unlike metals, can be deformed reversibly. Figure 1-1. Common conjugated poly mers include (a) poly(acetylene), (b) poly(pyrrole), (c) poly(thiophene), (d) po ly(3,4-ethylenedioxythiophene), (e) poly( p-phenylene), (f) poly( p-phenylene vinylene), (g) poly(aniline), (h) poly(fluorene), and (i) poly(carbazole). Because these new materials presented so many advantageous properties, the ensuing decades witnessed the exploration of conjuga ted polymers families including those based on poly(pyrrole), poly(thiophene), poly(3,4-ethylenedioxythiophene), poly( pphenylene), poly(furan), poly( p-phenylene vinylene), poly(aniline), poly(fluorene), and poly(carbazole).

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16 While none of these conjugated polymers exhibi t conductivities as high as polyacetylene, many showed greater environmental stability, tunable electrochromic propertie s and other interesting characteristics such as photoor electroluminescence.6 Poly(thiophene) was first prepared in the early 1980s7 and its abundant number of derivative s remain among the most widely studied polyheterocycles, due in large pa rt to the huge variety of elect roactive properties this large family of polymers exhibits. Light-emitti ng polymers were discovered in 1990 by Friend and coworkers8, which ushered in a widespread effort toward the development of organic electronics and displays. Twenty-three years after the fi rst demonstration that polymers could se rve as electrical conductors, Alan J. Heeger, Alan G. MacDiarm id and Hideki Shirakawa were awarded the Nobel Prize for "the discovery and development of conducting polymers." Electronic Properties of Conjugated Po lymers and the Conductivity Theory The structural feature that de fines a conjugated polym er is a backbone of atoms connected by -bonds that have continuous overlapping -orbitals throughout the chain. Conductivity in conjugated polymers is a complex material property that depends on both intraand intermolecular charge transport. In linear conducting polymers, the intermolecular charge transport is largely determined by relative chain arrangement, which depends on regional morphology. The nature of chain-to-chain intera ctions is governed by mi croscopic interactions such as chain alignment, packing forces and phase morphology.9,10 The behavior of electrons in an isolated at om depends on interaction with the nucleus and other electrons of that particul ar atom and is described by atom ic orbital wave functions. As atoms are brought together into molecules, molecular orbitals are produced in proportion to the number of atoms coming together. As a great number of atoms (> 1020) are brought together in

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17 an ordered array to form crystals, the number of associated orbitals becomes exceedingly large, and, as the energy difference between these orbi tals becomes smaller, continuous energy bands evolve. Some intervals of energy contain no or bitals, regardless of how many atoms aggregate; these energy differences are referred to as band gaps. As the interatomic distance becomes less than the spatial extension of the electronic wave function associated with a particular atom, the valence electrons within the bands are considered to belong to the entire material. Since the Pauli Exclusion Principle states that the electr ons of a certain atomic orbital cannot have the same energy, the electrons form a band of energy levels with small energy differences between individual levels.11 The distinction between metals and other materials (insulators and semiconductors) can be understood using band theory as illustrated in Figur e 1-2. Metals have an only partially filled highest occupied molecular orbital (HOMO), wh ile in semiconductors and insulators the HOMO is filled. A material's electrical properties directly depend on the energy difference between the HOMO and lowest unoccupied molecular orbital (LUMO) and the level of band filling and band overlapping; this leads to an unde rstandable distinction in electri cal conductivity between metals and insulators/semiconductors, in partic ular as temperatures approach 0 K.12 Electrical conductivity is described by the equation: = ne (1.1) where n is the density of charge carriers contributing to conductivity, e the charge of one electron, and is charge carrier mobility. Typical va lues of conductivity fo r metals are on the order of 105-106 S/cm based upon n = 1022 cm-3 = 103 cm2/Vs,13 and e = 1.602x10-19 C. For metals, n is temperature-independent as there is no band gap between the HOMO and LUMO, as seen in Figure 1-2. The mobility of charge ca rriers, and consequently the conductivity of a

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18 metal, increases as the temperature is decrease d, since the principal scat tering mechanism is the interaction between mobile electrons and thermal vibrations of lattice atoms. Figure 1-2. Band diagrams showing the diffe rence between metals, semiconductors and insulators. Semiconductors differ from metals since they have fill ed valence bands and e mpty conduction bands. Semiconductors are distinguis hed from insulators by the ease with which electrons can be excited from the valence band to the conduction band. This depends on the band gap, and is generally accepted as around 4 eV between semiconductors and insulators. At room temperature, semiconductors closely rese mble insulators, as few electrons have enough thermal energy to move from the valence band to the conduction band. In intrinsic sem iconductors, electrons in the va lence band can be thermally excited into the conduction band, creating a corresp onding hole in the valence band to allow both free electrons and free holes to contribute to conductivit y. Intrinsic semiconductors have a positive temperature dependence of conductiv ity, since the number of char ge carriers at the certain temperature dominates the overall temperatur e dependence of the conductivity, even though charge carrier mobility is affected by the same scattering process as in a metal.

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19 An extrinsic semiconductor is a semiconductor that has been doped, meaning the number of charge carriers has be increased by introduction of i mpurities, giving the material different electrical properties than the intrinsic (pure) semiconductor. The addition of dopant atoms changes the electron and hole carrier concentrations of the sem iconductor at thermal equilibrium. The carrier density is depende nt on the concentration and na ture of the dopant and less dependent on the tem perature. Extrinsic se miconductors are classi fied as either an n-type (negative) o r p-type (positive) based on domina nt carr ier concentrations. Electron density in insulators is highly localized around atoms, with little overlap between -orbitals of adjacent atoms and completely filled valence bands. The valence and conduction bands are separated by a large gap, which canno t be overcome by thermal excitation. An example of a good insulator is a polymer having only bonds, such as polypropylene. The mechanism of conduction in conjugated polymers has been the subject of considerable research. The most widely accep ted mechanism was proposed in 198114,15 and involves oneelectron oxidation upon initial charge injection (doping) to form a radical cation, or polaron, as seen in Figure 1-3. The charge is delocalized over several rings causing local distortion of the polymer chain, and in polythiophene, for example, charge is thought to be delocalized over at least five rings.16 Polaron formation is associated with an aromatic to quinoi d structural change, which increases overlap and gives rise to intragap energy levels.17 As the polymer becomes saturated with polarons, further doping induces stru ctural relaxation to form a dicationic species with no unpaired electrons, calle d a bipolaron. The bipol aron structure is also thought to be formed by the combination of two polarons, and ha s a greater quinoid charac ter than that of the polaron, which leads to a compression of the intr agap energy levels. The hopping mechanism is

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20 one model of conductivity and attributes the charge mobility of conducting polymers to interchain transfer of bipolarons.18 Figure 1-3. The doping mechanism in polythiophene. The first configuration depicts the neutral polymer. Initial oxidation results in polar on formation yielding a quinoidal structure with charge delocalized over four rings. Further oxidation results in the formation of a bipolaron. Since increased charge carrier mobility leads to higher conductivities, (Eq. 1.1), a major challenge is continuing to raise the carrier m obility and thus conductiv ity, which are currently limited by the defects in polymers. Charge tran sport along an ideal linear polymer chain can proceed no farther than the length of the fully extended chain; then the charge must hop to another chain. With improved or dering of the polymer chains, and appropriate selection of dopants and dopant concentrations conductivities could reach those of even the most conductive metals.

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21 Mechanism of Oxidative Electrochemical Polymerization Electroactive polym er films can be effectively prepared on conducting substrates using a technique known as electrochemical polymerizati on. The technique involves the application of an external potential to a solu tion of monomer in an electrolyt e. The potential required for effective polymerization depends upon the electron dens ity of the monomer; typically electronrich monomers are easier to oxidize, require m ilder conditions, and minimize the occurrence of side reactions such as overoxidation.19 Figure 1-4. Mechanism of electrochemical polymerization in polythiophene. The mechanism is detailed for thiophene in Fi gure 1-4, and occurs in an analogous fashion for other unsubstituted polyheterocycles. Polymerization begins with the one-electron oxidation of the monomer to form reactive, but resonance-stabilized radical cations, at a potential referred to as the monomer oxidation potential. Two radi cal cations can couple, or a radical cation can

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22 couple with a neutral monomer, to form the dicati onic dimer. This species rearomatizes with the loss of two protons, and the process continues to form oligomers and ev entually polymer. Figure 1-5. Coupling pathways dur ing thiophene polymerization. Since the carbon adjacent to the heteroatom in a heterocycl e is the most electron-rich, this is the favored site for coupling of the radical ca tions that are formed. However, while coupling at the 2and 5-positions ( -coupling) for thiophenes (or analogous monomers) may predominate, even a small number of couplings that proceed via the 3and 4-positions ( or -coupling) sacrifices lin earity in the polymer backbone and forfeits many of the associated electronic properties.20 The mechanism of these undesirable couplings is depicted in Figure 1-5.

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23 Such undesired coupling can be easily avoide d by substitutions on the 3and 4-positions of the heterocycle. The mechanism for the oxi dative polymerization of EDOT, whose 3and 4positions are blocked by an ethylenedioxy bridge, illu strates that if both the 3and 4-positions are blocked, only -coupling is possible. Poly (3,4-Dioxythiophene) Based Polymers Poly(thiophene) em erged from among the fir st generation of c onducting polymers as a material of considerable interest due to its stru ctural versatility and stab ility in both the neutral and p-doped states. Since poly(thiophenes) are unstable at the potenti als required for their electrochemical deposition, the monomer has been modified to obtain an extensive family of more easily polymerizable species.21,22 On substitution of the thiophene with an alkylenedioxy bridge, the resulting polymer is afforded a high degree of regioregularity. Polymers based on ethyl enedioxythiophene (EDOT) also have lower oxidation poten tials, desirable electrochromic properties, highe r conductivities and greater environmental stability.23,24 The dioxythiophene bridge in particular is key to the monomers and resulting polymers properties, as simple dialkyl substitution at the 3and 4positions, while preventing undesired coupling, results in steric hindrance that decreases conjugation.25 The monomer oxidation peak for EDOT is found at +0.88 V vs ferrocene (Fc/Fc+)26 while thiophene monomer oxidation27 was reported at +1.22 V vs Fc/Fc+ (assuming the half-wave potential (E1/2) of Fc/Fc+ = 0.38 V vs the saturated ca lomel electrode (SCE) and E1/2 of Ag/Ag+ = 0.26 V vs SCE).28 The alkylenedioxy bridge in PEDOTs also affords a high degree of order due to the lack of or -coupling, resulting in a high conductivity (300-400 S/cm).29 In contrast, lower conductivities were ob served with long alkoxy subsituents, likely due to interactions between adjacent side chains.30 The monomers with these 3-, 4-dialkoxy

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24 substituents were also not easily oxidatively polymerized due to stabilization caused by conjugation with the thiophene.31 The ethylenedioxy bridge main tains the benefits associated with electron-donating groups, while eliminating steric hindrances that other substitutions present. In addition to EDOTs electron-donor prope rties and substituted positions, the oxygen atoms at the 3-and 4-positions of the thiophene ring induce planarity of the -conjugated chain by non-covalent intramolecula r sulfuroxygen interactions.32 Despite these positive characteristics, in order to make EDOT more soluble, substitution of an sp3 carbon atom of the ethylenedioxy bridge is necessa ry. This has several undesired consequences, namely the possible creation of a stereogenic center, a nd the non-coplanarity of the substituent and conjugated system, which increases interchain dist ances and thus alters the conductive properties of the resulting polymer. These limitations can be overcome with a modification of the EDOT structure in order to attach the solubilizing group to an sp2 carbon by replacing the ethylene bridge of EDOT with a phenyl ring. In th e resulting 3,4-phenylenedioxythiophene (PheDOT)33 monomer, the ethylene bridge of EDOT is re placed by a 1,2phenylene moiety allowing further coplanar substitution at the sp2 carbon atoms of the benzene ri ng system. Roncali and coworkers have synthesized the PheDOT (Figure 1-7) monomer by two different routes.34 OO S OO S OO S OO S Figure 1-6. Poly(3,4-phenylen edioxythiophene) (PPheDOT)

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25 Electrochemical and theoretical results fr om the Roncali group indicate that the introduction of a phenyl ring stabilizes the ra dical cation, causing the el ectropolymerization of PheDOT to be considerably more difficult and less efficient than that of EDOT. Analysis of the electrochemical properties of the resulting poly mer reveals a higher oxid ation potential than EDOT. However, PheDOTs co mbinination of the strong donor properties of EDOT analogues with possible solubilization by substitution of sp2 carbons with compact packing arrangement in the solid state makes it an interesting platfo rm for the development of new classes of conjugated systems. Because of the challenges so far with PheDOT electropolymerization, films of sufficient quality for conductivity measurements have not be obtained electrochemically. Some information on the electrical properties of PPheD OT is known from the chemical polymerization using ferric chloride in chloroform.35 Sugimoto and coworkers re ported four-probe conductivity measurements of a compressed pell et on the order of 1 S/cm. PheDOT dimer and trimer have also recently been found to undergo electrochemical polymerization. CV data of the obtained polymers suggest that the increase of the chain length of the precursor increases the effective conjuga tion length of the resulting polymer, which should increase the polymers conductivity. Theoretical calculations indicate th at while delocalization of the unpaired electron over the PheDOT molecule lowers the r eactivity of the radical cation, extension of the conjugated chain to the dimer and trimer leads to a distribution of the singly occupied molecular orbital (SOMO) until it begins to resemble that of EDOT oligomers. The dimer and trimer of PheDOT were found to ha ve lower monomer oxidation potentials than the PheDOT monomer.36

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26 Applications In addition to the acad emic curiosity that co ntinues to attract chemists, physicists, and materials scientists to understa nd the fundamental characteristic s of conjugated electroactive polymers, this unique class of materials presen ts industry and government with a set of highly tunable properties not available in other materials. The commercialization of conducting polymer technologies is a vast a nd varied effort, with new polym ers finding applications both as replacements for existing material and as thrust toward the development of brand-new technologies. Conducting polymers are found as electrode materials for solid-state electrolyte capacitors37 and are also of particular interest in the supercapacitor field, as supercapacitors require not only high capacitance but rapid charge/discharge rates. Polymeric materials have so far shown promise over the more state-of-the-art carbon materials. 38 Another unique application is the development of polymer batteries39,40 and electrode materials for conventional capacitors.41 The first prototype of commercial ba tteries using conducting polymers was based on Li/PAni (BASF/Varta). Other interesting uses for conducting polymers include antistatic coatings,42 transparent electrodes,43 and chemical sensors,44 which respond to analytes based on changes in the polymers conductivity. Biosensor applications45 have recently been studied extensively in response to both military and medicinal need s. The development and exploitation of electrochromic polymers, and the mechanical fl exibility and light weight of these materials allows for fabrication of flexible electronic devi ces and displays. These applications, along with corrosion protection, controlled drug release, membrane and ion exchange, lasers, etc. will continue to endow the conducting polymer field with well-deserved importance.46

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27 Alan Heeger proposed two explanations in hi s Nobel Lecture of the importance of the discovery of conducting polymers: they didnt exist before, and they offer a unique combination of properties not available from any other known materials. Conjugated polymers will certainly continue to enjoy intense academic and commercial interest in the years to come, and allow for the integration of unique, functional, and unprecedented materials into nearly every conceivable aspect of daily life.

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28 CHAPTER 2 EXPERIMENTAL TECHNIQUES The inten t of this chapter is to provide background on the experimental techniques employed in the characterization of conducting poly mers used in this study. These techniques will be referenced in the re mainder of this thesis. Chemicals, Materials and Instrumentation Propylene carbonate (P C) was obtained from Acro s Organics and used as received (99.7%, anhydrous). Tetrabutylammonium perchlorate (TBAP) was prepar ed by mixing a 1:1 mole ratio of tetrabutylammonium bromide dissolved in wa ter with perchloric aci d. The precipitate was filtered, recrystallized from a 1:1 molar ratio ethanol and water and dried in the vacuum oven for 24 hours at 60C. Tetrabutylammo nium tetrafluoroborate (purity 98.0%) (TBABF4) and tetrabutylammonium he xafluorophosphate (purity 99.0%) (TBAPF6) were obtained from Fluka Chemika. Ferrocene (Fe(C5H5)2) used in reference electrode ca libration was obtained from Fluka Chemika and used in 5 mM concentration EDOT monomer (Baytron M V2) was provided by H.C. Starck and distilled under vacuum from CaH2. PheDOT and PheDOT derivatives were synthesized by other members of the Reynolds group and characterized by NMR and mass spectrometry to ensure purity. Indium-tin-oxide (ITO) -coated polished floa t glass slides CG-51IN-CUV (7 50 0.7 mm, Rs= 8-12 ) and CG-51IN-S107 (25.7 mm Rs = 8-12 ) were obtained from Delta Technologies, Ltd. ITO-coated glass slides were wiped with acetone and air dried prior to use. Platinum wire and sheets (metals basis 99.5%) we re purchased from Alfa Aesar, and platinum flag electrodes assembled in the Chemistry Department Machine Shop. Silver wire (0.5 mm diameter, 99.5%) and silver conductive a dhesive paint (Sheet resistance: 0.025 /square @

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29 0.001 thick) were purchased from Alfa Aesar (www.alfa.com). Platinum button and glassy carbon button electrodes and th e electrode polishing kit we re purchased from BASi Figure 2-1. Monomers used in electrochemical studies. (www.bioanalytical.com). Glassy carbon plate electrodes (25 x 25 x 2 mm) were purchased from SPI Supplies ( www.2spi.com ) and further cut to size using a diam ond knife in the Chemistry Department Machine Shop. Contact s to the plate electrodes were made using conductive copper tape (1131) purchased from 3M Quartz cuvettes with 10 mm path length were used in spectroelectrochemical measuremen ts and preliminary film formation studies and were obtained from Starna Cells. Potentials and currents for electrochemical studies were controlled using an EG&G Princeton Applied Research Model 273 potentiostat under control of Corrware II software from Scribner in a three-electrode cell configura tion. A Keithley 197 Autoranging Microvolt DMM was used to measure resistances us ed to calculate conductivities. The electrode polishing kit, obtained from BA Si, consists of two disk pads, a brown velvety Texmet pad and a white nylon pad. After dampening the disk pads with distilled water, several drops of the 1m diamond polish and a several drops of alumina suspension were added EDOT O O S MolecularWeight:190.22 O O S S O O MolecularWeight:378.42PheDOT-Et2 PheDOT biPheDOT

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30 to the nylon and Texmet pads, respectively. The electrode surface was placed on the nylon pad and polished using circular motions. The same procedure was followed on the Texmet pad. Then the electrode was rinsed with methanol or acetone.47 Electrochemical Methods Cyclic Voltammetry for Polymer Deposition A wide variety of electrochem ical methods can be used to study conducting polymer films.48 Cyclic voltammetry (CV) is an effectiv e technique for fundamental electrochemical studies and film formation due to its flexibility and the wide variety of information generated from a single experiment. Reduction and oxidation potentials for both the monomer and polymer can be located with CV, as well as information regarding the stability of the product during multiple switching cycles. The potential scan rate can be controlled, allowing for monitoring of both fast and slow reactions. CV allows for the formation of polymer film and subsequent characterization in one experiment, as the firs t forward scan generates a new redox species and subsequent scans characterize th e outcome of this reaction. In CV, the reducing or oxidizing strength of the working electrode is precisely controlled by the applied potential. Polymerization of electr on-rich monomers starts at low potentials. As the potential is increased, the m onomer oxidizes to its radical ca tion, which couples with another radical cation or neutral species to afford oligom ers in the vicinity of the electrode. As the oligomers increase in length, they precipitate on to the surface of the work ing electrode. Polymer deposition can be observed by the increase in the polymers anodic peak current (ipa), and cathodic peak currents (ipc) during repeated scans.

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31 Electrochemical Cell Setup Electrochemical studies and polym erizations were performed using a three-electrode cell configuration potentiostatically, galvanostically or potentiodynami cally. The potentiostat applies potential between a working electrode (WE) and a reference electrode (RE). The three-electrode cell configuration eliminates th e potential drop error that arises because of the resistance of solution. The reaction being studi ed takes place on the surface of the WE, while the current response is recorded through the working and counter electrodes. The schematic of the threeelectrode cell is given in Fi gure 2-1. A small glass cylindric al cell is filled with 0.1 M electrolyte/monomer solution and the working, counter and refere nce electrodes are immersed in the solution. The platinum butt on or glassy carbon button elec trode was polished prior to use using the polishing kit. The pa rticular platinum flag counter electrode selected for each experiment had a surface area gr eater than that of the worki ng electrode and was cleaned by firing in a butane torch. Due to the variety of solvents used in this series of el ectrochemical studies, a pseudo reference Ag wire electrode was used as the reference electrode. Ag wire was cleaned using sandpaper, rinsed with acetone, im mersed directly in solvent, and calibrated before and after each experiment with a solution of ferrocene/ferrocinium (Fc/Fc+) by using 5 mM ferrocene/0.1 M electrolyte solution. Before and after each ex periment, the cell was filled with the ferrocene solution and the potential ra nge scanned twice. The average of the anodic (Epa ) and cathodic (Epc) peak potentials yielded the half-wave potential of the Fc/Fc+ versus the Ag wire. This unique E1/2 value was subtracted from the experimental potentials for each experiment to convert potentials relative to Ag wire to Fc/Fc+. Prior to electropolymeriz ation, solutions were de-

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32 oxygenated by bubbling argon for ten minutes, and the entire cell was maintained under an argon blanket during the experiment. Figure 2-2. Schematic of three-electrode electrochemical cel l configuration. Film Formation Potentiostatic, galvanostatic and potentiodyna m ic electrochemical pol ymerization studies were carried out in a 0.1 M el ectrolyte/10 mM mono mer solution unless otherwise noted. For electrodeposition on a platinum button (0.02 cm2) electrode or glassy carbon button (0.07 cm2) electrode, the three-elec trode cell with a platinum flag c ounter electrode, and silver wire reference electrode was used. The polymers presented in Chapter 3 were prep ared electrochemically using 2 cm x 2 cm glassy carbon plate as the working electrode (F igure 2-3). During polymerizations, the area submerged in monomer/electrolyt e solution was 2 cm x 1 cm. Pr ior to use, glassy carbon plate electrodes were polished using the polishing k it, immersed in acetone, then washed with deionized water and dried under argon. Leads were attached to the glassy carbon plates using copper tape which covered the enti re width of the electr ode. A silver wire reference was used, and a large platinum flag or stainless steel coun ter electrode, with a surface area equal to or

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33 greater than that of the worki ng electrode. The elec trochemical cell was kept under a blanket of argon during polymerizations. Figure 2-3. Electrochemical cell showing se tup for film formation experiments. Low temperature polymerizations were conducted at 0C using an ice bath and at C by using a dry ice/ACN bath. For low temperatur e film deposition requiring long polymerization time (up to 24 hours), the electroche mical cell and cooling bath were kept in a cooler to maintain a constant temperature overnight. Due to both the increased surface area of the electrode and goal of forming free-standing th ick films, a higher monomer c oncentration is required than common for thin film deposition on ITO (10mM). Accordingly, free-standing films were prepared from 60 mM monomer in 0.1 M electrolyte dissolved in a low vapor pressure solvent, usually propylene carbonate (PC), which acts as a plasticizer for the resulting film. If the monomer was found to have reduced or limited sol ubility in the solvent, the concentration was limited to 10 mM or 20 mM monomer. Films we re removed from the el ectrode using a razor blade, either directly after polymerization or after drying on the electrode. The films were washed in solvent to remove monomer, placed between two Kimwipes and pressed between glass slides to flatten.

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34 Conductivity Measurements All film s were measured with two probe c onductivity measurements in order to identify the most highly conductive films, and determine whether the timing of the measurement (five minutes after removal, after dr ying, after heating in oven, and ove r time) had an effect on the conductivity. Standard Two-Probe Conductivity Measurements Two-probe conductiv ity measurements are derived from Ohms Law. The surface resistance (Rs) of free-standing PEDOT films and PPh eDOT films on non-conducting glass was measured. The surface resistance (Rs) is the ratio of potential (V) applied to two parallel electrodes to the current flowing between the electrodes. Silver paint contacts were deposited on the films in parallel to each ot her. The resistance between the parallel silver paint electrodes was measured with a Keithley 197 Autoranging Mi crovolt DMM and used to calculate the conductivity. Rs a b t Figure 2-4. Schematic for a standard twoprobe surface resistivity measurement. Four-Probe Conductivity Measurements Four-probe measurements have several advant ages for measuring electrical properties of conducting polymers. This method eliminates errors caused by contact resistance, as the current is passed through different contacts than those through which the voltage is measured. Measurements can be made on both free-standing rigid films and thin films on non-conducting substrates. Electrochemical formation of high-quality free-standing films is of great interest since many of these

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35 polymers are not soluble. Removal of the deposited material from the working electrode can be a challenge if not enough material has deposited on the electrode or the film is not of high enough quality to be removed intact. In these cases, it can often be removed using adhesive tape. Three methods of four probe conductivity techniques can be employed in the study of conducting polymers: Van der Pauw49, four-wire50 and four-point probe or Signatone.51 The selection of a particular method depends on sample quality and geometry and the instrumentation that is available. Particularly when temperature dependent conductivity measurements are to be taken, and the entire setup placed in a cryostat, a four-probe technique is appropriate. For room temperature measurements, conductive silver adhesive paint (Alfa Aesar) was used to attach four thin silver wires (Alfa Aesar, 0.5 mm diameter, 99.9%) to the film. The distance between contacts 2 and 3 (see Figure 2-5) is maximized relative to the distance between contacts 1 and 2, or 3 and 4. Volume conductivity is calculated from the following equation: = l/Rtw (2.1) where l is the distance between inner leads 2 and 3, t is the film thickness and w is the width of the sample. Sample thicknesses were measured with a micrometer. Keithley 2400 source measurement unit (SMU) was used in four-probe measurements. Figure 2-5. Setup for a four-w ire conductivity measurement.

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36 Temperature Dependent Conductivity Measurements Data was acquired at Crosslink USAs JVIC Laboratory in S pringfield, Missouri using a Temperature Dependent Electrical Transport Pr operties System consisting of three main components: a cryostat dewar, two measuremen t devices, and the computer interface for automation. The cryostat dewar was acquired fr om Cryomagnetics, Inc. and holds liquid cryogen as well as the sample holder insert. Measurement were taken using a Lakeshore Model 340 Temperature controller and a Keithley Model 2400 SMU. LabView software was used for autonomous data acquisition via the two meas urement units and the computer interface. The sample width used to acquire four probes measurements was 0.64 cm and the length was 0.16 cm.

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37 CHAPTER 3 RESULTS AND DISCUSSION The chapter provides experim ental results for electrochemical polymerization studies and conductivity measurements of EDOT, PheDOT, PheDOT-Et2 and bi-PheDOT. 3,4-Ethylenedioxythiophene (EDOT) Studies The electrochem ical polymer ization of thiophene deri vatives involves numerous experimental variables including solvent, concentr ation of monomer or el ectrolyte, temperature, and applied electropolymerization methods. Electrosynthesis condi tions have a great influence on the morphology and conducting properties of the resulting polymer. In order to determine optimal conditions for electrochemical polymeriz ation of EDOT, and formation of PEDOT freestanding films on large surface area electrodes, fundamental electrochemical studies were performed on a platinum bu tton electrode (A = 0.02 cm2). Various electrochemical polymerization methods have been applied: potentiodynamic, potentiostatic and galvanostatic techniques. Different s upporting electrolyte systems such as 0.1 M TBAP, TBABF4, TBAPF6, LiBTI and EMI/BTI (ionic liquid) in ACN and PC were used to pr obe the effect of counterions and solvent nature on electrochem ical properties of PEDOT films. Electrochemical synthesis was carried out at room temperature and reduced temperatures. Details for EDOT studies are given in the remainder of this section. All solv ents were distilled and pur ged with argon prior to use; electrolyte salts were recrystallized and dried in a vacuum oven. Electrolyte and monomer purity can be a significant cont ributor to resulting polymer pr operties. In addition, fresh monomer solutions were prepared at the start of each days expe riments due to the often highly reactive nature of electr on-rich heterocyclic monomers. PEDOT was used to help determine the best polymerization conditions for film forma tion before moving into the PheDOT, PheDOT-Et2 and bi-PheDOT monomers. EDOT is commerically available and well-characterized.

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38 Monomer Properties and Electrochemistry on Pt Button Electrode PEDOT was electrochem ically deposited on a platinum button working electrode in a standard three-electrode setup with a platinum flag count er electrode, and a s ilver wire reference electrode calibrated to Fc/Fc+. The three electrodes were subm erged in monomer solution. Prior to each experiment, the solution was purged by bubbling with argon, and the cell was maintained under an argon blanket during polymerization studie s. Electrochemical deposition for studies on a platinum button electrode were carried out in 10 mM EDOT in 0.1M TBAP/ACN. Potentiodynamic deposition Figure 3-1A shows the first scan of EDOT monomer polymerization in 10 mM EDOT in 0.1M TBAP/ACN. Monomer oxidation is marked by the sharp increase in current, followed by a peak, which is referred to as the monomer oxidation potential, determined to be at +1.1 V vs Fc/Fc+. The reverse scan demonstrates two featur es that are indicative of the deposition of electroactive species. The first is the nucleation loop, commonly s een in the first CV scans of potentiodynamic preparation of conducting polymers. This phe nomenon arises because of polymer having deposited onto the working electrode, increasing the c onductive surface area. The second feature is the increas e in current response between -0.2 V and -0.7 V in the reverse scan and is indicative of reduction of the polym er that was deposited on the working electrode. Repeated scans show evidence of thin insol uble polymer film formation on the electrode surface. Figure 3-1B shows the first to sixt h scans of PEDOT polymerization. Continually increasing current responses for polymer oxidati on and polymer reduction indicate deposition of a porous, conductive polymer film on the electrode surface.

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39 -1.5-1.0-0.50.00.51.01.52.0 -5 0 5 10 15 20 25 Nucleation Loop EpA 1.1 V vs Fc/Fc + Current Density (mA/cm 2 )Potential (V vs Fc/Fc + ) (A)-1.5-1.0-0.50.00.51.01.52.0 -30 -15 0 15 30 45 60 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) (B) Figure 3-1. (A) First CV scan of EDOT deposition on a platinum button (A = 0.02 cm2). The monomer oxidizes at +1.1 V vs. Fc/Fc+ in 10mM EDOT/0.1M TBAP/ACN with a scan rate of 50 mV/s. (B) First through sixth CV scans of EDOT deposition from 10mM EDOT/0.1M TBAP/ACN with a scan rate of 50mV/s on a platinum button (A = 0.02cm2). The working electrode was removed from monomer solution after repeated scan electropolymerization and rinsed with monomer-fr ee electrolyte so lution. Dark blue film was visible on the surface of the platinum button elect rode. The rinsed electrode was replaced in monomer-free electrolyte solution. The region of polymer oxidation and reduction was isolated and scanned using CV to ascertain the scan rate dependence as shown in Figure 3-2A. In scan rate dependence experiments, the polymer, st ill adhered to the pl atinum button working electrode, is cycled between oxidi zed and reduced states at various scan rates while the peak anodic and cathodic current responses are monitored. Since the redox processes of electrode-bound conjugated polymers are not diffusion controlled, both the anodic and cath odic current responses will scale linearly with scan rate as given by Equation 3-1. ip = n2F2A / 4 RT (3-1)

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40 where F is Faradays constant (96,485 C/mol), n is the number of electrons, A is the surface area of the wo rking electrode (cm2), is the scan rate (V/s), and is the concentration of surface bound electroactive centers (mol/cm3). -1.5-1.0-0.50.00.51.0 -80 -60 -40 -20 0 20 40 60 80 Current Density (mA/cm 2)Potential (V vs Fc/Fc + ) 10 mV/s 200 mV/s(A)0.000.050.100.150.200.25 0 15 30 45 60 75 90 Current Density (mA/cm 2 ) Scan Rate (V/s) (B) Figure 3-2 (A) CV of PEDOT film deposited on a Pt button electrode at scan rates between 10 and 200 mV/s demonstrated increasing current response at increas ing scan rates. Film was deposited in a 10mM EDOT/0.1M TBAP/ACN solution and switched in a background solution of 0.1M TBAP/ACN. (B) Linear dependence of peak current on the scan rate for PEDOT deposition in 10mM EDOT/0.1M TBAP/ACN on a platinum button (A = 0.02cm2). As plotted in Figure 3-2B, the linear dependenc e of peak current re sponse on scan rate indicates the redox process is non-diffusion controlled, and the el ectroactive centers of the polymer are well adhered to the working electrode surface. Since switching speeds are dependent upon f ilm thickness, electrolyte, and polymer structure, scan rate dependence studies are im portant in understanding wh ether a polymer can be switched between redox states ra pidly without loss of current response or switching stability. Galvanostatic deposition PEDOT was also deposited on a platinum button galvanostatically at applied currents of 1.0, 2.5, 5.0 and 10 mA/cm2 as shown in Figure 3-3. In th is experiment, the potential was monitored at a constant current. The observed po tential initially increased rapidly followed by a

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41 gradual decrease, since the conductive PEDOT depos its more efficiently on polymer rather than the bare electrode. 051015202530 0.5 0.6 0.7 0.8 0.9 Potential (V vs Fc/Fc+)Time (s) 10 mA/cm25 mA/cm21 mA/cm22.5 mA/cm2 Figure 3-3. Galvanostatic deposition of PE DOT in 10mM EDOT/0.1M TBAP/ACN on a platinum button (A = 0.02cm2). Potentiostatic deposition Potentiostatic deposition of EDOT ranging from 0.82 V to 1.25 V is shown in Figure 3-4. In this method, the current density is measured as a function of a constantly applied potential. The potentiostatic regime for PEDOT wa s found between 0.82 and 1.25 V vs Fc/Fc+. Peak current densities of PEDOT films de posited potentiostatically between 0.82V and 1.25 V vs Fc/Fc+ for three minutes are shown in Figure 3-5 and indicate that more polymer deposited during the experiments in which the film was formed at higher pot entials. Decreasing cathodic and anodic currents and the shape of voltammogram starting from films formed above 1.2 V indicates start of the over-oxidation process of the film.

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42 0255075100125150175200 0 20 40 60 80 100 120 140 0.82V 0.85V 0.90V 0.95V 1.0V 1.05V 1.1V 1.15V 1.20V 1.25VCurrent Density (mA/cm2)Time (s) Figure 3-4. Potentiostatic deposition of EDOT on Pt button (A = 0.02 cm2) revealed a potentiostatic regime between 0.82 and 1.25 V vs Fc/Fc+ for 180 seconds in 10mM EDOT/0.1M TBAP/ACN. 0.80.91.01.11.21.31.4 0 5 10 15 20 25 30 35 40 45 Current Density (mA/cm 2 )Potential (V vs. Fc/Fc + ) Figure 3-5. Peak current densities of PEDOT films deposited potentiostatically between 0.82V and 1.25 V vs Fc/Fc+ for 180 seconds at 10mM monomer concentration on a Pt button (A = 0.02 cm2) in 0.1M TBAP/ACN (scan rate of 50mV/s). The larger current responses for films deposited at higher pot entials indica ted that more polymer had deposited.

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43 Film Formation Experiments Many experim ental variables including monomer concentration, polymerization temperature, solvent and electrolyte choice, cell geometry and elec trode material are involved in the electropolymerization of thiophe ne derivatives. Since these parameters can be both widely varied and closely controlled, electrosynthesis conditions can have a large influence on the morphology and electrical properties of the resulti ng polymer. These experimental variables are naturally interdependent and optimization st udies are complex, requiring relatively large amounts of monomer. PEDOT was used to de termine the best geometric conditions for the formation of free-standing film with a functional si ze surface area for conductivity measurements while minimizing the amount of solution and t hus the amount of monomer necessary for each electrochemical polymerization. EDOT film formation was completed pot entiostatically, galvanostatically, and potentiodynamically. Potentiostatic and galvanostatic methods yielded films with large areas of uneven polymer deposition, even at lo w temperatures with optimized solvent/electroly te systems. Potentiodynamic deposition was found to be more effective and free-standing films were successfully formed using this method at both room and low temperatures. Galvanostatic and potentiostatic film formation Initial film s were formed by slow galvanostatic deposition at an applied current of 0.02 to 0.04 mA/cm2. The films were deposited in a round el ectrochemical cell in which a glassy carbon working electrode and stainless st eel counter electrodes are suspe nded in parallel and separated by 2 mm. Polymer was deposited using all co mbinations of PC or ACN and TBAP, TBABF4 or TBAPF6 and could be removed from the surface of the glassy carbon electrode using tape. Free-standing films were not obt ained using this method, with deposition times up to 14 hours. The current densities observed were much lower than for the platinum button experiments, which

PAGE 44

44 can likely be attributed to the electrode material Current densities on th e order of those seen on the platinum button were observed when a large platinum flag was used for film formation experiments. Initial film formation was attempted by potentios tatic deposition at an applied potential of 1.2 V vs Fc/Fc+. The films were deposited in a round el ectrochemical cell in which a glassy carbon working electrode and stainless steel counter electrodes were susp ended in parallel and separated by 2 mm. Polymer de position was attempted using all combinations of PC or ACN and TBAP or TBAPF6 and was unsuccessful as the potential was not able to be held constant for more than several minutes. Solvent and electrolyte effects A viable solvent m ust have a high dielectric constant, which ensures ionic conductivity of the electrolyte in solution, and a sufficiently hi gh electrochemical break down potential to guard against decomposition at the pot entials required for monome r oxidation. Most conductive polythiophene derivatives have been prepared in low nucleophilicity, high dielectric constant, anhydrous, aprotic solvents including benzonitrile, acetonitrile, nitrobenzene and propylene carbonate.52 In ACN solvent systems, PEDOT showed an electrochemical response indicative of polymer deposition. However, the ACN swelle d any film that formed and the deposited material, which dried rapidly, delaminated unevenl y from the surface of the electrode. The use of PC as a solvent prevented this rapid drying an d gave more even film formation and eventually glossy films. PEDOT was electrosynthesized in the presence of small anions derived from the tetrabutylammonium salts of ClO4 -, PF6 -, BF4 -, CF3SO3 -, lithium bistrifluoromethylsulfonylimide (Li-BTI) and ethyl-m ethyl-imidazolium BTI (EMI-BTI). Because

PAGE 45

45 the ions are incorporated into the polymer, the nature of the dopant affects the morphology and the electrical properties of the resulting film. Figure 3-6 shows the first ten scans of a CV of PEDOT deposited potentiodynamically on the 2 cm2 glassy carbon plate electrode in 0.1M TBAP/PC at a scan rate of 50mV/s at 0C. After ten scans, current densities over 1 mA/cm2 were observed. Current densities continued to gradually increase for films formed with up to ov er 800 CV scans, as shown in Figure 3-7. The highest current densities for film formation were observed usi ng TBAP as the electrolyte; as seen in Figure 3-8, after ten scans using TBAPF6 as the electrolyte in the same conditions, current densities were still less than 1 mA/cm2. Lower current densities can likely be attributed to electrode material, as film formation on 2 cm2 platinum flag working electrodes yielded higher current densities than glassy carbon, but polymer ized material cannot be delaminated from platinum. -0.50.00.51.01.5 -1 0 1 2 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) Figure 3-6. CV showing EDOT deposited potenti odynamically on the large GC plate electrode. 0.1M TBAP/PC (scan rate of 50mV/s) at 0C. Current densities over 1 mA/cm2 were observed after ten scans.

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46 -0.50.00.51.01.5 -8 -6 -4 -2 0 2 4 6 8 Current Density (mA/cm 2 )Potential (V vs Fc/Fc + ) 800 Scans Figure 3-7. CV showing the continued deposition of PEDOT on a large GC plate electrode. 0.1M TBAP/PC (scan rate of 50mV/s) at 0C. Current response increased dramatically after 800 sc ans as large amount of material had deposited. -0.50.00.51.01.5 -1 0 1 2 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) Figure 3-8. CV showing EDOT deposited potenti odynamically on the large GC plate electrode. 0.1M TBAPF6/PC (scan rate of 50mV/s) at 0C. Current densities under 1 mA/cm2 were observed after ten scans.

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47 Polymerization temperature effects Initial g alvanostatic and potentiostatic experiments performed by immersing the electrochemical cell in a liquid nitrogen/ACN bath (C) or and ice bath (0C) presented significant challenges with current or potential dr ift. This effect seemed to be exacerbated by lowering the temperature. However, lowering the temperature to 0C prove d to be the best for CV film formation. Polymerization in an ice bath allowed for the potentiodynamic formation of the first freestanding films. Lowering the temperature allowed for the formation of films with a longer mean conjugation length and higher condu ctivities, as the act ivation energy of unde sirable reactions (termination, irregularities in th e polymer backbone) is higher th an the activation energy for the polymerization reaction. Conductivity Measurements Room temperature measurements The results obtained for PEDOT film s elect ropolymerized in various conditions are presented in Table 3-1. The valu es reported are for the highest quality films obtained with the specified set of parameters. Unless otherwis e indicated, conductivity measurements of freestanding films were taken by four probe method using the resi stance output from the Keithley 2400 in four-point probe me thod and calculated using = l/Rtw. The sample width was kept at 1 cm +/0.3 cm, with the length between 0.2 and 0.4 cm. The conductivity of the film depends strongl y upon polymerization te mperature, solvent choice and dopant ion choice. Films produced at 0C had markedly higher conductivities than the films produced at room temperature. Since the activation energy of si de reactions such as termination, backbone irregularities, etc., is higher than the activation energy for the polymerization reaction, thus films deposited at low temperatures (0C) have a longer mean

PAGE 48

48 conjugation length and consequent ly higher conductivities. Po lymer deposition occurs more slowly at lowered temperature which allows fo r more even film formation, and film morphology has been shown in a number of studies to have a marked effect on conductivity. Furthermore, solvent choice has a drastic impact on conductivity. When PEDOT was polymerized in ACN, it formed powdery, brit tle films which were not free-standing and exhibited conductivities on the order of 10-6 to 10-4 S/cm. PEDOT films deposited in PC showed the best conductiviti es, up to 120 S/cm. Regardless of solvent choice or polymerization temperature, th e nature of the counter ion strongly affects the cond uctivity of the film. PEDOT films prepared in the presence of ClO4 show higher conductivities than the ones prepared with other dopants. The size of the ClO4 may facilitate more ordered packing of the polymer chains, increa sing the interchain transport contribution to the overall conductivity. The conductivity values follow the sequence ClO4 > PF6 >BF4 >EMI-BTI/Li-BTI, for deposition at both room temperature and low temperature. Films were not successfully deposited in CF3SO3 Table 3-1. Conductivity measurements for PEDOT film s formed using 800 CV cycles. Monomer Solvent Polymerization Temperature Anion Thickness (microns) (S/cm) EDOT ACN 27C or 0C PF6 Not free-standing 10-4 EDOT ACN 27C or 0C ClO4 Not free-standing 10-4 EDOT ACN 27C or 0C BF4 Not free-standing 10-5 EDOT ACN 27C or 0C Li+BTI-, EMIBTI Not free-standing 10-6 EDOT PC 0C PF6 200-300 70 EDOT PC 27C PF6 200-300 15 EDOT PC 0C ClO4 300-400 120 EDOT PC 27C ClO4 300-400 22 EDOT PC 0C BF4 200-300 60 EDOT PC 27C BF4 100-200 12 EDOT PC 0C CF3SO3 Not free-standing 10-1 EDOT PC 0C Li+BTI-, EMIBTI Not free-standing 10-2

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49 Low temperature measurements The m ost straightforward method to investigat e the possibility of metallic conductivity in conducting polymers is to measure the conductiv ity over a broad range of temperatures. Semiconductors and insulators exhibit behavior in which the electrical conductivity increases with increasing temperature, as more charge ca rriers thermally excited across the band gap are able to contribute to the conductiv ity. In contrast, the conductivity of a metal decreases as the temperature increases, since increased electronlattice scattering decreases the charge carrier mobility. Figure 3-9 shows the temperature depe ndence of PEDOT films formed under various conditions. The conductivity values were meas ured using the four-point probe method and calculated using = l/Rtw. The sample width was kept at 0.64 cm, with the sample length at 0.16 cm. 50100150200250300 0.0 0.2 0.4 0.6 0.8 1.0 (T)/(300K)T (K) Deposited in PC/TBAP at 27oC Deposited in PC/TBAP at 0oC Deposited in PC/TBAPF6 at 0oC Figure 3-9. Temperature dependence of conductivity of PEDOT deposited potentiodynamically in various conditions. ( 300 K, = 65 S/cm, = 100 S/cm, = 60 S/cm)

PAGE 50

50 The conductivity of PEDOT deposited in 0.1M TBAP/PC does not decrease too much at low temperatures, maintaining over half of its room temperature conduc tivity value as the temperature approaches 50 K. This suggests th at the sample has a lower activation energy. Changing the dopant to TBAPF6 leads to more disordered materials with a stronger temperature dependence of the conductivity and loss of nearly 90% of the room temperature conductivity as the temperature approaches 50 K. Diethyl Phenylenedioxythiopene Studies PheDOT-Et2 was electrochemically polymerized in ACN and PC using TBAP, TBABF4, TBAPF6, LiBTI, and EMI/BTI (ionic liquid) as the electrolyte. Th e strongest current responses for PheDOT-Et2 were seen using an ACN/TBAP/monomer system. While monomer was successfully deposited on GC plate electrodes, high-quality free-standing films were not obtained using PheDOT-Et2. Details for PheDOT-Et2 studies are given in the re mainder of this section. Electrochemistry on Pt Button El ectrode and Monomer Properties PheDOT-Et2 was studied using cyclic voltammetry in a standard three-electrode setup with a platinum button or gla ssy carbon working electrode, a pl atinum flag counter electrode, and a silver wire reference electrode calibrated to Fc/Fc+. Solutions were purged with argon, and maintained under an argon blanket during polymer ization. Unless otherwise indicated, monomer concentration was 10 mM for button electroche mistry and 60 mM for film formation and electrolyte concentration was 0.1 M. Figure 3-10 shows the deposition of 10 mM PheDOT-Et2 in 0.1 M TBAP/ACN. The inset of the first scan shows monomer oxi dation occurring at +1.1 V vs Fc/Fc+. In contrast to a similar first scan of EDOT, the PheDOT-Et2 did not demonstrate the charac teristic nucleation loop often seen on first CV scans of conducting polymers. The increase in current response on the reverse scan was also not yet evident on the first scan, indicating that not enough material had deposited

PAGE 51

51 on the electrode for current dens ities to reflect polymer reducti on. However, increasing scans indicated polymer deposition, with polymer oxi dation and reduction yielding peak anodic and cathodic currents of 6 mA/cm2 after twenty scans. CVs of PheDOT-Et2 deposition indicate that the cati on radical that forms on monomer oxidation has a reduction back to th e neutral form and is not coupling at a sufficiently fast rate. This is evident on both the first (Figure 3-10 inse t) and subsequent scans and may contribute to challenges encountered later during film formation experiments. -1.0-0.50.00.51.01.5 -10 -5 0 5 10 15 20 25 30 Current Density (mA/cm 2 )Potential (V vs Fc/Fc + ) Cation radical reduction Figure 3-10. First (inset) through tw entieth CV scans of PheDOT-Et2 deposition (10mM PheDOT-Et2/0.1M TBAP/ACN) with a scan rate (Vs) of 50mV/s.on a platinum button (Area = 0.02 cm2). The increase in anodic a nd cathodic current responses is indicative of polymer deposition. In TBAP /ACN, which was found to be the optimal system for PheDOT deposition (6 mA/cm2 peak polymer current density response after 20 scans) on a Pt Button electrode, th e forming polymer yielded current density responses about one-qua rter those of EDOT. The PheDOT-Et2 polymerized onto the working elec trode was removed from monomer solution after ten scans of de position and rinsed with monome r-free electrolyte solution.

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52 Translucent blue film was visible on the surface of the platinum button electrode. The rinsed electrode was replaced in monomer-free electrolyte solution. The region of polymer oxidation and reduction was isolated and scanned using CV to study the polymer switching stability as shown in Figure 3-11. PheDOT-Et2 showed good stability to 200 scans on a platinum button in ACN/TBAP, switching between re dox states rapidly without loss of current response or switching stability. -1.0-0.50.00.51.0 -4 -3 -2 -1 0 1 2 3 4 5 6 Current Density (mA/cm 2 )Potential (V vs Fc/Fc + ) First switch 200 switches Figure 3-11. CV scans of PheDOT-Et2 switching in 0.1M TBAP/ACN with a scan rate of 50mV/s indicate stability for up to 200 cycles on a platinum button (Area = 0.02 cm2). CV was used to determine the sc an rate dependence of PheDOT-Et2 in ACN/TBAP as shown in Figure 3-12. The PheDOT-Et2 deposited on the platinum button working electrode was cycled between oxidized and reduced states at scan rates between 50 and 200 mV/s while the peak anodic and cathodic current resp onses were monitored. PheDOT-Et2 shows a linear

PAGE 53

53 dependence of peak current respon se on scan rate which indicates the electroactive centers of the polymer are well adhered to the working electrode surface. -1.0-0.50.00.51.0 -15 -10 -5 0 5 10 15 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) 200 mV/s 50 mV/s Figure 3-12. CV of PheDOT-Et2 film deposited on a Pt button be tween 50 and 200 mV/s indicate increasing current response at increasing scan rates. Film was deposited in a 10mM PheDOT-Et2/0.1M TBAP/ACN solution and sw itched in a background of 0.1M TBAP/ACN. Scan rate dependence of the first CV of deposition confirms that the PheDOT-Et2 cation radical couples much more slowly than the EDOT cation radical. Figure 3-13 shows a detail of the first CV scan of PheDOT-Et2 on a platinum button electrode at scan rates of 5, 10, 25, 50, 75, and 100 mV/s. The cation radical reduction becomes less prominent at slower scan rates, which reflect the slower cat ion radical coupling. PheDOT-Et2 deposition on a platinum button wa s carried out using TBAP, TBAPF6, TBABF4, TBA-triflate, Li-BTI, and EMI-BTI to determine the electrolyte that yielded the highest current density and the mo st amount of polymer deposition.

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54 0.5 1.0 0 5 10 Current Density (mA/cm2)Potential (V vs Fc/Fc+) First scan at 100 mV/s First scan at 5 mV/s 5 mV/s 100 mV/s Figure 3-13. Detail of PheDOT-Et2 monomer oxidation on the first CV scan at scan rates of 5, 10, 25, 50, 75, and 100 mV/s on a platinum button (Area = 0.02 cm2). The cation radical reduction becomes less prominent at slower scan rates, indicating the coupling is taking place, but at a much slower rate than for EDOT. Figure 3-14 shows twenty scan s of a CV of PheDOT-Et2 deposited on a platinum button using LiBTI as the electrolyte. Polymer depos ition on the button electrode is evident from the increase in cathodic and anodic currents with current densiti es reaching 1 mA/cm2 after 20 scans. These current densities are 25% of those observe d using TBAP as the electrolyte of PheDOT-Et2 polymerization in the same conditions. Between -0.2 V and 0.8 V vs Fc/Fc+, PheDOT-Et2 shows capacitive response in this so lvent-electrolyte system. PheDOT-Et2 deposition occurs much more slowly usi ng an ionic liquid as the electrolyte. Figure 3-15 shows PheDOT-Et2 deposited using the ionic liq uid EMI-BTI produced a slower increase in cathodic and anodic cu rrents than other electrolytes. After 20 scans, current densities reached only 0.2 mA/cm2, or 5% of those observed using TBAP for PheDOT-Et2 polymerization in the same conditions.

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55 -0.40.00.40.81.2 -3 0 3 6 9 12 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) Figure 3-14. First through twentieth CV scans of PheDOT-Et2 deposition (10mM PheDOTEt2/0.1M LiBTI/ACN) with Vs = 50mV/s on a platinum button (Area = 0.02cm2). The increase in anodic and cathodic current responses indicated polymer deposition. In ACN/LiBTI, the forming polymer yielded current density responses of 1 mA/cm2 after 20 scans. -0.50.0 0.5 1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Current Density (mA/cm 2) Potential (V vs. Fc/Fc+) Figure 3-15. First through twentieth CV scans of PheDOT-Et2 deposition (10mM PheDOTEt2/0.1M EMI-BTI/ACN) with a scan rate (V s) of 50mV/s.on a pl atinum button (Area = 0.02cm2). The increase in anodic and cathodi c current responses indicated polymer deposition. In ACN/EMI-BTI, the forming pol ymer yielded current density responses of under 0.25 mA/cm2 after 20 scans.

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56 Figure 3-16 shows ten scans of a CV of PheDOT-Et2 deposited on a platinum button using TBAPF6 as the electrolyte. Polymer deposition on the button electrode is evident from the increase in cathodic and anodic currents with current densiti es reaching 1 mA/cm2 after ten scans. Further deposition to twenty scans, as seen in Figure 3-17, yielde d current densities of 2 mA/cm2, or 50% of those observed using TBAP as the electrolyte of PheDOT-Et2 polymerization in the same conditions. After twenty scans, the PheDOT-Et2 polymerized onto the working electrode was removed from monomer solution and ri nsed in monomer-free TBAPF6 solution. The translucent blue film was visible on the surface of the platinum button electrode. Afte r replacing the rinsed electrode in monomer-free electrolyte solu tion, the region of polymer oxida tion and reduction was isolated and the polymer film switched for 100 cycles as shown in Figure 3-18. The film showed little loss of current response af ter 100 switches on a platinum button in ACN/TBAPF6. 0.0 0.5 1.0 -2 0 2 4 6 8 10 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) Figure 3-16. First through te nth CV scans of PheDOT-Et2 deposition (10mM PheDOT-Et2/0.1M TBAPF6/ACN) with a Vs = 50mV/s on a platinum button (A = 0.02cm2). The increase in anodic and cathodic current re sponses indicated polymer deposition. In TBAPF6/ACN, the polymer yielded current density responses of 1 mA/cm2 after 10 scans.

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57 0.0 0.5 1.0 -4 0 4 8 12 16 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) Figure 3-17. First through twen tieth CV scans of PheDOT-Et2 deposition (10mM PheDOTEt2/0.1M TBAPF6/ACN) with a scan rate (V s) of 50mV/s.on a pl atinum button (Area = 0.02cm2). The continued increase in anodic and cathodic curre nt responses is indicative of further polymer deposition. In TBAPF6/ACN, the forming polymer yielded current density responses of 2 mA/cm2 after 20 scans. -1.0-0.50.00.51.0 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) 100 Switches First switch Figure 3-18. CV scans of PheDOT-Et2 switching in 0.1M TBAPF6/ACN indicate film stability for up to 100 cycles on a pl atinum button (Area = 0.02cm2).

PAGE 58

58 The scan rate dependence of PheDOT-Et2 in ACN/TBAPF6 is shown in Figure 3-19. Between 25 and 200 mV/s, PheDOT-Et2 shows a linear dependence of peak current response on scan rate, indicative of good adhe sion of polymer electroactive centers on the platinum button electrode surface. -0.5 0.0 0.5 1.0 -3 -2 -1 0 1 2 3 200 mV/sCurrent Density (mA/cm2)Potential (V vs Fc/Fc +)25 mV/s 306090120150180210 0.0 0.5 1.0 1.5 2.0 2.5 Current Density (mA/cm 2 )Scan Rate (mV/s) (B) Figure 3-19. (A) CV of PheDOT-Et2 film deposited on a Pt button between 25 and 200 mV/s indicates increasing current res ponse at increasing sc an rates. Film was deposited in a 10mM PheDOT-Et2/0.1M TBAPF6/ACN solution and switched in a background of 0.1M TBAPF6/ACN. (B) Linear relationship betw een scan rate and peak current density. Film Formation PheDOT-Et2 electrochemical polymerizations were also conducted at room temperature and 0C. Before each polymerization, conditions were tested with a platinum button electrode, glassy carbon button electrode a nd large platinum flag as the working electrode. Oligomers of PheDOT-Et2 were soluble in propylene carbonate, so studies were carried out in ACN, and various mixtures of DCM and toluene. Elect rochemical polymerizations were accomplished using cyclic voltammetry and depositing pol ymer potentiodynamically for 500-1000 scans, generally potentials ranging fr om -0.2 V to 1.2 V vs. Fc/Fc+. Polymer deposited on glassy carbon plate electrodes in ACN/TB AP, as seen in Figure 3-20, but formed a powdery layer

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59 instead of a glossy film, and could be removed from the electrode only using adhesive tape. Removal using adhesive tape yielded a powdery film on the tape, which has resistances in the mega-ohm range, most likely because the PheDOT-Et2 is not continuous. Films of similar quality were obtained using DCM, or DCM and to luene in a 1:1 mixture as the solvent. -0.40.00.40.81.2 -2 0 2 4 Current Density (mA/cm 2)Potential (V vs Fc/Fc +) Figure 3-20. PheDOT-Et2 deposition on 2 cm2 glassy carbon plate electr odes in ACN/TBAP at 0C. This figure shows up to twenty scans. Room Temperature Conductivity Measurements Solvent cho ice has presented a challenge in th e formation of large films using PheDOT-Et2 monomer. Similar to PEDOT, these compounds showed good electrochemical response in ACN on the platinum and glassy carbon small button st udies. However, ACN as a solvent for the larger glassy carbon electrode setup rendered the deposited po lymer far too powdery for any conductivity measurement. PC, which had yielde d such glossy, thick free-standing films for PEDOT studies, was not a viable system for PheDOT-Et2 monomer as the oligomers were soluble, even at low temperature.

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60 When material was successfully deposited on th e glassy carbon worki ng electrode, the film was dark indigo and could be removed using a dhesive tape, often with areas having visible irregularity. The most conductive films obtained were formed from a solution of 60 mM PheDOT-Et2/0.1 M DCM/TBAP. As in the PEDOT studies, the ClO4 counterion facilitated the formation of the most conductive films. However, PPheDOT-Et2 films were not free-standing and were able to be removed from the electrode su rface only using adhesive tape. In an effort to determine whether the alkyl chains contributed to the poor quality of the films, unsubstituted PheDOT monomer was also polymerized in identical conditions; the results were not significantly different from those of PheDOT-Et2. Table 3-2. Conductivity measurements for Ph eDOT and PheDOT-Et2 film formation experiments. Films were formed potentiodynamically with 800 scans of CV at 0C. Monomer Solvent Anion Film Quality (S/cm) PheDOT ACN PF6 -, ClO4 -, BF4 Not free-standing Not observed PheDOT PC PF6 -, ClO4 -, BF4 Oligomers soluble Not observed PheDOT DCM ClO4 Not free-standing 10-4 PheDOT Toluene/ DCM (1:1) ClO4 Not free-standing 10-5 PheDOT (Et)2 ACN PF6 -, ClO4 -, BF4 -, Li+BTI-, EMIBTI Not free-standing Not observed PheDOT (Et)2 PC PF6 -, ClO4 -, BF4 -, Li+BTI-, EMIBTI Oligomers soluble Not observed PheDOT (Et)2 DCM ClO4 Not free-standing 10-4 Phenylenedioxythiophene Dimer Studies Bi-PheDOT is known to undergo el ectrochem ical polymerization,53 and the increase of the chain length of the precursor thought to in crease the effective conjugation length and conductivity. The dimer and trimer of PheDOT have also been found to have lower monomer oxidation potentials than the PheDOT monomer.54

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61 Monomer Electrochemistry Figure 3-21 shows the deposition of 10 m M bi-PheDOT in 0.1 M TBAP/DCM on a platinum button electrode. The inset of the first scan shows monomer oxidation occurring at +0.65 V vs Fc/Fc+. In contrast to a similar first scan PheDOT-Et2, bi-PheDOT oxidizes at a lower potential. However, increasing scans indicated polymer deposition, with polymer oxidation and reduction yiel ding peak anodic and cathodic currents of 1 mA/cm2 after twenty scans. The first scan is irreversible, indi cating the bi-PheDOT deposits more rapidly than PheDOT-Et2 at similar scan rates. -0.6-0.4-0.20.00.20.40.60.8 1 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 Current Density (mA/cm2)Potential (V vs Fc/Fc+) Cycle 1 Cycle 5 Cycle 10 Cycle 15 Cycle 20 Figure 3-21. First through tw entieth CV scans of bi-PheDOT deposition (10mM biPheDOT/0.1M TBAP/DCM) with Vs = 50mV/s on a platinum button (A = 0.02 cm2). Film Formation Bi-PheDOT electrochem ical polymerizations were also conducted at room temperature and 0C. Before each polymerization, conditions we re tested with a platinum button electrode, glassy carbon button electrode and large platinum flag as the working electrode. Similar to what was observed with PheDOT-Et2, oligomers of bi-PheDOT were soluble in propylene carbonate,

PAGE 62

62 so studies were carried out in ACN, and various mixtures of DCM and toluene. Electrochemical polymerizations were accomplished using cy clic voltammetry and depositing polymer potentiodynamically for 500-1000 scans, generall y potentials ranging from -0.4 V to 0.8 V vs. Fc/Fc+. Polymer deposited on glassy carbon plate electrodes in 0.1M TBAP/ACN, but formed a powdery layer instead of a glossy film, and c ould be removed from the electrode only using adhesive tape. However, as shown in Fi gure 3-22, in 0.1M TBAP/DCM, 60 mM bi-PheDOT deposition led to the formation of free-standing small flakes of film. While the materials brittleness prevented removal in one 2 cm2 continuous piece (as for PEDOT), smaller 2-5 mm sections of film could be removed using a razo r blade. The material was too delicate to be removed from the electrode prio r to complete drying. As evid enced in the button electrode deposition, bi-PheDOT radical cations couple more rapidly than PheDOT-Et2 radical cations, which may contribute to the formation of highe r quality electrochemically deposited films. -0.40.00.40.8 -0.4 0.0 0.4 0.8 Current Density (mA/cm 2 )Potential (V vs. Fc/Fc+) Figure 3-22. CV showing bi-PheDOT deposite d potentiodynamically on a large GC plate electrode in 60 mMol bi-Phe DOT/0.1M TBAP/DCM (scan rate of 50 mV/s) at 0C. Current response increased dram atically after 800 scans as large amount of material had deposited.

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63 Room Temperature Conductivity Measurements Solvent cho ice continued to present a challenge in the formation of large films based on biPheDOT compounds. Similar to EDOT and PheDOT-Et2, biPheDOT showed good electrochemical response in ACN on the pla tinum and glassy carbon small button studies. However, ACN was not effective for polymer deposition on the larger glassy carbon electrode. Oligomers of bi-PheDOT were also solubl e in PC, even at low temperature. Table 3-3. Conductivity measurements for biPheDOT film formation experiments. Films were formed potentiodynamically with 800 scans of CV at 0C. Monomer Solvent Anion Film Quality (S/cm) Bi-PheDOT ACN ClO4 Not free-standing Not observed Bi-PheDOT PC ClO4 Oligomers soluble Not observed Bi-PheDOT DCM ClO4 Small intact flakes10 Bi-PheDOT DCM PF6 Small intact flakes10-4 When material was successfully deposited on th e glassy carbon worki ng electrode, the film was dark indigo and could be removed using adhesive tape, often with areas visible The most conductive films obtained were 10 S/cm and we re formed from a solution of 60 mM biPheDOT/0.1 M DCM/TBAP. As in the PEDOT studies, the ClO4 counterion facilitated the formation of the best quality and most highly conductive films. Conclusions The goal of the work assembled in this thes is has been to gain an understanding of the fundamental electrochemical properties and con ductivity of polymers based on electrochemically polymerized EDOT, PheDOT-Et2 and bi-PheDOT. To this end, the well-characterized EDOT monomer was studied electrochemically and used to optimize conditions for the formation of the most highly conductive films.

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64 Free-standing films of PEDOT were pr epared potentiodynamically in propylene carbonate using various el ectrolytes. The optimal conditions for free-standing films formation were determined to be at 0C. The most hi ghly conductive films of PEDO T were formed in an electrolyte-solvent combination of 0.1 M TBAP /PC. The conductivity of PEDOT deposited in these conditions maintained over half of its room temperature c onductivity value as the temperature approached 50 K. Fundamental electrochemical studies on pl atinum button electrodes indicated that PheDOT-Et2 can be polymerized on a platinum button electrode, with optimal current density response seen in a 0.1 M TBAP/ACN solvent-elec trolyte system. However, in all solvent systems studied, at scan rates of 50 mV/s, PheDOT-Et2 cyclic voltammograms indicated the cation radical formed upon mono mer oxidation was not coupling rapidly enough and returning to the neutral monomer state instead. Scan rate dependence studies of the first scan of polymer deposition confirmed that the cat ion radical only coupled at scan rates as low as 5-10 mV/s. PheDOT-Et2 oligomers were also soluble in PC, the solvent which, based upon EDOT studies, was most likely to yield glo ssy, free-standing films. Results from film formation experiments for PheDOT-Et2 echoed these challenges. While dark indigo film was successfully deposited on the glassy carbon work ing electrode, ev en at scan rates of 50 mV/s, likely solvent limitations did no t allow for the delamination of free-standing films. The most conductive material formed were from 0.1 M DCM/TBAP, but exhibited resistances high enough to lead to conductivities on the order of 10-5 to 10-4 S/cm. Much more encouraging were experiments carried out using bi-PheDOT, which exhibited electropolymerization results and redox switching of a system that was polymerizing more efficiently. As expected based on the compound s structures, in c ontrast to PheDOT-Et2, bi-

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65 PheDOT oxidized at a lower potential. The irreve rsibility of the first scan of polymerization indicated bi-PheDOT deposited more rapidly than PheDOT-Et2 at similar scan rates. Bi-PheDOT films formation experiments also yielded more promising results than PheDOT-Et2. While similar solvent challenges we re encountered, bi-PheDOT deposition in 0.1M TBAP/DCM led to the formation of free-sta nding small flakes of f ilm with co nductivities measured on the order of 10 S/cm. The lower oxidation potential of bi -PheDOT, sufficiently rapid cation radical coupling rate and encouraging results of f ilm formation experiments make this a promising system for continued study. Further investigation of the electrochemical preparati on of conductive films from PheDOT-based monomers should include contin ued optimization of solvents, electrolytes, polymerization conditions and post-polymerizatio n treatment. The comparison of bi-PheDOT functionalized with, for example, short-chain alkyl groups, could further the understanding into what features of the monomer are contribu ting to highly conductive films. Increased conductivities obtained over the course of th is study support continued development of conductive, free-standing electrochemically synthesized films based on phenylene dioxythiophene monomers.

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69 BIOGRAPHICAL SKETCH Laura was born in 1981 to Jim and Suzanne Moody, who taught elementary school in Bad Kreuznach, Germany. She spent her childhood in Augsburg, Germany, and attended school there until she was thirteen. Her high school ye ars were completed in Misawa, Japan, where she graduated in 1999 and moved, for the first time, to the U.S. and her mothers home state of North Carolina, to attend Duke University. She fini shed Duke with an undergraduate degree in chemistry in 2003 and a commission into the U.S. Ai r Force. Laura spent the ensuing four years in Californias Mojave Desert enjoying the near -constant sunshine at Edwards Air Force Base, where three years of research focused on developm ent and testing of materials for rocket motor casings and space survivable coatings. She was fortunate enough to spend her final year at Edwards working on the base commanders staff, until moving to Gainesville in August 2007 to join the Reynolds Group. Laura leaves UF fo r Kirtland AFB in Albuquerque, NM, to work for the Defense Threat Reduction Agency.